PROCESS OF FORMING A INFUSIBLE UNIDIRECTIONAL FABRIC
A process of forming infusible, unidirectional fabric containing the steps of arranging a plurality of unidirectional fibers into a unidirectional fabric, forming an emulsion or suspension of a solvent, a bridging polymer, and a film-forming preventing agent, applying the emulsion or suspension to the unidirectional fabric, removing the solvent, and removing the film-forming preventing agent to form an infusible, unidirectional fabric. The infusible, unidirectional fabric contains a plurality of unidirectional fibers, a plurality of bridges, and a plurality of void spaces between the unidirectional fibers. Each bridge is connected to at least 2 unidirectional fibers and at least 70% by number of fibers have at least one bridge connected thereto forming a bridged network of unidirectional fibers. The void spaces are interconnected and the fabric has a volume fraction of voids of between about 8 and 70% and a volume fraction of fibers of between about 35 and 85.
This application is a divisional application of co-pending U.S. application Ser. No. 14/085,095 filed on Nov. 20, 2013, which claims priority to U.S. provisional application 61/730,677, filed Nov. 28, 2012, the contents of both applications are incorporated herein by reference in their entirety.
FIELD OF THE INVENTIONThe present invention generally relates to processes for forming infusible, unidirectional fabrics.
BACKGROUNDThe development of more structurally efficient composite materials enables higher performance and more cost competitive solutions across a range of markets which use these materials. The traditional forms used for introducing dry fibers such as glass roving or carbon tow into a composite system are fabrics such as woven fabrics (having crimping) or multi-axial knit fabrics (with minimal crimping). These fabric forms typically impart performance penalties on the final composite system.
Composites reinforced with woven fabrics are known to exhibit lower modulus and strength due to the extensive fiber crimping which occurs as opposing direction fibers cross over each other. In the case of multi-axial knits, the layers of reinforcing fibers do not interpenetrate each other. The knitting process employs a stitch yarn which is looped around the reinforcing fibers tying the fibers together and providing stability to the fabric. The stitch yarn creates local deviations in yarn direction and imparts a subtle waviness along the fiber axis direction. The stitch yarns typically create a separation or gap between rovings or tows within a fabric and between layers of fabric while not offering any improvement in mechanical properties. Furthermore, the gaps created by the presence of the stitch yarns reduce the maximum achievable fiber volume fraction of a composite made with such reinforcement. Finally, the fiber waviness negatively impacts several structural properties of composites reinforced with such systems such as tensile modulus and compression strength.
There is an opportunity to develop infusible composite fabrics that offer high fiber volume fractions, a high degree of fiber alignment and straightness with excellent fiber distribution uniformity. These fabrics should be convertible into composite parts through common composite molding operations such as vacuum infusion or resin transfer molding. These characteristics enable superior structural properties while preserving the cost advantages of well-established resin infusion processing. A new approach for delivering a composite preform with these attributes is described.
BRIEF SUMMARYA process of forming infusible, unidirectional fabric containing the steps of arranging a plurality of unidirectional fibers into a unidirectional fabric, forming an emulsion or suspension of a solvent, a bridging polymer, and a film-forming preventing agent, applying the emulsion or suspension to the unidirectional fabric, removing the solvent, and removing the film-forming preventing agent to form an infusible, unidirectional fabric.
The infusible, unidirectional fabric contains a plurality of unidirectional fibers, a plurality of bridges, and a plurality of void spaces between the unidirectional fibers. Each bridge is connected to at least 2 unidirectional fibers and at least 70% by number of fibers have at least one bridge connected thereto forming a bridged network of unidirectional fibers.
The void spaces are interconnected and the fabric has a volume fraction of voids of between about 8 and 70%, a volume fraction of fibers of between about 35 and 85%, and at least 50% by number of the bridges have a bridge width minimum less than about 2 millimeters.
The unidirectional fabric may be any suitable width and in any suitable shape. In some embodiments where the width of the fabric is smaller, typically between about 2 and 300 mm, the fabric may be referred to as a unidirectional tape or fabric band.
Once the infusible, unidirectional fabric 10 is infused with resin and cured, an infused, unidirectional composite 400 illustrated in
A fabric with the above described structure will be infusible in vacuum assisted resin transfer molding (also called vacuum assisted resin infusion) process. The word “infusible”, in this invention, refers to fabrics having the following characteristics: The fabrics can be used to make fiber reinforced polymer composites having a thickness greater than 2 mm by using a standard vacuum assisted resin transfer molding (also called vacuum assisted resin infusion) method and low viscosity infusion grade thermoset resin. The infusion process has a typical processing time scale ranging from minutes to hours. Preferably, the finished composite with the infusible fabric typically has a void content as measured by a standard test such as ASTM D2734 of less than 5%, more preferably less than 2%.
A simple method to predict whether a fabric is infusible or not can be described as follows. Several water droplets with 0.01% water soluble color dye (for example, Acid Blue 9) are dropped on the surface of fabric by using a 5 mL transfer pipette. The time duration required for the droplets to completely infuse into the fabric is used as an indication of infusibility. By definition, in this method, “completely infuse into the fabric” means that more than 99% by mass of the water from the original droplet has been absorbed between the upper inner surface and lower inner surface of the fabric. By definition, a fabric is considered “infusible” in this invention if the average water droplet infusion time is less than 1 minute. This method is a method to indicate that the fabric is “infusible”, if the average water droplet infusion time is longer than 1 minute, it is not necessary to mean that the fabric is not resin infusible due to the hydrophobic nature of most thermoset resin. This test method may not be accurate if there is coating strong hydrophobic tendencies on the fabric.
Preferably, the infusible, unidirectional fabric 10 is self-supporting. “Self-supporting”, in this invention, means that the fabric is dimensionally stable, and the fibers in the fabric will not fall apart due to their own weight under gravity. The fabric has a well-defined width and thickness. Additional components may be attached to the fabric but are not required. Preferably, additional stabilizing means such as stitching, scrims, films, and the like are not needed to handle and convey the infusible, unidirectional fabric 10.
Within the infusible, unidirectional fabric 10, the void spaces are interconnected and the fabric has a void fraction of preferably between about 8 and 70%, more preferably between about 10 and 70%. The infusible, unidirectional self-supporting fabric preferably has a fiber volume fraction between 35% and 85%, preferably between 45% and 80%, more preferably between 50% and 80%. A fiber volume fraction less than about 30% may make the fiber reinforcement less practical as a composite reinforcement. A fiber volume fraction greater than 85% could have negative consequences as it may slow down the resin infusion process, reduce the mechanical properties perpendicular to the fiber direction, or reduce the fatigue durability of the composite. If the void spaces are not interconnected, there may be to too few channels for resin infusion. If there is not enough void content in the fabric, resin infusion may be very slow and difficult.
The fiber volume fraction can be measured using a first method (for fabric made by inorganic fibers) where one measures the total mass (m0), thickness (D), width (W), and length (L) of given piece of fabric, and then calculates the total volume (V0) of the given piece of fabric by V0=D×W×L. Next, the sample (piece of fabric) is placed in an oven, heated at 700° C. for 4 hours to burn off all organic content in the fabric, and the mass of inorganic component is measured (mass (mf)) after this burn off step. The fiber volume fraction (Vf %) is calculated by Vf %=(mf/ρf)/V0, where ρf is the density of the material which made the fiber. ρ can be measure by any suitable density measurement methods, or obtained from technical data sheet of the fiber material. The method works only when there is no or very small amount (less than 1%) other inorganic component such (for example, silica nanoparticles) in the fabric.
Another method to measure the void content in the fabric can be described as the following: use the infusible unidirectional fabric to make a fiber reinforced composite material by using vacuum assisted resin infusion method (detail description of this method is described in the Example section below) and do a SEM or optical imaging to a typical cross section of the composite, where the cross section is perpendicular to the fiber direction. The void content can be calculated by measuring the total cross section area of infused resin, divided by the total cross section area of the composite. To help identifying the infused resin area, about 0.01% to 0.1% by weight of color dye or fluorescent dye can be added into the resin before resin infusion.
The infusible, unidirectional self-supporting fabric also comprises polymer bridges, where the volume ratio of polymer bridges to fibers is between 1:370 and 1:2, more preferably between 1:40 and 1:4, more preferably between 1:12 and 1:4. The polymer bridges are a main source of support to the fabric structure and help prevent fibers fall apart due to gravity. The overall polymer bridge structure will not be strong enough to support the fabric structure if there are too few polymer bridges in the fabric. If there is too much polymer in the fabric, there may not be enough void space for resin infusion. The total volume of polymer bridges can be calculated by knowing how much mass (mp) polymer bridge material(s) have been added into the fabric during manufacturing, or using a burn off test (Described in the first method above for measuring fiber volume) to estimate the mass of polymer bridges by mp=(m0−mf). The volume of polymer bridges (Vp) is calculated by Vp=mp/ρp, where ρp is the density of polymer material.
The infusible, unidirectional fabric 10 (and composite 400) contains a bridging polymer which forms bridges 200 between and connected to at least a portion of the fibers 110. This is shown in both
Therefore, in a given cross-section, preferably between about 10 and 100% by number of fibers contain bridges to one or more fibers within the bridged network of unidirectional fibers 100 (composite 400). In another embodiment, between about 15 and 100% by number of fibers in a given cross-section contain bridges to one or more fibers, more preferably between about 50 and 100%, more preferably between about 60 and 100% more preferably between about 75 and 100% by number of fibers in a given cross-section.
Within the bridged network of unidirectional fibers 100, there are a plurality of bridges 200 between and connected to at least a portion of fibers 110. The bridging between fibers 110 helps control the position of the fibers 110 relative to other fibers and the fabric. The bridging attaches the fibers together and creates a stable fabric form. These bridges are connected and adhered to the surface of the fibers 110. A bridging polymer that extends between at least two fibers 110 but is not attached to at least two fibers 110 is not a bridge as defined in this application. The bridging increases the interaction between fibers 110 while still allowing resin to flow between and around the fibers 110. The bridging polymer preferably has an elasticity which is characterized as elongation at break at least about 50%, more preferably higher than 100%, and more preferably higher than 300%. The elasticity of the bridges helps the fabric remain flexible (able to conform to curved mold shapes) and helps the bridges survive bending or folding of the fabric.
The bridging polymer may be physically or chemically bonded (through there may be in some embodiments a thin layer between anchoring surface and fiber surface, for example, a coating layer or sizing) to the surface of the fiber 110 through interactions including but not limited to hydrogen bonding, van der Waals interactions, ionic interactions, electrostatic interactions, mechanical interlocking, or a portion of the anchoring surface may chemically react with the surface of the fiber 110 to form covalent bonds between the fiber and the anchoring surface. The anchoring surface may be physically or chemically bonded to a coating or sizing that was previously applied to the fiber, through interactions including hydrogen bonding, van der Waals interactions, ionic interactions, electrostatic interactions, or a portion of the anchoring surface may chemically react with the coating or sizing on the surface of the fiber to form covalent bonds between the coating or sizing on the fiber surface and the anchoring surface. If the fiber or coating or sizing on the fiber is porous or if the precursors to the bridge can diffuse or penetrate into the surface of the fiber, then the anchoring surface may interpenetrate with the fiber surface on a nanometer or micrometer length scale. It is important that the bridging polymer has good adhesion to fiber surface, because all the fibers in the unidirectional fabric structure are held together by the bridges.
In one embodiment, at least a number of the bridges contain a width gradient, where the width of the bridge is greatest at the anchoring surface and decreases in a gradient away from the anchoring surface. The greater width at the anchoring surface helps increase the strength of the adhesion between the bridge and the fiber, and a narrower width away from the anchoring surface leaves more void space in the fabric 10 for resin infusion. An optimized system is preferred which has sufficient strength for maintaining fabric integrity during handling while minimizing the time required to infuse the structure with resin.
Additionally, preferably at least 50% of the bridges have a bridge width minimum narrower than 2 mm, more preferably narrower than 0.5 mm, more preferably narrower than 0.2 mm. The bridge width minimum is defined as the minimum width of the bridge (in the direction of fiber length) from surface of the first fiber to the surface of the second connected fiber. In one embodiment, the bridges typically have approximately the same width along the fiber direction from the surface of one fiber to the connected fiber. In this case, the bridge width is approximately constant in the bridge. In another embodiment, the bridge is wider where the bridge attaches to the fibers and is the narrowest (and has the minimum width) between the two fibers.
The width of the bridges in fiber direction can be measured by optical microscopic image or SEM image. In this measurement, dry fabric (before resin infusion) is preferred to be used to take images. The images are taken from the cross section which is parallel to fiber direction.
In one embodiment, the bridges 200 preferably form between about 0.1 and 60% of the effective cross-sectional area of the infusible, unidirectional fabric 10 (and infused, unidirectional composite 400). In another embodiment, the bridges 200 form between about 0.1 and 30% of the effective cross-sectional area of the fabric and composite, more preferably between about 0.3% and 10%, more preferably between about 0.5% and 5%. “Effective cross-sectional area”, in this application, is measured by taking a cross-sectional image of the fabric and calculating the area of bridge. If the cross-sectional area of bridges is less than about 0.1%, there may not be enough bridges to enhance the mechanical properties of the composite. If the cross-sectional area of bridges is larger than 30%, there may not be enough porosity in the fabric for resin infusion leading to lower performance due to dry spots or voids in the composite systems.
Where bridging occurs in the fabric 10 depends on a number of factors including but not limited to the type of bridging polymer, solvent, film forming preventing agent, surface chemistry of fiber, separation distance between fibers, coating process conditions, drying conditions, post mechanical treatment during and after drying. The time required for bridging to occur also depends on concentration of bridging polymer, concentration of co-stabilizer, concentration of surfactant, surface chemistry of fiber, initial size of dispersed phase in the emulsion, temperature, solidification time of the bridging polymer, separation distance between adjacent fibers, and coating process conditions,
In one embodiment, the bridging polymer forms between about 1% and 20% by weight of the infusible, unidirectional fabric. In another embodiment, the cross sectional area of the fibers is between 30% and 80% of the total cross sectional area of the fabric, and the ratio by cross sectional area of polymer:void is between 1:0.5 and 1:93.
The anchoring surfaces of bridges cover less than 100% of the fiber surfaces (this includes all of the surface area of the fiber). The uncovered fiber surfaces can bond to the resin directly in composites and increase the interaction between fibers and infused resin in composite. In one embodiment, the anchoring surfaces of bridges cover about 10% to 99% of the fiber surface. Preferably the anchoring surfaces of bridges cover about 30% to 90% of the fiber surface.
The bridges in the infusible, unidirectional fabric are formed from a bridging polymer including but not limited to thermoset resin, thermoplastic resin, ionomer, dendrimer, and mixtures thereof. Thermoset resins, such as epoxy, polyurethane, acrylic resin, rubbers, and phenolic, are liquid resins which harden by a process of chemical curing, or cross-linking, which takes place during the coating process. Thermoplastic resins, such as polyethylene, polypropylene, polyolefin copolymer elastomer, thermoplastic polyurethane, polyvinyl alcohol (PVA), PET and PEEK, are liquefied by the application of heat prior to coating and re-harden as they cool within the fabric. Preferably, the bridging polymer has good adhesion on fiber surface. Preferably, the bridging polymer (or the polymer in organic solvent solution, or the chemicals that form polymer during process) can be uniformly dispersed in water before coating. In one embodiment, the bridging polymer is ethylene vinyl acetate (EVA) copolymer, styrene butadiene rubber (SBR), water borne polyurethane, polyolefin elastomer (POE), or a mixture thereof. SBR and polyurethane are preferred due to its moderate cost, good mechanical properties, and good adhesion to fibers.
In one embodiment, the polymer bridges are formed beginning with a polymer in water dispersion or polymer water solution. SBR latex or water borne polyurethane are preferred due to its moderate cost, good mechanical properties, good adhesion to fibers. Film-forming preventing agents are preferred to be added in to the polymer water dispersion or polymer water solution, because the film-forming preventing agents can create void space and channels between fibers by preventing the polymer forming continuous film.
In one embodiment, the film-forming preventing agent is solid or liquid particles which can be dispersed or dissolved in the polymer in water dispersion or polymer water solution. This type of film forming preventing agent will be removed from the fabric after the polymer solidified. Silica particles are one of the examples. In one embodiment, the film-forming preventing agent is water soluble material, which can phase separate from the polymer and form continuous phase during water evaporation. One requirement of the water soluble materials is that they don't make the polymer in water dispersion or polymer water solution unstable. In one embodiment, sugar or other water soluble non-ionic materials are preferred. In another preferred embodiment, glycerin or propylene carbonate is used as a film forming preventing agent to create the void space. After water evaporation and polymer solidified, the film forming preventing agent rich phase will be removed from the fabric, leaving voids and channels in the fabric.
In one embodiment, the film-forming preventing agents are a combination of blowing agents, and frothing agents or foaming agents. The blowing agent can be any suitable material that can create bubbles during coating process. In one embodiment, the blowing agent is water. Water can quickly evaporate under heat and creates bubbles. In another embodiment, the blowing agent is carbon dioxide that has dissolved in water. In another embodiment, the blowing agent is low boiling point organic liquid. In another embodiment, the blowing agent can chemically decompose and release gas under heat. This type of blowing agent includes but not limit to NaHCO3, azodicarbonamide, and p-p′-oxbis(benzenesulfonyl hydrazide). The frothing agents or foaming agents include but not limited to ionic surfactant such as sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate (NaDDBS), or non-ionic block copolymer such as ethylene oxide and propylene oxide copolymer. One example of the block copolymer is Pluronic® from BASF. A gelling agent is also preferred to be added to stabilize the polymer foam. The gelling agent includes but not limited to acacia, alginic acid, bentonite, carbomers, carboxymethylcellulose. ethylcellulose, gelatin, hydroxyethylcellulose, hydroxypropyl cellulose, magnesium aluminum silicate (Veegum®), methylcellulose, Pluronics®, polyvinyl alcohol, sodium alginate, tragacanth, and xanthan gum. A gelling agent with lower critical solution temperature (LCST) is preferred because it is soluble in cold water and gels in hot water. One example of the gelling agent with LCST characteristic is Pluronics® F-127.
In one embodiment, sugar is used as the film-forming preventing agent. The polymer solid content is between about 1% and 60% of the polymer in water dispersion or polymer in water solution. More preferably the polymer solid content is between about 3% and 20%. The sugar to polymer solid content ratio by weight is between about 0.5:1 and 10:1, more preferably between 1:1 and 5:1. Too little sugar cannot prevent polymer forming films and cannot create enough void space and channels in the fabric; Too much sugar makes the polymer bridges weak.
In one embodiment, glycerin is used as the film-forming preventing agent. The polymer solid content is between about 1% and 60% of the polymer in water dispersion or polymer in water solution. More preferably the polymer solid content is between about 3% and 20%. The glycerin to polymer solid content ratio by weight is between about 0.5:1 and 20:1, more preferably between 1:1 and 10:1. Too little glycerin cannot prevent polymer forming films and cannot create enough void space and channels in the fabric; too much glycerin makes polymer dispersion unstable and the polymer bridges are weak.
In another embodiment, foaming agents and gelling agents are used as the film-forming preventing agent. The polymer solid content is between about 1% and 60% of the polymer in water dispersion or polymer in water solution. More preferably the polymer solid content is between about 3% and 20%. The frothing agent is between about 0.1% and 20% of the total weight, more preferably between about 1% and 10% of the total weight. The gelling agent is between about 0.1% and 40% of the total weight, more preferably between 1% and 10% of the total weight. In one embodiment, Pluronics® F-127 is used as a foaming agent and also a gelling agent, preferably between 1% and 15% by weight in the coating mixture, more preferably between 3% and 10% by weight in the coating mixture.
In one embodiment, the bridging polymer and the resin 300 have different chemical compositions. Having a different chemical composition, in this application, means that materials having a different molecular composition or having the same chemicals at different ratios or concentrations. Having different chemical compositions may be able to help redistribute stress in composites. In another embodiment, the bridging polymer and the resin 300 have the same chemical compositions. Having the same compositions may make the infusing resin wet the fabric more easily.
The bridged network of unidirectional fibers 100 may be any suitable fibers for the end product. “Unidirectional fibers”, in this application means that the majority of fibers aligned in one direction with the axis along the length of the fibers being generally parallel. The composite 400 may contain a plurality of fibers in a bundle (the bundles may be part of a textile layer including but not limited to a woven textile, non-woven textile (such as a chopped strand mat), bonded textile, knit textile, a unidirectional textile, and a sheet of strands.) In one embodiment, the bridged network of unidirectional fibers 100 are formed into unidirectional strands such as rovings and may be held together by bonding, knitting a securing yarn across the rovings, or weaving a securing yarn across the rovings. In the case of woven, knit, warp knit/weft insertion, non-woven, or bonded the textile can have fibers that are disposed in a multi- (bi- or tri- or quadri-) axial direction. In one embodiment, the bridged network of unidirectional fibers 100 contains an average of at least about 2 fibers, more preferably at least about 20 fibers. The fibers 110 within the fabric 10 generally are aligned and parallel, meaning that the axes along the lengths of the fibers 110 are generally aligned and parallel. Each fiber has a fiber surface defined to be the outer surface of the fiber and a fiber diameter.
Preferably, the infusible, unidirectional fabric 10 contains unidirectional fibers 110 that are spaced uniformly in the unidirectional fabric 10. “Spaced uniformly” or “uniformly spaced”, in this application, means that in a typical fabric cross section, within the bridged network of unidirectional fibers, there is no clear boundary of any fiber bundle, yarn, roving, or tow.
For the purpose of this invention, fiber distribution uniformity can be measured by the following method. A typical cross section image of the unidirectional fabric or composite made thereof is prepared by standard microscopy mounting and imaging techniques. Unidirectional fabrics are typically encapsulated in a polymer such as mounting epoxy and cut with a diamond wafer saw orthogonal to the fiber direction through the sample. Composites can often be sectioned without requiring mounting because the fibers are already stabilized by the composite matrix polymer.
After sectioning, the surface of the cross section to be viewed is ground and polished to enable unobstructed viewing of the sample through optical or electron microscopy. The polishing process is repeated until the contrast between fiber and matrix in the images at the target resolution is sufficient to compute the fiber area fraction within the cross section. The perimeter of each fiber should be clearly distinguishable.
For measuring fiber distribution uniformity via the fiber area ratio method described herein, the image must be of a sufficient size scale to encompass the entire thickness of at least one layer of the fabric. An example image of a composite reinforced with two layers of unidirectional fabric, 501 and 501, comprising glass fibers is shown in
Within the unidirectional region of the cross section image located between the upper inner surface and the lower inner surface, a grid of squares is overlaid onto the image, 510. The grid contains a square pattern of non-overlapping connected squares which share edges and corners, 520. Each square in the grid has sides of length tb/2. The image must be of sufficient size to contain at least four such squares. The number of grid squares should be the maximum possible within the cross section area of the fabric where each sub-region remains fully within the fabric cross section. Each area of the image within the borders of each square, 521, is defined as a sub-region.
For each sub-region, the fiber area fraction is computed. The fiber area fraction for a sub-region is the ratio of the area within the sub-region that is occupied by fiber divided by the total area of the sub-region. This calculation is readily done by standard image processing algorithms based on the image contrast or color difference between the fiber and the matrix region.
After all sub-regions of the typical cross section image have been analyzed, the overall average fiber area ratio can be computed. A uniform distribution is defined as one in which at least 85% of the sub-regions have a fiber area ratio value that falls within the range defined by ±15% of the overall average fiber area ratio. More preferably the distribution is characterized by at least 95% of the sub-regions having a fiber area ratio value within the range defined by ±15% of the overall average fiber area ratio. Most preferably, the distribution is characterized by at least 98% of the sub-regions having a fiber area ratio value within the range defined by ±15% of the overall average fiber area ratio.
In some embodiments, a composite contains more than one fabric or a group of fabrics. The same definition of “uniform distribution” can be applied across cross section images containing regions of more than one unidirectional fabric. A grid as described above is created within one layer of the reinforcement, and then extended to encompass the entire unidirectional region of the composite less any residual area that does not fit within a full square defined by the grid. Fiber area ratios are computed within each sub-region. After all sub-regions of the typical cross section image have been analyzed, the overall average fiber area ratio can be computed. A uniform distribution is defined as one in which at least 85% of the sub-regions have a fiber area ratio value that falls within the range defined by ±15% of the overall average fiber area ratio. More preferably the distribution is characterized by at least 95% of the sub-regions having a fiber area ratio value within the range defined by ±15% of the overall average fiber area ratio. Most preferably, the distribution is characterized by at least 98% of the sub-regions having a fiber area ratio value within the range defined by ±15% of the overall average fiber area ratio.
A composite comprising multiple layers of conventional unidirectional fabrics may not be considered to have a uniform fiber distribution if the gaps created between the unidirectional yarns or rovings or tows within the fabric or the gaps created between the layers of fabric are large enough to prohibit satisfying the criteria requiring at least 85% of the sub-regions have a fiber area ratio value that falls within the range defined by ±15% of the overall average fiber area ratio.
In this definition, a fabric having yarns or threads woven into the unidirectional fibers in the direction perpendicular to the unidirectional fibers would fall under the definition of spaced uniformly as typically the gap between rovings or bundles are about 4 times of the fiber diameter. If a typical bundle of rovings is used, then the unidirectional fibers are grouped into bundles where the fibers in those bundles are held closer together and there is typically a space between bundles where little to no fibers reside. Preferably, there are no additional fibers or yarns holding the unidirectional fibers together.
The strength and free-standing nature of the bridged network of unidirectional fibers 100 is due mostly to the bridges 200. Preferably, the bridged fibers (containing no additional reinforcements besides the bridges) have enough tensile strength to be handled in a manufacturing process without any additional reinforcement fabrics or layers. In another embodiment, the bridged fibers have a tensile strength of at least 200 Pa in the direction perpendicular to the length direction of the unidirectional fibers. In another embodiment, the bridged fibers have a tensile strength of at least 700 Pa, more preferably higher than 10 kPa in the direction perpendicular to the length direction of the unidirectional fibers. The tensile strength of the fabric is measured by gripping two ends of a rectangular piece fabric in a tensile strength test machine (for example, Instron), while the tensile test direction is perpendicular to the unidirectional fiber direction. The fabric is then stretched under a constant speed (typically about 1˜10 cm/minute). The tensile strength is calculate by measuring the maximum tensile force before fabric is broken, divided by the area of cross section of the fabric. In another embodiment, the fabric does not suffer significant structural damage under a peel strength of 0.25 lbf/inch (0.44 N/cm) in a peel strength test between the fabric and an adhesive tape. In this test, a piece of adhesive tape about 6˜10 inch long is adhered to the fabric surface in the fiber direction at room temperature, and the peel strength between the tape and fabric is tested. The details of peel strength test can be found in ASTM D5170. “Not suffer significant structural damage”, in this invention, means that most fibers are still keeping their relative position in the fabric during peel strength test, and after the peel strength test, less than 20 fibers, preferably less than 10 fibers, more preferably zero fiber, are sticking on the adhesive tape. The fibers (if there is any) which are sticking on the adhesive tape are originally located on the surface of the fabric. This means that the interface between the tape and the fabric failed before the fabric cohesively failed. In one embodiment, the fabric contains no additional stitching fibers, reinforcement layers, or reinforcement fabrics such as stitching yarns or scrims. Thus, the infusible unidirectional fabric has enough strength to be used as a stand-alone fabric, for example allowing the fabric to be placed in the mold before infusion with resin. Because additional stitching fibers, reinforcement layers, or reinforcement fabrics usually creates gap or space with very few fibers, as a result, the fibers may not be spaced uniformly.
The fibers 110 may be any suitable fiber for the end use. “Fiber” used herein is defined as an elongated body and includes yarns, tape elements, and the like. The fiber may have any suitable cross-section such as circular, multi-lobal, square or rectangular (tape), and oval. The fibers may be monofilament or multifilament, staple or continuous, or a mixture thereof. Preferably, the fibers have a circular cross-section which due to packing limitations intrinsically provides the void space needed to host the bridges. The fibers 110 can have an average length of at least about 3 millimeters. In another embodiment, the fiber length is at least about 100 times the fiber diameter. In another embodiment, the average fiber length is at least about 10 centimeters. In another embodiment, the average fiber length is at least about 1 meter. Preferably, the fibers are continuous. The fiber lengths can be sampled from a normal distribution or from a bi-, tri- or multi-modal distribution depending on how the fabrics are constructed. The average lengths of fibers in each mode of the distribution can be selected from any of the fiber length ranges given in the above embodiments.
The fibers 110 can be formed from any type of fiberizable material known to those skilled in the art including fiberizable inorganic materials, fiberizable organic materials and mixtures of any of the foregoing. The inorganic and organic materials can be either man-made or naturally occurring materials. One skilled in the art will appreciate that the fiberizable inorganic and organic materials can also be polymeric materials. As used herein, the term “polymeric material” means a material formed from macromolecules composed of long chains of atoms that are linked together and that can become entangled in solution or in the solid state. As used herein, the term “fiberizable” means a material capable of being formed into a generally continuous or staple filament, fiber, strand or yarn. In one embodiment, the fibers 110 are selected from the group consisting of carbon, glass, aramid, boron, polyalkylene, quartz, polybenzimidazole, polyetheretherketone, basalt, polyphenylene sulfide, poly p-phenylene benzobisoaxazole, silicon carbide, phenolformaldehyde, phthalate and napthenoate, polyethylene. In another embodiment, the fibers are metal fibers such as steel, aluminum, or copper.
Preferably, the fibers 110 are formed from an inorganic, fiberizable glass material. Fiberizable glass materials useful in the present invention include but are not limited to those prepared from fiberizable glass compositions such as S glass, S2 glass, E glass, R glass, H glass, A glass, AR glass, C glass, D glass, ECR glass, glass filament, staple glass, T glass and zirconium oxide glass, and E-glass derivatives. As used herein, “E-glass derivatives” means glass compositions that include minor amounts of fluorine and/or boron and most preferably are fluorine-free and/or boron-free. Furthermore, as used herein, “minor amounts of fluorine” means less than 0.5 weight percent fluorine, preferably less than 0.1 weight percent fluorine, and “minor amounts of boron” means less than 5 weight percent boron, preferably less than 2 weight percent boron. Basalt and mineral wool are examples of other fiberizable glass materials useful in the present invention. Preferred glass fibers are formed from E-glass or E-glass derivatives.
The glass fibers of the present invention can be formed in any suitable method known in the art, for forming glass fibers. For example, glass fibers can be formed in a direct-melt fiber forming operation or in an indirect, or marble-melt, fiber forming operation. In a direct-melt fiber forming operation, raw materials are combined, melted and homogenized in a glass melting furnace. The molten glass moves from the furnace to a forehearth and into fiber forming apparatuses where the molten glass is attenuated into continuous glass fibers. In a marble-melt glass forming operation, pieces or marbles of glass having the final desired glass composition are preformed and fed into a bushing where they are melted and attenuated into continuous glass fibers. If a pre-melter is used, the marbles are fed first into the pre-melter, melted, and then the melted glass is fed into a fiber forming apparatus where the glass is attenuated to form continuous fibers. In the present invention, the glass fibers are preferably formed by the direct-melt fiber forming operation.
In one embodiment, when the fibers 110 are glass fibers, the fibers contain a sizing. This sizing may facilitate processing of the glass fibers into textile layers and enhances fiber-polymer matrix interaction. In another embodiment, the fibers 110 being glass fibers do not contain a sizing. The non-sizing surface may help to simplify the coating process and give better control of polymer-fiber interaction. Fiberglass fibers typically have diameters in the range of between about 10-35 microns and more typically 17-19 microns. Carbon fibers typically have diameters in the range of between about 5-10 microns and typically 7 microns, the fibers (fiberglass and carbon) are not limited to these ranges.
Non-limiting examples of suitable non-glass fiberizable inorganic materials include ceramic materials such as silicon carbide, carbon, graphite, mullite, basalt, aluminum oxide and piezoelectric ceramic materials. Non-limiting examples of suitable fiberizable organic materials include cotton, cellulose, natural rubber, flax, ramie, hemp, sisal and wool. Non-limiting examples of suitable fiberizable organic polymeric materials include those formed from polyamides (such as nylon and aramids), thermoplastic polyesters (such as polyethylene terephthalate and polybutylene terephthalate), acrylics (such as polyacrylonitriles), polyolefins, polyurethanes and vinyl polymers (such as polyvinyl alcohol).
In one embodiment, the fibers 110 preferably have a high strength to weight ratio. Preferably, the fibers 110 have strength to weight ratio of at least 0.7 GPa/g/cm3 as measured by standard fiber properties at 23° C. and a modulus of at least 69 GPa.
Textiles or other assemblies of the infusible, unidirectional fabric 10 can be further processed to create composite preforms. One example would be to wrap the fabric 10 around foam strips or other shapes to create three dimensional structures. These intermediate structures can then be formed into composite structures 400 by the addition of resin in at least a portion of the void space 120 in the fabric 10.
The infusible, unidirectional fabric 10 can be further processed into an infused, unidirectional composite 400 as illustrated in
The infusible, unidirectional fabric 10 is impregnated or infused with a resin 300 which flows, preferably under differential pressure, through the fabric 10 at least partially filling the void space creating the infused, unidirectional composite 400. The infused, unidirectional composite 400 could also be created by other wetting or composite laminating processes including but not limited to hand lay-up, filament winding, and pultrusion. Preferably, the resin flows throughout the infusible, unidirectional fabric 10 (and all of the other reinforcing materials such as reinforcing sheets, skins, optional stabilizing layers, and strips) and cures to form a rigid, composite 400.
It is within the scope of the present invention to any type of hardenable resin to infuse or impregnate the porous and fibrous reinforcements of the cores and skins. Thermoset resins, such as unsaturated polyester, vinyl ester, epoxy, polyurethane, acrylic resin, and phenolic, are liquid resins which harden by a process of chemical curing, or cross-linking, which takes place during the molding process. Thermoplastic resins, such as polyethylene, polypropylene, PET and PEEK, are liquefied by the application of heat prior to infusing the reinforcements and re-harden as they cool within the panel. In one embodiment, the resin 300 is an unsaturated polyester, a vinylester, an epoxy resin, a polyurethane resin, a bismaleimide resin, a phenol resin, a melamine resin, a silicone resin, or thermoplastic PBT or Nylon or mixtures thereof. Unsaturated polyester and epoxy are preferred due to their moderate cost, good mechanical properties, good working time, and cure characteristics.
In some commercial applications, the epoxy based resins have higher performance (fatigue, tensile strength and strain at failure) than polyester based resins, but also have a higher cost. The uniformly spaced fibers in the fabric 10 may increase the performance of a composite 400 using an unsaturated polyester resin to levels similar to the performance levels of the epoxy resin composite, but with a lower cost than the epoxy resin system.
Having the resin 300 flow throughout the infusible, unidirectional fabric 10 under differential pressure may be accomplished by processes such as vacuum bag molding, resin transfer molding or vacuum assisted resin transfer molding (VARTM). In VARTM molding, the components of the composite are sealed in an airtight mold commonly having one flexible mold face, and air is evacuated from the mold, which applies atmospheric pressure through the flexible face to conform the composite 400 to the mold. Catalyzed resin is drawn by the vacuum into the mold, generally through a resin distribution medium or network of channels provided on the surface of the panel, and is allowed to cure. Additional fibers or layers such as surface flow media can also be added to the composite to help facilitate the infusion of resin. A series of thick yarns such as heavy rovings or monofilaments can be spaced equally apart in one or more axis of the reinforcement to tune the resin infusion rate of the composite.
As an alternate to infusion of the infusible, unidirectional fabric 10 with liquid resin, the fabric may be further pre-impregnated (pre-pregged) with partially cured thermoset resins, thermoplastic resins, or intermingled with thermoplastic fibers which are subsequently cured (or melted and solidified) by the application of heat.
The infused, unidirectional composite 400 may be used as a structure or the composite 400 have additional processes performed to it or have additional elements added to form it into a structure. It may also be bonded to other materials to create a structure including incorporation into a sandwich panel. In one embodiment, skin sheet materials such as steel, aluminum, plywood or fiberglass reinforced polymer may be added to a surface of the composite 400. This may be achieved by adding the additional reinforcement layers while the resin cures or by adhesives. Examples of structures the composite may be (or be part of) include but are not limited to wind turbine blades, boat hulls and decks, rail cars, bridge decks, pipe, tanks, reinforced truck floors, pilings, fenders, docks, reinforced beams, retrofitted concrete structures, aircraft structures, reinforced extrusions or injection moldings or other like structural parts. In many of the above mentioned structures, fatigue life is an important consideration. The infused, unidirectional composite 400 may improve the fatigue performance of these structural parts.
Composites incorporating a bridged network of unidirectional fibers 100 can realize higher fiber volume fractions compared to those made with conventional reinforcements. Higher fiber volume fractions increase the modulus and strength of the composites, particularly in the direction of the fiber axis. The uniformity of fiber distribution and lack of fiber crimp due to stitching or off-axis fibers enables higher compression strength and enhanced fatigue durability. Composites with these characteristics are also resistant to delamination and therefore provide significant damage tolerance. These benefits can allow for longer, lighter, more durable and/or lower cost structures in numerous applications including wind turbine blades.
One benefit of the fabric with infusible uniformly spaced fibers is the opportunity to utilize the fabric in specific subsections of the structure where the demonstrated performance benefit is most applicable.
Wind turbine blades are an example of a large composite structure that can benefit from use of infusible, unidirectional fabrics in specific areas. The loading patterns on wind turbine blades are complex, and the structure is designed to satisfy a range of load requirements. For example, wind turbine blades are designed using at least four different design criteria. The blade must be stiff enough to not strike the turbine tower, strong enough to withstand the maximum expected wind gust loads, durable enough to tolerate hundreds of millions of cycles due to the rotation of the generator, and sufficiently resistant to buckling to avoid collapsing when flexed under the combined stress induced by the blade itself and the wind loads.
Sectional view A-A in
The blade shells generally consist of one or more reinforcing layers 1816 and may include core materials 1818 between the reinforcing layers for increased stiffness.
During the wind turbine blade design process, different sections of the structure are optimized based on the most critical design criteria for that section. For example, in blades using fiberglass reinforced spar caps, the size of the spar caps can be based on the stiffness requirements to avoid hitting the turbine tower or the fatigue requirements over which the spar cap can be expected to remain intact over hundreds of millions of load cycles. The nature of the design process and the requirements imposed on the various sections of the blade can benefit from materials which offer the opportunity to be deployed locally within that section. A spar cap reinforcement material with improved fatigue resistance could allow more optimized wind turbine blades when fatigue performance dictates the size and weight of the spar caps.
The infusible, unidirectional fabric 10 may be formed by any suitable manufacturing method. One method to form the infusible, unidirectional fabric begins with forming the fabric, or fiber tows. The fabric contains a plurality of fibers and void space between the fibers. Preferably the fabric then goes through one or multiple fiber tow spreading devices, which spread a fiber bundle into a fabric sometimes in the form of a fiber tape or fiber band. This step can break the binder which has already existed in the fiber bundle and re-distribute fiber space more uniformly. The tow spreading device can be any suitable design. In one preferred embodiment the tow spreading device(s) comprising several football-shaped rolls, and the fabric is spread when it is pushed against the football shaped rolls. In another embodiment, the fabric is spread by blowing air to the bundle. In another embodiment, the fabric is spread by immersing into water and nipped under pressure.
After spreading, preferably the fabric is then combined with other spread bundles of fibers in the fiber direction to form a heavier or wider unidirectional fiber tape, fiber sheet, fiber band, or fabric. In one embodiment, two 9600 Tex (Tex is a unit of measure for the linear mass density of fibers and is defined as the mass in grams per 1000 meters) bundles of fibers are spread independently and then combined together to form a 25.4 mm wide fabric or tape. In one embodiment, eight 9600 Tex bundles of fibers are combined together to form an approximately 500 gsm, 150 mm wide fabric. In another embodiment, multiple 9600 Tex bundles of fibers are combined together to form an approximately 1000 gsm, 400 mm wide fabric. In another embodiment, multiple 4800 Tex bundles of fibers are combined together to form the unidirectional fabric.
The fabric (in the form of a fiber tape, fiber band or fabric) is then coated with a coating liquid that contains the bridge polymer or the chemicals that can react and make the bridge polymer. In one embodiment, the polymer bridges are formed beginning with a polymer in water dispersion or polymer water solution. Preferably the polymer in water dispersion is an emulsion. The emulsion contains both a continuous solvent phase and a discontinuous dispersed liquid phase. The two phases are chosen so that the discontinuous dispersed phase is sufficiently stable that it does not agglomerate or solidify on the time scale required for emulsion preparation and coating at typical emulsion preparation and coating temperatures. This typically requires the resin to be stable for a period of at least several minutes. SBR latex or water borne polyurethane are preferred due to their moderate cost, good mechanical properties, good adhesion to fibers. In one embodiment, the average size of the particles in the dispersed phase (called dispersed particles or micelles or referred to as the discontinuous phase) in the emulsion is less than 50 μm, preferably less than 10 μm. These dispersed particles make up at least about 0.5% by weight of the emulsion, more preferably at least about 1% by weight, more preferably at least about 3% by weight. In another embodiment, the emulsion contains between about 3 and 10% by weight of dispersed particles. The continuous phase of the emulsion can contain an aqueous, a non-aqueous liquid, or a mixture of both. Preferably the solvent is aqueous or polar because of the cost and environmental concerns, wettability of the fiber, flammability issues and ability to create an emulsion with the dispersed phase. The solvent may also contain a surfactant, which may improve the stability of the dispersed phase after emulsification or may make emulsification a more reliable and efficient process.
In one embodiment, it is preferred to add film-forming preventing agents in to the polymer water dispersion or polymer water solution, because the film-forming preventing agents are able to create void space and channels between fibers by preventing the polymer forming a continuous film.
In one preferred embodiment, the film-forming preventing agent is a water soluble material, which can phase separate from the polymer and form solid or liquid phase during water evaporation. Preferably, the water soluble materials do not make the polymer in water dispersion or polymer water solution unstable. Sugar (solid or in liquid form) or other water soluble non-ionic materials are preferred. After water evaporation and polymer solidified, this type of film forming preventing agent will typically be removed from the fabric, leaving voids and channels in the fabric.
In one embodiment, sugar is used as the film-forming preventing agent. In this embodiment, the polymer solid content is between about 1% and 60% of the polymer in water dispersion or polymer in water solution. More preferably the polymer solid content is between about 3% and 20%. The sugar to polymer solid content ratio by weight is between about 0.5:1 and 10:1, more preferably between 1:1 and 5:1. Too little sugar may not prevent the polymer from forming films and may not create enough void space and channels in the fabric; too much sugar may make the polymer bridges weak.
In another embodiment, glycerin is used as the film-forming preventing agent. In this embodiment, the polymer solid content is between about 1% and 60% of the polymer in water dispersion or polymer in water solution. More preferably the polymer solid content is between about 3% and 20%. The glycerin to polymer solid content ratio by weight is between about 0.5:1 and 20:1, more preferably between 1:1 and 10:1. Too little glycerin may not prevent the polymer from forming films and may not create enough void space and channels in the fabric; too much may make the polymer bridges weak.
The polymer in water dispersion or polymer in water solution may be applied to the fiber bundles by any suitable coating method that results in the coating liquid filling the void spaces between the fibers and wetting the surface of the fibers. The fiber tape, fiber band or fabric is then treated to cause solidification of the bridge polymer and forming phase separation between bridge polymer and this type of film forming preventing agent. The bridge polymer chemical(s) can solidify by undergoing chemical reaction, cooling below its(their) melt point, precipitating, crystallizing, or evaporation of a portion of the mixture. In one preferred embodiment, this phase change occurs because of evaporation of water. In another preferred embodiment, this phase change occurs because of a chemical reaction, such as polymerization or crosslinking of mixture that may contain monomers, oligomers, cross-linkers, and initiators; these are commonly available as thermosetting resins that are paired with either a hardener or initiator. The liquid may also contain catalysts which may affect the rate of solidification of the polymer. It may also contain other solvents that affect the stability of emulsion, the rate of solidification. After the bridge polymer rich phase has solidified, the fiber tape, fiber band or fabric is treated to remove the film forming preventing agent phase and leave an infusible, unidirectional fiber tape, fiber band or fabric.
In another preferred embodiment, the film-forming preventing agents are a combination of blowing agents and frothing agents (or foaming agents). When the combination of blowing agents and frothing agents are used, preferably a gelling agent is also added to stabilize the polymer foam. The blowing agents may be any suitable materials that can generate small air bubbles when exposed to a stimulus after coating the polymer in water dispersion or polymer in water solution onto the fabric. Frothing agents or foaming agent in the coating liquid help stabilize the air bubbles, making the bubbles stable for longer periods of time and also allowing them to grow bigger (with the help from the blowing agents). During or after the foaming stage, bridge polymer chemical(s) start to solidify by undergoing chemical reaction, cooling below its melt point, precipitating, crystallizing, or evaporation of a portion of the mixture. In one preferred embodiment, this phase change occurs because of evaporation of water. In another preferred embodiment, this phase change occurs because of a chemical reaction, such as polymerization or crosslinking of mixture that may contain monomers, oligomers, cross-linkers, and initiators; these are commonly available as thermosetting resins that are paired with either a hardener or initiator. The liquid may also contain catalysts which may affect the rate of solidification of the polymer. It may also contain other solvents that affect the stability of emulsion, the rate of solidification, the structure of the resulting bridges, or the surface of the bridges. The gelling agent can increase the viscosity of the liquid, transfer the solvent from liquid state to a gel state. It can help to further stabilize the bubbles and polymer foam, and locks the phase structure of the coating material during the polymer solidify step.
In one embodiment, the blowing agent is water. Water can quickly evaporate under heat and creates bubbles. In another embodiment, the blowing agent is carbon dioxide that has dissolved in water. In another embodiment, the blowing agent is low boiling point organic liquid. The frothing agents or foaming agents include but not limited to ionic surfactant such as sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate (NaDDBS), or non-ionic block copolymer such as ethylene oxide and propylene oxide copolymer. One example of the block copolymer is Pluronic® from BASF. A gelling agent is also preferred to be added to stabilize the polymer foam. The gelling agent includes but not limited to acacia, alginic acid, bentonite, carbomers, carboxymethylcellulose. ethylcellulose, gelatin, hydroxyethylcellulose, hydroxypropyl cellulose, magnesium aluminum silicate (Veegum®), methylcellulose, Pluronics®, polyvinyl alcohol, sodium alginate, tragacanth, and xanthan gum. A gelling agent with lower critical solution temperature (LCST) is preferred because it is soluble in cold water and gels in hot water. One example of the gelling agent with LCST characteristic is Pluronics® F-127. In one embodiment, foaming agents and gelling agents are used as the film-forming preventing agent. The polymer solid content is between about 1% and 60% of the polymer in water dispersion or polymer in water solution. More preferably the polymer solid content is between about 3% and 20%. The frothing agent is between about 0.1% and 20% of the total weight, more preferably between about 1% and 10% of the total weight. The gelling agent is between about 0.1% and 40% of the total weight, more preferably between 1% and 10% of the total weight. In one embodiment, Pluronics® F-127 is used as a foaming agent and also a gelling agent, preferably between 1% and 15% by weight in the coating mixture, more preferably between 3% and 10% by weight in the coating mixture.
The coating mixture with the blowing agent, frothing agent (or foaming agent) and gelling agent can be applied to the fiber tape, fiber band or fabric through many coating methods that are typically used to apply coating mixture to fiber bundles or fabrics. The emulsion can be applied using dip, nip, roll, kiss transfer, spray, slot, slide, die, curtain, or knife coating processes among others. The coating should be applied so that it fills the void spaces within the fiber bundles and so that it does not destabilize the coating mixture during the coating process. Mechanical action, such as passing over a series of rollers, passing over a roller with a patterned surface, pumping the emulsion through the fiber bundles, repeated saturation of the bundles with the emulsion, sonication or oscillating the fiber bundle tension may aid in homogeneously filling the void spaces between fibers within the fiber bundle. The amount of applied coating mixture can be metered using routinely practiced metering methods available for the aforementioned coating methods.
After coating the fabric, the blowing agent is activated by exposing to a stimulus to generate bubbles. In one preferred embodiment, water is used as the blowing agent. The coated fiber tape, fiber band or fabric is exposed to heat, resulting rapid vaporization of water and bubble formation in water. Preferably the wet fibers are directly contact on a hot surface. Preferably the temperature of the hot surface is at least 100° C., more preferably the temperature of the hot surface is at least 120° C., more preferably the temperature of the hot surface is at least 150° C. The bubbles that are generated by the blowing agent are stabilized by the frothing agent or foaming agent, and are further stabilized by the gelling agent. In one preferred embodiment, Pluronics® F-127 is used as a foaming agent and also a gelling agent, preferably between 1% and 15% by weight in the coating mixture, more preferably between 3% and 10% by weight in the coating mixture. During or after the activating the blowing agent, the chemicals in the coating mixture is solidified to form the bridge structure. This bridge forming process has been shown to impact the formation of the bridge structure. An important part of the bridge forming process is to allow enough heat to transfer to the blowing agent rapidly to generate enough bubble before polymer has had time to solidify. And another important part of the bridge forming process is to stabilize the foam during polymer solidification. The size of the void space or channel in the fiber tape, fiber band or fabric, is controlled by the concentration of the polymer in the coating mixture, the concentration of the film-forming preventing agents, and the method to dry the fibers. The more effective of the activating the blowing agent, the better bridge structure can be made. The foaming agent and gelling agent is critical to prevent the polymer forming a film. If the blowing agent is not functioning well before the polymer has solidified, the polymer will want to form a continuous film on and between fibers. If the foam structure is not stable enough and breaks before the polymer has solidified, the polymer will also want to form a continuous film on and between fibers.
Likewise, if the water is removed from the system before the polymer has solidified, the polymer will want to spread out onto the functionalized glass fibers. This favorable surface interaction will cause the polymer to form films on and between the fibers, greatly reducing the ability of the fabric to be infused into a composite material using standard resin infusion techniques.
During or after the discontinuous phase has solidified, the coated fabric may be dried to remove the residual solvent. The drying process has been shown to impact the performance of the infusible, unidirectional fabric in composite. To increase the production rate it is preferable to dry the fiber bundles at a temperature above room temperature, preferably at or above the boiling point of the solvent, provided that the drying temperature and time are below a temperature and time combination that causes the structure of the bridges to change, for example by decomposing the material forming the bridges, causing them to flow, or causing the bridges to become significantly less fatigue resistant.
In one embodiment, the coated fabric is dried at a temperature between about 80 and 150° C. for a time of between about 3 and 60 minutes. In one particular embodiment, the coated fabric is dried at temperature of 150° C. for 3 minutes. In another embodiment, the surface temperature of fiber bundles immediately after drying is at least 110° C. The energy imparted to the fabric is sufficient to remove at least 90% of the solvent by weight, preferably at least 99.7% by weight. After drying in one embodiment, the solvent content in the fabric is preferably less than 1% by weight, more preferably less than about 0.1% by weight.
Mechanical action may also be used during various steps of production. Mechanical action may be used only once in the process, or many times during different steps of the process. Mechanical action may be in the form of sonication, wrapping the fabric around a roller under tension, moving the fabric normal to its uniaxial or machine direction in the coating bath, compressing/relaxing fabric, increasing or reducing the tension of the fabric, passing it through a nip, pumping the coating liquor through the fabric, using rollers in the process with surface patterns. These surface patterns can have similar characteristic dimensions to the diameter of the fiber, the outside diameter of the fiber bundle, or the width of the fabric. It has been found that the addition of mechanical action during production of the infusible, unidirectional fabric may temporarily increase or decrease the space between fibers either once or multiple times, provide a pressure gradient to increase flow of the emulsion or suspension into, throughout and out of the bundle, and homogenize the distribution of dispersed polymer phase within the bundle. In one embodiment, the coated fabric is subjected to mechanical action after the coating step. In another embodiment, the coated fabric is subjected to mechanical action during the drying step. In another embodiment, the coated fabric is subjected to mechanical action after the drying step. The mechanical action may help to soften the fabric and create additional discontinuity in the coating by breaking large polymer bridges into smaller pieces.
After the infusible, unidirectional fabric is formed, it may be further processed into a bridged composite using the infusing the infusible, unidirectional fabric with resin as described previously.
EXAMPLESThe invention will now be described with reference to the following non-limiting examples, in which all parts and percentages are by weight unless otherwise indicated.
Fatigue Testing MethodDuring testing, fatigue loads are normally characterized by an R value which is defined as the ratio of minimum to maximum applied stress. By convention, compressive stress is taken to be a negative number and tension stress is taken as a positive number. Full characterization of fatigue performance involves testing over a range of R values such as R=0.1, −1, and 10, which corresponds to tension-tension, tension-compression, and compression-compression fatigue cycles respectively. Tension-tension fatigue with R=0.1 is a key metric of fatigue performance and was used to quantify the fatigue behavior of composite systems herein.
The fatigue performance of the composite materials made with the fusible, unidirectional fabric was measured using a standard tension-tension fatigue test. After composite panels were infused, composite tabs (1.6 mm thick) were bonded to the surfaces of the panels in the appropriate locations to establish the specimen gage length. Specimen details and dimensions were similar to those specified in ISO 527-5 with straight sided specimens 25 mm wide.
The specimens were environmentally conditioned for 40 hours at 23° C.+/−3° C. and 50%+/−10% relative humidity.
Using a servohydraulic test machine equipped with hydraulic wedge grips, the specimens were gripped using the minimum pressure required to avoid slipping. The machine was programmed to load the specimen in sinusoidal fashion using a specified frequency, mean load, and load amplitude. Cyclic loading continued until the specimen failed.
Typical schemes employ testing at a given R value with peak stress values chosen for the different tests of 80%, 60%, 40%, and 20% of the quasi-static strength. Test frequency is chosen to accelerate testing while ensuring the specimen temperature does not increase significantly (less than 35° C. for room temperature testing). This means that lower stress level testing can be done at higher frequencies than higher stress level tests.
The output of a typical fatigue testing regimen at a given R value is known as an S-N curve which relates the number of cycles a material can survive to specified loading conditions. S-N curves provide the most common comparison tool for basic fatigue performance evaluation. S-N curves for well-defined conditions are frequently used to compare the fatigue performance of different composite systems under similar loading. Improvement in R=0.1 fatigue testing, generally indicates a significant change in the fatigue behavior of a composite material.
Wind blades are generally designed to withstand over 108 loading and unloading cycles, however testing materials to such extremes is an impractical exercise. Comparisons are often made among materials at intermediate points such as the one million or 106 cycle performance. In order to screen samples, a specific peak loading level of 800 N/mm of specimen gage section width was applied with an R value of 0.1 (tension-tension fatigue) and the number of cycles to failure was measured for each sample. This loading was chosen to balance the amount of time required to perform an experiment with the reliability of the data for predicting fatigue performance at more typical levels of strain. The same loading levels of 800 N/mm were also applied to a control composite samples made from traditional reinforcing fabrics.
Sample Layup ProcedureThe layup procedure was to stack the layers on top of a flat glass tool prepared with a mold release and covered with one layer of release fabric (peel ply). A laser crosshair was used to provide a fixed reference for alignment of the fibers in each layer. Both pieces of fabric were placed so that the fibers on the top surfaces ran in the same direction. Then a 900 layer of the unidirectional fabric was aligned with the crosshair and placed with the unidirectional tows up. This was followed with a 090 layer of unidirectional fabric that was aligned and placed with the unidirectional side down. The next 900 layer of unidirectional fabric was placed with the unidirectional tows up and a final 090 layer was placed with the unidirectional tows facing down. The last two layers of ±45 fabric were placed so that the fibers on their top surface ran perpendicular to the fibers on the top surface of the ±45 fabric on the bottom two layers of the fabric stack. Finally, the laminate stack was covered with another layer of release fabric (peel ply).
The vacuum infusion molding process was used to impregnate the laminates with resin. On top of the release fabric for each laminate, a layer of flow media was used to facilitate resin flowing into the reinforcement plies. The entire laminate was covered with a vacuum bagging film which was sealed around the perimeter of the glass mold. Vacuum was applied to the laminate and air was evacuated from the system. Resin was then prepared and pulled into the reinforcement stack under vacuum until complete impregnation occurred. After the resin was cured, the composite panel was removed from the mold and placed in an oven for post-curing.
Example 1An unsaturated polyester control sample was made using the sample layup procedure using the 090 fabric and the ±45 fabric. The stacked textiles were infused in a standard vacuum infusion apparatus at a vacuum of less than 50 mbar with unsaturated polyester resin (Aropol Q67700 available from Ashland) and 1.5 parts per hundred resin (phr) methyl ethyl ketone peroxide (MEKP). The resin flow direction was along the 0° direction of the 090 fabric. The panel was cured at room temperature for more than 8 hours and further post cured at 80° C. for more than 4 hours. Fatigue testing of the unmodified glass reinforced unsaturated polyester composite at R=0.1 with a load of 1450 N/mm of specimen gage section width measured a lifetime of approximately 1×104 cycles.
Example 2 to Example 7 showed how the film forming preventing agents affect the infusibility of the fiber fabric. The fiberglass fabrics used in Examples 2-6, and 8 were in small widths so will be referred to herein as fiberglass tapes.
Example 2A fiberglass tape was made in the following manner. First, a 9600 Tex fiberglass tow from PPG (HYBON® 2026) was spread into a 20 mm wide tape by a fiber tow spreading device. Next, four of the 20 mm wide tapes were combined and aligned in the same direction to form a 40 mm wide tape with twice the original tape thickness. A SBR latex (GENCAL® 7555 from OMNOVA) was mixed with water at a SBR latex to deionized water ratio of 1:4. The fiber tape was then dipped in the coating mixture and dried in an oven at 150° C. for 30 minutes. Next, the fiber tape was washed using deionized water and dried in oven at 150° C. for 15 minutes.
Example 3A 40 mm wide, fiberglass tape was made using the same fiberglass materials and process as Example 2. A SBR latex (GENCAL® 7555 from OMNOVA) was mixed with water and glycerin at a SBR latex to deionized water to glycerin ratio of 1:2:2. The fiber tape was dipped in the coating mixture and dried in an oven at 150° C. for 30 minutes. Next, the fiber tape was washed by deionized water and dried in oven at 150° C. for 15 minutes.
Example 4A 40 mm wide, fiberglass tape was made using the same fiberglass materials and process as Example 2. A SBR latex (GENCAL® 7555 from OMNOVA) was mixed with glycerin at a SBR latex to glycerin ratio of 1:4. The fiber tape was dipped in the coating mixture and dried in an oven at 150° C. for 30 minutes. Next, the fiber tape was washed by deionized water and dried in oven at 150° C. for 15 minutes.
Example 5A 40 mm wide, fiberglass tape was made using the same fiberglass materials and process as Example 2. A SBR latex (GENCAL® 7555 from OMNOVA) was mixed with water and glycerin at a SBR latex to water to glycerin ratio of 1:1:8. The fiber tape was then dipped in the coating mixture and dried in an oven at 150° C. for 30 minutes. Next, the fiber tape was washed by deionized water and dried in oven at 150° C. for 15 minutes.
Example 6A 40 mm wide, fiberglass tape was made using the same fiberglass materials and process as Example 2. A waterborne polyurethane (SYNTEGRA® YM 2000 from Dow Chemical) was mixed with water at a YM 2000 to deionized water ratio of 1:6. The fiber tape was then dipped in the coating mixture and dried in an oven at 80° C. for 4 hours. Next, the fiber tape was washed by deionized water and dried in oven at 80° C. for 12 hours.
Example 7A fiberglass tape was made in the following manner. First, a 4800 Tex fiberglass tow from PPG (HYBON® 2002) was wrapped on a piece of plastic to form a roughly 1000 gsm fabric. A waterborne polyurethane (SYNTEGRA® YM 2000 from Dow Chemical) was mixed with sugar and water at a YM 2000 to sugar to deionized water ratio of 1:2.7:6. The fiber tape was then dipped in the coating mixture and dried in an oven at 80° C. for 4 hours. Next, the fiber tape was washed by deionized water for 2 days and dried in oven at 80° C. for 12 hours.
The infusibility of the fiber tape for Examples 2-7 were characterized in the following manner: Several water droplets with 0.01% water soluble color dye Acid Blue 9 were dropped on the center surface of fiber fabric by using a 5 mL transfer pipette, and the time that how long it took for the droplets to completely infuse into the fabric was used as an indication of infusibility of the fiber tape. In this method, “completely infuse into the fiber fabric” means that more than 99% water from original droplets is staying between the upper inner surface and lower inner surface of the fabric.
For the fiber tape in Example 2, the droplets stayed on the surface of the tape and cannot infuse into the tape. For the tape in Example 3, the droplets took about several seconds to infuse into the tape. For the tape in Example 4 and 5, the droplets immediately infused into the tape. The differences between these three examples showed how the film forming preventing agent (glycerin in Example 2, 3 and 4) affected the infusibility of the final article.
For the fiber tape in Example 6, the droplets stayed on the surface of the tape and did not infuse into the tape. For the tape in Example 7, the droplets took about half minute to infuse into the tape. The differences between these two examples showed how the film forming preventing agent (sugar in Example 6) affected the infusibility of the final article.
An optical microscope image of the SBR coating in Example 5 was taken and determined that most of the bridges had a bridge width narrower than 60 microns.
Example 8A fiberglass tape was made in the following manner. A waterborne polyurethane (SYNTEGRA YM 2000), a blocked isocyanate based cross-linking agent (Milliken MRX), sugar and water were mixed at a mass ratio of 103:5.6:277:620. A fiber roving (PPG HYBON® 2002) was spread to form a roughly 500 gsm fabric (sometimes referred to as a tape as it is has a low width). Then the fiber tape was dipped into the coating mixture. The fabric was dried at 80° C. for 4 hours and washed by water for 12 hours. Next, the fiber tape was dried in oven at 80° C. for 12 hours.
The
A fiberglass fabric was made in the following manner. 7.6 g waterborne polyurethane (BONDTHANE J-884-A from Bond Polymers International), 0.2 g crosslinking agent (Milliken MRX), 9 g sugar and 100 g water was mixed to made the coating solution. A total mass of about 150 g fiber rovings (HYBON® 2002) from PPG were uniformly fixed on an 8″ by 24″ plate by holing ends of rovings under tension. A piece of SPUNFAB lightweight adhesive web was put on top of the rovings. The coating mixture was pulled onto the rovings and a rubber roller was used to apply uniform coating and nip the excess liquid. The 8″ by 24″ plate together with rovings was dried at 80° C. overnight. Next, the whole fabric was removed from the plate and immersed in water for 24 hours and then dried at 80° C.
Example 10A stack of textiles was formed in order: Two (2) layers of the infusible fabric of Example 9, the fibers in the two layers were parallel. The side having SPUNFAB web was on the outer side. The stacked textiles were infused in a standard vacuum infusion apparatus at a vacuum of −25 in. Hg (about 169 mbar) with 98.77% wt unsaturated polyester resin (Aropol G300 available from Ashland) and 1.33% wt methyl ethyl ketone peroxide (MEKP 925H available from Norox). The resin flow direction was along the fibers. The panel was cured at room temperature more than 8 hours and further post cured at 80° C. for more than 4 hours forming the composite.
A fiberglass fabric was made in the following manner. 8 g waterborne polyurethane (SYNTEGRA YM 2000 from Dow Chemical), 0.5 g crosslinking agent (Milliken MRX), 13.5 g sugar and 150 g water was mixed to made the coating solution. A total mass of about 260 g fiber rovings (HYBON® 2002) from PPG were uniformly fixed on an 14″ by 24″ plate by holing ends of rovings under tension. A piece of SPUNFAB lightweight adhesive web was put on top of the rovings. Another total mass of about 260 g fiber rovings (HYBON® 2002) from PPG were uniformly put on top of the SPUNFAB lightweight adhesive web and fixed on the same 14″ by 24″ plate by holding ends of rovings under tension. In both layers, all fibers are in the same direction. The coating mixture was pulled onto the rovings and a rubber roller was used to apply uniform coating and nip the excess liquid. And then the whole 14″ by 24″ plate together with rovings was dried at 80° C. overnight. Next, the whole fabric was removed from the plate, immersed in water for 24 hours, and then dried at 80° C.
Example 12The fabric in Example 11 was infused in a standard vacuum infusion apparatus at a vacuum of 25 in. Hg (about 169 mbar) with 98.77% wt unsaturated polyester resin (Aropol G300 available from Ashland) and 1.33% wt methyl ethyl ketone peroxide (MEKP 925H available from Norox). The resin flow direction was along the fibers. The panel was cured at room temperature more than 8 hours and further post cured at 80° C. for more than 4 hours. This formed the composite. The tensile modulus of the composite is 5% higher than Example 1 which comprises traditional stitched unidirectional fabric. The peak stress and peak strain in static tensile test of the composite is about 20% higher than Example 1. In the R=0.1 tensile fatigue test, the cycles to failure of this composite is about 12 times of the control which comprises traditional stitched unidirectional fabric.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
Claims
1. A process of forming infusible, unidirectional fabric comprising:
- arranging a plurality of unidirectional fibers into a unidirectional fabric, wherein the unidirectional fibers are spaced uniformly within the unidirectional fabric;
- forming an emulsion or suspension of a solvent, a bridging polymer, and a film-forming preventing agent, wherein the bridging polymer is dissolvable or dispersible in the solvent, wherein the film-forming preventing agent is dissolvable or dispersible;
- applying the emulsion or suspension to the unidirectional fabric;
- removing the solvent;
- removing the film-forming preventing agent to form an infusible, unidirectional fabric, wherein the infusible, unidirectional fabric comprises:
- a plurality of unidirectional fibers having a diameter and a length, wherein the unidirectional fibers are spaced uniformly in the unidirectional fabric;
- a plurality of bridges, each bridge being connected to at least 2 unidirectional fibers and wherein at least 70% by number of fibers comprise at least one bridge connected thereto forming a bridged network of unidirectional fibers, wherein the bridges comprise a bridging polymer, wherein between the unidirectional fibers the bridges each have a width and a bridge width minimum, and wherein at least 50% by number of the bridges have a bridge width minimum less than about 2 millimeters; and,
- a plurality of void spaces between the unidirectional fibers, wherein the void spaces are interconnected, wherein the fabric has a volume fraction of voids of between about 8 and 70%, wherein the fabric has a volume fraction of fibers of between about 35 and 85%.
2. The process of claim 1, wherein the solvent is water.
3. The process of claim 1, wherein the film-forming preventing agent is a liquid.
4. The process of claim 1, further comprising infusing and curing a resin into the infusible, unidirectional fabric.
5. The process of claim 1, wherein the infusible, unidirectional fabric does not comprises any stitching fibers or yarns.
6. The process of claim 1, wherein the bridging polymer forms between about 0.1 and 30% of the effective cross-sectional area of the infusible, unidirectional fabric.
7. The process of claim 1, wherein the film-forming preventing agent is sugar.
8. The process of claim 1, wherein the unidirectional fibers comprise fiberglass fibers.
9. A process of forming infusible, unidirectional fabric comprising:
- arranging a plurality of unidirectional fibers into a unidirectional fabric, wherein the unidirectional fibers are spaced uniformly within the unidirectional fabric;
- forming an emulsion or suspension of a solvent, a bridging polymer, a blowing agent, a foaming agent and a gelling agent, wherein the bridging polymer is dissolvable or dispersible in the solvent;
- applying the emulsion or suspension to the fabric;
- activating the blowing agent forming bubbles in the emulsion and suspension, wherein the foaming agent and gelling agent stabilize the bubbles;
- removing the solvent forming an infusible, unidirectional fabric, wherein the infusible, unidirectional fabric comprises:
- a plurality of unidirectional fibers having a diameter and a length, wherein the unidirectional fibers are spaced uniformly in the unidirectional fabric;
- a plurality of bridges, each bridge being connected to at least 2 unidirectional fibers and wherein at least 70% by number of fibers comprise at least one bridge connected thereto forming a bridged network of unidirectional fibers, wherein the bridges comprise a bridging polymer, wherein between the unidirectional fibers the bridges each have a width and a bridge width minimum, and wherein at least 50% by number of the bridges have a bridge width minimum less than about 2 millimeters; and,
- a plurality of void spaces between the unidirectional fibers, wherein the void spaces are interconnected, wherein the fabric has a volume fraction of voids of between about 8 and 70%, wherein the fabric has a volume fraction of fibers of between about 35 and 85%.
10. The process of claim 9, further comprising infusing and curing a resin into the infusible, unidirectional fabric.
11. The process of claim 9, wherein the infusible, unidirectional fabric does not comprises any stitching fibers or yarns.
12. The process of claim 9, wherein the film-forming preventing agent is sugar.
13. The process of claim 9, wherein the unidirectional fibers comprise fiberglass fibers.
Type: Application
Filed: Oct 13, 2015
Publication Date: Feb 4, 2016
Inventors: Xin Li (Boiling Springs, SC), Ryan W. Johnson (Moore, SC), Joseph E. Rumler (Greenville, SC)
Application Number: 14/881,877
