3. Lacquer systems and their properties
One- or multi-component lacquers
1k lacquers
One-component systems (1k lacquers) consist of binders, pigments and additives and are either a dispersion in an aqueous solution or dissolved in an organic solvent. As soon as the 1k lacquer is applied, the solvent begins to evaporate, and the binder creates crosslinks which then form a dry film on the substrate. 1k lacquers lack an extra hardener component which thereby distinguishes them from 2-component (2k) lacquers. The difference is mainly in the performance of the dried/cured lacquer where 2k systems generally have better abrasion properties and chemical resistance.
2k lacquers
Two-component systems (2k lacquers) also consist of binders and solvents, but immediately before the application, a hardener is added. As soon as the components are mixed, the hardener reacts with the binder. The binder molecules create crosslinks with the hardener molecules and a molecular network is formed. The process is also called crosslinking and the supply of heat accelerates this process. The process is not reversible and makes 2k paints very stable products.
AMB mainly uses 2k systems that form polyurethane. These have excellent properties in terms of abrasion resistance, chemicals, corrosion, and various weather conditions.
One or more coating layers
Lacquers can be applied in several layers that can have many different functions. In addition to what is described below, there are also special lacquers with other effects that may have a different structure.
The total layer thickness for a three-layer lacquer system is normally at AMB 30–70 µm and in the vast majority of cases these layers are no obstacle to joining or other functions in the final product. In general, however, the smaller a detail, the larger the layer thickness can affect. Especially at edges and holes, the construction sometimes has to be adapted with radii.
1 layer lacquers
Sometimes it is enough for a lacquer to be applied in one layer. This lacquer can be uncoloured, transparent, glossy or matt. The colour of the substrate can then be seen through. However, this type of lacquer is almost always pigmented or contains a soluble dye that gives the lacquer its colour. Such a lacquer can be compared with the lacquers that can be bought in spray cans. However, AMB’s 1-layer lacquers are usually more sophisticated. In our manual lacquering we use 1k lacquers and in our automatic lines of 2k laquers with hardener.
Lacquer systems for metallized surfaces
The high-gloss metal layers that AMB creates through vacuum metallization (PVD: physical vapor deposition) can be used to achieve very fine metallic effects that cannot be achieved with metallic lacquers alone. (See also section 2: Lacquering and metallization in combination.)
For the extremely thin metal layer (<100 nm) to come into its own, a primer which gives a smoother plastic surface, is almost always needed first. The high-gloss metal layer can then be applied on this. The third layer consists of a protective transparent or coloured topcoat. Through this you can create the effect of chrome, nickel, copper or other metallic colours.
Primer
Depending on the choice of material in the substrate, a primer is sometimes needed that will both create a good adhesion to the substrate and at the same time be able to bind to additional layers of paint. The primer can but does not have to be pigmented. However, a pigmentation may sometimes be necessary, among other things to give subsequent layers the right colour.
The layer thickness varies but is approx. 10–15 µm.
Basecoat
The colouring layer is also called base coat. It is applied either to the primer or directly to the substrate and is pigmented or contains a soluble dye in the case of metallized parts. The base coat should then be able to adhere well to the substrate and at the same time allow a topcoat to bond to its surface. The layer thickness depends on the desired colour and effect but is usually 10–15 µm.
Topcoat
The main task of the topcoat is to protect the underlying layers and thus the entire part. The layer is usually slightly thicker than for the underlying layers and for AMB’s applications is usually around 20 µm. The topcoat can also have different gloss values, reaching from ultra-matt to high gloss (piano lacquer). In addition, like all surfaces, it has haptic properties, i.e. it contributes to how the lacquered surface “feels”.
Different properties of lacquer to consider
Colour
The choice of colour for the product can be made in many ways and can be influenced by several factors. It must always be remembered that the geometry and surface structure of the final product can give completely different colour experiences than other reference materials and geometries.
Most lacquers that AMB applies in the automated lines consist of three layers: primer, base coat and top coat. The colour choice then usually refers to the base coat. It is important not only to produce samples with a base coat but also to apply the intended top coat as it affects the effect of the base coat in different ways. The top coat is an extra and protective layer that thus also refracts light and the underlying colour from the base coat can then be experienced differently than without a top coat.
There are various so-called colour fans or colour charts with standardized colours from the most famous colour catalogues and which you can buy. There are also many different colour samples in colour shops, but they have a limited value because they are two-dimensional and printed on at most two types of paper: matte or glossy. The format and the paper as a substrate can give a totally wrong impression if the intended product is three-dimensional, plastic-based and has a different surface finish. In addition, they rarely have the shine you strive for and as haptic (how it feels to touch them) samples they are not good at all. But as inspiration, they can be of great help.
In addition, you can get help in various ways from, for example, the NCS system.
Design agencies are also good at developing suggestions for colours that suit the product. As inspiration, you can also read in our knowledge blog how, over time, colours have been developed for medical technology products regarding the environment in which they are to be used: How to choose colours for medical technology products.
If you have a model (such as an existing product) with a colour that you would like to match or be close to, you can sometimes scan the colour or, which is often a better way, have AMB send it to a suitable supplier who can match the colour for a fee. However, a match cannot always reach all the way.
Preferably, the matching should be done on untreated pre-designed parts in the right material. If this is not possible, such as early in the product development process, you can buy test plates in the intended material, but it may take time to obtain these. Therefore, the matching result is usually delivered on a standard plate that is high-gloss polished and because of that probably provides a different experience than on the intended product. Again, it is therefore important to remember that the geometry and surface finish of the final product can provide very different colour experiences than other reference materials and geometries. The next section on the degree of gloss is also important to consider, as the gloss of a top coat can greatly affect the chosen colour.
Degree of gloss
Lacquers have different gloss values. This is especially true for multilayer lacquers where the outer layer, the top coat, can be anything from very matte to high gloss. The gloss of the top coat also affects how the base coat is experienced. In addition, different degrees of gloss also change the haptic, i.e. how the surface feels.
On some metallic colours in one layer, an extra topcoat can reduce the metallic effect, even though the topcoat may be necessary for the product’s performance. Our recommendation is to make different samples if you are unsure.
A matt lacquer can to a certain extent hide very minor unevenness of the substrate surface, but it should always be assumed that a coating in principle emphasizes unevennesses rather than hides them.
Parts with a very high gloss are extremely sensitive and require very good quality and high purity of the plastic substrate. In addition, these high-gloss lacquered parts often must be cured extra after the normally sufficient curing in the lacquer line so that they are handable. Because this type of lacquer is so sensitive, it also has higher yield losses in relation to duller variants.
Biocompatibility
If a lacquered surface is to have skin contact over a long period of time, the lacquer must be tolerated by the skin and must not emit harmful substances. The biological valuation of medical devices is regulated by ISO 10993
Neither AMB nor the suppliers of the lacquer material can be responsible for this investigation. The lacquers used by AMB and which have direct skin contact over a longer period of time are all approved according to the customers’ tests and standards.
Measurement parameters
The coatings must meet certain specifications. There are many different requirements specifications and AMB follows the paint manufacturers’ general recommendations based on their tests performed according to different standards. AMB therefore normally performs only a few tests and measurements that are sufficient for most customer requirements. If the customer has specific wishes and makes test equipment available, more tests can also be done.
Adhesion
First and foremost, it is important to ensure that the coating adheres to the substrate, i.e. that the adhesion is good. Since a lacquer can consist of different layers, the primer must adhere to the substrate material, but also the subsequent layers with each other.
AMB checks the adhesion through a lattice test that is standardized by Paints and varnishes – Cross-cut test (ISO 2409:2020)
Layer thickness
Some lacquers require a certain layer thickness for the cured lacquer that the lacquer manufacturer recommends for correct performance. Normally, the layer thickness is checked during the development of the lacquering program in our lacquering systems and then the parameters are locked so that the application takes place in the same way through the robots.
The inspection is mainly done by lacquering a plastic film at the same time as the parts and then measuring the various layers in micrometres. Alternatively, and where possible, the substrate is appropriately cut and measurements are made under a microscope.
There are also various devices for layer thickness measurements, but they have their limitations and currently none of these are used at AMB.
It should be remembered, however, that the geometries of the substrate always affect the layer thickness, even though our painting programs have been optimized for as even a coating as possible. It is physically impossible to get a completely even coating on all parts of the surface if it is not completely flat and even then, the layers become a little thicker along the edges.
Layer thickness on metallized surfaces
Normally the metal layer lies between two lacquer layers. The lacquer layers can then be measured, but we cannot measure the metal layer of our decorative and reflective thin film coatings by metallization as these are only a maximum of 100 nm thick. Instead, a visual inspection is made of the surface against a golden/master sample.
Colour measurement
Colours can be measured in different ways and with different devices, but all have certain limitations. The human eye is usually superior to see if a colour from an approved sample (see below) matches the lacquered part. A comparison is then made under standardized lighting conditions in a light cabinet with different lighting sources.
Gloss measurement
Depending on the geometry of the part, the degree of gloss can be measured at different angles in different ways with instruments. However, this is rarely a customer requirement, and the measuring instruments are mostly suitable for relatively large and even surfaces. AMB makes some such measurements directly on the part at different angles where possible or on reference material depending on customer requirements.
Visual assessment against golden/master samples is also common.
Golden or Master Samples
Although AMB documents working methods and parameters for different processes, approved so-called Master or Golden Samples are also needed and used at each production occasion.
The outcome from the first series production is sent to the customer who approves and signs the samples, keeps a few pieces and sends the rest back to AMB so that these can be used as a reference in production.
