Synthesis of Epoxy Resins

An outline of the general method of synthesis of epoxy resin and novolac epoxy resin are shown in Schemes 4.9 and 4.10 respectively. 

The ratio of the main ingredients used in the synthesis of epoxy resins (epichlorohydrin bisphenol A) determines the extent of the reaction and the molecular weight (or value of n repeating units in the molecular chain). The addition of bisphenol A to the reaction mix will advance the molecular weight of the resin and the value of n. As n increases, the viscosity or melting point of the resin also increases. Also as the value of n increases, the number of hydroxyl groups increases while the number of epoxy groups remains constant. 

Polymerization of epoxy monomers, requiring probably the simultaneous participation of the phenolic hydroxy group, and the production of polyurethanes are the main applications of the abovementioned catalysts. In particular, their role in the trimerization of the i.scx yanate group " appears quite relevant and several investigations of the synthesis of epoxy resins-- " - and polyurethanes- arc reported. A different type of catalyst is constituted by acetophenone-derived p-aminoketones employed with photocurable resins. -- ... 

Similarly, in the preparation of low molecular weight analogue, bis-phenol F, preferential reaction to form para-para links is achieved using acid catalysis. Bis-phenol F is an important intermediate in the synthesis of epoxy resins (see Section IV.B). 

Epichlorhydrin which is used for the synthesis of epoxy resins is a suspected carcinogen in animals. Thanks to the well developed manufacturing processes, practically all the current commercially available epoxy resins contain only the smallest traces of epichlorhydrin (Occupational Hygiene Advices for Manufacturing and Processing Plastics of CIBA, Company brochure). 

Thermal synthesis of methylene bridged, substituted phenols in the absence of a catalyst occurs in a melt with paraformaldehyde at temperatures between 150-200°C [33]. A representative reaction is 4,4 -(l-methyl ethylidene)bisphenol reacting with paraformaldehyde at 200°C to form novolacs with the structure drawn in equation 12 [33]. The acidity of the phenolic group is probably sufficient to catalyze the formation of these special novolacs, all of which have a yellow color. These products are prepolymers for the synthesis of epoxy resins [63]. 

A variety of poly/dihydric oils are used for the preparation of glyddyl ether-type epoxy resins. These include bisphenols, namely bisphenol-A (BPA), bisphenol-F (BPF), bisphenol-S (BPS) and bisphenol-H (BPH) and so on. Other aromatic diols and polyols such as phenolic resin, MF resins and hyperbranched polyol may also be used in the preparation of vegetable oil-based epoxy resins. Bisphenol-A (2,2-bis(4-hydroxyphenyl)propane) is one of the most widely used aromatic diols for the synthesis of epoxy resin. The resins are commonly used as lacquers for coated metal products such as food cans, bottle tops and water pipes. There are also reports on the use of bisphenol-S (BPS) (bis(4-hydroxyphenyl) sulphone), in the synthesis of epoxy resin. The advantages of resistance to deformation by heat and improvement of thermal stability were observed for such epoxy resins. The presence of sulphone group (BPS-based epoxy resin) in the epoxy resin exhibits better gel time than BPA-based epoxy. Another important diol, namely bis(4-hydroxydiphenyl)methane or bisphenol-F (BPF) is used for the synthesis of low viscosity epoxy resins. BPF generally comprises several isomers such as bis(2-hydroxylphenyl)methane (i.e. ortho-ortho isomer), bis(4-hydroxylphenyl)methane (i.e. para-para isomer) and... 

Figure 4.21 Synthesis of epoxy resins from resin acid dimer adduct with aciyUc acid.

The synthesis of epoxy resins involves an addition process and usually involves the reaction of a primary aromatic or aliphatic amine with a di- or higher function epoxy compound. The initial reaction will create a linear molecule that contains secondary amine functions. These secondary amine functions are less reactive than the primary amines and the result of the initial reaction is to grow... 

Bisphenol A is used with epichlorhydrin for the synthesis of epoxy resins bisphenol-A type. It leads to bisphenol-A diglycidyl ether, which is the monomer of bisphenol-A based epoxy resins. Reports of bisphenol-A sensitization, particularly in workers at epoxy resin plants, are controversial. Bisphenol-A was also reported as an allergen in fiberglass, semisynthetic waxes, footwear and dental materials. 

Peltonen [159] applied TGA followed by GC and MS to the determination of volatile compounds arising from epoxy powder paints during the curing process carried out at 200 "C. The volatile compounds may be residnes from the synthesis of epoxy resin or they can be constituents of the paint. Two types of epoxy powder paint were involved in this study. They differ from each other in the form of hardener used. 

Epichlorohydrin assumes difunctionality in the synthesis of epoxy resins due to its readiness to dehydrohalogenate with alkaline reagents. 

Figure 7.15 Synthesis of epoxy resins from bisphenols and epichlorohydrin (ECH).

The literature on the synthesis of epoxy resins is extensive, and several good sources are available [11,48]. Using Preparation 2-1, additional epoxy resins have been prepared as shown in Tables II and III. 

Three variations on the synthesis of epoxy resins from bisphenol-A and ECH exist to date, differing in the ratio between these monomers and the viscosity of the produd resins, as determined by the removal of the sodium chloride formed. 

Tertiary amines like benzyldimethylamine, pyridine, and imidazole have been widely used as a base to initiate the anionic polymerization of PGE and its derivatives as well as for the synthesis of epoxy resins of diglycidyl ether of bisphenol A (DGEBA). Even if initiation occurs with amine alone, the introduction of an alcohol is a common procedure to suppress the observed induction period and increase the polymerization rate. Two initiation mechanisms have been proposed (Scheme 18) (1) direct nucleophilic attack of the amine onto the cyclic monomer to yield the zwitterion (a) and (2) formation of alkoxide (b) via proton transfer in the presence of alcohol. Fast exchange between dormant alcohol and active alkoxide allows chain growth from both initial amine (a) and alcohol (b). Poly(PGE) oligomers whose degree of polymerization does not exceed 5 are obtained. The presence of terminal double bonds indicates significant transfer to the monomer via... 

Figure 3 Synthesis of epoxy resins by addition of l-chloro-2,3-epoxypropane 7 to bisphenol-A 8 providing intermediate compound 9. Subsequent dehydrochlorination gives a mixture (e.g. Shell Epon 828) of DGEBA 10, low molecular weight oligomers 11, and chlorine-containing species 12 and 13. Cycloaliphatic epoxy resins 15 and 17, such as Degussa FI26 , are prepared by oxidation of unsaturated compounds 14 and 16 by means...

Allylic chlorinations are important in industry because chlorine is relatively cheap. For example, 3-chloropropene (allyl chloride) is made commercially by the gas-phase chlorination of propene at 400°C. It is a building block for the synthesis of epoxy resin and many other useful substances. 

---