1. C.K.S. Pillai, Natural Monomers and Polymers: Can India use its Abundant Agricultural and Forest Resources as Alternative Sources for the Production of Polymers: Part 1, Pop. Plast. Packaging, 41 (8), August Issue, 64 (1996).
2. C.K.S. Pillai, Natural Monomers and Polymers: Can India use its Abundant Agricultural and Forest Resources as Alternative Sources for the Production of Polymers: Part 2, Pop. Plast. Packaging, 41 (10), October Issue, 69 (1996).
3. C.K.S. Pillai, Natural Monomers and Polymers: Can India use its Abundant Agricultural and Forest Resources as Alternative Sources for the Production of Polymers: Part 3, Pop. Plast. Packaging, 41 (11), November Issue, 67 (1996).
(Reprint not available: computer copy as a single article given below)
Renewable Resource based Polymers: CAN INDIA USE ITS ABUNDANT AGRICULTURAL AND FOREST RESOURCES AS ALTERNATIVE SOURCES FOR THE PRODUCTION OF POLYMERS (Part1, Part2 & Part3)
C.K.S.PILLAI
Regional Research Laboratory,
Thiruvananthapuram - 695 019
INTRODUCTION
This century has witnessed the spectacular growth and development of polymeric materials whose status is now considered in par with advanced metals and ceramics. In certain properties, some of the so called high performance polymers have exceeded those of even steel and aluminium. Great strides of developments are predicted in the further development of polymers in next century. Apart from the host of polymers with unprecedented qualities, one of the greatest developmental outcome is the generation of information necessary for the tailor making of polymers to meet specific property profiles for any conceivable application 1-4. There have been many attempts to apply these information to natural polymer systems to achieve desired property profiles with varying degrees of successes which have given / are giving it a new outlook as a possible alternative source for the production of polymers 5-22. This article will discuss briefly a few of the significant developments in the area taking particular emphasis on chemical transformations on natural monomers and polymers made to impart high performance / functional properties so that they stand as alternate sources for the production of polymers. Options of biomass as alternative resource to petrochemicals for possible substitution will be considered.
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A variety materials made from wood, bark, animal skin, cotton, wool, silk, natural rubber etc. were essentially playing key roles in early civilizations 23. Mechanical modifications on them could convert them into useful tools and materials (stone, axes, wood carvings, animal skin, the twisting of cotton, wool and flax to form threads, weaving, preparation of thin skinned papyrus and vegetable tissues for writing, etc). Embalming of corpses was one of the earliest practices which involved a chemical modification (cross-linking of proteins by formaldehyde). With the growth of human civilizations, the use and applications of materials extended to metals and ceramics and by 19th century, there were eight classical materials- metals, stones, woods, ceramics, glass, skins, horns and fibres - of which woods, skins, horns and fibres are organic polymers. During the last century and a half, two more materials were added to the list, rubber and plastics both of which are polymeric in nature. However, by 1900, there were only a few plastics in use for eg. shellac, gutta percha, ebonite and celluloid. Although a variety of chemical modifications came into vogue, four great discoveries paved the foundations of the industrial utilization of natural polymers and even to-day form the basis of continued support and maintenance of these industries against stiff competition from synthetics. They are : the vulcanization of natural rubber, the mercerization of cotton, hemp and flax, the tanning of leather and the loading of silk . These discoveries even to-day are the basis of many modern industries based on natural polymers that produce standard materials with a definite specification. The advent of synthetic polymers based on petrochemicals affected adversely many of the thriving industries based on natural polymers. But the time has come for regeneration.
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There are a number of factors that favour the use of natural polymers as a source for polymer production. The concerns on environmental pollution, tensions in the gulf countries over oil, the fear of a possible future depletion of oil etc. add to give credence to the move to a bio-based material policy 24 . The over dependence of chemical industry on petroleum resources could be reduced by the full utilization of renewable natural resources where ever it is possible. Ranging from algae to wood, its availability is limited only by the photosynthetic efficiency of the plants 25. Table 1 gives the availability of carbon over earth of which the total biomass is substantially high ( more than the total production of polymers) so that there will be sufficient agro -by- products and related materials for utilization 26. These materials are renewable and is also adaptable through genetic manipulations 27. Its availability is flexible through crop switching. Apart from naturally existing polymers, there are a variety of naturally occurring monomers that exist in free or in combined form and could be obtained by extraction, cleavage or depolymerization from biomass 2,7,16,26-33. The abundant availability and structural variety of agricultural and forest products lend support to the need to have a fresh look into their utilization . The major limitations will be, unlike petrochemicals , the variation in properties from source to source, the presence of contaminants, lack of uniformity in composition etc. which can be solved with a new approach based on application of the information gained from the synthetics as indicated earlier 1,3-5
II . Advanced Synthetic Structures vs Natural Polymers
One may ask now whether natural systems have the necessary structural evolution
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needed to incorporate high performance properties. So an attempt is made here to compare the structure of some of the advanced polymers with a few natural polymeric materials. Fig. 1 gives the cross-sectional microstructure of Kevlar@, an advanced polymer with high performance applications 34. A hierarchically ordered arrangement of fibrils can be seen. This is compared to the microstructure of tendon 35(Fig.2). The complexity and higher order of molecular arrangement of natural materials are evident. Tendon and similar materials are examples of several orders of hierarchical orders of molecular arrangement evolved over millions of years to specifically cater to specific functions. This can be explained by taking the structure of haemoglobin. There are four levels of structural hierarchy increasing the complexity and improving the specificity in function. The primary structure involves the covalent bonded polypeptide chain which by hydrogen bonding and helical arrangement forms the secondary structure. The tertiary structure is formed by folding resulted through the disulphide bonds giving rise to a myoglobin; four units of which combines to form quartenary structure of haemoglobin that carries oxygen with out fail to its destination. It was not possible to find a man made polymer with a similar structural complexity and functional specificity ( the honey combed structures of ablatives used in the space ships can be said to have a higher complexity in structure and function) 35. Possibly the structures self-assembling polymers might have an higher order complexity. It is well known that one of the main factors in the high strength of Kevlar@ originates from the hydrogen bonded structure of the polyaramide 36. The structural stability and strength of many of the natural polymers owe their source to hydrogen bonding which need not be emphasised further. From materials point of view, the strength of cellulose owe much to the hydrogen bonded structure made possible by the ß- glycosidic linkage in comparison to that of starch also made
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from glucose but with alpha glycosidic linkage. Comparisons can be made on the helical structures of synthetic and natural polymers to emphasise the role of advanced structures on properties. A number of such comparisons can be made available to show that the natural polymers are much more complex than synthetics and therefore, what is required is more imaginary chemical approaches to deal with natural systems.
III BIOMASS
Biomass is generally constituted of polymers, oligomers, monomers, and other non-polymerizable simple organic compounds including metallic salts and complexes 2,16,29,30. Polymers are , of course, the major constituents and have been serving human civilizations from time immemorial. The outstanding aspect of natural polymers is their wide variety which gives innumerable opportunities for structural modifications and utilization. A rough classification of biomass is given in Fig. 3 to indicate the sources from which natural monomers and polymers could be derived.
Fig. 3
Classification of Biomass
There are numerous natural materials, but a selected list which is important from the point of view of developing into useful polymers is given in Table 1.
Table 1
Plant types and Components
Components / Plant typesLignocellulosics / Trees, Straw
Starch / Corn, Wheat, Potatoes etc
Sugar / Cane, Beet
Proteins / Legumes, Foliage rich plants
Isoprinoid products / Rubber plant, Gauyule
Algae / Algae
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Cellulosics, starch and sugars come under the broad category of carbohydrates. The chemical constituents of the plant components are given in Table 2
Table 2
Chemistry of Plant Constituents
Plant Components / Chemical ConstituentsCarbohydrates / Simple: Glucose, Sucrose, Fructose, Lactose, Galactose, Other Hexosans, Pentosans
Complex : Cellulose, Hemicellulose, Starch, Pectin, Chitin, Glycogen etc.
Phenolic / Lignin, Flavanols, Polyphenols, Phenols, Resorcinols, Anthraquinnes, Napthaquinons, Hydragenol, Stibenes, Coumarines
Lipids / Fatty acids, Rosin acids, Sterols, Fatty alcohols, Phosphoglycerides, Sphingolipids, Lipoproteins, Proteolipids, Phosphatodilipids
Hydrocarbons / Rubber, Polyisoprenes
Proteins, Peptides, Nucleic acids / Enzymes, Transport and storage proteins, antibodies, Structural proteins( eg.Collagen, Keratin, etc), Hormones, Aromatic amino acids, Purines, Pyrimidines, Nucleosides, Nucleotides
Alkaloids / Pyrrolidine, Pyridine, Pyrrolizidine, Tropane, Quinolizidine, Isoquinoline, Piperedine, Indole, Terpenoids, Quinazoline etc.
Drying Oils and Alkyd Resins / Linseed, Cotton seeds, Castor, Tung, Soyabeen, Oiticia, Perilla, Menhaden, Sardine, Corn, Saflower, Vernonia, Fossil resins - amber, Kauri, Congo, Oleoresin- damar, Ester gum,
Steroids / Cholesterol, Andrenocortical, Bile acids, Ergosterol, Agnosterol, Desmosterol
Tannins / Tannin
The majority of plant components noted in table is of polymeric in nature and be classed as natural polymers. Of these materials , only the following are available for large scale production :
Carbohydrates, Drying Oils, Alkyd resins, Lignins, Polyisoprenes, Proteins ,Terpenes and Terpenoids
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IIIA. NATURAL POLYMERS
III A 1 CLASSIFICATION
Natural polymers can be classified in a variety of ways 7,16, 31, but it appears that a classification based on structural hierarchy is most appropriate. Thus, depending on the nature of the hetero atom inserted in the main chain, the polymers can be classified into four major types as hydrocarbon polymers(eg. natural rubber), carbon-oxygen (eg. carbohydrates- cellulose, starch etc.; phenolics - lignin, humus etc. and polyesters- shellac), carbon-oxygen -nitrogen/ sulphur ( eg. proteins with the exception of phospho proteins) and carbon-oxygen-nitrogen-phosphorus (eg nucleic acids) containing polymers. Of these polymers, polysaccharides, proteins and nucleic acids are grouped as polymers having pronounced physiological activity. When material applications are considered, nucleic acids can be excluded except some recent genetic engineering outlets with, of course, is a promising growth mode 16. So based on material applications, the following polymers are important : natural rubber, coal, asphaltenes (bitumens), cellulose, chitin, starch, lignin, humus, shellac, amber and certain proteins. Fig. 4a and b gives the primary structures of some of the above polymers. For detailed information on their occurrence, conventional utilization etc., the references cited above may be referred.
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Fig. 4a. Structures of some Natural Polymers
Fig. 4b. Structure of Natural Polymers : Lignin
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III A 2 NATURAL POLYMERS AND MONOMERS : THEIR AVAILABILITY
Table 3 gives the total availability of carbon over earth 37. The living organic matter in land accounts for 0.08 x1012 tonnes.
Table 3
Carbon in Biosphere
For m / Tonnes x1012Carbon in Sediment / 18,000
Organic Carbon in Sediment / 6,800
CO2 in Atmosphere / 0.65
Living Matter on Land / 0.08
Dead Organic Matter on Land / 0.70
CO2 in Ocean / 35. 40
Living Matter in Ocean / 0.008
Dead Organic Matter in Ocean / 2.7
The total approximate annual production of biomass is estimated to be 1.2 x 1011 tonnes in terms of dry matter. Half of this amount is in the form of wood of which another 50% is burnt as fuel. The balance of the biomass is distributed as forest canopy, marine algae, agricultural crops etc. The annual availability of wood is estimated to be around 1.3 x 1010 tonnes. The composition of wood is approximately 40-50% cellulose, 20-30% lignin and 20-30 % hemicellulose depending on the species and location. The potential yield of cellulose is of the order of 1010 tonnes. The production
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of wood pulp ( 1.2x108) alone is only below 1% of the total wood availability. The total world production of cereal crops is of the order of 109 - 10 10 tonnes. The average starch content is about 70% which makes starch as the next most abundant material after cellulose. The total availability of starch is around 1 x 109 tonnes per annum. The next most abundant carbohydrate is sucrose, the total production of which is touching 106 tonnes per annum. The total world production of polymers comes to around only 150 million tonnes 1. The production projected for 2,000 AD is around 425 million tonnes. The total annual polymer production in India is only near 550 kilotonnes and 2000 AD it is near 1500 kilotonnes. As starch and sugar go as food for major outlets, the portion available for other uses will be comparatively small. The total arable land area is estimates to be 17.6 % out of which only 11% is used. However, out of pressure for increased food production, this area will be cultivated and used and may not be available for crops for energy and materials. But, there is the agricultural wastes and residues that itself may amount to large volumes. The total availability of agricultural and animal waste in India comes to 22,683.3 lakh tonnes per annum of which crop residues and byproducts come to around 1599 lakh tones, forest residues 175 lakh tonnes aquatic biomass 30 lakh tonnes and algae etc. 640 lakh tonnes 38. Table 4 gives the split up of waste residues per anum of agricultural, animal, and forest products in India. It can be noted that the availability of such renewable materials in India for higher than the demand and production of polymers in India. So, it could be, processed into polymers, if appropriate technology is available for such a transformation. This list has considered only the waste products. Even products such as wood, fibres etc. that have found useful applications can be processed further for improved performance and better utilisation. India has only 22.7 % of the land as forest cover. The total growing stock of Indian forest is estimated to be 2400 million cm3. The annual increment is