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.
Matrix Ligament Thickness
Meaning
The minimum distance between the edges of adjacent rubber modifier particles dispersed within a polymer matrix determines whether the material will fail in a ductile or brittle manner under high-speed loading. This matrix ligament thickness must be kept below a specific critical value to allow the matrix to undergo plastic deformation instead of cracking. This value is characteristic of each polymer type and varies with the inherent stiffness of the polymer.
Maintaining the correct thickness is key to the design of high-impact polymers.
Deformation Mechanism
The physical state of the polymer between the rubber particles shifts from triaxial tension to plain shear when the matrix ligament thickness is reduced below the critical limit. In this state the stress fields around the particles overlap and trigger shear yielding throughout the matrix. This yielding absorbs the energy of an impact and prevents the formation of cracks.
If the thickness is too large the stress fields do not overlap and the matrix remains in a brittle state. This explains why the size and spacing of the modifier particles are so important for polymer toughness.
Material Formulation
Achieving the desired spacing requires careful control of the rubber content and the size of the dispersed particles during the compounding process. To keep the matrix ligament thickness low compounding extruders must break the rubber down into very small particles and spread them evenly through the matrix. This requires a high-shear screw configuration and proper viscosity matching.
If the rubber particles are allowed to coalesce the thickness of the matrix between them will increase which leads to a dramatic drop in impact strength. This drop can happen even if the total amount of rubber remains the same.
Production Parameter
The moulding process itself can affect the spacing of the modifier particles in the finished part. High melt temperatures or excessive dwell times can cause the rubber particles to run together which increases the matrix ligament thickness and makes the part brittle. Moulders must also control the amount of regrind they use since recycled materials can contain degraded rubber that cannot maintain the correct spacing.
Regulating the injection speed and the pack pressure helps to prevent the rubber particles from migrating or aligning in ways that increase the spacing in critical areas. Maintaining a stable and consistent moulding cycle is the best way to ensure that the material retains its high impact properties. If the cooling rate of the mold is too slow it can give the rubber particles more time to coalesce before the plastic freezes which can further hurt the toughness.
For this reason water cooling channels must be designed to keep the cooling rate high and uniform.