Phenolic cured epoxy reaction

Shechter and Wynstra (14) studied the uncatalyzed phenolic cured epoxy reaction and proposed the following mechanism,... 

The strong base catalyzed phenolic cured epoxy reaction proceeds via the phenoxide ion which reacts with epoxide, generating the alkoxide ion. The high basicity of the alkoxide ion reacts with phenolic to regenerate the phenoxide ion and the reaction repeats itself. 

The nitrogen-containing Lewis base benzyldimethylamine was found to be a better catalyst than potassium hydroxide. Furthermore, evidence shows that the quaternary compound of benzyldimethylamine was even a better catalyst (14). The phenolic cured epoxy reaction is a first order kinetics. The following Lewis bases have been used as catalysts for the epoxy-phenolic reaction (15) ... 

There is considerable evidence for the push-pull concerted effect in the reaction mechanism by using lead salts and DMP-30 salts as catalysts in the phenolic cured epoxy reaction. The complex formation is also supported by Alvey (13) and Son (23). 

Polymer systems are now available which may be cured by reaction of epoxy resin compounds with the phenolic hydroxyl groups. Such reactions do not evolve volatile by-products. These materials are showing promise in the area of heat-resisting electrical insulation laminates. 

For commercial application, diepoxides such as those derived from bisphenol A are employed, and they are cured via ring-opening crosslinking reactions, into which the epoxy group enters readily. Bisphenol A is so-called because it is formed from two moles of phenol and acetone (Reaction 1.7). 

Bisphenol F Resins. Diglycidyl ether resins based on bisphenol F (DGEBF) have been developed to provide cured epoxy resins with greater flexibility and lower softening temperatures than conventional DGEB A epoxy resins. The preparation of bisphenol F resins is from formaldehyde and phenol. Three isomers are possible because substitution can occur at the ortho-, meta-, or para- positions. The proportion of isomers depends upon the pH of the reaction medium. 

The use of Mannich ba.scs mixed with other monomers or oligomers as modifiers of phenolic," ureic, " epoxy, etc., resins is reported. For instance, crosslinked structures of type 425 may be envisaged for the rapid curing of hydrophilic resins " obtained by reaction of acetone/formaldchyde/resorcinol oligomers (moiety A) with aminophcnol/ formaldehyde oligomers (moiety B). 

In many cases the use of epoxy materials in so-called field conditions (for industrial, construction sites, etc.) demand an increase in the reaction velocity, which is usually achieved by adding accelerators. At present, the widely used accelerators include alkyl-substituted phenols, benzyl alcohol, carboxylic acids (in particular, salicylic acid), and others. A major disadvantage of these accelerators is their tendency to migrate from the cured epoxy matrix during the exploitation, which could lead to a change in the physical properties of the polymer. They also act as a plasticizer of epoxy-based polymers, and as a result reduce the polymer s chemical resistance. Thus, there is a need for new accelerator-modifiers that can provide faster curing of epoxy-amine compositions without negative side effects, and also improve the properties of the finished product. 

As previously mentioned, several commercial hybrid thermosets are known to be co-reacting thermosets, i.e. when the mixture of two different thermosetting monomers or prepolymers is cured, there is a simultaneous graft or co-reaction between the components along with the crosslinking reactions. These systems may therefore be considered as co-polymerizing thermosets and not as true blends. Examples of such systems are phenolic novolak/epoxy resin (or epoxy novolaks) ... 

Tris (dimethylamlnomethyl) phenol accelerator, epoxy cures Dlazabbydoundecene accelerator, epoxy-amine reactions... 

Other curatives, which react through addition mechanisms are phenolic resins, particularly if the hydroxyl/epoxy reaction is catalysed using a tertiary amine (usually accomplished at elevated temperatures), thiols, polysulphides and mercaptans (can be formulated to give very rapid cures), polyetheramines (relatively slow cures, which can be accelerated with nonyl phenol), polyamides (less reactive than their amine counterparts) and amidoamines (characterized as having very long pot lives). 

Competing ideas drawn from the literature on the catalyzed epoxy-phenolic curing mechanism are reviewed here and will be evaluated in subsequent chapters with regards to the results of experiments carried out in this study the most common catalysts employed are Lewis bases. Many reaction mechanisms have been suggested for the Lewis-base catalysis of the epo -phenolic reaction. " The basic reactions these authors attempted to describe are shown in Rxns 1 and 2. These models differ significantly in their description of which elementaiy molecular reactions occur to achieve the net reaction. In some of these models, the Lewis-base acts as a true catalyst in others, it first forms a complex with one or two of the reactants before the catalytic nature of the complex is activated. 

Phenolics or phenol-formaldehyde structural adhesives are chemically reactive systems that cure to form thermosets. In one-component systems, meltable powders (resols) are used as binders for particle board or as alloys (including nitrile-phenolics, vinyl - phenolics, and epoxy-phenolics), which are used in the structural bonding of metals. In two-component systems, the resin and catalyst are mixed and then heated to initiate curing. Both systems cure by a condensation reaction that produces a byproduct. 

In an analogous thermally cured system, high MW epoxy resins are cured by reaction of their hydroxyl groups with methylol or alkylated methylol groups of amino resins (Figure 2.56). Melamine formaldehyde and urea formaldehyde systems are most common. While chemical resistance of these systems is lower than those of phenolic cured systems, their lower curing temperature and better color compared to phenolic systems is taken advantage of when faster cure speed is important. 

Xylox Resin n Trade name for a family of heat-resistant thermosetting resins made by the condensation of aralkyl ethers and phenols, resulting in hydroxyphenylene-p-xylene prepolymers that can be cured to hard, intractable resins by reaction with hexamethylenetetramine or epoxy compounds. These thermosetting resins have the good qualities of phenolics and epoxies, with superior mechanical and electrical properties at elevated temperatures. 

The reaction is completed after 6—8 h at 95°C volatiles, water, and some free phenol are removed by vacuum stripping up to 140—170°C. For resins requiring phenol in only trace amounts, such as epoxy hardeners, steam distillation or steam stripping may be used. Both water and free phenol affect the cure and final resin properties, which are monitored in routine quaHty control testing by gc. OxaHc acid (1—2 parts per 100 parts phenol) does not require neutralization because it decomposes to CO, CO2, and water furthermore, it produces milder reactions and low color. Sulfuric and sulfonic acids are strong catalysts and require neutralization with lime 0.1 parts of sulfuric acid per 100 parts of phenol are used. A continuous process for novolak resin production has been described (31,32). An alternative process for making novolaks without acid catalysis has also been reported (33), which uses a... 

Other modifications of the polyamines include limited addition of alkylene oxide to yield the corresponding hydroxyalkyl derivatives (225) and cyanoethylation of DETA or TETA, usuaHy by reaction with acrylonitrile [107-13-1/, to give derivatives providing longer pot Hfe and better wetting of glass (226). Also included are ketimines, made by the reaction of EDA with acetone for example. These derivatives can also be hydrogenated, as in the case of the equimolar adducts of DETA and methyl isobutyl ketone [108-10-1] or methyl isoamyl ketone [110-12-3] (221 or used as is to provide moisture cure performance. Mannich bases prepared from a phenol, formaldehyde and a polyamine are also used, such as the hardener prepared from cresol, DETA, and formaldehyde (228). Other modifications of polyamines for use as epoxy hardeners include reaction with aldehydes (229), epoxidized fatty nitriles (230), aromatic monoisocyanates (231), or propylene sulfide [1072-43-1] (232). 

---