Sealant characteristics

Besides water vapor and gas diffusion, other requirements for good edge sealants are water resistance, uv resistance, heat/cold resistance (—40 to 80°C), adhesion to glass and metal, and good characteristics for appHcation. Polysulfide sealants have maintained an exceUent record in use since the 1950s. Development of new polysulfides and sealant formulations continues in order to meet market needs of the 1990s. 

Adhesion Life. A second key factor in determining the durabHity of a sealant is the abHity of the sealant to adhere to the substrate through its lifetime. A sealant may have exceHent resistance to uv effects, but if it has poor adhesion performance and faHs adhesively, it is of Httie use. The same can be said of a sealant with superior adhesion characteristics but poor resistance to uv. Either situation results in a short performance life. 

A chemical property of silicones is the possibility of building reactivity on the polymer [1,32,33]. This allows the building of cured silicone networks of controlled molecular architectures with specific adhesion properties while maintaining the inherent physical properties of the PDMS chains. The combination of the unique bulk characteristics of the silicone networks, the surface properties of the PDMS segments, and the specificity and controllability of the reactive groups, produces unique materials useful as adhesives, protective encapsulants, coatings and sealants. 

When formulating a silicone adhesive, sealant, or coating, based on hydrosilylation addition cure, one must consider the following properties of the uncured product pot life, dispensing technique, rheology, extrusion rate, cure performance. These characteristics directly affect the processing properties of the polymer base or crosslinker parts. The degree of cure conversion at the temperature of interest is determined by properties such as tack free time, cure profile and cure time. Once... 

Leaching characteristics can be improved, where necessary, by coating the resulting product with a sealant. 

In our study, the effect of moisture over the nonneutral pH range of 3-11, direct sunlight, ozone at a concentration level of 6000 ppm, and the effects of loading stresses, were investigated for the three commercial sealants. A characteristic variation of crosslink density for the typical silicone sealants is shown in Fig. 29. This figure depicts the results for the coupons exposed to moisture and sunlight. Initially upon exposure, the crosslink density of the sealants exhibit an increase due to the availability of residual uncurred crosslink sites... 

We can tune the physical characteristics of polyurethanes over a wide range by careful selection of their components. Polyurethanes are used extensively as foams, which range from soft and resilient to rigid. Solid polyurethanes range from soft moldings and elastic fibers to rigid injection molded items. Other uses of polyurethanes include coatings, sealants, and adhesives. 

Dielectrics and sealants. Fligh dielectric characteristics of silicone liquids allow them to be widely used in condensers and other electric and radio equipment, in airplanes and radars. High arc resistance of organosilox-anes is due to the formation of carbon dioxide, a good dielectric, when they disintegrate. 

The adhesive base or binder is the principal component of an adhesive. The binder provides many of the main characteristics of the adhesive such as wettability, curing properties, strength, and environmental resistance. The binder is often by weight the largest component in the adhesive formulation, but this is not always the case, especially with highly filled adhesives or sealant systems. The binder is generally the component from which the name of the adhesive is derived. For example, an epoxy adhesive may have many components, but the primary material or base is an epoxy resin. Once the binder is chosen, the other necessary ingredients can be determined. Chapter 4 describes in detail the various polymeric resins that are commonly used as bases or binders in epoxy adhesive formulations. 

Control of Viscosity. To ensure that adhesives and sealants function well during their application and end use, the formulator must be able to control the flow properties of the product. The challenge that faces the formulator is that the adhesive or sealant may need different flow characteristics at different times. For example, adhesives must flow readily so that they can be evenly applied to a substrate and wet out the surface. Yet, there should not be an excess of penetration into porous substrates, nor should the adhesive run or bleed to create a starved joint. Certain adhesives and sealants must be capable of convenient flow application by trowel or extrusion, but they must also exhibit sag and slump resistance, once applied. Therefore, the flow properties, or rheology, of the material must fit the desired method of application. 

Several types of polymeric fibers are used as thixotropes in both adhesive and sealant formulations. These are cellulose, polyolefin, and aramid fibers. A fiber that exhibits good thixotropy usually has several key characteristics including... 

It is interesting to note that these characteristics also are necessary to impart physical reinforcement and resulting cohesive strength improvement to the adhesive or sealant. 

Primers and adhesion promoters work in a similar fashion to improve adhesion. They add a new, usually organic, layer at the interface. The new layer can be bifunctional and bond well to both the substrate and the adhesive or sealant. The new layer is very thin so that it provides improved interfacial bonding characteristics, yet it is not so thick that its bulk properties significantly affect the overall properties of the bond. 

For an adhesive or sealant bond to be useful, not only must it withstand the mechanical forces that are acting on it, but also it must resist the service environment or the chemical forces that are applied. Thus, one of the most important characteristics of an epoxy adhesive or sealant is its endurance to the operating environment. 

These low-temperature environmental effects can be significant factors that contribute to an adhesive system s durability and life. This section discusses the characteristics of epoxy adhesive joints exposed to low temperatures and to thermal cycling and suggests formulations for improving the resistance of adhesives and sealants to these conditions. 

Most conventional low-modulus adhesives and sealants, such as polysulfides, flexible epoxies, silicones, polyurethanes, and toughened acrylics, are flexible enough for use at intermediate low temperatures such as -40°C. Low-temperature properties of common structural adhesives used for applications down to -129°C are illustrated in Fig. 15.9, and the characteristics of these adhesives are summarized in Table 15.12. 

An adhesive or sealant specification, like all material specifications, is a document that specifies values for all the important properties, together with limits of variability and methods for determining these values. There are many adhesives and sealant specifications, of which the most prominent are the industrial and government specifications. These describe and establish the technical and physical characteristics or performance requirements of adhesive materials. The most common sources of standards and specifications for the adhesives and sealants industry are the following ... 

The viscosity of thixotropic materials that exhibit a shear rate dependency is usually determined by the procedure described in ASTM D 2556. The viscosity is determined at different shear rates, and from this plot, apparent viscosity associated with a particular rotational speed and spindle shape can be obtained. Materials with thixotropic characteristics include Vaseline jelly and toothpaste. They are materials that tend to have very high viscosity characteristics and exhibit no flow at low shear rates. However, when pressure is applied (higher shear rates), the material flows easily, exhibiting a characteristic of lower viscosity. Such materials are very common in the adhesive and sealant industries. Thixotropic materials can be pumped through a nozzle, mixed, or applied to a surface with little resistance. However, when applied to a vertical surface, they will not flow under their own weight. 

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