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DryAdd for Epoxy Resins

The following examples for epoxy resins illustrate the range which DryAdd can tackle. With its ability to create one or two new reactive product sites from each ring-opening reaction, its capacity to mimic substitution effects on reacted monomers, and its full range of reports, plots and analysis functions, DryAdd can give you new insights into your own problems.

DryAdd has been validated against detailed experimental work and Miller-Macosko network models. For epoxy systems, cure by amines (primary and tertiary) , by radical initiation and by attack by Lewis acids and bases can be modelled. Some illustrative examples are given below.

Throughout this page, if you click on the pictures, you can expand them to get a better look.

A textbook example is the well-studied reaction of DGEBA with DDS. The epoxy group is ring-opened by a primary amine, forming a secondary amine and a hydroxyl group during the reaction. DryAdd includes a reaction type which lets both product sites be set up in the initial reaction, ready for further reaction.

The secondary amine opens an epoxy group with a rate which is about 40% as fast as the reaction with the primary group. DryAdd can account for these relative rates. You have the option of including a substitution effect as well, but this has not been done here.

Depending on conditions, the hydroxyl groups formed in the ring-opening can go on to react with more rings, causing them to open too. The relative rate of the hydroxyl group is often taken to be about 10% of that for the primary amine.

The point at which the system gels (i.e. crosslink density starts to rise rapidly) is crucially dependent on the amount of DDS amine to DGEBA epoxy. With too much of the DDS hardener (2 moles of DDS to 1 mole of epoxide) then gelation does not occur. Ultimate crosslink density (and mechanical strength) depends on relative stoichoimetries.

Tertiary amines act as catalysts for the epoxide ring-opening. The tertiary amine attacks the ring, leaving an O^--like species. This goes on to attack more rings, giving a highly crosslinked system with many ether linkages.

A typical example is the reaction of diethyl amino propylene (DEAP) with the difunctional epoxy DGEBA. DEAP is set up with one group, which we called initiator. A reaction is set up, in which initiator attacks the epoxy group. There are two epoxy groups on DGEBA, and we also set up a phantom group Oxygen Ion which is formed in the reaction. In subsequent reactions, the Oxygen Ion group can react with more epoxide groups, forming new Oxygen Ions.

The degree of crosslinking, and the gel point, depend on how much tertiary amine is present at the outset. If there is a lot, gelation is delayed.

The report in the screenshot above shows a fragment from the Network Analysis, which looks at the gel, the elastically effective component and the pendant fraction. Note that this Network Analysis is available in both DryAdd-Pro and DryAdd-Pro+.

This follows a similar reaction scheme to that for the tertiary amines. The assumption is made that a percentage of the epoxy (e.g. DGEBA) exists in a free radical form, following reaction with the free radical initiator.

Any or all of the above reaction schemes can be set up simultaneously. For example, we have set up a system containing DGEBA which is cured by a tertiary amine initiator, and which contains the primary amine DDS as well. The reaction of the initiator produces an anion site (which is like an O- ion) on DGEBA; this can go on to react with further DGEBA to give more anion sites, in a catalytic reaction. Ether links are produced.

At the same time, the primary amine on DDS can react with DGEBA, to give a secondary amine and a hydroxyl group. Both of these are capable of further reaction to ring-open more epoxide groups.

In this way, complex dual cure systems can be modeled realistically. And for this problem, DryAdd's analysis capabilities let you assess the relative occurrence of ether and amine linkages.

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