Curing system, vapours

Photoinitiators. Derivatives of benzoin and benzil are added in amounts of 1-3% as photoinitiators in UV-curing systems. These have differing effectiveness in the UP systems [2.99], Special initiators are available for pigmented systems. The UV light splits them into radicals which in turn initiate polymerisation. The UV radiation is generated using superactinic fluorescent lamps and/or high-pressure mercury vapour lamps [2.109]. 

Some polymerizable esters can be used as a copolymerizable internal plasticizer in technical applications. The best known of the group is diallyl phthalate (DAP), which is used to replace styrene, divinyl benzene, or methyl methacrylate in unsaturated polyester resins. It has a very low vapour pressure (300°C boiling point), leading to significant reduction in loss through evaporation. It considerably improves properties such as hardness, chemical resistance, hydrolysis resistance, electrical properties, and product life. It is particularly used in electrical applications, can be employed (after suitable preparation) in cold-cure systems, and shows high affinity to glass fibre. DAP can also be used as a reactive plasticizer with PVC resins. 

Moisture-curing systems are based on isocyanate-terminated branched prepolymers which cure by interaction with atmospheric moisture. The prepolymers are prepared by reaction of an excess of diisocyanate (usually tolylene diisocyanate) with a polyol (polyester, polyether or castor oil). It is important that the residual free tolylene diisocyanate content should be low to reduce toxic vapour hazards. When the pre-polymer is exposed to moisture, the following reactions occur and cure is effected ... 

This reaction is extremely slow in the absence of catalysts but can be serious in the presence of either acid or base catalysts. Polymers should therefore be free of ionic polymerization catalyst residues and their vulcanizates free from acidic residues of curing systems. It has been found that in a closed system in which water is present the depolymerization proceeds until an equilibrium molecular weight is reached which is inversely proportional to the water vapour pressure. 

If water-soluble substances are introduced when compounding the rubber, moisture will eventually diffuse to the water-soluble particles and form solution droplets within the rubber matrix. The difference in the vapour pressure of the internal solution and the vapour pressure of any external water phase then becomes the driving force for further infusion of water. When the rubber matrix is vulcanised, the elastic modulus of the rubber network exerts pressure on the internal solution, and when this pressure equals the osmotic pressure of the internal solution, equilibrium is established and absorption ceases.Consequently, when compounding for minimum water absorption it is important to minimise the introduction of electrolytic substances into the compound and to select the grade of butyl rubber and curing system which will give the highest elastic modulus consistent with other performance needs. 

S2C12, a by-product in the manufacture of carbon tetrachloride from carbon disulphide. Was used, dissolved in solvent naphtha, in the vulcanising of mbber by the cold cure process and the vapour cure process. The process was fraught with health and safety problems and has been superseded by low temperature accelerators and room temperature vulcanising (RTV) systems for silicone and polyurethane. 

The Royal (or Royale) process was originally developed as a method for drying timber, in which the wood is heated in oil under vacuum. The temperatures used are low (60-90 °C) compared to other thermal oil treatments, and although sufficient to lead to some curing of the oil itself, there is no direct modification of the wood as a result of this process. The oil does not penetrate the cell wall. In this process, wet timber is placed in a treatment vessel and oil is then introduced, which is heated to the desired temperature, whilst a vacuum is applied. Water is removed from the timber and the vapour is transported away by the vacuum system. When the wood has reached the desired MC, the oil is removed from the treatment vessel. After this, a vacuum is applied to removed excess oil from the wood. Some dimensional stability is imparted to the timber due to the water repellency of the oil. This treatment is marketed by Osmose as the Royale process. 

Investigation of the released vapours during the cure of epoxy resin system by TGA -... 

The NH3 and phenolic-OH concentrations versus time/ Temperature as measured by FTIR in the vapours released during the degradation experiment of a cured GEPN/dicy system sample in the TGA. 

UV curing is carried out at a conveyor speed of approximately 3 m/min. The conveyor speed is usually calculated on the basis of a high-pressure mercury vapour lamp with an output of 80 W/cm. Conventional curing of spray fillers in a circulating air oven takes 10 min at 80 °C. These systems provide virtually ideal substrates for a number of top coats as, unlike other systems, they have good hold-out. 

The diamine used is MDA salt, which is a sodium chloride complex of methylene dianiline. Above 120 C the salt dissociates, to give a very fast cure - 3 min. An example of the difficulties which can arise in specific industries and applications is given by the fact that the MDA salt is supplied as a 30% solution in dioctyl phthalate (DOP). DOP is incompatible with the developed PU system, and can exude under mechanical pressure to attack the primer. No satisfactory solution to this problem has yet been found, because MDA, like its predecessor methylenediorthochloroaniline (MOCA) is regarded as a toxic material, and is only supplied in this form. DOP is a very non-volatile high-boiling point ester, and this minimizes the vapour pressure and hence concentration in the air of MDA during transport and use. 

When rubber swelled in benzene (49), or when paraffin (50 was absorbed by cured rubber and by rubber gum, the positive temperature coefficients of the sorption velocity suggested that in these and other organic solids vapours are sorbed by processes of activated diffusion. The occurrence of activated diffusion has already been established for gas-rubber (52), cellulose, -bakelite and similar systems (Chap. IX). In liquids also, diffusion constants (53) may be expressed by the equation D = although the values of E are usually smaller. 

Unsubstituted and nonheat-reactive phenolic resins are used to a very limited extent due to their nonfilm-forming character. These resins are generally used as hardeners with epoxy resins to produce thermoset systems with high-quality engineering plastic properties. The base-catalyzed curing produces a crosslinked polyether structure, as shown in reaction (30), which is resistant to chemicals and heat and shows good barrier properties against moisture vapour. 

Recently, it has been reported that curing of PU resins can also be performed in the vapour phase (VPC). It has been shown that isocyanate-terminated prepolymers applied on various substrates can be rapidly cured by exposing the coated substrate to the vapours of ammonia and primary monofunctional or difunctional aliphatic amines. The major advantages of VPC curing are the relatively low capital cost for equipment and room-temperature curing at fairly high speeds (at least for aromatic-isocyanate-based systems) for both clear and pigmented films. 

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