Ethylene Propylene Rubber Domain Morphology Tuning for Low Temperature Container Impact Survival
Tuning EPR domains to 0.4-0.8 microns with 45 percent ethylene content ensures sub-zero container shock survival at minus 30 degrees Celsius.
Interparticle Distance
Meaning
The average spatial separation between the surfaces of dispersed elastomeric or mineral modifier domains within a polymer matrix determines the transition between brittle and ductile failure under impact. This interparticle distance is a critical design variable for impact-modified plastics like rubber-toughened nylon or polypropylene. If this spacing is kept below a critical value the stress fields around the particles overlap and allow the matrix to deform plastically.
When the spacing exceeds this value the material behaves in a brittle manner.
Stress Interaction
The physical stress state in the matrix between the modifier particles change from triaxial tension to plain shear as the particles get closer together. Maintaining a small interparticle distance means that the stress fields created when the part is stretched will overlap with each other. This overlap triggers shear yielding in the polymer matrix which absorbs a lot of energy and stops cracks from growing.
If the distance is too large each particle acts in isolation and the matrix between them fails through brittle cracking. This interaction explains why simple rubber content is less important than how the rubber is distributed.
Compounding Influence
The final spacing between the dispersed phases is determined by the formulation and the mixing efficiency of the compounding extruder. To achieve a low interparticle distance the modifier must be broken down into small droplets and spread evenly throughout the host polymer. This requires high shear screws and proper viscosity matching between the rubber and the polymer matrix.
If the compounding step is not done right the rubber particles can clump together which increases the average distance between them. This clumping lowers the impact strength of the finished part even if the overall rubber percentage remains the same.
Processing Variable
Moulding conditions can alter the spacing and distribution of the toughening phase in the finished part. When melt temperatures are too high or when excessive regrind is used the rubber domains can coalesce which increases the interparticle distance and reduces toughness. High shear during injection can also cause the particles to align in the flow direction which changes the distance depending on the direction of measurement.
Moulders must monitor and control parameters like screw speed and injection pressure to make sure the rubber phase does not clump together. Maintaining a stable process is key to keeping the impact properties consistent across the entire production run. When the cooling rate is too slow the resin stays hot for too long which can also cause the dispersed phases to move and coalesce.
Therefore fast and uniform cooling is necessary to freeze the desired distribution in place.