Research Progress on Organic Silicone Adhesives

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Research Progress on Organic Silicone Adhesives In 1941, the Rochow Company synthesized organic chlorosilanes using a direct method, laying the foundation for the production and development of silicones. In 1943, Dow Corning was established to focus on the research and production of silicone, and it grew very rapidly. Currently, the companies that lead in terms of scale and technology are Dow Corning, GE, Shin-Etsu Chemical, Walker, Degussa Crompton, Rhodia, and several other large firms. In 2000, their market shares were 35%, 17%, 14%, 11%, 9%, 6%, and 5% respectively. 　　In the 1980s, China developed organic silicone adhesives; currently, these are mainly produced by a dozen or so enterprises and research institutions with strong technical capabilities and advanced equipment. Development has been rapid in recent years, with an annual growth rate of no less than 10%, which is higher than the growth rates in developed countries. During the 10th Five-Year Plan period, building sealing materials were mainly silicone and polyurethane-based, with an annual consumption of around 100,000 tons. 　　Silicone adhesives possess excellent resistance to high and low temperatures, weathering, electrical insulation, water repellency, and chemical reagents, and are widely used in industries such as electronics, machinery, aviation, construction, healthcare, and communications. The adhesive component of organic silicone adhesives consists of high-molecular-weight linear or cross-linked polysiloxanes, which are produced from one or several types of linear, cyclic, or branched siloxanes through reactions such as balanced addition or condensation, under the action of catalysts. Organic silicone adhesives are mainly divided into two types: rubber-type and resin-type. 1 Silicone Rubber 1.1 Overview Silicone rubber is an elastomer with a network structure, whose main structural units are Me2SiO and MeRSiO (where R is PHCF3CH2CH3, Vi, H, etc.). Based on the vulcanization temperature, they can be classified into the following types: 1) High-temperature vulcanized silicone rubber, which includes cross-linking initiated by organic peroxides as well as cross-linking through hydrosilylation; 2) Low-temperature vulcanized liquid silicone rubber (LTV), which is based on polydimethylsiloxane containing MeViSiO and MeHSiO units, uses platinum as a catalyst, and comes in two forms: two-component addition-type and one-component addition-type; 3) Room-temperature vulcanized liquid silicone rubber (RTV), which is available in both one-component and two-component versions. Single-component room-temperature vulcanizing silicone rubber (RTV-1) mainly includes types such as acetate-decomposing, alcohol-decomposing, dehydrogenation-type, dehydration-type, hydroxylamine-decomposing, and addition-type; two-component room-temperature vulcanizing silicone rubber (RTV-2) mainly includes types such as alcohol-decomposing, dehydrogenation-type, dehydration-type, hydroxylamine-decomposing, and addition-type. Before being filled and cross-linked, silicone rubber has a very low tensile strength, of around 0.35 MPa. Research has shown that increasing the cross-linking degree of silicone rubber, modifying it, or adding reinforcing fillers can all enhance its strength; products with a tensile strength of over 20 MPa are now available. High-temperature vulcanized silicone rubber is mainly used to manufacture vulcanized products. One-component room-temperature vulcanizing silicone rubber exhibits excellent adhesion to most substrates; when tackifiers are added, two-component room-temperature vulcanizing silicone rubber and low-temperature vulcanizing silicone rubber can also achieve good adhesion. In addition to exhibiting good adhesion to metallic and non-metallic materials, liquid silicone rubber adhesives also have good adhesion to materials with low surface energy that are difficult to bond, such as polytetrafluoroethylene, polyolefins, and silicone rubber. The silicone rubber currently under development has seen improvements to varying degrees in terms of adhesiveness, thermal conductivity, and curing properties. 1.2 Adhesiveness 1.2.1 Modification of silicone rubber Polmarrlerve found that increasing the content of X-links in polydimethylsiloxane molecules, and then mixing this material with peroxides as well as silica treated with hexamethyldisiloxane and isopropanol, results in an adhesive that exhibits good adhesion to steel. In the development of coating printing adhesives, Ding Zhengxue and others used hydrogenated polymethylsiloxane-modified acrylates to create adhesives; compared with pure acrylate coating printing adhesives, the friction fastness of these adhesives improved by one grade. The method used by Matsumoto et al. to modify polyorganosiloxanes with acrylates involves adding methyl methacrylate, butyl acrylate, acrylic acid, and styrene to a polyorganosiloxane dispersion; using potassium persulfate as an initiator and adjusting the pH value, a graft-modified polyorganosiloxane is obtained through a polyaddition reaction. In the early 1970s, organofunctional silane-terminated polyurethane prepolymers were already used in the formulation of adhesives and sealants, resulting in prepolymers containing free radical NCO groups that exhibited good durability and adhesion properties. Misty’s research shows that the properties of organofunctional silane-terminated polyurethane prepolymers can be adjusted by changing the n(NCO)/n(OH) ratio in the polyol and by using different silane terminators. The use of siloxanes can improve the adhesion properties of adhesives and the mechanical properties of sealants. 　　Tsinghua University synthesized alkoxysilane-modified one-component polyurethanes using polyether polyols, TDI, BIDL, KH-550, fillers, plasticizers, and other additives. The results show that as the proportion of alkoxysilane capping increases, the NCOR content decreases, but the tensile strength of the sealant increases. The proportion of alkoxysilane capping should be around 10%, while the proportion of triols in the polyurethane prepolymer should not exceed 50%. Mine and Imai et al. incorporated epoxy-containing compounds into silicone raw rubber to achieve good bonding results. Some people also incorporate acrylate compounds, silicones containing amines and sodium sulfonates, and boron-containing compounds into silicone rubber. In addition, Pan Huiming and others used water glass and hexamethyldisiloxane as basic raw materials to synthesize methyl MQ and MTQ resins, which were then applied in RTV silicone rubber adhesives to improve adhesion. 1.2.2 Coupling agents: Rittenhouse added NH2(CH2)3Si(OEt)3 and epoxy resin to surface treatment agents containing Rsi(OEt)3 (where R is methyl, phenyl, or vinyl), in order to treat aluminum sheets and stainless steel. Yang Weisheng et al. incorporated tetravinylsilane and tetraallyl groups into HTV silicone rubber to improve adhesion to metals. The curing-type organic silicone adhesive developed by Shin-Etsu Chemical can achieve improved adhesion to films such as polyethylene terephthalate, polyimide, and polyphenylene sulfide when the primer X-40-3501 is used. The company has also developed RTV silicone adhesives that can bond butyl terephthalate, as well as adhesives that can bond polyamide resins without sticking to metal mold surfaces. 1.3 The curing process in the curing production line imposes high requirements on the curing speed of adhesives. 　　Shin-Etsu Chemical’s room-temperature fast-curing polysiloxane sealant is composed of hydroxyl-terminated polydimethylsiloxane along with tetramethylsilane, propylene, n-butylamine, and dibutyltin dilaurate; after curing at 20°C and 55% RH, its tensile strength can reach 2.5 MPa. Dow Corning 9-1359 has a surface drying time of 5–10 minutes at 25°C and 50% RH, and it possesses high initial adhesion strength. SS-67B from Silicone Solution Company exhibits excellent adhesion to silicone rubber; when used after being treated at 200°C for 30 seconds, it cures within 1 minute. Qi Shicheng et al.] used hydroxyl-terminated polydimethylsilane, zinc oxide, hydrogenated silicone oil, and a catalyst as raw materials to prepare a three-component room-temperature vulcanizing foam silicone sealant. The shelf life of this sealant is 50 minutes, and it retains its elasticity even after being placed at 300°C for 100 hours. In recent years, extensive research has been conducted on ultraviolet-cured silicone rubber. If the research is successful, it will be possible to achieve rapid vulcanization and reduced energy consumption. 1.4 Thermal Conductivity The thermal conductivity of general-purpose room-temperature vulcanized silicone rubber can reach 0.16 W(m•K)-1, which is approximately twice that of synthetic rubbers. The thermal conductivity can be further increased by adding thermally conductive fillers to the rubber compound. Dalbe et al. used two fillers with different particle size distributions, and at addition levels of 35%–70%, they produced silicone rubber with a thermal conductivity exceeding 1.2 W(m•K)-1. Wang Qian et al. studied the effect of the particle size distribution of Al2O3 and SiC2-type thermal conductive fillers on the thermal conductivity of RTV silicone rubber. Dow Corning has newly introduced several products that can be used for bonding and sealing heat-conducting materials; Dow Corning 3-6658 is used for seals with high thermal conductivity. Dow Corning SE4420, SE4422, and SE4486 are used for bonding integrated circuit boards to heat sinks, while Dow Corning SC102 is used as a thermally conductive encapsulation material. 2 Silicone resin Silicone resin is a thermosetting polysiloxane that has a highly cross-linked structure, with R2Si2O3 and RR’ (where R is Me, Ph, H, Vi, etc.) as its main structural units. Based on the main chain composition, they are divided into pure resins and silicone-modified resins. The adhesiveness of silicone resin is better than that of silicone rubber. The properties of silicone resins are related to the ratio of R to Si (where R refers to the average number of organic groups attached to a silicon atom, mainly methyl and phenyl groups) as well as the type of organic group R. The ratio of R to Si is generally between 1.0 and 1.7; if this ratio is low, the resin has good drying properties, low flexibility, and high hardness. The higher the phenyl content in the organic group, the lower the thermoplasticity and the greater the hardness. The heat resistance is best when the phenyl content is 20%–60%. Changing the phenyl content can also improve adhesion to various substrates. Pure silicone resin can bond various metal alloys, ceramics, composites, etc.; it is an adhesive with good heat resistance. However, its curing typically requires high temperatures (>200°C) and pressure, and it has low toughness after curing, which makes it unsuitable as a structural adhesive. Furthermore, its adhesion to copper is poor; therefore, it has been proposed to combine silicone resin with other resins to create a modified silicone resin that possesses the excellent properties of both. Resins used in modifying silicone in recent years include epoxy resins, polyester resins, polyurethanes, acrylic resins, alkyd resins, phenolic resins, and others. Contact adhesives produced by Kaneka Corporation using a copolymer formed from butyl acrylate, methyl methacrylate, octadecyl methacrylate, and γ-mercaptopropyl dimethoxysilane, as well as silicone-containing polyoxypropylene prepared by reacting 2-methallyl-terminated polyoxypropylene with dimethoxysilane, exhibit a holding time of 120 minutes and a peeling strength of 75 N•(25 mm)-1. Wu Runde et al. used polysiloxane sols obtained from the partial hydrolysis of ethyl orthosilicate to graft-modify acrylic resins; the modified silicone resins exhibited superior properties in terms of curability, adhesion, and solvent resistance compared to pure silicone resins. Yang Ming and others modified epoxy resin E-20 with silicone resin to use as the resin matrix; the adhesive thus prepared exhibits a shear strength of over 20 MPa at room temperature and can be used for extended periods at 300°C. 3 Silicone pressure-sensitive adhesives Silicone pressure-sensitive adhesives play an important role among adhesives; they are composed of silicone rubber, MQ resin, crosslinking agents, curing agents, and organic solvents. Among them, MQ resin is a key component of pressure-sensitive adhesives; it should be soluble in organic solvents, have a low molecular weight and M/Q value, as well as an appropriate amount of terminal silanol groups. Silicone pressure-sensitive adhesives mainly include solvent-based, hot-melt type, emulsion type, radiation-cured types (UV, EB), resin-modified types, and high-solid-content types. 　　Currently, solvent-based silicone pressure-sensitive adhesives are widely used, with common solvents including benzene, toluene, dichlorotoluene, petroleum ether, and their mixtures. Li Maoguo developed a room-temperature curing transferable silicone pressure-sensitive adhesive. After being transferred onto a silicone rubber foam pad, this adhesive was applied to aluminum blocks under pressure at 60°C and held in place for 48 hours before being removed; no residual adhesive remained at the bonding site, meeting military standard requirements. However, the solvents used in this adhesive cause environmental pollution, and reducing the content of volatile organic compounds (VOCs) as well as increasing the solid content are two issues that need to be addressed urgently for this adhesive. Boardman developed a silicone pressure-sensitive adhesive with a solid content of 95%, in which the MQ resin contained 1% to 4% SiH groups. In recent years, Pan Huiming and others have conducted extensive research on room-temperature silicone rubber sealants [42]. The HP-1 deprotonated homocrosslinking agent developed in these studies exhibits high reactivity, allowing for rapid crosslinking and curing using only small amounts of fatty amines; this eliminates the need to use toxic organotin catalysts that require high temperatures to be effective. Ultra-fine calcium carbonate was used to fill the deprotonated homocrosslinked RTV-1 silicone rubber, and the effect of particle size on the mechanical properties and rheology of the silicone rubber was studied. By employing a chain-extension/crosslinking vulcanization system to control the crosslinking density and distribution, RTV silicone rubbers were produced that possess good processability and extrudability, high elongation rates (898%), and low moduli; moreover, they exhibit low tensile strength after vulcanization (0.13 MPa). Methods for introducing stable acid or base groups into the silicone rubber macromolecules were explored, and in line with market demands, two types of silicone rubber sealants with acid-base functional properties were developed: SGS689, a high-adhesion silicone sealant, and SWS683, a weather-resistant flexible silicone sealant. In addition, they also conducted research on cure-type silicone rubber systems, such as the preparation and activity study of platinum catalysts, the effect of electromagnetic radiation on the properties of silicone rubber, and the preparation of cure-type silicone rubber. 　　With the rapid development of the electronics, construction, and automotive industries, the demand for silicone rubber has surged, and silicone rubber products account for the largest number among all silicone-based products. Silicone resin was the earliest to be developed among silicone products, but its development was slow. Both silicone rubber and silicone resins possess excellent properties that other organic adhesives lack. However, factors such as higher costs, lower mechanical strength, and poor curing characteristics limit their further use, and these aspects require further improvement.