Natural stone is often used as building decoration material because of its colorful and natural appearance, but its use is limited to a great extent because of its heavy quality, increasing wall load, complex construction, many unsafe factors, high cost, over-exploitation and destruction of the environment. It is mainly composed of genuine natural or artificial coloured sand, high-performance emulsion and various auxiliaries. It has a distinct advantage in construction, environmental protection and color diversity because of its resembling marble and granite. It has been vigorously promoted in China since 2010, and is gradually replacing traditional stone facing materials. In recent years, it has been speeding up, and has become the largest output in China's architectural coatings.
With the widespread application of real stone paint, it is easy to produce color difference, which leads to blooming, yellowing, poor stain resistance and whitening of paint film. Light blooming caused by color difference will affect the appearance of the paint, and heavy rework and re-painting will cause serious economic losses, which is the practical application of real stone paint. The most common and thorny issue.
There are many reasons for color difference of real stone paint, such as batch stability of colored sand (natural colored sand is difficult to achieve the same color between different batches), substrate treatment (substrate color and porosity are different may lead to different colors of real stone paint), construction conditions (temperature, humidity and wind speed during construction and maintenance) and so on. The same may lead to different colors, construction methods (different construction habits and scaffolding or hanging basket construction and other differences may lead to different film thickness, surface texture difference caused by surface floating sand or slurry; uneven gloss caused by the cover caused by different color difference) It is possible to produce chromatic aberration and cause flower problems. For these many factors caused by the hair problem can only be specific analysis of the causes of each case color difference in order to find a solution accordingly.
Temperature discoloration is the most complicated and thorny problem in the flowering caused by the color difference of real stone paint, that is, the color difference caused by the wall temperature or the construction environment temperature. Based on the analysis of the causes of temperature discoloration, this paper puts forward some methods to solve the problem of temperature discoloration of real stone paint from the rationality of paint formulation design.
Thermo chromatic aberration of real stone paint
One of the most common and difficult phenomena of color difference in real stone paints is temperature discoloration, i.e. the same batch of true stone paints are constructed by the same builder according to the correct construction procedures on the same wall surface, but because of the different wall temperature and environmental temperature and resulting in different color phenomena. For example: the same morning, noon and afternoon construction of the same real stone paint to get different colors; the same house may appear in the sun and back shade color difference; sunny and cloudy construction may appear color difference.
Usually low-cost dark real stone paint is the most prone to temperature discoloration, such as brown, brick red, dark gray and other colors. The change rule of temperature discoloration of real stone paint is usually the higher the temperature is, the darker the color is, the lower the temperature is, the lighter the color is. This change law is very similar to that of ordinary latex paint, ordinary latex paint when PVC close to or greater than CPVC also easy to produce temperature discoloration phenomenon, as shown in Figure 2 for the same exterior wall latex paint at different temperatures to obtain different colors, and the higher the temperature, the darker the color, the lower the temperature, the lighter the color.
Real stone paints are usually used in construction sites. It is impossible to keep the wall temperature and ambient temperature unchanged during the construction period. The temperature difference between day and night can sometimes be as high as 30 degrees Celsius at alternating seasons, while the wall temperature difference may be larger. It is impossible for us to ask for and do not have to seek true stone lacquer construction at constant temperature. As shown in Figure 3, a brand of high-quality true stone paint TX cured at 10 C and 35 C respectively, the final color is almost the same. It can be seen that the problem of temperature difference color of real stone paint can be solved by optimizing the formula of real stone paint.
Cause analysis of temperature difference of real stone paint
It is well known that many properties of coatings change dramatically at the critical color volume concentration (CPVC), such as internal stress, porosity, covering force, water permeability, etc. The achromatic force of paint will also change at CPVC. As shown in Figure 4, with the increase of PVC, the achromatic force changes very little before CPVC. Therefore, high quality and low PVC latex paints are not easy to get temperature difference. When the PVC is close to CPVC, if the temperature and other external conditions change, the porosity of the film will increase rapidly. As the covering power increases, the achromatic power increases sharply, the color becomes lighter, and the brightness value L also increases sharply. Therefore, the temperature discoloration usually occurs when PVC is close to or larger than CPVC medium and low grade latex paint. In Fig. 2, the orange ordinary exterior wall coating becomes lighter when the temperature is lower because the PVC of the coating is close to CPVC, the dosage of film-forming additives is less, the film-forming efficiency is reduced, the CPVC is reduced, the PVC of the coating is higher than CPVC, the porosity in the film is greatly increased, the achromatic power is increased and the color becomes lighter.
Generally speaking, PVC is only applicable to the common coating system with smaller scale and more than 400 mesh. The term PVC is generally not appropriate for putty with coarse filler and for true stone paint with colored sand, but this does not prevent us from understanding and explaining the principle of the covering force of the film in ordinary latex paint due to light scattering.