MDME: MANUFACTURING, DESIGN, MECHANICAL ENGINEERING 

POLYMERS

Polymers are commonly known as plastics - which more specifically means thermoplastics (polymers that soften with heat). Polymers that do not soften are called thermosets.

Image Video Lesson Description and Link Duration Date Download
  Polymers 43:40 min 20140807  

Polymers

What Are Polymers?

Polymers Notes (4MB)

A polymer is a material whose molecules contain a very large number of atoms linked by covalent bonds, which makes polymers macromolecules. Polymers consist mainly of identical or similar units joined together. The unit forming the repetitive pattern is called a "mer" or "monomer".

Schematic representations of polyethylene showing the 'mer' unit ethylene (in red box)

The zigzag backbone structure of the actual  polyethylene macromolecule shown below right.

There are big differences in polymer properties depending on how the atoms and chains are linked together in space. Polymers that have a 1-D structure (g.g. Polyethylene) will have different properties than those that have either a 2-D (rare - e.g. Graphite) or 3-D structure (Epoxy). Most engineering polymers are based on a 1D structure with bits hanging off them.

Polymers also occur naturally in the form of proteins, cellulose(plants), starch(food) and natural rubber. Engineering polymers, however, are usually synthetic polymers - usually called plastics, and usually sourced from crude oil or plant material. The interest in engineering polymers is driven by their manufacturability, recyclability, mechanical properties, and lower cost as compared to many alloys and ceramics. Other special properties include transparency, thermal insulation, electrical insulation, flexibility, colour and surface finish, and special things like biocompatibility (tubes for drug delivery, artificial tendons and joints etc).

By using different starting materials and processing techniques, we can produce polymers having different molecular structures.
Polymer Structures

(a) Linear. Many van der Waals bonding between the chains hold it together. Examples of linear polymers are polyethylene, polyvinyl chloride, polystyrene, nylon and the fluorocarbons.
(b) Branched. Side-branch chains connect to the main ones during synthesis of the polymer. These reduces the packing efficiency, so lower density.
(c) Crosslinked. Adjacent linear chains are actually connected - covalently bonding the chains. Many of the rubber materials consist of polybutadiene crosslinked with S atoms, the process is called vulcanisation
(d) Network. Mer units with three active covalent bonds form 3D networks. E.g.Epoxies

Link: (Nice introduction on polymers): http://www.chemistryland.com/PolymerPlanet/Polymers/PolymerTutorial.htm

Amorphous and Crystalline

Amorphous polymers have molecular chains arranged randomly with no long term order. They soften rather than melt suddenly.E.g. Polystyrene. 
Crystalline polymers have a regular order or pattern of molecular arrangement, so they have a sharp melting point. HDPE is semi-crystalline.

Thermoplastics

A thermoplastic is a polymer that melts when heated. It will also get harder and brittle (glassy) when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene).
  • Thermoplastics become soft, remoldable and weldable when heat is added.
  • They are usually difficult to glue.
  • They tend to be fairly lightweight, although some high performance engineering thermoplastics are quite dense.
  • Easy to process - mould, extrude, blow, roll
  • Tough, flexible, good surface finish
  • Can be cheap
  • High tendency to creep, low stiffness
  • Degradation in sunlight (UV), some with solvents (e.g. PVC).


Get Full Version here:  Polymer_Periodic_Table.pdf

A mixture of plastics is called a copolymer. (Also sometimes called an alloy). ABS is a copolymer of the three monomers acrylonitrile, butadiene and styrene. Polystyrene is too brittle, but the butadiene makes it tougher.

Common Thermoplastics

Code Chemical Name Trade
Names
Description Examples Typical
Cost
PP Polypropylene  
 
Scratch resistant lightweight
Living hinges
Good chemical resistance

End caps
Bottles
 
0.50 to 1.25
$/lb
PMMA Acrylic Lucite
Plexiglas
Perspex
Transparent or opaque   Display stands. 1.00 to 2.00
$/lb
ABS Acrylonitrile, Butadiene, Styrene Cycolac Strong, flexible uses rubber as a filler Toys (Lego), automotive, gauges, and boxes. Appliances 1.00 to 3.50
$/lb
PA6/6 Nylon 6/6 Zytel Chemical resistant & strong Chain saws connectors housings
Zip ties. gears
2.50 to 3.75
$/lb
AC Acetal Delrin
Celcon
Wear resistant Slide guides
Rotors, bearings
2.50 to 4.00
$/lb
PC Polycarbonate Lexan Bullet proof Safety shields, electrical 3.00 to 7.00
$/lb
PBT / PET Polyester Valox Withstands high heat Valves   2.75 to 7.00
$/lb
PPS Polyphenylene sulfide Ryton
Fortron
Possesses many properties of Thermoset materials, i-e. Dielectric, heat resistance, etc.
Withstands the highest heat
Often replaces Thermoset in some cases
Covers, switches, and shields
4.00 to 9.00
$/lb
PEI
PEI+PC
  Ultem Possesses many properties of Thermoset materials, i-e. Dielectric, heat resistance, etc.   Often replaces Thermoset in some cases.
Surgical tools
Covers, switches, and shields
8.00 to 15.00
$/lb
PEEK   Polyetheretherketone  
 
 
Chemical resistant & strong   Aerospace
Mil-Spec connectors
30.00 to 90.00
$/lb
PS   Polystyrene  
 
 
Easy moulding, good finish . Suits vac forming. Glueable Fridge interior, CD cases, cups. Also foamed PS $2/kg  link

PE   Polyethylene  
 
 
Tough, cheap , all processes. High chemical resistance  LDPE: Soft plastic bags, squeeze bottles.
HDPE: harder bottles, m
ilk bottles, freezer tubs and bins
UHMWPE: Hard gas pipes
0.50 to 1.25
$/lb
PVC   Polyvinylchloride  
 
 
Tough, gluable. Can be plasticised  UPVC: Unplasticised PVC- Plumbing and electrical pipes.
Plasticised PVC: Hoses, wire insulation

Recycling

Only thermoplastics can be recycled - and only a certain percentage is used and mixed with new polymer (depending on the application). The application can also prohibit recycling - e.g. drinking containers are not allowed to be made from recycled material in case of contamination.
Numbers are used to identify common thermoplastics for the purpose of re-cycling (so they don't get mixed up).

Code Unicode Abbreviation Polymer name Uses

U+2673 PETE or PET Polyethylene terephthalate Soft drink bottles, Polyester fibres and rope, thermoformed sheet, strapping.

(See also: Recycling of PET bottles)

U+2674 HDPE High density polyethylene Bottles, grocery bags, milk bottles, recycling bins, agricultural pipe, base cups, rotationally moulded playground equipment
U+2675 PVC or V Polyvinyl chloride Pipe, electrical wire insulation, building products, non-food bottles
U+2676 LDPE Low density polyethylene Plastic bags, 6 pack rings, various containers, dispensing bottles, wash bottles, tubing, and various molded laboratory equipment
U+2677 PP Polypropylene Auto parts, industrial fibers, food containers, and dishware, buckets and bins, toys, rope, strapping
U+2678 PS Polystyrene Desk accessories, cafeteria trays, plastic utensils, toys, video cassettes and cases, and insulation board and other expanded polystyrene products (e.g., Styrofoam)
U+2679 OTHER or O Other plastics, including acrylic, acrylonitrile butadiene styrene, fiberglass, nylon, polycarbonate, and polylactic acid Bottles, engineering uses (gears, bearings), lenses 



Thermosets

A thermoset is a polymer that cannot be remelted and remoulded (Bakelite; vulcanized rubber). This is becasuse joints are formed between polymer molecules that hold them permanently together. (E.g. Vulcanization of rubber, curing of epoxy). This process is called polymerisation.
  • Thermosets cannot be welded or remolded when heated, they will simply burn instead. 
  • Thermosets tends to be stronger and harder than thermoplastics.
  • Stiffer (high modulus)
  • Handles higher usage temperatures
  • Low ductility
  • Lower shrinkage during cure
Sometimes another division is used - elastomers (elastic or rubbery polymers). The traditional elastomers were thermosets (like natural rubber), but now there are many thermoplastic elastomers. A lot of these are not true elastomers like rubber, but in between rigid and elastomeric materials.

Polyester: Fibre glass reinforcing of boats, car panel filler etc. Glass gives stiffness and heat resistance.
Epoxy: Strong, corrosion resistant, low shrinkage, good electrical properties, good adhesion, variation of properties - transparent, softeners. Used for IC's, electrical fittings, matrix for carbon fibre composite.
Formaldehydes: Phenol-formaldehyde (Bakelite): Black or brown, electrical, adhesives, pot handles. If other colour needed - like benchtops etc use urea-formaldehyde or melamine-formaldehyde (which can handle outdoors)
Polyurethane: 2 part rigid PU is often used for casting/moulding of prototypes since it is tougher than other thermosets. Also comes as elastomer.

Composites

Reinforcing Fibres

Glass: Heavy, good stiffness. Density 2.56. E=70GPa (similar to Aluminium). Medium brittle/tough.
Carbon: Lighter, very high stiffness and strength. There are several categories of carbon fibers: standard modulus (240 GPa) , intermediate modulus (300 GPa), high modulus (> 300 GPa). Steel has E=200GPa. The tensile strength of different yarn types varies between 2000 and 7000 MPa. Density 1.6-1.7. Brittle.
Aramid (Kevlar, Twaron): Light, good toughness but not as stiff as carbon. Aramid is derived from polyarylamide. Kevlar (DuPont) reinforcement, and Twaron (Akzo) reinforcement, Nomex (DuPont) for paper and honeycombs. E=140GPa, Stress=3600MPa, Strain 2-4%. Fibres absorb water and degrade in sunlight. Density 1.44. Tough.
Polyethylene fibres: (Dyneema, Spectra). Made from ultra-high molecular weight polyethylene (UHMWPE)
trade names Dyneema (DSM), and Spectra (Allied Corporation). Modulus similar to aramid but strength-to-weight higher, almost matching HM carbon fibre. Melts at 150C and too slippery to stick to anything.
Natural fibres: Flax, hemp, jute, kenaf, sisal


Matrix Materials for Fibre Composites

The job of the matrix material is to bond the fibres together and form the body of the part. Any polymers that can be mixed with the fibres can work as a matrix material, but certain combinations are popular:
2 part Polyester resin: Most often used with fibreglass becasue it is cheap and cures easily.
2 part epoxy resin: More often used with carbon fibre because of the high performance. Can be quite exothermic (gives off a lot of heat, so a thick epoxy part is in danger of overheating during cure.
Short chopped fibre mixed with thermoplastics: Commonly mixed with nylon to produce very rigid and heat resistant injection mouldings for use in automotive and other high volume application - typically as substitute for a thermoset or zinc die-cast part. Massive increase in rigidity and creep resistance. Glass does abrade the mould though, so limited moulding run.

Adhesives

  1. Hot melt. Heated plastic with low MP. Hardens by cooling. 
  2. Plastic welding. Hot air gun with filler rod of same or compatible (plastic) material.
  3. Water based. Hardens when water dries off. (PVA wood glue)
  4. Acrylic based. Many uses from caulking to sticky paper.
  5. Rubber based. High tack (stickiness)
  6. Solvent based. Solvent (thinner) evaporates and leaves rubbery adhesive. (Contact Adhesive)
  7. 2 part curing. Any 2 part thermoset, esp Epoxy and Polyurethane (Araldite=Epoxy)
  8. Surface curing. Cyanoacrylate. (Super glue)
  9. Anaerobic: Hardens when air excluded. (Loctite 600 series)
  10. Silicone: In it's own category as usual. Cures by contact with air/moisture. Acetic or neutral cure types.
  11. Special curing. UV Light curing epoxy, various heat curing compounds - e.g Bakelite

These above adhesives are from US, solvent based are Super77, Fastbond, Goop, E-6000

DVDs

  Warriewood, NSW : Classroom Video, c2003.   1 videodisc (21 min.) : sd., col. ; 4 3/4 in. + teacher's notes.
  Explains injection moulding, rotational moulding, and vacuum and pressure forming of plastic products.

DVD 668.412/PLAS 35555076215658

 

  Chadstone, Vic. : Double D Technical Productions, c2002.   1 videodisc (29 min.) : sd., col. ; 4 3/4 in.
  This video examines compression and injection moulding of thermosetting materials, physical properties of polymers and how these vary with temperature changes, compounding, extrusion of sheet tube, cross head and co extrusion, blow moulding, blown film and printing, production of fibres and printing on plastics.

DVD 668.412/FORM PART A

 

  Chadstone, Vic. : Double D Technical Productions, c2002.   1 videodisc (29 min.) : sd., col. ; 4 3/4 in.
  This video examines the injection moulding process for a range of products, injection blow moulding, using preforms, production of flexible foams, rotational moulding, extrusion compression moulding, thermoforming and the computer modelling and production of stadium seat.

DVD 668.412/FORM PART B

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