The color art of pearlescent pigments and coatings

1 Since soldanush, a German paint scientist, first applied mica titanium pearlescent pigment to the metal flash car finish in 1978, it has greatly promoted the progress and development of the world automotive coatings and coating technology In recent years, more and more attention has been paid to the application of high decorative pearlescent coatings made of pearlescent pigments Especially for the car with high decorative color finish, can create a new color art effect At present, new varieties of cars of various brands painted with various kinds of pearlescent paints are constantly appearing on the exhibition stand of international automobile and paint exhibition, giving the majority of automobile lovers and consumers a fresh feeling From the aesthetic point of view, it is no exaggeration to call mica titanium pearlescent coating "the art of modern coating fine chemical industry" and "a revolution of car coating" On the basis of summing up the previous work and our own practice, this paper discusses how to create a brand-new artistic effect of paint color by using pearlescent pigments 2 the structure and types of pearlescent pigment the pearlescent pigment is a series of mica titanium pearlescent pigment, also known as "Rainbow pearlescent pigment" This kind of pigment has no color itself The color it displays is the same as the color produced by the natural rainbow It is a typical pseudo color It is the result of multiple reflection and refraction of the incident light by the pigment sheet On the basis of making silver white pearlescent pigment, the magic color pearlescent pigment is obtained by re coating, adjusting and controlling the layer and thickness of titanium dioxide polycrystalline film on mica sheet At present, the domestic and imported pearlescent pigments can be divided into five kinds: golden yellow, golden violet, aubergine purple, sapphire blue and olive green according to the pseudo color phase displayed At the same time, with the increase of the thickness of titania polycrystalline film on mica sheet, a series of transition phase pearlescent pigments may be produced between these main pseudo color phases However, due to the difficulty in controlling the reaction parameters and ensuring the geometric thickness of titanium dioxide polycrystal film in the manufacture of this kind of pearlescent pigment with transition color phase, the reproducibility is poor, and the visual sense is poor, so the manufacturer seldom goes on the market as a product The relationship between the optical thickness and the geometric thickness of titanium dioxide coated on mica sheet is as follows: a the geometric thickness = the optical thickness / refractive index; B the refractive index of titanium dioxide: anatase type is 2.52; rutile type is 2.71; 3 The optical properties of the pearlescent pigments are mainly shown in the following five aspects: 3.1 Pearlescent effect pearlescent effect refers to the soft pearl like color Pearl color is the result of multiple reflection and transmission of incident light by titania polycrystalline film coated on mica substrate For pearlescent pigments with small particle size, the titanium dioxide polycrystalline film coated on the surface of mica flakes is divided into many micro layers like the internal structure of natural pearls, so that when the natural light hits the surface, it will present a soft color similar to silk, which is called "pearlescent" This optical property is called "pearlescent effect" Pearlescent effect is the basis of creating the artistic color of pearl luster 3.2 in the 1920s and 1930s, a decorative metal pigment appeared in the industry It is to achieve decorative purposes by virtue of the specular reflection of light on the metal pigment sheet Under the irradiation of natural light, aluminum flashes blue light, copper (brass) flashes yellow light, stainless steel flashes white light, and zinc copper alloy flashes strong golden light This kind of metallic surface's specular reflection of light on the human eye is called "metallic flash effect", and mica titanium pearlescent pigment just has this characteristic A series of metal colors can be obtained by coating pure natural mother flakes with titanium dioxide and other metal oxides or treating them with colorants For example, mica titanium pearlescent pigment coated with titanium dioxide alone can show strong silver white metallic luster; silver pearlescent pigment can be colored with carbon black or calcined in reducing atmosphere to form titanium modified oxide, which can show It shows the lead metallic luster in deep tone, the silver white pearlescent pigment in chrome oxide flashes the bronze luster, and the Yellow pearlescent pigment in iron oxide shows the strong and realistic gold luster 3.3 the angle flash effect is due to the fact that the mica titanium pearlescent pigment is composed of a transparent layer of metal oxide coated on the surface of the transparent mica sheet, whose refractive index is much higher than that of the base mica When the light refracts and reflects at the transparent interface with different refractive index, and when the light partially absorbs and penetrates, the interference phenomenon of light will inevitably occur among the parallel reflected light, which will produce the illusion color For the observer, when the line of sight is at the reflection angle of the light, he can see the strongest interference color, while when the line of sight deviates from the reflection angle, he can only see pearl white or other colors The phenomenon that different interference colors are seen according to the different viewing angles of the observer is called "visual flash effect" 3.4 color transfer effect the continuous coating made of the pearl pigment with interference color can display two different colors at the same time, which is called "color transfer effect" For example: the color of a car painted with pearlescent paint prepared with a magic color pearlescent pigment will change with the curvature of the car body The color transfer effect will change from blue to orange and from yellow to purple That is to say, the reflection color of the pearlescent pigment changes to its complementary color It is because of the unique color transfer effect of mica titanium pearlescent pigment that it is possible for people to design and create various wonderful and dreamlike decorative effects according to different needs 3.5 additional color effect mica titanium pearlescent pigment can match transparent organic and inorganic pigments, or directly use these pigments to color pearlescent pigments, resulting in more attractive colors For example, copper phthalocyanine blue and green, transparent iron oxide red mixed with the same color of the magic pearl pigment will not weaken its color, but will enhance it This is the unique "additional color effect" of pearlescent pigments, also known as "hyperchromic effect" We know that one of the biggest disadvantages of conventional metallic pigments, such as aluminum, is that once it is mixed with resin, the saturation of its color will be greatly reduced, so that the color becomes dark and dim However, the color of pearlescent paint or coating after construction will become more bright and bright 4 Standard base paint - pearlescent varnish, topcoat system 4.1 pearlescent varnish Here, pearlescent varnish refers to a kind of surface paint which is made of various transparent resins without adding any physical pigments or coloring pigments It is just mixed with pearlescent pigments and made into a similar varnish for light cover The pearlescent pigments contained in varnish are very few, usually only accounting for 0.5% - 1% or less of the quality of resin base material The reason why the formula with low color to base ratio is adopted is to reduce its covering power as much as possible, to make the color of the primer exposed through the thin surface coating, or to interfere with the color light of the primer, so as to produce new color effects In addition to the solid adhesion with the base material, the formula used for the base color paint should also be matched with the varnish to avoid undercut and other paint diseases The varnish shall adopt low solid content and add leveling agent to ensure the directional arrangement of flake pearlescent pigments in the varnish Due to the low viscosity of varnish, a small amount of thickener, such as polyethylene wax paste, may be added during the construction of "sagging" For example: for room temperature curing, the pearlescent varnish formula of two-component polyurethane with - oh and - NCO cross-linking film can also be made into baking type heat curing cross-linking film-forming single-component paint The color phase of the primer can be selected freely according to the needs, but it requires high covering power; it has good rheological property during construction; it has good interlayer adhesion with pearlescent varnish; and it is not dissolved by the strong solvent in the varnish during the "wet touch" construction It is worth pointing out that the pearlescent coating made by using the pearlescent pigment matched with the conventional base color paint is extremely bright in color No matter its hue, lightness or color saturation, it is impossible and incomparable to use conventional color matching Pearlescent varnish is usually used in the double coating system of base coat and finish coat It can play a dual role of decoration and finishing at the same time 4.2 color topcoat pearlescent topcoat refers to a kind of topcoat which mixes resin base material with pearlescent pigment after being colored with transparent organic or inorganic pigment The hue of the coloring pigment can be the same as the interference hue of the magic pearlescent pigment, or it can be two different hues The pearlescent paint, which is made up of pearlescent pigments and transparent organic or inorganic pigments, can produce distinct "color transfer effect" For example, the pearlescent top coat made of the resin colored with phthalocyanine green and then added with the olive green pearlescent pigment has a very attractive green tone The film formed by applying the top coat on the conventional black primer has a positive viewing angle of dark green, while a bright olive green pearly light is displayed The pearlescent top coat, which is made by the combination of sapphire blue pearlescent pigment and phthalocyanine blue, is applied on the conventional white primer Its positive angle of view is milky white, while the grazing angle of view shows the sky blue pearlescent similar to the ultramarine color The following points must be paid attention to in the color matching of pearlescent finish: first, pearlescent pigment should be compatible with transparent pigment; remember not to mix with opaque pigment, such as titanium white, chromium pigment, cadmium pigment, iron oxide pigment and metal pigment; second, the color phase cannot complement each other; third, two different kinds of pearlescent pigments should not be used at the same time A higher ratio of color to base should be used to prepare pearlescent finish Generally, the mass ratio of pearlescent pigment in resin matrix should be about 6% but not more than 8%, otherwise, it will affect the directional arrangement of pigment flakes in the film and reduce its pearlescent effect Pearlescent topcoat is generally used in the three-layer coating system of base coat, top coat and finishing coat 5 Color pearlescent art coating, also known as pearlescent cloud coating or pearlescent dreamlike coating, is the latest generation of decorative art coating, which has been widely used in the external coating of racing vehicles, rowing, aerial ropeway vehicles, leisure and entertainment facilities The surface tension theory, solution volatilization gradient theory and solution swelling theory in modern physical chemistry of coatings are fully used in the illusory pearlescent art coatings The paint defects such as floating color, shrinkage cavity, undercutting, blooming and color penetration, etc produced during the coating process are utilized, and the illusory pearlescent pigments are used as the effect pigments The color transition of the pattern pattern made of this kind of magic pearlescent art coating is very natural and lifelike It can not only make natural marble pattern or cloud pattern according to the needs, but also make abstract art pattern with multiple light color effects From the construction point of view, the magic pearlescent art coating is usually composed of different colors of base coating, middle coating and surface coating There are also several layers and several colors of midcoat The principle and method of producing pattern are as follows 5.1 add surfactant to reduce the surface tension of the surface coating to the minimum When the middle coating is not completely dry, spray the surface coating on the middle coating with a large diameter spray gun nozzle into a cloud.