MORE SUSTAINABLE NON-WOVEN FABRIC COMPOSITES FOR AUTOMOTIVE USING COIR (COCONUT) FIBERS

D. Stanton Greer, Whole Tree Inc., Waco, TX 76712 Walter L. Bradley, Baylor University, Waco, TX 76798

Danny Natividad, Hobbs Bonded Fibers, Waco, TX 76710 Ryan J. Vano, Baylor University, Waco, TX 76798

Abstract

More environmentally friendly composite materials for automotive manufacturing have been made by substituting coir fibers for the widely used polyester fibers to make non-woven fabric composites of coir fibers and recycled polypropylene fibers. This more environmentally friendly composite that can be compression molded into a wide range of parts has a greater bending stiffness, is more resistant to fire, less expensive and without the odor problems that accompany many natural fibers.

Introduction

Consumers today are increasingly interested in purchasing products that have been manufactured using more sustainable, environmentally friendly materials, and the automobile industry is a major consumer of these materials. Non-woven fabric composites are widely used in industry with over 500 million kg/yr consumed [1]; for example, to make trunk trim, door panels, ceilings and dash boards for automobiles or interior panels for buildings.

A traditional non-woven fabric composite Non-woven fabric composites are made from two or more fibers that are physically

intertwined without weaving. Commonly, the fibers can be layered by either mechanical carding or air deposit and then combined through mechanical needle punching, thermal bonding, or spray glue. The non-woven fabric can then be impregnated with resin, combined with different fabrics, or compression molded into a variety of shapes. Trunk lining parts are often compression molded from fabric rolls made of a 50:50 mixture of polyester and recycled polypropylene fibers. To produce the rolls, the fibers are pre-cut to 5-7 cm lengths, blended, randomly laid down in layers, compression rolled to a desired density (or thickness) and consolidated using needle punching (not weaving). The resulting felted material has a density of 0.1-0.3 g/cm3, a low tensile strength that is sufficient for handling and a high degree of flexibly for subsequent compression molding. Figure 1 shows a micrograph (~10X) of felted material comprised of polypropylene fibers (light colored) and polyester fibers (dark colored) prior to compression molding.

The resulting felted material, as seen in Figure 2, can be compression molded into parts as

seen in Figure 3. The part has a microstructure that reflects the flow of the lower melting point fibers (PP) effectively connecting the higher melting point fibers (PET) into a web, as seen in Figure 4. This web of PET fibers reinforces the resulting matrix of melted PP fibers.

The processing window for the compression molding of non-woven fabric composites with

two fibers is between the melting temperatures of the lowest melting temperature fibers and the highest melting temperature fibers (or degradation temperature when it is lower than the melting temperature). For polyester/propylene (PET:PP) non-woven fabric composites, this would be between 180C and 250C.

The density of compression molded PET:PP parts can range from 0.3 g/cm3 to 1.2 g/cm3 [3]. This is 25-100% of the ideal density of the composite, which is ~1.2 g/cm3, since it is comprised of 50% polypropylene fibers with a density of 0.91 g/cm3 and 50% polyester fibers with a density

of 1.4 g/cm3. The actual density achieved during compression molding depends on the compression molding temperature, the molding pressure, the time at temperature and the fibers used. For a given amount of PP flow, the bending stiffness of the higher melting point fiber, which is determined by the diameter and flexural modulus, is very important in the resulting density of the compression molded part. A wide range in density will give a significant variation in the mechanical properties of the various non-woven fabric composite materials produced by compression molding [3], allowing the mechanical properties to be “tuned” for various applications.

The total market for PET:PP non-woven fabric composites exceeded 400,000 kg/yr in 1998

and has continued to grow since [2]. If natural fibers could be developed to replace the high melting point PET fibers, then more sustainable automobile parts would be available for the industry that eliminate 1.8 million pounds of CO2 annually.

Replacing PET with natural fiber(s) The emphasis on sustainable and environmentally friendly products has created an

opportunity to consider substituting natural fibers for synthetic fibers [5] in non-woven fabric composite materials. Natural fibers can only be substituted for the high melting temperature fibers in the non-woven mat since they normally degrade by oxidation before they reach a temperature where they can flow, rendering them unsuitable to be the lower melting temperature fiber and eventual composite matrix.

There are at least 15 natural fibers that could be considered to make non-woven fabric

composites. Their mechanical and physical properties vary widely depending on the relative amounts of lignin, cellulose and hemi-cellulose in their respective biomass. An excellent summary of their properties and how they are produced may be found at a website sponsored by the United Nations entitled, 2009: The International Year of the Natural Fiber [4]. Their quality and costs can also vary widely. The fibers that have received the most attention to date include flax, jute, hemp, sisal and kenaf. The advantages of some of these natural fibers compared to polyester fibers include lower costs, lighter weight, better dimensional stability, and enhanced sustainability and ease of recycling. Disadvantages include their variability in quality and sometimes properties, hydrophilic nature, and unreliable supply chains, which are the reasons these fibers have lost much of their market share to synthetic fibers in the last half-century.

Using coir fibers from coconut husks The objective of this investigation has been to determine the efficacy of coir fiber that is

derived from the husk of coconuts as a candidate for substitution for polyester fibers in non-woven fabric composites made with polypropylene as the low-melting point fiber. Potential automotive applications include package trays (behind rear seats) and trunk trim. Coir fiber was selected for this study for four reasons. First, coir fibers have an attractive combination of strength, stiffness and elongation (and by implication, impact strength) [5]. Second, coir fibers have the highest lignin content (35%) of any natural fiber [6,7], making them quite resistant to burning, odor producing decay due to fungal attack or degradation induced by other microorganisms. Third, coir fibers are also generally very durable in wet environments, which are well demonstrated by their widespread uses in the production of large diameter ropes that are used to tie-up ocean-going vessels. Fourth, there are 50 billion coconuts that grow each year and the vast majority of them are burned as trash, damaging the environment and

depriving poor coconut farmers of some additional income to the $500/year on average that they currently make [8].

Another interesting aspect of this investigation is the indirect impact that the fiber diameter

has on the mechanical properties through its effects on compression molding [4]. A non-woven fabric composite made with polypropylene fibers with a diameter of ~40 μm combined with polyester fibers with a small diameter of ~40 μm into a felted material compression molds differently than a felt with polypropylene combined with a large diameter natural fiber such as coir fibers (d~170μm). The coir fibers will be much stiffer, limiting the maximum density achievable, giving a lower density for similar compression molding PP flow, and therefore, different mechanical properties. The lower densities are due to the coir fibers' inability to flex and bend during processing to consolidate out all of the empty air space. The naturally lower consolidation means that for a given degree of PP flow, the coir:PP will have a different set of mechanical properties than the typical PET:PP composite.

Natural fibers are also much more hydrophilic than synthetic fibers. This often results in poor

chemical compatibility and interfacial adhesion between natural fibers (high melting point fiber) and synthetic fibers (low melting point fibers), which can diminish the mechanical properties of the composite [7, 9-10]. Coir fibers are less hydrophilic than other natural fibers because of the high lignin content.

In what follows, an experimental plan and procedures will be outlined for substituting coir

fibers from coconuts husks for polyester fiber in non-woven fabric composite materials made with polypropylene fibers, to determine their effect(s) on the processing and mechanical properties of the resultant composite. Experiment results from the executed plan will then be presented and discussed. Finally conclusions drawn from this work will be summarized.

Experimental Plan and Procedures

The experimental plan included the following steps: 1. Secure from the Philippines 1500 kg of coir fiber, carefully cleaned to remove all pieces of

the skin on the outer surface of the husks, cut to 5-7.5 cm lengths, and delivered in bales as seen in Figure 5.

2. Mix this fiber with recycled (post industrial) polypropylene fiber, also cut to 5-7.5 cm length.

Card and needle punch 1.5m x 10m rolls at Hobbs Bonded Fibers (Figure 6) with an areal density of 1000 g/m2.

3. Photograph in Leica binocular micrographs before and after compression molding (Figure 7). 4. Hot press 9in x 12in pieces of felt cut from the larger rolls, using different combinations of

temperature (170C to 230C), pressure (0.33 to 1 MPa) and time (15-60s) to produce rigid plaques.

5. Measure the density of each plaque and then cut each into 10 flexural test specimens. For

each specimen measure the density, determine the flexural stiffness on a table-top Instron, cut tensile specimens from the flexural specimens and determine the tensile strength and elongation on the Instron.

6. Repeat for 50:50 PET:PP non-woven fabric composite fiber mixture from Hobbs Bonded

Fibers and compression molded to different densities.

Experimental Results and Discussion

Mechanical test results are presented in Figures 8-10, comparing the tensile strength, flexural modulus and flexural rigidity as a function of density for coir:PP and PET:PP composites. The PET:PP compression molded to a much higher density than the coir:PP at the same PP flow because the two small diameter fibers are much more flexible than the large diameter coir fiber, allow them to fill the “nooks and crannies” more efficiently. Since the polypropylene does the flowing in both non-woven fabric composites, the processing variable must be the difference in fiber stiffness. The bending stiffness is given be Ef * I, where I = π r4/4. Since the coir fiber has a diameter that is more than 4X the diameter of the polyester fiber, it is clear that the bending stiffness E*I is much greater for the coir fiber, even though its tensile modulus is less than that for polyester. Note that the flexural modulus and tensile strength increase monotonically with increasing density. This make sense as a denser material has a greater total load-bearing cross-sectional area of fibers to support the load in tension or bending at the higher densities, which results in a higher macroscopic strength and stiffness. The most interesting result of the study is seen in Figure 10. Because the specimens all had the same areal density of 1000 g/m2 , the specimens with a lower density after compression molding were thicker, and thicker specimens gave a higher “I” value in the bending stiffness term Ef *I, even though it has a lower flexural modulus Ef . If the extra stiffness is not needed, then a lower areal density felt could be compression molded to get the equivalent Ef*I value to that for the PET:PP, reducing the weight and cost by using less material. A comparison of the properties of the PET:PP to the coir:PP non-woven fabric composites is shown in Table I. Since most applications are driven more by compression strength and bending stiffness than tensile strength, the coir:PP looks very favorable in the comparisons. The results of other comparisons such as burn tests results that have been completed as part of this study are also included in Table I. The results of this study have lead to the filing of a utility patent; Serial Number 12/574,518 entitled “Non-woven Fabric Composites from Lignin-rich, Large Diameter Natural Fibers” by Baylor University, with an exclusive license agreement between Baylor University and Whole Tree Inc. (www.wholetreeinc.com) for commercialization of the technology.

Summary

1. Non-woven fabric composites made using coir fiber and polypropylene have been found to provide a very attractive family of physical and mechanical properties when compared with the widely used PET:PP non-woven fabric composite.

2. The large fiber diameter for coir fiber and its high lignin content gives coir fiber a competitive

advantage over other natural and synthetic fibers for some applications.

3. Because coconuts are an abundant, renewable resources that is currently under-utilized, they provides an excellent resource for making more environmentally friendly, non-woven fabric composite materials at very competitive price points.

Acknowledgments

Financial support of the National Science Foundation through SBIR: 0912360 is gratefully acknowledged.

References

1. D. Harrison, “Synthetic Fibers for Non-wovens Update”, Nonwovens Industry, 28 (6) 32-39 (1997).

2. D. Harrison, “Shipment of Fibers to Non-wovens Reported for 1998”, Nonwovens Industry, 29 (6), 52 (1998).

3. D.S. Greer and W.L. Bradley, unpublished data from work done at Baylor University, Waco, TX 76798.

4. 2009: The International Year of the Natural Fiber, United Nations Website, www.naturalfibres2009.org/en/fibres/coir

5. K.M.M. Rao and K.M. Rao, “Extraction and tensile properties of natural fibers: Vakka, date and bamboo”, Composite Structures 77, 288-295 (2007).

6. K. Sindhu and J. Kuruvilla, “Degradation Studies of Coir Fiber / Polyester and Glass Fiber / Polyester Composites Under Different Conditions”, Journal of Reinforced Plastics and Composites, prepublication copy, (2007).

7. Polyolefin Composites, 2008, edited by Domasius Nwabunma and Thein Kyu. (New York: Wiley Interscience, A John Wiley & Sons, Inc, Publications)

8. R.N. Arancon, Jr, Asian and Pacific Coconut Community, www.apccsec.org. 9. K.M.M. Rao, K.M. Rao, “Extraction and Tensile Properties of Natural Fibers”, Journal of

Composite Structures, 288-295, (2005). 10. S. Fakirov and D. Bhattacharyya, eds., Handbook of Engineering Biopolymers:

Homopolymers, Blends and Composites, 2007, (New York: Hanser Gardner)

Figure 1 - Unpressed PET:PP nonwoven (10x) Figure 2 - Dark PET, white PP, resulting nonwoven

Figure 3 - Compression molded automoblile part Figure 4 - Compression molded PET:PP non-woven

Figure 5 - Bales of coir (coconut husk) fiber Figure 6 - Exmple of rolled material

Figure 7 - Coir:PP nonwoven (~10X). Left: uncompressed, Center: pressed at 180C, Right: pressed at 220C

Figure 8 - Tensile strength vs density for coir:PP and PET:PP compression molded non-woven composites

Figure 9 - Flexural modulus vs density for coir:PP and PET:PP compression molded non-woven composites

Figure 10 - Flexural rigidity vs density for coir:PP and PET:PP compression molded non-woven composites

Table I - Comparison of non-woven fabric composites of coir:PP fiber and PET:PP fiber

Coir:PP PET:PP Areal density 1000 1000 g/m^2

Unpressed Bulk Density 0.16 0.32 g/cm^3 Pressed Thickness 0.20 0.10 cm

Pressed Bulk Density 0.5 1.0 g/cm^3

Tensile Strength UTS 12 36 MPa

Specific Tensile Strength UTS/SG* 24 36 MPa

Tensile Modulus Et 690 2500 MPa

Specific Tensile Modulus Et/SG 1380 2500 MPa

Elongation L/L 20 27 %

Flexural Modulus Ef 470 1400 MPa

Specific Flexural Modulus Ef/SG 950 1400 MPa

Flexural Rigidity** EfI 330 120 N-cm^2

Specific Flexural Rigidity** EfI/SG 660 120 N-cm^2

Burn Testing for GM9070P 1.00 1.25 in/min.

* SG is specific gravity = material / water

**Moment of Inertia (I) calculated for 1in wide for indicated pressed thickness