CRYSTALLISATION RATE

The stage of rapid crystallisation ends when crystallinity reaches approximately 0.5—0.7, ie, when HDPE is 50—70% crystallised. After that, the crystallisation rate falls drastically. The slower process that follows is known as secondary crystallisation (20), which can be described by the empirical equation CR = A - - S-log(t). Secondary crystallisation can be accelerated if HDPE is heated. 

Pecrystallisation of a copolymer having 15 wt % VC has been found to be nucleated by material that survives the melting process plus new nuclei (74). The maximum crystallisation rate occurred at 373 K the maximum nucleation rate at 283 K. Attempts to melt all the polymer led to degradation that interfered with recrystallisation. 

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. 

Eoams were extruded from low density polyethylene (LDPE) and blends of LDPE with syndiotactic polypropylene (sPP), using isobutane as the blowing agent. The extruded materials were characterised by measurement of dimensional stability at room temperature, density, tensile properties, dynamic stiffness, and crystallinity determined by differential scanning calorimetry. The sPP, with a slow crystallisation rate, did not interfere with the expansion of the LDPE, and enhanced the temperature resistance by in-situ crystallisation. The blends were flexible, dimensionally... 

The ratio of the crystallisation rates is 98/23 = 4.26, the ratio of the molar masses is 143/87 = 1.64. The ratio of the melt viseosities (which govern the erystal growth) amounts to 1.64 1 = 5.38. Apparently the rate of crystallisation is, next to the rate of nucleation, governed by the melt viscosity. 

In contrast to isotactic polystyrene, syndiotactic polystyrene crystallises very rapidly (it contains ca 72% crystalline domains) with a crystallisation rate... 

Tietz and Hartel (2000) studied the effects of removing or adding minor components naturally present in milk fat on the crystallization of milk fat-cocoa butter blends. They suggested that at low concentrations, minor lipids act as sites for nucleation and promote the rate of crystallisation and at higher concentrations inhibit crystallisation. They concluded that the presence of minor lipids, at the concentrations naturally occurring in milk fat, were sufficient to affect crystallisation rates, chocolate microstructure and fat bloom formation in chocolate. 

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. 

Fig. 8.29. The influence of temperature cm the crystallisation rate of (a) zeolite A and (b) ZSM5. From...

Polyester is not a fast crystallising polymer. The rate of crystallisation is maximum for polyester at around 180 C. The maximum crystallisation rate for polyester is about 0.016/sec, while it is 0.14/sec for nylon 6. The following changes take place as a function of increasing temperature in polyester when heat-set under free to relax conditions (i.e. free annealing) and when held taut at constant length (i.e. taut annealing) ... 

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