While it is easy to achieve high levels of bond strength on certain plastics, such as polyamide, it is by no means so easy with other plastics. In the second part of our series on ”materials that are difficult to bond”, we are looking at LCP and aim to show the products and methods that can be used to achieve durable and reliable bondings.
LCP is a family of high-performance plastics with very good mechanical properties. It is also inherently flame-resistant and remains dimensionally stable at temperatures of up to and beyond 300 °C. They are also extremely well flowable, enabling them to fill injection molds with long, narrow, and complex flow paths with minimal warpage. That is why they are well suited for use in filigree mold components needed for microelectronics.
What Kind of Plastic Is LCP?
The abbreviation LCP does not denote any specific chemical composition in the way that PC stands for polycarbonate. Instead, it stands for 'liquid crystal polymers'. These are polymers with liquid crystalline properties. Certain areas of the material exist in the form of crystalline structures surrounded by amorphous areas.
The mechanical properties of liquid crystal polymer can be adjusted across a relatively broad spectrum by changing the ratio of crystalline and amorphous phases and the level of interlinking. In addition, the shape of these crystalline structures affects the properties of the material. LCP therefore comes in different guises. As a result, the adhesion level achieved by the same adhesive on different grades of LCP such as E130i, E463i, or E525T can vary significantly.
Having said that, LCP is a material difficult to bond. This is due to its chemical structure and to the characteristic 'injection molding skin' that forms on the surface. Another bonding challenge is that LCP is frequently filled to approx. 30 % with minerals or glass fibers, in part to achieve a low coefficient of thermal expansion (CTE of ~12 ppm/K). Since the second component to be joined often has a much higher CTE, the adhesive has to equalize additional tensions.
Low Pressure Plasma as a Magical Invocation
As the initial compression shear strength figures in the diagram illustrate, adhesion to LCP is low in the beginning. This is especially true after climatic storage at higher temperatures and high air humidity. The same diagram also shows that adhesion can be improved significantly through plasma pretreatment. As the results show, LCP is easy to bond after all.
The values remain at a high level even after aging. This improved adhesion is achieved by the plasma pretreatment, which breaks up the surface of LCP and exposes the glass fibers more strongly, as illustrated by these scanning electron microscopy (SEM) micrographs. However, excessive pretreatment can lead to poorer adhesion because the glass fibers are then no longer embedded sufficiently in the LCP matrix. Generally, low pressure plasma is the LCP pretreatment method of choice, and the method gas used (air, oxygen, or nitrogen) does not have any decisive role to play.
Adhesives Suitable for LCP from DELO
Over the last few years, DELO has developed a range of adhesives that demonstrate great performance on LCP. Since most of them are designed for consumer electronics, they not only exhibit good adhesion, but also high light fixation strength, very good drop test and humidity resistance, and are suitable for low temperature curing at 80 °C. Alongside these, there are products for die attach applications that demonstrate good reflow stability.
Here is a selection from DELO’s product portfolio: