Curing polyurethane sealants

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, 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. 

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. 

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. 

Adhesives and Sealants. Polyurethanes can be conveniently applied in liquid or paste form and then polymerized/cured in place without evolution of volatile by-products, a very convenient feature in making enclosed adhesive bonds. Their mechanical strength, flexibility, adhesion, and chemical resistance make them attractive in many applications. Typical applications of polyurethane sealants are in expansion joints, aerospace, architectural, electronic, and marine products. 

Figure 6. Curing of atypical one-part polyurethane sealant. (Reprinted with permission.) ...

Solvents are used principally to reduce the viscosity and improve surface wetting of the substrate. Although the trend is to reduce or eliminate solvents for most polyurethane sealants, the solvent content must be low (0 to < 10%) in order to maintain properties after cure otherwise the polymer will develop stresses at the interface, leading to debonding. Typical solvents are ... 

Plasticisers act under the same phenomenon as solvents, increasing the free volume of the polymer but without producing complete dissolution. Both are governed by the same laws of solubility. However, each plays a different role. While solvents serve mostly as viscosity modifiers, plasticisers modify the curing properties of the sealant, softening and lowering the glass transition temperature. Common plasticisers in polyurethane sealants are ... 

Additionally, the classification of a product that has both adhesion and sealing capabilities is obscure. For this reason, the classification does not have a well-defined demarcation line. In some respects, the physical properties of urethane adhesives differ from those of urethane sealants. In line with that, this section presents the main types of polyurethane sealants and adhesives, their characteristics, advantages and disadvantages according to the form in which they are found, i.e. 100% solids, solventbome, waterborne, and the cure characteristics whether one or two-component material. 

There are many different formulations for two component polyurethane sealants dependent of the sealant properties required. The usual NCO/OH equivalent ratio is 1.05 to 1.10. One component is a liquid isocyanate-terminated prepolymer containing pigments, fillers such as calcimn carbonate or talc, and an antisag agent such as fimied silica. The second component is a hydroxyl terminated polymer, pigment, and a catalyst such as methylene dianiline. The individual components are mixed at elevated temperatures under dry conditions so that premature crosslinking does not occur. Prior to use, the two components are mixed well and have a pot life of about 1 to 4 hours. Cure time to develop full sealant properties is 1 to 2 days. 

As stated in Section 8.12.5.3, one can get a marvelous variety of products based upon this chemistry. The chemistry of these materials is as described above with the exception that the material must remain an elastomer after cure in order to perform as a sealant. The same diisocyanates, diols, and chain extenders that are used in the manufacture of adhesives are used to make sealants, with the proviso that the result should be an elastomer. Polyurethane sealants have a 25% movement capability and are thus considered to offer premium performance. 

Operating conditions are important determinants of the choice of filter media and sealant used in the cartridges. Some filter media, such as cellulose paper filters, are useful only at relatively low temperatures of 95 to 150"C (200 to 300°F). For high-temperature flue gas streams, more thermally stable filter media, such as nonwoven polyester, polypropylene, or Nomex, must be used. A variety of commercially available sealants such as polyurethane plastic and epoxy will allow fabric operating temperatures up tol50°C (300°F). Selected sealants such as heat cured Plasitcol will withstand operating temperatures up to 200°C (400°F). 

The rate of reversion, or hydrolytic instability, depends on the chemical structure of the base polymer, its degree of crosslinking, and the permeability of the adhesive or sealant. Certain chemical linkages such as ester, urethane, amide, and urea can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride cured epoxies. Generally, amine cured epoxies offer better hydrolytic stability than anhydride cured types. 

In this book I have confined discussion to those polymeric materials which are cured by chemical reaction and which have found widespread application in the construction industry. As such, the book covers materials based on epoxies, polyurethanes, silicones, polysulphides, alkyds and polyesters. In addition, there is a chapter on hybrid polymer systems and one on acrylics. It is true that acrylic emulsions are not strictly thermosetting polymer systems, but their widespread use and importance made their exclusion difficult. These materials find use as coatings, sealants, adhesives, grouts, flooring compounds, repair compounds and waterproofing agents. 

The mechanism of cure has an important influence on the filler choice. In reactive systems such as polyurethanes, water present in formulation adversely affects sealant stability (shelf-life) which imposes the additional selection criterion of water content. If water is present in the filler it must be removed by additional operations either by an expensive drying process or through the use of chemical moisture scavengers which is also an expensive approach. 

Two sealants were shown to have the most flexibility or toughness at 76 K and were subsequently used as the basis for modification studies to improve their low-temperature mechanical and physical properties. One was a polyurethane manufactured by duPont and designated Adiprene L-100. The other was a silicone manufactured by General Electric having both methyl and bulky radicals on the polymer backbone and designated RTV X-511. These sealant formulations were prepared and cured as shown in Table I. 

From the performance in bending of present polymer systems usable for cured-in-place sealants, a polyurethane and a noncrystallizing silicone appear to be most suitable as cryogenic sealants. They must be reinforced with a woven glass fabric to provide the strength necessary for expected service conditions on launch vehicles. Tests are underway to determine if they will also remain serviceable down to 20°K. 

As previously noted, the primary use for this elastomer has been as solvent-based adhesives and sealants. Solvent-based products are losing market share to water-based poly-chloroprenes, to other polymer types such as acrylics and polyurethanes, and to hot melt adhesives. However, where the processing facility is able to contain the vapor emissions, a solvent-based adhesive or sealant is preferred because of better wetting of surfaces, faster drying, and higher performance of the cured or dried product. Many rubber bonding... 

Uses PU curing agent producing high-performance R.T. cast and cured elastomers, cast prototypes and elastomers, coatings, adhesives, sealants, and spray systems epoxy flexibilizer Features Eor use with TDI or MDI prepolymers Properties Liq. equiv. wt. 225 Versalink P-650 [Air Prods./Polyurethanes]... 

Besides durability, premium sealants are judged by special properties as shown in Table 4. The ability to take on greater elongation and compression is measured by movement capability in terms of joint width. The stability to UV exposure is important for those glazing and insulation compounds used in modern high-rise structures. Thermal stability is in demand for solar collectors, or for other structural materials. On the basis of these evaluations, we can foresee future trends of sealants as shown in Table 4. Silicones appear to out-perform others. In the meantime, technical advances will provide low-modulus polysulfides, and better movement ability for both polysulfides and polyurethanes. Their cure time will be decreased and the UV stability will be improved to match or compete with silicones. All three will be developed for better adhesion under the un-primed conditions. 

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