Latex curing processes

When rubber products are made many inorganic ions get into the system. Natural rubber latex, which is a secretion from a tree, will contain traces of metals. Much larger concentrations are added during the curing process as accelerator or plasticizers and to keep the rubber from sticking to the molds. Many steps in the manufacture of a tire or other rubber products involve inorganic ions. 

Many polymer and coating formulations are reactive because a free radical is generated to do the polymerisation or cure. The comprehensive article explains the latex polymerisation process which requires an initiator radical. The various uses of the free radicals are examined in detail, together with their sources, and comprehensive examples of commercial initiators are supplied. The article provides a comprehensive analysis of the function and limits of the free radical in the polymerisation process. 6 refs. 

An aqueous solution of RF resin is used with different rubber latexes to make the RFL dip needed to treat the textile cord to allow it to achieve rubber-to-cord adhesion during the curing process. It is the RF resin portion of the RFL dip that is responsible for the good adhesion with the textile cord. 

While carboxylated latexes are widely accepted as not needing a cure system, those described above for SBRs are applicable, as are melamine formaldehyde or other formaldehyde condensates. Multivalent metal compounds may also be used with carboxylated systems the most common of these is zinc oxide, but other materials such as zirconium ammonium carbonate may also be used. Some of these types of materials have the advantage that they are effective at room temperature. Some functional SBR latexes have their own cure system built into the polymer and are often referred to as self-crosslinking y or as heat-reactive in instances where heat is involved in the curing process. 

With dried coatings, reflection techniques can be employed to obtain infrared spectra. An emulsion or latex can be examined by similar techniques to those described above. Solvent-based coatings can be examined either directly in liquid cells or as dried films. Usually the evaporation of the solvent can be monitored spectroscopically. Alkyd resins which are solvent based still form a substantial part of the commercially available coatings. The band due to the C=C stretching vibration may be used to follow the curing process. 

A series of SEM photographs of the fractured surface were taken and shown in Fig. 12-13. These photographs, especially (b) and (c) demonstrate the honeycomb structures of the SBR polymer formed around asphalt particles. Some latex polymers are also adhering on the aggregate surface as seen in (c). It is important to realize that the latex polymers should remain in the aqueous phase, not in the asphalt, and transform to a continuous polymer film during the curing process. Since Portland cement particles also remain in the aqueous phase, the flexible polymerhoneycomb structures. In contrast, the honeycombs made only with Portland cement would also be very brittle and this would be the case when the polymer is present in the asphalt phase. 

The polymei latex is then coagulated by addition of salt oi acid, a combination of both, oi by a fiee2e—thaw process. The cmmb is washed, dewatered, and dried. Since most fluorocarbon elastomer gums are sold with incorporated cure systems, the final step in the process involves incorporation of the curatives. This can be done on a two-roU mill, in an internal mixer, or in a mixing extmder. 

The choice of coagulant for breaking of the emulsion at the start of the finishing process is dependent on many factors. Salts such as calcium chloride, aluminum sulfate, and sodium chloride are often used. Frequentiy, pH and temperature must be controlled to ensure efficient coagulation. The objectives are to leave no uncoagulated latex, to produce a cmmb that can easily be dewatered, to avoid fines that could be lost, and to control the residual materials left in the product so that damage to properties is kept at a minimum. For example, if a significant amount of a hydrophilic emulsifier residue is left in the polymer, water resistance of final product suffers, and if the residue left is acidic in nature, it usually contributes to slow cure rate. 

The dipping process, as the name implies, involves the immersion of a former into either a latex, or a rubber solution, and then its slow withdrawal to leave a uniform deposit on the former. Subsequent processes involve, amongst others, drying the deposit, and curing it, if this is required. 

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. 

Figure 7 shows the representative bright field HRTEM images of nanocomposites of NR and unmodified montmorillonite (NR/NA) prepared by different processing and curing techniques. It is apparent that the methodology followed to prepare the nanocomposites by latex blending facilitates the formation of exfoliated clay structure, even with unmodified nanoclays. It has been reported in the literature that hydration of montmorillonite clay leads to extensive delamination and breakdown of silicate layers [94, 95]. It has also been shown that NA disperses fully into the individual layers in its dilute aqueous dispersion (clay concentration <10%)... 

Fig. 7 Bright fiefd HRTEM images showing the deveiopment of morphoiogy in 4 phr NA-filled NR nanocomposites under different processing and curing conditions a latex-biended uncured NC (NLUNA) b prevuicanized NC (NLPNA) and c conventionaliy cured NC (NMNA). d X-ray diffractograms of NA and its nanocomposites...

Morphology evolution is thus found to be dependent on the processing technique applied to disperse the nanoparticles. The latex-blended and prevulcanized nanocomposites show predominant exfoliation with some intercalation, especially in uncured and prevulcanized samples. In conventionally cured but latex-blended nanocomposites, realignment of NA particles is visible, with a greater tendency of NA platelets towards agglomeration. In solid state mixing, the dispersion is still poorer. XRD studies also corroborate the above observations. 

---