Introduction to Polymer Science and Technology

Polymer processing

or curing agent) is placed into the open mould; it is then closed and pressure is applied to cause the material to flow around the cavity. For SMC, the charge is cut into small pieces, weighed and placed in the mould cavity and pressure is applied to the hot mould to force the material to take up the shape of the cavity. After compression the mould remains closed until curing is complete and the prepolymer has cross-linked. Compression and transfer moulding, covered below, are both batch processes.

Figure 3.52Illustration of compression moulding

3.3.2 Transfer moulding

Transfer moulding (Figure 3.53) is a variant on compression moulding, in which there is a reservoir of catalysed molten resin, an amount of which is 'transferred' to the mould cavity at an appropriate point in the cycle, in a fairly homogeneous state. It is better suited than the compression moulding for the applications where delicate inserts or hollow cores are required in a moulding, because the cavity is not being directly pressed. Electrical components are usually produced by transfer moulding.

PIat en

moulding-

cull & runners

Figure 3.53Illustration of compression moulding

Introduction to Polymer Science and Technology Polymer processing

3.3.3 Injection moulding

Injection moulding of thermosetting plastic is similar to the injection moulding of thermoplastics except that the plasticisation is achieved at low temperature (60-90 °C), and the curing occurs in the mould which is set at a temperature that produces rapid cross-linking (140-200 °C). The material remains in the mould until it is cured sufficiently to be stable in shape (i.e., has green strength), when it can be demoulded, although still hot. Cycle times are longer for thermosets than for thermoplastics due to the chemical reaction.

The processes of injection, compression and transfer moulding are high pressure moulding techniques, there are also low pressure mouldingtechniques that rely on specially formulated liquid resins that are used as a matrix for fibre reinforced composites. Vacuum assisted resin injection (VARI), resin transfer moulding (RTM) or vacuum infusion processes are variants of low-pressure moulding, where the catalysed resin is injected into matched moulds or bagged tools in which the reinforcement has been placed prior to mould closure. Application of vacuum ahead of resin injection assists mould filling. All these techniques depend on the easy flow of the prepolymer and good wetting of the reinforcement. For detailed information and illustration/animation of RTM and other processes, such as pultrusion and SMC, associated with fibre reinforced composites visit the BPF web site.

Many thermosetting polymers (e.g., polyurethanes, epoxides, silicone, modified polyester, phenol-formaldehyde and amino resins), and some thermoplastics (e.g., modified Polyamide 6 and certain acrylics) can be shaped directly from low-viscosity liquid monomeric or prepolymer components, which have to be mixed in stoichiometric proportion immediately before processing. Dispensing units include facilities for metering and mixing of ingredients.

Introduction to Polymer Science and Technology Polymer processing

In general, thermosetting polymers have low viscosity, so when the material fills the mould cavity under pressure, some of the material will leak between the two halves of the mould and also from the vent holes for gas and air escapement, generating flash. While this problem can normally be prevented in TP injection moulding by proper mould construction and processing parameters, it cannot be avoided in TS injection moulding and needs to be removed as a secondary operation. When possible, the parts are de-flashed in an automatic tumbling operation rather than hand de-flashing to save costs. Thermoset scrap cannot be reground and mixed with virgin material for reprocessing and, therefore, when designing for recyclability, thermoplastics should be the preferred choice.

3.3.4 Expanded plastics

There are four basic means of expanding plastics: chemically - gas that causes foaming is generated by means of a chemical reaction; physically - gas is generated by means of change in the physical state of the chemicals used (e.g., boiling of the foaming agent); mechanically - rigorous mixing/agitation of the resin with air, and introduction of hollow micro spheres into the resin.

Cellular plasticsfind uses in industries such as furniture and bedding, flooring, automotive, building and aerospace (sandwich panels, memory foam). Polyurethane (PU) foam or expanded PS (EPS) are most widely used, phenolics, urea-formaldehyde, silicone, PVC and acrylics are also used as foams, e.g., "Rohacell" structural foam (closed-cell rigid expanded foam based on polymethacrylimide) in aircraft sandwich structures.

By way of an example, the basic chemistry of PU foam is briefly described here: a urethane link is produced as a result of the reaction between an isocyanate and a hydroxyl group.

R-N = C = O + R'-O-H-> R-NHCOO-R

urethanelink

Polyurethanes are obtained if the simple isocyanate is replaced by a diisocyanate and the simple alcohol by a polyol, more generally, when isocynates react with materials having active H atoms, e.g., OH-compounds, amines, water, and carboxylic acids.

Di-isocyanates:

Introduction to Polymer Science and Technology Polymer processing

ocn - O); - сн₂ - \CJ/ - nco

Diphenylmethane -4,4' - diisocyanate (MDI), which is a lesser health hazard. There are other aromatic as well as aliphatic isocyanates that are used, e.g., hexamethylene diisocyanate.

Polyols:there is a wide range of polyether or polyester based polyols. For flexible foam production, high molecular weight, approximately 3000, polyether or polyester prepolymer triols are used. For rigid foam, the polyols are of higher functionality and lower molecular weight (300-1000).

Catalysts:amines or organometallic compounds, e.g., organotin, are used to promote the reaction.

Expanding (blowing)agents: CFCs such as "Freons 11 and 12" (dichlorofloromethane) were popularly used but they are now banned, because they possess high ozone depletion potential (ODP) and destroy ozone in the stratosphere, causing "ozone-layer depletion", and they also pose a very high global warming potential (GWP). Alternative blowing agents include hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), which contain C-H bonds that break down in troposphere, but these are now also scheduled to be phased out, as they both present high GWP and HCFCs also present ODP.

Other recent auxiliary blowing agents include low boiling point (< 50 °C) hydrocarbons (methylene chloride and isomers of butane and pentane) and CO₂ (liquid or gas injection). The boiling point needs to be above room temperature in some applications where a liquid foam blowing agent is required. Hydrocarbons such as cyclopentane, isopentane, and normal-pentane as well as butane isomers, which are mostly used as co-blowing agents offer the advantages of being low cost, low GWP and commercially available. The disadvantages of hydrocarbons are that they are highly flammable and they are volatile organic compounds (VOCs), and maybe regulated. For further information on alternative foam blowing agents refer to Wu & Eury (2002).

Water also reacts with isocyanate and produces CO₂, thus, controls the foam density, and contributes to producing a more cross-linked rigid micro-structure.

Surfactant:usually a silicone oil to control foam cell structure and cell homogeneity.

For processingpurposes, the raw materials of polyols, catalysts, expanding agents and surfactant are mixed together and degassed; making up Part A. Isocyanate constitutes Part B. During the production Parts A and В are mixed together and the mixture is immediately dispensed either into moulds or onto a moving belt for the slab stockproduction (Figure 3.54). The mixing in a dispenser unit is either by mechanical agitation, with a static mixer (relying on the flow of chemicals through tortuous paths) or by high speed impingement of the two streams of chemicals. The mixing/dispensing head must be immediately purged with solvent to avoid clogging.

Introduction to Polymer Science and Technology

Polymer processing

Figure 3.54Illustration of foam-slab-stock process

Figure 3.55 shows rollers for agricultural machinery that are rotational moulded and then filled with rigid PU using a dispensing machine. The foam keeps the metal frame-shaft insert in place and avoids it from being loosened and detached from the roller with usage.

Introduction to Polymer Science and Technology

Polymer processing

Figure 3.55Rigid-PU foam filled agricultural rollers (courtesy of JFC Manufacturing Ltd.) Other types of foam, besides polyurethane-based rigid and flexible foams, include:

Polystyreneand polyethylene foam:these thermoplastic foams are extensively used in packaging and building applications. Polymerisation of PS beads (by suspension polymerisation) for expansion includes pentane blowing agent in its formulation. Approximately 6 % pentane is added with monomer and following polymerisation gets trapped in polymer beads. Nucleating agent (sodium bicarbonate or citric acid) is also used to promote uniform beads with uniform pore size and structure to ensure the encapsulation of pentane gas in beads. The beads/granules are steam («120 °C) expanded moulded into products of various shapes. PS foam can also be extruded in sheet form: in a post-polymerisation process, the beads and foaming agents (hydrocarbons, HCFCs, HFCs and CO₂ or their blends) are fed into an extruder where the beads expand and incorporate the blowing agent and are extruded through a flat die. The sheets can be subsequently shaped, e.g., by vacuum forming.

PE foamis mainly produced through extrusion technologies: the foam is produced by dissolving and mixing a gas (iso-butane or pentane) into the molten PE, where they form small gas bubbles or cells and expand the polymer and finally cooling of the expanded polymer produces the foam. By using a suitable extrusion die, the foam can be shaped into different semi-finished products such as tubes, profiles, sheets and blocks. The expansion results in a substantial reduction of the polyethylene density: LDPE has a specific gravity of approximately 0.920, which reduces to 0.030 when it is foamed. This weight reduction is obtained by expanding the PE approximately 30 times.

Urea-formaldehyde (UF) foam:foaming is achieved by mechanical agitation. It readily absorbs a variety of liquids. Therefore used in flower arrangement: the stems of flowers easily pierce UF foam and the foam absorbs water to keep the flowers fresh.

Hollow beadsmay be added to plastics (as an alternative to foaming) to reduce weight. Phenol-formaldehydeis used to produce inherently fire retardantfoam.

Date: 2015-12-11; view: 847