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The elastic recovery mechanism of TPV is consistent with the deformation mechanism (see the basic performance of TPV (1)). Based on the deformation behavior of TPV, the physical causes of elastic recovery are as follows. During the solid state deformation in the stretching process, the PP phase is only partially deformed. The thin PP layer is deformed in the equatorial region (perpendicular to the applied load) and the rest of the PP phase is not affected. During tensile recovery, part of PP is pulled back by elastic EPDM, and a small part of PP undergoes irreversible deformation. The relationship between microstructure and elastic properties of TPV provides guidance for the preparation of high elastic TPV. The elasticity of TPV is mainly affected by the crosslinking degree of rubber phase, the size of rubber particles in TPV, the composition ratio of R/P, plasticizer, filler and processing conditions.
I. Influence of R/P Composition Ratio on Elastic Properties.
In general, an increase in the composition ratio of R/P results in an increase in the elasticity of TPV. This is because the rubber phase has higher elasticity than the plastic phase. Kang et al. studied the effect of BPE content on TPV elasticity. The results show that high content of BPE leads to an increase in elongation at break of TPV, i.e. higher elasticity of BPE/PLA-TPV. Fu et al. showed that within a certain strain range, the residual strain and hysteresis loss decreased with the increase of EPDM/PP composition ratio in TPV, indicating that the elasticity of TPV increased.
Second, the influence of rubber particle size and plastic matrix thickness on elastic properties.
Research shows that the decrease of dn and IDpoly can improve the elasticity of TPV. For example, the dn and IDpoly of BIIR particles decrease with the increase of DV time, resulting in an increase in the strength of the rubber network, thus increasing the elasticity of BIR/PA12-TPV. For EPDM/PP TPV aggregates with EPDM nanoparticles, more and smaller rubber particles in the PP matrix form a stronger rubber network structure, which reduces the aggregation of EPDM nanoparticles and thus increases the elasticity of TPV, as shown in figs. 1-1(a) and (b). Research by L'bee et al. shows that the elasticity of EPDM/PP-TPV with larger rubber particles (dn> 40 μm) is similar to that of pure PP, regardless of the rubber particle size. In smaller cases, the elasticity increases significantly with the decrease of dn(1 μm<dn<40 μm). As shown in figure 1-1(c). This study proves that the rubber particles have a critical influence on the elasticity, and the rubber particles above the critical will not affect the elasticity of TPV. Cyclic stress-strain curves (B) hysteresis loss and permanent deformation (C) compression permanent deformation of EPDM/PP-TPV with different dn.
Fig.1-1 (a) Tension-recovery stress-strain curves of EPDM/PP-TPV with different dn and (b) Hysteresis loss and permanent deformation as a function of dn and (c) Compression setof the pure PP and EPDM/PP-TPV as a function of dn.
Third, the effect of adding compatibilizer on elastic properties.
The addition of compatibilizer can reduce the interfacial tension and reduce the size of rubber phase, thus improving the elasticity of TPV. For example, HIPS/HVPBR-TPV shows higher elasticity after adding compatibilizer. Research shows that with the development of DV, the compatibility between rubber phase and plastic phase in TPV is improved, resulting in the reduction of rubber particle size, thus improving the elasticity of TPV. Reactive compatibilization results in good adhesion between the matrix and the rubber phase, thus contributing to the improvement of TPV elasticity.
Fourth, the effect of adding oil and filler on elastic properties.
Since the decrease in the content of TPV crosslinked rubber will lead to more plastic deformation, elasticity can be improved by adding oil. The results show that the increase of oil content improves the elasticity of TPV. L'Abee studied the effect of oil content on the elasticity of PE/ poly (alkyl methacrylate) TPV. The results show that when the oil content is increased, the elasticity of TPV is improved due to the increased mobility of PE matrix and the formation of submicron rubber particles in TPV. Le et al. have studied the influence of polar oil and nonpolar oil on the elasticity of TPV. The results show that after DV, with the increase of polar oil and nonpolar oil content, the crystallinity of PP decreases due to the oil in TPV, thus improving the elasticity of NBR/PP-TPV. The research shows that the elasticity of EPDM/PP-TPV increases with the increase of carbon nanotubes (CNTs) content, showing a decrease in permanent deformation and hysteresis loss with the increase of CNTs content (see fig. 1-2). The reason is that CNTs network dispersed in TPV acts as a nano-network structure in the low strain region.
Figure 1-2 Effect of CNTs Dosage on Elasticity of CNTs/TPV Composites. (a) cyclic tensile stress-strain curve (b) hysteresis loss and permanent deformation.
Fig.1-2 Elasticity of CNTs/TPV composites with different contents of CNTs. (a) tension recovery stress–strain curves; (b) hysteresis loss and permanent set.
Rheological Properties and Influencing Factors of TPV
Rheological properties determine the processability of TPV. Rheological behavior of TPV is generally measured by capillary rheometer and RPA. Due to the strong rubber network formed by crosslinked rubber particles, the initial value of complex viscosity of TPV is much higher than the initial value of plastic component. Research shows that the rheological properties of TPV are controlled by rubber phase in low frequency region and plastic matrix in high frequency region. Due to its strong rubber network and molecular entanglement, the rheological behavior of TPV has rubber elasticity at low strain and low frequency, but the collapse and deformation of rubber network at high strain and high frequency make TPV have good melt processability. TPV materials usually exhibit pseudoplasticity and shear thinning behavior, and the viscosity of TPV decreases with the increase of shear stress or frequency, as shown in Figure 1-3.
I. Effect of R/P Composition Ratio on Rheological Properties.
As the shear viscosity of the cross-linked rubber phase is higher than that of the plastic phase, the viscosity of TPV increases with the increase of the rubber content, and with the increase of the rubber content, the number and density of rubber particles in TPV increase, forming a stronger rubber network, resulting in the reduction of the melt processability of TPV.
Second, the influence of crosslinking agent content and type on rheological properties.
Research shows that increasing the content of crosslinking agent will increase the degree of crosslinking, thus increasing the viscosity of TPV, but will be accompanied by a decrease in processability. On the one hand, with the increase of crosslinking degree, the rigidity of rubber particles increases and the deformation ability decreases, resulting in instability of rubber network in TPV. On the other hand, the increase of crosslinking agent content leads to the formation of smaller rubber particles in TPV, which leads to the increase of viscosity of TPV. Influence of crosslinking agent type. Nakason et al. studied the effects of different crosslinking systems on the rheology of TPV. The results show that at a given shear rate, because sulfur and peroxide both produce crosslinking reaction of NR phase and peroxide also produces crosslinking reaction to a certain extent in HDPE phase, the highest shear stress is reached in NR/HDPE-TPV crosslinked by the mixed crosslinking system of sulfur and peroxide. In sulfur crosslinked TPV, crosslinking reaction only occurs in NR phase, resulting in the lowest apparent shear stress.
III. Influence of Rubber Particles dn on Rheological Properties.
Research shows that the decrease of dn increases the interfacial surface area and decreases the distance between rubber particles, both of which tend to strengthen the rubber network. Therefore, at the same shear stress or shear frequency, the viscosity of TPV increases with the decrease of the rubber phase dn, as shown in fig. 1-3. In BIIR/PP-TPV, although the size of BIIR particles decreases with the increase of DV time, the aggregation of single rubber nano-aggregates in TPV deteriorates the rubber network, thus improving the melt processability of TPV.
Complex viscosity of bio-based BPE/PLA-TPV with different rubber-plastic ratio at 180℃ (b) Shear viscosity of TPV with different dn after 100 s creep at 210℃ (c) Storage modulus and complex viscosity of EPDM/PP TPV with different dn as a function of angular frequency at 210℃ (d) Storage modulus and complex viscosity of BIIR/PP-TPV samples.
4. The effect of compatibilizer in-situ compatibilization on rheological properties of TPV.
In most cases, the compatibility of TPV can reduce the size of rubber particles and increase the density of rubber particles in TPV, thus increasing the viscosity of TPV. Nakason et al. studied the effect of the dosage of PP-g-MAH on the rheological properties of TPV. The results show that at a certain shear rate, due to the chemical interaction between PP-g-MAH and ENR, the apparent shear viscosity of ENR/PP TPV increases with the increase of PP-g-MAH content, and reaches the maximum when PP-g-MAH content is 7.5%, as shown in Figure 1-4. Goharpey and other research results show that the compatibilization of organoclay and maleic anhydride leads to an increase in interlayer thickness, thus reducing interlayer slip and increasing viscosity.
