Blocked polyisocyanates

An interesting variation of urethane technology is that of the blocked polyisocyanates. These are used when chemical-cure urethane chemistry is desired but, for technical or economical reasons, a two-pack coating is not an option. Heat is needed for deblocking the isocyanate, so these coatings are suitable for use in workshops 

Creation of the general chemical composition consists of two steps  

An example of the application of blocked polyisocyanate technology is polyurethane powder coatings. These coatings typically consist of a solid, blocked isocyanate and a solid polyester resin, melt blended with pigments and additives, extruded and then pulverized. The block polyisocyanate technique can also be used to formulate waterborne polyurethane coatings [8]. 

Additional information on the chemistry of blocked polyisocyanates is available in reviews by Potter et al. and Wicks [13-15]. 

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Blocked polyisocyanates, which are addition compounds of labile hydrogen-containing compounds with polyisocyanates, are shown below ... 

Blocked polyisocyanates are inert compounds at ambient temperature, and they generate free polyisocyanates at elevated temperatures by thermal dissociation (208). Blocked polyisocyanate technology is used in one-component urethane coatings. 

Polyurethanes (single-component system) Block polyisocyanates and hydroxyl polymers Phenol-blocked isocyanates and hydroxyl groups Unblocking or removal of phenol to form a free isocynate followed by addition of hydroxyl groups to form urethane linkages 8 ... 

OH branched, low molecular mass, hydroxy-functional 1000- 5000 melamine resins, free/blocked polyisocyanate resins automotive/ industrial paints, polyurethane/ powder coatings... 

Polyurethane systems based on hydroxypolyester resins and free polyisocyanate resins (two-pack polyurethane paints. Section 2.9) are used particularly for finishes of road vehicles, railroad vehicles, aircraft, and plastics. Blocked polyisocyanate resins are available for formulating one-pack stoving enamels which are particularly suitable for coil and can coatings [2.83]. 

Surfacers are mainly produced from low molecular mass polyester resins cross-linked with amino resins. Blocked polyisocyanate resins may also be used for high stone chip resistance. Water-soluble polyester resins are increasingly used for water-thinnable stoving surfacers. Spray and dip paints based on polyester resins are used to coat automobile accessories (e.g., windscreen wipers, axle parts). 

Hydroxy-functional resins for combination with blocked polyisocyanate resins (based on isophorone diisocyanate) [2.96]... 

Blocked polyisocyanates are obtained by chemical addition of compounds that contain acidic or potentially acidic hydrogen atoms (e.g., alcohols). These isocyanates are not reactive at room temperature since they do not contain free isocyanate groups. Such polyisocyanates can, however, be regenerated by heating to eliminate the blocking agent. 

Water-soluble binders (e.g., polyesters, polyacrylates) can also be chemically cured with cross-linking agents at elevated temperature (oven drying). These binders can be reacted with water-soluble or water-miscible amino resins (especially fully or partially methylated methoxymethylmelamine resins) or with water-soluble or water-miscible blocked polyisocyanates, which only react after the isocyanate groups have been unblocked. 

R. Gras. Blocked polyisocyanates, a process for their preparation, and then-use. US Patent 6437 074, assigned to Huels Aktiengesellschaft (Marl, DE), August 20, 2002. 

Chem. Descrip. Phenol-blocked polyisocyanate Uses Binder for wire enamels... 

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. Blocked polyisocyanate adduct Uses Raw material in one-component heat-curing systems for adhesives and coatings... 

Biobased polyols with high functionality have been synthesized by ring-opening epoxidized sucrose esters of soybean oil with methanol under acidic conditions, and were subsequently formulated with blocked isocyanates to form one package PUs (16). Biobased PU coatings were prepared by formulating the polyols with blocked polyisocyanates based on isophorone diisocyanate and hexamethylene diisocyanate (HDI). 

This technique has been adapted for PLA/PEG blends investigated in the range of 100/0 to 70/30. Cross-linking occurred by adding blocked polyisocyanate. 

L. (2012) Influence of blocked polyisocyanate on thermomechanical, shape memory and biodegradable properties of poly(lactic acid)/poly(ethylene glycol) blends. Iran. Polym. J., 21 (5), 317-323. 

Certain textile cords, such as polyester, do not have sufficient surface activity to properly react with the RFL dip. Therefore, these special textile cords require a predip in a proprietary blocked polyisocyanate and/or an epoxy dip, after which they are heat treated and dried before the RFL dipping procedure. 

Most of the aramid adhesive systems initially used were based on a first-step dip of either an epoxide or a blocked polyisocyanate and then a second or top coat of RFL. Wenghoefer in his patent disclosed a somewhat different first step dip which consisted of bis(j3-azido-formyl oxyethyl)isophthalate. After drying, the cord thus treated, was then dipped in a phenolic adhesive and cured at 230 C for 60 seconds. A patent issued to Van Gils uses a one-step dip based on vinyl pyridine latex, trimethylol-phenol, resorcinol-formaldehyde resin, and... 

Blocked isocyanates are used extensively in many CASE applications mostly for technical and economic reasons when the presence of free isocyanate must be eliminated, due to the potential toxic hazards associated with their use, and heat curing is possible. Blocked polyisocyanates are also used to crosslink both solvent-borne and water-borne resins offering a wide spectrum of possible formulations (refer to Sections 3.S.2.2 and 3.5.2.3). 

For instance, blocked polyisocyanates based on aromatic polyisocyanates dissociate at lower temperatures than those based on aliphatic ones. Generally, the dissociation temperatures of blocked polyisocyanates based on commercially utilised blocking agents decrease in the order alcohols > s-caprolactam > phenols > methyl ethyl ketoxime > active methylene compounds. 

One-component water-borne polyurethane systems can be derived from polyurethane dispersions or blocked polyisocyanates (refer to Section 3.2.1). Blocked isocyanates are added to the co-reactant resins providing one-component systems with excellent shelf life. This type of adhesives is principally used for non-porous materials and the bonding of unlike metals such as aluminium to steel, and stainless steel to mild steel. They are also usefiil in bonding some of the high pressure laminates such as those based on phenoUcs and melamine [39]. Systems based on water-borne blocked polyisocyanate crosslinkers and suitable waterborne polymers approach the performance levels previously obtained only by solvent-borne systems [16]. 

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