Crystallisation blow moulding

Processing temperatures should not exceed 180°C, and the duration of time that the material is in the melt state should be kept to a minimum. At the end of a run the processing equipment should be purged with polyethylene. When blow moulding, the blow pin and mould should be at about 60°C to optimise crystallisation rates. Similarly, injection moulds are recommended to be held at 60 5 C. 

Starch can be destructured in the presence of more hydrophobic polymers such as aliphatic polyesters. Aliphatic polyesters with low melting points are difficult to process by conventional techniques such as film blowing and blow moulding. Films such as polycaprolactones (PCL) are tacky as extruded and have a low melt strength (over 130 °C). Also, the slow crystallisation of the polymer causes the properties to change with time. Blending starch with aliphatic polyesters improves processability and biodegradability. 

This monomer (diol) is used to broaden the processing window, increase line speeds, slow the crystallisation rate and reduce stress in biaxially orientated structures. It is used in concentrations ranging from 1% up to about 18%. The higher concentration PET is usually referred to as PETG and is mainly used for extrusion blow moulding and sheet extrusion. Polymers from PTA and CHDM are approved in the US by 21 CFR 177.1315 recently the use of CHDM as comonomer in PTA-EG polymers has been listed in the US FDA regulation for PET (21CFR 177.1630). Moreover, the use of CHDM has been approved in Europe for plastics intended for food contact applications since the first directive 90/128/EC. 

In an extrusion blow-moulded polyethylene container with a 1 mm wall, about 5 s elapse before crystallisation takes place at the inner surface (Eq. 5.2). During this time, the melt tensile stresses relax, so the microstructure will be spherulitic, even though the spherulites may be somewhat distorted. In contrast there is high orientation in the wall of a stretch blow-moulded PET bottle (Section 2.4.7) because crystallisation occurred while the preform was stretching. 

These materials have better resistance to stress cracking that PE or PP, superior low-temperature toughness compared with PP or PE, better fatigue resistance than PP, and are of low density. The disadvantages are that crystallisation continues for 50 hours after moulding, and the low surface hardness. Applications include wire covering, film, and blow-moulded bottles. 

The crystallisation behaviour of blow moulded PETP bottles, which helps determine the product s transparency, was investigated by DSC dynamic cooling experiments that simulated the cooling that occurs in the injection blow moulding manufacturing process. DSC measurements were used to obtain information on related aspects, such as the ease of crystallisation from glassy and molten states and crystallinity in the products. An Avrami equation was used for calculation of the crystallisation kinetic parameters. 40 refs. 

Biaxial orientation of PET containers by mechanical means - axially by a stretch rod, radially by high-pressine air- during injection stretch blow moulding process introduces strain-induced crystallinity. Strain-induced crystallisation has a direct influence on the level of crystallinity of the final product. Factors, which influence the crystallinity level and the morphology, include the ISBM mould temperature, blow pressure and time to stretch the preform. 

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