Natural Polymer:
- Fibres commonly used in composites materials, carpet, ropes, coir and geo textile.
- Natural fibres such as jute and coir have been computing the properties of cellulose fibres.
- Cellulose fibres are obtained from different parts of vegetables plants for e.g. jute are obtained from the stem; sisal, banana and pineapple from leaf, cotton from seed, coir from fruit etc.
Chemical Composition:
Namely Constitutes:
1. Cellulosee
2. Hemi-cellulose
3. Lignin
Cellulose: Cellulose is a linear condensation polymer consisting of D-anhydroglucopyranose units joined together by -1, 4-glycosIdic bonds. The pyranose rings are in chair conformation. The crystallinity in natural fibres is mainly due to the crystalline cellulose; as cellulose molecule is stiff and straight because of its internal hydrogen bond and linkages.
Hemicelluloses: hemicelluloses are subdivided into (l) cellulosans and (2) incrusting hemicelluloses. Cellulosans are regarded as integral and structural part of cellulosic aggregate and are oriented in the same way as true cellulose.
1. The cellulose an associated with most cellulosic xylan. The incrusting hemicelluloses on the other hand are loosely bound, being readily removed from the fibre.
2. The predominant polysaccharide of Jute is however composed of a backbone of-D-xylopyranose units with approximately every seventh unit carrying a terminal 4-O-methyl -D-glucuronic acid residues linked through position.
Lignin: Lignin is a group of high molar mass, amorphous compounds with high methoxyl contents that are present in wood and some other plants.
The monomers of lignin are aromatic alcohols with phenyl propane backbone p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol.
The high Tg and rigidity of lignin may be due to cross-linking and intermolecular interaction. Amorphous lignin structure, under some conditions may adopt a quasi-ordered arrangement.
The hemicellulose is linked to lignin hydroxyl via its uronic groups in an ester linkage. It has been suggested that the hydroxyl group in the propyl side chain of phenyl propane unit of lignin may be involved in this linkage.
Amongst the naturally occurring lignocellulosic fibres, jute contains one of the highest proportions of stiff natural cellulose.
Applications:
1. In the textile industry, the high stiffness of Jute is often a limitation because of difficulties in the fine spinning, thus leading to only harsh and coarse fabrics.
2. However, in fibrous composites, stiffness of fibre is an important basis of design criteria since structures are designed on the basis of minimum deflection under load.
3. Coir is also a hard fibre of low density. The best application of such lightweight hard fibres can be in the area of composite materials
Natural Resin: “Natural organic substances that are usually transparent or translucent and yellowish to brown, are formed especially in plant secretions”.
1. They are soluble in organic solvents but not in water, are electrical nonconductors, and are used chiefly in varnishes, printing inks, plastics, and sizes and in medicine.
2. The resins arise from condensation of naturally occurring chemical compounds such as the terpenoids and flavonoids contained in trees
3. Being mixtures, the resins do not exhibit simple melting or boiling points, although they do exhibit a predominant glass-transition temperature (Tg).
4. The Tg of natural resins ranges from approximately 0 to about 100°C, which is high for low molecular weight condensation polymers. Chemical modification increases the molecular weight and broadens the range of Tg
Rosins: Rosins are known as gum, wood, or tall oil rosin, based on the method of isolation and the source. Rosin is isolated from pine trees, principally from longleaf Pinus palustris, Pinus ellioti.
Composition: Rosin is primarily a complex mixture of monocarboxylic acids of alkylated hydrophenanthrene nuclei.
1. The resin acids are subdivided into two types based on their skeletal structure.
2. The abietic-type acids contain an isopropyl group pendent from carbon 13.
3. The pimaric-type acids have a methyl and vinyl group pendent from the same carbon atom.
Applications
1. The carboxyl group reacts with metal oxides, hydroxides, or salts to form rosin soaps or salts.
2. The soaps of alkali metals, such as sodium and potassium, are useful in paper sizing and as emulsifiers in rubber polymerization.
3. The salts, primarily of calcium and zinc, are used in printing ink formulations.
4. The carboxyl group is converted to the alcohol by catalytic hydrogenation.
5. Specialty paper is treated with stabilized rosins to minimize yellowing during aging. Treatment are used in concentrations of 0.1-3.0%.
6. Rosin acid esters find wide application in printing ink formulations.
7. The resins provide pigment wetting, resistance to penetration, adhesion to substrates, hardness, and gloss.
8. Rosin-derived resins dispersed in linseed oil are used as vehicles for letterpress inks, which dry to a polymeric film by oxidation.
9. Metal salts of rosins, modified rosins, and polymerized rosins are used inexpensive gravure inks. Rosin ester resins are used extensively in pressure-sensitive adhesives as tackifiers.
10. Rosin, modified rosins, and derivatives are also used in hot-melt adhesives.
11. Rosin ester resins are used as modifiers in the formulation of chewing gum.
12. The soap of modified rosin has a long history as an emulsifier for the polymerization of styrene-butadiene rubber.
Shellac: Unlike other natural resins, shellac does not originate from plants, but is produced from the secretion of an insect. It is a purified form of lac and is the most widely known as lac product. Lac is produced by the larvae of Kerria laeea (Laccifer lacca).
Composition: Shellac is primarily a mixture of aliphatic poly hydroxy acids in the form of lactones and esters.
It has an acid number of 70, a saponification number of 230, a hydroxyl number of 260, and an iodine number of 15. Its average molecular weight is about 1000.
Applications
1. Shellac still has significant sales to the protective coatings industry.
2. Unpigmented shellac is used on floors, woodwork, and panelling.
3. White, pigmented shellac is used as a primer-sealer for interior applications.
4. Shellac is used as a protective coating for pharmaceuticals.
5. Candy is coated with shellac to seal in moisture and keep the product fresh.
6. Shellac is used in electrical applications. Paper coated with it serves as cores for motor windings. Shellac is used as the binder for manufacturing electrical insulation board based on flaked mica.
Rubber:
Natural Rubber:
Natural rubber (NR) (cis-1,4-polyisoprene) occurs in over 200 species of plants. The Hevea brasiliensis tree accounts for over 99% of the world's natural rubber production
Latex Composition
1. Freshly tapped Hevea latex has a pH of 6.5-7.0, a density of 0.98 g/cm3, and a surface free energy of 4.0-4.5 J/cm2 (0.96-1.1 cal/cm2).
2. The total solids of fresh field latex vary typically from 30 to 40%, depending on the clone, weather, stimulation, tapping frequency, and other factors.
3. The dry rubber content is 3 wt % less than the total solids.
4. The rubber phase typically contains 96 wt % rubber hydrocarbon, 1 wt % protein, and 3 wt % lipids along with trace amounts of magnesium, potassium, and copper
Processing:
1. Latex was collected from tapping cups 2-3 hrs after tapping and taken immediately to the laboratory without ammoniation. In order to minimize tree-to-tree variation, the latex from 8-15 trees was blended together.
2. It was shown also that rubber with a high initial viscosity hardened less on storage.
3. The molecular weight distribution as determined by Mw/Mn is extremely wide for natural rubber. Molecular weights range from 3 x 104 to an estimated upper limit of 107.
4. The high molecular weight fraction imparts strength and other important physical properties; the low molecular weight fraction contributes to processibility.
Types and Grades
The type of rubber is defined by the raw material and the method of production, whereas the grade refers to quality subdivisions within a type
Visually Graded Rubber
Visual inspection is the oldest method of grading rubber; eight types are produced from the latex of the Hevea tree:
1. Ribbed smoked sheets (RSS)
2. White and pale crepes
3. Estate brown crepes
4. Compo crepes
5. Thin blanket crepes (remills)
6. Thick blanket crepes (ambers)
7. Flat bark crepes and
8. Pure smoked blanket crepes.
Technically Specified Rubber (TSR):
Technically specified rubber (TSR) was introduced by the Malaysians in 1965 under the Standard Malaysian Rubber (SMR) scheme.
Other natural rubber- producing countries soon followed with their own version: - Indonesia (SIR), Singapore (SSR), and Thailand (TTR). The introduction of TSR brought innovations in processing, packaging, and quality control to the natural rubber industry.
- All versions of TSR are analysed with the same set of tests to determine quality, but small differences exist in the specification limits and the permissible raw materials.
- The standard test methods for determining SMR are recognized by all natural rubber producers and consumers, with minor differences.
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