|Crosslink density|| |
The engineering stress versus extension ratio curves were plotted for these double networks, where extension ratios (kt ) were calculated based on the final equilibrium length of these double networks. These stress versus extension ratio curves were used to calculate the effective crosslink density, which contains a contribution attributed to chain entanglements and loose chain ends acting as crosslinks in rubber. The effective crosslink density was calculated according to the Mooney–Rivlin equation.
The crosslink density of the vulcanized rubber has a strong impact on the dynamic properties of the material. Along with an increasing number of crosslinks in rubber, the energy dissipation is decreasing, which can be easily noticed from the tangent δ
|Influence of different crosslinking systems on the mechanical properties of thermoplastic vulcanizates|
The crosslink density of the vulcanized rubber has a strong impact on the dynamic properties of the material. Along with an increasing number of crosslinks in rubber, the energy dissipation is decreasing, which can be easily noticed from the tangent δ. Loss Factor or Offset Tangent (tan δ) = E '' / E 'Indicates the damping capacity of the material and its viscoelastic balance.
Las propiedades físicas de los TPV curados con peróxido a una relación PP / EPDM tanto fija como variada cambian significativamente con la naturaleza química de los peróxidos, el grado de reticulación de la fase EPDM y el grado de degradación de la fase PP. El motivo del aumento de la resistencia a la tracción, el módulo y la dureza con la cantidad creciente de PP es el componente duro termoplástico aumentado en las mezclas. La velocidad de curado, la densidad de reticulación final, la estabilidad térmica de las reticulaciones formadas, las características de seguridad, salud y medio ambiente de los productos químicos utilizados y el precio de costo son parámetros relevantes para la elección final del sistema de reticulación.
|Influence of different crosslinking systems |
The effects of three curing systems, peroxide, peroxide–phenolic combination, and phenolic. The changing curing system from peroxide to peroxide–phenolic and phenolic increased the glass transition temperature of the filled cured rubbers between 3 and 5 8°C. Unlike of peroxide curing system, has a dual phase then peroxide–phenolic and phenolic cure systems. The phenolic cure system deteriorated mechanical properties for both (PP and EPDM) , aged and unaged cured rubbers. Increasing the amount of diene monomer in EPDM structure was beneficial for phenolic rubber cure system.
The physical properties of peroxide-cured TPVs at a fixed as well as at varied PP/EPDM ratio change significantly with the chemical nature of peroxides, the extent of crosslinking of the EPDM phase and the extent of degradation of PP-phase. The reason of increased tensile strength, modulus and hardness with the increasing amount of PP is the increased thermoplastic hard component in the blends. The cure rate, the final crosslink density, the thermal stability of the crosslinks formed, the safety, health and environmental characteristics of the chemicals used and the cost price are relevant parameters for the final choice of the crosslinking system.
|Amount of phenolic resin (for TPVs)|
The dispersed EPDM phase in TPVs is responsible for the elastic properties of the blends. The purpose of changing the amount of phenolic resin is to alter the crosslink density of the EPDM phase and study its influence on the tensile and elastic properties. Another consequence of varying the amount of phenolic resin would be, to alter the viscosity ratio between EPDM and PP. This should have an influence on the morphology of TPVs. In fact, we can see , that the EPDM particle size distribution is narrowed by increasing the crosslink density of the EPDM phase.
Increasing the amount of phenolic resin results in an increase of hardness, E-modulus, tensile strength and a decrease of elongation at break values and tension set of the TPVs. The changes in the mechanical properties are as expected on the basis of a higher crosslink density of the EPDM-phase.
|Amount of Peróxido resin (for TPVs)|
The more peroxide is added, the more radicals are formed to cross-link the EPDM phase, resulting in higher network density. With higher network density, fewer solvent molecules have the chance to get between the macromolecules and drive them apart (swelling). The same is also valid for phenolic cross-linked TPVs. But here, of course, no radical reaction takes place. During the crosslinking reaction, first ether bridges are split, yielding mono-phenolic units having benzylic cations. These benzylic cations then react with the unsaturation of EPDM rubber to accomplish the crosslinking. So with higher phenolic resin concentration, a higher network density is achievable. Overall, with the phenol resin as cross-linking agent, higher crosslink densities are achievable compared to peroxide cross-linked TPVs. The reason for this is, that the peroxide only starts the chain reaction and does not take part in the cross-linking step, whereas the phenolic resin is an active reactant of the cross-linking process.
The compression set is a measurement of the ratio of elastic to viscous components of an elastomer’s response to a given deformation. Table I shows compression set values obtained for the PP/EPDM blends. With increasing peroxide concentration, or in other words increasing cross-link density, the compression set of the peroxide cross-linked TPVs decreases. A similar trend can be found in batch processing. The compression set of phenolic resin crosslinked TPVs also decreases perceptibly when adding 0.5 wt. % of phenolic resin, but after the first distinct decrease, the compression set decreases only slightly with increasing phenolic resin concentration. With only 1.5 wt. % of peroxide an improvement in compression set of 50 % is achievable. For the same improvement, almost twice the concentration of phenolic resin is necessary to build a strong network. At a higher cross-link density, the resulting networks provide the necessary stability and the appropriate resilience. A strong and dense network also prevents creeping of the material
|Resistencia a la tracción|
The tensile strength and the elongation at break as a function of cross-linking agent concentration (peroxide and phenolic resin). The curves show an increase in tensile strength and elongation at break with an increase in the concentration of the peroxide, because of cross-linking in the EPDM phase. This contributes to the dissipation of the large amount of energy. The tensile strength reaches a maximum at 0.5 wt. % followed by marginal decrease with further increasing peroxide concentration. The elongation at break reaches a maximum at 0.2 wt. % peroxide and shows a continuous decrease with increasing concentration of peroxide. This can be attributed to a higher degradation of the PP phase at a higher peroxide concentration (more free radicals). Overall, the relatively high elongation at break values are also due to the creeping of the material. Similar trends are also found in batch processing. In comparison, the phenolic resin has no influence on the PP phase. Therefore, and on the basis of the similar TEM-pictures, the tensile strength of the phenolic resin cross-linked PP/EPDM blends stays nearly the same with increasing phenolic resin concentration. The continuously decrease in elongation at break can be explained by an inextensibility of highly cross-linked rubbery particles and a deterioration of the interface interactions between the cross-linked rubber particles and the matrix.