Thermal and Dynamic-mechanical Properties of Wood-PVC Composites

The influence of maleation on thermal and dynamic-mechanical properties of wood-PVC composites was investigated in this study. Experimental results indicated that maleation had a significant influence on most of the properties for wood-PVC composites. Storage modulus (E’), loss modulus (E”), and complex modulus (E*) of resultant wood-PVC composites were related to the retention levels and graft rate of maleated polypropylene (MAPP). However, tanδ was independent of MAPP retention and graft rate. Interfacial bonding strength increased with the increase of E’ and E* at low MAPP retention, but was not so sensitive to these moduli at high retention levels. Compared with PVC, wood, and wood-PVC composites without MAPP, maleated wood-PVC composites had significant shifts in most DMA, TGA, and DSC spectra. Also, there was a difference between E-43 and G-3015 on the thermal and dynamic-mechanical properties. All these were due to maleation and the interfacial structure in wood-PVC composites. INTRODUCTION Chemical coupling agents usually act as bridge to link polar wood fiber and non-polar thermoplastics. This helps transfer the stresses between wood and thermoplastics, thus improving the interfacial bonding strength in wood fiber and polymer composites (WFPCs) (Woodhams et al. 1984; Dalväg et al. 1985). The coupling forms include covalent bonds, secondary bonding (such as hydrogen bonding and van der Waals’s forces), polymer molecular entanglement, and mechanical interblocking. Although the coupling action in wood and polymer composites is complicated, the primary forms of covalent bonds for coupling agents are esterification, etherification, and carbon-carbon double bonds (Lu et al. 2000). Coupling agents (such as maleated polypropylene) create a new structure at the interface, which influences morphology, crystallization, rheology, and mechanical, thermal, and other properties of wood and polymer composites (Kolosick et al. 1992; Quillin et al. 1993; Collier et al. 1995). Thermal analysis has been extensively applied to investigate the thermal behavior of various materials as a function of temperature (Hatakeyama and Quinn 1994). A number of researches on thermal properties of WFPCs have been reported (Simenson and Rials 1991; Oksman and Lindberg 1995). Crystallization and morphology in WFPCs have been investigated with many thermal methods by a number of research workers (Felix and Gatenholm 1994). Weight or volume ratios of wood fiber greatly influenced glass transition 2 Proceedings Title (to be inserted by the publisher) temperatures and storage moduli of the resultant composites (Gatenholm et al. 1993). However, it is not clear how the interphase influences the thermal behaviors of resultant wood and polymer composites and whether there is any relationship between coupling agent performance and thermal properties. As a continuation of our early paper on the influence of maleation on graft polymerization, wettability, and interfacial adhesion in wood-PVC composites (Lu et al. 2002), thermal properties of wood-PVC composites with chemical coupling were investigated in this work. The objectives of this study were to investigate thermal characteristics of woodPVC composites with maleation and the relationship between thermal properties and coupling agent performance in the resultant composites. EXPERIMENTAL Materials Yellow poplar veneer (610 mm × 610 mm × 0.91mm) was obtained by Columbia Forest Products Co., VT. Wood veneer was cut into 50.8 mm by 25.4 mm in size. Moisture content of all wood specimens was between 5% and 6%. Clean and rigid polyvinyl chloride sheets (508 mm × 1270 mm × 0.0762 mm) with a density of 1,390 kg/m were purchased from Curbell Plastics Co., AZ. The glass transition and melting temperatures of the PVC sheets are 81C and 175C, respectively. It has a tensile strength of 55 MPa and a tensile modulus of 2,800 MPa (Delassus and Whiteman 1999). Before manufacture of wood-PVC composites, PVC sheets were cut into a dimension of 25.4 mm by 12.7 mm for shear testing and 50.8 mm by 25.4 mm for DMA testing, respectively. Two maleated polypropylenes (MAPPs), Epolene E-43 and Epolene G-3015, were used as coupling agent. E-43 has an average weight molar mass (Mw) of 9,100, and its acid number is between 40 and 55. G-3015 has a high molecular weight of 47,000 g/mol, but has a low acid number (between 12 and 18). E-43 contains more maleic anhydride groups than G-3015. Benzoyl peroxide (BPO) was used as initiator, and toluene was used as solvent for both MAPPs. Sohxlet Extraction Sohxlet extraction was conducted on all wood specimens according to the ASTM standard D1105-96 to reduce the influence of extractives on chemical coupling. The wood samples were first extracted with a 120 ml mixing solution of toluene and ethyl alcohol for 4 hours. They sequentially underwent the second extraction with a 120 ml of ethyl alcohol for 4 hours. The extracted wood specimens were finally oven-dried at 70C for 24 hours to reach a constant weight. The oven-dried weight of each sample was measured. Secondary Sohxlet extraction was conducted to determine the graft rate of MAPP on wood specimens. All treated specimens were continuously extracted with toluene for 24 hours (Lu et al. 2002). The extracted specimens were then oven-dried at 70C for 24 hours to reach a constant weight. Coupling Treatment The procedure of coupling treatment for wood specimens was described in Lu et al. (2002). Wood specimens were dipped in coupling solution at 100C for 5 min under a continuous stirring with a magnetic stirrer. The concentration levels of MAPP were designed to be 0, 12.5, 25, and 50 g/L. The weight ratio between BPO and MAPP was 0.5. The treated Proceedings Title (to be inserted by the publisher) 3 specimens were removed from the solution and cooled down to room temperature. All treated specimens were finally oven-dried at 70C for 24 hours to reach a constant weight. Retention and graft rate of coupling agent for treated wood specimens were calculated as: % 100 / (%) 0 1 × ∆ = W W Rt [1] % 100 / (%) 0 2 × ∆ = W W Gr [2] where, Rt = retention of coupling agent in a specimen (%); Gr = graft rate of coupling agent in a specimen (%); W0 = oven-dry weight of extracted wood samples before coupling treatment (g); ∆W1 = oven-dry weight of polymer deposited in wood after coupling treatment (g); and ∆W2 = oven-dry weight of polymer remained in wood after secondary Sohxlet extraction (g). Manufacture of Wood-PVC Composites The manufacture of wood-PVC composites followed the procedure given in Lu et al. (2002). To create a wood-PVC laminate, a piece of PVC sheet was inserted between two MAPPtreated wood specimens. The assembly was temporily fixed with two pieces of narrow Scotch tape on each side. The assembly was then hot-pressed with a small-scale press under a pressure of 0.276 MPa for a shear specimen and under a pressure of 0.552 MPa for a DMA specimen. The pressing cycle for the wood-PVC assembly consisted of a 3 min heating at 178C and a 1 min cooling under pressure. At the end of the heating period, the press platens were cooled with running tap water to 70C. The laminate was allowed to cool to room temperature (Lu et al. 2002). Shear Strength Measurements Shear tests were conducted with a Model 1125 INSTRON machine according to ASTM standards D3163 and D3165. Two mechanical tensile grips were used to clamp the sample to the loading frame. The span between the two clamps was 50.8 mm. Each sample was tested to failure at a loading speed of 2.54 mm/min. Shear strength (Pa) was calculated as a ratio of the maximum failure load (N) to the bonding area (m). Thermal Analysis Dynamic Mechanical Analysis (DMA): A DMA system (Seiko Model DMS 110) was used to conduct dynamic mechanical analysis for maleated wood-PVC composites. The specimen size was 50 mm by 12 mm by thickness. The DMA testing procedure consisted of three cycles: first heating, first cooling, and second heating (Table 1). A test specimen was subjected to sinusoidal stress under a three-point bending mode. The span between the load and each supporting point was 20 mm. The oscillating frequencies of the load acted on the specimens were 0.01 Hz, 0.1 Hz, 1 Hz, 10 Hz, and 100 Hz. Testing temperatures changed according to test materials. Started from room temperature, the maximum heating temperature was 220C for wood, 100C for PVC, and 150C for wood-PVC composites. The heating rate was 0.5C/min, while the cooling rate was 0.25C/min (Table 1). Thermogravimetric Analysis (TGA): A modulated thermogravimetric analyzer (TA Instruments Model TGA2950) was used to characterize the decomposition and thermal stability of wood-PVC composites with coupling treatment. A specimen was first placed into 4 Proceedings Title (to be inserted by the publisher) a Seiko Al sample pan on the Pt basket in the furnace, and then heated from room temperature to 600C. The heating rate was 5C/min. During testing, the heating unit was flushed under a continuous nitrogen flow at a pressure of 8 KPa. To separate possible overlapping reactions during measurements, derivative thermogravimetric (DTG) analysis was also conducted to measure the mass change of a specimen with respect to temperature (dm/dT) using the same TGA system. Table 1. DMA test cycles for wood, PVC, and wood-PVC composites Temperature [C] Specimen Test mode Test cycle Start Stop Rate [°C/min] Wood Bending First heating First cooling Second heating 20 220 30 220 30 220 0.50 0.25 0.50 PVC Bending First heating First cooling Second heating 20 100 30 100 30 100 0.50 0.25 0.50 Woo-PVC composites Bending First heating First cooling Second heating 20 150 30 150 30 150 0.50 0.25 0.50 Differential Scanning Calorimetry (DSC): A modulated DSC analyzer (TA Instruments Model DSC2920) was used to determine the thermal complex transitions of maleated woodPVC composites. A specimen pressed into an aluminum sample pan was placed into the heating chamber in DSC. For comparisons of wood, PVC, and resultant composites, the maximum temperature was controlled at 150C for all specimens. The heating rate was 5C/min. During measurements, the heating chamber was flushed with a continuous nitrogen flow at a pressure of 8 KPa. Each specimen was measured three times. RESULTS AND DISCUSSIN Retention and graft rate of MAPP on wood after coupling treatment are listed in Table 2. The primary thermal and mechanical properties of wood-PVC composites are summarized in Table 3. Dynamic Mechanical Analysis Wood-PVC composites treated with coupling agents presented different thermal behaviors from wood and PVC. As shown in Table 3 and Figure 1, the glass transition of PVC was about 80°C, while the glass transition of yellow poplar was close to 160°C. From the first heating, the glass transition of wood-PVC composites was around 89°C at the frequency of 1 Hz. Therefore, the glass transition of wood-PVC composites was between those of wood and PVC. Frequency of the oscillating load greatly influenced the glass transition of wood-PVC composites (Figure 2). From the second heating, the glass transition of wood-PVC composites with 6.83% E-43 shifted about 20C from 0.01 Hz to 100 Hz. Similar trends also occurred in wood-PVC composites with other retention levels of E-43 and G-3015 in the same heating procedure and with all retention levels in other two procedures. Thus, the larger the frequency used, the higher the glass transition of wood-PVC composites. Proceedings Title (to be inserted by the publisher) 5 Table 2. Retention and graft rate of MAPP on wood after coupling treatment 1 MAPP Concentration in toluene [g/L] Retention [wt%] 2 Graft rate [wt%] b E-43 0 12.5 25.0 50.0 0 2.95 (0.15) 4.12 (0.94) 6.83 (1.08) 0 1.96 (0.24) 2.42 (0.21) 2.86 (0.36) G-3015 0 12.5 25.0 50.0 0 2.17 (0.07) 3.64 (0.81) 6.35 (1.03) 0 2.06 (0.18) 2.63 (0.35) 3.08 (0.78) 1 The values in parentheses are standard deviations. 2 wt% means the weight percentage of oven-dried wood samples. Table 3. Thermal and mechanical properties of wood-PVC composites Material E’ [Gpa] E” [Gpa] 1 Glass transition [C] tanδ 1 Shear Strength [MPa] Enthalpy [J/g] TG at 600C [%] DTGmax [%/C] PVC Wood Wood-PVC composites: 0% MAPP 2.95% E-43 4.12% E-43 6.83% E-43 2.17%G-3015 3.86%G-3015 6.35%G-3015 5.73 10.43