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UTILIZATION OF CARPET WASTE AS A MATRIX IN NATURAL FILLER FILLED ENGINEERING THERMOPLASTIC COMPOSITES FOR

AUTOMOTIVE APPLICATIONS

Alper Kiziltas1, 2 & Douglas J. Gardner1 1Advanced Engineered Wood Composite (AEWC) Center, University of Maine, Orono,

ME 04469, USA

2Department of Forest Industry Engineering, Faculty of Forestry,

University of Bartin, 74100 Bartin, TURKEY

Abstract

Recycled nylon 6, 6 from carpet waste is a potential material for use in the production of composites. There has also recently been increased interest in the use of natural fillers in numerous automotive interior and exterior parts. The aim of this proposed research is to explore the use of carpet waste, recycled nylon 6, 6 (RPA 6, 6) and natural filler (microcrystalline cellulose (MCC)) for certain under-the-hood applications in the automobile industry where conditions are too severe for commodity plastics to withstand. In this study, engineering thermoplastic composites were fabricated that were reinforced with MCC at up to 30 percent MCC to RPA 6, 6 weight ratios. The tensile strength increased by 110% at 20 wt. % of MCC loading level. Tensile and flexural modulus of elasticity increased with the addition of microcrystalline cellulose. Incorporation of MCC particles into RPA 6, 6 also resulted in a considerable decrease of creep compliance. Differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) were used to determine thermal properties of the composites. The thermal expansion of the composites also decreased with increased filler loading. The mechanical and thermal properties of the composites suggest that composite components could be especially relevant in thermally challenging areas such as the manufacture of under-the-hood automotive parts.

Background

Carpet, which is extensively used in homes, commercial buildings, automobiles and aircraft, is a textile floor covering consisting of an upper layer of "pile" attached to a backing. Carpet can be made from many single or blended natural and synthetic fibers and is specifically manufactured to be durable and long lasting, but after a certain period of time it needs to be replaced, and old carpet is disposed of in landfills [1-3]. According to U.S. carpet industry statistics, the rate of carpet disposal is about 2-3 million tons per year in the US and about 4-6 million tons per year worldwide. Only less than 5% of this amount is being recycled and less than 1% is reused [4-7]. Nearly all the rest of the waste, approximately 95%, is disposed of in landfills [1].

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Total carpet waste accounts for approximately 1 wt% and 2 vol% of the municipal solid-waste stream in the U.S. and is increasing every year because of the average life expectancy of a carpet. About 70% of the carpet produced replaces old carpet, which typically is between 8-11 years old [1, 5]. The landfilling of carpet waste has become a growing environmental problem because carpet waste is nonbiodegradable and diminishes the availability of landfills for other uses [1, 9, 10]. In addition to environmental issues, there are also economical aspects associated with carpet waste related to the direct and indirect costs for its disposal. The direct cost includes the cost for discarding it to the landfill (tipping fee) which is approximately $55/ton. The indirect cost includes the loss of potential resources and energy from this carpet waste. Disposal of carpet includes the discarding of valuable raw materials in the form of high engineering value fibers such as nylon 6, nylon 6, 6, and polyester [1, 9]. Therefore, government and industry have taken note of the millions of tons of carpet being disposed of each year, and efforts are underway to divert this volume from landfills [11].

All carpet is a composite product generally containing a carpet face fiber and carpet backing. Based on face fiber material, carpet is classified as nylon 6, nylon 6, 6, polypropylene, olefin, polyester and polyvinyl chloride. Nylon fiber is currently more recycled than other fibers and represents approximately 65 percent of all carpet sold in the USA [1, 3]. Nylon performs the best among the all synthetic fibers as a carpet face fiber and nylon ($2.5/kg) and is also more expensive than other synthetic fibers (polyester (($1.2/kg), polypropylene ($0.75/kg )). The price provides a perspective on the economics of recycling and also explains why most of the recycling effort focuses on nylon recovery [5, 8]. If the polymers are cleaned and separated, the recycled polymers from carpet waste can achieve reasonable property levels to qualify for variety applications such as automobiles, low-cost containers, civil infrastructure components, and materials for impact protection [1, 4, 12].

The automobile industry is one of the most resource intensive industries of all major industrial systems and the largest manufacturing enterprise in the world [13]. Because of its versatility, moldability and resistance to high temperatures and harsh chemicals, nylon 6, 6 is the most frequently used engineering thermoplastic in the automotive industry today. Under-the-hood nylon 6, 6’s performance properties make it a rising star. In 1960, the average car used a total of 0.2 kg of nylon, most of which avoided the harsh under-the-hood environment. By 2000, the average car used 5 kg of nylon in under-the-hood applications alone. This rapid growth can be attributed to government regulations requiring increases in fuel economy, the need to reduce component costs and minimize overall vehicle weight [14]. Overall, worldwide automotive industry demand for nylon is expected to grow. Nylon 6.6 from carpet waste could potentially fill this demand. It is forecast that the demand for nylon 6, 6 will be 1.1 million metric tons in 2012 and expand at a 4% rate. Demands for nylon 6, 6 will grow on the backs of the major sectors they serve. To expand the demand, producers of engineering polymers must replace nylon 6, 6 with traditional materials like metal and glass. The need to boost more fuel-efficient vehicles to reduce energy consumption and air pollution presents an opportunity for nylon makers [15-16]. According to an auto industry rule of thumb, the reduction for every 10 percent of the weight of a car can decrease fuel usage by 6 to 8 percent. It is also predicted that the fuel-efficiency trend will continue as the U.S. rolls out Corporate Average Fuel Economy (CAFE) standards, which will rise from less than 35 mpg in 2010 to more than 54 mpg in 2025 [15-16].

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The first automotive product of its kind manufactured from post-consumer recycled plastic is cylinder head covers used by Ford Motor in its Escape, Fusion, Mustang, and F-150 vehicles. Ford Motor utilizes Wellman Engineering Resins' EcoLon material, a nylon comprised of 100% recycled carpet diverting 4.1 million lb of carpet that would have gone to landfills in 2010. In addition to their recycled content, the cylinder head covers are environmentally friendly and show great fuel economy due to weight savings of almost 20% when compared to aluminum die-cast cylinder head covers [17].

Pressured by government demands increases in fuel economy and the need to reduce component costs, minimize overall vehicle weight and emission reductions, automakers are warming to innovative solutions and looking at more costly materials such as carbon fiber composites and nanocomposites to achieve light material targets [18]. It is possible to make low cost, high performance materials to achieve, higher fuel economy performance and lower CO2 emissions using microcrystalline cellulose (MCC) filled recycled nylon 6, 6 composites from carpet waste for high temperature circumstances, like under-the-hood applications in the automobile industry. Cellulose filled composites have been extensively studied and continue to grow commercially in many industrial markets. The most common cellulose filled composites contain a polyolefin based polymer such as polyethylene and polypropylene matrices because of their low cost and easy processing [19]. Higher melting engineering thermoplastics (melting

points (MP) above 200 C) are believed not to be effectively reinforced by cellulose fillers due to their lower thermal degradation temperature. Kiziltas et al. found that MCC showed greater thermal stability compared to wood fiber and did not show significant initial degradation under 300°C. Therefore, MCC-filled composites could be used for high temperature circumstances, such as those in the automobile industry [20-21].

The lack of available landfill spaces, demands for resource conservation, environmental issues, the need for relief from foreign oil dependence and demands for nylon 6, 6 establish the importance of recycling carpet. There has also been increased interest in the use of natural fibers in numerous automotive interior and exterior parts recently. In this context, natural fibers (MCC) filled engineering thermoplastic (carpet waste (nylon 6, 6)) composites will be produced in this research for certain under-the-hood applications in the automobile industry where conditions are too severe for commodity plastics to withstand.

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Experimental Approach

Recycled nylon 6, 6, from carpet waste, was supplied in the form of homopolymer pellets from a commercial source; this nylon 6, 6 has a melting point of approximately 260 C. The MCC powder with a particle size range from 26 μm to 96 μm was used as the reinforcement. The average particle size was 50 μm. The MCC is highly crystalline cellulose and was supplied by Sigma Aldrich Co. The lubricant (TPW113) used as an additive to improve processing conditions, was supplied by Struktol Co. The MCC and recycled nylon 6, 6, was dried to moisture content of less than one percent using an oven at 105°C for 16-hours. The matrix polymer, recycled nylon 6, 6, was mixed with the MCC. The compounding was conducted using a Brabender Prep-mixer® equipped with a bowl mixer and the process temperature and torque changes were measured in real time. The temperature was set to 270°C and rotor speed at 60 rpm. MCC was added to the mixer when the polymer melt appeared well mixed. MCC residence time, approximately three minutes, was recognized as a relatively safe temperature range to prevent severe thermal degradation with a guarantee of composite processability. The recycled nylon 6, 6-MCC compounds were granulated using a lab scale grinder. The ground particles were dried in an oven at 105°C for 16 hours before being injection molded into ASTM test specimens. All materials were injection molded using a barrel temperature of 270°C, mold temperature of 270°C, and injection pressure of 2500 psi. The compositions of composites are shown in Table 1. Table 1: Composition of the MCC-filled nylon 6, 6 composites.

Sample Code MCC Content Recycled Nylon 6,6 Lubricant

RPA66 0 95 5

5 5 90 5

10 10 85 5

20 20 75 5

30 30 65 5

Values are percentage by weight (wt. %) The tensile and flexural strength, tensile and flexural modulus and elongation at break were compared using a one-way analysis of variance followed by Tukey-Kramer Honestly Significant Differences (HSD) test with JMP statistical analysis program [22].

Mechanical and Thermal Properties

All the tension tests were conducted according to the American Society of Testing and Materials (ASTM) standard D 638-03. The tensile behaviors of composites were measured using an Instron 8801 with a 10 kN load cell. All the tension tests were tested at a rate of 0.2 in/min. At least six specimens were tested for each composition, and the results are presented as an average for tested samples. The flexure tests were conducted according to ASTM D 790-03, using an Instron 8801 with a 4.48 N load cell. The support span was 50 mm. and tests were run at a test speed of 0.05 in/min. At least six specimens were tested for each composition and the results are presented as an average for tested samples.

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The viscoelastic properties of the composites were determined with a rheometric scientific dynamic mechanical thermal analysis (DMTA) IV. The experiments were conducted in three point bending mode under isochronal conditions at a frequency of 1 Hz. The strain amplitude was fixed at 0.01% to be in the domain of the linear viscoelasticity of the composites. The samples were rectangular with dimensions about 42 mm x 3.2 mm x 2 mm. The temperature range was from -50 to 260°C at a scanning rate of 5°C/min. At least three samples were tested for each composition, and the results are presented as an average for tested samples. Additional experiments were conducted to monitor the thermal expansion behavior using DMTA. The specimens (rectangular with dimensions about 20 mm x 3.2 mm x 2 mm) were subjected to a stress of 0.1 MPa in tensile mode. Tests were conducted at a frequency of 0.3 Hz in the temperature range of -20 to 200°C at a scanning rate of 5°C/min. Short time creep tests were made at different temperatures (30, 45 and 60°C) using a dynamic mechanical analysis (DMA) Q800 apparatus (TA instruments). The creep compliance was determined as a function of the time (t creep: 167 min.). The tensile stress applied was 10 MPa. The specimens’ dimensions were 63.5 mm x 12.5 mm X 2 mm). Differential scanning calorimetry (DSC) analysis was carried out using a Perkin Elmer Instrument Pyris DSC with a sample weight of 8 to10 mg. All samples were held at 20°C for 5 min, heated at a rate of 10°C/min to 300°C, subsequently held for 5 min to erase thermal history, then cooled at a rate of 10°C/min to 0°C, subsequently held for 5 min and heated again at a rate of 10°C/min to 300°C under a nitrogen atmosphere.. Melting temperature (Tm) was determined from a second scan. The Tm was taken as the peak temperature of the melting endotherm. At least three randomly picked samples from ground samples were tested for each composition, and the results are presented as an average for tested samples. Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo analyzer on samples of about 10 mg. Each sample was scanned over a temperature range from room temperature to 600°C at a heating rate of 10°C/min under nitrogen with a flow rate 20 ml/min to avoid sample oxidation. The samples used for the TGA measurement were 5 randomly selected individual samples from ground samples.

Results and Discussions

The tensile strength, tensile modulus of elasticity and elongation at break were determined from the stress and strain curves and the results are presented in Table 2. It was observed that recycled nylon 6, 6 exhibits a nonlinear elastic behavior with a tensile strength of 26.2 MPa and an elongation at break of 0.83 %. None of composites including the recycled nylon 6, 6 showed signs of stress yielding. The composite reinforced with MCC displayed enhanced tensile properties in comparison with the recycled nylon 6, 6. Because of better stress-transfer properties, the tensile strength of the composites was greater (reaching values of 54.9 MPa with the addition of 20 wt% MCC). Tensile strength increased by 109% with 20% MCC addition. After 20 wt% MCC addition, the tensile strength decreased but it was greater than recycled nylon 6, 6. The reason why tensile strength decreased after 20 wt% MCC addition was attributed to very little or no stress-transfer properties in the higher weight percent MCC-filled composite [23]. In Table 2, elongation at break of composites was greater (reaching values of 1.98 % with addition of 20 wt% MCC). The reason for this might be better dispersion below 20 wt% MCC addition. With high filler contents, the degree of MCC-MCC interaction became more prominent and, as a consequence, a reduction in elongation at break was observed [24]. Kiziltas et al. and Kiziltas observed similar phenomena with MCC filled PET-PTT blend and nylon 6 composites [25-26].

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Table 2: Summary of mechanical properties of recycled Nylon 6, 6 and MCC filled composites.

Properties RPA66 5 10 20 30

Tensile Strength (MPa) C, 26.2 C, 34.6 (NC) B, 44.2 (68.4%) A, 54.9 (109.3%) A, 53.9 (105.6%) TMOE (GPa) D, 3.2 CD, 3.4 (NC) C, 3.5 (7.6 %) B, 3.9 (19.4 %) A, 4.4 (35.8 %) EAB (%) D, 0.8 CD, 1.1 (NC %) BC, 1.3 (59.1 %) A, 1.9 (140.4 %) B, 1.5 (81.1 %) Flexural Strength (MPa) C, 84.3 B, 89.6 (6.3 %) A, 95.9 (13.9 %) B, 91.9 (9.1 %) C, 84.2 (NC) FMOE (GPa) E, 2.2 D, 2.5 (15.4 %) C, 2.8 (28.3 %) B, 3.30 (50.3 %) A, 3.7 (70.0 %)

The same letters indicate no statistical difference between properties of composites and those around it. NC is no significant change upon the addition of natural fiber blends (α=0.05) and parenthesis show the effect of natural fiber blends loading on the mechanical properties of composites in comparison with the neat Nylon 6. FMOE: Flexural modulus of elasticity, TMOE: Tensile modulus of elasticity and EAB: Elongation at break. The tensile modulus of elasticity of MCC-filled composites systemically increased with increasing MCC loading (reaching values 4.40 GPa with the addition of 30% MCC) in Table 2. Tensile modulus of elasticity increased by 36% with 20% MCC addition. Tensile measurements show that the effect of MCC is more pronounced on the tensile modulus of elasticity than tensile strength and MCC acts as a mechanical reinforcement of the polymer chains. A similar effect was also reported by Caulfield et al. for nylon 6, 6/hardwood and softwood fiber composites and Kiziltas et al. for PET-PTT blend/MCC composites [25, 27]. Table 2 also shows flexure strength and flexural modulus of elasticity of the recycled nylon 6, 6 and MCC filled composites. The composite reinforced with MCC displayed comparable or higher flexural strength in comparison with the recycled nylon 6, 6. As can be seen in Table 2, the flexural modulus of elasticity of composites was higher than the recycled nylon 6, 6. The modulus of elasticity also increased with increasing MCC loading (reaching values 3.74 GPa with the addition of 30 wt% MCC). Flexural modulus of elasticity increased by 70% with 30% MCC addition. A similar effect was also reported by Kiziltas, Kiziltas et al, Xu and Sears et al. [25-26, 28-29]. The important step in understanding the behavior of nylon 6, 6/MCC in under-the-hood applications is to have basic knowledge of the effects of temperature on their mechanical performance. The range of temperatures for this application is -40 to 125°C and the highest tolerable temperature is 150°C. Figure 1 shows the temperature dependence of the storage modulus of recycled nylon 6, 6 and MCC filled composites. The recycled nylon 6, 6 shows a typical behavior of semi crystalline polymer. MCC filled composites show a similar behavior and have a higher modulus for all temperature ranges especially temperature ranges for under-the-hood application compared to the unfilled composites. It can be assumed that the increase in the composite modulus should be related to the reinforcing effect of MCC. Also, it can be seen from the Figure 1 that the composites with MCC exhibit better temperature stability than the recycled nylon 6, 6 in the rubbery region.

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Figure 1: Storage modulus of recycled nylon 6, 6 and MCC filled composites from -50 to 150°C as a function of temperature.

The typical DMA curve for the thermal expansion of MCC filled recycled nylon 6, 6 composites is shown in Figure 2. The thermal expansion of the composites decreased with increasing MCC content. Thus, MCC filler is a suitable material for preventing the thermal expansion of the composite materials caused by cold and warm atmospheric changes. This means that this material can be used under-the-hood applications in automobile industry. The thermal expansion of the composites varied only slightly up to 40°C. Subsequently, the slope was steep up to 70°C and than gentle up to 200°C. The thermal expansion of the recycled nylon 6, 6 and MCC filled composites increased with increasing temperature because of the increased polymer chain mobility at higher temperature.

Temperature (°C)

0 50 100 150 200

Dis

plac

emen

t (m

m)

0.000

0.001

0.002

0.003

0.004

RPA66

5

10

20

30

Figure 2: Thermal expansion of the recycled Nylon 6, 6 and MCC filled composites.

Glassy

Glass Transition

Rubbery

Plateau Rubbery

Flow

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Figure 3 shows the creep compliance of recycled nylon 6 and MCC filled composites at various temperatures at 167 min. It can be clearly seen that with an increase in temperature the creep compliance increases for recycled nylon 6, 6 and MCC filled composites, which indicates the temperature-activated softening of polymer matrix as a result of reduction of stiffness of entangled networks of polymer chains [30]. The creep compliance decreased with the incorporation of MCC at all test temperatures. The magnitude of creep compliance undergoes a large reduction with the increase in MCC content. This fact can be observed, for example, from the drop in creep compliance at 60°C by about 61 and 68 % for the composites with 20 and 30 wt.-% of MCC content, respectively, compared to recycled nylon 6,6, which indicates enhanced creep resistance. This is most probably attributed to larger crystallites and the absence of any MCC-recycled nylon 6, 6 interactions [30].

Filler Loading (%)

RPA66 5 10 20 30

Cre

ep C

ompl

ianc

e (µ

m²/N

)

0

1000

2000

3000

4000

30°C

45°C

60°C

Figure 3: Creep compliance at different temperatures at 167min.

The effect of the microcrystalline cellulose on the thermal properties of the thermoplastic polymer composites was examined in non-isothermal DSC experiments. The values of melting temperature (Tm), crystallization temperature (Tc) and corresponding melting enthalpies (∆Hm) and crystallization enthalpies (∆Hc) are presented in Table 3. The DSC thermograms of heating and cooling of recycled nylon 6, 6 and composites are presented in Figure 3.1. Table 3: Summary of Tm, Tc, ∆Hm (J/g) and ∆Hc (J/g) for the recycled Nylon 6, 6 and MCC filled composites.

Sample Code Tc(0C) ΔHc(J/g) Tm (0C) ΔHm(J/g)

RPA66 234.82 (0.17) 60.56 (4.71) 258.70 (0.22) 76.41 (3.26)

5 234.48 (0.06) 53.20 (1.91) 258.30 (0.08) 65.48 (2.98) 10 234.10 (0.15) 48.14 (1.84) 257.80 (0.22) 65.41 (1.49)

20 233.24 (0.11) 47.85 (4.11) 256.50 (0.04) 58.25 (1.73)

30 231.49 (0.14) 42.02 (5.65) 255.20 (0.13) 49.68 (5.26)

Parenthesis indicates standard deviation. ΔHm and ΔHc are calculated based on total area of endothermic peak and exothermic peak respectively. The MCC mass was taken into account when getting the total area of the endothermic and exothermic peaks of Nylon 6, 6 composites from the DSC thermograms.

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Temperature (°C)

0 50 100 150 200 250 300

Hea

t F

low

Exo

Up

(m

W)

-10

0

10

20 RPA66

5%MCC

10%MCC

20%MCC

30%MCC

Figure 4: The DSC thermograms of heating and cooling course of recycled nylon 6, 6 and MCC filled composites.

The recycled nylon 6, 6 and MCC filled composites displayed similar thermograms and small differences in their melting and crystallization temperatures within the experimental precision of the equipment. It can be seen in Table 3 that the melting point of recycled nylon 6, 6 is about 259 °C and the melting points of composites are between 255°C and 258°C. The addition of the MCC to composites has only a marginal effect on the melting temperature and decreased slightly with the MCC addition. Figure 1 and Table 3 also show the crystallization temperatures of recycled nylon 6, 6 and composites. Figure 1 illustrates that adding MCC does not change the crystallization temperature of the composites or it has a slight effect at most. However, increasing the MCC content in the composite, in all cases, results in smaller crystallization enthalpies (∆Hc) and melting enthalpies (∆Hm) in Table 3. A reason for this is that polymer chain mobility is hindered in highly filled composites. Because of greater thermal stability compared to wood flour, microcrystalline cellulose can be used with engineering thermoplastics. Kiziltas provides evidence that MCC is thermally more stable and has less weight loss in comparison with similar sized wood filler particles at 270°C for 1 hour [26]. The TGA curves for recycled nylon 6, 6 and MCC filled composites are shown in Figure 5. The results show that as the filler loading increased, the thermal stability of the composites slightly decreased as the MCC content increased because of the lower thermal stability of MCC compare to the recycled nylon 6, 6. Whereas the thermal degradation of the composites was retarded above 490°C, due to increased ash content from around 4 % for the recycled nylon 6, 6 to 11% for 30% MCC addition. Thermogravimetric analysis also indicated

that the MCC did not show significant initial degradation under 300C in Figure 5. In other words, the TGA results demonstrated thermal stability decreased slightly with the addition of MCC but still within temperature ranges for the automobile industry.

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Temperature (°C)

100 200 300 400 500 600

Mas

s C

hang

e (%

)

0

20

40

60

80

100

MCC

RPA66

5% MCC

10% MCC

20% MCC

30% MCC

Figure 5: TGA curves of recycled nylon 6, 6 and MCC filled composites.

Conclusions

It is possible to produce composites of MCC in high melting engineering thermoplastics, nylon 6, 6 with melt compounding followed by injection molding without compatibilizers and other additives. The tensile strength of MCC filled composites is 54.9 MPa with the addition of 20 wt% MCC, which is 109% higher than the strength of recycled nylon 6, 6. The flexural modulus of elasticity increased with increasing MCC loading. Flexural modulus of elasticity increased by 70% with 30% MCC addition. MCC filled composites show a similar behavior to recycled nylon 6, 6 in DMTA experiments and have a higher modulus for all temperature ranges especially temperature ranges for under-the-hood application compared to unfilled composites. The thermal expansion of the composites decreased with increasing MCC content. Thus, MCC filler is a suitable material for preventing the thermal expansion of composite materials caused by atmospheric temperature changes. The magnitude of creep compliance undergoes a large reduction with the increase in MCC content. This fact can be observed, for example, from the drop in creep compliance at 60°C by about 61 and 68 % for composites with 20 and 30 wt.-% of MCC content, respectively, compared to recycled nylon 6,6 ,which indicates enhanced creep resistance. There was not a significant change in the crystallization (Tc) and melting (Tm) temperatures of the composites with increased MCC loading. Thermogravimetric analysis

indicated that the MCC did not show significant initial degradation under 300C. Therefore, MCC-filled composites could be used for high temperature circumstances, such as in the automobile industry. Recycled nylon 6, 6 from carpet waste not only offers the special capabilities required for under-the-hood environments, but can also help to improve environmental conditions. The mechanical and thermal properties of composites will suggest that composite components could be especially relevant in thermally challenging areas such as the manufacture of under-the-hood automobile components. More work needs to be done before cellulose filled engineering thermoplastic composites can literally, be rolled out in a production vehicle.

With decreasing MCC content

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