Latent adhesive

Reactivity can also be increased by externally heating the epoxy formulation to a preselected curing temperature. Epoxy resin reactions roughly obey Arrhenius law that for every 10°C rise in temperature, the reaction rate doubles. Certain epoxy resin systems must be heated for any reaction to take place at all. This is beneficial in that these latent adhesive formulations are one-component products that do not require metering or mixing yet have long, practical shelf lives. 

Esters. Most acryhc acid is used in the form of its methyl, ethyl, and butyl esters. Specialty monomeric esters with a hydroxyl, amino, or other functional group are used to provide adhesion, latent cross-linking capabihty, or different solubihty characteristics. The principal routes to esters are direct esterification with alcohols in the presence of a strong acid catalyst such as sulfuric acid, a soluble sulfonic acid, or sulfonic acid resins addition to alkylene oxides to give hydroxyalkyl acryhc esters and addition to the double bond of olefins in the presence of strong acid catalyst (19,20) to give ethyl or secondary alkyl acrylates. 

The blocked isocyanate systems (with curative present) are latent cure systems. In order to create a latent curing adhesive, the blocked isocyanate is added to a catalyzed polyol component without a reaction occurring at room temperature. In theory, the blocked adhesive system is relatively stable at room temperature. When this system is heated to the unblocking temperature, the chemical reaction, which... 

Interestingly, this same effect has been observed for the addition of a rubber toughening agent to ethyl cyanoacrylate-based adhesives, as was reported previously. The rubber must contain enough latent acid functionality on the polymer backbone or in an additive to inhibit the thermally activated decomposition of the alkyl cyanoacrylate adhesive polymer. 

Vinyl substituted cyclic hemlamidals 2 and their Interconvertible acetal precursors (eg. acrylamldo-butyraldehyde dimethyl acetal 1) were Incorporated as latent crosslinkers and substrate reactive functional comonomers In solution and emulsion copolymers. Some use and applications data for copolymers prepared with these new monomers are presented. They show low energy cure potential, long shelf life and high catalyzed pot stability In solvent and aqueous media, good substrate reactivity and adhesion, and good product water and solvent resistance. They lack volatile or extractable aldehyde (eg. formaldehyde) components and show enhanced reactivity and hydrolytic stability with amines and diol functional substrates. 

Dicyandiamide Latent cure Good elevated-temperature properties Good chemical resistance Good combination of tensile and peel strength Long elevated-temperature cure Insoluble in resin One-component adhesives Powder coatings Film and solid adhesives Laminates and other composites... 

Latent imidazole catalysts have also been developed to provide cure rates considerably faster than those of dicyandiamide cured epoxy resins.18 They also exhibit excellent adhesive characteristics and heat and chemical resistance. 

A significant amount of development is currently occurring relative to latent catalysts because of interest in their long shelf life, high reactivity, and single-component adhesive formulations. Present technologies involve absorption of acidic or basic catalysts in molecular sieves, formation of Lewis acid salts or other amine salts, microencapsulation of amines, and other novel segregation methods. 

Novel, toughened one-component epoxy structural adhesives based on epoxy-terminated polyurethane prepolymer incorporating an oxolidone structure were developed to provide improved toughness, fracture resistance and adhesive properties with good chemical and moisture resistance.21 The hybrid resin cures with a standard latent curing agent/accelerator. 

Whereas most room temperature curing epoxy adhesives are cured with aliphatic amines, polyamides, or amidoamines, most elevated-temperature curing epoxy adhesives are cured with aromatic amines, modified aliphatic amines, alcoholic and phenolic hydroxyls, acid anhydrides, Lewis acids, and a host of other curatives. Latent curing agents, such as dicyan-diamide and imidazoles, are typically used in one-component epoxy adhesives systems. 

Dicyandiamide is a true latent catalyst for epoxy resin curing. It is also considered to be the workhorse of one-component adhesives due to its ease of use, excellent performance properties, long shelf stability, and low toxicity. In certain admixtures with DGEBA, it has demonstrated a room temperature storage life in excess of 4 years. Dicyandiamide is usually added to the solid epoxy resin in concentrations of about 3 to 6 pph. It melts at about 150°C. Cures can be conducted in the range of 120 to 175°C but are very slow at the lower temperatures. As a result, it is common practice to add accelerators such as benzyl-dimethylamine (BDMA) and mono- or dichlorophenyl substituted ureas to these systems. 

Other latent curing agents that are used in solid adhesives are dihydrazides and BF3-MEA complexes. These compositions are also stable at room temperature but cure when heated. Solid anhydrides can be used in one-component powder blends (e.g., 10 pph of trimellitic anhydride accelerated with 0.5 pph of 2-methylimidazole). Solid systems with aromatic diamines are prepared by comelting the solid epoxy with the amine. Typically 30 pph of curing agent is used.1... 

Similar information is presented in Table 14.5 for starting adhesive formulations made from an epoxy resin emulsion and dicyandiamide latent curing agent. This adhesive has exceptionally good water resistance when cured. The adhesive was applied to the indicated substrates in a manner similar to that described above, and it was cured for 3 min at 65°C followed by 10 min at 175°C. 

Chelating agents such as triethanolamine borate also behave as latent catalysts at elevated temperatures. Other boron-containing compounds, cadmium or zinc bromide dieth-ylenetriamine, and salts of aluminum acetoacetic ester have also been suggested as curing agents for high-temperature epoxy adhesives. 

Adhesives for integrated circuits, requirements, 14 AgCl-As2S3, 172 Ag2Se-GejSei., resists development of latent image, 174 anisotropic wet chemical etching, 177/ chemical basis, 174... 

Whatever the truth of the matter, G-CSF should be regarded as a possible cause of pulmonary complications. The abrupt increase in the number of activated neutrophils after G-CSF may account for exacerbation of latent chemotherapy-induced pulmonary damage. Endothelial damage subsequent to increased neutrophil activity (that is, enhanced superoxide release and increased adhesion molecule expression and adherence) or the release of cytokines (IL-1, IL-6, TNF) has been advanced as possible mechanisms. In addition, transient slight hypoxia was found in G-CSF users, although no relation with specific cytotoxic drug treatment or previous radiotherapy was identified (32). A sudden increase in neutrophil count, a rise in LDH and C reactive protein, and the occurrence of dyspnea or fever in G-CSF-treated patients were proposed as possible early signs of the subsequent development of interstitial pneumonia. 

Surface preparation depends on coating formulation and intended usage. This could vary on steel substrates from a commercial to a white metal blast. Profile is also essential to obtain ultimate adhesion. The depth of profile required is proportional to the total dry film thickness of the coating system. On concrete surfaces, the general requirement is to remove latents and contaminants and this can be accomplished by chemical preparation, scarifying or abrasive blasting. 

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