Rubber compounding filler systems

Report 79 Rubber Compounding Ingredients - Need, Theory and Innovation, Part I Vulcanising Systems, Antidegradants and Particulate Fillers for General Purpose Rubbers, C. Hepburn, University of Ulster. 

A rubber to fabric bonding system comprising resorcinol, a formaldehyde donor and a hydrated silica filler. Incorporated as dry ingredients in the rubber compound and activated by heat. 

Resorcinol formaldehyde latex (RFL) cord dips have only a limited application within the general rubber goods industry and for adhesion to be achieved with synthetic fibres it is necessary to use the systems developed by Bayer and Degussa. These systems are based upon a combination of resorcinol, a formaldehyde donor and a hydrated silica filler (commonly called the RFK system). This system is incorporated as dry ingredients into the rubber compound and is activated by the application of heat. 

Advances in quality control requirements for rubber compounds have demanded that special care is exercised in the weighing of the various fillers and critical small volume powders necessary for the formulation. Ingredient weighing systems are available which can be tailored to the specific requirements of individual factories. Additional components can include conveyors, bagging units and pneumatic conveyance systems. 

The basic compounding formulation specifies the minimum requirement of fillers, vulcanizing agents, and other substances that must be added to the rubber compound to achieve the desired properties. After the rubber, cure system and reinforcing filers have been selected it will be necessary to make several adjustments before all requirements are satisfied. It is generally sensible to start with the simplest mix formula for meeting the requirements. The recipe or the formula is usually written on the basis of hundred parts of rubber. For example if 5 parts of zinc oxide is to be added it is denoted as 5 phr (five parts per hundred rubber). Elementary compounding... 

The proven compounding practice for the natural rubber layer is to load it with 30 phr of graphite, 30 phr of semi-reinforcing carbon black filler and 50 phr of inert filler, china clay. A low sulfur curing system is adopted for heat resistance with a suitable antioxidant to prevent flex-cracking. A typical chlorine resistant soft natural rubber compound formula is given in Table 3.2. 

Figures 3.4 and 3.5 demonstrate the change in shear rate with viscosity at different temperatures of compounds A and B. The shear rate in injection moulding varies through the system according to the design of the machine and mould, and may range from 100 to >10,000 sec. The addition of small amounts of a process aid and variation of the mixing procedure for the two compormds have brought about these differences in properties. This demonstrates that it is possible to compound and process rubber compounds to suit moulds through diligent selection of polymers, fillers and process aids.

The carbon black critical loading factor and the optimum loading level tvill be a function of the specific carbon black grade and specific grade of polymers in the rubber compound blend, and are therefore unique for each polymer and filler system. 

Zinc oxide, which is formed from the burning of zinc metal, was the first nonblack filler nsed for reinforcement of rubber compounds. Although zinc oxide and magnesinm oxide are still used as reinforcing fillers in some specialty compoimds, particnlarly those that reqnire heat resistance, their role in rubber compoimding in the last several decades is that of an activator for the sulfur cure system or as curatives for chloroprene rubber compounds. 

Many rubber products must operate in contact with fluids of various types. An important part of compound design is formulation for resistance to fluids. Selection of suitable polymer is important, (e.g., use of a polar rubber for oil resistance), while for minimal swelling it is desirable to have a high crosslink density. Other design considerations are also important, such as the effect of the liquid on the filler and plasticiser systems. Plasticisers will often be extracted by fluids and occasionally replaced by the fluid within the compound. Filler effects are discussed in Section 7.6.3.5. 

The system should not interact in an adverse manner with other components of the rubber compound such as fillers or antioxidants. 

Hydrated precipitated silica has been used in the rubber industry over 40 years. Not only is hydrated precipitated silica a major inorganic filler used in rubber compounding to impart reinforcement, it is also commonly used as part of the HRH in situ rubber-to-metal adhesion systems just discussed. 

The hydrated precipitated silica used with general-purpose elastomers in HRH adhesive systems (and also as fully reinforcing filler in rubber compounding) comes from water glass (sodium silicate). This is made from natural silica (sand) that is reacted either with sodium hydroxide or sodium carbonate at very high temperatures see Figure 8.6. 

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