Urethane adhesives moisture resistance

The specialty class of polyols includes poly(butadiene) and polycarbonate polyols. The poly(butadiene) polyols most commonly used in urethane adhesives have functionalities from 1.8 to 2.3 and contain the three isomers (x, y and z) shown in Table 2. Newer variants of poly(butadiene) polyols include a 90% 1,2 product, as well as hydrogenated versions, which produce a saturated hydrocarbon chain [28]. Poly(butadiene) polyols have an all-hydrocarbon backbone, producing a relatively low surface energy material, outstanding moisture resistance, and low vapor transmission values. Aromatic polycarbonate polyols are solids at room temperature. Aliphatic polycarbonate polyols are viscous liquids and are used to obtain adhesion to polar substrates, yet these polyols have better hydrolysis properties than do most polyesters. 

The two-component waterborne urethanes are similar in nature to the one-component waterborne urethanes. In fact, many one-component PUD s may benefit from the addition of a crosslinker. The two-component urethanes may have higher levels of carboxylic acid salt stabilizer built into the backbone than is actually needed to stabilize the urethane in water. As a result, if these two-component urethane dispersions were to be used as one-component adhesives by themselves (without crosslinker), they would show very poor moisture resistance. When these two-component urethane dispersions are used in conjunction with the crosslinkers listed in Fig. 8, the crosslinkers will react with the carboxylic pendant groups built into the urethane, as previously shown in the one-component waterborne urethane section. This accomplishes two tasks at the same time (1) when the crosslinker reacts with the carboxylic acid salt, it eliminates much of the hydrophilicity associated with urethane dispersion, and (2) it crosslinks the dispersion, which imparts solvent and moisture resistance to the urethane adhesive (see phase V in Fig. 5). As a result of crosslinking, the physical properties may be modified. For example, the results may be an increase in tensile properties and a decrease in elongation. Depending upon the level of crosslinking, the dispersion may lose the ability to be repositionable. (Many of the one-component PUD s may... 

As previously mentioned, some urethanes can biodegrade easily by hydrolysis, while others are very resistant to hydrolysis. The purpose of this section is to provide some guidelines to aid the scientist in designing the desired hydrolytic stability of the urethane adhesive. For hydrolysis of a urethane to occur, water must diffuse into the bulk polymer, followed by hydrolysis of the weak link within the urethane adhesive. The two most common sites of attack are the urethane soft segment (polyol) and/or the urethane linkages. Urethanes made from PPG polyols, PTMEG, and poly(butadiene) polyols all have a backbone inherently resistant to hydrolysis. They are usually the first choice for adhesives that will be exposed to moisture. Polyester polyols and polycarbonates may be prone to hydrolytic attack, but this problem can be controlled to some degree by the proper choice of polyol. 

Epoxy-urethane adhesives provide properties when cured that are similar to those of epoxynylon adhesives except they offer a major improvement in moisture resistance. Isocyanate monomers and prepolymer react with the hydroxyl groups on epoxy resins to give tough,... 

Another urethane one-part adhesive (Urethane Bond) developed by Dow Corning is cured by moisture in the air at room temperature. This material requires a thin glue line and clamping to produce the strongest joints. The resultant bonds are moisture resistant and are claimed to work well on polystyrene, PVC, and acrylics, and fair on polyethylene.There are excellent references for additional study of this adhesive family. 

Performance and durability of wood products bonded with isocyanate and urethane adhesives have been the subject of several investigations. The strength of composition board is generally determined by its modulus of rupture (MOR), which is a flexure to break test its modulus of elasticity (MOE), or stiffness and its internal bond strength (IB), or tensile strength. These tests are run on dry and on water-soaked or wet samples, and are described in ASTM test method D 1037. As compared to phenol-formaldehyde adhesives, composition boards prepared with isocyanate or urethane adhesives are reported to display similar or superior initial strengths. Moisture resistance of isocyanate-bound par-... 

As a class, urethane adhesives have somewhat poorer thermooxidative and moisture resistance than acrylic and epoxy structural adhesives. This has historically limited their expansion into certain areas of use. A 2K adhesive having the ability to survive automotive paint oven temperatures, which rim as high as 205° C,... 

Wattle tannins have been used as fortifiers in starch-based adhesives for corrugated carton manufacture in order to improve the moisture resistance of boxes used in shipment of fruit, for example (144, 217). Addition of 4% wattle tannin based on resin solids substantially improved the durability of these cartons. Wattle tannin urethane-based varnishes with excellent durability have been developed by Saayman (214). Other adhesive applications based on reactions of wattle tannins with isocyanates have been developed for specialty applications such as the bonding of aluminum (181). 

Silane-modified urethane adhesives offer moisture-resistance. The gain of peel strength at high R.H. is remarkable. A mercapto-silane is used to react with the urethane prepol3mier. 

Urethanes Urethane adhesion promoters are used widely to bond PVC to polyester, nylon, and other fabrics, including sized glass fiber. In multicoat operations, the additive is used only in the first coat. With, polyester fabric, the isocyanate component in urethane formation can be used alone. Solvent or plasticizer solutions of oligomerized toluene diisocyanate (TDl) are available under the trade names Vulcabond (Akzo Nobel), Vanchem HM-50 (Vanderbilt), Thanecure T9 (TSE Industries), and Desmodur (Bayer). For improved resistance of the bond to heat and moisture, products based on bisphenyl diisocyanate (MDl) are also used. These include Vanchem HM-4346 and Rubinate 9480 (Huntsman). These are typically used at 3-7 phr levels in plastisols for coating polyester fabric. The second or subsequent coats should contain titanium dioxide and UV light stabilizer for outdoor service. With translucent products, an aliphatic isocyanate such as Desmodur N-lOO or TMl META (Cytec) should be used. 

Sihcones (qv) have an advantage over organic resias ia their superior thermal stabiUty and low dielectric constants. Polyurethanes, when cured, are tough and possess outstanding abrasion and thermal shock resistance. They also have favorable electrical properties and good adhesion to most surfaces. However, polyurethanes are extremely sensitive to and can degrade after prolonged contact with moisture as a result, they are not as commonly used as epoxies and sihcones (see Urethane polymers). 

Urethane Asphalt Adhesive/Membrane A urethane asphalt elastomer serves as both an adhesive and a membrane to protect the substrate. It is a two-component material which bonds the blocks to each other and to carbon steel, alloy steel, concrete or other organic linings and also functions as a moisture and chemical-resistant barrier (or membrane) between the block and the substrate. 

Uses Soft segment in formation of elastomers such as thermoplastic urethanes, polyether esters, polyether amides, coatings, adhesives and sealants, casting resins, and urethane foams Features Produces elastomeric end-prods, with hydrolytic stability, resist. to microbial attack, low temp, flexibility and elasticity Properties APHA 40 max. wax-like solid sol. in most org. solvs. barely sol. in water m.w. 650 sp.gr. 0.977 (40 C) acid no. 0.05 hyd. no. 166.2-179.5 flash pt. 215 C 0.025% water Storage Hygroscopic protect from moisture and air dry nitrogen blanket should be applied to open containers before resealing Polythix M [Poly-Resyn]... 

PENNGUARD Adhesive/Membrane is a two-component urethane asphalt composition used in the installation of the PENNGUARD Block Lining System. As an adhesive, it serves to bond the PENNGUARD Block to a prepared substrate, and as a membrane, it serves as a moisture and chemical-resistant barrier. TUFCHEM II Membrane is an improved, two component, urethane asphalt based membrane system, that can be used, within its limitations, to protect concrete and steel surfaces from chemical attack and abrasion. 

The new steel panels used in vehicles are not as heavy as in older cars but, being thinner, are not as strong. The use of PU windshield adhesive/sealants to replace the traditional butyl sealants has enabled the windshield to become a structural part of the vehicle because it supports the roof. OEM adhesives are typically one-component moisture-cured urethanes that take several hours to cure and give bonds with tensile shear strengths of 5-8 MPa. The products are pigmented black to give resistance to UV. After-market adhesives... 

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