Vulcanization

In the vulcanization of polychloroprene rubber, the zinc oxide is even more essential. It must always be used, accompanied by magnesium oxide (MgO). Vulcanization of trans-l,4-polychloroprene, the main constituent of the polychloroprene rubbers, proceeds much more sluggishly than is the case with natural rubber or SBR. This is because the chlorine substituent inhibits the activity of the carbon atoms in the alpha position to the double bond. The vulcanization sites for practical purposes are confined to the small amount of 1,2 polymer. Fig. 20.9, which forms adventitiously during the polymerization process. This polymer has a more active tertiary allylic site, indicated in Fig. 20.9, and it is here that crosslinking occurs. 

Fairly normal sulphur and organic accelerator systems are used. The special function of the metal oxides is to act as a scavenger for chlorine atoms. 

Other systems for vulcanizing polychloroprenes are in use they include the use of the metal oxides alone, and the use of thiourea derivatives. In the production of the Selby conveyor belt the sulphur system was selected in preference to thiourea because it offered greater processing safety before the vulcanizing stage. 

A typical vulcanizing system for polychloroprene rubber is given below. Chemical structures of the organic accelerators appear in Fig. 20.10. 

Name Tetramethyl thiuram monosulphide Common Abbreviation TMTM 

As noted above, natural rubber would have limited usefulness if it could not be cured or vulcanized. The two terms are used interchangeably the word vulcanization was derived from Vulcan, the Roman god of fire, as vulcanization is rarely effected without heat. The ASTM definition of vulcanization is (2)  

Natural rubber is vulcanized commercially in three ways (all using heat to promote the reaction) 1) mixed with sulfur or sulfur donors, 2) mixed with peroxides, or 3) mixed with urethane type crosslinkers. 

The rubber compound, which usually contains other ingredients as well as the vulcanizing agent, is molded or otherwise formed into the shape desired, and heat is applied until the desired properties are achieved. 

Sulfur in the cyclic mode - the Sy sulfur - does not contribute to the qualities desired by vulcanization. The Sx bridge or crosslink may have one to eight atoms of sulfur. Heat resistance of the compound is improved if the number of atoms in the Sx crosslink is kept low - say one or two atoms. This is more easily accomplished by using sulfur donors rather than elemental sulfur. The amount of sulfur bound to the rubber is relatively small, perhaps 2 to 3 parts per 100 of rubber. With this ratio, only about 1 in 200 monomer units in a chain is crosslinked. Natural rubber can react with much larger quantities of sulfur to form hard ebonite products, such as combs, which may contain 30 parts of sulfur per 100 of rubber. 

Peroxide curing of natural rubber might be used, for example, where excellent heat resistance is needed in the rubber product. This ruggedness occurs because there are direct carbon-to-carbon links from one chain to another, there is no intervening atom, and the bond energies between carbon atoms is greater than the carbon-sulfur bond or the bond between two sulfur atoms. The type of bond is illustrated in Fig. 10.4.1 (B) and the reactions involved in Fig. 10.4.1 (C). 

The process that makes the chemistry, properties, and applications of elastomers so different from other polymers is cross-linking with sulfur, commonly called vulcanization. The modem method of cross-linking elastomers involves using a mixture of sulfur and some vulcanization accelerator. Those derived from benzothiazole account for a large part of the market today. Temperatures of 100-160°C are typical. 

Zinc oxide and certain fatty acids (R—COOH) are also added. Although this mechanism is by no means completely understood, it is proposed that the benzothiazole and zinc oxide give a zinc mercaptide, and this forms a soluble complex with the fatty acid. 

Reaction of this with Sg molecules gives a persulfidic complex (X benzothiazole). 

Interchange with the original complex leads to the formation of a mixture of polysulfidic complexes, which are considered to be the active sulfurating species. 

These complexes then react with the allyl carbons of rubber, the most reactive sites in the polymer. 

Because there is no directionality in the metallic bond and because the charge on the ion cores is screened by the intervening sea of electrons, metals tend to form close-packed structures. The two close-packed structures that can fill all space are the fee and the hep. The two structures differ in the packing sequence of hexagonal layers, hep packs as ABAB, 

Vulcanization of ds-polyisoprene. The addition of sulfur opens the carbon double bond and forms a covalent sulfur bridge between two adjacent chains which greatly strengthens the final product. 

Covalently bonded systems, because of the directionality of the covalent bond, tend to form open systems with low coordination numbers. Like the metals, they can form cubic and hexagonal systems by altering their stacking sequence. 

The various covalent and ionic structures are summarized in Table 5.2. 

Structure Type Lattice Unit Cell Description Examples 

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The adsorption of the surfactant Aerosol OT onto Vulcan Rubber obeys the Langmuir equation [237] the plot of C/x versus C is linear. For C = 0.5 mmol/1, C/x is 100 mol/g, and the line goes essentially through the origin. Calculate the saturation adsorption in micromoles per gram. 

Sulfur is a component of black gunpowder, and is used in the vulcanization of natural rubber and a fungicide. It is also used extensively in making phosphatic fertilizers. A tremendous tonnage is used to produce sulfuric acid, the most important manufactured chemical. 

Thiazole disulfides absorb at 235 and 258 nm (320-322) and characteristic infrared bands are reported in Ref. 320. The activities of 2-cyclo-hexyldithiomethylthiazoles as vulcanization accelerators have been correlated with their mass-spectral fragmentation patterns (322). 

These compounds are commercially important as accelerators in the vulcanization of rubber (Scheme 83). 

Copolymerization can be carried out with styrene, acetonitrile, vinyl chloride, methyl acrylate, vinylpyridines, 2-vinylfurans, and so forth. The addition of 2-substituted thiazoles to different dienes or mixtures of dienes with other vinyl compounds often increases the rate of polymeriza tion and improves the tensile strength and the rate of cure of the final polymers. This allows vulcanization at lower temperature, or with reduced amounts of accelerators and vulcanizing agents. 

Originally, vulcanization implied heating natural rubber with sulfur, but the term is now also employed for curing polymers. When sulfur is employed, sulfide and disulfide cross-links form between polymer chains. This provides sufficient rigidity to prevent plastic flow. Plastic flow is a process in which coiled polymers slip past each other under an external deforming force when the force is released, the polymer chains do not completely return to their original positions. 

The polymer can be vulcanized to give a rubber with very good chemical (solvent) resistance, excellent resistance to aging and weathering, and good color retention in sunlight. 

Ethylene-propylene-diene rubber is polymerized from 60 parts ethylene, 40 parts propylene, and a small amount of nonconjugated diene. The nonconjugated diene permits sulfur vulcanization of the polymer instead of using peroxide. 

Natural rubber, cis-1,4-polyisoprene, cross-linked with sulfur. This reaction was discovered by Goodyear in 1839, making it both historically and commercially the most important process of this type. This reaction in particular and crosslinking in general are also called vulcanization. 

FLALffiRETARDANTS - PHOSPHORUS FLALffi RETARDANTS] (Vol 10) -vulcanization accelerator [RUBBERCITEMICALS] (Vol 21)... 

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