Thermosoftenting Plastics:

Definition: The polymer which become soft on heating and hard on cooling, reversibly are known as thermo softening polymers.

Eg. PVC, PE.

1. Generally formed by addition or chain polymerization reaction.
2. It consist of a linear chain with negligible crosslink’s
3. Monomers used are bifunctional in form of C=C
4. Relatively lower molecular wt. of polymer.
5. Soluble in some organic solvent.
6. These can be reclaimed from waste.
7. Shape can be changed number of times.
8. Soft, week, less brittle

Thermosetting plastics:

Definition: The Polymeres which becomes hard on heating at the softening temperature are known as thermosetting polymer / plastics.

E.g. UF, PF, Nylon.

1. Generally formed by condensation polymerization reaction.
2. They are three dimensional due to crosslink’s
3. Monomers used are with higher functionality or there is use of cross-linking agent.
4. Relatively very higher molecular wt. of polymer.
5. Insoluble due to strong bonds and cross links.
6. Can not be reclaimed.
7. Shape once given cannot change, only sawn, cut, and drilled.
8. Hard strong more brittle.

addition polymerization:

• 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.

E.g.

l)       n(CH₂-CH₂) --→-(CHl₂-CH₂-CH₂Cll₂-)-n   (Polyethylene)

Ethylene

2)      n(CF₂-CF₂) ----→ -(CF₂-CF₂-CF₂-CF₂-)-n   (Teflon)

Tetrafloroethylene.

Condensation/step Reaction:

• 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 Na₂S_x --- heat & catalyst ---→

Cl-(-CH₂-CH₂-S_x-)-Na + nNaCl

1, 2-dichloro ethane Na-polysulphid polysulphide byproduct

Glass-transition temperature (T_G):

• 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) = +100^oC.

• 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.