Curing cross Unking

For this reason formulators usually use polymer systems that cure by evaporation and not by reactive cross Unking. Cross linked systems have been used but care must be taken to have a mechanism where the curing is inhibited in the print head and takes place only after jetting. 

The DADC monomer has been copolymetized with small amounts of polyfunctional methacryhc or acrylic monomers. For example, 3% triethylene glycol dimethacrylate was used as a flexibilizing, cross-Unking agent with a percarbonate as initiator (26). CR-39 and diethylene glycol diacrylate containing isopropyl percarbonate were irradiated with a mercury lamp to a 92% conversion and then cured at 150°C (27). By a similar two-step process DADC was copolymetized with methyl methacrylate and tetraethylene glycol dimethacrylate (28). 

Epoxy resins can also be cured with the aid of ionic liquids [136]. The cross-Unking reaction takes place at elevated temperatures... 

In either case, the torque is plotted against time to give a so-called rheometer chart, rheograph, or cure curve (Fig. 4.19). The cure curve gives a rather complete picture of the overall kinetics of cross-link formation and even cross-Unk disappearance (reversion). In some cases, instead of reversion, a long plateau or marching cure can occur. 

In a typical formulation, an ethylene-n-butylacrylate-carbon monoxide (60/30/ 10) terpolymer (60 wt%) is melt compounded in a twin-screw extmder with PVC (30 wt%) along with an optional, nonvolatile plasticizer such as trioctyl trimellitate (10 wt%) such that the ethylene terpolymer dispersion was cured in situ during the mixing by catalytic amounts of a suitable peroxide (0.3 %) and a bismaleimide crosslink promoter (0.2 %). It is believed that the initial homogeneous miscible melt blend later forms the micro phase-separated mbber domains as the selective rubber cross-Unking progresses. Currently, such TPV blends are commercially sold as Alcryn melt-processable rubbers by Advanced Polymer Alloys division of Ferro. 

Curing agents catalyze the cross-Unking of binder systans. 

Uniform and consistent cure kinetics. The cross-linking chemical reaction is an exothermic polymerization. The length of the cure cycle depends on the type of resin system and temperature. The degree of cure is proportional to the cross-Unked material, and can be characterized by measuring the fractional exothermic heat of the chemical reaction. The cure kinetics are usually characterized separately and may not reflect the interaction with the sizings on the fiber surfaces which may change the cure kinetics. 

When an allylated novolac/BMI resin was compounded with polyhedral oligomeric silsesquioxane (BOSS), in the presence of octamaleimidophenyl polyhedral silsesquioxane (OMPS) as a co-curing reagent, materials with POSS cages uniformly distributed in the network resulted (Li et al. 2007). Their enhanced thermal stability is the result of the higher cross-Unk density and the inorganic nature of POSS. 

Standard Epoxies. Two resin systems are used to make GF laminate difunctional and tetrafnnctional. These systems are distinguished by the nature of the epoxy cross-linking. In a difnnctional system, the epoxide molecule has two cross-Unking sites, and the cured epoxy contains long linear molecular chains. Pure difnnctional laminates have excellent physical properties, and for many years were the mainstay of the industry. They have aTg of 120°C, which is adequate for most use environments, but is low for some apphcations and too low for LFA. 

The cross-Unking in these bioelastomers is carefully controlled by nature, using techniques very unUke those usually used to cure commercial elastomers [9,162]. The number and spacing of the cross-Unks are fixed by the ribosome-controlled... 

Since the cross-Unking of polyester-styrene system occurs by a free-radical chain-reaction mechanism across the double bonds in the polyesters with styrene providing the cross-links, the curing reaction does not give rise to volatile by-products (unlike phenolic and amino resins) and it is thus possible to cure without applying pressure. This fact as well as that room temperature cures are also possible makes unsaturated polyesters most useful in the manufacture of large structures such as boats and car bodies. 

Where shorter cure times are desired for maximum economics, higher cure temperatures can be employed. The maximum cure temperature for a given peroxide can be limited by its scorch time or the time before the onset of cross-Unking. Plots of scorch times for the peroxides listed in Table 13.1 are shown in Figure 13.3. 

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