Topic 3 - Polymers

Define polymer

1. Long chain organic macromolecules

2. Macromolecules formed by the chemical bonding of large numbers of smaller molecules

Define monomer

A molecule which each can be linked to other monomers to form polymer chain

What are some naturally occuring polymers?

Hair

DNA

Spider silk

How are synthetic polymers produced?

They are produced by a polymerization reaction

What happens during polymerisation

1. C=C breaks apart

2. 2 free electrons from the broken double bond combine with 2 other ethene molecules to form a chain

3. As it progresses, more and more ethene molecules are added to the chain resulting in a macro-molecule

Define polymeric materials

• combination of carbon with oxygen, hydrogen, nitrogen and other organic/ inorganic elements.

• Easily shaped into forms under heat and pressure

Define plastic and the two main types of plastic

Plastic: Large and varied group of synthetic materials formed or molded into shape

Thermoplastic: Polymer that can be melted

Thermoset: Cross-linked polymer that cannot be melted [silicone,, phenolics]

Define elastomers

Lightly crossed-linked macromolecules with rubbery viscoelastic properties [extremely flexible] [eg: Butyl, natural rubber, EPDM, neoprene, nitrile]

Thermoplastic

• Soften when heated and harden when cooled [Reversible]

  • Melt and solidify without chemical changes and significant loss in mechanical properties

• FOA between polymer chains can be overcome by heating thereby allowing the chain to slide over one another

• Most linear polymers and some branched polymers are thermoplastics

• Support hot-forming methods such as injection-moulding and FDM

• Poor resistance to high temperature

• Low strength but ductile

Thermosets

• Harden the first time they are heated, do not soften after subsequent heating

• During the initial heat treatment, covalent linkages are formed between chains [chains become cross-linked]

• Do not melt with heating but will degrade at high enough temperatures

• Networked/crosslinked polymers are typically thermosets

• High thermal stability

• Strong, rigid, hard, somewhat brittle

Name a few examples of thermoplastics

• Polyethene (PE)

• Polypropene(PP)

• Polyvinyl chloride (PVC)

• Polymethyl methacrylate (PMMA)

• Polystyrene (PS)

• Polyamide (PA-nylon)

Name a few examples of thermoset plastic

• Epoxy

• Silicone rubber

• Polymelamin

• Polyurethanes

What is a homopolymer?

polymer formed from one monomer [All repeating units are the same]

What is a copolymer?

Polymer made of 2 or more monomers

What are the 4 main structures of polymers?

1. Linear polymers [HDPE, PVC, Nylon]

2. Branched polymers [LDPE]

3. Crosslinked polymers [Rubber]

4. Network polymers [Kevlar, Epoxy]

Describe the layout of an amorphous structure

• No orderly arrangement of molecular chain in the solid state

• When heated, shows no definite melting temp, but progressively becomes less rigid

Describe the layout of a crystalline structure

• Ordered arrangement of the molecules in the solid state

• distinct melting point, higher melting point

What are the effects of crystallinity on the mechanical properties and physical properties of a polymer?

The higher the crystallinity of polymer, the higher the melting point, the higher the tensile strength and modules of elasticity [resistance to deformation] of polymer

Define molecular weight

The mass of 1 mole of molecular chain [1 mole = 6.02×10²³]

What are the effects of a higher molecular weight on properties of polymers?

1. Increases ductility

   • The higher degree of entanglement allows the material to be pulled further before the chains break

2. Increases the tensile & impact strength of the material

   • The higher degree of entanglement means that in order to rupture, more polymer bonds need to be broken, this means that the polymer can absorb more energy before failing

3. Increases the viscosity of the material

   • Harder to process the material using conventional methods. The longer the chains, the harder it is ti get them to flow because they are more tangled

4. Increases the chemical resistance

   • Takes more damage to the main chains of the molecules before it will affect the strength of the material

Explain what is glass transition temperature [T_^g]

When an amorphous polymer is heated, the temperature at which it changes from a glass to the rubbery form is called T_g. This is the temp below which an amorphous soild (such as glass, polymers, tire rubber or cotton candy) goes from being ductile to brittle

Explain what is melting temperature [T_m]

For Crystalline or Semi-Crystalline materials, the melting temperature (Tm) is the temperature at which the crystals melt. Amorphous materials don’t truly have a T_m. They just continue to soften more until they behave more like a liquid. When we refer to the melt temperature for amorphous materials, it is usually the temperature at which we can process it.

What are the additives added to polymers to change the mechanical, chemical and physical properties?

1. Filler

   • Increases bulk and reduce cost

   • improve properties such as heat and chemical resistance

2. Plasticiser

   • Lowers T_^g to improve ductility and softness

3. Stabiliser

   • Prevent degradation by heat and light

4. Colarant

   • Organic dye or inorganic pigment to form color

5. Lubricant

   • Reduce friction during processing or prevent sticking to molds

6. Flame retardant

   • Reduces the flammability of plastic

State the properties and applications of polyethylene

Properties:

• Cheap

• tough

• flexible

• Good chemical resistance

Applications:

• LDPE: Bread bags, frozen food bags, grocery bags

• HDPE: Milk, water and juice containers, toys, liquid detergent bottles

State the properties and applications of polypropylene

Properties:

• Strength similar to HDPE, but easier to injection mold

• Good fatigue properties

• Excellent chemical resistance

Applications:

• Gasoline tanks, chemical tanks, luggage, battery cases, ropes, fibers or filaments

• Consumer products: Ketchup bottles, yogurt containers and margarine tubs, medicine bottles

State the properties and applications of Polyvinyl Chloride

1. Plasticised — low strength

   • Applications: decorative coatings, wire coating, imitation leather

2. Rigid — much stronger, tough resistance to grease/oil, resistance to chemicals, clarity, low cost

   • Applications: Chemical storage tank, piping, ducting

State the properties and application of polystyrene

Properties:

• Brittle transparent material

• Excellent electrical insulator

Application:

• toy boxes, casing, radios

State the properties and application of polyamide

Properties:

• strong and tough

• relatively high melting point

• low friction

Applications:

• Gears, bearings, toothbrush bristles

State the properties and application of acrylonitrile-butadiene-styrene

Properties:

• Tough, stiff, and abrasion resistance

• Moderate rensile and compressive strengths

• Remarkable dimensional stability

Applications:

• Instrument panels

• safety helmets

State the properties and application of acrylics

Properties:

• Completely transparent themoplastics

• Stiff strong with outstanding weather resistance

Applications:

• Lenses for car lights, signs and nameplates

State the properties and application of Alkyds

Properties:

• Hard and stiff

• Good mechanical and electrical properties, dimensional stability

Applications:

• Encapsulation of small electronic parts

State the properties and application of Phenolics

Properties:

• High stiffness, impact, and heat resistance

Applications:

• electrical plugs, sockets, switches

State the properties and application of Silicones

Properties:

• excellent chemical inertness

• low toxicity

Applications:

• Water repellent properties

What are the differences between the processing of thermoplastic and thermoset?

Thermoplastic:

• Raw polymeric materials are often in the form of pellets, granules, flakes or powders

• Easily remelted and formed to the desired shapes

• Examples: Injection Moulding, plastic extrusion

Thermoset:

• Permanently set, with heat and catalysts, upon polymerization

• Reheating will not change their shape aand heating at high temp will only cause disintegration and damage to the polymer

• Raw material form monomer solutions or rod and sheet stocks

• Examples: Compression Moulding, Transfer moulding

How does an injection mould work?

1. Polymer granules feed via hopper, polymer melts into molten state using heater bands and friction action of a reciprocating screw barrel

2. Heated plastic fills barrel, motorised screw pushes molten polymer into a mould

3. Injects molten polymer at high pressure into mould via gates and hot runners

4. The plastic product is allowed to cool and solidified before being ejected from the opened mould.

What are the advantages and disadvantages of injection moulding?

Explain the process of plastic extrusion

Plastic Extrusion Process is an operation for transforming a polymer using an extruder from the solid to the easily mouldable molten state, then discharging it through an extrusion die in a pre determined cross-sectional shape and re-solidifying by cooling.

What are the advantages and disadvantages of plastic extrusion

Explain the process of compression moulding

Compression moulding is the simplest and most widely used process for making thermoset polymer products. Compound or blend is placed in mould and heated under pressure within the platens of a steam-heated press. When reaction is complete, product is cooled and ejected

State the advantages and disadvantages of compression moulding

Explain the process of transfer moulding

1. Athermosetting charge (preform) is loaded and heated inside a chamber immediately ahead of the mould cavity

2. Pressure applied on the heated material using a plunger to force the softened polymer to flow into the heated mould

3. Pressure applied on the heated material using a plunger to force the softened polymer to flow into the heated mould

4. Athermosetting charge (preform) is loaded and heated inside a chamber immediately ahead of the mould cavity.

5. Pressure applied on the heated material using a plunger to force the softened polymer to flow into the heated mould.

6. The polymer within the mould cavities are permitted to cure before the final part is ejected from the mould.

State the advantages and disadvantages of transfer moulding