Effects of fiber size on short‐term creep behavior of wood fiber/HDPE composites

When natural fiber‐thermoplastic composites are used in long‐term loading applications, investigating creep behavior is essential. The creep behavior of high‐density polyethylene (HDPE)‐based composites reinforced with four sizes of wood fibers (WFs) (120–80, 80–40, 40–20, and 20–10 mesh) was investigated. The instantaneous deformation and creep strain of all WF/HDPE composites increased at a fixed loading level when the temperature was increased incrementally from 25 to 85°C. At a constant loading level, composites containing the larger‐sized WFs had better creep resistance than those containing smaller‐sized fibers at all measured temperatures. The creep properties of composites with smaller‐sized WFs were more temperature‐dependent than those with larger‐sized WFs. Two creep models (Burger's model and Findley's power law model) were used to fit the measured creep data. A time–temperature superposition principle calculation was attempted for long‐term creep prediction. The Findley model fitted the composite creep curves better than the four‐element Burger's model. From the predicted creep response of the WF/HDPE composites, two groups of small fibers (120–80 and 80–40 mesh) had the lowest creep resistance over long periods of time at the reference temperature of 25°C. The largest WFs (10–20 mesh) provided the best composite creep resistance. POLYM. ENG. SCI., 55:693–700, 2015. © 2014 Society of Plastics Engineers     

Creep analysis of bamboo high‐density polyethylene composites: Effect of interfacial treatment and fiber loading level

The effect of chemical treatment at fiber–plastic interface and fiber loading level on creep property of bamboo fiber high‐density polyethylene (BF/HDPE) composites was investigated. For single modifier systems, the use of maleic anhydride grafted polyethylene (PE‐g‐MA) as a coupling agent helped reduce the creep and achieved the optimum effect at the 5.7% loading level. The addition of either a semicrystalline or an amorphous MA grafted ethylene propylene rubber (sEPR‐g‐MA or aEPR‐g‐MA) as an impact modifier increased the creep. For the combined modifiers, the use of PE‐g‐MA in EPR‐g‐MA modified composites gradually improved creep performance. Four‐element Burgers model was shown to fit measured creep data well only within the specified test period. However, both partially stretched Burgers (PSB) model and fully stretched Burgers (FSB) model could be applied for characterization and prediction when the stretching exponent was fixed at certain given values. The FSB model offered a better long‐term prediction based on the short‐term creep data. Time‐temperature superposition technique produced smooth master creep curves through horizontal shifts, but it slightly over‐predicted the long‐term creep for most composite systems. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Creep resistance of wood‐filled polystyrene/high‐density polyethylene blends

Creep, the deformation over time of a material under stress, is one characteristic of wood‐filled polymer composites that has resulted in poor performance in certain applications. This project was undertaken to investigate the advantages of blending a plastic of lower‐creep polystyrene (PS) with high‐density polyethylene (HDPE) at ratios of 100:0, 75:25, 50:50, 25:75, and 0:100. These various PS–HDPE blends were then melt blended with a short fiber‐length wood flour (WF). Extruded bars of each blend were examined to measure modulus of elasticity and ultimate stress. Increasing the ratio of WF increased modulus of elasticity in all composites, except between 30 and 40% WF, whereas the effect of WF on ultimate stress was variable, depending on the composite. Scanning electron microscopic images and thermal analysis indicated that the wood particles interacted with the PS phase, although the interactions were weak. Finally, creep speed was calculated by using a three‐point bending geometry with a load of 50% of the ultimate stress. Creep decreased only slightly with increasing WF content but more significantly with increasing PS content, except at pure PS. The WF/75PS–25HDPE blend showed the least creep. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 418–425, 2001     

Tensile creep of a long‐fibre glass mat thermoplastic (GMT) composite. II. Viscoelastic‐viscoplastic constitutive modeling

In Part I of this article, the short‐term tensile creep of a 3‐mm‐thick continuous long‐fibre glass mat thermoplastic composite was characterized and found to be linear viscoelastic up to 20 MPa. Subsequently, a nonlinear viscoelastic model has been developed for stresses up to 60 MPa for relatively short creep durations. The creep response was also compared with the same composite material having twice the thickness for a lower stress range. Here in Part II, the work has been extended to characterize and model longer term creep and recovery in the 3‐mm composite for stresses up to near failure. Long‐term creep tests consisting of 1‐day loading followed by recovery were carried out in the nonlinear viscoelastic stress range of the material, i.e., 20–80 MPa in increments of 10 MPa. The material exhibited tertiary creep at 80 MPa and hence data up‐to 70 MPa has been used for model development. It was found that viscoplastic strains of about 10% of the instantaneous strains were developed under load. Hence, a non‐linear viscoelastic–viscoplastic constitutive model has been developed to represent the considerable plastic strains for the long‐term tests. Findley's model which is the reduced form of the Schapery non‐linear viscoelastic model was found to be sufficient to model the viscoelastic behavior. The viscoplastic strains were modeled using the Zapas and Crissman viscoplastic model. A parameter estimation method which isolates the viscoelastic component from the viscoplastic part of the nonlinear model has been developed. The model predictions were found to be in good agreement with the average experimental curves. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Critical stress of high‐density polyethylene during stress and photo‐oxidative aging

The long‐term photo‐oxidative aging behavior of high‐density polyethylene (HDPE) under different tensile stress was studied using a stress‐aging apparatus. The aging behavior was investigated through the methods of the surface morphology observation, gel content measurement, Fourier transform infrared spectroscopy, and creep behavior. It was found that stress has influence on the development of cracks and stress induces cracking through creep deformation. With increasing stress, the cracking time decreases in a reversed S‐shape curve way, and there is a critical stress near 7 MPa where the cracking time has a maximum decreasing rate. Meanwhile, the creep deformation increases rapidly when the stress exceeds the critical stress. The critical stress of HDPE is about 20–25% of breaking strength, and HDPE with low comonomer content has good dimensional stability when the stress is less than the critical stress, while HDPE with high comonomer content has a good performance when the stress exceeds the critical stress. This study may be useful for the rational selection of HDPE for the sheath material of bridge cable. POLYM. ENG. SCI., 55:2277–2284, 2015. © 2015 Society of Plastics Engineers     

Characterization of rheological and thermophysical properties of HDPE–wood composite

The objective of this study is to develop a new biocomposite material with high deformation ability. In this regard, the thermal, rheological, and thermophysical properties of this new composite were characterized as a function of temperature and filler concentration. High density polyethylene (HDPE) was the matrix of this new composite which was reinforced with six sawdust concentrations 0%, 20%, 30%, 40%, 50%, and 60%. Maleic anhydride grafted polyethylene (PE‐g‐MA) was used as coupling agent. Addition of sawdust with PE‐g‐MA increased significantly the complex viscosity, the storage modulus (G′), and loss modulus (G″) of the matrix. The superposition of the complex viscosity curves using temperature dependent shift factor, allowed the construction of a viscosity master curve covering a wide range of temperatures. Arrhenius law was used for the relationship of the shift factor to temperature. Furthermore, method of Van Gurp and Palmen (tan delta vs. G*) is also used to control the time–temperature superposition. The experimental results can be well fitted with the cross rheological model which allowed the prediction of the thermorheological properties of the composites over a broad frequency range. By increasing wood concentration, both the activation energy and relaxation time for the biocomposites determined using, respectively, the Arrhenius law and the cole–cole rule increased. By contrast, specific heat of the matrix decreased with sawdust addition while its dimensional stability improved. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40495.     

Morphology and viscoelastic properties of melt‐spun HDPE/hydrotalcite nanocomposite fibers

Viscoelastic properties of nanocomposite fibers of high density polyethylene (HDPE) and organically modified hydrotalcite were studied. Neat and nanofilled HDPE fibers (with nanofiller content between 0.5 and 3 wt%) were produced by melt spinning and hot‐drawing at different draw ratios up to 20. Effect of temperature on storage modulus, loss modulus, and creep compliance were compared. Rising nanofiller content and/or drawing ratio accounted for an increase in storage modulus in the glassy (i.e., below the γ transition at −100°C) as well as in the rubbery state of non‐crystalline regions. The α relaxation temperature read‐off for the maximum of the loss modulus peak ranged from 20 to 60°C being dependent on frequency, filler content and draw ratio. Sumita model was successfully applied to evaluate the effective volume fraction of the dispersed phase; maximum fraction of immobilized matrix was observed for the composite with 1 wt% of nanofiller. Creep behavior was evaluated by fitting experimental data with the Burgers model. The addition of a small amount of well‐dispersed hydrotalcite (0.5–1 wt%) had a beneficial effect on the creep resistance of drawn fibers at room temperature as well as at 70°C. TEM analysis evidenced a good dispersion of 0.5% nanofiller in as‐spun fibers and improved interfacial adhesion after drawing. The best mechanical properties were observed for the composition with 1 wt% of hydrotalcite, due to combined effects of nanofiller reinforcement and stiffening produced by hot drawing. POLYM. COMPOS., 288–298, 2016. © 2014 Society of Plastics Engineers     

A constitutive model for non‐linear behavior of SMC accounting for linear viscoelasticity and micro‐damage

An approach for modeling sheet molding compound (SMC) composites as viscoelastic damageable material is presented. Continuum damage mechanics theory by Chow and Wang (Int. J. Fract., 33, 3 (1987)) was used in combination with linear viscoelasticity. The model was applied to a modern SMC composite material containing both hollow glass spheres for low density and toughening additive for improved impact resistance. Tensile tests and uniaxial creep test were employed to build the constitutive model. Validation was done by comparing test data with simulations of uniaxial creep on material with different degrees of damage. The model has good accuracy at moderate damage levels under controlled time‐dependent crack propagation. Tensile testing at two different fixed strain rates was simulated using quasi‐elastic method to calculate relaxation modulus. The model predicts the stress‐strain curve with good accuracy until the region is close to failure, where new mechanisms not accounted for are taking place. Finally, a simulation of a cyclic tensile test with increasing maximum strain per cycle was performed, and since both damage and viscoelasticity are included in the model, the slope change, accumulation of residual strain, and hysteresis in the stress‐strain, loading‐unloading curve are predicted. POLYM. COMPOS., 26:84–97, 2005. © 2004 Society of Plastics Engineers     

Influence of acetylation on the tensile properties,water absorption,and thermal stability of (High‐density polyethylene)/(soya powder)/(kenaf core) composites

Composites comprising high‐density polyethylene (HDPE)/(soya powder)/(kenaf core) were prepared by incorporating different loads of kenaf core powder into HDPE/(soya powder) composites with an internal mixer (Haake PolyLab) for 12 min at 180°C and 50 rpm. The composition of the soya powder used was soluble carbohydrates, 15%; insoluble carbohydrates, 15%; oil, 18%; protein, 38%; and moisture, ash, other, 14%. The effect of kenaf core acetylation on the properties of the HDPE/(soya powder)/(kenaf core) composites was investigated by using a water‐absorption test, a tensile test, thermogravimetric analysis, Fourier‐transform infrared spectroscopy, and field emission scanning electron microscopy (FESEM). The acetylation of the kenaf core caused a significant increase in the water resistance, thermal stability, and tensile properties of the composites. The FESEM results showed that better interaction occurred between acetylated kenaf core and HDPE relative to unmodified kenaf core. J. VINYL ADDIT. TECHNOL., 2011. © 2011 Society of Plastics Engineers     

Effect of mold temperature on the long‐term viscoelastic behavior of polybutylene terepthalate

The effect of mold temperature variation during injection molding on the long‐term viscoelastic behavior of polybutylene terepthalate (PBT) was studied by dynamic mechanical thermal analysis (DMTA) and flexural creep tests. The time–temperature superposition (TTS) principle was applied to the experimental data and the master curves were created to predict their long‐term behavior. The WLF and Arrhenius models were verified for the shift data in the investigating temperature range and the activation energies for the deformation process were calculated based on the Arrhenius equation. Further a four‐element Burger model was applied to the creep results to represent the creep behavior of the PBT processed at two different mold temperatures and to better understand the deformation mechanism. Differential scanning calorimetry (DSC) and density measurements were accomplished to characterize the process‐dependent microstructures. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Effect on Mechanical Performance of UHMWPE/HDPE-Blend Reinforced with Kenaf,Basalt and Hybrid Kenaf/Basalt Fiber

The aim of this study was to investigate the performance of UHMWPE/HDPE-reinforced kenaf, basalt and hybrid kenaf/basalt composites. Mechanical testing of these samples was carried out such as tensile, flexural (three-point bending) and an impact test (Charpy). Pure resin (UHMWPE/HDPE) samples were tested and compare with reinforced 10% weight fraction of kenaf, basalt and hybrid kenaf/basalt samples to identifying their contribution and potential in this new composite material. UHMWPE/ HDPE sample was produced in constant ratio 60:40 respectively via extrusion process. Basalt reinforced UHMWPE/HDPE generates the highest elastic modulus result compared to kenaf and hybrid kenaf/basalt as a reinforcement material. The tensile results of kenaf reinforcement UHMWPE/HDPE samples are significantly higher (20%) than pure blend resin, which is an indication for good performance of kenaf, basalt and hybrid kenaf/basalt to be used in UHMWPE/HDPE-blend polymers. The flexural and Charpy strengths show the drawback results, where performance of polymer is reduced 5% with the absence of kenaf. It can be concluded that kenaf, basalt and hybrid kenaf/basalt fiber successfully increase the UHMWPE/HDPE blends performance especially under tensile loading.     

Tensile creep behavior of unidirectional glass‐fiber polymer composites

The tensile creep behavior of unidirectional glass‐fiber polymer composites was studied at three different temperatures, namely 298, 333, and 353 K. Testing was performed on the pure epoxy matrix, the 0° specimens as well as off‐axis at 15, 30, and 60 degrees in respect to the axis of tension. The creep strain rate was negligible at room temperature, while it was considerable at the higher temperatures examined. The materials exhibit nonlinear viscoelastic behavior, and the creep response of the composites was treated as a thermally activated rate process. The creep strain was considered to include an elastic, a viscoelastic and a viscoplastic part. The viscoplastic part was calculated through a functional form, developed in a previous work, assuming that viscoplastic response of polymer composites arises mainly from the matrix viscoplasticity. The model predictions in terms of creep compliances were found to be satisfactory, compared with the experimental results. POLYM. COMPOS. 26:287–292, 2005. © 2005 Society of Plastics Engineers.     

The effect of fiber concentration on mechanical and thermal properties of fiber‐reinforced polypropylene composites

A novel composite material consisting of polypropylene (PP) fibers in a random poly(propylene‐co‐ethylene) (PPE) matrix was prepared and its properties were evaluated. The thermal and mechanical properties of PP–PPE composites were studied by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) with reference to the fiber concentration. Although, by increasing PP fiber concentration in PPE, no significant difference was found in melting and crystallization temperatures of the PPE, the storage, and the tensile and flexural modulus of the composites increased linearly with fiber concentrations up to 50%, 1.5, 1.0, 1.3 GPa, respectively, which was approximately four times higher than that for the pure PPE. There is a shift in glass transition temperature of the composite with increasing fiber concentration in the composite and the damping peak became flatter, which indicates the effectiveness of fiber–matrix interaction. A higher concentration of long fibers (>50% w/w) resulted in fiber packing problems, difficulty in dispersion, and an increase in void content, which led to a reduction in modulus. Cox–Krenchel and Haplin–Tsai equations were used to predict tensile modulus of random fiber‐reinforced composites. A Cole–Cole analysis was performed to understand the phase behavior of the composites. A master curve was constructed based on time–temperature superposition (TTS) by using data over the temperature range from −50 to 90°C, which allowed for the prediction of very long and short time behavior of the composite. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2260–2272, 2005     

Dynamic mechanical properties of extruded nylon–wood composites

Dynamic mechanical properties determine the potential end use of a newly developed extruded nylon–wood composite in under‐the‐hood automobile applications. In this article, the dynamic mechanical properties of extruded nylon–wood composites were characterized using a dynamic mechanical thermal analyzer (DMTA) to determine storage modulus, glass transition temperature (T_g), physical aging effects, long‐term performance prediction, and comparisons to similar products. The storage modulus of the nylon–wood composite was found to be more temperature stable than pure nylon 66. The T_g range of the nylon–wood composite was found to be between 23 and 56°C, based on the decrease in storage modulus. A master curve was constructed based on the creep curves at various temperatures from 30 to 80°C. The results show that the relationship between shift factors and temperature follows Arrhenius behavior. Nylon–wood composites have good temperature‐dependent properties. Wood fillers reduced the physical aging effects on nylon in the wood composites. The comparison of the nylon–wood composite with other similar products shows that nylon–wood composites are a promising low cost material for industrial applications. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Experimental analysis and prediction of viscoelastic creep properties of PP/EVA/LDH nanocomposites using master curves based on time–temperature superposition

Prediction of viscoelastic behavior of polymers over a long‐term period is of vital importance for engineering applications. An attempt was made to uncover the interplay between the morphology and viscoelastic behavior of compatibilized polypropylene/ethylene vinyl acetate (EVA) copolymer blends in the presence of layered double hydroxide (LDH) nanoplatelets. The time–temperature superposition (TTS) principle and WLF equations were merged to obtain master curves of storage modulus at defined reference temperatures enabling prediction of storage modulus at high frequency ranges which are not experimentally measureable. Moreover, the creep compliance master curves were acquired for different reference temperatures to predict the creep compliance of nanocomposites over long period of times. It was found that the presence of LDH decreases the creep compliance at long period of times while it decreases the unrecoverable deformation of EVA domains. A simple mechanism was proposed to explain the creep and recovery behavior of samples blend at different temperatures. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46725.     

Impedance spectra of carbon black filled high‐density polyethylene composites

Carbon black (CB) filled high‐density polyethylene (HDPE) composites are prepared by ordinary blending for use as an electrical conductive polymer composite. The composite changes from an electrical insulator to a conductor as the CB content is increased from 10 to 20 wt %, which is called the percolation region. For explanatory purposes, three models, namely, “conduction via nonohmic contacting chain,” “conduction via ohmic contacting chain,” and a mixture of them corresponding to the conductions in the percolation region, high CB loading region, and limiting high CB loading are proposed by the reasonable configurations of aggregate resistance, contact resistance, gap capacitance, and joining aggregates induction. The characters of the impedance spectra based on the three models are theoretically analyzed. In order to find some link between the electrical conductivity and the CB dispersion manner in the composites, the impedance spectra of three samples, HDPE/15 wt % CB (the center of the percolation region), HDPE/25 wt % CB (a typical point in the high CB loading region), and HDPE/19 wt % CB (the limiting high CB loading region), are measured by plotting the impedance modulus and phase angle against the frequency and by drawing the Cole–Cole circle of the imaginary part and real part of the impedance modulus of each sample. The modeled approached spectra and the spectra measured on the three samples are compared and the following results are found: the measured impedance spectrum of HDPE/15 wt % CB (percolation region) is quite close to the model of conduction via nonohmic contacting chain. The character of the measured spectrum of HDPE/25 wt % CB consists of the form of the model of conduction via ohmic contacting chain. The impedance behavior of HDPE/19 wt % CB exhibits a mixture of the two models. From the comparisons, it is concluded that the electrical conducting network in the percolation region of the CB filled HDPE composite is composed of aggregate resistance, nonohmic contact resistance, and gap capacitance, and that of the high CB loading region consists of continuously joined CB aggregate chains, which are possibly wound and assume helix‐like (not straight lines) conductive chains, acting as electrical inductions as the current passes through. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1344–1350, 2005     

Temperature and frequency‐dependent creep and recovery studies on PVDF‐HFP/organo‐modified layered double hydroxides nanocomposites

A series of poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) loaded with various contents of layered double hydroxides (LDHs) nanoparticles were prepared via a melt mixing method. Detailed investigations on LDH dispersion state in the polymeric matrix conducted by TEM revealed intercalated/exfoliated, and agglomerated structures at low (1 wt %) and high (>3 wt %) loadings of LDH contents, respectively. Wide angle X‐ray scattering and DSC results showed that incorporation of LDH into PVDF‐HFP matrix reduced its overall crystallinity and helped to form polar crystallites, while the crystal size at 020 crystallographic directions was found to be most affected by presence and dispersion state of LDH in the matrix. TGA results showed LDH improved thermal stability of matrix however, unlike many other nanomaterials it significantly reduced the residual mass which highlights catalytic role of LDH in degradation of residual carbon char. Detailed analysis on creep and recovery data over wide range of selected temperatures revealed that the creep compliance of nanocomposites are basically controlled by crystallinity and presence of LDH at low and high temperatures, respectively. Based on obtained storage modulus and creep compliance master curves it was also found that the influence of LDH on decreasing the creep compliance and improving viscoelastic properties of PVDF‐HFP over long time period and over high frequency ranges becomes more pronounced. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46352.     

Dynamic rheological properties of high‐density polyethylene/polystyrene blends extruded in the presence of ultrasonic oscillations

The linear rheological properties of high‐density polyethylene (HDPE), polystyrene (PS), and HDPE/PS (80/20) blends were used to characterize their structural development during extrusion in the presence of ultrasonic oscillations. The master curves of the storage shear modulus (G′) and loss shear modulus (G″) at 200°C for HDPE, PS, and HDPE/PS (80/20) blends were constructed with time–temperature superposition, and their zero shear viscosity was determined from Cole–Cole plots of the out‐of‐phase viscous component of the dynamic complex viscosity (η″) versus the dynamic shear viscosity. The experimental results showed that ultrasonic oscillations during extrusion reduced G′ and G″ as well as the zero shear viscosity of HDPE and PS because of their mechanochemical degradation in the presence of ultrasonic oscillations; this was confirmed by molecular weight measurements. Ultrasonic oscillations increased the slopes of log G′ versus log G″ for HDPE and PS in the low‐frequency terminal zone because of the increase in their molecular weight distributions. The slopes of log G′ versus log G″ for HDPE/PS (80/20) blends and an emulsion model were used to characterize the ultrasonic enhancement of the compatibility of the blends. The results showed that ultrasonic oscillations could reduce the interfacial tension and enhance the compatibility of the blends, and this was consistent with our previous work. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3153–3158, 2004     

Tensile creep of a long‐fiber glass mat thermoplastic composite. I. Short‐term tests

This work is part of a larger experimental program aimed at developing a semi‐empirical constitutive model for predicting creep in random glass mat thermoplastic (GMT) composites. The tensile creep response of a long‐fiber GMT material has been characterized for 3‐ and 6‐mm thick material. Tensile tests showed that the variability within and between plaques are comparable with an overall variability of about 6% and 8% for the 3‐ and 6‐mm thick materials, respectively. The thicker material exhibited slightly higher variability and directional dependence due to greater flow during molding of the plaques. Short‐term creep tests consisting of 30 min creep and recovery, respectively, were performed over the stress range between 5 and 60 MPa. Three tests for determining the linear viscoelastic region were considered which showed that the 3‐ and 6‐mm thick GMT are linear viscoelastic up to 20 and 25 MPa respectively. The 6‐mm thick GMT consisting of a higher fiber weight fraction was linear over wider stress range. Furthermore, it was found that plastic strains were accumulated during creep, which suggests that a nonlinear viscoelastic–viscoplastic model would be more appropriate for long‐term creep at relatively high stresses, which will be presented in our companion paper. The magnitude of the plastic strains developed in the creep tests presented here was lower because a single specimen was loaded at multiple stress level over short durations. Hence, a nonlinear viscoelastic constitutive model has been developed for the two thickness materials. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Long‐term water uptake behavior of lignocellulosic‐high density polyethylene composites

Composites of different lignocellulosic materials and high‐density polyethylene were prepared and their long‐term water absorption behaviors were studied. Wood flour, rice hulls, newsprint fibers, and kenaf fibers were mixed with the polymer at 25 and 50 wt % fiber contents and 1 and 2% compatibilizer, respectively. Water absorption tests were carried out on injection‐molded specimens at room temperature for five weeks. Results indicated a significant difference among different natural fibers with kenaf fibers and newsprint fibers exhibiting the highest and wood flour and rice hulls the lowest water absorption values, respectively. Very little difference was observed between kenaf fiber and newsprint composites and between rice hulls and wood flour composites regarding their water uptake behavior. The difference between 25 and 50% fiber contents for all composite formulations increased at longer immersion times, especially for the composites with higher water absorption. Kenaf fiber composites containing 50% kenaf fibers exhibited the highest water diffusion coefficient. A strong correlation was found between the water absorption and holocellulose content of the composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3907–3911, 2006