Polymer Degradation

What is polymer degradation?

The degradation in the properties of polymer which is characterized by an uncontrolled changed in the molecular weight or constitution of the polymer is called polymer degradation.Conventionally the term degradation is taken to mean a reduction in the molecular weight of polymer.

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, alkali and some salts. These changes are usually undesirable, such as cracking and chemical disintegration of products or, more rarely, desirable, as in bio-degradation, or deliberately lowering the molecular weight of a polymer for recycling. The changes in properties are often termed "aging".

Degradation can be useful for recycling/reusing the polymer waste to prevent or reduce environmental pollution. Degradation can also be induced deliberately to assist structure determination.

Polymeric molecules are very large (on the molecular scale), and their unique and useful properties are mainly a result of their size. Any loss in chain length lowers tensile strength and is a primary cause of premature cracking.

Almost 50% of failures of engineering plastics result from environmental degradation. The increasing utilization of plastics in more exacting applications and the pressure for increased life, without uneconomic over design, are imposing a requirement for improved characterization of the performance of polymeric materials. The major challenge is to predict long term behavior from short term laboratory or field exposures.

The most common mode of environmental degradation is caused by exposure of engineering polymers to chemicals, resulting in environmental induced stress cracking. The steps in the degradation process involve stress enhanced absorption and concentration of the chemical molecules at susceptible micro structural sites. Localized plasticization then ensues, leading to crazing and subsequent crack development.

Failure may often be associated with exposure to secondary fluids, such as cleaning agents or lubricants, rather than the primary design environment, and with residual molding stresses.

Stage of degradation:

A polymer can suffer degradation mainly two stages.

                      First — During Fabrication

                      Second — It’s daily usage

Normally polymer degradation is undesirable but sometime it is desirable.

DEGRADING AGENTS:

• Atmospheric oxygen
• Moisture
• Scission
• Depolymerization
• Cross-linking
• Changes in side groups

1.  When plastics, melt extruding and the macro molecule solution beat up acutely, macro molecule chain may rupture and degrade.
2. When polymer degrade mechanically, the molecular wt decrease with
3. Time.Mechanical energy transferred to a polymeric system can be dissipated via relaxation processes without chemical changes.
4. Degradation will occur if relaxation is impeded
5. Strain is a pre-requisite for bond rupture in polymer chains
6. Stressed reaction:
1. High speed stirring
2. Shaking
3. Turbulent flow
7. The larger the initial molecular wt, the greater the molecular wt drop due to mechanical degradation.

The mechanism of mechanical degradation:

• 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 sterilized.
• 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.
• 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.

AGING:

          The polymer effect by physical and chemical factor, the main reaction is degradation, sometimes the cross linking occur.

Aging symptom:

1. Appearance: change color, distortion, chap, fleck and soon ;

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

Physical degradation:

1. Thermal degradation
2. Mechanical degradation
3. Chemical Degradation
4. Biological degradation
5. UV Degradation
6. Oxidation Degradation
7. Hydrolytic Degradation
8. Ultrasonic wave degradation

Thermal Degradation:

Thermal degradation of polymers is molecular deterioration as a result of overheating. At high temperatures the components of the long chain backbone of the polymer can begin to separate and react with one another to change the properties of the polymer. Thermal degradation can present an upper limit to the service temperature of plastics as much as the possibility of mechanical property loss. Indeed unless correctly prevented, significant thermal degradation can occur at temperatures much lower than those at which mechanical failure is likely to occur. The chemical reactions involved in thermal degradation lead to physical and optical property changes relative to the initially specified properties. Thermal degradation generally involves changes to the molecular weight of the polymer and typical property changes include reduced ductility and embrittlement, chalking, color changes, cracking, general reduction in most other desirable physical properties.

The Mechanism of Thermal Degradation

Most types of degradation follow a similar basic pattern. The conventional model for thermal degradation is that of an autoxidation process which involves the major steps of initiation, propagation, branching, and termination.

Initiation

The initiation of thermal degradation involves the loss of a hydrogen atom from the polymer chain as a result of energy input from heat or light. This creates a highly reactive and unstable polymer ‘free radical’ (R•) and a hydrogen atom with an unpaired electron (H•).

Propagation

The propagation of thermal degradation can involve a variety of reactions and one of these is where the free radical (R•) reacts with an oxygen (O2) molecule to form a proxy radical (ROO•) which can then remove a hydrogen atom from another polymer chain to form a hydroperoxide (ROOH) and so regenerate the free radical (R•). The hydroperoxide can then split into two new free radicals, (RO•) + (•OH), which will continue to propagate the reaction to other polymer molecules. The process can therefore accelerate depending on how easy it is to remove the hydrogen from the polymer chain.

Termination

The termination of thermal degradation is achieved by ‘mopping up’ the free radicals to create inert products. This can occur naturally by combining free radicals or it can be assisted by using stabilizers in the plastic.

The Research Methods of Thermal Degradation of Polymers

TGA

Thermogravimetric Analysis (TGA) refers to the techniques where a sample is heated in a controlled atmosphere at a defined heating rate whilst the samples mass is measured. When a polymer sample degrades, its mass decreases due to the production of gaseous products like carbon monoxide, water vapour and carbon dioxide.

DTA and DSC

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC): Analyzing the heating effect of polymer during the physical changes in terms of glass transition, melting, and so on.These techniques measure the heat flow associated with oxidation.

Ways of Polymer Thermal Degradation

Depolymerisation

Under thermal effect, the end of polymer chain departs, and forms low free radical which has low activity. Then according to the chain reaction mechanism, the polymer loses the monomer one by one. However, the molecular chain doesn’t change a lot in a short time. The reaction is shown below.This process is common for polymethymethacrylate (Perspex).

Random Chain Scission

The backbone will be break down randomly, could be occurred at any position of the backbone. The molecular weight decreases rapidly, and cannot get monomer in this reaction, this is because it forms new free radical which has high activity can occurs intermolecular chain transfer and disproportion termination with the CH2 group.

Side-Group Elimination

Groups that are attached to the side of the backbone are held by bonds which are weaker than the bonds connecting the chain. When the polymer was being heated, the side groups are stripped off from the chain before it is broken into smaller pieces. For example the PVC eliminates HCL, under 100-120oC.

Mechanical degradation:

Chemical Degradation:

Chemically Degradation of Polymers is a polymer degradation that involves a change of the polymer properties due to a chemical reaction with the polymer’s surroundings. There are many different types of possible chemical reactions causing degradation however most of these reactions result in the breaking of double bonds within the polymer structure.

1. Solvolysis
2. Ozonolysis
3. Oxidation
4. Galvanic action
5. Chlorine-induced cracking

Solvolysis

Step-growth polymers like polyesters, 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. 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.

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, with Natural rubber, polybutadiene, Styrene-butadiene rubber and NBR being most sensitive to degradation. 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. 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.

Oxidation

IR spectrum showing carbonyl absorption due to oxidative degradation of polypropylene crutch moulding.

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. 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). 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.

Galvanic action

Polymer degradation by galvanic action was first described in the technical literature by Michael C. Faudree in 1990.[2][3]. This was the discovery that "plastics can corrode", i.e. polymer degradation may occur through galvanic action similar to that of metals under certain conditions. Normally, when two dissimilar metals such as copper (Cu) and iron (Fe) are put into contact and then immersed in salt water, the iron will undergo corrosion, or rust. This is called a galvanic circuit where the copper is the noble metal and the iron is the active metal, i.e., the copper is the cathode or positive (+) electrode and the iron is the anode, or negative (-) electrode. A battery is formed. It follows that plastics are made stronger by impregnating them with thin carbon fibers only a few micrometers in diameter known as carbon fiber reinforced polymers (CFRP). This is to produce materials that are high strength and resistant to high temperatures. The carbon fibers act as a noble metal similar to gold (Au) or platinum (Pt). When put into contact with a more active metal, for example with aluminum (Al) in salt water the aluminum corrodes. However in early 1990, Michael C. Faudree discovered that imide-linked resins in CFRP composites degrade when bare composite is coupled with an active metal in salt water environments. This is because corrosion not only occurs at the aluminum anode, but also at the carbon fiber cathode in the form of a very strong base with a pH of about 13. This strong base reacts with the polymer chain structure degrading the polymer. Polymers affected include bismaleimides (BMI), condensation polyimides, triazines, and blends thereof. Degradation occurs in the form of dissolved resin and loose fibers. The hydroxyl ions generated at the graphite cathode attack the O-C-N bond in the polyimide structure. This phenomenon, that polymers can undergo galvanic corrosion like metals do has been referred to in the field as the "Faudree Effect". Standard corrosion protection procedures were found to prevent polymer degradation under most conditions although research is ongoing.

Chlorine-induced cracking

Another highly reactive gas is chlorine, which will attack susceptible polymers such as acetal resin and polybutylene pipework. 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. 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. Discolouration 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 polybutylene pipework, and led to the material being removed from that market, although it is still used elsewhere in the world.

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.

Polymer chemistry and material selection.

When investigating the selection of the polymer for biomedical applications, important criteria to consider are-

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.[5] 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.

Processing         

Biodegradable polymers can be melt processed by conventional means such as compression or injection molding. Special consideration must be given to the need to exclude moisture from the material. Care must be taken to dry the polymers before processing to exclude humidity. As most biodegradable polymers have been synthesized by ring opening polymerization, a thermodynamic equilibrium exists between the forward polymerization reaction and the reverse reaction that results in monomer formation. Care needs to be taken to avoid an excessively high processing temperature that may result in monomer formation during the molding and extrusion process.

Degradation

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.

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.

An example of the structure of some of the types of polymer degradation can be viewed in Figure one in this article.

Stabilisers

Hindered amine light stabilisers (HALS) stabilise against weathering by scavenging free radicals that are produced by photo-oxidation of the polymer matrix. UV-absorbers stabilises 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.

UV Degradation

Many natural and synthetic polymers are attacked by ultra-violet radiation and products made using these materials may crack or disintegrate (if they're not UV-stable). The problem is known as UV degradation, and is a common problem in products exposed to sunlight. Continuous exposure is a more serious problem than intermittent exposure, since attack is dependent on the extent and degree of exposure.

Many pigments and dyes can also be affected, when the problem is known as photo tendering in textiles such as curtains or drapes.

Susceptible polymers

Common synthetic polymers which may be attacked include polypropylene and LDPE where tertiary carbon bonds in their chain structures are the centres of attack. The ultra-violet rays activate such bonds to form free radicals, which then react further with oxygen in the atmosphere, producing carbonyl groups in the main chain. The exposed surfaces of products may then discolour and crack, although in bad cases, complete product disintegration can occur.

In fibre products like rope used in outdoor applications, product life will be low because the outer fibres will be attacked first, and will easily be damaged by abrasion for example. Discolouration of the rope may also occur, so giving an early warning of the problem.

Polymers which possess UV-absorbing groups such as aromatic rings may also be sensitive to UV degradation. Aramid fibres like Kevlar for example are highly UV sensitive and must be protected from the deleterious effects of sunlight.

Detection

The problem can be detected before serious cracks are seen in a product using infra-red spectroscopy, where attack occurs by oxidation of bonds activated by the UV radiation forming carbonyl groups in the polymer chains.

In the example shown at left, carbonyl groups were easily detected by IR spectroscopy from a cast thin film. The product was a road cone made by rotational moulding in LDPE, which had cracked prematurely in service. Many similar cones also failed because an anti-UV additive had not been used during processing. Other plastic products which failed included polypropylene mancabs used at roadworks which cracked after service of only a few months.

Prevention

UV attack by sunlight can be ameliorated or prevented by adding anti-UV chemicals to the polymer when mixing the ingredients, prior to shaping the product by injection moulding for example.

UV Stabilizers in plastics usually act by absorbing the UV radiation preferentially, and dissipating the energy as low level heat. The chemicals used are similar to those used in sunscreen cosmetic products, which protect skin from UV attack.

UV degradation in material testing

The effects of UV degradation on materials that require a long service life can be measured with accelerated exposure tests. With modern solar concentrator technologies it’s possible to simulate 63 years of natural UV radiation exposure on a test device in a single year. In Solar Power Modules degradation is a longterm issue according to material testings; the efficiency loss by UV degradation is about 6.81 to 9.54 % in 20 years

Oxidative Degradation

In the presence of oxygen or ozone, as soon as free radicals form, oxygenation of the radicals gives rise to peroxy radicals, which through a complex series of reactions result in polymer degradation. Oxidative degradation may occur at moderate temperature (thermal oxidation) or under the influence of ultraviolet radiation (photooxidation). Unsaturated polyolefins are particularly susceptible to attack by oxygen or ozone.

Hydrolytic Degradation

Hydrolysis is a major degradation mechanism in which vulnerable bonds in a polymer chain react with water molecules, break up, and result in smaller chains. Chemical reactivity of polymer bonds, diffusion rates of reactants and products including water, polymer bonds, ions in testing media, and small polymer segments, and polymer-water thermodynamic interactions all are involved. In order to understand each fundamental step of polymer hydrolysis, studies should be designed such that these factors are decoupled. In this review, hydrolytic degradation will be discussed at three levels to decouple those factors. The first part is at the molecular level. Polymer solutions are used so the diffusion of reactants and the polymer-water interactions are not limiting factors. Therefore, hydrolysis is controlled by chemical reactivity only. A second part also is at the molecular level but the molecular mobility and water-polymer interactions are added to the degradation processes by using bulk samples. A third one is at macroscopic level. The focus is on the diffusion-reaction competition at the macroscopic scale. Surface erosion is at this length scale and is discussed. In all these discussions, a polylactide sample (PLA) composed of 70% l-lactic acid and 30% d,l-lactic acid is used as the example. Other polymers such as poly(anhydride), polyorthoesters, and poly(amino acid) may have different degradation kinetics, but the same principles should apply.

Hydrolysis in Solutions and Gels–Chemistry of Hydrolysis

In hydrolytic degradation, polymer bonds react with water molecules, break up, and produce new chain ends. The original chains break up into smaller segments, resulting in polymer degradation. Most chemical groups that react with water contain O, N, S, P, and other non-carbon atoms. These atoms cause the adjacent carbon atoms to be positively charged. Electron-withdrawing oxygen atoms of water molecules attack these positively charged carbon atoms through a 2nd order nucleophilic substitution reaction resulting in the formation of two new chemical spices. A general reaction mechanism is illustrated in Scheme 1.

The reactant molecules, X, Y, Z, and R, can be C, O, N, and other atoms. The charge value of the reacting C atoms is a primary factor affecting the hydrolysis reactivity. Charge values of a few common groups have been calculated using Accelrys Material Studio® and are listed in Table 1. The chemical groups such as esters, anhydrides, orthoesters, etc. are usually more susceptible to hydrolysis than C-C, C-O-C moieties, etc. The charge values of these two types of chemical groups seem to be distinguished by a value of 0.3 electron charges. The chemical groups having charge higher than 0.3 electron charges seem to be hydrolytically more active than those with charge lower than 0.3. However, hydrolytic activities of chemical groups also depend on many other factors such as conjugate structures that stabilize chemical groups. For example, urethane, carbonate, and aromatic ester groups have large conjugate structures; so they are generally more stable than aliphatic esters against hydrolysis, even when their charge values are high. The side groups also reduce hydrolytic activity of carbons via steric effects. For example, the silicon atoms in polydimethyldiloxane (PDMS) have 0.62 electric charges, but the two methyl groups closely protect the silicon atoms and PDMS is rather stable towards hydrolysis. For the same reason, poly(lactic acid) (PLA) is less reactive than poly(glycolic acid) (PGA). On the other hand, polyorthoester, even with a relatively low charge at its central carbon atoms, reacts with water rather fast. This is due to the high bond tension caused by the three RO- groups around the carbon atoms. Besides these, the dielectric constant, degradable bond concentration, and water solubility also affect the hydrolytic activities of polymers. Overall, all chemical groups, except pure C-C bond, can undergo hydrolysis reaction. The only difference is that they react at different rates.

Ultrasonic wave degradation

Ultrasonic waves are a very high frequency (above 20,000 Hz) which is beyond the available range of the human ear. When dilute solution of a high molecular wt polymer is subjected to ultrasonic waves, the polymer begins to degrade. As ultrasonic waves pass through the solution, the localized shear gradients produced tear off the polymer molecules leading to chain scission. As a result average molecular wt of the sample goes down. The decreased in molecular wt does not go below a certain value even after a long exposure. In this process bigger molecules are affected more than the smaller one.