Polyurethane sealant

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. 

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. 

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, tillers, plasticizers, adhesion promoters, and other additives. 

Water scavenger Acts as a water scavenger or drying agent by forming polymeric titanium oxide hydrate or other compounds Used as an additive in silicone or polyurethane sealants (0.5-5% by weight)... 

PAROIL 57-61 especially good for polysulfide and polyurethane sealants... 

Whereas epoxy crack injecting systems are applied in static cracks, polyurethane systems are suitable for cracks where some movement is anticipated. There are other applications where polymeric materials are considered as competitors to each other. For example, neutral cure silicone sealants and polyurethane sealants are both suitable for use in construction joints. The former type offers greater durability and ageing resistance. However, polyurethane sealants are widely used because of their cost-effectiveness. Furthermore, some products are preferred over their counterparts for environmental reasons. 

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. 

Polyurethane sealants are available as in both single-component and multicomponent formulations and in each case a variety of performance... 

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. 

Polyurethane sealants are available in a non-sag formulation suitable for vertical joints and in a self-levelling formulation, useful for horizontal joints. The latter class of sealants flow into the joint giving a smooth level surface. They do not require tooling. 

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

Self-levelling polyurethane sealants are used to seal horizontal movement joints and construction joints in a variety of materials subject to light traffic such as concrete pavements (Figure 4.4), access roads, yards, parking areas, between rails and metal enclosures and around equipment and machinery. 

Pitch-polyurethane sealants. Polyurethane sealants can be formulated in combination with pitch or coal tar in order to impart the property of fuel and oil resistance. Such sealants comply with British Standard BS 5212 and US Federal specification SS-S-200E. 

Figure 4.1 The use of non-sag polyurethane sealants in an expansion joint (a) applieation (b) tooling. Reproduced with permission from Fosroc...

Figure 4.2 Precast joints applied with polyurethane sealant. Reproduced with permission...

R.M. Evans, Polyurethane Sealants, Technology and Applications, 2nd Edition, Technomic Publishing, Lancaster, PA, USA, 1993. 

Great Stuff Sealant. [Insta-Foam Prods.] One-component polyurethane sealant in aerosol can. 

Oxazolldene/allphatlc diisocyanaes can be used as one pot resin systems which cure in the presence of water at ordinary temperatures and pressure.— Flexible bubble-free coatings of polyurethanes have been produced by the use of vacuum centrifugal coating techniques.— New amine catalysts may be used to reduce curing time of polymerization for the production of cast urethanes.— Additional information on polyurethanes sealants is supplied Chapter 4. 

The life expectancy of polyurethane sealants has been considered to be 10 years while that of butyl rubber, acrylics, polysulfides and silicones has been stated to be 15 years. Additional information on application techniques2 l and tests for sealants is readily available. 

Since their introduction less than a century ago, the use of polyurethane sealants has grown tremendously. The use of polyurethane sealants2S 2 and ways of minimizing toxicity hazards characteristic of these materials have been outlined.22. 

Polyurethane Sealants Technology and Applications, by R.M. Evans, 1993, 186 pp., softcover, Technomic Publishing. Written for the compounding chemist, with four chapters on applications and live on materials. 

Sealants are divided into groups according to the generic names of polymer base. The main groups include polyurethanes, silicones, acrylics, polysulfide and others (PVC, polybutylene, styrene-butadiene-styrene copolymers, polychloroprene, and several others). The amount of solvent used in sealants is controlled by the standards which previously divided sealants into two groups these below 10% VOC and those above. Recently, a provision was made to include water-based acrylics and the limit of VOC for class A sealants was increased to 20%. Polyurethane sealants and structural adhesives can be made without solvent (the first solvent-free polyurethane sealant was made in 1994). Solvents are added to reduce sealant viscosity and to aid in the manufacture of polymer. Typical solvents used are mineral spirits, toluene, and xylene. A small amount of solvent is emitted from curatives... 

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