Biodegradable polymer
Biodegradable polymers are polymers that can be decomposed by the action of living organisms. Whereas most polymers are designed for longevity, biodegradable polymers are not. Biodegradable polymers can be derived from renewable raw materials, petrochemicals, or combinations thereof.
Polymers are the majority component of most plastics, so the discussions of biodegradable plastics and polymers are intimately related. While the words "bioplastic" and "biodegradable polymer" are similar, they are not synonymous. Not all bioplastics are biodegradable, and some biodegradable plastics are fully petroleum based. As more companies are keen to be seen as having "green" credentials, solutions such as using bioplastics are being investigated and implemented more. The definition of bioplastics is still up for debate. The phrase is frequently used to refer to a wide range of diverse goods that may be biobased, biodegradable, or both. This could imply that polymers made from oil can be branded as "bioplastics" even if they have no biological components at all. However, there are many skeptics who believe that bioplastics will not solve problems as others expect.
History
Very early work on biodegradable materials necessarily preceded the era of synthetic polymers, which require petrochemicals. This early work focused on natural polymers or their derivatives. One of the first medicinal uses of a biodegradable polymer was the catgut suture, which dates back to at least 100 AD. The first catgut sutures were made from the intestines of sheep, but modern catgut sutures are made from purified collagen extracted from the small intestines of cattle, sheep, or goats.In the 1830's, cellulose was converted to gun cotton and then cellulose acetates, which are probably the first biodegradable polymers. Early studies on the biopolymers polyhydroxyalkanoate provided the groundwork for its commercial production. Follow-up efforts by W.R. Grace & Co. failed. When OPEC halted oil exports to the US to boost global oil prices in 1973, Efforts to produce PHB using the strain Alcaligenes latus by Imperial Chemical Industries also collapsed. The specific PHA produced in this instance was a scl-PHA. Efforts continue. Related to PHA is polylactic acid. Studies on its polymerization of lactic acid and its derivatives began at DuPont in the 1930's. In the 1970's, a copoiymer of PLA and polyglycolic acid led to the commercialization of Vicryl, resorbable suturing material.
The concept of synthetic biodegradable plastics and polymers was first introduced in the 1980s. In 1992, an international meeting was called where leaders in biodegradable polymers met to discuss a definition, standard, and testing protocol for biodegradable polymers. Also, oversight organizations such as American Society for Testing of Materials and the International Standards Organization were created. Some clothing and grocery store chains have pushed to utilize biodegradable bags in the late 2010s.
Industrial production of biodegradable polymers commenced on scale in the late 1990's.
Types of biodegradable polymers
Most biodegradable polymers are polyesters. The ester group is susceptible to hydrolysis by both chemical and enzymatic action. In addition to polymers, the biodegradability of additives requires attention.Bio-based polymers
Biologically synthesized polymers are produced from natural origins, such as plants, animals, or micro-organisms.Polyhydroxyalkanoates (PHAs)
are a class of biodegradable plastic naturally produced by various micro-organisms. Specific types of PHAs include poly-3-hydroxybutyrate, polyhydroxyvalerate and polyhydroxyhexanoate. The biosynthesis of PHA is usually driven by depriving organisms of certain nutrients and supplying an excess of carbon sources. PHA granules are then recovered by rupturing the micro-organisms.PHA can be further classified into two types:
- scl-PHA from hydroxy fatty acids with short chain lengths including three to five carbon atoms are synthesized by numerous bacteria, including
Polylactic acid (PLA)
is thermoplastic aliphatic polyester synthesized from renewable biomass, typically from fermented plant starch such as from maize, cassava, sugarcane or sugar beet pulp. In 2010, PLA had the second-highest consumption volume of any bioplastic of the world.PLA is compostable, but non-biodegradable according to American and European standards because it does not biodegrade outside of artificial composting conditions.
Starch blends
Starch blends are thermoplastic polymers produced by blending starch with plasticizers. Because starch polymers on their own are brittle at room temperature, plasticizers are added in a process called starch gelatinization to augment its crystallization. While all starches are biodegradable, not all plasticizers are. Thus, the biodegradability of the plasticizer determines the biodegradability of the starch blend.Biodegradable starch blends include starch/polylactic acid, starch/polycaprolactone, and starch/polybutylene-adipate-co-terephthalate.
Others blends such as starch/polyolefin are not biodegradable.
Cellulose-based plastics
bioplastics are mainly the cellulose esters, and their derivatives, including celluloid. Cellulose can become thermoplastic when extensively modified.Petroleum-based plastics
The most widely used petroleum-based plastics are polyethylene terephthalate, polyethylene, polypropylene, and polystyrene, polyvinyl chloride are not biodegradable. In fact, they are desirable for their resilience. For example, PVC plumbing is used extensively for sewage, which is corrosive. Otherwise many petrochemicals are use to produce biodegradable polymers.Polyesters
is a thermoplastic polymer derived from the hydroxycarboxylic acid glycolic acid. PGA is often used in medical applications such as PGA sutures for its biodegradability. The ester linkage in the backbone of polyglycolic acid gives it hydrolytic instability. Thus polyglycolic acid can degrade into its nontoxic monomer, glycolic acid, through hydrolysis, This process can be expedited with esterases. In the body, glycolic acid can enter the tricarboxylic acid cycle, after which can be excreted as water and carbon dioxide. Closely related to PGA is polylactic acid. Since lactic acid is obtained biologically, it is discussed as a bio-derived biodegradable material.Polybutylene succinate is derived from succinic acid and 1,4-butanediol. It a thermoplastic polymer resin that is used in packaging films for food and cosmetics. In the agricultural field, PBS is used as a biodegradable mulching film. PBS can be degraded by Amycolatopsis sp. HT-6 and Penicillium sp. strain 14-3. In addition, Microbispora rosea, Excellospora japonica and E. viridilutea have been shown to consume samples of emulsified PBS.
Polycaprolactone is obtained by ring-opening polymerization of the monomer caprolactone. It is a prominent implantable biomaterial. It has been shown that Bacillota and Pseudomonadota can degrade PCL. Penicillium sp. strain 26-1 can degrade high density PCL, although not as quickly as thermotolerant Aspergillus sp. strain ST-01. Species of clostridium can degrade PCL under anaerobic conditions.
Polybutylene adipate terephthalate is another biodegradable copolymer. It is derived from butanediol and two kinds of dicarboxylic acids, adipic acid and terephthalic acid.
A prominent omission from the list of biodegradable polyesters is polyethylene terephthalate, of which >80M tons/y are produced. Bacteria and their associated enzymes that degrade PET have been identified, but the conversions are slow.
Poly(vinyl alcohol) (PVA, PVOH)
is one of the few biodegradable vinyl polymers that is soluble in water. Due to its solubility in water, PVA has a wide range of applications including 3d printing, food packaging, textiles coating, paper coating, and healthcare products.Other biodegradable polymers
Motivated significantly by medical applications, many bio-derived and totally synthetic polymers have been developed with varying degrees of biodegradation. Some of these polymers are polyanhydrides, polyacetals, poly, polyurethanes, polycarbonates, and polyamides.Biodegradation pathways and mechanisms
Most biodegradable polymers are polyesters. They break down by hydrolysis, i.e., a cleavage with water to give a carboxylic acid and an alcohol :The cases of polyesters derived from hydroxyl carboxylic acids, the alcohol and the carboxylic acid are part of the same monomer, so the equation for hydrolysis is simplified:
To be even more precise, at neutral pH, the carboxylic acid exists as the carboxylate:
Hydrolysis can induced by "chemical" routes or enzyme-catalyzed. These enzymes are exported from a cell or result from the rupture of some cell. Polymers are too large to enter cells. Chemical hydrolysis can be very slow, but the presence of acids, bases, and mineral surfaces promote the process. Once the polyesters are fully hydrolyzed, the monomers are suited for complete degradation by entering the cellular environment and being metabolized. Microbial degradation is sometimes considered as a 3-step process. Ultimately the biodegradation affords H2O and CO2.
Biodegradability is a "system property". That is, whether a particular plastic item will biodegrade depends not only on the intrinsic properties of the item, but also on the conditions in the environment in which it ends up. The rate at which plastic biodegrades depends on a wide range of environmental conditions, including temperature, its physical size, and the presence of specific microorganisms. Synthetic polyolefins are some of the least degradable.
Depressed plastics recovery rates can be attributed to conventional plastics are often commingled with organic wastes, leading to accumulation of waste in landfills and natural habitats. On the other hand, composting of these mixed organics is a potential strategy for recovering large quantities of waste and dramatically increasing community recycling goals. As of 2015, food scraps and wet, non-recyclable paper respectively comprise 39.6 million and 67.9 million tons of municipal solid waste.