Isocyanate crosslink

A more complete discussion of the kinetics of isocyanate crosslinking in the presence of humidity along with experimental verification of the rate equations used above has been given by van der Ven, et al. (18). Equations 38 and 39 can be integrated numerically for different values of isocyanate to hydroxy ratio and H. Plots of the consumption of hydroxy and isocyanate functionality are shown in Figure 2 for H-0 and H-0.66 assuming equal Initial Isocyanate and hydroxy levels. High humidity increases the rate of consumption of isocyanate and reduces the consumption of hydroxy. 

Figure 7 illustrates the use of HPGPC to aid a resin chemist in developing an in-house isocyanate crosslinker for a powder coating system. Isocyanate crosslinker X-02 gave desired properties and is considered the standard. At the early stage of the development, resin X-03 was initially made. By changing the types of reactants, molar ratio of reactants and reaction conditions, resin X-36 was the next iteration in the resin synthesis process. Finally, X-36 was fine-tuned to produce X-38 which matched X-02 in both its chemical reaction properties and its MWD. 

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. 

Another example involved a batch of isocyanate crosslinker which was too tacky. Upon comparing the HPGPC trace of this sample with that of a control as shown in Figure 9, it is seen that the major difference between these two samples was the level of free caprolactam. The high content of free caprolactam in sample CX-006 depressed the glass transition temperature (Tg) of the sample to such an extent that CX-006 became too tacky. This method of analysis has proved to be a reliable and useful technique for detecting low levels of free caprolactam in this type of oligomeric crosslinker. 

The data on isocyanate crosslinked polybutadiene (B2) is given in Table II, and for this the molecular weight (Mc) was calculated using eqn. (2) (Mc=l0,1OOF3). The expected value of Mc as obtained from GPC on uncrosslinked polymer should be somewhere in the range of Mn (5100) to Mw (7400). Comparison of GPC and swelling data yields that F3 is in the range of 0.51-0.73, which in turn determines the extent up to which the chains deform non-affinely. 

Isocyanate Crosslinkers. A wide variety of both aromatic and aliphatic Isocyanate crosslinkers are used in coatings (4). Aliphatic isocyanates are used when external durability is required. The isocyanate crosslinker studied in this work is the biuret of hexamethylene diisocyanate (Figure 1). Although resins based on triisocyanurates have been claimed to be superior in durability (,22.) > biuret based triisocyanates are more commonly used. Urethane coatings are generally formulated with a ratio of isocyanate to hydroxy of around 1 1. 

The calculation requires measurements of the extent of reaction and of the distribution of functionality on the polymer and crosslinker. The functionality of the isocyanate crosslinker is formally three although it must be recognized that all of these materials have a distribution of functionality. Fully alkylated melamines have a functionality of 6. Coatings are formulated with an excess of alkoxy to hydroxy in order to achieve rapid cure (2). Thus, not all of the melamine alkoxy groups will react (ignoring side reactions) and the effective functionality will be less than six. 

The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used in the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps... 

In most cases the Ta values are related directly to the lignin content of the network. Soft segment-free networks show more scatter than those containing alkylether components, and this may be attributed to the crosslink density of the materials. The method of soft segment incorporation, simple addition or covalently bonded, has a minimal impact on Ts. A comparison of the HDI and aromatic isocyanate-crosslinked networks, at a given lignin content, shows that the HDI-crosslinked polyurethanes generally have a lower Tg than the aromatic isocyanate-crosslinked networks. 

On the other hand, substituted p-nitroaniline compounds are preferred as stabilizers for isocyanate crosslinked CMDB propellants. The author of this patent claimed that such compounds have good solubilities in the propellant matrix and thereby impart good stabilization properties [224]. 

Uses Polyol for two-component waterborne PU concrete coatings Features Zero VOC produces coatings with very fast cure profile, good pot life should be cured with a water-disp. aliphatic isocyanate crosslinker Regulatory DOT nonregulated SARA 312 immediate health hazard, 313 nonreportable... 

Features For use with water-disp. isocyanate crosslinkers provides tough, chem. resist., glossy coatings with exc. exterior weathering durability and humidity resist. 

Chem. Descrip. Aliphatic urethane diol resin modifier Uses Modifier for amino or isocyanate crosslinked coatings, elastomers, and adhesives, automotive, aerospace, wood/paper, and general industrial coatings, and inks Features Water-sol. reduces VOCs... 

Uses Blocked catalyst for use in isocyanate-crosslinked polyols like solv.-based and aq. hydroxy acrylates and polyesters, e.g., forced drying two-pack lacquers... 

Use Level 1-3% (on polyol in conventional isocyanate crosslinking systems) 0.5-2.0% (aq. systems)... 

Chem. Descrip. Catalyst in butyl acetate/polyglycol Uses Catalyst reducing drying time in solv.-based and aq. isocyanate-crosslinked polyols, hydroxy acrylates, and polyesters, lacquers Properties YIsh. clear to si. turbid liq. vise. < 20 mPa s (20 C) flash pt. 34 C 30% act. 

Outstanding properties NOR alkoxyamine, radical scavenger, hydroxyl group permits condensation with melamine or isocyanate crosslinkers (resistance to migration), low basicity, very low volatility... 

Luo H, Churu G, Fabrizio EF, Scbnobrich J, Hobbs A, Dass A, Mulik S, Zhang Y, Grady BP, Capecelatro A, et al (2008) Synthesis and characterization of the physical, chemical and mechanical properties of isocyanate-crosslinked vanadia aerogels. J Sol-Gel Sci Technol 48 113-134... 

Zhang G et al (2004) Isocyanate-crosslinked silica aerogel monoliths preparation and characterization. Journal of Non-Crystalline Solids 350 152-164. 

Figure 13.3. A. Mechanical characterization by a short beam 3-point bending (see inset) of polyurea-crosslinked silica aerogel monoliths and their noncrosslinked (native) counterparts a, 0.63 gcm b, 0.44 gcm c, 0.38 gcm , and d, 0.28 gcm . Native samples do not register in the load-force scale shown. B. Cumulative data. Dark blue triangles and the dark blue line concern two-step aerogels made by acid-catalyzed hydrolysis and base-catalyzed gelation. All other samples use one-step base-catalyzed silica with different isocyanates. Multiple lines for crosslinked samples correspond to different di- and tri-isocyanate crosslinkers.

In analogy to isocyanate crosslinking, which mimics polyurethane/polyurea chemistry, epoxy glue chemistry has also been adopted for crosslinking, whereas amine-modified silica reacts with multifunctional epoxies [61]. Epoxide-crosslinked aerogels based on the... 

P, Scheiman D A, McCorkle L, Palczer A, Johnston J C, Meador M A B, Leventis N (200A) Isocyanate-crosslinked... 

Katti A, Shimpi N, Roy S, Lu H, Fahrizio E F, Dass A, Capadona L A, Leventis N (2006) Chemical, physical and mechanical characterization of isocyanate-crosslinked amine-modified silica aerogels. Chem Mater 18 285-296... 

Vivod S L, Meador M A B, Nguyen B N, Quade D, Randall J (2(X)8) Di-isocyanate crosslinked aerogels with l,6-bis(trimethoxysilyl)hexane incorporated in silica backbone. Polym Preprints 49 521-522 Vivod S L, Meador M A B, Capadona L A, Sullivan R M, Ghosn L J, Clark N, McCorkle L, Quade D J (2(X)8) Carbon nanofiber incorporated silica based aerogels with di-isocyanate cross-linking. Polym Preprints 49 306 307... 

Scheme 15.6. Synthesis of MTMS-based aerogels with BTMSPA used as both a flexible linking group and site for tri-isocyanate crosslinking. Reprinted from [38], Cop3uight 2010 American Chemical Society.

Zhang G, Dass A, Rawashdeh A-M M, Thomas J, Counsil J A, Sotiriou-Leventis C, Fabrizio E F, Uhan F, Vassilaras P, Scheiman D A, McCorkle L, Palczer A, Johnston J C, Meador M A B, Leventis N (2004) Isocyanate-crosslinked silica aerogel monoliths preparation and characterization. J. Non-Cryst SoUds 350 152-164. 

Figure 22.2. Scanning electron micrographs. A. A native (noncrosslinked) amine-modified silica aerogel (Pb = 019 g/cm ) B. a Desmodur N3200 isocyanate crosslinked amine-modified silica aerogel (pb = 0.48 g/cm C. a native vanadia (VO ) aerogel (bulk density pj, = 0.08 g/cm ) D. a crosslinked vanadia (X-VO aerogel (Pb = 0.43 g/cm ).

Isocyanate Crosslinking agent Isocyanate Part polyhydroxyl Compound... 

Ley DA, Fiori DE, Quinn RJ. Optimization of acrylic polyols for low VOC two-component water reducible polyurethane coatings using tertiary isocyanate crosslinkers. 

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