Curing humidity epoxy resin

The most widely used epoxy systems are those which are based on pure epoxy resins, hardened with a curing agent. Curing of epoxy resins containing two epoxy groups per molecule can be readily accomplished by the addition of primary polyamines, such as ethylene diamine, diethylene triamine, triethylene tetramine, tetra-ethylene pentamine, etc. Aliphatic polyamines produce cured resins with the greatest chemical resistance. However, these systems have inadequate durability, weather resistance and film-forming properties. They are sensitive to humidity, errors in addition rates are quite possible, and the catalysts are relatively toxic. 

The main disadvantages of the use of poly amines are (1) coating systems blush or sweat under conditions of high humidity, (2) polyamines are skin sensitizers and need special handling and (3) they need precise mixing because small amounts of polyamines are used for the curing of epoxy resins. 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

Hydroxy-terminated polybutadiene (HTPB) is considered to be the best binder for obtaining high combustion performance, superior elongation properties at low temperatures, and superior mechanical strength properties at high temperatures. This combination of properties is difficult to achieve in double-base propellants. HTPB is characterized by terminal -OH groups on a butadiene polymer. The other type of butadiene polymer used is carboxy-terminated polybutadiene (CTPB), which is cured with an imine or an epoxy resin. It should be noted that CTPB is somewhat sensitive to humidity, which has an adverse effect on its ageing charac-... 

Figure 3. Effect of EME 58 (58 wt% mercaptoester units co-polymer) coupling agent concentration on the peel strength of flexible epoxy (amine-cured)/AD = acetone-degreased steel test panels following (a) I day and (b) 3 day exposure to 57°C condensing humidity. See Appendix 4 for epoxy resin and cure description.

Similar to epoxy novolac resins, tetraglycidyl ether of tetraphenolethane resin is usually employed in a blend with lower-MW liquid resins. The high aromatic ring content combined with polyfunctionality provides increased thermal stability for high-temperature applications. The chemical resistance and humidity resistance of cured tetraglycidyl ether of tetraphenolethane epoxy resins are also outstanding. 

Tetraglycidyl ether of tetraphenolethane is an epoxy resin that is noted for high-temperature and high-humidity resistance. It has a functionality of 3.5 and thus exhibits a very dense crosslink structure. It is useful in the preparation of high-temperature adhesives. The resin is commercially available as a solid (e.g., EPON Resin 1031, Resolution Performance Polymers). It can be crosslinked with an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur. 

The tertiary amine salts are claimed to provide epoxy formulations with very good adhesion to metal. The cured resins also show a hydrophobic effect when in contact with water or at high humidities. The strength, toughness, and elongation (4.7 percent) of the cured epoxy resin are very good. However, heat distortion temperature is only in the range of 70 to 80°C, and chemical resistance is relatively poor for an epoxy. The physical properties fall off rapidly with any rise in temperature. 

As a family of curing agents for epoxy resins, the amidoamines are lower in viscosity than the polyamides. They exhibit very good adhesive properties due to their chemical structure and easy penetration. Amidoamine cured epoxy adhesives have shown very good properties on concrete and other porous substrates. They cure extremely well under humid conditions. In fact amidoamine cured epoxy formulations have been used to cure underwater in certain applications. A typical general-purpose room temperature curing epoxy-amidoamine system is described in Table 11.7. This adhesive is used as a general-purpose metal-to-metal adhesive and body solder in the automotive industry. 

Acetylene terminated (AT) resins are being studied as a new technology to form the matrix of high performance structural composites (1). This class of resin cures by an addition reaction mechanism. Because of the absence of volatile by-product during cure, these resins are easy to process to yield void-free components. These resins also have the advantage over epoxy resins in terms of resistance to deterioration of properties at elevated temperature due to exposure to humid environments (2). 

Note Cure schedule, gelled at 25 C plus 3 h at 120 °C Test conditions, 23,6 °C, 50% relative humidity, conditioned 96 h Epoxy resin =190 eew. 

A filled epoxy resin used as a structural adhesive and based on the diglycidyl ether of bisphenol A cured with dicyandiamide was subjected, in its bulk form, to ageing at 40, 55 and 70C and about 100% relative humidity. Gravimetric viscoelastomeric and FTIR studies were effected after various times of exposure to monitor water ingress and to link variations in mechanical behaviour with chemical modifications occurring in the polymer. 34 refs. 

External parameters such as relative humidity also play a very important role in void content in the laminate. Generally, epoxy resin tends to absorb moisture in the air and the trapped moisture is very difficult to remove under VBO processing. Influence of relative humidity on void content of VBO processed laminate has been systematically studied by Nutt et alJ Cure cycle recommendations from the manufacturer for the curing of Cycom 5320 are ... 

Ketimines. Ketimines are the reaction products of ketones and primary aliphatic amines. In the absence of reactive hydrogens, they do not react with epoxy resins. They can be considered blocked amines or latent hardeners, since they are readily hydrolyzed to regenerate the amines. They have low viscosity, cure rapidly when exposed to atmospheric humidity, and are usefiil in high solids coatings. Similar products have been obtained with acrylonitrile. 

Adhesion studies Rattana and co-workers [231] and Watts and co-workers [232] have studied the adhesion of aluminium and epoxy resins nsing amine and organosilane primers. Leadly and co-workers [261] give details of coating delamination in a hot, humid environment for a cationic radiation-cured coating of cycloaliphatic epoxy resin. XPS and ToF mass spectroscopy of the delaminated snrface showed that the phosphorus hexafluoride anion of the photoinitiator segregates to the interface. The durability of the coating was improved by reformulation with a rednced concentration of photoinitiator. 

Thus, for using chemically curing adhesives only a few systems are applicable such as two-part systems consisting of a resin and a hardener, for example, epoxy resins. Another appropriate approach is the application of, for example, urethanes and silicones that cure by humidity and do not need complex hardware. However, the bi est disadvantage of using these systems is the expanded necessary curing time. 

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