Isocyanate reaction blocked

It is generally accepted that the tin compounds do not assist the elimination step for blocked isocyanates that function by the elimination-addition mechanism, but it is conceivable that tin catalysts can act as classic Lewis acids to promote the transesterification, in the case where the addition-elimination mechanism is operative. Tin catalysts have been shown to act synergistically with other metal catalysts active in promoting blocked isocyanate reactions such as metal acetylacetonate. Tin catalysts have also been found to act synergistically with amines in circumstances where the elimination-addition mechanism was confirmed.20... 

The catalyst selection for blocked isocyanates depends on the nature of the blocking agent, the desired cure temperature, the polyol structure, and the application. Mono-, di- and trialkyltin compounds are effective catalysts for many blocked isocyanate reactions with hydroxyl compounds. In most solvent-borne coatings, DBTDL is used as a catalyst. 

Table 6.2.9 shows the cure temperature obtainable with a range of blocked isocyanates. Co and Mn carboxylates are very effective catalysts, but in most formulations are not acceptable because of the purple or brown color they produce. Some of the lower cure temperatures reported in the literature are onset of cure, or cure in very thin films. Many blocked isocyanate reactions are reversible, and the rate of cure depends on the rate of evaporation of the blocking agent. In most published reports the effectiveness of a catalyst is related to the structure of the blocked isocyanate, and not much attention is paid to the... 

With this system and the building blocks 19,28, and 45 shown in the scheme up to a branched hexasaccharide W-glycan was successfully obtained (11 steps on solid phase, 19% overall yield 86% per step) (see 34). This was a major improvement over the results reported in Scheme 4. A careful study of this reaction sequence revealed that the second glycosylation step is incomplete, thus leading also to byproduct formation (48). Therefore, a capping step of unreacted hydroxy groups based on benzoyl isocyanate reaction was introduced (25). As expected and shown in Scheme 11, this way clean product formation could be gained, however the overall yield was not increased. 

Resins obtained using alternative means to phenyl isocyanate for blocking the hydroxyl functionality of a sucrose "penta" ester, e.g. reaction with aluminium diisopropoxide monoacetylacetonate or cyclic anhydride/ epoxide also show promise. 

Keywords Additives Aliphatic Aliphatic isocyanates Amine-isocyanate reaction Aromatic isocyanates Blocked isocyanates Chain extenders Construction (application in) Glass transition temperature Hydroxyl-isocyanate reactions Hot melt reactive PU Isocyanates MDl Moisture curing PU One-component PU Packaging (application in) Plastic and composites (application in) Polyester Polyether polyols Polyisocyanates Sealants Silane PU hybrids TDI Testing of PU adhesives testing of PU sealants Testing standards TPU thermoplastic PU Transportation (application in) Two-component PU Water-borne PU... 

Another group of polyisocyanates commonly used are blocked isocyanates. A blocked isocyanate is an isocyanate which has been reacted with a material, e.g. monofunctional alcohols or amines, to prevent its reaction at room temperature, with compounds that conventionally react with isocyanates, but will permit that reaction to occur at higher temperatures [17]. Less commonly used terms to describe them are capped , heat latent , thermally liable , masked , and splitters . Their chemistry has been extensively reviewed by Wicks et al. [17-20]. 

One-part polyurethane adhesives can also correspond to prepolymers with isocyanate functions blocked reversibly (due to reaction with oximes, phenols, etc.). The isocyanate groups are then regenerated, either by heating to a more or less high temperature, or by substitution of the blocking agent by a diol (oligomer, at least partly) (O Fig. 14.18). 

This simple reaction is the bedrock of the polyurethane iadustry (see Urethane polymers). Detailed descriptions of the chemistry and process have been published (65—67). Certain carbamates are known to reversibly yield the isocyanate and polyol upon heating. This fact has been commercially used to synthesize a number of blocked isocyanates for elastomer and coating appHcations. 

Developments in aliphatic isocyanates include the synthesis of polymeric aliphatic isocyanates and masked or blocked diisocyanates for appflcafions in which volatility or reactivity ate of concern. Polymeric aliphatic isocyanates ate made by copolymerizing methacrylic acid derivatives, such as 2-isocyanatoethyl methacrylate, and styrene [100-42-5] (100). Blocked isocyanates ate prepared via the reaction of the isocyanate with an active hydrogen compound, such as S-caprolactam, phenol [108-95-2] or acetone oxime. 

One-part urethane sealants (Table 3) are more compHcated to formulate on account of an undesirable side reaction between the prepolymer s isocyanate end and water vapor which generates carbon dioxide. If this occurs, the sealant may develop voids or bubbles. One way to avoid this reaction is to block the isocyanate end with phenol and use a diketamine to initiate cure. Once exposed to moisture, the diketamine forms a diamine and a ketone. The diamine reacts with the isocyanate end on the prepolymer, creating a cross-link (10). Other blocking agents, such as ethyl malonate, are also used (11). Catalysts commonly used in urethane formulations are tin carboxylates and bismuth salts. Mercury salt catalysts were popular in early formulations, but have been replaced by tin and bismuth compounds. 

When polyols, di-isocyanates and glycols are reacted together as described in the previous section they do in fact tend to produce block copolymers as can be seen from the following reaction mechanism ... 

Blocked isocyanate, for our purposes, will refer to the reaction product of a diisocyanate or isocyanate-terminated prepolymer in which the isocyanate functionality has been reacted with a blocking agent . Once blocked , the diisocyanate can be added to polyols or certain chain extenders, and these materials will not react at room temperature. The concept is shown in the sixth item of Fig. 1. An adhesive formulated with a blocked isocyanate is basically a two-component adhesive that does not react until heated to the activation temperature. When an adhesive is made with a blocked isocyanate together with hydroxyl-containing curatives, the adhesive has a good long shelf life at room temperature. However, once heated... 

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. 

The blocked isocyanate systems (with curative present) are latent cure systems. In order to create a latent curing adhesive, the blocked isocyanate is added to a catalyzed polyol component without a reaction occurring at room temperature. In theory, the blocked adhesive system is relatively stable at room temperature. When this system is heated to the unblocking temperature, the chemical reaction, which... 

Polyaddition reactions based on isocyanate-terminated poly(ethylene glycol)s and subsequent block copolymerization with styrene monomer were utilized for the impregnation of wood [54]. Hazer [55] prepared block copolymers containing poly(ethylene adipate) and po-ly(peroxy carbamate) by an addition of the respective isocyanate-terminated prepolymers to polyazoesters. By both bulk and solution polymerization and subsequent thermal polymerization in the presence of a vinyl monomer, multiblock copolymers could be formed. 

Polyurethanes are manufactured by the mixing of various resins, isocyanates and catalysts to produce an exothermic reaction, which liberates the foaming agent and causes the mix to expand. They are made in large block molds as a batch process or are continuously foamed onto a paper or polythene substrate on a conveyor system. 

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