Water bond strengths

Figure A3.5.10. Bond strengths of water clustering to various core ions as a fiinction of the number of water molecules.

Pentaerythritol in rosin ester form is used in hot-melt adhesive formulations, especially ethylene—vinyl acetate (EVA) copolymers, as a tackifier. Polyethers of pentaerythritol or trim ethyl ol eth an e are also used in EVA and polyurethane adhesives, which exhibit excellent bond strength and water resistance. The adhesives maybe available as EVA melts or dispersions (90,91) or as thixotropic, one-package, curable polyurethanes (92). Pentaerythritol spko ortho esters have been used in epoxy resin adhesives (93). The EVA adhesives are especially suitable for cellulose (paper, etc) bonding. 

In addition to polyamide, lamination inks ordinarily contain modifiers such as polyketone resin, plasticizer, and wax to impart specific properties such as block resistance and increased bond strength. Because laminating inks are usually reverse-side printed and end-up sandwiched between films, gloss is not a primary requirement. Water-base laminating inks that will meet the U.S. EPA emission requirements and have the correct functional properties are currently under development. 

Different phenoHc resins are used for different types of wood for example, plywood adhesives contain alkaline-catalyzed Hquid resole resins. Extension with a filler reduces cost, minimizes absorption, and increases bond strength. These resins have an alkaline content of 5—7% and are low in free phenol and formaldehyde. Because many resins have a high water content and limited storage stabiHty, they are frequently made at or near the mill producing the plywood product. The plywood veneers are dried, coated with resin, stacked for pressing, and cured at 140—150°C. 

The high heat resistance produced by adding phenolic resins to solvent-borne CR adhesives is due to the formation of the infusible resinate, which reduces the thermoplasticity of the adhesive and provides good bond strength up to 80°C (Table 11). The resinate also increases the adhesive bond strength development by accelerating solvent release. 4 phr of magnesium oxide for 40 phr of phenolic resin are sufficient to produce a room temperature reaction. A small amount of water (1-2 phr) is necessary as a catalyst for the reaction. Furthermore, the solvent... 

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. 

The deterioration of a bond line can occur due to (1) the failure of the resin (low hydrolysis resistance, degradation of the hardened resin causing loss of bonding strength) (2) the failure of the interface between resin and wood surface (replacement of secondary forces between resin and reactive wood surface sites by water or other non-resin chemicals) (3) the breaking of bonds due to mechanical forces and stresses (the influence of water will cause swelling and therefore movement of the structural components of the wood-based panels). 

A term that is widely used (and sometimes abused) in discussions about metal-water interactions is hydrophilicity. By this term is meant the strength of interaction between a metal surface and water molecules in contact with it, and the term usually implies chemical bond strength. However, there is a problem with the way hydrophilicity scales are built up. Various quantities (capacitance, adsorption energy, etc.) are used to rank the metals, and the hydrophilicity scale may differ for different parameters. 

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