The DIY sector expects more and more premium class paints. Latex paints for interior painting are at the forefront of these expectations. Excellent hiding power, low or zero-VOC, no reactive VOC, perfect application properties in the hands of a non-professional painter are only part of the customer’s expectations. The parameters of the durability of the coatings, different levels of gloss and sheen, the resistance of the coating to scrubbing as well as the lack of spattering during painting, no sagging or odorless are the distinguishing features of cheap paints.
But what are such premium paints made of? How are they tested and what is their quality compared to to determine if they are really premium class? We invite you to read the article about matt latex paints and their ingredients as well as tests in the premium segment dedicated to the US market.
Premium class and formulations
High-quality latex paints are created by their recipe, and more precisely by how the raw materials were selected and what their quality is based on research. It is often believed that low PVC is the key to high-quality latex paint, and this is not true. The level of PVC has an impact on the obtained quality parameters, however, the use of raw materials of insufficient quality and not interacting effectively with each other in the formulation will not ensure the quality of premium class paints, even with low PVC.
Formulation of paints with different gloss and sheen requires the use of PVC in the range from PVC 15 for high gloss paints to PVC 60 for flat paints, however, this level should be determined appropriately with the participation of individual raw materials ensuring appropriate hiding power, sheen and gloss, rheological properties, volume solids (minimum 40%) and the scrub resistance of the coating. It is very important to use individual additives ensuring extended open time (OTE-additives), in-can biocides, fungicides, but also very often flame retardants or corrosion inhibitors to protect e.g. nail heads during painting in places where they occur on the walls. The final selection of PVC depends on the quality of the paint and the quality shelf we target our recipe. Often, flat paints have low PVC and the gloss (sheen at 85° < 10) is obtained by using appropriate fillers and matting agents.
That is why the stage of formulating the recipe is so important, which should be supported by laboratory tests carried out by the R&D department of the paint manufacturer. These, in turn, should be supported by the knowledge provided by the producers of raw materials in the form of starter formulations showing the results of tests and analysis of the use cases of a given raw material in various ranges of PVC, different doses (ladder tests) and compatibility with other raw materials, which usually harmonize with other ingredients of the recipe (e.g. . low-mid-high shear forces thickeners between each other, polymer dispersions with coalescing agents, polymer dispersions and dispersants with defoamers, dispersants with wetting agents, etc.)
Pigments & Fillers
Basic fillers – such as ground calcium carbonate, for example, can be used in premium paints with little or no use. Calcium carbonate fillers in high-quality paints should not only act as a filler, but also provide additional efficiency. Therefore, the use of precipitated calcium carbonate (PCC) or modified calcium carbonate (MCC) is most desirable for whiteness, hiding power enhancement and sheen reduction.
Functional fillers in high-quality paints are the most important group of fillers. Their task is to build together with the binder and volume solids additives to create the appropriate residue on the substrate – the coating. The share of functional fillers forces the PVC to be lowered in a way to ensure proper binding properties of the latex binder. The demand for the binder increases with the increase in the proportion of fillers with a large specific surface area, and although oil absorption is not entirely an indicator of the binder demand, it indicates the need to use an increased amount of polymer dispersion, and thus lower PVC, even in mat paints. Below in the article I also discussed an example of a common mistake made when formulating paints with the use of functional fillers.
The use of faceted fillers must always be very careful. Firstly, because of the cost of the batch for the recipe, and secondly, because of its effective operation. Many functional fillers have a great effect on improving the parameters, however, their use depends on the possibility of their application in a specific range of PVC, the coordination of several functional fillers in the recipe and the operation of additives that allow them to be grinded and stabilized over time.
An example of the most commonly used functional fillers in premium class paints are, among others nepheline syenite, which is largely responsible for increasing the resistance to wet scrubbing, reducing dirt pick-up, as well as the hardness of the coating. Like hard ground quartz or cristobalite. Fibrous wollastonite is used in matte paints, especially to build a single-layer coating. In turn, talc is used to regulate gloss to a certain extent, as well as reduce mudcracking. In order to improve the opacity and also effective matting, various types of natural kaolin are used, although in most cases calcined. The type of kaolin used is crucial as not everyone provides the appropriate level of opacity. As an example, the photo below shows two types of kaolins used in the same latex paint formulation.
It is very important to use fillers with the appropriate grain size, depending on the gloss of the coating. In matt paints, you can use slightly thicker fillers, because the matt coating will visually hide the thicker particles (of course without a large spread between individual fillers). In low-sheen, semi-gloss, satin-gloss, gloss, high-gloss paints, the use of grain sizes and types of fillers is significantly reduced. In such paints it is important to use suitable white pigments with a high refractive index, such as titanium dioxide and zinc oxide.
White pigments known as prime pigments provide opacity and hiding power with appropriate cooperation with functional fillers, PVC and volume solids. This is important for building the quality of the formulation. In premium class paints, rutile titanium dioxide should be used, tested to meet durability parameters in water-based paints and classified as grade II (according to ASTM D476) to low-medium PVC (flat, low-sheen and semi-gloss paints), as well as grade V of premium class paints for satin gloss and high gloss.
About the differences in the classification of TiO2 in glossy paints, I recommend my article in PPCJ at the link: https://flickread.com/edition/html/free/61a9fe2971ad0#29
Latex binders and coalescing agents
Polymer dispersions are responsible for combining all the ingredients of the coating. Their correlation with fillers and pigments in the volume ratio determines the PVC of the coating. Premium-class paints must use high-quality latex binders in terms of coating strength, excellent pigment and filler binding capacity, very low content of residual monomers, VOC-free, APEO, etc. Therefore, pure acrylic polymer dispersions are used for premium paints, but also VAE (vinyl-acetate ethylene), as well as vinyl acetate copolymers with acrylic monomers. The choice of the polymer dispersion must also be supported by laboratory tests. It is very important that the paint that meets the requirements of the premium class has adequate scrub resistance. Not every acrylic polymer dispersion is by definition good for binding in paints, especially flat paints, where there are many different fillers with a large specific surface, such as talc, kaolins, nepheline syenite, quartz or silica. As an example, a paint formulation with PVC 40 can be mentioned, which was prepared with two different dispersions of acrylic polymers. The photo below shows the scrubbability test result according to ASTM D2486.
The picture was taken with less than 140 scrubbing cycles with an abrasive medium. This result shows that despite the low PVC, the paint at the bottom of the photo withstood about 15% of the scrub resistance of the paint on the polymer dispersion at the top of the photo. This is due to the misinterpretation that relatively low PVC equals high coating strength. This is due to the inferior ability to bind fine fillers with high binder requirements.
Coalescing agents must inherently occur with polymer dispersions. This applies to polymeric binders with film forming temperatures greater than 39 °F. Many binders, including VAE, do not require the use of coalescing agents for proper coating formation, however, purely acrylic dispersions with a Tg of about 68 °F require the use of appropriate film-forming aids. It is very important that the selection of coalescents is based not only on the dose read from the MFFT curve, but also on the basis of studies on the effect of the dose and the type of coalescent on the formation of paint coatings at low temperature on an absorbent and non-absorbent substrate, i.e. perform the LTC Test (Low Temperature Coalescence Test). I recommend my PCI magazine article on selecting coalescents. Link to the article: https://digitaledition.pcimag.com/december-2021/feature-spektrochem/?oly_enc_id=8519D4501489E4C&_ga=2.54668847.508521587.1639667082-1364750107.1620916933
Additives are the raw materials that allow the formulated paint to be not only stable over time, but also the durability of the coating, and during application, appropriate painting properties that make premium not only a durable coating, but also the pleasure of painting, and odorless during drying, open time for execution corrections, or lack of spattering when painting with a roller. The additives used for premium paints must also be zero-VOC (<1500 ppm), formaldehyde-free and APEO-free.
The additives are also to ensure long-term storage of the paint without the development of bacteria or mold in it (thanks to in-can preservatives, preferably MIT-free) and to ensure no mold growth on the coating (thanks to fungicides, e.g. biocides containing ziram). The stability of premium paints over time does not only mean ensuring adequate microbiological purity, but also viscosity stability, as well as the lack of syneresis, which spoils the aesthetics of the paint after opening the can and requires intensive mixing. These properties, as well as settling elimination, are influenced by many well-chosen additives, ranging from wetting agents, through dispersants, to rheological additives ensuring viscosity not only in the low and mid-shear forces areas (Boorkfield and Stormer viscosity) responsible for sagging, settling or the feeling of can-viscosity, but also affecting the appropriate application properties in the area of high-shear forces (ICI viscosity), ensuring easy application by roller, no spattering, which is especially important with DIY paints (paints for non-professional painters).
Therefore, in premium class latex paints you can find various acrylic (ASE, HASE), polyurethane (HEUR) and cellulose additives – especially hmHEC (hydrophobically modified hydroxyethyl cellulose), microfibrillated cellulose (MFC), as well as other additives responsible for e.g. for the elimination of foam (defoamers), matting waxes (allowing to design the desired level of sheen and gloss), corrosion inhibitors (preventing corrosion of metal cans and metal elements in walls, e.g. nails) or additives adjusting open time (open time extenders). The selection of all of them must be supported by appropriate tests that are imposed on the R&D departments of paint producers. Of course, they are much easier to do when the producer of the raw material provides appropriate case studies showing how a given additive works in a different range of PVC, with different polymer dispersions, with different fillers, etc.
Lab-scale preparation of samples
Laboratory preparation of samples for the evaluation of raw materials in terms of their effectiveness requires appropriate equipment. High-speed cowles dissolver with serrated disc should be used for dispersing and grinding process, with the relationship between the vessel geometry and the disc in accordance with ASTM D6619. The vessel should also have a cooling jacket that will allow it to maintain a constant temperature during dispersing and grinding, where temperatures are often released. The batch temperature should not exceed 86 °F as many raw materials destabilize their performance at higher temperatures.
Premium latex paints are best produced in the two-step process of combining slurry with a binder – polymer dispersion. The first step is to prepare slurries from each pigment and filler (or one slurry dedicated to one paint) by grinding with lab-scale cowles dissolver, and then in the second step to add the slurries to the latex vessel.
You do not add latex to the slurry, as mixing is much more difficult due to the higher density of the slurry than the density of the polymer dispersion. Adding latex to the heavier slurry while mixing allows for additional natural blending and spreading the latex over the entire slurry, a process called let-down. The same process is used in the industrial manufacture of latex paints. We discussed paint production in the let-down process on our blog here: https://paintlaboratory.wordpress.com/2020/06/28/latex-paint-production-in-slurry-process/
The prepared slurries are usually checked for fineness of grind. The test is carried out using a grindometer, which easily assesses the degree of grinding of all pigment and filler particles. Grinding evaluation is very important at the stage of R&D works for the selection of appropriate wetting and dispersing additives, as well as stabilizing additives, thanks to which it is possible to assess the slurry stability over time and determine the rate of re-agglomeration of pigment particles and fillers.
Below, see our video on the YT channel in which we show the next steps in the preparation of premium class latex paint from the raw materials that we describe here.
Let’s move on to the tests of paints prepared in our laboratory and presented in the video as an example. It is a paint in which raw materials are selected for testing in terms of case studies of fillers used to obtain flat paint dedicated to the US market, in which we test various types of fillers in terms of obtaining the desired gloss and compared to the reference paint recognized as a premium class on the US market.
Test results – case studies
What does premium paint mean? It is not easy to define the requirements that must be met. The simplest solution is to compare the test results of the prepared flat paint (or a series of paints) with paints recognized on the market as high-class or premium-class flat paints. This is also how the paint sample, the preparation of which we presented in the video, was compared.
The first test is to determine the consistency with a Stormer viscometer. US paints should have a viscosity KU within the conventional range of 90-110 KU. The reference paint is Stormer viscosity 102 KU and the paint prepared in our laboratory is 108 KU. Measurement was performed using a Stormer digital viscometer in accordance with ASTM D562.
The next step was to determine the ICI-viscosity in the high-shear forces range. Viscosity comparison was performed at 77 °F with a shear rate of 10,000 [1/s] according to ASTM D4287 using a BYK CAP2000 + (cone-plate) viscometer. The viscosity of the reference market paint was 2.9 P., while the paint prepared in the laboratory was 2.8 P.
In the Anti-Sag Index comparison, both paints scored 24 mils-gap without sagging when tested according to ASTM D4400 (with pre shearing by syringe and needle).
The spattering test is an assessment of the extent to which the paint will leave drops and dots all over the place when rolled over. This test is performed in accordance with ASTM D4707 using the notched spool-roller and is rated from 0 to 10, with 0 being the worst and 10 being the best. For the prepared paint and the reference paint, the spattering test result was 10 – no spattering.
It is very important that premium paints have a high volume solids content and is expected to be a minimum of 40%. Test performed in accordance with ASTM D2697 showed 42% volume solids for the reference paint and 43% for the paint prepared for testing the filler formulation in our laboratory.
Volume solids is a much more important parameter than non-volatile matter measured as percentage by weight. It is true that in order to determine volume solids, it is necessary to mark non-volatile parts by weight to use in the calculation of volume solids, however, in the technical data sheets of premium class paints, the nominal value of volume solids is indicated.
The next test was to determine weight per gallon. In the case of the prepared paint and the reference paint, these were not paints with light fillers, but conventional latex paints. The density of the reference paint was 11.7 lbs per gallon, and the density of the paint prepared in our laboratory was 11.5 lbs per gallon.
Let us move on to the coating tests which were performed after 7 days of conditioning at temperature 73.5 °F ± 3.5 °F and 50% ± 5% RH after the automatic drawdown procedure performed with a spreading rate of 450 sq.ft./gallon.
The tested paint was prepared in off-white color, similar to Swiss coffee, in which the reference paint was present. The first test was to evaluate hiding power as contrast ratio for coatings obtained with a spreading rate 450 sq.ft./gal. The contrast ratio for the coating of the test paint prepared in our laboratory and the reference paint was 98.2% (test paint) and 98.4% (reference paint). The contrast ratio was measured with a reflectometer in accordance with ASTM D2805.
The gloss of the coatings was determined by ASTM D523 at an angle of 60° (gloss) and 85° (sheen). The reference paint scored at 60° – 2.2, and at 85° – 4.5. The tested paint showed a slightly higher sheen at 85° bo amounting to 7.2 and at an angle of 60 ° – 3.3. These results show that in the case of sheen it could be slightly lowered, although according to the MPI gloss level 1 (flat), the paint is classified as flat (gloss < 10 at 85° and < 5 at 60°).
Scrubbability was carried out in accordance with ASTM D2486 with a shim under coating and with Leneta SC-2 abrasive medium, obtaining the results of 1,000 cycles ± 80 cycles for both paints (test paint and reference paint).
Open-time was also tested in accordance with ASTM D7488, which for both paints was 40 minutes ± 10 minutes for both the test and reference paints. Test conducted at 73.5 °F ± 3.5 °F temperature and 50% ± 5% relative humidity
The low temperature test was performed in accordance with ASTM D7306 at 39 ° F on a sealed and unsealed substrate by applying the paints with a 12 mils applicator to aid low temperature drying. Both paint samples obtained the result of 10, which is the best (no cracking in both areas with different absorbency).
Formulating recipes for premium class paints is not an easy task. The necessity to correlate many raw materials with each other, ensuring appropriate parameters of liquid paint as well as coating parameters, as well as adjusting the cost of raw materials in the recipe is a real challenge for the R&D departments of paint manufacturers. To support them in this tedious and difficult process of formulating recipes, laboratories such as Spektrochem provide producers of raw materials with case studies in the form of technical reports from laboratory tests, which then constitute a tool for interpreting the effectiveness of raw materials in recipes. This tool allows for a better understanding of the operation of the raw material in various recipes, especially when it comes to the formulation of premium class paint raw materials.
The raw material information as well as the test results presented here have been described very sparingly to draw attention to the issue, however, I am aware that they are not exhaustive. For this purpose, there will soon be many extensive publications in which we will provide more details.