Vulcanisation bonding

Rubber range - vulcanised butyl rubber, CP, EPDM, CSM, NR, NBR and SBR rubbers. Substrate range - using Chemosil 211 primer, the PV bonding agents will bond ferrous and non-ferrous metals and plastics. They can be used as one-coat systems to bond vulcanised rubbers to themselves or to other vulcanised rubbers. 

ASTM D429-02, Method D, Post Bond Vulcanisation butt test [16]. Parts are clamped in a jig and put into an autoclave or oven. 

Neoprene-based adhesive is also found to be effective in bonding vulcanised pieces and a typical formulation is given in Table 5.3. It is used as a 15 - 20% solution in toluene. 

Table 5.4 Formulation of a NR-based bonding strip compound for bonding vulcanised rubber pieces ...

Polyurethane and CR adhesives are widely used for bonding vulcanised rubber. Urethane adhesives perform better in some respects compared to neoprene adhesives on greasy leather as excessive grease and fatty acid present can affect neoprene adhesives more adversely [35]. Vulcanised SBR can be bonded satisfactorily using neoprene adhesive if the surface is freshly prepared. Urethane adhesives used are of three types ... 

MIL-A-1154 Adhesive-Bonds Vulcanised Synthetic Rubber Parts. 

Peroxides. Peroxides are probably the most common materials used after sulfur because of their abiUty to cross-link a variety of diene- and non diene-containing elastomers, and their abiUty to produce thermally stable carbon—carbon cross-links. Carbon—carbon bonds are inherently stronger than the carbon—sulfur bonds developed with sulfur vulcanisation (21). 

The terminal double bond is active with respect to polymerisation, whereas the internal unsaturation remains in the resulting terpolymer as a pendent location for sulfur vulcanisation. The polymer is poly(ethylene- (9-prop5iene- (9-l,4-hexadiene) [25038-37-3]. 

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

In the case of polychloroprene the chlorine atom so deactivates both the double bond and the a-methylenic group that a sulphur-based vulcanisation system is ineffective and special techniques have to be employed. 

The proximity of the methyl group to the double bond in natural rubber results in the polymer being more reactive at both the double bond and at the a-methylenic position than polybutadiene, SBR and, particularly, polychlor-oprene. Consequently natural rubber is more subject to oxidation, and as in this case (c.f. polybutadiene and SBR) this leads to chain scission the rubber becomes softer and weaker. As already stated the oxidation reaction is considerably affected by the type of vulcanisation as well as by the use of antioxidants. 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

Although the elastomer phase is essentially in particulate form, the tensile strength of the blend can be increased five-fold by increasing the cross-link density from zero to that conventionally used in vulcanisation processes, whilst tension set may be reduced by over two-thirds. Since the thermoplastic polyolefin phase may be completely extracted by boiling decalin or xylene, there is apparently no covalent chemical bonding of elastomer and thermoplastic phases. 

Modern bonding systems usually consist of a primer coat, often with a secondary tie coat, plus a tacky solution to assist in the application of the rubber. The bonding systems currently in use are usually suitable both for autoclave vulcanisation and vulcanisation at 100°C with atmospheric pressure steam or hot water. Ambient vulcanisation bonding systems have to be chemically active at the lower temperatures and are therefore specialist in nature. 

Unless test coupons are produced alongside the lining, the only method of testing the vulcanisation state is with a hand hardness meter. A Shore A or IRHD meter is used for soft rubber linings and a Shore D meter for ebonites. The usual specification is that the hardness has to conform to 5° of the specified hardness. There is no quantitative non-destructive test for the strength of the bond between the lining and the substrate and so such tests are usually carried out in the laboratory on a sample prepared from the materials used. 

Thermoset polymers (sometimes called network polymers) can be formed from either monomers or low MW macromers that have a functionality of three or more (only one of the reagents requires this), or a pre-formed polymer by extensive crosslinking (also called curing or vulcanisation this latter term is only applied when sulfur is the vulcanising or crosslinking agent.) The crosslinks involve the formation of chemical bonds — covalent (e.g., carbon-carbon bonds) or ionic bonds. 

The joining of polymer molecules to each other by valency bonds. In very long chain-like elastomer molecules crosslinking introduces lateral links between either two separate molecules or different parts of the same molecule. See Vulcanisation. 

A dusting agent which is soluble in rubber and thus does not impair the vulcanised bond between rubber components of a composite product. It is also an activator combining the functions of zinc oxide and stearic acid, of particular value in transparent rubbers since it does not produce the same opacity as zinc oxide. 

Due to the absence of double bonds in the main chain, these materials can only be crosslinked by the action of peroxides or radiation. It is recommended that metal oxides are added to act as acid acceptors during vulcanisation, the oxides of magnesium and lead being used zinc oxide is not used as it decreases the stability of the polymer. 

A number of systems are being evaluated to reduce levels of cobalt in the complexes with resins. Organic borates and post vulcanisation stabilisers are being examined. Chlorotriazine-based bonding systems can reduce the adhesion system from three components to one. 

Cellulose fibres produced from hardwoods, with various chemical surface treatments to ensure that they are compatible with rubbers, can be used to produce high modulus vulcanisates. The bond between rubber and fibres is created during vulcanisation. These fibres can be used to reinforce extruded hoses gaining orientation in the direction of flow. There is a range of fibres available which are compatible with different rubber types. 

Metals, especially the more common iron and steel types, come from the foundry coated with oil, grease and most often a generous layer of oxide formed on the exposed surfaces. All these materials must be removed from the surfaces and from the pores of the metal to ensure that the oils and greases cannot exude under the increased temperature of vulcanisation, when they become more mobile or volatile. Surface oxides also must be removed for they are often only loosely attached to the metal substrate and will detach themselves under duress in service, after bonding. 

A pre-cured tread with pattern is prepared by moulding. A thin strip of unvulcanised cushion rubber compound is inserted between the casing and the tread rubber to form the bonding agent between the new and old components. The tread is then applied to the casing and consolidated by pressure. The cushion rubber is then vulcanised. This is also known as cold retreading . 

Ultrasonic vulcanisation also tends to change the interfacial property of the rubber and the reinforcing materials to improve bonding. Improved wetting and flow characteristics produced by ultrasonic vulcanisation have the potential to increase the interfacial bond strength between the rubber and the reinforcing materials currently used. 

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