Powder or liquid coating: which to choose?
Powder or liquid coating:
which to choose?
25 November 2021
The first approach to the world of industrial coating also involves understanding one of the biggest differences in the industry: what are the differences between powder and liquid coating? When to choose one or the other? We will try to clarify this by illustrating the characteristics and peculiarities of the two coating processes.
Coating an article means finishing it, improving it and protecting it. This is why choosing the most suitable coating cycle is essential to extend its life and guarantee the qualities and specifications that make it suitable for the activity for which it was made.
Leaving aside the question of e-coating or galvanisation, let’s address one of the main choices offered by the sector: when to rely on powder coating and when to rely on liquid coating? Why powder coating for certain articles and liquid coating for others?
Over the next few lines, we will try to answer these and other questions, clarifying the characteristics and differences between these two surface treatment technologies.
The characteristics of powder coating
Powder coating is an established standard in industrial coating. To formulate these coatings, binders are created through an extrusion process in which solid components are mixed and melted together, into which pigments are then dispersed and finally mixed. The mixture is then pulverised so that the paint has an appropriate particle size distribution curve.
In the vast majority of cases, with powder coating, the substrate is coated using manual and/or automatic electrostatic guns, the ultimate goal being a continuous, homogeneous film.
Once coated, the articles enter the baking oven, where the paint is polymerised and permanently adhered to the substrate for a relatively short time at a temperature of 180 to 200 Celsius degrees.
Both thermosetting resins and plastics can be used for this type of coating, although the former is the most widely used and popular.
There are several classes of thermosetting resins used for powder coating, which can be categorised into:
- UV radiation cure
We mentioned earlier that this is an electrostatic process, but how does it work?
The guns that direct the flow of paint particles also charge the powder electrostatically via a generator connected to the sprayer. The powder emitted by the guns will attach itself to the articles by means of the potential difference between the “target” and the powder. In this case, the article to be treated acts as the positive pole and the powder as the negative pole.
The characteristics of liquid coating
Liquid coatin is a type of historically well-established surface treatment, which consists of reducing the coating liquid into more or less fine droplets. Various technologies enable this to be done, including pneumatic, compressed air, mixed and electrostatic.
The substrate to be treated is hit by the miniaturised liquid through special guns, typically through an external conductor (compressed air). The role of the gun is precisely to break up the jet of paint into very fine droplets and deposit them on the article.
It should be noted that it can be applied to both metal and plastic articles.
As mentioned above, there are various application techniques, which must be evaluated according to the article to be painted. The main ones are:
- High-pressure spraying (airless): with sufficiently high pressure and a small gun nozzle, the paint is atomised. This technique is used for items that do not require an excellent or perfect finish, but focus on the adhesion of the coating to the substrate. When used with precision, due to the high flow rate of the paint used, processing times are speeded up, as a dense paint can be applied in a single coat without the risk of dripping, which is deposited at low speed (i.e. with less bounce). This also means using a thinner paint with less solvent vapor.
- Mixed spraying (airmix): also called mixed-air spraying, it allows industrial coatings to be applied with a minimum of spray mist, while maintaining excellent quality and a normal coating thickness. It is a technique patented under the name Airmix®. Starting with high-pressure spraying, the spray gun was modified, retaining the outlet nozzle but inserting a pair of air-powered ears at the sides.
- Electrostatic spraying: this technique means that the guns used, as well as atomising the paint into tiny droplets, charge the paint electrically and create an electrical force field that causes the paint to adhere to the article being treated. This method reduces the application time, but on the other hand it also involves a greater use of solvents and consequent pollution of the workplace.
Again, after the coating phase, the articles enter the oven: unlike powder coating, the curing temperatures are lower and vary between 60 and 80 Celsius degrees.
Differences, advantages and disadvantages of powder coating and liquid coating
Some substantial differences have already emerged in the following lines, starting with the articles that can be coated: where the liquid coating can be applied to plastic or metal articles, powder coating is reserved for metal articles only. This is because of the temperatures reached by the baking ovens: as we have seen, liquid operates within a range of 60-80 degrees, while powder goes up to 180-200 Celsius degrees.
Let’s take a look at some of the most common advantages and disadvantages of these two surface treatments.
Advantages of powder coating:
- It is more environmentally friendly as it does not use solvents;
- great ease of recovery and reuse of powders that fall and do not adhere to the substrate;
- excellent coverage of the edges of the substrate;
- absence of possible leakage points due to vertical sliding of the coating;
- low operating costs;
- easy control of coating thickness.
Disadvantages of powder coating:
- The colour change is not immediate and has to be changed throughout the production line;
- it can only be applied to metal articles.
As far as the liquid coating is concerned, the pros and cons must be weighed up against the application technique chosen. Let us, therefore, try to address this issue in general terms.
Advantages of liquid coating:
- Historic and robust painting system;
- possibility of retouching, as even a small amount can be achieved;
- usable for jobs requiring little painting;
- the rapidity of execution.
Disadvantages of liquid coating:
- If not executed perfectly, there is a risk of getting drips and bubbles;
- lower costs compared to powder coating;
- use of solvents and the resulting pollution;
- to avoid impurities, the working environment must be perfectly clean.
At the end of our overview, it is difficult to say which of the two industrial coating techniques is the best. Experience has shown that it is best to examine each case, need by need, before deciding on which type of coating to choose and apply.
Some industries prefer one over the other, while others seek a combination of both to create optimum aesthetic and protective effects. The important thing is to carry out preliminary assessments, tests and coating trials so that you know exactly which technique will produce the desired result.
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