Sealants degradation

A partially cross-linked, isobutylene—isoprene—divinylbenzene terpolymer containing some unreacted substituted vinylbenzene appendages is commercially available from Polysar Division, Bayer AG. Because of the residual reactive functionality, it can be cross-linked by peroxides that degrade conventional butyl mbbets. It is employed primarily in the manufacture of sealant tapes and caulking compounds (31). 

Shephard, N.E., Klosowski, J.M. and Wolf, A.T., Effects of degradation factors on sealant adhesion, in press. 

Most elastomeric sealants used in structural glazing applications are organic polymers or elastomers. Because organic materials degrade, some changes in the properties of the elastomer can be expected to occur with pas-... 

Sealants obtained by curing polysulfide liquid polymers with aryl bis(nitrile oxides) possess stmctural feature of thiohydroximic acid ester. These materials exhibit poor thermal stability when heated at 60°C they soften within days and liquefy in 3 weeks. Products obtained with excess nitrile oxide degrade faster than those produced with equimolar amounts of reagents. Spectroscopic studies demonstrate that, after an initial rapid addition between nitrile oxide and thiol, a second slower reaction occurs which consumes additional nitrile oxide. Thiohydroximic acid derivatives have been shown to react with nitrile oxides at ambient temperature to form 1,2,4-oxadiazole 4-oxides and alkyl thiol. In the case of a polysulfide sealant, the rupture of a C-S bond to form the thiol involves cleavage of the polymer backbone. Continuation of the process leads to degradation of the sealant. These observations have been supported by thermal analysis studies on the poly sulfide sealants and model polymers (511). 

Construction. Polymeric materials such as adhesives, sealants, and composites have been used considerably in the last several decades for the construction, repair, and rehabilitation of our transportation infrastructures. Even though most processes were experimental until recently, they have evolved to the point where many are now standardized and well accepted. Table 1.6 hsts several common applications for advanced polymeric materials (as well as the polymeric resins that are most commonly employed). In the construction or repair of roads and bridges, epoxy adhesives have primarily been used for bonding concrete and for bonding stiffening members or repair structures to degrading concrete facilities. 

Moisture can degrade the properties of the bulk adhesive or sealant itself. 

Internal degradation within the bulk adhesive or sealant occurs primarily by absorption of water molecules into the polymer structure. All polymers will absorb water to some extent. Moisture can also enter by wicking along the adhesive-adherend interface or by wicking along the interfaces caused by reinforcing fibers and the resin. Deterioration may occur more quickly in a 100 percent relative humidity (RH) environment than in liquid water because of more rapid permeation of the vapor. 

Another way moisture can degrade the strength of adhesive joints is through hydration or corrosion of the metal oxide layer at the interface. Common metal oxides, such as aluminum and iron, can undergo hydration. The resulting metal hydrates become gelatinous, and they act as a weak boundary layer because they exhibit very inadequate bonding to their base metals. Thus, the adhesive or sealant used for these materials must be compatible with the firmly bound layer of water attached to the surface of the metal oxide layer. 

Certain chemical linkages are susceptible to hydrolytic attack and, if present in an adhesive or sealant, are potential sites for irreversible reaction with water that has diffused into thejoint. Such hydrolytic (chemical) degradation causes a permanent reduction in the cured physical properties. The functional groups present in the chains are hydrolyzed, resulting in both chain breaking and loss of crosslinking. 

Adhesive systems may be composed of low-molecular-weight constituents that can be extracted from the bulk adhesive when exposed to a vacuum environment. If these low-molecular-weight constituents also have a low vapor pressure, they may migrate out of the bulk because of exposure to elevated temperatures with or without the presence of a vacuum. This results in an overall weight loss and possible degradation of the adhesive or sealant. 

The heat and ozone resistant [126] EPR was made by incorporating acrylic rubber, dicumyl peroxide, triaUyl cyanurate, ZnO and carbon-black into the matrix. Triallyl cyanurate increases the crosslinking efficiency probably due to an addition reaction between polymeric and aUyl radicals and leads to stable chemical crosslinks. Thus ozone because there is no unsaturation cannot initiate a degradation reaction. Digteva et al. [127] prepared sealants for use at high temperature by adding aromatic diaminodisulfide, MgO, ZnO and carbon black in EPR. The aromatic diaminodisulfide is an antiozonant and functions both as an antioxidant and a... 

Thermal degradation studies of polysulphide sealants have shown the evolution of gases such as hydrogen sulphide, ethylene, sulphur dioxide and carbon dioxide, but the rate of evolution of these compounds was found to be below the level considered hazardous (Berenbaum, 1969). 

To reduce or eliminate debonding, a UV-polymerized, hydro-phobic sealant was painted around the dentin-methacrylate interface. Thus, it was hoped that hydrolytic degradation of the nd would be minimized. The rationale for this procedure is that the coating would seal off the adhesive from contact with water, or at least the water permeation through the protective film to the adhesive surface would be reduced. Suitable protective formulations should (1) cure within 1 minute, (2) be hydrophobic,... 

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