Taffy process

Tafel equation Tafel kinetics Tafel slope Taffy process Taft s SV function Tagamet [51481-61-9] d-Tagatose... 

Polyfunctional aliphatic resins have exhibited high reactivity and degrees of cure with amines but problems of toxicity have diminished their usehilness and commercial interest. SoHd epoxy resins can be prepared by the taffy process or the advancement process. 

Taffy Process. Bisphenol A reacts direcdy with epichlorohydrin in the presence of a stoichiometric amount of caustic. The molecular weight of the product is governed by the ratio of epichlorohydrin—bisphenol A. 

In practice, the taffy process is generally employed for only medium molecular-weight resins (1) (n = 1-4). The polymerization reaction results in a highly viscous product (emulsion of water and resin) and the condensation reaction becomes dependent on agitation. At the completion of the reaction, the heterogeneous mixture consists of an alkaline brine solution and a water—resin emulsion and recovery of the product is accompHshed by separation of phases, washing of the taffy resin with water, and removal of water under vacuum. 

Gel-permeation chromatography studies of epoxy resins prepared by the taffy process shown n values = 0, 1, 2, 3, etc, whereas only even-numbered repeat units are observed for resins prepared by the advancement process. This is a consequence of adding a difunctional phenol to a diglycidyl ether derivative of a difunctional phenol in the polymer-forming step. 

SERs are prepared by two processes the taffy process and the advancement or fusion process. The first is directly from epichlorohydrin, bisphenol A, and a stoichiometric amount of NaOH. This process is very similar to the caustic coupling process used to prepare liquid epoxy resins. Lower epichlorohydrin to bisphenol A ratios are used to promote formation of high MW resins. The term taffy is derived from the appearance of the advanced epoxy resin prior to its separation from water and precipitated salts. 

In the taffy process, a calculated excess of epichlorohydrin governs the degree of polymerization. However, preparation of the higher molecular weight species is subject to practical limitations of handling and agitation of highly viscous materials. The effect of epichlorohydrin-bisphenol A (ECH-BPA) ratio for a series of solid resins is shown in Table 6. 

In commercial practice, the taffy method is used to prepare lower MW solid resins, ie, those with maximum EEW values of about 1000 (type 4 ). Upon completion of the polymerization, the mixture consists of an alkaline brine solution and a water-resin emulsion. The product is recovered by separating the phases, washing the taffy resin with water, and removing the water under vacuum. One disadvantage of the taffy process is the formation of insoluble polymers, which create handling and disposal problems. Only a few epoxy producers currently manufacture SERs using the taffy process. A detailed description of a taffy procedure follows (24). 

The difference between taffy-processed and fusion advancement solid resin can be noted in HPLC chromatograms. In the advancement process, the even-membered oligomers predominate, whereas taffy-produced resins exhibit both even- and odd-numbered ohgomers. Compounds that contribute to hydrolyzable chloride and a-glycol content can be quantified by HPLC. The presence of branched chain components is detectable in studies using an improved reversed-phase gradient HPLC method (92,93). Excellent reviews of applications of chromatographic techniques to the analysis of epoxy resins are available (94). 

Epoxy reins forms the second largest group of the end use of BPA (30% of the global production volume). The Bisphenol-A-based resins are the most common of commercial epoxies. They are produced by the two-step Taffy process shown in Figure 7.15. 

These resins are best prepared by the reaction of epichlorohydrin, bisphenol A, and a stoichiometric amount of sodium hydroxide. The process has been referred to as the taffy process. 

The so-called taffy process consists in the two-phase reaction of an alkaline solution of bisphenol A with ECH in stoichiometric excess [241]. The main reaction as described above is accompanied by side reactions, such as hydrolysis and alcoholysis of chlorine and epoxides in ECH. These reactions create molecules with functionality one or even zero, and must of course be minimized. 

Molar ratio of building blocks in product. Appbcable to Taffy process for the 1001 type. 

Epoxy resins make use of several different reactions. To make the simplest man-ber of the series, 2 mol of epichlorohydrin is reacted with 1 mol of bisphenol A at 60°C with flake NaOH added to neutralize the HCl that is formed. Actually, more than 2 mol of epichlorohydrin is charged, and the excess is removed after the reaction. Higher molecular-weight resins call for sturdy equipment to agitate the molten polymer. In the taffy process (Figure 17.2), epichlorohydrin and bisphenol A are... 

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