Rubber
Tree:
Natural Rubber Production
Rubber trees are
perennial plants of economic importance that belong to the genus Hevea.
Propagation of this tree crop is by seeds or vegetative production with clones,
resulting in early development and establishment in plantation of genetically
superior genotypes, in productivity, quality of latex, resistance to the pest
and diseases among other interesting traits. The rubber is valuable for furniture
purposes and making its latex and also it latex in the production of natural
rubber.
Rubber Tree
Hevea brasiliensis, the Para’ rubber tree often
simply called rubber tree, is a tree belonging to the family Euphorbiaceae, and
the most economically important member of the genus Hevea. It is of major
economic importance because of it sap-like extract (known as latex) is the
primary source of natural rubber.
Hevea
brasiliensis originated in the Brazilian Amazon basin; the latex of this tree
was already known to ancient Indian
tribes in Ecuador and Brazil who harvested the latex from wild trees.
Early
explorers, including Columbus noticed the use of rubber by the Indians, but for
more than three centuries it was considered a curiosity and more neglected. The
French called rubber ‘caoutchouc’, after ‘ca-o-chu’, the name given to the
rubber tree by the South American Indians, meaning ‘weeping tree’.
The
name ‘rubber’ was given to the latex product by the British. They found out
that a text, written with a pencil on paper, could be removed by rubbing it
with the latex product and subsequently called it ‘rubber’. In the 18th
century, domestication of hevea rubber began and plantations were developed.
That was relatively late in history; hevea rubber can be seen as one of our
youngest domesticated major crops.
Gradually,
the cultivation of hevea rubber in South America was seriously impeded by a
fungal disease caused by Microcyclus ulei. Subsequently, the British introduced
H. brasiliensis seedlings via Kew Gardens into South-east Asia, where
subsequently many plantations were established. Due to technical innovations,
the rubber industry began to develop in the 19th century. The
invention called ‘masticator’ in 1820 made it possible to soften, mix, and
shape natural rubber. In 1839, Hayward and Goodyear discovered that the elastic
properties of rubber could be improved considerably by treating the rubber with
sulphur and heat, which process is called ‘vulcanization’. After the invention
of the combustion engine in the late 19th century, the demand for
rubber increase greatly due to the expanding need for tyres for motor vehicles
and aircraft, and also for the production of other goods such as bicycle and
raincoats.
As
well as H. brasiliensis, at least eight other Hevea species occur that yield
latex which can be used for producing rubber. However, the yield and quality of
the latex of H. brasiliensis are far superior to all other species, which is
why about 99% of the world’s natural rubber production is obtained from H.
brasiliensis. Since 1979 latex allergy has evolved in Europe and North America,
probably due to the use of inferior hevea rubber. For several people it means
that touching hevea rubber may cause an allergic reaction or even a severe
allergic shock. Therefore a number of rubber products are made from the milky
sap obtained from the totally different species Parthenium argentatum
(Compositae), an herb grown in Mexico, also called ‘guayule’. The advantage of
this crop is also that harvesting can be fully mechanized, contrary to hevea
rubber.
As
wild Hevea rubber is native to the tropical rainforest of the Amazon basin, the
species is adapted to tropical lowland conditions with a hot and humid climate.
It thrives best at a well-distributed annual rainfall of 2000-2500 mm or more.
A rain gauge (also known as a udometer or a pluviometer or an ombrometer
or a cup) is used, a type of
instrument used by meteorologists
and hydrologists
to gather and measure the amount of liquid precipitation
over a set period of time. Rain should fall in the late afternoon and during
the night, because bark cannot be tapped when the bark is still wet. The plant
is sensitive to strong winds. Optimum day temperatures range from 25 to 35
degrees Celsius. Rubber tolerates a wide range of soils but thrives best in
deep well-drained, moist, loamy soils, with an adequate moisture storage
capacity, and pH 5-6. The plant can stand a short period of water logging,
provided that the water is not stagnant.
Hevea
rubber is propagated by seed of by budding. Seeds have to be sown fresh because
they may lose viability in 10-20 days. This period can be extended to about 5
weeks if seeds are stored in moist charcoal of sawdust in perforated polythene
bags. To avoid too much variability between seedling trees and seedling
rootstocks, so-called ‘clonals’ are used, which are seeds from clonal rubber
plantings. Smallholders, the main producers of rubber, plant mainly seedling
trees while commercial plantations use budded rootstocks as planting material.
Successful bud-grafting was developed by Van Helten and Maas on Java in 1916
and on Sumatra in 1917, respectively. As a source of bud-wood the highest-
yielding mother trees are chosen. Seeds are at first germinated on shaded beds
and subsequently transferred to a nursery and planted in the ground. Bud-grafting takes place when seedlings are
12 to 18 months old. Prior to transplanting to the field, plants are uprooted
and both the stem and the taproot are cut back to a length of about 50 cm. The
bare root stumps are planted at the beginning of the rainy period in large
planting holes with a mixture of surface soil, phosphate and manure. A more
recent development is raising nursery seedlings in polythene bags and so-called
‘green budding’ of 4 to 6-month-old rootstocks, which shortens the nursery
period but requires greater skill.
In the field the young trees are pruned to
restrict development to one single stem, free from any branches, up to 3 m to
ensure enough bark that can be tapped. Plant densities may vary at first, but
after thinning, the final number of trees is 250-300/ha, which means spacing
about 6-7 m apart. In some countries, wider spacing is used to enable
intercropping, for example with coffee or cacao. In new rubber plantations, the
natural ground cover is sometimes maintained and controlled by periodic
slashing. Weeding is usually only required in a circle around the young tree,
until the weed is shaded out. Instead of maintaining the natural cover,
leguminous cover crops are particularly advantageous because the fix nitrogen,
although inoculating the seed or the soil with the proper Rhizobium bacteria
may be required. Due to Nitrogen fixation, no N fertilizer may be needed.
However, on some soil types, P, 60 kg K and 20 kg Mg. If no leguminous crop is
used, 50 kg M may be given. In general, hevea rubber is less demanding in terms
of soil fertility than most other tree crops.
Harvesting
begins when the trees are 5-8 years old, while maximum production is reached
when the tree is about 15 years old. Commercial latex production is sustained
for about 25 years.
Tapping
rubber trees is done by hand. The latex is located in the phloem of the inner
bark. To harvest the latex, a tapping cut is made an angle of 30 degrees from
the horizontal, from high left to low right. Usually only the basal part of the
tree (1.5 m) is tapped. Special knives are used to cut to the correct depth. A
common practice is to use a half-spiral cut and to tap on alternate days. The
bark above the cut is renewed from the cambium and can be retapped after about
10 years. Latex yield can be stimulated by applying ethylene-releasing
chemicals on the bark of the tapped tree. This is mainly a labor-saving
practice for obtaining reasonably high yields at a lower tapping frequency. The
latex is collected in a cup below the incision. When the latex arrives at the
factory it is filtered and then either coagulated with formic acid for the
production of sheet rubber, crepe rubber of block rubber, or just concentrated
by centrifugation.
Hevea
rubber is marketed as natural raw rubber. In South-east Asia the average annual
estate latex yield is about 1.5 t/ha. On smallholdings yields are about half
this amount. In 2004, the total latex production of rubber was about 9 million
t. This is about 40% of the total world consumption of rubber. The remaining
60% is synthetic rubber. At present, Thailand, Indonesia, and Malaysia are the
major producers of natural rubber, together accounting for about 80% of the
world total production, while the production from South and Central America is
at present less than 3%.
Development of Natural Rubber
Industry
If
latex is allowed to evaporate naturally, the film of rubber that forms can be
dried and pressed into usable articles such as bottles, shoes, and balls. South
American Indians made such objects in early times: rubber balls, for instance,
were used in an Aztec ceremonial game (called ollama) long before
Christopher Columbus explored South America and the Caribbean. On his second
voyage to the New World in 1493–96, Columbus is said to have seen natives in
present-day Haiti play a game with balls made from the gum of a tree. In 1615 a
Spaniard related how the Indians, having gathered the milk from incisions made
in various trees, brushed it onto their cloaks and also obtained crude footwear
and bottles by coating earthen molds and allowing them to dry.
The
first serious accounts of rubber production and the primitive Native American
system of manufacture were given in the 18th century by Charles-Marie de La
Condamine, a member of a French geographic expedition sent to South America in
1735. La Condamine described “caoutchouc” (the French spelling of a native term
for “weeping wood”) as the condensed juice of the Hevea tree, and in
1736 he sent rubber samples to Europe. Initially the new material was merely a
scientific curiosity. Some years later the British scientist Joseph Priestley
remarked on its usefulness for rubbing pencil marks from paper, and so the
popular term rubber was coined. Other applications gradually developed, notably
for waterproofing shoes and clothing.
Important
progress toward a true rubber industry came at the beginning of the 19th
century from the separate experiments of a Scottish chemist, Charles Macintosh,
and an English inventor, Thomas Hancock. Macintosh’s contribution was the
rediscovery, in 1823, of coal-tar naphtha as a cheap and effective solvent. He
placed a solution of rubber and naphtha between two fabrics and in so doing
avoided the sticky surfaces that had been common in earlier single-texture
garments treated with rubber. Manufacture of these double-textured waterproof
cloaks, henceforth known as “mackintoshes,” began soon afterward.
The
work of Hancock, who became Macintosh’s colleague and partner, is of even
greater importance. He first attempted to dissolve the rubber in turpentine,
but his hand-coated fabrics were unsatisfactory in surface texture and smell.
He then turned to the production of elastic thread. Strips of rubber were cut
from the imported lumps and applied in their crude state to clothing and
footwear. In 1820, in an effort to find a use for his waste cuttings, Hancock
invented a masticator. Constructed of a hollow wooden cylinder equipped with
teeth in which a hand-driven spiked roller was turned, this tiny machine,
originally taking a charge of two ounces of rubber, exceeded Hancock’s greatest
hopes. Instead of tearing the rubber to shreds, it produced enough friction to
weld the scraps of rubber into a coherent mass that could be applied in further
manufacture.
Macintosh’s
and Hancock’s efforts resolved the initial problem of handling the raw
material, but there remained one principal obstacle to the full exploitation of
natural rubber: it softened with heat and hardened with cold (particularly
annoying in North America, where the climate was more extreme than in Britain).
It also was tacky, odorous, and perishable. These fundamental weaknesses were
removed by the invention of vulcanization in 1839 by Charles Goodyear.
Developing a compound of rubber, white lead, and sulfur and a heat treatment
(or curing) process, Goodyear created a product—at first called fireproof gum,
afterward vulcanized rubber—that exhibited impressive durability.
Vulcanization
made the modern rubber industry possible by permitting use of the substance in
machinery and in tires for bicycles and, later, for automobiles. Though
subsequent discoveries have refined Goodyear’s original techniques, the
vulcanization process remains fundamentally the same as it was in his day. With
the advent of the bicycle and, somewhat later, the automobile and the invention
of the solid and later the pneumatic rubber tire, demand for rubber grew
rapidly. By 1900 more than 40,000 tons were used each year, about one-half from
Brazil and one-half from Central Africa, where rubber was obtained principally
from Landolphia vines. However, as an important industrial material,
rubber was required in larger amounts than could easily be obtained from wild
and widely dispersed trees in the Brazilian jungle or from African vines that
produced only about one kilogram per hectare and were destroyed to obtain the
rubber. With a view to cultivating rubber trees elsewhere, in 1876 seeds of the
Hevea brasiliensis tree from the upper Orinoco basin were taken from
Brazil to England at the instigation of the British India Office. Seedlings
were raised at Kew Gardens and shipped to Ceylon (Sri Lanka) and Singapore.
These trees were the origin of the rubber plantation industry in Asia, which
now produces more than 90 percent of the world’s supply. The industry developed
largely as a result of the work of Henry N. Ridley, director of the Singapore
Botanic Gardens from 1888 until 1912. Ridley introduced horticultural and
tapping methods that are still used today. Total world natural rubber
production is now approximately 6.5 million reached 3 million metric tons per
year , of which some 70 percent is used in automobile and truck tiresin the
early 1970s, surpassed 4 million metric tons per year in the early 1980s, and
reached 10 million metric tons per year in 2008. The principal rubber-producing
countries are Thailand, Indonesia, and Malaysia, each producing more than one
million tons per year. Smaller amounts are produced in followed by the Asian
producers China, India, the Philippines, Vietnam, and Sri Lanka, and the West
African states of Nigeria, Côte d’Ivoire, Cameroon, and Liberia.
The
first decade of the 20th century saw the establishment of the motorcar in
Europe and North America, and the automotive industry remained entirely
dependent on natural rubber for its tires and other components until World War
II. After Japan entered the war in 1941, Asian sources, except for Sri Lanka,
were cut off from the Allies. In response, the United States and the Soviet
Union attempted to cultivate alternative sources of natural rubber, such as the
guayule shrub and the Russian dandelion. These attempts met with little
success, but far better results were obtained from synthetic rubber. The United
States in particular developed a synthetic rubber industry almost overnight,
achieving a production of 800,000 tons per year. At the war’s end, with natural
rubber again available, the U.S. synthetic rubber industry went into a sharp
decline, but by the early 1950s superior and more uniform synthetics had become
available. The export of these materials stimulated development of a synthetic
rubber industry in Europe. In the early 1960s production of natural rubber was
surpassed by that of synthetic elastomers, which now represent some 60 percent
of the total.
Uses
The first use of rubber was by the
Olmecs, who centuries later passed on the knowledge of natural latex from the
Hevea tree in 1600 BC to the ancient Aztecs and Mayas.
For the Aztecs and Mayas, rubber was
religiously and socially and important material. They also produced bouncing
rubber balls for a ball game called ‘tlachtli’. Indian tribes also tapped
rubber from other plant species.
Other significant uses of rubber are
door and window profiles, hoses, belts, matting, flooring and dampeners (ant
vibrations) for the automotive industry in what is known as the “under the
bonnet” product. Gloves (medical, household and industrial) and toy balloons
are also large consumer of rubber.
Additionally, rubber produced as a
fiber sometimes called elastic, has a significant value for use in the industry
because of its excellent elongation and recovery properties.
Processing
Rubber processing consists of four basic steps: (1)
mastication, when the elastomer is sheared and the molecules are broken down to
give easier flow, (2) mixing, usually carried out immediately after
mastication, when additives are incorporated, (3) shaping of the viscous mass,
for example, by extrusion or molding, and (4) curing, when the polymer
molecules become interlinked and the shape is fixed.
Mastication and softening are usually carried out in
batches. The operation is done either in large enclosed mixing machines or on
rubber mills. The preeminent example of an enclosed machine is the Banbury
(registered trademark) mixer, consisting of heavy steel counter rotating
paddles in an hourglass-shaped chamber, holding up to one-half ton of rubber.
Rubber mills have two large horizontally opposed, closely spaced steel
cylinders, up to 3 meters (10 feet) long, that are rotated slowly in opposite
directions and at somewhat different speeds. Rubber is sheared and softened in
the gap between the paddles and wall of the Banbury mixer and in the gap
between the two cylinders in the roll mill.
Mixing is carried out on machines similar to those used in
mastication, sometimes immediately after softening. Reactive materials,
fillers, oils, and protective chemicals of various kinds, as described above,
are incorporated into the base elastomer by a combined shearing and mixing
action. An enclosed Banbury-type mixer can produce up to one-half ton of mixed
compound in a few minutes. The compound is then sheeted out, coated with a
release soap to prevent sticking, and stored until use on steel pallets that
can hold up to one ton of rubber.
Shaping of the mixture into the desired form takes place in
several ways. Extruders are used to produce long continuous products such as tubing,
tire treads, and wire coverings. They are also used to produce various profiles
that can later be cut to length. Multiroll calenders are used to make wide
sheeting. In transfer and injection molds, the rubber mix is forced through
channels into a mold chamber of the required shape, where it is cured under
pressure. Tires are made of several components: bead wire, sidewall compound,
inner liner, cord plies, belt package, and tread; these are brought together
and assembled as a complete tire before being transferred to the curing press.
Curing is carried out in pressurized steel molds, which are
heated by steam or electricity to temperatures at which the interlinking
reaction takes place. Typical cure conditions are several minutes at a
temperature of 160 °C (320 °F). Because heat penetrates rubber slowly, thick
articles must be allowed longer curing times, up to several hours, at lower
temperatures. Pressures of 1 megapascal (145 pounds per square inch) or more
are normally imposed in order to maintain the desired shape and to force
trapped air to dissolve in the compound. Other methods of curing the rubber mix
after it has been shaped include steam heating in autoclaves, microwave
irradiation, and passage through a heated bath of molten metal salts or a
fluidized bed. In these cases curing is carried out at near-atmospheric
pressure.
Additives
A number of ingredients are added to both natural and
synthetic rubber in order to obtain certain desirable properties. By
convention, mix formulations begin with the amount of the designated
elastomer—for instance, natural rubber (NR), butadiene rubber (BR), or
styrene-butadiene rubber (SBR)—given as 100 parts by weight. The amount of each
other ingredient is then expressed in parts by weight added per 100 parts by
weight of the elastomer. If two or more elastomers are used, then they are
shown in the recipe as fractions of 100 parts—for example, “NR, 60 parts; BR,
40 parts.” When the elastomer contains oil already added by the producer,
allowance is made for this dilution in the recipe. For example, if SBR 1702 is
used, the mix formulation may begin “SBR 1702, 137.5 parts by weight,” because
that amount of SBR 1702 contains 37.5 parts by weight of oil and 100 parts by
weight of SBR elastomer.
The most important ingredients are those, known as the cure package
that causes interlinking reactions to take place when the mix is “cured.” In
order to minimize the risk of premature cure, they are usually added at the end
of mixing. The cure package usually consists of sulfur and one or more
“accelerators” (e.g., sulfenamides, thiurams, or thiazoles), which make the
sulfur interlinking reaction occur faster and more efficiently. When the ratio
of sulfur to accelerator is less than one, the recipe is known as an “efficient
vulcanization” (EV) system and gives products with sulfur interlinks of shorter
length. EV products have improved resilience but lower strength.
Two other ingredients that play an important role in
vulcanization chemistry are known as “activators,” commonly zinc oxide and
stearic acid. These compounds react together and with accelerators to form zinc
sulfurating compound, which in turn is the key intermediary in adding sulfur to
a diene elastomer and creating sulfur interlinks.
Other less widely used interlinking reagents are sulfur
compounds known as sulfur donors—e.g., tetramethylthiuram disulfide—which
introduces monosulfide interlinks between polymer molecules, and peroxides,
notably dicumyl peroxide. Peroxides decompose on heating to form radicals,
which abstract hydrogen from groups on the polymer molecules. Carbon radicals
formed in this way on different molecules then combine to create carbon-carbon
interlinks. Although products with C−C interlinks are more resistant to heat
and oxidative attack, their strength is lower than products with sulfur
interlinks. In addition, monosulfide links give weaker products than
polysulfide links. This paradoxical result—that inherently strong C−C
interlinks give the weakest products, whereas inherently weak polysulfide links
give the strongest products—is attributed to the fact that weak interlinks will
break under stress before the main chain does, so failure of the elastomer
molecule itself is delayed.
Almost every conceivable material has been added to rubber
in attempts to cheapen and stiffen it. Two particulate fillers are outstanding
because they also strengthen elastomers to a remarkable degree. The most
important, used almost universally, is finely divided carbon black, prepared by
incomplete combustion of oil or gas. Carbon black consists of small spherical
particles having diameters of only 10–100 nanometers (10–100 billionths of a meter)
and made up of concentric graphitic layers of carbon. The surface of the
particles also contains some oxygen and hydrogen. During manufacture, chains of
particles become fused together to create extended open “structures,” still
very small in size.
Another reinforcing filler with particles of similar shape
and size is finely divided silica (silicon dioxide, SiO2), prepared either by
burning silicon tetrachloride or by acid precipitation from a sodium silicate
solution.
Both carbon black and silica, when added to a mix compound
at a concentration of about 30 percent by volume, raise the elastic modulus of
the rubber by a factor of two to three. They also confer remarkable toughness,
especially resistance to abrasion, on otherwise weak materials such as SBR. If
greater amounts are added, the modulus will be increased still further, but the
strength will then begin to fall. Disadvantages of reinforcement with carbon
black or silica are lower springiness (resilience) and a decrease in the
initial high stiffness after flexing.
For a filler to be reinforcing, it appears that the
fundamental particles must be small—for instance, 10–50 nanometres in
diameter—and that the elastomer must adhere well to them. If either of these
conditions is absent, the reinforcing power will be lessened. Indeed, the
smaller the particle size (and hence the greater the surface area), the greater
the observed reinforcing effect. It is still not understood how fine particles
are able to confer high strength and toughness on elastomer compounds.
Strengthening and toughening are possibly associated with debonding of highly
stressed elastomer molecules from the filler particles, reducing the stress on
the polymer chains and delaying catastrophic fracture.
Certain additives confer resistance to heat, sunlight,
oxygen, and ozone. Amines, particularly paraphenylene diamines, are powerful
retarders of oxidation, or antioxidants. Added to rubber compounds in small
amounts (1–2 percent), they appear to disrupt the free-radical oxidation
reactions that lead either to molecular rupture and softening or to increased
interlinking and hardening as rubber ages. Hindered phenols, another
antioxidant class, are less powerful than amines but have fewer tendencies to
stain light-colored rubber compounds. Small amounts of certain metals, notably
copper, manganese, and iron, act as powerful catalysts of oxidation;
sequestering agents are therefore used to block the action of these elements if
their presence is unavoidable.
Atmospheric ozone reacts readily with elastomers containing
C=C double bonds, leading to breakage of molecules lying in the surface. As a
result, small, deep fissures, termed ozone cracks, are formed if the rubber is
stretched slightly (by more than about 10 percent). Cracks one millimeter long
appear in unprotected rubber after only a few weeks of exposure to a typical
outdoor concentration of ozone, about 5 parts per 100 million. However, certain
demines (e.g., alkyl-aryl paraphenylene diamines) prevent cracking, probably by
competing with the C=C bonds in rubber for reaction with ozone. These
antiozonants “bloom” to the surface and react there, protecting the rubber. A
few percent of an antiozonant is therefore commonly included in the mix
formulation of rubber compounds based on unsaturated elastomers. An alternative
method of protection, often employed simultaneously, is to include a few
percent of a microcrystalline paraffin wax in the mix formulation. Because it
is incompatible with the elastomer, the wax blooms to the surface and forms a
protective skin. Liquids are added to elastomer mixes in order to soften and
plasticize the compound, either in processing or later in use. For example,
elastomers with high glass transition temperatures (and correspondingly slow
molecular motions) can be improved by adding low-temperature plasticizers—i.e.,
compatible liquids that act as internal lubricants. Plasticizers must have low vapor
pressure and a high boiling point in order to be retained within the compound
over long periods of service. Examples are aliphatic esters and phthalates.
Phosphate plasticizers also confer a measure of flame resistance. Other liquids
are added to rubber compounds as processing aids in order to make mixing and
extrusion easier. Typically, 5 percent of petroleum oil is used.
Properties
Rubber exhibits
unique physical and chemical properties. Rubber's stress-strain behavior
exhibits the Mullins effect, the Payne
effect, and is often modeled as hyperelastic.
The Mullins effect is a particular aspect
of the mechanical response in filled rubbers in which
the stress-strain curve depends on the maximum loading previously encountered.
The phenomenon, named for rubber scientist Leonard Mullins, working at
the NRPRA in Hertford, can
be idealized for many purposes as an instantaneous and irreversible softening
of the stress-strain curve that occurs whenever the load increases beyond its
prior all-time maximum value. At times, when the load is less than a prior
maximum, nonlinear elastic behavior prevails.
Although the term
"Mullins effect" is commonly applied to stress softening in filled
rubbers, the phenomenon is common to all rubbers, including "gums"
(rubber lacking filler). As first shown by Mullins and coworkers, the
retraction stresses of an elastomer are independent of carbon black
when the stress at the maximum strain is constant. Mullins softening is a
viscoelastic effect, although in filled rubber there can be additional
contributions to the mechanical hysteresis from filler particles debonding from
each other or from the polymer chains.
A number of constitutive models have been proposed to
describe the effect. For example, the Ogden-Roxburgh model is
used in at least one commercial finite
element code.
The Payne effect is a particular feature
of the stress-strain behavior of rubber, especially rubber compounds containing fillers such as carbon
black. It is named after the British rubber scientist A. R. Payne, who made extensive
studies of the effect (e.g. Payne 1962). The effect is sometimes also known as
the Fletcher-Gent effect, after
the authors of the first study of the phenomenon (Fletcher & Gent 1953).
The effect is
observed under cyclic loading conditions with small strain amplitudes, and is
manifest as a dependence of the viscoelastic
storage modulus on the amplitude of the applied strain. Above approximately
0.1% strain amplitude, the storage modulus decreases rapidly with increasing
amplitude. At sufficiently large strain amplitudes (roughly 20%), the storage
modulus approaches a lower bound. In that region where the storage modulus
decreases the loss modulus shows a maximum. The Payne effect depends on the
filler content of the material and vanishes for unfilled elastomers.
Physically, the
Payne effect can be attributed to deformation-induced changes in the material's
microstructure, i.e. to breakage and recovery of weak physical bonds linking
adjacent filler clusters. Since the Payne effect is essential for the frequency
and amplitude-dependent dynamic stiffness and damping behavior of rubber
bushings, automotive tires and other products, constitutive models to represent
it have been developed in the past. Similar to the Payne effect under small
deformations is the Mullins effect that is observed under large
deformations.
Benefits
Its ecological benefits provide a green leguminous ground
cover and green umbrella above the soil. Rubber tree has almost all the
attributes of a forest species which purifies the atmosphere through carbon
sequestration and it improve soil properties through addition of organic matter
keeps the soil cool, enriches fertility porosity and water intake capacity.
In north-east India, rubber planting promotes gainful
self-employment and sustainable livelihood for the youth. It generates direct
employment around 1000 man-days per hectare during immature phase of the tree
and there permanent jobs for seven persons per ten hectares in mature phase of
the tree. Indirect employment also occurs in the nursery, production and distribution
of plantation inputs, intercropping, processors, rubber wood cutting, sales,
processing and furniture making.
In conclusion, the rubber tree is a plant of economical
value, with properties that make it ecologically friendly. For this reason, it plays
an important part in numerous programs aimed at sustainable agriculture
systems. The tree has a capacity for sequestrating atmospheric carbon and hence,
could be now explored for this function. Another positive factor is fixation of
rural areas. Rubber trees put a high demand on hand labor during the different
phases of development of the crop. This guarantees more job opportunities,
improvement of living standards of communities, through amassed incomes from
agricultural.
Bibliography
=Natural+rubber +production
Elzebroek, Ton and Koop Wind. Guide to Cultivated Plants. CAB
International,
2008.
132-137
