Cross-linker , isocyanate

Cross-linker Isocyanate silane Amine adduct None Amine adduct... 

Figure 7. HPGPC chromatograms of four isocyanate cross-linkers...

Figure 8. HP GPC chromatograms of two isocyanate cross-linkers from different...

Figure 1. Typical melaminc-fonnaldehydc and isocyanate cross-linkers.

The synthesis of the module is provided in Scheme 10.5 (Kushner et al. 2007). Double alkylation of ethyl acetoacetate followed by guanidine condensation afforded alkenyl-pyrimidone intermediate 24 (Kushner et al. 2007). Isocyanate 25 was coupled to pyrimidone 24 to yield 26. Upon dimerization in DCM, RCM effectively cyclized the two UPy units (Mohr et al. 1997 Week et al. 1999). A one-pot reduction and deprotection through hydrogenation using Pearlman s catalyst gave diol module 27. Finally, capping 27 with 2-isocyanatoethyl methacrylate at both ends provided the UPy sacrificial cross-linker 28, which was thoroughly characterized by H- and C-NMR, Fourier transform IR (FTIR), and mass spectrometry. 

A primer surfacer, composed most often of a polyester to be cross-linked with a blocked isocyanate or melamine cross-linker, is applied next at a dry film thickness of approximately 1.0 mil by conventional spray application. The function of this coating is to provide filling and leveling properties as well as some stone-chip resistance. Cure conditions range from 250 to 325°F for 20 min. 

The clear-coat is referred to as a two-component (2K) clear-coat when isocyanate is used as the cross-linker. The isocyanate must be added to the hydroxy functional acrylic just prior to spray application to prevent premature cross-linking (or gelation). For this reason, the two-part mixing is referred to as 2K. Cure conditions for isocyanate-cured clear-coat range from room temperature (car refinish) to 250°F for 30 min. 

The production of polyurethane involves the controlled polymerization of an isocyanate, a long-chain-backbone polyol and a shorter-chain extender or cross-linker. The reaction rates can be controlled through the use of specific catalyst compounds, well known in the industry, to provide sufficient time to pour or otherwise transfer the mix and to cure the polymer sufficiently to allow handling of the freshly demolded part. The use of blowing agents allows the formation of a definite cellular core (thus the term microcellular elastomer ) as well as a non-porous skin, producing an integral sandwich-type cross section. 

Otherwise a co-network would have been obtained. Thus PDMS network was formed starting from a,co-(3-hydroxypropyl)polydimethylsiloxane (diOH-PDMS oligomer) by DBTDL catalyzed addition between the hydroxy end groups and an pluri-isocyanate cross-linker (Scheme 2). No side reaction has been detected between these precursors. 

To the volumetric flask (5 mL) add the isocyanate cross-linker MBPI (5 mg) and make up the solution using dry dichloromethane (5 mL). 

Raw Materials for Urethane Coatings Isocyanate Components Di- and Polyhydroxy Components Chain Extenders and Cross-linkers Catalysts... 

Other water-soluble, blocked isocyanate cross-linkers have been prepared by reaction of diisocyanate with a,a-dimethylolpropionic acid followed by blocking with caprolactam and subsequent solubilization in water by means of a tertiary amine (81) or 2-ethylimidazole (82). 

Chain Extenders and Cross-linkers. In addition to the two principal components of most urethane coatings, isocyanate and polyol components, a number of di- or polyfunctional, active hydrogen components may be used as chain extenders or cross-linkers. The most important classes of compounds for this use are diols or polyols (monomers or oligomers), diamines, and alkanolamines. Typical examples of diols are ethylene, dlethylene, dlpropylene glycol, 1,4-butanedio1, 1,5-hexanediol, neopentyl glycol,... 

In the U.S. converting industry, one- and two-component solvent-based adhesives are still the most widely used, accounting for nearly 80% of adhesive formulations. Many of these adhesives are typically polyester-or polyether-based urethanes (see Chapter 4) with isocyanate fimctionaHty that cure by reacting either with atmospheric moisture or a cross-linker. Polyether-based urethanes, while less expensive than polyester-based, do not provide the same strength and do not perform well in some processing environments or end uses. 

The most conunonly used cross-linkers for CS membranes are epichlorohydrin (ECH) and glutaraldehyde (GA, Rgure 16.11). In literature, one can also find the other cross-linkers like toluene isocyanate and sodium tripolyphosphate (TPP). Most recently, Mio et al. [55] prepared amphoteric thin film NF membrane using sulfated CS as the active layer on to the surface of PSf UF support followed by cross-linking using ECH. The membranes were tested for salt rejection application. The membranes showed a maximum of 90.8% Na2S04 (with 22.9 kg h permeate flux) and 32.5% NaCl (with 58.4 kg-2 h- permeate flux) rejection, respectively, at 0.4 MPa pressure for 1000 mg/L feed concentrations. Thus, they concluded that the resultant cross-linked SCS/PSf membranes... 

Emulsion Polymer Isocyanate (EPI) adhesives are two-component adhesive systems consisting of water-based emulsions cured with an isocyanate cross-linker. The first EPI adhesives were developed in Japan in the early I970 s and this adhesive type has been extensively used in Japan since then [1-3]. 

Originally the adhesive system consisted of an aqueous poly(vinyl alcohol) solution with an isocyanate cross-linker. At this stage the common name was Aqueous Polymer Isocyanate (API) [3]. Further developments including use of different types of polymer emulsions, like poly(vinyl alcohol) (PVA), ethyl(vinyl acetate) (EVAc), styrene butadiene rubber (SBR) or acrylic-styrene (AcSt) emulsion, led to adhesives systems with improved performance, hence today the common abbreviation is EPI [4, 8]. The isocyanate cross-linking agent has also been further developed to improve the compatibility and the reactivity with the water-based component, thus several different types are now available for use in EPI adhesives. 

EPI adhesives are two-component systems. In the following the emulsion/polymer based component (the adhesive component) and the isocyanate based cross-linker component will be discussed separately. 

Isocyanates are used as a cross-linker in EPI adhesive systems. Theoretically, any isocyanate with two or more NCO groups would be suitable. In practice two main parameters are important for the choice of isocyanate The volatility and reactivity of the isocyanate [1, 12]. The use of isocyanates with low volatility, low vapor pressure, is preferred to minimize the health risks concerned with the use and handling of the isocyanate. Isocyanates that are typically used, or have been used, as cross-linkers in EPI adhesive systems are based on the following isocyanate monomers toluene diisocyanate (TDl), diphenylmethane diisocyanate (MDl), hexamethylene diisocyanate (HMDI), 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI) and triphenylmethane-triisocyanate (TTl)... 

Aziridine compounds having two or more pendant aziridine rings have been described as alternative cross-linkers for EPI adhesives, offering approximately the same reactivity as isocyanates [4]. To the best of oixr knowledge these compounds are not frequently used. 

Since the EPI adhesives are emulsion based but cross-linked with isocyanate, they share characteristics with both thermosetting and thermoplastic adhesives. The adhesives are multi-phase systems comprising emulsion particles, polymer solution, cross-linker droplets and filler particles. Just as for other emulsion adhesives, the coalescence of the emulsion particles [18, 19] and the distribution of these in the glue film is important for the bond quality. The cross-linking in the adhesive film is also of great importance for the bond quality as well as for the moisture resistance and heat resistance of the adhesive. 

The amount of isocyanate used in the EPI adhesive will sigiuficantly influence the adhesive properties. Krystiofiak et al. [22] showed that both the hardness of the glue film and the moisture- and heat-resistance of the glueline increased with increasing amounts of cross-linker in a series of tests where 10-20 parts cross-linker... 

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