Natural rubber is a naturally occurring polymer obtained from the latex of rubber trees. It is a cis-1,4-polyisoprene polymer. Latex is extracted from rubber trees by making cuts in the bark. Natural rubber can be emulsion polymerized using Ziegler-Natta catalysts or anionically. It is used to make many products due to its elasticity, strength and chemical resistance including medical gloves, tubing and other medical products. Processing involves compounding, mixing, shaping and vulcanization to introduce crosslinks. Natural rubber is commercially graded according to quality standards.
3. 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.
5. Cis-1,4 polyisoprene structure
Hevea brasiliensis
It is the main constituent
of natural rubber.
Synthetic cis-
polyisoprene and natural
cis-polyisoprene are
derived from different
precursors.
8. Chemical properties of raw NR
• If 300°c destructive distillation 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 o2 or o3)
• Other reactions with hydrogen leads to formation of
hydrogenated rubber
• Reacts with chlorine to form chlorinated rubber; reacts with
HCL to form hydro-chlorinated rubber; cyclization reactions
also may occur
• Since NR contains some amount of antioxidants, it is stable for
longer storage periods in air at room temperatures and on
exposure to light, nr oxidizes by forming hydroperoxides
through oxygen radicals
• Unsaturation facilitates reaction with oxidizing agents like
peroxides, peroxy acids, potassium permangnate, ozone and
chlorine.
9. Physical properties of raw NR
Specific gravity : 0.932 at 20°C
Specific heat : 0.502 at 20°C
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 10^15 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.
10. 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.
11. 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.
20. METHOD OF NATURAL RUBBER LATEX
Production of rubber is followed by processing into final product
consist of:
Compounding
Mixing
Shaping
Vulcanizing
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.
21. 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.
22. SHAPING
Shaping processes for rubber products can be divided into four
categories:
Extrusion
Calendering
Coating
Molding and Casting
23. 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.
24. 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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26. 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.
27. Coagulation process can occur by:
a) Addition with acid(fast reaction)
b) Action of bacteria(slow reaction)
28. (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.
29. FORMIC ACID
Advantages:
a) Formic Acid’s acidity is ideal for transforming latex
into homogeneous dry rubber
b) No effect on rubber elasticity
c) Cost efficient
30. (b) Action with bacteria
Bacteria from the air enter the latex.
The growth of bacteria produces lactic acid.
Lactic acid causes coagulation of latex.
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36. 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. Inexpensive
2. Easy to obtain
3. Volatile
Disadvantages of ammonia :
1. Smelly
2. Sensitive to zinc oxide
3. Cause health hazards if being inhaled for long period
37. METHOD OF POLYMERIZATION
Monomer: 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.
38. 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.
39. 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
(𝜇m)
40. COMMERCIAL GRADE
In 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.
42. 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
43. LATEX TUBINGVacuum 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
44. 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.
