2015/09/29

Adherence in plastic pieces coatings

Category: Plastics in the automotive and transport industries

The automotive sector is especially demanding when talking about quality and resistance of the coating applied to plastic pieces, both indoors and outdoors of the car. Among the amount of tests that can be made to coatings in the laboratory in order to check their suitability, one of the most used ones is the coating adherence to the plastic substrate. It is essential that the coating is perfectly adhered to the plastic surface to fulfil all the functions already mentioned. A bad adhesion to the substrate or between the different layers that form the coating can cause the lacing and the coating detachment, thus affecting the aesthetic appearance and the substrate’s protection. In order to overcome this problem, some plastic surface activation techniques are used. These are flame treatment and corona and plasma treatment, among others.

One of the most common tests to evaluate the coating adherence to the substrate is the grating cutting. This test consists of carrying out ―and reaching the substrate ― a grid on the coating with parallel and perpendicular ―or at an angle, depending on the normative ― incisions, by means of a blade and cutting guides. The number of incisions and the space between them is established depending on the film thickness and the testing normative applied. Then, the grid is planned and by means of a magnifier we observe if there is a detachment. If you want to check it deeply, you have to cover the grid with a self-adhesive tape and pull it after in order to detach it with a fast movement. The level of adherence is checked by comparing the results with a standard classification.

Moreover, adherence is one of the characteristics that is usually evaluated to check the coating resistance after being subjected the piece to degradation or ageing controlled conditions (extreme temperature values, temperature and humidity cycles, luminous radiation…). A usual procedure is to do an incision or a St Andrew’s cross in the coating before the ageing process. Being finished this stage, we check the capacity of the coating adhesion after being overcome these extreme conditions by observing the amount of detached coating when an adhesive tape and other kind of pressure staff is applied on the incision.

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