Sealant performance

Solvent-based acrylic sealants are semi-elastomeric because of their thermoplastic nature. The movement accommodation factor is approximately 20%. However, these sealants perform best in the area from 7.5%... 

Other than cure rate, the effect on the sealant performance of changes in the R group on the silane crosslinker is quite smalL When R is changed for a 4th functional group [e.g., MeSi(0Ac)3 replaced with Sl(0Ac)i, (both commonly used)] the reactivity Increases dramatically but so does the crosslink density. 

Usable in Hot Climates. Most organic sealants perform satisfactorily when temperatures are warm. But silicones remain effective in such hot locations as the sandwich panel of a solar collector or around heat ducts or hot pipes. However, the designer must choose the right silicone, since some perform up to 150 C, some to 200 C and 250 C. These temperatures can be contrasted to the 70 C to 120 C operating maximum of nonsilicones. 

The failure envelope is used in the literature to characterize polymers because it is independent of time and temperature, but its usefulness is limited with sealants. From the point of view of sealant performance, the projection of the failure points into the log strain-log time plane is the most important characterization it is the strain that is imposed on the sealant by the movement of the joint and the stress develops as a consequence of the imposed strain. Consequently, the design of a sealed joint is usually based on an estimate of strain, not of stress, and the sealant is chosen according to its movement capability, that is, the per cent movement the sealant can take without failure in a yearly movement. Stress has to be considered only in those rare cases where the substrate is a fragile, porous material whose tensile strength approaches that of sealants. In this case, a sealant with the lowest strength possible has to be chosen. 

Sealant Performance Characteristics, Strucutral Sealant Newsletter, No. 18, Jan. 1968, Thiokol Chemical Corp. 

Sealant performance is improved under stress if it adheres only to the sides of the joint and not to the bottom. 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

Typical specifications for the polysulfide polymers are summarized in Table 1. Specifications for the sealants vary widely depending on the specific apphcation and the needs of the appHcators. Standards for sealant testing vary in different countries. Ultimately, the tests should simulate the environment the sealants will be exposed to and measure their performance under these conditions. 

Aircraft Sealants. Polysulfides have been used for sealing fuel tanks and aircraft stmctural components since the 1940s. There are stringent requirements for these sealants. They must have outstanding resistance to fuels and exceUent adhesion to many different materials such as various metal aUoys and protective coatings used in aircraft constmction. The sealants must also perform in extremely variable weather conditions (10). The polysulfides have an exceUent performance record. 

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. 

Another way to determine durabHity is to find successfuHy sealed, existing field instaHations. Sealant manufacturers often have case histories of successful instaHations. Adhesion performance can vary widely with sealant type, substrate type, and cleaning method. Eor this reason, it is important to understand the sealant s sensitivity to cleaning practices. Often with difficult-to-adhere substrates, a primer is used. Sealant manufacturers can provide recommendations as to which substrates require primers, what type of primers should be used, and how they should be appHed. 

Future developments are likely to feature the production of more siUcone sealants that do not pick up dirt, more latex acryUc sealants that have high performance properties, urethanes that have improved uv stabiUty, and high performance polysulfides that are made in the United States. 

M. A. Shewia, "High Performance AcryUc Latex Sealants," ia Ref. 9. 

The surface of the substrate, the silicone/substrate interface, and the bulk properties of silicones all play significant and influential roles that affect practical adhesion and performance of the silicone. The design of silicone adhesives, sealants, coatings, encapsulants or any products where adhesion property is needed requires the development chemist to have a thorough understanding of both silicone chemistry and adhesion phenomena. 

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... 

If outdoor air can easily leak into and through homes, both comfort and energy-efficient performance will be difficult to maintain. Today, off-the-shelf technologies that contribute to airtight construction include a variety of house wraps, sealants, foams, and tapes. In energy-efficient homes, builders use these tools to seal the myriad of cracks... 

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