Rubber Stock

Two-roll mills are used rnainlv for preparing color pastes for the ink, paint, and coating industries. There are a few applications in heaw-diitv blending of rubber stocks, for which corrugated and masticating rolls are often used. 

Figure 12.12 Coupled SEC-RPLC separation of Plioflex rubber stock (a) SEC (b) RPLC ti ace of fraction 1, Wingstay 100 (Eive-peak pattern is representative of diarylphenylenedi-amine isomers) (c) RPLC ti ace of fraction 2, mixed disulfide and MBTS (2,2 -thiobis (ben-zothiazole)). Obtained under the same conditions as given for Eigure 12.11. Reprinted from Journal of Chromatography, 149, E. L. Johnson et al, Coupled column chromatography employing exclusion and a reversed phase. A potential general approach to sequential analysis , pp. 571-585, copyright 1978, with permission from Elsevier Science.

Glass-seal ampoules provide the most impervious barrier for gas transmission. A butyl rubber stock is used with rubber-stoppered products that are sensitive to oxygen because it provides better resistance to gas permeation than other rubber stocks. 

MgC03 is used as a reinforcing filler for white and light-coloured rubber stocks. Certain types have almost the same refractive index as rubber and are therefore useful in making transparent and translucent vulcanisates. 

The vacuum extruder was developed to eliminate unwanted gases from rubber compounds, so that the profiles produced from rubber stocks could be vulcanised at atmospheric pressure without problems of porosity occurring. Removal of the volatiles is achieved in principle by combining the actions of two individual machines in series, into one machine. The first part of the extruder screw... 

After compounding, the stock is sheeted out in a roller mill and extruded into sheets or pelletized. This new rubber stock is tacky and must be coated with an antitack solution, usually a soapstone solution or clay slurry, to prevent the sheets or pellets from sticking together during storage. 

The rubber stock, once compounded and mixed, must be molded or transformed into the form of one of the final parts of the tire. This consists of several parallel processes by which the sheeted rubber and other raw materials, such as cord and fabric, are made into the following basic tire components tire beads, tire treads, tire cords, and the tire belts (fabric). Tire beads are coated wires inserted in the pneumatic tire at the point where the tire meets the wheel rim (on which it is mounted) they ensure a seal between the rim and the tire. The tire treads are the part of the tire that meets the road surface their design and composition depend on the use of the tire. Tire cords are woven synthetic fabrics (rayon, nylon, polyester) impregnated with rubber they are the body of the tire and supply it with most of its strength. Tire belts stabilize the tires and prevent the lateral scrubbing or wiping action that causes tread wear. 

Transfer molding involves the forced shifting of the uncured rubber stock from one part of the mold to another. The prepared rubber stock is placed in a transfer cavity where a ram... 

The scorch behaviour of compounds is generally studied with the help of a mooney viscometer. The test is usually conducted at temperatures encountered during processing of the rubber stock. Typically this is in the temperature range of 250°F to 275°F. From a chart of mooney units vs test time, the time required for the... 

Buist and Davies during a study of natural rubber and chloro-prene rubber stocks containing various carbon blacks point out that certain properties are closely related. For example, for natural rubber the following groups are highly correlated. 

It can be seen that every type of vulcanization system differs from every other type in the kind and extent of the various changes that together produce the vulcanized state. In the vulcanization processes, consideration must be made for the difference in the thickness of the products involved, the vulcanization temperature and thermal stability of the rubber compound. The word cure to denote vulcanization is believed to have been coined by Charles Goodyear and the same has been a recognized term in rubber industry circles [2]. The conditions of cure will vary over a wide range according to the type of vulcanizate required and the facilities available in a rubber factory. Many factors must be predetermined, including the desired hardness of the product, its overall dimensions, the production turnover required and the pretreatment of the rubber stock prior to vulcanization. Hardness will normally be determined by the composition of the stock but it can also be influenced by the state of cure. 

Tyres are normally cured in a modification of the compression mould where a bladder or an inflated airbag forces and holds the green rubber stock of the tyre against the mould surface during vulcanization. This force reproduces the design of the tyre tread and the heat from the steam is introduced into the bladder to effect the vulcanization. Small size rubber expansion joints used in piping systems are moulded by compression moulding... 

Air blisters generally occur due to trapped air during the processing of the rubber in the calender or extruder or during hand building up of the products. The remedial measures to be undertaken are 1) Modify process temperature or pressure, 2) Feed the calender or extruder with adequate rubber stock, 3) Prick out visible blisters when the compound is in unvulcanized state, and 4) Use bleeder cords between metal and the rubber sheets applied at the welded places in the rubber lining process. 

The frames of the machine should be sturdy and made of closely grained cast iron of heavy design, free of blowholes which usually form during casting, to withstand the load exerted by the rubber stock. 

When the rubber stock is passed through the rolls, the pressure between them causes deflection which results in parallel openings or nips between the rolls as shown in figure 13.2. 

The contouring of the calender rolls by grinding in order to counteract the natural deflection and the deflection set up by the work being done with rubber stock load is a precision job and in most cases it is done by trial and error method. The nip pressure per inch width of the roll for most rubber compounds has been extensively studied by scientists in the Dunlop Rubber Company [ref Paper "Estimation of the shear deformation exerted by a roll-mill upon a rubber compound" by T.S. Ng, G. Angerer, Dunlop Forschung, Dunlop-Strasse 2, Hanau, Federal Republic of Germany, Presented at the Jahrestagung der... 

Choosing the correct flow period is a very important aspect in the forming and shaping process. Mooney cure curves give good data on the flow period. The flow period also depends on the flow distances in the cavities of the mould and the viscosity of the rubber stock and the repeat pressure application cycles, called "bumping" cycles on the shop floor. 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

Composite - A homogeneous lining created by the assembly of two or more rubber stocks with other materials like fabrics. 

Green strength -1) The resistance to deformation of a rubber stock in the uncured state. 2) Uncured adhesion between plied or spliced surfaces. 

The compounded rubber stock will be further processed for use. The process could be injection or transfer molding into a hot mold where it is cured. Tire curing bladders are made in this fashion. Extrusion of the rubber stock is used to make hose or tire treads and sidewalls. Another common process is calendaring, in which a fabric is passed through rolls where rubber is squeezed into the fabric to make fabric-reinforced rubber sheets for roofing membranes or body plies for tires. The actual construction of the final product can be quite complex. For example, a tire contains many different rubber components some of which are cord or fabric reinforced. All of... 

Extrusion of tubes, hose, and profiles is done on standard extruders for rubber. The usual temperature pattern is a gradual increase of temperature from the feed zone to the die. The die temperature is typically 100°C (212°F), and the screw temperature is approximately the same as the temperature of the feed zone [44]. Processing aids are almost always required to improve the surface appearance and to increase the extrusion rate. Extrusion represents only a small proportion (about 10%) of the total consumption of fluorocarbon elastomers [52].Cold feed extruders typically used for extrusion of rubber stocks are also suitable for the extrusion of fluorocarbon elastomers. Extrusion should be carried out at temperatures below 120°C (250°F) to avoid scorch. In most cases a breaker plate and screen pack are used to generate backpressure on the screw and to remove foreign particles from the stock. A straight head is used for the extrusion of profiles or tubing, whereas a cross-head die is used for... 

Ram and piston injection molding units are used in the rubber iudustry [57]. The rubber stock is fed to a heated cyliuder, which is heated to the required temperature aud from there is forced by a hydraulic ram through a nozzle, mold runners, and... 

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