Crosslinking polyisocyanates

Crosslinkers. Polyisocyanates with functionality greater than two (such as p, p", p " -triisocyanate triphenylmethane, thiophosphoric acid tris (p-isocyanatephenyl) ester) can be added to improve specific adhesion and heat resistance of the polyurethane adhesives (especially those with slower crystallization rate). Most of them have 20-30 wt% solids, 5.4-7 wt% free NCO and the most common solvent is ethyl acetate. 5-10% polyisocyanate is generally added in the adhesive solution just before appHcation on the substrate, and it acts as cross-linking agent. Since all isocyanates react with the residual hydroxyl groups on the polyurethane, in solution they all yield adhesive systems with a limited pot life (1-2 h to several days). Addition of polyisocyanate improves the spotting tack and heat activation of the freshly dried adhesive films, but after several days they lose their capacity to be reactivated. 

Two-component waterborne urethane dispersions are similar to the one-component PUD s in that a polyurethane dispersion comprises one of the two components. The second component is usually a crosslinker from the following classes of materials (a) polyisocyanates, (b) aziridines, (c) polycarbodiimides, and (d) epoxies. Many of the crosslinkers are not inherently water-soluble or water-dispersible. Therefore, they must be modified with surface active agents themselves, so as to become emulsifiable in water. 

Even with elastomers, it is often desirable to phase mix the material for certain combinations of physical properties. For example, elastomers with high hardness and flexural modulus are prepared from polyisocyanates, polyols, and polyamines of high functionality, creating a high crosslink density. Although polyol chains... 

We can make polyurethanes via one- or two-step operations. In the single-stage process, diols and isocyanates react directly to form polymers. If we wish to make thermoplastic linear polymers, we use only diisocyanates. When thermosets are required, we use a mixture of diisocyanates and tri- or polyisocyanates residues of the latter becoming crosslinks between chains. In the first step of the two-stage process, we make oligomers known as prepolymers, which are terminated either by isocyanate or hydroxyl groups. Polymers are formed in the second step, when the isocyanate terminated prepolymers react with diol chain extenders, or the hydroxyl terminated prepolymers react with di- or polyisocyanates. 

This plastic includes a large group of polyaddition polymers which are formed through the reaction of bifunctional or trifunctional alcohols with di- or polyisocyanates. By varying the starting materials, linear as well as crosslinked macromolecules with correspondingly different properties are formed. Alcohols with three functional groups and/or triisocyanate are used to make crosslinked polyurethane elastomers (PUR). 

Maniiich bases (see ref. 23) ore reported to crosslink in film form at ambient ten erature, and are worthy of further appC lication investigation. Polyisocyanates have been used to react with vinyl polymers containing active hydrogen 5 ), but the subject is too vast for detailed consideration here. 

Example (1) Unsaturated monoalcohol (acryloesterol) and styrene monomer are reacted with a polyisocyanate in the presence of a urethane catalyst and a radical polymerization catalyst to form hybrid resins (52). Styrene monomer acts as a crosslinker and at the same time, acts also as a reactive diluent. The trade name of a commercial product of such systems is Arimax (Ashland Chemical) (107). 

These compounds can be copolymerized with styrene to form crosslinked copolymers. When polyisocyanates are incorporated in these reaction systems, the resulting polymers are hybrid polymers containing polyurethane linkages. Rigid foams can be obtained by using the above reaction in the presence of a blowing agent. 

In the case of using aromatic polyisocyanate in making polyimide foams, crosslinking is increased by adding a highly functional polyol to react with part of the isocyanate so that the foam contains some urethane structure. A typical foam prepared in this way would have a density of 3.97 Ib/ft (63.5 kg/m ) and a K-factor of 0.26 Btu/(h) (fP) ( F/in). Foams of this type have been prepared in a range of densities of 2.5 to 18.5 Ib/ft (40 to 296 kg/m ) with compressive strengths of 25 to 1340 psi (172 to 9239 kPa) (8). 

The main component of these adhesives consists of pre-crosslinked, high molecular weight polyurethanes. These prepolymers (Section 2.1.3) exist in a liquid or paste-like state and still show free isocyanate groups (so-called polyisocyanate-polyurethane). With these isocyanate groups the -O-H group... 

Polyure- thanes PUR Di- or polyisocyanates react with diols or polyols to form crosslinked hard or soft (usually elastic) products -N=C=0+H0- i -NH-CO-0- Isocyanates (e. g., MDI, TDI, Desmo-dur) and OH-con-taining compounds (different polyols) are reacted in the liquid or molten state... 

Chem. Descrip. Aliphatic acrylic resin in MAK Uses Designed to crosslink at room temps, with polyisocyanates to produce high-performance coatings... 

Features Hydroxy-functional crosslinkable with polyisocyanates pigment compat. high film hardness, flexibility, abrasion resist. exc. diem, resist. 

Uses Polyester blending resin for use with other resins in solv.-based or water-based paints, one-pack enamels, polyisocyanate-crosslinking two-pack systems... 

Chem. Descrip. Polyester resin (78%) in butyl acetate Uses Polyester tor ultra high solids, two-pack, clear and pigmented top coats tor automotive repair and industrial lacquers Features Hydroxy-tunctional crosslinkable with polyisocyanates cured films provide high hardness, exc. solv.- and chem. resist., superior weathering stability can produce dear coats with VOC 350 g/l, and pigmented top coats with VOC 420 g/l... 

Features Hydroxy-functional crosslinkable with polyisocyanates exc. 

Features Hydroxy-functional crosslinkable with polyisocyanates high gloss exc. mech. props., adhesion to steel and noniron metals (Al, Zn) Properties Hazen < 200 color dilutable with toluene, xylene, acetone, MEK, MIBK, methoxypropyl acetate, ethyl acetate, butyl acetate dens. = 1.0 g/cm (20 C) dynamic vise. 1100-2500 mPa s hyd. no. 40-50 flash pt. 28 C 58-62% NV... 

Chem. Descrip. Acrylic resin (65%) in methoxypropyl acetate Uses Acrylic for air-drying and stoving two-component industrial coatings, wood coatings, metal coatings, plastic coatings Features Hydroxy-functional crosslinkable with polyisocyanates exc. 

Uses Acrylic for fast drying two-pack coatings, industrial lacquers Features Hydroxy-functional crosslinkable with polyisocyanates Properties Hazen < 80 color dens. = 1.00 g/cm (20 C) dynamic vise. 

Chem. Descrip. Aromatic polyisocyanate in toluene Uses Adhesion promoter in adhesives primer or adhesive additive for adhering elastomeric coatings to syn. fiber fabrics coated with syn. rubbers crosslinking agent for elastomers and plastics Features No ozone-depleting substances Properties Dk. brn. liq. mod. sol. in alcohol, toluene, chloroform si, sol. in petrol, ether pract. insol. in water dens. 1,05 0.02 mg/m flash pt. (PM) 4 C min,... 

Chem. Descrip. Blocked polyisocyanate Uses Crosslinking agent for polyurethane powd. coatings Properties Pellets bulk dens. 630 kg/m 100% solids Vestagon B1530 [Degussa AG/Marl]... 

Chem. Descrip. Polyisocyanate prepolymer Uses Crosslinking agent for two-component polyurethane paints Properties SI. yel. liq. sp.gr. 1.06 g/cm (15 C) 70% solids in n-butyl acetate... 

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