Two-Pack Systems

Polyisocyanate and Polyhydroxyl Systems. Type 8 in Table 2.5. This is by far the 

Two-pack polyurethane paints can be applied by all conventional methods apart from dipping. Curing takes place at ambient temperature, but can be accelerated if necessary by heat. Stoving is not necessary (energy saving). 

The reaction between the isocyanate and the hydroxyl groups results in 100% cross-linking if the reactants are converted in stoichiometric proportions. Optimum paint films are generally formed when NCO OH equals 1. However, it may be advantageous to deviate from this ratio (under- or over-cross-linking). With aliphatic polyisocyanates, a catalyst (accelerator) is generally necessary. Small amounts (0.05-0.5%) of tertiary amines or metal-containing compounds are normally added 

Esters, ketones, and ether esters of polyurethane grade (i.e., absence of reactive constituents and a maximum water content of 0.05%) are required for solvent-containing paints. Aromatic hydrocarbons are suitable as diluents. High-solids two-pack paints containing 30-40 wt% of volatile components are becoming more popular on account of their low solvent emission. 

Solvent-free two-pack systems require liquid reaction components and are normally based on MDI and its homologues. In contrast to solvent-containing paints, pigmented solvent-free two-pack systems require the use of water-binding agents (e.g., zeolites) to obtain thick, bubble-free coatings. 

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Two-pack systems (sometimes known as RTV-2 rubbers). These are widely used for making flexible moulds, particularly for craft work. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Moisture-curable urethane systems (one-pack) can be considered as two-component systems which use atmospheric moisture as the second component. One-pack urethane coatings can be produced that are similar in physical properties to the two-pack systems for almost all applications. These highly complex systems can have a great deal of flexibility. Claimed advantages are a one-pack system, rapid cure, even at low temperatures, excellent chemical and abrasion resistance and good flexibility. Although these systems have been available for some time in other countries of Europe, they are only recently beginning to be of interest in the UK. 

Unsaturated polyester finishes of this type do not need to be stoved to effect crosslinking, but will cure at room temperature once a suitable peroxide initiator cobalt salt activator are added. The system then has a finite pot life and needs to be applied soon after mixing. Such a system is an example of a two-pack system. That is the finish is supplied in two packages to be mixed shortly before use, with obvious limitations. However, polymerisation can also be induced by ultra violet radiation or electron beam exposure when polymerisation occurs almost instantaneously. These techniques are used widely in packaging, particularly cans, for which many other unsaturated polymers, such as unsaturated acrylic resins have been devised. 

The reactive system can be a one- or a two-pack system. Normally end users prefer a one-component system with a long pot life at room temperature and a high reactivity in the mold at a temperature as lower as possible. These two conditions are in contradiction with the Arrhenius law that can, however, be bypassed by the use of blocked reactants or by reactions activated by radiations. 

Two-Pack Systems 8 Reaction of NCO groups with OH groups solvent-containing, solvent-free, or aqueous up to 130 C... 

Systems with Polyisocyanates and Blocked Reactants. Type 9 in Table 2.5. In special cases, compounds with blocked amine groups (e.g., ketimines) can be used as reaction partners for polyisocyanates. The chemical curing of these two-pack systems is initiated by humidity which causes regeneration of the reactive amine groups. 

Transportation and Storage. Containers and drums with a polyethylene insert or resistant lining are suitable. The binders should be stored at as low a temperature as possible and under absolute exclusion of moisture. The storage time of two-pack systems varies and is specified by the manufacturer. Commercial products are stable for about 9 months when stored at 20 C. 

Epoxy paints (see also Section 2.10) are usually applied as two-pack systems. Curing with amines is preferred in high-solids systems. Epoxy resins may also be used as coreactants or reactive diluents in low-solvent paints based on acrylic, polyester, or alkyd resins. 

Side and cross members are pretreated by blasting and phosphating. An electrode-posited primer coating follows and is sometimes immediately overcoated with a topcoat based on an air-drying alkyd or epoxy ester, or on an oven-drying alkyd-melamine resin combination. After assembly of the truck, a third layer (i.e., a second topcoat) is applied that serves as a supply finish. These finishes are mostly two-pack acrylic aliphatic isocyanate topcoats. Water-based two-pack systems and powder coatings are the most recent developments. 

Various surface effects can be produced by varying the viscosity and adding coarse, possibly colored extenders or fibers. Two-pack systems based on polyurethane resins or epoxy resins are used for wall coatings that require a good resistance to agents used for chemical cleaning and decontamination. 

Uses Polyester blending resin for use with other resins in solv.-based or water-based paints, one-pack enamels, polyisocyanate-crosslinking two-pack systems... 

Uses Acrylic for fast drying two-pack systems, primer surfacers for car repair and general Industrial applies., automotive refinishinq, hlqh-qrade Industrial paints... 

Uses Binder, film-former in paints (printed circuit board coatings, coil, container, conosion-protection, marine, metal, plastics, sealers, tarp, wood varnishes), printing inks (screen printing), primers (adhesion pro-motion/corrosion protection), ceramic compds., sealers, heat-sealable lacquers, two-pack paints, resinous/polymeric food-contact coatings reactive resin in two-pack systems, e.g., crosslinked with melamine/phenolic resins... 

Chem. Descrip. Polyurethane in water and emulsifier Uses Thickener, rheology control agent, flow aid for aq. coatings, acrylic gloss paints, anticorrosion paints, styrene-acrylate, S/B, water-emulsifi-able two-pack systems, air- and force-drying water-emulsifiable binders Features Low-vise. 

Definition Elastomer made by reaction of an isocyanate prepolymer with a short chainextending agent at ambient or high temp. one-and two-pack systems avail. 

One-pack or two-pack polyurethane surface coatings, which may sometimes be combined with acrylic resin systems. The one-pack system, which has a long shelf life, is an adduct of resin with polyisocyanate dissolved in a moisture-free solvent. When applied to a surface the film cures and hardens, as moisture in the air reacts with the isocyanate groups in the resin. The two-pack system consists of a polyisocyanate (frequently derived from an aliphatic isocyanate monomer) and a polyol as separate components. Once the two components are mixed, the shelf life is very limited. 

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