Rheology of polyamide 6 composites reinforced with microcrystalline cellulose

Engineering thermoplastics incorporating cellulose fibers is challenging, because the thermoplastics have higher melting points and the cellulose has low thermal stability. Although recent efforts have demonstrated the feasibility of producing cellulose-filled engineering thermoplastic composites, only a few studies have been…

Engineering thermoplastics incorporating cellulose fibers is challenging, because the thermoplastics have higher melting points and the cellulose has low thermal stability. Although recent efforts have demonstrated the feasibility of producing cellulose-filled engineering thermoplastic composites, only a few studies have been done. It is difficult to produce the composites without considerable thermal degradation of the cellulose, and the cost of engineering thermoplastics is high compared with commodity plastics.1–7The flow or rheological behavior of polymer composites is very important in analyzing and designing processing operations as well as understanding the relationship between structures and the polymer properties of polymer composites.8, 9 The rheological properties of cellulose-reinforced composites are related to the material’s microstructure, the state of cellulose dispersion in the matrix, as well as the interactions between cellulose and polymer chains.9 Most research in the literature related to cellulose-filled engineering thermoplastic composites has been on the composites’ mechanical and thermal properties. By contrast, the rheological properties have rarely been investigated.We prepared composites to investigate the influence of microcrystalline cellulose (MCC) on the rheological properties and crystallization behavior of polyamide 6 (PA6) composites. Current studies did not use compatibilizers or coupling agents because of the complicated rheological data. Such composites contain several types of materials, such as coupling agents and MCC. Our present study is the starting point for future research that will help us understand the relationship between structures and the polymer properties of cellulose-filled engineering thermoplastic composites as well as easily compare the various properties of initial composites with compatibilizers or other additives.Figure 1.Complex viscosity of samples as a function of frequency. MCC: Microcrystalline cellulose. PA6: Polyamide 6. Pa.s: Pascal seconds. We mixed the MCC and PA6 at various composition ratios (2.5, 10, 20, and 30wt% with respect to PA6) using a Brabender Prep-Mixer equipped with a bowl mixer. The rheological properties of the PA6 and MCC composites were measured on a stress-controlled Bohlin Gemini rheometer (25mm diameter, 1.0mm gap) at a temperature of 235°C to characterize the linear viscoelastic region to obtain the complex viscosities (η# ) and the storage moduli (G ′ ). Steady rate sweep tests were also performed at low shear rate range to obtain the apparent viscosities (η) for the neat PA6 and MCC composites.10Figure 1 shows η* for PA6 and its composites as a function of frequency (ω). The complex viscosities decreased with increasing ω, indicating a shear thinning behavior of the PA6 and MCC composites. That behavior can be attributed to the orientation of the rigid MCC molecular chains, which disturb the formation of PA6 chain entanglements in the composites during the applied shear force. The higher degree of PA6-MCC interaction requires higher shear stress and longer relaxation times for the composites to flow. Compared to PA6, the composites have a higher melt viscosity and the cellulosic phase’s contribution becomes apparent.10It is clear that the storage moduli of the composites were larger relative to the PA6 matrix because of the intrinsic rigidity of MCC (see Figure 2). The storage moduli of composites are higher than the pure matrix, especially for the highest MCC content, indicating that stress transfers from the matrix to the MCC. The slope of the storage modulus curves as a function of frequency decreases a small amount with increasing amounts of MCC. The decrease in the slopes of the storage modulus for the composites compared with PA6 can be explained by the microstructural changes of the polymer matrix owing to the incorporation of MCC. The MCC-MCC and strong PA6-MCC (interfacial) interactions rise with increasing MCC content.10,11Figure 2.Storage modulus (G ′ ) of samples as a function of frequency. Figure 3.Steady shear viscosity of samples as a function of shear rate. We found that the PA6-MCC composites behave as pseudoplastic fluids (see Figure 3). The higher the MCC content of the composite, the higher the shear viscosity. This behavior is attributed to the MCC’s hindering the movement of the matrix’s macromolecular chains.12 The pseudoplastic’s behavior becomes more important as the MCC content in the composite increases.10Our future research will focus on new applications of PA6 composites with MCC for the automobile industry, especially for under-the-hood applications. Development of surface modifications for cellulose, new compounding techniques, and the use of new engineering thermoplastics with melting temperatures higher than PA6 will extend the application of MCC-filled engineering thermoplastic composites because of the environmental and economic benefits. Knowledge gained from this work and future studies could eventually result in cellulose replacing glass and mineral fillers in the automobile industry.As a next step, we plan to investigate and compare the rheological behavior of cellulose-filled engineering thermoplastics with melting temperatures higher than PA6, such as nylon 6,6 and styrene maleic anhydride copolymer.