Programmed Degradable Plastics

Programmed degradable plastics are designed for particular application, and differ from readily degradable plastics with the degree of control of degradation in terms of timing and also the shape of the degradation curve. This approach aims at eliminating the litter problem. Exposure to sunlight is one such application, which is common in agricultural applications. The polymer resin in such case is modified to promote UV radiation. As explained below, the strategy is to attach a photosensitizing group to the polymer chains by chemical means.

In more general terms, oxidative programmed degradation is a programmed degradation that results from oxidation. Although fragmentation is not same as total degradation, some plastics are programmed to fragment after useful lifetime. Fragmentation of the polymer chains in the plastic makes them more susceptible to other modes of degradation, possibly including some biodegradation activity. In any case, it is important to ensure that fragmentation leads to biodegradation without any ecotoxicity effects and would leave no remains, whether invisible or not, in soil within a reasonable time period to avoid accumulation in the soil.

Photodegradation
It is the process of decomposition of the material upon exposure to radiant energy such as the action of light. Most plastics tend to absorb high-energy radiation in the ultraviolet portion of the spectrum, which activates their electrons to higher reactivity and causes oxidation, cleavage, and other degradation. The UV energy absorbed by plastics can excite photons resulting in formation of free radicals. A variety of additives or UV stabilizers are used to protect the plastics from UV radiation and ultimately from the long-term degradation effects from light [4], while on the other hand, for particular applications, there has been intentional addition of photosensitive molecular structures by different techniques to induce degradation via the medium of light. This strategy is commonly used in applications such as packaging, plastics bags and agricultural films.

Thermal degradation
When the polymer gets overheated at high temperatures, the molecules of the long chain polymer backbone start to separate thus resulting in molecular scission and altering the properties of polymer like molecular weight/molecular weight distribution as well as some mechanical properties.

Thermal degradation proceeds through a variety of ways such as depolymerisation in which the polymer starts losing its monomers one by one, random chain scission in which there is a random breakdown of polymer backbone leading to decline in molecular weight, side group elimination in which the side groups are stripped off from the backbone chain before it breaks up into smaller pieces and oxidation of polymer.

Chemical degradation
It is one of the most important types in abiotic degradation of the polymers. Chemical degradation occurs via simple hydrolysis or enzyme catalyzed hydrolysis (biologically mediated degradation). Such type of mechanism is much more significant in biodegradable polymers due to the presence of hydrolysable bonds in their chain [6]. Some of the groups like esters, ethers, anhydrides, amides, carbamide, esteramide etc. contain such covalent bonds that are able to be split by water molecules.

Generally enzymatic degradation involves loss of material starting from the surface inwards while the simple chemical hydrolysis can occur throughout the cross section of the polymer with the exceptions of very hydrophobic polymers. Various factors such as type of chemical bond, pH, temperature, copolymer composition and water uptake greatly affect the chemical polymer degradation.

Polymer erosion which is a type of chemical degradation is a common property shared by all the degradable polymers. The biodeterioration of these materials proceeds by two different mechanisms; i. Bulk or homogenous and ii. Surface or heterogeneous eroding materials which form the basis of classification of degradable polymers.