Formaldehyde equilibrium concentration

The difference in formaldehyde equilibrium concentration between homogeneous and heterogeneous polymerization is large enough to indicate a difference in the physical state of cationic chain ends in the dissolved and in the crystalline polymer. Thus, Model B is ruled out. In the homopolymerization of trioxane and in the heterogeneous copolymerization with small amounts of dioxolane the active centers of chains which have precipitated from the solution predominantly are directly on the crystal surface (Model A). According to Wunderlich (20, 21), this is the first case in addition polymerization where Model A—simultaneous polymerization and crystallization—has been proved experimentally. 

The observed difference in formaldehyde equilibrium concentration between dissolved and crystalline copolymer chains may affect the copolymer composition in still another manner part of the formaldehyde,... 

One of the most prominent features in the heterogeneous copolymerization of trioxane is the occurrence of two different kinds of active centers—dissolved and crystalline copolymer cations. They have different copolymer reactivity ratios and different tendencies to depolymerize, i.e., different formaldehyde equilibrium concentrations. At first the formation of soluble copolymer with high dioxolane content did not raise much hope for obtaining a crystalline copolymer of good thermal stability from trioxane and dioxolane but the gradual depolymerization of the soluble copolymer proved to be a useful side reaction which greatly improved the situation. Eventually, the entire complicated process turned out to be quite favorable for the formation of a stable crystalline copolymer with the desired random distribution. 

A schematic representation of the solid—liquid relationship is given in Figure 21. The position of the formaldehyde equilibrium concentration... 

Zavitsas et al. account for the effects of water in their calculations. Water promotes depolymerization of the paraformaldehyde as well as the hemiformals. Their modifications correct for the apparent reduction in methylolation rate as the extent of reaction proceeds, in that the hemiformals remove formaldehyde reactivity from the reaction mixture. Their rate constants look large because they are written for phenate concentrations rather than phenol and because of the formaldehyde equilibrium adjustments. They note that unsalted phenol is a by-... 

The N-nitrosation reaction is usually very slow at neutral or alkaline pH due to the low equilibrium concentration of anhydrous nitrous acid. However, in the presence of formaldehyde or chloral as a catalyst (21), appreciable nitrosation occurs, even at pH 6 to 11. Similarly, Keefer (22) showed that some metal ions could catalyze the reaction under basic conditions. 

Trioxane polymerizations proceed with induction periods, which correspond to the buildup of the equilibrium concentration of formaldehyde [Lu et al., 1990]. This also corresponds to a buildup in 1,3,5,7-tetroxocane, apparently by insertion of formaldehyde into... 

Fahey presents the products of (17) as uncomplexed formaldehyde and HCo(CO)3 rather than a bound-formaldehyde species (43). Free formaldehyde is a thermodynamically unfavorable product from H2 and CO (8), and significant stabilization may be expected as the result of coordination in a metal complex. However, thermodynamic calculations are presented which indicate that small equilibrium concentrations of formaldehyde could be present under the conditions of these cobalt-catalyzed reactions (43). Although small amounts of uncoordinated formaldehyde are indeed expected as a result of the following endothermic (36, 37) equilibrium ... 

As discussed previously, thermodynamics indicate that free formaldehyde will not be a major product of this reaction, although a small equilibrium concentration may be formed.)... 

The experimental system is shown in Figure 4.9. The tested samples are placed in an airtight chamber whose volume is 301. By using a water bath, the chamber and air temperature can be maintained at the desired temperature. Tests were conducted at four air temperatures 18 0.5 °C, 30 0.8 °C, 40 0.8 °C and 50 0.6 °C, with air humidity uncontrolled but in the range 60 8%. After the system reaches thermal equilibrium, a dose of saturated formaldehyde vapor is injected into the chamber and thereafter the instantaneous concentrations of formaldehyde in the chamber are continuously recorded by an INNOVA-1312 until the equilibrium concentration C(y is reached at the equilibrium time, There are several reasons for applying a real-time photo-acoustic monitor to measure the instantaneous chamber compound concentrations for this research. First, its sampling volume is small and the air can be returned into the chamber... 

The copolymerization of trioxane with cyclic ethers or formals is accomplished with cationic initiators such as boron trifluoride dibutyl etherate. Polymerization by ring opening of the six-membered ring to form high molecular weight polymer does not commence immediately upon mixing monomer and initiator. Usually, an induction period is observed during which an equilibrium concentration of formaldehyde is produced. 

In the copolymerization of trioxane with dioxolane, reactivity ratios of dissolved copolymer cations are quite different from those of active centers in the crystalline phase. The former strongly prefer addition of dioxolane. The difference in reactivity ratios between dissolved and precipitated active centers is attributed to the fact that in the solid phase, polymerization and crystallization of the copolymer are simultaneous. The cationic chain ends are assumed to be directly on the crystal surface. Determination of the equilibrium concentrations of formaldehyde confirms this conclusion dissolved copolymer has a higher tendency to cleave formaldehyde than crystalline polyoxymethylene. In the latter stages of copolymerization the soluble copolymer is degraded gradually to the dioxolane monomer which is incorporated into the crystalline copolymer in an almost random distribution. 

Formaldehyde is cleaved from the cationic chain ends much faster than chains propagate by addition of trioxane (13). At the end of the induction period formaldehyde reaches its equilibrium concentration, and from then on the rate of formaldehyde addition at the active chain ends equals the rate of formaldehyde production. 

On the other hand copolymer with a trioxane unit at the cationic chain end (Pi+) may be converted intp P2+ by cleavage of several formaldehyde units. These side reactions change the nature of the active chain ends without participation of the actual monomers trioxane and dioxo-lane. Such reactions are not provided for in the kinetic scheme of Mayo and Lewis. In their conventional scheme, conversion of Pi+ to P2+ is assumed to take place exclusively by addition of monomer M2. Polymerization of trioxane with dioxolane actually is a ternary copolymerization after the induction period one of the three monomers—formaldehyde— is present in its equilibrium concentration. Being the most reactive monomer it still exerts a strong influence on the course of copolymerization (9). This makes it impossible to apply the conventional copolymerization equation and complicates the process considerably. 

On the other hand, no difference in equilibrium concentration is expected between Models C and B. In the latter only the "dead chain segments are crystallized while the cationic active centers, at which depolymerization and polymerization of formaldehyde takes place, are in solution. 

In the experimental investigations the soluble copolymer from trioxane and 25 mole % of dioxolane was used as Model C. This is permissible because dioxolane does not cleave off formaldehyde during polymerization. The experimental determination of equilibrium concentrations of formaldehyde during trioxane polymerization is somewhat problematic. Up to now the only available method is extraction of... 

In Zone A, at monomer concentrations below the equilibrium concentration solid polymer dissolves. This is equivalent to the dissolution of a crystalline low molecular wt. compound in a good solvent. In Zone C the solution is super-saturated, it is above the stability limit, spontaneous nucleation occurs and polymer precipitates from the clear solution. There is a Zone B, about 4—8% above the equilibrium concentration of formaldehyde, where super-saturation is insufficient to cause spontaneous precipitation of polyoxymethylene but where seeds or nuclei of polyoxy-m ethylene can grow when added to the clear solution and can increase in weight and molecular weight. This is the desirable range for the preparation of high molecular weight polyoxymethylene in hydroxylic media. 

In dynamic (ventilated) chambers, release rate coefficients were increased by a factor of 4.4 for particle board and 2.2 for plywood at loadings of 1.4-1.6 m /m over values at loadings of 9-11 m2/m3 (Table IV). Increased pressure of formaldehyde in the chamber was associated with reduced release of formaldehyde from wood products, as indicated by comparing equilibrium concentrations of formaldehyde (H). 

Reduced sample loadings in the dynamic chamber led to decreased formaldehyde concentrations in the chamber as noted or predicted previously by others (17, 20-22). This resulted in increased release rate coefficients (yg m 2 day"b. Samples analyzed at 1.4 and 1.6 m2 of product surface area/m of chamber volume chamber loadings had formaldehyde chamber concentrations of 28-32% of the calculated equilibrium air concentrations of formaldehyde (17), suggesting better relative ventilation than that at higher chamber loadings. 

These compounds form rapidly at room temperature with an equilibrium concentration depending on total and relative concentration of all reagents. The reaction is reversible and releases formaldehyde upon dilution. The resulting 13C-NMR shifts are shown in Figure 2 and are included in Table II. 

The curve shown in Figure 2 is a logarithmic function, which means that a straight line is obtained, if the logarithm of the driving force - this is the difference between the equilibrium concentration and the current formaldehyde concentration (Cg - Cg) - is plotted against time. 

Out of the results of the intersection should follow an equilibrium concentration of 0.35 mg/m, which is not in accordance with the determined equilibrium value. So this experimental set up is a case of a situation which is not well defined and therefore not suitable for measurement of the relevant formaldehyde release parameters of the particleboard. 

As soon as the formaldehyde concentration in the air becomes greater than the equilibrium concentration of one of the two boards, this board will start to absorb formaldehyde instead of emitting it. (For deduction of the mathematical equations, see appendix 2.)... 

The purpose of this study was to evaluate laboratory formaldehyde release test methods for predicting real-life formaldehyde air concentrations human exposure levels, and health risk. Three test methods were investigated the European perforator test, the gas analysis method at 60 C and 3% RH, and the gas analysis method at 23 C and 55% RH. Different types of particleboard bonded with urea-formaldehyde and urea-melamine-formaldehyde resins were tested. The results were used to rank boards as a function of test method, conditioning, short-term humidity, and temperature variations during storage. Additional experiments were conducted in small experimental houses at a Dutch research institute. Our conclusions are that relative ranking of products is influenced by the test method and by change in relative humidity. The relationship between test method and release in real-life situations is not clear. In fact, it seems impossible to use laboratory measurements to predict real-life product performance of board if the board is not fully in equilibrium with the atmosphere. 

It is important to distinguish between those emission tests that measure the emission in a closed, or unventilated, system and those that measure in a ventilated system. If a particleboard is kept in an unventilated system, the formaldehyde concentration will increase until it levels off at an equilibrium concentration which will depend on the formaldehyde content of the board under test, the temperature and the relative humidity. The particleboard loading, on the other hand, will not influence the equilibrium concentration, just the time it takes to reach it. The time to reach the equilibrium concentration is also influenced by the mass... 

In a ventilated system the exhaust air will remove some of the emitted formaldehyde, and a steady state concentration will be established. The steady state concentration will be lower than the equilibrium concentration. How much lower, will depend on the ventilation rate, the particleboard loading and the mass transfer coefficient. 

The Bell method can be used to determine the equilibrium concentration of formaldehyde, C in the model above. When the formaldehyde concentration in the Bell system is plotted against time, the initial slope of the resulting curve can be used to determine the mass transfer coefficient, kg in the same model. 

Affect the equilibrium concentration in an unventilated system, C. A coating containing a formaldehyde scavenger would act by binding formaldehyde, thus reducing the equilibrium concentration. On the other hand some surface finishes will introduce extra formaldehyde, and may thus increase C. ... 

Under neutral conditions and at 0 °C, indole reacts with a mixture of formaldehyde and dimethylamine by substitution at the indole nitrogen. This A-substitution may involve a low equilibrium concentration of the indolyl anion (20.4.1) or may be the result of reversible kinetic attack followed by loss of proton. In neutral solution at higher temperature or in acetic acid, conversion into the thermodynamically more stable... 

The thermodynamics of TXN polymerization also influences the polymerization kinetics. Kern 92) has proposed that induction periods, frequently observed in the cationic polymerization of TXN, are due to the build up of the equilibrium formaldehyde concentration, which has to occur before polymer can be formed. According to other authors, it is not formaldehyde but TTXN that must be formed prior to polymerization 93). We conclude that formation of polymer may require that the equilibrium concentration of any monomeric species which is in equilibrium with polymer is reached first. 

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