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 the Polymer Processing Society Regional Meeting, Florianopolis, Brazil, 7th-10th November 2004

O K Ademosu and S F Bush

Abstract

Recent papers ^2, 3 have described the Rotofoam© process which combines in one stage the rotomoulding of hollow shapes with the in situ generation of foam to fill the cavities. The foam is usually polystyrene driven by a combination of blowing agents, but other systems may be used. This paper defines the material quantities and the processing variables needed to reconcile three competing objectives in making panels for applications in the storage and transportation of chilled and frozen foods. These objectives are maximum bending stiffness to weight ratio, maximum thermal insulation, and minimum manufactured cost. Results of experiments at both the laboratory and industrial scales are reported. Panel stiffness, short and long-term resistance to indenting, density and thermal conductivity measurements have been done to generate an algorithm for the overall optimisation of the process and product.

References

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

[2] S F Bush and O K Ademosu, Combined Foaming and Rotomoulding: the Rotofoam Process, Eurofoam Conference 7-10 July 2002, Manchester, UK.

[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 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.

Paper to Eurofoam 2002 Conference, 7th-10th July 2002

S F Bush with O K Ademosu

Abstract

Polymer foams are widely used in the manufacturing and construction industries to provide thermal insulation and/or to increase bending stiffness to weight ratios. The principal polymer foam systems used are expanded polystyrene and polyurethane. The former is commonly found in polystyrene cups where the combination of heat insulation and adequate stiffness are a great improvement over the PVC alternative. Polyurethane foams are used in shoe soles where their friction properties are the best of any common material, and in refrigeration where special formulations provide a “self-skinning” property. In this case, the reaction of the isocyanate and polyol constituents is controlled by the catalyst used to ensure that for a given blowing agent, a solid (closed cell) surface is obtained next to the metal of the refrigerator cavities being fitted. It is thus a highly convenient manufacturing process.

Rotomoulding is an established process for forming relatively large hollow structures by rotating a mould containing polymer powder (typically polyethylene) about two axes. The powder is distributed more or less uniformly over the mould surface by a combination of gravity and centrifugal forces. If the mould temperature is set with appropriate regard to the melting temperature range of the polymer powder, a viscous molten layer is built up next to the mould surface. The free surface of this layer gradually extends inward. Reducing the mould temperature to below the polymer melting point will give a solid skin which allows the hollow form to be removed. Footballs, and hollow planking are made this way. Dustbins are made by cutting one end off and using it as the lid. In the UMIST laboratory, a two axial rotational moulding rig was designed by W G Neilson.

Up to now, if a rotomoulded artefact is needed to be foam-filled to promote its strength and stiffness without sacrificing too much the insulating properties of a hollow interior, the hollow form has had to be made first in one operation, then, as a comparatively costly second step, removed from the mould and filled with polyurethane foam. The UMIST Rotofoam process allows the foaming step to proceed at the same time as the moulding step, giving a a solid outer skin of one material and a foamed interior made of another. Besides the sharp reduction in overall process time, and the avoidance of the special equipment needed in the present foaming setup, the principal materials used – polyethylene and expanded polystyrene – are considerably cheaper than self-skinning polyurethanes, and without their chemical hazards. A composite variant has also been devised [Ref 1].

The paper will describe experiments on the Rotofoam process at both the 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 and cold store doors all made on our industrial collaborator’s plant (Micropol Ltd in Stalybridge) will illustrate the results obtained in practice from this new industrial process.

Reference

[1] O K Ademosu and D R Blackburn, “An Investigation of the Production of Rotationally Moulded Composites”, 9th International Eurofoam Conference on Fibre Reinforced Composites, 26th-28th March 2002.

Paper to 16th Annual Meeting of the Polymer Processing Society, Shanghai, China, 18th-23rd June 2000

S F Bush with O K Ademosu, S A Harrison, J M Methven and S Smith

Introduction

The generally poor fire resistance of hydrocarbon polymers has greatly inhibited their application in places where human beings are expected to congregate in confined spaces. Underground transportation systems, leisure centres, and apartment blocks are all examples where the flammability of polymers has been implicated in substantial loss of life in different parts of the world.

Public concern is also focussed on the recycling and reuse of polymers issue. Arguably, used tyres constitute the single largest, most predictable and most intractable recycling problem in the mass car-owning parts of the world. This is because of the numbers involved (around one tyre per adult per year in the industrialised world) and the fact that as a thermoset, rubber cannot be reprocessed by melting into a new tyre or some other object.

The present paper introduces a new technology which aims to provide potential large scale applications of rubber from used tyres and to provide sufficient fire resistance to allow use in public areas. Besides the various mixing sequences involved, the technology encompasses in some of its forms the chemical grafting of the rubber on to other hydrocarbon matrices and the use of different fire retardants. This laboratory’s long-fibre reinforcement technology is also used to provide another degree of freedom in meeting both technical and economic goals^[1].

Fire resistance is typically characterised by the Limiting Oxygen Index (LOI) test^[2] and mechanical properties are characterised by tensile strength and stiffness.

References

[1] S F Bush, Long glass fibre Reinforcement of Thermoplastics”, International Polymer Processing 14 (3) 1999, 282-290

[2] Determination of flammability by Oxygen Index BS 2782 Part 1: Method 141: 1986m ISO 45891 – 1984

Paper to the Institution of Chemical Engineers Research Event/First European Conference, Edinburgh, 104, 4th-6th January 1995.

S F Bush, with O K Ademosu

Abstract

Process Pathways Analysis generalizes the familiar concept of reaction pathway to allow changes in physical state to be described in broadly the same way as chemical changes. While applicable to all chemical processes, the approach is particularly relevant to polymerization, where the product is required in specific physical as well as chemical forms, and where the phases in which reactions may be carried out – solution, suspension, melt, liquid, gas – greatly affect the viability of candidate processes. Five processes for making Styrene-Maleic Anhydride copolymers have been explored using the PPA approach. Experimental and predicted results are compared for key chemical and physical properties.

Paper (6-2-8) to the 10th International Annual Meeting of the Polymer Processing Society, Akron, USA, 5th-8th April 1994.

S F Bush with D R Blackburn, O K Ademosu, F B Yilmaz and P F Zhang

Introduction

Earlier papers^{(1, 2, 3, 4)} have described the factors affecting fiber-matrix cntacting, the organization of long fibers into coherent lace-like or mat-like structures, and the dependence of tensile strength on these two classes of variable. Equations for the mean number of touches¹ in the fiber structure and tensile strength² have been proposed which allow for the main variables present in moldings and extrusions, including differing matrix properties. Fiber management technology developed under the generic acronym SAFIRE⁴ (Self Assembling Fiber Reinforcement) has recently been applied commercially to practical moldings such as hard hats and pallets, in both of which examples impact strength is a key property.

Accordingly, a wide series of experiments has been carried out using the Izod method to determine the variation of impact strength as a function of the fiber length, fiber concentration, the fiber-matrix interface, fiber reinforcement structure and matrix properties. Both commercially available long-fiber granules and laboratory-compounded types have been used with different mold configurations and molding conditions for a variety of polypropylene and polyethylene matrices.

References

[1] S F Bush, Control of Fiber Structures in Melt Extrusion, 36 Ann Mtg Can Soc Chem Eng, Sarnia, Canada (1986) paper 32d.

[2] S F Bush, O K Ademosu, D R Blackburn, F B Yilmaz, Factors Affecting the Strength of Long-Fibre Reinforced Injection Moldings, Poly Proc Soc Eur Mtg, Prague (1992) paper 6-06.

[3] D R Blackburn and O K Ademosu, Factors Affecting Fiber-Matrix Contacting in Fiber-filled Granules, Poly Proc Soc, 9th Ann Mtg, Manchester 1993, Paper 6-14.

[4] S F Bush, Self Assembling Fibre Reinforcement (SAFIRE) processes, “Textile-reinforced Composites for Engineering”, Bolton Institute, Bolton, England, 12-14 January, 1994.

Paper at the Polymer Processing Society European Meeting in Prague, 21st-24th September 1992, Paper 6-06.

S F Bush with O K Ademosu, D R Blackburn and F Yilmaz

Introduction

To view the introduction, please click on the link: LFRmoldings