Rubber techniques

Density of trichomes and stomata -- Densities of trichomes and stomata were determined from positive leaf impressions by the silicone rubber technique of Sampson (1961). A total of eight areas were sampled randomly on each of four leaf replicas. 

Mixing. Fluorocarbon elastomers may be compoimded by any standard rubber technique. Open mills may be used, since most commercial gums mix well, except very low viscosity gums, which stick to the rolls, and high viscosity gums, which are very tough. 

The technique of the filtration of hot solutions has already been described in Section 11,28. The filtration of cold solutions will now be considered this operation is usually carried out when it is desired to separate a crystalline solid from the mother liquor in which it is suspended. When substantial quantities of a solid are to be handled, a Buchner funnel of convenient size is employed. The ordinary Buchner fimnel (Fig. 11,1, 7, a) consists of a cylindrical porcelain funnel carrying a fixed, flat, perforated porcelain plate. It is fitted by means of a rubber stopper or a good cork into the neck of a thick-walled filtering flask (also termed filter flask, Buchner flask or suction flask) (Fig. 11,1, 7, c), which is connected by means of thick-walled rubber tubing (rubber pressure tubing) to a similar flask or safety bottle, and the latter is attached by rubber pressure tubing to a filter pump the safety bottle or trap is essential since a sudden fall in water pressure may result in the water sucking back. The use of suction renders rapid filtration possihle... 

One of our previous complaints was that we had more parameters than we knew what to do with Eq. (2.33) makes this problem even worse. It turns out, however, that using only two or three terms of Eq. (2.33) results in a usable equation with improved curve-fitting ability. Techniques have been developed for extracting acceptable parameters from experimental data in these cases (see Problem 4). Figure 2.9, for example, shows data collected from a sample of natural rubber, analyzed according to a two-term version of Eq. (2.33). The line in Fig. 2.9 is drawn according to the equation... 

The mbber was ground and treated. It was then washed and the mbber steam devulcani2ed and formed to shape where it could then be reincorporated into virgin mbber compounds. An improvement on this technique was the use of alkaH digestion (see Recycling, rubber). 

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutjiene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural mbber or SBR, cure system design generally requites higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, thein attributes, and principal appHcations have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl mbber as shown in Table 7 (21). 

The compounding technique for latex differs from that of dry mbber and is fundamentally simpler. A critical factor of colloidal stabiUty makes necessary that each ingredient is of optimum particle size, pH, and concentration when added as an aqueous dispersion to the latex. Rubber latex is a colloidal aqueous emulsion of an elastomer and natural mbber latex is the milky exudation of certain trees and plants that of greatest commercial importance is the... 

M. Roland, Uiscoelasticity—Material Behavior and Measurement Technique, at the CL Meeting of the 147th Rubber Division, Philadelphia, Pa., May 2—5, 1995, American Chemical Society, Washington, D.C. 

Deformation of a polymer in the rubber state—of importance in vacuum forming, pressure forming and warm forging techniques. 

Polybutadiene was first prepared in the early years of the 20th century by such methods as sodium-catalysed polymerisation of butadiene. However, the polymers produced by these methods and also by the later free-radical emulsion polymerisation techniques did not possess the properties which made them desirable rubbers. With the development of the Ziegler-Natta catalyst systems in the 1950s, it was possible to produce polymers with a controlled stereo regularity, some of which had useful properties as elastomers. 

Subsequently, much improved thermoplastic polyolefin rubbers were obtained by invoking a technique known as dynamic vulcanisation. This process has been defined (Coran, 1987) as the process of vulcanizing elastomer during its intimate melt-mixing with a non-vulcanizing thermoplastic polymer. Small elastomer droplets are vulcanized to give a particulate... 

It was found that the amount of chlorine that could be removed (84-87%) was in close agreement to that predicted by Flory on statistical grounds for structure Figure 12.10(a). It is of interest to note that similar statistical calculations are of relevance in the cyclisation of natural rubber and in the formation of the poly(vinyl acetals) and ketals from poly(vinyl alcohol). Since the classical work of Marvel it has been shown by diverse techniques that head-to-tail structures are almost invariably formed in addition polymerisations. 

By reduction in the degree of polymerisation. To produce processable rubbers the original polymers are masticated with substances such as benzothiazole disulphide and tetramethylthiuram disulphide. The more severe degradation techniques to produce liquid polysulphides are mentioned below. 

Some typical properties of a Vulkollan-type polyurethane cast rubber and a black-reinforced polyurethane rubber processed by conventional techniques are compared with black-reinforced natural and nitrile rubbers in Table 27.2 ... 

The pieces of cloth are then plied up and moulded at about 170°C for 30-60 minutes. Whilst flat sheets are moulded in a press at about lOOOlbf/in (7 MPa) pressure, complex shapes may be moulded by rubber bag or similar techniques at much lower pressures ( 15 Ibf/in ) (0.1 MPa) if the correct choice of resin is made. A number of curing catalysts have been used, including triethanolamine, zinc octoate and dibutyl tin diacetate. The laminates are then given a further prolonged curing period in order to develop the most desirable properties. 

Isocyanates can be added to solvent-borne CR adhesive solutions as a two-part adhesive system. This two-part adhesive system is less effective with rubber substrates containing high styrene resin and for butadiene-styrene block (thermoplastic rubber) copolymers. To improve the specific adhesion to those materials, addition of a poly-alpha-methylstyrene resin to solvent-borne CR adhesives is quite effective [76]. An alternative technique is to graft a methacrylate monomer into the polychloroprene [2]. 

Variations on this basic process are (i) vacuum bag moulding and (ii) pressure bag moulding. In the former process a flexible bag (frequently rubber) is clamped over the lay-up in the mould and a vacuum is applied between the moulding and the bag. This sucks the bag on to the moulding to consolidate the layers of reinforcement and resin. It also squeezes out trapped air and excess resin. The latter process is similar in principle except that pressure is applied above the bag instead of a vacuum below it. The techniques are illustrated in Fig. 4.67(b) and (c). 

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