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STUDIES IN THE DEPROTEINIZATION POTENTIAL OF COCONUT WATER (COCOS NUCIFERA) NATURAL RUBBER LATEX

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Abstract

The use of untreated natural rubber latex for the production of dipped goods has suffered a major setback due to numerous complaints of allergy traced to inherent protein content, a number of conventional treatment substances have been reported such as urea, papain and bromelain. The challenges in the use of these substances range from the cost to environmental problems. The use of coconut water being a natural product as a potential deproteinizing agent in natural rubber latex has therefore been investigated. Amount of coconut water ranging from 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 1.0 % v/v in 1% ammonia preserved latex were analysed. The results obtained showed that coconut water has potential for use as deproteinizing agent as they were reductions in protein content (%) for all samples except for 0.2 and 0.3 v/v which remained indifferent. The most efficient formulation gave a value of 48% reduction in protein content corresponding to 0.5% v/v coconut water in latex. These result sugg

1.0                                                            INTRODUCTION

Natural rubber, also called India rubber or caoutchouc as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds, and water. Malaysia and Indonesia are two of the leading rubber producers in the world. Forms of polyisoprene that are used as natural rubbers are classified as elastomers. Natural rubber has characteristic properties such as large enlongation, high elasticity, high tensile strength and tear strength and satisfactory coating film strength. Natural rubber is utilized in wide variety of products including household materials such as gloves for surgeries and contraceptives. Rubber is harvested mainly in the form of the latex from the rubber tree. Raw rubber comes from the field or plantation in two basic forms: field latex and field coagula.

Natural Rubber Latex contains a small amount of non-rubbers, which include a variety of protein and which have played a role in the biosynthesis and stabilization of rubber latex within the vessels of the tree. The latex of Hevea brasiliensis is a complex colloidal dispersion of biochemical components consisting of polyisoprene latex rubber particles and non-rubber components in an aqueous serum phase. Some adverse effects of these non-rubbers are well documented. These non-rubbers continue to play a role in the subsequent processing behavior of the Natural Rubber Latex, its long-term stability, and catalyzing crosslinking reactions through free radical and ionic mechanisms resulting in covalent bonds. The protein sheath, which may be amphoteric in nature, around the latex particle is conjectured to facilitate the movement of curatives, usually carbon-, sulfur-, and nitrogen-containing materials, into the latex particles by providing an intermediate transport mechanism from the water phase to the rubber phase. The removal of the non-rubbers in treated Natural Rubber Latex slows down the maturation process, hence translating into a longer “pot life.”

Rubber is used as raw material in many products today, which can be divided into two main types, i.e. natural and synthetic rubber. Raw natural rubber involves processing fresh latex or field coagula obtained by cutting the rubber tree, into a raw material ready for use in making rubber products further on. Raw materials in the natural rubber industry consist of fresh (field) latex, obtained by cutting rubber trees, and field coagula, which include cup coagula, cup lump, tree lace, bark scraps, earth scraps and smoked sheet cuttings, etc.

 

1.1 NATURAL RUBBER LATEX

The latex is a sticky, milky colloid drawn off by making incisions in the bark and collecting the fluid in vessels in a process called “tapping”. The latex then is refined into rubber ready for commercial processing. In major areas, latex is allowed to coagulate in the collection cup. The coagulated lumps are collected and processed into dry forms for marketing. There are different rubbers such as Hevea brasilienisis, congo rubber, dandelion e.t.c.

Latex is the polymer cis-1, 4-polyisoprene with a molecular weight of 100,000 to 1,000,000 daltons. Typically, a small percentage (up to 5% of dry mass) of other materials, such as proteins, fatty acids, resins, and inorganic materials (salts) are found in natural rubber. Polyisoprene can also be created synthetically, producing what is sometimes referred to as “synthetic natural rubber”. Natural rubber is an elastomer and a thermoplastic, but as rubber is vulcanized, it turns into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both; i.e., if it is heated and cooled, it is degraded but not destroyed. Naturally coagulated rubber (cup lump) is used in the manufacture of TSR10 and TSR20 grade rubbers. Processing of these grades is a size reduction and cleaning process to remove contamination and prepare the material for the final stage of drying. The dried material is then baled and palletized for storage and shipment (Hosler et al.,1999).

 

1.2 PRODUCTION OF NATURAL RUBBER LATEX

Rubber tree is an important industrial crop for natural rubber production. Latex comes from the Hevea brasiliensis tree, which grow in tropical regions. They typically reach 20-30 meters in height in the rubber plantations, and are able to produce commercial quantities of latex at about 7 years of age, depending on climate and location. Hevea latex is collected through tapping of trees in the natural forest.

 

1.3 GENERAL PROPERTIES OF NATURAL RUBBER LATEX

Rubber exhibits unique physical and chemical properties. Some of the properties are stated below:

• Natural rubber combines high strength (tensile and tear) with outstanding resistance to fatigue.
• It has moderate resistance to environmental damage by heat, light and ozone which are one of its drawbacks.
• The natural rubber has excellent adhesion to brass-plated steel cord, which is ideal in rubber tyres.
• It has low hysteresis which leads to low heat generation, and this in turn maintains new tyres service integrity and extend retreability
• The natural rubber has low rolling resistance with enhanced fuel economy.
• It has high resistance to cutting, chipping and tearing.
• Macromolecules of natural rubber are long, regular, flexible and practically linear, thus it has very good elastic properties.
• It has chemical structure of cis-1,4-polyisoprene
• It has a molecular weight beyond 100,000
• Typically, a small percentage (up to 5% of dry mass) of other materials, such as proteins, fatty acids, resins and inorganic materials (salt) are found in natural rubber.
• Elasticity is the fundamentally important property of natural rubber. Rubber is unique in the extent to which it can be distorted, and the rapidity and degree to which it recovers to its original shape and dimensions.
• It is strong and tough. Because of this property it can be put to use even under abnormal conditions
• It is highly impermeable to both water and air and therefore, it can be used to retain water as in rubber bottles, hoses etc.
• It exhibits a great resistance to abrasion, tearing and cutting over a wide range of temperature
• It is a bad conductor of heat
• It can contain liquid and gas
• When two fresh surface of milled rubber are pressed together, they coalesce to form a single piece. This property, known as tackiness of rubber, makes the manufacturing of composite articles, such as tyres possible.
• The plasticity of rubber makes it pliable and amenable to all manufacturing processes
• By vulcanizing the rubber, its mechanical properties can be considerably improved

The electrical insulating properties of pure rubber are inferior to those of vulcanized rubbers (Leonid Viktorovich, 2009)

 

1.4   APPLICATIONS OF NATURAL RUBBER LATEX

 

1. Natural rubber forms an excellent barrier to water.
2. This is possibly the best barrier against pathogens such as the AIDS virus (HIV). That is the reasons why latex is used in condoms, catheters, surgical or medical gloves.
3. Natural rubber is an excellent spring materials
4. Natural rubber latex is also used in balloons, medical tubes, elastic thread, and also in some adhesives.
5. It is used by automotives industry
6. The flexibility of rubber is appealing in hoses, tyres and rollers used in printing
7. It elasticity makes it suitable for various kinds of shock absorbers and for specialized machinery mounting designed to reduce vibrations.

Because of their electrical resistance, soft rubber goods are used as insulation and for protective gloves, shoes and blankets; hard rubber is used for articles such as telephone housings, parts for radio sets, meters and other electrical instrument (Leonid Viktorovich, 2009).

1.5   PROTEIN IN NATURAL RUBBER LATEX

 

The total protein content in latex has been estimated to be about 2-3%, however, discrepancies in the distribution of the proteins between the major phases of latex exist. Report shows that 20% of the total proteins were absorbed on the rubber surface, 66% in the C-serum and 14% in the bottom fraction.

1.5.1   Proteins on the surface of the rubber particle

The existence of proteins in association with phospholipids on the surface of rubber particles was recognized as early as 1953 by Bowler. The attributed that this protein phospholipid layer imparted a net negative charge to the colloidal stability of these particles. By measuring the iso-electric points of various latex samples, he concluded that there was more than one protein adsorbed on the rubber surface and that the relative proportion of the adsorbed proteins varied with clones.

1.5.2 Protein in the C-serum

Nearly half of the enzymes examined in natural rubber latex glycolytic appeared to be located in the C-serum of latex. These include enzymes for the pathway as well as many of the enzymes for rubber biosynthesis. The first protein to be isolated from natural rubber latex was from C-serum and it was named ?-globulin.

 

1.5.3 Protein in the bottom fraction

Proteins in the bottom fraction are essentially studied as the soluble proteins in B-serum. These have been examined with various techniques, including paper electrophoresis, starch gel electrophoresis etc. (Ferreira et al., 2009).

 

1.6 DEPROTEINIZATION OF NATURAL RUBBER LATEX

Denaturation helps in protein reduction and removal of undesirable residual protein, and there are several ways of doing this which would be reviewed here. The different methods include saponification, surfactant washing and enzymatic treatment.

The cleavage of proteins in NR latex was found to proceed with concomitant formation of low molecular weight polypeptides. This results in lowering gel formation of the enzyme-treated latex, indicating modification of the remaining proteins at the rubber chain-end. Washing NR latex with surfactant would efficiently reduce and remove proteins from NR latex particles through denaturation and transferring them to the serum phase. The relatively stable gel formed during storage of surfactant-washed NR latex is an indication of the absence of branch formation of proteins at the rubber molecule terminal. Saponification by strong alkali would hydrolyze the proteins and phospholipids adsorbed on the latex particle surface. The reason of the significantly higher gel formed in saponified NR latex is still not clear. The present study shows that deproteinization treatments result in modification of the proteins at the surface of NR latex particles and also those freely-suspended in the serum. The cleavage or the denaturation of the rubber proteins during purification by washing has a profound effect on the properties of the deproteinized NR latex upon storage, in particular the thermal oxidative aging properties of the rubber obtained.