PROCESS FOR MANUFACTURING POLY(LACTIC ACID) BIO-COMPOSITES....

Abstract

Disclosed is a method for manufacturing bio-composites, in which biodegradable poly(lactic acid) (PLA) fibers are mixed, optionally along with general-purpose polypropylene fibers, using a carding process, and compression molded into bio-composites which overcome the problems of the PLA bio-composites manufactured by injection molding, and the PLA bio-composites manufactured thereby.

Description

TECHNICAL FIELD

The present invention relates, in general, to a method for manufacturing bio-composites and, more particularly, to a method in which bio-degradable poly(lactic acid) (PLA) fibers are mixed, optionally along with general-purpose polypropylene fibers, using a carding process, and compression molded into bio-composites which overcome the problems of the PLA bio-composites manufactured by injection molding. Also, the present invention is concerned with the PLA bio-composites manufactured by the method of the present invention.

BACKGROUND ART

With an increasing concern being taken in global environmental problems, such as the treatment of waste plastics, and the sway of the Climatic Change Convention, new environmental regulations, etc., intensive attention has been paid to new environmentally friendly materials including bio-composites. Bio-composites are generally composed of natural cellulosic flour, such as wood flour, chaff flour, bamboo flour, etc., and reinforcements made of natural fibers such as wood fibers, linen, hemp, etc. As such, bio-composites are used as substituents for conventional polymer composites composed of inorganic materials such as carbon fiber using glass fibers as reinforcements. Compared to the inorganic fillers, bio-composites have the advantage of being bio-degradable and friendly to the environment.

DISCLOSURE

Technical Problem

Most of the currently used or studied bio-composites are based on polyolefins (PP, PE, PS), which are most widely used in the polymer industry, with environmentally friendly, biodegradable natural fibers or flour used as a reinforcing agent. These bio-composites are developed for use in and can be found in deck construction materials, structuring materials, packaging materials and materials used in the interiors of cars. However, polyolefin-based composites, although partially environmentally friendly, are not regarded as being environmentally friendly due to the non-biodegradability of the polyolefin base. In the future, thus, completely biodegradable bio-composites based on biodegradable polymers are expected to find various applications in industry. Active studies are now being conducted to bestow on bio-composites physical properties as good as those of the currently used general-purpose resins.

Of the many biodegradable polymers, PLA (poly(latic acid)) attracts intensive attention. The use of biodegradable PLA as a base polymer for bio-composites is advantageous in that when buried in a landfill the polymer is degraded to non-toxic materials. In addition, PLA is a sustainable bio resource which can substitute for exhausting petroleum resources. However, the high brittleness of PLA fibers at room temperature causes problems when prepared into bio-composites through injection molding. That is, when PLA in mixture form with natural fillers having a higher Young's modulus is injection-molded, the resulting bio-composites are apt to break. In addition to this brittleness, the natural material shares the problems attributable to the flour processing needed by injection molding.

In order to overcome the problems occurring when PLA fiber-based bio-composites are prepared by injection molding, the present inventors suggest a compression molding using a carding process. Further, the bio-composites prepared from PLA in combination with the general-purpose polymer PP by compression molding score well on various physical indexes and overcome the brittleness of PLA.

Technical Solution

It is an object of the present invention to provide a method for manufacturing a bio-composite through compression injection using a carding process by which the physical properties overcome the brittleness of PLA.

It is another object of the present invention to provide a method for manufacturing from PLA fibers a bio-composite improved in physical properties by manufacturing it in combination with the general-purpose polymer PP through compression injection using a carding process.

It is a further object of the present invention to provide bio-composites which can find application in various industrial fields requiring mechanical strength, including cases for electronic appliances and the interiors for cars.

ADVANTAGEOUS EFFECTS

The bio-composite manufactured using the method comprising: mixing poly(lactic acid) (PLA) fibers as a base polymer and natural fibers as a reinforcement through a carding process to give webs; processing the webs under a predetermined pressure into a mat; and compression molding the mat to a bio-composite plate which overcomes the problem of high brittleness of PLA and meets required physical properties including tensile strength, flexural strength and impact strength.

In addition, the method of the present invention guarantees strength and biodegradability in the bio-composites even if PLA fibers are used in combination with PP fibers. Having a certain degree of mechanical physical properties, the bio-composites of the present invention find application in various industrial fields requiring mechanical strength, including cases for electronic appliances and the interiors for cars.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram schematically showing processes of manufacturing bio-composites in accordance with the present invention;

FIG. 2 is a schematic view showing a method of measuring the flexural strength of bio-composites in accordance with the present invention;

FIG. 3 is a graph showing tensile strengths of bio-composites according to types of base polymer and the contents of natural fibers;

FIG. 4 is a graph showing tensile strengths of bio-composites according to types of base polymer;

FIG. 5 is a graph showing flexural strengths of bio-composites according to types of base polymer;

FIG. 6 is a graph showing impact strengths of bio-composites according to types of base polymer and the contents of natural fibers; and

FIG. 7 is a graph showing impact strengths of bio-composites according to types of base polymer.

BEST MODE

In order to above objects, there is provided a method for manufacturing a bio-composite, comprising: mixing poly(lactic acid) (PLA) fibers as a base polymer with natural fibers functioning as a reinforcing material in a carding process to produce webs; processing the webs under a predetermined pressure into a mat; and compression molding the mat to a bio-composite plate.

Also, provided is a method for manufacturing a bio composite, comprising: mixing poly(lactic acid) (PLA) fibers as a base polymer with natural fibers functioning as a reinforcing material in combination with polypropylene fibers in a carding process to produce webs; processing the webs under a predetermined pressure into a mat; and compression molding the mat to a bio-composite plate.

The bio-composites overcome the problem of high brittleness of PLA and meet required physical properties including tensile strength, flexural strength and impact strength.

In addition, the method of the present invention guarantees strength and biodegradability in the bio-composites even if PLA fibers are used in combination with PP fibers. Having a certain degree of mechanical physical properties, the bio-composites of the present invention find various industrial applications including electronic appliances and interior materials of cars.

A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as limiting the present invention

First, a description is given of the components of the bio-composite according to the present invention, and then of the bio-composites and experiments therewith. As used herein, the term “carding process”, means a process by which raw fibers in small aggregates are separated into individual yarns with the removal of unwanted materials or unwanted short lengths, and then they are placed in parallel to each other and prepared into slivers. The carding process is conducted using a carding machine.

In the present invention, fibrous poly(lactic acid) (PLA) is used as a bio-degradable polymer. It is 30 mm long and ranges in melt index from 10 to 30 g/10 min (190° C./2,160 g) with a density of 1.22 g/cm3. PLA is represented by the following Chemical Formula 1.

A non-biodegradable material used in combination with PLA is polypropylene (PP) fibers from Kolon, Korea. The PP fibers have a density of 0.91 g/cm³ and an MFI of 12 g/10 min (230° C./2,160 g) and are 30 mm long.

In accordance with the present invention, PLA and PP fibers may optionally be used together with the natural fibers kenaf and jute (imported by Soo Trading Company, Korea) as a reinforcement for the bio-composites of the present invention. Additionally, natural fibers 50˜70 mm in length may be used as a reinforcement.

Example 1

Manufacture of PLA Bio-Composites

PLA bio-composites were manufactured using the following compression-molding process. First, the bio-degradable PLA fibers and/or non-biodegradable PP fibers and the kenaf or jute fibers are mixed using a carding machine. After being punched with a needle punch, the mixed web thus obtained is processed at 120° C. under a predetermined pressure into a mat which was then subjected to compression molding. For this compression molding, the temperature was set at 200° C. under a pressure of 70 kgf/cm2. Throughout this process, bio-composite plates were constructed. They were stored in polyethylene bags in order to prevent moisture from penetrating thereinto. FIG. 1 shows the manufacturing processes of PLA bio-composites comprising a carding process. The compositions of the bio-composites manufactured by the processes are summarized in Table 1. The mixture ratios of the component fibers are represented by wt %.

TABLE 1
Compositions of Bio-Composites (wt %)
			Natural Fiber
Nos.	PP Contents	PLA Contents	Contents
1	70		Kenaf, Jute,
			Abaca, 30 each
2	50		Kenaf, Jute,
			Abaca, 50 each
3	30		Kenaf, Jute,
			Abaca, 70 each
4		90	Jute 10
5		70	Jute 30
6		50	Jute 50
7		30	Jute 70
8	35	15	Jute 50
9	25	25	Jute 50
10	15	35	Jute 50

Experimental Example 1

Preparation of Specimens

From the dried plate, specimens for use in tensile strength and flexural strength tests under 5 MPa were prepared using a punching press. They were incubated for 40 hours at a temperature of 23±2° C. and an RH of 50±5%.

Experimental Example 2

Measuring Tensile Strength of Bio-Composite

The specimens were measured for tensile strength so as to examine the effects of the carding process and the polymer materials on the bio-composites. Tensile strength was measured according to ASTM D638-03 using a Universal Testing Machine (Zwick Co.) at room temperature with a cross-head speed set at 5 mm/min. An average value of five measurements was obtained for the tensile strength.

Experimental Example 3

Measuring Flexural Strength of Bio-Composite

The specimens were measured for flexural strength so as to examine the effects of the carding process and the polymer materials on the bio-composites. Flexural strength was measured according to ASTM D790-03 using a Universal Testing Machine (Zwick Co.) at room temperature with a compression speed set at 5 mm/min. An average value of five measurements was obtained for the tensile strength. FIG. 2 illustrates a three-point flexural test.

Experimental Example 4

Measuring Impact Strength of Bio-Composite

Impact strength is the energy per area or length which is required to fracture a specimen subjected to shock loading. According to ASTM D256, impact strength is represented by energy per width of impact side. Measurement is generally conducted in three manners: a notch process in which a measurement is achieved in the same direction as a notch, an un-notch process in which a measurement is achieved in the direction opposite to a notch, and a final process wherein an impact is loaded on a vertically-standing specimen. In this example, impact strength was tested using a notch process.

1. Tensile Strength According to Polymer Material

FIG. 3 shows tensile strengths of bio-composites composed of PP plus natural fibers or PLA plus jute fibers. As seen in this graph, the bio-composite composed of natural fiber and PP decreased in tensile strength with increasing content of the natural fibers. The decrease was gradual when the content of the natural fiber was below 30%, but steep when more contents were used, indicating that when a lot of natural fibers were added as a reinforcement, the bond between PP and the natural fibers was weakened and the tensile strength of the bio-composite was greatly decreased. In addition, higher contents of natural fibers increase void volumes within the bio-composite, resulting in a decrease in tensile strength, as elucidated in Table 2 below. Void volumes were found to increase with an increasing content of natural fibers. Void volumes interfere with the bonding of PP to jute fibers and block the transmission of stress, thus causing a decrease in tensile strength. The content of voids' interfaces comprised voids at interfaces between natural fibers and within natural fibers. When prepared into specimens with a low content of natural fibers, the bio-composite based on PLA was apt to crack on the surface of the specimens due to the brittleness of PLA. As a result, low strengths were obtained in the bio-composite. On the other hand, bio-composites with a natural fiber content of 50% or 70% were measured to be rather high in strength. Further, their strength was higher than that of the bio-composites based on PP alone. These results explained one of the reasons why a PLA polymer was selected as a base polymer for bio-composites.

TABLE 2
Void Contents According to Contents of
Natural Fibers in Jute/PP Composites
Types of Bio-	Measured	Theoretical
Composite	Density(g/cm3)	Density (g/cm3)	Void Contens(%)
PP fiber 70 + Jute	0.99	1.03	3.2 ± 0.4
fiber 30
PP fiber 70 + Jute	1.03	1.11	7.4 ± 0.3
fiber 30
PP fiber 70 + Jute	1.03	1.18	12.3 ± 2.7
fiber 30

FIG. 4 shows tensile strengths according to base polymers used as bio-composites. PP, a mixture of PP and PLA, and PLA were used as respective base polymers (Type 1: PP fiber 50%+Jute fiber 50%, Type 2: PP fiber 35%+PLA fiber 15%+Jute fiber 50%, Type 3: PP fiber 25%+PLA fiber 25%+Jute fiber 50%, Type 4: PP fiber 15%+PLA fiber 35%+Jute fiber 50%, Type 5: PLA fiber 50%+Jute fiber 50%). The tensile strength was measured to be decreased only in Type 5 which used PLA as a sole base polymer, but showed similar values over the other bio-composites without significant reduction, indicating that PLA can partially substitute for PP without a significant decrease in tensile strength. That is, bio-composites based on PLA and PP show tensile strengths similar to those of PP-based composites, but are more biodegradable, and thus can be used as more environmentally friendly materials.

Also, as proven by the above results, the PLA bio-composites manufactured through compression molding using a carding process overcome the brittleness problem which the bio-composites manufactured through injection molding suffer from. In addition, although a portion of the PP fibers was substituted for by PLA, the bio-composites showed sufficient physical properties.

2. Flexural Strength According to Polymer Material

Flexural strengths of the bio-composites manufactured in Example 1 were measured and plotted in FIG. 5. As shown, even the use of Jute fibers at a content of 50% in bio-composites composed of PP and PLA makes no difference in strength over the bio-composites composed of PP and PLA, suggesting that a combination of PP and PLA as well as PLA only can be useful as base polymers for bio-composites.

3. Impact Strength According to Polymer Material

With reference to FIG. 6, the impact strength of the bio-composites increased as the content of natural fibers increased. The improvement of impact strength translates into meaning that the bio-composites overcame the brittleness of the base polymer. The impact strengths of the bio-composites composed of mixed base polymers are shown in FIG. 7. As apparent from the data of the graph, the bio-composites composed of various mixtures of PP and PLA plus jute fibers were found to have sufficient impact strength.

Claims

1. A method for manufacturing a bio-composite, comprising:

mixing poly(lactic acid) (PLA) fibers as a base polymer with natural fibers functioning as a reinforcing material in a carding process to produce webs;

processing the webs under a predetermined pressure into a mat; and

compression molding the mat to a bio-composite plate.

2. The method according to claim 1, wherein PLA fibers are used in combination with polypropylene fibers to give a web.

3. The method according to claim 1, wherein the natural fibers are kenaf or jute fibers.

4. A poly(lactic acid) bio-composite material, manufactured by the method of claim 1.

5. The method according to claim 2, wherein the natural fibers are kenaf or jute fibers.

6. A poly(lactic acid) bio-composite material, manufactured by the method of claim 2.

7. A poly(lactic acid) bio-composite material, manufactured by the method of claim 3.

8. A poly(lactic acid) bio-composite material, manufactured by the method of claim 5.