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Action Mechanism and Characters of Coupling Agent

To learn the relationship between the titanate structure and property will help you make the right choices of all kinds of products (varieties).

Tetravalent elements are the best molecular builders, for example, tetravalent titanic carbon forms the basis of lives. In the same way, the Titanium chemistry indicates that tetravalent Titanium makes it possible for the chemist to synthesize all kinds of titanic acid ester as coupling agent. They not only bring good coupling effect to different filler or polymer system, but also manifest other functions.

The molecular of titanate coupling agent can be divided into six functional zones, and each functional zone has its own effect in the coupling mechanism. The six mechanism zones are demonstrated as following figure:

Function zone a (RO)m--------Group to couple between inorganic substance and Titanium.

Titanate coupling agent uses its alkoxy group to directly couple with a small amount of hydroxyl or carboxyl (through chemical action) absorbed on the surface of filler or pigment.

Owing to the difference of functional zone a, different type of titanate coupling agent have been developed. Each type of titanate coupling agent has its own selectivity towards the moisture content of filler's surface.

The character of different types:

1. Monoalkoxy titanate coupling agent

Monoalkoxy titanate coupling agent causes chemical combination in the interface between inorganic powder and matrix resin. The unique property of this type of agent is that it can form monomolecular film on the surface of inorganic powder. So there is no multimolecular film on the surface of inorganic powder.

Because it still has the chemical structure of titanic acid ester, the excess coupling agent may cause the change of surface energy and great drop of viscosity. Then coupling agent can be coupled in the phase of matrix resin due to the 3 functional radicals of the coupling agent and transesterification reaction. So this is easy for the change-type of titanate molecule and selecting titanic acid ester and filling polymer system. This kind of coupling agent (except pyrophosphoric type coupling agent) is especially suitable for dry filler system (e.g. CaCO3, hydrated alumina) which only contains chemical/physical bonded water and doesn't contain free water. The mechanism of coupling is: the typical is TTS.

2. Mono-alkoxy pyrophophate coupling agent This kind of coupling agent is suitable for the filler system of high moisture content such as clay, talc and etc. when coupling with this kind system, the monoalkoxy can react with the hydroxyl radical on the surface of filler, the pyrophospboric radical can decompose into phosphate radical, combined with part of water.

3. Coordination type coupling agent This kind of coupling agent can avoid side-reaction between titanic acid ester and some system. (e.g. coupling agent reacts with hydroxyl in epoxy resin; reacts with polyalcohol or isocyanester in polyurethane.) And this kind of coupling agent is suit for many kinds of filler system and it may bring good coupling effect. The coupling mechanism of this kind of agent is like that of monoalkoxy type coupling agent.

4. Chelate type coupling agent This kind of agent is suit for filler of high moisture content or water containing polymer system such as wet SiO2, clay, talc, Al2(SiO3)3, water treated glass fiber, lamp black and etc. In such systems, chelate type coupling agent will behave well for it is in the situation of hydrolytic stability.

Functional zone b ----(---O-----)------it has the function of coupling and transesterification

This functional zone can cause the transesterification reaction with polymers which have carboxyl, or esterification reaction with carboxyl in epoxy resin, to make the coupling of three materials (filler, titanate acid ester and polymer).

The activity of transesterification reaction is controled by following factors:

  1. The structure of coupling part between titanic acid and molecule inorganic substance ;
  2. The chemical structure of OX radical in Functional zone c;
  3. The structure of organic polymer
  4. The chemical properties of other assistants (e.g. plasticizer)

When titanate is applied into thermoplastic polymer (e.g. polyolefin system), there is no transesterification reaction. But in polyesters, epoxy resins or polyvinyl chloride which contains plasticizer, the transesterification reaction has great influence. The high activity of transesterification reaction will cause the bad consequence. For example, when KR-9S (a titanate of high activity of transesterification) is added into polymers, there will comes transesterification reaction immediately and the infantine viscosity of the system will hoik. So the filler content will drop by a large margins. But when KR-12 (a titanate of low activity of transesterification) is added into polymers, there will be no effect of infantine viscosity. The transesterification will proceed along with the time. This not only causes good infantine viscosity, but also improves the filler content by a large margin.

In coating industry, we can use the titanate's transesterification mechanism to cross-link and solidify saturated polyester and alcohol acid resin. Then we can get a kind of material which doesn't become yellow color for there are no unsaturated structure in the material. Because the transesterification can bring thixotropy to the materials, the KR-9S

(relatively high activity of transesterification) has the effect of thixotropy and TTS has some ability of transesterification.

Function zone c  - OX

Radical linking with Ti atom The different OX radical may have the different effect on the coupling agent's property. For example, carbonyl may improve the solubility with semi-polar material; sulfonic group has thixotropy; sulphone radical may improve activity of transesterification; phosphate radical may improve flame-retarded property (or softening property of PVC); pyrophophato radical can absorb moisture and improve impact strength of hard PVC; phosphite ester radical can improve the antioxidation and reduce the viscosity of polyesters or epoxy resin.

Function zone d - R

Long-chain entanglement radical of the thermoplastic polymer and the organic framework of the titanate molecule.

A large number of long-chain carbon atoms improves the solubility with high polymer system and causes the change of surface energy in the interface of inorganic substance. And it has the functions of toughness and stress transferring so as to cause the reaction of the self-lubrication and largely reduce the viscosity of the system. It can also improve the manufacturing process and the elongation and tearing strength and impact property of the products. And if the R radical is aromatic radical, the solubility between aromatic polymers and titanate will be improved.

Function zone e - Y

Reaction radical of thermosetting polymers. If this kind of radical is linked with the organic framework of titanium, it will connect coupling agent with organic substance through chemical reactions. For example, double bond can cross link and solidify with unsaturated material and amino radical can cross link with epoxy resin.

Function zone f - n

It represents the titanate's degree of functionality (n may be from 1 to 3 and can be adjusted if necessary). So it can produce different effect to organic substance. In this aspect, it has more flexility than coupling agent which has three alkoxy radicals and one functional zone (e.g. silane).

From the description of above six functional zones, we can draw the conclusion that the tiatnate coupling agent is of good flexibility and multi-function. The titanate itself is coupling agent and it may be used as dispersant, wetter, bond, cross-linking agent and catalyst. And it even has the functions of rust prevention, anti-oxidation and flame-retarded. So it is better than other coupling agents and has wide range of application.