Setting times

Set times for aEyl esters are similar to those for ethyl esters, as are bond strengths to steel, aluminum, ABS, PS, and PC. 

To control the degree of moisture evaporation and setting time, freshly poured concrete is sprayed with solvent solutions of aromatic, dicyclopentadiene, or aHphatic resins (see Cement). 

Duromers (cross-linked polymers) based on highly reactive resins with short setting times. 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

Accelerator in Ready-Mix Concrete. Calcium chloride accelerates the set time of concrete giving it a high early strength development. It is not an antifreeze, but by using it duriag cold weather protection can proceed in a timely manner (31—34). In Russia, calcium chloride forms a component of several antifreeze admixtures (33). Reviews of the concerns and possible remedies of calcium chloride corrosion problems in concrete are available (21,35). There is no consensus on what the safe levels of calcium chloride in concrete are. 

Fegulated-set cement, called jet cement in Japan, is formulated to yield a controlled short setting time, <1 h, and very eady strength (73). It is a modified cement that can be manufactured in a conventional Pordand cement kiln. It incorporates set control and eady strength development components. 

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

Impression plasters are formulated to produce a thin, fluid slurry when mixed with the proper amount of water. A satisfactory impression plaster should have a setting time of 4 1.5 min fineness, ie, 98% should pass a number 100 sieve (ca 0.15 mm), and 90% pass a number 200 sieve (ca 0.07 mm) setting expansion at 2 h should be <0.15% the compressive strength at one hour should be 5.9 2 MPa (855.5 290 psi) and testing consistency as determined by the diameter of the slump in the consistence test should be 90 3 mm. 

Impression plasters are manufactured from the finest finishing plasters, selected for color and purity. Setting time accelerators, setting expansion control agents, fillers, flavors, colors, or other special modifying agents may be added, eg, starch, to cause disintegration of the plaster impression when it is boiled. 

Model Plasters. Model plaster should have a setting time of approximately 10 minutes. The fineness of the powder should be such that 98% passes a number 100 sieve (ca 0.15 mm) and 90% passes a number 200 sieve (ca 0.07 mm). Setting expansion should be less than 0.30%, compressive strength at the end of one h should be a minimum of 8.8 MPa (1276 psi), and the consistency should form a disk during the slump test of 30 2 mm diameter. 

Model plasters are manufactured from select finishing plasters with special emphasis on a clean, white color. Setting-time control agents, setting-expansion control additives, fillers, and pigments may be added. 

Mixes of improved dental stone (type IV) using 22 parts of water to 100 parts of powder produce a mass that is not fluid and pourable but can be easily vibrated into place. The physical properties of the improved dental stone include a setting time 10 3 min, fineness of powder such that 98% passes a 100 sieve (ca 0.15 mm) and 90% passes a 200 sieve (ca 0.07 mm) setting expansion at 2 h limited to a max of 0.10% compressive strength at 1 h of at least 34.3 MPa (4974 psi) and a disk formed in the slump test for consistency of a 30 2 mm diameter. 

The polysulfide impression materials can be formulated to have a wide range of physical and chemical characteristics by modifying the base (polysulfide portion), and/or the initiator system. Further changes may be obtained by varying the proportion of the base to the catalyst in the final mix. Characteristics varied by these mechanisms include viscosity control from thin fluid mixes to heavy thixotropic mixes, setting-time control, and control of the set-mbber hardness from a Shore A Durometer scale of 20 to 60. Variations in strength, toughness, and elasticity can also be achieved. 

The accumulator holdup will often be the limiting item setting time required for samples to be representative of the column operating conditions. One changeout or turnover of the accumulator may be insufficient time, if test conditions are much different from conditions prior to the test. The following example will serve to show this. 

PVAc is another important type of adhesive, especially in furniture manufacturing and for carpentry. They form the bond line in a physical process by losing their water content to the two wooden adherends. PVAc adhesives are ready to use, have short setting time and give flexible and invisible joints. They are easy to clean and show long storage life. Limitations are their thermoplasticity and the creep behavior. 

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