Thermochromic paint: composition, characteristics, application

What kind of paint is this?

One of the main characteristics of thermochromic paint is that it changes color when exposed to heat or cooling. The idea is that the pigment reacts to the ambient temperature. The sensitivity threshold of a substance mainly depends on the area in which it is used. Most often, this ink is used for printing labels and producing packaging. Moreover, depending on the current temperature, the image will either disappear or appear.

Hydrochromic paint

Hydroenamel changes color when water comes into contact with it, which is logical to assume from the name. The composition of this paint and varnish material includes microgranules, which, when dry, give the paint its normal color. However, when exposed to water, the solid color turns into transparent, and then, as if by magic, the color of the paint applied underneath, under the hydroenamel, will appear underneath.

Hydrochromic enamel does not contain toxic substances and is safe for human health and the environment.

This option for painting a car gives less room for imagination and activity than painting using thermochromic enamels, which will be discussed below. However, some car owners, not without success, resort to this method of painting - hydroenamel, as an option, is applied to certain areas of the body, masking patterns that will become visible when water gets on the car.

Characteristics and composition of thermochromic paint

The paint is made from powder of special thermochromic microcapsules. They are round in shape and range in size from three to ten micrometers. The color pigments contained in the paint can be presented either in the form of liquid crystals or in the form of leuco dyes. The reactive element itself can be mixed with various solutions. For example, with acrylic, rubber or oil-based paints. The proportion of the active substance, as a rule, ranges from five to thirty percent of the total mass of the finished product. This figure depends on the required result.

Types of paint

There are two main types of heat-sensitive pigment:

1. Reversible paints. They acquire a different color at a certain temperature. When it changes in the opposite direction, the pigment returns to its original shade. That is, this is a reversible color transformation.

2. Irreversible paints. They do not have such a feature. When the temperature changes again, the original shade is not restored. That is, the pigment can only transform color once.

The most popular, of course, are reversible thermochromic paints. They are most often used in medicine to control the sterilization of surgical instruments. Additionally, phosphorescent pigment can be added to the paint.

Usage

Basically, such paint is applied to textiles or ceramics, less often to plastic and glass items, and very rarely to paper. Recently, it has also become possible to apply thermochromic paint to cars. This not only improves the appearance of the car, but also reduces the heating of the metal on hot summer days. In addition, this is an ideal option for those who like a dark car color, but do not want the interior to overheat in the summer.

The most popular option using thermochromic paint is the design on ceramic and glass mugs. Almost every man at least once in his life was given such a dish where, when pouring hot drinks, part of the image disappeared or appeared. This development has also found its application in clothing. In addition to interesting decorative and gift options, such things can warn of freezing or increased body temperature. Similar patterns are also applied to workwear.

Thermochromic paint has also found its application in the production of goods for children. For example, there are warning signs on dishes and bath toys that tell you what the temperature of food or water is. Many drinks that are meant to be enjoyed chilled are also marked with this ink. They can be applied both to bottles and cans themselves, and to labels.

Another advantage that thermochromic paint has is price. It is quite low, taking into account the properties of this material (1,500 rubles for a 25-gram jar, which lasts for a long time). Such solutions attract customers and are an excellent advertising ploy. Fashion magazines and advertising literature also often use paint that changes color at different temperatures. Another area of ​​application of thermochromic dyes is the creation of protective elements on medicines and cosmetics.

Types of Heat Sensitive Pigments

Based on the way they change color, pigments that react to temperature can be divided into three types:

1. Initially invisible. Usually, when heated to 50-60 degrees, they turn a certain color. For example, black, red, blue or green. After cooling, the pigment becomes colorless again.
2. Initially visible. When a certain temperature is reached (from 7 to 60 degrees), the color becomes transparent. That is, it is not visible, but after that the pigment comes back. Typically, such heat-sensitive inks are used for offset and screen printing.
3. Multi-colored pigment. One shade changes to another and back again. Such coloring agents are used when an encrypted message or image needs to be applied to the packaging. It only appears when heated or cooled.

Types of thermochromic paints

One of the components of temperature-sensitive paint is a thermochromic pigment, which reacts to temperature changes. In other words, when such paint comes into contact with a cold or hot object, its color undergoes a change (possibly complete discoloration). The pigments of this paint are liquid crystals that are located in microscopic capsules, provoking a color change that affects the entire painted object. The shell of such microcapsules ensures the preservation of the crystals in a liquid state, so this type of coloring matter can be easily mixed with other types of paint (for example, acrylic paint), with transparent varnishes, printing inks, rubber, plastic or resin, to obtain the desired properties of the painted material. surfaces.

Temperature-sensitive paint has the following types:

Reversible (returnable) paint

If the surface on which the reversible paint is applied does not absorb well, then the paint (80%) is diluted with varnish (20%) on a water-dispersion basis or with another acrylic polymer emulsion (this emulsion consists of polyacrylics and polymethacrylics and is the cheapest of the polymers ) white. The emulsion will act as a connecting element between water and the coloring pigment and will fix the coloring pigment on the surface to be painted. It is made from water without using harmful chemicals. The coloring particles do not dissolve completely, but remain in a suspended state. Because of this, after painting, the water will evaporate from the surface being painted, but the dyes will remain, resulting in a thin film of polymers. In a few minutes it will dry, and if you apply a stream of hot air, the drying process will significantly speed up.

Due to the fact that the paint layer with a thermochromic effect does not have mechanical strength and chemical resistance, ultraviolet varnish is applied on top of it. This will make the coating of this paint last longer.

Irreversible (non-returnable) paint

Does not restore color after the end of the influence of temperature (from 120 degrees Celsius to 130 degrees Celsius) on the painted object. It is used as an indicator for packaging material for food products if they are subjected to heat treatment in this packaging material. The color changes from black to red when heated, and when cooled, the reverse transition from red to black no longer occurs.

Impact of temperature

Paints that are classified as returnable paints return their pigment to its original state after the end of heat or cold. Moreover, each type of enamel has its own characteristic temperature activation. The permissible spread is from - 15 to + 70 degrees Celsius.

Let's consider which temperature regime is suitable for certain cases:

• Up to +20 degrees. Used for applying paint to containers intended for soft drinks.
• From +29 to +31 degrees. In this temperature range there are surfaces that change color under the influence of body temperature (in contact with the skin). This effect is widely used for advertising purposes, in magazines, on booklets or T-shirts.
• More than +43 degrees. This temperature is used for products that will interact with high temperatures. Most often, printing is done on mugs and other utensils for hot drinks. In this case, the color changing effect serves both a decorative and warning function.

The highest exposure temperature can be 280 degrees Celsius. But experts recommend treating surfaces at lower temperatures (about 230 degrees). This is especially necessary if heat-sensitive materials such as textiles or plastic are used.

Properties of thermochromic paints

The thermochromic type of paint and varnish products does not harm human health (they do not contain toxic substances, radiation or other dangerous elements). These paints can be mixed with each other or with other dyes.

Paint particles range in size from 1 to 10 microns, have a temperature resistance of 230 degrees Celsius to 280 degrees Celsius, and their specific gravity is 0.26 grams per cubic centimeter.

Paint that is sensitive to temperature changes is also sensitive to the effects of ultraviolet rays. Because of this, it cannot be stored in direct sunlight (it will lose its properties in just 7 days).

Color palette

The shade range that is used for heat-sensitive pigments is quite wide. The most popular colors are: red, blue, pink, cyan, yellow, green, orange, purple, black and brown. If different types of substances are used, the pigment changes not into one, but into several colors depending on the intensity of exposure to temperature.

Scope of application

There are several main practical tasks for paint that changes color depending on temperature:

• Inscriptions and pictures that appear during cooling or heating may indicate a violation of storage conditions during prolonged exposure to temperature.
• The heat-sensitive pigment is a way to protect against counterfeiting. For example, a hidden word appears on a document passed through a photocopier.
• A certain color applied to the label can be an indicator of the optimal temperature for consuming drinks and other food products.

Pigments that react to cold or heat are most popular in the food industry. An image covered with such paint is placed on the labels. It informs you whether the product has reached the required temperature while in the oven or refrigerator. Thermochromic enamel is also used by manufacturers of alcoholic beverages and beer products. Labels, bottles, stickers and other attributes indicate whether the drink is cool enough. Thermochromic paint has also been used in other industries. It is used to make ceramic dishes (glasses, cups, plates). The pigment is used in various types of plastic, silicone rubber and other materials for extrusion, injection molding, flexography, silk-screen printing, screen and offset printing.

Areas of use

Returnable thermochromic paints are used more widely than their “disposable” counterparts. These materials have gained great popularity among car owners who want to make their car original in its external design.

Covering for cars

Heat-sensitive paint is a godsend for those who like to experiment and take a creative approach to car care. Everyone can create a new interesting image for their iron horse with their own hands, because working with paint that changes color is not difficult. It can even be applied with a regular paint brush or roller, although the best option for painting a car body is, of course, a paint sprayer.

Thermochromic material can not only be a highlight of the decor, but also have an important practical function: if, when heated, the car’s coating begins to turn white or another light shade, then in hot weather the body will be able to reflect the sun’s rays and the surface of the car will overheat less.

To create a complex visual effect, you can use the following technique: paint the car in several layers of thermal paint, using compositions with different temperature thresholds. Drawings made using a stencil or applied by hand (if you have the makings of an artist) will help add “magic.” A car, for the design of which heat-sensitive paints were expertly used, simply cannot go unnoticed in the stream of other cars!

While you rejoice at the opportunity to decorate your vehicle, you should still know that thermochromic paint also has some disadvantages:

• low light resistance: in order to protect the car body coating from the destructive effects of ultraviolet radiation, you will have to apply a layer of special varnish and equip the parking area with a canopy (the best option is a garage);
• in case of mechanical damage, a complete repainting of the car will be required;
• difficulties when registering a car that does not have a permanent color;
• heat-sensitive paint is an expensive material.

Dishes that change color

A tea or coffee mug, on the surface of which a funny inscription or design appears when a hot drink gets into it, is a good memorable gift. An appetizer dish with an emerging pattern is an interesting detail in table setting. And a variety of baby dishes that give a visual signal when porridge or milk is too hot is a useful thing in the everyday life of young mothers.

Important: thermochromic paints do not contain toxic substances, and dishes painted using these materials are safe for health.

Cloth

The textile industry also uses compounds that change color depending on temperature. So, a plain T-shirt worn on the body can surprise you with the fashionable print that appears, and a stylish pattern or label will appear on your jeans.

Souvenirs and decorative elements

In this industry, there is an unusually wide scope for the use of thermochromic materials: Christmas tree decorations and garlands, other holiday paraphernalia, original lamps and candlesticks, key rings, gift stationery, etc. The great thing is that you can do and paint a lot of things yourself, for example, draw a picture or create a panel with a “secret”.

Printed products

Business cards that “come to life” at the touch of warm hands, advertising brochures or magazines promoting perfume (rub the page!), children’s picture books, postcards - all this is often produced using heat-sensitive compounds, fortunately their color palette is quite rich.

In general, anyone can find original uses for these unusual materials in everyday life on their own, using imagination and a little effort.

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Paint with reflective elements

Application of heat-sensitive enamel

The coloring pigment can be applied to various surfaces in various ways: using a brush, sponge, roller or spray gun. That is, the principles of application are no different from coating with conventional enamel. The highest quality results with minimal product consumption and uniform coverage can only be achieved when working with a sprayer. However, this method requires some skills and is most often used by professionals.

As a rule, 65 milliliters of thermochromic enamel is enough for about a square meter of surface. But the final consumption will depend on the required color saturation. If the pigmented product is applied to non-absorbent materials (for example, for printing on mugs), then it is recommended to dilute it with water-dispersion varnish or any acrylic emulsion. In this case, after this the paint must be given a few minutes to dry. You can also reduce the waiting time by using a jet of hot air.

It should be noted that thermal paint has a weak level of UV protection. Therefore, it is recommended to additionally use varnish, which will protect the coating and increase the service life of the painted layers.

Sheetfed printing is also worth mentioning. Since such pigments have low intensity, they should be applied to a good layer of paint. To increase the thickness of the substrate, the image is printed in several passes. Neutral white media are most often used. They allow you to achieve maximum color contrast. For this reason, it is not recommended to use paper with a grayish or yellowish undertone for offset thermochromic printing.

Thermochromic paint for cars

The thermochromic paint is mainly used in the textile industry and in the ceramics industry. Sometimes this paint is applied to objects made of plastic or glass and extremely rarely to paper products. The printing process with this paint is quite easy and cheap, but the result is good. And an additional protective layer will make the design reliable and durable.

Currently, thermochromic material is used to paint cars, which not only makes the car more beautiful, but also reduces the level of heating of the metal structures of the car in hot weather (especially important for owners of dark-colored cars).

Most often, this paint is applied to mugs made of glass and ceramics (they change color (a pattern appears) when a hot drink gets into them). On jars and bottles with chilled drinks, such paint is applied to the labels or to the container itself. When applied to fabric, in addition to the beautiful appearance of clothing and an original gift, the thermochromic property of the paint signals that a person is cold or has a fever. A pattern is also applied to workwear using this paint. It is also used in children's products (indicators on bathing toys and children's dishes). Protective elements are applied to the packaging of medicinal and cosmetic products using non-returnable thermochromic substances.

This type of paint is inexpensive, given its properties (about 1,500 rubles for a 25 gram jar, it can be used for a long time).

Thus, we can conclude that: paints with a thermochromic effect based on acrylic emulsion are non-toxic, economical and are needed not only for decoration, but also as a temperature indicator. They are used in various fields. This type of paint has proven itself well on surfaces with poor absorption, and for reliability, such a surface after painting is additionally coated with ultraviolet varnish.

Table. Palette of thermochromic paints for cars.

Lightfastness of thermal paint

It should be noted that thermochromic enamels are extremely sensitive to ultraviolet radiation. Practice shows that due to sunlight, the product loses all its properties in just a week. Moreover, the paint is negatively affected by external factors. For these reasons, experts recommend applying a layer of protective UV varnish over the thermochromic pigment (especially for outdoor use). This will significantly extend the life of the product.

Storing paint that changes color

For the reasons described above, it is recommended to store thermal paint away from direct sunlight. Otherwise, it will lose its ability to change color. The ideal place for storage would be a dark and dry room. It is also worth considering that the shelf life of high-quality heat-sensitive enamel is only two months. After this, the paint begins to transform into a gel-like substance, which is no longer suitable for its intended use.

Despite its interesting and unusual properties, thermochromic paint has not become widespread. One of the main reasons for this is the rather high price and specificity of use. In some cases, this is a complicated application technology. We can only hope that in the future the situation will change and unique dyes will become more accessible to the population.