Silane coupling agent and its application in Composites

Silane coupling agent is one of the important and widely used treatment agents. It was originally developed as a treatment agent for fpr glass fibers [1]. Later, with the development of new compounds, they were gradually applied in various fields. Now, silane coupling agents are basically applicable to the connecting surfaces of all inorganic and organic materials [2]. This paper mainly introduces silane coupling agent and its application in composites.

1. Silane coupling agent silane coupling agent is a kind of silicone compound represented by the following formula, which is characterized by having more than two different reaction groups in the molecule. General formula: y-r-si-x3r: alkyl or aryl; 10: Methoxy, ethoxy, chlorine, etc; Y: Organic reactive groups (vinyl, epoxy, amino, mercapto, etc.). The hydrolytic group represented by X can be combined with inorganic materials (glass, silica, metal and its oxides, clay, etc.), and the reactive group represented by y can be chemically combined with organic materials. Therefore, silane coupling agents play a bridge role at the interface of inorganic materials and organic materials, and are widely used in the modification of composite materials. At present, there are many kinds of silane coupling agents at home and abroad, and the commonly used ones are listed in Table 1.

2. Action mechanism in composite materials people have done a lot of research on its action mechanism and put forward various theories, but so far there is no complete and unified understanding.

2.1 chemical bond theory this theory believes that the X Group in silane can react with the hydroxyl group on the surface of inorganic materials to form chemical bonds, and the Y group can react with the resin to form chemical bonds. These two materials with very different properties are "coupled" by chemical bonds to obtain good adhesion, which is also the reason why these compounds are called coupling agents. Chemical bond theory has been widely used to explain the role of coupling agents, especially how to choose coupling agents has certain practical significance.

2.2 wetting effect and surface energy effect in the manufacture of composite materials, good wetting of liquid tree fingers and adherends is of primary importance. If complete infiltration can be achieved, the adhesion strength of the physical adsorption of the resin on the high-energy surface will be much higher than the cohesion strength of the organic resin. Treating the surface of glass fiber (or other inorganic materials) with a suitable silane coupling agent will increase its surface tension, thus promoting the infiltration and expansion of organic resin on the surface of inorganic materials.   

2.3 morphology theory the silane treatment agent on the inorganic material will change the morphology of the adjacent organic polymer in some way, so as to improve the bonding effect. According to the deformable layer theory, a flexible resin layer can be produced to alleviate the interface stress; According to the confinement layer theory, silane can "tightly bind" the polymer structure in the interphase region.   

2.4 the coupling agent on other theoretical interfaces may play a variety of functions, such as producing a lubricating effect to protect inorganic materials from wear during manufacturing; It is also possible to protect the surface of the inorganic material from stress corrosion by water. In addition, there are acid-base reaction theory, reversible hydrolytic bond mechanism and so on.   

(3) application in composite materials

3.1 thermosetting resins, inorganic fillers and inorganic reinforcing materials are widely used in making composite materials together with thermosetting resins, and silane coupling agents are also the earliest and most mature in this field.   

3.1.1 unsaturated polyester was compared with a variety of unsaturated silane coupling agents on the glass fiber in the polyester laminate [4], many of which are very effective coupling agents. See Table 2 for their superior performance and applications. For most general purpose polyesters, silane coupling agents containing methacrylate are often used. In typical polyester casting with fillers, various fillers and Methacryloyloxy functional silanes can improve the properties to different degrees.

3.1.2 epoxy resins many silanes are quite effective for epoxy resins, but some general rules can be formulated to select the most suitable silanes for a specific system. The reactivity of the coupling agent is at least equivalent to that of the specific curing system used for the epoxy resin. For epoxy resins containing glycidyl functional groups, glycidyl oxypropyl silane is obviously the best choice. For epoxy resins cured with ALICYCLIC EPOXIDES or anhydrides, alicyclic silane is recommended. In practical application, the application mechanism of silane coupling is not always clear, but it can be selected in combination with application experience. For example, silane with primary amine group can obtain the best performance of room temperature cured epoxy resin, but it can not be used for anhydride cured epoxy resin; Silane containing chloropropyl functional group is a reliable coupling agent for epoxy resin cured at high temperature; Silane containing methacrylate is an effective coupling agent for dicyandiamide cured epoxy resin.  

3.1.3 phenolic resin silane coupling agent can be used to improve almost all composite materials containing phenolic resin. Aminosilane can be used in glass fiber insulation materials together with phenolic resin binder; It is used in glass fiber tire cord together with resorcinol formaldehyde resin or phenolic resin in resorcinol formaldehyde latex impregnation solution, and used as binder for sand core for metal casting together with furan resin and phenolic resin; Aminosilane and phenolic resin can be used to fix sand layer in oil well, and the effect is ideal.

3.2 thermoplastic resin treating granular inorganic fillers with silane can significantly improve the rheological properties of thermoplastic resins containing fillers, and protect fillers from mechanical damage in high shear operations such as mixing, extrusion or injection molding.  

3.2.1 polyolefin filler containing polyethylene used for cable cladding by extrusion method can be modified with silane to improve the electrical performance of the composite material in wet state. The properties of polyethylene composites filled with clay, calcium silicate and quartz were improved obviously after adding silane.

3.2.2 thermoplastic engineering plastics are suitable for silane, an organic functional group of epoxy resin, and can also produce good effects in nylon filled with inorganic fillers. Aminosilane can be used in a large number of thermoplastics, such as ABS, acetal resin, nylon, polycarbonate, polysulfone, polystyrene, polyester, polyvinyl chloride, styrene acrylonitrile copolymer, etc.   

3.3 elastomer silane has been used in rubber to treat carbon black, silica and other inorganic fillers for many years. Silane coupling agents have been widely used in rubber products such as radial tires, rubber rollers and high-grade soles, and sulfur-containing silane WD-40 has become an indispensable treatment agent in these rubber formulations. The results show that when silane is added to various types of rubber, other properties will change with the increase of adhesive strength, and the changes vary with the type of rubber.