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Natural Rubber (material)
It has been estimated that some 2000 plant species yield polymers akin to that of
the natural rubber molecule and that rubbers of some sort have been obtained
from some 500 of them.
Natural rubber has a number of special features
are:
(1) Its mastication behaviour.
(2) Its ability to crystallise.
(3) Its high resilience.
(4) Its reactivity with oxygen and sulphur.
Because of its highly regular structure natural rubber is capable of
crystallisation. Quoted figures for T, are in the range 15-50°C which means that
for an unfilled unvulcanised material there is some level of crystallinity at room
temperature.
Natural rubber is generally vulcanised using accelerated sulphur systems
although several alternatives have been used. At the present time there is some
limited use of the cold cure process using sulphur chloride in the manufacture of
rubber proofings
A further deficiency of natural rubber, compared with the synthetics, is its very
high molecular weight coupled with a variable microgel content.
( Plastics Materials - J.A. Brydson pg.285-289 )
Natural Rubber (material) (OLD)
As IS WELL KNOWN, natural rubber was the only polymer from which rubber
products could be made until the first synthetic rubber, Neoprene, was
invented and became commercialized in the 1930s. Natural rubber was the
only polymer used in large ticket items such as tires until the Second World
War. When the natural rubber supply was cut off in the early 1940s, the
mainstay synthetic, SBR (Styrene Butadiene Rubber) was developed and
cormnercialized.
Natural rubber is an agricultural product. Tests to determine the so
called "quality" of natural rubber parallel the increased sophistication and
growth of the rubber industry. Tests were developed to distinguish good
from bad. These tests, however, were mostly used internally by large com-
panies to assist in product development. In other words, they were the tools
that compounders used and had little commercial impact because natural
rubber was bought and sold only against visual standards. The introduction
of the technically specified rubber (TSR) concept by Malaysia in the late 19603
dianged the commercial aspect of natural rubber. ASTM tests became im-
portant, and were referenced in commercial purchase contracts. Referee lab-
oratories were established on most continents. Also up to the 1970s there
were 22 official grades of natural rubber, mostly differentiated by different
production and packing methods. All of them were visually inspected to de-
termine conformance to requirements. Table 3.1 describes some of the more
commonly used grades. All the ribbed smoked sheets are visually graded to-
day and the purchase contracts stipulate accordingly.
As IS WELL KNOWN, natural rubber was the only polymer from which rubber
products could be made until the first synthetic rubber, Neoprene, was
invented and became commercialized in the 1930s. Natural rubber was the
only polymer used in large ticket items such as tires until the Second World
War. When the natural rubber supply was cut off in the early 1940s, the
mainstay synthetic, SBR (Styrene Butadiene Rubber) was developed and
cormnercialized.
Natural rubber is an agricultural product. Tests to determine the so
called "quality" of natural rubber parallel the increased sophistication and
growth of the rubber industry. Tests were developed to distinguish good
from bad. These tests, however, were mostly used internally by large com-
panies to assist in product development. In other words, they were the tools
that compounders used and had little commercial impact because natural
rubber was bought and sold only against visual standards. The introduction
of the technically specified rubber (TSR) concept by Malaysia in the late 19603
dianged the commercial aspect of natural rubber. ASTM tests became im-
portant, and were referenced in commercial purchase contracts. Referee lab-
oratories were established on most continents. Also up to the 1970s there
were 22 official grades of natural rubber, mostly differentiated by different
production and packing methods. All of them were visually inspected to de-
termine conformance to requirements. Table 3.1 describes some of the more
commonly used grades. All the ribbed smoked sheets are visually graded to-
day and the purchase contracts stipulate accordingly.
Synthetic Rubber (New)
Synthetic rubber is produced from petroleum-based hydrocarbons such as isoprene, butadiene, chloroprene, isobutylene, and styrene.Hundreds of synthetic rubber compounds have been developed for many diverse applications. Polysulfide rubbers were among the first commercial synthetic rubbers produced.
Synthetic Rubber is resistant to natural oxidants such as oxygen and ozone and to organic solvents such as oils and gasoline. This makes it useful for engine O-rings, gaskets, and hoses which may come in contact with oils.
(Russo, Tom and Mark Meszaros, Thiokol Rubber, Vail Organic II, Flinn scientific, Inc., Batavia, IL, 2001)
4) Synthetic Rubber (old)
Although synthetic elastomers use a number of monomers identical to those
of plastics for their synthesis, such as ethylene, propylene, butadiene and
styrene, they are different in their physical properties, synthesis processes
and transformations.
The main qualities desired in an elastomer are high elasticity, high tensile
strength, low energy dissipation by hysteresis, good abrasion resistance, good
ageing behavior, and good resistance to chemical degradation.
Any polymer substance which, after being stretched to at least twice its
initial dimension, can rapidly recover its initial size after the release of the
stretching force, satisfies the definition of an elastomer. However. very few
polymers, if any, actually correspond to this definition in the raw state. The
macromolecules which form a viscous mass slide over each other under the
effect of a stretching force. If this force is removed. the substance in fact
does not return to its initial state. The insoluble three-dimensional network
which meets the definition of the rubbery state is obtained by a chemical
reaction: vulcanization. This operation is generally conducted by the addition
of chemical substances such as sulfur. Since most synthetic elastomers are
unsaturated compounds, the sulfur is added to the a of the double bond. but
the unsaturation is not eliminated. This change is irreversible, and this char-
acteristic is related to the transformation undergone by thermosetting polymers.
(
Although synthetic elastomers use a number of monomers identical to those
of plastics for their synthesis, such as ethylene, propylene, butadiene and
styrene, they are different in their physical properties, synthesis processes
and transformations.
The main qualities desired in an elastomer are high elasticity, high tensile
strength, low energy dissipation by hysteresis, good abrasion resistance, good
ageing behavior, and good resistance to chemical degradation.
Any polymer substance which, after being stretched to at least twice its
initial dimension, can rapidly recover its initial size after the release of the
stretching force, satisfies the definition of an elastomer. However. very few
polymers, if any, actually correspond to this definition in the raw state. The
macromolecules which form a viscous mass slide over each other under the
effect of a stretching force. If this force is removed. the substance in fact
does not return to its initial state. The insoluble three-dimensional network
which meets the definition of the rubbery state is obtained by a chemical
reaction: vulcanization. This operation is generally conducted by the addition
of chemical substances such as sulfur. Since most synthetic elastomers are
unsaturated compounds, the sulfur is added to the a of the double bond. but
the unsaturation is not eliminated. This change is irreversible, and this char-
acteristic is related to the transformation undergone by thermosetting polymers.
(
Synthetic rubbers:
processes and economic data, Jean-Pierre Arlie, 1992, page: 3)
Viscoelasticity (Material Property)
Polymers are known to exhibit viscoelastic behavior, meaning the relationship between stress and strain in these materials depends on time. The primary source of viscoelasticity comes from the thermally activated motions of polymer chains and side groups. Different relaxation processes become apparent at very different time scales and temperatures. Therefore, complete characterization of the viscoelastic behavior in polymers requires measurements over a wide range of time scale and temperatures. Many types of experiments are used to assess viscoelasticity in polymers.Creep is a common example of a transient type of experiment, in which a constant stress is applied and the increase in strain is measured with time. In a dynamic experiment, the response of a material to cyclic loading at various applied frequencies is analyzed.
Jakes, J. E., Lakes, R.S., Stone, D., "Broadband nanoindentation of glassy polymers I: Viscoelasticity", Journal of Materials Research,pg.463
Viscoelasticity (old)
Viscoelasticity is a generalization of elasticity and viscosity. It is characterized by the phenomenon of creep which manifests itself as a time dependent deformation under constant applied force. In addition to instantaneous deformation, creep deformations develop which generally increase with the duration of the force. Whereas an elastic model, bydefinition, is one which has the memory only of its reference shape, the instantaneous deformation of a viscoelastic model is a function of the entire history of applied force. Conversely, the instantaneous restoring force is a function of the entire history of deformation. The ideal linear viscous unit is the dashpot The rate of increase in elongation or contraction e is proportional to applied force f: Wd = f, where i/is the viscosity constant (the overstruck dot denotes a time derivative). The elastic and viscous units are combined to model linear viscoelasticity, so that the internal forces depend not just on the magnitude of deformation, but also on the rate of deformation. The stress-strain relationship for this assembly has the general form ~z2E+a,~+aoe=b~]+b,j +bof, (1) where the coefficients depend on the spring and viscosity constants..
(Terzopoulos, D., Fleischer, K., Modeling Inelastic Deformation: Viscoelasticity, Plasticity, Fracture, pp. 271-272)
Prototyping (new better)
Prototyping is a method used by designers to acquire feedback from users about future designs.
The process of prototyping, on the other hand, can be characterised as explorative, experimental, or evolutionary.
An explorative prototype is used to explore system requirements in cooperation with users and can, as such, be seen as a communication medium and facilitator between user and designer in the same way as a mock-up.
An experimental prototype is the type of prototype that is closest to the classical concept of prototype . It is experimental in the sense that it is built to try to determine whether the planned system will be adequate and acceptable when finished. Experimental prototypes can be used as requirements specification.Lastly, prototypes can be evolutionary, meaning that a system evolves through multiple generations/prototypes succeeding each other. Thus, each prototype is an early version of the system that is further worked upon until the prototype has evolved into a finished system.
Floyd, Christiane (1984): A Systematic Look at Prototyping. In: Budde, R.,Kuhlenkamp, K., Mathiassen "Approaches to Prototyping". Springer Verlag pp. 1-17
Prototyping (old)
Although dictionaries define prototype as a noun only, in product development practice the word is used as a noun, a verb, and an adjective.
We define prototype as "an approximation of the product along one or more dimensions of interest." Under this definition, any entity exhibiting at least one aspect of the product that is of interest to the development team can be viewed as a prototype.This definition deviates from standard usage in that it includes such diverse forms of prototypes as concept sketches, mathematical models, simulations, test components, and fully functional preproduction versions of the product. Prototyping is the process of developing such an approximation of the product.
(Kalpakjian S. , Schmid S.R. , Manufacturing Engineering and Technology, p. 247)
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