Aqueous polyurethane adhesives

Acidification of chloramine T with sulfuric acid produces the formation of dichloramine T (DCT) and hypochlorous acid (HCIO), species which react with C=C bonds of the butadiene units. The effectiveness of the treatment is ascribed to the introduction of chlorine and oxygen moieties on the mbber surface. A decrease in the pH of the chloramine T aqueous solutions produced more extended surface modifications and improved adhesion properties in the joints produced with waterborne polyurethane adhesive (Figure 27.9). The adhesive strength obtained is slightly lower than that obtained for the rubber treated with 3 wt% TCI/MEK, and its increases as the pH of the chloramine T solution decreases (Figure 27.9). A cohesive failure in the rubber is generally obtained. 

FIGURE 27.9 T-peel strength values of styrene-butadiene rubber (SBS) treated with chloramine T aqueous solutions with different pH/waterbome polyurethane adhesive/roughened leather joints, as a function of the pH value of the chloramine T aqueous solutions. A adhesion failure to the rubber, M cohesive failure in tbe rubber. (From Navarro-Banon, M.V., Pastor-Bias, M.M., and Martm-Martinez, J.M., Proceedings of the 27th Adhesion Society, Wilmington, NC.)... 

The general formula of an emulsified polyurethane or aqueous dispersion used for adhesive purposes is shown in Fig. 8.11. Aqueous polyurethane dispersions usually have 40% solids of a thermoplastic type (for aqueous inertness), aliphatic diisocyanate polymer (for light stability) and an anionic surfactant is used. However, if a polyurethane ionomer is prepared this does not contain added emulsifier, the polyurethane itself having its own emulsifier function. The incorporation of this function in the polyurethane chains as (a) sodium sulphonate or (b) sodium carboxylate groups is seen in Fig. 8.11. 

The elimination of organic solvents in the application of coatings, saturants, sealants and adhesives, and its importance in environmental pollution abatement and worker health and safety, has already been identified in connection with aqueous polyurethane latex and polyurethane... 

Solutions (5-25% solids) of fully reacted polyurethanes based on MDI or IPDI are also used in coating applications. These coatings are used mostly as finish coats on fiexible substrates. Because of concern over airborne solvents aqueous polyurethane dispersions are used today. These polymers are prepared by incorporating ionic groups into their backbone, and both cationic and anionic groups are utilized in the preparation of the ionomers. The solvent and water resistance of films formed from aqueous dispersions can be improved by cross-linking. Ionomers are also used in the formulation of speciality adhesives. 

Aqueous dipersion adhesives are high-molecular-weight polyurethanes dispersed in water. They are applied and used in exactly the same manner as solvent-borne systems with the benefit of not using hazardous or flammable solvents. 

Only polyurethane adhesives should be used to bond PVC soles. The adhesion problems of PVC derive from the presence of plasticizers and stabilizers (stearate type) able to migrate to the surface impeding the contact with the adhesive (creation of weak boundary layers). A solvent wiping on the PVC surface is usually effective in improving adhesion and solvents such as MEK are adequate. On the other hand, treatment with 10 wt% aqueous solutions of sodium hydroxide has been successfully applied to increase the adhesion performance of plasticized PVC soles (Abbott et al. 2003). 

The prime advantage of aqueous polyurethane and polyacrylate dispersions over solvent-containing systems is that recovery or disposal of significant amounts of solvent is unnecessary. In the packaging industry, potential for residual solvent traces in the adhesive and subsequent migration into food are also major concerns. Therefore, solvent-containing adhesives have already been replaced by environmentally friendly water-borne adhesives in a number of applications. 

Sorbitol is manufactured by the reduction of glucose in aqueous solution using hydrogen with a nickel catalyst. It is used in the manufacture of ascorbic acid (vitamin C), various surface active agents, foodstuffs, pharmaceuticals, cosmetics, dentifrices, adhesives, polyurethane foams, etc. 

A Methylolhydantoins. l,3-Bis(hydroxymethyl)-5,5-dimethyIhydantoia [6440-58-0] is used extensively as a preservative in cosmetic and industrial appHcations, and carries EPA registration for the industrial segment. It is available in soHd and in aqueous solution forms, including low free formaldehyde versions of the latter. A related derivative, l,3-bis(hydroxyethyl)-5,5-dimethyIhydantoia [26850-24-8] is used in the manufacture of high temperature polyesters, polyurethanes, and coatings, offering improved heat resistance, uv stabiUty, flexibiUty, and adhesion. 

Aqueous dispersions of polyurethanes have also become available which may be used instead of solutions in organic solvents for such applications as leather treatment, adhesives and surface coatings. 

Industrially, silicone surfactants are used in a variety of processes including foam, textile, concrete and thermoplastic production, and applications include use as foam stabilisers, defoamers, emulsifiers, dispersants, wetters, adhesives, lubricants and release agents [1]. The ability of silicone surfactants to also function in organic media creates a unique niche for their use, such as in polyurethane foam manufacture and as additives to paints and oil-based formulations, whilst the ability to lower surface tension in aqueous solutions provides useful superwetting properties. The low biological risk associated with these compounds has also led to their use in cosmetics and personal care products [2]. 

Therefore, die polarity and solubility of polymer can be modified deliberately by varying the nature of the components. High aqueous solubilities of polyamides and polyurethane threaded with crown ethers or CD are intriguing, because this observation implies potential applications of the polyrotaxane concept in coatings, adhesives, and water-borne processing. The observation of the emulsification of... 

Other applications. Phenylmercuric acetate has been used in aqueous preparations such as inks, adhesives, and caulking compounds, as a catalyst for the manufacture of certain polyurethanes, and as a fungicide in seed dressings and interior and exterior paints (IARC 1993 Reese 1990). Dimethylmercury is used to prepare mercury nuclear magnetic resonance standards (Blayney et al. 1997) and mass spectrometer mercury calibration standards (Toribara et al. 1997). 

We have studied the adhesion of particles to painted surfaces that have been pretreated with aqueous solutions of various types of surfactants (nonionic, cationic, and anionic) [232]. Data are shown below for the adhesion (7 ) of spherical glass particles (SO-lOO-jLim diameter) to a surface that had been painted with polyurethane enamel U-21 and then treated with various surfactants... 

In Fig. VIII.2 we show the adhesion of spherical glass particles (80-100-jum diameter) to a surface painted with a polyurethane enamel, in relation to the concentration of the aqueous Gardinol solution used to treat the surface. The particles were detached by means of vibration after the film had been dried. 

Thermoplastic linear polyurethanes which are usually chain-terminated so that no unreacted free NCO groups remain available. Environmental considerations direct growing attention to these newer non-polluting urethane adhesive forms, e.g. powders, films, aqueous dispersions and 100% solids reactive systems. Some systems do possess blocked diisocyanates which are activated on heating to produce chemically reactive solid systems. 

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