Solutions, formaldehyde Polymer precipitation

Aqueous formaldehyde, known as formalin, is usually 37 wt % formaldehyde, though more concentrated solutions are available. Formalin is the general-purpose formaldehyde of commerce suppHed unstabiLized or methanol-stabilized. The latter may be stored at room temperature without precipitation of soHd formaldehyde polymers because it contains 5 —10% methyl alcohol. The uiiinhibited type must be maintained at a temperature of at least 32°C to prevent the separation of soHd formaldehyde polymers. Large quantities are often suppHed in more concentrated solutions. Formalin at 44,... 

The concentration of open-chain polymers, similar to what was observed in the homopolymerization, reached a steady state and could be ascertained easily. However, the concentration of CH20 increased rapidly as the polymerization progressed, reaching a maximum immediately before precipitation of the polymers. An attempt to measure the maximum formaldehyde concentration was made by integrating the spectra immediately before solidification. This was somewhat difficult since the polymer precipitates from solution rapidly. Therefore, quantitative estimates may not represent the maximum concentrations of CH20 but are a close approximation. 

Figure 11-10 shows the laboratory recipe for the process. We first form solutions of the separate polymers. Then the active material is dispersed as droplets of the desired size in one of the polymer solutions. The two solutions are mixed and the pH is adjusted to let the polymers precipitate. The polymer wets the active drops and forms a weak layer around them. After cooling we cross-link the polymer with an excess of formaldehyde to obtain capsules with a good strength. These need to be filtered, and the process liquid washed out. The capsules can then be dried. 

In solvents of high dielectric constant, (e.g. dimethylformamide), formaldehyde polymerized sluggishly and polymers were formed in low yield. In similar solvents, aliphatic aldehydes could not be polymerized. Precipitated aldehyde polymers have the tendency to absorb monomers. The monomer concentration near the propagating site may be much hi er than that in the surrounding solution. This occurs with n-butyraldehyde polymerizations in pentane [5] the polymer precipitated during the polymerization is highly swollen by the monomer. 

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. 

Other polycondensation reactions which lead to finely dispersed polymers in liquid polyethers are the polycondensation reactions of urea and melamine with aqueous formaldehyde [92-95]. The reaction medium is usually polyether polyols, PO homopolymers or PO-EO copolymers (random or block copolymers), with MW of 3000-5000 daltons. During the polycondensation reaction, the aminoplast polymer precipitates, being insoluble in polyether and water (water from formaldehyde solution and reaction water), is eliminated by vacuum distillation. A variant of this reaction is to develop the polycondensation in water, and water containing the aminoplast polymer (as a viscous solution) is added to a polyether polyol, under vacuum, and at high temperature (100-130 °C), water being continuously eliminated from the reaction medium. The aminoplast insoluble polymer precipitates in the form of fine particles. 

The reaction can continue to produce chains of polyoxymethylene glycol. To prevent precipitation of the formaldehyde polymer, known as paraformaldehyde, the solution can be stabilized by adding methyl alcohol. A 37% solution of formaldehyde must be maintained at a temperature of at least 32 C to prevent precipitation of the polymer. By adding 5-10% methanol, the solution will remain clear if stored at room temperature. 

Commercially, formaldehyde is manufactured and marketed chiefly in the form of an aqueous solution containing 37 per cent by weight dissolved formaldehyde (CH 0) plus sufficient methanol (8 to 15 per cent) to prevent precipitation of pohmer under ordinaiy conditions of transportation and storage. Solutions containing less than 1 per cent methanol are also knoA n to commerce, but can be employed only where conditions permit their transportation and sale in bulk, since they must be kept warm to prevent pohmierization. Thirty per cent formaldehyde solution containing less than 1 per cent methanol has a limited application. This solution does not precipitate polymer at ordinary temperature and does not require the presence of methanol or other stabilizers. ... 

Occasionally formaldehyde containing little or no methanol has been marketed at a concentration of 30 per cent, since a solution of this concentration does not have to be kept at an elevated temperature to prevent polymer precipitation. At 1S°C, this solution has a density of U0910 and a refractive index of 1.3676 . 

Solution Stabilizers, The action of methanol in preventing polymer precipitation in formaldehyde solutions is probably due to the formation of hemiacetals which exist in a state of chemical equilibrium wuth the hydrated... 

Appearance. Pure fonnaldehyde solutions are clear and colorless. However, according to Wiegand, they show a distinct fluorescence under idfawdolet radiation. Cloudiness or opalescence in formaldehyde is due to polymer precipitation and is discussed in detail under this heading (page 56).. ... 

When the equihbrium formaldehyde concentration is reached, polymer begins to precipitate. Further polymerisation takes place in trioxane solution and, more importantly, at the surface of precipitated polymer. 

Kamogawa, and Sekiya (54) studied the graft polymerization of acrylamide onto cotton fabric using ceric ammonium nitrate as the catalyst. Similarly to Kulkarni et al. (35) the authors performed subsequent cross-linking with formaldehyde amd methylol compounds. From precipitation studies by acidification of cuprammonium solutions on mixtures of polyacrylamide and cellulose on the one hand and polyacrylamide-cellulose grafts on the other the authors conclude that chemical bonds must exist between the two polymers in the grafted product. 

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. 

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