Blocked urethane adhesives

Blocked urethane adhesives are termed blocked by virtue of the fact that one of the key reactants is chemically blocked to prevent reaction. The concept of a blocked isocyanate has already been discussed. 

Applications for blocked urethane adhesives are small. However, they may be used as flocking adhesives or as crosslinkers for solvent-borne adhesives. Blocked urethane adhesives are also used as splicing adhesives for belts. Blocked isocyanates based on TDI, IPDI, and derivatives of hexamethylene diisocyanate are most commonly commercially available. 

BOROSILICATE GLASS BLOCK ON ASPHALT-URETHANE ADHESIVE MEMBRANE. 

Uses Polyester for coil coatings, appliance coatings, adhesives, blocked urethane systems... 

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. 

In the next paper, Illinger, Lewis and Barr find that the morphological structure of polyurethane adhesives affects both the optical clarity and the energy-absorbing properties of acrylic-poly-urethane adhesive-polycarbonate laminates. The morphological structure of the adhesive is in turn related to the composition of the hard and soft segments of the blocked polyurethane adhesives. 

Desmocap Blocked Urethane Oligomer For Improved properties for Coatings and Adhesives... 

Aminoamides Polyaminoamides are used to improve bonding of PVC plastisols to metal surfaces. Used at 3-5 phr, generally dissolved in solvent or plasticizer, such products are available under the trade names Euretek (Schering) and Versamid (Henkel = Cognis). The bond formed is to the surface metal oxide layer. Combinations with urethane adhesion promoters should not be used, since the aminoamides are strong urethane catalysts. A topcoat (without adhesion promoter) is usually desirable it should be formulated to resist amine stain. Phenolic antioxidants used with aminoamides (or other amine additives) should have all ortho and para positions blocked to prevent color development on aging. Similarly, aliphatic phosphites are the best choice. 

Block copolymers can contain crystalline or amorphous hard blocks. Examples of crystalline block copolymers are polyurethanes (e.g. B.F. Goodrich s Estane line), polyether esters (e.g. Dupont s Hytrel polymers), polyether amides (e.g. Atofina s Pebax grades). Polyurethanes have enjoyed limited utility due to their relatively low thermal stability use temperatures must be kept below 275°F, due to the reversibility of the urethane linkage. Recently, polyurethanes with stability at 350°F for nearly 100 h have been claimed [2]. Polyether esters and polyether amides have been explored for PSA applications where their heat and plasticizer resistance is a benefit [3]. However, the high price of these materials and their multiblock architecture have limited their use. All of these crystalline block copolymers consist of multiblocks with relatively short, amorphous, polyether or polyester mid-blocks. Consequently they can not be diluted as extensively with tackifiers and diluents as styrenic triblock copolymers. Thereby it is more difficult to obtain strong, yet soft adhesives — the primary goals of adding rubber to hot melts. 

An interesting blocked room-temperature curing urethane windshield adhesive is described by Barrons [63] ... 

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

Although polyethers are the main building blocks of the prepolymer, other materials such as polyesters, polythioethers, and polybutadienes are also used. Most urethanes use blends of polymers to achieve desired properties. Urethane sealants have pood inherent adhesion to most substrates, but silane adhesion promoters are often used to unprove this adhesion,... 

Blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines.18,19 These blocked prepolymers will initially react with the amines to form amine-terminated prepolymers that crosslink the epoxy resin. Urethane amines are also offered commercially for use with epoxy resins to develop hybrid adhesive systems.20... 

Organic isocyanates are the major building blocks of urethane structural adhesives. They can be synthesized by a variety of routes, but most of the commercially available isocyanate compounds used in adhesives are made by the reaction of a precursor primary amine or amine salt with phosgene, followed by dehy-drohalogenation. The reaction with phosgene usually is carried out at a relatively low temperature, less than 60°C, and then the temperature is raised to 100-200°C to remove the HC1. 

There are two different bonding systems employed with the block a urethane asphalt adhesive/membrane or a special inorganic silica-based mortar. The choice depends on mechanical considerations and on the chemical and thermal environment. 

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. 

Figure 17-2 Foamed borosilicate glass block and urethane asphalt adhesive/membrane over fiberglass reinforced plastic.

The insulating power and chemical resistance of the glass block allows for a thinner lining and better membrane protection for pickle tanks where stability is not a concern. The tank is first lined with the rubber membrane, the block is bonded over that with its urethane asphalt adhesive/membrane and finally a course of acid-resistant brick laid in acid-resistant mortar is placed over the block for mechanical protection. 

In the early 1970s, light, closed-cell foamed borosilicate glass block (see Chapter 17) were developed. These block, with a mass of only 12 Ib/cu.ft., will adhere to the underside of a concrete or steel cover by using a urethane asphaltic adhesive all joints between them are made the same way. Insulation of this type has been applied to the underside of both concrete and steel covers and has functioned successfully for a decade. 

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