Crosslinking agent

Silane-XLPE is commonly used for low-voltage cable insulation, and it also can be used for thick products. Improved crosslinking processes and properties, along with new metallocene POs, have reportedly allowed its use in more rigorous apphcations, such as under-the-floor hot-water heating pipes and better foams [15-2, 15-4). 

Alternatively, as in Wacker Chemie s MONOSIL process, the extrusion steps can be combined into a one-step process for aeating a readily crosslinkable compound. Other suppliers such as Dow Coming have also tried to make one-step crosslinking easier for processors by offering preblended formulations that contain the aosslinking catalyst, peroxide and silane agent in one PE compoimd [15-3, 15-4). 

Outside Emope and North America, a one-step infusion process is reportedly becoming more popular for lowa-cost silane aosslinking of cable coverings. This process uses a aossUnkable PO compound that already incorporates a aosslinking catalyst. The complete formulation is mixed in a separate machine near or in-line with the cable extmda, where the grafting 

E-beam crosslinking does provide some advantages over chemical aosslinking agents, however. In extruded pipe, researchers have found that radiation-crosslinked PE has equal or better burst resistance at high temperatures than 

Given the differences in methods and materials mentioned above various factors influence the choice of crosslinking approach  

The great utility of epoxy resins lies in the fact that the epoxide group is reactive with a large number of molecules which form tough thermoset networks without the evolution of by-products. The reactants can be classified as either direct participants in the crosslinked network or those which promote crosslinking catalytically. 

The catalytically functioning curing agents do not directly participate in the crosslinked network but promote reactions between epoxy groups themselves. Tertiary amines as well as boron trifluoride type complexes are effective catalytic agents. Excellent discussions of the specific curing agents, their reactivity with epoxy and their effect on epoxy mechanical properties are available in the literature 4 6I. 

It is clear that other components quite different chemically from the main constituents of the epoxy resin system may be present in the starting material. The structure of the cured epoxy may or may not incorporate these components. To the extent that these other species are not part of the crosslinked epoxy network, they can be concentrated at the interphase or they may be able to migrate to the interphase during the curing process. 

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Coacrete can also be made water-repeUent by the polymerisation of vinyl monomers on the surface (85). Polymerisation can be iaitiated with peroxides, and polyfunctional methacrjiates can be used as crosslinking agents. These treatments have a tendency to produce changes ia color and gloss. 

The original compound, maleimide (2,5-dioxo-A -pyrroline), is synthesized by the cyclo-condensation of ammonia and maleic acid. Similarly, primary amine is added to maleic anhydride, followed by cyclocondensation, to form N-substituted maleimide (Fig. 2). This reaction is applied to the preparation of bis-maleimides (BMl) [1]. At first, BMI was used as a crosslinking agent for natural rubber (NR). An o-dichlorobenzene solution of NR was crosslinked by BMI at I08-150°C in the presence of peroxides. The radicals generated from peroxides react with the double bonds of both BMI and NR [ 1 ]. 

BMI was also used as a crosslinking agent for poly(iminoethylene). The Michael addition takes place with the nucleophilic nitrogen of the imino group and the double bonds of the electrophilic BMI. The Michael addition of BMI is now adopted as a crosslinking reaction for polymers with amino end groups [2]. 

Murray, G.S., An Investigation into the Chemistry of the Reactions of Phenol-Fonnaldehyde Compounds with Novel Crosslinking Agents, Ph.D. The.sis. Portsmouth University, Portsmouth, 1993. 

Shellac [124,125] is a natural resin used in very old times for varnishes and moulding compounds. The resin secreted by the lac insect, Kerriar paca, is collected by scraping the shellac-encrusted trees found in southern parts of Asia. It consists of a complex mixture of cross-linked polyesters derived from hydroxy acids, principally aleuritic acid (9,10,16-trihydroxyhexadecanoic acid). From a structural point of view, it appears that this material can be used as a crosslinking agent and/or as a monomer for developing dentrite-like polymers. The question is whether it is possible to produce this material from shellac by controlled hydrolysis. 

Interfacial adhesion and, thereby, compatibility can be enhanced by the selective crosslinking reaction in polymer blends. Inoue and Suzuki [26] reported the properties of blends dynamically crosslinked PP-EPDM blends. The crosslinking agent was yV,N -/w-phenylene-bismaleimide - poly(2,2,4 - trimethyl - 1,2-dihydroquino -line) system. Increase in interfacial adhesion leads to... 

Conventional crosslinking agents, such as sulphur, accelerators, and peroxide, etc., used in dynamic vulcanization and melt-mixing of two polymers is the first step in the preparation of an EA. The dynamic vulcanization is done by dynamic shear at a high temperature to activate the process. 

Caprolactam, a white solid that melts at 69°C, can be obtained either in a fused or flaked form. It is soluble in water, ligroin, and chlorinated hydrocarbons. Caprolactam s main use is to produce nylon 6. Other minor uses are as a crosslinking agent for polyurethanes, in the plasticizer industry, and in the synthesis of lysine. 

The formation mechanism of structure of the crosslinked copolymer in the presence of solvents described on the basis of the Flory-Huggins theory of polymer solutions has been considered by Dusek [1,2]. In accordance with the proposed thermodynamic model [3], the main factors affecting phase separation in the course of heterophase crosslinking polymerization are the thermodynamic quality of the solvent determined by Huggins constant x for the polymer-solvent system and the quantity of the crosslinking agent introduced (polyvinyl comonomers). The theory makes it possible to determine the critical degree of copolymerization at which phase separation takes place. The study of this phenomenon is complex also because the comonomers act as diluents. 

For flexible chain copolymers based on acrylic and methacrylic acids (AA and MA) crosslinked with a polyvinyl component, the inhomogeneity of the structures formed depends on the nature of the crosslinking agent, its content in the reaction mixture and the thermodynamic quality of the solvent [13,14],... 

Fig. 3. Change in relative swelling coefficient Kw during ionization of macroreticular MA-EDMA copolymers obtained in 30% Ac A solution (1-4) and heteroreticular MA-EDMA copolymers obtained in 5% Ac A solution (5,6) with different amounts of the crosslinking agent — EDMA (mol%) /) 1.0 2) 1.5 3) 2.5 4) and 5) 4.0 6) 10.0...

Fig. 4. Swelling coefficients of crosslinked MA-DMEG copolymers as a function of the content of the crosslinking agent 1) pH 2.8 2) pH 12.5...

Fig. 6. Electron microscopy of carboxylic CP MA-DMEG copolymers with different contents of crosslinking agents (mol%) 1) 9 2) 10.5 3) 13.5...

Fig. 7. Quantity of enzymes (m) ]) terrilytin and 2) pancreatic ribonuclease, bonded by crosslinked MA-DMEG copolymers with different contents of the crosslinking agent...

The application of the above methods of calculation has shown that the quasi-diffusion of organic counterions is profundly affected by both the amount of the crosslinking agent in a crosslinked polyelectrolyte and the method of formation of the crosslinked structure [109-112]. Fig. 28 shows the dependence of diffusion coefficients for streptomycin ions on the amount of the crosslinking... 

Serious deviations of the polymer network structure from the ideal one can have several causes. One of them is the crosslinking agent involvement in intramolecular cycle formation. The contribution of this reaction grows with the system dilution as well as when the crosslinker units in the chain are close one to the other, i.e. its fraction in the copolymer increases. All this is in good agreement with the observed trend. 

Fig. 1. Comparison of experimental and theoretical values of Mc at free-radical copolymerization of AAm with MBAA as a crosslinking agent CT — total concentration of monomers, C — that of MBAA C = 10 wt% (/), CT = 6.7 g dl-1 (2). From Baselga et al. [18]...

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