Selecting a Curing System

A number of factors must be considered in processing conditions, in descending order of importance  

Thick-walled castings or thermally insulated mouldings, which can reach a peak exotherm of over 200°C, producing cracks from internal stress or shrinkage, can be moulded better with less-active curing systems. Surface coatings (with no exotherm and slow cure with possible air inhibition) can better use very active curing systems. Where colour is important, accelerators must be kept to a minimum amines are not suitable, ketone peroxides/metal salts are preferable. 

Ketone peroxides for cohalt curing Methyl ethyl ketone peroxide liquid, high activity 

Acetyl acetone peroxide liquid, normal activity 

Versatile type for all resins, particularly vinyl A. c. D. E. G esters relatively long gel times, short mould release times good for large parts and/or use in hot countries 

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IC Dupuis. Basic Compounding of Hypalon. Selecting a curing system. Publication HP-320.1 Delaware Du Pont de Nemours, 1970. 

Sulfur Donor or Semi-EV cures for nitrile rubber are given in Table 2.21 [13]. When selecting a cure system to obtain good heat and compression set resistance, this type of cure should be selected. Process safety and cure rate need to be considered in order to satisfy factory conditions and economics. Although dynamic properties are not as good as with normal sulfur cures, some combinations such as sulfur 0.5,... 

The sulfonyl chloride cure sites in CSM provides a wide range of choice in selecting a curing system, depending upon the end properties needed and factory process conditions [7]. A curing system combination requires an acid acceptor plus a cross-linking mechanism, with or without an accelerator. 

DuPont Hypalon Technical Information HPE-H68578 X)-C1102, Selecting a Curing System Rev. 2 November, 2002. 

Vulcani2ation is a chemical process for improving an elastomer compound s performance. However, in most cases not all of the desired properties reach their optimum levels simultaneously. One of the mbber compounder s key responsibiHties is to achieve a balance of the most important property requirements by the proper selection of cure system (chemical) and time—temperature cure cycle (physical). 

The chemical bonding theory of adhesion applied to silicones involves the formation of covalent bonds across an interface. This mechanism strongly depends on both the reactivity of the selected silicone cure system and the presence of reactive groups on the surface of the substrate. Some of the reactive groups that can be present in a silicone system have been discussed in Section 3.1. The silicone adhesive can be formulated so that there is an excess of these reactive groups, which can react with the substrate to form covalent bonds. It is also possible to enhance chemical bonding through the use of adhesion promoters or chemical modification of the substrate surface. 

Because there is a large variety of different materials that are excellent for specific applications, the right polyurethane for the particular application has to be selected. The base chemistry used in producing the polyurethane system can have a profound influence on the final properties of the part produced. The mechanical, thermal, and chemical properties of a cured system have to be selected for each application. A typical example is that certain amines cannot be used in applications where there is contact with food. 

Primers for adhesive bonding chemically functionalize the substrate surface to provide pathways for chemical bonding with a selected silicone cure system. The increase of chemical bond improves adhesion durability. The main disadvantage of priming the substrate is the addition of an extra step in the whole process of adhesive application. The primers are usually Silane adhesion promoters, a reactive alkoxy silane molecule, oligomer, or a mixture of two or more different silanes. 

Liquid curing medium (LCM) is a practical method provided that a rapid cure rate system and a high viscosity CSM are selected. A peroxide system may leave a tacky surface unless complete submersion or rain of the LCM on the part is provided. 

The role of accelerators in the curing and mechanical properties of EPDM/SBR blends is investigated. In this study, blend properties were optimised by selecting accelerators with a shorter scorch time and a faster cure rate in the EPDM phase than in the SBR phase. The accelerators investigated included a sulphenamide-based accelerator and a system composed of a combination of a thiuram and thiazole-based accelerator. By use of techniques described in this work, sulphur vulcanisates with compound properties comparable to those cured with a cure system composed of peroxide and sulphur coagent were obtained. The effects of the accelerators in final compound properties are discussed, with reference to mechanical properties, ozone resistance, heat ageing, and compression set. 13 refs. 

Catalyst Selection. The low resin viscosity and ambient temperature cure systems developed from peroxides have faciUtated the expansion of polyester resins on a commercial scale, using relatively simple fabrication techniques in open molds at ambient temperatures. The dominant catalyst systems used for ambient fabrication processes are based on metal (redox) promoters used in combination with hydroperoxides and peroxides commonly found in commercial MEKP and related perketones (13). Promoters such as styrene-soluble cobalt octoate undergo controlled reduction—oxidation (redox) reactions with MEKP that generate peroxy free radicals to initiate a controlled cross-linking reaction. 

Nitrile mbber compounds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from —30 to 125°C. The compounds are also plasticized using polar ester plasticizers. The main dilemma is the selection of a heat-stable, nonfugitive plasticizer that also gives good low temperature properties. 

The selection of the cure system in these applications is directed by constraints such as location of the adhesive in terms of confined space, speed and depth of cure, etc. The volumes of silicones typically applied are relatively small. In general, the uncured adhesive needs to be dispensed in a well-defined and limited area, and needs to stay in place without flowing during cure. No by-products of the cure reaction are acceptable as they may contaminate other sensitive areas of the devices. These constraints often direct the choice to the platinum-catalyzed hydrosilylation cure system that is relatively expensive. 

Our study was also extended to obtain the effect of a selective curing system on NBR-PVC compositions. 

For making compatible blends, the polymers should have comparable polarities and viscosities. The oil needs to be selected properly so that its solubility parameter is close to those for blend components. The cure system should be efficient for all constituent rubbers and the filler system needs to be appropriate. Finally, cost consideration should be taken into account to provide a commercially viable product. 

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