Properties and Applications of some Thermoplastics

 Some of the important thermoplastics, their trade names, properties, and typical applications are given in Table 4.4.

PROPERTIES AND APPLICATIONS OF SOME THERMOPLASTICS

Some of the important thermoplastics, their trade names, properties, and typical applications are given in Table 4.4.

Following are some comments about several thermoplastics listed in Table 4.4.

1. Hydrocarbon Plastics

The major hydrocarbon thermoplastic polymers are polyethylene, polypropylene, and polystyrene.

1. Polyethylene (PE)

✓ Polyethylene, also known commonly as polythene, is made by the polymerisation of ethene i.e., ethylene (CH2=CH2).

✓ Polyethylene is made from petroleum or natural gas feed stocks.

✓ The properties and application of PE vary over wide ranges depending on the molecular weight, the method of manufacture, and differences in structure and density.

✓ Types of polyethylenes: Based on density, there are four classifications of polyethylene :

1. Low density polyethylene (LDPE),

2. High density polyethylene (HDPE),

3. Linear low density polyethylene (LLDPE), and

4. Ultra-high-molecular weight polyethylene (UHMWPE). 

✓ Characteristics of PES:

1. They have excellent resistance to most solvents and chemicals.

2. They are tough and flexible over a wide range of temperature.

3. They are non-toxic.

4. They possess good electrical insulation properties.

✓ Applications: Typical applications of polyethylene include flexible bottles, toys, packing films, seals and gaskets, bowls and buckets, etc.

2. Polypropylene (PP)

✓ Polypropylene is formed from the monomer propene i.e., propylene (CH2=CH2 — CH3).

✓ It is similar to high-density polyethylene (HDPE), but its mechanical properties make it more suitable for moulded parts than polyethylene.

✓ Characteristics of PPs:

1. They are stiffer, harder, and often stronger than polyethy- lenes.

2. They have excellent fatigue resistance and a higher use temperature.

3. They are lighter in weight.

4. They have good chemical and thermal resistance. But they have poor resistance.

5. They are relatively low cost to ultra-violet light.

✓ Applications: Typical applications of polypropylene include low-cost semi-rigid moulded products such as housewares, car interior components; bottle caps; extruded pipes; carpet fibres, etc.

Note 

Polypropylene is brittle at low temperatures. To overcome this, a polyallomer is commercially used. A polyallomer of commercial significance is a block copolymer of polyethylene and polypropylene.

3. Polystyrene (PS)

✓ Polystyrene is made from ethyl-benzene. It accounts for about 20% of all thermoplastics in commercial use.

✓ Characteristics of polystyrenes :

1. They are low cost, brittle, glassy, transparent polymers.

2. They are easy to process; have excellent moldability.

3. They have good dimensional stability.

4. They have a poor chemical and corrosion resistance.

5. They are susceptible to ultra-violet lights.

✓ Types of polystyrenes: Some of the commonly used polystyrenes are:

1. HIPS (High Impact Polystyrene),

2. ABS (Acryknitrile-Butadiene-Styrene), and

3. SAN (Styrene-Acrylonitrile).

✓ Applications: Typical applications of polystyrene include packaging and insulating foams, lighting panels, appliance components, egg boxes, wall tiles, battery cases, etc.

2. Chlorocarbon and Flurocarbon Polymers

4. Polyvinyl chloride (PVC)

✓ Polyvinyl chlorides (PVC) is made by polymerisation of the monomer vinyl chloride (CH2=CHCl).

✓ PVC is one of the most widely used plastics in terms of volume produced.

✓ Characteristics of PVCS:

1. They are good low-cost, general purpose materials.

2. They are ordinarily rigid, but can be made flexible with plasticizers.

3. They are often copolymerised.

4. They are susceptible to heat distortion.

5. They have good flame, electrical, chemical, oil, abrasion, and weather resistance in various grades.

✓ Types of PVCs: The various grades of PVC available are : 

1. UPVC (Unplasticised PVC): UPVC is a strong, rigid and tough material with good resistance to ultra-violet light. 

2. PPVC (Plasticised PVC) : The raw polymer is compounded with a plasticising agent to form PPVC. This makes PPVC a soft, flexible, rubbery material.

3. CPVC (chlorinated PVC): It is a polymer of dichloroethylene.

4. PVDC (Polyvinylidene chloride).

✓ Applications: Typical applications of PVC include pipes, valves, fittings, floor tiles, wire insulations, toys, phonograph records, vinyl automobile roofs, etc.

5. Polytetrafluoro ethylene (PTFE)

✓ Polytetrafluoro ethylene (PTFE) is the most widely used fluorocarbon polymer.

✓ Characteristics of PTFES:

1. They are chemically inert in almost all environments.

2. They have excellent electrical properties.

3. They have low coefficient of friction.

4. They possess relatively weak and poor cold-flow properties.

✓ Other fluorocarbon polymers are:

1. PVDF (polyvinylidene fluoride),

2. PVF (polyvinyl fluoride), and

3. PFA (perfluoroalkoxy ethylene).

✓ Applications: Typical applications of PTFE include anti- corrosive seals, chemical pipes and valves, bearings, non-stick coatings, and high temperature electronic parts.

3. Acrylic Materials

Acrylic materials are thermoplastic polymers based on the polymerisation of esters of acrylic acid and/or methacrylic acid. The most commonly used acrylic polymers are :

1. PMMA (polymethyl methacrylate), and

2. PAN (polyacrylonitrile).

6. PMMA

✓ PMMA (polymethyl methacrylate), also commonly known as perspex or plexiglass, is produced by the addition polymerisation of methyl methacrylate.

✓ Characteristics of PMMAs.

1. They are hard, rigid, and high impact strength thermoplastic. 

2. They are highly transparent to light.

3. They can be easily formed.

4. They can be readily coloured and they have excellent decorative properties.

✓ Applications : Typical applications of PMMAs include camera lenses, flash lights, safety glasses, drafting equipments, instrument panels, display signs, transparent aircraft enclosures, and windows.

7. PAN

✓ PAN (polyacrylonitrile) is an acrylic polymer made by the addition polymerisation of acrylonitrile.

✓ Characteristics of PANs:

1. They are highly stable material.

2. They have good resistant to oils and greases.

✓ Applications: The PAN is mainly used for synthetic fibre manufacture.

4. Polyamides (or Nylons)

8. Polyamides (PA)

✓ Polyamides (PA), also known as nylons, are the products of condensation reactions between an amine and an organic acid. 

✓ There are number of common polyamides. They are usually designated as nylon 6, nylan 6/6, nylon 6/10, nylon 6/12, nylon 11, and nylon 12. These suffixes refer to the number of carbon atoms in each of the reacting substances involved in the condensation polymerisation process. Nylons with a slash (/) between the numbers are copolymers; for example, nylon 6/12 is a copolymer of nylons 6 and 12.

✓ Characteristics of nylons:

1. They are crystalline thermoplastics with good mechanical properties.

2. They are very strong and tough.

3. They possess good abrasion resistance.

4. They are flexible and have high impact strengths.

5. They possess high softening temperatures (in excess of 200°C), so moulding is difficult.

6. They possess a good resistance to most solvents and chemicals, but are affected by phenols.

7. They tend to absorb water, with a consequent reduction in strength.

✓ Applications: Typical applications of nylons include bearings, gears, cams, fastenings, many automotive parts, bushings, electrical components, fibres, and ropes.

5. Heterochain Polymers

Heterochain polymers are those polymer materials which possess linear molecular chains with some of the atoms in the chain being of elements other than carbon. The major heterochain polymers are: acetal (POM), polycarbonate (PC), and polyethylene teraphthalate (PET).

9. Acetal (POM)

✓ Acetal, also known as polyacetal and polyoxymethylene, is based on the polymerisation of formaldehyde.

✓ Characteristics of acetals :

1. They possess good strength, stiffness and toughness.

2. They have a low coefficient of friction.

3. They have outstanding fatigue life and exceptional solvent resistance.

4. They retain their properties at temperatures of upto 120°C. 

✓ Applications : Typical applications of acetals include plumbing fixtures, pens, bearings, gears, cams, levers, fan blades, pump parts, water taps, and piping.

10. Polycarbonate (PC)

✓ Polycarbonate is a linear heterochain polymer made from the condensation of bisphenol A and carbonic acid.

✓ Characteristic of polycarbonates:

1. They have very good impact resistance and ductility.

2. They are dimensionally stable.

3. They are transparent and low water absorption materials.

4. They have low fatigue and weak resistance.

5. They can be attacked by some organic solvents and are susceptible to stress cracking.

✓ Applications: Typical applications of polycarbonates include safety helmets, shields and goggles, lenses, glazing, lighting fittings, CDs, car headlamp mouldings, kettle mouldings, instrument casings and machine housings, sterilisable medical components, and kitchen-wares.

11. Polyethylene teraphthalate (PET)

✓ PET (polyethylene teraphthalate), also known as polyester or PETP, is a linear polyester made by the condensation poly- merisation of ethylene glycol and teraphthalic acid.

✓ Characteristics of PETS:

1. They are high strength, high stiffness thermoplastics.

2. They are produced as fibres, as transparent films, and as moulding materials.

3. They have excellent fatigue and wear strength.

4. They possess good resistance to humidity, acids, greases, oils, and solvents.

5. They can be reinforced with glass fibre.

✓ Applications: Typical applications of PET include fibres, photographic films, recording tapes, boil-in-bag containers, beverage containers, soft drinks bottles, electrical connectors, parts for domestic goods and automotive components.

12. Polyether ether ketone (PEEK)

✓ PEEK (polyether ether ketone) is a linear-crystalline hetero- chain polymer. It is a high-temperature plastics, which is generally used for service of high temperatures.

✓ Characteristics of PEEKs :

1. They have a melt temperature as high as 330°C.

2. They have low flammability and lowest smoke emission ratings.

3. They have good fatigue and chemical resistance.

4. They have good dimensional stability at elevated temperatures.

5. They possess better mechanical properties.

✓ Applications: Typical applications of PEEK include high- temperature engineering components, high-temperature electrical insulation and coatings, and aerospace applications.

13. Polyphenylene oxide (PPO)

✓ PPO (polyphenylene oxide) is an amorphous linear heterochain polymer.

✓ Characteristics of PPOs:

1. They possess good mechanical properties and a high impact strength over a wide range of temperature (-40°°C to 150°C).

2. They have a low mould shrinkage and they show very good dimensional stability over a wide range of temperatures.

3. They have good resistance to hydrolysis but not to many solvents.

4. They are suitable for continuous use at temperatures upto 80°C.

✓ Applications: Typical applications of PPO include computer components, high-temperature consumer electrical components, automotive parts, components for dishwashers and washing machines.

14. Polyphenylene sulphide (PPS)

✓ PPS (polyphenylene sulphide) is also a linear heterochain polymer. In many ways, the PPS is similar to PPO.

✓ Characteristics of PPSs :

1. They have good resistance to chemicals and solvents.

2. They also possess good flame resistance.

3. They are difficult to pigment, therefore they are available only in dark colours.

4. They are suitable for continuous use at temperatures upto 190°C.

✓ Applications: Typical applications of PPS include coatings, fluid handling components (such as pumps, pipes and gaskets), electronic components, hair dryer components. However the main uses of PPSS are similar to PPOs but usable at higher temperatures.

15. Polyimides (PI)

✓ The polyimides are a group of linear aromatic polymers. They are produced by a condensation reaction between pyromellitic dianhydride and a diamine.

✓ Characteristics of PIs:

1. They possess good mechanical properties with a low coefficient of friction.

2. They possess excellent thermal resistance upto about 250°C. 

3. They have good resistance to organic solvents but are attacked by alkalis and concentrated acids.

4. They are transparent to microwaves and are not affected by radiation.

✓ Applications: Typical applications of polyimides include bear- ings and seals for high-load/high-speed applications, printed circuit boards, high temperature electrical cabie insulations, fibres for space shuttle, and adhesives.

16. Polyamide-imide (PAI)

✓ PAI (polyamide-imide), similar to PI, is also a linear aromatic polymer for use at high temperatures.

✓ Characteristics of PAIs: The properties of PAIS are approaching those of PIs. However the PAIS are expensive and are attacked by alkalis.

✓ Applications: Typical applications of PAIS include valves, bearings, electrical connectors, printed circuit boards, and components for gas turbine and spark ignition engines.

Note. 

In addition, there are so many thermoplastics available. Some of them are :

1. Cellulosics: 

✔ Cellulose, is a naturally occurring high-polymer, formed by the photosynthesis of glucose, makes up a significant portion of all plant life.

✔ The most commonly used cellulose polymers are cellulose acetate (CA), cellulose acetate butyrate (CAB), cellulose nitrate (CN), cellulose propionate (CP), ethyl cellulose (EC), viscose and viscose rayon.

2. Polysulphones and poly arylates: 

✔ These polymers are tough, glassy thermoplastics. They are both heterochain aromatic polymers.

✔ The main polymers of these groups are polysulphone (PSU), polyether- sulphone (PES), poly phenylene sulphone (PPSU), polyaryl sulphone (PAS), and polyaryl ether (PAE).