Silicone adhesives, characteristics

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... 

Adhesion of polyimides to inorganic substrates is of great importance to the microelectronics industry [1, 2]. The polyimide films are deposited most often by spin coating the polyamic acid (PAA) usually from a TV-methylpyrrolidone (NMP) solution onto the substrate surface followed by thermal imidization at temperatures up to 400<>C. The most studied polyimide is the pyromellitic dianhydride-oxydianiline (PMDA-ODA), which exhibits excellent mechanical and dielectric properties, but not so good adhesion characteristics. The latter has been generally overcome by application of an adhesion promoter, such as y-aminopropyltriethoxysilane [3-7]. The reactions of APS (coated from water solution) with the silicon dioxide surface as well as with polyamic acid have been well characterized by Linde and Gleason [4] however, we do not have such detailed information available on APS interaction with other ceramic surfaces. 

The introduction of functional groups (halogen, CN, COOH, NH2, etc.) into the organic part of the molecule is a way for a controlled modification of silicone polymers. These groups increase the polarity of the molecule and the intermolecular forces of interaction between the chains, which has a positive effect on the mechanical and adhesive characteristics of the polymers. 

Planarization and Adhesion Characteristics. The adhesion of the SOG films on unpatterned surfaces of silicon, thermal Si02 and aluminum was qualitatively evaluated by placing cured SOG films on the various substrates in boiling water for 30 min. No detachment or lifting-off of either film from any substrate was observed. However, it was found that SOG 203 spun on aluminized substrates had a tendency to form cracks (craze) upon cure at 400°C indicating poor adhesions, or, more likely, mismatch of thermal expansion coefficients of the two materials. 

The rheological properties of adhesives and sealants are important in many applications. When these products must be pumped or applied through automated equipment, the flow characteristics at pertinent shear rates are critical. Sophisticated rheological measurements can be performed to predict performance. The rheology of silicone adhesives and sealants can be tailored through adjustment of polymer viscosity, filler loading, and incorporation of various additives. 

Table 5.1 summarizes some of the characteristics of silicone adhesives. 

Chapter 1 Construction Sealants, by Philippe Cognard, including technical characteristics, different chemical types, applications in building, standards and regulations. This chapter will complement the chapters Silicone Adhesives and Sealants, MS Polymer Adhesives and Sealants, Polyurethane Adhesives and Sealants, Civil Engineering Sealants, which will be discussed in Volume 3 and also in later volumes. 

The adhesion-in-peel test was used to determine peel strength and adhesion characteristics of a cured-in-place silicone elastomeric joint sealant on aluminium substrates. The sealant used was Dow Corning Type 3145 RTV Adhesive Sealant. The results showed that the silicone sealant had poor adhesive bonding to the untreated aluminium. Plasma polymerisation of hexamethyl-disiloxane(HMDS) onto the aluminium was shown to move the locus of adhesive failure to between the plasma film and the silicone. Plasma polymerisation of HMDS... 

The term Structural Adhesive covers many basic chemical classes of adhesives. Also, within chemical classes there may be both structural and non-structurai adhesives, e.g. not all polyurethane adhesives are structural some are sealants having some adhesion characteristics. As such, information is restricted to those types of adhesives used, or promoted for use, in structural applications. In some cases, the dividing line between sealant and adhesive becomes blurred . For example silicone-based materials may be used as adhesives or as sealants. In many applications, the ability of an adhesive to also seal a joint against the environment is an important advantage, avoiding leakage or crevices where corrosion could occur. 

These versatile polymers can be compounded with other chemicals and fillers into an enormous variety of products that serve in a multitude of applications. This chapter will describe the product types, chemistry, and applications of only one major class of silicone products, the adhesive sealants. In addition, the class of silicone adhesive products will be described and discussed these materials are employed where release characteristics are desired, rather than adhesion. 

Finally, due to the nature of -Si-O- sequences, silicones have an extremely low surface energy that induces anti-adhesive characteristics and a damp-proof character which are used in multiple applications. 

Dow Corning Corp. offers an improved silicone adhesive/sealant for high-temperature use. This is a one-part, non-slumping paste that cures to a tough, rubber solid at room temperature on exposure to water vapor in the air. This material is said to perform at temperatures ranging from —65 °C to 260 °C for continuous operation and to 316 °C for intermittent exposure. This material wiU meet the requirements of MIL-A-46106A (2), Type 1 (see http //mil-spec-industries.com). The adhesive/sealant is acid cured and acetic acid is evolved during cure. Table 8.1 summarizes some of the characteristics of silicone adhesives. 

One of the important breakthroughs in the silicone PSA industry has been the development of new release liners based on fluorosilicones [116]. These new materials significantly improve the release characteristics of the silicone PSAs, something that has been difficult to achieve because of the high adhesion silicone PSAs show to low energy substrates, including silicone rubbers and liners. 

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. 

Polyphenylsilsesquioxane varnishes are used as binding agents for heat-resistant, protective and decorative enamels. To increase low adhesion, which is characteristic of silicone varnishes, enamels are often supplemented with various organic modifiers, pigments and fillers used in the vamish-and-paint industry. 

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