Thread Content
Rubber Technology Network l Introduction With the rapid development of industries such as automobiles, electronics and electrical equipment, construction, and transportation in China, this has promoted the use and development of special rubbers in the country. In recent years, significant progress has been made in the production technologies and applications of special rubbers in China. As the capacity for producing and the demand for special rubbers continue to grow, higher requirements are placed on the additives used in their production, processing, and application. High-performance new materials require the use of new and efficient additives. Some of these processing additives are traditional types, while others are new and specialized additives. The development and production of special rubber additives can not only effectively promote the healthy development of China’s special rubber industry; moreover, some new types of additives offer considerable potential for growth and profitability, making them worthy of attention from domestic rubber additive manufacturers. 2 Processing and compounding additives In 2005, China’s production capacity for fluororubber was approximately 3,500 tons, while domestic consumption amounted to around 6,500 tons. It is estimated that by 2010, the automotive industry alone will consume about 7,500 tons of fluororubber; when combined with demand from other sectors, the total demand is expected to reach roughly 11,000 tons. Currently, there are three fluororubber vulcanization systems developed worldwide: 1) diamine compounds containing two amino groups in their molecules ； 2) Polyol compounds containing 2 hydroxyl groups ； 3) Peroxides and polyfunctional compounds. The most widely used system is the polyol vulcanization system, in which the polyol used is limited to biphenol AF. Compared with traditional diamine vulcanization systems, the polyol system offers two advantages: low compressive set and high resistance to scorching. There are many processing aids for fluororubber, and their addition amounts and functions vary depending on the vulcanization system and the type of fluororubber. Currently, polyol-based vulcanization systems are primarily used both domestically and internationally; therefore, this section introduces some of the processing additives used for fluororubber in such vulcanization systems. For plasticizers, stearates or low-molecular-weight fluororubbers are commonly used in China. The usage amount of the former should not exceed 2%, while that of the latter depends on specific requirements; it should not exceed 5% at most, otherwise it will affect the properties of the rubber. Antioxidants are needed because scorching tends to occur during extrusion and injection molding processes where large amounts of rubber compound are used and a high degree of automation is employed. A certain amount of antioxidant must therefore be added; phenol, p-nitrophenol, o-hydroxyphenol, and the antioxidant NA are commonly used, with amounts ranging from 0.4 to 0.5 parts. The addition of an appropriate amount of antioxidant not only prevents scorching but also improves the flow properties and physical characteristics of the rubber compound. For accelerators used in polyol vulcanization systems, it is required that they exhibit good scorch resistance during the mixing and processing stages, as well as a relatively fast vulcanization rate. Currently, the better promoters are quaternary phosphonium salts, such as 1-phthalimidoacid-4-butyltriphenylphosphonium bromide, bis(benzyltriphenylphosphine)imine chloride, and triphenylbenzylphosphonium chloride; the amount used is generally 0.4–0.7 parts. Of course, composite promoters can also be employed – in addition to quaternary phosphonium salts, quaternary ammonium salts can be added as well, such as tetrabutylbenzothiazolium sulfide and bis-cinnamylpropanediamine. Activators are required to be able to promote the vulcanization reaction as well as to act as acid absorbers; commonly used activators include magnesium oxide, lead oxide, calcium hydroxide, and zinc oxide. Generally, highly active magnesium oxide improves heat resistance ； Calcium hydroxide improves resistance to compressive set ； Zinc oxide can improve water resistance ； Lead oxide can improve acid resistance. The amount of calcium hydroxide used is generally 3–6 parts; 6 parts are chosen in polyol vulcanization systems ； The amount of highly active magnesium oxide used is 3 parts, while the maximum amount of lowly active magnesium oxide that can be used is 15 parts. When the amount of activator is too high, the rubber compound tends to form bubbles. As other additives, in order to increase the strength and hardness of the rubber compound, reduce its elongation rate, and improve its wear resistance, heat resistance, and tear resistance, MT carbon black (medium-particle thermal black) is commonly used as a reinforcing agent. The typical dosage is 20 parts. Of course, calcium fluoride, calcium carbonate, highly dispersible barium sulfate, etc., can also be added and used in combination as reinforcing agents. Low-molecular-weight polyethylene is added as a release agent. 2.2 Acrylate rubber: Also known as ACM, acrylate rubber is primarily used in the automotive industry and is regarded as a \"high-performance automotive rubber\". By 2005, the domestic automotive industry’s demand for ACM is expected to reach 12,500 tons per year, without taking into account the consumption of ACM in seal products manufactured for export. The next 3–5 years will represent a peak period for demand for ACM in China. It is estimated that the domestic demand for ACM in 2010 will exceed 15,000 tons. The copolymer monomers of ACM can be divided into three categories: main monomers, low-temperature oil-resistant monomers, and vulcanization-point monomers. Among the main monomers, commonly used ones include methyl acrylate, ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate ； Low-temperature oil-resistant monomers mainly include alkyl ether acrylates, methoxyethyl acrylate, polyethylene glycol methoxyacrylate, dimethoxyethyl maleate, etc ； Vulcanization-point monomers, which are used to facilitate the vulcanization process of ACM; the main types that are currently in industrial use include chlorinated variants such as vinyl chloroacetate, epoxy variants such as glycerol methacrylate, allyl glycidates, double-bonded variants such as 3-methyl-2-butene ester, and ethylenyl norbornene ； Carboxylic acid-type monoesters of maleic acid or itaconic acid, etc. The ACM products available on the market today are mainly those based on active chlorine. The most common vulcanization systems used for ACMs containing active chlorine include: 1) a soap/sulfur combined vulcanization system. This system features good processability, a rapid vulcanization rate, and good storage stability of the rubber compound; however, its thermal aging resistance is somewhat poor, and it exhibits significant compressive set. Common soaps used in this system include sodium stearate, potassium stearate, and sodium oleate ； 2) It is an N,N’-di(benzylidene-1,6-hexanediamine) vulcanization system. Rubbers vulcanized using this system exhibit good thermal aging resistance and low compression set. However, its processability is somewhat poor; sometimes mold sticking occurs. The storage life of the compounded rubber is relatively short, and the degree of vulcanization is not high, so secondary vulcanization is generally required ； 3) It is a TCY (1,3,5-trithiyo-2,4,6-triazine) vulcanization system; this system features fast vulcanization speed. The vulcanized rubber exhibits good thermal aging properties and low compressive set. Its processability is average, but it causes significant corrosion to molds, the mixed rubber has a short storage life, and it tends to scorch easily. As a filling reinforcement material, ACM is an amorphous rubber; the mechanical strength of pure raw rubber is only around 2 Mpa, so reinforcements must be added to make it usable. Tests have shown that for ACM compounds, high-wear-resistant carbon black, fast-extrusion carbon black, semi-reinforcing carbon black, and spray carbon black are suitable choices. Generally, as the carbon black particles become finer, the reinforcing performance improves, but the compression deformation also increases. ACM should not be used with acidic reinforcing fillers, such as channel black and fumed silica. The rubber compound used for oil seals is generally light in color; therefore, neutral or slightly alkaline white fillers such as precipitated silica, calcium silicate precipitate, diatomaceous earth, and talc can be used as reinforcing agents. For other products, these fillers merely serve as inert fillers; only precipitated silica has significant reinforcing properties. However, the pH value of fumed silica significantly affects the vulcanization rate. Therefore, care must be taken when selecting white fillers. The higher the pH value of fumed silica, the faster the vulcanization rate becomes, resulting in an increased crosslink density ； When using silica within a high pH range, care must be taken regarding the heat generation during mixing. When the compound temperature exceeds 150°C, there is a risk of a rapid increase in viscosity, which prevents the silica and ACM from bonding firmly together after molding; consequently, the reinforcing effect is poor. To address this issue, the addition of silane coupling agents is generally very effective in improving interfacial adhesion. Amine-based, ethylene-based, and epoxy-based silane coupling agents can be used; in practice, KH-550 and Si60 are commonly employed, with usage amounts typically ranging from 0.5 to 3.0 parts. As the amount of silane coupling agent increases, the strength and hardness of the rubber compound increase, while its tear elongation and resilience decrease. To improve the wear resistance of the rubber compound, lubricating fillers such as graphite powder, molybdenum disulfide, and carbon fibers can be added. When the amount of carbon fiber used is 5–10 parts, it can also help improve the tensile strength and heat aging resistance of the rubber compound, but the application cost is relatively high ； The amount of graphite used is 5–30 parts; an excessive amount will affect the adhesion between the rubber compound and the matrix material ； The amount of molybdenum disulfide used is 1–5 parts; higher amounts can also have an adverse effect on vulcanization. Antioxidants: ACM itself possesses good resistance to thermal aging. Given that the main products made from ACM are used for extended periods in high-temperature and oily environments, and that the operating temperature of ACM-sealed components is typically between 150°C and 170°C, it is necessary to add a certain amount of antioxidant. The choice of antioxidant should be based on those types that do not volatilize at high temperatures and do not get extracted in oily environments. Currently, abroad, the mainly used antioxidants are Naugard 445 produced by the American company Uniroyal and Nocrac #630F manufactured by Japan’s Ouchi Shinko Company. In the domestic market, it is primarily Naugard 445 that is sold. This product belongs to the class of diphenylamine-based rubber antioxidants; it protects rubber from damage caused by heat and oxygen, and is regarded as the best heat- and oxidation-resistant antioxidant for acrylate rubbers. Since imported products can sometimes be difficult to obtain, domestic processing enterprises opt for quinoline-based antioxidants such as RD, diphenylamine-based antioxidants like BLE, or high-performance p-phenylenediamine-based antioxidants such as 4010NA and 4020. However, since ACM does not have high requirements regarding the performance of antioxidants – the main requirements being that they should not volatilize at high temperatures nor leach out in oily environments – ordinary antioxidants such as BD, RD, and BLE are sufficient. These antioxidants exhibit good anti-aging properties at 150°C, but their anti-aging performance declines significantly when the temperature reaches 175°C, which prevents ACM from demonstrating its full high-temperature resistance. It is worth noting the TK—100 rubber anti-aging agent developed by Suining Qinglong Acrylate Rubber Factory in China for use in ACM under high-temperature conditions. This agent is synthesized through a multi-component process using aniline compounds as raw materials. Extensive use has shown that TK—100 provides excellent resistance to high-temperature aging for ACM containing active chlorine, making it particularly suitable for use at temperatures above 150°C ； Suitable for the various vulcanization systems commonly used in reactive chlorine-type ACM. For plasticizers, generally 5–10 parts of polyester-based plasticizers are added to ACM. The effectiveness of the plasticizer used is evaluated by assessing the weight change of the rubber compound after high-temperature thermal aging; this indicates its volatility at high temperatures ； The volume change of the rubber compound after immersion in oil determines the extent to which it is extracted by the oil. Lubricants: When processing ACM using open-type rubber mixing machines, it is necessary to add certain amounts of neutral lubricants such as white mineral oil, polyethylene glycol, and methylsilicone oil. These lubricants can significantly reduce the stickiness of the rubber compound, lower its viscosity, and improve its fluidity. They also have a positive effect on the cold resistance of ACM rubber compounds ； The effect is particularly pronounced with methyl silicone oil. Many sources report that sorbitan monostearate, as a neutral lubricant, has no delaying effect on vulcanization, and can improve the stickiness of the rubber compound to the rolls as well as reduce its viscosity ； Stearamide derivatives can improve demolding properties ； Glycidyl epoxy resins not only act as plasticizers for ACM but also can reduce the viscosity of the adhesive compound, improving compressive deformation without affecting low-temperature resistance and oil resistance ； A small amount of low-molecular-weight polyethylene (1–3 parts) can increase the extrusion speed of the rubber compound and the smoothness of the product surface, as well as improve the flowability of the rubber compound within the mold cavity ； When selecting a lubricant, care must be taken to ensure that it does not affect the self-adhesion of the rubber compound and its adhesion to metals, which is crucial for oil seal rubber compounds with metal frames. Antioxidant: Compared to other adhesives, ACM is prone to scorching; the most commonly used antioxidant is N-cyclohexylthiotitanoyl imide (antioxidant CTP) ； Derivatives of sulfimide, a anti-coking agent introduced by Nippon East Asia Paints, exhibit good anti-coking performance for both chlorinated and epoxy-type ACMs. Additionally, the addition of vulcanization activators such as triethanolamine and R-aminopropyltriethoxysilane during the vulcanization process can overcome the adverse effects of white and black substances on vulcanization, reduce the scorching time of the rubber compound, and improve properties such as tensile strength and stress at fixed elongation. In recent years, many new types of additives have also been increasingly used in ACM rubber compounds. The use of processing aids such as FL (a product of German company KETTLITZ), the internal rubber release agent Modellit 955P, and the dispersant Z-78 (a product of the Beijing Rubber Industry Research and Design Institute) is highly beneficial for ACM rubber compounds, as it reduces the power required for mixing, improves fluidity, enhances resistance to compression set, and lowers the rubber content. 2.2.3 Chlorosulfonated polyethylene Chlorosulfonated polyethylene, abbreviated as CSM, is primarily used in the anti-corrosion field. It also finds extensive applications in the automotive and electrical/electronic industries. In 2005, China’s production capacity was approximately 10,000 tons, while domestic demand stood at around 12,000 tons. It is estimated that by 2010, domestic demand will reach approximately 16,000 tons. The physical properties and various aging properties of CSM also vary depending on the type and amount of additives added; among these, the vulcanization system has a significant influence on its properties. There are a wide variety of vulcanization systems for CSM. Representative ones include formulations using metal oxides for general-purpose products: the magnesium oxide vulcanization system (used for white and colored products, with good coloring properties but poor water resistance); the magnesium oxide/polyol vulcanization system (suitable for white or colored products, with poor resistance to water and oils); the lead monoxide vulcanization system (a formula specifically designed for black products, with excellent water and chemical resistance); the lead monoxide/magnesium oxide vulcanization system (also for black products, with good heat and oil resistance); and the lead trisulfite maleate vulcanization system (offering electrical insulation properties, aging resistance, and good processability) ； Special formulations are used for specialty products: epoxy resin vulcanization systems (with good water, chemical, and oxidation resistance, as well as strong adhesion to metals); peroxide vulcanization systems (offering excellent compressive set resistance, along with good heat and oil resistance); and maleic anhydride imine vulcanization systems (providing high tensile strength, as well as excellent heat and oil resistance). The commonly used magnesium oxide vulcanization system is suitable for formulating light-colored CSM products, but the vulcanized rubber has poor water resistance. To address this issue, the use of hydrated magnesium aluminate carbonate-based vulcanizing agents proves effective; although it does not achieve the same water resistance as the lead monoxide vulcanization system, it still meets the requirements for light-colored rubber products. To enhance the performance and applications of CSM, extensive research has been conducted abroad on curing agents for it; for example, trihydrogenazine developed in Japan is used as a curing agent to produce CSM sponges that are resistant to charring, fading, and staining ； Rubber compounds were prepared using benzene-1,2-dimaleimide, alkaline earth metal (hydroxide) hydroxides, ammonia water, and nickel dithiocarbamate as a CSM composite vulcanization system, exhibiting good resilience, tensile strength, and ozone resistance ； Adding magnesium hydroxide to the thiadiazole vulcanization system can significantly improve the anti-scorch stability of the rubber compound and alter the vulcanization rate of the system ； The newly developed peroxide vulcanizing agent F, created in the United States, offers advantages such as higher cross-linking efficiency compared to other organic peroxides, a weaker odor, cost-effectiveness, safe handling, and a longer time before scorching occurs. The most commonly used reinforcement agent in CSM is carbon black. Vulcanized rubber containing fine-particle carbon black possesses high physical strength, but it has poor performance in terms of handling safety, rubber flow properties, and the elasticity of the vulcanized product. Therefore, soft carbon blacks are generally used in CSM, such as rapidly cooled furnace black, semi-reinforcing furnace black, and fine-particle thermally cracked carbon black. Since CSM is used to manufacture light-colored and colored rubber products, apart from silica, few other inorganic fillers possess reinforcing properties; they are mostly used as fillers. Materials such as calcium carbonate and titanium dioxide can improve the affinity of CSM, reduce frosting, and enhance its heat resistance and transparency, making them important fillers for CSM. The hardness of CSM pure rubber combined with vulcanized rubber is higher than that of diene rubbers such as neoprene and natural rubber; therefore, more softeners are required to achieve the same hardness as vulcanized rubber, and generally aromatic softeners with low cost and good compatibility are used ； However, for products requiring good low-temperature performance or colored products, it is preferable to use ester-based plasticizers ； For rubber products that require heat resistance, high-molecular-weight esters and chlorinated paraffins are more suitable. CSM molecules contain very few active sites that can lead to aging; therefore, quite good aging resistance can be achieved even without adding anti-aging agents. However, in order to obtain even better heat and weather resistance, anti-aging agents are generally added. Commonly used anti-aging agents include Anti-Aging Agent NBC (nickel dibutyldithiocarbamate) and Anti-Aging Agent $00 (4,4’-butylidenebis(3-methyl-6-tert-butylphenol)), etc. To prevent the rubber compound from sticking to the rolls during CSM processing, microcrystalline wax, low-molecular-weight polyethylene, polyethylene glycol, stearic acid, polybutadiene, and fatty acid esters can generally be added. CSM is a synthetic rubber that is difficult to process; its compounded rubber presents many problems such as scorching, roller sticking, and mold contamination. In particular, the issue of scorching is influenced not only by temperature but also by humidity, which can accelerate its premature cross-linking. Therefore, special attention must be paid to temperature and humidity when storing the compounded rubber. Additionally, during the vulcanization of CSM, especially in high-temperature processes like injection molding, mold contamination and corrosion, as well as the formation of free chlorine and sulfur dioxide, require particular attention. 2.4 Chlorinated polyethylene Chlorinated polyethylene, abbreviated as CPE, encompasses a range of polymers—from soft plastics and elastomeric plastics to cross-linked elastomers and coating resins—depending on its chlorine content. The rubber-type chlorinated polyethylene is referred to as CM. In recent years, China has seen rapid growth in its CPE production capacity; in 2005, the country’s total CPE production capacity soared to approximately 350,000 tons per year, of which the production capacity for CM was around 30,000 tons per year. Given that supply falls short of demand, CPE holds promising development prospects. Vulcanizing agents: The molecular structure of CPE elastomers is saturated, and the chlorine atoms bonded to the secondary carbon atoms lack high reactivity; as a result, CPE has low chemical activity and its vulcanization systems are limited. Typically, four types of vulcanization systems are used: peroxides, amines, thioureas, and triazines. When peroxides are used as vulcanizing agents, they are generally used in combination with multifunctional monomers (as cross-linking agents). Common cross-linking agents include ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, and triallyl cyanurate. For amines, hexamethylenediamine adipate and hexamethylenediamine sebacate are generally used. For the thiourea vulcanization system, vinyl thiourea is generally chosen. Triazine-based sulfiding systems, such as trithiopropyl isocyanate, are generally not used alone and require accelerators. Like polyvinyl chloride, CPE also releases hydrogen chloride under the action of light and heat. Therefore, PVC stabilizers must be used as stabilizers for CPE; examples include organic phosphates and stearates. Plasticizers: When a peroxide vulcanization system is used, plasticizers can include chlorinated paraffins, dioctyl sebacate, dioctyl adipate, epoxy soybean oil, and paraffin oils with low aromatic content ； When amines, thioureas, and sulfur are used as vulcanizing agents, stearic acid and petroleum aromatic plasticizers are employed. ` Fillers: Apart from rubber compounds vulcanized with peroxides, which use alkaline or neutral fillers, common fillers include calcium carbonate, clay, carbon black, and kaolin ； For light-colored rubber compounds, magnesium silicate, silica, etc. can be used. High-wear-resistant carbon black, spray carbon black, and calcium carbonate are generally used together as a filler system for CPE elastomers. 2.5 Hydrogenated Nitrile Rubber Hydrogenated nitrile rubber, abbreviated as HNBR, is a modified version of nitrile rubber (NBR). It retains the excellent oil and chemical resistance of nitrile rubber, while also exhibiting significantly improved cold resistance and resistance to permanent compression set. It is currently seeing rapid adoption in industries such as the automotive and aerospace sectors; in China, Lanhua Company has manufacturing facilities for this material. The vulcanizing agents used for HNBR are the same as those for NBR: sulfur and peroxides can be employed, with Petro*mon F—40 being a common choice for peroxides; the accelerator is HVA—2 (m-phenylene bismalimide). However, when vulcanized with sulfur, since the polymer backbone must contain double bonds, its use in HNBR with a hydrogenation degree below 95% is limited. For HNBR with a hydrogenation rate of over 95%, it is difficult to obtain products with sufficient rubber elasticity even through sulfur vulcanization. HNBR with a high degree of hydrogenation must be vulcanized with peroxides. Compared to NBR, since it is difficult to increase the crosslinking density of HNBR, the amount of peroxide required is also about 3-5 times that needed for NBR. At the same time, to improve cross-linking efficiency, it is very effective to use multifunctional monomers such as trimethylolpropane, trimethacrylate (TMPT), or triallyl isocyanurate (TAIC) as co-cross-linking agents. However, the addition of TMPT causes the problem of rubber scorching; therefore, it is necessary to pay attention to controlling the processing temperature and molding temperature. Compared to sulfur vulcanization, peroxide vulcanization offers superior heat resistance and compressive set resistance, which is why it is mostly used for the vulcanization of molded products. What is commonly used for HNBR? The antioxidant is ZMTI (Vulkanox MB2), with the chemical name being **zinc bisimidazole salt** ； Plasticizers include high-molecular-weight, low-volatility substances such as TOTM (tri-octyl phthalate) and TINTM (tri-isononyl phthalate) ； Reports indicate that incorporating zinc polymethacrylate into HNBR improves its strength and durability. In addition to these, other types of additives for HNBR include reinforcing and filling agents such as black and white carbon black ； Processing aids such as organic fatty acids ； Plasticizers used to improve processability and provide cold resistance ； Used to send heat-resistant and ozone-resistant antioxidants ； Metal oxides and similar substances that act as vulcanization aids during sulfur vulcanization. 3 Conclusion With the rapid development of China’s automotive, electronics and electrical, aerospace, and construction industries, there are increasing demands regarding the quantity, variety, and performance of special rubbers. In recent years, China’s production capacity for such rubbers has grown rapidly, yet the range of available varieties and grades remains limited ； Product quality is unstable, and there is significant pressure related to environmental protection ； There are few specialized compounding agents; traditionally used antidegradants and processing aids are primarily employed, which is highly detrimental to improving the quality of specialty rubbers and their products ； Therefore, in the future, while improving the synthesis technology for special rubbers in China, it is necessary to accelerate research in areas such as the application technologies of these rubbers and the use of additives, in order to produce more and better special rubber products to meet the demands of the domestic market.