Viscosity pot life

The specific equipment and technique can vary by the material type used.The method chosen to apply the material in the holes can be influenced by many variables such as material cost, pot life, viscosity, equipment resources, and the quantity of panels to be processed. Some hole-fill materials lend themselves to a variety of fill methods. Some of the common hole-fill... 

The physical properties (e.g., color, density, solids or filler content, electrical properties, etc.) and working properties (storage life, pot life, viscosity, etc.) often need to be considered and subjected to screening testing too. 

Pot Life (Viscosity) ASTM D 1200 Ease of Application SSPC-PA-2... 

A hardener consisting of a blend of the two reaction products shown in the above equation is a low-viscosity liquid giving a 16-18 minute pot life for a 500 g batch at room temperature. 

The greater the degree of cyanoethylation the higher the viscosity of the adduct, the larger the pot life and the lower the peak exotherm. The products are skin sensitive. 

Progressive replacement of amine hardener by a low-viscosity flexibiliser will reduce mix viscosity, increase pot life and reduce the heat distortion temperature of the cured system. Higher impaet strengths are achieved using approximately equivalent amounts of hardener and flexibiliser. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Pot life is several hours versus several days for conventional non-reactive hot melts. A good reactive urethane is one which exhibits a viscosity rise of less than 10%/h. The slow increase in viscosity with urethane adhesives is due to chain extension via the slow reaction of the active hydrogen of the urethane groups with... 

Two-component systems consist of (1) polyol or polyamine, and (2) isocyanate. The hardening starts with the mixing of the two components. Due to the low viscosities of the two components, they can be used without addition of solvents. The mass ratio between the two components determines the properties of the bond line. Linear polyols and a lower surplus of isocyanates give flexible bond lines, whereas branched polyols and higher amounts of isocyanates lead to hard and brittle bond lines. The pot life of the two-component systems is determined by the reactivity of the two components, the temperature and the addition of catalysts. The pot life can vary between 0.5 and 24 h. The cure at room temperature is completed within 3 to 20 h. 

Usually spray-dried powders are sold. A purification step is not usually done on the industrial scale. The modification of the extracts are especially aimed at decreasing the sometimes too high viscosity to achieve better handling and application, but also a longer pot life and a better crosslinking [16,17,144]. 

For the above polyol blend viscosity (Brookfield, ASTM D-2196) = 1500 mPa-S at 23° C. For the reaction mixture working (pot) life 20 min Gardner circular dry times [72°F, 54% relative humidity (RH)] surface dry = 1.0 h, hard dry = 2.0 h, mar free = 3.5 h. For the finished coating gloss (ASTM D-523) = 90+ at 60° impact (ASTM D-2794) = 60 in.-lb direct, 10 in.-lb reverse Tabor abrasion (ASTMD-4060,1000 g load, 1000cycles, CS-17 wheel) = 95.6 mg pendulum hardness = 180 s MEK double rubs (ASTM D4752-95, 50 double rubs) = softened. 

Formulation Requirements. In order to penetrate the mass of fiber at one end of the bundle, the formulation must have sufficiently low viscosity to move easily through the bundle completely wetting all fiber surface area. Typically, formulations of viscosity less than 8000 poises have been successful. Too low viscosity or too rapid delivery of the formulation can result in the occlusion of air and the ultimate development of voids with loss of mechanical integrity. Our process demands that formulation be delivered and partially cured to an intermediate plateau termed green state. This requires a minimum pot life of 30 minutes after blending of resin and curative. The physical chemistry of the composite membrane requires that the initial exotherm not exceed approximately 150 C. 

The typical resin systems include thermoset polyesters, vinyl esters, epoxies, polymi-dies, bismaleimide, and phenolics. Thermoplastics are also finding their way into filament winding. Wet thermoset filament winding requires a resin with viscosity in the range of 1000-3000 cpoise. Resin components are chosen on the basis of pot life, winding temperature, viscosity, gel time, and cure time. 

In the plastisol propellant process, it is essential that the resin particles not solvate too rapidly at processing temperature since a rapid increase in viscosity of the propellant mix interferes with the mixing and casting operation. There must be adequate pot life of the mixed propellant. The resin-plasticizer system itself is the dominating influence on pot life, and for this reason certain combinations cannot be used in the plastisol process. 

The most commonly employed curative for HTPB is TDI which is comparatively more reactive than other curatives except MDI. Due to this, the pot-life of TDI-cured propellant is only 4— 5h. IPDI is another curative which is preferred to TDI mainly on account of its slow reactivity resulting in extended pot-life, that is, 15-18h and low toxicity [91]. It is only recently that a bicurative system consisting of TDI and IPDI (70 30) has been developed and compared with TDI and IPDI-based systems. Compared with pot-life of TDI system (4—5h), bicurative system has got 7-8 h pot-life. At the same time, viscosity build-up is less in bicurative system revealing an edge over conventional TDI-based system for smooth processing of a propellant slurry [92]. 

Once the prepolymer and catalyst are added together, the chain extension (curing) reactions will commence. The time taken to mix must be carefully monitored. It must be sufficient to allow complete mixing, but there must be enough pot life left to allow pouring into the mold while the material is at the lowest viscosity possible. This is to allow the polyurethane to fill the mold completely and any entrapped bubbles to reach the surface. 

Nonreactive plasticizers will soften the polyurethane as well as extend the polyurethane s pot life. The mixture has a reduced viscosity, which assists in the ease of flow of the mixture and the removal of any entrapped air. This is of great importance in the casting of rolls where the final product must be bubble free. 

Short pot life Faster increase in pot viscosity Curative contamination Incorrect temperature... 

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