Two-component sealants

In two-component sealants the components consist of a base, a hydroxyl-terminated compound and a liquid isocyanate-terminated prepolymer. The prepolymer reacts with the base in the presence of a catalyst to form the polyurethane ... 

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

Both one-component and two-component silicone sealants contain polydimethyl siloxane as the base polymer along with fillers such as calcium carbonate and/or fumed silica fillers, plasticisers (silicone oil) and a variety of cross-linking agents and adhesion promoters. Two-component sealants utilise a catalyst such as dibutyl tin dilaurate, alkyl silicate esters and metallic salts (Maslow, 1982). 

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. 

THERMOSETTING POLYMERS IN CONSTRUCTION APPLICATIONS 7.3.1 Two-component sealants... 

Reference standards have been adopted for polysulphide sealants in countries such as Australia (Doc. 1572-1970), Canada (19 GP 13a-1970), Japan (JIS-5724-1969) and other countries. However, a good two-component sealant can meet British Standard BS 4254, US Federal Specification TT-S-00227 E and ASTM C-920. Fuel-resistant two-part polysulphide sealants comply with BS 5212-75 and US Federal Specification SS-S-200 D. Sealants used for water retaining and excluding meet the requirement of BS 6920. Polysulphide sealants used for fire protection comply with BS 4254-1983. Additionally these sealants are non-combustible to BS 476, part 4 1984. All these categories will be discussed under separate head-... 

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. 

The multicomponent products generally do not rely on moisture penetration for cure. Their chief attribute is fast cure in very deep sections. Thus many industrial production lines that demand fast cure use a two-component sealant (including the use of silicone encapsulants for electrical components). Cure of these two-part systems can be accelerated further by additional catalyst or exposure to elevated temperatures. 

The two-component products (structural epoxydes, polyurethane adhesives and sealants, other two-component sealants) may be extruded from two-component cartridges (Fig. 15) and then mixed in a mixing device called static mixer immediately at the exit of the cartridge. 

The two-component sealants are cured by a chemical reaction between a free isocyanate group of part (A) and the active hydrogen group of part (B). To accelerate the curing, metallic ortertiary amine catalysts can be incorporated in part (B). The curing ofthe two-component systems is via the formation of a urethane linkage ... 

Two-Component Sealants.Two-component butyl sealants are room temperature vulcanized materials. Low in moisture vapor transmission and easy working because of low molecular weight and semi-liquid form, these sealants are filled with calcium carbonate and titanium dioxide, plasticized with polybutene, and may be modified with epoxy and silane adhesion promoters. The cure system is p-qui-none dioxime activated with a peroxide. 

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. 

Fig. 4. Application of fibrin sealant (arrow points to spray head for applying two-component adhesives) at the time of mastectomy and axillary dissection to reduce seroma formation.

An interpenetrating polymer network (IPN) consisting of an epoxy and an elastomer has been developed by Isayama.29 This is a two-component adhesive-sealant where the components are simultaneously polymerized. It consists of the MS polymer, developed in Japan by Kanegafuchi and commonly used in sealant formulations, with the homopolymerization of DGEBA using a phenol catalyst and a small amount of silane as a graft site to connect the MS polymer and epoxy homopolymer networks. 

Polyurethane sealants have taken a good share in today s sealant market. Polysulphide sealants are in greater use for water-retaining structures, and silicone sealants for structural glazing purposes but polyurethanes have a greater market share as construction sealants. These sealants were first introduced to the construction industry in the early 1970s as two-component materials. 

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

Two-component materials often use water as the chemical curing agent, by using wet fillers or by adding water to one of the components (SWRI, 1995). A fungicide can be incorporated in sealants used for moist environments (i.e. sanitary sealants). 

Polysulphide sealants were used in building structures for a long time before a standard specification was developed for such uses. ASA-A116.1 was the first specification for building sealants written by the Building Trade Specification Committee in 1956. Later, several specifications were designated by different societies for one-component and two-component systems. The specifications will be mentioned in the following. Apart from polysul-phides these specifications also refer to urethane and silicone sealants... 

In this type of sealant, the sealant consists of two parts, one containing polymer (also called the base) and a separate component containing the curing agent. These are combined by a mechanical mixer prior to application. Proper mixing is attained in approximately five minutes. Incomplete mixing is indicated by the appearance of streaks in the mix (if the two components are of different colour). At ambient temperatures the pot-life is about three hours. The pot-life decreases with increase in temperature. It is possible to increase or decrease the pot-life by incorporation of an accelerator or a retarder, respectively. At ambient temperatures the sealant cures to form a rubber-like solid. 

Table 7.7 Reference standards for two-component polysulphide sealants...

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. 

The coating is two-component and needs mixing on-site prior to application. It can be applied either using conventional airless spray equipment or by brush or roller. A final thickness of 1 mm to 2 mm can be obtained with two coats. Pretreatment of the substrate with an epoxy primer ensures good adhesion. Defects in the concrete or brickwork are sealed with an elastomeric sealant which is resistant to the fluid being stored in the tank. 

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