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Though polyethylene is widely used for rotational moulding process, rotational moulding sector is a small application of polyethylene. Rotational moulding process requires a very long processing time. In this process, the material remains in contact with oxygen present in the air that is present in the mould. Longer processing time would induce oxidative degradation due to the reaction of oxygen with PE chains. Impact strength particularly of large rotational moulded products such as water tanks is very essential. PE grades that are suitable for rotational moulding could have good initial impact strength. However, the impact strength could drop very drastically due to oxidative degradation. Many times rejection takes place after the product is put into market. Besides good antioxidant system the grade should have broad molecular weight distribution in order to have broad processing window.
The grade suitable for rotational moulding process needs to be very carefully balanced with a large dosage of process antioxidant in the material. The performance antioxidant is also essential to provide adequate life span to the rotational moulded products. Typically, medium density PE with density ranging from 0.935 to 0.945 is used with MFR varying from 2-6. Lower MFR provides better properties and is used for large articles, while higher MFR material is used for small articles and for improvement of productivity.
Processors need to pay sufficient attention to the antioxidant system in order to have good impact strength of rotational moulded products not only in the initial phase but even in actual use. |
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