Classification of Rubber

Natural Rubber

Natural rubber is a solid product obtained through coagulating the latex produced by certain plants, particularly the Brazilian rubber-tree (Hevea Brasiliensis). This raw material is usually tapped from the rubber tree, which is native to Amazonia. Although there a large number of species that exude secretions similar to latex when the bark is cut, only a few produce sufficient quantities of a quality adequate for exploitation on economic bases.

The structural formula for the molecule of natural rubber may be represented by the simple unit C₅H₈ multiplied many 1000 times. Rubber  is an addition polymer of a diene  monomer(a hydrocarbon molecule containing two double bonds) called isoprene or 2-methyl-1,3-butadiene with the following formula:

CH2=C(CH3)-CH=CH2

The repeating unit shown above would, therefore, be called the monomer.  On polymerization of isoprene, we get polyisoprene, which is the chemical name of natural rubber

Poly isoprene

Synthetic rubbers

SOME EXAMPLES FOR SYNTHETIC RUBBER:

SBR(STYRENE BUTADEINE  RUBBER)

Styrene-butadiene or styrene-butadiene rubber (SBR) describe families of  synthetic rubbers derived from styrene and butadiene. The co monomers for SBR production, styrene and butadiene, are today invariably produced from petroleum sources.

STRUCTURE OF SBR

The bulk of SBR is produced by free radically initiated emulsion polymerisation. Solution polymerization is also used for preparation of SBR. Copolymers prepared by use of lithium catalyst systems first came into existence in early 1960 and varieties of types are now commercially available. Polymerization is carried out in solution and with these systems the reactivity ratios are quite different to observed with free radical polymerizations.

Properties

Property	S-SBR	E-SBR
Tensile strength (MPa)	18	19
Elongation at tear (%)	565	635
Mooney viscosity (100 °C)	48.0	51.5
Glass transition temperature (°C)	-65	-50
Polydispersity	2.1	4.5

Applications

The elastomer is used widely in pneumatic tires, shoe heels and soles, gaskets and even chewing gum. It is a commodity material which competes with natural rubber. Latex (emulsion) SBR is extensively used in coated papers, being one of the most cost-effective resins to bind pigmented coatings. It is also used in building applications, as a sealing and binding agent behind renders as an alternative to PVA, but is more expensive. In the latter application, it offers better durability, reduced shrinkage and increased flexibility, as well as being resistant to emulsification in damp conditions. SBR can be used to 'tank' damp rooms or surfaces, a process in which the rubber is painted onto the entire surface (sometimes both the walls, floor and ceiling) forming a continuous, seamless damp proof liner; a typical example would be a basement.

NBR

Nitrile rubber, also known as Buna-N, Perbunan, or NBR, is a synthetic rubber copolymer of acrylonitrile(ACN) and butadiene. Trade names include Nipol, Krynac and Europrene.

STRUCTURE OF NBR

Nitrile butadiene rubber (NBR) is a family of unsaturated copolymers of 2-propenenitrile and various butadiene monomers (1,2-butadiene and 1,3-butadiene). Although its physical and chemical properties vary depending on the polymer’s composition of nitrile, this form of synthetic rubber is generally resistant to oil, fuel, and other chemicals (the more nitrile within the polymer, the higher the resistance to oils but the lower the flexibility of the material).

Production

Emulsifier (soap), 2-propenenitrile, various butadiene monomers (including 1,3-butadiene, 1,2-butadiene), radical generating activators, and a catalyst are added to polymerization vessels in the production of hot NBR. Water serves as the reaction medium within the vessel. The tanks are heated to 30–40 °C to facilitate the polymerization reaction and to promote branch formation in the polymer. Because several monomers capable of propagating the reaction are involved in the production of nitrile rubber the composition of each polymer can vary (depending on the concentrations of each monomer added to the polymerization tank and the conditions within the tank). One repeating unit found throughout the entire polymer may not exist. For this reason there is also no IUPAC name for the general polymer. The reaction for one possible portion of the polymer is shown below:

1,3-butadiene + 1,3-butadiene + 2-propenenitrile + 1,3-butadiene + 1,2-butadiene → nitrile butadiene rubber

Monomers are usually permitted to react for 5 to 12 hours. Polymerization is allowed to proceed to ~70% conversion before a “shortstop” agent (such as dimethyldithioarbamate and diethyl hydroxylamine) is added to react with the remaining free radicals. Once the resultant latex has “shortstopped”, the unreacted monomers are removed through a steam in a slurry stripper. Recovery of unreacted monomers is close to 100%. After monomer recovery, latex is sent through a series of filters to remove unwanted solids and then sent to the blending tanks where it is stabilized with an antioxidant. The yielded polymer latex is coagulated using calcium nitrate, aluminium sulfate, and other coagulating agents in an aluminium tank. The coagulated substance is then washed and dried into crumb rubber.

The process for the production of cold NBR is very similar to that of hot NBR. Polymerization tanks are heated to 5–15 °C instead of 30–40 °C. Under lower temperature conditions, less branching will form on polymers (the amount of branching distinguishes cold NBR from hot NBR).

Applications

The uses of nitrile rubber include non-latex gloves for the healthcare industry, automotive transmission belts, hoses, O rings, gaskets, oil seals, V belts, synthetic leather, printer's roller, and as cable jacketing; NBR latex can also be used in the preparation of adhesives and as a pigment binder.

Unlike polymers meant for ingestion, where small inconsistencies in chemical composition/structure can have a pronounced effect on the body, the general properties of NBR are not altered by minor structural/compositional differences. The production process itself is not overly complex; the polymerization, monomer recovery, and coagulation processes require some additives and equipment, but they are typical of the production of most rubbers. The necessary apparatus is simple and easy to obtain. For these reasons, the substance is widely produced in poorer countries where labour is relatively cheap. Among the highest producers of NBR are mainland China and Taiwan.

BUTYL RUBBER

Butyl rubber is a synthetic rubber, a copolymer of isobutylene with isoprene. The abbreviation IIR stands for Isobutylene Isoprene Rubber. Polyisobutylene, also known as "PIB" or polyisobutene, (C₄H₈)_n, is the homopolymer of isobutylene, or 2-methyl-1-propene, on which butyl rubber is based. Butyl rubber is produced by polymerization of about 98% of isobutylene with about 2% of isoprene. Structurally, polyisobutylene resembles polypropylene, having two methyl groups substituted on every other carbon atom. Polyisobutylene is a colourless to light yellow viscoelastic material. It is generally odourless and tasteless, though it may exhibit a slight characteristic odour.

Butyl rubber has excellent impermeability, and the long polyisobutylene segments of its polymer chains give it good flex properties.

The formula for PIB is: –(–CH₂–C(CH₃)₂–)_n–

The formula for IIR is: 

It can be made from the monomer isobutylene or CH₂=C(CH₃)₂ only via cationic addition polymerization.

APPLICATIONS

Butyl rubber and halogenated butyl rubber are used for the inner liner that holds the air in the tire.

Chloroprene rubber

 Among the speciality elastomers polychloroprene [poly(2-chloro-1,3-butadiene)] is one of the most important.  First production was in 1932 by DuPont (“Duprene”, later “Neoprene”) and since then CR has an outstanding position due to its favourable combination of technical properties.

  

The basic polymerization scheme leads to incorporation of the monomer into a polymer consisting of different structural units. The physical, chemical and rheological properties of the different grades of commercial polychloroprene are dependent on the ability to change the molecular structure by changing polymerization conditions, e.g. polymerization temperature or monomer conversion, polymerization aids (comonomers, type and amount of molecular weight modifier and emulsifier) and conditions during finishing.

The high amount of trans-1,4-units in the polymer (about 90 % at standard polymerization conditions) leads to synthetic rubber, which has crystallization as an inherent property.

Properties

CR is not characterised by one outstanding property, but its balance of properties is unique among the synthetic elastomers. It has:

Good mechanical strength

High ozone and weather resistance

Good aging resistance

Low flammability

Good resistance toward chemicals

Moderate oil and fuel resistance

Adhesion to many substrates

Polychloroprene can be vulcanized by using various accelerator systems over a wide temperature range.

 Application areas in the elastomer field are widely spread, such as moulded goods, cables, transmission belts, conveyor belts, profiles etc.