Chapter 6 Polymer Degradation 6.3 the Degradation of polymer 6.4 the aging and anti-aging of polymer

6.3 Degradation of polymers 6.3.1 thermal degradation 6.3.2 mechanical degradation 6.3.3 oxidative degradation 6.3.4 chemical degradation and biodegradation

6.3 Degradation of polymers What is the degradation？ The reaction during which the molecular weight of polymer reduce is called degradation, includes depolymerization, random chain scission and the remove of micromolecule The influencing factors of degradation： the heat, light, oxygen, mechanical force, ultrasonic, chemical medicine and microorganism. Sometimes it is subjected by the combined influence of several factor. What is the aging？ the polymer effect by physical and chemical factor, the main reaction is degradation, sometimes the cross linking occur.

6.3.1 thermal degradation The classical research technique of thermal degradation （1）thermo-gravimetry analysis: heat the polymer in vacuum, note the change of weightlessness with temperature. Then gain the thermal weightlessness-temperature curve. It is used to study the thermal stability or decomposition.

Figure 6.1 thermal weightlessness-temperature curve 1-polyα-methylstyrene 2-polymethyl methacrylate 3-polyisobutylene 4-polustyrene 5-polybutadiene 6-polyformaldehyde 7-polytetrafluoroethylene 8－polyvinyl chloride 9－poly-acrylonitrile 10－poly-vinylidene chloride Figure 6.1 thermal weightlessness-temperature curve

Constant temperature heat: 6.3.1 thermal degradation Constant temperature heat: Heat the polymer in vacuum and constant temperature for 40~45 min, and estimate the thermal stability of polymer with the temperature that cut weight by half (Th) commonly，polymer with high Th have good thermal stability

6.3.1 thermal degradation differential thermal analysis ： study the physical change (gamma transition, crystallization, melt of crystal) and chemical change (oxidation, thermal decompose) in raise temperature show as differential thermal analysis curve

Figure 6.2 differential thermal analysis curve crystallization oxidation vitrification melt thermal decomposition reference line temperature

6.3.1 thermal degradation Many reactions occur in the polymer thermal degradation, as show below： Main-chain scission side group scission disappear depolymerization

6.3.1 thermal degradation cyclization Many reactions occur in the polymer thermal degradation, as show below： cyclization

6.3.1 thermal degradation Many reactions occur in the polymer thermal degradation, as show below： cross link

6.3.1 thermal degradation The instantiation of depolymerization The key factors that influence the product of thermal degradation are the reactivity of free radical in the thermal degradation process and the activity of hydrogen atomics which are concerned in the chain transfer.

Several examples of depolymerization: A Several examples of depolymerization: A. The monomer yield of all the polymers which contain active hydrogenous such as polyacrylate, branched polyethylene, is very low in thermal degradation. B. If the free radicals created after degradation are stabilities by substituent, such as polymethyl methacrylate, the reaction is degradation mechanism © Because the free radicals created by the degradation of polystyrene conjugate with the benzene ring and stabilization, it have monomer yield(depolymerization and random chain scission) despite of the existence of active hydrogen.

Depolymerization from terminal hydroxy group 6.3.1 thermal degradation Poly-formaldehyde is easily thermal depolymerized, but it isn’t free radical mechanism. The depolymerization often begin from terminal hydroxide group Depolymerization from terminal hydroxy group So the production method of poly-formaldehyde is blocking terminal group that esterify or etherification the terminal hydroxy group to increase the thermal stability.

6.3.1 thermal degradation (2) random chain scission What is the random chain scission? When the polymer is heated, the main chain rupture random. The molecular weight decline quickly, but monomer yield is very low. This is random chain scission. The demonstration of random chain scission： Such as polyethylene, the activity of free radical form after chain scission is high. And the molecule contain many active para hydrogen which make chain transfer and bi- radical disproportionation end easily, so the monomer yield is very low

6.3.1 thermal degradation The reaction of polythene random chain scission： disproportionation cessation

6.3.1 thermal degradation （3） the remove of substitutable group When PVC,PAN, PVAc and PVF are heated, the remove of substitutable group will occur. Therefore the flat roof is often appeared at evening on the thermal weightlessness curve. At 100～120℃, PVC begin to remove HCl. At 200℃ the speed of HCl removing is very fast. So when it is machining（180～200℃）, the phenomena such as the colour and lustre deeper, the strength reduce occur. The reaction as show below ：

2 6.3.1 thermal degradation The free HCl catalyze the remove of HCl. The reaction of PVC substitutable group remove 2 The free HCl catalyze the remove of HCl. Metal chloride can catalyze the reaction, too.

6.3.1 thermal degradation the influencing factor of PVC substitutable group removing . （1）There are unstable structure allyl chloride in molecular chain [in PVC every 1000 carbon atom contain 0.2～1.2 double bond，the chlorine beside double bond is allyl chloride. More double bonds lead to instability.] （2）The molecular weight of PVC also affect the thermal stability [When PVC is hot machining, n% acid absorbent should be added to increase the thermal stability]

6.3.2 Mechanical degradation when plasticate, melt extruding and the macromolecule solution beat up acutely or ultrasonic, macromolecule chain may rupture and degradate When polymer degrade mechanically, the molecular weight decrease with time. As figure 6-3。

Figure 6-3 the relation between inherent viscosity and abrasive time of polystyrene ×-20℃； ○-40℃； ·-60℃

6.4 the aging and anti-aging of polymer Aging symptom ： 1.appearance：change color, distortion, chap, fleck and so on； 2.physical chemistry property: specific gravity, melting point, solubility, molecular weight, thermo-stability, chemistry corrosion resistance and so on； 3. mechanical strength ：tensile strength, impact strength, rigidity, elasticity, abrasive resistance 4. electrical property ：insulation resistance 、dielectric loss 、breakdown voltage and so on