Moisture-curing

OxazoHdines, formed by reaction of alkanolamines with aldehydes, ate useful as leather tanning agents and ate effective curing agents for proteins, phenoHc resins, moisture-cure urethanes, etc. They also find use as antimicrobial agents. 

Silicones. Commercially avaHable sHicone sealants are typicaHy one of three curing types moisture-reactive (curing) sealants, moisture-releasing (latex) sealants, and addition-curing sealants. Of these three types, moisture-curing sHicones make up the vast majority of sHicone sealants sold. 

Moisture-Curing Silicones. The formulation of moisture-curing sHicones includes a sHicone polymer, filler, a moisture-reactive cross-linker, and sometimes a catalyst. The most common sHicone polymer used in sealant formulations is an alternating sHicon—oxygen backbone with methyl groups attached to the sHicon such as the sHicone polymer (1). 

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

Other modifications of the polyamines include limited addition of alkylene oxide to yield the corresponding hydroxyalkyl derivatives (225) and cyanoethylation of DETA or TETA, usuaHy by reaction with acrylonitrile [107-13-1/, to give derivatives providing longer pot Hfe and better wetting of glass (226). Also included are ketimines, made by the reaction of EDA with acetone for example. These derivatives can also be hydrogenated, as in the case of the equimolar adducts of DETA and methyl isobutyl ketone [108-10-1] or methyl isoamyl ketone [110-12-3] (221 or used as is to provide moisture cure performance. Mannich bases prepared from a phenol, formaldehyde and a polyamine are also used, such as the hardener prepared from cresol, DETA, and formaldehyde (228). Other modifications of polyamines for use as epoxy hardeners include reaction with aldehydes (229), epoxidized fatty nitriles (230), aromatic monoisocyanates (231), or propylene sulfide [1072-43-1] (232). 

Type of urethane used is one of three (1) Type II, moisture-cured (2) Type IV, two-package catalyst or (3) Type V, two-package polyol. (Ref. ASTM C16.)... 

Scheme 9. Crosslinking reaction of a two-part moisture cure silicone adhesive system.

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

Fig, 17. (a) Isocyanate-terminated polyurethane prepolymer synthesis, (b) Moisture cure of isocyanate-terminated polyurethane prepolymers, (c) Side reactions of isocyanate-terminated polyurethane prepolymers. 

Catalysts such as dibutyl tin dilaurate or tertiary amines are added to promote the urethane reaction and/or subsequent moisture cure. Dimorpholine diethyl ether is particularly effective at promoting moisture cure without promoting allophanate side reactions at the application temperature (which leads to instability in the hot melt pot) [29]. 

Effect of moisture cure on properties of a urethane-based hot melt adhesive... 

EVA-based adhesives machine more cleanly and are less expensive. For optimum lay-flat and low temperature performance, moisture-curing polyurethane-... 

EVA-based Rubber-based Moisture-cure urethane... 

To improve on penetration and heat resistance (properties normally in opposition when formulating hot melts), reactive hot melt systems are being developed. Moisture curing urethanes have already gained wide acceptance in a number of wood and automotive applications. UV cure is slowly gaining acceptance in PSA, labeling, and other markets. 

For all its benefits, the water/isocyanate reaction can be troublesome as well. All raw materials that go into a one-component moisture-cured adhesive must have very low moisture content, usually less than 0.05% water. If higher water levels are present, the adhesive can start curing in the reactor, causing an increase in viscosity, or, in extreme cases, gelation. 

The amorphous polyesters are becoming increasingly important for one-component 100% solid moisture-curing adhesives. These materials are usually viscous, amorphous liquids. Poly(2-methylpropane adipate), an example of an amorphous polyester, is a liquid even at a molecular weight of 2000. The amorphous polyesters are usually asymmetrical in structure. In the poly(2-methylpropane adipate) example, the pendant methyl group would be expected to interfere with chain packing, thereby preventing crystallization [27]. 

The soft segments made from asymmetrical (amorphous) polyols are important for two-component structural adhesives and one-component moisture-curing adhesives. These materials are applied and usually cured at room temperature. 

Several large applications for one-component moisture-cure urethane adhesives are available. Polymeric MDI is an exceptional binder for wood products, such as oriented strand board and particleboard. One-component urethane windshield adhesives are used almost exclusively in both the OEM and automotive aftermarket. One-part urethane adhesives are used to assemble the sidewalls for recreational vehicles (RV s), manufactured housing, and mobile homes. In construction applications, one-part urethanes are used to bond metal doors, hardwood flooring, panels, and partitions. 

The basis of most one-component, moisture-curing urethanes is the synthesis of an isocyanate-terminated prepolymer, as shown ... 

Most moisture-curing liquid adhesives utilize poly(oxypropylene) (PPG) polyols, as shown above. These raw materials produce among the lowest-viscosity prepolymers but may not have sufficient modulus at higher temperatures for some applications. A certain percentage of polyester polyols may also be utilized to boost performance, but these may cause a large increase in viscosity, and so they are more often used in conjunction with polyether polyols to provide a high-performance adhesive with workable viscosities. Poly(butadiene) polyols may be utilized for specific adhesion characteristics. 

Catalysts serve a dual purpose in one-component moisture-curing urethanes. The first purpose is to accelerate the prepolymer synthesis. The second purpose is to catalyze the curing reaction of the adhesive with moisture. The most common catalysts used to promote both prepolymer formation (NCO/OH) and later the adhesive curing reaction (NCO/H2O) are dibutyltin dilaurate and DMDEE ((tertiary amine. A stabilizer such as 2,5-pentanedione is sometimes added when tin is used, but this specific stabilizer has fallen from favor in recent years, due to toxicity concerns. DMDEE is commonly used in many one-component moisture-curing urethanes. DMDEE is one of the few tertiary amines with a low alkalinity and a low vapor pressure. The latter... 

Most moisture-curing hot-melt adhesives utilize a crystallizable backbone and are based almost exclusively on monomeric MDI at NCO/OH ratios of 1.5 to 2.2. Poly(hexamethylene adipate) polyol is the workhorse of the curing hot-melt adhesives. 

Moisture-curing hot melts are a small but fast growing segment of the urethane adhesive market. They are used mostly in construction and furniture assembly applications. Recent applications include RV sidewall assembly and other OEM automotive applications. Smaller applications include bookbinding and footwear. A typical adhesive is shown below ... 

Curing hot melts usually have less fixturing strength than do standard hot-melt adhesives but eventually develop structural strength. Once the moisture cure is complete, the curing hot melt has good structural properties and has better elevated heat strength than does a traditional hot melt such as EVA. 

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