Solutions, formaldehyde Viscosity

Naturally occurring polyphenollc compounds such as tannin extracts can be gelled with a source of formaldehyde such as paraformaldeyde and other additlves . At the levels of 5-15% concentration, these solutions showed viscosities in the range of 6-8 cP at 18000 sec-1 prior to In-situ setting. The gels have shown stability at 275 C (527 F) for at least several months. ... 

The polymer compatibility was evaluated in terms of viscosity reduction and filterability of dilute polymeric (500 ppm) solutions. The viscosity reduction curves are plotted in Figure 24 for different concentrations (10, 50, and 100 ppm) of formaldehyde. The curves indicate that viscosity is reasonably insensitive to con-... 

TABLE 9 - EFFECT OF FERROUS IRON SALTS ON PAA SOLUTIONS CONTAINING FORMALDEHYDE, VISCOSITY LOSS (%)... 

Viscosities have been measured for representative commercial formaldehyde solutions (21). Over the ranges of 30—50 wt % HCHO, 0—12 wt %... 

Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

The type of polymer obtained depends on factors such as the pH and temperature of reaction, the ratio of melamine to formaldehyde, and the type of catalyst employed. For decorative laminates, melamine-formaldehyde is prepared by reacting melamine in stainless steel kettles under reflux, alkaline conditions with 37% to 46% formaldehyde in aqueous solution. The reaction temperatures used vary from 80 to 100°C and are maintained until the condensation has reached the desired end point—that is, reacted sufficiently but still water-soluble. The end point is checked by measurements of viscosity, cure time, and water tolerance. Depending on the type of laminate to be produced, other constituents (surfactants, plasticizers, release and anti-foam agents) normally are added to the base resin before impregnation of the surface papers. It is common practice also at this stage to adjust the pH by adding acid catalysts. 

The last decade has seen quite remarkable advances in our knowledge of the structure and properties of the proanthocyanidins. Viscosity measurements were made of solutions of procyanidins isolated from Theobroma cacao and Chaenomeles speciosa with number-average degrees of polymerization of 6.1 and 11.8, respectively, in water and 1% sodium hydroxide at 25 °C. Procyanidins are apparently completely crosslinked by formaldehyde up to a chain length of 6 units, but few units are crosslinked in polymeric procyanidins. The second order rate constants observed for the formaldehyde reaction with catechin or epicatechin are approximately six times higher than that observed for the C. speciosa polymer. 

The current study seeks to extend our knowledge of the behavior of procyanidins in two areas important to their industrial utilization 1) the viscosity of procyanidin polymers in aqueous solutions, and 2) the stoichiometry and rate of reaction of procyanidins with formaldehyde. 

Tannins. Herb Hergert s introduction (Chapter 12) to the use of condensed tannins in adhesives is especially interesting because he provides some reasons why commercial success is lacking in the use of condensed tannins from conifer barks despite substantial effort worldwide to parallel the South African success in the use of wattle tannins. Much of the problem in the use of conifer bark tannins remains centered on our inadequate understanding of the fundamental chemistry of these polymers. For example, Lawrence Porter (Chapter 13) provided the first measurements of the viscosities of solutions of purified condensed tannin isolates of known molecular weight and the reactions of these polymers with formaldehyde. It is incredible that this has not been done previously considering the hundreds of papers that have been published on tannin use in wood adhesives. Further evidence for the comparatively limited knowledge... 

This is a point to consider process improvements. One problem is that the biopolymers we can buy are not of constant quality. Would it be possible to get other materials that are more reliable (This would mean a major redo of the work in the lab.) At this point you should also obtain a good idea of the prices of the ingredients and the energy used in the process. These may force you to go back to the lab to change the process. A fairly obvious improvement is to reduce the amount of water. This will reduce the size of equipment, the energy requirement and the amount of waste. However, you may be limited by a low solubility of the polymers, or by too high a viscosity of the solutions produced. The stream of waste water is a nuisance, especially because it contains formaldehyde. Could we separate the formaldehyde and recycle it This might be possible with a membrane or with distillation. 

TEXAPRET S is the approx. 55% solution of a urea-formaldehyde compound. It has medium viscosity and a neutral reaction. 

Fig. 12. Depemlence of viscosity, loss angle tangent and optical density on time of curing of a melamine-formaldehyde resin in solution and in bulk [81]. T = 80°. Concentration of fte solution 40% (1) 50% (2) 57% (3)...

Experimental verification of Eq. (12) was carried out for a number of quite different systems curing both in a solution and bulk such as epoxy, epoxy silicone and silicone oligomers, melamine-formaldehyde and carbamide resins, derivatives of furan resins [11,28,32,63, 81]. Typical results plotted according to Eq. (12) are given in Fig. 18. After phase segregation (t > tp), the experimental dependences fl(t) are completely described by formula (12). The viscosity variation of the dispersion medium is a function of molecular mass andis calculated on the basis of Eq. (12). 

Phase segregation affects the dependence of the viscosity growth on the applied stress of shear rate [88], It is possible to see Such an effect is seen experimentally in a sufficiently extended part of flow ability after a microphase segregation i.e. the ratio ijt is relatively small, for example, in curing melamine-formaldehyde resins in solution [87], However, the case of tp/t close to unity, this effect may not always be revealed due to experimental difficulties. 

Reaction Variables. An excess of paraformaldehyde was necessary (15 to 35 mole % excess was used) when the prepolymers were built up in solution. An excess evidently was not necessary when the prepolymers were built up in the solid phase. In experiments with tetramethylcyclobutanediol, a polymer with an inherent viscosity of 1.2 was obtained when the diol was present in 5 mole % excess, but the polymer was brown. When the paraformaldehyde was in excess, the amount was not critical, but it was necessary to use a minimum of about 10 mole % excess in order to obtain white polymers. Polyformals with inherent viscosities of 0.5 or greater were obtained when the molar excess of paraformaldehyde ranged from 2 to 40%. The highest inherent viscosities (above 1.0) were obtained when the excess was about 10 mole %. When trioxane was used as the formaldehyde source instead of paraformaldehyde, reaction did not take place (no water was obtained). 

Alternatives to alkaline solutions of carbon disulfide are combinations of formaldehyde and DMSO various alkyl silanes, and urea and alkali. The latter is currently being promoted for industrial applications. It is known under the term carbamate process. The carbamate process has advantages over the viscose rayon process while being able to use the same process technology. 

Polystyrene Although polystyrene is usually bonded by solvent cementing, it can be bonded with vinyl acetate/vinyl chloride solution adhesives, acrylics, polyurethanes, unsaturated polyesters, epoxies, urea-formaldehyde, rubber-base adhesives, polyamide (Versamid-base), polymethylmethacrylate, and cyanoacrylates. The adhesives should be medium-to-heavy viscosity and room-temperature and contact-pressure curing. An excellent source is a Monsanto Company technical information bulletin which recommends particular commercial adhesives for bonding polystyrene to a number of different surfaces. Adhesives are recommended in the fast-, medium-, and slow-setting ranges (10). 

With all these possible additives the end product is quite different from the original phenol-formaldehyde concept. While Geoseal does succeed in providing gel time control without dilution of the phenol (see Fig. 11.41) and can be used at short gel times (as low as 10 s, according to the manufacturer s literature) the viscosity of the initial solution is dependent on concentration, varying from 2 to 10 cP for field use (resorcinol-formaldehyde solution viscosities are relatively independent of solids content) the viscosity increases continuously from catalyzation to gel and the final gel is considerably weaker than the resorcinol-formaldehyde formation. Figure 11.42 shows the relationship between strength and concentration. 

Urea solutions have very low viseosities, similar to the acrylics and phenoplasts. The reaction with formaldehyde, in addition to requiring elevated temperatures, is rapid and difficult to control. However, there are intermediate stages in the reaction when the urea is still soluble in water. Such materials, called precondensates or prepolymers, are readily available from industry, since urea formaldehydes are used in large quantities as adhesives. Of course, prepolymers are more viscous than the initial urea solution, but products are made which permit the final grout to be used at viscosities in the 10 to 20 cP range. The trade-off in viscosity is made to obtain a product easy to handle, with good gel time control. 

The viscosity loss from the static mixer to the injection wellhead was mainly caused by chemical degradation due to It was found that concentrations at the static mixer, injection well, and in the solution returned from the injection well were 0.3, 0.6, and 10 mg/L. Experimental data showed that the viscosity loss reached 77% when the solution had 2 mg/L Fe. If 100,400, or 800 mg/L formaldehyde was added, the viscosity loss was 67%, 56%, or 36%, respectively (Pang et al., 1998a). 

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