Special Purpose Synthetic Rubbers

Properties or combination of properties which make them

suitable for a particular application e.g.

Oil resistanceChemical resistanceExtreme temperature resistance

Important special purpose rubbers are

Nitrile rubberChloroprene rubberChlorosulphonated polyetylne rubbersSlicone rubbersPolysulphide rubbersFlurocarbon rubbersPolyurethane rubbers

Nitrile rubber is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene.

Acrylonitrile 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 acrylonitrile (the more acrylonitrile within the polymer, the higher the resistance to oils but the lower the flexibility of the material), this form of synthetic rubber is generally resistant to oil, fuel, and other chemicals. Contd………

Its resilience makes NBR the perfect material for disposable lab, cleaning, and examination gloves. In the automotive industry, it is used to make fuel and oil handling hoses, seals and grommets. NBR’s ability to withstand a range of temperatures from -40°C to +120°C makes it an ideal material for extreme automotive applications. Acrylonitrile butadiene is also used to create moulded goods, footwear, adhesives, sealants, sponge, and floor mats. Compared to natural rubber, nitrile rubber is more resistant to oils and acids, but has inferior strength and flexibility. Nitrile rubber is generally resistant to aliphatic hydrocarbons. However (like natural rubber), it can be attacked by ozone, aromatic hydrocarbons, ketones, esters and aldehydes.

Production Process

In the production of hot NBR, emulsifier (soap), 2-propenenitrile (acrylonitrile), various butadiene monomers (including 1,3-butadiene, 1,2-butadiene), radical generating activators, and a catalyst are added to polymerization vessels. Within the vessel, water serves as the reaction medium. The tanks are heated to 30°C-40°C to facilitate the polymerization reaction and to promote branch formation in the polymer. Because several monomers capable of propagation 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). Contd……

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 => acrylonitrile 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. Contd………….

Environmental Concerns

Acrylonitrile is considered an environmentally hazardous substance that cannot be freely ejected into the environment (it must be disposed through a rotary kiln, fluidized bed, liquid injection incineration, or underground injection). Because the compound is quite volatile and readily soluble in water, its release to the environment from waste sites is a concern. 1,3 – butadiene is very reactive in the presence of hydroxyl radicals, and therefore possess high ozone creation potential. As mentioned previously, however, unreacted monomer recovery is nearly 100% in the process outlined above. Since unused monomers can easily be recycled, its disposal is not an unbearable concern. Thus, despite more stringent disposal restrictions, acrylonitrile butadiene rubber is produced throughout North America.

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 chloride, aluminum sulfate, and other coagulating agents in an aluminum 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°C- 15°C instead of 30°C to 40°C. Under lower temperature conditions, less branching will form on polymers (the amount of branching distinguished cold NBR from hot NBR).

Better known as Nitriles, reinforced NBR rubbers have good oil and gasoline resistance, tensile strength, elongation properties, heat resistance and low compression set. Special compounding is required for good weatherability.

Typical application for these medium priced rubbers are seals, O-rings, gaskets, diaphragms, pipe gaskets, tank linings, boots and bellows. A carboxylated version of high-acrylonitrile butadiene copolymer (XNBR) is available for application requiring the ultimate in wear resistance. In the reinforced state, this medium-priced rubber also offers high tensile strength and elongation, as well as, high load bearing capability. These properties are obtained without any sacrifice in the other properties of conventional, high-acrylonitrile copolymers.

Nitrile Rubber

By the emulsion polymerisation of butadiene and acrylonitrile.

CH2=CH–CH=CH2 CH2=CH CN

CH2=CH–CH=CH2 CH2=CH CN

+

n

Nitrile rubber

Rubber Production

Dry rubber Latex

PolymerisationCoagulationWashingDrying

PolymerisationStabilization Concentration

Nitrile Rubber Production

Hot polymerization ( 25- 50 oC )

Cold polymerization ( 5oC )

High gel contentToughDifficult to process

LinearNo gelEasy Processability

Butadiene 67

Acrylonitrile 33

Water 200

Emulsifier 3.5

Modifier 0.5

Electrolytes 0.3

Activator 0.05

Short stop 0.1

Stabilizer 1.25

Controlling factors•Monomer ratio•Temperature•Polymerisation nature•Amount of modifiers and emulsifiers

Polymerization

Producer Trade Name

M/s. Synthetics & Chemicals Chemaprene

Bayer, Germany Perbunan

Polymer Corporation, France Polysar

Aku-Goodrich, Netherlands Hycar

Japan Synthetic Rubber Co.Ltd, Japan

JSR

Fire-stone tyre &Rubber Co Butaprene

Nitrile Rubber is available in several grades depending on the acrylonitrile content

As the acrylonitrile content increases

•Oil resistance improves•Fuel resistance increases•Tensile strength increases•Hardness increases•Abrasion resistance improves•Gas permeability increases•Heat resistance increases•Low temperature resistance decreases•Resilience decreases•Plasticizer compatibility decreases

Nitrile Rubber

SlabSheetCrumbPowderLiquid

Widely used forms

Adhesives

Blends eg. NBR/PVC

Properties•Low tensile strength•Better heat resistance•Poor ozone resistance

Oil resistanceTendency to retain the physical properties while in contact with oils and fuels

Oil resistance depends on•Proportion of acrylonitrile•Chemical nature of fuel or chemical

•Degree of crosslinking•Polymerization temperature•Homogeneity of the polymer•Lower the aniline point higher will be swell

Compounding of NBR•Preliminary breakdown of NBR is most important compared to NR•Less plastic than NR and will develop more heat.•Breakdown on a cold tight mill for 5-10 min is ideal

Activation• ZnO 5 parts is used for activation

Sulphur•Solubility of sulphur is less•1-2 parts of sulphur is sufficient

Fillers•Semi reinforcing carbon black is generally used•Phenolic resins and PVC are also used• (better ozone resistance, good weathering, better gloss, •high abrasion and oil resistance, bright colour)

Plasticizers•To improve the mixing and processing properties •To reduce the hardness•Ester and phenolic plasticizers are used•Aromatic oil can also be used

Accelerators•TMTM, MBTS and MBT are generally used•1.5 MBTS is sufficient•Peroxides can also be used for curing

Antioxidants and stabilisers•1-3 phr antioxidant/antiozonants is to be added •Blending with PVC also impart good ozone resistance

Lubricants1 part of stearic acid is added for activation and processing

Applications

•High acrylonitrile grade is for resistance to aromatic oils, fuels• and solvents eg. Oil well parts, fuel cell liners•Medium grades are used with oil of lower aromatic content•Low acrylonitrile is used where low temperature flexibility is required

Chloroprene Rubber

CH2= CCl –CH = CH2 CH2- CCl =CH- CH2 n

Properties• Medium oil resistance• Metal oxides are used for vulcanisation• Good ozone/weather resistance• Good flame resistance

Producer Trade name

Du pont Neoprene

Bayer Bayprene

Distugil Butachlor

Rubber Production

General purpose Special purpose (AC & CG

Sulphur modified(C- type)

Mercaptan modified(W-type)

Rapid cureHigh resilienceGood tear resistance

Excellent storage StabilityCrystalization resistant

AdhesivesPaints

Compounding

5 phr Zno + 4 phr MgO is the vulcanising agentEthylene thiourea (NA 22) also usedFor W-type sulphur/accelerator is essential

Type GN 100 -

Type W - 100

PBN 1 1

MgO 4 4

ZnO 5 5

NA 22 - 0.5

Antioxidant•Diaryl P-phenylene diamine is used as the antiozonant

FillersAll types of fillers are suitablePlasticisers are essential Petroleum oil as softeners

ApplicationsWire and cable industryHoses and beltingsAutomobile parts

Chlorosulphonated Polyethylene

Made by substituting chlorine and sulphonyl chloride groups into polyethyleneThis modification changes the stiff plastic into a flexible rubbery polymer and the sulphonyl chloride groups provide reactive centres for cross linking

5 grades of hypalon are available. Their chlorine content varies Fron 29-43% and sulphur content from 1-1.4%.

Properties•Outstanding ozone resistance•Light stability•Heat resistance•Weather resistance•Oil resistance•Abrasion resistance•Excellent storage stability

Compounding•Sulphonyl chloride + Traces of water + pentaerythritol+ divalent metallic oxide•Accelerator+alky chloride+metalic oxide

Hypalon 100 100 100

Litharge 25 20 -

MBTS 0.5 0.5 -

Tetrone A 2.0 0.75 2.0

Magnesia - 10 4

Pentaerythritol - - 3

Recommended use General purpose

Heat resistant

Non-black

Compounding

Applications

•Many domestic and industrial articles•Automobile industry•Conveyor belting•Coated fabrics•Wires and cable sheathing•Industrial roll covering•Discharge hoses

Polysulphide rubbersReaction of a dithalide and sodium or calcium polysulfides

Properties•The properties of polysulphide rubbers depend upon the structure of polysulphide rubbers.•The vulcanizing agents are ZnO, Calcium peroxide and lead peroxide

Applications•Printer rolls•Seals•Gaskets•Adhesives

Silicone RubbersThe polymer backbone is formed by an alternating row of silicon and oxygen atoms

ApplicationsHigh temperature applicationsCables, wires and general electrical goodsIlluminationIn medicine

Fluoro Carbon RubbersAmines and MgO, PbO or ZnO are used as crosslinking agents

ApplicationsSealsIn aerospaceChemical industry

Urethane ElastomersMade from low molecular weight polyester or polyether

ApplicationsSolid tyresSeals and bootsCalendered sheetingGeneral engineering mechanical goods