Crosslinking blocked isocyanates

Chem. Descrip. Branched polymer with ether and urethane groups, contg. crosslinking, blocked isocyanate groups Uses Raw material, flexibilizer for epoxy resins, flexible coatings and sealing compds., PU systems Features Solv.-free... 

Chem. Descrip. Branched PU prepolymer with crosslinking blocked isocyanate groups... 

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 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. 

FIGURE 4 EXPERIMENTAL DATA AND CALCULATED ISOTHERMAL CURE CURVES (A PREDICTEDD [BLOCKED ISOCYANATE], 0 EXPERIMENTAL [NCO], PREDICTED [NCO], + PREDICTED [CROSSLINKS])... 

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... 

Conventional Solvent-Based Coatings Aliphatic- Aromatic Hydrocarbons Melamine Epoxy Blocked isocyanates Polyester Acrylic Epoxy Urethane 40-60% solvent 30-40% polymer 10-20% crosslinking resin... 

Powder — Melamine epoxy blocked isocyanate Polyester acrylic epoxy 10-20% crosslinking resin... 

Neat blocked isocyanates (IPDI) were used in this study to avoid competing reactions between the isocyanate functionality and hydroxy groups which are present in typical crosslinking reactions. Thermal dissociation of blocked isocyanates is thought to proceed by one of two mechanisms . In the first (Scheme 2), thermal dissociation of the blocked isocyanate is followed by crosslinking with available functional groups. 

The blocked isocyanate function of aminimides also allows their use as polyurethane adhesives. Thus bis(aminimides) crosslink with polyester or polyether diols to yield a range of elastomers dependent upon the structure of the bis(aminimide) and the co-reactant chosen. For example, bis(aminimides) mixed with polyhydroxyl components can provide stable, single-package prepolymer compositions which yield polyurethanes on heating without the problem of isocyanate moisture sensitivity which can plague polyurethane applications. 

Polyester resins for powder coating are brittle solids with a relatively high Tg (50-60°C) and so the powder coating does not sinter during storage. Terephthalic acid (TPA) and NPG are used as the principal monomers with smaller amoimts of other monomers to increase fn and to reduce Tg to the desired level. Both hydroxy-and carboxy-terminated polyesters are used. The former are most commonly cross-linked with blocked isocyanates and the latter with epoxy resins. Other crosslinkers include 2-hydroxyalkylamides and tetramethoxymethylglycolmil. 

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). 

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]. 

Blocked Isocyanates which can be Used as Crosslinkers in Electrodeposition Paints... 

Thermosetting (crosslinking) acrylics normally require a crossUnker to be present. Their actual cure behaviour is dependent upon the pendant (side chain) functionality of the copolymerised co-monomers. Acrylamide will crosslink with itself, epoxy or amino resins, whilst amino or melamine crosslinkers will react with hydroxyl or acid groups. Isocyanates can react with hydroxyl groups at room temperature. To reduce the hazards of isocyanates and the inconvenience of two pack systems, blocked isocyanates can be used. At elevated temperatures used for stoving they can unblock generating an alcohol and an isocyanate which will react with any hydroxyl or acid groups on the acryUc resin. 

Hydroxyl functional acrylics can be crosslinked by either blocked isocyanates or aminoplasts, particularly melamines and benzoguanamines. Some acid functionality will be present to improve adhesion. Hydroxy acrylates or methacrylates can both be used and the choice may depend upon cost and Tg required. The former tend to give softer films. Stoving conditions will vary according to the crosslinking mechanism, but the commonly encountered conditions for other systems are used. 

Disperse items 1 2 by a Cowles mixer or bead mill. Uralac SN808 S2G3-50 (Ex DSM) is a saturated polyester resin specially designed for metal decorating and for cure by a benzoguanamine in the ratio 90/10 - 80/20 (solid/solid). In this example a ratio of 87/13 is used. To optimise flexibility, crosslinking with benzoguanamine, and also blocked isocyanate Desmodur BL 3175, are used to increase adhesion. Adhesion is fiuther enhanced by the use of an epoxy resin (Epikote 836 (Ex Shell)). 

Table 10 shows the attribute comparison of IK melamine crosslinked clearcoats as compared to 2K isocyanate crosslinked clearcoats. Figure 12 graphically shows selected attributes from this comparison. Blocked isocyanate clearcoats also exist, but these are not very common over plastic substrates presumably due to the higher temperatures needed to cause the unblocking (95). On the other hand, the use of low imino, methylated melamine resins (96) can provide cure at temperatures as low as 82°C, opening up the opportunity for use on heat-sensitive substrates. 

Z He, WJ Blank. Crosslinking with malonate blocked isocyanates and with melamine resins. J Coatings Technology 71(889) 85—90, 1999. 

The types of resins that comprise IK crosslinkers are generally of the aminoplast family of blocked isocyanates. 

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