Polymer processing film blowing

Andre, J. M. ct al., 1998. Numerical modelling of the polymer film blowing process. Int. J. Forming Processes 1, 187-210. 

H. W. Jung and J. C. Hyun, Fiber Spinning and Film Blowing Instabilities, in Polymer Processing Instabilities - Control and Understanding, S. G. Hatzikiriakos and K. B. Migler, Eds., Marcel Dekker, New York, 2004, Chapter 11. 

V. Sidiropoulos and J. Vlachopoulos, Numerical Study of Internal Bubble Cooling in Film Blowing, Int. Polym. Process., 16, 48-53, (2001). 

Multi-layer films can be produced, composed of different polymers. For each polymer a separate extruder is applied the extruders discharge into an accumulator, in which the separate streams are guided to their own position in the film. A laminated film can thus be made, in which one of the layers provides the strength, another one acts as a gas barrier, other layers promote adhesion and weldability, etc. This laminating process is applied with flat film extrusion as well as with film blowing. 

Hong, Y., Coombs, S. J., Cooper-White, J. J., Mackay, M. E., Hawker, C. J., Malmstrom, E., and Rehnberg, N. 2000. Film blowing of linear low density polyethylene blended with a novel hyperbranched polymer processing aid. Polymer, 41, 7705-7713. 

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. 

Biodegradable polymers are similar in terms of their chemical structure to conventional thermoplastics such as polyethylene, polypropylene and polystyrene. They can be processed using standard polymer processing methods such as film extrusion, injection moulding and blow moulding. 

During stretching flow, material is drawn from one cross-sectional area to another. This type of flow dominates fibre, film, blow molding, and vacuum forming processes (57). Let us assume that a filament of a molten polymer is hauled off under a force F (see Fig. 13.40). Taking the die of the extruder as the origin of the reference frame, the cross-sectional area A of the filament... 

In the film blowing process where a continuous stable parison is blown from an annular die, it is crucial that the molten polymer exhibits certain elastic extensional properties and it is here that the viscoelastic nature of the polymer is beneficial.If, however, the manufacturer is concerned with profile and surface finish of an extrudate, viscoelastic effects of the polymer may well present difficulties. Both die swell and most polymer extrusion instabilities are linked to viscoelastic effects and as such different levels of viscoelasticity give rise to different extrusion characteristics. 

Randomly branched polymers are of enormous importance for certain polymer processing operations, such as blow moulding and film blowing. The molar mass distribution of all randomly branched polymers is described by percolation models. For commercial randbmly branched polymers, the critical percolation model applies only very close to the gel point. The branching chemistry used in commercial randomly branched polymers is usually stopped far short of the critical region. While critical percolation does not apply to these polymers, the mean-field percolation model does a superb job of describing the molar mass distribution of randomly branched commercial polymers. 

Cellulose derivatives were the first thermoplastics formed in the film blowing process in the beginning of the 20th century. By the 1930 s, biaxially oriented films of cellulose acetate were commercially available. Forming of synthetic high polymers, especially LDPE, by the film blowing process began only in the 1940 s. 

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