ABOUT POLYMER SUBLIMATION DECORATION

The modern world is created from various materials, including glass, metals and alloys, moreover, they are one of the main links in modern building structures. At the same time, materials are used both in a natural color palette and in a man-made one, the task of which is to harmonize the aesthetic and functional qualities of the object environment in order to form a beneficial effect on a person.

The concept of “aesthetic qualities” implies that the object in question has individual originality, unusualness and, most importantly, has visual harmony, assessed through the visual perception of objects, which in turn occurs through an analysis of the characteristics of their surface, in particular, its texture and color.

So the main task of decorating is to achieve a harmonious combination of the material and technical qualities of the object with the principles of aesthetic value. Its successful solution is directly related to the constant development and complication of artistic techniques used in design, as well as the use of progressive innovative technologies for their implementation.

Over the past 30 years, numerous studies have been carried out and various methods and technologies for decorating metal products have been developed.

THE MAIN OF THEM:

applying an image by destroying the surface layer (all types of engraving);
applying an image to the metal structure using Metalphoto, Gedacolor, Polychromal technologies;
surface application of an image on metal: silk screen, sublimation printing on metal Grawerton and Colorton;
introduction of an organic dye into the pores of the metal using the technology of metallography;
forming a pattern using sublimation heat transfer technology;
and the most obvious use of different coating techniques.

Each of the technologies is unique and has its own application environment. Therefore, the choice of one or several technologies is primarily determined by specific requirements for the quality of the coating and their operational characteristics.

If we consider aluminum as a building material (window and door, facade, sliding systems, etc.), then it is primarily an alternative to wood. Therefore, when choosing a method of decorating the specified systems, the requirements to ensure maximum visual similarity with large-format wooden elements and to meet the conditions of external use were prioritized.

At first look, these requirements are met by the technology of combined powder coating (the so-called “wet-on-wet” technology).However, there are limitations on the appearance and texture of the drawing and there is practically no repeatability.

As research has shown, the method of polymer decoration using the principle of sublimation heat transfer is not difficult for mass production application and optimally fits into the technological process of powder coating. Enterprises engaged in powder painting can easily implement this technology in their production line, and at the same time diversify their production and increase its profitability.

Sublimation technology is a method of the image transfer.

First, the image in the mirror look is formed with ink on an temporary carrier, as a rule, it is a sublimation film. Then, with the help of temperature and pressure, the ink changes from a solid state to a gaseous state, with the penetration of the colorant into the polymer structure of the specially treated surface of the base.

A prerequisite for transferring the image to the base is the polymer structure of the material itself, or the presence of a polymer coating on its surface.

The name “sublimation heat transfer” implies the use of the physical phenomenon “sublimation”, i.e. a change in the aggregate state of a substance (phase transition of the first kind).

Then the decoration mechanism can be described as follows: First, upon heating, the kinetic energy of the colorant molecules increases. As soon as it becomes greater than the energy of sublimation Eo, which is equal to work, by overcoming the forces of molecular attraction, the particles are able to leave the surface of the colorant.

where: Cc – is a constant, depending on the material of sublimation paper, ink, as well as on the amount and range of dispersion of colorant in their composition;
T – is temperature;
k – is the Boltzmann constant.

The molecules of colorant that make up the sublimation ink have the ability to diffuse into the interior of the structure of certain materials (for example, polyesters).
Therefore, when the molecules, when heated, are “sandwiched” between the base, which is painted with powder coating and sublimation paper, the surface pretreatment of which does not allow the dye to penetrate into its structure, the process of contact transfer of the pattern itself occurs with the preservation of geometry and contrast due to their diffusion.

In this case, the dependence of the diffusion coefficient D0 on temperature

where:
𝑪𝒅 – is a constant depending on the physicochemical characteristics of the polymer substrate;
𝜺𝟎 – is the activation energy.

Formulas (1) and (2) show only the general character of the behavior of the curves of the two processes. The choice of optimal modes, taking into account all the variable factors, is already a task far beyond the scope of our review. Having penetrated into the structure of the polymer, the disordered movement of molecules decreases, since the packing of particles in the polymer is denser than in gases.

In this case, the molecules begin to approach each other and the attractive forces that arise in this case contribute to their ordered packing. As a result, a durable color image that meets high requirements in the field of visual perception is formed inside the structure of the polymer layer, completely protected from external influences, which is practically impossible to achieve using other alternative methods.

THE MAIN STAGES AND DECORATION TECHNOLOGY

Based on the main provisions, the process of thermal transfer sublimation can be divided into the following stages:

creating a polymer coating on the surface of the product;
formation of an image on an temporary carrier;
positioning of the temporary carrier relative to the surface to be decorated;
ensuring a uniform, maximum pressing of the temporary carrier with the surface to be decorated;
thermal exposure with maintaining the specified temperature and pressure over the entire contact surface of the temporary carrier with the product surface;
removal of the temporary carrier;
assessment of the quality of the resulting image.

Now let’s look at it step by step:

a polymer coating on the surface of the product is created according to the standard technology of powder coating;

an image on an temporary carrier, and, as a rule, either transfer paper or a special polymer film is used – it is formed in one of the following ways: inkjet printing, screen printing, offset printing. Until recently, a certain “vacuum” has formed around the technology of sublimation decoration of metal products, in the sense of design activity. And this is a segment for which manufacturers of sublimation films are responsible, to a greater extent. There is a clear distance between the rapidly progressing development of the technical tooling of equipment and the relative “stagnation” in the improvement of approaches and technical means for implementing the design qualities of sublimation transfers. An analysis of the overall product range of the main manufacturers of sublimation films showed that over the past 15 years there has been no intensive development of new technologies. The only manufacturer that we have singled out from the general list is the Italian company MIROGLIO TEXTILE SRL – SUBLITEX, which pays special attention to the search for new trends in the field of creating a product with fundamentally different design quality indicators, due to their enhancement with tactile sensations. Our opinion: the company “MIROGLIO TEXTILE SRL – SUBLITEX”, at the moment, is a leader in innovative developments of transfer printing and this is confirmed by their new product – films for sublimation decoration based on a superposition of sublimation colorant and polymers that form an image in 3D format, the so-called – SUBLITOUCH technology. The decorated surface has both a visual and a tactile 3D effect. This technology is patented and in 2012 recognized as the “Product of the Year”;

positioning of the carrier and clamping, when decorating profiled products, is provided by wrapping and fixing the sublimating film on the part (the “stocking” method), followed by vacuuming the internal space between the film and the part. For flat products, the “membrane press” method is used, when maximum contact between the film and the surface is ensured by creating a vacuum betwen top and lower covers which are made of the thinnest silicone membrane. Vacuuming occurs throughout the entire technological cycle until the product leaves the oven;

the final stage of decoration is the actual transfer of the image. For this purpose, special ovens are used that provide a temperature range from 150°C to 220°C. The duration of exposure is from 1 to 10 minutes;

and the last: an assessment of the quality of decoration.

Since there are no quantitative criteria, a visual analysis of the generated image is performed according to the following parameters:

deviation in color reproduction;
vague contours;
uneven staining;
incomplete transfer of the inks from the temporary carrier.

The practical side of optimizing the characteristics of equipment and conditions for decorating, aimed at improving its quality, assumes that the parameters of all the elements that make up the technological cycle are known with proper accuracy. Along with a fairly complete description of the transfer characteristics of the coating, vacuum, thermal links, there is currently no equally detailed information about the similar characteristics of the temporary image carrier.

Belmar LTD conducts experimental studies to measure the threshold characteristics of sublimation films by single imprinting of a special sinusoidal target, which allows you to determine several points of the threshold contrast curve at once, up to frequencies corresponding to the resolution. Continuing work in this direction, apparently, will make it possible to determine more complete practical recommendations for decorating technology, taking into account the selected criterion for the quality of the output image.

ASSESSMENT OF THE INFLUENCE OF VARIOUS FACTORS ON THE PHYSICO-CHEMICAL CHARACTERISTICS OF COATINGS

Our studies on the evaluation of the quality of the coating, as well as the image obtained during polymer decoration, indicate the complexity and diversity of issues related to the problem of taking into account all patterns.
An aggravating factor is also the almost complete absence in the literature of the necessary theoretical and experimental data. Therefore, individual conclusions and conclusions expressed by the authors follow from logical premises and are a subject for further research.

Firstly polymer decoration, according to the physical principles used, is a synthesis of two technologies – powder polymer coating and sublimation thermal transfer. Each technology has its own criteria for assessing the quality of the output effect. And if there is national standard for the polymer coating – then there are no regulatory documents for decoration.

Question: How to evaluate the influence of various factors on an aluminum window and door profile decorated using sublimation thermal transfer technology?

On the one hand – this is a protective coating, on the other hand, we are faced with the pictorial properties of the formed pattern, that is, the image. In the light of modern ideas, the process of building an image can be described using the apparatus of spatial or frequency characteristics. In this case, a transfer should be considered as an original, that is, a print made on an temporary carrier, and an image is a pattern obtained on a substrate using sublimation thermal transfer technology.
Then the resolution of the decorating setting is determined as the abscissa of the intersection point of the resulting contrast-frequency characteristic (CFC) and the threshold contrast of the visual analyzer. The resulting CFC is the product of the CFC of the constituent links.

In the first approximation, we can distinguish the main elements that determine the visual properties of the decorating system:

polymer base, (powder layer thickness, type of powder, forming temperature, layer texture);
thermal vacuum transfer process (absolute temperature, pressing force, exposure time, heating method, parameter stability);
sublimation film (material, thickness, elasticity characteristics, print quality, specific output of sublimation ink during thermal transfer, ambient temperature and humidity).

Obtaining a mathematical expression for the transfer characteristics of each link, taking into account the influence of various factors, will require a decent amount of physical experiments and, at the same time, there is no full confidence that the obtained empirical dependencies can be effectively used in real working conditions.

Therefore, all this can be considered as recommendations for finding ways to realize the potential of decorating systems. As the experimental results are processed, we will try to inform the reader.

Today, in practice, the assessment of the quality of decoration, taking into account the degree of influence of various factors, the search for ways to eliminate or correct various kinds of distortions, in our opinion, it is advisable to produce in relation to the visual perception of the similarity of the formed image and the “primary”.

If you need to obtain any quantitative results, you can use the method of statistical processing of expert estimates.

At the same time, we do not deny other possible approaches.