Main sealants

The general composition of a sealant consists of a base polymer and additives. However, there is an inclination to classify sealants in terms of their chemical composition or physical properties. Based on this, the main sealant groups are Pi... 

In the United States approximately 50% of the 40,000 t of chloriaated paraffins consumed domestically are used in metal-working lubricants. Approximately 20% are consumed as plastic additives, mainly fire retardants, and similarly 12% in mbber. The remainder as plasticizers in paint (9%) and caulks, adhesives, and sealants at 6%. 

Adhesion promoter. Epoxy silane can be added to increase adhesion to glass (mainly for BR sealants). In certain textile applications, isocyanates can be added to solvent-borne BR cements. 

The main source of human exposure to cyclic siloxanes is PCPs. Other less significant sources are rubber products, sealants, cookware, silicone grease, pharmaceuticals, medical devices, electronics, dust, and even indoor and outdoor air [277, 281]. The highest concentrations of cyclic siloxanes reported by Lu et al. [282] in PCPs from China were 72.9 pg/g D4 and 1,110 pg/g D5 in shampoos and... 

Exposure through inhalation As most of the quantity of the cyclic siloxanes evaporate shortly after application of the PCPs or volatilize from electronics, adhesives and sealants, polishes and surface cleaners, etc., the main pathway of exposure is through inhalation. Plotzke et al. [286] studied the inhalation exposure to D4 by exposing rats to C-DA. Using liquid scintillation counting, it was determined that the retention of inhaled D4 in the body of the rats was 5-6%. The radioactivity reached maximum concentrations in the fat 24 h after exposure, but in the plasma and other tissues (except for fat) in only 3 h. The fat tissue acted as a depot because the elimination of the radioactivity from it was slower than from other tissues. 

It is mainly used in the industrial production of rigid polyurethane foams. Many other uses are in the fields of coatings, adhesives, sealants and elastomers such as paints, adhesives, weather-resistant sealing materials and foohvear. There is use also in the production of particle board (bonding of wood) and mould cores for the foundry industry (European Union, 1999). 

Although polyethers are the main building blocks of the prepolymer, other materials such as polyesters, polythioethers, and polybutadienes are also used. Most urethanes use blends of polymers to achieve desired properties. Urethane sealants have pood inherent adhesion to most substrates, but silane adhesion promoters are often used to unprove this adhesion,... 

Table 23. Main properties and applications of silicone sealants and compounds...

Table 24. Main properties of Elastosil glue sealants...

In the process of vulcanisation in air Elastosils form rubberlike materials and have good adhesion to steel, copper, aluminum, wood, ceramics, concrete, polymethylmetaciylate, glass and other materials. Consequently, they do not require the use of any special sublayer. Optimal physicochemical properties of Elastosils are achieved after 5-7 days of solidification at 60-75% humidity in air. The main properties of glue sealants Elastosil are given in Table 24. 

The analogous large-volume series ZSK MEGAvolume with 8.7 Nm/cm3, a diameter ratio of 1.8, and screw speeds of up to 1800 rpm is used for compounds with a high solid matter content, such as those often found in the chemical/food/pharmaceutical industries. The maximum speeds of up to 1800 rpm are used mainly for low-viscosity products, such as pressure-sensitive adhesives and silicon sealants because they provide better elastomer and filler dispersion. Other processes often require large volume only and operate at very moderate speeds of 100 to 600 rpm to protect the product or for direct extrusion into films, sheets, or profiles. 

Several organic sealants such as epoxy resins, butyl rubber or silicones prove to be more or less permeable and the tiny amount of solvent in the cell is rapidly lost. Suitable organic sealing materials for this technology turn out to be thermoplastic materials, like polyethylene/carboxylate copolymers. So far, Surlyn 1702 ionomer from Dupont has been the main substance used to optimize cell performance and build module prototypes. However, the softening point of Surlyn is rather low (65° C) and at elevated temperatures (> 70°C), serious solvent loss is observed because the bond between Surlyn and TCO-coated glass is substantially weakened [7]. 

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. 

Nondestructive Testing. Nondestructive testing (NDT) is far more economical than destructive test methods, and every assembly can be tested if desired. Several nondestructive test methods are used to check the appearance and quality of structures made with adhesives or sealants. The main methods are simple ones such as visual inspection, tap, proof, and more advanced physical monitoring such as ultrasonic or radiographic inspection. The most difficult defects to find are those related to improper curing and surface treatments. Therefore, great care and control must be exercised in surface preparation procedures and shop cleanliness. 

Polyethylene foam sheet is used mainly in package cushioning, and also in roof insulation and extruded profile is also used as construction sealants and pipe insulation. It is made mainly by tandem extrusion. The first extruder melts the polyethylene and blends in liquid volatile organic foaming agent. The second extruder cools it to the optimum foaming temperature, and pumps it through a die into room-temperature air, where it expands into foamed sheet. 

---