Sealant rheological properties

Almost all sealants contain a mixture of a powdered filler incorporated into a viscous liquid, which results in a viscous liquid sealant having a paste-like consistency, Processing conditions can have a dramatic effect on sealant rheology, cure time, and physical properties. Typical processing variables are mixer speed (rpm), time, temperature, and vacuum. Order of ingredient addition is also important. 

Cab-O-Sil Fumed Silica in Adhesives and Sealants. Cahot Corp. Publication Jun. 1990. McNeilly K, lost EM, Sexton P. Effect of Lubricant Solvent System on Rheological Properties of Silver-Filled Polymers. ISHMProc. Sthintl. Microelectronics Conf. 1994 308-317. Korkin A, Gusev E, Labonowski JK, Luryi S, eds. Nanotechnology for Electronic Materials and Devices. Springer 2007. 

Fillers increase the strength, impart needed rheological properties, and reduce the cost of sealants. Tensile properties are increased significantly, depending on the type of filler, its particle size, and the type of cure. Improper filler selection can ruin the performance of a polysulfide sealant. Calcium carbonates (wet or dry ground limestone, precipitated), carbon blacks (furnace, thermal), calcined clays, silica and silicate fillers, and rutile titanium dioxide are typical fillers used in polysulfide sealants. Generally, combinations of fillers are used in formulation. Plasticizers improve the working properties while... 

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. 

One of the features of this formulation is a broad use of different fillers in combination to regulate rheological properties required by sealant. 

Today, flowable materials are used not only for most classes of restoring teeth, but also as fissure sealants. Current products differ considerably in formulation and filler content (Beun et al., 2008). Fissure sealants contain less than 25% inorganic filler, while conventional flowable filling materials have about 50-70% filler. Consequently there are considerable differences in properties between commercially available products. Beun et al. (2008) have studied the effect of these variations on rheological properties including storage modulus, and classified the products into three groups, based on their sohd-like behavior. 

Beun S, Bailly C, Devaux J, Leloup G (2008) Rheological properties of flowable resin composites and pit and fissure sealants. Dent Mater, 24, 548-555. 

Latex products with high solids content and acceptable rheological properties are desirable in many industrial applications such as adhesives, coatings, and caulks and sealants. Such concentrated polymer colloids are generally manufactured by semibatch emulsion polymerization processes and characterized by the extremely efficient packing of the polymer particles with broad particle size distributions. Thus, how to effectively manipulate the complex... 

Adhesives and sealants - Kaolin is used to control flow, penetration, and specific adhesion on application, and adhesive strength, tear strength, tensile strength, and elongation after cure. For aqueous systems the choice of airfloated vs. water-washed clay is dictated by cost, color, abrasion, and rheological properties pre-cure and reinforcing properties post-cure. Stearate-coated clays are used for improved compatibility in nonaqueous systems. 

Examples of die rheological properties of silicone sealants reinforced with precipitated and fumed silicas are shown in Table 4 and Figure 5 (Cabot, 1994). 

Good dispersion of the precipitated calcium carbonate in the adhesive or sealant is essential to optimize the reinforcement and rheological properties. Frequently blends of ground and precipitated calcium carbonates are used to optimize cost/performance. The ratio of ground to precipitated typically ranges from 1 1 to 3 1. 

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... 

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. 

Other Thixotropic Additives. Microcellular fillers such as glass and plastic are also used to provide nonsag properties to adhesives and sealants. However, their mode of operation is very much different from that of the rheological modifiers mentioned above. Microcellular fillers work because they drastically reduce the specific gravity of the product. Thus, because the adhesive or sealant is lighter, there is less stress on the adhesive or sealant due to the forces of gravity. 

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