Soap/sulfur systems

Resin-curing systems Soap/sulfur systems Urethane crosslinking agents Bisphenol curing agents... 

One would expect that soap/sulfur, or sulfur donor or triazine cure systems may be used because of the presence of the chlorine cure site, however, the shelf stability is only fair and the cure rates are slow. The physical properties, heat aging and compression set obtained are good with the soap/sulfur combinalion but better properties may be obtained with other curatives such as NPC, hence these are not recommended. Diak 3 gives properties similar to those of the soap/sulfur system, but has the added disadvantage of a poor compression set and a higher hardness. 

Because of the different vulcanization chemistry involved in each commercial ACM, a vulcanization system specific to the cure site present has to be adopted. Many cure systems for labile chlorine containing ACM have been proposed (45). Among these the alkali metal carboxylate—sulfur cure system, or soap—sulfur as it is called in the United States, became the mainstay of acryflc elastomer technology in the early 1960s (46), and continues to be widely used. 

The original, and currently extensively used, cure-site group is chlorine based. A number of cure systems may be used, but the most common is the soap-sulfur one using sodium and potassium stearate with sulfur or a sulfur donor. Very good physical properties and heat resistance may be obtained with the chlorine cure sites, however compression set resistance is only fair. 

Naugard 445 is also the recommended antioxidant for imparting the best high temperature resistance in ACM. Various antioxidant synergists were compared to supplement the performance of Naugard 445. They included a phosphite (Naugard P), a toluimidazole (ZMTI), a thioester, and sodium ethyUiexyl sulfate in either a trithiocyanurate or a soap/sulfur cure system. 

It is convenient to classify sols into three types (I) tvophilii (solvent loving) colloids, for example, are solutions or gelatin or starch in water (2 association colloids, of which a solution of Soap in water at moderate concentration is an example and (3) Iwphohic (solvent repelling) colloids, for example, sulfur in water. Both lyophilic and association colloids can be prepared in thermodynamic equilibrium, so that when solvent is removed and then returned to the system, the original properties of the system are regained. 

The ionizable carboxyl groups in the resin ionize completely in alkaline solution to stabilize the pitch dispersion. However, in the neutral conditions in paper mills, the multivalent metal ions, such as calcium or magnesium, when inadequately washed from the pulp, precipitate the resin and fatty acids as soaps that are difficult to dissolve and remove. Within the normal pH range in the paper mill system, aluminum soaps are charged positively and partly stabilize the dispersion and partly enable the soap particles to be drawn to the negatively charged fiber and thus removed from the system with the paper. Care is needed with the addition of alum, which decreases the pH value and increases corrosion, lowers the strength of the paper, and affects other paper properties. Nevertheless acid conditions of pH 4.5-5.5 can considerably reduce the pitch problem encountered at pH 6-8 and, for that purpose, sulfuric acid alone has been added. 

Differences in the property profile of commercially available S-grades are caused by different combinations of sulfur level, comonomers, soap system, polymerization and peptization reactions, and staining or nonstaining stabilizers. 

Vulcanization accelerator systems, based on rub-ber-sulfur-organic accelerator, need activators, e.g., ZnO and ->stearic acid or Zn-stearate (- metallic soaps). - Fatty acids also improve processability. They may be already contained in synthetic rubbers, where their salts are used as - polymerization additive similar to the use of the salts of dis-proportionated rosin acid (- rosin), > ch impart improved tackiness to the rubber compound. Another example of product combination is the coating of fillers with - stearic acid to improve distribution. 

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