Resinification

On further heating the methylolmelamines condense and a point is reached where hydrophobic resin separates out on cooling. The resinification is strongly dependent on the pH and is at a minimum at about pH 10.0-10.5. An increase or decrease of pH from this value will result in a considerable increase in resinification rates. 

There is some evidence that the principal resinification reaction involves methylol-methylol condensations.  

Methylene links may also be formed by the following reactions 

In commercial practice the resin is condensed to a point close to the hydrophobe point and then either applied to the substrate, or converted into a moulding powder, before proceeding with the final cure. 

In a typical process a jacketed still fitted with a stirrer and reflux condenser in charged with 240 parts 37% w/w (40% w/v) formalin and the pH adjusted to 8.0-8.5 using sodium carbonate solution with the aid of a pH meter. One hundred and twenty six parts of melamine (to give a melamine formaldehyde ratio of 1 3) are charged into the still and the temperature raised to 85°C. The melamine goes into solution and forms methylol derivatives. For treatment of fabrics, paper and leather this product may be diluted and cooled for immediate use. It may also be spray dried to give a more stable product. Cooling the solution would yield crystalline trimethylolmelamine, which may be air dried but which is less soluble in water than the spray-dried produet. 

---

Nate 2. The reaction probably proceeds via the complex of CuCN and KCN, affording a mixture of propargyl cyanide and cyanoallene. The acetylene isomerizes under the influence of the slightly alkaline KCN. If the latter is added at too fast a rate, resinification of the allene may occur. Care should therefore be taken that some CuCN is always present. 

Note 2. If the addition is performed at too fast a rate, all of the copper cyanide may dissolve temporarily. The free KCN, present in the solution may cause partial resinification of the allenic nitrile. 

Several early interpretations of the polymerization mechanism have been proposed (1,17,29—31). Because of the complexity of this polymerization and insoluble character of the products, key intermediates have not ordinarily been isolated, nor have the products been characterized. Later work, however, on the resinification of furfural (32,33) has provided a new insight on the polymerization mechanism, particularly with respect to thermal reaction at 100—250°C in the absence of air. Based on the isolation and characterization of two intermediate products (9) and (10), stmcture (11) was proposed for the final resin. This work also explains the color produced during resinification, which always is a characteristic of the final polymer (33). The resinification chemistry is discussed in a recent review (5). 

Reaction of one mole of acetaldehyde and excess phenol in the presence of a mineral acid catalyst gives l,l-bis(p-hydroxyphenyl)ethane [2081-08-5], acid catalysts, acetaldehyde, and three moles or less of phenol yield soluble resins. Hardenable resins are difficult to produce by alkaline condensation of acetaldehyde and phenol because the acetaldehyde tends to undergo aldol condensation and self-resinification (see Phenolic resins). 

The second step is a condensation reaction that involves the linking together of monomer units with the Hberation of water to form a dimer, a polymer chain, or a vast network. This is usually referred to as methylene bridge formation, polymerization, resinification, or simply cure, and is illustrated in the following equation ... 

Modification of urea-formaldehyde resins with other reagents gives rise to a number of useful materials. For example, co-condensation of urea-formaldehyde and a monohydric alcohol in the presence of small quantities of an acidic catalyst will involve simultaneous etherification and resinification. n-Propanol, n-butanol and isobutanol are commonly used for this purpose. As an example n-butanol will react with the methylol urea as shown in Figure 24.4. 

Comparatively little is known of the chemistry of resinification of either furfuryl alcohol or furfural. 

Polymerization of raw feedstock. Aliphatic hydrocarbon resins. Raw feedstock contains straight-chain and cyclic molecules and mono- and diolefins. The most common initiator in the polymerization reaction is AICI3/HCI in xylene. The resinification consists of a two-stage polymerization in a reactor at 45°C and high pressure (10 MPa) for several hours. The resulting solution is treated with water and passed to distillation to obtain the aliphatic hydrocarbon resins. Several aliphatic hydrocarbon resins with different softening points can be adjusted. 

Kinetic observations of the homogeneous part of the reaction in water12,13 do not provide any substantially new element to the knowledge of this system. The obvious observations that the rate of resinification increases with increasing temperature and decreasing pH of the mixture only provide technically useful correlation parameters and the zero-order of reactions carried out to small conversion of 2-furfuryl alcohol13 does not indicate anything except an elementary kinetic approximation (the use of colour build-up as a criterion for the extent of alcohol consumed is also questionable since no firm relationship has ever been established between these two quantities). 

The resinification induced by 7-alumina11 seems to proceed by a somewhat different mechanism, probably because of the higher temperatures involved. Side reactions are more prominent from the beginning and it has been suggested, but not proved, that the C-3 and C-4 positions of the ring are vulnerable under these conditions to substitution reactions. A new compound, viz. 4-(2-furfuryl)-2-pentenoic acid-7-lactone, which is an isomer of the condensed dimers, was identified among... 

The resinification of 2-furaldehyde promoted by acidic substances or by heat has been known to chemists since the end of last century, and attempts to explain the mechanism leading to the formation of black, insoluble resins have been published... 

Two intermediate products were isolated in the study of the thermal resinification of 2-furaldehyde24,25 They were characterized by spectro opic and other standard techniques and their structures are given below ... 

The catalytic effect of various surfaces was also investigated and showed that electron-deficient sites were responsible for promoting the condensation. Basic substances and small amounts of water were found to considerably reduce the rate of resinification. 

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

Among the various agents which provoke the resinification of 2-furaldehyde, ultraviolet light has been the least studied. Some comments have already been made on... 

The gas-phase photolysis of 2-furaldehyde in the it -n and ir <-it transitions76 proceeds with fragmentation to CO, furan and C3-hydrocarbons, but a certain amount of resinification is also noted (about 5% quantum yield with excitation of the it - n transition). The latter observation prompted a study of the vacuum liquid-phase photolysis by sunlight or by light from a medium-pressure mercury arc at room temperature24 7S. The resin obtained was submitted to fractionation and structural analysis. On the basis of the results obtained and other mechanistic evidence, the following sequence of events was postulated for the photopolymerization ... 

This section deals with investigations specifically aimed at producing homopolymers and copolymers of furan carbonyl compounds by the selective opening of the carbonyl bond. The many reports on polymerization of 2-furaldehyde which in fact deal with complicated acid-catalysed resinification reactions which involve both the formyl group and the furan ring are reviewed in Chapter VI. 

Compounds which are rather unstable. Typical members of this class are 2-furaidehyde, 2-furfuryl alcohol and 2-alkyl furans, the latter being more resistant than the former. The action of acids or oxygen on these derivatives produces appreciable resinification, but, if properly purified and stored in vacuo, they are indefinitely stable25 16s. ... 

---