Coupling agent refers to a type of substance that can improve the interfacial properties between fillers and polymer materials. There are two functional groups in its molecular structure: one functional group can undergo chemical reactions with the polymer matrix or at least have good compatibility; Another functional group can form chemical bonds with inorganic fillers. Coupling agents can improve the interfacial properties between polymer materials and fillers, enhance interfacial adhesion, and improve the performance of filled or reinforced polymer materials.
Coupling agents were first developed by Union Carbide Corporation (UCC) in the United States for the development of glass fiber reinforced plastics. As early as the 1940s, when glass fiber was first used as a reinforcing material for organic resins to prepare the widely used fiberglass, it was found that when they were left in moisture for a long time, their strength would significantly decrease due to the debonding of resin and hydrophilic glass fiber, and thus water-resistant composite materials could not be obtained. Given that organic silicon materials containing functional groups are a hybrid of organic and inorganic materials that have both amphiphilic relationships with silicon dioxide (the main component of glass fibers) and resin, using them as "adhesives" or coupling agents to improve the adhesion between organic resins and inorganic surfaces and achieve the goal of improving polymer properties has become a major idea for scientists and has achieved good results in practical applications. Therefore, from the early 1940s to the 1960s was a period of the emergence and development of coupling agents, and the first generation of silane coupling agents was formed. At present, coupling agents used in industry can be classified according to their chemical structure into silane, titanate, aluminate, organochromite, boride, phosphate, zirconate, stannate, etc. They are widely used in the field of polymer materials such as plastics and rubber.
1. Silane coupling agent
Among the numerous types of coupling agents, silane based coupling agents are one of the earliest studied and widely used varieties. The general formula for this type of coupling agent can be written as RSiX3, where R is an active functional group that has affinity and reactivity with polymer molecules, such as vinyl, aminopropyl, epoxy, methacryloyl, amino, and thiol groups; X is an alkoxy group that can be hydrolyzed, such as methoxy, ethoxy, etc. The coupling effect of silane is often simply described as the formation of covalent bridges between neatly arranged silane based molecular layers of polymer and filler. Silane coupling agents have a more significant coupling effect on filling systems containing polar groups, but have no significant effect on non-polar systems, and have poor effect on calcium carbonate filling composite systems.
2. Titanium ester coupling agent
Titanium ester coupling agent is a new type of coupling agent developed by Kenlich Petrochemical Company in the late 1970s in the United States. For thermoplastic polymers and dry fillers, there is a good coupling effect, and this type of coupling agent can be used with the general formula: (ROO (4-n) Ti (OX-R)&Y) n (n=2,3) represents. Among them, RO - is a hydrolyzable short carbon chain alkoxy group that can react with inorganic surface hydroxyl groups to achieve chemical coupling; OX - can be a 0 group, alkoxy group, sulfonic acid group, phosphorus group, etc. These groups are important and determine the special properties of titanium acid esters, such as the sulfonic acid group endowing organic polymers with certain thixotropy; Pyrophosphoryloxy has flame retardant, rust proof, and adhesive enhancing properties; Phosphinoxy groups can provide antioxidant and flame retardant properties, so by selecting OX -, titanium esters can combine coupling and other special properties; R&'-' is a long carbon bonded alkyl hydroxyl group, which is relatively soft and can bend and entangle with organic polymers, improving the compatibility between organic and inorganic substances and enhancing the impact strength of materials; Y is a hydroxyl, amino, epoxy, or double bond containing group that is attached to the end of the titanium ester molecule and can react chemically with organic polymers to bond together. Titanium ester coupling agents further expand the scope of use of silane coupling agents, significantly improving the coupling effect of non-polar calcium plastic filling systems. In addition, according to the different special functional groups, it can be divided into three types: monoalkoxy, chelating, and coordinating. The above two types of coupling agents are currently widely produced and used.
3. Aluminum ester coupling agent
This type of coupling agent is a domestically developed variety that can improve the physical and mechanical properties of products, such as increasing impact strength and thermal deformation temperature, comparable to titanium ester coupling agents. In addition, its cost is low, with a price only half of that of titanium ester coupling agents. It has the characteristics of light color, non toxicity, and easy use. Its thermal stability is even better than titanium ester coupling agents. The biggest difference between it and titanium based coupling agents is its excellent dispersibility of pigments such as carbon black. Therefore, it is widely used in coatings.
4. Organic chromium complex coupling agent
This type of coupling agent was developed by DuPont in the United States and is a coordination complex formed by 0 acid and trivalent chromium chloride, with the brand name Volan. For many years, the complex of chromium and methacrylic acid has been used as a standard treatment agent for glass fibers reinforced with polyester and epoxy resins. The Volan treatment agent for glass fibers can also endow them with excellent anti-static properties and other process properties. Therefore, the green color produced by chromium is generally regarded as the standard for "coupled" glass fibers reinforced with plastics.
5. Other types of coupling agents
Various inorganic acid salts have been used as coupling agents to enhance plastics and have been applied. They include phosphate esters, borate esters, stannate esters, zirconate esters, and zirconate aluminate esters. At the same time, some people have used butyl acrylate as a surface treatment agent to treat calcium carbonate in polypropylene composites filled with calcium carbonate, which can improve the dispersibility and compatibility of calcium carbonate in polypropylene and enhance the performance of the composite material. If graft copolymers of polypropylene with maleic anhydride or butyl acrylate are added separately to the above composite system, the dispersibility and compatibility of the composite system can also be improved, as well as various properties of the composite material.
