Fabian Gyger – ZSCHIMMER & SCHWARZ
Most water-based acrylic polymers used as binders in paints and coatings are anionically charged. With these polymers, most of the available paint ingredients are well compatible. However, certain applications benefit from the use of cationic acrylic polymers. These are mostly made by protonating an amine-containing polymer by acids or using monomers with quaternary ammonium functions.
IPN polymer wet and dry phase
For some applications this class of polymers brings outstanding performance for example in adhesion to surfaces like wood, metal, old alkyd paints or in general anionically charged surfaces.
These polymers are also broadly used in water-based stain blocking primers to prevent migration of stains like nicotine, wood tannins or dyes. Cationic acrylates complexify anionically charged stains or tannins, thus locking them in the primer layer and preventing migration and discoloration of the topcoat. The performance is comparable with solvent-based alkyd primers or zinc oxide-based systems, which, however, must be viewed critically for environmental protection reasons.
Formulation of IPN as a clear stain blocking primer
A major drawback of cationic polymers is their incompatibility with anionic components. This severely limits the choice of ingredients for the formulation. It can also cause considerable difficulties during production, even in subsequent batches of other products, so that many paint manufacturers completely avoid cationic acrylates or produce in separate vessels. Cationic polymers are only stable at low pH which is also a challenge to formulation as for example calcium carbonate, a common and inexpensive filler, is soluble at low pH and not usable in cationic formulations.i
INTERPENETRATING POLYMER NETWORK (IPN)
An interesting class of polymers that was introduced recently by Zschimmer & Schwarz are amphiphilic interpenetrating networks (IPN) that take advantage of the benefits of cationic functions without their major drawbacks. The IPN involves two different ionic structures in the same polymer network at a neutral pH value.
The overall polymer behaves neutral in the wet phase which means that they can be combined with conventional paint ingredients like waterborne anionic acrylics, polyurethane dispersions, or additives. Thus, unlike cationic acrylics they can also be used as co-binders in formulation for example as adhesion promoters. Thus, there is also no cross-contamination occurring in production.
Substrate: wood wetted with tannin solution
Upon drying cationic functionalities are formed, which provide similar benefits as pure polycationic acrylics, e. g. excellent tannin, stain and dye blocking without the compatibility problems common to cationic polymers. Adhesion to various substrates such as aluminium, PVC or wood is also improved.
A very simple but effective formulation example of a soft type IPN for a clear stain blocking primer is given in table 1. These types of formulation usually work best with minimal and well selected co-solvents since solvent retained in the final film after drying will facilitate an additional migration mechanism for the tannins.
dE on oak with tannin solution**
*after 2 weeks of aging at room temperature
**dE zero reference: topcoat on Leneta Chart
Tannin blocking tests were carried out on different wood types. In addition, the wood was soaked with a tannin solution. Results are given in figure 2 and 3. The blocking performance is clearly visible in the photo but also measurable with minimal colour change within two weeks after application.
Apart from using only softer IPN types, but also harder types or mixtures in the formulation another advantage over cationic acrylics becomes visible: very good stain resistance properties.
Exchanging 20% of the soft IPN against a hard type (MFFT 52°C) in our test formulation gave very good water, ammonia and stain resistance with results even passing a 16 h coffee staining test. Thus, combined with good scratch resistance these polymers cannot only be used in primer applications but also in topcoats or both at the same time. In addition to these properties, they exhibit excellent adhesion due to the available cationic functionality.
IPN stain resistance: use as primer and topcoat
IPN – HIGH PERFORMANCE and HIGH COMPATIBILTY
To conclude, amphiphilic IPNs have many advantages of cationic acrylics without the usual compatibility issues. Excellent tanning, stain and dye blocking is achieved with a zinc-free, waterborne acrylic formulation. In addition to good adhesion, this class of polymers offer good resistance against stains and chemicals which broaden their usage further, compared to the cationic acrylics.
These Zschimmer & Schwarz’s polymers in Italy are distributed by Eurosyn Spa
iAkkerman, Jaap, Mestach, Dirk, Biemans, Toine, Corten, Cathrin, Hövelmann, Class, Krakehl, Joachim, Leute, Martin and Warnon, Jacques. Resins for Water-borne Coatings, Hannover, Germany: Vincentz Network, 2021. https://doi.org/10.1515/9783748605249