Blocked isocyanate

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. 

The use of polystannoxane catalysts with improved cure performance over dibutyltin or other stan-noxane compounds has been claimed with a range of blocked isocyanates iron, zinc, zirconium, nickel, and bismuth salts are claimed as cocatalysts. The improved catalytic activity appears, to some extent, related to increased solubility of the polystannoxanes. 

Stable one-pack polyurethane compositions containing the oxadiazinetrione ring are catalyzed and cured using l,3-bis(acetyloxy)-l, 1,3,3-tetra-n-butyl-distannoxane.  

Phenol blocked isocyanates also have been used to prepare hyperbranched polyurethanes by a step-growth polycondensation mechanism, using DBTDL as a catalyst. 4 

Because of the toxicity of isocyanates, there is interest in preparing blocked isocyanates and polyurethanes by a non-isocyanate process. Reaction of aliphatic diamines with cyclic carbonates (l,3-dioxolan-2-ones) results in the formation of /3-hydroxyalkyl carbamates. 

The isocyanate moiety is highly reactive towards poly(ol)s. This reaction is utilized in the preparation of poly(urethane)s (PU)s. With catalysts such as amines, the reaction occurs rapidly at room temperature. Consequently, urethane polymers are mostly prepared from two components, one containing the isocyanate component, and the other containing the polyol component. The components are nuxed prior to curing. 

However, there is one component formulations available that can be cured by heating. PU powder coatings consist essentially of a polyol and a poly(isocyanate), whose NCO groups are partially or completely masked with a blocking agent, so that the polyaddition reaction is inhibited at temperatures below 140°C. 

An alternative blocking agent is 1,2,4-triazole. Mixtures of diisopropylamine and 1,2,4-triazole may also be applied for blocking. 

The crosslinking temperature of aliphatic poly(isocyanate)s, based on 1,6-hexane diisocyanate, which are blocked with equimolar quantities of diisopropylamine and 1,2,4-triazole is 139°C, without the addition of any catalyst. Aliphatic poly(isocyanate)s, which are bloeked with equimolar quantities of diisopropylamine and malonic acid diethyl ester have a cross-linking temperature of 130°C. ° 

Triazole blocked isocyanates are stable up to 130-140°C. The blocking reaction can be performed at a temperature of about 15-25 C below the unblocking temperature. The storage stability is distinctly improved in comparison to those formulations that are exclusively blocked with CH-acidic esters, such as malonic acid diethyl ester.  

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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. 

Acryflc coating powders have achieved some success in Japan utilizing resins having gflcydyl methylacrylate functionality cured with C q—0 2 dicarboxyflc acids (49). Hybrid polyester—acryflc coating powders have also been reported in which an acid functional polyester resin coreacts with a glycidyl-containing acryflc polymer (50). Hydroxyl functional acryflc resins cured with blocked isocyanates have also been available for many years in the United States and achieved some commercial success as appliance finishes. 

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 isocyanates are particularly helpful in dual cure mechanisms. In one instance, UV light first polymerizes an acrylate polymer containing hydroxyl groups. The system also contains a malonate ester-blocked isocyanate. The one-component system is heated, which starts the polymerization of the acrylate. Higher temperatures unblock the isocyanate, permitting the cure of the urethane to proceed [15]. 

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 concept of utilizing a blocked isocyanate in a one-component adhesive system is shown below ... 

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. 

The blocked isocyanate does not react with the polyol at room temperature because it has already reacted with the blocking agent. Normally, if one adds polyols to the blocked isocyanates, the viscosity of the system is stable over long periods of time at room temperature. A good review of blocked isocyanates is found elsewhere [61]. 

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... 

Most blocked isocyanates are solids at room temperature and thus may require the use of solvent. The unblocking temperatures are often fairly high and are energy intensive. Furthermore, certain blocking agents may qualify as volatile organic compounds. For these reasons, the blocked isocyanate adhesives occupy a small, but important segment of the adhesive marketplace. 

Phenolic-blocked isocyanate prepolytner Thixotrope (Thixin R)... 

Phenol-Blocked Isocyanate Prepolymer + Ketimine (PBIP)... 

Fig. 7. Room-temperature curing concept for a phenol-blocked isocyanate-terminated prepolymer.

The particles therefore lose their charge. Since the charge provides the colloidal stability, the colloidal paint destabilises and deposits on the nearest surface, the car body. Primer coatings 12-35 /im thick are applied according to primer type. Each particle also contains a crosslinker for the resin, usually a blocked isocyanate. After rinsing, the primed article is passed into a hot... 

HPGPC also was used for quality control of incoming raw materials. Figure 8 shows the chromatograms of two different batches of blocked isocyanate crosslinkers. One was acceptable and the other was too reactive. As can be seen from the HPGPC traces, the level of the component eluted at retention volume 40 is much higher for CX-46 than for CX-48. This component was associated with free isocyanate functionality which in excess would make CX-46 too reactive. With this information, either the necessary adjustment for the presence of excessive free isocyanate functionality could be made or this particular batch from the supplier could be rejected. 

The use of blocked isocyanates to cure hydroxyl containing coatings is an example of a complex system having many practical applications. The chemistry of blocked isocyanates has been reviewed previously by Wicks (J.,.2). The cure reaction proceeds via consecutive first and second order reactions ... 

The effective development of blocked isocyanate based coatings requires a complete understanding of the cure chemistry. Many materials have been identified which will improve cure response. Often these are reported as deblocking catalysts even though no direct evidence exists to support this claim. Most of these materials are well known catalysts of the cure reaction between hydroxyl and isocyanate and may be improving cure response solely by catalyzing this reaction. Effective development of catalyst systems requires a better understanding of the effect of catalysts on cure. 

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