Relative rates of curing

Figure 4. Relative Rates of Curing of MFAs Compared with NVP in admixture (1/1, v/v) with Urethane Acrylate and 0 (1% w/v) using UV Initiation (20).

Clear or pigmented liquid photocurable coatings can be applied to a flexible substrate (metal, polyethylene or filter paper) and the relative rate of cure or network formation as a function of changes in dynamic mechanical properties versus exposure time can be determined. 

Relative rate of cure Medium Slow East Slow -1 K Medium -2 K Very fast Past fixture but slow cure... 

In order that the rate of cure of phenolic moulding compositions is sufficiently rapid to be economically attractive, curing is carried out at a temperature which leads to the formation of quinone methides and their derivatives which impart a dark colour to the resin. Thus the range of pigments available is limited to blacks, browns and relatively dark blues, greens, reds and oranges. 

Monitoring the polymerization of each substrate provided an informative picture of the effect of both substituents and of isomer distribution on the curing process. We first addressed the question of the relative rates of substrate isomerization and polymerization. We found that, for the parent monomer (PN and PX), the rate of isomerization greatly exceeds the rate of polymerization. Under conditions where PN and PX are fully equilibrated (195°C/15 hrs or 250 /l hr) there is still less than 20% polymer formation in the neat sample. We conclude that for PN or PX the composition of the mixture undergoing polymerization is essentially independent of the starting isomer. The observation that fully cured samples of either PN or PX show identical and C NMR spectra and indistinguishable SEC analyses, is consistent with this contention. 

They suggested that three possible trajectories of the matrix and domain composition during curing of the modified epoxy (Fig. 3.5) [39]. The trajectory is determined by the relative rates of phase separation and polymerization. 

Rate of cure - The relative time required to reach a predetermined state of vulcanization under specified conditions. 

Figure 9. Variation of log rate of cure with log relative intensity of ultraviolet light irradiation (56)...

Epoxides which cure relatively slowly or not at all at ordinary temperatures can be used as components of mixtures containing 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxyl ate (ECC), in which case the rate of cure is dependent upon the amount of ECC in the mixture. 

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

While emphasis has been given to the use of anti-metabolites in the treatment of acute lymphoblastic leukaemia in children, they are also used with varying success with other types of cancer. For the other common forms of leukaemias - acute myeloid and monocytic leukaemias - which affect adults in the age range 55-75, a cocktail of drugs that includes cytarabine, 6-thioguanine (an analogue of 6-mercaptopurine) and daunorubicin (a structural relative of doxorubicin, discussed later) can induce remission in up to 80% of patients. Bone-marrow transplantation has also become a standard part of the treatment, once remission has been achieved, and the resultant rate of cure is usually around 50%, especially in patients under 65 years. 

Veith [l. j reviewed the current state of precision testing using some ASTM test methods in the rubber manufacturing industry in 1987, Mooney viscosity (ISO 289. ASTM D1646), a widely used test for quality assessment of raw rubbers, gave reasonably good relative precision. Type 1 (r) pooled values of 3,0 percent for several clear rubbers, and good pooled R) values of 3,8 percent on the same basis. See Annex D for the definition of Type 1 and 2 precision and (r) and (/ ), For a widely used rate-of-cure... 

Moisture-curing adhesives are formulated with resins that react with atmospheric moisture to form a curedpolymer. Most ofthese resins are one-part cyanoacrylates, silicones, or polyurethanes. The key advantage, of course, is their ability to cure without the use of heat or other external form of energy. However, in many cases, several days of ambient exposure are required to achieve full properties since the rate of curing is dependent on the relative humidity and the thickness of the adhesive. If there is too much moisture on the surface, curing of polyurethanes occurs so quickly that... 

The rate of cure is temperature dependent and many formulations stop curing altogether below a temperature of about 5 °C. If carefully formulated the change in volume between the uncured resin-hardener system and the fully cured polymer can be very low. This property, together with their relatively high strength and claimed resistance to moisture and chemical attack, forms the basis of the use of epoxy resins as structural adhesives. 

Fundamental kinetic studies are by preference performed in isothermal rather than in non-isothermal reaction conditions because frequently, as cure proceeds, parallel reactions with different activation energies occur, changing the relative rates of reactions with temperature. In theory, one non-isothermal experiment comprises all the kinetic information normally enclosed in a series of isothermal experiments, which makes the kinetic analysis of non-isothermal DSC data very attractive. The criteria forjudging the kinetic parameters derived from non-isothermal experiments must be its... 

A study by Comyn et al. [8] indicated that low (or no) cure took place in the interphase between an amine cured epoxy and aluminum because the amine was preferentially adsorbed onto the aluminum oxide on the aluminum. Garton et al. [9] showed that the acidic surface of a carbon fiber selectively adsorbed amine and catalyzed the reaction between the amine and an epoxy resin. Nigro and Ishida [10] found that homopolymerization of epoxy resin was catalyzed by a steel surface. Zukas et al. [11] discovered, in a model system of an amine cured epoxy resin and an activated aluminum oxide, a change in the relative rates of the reactions leading to crosslinking of the epoxy, so that the material in the interphase was structurally different from that in the bulk. 

Cyanoacrylate adhesives are relatively low viscosity fluids based on acrylic monomers and characterised by extremely fast rates of cure. When placed between closely fitting surfaces, some will cure to give a strong joint in two to three seconds. 

The detailed microstructure of free-radical cured polymers determines their overall properties, and is governed by the rate of cure and relative rates of various competitive polymerisation processes such as initiation, propagation, combination and termination. 

In studies of model systems based on squalene [34], it was shown that cobalt salts undergo a dissociation reaction to form Co ions at the interface and the relative rate of formation and amount of cobalt deposited on the surface depends on the nature of the organic anion. It was also shown how the type of anion affects ageing properties by influencing cure and complex stability and hence the amount of cobalt available to maintain bond strength during ageing. Therefore by judicial choice of the anionic complex, optimmn performance was achieved. 

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