Review articleMechanistic implications of plastic degradation
Introduction
Plastics due to their versatility become the essential ingredients to provide a quality to life. These are now rival metals in breadth of use and in severity of applications because of their flexibility, toughness, excellent barrier and physical properties and ease of fabrication [1], [2], [3], [4], [5]. The accumulation of plastics in the environment is a matter of great concern leading to long-term environment, economic and waste management problems. Degradation of waste plastics through various means becomes one of the alternatives to deal with such problems [6], [7]. A wide variety of synthetic polymers absorb solar ultraviolet (UV) radiation and undergo photolytic, photo-oxidative, and thermo-oxidative reactions that result in the degradation of these materials [8], [9]. The propensity of plastic products to undergo solar UV radiation induced degradation/ozone-induced degradation has been increased by addition of some additives in these polymers [10], [11]. Besides these degradations, biodegradation offers another most efficient and attractive route to environmental waste management. The mechanisms involved in the biodegradation are complex due to the interaction of different oxidative processes which are caused by the oxygen present in the air either by the microorganisms or by the combination of the two [12].
On the other hand, sometime the product needs stability instead of degradation. Therefore, depending upon the product applications plastic needs controlled stability. In order to increase the outdoor uses of plastics, the development and production of polymers with increased weathering resistance is required similarly for the use in high temperature applications, polymers need thermal stability [13], [14], [15]. To control the stability, understanding of the mechanism is the primary requisite. Keeping in view the facts for the plastic degradation, in this review article, we have mainly discussed the various types of polymeric degradations (photo-oxidative degradation, thermal degradation, ozone-induced degradation, mechanochemical degradation, catalytic degradation and biodegradation) and mechanism followed by these degradations. We have also discussed the various methods used to study these degradations and various factors which affect these degradations.
Section snippets
Types of plastics
Plastics can be synthesized via the polymerization (polyaddition or polycondensation) of small molecules and are in general classified into two groups i.e. thermoplastics and thermoset plastics [16]. Thermoplastics are linear chain macromolecules where the atoms and molecules are joined end-to-end into a series of long, sole carbon chains. The bi-functionality necessary to form a linear macromolecule from vinyl monomers can be achieved by opening the double bond and reaction proceeds by a free
Types of plastic degradation
Changes in polymer properties due to chemical, physical or biological reactions resulting in bond scissions and subsequent chemical transformations are categorized as polymer degradation [22]. Degradation reflects changes in material properties such as mechanical, optical or electrical characteristics in crazing, cracking, erosion, discoloration and phase separation [23]. Depending upon the nature of the causing agents, polymer degradations have been classified as photo-oxidative degradation,
Factors affecting polymer degradation
Degradation is defined as a process which leads to a deterioration of any physical property of a polymer [17]. In general, the degradation process affects the thermal stability, mechanical properties, crystallinity and lamellar thickness distribution and begins in the amorphous/crystalline interface [56]. Degradations of plastic affected by various factors are given below:
Plastic materials: future perspectives, challenges and other issues
Various types of plastics may be of use in packages, biomedical products, electronic components, non-returnable goods, construction materials, etc. However, an important aspect of these materials for a sustainable society is to predict their lifespan. How a material affects its environment is always important [226], [227]. Sustainability, industrial ecology, eco-efficiency, and green chemistry are the new principles that are guiding the development of the next generation of plastic products and
Conclusion
It is concluded from the foregone discussion that understanding about the mechanism can go a long way in helping the researchers and the technologists to induce the different types of degradation in the plastic. These degradations can further be enhanced by the addition of the additives in the plastic and by understanding the various factors responsible for these degradations. It is also concluded from this discussion that plastic degradation could be enhanced by its modification with natural
Acknowledgement
This article is prepared from the literature collected for the project sponsored by the Ministry of Environment and Forest. The authors wish to thank the Ministry of Environment and Forest, Government of India, for providing the financial assistance for the project.
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