Very fast cure rate

Two-part sprayable polyurethanes are used in the production of chemical-resistant paints and coatings. The polyurethanes can be let down in some solvent to ease in the spraying. Polyureas are becoming important in this section of the market because of their very fast cure rates. 

ZnDBC is commonly referred to as an ultra-accelerator because the dithiocarba-mate class of rubber accelerators is known for imparting a very fast cure rate (just like the thiuram accelerator class). 

HyTemp NPC-25 is the cure system of choice as it contributes a very fast rate of cure excellent shelf stability, and fair scorch safety. Processing conditions must be carefully controlled to maintain a maximum temperamre of 85°C in the compound to prevent scorch. The very fast cure rate limits the use of the NPC cure system for injection or transfer molding. The NPC cure system does work well for injection or transfer molding operations provided that care is taken to process the compound under 85°C and control runner temperatures. Very good compression set with out post-cure offers a major advantage and if even lower compression set is required, a post-cure will give values of less than 15%, 70 h at 150°C. 

The HyTemp SC-75 cure package also will provide very fast cure rates with excellent shelf stability and fair scorch safety, but does require a post-cure to obtain desired physical properties. This cure system is recommended for injection and transfer as well as compression molding. The physical properties are good with a higher elongation and lower hardness than with other mechanisms. The heat aging and compression set are excellent, making this a very usefid product if process conditions are hard to control and a post-cure can be tolerated. 

This gives a very fast cure rate and is used in the continuous vulcanisation of both solid and cellular profiles, particularly in the latter to obtain a sufficiently fast cure rate for good skin formation. Disadvantages arising from the use of this material are poor processing safety and a lacrimatory effect on the operator s eyes necessitating efficient ventilation on the curing unit. 

Halogenation of saturated hydrocarbon polymers can hardly be controlled and is frequently assodated with chain degradation phenomena In contrast, the presence of randomly distributed olefinic unsaturations, allows selective halogenation reactions by adopting appropriate conditions. For instance, butyl rubber can be chiorinated or brominated in allylic positions and chloro-butyl or bromo-butyl rubber results The latter polymers are very interesting since they exhibit fast curing rates when sulfur and ZnO are introduced in the formulations. 

The main family of bioerodible polymers is represented by poly(alkylcya-noacrylate)s (PACA). These polymers have a long history since 1947 as adhesives, especially in areas where a fast cure rate is needed. Polymerisation of alkylcyanoacrylates (ACA) can be initiated in the presence of bases as weak as the hydroxyl ions of water. The propagation rate decreases when the size of the lateral alkyl chain increases. As polymerisation can be very fast, ACAs have been used as surgical glue, as tissue adhesive and for embolisation purposes. As shown in Section 4.4.6, ACAs have also been used extensively for preparing nanospheres and nanocapsules for drug delivery. 

CBS is another very high volume accelerator used by the tire industry as well as other segments of the rubber industry. Just as with TBBS, CBS has a very high use because it imparts a significant level of scorch safety to the compound, a reasonably high ultimate crosslink density, and a relatively fast cure rate. CBS is similar to TBBS in that it too is based on a primary amine, which minimizes the nitrosamine problem in the factory. Also, the cure rate and ultimate state of cure are only slightly less for CBS vs. TBBS as discussed earlier. 

A combination of Vanax 829 and TBTD in an ACM gives excellent scorch safety and fast cure rates but the shelf stability is sensitive to moisture. The physical properties after a post-cure are very good, but the compression set is only fair. Aging properties with this cure combination are excellent. 

The Zeonet system does provide very good compression set without post-cure, good bin stability, reasonably fast cure rates, and it may be cured in open steam. The vulcanizates have good water and corrosion resistance. 

Decreasing the part thickness to obtain W3 =10 produces a dramatic change in the way in which the part cures, as observed in Fig. 9.8. The material located at the core cures first, generating conversion and temperature fronts advancing at a very fast rate to the wall, which acts as a heat sink (notice the short period of time in which most of the material is cured). The maximum temperature is observed at an intermediate position and is lower than m the previous case. Tq -l- A l ad Tj ax Tw + ATad. 

It is interesting to observe variations due to different reaction rates. Fast hydrolysis and condensation are reported on Fig. 6, slow hydrolysis and condensation are reported on Fig. 7. When the chemical reactions are fast compared to the diffusion of water, there is a very fast sur ce curing. The crosslinking is controlled by the water diffusion. As soon as the water diffuses in the uncured sealant, it reacts and is consumed by the hydrolysis reaction. It does not difiuse further until the sealant is fully cured. At the surface the full cure is quickly reached. A full cute at 5 mm is obtained in 100 h, diis is close to the experimental data observed for an acetic type sealant (Fig. 8). 

Typical adhesive spreads for plywood are 100-170 g m , with curing temperatures of 100-160 °C, and bonding pressure of 1-1.6 MPa. For particle board, the adhesive content is around 8 g of solid resin per 100 g of dry chips, and 12 g solid resin per 100 g dry fibres in the case of MDF. Curing rates are very fast, typically 8-10 s mm of board thickness for particle board. 

Most amines react too rapidly for use in 2K coatings however, hindered amines have been developed that permit their use. Polyaspartic esters are hindered amines that are used in high solids coatings (12). The aminoesters are synthesized by the addition of a diamine such as 4,4 -(aminophenyl)methane to diethyl maleate. The pot life of these combinations is increased by addition of DBTDL. On the other hand carboxylic acids and water accelerate reaction rates. By proper selection of isocyanate and aspartate and adjustment of catalysts, it is possible to formulate very high solids 2K coatings with a reasonable pot life and fast curing. 

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