Mehdi Tavakoli, after gaining a BSc in chemistry, studied for an MSc in polymer science and technology at Aston University. He then completed a PhD in the same field at Aston and graduated in 1981. He continued research into polymer degradation and stabilisation. Prior to joining TWI in 1989 as a principal polymer scientist he worked at Bath University on polymer composite manufacturing processes including RTM, fibre/resin compatibility, stress rupture behaviour and damage mechanisms in phenolic resin composites. Since joining TWI, Mehdi has been working mainly on novel surface preparation techniques, including the application of power beams for adhesive bonding, and mechanisms of degradation in polymer welds and adhesively bonded joints.
While many polymer scientists are developing ways of extending the life of polymers, others are trying to make them degrade as soon as their useful life is over. Recent work at TWI has shown that thermoplastic welds degrade faster than their parent material. Mehdi Tavakoli examines the mechanisms and significance of degradation in polymeric systems.
As consumption of polymeric materials increases and their applications expand, problems of durability and disposal become more important. These materials are prone to degradation when exposed to oxygen, heat, light, mechanical forces, biological environments, or other sources of energy, and irreversible processes can occur leading to loss of useful properties and to failure. There are many different types of degradation depending on the individual or combined effects of environmental agents.
For many years polymer scientists and technologists have investigated the resistance of polymers to degradation and developed materials with improved performance. However, in recent years concern regarding disposal of wastes, particularly those used in packaging, has become a major environmental issue. To overcome these problems, polymer producers are developing materials which can be recycled, or degraded. Furthermore, new polymers with controlled service life have been developed which decompose after fulfilling their function.
Engineering applications frequently require welding of polymeric materials, but recent work at TWI has shown that thermoplastic welds degrade faster than the corresponding parent material. Welding, like other processing operations such as injection moulding and extrusion, exposes polymers to elevated temperatures and mechanical forces which may initiate degradation. The initial oxidised products and/or residual stresses may then accelerate degradation during service, leading to deterioration of properties at the joint. Further work is in progress to study degradation mechanisms and ageing properties of joints welded by different techniques. This will lead to improvements in selection of material, welding procedures and improved long-term durability of welded joints.
This article deals with general mechanisms of degradation in polymers, to provide an introduction to a technology likely to be unfamiliar to many Bulletin readers. Future issues will focus on effects of environmental agents on durability, and mechanisms of degradation in welded and adhesively bonded joints.
The environment
Both chemical reactants and energy sources contribute to degradation of polymeric materials. Oxygen and water are two of the most important chemical reactants responsible for polymer deterioration. All polymers react with oxygen particularly under extreme conditions, i.e. at elevated temperatures, and many oxidise even at low temperatures. Reaction with oxygen accounts for the vast majority of polymer failures occurring during service. This includes attack of ozone on unsaturated polymers under stress.
Water reacts with certain polymers containing hydrolysable groups, such as ester and amide groups, and adversely affects important properties, polymer hydrolysis often being catalysed by traces of bases and acids. In addition to oxygen and water, there are many other chemicals to which polymers may be exposed during service. These include solvents, lubricating fluids, detergents and other foreign materials.
Acceleration of degradation by heat is a general phenomenon responsible for deterioration of polymers either in the presence or absence of chemical reactants. Various forms of radiation and mechanical deformation can also contribute to polymer failure.
The major types of degradation of polymers shown in Fig.1 are described below.
Degradation
Thermal degradation
Heat is a major factor in polymer deterioration. Exposure to elevated temperatures during processing or during service accounts for failure of many different types of polymer. Absorption of thermal energy is the primary environmental factor responsible for pyrolysis in an inert atmosphere or in vacuum. Dependent on polymer structure, thermal degradation in the absence of oxygen is usually associated with elimination of low molecular weight, volatile fragments, but rupture of polymer chains can also occur by depolymerisation or by a random process initiated at weak bonds in the chain.
Thermal-oxidative degradation
All organic materials are prone to oxidative degradation. Natural and synthetic polymers are very sensitive to reaction with oxygen, particularly at elevated temperatures. During compounding and processing operations and conversion of polymers to fabricated end products, they are normally exposed to high mechanical deformations and elevated temperatures. These operations inevitably affect chemical structure to some degree, possibly with significant influence on long-term serviceability. Hydroperoxides are the initial product of thermal-oxidative degradation, and these may initiate and accelerate further degradation.
Burning is an extreme case of thermal-oxidative degradation, and flammability is a major limiting factor in replacement of metals and other traditional materials by polymers. Polymeric materials are mainly organic in nature and so they burn when the temperature is high enough.
Polymers are being increasingly used as construction materials in areas such as domestic and industrial buildings, domestic appliances and vehicles. When a fire strikes a high rise building, a chemical plant, a train or an aircraft, heat and smoke can burn and suffocate occupants, and destroy or damage the structural materials with catastrophic consequences. But the flammability of polymers and the associated damage to buildings are not the main problem. Casualties in a fire are mainly caused by suffocation by smoke or poisoning by noxious gases, rather than as a direct result of burning. Smoke can also limit visibility and make escape more difficult.
Fire retardance in polymers is known to result from the presence of one or more essential elements, particularly bromine, chlorine, phosphorus, nitrogen, aluminium, antimony, or a volatile ingredient ( e.g. water of hydration). The main types of flame retardants used in polymeric systems include halogenated products ( e.g. chlorinated paraffins, bromo- and chloro-aromatic additives), inorganic fillers and additives such as alumina trihydrate (Al 2O 3 3H 2O) and antimony trioxide (Sb 2O 3).
Photo-degradation
Radiation effects in polymer degradation are mainly based on the influence of ultraviolet light or ionising radiation. X-rays and γ-rays are the principal ionising radiations responsible for deterioration and the ultraviolet region of the electromagnetic spectrum (solar radiation) provides the energy for photoreactions in normal outdoor weathering conditions.
Photo-oxidation is the principal reaction in degradation of polymers during exposure to ultraviolet radiation. Photo-oxidation is initiated by energy absorbed from light sources, either artificial light or sunlight.
The intensity of ultraviolet light to which polymers are exposed in natural weathering is not constant. In addition to obvious variations that occur with season and location, smog density, cloud cover and temperature can affect the intensity of incident radiation and therefore the rate of photo-oxidation.
The combination of lower density and strength makes polymers of special interest in space applications, both as dielectric materials and as structural components. However, the high energy radiation encountered in space places a serious limitation on these applications.
A study of effects of photo-degradation on welded and adhesively bonded joints is proposed for TWI's future Core Research Programme.
Chemical degradation
Apart from oxygen and ozone, a wide variety of chemical reactants can contribute to polymer deterioration. Some, particularly water or other atmospheric contaminants, are often present in a normal environment. Effects of water are most evident in hydrolysis of condensation polymers.
Resistance of a polymer to chemical attack is primarily dependent upon the reactivity and availability of the degradant, and on the chemical and physical structure of the polymer, and the effect must be recognised when selecting a polymer for a specific environment.
Mechano-degradation
Polymer degradation initiated by mechanical forces is called mechano-degradation. This occurs during processing, milling, grinding or when polymers are subjected to shear, tension or compression during processing or during service. Such degradation is important during compounding of rubber, when it is used to reduce molecular weight and viscosity. Efficient mixing of vulcanising ingredients can only be achieved after plasticisation of rubber by this process.
Mechano-chemical degradation
Polymers may be used under high static or cyclic stresses. The term fatigue is used in the rubber field in relation to long-term degradation under the action of stresses and chemical agents, particularly oxygen and ozone. This phenomenon is of paramount practical importance particularly for polymers that undergo cyclic stresses in use.
In a motor car tyre, for instance, the sidewall and tread are exposed to cyclic stresses during normal operation. Because of application of repeated mechanical stresses, rupture of the polymer chains occurs with formation of free radicals and consequent modification of chemical structure. This is usually associated with formation of cracks, deterioration of strength and finally failure of the rubber component. Fatigue of polymers is therefore regarded as a mechano-chemical process.
Mechano-chemical processes take place also during wear of polymers, causing chemical and structural changes on the surface.
Environmental effects in fracture of metals are well-known, e.g. hydrogen embrittlement, stress corrosion cracking, liquid metal embrittlement. However, effects of environment and stress are also highly significant for polymeric materials. For instance, it is not uncommon for a ductile polymer to fail in an apparently brittle manner at a fraction of its ultimate strength if exposed to certain environments ( e.g. stressed polycarbonate in contact with NaOH and ethanol).
Environmental stress cracking (ESC) or solvent crazing may be regarded as a type of mechano-chemical degradation. ESC, which is normally limited to partially crystalline polymers or fibre-reinforced thermoset based composites, occurs when small amounts of certain non-solvents contact stressed polymer surfaces. Residual stresses, which may be generated during moulding, welding or other processing operations, are often sufficient to initiate environmental stress cracking. Immersion of moulded parts in certain chemical environments is a widely used, but destructive, method for detecting residual tensile stresses.
Solvent crazing occurs in both amorphous and crystalline polymers when small amounts of solvents are in contact with surfaces of stressed polymers. The crazing process is usually reversible, since crazes disappear when the stress is removed. The rate of application of the solvent and the level of stress are the most important parameters in solvent crazing.
Biodegradation
When polymeric implants are placed into a living organism, an immune reaction is initiated. Oxygen concentration as well as the concentration of physiologically active substances, the pH of the environment and the concentration of metallic ions all change. Durability of a polymer introduced into a living body is mainly influenced by the presence of salts and water, the pH of the environment and the action of enzymes.
TWI is also developing an interest in joining of biomaterials, particularly biomedical polymers.
Polymers are used as carriers for controlled release of bioactive agents. In a controlled release system, a drug, pesticide, or other bioactive agent is incorporated into a carrier, generally a polymeric material. The rate of release of the substance is determined by the properties of the polymer itself and is only weakly dependent on environmental factors such as the pH of the body fluids. Controlled release systems are capable of delivering substances gradually and continuously for up to several years. Controlled release formulations were first used in agricultural applications for low molecular weight fertilisers, antifoulants and pesticides and then extended into the medical field.
Polymers used for implants in living bodies are expected to be bioresistable. However, heterochain polymers, particularly those containing nitrogen and/or oxygen in the main chain, are generally prone to hydrolysis. Depending on the chemical structure of the polymer, this can be accelerated by either alkaline or acidic conditions. Hydrolysis of many heterochain polymers may occur in the aqueous environment of the body, particularly in the presence of hydrolytic enzymes. Polymers known to degrade by hydrolysis are polyesters, cyanoacrylates, some polyurethanes and polyamides including nylons.
The hydrolytic instability of certain bonds can be utilised for design of intentionally degradable polymers in drug delivery systems. Another example of this principle is use of the aliphatic polyesters, polyglycolic and polyactic acids for absorbable sutures as alternatives to catgut. The non-hydrolysable polymers employed in surgery, such as fluorinated polymers, ( e.g. PTFE), some polyolefines and polyether-based polyurethanes may degrade in the presence of heat, light (UV or high energy radiation) and oxygen. Although these conditions are not encountered in a living body, slow degradation of these polymers is known to occur. This could be caused by initiation of degradation during fabrication or sterilisation of the implants which then may continue after implantation. Evaluation of the kinetics and mechanisms of degradation of biomedical polymers is becoming very important because of the need to quantify predictions of an implant's lifetime.
Degradable polymers
Interest in degradable polymers has been increasing significantly in recent years as a result of problems associated with their disposal and their effects on the environment. The solid waste problems and litter in the countryside, cities and marine environments have intensified efforts to produce degradable polymers, particularly those used in packaging and disposable applications.
In some applications, such as sutures used in surgery, they do not cause any problem.
There is also interest in using degradable polymers in agricultural planting containers and agricultural mulches: ultimate biodegradability, as in composting, would allow degraded polymers to combine with other biodegradable materials and transform into useful soil-improvers. Application of degradable polymers as agricultural mulches enables growers to use polymer films to help with plant growth, then photo-degrade in the fields and thus avoid the cost of removal. Polymers for this purpose normally contain photo-sensitising additives which make the polymer undergo photodegradation. For example, polycaprolactone is being used as plant containers for automated machine planting of tree seedlings, and within only 6-12 months in soil the containers undergo significant biodegradation.
Two main types of degradation are usually found in degradable polymers - biodegradation and photo-degradation - and these two processes may occur individually or in combination. In fact, most applications of photodegradable polymers rely on biodegradation to consume the polymer oxidised products resulting from initial photo-degradation.
A degradable polymer under the trade name Biopol was introduced by ICI last year. Biopol is a biodegradable thermoplastic polyester which is also known as poly(hydroxybutyrate-co-hydroxyvalerate) or PHB/V. The ester linkages in the polymer backbone are prone to hydrolysis as well as to biological attack from enzymes secreted by micro-organisms. As shown in Fig.2, carbohydrates generated during photosynthesis of crops, such as cereals and sugar beet, are the raw materials from which Biopol is produced. The carbohydrate is converted into Biopol by a specific micro-organism, 'alcaligenes eutrophus', which occurs widely in water and in soil. At the end of fermentation, Biopol is produced by extracting and purifying the polymer. The product can be processed by conventional techniques into various articles such as bottles, films and fibres. ICI states that it can be safely incinerated or biodegraded and converted into CO 2 and water after use. Figure 3 shows Biopol bottles at various stages of biodegradation after disposal.
