Chemical Engineering Materials - Degradation of polymers. Composite materials. Selection of materials
Chemical Engineering Materials
Assignment No. 4
Degradation of polymers. Composite
materials. Selection of materials
Name-Ajinkya Sanjay Khandizod
Class: B (S.E)
Polymer degradation :-
•Is a change in the properties—tensile strength, color, shape, etc.—of a polymer or
polymer-based product under the influence of one or more environmental factors
such as heat, light or chemicals such as acids, alkalis and some salts.
•These changes are usually undesirable, such as cracking and chemical disintegration
of products or, more rarely, desirable, as in biodegradation, or deliberately lowering
the molecular weight of a polymer for recycling. The changes in properties are often
1. Photoinduced degradation
• Most polymers can be degraded by photolysis to give lower molecular weight
• Electromagnetic waves with the energy of visible light or higher, such as ultraviolet
X-rays and gamma rays are usually involved in such reactions.
• Chain-growth polymers like poly(methyl methacrylate) can be degraded
by thermolysis at high temperatures to give monomers, oils, gases and water.
• Step-growth polymers like polyesters, polyamides and polycarbonates can be
degraded by solvolysis and mainly hydrolysis to give lower molecular weight
• The hydrolysis takes place in the presence of water containing an acid or
a base as catalyst. Polyamide is sensitive to degradation by acids and
polyamide mouldings will crack when attacked by strong acids.
• For example, the fracture surface of a fuel connector showed the progressive
growth of the crack from acid attack (Ch) to the final cusp (C) of polymer. The
problem is known as stress corrosion cracking, and in this case was caused
by hydrolysis of the polymer. It was the reverse reaction of the synthesis of
Ozone cracking in Natural rubber tubing
• Cracks can be formed in many different elastomers by ozone attack. Tiny traces
of the gas in the air will attack double bonds in rubber chains, with Natural
rubber, polybutadiene, Styrene-butadiene rubber and NBR being most sensitive
• Ozone cracks form in products under tension, but the critical strain is very
small. The cracks are always oriented at right angles to the strain axis, so will
form around the circumference in a rubber tube bent over.
• Ozone cracks were commonly seen in automobile tire sidewalls, but are now
seen rarely thanks to these additives. On the other hand, the problem does
recur in unprotected products such as rubber tubing and seals.
A Composite material (also called a composition material or
shortened to composite, which is the common name) is a
material made from two or more constituent materials with
significantly different physical or chemical properties that,
when combined, produce a material with characteristics
different from the individual components.
Typical engineered composite materials include:
a) mortars, concrete
b) Reinforced plastics, such as fiber-reinforced polymer
c) Metal composites
Ceramic composites (composite ceramic and metal matrices)
Composite materials are generally used for buildings, bridges, and structures
such as boat hulls, swimming pool panels, race car
bodies, shower stalls, bathtubs, storage tanks, imitation granite and cultured
marble sinks and countertops. The most advanced examples perform routinely
on spacecraft and aircraft in demanding environments.