Polymerization of gaseous formaldehyde

Very little work has been done on the polymerization of liquid, neat formaldehyde, although it had long been known that it is almost impossible to keep liquid formaldehyde from polymerizing. It was early suspected that bases especially amines and relatively weak acids were active initiators for formaldehyde polymerization. Water, which is the most important impurity in any formaldehyde system, was an active initiator, later shown to be an anionic initiator which caused initiation by the HO ion. 

A simpler system than polymerization of formaldehyde in the liquid state is the polymerization of formaldehyde from the gaseous state. This was investigated in three distinct periods the early period [44—46] was followed by the elegant work of Norrish et al. [13, 47]. Recently some additional work on the kinetics of gaseous formaldehyde was published which adds but little extra knowledge to that from Norrish s work. 

The early work on polymerization from the gas phase was done on what was then thought to be pure formaldehyde. Solid polymer formed on the cold surfaces but the data were very irreproducible. Unknown kinds and unknown amounts of impurities in the monomer and on the glass surfaces made polymerization results erratic. The polymerization of formaldehyde from the gas phase has one advantage over polymerization in solution for kinetic studies. The rate of monomer disappearance can be followed readily manometrically in addition, additives can be added simply and very accurately to gaseous formaldehyde. 

When a few percent of formic acid was added to gaseous formaldehyde at about 500 mm pressure a rapid polymerization was observed, the velocity was some hundredfold greater than with pure formaldehyde. It appeared that formic acid was a powerful initiator of formaldehyde polymerization under these conditions. The polymerization was confined to the surface of the vessel and the kinetics were those of a heterogeneous system. Because of the much faster formaldehyde polymerization promoted by formic acid the purity of the formaldehyde became less important. The erratic results of earlier investigators were best explained by varying degrees of purity of earlier preparations of formaldehyde monomer. 

Carruthers and Norrish [13] found that no polymerization occurred if the whole system was held above 100°C, even at 30 torr formic acid pressure. If one portion of the apparatus was cold, polymerization occurred in this portion. If the cold portion was cooled throughout the polymerization, this went to completion if the cooling was insufficient and the surface was exposed to the bombardment of hot gases depolymerization occurred and an equilibrium between polymer and monomer was established. 

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An apparatus was designed which allowed the experimenters to prepare mixtures of formaldehyde and various gaseous initiators without any undesired polymerization. The whole apparatus was heated to 100°C or slightly higher. The actual polymerization portion of the apparatus was kept at 20°C, the commonly used temperature at which the polymerization of gaseous formaldehyde was studied. 

Fig. 17. Effect of hydrogen chloride upon the polymerization of gaseous formaldehyde.

Toby et al. studied the polymerization of gaseous formaldehyde, using formaldehyde containing a known level of impurities and in apparatus made from virgin quartz which had been flamed out and cooled in vacuum. No grease on connectors other than glass seals were used in this apparatus. When the apparatus was rinsed with detergent, the results became erratic and unreliable. Emphasis in Toby s work was on the surface to volume ratio (S/V) of the reaction vessel, which was 1.0, 2.6 and 5.0 cm . Basically two types of surface of the reaction vessel were used one the flamed out surface, the other the surface of already deposited polymer. In other words the polymer deposit was not removed between runs. 

Although it is generally accepted that AH and AS do not substantially depend on temperature, Berlin et al. calculated this dependence for polymerization of gaseous formaldehyde to crystalline polymer. Such a calculation could seldom be found in the polymer monographs and textbooks. 

Cflof/on Because of the formation of gaseous formaldehyde the polymerizations have to be carried out in a closed hood. 

In addition to the initiation of gaseous formaldehyde with formic acid, HCl, boron trifluoride and stannic chloride were studied and found to be more active than formic acid (Fig. 17). The rate of gaseous formaldehyde polymerization with the initiator was measured under the same conditions as the formic acid initiated polymerization by determining the decrease in pressure of formaldehyde. HCl as initiator (at about 3—4% in the mixture) caused kinetic branching, i.e. a rapid increase in the rate of polymerization. 

The polymerization is a precipitation polymerization from gaseous formaldehyde in an inert solvent such as cyclohexane at fairly low temperatures to prevent depolymerization reactions. Amines such as tri-n-butyl amine are used as initiators. The polymer precipitates in the dispersing agent as powder. After completion of the polymerization, it is necessary to stabilize the hydroxyl end groups, for example by esterification with acetic anhydride to prevent the unzipping reaction of Eq. (49). 

Like formaldehyde, acetaldehyde easily forms polymers, in this case paraldehyde and metaldehyde. Paraldehyde will form when hydrochloric or sulfuric acid is added to acetaldehyde. Polymerization of acetaldehyde to mec-aldehyde occurs in the gaseous phase in the presence of aluminum oxide or silicon dioxide catalyst. 

Both liquid and gaseous formaldehyde polymerize readily at low temperatures and can be kept in the pure monomeric state only for a limited time. Because of these facts, formaldehyde is sold and transferred either in solution or in polymerized form, such as paraformaldehyde and trioxane, described here under Formaldehyde polymers Commercial, 37% solution of formaldehyde (So-called Methanol-free)... 

A high pressure stream of argon was needed to prevent the gaseous formaldehyde from polymerizing to paraformaldehyde in the transfer flex-needle. 

Formaldehyde polymerization has been studied in the liquid state, in solution of protic or aprotic solvents and in the gaseous state where gaseous formaldehyde forms directly crystalline polymer. It has been studied with anionic and cationic initiators and by high energy radiations. Although there are more than 100 MM lbs. of poly form aldehyde produced per year, very few papers have been published that are actually concerned with the kinetics of formaldehyde polymerizations. The reason for this lack of detail is understandable when one realizes how difficult it is to obtain pure formaldehyde (with impurities of less than 100 p.p.m). Even pure formaldehyde undergoes side reactions and self condensation which cause new introduction of impurities. 

Early kinetic studies with gaseous formaldehyde [13] showed that the polymerization may be initiated by relatively small amounts of formic acid and can become almost explosive at higher formic acid concentrations. The rate of polymerization was found to be greater at lower temperatures and at 100°C, even with high HCOOH concentrations, no polymerization was observed, indicating an early observance of the now well established ceiling temperature phenomena. 

Fig. 16. Effect of formic acid upon the polymerization of 500 torr gaseous formaldehyde (plotted from result of Carruthers and Norrish [13]).

The actual mechanism of these gaseous formaldehyde polymerizations with HCl and formic acid as initiators is not very well understood because the work was analysed only from the kinetic point of view. It is not... 

Formaldehyde polymerization data have also been obtained by Sauterey [48] and in the early work of Toby and Rutz [49]. More recently, Boyles and Toby [50, 51] reinvestigated the kinetics of noncatalysed gaseous formaldehyde polymerization on the basis of careful attention to the purity of the monomer formaldehyde, a study of the surface to volume ratio of the vessel and the purity of the surface on which the polymer was deposited. 

Carbonaceous materials (CMs) are sometimes also named polymeric carbons. They are mostly prepared by thermal decomposition of organic precursors. One strategy is pyrolysis of gaseous or vaporized hydrocarbons at the surface of heated substrates, a second is heating (pyrolysis) of natural or synthetic polymers, both in an inert atmosphere. The latter is of special interest, and according to Miyabayashi et al. [374], precursors such as condensed polycyclic hydrocarbons, polymeric heterocyclic compounds, phenol-formaldehyde resins, polyacrylonitrile or polyphenylene are heated to 300-3000 °C for 0.15-20 h. Sometimes, a temperature/time profile is run. The temperature range must be divided into two domains, namely... 

Even though formaldehyde release from UF-bonded wood products has been studied for more than 25 years, only very little Is known about how formaldehyde Is stored In UF-bonded wood products. In fact. It is not even known whether storage of formaldehyde is a physical or a chemical process. Formaldehyde is gaseous at room temperature, but it can polymerize forming para-formaldehyde, and it readily dissolves In water forming methyIenegIycoI (2). The most likely physical storage process is absorption by moisture. Water is present in wood in two forms free water In the cell cavities In form of liquid... 

Formaldehyde has been used as a sterilizing agent for a long time. Although this method is relatively inexpensive, however, it has a number of drawbacks which also apply to EO sterilization. In addition, it is difficult to generate and distribute formaldehyde gas and there is a potential for the polymerization of the gaseous monomer. For most practical purposes, formaldehyde is also a surface sterilant as discussed above for EO. 

The preferred method of generating formaldehyde gas is by heating powdered or flake paraformaldehyde. Paraformaldehyde will de-polymerize and convert to the gaseous state when heated to a temperature above 150°C. There are various practical methods for heating the paraformaldehyde to above 150°C, but a household electric frying pan equipped with a thermostat is one of the simplest (451). The electric cord of the flying pan can be equipped with a one-hour timer so that the pan can be placed in the space to be treated and, after the... 

Various polymeric materials were tested statically with both gaseous and liquefied mixtures of fluorine and oxygen containing from 50 to 100% of the former. The materials which burned or reacted violently were phenol-formaldehyde resins (Bakelite) polyacrylonitrile-butadiene (Buna N) polyamides (Nylon) polychloroprene (Neoprene) polyethylene polytriflu-oropropylmethylsiloxane (LS63) polyvinyl chloride-vinyl acetate (Tygan) polyvinylidene fluoride-hexafluoropropylene (Viton) polyurethane foam. Under dynamic conditions of flow and pressure, the more resistant materials which binned were chlorinated polyethylenes, polymethyl methacrylate (Perspex) polytetraflu-oroethylene (Teflon). 

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