SAG RESISTANCE

Amino resins are used by the paper industry in large volume for a variety of apphcations. The resins are divided into two classes according to the mode of appHcation. Resins added to the fiber slurry before the sheet is formed are called wet-end additives and are used to improve wet and dry strength and stiffness. Resins appHed to the surface of formed paper or board, almost invariably together with other additives, are used to improve the water resistance of coatings, the sag resistance in ceiling tiles, and the scuff resistance in cartons and labels. 

Miscellaneous. Talc is used in gypsumboatd joint compounds as a high end filler to promote smoothness, sandabiHty, and sag resistance. It is used in automotive primers and polyester body repair compounds to promote sandabiHty. It is used in a wide variety of caulking compounds to improve theology and sag resistance. 

Fumed silicas (Si02). Fumed silicas are common fillers in polychloroprene [40], natural rubber and styrene-butadiene rubber base adhesives. Fumed silicas are widely used as filler in several polymeric systems to which it confers thixotropy, sag resistance, particle suspension, reinforcement, gloss reduction and flow enhancement. Fumed silica is obtained by gas reaction between metallic silicon and dry HCl to rend silica tetrachloride (SiCU). SiC is mixed with hydrogen and air in a burner (1800°C) where fumed silica is formed ... 

The isocyanurate reaction can be both beneficial and troublesome. It can be the bane of production engineers. Low levels of alkaline impurities present in urethane raw materials such as polyols, tackifiers, etc., can cause problems in prepolymer production, resulting in high viscosity products at levels of 5 ppm or less. At higher levels of alkaline impurities, more serious problems can occur, including poor shelf life, poor caulkability, or poor sag resistance. At levels of 15 ppm or higher, the alkalinity can cause an isocyanurate reaction in a prepolymer that can result in a gelled reactor. 

Curing compound can be applied to vertical surfaces due to enhanced sag resistance. 

It can provide sag resistance (thixotropy) for adhesives that are applied to vertical surfaces. 

Thixotropy provides a shear thinning effect that is, viscosity decreases as the shear rate increases, and vice versa. This not only allows easy pumping, dispensing, and mixing of the adhesive, but also provides sag resistance once the adhesive is applied. The thixotropic fillers work by forming a temporary structure in the mixture, which can be broken down at high rates of shear. This structure is generally the result of van der Waals forces between molecules. 

TABLE 9.7 Comparative Sag Resistance Properties of Fumed Silicas11... 

Comparative sag resistance properties of various commercial types of fumed silica are shown in Table 9.7. In most liquid epoxy resin systems, fumed silica is only employed at a 1 to 3 wt percent basis to provide thixotropic characteristics. 

Flow characteristics can be regulated by the incorporation of fillers, by the use of scrims or woven tapes as internal shims within the adhesive itself, or by the careful regulation of the cure cycle. All these options along with a few more are available to the adhesive for-mulator and end user. Generally, fillers are incorporated to control the viscosity of the adhesives as well as other properties such as thixotropy and sag resistance. 

Goel, A. B., Sag Resistant Urethane Adhesives with Improved Antifoaming Property, U.S. Patent No. 4,728,710,1988. 

In industrial products, the use of fumed silica will confer thixotropy, sag resistance, particle suspension, emulsifiability, reinforcement, gloss reduction, flow enhancement of powders, anti-caking, anti-slip, anti-blocking, etc. Because of its effect on these important properties, fumed silica is widely used in many industries. 

The overwhelming majority of filaments are made of NS-W (sag-resistant tungsten). Only for special shock and vibration resistant lamps are W-Th02 or W-Re wires used. Every year, about 20 billion meters of lamp wire are drawn, a length which corresponds to about 50 times the earth-moon distance [7.8]. 

About 1975 the ASTM specifications for gypsum board were changed to eliminate arbitrary weight limits and substitute performance tests. The tests added were humidified sag resistance core, end, and edge hardness and nail pull... 

With the exception of sag resistance, these tests are run by using a testing machine that can apply force to a specimen and measure the force applied. The device used was designed about 1922. It is slow and laborious to use and is not commercially available. 

Si G Sag Resistance distance-geometric distance combined matrbc... 

Cel time In two-component systems, the period of time in which a ready-to-use adhesive mix passes from the free-flowing to the sag-resistant state. 

The primary purposes of adding reinforcement are to increase stiffness, improve sag resistance at 163°C and lower... 

Commercial PBT/copoly(ether-ester) blends are generally richer (> 50%) in the copoly(ether-ester) elastomer content. These blends were designed for the automotive, flexible bumper fascia market (Bexloy V, DuPont). Typical properties of these blends are shown in Table 15.32. Typically they exhibit low moduli (< 800 MPa), high elongation and toughness. The purpose of PBT in the blend is to lower the cost and improve the heat sag resistance required for paint oven capability. 

HMHEC polymers are improved rheological modifiers for latex paints. They retain the benefits of traditional cellulosic ethers (compatibility with a wide range of coatings ingredients, storage stability, better sag resistance, etc.) but overcome many deficiencies of synthetic associative thickeners (sensitivity to latex systems and pH of paint formulations, poor sag resistance, etc.). The combined advantages offered by HMHEC are best suited for interior and exterior flat paints (5). 

Table I compares typical test results for low-, mid-, and high-effective molecular weight thickeners in a vinyl-acrylic latex paint. Many of the same trends are present with cellulosic and HE UR thickeners when the effective molecular weight designation for HEUR thickeners is used. As the effective molecular weight of an HEUR increases, Stormer thickening efficiency, roller spatter, and water resistance increase, whereas ICI viscosity decreases. The major difference between these two thickener types is in their low-shear behavior. As effective molecular weight increases, cellulosic paints improve in leveling and decrease in sag resistance. HEUR paints decrease in leveling and improve in sag resistance. Other property differences between these two thickener types are highlighted in Table II.

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