JEC COMPOSITES MAGAZINE - Issue #87 - March 2014 Special JEC Europe - 125

Handheld FTIR analyzers for analysis of composites
CFRP part, inevitably some of the
release agent is left on the part and
this must be treated and the surface
activated to achieve optimal bonding.
Surface treatment and activation can
be conducted using radio frequency
generated plasma. If the CFRP part
is either undertreated or over-treated
during the plasma cleaning process,
the result will be a sub-optimal surface for bonding. Undertreatment
does not cause enough changes in
the release agent while overtreatment
will change the release agent but
additionally causes thermal damage
to the CFRP. Undertreatment causes
the more severe adhesive failure
with greatly reduced strength whereas over-treatment results in CFRP
damage, but only moderate adhesive
peel strength decrease. Therefore a
fine balance of treatment is required
to ensure that the surface is properly
prepared and the degree of thermal
damage in the CFRP is minimized.
The effectiveness of the plasma
treatment is highly dependent on
the speed at which the plasma nozzle
traverses the CFRP surface as well
as the distance the nozzle is from
the surface. Small changes in either
variable can lead to a significant diminishment in the effectiveness of the
treatment. The intention of this study
is to demonstrate that a handheld
FTIR (figure 1) can non-destructively
detect and measure chemical changes
in CFRP surfaces induced by plasma

Fig. 1: Agilent 4100 ExoScan FTIR equipped
with diffuse reflectance sample interface for
the analysis of composites and polymers.

Based on the characteristic mid-infrared spectrum of materials, FTIR spectroscopy offers unique
information about the chemical structure of composites and polymers, providing non-destructive
molecular analysis of these materials. Since chemical changes may precede physical changes,
FTIR is complementary to other, more classical, NDT methodologies.
In 2008, the first handheld FTIR system, the 4100 ExoScan , became commercially available
and a collaborative project funded by a Federal Aviation Administration research contract enabled
a detailed study of thermal exposure in composites used in commercial aircraft. Information
gained from that project, as well as a later project funded by ENCOMB for the study of composite
bonding, is leading to the development of next generation handheld FTIRs optimized for at-site
measurement of composite and polymers.

treatment and
subsequently provide an objective
measurement of the
effectiveness of the
plasma treatment.

FTIR analysis
of plasma
treated CFRP
Changes in the
FTIR spectra were
measured as a
function of the
Fig. 2: Plasma nozzle traverses CFRP coupon at a fixed speed and contitemperature flux
nually decreasing distance from surface resulting in an increasing temperature. As a result, the smallest plasma nozzle gap from the CFRP surface
experienced by
results in the highest temperature conditions. Adhesive peel strength
the CFRP coupon
measurements were made with Instron 5566 Universal Testing Machine on
a variety of perpendicularly lathed sub-samples that were adhered to one
surface as a result
another and cured before testing.
of the plasma
treatment. These spectral changes
after removal from the mold. The
resulted from the plasma nozzle
plasma nozzle traversed each CRFP
traversing the coupon surface at a
coupon at a constant speed of 6 cm/
constant velocity, while the nozzle
sec and the full sweep of the CFRP
height was varied. The distance
took approximately 4.5 sec. The
traveled was directly linked to the
distance of the nozzle from the
nozzle height, and therefore with the
coupon surface was pre-programmed
temperature flux to which the surface
to change from 20.5 mm to 5 mm
was exposed at any given point. In
(figure 2).The largest nozzle distance/
this manner, over-treated, optimally
height (greatest gap size) resulted
treated and under-treated portions of
in a condition in which the surface
the CFRP surface were evaluated by
was undertreated and remained just
FTIR analysis. CFRP surfaces treated
under 100oC in temperature. The
with either PDMS or Wax based
smallest gap sizes, for which the
release agents were tested using this
nozzle was very close to the surface,
protocol. The effectiveness of plasma
caused an over-treated condition,
treatment was independently assessed
and the transient temperature at the
using standard adhesive peel-strength
surface exceeded 260oC. Twenty
testing (from which the G1c values
discrete infrared measurements were
are calculated) of CFRP-to-CFRP
made along the coupon surface and
bonded samples.
these spectra could be correlated to
the different temperature conditions
CFRP samples were plasma treated
experienced at the coupon surface
No87 March 2014

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JEC COMPOSITES MAGAZINE - Issue #87 - March 2014 Special JEC Europe

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