JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 44

bonding

Laser pre-treatment of CFRP
for adhesive bonding
Pre-treatment of CFRP is essential for structural or ageing-resistant adhesive joints. Laser
technology makes it possible to remove release
agents/contaminants without damaging the
joints, thus utilizing the full potential of adhesive
joints while overcoming the disadvantages of
conventional pre-treatments.

By
Dr. rer. nat Fabian Fischer, Head of the Adhesive Bonding
and Composite Technologies Department,
Dipl.-Ing. Stefan Kreling,
Prof. Dr.-Ing. Klaus Dilger,
Director of the Institute for Joining and Welding,
Institute for Joining and Welding, TU Braunschweig

ightweight construction plays a more and more important
part in the course of the ever-increasing efforts to reduce the CO2 emissions of airplanes and vehicles. In the
past few years, fibre-reinforced plastics have become the main
focus of interest. In order to make the best possible use of the
potential of these lightweight materials, the bonding techniques
and the corresponding design of the bonding areas need to be
carefully selected.
The most frequently used bonding technique – riveting – has
the essential disadvantage that the fibres carrying the main load
are disrupted by the required bores, and stress peaks occur at
the margins of the joint elements. Additionally, the net weight
of the rivets makes lightweight constructions with several
rows of rivets significantly heavier. Thus, a great potential lies
in adhesive bonding. Due to the achievable flat force input,

which is done without damaging the fibres, stress peaks can
be avoided, thus enabling a clearly improved utilization of the
material. The pre-treatment of the adhesive surfaces is essential
for a structural and ageing-resistant adhesive joint. Because of
the release agent residues frequently remaining on the surfaces
from the manufacturing process, this point requires a high level
of attention, especially with fibre-reinforced materials.

Conventional procedures
Manual abrading is a well-known method for the adhesive
bonding pre-treatment of fibre-reinforced polymers. This
implies mechanical removal of the top matrix layer and thus of
the contaminants present on this layer. The essential disadvantages of this process are, for one thing, the execution by hand
involving high process and labour costs, and, secondly, the
comparatively low process control and reproducibility. Fibre
damage cannot be excluded in the case of manual abrading.
Conventional particle beam procedures can also be used to
remove the top matrix layer mechanically, in which case some
damage of the fibres can, again, not be excluded. Because of the
formation of dust, an enclosure and subsequent cleaning are
necessary. Depending on the beam pressure and the particles
used, the particles can be included in the surface and thus cannot be removed by simple cleaning procedures such as brushing
or wet cleaning.
Special fabrics, so-called peel plies, are frequently used for
adhesive pre-treatment, especially in the aircraft industry. These
fabrics are inserted into the surface during component production and removed mechanically just before adhesive bonding.
Thus, a surface with a highly defined topography is formed, and
the surface contaminants are removed together with the fabric
and the top matrix layer. One disadvantage of this method is
that the use of an additional semi-finished product is required.
Also, residues of peel ply and release agents may be present on
the fabric, as reported in various publications. These can lead to
a deterioration of the bonding strength. Another considerable
disadvantage of this method is the fact that it is not suitable for
repair, as peel ply cannot be subsequently added to the bonding
surface.

Laser pre-treatment
For these reasons, the use of modern laser technology for the
adhesive bonding pre-treatment of fibre-reinforced polymers
was thoroughly investigated under the MoLaB project of the
Central Innovation Programme SME (ZIM) of the German

44 jec composites magazine / No78 January - February 2013

JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013

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