How Plastics are Made… Understanding the Physical Properties of Plastics

1. How Plastics are Made… Understanding the Physical Properties of Plastics Prepared by the IAPD Education Committee (Module 1) Presented courtesy of Modern Plastics, Inc. The IAPD Plastics Primer, Module 1

2. Why use plastics • Plastic are easily formed materials. • The advantage to the manufacturer is that plastic products can be mass-produced and require less skilled staff. • Plastics require little or no finishing, painting, polishing etc. Plastic is referred to as a self-finishing material. Particular finishes can be achieved at relatively low cost. • Plastics can be easily printed, decorated or painted. • Plastics are corrosion resistant, and generally waterproof although certain types of plastics such as UPVC can become brittle and it is possible for the sun’s rays to cause the colour of the plastic to fade. It becomes bleached. • Plastics are lighter than metals, giving deeper sections for a given weight, and hence stronger sections.

3. Origins of Plastics - synthetic plastics. • The main source of synthetic plastics is crude oil. • These gases are broken down into monomers. Monomers are chemical substances consisting of a single molecule. • A process called Polymerisation occurs when thousands of monomers are linked together. The compounds formed as called polymers. • Combining the element carbon with one or more other elements such as oxygen, hydrogen, chlorine, fluorine and nitrogen makes most polymers.

4. Thermoplastics • There are a wide range of thermoplastics, some that are rigid and some that are extremely flexible. • The molecules of thermoplastics are in lines or long chains with very few entanglements. When heat is applied the molecules move apart, which increases the distance between them, causing them to become untangled. This allows them to become soft when heated so that they can be bent into all sorts of shapes. • When they are left to cool the chains of molecules cool, take their former position and the plastic becomes stiff and hard again. The process of heating, shaping, reheating and reforming can be repeated many times. Long chain molecules

5. Plastic types: Thermoplastics General properties: low melting point, softer, flexible. Typical uses: bottles, food wrappers, toys, … Examples: Polyethylene: packaging, electrical insulation, milk and water bottles, packaging film Polypropylene: carpet fibers, automotive bumpers, microwave containers, prosthetics Polyvinyl chloride (PVC): electrical cables cover, credit cards, car instrument panels Polystyrene: disposable spoons, forks, Styrofoam™ Acrylics (PMMA: polymethyl methacrylate): paints, fake fur, plexiglass Polyamide (nylon): textiles and fabrics, gears, bushing and washers, bearings PET (polyethylene terephthalate): bottles for acidic foods like juices, food trays PTFE (polytetrafluoroethylene): non-stick coating, Gore-Tex™ (raincoats), dental floss

6. Plastic types: Thermosets General properties: more durable, harder, tough, light. Typical uses: automobile parts, construction materials. Examples: Unsaturated Polyesters: lacquers, varnishes, boat hulls, furniture Epoxies and Resins: glues, coating of electrical circuits, composites: fiberglass in helicopter blades, boats, …

7. Plastic types: Elastomers General properties: these are thermosets, and have rubber-like properties. Typical uses: medical masks, gloves, rubber-substitutes Examples: Polyurethanes: mattress, cushion, insulation, toys Silicones: surgical gloves, oxygen masks in medical applications joint seals

8. Clear Acrylic (Perspex) • It was first used to make aircraft canopies. It is ten times more impact resistant than glass.

9. Polystyrene • Polystyrene is used to make plates, cutlery and model kits. • It is stiff hard and comes in a wide range of colours.

10. Nylon • Nylon is hard, tough, self-lubricating, has a high melting point and has very good resistance to wear and tear. • It has been used to make fibres, clothing, bearings and propellers.

11. PVC • The rigid type is used to make pipes, guttering and roofing. It is very lightweight and is resistant to acids, alkalis & the weather. • The plasticised type is used for suitcases, hosepipes, electrical wiring and floor coverings.

12. Polythene • High-density polythene has been used to manufacture milk crates, bottles, buckets, bowl and gear wheels. • It is stiff, hard, can be sterilised and is dense.

13. The Performance of a Plastic Part is Affected By: • Type of load • Size and application of load • Frequency of application of load • Speed of load • Temperature the part will see, and for how long • Use and environment of load

14. Mechanical Properties • Torsional strength • Modulus • Impact strength • Specific gravity • Water absorbtion • Coefficient of Friction (COF) • Tensile strength • Elongation • Compressive strength • Creep • Shear strength • Flexural strength

15. Tensile Strength

16. Elongation

17. Compressive Strength • Measured in Kg/m2 • Higher Kg/m2 = harder to compress

18. Creep • Associated with compressive strength • Creep at room temperature is called “cold flow”

19. Shear Strength The IAPD Plastics Primer, Module 1

20. Flexural Strength

21. Torsional Strength The IAPD Plastics Primer, Module 1

22. Modulus • Also referred to as “stiffness” • Used in conjunction with strengths (flexural modulus, tensile modulus, etc.) • Higher modulus = stiffer material • Measured in N/m2 (Kg/m2)

23. Izod Impact Impact Strength

24. Specific Gravity • Related to the density of material • Can be used to determine the weight of material • Specific gravity of less than 1.0 will float in water The IAPD Plastics Primer, Module 1

25. Water Absorption • Measured by the percentage of swell • Think of a sponge as having high percentage absorption

26. Chemical Resistance • Typically expressed in terms of alkali or acid resistance • Inertness; or the ability not to dissolve or react to chemicals

27. Coefficient of Friction (COF) • Resistance to sliding (slickness) • Low COF = more slippery (think of “wet ice” as having lowest COF) • Static COF refers to initial movement from rest • Dynamic COF refers to being already in motion

28. Thermal Properties • Coefficient of thermal expansion • Heat deflection temperature • Continuous service temperature • Melting point • Thermal conductivity

29. Coefficient of Thermal Expansion (CTE) • Change in size as temperature changes • Lower value = less change with temperature

30. Continuous Service Temperature (CST) • Highest temperature a material can withstand and still retain at least 50% of its properties • Measured in degrees Fahrenheit, in air • In high temperatures, both CST and HDT must be considered

31. Melting Point Temperature at which a crystalline thermoplastic changes from solid to liquid

32. Thermal Conductivity • How much heat a material will conduct • Most plastics are good “insulators” (do not conduct heat well) • Higher value = more heat conducted • Thermal conductivity of plastics is 300 to 2,500 times less than most metals

33. Mouldability • The ability of a plastic to flow into a complex mould • Determined by its liquid viscocity, & • Rate of cooling

34. Electrical Properties • Volume resistivity • Surface resistivity • Dielectric constant • Dielectric strength • Dissipation factor • Arc resistance • Flammability

35. Flammability • Most favorable ratings are given to materials that extinguish themselves rapidly, and do not drip flaming particles • Scale from highest burn rate => most flame retardant is HB, V-2, V-1, V-0, 5V

36. Cost… • Ranging from PP & PE @ ~ £0.90/kg • & Nylon @ £3 - £4/kg • to PTFE @ £10/kg …

37. Processing • Hand (or spray) lay up • Laminating • Filament winding • Polymer orientation • Injection molding • Extrusion • Ram extrusion • Screw extrusion • Coextrusion • Casting • Compression molding • Rotational molding • Transfer molding • Calendering

38. Injection Moulding The IAPD Plastics Primer, Module 1

39. Blow moulding The IAPD Plastics Primer, Module 1

40. Extrusion • Ram • Screw • Coextrusion

41. Casting Base Material Reactive Additive Oven Casting Mold

42. Mold Lid Mold Resin Compression Moulding • Sheet and block moulding • Parts moulding

43. Rotational Moulding • Low cost • Low pressure • Used in many markets • Easily adapted for short production runs

44. Transfer Moulding Widely used in the semiconductor industry… Ideal for thermosets but slow…

45. Calenderingfor films & sheets…

46. Hand (or Spray) Lay Up • Used to make large parts • Used to produce fiberglass boats and camper shells

47. Laminating Resin Binder Reinforcing Substrate Heat and pressure applied to the top and bottom of the material

48. Filament Winding • Used to make containers and tubes • Items used for transportation of liquids or gasses

49. Polymer Orientation Used to manufacture polymer fibers, strapping, webbing, film, sheet and profiles

50. Plastics Fabrication • CNC machining centers • CNC lathes • Cutting and drilling • Profiling • Routing and milling • Thermoforming • Forging • Milling • Welding • Bending • Bonding