Liquid DGEBA Resins

Liquid crystal thermoplastics, 10 8 Liquid dessicants, 8 365-366 Liquid DGEBA resins, 10 460 Liquid diffusion, 9 109, 110-112 Liquid discharge treatment, ion-exchange, 14 422... 

A majority of the world s epoxy resin market consists of the DGEBA type. The liquid DGEBA resins are typically used where low viscosity, high reactivity, and high crosslink density are required. The low viscosity allows them to be conveniently and easily compounded with fdlers and other additives. The relatively high functionality allows them to... 

DADS melts at 135°C and is employed stoichiometrically with DGEBA at 33.5 pph. Fortunately, it is relatively unreactive so it can be mixed with epoxy resin at elevated temperatures. It can also be used in epoxy solutions to provide an adhesive formulation for manufacturing supported or unsupported film with long shelf life. Because of the low reactivity of the system, DADS is generally employed at a concentration that is about 10 percent greater than stoichiometry, or an accelerator, such as BF3-MEA, is employed at about 0.5 to 2 pph. When DADS is mixed with liquid DGEBA resin, it provides a pot life of 3 h at 100°C and requires a rather extended high-temperature cure to achieve optimal physical properties. 

Dicyandiamide has an activation temperature of about 177°C. However, this can be reduced to about 120°C by the use of accelerators. When mixed into liquid DGEBA resins, dicyandiamide has very good shelf life due to its latency. The shelf life of unaccelerated systems is about 6 months at room temperature or longer if refrigerated. The shelf life is reduced significantly with the addition of accelerators. 

Dynamic mechanical properties of liquid DGEBA resin cured with Hf-PA... 

Data from U.S. International Trade Commission, Synthetic Organic Chemicals. Data include modified and unmodified epoxy resins. Modified epoxy resins include solid epoxy resin (SER), vinyl ester resins, epoxy acrylates, etc. There appear to be some discrepancies in epoxy resin production and market data as reported by different publications and organizations (7). This is primarily due to the fact that some epoxy resins such as liquid DGEBA resins and epoxy novolacs are used as raw materials to produce modified or advanced epoxy resins, which may be further converted to end-use products. Some pubhcations report only unmodified epoxies. 

The description of liquid DGEBA resins presented so far is oversimplified. In reality, side reactions result in the formation of low levels of impurities that both decrease the epoxide content from the theoretical amount of 2 per molecule and affect the resins properties, both before and after curing (22). The five common side reactions are as follows ... 

In a typical advancement process, bisphenol A and a liquid DGEBA resin (175-185 EEW) are heated to ca 150-190°C in the presence of a catalyst and reacted (ie, advanced) to form a high MW resin. The oligomerization is exothermic and proceeds rapidly to near completion. The exotherm temperatures are dependent upon the targeted EEW and the reaction mass. In the cases of higher MW resins such as type 7 and higher, exotherm temperatures of >200°C are routinely encountered. 

Table 18. Typical Properties, Chemical Resistance, and Thermal Degradation of Liquid DGEBA Resin (185 EEW) Cured With Common Hardeners (Dow Chemical Data)...

Aromatic hydrocarbons, such as toluene or xylene, significantly reduce the viscosity of liquid DGEBA resins, but their use can be accompanied by a 15-25% decrease in compressive yield strength and a 10-20% reduction in compressive modulus (Fig. 11). If the solvent is trapped in the cured system, solvent resistance is reduced and cracks develop if the resin is used in heat-cured castings. The use of solvents and reactive diluents in epoxy systems is reviewed in References 192 and 193. 

The cycloaliphatic epoxy resins are characterized by the saturated ring in their chemical structure. They are almost water-white, very low-viscosity liquids. They provide excellent electrical properties such as low dissipation factor and good arc-track resistance, good weathering, and high heat distortion temperature. They are also free of hydrolyzable chlorine, sometimes present in DGEBA resins, which adversely affects certain electronic applications. 

With liquid DGEBA epoxy resins, DETA is normally used at the stoichiometric concentration of 10 to 11 parts per hundred (pph), and TETA is used at a concentration of 14 pph. However, both curing agents can be used at mix ratios as low as 70 to 75 percent of stoichiometry for greater toughness and increased pot life at the sacrifice of heat and chemical resistance. The effect of the mix ratio of DETA and TETA on the heat deflection temperature of castings is shown in Fig. 5.3. 

DGEBA liquid epoxy resin alone (e.g., EPON 828, Resolution Performance Products, LLC) 44-49... 

Nadic methyl anhydride (NMA) is the most versatile of all the anhydrides. NMA is a liquid of viscosity about 200 cP at room temperature, and it is readily soluble in epoxy resins. The mix ratio is 60 to 90 pph when used with a liquid DGEBA epoxy resin. At 60 pph and with no catalyst, the working life is about 2 months and with the incorporation of 0.5 pph DMP-30 as an accelerator, the working life reduces to 4 to 5 days. 

Generally, when used as a sole catalyst, tertiary amines are used only in specialty applications where short pot life can be tolerated and where maximum physical or chemical properties are not required. DMP-10 and DMP-30 are used at concentrations of 4 to lOpph with liquid DGEBA epoxy resins. They achieve fairly fast cures overnight, even at room temperatures since the hydroxyl groups present in the epoxy molecule enhance the catalytic activity of the tertiary amine groups. 

Tertiary amine salts of DMP-30 provide extended room temperature pot life (6 to 10 h at 20°C) when used at concentrations of 10 to 14 pph in liquid DGEBA epoxy resins. They cure at moderately elevated temperatures (4 to 8 h at 60°C), or even at room temperature with a heat bump. The acid moiety blocks the tertiary amine centers and deactivates them. The salt then dissociates on heating, freeing the amine groups, which are then able to react with the epoxy group. 

FIGURE 6.3 Effect of various diluents on the viscosity of a standard DGEBA liquid epoxy resin.6... 

There are also several reactive diluents that do not contain epoxy groups. These are represented by triphenyl phosphite and y-butyrolactone (Fig. 6.7). The phosphite is a low-viscosity colorless liquid, which is sensitive to moisture. It reacts with hydroxyl groups in the resin. The y-butyrolactone is a very effective viscosity reducer. It can reduce the viscosity of a liquid DGEBA from about 15,000 to 2000 cP with only 10 pph. In the curing reaction with amines (Fig. 6.8), the lactone forms an amide, which can then crosslink with the polymer via the hydroxyl groups. 

The epoxy-PVC plastisol type is a mixture of a plastisol-grade PVC powder, primary PVC plasticizers (e.g., dioctyl phthalate), a liquid DGEBA epoxy resin, thickeners, stabilizers, surfactants, and other additives. The epoxy serves as a secondary plasticizer, acts as a stabilizer (acid scavenger), and helps to fortify the plastisol by crosslinking during cure. 

As with amidoamine and polyamide cured adhesives, epoxy resins cured with aliphatic amines exhibit tensile shear strength that is dependent on the type of filler and concentration. Table 11.10 shows the effect of filler loading on strength of a simple general-purpose, room temperature curing epoxy adhesive composed of liquid DGEBA epoxy mixed with 10 pph of a tertiary amine. 

Probably the most effective accelerator for dicyandiamide systems is the substituted ureas because of their synergistic contribution to the performance properties of the adhesive and their exceptionally good latency. It has been shown that adding 10 pph of a substituted urea to 10 pph of dicyandiamide will produce an adhesive system for liquid DGEBA epoxy resins that can cure in only 90 min at 110°C. Yet this adhesive will exhibit a shelf life of 3 to 6 weeks at room temperature. Cures can be achieved at temperature even down to 85°C if longer cure times are acceptable.10... 

A general-purpose two-component adhesive that will provide high tensile shear strength up to 150°C is described in Table 12.11. The base epoxy resin in this formulation is a mixture of an epoxy novolac and a liquid DGEBA epoxy resin. 

Hycar CTBN-Bisphenol A-Epon 828 Cast Samples. A standard recipe follows 100 parts DGEBA resins (Epon 828), 24 parts bisphenol A, five parts liquid rubber (CTBN), and five parts piperidene. It is often desirable to have a stock mixture of bisphenol A and Epon 828. Since the shelf stability of the mixture is poor at room temperature, it should be refrigerated. Melt the bisphenol A in Epon 828 with stirring... 

Ryan and Kamal (1976) discuss a combined chemoviscosity model as described in Equation (5.31) for an unfilled liquid epoxy resin (DGEBA/MPDA and Fiberite industrial resin), namely... 

The DC properties of anti-static epoxy GFR pipes The use of glass-fibre reinforced (GFR) epoxy resin pipes is, especially in tankers, hampered by the bad electrostatic properties of these pipes. The possibility to decrease the volume resistivity to an acceptable level using only a small amount of carbon black (see 5.3.2) resulted in the development of the WAVIMAR anti-static GFR pipe system by Wavin BV. This pipe system is based on a liquid DGEBA/MDA (100/27) and cured for two hours at 120°C, containing about 1.5 %wt. (on the resin phase) of Ketjen black EC-2000 carbon black. The specific volume resistivity of a sample of such a pipe was measured as a function of the direction, the field strength and the pipe wall thickness. 

---