Sealants one-component

Polysulphide sealants are of two types, namely two-component sealants and one-component sealants. These will each be discussed in turn in the following two sections. 

Two-part polysulphide sealants in the cured state have elastoplastic and elastic characteristics. A movement accommodation factor of 25%-30% is possessed by low-modulus sealants and one of 10%-20% by high-modulus sealants. The life expectancy is 20-25 years. They are more suitable for sealing larger joints and also joints that are subject to a somewhat larger degree of movement than are one-component sealants. 

Completely cured one-component polysulphide sealants are elasto-plastic and elastic in character. They have a movement accommodation factor of 20%-25%. These types of sealants are particularly suited for the perimeter sealing of window joints [Figure 7.2(a)], joints between precast concrete panels [Figure 7.2(b)], movement joints in brickwork where the movement is likely to be relatively slow, stone claddings, etc. The life expectancy for one-component sealants is, like that of two-component sealants, 20-25 years. 

EN 2839 1990 Building construction jointing products. Sealants determination of extrudability of one-component sealants (ISO 8394 1988). 

CuO, ZnOi, BaO, Water, amines, sulfur, te-tramethyl- thiouramide- sulfide Molecular sieve For one-component sealants... 

One-component sealants are based on isocyanate-terminated pre-pol miers with an isocyanate equivalent of 1000-2000, which are made from isocyanate and active hydrogen materials with an excess of isocyanates. The curing relies on the atmospheric moisture and proceeds with the formation of urea linkages ... 

In the global market, the acetoxy, methoxy, and oxime curing systems are the commercially predominant curing chemistries for one-component sealants. The tetrafunctional ethylor-thosilicate is used in two-component formulations. 

One-Component Silicones. The liquid silicone prepolymers used in one-component sealants are usually methyl-substituted polysiloxanes having more than two blocked, hydrolyzable end groups. These groups may be acetoxy, ketoxime, alkylamino, or benzamido. When the composition is extruded into a joint... 

The isocyanate terminated prepolymer used in the manufacture of polymethane sealants can be cured in two ways in two component sealants by the addition of hydroxyl containing coreactant such as polyester or polyether or in one component sealant by water absorption followed by polymea formation with the production of carbon dioxide as a by-product. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fiUers, plastici2ers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utili2e diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, siUcones, or polysulfide, are also used. 

Windshield sealants comprise some of the most significant applications of one-component urethane adhesives. Several years ago, the requirements for these sealants were increased. In an accidental rollover, in certain automobiles, the windshield becomes a structural part of the car. Therefore, the windshield adhesive, which holds the windshield in place, must have structural strength. An example of a one-component windshield sealant is shown below [48] ... 

Example 11. One-Component, Moisture-Cure Polyurethane Sealant. This example is of a low-hardness, high-elongation, moisture-curable polyurethane sealant. The material is based on a low-%NCO prepolymer made from 4,4 -MDI and a low-unsaturation (low-monol-content) Acclaim polyol from Bayer. It is adapted from (a) J. Lear et al., Adhesives Age, February 1999, pp. 18-23 and (b) B. Lawrey, et al., presented at UTECH 2000, The Hague, The Netherlands, March 30, 2000, Crain Communications London, 2000. 

Similarly to sealants, the compound paste is mixed before use with a metalorganic or silicone catalyst as a result, even at room temperature the paste switches from a thick-flowing to a rubberlike state and solidifies. The compound paste can be prepared with a catalyst, i.e. as a one-component system. In this case the equipment should be particularly airtight, especially the equipment used to load the compound into containers. 

All the materials to be tested for shelf life are stored in unopened containers. Storage in containers that have been opened exposes the resin to oxygen and humidity that, depending on the type of resin, could drastically reduce the shelf life and affect final properties. Most polymeric adhesives and sealants and their components have a shelf life greater than 6 months at room temperature. However, some one-component adhesives need to be stored at refrigerated conditions to have a practical shelf life. 

One-component polyurethane sealants are based on a polyether or polyester isocyanate prepolymer end-capped with a room-temperature-curable group such as trialkoxy silane. Free isocyanate groups are not used in one-part systems because carbon dioxide is evolved on reaction with moisture from the atmosphere which results in bulging of the sealant in the joint. 

Table 4,3 Properties of one-component and two-component polyurethane sealants...

Emulsion sealants low cost one component environmentally friendly light construction internal joins, fixtures and filling... 

Solvent-based sealants one-component non-staining good weathering good chemical resistance perimeter pointing cladding pre-cast joints... 

Water-based acrylic sealants are also referred to as acrylic emulsion sealants or latex sealants. These are one-component gun-grade material with a solid content ranging from 80%-85%. Curing occurs by evaporation... 

Although in this section one-component and two-component sealants are going to be discussed, the text is inclined towards one-component... 

As already mentioned, one-component silicone sealants cure by moisture pick-up from the atmosphere to form permanently elastic rubber. There are different types of curing systems for one-part silicone sealants (Table 6.5). However, the most commonly available systems are the acetoxy and neutral curing systems. 

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