Balancing Stiffness with Toughness
The chemical structure of Polypropylene means that it inherently shows high stiffness but can be brittle and not exhibit the impact performance of Polyethylene.
RMPP141 has been designed to optimise these properties, giving moulded products both good stiffness and impact performance.
All-Round Mechanical Performance
The Tensile Modulus of a polymer shows its resistance to elastic deformation under load.
Polypropylene has a superior tensile modulus to polyethylenes. The combined properties of impact and stiffness result in RMPP141 leading the way when compared to other rotational moulding grades of polypropylene.
Heat Deflection Temperature (HDT) is the temperature at which a polymer sample deforms under a specified load.
The HDT of polyethylene is lower than that of polypropylene and should not be used in high temperature environments. The HDT of RMPP141 has been tested to show that it is at the upper level for rotomouldable polypropylene.
The Association of Rotational Moulders-designed Impact Test requires a machined dart to be dropped onto a rotomoulded plaque. The drop height is increased until the sample plaque breaks under the dart impact.
This test is performed on test plaques which have been conditioned at room temperature, freezing point and below freezing point.
Polypropylene does not exhibit the low-temperature impact performance of polyethylene, but RMPP141 has been shown to perform better than most polypropylenes commonly used in rotational moulding.
The Full Notch Creep Test (FNCT) is a widely used method to classify materials in regard to their slow-crack growth behaviours under accelerated conditions. The aim of the FNCT is to assess resistance to brittle failure.
This test shows that polypropylene, and particularly RMPP141, has vastly superior resistance when in contact with surfactants. It is essential that testing is conducted with any chemical that will be in contact with the finished product at its operating temperature.
Pipe Pressure Test to ISO 21138
A DN63 pipe made from RMPP141 was subjected to hoop stress of 3.9 MPa at 60oC (140oF) and has exceeded 4,000 hours. This demonstrates that RMPP141 greatly exceeds the minimum pressure requirements nominated in the ISO standard for rotational moulding grades of polyethylene.
This indicates that RMPP141 can withstand higher pressures, temperatures and top loading (long term creep) than polyethylene.
Long Term Creep (Load) Strength
Polymer materials exhibit time-dependent behaviour. When a plastic material is subjected to a constant load it deforms continuously. The material will continue to deform slowly or until rupture or yielding causes failure. All plastics creep to an extent.
Polypropylene has inherently better resistance to creep than polyethylene.
The creep test allows for extrapolated data which shows that, even after 20 years, RMPP141 remains superior to polyethylene after 1 year.
The (short term) Ring Stiffness is determined by measuring deflection and force applied to a section of pipe, manhole or tank. It is compressed in a radial direction at a constant rate.
Polypropylene, being inherently higher in modulus than polyethylene, has higher ring stiffness. RMPP141 outperforms polyethylene to such an extent that, for the same ring stiffness, 30% less material can be used.
Testing the integrity and strength of a tank is best carried out by way of a vacuum test.
This demonstrates the exceptional performance of RMPP141 and its superiority over polyethylene for tanks.
Gloss, Hardness, Abrasion and Scratch-Resistance
Scratch-resistance is better in a material with higher surface hardness and higher tensile strength, resulting in reduced rate of abrasion. Polypropylene is superior to polyethylene in these attributes.
RMPP141 also displays higher gloss in moulded parts than polyethylene. With the difficulty in painting polyolefins, this becomes important for aesthetic applications.