1. Structure, General Properties, and Applications
Polymers:
Structure, General Properties, and Applications

2. Polymer Plastics meaning can be molded and shaped
Most products have a component made of some type of polymer.

3. generally characterized by:
Low density.
Low strength and stiffness.
Low electrical and thermal conductivity.
Good resistance to chemicals.
High coefficient of thermal expansion.

4. (cont’d) Many colors and transparencies Low in cost.
Can be machined, cast, formed, and joined
Minimal surface-operations required -advantage over metals

5. Structure of Plastics
Plastics are essentially a series of linked or cross-linked molecules
Poly-mer, meaning many-units (mers).

6. Monomer:
basic building block of polymers

7. Polymerization:
Chemical reaction where monomers are linked to form larger molecules.

8. Bonding
Determines overall strength of a polymer

9. Classification of Polymers
Thermoplastics
Thermosets
Elastomers

10. Thermoplastics
Can be reshaped after heated to glass temperature, returning to original hardness and strength after cooled.

11. Repeated heating/cooling leads to degradation (thermal aging).
Can undergo large uniform deformations in tension before fracture, which enables the forming of complex shapes.

12. Like metals, when deformed, molecules align in the direction of the elongation, becoming stronger along the stretched direction, yet weaker in the transverse direction (anisotropic behavior).
Deformation leads to strength in the direction of elongation

13. Thermosets
Curing reaction is irreversible, shape is permanently set, thus the term thermosetting plastic.

14. Behavior of Thermosets
Generally, thermosetting plastics have better mechanical, thermal, and chemical properties, electrical resistance, and dimensional stability than thermoplastics.

15. If temperature is increased sufficiently, thermosetting polymers burn, degrade, and blacken.

16. Elastomers (Rubber)
Amorphous polymers (low glass-transition temperature).
Ability to undergo large elastic deformation without rupture, recovering original form and dimension after loads have been removed, even after large deformations.

17. Elastomers are Cross-linked at elevated temperatures (vulcanization), cannot be reshaped.

18. Types of Elastomers

19. Natural rubber
Latex base, sap obtained from tropical tree bark, good resistance to abrasion and fatigue, low resistance to oil, heat, ozone, sunlight.

20. Synthetic rubber Further developed natural rubber
improved resistance to heat, gasoline, and other chemicals.

21. Additives
Used to impart specific properties to polymers

22. Fillers Reduce cost, improve properties
fine sawdust, silica powder, clay, cellulose, glass, asbestos

23. Plasticizers
impart flexibility and softness by lowering the glass transition temperature.

24. Antioxidants
Used to prevent or slow degradation from ultraviolet radiation and oxygen.

25. Colorants

26. Flame Retardants
Chlorine, bromine, phosphorus

27. Lubricants
Reduce friction during processing
Linseed oil, waxes, soaps

28. Traditional Thermoplastic Processing Techniques

30. Extrusion Screw extruders Feed section
Melt section – friction and heating elements
Metering or pumping section – pressure buildup at the die
Water and air cooling often used after the extrusion

33. Extrusion Processes Plastic tubes and pipes Use of a spider die
Coextrusion
Simultaneous extrusion of 2 or more polymers
Plastic coating electrical wire
Electrical cables and paper clips
Polymer sheets and films
Flat extrusion die (coathanger die)
Sheet = thicker than .5 mm

36. Extrusion Processes Cont.
Thin film
Film = thinner than .5 mm
Blown film process
Plastic films
Such as teflon
Produced by shaving the circumference of a solid plastic billet
Much like veneers from logs
Called “skiving”
Pellets
Used as the raw material for many thermoplastics processing techniques
Small diameter is extruded and chopped into pellets

38. Injection Molding
Use of hydraulic plunger or rotating / reciprocating screw (extruder)

40. 250 Ton Machine

41. Molds complex, several cavities
“multicomponent injection molding – various colors and shapes (rear light covers – autos)
Insert molding – metal hardware – (auto elect components
Overmolding – process of making hinges and ball & socket joints in one operation
Use of different materials that won’t bond
Use of cooling inserts between the material to avoid bonding called “ice cold molding”

43. Blow Molding Modified extrusion and injection molding
Extrusion blow molding
Tube (preform) is extruded into mold cavity
clamped in mold cavity with a larger diameter
blown outward with hot air blast to fill mold cavity
Drums up to 5oo gal.

44. Extrusion Blow Molding

45. Injection Blow molding
Parison is injection molded (may be stored for later use)
Mold opens – parison loaded and mold closed
Hot air blown into parison and expands to parison to mold walls
Beverage bottles, shampoo, etc.
Multilayer blow molding
Coextruding tubes (parisons) for multilayer structures
Packaging for food and beverages
Odor, taste, aroma protection, scuff resistance, capabilty for printing

46. Injection Blow Molding

47. 3 Station Injection Blow Molding

48. Rotational Molding Large hollow parts
Premeasured powdered polymer placed inside warm mold
Mold is heated and rotated in oven
Catalyst is added for thermosets
Inserts may be molded integrally
Trash cans, large hollow toys, carrying cases, footballs

50. Thermoforming
Forming thermoplastic extruded sheets or films over a mold
Use of heat and pressure
Sheet is clamped
Heated to sag point
Forced against mold by vacuum or air pressure
Packaging trays for cookies & candy
Advertising signs, refrigerator liners, shower stalls
No parts with holes

52. Casting
Thermoplastics and thermosets cast into rigid or flexible molds
Typcical parts are gears, bearings, wheels, lenses, etc.
Potting and encapsulation
Casting the polymer around an electrical component to embed it in plastic (coils or transformers)
Potting: housing becomes part of final product
Encapsulation: component is coated with a layer of polymer, surrounding it completely
Both of these processes - the plastic serves as a dielectric (nonconductor)

54. Foam Molding Raw material is expandable polystyrene beads
Polystyrene beads obtained by polymerization of styrene monomers are placed in a mold
Exposed to heat (steam)
Beads expand as much as 50 times original size to fill mold cavity
Thermoplastics & thermosets can be used but thermoplastics are in liquid form so the process is much like RIM
Products: styrofoam cups, food containers, insulating blocks, packaging materials

55. Foam Molding Cont. Structural foam molding
Thermoplastics are mixed with a blowing agent (inert - nitrogen)
Injection molded into cold molds
Rapid cooling against the cold mold produces a rigid skin and a cellular core
Products: furniture components, computer and business machine housings, construction trim and moldings

56. Composite Materials
Combination of two or more chemically distinct and insoluble phases whose properties and structural performance are superior to those of the constituents acting independently.

57. Examples
addition of straw (reinforcement) to clay (matrix) for making mud huts and bricks for structural usage.
Concrete reinforced with re-bar or wire mesh

58. Consist of a matrix and reinforcement combined

59. Categories Polymer matrix composites Metal matrix composites (MMC)
Thermoplastics
Thermosets
Metal matrix composites (MMC)
Ceramic matrix composites

60. Types of Polymer Matrix materials
Polyester
Epoxy
Phenolics
Carbon

61. Functions of the matrix material
Support the fibers in place
Protects the fibers
Transfer stresses

62. Reinforcements Give composites their directional qualities
Very anisotropic
Fibers very strong and stiff in tension
Molecules oriented in long direction

63. Types of Reinforcements
Glass
Carbon
Aramid (Kevlar)

64. Reinforcement Forms Fibers (dia usually less than .0004” ) Particles
Whiskers (tiny needlelike single crystals grown)

65. Also classify fibers as:
Continuous
Discontinuous

66. Fiber forms
Chopped
Yarns and Roving – twisted and untwisted strands of fibers
Woven fabric
Mats

67. Other Composites

68. Metal Matrix Composites
high resistance to elevated temperatures.
high ductility and toughness
Aluminum, aluminum lithium, magnesium, titanium

69. Ceramic Matrix Composites
resist high temperatures and corrosive environments.

70. Honeycomb Structures
core of honeycomb, or other corrugated shape, bonded (by adhesives to thin outer skins)
High strength to weight ratio
Core carries compressive loads, faces carry the bending loads

71. Applications of composites
Aircraft / Aerospace
Sporting goods
Construction
Transportation
Marine
Biomedical – Artificial limbs

72. Composites Processing Techniques
Thermosetting Polymers

73. Composites Raw Material Forms
Prepregs
Fibers pre-coated in resin and rolled into sheet or tape.
Sheet Molding Compounds (SMC)
Chopped fibers deposited on layer of resin, then pressed on rollers.

75. Reinforced Plastics (Cont.)
Bulk molding compounds (BMC)
Similar to SMC but in billets. Formed like dough
Thick molding compounds (TMC)
Combines characteristics of BMC (cheep) with SMC (strength) and injection molding.
Electrical components.

76. TMC

77. Molding techniques

78. Contact Molding
Parts with high surface area-to-thickness ratios (swimming pools, tubs, shower units),
usually hand layup or spray- up

81. Vacuum-Bag Molding
Vacuum used to form shape of plastic (prepregs) to mold

82. Automated Tape lamination
Multiaxis CNC Machines

83. Compression molding Also called matched die molding open mold process
material placed between molds & pressure applied

84. Resin Transfer Molding
Resin mixed with a catalyst is forced through a piston pump into a mold cavity with fiber reinforcements.

86. Reaction injection molding (Thermosets)
Two reactive resin components are mixed and then injected into a closed mold
Typical parts
Auto bumpers, fenders, steering wheels, instrument panels, water skis, etc.
Parts up to approx 110 lbs

88. Filament Winding Resin and fibers combined and wrapped around mandrel.
pipes, tanks.

92. Pultrusion
Resin and fibers pulled through a thermosetting polymer bath and a heated die.
Polyester and glass reinforcements are the most commonly used materials.
Production of long shapes with various continuous profiles
rods, tubing, ladders, golf clubs, handrails.

95. Pulforming
After resin and fabric (fibers) are pulled through a polymer bath, product is clamped by two dies and then cured.
Non constant cross sections.

96. Metal Matrix Composites
Liquid Phase Processing
Casting of liquid matrix and solid reinforcement.
Solid phase processes
Fibers and powder metals
tungsten carbide in cobalt matrix for tools.

97. Ceramic Matrix Composites
Slurry Infiltration
Fiber preform is hot pressed and impregnated with a matrix