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ionomer composites
Composites: Part A 38 (2007) 1–12 www.elsevier.com/locate/compositesa
Mechanical properties of wood flour/HDPE/ionomer composites Tieqi Li, Ning Yan
*
Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ont., Canada M5S 3B3 Received 14 March 2005; received in revised form 30 January 2006; accepted 6 February 2006
Abstract The structure and mechanical properties of wood flour composites with HDPE/ionomer blends as matrices were studied at a fixed wood loading of 60% by weight. It was found that toughness and strength properties of the composites can be improved significantly by adding ionomers of different types and contents. The enhancement in the interfacial interaction was observed through short-time creep analysis. The interfacial interaction and the structure of the matrix phase were characterized through the melting behavior using differential scanning calorimetry (DSC) and with small strain oscillatory tests on the melts using a Dynamic Mechanical Analyzer. Both the sodium and zinc ionomers were found to be immiscible with the HDPE in matrix. The immiscible characteristic was correlated with the interfacial load transfer efficiency as revealed by the creep tests. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Polymer–matrix composites (PMCs); A. Thermoplastic resin; A. Wood; B. Mechanical properties; B. Creep; D. Thermal analysis; D. Electron microscopy
1. Introduction The past decade has seen fast and steady growth of wood plastics industry. Among many reasons for the commercial success, the low cost and reinforcing capacity of the wood fillers provide new opportunities to manufacture composite materials. Certain problems, however, are challenging the further application of the wood plastics technology owing to some intrinsic properties of wood such as its hydrophilic nature and relatively poor thermal stability of the ligninocellulose components. Efforts are being made to improve the compatibility between the wood filler and the matrix polyolefin resins [1–5]. Moisture sensitivity and related dimensional stability and aging performance are also topics of intensive research [6–9]. It is especially worth noting that commercial wood plastics products are typically formulated at high wood loading levels, for example, of 40–70% by weight. The low strain limit and impact resistance associated with the high filler *
Corresponding author. Tel.: +1 416 946 8070; fax: +1 416 978 3834. E-mail address: [email protected] (N. Yan).
1359-835X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2006.02.003
content were frequently documented [2,10–15]. Elastomers were shown to be effective in modifying the impact resistance [11–14], but, they typically are too expensive to be practical. The choice of fillers of high aspect ratio [14,15] was promising but would be limited by the increased difficulty in processing. Exploration of new matrix systems for improving both the static and impact resistance of wood plastics will be helpful for the further development of this type of products. Among many candidates other than the resins currently used for wood plastics, ionomers based on copolymers of acids and olefin monomer units are worthy of exploration. These ionomers are amphiphilic and can be tailored to be compatible with both the matrix and wood and hence could also act as coupling agents. Many of the ionomers are known to have extraordinary toughness [16] and were recently found to be able to improve interfacial bonding strength [17] and modify interlaminar toughness [18] in advanced composites. The large strain capacity should be applicable in improving the toughness of wood composites for higher performance. The versatility in structure and properties and the relatively low cost compared to
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engineering resins and other functional polymers present a potential to tailor the wood composites at reasonable cost. In this work, wood composites made from blends of HDPE and poly(ethylene-co-methacrylic acid) ionomers as matrices are studied. Wood flours, the most popular fillers in wood plastics industry, were used as the reinforcement. Off-the-shelf Surlyn ionomers were used because of their commercial availability. The selected ionomer resins were two sodium-neutralized ionomers with different stiffness and two zinc ionomers with various flow properties. Flexural and impact tests were carried out to examine the improvements in toughness and strength properties. The short time, low stress creep experiments were performed to evaluate the interfacial interaction in wood composites with ionomers. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to characterize the phase structure of the ionomer/HDPE matrix. Fracture surfaces of the ionomer composites were studied with a scanning electron microscope for understanding the deformation and failure behavior. The principles for creep and dynamic tests for structure characterization are reviewed briefly in the following section. 2. Theoretical 2.1. Creep in linear regime Creep performance of wood plastics composites has been extensively studied for application purposes [19–23]. Extruded wood plastics composites at high wood loadings have been observed to consist of two different stages of creep deformation depending on load [23]. The creep was considered linear viscoelastic in nature at lower stress levels and contain extra contribution from damages at higher loads. While most of the published creep experiments are on long term performance for practical purposes, they do not differentiate the contribution from the damage caused by microscopic fracture from the plastic flow due to the viscoelastic nature. When a creep test is performed at a stress much lower than the ultimate stress, the creep deformation of a similar wood plastic composite was shown to be mainly of viscoelastic nature and contained no contribution from damage. Under such conditions, the time dependent compliance can be expressed in a power-law form as [24] DðtÞ ¼ D0 þ D1 tn
ð1Þ
where the constants D0, D1, and n are material parameters typically constant for both the creep and recovery processes. In general, the rate-dependence nature of polymeric materials should result in different compliance and exponent parameters [24]. The time-dependent strain is hence written as e ¼ ðD0 þ Dc tnc Þr0 ; nr
0 < t 6 t0 nr
e ¼ Dr ðt ðt t0 Þ Þr0 ;
t0 < t
ð2Þ ð3Þ
where t0 is the time when the creep begins to recover and subscripts ‘c’ and ‘r’ denote creep and recovery respectively. The power-law relationship in the same form as Eq. (2) was shown by Findley [25] to also govern the very long term creep of polyethylene and poly(vinyl chloride). It was hence referred to as Findley equation in most other creep studies on wood plastics composites [19–22], where the stress is often above half of the expected ultimate stress. The creep under low stress and short-time intervals and the subsequent recovery are of viscoelastic nature and free from the effects of damage in wood plastics with high wood loadings [23]. Despite of the approximating nature of the power-law relations, the parameters Dc, Dr, nc and nr from the low stress, short-time span creep study can reveal the viscoelastic behavior and provide information on the wood–polymer interfacial interaction. The parameter Dc is elastic in nature and correlates inversely with the modulus. Dc, Dr, nc and nr describe the time-dependent deformation. The exponents nc and nr relate to the spread of the retardation spectra while Dc and Dr contain the contribution from both the characteristic time and the spread of the spectra [26]. A decreased nc or nr value would usually indicate broader spread of the spectrum and could reveal different contribution from the wood reinforcement. As the data sampled on the short-time frame usually are not likely sufficient to provide detailed relaxation spectra, the power-law analysis still can be used as a simple way to compare the viscoelastic properties of the composites. 2.2. Small-amplitude oscillatory experiments Small amplitude, dynamic mechanical experiments are among the most frequently used methods for characterizing the structure of polymeric materials. When the deformation is within the range of the linear viscoelasticity, the responses to the applied oscillatory strain or stress provide rheology characterization of the studied materials. For example, the isothermal frequency spectrum obtained in the linear regime can be used to predict deformation of all other type within the linear viscoelasticity limit through various transformations. Difficulties are encountered, however, in applying the dynamic experiments to wood plastic composites. Similar to many other suspensions with high filler contents, the filled wood plastic composites typically are non-linear in conditions commonly used for neat polymer melts [27]. The non-linear feature prevents the direct application of experimental results based on more conventional methods such as the frequency sweep tests owing to the dependence of the measured response on time and strain history. Despite the problems associated with the highly non-linear responses, the dynamic tests are still informative for exploring the structure of the wood plastic composites. As revealed through the stress relaxation experiments of HDPE/maple composites [27], the strain dependence of the relaxation modulus approaches its linear viscoelasticity limit at below 0.1% strain for the HDPE/maple composites
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The composites containing 60% by weight of wood fillers and the ionomer of either 0%, 2%, 4%, 8%, 12%, 16%, 24%, 32%, or 40% based on the total weight of the composites were compounded with a Brabender mixer with roller rotors and then compression molded into plates. The neat resin mixtures were first introduced into the mixer soaked at 180 C for melting. The wood flours were then added into the melted resin mixture with the rotor speed varying between 5 and 20 rpm. The composition was subsequently mixed for 3 min at 35 rpm and 3–5 min at 20 rpm after the ampere meter gets stabilized. The compound was then compression-molded into plaques of 4 mm thickness at 180 C with a maximum nominal pressure of 7.5 MPa using a non-matched mold.
are studied for Modulus of Rupture (MOR), strain at break (eb) and the strain energy at break (Emax). Notched Izod impact experiments were carried out following the ASTM D256 standard. The room temperature short-time creep properties were studied by a TA Q800 Dynamic Mechanical Analyzer using a three-point bending fixture of 50 mm span. The sample of 12.7 mm width was brought to the fixed stress level of 5 MPa in shorter than 6 s, let deformed at the stress level for 10 min, and then allowed to recover for 20 min. Regression based on Eqs. (2) and (3) were performed based on the Levenberg–Marquardt algorithm using a Macrocal Origin software and double checked with a Wolfram Research Mathematica package. To avoid the sensitivity of regression based on Eq. (2) to initial values, the parameters were first obtained via fitting to Eq. (3). Value of nr was then used as the initial value of nc for fitting for D0 and D1 through Eq. (2). Differential scanning calorimetry studies were performed using a TA1000 instrument on the slices of ca. 0.2 mm thickness cut from the compression-molded plates perpendicular to the molded surfaces. The sample was brought from room temperature to 200 C and then cooled to 70 C at a 5 C/min rate and then heated to 200 C at 10 C/min to obtain information about melting. Strain sweep tests at 1 Hz were also carried out for rheology properties using the TA Q800 instrument with the shear-sandwich fixture under the multi-strain mode. Samples with 10 mm · 10 mm area were load into the sandwich fixtures and let soak sufficiently at 180 C. A strain ramp was then performed from 1 to 100 lm at the 5 point/decade interval. The fracture surfaces of both ASTM D790 and D256 specimens were gold-coated and examined with a Hitachi S520 SEM using the acceleration voltage of 20 kV.
3.3. Mechanical testing and structure characterization
4. Results and discussion
Flexural tests were performed on the samples of 12.7 mm in width and the as-molded thickness of around 4 mm conforming to the ASTM D790 standard using 6 defect-free specimens from the same plate for each formulation. The modulus of elasticity (MOE) was determined on the strain range between 0.1% and 0.5%. Only the results of specimens that break below or close to 5% strain
4.1. Flexural and impact properties
at 40–60% wood contents. The dynamic moduli at the lowest available strain amplitude hence provide a useful approximation to the linear viscoelastic properties. The response is evaluated at the fixed level of stress amplitude for preventing the possible effects of interfacial slip on matrix strain field [28,29]. 3. Experimental 3.1. Materials Wood fillers used in this work are grade 14010 maple flour from American Wood Fiber. The HDPE and ionomer resins are listed in Table 1. The properties were provided by the suppliers unless stated otherwise. The HDPE resin and the wood flour were used as received. The ionomer resins were used immediately after being taken out of the package without drying. 3.2. Compounding and molding
Fig. 1 reports the typical stress–strain curves of the maple flour/HDPE/ionomer composites in the flexural tests as compared to the neat HDPE/maple used. As shown in the figure, the composites containing 4% of the sodium ionomers deform and break in the way similar to the
Table 1 Polymers used in the study Legend
HDPE NaR NaS ZnL ZnH a
Melt index (g/10 min) ASTMD-1238 0.3 0.9 1.0 1.0 4.5
Density (kg/m3)
Melting point (C)
Vicat softening point (C)
D-792
Flexural modulus (MPa) ASTMD-790
946 940 950 960 970
940a 49 30 178 358
– 78 70 86 82
– 51 47 60 57
Measured in this laboratory on compression-molded specimens of 4 mm thickness.
Elongation at break ASTM D638 660 555 510 335
Supplier and resin grade Equistar LB01000 Dupont Surlyn 8120 Dupont Surlyn 8350 Dupont Surlyn 9120 Dupont Surlyn 9150
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Fig. 1. Typical stress–strain curve of the composites with 4% ionomer compared to neat HDPE/maple compound.
straight blend of HDPE and maple. It was found the composites with 2–4% of ionomers show whitening and multiple cracks starting near the surface in tension when the maximum load is reached. The subsequent decrease of load corresponds to the increase in the number of such damages and the quasi-stable propagation and merging of the damages that form the main crack. With the increase in ionomer content, more compression-related failure is observed. With 12% of NaR or 8% of NaS ionomer, the fracture is mainly along the shear plane. Composites with more sodium ionomers do not break below 5% of the flexural strain. The zinc ionomers, however, change the load– deflection curve significantly even at the low content of 2%. The composites with the zinc ionomers generally show higher stiffness and breaks along the shear plane, causing the drastic drop of the load as exemplified in the figure. The maximum load point is referred to as break hereafter for all composites. The corresponding stress, strain, and strain energy are reported as MOR, eb, and Emax. The difference in fracture mechanism will be further shown with SEM fractographs later in this paper. Fig. 2 illustrates the MOE and MOR of the composites with different ionomer contents. From Fig. 2a, it can be seen that the ionomers improve MOE at low ionomer contents. With the further increase in the ionomer content, there is a general trend for MOE to decrease with increasing ionomer content for all the ionomers studied. When ionomer content is higher than 24% in the matrix, the composites with the NaR ionomer, which has higher modulus, show higher MOE than the composites with the NaS polymer. MOE of the composites with the zinc ionomers is also higher than that of the sodium counterparts. There seems to be a general trend of increase in composite MOE with the modulus of the ionomer component, agreeing with the rule of mixture at these higher ionomer contents. However, such a dependency on component modulii is not followed when the composites with the two zinc ionomers are compared. The softer ZnL ionomer results in composites with higher MOE. Structure details such as the molecular weight, ionic monomer content and degree of neutraliza-
Fig. 2. MOE (a) and MOR (b) of the wood plastics composites as a function of ionomer content. The arrows indicate the contents above which the materials are too ductile for the MOR to be evaluated with the method.
tion of the ionomers remain to be studied for future application of the ionomer techniques. In contrast to the case of MOE, MOR of the composites shows a simpler dependence on the moduli of the ionomers. As shown in Fig. 2b, the zinc composites have higher MOR values than the sodium composites. The ZnL polymer, which has the highest modulus among the ionomers studied, results in the highest MOR for the composites while the softest ionomer, NaS, gives the lowest MOR value for the composites. The higher toughness of the ZnL ionomer than its ZnH counterpart, seen as the higher tensile strain in Table 1, may be responsible for the higher MOR values observed with the ZnL composites. In contrast to the decrease in MOE with the increase in ionomer content, the MOR values for sodium composites remain unchanged with the changing ionomer content and the two zinc composites show the increases in MOR at higher ionomer contents. While the decrease in MOE can be expected from the addition of the ionomers of the lower moduli, the increase in MOR suggests the toughness of the ionomers has some contributions. Fig. 3 illustrated the eb and Emax of the ionomer composites. As shown in the figure, both break strain and energy of the ionomer composites increase with the ionomer content. As shown in Fig. 3a, lower ionomer content is required to achieve a similar level of eb for composites containing
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Fig. 4. Izod strength of wood plastics as a function of ionomer content. Trend lines are second-order polynomial fit for composites containing ionomers.
Fig. 3. Strain at break (a) and Energy at maximum load (b) as a function of ionomer content (trend lines are second-order polynomial fit).
sodium ionomers, indicating that the sodium ionomers are more effective than the zinc ionomers in improving the toughness. A similar trend is also observed for the energy at break, Emax (Fig. 3b). Comparing the two zinc ionomers, the softer ZnL ionomer seems to be more effective in improving both eb and Emax. The static toughness in terms of the eb and Emax relates more to the deformation capacity of the ionomers than the stiffness as revealed by MOE. The strain capacities of the ionomers are also found to affect the impact properties. Fig. 4 illustrates the notched Izod strength of the composites with different ionomers. As shown in the figure, the composites with the sodium ionomers have much higher toughness than the zinc ionomers especially at the high ionomer contents. Appreciable toughening effects are observed with the softest NaS ionomer at 8–12% of loading, consistent with the value observed for similar HDPE/wood composites containing a maleated styrene–ethylene/butylenes–styrene (SEBS) compatibilizer [3]. In contrast, the toughening effect of the more rigid NaR ionomer is evident above 16%. The effects of zinc ionomers on impact resistance are not discernable. The structure characterization in the following sections will shed some light on the different roles played by the sodium and zinc ionomers.
the creep behavior of the wood composites with and without the ionomers can be modeled with the power-law relationship. However, the parameters for recovery were found generally different from those for creep. Fig. 5 exemplifies the applicability of Eqs. (2) and (3) to the short creep behavior. It was found that the parameters for creep (Dc) are typically an order of magnitude higher than Dr for the recovery stage. The two quantities are shown in Fig. 6 in terms of D1 and D1 in comparison with the r c MOE values. It is evident that D1 is close to MOE but c D1 r is much higher. The samples seemed to be much stiffer in recovery than in creep. The exponents in Eqs. (2) and (3) were also found to be different. The exponent in recovery, nr, were around 0.3, close to the value for creep of unfilled polymers [24]. But the values for creep (nc) were lower than 0.1 and well below the values shown in recovery. Given the moderate agreement between D1 c and MOE, the compliance Dc and nc of composites with varying ionomer type and contents are compared in Fig. 7. From Fig. 7a, it can be seen that Dc of composites containing the two sodium ionomers increases with the ionomer content due to the lower modulus of the ionomers. The increase of Dc for the zinc composites, however, is
4.2. Linear creep properties In agreement with what was observed for neat HDPE/ wood composites [23], the present experiments show that
Fig. 5. Creep and recovery of the HDPE/maple composite with powerlaw fit. The solid line is the best fit to the data in creep and recovery stages according to Eqs. (2) and (3).
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than the ZnL ionomer, suggesting the same mechanism responsible for the broader spread of the relaxation spectra. 4.3. Melting behavior
Fig. 6. Dc and Dr compared with MOE. Line shows value for 1/D0 = MOE.
Fig. 7. Model parameters Dc and nc of the wood plastics with ionomer matrices.
much less significant, confirming the less change in MOE of the composites shown in Fig. 2a. As shown in Fig. 7b, the exponent n of composites with the sodium ionomers tends to be higher than for the neat HDPE/maple composite. The softer sodium ionomer (NaS) appears to cause higher exponent, which would suggest broader relaxation spectra for the composites. The result is reasonable since the wood reinforcement is expected to cause more diversified conformations when the matrix is softer. Similar to the sodium ionomers, the more rigid zinc ionomer (ZnH) also results in lower n values
Fig. 8 compares the melting curves of the composites with the ionomers of different ions. Two melting peaks can be seen for the ionomer composite, one at around 82 C for the ionomer [29–31] and the other one near 130 C for the HDPE resin. In contrast, both the neat sodium and zinc ionomer resins show only the melting peak in the lower temperature range and do not have the polyethylene melting peak at around 130 C. The enthalpy DH and peak temperature Tm are compared with those of the neat ionomers in Fig. 9. From Fig. 8, it can be seen that the HDPE melting peak decreases in area with the increase in the ionomer content. As shown in Fig. 9a, the enthalpy of the polyethylene melting peak tends to decrease with the increasing ionomer content as expected for both the sodium and zinc ionomers. The slight negative deviation of the dependence of DHm on the sodium ionomer content from the linear additive prediction suggests that the ionomer phase has a limited effect on the crystallization or melting of polyethylene, indicating certain level of miscibility in amorphous phase between the HDPE and the sodium ionomer. In comparison, the zinc ionomer composites also show the decrease in DHm but the enthalpy stays closer to the linear prediction than for the sodium ionomer composites. At higher ionomer contents, the polyethylene melting peak does not disappear as for the sodium composites but remains at an appreciable level that is similar to that of the composites even up to 80% of ionomer in matrix. There seems to be less interruption from the zinc ionomer phase in the composites than the case with the sodium ionomer. The inhomogeneous structure can always be expected for both zinc- and sodium-neutralized E-MAA copolymers according to recent studies by Kuzumizu et al. [32] and Farrell and Grady [33]. However, the ionomer resins themselves do not show the polyethylene melting endotherm in un-filled form. The appearance of the polyethylene endotherm for the ZnL ionomer in the wood composites suggests the ionomer experiences further phase separation that is significant enough for the ethylene segments to enter the polyethylene crystalline domains. The zinc ionomer is hence proved to be less stable in structure than its sodium counterpart in wood composites. The different miscibility with HDPE is further revealed through the melting temperature, Tm. As shown in Fig. 9b, Tm of the HDPE melting peak tends to decrease with ionomer content for both the sodium and zinc ionomers. For the ionomer melting endotherms, however, Tm shows a different dependence on the ionomer content. With the sodium ion, the ionomer melting peak shifted to a higher temperature with the decrease in the ionomer content while the zinc endotherm does the opposite. An increase in the melting temperature of the sodium endo-
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Fig. 8. DSC melting curves of composites with NaS (a) and ZnL (b) ionomers compared with neat ionomer resins.
Fig. 9. Melting enthalpy (a) and temperatures (b) composites with NaR and ZnL as function of ionomer content.
therm may be a result of the increase in the sequence length of the crystalline chain segments in the ionomer phase or co-crystallization with the HDPE, which melts at a higher temperature. Both possibilities mean good compatibility between the sodium and HDPE in solid phase, agreeing with the miscibility observed through the negative deviation of HDPE melting enthalpy in Fig. 9a. For composites with the zinc ionomer, the independence of the zinc Tm on the ionomer content at the higher ionomer contents reveals the immiscible nature between the zinc ionomer and HDPE. The decreasing Tm at the lower zinc ionomer contents, on the other hand, indicates more imperfect crystals resulting from stronger ion-ion association as the consequence of the phase separation of the zinc ionomer component. The zinc ionomer proves to be less miscible with HDPE and the different miscibility should account for
the different mechanical responses between the two different types of ionomers. The immiscible nature explains why the exponent n from the creep tests tends to saturate beyond the 16% ionomer content. With the increase of ionomer content, the ionomer gradually becomes the continuous phase that governs the load transfer among the neighboring wood particles. The ionomer content of 40% based on matrix weight seems to be reasonable for the ionomer continuous phase to form, especially in the case when the space between the wood particles in these composites is small. 4.4. Rheology properties of the composite melts The difference between the matrix morphology of composites was further examined through the rheology
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Fig. 10. Complex modulus (a) and tan d (b) of the composites with NaR (red) and ZnL (black) ionomers at 180 C at 10 kPa stress amplitude. The lines are linear connection between the responses of neat HDPE and ionomer as matrix detailed in text. (For interpretation of the references in color in this figure legend, the reader is referred to the web version of this article.)
observations on the rigid ionomer of sodium ions (NaR) and the ionomer with a similar melt index but contains zinc ions (ZnL). Fig. 10 compares the complex viscosity and tan d of the ionomer composites at 10kPa. As shown in the Fig. 10a, the dependence of the rheology properties on the ionomer content is different between the two ionomers. Though both ionomers cause negative deviation (NDV) from the rule of mixture [28], the melt with the zinc ionomer shows two local minimums and the sodium system does not show any absolute minimum. The different NDV behaviors can be interpreted as the difference in matrix morphology if the effects of the wood particles are assumed to be the same. For the sodium composites, the negative deviation of g* occurring at a low ionomer content would suggest immiscibility between the sodium and HDPE melts when the ionomer is the minor component in the matrix. The decreasing level of NDV behavior with the increase in the sodium ionomer content may result from the similarity between the ethylene segments in ionomer and HDPE, which makes the ionomer phase more miscible with the HDPE. On the other hand, two local minimums shown by the melts of zinc composites support the immiscibility between the zinc ionomer and HDPE, agreeing with the DSC observation. Combined with the DSC observation shown in Figs. 8 and 9, the result on melting temperatures reveal more phase-separation for the matrix in zinc composites than for composites with the sodium ionomers. From Fig. 10b, positive deviation (PDV) of tan d from the linear additive rule is clearly shown and the tan d values of the composites with the ionomers are much closer to the
level of the composites with pure ionomer matrix than to the value of neat HDPE/maple composites. The viscous damping of the composites is affected more by the interface than the modulus. It is reasonable to assume that the significant difference in tan d between the composites with and without the ionomers originates from the interfacial dissipation. The tan d values at 2% of NaH and 4% of ZnL may hence be taken as the reference value for the HDPE matrix with strong restriction at boundary by the wood reinforcement. In such, the ionomer content dependence would again show the NDV nature, confirming the immiscible characteristic of the HDPE/NaH blends in the matrices of the wood composites. The ZnL composites, in contrast show NDV characteristic at the low ionomer contents and PDV behavior when the zinc ionomer becomes the continuous phase. The mixed NDV-PDV behavior is consistent with the variation in DSC enthalpy shown in Fig. 9a. The entry of ethylene segments from the ionomer into polyethylene crystalline domains causes higher melting enthalpy than the prediction based on added HDPE content and the rule of mixture. The higher content of the more ordered ethylene phase results in a lower tan d value. 4.5. Fracture mechanism Fig. 11 illustrates the fracture surfaces of the sodium and zinc composites near the tensile and compressive edges, respectively. Near the tensile edge, there are considerable differences between the two ionomers at the low ionomer
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content of 2%. Both composites show remainder of cold drawing during the flexural loading but the process seems to be more intensive for composite with ZnL (Fig. 11a and b). Near the compressive edge, the composite with the 2% NaR ionomer experiences (Fig. 11c) a much less extent of drawing than near the edge in tension (Fig. 11c). Its ZnL counterpart, however, shows the characteristic of large scale planar plastic flow (Fig. 11d). The difference in the cold drawing on the tensile edge is associated with the immiscible nature between the ionomer and HDPE because the ionomer content in this case is not high enough to allow such drastic changes if they form a single phase with HDPE. The higher rigidity of the zinc ionomer is hence amplified owing either to its enrichment at the wood–plastic interface or the phase-separated structure in matrix. In Fig. 11e–f, the fibrils from cold drawing is still visible at the ionomer content of 16%. However, the difference between the sodium and zinc composites is not discernable for the tensile fracture surface. The fracture surfaces are
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more flat than at the lower ionomer content of 2% for both the NaR and ZnL composites. It is evident that shear failure dominates at the higher ionomer content. The premature tensile failure typically found for straight blend of HDPE and wood at this high wood loading has been fully prevented owing to the strong wood–plastic interaction by the coupling role of the ionomer. The matrix stiffness, which is higher for the zinc ionomer/HDPE blend than the sodium ionomer/HDPE blend, governs the MOR as shown in Fig. 2b. Because of the sufficient interfacial bonding and tough nature of the ionomers, higher MOR is achieved with the more rigid matrix. Fig. 12 shows the typical morphology of the Izod specimens after fracture. It is evident that there are no longer fibrils showing extensive cold drawing as seen on the flexural fracture surfaces. Plastic deformation of matrix is still evident, seen as the thin ligament of sub-micron thickness at the ionomer contents of 2% (Fig. 12a and b) and 16% (Fig. 12c and d) and rough surfaces outside of the wood particles for the composites with neat ionomers as matrix
Fig. 11. SEM pictures of flexural fracture surfaces (a)–(d) near the edge in tension and (e)–(f) near the edge in compression. Ionomer type and content are indicated beneath each figure.
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Fig. 11 (continued )
(Fig. 12e and f). The difference between the composites with ionomer/HDPE blend matrices and those with only ionomer matrices confirms the immiscible nature between the ionomers and HDPE observed in the previous section. There is indication as shown in Fig. 12e and f that the zinc ionomer matrix and its interface with wood fail in more brittle way than its sodium counterpart. However, the difference between the two systems at lower ionomer contents is not discernable with the SEM pictures. Further investigation is required to check how the composite impact strength depends on the ionomer impact toughness and the miscibility with HDPE.
5. Conclusions The wood composites with HDPE/ionomer blends as matrices have been studied for the fixed wood loading of 60% by weight. It has been shown that a wide spectrum of mechanical properties can be achieved with the iono-
mers. All studied ionomers improve the static strain capacity in terms of strain energy at break. The two sodium ionomers result in a decrease in MOE but an increase in both strain at break and Izod impact strength. The more rigid zinc ionomers at low contents are less effective in toughening the HDPE/wood composites but are proved to be useful in achieving significantly higher MOR. The viscoelasticity and structure of the wood/ionomer/ HDPE composites have been characterized using the creep, DSC and DMA experiments. It was shown that the ionomers modified the wood–polymer interface and formed immiscible matrix morphology. A comparison between the matrix blend morphology and the creep test results indicates that the immiscible nature between HDPE and the ionomers can be beneficial. The immiscible nature may allow the ionomer to connect the wood particles at the contents lower than what is necessary to fill in all spaces and hence play its role in a more cost-effective way than in the cases where the ionomer alone is used as matrix.
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Fig. 12. Typical SEM views of fracture surfaces of composites with NaR or ZnL ionomer. Ionomer type and content are indicated beneath each figure.
Though the importance of the ionomer structure has also been identified, further studies are necessary to correlate the coupling role with the ionomer structural details, such as molecular weigh, ionic content and neutralization level,
for a better understanding of the effect of the ionomers. On the other hand, the phase separation between polyethylene zinc domains is identified for the ZnL zinc ionomer in the wood composites with the creep, DSC and rheology
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characterization. The implication of the lower miscibility remains to be a subject for future study in order to fully utilize the capacity of zinc ionomer for high MOR values. Acknowledgements The authors acknowledge Dupont Chemical, Inc. for supplying the Surlyn ionomer resins and Equistar for the HDPE polymer. Financial support from NSERC and CFI/OIT are also acknowledged. References [1] Geng Y, Li K, Simonsen J. Effects of a new compatibilizer system on the flexural properties of wood–polyethylene composites. J Appl Polym Sci 2004;91:3667–72. [2] Li TQ, Ng CN, Li RKY. Impact behavior of sawdust/recycled polypropylene composites. J Appl Polym Sci 2001;81:1420–8. [3] Lai SM, Yeh FC, Wang Y, Chan HC, Shen HF. Comparative study of maleated polyolefins as compatibilizers for polyethylene/wood flour composites. J Appl Polym Sci 2003;87:487–96. [4] Nitz H, Semke H, Landers R, Mulhaupt R. Reactive extrusion of polycaprolactone compounds containing wood flour and lignin. J Appl Polym Sci 2001;81:1972–84. [5] Chen MJ, Meister JJ, Gunnells DW, Gardner DJ. A process for coupling wood to thermoplastic using graft copolymers. Adv Polym Technol 1995;14(2):97–109. [6] Wolcott MP. Formulation and process development of flat-pressed wood–polyethylene composites. Forest Products J 2003;53(9):25–32. [7] Cantero G, Arbelaiz A, Mugika F, Valea A, Mondragon I. Mechanical behavior of wood/polypropylene composites: effects of fiber treatments and ageing processes. J Reinf Plast Compos 2003;22: 37–50. [8] Stark N. Influence of moisture absorption on mechanical properties of wood flour-polypropylene composites. J Thermoplast Compos Mater 2001;14(5):421–32. [9] Bledzki AK, Reihmane S, Gassan J. Thermoplastics reinforced with wood fillers: a literature review. Polym-Plast Technol Eng 1998;37: 451–68. [10] Oksman K, Clemons C. Mechanical properties and morphology of impact modified polypropylene–wood flour composites. J Appl Polym Sci 1998;67:1503–13. [11] Park BD, Balatinecz JJ. Effects of impact modification on the mechanical properties of wood–fiber thermoplastic composites with high impact polypropylene (HIPP). J Thermoplast Compos Mater 1996;9:342–64. [12] Dingova E, Djiporovic M, Miljkovic J. Effects of EPDM modification on some properties of polypropylene wood flour composite. Mater Sci Forum 1998;282–283:303–8. [13] Park BD, Balatinecz JJ. Mechanical properties of wood–fiber/ toughened isotactic polypropylene composites. Polym Compos 1997;18:79–89.
[14] Balasuriya PW, Ye L, Mai YW. Mechanical properties of wood flake-polyethylene composites. Part I: Effects of processing methods and matrix melt flow behavior. Composites: Part A 2001;32A: 619–29. [15] Li TQ, Li RKY, Zeng HM. Fracture toughness and interfacial structure of PP/Wood composites. In: Hong CS, Kim CG, editors. Composites technologies for the new millennium (Proceeding of the ACCM 2000), vol. 1. 18–20 August, 2000, Kyongju, Korea, August, 2000. p. 611–6. [16] Akimoto H, Kanazawa T, Yamada M, Matsuda S, Shonaike GO, Murakami A. Impact fracture behavior of ethylene ionomer and structural change after stretching. J Appl Polym Sci 2001;81:1712–20. [17] Sadiku ER, Sanderson RD. Variables that determine the fiber–matrix bond strength in ethylene-type ionomer composites. Macromol Mater Eng 2001;286:472–9. [18] Matsuda S, Hojo M, Ochiai S, Murakami A, Akimoto H, Ando M. Effect of ionomer thickness on mode I interlaminar fracture toughness for ionomer toughened CFRP. Composites: Part A 1999;30A: 1311–9. [19] Chia LHL, Teoh SH, Boey FYC. Creep-characteristics of a tropical wood–polymer composite. Radiat Phys Chem 1987;29(1):25–30. [20] Sain MM, Balatinecz J, Law S. Creep fatigue in engineered wood fiber and plastic compositions. J Appl Polym Sci 2000;77:260–8. [21] Xu B, Simmonsen J, Rochefort WE. Mechanical properties and creep resistance in polystyrene/polyethylene blends. J Appl Polym Sci 2000;76:1100–8. [22] Brandt CW, Fridley KJ. Load-duration behavior of wood–plastic composites. J Mater Civil Eng 2003;15(6):524–36. [23] Rangaraj SV, Smith L. The nonlinearly viscoelastic response of a wood–thermoplastic composite. Mech Time-Depend Mater 1999;3: 125–39. [24] Ward IM, Hardley DW. An introduction to the mechanical properties of solid polymers. Chichester: Wiley; 1993, p. 195. [25] Findley WN. 26-year creep and recovery of poly(vinyl chloride) and polyethylene. Polym Eng Sci 1987;27(8):582–5. [26] Tomlins PE. Comparison of different functions for modeling the creep and physical ageing effects in plastics. Polymer 1996;37:3907–13. [27] Li TQ, Wolcott MP. Melt rheology of HDPE/woodfiber composites with matrix of different molecular weight. In: Proceedings of the progress in woodfibre-plastic conference 2004, Toronto, May 2004. [28] Utracki LA. Polymer Alloys and Blends. Munich: Hanser; 1989. [29] Han CD. Multiphase flow in polymer processing. New York: Academic Press; 1981. [30] Kuwabara K, Horii F. Solid-state NMR analyses of the crystalline– noncrystalline structure and its thermal changes for ethylene ionomers. J Polym Sci Part B: Polym Phys 2002;40:1142–53. [31] Ishioka T. Brillouin and Raman study on the melting process of poly(ethylene-co-methacrylic acid) ionomers. Macromolecules 1996; 28:1298–305. [32] Kutsumizu S, Goto M, Yano S. Electron spin resonance studies on sodium-neutralized ethylene ionomers: microphase-separated structure and thermal behaviors. Macromolecules 2004;37:4821–9. [33] Farrell KV, Grady BP. Effects of temperature on aggregate local structure in a zinc-neutralized carboxylate ionomer. Macromolecules 2000;33:7122–6.
Mechanical properties of wood flour/HDPE/ionomer composites Tieqi Li, Ning Yan
*
Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ont., Canada M5S 3B3 Received 14 March 2005; received in revised form 30 January 2006; accepted 6 February 2006
Abstract The structure and mechanical properties of wood flour composites with HDPE/ionomer blends as matrices were studied at a fixed wood loading of 60% by weight. It was found that toughness and strength properties of the composites can be improved significantly by adding ionomers of different types and contents. The enhancement in the interfacial interaction was observed through short-time creep analysis. The interfacial interaction and the structure of the matrix phase were characterized through the melting behavior using differential scanning calorimetry (DSC) and with small strain oscillatory tests on the melts using a Dynamic Mechanical Analyzer. Both the sodium and zinc ionomers were found to be immiscible with the HDPE in matrix. The immiscible characteristic was correlated with the interfacial load transfer efficiency as revealed by the creep tests. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Polymer–matrix composites (PMCs); A. Thermoplastic resin; A. Wood; B. Mechanical properties; B. Creep; D. Thermal analysis; D. Electron microscopy
1. Introduction The past decade has seen fast and steady growth of wood plastics industry. Among many reasons for the commercial success, the low cost and reinforcing capacity of the wood fillers provide new opportunities to manufacture composite materials. Certain problems, however, are challenging the further application of the wood plastics technology owing to some intrinsic properties of wood such as its hydrophilic nature and relatively poor thermal stability of the ligninocellulose components. Efforts are being made to improve the compatibility between the wood filler and the matrix polyolefin resins [1–5]. Moisture sensitivity and related dimensional stability and aging performance are also topics of intensive research [6–9]. It is especially worth noting that commercial wood plastics products are typically formulated at high wood loading levels, for example, of 40–70% by weight. The low strain limit and impact resistance associated with the high filler *
Corresponding author. Tel.: +1 416 946 8070; fax: +1 416 978 3834. E-mail address: [email protected] (N. Yan).
1359-835X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2006.02.003
content were frequently documented [2,10–15]. Elastomers were shown to be effective in modifying the impact resistance [11–14], but, they typically are too expensive to be practical. The choice of fillers of high aspect ratio [14,15] was promising but would be limited by the increased difficulty in processing. Exploration of new matrix systems for improving both the static and impact resistance of wood plastics will be helpful for the further development of this type of products. Among many candidates other than the resins currently used for wood plastics, ionomers based on copolymers of acids and olefin monomer units are worthy of exploration. These ionomers are amphiphilic and can be tailored to be compatible with both the matrix and wood and hence could also act as coupling agents. Many of the ionomers are known to have extraordinary toughness [16] and were recently found to be able to improve interfacial bonding strength [17] and modify interlaminar toughness [18] in advanced composites. The large strain capacity should be applicable in improving the toughness of wood composites for higher performance. The versatility in structure and properties and the relatively low cost compared to
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engineering resins and other functional polymers present a potential to tailor the wood composites at reasonable cost. In this work, wood composites made from blends of HDPE and poly(ethylene-co-methacrylic acid) ionomers as matrices are studied. Wood flours, the most popular fillers in wood plastics industry, were used as the reinforcement. Off-the-shelf Surlyn ionomers were used because of their commercial availability. The selected ionomer resins were two sodium-neutralized ionomers with different stiffness and two zinc ionomers with various flow properties. Flexural and impact tests were carried out to examine the improvements in toughness and strength properties. The short time, low stress creep experiments were performed to evaluate the interfacial interaction in wood composites with ionomers. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to characterize the phase structure of the ionomer/HDPE matrix. Fracture surfaces of the ionomer composites were studied with a scanning electron microscope for understanding the deformation and failure behavior. The principles for creep and dynamic tests for structure characterization are reviewed briefly in the following section. 2. Theoretical 2.1. Creep in linear regime Creep performance of wood plastics composites has been extensively studied for application purposes [19–23]. Extruded wood plastics composites at high wood loadings have been observed to consist of two different stages of creep deformation depending on load [23]. The creep was considered linear viscoelastic in nature at lower stress levels and contain extra contribution from damages at higher loads. While most of the published creep experiments are on long term performance for practical purposes, they do not differentiate the contribution from the damage caused by microscopic fracture from the plastic flow due to the viscoelastic nature. When a creep test is performed at a stress much lower than the ultimate stress, the creep deformation of a similar wood plastic composite was shown to be mainly of viscoelastic nature and contained no contribution from damage. Under such conditions, the time dependent compliance can be expressed in a power-law form as [24] DðtÞ ¼ D0 þ D1 tn
ð1Þ
where the constants D0, D1, and n are material parameters typically constant for both the creep and recovery processes. In general, the rate-dependence nature of polymeric materials should result in different compliance and exponent parameters [24]. The time-dependent strain is hence written as e ¼ ðD0 þ Dc tnc Þr0 ; nr
0 < t 6 t0 nr
e ¼ Dr ðt ðt t0 Þ Þr0 ;
t0 < t
ð2Þ ð3Þ
where t0 is the time when the creep begins to recover and subscripts ‘c’ and ‘r’ denote creep and recovery respectively. The power-law relationship in the same form as Eq. (2) was shown by Findley [25] to also govern the very long term creep of polyethylene and poly(vinyl chloride). It was hence referred to as Findley equation in most other creep studies on wood plastics composites [19–22], where the stress is often above half of the expected ultimate stress. The creep under low stress and short-time intervals and the subsequent recovery are of viscoelastic nature and free from the effects of damage in wood plastics with high wood loadings [23]. Despite of the approximating nature of the power-law relations, the parameters Dc, Dr, nc and nr from the low stress, short-time span creep study can reveal the viscoelastic behavior and provide information on the wood–polymer interfacial interaction. The parameter Dc is elastic in nature and correlates inversely with the modulus. Dc, Dr, nc and nr describe the time-dependent deformation. The exponents nc and nr relate to the spread of the retardation spectra while Dc and Dr contain the contribution from both the characteristic time and the spread of the spectra [26]. A decreased nc or nr value would usually indicate broader spread of the spectrum and could reveal different contribution from the wood reinforcement. As the data sampled on the short-time frame usually are not likely sufficient to provide detailed relaxation spectra, the power-law analysis still can be used as a simple way to compare the viscoelastic properties of the composites. 2.2. Small-amplitude oscillatory experiments Small amplitude, dynamic mechanical experiments are among the most frequently used methods for characterizing the structure of polymeric materials. When the deformation is within the range of the linear viscoelasticity, the responses to the applied oscillatory strain or stress provide rheology characterization of the studied materials. For example, the isothermal frequency spectrum obtained in the linear regime can be used to predict deformation of all other type within the linear viscoelasticity limit through various transformations. Difficulties are encountered, however, in applying the dynamic experiments to wood plastic composites. Similar to many other suspensions with high filler contents, the filled wood plastic composites typically are non-linear in conditions commonly used for neat polymer melts [27]. The non-linear feature prevents the direct application of experimental results based on more conventional methods such as the frequency sweep tests owing to the dependence of the measured response on time and strain history. Despite the problems associated with the highly non-linear responses, the dynamic tests are still informative for exploring the structure of the wood plastic composites. As revealed through the stress relaxation experiments of HDPE/maple composites [27], the strain dependence of the relaxation modulus approaches its linear viscoelasticity limit at below 0.1% strain for the HDPE/maple composites
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The composites containing 60% by weight of wood fillers and the ionomer of either 0%, 2%, 4%, 8%, 12%, 16%, 24%, 32%, or 40% based on the total weight of the composites were compounded with a Brabender mixer with roller rotors and then compression molded into plates. The neat resin mixtures were first introduced into the mixer soaked at 180 C for melting. The wood flours were then added into the melted resin mixture with the rotor speed varying between 5 and 20 rpm. The composition was subsequently mixed for 3 min at 35 rpm and 3–5 min at 20 rpm after the ampere meter gets stabilized. The compound was then compression-molded into plaques of 4 mm thickness at 180 C with a maximum nominal pressure of 7.5 MPa using a non-matched mold.
are studied for Modulus of Rupture (MOR), strain at break (eb) and the strain energy at break (Emax). Notched Izod impact experiments were carried out following the ASTM D256 standard. The room temperature short-time creep properties were studied by a TA Q800 Dynamic Mechanical Analyzer using a three-point bending fixture of 50 mm span. The sample of 12.7 mm width was brought to the fixed stress level of 5 MPa in shorter than 6 s, let deformed at the stress level for 10 min, and then allowed to recover for 20 min. Regression based on Eqs. (2) and (3) were performed based on the Levenberg–Marquardt algorithm using a Macrocal Origin software and double checked with a Wolfram Research Mathematica package. To avoid the sensitivity of regression based on Eq. (2) to initial values, the parameters were first obtained via fitting to Eq. (3). Value of nr was then used as the initial value of nc for fitting for D0 and D1 through Eq. (2). Differential scanning calorimetry studies were performed using a TA1000 instrument on the slices of ca. 0.2 mm thickness cut from the compression-molded plates perpendicular to the molded surfaces. The sample was brought from room temperature to 200 C and then cooled to 70 C at a 5 C/min rate and then heated to 200 C at 10 C/min to obtain information about melting. Strain sweep tests at 1 Hz were also carried out for rheology properties using the TA Q800 instrument with the shear-sandwich fixture under the multi-strain mode. Samples with 10 mm · 10 mm area were load into the sandwich fixtures and let soak sufficiently at 180 C. A strain ramp was then performed from 1 to 100 lm at the 5 point/decade interval. The fracture surfaces of both ASTM D790 and D256 specimens were gold-coated and examined with a Hitachi S520 SEM using the acceleration voltage of 20 kV.
3.3. Mechanical testing and structure characterization
4. Results and discussion
Flexural tests were performed on the samples of 12.7 mm in width and the as-molded thickness of around 4 mm conforming to the ASTM D790 standard using 6 defect-free specimens from the same plate for each formulation. The modulus of elasticity (MOE) was determined on the strain range between 0.1% and 0.5%. Only the results of specimens that break below or close to 5% strain
4.1. Flexural and impact properties
at 40–60% wood contents. The dynamic moduli at the lowest available strain amplitude hence provide a useful approximation to the linear viscoelastic properties. The response is evaluated at the fixed level of stress amplitude for preventing the possible effects of interfacial slip on matrix strain field [28,29]. 3. Experimental 3.1. Materials Wood fillers used in this work are grade 14010 maple flour from American Wood Fiber. The HDPE and ionomer resins are listed in Table 1. The properties were provided by the suppliers unless stated otherwise. The HDPE resin and the wood flour were used as received. The ionomer resins were used immediately after being taken out of the package without drying. 3.2. Compounding and molding
Fig. 1 reports the typical stress–strain curves of the maple flour/HDPE/ionomer composites in the flexural tests as compared to the neat HDPE/maple used. As shown in the figure, the composites containing 4% of the sodium ionomers deform and break in the way similar to the
Table 1 Polymers used in the study Legend
HDPE NaR NaS ZnL ZnH a
Melt index (g/10 min) ASTMD-1238 0.3 0.9 1.0 1.0 4.5
Density (kg/m3)
Melting point (C)
Vicat softening point (C)
D-792
Flexural modulus (MPa) ASTMD-790
946 940 950 960 970
940a 49 30 178 358
– 78 70 86 82
– 51 47 60 57
Measured in this laboratory on compression-molded specimens of 4 mm thickness.
Elongation at break ASTM D638 660 555 510 335
Supplier and resin grade Equistar LB01000 Dupont Surlyn 8120 Dupont Surlyn 8350 Dupont Surlyn 9120 Dupont Surlyn 9150
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Fig. 1. Typical stress–strain curve of the composites with 4% ionomer compared to neat HDPE/maple compound.
straight blend of HDPE and maple. It was found the composites with 2–4% of ionomers show whitening and multiple cracks starting near the surface in tension when the maximum load is reached. The subsequent decrease of load corresponds to the increase in the number of such damages and the quasi-stable propagation and merging of the damages that form the main crack. With the increase in ionomer content, more compression-related failure is observed. With 12% of NaR or 8% of NaS ionomer, the fracture is mainly along the shear plane. Composites with more sodium ionomers do not break below 5% of the flexural strain. The zinc ionomers, however, change the load– deflection curve significantly even at the low content of 2%. The composites with the zinc ionomers generally show higher stiffness and breaks along the shear plane, causing the drastic drop of the load as exemplified in the figure. The maximum load point is referred to as break hereafter for all composites. The corresponding stress, strain, and strain energy are reported as MOR, eb, and Emax. The difference in fracture mechanism will be further shown with SEM fractographs later in this paper. Fig. 2 illustrates the MOE and MOR of the composites with different ionomer contents. From Fig. 2a, it can be seen that the ionomers improve MOE at low ionomer contents. With the further increase in the ionomer content, there is a general trend for MOE to decrease with increasing ionomer content for all the ionomers studied. When ionomer content is higher than 24% in the matrix, the composites with the NaR ionomer, which has higher modulus, show higher MOE than the composites with the NaS polymer. MOE of the composites with the zinc ionomers is also higher than that of the sodium counterparts. There seems to be a general trend of increase in composite MOE with the modulus of the ionomer component, agreeing with the rule of mixture at these higher ionomer contents. However, such a dependency on component modulii is not followed when the composites with the two zinc ionomers are compared. The softer ZnL ionomer results in composites with higher MOE. Structure details such as the molecular weight, ionic monomer content and degree of neutraliza-
Fig. 2. MOE (a) and MOR (b) of the wood plastics composites as a function of ionomer content. The arrows indicate the contents above which the materials are too ductile for the MOR to be evaluated with the method.
tion of the ionomers remain to be studied for future application of the ionomer techniques. In contrast to the case of MOE, MOR of the composites shows a simpler dependence on the moduli of the ionomers. As shown in Fig. 2b, the zinc composites have higher MOR values than the sodium composites. The ZnL polymer, which has the highest modulus among the ionomers studied, results in the highest MOR for the composites while the softest ionomer, NaS, gives the lowest MOR value for the composites. The higher toughness of the ZnL ionomer than its ZnH counterpart, seen as the higher tensile strain in Table 1, may be responsible for the higher MOR values observed with the ZnL composites. In contrast to the decrease in MOE with the increase in ionomer content, the MOR values for sodium composites remain unchanged with the changing ionomer content and the two zinc composites show the increases in MOR at higher ionomer contents. While the decrease in MOE can be expected from the addition of the ionomers of the lower moduli, the increase in MOR suggests the toughness of the ionomers has some contributions. Fig. 3 illustrated the eb and Emax of the ionomer composites. As shown in the figure, both break strain and energy of the ionomer composites increase with the ionomer content. As shown in Fig. 3a, lower ionomer content is required to achieve a similar level of eb for composites containing
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Fig. 4. Izod strength of wood plastics as a function of ionomer content. Trend lines are second-order polynomial fit for composites containing ionomers.
Fig. 3. Strain at break (a) and Energy at maximum load (b) as a function of ionomer content (trend lines are second-order polynomial fit).
sodium ionomers, indicating that the sodium ionomers are more effective than the zinc ionomers in improving the toughness. A similar trend is also observed for the energy at break, Emax (Fig. 3b). Comparing the two zinc ionomers, the softer ZnL ionomer seems to be more effective in improving both eb and Emax. The static toughness in terms of the eb and Emax relates more to the deformation capacity of the ionomers than the stiffness as revealed by MOE. The strain capacities of the ionomers are also found to affect the impact properties. Fig. 4 illustrates the notched Izod strength of the composites with different ionomers. As shown in the figure, the composites with the sodium ionomers have much higher toughness than the zinc ionomers especially at the high ionomer contents. Appreciable toughening effects are observed with the softest NaS ionomer at 8–12% of loading, consistent with the value observed for similar HDPE/wood composites containing a maleated styrene–ethylene/butylenes–styrene (SEBS) compatibilizer [3]. In contrast, the toughening effect of the more rigid NaR ionomer is evident above 16%. The effects of zinc ionomers on impact resistance are not discernable. The structure characterization in the following sections will shed some light on the different roles played by the sodium and zinc ionomers.
the creep behavior of the wood composites with and without the ionomers can be modeled with the power-law relationship. However, the parameters for recovery were found generally different from those for creep. Fig. 5 exemplifies the applicability of Eqs. (2) and (3) to the short creep behavior. It was found that the parameters for creep (Dc) are typically an order of magnitude higher than Dr for the recovery stage. The two quantities are shown in Fig. 6 in terms of D1 and D1 in comparison with the r c MOE values. It is evident that D1 is close to MOE but c D1 r is much higher. The samples seemed to be much stiffer in recovery than in creep. The exponents in Eqs. (2) and (3) were also found to be different. The exponent in recovery, nr, were around 0.3, close to the value for creep of unfilled polymers [24]. But the values for creep (nc) were lower than 0.1 and well below the values shown in recovery. Given the moderate agreement between D1 c and MOE, the compliance Dc and nc of composites with varying ionomer type and contents are compared in Fig. 7. From Fig. 7a, it can be seen that Dc of composites containing the two sodium ionomers increases with the ionomer content due to the lower modulus of the ionomers. The increase of Dc for the zinc composites, however, is
4.2. Linear creep properties In agreement with what was observed for neat HDPE/ wood composites [23], the present experiments show that
Fig. 5. Creep and recovery of the HDPE/maple composite with powerlaw fit. The solid line is the best fit to the data in creep and recovery stages according to Eqs. (2) and (3).
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than the ZnL ionomer, suggesting the same mechanism responsible for the broader spread of the relaxation spectra. 4.3. Melting behavior
Fig. 6. Dc and Dr compared with MOE. Line shows value for 1/D0 = MOE.
Fig. 7. Model parameters Dc and nc of the wood plastics with ionomer matrices.
much less significant, confirming the less change in MOE of the composites shown in Fig. 2a. As shown in Fig. 7b, the exponent n of composites with the sodium ionomers tends to be higher than for the neat HDPE/maple composite. The softer sodium ionomer (NaS) appears to cause higher exponent, which would suggest broader relaxation spectra for the composites. The result is reasonable since the wood reinforcement is expected to cause more diversified conformations when the matrix is softer. Similar to the sodium ionomers, the more rigid zinc ionomer (ZnH) also results in lower n values
Fig. 8 compares the melting curves of the composites with the ionomers of different ions. Two melting peaks can be seen for the ionomer composite, one at around 82 C for the ionomer [29–31] and the other one near 130 C for the HDPE resin. In contrast, both the neat sodium and zinc ionomer resins show only the melting peak in the lower temperature range and do not have the polyethylene melting peak at around 130 C. The enthalpy DH and peak temperature Tm are compared with those of the neat ionomers in Fig. 9. From Fig. 8, it can be seen that the HDPE melting peak decreases in area with the increase in the ionomer content. As shown in Fig. 9a, the enthalpy of the polyethylene melting peak tends to decrease with the increasing ionomer content as expected for both the sodium and zinc ionomers. The slight negative deviation of the dependence of DHm on the sodium ionomer content from the linear additive prediction suggests that the ionomer phase has a limited effect on the crystallization or melting of polyethylene, indicating certain level of miscibility in amorphous phase between the HDPE and the sodium ionomer. In comparison, the zinc ionomer composites also show the decrease in DHm but the enthalpy stays closer to the linear prediction than for the sodium ionomer composites. At higher ionomer contents, the polyethylene melting peak does not disappear as for the sodium composites but remains at an appreciable level that is similar to that of the composites even up to 80% of ionomer in matrix. There seems to be less interruption from the zinc ionomer phase in the composites than the case with the sodium ionomer. The inhomogeneous structure can always be expected for both zinc- and sodium-neutralized E-MAA copolymers according to recent studies by Kuzumizu et al. [32] and Farrell and Grady [33]. However, the ionomer resins themselves do not show the polyethylene melting endotherm in un-filled form. The appearance of the polyethylene endotherm for the ZnL ionomer in the wood composites suggests the ionomer experiences further phase separation that is significant enough for the ethylene segments to enter the polyethylene crystalline domains. The zinc ionomer is hence proved to be less stable in structure than its sodium counterpart in wood composites. The different miscibility with HDPE is further revealed through the melting temperature, Tm. As shown in Fig. 9b, Tm of the HDPE melting peak tends to decrease with ionomer content for both the sodium and zinc ionomers. For the ionomer melting endotherms, however, Tm shows a different dependence on the ionomer content. With the sodium ion, the ionomer melting peak shifted to a higher temperature with the decrease in the ionomer content while the zinc endotherm does the opposite. An increase in the melting temperature of the sodium endo-
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Fig. 8. DSC melting curves of composites with NaS (a) and ZnL (b) ionomers compared with neat ionomer resins.
Fig. 9. Melting enthalpy (a) and temperatures (b) composites with NaR and ZnL as function of ionomer content.
therm may be a result of the increase in the sequence length of the crystalline chain segments in the ionomer phase or co-crystallization with the HDPE, which melts at a higher temperature. Both possibilities mean good compatibility between the sodium and HDPE in solid phase, agreeing with the miscibility observed through the negative deviation of HDPE melting enthalpy in Fig. 9a. For composites with the zinc ionomer, the independence of the zinc Tm on the ionomer content at the higher ionomer contents reveals the immiscible nature between the zinc ionomer and HDPE. The decreasing Tm at the lower zinc ionomer contents, on the other hand, indicates more imperfect crystals resulting from stronger ion-ion association as the consequence of the phase separation of the zinc ionomer component. The zinc ionomer proves to be less miscible with HDPE and the different miscibility should account for
the different mechanical responses between the two different types of ionomers. The immiscible nature explains why the exponent n from the creep tests tends to saturate beyond the 16% ionomer content. With the increase of ionomer content, the ionomer gradually becomes the continuous phase that governs the load transfer among the neighboring wood particles. The ionomer content of 40% based on matrix weight seems to be reasonable for the ionomer continuous phase to form, especially in the case when the space between the wood particles in these composites is small. 4.4. Rheology properties of the composite melts The difference between the matrix morphology of composites was further examined through the rheology
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Fig. 10. Complex modulus (a) and tan d (b) of the composites with NaR (red) and ZnL (black) ionomers at 180 C at 10 kPa stress amplitude. The lines are linear connection between the responses of neat HDPE and ionomer as matrix detailed in text. (For interpretation of the references in color in this figure legend, the reader is referred to the web version of this article.)
observations on the rigid ionomer of sodium ions (NaR) and the ionomer with a similar melt index but contains zinc ions (ZnL). Fig. 10 compares the complex viscosity and tan d of the ionomer composites at 10kPa. As shown in the Fig. 10a, the dependence of the rheology properties on the ionomer content is different between the two ionomers. Though both ionomers cause negative deviation (NDV) from the rule of mixture [28], the melt with the zinc ionomer shows two local minimums and the sodium system does not show any absolute minimum. The different NDV behaviors can be interpreted as the difference in matrix morphology if the effects of the wood particles are assumed to be the same. For the sodium composites, the negative deviation of g* occurring at a low ionomer content would suggest immiscibility between the sodium and HDPE melts when the ionomer is the minor component in the matrix. The decreasing level of NDV behavior with the increase in the sodium ionomer content may result from the similarity between the ethylene segments in ionomer and HDPE, which makes the ionomer phase more miscible with the HDPE. On the other hand, two local minimums shown by the melts of zinc composites support the immiscibility between the zinc ionomer and HDPE, agreeing with the DSC observation. Combined with the DSC observation shown in Figs. 8 and 9, the result on melting temperatures reveal more phase-separation for the matrix in zinc composites than for composites with the sodium ionomers. From Fig. 10b, positive deviation (PDV) of tan d from the linear additive rule is clearly shown and the tan d values of the composites with the ionomers are much closer to the
level of the composites with pure ionomer matrix than to the value of neat HDPE/maple composites. The viscous damping of the composites is affected more by the interface than the modulus. It is reasonable to assume that the significant difference in tan d between the composites with and without the ionomers originates from the interfacial dissipation. The tan d values at 2% of NaH and 4% of ZnL may hence be taken as the reference value for the HDPE matrix with strong restriction at boundary by the wood reinforcement. In such, the ionomer content dependence would again show the NDV nature, confirming the immiscible characteristic of the HDPE/NaH blends in the matrices of the wood composites. The ZnL composites, in contrast show NDV characteristic at the low ionomer contents and PDV behavior when the zinc ionomer becomes the continuous phase. The mixed NDV-PDV behavior is consistent with the variation in DSC enthalpy shown in Fig. 9a. The entry of ethylene segments from the ionomer into polyethylene crystalline domains causes higher melting enthalpy than the prediction based on added HDPE content and the rule of mixture. The higher content of the more ordered ethylene phase results in a lower tan d value. 4.5. Fracture mechanism Fig. 11 illustrates the fracture surfaces of the sodium and zinc composites near the tensile and compressive edges, respectively. Near the tensile edge, there are considerable differences between the two ionomers at the low ionomer
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content of 2%. Both composites show remainder of cold drawing during the flexural loading but the process seems to be more intensive for composite with ZnL (Fig. 11a and b). Near the compressive edge, the composite with the 2% NaR ionomer experiences (Fig. 11c) a much less extent of drawing than near the edge in tension (Fig. 11c). Its ZnL counterpart, however, shows the characteristic of large scale planar plastic flow (Fig. 11d). The difference in the cold drawing on the tensile edge is associated with the immiscible nature between the ionomer and HDPE because the ionomer content in this case is not high enough to allow such drastic changes if they form a single phase with HDPE. The higher rigidity of the zinc ionomer is hence amplified owing either to its enrichment at the wood–plastic interface or the phase-separated structure in matrix. In Fig. 11e–f, the fibrils from cold drawing is still visible at the ionomer content of 16%. However, the difference between the sodium and zinc composites is not discernable for the tensile fracture surface. The fracture surfaces are
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more flat than at the lower ionomer content of 2% for both the NaR and ZnL composites. It is evident that shear failure dominates at the higher ionomer content. The premature tensile failure typically found for straight blend of HDPE and wood at this high wood loading has been fully prevented owing to the strong wood–plastic interaction by the coupling role of the ionomer. The matrix stiffness, which is higher for the zinc ionomer/HDPE blend than the sodium ionomer/HDPE blend, governs the MOR as shown in Fig. 2b. Because of the sufficient interfacial bonding and tough nature of the ionomers, higher MOR is achieved with the more rigid matrix. Fig. 12 shows the typical morphology of the Izod specimens after fracture. It is evident that there are no longer fibrils showing extensive cold drawing as seen on the flexural fracture surfaces. Plastic deformation of matrix is still evident, seen as the thin ligament of sub-micron thickness at the ionomer contents of 2% (Fig. 12a and b) and 16% (Fig. 12c and d) and rough surfaces outside of the wood particles for the composites with neat ionomers as matrix
Fig. 11. SEM pictures of flexural fracture surfaces (a)–(d) near the edge in tension and (e)–(f) near the edge in compression. Ionomer type and content are indicated beneath each figure.
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Fig. 11 (continued )
(Fig. 12e and f). The difference between the composites with ionomer/HDPE blend matrices and those with only ionomer matrices confirms the immiscible nature between the ionomers and HDPE observed in the previous section. There is indication as shown in Fig. 12e and f that the zinc ionomer matrix and its interface with wood fail in more brittle way than its sodium counterpart. However, the difference between the two systems at lower ionomer contents is not discernable with the SEM pictures. Further investigation is required to check how the composite impact strength depends on the ionomer impact toughness and the miscibility with HDPE.
5. Conclusions The wood composites with HDPE/ionomer blends as matrices have been studied for the fixed wood loading of 60% by weight. It has been shown that a wide spectrum of mechanical properties can be achieved with the iono-
mers. All studied ionomers improve the static strain capacity in terms of strain energy at break. The two sodium ionomers result in a decrease in MOE but an increase in both strain at break and Izod impact strength. The more rigid zinc ionomers at low contents are less effective in toughening the HDPE/wood composites but are proved to be useful in achieving significantly higher MOR. The viscoelasticity and structure of the wood/ionomer/ HDPE composites have been characterized using the creep, DSC and DMA experiments. It was shown that the ionomers modified the wood–polymer interface and formed immiscible matrix morphology. A comparison between the matrix blend morphology and the creep test results indicates that the immiscible nature between HDPE and the ionomers can be beneficial. The immiscible nature may allow the ionomer to connect the wood particles at the contents lower than what is necessary to fill in all spaces and hence play its role in a more cost-effective way than in the cases where the ionomer alone is used as matrix.
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Fig. 12. Typical SEM views of fracture surfaces of composites with NaR or ZnL ionomer. Ionomer type and content are indicated beneath each figure.
Though the importance of the ionomer structure has also been identified, further studies are necessary to correlate the coupling role with the ionomer structural details, such as molecular weigh, ionic content and neutralization level,
for a better understanding of the effect of the ionomers. On the other hand, the phase separation between polyethylene zinc domains is identified for the ZnL zinc ionomer in the wood composites with the creep, DSC and rheology
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characterization. The implication of the lower miscibility remains to be a subject for future study in order to fully utilize the capacity of zinc ionomer for high MOR values. Acknowledgements The authors acknowledge Dupont Chemical, Inc. for supplying the Surlyn ionomer resins and Equistar for the HDPE polymer. Financial support from NSERC and CFI/OIT are also acknowledged. References [1] Geng Y, Li K, Simonsen J. Effects of a new compatibilizer system on the flexural properties of wood–polyethylene composites. J Appl Polym Sci 2004;91:3667–72. [2] Li TQ, Ng CN, Li RKY. Impact behavior of sawdust/recycled polypropylene composites. J Appl Polym Sci 2001;81:1420–8. [3] Lai SM, Yeh FC, Wang Y, Chan HC, Shen HF. Comparative study of maleated polyolefins as compatibilizers for polyethylene/wood flour composites. J Appl Polym Sci 2003;87:487–96. [4] Nitz H, Semke H, Landers R, Mulhaupt R. Reactive extrusion of polycaprolactone compounds containing wood flour and lignin. J Appl Polym Sci 2001;81:1972–84. [5] Chen MJ, Meister JJ, Gunnells DW, Gardner DJ. A process for coupling wood to thermoplastic using graft copolymers. Adv Polym Technol 1995;14(2):97–109. [6] Wolcott MP. Formulation and process development of flat-pressed wood–polyethylene composites. Forest Products J 2003;53(9):25–32. [7] Cantero G, Arbelaiz A, Mugika F, Valea A, Mondragon I. Mechanical behavior of wood/polypropylene composites: effects of fiber treatments and ageing processes. J Reinf Plast Compos 2003;22: 37–50. [8] Stark N. Influence of moisture absorption on mechanical properties of wood flour-polypropylene composites. J Thermoplast Compos Mater 2001;14(5):421–32. [9] Bledzki AK, Reihmane S, Gassan J. Thermoplastics reinforced with wood fillers: a literature review. Polym-Plast Technol Eng 1998;37: 451–68. [10] Oksman K, Clemons C. Mechanical properties and morphology of impact modified polypropylene–wood flour composites. J Appl Polym Sci 1998;67:1503–13. [11] Park BD, Balatinecz JJ. Effects of impact modification on the mechanical properties of wood–fiber thermoplastic composites with high impact polypropylene (HIPP). J Thermoplast Compos Mater 1996;9:342–64. [12] Dingova E, Djiporovic M, Miljkovic J. Effects of EPDM modification on some properties of polypropylene wood flour composite. Mater Sci Forum 1998;282–283:303–8. [13] Park BD, Balatinecz JJ. Mechanical properties of wood–fiber/ toughened isotactic polypropylene composites. Polym Compos 1997;18:79–89.
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