- Polyether polymers are the most diverse of the non vinyl polymer in the structural variety that have achieved commercial prominence.They includes polyacetals and PPO.
- The polyacetal is synthesised by chain-reaction polymerisation. Sometime this ether is also prepared by ring-opening polymerisation.
- PPO (Polyphenylene oxide) is prepared by step-reaction polymerisation.
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Polyoxymethylene (POM), also known as acetal,[2] polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic polymers, it is produced by different chemical firms with slightly different formulas and sold variously by such names as Delrin, Ultraform, Celcon, Ramtal, Duracon, Kepital, Polypenco, and Hostaform.
- The material was produced by the polymerisation of formaldehyde, which was isolated by first Butlerv in 1859 in polymeric form. It was not commercially available until 1952.
- The first commercially available acetal resin was marketed by Du Pont in 1952 under the trade name of Derlin.
- The Du Pont monopoly was usually shortlived as Celcon, as acetal copolymer produced by the Celanese Corporation in 1960. It was commercialized in 1962. In the same year it was also available as Hoste form from Farb Werke Hoechst Co. Germany.
- In 1963, it was also available from Danippon celluloid Co., of Osaka, Japan and Imperial Chemical industries (ICI), Britan when Celanese joined them. In the early 1970’s ultra form GmBH (A Joint venture of BASF and Degussa) introduced copolymer under the name of ultra form.
- In the early 1970’s Japanese Co., Ashai chemicals introduced the homopolymer under the name Tenal.
- By the late 1990’s the main manufacturers where the American based Du Pont, the Japanese based PolyPlastics and the European based Ticone. Among atleast 8 plants in Asia those of Mitsubishi Gas and Ashai were significant as was also that of BASF.
POM is characterized by its high strength, hardness and rigidity to −40 °C. POM is intrinsically opaque white, due to its high crystalline composition, but it is available in all colors. POM has a density of 1.410–1.420 g/cm3.
Typical applications for injection-molded POM include high-performance engineering components such as small gear wheels, eyeglass frames, ball bearings, ski bindings, fasteners, guns, knife handles, and lock systems. The material is widely used in the automotive and consumer electronics industry.
Monomer Ingredients for Polyacetal:
- Polyacetal is made from formaldehyde (HCHO).
- Copolymer of polyacetal is made from trioxane, the cyclic trimer of formaldehyde.
Chemistry of Preparation of Polyacetal
Formaldehyde polymerise in the following ways,
1. The cyclic trimer (trioxane) and tetramer are obtained by a trace or sulphuric acid on hot formaldehyde vapor (type i)
2. Linear polymer with degrees of polymerisation of about 50 and a terminal hydroxy group are obtained by evaporation of aqueous solution of formaldehyde (type ii)
3. In the presence of strong acid, the average chain length may be doubled. Evaporation leads to products (type iii)
4. In the presence of lime water more complex reactions occur, leading to the formation of aldose and hexose (type iv)
Relations of Structure and Properties of POM
- Due to structural similarity properties of acetal polymers are compared with those of polyethylene.
- Both polymers are linear with a flexible chain backbone and are thus both thermoplastic.
- Both the structures are regular and since there is no question of tacticity arising both polymers are capable of crystallization.
- In the case of both materials polymerization conditions may lead to structures which slightly impede crystallization; with the polyethylene, this is due to a branching mechanism, whilst with the polyacetals this may be due to co-polymerization.
- The acetal polymer molecules have a shorter backbone (-C-O-) bond and they pack more closely together than those of polyethylene. The resultant polymer is thus harder and has a higher melting point.
Characteristics of POM (for identification)
The characteristics of POM are,
- The material is semicrystalline and maintain high dimensional stability and it is sensitive to UV light
- It is opaque
- It is identified by the strong smell of formaldehyde, when burned, faint color flame, melt and drips
- Its melting point is 165-175°C
- Its short term and long term service temperatures are respectively 160 - 140°C and 90 - 100°C.
Characteristics of POM
- Good appearance
- Homopolymer is resistant to mid acids and bases
- Good electrical properties but affected by moisture
- Stiff and rigid
- Good toughness
- Notch sensitive
- Excellent fatigue resistance under repeated load
- Excellent creep resistance under continuous load
- Low coefficient of friction
- Good abrasion resistance
- Maintains the mechanical, chemical and electrical properties over broad temperature range and time
- High resistance to thermal and oxidative degradation
- Very good resistance to stress relaxation
- Excellent dimensional stability
- Good processability
- Copolymers have better thermal stability
- Burn slowly without smoke generation
- Susceptible to UV degradation
- Attacked by phenol and aniline
- Difficult to electroplate
- Degradation at high processing temperature and liberate formaldehyde
Properties of Polyacetals (special features)
The principal features of acetal resins leading to commercial application may be summarized as follows.
- Stiffness
- Fatigue endurance
- Resistance to creep
- Low co-efficient of friction
- Good appearance
Mechanical Properties
The stress- Strain behaviour of polyacetal is such that it could be used to replace metallic materials in many precision engineering applications.
Thermal Properties
- Acetals have a heat distortion temperature in excess of 110°C and can be used in applications upto this temperature intermittently.
- However, acetal can loose strength and toughness after long exposure to hot environments.
- Homopolymers resist deterioration upto one and a half years at 82°C in air while the copolymers may be used continuously at temperature upto 104°C in air.
- Mouldings of acetal remain dimensionally stable over the recommended use temperature range.
Electrical Properties
- The electrical insulation properties of the acetal resins may be described as good but not particularly outstanding.
- However, application where impact toughness and rigidity are required along with good electrical insulation characteristics they be used.
Water absorption
The water absorption of polyacetals is low, 15 mg after immersion for 24 hours and 30 mg after 96 hours at 200° C.
Optical properties
- Polyacetal moldings are translucent to white.
- The light transmission of 2mm thick injection molded panels is 50%, the refractive index is 1.48.
- The gloss of the moldings depends on the surface finish of the mold.
Permeability to gases and vapours
- The permeability of polyacetal is very low compared with that of other plastics.
- This applies to both aliphatic and halogenated hydrocarbons.
- Polyacetal is resistant to fuel gases and is therefore suitable for use in gas fittings and aerosol containers.
Chemical properties
- Polyacetals are resistance to weak acids, weakly alkaline solutions (strongly alkaline solutions only for copolymers), gasoline, benzene, alcohols, oils, grease, halogenated hydrocarbons, water, detergents. They are not resistance to strong acids and oxidising agents.
- Polyacetals are not susceptible to stress cracking.
Weathering resistance
- Polyacetals are damaged by UV radiation. Resultant changes in properties occur more rapidly with smaller wall thicknesses. Degradation can be delayed by light stabilizers.
- Active carbon black has proved to be the most effective stabilizer. Less effective are organic light stabilizers used for natural or colored material.
- Some pigmented grades exhibit good weathering resistance with added UV absorber.
- Resistance to high energy radiation:- polyacetal molding should be used in situation where the total radiation dosage exceeds approx. 3.104 kg-1 /3mrad. Yellowing and embrittlement occur at higher dosages.
Flammability
- As polymerization products of formaldehyde, polyacetals are flammable.
- They burn with a weak bluish flame and drip.
- After extinguishing or incomplete combustion there is a choking smell of formaldehyde.
Toxicity and Sterilization
Toxicity
- Polyacetals are free of smell and taste.
Sterilization
- Items made of plastics are usually sterilized using a dosage of 25.104 Jkg-1 /2.5 Mrad. This cause some degradation which is associated with a decrease in toughness.
Properties
1. High tensile, impact, and stiffness
2. Outstanding fatigue endurance
3. Excellent resistance chemicals
4. Excellent dimensional stability
5. Good electrical insulating characteristics
6. Good resilience and resistance to creep
7. Good abrasion resistance
8. Natural lubricity
9. Wide end-use temperature range
Applications of POM
1. Gears, Rollers, Pulleys, bolts, nuts, shelf support brackets, detergent pumps, spray nozzles, mixing blades
2. Sprinklers, pump housings, impellers, pistons
3. Cooling fans, Filter bodies & valves, water meters, tool holders.
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