Extenders and Crosslinking Agents

Reactions (1.4)-(1.10) and those of Figs 1.5 and 1.6 may be greatly influenced by use of appropriate catalysts, so that here, as in the choice of initial reactants, it is possible to affect both the rate and the direction of the polymer-forming process. The catalysts most widely used commercially in polyurethane processes are tertiary amines and organotin compounds. In the case of the amines, promotion of urethane links is related to the strength of the base but structural effects can be important and, as shown in Table 1.7 the relatively weakly basic triethylene diamine, where there is little or no steric hindrance, is an extremely powerful catalyst compared with triethylamine. 

While a number of organometal compounds, e.g. ferric acetylacetonate and cobalt naphthenate, will catalyze isocyanate reactions, those based on tin show remarkable activity, promoting reactions of NCO with OH groups in preference to reaction with water. For this reason, dibutyltin dilaurate and tin octoate are widely used they are readily soluble in the reaction mixtures and have the advantage of low volatility and little odour. 

Temperatures of reaction can, of course, be important. At up to SC C the linear chain-forming reaction predominates but as higher temperatures (up to 150°C) are reached then biuret and isocyanurate formation become effective and branching occurs. At above 150°C some of the less stable links are affected and reversion or degradation can then take place. It must be stressed that the isocyanate reactions are highly exothermic, and under conditions where heat transfer is slow appreciable temperature rises can be experienced care is necessary to minimize their occurrence, otherwise deterioration in properties results. 

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Simultaneous PU/EP IPNs were prepared by a casting technique. The polyol was heated to 100-110 C at reduced pressure for one hour. It was cooled to 60 C and the isocyanate and epoxy resin were added. The preheated, premixed mixture of extender and crosslinking agents for polyurethane and epoxy were then added and mixed thoroughly. The mixture was poured into a preheated mold, allowed to gel, and cured for 16 hours at 110 C. Rods of approximately 3/16 diameter X 12 were also produced for dynamic mechanical testing. 

Uses Polymer building block polymerizes with alcohols and polyols to yield plasticizers, lubricating oils, hydraulic fluids intermediate for paints corrosion inhibitor as extender and crosslinking agent for high polymeric systems comonomer for alkyd/epoxy ester/PU resins aux. emulsifier for chloroprene rubbers... 

Uses Corrosion inhibitor intermediate whose derivs. are used as syn. lube components, corrosion inhibitors for petrol, processing, as extenders and crosslinking agents for high polymeric systems mildness additive for detergents Preperties Liq. 100% cone. 

Isocyanates also react with primary and secondary amine compounds. Tertiary amines cannot react with isocyanates because they do not contain active hydrogen atoms, but they are powerful catalysts for many other isocyanate reactions. Diamines are frequently used as chain extenders and curing agents in PU manufacture. The addition of a diamine to the reaction mixture increases the overall reactivity during polymerization. The reaction between an isocyanate group and an amine results in the formation of a urea bond. The polyurea segments present in the finished PU serve to increase the potential for both covalent and hydrogen bond crosslinks within the polymer. 

U.S. Pat. No. 5,516,472 [7] (Strandex Corporation) discloses a composite having approximately 26% HDPE and 65% wood flonr, extended in the presence of zinc stearate (2%) as a Inbricant along with phenolic resin and polynrethane as minor additives and crosslinking agents (4-1.3%, respectively). 

The application of this method is visualized using the example of a tricomponent system composed of a telechellc polymer with end groups of independent reactivity, a trlfunctlonal crosslinking agent and a blfunctlonal chain extender where the latter two compounds... 

Early attempts to use mimosa tannin in particleboard adhesives involved high-temperature alkaline treatment of the extract to reduce viscosity of the 40% solids level needed (43,44) Subsequent improvements followed the same course as with plywood, namely the use of phenol-formaldehyde or phenol-resorcinol-formaldehyde as crosslinking agents (45) and the use of catalysts or mix modifications to reduce press temperature requirements and to extend pot life. Recent work (46) has shown that exterior chipboard adhesives can also be prepared by crosslinking of mimosa tannins with 4,4-diphenylmethane diisocyanate. 

The irradiation of ammonium dichromate in gelatin results in crosslinking via two mechanisms. Oxidative crosslinking can occur, and the Cr +, which is formed by photochemical reduction, can also act as a crosslinking agent for the protein. The spectral sensitivity extends past 500 nm, and the exposure is typically a few mJ/cm. Resolution is very high, at least 5000 lines/mm. 

The length of the post-treatment time is dependent on the type of crosslinking agent used and the peak of ISC formation. A time-course experiment should be performed to ascertain this. For example, following a 1-h treatment the peak of ISC for chlorambucil is reached following a 3-h post-incubation, while cisplatin requires a post-incubation of 6-12 h. For repair experiments, the post-treatment time can be further extended. 

Polymers, fillers, softeners and some additives are normally contained in the rubber base, whilst crosslinkers and tin catalysts, as well as extenders and dyes or pigments (if added for visual mixing control), are included in the curing agent. 

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