Adhesion to Aluminium of PUR Coating for LED encapsulation

Only a protection against "environmental influences" and disturbing electrical currents will allow a broad use of mobile electronic devices and sensors even under extreme conditions.

Anbieter zum Thema

SPEA GmbH Systeme f. professionelle Elektronik und Automation

MEV Elektronik Service GmbH

Shanghai Yongming Electronic Co., Ltd.

Casting compounds based on polyurethane resins are distinguished, on the one hand, by their low exothermic reaction when curing and the low reaction shrinkage related to this, and by their moderate elasticity modules on the other. The latter are a key size for the safe operation of electronic assemblies under changing temperature conditions. Due to this, PUR casting compounds enjoy a constantly expanding market presence when it comes to encapsulating electronic assemblies.

With LED technology constantly developing, a new promising field of application has been created for which PUR casting compounds are well-predestined. LEDs that were, at an early stage, mainly used in the fabrication of display elements such as status indicators, are nowadays not only more and more applied as lighting units in automotive engineering, but also tend to displace conventional light sources in the area of general indoor and outdoor lighting.

Thanks to the pointed encapsulation of complete LED circuits, PUR casting compounds, in view of a permanent resistance of the LED against environmental influences, are capable of performing protective functions that go far beyond a single light emitting diode encapsulation. Special PUR casting compounds developed for this application can, in particular, fulfill the optical requirements such as high transparency along with a fully colourless encapsulation that, in addition, ensure the optical properties under the influence of moisture and sunlight, thus not yellowing and/or chalking.

In architecture, most light strips used for the lighting technology consist of a plastics or aluminium U-profile with embedded LEDs where PUR casting compounds enclose complete LED circuits without significantly affecting their optical properties. In this context, aluminium is constantly gaining importance on account of its low density and the high elasticity module, being a very stretchy and easily processable metal.

PUR casting compounds must exhibit good adhesion

In co-operation with the Faculty of Chemistry of the Niederrhein University of Applied Sciences, the conditions of a good adhesion of 2-pack PUR casting compounds on aluminium alloy were examined as part of a bachelor thesis in the labs of Lackwerke Peter. Given the fact that the light emitting diode circuits are fully enclosed in the casting compound, the latter should not weaken or alter the luminescence or colour of the LED.

The adhesion of a coating material is a property that depends on cohesion, for example. Internal stress may be due to various causes - one may stem from reaction shrinkage - but they can also originate from ageing or degradation processes. Furthermore, diffusion processes and accumulations of the diffunding material on the interface may lead to a change in adhesion.

The wet grip is of major importance for the level of reliability

If water is the diffunding material, one would speak of wet grip. The wet grip can be determined by methods similar to those applied for adhesion in a normal climate, provided that the method used does not have an influence on the moisture level of the film. Especially in case of material combinations which are exposed to moisture at varying temperatures over long periods of time, the wet grip is of major importance for reliability.

The adhesion – meaning the grip of two materials – is an effective variable in molecular dimensions. The molecular adhesion is not directly measurable in general; therefore it is determined as an integral value through an area-related evaluation. For this purpose, a set of interacting forces is always determined along with a more or less unknown load distribution. Any deformation in the coating material and/or the substrate is likely to affect the distribution of loads and thus partially the result.

The direction of the load, either in parallel or vertically from the surface, has an influence on the result. The adhesive strength, being a measure of the resistance according to DIN EN ISO 55945 that is opposed by the coating to a mechanical separation from the substrate due to its grip, is used as a measurand to be determined through experiments. For examining the adhesion, different measuring principles are available that can be divided into three categories:

• sability testing

• direct measurement and test methods

• indirect test methods

How to evaluate the adhesive strength

A test procedure based on the Z 050 tensile test machine by the German company Zwick and Roell was developed, in accordance with the pull-off test described in DIN EN ISO 4624. This procedure is also referred to as „butt-joint test“. Within this test, PUR casting compounds were casted on aluminium sheet in constant layer thicknesses, followed by the separating of a pre-defined area of casting compound from the surrounding coating material. This may produce various fracture points, due to the fact that the fracture will occur in the weakest point of the system.

Any conclusions on the adhesive strength between the casting compound and the aluminium sheet can only be drawn after an adhesion fracture has actually occurred. In all other cases, adhesion can only be defined by a value exceeding the one received. The pull-off strength required for calculating the adhesion (formula) is equivalent to the maximum load value in the diagram created for the respective measurement.

σ: adhesive strength [MPa]

F: pull-off strength [N]

A: pull-off area [mm²]

To begin with, the influence of the binding agent on the adhesion to pure aluminium substrates was determined by modifying the composition of the binding agent - based on various polyols and polyisocyanate classes. This revealed no systematic relationship between the chemical structure of the binding agent components and the adhesion. However, it was possible to improve the adhesion to the aluminium substantially through the choice of the binding agent. The adhesion values reached in these tests varied between 0.5 MPa and 15MPa.

With an adhesion of approx. 15 MPa, an outstanding result for casting compounds was finally achieved. For corrosion protection in structural steel engineering, butt-joint values of less than 1 MPa are considered a poor result while values of between 5 and 15MPa are midrange.

Sources

[1] Suppa, M: Schutzlacke für elektronische Baugruppen- Anwendungsgebiete- Anforderungsprofile- Merkmale- Verarbeitung, Stünings Medien GmbH, Krefeld, 2010

[2] DIN EN ISO 55945: 2007- 03, Beschichtungsstoffe und Beschichtungen- Ergänzende Begriffe zu DIN EN ISO 4618

[3] DIN EN ISO 4624:2002, Abreißversuch zur Beurteilung der Haftfestigkeit;

Deutsche Fassung

[4] DIN EN ISO 2409: 2007, Beschichtungsstoffe- GItterschnittprüfung

[5] Funke, W. / Fischer, E: Farbe und Lack 95 (1989) 403

* * Stefanie Schäfer Beng and Dr. Manfred Suppa ... are experts at Lackwerke Peters GmbH + Co KG in Kempen, Germany

(ID:29650030)