CHAPTER 5 DEGDRADATION OF POLYMERS

1. CHAPTER 5DEGDRADATION OF POLYMERS DR. MOHD WARIKH BIN ABD RASHID FKP

2. OUTLINE CONTENT • Chemical Degradation • Biological Degradation • Stabilizers

3. 5.4: Chemical Degradation • Solvolysis • Step-growth polymers like polyester, polyamides and polycarbonates can be degraded by solvolysis and mainly hydrolysis to give lower molecular weight molecules; • 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.

4. 5.4: Continue • 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 the polymer:

5. 5.4: Continue • (ii) Ozonolysis • 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; • Ozone cracks form in products under tension but the critical strain is very small; • Cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over;

6. 5.4: Continue • Such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe and fuel leakage and fire may follow; • The problem of ozone cracking can be prevented by adding anti-ozonants to the rubber before vulcanization. • 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.

7. 5.4: Continue • (iii) Oxidation • Polymers are susceptible to attack by atmospheric oxygen, especially at elevated temperatures encountered during processing to shape. • Many process methods such as extrusion and injection moulding involve pumping molten polymer into tools, and the high temperatures needed for melting may result in oxidation unless precautions are taken; • For example, a forearm crutch suddenly snapped and the user was severely injured in the resulting fall.

8. 5.4: Continue • The crutch had fractured across a polypropylene insert within the aluminium tube of the device, and infra-red spectroscopy of the material showed that it had oxidised, possible as a result of poor moulding. • Oxidation is usually relatively easy to detect owing to the strong absorption by the carbonyl group in the spectrum of polyolefins. Polypropylene has a relatively simple spectrum with few peaks at the carbonyl position like polyethylene.

9. 5.4: Continue • Oxidation tends to start at tertiary carbon atoms because the free radicals formed here are more stable and longer lasting, making them more susceptible to attack by oxygen. • The carbonyl group can be further oxidised to break the chain, this weakens the material by lowering its molecular weight, and cracks start to grow in the regions affected.

10. 5.4: Continue • (iv) Chlorine-induced craking • Another highly reactive gas is chlorine, which will attack susceptible polymers such as acetal resin and polybutylenepipework. • There have been many examples of such pipes and acetal fittings failing in properties in the US as a result of chlorine-induced cracking. • In essence, the gas attacks sensitive parts of the chain molecules (especially secondary, tertiary, or allylic carbon atoms), oxidizing the chains and ultimately causing chain cleavage.

11. 5.4: Continue • The root cause is traces of chlorine in the water supply, added for its anti-bacterial action, attack occurring even at parts per million traces of the dissolved gas. • The chlorine attacks weak parts of a product, and in the case of an acetal resin junction in a water supply system, it is the thread roots that were attacked first, causing a brittle crack to grow.

12. 5.4: Continue • Discoloration on the fracture surface was caused by deposition of carbonates from the hard water supply, so the joint had been in a critical state for many months. • The problems in the US also occurred to polybutylenepipework, and led to the material being removed from that market, although it is still used elsewhere in the world.

13. 5.5: Biological Degradation • Biodegradable plastics can be biologically degraded by microorganisms to give lower molecular weight molecules. • To degrade properly biodegradable polymers need to be treated like compost and not just left in a landfill site where degradation is very difficult due to the lack of oxygen and moisture.

14. 5.5: Continue • Many opportunities exist for the application of syntheticbiodegradablepolymers in the biomedical area particularly in the fields of tissue engineering and controlled drug delivery. • Degradation is important inbiomedicinefor many reasons. Degradation of the polymeric implant means surgical intervention may not required for removal at the end of its functional life, eliminating the need for a second surgery.

15. 5.5: Continue • In tissue engineering, biodegradable polymers can be designed such to approximate tissues, providing a polymer scaffold that can withstand mechanical stresses, provide a suitable surface for cell attachment and growth, and degrade at a rate that allows the load to be transferred to the new tissue. • In the field of controlled drug delivery, biodegradable polymers offer tremendous potential either as a drug delivery system alone or in conjunction to functioning as a medical device.

16. 5.5: Continue • In the development of applications of biodegradable polymers, the chemistry of some polymers including synthesis and degradation will discuss later. • A description of how properties can be controlled by proper synthetic controls such as copolymer composition, special requirements for processing and handling, and some of the commercial devices based on these materials are discussed.

17. 5.5: Continue • When investigating the selection of the polymer for biomedical applications, important criteria to consider are; • * The mechanical properties must match the application and remain sufficiently strong until the surrounding tissue has healed. • * The degradation time must match the time required. • * It does not invoke a toxic response. • * It is metabolized in the body after fulfilling its purpose. • * It is easily processable in the final product form with an acceptable shelf life and easily Sterilization (microbiology) sterilized.

18. 5.5: Continue • Mechanical performance of a biodegradable polymer depends on various factors which include monomer selection, initiator selection, process conditions and the presence of additives. • These factors influence the polymers crystallinity, melt and glass transition temperatures and molecular weight. • Each of these factors needs to be assessed on how they affect the biodegradation of the polymer;

19. 5.5: Continue • Biodegradation can be accomplished by synthesizing polymers with hydrolytically unstable linkages in the backbone. • This is commonly achieved by the use of chemical functional groups such as esters, anhydrides, orthoesters and amides. • Most biodegradable polymers are synthesized by ring opening polymerization

20. 5.5: Continue • Once implanted, a biodegradable device should maintain its mechanical properties until it is no longer needed and then be absorbed by the body leaving no trace. • The backbone of the polymer is hydrolytically unstable. That is, the polymer is unstable in a water based environment. • This is the prevailing mechanism for the polymers degradation. This occurs in two stages

21. 5.5: Continue • (a) Water penetrates the bulk of the device, attacking the chemical bonds in the amorphous phase and converting long polymer chains into shorter water-soluble fragments. This causes a reduction in molecular weight without the loss of physical properties as the polymer is still held together by the crystalline regions. Water penetrates the device leading to metabolization of the fragments and bulk erosion. • (b) Surface erosion of the polymer occurs when the rate at which the water penetrating the device is slower than the rate of conversion of the polymer into water soluble materials.

22. 5.5: Continue • Biomedical engineers can tailor a polymer to slowly degrade and transfer stress at the appropriate rate to surrounding tissues as they heal by balancing the chemical stability of the polymer backbone, the geometry of the device, and the presence of catalysts, additives or plasticisers.

23. 5.6: Stabilisers • Hindered amine light stabilisers (HALS) stabilise against weathering by scavenging free radicals that are produced by photo-oxidation of the polymer matrix. • UV-absorbersstabilises against weathering by absorbing ultraviolet light and converting it into heat. • Antioxidants stabilize the polymer by terminating the chain reaction due to the absorption of UV light from sunlight. The chain reaction initiated by photo-oxidation leads to cessation of crosslinking of the polymers and degradation the property of polymers.

24. 5.6: Continue • Stabilizers for polymers are used directly or by combinations to prevent the various effects such as oxidation, chain scission and uncontrolled recombinations and cross-linking reactions that are caused by photo-oxidation of polymers. • Polymers are considered to get weathered due to the direct or indirect impact of heat and ultraviolet light. • The effectiveness of the stabilizers against weathering depends on solubility, ability to stabilize in different polymer matrix, the distribution in matrix, evaporation loss during processing and use. • The effect on the viscosity is also an important concern for processing.

25. 5.6: Continue • (i) Antioxidants • Antioxidants are used to terminate the oxidation reactions taking place due to different weathering conditions and reduce the degradation of organic materials. For example, synthetic polymers react with atmospheric oxygen; • Organic materials undergo auto-oxidizations due to free radical chain reaction. • Oxidatively sensitive substrates will react with atmospheric oxygen directly and produce free radicals. Free radicals are of different forms, consider organic material RH

26. 5.5: Continue • This material reacts with oxygen to give free radicals such as R• , RO• , ROO• , HO•[1]. • These free radicals further react with atmospheric oxygen to produce more and more free radicals. • For example, • R• + O2 -> ROO• ROO• + RH -> ROOH + R• • This can be terminated using the antioxidants. Then this reaction comes to, • 2R• -> R----R ROO• + R• -> ROOR 2ROO• • Non-radical products[1] Weathering of polymers is caused by absorption of UV lights, which results in, radical initiated auto-oxidation

27. 5.5: Continue • This produces cleavage of hydro peroxides and carbonyl compounds. • This is because of the weak bond in hydro peroxides which is the main source for the free radicals to initiate from. • Homolytic decomposition of hydro peroxide increases the rate of free radicals production. Therefore it is important factor in determining oxidative stability. • The conversion of peroxy and alkyl radicals to non-radical species terminates the chain reaction, thereby decreasing the kinetic chain length.

28. 5.5: Continue • Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with organic substrate (RH) for peroxy radicals, thereby terminating the chain reaction and stabilizing the further oxidation reactions. • At K17, ROO• + AH -> ROOH + A• • At K6, ROO• + RH -> ROOH + R• • Here K17 is larger than K6, therefore AH can be at low concentrations. At low concentrations AH are more effective because the usual concentration in saturated plastic polymer range from 0.01 to 0.05% based on the weight of the polymer.

29. 5.5: Continue • Benzofuranones is another most effective antioxidant, which terminates the chain reaction by donating weakly bonded benzylic hydrogen atom and gets reduced to a stable benzofuranyl (lactone). • Antioxidants inhibits the formation of the free radicals thereby enhancing the stability of polymers against light and heat.

30. 5.5: Continue • (ii) Hindered Amine Light Stabiliser (HALS) • A The ability of hindered amine light stabiliser (HALS) to scavenge radicals which are produced by weathering, may be explained by the formation of nitroxyl radicals through a process known as the Denisov Cycle. • The nitroxyl radical(R-O•) combines with free radical in polymers: • R-O• + R'• -> R-O-R'

31. 5.5: Continue • B Although they are traditionally considered as light stabilizers, it can also stabilize thermal degradation. • Even though HALS are extremely effective in polyolefins, polyethylene and polyurethane, they are ineffective in polyvinyl chloride (PVC). • It is thought that their ability to form nitroxyl radical is disrupted. HALS act as base and become neutralized by hydrochloric acid (HCl) that is released by photooxidation of PVC. • The exception is the recently developed NOR HALS which is not a strong base and is not deactivated by HCl ;

32. 5.5: Continue • Benzofurano The UV absorbers dissipate the absorbed light energy from UV ray as heat by reversible intramolecular proton transfer. • This reduces the absorption of UV ray by polymer matrix and hence reduces the rate of weathering. • Typical UV-absorbers are oxanilides for polyamides, benzophenones for PVC, benzotriazoles and hydroxyphenyltriazines for polycarbonate.

33. 5.5: Continue • Strongly light-absorbing PPS is difficult to stabilize. • Even antioxidants fail in this polymer since the polymer is electron-rich and behaves as antioxidant. • The acids or bases in the PPS matrix can disrupt the performance of the conventional UV absorbers such as HPBT. • PTHPBT which is modification of HPBT are shown to be effective even in these conditions;

34. 5.5: Continue • Antiozonant • Antiozonants prevent or slow down the degradation of material caused by ozone gas in the air (ozone cracking). • (iv) Organosulfurcompounds • Organosulfur compounds are efficient hydroperoxide decomposers, which thermally stabilize the polymers. Sulfuric acids are produced as the product of decomposition, which catalyse further hydroperoxide decomposition;