Acceleration mixture

Total carbon in beryUium is determined by combustion of the sample, along with an accelerator mixture of tin, iron, and copper, in a stream of oxygen (15,16). The evolved carbon dioxide is usuaUy measured by infrared absorption spectrometry. BeryUium carbide can be determined without interference from graphitic carbon by dissolution of the sample in a strong base. BeryUium carbide is converted to methane, which can be determined directly by gas chromatography. Alternatively, the evolved methane can be oxidized to carbon dioxide, which is determined gravimetricaUy (16). 

Frequently, pure liquid samples produce charts that are essentially a perfect match for each peak, both by retention and quantity. This allows a quick and certain identification. However, as individual samples of accelerants are "weathered" by exposure to fire and air the more volatile, low-boiling fractions are consumed or lost through evaporation. The compositions of some accelerant mixtures such as gasoline are rapidly altered. With passage of time., "weathered" accelerant mixtures become more difficult to compare with charts of standard mixtures. 

In addition, the mixing of two or more accelerants may produce a sample that is virtually impossible to identify. The chromatogram produced from such a sample is difficult to associate with a single original accelerant or with a known mixture of accelerants prepared by the forensic chemist since he is estimating the ratios of accelerant mixtures he suspects to be present in the questioned sample. 

In maintenance applications and other in-place coating work it is often desirable to have a formulation which cures at temperatures below the normal room temperature. For this reason it was of interest to investigate the cure rates of various epoxy resin-curing agent-diluent-accelerator mixtures at lower temperatures. 

The hardening of MUF resins can be enhanced by the addition of formaldehyde-based accelerator mixtures and monitored via rheology, gel time measurements, as well as the so-called ABES tests (98,99). Analyses indicate that cured MUF resins are mainly composed of separate MF and UF networks. Thus particleboards glued with an MUF/accelerator mixture exhibit improved mechanical properties compared to boards produced with commercially used MUF adhesives. The swelling properties of particleboards glued with an MUF/accelerator mixture are comparable to boards made from a commercial MUF resin (100). 

Figure 14.25 Plots of dg/df versus cure time for Aropol 7030/25 wt % CaCOg/BPO/accelerator mixture at various isothermal cure temperatures (°C) (O) 30, (A) 40, ( ) 45, (V) 50, (O) 55, and (O) 60. The accelerator employed was A/JV-dimethylaniline diluted in styrene. (Reprinted from Lem and Han, Journal of Applied Polymer Science 28 3185. Copyright 1983, with permission from John Wiley Sons.)... 

Figure 14.26 Plots of da/df versus a for Aropol 7030/25 wt % CaC03/ BPO/accelerator mixture at various isothermal cure temperatures C C) ...

Figure 14.28 Plots of log t] versus a at 60 °C for Aropol 7030/50 wt % CaC03/BPO/accelerator mixture at various shear rates (s ) (O) 0.0, (A) 0.11,...

It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

The developer is generally a solvent in which the components of the mixture are not too soluble and is usually a solvent of low molecular weight. The adsorbent is selected so that the solvent is adsorbed somewhat but not too strongly if the solvent is adsorbed to some extent, it helps to ensure that the components of the mixture to be adsorbed will not be too firmly bound. Usually an adsorbate adheres to any one adsorbent more firmly in a less polar solvent, consequently when, as frequently occurs, a single dense adsorption zone is obtained with light petroleum and develops only slowly when washed with this solvent, the development may be accelerated by passing to a more polar solvent. Numerous adsorbat are broken up by methyl alcohol, ethyl alcohol or acetone. It is not generally necessary to employ the pure alcohol the addition from 0 5 to 2 per cent, to the solvent actually used suffices in most cases. 

Semicarbazones. Dissolve 1 g. of semicarbazide hydrochloride and 1 5g. of crystallised sodium acetate in 8-10 ml. of water add 0 - 5-1 g. of the aldehyde or ketone and shake. If the mixture is turbid, add alcohol (acetone-free) or water until a clear solution is obtained shake the mixture for a few minutes and allow to stand. Usually the semicarbazone crystallises from the cold solution on standing, the time varying from a few minutes to several hours. The reaction may be accelerated,... 

In a 1 litre flask mix 53 -5 g. of p-toluidine and 400 ml. of water, and then add cautiously 98 g. (53 6 ml.) of concentrated sulphuric acid warm until the p-toluidine dissolves. Cool the flask in a bath of ice and salt to 0-5° add about 100 g. of crushed ice to the contents of the flask in order to accelerate the cooling. Add slowly and with frequent shaking a solution of 35 g. of sodium nitrite in 60 ml. of water until a slight excess of sodium nitrite is present (.see Section IV,60) keep the temperature of the mixture below 10°. 

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

Sulphuric acid catalysed nitration in concentrated nitric acid, but the effect was much weaker than that observed in nitration in organic solvents ( 3.2.3). The concentration of sulphuric acid required to double the rate of nitration of i-nitroanthraquinone was about 0-23 mol 1, whereas typically, a concentration of io mol 1 will effect the same change in nitration in mixtures of nitric acid and organic solvents. The acceleration in the rate was not linear in the concentration of catalyst, for the sensitivity to catalysis was small with low concentrations of sulphuric acid, but increased with the progressive addition of more catalyst and eventually approached a linear acceleration. 

A mixture of E and Z forms is obtained by the reaction of the fZ)-alkenyl bromide 33. Z to E isomerization takes place. However, the reaction is remarkably accelerated by using K2CO3 instead of KHC03[17] and BU4NCI in DMF, and the reaction of the (Z)-iodide 34 proceeds rapidly at room temperature without isomerization[37]. 

Carboxylic acids react with butadiene as alkali metal carboxylates. A mixture of isomeric 1- and 3-acetoxyoctadienes (39 and 40) is formed by the reaction of acetic acid[13]. The reaction is very slow in acetic acid alone. It is accelerated by forming acetate by the addition of a base[40]. Addition of an equal amount of triethylamine achieved complete conversion at 80 C after 2 h. AcONa or AcOK also can be used as a base. Trimethylolpropane phosphite (TMPP) completely eliminates the formation of 1,3,7-octatriene, and the acetoxyocta-dienes 39 and 40 are obtained in 81% and 9% yields by using N.N.N M -tetramethyl-l,3-diaminobutane at 50 in a 2 h reaction. These two isomers undergo Pd-catalyzed allylic rearrangement with each other. 

Copolymerization can be carried out with styrene, acetonitrile, vinyl chloride, methyl acrylate, vinylpyridines, 2-vinylfurans, and so forth. The addition of 2-substituted thiazoles to different dienes or mixtures of dienes with other vinyl compounds often increases the rate of polymeriza tion and improves the tensile strength and the rate of cure of the final polymers. This allows vulcanization at lower temperature, or with reduced amounts of accelerators and vulcanizing agents. 

The reaction of an aryl diazonium salt with potassium iodide is the standard method for the preparation of aryl iodides The diazonium salt is prepared from a primary aro matic amine m the usual way a solution of potassium iodide is then added and the reac tion mixture is brought to room temperature or heated to accelerate the reaction... 

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