Solvent adhesive energy

Solvent-borne polychloroprene adhesives are unsuitable for bonding low-energy substrates, such as PVC. However, water-borne polychloroprene adhesives display good peel adhesion to vinyl substrates. Addition of an accelerator such as zinc oxide is essential for improved hot bond strength. 

Most blocked isocyanates are solids at room temperature and thus may require the use of solvent. The unblocking temperatures are often fairly high and are energy intensive. Furthermore, certain blocking agents may qualify as volatile organic compounds. For these reasons, the blocked isocyanate adhesives occupy a small, but important segment of the adhesive marketplace. 

The other problem that can occur with metals is the presence of various machine oils on the surface, such as might be used in the automotive area. Oil serves as a low surface energy barrier to adhesion, in most instances. Abrading and solvent wipe are recommended or else degreasing. Removing the surface oil is... 

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

For instance, in 1996, 3M company announced a breakthrough in the process for making medical adhesive tapes, a process that reduces energy consumption by 77 percent. The new process also cuts solvent use by 2.4 million pounds, lowers manufacturing costs, and cuts mannfacturing cycle time by 25 percent. The proprietary process took researchers nine years from conception through final implementation. 

Traditionally, UV curable polymers have been utilized as coatings for wood and vinyl floors, but their applications have increased dramatically over the last twenty years to encompass many diverse areas, including optical fiber coatings (7), adhesives (2), disc replications (3-5), and microelectronics (6). This widespread use of UV cross-linked systems is attributed to their rapid, energy efficient curing and their solvent free, one piece formulations. Typically, UV curable systems require only a small fraction of the power normally utilized in thermally cured systems and their solvent free nature offers an environmentally safer alternative. 

The study of inverse adhesive emulsions has revealed the same features as direct emulsions [112,113]. Here again, it was shown that adhesion is favored when the surfactant becomes less soluble in the continuous phase [113]. This can be tested experimentally by using binary mixtures of oils, one in which the surfactant is soluble and another one in which the surfactant is insoluble. For example, water droplets can be stabilized in mineral oil by sorbitan monooleate (Span 80). This surfactant is soluble in dodecane whereas it is not in silicon oil. The affinity of the surfactant for the organic solvent can be tuned by mixing dodecane and silicon oil. As shown in Fig. 2.38, the energy of adhesion between water droplets strongly varies as the ratio of the mixture is changed. A sharp rise is noted as the surfactant... 

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. 

Finally, the solubility parameter of the adhesive and the substrate must be close. Without getting too teehnieal, the solubility parameter is a rough estimate of polarity. The old saying like dissolves like can be extended to like bonds like. More aeeurately, the solubility parameter is the ealeulated potential energy of 1 em of material for eommon solvents. Polymers are assigned solubility parameters of solvents in which they are soluble. Table 19.3 lists solubility parameters for various solvents and polymers. As an example of how to use this table, butadiene-acrylonitrile rubber with 6= 9.5 bonds natural rubber (6= V.9-8.3) to phenolic plastics (6= 11.5). Note that its solubility parameter is between that of the two substrates. 

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