(slide 4)
I’ll briefly describe the chemical constitution and physical state that we find natural rubber in first. Firstly it’s a polymer. It is found in a viscous, high-elastic form, (a thick gooey mess in other words) these conditions depend on long hydrocarbon chains (don’t worry the chemistry part is brief!), which are normally coiled and tangled. The high elasticity of rubber is made possible by the ability of the chains to straighten and recoil. The bonds between the molecules in the rubber are what cause this structure but I’ll just refer to these bonds as its cohesion for the benefit of those of you who don’t do chemistry. On breaking down the cohesion between the chains by various means, normally by heating, rubber has some of the characteristics of a thick oil (due to its hydrocarbon structure); whilst, on the other hand, certain conditions, especially those of intense cooling, cause rubber to behave in many respects like a crystalline material. So that makes quite an interesting material chemically.
I’ll talk to you now a bit about natural rubber’s advantageous physical properties. One of which is its long fatigue life, due to this it is commonly first choice for spring applications as it can be stretched compressed and distorted many times before becoming deformed. It also has a high strength without the need for reinforcing fillers like many other polymers- (Strength in this instance being an indication of its yield stress, which just to refresh your memory is the stress at the point when the material has reached its elastic limit.) It can also be compounded to give a wide range of hardness. It has a very good low temperature flexibility which means its form can be manipulated easily without needing to be heated, to think of a commonplace example ‘rubber gloves need to be misshapen at room temperature without becoming permanently deformed.
(slide 4)
And now for synthetic rubbers.
I’ll now briefly talk about synthetic rubber.
There are more than one dozen major classes of synthetic rubber are made of raw material derived from petroleum, coal, oil, natural gas, and acetylene. Many of them are copolymers, i.e., polymers consisting of more than one monomer making a compound. By changing the composition it is possible to achieve specific properties desired for special applications. These are some of the earliest synthetic rubbers (show list on slide 4) styrene-butadiene copolymers, Buna S and SBR, whose properties are closest to those of natural rubber. SBR is the most commonly used elastomer because of its low cost and good properties; it is used mainly for tires.
These specialty elastomers have excellent oil resistance and are widely used for flexible couplings, hoses, and washing machine parts they are therefore valuable because of their good resistance to abrasion, low gas permeability, and high dielectric strength, all these properties combined make rubber ideal for use in tyres. Another form of rubber-Neoprene (polychloroprene) is particularly useful at elevated temperatures and is used for heavy-duty applications. Other automobile parts, hose electrical insulation and footwear are normally made up of ethylene-propylene rubbers (RPDM) due to their high resistance to weathering and sunlight.
(slide 5)
Some synthetic materials have been developed to replace rubber. Urethane elastomers are called spandex and they consist of urethane blocks and polyether the urethane blocks give strength and heat resistance, the polyester and polyether blocks provide elasticity; they are the most versatile elastomer family because of their hardness, strength, oil resistance, and aging characteristics. Other uses range from airplane wheels to seat cushions. Other synthetics are highly oil-resistant, but their high cost limits their use. Silicone rubbers are organic derivatives of inorganic polymers, they are very stable and flexible. Over natural rubber: better aging and weathering, more resistance to oil, solvents, oxygen, ozone, and certain chemicals, and resilience over a wider temperature range. The advantages of natural rubber are less build-up of heat from flexing and greater resistance to tearing when hot.
So basically although rubber in its natural state has many physical properties that make it a useful material we can modify and manipulate this natural material to make high performing compounds that can be found in everyday use.
