Study On Rheological Behaviors And Crystallization Properties Of Highly Filled Wood Plastic Composites

Wood plastic composite (WPC) is an intimate mixture of wood and polymers, processed by techniques similar to the plastics industry. The benefits of WPC, including raw material widespread availability, low maintenance, easily molding, excellent weather ability and much more aesthetics, have been gaining interest in wood replacement applications for architecture purposes, automotive industry and furniture manufacturing. It is the natural incompatibility between the polar hydrophilic wood flour and the non-polar hydrophobic polyolefin matrix. Phase incompatibility yields very weak interactions and thus a weak interface between the wood flour and the matrix. In practice, most of the commercial WPC is highly filled with wood particles, e.g., 50–60 wt%, which is close to the maximum packing content. So it usually leads to poor processability and weak mechanical properties of the products.Many domestic and foreign scholars and specialist pay highly attention to modify the wood surface with coupling agents to improve the properties and characteristics. In a great number of studies, the mechanical properties are used to evaluate the properties and characteristics of the WPC products. In fact, the mechanical properties of a wood plastic composite depend on many factors. The crystallinity and the crystal form of polyolefin, the shape, size and agglomeration of wood particles, and the interactions between wood particles and the polymers play important roles in mechanical strength. This makes it improper to evaluate the wood particle-polymer interaction directly from mechanical properties. It is then desirable to characterize the particle-polymer interaction in melt state, the crystal structure near the interface and the solid-state morphology, where rheology and crystallization kinetics becomes the effective tools.Although the rheology has been used efficiently to characterize the interfacial properties in the immiscible blends, few studies can be found to systematically analyze the relationship between the rheological behavior and the structure of the WPC. At the same time, it is also unclear in such highly filled systems if there are changes in crystallization behavior or crystal morphology around the wood particles with the addition of maleic anhydride grafted polyolefin. So the rheological behavior and crystallization properties of highly filled WPC were studied in this thesis. The research contents and main conclusions were introduced as follows:1. WPC melt had the wider relaxation time spectrum and multiple relaxation mechanism owing to the system being provided with multiple structures, such as the matrix, phase interface between the PP melt and wood particles and wood powder particles network. Highly filled WPC usually exhibited the nonlinear characteristics ( I 31 and?3 1)and elasticity behavior (such as edge fracture , sharkskin extrusion distortion, wall slip, secondary extrusion distortion etc). Those theological behavior of WPC is closely associated with the relaxation and evolution of the microstrure involved in WPC as well as the damped relaxation processes under macro deformation.2. The rheological behaviors of wood plastic composite with respect to wood particle content and maleic anhydride-grafted polypropylene (MAHPP) compatibilizer is studied by linear rheological methods. The relaxation spectrum was calculated from storage modulus G?and loss modulusG?data by GENEREG program. For WPC samples, they showed an additional longer characteristic relaxation process other than that of linear PP molecule in matrix, although the longer relaxation process cannot be captured completely in the the limited frequency range. This indicates that there is an additional longer characteristic relaxation process in the WPC, which can be ascribed to relaxation process of the wood particle agglomerates and the heterogonous interface. In other words, if we want observe the longer characteristic relaxation behavior, it is necessary to use lower frequency and spend more time.3. The relaxation of large scale structure can be excited by a large stress which is out of the linear regime,, i.e., when the input shear rate exceeds the inverse of the longest relaxation time of the system, the large-scale structure can be investigated in nonlinear region. The Fourier transform rheology (FTR) by the stress control mode is found to be an effective method for further investigating the structure with long relaxation time in WPC system. The plateau value of I 31at high stress and the range of ?3 1 are proved to be corresponding to the content of wood particle agglomerates in the WPC melts and the type of interfacial hydrodynamic interaction. As the MAHPP content increases, the WPC system displays significant decrease in the value of I 31 and the variation range of ?3 1 in nonlinear region. Since the gradual collapse of the local wood particle agglomerates and the disentanglement of PP interfacial layers near the orientation wood particles are the origin of the nonlinear properties in WPC system, the decreased value of I 31 and ?3 1are believed to be related to the stability of structures when MAHPP is added.4?The phenomena of wall slip on flow instabilities and extrudate distortions of WPC has been stuied by the double barrel capillary rheological instrument. The results showed that in the range of experimental extrusion speed, the pure PP melt exhibited stable flow behaviour. A higher extrusion speed is beneficial for the production of smoother extrudates. This is probably due to the wall slip effect in capillary flow, which implys the WPC can be product with higher extrusion speed. Mooney analysis, which was shown to be effective to characterize wall slip phenomena of the WPC melts, was then performed to compare the different roles of the wood content and MAHPP effect. The relationship between the multiple relaxation characteristic and the processability of the WPC melt in high extrusion speed have also been analysed in details. The characteristic relaxation time of the WPC increased with the increasing of the wood content, thus the relaxation process of the microstrure involved in WPC fell behind the macro deformation of the WPC melt in the extrusion. So WPC system was inclined to storage more elastic energy and exhibit flow instability such as sharkskin and secondary extrudate distortions. The MAHPP compatibilizer is generally bifunctional molecules with the side chains capable of adhering to the wood particles and the main chains entangled with the polymer. With small amount of MAHPP can basically alter the character of the interface and reduce the agglomerated particles. Hence the relaxation time of the WPC was reduced after adding small amount of MAHPP. The wood particle agglomerates and the interface do get more opportunity to relaxation in macro deformation and compatibilizer can really stabilize the system in the process of extrusion. The results also illustrated the morphological features of the WPC melts in dependence on the L/D of the die at different apparent shear rates. It can be seen that during extrusion of 40 wt% filled WPC (the other composites showed similar trend), the extrudates emerge from the 0.25 mm length die with much smoother surface whereas increasing the die length results in tearing of the extrudates. The most severe extrudate tearing and surface roughness were observed in the case of flow through the 24mm length die.5. The flow instability phenomenon of the high filled WPC system has also been observed at very low shear rates in parallel plate. In order to obtain the relationship between the microstructure of WPC and the limited shear rate, the steady rheological properties of WPC with different gap were studied. The real-time tracking method by the high speed camera system was used. Experiment results showed that the phenomena of edge fracture likely took place in the process of steady stress sweep by using of serrated plate. The value of critical elastic energy storage is mainly affected by the interfacial tension between PP melt and air and slightly was influenced the content of wood flour of and a small amount of MAHPP. Since relaxation time of the WPC increasing with the addition of wood particles and it inclined to storage more elastic energy during the viscometric flow, the edge fracture phenomena inclined to happen for the system containing much more wood particles. Because MAHPP can improve interface interaction between the wood particles and PP melt, and reduce wood particles network. So in the same shearing rate, the WPC system containing MAHPP can flow steadily in higher shear rate and bear markedly successive deformation.6?The isothermal crystallization behavior and crystal structure of the polypropylene component in WPC with respect to wood particle content and MAHPP compatilizer were studied by means of polarized optical microscopy (POM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) in this part. The relative integral intensities of ? crystal form planes is changed in WPC system, and the addition of wood particles resulted in a prompt PP chains to arrange at (040) plane. It was found that under the experimental conditions of this research, the speed of crystallization of PP was faster in WPC with MAHPP than that in composites without MAHPP. This is ascribed to the difference in undercooling due to the change in the equilibrium melting temperatures ( Tm 0) of the PP component in WPC due to the addition of wood flour and MAHPP compatibilizer. Tm 0 decreased with the increase of wood particle content, and it decreased more severely with the addition of wood flour than the addition of compatibilizer. The half crystallization time was smallest in PP/wood composites, intermediate in PP/wood/compatibilizer system and largest in pure PP under the same undercooling. According to the LH theory, the values of K g for the WPC sample were lower than that for pure PP, implying the presence of wood particles accelerated the nucleation rate of PP. In addition, WPC without MAHPP had smaller value of e for nucleation than the WPC samples with compatibilizer, which further proves the MAHPP was inclined to adhere to the surface of the wood particles.