Indian Journal of Chemistry Vol. 388, May 1999, pp. 525 - 537

Advances in Contemporary Research

Environmentally degradable-thermoplastics: An overview

M Anuradha* & V G Kumar Hindustan Lever Research Centre, Chakala, Andheri(E)

Mumbai 400 099, India

One of the key environmental issues facing the world today is the pl astic waste disposal problem and one of the approaches in solving this problem has been the development of environmentally degradable thermoplastics. A concise review of environmentally degradable materials is provided in the report . Both mechanisms of degradability, photodegradation and biodegradation are provided. The nature of the study necessitates a deeper understanding of biodegradable materials. Hence, materials, criteria, mechanism of degradation and evaluation methods are discussed. The objective in undertaking a review of environmentally degradable thermoplastics has been two-fold: (i) to establish a database on the development of environmentally degradable thermoplastics and (ii) more fundamentally, to look at the basic mechanism and criteria for biodegradability.

Traditional applications of synthetic polymers are based on their relative inertness to the environment. The largest tonnage plastics like polyolefins, polystyrene and poly(vinyl chloride), which have attractive properties and find a variety of applications, are all non-biodegradable. Addressing the issue of plastic waste remains a key environmental issue to date.

Several oplions are available for tackling plastic waste, among them incineration , recycling and the development of degradable polymers have been the most investigated methods ' . Incineration and recycling are viable, expedient alternatives for plastic waste management that are stil the focus of much research activity. The two methods are however outside the scope of this review. As opposed to other approaches, the development of environmentally degradable polymer molecules allows for a cradle to grave approach in managing plastic waste. The review provides a critical analysis of environmentally degradable polymer molecules.

Polymer Degradability

Several reasons have been put forward to explain the resistance of polymers to environmental degradation. These include the presence of long chains in polymers and the unavailability of chain ends for microbial attack. The necessary conditions for microbial activity like water activity and proper pH may also be absent in hydrophobic polymers. The history of development of environmentally degradable polymers has therefore involved the exploitation of

factors that are conducive to or can induce degradation. In this section the basic definition of degradability, modes of degradation and the methodology for development of degradable materials have been discussed.

The definition of a degradable plastic has been much debated . A comprehensi ve definition is provided by the ASTM Committee on Plastic Terminology. The committee defines a degradable plastic as one designed to undergo a significant change in its chemical structure under specific environmental conditions, resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the application in a period of time that determines its classification2

.

This degradation can take place by the action of li ght , water, wind or microorganisms. The chemical processes involved in degradation are usuall y hydrolysis or oxidation . However, a working definition of polymer degradation would be any deleterious change in the properties of the polymer due to change in chemical structure.

Whatever be the mode of degradation, the singl e most important parameter involved is the molecu lar weight. The degradation mechanism must cause cleavage of the main chain with fragmentation of the polymer molecule and consequent decrease in molecular weight. The resulting molecular weight loss may also be associated with a variety of chemical and physical effects, which can be readily measured (eg . by-product formation, weight loss, surface erosioll. etc.).

526 INDIAN J CHEM, SEC B, MAY 1999

Significant property changes are also associated with the decrease in molecular weight. Physical properties such as glass temperature, degree of crystallinity and viscosity and mechanical properties such as tensile strength , modulus and impact strength de teriorate with decrease in molecular weight. This decrease is often taken as the fu ndamental characteristic of polymer degradationJ

.

Two methods are commonly lI sed to bring about degradati on. These are photodegradation and biodegradation. Photodegradable polymers have been discussed briefly while biodegradable materials and the mechani sm of degradability have becn discussed i Jl more detai I.

Photodegradable Polymers

Photodegradation is th e degradat ion of the polymer resulti ng from the act ion of natural li gh t in the presence of air (oxygen). Photodegradable material s are vinyli c polymers (C-C backbone) like polyethylene and polystyrene which arc modified. These polymers are not susceptible to hydrolysis and their degradation req uires an oxidati on process.

Polymers li ke po lystyrene and polyethylene are chemically modified by the introduct ion of carbonyl groups during the polymerization reaction. The C=O

group can be a part of the main c ain or directly linked to the main chain. To incorporate the carbonyl group, carbon monoxide or vinyl ketones can be introduced during the polymerisation process. The C=O groups provide targets for accelerated photochemical degradation4.

The ini tia l degradation step involves the action of li ght ; however, once the polymer is broken up into small er fragments, further degradation is by the ac tion of microorganis ms. An example of this dual mechanism is provided by modified polyethylene. At molecular w(';ghts above 500 poly(ethylene) is not degraded by the act ion of microorganisms. However, polye thylene of molecular weight below 500 daltons is readil y degraded by micoorganisms. Hence, the in trod uct ion of carbonyl groups in polyethylene provides weak trigger sites for in it ial photodegra­dation , which is followed by biodegradation of low molecular weight poly(e thylene) fragments3

. •

Carbonyl groups are introduced at random in the molecu le since ketone groups on exposure to UY light, take part in two-bond breaking reactions. Non'ish I and II reactions, resulting in degradation of the polymer molecul e (Figure 1).

Anuradha Mou lee was born in Mumbai , Maharashtra in 1965. She rect:i \-eu her Master's degree in 1987 fro m liT, Mumbai and her Ph .D. degree from the same institute in 1993 in the area of polymer synthesis and charat:terisat ion. She worked brieOy with Gharda Chemicals before joining Hi ndus tan Lever Research Centre in 1993 where she curren tl y works as a Rescarch Scienti st.

,. I .. :.

"'I • .y.y'it;::IY

Her current interests are in the area of polymer solution behaviour and polymer-surfactant interactions as we ll as the synthesis and characterisation of polysacchari de derivat ives.

V. Gopa Kumar was born in Tri vandrum, Kerala in 1952. He received his Master' s degree in Chemistry in 1972 from the University of Kerala. He took his Ph.D. in 1981 from Technical University of Graz, Austri a.

He worked for a long period with Vikram Sarabhai Space Centre, Trivandrum before joining Hindustan Lever Limited in 1987. He is currently Head , Chemicals Research Group at Hindustan Lever Research Centre.

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ANURADHA el al.: ENVIRONMENTALLY DEGRADABLE-THERMOPLASTICS: AN OVERVIEW 527

o II

NoN' CH2CH 2 C CH2 CH2CH2 ..wN'

! NORRISH I

REACIlON

o II

•

.wow CH 2CH2C. + • CH2CH2CH2 NM'

! o

" CH2CH2C CH3 +

H3C = CH ......

Figure I-Norrish I and II reactions

The Norrish I reaction generates carboxylic acid and hydroxyl terminated oligomers which undergo further enzymatic degradation.

In Norri sh II reactions, fragments with vinyl and methyl ketone and groups are formed. Further photochemical and oxidative reactions form low molecular weight polymers or oligomers with carboxylic acid end groups which undergo further biodegradation.

Photodegradability can also be brought about by the addition of catalysts. Catalysts used are metal (Co, Mg, Fe, Zn, etc.) salts of unsaturated fatty acids. Free radicals can be formed on the C-C backbone which initiates further degradation (Figure 2).

The synthesis of photodegradable polymers is of interest as it is based on the modification of commercial, large tonnage polymers. Several photodegradable products are commercially available (Table I). The use is currently limited due to the high cost. The materials also have to degrade predictably after a fixed exposure time to light.

Photodegradation requires the presence of natural light. Hence, this approach may not entirely solve the waste problem as discarded materials in landfills may not be exposed to light. Useful applications are envisaged in agricultural mulch film.

Biodegradable Polymers

Definition alld mechanism

The controlled biodegradation of polymeric materials is a potentially interesting approach, which has been the basis of research over the past few years.

Biodegradation is a term used to define chemical degradation resulting from biochemical reactions,

Figure 2--Photodcgradation by addition of catalysts

Table I-{:ommercially avai lable photodegradable polymers

Product

Ethylene CO copolymer

Vinyl ketone/PE, PS, PP copolymer

Application

Beverage rings

Plastic bags

Manufacturer

Dow, DuPont Union Carbide

Eco Plastics (italy)

especially those catalysed by enzymes produced by microorganisms under either aerobic or anaerobic conditions3

. Microorganisms which can provide the enzymes for such processes include bacteria, fungi , yeasts, algae and others. Of these, bacteria and fungi are of particular interest in the biodegradation of polymers.

By this definition, the biodegradation of polymers involves primarily, enzyme catalysed chemical reactions, and such reactions can occur either by random attack along the polymer chain backbone or by specific attack at the polymer chain ends. The former results in random chain cleavage with a concomitant substantial decrease in molecular weight (with consequent formation of oligomers and low molecular weight polymers) while the latter involves removal of terminal groups (resulting in the formation of monomer, dimer or trimers) with slight decrease in molecular weight of the polymer chain.

Endo and exo-molecules act in consortium to degrade polymer molecules. Microorganisms can also secrete reactive reagents into the environment. These reagents, particul arly acids, can cause degradation reactions. Some enzymes can catalyze the formation of reactive reagents in the environment (especially peroxides) that can degrade polymers. Though the enzyme plays an indirect role, these mechanisms are also considered in biodegradation.

528 INDIAN J CHEM. SEC B. MAY 1999

It is likely that once an enzyme molecule associates with the polymer chain and causes the first chain cleavage, either by exo- or endo- cleavage, it remains associated with the fragmented chain and can catalyse the hydrolris of several more units before dissociating. Little quantitative information is available on the degradation.

Degradation mechanisms have been studied in various environments--soil, aquatic and landfills. Soil microflora have been extensively evaluated in biodegradation tests. Aerobic soil environments generally contain consortia of several different types of degrading bacteria. The final products of aerobic degradation are CO2 and water. Anaerobic environments exist below the surface of soil or in muds and sediments. They also contain primary and secondary microorganisms to facilitate biodegradation. Here, a variety of final products are formecJ--.-..C02, hydrogen, methane, H2S and NH3.

Biodegradation In aerobic and anaerobic environments can be represented chemically the following two equations. It is assumed that the polymer is a hydrocarbon polymer.

Aerobic degradation:

-CH2-CH2-CHr CHr + O2 ~ CO2 + H20 + biomass + residue

Allaerobic degradation :

-CH 2-CH2-CHr CHr ~ CO2 + CH4 + H20 + biomass + residue

The equations can be readily modified to include other elements that may be present in a polymer. Most of the work so far has invol ved aerobic degradation and in this overview too, unless otherwise mentioned, degradation refers to anaerobic degradation. Anaerobic degradation, however, is becoming incresingly pertinent for water sol uble polymers.

Enzyme catalysed biochemical reactions responsible for biodegradation can take place on a variety of polymeric material s. These can be broadly class ified as:

I . Natural polymers

2. Synthetic polymers

3. Bacterial polymers

Biodegradable thermoplastics are based on any of the ::bove three c lasses and include modified natural po lymci"';, modified :-;ynthetic po lymers and bacterial

polymers. These will be discussed in detail in later sections.

Natural Polymers

Classification and degradation mechanism

Natural polymers are polymers formed in nature during the growth cycles of all organisms. Natural polymers serve in nature as either structural or reserve cellular materials. Their syntheses involves enzyme­catalysed, chain-growth polymerisation of activated monomers, which are generally formed within the cells by complex metabolic processes. The most prevalent structural and reserve biopolymers are the polysaccharides, examples of which are starch, cellulose, chitosan, guar gum, etc.

Of these, cellulose and starch have been the most commonly exploited natural polymers for material applications. There has also been much research actiVIty in modifying chitosan for materials applications5

.

Both cellulose and starch are composed of repeat units of D-glucopyranoside units. In starch, the glucopyranoside units are linked together by l,4-a acetal linkages, while in cellulose the repeating units are linked together by 1 ,4-~ linkages. The enzymes that catalyse the acetal hydrolysis reactions in the biodegradation reactions of these two polysaccharides are different.

Starch is the major polysaccharide reserve material of photosynthetic tissue and of many types of storage organs such as seeds, swollen stems and roots. It can be chemically or physically modified and is used for various applications in food, textile and paper industries. In starch degradation, both endo-amylases and exo-amylases are produced by microorganisms. The endo-amylases generally hydrolyse only main chain acetal bonds in amylose or amylopectin but are not active on branch points of amylopectin. Exo­amylases can cleave either main chain or branch bonds6

•

Cellulose is the most abundant polysaccharide, with about 1015 kg being synthesised or degraded every year. It is primarily a structural polymer in plants and trees. Cellulose is also produced by bacteria in the form of exo-cellular microfibrils. In all cases, cellulose is a highly crystalline, high molecular weight polymer which is insoluble in most comon solvents and infusible. Cellulose is readily hydrolysed at the acetal posi tion by specific enzymes , the

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ANURADHA et af.: ENVIRONMENTALLY DEGRADABLE-THERMOPLASTICS: AN OVERVIEW 529

cellulases, which are secreted by various microorganisms.

Many fungi and bacteria, both aerobic and anaerobic, secrete cellulolytic enzymes under a wide range of conditions. Atleast ten different types of anaerobic bacteria and thirty different types of aerobic bacteria also show cellulolytic activity. The bacteria secrete both endo- and exo-enzymes, some of which form complexes that act jointly in degrading cellulose to form carbohydrate nutrients which microorganisms need for survival7

• Cellulases, produced by fungi for degrading the cellulose, include three principal types of enzymes, endo-l,4-~-glucanases, which randomly attack internal 1 ,4-~-glucosidic bonds along the main chain to generate lower molecular weight polymers and oligomers (cellodextrins), exo-l ,4-~-glucanases which selectively hydrolyse terminal units on the chain, always at the non-reducing end of the chain, to form monomers, dimers, trimers and oligomers, and 1 ,4-~-glucosidases, which catalyse the hydrolysis of cellobiose and cellodextrins to glucose.

Cellulose is also found in nature in the lignocellulosics such as straw, wood, jute, kenaf, etc . Lignocellulosics are plant based materials which contain. in addition to. cellulose, hemicellulose and lignin in various proportions and have been evaluated for biodegradable polymer applications. In Iignocellulosics, fungal species may degrade cellulose alone or operate together to degrade the three components of lignocellulosics, cellulose, lignin and hemicellulose. Lignocellulosic degrading fungi are of three types: (i) brown rot fungi which produce enzymes primarily for degradation of cellulose and hemicelluloses, (ii) white rot fungi which secrete exo­cellular peroxidases to degrade lignin and to a lesser extent cellulases to degrade polysaccharides, and (iii) soft rot fungi which act principally on cell uloses and hemiceUuloses.

Bfodegradable polymers based on natural pol~mers

The development of biodegradable thermoplastics based on polysaccharides (e.g. starch and cellulose) is of interest as polysaccharides are a cheaply available, renewable and biodegradable resource. However, in mose- cases they do not possess thermoplastic properties in the unmodified fonn. There have been some attempts, however, to obtain thermoplastics based on unmodified polysaccharides. Starch containing 10-20% water (water acts as a plasticiser)

can be melt processed and is termed thermoplastic starch. Extrusion gives brittle pellets or rOOs ll

•

In a limited number of cases, starch may also be blended with other natural polymer based systems like modified cellulose or chitosan. Materials can be extruded to give foamed, expanded products9

•lo which

have applications as disposable, fresh food trays.

In a majority of cases, however, the polysacchari~.; has to be chemically modified to be processed into useful products. In modified polysaccharides, the synthetic component is expected to cause thermoplastic behaviour while the polysaccharide causes degradation of the plastic part. Whatever be . the method of modification, it is postulated that the biodegradable component is removed by microorganisms in the waste disposal environment, while the plastic or film containing the remaining inert components loses its integrity, disintegrates and disappears. When a sufficient amount of the polysaccharide is removed, pits and voids are formed with a concomitant loss in strength and continuity . UsuaHy, such materials are termed biodisintegrable II. However, it is clear that both degradation and modification are in opposition and a balance of properties is obtained'.

Modification can be brought about in the following ways:

(1) Composites of polysaccharides with natural and synthetic polymers. The modification is physical in nature.

(2) Chemically modified polysaccharides. Examples of these are cellulose esters like cellulose acetate, cellulose ethers, etc.

(3) Macromolecularly modified polysaccharides. These include block and graft copolymers where synthetic polymer chains are anchored onto the backbone.

Polysaccharide composites with synthetic polymers

Polysaccharides can be mixed with a synthetic polymer to give composite materials. The mixing is usually achieved by melt mixing the two components.

Blends or composites of synthetic polymers with a polysaccharide, starch, is a commonly used route for the development of biodegradable thennoplastics. The limitation in this method has been the poor compatibility of starch with some of the- synthetic polymers as well as the poor moisture resistance~of

530 INDIAN J CHEM, SEC B, MAY 1999

starch. Compatibility problems are often circumven­ted by adding compatibilisers.

(a) Starch-polyethylene blends are the most commonly used blends. Granular starch, in virgin form or chemically modified on the granule surface to increase its compatibility with the polymer matrix is used. They are used in agricultural mulch film and replace polyethylene. 6-10 % starch is used in these composites. It is theorised that the starch matrix would degrade first. The complete biodegradability of such compositions is controversia14

•

FERTEC (Ferruci Technologies), however, report upto 50 % starch incorporation in their products. They claim enhanced biodegradability.

(b) Ethylene/acrylic acid copolymer-starch blends are also reported 12. Here, the synthetic polymer is functionalised to give increased compatibility with starch. These composites can be extrusion blown into films used as agricultural mulch.

(c) Starch-poly(vinyl alcohol) fi lms also fall in the first category. Their special attributes are expandability and heat sealability and find use in packaging. Mater-Bi's range of expandable products are based on starch-poly(vinyl alcohol) blends 13

•

(d) Blends are also reported with other biodegradable polymers like poly(capro­lactone) (peL) and poly(hydroxybutyrate) (PHB).

Composites with lignocellulosics are also known. They provide both biodegradable composites as well as reinforcement and can be used to replace glass fibres. Jute, kenaf, cotton, hemp have all been investigated as composites and are generally used in automotive applications 14.15 . Poly(propylene) and poly(ethylene), both commodity plastics, are usually used as the synthetic polymer. There is some property reduction in comparison to the synthetic polymer.

Composites, however, lack cohesion and have poor properties. To improve cohesion, two methods have been evaluated. Steam explosion of polysaccharides and hydrophobic modification of lignocellulosics are the two methods commonly used to improve cohesion and subsequent composite properties. Modification is carried out by introducing hydrophobic groups, an example being the introduction of acetyl substituents.

Steam explosion of polysaccharides is a relatively new technique to improve cohesion between the two components l6

. Here, the material is subjected to high pressures at a temperature of 220-230 cC followed by a sudden release of pressure. The fibrillated polysaccharide obtained shows improved chemical reactivity and adhesion as more hydroxyl groups on the polysaccharide are exposed. It also removes amorphous components like lignin and hemicelluloses. The method is particularly useful for lignocellulosics.

Chemical modification of polysaccharides

Chemical modification of the polysaccharide can be achieved in a variety of ways. Typically,. These would include esterification, etherification, hydroxy­propylation, cyanoethylation, etc. These modifications enhance certain properties, particularly for textile applications.

Derivatised polysaccharides find plastic applications. Ecofoam from National Starch is a 95% hydroxypropylated, 50% amylose starch and is used in expandable, foamed products.

Of all chemically modified polysaccharides, cellulose esters find wide applications as thermoplastics. Cellulose acetate, cellulose butyrate, cellulose acetate-butyrate are all well known thermoplastics. They are transparent, water resistant and have a wide range of properties. They may also be processed in a variety of ways. Their drawbacks are susceptibility to strong acids and alkalies as also to some strongly polar organic compounds like ketones, esters and chlorinated hydrocarbonsl7. There are reports of the partial biodegradabil ity of cellulose acetate l8

• The degree of substitution has to be ~ 2.5 to be biodegradable. Cellulose diacetate is also reported as a thermoplastic that is processable by conventional methods like extrusion and injection moulding, and is biodegradable l9

•

Polysaccharides or lignocellulosics may also be modified by anchoring long polymer chains on the polysaccharide backbone. This can be achieved by two comonly used synthetic techniques: block and graft copolymerisation.

For polysaccharides, graft copolymerisation is the common method of modification, though block copolymers have also been evaluated. Free radicals are generated on the polysaccharide backbone by using redox initiators. [ceric ammonium nitrate (CAN) and ferrous ammonium sulphate (FAS)20.21] or

ANURADHA e/ af.: ENVIRONMENTALLY DEGRADABLE-THERMOPLASTICS: AN OVERVIEW 531

irradiation. Once free radical sites are generated on the polysaccharide backbone they can initiate further polymerisation of the monomer.

Due to free radical nature of grafting, the monomers used are often vinylic in nature. Cyclic monomers may also be used and their polymerisation is initiated at the carbonyl groups (polysaccharides like cellulose ex ist in a slightly oxidised form). For grafting to induce thermoplasticity in the polysaccharides, it is required that the synthetic polymer itself be a thermoplastic. The acrylate class of polymers possess a wide range of behaviour from elastomeric to rigid thermoplastic22

• Hence, they are commonly evaluated for grafting. The synthetic component in the total graft is exp..:cted to be more than 50% to generate continuous plastics which find thermoplastic application.

However, the grafting process is not always reproducible. The molecular weight distribution of the side chains and grafting frequency is difficult to control. All these factors may pose some difficulty in their use as thermoplastic materi als23

.

The literature on the thermoplastic behaviour of these grafts is limited. In the few reported cases, styrene and acry lates like methyl methacrylate (MMA) and methyl acrylate (MA) are the monomers of choice.

Grafting on starch gives thermoplastics with good biodegradability, and many studies are available on these. Starch-g-poly(methyl acrylate) gives translucent, leathery films which are biodegradable. Applications are in agricultu ral mulch film24. Gelatinised starch-g­poly(methyl acrylate) could be extruded and showed good fungal susceptibility . Starch-g­polystyrene is more brittle and can be processed by both extrusion and injection moulding25

.

2 Grafted starches are also utilised as biodegradable fillers for thermoplastics26

.

3 Shiraishi et al 27 report thermoplasticisation of pure cellulose and wood. Unlike in other examples, grafting is caITied out in a non­aqueous medium. S02/DEAlDMSO, a solvent for cellulose, is used. Grafting is done by irradiation. The grafting reaction yielded thermoplastic materials. Grafting was also carried out on acylated woods and the products showed thermoplas tic behaviour. Consequent grafting on acylated wood also gave products

which could be moulded. However, there is no evidence presented for biodegradation. Commercial viability of this method is limited due to the following factors:

4 Arthur28 also reports mouldable compositions of cellulose-PS grafts with a glass transition of 58-78 °C.

5 Block copolymers of amylose triacetate with diisocyanates are reported. Poly(propylene glycol) or poly(butadiene diol) may also be included. Enzymatic hydrolysis of these block copolymers is reported and is monitored by . . . . . 29 mtnnsic VISCOSIty measurements .

6 Graft copolymers of cellulose and starch acetates have been synthesi sed wherein control over the molecular weights of the graft, degree of substitution and control over the backbone-graft linkage have been achieved. The grafting is carried out on starch acetate or cellulose acetate (degree of substitution 2.4-2.5) using living polymeri sation techniques as opposed to free radical mechanisms that operate in most reactions. Polystyrene is the synthetic polymer. Tre synthetic procedure yields uniformly grafted materials that could also be used as compatibilisers for starch-synthetic polymer blends and are potentially biodegradable30J I

.

Applications

Most applications of graft copolymers are as compatibilisers in blends of polysaccharides with synthetic polymers. These blends show superior properties as compared to unmodi fied polysaccharide/synthetic polymer blends26

• As mentioned earlier, only scattered reports exist on the use of the graft itself as a thermoplastic. No commercial product is available.

Biodegradable Synthetic Polymers

Synthetic polymers are by and large non­degradable in nature. Polymers which contain a C-C backbone are as a rule not degradable. Materi als which contain heteroatoms like polyamides and polyesters are also not degradable. The non­degradability is attributed to crystallinity and the presence of identical repeating units (proteins which are polyamides are degradable due to different repeating units that may be present in a single chain).

However, some synthetic materials are seen to be ?iodegradable, based on standard methods of

532 INDIAN J CHEM, SEC B, MAY 1999

evaluation. Examples of these are polycaprolactone (PCL), polyurethanes (based on polyesters), polyvinyl · alcohol (PVA) and ethylene-vinyl alcohol copolymer (EV AI) with high vinyl alcohol content32

•

The mechanism of degradation in a biological environment for synthetic polymers is similar to that of natural polymers. Materials may be used alone or in combination with other materials to enhance properties.

Polyesters

Typically, only aliphatic polyesters are seen to be biodegradable. Of all biodegradable polyesters, poly)caprolactone) (PCL) is one of the most explored materials and its biodegradability IS weB established33

.

Degradation takes place in the presence of lipases secreted by fungi like Rhizopus delemar and Rhizopus arrbizus. The amorphous zone is attacked preferentially. In enzymatic hydrolysis, it was seen that polymer on the sample surface is degraded and low molecular weight degradation products are removed from the sample by solubilization in the surrounding aqueous medium. Soil burial tests show that chemical and enzymatic hydrolyses play a role and take place throughout the film.

PCL is usually used in blends with other synthetic polymers. Blends of PCL with poly(propylene) are reported34

• The blends show biodegradability only when PCL is the continuous phase. Blends of PCL with starch and PHB are also reported.

Poly(vinyl alcohol) (PVAI)

PV AI is one of the few polymers with a carbon­carbon backbone that is cO!f1pletely biodegradable. It can be readily degraded in a variety of environments, particularly by soil bacteria like Pseudomonads. The basic mechanism of cleavage is believed to be a random chain cleavage process on the polymer balckbone.

PVAI is obtained by the hydrolysis of poly(vinyl acetate). Controlled hydrolysis can give copolymers of varying compositions. At high vinyl alcohol contents these are readily biodegradable.

Inspite of the identification of biodegradable synthetic materials, their applications are limited. The

polymers have been investigated in blends with other synthetic polymers. Polymer Modification to Facilitate Biodegradation

Most approaches to modify existing synthetic polymers for biodegradable applications are based on synthetic procedures which introduce 'weak' links or trigger points for degradation. Photodegradable polymers, which have been discussed in some detail earlier are an example of such synthesis. The literature also affords other such examples. A common approach has been the introduction of ester groups (Figure 3). Ester groups have been demonstrated to undergo hydrolysis and hence provide sites for molecular fragmentation. Biodegradable polymers with ester linkages are readily cleaved by chemical hydrolysis35.

Apart from introducing weak links, another approach has been to use pH control as a trigger. Base stable polycarboxylic acids derived from glyoxylic acids can degrade in neutral or acidic water as shown in Figure 4 35,

In polymers like nylon 6, the non-degradability of the material is attributed to the presence of single repeat units in the chain that contribute to crystallisation. In contrast, polyamides like proteins which are built up with several amino acid residues are biodegradable. Hence, glycine (basic repeat unit in Nylon 2) units have been incorporated in a nylon 6 polymer to induce biodegradability (Figure 5).

Bacterial Polymers

A wide variety of bacteria produce natural

Figure 3-Insertion of ester linkages in a p:>lymer molecule

H H I I

-C-O-C- -----i .. ~ CHO

I I C~Na

Figure 4-Acid hydrolysis of poly(carboxylic acid)

ANURADHA et al.: ENVIRONMENTALLY DEGRADABLE-THERMOPLASTICS: AN OVERVIEW 533

~H-tCH~ Cora ~H ?H2CO+~H (CHm cor;jn NYLON 6 GLYCINE

Figure 5-Synthesis of biodegradable Nylon 6

polyesters, which are intracellular reserve materials. These polyesters are biodegradable, melt processable, obtainable from renewable resources and have been the subject of much research activity.

The members of this family of thermoplastic biopolymers have the structure as shown in Figure 6.

Material properties vary from rigid brittle plastics to flexible plastics with good impact properties to tough elastomers, depending upon the size of pendant alkyl group R, and the composition of polymer36

•37

The commonly used polyesters, however, are poly(hydroxy butyrate) (PHB) and poly(hydroxy butyrate-co-valerate) (PHBVl PHB itself is brittle so PHBV replaces it in certain applications, Molecular weight can be controlled and ranges from hundred thousand to a million.

Bacterial polyesters result when cells are grown under stressed conditions. This can result from the limitation of some vital components needed by the microorganisms for their normal metabolic processes. Under normal or balanced growth conditions, the appropriate bacterium would completely utilise the organic substrate available for energy and for creating cellular materials. Under biologically stressed conditions, however, cells will interrupt their normal metabolic process to convert intermediates generated from such substrates into the polyester.

Enzymatic reactions involved in the production of PHB are as follows:

Normal Iv1etabolic Balanced

Organic ~ CH3COOH ~

S"b"r" "m::::"" Gmw'h . ~ PHB

Growth

Energy + Cellular Materials

PHB formation is shown in Figure 7.

The enzymatic hydrolysis of PHB takes place by several different bacteria which are known to secrete active esterases. PHB can be completely biodegraded

Figure 6-Structure of poly(hydroxy alkanoates)

-[ °t . II 0yH CH 2 -C

CH3

OH ° E4 • I II

.-. CH3 CHCH2C CoA

Figure 7-Reaction scheme for PHB formation

to CO2, water and energy. Recent work also indicates that PHB is susceptible to fungal biodegradation38

.

Commercial exploitation of these materials is currently limited due to high costs. There are reports of alloys of PHB or PHBV with a natural polymer or a biodegradable synthetic polymer like polycarpro­lactone (PCLl Properties are comparable with that of the pure bacterial polyester. PHB-PCL diblocks have been synthesised39 which could act as compatibilisers for these blends, improving properties.

PHB can be blended with steam-exploded lignocellulosic fibres like straw. Steam explosion removes lignin and other amorphous components improving adhesion between the two polymers. Straw interfers with PHB crystallisation thus improving properties. Up to 20% straw may be incorporated to toughen PHB4o.

Many different types of polysaccharides are produced and excreted by bacteria into their surroundings (e.g. xanthan, dextran, pull ulan, cellulose, etc.)41. Applications are typically 111

cosmetics, as adhesives and protective gels. No application as thermoplastic is reported.

Other Materials

Lactic acid is produced by Lactobacillus fermentation of carbohydrates. It has been demonstrated that ' lactic acid can be chemically polymerised to high molecular weight poly(1actic acid) (PLA). These materials have potential

534 INDIAN J CHEM. SEC B. MAY 1999

biomedical applications, being degradable in vivo 42

PLA has also been explored for thermoplastic applications, its properties are comparable to PS and it decomposes in three weeks43

.

Factors that Affect Biodegradation

The examples of polymers that readily degrade indicate that the important structural factors that affect the rate of degradation of polymers in a biological environment are3

:

Certain kinds of functional groups provide sites for degradation. The presence of easily hydrolysable and/or oxidisable groups appears to be an essential criteria (eg ester, amide, urethane, urea, ketone, etc.)

2 Hydrophilic polymers are likely to undergo degradation faster than hydrophobic polymers. However, not all water soluble polymers are biodegradable.

3 Addition polymers with a C-C backbone and molecular weight above 500 do not biodegrade.

4 Synthetic addition polymers with heteroatoms may degrade.

S Synthetic heteroatom contammg polymers prepared by step growth or condensation are likely to degrade.

6 Lower molecular weight facilitates biodegradation. However, there seems to be no agreed specified limit.

7 Crystallinity inhibits biodegradation. In semicrystalline polymers, the amorphous zone degrades more rapidly than the crystalline zone. However, not all amorphous polymers are biodegradable.

Evaluation of Biodegradation

Because of the complexity of the relationships between, and the consequences of the chemical, physical and property changes for different polymers in different types of biochemical degradation processes and environments, it is difficult to specify un ique or even standard tests and speci fi c or standard des :;rip t i on~ of biodegradation.

Within these constraints , tesi practices are in exis tence to determine aerobic and anaerobic

degradation in municipal sewage & degradation in aquatic, composting and soil envi ronments. The ASTM committee D20.96 has set speci fic conditions and specific test procedures for evaluating aerobic and anaerobic biodegradation in a variety of environments. Soil, freshwater and marine environments need to be evaluated to establish biodegradation.

Methods commonly used to evaluate biodegradation44 are discussed. The protocols are both quantitative and qualitative in nature. Tests are also being designed to determine environmental fate of the polymer and the effect of polymer residues in the environment. Qualitative tests are essentially screening tests for biodegradation.

Soil burial

This is the traditional way of evaluating biodegradatIOn as it is similar to actual conditions of waste disposal. Generally, samples are buried for a period of two years. At the end of this period,. changes in weight, mechanical strength, shape, etc. are studied. The method is merely a qualitative tool due to its lack of reproducibility (si nce climatic conditions and involved biological populations are both variables) .

Microbial degradation

Biodegradation is studied using polymers as C and N sources for the growth of microorganisms. Fungi are used more commonly than bacteria. The degree of deterioration is determined by observing colony growth, production of CO2, oxygen consumption and cell count (when bacteria are used). To determine fungal degradation polymeric substrates are inocculated with 4-5 different spore suspensions and incubated. Normal incubation period .is upto 8 weeks. Physical properties like molecul ar weight, solution viscosity, tensile strength and change in morphology are monitored to provide an indirect estimation of degradation.

ASTM Method

A brief mention of the ASTM protocol (based on microb ial degradation is necessary).

ASTM uses the specific biometer assay to determine biodegradabiliti. The method meas 'lres utilization of the plastic substrate by microorganisms resulting in the production of C02/CH4 and stable

ANURADHA et al.: ENVIRONMENTALLY DEGRADABLE-THERMOPLASTICS: AN OVERVIEW 535

humic material. The use of positive controls (biodegradable material like cellophane) and negative controls (recalcitrant materials like polystyrene) provides a quantitative measurement of biodegradability.

In the ASTM method for fungal degradation45, a sample of molded plastic or film is placed in contact with a standard set of microorganisms, particularly fungi, in a favourable medium for growth, and the density of the growth of microorganisms on the su rface of the sample is estimated visually. Degradation is measured by assigning numbers from o (no visible growth) to 4 (from 60-100%) for the ex tent of plastic covered.

Apart from the standard method of evaluation, fate analysi s of polymers in a waste environment is essential. It is a necessary criteria that the breakdown products of degradation should not be toxic in nature.

Fate analysis

Fate analysis of polymers 10 determine the effect of degraded residues in the environment has been extensively investigated for water soluble polymers in the detergent industry. The methodology has also been extended to thermoplastic materials. The C balance of the process for both aerobic and anaerobic environments is established to assess biodegradation3

.

In aerobic environments:

Cc = CO2 + Cc + Cr

where Ce, carbon content of the polymer Cc, carbon content of the polymer converted to celular matter and

Cr , residual carbon content in the environment. When C, is 0 complete biodegradation takes place, when Cr > 0 the biodegradation is not complete (hence an analysis of possibly toxic residues is necessitated) and when Cr = C., the material is not biodegraded.

In anaerobic degradation, CH4 is also introduced in the environment.

Commercial Biodegradable Thermoplastics

The exploitation of biodegradable materials for plastic applicat ions i~ baseu on nat ral polymer composites or bacterial polyesters. While bacterial polymers are .ompletely biodegradable, starch

composites are only partially degradable and there has been some controversy over their degradability.

In its application in biodegradable plastics, starch is either physically mixed with the native granules kept intact or melted and blended on a molecular level with the appropriate polymer46,47. PHBV is the bacterial polyester that is commonly in use.

Compositions currently marketed (for mainly packaging applications) are as shown in Table II.

Application Areas

Targeted areas for degradable polymers are generally for packaging materials and use and throw materials where degradability is an attractive criteria. Potential applications are as shown in Table III 48. However, the list is not exhaustive in itself and the use of degradable plastics needs to be extended to all plastic applications.

Current Research

With degradability becoming an important criteria, the development of biodegradable molecules has assumed importance. Typically, any of the three categories can be manipulated to tailor these

Table II--Commercially available biodegradable polymers

Product

StarchIPolyethylene

StarchiPoly(vinyl alcohol)

Ecofoam (hydroxypropylated starch)

Starch-PCL blend

Manufacturer

Corollo(UK)

FERTEC (Italy)

Churchi ll Technology

Mater-Bi

NSCC

Melitta

Copolyester Eastman Chemical

Cellulose Acetate

Copolyester (glycol+aliphatic Showa Highpolymer Dicarboxy lic acids)

PHB/PHBV ICI, Zeneca

Poly(Jact ic acid) Shimadzu

Table III--Targeted applicat ion areas for biodegradable polymers

Film products

Retai l and refuse bags

Agricul tural mulches

Beverage rings

Bottles and drums Diaper lin ings

Foam products

Disposable f:lst food service item.

Egg cartons

Containers

536 INOlAN J CHEM, SEC B, MAY 1999

materials. Areas currently being explored are:

Modification of naturally occurring materials. These include, principally starch, cellulose, lignocellulosics, lignin and materials like chitosan. Modification may be chemical or physical in nature.

2 Bacterial polyesters are also the subject of much research. As already mentioned, the limiting factors in their use are poor properties and high cost. Blends of these polyesters with synthetic biodegradable polymers like PCL as also composite with straw and other fibres are under investigation.

3 Synthetic biodegradable thermoplastics like PCL, PYAI do not have large scale thermoplastic applications. Hence, their blends with high tonnage materials like polyethylene and polystyrene are under investigation .

4 Tailoring completely new polymers using different synthetic routes. The introduction of weak chemical li nks in the polymer chain or pH sensitive groups in the polymer chain have been some of the attempted chemical aspects.

None of the current approaches, however sati sfy completely, both performance and degradability criteria and are compromise approaches. They still do not replace commodity plastics and fi nd mainly niche applications. However, environmental concerns will provide a push for the development of biodegradable thermoplas tics. Of all approaches, bacterial polymers sat isfy best the "cradle to grave" approach to degradability . Cost and properties still remain a matter of concern.

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