Thermosofetning and Thermosetting
Definition: The polymer which become soft on heating and hard on cooling, reversibly are known as thermo softening polymers.
Eg. PVC, PE.
- Generally formed by addition or chain polymerization reaction.
- It consist of a linear chain with negligible crosslink’s
- Monomers used are bifunctional in form of C=C
- Relatively lower molecular wt. of polymer.
- Soluble in some organic solvent.
- These can be reclaimed from waste.
- Shape can be changed number of times.
- Soft, week, less brittle
Definition: The Polymeres which becomes hard on heating at the softening temperature are known as thermosetting polymer / plastics.
E.g. UF, PF, Nylon.
- Generally formed by condensation polymerization reaction.
- They are three dimensional due to crosslink’s
- Monomers used are with higher functionality or there is use of cross-linking agent.
- Relatively very higher molecular wt. of polymer.
- Insoluble due to strong bonds and cross links.
- Can not be reclaimed.
- Shape once given cannot change, only sawn, cut, and drilled.
- Hard strong more brittle.
- Polymer is made by addition /chain reaction of monomers.
- All vinylic (CH2 = CH-R) monomers undergo the addition polymerization reaction.
- This is fast & exothermic type of reaction.
- There is no by product of this reaction, polymer is multiple of that monomers.
l) n(CH2-CH2) --→-(CHl2-CH2-CH2Cll2-)-n (Polyethylene)
2) n(CF2-CF2) ----→ -(CF2-CF2-CF2-CF2-)-n (Teflon)
- These polymers are obtained by stepwise growth of monomers in polymerization reaction & polymer is formed along with byproduct.
- Monomers have reactive functional group.
- It is slow & endothermic reaction.
E.g. Cl-CH2-CH2-Cl + n Na2Sx --- heat & catalyst ---→
Cl-(-CH2-CH2-Sx-)-Na + nNaCl
1, 2-dichloro ethane Na-polysulphid polysulphide byproduct
Glass-transition temperature (TG):
- The temperature below which polymer becomes hard, brittle & glassy and above which it is softer & flexible, is known as glass transition temperature.
e.g. Polyethylene glass transition temp. (Tg) = –1250C, polystyrene (Tg) = +100oC.
- Hard brittle state is glassy state and soft-flexible state is the rubbery state / viscoelastic state, if viscoelastic state further heated polymer starts flowing.
- Glass transition temp. Is not definite temp. But it is normally rang of 10-20°C.
- Tg gives idea about the softening temperature of polymers.
- Tg depends upon the structure of polymers, the side groups attached to the chain.
- Polarity, frequency & size of side groups increase the Tg of polymer.
- It is determined by using Dilatometer.
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