Coupling agents are key additives for improving the performance of polymer composite materials and ideal excipients for reducing the cost of polymer composite materials. Coupling agents, as surface modifiers for inorganic fillers, improve the compatibility between fillers and resins. Fillers are more easily dispersed in resins, reducing melt viscosity, improving processing performance, increasing production efficiency, and reducing mechanical wear. They play an important role in achieving high filling while reducing resin usage and production costs. At present, coupling agents used in industry can be divided into silane coupling agents, titanium ester coupling agents, chromium system coupling agents, zirconium aluminum system coupling agents, aluminum ester coupling agents, and aluminum titanium composite coupling agents according to their chemical structures. Titanium ester coupling agent is currently a widely used type of coupling agent, especially in PVC filled plastics where it has the best practical value.
Coupling agents are substances that can combine inorganic and organic materials, or both, in different organic material composite systems through chemical reactions, or improve their affinity through chemical reactions, thereby enhancing the functionality of composite materials. Some of the functional groups in its molecules can react with the chemical groups on the surface of inorganic materials to form strong chemical bonds, while others have organic friendly properties and can react or physically entangle with organic molecules, thereby firmly bonding two materials with different properties, that is, connecting the interface between inorganic materials (fillers) and polymer materials (PVC resin). Titanium ester coupling agent is a new type of coupling agent developed by Kenrich Petrochemical Company in 1975 in the United States. It has a unique structure and good coupling performance for thermoplastic polymers and dry fillers.
（1） According to the coupling mechanism between molecular structure and filler surface, titanium ester coupling agents can be divided into four basic types.
1. Monoalkoxy type
This type of coupling agent is particularly suitable for dry filler systems that do not contain free water, but only contain chemical or physical bonding water, such as calcium carbonate, hydrated alumina, etc. The typical variety is isopropyl tristearate titanate, which is currently the most widely used titanate coupling agent.
2. Monoalkoxy pyrophosphate ester type
This type of coupling agent is suitable for filling systems with high moisture content, such as clay, talc powder, etc. In these systems, in addition to the coupling formed by the reaction between the monoalkoxy group and the hydroxyl group on the surface of the filler, the pyrophosphate group can also decompose to form a phosphate group, which combines with some water. The typical variety of this type of coupling agent is isopropyl tris (dioctylpyrophosphayloxy) titanate.
3. Chelating type
This type of coupling agent is suitable for high-temperature fillers and aqueous polymer systems, such as wet process silica, clay, talc powder, aluminum silicate, carbon black, etc.
In high-temperature systems, typical monoalkoxy titanium esters have poor hydrolysis stability and low coupling effects. Chelated titanium esters have excellent hydrolytic stability and are suitable for use at high temperatures. Their representative variety is di (dioctylpyrophosphayloxy) oxoester titanium.
4. Coordination type
This is developed to avoid the reaction of tetravalent titanium esters in certain systems. This type of coupling agent is suitable for many filling systems, and its coupling mechanism is similar to that of monoalkoxytitanate. Its representative variety is diisopropyl di (dioctyl phosphate) titanate.
5. Types and chemical structures of typical titanium ester coupling agents
The molecular structure of titanium ester coupling agent can be divided into six functional zones, and the performance and efficacy of the coupling agent vary depending on the functional zone. Its general formula is: (Ro) m-Ti - (OX-R)&In the prime; - Y) n equation: 1≤ ｍ&Le; 4m+n≤6Ｒ— Short carbon chain alkyl group; Ｒ′— Long carbon chain alkyl group; Ｘ— C. Elements such as N, P, S, etc; Ｙ— Hydroxyl, amino, epoxy, double bond and other functional groups.
（2） Coupling mechanism
The role and effect of coupling agents have been recognized and affirmed by the world, but there is currently no complete theory about the mechanism of action of coupling agents. There are existing theories such as chemical bond theory, wetting effect and surface energy theory, deformable layer theory, and confinement layer theory, among which chemical bond theory is considered a relatively complete theory. The chemical bond theory suggests that coupling agents contain a chemical functional group that interacts with protons on the surface of inorganic fillers to form covalent bonds. In addition, coupling agents also contain at least one different functional group that bonds with polymer molecules, thereby serving as a bridge between the inorganic and organic phases, resulting in strong interfacial bonding.
The influence of different types of titanium ester coupling agents on the coupling performance of PVC
Due to the different molecular structures of titanium ester coupling agents, their functions vary and they have their own characteristics in coupling. When selecting coupling agents, we should choose them based on different resins and fillers. The main application scope of titanium ester coupling agents is that they react chemically with trace hydroxyl or proton groups adsorbed on the surface of fillers through alkoxyl groups, forming a monolayer on the surface of the coupling agent fillers and releasing isopropanol.
