A level Product Design Unit 2 Polymers A level Product Design Unit 2

Whats the difference between a plastic & a polymer ? Plasticity is a material property & not a material “the ability to be shaped or formed” Plastic Materials Bone Horn Clay Concrete A polymer is a certain type of material There are natural & synthethic polymers We are only interested in synthetic polymers Whats the difference between a plastic & a polymer ?

an individual chemical molecule is called a monomer When many monomers are linked together to form a chain, a polymer molecule is formed A plastic consists of many large polymer molecules What is a Polymer ?

A chemical reaction forms the polymer molecule (makes the chain) polymerisation Normally, chains are randomly arranged and form a 3d pattern Imagine a piece of string scrunched up into a ball What is a polymer ?

Fully uncoiled, one polymer molecule’s chain length may be 10,000 times the diameter of the chain The length of the chain gives the material flexibility Many other material properties are dependent on the chain length Tg = glass transition temperature Temperature at which polymer moves from a hard glass like state to a softer rubbery state Polymer Properties

Plastic Additives Polymer molecules Reinforcement Plasticizer Filler Added to improve material strength properties Plasticizer Added to change natural material properties Makes material easier to process Filler Added to replace polymer Cost reduction measure Stabilizers Stop deterioriating over time Heat, UV, Biodegradation Foamants Increases volume Gives more elasticity under compression Plastic Additives Typical PVC Content

Synthetic Polymer Categories Long chain polymers Crosslinked polymers Thermoplastics Thermosetting Plastics Elastomers Semi-crystalline amorphous Synthetic Polymer Categories

Crosslinked Polymers Very strong Level of crosslinking determines Tg Strong bonds between chains (crosslinks) and intra-chain Level of crosslinking determines Tg Thermosetting plastics (eg Araldite) are highly linked Chemical reaction forms the links One way process – cannot be reversed Will not soften with heat - very high Tg Araldite is a tradename for a resin based polymer The resin is mixed with an activator to start curing Materials like rubbers are lightly crosslinked Tg is below freezing ie. Is in a rubbery state at all temperatures above 0 Below Tg, material is hard & brittle (useless) Crosslinked Polymers

Thermoplastics Strong covalent bonds Weak bonds between chains Bonds between individual chain elements Weak bonds between chains These bonds can be easily broken with heat Breaking all bonds causes return to original shape Recyclable by re-heating Energy cost ! Contamination is a big issue Thermoplastics

Thermoplastics Amorphous thermoplastics Semi-crystaline thermoplastics Chains are randomly entangled Generally transparent Eg. PolyCarbonate (CDs) Glass transition temperature (Tg) is a key property Semi-crystaline thermoplastics Cooling causes molecules to fold in a regular structure and form a crystal Crystal is dense and so plastic is opaque Eg. Polyethylene (gas and water pipes) Have a definate meting point (Tm) Thermoplastics

Semi-Crystalline Thermoplastics The more crystalline a material is: The stronger it gets It has more resistance to solvents Solvents need to dissolve into a material Amorphous plastics have greater free space Higher density Higher modulus (stiffness) Higher melting point (Tm) The lower its transparency Reduced impact resistance (brittle) Reduced ductility Ability to be deformed without cracking Semi-Crystalline Thermoplastics

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