There have been numerous definitions proposed for environmentally degradable and biodegradable water-soluble polymers and plastics; almost every article written on the subject includes the author's favorite definitions. All tend to encompass the same broad concepts but are slightly different in phraseology depending on the author's perspective and discipline, that is, chemist, biochemist, layman, lawyer, legislator, and so forth. This is both fortunate and unfortunate since, on the one hand, it should ensure that there is broad understanding of the problem but, on the other hand, it may also mean that we will never reduce to words an acceptable definition that has worldwide consensus. Consensus may be better reached by addressing the specifications for polymers in various degradation pathways as established by the testing protocols to be discussed later. However, definitions are important because they are indicative of general expectations for the acceptance of environmentally degradable and biodegradable polymers and of the types of testing protocols that are needed to establish the acceptability of polymers in a particular environment. At this time, the definitions developed by the American Society for Testing and Materials (ASTM)  for degradable, biodegradable, photodegradable, hydrolytically degradable, and oxidatively degradable plastics, indicated below, are probably the most widely accepted as written or in some slightly modified form. They are equally applicable to water-soluble polymers and are used as such in this chapter.
Degradable plastic, a plastic designed to undergo a significant change in its chemical structure under specific environmental conditions resulting in a loss of some properties as measured by standard test methods appropriate to the polymer Biodegradable plastic, a degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi, and algae
Hydrolytically degradable plastic, a degradable plastic in which the degradation results from hydrolysis
Oxidatively degradable plastic, a degradable plastic in which the degradation results from oxidation
Photodegradable plastic, a degradable plastic in which the degradation results from the action of natural sunlight
It is immediately apparent that these definitions do not quantify the extent of degradation in any of the pathways defined; they are only indicative of the mechanism that is operating to promote degradation. While this is acceptable in a scientific sense to define a process, the definitions do not satisfy the requirement for environmentally acceptable degradable and biodegradable polymers, which, in the minds of legislators and laypeople, is the key issue. As indicated above, specifications for acceptability have to be set and then monitored by the standard testing protocols to be discussed later. The environmental degradation processes are interrelated, as shown schematically in Figure 12.1.
All four environmental degradation pathways for polymers, biodegradation, oxidation, hydrolysis, and photodegradation initially give intermediate products or fragments. These may (bio)degrade further to some other residue, biodegrade completely and be removed from the environment entirely, ultimately mineralized as indicated in Figure 12.1, or remain unchanged in the environment. It should be noted that mineralization is a slow process that refers to complete conversion of a
Biodegradation Photodegradation Oxidation Hydrolysis
Biodegradation Photodegradation Oxidation Hydrolysis
Fate and effects
Figure 12.1. Interrelationships of environmental degradation pathways for polymers.
polymer (or any organic compound) to carbon dioxide and/or methane (depending on aerobic or anaerobic environment), water, and salts. It is used here loosely to indicate complete or total removal from the environment to carbon dioxide or methane, water, and biomass. In the instances where residues remain, these must be established as harmless in the environment by suitably rigorous fate-and-effect evaluations. Clearly, only biodegradation has the potential to remove polymers completely from the environment, and this should be recognized when developing and designing polymers for degradation by any of the other pathways. The final degradation stage should preferably be complete biodegradation and removal from the environment with ultimate mineralization. In this way, the polymers are recycled through nature into microbial cells, plants, and higher animals  and then back into renewable resource chemical feedstocks.
Based on the above arguments, an acceptable environmentally degradable polymer may be defined as one that degrades by any of several, defined mechanisms—biodegradation, photodegradation, oxidation, or hydrolysis — to leave no harmful residues in the environment. This definition has the advantage of not limiting the rate or degree of degradation for a particular polymer but requiring sufficient testing of fragments and degradation products that are incompletely removed from the environment to ensure no long-term damage or adverse effects to the ecological system. Polymers meeting this definition should be acceptable for disposal in the appropriate environment anywhere in the world. But, it should be emphasized that the most significant goal for water-soluble polymers should be environmental biodegradation, regardless of the mechanisms involved prior to the biodegradation stage, with complete removal from the disposal environment. This categorically precludes any unwanted adverse environmental impacts and prolonged fate-and-effects evaluations.
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