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 Professor Tavakoli has worked mainly on adhesion and adhesive bonding, novel surface preparation techniques, including the application of power beams for adhesive bonding, and mechanisms of degradation in polymer welds and adhesively bonded joints. In recent years he has been particularly involved in research on medical and biomaterials and related joining processes. He is a Visiting Professor of Medical Materials at Queen Mary/University of London. Recently he has also been appointed as a Visiting Professor of Medical Devices at the Bioengineering Research unit of the University of Strathclyde. He is currently the Programme Manager of the Medical Devices Faraday Partnership.
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 environment, or other sources of energy, and irreversible processes can occur leading to loss of useful properties and indeed failure. As Mehdi Tavakoli discusses, there are many different types of degradation depending on the individual or combined effects of environmental agents.
For many years engineers, polymer scientists and technologists have investigated the resistance of polymers to degradation and developed materials with improved performance. It has been a subject of crucial interest to many of TWI's Industrial Member sectors ( eg aerospace, automotive, microelectronics and medical) which have been introducing polymers for many new applications or for replacing other materials.
However, recent concern regarding disposal of wastes, particularly those used in packaging, has become a major environmental issue. To overcome these problems, polymer producers have been developing materials that can be recycled, or degraded. Furthermore, new polymers with controlled service life have been developed which decompose after fulfilling their function.
Many engineering applications, like gas and water pipe welding, frequently require welding of polymeric materials, but related 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 that 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.
This feature revisits a topic, first tackled in Bulletin 14 years ago and which has changed appreciably during that period.
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, ie at elevated temperatures, and some may oxidise even at low temperatures with time. 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 a 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 (unzipping reactions, the reverse of chain polymerisation) or by a random process initiated at weak chemical bonds in the chain. Exposure to low temperatures may also be responsible for changes in properties of a polymer.
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 known to be 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.
Polymeric materials are being increasingly used as construction materials in areas such as domestic and industrial buildings, domestic appliances and vehicles, in furniture manufacture, in the electronics industry ( eg as one of the main constituents of printed circuit boards) as well as in food packaging. 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, particularly in a confined environment, are primarily 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 retardancy in polymers is generally known to result from the presence of one or more essential elements, particularly bromine, chlorine, phosphorus, nitrogen, aluminium, antimony, or a volatile ingredient ( eg water of hydration). The main types of flame retardants used traditionally in polymeric systems include halogenated products ( eg chlorinated paraffins, bromo- and chloro-aromatic additives), inorganic fillers and additives such as alumina trihydrate (Al 2 O 3 H 2 O), magnesium hydroxide, and antimony trioxide (Sb 2 O 3 ).
In recent years the use of halogen free systems such as alumina trihydrate, and new approaches such as development of intrinsically fire-resistant polymers, new fire resistant coatings, the uses of some nanocomposites and carbon nanotubes and nitrogen compounds have found more acceptance due to new environmental legislations.
Photo-degradation
Radiation effects in polymer degradation are mainly based on the influence of ultraviolet light or ionising radiation. X-rays and g-rays are the principal ionising radiations responsible for deterioration and the ultraviolet region of the electromagnetic spectrum (solar radiation) provides the energy for photo-reactions 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.
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 polymeric material to chemical attack is primarily dependent upon the chemical and physical structure of the polymer and the nature and the effectiveness of the stabilising system used. All the relevant parameters need to be considered carefully before selecting a polymer for a specific application in a chemical environment.
Mechano-degradation
Polymer degradation initiated by mechanical forces is regarded as 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 in a controlled manner is important during compounding of rubber, when it is used to reduce molecular weight and viscosity and plasticise the rubber prior to curing and vulcanisation. Efficient mixing of vulcanising ingredients can only be achieved after plasticisation of rubber by this process. However, mechano-degradation or stress-induced reactions could also lead to deterioration of useful properties.
Mechano-chemical degradation
Polymers may be used under high static or cyclic stresses. The term fatigue is particularly relevant 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 service.
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 may occur 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. A typical example of the wear of polymers is wear of unmodified polyethylene, as in the cup-shaped hollow socket, or acetabulum, used in some hip implants. A process called Steolysis, commonly known as particle disease, may occur causing complications with the patient using the implant. For this reason considerable effort has been devoted in recent years to using alternative materials ( eg ceramics) or more wear resistant polymers ( eg cross-linked polyethylene).
Environmental effects in fracture of metals are well-known, eg 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 ( eg 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 may occur 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.
Biodegradable polymers are finding a new and important role in many applications as medical or biomaterials. TWI has been particularly active in investigating novel joining and associated processes ( eg surface modification) related to medical or biomaterials.
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 formulations were first used in agricultural applications for low molecular weight fertilisers, antifoulants and pesticides and then extended into the medical field. As shown in Fig.2 the pH condition of the human body varies significantly. Some pharmaceutical polymers ( eg anionic polymers of methacrylic acid and methacrylates) which break down in specific pH environments, are of significant importance in many drug delivery applications.
Polymers used for implants in living bodies are expected to be bio-resistable. 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. Typical polymers known to degrade by hydrolysis are polyesters, cyanoacrylates, some polyurethanes and polyamides including nylons.
Other examples of biodegradable polymers include aliphatic polyesters, polyglycolic and polyactic acids for absorbable sutures as alternatives to catgut.
In biodegradable polymers the hydrolytic instability of certain linkages can be used for design of intentionally degradable materials such as in drug delivery systems. The main types of natural polymers prone to biodegradation are materials such as starch and cellulose. These materials which are prime candidates as packaging materials have the additional advantage of being renewable sources of polymeric materials.
Recently there has been significant interest in using polymers for short and long-term implantable applications in medical devices. An example of this type of polymer is a specific grade of polyetheretherketone (PEEK-OPTIMA). PEEK can resist biodegradation as well as degradation in many environments including exposure to a range of sterilisation conditions.
The non-hydrolysable polymers used as medical or biomaterials, such as fluorinated polymers, some polyolefins and polyether-based polyurethanes may degrade in the presence of heat, light 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 continued to increase 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.
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 photo-degradation. For example, polycaprolactone was 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 photo-degradable polymers rely on biodegradation to consume the polymer oxidised products resulting from initial photo-degradation.
A typical example of a degradable polymer under the trade name Biopol was first introduced by ICI in 1983. Biopol is a biodegradable thermoplastic polyester which is also known as poly (3-hydroxybutyrate-co-3-hydroxyvalerate) or PHBV.
This material is part of Polhydroxyalkanates (PHAs) which can be prepared as a source of natural polymer in cell plants or synthesised using a biochemical fermentation. 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.3, 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. It was reported that this product can be safely incinerated or biodegraded and converted into CO 2 and water after use. Figure 4 shows Biopol bottles at various stages of biodegradation after disposal.
Conclusions
Usage of polymeric materials is expected to continue to grow as their unique properties offer advantages to their inorganic counterparts in many new applications. This expectation is mainly based on their unique and diverse properties, their readiness to be processed by a wide range of techniques, low density and their cheapness compared with other materials.
Polymers are unique in terms of offering materials which can compete with some metals and other materials in load bearing, structural applications. But they can also be used in short and some long-term medical device applications as well as being edible as part of a drug delivery system. Future developments in degradable polymers will depend upon further fundamental understanding of the mechanisms of degradation and factors promoting disintegration of polymeric components into harmless chemical species which can be dispersed within the natural environment or the human body.
Since Bulletin's first coverage of this issue considerable advances have been made in developing new grades of polymers which can be degraded or recycled. However, one of the key remaining issues is the high cost of biodegradable polymers compared with the commodity polymers preventing their use in general packaging applications.
The susceptibility of degradable polymers to factors such as bacteriological attack and inferior mechanical properties makes them difficult to use in large volume packaging applications. However the use of these materials in many medical applications is growing significantly. One of the major areas of interest still remains on tailoring of traditional polymers, or developing new polymers with pre-determined service life, which can have all the properties required for their intended service but could degrade or recycle on demand.
Some polymers are used under aggressive environmental conditions involving exposure to elevated temperatures, hot fluids, high-energy radiations and stress, which adversely affect their service properties. Attempts to prolong their serviceability will also continue. This will include designing polymers with chemical structures and inherent resistance to degradation and incorporation of new stabilisers and other additives to improve durability and service performance.
