Composite Materials Journal of Thermoplastic

Journal of Thermoplastic Composite Materials http://jtc.sagepub.com

Effect of Laboratory Aging on the Physical and Mechanical Properties of Wood-Polymer Composites Sreekala G. Bajwa, Dilpreet S. Bajwa and Alexander S. Anthony Journal of Thermoplastic Composite Materials 2009; 22; 227 DOI: 10.1177/0892705708091857 The online version of this article can be found at: http://jtc.sagepub.com/cgi/content/abstract/22/2/227

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Effect of Laboratory Aging on the Physical and Mechanical Properties of Wood-Polymer Composites SREEKALA G. BAJWA* Biological & Agricultural Engineering Department, University of Arkansas, 203 Engineering Hall, Fayetteville, AR 72701, USA DILPREET S. BAJWA Greenland Composites, Fayetteville, AR 72701, USA ALEXANDER S. ANTHONY Physics Department, California Institute of Technology Pasadena, CA 91125, USA ABSTRACT: The long-term performance of wood-polymer composites (WPC) under severe weather conditions is not well known. This study evaluates the changes in physical and mechanical properties of three commercially available WPC and treated southern yellow pine (SYP) under a modified 6-cycle accelerated aging process. The accelerated aging causes warping, splitting, discoloration, and significant changes in physical and mechanical properties of SYP. The compressive and flexural strength of the WPCs show negligible changes whereas stiffness, hardness, and screw withdrawal force show considerable deterioration and some recovery during accelerated aging. The composition and manufacturing process influence the performance of WPC under accelerated aging. KEY WORDS: wood-polymer composites, accelerated aging, physical properties, mechanical properties.

INTRODUCTION

W

OOD-POLYMER COMPOSITES (WPC) have become popular in the last decade as a substitute for treated wood in nonstructural building

*Author to whom correspondence should be addressed. E-mail: [email protected] Figure 1–10 appear in color online http://jtc.sagepub.com

Journal of THERMOPLASTIC COMPOSITE MATERIALS, Vol. 22—March 2009 0892-7057/09/02 0227–17 $10.00/0 DOI: 10.1177/0892705708091857 ß SAGE Publications 2009 Los Angeles, London, New Delhi and Singapore


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applications. The WPC is a composite matrix of a thermoplastic substrate reinforced with the wood fiber. Since wood and other natural cellulosic fibers are abundant in the nature and exhibit good strength properties, they are ideal as fillers in a thermoplastic matrix to produce composites intended for building applications [1]. Typically, polyethylene is used as the thermoplastic substrate. The WPCs combine the beneficial properties of both wood and polyethylene to overcome the drawback of treated wood for outdoor building applications. The drawback of treated wood for outdoor applications is its poor weathering. Treated lumber deteriorates fast when exposed to the temperature fluctuations and moistures that are typical under outdoor conditions. Unlike wood, polymers such as polyethylene have good weathering properties. But the mechanical (strength) properties of polyethylene are not adequate for building applications. The main attractiveness of WPC is that it combines the superior weathering properties of polyethylene [2], with the strength properties of wood fiber such that the final product is more durable than wood, and stronger than polyethylene. The primary use of WPC is in nonstructural applications such as deck boards (46%), molding and trim (22%), fencing (16%), door and window components (7%), railing, automotives and others (9%) [3]. Several studies in the last 2 years have suggested a relative stability in the decking demand [3]. Therefore, there is a thrust for finding new applications for WPC. The door and window industry is showing a growing acceptance of this material, primarily for door frames, door jambs, thresholds, and sill plates. The strength properties are not as critical for these nonstructural applications as in the case of structural components. The more critical aspect is the ability to weather without significant product deterioration. Majority of the products such as decks, fencing, railing, as well as door and window components that are being replaced by WPC are exposed to the temperature fluctuations and moisture in the environment, making them susceptible to rot and decay defects. Such susceptibility to damage when exposed to weather fluctuations pose a risk to the manufacturers in providing extended warranty for these products, while increasing the maintenance cost for the customers. The polymer encapsulation of the ligno-cellulosic fibers in the WPC protects it from the moisture in the environment, thus leading to minimal decay, long life, and better anticipated long-term performance compared to treated lumber. Therefore, WPC products are sold as low maintenance products, with manufacturer’s warranty for 15–50 years. Although the polymer base of WPC provides better weathering capabilities, its long-term performance when exposed to the extremities of weather are not adequately studied. A few of the reported studies indicated that accelerated UV aging and freeze–thaw cycles reduced mechanical properties considerably [1,4–11]. Considering the fast growing market for


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current and new WPC products, extended warranty on existing products, and the intended applications where long-term performance is critical, there should be more research to quantify the long-term performance of WPC when subjected to outdoor weather conditions. The different raw materials and manufacturing processes may also affect the weatherability of these products. Therefore, this research was undertaken with the goal of studying the long-term aging performance of commercially available WPC in comparison to southern yellow pine (SYP) treated with chromated copper arsenate (CCA), which is often substituted by the WPC products. The specific objectives of the study were to: 1. understand the changes in physical and mechanical properties of three commercially available WPCs with respect to CCA treated SYP when subjected to accelerated weathering; 2. understand the effect of different manufacturing processes (materials and processes) on product performance under accelerated weathering. MATERIAL AND METHODS A laboratory experiment was conducted with four products used in decking applications by subjecting them to six accelerated aging treatments. The 4 products, together with 6 accelerated aging cycles, resulted in 24 treatment– product combinations. The four products included three commercially available WPCs (WPC1, WPC2, and WPC3) and traditional CCA-treated SYP. The SYP was included as a treatment since WPC is manufactured as a more durable substitute for SYP. Each WPC product was different in its formulation as well as the manufacturing process. For example, WPC1 was compression molded, WPC2 was produced by direct extrusion, and WPC3 was extruded from pelletized raw materials. The pelletization process included premixing of the ingredients (wood flour, polyethylene, and additives), followed by pelletization. The premixed pellets were then extruded to manufacture the final product or the deck boards. Therefore, WPC3 was subjected to better mixing than the other two WPCs. The WPC products were also different in the type of raw materials, particularly the polymer substrate. While WPC1 and WPC3 used recycled high-density polyethylene (HDPE), WPC2 used virgin HDPE. Although the exact formulations of the three WPCs were only known to the manufacturers, a composition analysis showed that WPC1 and WPC2 were fiber rich, with approximately 50% wood fiber and 35% polyethylene; whereas WPC3 was a relatively polymer-rich composite with approximately 45% wood fiber, and 39% polyethylene. The WPC and SYP samples were subjected to a modified 6-cycle accelerated aging treatment, which included accelerated aging of samples


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through a 5-step process for 0–5 times, respectively. The modified 6-cycle accelerated aging was based on the ASTM accelerated aging test specified in D 1037 standard. The samples assigned to zero cycle were the control samples, and they were not subjected to the 5-step accelerated aging process. All other samples were subjected to the 5-step accelerated aging cycles for 1–5 times or cycles. Each cycle of accelerated aging included the following five processes in the order given: 1. A 24-h water soak process at room temperature (258C) to simulate moisture absorption and swelling of samples exposed to outdoor environment. 2. A 24-h freeze process at 8.58C to simulate winter temperature, and freeze expansion in the samples. 3. A 2-h (þ30 min warm up) steam segment at 1218C to simulate thermal degradation due to simultaneous exposure to heat and moisture. 4. A 14-h oven-drying segment at 718C to simulate shrinkage under summer weather conditions. 5. A 24-h conditioning at room temperature to bring back samples to equilibrium conditions. The 5-step accelerated aging cycle was used to simulate the annual cycles of heat, rain, freezing, and thawing in outdoor weather conditions. The extent to which the samples were heated was beyond the normal temperature extremes present under outdoor conditions. Such extreme combinations of temperature and moisture were used to accelerate the deterioration process such that long-term performance of the products can be evaluated using short duration experiments in the laboratory. The ASTM 6-cycle accelerated aging test is considered as a standard test for evaluating long-term performance of woodbased products. The 6-cycle accelerated aging test is assessed to simulate an equivalent of 20 years of natural weathering in the case of railroad cross ties. Many scientists [12,13] have used a modified 6-cycle accelerated aging test for studying long-term performances of hardwood reconstituted structural panels, particle boards, and other wood-based products. Commercially available 1 6 deck boards come in either 3.6 or 4.8 m lengths. These boards were cut into 1.2 m long sections before subjecting them to the accelerated aging treatments, since the autoclave and oven limited the length of the sections that can be fitted inside. Two of the 1.2 m sections of each product were randomly assigned to each of the six accelerated aging cycles. After completing each segment of an aging cycle, the sample dimensions were measured, and samples were examined for surface defects such as surface color appearance, warping, cracking, splitting, flaking, and chalking. After completion of a specific aging cycle,


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the set of samples assigned to that particular aging treatment were as removed for testing. The samples that were subjected to the assigned number of accelerated aging cycles (0–5) were first tested for surface defects, and then cut into sample coupons to test for physical and mechanical properties. The properties tested include specific gravity, water absorption, thickness swelling, coefficient of linear thermal expansion (CLTE), flexural modulus of elasticity (MOE), flexural modulus of rupture (MOR), compressive strength, screw withdrawal strength (SWS), and hardness (Table 1). Under each product–aging cycle combination, 2–5 samples were tested for each property. The tests were performed following the guidelines set in ASTM D 7032 primarily. ASTM D 1037 was used for testing properties that were not covered in ASTM D 7032. The details of standards used for each tests is shown in Table 1. Mechanical properties such as flexural MOE and MOR, SWS, and hardness were tested with an Instron universal testing machine, with a 5000 N load cell. Compressive strength was tested with an Instron with a 40 kN load cell. For flexural test, a 4-point testing jig was used. Sample coupons of 25.4 25.4 mm cross section, and 0.5 m length were cut from the 1 6 sections for bending test. Each treatment was replicated five times for testing MOE and MOR, and three times for testing SWS and hardness. For CLTE measurement, sample coupons with 25.4 25.4 mm2 crosssectional profile, and 0.305 m length were used. The samples were subjected to a low temperature of 8.58C for 12 h, followed by 12 h in an oven at 498C, and the change in the linear dimensions were recorded at multiple marked points. The CLTE was measured as the thermal deformation per unit length of the sample per unit change in the temperature. Table 1. Testing standards followed to evaluate physical and mechanical properties of WPC under aging. Property tested

Standard used

Test sample size

1. Specific gravity 2. Water absorption 3. Coefficient of linear thermal expansion 4. Flexural modulus of elasticity 5. Flexural modulus of rupture 6. Compressive strength 7. Hardness 8. Screw withdrawal force

ASTM D 1037 ASTM D 1037 Modified ASTM D 1037 ASTM D 7032 ASTM D 7032 ASTM D 7032 ASTM D 7032 ASTM D 1471

25.4 mm 25.4 mm 25.4 mm 25.4 mm 25.4 mm 25.4 mm 25.4 mm 25.4 mm 0.305 m 25.4 mm 25.4 mm 0.5 m 25.4 mm 25.4 mm 0.5 m 25.4 mm 25.4 mm 50 mm 50 mm 90 mm 100 mm 50 mm 90 mm 100 mm


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RESULTS AND DISCUSSION Surface Appearance All four products showed color change when subjected to aging (Figure 1). The WPC samples showed progressive color change into a darker brown, with the number of aging cycles. The treated SYP samples changed from its original light greenish to a lighter brown and then darker brown progressively. The SYP samples also showed extensive surface defects such as cracking and splitting after two cycles of accelerated aging in the laboratory (Figure 1). The WPC products did not show surface defects other than color change, with one exception. The exception was that some of the WPC1 samples showed some charring on the surface during the steam segment of the accelerated aging cycle. Physical Properties The specific gravity of all three WPCs was above unity, while the treated SYP showed an average specific gravity of 0.5 (Figure 2). The specific gravity of the compression-molded product was slightly lower than the two extruded WPCs. This is expected since the compression-molded product is typically subjected to a relatively lower temperature and pressure during the

Figure 1. Side by side display of SYP and WPC3 samples after 0, 3, and 5 cycles of aging (left to right). Both samples underwent significant changes in color during the accelerated aging process.


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production process, compared to the extruded products. Therefore, the compression-molded products usually have lower material density, resulting in lower specific gravity than extruded products. Laboratory-based accelerated aging did not affect the specific gravity of WPC products, as the maximum change in mean specific gravity was 5% or less for all three WPCs. The treated SYP showed a maximum variation of 9% change in specific gravity during the 6-cycle accelerated aging process. All three WPC products assigned to control treatment exhibited excellent water absorption rates of 4% or less (Figure 3). In contrast, the treated SYP showed a high water absorption capacity that fluctuated from 32% to 55% as it was subjected to repeated cycles of accelerated aging (Figure 4). The compression-molded product (WPC1) absorbed significantly higher amount WPC 1

WPC 2

WPC 3

SYP

Specific gravity (ratio)

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 2. Effect of accelerated aging cycles on specific gravity of three WPCs and CCAtreated SYP.

8

WPC 1

WPC 2

WPC 3

Water absorption (%)

7 6 5 4 3 2 1 0 Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 3. Effect of accelerated aging on water absorption of 3 commercially available WPCs.


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Water absorption (%)

60 50 40 30 20 10 0 Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 4. Effect of accelerated aging on water absorption of CCA-treated SYP.

of water, compared to the two extruded WPCs. The reason for such large amount of water absorption by the compression-molded WPC is that one surface of this product is usually machined, resulting in exposed wood fibers. The exposed fibers absorb high amount of water, resulting in high water absorption rates for this product. The extruded WPCs (WPC1 and WPC2) absorbed 52.5% of water, with the mean water absorption per treatment of 51%. The WPC3 showed the lowest water absorption rates. The reason for the lower water absorption rate of WPC3 is that it had the lowest proportion of wood fibers in the composite matrix. The fiber filler is the fraction in WPC that absorbs water, and hence it is natural that the product with the lowest proportion of fiber fillers had the lowest water absorption rates. All four products showed some fluctuation in water absorption with respect to aging cycles (Figures 3 and 4). The WPC products showed a dramatic increase in percentage water absorption after one cycle of accelerated aging, compared to the control. After two cycles of accelerated aging, all WPC products showed decrease in water absorption, followed by a small increase again in cycle 4. This fluctuation was particularly pronounced for WPC1. In compression-molded products, the cellulosic fibers are not mixed as thoroughly as in the case of extruded products. The relatively poor mixing between polymer and fiber filler, along with the exposed fibers on the machined surface have contributed to the large amount of water absorption observed in the first two cycles of accelerated aging. From cycle 2 through 5, the water absorption of the three WPC products decreased and stayed significantly lower than the corresponding values at cycles 0 and 1, while SYP showed significantly higher values. The breakdown of the long polymer chains into smaller sized molecules [10,14] during the first few cycles of aging perhaps resulted in reorientation and better coupling of the polymer with cellulosic fibers, leading to limited direct fiber contact area for


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Effect of Laboratory Aging on Wood-Polymer Composites WPC 1

Thickness swelling (%)

3.0

WPC 2

WPC 3

SYP

2.5 2.0 1.5 1.0 0.5 0.0 Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 5. Thickness swelling of four products plotted against accelerated aging cycle.

water absorption. Past studies [9] have only reported the initial increase in water absorption. It is the first time that such a dramatic decrease in water absorption after repeated cycles of aging was observed. Similar to the water absorption property, thickness swelling of the WPC showed fluctuations with aging cycle (Figure 5). The compression-molded product (WPC1) had the highest amount of thickness swelling, which increased dramatically after the first cycle of laboratory aging. Starting from the second cycle of accelerated aging (cycle 2), the thickness swelling decreased to less than half of the initial values for all WPC products. However, the treated SYP showed a different trend. Unlike its water absorption rate that showed sinusoidal fluctuation with aging cycle, the SYP showed a parabolic variation in thickness swelling with respect to aging cycle, with highest value occurring at cycle 3. Both the thickness swelling and water absorption of the aged WPC products were substantially lower than the SYP starting from cycle 2, indicating that the plastic substrate imparted excellent thickness swelling properties to the WPC under aging. Lower water absorption and thickness swelling over time can protect the material from surface cracking, splitting and bacterial rot, leading to longer life compared to wood. Mechanical Properties Mechanical properties tested included MOE and MOR under flexure, compressive strength, screw withdrawal force, hardness, and CLTE. Most of the strength properties showed sinusoidal fluctuations with respect to aging cycles. Analysis of variance (ANOVA) on various measures of strength indicated that product, aging, and their interaction were highly significant in explaining the strength properties exhibited by the four products under the six aging cycles (Table 2).


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Table 2. Analysis of variance (ANOVA) showing effect of product and aging treatments on mechanical properties of three commercial WPC products and SYP. Treatment variables No. of samples (N) P-value for Product P-value for Aging P-value for Product Aging R2 value

Flex. MOE

Flex. MOR

Hardness extension

Screw withdrawal

CLTE

120 50.0001 50.0001 50.0001 0.97

120 50.0001 50.0001 50.0001 0.96

120 50.0001 50.0001 50.0001 0.96

72 50.0001 50.05 50.05 0.68

72 50.0001 50.0001 50.0001 0.92

As expected both flexural MOE and MOR of the WPC products were much lower than the CCA treated SYP (Figures 6 and 7). Under control treatment, the average MOE of the WPC products was only 30–50% of the MOE exhibited by SYP, which was 10 GPa. This is expected, as the polymer substrate in the WPC tends to reduce the mechanical strength properties of the WPC in comparison to wood. The treated SYP was stronger (higher MOR) and stiffer (higher MOE) after five cycles of aging, compared to the control samples. On the contrary, the aged WPC products lost some of its MOE or stiffness compared to the control treatment. The five cycles of aging decreased the MOE of WPC1, WPC2, and WPC3 by 30, 22, and 13%, respectively. Similar results were also reported by Cantero [11] under a UV aging study on WPC products with a different proportion of wood fiber and polymer. The pelletized and extruded WPC3 under control treatment had a relatively low MOE of 3.2 GPa compared to WPC1 (4.5 GPa) and WPC2 (5 GPa), mainly due to the lower amount of fiber filler it contained. However, the loss of stiffness due to aging was much less in WPC3, leading to comparable MOE for WPC1 and WPC2 after five cycles of aging. Unlike the WPC products, CCA-treated SYP showed significant increase in its MOE (by 23%) after the five cycles of accelerated aging. The CCA-treated SYP was 2–3 times stronger than WPC under flexural loading (Figure 7). The WPC products (WPC 1–3) showed small fluctuations of 13, 6.2, and 2.5% respectively in the MOR compared to the control. After five cycles of aging, WPC1 showed the largest decrease in MOR by 13%, whereas WPC2 gained MOR by 6.2%, and WPC3 lost MOR by 2.5%. In comparison, the CCA treated SYP showed a 53% increase in its MOR after five cycles of accelerated aging. Such increase in MOR is common in treated lumber as the moisture content becomes low. The SYP showed some fluctuations in its MOR, with an increasing trend up to cycle 2, then decreasing slightly at cycle 3, and again increasing up to cycle 5.


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Modulus of elasticity (MPa)

14000

WPC 1

WPC 2

WPC 3

SYP

12000 10000 8000 6000 4000 2000 0

Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 6. Flexural modulus of elasticity of four products plotted against accelerated aging cycle.

Modulus of rupture (MPa)

120

WPC 1

WPC 2

WPC 3

SYP

100 80 60 40 20 0 Cycle Cycle Cycle Cycle Cycle Cycle 0 1 2 3 4 5

Figure 7. Flexural modulus of rupture of four products plotted against accelerated aging cycle.

The compressive strength of SYP samples increased till cycle 2, then showing a decrease till cycle 4, with some recovery in cycle 5 (Figure 8). There was a 22% of loss in compressive strength of SYP between cycle 1 and cycle 4, whereas only 5% difference existed between cycle 1 and 5. Unlike the flexural MOE, the compressive strength of the WPC products did not show a significant variation with respect to aging cycle. Between cycle 1 and 5, the average compressive strength of WPC1 increased by 1.6%, whereas the compressive strength of WPC2 and WPC3 decreased by 5.2 and 2%. The lack of significant changes in compressive strength and MOR of WPC under aging may be due to the fact that the re-crystallization of the polymer and


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Compression strength (MPa)

45

WPC 2

WPC 3

ET AL.

SYP

40 35 30 25 20 15 10 5 0 Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 8. Compression strength of four products plotted against aging cycle.

better bonding between the filler and polymer during the heat–thaw phases of accelerated aging may have compensated for any deterioration that may have occurred in the fibers. The screw withdrawal strength of CCA-treated SYP was significantly lower than that of the WPC products (Figure 9). All four products showed some fluctuations in the screw withdrawal force with respect to aging cycle. The compression molded WPC (WPC1) showed a relatively high screw withdrawal strength of 3422 N prior to aging. The pelletized and extruded WPC3 showed the lowest screw withdrawal force of 2491 N among the WPC. In comparison to WPC1 and WPC2, WPC3 had the lowest proportion of wood fiber and highest proportion of polymer, which may have contributed to the low screw withdrawal force. After five cycles of aging, the screw withdrawal strength of WPC3 increased by a substantial 46% compared to a modest 5% in WPC1. The WPC3 exhibited the highest screw withdrawal strength starting at cycle 1, with the only exception of cycle 3, where the difference between WPC1 and WPC3 were insignificant. Such gains in screw withdrawal strength show that the danger of screw loosening because of weathering alone in a building application are nonexistent for these two products. The direct extrusion product, WPC2, however, showed a gradual decline in screw withdrawal strength, as much as by 15%, after five cycles of aging. This deviation in the behavior of WPC may be attributed to the different additives such as coupling agents [1,11], the type of polyethylene (virgin vs. recycled), the proportion of various ingredients in each commercial product, and the interaction between the polymer and the wood fiber filler. Although hardness is usually measured as the force required to make an indentation of a certain depth, the set up used in this study measured the


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Screw withdrawal force (N)

4500

WPC 1

WPC 2

WPC 3

SYP

4000 3500 3000 2500 2000 1500 1000 500 0 Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 9. Screw withdrawal force of four products plotted against aging cycle.

indentation depth or hardness deformation at 5250 N load since several of the samples did not reach the preset indentation depth under the maximum load capacity of the Instron. Hardness is inversely proportional to the hardness extension. The lower the hardness deformation, the harder that material would be. The SYP was much softer than the three WPCs. Both WPC1 and WPC2 were relatively hard, with the hardness extension 53 mm under 5250 N load, which did not change with aging (Figure 10). However, WPC3 resulted in deeper indentation under the same load, indicating a slightly softer material. The higher proportion of polymer in WPC3 compared to the other two WPCs may have contributed to the softness of the material. The indentation depth of WPC3 showed a parabolic trend with aging cycle, with a peak value of 8.2 mm after two cycles of aging (cycles 2 and 3). Hardness is a desirable property for decking applications since a harder surface retains the surface characteristics for longer period and also reduces the wear and tear over time. Coefficient of linear thermal expansion is another important property considered in building products. All WPC products showed positive CLTE compared to the negative values exhibited by SYP (Figure 11). The softening and expansion of polymer substrate in WPC causes expansion of WPC when subjected to heat after cold temperature, resulting in positive CLTE. On the other hand, the loss of water from SYP caused it to shrink when exposed to heat after cold temperatures, resulting in negative CLTE. As the products were aged, the CLTE increased dramatically for SYP. WPC1 and WPC3 showed peak CLTE values at cycle 2, where as WPC2 showed the peak value at cycle 5, indicating a need for adding more cycles to the experiment in the future.


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Hardness extension (mm)

12

WPC 1

WPC 2

WPC 3

ET AL.

SYP

10 8 6 4 2 0 Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 10. Depth of indentation corresponding to a load of 5250 N applied with a modified Janka ball test jig for hardness testing against aging cycle.

80

WPC 1

WPC 2

WPC 3

SYP

CLTE (10−6 mm/mm)

60 40 20 0 −20 −40

Cycle 0

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Figure 11. Coefficient of linear thermal expansion (CLTE) of four building products plotted against aging cycle.

DISCUSSION The accelerated aging protocol followed in this study tried to emulate the annual cyclic variations in environmental conditions such as heat, moisture, freezing, and thawing. All building products intended for outdoor application are subjected to these annual cyclic variations in weather conditions. Therefore, it is important to evaluate the long-term performance of these products under similar accelerated aging process. Some of the physical (water absorption, thickness swelling) and mechanical properties (MOE, screw withdrawal force, hardness extension,



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CLTE) of majority of the products tested in this study showed sinusoidal fluctuations, with a peak or valley occurring after 1–3 cycles of aging. Although the molecular structure of the aged samples were not investigated to understand the reasons for this trend, past research [10,11] sheds some light into the phenomena of thermal degradation. Accelerated aging can lead to oxidation and chain scission of the polymer molecules [1,10,14]. The breakdown of the long molecular chains and the resulting entangled molecular network reduces the toughness and ability to deform plastically, resulting in a decrease in all the strength properties. The reported studies [1,10,14] on WPC aging were primarily based on UV aging or heat–thaw cycles under temperatures well below 1008C, or higher temperatures (up to 1908C) for a short duration of time (530 min). The UV aging protocol primarily affect the surface layer since the UV radiation does not penetrate very deep into the composite matrix. Therefore, these studies noticed that only a thin layer on the surface was affected by the chemicrystallization process of oxidation, chain scission, and re-crystallization. Unlike these past studies, the accelerated aging set up in this study exposed the samples to an extremely high temperature–pressure–moisture combination for 2 h (plus 30 min warm up) during the steam segment of each accelerated aging cycle to simulate accelerated thermal degradation of the entire matrix. Therefore, it is theorized that the entire material, not just the surface layer, experienced the high temperatures, which may have caused oxidation and chain scission in much greater depth than what is reported in [10], decreasing the strength properties more than what was observed in [10] after the first one to three cycles of aging. This is the reason for the deterioration in the strength properties of WPC during the first few cycles of accelerated aging while the SYP showed some gains. The cellulosic fibers that are exposed to repeated cycles of wetting, freezing and drying become brittle and stiffer, resulting in net gains in MOE and strengths. Most strength properties deteriorated initially, and then improved after 3–4 cycles of accelerated aging. Accelerated aging decreases the melting temperature (form 165 to 1338C for 50% wood fiber) as well as crystallization temperature (from 1258C to 1078C for 50% wood fiber) of WPC at a slow rate initially and then at a rapid rate [10]. Therefore, it is possible that the laboratory aging process used in this study may have decreased the crystallization temperature of the polymer in the WPC to well below the 1218C it was exposed during the steam segment of each aging cycle, after the first 2–3 cycles of accelerated aging. Therefore, the smaller polymer molecules resulting from chain scission on the entire composite matrix may have re-crystallized during the steam segment in later aging cycles (cycle 4 or 5). The temperature at the inner core of the composite matrix may have been elevated to 1218C, leading to re-crystallization of the entire composite


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matrix relatively uniformly, thus increasing the strength properties after approximately three cycles of aging. That may be the reason for the sinusoidal pattern observed in strength properties of WPC when subjected to the 6-cycle modified accelerated aging process described in this study. If the aging were to continue well beyond the five cycles, it is anticipated that the repeated oxidation, chain scission, and chemicrystallization will eventually lead to a uniform deterioration in mechanical properties. Further studies are required with fractography, infrared spectroscopy, and differential scanning calorimetry across the cross-sectional profile to understand and confirm the theory on the degradation and crystallization process across the entire cross-section of the WPC samples. CONCLUSIONS All three commercial wood plastic composites exhibited some loss in physical and some mechanical properties with accelerated aging. Treated SYP exhibited changes in surface characteristics such as splitting and cracking. Specific gravity was unchanged by the aging process. Water absorption was the highest in SYP, followed by compression molded WPC, and then by the extruded WPC, respectively. Both water absorption and thickness swelling increased dramatically in the WPCs after one cycle of accelerated aging compared to the control specimens, then showing a dramatic decrease starting at two cycles of accelerated aging. Product type, aging cycles and its interaction had a significant effect on strength characteristics and CLTE. Accelerated aging improved the flexural modulus of elasticity and modulus of rupture in CCA treated SYP. However, MOE decreased with aging in all three WPC. The effect of aging of WPC on compressive strength and MOR were relatively negligible. The screw withdrawal strength increased in SYP and WPC3 with aging cycle whereas it was unchanged in the other two WPC. SYP showed negative values for CLTE, indicating shrinkage, whereas WPC showed positive values (expansion), which increased with aging cycles. REFERENCES 1. Gauthier, R. (1998). Interfaces in Polyolefin/Cellulosic Fiber Composites: Chemical Coupling, Morphology, Correlation with Adhesion and Aging in Moisture, Polymer Composites, 19(3): 287–300. 2. Faulk, R.H., Vos, D. and Cramer, S.M. (1999). The Comparative Performance of Woodfiber-Plastic and Wood-based Panels, Proc. of the Fifth International Conference on Woodfiber-Plastic Composites, pp. 269–274, Forest Product Society, Madison, WI. 3. The Freedonia Group, Inc. (2006). 2005 Wood and Competitive Decking Study, Jan–Feb 2006 Issue, Professional Deck Builder, Williston, VT.



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4. Faulk, R.H., Lundin, T. and Felton, C. (2000). The Effect of Weathering on Woodthermoplastic Composites Intended for Outdoor Applications, In: Proc. of the 2nd Annual Conference on Durability and Disaster Mitigation in Wood-Frame Housing, pp. 175–179, Forest Product Society, Madison, WI. 5. Maldas, D. and Kokta, B.V. (1989). The Effect of Aging Conditions on the Mechanical Properties of Wood Fiber-polystyrene Composites: I. Chemithermomechanical Pulp as a Reinforcing Filler, Composite Science and Technology, 36: 167–182. 6. Raj, R.G. (1989). Performance of PP-wood Composite Materials Subjected to Extreme Conditions, Proc. of the ACS Division of Polymeric Materials, Science and Engineering, 60: 690–694. 7. Caraschi, J.C. and Leao, A.L. (2002). Woodflour as Reinforcement of Polypropylene, Materials Research, 5(4): 405–409. 8. Panthapulakkal, S., Law, S. and Sain, M. (2006). Effect of Water Absorption, Freezing and Thawing, and Photo-aging on Flexural Properties of Extruded HDPE/rice Husk Composites, Journal of Applied Polymer Science, 100(5): 3619–3625. 9. Pilarski, J.M. and Matuana, L.M. (2005). Durability of Wood Flour-plastic Composites Exposed to Accelerated Free-thaw Cycling, Journal of Vinyl and Additive Technology, 11(1): 1–8. 10. Selden, R., Nystrom, B. and Langstrom, R. (2004). UV Aging of Poly(propylene)/WoodFiber Composites, Polymer Composite, 25(5): 543–552. 11. Cantero, G. (2003). Mechanical Behavior of Wood/Polypropylene Composites: Effects of Fibre Treatments and Aging Processes, Journal of Reinforced Plastics and Composites, 22(1): 37–50. 12. Chow, P., Janowiak, J.J. and Price, E.W. (2007). The Internal Bond and Shear Strength of Hardwood Veneered Particleboard Composites, Wood and Fiber Science, 18(1): 99–106. 13. Kajita, H., Mukudai, J. and Yano, H. (2004). Durability Evaluation of Particleboards by Accelerated Aging Tests, Wood Science and Technology, 25(3): 239–249. 14. Anton-Prinet, C., Mur, G., Gay, M., Audouin, L. and Verdu, J. (1999). Change of Mechanical Properties of Rigid Poly(vinylchloride) During Photochemical Ageing, Journal of Materials Sciences, 34(2): 379–384.
