Increasing Dicumyl Peroxide Content Research Articles

Tailored rheology of a metallocene polyolefin through silane grafting and subsequent silane crosslinking

Polymer modification through silane grafting and its subsequent crosslinking allows the rheological properties of a polymer to be tuned from those of a viscous melt to those of a crosslinked elastic network. In this study, a metallocene polyolefin resin is grafted with vinyl trimethoxy silane (VTMS) using dicumyl peroxide (DCP) as the initiator and is subsequently crosslinked in an oxidative environment. Dynamic rheological experiments are conducted to elucidate the effects of DCP and VTMS concentrations on the grafting and ensuing crosslinking processes. We find that the addition of VTMS alone to the polymer produces no grafting. In contrast, the presence of DCP by itself leads to direct crosslinking between polymer chains as suggested by an increase in elastic modulus and complex viscosity. Samples containing both DCP and VTMS undergo silane grafting, with the extent of grafting increasing with increasing DCP concentration. This conclusion is borne out by both rheological and Fourier transform infrared measurements. The grafted samples undergo silane crosslinking only in an oxidative environment and at temperatures equal to or greater than 190 °C. During crosslinking, the samples undergo a transition from a viscous melt with frequency-dependent moduli to a gel exhibiting frequency-independent moduli with the elastic modulus exceeding the viscous modulus. However, the kinetics of crosslinking and the extent of the modulus increase are a function of the DCP concentration, with both exhibiting a maximum at a specific DCP and VTMS combination.

Influence of grafting formulations and extrusion conditions on properties of silane-grafted polypropylenes

The melt grafting of unsaturated silanes onto powdered polypropylene (PP) in a Haake TW100 twin-screw extruder and curing in hot water were studied. The influence of grafting formulations and extrusion conditions on the melt flow rates of grafted PP and the gel percentages of crosslinked PP was investigated. The gel percentages of methacryloylpropyltrimethoxysilane (VMMS)-grafted PP were markedly higher than those of vinyltriethoxysilane (VTES)- and vinyltrimethoxysilane (VTMS)-grafted PP, while significantly less degradation of PP during grafting was observed for VMMS-grafted PP. When benzoyl peroxide (BPO) was used as an initiator, no degradation of PP during grafting was observed, and the melt flow rates of grafted PP decreased with increasing BPO concentration. In contrast, use of dicumyl peroxide (DCP) as an initiator resulted in severe degradation of PP, and the melt flow rates of grafted PP increased gradually with increasing DCP concentration. BPO resulted in higher gel percentages than those of DCP at a fixed initiator concentration. Introduction of styrene into the grafting system greatly improved the gel percentage of crosslinked PP and reduced the degradation of PP during grafting. The optimum molar ratio of styrene to monomer is at about 1.5:1. Relatively low processing temperatures and high screw speeds are favorable. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1233–1238, 2000

Preparation and Characterization of a Branched Bacterial Polyester

The preparation, rheological behavior, and solution properties of branched poly(3-hydroxy butyrate)-co-(3-hydroxy valerate), PHBV, are described. Branching is produced by the use of dicumyl peroxide (DCP) during the reactive extrusion of PHBV. The shear viscosities of reactively extruded PHBV and the straight-chain polymer of similar molecular weight are nearly identical, but the reactively extruded PHBV's elongational viscosity is considerably higher. High elongational viscosity of these polymers results from a branched structure produced when extrusion is carried out in the presence of small amounts of DCP (0.025−0.3 wt %). Strain hardening is observed in the elongational response of the branched PHBV at all deformation rates and increases with increasing concentrations of DCP. Study of the solution properties of the linear and branched PHBV indicates larger hydrodynamic volume compared to that of polystyrene of comparable molecular weight. Mark−Houwink plots, observed for PHBV, exhibited increasing nonlinearity with increasing DCP content. Trifunctional branching frequencies computed on the basis of intrinsic viscosity ratios indicated explosive growth in branching, possibly due to the generation of microgel, when branching reactions are initiated with a DCP level exceeding 0.2 wt %.

Effects of curing conditions on the properties of the dynamically cured EPDM/HDPE blends

Effects of curing conditions on the properties of the dynamically cured blends of the high density polyethylene (HDPE) and ethylenepropylene-diene terpolymer (EPDM) were studied. The blends were prepared using a curing die where EPDM was cured with dicumyl peroxide (DCP) in the presence of HDPE at different shear rate. The pressure drop for the constant flow rate across the curing die increased with increasing DCP concentration and EPDM content and decreased with increasing shear rate in curing die. The melt viscosity obtained by Capillary Rheometer increased with increasing DCP concentration for all compositions and decreased with increasing shear rate in curing die. The flow behavior index decreased with increasing DCP concentration and increased with increasing shear rate in curing die. The mechanical strength increased with increasing DCP concentration. The morphology of the blends was investigated by the scanning electron microscope (SEM).

Tensile properties and morphology of the dynamically cured EPDM and PP/HDPE ternary blends

AbstractThe tensile properties and morphology of the polyolefin ternary blends of ethylenepropylene–diene terpolymer (EPDM), polypropylene and high density polyethylene were studied. Blends were prepared in a laboratory internal mixer where EPDM was cured in the presence of PP and HDPE under shear with dicumyl peroxide (DCP). For comparison, blends were also prepared from EPDM which was dynamically cured alone and blended with PP and HDPE later (cure–blend). The effect of DCP concentration, intensity of the shear mixing, and rubber/plastics composition was studied. The tensile strength and modulus increased with increasing DCP concentration in the blends of EPDM‐rich compositions but decreased with increasing DCP concentration in blends of PP‐rich compositions. In the morphological analysis by scanning electron microscopy (SEM), the small amount of EPDM acted as a compatibilizer to HDPE and PP. It was also revealed that the dynamic curing process could reduce the domain size of the crosslinked EPDM phase. When the EPDM forms the matrix, the phase separation effect becomes dominant between the EPDM matrix and PP or HDPE domain due to the crosslinking in the matrix.

Rheological properties and crystalline structure of the dynamically cured EPDM and PP/HDPE ternary blends

AbstractThe rheological properties and crystalline structure of the polyolefin ternary blends of EPDM/polypropylene/high density polyethylene were studied. Blends were prepared in a laboratory internal mixer by two different methods. In blend–cure process, blending and curing were performed simultaneously and EPDM was cured by dicumyl peroxide (DCP) in the presence of PP/HDPE under shear. The cure–blend was to cure EPDM alone first under shear (dynamic curing) and then mix the cured EPDM with PP and HDPE. The effect of DCP concentration, intensity of the shear mixing, and the rubber/plastic composition were studied using capillary rheometer and X‐ray diffractometer. The PP‐rich ternary blends showed the effect of the mechanooxidative degradation of PP by shear and peroxide. The melt viscosity increased with increasing DCP concentration in blends of EPDM‐rich compositions. X‐ray diffraction studies revealed that the inclusion of 25 wt % of linear EPDM in the PP/HDPE mixture for the PP‐rich ternary blends changed the crystal structure of polypropylene component in the ternary blends. However, the dynamic curing did not alter the crystal structure of PP or HDPE in the blends.

Structure and properties of dynamically cured EPDM/PP blends

AbstractThe structure and properties of polyolefin blends of ethylene–propylene–diene terpolymer (EPDM) and polypropylene were studied. Blends were prepared in a laboratory internal mixer where EPDM was cured with PP under shear with dicumyl peroxide (DCP) at different shear conditions (blend–cure). Blends were also prepared for comparison from EPDM which were dynamically cured in the absence of PP and blended later (cure–blend). The effect of DCP concentration, intensity of the shear mixing, and rubber/plastic composition were studied. In blend–cure, the melt viscosity increased with increasing DCP concentration in blends of 75% EPDM and 25% PP, but it decreased with increasing DCP concentration in blends of 75% PP and 25% EPDM. In cure–blend, however, the melt viscosity increased with increasing DCP concentration for all compositions. The melt viscosity decreased with increasing intensity of the shear mixing presumably due to the formation of the smaller segregated microdomain of the crosslinked EPDM gels in both blend–cure and cure–blend materials. The crystallization rate was higher in EPDM/PP blends than in PP homopolymer. The crystallization rates for various blending conditions were also compared.