Epoxies hydrolytic stability

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Siloxane containing interpenetrating networks (IPN) have also been synthesized and some properties were reported 59,354 356>. However, they have not received much attention. Preparation and characterization of IPNs based on PDMS-polystyrene 354), PDMS-poly(methyl methacrylate) 354), polysiloxane-epoxy systems 355) and PDMS-polyurethane 356) were described. These materials all displayed two-phase morphologies, but only minor improvements were obtained over the physical and mechanical properties of the parent materials. This may be due to the difficulties encountered in controlling the structure and morphology of these IPN systems. Siloxane modified polyamide, polyester, polyolefin and various polyurethane based IPN materials are commercially available 59). Incorporation of siloxanes into these systems was reported to increase the hydrolytic stability, surface release, electrical properties of the base polymers and also to reduce the surface wear and friction due to the lubricating action of PDMS chains 59). 

Many papers have shown that the water absorption of a syntactic foam is proportional to that of its binder. Polyester syntactic foams, for example, absorb more water, even with dressing additives (silanes, vide infra) than do epoxy syntactic foams (Fig. 11)1. The hydrolytic stability of epoxy foams is increased when the glass microspheres are replaced by carbon ones (Table 19)40). 

Table 19. Hydrolytic stabilities of epoxy syntactic foams 40) (12% filler)...

The toughness of an epoxy-nitrile adhesive is nearly equivalent to that of an epoxynylon adhesive. However, the epoxy-nitrile system has much better hydrolytic stability. Also, the low-temperature properties of an epoxy-nitrile adhesive are superior to those of epoxy-nylon adhesive. Table 7.3 illustrates the effect of nitrile addition on tensile shear and peel strength. 

The rate of reversion, or hydrolytic instability, depends on the chemical structure of the base polymer, its degree of crosslinking, and the permeability of the adhesive or sealant. Certain chemical linkages such as ester, urethane, amide, and urea can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride cured epoxies. Generally, amine cured epoxies offer better hydrolytic stability than anhydride cured types. 

Since the reaction between an epoxy resin and an acid anhydride curing agent also produces an ester linkage, anhydride cured epoxies have poorer hydrolytic stability than do... 

Antipova and coworkers [193] increased the hydrolytic stability and adhesive strength of EPR by adding an epoxy resin, dicarboxylic acid anhydride and dialkyl tin dicarboxylate. The dialkyl tin dicarboxylate does not react with the double bonds in keto allyl groups but at radical sites, thus the replacement of labile hydrogen atoms by the two electronegative groups of organo tin ompounds results in an increase in EPR stability. 

Earlier in this chapter, it was indicated that the presence of halide ions (e.g., Cl and Br ) is undesirable because they tend to promote the corrosion of parts of the IC. Low-flammability requirements for these encapsulants are realized through the use of brominated epoxy resins in combination with antimony oxide (Sb203). For some time the brominated epoxies have been suspected as a possible source of Br ions. Since it is difficult to achieve low flammability for these encapsulants without the bromine-containing species, efforts have been made to arrive at bromine-containing flame retardants of greater hydrolytic stability so that the flame retardant does not inadvertently diminish the corrosion protection these encapsulants are expected to provide. 

In electronic encapsulation applications, epoxy derivatives of novolacs containing meta-bromo phenol have exhibited substantially better physical and performance properties compared to the conventional tetrabromo bisphenol-A epoxies and brominated epoxy novolacs. The meta-bromo phenol moiety contributes the expected improvements in thermal and hydrolytic stability to the formulation while providing fire retardancy properties (13). 

The hydroxyl terminated polybutadienes lead to PU with physico-mechanical properties significantly lower than those of PU based on polyether or polyester polyols. The nonpolar polymeric chain and the extraordinary hydrophobicity mean that hydroxy terminated polybutadienes are used for special applications, due to their excellent electrical insulation properties which are equal or superior to epoxies or silicone elastomer systems. The hydrolytic stability of PU elastomers derived from hydroxyl terminated polybutadiene is superior to the majority of other types of PU. Thus, some specific applications of... 

Uses Polymer additive for PVC and rubber, polyolefin and ABS processing, engineering plastics, PU rigid foams and elastomers, polyamides and polyesters, polyurethanes and epoxies modifier (increases flexibility, impact resist., toughness, hydrolytic stability) for industrial coatings (automotive, coil, textile, leather, wood lacquers)... 

Two types of environmentally friendly jute fibre reinforced green composites have been studied. These are based on Mesua ferrea L. seed oil-based poly(urethane ester) and poly(urethane amide) resin blends with commercially available partially butylated melamine-formaldehyde and epoxy resins by solution impregnation and hot-curing methods. The composites were cured at a temperature of about 130-140°C under a pressure of 35.5 kg cm for around 2 h.The physical, mechanical and chemical properties of the epoxy-modified polyurethane composites were better than those of the MF-modified composites. They also possessed excellent chemical resistance and hydrolytic stability in water, acid and salt solutions, making them useful for low load-bearing applications. 

Figure H.6 Hydrolytic stability of potting compounds. Materials showing rapid loss of hardness in this test soften similarly after 2-4 years in high temperature, high humidity climate zones. (Ref Bolger, J.C., New One Part Epoxies are Rexible and Reversion Resistant , Insulation, October, 1969).

Additives can also be used to improve the hydrolytic stability of polyester polyurethanes. Glycol-cured materials are reported to be stabilized with cyclic imines. Substituted ureas or thioureas ethylene-diaminetetraacetic acid-derived chelating agents oxycarbonyl isocyanates epoxies carbodiimides, alone " or in combination with silica silanes " substituted phthalimides and an undisclosed combination of additives have all been cited as stabilizers for polyester polyurethanes. 

Figure 7. Hydrolytic stability of TGMDA epoxy resins at 60°C/96% RH.

DeNicola and Bell have reported attempts to improve durability of epoxy-bonded steel joints using coupling agents containing )8-diketone and polyfunctional mercaptoester chelating groups. The success of this approach again depends on the hydrolytic stability of the chemical bonds formed with the oxide surface. 

This compound has enhanced hydrolytic stability in addition to low volatility. It is useful in many types of flexible foam, as well as in adhesives and epoxy- or phenolic-based laminates. 

The acrylic modified epoxy copolymer can be neutralized via the pendant carboxyl groups to provide a water-dispersible system with good hydrolytic stability. The grafting is presumed to occur onto the methylene hydrogens of the glycidyl group in the epoxy resin ... 

Hydrolysis (Figure 5.303) of polycarbonate is of decisive importance both for processing and for applications involving contact with water at elevated temperatures. To suppress this reaction, it is necessary to remove all catalyzing acid and base residues from material manufacture and to minimize water content to less than 0.05 wt.% by careful drying (e.g., 2 - 4 h at 120 °C) prior to melt processing. If the application requires additional hydrolytic stabilization, it is also possible to use neutral or hydrolytically stable stabilizers, such as sterically hindered phosphites or to add buffer substances, such as epoxies [673]. 

Many of the primers developed during the past 20 years contain corrosion inhibitors. These primers have a second function to further improve hydrolytic stability at the adhesive to metal interface and prevent corrosion during exposure to salt spray. Table 1 shows the difference in strength retention of 2024-T3 alclad aluminum lap shear coupons exposed to salt spray. Both sets of coupons were bonded with a 250°F cure elastomer modified epoxy adhesive. One set of coupons was primed with BR 123 primer (a non-corrosion inhibiting primer) while the other set was primed with... 

The electronics industry desires improved flame suppressant additives for microelectronic encapsulants due to bromine induced failure. Epoxy derivatives of novolacs containing meta-bromo phenol have exhibited exceptional hydrolytic and thermal stability in contrast to standard CEN resins with conventional TBBA epoxy resins. When formulated into a microelectronic encapsulant, this stable bromine epoxy novolac contributes to significant enhancements in device reliability over standard resins. The stable bromine CEN encapsulant took about 30% more time to reach 50% failure than the bias pressure cooker device test. In the high temperature storage device test, the stable bromine CEN encapsulant took about 400% more time to reach 50% failure than the standard compound. Finally, the replacement of the standard resins with stable bromine CEN does not adversely affect the desirable reactivity, mechanical, flame retardance or thermal properties of standard molding compounds. 

Figure 12.13 The rubber storage modulus of two cyanate ester resins as a function of moisture absorption with thermal spiking temperature illustrating the hydrolytical instability of the thermally cured AroCy LIO ( ) compared to the stability of an epoxy-cured AroCy LIO (o) [7,19].

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