Over cure

A cure which is longer than optimum is over cure. Over cures may be of two types. In one type the rubber continues to harden, the modulus rises and tensile strength and elongation fall. In certain cases, including most natural rubber compounds, reversion occurs with over cure leading to decrease in tensile strength and modulus. 

Processibility is dependent on the viscosity or plastic flow of the rubber compound,i.e., resistance to flow. Plasticity or viscosity determines the energy requirement of the rubber during milling, calendering or extrusion while the time to the onset of curing, i.e., scorch time, indicates the amount of heat history which can be tolerated before the rubber is converted from the plastic to the elastic state at which time processing becomes virtually impossible. 

The rate at which the rubber compound is deformed has a significant effect on its apparent plasticity. The various processes such as mixing or milling, extrusion, calendering and moulding 

---

The action of redox metal promoters with MEKP appears to be highly specific. Cobalt salts appear to be a unique component of commercial redox systems, although vanadium appears to provide similar activity with MEKP. Cobalt activity can be supplemented by potassium and 2inc naphthenates in systems requiring low cured resin color lithium and lead naphthenates also act in a similar role. Quaternary ammonium salts (14) and tertiary amines accelerate the reaction rate of redox catalyst systems. The tertiary amines form beneficial complexes with the cobalt promoters, faciUtating the transition to the lower oxidation state. Copper naphthenate exerts a unique influence over cure rate in redox systems and is used widely to delay cure and reduce exotherm development during the cross-linking reaction. 

Although not in wide use, a fast crystallizing TPU adhesive can be used in the shoe industry as an alternative to solvent-borne urethane adhesives. The TPU adhesives have good holding strength soon after crystallization, which can be a distinct advantage over curing hot-melt adhesives. TPU adhesives normally have... 

Extreme heat In the manufacturing process, belts are cured with scientifically controlled heat for given periods of time. If standard belts are operated below 140°F, their materials of constmction are not affected. However, at temperatures above 140°F, over-curing will occur and belt life will be shortened. Therefore, the use of standard V-belts above this temperature should be avoided. Often, adequate shielding between the heat source and belts can be provided. 

Kirschner, E., Prevention Takes Priority Over Cure, Chemical Week, p. 2732 (June 2, 1993)... 

Chem. Descrip. Low m.w. hydroxy-functional silicone-based additive Uses Mar resist, aid, slip agent, flow aid for powd. coatings based on epoxy, polyester, acrylic, and PU resins Features Does not reduce gloss or heat stability, even at over cure Properties Grans. sp.gr. 1.23 g/cm m.p. 85 C 96.6% act. 

Features Benzoin replacement eliminates pinholes easy to disperse does not evaporate (fume) during baking process very good compat. and yellowing resist, even at over cure 200 C Properties Wh. grans. soften, pt. 95-105 C 100% act. 98% min. NV Use Level 0.5-4.0%... 

The CNT/NR nanocomposites were fabricated through solvent mixing assisted with a two-roll mill. Compared to the neat NR and traditional CB/NR composite, the over-curing reversion of CNT/NR nanocomposite abated. The dispersion of the treated CNTs in the rubber matrix and interfacial bonding between them were satisfactory. 

With bromobutyl/butyl rubber blends, elastomer structures are essentially the same, but the different reactive functionalities provide different vulcanization chemistry. Since bromobutyl rubber has greater cure reactivity, accelerators that will over-cure the bromobutyl rubber phase should be avoided. Briefly... 

Aftercure n. A continuation of the process of curing or vulcanization after the cure has been carried to the desired degree and the source of heat removed, generally resulting in over-cure and a product less resistant to aging than properly cured products. 

Heat transfer is fairly rapid and in those cases where the controlled temperature of the oil bath is not too high, over-curing the adhesive is not likely to occur. This system would be most effective on metallic substrates. Certainly, the use of oil baths with porous materials would not be suitable. 

Figure 3.13 Pyrogram of (a) under cured rubber 30 gg (b) optimally cured rubber 20 pg (c) over cured rubber 20 pg - Continued overleaf. (Source Author s own files)...

Over-cure a degree of cure greater than the optimum causing some desirable properties to be degraded. 

An example of epoxy resins that cure when a strong acid is liberated is given in Scheme 16.35 (see Section 16.8.5, on UV curing). Press moldings from epoxy resins, catalyzed by tertiary amines for cure, are used in automotive and electronic components. As they only react with their glycidyl end groups, they are relatively insensitive to over-cure. They are furthermore dimension-stable, and are very resistant to heat and chemical influences. 

Urea molding compounds are processed at temperatures lower than those for phenolic resins mold temperatures here range between 145 and 155 °C. The processing window is clearly narrower, i. e., under- and over-curing have a much stronger detrimental effect on quality. 

Cure temperature and cure duration (time) are also important parameters relative to paint adhesion. For conservation reasons, lower temperatures and shorted times are most desirable. In Table 10 are listed the results of adhesion testing on DPTPO-3 that show that both under cure and over cure can be undesirable, and that under cure is perhaps the less desirable from the standpoint of obtaining acceptable paint adhesion. This is not inconsistent with the results seen using TPO and adhesion promoter. 

---