COMPOSITES 2010

COMPOSITES 2010 American Composites Manufacturers Association February 9-11, 2010 Las Vegas, Nevada USA

Natural Fiber Reinforced Pultruded Composites

by

Bhyrav Mutnuri, Bedford Reinforced Plastics Can (John) Aktas, University of Pittsburgh

Joe Marriott, University of Pittsburgh Melissa Bilec, University of Pittsburgh

Hota Gangarao, West Virginia University

ABSTRACT

Feasibility of pultruding composites and their cha-racterization using bio-based constituent materials are discussed to address the growing demand for sustainable materials. Various mechanical and physical properties of Natural Fiber Reinforced Polymer (NFRP) Composites and NFRP/E-Glass hybrid composites, and life cycle as-sessment including fiber treatment are reported herein with a view to develop green composite sound wall pa-nels and other applications. Based on the experimental results and observations, it is concluded that hybrid (NFRP/E-glass) composites are better than NFRP com-posites. A hybrid composite made for this project proved to be a good thermo-acoustic insulator and for other non-load bearing applications. This paper identifies the need for high strength and stiffness (high grade) natural fibers with properties on par with glass fibers to originate load bearing green composites.

INTRODUCTION

Pultruded glass fiber reinforced polymers are used in a wide range of non-structural to structural applica-tions. In a study comparing materials used for bridges, Glass FRP (GFRP) composites showed less than half the embodied energy (overall energy required in a process to make a product) than aluminum, stainless steel and structural steel [1]. Keeping in view of durability, sustai-nability, energy efficiency in manufacturing and other advantages, a growing number of architects and building owners are now choosing FRP based building products over conventional materials. Moreover, increasing awareness about LEED ratings created increasing de-mand for innovative eco-friendly materials. Several case studies have proven that fiberglass components for build-ing construction are good thermal insulators, economical,

strong, dent resistant, scratch proof, with good acoustic barrier properties and user friendliness [2].

Present glass fiber reinforced polymer (GFRP) composites are made from fossil fuel based polymers and synthetic fibers. With growing interest in developing an environmental friendly and efficient embodied energy alternative to conventional glass reinforced composites, plant-based naturally renewable fiber reinforcement with bio-based resin system to make “green” composites has been in consideration for the last few years. Research showed that natural fibers such as flax are available for half the cost of conventional E-glass fiber and minimizes the emission of CO and CO2 [3]. In the process of serv-ing customers’ requirements and to provide basic infor-mation to manufacture “green” composites, Bedford Reinforced Plastics Inc (BRP) has explored processing methods and thermo-mechanical responses for an indus-trial scale pultrusion of environmentally benign natural fiber composites using commercially available bio-based resin system and natural fibers, as reported herein. Moreover, the possibilities of using “green” composites in various applications such as thermal insulators and sound transmission barriers are also investigated for their feasibility in FRP markets.

NATURAL FIBERS

Natural fibers used in composites are mostly de-rived from plant fibers. Among the natural fibers, stem-based fibers such as flax, kenaf, jute, hemp, and leaf-based fibers such as abaca rattan and sisal are considered important with respect to their specific properties and compatibility for composite manufacturing [4]. Amongst these fibers, high grade flax fiber’s mechanical proper-ties are nearly on par with conventional E-glass fiber for composite reinforcement [4,5,6,7] and research. A com-parison between natural fibers and conventional fibers with respect to their mechanical properties and cost ratio are presented in Table 1.

In this study, commercially available flax with two different densities 225 gsm and 685 gsm have been used. Due to unavailability of readily purchasable flax fabric in USA, this reinforcement was imported from Asia and the cost of the fabric including shipping is approximately three times higher than conventional E-glass fibers.

BIO-BASED RESINS

With an imperative need to curb the usage of petro-leum based thermoset resins due to environmental as well as economic and resource sustainability issues, re-searchers have been developing bio-based and sustaina-ble resin systems for composite manufacturing. Upon processing, plant based materials such as soy, crambe, linseed and castor oil produce unsaturated triglycerides.

COMPOSITES 2010

The triglycerides constitute unsaturated and saturated fatty acids which can be polymerized to form elastomeric network which can replace petro-based resins [4,11]. With constant emphasis from markets pertaining to LEED and US Department of Agriculture’s BioPreffered Program, Ashland, Inc. Chemicals Company initiated production of soy-based/ bio-based resins for commer-cial applications [12]. BRP has used ENVIREZ 70301 resin, which contains 22% bio-derived content and is compatible to run for pultrusion process.

SAMPLE PREPERATION

Alkali Treatment

Although natural fibers show excellent engineering properties, they still have limitations such as moisture retention, inadequate interfacial bonding with polymers, limited processing temperature tolerance, and low di-mensional stability [8,9,10]. To counteract these limita-tions, natural fibers (especially the surface of natural fi-bers) need chemical modification, and that process is called “sizing” the fibers. After preliminary decortication stages, depending on fiber type and ultimate require-ments, natural fibers are subject to a series of chemical treatments. Out of these chemical treatments, alkali treatment, acetalization and saline treatment are primari-ly considered for enhancement of mechanical and ther-mal properties by improving interfacial bonding, mois-ture resistance, thermal tolerance and dimensional sta-bility[6,8,9,10].

By making use of available research on successful sizing of flax fibers [8,9], alkalization process was adopted in this study to develop sizing for these natural fibers. Alkali treatment (Equation 1) purges natural and artificial impurities, reduces fiber diameter and thus in-creases aspect ratio and develops rough surface topogra-phy. Increase in aspect ratio leads to increase in mechan-ical properties [6,13] and rough fiber surface enhances mechanical interlocking and cellulose exposure on the fiber.

Fiber-OH + NaOH → Fiber –O-Na

+ + H2O Eq(1)

In this project, flax fabrics with both the densities 225 gsm and 685 gsm were chemically treated with 2% and 10% NaOH solution for 10 mins and 30 mins. Com-posite samples were made using these chemically treated fabrics with ENVIREZ resin using hand lay-up tech-nique. For mechanical property characterization of these samples, coupon samples were prepared for tensile and flexural testing. The test results are given in Table 2,Table 3 and analyzed.

Two types of samples were pultruded to compare between 100% natural fiber composites and hybrid com-posites with natural fiber and E-glass. Varieties of tests were performed on these samples and the details are giv-en below:

PULTRUDED NATURAL FIBER COMPOSITE (NFRP)

To evaluate the mechanical properties of flax com-posites, a 0.25 inch thick flat sheet with 24" width was pultruded using 685 gsm flax fabric and ENVIREZ resin. Lack of linear reinforcement (flax yarns or glass rovings) and stiffness in the fabric to withstand the pulling force of the caterpillar type puller, resulted waviness and shrinkage of part in width direction and also decreased the pull speed by approximately 40% in relation to fiber-glass pultrusion. Shrinkage of panel size when compared to Hybrid composite is shown in Figure 1. Coupon sam-ples were prepared to characterize tensile (length wise and crosswise), flexural (length wise and cross wise) and interlaminar shear strengths. In addition, water absorp-tion, thermal conductivity properties were established and reported herein. Constituent material details of the NFRP composite and NFRP/Glass composite are given in Table 4.

PULTRUDED NATURAL FIBER/GLASS HYBRID COMPOSITE

To evaluate the mechanical properties and advan-tages of a natural fiber/E-glass hybrid composite, a 0.25 inch thick by two feet wide flat sheet was pultruded us-ing 685 gsm flax fabric/glass rovings and ENVIREZ re-sin. This panel was used for testing sound transmission loss using ASTM E 90-09, tensile (length wise and crosswise), flexural (length wise and cross wise) and thermal conductivity properties. The details of these tests are furnished in Table 5, Table 6, Table 7, Table 8 and Table 9 providing data on flexural, sound transmission loss and thermal conductivity. Presence of rovings in Hybrid composite increased the processing speed, de-creased the shrinkage and waviness of the product when compared to NFRP composite (See Figure 1).

TESTING METHODS AND INSTRUMENTA-TION

Test methods used for coupon samples prepared from hand lay-up, pultruded flax reinforced composite, pultruded flax/glass hybrid composite are ASTM D 638, ASTM 790, ASTM D 2344, ASTM D 570-95, ASTM E 1530 and ASTM E 90-09 respectively for tensile, flexur-al, inter-laminar shear, thermal conductivity and sound transmission loss.

COMPOSITES 2010

For tensile testing, Tinius-Olsen universal testing machine is being used along with an extensometer to measure the strain values. The coupons for tensile tests are cut to a size of 10 in x 0.5 in with a gage length of 2 inches.

For flexural testing, Instron universal testing ma-chine was used and the size of the samples is 5 in x 0.5 inch with 3 and 4 inch simply supported span.

Interlaminar shear test (short beam shear test) was performed by using Instron machine. The coupon sam-ples for this test were cut to a size of 1.5 in x 0.25 in x 0.25 in.

24 hour water absorption test was conducted ac-cording to ASTM D 570-95 using a sample size of 3 in x 1 in x ¼ in.

To measure the thermal conductivity of natural fi-ber and Hybrid composites, UNITHERM 2022 thermal conductivity measuring device (Constructed Facilities Center – West Virginia University) was used. According to the test protocol, a 2 inch diameter disk was cut from the panels to measure conductivity at 30oC.

SOUND TRANSMISSION LOSS TESTING

A 4 feet x 8 feet x 0.25 inch thick natural fiber-glass hybrid composite was used to evaluate the sound barrier characteristics of pultruded hybrid composite. This testing was performed by Owens Corning Acoustic and Insulation Product Testing Laboratory in accordance with ASTM Standard Test Method for Laboratory Mea-surement of Airborne Sound Transmission Loss of Building Partitions and Elements: ASTM E 90 – 09 and Classification of Rating Sound Insulation ASTM E 413-04.

RESULTS AND DISCUSSION

Alkali Treatment

Alkali treatment to the flax fabric used for this study caused no significant improvement to the mechani-cal properties of the composite. Alkali treatment would remove contaminants from the surface and increase the surface roughness of the fiber, but excessive exposure to alkali treatment might lead to cracks on the fiber surface [14] which could result in deterioration of mechanical properties. As the flax was processed by the flax fabric manufacturer using hot water and bleach before weaving it into a fabric, the elemental composition of the fiber is unknown and advanced techniques such as SEM, ATR-IR spectroscopy, X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) are needed to research

further on the effect of sizing or any chemical treatment to natural fibers.

Tensile Properties

Tensile properties of both NFRP and NFRP/Glass hybrid samples were tested in both lengthwise and cross wise directions. The tensile tests of flax composites showed inferior properties when compared to some of the published literature [7]. This might be due to the dif-ference in grade of the flax fiber, inconsistency in processing, sizing (chemical treatment) of the fiber etc. Although the flax fabric used for pultrusion in this project (685 gsm) is equally dense in both weft and warp directions, there is approximately 43% reduction in modulus value in cross wise direction than in longitudin-al direction. Similar trend was observed for maximum tensile strength value. This change in modulus value might have occurred due to the waviness caused in cross wise direction during the pultrusion. This waviness is clearly visible on the surface of the sample.

Flexural Properties

To evaluate the flexural properties of NFRP and Hybrid composite samples, bending tests were conducted on 3 and 4 in spans. It was observed that the NFRP samples with 75% flax by volume demonstrated higher flexural modulus and stress values than hybrid samples with 60% flax and 6% glass rovings by volume. This disparity of modulus value can be attributed to the placement of sin-gle band of rovings (6% Vf) in the mid layer (neutral axis region). The volume of flax fabric was decreased by 15% to accommodate glass rovings by 6%. In this test, a bi-linear load-deflection curve was observed for both NFRP and Hybrid composites. This phenomenon might be due to initiation of micro-cracking in the fabric during load-ing in the samples. Further research is needed in this area to understand this failure phenomenon. The results of the test and graphs including the slopes and correlation coef-ficient of the bi-linear segments are given in Table 7 and Figure 3. When compared with NFRP samples, hybrid samples exhibited better ductility before final rupture by deflecting more. It was observed that the failure trend for NFRP and hybrid composite samples is consistent for all the five samples tested in each case. For comparison purpose, a generic FRP sample was also tested along with NFRP and Hybrid samples and reported.

Sound Transmission Loss

The test results showed that the tested hybrid panel belongs to a sound transmission class (STC) of 31 (See Table 8). Available literature shows that a typical stan-dard stud wall with one layer of 0.625 inch (5/8th inch) thick plaster board (drywall) has a STC of 28 [15]. It can be inferred that this present hybrid wall panel is compa-

COMPOSITES 2010

rable or better than a standard drywall panel for commer-cial building acoustic barrier applications. The other ap-plications of this hybrid panels could range from floor panels to highway sound wall panels. Further research and testing is needed to evaluate the hybrid panels for those specific applications.

Water Absorption Test:

A 24 hour water absorption test confirmed the hy-drophilic nature of the natural fiber composites. The test showed that both NFRP and Hybrid composite samples absorbed up to 2.21% and 2.15% of water in 24 hours time as opposed to 0.45% max for a typical GFRP com-posite.

Thermal Conductivity Results:

Although the thermal conductivity results of NFRP, Hybrid and GFRP are similar with minimal differences, NFRP sample with 75% Vf showed better thermal insu-lation when compared to hybrid and GFRP composites. The details of thermal conductivity test at 30o C are re-ported in Table 7.

Life Cycle Energy Assessment (LCEA)

Life Cycle Energy Assessment analysis was per-formed comparing a traditional glass fiber product with respect to a new composite made by pultruding flax fi-bers in the form of fabric together with a bio-resin. It was calculated that flax fiber composites resulted in approx-imately 40-60% less energy consumption when com-pared to glass fiber composites. Transportation, raw ma-terials extraction and processing, and manufacturing of the composite were considered in the comparative analy-sis. Approximate embodied energies involved in produc-ing GFRP, Hybrid and NFRP samples for this project are reported as 1500-1850 Btu/in3, 650-800 Btu/in3 and 400-630 Btu/in3 respectively. Further details about research accomplishments and procedures will be published in a separate article by research partners on the same project from University of Pittsburgh.

Summary and Conclusions

To meet the immense demand for green composites, in this research, usage of natural fibers/fabric as reinforce-ment was evaluated for availability, processibility, ther-mo-mechanical advantages and challenges. Albeit supe-rior mechanical properties, a reliable source of commer-cial grade flax supplier is unavailable in USA. This re-sulted in importing the raw material which led to higher raw material cost (by three times) and added embodied energy to the final product. Moreover, pre-processing of flax material using hot water and bleach by fabric manu-

facturer to make flax fabric hinders the opportunity to surface treat the material and evaluate the advantages of any chemical treatment to the raw material. This empha-sized the need for research and development of high grade flax fiber processing exclusive for composites manufacturing.

From a pultrusion process stand point, even though NFRP composites yield lower embodied energy and comparable material properties, a hybrid composite with E-glass rovings proved as a better green alternative for non-load bearing applications because of advantages such as ease in processing, better consistency in the di-mensions of the final product and superior mechanical properties. Overall, NFRP and Hybrid composites proved to be better insulating materials than GFRP com-posites and Sound Transmission Loss testing demon-strated superior sound insulation properties for Hybrid composites. Life cycle energy assessment results proved that by replacing GFRP with a hybrid composite could save up to 40-60% of embodied energy required to make a GFRP composite. It is evident from this project that hybrid composites can be commercially produced and used in non-load bearing applications.

Acknowledgements

This research was funded in part by a grant from the Green Building Alliance in collaboration with the Pennsylvania Green Growth Partnership, an initiative funded in part by the Commonwealth of Pennsylvania, Ben Franklin Technology Development Authority, the Richard King Mellon Foundation, and The Heinz En-dowments. BRP thankfully acknowledges the support provided by Dr. Kent Harries from University of Pitts-burgh, Dr. Ray Liang, Siddalingesh Kalligudd and other students from Constructed Facilities Center-West Virgin-ia University for testing equipment and voluntary testing of materials, special thanks to Brian Caulkins, Don Hill and Dave Burd from Owens Corning for acoustic testing and expert suggestions from Dr. Holser and Dr. Foulk of USDA and Kathy Kitchen from Ashland Inc for her ex-pert advices and LCA information. The sample prepara-tion and testing work performed by BRP employees Odd-Arne Hogset, Bridgett Diehl, Stanley Michuk and Dave Lichvar is gratefully acknowledged.

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REFERENCES

1. Technical Sheet 07/07, “FRPs-Environmental Impact and Embodied Energy”, Network Group for Composites in Construction www.ngcc.org.uk 2. B. A Nadel, “Fiberglass Fenestration: A Dura-ble, Sustainable, and Economic Alternative for Windows and Doors”, McGraw Hill Construction-Continuing Education Center, June 2006 3. L B Manfredi, E S Rodriguez, M Wladyaka-Przybylak, A Vazquez, “Thermal Degradation and Fire Resistance of Unsaturated Polyester, Modified Acrylic Resins and their Composites with Natural Fibers”, Journal of Polymer Degradation and Sus-tainability 91 (2006) 255-261 4. L T Drazl, A K Mohanty R Burgueno and M Mishra “Biobased Structural Composite Materials for Housing and Infrastructure Applications: Op-portunities and Challenges” www.pathnet.org/si.asp?id=1076 5. S V Joshi, L T Drzal, A K Mohanty, S Arora, “Are Natural Fiber Composites Environmentally Superior to Glass Fiber Reinforced Composites?”, Composites: Part A 35 (2004) 371-376 6. D H Mueller, A Krobjolowski, “New Discovery in the Properties of Composites Reinforced with Natural Fibers”, J. Industrial Textiles, Vol. 33, No.2- October 2003 111-130 7. G B Gaceva, M Avella, M Malinconico, A Bu-zarovska, A Groxdano, G Gentile, M E Errico, “Na-tural Fiber Eco-Composites”, Polymer Composites, 2007 pp 98-107 8. D N Saheb and J P Jog, “Natural Fiber Polymer Composites- A Review”, Advances in Polymer Technology, Vol 18 No. 4, 351-363 (1999) 9. I V Weyenberg, J Ivens, A D Coster, B Kino, E Baetens, I Verpoest, “Influence Of Processing And Chemical Treatment Of Flax Fibres On Their Com-posites”, Composites Science and Technology, 63 (2003) 1241-1246+ 10. M J John, R D Anandjiwala, “Recent Develop-ments in Chemical Modification and Characteriza-tion of Natural Fiber Reinforced Composites”, Po-lymer Composites, 2008 pp 187-207 11. K Chandrashekara, S Sundararaman, V Flani-gan S Kapila, “Affordable Composites Using Re-newable Materials”, Materials Science and Engi-neering A 412(2005) pp 2-6 12. Composites Technology, “Bio-Composites Up-date: Bio-Based Resins Begin to Grow”; 04/1/08 http://www.compositesworld.com/articles/bio-composites-update-bio-based-resins-begin-to-grow.aspx 13. Z. Liu, S Z Erhan, D E Akin and F E Barton, “ “Green” Composites from Renewable Resources: Preparation of Epoxidized Soybean Oil and Flax Fi-ber Composites”, J. Agric. Food Chem. , 2006, 54, 2134-2137

14. T Nelson, M Hosur, A Netravali, M Theoodre S Jeelani, “Physical and Mechanical Characterization of Alkali Treated Kenaf Fibers”, SAMPE 2009 15. http://www.saecollege.de/reference_material/pages/STC%20Chart.htm

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Table 1. Mechanical Properties and Cost Ratio of Natural Fibers [7]

Table 2. Flexural Property Comparison

Description Max.

Stress, psi

SD Flex. Mod, msi

SD FlaxW%

1 Control Sample 225 g/m² 10150 332 0.44 0.023 35 2 2% NaOH, 10 min, 225 g/m² 11130 359 0.47 0.024 31 3 2% NaOH, 30 min, 225 g/m² 11530 343 0.47 0.038 31 4 10% NaOH, 10 min, 225 g/m² 10530 458 0.46 0.017 30 5 10% NaOH, 30 min, 225 g/m² 18420 1063 1.15 0.031 31 6 Control sample 685 g/m² 9470 199 0.44 0.011 40 7 2% NaOH, 10 min, 685 g/m² 8610 67 0.40 0.037 36 8 2% NaOH, 30 min, 685 g/m² 9290 661 0.42 0.024 36 9 10% NaOH, 10 min, 685 g/m² 8550 249 0.41 0.011 35

10 10% NaOH, 30 min, 685 g/m² 9270 461 0.44 0.031 36

Table 3. Tensile Property Comparison

Description Max.

Stress, psi

SD Tens. Mod, msi

SD FlaxW%

1 Control Sample 225 g/m² 8520 268 0.775 0.033 35 2 2% NaOH, 10 min, 225 g/m² 9540 138 0.840 0.052 31 3 2% NaOH, 30 min, 225 g/m² 9080 468 0.917 0.112 31 4 10% NaOH, 10 min, 225 g/m² 7490 975 0.743 0.072 30 5 10% NaOH, 30 min, 225 g/m² 7890 1642 0.841 0.076 31 6 Control sample 685 g/m² 8000 116 0.699 0.070 40 7 2% NaOH, 10 min, 685 g/m² 5650 307 0.560 0.004 36 8 2% NaOH, 30 min, 685 g/m² 6910 285 0.635 0.004 36 9 10% NaOH, 10 min, 685 g/m² 5170 216 0.618 0.095 35 10 10% NaOH, 30 min, 685 g/m² 6160 251 0.679 0.077 36

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Table 4. Details of NFRP/Hybrid and NFRP Pultruded Composites NFRP/Glass Hybrid

For 685 gsm lb/ft Wf, % density, lb/in3 Vf , % E, msi Specific Modulus Flax (6 layers) 1.67 40.34 0.032 53.73 1.78 54.99*

Glass (18 ends) 0.59 14.25 0.093 6.60 10.40 111.83 Resin (70301) 1.88 45.41 0.049 39.67 0.47 9.61

Total 4.14 100.00 0.058 100.0 1.83

NFRP For 685 gsm lb/ft Wf, % density, lb/in3 Vf , % E, msi Specific Modulus

Flax (8 layers) 2.23 59.73 0.032 69.38 1.88 58.02* Resin (70301) 1.50 40.27 0.049 30.62 0.47 9.61

Total 3.73 100.00 0.052 100.00 1.45 * Specific modulus values are based on density of bi-axial fabric, if only the fiber acting in tension

direction is considered, the specific modulus has to be nearly doubled.

Table 5. Flexural, Tensile and Short Beam Shear Properties of 685 gsm NFRP and Hybrid composite in Longitudinal Direction

Flexural Properties Flexural Properties, Length Wise

Hybrid SD NFRP SD Max Stress, psi 15941.62 724.6 16355.12 609.6 Modulus, psi 0.69E+06 0.04E+06 1.01E+06 0.08E+06

Tensile Properties Tensile Properties, Length Wise

NFRP/Glass SD NFRP SD Max Stress, psi 26000.00 465.00 12310.00 265.00 Modulus, psi 1.83E+06 7.91E+04 1.45E+06 1.40E+04

Short Beam Shear Short Beam Shear, Length Wise

NFRP/Glass SD NFRP SD Max. Stress, psi 3250.00 76.20 3060.00 102.00

Table 6. Flexural, Tensile Properties of 685 gsm NFRP and NFRP/Glass Hybrid Composite in Cross wise Direction

Flexural Properties Flexural Properties, Cross Wise

NFRP/Glass SD NFRP SD Max Stress, psi 11170.00 1010.00 8960.00 151.80 Modulus, psi 5.37E+05 2.89E+04 4.71E+05 1.63E+04

Tensile Properties Tensile Properties, Cross Wise

NFRP/Glass SD NFRP SD Max Stress, psi 6430.00 173.70 6110.00 151.90 Modulus, psi 1.15E+06 1.12E+05 8.26E+05 2.03E+05

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Table 7. Flexural test results Sheet for NFRP, Hybrid composites on a span of 3 and 4 inches

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Table 8. Sound Transmission Loss Class Details of NFRP/E-Glass Hybrid Composite

Table 9. Thermal Conductivity of NFRP and NFRP/Glass Hybrid Samples at 30o C Material W/m-K

Pultruded 685 gsm Hybrid 0.30 Pultruded only flax 685 gsm 0.22

Pultruded E-glass/vinyl-ester 0.31

Figure 1. 24 inch wide Pultruded NFRP and Hybrid Samples showing shrinkage of panel and wavi-ness of fabric in transverse direction in NFRP composite

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Figure 2. Failure Mode of NFRP and Hybrid Samples in Flexure

Figure 3. Typical Load-Deflection Charts Showing Bi-Linear Curve in Flexural Testing

Figure 4. Load Deflection Trend of NFRP and Hybrid Coupon Samples in Flexural Testing