Urethanes crosslinking

NR can be cross-linked by a blocked diphenyl methanes diisocyanate to produce urethane crosslinks. The cross-linking agent dissociates into two quinonedioxime molecules and one diphenyl methane diisocyanate. The quinone reacts with the rubber via a nitroso group and forms cross-links via diisocyanato group. The performance of this system in NR is characterized by excellent age resistance and outstanding reversion resistance. 

One example of the chemical crosslinking strategy is the formation of urethane linkages between polyoxyethylene chains. Fig. 5.17 (Killis et ai, 1982). The urethane crosslinked polyether systems were exploited by Killis et al. in their extensive studies of the relation between conductivity and bulk mechanical properties of network electrolytes. Other chemically... 

Materials. The adhesives and primers used in this study were model and commercial materials that were cured according to conditions appropriate for the specific adhesive chemistry. Adhesives A and B were conventional epoxy/Versamid and epoxy/dlcyandiamlde adhesives, respectively. Adhesives C and D were commercial urethane and epoxy/polyamlde adhesives, respectively. Adhesive E was a conventional two-part epoxy/amldoamlne adhesive. Adhesive F was a vinyl plastlsol adhesive. The adhesive primers used in this study were a urethane crosslinked epoxy electrocoat primer and spray primers based on tall oil modified epoxy ester, and polyesterpolyol/isocyan-ate chemistry. Dicyandlamlde was obtained from Aldrich Chemical Company. Epon 828 was obtained from Shell Chemical Company. 

Differences In Crosslink Chemistry. The main crosslinking reaction In Isocyanate-polyol coatings Is the reaction of the Isocyanate group with hydroxy groups to form a urethane crosslink. 

The rate limiting step In this side reaction Is the reaction of Isocyanate and water. The Importance of this reaction depends on the polarity of resin, the level and type of catalyst, and the humidity In the bake oven. The effect of this reaction Is to reduce the number of urethane crosslinks but Increase the number of urea crosslinks. The total crosslink density will likely decrease since two Isocyanate groups are necessary to form one urea link while only one isocyanate group is required to form a urethane link. If excess isocyanate groups are present, this reaction will consume the excess Isocyanate and Increase the crosslink density. This reaction can occur not only in the bake oven but also on aging of the coating if unreacted isocyanate groups are present after cure. 

Ethyl-2-methyl-2-(3-methylbutyl)-1,3-oxazolidine. Oxazolidine, 4-ethyl-2-methyl-2-(3-methylbutyl)-. scavenger urethane crosslinker. Angus. 

Three urethane-crosslinked polybutadiene elastomers (TB-1, TB-2, and TB-3) of varying crosslinking levels, along with a similarly crosslinked styrene-butadiene copolymer (HTSBR) and two polybutadiene polymers randomly crosslinked with dicumyl peroxide (PB-1 and PB-2), have been investigated to determine their viscoelastic behavior. Elsewhere, TB-1, TB-2, and TB-3 have been designated as HTPB-1, HTPB-2, and HTPB-3, respectively. 

Torsional creep measurements were made on the urethane-crosslinked polybutadiene elastomers at temperatures between —68 C and 25 C. The average molecular weight of a networks chain. Me, is 3400, 5200, and 8300 for TB-1, TB-2, and TB-3, respectively. The reduced shear creep compliance Jpit/ax) curves obtained for the three samples are shown in Figure 5.13. The reference temperatures are chosen to be 7.4°C, 0.0 C, and 17.0°C for TB-1, TB-2, and TB-3, respectively, so that superposition is achieved at shorter times... 

Torsional creep measurements were made on the urethane-crosslinked polybutadiene elastomers at temperatures between -68 and 25°C.The average molecular weight of a networks chain. Me, is 3400, 5200, and 8300 for TB-1, TB-2, and TB-3 respectively. The reduced shear creep compliance Jp(t/ar) curves obtained for the three samples are shown in Fig. 18. The reference temperatures are chosen to be 7.4,0.0, and 17.0°C for TB-1, TB-2, and TB-3 respectively so that superposition is achieved at shorter times in the primary softening dispersion. The most loosely urethane-crosslinked TB-3 has the largest 7e = 2.5 x lO Pa There is a plateau intermediate between the glassy compliance 7g (not reached in these measurements) and /e, and its level is about 2.0 X 10 Pa in all three samples. The network chain density does not affect the form of the time-dependent response up to and including the intermediate plateau in the Jp(t) curves. Only the terminal dispersion, i.e., the approach to /e, is influenced. The shift factors, Uj, that were used to obtain the... 

The logarithm of the reduced shear creep compliance Jp t/aj) of several elastomers are compared at 0°C in Rg. 20 as a function of the logarithm reduced time-scale log(i/flr)- The elastomers include (1) the urethane-crosslinked polybutadiene elastomer, TB-2, described in Subsection B (2)... 

B. C., Poly(siloxane-urethane) Crosslinked Structures Obtained by Sol-Gel Techniques. J. Polym. Set, Part A Polym. Chem. 2011,49,1708-1718. 

Urethanes crosslink as well as undergo chain extension to produce a wide variety of compounds. They are available as castable liquids and solids or gums to be used on conventional rubber processing equipment. They are also available as thermoplastic resins, which can be processed similar to polyvinyl chloride. 

Poly (ester-urethane) Crosslinking of polyester triols prepared from D.L-lactide, c-caprolactone, or comonomer mixtures with ethyl 2,6-diisocyanatohexanoate. Chemical hydrolysis of polymer backbone. Storey et al, 1994... 

Resin-curing systems Soap/sulfur systems Urethane crosslinking agents Bisphenol curing agents... 

A more recent development has been the use of urethane crosslinkers. These are expensive but may be used as co-vulcanizing agents with sulfur for economy. They impart excellent fatigue resistance on aging. Urethane crosslinkers are formed from nitrosophenols and diisocyanates the resultant diurethanes are stable at processing temperatures but spilt into their components at curing temperatures. Pendent amino-phenol groups are formed on the rubber molecules, which are then linked by the diisocyanate. 

The analogous reaction between castor (or lesquerella) oil and sebacic acid also forms an aliphatic polyester. Unlike the epoxy-acid reaction, water is formed as a by-product, which must be removed from the reaction medium in order to obtain complete reaction and a void-free material. The reaction may be carried out with deliberate stoichiometric imbalance to form a high molecular weight hydroxy endcapped prepolymer, which may be thoroughly dried, and then crosslinked with a diisocyanate, which form urethane crosslinks without the release of water. Of course, hydroxy functionalized oils can also be reacted from the start with a diisocyanate, forming a complete polyurethane network,... 

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