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DryAdd for Amino-Formaldehyde Resins

The following examples for amino formaldehyde resins show some of the problems to which DryAdd can be applied. Both urea formaldehydes and melamine formaldehydes are discussed. To see the pictures more clearly, click on them to get bigger versions.

DryAdd uses Monte Carlo methods to model what happens in a real system during polymerization and crosslinking. Just set up the materials which are present, and the reactions between different functional groups. DryAdd enables you to see when a particular system will cure, and to predict trends for the ultimate mechanical strength which will be developed. A detailed examination of the chemical architecture of the system can be undertaken. DryAdd can be used for linear and crosslinked polymer systems. DryAdd has been validated by comparison with detailed experimental work and, for crosslinked systems, with Miller-Macosko network models.

This application note discusses the cases of urea and melamine formaldehydes. For these chemistries, the key reaction is between the amine group and formaldehyde, giving methylol sidegroups. Subsequent reaction and condensation of these methylol groups gives crosslinked thermosets with ether and methylene bridges Although separate examples are given for the urea and melamine cases, it would of course be possible to set up a system like Melurac ^TM in which melamine, urea and formaldehyde are mixed.

Urea-Formaldehyde Resins

In urea-formaldehydes, the initial reaction is usually carried out in aqueous, slightly alkaline, conditions. The urea reacts with formaldehyde to give methylol groups. Depending on the U:F ratio, mono, di or tri-substituted methylolamines are found; the tetramethylol species is not observed. The system is then acidified, and the methylol groups condense with unreacted amides to give methylene bridges. Ether bridges can also be found when methylol groups react with other methylols.

In the materials workscreen, urea is set up with two primary amine groups, and 'phantom' secondary amines which will be formed in the reaction. The secondary amine reacts less quickly than the primary amine; this can be accounted for in the relative reaction rates, as illustrated in the screenshot.

In order to prevent the formation of tetramethylol ureas (which are not favoured for steric reasons) we used the Block Reactions feature of DryAdd-Pro+ in the first stage of the reaction. This lets us stop all molecules that are already tri-substituted from further reaction.

In the second stage of the reaction, this block can be lifted, allowing crosslinking to take place. This can be done by saving the results from Stage 1 as a pre-polymer, then loading this into a new dataset for further reaction. If good kinetic parameters are available, the relative balance between ether and methylene bridges can be studied. Since these groups show different environmental resistance, getting the right balance can be crucial to get the correct end-use properties.

Melamine-formaldehyde resins

Melamine contains three NH₂ groups; each can react with two formaldehyde molecules to give two methylol groups, provided sufficient formaldehyde is available.

In practice, most commercial melamine-formaldehyde resins use a ratio M:F of 1:1.5 to 1:3, so that dimethylol (with some mono and some trimethyol) species are found. Synthesis occurs in aqueous, often slightly alkaline, conditions.

The methylol groups can condense, upon further heating, to form bridges. Alternatively, condensation can be initiated by acidifying the system, since the methylol groups become less stable in an acid environment. The condensations are of two types:

1. reaction of methylol groups with unreacted amines, to give methylene bridges. Water is lost.

2. condensation of two methylol groups to give an ether bridge, with loss of a water molecule.

The reaction schemes are shown in the screenshot above. Relative reaction rates have been assumed on the basis that the secondary amine will be half as reactive as the primary amine, for steric reasons.

As illustrated in the two screenshots, DryAdd shows that as the M:F ratio is increased, then gelation is delayed, and the thermoset is less tightly crosslinked. Conversion has been measured with respect to consumption of the formaldehyde.

A Network Analysis can be carried out, giving the average functionality of all the monomers which go into the gel (in the simulation, the largest molecule). For the case when M:F is 2:3, then there are mainly 2-functional network active junctions on the melamine - albeit with a significant smattering of 3,4 and 5-functional junctions. The average functionality of the branchpoints is 3.5.

When M:F is 1:3, then most of the junctions are 5-functional, with 4 and 6 functional junctions found in abundance as well. The average functionality of the branch points is 4.9. Mc, the average molecular weight between crosslinks, is small - indicating a fairly brittle material with a high Tg.

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