The hardness parameter is usually associated with floor paints, parquet varnishes and other coatings that require high resistance to deformation under the pressure exerted on them by harder objects. However, hardness as a strength parameter also applies to coatings in other applications and is an indicator of how the coating acquires its parameters, and the development of this hardness is particularly important when selecting certain raw materials which leave the coating during evaporation and make it harder.
Such raw materials are additives ensuring the formation of a film of water-borne polymer dispersions – coalescents. It is about them that this short insight into showing how coalescing additives can affect the hardness development of low-medium PVC dispersion paints based on acrylic latex polymers.
The coatings of polymer dispersions dry out by the evaporation of water and the process of combining individual polymer particles, which is called coalescence. It takes place above a certain temperature called MFFT (Minimum Film Forming Temperature), for the lowering of which coalescing additives are used. You can read about how they are selected on the blog in the article at the link here.
The coalescence process takes place as long as the coalescent evaporates, and also as long as its plasticizing effect is maintained. The polymer particles are then still plastic and the coating does not immediately acquire its hardness. That is why the process referred to as hardness development is so important. But what exactly is hardness?
Hardness – property of solids, which generally is described as the static resistance of their surface against an indenter [Paolo Nanetti, Coatings from A to Z]
In practice, hardness thus means the resistance of the coating to deformation of its surface under the influence of static pressure by an object, usually of a higher hardness, which exerts this pressure. For floor coverings, this usually means deformation due to chair legs, furniture, heels, and for window sill coatings, for example pressure from flower pots.
While the hardness of each coating of paint or dispersion enamel is developed over time and is not obtained immediately after the evaporation of water is completed, the knowledge of the influence of the raw materials present in the recipe on the development of this hardness over time provides a lot of valuable information on how the coating acquires its mechanical strength and at what time it can be obtained. For this purpose, it can be used from the simplest method of determining hardness – pencil hardness tester (ASTM D3363, ISO 15184), through the determination of indentation by Buchholz (ISO 2815), to the determination of hardness as the damping time of the Persoz or König pendulum (ASTM D4366, ISO 1522).
For the purposes of this article, case studies have been prepared with the use of a pendulum damping tester as a device for measuring the development of hardness.
Pendulum hardness tester – a device for measuring the hardness of a dry film, based on the damping time required for a specified decrease in oscillation (swing) amplitude. The shorter the damping time the lower the hardness [ASTM D16 Terminology].
Two coalescents were tested: Butyl CELLOSOLVETM Solvent and Butyl CARBITOLTM Acetate, which were used as coalescents in the water-borne gloss acrylic dispersion enamel, in two doses: dose I – providing MFFT around 39 °F / 4 °C and dose II – increased to provide LTC (low temperature coalescence on a porous and absorbent substrate). The hardness was determined on drawdown-prepared coatings on glass plates. Hardness was measured 24 hours after application and after 7 days using a Persoza pendulum. The results are shown in the chart below.
In the case of the Butyl CELLOSOLVET Solvent coalescent, the hardness after 24 h was slightly higher for dosing II (increased dose), and after 7 days the hardness of the coating was achieved at a slightly lower level compared to the dose I. Coalescent Butyl CARBITOL Acetate was higher after 24 h. hardness for dose I compared to dose II, while after 7 days the hardness had evened out to the same level. The presented diagram shows that depending on the coalescent agent used and its dose, different values can be obtained and the increased dosage does not mean that the hardness is lower or higher in an obvious and unambiguous way. It depends on the evaporation of the coalescing agent, its interaction with the polymer dispersion and other ingredients of the formulation.
The most interesting is the analysis of the results between the coalescent Butyl CELLOSOLVE Solvent dose I vs. Butyl CARBITOL Acetate dose I, where the hardness of the Butyl CELLOSOLVE Solvent coalescent can be seen in the initial period of drying of the coating (first 24 hours), is almost 50% higher than that of the coating with Butyl CARBITOL Acetate.
In turn, the comparison of these two coalescents in dose II in the first 24 hours of drying gives a result over 60% higher for Butyl CELLOSOLVE Solvent. Such an analysis allows extending the process of selecting coalescents not only in relation to MFFT or LTC, but also allows for a more detailed understanding of the relationship between the type of coalescent, its dose and the obtained hardness.
These case studies are part of the Coalescing Agents MFFT + LTC 2022 project carried out at the Spektrochem Paint Technical Center. Do you want to know more test results and more dependencies? Please contact us directly.