Reports (1) and (2) to DTI Project No. 3530, Techno-economic and Process Models, 2008-2011

S F Bush

To see Report (1) please click on the link Nercor.

To see Report (2) please click on the link Nercor2.

Paper published by Colloids and Surfaces, A:Physicochemical and Engineering Aspects, Volume 263 (2005) 370-378

S F Bush and O K Ademosu

Abstract

Solid polymer foams are well-known materials used to provide insulation, packaging impact protection, low slip shoe soling and so on. This paper examines the nature of the foams produced when combined with the process of rotomoulding, long established as the means by which hollow polymer shapes are made. Rotomoulding refers to the fact that a mould with a meltable or sinterable powder inside it is heated and rotated about two axes at right angles to distribute the powder over the inside of the mould to form a skin. This heating phase is followed by a cooling phase.

The aim of the research reported here is to determine the conditions under which a holow moulding with a skin made from one polymer powder, in this case low density polyethylene, can be made at the same time as a foam made from another polymer is formed to fill the cavity but not to penetrate through the skin. The foam in this case is polystyrene with around 6% ^w/_w n-pentane pre-absorbed. The whole system is referred to as the Rotofoam© process.

Experiments on both the laboratory and the full industrial scales are reported. The Rotofoam laboratory kinetics rig allows the foam development to be seen by eye and by camera as a glass mould undergoes the two axes rotations. Temperatures inside the foam and in the mould are monitored via a system of slip rings and hollow axles.

Examination by SEM allows the micro-development of the foam to be seen and linked to a simple shoebox-like model of a foam cell which correlates well with overall foam density measurements. The model also ties together the heat flow needed to expand the foam and heat the polystyrene and polyethylene, with the heat transfer rates calculated from the material conductivities, the material path lengths and the imposed temperature difference between mould and foam.

Finally, the paper reports the results obtained by the use of foam control agents – hydrated salts in this case – which by release of steam during the heating phase act to retard the pentane-driven foam expansion until the polyethylene skin is formed. The diffusion of the steam through the cell walls into the foam cavities is briefly discussed.

Paper to EUfoam Conference, Paris, 5th-8th July 2004

S F Bush with O K Ademosu

Background

Rotomoulding is a mechanically simple, low pressure plastic processing operation for forming relatively large hollow structures by rotating a mould containing polymer powder either about two perpendicular axes or about one axis combined with a reciprocating back and forth motion along the second perpendicular axis. Polyethylene (PE) has been the principal polymer powder used owing to its low cost and easy availability. However, PE has a relatively low inherent stiffness compared with other thermoplastics although these have higher material cost. The use of fibre reinforcements to increase stiffness has recently been described [Ref 1]. However, the prospect of multi-layer mouldings using polymer foams within the hollow rotomoulding artefact to provide thermal and sound insulation as well as increasing bending stiffness to weight ratio is potentially very attractive.

The principal polymer foam systems used for sandwich structures are based on polystyrene or polyurethane. Solid polystyrene foam is created from solid beads containing dissolved gas which is released over a defined temperature range. Solid polyurethane foams are formed by the rapid reaction of an isocyanate with polyol in the presence of a blowing agent. Both systems currently involve a second state of manufacture after the hollow mouldings have been removed from their moulds.

As previously described [Refs 2, 3] the UMIST ROTOFOAM© process enables the foaming step to proceed without demoulding first, either in parallel with skin formation or immediately after. A major objective is to produce ultra light integral foams so that the panel bending strength to weight ratio is maximised. This is important in a host of industrial and commercial applications.

References

[1] O K Ademosu and D R Blackburn, Fibre-impregnation in Rotational Moulding & its effect on Mechanical & Thermal Properties, Advances in Materials and Processing Technologies, 2003.

[2] S F Bush and O K Ademosu, Combined Foaming and Rotomoulding: the Rotofoam Process, Eurofoam, 2002.

[3] S F Bush and O K Ademosu, Combined foaming and rotomoulding in the Rotofoam Process, Polymer Processing Society 19th Annual Meeting, Melbourne, Australia, 7-10 July 2003.

Paper to the 19th Annual Meeting of the Polymer Processing Society, Melbourne, Australia, 7th-10th July 2003

S F Bush with O K Ademosu

Abstract

Rotomoulding is an established process for forming relatively large hollow structures by rotating a mould containing polymer powder (typically polyethylene) either about two perpendicular axes or about one axis combined with a rocking back and forth motion along the second, perpendicular axis. Up to now, if such a rotomoulded hollow form needed to be foam-filled, the hollow form has had to be made first in one operation, demoulded, and then, as a comparatively costly second operation, taken to another station where it is filled with polyurethane foam. The UMIST Rotofam process allows the foaming step to proceed at the same time as the moulding step, giving a solid outer skin of one material and a foamed interior made of another. The paper describes experiments on the Rotofoam process at both laboratory scale and full-scale as rotation speeds, feed materials and temperature-time profiles are varied. Large bore steam pipe insulators, damage resisting post covers, cold store doors, harbour fenders and pallets, all made on our industrial collaborators’ plants will illustrate the results obtained in practice from this new industrial proces. A further variant – Rotofil – in which long glass fibre filaments are distributed into the polyethylene skin will also be briefly described.