Degradable Polymers

Polymers have gained a unique position in modern materials technology for a number of quite different reasons. The development of the inflatable rubber tyre in modern transport would not have been possible without the use of natural, and later synthetic, rubbers as the energy absorbing components. ‘Plastics’, have largely replaced traditional materials used in packaging because of their better physical properties, notably strength and toughness, lightness and barrier properties. Their ability to protect perishable commodities against spoilage at minimal cost has led to a revolution in the distribution of foodstuffs to the extent that they are now indispensable in modern retailing [1].

It is now widely recognized that, along with the importance of synthetic polymers possessing long-term stability, there is also a need for polymers that break down in a controlled manner. Biodegradable macromolecules can be tailored specifically for controlled degradation under the inherent environmental stress in biological systems either unaided or by enzyme-assisted mechanisms. Medical applications of these materials have led to significant developments, such as the controlled release of drugs, fertilizers and pesticides, absorbable surgical implants, skin grafts and bone plates. Many studies of the mechanisms of biodegradation of synthetic polymers were motivated by this and will be initially discussed. Recent interest in polymer waste management of packaging materials has and incentive to the research and will be discussed in a later section.

Synthetic polymers appeared about sixty years ago and immediately medical people realized that this new class of materials was of interest for therapeutic applications. For example, isotonic aqueous solutions of polyvinylpyrrolidone (PVP) were used as plasma expander during World War II and although this compound was far from ideal, it stayed in use for years before substitutes appeared [1]. Since then, many polymers have been evaluated as candidate biomaterials [2]. However, only a number of them have reached the stage of clinical applications and commercial availability.

In nature, starch represents a link with the energy of the sun, which is partially captured during photosynthesis. Starch serves as a food reserve for plants and provides a mechanism by which non-photosynthesizing organisms, such as man, can utilize the energy supplied by the sun.

The current utilization of natural resources cannot be sustained forever. Most of the fuel utilized in our societies comes from fossil fuel, such as oil that, other than being subjected to price fluctuations, must eventually be depleted. Rising atmospheric carbon dioxide levels from combustion of fossil fuels are thought to be increasing global temperature that, in turn, may cause droughts, crop losses, storm damage, etc [1]. Fuel shortage and the waste accumulation in the environment are generating a worldwide interest in alternative resources and particularly for the use of renewable resources both as an energy source [2] and as raw materials for polymers and plastics [3]. There is increasing pressure for a wider utilization of biomass feed-stocks for specialty items. The total biomass produced on earth is estimated as approximately 170 billion tons, of which a very small portion, less than 4%, is used. [4].

Polyhydroxyalkanoates (PHAs) are homo- or heteropolyesters synthesized and intracellularly stored by numerous prokaryotes. They can be produced in large quantities from renewable resources by means of well known fermentation processes and the imposition of particular culture conditions, and a number of physical or chemical methods are known to extract them from the producing biomass. Production processes such as batch, semi-batch and continuous fermentation are all known to work. PHAs have properties similar to those of some polyolefins. This, combined with the fact that they are fully and rapidly biodegraded under the appropriate conditions, has generated a high interest in them as substitutes to petroleum-based polymers in many applications [1].

Polyhydroxy alkanoates (PHAs) comprise a group of biodegradable polyesters produced by bacteria. Their primary function is energy storage and they are used as an energy reserve for bacteria, similar to the role of polysaccharides or polyphosphates in living cells. PHAs can be produced by relative simple and efficient procedure based completely on biotechnology utilizing fully renewable resources (see Chapter 8). Variations in this procedure, mainly consisting in changes in the composition of the food supplied to the bacteria, lead to a production of modified PHAs as homo or copolymers, containing different functional groups. The original or modified PHAs seem to be ideal for applications in various short-term plastic products, especially packaging. In spite of their attractive potential, the applications of PHAs are at present marginal, because they possess several serious drawbacks that prevent high volume production and application.

Biomaterials are substances that are used in prostheses or in medical devices designed for treatment, augmentation, or replacement any tissue, organ or function of the body. Both natural and synthetic materials are used as biomaterials.

Environmentally biodegradable water-soluble polymers represent a very important goal in the drive towards responsible waste management and environmental protection in a major segment of the polymer industry. The water-soluble polymer segment is often forgotten or neglected when environmental issues associated with the disposal of polymers and plastics in general are evaluated. Water-solubility is sometimes magically considered to confer environmental acceptance and biodegradabilityon such polymers: this is categorically untrue as indicated in this chapter. The scope of this chapter is broad to include and address the key issues of definitions and testing protocols, that need to be addressed before a polymer may be labelled as biodegradable. Currently acknowledged biodegradable water-soluble polymers and synthetic approaches to new polymers are also covered. Structurally similar water-soluble biodegradable polymers designed for biomedical and controlled drug delivery applications rather than environmental degradation are not included in this chapter. However, necessary comparisons and correlations with the extensive work and literature on biodegradable plastics are included where essential for the understanding of biodegradable water-soluble polymers and their development.

Over the past half-century, synthetic plastics have become the major new materials for everything from replacements for human body parts to the construction of supersonic aircraft and spacecraft. Much of this growth has taken place at the expense of more traditional materials, such as steel, aluminum, paper and glass. Quite understandably, the industries associated with their manufacture have fought back through public relations campaigns intended to protect their own specific markets. Unfortunately, advertising agencies are not the most reliable sources of scientific information, and as a result, the public perception of the role of plastics in society is based more on what can only be described as “mythology” than on demonstrable facts. It is the purpose of this chapter to deal with a number of the misconceptions about plastics generally and about the role of degradable plastics in particular. The first of these will involve resource considerations.

Wastes are by-products of nature’s productive activities including human activity. Most naturally occurring wastes are not normally perceived to cause environmental problems; for example, even when natural polymers become durable litter, as in the case of trees, branches etc., they are eventually incorporated into the biological carbon cycle [1]. By contrast the by-products of human activity are not seen in this way although they rarely remain in the outdoor environment as long as fallen trees [2]. Firstly, synthetic polymers look different from nature’s wastes and many man-made products, particularly those manufactured from non-renewable resources are not considered to be bioassimilable into the natural cycle. The latter view, which is popular among environmentalists, is in fact a misunderstanding since there are very few man-made carbon-based polymers that are not ultimately bioassimilated and those that are not degraded are so stable that they cause no environmental hazard. The second problem with man-made wastes is that they are produced mainly in cities and towns where acceptable disposal becomes a logistical challenge.