Coupling agents unsaturated acids

The introduction of a reactive functionality into the polymer, such as unsaturation, offers the potential to use organic acids as coupling agents, but mixed results have been reported. A number of potentially suitable unsaturated products exist, notably, maleic acid and anhydride, acrylic and methacrylic adds, and unsaturated versions of fatty acids such as oleic acid. The acidities and double-bond reactivities of these compounds widely vary depending on their structures, and this probably accounts for the marked differences in their performance. Compounds such as acrylic acid have both high addity and high double-bond reactivity due to the proximity of the carbonyl group to the double bond. The only commercial products specifically developed for use with filled polymer systems are from Lubrizol Advanced Materials (SOLPLUS). 

Table 6.1 Effects of an unsaturated acid coupling agent on the mechanical properties of calcium carbonate-filled unsaturated polyester resin [15] (filler level not given).

Table 6.2 A comparison of various unsaturated acids and three organofunctional silane coupling agents as surface modifiers in 60wt% ATH -filled EVA [16],...

Selected unsaturated carboxylic acids are particularly useful as coupling agents for calcium carbonate-type fillers. They interact strongly with the basic surface of the carbonate, unlike silanes that only show a weak interaction. In addition, unlike simple saturated carboxylic acids, which just have a beneficial effect on impact strength, unsaturated carboxylic acids may increase tensile strength. Table 6.5 illustrates this effect in a 60% filled CaCOs/PP homopolymer system. This system is heavily stabilized with both hindered phenol and phosphite-type antioxidants, and a small amount of peroxide was therefore added. 

Cyclic anhydrides, such as maleic and succinic anhydride, appear to readily react with filler surfaces. This reaction has not been studied in detail but appears to be acid formation through hydrolysis, followed by salt formation. Most interest has been in anhydrides carrying some unsaturation, such as maleic anhydride, that have the potential to act as coupling agents. Some of the work with maleic anhydride has already been mentioned [17,18]. Modeling shows that the area occupied at the filler surface is about 0.32 nm for the anhydride and 0.45 nm for the diadd resulting from hydrolysis [23]. It is not dear as to whether the reaction with the filler surface involves one or both acid groups. 

The surface energy of fibres is closely related to the hydrophility of the fibre. Some investigations are concerned with methods to decrease hydrophility. The modification of wood-cellulose fibres with stearic acid [49] causes those fibres to become hydrophobic and improves their dispersion in PR As can be observed in jute reinforced unsaturated polyester resin composites, treatment with polyvinylacetate increases the mechanical properties [50] and moisture repellence. Silane coupling agents may contribute hydrophilic properties to the interface, especially when amino-functional silanes, such as epoxies and urethane silane are used as primers for reactive polymers. The primer may supply much more amine functionality than can possibly react with the resin at the interphase. Those amines, which could not react, are hydrophilic and therefore responsible for the poor water resistance of the bonds. An effective way to use hydrophilic silanes is to blend them with hydrophobic silanes such as phenyltrimethoxysilane. Mixed siloxane primers also have an improved thermal stability, which is typical for aromatic silicones [48]. 

These are an interesting class of surface modifiers, now finding significant commercial uses and capable of being used as dispersants and coupling agents with a wide range of filler types. With these products, the unsaturated acid, or pre-cursor such as anhydride, is pre-reacted onto a suitable polymer backbone. Two main classes of product are available, one with a saturated hydrocarbon backbone, the other with an unsaturated one. 

The use of such materials was prompted by the need to find coupling agents for calcium carbonate, as silanes were largely ineffective with this filler. Simple unsaturated acids also appeared to have little effect, but Hutchinson and Birchall found unsaturated polymeric acids to be excellent treatments for precipitated calcium carbonate [39]. The concept was developed and extended by Evans and co-workers [40, 41]. 

As mentioned earlier, these were the original coupling agents for use in polymer composites. Salts of trivalent chromium can be reacted with certain organic acids to produce cyclic complexes, such as shown in Fig. 22.12. If the acid carries a polymer reactive functionality, this can be incorporated into the complex. The reaction between chromium chloride and methacrylic acid is particularly suitable, resulting in a complex with reactive unsaturation. This complex can react with inclusion surfaces through the chromium functionality and with suitable polymers through the unsaturation. 

Two important widely used sulfonic acids are known as TwitcheU s reagents, or as in Russia, the Petrov catalysts. These reagents are based on benzene or naphthalene ( ) and (12), [3055-92-3] and [82415-39-2] respectively. The materials are typically made by the coupling of an unsaturated fatty acid with benzene or naphthalene in the presence of concentrated sulfuric acid (128). These sulfonic acids have been used extensively in the hydrolysis of fats and oils, such as beef tallow (129), coconut oil (130,131), fatty methyl esters (132), and various other fats and oils (133—135). TwitcheU reagents have also found use as acidic esterification catalysts (136) and dispersing agents (137). 

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