Masticated Rubber

Natural rubber displays the phenomenon known as natural tack. When two clean surfaces of masticated rubber (rubber whose molecular weight has been reduced by mechanical shearing) are brought into contact the two surfaces become strongly attached to each other. This is a consequence of interpenetration of molecular ends followed by crystallisation. Amorphous rubbers such as SBR do not exhibit such tack and it is necessary to add tackifiers such as rosin derivatives and polyterpenes. Several other miscellaneous materials such as factice, pine tar, coumarone-indene resins (see Chapter 17) and bitumens (see Chapter 30) are also used as processing aids. 

Chlorinated rubber is usually prepared by bubbling chlorine into a solution of masticated rubber in a chlorinated hydrocarbon solvent such as carbon tetrachloride. Hydrochloric acid is removed during the reaction. The solvent may be removed by vacuum or steam distillation or by precipitation of the derivative by a non-solvent such as petroleum. 

A roll of masticated rubber cut from an open mill, also known as a pig. 

A machine for masticating rubber, mixing rubber compounds and for warming up rubber compounds prior to calendering, extrusion, etc. See Open Mill. 

A machine for converting masticated rubber or mixed compound into pellet form. It consists of an extruder screw which forces the rubber through a drilled plate, the rods so formed being cut into small lengths and treated with a lubricant to prevent sticking. In some arrangements the cutting is performed underwater to improve partition. 

A machine for masticating rubber. The principle is that of an extruder, the rubber being softened by working between the threads of the screw and the surface of the barrel. The best known type is the Gordon Plasticator . Now obsolete. Plasticiser... 

Thomas Hancock invented a method of masticating rubber so that it could be easily moulded under pressure but it could not set into a shaft. 

The mixing of the masticated rubber with the chemicals is done in the same two-roller mixing mill. The following things should be considered in the mixing process sequence of addition of chemicals, the nip adjustment between the rolls, the differential temperature... 

Usually in the manufacture of rubber compounds for tank linings, the Mooney viscosity of the masticated rubber is maintained at 30-35 units, which is achievable, either by premastication or by the use of peptisers. The premastication normally takes about 30-40 minutes at 70 °C in the mixing mill. With the addition of peptisers this time can be reduced to 15—20 minutes. 

Rubber that has been masticated is more soft and flows more readily than the unmasticated material also allows preparation of solutions of high solids content because of the much lower solution viscosity of the degraded rubber. Rubber is also rendered tacky by mastication, which means that the uncured rubber sticks to itself readily so that articles of suitable thickness can be built up from layers of masticated rubber or rubberized fabric without the use of a solvent. 

The decrease of Vo, when maturing viscose or when masticating rubber show, that for both these products the technical operations involved are accompanied by a breakdown of the macromolecules. 

Typical adhesives in each class are Liquids 1. Solvent—polyester, vinyl, phenolics acrylics, rubbers, epoxies, polyamide 2. Water—acrylics, rubber-casein 3. Anaerobics—cyanoacrylate 4. Mastics—rubbers, epoxies 5. Hot melts-polyamides, PE, PS, PVA 6. Film—epoxies, polyamide, phenolics 7. Pressure-sensitive—rubbers. 

As will be described subsequently it is usually necessary to lower the molecular weight of natural rubber for subsequent processing by the technique of mastication. As with the raw rubber two masticated rubbers with the same bulk viscosity may differ in solution viscosity. It is also to be noted that microgel is not easily broken down on mastication. 

The temperature set points of the mixer are adjusted to 100°C and the torque calibration procedures are carried out with the rotors in motion (e.g., at 40 rpm). The rubber is then added (with the rotors running) and masticated for 2 min, after which the rotors are run in the reverse direction (reverse jog) to remove 10.0 +0.2 g of the rubber. The 10.0 g of masticated rubber is put aside in a paper cup to keep it relatively warm for readdition a little later in the procedure. 

The larger batches, each weighing 275.35 g, were mixed in a Brabender Prep Mixer measuring head, electrically heated, air cooled, with stock temperature thermocouple, fitted with Banbury-type rotors) at 40 rpm. The start-up procedure was similar to that used with the smaller mixing head however, the amount of masticated rubber temporarily removed from the mixer (to make room for the addition of the bulky carbon black) was scaled up from 10 g (taken from the 70-g batch) to 40 g. Fatigue resistance was measured by the Monsanto Fatigue to Failure Tester (ASTM D 4482). 

ESR can equally be used for detection of radicals in masticated rubber their identification in relation to the chemical structure might be approached with specific techniques such as electron nuclear double resonance (ENDOR). ESR studies also contribute to the understanding of the char forming process of various polymers [815], to the study of mechanical fracture, which produces free radicals, grafting reactions, etc. Pedulli et al. [816,817] have determined the bond dissociation enthalpies of a-tocopherol and other phenolic AOs by means of ESR. The determination of the O—H bond dissociation enthalpies of phenolic molecules is of considerable practical interest since this class of chemical compounds includes most of the synthetic and naturally occurring antioxidants which exert their action via an initial hydrogen transfer reaction whose rate constant depends on the strength of the O—H bond. 

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