The discussion of the problem of latex paint snail pattern on the outer wall and the study of a new semi-quantitative test method.

Abstract:
has developed a qualitative and semi-quantitative detection method that can be used to analyze the raw material composition and content of latex paint on the outer wall that can easily cause problems with bright spots (snail lines), and evaluates the rain-resistant effect of a newly developed special polymer dispersant, Disergen®PLF 100, in this application.
key words: rain marks; snail patterns; semi-quantitative; polymer dispersants.
of latex paint on the exterior wall is one of the most common paint film problems in architectural coatings, which can seriously affect the appearance

. Newly painted wall paint is vulnerable to weather, especially in spring and autumn, in 2 to 24 hours encountered low temperature and high humidity environmental conditions, the surface is prone to condensation, or drizzle, latex paint film process in this condition significantly delayed. The hydropho material in the paint film is easier to migrate to the surface of the lacquer film, forming water droples rich in hydrophosphoric substances on the wall, which flow down under the effect of gravity to form a waterline hanging on the wall. When the moisture is completely volatile, hydrophobic organic matter can not be volatile, rich out stranded on the surface of the paint film, it forms a shiny white traces, often referred to as bright spots of rain marks or the international common name snail trails, that is, "snail pattern".
Is still unable to fully understand in theory which ingredients are easy to migrate from the coating, leading to the appearance of snail lines, but surfactants are often considered to be one of the main "starters" because of their water solubility and susceptibility. They are generally introduced into coating formulations as emulsifiers and wetting dispersants, etc. through ingredients such as emulsions and slurry. Nevertheless, since various additives do play a key role in coating manufacturing and application, the hope is that the design of a coating formulation that does not contain any surfactants at all is technically unlikely or cost-effective.
effects of snail patterns are particularly evident on dark exterior paint surfaces: because dark paints generally add more slurry, color paint introduces a large number of water-soluble ingredients. This shiny snail print contrasts sharply with the dim dark paint surface. Rain marks often result in customer complaints, and even in the worst case, wall paint needs to be repainted, resulting in substantial compensation costs for paint manufacturers. Generally speaking, because the human eye is the most sensitive to gray, dark gray in exterior paint is the most likely to produce snail-print complaints.
the problem of rain marks in latex paint on the exterior walls is common, systematic reporting in this area is rare. At present, there are no standard detection methods for anti-snail-print performance in relevant standards at home and abroad, and the "outdoor rain stain test" in the "outdoor rain stain test" in the "GB/T 31815-2015 building exterior surface self-cleaning paint" is used to detect the anti-pollution rain mark performance of the exterior paint, and cannot be used to evaluate the anti-snail pattern of the exterior paint. The study of rain marks on surfactants in the American Society for Testing Materials standard D7190-10 (2015) also showed only pictures of water droplets stuck on the surface of the coating, with no quantitative data. After discussion, it is found that the "direct method" commonly used by domestic paint industry peers - brushing the surface of the paint film with cold water strips, "dewing method" - creating suitable warm and humid conditions for the surface of the paint film to condense, "flushing method" - access to cold water pipe flushing Shower simulation of light rain environment, "ice block method" - with ice cubes placed above the vertical lacquer plate to make the paint film naturally dew or a small amount of washing, these detection methods are visual comparison, belong to qualitative analysis, can not obtain more accurate quantitative analysis results.
//Newly developed testing methods help trace the root causes of snail marks
In order to gain insight into the causes of snail patterns from a deeper perspective, Clariant Chemical Co., Ltd. has developed a new test method to qualitatively and semi-quantitatively study this rain mark phenomenon. Similar methods have been published in the study of emulsions used in coatings, or the environmental impact of coatings, on the basis of which Clariant's technicians have borrowed and made further in-depth studies. Among them, the qualitative test method part, can quickly screen the paint formula, in order to first determine some of the easier to migrate out of the ingredients. Semi-quantitative test methods, by comparing with the standard reference (internal standard), can further confirm the corresponding specific components. The same components can be quantitatively analyzed by comparing them with the comparison of one-to-one markers in the reference.
//qualitative analysis - does a paint formula produce snail marks?
for qualitative analysis, we have designed a unique rain mark test box, which is very convenient to use. Latex paints that are interested in quickly learning if snail prints are produced can be used as a substrate by coating standard scrub-resistant cardboard in black (PVC/vinyl acetate co-polymer; according to DIN 53778). The wet film thickness of the coating is approximately 400 m (similar to the way two coats are constructed with wall paint brushes), while 3-4cm blank areas are reserved on each side of the upper and lower wide sides of the cardboard. When the coating is dry, a piece of cardboard is cut into two pieces of the same size for parallel testing. The blank area of the cardboard can be bent and rolled into a "bag" and secured with transparent tape to collect the leachate that migrates from the coating to form a snail pattern and flows down (see Figure 1). This thin coated cardboard can also be easily drilled. Well-drilled holes can be hung vertically in the test case with small hooks and simulate the vertical directional distribution of wall paint. In addition, custom hooks ensure that the painted surface is directed towards the external glass wall of the test case, exposing it to sufficient moisture and making it easy to observe.
Figure 1 - Prepare the coating plate and the coating plate with the collection bag and place it in the Kesternich test box
After entering moist air according to the DIN 50017 standard test method, the newly coated coating surface can be tracked throughout the curing film phase of the coating. Especially for color paint, the presence of snail patterns can be observed quickly. With handheld optical microscopes, snail patterns can be further clearly enlarged (see Figure 2). In addition to the coating surface, in the concentrate collection bag, can be used for snail-print observation: after drying, the presence of residue can be indicated in the water-soluble ingredients, these components may not be observed in the coating part of the area, because the coating plate length and area is limited, after all, but may occur in the actual application of higher external wall coating. In our standard test, each coating plate is tested in parallel in the test box, so that three paint samples can be tested simultaneously. This method of observing test boxes can be widely used in a variety of coating formulations.
Figure 2 - An optical microscope image of snail print on a paint coating board
// Different lotions are introduced into the paint formulation for evaluation
To prove the theoretical assumption about the source of the rain marks, we have developed a very simple white paint formula to assess the effects of different lotions. That is, first in accordance with a coating formula grinding preparation of the base pigment mixture, and then divided into the same amount of several parts, respectively, added quantitative different lotions, to avoid repeated weighing and preparation process of large differences. Some of the ingredients suspected of being prone to snail prints have been carefully excluded in the first place: this coating formulation does not contain desmoulating agents and does not add additional fluid modifiers (hydrophobic group-modified alkali-soluble thickeners) after the grinding phase. And the wetting agent is also reduced to the minimum dosage, only to ensure that the necessary amount is added to grind titanium dioxide and fillers.
first studied the different monosome compositions and minimum film-forming temperatures in the emulsion (see Table 1- Coating Formula, and Table 2-Emulsion). Different results are obtained in challenging high PVC coating formulations. Lotions 3 and 4 show little snail print, while the collection bag is clarified, while emulsions 1 and 2 are on the coated surface, clearly showing a shiny snail-print appearance, as well as the residue in the bag (Figure 3). This indicates that emulsions 1 and 2 need to be moderately increased in the formulation to better close the coating surface to obtain similar results to emulsions 3 and 4 in high PVC coating formulations. Because emulsions 1 and 2 are relatively high film-forming temperatures, the coating formulation also needs to be adjusted appropriately to add a small amount of film-forming additives to obtain better film-forming.table
table 1 - Simplified coating formula for snail pattern research
Table 2 - Selection of emulsions in qualitative studies
Figure 3 - Qualitative testing of emulsion 2
// Quantitative testing: Further study of the root causes of snail patterns
The above test box helps to determine whether a paint formula is prone to snail patterns, but it does not reveal which of the more complex formulations, which raw materials in the formula are the cause of snail prints. To answer this question, more complex testing and evaluation methods need to be developed. For known coating formulations, the ingredients that are suspected to be the cause of snail prints are retained according to their high-efficiency liquid chromatography (HPLC): used to analyze small molecular chemicals, or gel chromatography (GPC): used to analyze polymeric chemicals. Paint formulations are applied to the black surface of porcelain pieces (bathroom tiles of a certain size) as inorgeable substrates. Coatings are coated with a scraper and the weight is recorded, requiring approximately 11g of wet weight paint (corresponding to a coating approximately 400 m thick on thin cardboard) on each piece of porcelain sheet (14x14cm).
test case is made up of a modified glass fish tank plus an attached cold water evaporation condensator (Figure 4). Cold water is used to simulate the low temperature and high humidity conditions that snail patterns can easily form on the morning of spring and autumn festivals. In this modified glass fish tank, four plates can be placed at the same time for parallel comparison studies. When coated with a plate and evenly cut in half, this means that two paint formulations can be compared simultaneously in the test cabinet. The leaching liquid is collected at the bottom with a glass dish. At the end of the "rain mark cycle" test, the leachate collected in the glass dish is weighed and dehydration concentrated in order to reach the lower detection limit of the instrument for quantitative analysis. When the sample is injected into the analytical equipment, the retention time of each generated peak can be compared with some predetermined internal labels, so that the specific ingredients migrated out of the coating formulation can be found. This test method itself is more cumbersome than visual observation alone. However, it does have value in determining the cause of the rain marks.
Figure 4 - A modified glass fish tank plus a test case for cold water evaporation condensers to study rain marks
it is important to note that in a semi-quantitative way of testing, the amount of leaching ingredients can be compared to each other. Weighing during the transfer and concentration of leachate can cause some weighing errors. In addition, there are some deviations in the coating weight of each sheet surface, and errors can occur when weighing to determine the wet and dry weight of the coating. All of these system errors result in us not being able to do a completely 100% accurate quantitative analysis. At this stage, because our study of the causes of rain marks in many different coating formulations is only in its infancy, and there is no need to pursue more accurate and accurate quantification.
//case: Dispersants study
a prominent representative of the snail pattern phenomenon in coating formulations is dispersants. So we studied a new specialty polymer dispersant, Dispersogen®PLF 100, developed by Clariant, compared to traditional small molecular dispersants. In theory, polymer-type dispersants are generally not easy to migrate out of coating formulations because of the slower Brown movement of polymers relative to smaller molecules and the entanglement of polymer chains with other coating components. In order to study this effect, we prepared two kinds of color slurry, respectively, using a new polymer type dispersant and ordinary small molecule surfactant type dispersant for comparative experiments. Both colors are added to the same amount in white paint. We chose iron oxide red (P.R.101) inorganic pigments, because as mentioned earlier, snail patterns are usually more likely to appear on dark paint surfaces, while the outer wall latex coating formula based on weather resistance and cost considerations, more use of inorganic pigments. The slurry formula is shown in Table 3. This formula has also been simplified to eliminate interference from some other ingredients. Add 8wt% of the paste to the base white paint to tone it.
Table 3 - Simplified coating formulations for snail pattern research
A visual inspection of the test box observed a significant snail pattern in the color paint coating of the selected surfactant type dispersant.
, the color paint with the new polymer dispersant did not appear snail pattern from beginning to end in the test. In order to quantitatively test the migrating ingredients in the leachate of this formula, we also performed a gel chromatography (GPC) test.
two different types of dispersants can be tracked and quantitatively detected by instruments. As can be seen from Figure 5, the surfactant-type dispersant peaks at the position of the high-efficiency liquid chromatography (HPLC) 15 minute retention time. Polymer-type dispersants, because of their high molecular weight, need to be detected by gel chromatography (GPC). It is retained for about 20 minutes. When the collected leachate is injected into an efficient liquid chromatography instrument, a large peak can be found during the 15-minute retention time of the chromatography. In the gel chromatography, only a small peak appears in the position of the polymer dispersant calibration. After processing with the correction curve, the approximate quantitative results of the two dispersants leached can be obtained. For surfactant-type dispersants, approximately 2.1g/L is leached, while for polymer-type dispersants, only 0.4g/L is detected. These figures are not suitable as an absolute end result, but can be compared with each other. Using two numbers to calculate the ratio, it can be found that under the same amount of addition, polymer-type dispersants only about one-fifth of the amount of surfactant-type dispersant leaching.
Figure 5 - High-efficiency liquid chromatography tracks surfactant-type dispersants as internal targets, and in the light of the controls
/// Conclusions
in the coating, we have developed a new, more systematic testing method in the laboratory to help predict the likelihood of snail patterns in the coating. Among them, qualitative method is used to quickly judge the possibility of snail pattern phenomenon in any latex paint formula, and the subsequent semi-quantitative analysis method can confirm the specific coating parts leading to snail pattern, and its approximate migration and analysis of the content. It has been found that Clariant's newly developed specialty polymer dispersant Dispersogen®PLF 100 can be applied in coating formulations to reduce snail pattern problems. In addition, Clariant's cooperation with domestic and foreign customers is under way for further in-depth research on issues such as rain marks.
references to the
. 1. Schoff, C.K.