JEC COMPOSITES MAGAZINE - Issue #131 - Novembre/December 2019 - 65

Drilling and trimming CFRP materials using a solid lubricant film

Tab.1: Surface free energy of CFRP before and after removing SLE®
Sample

Water (°)

Surface free
energy (mN/m)

"Fresh" CFRP
(IPA cleaned)

68 ±1

~ 41 ±1

CFRP after SLE® removal
(uncleaned)

82 ±1

~ 30 ±1

After SLE® removal next to drilling
hole (uncleaned)

78 ±1

~ 30 ±1

CFRP after wiping with IPA tissue

79 ±1

~ 35 ±1

The SLE® material was provided by MGC in the FS version, offering
the strongest possible adhesion by the tape during the evaluation. The
mechanical cutting, drilling and trimming steps were achieved with and
without an SLE® sheet attached to the surface of the CFRP plate. The
CFRP plate was then cut into smaller segments (samples, see Figure 1)
for further analysis via wetting behaviour, IR and XPS analysis.
Surface analysis
The samples were produced and SLE® sheets were attached, before drilling various numbers of holes and trimming edges over a length of several
metres. After preparing smaller samples out of the larger original samples, the SLE® sheet was removed right before further surface analysis.
The wetting behaviour results in Table 1 show a difference before and
after using SLE®. Water more easily wets the surface after cleaning with
isopropanol (IPA) (by wiping with a tissue) than after simply removing
the SLE® sheet without cleaning. For processing, there is no difference in
behaviour if a hole is drilled or an edge is cut. Simple cleaning by wiping
once with IPA is not sufficient for total removal, as Table 1 clearly indicates, but a cleaning effect is also obtainable.
For a deeper understanding, infrared (IR) spectroscopy in ATR mode
was used on the resin surface of the CFRP part with and without SLE®
(adhesive side). Figure 2 shows an example of the samples used for the
IR evaluation (point of measurement indicated by a red dot on the
CFRP). An area close to the trimming line was chosen to find out if the
mechanical stress and/or heat induced by the mechanical processing
step impact the residues on the surface of the CFRP.

Fig. 2: CFRP sample after drilling tests
with SLE®, after removal of SLE®.
Position of IR measurement is marked
by a red dot.

A comparison of the surface of a clean (fresh after production) CFRP
with a surface after mechanical processing with SLE® is presented in
Figure 3. The spectra look quite similar, except for the peak at around
1,700-1,750 cm-1, which is only present on the CFRP surface after removal of the SLE®. A direct comparison of the IR spectra of the adhesive
from the SLE® and the CFRP surface after SLE® removal made the origin
of the residue quite clear.

Fig. 3: Comparison of cleaned pure CFRP (blue) with CFRP next to trimming edge
after SLE® removal (dark red). An additional absorption peak is visible at ~1,7001,750 cm-1, indicating a slight adhesive tape residue remaining on the CFRP after
SLE® removal

Without a cleaning step, there is either a thin layer of residues or an island-like structure of residues originating from the adhesive tape. But
as the SLE® does not need to be placed on the whole surface, cleaning
areas should be areal limited. When wiping the surface with an IPA impregnated tissue, the spectrum shows a less intense peak but still some
presumable residues - so a more intense cleaning procedure with e.g.
a different cleaning solvent or a more intense wiping would be needed.
Nevertheless, no clear visible residues are identified on the surface after
SLE® removal. To evaluate the situation in more detail, an X-ray photospectroscopy analysis was carried out on the same type of samples. Table
2 shows the results of this evaluation, focusing on the main atomic elements on each surface in order to evaluate the degree of contamination
of the surface after using SLE®.
Tab.2: XPS results of CFRP surfaces before and after use of
SLE® (with and without mechanical processing/cleaning)
Sample

C
(at %)

O
(at %)

N
(at %)

Sum of
other
elements

Pure CFRP -
IPA cleaned

72.1

21.4

4.5

2.0

Adhesive side of
SLE® (FS)

80.0

19.3

0.2

0.5

CFRP surface after
SLE® removal

78.3

19.0

2.0

0.7

CFRP surface after
SLE® removal -
IPA cleaned

77.5

18.5

3.0

1.0

CFRP next to
drilling hole after
SLE® removal

76.9

19.6

2.9

0.6

CFRP next to
drilling hole after
SLE® removal -
IPA cleaned

77.4

18.0

3.4

1.2

Due to the composition of the epoxy resin matrices, nitrogen (from the
resin/curing compound in the epoxy system) can be taken as a suitable
marking element among the elements measured for the level of cleanliness of the surfaces. As there is only one marking element, the provability of the statement is limited. But it can be stated that no other significant
measurable element (XPS measurement significantly higher than 1%) is

N°131 November - December 2019

/ jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #131 - Novembre/December 2019

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