Polymer processing

11.8 Polymer processing

Processing is extremely important in polymer technology in order to develop the optimum properties for the product. Starting with polymer powder/granules, it is the control of the various processes which develops the individual molecules and arranges them to maximum advantage. High rates of shear and close temperature control are a feature of all the methods shown in Figure 11.18 which are commonly used in thermoplastics. These include (a) injection molding of thermoplastics (e.g. PE, PS), (b) extrusion into bars, tubes and sheet, and (c) thermoforming into sheet and thin-walled hollow shapes, such as buckets, baths, car body components, boat parts and aeroplane accessories, with ABS, PS, PVC and PMMA.

Thermoplastics are highly viscous when subjected to the shear stress conditions during processing as they flow through the die. Typical rates of shear are 10–10 3 s −1 for extrusion and 10 −5 –10 −3 s −1 for injection molding. Flow is non-Newtonian and the shear stress/shear strain ratio (apparent shear viscosity) falls, giving rise to a pseudo-plastic behavior. Fluidity as measured by a melt flow index test

(MFI) 3 indicates that at the high stress a broad distribution of molecular mass leads to less pseudo- plastic behavior. Quantitatively τ

= cγ n , where c and n are constants, and since τ = ηγ, the viscosity η = cγ n −1 . The index n ranges from unity for Newtonian flow to <0.2 depending on the polymer.

The index decreases as the shear rate increases and the thermoplastic melt behaves increasingly pseudo-plastic.

In blow molding and filament drawing it is tensile stresses rather than shear stresses that predom- inate. In this case it is tensile viscosity, i.e. tensile stress/strain rate, which governs the dimensional stability of the product. During blow molding of LDPE the tensile viscosity rises with increase of stress and counteracts any tendency for wall thinning, whereas for HDPE and PP the reverse is true and wall thinning and rupture are a possibility. Dimensional stability can be a problem for extruded products, as strain energy is released on exiting from the die and elastic recovery takes place. This has to be allowed for in product design. For polymers with a wide molecular mass distribution, the elastic shear modulus is low and elastic recovery is appreciable but slow. For a narrow molecular mass distribution recovery is less but quicker.

3 The MFI is the weight (g) collected in 10 minutes when the heated polymer is extruded through a standard die.

Non-metallics II – Polymers, plastics, composites 563 Table 11.3 Common applications of polymers.

Useful properties Polyethylene

Blow molded containers,

Chemical resistant,

Injection molded crates,

electrically insulating,

Extruded pipes

tough, low coefficient of friction

Linear LDPE

Agriculture, horticulture and construction (silage sheets, tunnel housing, cloches, damp-proof membranes)

Ultra-high MPE

Artificial joints, surgical

Purity, wear resistant, tough

prostheses

PVC

Building and construction

Stiff, hard, low thermal

(window frames, cladding,

conductivity, weather

resistance PB

underground pipes)

Piping for underfloor

Heat resistant, degraded

by Cl − PCarbonate

heating

Transparent roofing

Dimensionally stable, water resistant, good impact properties

PP

Automotive components

Fatigue strength, stiff,

(bumpers, panels, fans,

tough, resistant to heat,

creep and chemicals PS

ducts, etc.)

Rigid heat-insulating foam

Dimensional stability, cheap, resists crazing

Resistant to chemical attack Polychloroprene

SSR

Synthetic rubbers, car tyres

Automotive seal, water

Resistant to oil-heat

circuit pipes

Silicone rubber

Medical implants,

Resilience, chemical

gaskets seals, coatings

stability, insulating

Defects can occur in polymer products as a result of manufacture. Heating to the processing temperature significantly reduces the polymer density and on cooling shrinkage defects may occur, particularly in crystalline polymers, which pack more closely than amorphous polymers. Thick sec- tions are particularly prone to shrinkage cavities. Voids may also form when the surface cools faster and becomes more rigid. Polymers have a long thermal conductivity and this accentuates relative rates of cooling to produce both surface and internal defects.

Thermosets are more difficult to process because of the need to incorporate a curing process. Com- pression molding is similar in principle to mounting a metallographic specimen. In transfer molding, which can produce more complex shapes, resin is melted in a primary vessel and then transferred to a final, vented mold for final curing. In reaction injection molding (RIM) with polyurethyane, or nylon, polymerization takes place during forming in about 30 seconds after two or more streams of chemical reactants are injected into a mixing chamber. Moldings manufactured for ‘higher’ temperature service are stabilized or post-cured by heating at 100 ◦

C for a short time. The RIM process has now been developed to such an extent that cycle times are comparable to those for thermoplastics.

564 Physical Metallurgy and Advanced Materials

Moving mold

Heater bands half Nozzle

Screw rotation

Screw movement

Non-return Injection unit

Hydraulic system

Cooling water out

Extruder Cooling

Die spider legs

water in

Brass sizing

Cooling water

(b)

Heater Plastic

Mold Mold table

Air Table raised

Vacuum Table locked up

(c)

Figure 11.18 (a) Injection-molding machine. (b) Production of plastic pipe by extrusion. (c) Thermoforming of plastic sheet ( from Mills, 1986; by permission of Edward Arnold).