Higher strength, lower weight materials - maximising the benefits of reinforced thermoplastic foams

TWI Bulletin, March/April 1991

Gareth McGrath joined TWI's Engineering Department in 1989. He has been involved with adhesives, composites and polymers for the last six years, after an initial career in metallurgy. His move to composite technology began during a research assistantship at Sheffield City Polytechnic where his thesis was devoted to reclamation prospects for advanced thermoplastic composites. This concluded with a predictive model and component fabrication. During this period he also completed a number of industrial research contracts on polymers.

Since joining TWI, Gareth has been developing techniques necessary to explain composite failure. These have included fracture toughness measurement techniques, impact damage analysis, and study of the time dependent response of joints in thermoplastics and composites. He has recently been appointed to a technical co-ordination function for the expanding adhesives activities at TWI.

Reinforced thermoplastic foams offer an attractive combination of properties in a range of engineering applications. Gareth McGrath outlines the advantages of these materials.

For many years glass fibre reinforced thermoplastics have successfully bridged the materials gap between reinforced thermoplastics and the more conventional engineering materials such as steel and wood. More recently structural foams have been developed to reduce the weight of plastic products, at the same time providing a one-step route to a twin-component structure which can optimise properties at minimum cost.

An interesting development of recent trends has been to consider a combination of the two concepts (fibre glass reinforcement and foaming) and to use, for example, short glass fibres to minimise or even reverse some of the property sacrifices incurred in structural foaming. A comparison of the results of this concept is shown in Table 1.

Table 1. Comparison of the properties of high-density polyethylene (HDPE), glass fibre reinforced HDPE and their foamed versions

The reinforced structural foam has mechanical properties superior to those of the rigid resin but at the same density as the unreinforced foam. This gives a higher strength-to-weight ratio. It must be emphasised, however, that to achieve satisfactory properties, the reinforcement needs treating with a suitable coupling agent to ensure transfer of stress from resin to fibre for the thermoplastic being considered.

Fabrication methods

Injection moulding is used almost exclusively for fabrication of foams, although extrusion is possible. Reinforced or unreinforced thermoplastic foams are prepared by introducing a gas into the melt and allowing the gas to expand, so displacing the thermoplastic resin. Close control of the process is essential to form a fine, uniform cell structure as there is always the possibility that it will collapse to form one large void.

Many foaming processes have been developed and are available commercially. Most of the processes are suitable for use with glass fibre reinforced thermoplastics, providing that the equipment manufacturer is advised of the intention to use glass fibres, so that the moulds and machines are built with steels suitably treated to prevent excessive abrasion. This largely depends upon the thermoplastic being processed. Polypropylene-glass fibre compounds are virtually non-abrasive, but when nylon-glass compounds are processed abrasion can be a problem because of the increased viscosity. These are the two extremes of the scale.

These processes involve either low or medium-to-high pressure systems. Typical pressures involved are approximately one tenth and one-third, respectively, of the solid injection moulding pressures.

Low pressure processes

These are used mainly in Europe: a quantity of melt, less than that required to fill the mould and containing a dissolved inert gas is injected so that expansion proceeds under lowered pressure and the fixed mould is filled. As the dissolved gases come out of the solution, foaming results. A solid skin is formed on the outside surface as the cells collapse in this region.

Three variations of the basic process dominate current practice:

1. Solid chemical blowing agent is mixed with polymer and processed in specially constructed moulding machines. The main features of these machines are an efficient shut-off nozzle system and low mould locking forces. The process is outlined in Fig.l. In this instance, the gas is generated by chemical decomposition of the blowing agent in the hot melt.
2. Solid chemical blowing agent is mixed with polymer and processed in a conventional high pressure injection moulding machine. Critical modifications are an efficient shutoff nozzle and good control over back pressure. Because of the high locking forces, which are unnecessary for foams, capital and running costs for producing reinforced thermoplastic foams are higher than if a specially designed machine is used. Against this is the versatility offered by a choice of thermoplastic output, i.e. solid or foamed components.
3. The Union Carbide process: an extruder which usually has injection of an inert gas midway along the barrel runs continually to feed the pressurised melt to an accumulator. Here it is held under pressure to keep the gas in solution until the charge volume is reached, whereupon it is injected into the mould which it fills by expansion.

Structural foams have an extended cooling period which may affect adversely the economics of the process. One way of overcoming this is to use a multi-station rotary machine, which is particularly efficient for production of smaller parts.

Medium-to-high pressure processes

Numerous other processes have been developed to improve surface finish, including the following:

1. The counter-pressure process: the cavity is pressurised by an inert gas prior to filling, so degassing is avoided. As injection proceeds a manifold is opened, thus allowing expansion of the thermoplastic melt and foam formation. The component densities are generally increased because of the increased skin thickness provided.
2. Thermal cycling of the mould, developed by BASF and Krauss-Maffei and known as the Vanthem process: formation of a solid skin is prevented by heating the mould during injection. The skin is then formed by rapid cooling of the mould after injection of the thermoplastic melt.

Structure

Thermoplastics are foamed by introduction of gas into a melt. Reinforced thermoplastic foams are characterised by a density gradient from the core of the foamed structure to the surface. The cooling conditions produce this density gradient and variation of the cooling rate produces different characteristics in the finished moulding.

Slow cooling allows greater time for cell growth and therefore the largest cells are found in the cooled region where heat dispersion has been slowest. On the contrary the cooler mould-melt interface promotes higher cooling rates, and this together with the high shear rate along the mould walls, gives a solid skin of high density. So a density gradient is created. As the cells become smaller towards the surface of the moulding the proportion of thermoplastic in the melt, and therefore its density, increases. Thus the physical properties of a foamed structure depend on the internal density gradient as well as the overall density.

It is essential to keep the cell formation under strict control at all times, to achieve a moulding with a high-density skin and a low-density core without large voids. The presence of glass fibres greatly aids this control. As the cells grow, the fibres are pushed back, mostly re-orientating themselves parallel to the cell walls. The orientation of fibres in the moulding is generally less ordered and more even than that found is solid injection mouldings. This produces more uniform properties in the finished component.

A growing bubble uses part of its energy to push back the fibres and therefore will grow less than it otherwise would in an unreinforced foam. A fine cellular structure, free from voids produced by cellular collapse, is promoted.

Although properties are more uniform for a constant cross-section moulding, where section changes are encountered, it is usual for the thicker sections to have somewhat lower densities than thinner sections or areas near gates. Parameters which have the greatest effect on this variation in density are the injection speed and final required weight of the moulding. Failure under tension in mouldings containing variations in cell density is nearly always at the point of lowest density, assuming constant cross-sectional area, and that failure does not occur at a void.

The density changes from core to surface, but this is not the complete picture, as the actual shape of the cells also varies, being near-spherical in the core and elongated nearer the surface with virtually no cells at all in the skin region.

Interaction between processing and microstructure

To understand the advantages and disadvantages of glass reinforced thermoplastic foams it is helpful to consider how the cellular structure is formed during the moulding process. The saturated solution of thermoplastic, glass fibre and gas is allowed to expand in the mould. While the expansion is taking place the melt is cooling and solidifying and this stabilises the cell structure. The final structure has a cell size distribution as seen in Fig.2, with the cells at their largest at the centre, decreasing in size towards the walls, and finally disappearing in the skin.

Reinforced glass fibres have two significant effects on the mechanism of growth as follows:

1. The apparent viscosity of the melt is increased and thus maintenance of a saturated melt solution is assisted since more energy is required for cell growth. This creates a fine cell structure and avoids cell collapse as the expansion proceeds.
2. A growing cell uses part of its energy to displace and partially to orientate the glass fibres and therefore the cell grows less than in an unreinforced foam, creating a finer cellular structure, free of central voids. As expansion proceeds the glass fibres are distributed in a partially random manner orientating themselves mostly tangentially to the cell walls as seen in Fig.3. This produces cell walls reinforced with glass fibres and thus increased compressive strength. A high strength-to-weight ratio is produced in the composite.

Toughening effect

The presence of a proportion of reinforcing fibres with a length equal to or greater than the critical length significantly improves the toughness of a thermoplastic foam. Economic consideration of this phenomenon results in the use of cheaper fibres, such as glass, to promote toughening while using stronger, more expensive fibres, such as carbon, to promote strengthening. Use of talc as a filler increases stiffness in polypropylene foams.

Reinforced thermoplastic foams absorb impact energy via the cellular structure and, because the cells are discrete energy absorbers, very little crack propagation is possible. This results in a very slight fall in impact properties over a large range of density reductions. Providing that suitable coupling agents are used with the reinforcements, the glass fibres may also act as crack arresters by means of crack blunting mechanisms.

Advantages of fibre reinforcement

Addition of glass fibres to a thermoplastic foam increases the apparent viscosity and the thermal conductivity resulting in improved cell size control, shorter moulding cycles and hence more uniform mechanical and physical properties.

Unreinforced thermoplastic foams are suitable only to replace metals and other materials in low-stressed applications, but addition of glass fibre reinforcements to the foam produces a composite ideal for more highly stressed conditions. The special features of these glass fibre reinforced thermoplastic foams are numerous:

1. Glass fibres increase the strength of the cellular structure, adding back the strength normally lost when unreinforced thermoplastics are foamed. The glass fibres orientate themselves at random tangentially to each cell wall ( Fig.3), thus reinforcing it individually, as explained above.
2. They are relatively free from moulding stresses. In production the mould is filled by internal expansion of the material, minimising orientation of the thermoplastic molecular structure and hence orientation and packing stresses in the component.
3. They are ready to mould on existing machines. Glass fibre reinforced thermoplastic foams can be moulded on existing injection moulding machines with a positive shut-off nozzle to prevent escape of gas from the cylinder. Chemical blowing agents compounded within the resin are activated in the moulding process and expand to form the cellular structure within the component.
4. They have high strength-to-weight ratio and excellent creep resistance. When reinforced thermoplastics are foamed, properties decrease proportionally to the amount of foaming, but remain several times superior to those exhibited by the same thermoplastic foam without reinforcement. The tensile strength of the foam may be increased by a factor of 1.6-2.5 by addition of glass fibres, depending on the particular thermoplastic being reinforced.
5. Short cycle times are possible. Glass fibre reinforcements stiffen the outer skin of the foam, a result of the higher modulus of glass fibre reinforced thermoplastics. This allows earlier ejection of the components which can then be cooled out of the mould. Post-blowing or cauliflowering of the component must however be avoided if this method is used.
6. Retention of nails and screws is improved. As the nail or screw enters the cellular structure of the component, cells collapse around it, and this creates better retention characteristics. Furthermore, as glass fibre reinforced thermoplastics tend to compact when impacted and therefore do not propagate cracks so readily as their unreinforced counterparts, they may be nailed together, where it would be disastrous to nail conventional thermoplastics.
   Components can be designed with fewer ribs and bosses because of the improved nail and screw retention characteristics. This also helps reduce cycle time.
7. Warp-free components are easily produced. When moulding with high pressures, as in conventional injection moulding, internal stresses are introduced into the components, and this frequently produces warping after ejection. However, the low pressures used to mould reinforced thermoplastic foams produce very little stress in the components and warp is almost eliminated.
8. Sink marks are virtually eliminated. The material is allowed to expand freely to fill the mould completely, and the resultant internal pressure in the melt produces a sink-free component.
9. Thermal insulation is increased. Frequently the insulation properties of the reinforced thermoplastic foam are better than those of the analogous solid moulding, but the significant reductions in thermal conductivity necessary for true insulation applications are only possible at great density reductions and this results in significant losses in physical and mechanical properties.
10. Acoustic damping is increased. Reinforced structural foam components provide useful acoustic damping which has obvious benefits. Vibration noise can also be minimised.
11. Lower tooling costs are possible. These may be achieved by use of cast aluminium as a mould material, this being possible because of the low internal pressures necessary in the mould. Particular advantage is obtained in short-run applications, where high tooling costs would otherwise preclude use of injection moulded reinforced thermoplastic foam. Glass fibres are, however, highly abrasive and this may offset any advantage gained from lower tooling costs if continuous or semi-continuous production is required because of the down time in mould changes. The savings do however depend on the thermoplastic being processed and the tolerances required in the final component.
12. Reinforced thermoplastic foams produced by low-pressure systems tend to have a characteristic swirled surface finish, which is desirable, when suitably pigmented, for wood grain effects. A textured mould finish also helps to complement this finish, but care must be taken not to introduce notches, which would act as stress raisers, reducing the impact properties of the component.

Finishing processes

It is vital to allow reasonable time between demoulding and finishing so that any residual pressure in the cellular structure equilibrates and does not disturb subsequent operations. This time is dependent upon the reinforced thermoplastic being produced.

Painting is preferred for finishing, but the swirled surface may be a hindrance to this, if it is necessary to produce a smooth finish. A smoother finish is produced by raising the mould temperature. More expensive and sophisticated machines are available which make possible further enhancement of surface finish. If a high quality paint finish is required then surface preparation, priming, high built base coats and even sanding before the final finishing coat are necessary. This is expensive and the economics of these operations often mean that it is cheaper to improve the surface finish by improved moulding techniques.

Thermoset or thermoplastic matrix

Processing of both reinforced thermoplastic and thermoset foams is similar, with relatively low mould pressures being involved in both systems, the working pressure arising from degassing of the melt or liquid and not from the injection pressure.

The most obvious difference between the two systems is that thermoplastics are used in pellet or powder form whereas thermosets are frequently liquid systems.

Most versatile and widely used of the thermosets are polyurethanes. The physical and mechanical properties can be tailored by suitable density reductions and by adding reinforcement to produce components suitable for a wide range of applications.

However, a comparison of the two types is complex and generalisations are misleading and unrealistic. Each application must be evaluated individually, with reference to surface quality, strength, and heat and flame resistance.

Thermoplastics have the advantage that, under normal operating conditions, any rejected components can be recycled by grinding and further processing with new polymer, particularly if the subsequent component surface is to be painted or foil coated.

Applications

The excellent mechanical and physical properties, together with the dimensional stability, of reinforced thermoplastic foams provide a wide variety of applications in fields which were originally thought to be the domain of unreinforced foams until inherent failings were fully realised, e.g. considerable losses in mechanical properties.

Addition of fibre reinforcements has offset many of these failings, and designers, moulders and material suppliers tailor the additives to achieve the most suitable composite for a given application. Common additives are glass fibres, talc, calcium carbonate and rubber modifiers. Not only are mechanical and physical properties improved, but additives also act as nucleation sites, promoting cell formation and uniformity during moulding. Short glass fibres are at present the main source of reinforcement in structural thermoplastic foams, although carbon and synthetic fibres are also used.

A rotating drum

With these factors in mind, the feasibility of using reinforced thermoplastics foam for moulding the cylindrical outer tank of a front-loading washing machine was considered by ICI, Philips and Cabinet Industries. The inherent design difficulties involved in the manufacture of a horizontally rotating drum in metal include large investments in presses, tools and welding equipment. Possible materials are vitreous enamelled mild steel or stainless steel suitably reinforced to carry the load.

To replace the separate fabrication operations, 16 in all, by one single moulding was obviously very attractive, lowering the capital investment considerably. But to do this the designers had to ensure that the tank was capable of fulfilling the necessary requirements of watertightness, corrosion resistance and strength to carry the washing load and ancillary equipment.

The material selected was fibre glass reinforced polypropylene, which has outstanding chemical resistance, good heat stability and resistance to abrasion and fatigue. An added advantage is a reduction in noise from the machine because of the acoustic deadness of polypropylene. This design has proved very successful and other washing machine manufacturers are now involved in development of reinforced thermoplastic drums, although not all developments are concerned with thermoplastic foam but with solid resin components. This is because of the possibility of achieving faster cycle times with certain designs of solid moulding.

Figure 4 compares the former vitreous enamel drum with a new reinforced polypropylene foam drum. Although this represents a large part of the present reinforced thermoplastic foam market, other applications are numerous and can be classified as below.

Static loading

Successful applications include a desk support moulded by Hollis Plastic Ltd giving a functional component with good rigidity and stiffness at reasonable cost, and the Sharna Ware wheelbarrow. Although mobile, the main loading is static and the wheelbarrow has a rigid framework with high stiffness. It is tough, resilient and light. The wheelbarrow is offered in a choice of colours, which remain attractive after prolonged outdoor use, all features difficult to achieve with metals. Chairs produced by Wetherell Plastics are intended for the large-quantity prestige market and need to be rigid with good stiffness while being cost effective and available in various styles.

Dynamic loading

Probably the first commercial application of glass reinforced thermoplastics foam was the ACE canoe paddle, designed by Robin Witter, the British double canoe slalom and Olympic coach. The major design features were low weight, good tensile and fatigue strength, flexural stiffness, impact resistance to sharp objects and weather resistance.

High temperature

The electrical industry also uses electric brake controllers, with resistors that can reach 200°C, manufactured from suitable glass fibre reinforced thermoplastic foams.

Combined conditions

Designers in the automotive industry have successfully used reinforced thermoplastic foam in several areas:

1. Glass fibre reinforced polycarbonate foam to manufacture door sills and certain roof sections;
2. Instrument panels, permitting reduced panel complexity;
3. Seating for trucks and tractors;
4. Bonnets and exterior panels.

A trend towards reduction of interior noise levels in commercial vehicles favours use of housings and panels manufactured from polypropylene foam because the stiffness and mechanical inertness of the panels effectively damp out transmitted vibrational noise to which metal panels are particularly prone. It is not, however, normally necessary to reinforce these mouldings.

Summary

Practically any foamable thermoplastic can be reinforced by up to 30% fibres. Suitable thermoplastics include high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), acrylonitrile-butadienestyrene (ABS), styrene-acrylonitrile (SAN), polycarbonate (PC), polyphenylene oxide (PPO) and nylon. Table 2 summarises the main applications.

Table 2. General applications of fibre reinforced thermoplastic foams

The future

Future growth in applications of reinforced thermoplastic foams depends primarily on the automobile and domestic appliance markets and these industries have a sufficient flow of new models to justify trials with new engineering materials. A significant move is the use of solid mouldings, wherever feasible, to reduce moulding times, particularly in automobile seating.

Generally the future of reinforced thermoplastic foams is uncertain. However, there is no doubt that where low densities combined with good mechanical properties are required, they are very strong candidate materials.