Thermoplastics

Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a rigid state 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). Thermoplastic polymers differ from thermosetting polymers (e.g. phenolics, epoxies) in that they can be remelted and remoulded. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene and polypropylene; others are productions of condensation or other forms of polyaddition polymerisation, such as the polyamides or polyester.

Theory

Thermoplastics are elastic and flexible above a glass transition temperature T_g, specific for each one — the midpoint of a temperature range in contrast to the sharp melting point of a pure crystalline substance like water. Below a second, higher melting temperature, T_m, also the midpoint of a range, some thermoplastics have crystalline regions alternating with amorphous regions in which the chains approximate random coils. The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity, as is also the case for non-thermoplastic fibrous proteins such as silk. (Elasticity does not mean they are particularly stretchy; e.g., polyamides/Nylons rope and fishing line.) Above T_m all crystalline structure disappears and the chains become randomly inter dispersed. As the temperature increases above T_m, viscosity gradually decreases without any distinct phasechange.

Some thermoplastics normally do not crystallize: they are termed "amorphous" plastics and are useful at temperatures below the T_g. They are frequently used in applications where clarity is important. Some typical examples of amorphous thermoplastics are PMMA, PS and PC. Generally, amorphous thermoplastics are less chemically resistant and can be subject to environmental stress cracking. Thermoplastics will crystallize to a certain extent and are called "semi-crystalline" for this reason. Typical semi-crystalline thermoplastics are PE, PP, PBT andPET. The speed and extent to which crystallization can occur depends in part on the flexibility of the polymer chain. Semi-crystalline thermoplastics are more resistant to solvents and other chemicals. If the crystallites are larger than the wavelength of light, the thermoplastic is hazy or opaque.

Semi-crystalline thermoplastics become less brittle above 'T'_g. If a plastic with otherwise desirable properties has too high a T_g, it can often be lowered by adding a relatively low molecular weight plasticizer to the melt before forming (plastics extrusion; molding) and cooling. A similar result can sometimes be achieved by adding non-reactive side chains to the monomers before polymerization. Both methods make the polymer chains stand off a bit from one another. Before the introduction of plasticizers, plastic automobile parts often cracked in cold winter weather. Another method of lowering T_g (or raising T_m) is to incorporate the original plastic into a copolymer, as with graft copolymersof polystyrene, or into a composite material. Lowering T_g is not the only way to reduce brittleness. Drawing (and similar processes that stretch or orient the molecules) or increasing the length of the polymer chains also decrease brittleness.

Thermoplastics can go through melting/freezing cycles repeatedly and the fact that they can be reshaped upon reheating gives them their name. This quality makes thermoplastics recyclable. The processes required for recycling vary with the thermoplastic. The plastics used for soda bottles are a common example of thermoplastics that can be and are widely recycled. Animal horn, made of the protein α-keratin, softens on heating, is somewhat reshapable, and may be regarded as a natural, quasi-thermoplastic material.

Although modestly vulcanized natural and synthetic rubbers are stretchy, they are elastomeric thermosets, not thermoplastics. Each has its own T_g, and will crack and shatter when cold enough so that the crosslinked polymer chains can no longer move relative to one another. But they have no T_m and will decompose at high temperatures rather than melt. Recently, thermoplastic elastomers have become available.

List of Thermoplastic Marerials

• Acrylonitrile butadiene styrene (ABS)
• Acrylic
• Celluloid
• Cellulose acetate
• Ethylene vinyl acetate (EVA)
• Ethylene vinyl alcohol (EVAL)
• Fluoroplastics (PTFEs, including FEP, PFA, CTFE, ECTFE, ETFE)
• Ionomers
• Liquid Crystal Polymer (LCP)
• Polyacetal (POM or Acetal)
• Polyacrylates (Acrylic)
• Polyacrylonitrile (PAN or Acrylonitrile)
• Polyamide (PA or Nylon)
• Polyamide-imide (PAI)
• Polyaryletherketone (PAEK or Ketone)
• Polybutadiene (PBD)
• Polybutylene (PB)
• Polybutylene terephthalate (PBT)
• Polyethylene terephthalate (PET)
• Polycyclohexylene dimethylene terephthalate (PCT)
• Polycarbonate (PC)
• Polyketone (PK)
• Polyester
• Polyethylene/Polythene/Polyethene
• Polyether Block Amide (PEBA)
• Polyetheretherketone (PEEK)
• Polyetherimide (PEI)
• Polyethersulfone (PES)- see Polysulfone
• Polyethylenechlorinates (PEC)
• Polyimide (PI)
• Polylactic acid (PLA)
• Polymethylpentene (PMP)
• Polyphenylene oxide (PPO)
• Polyphenylene sulfide (PPS)
• Polyphthalamide (PPA)
• Polypropylene (PP)
• Polystyrene (PS)
• Polysulfone (PSU)
• Polyvinyl chloride (PVC)
• Spectralon

Material properties of some thermoplastics

Properties of some thermoplastic materials

name	Symbol	Density [g/cm3]	Tensile strength [MPa]	Flexural strength [MPa]	Elastic modulus [GPa]	Elongation at rupture [%]	Thermal stability [°C]	Expansion at 20°C [10−6/°C]
High DensityPolyethylene	HDPE	0.95	31	40	1.86	100	120	126
Low DensityPolyethylene	LDPE	0.92	17	14	0.29	500	90	160
Polyvinyl Chloride	PVC	1.44	47	91	3.32	60	80	75
Polypropylene	PP	0.91	37	49	1.36	350	150	90
Polyethylene terephthalate	PET	1.35	61	105	1.35	170	120	70
Polymethylmethacrylate	PMMA	1.19	61	103	2.77	4	100	65
Polycarbonate	PC	1.2	68	95	2.3	130	120	66
Acrylonitrile butadiene styrene	ABS	1.05	45	70	2.45	33	70	90
Polyamide	Nylon 6	1.13	60	91	2.95	60	110	66
Polyimide	PI	1.38	96	143	3.1	7	380	43
Polysulfone	PSF	1.25	68	115	2.61	75	160	56
Polyamide-imide, electrical grade	PAI	1.41	138	193	4.1	12	260	30
Polyamide-imide, bearing grade	PAI	1.46	103	159	5.5	6	260	25
Polytetrafluoroethylene	PTFE	2.17	24	33	0.49	300	260	95
Polyetherimide	PEI	1.27	105	151	2.9	60	210	31
Polyether ether ketone	PEEK	1.32	100		3.6	50	343
Polyaryletherketone(strong)	PEAK	1.46	136	213	12.4	2.1	267
Polyaryletherketone(tough)	PEAK	1.29	87	124	3	40	190
Self-reinforcedpolyphenylene	SRP	1.19	152	234	5.52	10	151
Polyamide-imide	PAI	1.42	152	241	4.9	15	278

NOTE: Bulk properties of pure cast or hot formed materials. Properties could change considerably by mechanical treatment and cold forming. Fiber and foils are not considered.

Terminology

The literature on thermoplastics is huge, and can be quite confusing, as the same chemical can be available in many different forms (for example, at different molecular weights), which might have quite different physical properties. The same chemical can be referred to by many different tradenames, by different abbreviations; two chemical compounds can share the same name; a good example of the latter is the word "Teflon" which is used to refer to a specific polymer (PTFE); to related polymers such as PFA, and generically tofluoropolymers.

Testing

Testing of thermoplastics can take various forms.

Tensile tests—ISO 527 -1/-2 and ASTM D 638 set out the standardized test methods. These standards are technically equivalent. However they are not fully comparable because of the difference in testing speeds. The modulus determination requires a high accuracy of ± 1 micrometer for the dilatometer.

Flexural tests—3-points flexural tests are among the most common and classic methods for semi rigid and rigid plastics.

Pendulum impact tests—impact tests are used to measure the behavior of materials at higher deformation speeds. Pendulum impact testers are used to determine the energy required to break a standardized specimen by measuring the height to which the pendulum hammer rises after impacting the test piece.

Recycling

Thermoplastics are easily recyclable, compared to thermosets, because the polymer chain does not degrade when melted down. This is because the weaker interactions between polymer chains break down at much lower temperatures than the chemical bonds between monomers. This allows thermoplastics to be recycled indefinitely until the polymers are broken down to the point that the material loses structural integrity.

Brick and concrete are good analogies when comparing the properties of thermosets and thermoplastics. A thermoplastic is made of strong polymers, like bricks, with weak forces holding them together, like mortar. A thermoset is made of strong polymers that bind together and form one molecule, similar to concrete. Recycling or reforming a thermoplastic is like chipping out the mortar and re-laying the bricks; in contrast, a thermoset, like concrete, can never be broken down and reformed with the same strength. However, if individual bricks are damaged, relaying them will not restore the strength of the unbroken bricks, just as remelting a degraded thermoplastic will not restore damaged polymers.

In real-world recycling, thermoplastics have a limit recyclable lifespan due to degradation of the polymers and contamination during the recycling process. Contaminants can be inert materials, which act as fillers, or they can be other plastics, which alters the physical properties of the resulting material.