Anaerobic adhesives chemistry

Anaerobic adhesive chemistry has been reviewed [113], so only a few recent developments will be covered here. Many of the components of anaerobic adhesives. 

Robert W, R. Humphreys received his B,Sc, andPh.D. in Photochemistry and ESR Spectroscopy with Professor Donald R. Arnold at the University of Western Ontario. After 1 1/2 years postdoctoral work with Professor Cheves Walling at the University of Utah, he worked as a Research Chemist at Loctite Corporation in anaerobic adhesives chemistry. He recently joined Lever Research, Inc. as a Research Scientist. His current research interests include free radical and peroxide chemistry and metal catalyzed redox reactions. 

The cornerstone of anaerobic adhesive chemistry is the set of competitive chemical reactions... 

Anaerobic adhesives contain reactive monomer(s), accelerators, stabilisers or inhibitors, fillers, colorants, modifiers, tougheners (rubbers, butadiene graft, etc.). The chemistry of anaerobic adhesive is complex but the schematic brief of the reaction may be shown as follows ... 

Vernon Krieble, chairman of the Chemistry Department at Trinity College in Hartford, Connecticut, learned about the product through his son, Robert Krieble, who was employed at General Electric. Vernon Krieble found a chemical solution to the problem by using cumene hydroperoxide (I) (see Section XII for all structures) as the initiator and packaging in half-filled oxygen-permeable polyethylene bottles [8]. He licensed the GE patent and in 1954 founded the American Sealants Corporation, which later became Loctite Corporation [9]. At the present time anaerobic adhesives and sealants are manufactured or sold on every continent by more than a dozen companies. Applications in virtually every industry, and technological iimovation, as measured by patent activity, continue unabated. 

In the following sections/ we shall discuss adhesion chemistry/ adhesion physics / radiation-curable adhesiveS/ high-temperature adhesiveS/ anaerobic and structural adhesiveS/ hot-melt adhesives/ film adhesiyes/ waterborne adhesives/ aerospace structural adhesiveS/ conventional sealants/ advanced aerospace sealants/ and adhesives and sealants for solar collectors. 

Humphreys, of the Loctite Corporation, reported on the chemistry of accelerators for curing anaerobic adhesives. He showed that the reaction between N,N-dimethylaniline derivatives and cumene hydroperoxide is relatively slow even at lOO C. He concluded that the accelerated polymerization of anaerobic adhesives of ambient temperatures caused by cure systems containing combinations of tertiary aromatic amines, hydroperoxides, and sulfonimides does not result from the hydroperoxide-amine reaction. 

Chemistry of Accelerators for Curing Anaerobic Adhesives-Reaction of N, N-Dimethylaniline Derivatives with Cumene Hydroperoxide... 

Some of the chemistry of common accelerators used in curing anaerobic adhesives is discussed in detail. Emphasis is placed on the reactions of aromatic amines particularly in the presence of hydroperoxides. Product studies are presented for the reaction of a series of amines with cumene hydroperoxide (CHP) and plausible mechanisms for product formation are proposed. Relationships are drawn between these results and the chemistry which occurs in anaerobic adhesive formulations. 

Tertiary aromatic amines hydroperoxides and sulfonimides are important components of many of the common anaerobic adhesive cure systems. While various formulative aspects of these compounds are well understood, a detailed explanation for their dramatic effect on the rate of polymerization of the adhesive has been lacking. Our approach to the problem has been to study the chemistry of the isolated components of this cure system under well defined conditions and to apply the results to understanding the mechanism by which these compounds accelerate the polymerization of anaerobic adhesives. Herein, we report some of the results of our studies of the reactions of N,N-dimethylaniline derivatives, which are typical amines used in anaerobic formulations, with cumene hydroperoxide (CHP). Connections will be made between the chemistry of the isolated systems and that which occurs in anaerobic formulations, both during storage and cure. 

C. W. Boeder, Anaerobic and structural acrylic adhesives, in Structural Adhesives - Chemistry and Technology, S. R. Hartshorn, Ed., Plenum Press, New York, 1985, Chap. 5. 

Moane, S., Raftery, D. P., Smyth, M. R., Leonard, R. G. (1999). Decomposition of peroxides by transition metal ions in anaerobic adhesive cure chemistry. International Journal of Adhesion and Adhesives, 19(1), 49-57. 

Engineering adhesives are solventless, liquid, reactive, durable adhesives for bonding durable substrates. There are six recognized chemical types of engineering adhesives acrylic, anaerobic, cyanoacrylate, epoxy, silicone, and urethane. Strictly speaking, the first three in this list are varieties of acrylic, but only the first will be discussed here. There is, however, considerable overlap in the chemistry and properties of acrylic and anaerobic adhesives. Since considerable progress has been made in acrylic adhesives over the last two decades, those used in engineering applications are now referred to as modified acrylic adhesives. 

Further improvements in the reactivity of anaerobic adhesives were obtained with the introduction of hydrazide accelerators. However, incorporation of more active cure systems was made possible only by concurrent advances in stabilization chemistry. Early formulations employed quinone polymerization inhibitors which were effective at levels of 10-1000 parts per million. As more active formulations were developed, attempts to combat premature polymerization by addition of greater quantities of inhibitor served only to reduce performance. A significant breakthrough occurred when a method of removing trace amounts of contaminant metals from anaerobic formulations was... 

Curing acrylic adhesives are distinctly different from anaerobics, cyanoacrylates, and acrylic solution adhesives and emulsions. These related chemistries use different formulating materials, cure via different curing mechanisms, and often possess minimal high performance properties over long periods of time, or when exposed to aggressive environments. 

One property common to UV-cure.d acrylate formulations as was discussed earlier is relatively poor surface-cure due to oxygen inhibition (see Fig. 1). This effect is most predominant at low photoinitiator concentrations, in thin coatings. Adhesive formulations can benefit from this property by enhancing surface tack via the presence of residual uncured resin. Post-cure, from residual cross-linking, or other anaerobic chemistry, can later result in enhanced bond strength. 

So-called anaerobic polyurethane adhesives also belong to this category. This system combines urethane poly-addition chemistry with free-radical initiated addition polymerization. A polymerizable alcohol such as j -hydroxyethyl methacrylate is reacted with an equivalent amount of a diisocyanate such as TDI or with an isocyanate-terminated urethane prepolymer. An organic hydroperoxide is then added to such intermediates... 

Anaerobic and structural acrylic adhesives (acrylics) are closely related members of the large acrylic adhesive family. They are reactive systems which cure by redox-initiated free radical polymerization. It is difficult to make a clear distinction between the two classes of adhesives. Many aspects of the chemistry, compositions, and terminology overlap. 

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