Open-time bonding

While some of the earlier acrylics had a short open time (bonding window), the more recent entries permit the homeowner more flexibility in bonding time. Thus, good bonding may be achieved for hours after the clear point is reached. 

Open-time bonding In this method, the adhesive is applied to both surfaces and allowed to stand open until suitable tack is achieved. At least one of the adherends should be porous... 

In contact adhesives, the so-called tack open time is important. This can be defined as the time available after the adhesive is applied during which the surface remains tacky enough for the application of the adherend. It can be easily measured by applying a thin layer of fresh adhesive on Kraft paper and making a bond at different times until no bond is obtained. 

Solvents. Solvents affect adhesive viscosity, bond strength development, open time, cost, and ultimate strength. Blends of three solvents (aromatic, aliphatic, oxygenates, e.g. ketones, esters) are generally added, and in their selection environmental and safety regulations must be considered. A graphical method has been proposed to predict the most adequate solvent blends for solvent-borne CR... 

Bond strength can vary from a temporary bond (non-curing compound) to a substrate tearing bond (using phenolic-modified curing products). Solvent-borne CR adhesives can be formulated to have very short open times for fast production operations or to retain contact bond characteristics for up to 24 h. Heat and solvent reactivation can be used to re-impart tack to dried surfaces. 

A typical formula would consist of 18-28 parts of SBS rubber, 50-60 parts of an aliphatic/aromatic tackifier with about 15-30% aromaticity (for long open time), and 15-30 parts of a white oil or a very clean process oil. Formulations are designed to maximize open time, while maintaining adequate heat resi.stance (maintenance of bond strength upon aging at 40-55 C — warehouse conditions). 

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

Polychloroprene. Polychloroprene dispersions have a range of qualities similar to those of solvent-based polychloroprene adhesives and a similar range of uses. As an example, the bonding of vinyl materials with phenolic resin/paper decorative laminates often is carried out with these products. It is necessary to incorporate acid-acceptor dispersions of metallic oxides, and the dispersions in general do not provide such long open times as solvent-based polychloroprene adhesives. 

Due to the sequential mode of opening and closure, the bond opening rate of region II, defined as the number of bonds that open per unit time, dNs/dt, where Nb is the number of open LL bonds at any given time, is identical to the net opening rate of the outermost bond, This implies that the... 

Open time—the elapsed time between the application of the adhesive to one or both substrates and the bringing together of the two surfaces to give an effective bond. 

The range is the time over which an adhesive, once applied to a substrate, retains a useful degree of tack. It is a function of the adhesive formulation. For an adhesive to form a useful bond between two substrates, the range of the adhesive must be at least as long as the open time. 

Chem. Descrip. Pentaerythritol ester of dimeric resin acids Uses Resin in fast-drying soft-oil varnishes, in prod, of flushed colors and resin-bonded pigments, in heat set printing inks requiring extreme hardness, high gloss, and sol. in low-KB ink oils, and inshort-open-time, heat-resistant hot-melt adhesives... 

Polychloroprene latex. Anionic or non-ionic latices can be used. The polymer determines the initial tack and open time, the bond-strength development and hot bond strength, the application properties and the adhesive viscosity. Anionic latices are stabilized with rosin soaps. Carboxylated polychloroprene latex is stabilized with polyvinyl alcohol and provides better freeze-thaw stability than the anionic types [78]. 

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