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natural polymer. Cellulose itself is not a thermoplastic, since it will not melt. Yet, by
the viscose process, it is made into film (cellophane) and into fiber (rayon) that
compete with the products of its thermoplastic derivatives.
Butyrate tubes are transparent, rigid with excellent dimensional stability, high
impact strength, good chemical resistance, good clarity, sparkle and will take
decoration readily. Vacuum formed display units from CAB sheet for its deep-draw
properties, ease of formability, and impact resistance are popular.
Strengths: FDA compliant, Low water absorption, ease of fabrication, UV resistant,
good clarity, Impact resistant, Manufactured from a renewable resource (wood
pulp), resistant to most household chemicals.
Limitations: attacked by alcohol, alkalis, paint removers, and acetones
Applications: Sign panels, Packaging, tubing, Machine guards, Glazing, goggles,
Lacquers and coatings for automotive, Nail care, printing inks.
52) Polyoxymethylenes (POM):
Is a semi-crystalline thermoplastic polymer that is also known as also known as
ACETAL, Polyacetal and Polyformaldehyde. POM is available as a homopolymer
having high strength while copolymers with oxyethylene groups have better thermal
stability.
Density 1.41 - 1.42 g/ cm 3
Glass transition temperature - 60 to - 50 °C
Max/ Min continuous Service temperature 80 - 105 °C / - 40 °C
Melting Point 165 -170 °C
POM may be processed by injection moulding, extrusion, compression moulding,
rotational casting or blow moulding.
Processing temperature of 180 to 220°C recommended. Longer residence time in
the cylinder above 235 °C results in thermal damage. High thermal stress during
processing results in the formation of (highly odorous) formaldehyde.
Pre-drying is not necessary but can be carried out at 110°C for 2 hours if the material
has become moist or if the uniformity of the material needs to be improved.
Injection Moulding: A mould temperature of 50-120°C recommended. Mechanical
properties are determined by the degree of crystallisation, higher mould
temperatures lead to tougher and stiffer parts with less post-shrinkage. Has
relatively high shrinkage (~2%) due to its high crystallinity, thus longer hold time is
recommended.
Extrusion: single and multi screw extruders to produce round and flat rods, hollow
rods, slabs. L/D ratio of 15 to 25 is recommended.
Acetals, notable for their high degree of crystallinity are stiff, hard, tough and
resilient with excellent dimensional stability and high impact resistance even at very
low temperatures. Exhibits very good resistance to many organic liquids (fuel, oil,
grease, brake and transmission fluids), low water absorption, Low creep and very
good long term fatigue, abrasion and wear resistance, low friction coefficient and
very good insulating properties. Because of these properties, acetals often
compete with nylons for many of the same applications.
Homopolymers have greater rigidity, higher tensile strength, higher flexural
strength and a higher resistance to fatigue. Copolymer shows an exceptional

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