BY NORHAZLINA BT. AMONMECHANICAL ENGINEERING

DEPARTMENT

Topic 6 : PLASTIC

INTRODUCTIONWhat are polymeric materials?

Polymers are organic materials made of very large molecules containing hundreds of thousands of unit molecules called “mers” linked in a chain-like structure (repeated pattern)

Polymers are characterized by:

Low density materials (replace metals such as steel, aluminium etc)

Versatility in synthesis – processing – properties relationship Raw materials and processing are cost-effective Recycling is possible and practical

Applications of Polymers

PP

Polycabonate roof

Bottles extrusion blow molding

Classification of Polymers

POLYMERS

ELASTOMER (RUBBER)

NATURAL SYNTHETIC

THERMOPLASTICS

PE, PVC, PP, PS

THERMOSETS

Epoxy, phenolic resins

e.g; wood, cotton, leather, skin, hair

Chemical Composition of Polymers

Polymers are classified into:

1. Homopolymers Only 1 type of repeat unit

Chemical Composition of Polymers

2. Copolymers At least 2 types of repeat unit

Molecular Chain

Thermostetting polymers

Rubbers

Thermoplastics: PVC, acrylic, polyethylene

Polyethylene

1. Linear

2. Branched

3. Cross-linked

4. Network

Strength increases1 4

Structure of polymers consists of a large number of repeating molecules to form a long flexible chain.

Thermoplastics - Linear or branched polymers in which chains of molecules are not interconnected to one another.

Thermosetting polymers - Polymers that are heavily cross-linked to produce a strong three dimensional network structure.

Elastomers (Rubbers) - These are polymers (thermoplastics or lightly cross-linked thermosets) that have an elastic deformation > 200%.

Thermoplastics and Thermosettings

Polymers are basically divided into:

1. Thermoplastics

Solids at room temperature that are melted or softened by heating, placed into a mould and then cooled to give the desired shape

Can be recycled

2. Thermosets

Can be either liquids or solids at room temperature that are place into a mould and then heated to cure (set) or harden, thus giving the desired shape and solid properties

Thermosets cannot be reshaped by heating

Thermoplastic Uses Mechanical properties

Polyethylene(PE) Packaging bags, containers, piping, chemical equipment, coating for cables and wires.

Tough and flexible at temperatures. Good dimensional stability. Easily moulded. Good resistance to common solvents.

Polypropylene(PP) Moulding for hospital and laboratory equipment; chemical plant.

Similar properties to polythene, better heat resistance to chemical attack. Tough, rigid and light in weight.

Polystyrene(PS) Refrigerator trays, boxes and many articles of household hollowware. Toys, display figures, etc.

A tough, dense plastic-hard and rigid. Good dimensional stability. Moulds with high surface gloss.

Thermoplastic materials

PE

Thermoplastic Uses Mechanical properties

Polyvinyl Chloride (PVC)

Mouldings. Imitation leather cloth, table cloths, raincoats; etc. Piping and electrical cable covering.

Soft, flexible and rubbery. Good dimensional stability. Good resistance to water, acids, alkalis and most common solvent.

Polymethyl Methacrylate (Perspex)

Lenses, dentures, telephones, knobs and handles. Aircraft windows, sinks, building panels.

Excellent transmission of light. Strong and rigid, easily scratched. Attacked and many organic solvents.

Polyamides (Nylon) Plastic gears, valves, bearings, cams. Raincoats. Climbing ropes, fishing lines and textiles.

Very strong and tough. Good resistance to abrasion. Good dimensional stability. Deteriorates with long exposure to outdoor.

Thermoplastic materials

Thermoset materials Thermoset parts are made from polymer resins that are capable of forming

chemical crosslinks.

As the number of crosslinks increases, the stiffness of the material also increases. Thus, many thermosets are typically stiffer and more brittle than thermoplastics

The impact toughness can be increases by adding fillers or reinforcements (also increase strength)

Thermosets include:

Phenolics (PF) Amino plastics (UF and MF) Polyester thermosets (TS) Epoxies (EP) Thermoset polyamides

Thermoset Uses Mechanical properties

Phenol formaldehyde

Electrical equipment, handles buttons, radio cabinets, engine ignition equipment, cheap jewellery, ornaments.

Filled with fibrous materials to increase strength. Absorbs water but resistant to alcohol, oils and most common solvents.

Epoxides Sold as resins and syrup. Used as adhesives for gluing metal. Laminates (with glass fibre): boat hulls, table top.

Commonly used with glass fibre for increased strength. Good adhesion to metals. Good resistance to water and most solvents.

Melamine formaldehyde

As syrups and resins : surface coating of papers and textiles , laminates. Mouldings : electrical equipment, table ware.

Properties are generally similar to those of urea formaldehyde but with improved resistance to heat.

Thermoset materials

The advantages of using plastics as engineering materials:Good electrical insulation Excellent heat resistanceShock – resistantGood resistance to attack by acids and

alkalis.Self-lubrications.

The disadvantages of using plastics:Low range temperatures operation (150-

600 oF).Adversely affected by prolonged exposure

to ultra-violet light.Not suitable for outdoors.Ductile in low temperatures.Deteriorate with atmospheric moisture and

temperatures.

Processing of PlasticsPlastics can be formed from granular and pallets

into shape product such as sheet, rods, extruded sections, pipe, or finished moulded parts.

Process used depends on whether the plastic is a thermosetting or thermoplastic.

Among of the processes:Injection moulding Extrusion mouldingCompression mouldingTransfer mouldingBlow moulding

Injection MouldingWidely used process of forming thermoplastic

materials. The basic process involves six major steps in the

moulding cycle: The hopper is loaded with granular plastic materials.Heat is applied to the plastic until it becomes soft

enough to flow.The softened plastic is forced through a nozzle into

the mould cavity.When cool, the halves of the mould are separated.The part is ejected from the mould.Gates connecting the product to the runner system

are removed.

Processing Plastics - MoldingCompression and transfer molding

thermoplastic or thermoset

Adapted from Fig. 15.23, Callister 7e. (Fig. 15.23 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed.,John Wiley & Sons, 1984. )

Compression Moulding

This process was the first to be used to form plastics. It involves four steps:

1. Pre-formed blanks, powders or pellets are placed in the bottom section of a heated mould or die.

2. The other half of the mould is lowered and is pressure applied. 3. The material softens under heat and pressure, flowing to fill the mould. Excess

is squeezed from the mould. If a thermoset, cross-linking occurs in the mould. 4. The mould is opened and the part is removed.

When thermoplastics are used, the mould is cooled before removal so the part will not lose its shape.

When thermosets are used, they may be ejected while they are hot and after curing is complete. This process is slow, but the material moves only a short distance to the mold, and does not flow through gates or runners. Only one part is made from each mold.

This process makes parts of constant cross section like pipes and rods. liquid polymer goes through a die to produce a final shape. It involves four steps: 1. Pellets of the polymer are mixed with colouring and additives. 2. The material is heated to its proper plasticity. 3. The material is forced through a die. 4. The material is cooled.

Extrusion Moulding

Blown-Film Extrusion

Adapted from Fig. 15.26, Callister 7e. (Fig. 15.26 is from Encyclopædia Britannica, 1997.)

Transfer MouldingTransfer moulding is most generally used for thermosetting

plastics. Method is like compression moulding in that the plastic is

cured into an infusible state in a mould under heat and pressure.

In transfer moulding the plastic is heated to a point of plasticity before it reaches the mould and forced into a closed mould by means of a hydraulically operated plunger resulting in lower cavity pressure.

Blow Moulding ProcessThe blow moulding of thermoplastic

materials began during World War 11. Blow moulding is intended for use in

manufacturing hollow plastic products.

Blow Moulding ProcessThe basic steps of blow moulding process

are:Melt the materialForm the molten resin into a tube or parisonEnclose the hollow parison in the blow mouldInflate the parison inside the mouldCool the blow-moulded partRemove the part from the mouldTrim flash, as needed