NATURAL RUBBER LATEX

CONTENT REACTIONS AND METHODS POLYMERIZATION APPEARANCE,MECHANICALS AND CHEMICAL PROPERTIES COMMERCIALS GRADE APPLICATIONS

INTRODUCTION Rubber is a polymer which is a word that is derived from Greek meaning “many part”

Natural rubber is a naturally occurring polymer obtained from the latex of rubber trees.

Natural rubber(NR) is also known as 𝑐𝑖𝑠-1,4- ( )𝑝𝑜𝑙𝑦 𝑖𝑠𝑜𝑝𝑟𝑒𝑛𝑒 Natural rubber(NR) is found in the latex that comes from the rubber trees.

It is collected in a cup mounted on each tree, by slashing the bark to reach the latex vessels.

Natural Rubber Obtain from the latex (rubber tree) Poly(isoprene) Monomer : isoprene(2-methyl-1,3-diene)

Cis-1,4 polyisoprene structureHevea brasiliensis

It is the main constituent of natural rubber.

Synthetic cis-polyisoprene and natural cis-polyisoprene are derived from different precursors.

Trans-1,4-polyisoprene gutta-percha

Appearance of Natural Rubber Latex

Milky fluid Has odour Sticky

Chemical properties of raw NR• I f 300c destructive distil lation occurs

• Double bond in the isoprene unit and the -methylene groups are reactive for sulphur vulcanization

• May also lead to age degradation due to the presence of double bond (reacts with o 2 or o 3 )

• Other reactions with hydrogen leads to formation of hydrogenated rubber

• Reacts with chlor ine to form chlor inated rubber; reacts with HCL to form hydro-chlor inated rubber; cycl ization reactions a lso may occur

• Since NR contains some amount of antioxidants, i t i s stable for longer storage per iods in a ir at room temperatures and on exposure to l ight , nr oxidizes by forming hydroperoxides through oxygen radicals

• Unsaturation faci l i tates reaction with oxidiz ing agents l ike peroxides, peroxy acids, potassium permangnate, ozone and chlor ine.

Physical properties of raw NR Specific gravity : 0.932 at 20 Specific heat : 0.502 at 20 Purified NR after extraction with acetone has refractive index of 1.5215 to 1.5238. Electrical properties of NR are determined by its water soluble impurities. For instance the specific resistivity of sheets is 10^ and that of crepe is 2x10^15. When NR is stretched more than 80% of its original length, crystallization occurs. Un-crosslinked NR can be extended to 800 to 1000% of its original length without breaking. As the temperature increases forces required to deform the material becomes larger.

At high rate of strains, deformations are completely recoverable, whereas at low strain rates some deformation may remain. This deformation will however recover on heating of after some time. Because of this incomplete recovery the extension and retraction do not coincide. This hysteresis leads to a heat build up and is more pronounced in the first heat cycle and less in the subsequent cycles. Behavior in solvents :oWhen in contact with organic liquids like benzene, gasoline, vegetable oils, mineral oils, carbon tetrachloride, swell raw rubber to a considerable amount to form a highly viscous solution or gel.o Physical bonds are broken in this process.

The Glass Transition Temperature, Tg

The glass-transition temperature of NR is about -72˚C, but do not be surprised if other values are stated because the value of Tg depends on the method and technique used to determine its value.

The low Tg of NR is responsible for its high resilience, low rolling resistance, very good low-temperature flexibility, low heat generation and hence low heat build-up.

REACTION OF NATURAL RUBBER:LATEX

Coordination of polymerization isoprene using Ziegler-Natta Catalyst

MONOMETTALICInitiation step

Propagation Step

Termination Step

Bimettaliciniation step

Propagation step

Termination step

METHOD OF NATURAL RUBBER LATEXProduction of rubber is followed by processing into final product consist of:

CompoundingMixing

ShapingVulcanizing

Processing techniques for natural and synthetic rubbers are virtually the same, difference being in the chemicals used to effect.

COMPOUDING Rubber is always compounded with additives to satisfy the given application in terms of properties ,cost and processability.

Compounding adds chemicals for vulcanization. Mainly used reinorcing filler used in NR is carbon black.

MIXING

The additives must be thoroughly mixed with the base rubber to achived uniform dispersion of the ingredients.

A two-stage mixing process is usually employed. In the first stage,carbon black and other non vulcanizing additives are combined with the raw rubber.

After thorough mixing has been allowed,the second stage is carried out in which the vulcanizing agents are added.

Equipment for mixing includeds the Two-roll mill and internel mxers such as the Banbury mixer.

SHAPING

Shaping processes for rubber products can be divided into four categories:

Extrusion Calendering Coating Molding and Casting

VULCANIZATION Vulcanization is a process whereby chemical crosslinks are introduced to tie the molecules together to form a three dimensional network that prevents flow and provide high elasticity.

In practice, it is achieved by subjecting the ready mixed (compounded) rubber to heat and pressure enclosed in a mould.

The treatment that accomplishes cross-linking of elastomer molecules.

Makes the rubber stiffer and stronger but retain extensibility.

Vulcanization Sulfur react both at the double bonds and at allylic hydrogen atoms:

The reactive sites—"cure sites"—are allylic hydrogen atoms. These C-H bonds are adjacent to carbon-carbon double bonds.

During vulcanization, some of these C-H bonds are replaced by chains of sulfur atoms that link with a cure site of another polymer chain. These bridges contain between one and eight atoms.

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Vulcanization Reaction

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Coagulation process Latex is milk-like liquid obtained from tapped rubber tree Latex is a colloid (group) which contains suspension of rubber particles in water. Each group of particles of rubber is surrounded by a layer of protein membrane. This membrane is negatively charged and will repel with each other. So, natural rubber remain in liquid form.

Coagulation process can occur by:a) Addition with acid(fast reaction)b) Action of bacteria(slow reaction)

(a) Added with acid Acid (methanoic acid / ethanoic acid /formic acid) which consists of H+ ion is added into the latex.

The H+ ion from acids neutralize the negatively charged protein membrane.

The rubber particles collide with each other. Protein membrane of the rubber particle breaks. Rubber molecules clump together. Latex coagulates.

FORMIC ACID Advantages:a) Formic Acid’s acidity is ideal for transforming latex

into homogeneous dry rubberb) No effect on rubber elasticityc) Cost efficient

(b) Action with bacteria Bacteria from the air enter the latex. The growth of bacteria produces lactic acid. Lactic acid causes coagulation of latex.

AMMONIA Ammonia(NH3 is used to prevent coagulation of latex. It consists of OH-ion that neutralized the acid. The latex will remain negatively-charged thus preventing coagulation.

Advantages of ammonia :1. Inexpensive2. Easy to obtain3. Volatile Disadvantages of ammonia :1. Smelly2. Sensitive to zinc oxide3. Cause health hazards if being inhaled for long period

METHOD OF POLYMERIZATIONMonomer: isoprene

Polymerization: Coordination polymerization Ziegler-Natta catalyst. Anionic polymerization Why choose emulsion polymerization:oAnionic polymerization process lead to cis 1,4-polyisoprene,content of more than 90%.oTo prepare high-molecular weight elastomer.

EMULSION FOR SYNTHETIC POLYISOPRENE LATEX

•Polymer(polyisoprene) is dissolved in suitable organic solvent.eg:n-pentane

•The organic solution containing the polymer must be blended with an aqueous solution high shear emulsification unit (in situ preparation-chemical reaction takes place shortly, spontaneously during the preparation of emulsion).

•The resulting intermediate product is a highly diluted emulsion containing a high amount of organic solvent(VOC).eg:n-pentane solvent.

•The solvent contained in this emulsion is stripped and subsequently the diluted emulsion is higher concentrated to produce the polyisoprene latex with targeted properties.

•The total solids contents are similar but impurity level is higher in natural rubber lattices which is translated into a lower rubber content.

TYPICAL PROPERTIES LATEX ANIONIC(POLYISOPRENE) NATURAL RUBBER

Total solid content(%w) 65 Similar

Total rubber content(%w) 64 Lower

pH 10.3 Similar

Average particles size 1.5 0.9

Mechanical stability higher lower(m)

COMMERCIAL GRADEIn order to produce rubber that being that is high in quality as standardised as being graded by Standard Malaysian Rubber(SMR), tapping is the most vital step to obtain the latex.

Application of natural rubber

latex

MEDICAL GLOVES Rubber surgical gloves were first used in the healthcare sector

NRL is the material of choice for medical gloves due to :

1. Superior barrier protection

2. High elasticity

3. Chemical resistance.

4. High strength

LATEX TUBING Vacuum tubing Natural rubber tubing with thick walls is especially well suited for use as vacuum tubing.

Has Impermeable wall towards air. Commonly used in laboratories Medical Tubing -natural rubber tubing used in

medical department must be

treated to reduce the amount of

protein in the rubber specially for

patients who have

allergy to protein in latex

LATEX PROTEIN ALLERGIC Skin irritation occurred due to protein presence in natural rubber latex. Steps taken to improve allergy :

• Natural rubber latex (NRL) have taken steps to lower the protein level by leaching, chlorination or through enzyme treatment.

• Change from powdered NRL to free-powder of NRL to make gloves.

REFERENCE http://www.slideshare.net/MadihahRamly/natural-rubber-18978774?related=1

https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF8#q=polymerization+techniques+for+glove

https://www.google.com.my/?gws_rd=ssl#q=polymerization+techniques+for+natural+rubber+latex

http://docs.kraton.com/kraton/polyisoprene.com