Organoclays quaternary ammonium cations

Fig. 2. Diagram showing the intercalation of compact quaternary ammonium cations, such as trimethylphenylammonium (TMPA) into different smectites, giving rise to type I organoclays with a basal spacing of about 1.5 nm. SWa is a high-charge nontronite (iron-rich smectite) and SAz is a high-charge montmorillonite, while SAC is a low-charge montmorillonite. After Jaynes and Boyd (1991b).

Fig. 12. Scenario for the in-situ modification of subsoils or aquifers for pollution control. The organoclay formed by injecting a solution of long-chain quaternary ammonium cations (e.g. HDTMA) acts as a sorptive zone where organic contaminants dissolved in a plume from buried waste can be immobilised and degraded. After Xu et al. (1997).

In-situ PolYmerization of Otefns with Coordination Catalysts Supported on Clays 165 Table 3.1 Quaternary ammonium cations frequently used to prepare organoclays. 

Organoclays are the most widely used inorganic thickeners in the paint and coating industry. They are derived by modification of naturally occurring laminar silicates (clays) such as hectorite and bentonite. They have replaceable cations (Na Ca" or Lk) on the surface of the crystal that are replaced, through an ion exchange reaction, by organic quaternary ammonium cations such as [dimethyldioctadecylammo-nium] or [dimethylbenzyloctadecylammonium]. Occasionally a blend of quaternary amines is used to tailor the properties of the product. 

A typical polymer-modified organoclay for the preparation of PS nanocomposites could be a PS polymer with some cationic sites to be attached to the clay surface. Polystyryl quaternary ammonium salts (PSQAS) were synthesized via three different polymerization routes anionic polymerization (AP), free radical co-polymerization (CP), and chloromethy-lation of polystyrene (CM) [67] (Table 3.6). TGA results indicated that CP-PSQAS and CM-PSQAS were thermally stable, because the C-C bonds in aromatics are much stronger than those in aliphatic compounds. The polystyryl-modified clays exhibited intercalated or partially exfoliated structures. 

A commercially available organoclay based on a layered silicate (montmorillonite modified by dimethyldistearylammonium cation exchange) was used. The content of the quaternary ammonium compound was 38 wt%. Octadecylamine-modified montmorillonite is used to study the thermal stability of organoclay. 

Heat stability has been a major concern for organoclays when they are used in plastics, because of processing temperature and residence time. Traditional organoclays with quaternary ammoniums such as dimethyldistearylammonium as exchange cations were considered, with heat stability only up to 200 °C [43]. Even with special selection of chemicals and refined processing, the heat stability can only be improved to 240 °C. This is mainly because of the Hofman elimination associated with the quaternary ammonium. Increasing the heat stability has been a hot topic with emphasis on the use of quaternary phosphonium chemistry. 

Quaternary alkylammonium salts or alkylamines are the cationic surfactants most commonly used in the modification of layered silicates. They are synthesized by complete alkylation of ammonia or amines. Many efforts have been made to produce ammonium surfactants to improve the affinity between the clay mineral and the polymer [3]. Ammonium organoclays undergo thermal degradation at temperatures below or comparable to the melt-processing temperatures of many polymers. 

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