Polymerisation formaldehyde

Homopolymer. Formaldehyde polymerises by both anionic and cationic mechanisms. Strong acids are needed to initiate cationic polymerisation. Anionic polymerisation, which can be initiated by relatively weak bases (eg, pyridine), can be represented by the following equations Initiation... 

The amount of resorcinolic lipids in plant and microbial sources varies considerably depending on the source. The most prominent source of resorcinolic lipids (cardol and methylcardol), as well as of other phenolic lipids, is the oil obtained from extraction of the shell of cashew nuts, Anacardium occidentale, (up to 20% of resorcinolic lipids). The oily extract from the roasting of cashew nuts (Cashew Nut Shell Liquid) is one of the most important sources of these compounds for formaldehyde polymerisation in industry [1,2,11,114], Other plant sources contain amounts of resorcinolic lipids that vary from 0.01% to 0.1% with rye grains as the richest [101,115-120] while bacterial sources, depending on the family and strain contain up to 6% of various resorcinolic lipid derivatives [105]. 

The phenolic lipids of Anacardieum occidentale have been commercially exploited (ref. 174) and those in Rhus vernicifera to a lesser extent. Most of the technical cashew nut shell liquid (CNSL) which results from industrial processing is and has been employed as a phenolic source for formaldehyde polymerisation the products from which in compounded form have been the basis for friction dusts widely used throughout the world in vehicle brake and clutch linings (ref.175). Urushiol has had use over many centuries in the art of Japanese lacquering (ref. 176) and in more recent years has been sometimes supplemented with CNSL. Chemical uses are referred to later. 

Formaldehyde is a gas, b.p. — 21°, and cannot obviously be stored as such moreover, it polymerises readily in the liquid and the gaseous state. The commercial preparation, formalin, is an aqueous solution containing 35-40 per cent, of formaldehyde and some methyl alcohol. The preparation of a solution of formaldehyde may be demonstrated by the following experiment. 

Which giyes formaldehyde as one of the starting materials. Base-catalysed reactions with this yery reacfiye aldehyde often giye poor yields because of polymerisation and other side reactions. The Marmich reaction is used instead ... 

The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerisation so pervades the chemistry of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subde differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerisations of formaldehyde and trioxane are equiUbrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerisation (called unsipping) which begins at chain ends. 

Although there is a substantial body of information in the pubHc domain concerning the preparation of polyacetals, the details of processes for manufacturiag acetal resins are kept highly confidential by the companies that practice them. Nevertheless, enough information is available that reasonably accurate overviews can be surmised. Manufacture of both homopolymer and copolymer involves critical monomer purification operations, discussion of which is outside the scope of this article (see Formaldehyde). Homopolymer and copolymer are manufactured by substantially different processes for accomplishing substantially different polymerisation chemistries. 

In production, anhydrous formaldehyde is continuously fed to a reactor containing well-agitated inert solvent, especially a hydrocarbon, in which monomer is sparingly soluble. Initiator, especially amine, and chain-transfer agent are also fed to the reactor (5,16,17). The reaction is quite exothermic and polymerisation temperature is maintained below 75°C (typically near 40°C) by evaporation of the solvent. Polymer is not soluble in the solvent and precipitates early in the reaction. 

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. 

Fig. 6. Flow diagram of microencapsulation process that utilises acid-cataly2ed in situ polymerisation of melamine or urea with formaldehyde to form a...

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

In the above examples the polymerisation takes place by the opening of a carbon-carbon double bond. It is also possible to open carbonyl carbon-oxygen double bonds and nitrile carbon-nitrogen triple bonds. An example of the former is the polymerisation of formaldehyde to give polyformaldehyde (also known as polyoxymethylene and polyacetal) (Figure 2.3). 

Formaldehyde is a gas with a boiling point of -21 °C. It is usually supplied as a stabilised aqueous solution ( 40% formaldehyde) known as formalin. When formalin is used as the source of the aldehyde, impurities present generally include water, methanol, formic acid, methylal, methyl formate and carbon dioxide. The first three of these impurities interfere with polymerisation reactions and need to be removed as much as possible. In commercial polymerisation the low polymers trioxane and paraformaldehyde are convenient sources of formaldehyde since they can be obtained in a greater state of purity. 

Formaldehyde will polymerise in a number of ways, as indicated in Figure... 

The cyclic trimer (trioxane) and tetramer are obtained by a trace of sulphuric acid acting on hot formaldehyde vapour (i) Figure 19.1). Linear polymers with degrees of polymerisation of about 50 and a terminal hydroxyl group are obtained by evaporation of aqueous solutions of formaldehyde (ii). In the presence of strong acid the average chain length may be doubled. Evaporation of methanol solution leads to products of type (iii). 

In the early 1940s an intensive research programme on the polymerisation of formaldehyde was initiated by the Du Pont Company. As a consequence of this work polymers, both tough and adequately stable to processing conditions, were prepared and eventually marketed " (Delrin). 

Urea formaldehyde /alkyd blends Sloving Condensation polymerisation Aromatic hydrocarbons and alcohols Fairly good Fairly good Very good Good Fair Water white Gives white finishes of excellent colour... 

The initial step of the polymerisation process is reaction of the amine groups with formaldehyde to generate methylol units, as illustrated in Reaction 1.9. Further heating of the polymer then leads to a variety of reactions. For example, the methylol groups can undergo self-condensation (Reaction 1.10). 

It is marketed as a 35-40 per cent, solution in water (formalin). The rpactions of formaldehyde are partly typical of aldehydes and partly peculiar to itself. By evaporating an aqueous solution paraformaldehyde or paraform (CH O), an amorphous white solid is produced it is insoluble in most solvents. When formaldehyde is distilled from a 60 ptr cent, solution containing 2 per cent, of sulphuric acid, it polymerises to a crystalline trimeride, trioxane, which can be extracted with methylene chloride this is crystalline (m.p. 62°, b.p. 115°), readily soluble in water, alcohol and ether, and devoid of aldehydic properties ... 

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