NC State
BioResources
Tisserat, B., Reifschneider, L., Carlos López Núñez, J., Hughes, S. R., Selling, G., and Finkenstadt, V. L. (2014). "Evaluation of the mechanical and thermal properties of coffee tree wood flour - polypropylene composites," BioRes. 9(3), 4449-4467.

Abstract

Columbian coffee trees are subject to frequent replacement plantings due to disease and local climate changes, which makes them an ideal source of wood fibers for wood plastic composites (WPC). Composites of polypropylene (PP) consisting of 25% and 40% by weight of coffee wood flour (CF) and 0% or 5% by weight of maleated PP (MAPP) were produced by twin screw compounding and injection molding. Composites containing MAPP had significantly improved tensile and flexural properties compared to neat PP or composites without MAPP. Excellent mechanical properties were obtained with CF relative to conventional wood fillers. Izod impact resistances of CF composites were significantly lower than neat PP although WPC containing MAPP were superior to WPC without MAPP. Bio-based fiber composites made by mixing CF in equal portions with other fiber sources were evaluated to determine the compatibility of using CF with other sources of filler materials. Soaking of tensile bars of the various CF blends in distilled water for 35 days may alter their mechanical properties and result in weight gain. Differential scanning calorimetry and thermogravimetric analysis were conducted on the neat PP and bio-composites to evaluate their thermal properties as they relate to potential degradation during conventional thermoplastic resin processing.


Download PDF

Full Article

Evaluation of the Mechanical and Thermal Properties of Coffee Tree Wood Flour – Polypropylene Composites

Brent Tisserat,a,* Louis Reifschneider,b Juan Carlos López Núñez,c Stephen R. Hughes,d Gordon Selling,e and Victoria L. Finkenstadt e

Columbian coffee trees are subject to frequent replacement plantings due to disease and local climate changes, which makes them an ideal source of wood fibers for wood plastic composites (WPC). Composites of polypropylene (PP) consisting of 25% and 40% by weight of coffee wood flour (CF) and 0% or 5% by weight of maleated PP (MAPP) were produced by twin screw compounding and injection molding. Composites containing MAPP had significantly improved tensile and flexural properties compared to neat PP or composites without MAPP. Excellent mechanical properties were obtained with CF relative to conventional wood fillers. Izod impact resistances of CF composites were significantly lower than neat PP although WPC containing MAPP were superior to WPC without MAPP. Bio-based fiber composites made by mixing CF in equal portions with other fiber sources were evaluated to determine the compatibility of using CF with other sources of filler materials. Soaking of tensile bars of the various CF blends in distilled water for 35 days may alter their mechanical properties and result in weight gain. Differential scanning calorimetry and thermogravimetric analysis were conducted on the neat PP and bio-composites to evaluate their thermal properties as they relate to potential degradation during conventional thermoplastic resin processing.

Keywords: Mechanical properties; Flexural properties; Differential scanning calorimetry; Thermal properties; Injection molding

Contact information: a: Functional Foods Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University St., Peoria IL 61604 USA; b: Department of Technology, College of Applied Science and Technology, Illinois State University, Normal IL 61790 USA; c: National Federation of Colombian Coffee Growers, Bogota, Colombiad: Renewable Product Technology Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University St., Peoria IL 61604 USA; e: Plant Polymer Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University St., Peoria IL 61604 USA;*Corresponding author: Brent.Tisserat@ars.usda.gov

Introduction

Coffee is the largest food commodity traded internationally and is the second most valuable commodity traded behind only crude oil (Café de Colombia.com 2012). The distinctive mild taste of Columbian coffee is obtained from the beans of Coffea arabica L. (family Rubiaceae) trees grown in equatorial regions lower than 10o latitude at altitudes of 1000 to 1500 m (Wikipedia.com 2013). According to the National Coffee Association, USA (2013), Columbia is the fourth largest international supplier of coffee, ranking behind Brazil, Vietnam, and Indonesia. Colombia exported 9.5 million 60-kg bags of coffee in 2012 (Coffeereserach.org 2013). Coffee trees need to be replaced on a regular basis to provide high and consistent yields due to “aging” and diseases (Guarin and Pachón 2012). In addition, coffee trees are susceptible to changes in climate. Between 1980 to 2010, the average temperature in Columbian coffee regions has risen one degree Celsius, but the average precipitation increased 25%; this in turn has severely disrupted the specific climatic requirements of the Coffea arabica bean harvest and encouraged coffee leaf rust infestation (Café de Colombia.com 2012; Birbragher and Rodriquez 2013). To address these situations, new productive and resistant varieties are being developed to renew existing crop acreage (Guarin and Pachón 2012). The frequent replacement of coffee trees is creating a massive biomass resource. As new and improved coffee varieties are developed, the replacement rates will become even more frequent, occurring every 7 years or less. Colombia Coffee Growers Federation/Coffee System Information (Sistema de Información Cafetera/SICA) provides data that during the last five years (2008 to 2012) more than 1.2 millions of acres were renewed. The coffee crop in Colombia consists of approximately 2.5 million acres, and the annual renewal capacity could be one-fifth of the total (i.e., 500 thousand acres/year) depending on disease problems. Theoretically, this results in a minimum biomass equivalent of 3.2 million tons of dry wood per year (Café de Colombia.com 2012; Birbragher and Rodriquez 2013). This estimation is only for the stem biomass (approximately 23% from total dry plant material) from coffee trees and does not include the dry materials obtained from leaves and branches, which are 22% and 25%, respectively, of the dry tree weight (Riaño et al. 2004). Currently, most of this biomass, being unsuitable as lumber, is burnt on the spot to eliminate its obstruction to further coffee agricultural operations. However, higher value product utilization of this lignocellulosic source is sought.

The composite plastic industry is constantly seeking reinforcement materials that are more environmentally friendly, inexpensive, and improve the mechanical and physical properties compared to current products (Clemons et al. 2013). The global market for wood plastic composites (WPC), cellulosic plastics, plastic lumber, and natural fiber composites was 2.4 million metric tons (tonnes) in 2011 and by 2016 is estimated to grow to 4.6 million tonnes (Bccresearch.com 2011). From 2011 to 2016, the market for building materials and automotive applications are expected to grow 12.4% and 17.1% per yr, respectively (Bccresearch.com 2011). Because Colombia is a major oil producer and exporter in the world’s economy, it is not unreasonable to suggest that the glut of coffee tree biomass could be utilized as a wood fiber source for the manufacture of thermoplastic WPC. Coffee tree cellulosic fibers may provide a perennial supply of fibers for “sustainable exploitation” of an available natural resource without causing severe strain on the natural environment (i.e., the rain forest). For any long-term commercial development of a WPC, there must be a guaranteed supply of both the plastic and wood resources (Kim and Pal 2010). It is essential that both the thermoplastics and the filler materials used in the WPC be locally available, abundant, and inexpensive in order to be utilized. Unless a particular fiber has some advantage in the market, the lignocellulosic fibers used in WPC are those based on their availability at the locale they are manufactured (Kim and Pal 2010). Market advantage is based on availability, price, and performance of the fiber in the WPC. For example, jute fibers are commonly employed in eastern India and Bangladesh as an inexpensive source of reinforcements in polymer composites (Kim and Pal 2010). However, although jute fibers used at high fiber levels result in higher performance characteristics like modulus and recyclability, it is usually economically infeasible to export jute fibers to other countries as a WPC component (Kim and Pal 2010). One of the reasons that wood is the major source of agro-based fibers is its cost versus other agro-fibers. Wood has a higher density than other agro-fibers and is less expensive to grow than materials obtained from seasonal crops.

The objective of this study was to evaluate the mechanical, physical, and thermal properties of WPC obtained from blending coffee wood flour (CF) with polypropylene (PP). It is particularly important to determine if CF functions as either a filler or a reinforcement material in WPC. Fillers generally do not improve the mechanical properties of the composite but are an inexpensive way to increase bulk. On the other hand, reinforcements improve strength and increase stiffness (Clemons et al. 2013). If a bio-fiber can be inexpensive and provide a notable improvement in the mechanical properties, then the benefit of replacing the neat resin with a bio-sourced material is not compromised. Further, there is a particular interest in the utilization of CF derived from terminated “replaced” trees from the high altitude regions of Colombia since such short-rotation woody crop trees will likely be a renewable source of woody biomass available for Colombia in the future. This biomass resource will not interfere with other agricultural or forestry operations in its production. Hence, this study was conducted utilizing CF derived from replaced tree biomass (i.e., 7-year-old). In addition, because coupling agents have been commonly used for wood fiber PE composites (Carlborn and Matuana 2006; Lei et al. 2007; Clemons 2010), a maleated PP (MAPP) was employed as part of the scope of the project. Because CF is a bio-fiber and is subject to degradation by water, water immersion tests were administered on CF composites to evaluate their environmental durability. In the US, typical WPCs utilize commercial filler blends derived from various tree species (ash, oak, maple, birch). These blends are obtained by pulverizing and sieving lumber milling byproducts (wood shavings and sawdust). Therefore, tests were conducted to ascertain the potential compatibility value of mixing dissimilar fillers, i.e., Osage orange wood (OOW), pine wood (PINEW), and Camelina press cake (CAM) with CF and PP. Finally, differential scanning calorimetry and thermogravimetric analysis were conducted on CF composites to evaluate their thermal properties and the implications these may have on selecting processing conditions for the bio-fiber use.

EXPERIMENTAL

Materials

The PP employed as the matrix material was Pro-fax SB891 (Lyondellbasell, Equistar Chemicals LP, Houston, TX). It had a melt-flow index of 35 g/10 min, a density of 0.90 g/cm3, and a melt temperature of 165 oC. The coupling agent was a MAPP, supplied by Equistar Chemicals LP (product code NP 507-03). The MAPP had a melting point of 155 to 165oC with approximately 1% maleic anhydride by weight grafted on the polypropylene.

Coffee tree wood was obtained from 7-yr-old trees grown in the Caldas region of Colombia. Osage orange (Maclura pomifera (Raf.) Schneid., Family Moraceae) wood was obtained from 20-yr-old trees grown in Missouri. White pine (Pinus strobus L., family Pinaceae) wood was obtained from packaged bedding shavings (American Wood Fiber, Schofield, WI). Camelina (Camelina sativa L. Crantz, family Brassicaceae) seeds were grown in Peoria County, Illinois. Camelina seeds were ground, and oils were extracted with hexane and methanol using a Soxhlet extractor. Subsequently, bio-filler materials were milled with a Thomas-Wiley mill grinder, (Model 4, Thomas Scientific, Swedesboro, NJ). Each bio-fiber material was milled successively through 4-, 2- and 1-mm diameter stainless screens. Particles were then sized through a Ro-TapTm Shaker (Model RX-29, Tyler, Mentor OH) employing 203 mm diameter stainless steel screens. Sieve/Screens employed were #30, #40, #50, #60, #80, #140, and #200 US Standards (Newark Wire Cloth Company, Clifton, NJ). Bio-fiber mixtures composed of #40 mesh and finer were employed in the extrusion operation. The bio-fiber particle size composition of these mixtures for each species is shown in Table 1.

Table 1. Sieve Information and Particle Distribution Percentages

Preparations

Table 2 summarizes the various treatments conducted in this research project.

Table 2. Weight Percentages in Test Formulations

Composite blends were compounded and extruded into strands using a Micro-18 30/l L/D co-rotating twin-screw extruder (American Leistritz Extruder, Branchburg, NJ). The barrel had six different zones, each 90 mm long, which were controlled at the following temperatures (oC): 40, 75, 125, 175, 175, and 175, respectively. The strand die temperature was set at 175 oC. Premixed weight fractions of CF, PINEW, OOW, or CAM were fed into zone 1 using a feeder (Model 15C-CM plus/R1.0, Brabender Technologies, Mississauga, Ontario, Canada), while at the same time, premixed quantities of PP and MAPP were fed into the extruder in zone 1 using a twin-screw gravimetric feeder (Model KCL24K520, K-tron, Pitman, NJ). The compounding screw speed was set at 200 rpm. Extruded strands were cooled on a conveyor belt equipped with an air stream (Model 2100, Dorner Mfg. Corp., Hartland, WI) and were processed into pellets with a strand pelletizer (Model 4, Killion, Cedar Grove, NJ).

Test specimen pellets were injection molded using an ASTM test specimen mold that included cavities for an ASTM D790 flexural testing bar (12.7 mm W × 127 mm L × 3.2 mm thickness), an ASTM D638 Type I tensile testing bar, and an ASTM D638 Type V dynamic tensile testing bar. Molding was conducted with a 30-ton molding machine (Engel ES 30, Engel Machinery Inc., York, PA) with set point temperatures (C) for the four zone injection molding barrel set at: feed = 160; compression = 166; metering = 177, and nozzle = 191. The mold temperature was 37 C. The Type I bars were used for the tensile strength property tests. The flexural bars were used to evaluate flexural properties and also used to make impact resistance measurements. The Type V bars were used to evaluate changes due to prolonged exposure to water: weight change, color change, and changes in tensile mechanical properties of the composites.

Mechanical Property Measurements

Samples were conditioned for approximately 240 h at standard room temperature and humidity (23 oC and 50% RH) prior to any test evaluations. The ASTM D638 Type I tensile bars were tested for Young’s or tensile modulus (E), tensile strength (u), and elongation at break (%El) using an Instron universal testing machine, Model 1122 (Instron Corporation, Norwood, MA). The speed of testing was 50 mm/min. Three point flexural tests were carried out according to ASTM-D790 specification on the Instron Model 1122.

The flexural strength or maximum fiber stress (fm) and flexural modulus of elasticity (Eb) were calculated. The flexural tests were carried out using Procedure B with a crosshead rate of 13.5 mm/min. However, even Procedure B failed to achieve a maximum bending force within the required 5% of strain. The maximum bending load typically occurred between 5.5 to 8% of strain, and the calculated flexural strength reported in this study is based upon the maximum load. IZOD impact tests were conducted with a pendulum impact tester, Model Resil 5.5, P/N 6844.000 (CEAST, Pianezza, Italy) according to ASTM D256-84. Impact test specimens were obtained by cutting the flexural specimens in half to 12.7 mm W × 64 mm L × 3.2 mm thickness and then notched. Each mechanical test involved testing five specimens of each formulation. The average values and standard errors were reported. Following impact testing, the fractured surfaces of specimen bars were examined using a Wild Heerbrugg M5 Stereo dissecting microscope (Leica Microsystems GMbH, Wetzlar, Germany) to evaluate the dispersion of the wood flour particles in the PP matrix.

Water Absorption

The Type V tensile bars injection molded for each composite were dried in an oven for 24 hours at 100 2 oC and weighed. Tests were conducted in an incubator at 25 2 oC under a photosynthetic photon flux density of 180 μmol.m2.s-1 using a photoperiod of 12 h light/12 h dark. Tensile bars were placed in distilled water at room temperature for 872 h. At predetermined time intervals the specimens were removed from the distilled water, the surface water was blotted off with paper towels, and their wet mass and thickness were determined. Water absorption, measured as weight gain percentage, was computed using the following formula,

Weight gain (%) = (mt – mo)/mo ×100 (1)

where mo denotes the oven-dried weight and mt denotes the weight after soak time t.

Thermal Properties

Differential scanning calorimetry (DSC) of molded specimens was conducted with an Auto DSC-7 calorimeter with a TAC/DX controller (TA Instruments, New Castle, DE). Samples of 5 to 7 mg were weighed and sealed hermetically in aluminum DSC pans. First, the calorimeter was programmed to increase the temperature from 0 to 180 oC at a rate of 10 oC/min, then kept isothermal for 3 min. Second, the samples were cooled to -50 oC at a rate of 10 oC/min. Finally, the samples were heated to 180 oC from -50 to 180 oC at the same rate. Data from the second heating cycle were used to determine the melting temperature (Tm) and enthalpy of melting  for PP-CF blended samples. The heat flow rate corresponding to the crystallization of PP in composites was corrected for the content of the WF and MAPP. The value of crystallization heat was also corrected for the crystallization heat of MAPP. The degree of crystallinity  of the PP matrix was evaluated from the following relationship (López et al. 2012),

 (2)

where  is the experimental heat of fusion  or crystallization determined by DSC,  is the assumed heat of fusion or crystallization of fully crystalline PP (204 J/g), and Wf is the weight fractions of PP in the composites.

Thermogravimetric analysis (TGA) was performed to determine the thermal characteristics of the composites. The TGA was conducted using a Model 2050 TGA (TA Instruments) under nitrogen at a scan rate of 10 oC/min from room temperature to 600 oC. A sample of 7.5 mg was used for each run. Data were analyzed using the TA Advantage Specialty Library software (TA Instruments). The derivative TGA (wt %/min) of each sample was obtained from the software.

Results and Discussion

Mechanical Properties of CF Blends

The mechanical properties of the various composite blends are shown in Fig. 1. Pearson correlation coefficients comparing the mechanical properties for the composites conducted in this study are presented in Table 3. High positive correlations occurred between u and fm (0.951), E and Eb (0.992), and %El and impact resistance (0.964). In addition a negative correlation occurred between E and %El (-0.770).

Table 3. Pearson Correlation Coefficient Values for the Mechanical Properties of the Blends Conducted in this Study

The blending of 25% CF with PP produced a formulation (PP-25CF) that had a slightly lower or on par strength (u and fm) values to neat PP. The u of PP-25CF was 7% lower that of neat PP, and the fm of PP-25CF was 3% greater than neat PP, referring to Fig. 1. Modulus (E and Eb) values, however, increased significantly with the addition of CF. The E was improved by 72%, and the Eb was increased by 33% compared to neat PP, as shown in Fig. 1. The dramatic improvement of the composite modulus was due to the presence of the higher modulus wood fiber that impeded the relatively small scale deformation of the matrix. As with other fiber fillers, however, the addition of CF dramatically lowered the %El, a 91% reduction, and also lowered the impact resistance, a 65% reduction, compared to neat PP. These trends continued in blends containing 40% CF. The PP-40CF composite exhibited uE, %El, fmEb, and impact resistance values that were -9, +125, -94, +1, +121, and +70%, respectively, compared to neat PP. Other investigators have observed that the inclusion of wood or lignocellulosic flour into thermoplastics such as PLA, PE, or PP results in a significant decrease in u and %El values but increases E values (Stark and Berger 1997; Julson et al. 2004; Febrianto et al. 2006; Li and Sun 2011). Poor interfacial adhesion between the wood flour and the thermoplastic resin is responsible of decrease in the u and %El values (Petinakis et al. 2009). However, other investigators have been able to obtain u values in PLA-wood composites without the use of coupling agents comparable to neat PLA which suggests good interfacial adhesion is possible without coupling agents (Petinakis et al. 2009). CF formulations showed only a slight reduction in u values (≥10%) compared to neat PP, which can be attributed to relatively good interfacial compatibility occurring between the CF and PP.

The addition of MAPP in the PP-CF formulations significantly improved the strength, impact, and elongation properties, but did not significantly alter the modulus values. The MAPP had a pronounced effect on the strength, elongation, and impact properties because MAPP works to minimize the creation of microcracks between the dissimilar polar wood material and the nonpolar PP matrix (Myers et al. 1991; Clemons 2010; Rodríquez-Llamazares et al. 2011; Clemons et al. 2013; Ehrenstein 2001; Lopez et al. 2012; Tisserat et al. 2013). The ultimate strength and elongation are large-scale deformation processes that are disrupted by the formation of cracks (Ehrenstein 2001). The stress field that can be generated in a composite is a function of how well the composite can hold together during increased loading. If microcracks are present at the onset of loading, then higher stresses will function to open the cracks and lead to premature rupture of the composite. In effect, this lowers the u of the composite compared to neat resin. This trend is seen by comparing PP-25CF to PP in Fig. 1. The addition of MAPP reduces the potential for small cracks to form and thus helps to maintain the integrity of the composite during higher loading. This results in an increase compared to non-MAPP composites. Impact strength is a measure of how well a material can minimize the propagation of a crack during dynamic loading. The addition of MAPP functions to better bind the polar and non-polar components of the composite. This works to minimize the creation of cracks and thus retards crack propagation during impact thus improving impact strength. However, modulus (E and Eb) values are a measure of a small scale deformation processes thus the development of cracks have less effect on these properties.

CF conc1comp

Fig. 1. Effect of coffee wood concentrations with and without coupling agent (MAPP) on the mechanical, flexural, and impact resistance properties in coffee composites and compared to PP and PP-MAPP controls

The effect of MAPP is accentuated with increased loading of CF, as shown in Fig. 1. The 40% loading of CF has greater changes in u, %El, fm, and impact resistance when MAPP is added compared to similar changes 25CF to 25CF-MAPP. The mechanical properties of uE, %El, fmEb, and impact resistance of PP-40CF-MAPP were 74, 1.3, 94, 59, 2, and 66% higher than the same mechanical values of the PP-40CF formulation, referring to Fig. 1.

Generally, those composites that exhibit high fm also will exhibit high Eb (Zabihzadeh 2010a,b). This was similarly observed in the present study (Fig. 1). Flexural strength of the PP-25CF composite was statistically identical to neat PP. However, the Eb for the PP-25CF composite was greatly improved over the neat PP. Adding MAPP in the PP-25CF and PP-40CF formulations continued to improve the fm and Evalues compared to neat PP. Impact strengths of neat PP or PP-MAPP were significantly higher than any PP-CF formulation tested. Notched impact resistances of PP-CF formulations containing MAPP were significantly higher than the PP-CF formulations without MAPP. For example, the formulation PP-25CF-MAPP exhibited an impact energy that was 43% higher than PP-25CF formulation.

Optical observations of impacted tested fractured specimens provides information concerning the interaction of the matrix and filler components in response to a physical stress. It should be noted that the differences between composite blends were subtle. However, the fractured surface morphology of the PP-40CF-MAPP composite appeared more homogenous than that of the PP-40CF composite which was considerably rougher in appearance (Fig. 2).

Fig. 2. Optical microscope images of the PP-40CF (a-d) and PP-40CF-MAPP (e-h) composites following impact strength tests. Arrows designate visible wood particles and the bar in image h represents 1 mm.

This roughness was attributed to matrix disruption and higher occurrence of visible “uncoated” wood flour particles. We did not note any wood flour particle clumping in any composite blends examined but rather a random dispersion of wood flour particles throughout the matrix.

Wood particles were not as apparent in the PP-40CF-MAPP composite as they were in the PP-40CF composite. This suggests that greater coating of the wood flour particles by the PP matrix occurred in the PP-40CF-MAPP composite than in the PP-40CF composite.

Evidently, there was greater interfacial adhesion between PP matrix and the CF wood particles in composites containing MAPP than in composites without MAPP. Similar observations were made for CF composites containing lower filler loadings (i.e. PP-25CF versus PP-25CF-MAPP).

Overall, CF showed promising results when blended with PP, since the resulting formulations exhibited mechanical properties (i.e., uEfm, and Eb) that significantly exceeded that of neat PP. These observations suggest that CF materials can be considered to be reinforcement in WPC.

Comparing CF to Other Fillers

The mechanical properties of the PP-25CF-MAPP formulation compared well to other wood (PP-25PINEW-MAPP and PP-25OOW-MAPP) and press cake (PP-25CAM-MAPP) formulations (Fig. 3). Pine wood flour is a common commercial wood filler material employed in WPC and was included as an appropriate comparative filler/reinforcement source. OOW flour was included because it is a readily available hardwood tree common to the Midwest, USA where this study was conducted. Similarly, the CAM filler was included because it is also readily available in the Midwest. The uE, %El, fmEb, and impact resistance values of PP-25CF-MAPP were -1, -6, +32, +13, -7, and +30%, respectively, of that of the PP-25PINEW-MAPP formulation (Fig. 3). Likewise, CF formulations compared well to the OOW and CAM composites.

Every bio-polymeric material has its own unique set of chemical and physical properties which, when combined with thermoplastic resins, correspondingly results in a unique bio-composite. For example, wood fillers are composed primarily of cellulose (40 to 45%) and lignin (20 to 30%), and have some protein (~1 to 2%) and solvent extractables (~3 to 12%).

In contrast, press cakes are primarily composed of protein (20 to 35%), vegetable oils (~8 to 12%), and have much lower cellulose (11 to 25%) and lignin (3 to 15%) levels (Tisserat et al. 2013). Although press cakes are relatively abundant and inexpensive, few studies have been conducted attempting to employ them as a bio-filler with thermoplastic resins (Tisserat et al. 2013). Clearly, they provide an inferior filler compared to wood (Fig. 3).

The high concentration of protein and residual oils are probably undesirable characteristics of press cake fillers and interfere with binding with thermoplastic resins (Finkenstadt et al. 2007; Reifschneider et al. 2013). Nevertheless, mixture of CF and CAM in equal proportions improves the mechanical properties and suggests that CAM filler could be employed commercially.

Comparing wf1

Fig. 3. Comparing the mechanical, flexural, and impact resistance properties of individual bio-fibers and mixtures

Water Absorption Responses

Figure 4 shows the long-term water absorption plots of WF-based composites at room temperature, where weight gain (%) (i.e., water absorption) is plotted against immersion time (h). All composites absorbed water during the incubation period, and a saturation level did not occur for any of the fillers employed. PP and PP-MAPP exhibited less than a 1% increase in weight after the immersion incubation time (872 h). Inclusion of the MAPP coupling agent to the CF filler formulation produced a biocomposite that was more resistant to water absorption. For example, PP-40CF and PP-40CF-MAPP exhibited weight gains of 7.8 and 4.7%, respectively; inclusion of MAPP reduced weight gain by 40%. Others have reported that inclusion of maleated polyolefins in the composite considerably reduces water absorption when using bio-fillers of popular wood, loblolly pine wood, sisal fiber, or wheat straw (Joseph et al. 2002; Zabihzadeh 2010a,b).

As shown in Fig. 4, the PP-25CF-MAPP absorbed more water (2.9%) than PP-25PINEW-MAPP (2.4%) or PP-25OOW-MAPP (1.6%). The PP-25CAM-MAPP formulation exhibited a higher absorption rate, 4% of which may be attributed to the high content of protein present. Mixing CF and non-CF fillers resulted in “mixed” formulations that exhibit absorption rates manifesting the mixing properties of the two fillers (Fig. 3). The response of bio-composites to water soaking is related to the bio-filler’s chemical and anatomical properties (Joseph et al. 2002; Kord 2011; Zabihzadeh 2010a; Segerholm et al. 2012). Clearly, a species-related WF response to soaking was observed, since CF WPC exhibited a higher weight gain that PINEW or OOWF filler WPC. Absorption of water by composites is a crucial factor in determination of the ability of bio-composite to be commercially utilized (Zabihzadeh 2010a,b).