Latex Films Vulcanization

The process of latex films vulcanization can be presented in the following way particles of sulphur in the film are surrounded by latex globules and, until the moment of vulcanization, no marked diffusion of sulphur into rubber occurs. This is confirmed by the same size of sulphur particles in the dispersion and dried film. As the vulcanization process takes place at a temperature higher than the temperature of sulphur melting (114°C), the sulphur... 

Figure 3.22 Influence of HAF black loading on tensile strength of post-vulcanized NR latex film.

Examples of the use of blocked diisocyanates for rubber-fabric adhesion are as follows vulcanized polychloroprene and SBR can be adhered strongly to nylon and polyester fibre fabric by means of aqueous adhesive systems (Table 8.4). This combination is spread or roller coated on to the fabric which is then allowed to dry. Bonds to sheet rubber stock can be made immediately after the treated fabric is dried or at any time thereafter. When the sheet rubber is applied it should be held under moderate pressure to provide intimate contact with the treated fabric and to prevent lifting if any gases are emitted during cure. Press cures of 20-40 min at 140°C are sufficient to cure the adhesive and most elastomer compositions being adhered. If a latex film is applied to the treated fabric, the assembly can be cured in a hot-air oven at 120"C. A chemical bond results between fabric and the diphenylmethane-/7,/ -diisocyanate generated on the thermal cleavage of the blocked diisocyanate. 

Table 6. Some Performance Characteristics of Latex Films Cured With an Organic Peroxide Post-Vulcanization System...

Journal of Applied Polymer Science 64, No.3, 18th April 1997, p.553-66 PERMEABILITY AND MATERIAL CHARACTERISTICS OF VULCANIZED LATEX FILM DURING AND FOLLOWING CYCLIC FATIGUE IN A SALINE ENVIRONMENT Dillon J G Schroeder L W US, Food Drug Administration... 

Figure 7. Stress-strain curves of radiation-vulcanized NR latex films as a function of their composition.

In the sulphur vulcanization of latex films, the formation of cross-links proceeds only on the surface of the latex globules. The rate of reaction of vulcanization is determined by content of rubber in the surface layer of globules. The rate of reaction of vulcanization is zero order with respect to sulphur. Therefore it is possible, by regulating the amount of vulcanizing agents in the latex, to define the quantity of rubber taking part in the vulcanization reaction, and to calculate the average thickness of the surface reaction zone. The peculiar mechanical characteristics of these films are explained by their unusual network structure. 

Recently a ntmber of works have been published in which the peculiarities of the structure of vulcanized latex films are described in detail. By means of a number of different methods, it was determined that these films have a microheterogeneous structure which represents the combination of a highly vulcanized polymer on the surface layer of latex globules and an unvulcanized polymer inside the globules. The relation between the topology of the vulcanizing network of latex films and their properties has been described ". The present work is devoted to an investigation of the kinetics of vulcanization process of latex films. 

At the same time, the curve of the change of free sulphur content during vulcanization of latex films obtained from blends 1-5 is rather accurately described by a straight line in semilogarithm coordinates (Figure 3). Thus, depending on the sulphur content in the blend, vulcanization can proceed with 1-st order kinetics with respect to sulphur - as occurs for blends 1-5 - if the sulphur content is 2 pph or less or with zero order if the sulphur content is 2.5 pph or more. 

From all the experimental data presented thus far it is possible to imagine in detail the process of vulcanization in these latex films. The particles of sulphur are arranged in the inter-globular space and do not dissolve in the rubber until the vulcanization begins. At the vulcanization temperature, melting of the sulphur particles takes place, and they proceed to dissolve into surface layers of the latex particles. If the amount of sulphur is less or equal to the solubility of the sulphur in globules surface layers, all the sulphur comes into the reaction zone and the vulcanization reaction proceeds with 1-st order kinetics with respect to sulphur. If the quantity of sulphur exceeds its solubility in the surface layers, there will be progressive transport of sulphur into the reaction zone as it reacts with the rubber. 

The results of mechanical tests, presented in Figure 5, show that the tensile strength of vulcanized latex films surpasses considerably that of analogous vulcanized rubbers (30-35 kg/cm ). This fact, already determined repeatedly for other types of rubber, can be explained in terms of the peculiarity of network of latex films distribution. 

S. A. Shteinberg, "Some Peculiarities of Vulcanization Process and Structure of Latex Films", Ph.D. Thesis, Yaroslavl (USSR), 1975. 

The increase in the surface area of the latex film, as a result of vulcanization, was determined directly by means of electron micrographs which are shown in Figure 4. The comparatively smooth surface of non-vulcanized film, which was dried for 180 min. at 70 C, (Figure 4a) becomes rough with clearly visible pores, hollows and caverns after. vulcanization for 60 min. at 120 C (Figure 4b). It is clear that the surface of the substrate increases after vulcanization. 

Films deposited from compounded nitrile latices can be vulcanized with sulfur and accelerators, assisted by relatively high levels (ca 4.0—5.0 parts /100 DRC) of ziac oxide. For other uses, nitrile latices are sometimes used ia unvulcanized form. An appHcation of medium soHds nitrile latex, eg, Nitrex J-6849 and Polysar 845, has been ia preparation of oil-resistant foams for lubricants ia heavy-duty beariags, such as railroad-car journal boxes. 

Over the last few decades, the use of radiation sources for industrial applications has been widespread. The areas of radiation applications are as follows (i) Wires and cables (ii) heat shrinkable tubes and films (iii) polymeric foam (iv) coating on wooden panels (v) coating on thin film-video/audio tapes (vi) printing and lithography (vii) degradation of polymers (viii) irradiation of diamonds (ix) vulcanization of mbber and rubber latex (x) grain irradiation. 

The prevulcanization of natural rubber in latex form has also been a subject of much investigation. The cross-linking mechanism is not yet fully understood, but the water apparently plays a major role in it. Irradiation results in the cross-linking of the rubber molecules and in coarsening of the latex particles. A process of cross-linking of natural rubber latex has been developed to the point that it can be used for an industrial-scale application. The irradiation is performed in aqueous media by electron beam without a prorad (sensitizer) at a dose of 200 kGy (20 Mrad) or in the presence of n-butyl acrylate at considerably lower doses, typically 15 kGy. The cross-linked film exhibits physical properties comparable to those obtained from sulfur cured (vulcanized) film. As an alternative, the addition of a variety of chloroal-kanes makes it possible to achieve a maximum tensile strength with radiation doses of less than 5 Mrad (50 kGy). ... 

A process of cross-linking of natural rubber latex has been developed to where it should be soon ready for an industrial-scale process.149 The irradiation is performed in aqueous media by electron beam without a prorad ( sensitizer ) at a dose of 200 kGy (20 Mrad) or, in the presence of n-butyl acrylate at considerably lower doses, typically 15 kGy. The cross-linked film exhibits physical properties comparable to those obtained from sulfur-cured (vulcanized) film. 

To explain the fact that HSPAN swells in water to form gel sheets or macroparticles rather than disintegrating into a gel dispersion, we initially felt that chemical bonding must take place between individual particles of water-swollen gel as water evaporates. Although we cannot totally eliminate this possibility, the proposal of primary chemical bonding is not necessary to explain the behavior of these films and conglomerates. For example, Voyutskii (19) has reviewed the formation of films from vulcanized rubber latexes and concludes that film formation in these systems is observed because of interdiffusion of ends of individual macromolecules in adjacent latex particles. This diffusion can take place even though individual latex particles are crosslinked, 3-dimensional networks and the continuity of the resulting films, even when... 

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