COATED FABRICS
A fabric has a coating which comprises an elastomeric, heat curable silicone rubber composition containing a porous inorganic filler. The filler is at sufficiently high loading to produce a percolated porous structure, the percolated porous structure being permeable to water vapour.
The invention relates to coated fabrics and in particular to waterproof, breathable coated textiles that can be used, for example, in the manufacture of protective and leisure clothing, bags and luggage.
Waterproof, breathable coating and laminates for textiles are well known in the prior art, the terms “waterproof” and “breathable” relating to the coating or laminate being impervious to liquid water and permeable to water vapour respectively. This imparts a high degree of comfort to the wearer, preventing condensation of sweat inside the garment while providing a waterproof barrier to keep the wearer dry in all types of weather.
Materials sold under the registered trade mark GORE-TEX are based on an expanded polytetrafluoroethylene membrane (e.g. U.S. Pat. No. 3,953,583) to produce a microporous membrane that can be laminated to a fabric. The pores in the membrane are 20,000 times larger than a water molecule allowing the passage of vapour, while the extremely low surface energy of the membrane imparts high waterproofing properties.
Microporous coatings based on polyurethanes are also known in the art, as described by U.S. Pat. Nos. 4,560,611, 5,520,998, 5,626,950 and 5,692,936. These are produced by direct coating a polyurethane solution onto a fabric, then coagulating the solution to produce a network of micropores in the polymer structure.
Another group of coatings are based on hydrophilic polyurethanes. These are usually directly coated onto a fabric, and rely on the incorporation of hydrophilic segments in the polymer chains. Water molecules from perspiration are therefore able to diffuse from inside the clothing of the wearer through the polymer layer via a step-wise process and transported to the outside environment. The functionality of these hydrophilic layers relies on a humid environment building up inside the breathable garment. The higher the humidity inside the garment, the faster the rate of diffusion of water molecules through the polymeric layer. These types of polyurethane coatings usually contain both hydrophilic and hydrophobic segments to incorporate breathability and waterproofness respectively.
There are a number of problems associated with both the microporous membranes and the hydrophilic coatings. The performance of microporous membranes can deteriorate over time due to contamination of the pores by soil, detergents and body oils. Blocking of the pores in this manner reduces the breathability of the membrane and can also alter the surface chemistry of the membrane, resulting in the increased likelihood of liquid water penetration. Gore attempted to resolve this problem by the application of a very thin layer of hydrophilic polyurethane on top of the microporous PTFE as a protective layer to prevent the microporous layer from contaminants.
There are also problems associated with hydrophilic based polyurethane breathable coatings. The main one is their susceptibility to swelling in water. By their very nature, water molecules are attracted to the hydrophilic segments in the polymer. Water molecules are therefore able to “solvate” the hydrophilic segments within the polymer and swell the membrane. This can impart to a “clammy” feeling to the wearer as well as a loss of coating strength. In severe cases it can lead to a loss of adherence to the fabric itself, causing delamination. This is quite common when the garment is subjected to harsh aqueous environments.
The present invention has been made from a consideration of the above.
According to the present invention there is provided a coated fabric, wherein the coating comprises an elastomeric, heat curable silicone rubber composition containing a porous inorganic filler at sufficiently high loading to produce a percolated porous structure, the percolated porous structure being permeable to water vapour.
In a percolated system a gas is able to pass through the porous structure via a series of holes or pathways. The invention achieves porosity throughout the rubber coating. The very nature of the inorganic filler used in the formulation is that of particles containing a plurality of micron and sub-micron pores as an integral part of its structure. In the silicone rubber formulation, below a certain level of filler these particles are not in close proximity to each other. However, when a certain level, or amount of filler is incorporated into the rubber matrix, a mixed matrix compound is achieved where the porous particles are in sufficiently close proximity to each other to allow the passage of water vapour through each porous particle straight through the continuous mixed matrix coating i.e. the “percolation threshold” is reached to produce a true percolated structure throughout the mixed rubber/filler matrix. For any given system, below the “percolation threshold” a continuous connected component does not exist, but above the percolation threshold there exists a connected component to scale of the system size. Unlike other percolated systems, the invention tends not to allow the passage of liquid water (up to a hydrostatic head pressure of at least 2000 mm) due to the very hydrophobic nature and low surface energy of the silicone rubber.
In its simplest terms, the “percolated structure” of the proposed invention can be described by the interconnectedness of the porous filler particles throughout the mixed matrix compound that gives rise to continuous connected porous pathways to allow the passage of water vapour molecules throughout the rubber matrix, giving rise to its breathability.
The silicone composite solution can be coated on a natural or synthetic fabric which may be woven, nonwoven or knitted and comprise, for example, polyester or polyamide. The coating process may involve direct coating onto the fabric substrate. The coated fabric of the invention comprises a waterproof, breathable coated textile that can be used for the manufacture of a variety of articles, such as protective and leisure clothing, bags and luggage.
Silicone rubber exhibits gas permeability due to the zero energy of rotation about the silicon-oxygen bond in the backbone chain. Together with the plurality of methyl groups on the outside of the polymer chains (which in turn impart water repellent properties), intermolecular interaction is very low causing a large free volume between polymer chains, allowing the diffusion of gas molecules through the polymer matrix. However the level of permeability of silicone rubber alone is not sufficient to produce a water proof, breathable membrane on its own. The coated fabric of the invention therefore incorporates a porous filler which by creating a percolated structure within the silicone rubber composition, complements the permeable properties of the silicone rubber to produce a truly water-vapour permeable, liquid water impermeable membrane.
For the proposed invention of a waterproof, breathable silicone rubber composite coated onto a textile surface, it is proposed that different construction of coated fabric can be produced according to the end use. Single fabric coated structures are proposed along with multi fabric layer structures. The silicone rubber coating may be provided between adjacent fabrics in a multilayer structure and/or on the exterior of the structure. Some possible embodiments of the invention are mentioned below:—
1. Single texture (2-layer) comprising one fabric (e.g. polyester/polyamide (Nylon)) coated on one side with the silicone composite. This would be the most basic construction, intended for example for use in the manufacture of hiking, walking and camping jackets.
2. Double texture (tri-laminate) comprising an outer layer fabric (e.g. polyester/polyamide (Nylon)) laminated to a second, inner liner fabric (polyester) with the silicone composite layer in between. This would be a more durable fabric intended for example for water sport surface suits, dry suits and sailing/yachting suits/jackets. It is envisaged that this durable fabric construction can also be used in heavy duty work wear, for example, for outdoor workers.
3. Double texture (tri-laminate) fabric constructed from an outer fabric and an inner fabric (e.g. polyamide, polyester/polyamide (Nylon)) with the silicone coating in between. This type of construction would be useful for applications where durability and fire retardance are required.
The silicone rubber coating is a solution based formulation that is ideally coated directly onto a fabric. The silicone rubber composition contains the following polysiloxane and solid filler. Optionally the silicone rubber contains any of catalyst, adhesive and/or adhesion promoter and a solvent.
In one embodiment of the invention the various components of the rubber coating may be included as follows:—
The breathable silicone rubber coating of the invention has advantages over breathable polyurethane systems in as much that it is non-swelling in aqueous environments with no loss of adhesion in tri-laminate constructions. As it is an inherent property of silicone rubber, the coating also has excellent resistance to ageing (e.g. no hydrolysis, another issue with PU systems) and excellent flexibility and low temperature performance.
The breathable silicone rubber composite has an extremely porous nature due to the high porosity of the diatomaceous earth filler that is used in the silicone formulation. The high porosity and surface area of the composite raises the possibility of extra functionality being designed into the breathable coating. Possibilities include:
a) incorporation of essential oil anti-microbial agents into the silicone composite. The large internal volume of the porous filler particles may be able to act as a “reservoir” for essential oils that would be released slowly over time to impart antimicrobial properties to the breathable fabric. This would be extremely advantageous for garments that are susceptible to bacterial and fungal growth which causes unpleasant odours.
b) Similarly, insect repellent agents could also be incorporated into the filler particles. This would be advantageous for hiking and camping clothing in areas where a high population of insects is present.
The above areas are only possible due to the large porous internal volume of the filler particles. Traditional waterproof, breathable coatings fashioned from microporous PTFE, polyurethane and hydrophilic solid polyurethane membranes do not have this large, internal free volume to act as a reservoir for additional functional materials. This is one other distinct advantage that the present invention has over prior art breathable materials.
The invention will now be described further by way of example only with reference to the accompanying drawing:—
Referring to the drawing a coated fabric 10 comprises a first woven fabric 11 having a plain weave construction made from polyamide. This is direct coated on one side thereof with a silicone rubber composition 12. The silicone rubber composition has the following components:—
(A) a polysiloxane 13 that can be cross-linked by polyaddition, polycondensation or free radical means catalysed by platinum, organotin or peroxide compounds respectively.
(B) a porous filler 14. In one embodiment this comprises natural diatomaceous earth (or diatomite) which is a soft siliceous sedimentary rock crumbled into a fine powder. The porous filler, while typically natural diatomaceous earth, may also be calcined and flux-calcined. Calcination is the heat treatment of a material in the presence of air or oxygen. Flux-calcination is the heat treatment of a material in the presence of a fluxing agent.
(C) a catalyst (not shown), which can be either an organo-tin compound, platinum catalyst or peroxide. For the current formulation, C14-010, an organo-tin catalyst commercially available from Itac Ltd can be typically used.
(D) additionally (not shown), a mixture of glycidoxypropyltrimethoxysilane and vinyltriacetoxysilane adhesion promoter, such as the commercially available SYL-OFF 297 from Dow Corning. This is also supplied under the product code C14-025 from Itac Ltd.
(E) an aromatic or aliphatic solvent (not shown) to adjust the solids content of the coating solution. Toluene is typically used for this purpose, although SPB2 (a mixture of hexane and heptane) can also be used.
The ratio of mixing the components A-E is set out below.
A second plain weave fabric 15 also made form polyamide is situated on the other side of the coating such that the silicone coating is sandwiched between the two fabrics.
If using a polycondensation cure silicone rubber such as that mentioned above, excess moisture in the filler should be removed prior to mixing. Natural diatomaceous earth contains around 8% by weight of moisture that is held by the filler. Excessive moisture can interfere with the condensation reaction, leading to insufficient crosslinking and poor physical properties. The diatomaceous earth filler should therefore ideally be dried at temperatures of 100-120° C. for a minimum of 16 hours prior to mixing. Alternatively, a molecular sieve compound can be added to the coating formulation that will remove the moisture from the filler, thus obviating the necessity to pre-dry the filler before mixing the compound. If the latter alternative is to be employed, the molecular sieve compound needs to be added to the coating formulation and thoroughly mixed and left to stand for not less than two hours before coating onto the fabric. A typical molecular sieve compound employed for this purpose is a porous, crystalline aluminosilicate powder such as those marketed under the brand name “Sylosiv”.
The formulation should be mixed in a mechanical mixing device, such as a Z-blade mixer. Components (A) and (B) should be thoroughly mixed in the first instance for 20-30 mins to produce a homogenous “dough” mix, followed by components (C), (D) and (E). The complete formulation should then be mixed for a further 20-30 mins until a completely homogenous solution is produced. Measured solids content of the solution should be 35-45%. After the final mix, the formulation will have a useable pot life of 8 hours.
The water vapour pathway is in the direction of arrows A.
The invention will now be described further with reference to the following examples.
EXAMPLE 1 Tri-Laminate Coated Fabric (as Shown in FIG. 1)The silicone composite solution can be coated on a synthetic woven or knitted fabric e.g. polyester or nylon using direct coating (knife over roller) method. A multiple number of passes is required in order to achieve a coating weight of 70-100 gsm. Ideally a coating weight of around 85 gsm should be achieved. It is usual for at least three coating passes to be needed to achieve this weight. It is important to ensure the temperature is at a temperature that enables evaporation of the solvent, but is not too high as to cause premature cross-linking of the silicone composite. As a guide, a temperature of 60-80° C. should be maintained in the heated spreading chest. The uncoated side of the fabric will function as the “inner” lining of the waterproof, breathable fabric.
After the last coating of the silicone composite has been applied, a final coating of silicone based pressure sensitive adhesive (PSA) is required in order to laminate the coated fabric to an “outer” layer fabric to produce a tri-laminate construction. A commercial grade PSA from Dow Corning known as DC7358 (a peroxide cured adhesive) can be used for this purpose. Peroxide (typically dibenzoyl peroxide) needs to be premixed into the adhesive solution at a level between 0.5-2%. A light coating of adhesive (between 5-20 gsm depending on the level of adhesion required) is then applied over the silicone coating, the solvent is allowed to evaporate through the heated chest (temperature must not be over 75° C. or premature crosslinking of the adhesive will occur). The coated fabric is then laminated through two pressurised rollers against a second, uncoated fabric to produce the tri-laminate construction. The laminated fabric can then be cured off-line. The fabric coating can be cured for 30-45 min at a temperature of 120-140° C. Lower temperatures down to 100° C. can be used for longer curing times of up to 6 hours.
The tri-laminate water proof breathable fabric can be used (but not limited to) garments for water sport and marine applications such as surface suits, dry suits, sailing garments, etc.
EXAMPLE 2 Single Coated FabricThe single coated fabric is manufactured in a similar way to the tri-laminate. On this occasion the coated fabric serves as the outer fabric shell as opposed to the inner layer fabric, and the fabric is not then laminated to another fabric. There is therefore no need to apply a silicone adhesive layer. However, due to the non-stick properties of silicone rubber, it is necessary to apply a breathable, low melting point thermoplastic top coat on the breathable silicone layer in order to facilitate the adhesion of seam sealing tapes. A polyurethane coating solution such as Larithane BTH231 can be used for this purpose. Swelling effects of the polyurethane are not as critical as the fabric is not laminated to another outer fabric, where loss of peel adhesion could become an issue.
Water vapour transmission rate (breathability):
The breathability of the fabric has been internally tested according to BS7209 “Water vapour permeable apparel fabrics”. A circular test piece of the fabric is fixed over the rim of a circular aluminium dish that contains a measured amount of distilled water. The outside surface of the rim of the dish is then sealed so that the only pathway water vapour can take is through the fabric. The total weight of the dish, fabric and water is measured, whereupon the dish is then placed on a circular turntable that rotates to prevent a microclimate of humid air above the surface of the fabric. The dish is then left in atmospheric conditions of 65% relative humidity and 20° C. for at least 16 hours. The weight of the whole test dish is then measured to calculate the loss of water in the form of vapour through the dish. This measurement of weight loss is then calculated to calculate the “water vapour transmission rate” (WVTR) in units of g/m2/24 hours. The weight loss of water vapour through the test fabric is also measured against that from a reference fabric to calculate the water vapour permeability index (WVPI), quoted as a percentage of the reference fabric.
The waterproof breathable fabric as described in this specification will typically have a WVTR of 400-650 g/m2/24 hours, more typically around 500 g/m2/24 hours and a WVPI of 50-90%, more typically around 75%.
Hydrostatic Head pressure rating (waterproofness):
This is described as the water pressure required to leak (penetrate) through the fabric. This is determined by carrying out test BS3424-26, which subjects a test piece of fabric to pressure from either a column of water or water pressurised from a compressor. Any fabric that can withstand a pressure equivalent to a column height of 1000 mm is deemed “waterproof” although in practice ratings of 2,000-20,000 mm are expected from quality products.
The waterproof breathable fabric as described in this specification demonstrated a hydrostatic head rating of at least 2,000 mm according to internal tests.
Peeling (adhesion) strength—tri-laminate only.
The peeling strength is simply the force required to peel the laminated fabrics apart, as measured on a tensometer. The test piece is a 50 mm wide strip of the fabric with both face and back fabrics delaminated from each other. The force in N/50 mm is then measured that is required to peel the fabrics apart.
The tri-laminate fabric as described previously typically has a peeling strength of 4-8 N/50 mm, more typically around 6 N/50 mm. The peeling strength will remain unaffected after the test piece has been immersed in a 2% salt water solution for 24 hours.
It is to be understood that the above described embodiments are by way of illustration only. Many modifications and variations are possible.
Some possible modifications and variations are set out below.
a) The invention contemplates the optional addition of a thixotropic filler such as fumed silica to the base silicone rubber formulation. This would alter the rheological properties by increasing the viscosity of the coating formulation and reduce “strike-through” of the solution through the fabric. This leads to reducing the degree of coating penetration through the fabric leading to a better appearance and handle of the coated fabric.
b) An improvement in adhesion of silicone rubber to the fabric may optionally be achieved by applying a pre-treatment of an organic silane compound to the fabric prior to applying the rubber coating. The silane pre-treatment would typically be applied by a dipping technique into a silane solution followed by drying to evaporate the solvent.
c) As discussed previously the diatomaceous earth filler may be dried prior to mixing into the rubber formulation, as excess moisture present in the filler disrupts the condensation cross-linking reaction. The addition of a molecular sieve compound into the rubber/filler solution 2-3 hours prior to coating removes the moisture from within the coating solution, thus eliminating any requirement to pre-dry the filler.
d) One or more blowing agents may be used to increase the porosity of the adhesive. Increasing the elastomer content of the adhesive may improve the peel strength.
d) While the currently preferred porous filler used to create a percolated porous structure is natural diatomaceous earth, there are a range of other porous fillers which may be used in place of or in combination with diatomaceous earth. Such fillers may include kaolinite, amorphous silica, zeolites, metal organic frameworks, porous carbon blacks and montmorillonite clay.
e) Additional fillers may be incorporated into the composite formulation to improve the physical properties of the composite coating. These include, either alone, or in combination, any of calcium carbonate, barium carbonate, talc, mica, hydrotalcite, calcium sulphate, barium sulphate, aluminium hydroxide, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, and zinc oxide.
Claims
1. A coated fabric, comprising:
- a coating comprises an elastomeric, heat curable silicone rubber composition containing a porous inorganic filler at sufficiently high loading to produce a percolated porous structure, the percolated porous structure being permeable to water vapour.
2. The coated fabric of claim 1, wherein a fabric is selected from the group consisting of knitted, woven non-woven, and combinations thereof.
3. The coated fabric of claim 1, wherein a fabric is selected from the group consisting of cotton, polyester, polyamide, and combinations thereof.
4. The coated fabric of claim 1, wherein the silicone rubber composition is selected from the group consisting of polysiloxane (condensation), peroxide, additional cured silicone, and combinations thereof.
5. The coated fabric of claim 1, wherein the silicone rubber composition comprises a filler or combination of fillers with a porosity in the range from 2 nm to 1 μm.
6. The coated fabric of claim 5, wherein the silicone rubber composition comprises a filler or combination of fillers with a mean pore size of substantially −0.5 μm.
7. The coated fabric of claim 1, wherein the porous filler is selected from the group consisting of kaolinite, amorphous silica, diatomaceous earth, zeolites, metal organic frameworks, porous carbon blacks, montmorillonite clay, and combinations thereof.
8. The coating fabric of claim 1, wherein the porous filler comprises natural diatomaceous earth.
9. The coating fabric of claim 8, wherein the porous filler comprises natural diatomaceous earth which is calcined.
10. The coating fabric of claim 8, wherein the porous filler comprises natural diatomaceous earth which is flux-calcined.
11. The coated fabric of claim 8, wherein the particle size range of filler is in the range from 5 to 100 μm.
12. The coated fabric of claim 11, wherein the particle size range of filler is an average pore size of substantially 11 μm.
13. The coated fabric of claim 1, wherein one or more additional fillers are incorporated into the formulation for improving the rheological properties of the coating compound, and for improving the physical properties of the cured coating itself.
14. The coated fabric of claim 13, wherein the reinforcing filler or rheology modifying filler is selected from a group consisting of calcium carbonate, barium carbonate, talc, mica, hydrotalcite, calcium sulphate, barium sulphate, aluminium hydroxide, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, zinc oxide, and combinations thereof.
15. The coated fabric of claim 1, wherein the silicone rubber composition comprises from 50 to 150 parts by weight of porous filler per 100 parts by weight silicone rubber.
16. The coated fabric of claim 1, wherein the silicone rubber composition comprises catalyst.
17. The coated fabric of claim 16, wherein the catalyst is selected from a group consisting of organo-tin, platinum, peroxide, and combinations thereof.
18. The coated fabric of claim 16, wherein the silicone rubber composition comprises from 0.5 to 7.0 parts by weight of the catalyst per 100 parts by weight (solid) silicone rubber.
19. The coated fabric of claim 1, wherein the silicone rubber composition comprises an adhesion promoter.
20. The coated fabric of claim 19, wherein the adhesion promoter comprises at least one of an organosilicon compound containing an epoxy group, and an organosilicon compound containing a vinyl acetoxy group.
21. The coated fabric of claim 19, wherein the silicone rubber composition comprises from 2.25 to 4.5 parts by weight of adhesion promoter per 100 parts by weight silicone rubber.
22. The coated fabric of claim 1, wherein the silicone rubber composition comprises a solvent.
23. The coated fabric of claim 22, wherein the solvent comprises an aromatic or aliphatic organic solvent.
24. The coated fabric of claim 22, wherein the solvent is selected from a group consisting of toluene, xylene, hexane, heptanes, and combinations thereof.
25. The coated fabric of claim 22, wherein the silicone rubber composition comprises from 200 to 400 parts by weight of solvent per 100 parts by weight (solid) silicone rubber.
26. The coated fabric of claim 1, wherein the silicone rubber composition comprises at least one anti-microbial agent.
27. The coated fabric of claim 1, wherein the silicone rubber composition comprises at least one insect repellent agent.
28. The coated fabric of claim 1, wherein the coated fabric is breathable and waterproof.
29. The coated fabric of claim 1, having a coating weight of 70-100 gsm of the silicone rubber composition on the fabric.
30. A coated fabric structure, comprising a coated fabric comprising a coating which comprises an elastomeric, heat curable silicone rubber composition containing a porous inorganic filler at sufficiently high loading to produce a percolated porous structure, the percolated porous structure being permeable to water vapour;
- the coated fabric in combination with one or more further fabrics.
31. The coated fabric structure of claim 30, wherein adhesive is coated onto the coated fabric to laminate a second fabric to produce a trilaminate structure, wherein the adhesive coating has a coating weight of adhesive of 5-20 gsm.
32. A method of making a coated fabric comprising:
- providing a coated fabric comprising a coating which comprises an elastomeric, heat curable silicone rubber composition containing a porous inorganic filler at sufficiently high loading to produce a percolated porous structure, the percolated porous structure being permeable to water vapour;
- wherein the silicone rubber composition is coated onto the base fabric using a knife-over roller technique.
33. A method of making a coated fabric of claim 32, wherein a second fabric is laminated to an adhesive layer on the base fabric to provide a trilaminate coated fabric.
Type: Application
Filed: Jun 24, 2014
Publication Date: May 12, 2016
Inventor: Wayne John Hanison (Oldham lancashire)
Application Number: 14/901,139
