Speciality elastomers aid the formulation of polyolefin compounds.
Vistamaxx elastomers are compatible with both polyethylene and polypropylene in blends and Exxon says they can partially substitute for a variety of polymers including styrene block co-polymers and polyisobutylene, reducing compound cost and improving performance.
The elastic properties of Vistamaxx are a result of a predominantly amorphous ethylene propylene matrix laced with a network of fine, well-dispersed isostatic PP crystallites. This delivers elasticity, flexibility and impact toughness while maintaining clarity in PP random copolymer (RCP) and homopolymer (hPP) blends.
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A compound of Vistamaxx and RCP or hPP, has physical properties that will vary greatly depending on elastomer concentration. The compound becomes more flexible with increasing levels of the Vistamaxx elastomer (Figure 1). Tensile strength initially experiences a steep drop when the elastomer is added, due to the reduction in blend crystallinity (Figure 2). Conversely, there is a steep increase in elongation at break when the elastomer content is above 30 per cent due to it forming a co-continuous phase. Initial PP hardness is not greatly affected at low elastomer concentrations.
Adding Vistamaxx to RCP or hPP can improve the impact strength of the compound down to -20 degC. Even low addition levels provide increases in blend impact energy. At higher concentrations and temperatures above the glass transition temperature (-28 degC), a no break point in the Notched Izod test can be achieved. A high ethylene content and molecular weight are favourable characteristics for low temperature impact modification. Figure 3 highlights the impact strength for different blends at varying temperatures.
When the elastomer content is under 30 per cent, thermal properties are dominated by those of the PP. At this blend ratio, long term ageing does not significantly affect mechanical properties. Stickiness is observed, however, on high elastomer concentration blends (over 30 per cent) aged above 100 degC. The Vicat softening point of the blend is lower with increasing levels of elastomer concentration.
A low haze level compound can be produced when Vistamaxx is added to RCR Excellent mixing of dry blends during processing or melt blending is essential to optimise transparency. Vistamaxx can also help to reduce or eliminate stress whitening in hPP. Tests demonstrate that in concentrations at and above 30 per cent, stress whitening is eliminated.
Chemical resistance of Vistamaxx is described as excellent in terms of aqueous systems and non-hydrocarbon based fluids. Blend density has a significant influence on chemical resistance.
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Vistamaxx elastomers can be used as modifiers for styrenic block copolymer compounds. Their compatibility with PP, and with oil and filler loading capabilities, makes them particularly suitable for styrene ethylbutylene styrene (SEBS) compound modification. They increase both tensile and tear strength by up to 45 per cent, acting as a plasticiser and reducing raw material costs by replacing SEBS resin (Figures 4 and 5).
Elastomer grades with a high molecular weight and high ethylene content, such as Vistamaxx 6102, are recommended to maximise oil retention capability in compounds containing significant concentrations of process oil. Low molecular weight grades with high ethylene content, such as Vistamaxx 6202, are recommended to maintain processability of compounds containing small concentrations of process oil.
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ExxonMobil presented a webinar on using Vistamaxx for polymer modification and compounding during June. The webinar can be downloaded from www.ides.com/webinars.
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|Comment:||Speciality elastomers aid the formulation of polyolefin compounds.(compounding)|
|Publication:||British Plastics & Rubber|
|Date:||Jun 1, 2009|
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