Proceedings of the 4th Annual GRASP Symposium, Wichita State University, 2008
Evaluation of the Cure State and Water Content at
Carbon Fiber containing Composite Surfaces
Rama Gandikota*, Irish Alcalen, Sarah Hall, William T.K.Stevenson
Department of Chemistry, Fairmount College of Liberal Arts and Sciences
Abstract A technique called ‘Near Infrared diffuse reflectance spectroscopy’ is able to monitor the moisture content in resin
rich fiber reinforced composite surfaces and the cure state of resin at the surface of a resin rich fiber reinforced composite.
Measurement of water in composites made from 934 resin and T300 fibers was addressed using both normalized absorption
spectroscopy and using a “Chemometrics” second derivative partial least squares spectrum analysis. (Chemometrics is the
procedure of relating measurements made on a chemical system via application of mathematical or statistical methods). We
will show that interpretation of a diffuse reflectance near IR spectrum is more complex than interpretation of a transmission
near IR Spectrum, with the result that a partial least squares (Chemometrics) analysis gives better results than a
straightforward normalized Beer type plot. Calibration curves have been produced to relate diffuse reflectance near IR
spectra to water content for uptake and desorption of water in medium and high performance epoxy resins, high
performance adhesives, and carbon fiber reinforced composites. Calibration curves have also been produced to relate the
near IR diffuse reflectance spectrum to cure state in high performance adhesives and carbon fiber reinforced composites.
1.Introduction
Composite materials used in the aerospace industry need to be tested for extent of cure and water
content. While there are numerous methods employed to determine moisture content and degree of cure in
polymeric materials, they have been limited by the inconvenience of sample preparation and the invasive nature
of the procedure. It is vital that a technique for monitoring water content and cure state in polymers and
composite materials be developed without resorting to destructive and time consuming methods that cannot be
used for on-site analysis.
The reliability of near-infrared spectroscopy as a rapid, nondestructive and expedient method has been
boosted by the introduction of chemometric data evaluation techniques, diffuse reflectance measurements, and
light-fiber optics. It is a low-cost method that requires no sample preparation and with the advent of light-fiber
cables, it is now possible to take spectral data without physically removing parts from a structure or samples
from a processing line.
2. Experiment, Results, Discussion and Significance
Second derivatives of the near IR spectra were obtained to eliminate baseline deviations and then
subjected to partial least squares (PLS) analysis to build a calibration curve.
Two goals were achieved with the help of the calibration curves i.e, primarily water content in epoxy
resins for water uptake and desorption in medium and high performance resins and in high performance
adhesives and carbon fiber reinforced composites and secondly the degree of cure in high performance epoxy
adhesives and high performance carbon fiber reinforced composites.
Water in composites gives rise to an IR peak at 5215 cm-1 (Figure 1). This peak is free from
interference from resin hydroxyls and provides a quantitative measure of water. Figure 2 shows the calibration
curve obtained from partial least squares (PLS) analysis of spectral data. Actual percentage of water values are
acquired from agravimetric method.
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Proceedings of the 4th Annual GRASP Symposium, Wichita State University, 2008
1.00
0.90
Chemometrics Calibration Curve for Water uptake in
MY720/DDS resin
0.80
6
0.70
5
Calculated
0.60
0.50
0.40
0.30
2
R = 0.9919
4
3
2
1
0.20
0
0.10
0
0.00
10000
1
2
3
4
5
6
Actual
9000
8000
7000
6000
5000
4000
Wavenumbers(cm-1)
Figure 1. water absorption peak at 5215 cm-1
Figure 2. chemometric calibration curve for
water uptake of MY720-DDS resin
Epoxy groups produce a strong and well defined combination band at 4530 cm-1 as shown in Figure 3.
The calibration curve in Figure 4 are derived from PLS analysis of near IR spectra. The actual values of
degree of cure are determined from differential scanning calorimetry
C uring FM 377U adhesive
1.00
0.80
Chemometrics Calibration Curve for cure of
FM377U adhesive
e
c 0.60
n
a
b
r
o 0.40
s
b
A
0.20
0.00
5000
Calculated % cure
1.2
uncured
41% cure
98% cure
4800
4600
4400
4200
W avenum bers (cm -1 )
1
2
R = 0.987
0.8
0.6
0.4
0.2
0
4000
0
0.2
0.4
0.6
0.8
1
1.2
Actual % cure
Figure 3. epoxide peak at 4530 cm-1
Figure 4. chemometric calibration curve for cure of
FM377U adhesive
Both calibration curves (water content and degree of cure) show a good correlation between actual and
calculated values as indicated by the R2 value. These can then be an effective quantitative technique to
determined water content and cure state in composite materials
3. Conclusion
Diffuse reflectance near infrared spectrometry was shown to effectively quantify water content and
degree of cure in polymeric and composite materials without the hassles of sample preparation and time
consumption. Furthermore because the method is nondestructive in nature it can be applied to on site analysis of
parts such as in the aviation industry where these materials are mostly used.
4. Acknowledgements
Professor William Stevenson, Rama Gandikota, Irish Alcalen, Sarah Hall
This work is funded by Federal Aviation Administration
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