Physical processes, rubber abrasion

A physical model of rubber abrasion in unsteady state and the corresponding processes of tongue ruptured are shown in Figures 2 and 3 respectively. As seen from Figure 3,... 

From the physical processes of rubber abrasion and its mathematic description, it can be deduced that a general process of rubber abrasion might be regarded as consisting of three stages and expressed correspondingly in terms of a wear curve (Figure A) ... 

Oh the basis of the physical processes and mathematic description of rubber abrasion stated above, a theoretical relationship between N and can be obtained through the use of numerical calculation under the condition of = r, as shown in Figure 11. Moreover, it can be approximately represented as follows... 

Nitrile and Acrylic Rubber. Nitrile rubbers are made by the emulsion copolymerization of acrylonitrile (9-50%) and butadiene (21) and are abbreviated NBR (eq. 11). The ratio of acrylonitrile (ACN) to butadiene has a direct effect on the properties and the nature of the pol5nners. As the ACN content increases, the oil resistance of the poljnner increases (14). As the butadiene content increases, the low temperature properties of the polymer are improved. Nitrile rubber is much like SBR in its physical properties. It can be compoimded for physical strength and abrasion resistance using traditional fillers such as carbon black, silica, and reinforcing clays. The primary benefit of the polymer is its oil and solvent resistance. At a medium ACN content of 34% the volume swell in IRM 903 oil at 70°C is typically 25-30%. Nitrile rubber can be processed on conventional rubber equipment and can be compression, transfer, or injection molded. It can also be extruded easily. Nitrile rubber compoimds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from -30 to 125°C. The compounds are also plasticized nsing polar ester plasticizers. 

Zinc Sulfide and Zinc Oxides. Both materials are white but do not approach titanium dioxide for use as a tinting pigment or opacifier in plastics. Both materials can have nonpigmentary utility in plastics, such as providing whitening power at much lower abrasion levels than titanium dioxide. Zinc oxide, for instance, not only brings whitening to rubber products, but also performs as an accelerator in the vulcanization process. These products cannot compete directly with titanium dioxide when whiteness and opacity are the only criteria. However, they can play an important role when they contribute to chemical reactions and/or physical properties. 

The outstanding property of rubber in general is resilience, or low modulus of elasticity. The flexibility and physical properties of rubber account for its application in general engineering and automobile industries whereas its chemical, wear and abrasion resistance as a sacrificial material, plus its insulating properties are utilised in many corrosion and erosion applications in process industries. Rubber lined mild steel, pipes and tankages have been standard materials of construction for hydrochloric acid service for many years. 

Filler dispersion is a property that determines how well the filler partciles in a given rubber compound are dispersed as a result of the mixing process. This relates to carbon black dispersion as well as the dispersion of nonblack fillers such as silica, clay, calcium carbonate, titanium dioxide, etc. Also rubber curatives such as sulfur and accelerators can be poorly dispersed (commonly these ingredients are added late in the mixing cycle). Poor dispersion makes a mixed stock less uniform, and commonly the cured ultimate tensile strength will have more variability. Poor dispersion can affect other important cured physical properties such as abrasion, tear, and fatigue resistance, flexometer heat buildup, and other dynamic properties. 

Thermoplastic polyurethane elastomers are polymers that bridge the gap between rubbers and plastics. They can be used in a wide range of properties, from hard rubbers to soft engineering thermoplastics as they are elastic and melt-processable. They can be processed on extrusion as well as injection, blow and compression molding equipment. They can be vacuum-formed or solution-coated and are suited for a wide variety of fabrication methodologies. They provide a considerable number of physical property combinations high resilience, good compression set, resistance to abrasions, tears, impacts, weather, and even hydrocarbons. Such materials... 

Carbon black is by far the most important filler and reinforcement agent used in rubber compounding. It is commonly used with processing oil (the black oil balance) to control the durometer hardness of the rubber compound. Carbon black is many times used with oil to reduce the cost of the compound because carbon black and process oil are usually less expensive per pound than many raw elastomers. Also, carbon black is essential for compound reinforcement, for improvement in compound physical properties such as ultimate tensile strength, abrasion, wear, and tear resistance. 

To improve low temperature flexibility, nitrile rubbers are often blended with NR, SBR, and polybutadiene or blended with lower ACN content nitrile rubbers. Dioctyl adipate (DOA) is used for good compatibility between NBR and PVC. Carbon blacks are used for improving reinforcements, better compression set, and aging. Silica particles are used for improving reinforcement, abrasion resistance, and tear resistance. Clays are added into NBR compoimd for better processability, tear resistance, and physical properties as well as cost reduction of the compounds. Calcium carbonates are used for better processability and cost reduction... 

Since these elastomers are used for applications requiring high abrasion resistance and excellent physical properties they are normally compounded with lower levels of plasticizer. The base polymer is tougher than standard NBR and higher hardness and tensile strength is obtained with the base rubber alone, hence lower levels of filler are usually used. In addition a two pass mix is essential for better processing safety with the zinc peroxide and accelerators added in the second pass. 

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