PRODUCTION OF DISPERSION PAINTS
One of the most important ingredients in the dye dispersion formulation is the dispersive binder. The type and amount of dispersion used affects the ability of the coat and to resist the impact of the surrounding environment, including scouring and scrubbing. At the same time, it significantly influences the strength of the adhesion to the substrate, the resistance to UV radiation, the absorption and the permeability of the vapor, the lifetime of the coatings and its covering ability. One of the dispersion binders used to make high-quality dispersion coatings is the vinyl acetate dispersion. Such paints are generally dilutable with water, non toxic, environmentally and hygienically safe, free of organic solvents.
Dispersed paints generally
The coatings have two basic functions: aesthetic and protective. The aesthetic function of the coating is evident from the fact that the coating determines the appearance of the structure. The protective function is related to the surface protection of the masonry against physical, chemical and biological corrosion. The specific conditions in which the coating is to be applied determine the dominating function of the coating. The difficulty of the application divides coatings into two groups - indoor and outdoor coatings.
Indoor coatings are characterized by their application in areas that are not exposed to weather. While the aesthetic function of the coatings would seem to dominate indoors, there is a wide range of applications where their protective function also plays an important role. These are, for example, coatings in public areas (bus and train stations, office vestibules, restaurants, etc.), storage areas, production halls, shops, laundry facilities etc.
The most important properties of indoor paints are resistance to abrasion and adhesion to the substrate. Further, the optical properties (gloss) and application properties are evaluated. An important criterion for color quality is the minimum temperature at which paint is applied. It is generally required that the coating is applied at 5 ° C and allowed to dry at this temperature during the first 24 hours of maturation, so it can provide a continuous coating without visible faults or flaws.
Compared to indoor coatings, the outdoor surfaces are exposed to weather conditions and the demands on them are of course higher. Increased claims are reflected in the increased number of quality indicators tested by the Testing Authorities as well as by stricter requirements for their limit values.
An important function of the exterior paint, in addition to the aesthetic, is the masonry protection against moisture. Accumulation of moisture in the masonry is an undesirable phenomenon as it causes deterioration of its mechanical properties, increased thermal losses from the building and accelerates corrosion. Exterior coatings are used to solve the problem of moisture coming from rainwater and the interior. The two parameters determine their protective effect. The first is the ability to prevent the entering of flowing precipitation into the masonry and the second ability to allow diffusion of water vapor from the masonry to the surrounding area.
The exterior coating is exposed to weather conditions such as high and low temperature rotation, frost, sunlight, humidity, acid rain, smog, dust drops and soot. In an environment favorable to the growth of molds, mushrooms and algae, the coating should also resist these microorganisms. In addition, it is exposed to unintentional or even deliberate harm by people. A good exterior coating should retain its aesthetic and protective properties for a long period of time.
Generally speaking, the action of the dispersion binder greatly influences precisely these qualitative colour indicators. Therefore, choosing a suitable binder is a challenging and responsible task. Of course, by adding other components to the coating, it is also possible to adjust those parameters that are slightly below the required values. In this case, it is usually a price-quality compromise.
Properties of dispersion binders and their influence on the properties of building paints
Minimum film forming temperature (MFFT)
The minimum film forming temperature is the lowest temperature at which a clear continuous film is still formed from the dispersion. Under this temperature there is insufficient coalescence of the dispersed particles, the film can either not be formed, or it is cloudy and cracked. MFFT specifies the lowest temperature at which paint can be applied. The requirements for exterior coatings determine a minimum temperature of 5 ° C. Therefore, it is required that the MFFT dispersant binder be in the range of 0-5 ° C. The MFFT dispersant binder is closely related to the glass transition temperature (Tg). The glass transition temperature of the polymer depends on the monomers from which the polymer is prepared. It is characteristic of each homopolymer. It may be affected by the composition of the macromolecule or the addition of an external plasticizer - a low-molecular substance (plasticizer, coalescent). The glass transition temperature of the copolymers depends on the mutual ratio of the comonomer units that the copolymer forms. Using the dependence between the glazing temperature control and MFFT is called internal polymerization. Coalescents act temporarily and gradually evaporate after formation of the film. On contrary, plasticizers should affect quality of a film permanently and their evaporation or migration are not wanted.
Although the MFFT is related to the glassing temperature, it is often not identified with it.
The particle size of the dispersion
The particle size of the dispersion and its distribution depend on the polymerization conditions in which the dispersion was prepared. The choice of type and amount of emulsifiers and protective colloids used as well as the way of their addition to the reaction mixture during the preparation of the dispersion is of decisive importance. The advantage of larger particle dispersions is that the rheological properties of the dispersions and the coatings prepared therefrom are more suitable. The advantage of bonding with small particles is mainly the better ability of the small particles to penetrate into the thinned layers of the old coating and thus to improve the adhesion of the new coating to the surface.
The alkaline resistance of the dispersion is determined by the chemical nature of the polymer that forms it. It depends on the reactivity of the functional groups on the polymer chain in the hydrolysis reaction. Reactivity of acetyl groups in homopolymeric polyvinyl acetate is very high. Alkaline hydrolysis degrades the binder in the paint and causes damage due to degraded adhesion and abrasion resistance. Alkaline hydrolysis affects coatings that are in contact with moisture and alkaline surfaces at the same time. Homopolymeric polyvinyl acetate dispersions therefore cannot be used for exterior coatings, whereas in interiors where they are not exposed to moisture, their quality is completely sufficient. Dispersions consisting of vinyl acetate-acrylate copolymers or polyacrylate dispersions have alkali resistance better than polyvinyl acetate. The rate of reaction is about 10 times lower compared to homopolymers of vinyl acetate. Significant improvement in alkaline resistance is achieved by copolymerization of vinyl acetate with vinyl ester of versatile acid. Units containing branched bulky alkyl groups are incorporated into the polymer. These groups form a spherical barrier that protects the ester group from the anions of OH and thereby prevents hydrolysis.
With the increasing content of vinyl ester of the versatile acid in the copolymer, the hydrolysis rate constant can drop to a level 1000 times less than the polyvinyl acetate hydrolysis rate constant, i.e. to the level of the acrylate copolymers.
When comparing the alkaline resistance of selected dispersions, it has been found that resistance increases in order of:
Duvilax BD-20 <Duvilax KA-11 <Duvilax VV <Duvilax KA-9/50
Resistance to weathering (climatic degaradation)
Weathering of polymers is a complex process involving photochemical and oxidative reactions. There is a gradual destruction of the polymers, which results in a change in their colour and deterioration of the mechanical properties. Degradation of the polymer, which is a binder in paint, causes the chalking, deterioration of the adhesion, peeling and cracking of the coating as well as the change of its colour shade. Weather degradation is mainly exposed to polymers that contain functional groups in its structure, accelerating some of its partial processes, Benzene nucleus, styrene etc. Dispersions prepared on the basis of homopolymers and copolymers of vinyl acetate and polyacrylates do not include groups that accelerate weathering. It should be noted that the resistance of coating to weathering depends not only on the binder but also on the overall formulation of the coating (pigment concentration, type and amount of fillers and pigments etc).
Absorptivity and vapor permeability
The most important but not the only parameter influencing the absorption of water into a film prepared from a polymeric dispersion is the chemical composition of the polymer that forms the dispersion particles.
Polyvinyl acetate is a hydrophilic polymer, and it can be expected that dispersion films formed by polymers containing predominantly vinyl acetate units will absorb a large amount of water. Hydrophobic versate groups reduce absorption in virtually the same way as styrene segments in the styrene-acrylate dispersion.
Water vapor permeability as well as absorption depends mainly on the structure of the polymer. High permeability is characteristic for polymers with hydrophilic bodies as well as for polymers with hydrophobic, strongly branched bodies. The bulkier the substituent is, the greater the penetration of the polymer.
The presence of hydrophilic emulsifiers increases the vapor permeability of dispersion films. Their elution therefore causes a decrease in this value and the steady-state values measured after five dipping cycles and drying were about 15% lower than the initial.
The absorption and permeability of paints for vapours is decisively controlled by the volume concentration of pigments and fillers (OKP). Both variables depend on the extent to which the polymeric binder fills the gaps between the mineral components of the coating. At a critical volume concentration of pigments (KOKP), the binder content is such that it is just enough to fill all the gaps. If the OKP is higher than KOKP, there are loose gaps in the paint, and with the increasing OKP, the coating becomes porous. With the growth of OKP, it is therefore logical to expect sudden changes in absorbency, vapor permeability, and other coating properties.
Diffuse resistance, which is inversely proportional to the permeability of a pair, drops sharply at near KOKP values; on the contrary, the absorption begins to rise under these conditions. In the vicinity of KOKP, conditions can be found where the absorption and diffusion resistance remain low. At this optimal value of OKP, it is important to compare the absorbency and the vapor permeability of paints formulated with different dispersion binders. The choice of dispersant binder with low absorbency and high vapor permeability and therefore low diffusion resistance will favorably affect these properties in coatings with an optimal value of OKP.
Regarding formulations for interior paints, the amount of dispersion used ranges from 5 to 10%. This portion provides sufficient resistance to abrasion. By increasing the proportion to 15 - 20% it is possible to achieve the washability of the coating, respectively its plastifiability (structural coatings).
For façade - exterior coatings, the proportion of dispersion usually ranges between 20 and 30%.
Wood coatings require high adhesion and high resistance to mechanical stress. These properties will ensure the content of the dispersion in the formulation of approx. 50%.
The content of the dispersant component in the formulation of the universal coatings is strictly derived from the quality of the resulting coating, and therefore its determination is the result of the laboratories development of the coating.