Curing period

The reinforcing capacity of asbestos fibers in a cement matrix constitutes another key criteria for the evaluation of asbestos fibers. This property is assessed by preparing samples of asbestos —cement composites which, after a standard curing period, are tested for flexural resistance. The measured mpture modub are converted into a parameter referred to as the fiber strength unit (FSU) (34). 

Unsaturated polyesters are invariably susceptible to air inhibition and surfaces may remain undercured, soft and in some cases tacky if freely exposed to air during the curing period. The degree of surface undercure varies to some extent with the... 

The pieces of cloth are then plied up and moulded at about 170°C for 30-60 minutes. Whilst flat sheets are moulded in a press at about lOOOlbf/in (7 MPa) pressure, complex shapes may be moulded by rubber bag or similar techniques at much lower pressures ( 15 Ibf/in ) (0.1 MPa) if the correct choice of resin is made. A number of curing catalysts have been used, including triethanolamine, zinc octoate and dibutyl tin diacetate. The laminates are then given a further prolonged curing period in order to develop the most desirable properties. 

MUF resin is widely used as an adhesive in wood industries, coating technology, paper industries and a main material in kitchenware production. In various applications, different resin properties are needed to suit its application. Important resin properties are for example higher resin solubility, low curing period with lower temperature and catalyst amount, good stability for longer shelf life, and lower free formaldehyde emission, as formaldehyde is very toxic, and can cause cancer [1]. One of the factors that affecting the MUF resin properties is the mole composition. The mole composition is a ratio of formaldehyde to amino compoimd... 

The MUF resin pH was determined using pH meter model pH 340-A/SET l-MTM. The pH meter was calibrated before it was used to determine the pH of the resin. The viscosity was determined using the Cole-Parmer 98936-15 viscometer (R2 spindle, lOOrpm speed). The storage life was a test of shelf life of the MUF resin under the ambient environment. Resin was first stored at ambient room temperature. Viscosity of the resin was checked for every three to four days. The ratio of water that can be added into resin before it turned turbid or precipitated is called resin solubility. The resin solubility was determined by divide the weight of resin and the weight of water added into resin before it turned turbid or precipitated. The curing period of a resin was defined as the time period for the resin to be hardened after application in a 30°C and 1.0% of NH4CI powder (as hardener). 

Fig. 3. A 3-D surface plot for MUF resin curing period Fig. 4. A 3-D surface plot for MUF resin storage life...

The specimen was then cured at 100% relative humidity for 28 days and allowed to dry from one exposed face for 28 days at 38 °C and 40% relative humidity. At the end of the moist curing period, and after the drying period, ID MRI measurements were performed on the specimen. Figure 3.4.8 is the ID MRI profile of evaporable water content in the concrete specimen. The profiles show that significant amounts of water can be removed with extended drying periods. 

Quartz fine aggregates and normal Portland cement were used to prepare mortar at a w/c of 0.6. Mortar was cast into 30-mm diameter by 30-mm length cylinders and cured for 3 days. At the end of the curing period the specimens were oven dried at 105 °C until a constant mass was reached. Epoxy was then applied to the curved surface and the specimen was placed such that 1-2 mm of its height was immersed in a 20% NaCl solution. 

Some materials require a controlled curing period to develop full strength and prevent serious cracking in the future. 

Control of crosslinking is critical for processing thermoset plastics, both the reaction prior to the gel point and that subsequent to the gel point. The period after the gel point is usually referred to as the curing period. Too slow or too rapid crosslinking can be detrimental to the properties of a desired product. Thus, in the production of a thermoset foamed product, the foam structure may collapse if gelation occurs too slowly. On the other hand, for reinforced and laminated products the bond strength of the components may be low if crosslinking occurs too quickly. 

Depending on the extent of expansion achieved in the wet curing period and the rate of shrinkage soon after water curing ceases, significant contraction can occur (Fig. 6.13). 

Both surfaces to be cemented should be clean and fit accurately to one another. A minimum continuous layer of cement should be applied to each surface to be joined. Where catalytic resins are used, provision should be made to hold the members in firm contact for the curing period. When contact cements are used, mechanical means are desirable to assure that the elements are in the proper position and orientation when they make the first contact... 

For the sample cured in air for 20 h ( ), an additional stabilization is obtained. This stabilization is attributed to oligomerization of the silane molecules on the silica surface. Oligomerization occurs under the influence of air humidity. From the difference in position of the curves for 3 h (+) and 20 h ( ) cured sample, it can be concluded that there is a progressive oligomerization during the air curing period. 

A sample with a w/c ratio of 0.4 by weight (which is at the threshold water content required for complete hydration of OPQ was cured for 28 days in a dry desiccator with a relative humidity of <5%. All surfaces of the sample except the top face were sealed with Parafilm, to promote evaporation of water through this surface only. This was done to disrupt the hydration process in this part of the sample and hence introduce some inhomogeneity into the cured sample. Following the cure period, the Parafilm was removed and the sample was dried in an oven for 48 h at 105°C in order to remove all evaporable water. Water penetration studies were then carried out as described below. 

There has been, in fact, some movement beyond AIDS, though it mostly involves providing free or discounted medication and treatment, rather than researching new cures. Periodically, Merck and other pharmaceutical makers... 

Owing to their long curing period, one-component polysulphide sealants are not generally used in joints with dynamic movements. When a joint opens and closes under stress and under variations in temperature, a slow-curing sealant may experience cohesive failure as well as deformation. 

Residual products wj]] be presented to the target species over a range of time. The initially laid down deposit will be wet . This is followed by a curing period during which the product may become gradually more firmly attached to the substrate and therefore less easily picked up. Poor experimental formulations can result in extremely low control. 

Experimental. Films of B-staged BCB were coated on salt substrates by spray deposition and cured in air at 250°C for one hour. This cure period was determined from the minimum time necessary for the elimination of the reaction exotherm as measured by DSC. After the cure cycle the films were removed by dissolving the substrate in a water bath. The freely standing films so obtained were between 25 mm and 50 mm thick. 

Fig. 15.2.2. Solid state CP/MAS NMR spectra of a normal PF resin after various cure periods. SB = spinning side band.

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