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Artur Palasz, Spektrochem, discusses how important it is to characterise paint additives in terms of their impact on stability, as well as how important it is to consciously use special additives that can extend this stability, when formulating waterborne latex paints
Waterborne paints based on binders in the form of polymer dispersions, either acrylic or vinyl (VAE, vinyl-acrylic, with branched monomers, etc.) are generally a type of paint in which many raw materials are apparently incompatible with each other. Starting from insoluble monomer droplets polymerised and suspended in a homogeneous form of polymer dispersion thanks to surfactants and/or protective colloids, to the creation of the latex paint itself, where fillers and pigments are kept in stable suspension thanks to dispersants, thickeners and other raw materials, in which many raw materials are completely immiscible with water, such as coalescing agents or defoamers. Such behaviour and physical difficulties in combining these ingredients to form a stable suspension requires knowledge and appropriate raw materials – additives, but stability over time is very limited.
It is true that today’s latex paint formulations allow the production of paints that are stable for up to five years using appropriately selected raw materials, but during this time many situations may occur in which this stability will be disturbed. To prevent them from ending up in paint waste during their expected life, I have prepared this article to show how important it is to characterise paint additives in terms of their impact on stability, as well as how important it is to consciously use special additives that can extend this stability.
Factors causing stability disorders
If I ask you what can happen to waterborne paint when stored in a can in liquid form, the first thing that will probably come to your mind is microbiological deterioration. And it’s true, microbial contamination that causes bacteria to grow can destroy the paint, causing it to smell offensive and make it less suitable for painting. But this is not the only possible reason for eliminating it from further use, causing consumer dissatisfaction and the need to purchase a new paint.
Figure 1. Bacterial in a sample of bentonite gel in water – on the left: liquid sample, on the right: result on enterobacterial growth medium
It is often difficult to diagnose the real reason for the destabilisation of the paint, because insufficiently tested raw materials in a given formulation may not at first seem responsible for this or that behaviour of the paint. Therefore, it is so important to properly characterise the performance of raw materials in formulations, in correlation with binders, pigments, fillers and other additives, in order to provide formulators with knowledge about potential problems that should be avoided, as well as provide knowledge on how to extend stability by appropriate use of a given raw material, selection a specific dose or optimising compatibility with, for example, binders.
Factors that destabilise liquid paints are usually associated with poor storage conditions and here, mainly the temperature is too low (freezing) or too high (an increase in temperature accelerates the changes taking place in the paint). However, there are many more destabilising factors and they often appear spontaneously in the paint, even under appropriate storage conditions. The effects of such changes are most often:
- Viscosity changes, decrease or increase, gelation
- pH drift
- Poor tintability
- Microbial deterioration
In this article, I will discuss several examples, supported by laboratory tests and application studies, for formulations and raw materials in which their appropriate selection of dosage, as well as compatibility characteristics with other raw materials, can significantly contribute to improving the stability of paints in the liquid state. I will also discuss examples of additives that if characterised for effectiveness can help improve the stability of paints against freezing, as well as settling over time.
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Test methods and stability assessment
In order to assess whether the selected raw materials in a given formulation are stable or not, it is good to perform appropriate laboratory tests. This allows not only visual or sensory assessment, but also the ability to repeatably evaluate a series of samples. For such tests, packages of test methods are usually used, the scope of which depends on the key performance features we care about in order to identify potential symptoms of destabilisation. The test methods most commonly used in our laboratory for such tests are presented in Table 1.
|Table 1. Test methods used in stability testing|
|Test method||Test range||Test specification|
|ASTM D1849||Storage stability test||Accelerated storage stability test
(1 month at 52°C / 125°F).
The test is often shortened to 7 or 14 days to notice the first trend of changes occurring
|ASTM D869||Evaluation of settling||Scale of settling formation in paints from 0 to 10 assessed using a standardised spatula|
|Spektrochem’s rating of syneresis||Evaluation of degree of syneresis||Scale for assessing the degree of syneresis (from 1 to 5)
* In-house test method due to lack of other standard
|ASTM D2243||Freeze-thaw stability test||Freeze-thaw resistance assessment method from 1 to 5 cyclic freezes and thaws|
|ASTM D2574||Microbial attack test in a container||Assessment of paint susceptibility to microbiological contamination|
|ASTM D3925||Sampling||Method for collecting samples for testing|
Five areas of changes in storage stability are discussed below, which, apart from microbiological attack, are the most common in latex paints. The presented case studies come entirely from research projects carried out in our laboratory for the purpose of characterising raw materials for their producers at the stage of research on new additives or tests for existing additives in order to develop guideline formulations for them.
Changes in paint viscosity are the most common problem in stability testing. Viscosity can both increase and decrease. An increase in viscosity within certain limits is permissible but, more importantly, it is important whether this increase occurred in the low-, mid- or high-shear forces area and whether the paint is still applicable. An increase in viscosity usually leads to problems with flow, levelling, mixability, acceptance of pigment concentrates, etc. A decrease in viscosity, in turn, is very dangerous because a reduction in viscosity is associated with the loss of virtually all application properties and the occurrence of problems with spattering, sagging, etc. In the worst case, the viscosity can increase so dramatically that the paint fails completely (Figure 1).
The cause of viscosity drift is most often a disturbance resulting from the associative interaction of the thickener with the particles of the polymer dispersion and, more precisely, with its surfactants. However, this is not the only cause and often occurs in combination with other factors causing viscosity fluctuations. In the case of many thickeners, their effectiveness in building the spatial network and building viscosity depends on pH (cellulose thickeners, acrylic thickeners, some HEUR thickeners). As the pH changes in a sample during stability testing, the viscosity may also change. More on the causes of pH drift later in the article.
An example of viscosity drift may be the graph of changes in the viscosity of PVC paint 37, shown in Figure 2, based on three different polymer dispersions and all other raw materials identical in each formulation. This chart is in no way intended to generalise what type of polymer dispersion will be more or less stable in the formulation in terms of changes in viscosity, but to provide a moment of reflection for the formulator by showing that changes in viscosity increase can be significantly dependent on the polymer dispersion, its surfactants, acidity and alkalinity, redox potential, etc. As you can see in the graph, viscosity changes after 14 days of testing at 52°C can look very different.
Figure 2. Viscosity changes after storage stability tests of paints based on various polymer dispersions
Another example shows changes in the viscosity of latex paints in terms of tests of neutralising agents, which may be, for example, amines or their substitutes. Figure 3 and Table 2 show the viscosity changes after the storage stability test for the two neutralising agents tested. As you can see, neutralising agent No. 1 works very effectively in relation to the control sample, ensuring its viscosity stability (ASTM D562, KU viscosity), which was not ensured in the case of the control sample (without neutralising agent). However, it is also seen that the neutralising agent No. 2 caused a quite drastic drop in viscosity after the storage stability test, which will translate into deterioration of application properties. These results show that it is very important to compare raw materials and characterise them in different formulations, which provides extremely valuable value for the formulator and shortens the time of work on the final formulation.
Figure 3. Appearance of samples with tested neutralising amines showing the fluidity of paints after storage stability test
|Table 2. Changes in paint viscosity after neutralising additives tests|
|Test||Control sample||Neutralising agent #1||Neutralising agent #2|
|Initial viscosity||106 KU||105 KU||107 KU|
|Viscosity after 14 days at 52°C / 125°F
ASTM D1849 test
(D = 1 KU, ↑ 1%)
(D = –21 KU, ↓ 20%)
A decrease or increase in viscosity does not have to be caused by raw materials specifically responsible for viscosity and stability, such as thickeners or neutralising amines. It may be a completely side effect of the presence of additives that, at first glance, we would not suspect of affecting the viscosity.
Figure 4 shows photos from sagging resistance tests (ASTM D4400) taken for paint samples in which open time extending additives (OTE-additives) were tested. Tests were performed before and after storage stability to indicate whether any of the additives would alter sagging resistance. The results show that the impact is extremely visible and extremely diverse. This proves that tests of targeted additives for the specific property they are supposed to provide (such as here, extending open time) are extremely important, but we cannot forget to also provide the formulator with a guide to side parameters, the impact of which can sometimes be so drastic that ensuring good properties extending the open time may turn out to be less important.
Figure 4. Viscosity drift based on analysis of changes in sagging resistance (ASTM D4400) before and after storage stability test (14 days at 52°C /125°F) caused by OTE additives (open time extenders)
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pH changes are also a common cause of many paint stability problems. Starting from greater susceptibility to bacterial growth when the pH drops, through viscosity drift, to the formation of lumps and changes in the effectiveness of other raw materials (e.g. tendency to discolour pigments, coagulation of binders, disruption of the stabilising effect of dispersing agents).
Changes in pH may occur as a result of the decomposition of raw materials, disturbances in the acid-base balance, as well as progressive changes in ion exchange in the early period of paint storage, e.g. in the particular case of dispersing additives. Such an example is demonstrated in Figure 5, where four bio-based dispersant additives were tested along with a control sample using a conventional dispersant. As can be seen from the course of pH monitoring in the period from preparation to six months, two of the tested samples with bio-based dispersants showed a tendency to quite a drastic drop in pH in the period from 14 days to six months from sample preparation.
Figure 5. pH drift of paint samples with tested dispersants
This example shows how much attention should be paid to testing all new additives, especially when wanting to replace conventional dispersing and other additives with bio-based ones, the stability of which in formulations requires many tests and the development of appropriate guideline formulations showing case studies not only in the short term tests.
Syneresis occurs extremely often in latex paints and accompanies storage stability tests much more often than settling. This is the squeezing of water onto the surface under the influence of agglomeration and coagulation processes and is a signal that the stability of the paint is disturbed. While in many cases the appearance of syneresis does not disqualify the paint from maintaining its quality characteristics, because after mixing it returns to its original functional properties, the presence of syneresis does not look aesthetically pleasing, and additionally complicates the process of mixing the paint before use and introduces the risk of insufficient homogenisation of the paint by the painter, which may result in streaks, uneven gloss, or failure to mix the paint homogeneously with any sediment at the bottom that often appears with syneresis.
Figure 6. Syneresis in a pigment concentrate based on iron oxide – on the left: control sample, on the right: stabilisation with phyllosilicate thickener without increasing viscosity
In Figure 6 you can see what syneresis looks like at degrees 1/5 (for the sample on the left) and 3/5 (for the sample on the right). These are samples of pigment concentrates based on red iron oxide for tinting latex paints. The sample on the left does not contain any thickener or rheological stabiliser, while the sample on the right contains attapulgite, the dosage of which has been adjusted so as not to increase the viscosity of the pigment concentrate. This allowed to extend the stability of the prepared sample to syneresis and, as can be seen in Figure 6, after six months of observation the difference is very significant.
Settling is a phenomenon that occurs quite rarely in latex paints, but not so rarely that it can be ignored. It is usually associated with the settling of heavy fillers in a low viscosity system, however, settling may also be caused by the agglomeration of filler particles whose size is so large that even in a higher viscosity system a precipitate is formed, which is often difficult to mix. Settling usually occurs in pigment and filler suspensions used in the two-stage latex paint production process (grinding + let-down), i.e. in slurries that contain little or no thickener to ensure pumpability. Then, effective anti-setting additives are selected, which you can see in Figure 7 as an example of ladder-efficiency.
Figure 7. Results of anti-setting additive performance tests in calcium carbonate slurry
Settling in latex paints may occur as a result of disruption of the stabilising effect of dispersing additives or thickeners. A common cause is a change in coalescing agent, which contributes to a change in the polarity of the system and the initiation of the agglomeration process of pigment and filler particles. However, this is not strictly related to the type of coalescent, but the combination of several phenomena at the same time and the combination with the appropriate environment in the formulation and paint, which favours the formation of settling. In Figure 8 you can see how completely different stability results can be obtained in paints, based on two styrene-acrylic dispersions when the coalescent is changed.
Figure 8. Settling in latex paints based on two styrene-acrylic polymer dispersions and the differentiation of its formation depending on the coalescent used
These results show that it is very important to develop extended technical documentation for coalescents, the effectiveness of which assessed in the context of reducing MFFT is very important, but also as an additional effectiveness to ensure stability with various acrylic binders in the formulations, because good coating formation is nothing if the paint has previously formed a residue that makes it impossible to mix.
The factor that basically always completely destroys liquid paints is their freezing. The pressure of ice forming inside the can causes the particles of the polymer dispersion to deform, the viscosity increases dramatically, which causes coagulation, and the surfactants present in the polymer dispersion are unable to stabilise the particles. As a result, either a lot of coagulants are formed, which agglomerate particles of pigments and fillers, or the paint is completely destroyed into a solidified form or ‘cottage cheese’ form.
The challenge of making latex paints resistant to freeze-thaw is taken up by special surfactants that are able to reduce the lack of effective action of other surface-active compounds in the formulation. It should be remembered that such additives require very thorough testing, because their effectiveness depends very much on the dose, temperatures at which they will work effectively, as well as binders, coalescing agents, as well as the number of cycles to which the paints are tested in relation to ASTM D2243, which is used in such tests. It is also very important to be aware of the impact of such additives on many parameters of coatings, e.g. blocking and scrubbability. Their share must be selected on the basis of tests and guideline formulations because preventive additives added to paints to prevent potential accidental freezing during transportation heating failures must be supported by appropriate application studies (Figure 9).
Figure 9. Effectiveness of the freeze-thaw additive after ASTM D2243 tests (photos after the third cycle)
Now you know that not only in-can preservatives are responsible for extending the expiration date of paints. In principle, all raw materials present in the formulation and used in the production of latex paint must be properly tested and it is the responsibility of the raw material manufacturer to provide appropriate technical papers to properly recommend the use of particular raw materials for different ranges of PVC, different binders, used together with different coalescents, etc.
Such application studies and investigations in the laboratory described in the form of case studies, guides and recommendations for formulators not only save time during work at the R&D stage, but above all save on complaints caused by problems with paints after their production, when they are waiting for the buyer on the shelves in paint stores.
Artur Palasz, Ph.D.
e-mail: [email protected]