Cristiano Tunice – IMCD Italia
Epoxy resins stand out for their mechanical, dielectric, chemical, thermal resistance and adhesion to various substrates, especially metal surfaces. For these reasons, in the world of paints, they are well suited to the manufacture of flooring, composite materials and long-lasting anti-corrosive protective products.
On the other hand, they show a low resistance to light, which hampers their use as exterior finishes. It is known that resins are synthesized by crosslinking the Bisphenol A (or Bisphenol F) with epichlorohydrin. Working with a too large quantity of epichlorohydrin, n=0 is obtained, whereas reducing the epichlorohydrin/bisphenol A ratio causes an increase in the final molecular weight of the resin, up to a value of n>1, thus obtaining amorphous solids with high Tg values. Each epoxy resin is characterized by its epoxy equivalent (EEW), i.e. the quantity in grams of resin which contains one of this. For large n values, high epoxy equivalents are obtained, which involve such a limited presence of oxirane groups that the resins can be defined as simple polyalcohols, the so-called ‘phenoxy’.
Inserting Bisphenol F into the synthesis products with a lower final viscosity are obtained (the presence of two hydrogens replacing the methyl ones causes less steric hindrance) and a higher crosslinking speed, at the expense of greater yellowing. The presence of aromatic rings in the chain gives reason of the poor resistance to UV rays.
As a matter of consequence, hydrogenated epoxies can also be produced which, however, although they do not show a greater resistance in the exterior, they lack hardness and much lower Tg values. To crosslink epoxy resins, amine catalysts are needed. Four distinct types can be found in the market:
– aliphatic amines;
– cycloaliphatic amines (slower);
– mannich bases (fast even at low temperatures);
– polyamide amines (typical for anti-corrosion given their hydrophobicity).
The reaction that leads to the crosslinking of epoxy polymers with the corresponding amine catalysts is at page 11. Therefore, it could be understood that the more ‘reactive’ hydrogens are in the amine component, the greater the reactivity of the amine itself.
For reasons of toxicity, but not only, amines are often ‘adduct’, that is, partially crosslinked with a quantity of epoxy resin (below the stoichiometric value), so as to be less difficult to handle it faster in the final reaction and also providing fewer secondary problems (e.g. carbonation), and with a greater number of ‘reactive’ hydrogens in the chain, which helps to dose them more easily.
In fact, amines are characterized by the equivalent hydrogen (AEHW), calculated from the ratio between the molecular weight of the amine and the number of ‘reactive’ hydrogens found in the chain.
The amine/epoxy resin stoichiometric ratio is obtained from the following equation:
(AEHW/EEW) * 100
Solvent-based epoxies are widespread on the market, however, although still a niche, water-based epoxies can bring interesting technical advantages with similar performance:
– lower VOCs.
– Lower flammability (no dilution solvents).
– Final cleaning that can be done with water, therefore no VOCs and less toxicity for the workers.
– Lower evaporation risk as there are no bulk solvents.
On the other hand, resins can show
– lower shelf life.
In this article, the water-based resins in the ddchem product portfolio will be dealt with (we summarize the products in Tab. 1).
It should be underlined that, while Itapox 110 and 120 contain a very low % amount of organic solvent, the 201 version is totally free of it. The resins have been developed for two main sectors:
– anticorrosion.
– Flooring.
In this work the three resins have been compared using four different amine catalysts, whose characteristyics are reported in Table 2.
The resins were characterized using two types of tests:
– pencil hardness (ASTM D 3363).
– Drying speed on the dry recorder (ASTM D 5895-13).
The results obtained for each product are reported in Figure 1-2-3.
Some considerations from the analysis of the results:
– Itapox 201 shows the best final hardness with all catalysts after 1 week from the application, the 120 version, the worst.
– In 24 hours, Itamid Water 1419 always shows the best hardness development with each resin.
It is interesting to compare these data with those deriving from the dry recorder, reported in Figure 4 (three different times (hours) recorded on the machine were indicated:
T1 start of crosslinking, T2 deep polymerization, T3 completion of surface polymerization).
As it can be seen in Figure 5-6-7, Itapox Water 120 seems to be the fastest resin in crosslinking with all the amine catalysts tested, while Itapox Water 201 seems to be the slowest.
This combines well with the hardness development results; in fact, a fast crosslinking leads to a faster polymerization which, often, does not lead to a gradual development of the final crosslinking, crystallizing the system earlier and, consequently, developing less hardness over time.
The difference could also explained considering that 201 has a much lower epoxy equivalent than the others, so as a greater catalysis ratio is requested with the amines. However, considering 110 and 120, which have similar EEW, the same trend is found, i.e. 120 shows a greater reaction speed with a lower hardness development over time. Given these results, for a flooring system one could choose a combination of Itapox Water 201 Itamine Water 1419, whereas for anticorrosion Itapox Water 110 with Itamid Water 512 or 612. This is also a consequence of using polyamide amine catalysts, given their fatty acid nature, cause yellowing that wich could be avoided with 1419.
Regarding anticorrosion, a test was carried out on the combination shown above, which led to an excellent result, after having optimised the formulation with adequate dispersants and extenders.
The salt spray test was lead on steel at two different thicknesses (40 and 115 wet microns respectively) which highlighted its excellent resistance at the formulation level, as shown in Figure 8.
CONCLUSIONI
In this article, the characteristics of the new epoxy water-based resins in the ddchem product portfolio have been highlighted, through which it is possible to develop excellent painting systems for both the flooring and anti-corrosion sectors with a lower impact on the environment in terms of emissions both in the application phase and in the cleaning phase of the painting systems.
