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Development of Non-contact NDE Method for Autoclave Cure
Monitoring of Carbon-Phenolic Re-entry Shells of Missiles by On-line
Gas Chromatography Technique
Kammari Veera Brahmam, Vemana Venkateswara Rao
Advanced Systems Laboratory, DRDO, Kanchanbagh, Hyderabad- 500 058, INDIA
[email protected], [email protected]
Abstract
During re-entry phase of the long range missiles, they experience severe
aerodynamic friction and high surface temperatures. In this context, carbon-epoxy shell is
used as internal structural layer and carbon-phenolic shell is used as external layer for
production of re-entry vehicle structure. Autoclave curing is used for production of
external thermal shells of Re-entry vehicle structures of the missiles. Autoclave curing of
phenolic composites involves a complex regime of time – temperature -pressure cycle
where selection of pressure application point is the most important parameter, decides the
quality of the component. Early pressure application with respect the gelation produces
resin starved component, which degrade the thermal performance of the component
whereas late pressure application produces more porosity in the component due to
trapping of volatiles generated due to curing chemical reaction. Production of high resin
content and minimum void distribution is most essential and which is achieved by
analyzing the volatiles evolved during the process.
During curing process, m-phenol and water are the one of the indicators and the
concentrations are determined by suitable detectors as a function of temperature of the
component. Based on the diminishing trend of the curves the gelation region for pressure
application was determined. A new contact NDE method for cure monitoring of carbon-
phenolic composites was developed right from development of concept, realization,
development of proto type system for assessing the advancement of curing reaction and
for determination of gelation region for pressure application. The system contains gas
sample injection port, gas-chromatograph, detector unit, software for signal analysis. The
equipment and experimental details were presented in the paper.
National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014)
Vol.20 No.6 (June 2015) - The e-Journal of Nondestructive Testing - ISSN 1435-4934www.ndt.net/?id=17838
2
Key Words: Gas-chromatography, Carbon-Phenolic Composites, Cure Monitoring
Technique,
1. Introduction
Aerospace structures are made by embedding high strength fibers in a light weight
thermoset plastic by adding hardener or supplying heat energy. In this process, addition
or condensation polymerization type of thermoset resins are used. Epoxy resins used in
structural applications undergo addition polymerization without volatiles evolution and
hence control of curing process is easy. In case of resins undergoing addition
polymerization, the degree of cure is evaluated by conventional methods like differential
scanning calorimetry (DSC) or dielectric methods [1-6]. Carbon-phenolic(C-P)
composites are used as thermal protection layer in aerospace structures as ablative liners
[7-9]. Phenolic resins undergo condensation polymerization and produce water and
methylol phenol (M-phenol) by-products during curing. These by-products come out as
volatiles/vapors during curing process and form porosity in the component. The curing
reaction is a function of temperature, time and pressure. Therefore to control the curing
process the component is cured in autoclave by keeping in vacuum bag. Volatile
management with selection of vacuum levels, rate of heating and gelation region for
pressure application are crucial parameters to minimize the porosity and better
consolidation among the fabric layers. Among the above parameters identification of
gelation region for pressure application is most sensitive parameter which depends on the
advancement of resin/prepreg properties. Due to above criticalities, an on-line cure
monitoring system for selection of on-line pressure application point is most essential.
Cure monitoring is required to track the real-time changes in a chemical reaction
that occurs during advancement of the resin [8]. In case of epoxy resins, during curing
process reactions have been studied by many researchers with the help of (i) modeling the
cure process[2,6] (ii) in-situ monitoring and control of curing during the actual
manufacturing of composite products. Many authors reported the cure monitoring
techniques by measuring the specific property of the material [2-6] by embedding the
3
sensors in the component. But in case of C-P composites, the embedded sensors act as
hot spots, generate high temperature under thermal environment and the structure fails
easily. Therefore a contact and embedding sensor type of cure monitoring method is not
suitable. In view of the above circumstances, the author developed a low cost, non-
contact and non embedding sensor type of cure monitoring technique using on-line gas
chromatograph.
Phenolic resin is a thermoset type of resin with aromatic network and is obtained by
condensation of phenol with formaldehyde as shown in Fig.1. In the first step of curing
reaction, the M-phenol interacts with phenol and forms a polymer chain with
methylene-bridge. In the second step, the M-phenol reacts with M-phenol and forms a
polymer chain with ketone-bridge. Further continuous supply of heat energy forms a
3D-network of cured solid. Therefore the liquid phenolic resin initially in the low
molecular weight monomeric stage undergoes long chain pre-polymer formation and
subsequent gelation (rubbery state) to the final stage of chemical cross linking (solid
glassy state).
Fig.1. Condensation reaction in phenolic resin
In the present technique the evolved gaseous by-products as a mixture (with M-
phenol and water) are injected into the gas chromatograph, separated as individual
components and their concentrations are determined by suitable detectors. The
concentrations of M-phenol and water are monitored as a function of component
temperature by connecting gas chromatograph (G.C) to autoclave facility. Finally based
on the falling trend of M-phenol concentration and post analysis of laminate properties,
the criterion for pressure application is established.
M-phenol
OH
CH2O
H
OH
OH
CH2
CH2OC
H2
OH
OH
+
+
H2O
H2O
OH
HCHO +
Phenol Formaldehyd
e (Methylol –phenol)
(Water
) Cross-linked polymer
Stage- 1
4
2. Experimental
2.1. Sample Preparation
Laminates were made by hand lay-up process with 45 layers and subsequently
cured in autoclave by keeping the component in a vacuum bag. Teflon treated releasing
fabric is used as a separator ply followed by nine layers of bleeder material. The bleeder
material is used to absorb excess resin and to provide path to volatiles at vacuum ports.
The total lay-up was kept in a kapton made vacuum bag and cured in autoclave. The
laminates were cured at different pressure application points based on the falling trend of
M-phenol and the samples are designated as CP-HP-B, CP-1/3-C, CP-1/2-D and CP-3/4-
E.
2.2. Coupling of gas chromatograph equipment with autoclave facility
Fig.2 On-line gas chromatograph coupled with autoclave facility
Suction pump
Air N2 H2
RS 232
Main Vacuum
Pump Gas Hut
Autoclave
5
Chromatography is an analytical technique, which separates the gas mixture in to
individual components to identify and quantify the concentrations [10]. Fig.2 shows the
coupling of on-line gas chromatograph with autoclave facility.
The total experimental setup was developed indigenously and the setup contains four sub
parts as
i). Gas distribution panel: The nitrogen, oxygen and hydrogen gases from different
cylinders were purified by sending through the molecular sieves. Hydrogen and air
mixture is used to produce flame in the flame ionization detector (FID) and where as
nitrogen is used as a carrier gas.
ii). Sample connection line: The gas chromatograph is coupled to autoclave through a
suction pump followed by a gas-sampling valve (GSV). Sample line was maintained at
1400C to avoid condensation of gaseous sample mixture. The gas mixture from vacuum
bag is injected into gas chromatograph for every 4 minutes by operating (GSV) through
electronic timer circuit module and all electronic modules are interfaced with P.C
iii). Gas chromatograph: The gaseous mixture is separated into individual components
using 2 meter length Porapak-Q column. The separated components of M-phenol and
water concentrations are determined at the other end of the column with thermal
conductivity detector (TCD) and flame ionization detectors (FID) respectively.
Chromatograms are obtained at every four minute interval of time and the concentrations
of the M-phenol and water is determined from the area under the peaks.
iv).Electronic data acquisition and Interfacing: The electronic data acquisition module
converts the analog response into digital file format. The digital data is transferred to
computer through RS-232 interfacing cables and stored with .TCD and .FID extension
files. The process parameters are controlled through user-friendly “AUTOCHROWIN”
software. On-line cure monitoring of the process is carried out and the
evolution/concentration curves of M-phenol and water are obtained as a function of
component temperature/time.
6
2.3. Optimization of testing parameters in gas chromatograph
Separation of individual components from the gas mixture depends on the carrier
gas flow rate through the column, column temperature and sensitivity of the detector.
AUTOCHROWIN’ software is used for analysis of the retention times and area under the
peaks in the chromatograms. M/s. National Physical Laboratory (NPL) standards are used
for calibration of water and M-phenol peaks interms of retention time and area of the
peak. The retention times for water and m-phenol are calibrated as 0.4 minutes and 0.8
minutes respectively. The calibration process is carried out in a lab G.C and the same
calibration parameters are implemented for on-line analysis of the gaseous samples. The
following testing parameters are optimized in the calibration process to obtain consistent
and error free results. The area under the peaks in the chromatogram is measured as the
concentration of the constituents.
Oven Temperature : 1500C
Injector Temperature : 1200C
TCD Temperature : 1500C
FID Temperature : 1500C
GSV Temperature : 1400C
Filament current for TCD : 85mA
FID range : 100
Attenuation : 1 dB
Gas flow rates :
Carrier-L-TCD : 1.1 Kg/cm2
Carrier-R-TCD : 1.5 Kg/cm2
Hydrogen : 0.9 kg/cm2
Air (Oxygen) : 1.00 kg/cm2
Retention time for M-phenol : 0.70 min
Retention time for M-phenol : 0.38 min
7
2.4. Chemical analysis of composite samples
The laminates were prepared by varying pressure application point and small samples
from the laminates were subjected to chemical analysis as per the ASTM-D3171
standard for determination of solid resin content, void content and fiber volume fraction.
Resin bleed out volume is measured by weight difference of the laminate at before and
after curing.
3. Results and Discussion
3.1. Role of pressure application point in autoclave curing process
Phenolic resin undergoes condensation reaction, and produces condensation by-
products like M-phenol and water. These by-products along with the residual solvents are
sucked out of the vacuum bagged product by a suitable vacuum pump, while pressure is
applied inside the autoclave for consolidating the layered composite structure. Therefore
a complex temperature-vacuum-pressure regime needs to be carefully selected for
producing composite products of acceptable quality. It is important to note that early
pressure application (before gelation) tends to bleed more resin and forms low resin
content in the component; whereas, late pressure application (after gelation) tends to
generate defects like voids and de-laminations, leading to rejection of expensive
products. Hence an on-line monitoring of the curing reaction and identification of the
correct pressure-application point are of crucial importance.
3.2. Determination of broad gelation region from evolution curves
Fig.3. shows typical volatile evolution curves of m-phenol and water during curing
process. In all the experiments the trend of m-phenol evolution is consistent and
reproducible. Water evolution curves are found inconsistent due to moisture absorption
from ambience and the water by-products produced due to advancement of resin during
storage of prepreg. Hence, based on the trend of m-phenol evolution, the pressure
application criterion is decided.
8
The m-phenol evolution behavior is divided into three parts). I).Increasing
evolution region (AB), ii). Decreasing evolution region (BD) and iii). Constant evolution
region (DE) corresponding to liquid, gelation and solidification states of the resin
respectively, as shown in Fig.4.
20 40 60 80 100 120 140
0
50
100
150
200
250
M-Phenol Peak
Water Peak
Average component temparature(0C)
M-P
he
no
l Pea
k A
rea
(mV
.s)
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
Wa
ter p
ea
k are
a( m
V.s)
Fig.3. Typical volatile evolution curves - without pressure application
At the early stage of curing, the reaction rate of phenol with formaldehyde is slow
at low temperatures due to low M-phenol formation up to the point A. From A to B
region, free phenol reacts with formaldehyde and forms more M-phenol and the M-
phenol concentration increases up to B. In BE region, the M-phenol again reacts with M-
phenol and forms cross-linked polymer and shows a decreasing trend of M-phenol
concentration up to E. In EF region due to formation of three dimensional network of the
polymer, the reaction completes and evolution of M-phenol reaches to a saturation due to
solidification. Therefore a broad gelation region for pressure application is observed from
B to E. The broad gelation region (BE) is further divided in to four sub parts as BC, CD,
DE and EF, to identify the exact pressure application point. Based on the falling trend of
the m-phenol curve the BE region is divided as i). At highest peak (B point) ii). At 1/3
fall of highest peak area (C point) iii). At ½ fall of highest peak area (D point) and iv). At
9
¾ th
fall of highest peak area (E point). To identify the correct pressure application point,
four laminates were produced by applying the pressure at highest peak, 1/3 rd fall of
peak, at ½ fall of peak and at3/4 th fall of peak respectively and the cured laminates are
designated as CP-HP-B, CP-1/3-C, CP-1/2-D and CP-3/4-E. During the curing process
of the four laminates the M-phenol and water evolution curves are plotted as shown in
Figs. 5 and 6. The properties like resin content, void content, fiber volume fraction and
density of the different laminates are compared in the Table.1.
40 60 80 100 120
0
500
1000
1500
2000
2500
3000
3500
M-p
he
no
l P
ea
k A
rea
(mV
.s)
Average component tempearture (0C)
FID
Fig.4. Typical m-phenol evolution curve
A
B
D
E
F
C
20 30 40 50 60 70 80 90 100 110 120 130 140
0
50
100
150
200
250
300
350
400
450
500
550 CP-HP-B
CP-1/3-C
CP-1/2-D
CP-3/4-E
M-P
henol A
rea (
mV
sec)
Avg Component Temperature (0C)
10
Fig.5. M-phenol evolution versus pressure application point curves
3.3. Determination of on-line pressure application criterion - Post properties of
laminates
High resin content and low void content are preferable for better thermal
performance of the ablative components. High resin content in the carbon-phenolic
composites produces more pyrolysis gases and chars yield, which keeps the top surface of
the material cool and protects from the thermal environment [11-12]. Correlation of
laminate properties interms of resin content, void content, fiber volume fraction with
respect the pressure application point have been discussed in the following sections.
Fig.6. Water evolution versus pressure application point curves
3.3.1. Pressure application at highest peak on the curve
Pressure application at B-point (at highest concentration of M-phenol peak) on
the curve indicates starting of gelation region. At this point, the viscosity of the liquid
resin is low and it can flow easily through the reinforcement layers. Therefore due to
20 30 40 50 60 70 80 90 100 110 120 130 140
0
200
400
600
800
1000
1200
Wate
r A
rea (
mV
sec)
Avg Component Temperature (0C)
CP-HP-B
CP-1/3-C
CP-1/2-D
CP-3/4-E
11
more bleed of resin, resin content is low and fiber volume fraction is high. In this stage
the by-products in the vapor form can be removed effectively due to low shearing force
among the trapped gaseous molecules with respect the liquid molecules of the resin and
therefore the volatiles can be removed effectively without trapping in the component.
Hence CP-HP-B laminate possess minimum porosity. At this stage of pressure
application the bleeding of resin is more due to low viscous nature of the resin and
produces more bleed out of the resin compared to other cases of pressure application.
Therefore lowest resin content and minimum porosity is obtained in the samples, which is
also supporting the chemical analysis results of the samples as shown in the Table.1.
3.3.2. Pressure application at 1/3 fall of peak on the curve
Pressure application at C-point (at 1/3 fall of M-phenol concentration) on the
curve indicates increase in resin viscosity due to pre-polymers formation from its
monomeric state. Due to beginning of advancement of resin, the volume of resin bleed
from the component is very low compared to CP-HP-B case. Hence more resin content is
produced in the laminate. In this stage the by-products in the vapor form can be removed
effectively due to low shearing force among the trapped gaseous molecules with respect
the liquid molecules of the resin but slight increase in viscosity of the resin retards the
movement of the volatiles slightly. Therefore it produces minimum porosity. Therefore
high resin content with low porosity is observed in CP-1/3C laminate. Increase in resin
content will decrease the fiber volume fraction and the density of the laminate.
Property CP-HP-B CP-1/3-C CP- 1/2 -D CP- 3/4 -E
Solid resin content (wt%) 28.25 34.37 37.64 39.08
Fiber volume fraction (%) 65.38 55.95 56.02 56.66
Density (gm/cc) 1.50 1.45 1.40 1.41
Void content (%) 0.20 0.60 0.81 1.10
Bleed out ( kg) 1.30 0.72 0.20 0.10
Table.1. Laminate properties with respect to pressure application
12
3.3.3. Pressure application at 1/2 fall of peak on the curve
Pressure application at the point ‘D’ (at 1/2 fall of M-phenol concentration) on the
curve indicates further increase in resin viscosity due to long chain formation from pre-
polymer state of the resin. Due to further advancement of resin, the bleed of resin from
the laminate is relatively very low as compared to CP-1/3-C case. The increase in
viscosity of the resin further retards the movement of the volatiles through the gel stage
of the resin and produces considerable porosity. Therefore high resin content and
considerable porosity is produced in CP-1/2-D laminate.
3.3.4. Pressure application at 3/4th
fall of peak on the curve
Pressure application at the point ‘E’ (at 3/4 fall of M-phenol concentration) on the
curve indicates sudden increase in resin viscosity due to formation of solid state of the
resin from long chain of molecules. Due to formation of three dimensional network of
solid, the m-phenol evolution reaches to a minimum and saturates at above the point’ E’,
which indicates glassy state of the resin. At this point the advancement of resin has been
completed and the bleed of resin from the laminate is lowest compared to other cases. At
this stage the viscosity of the resin is very high and completely retards the movement of
the volatiles through the resin. Therefore the reaction by-products are completely trapped
in the component and produces accumulated porosity (voids) and delaminations.
Therefore high resin content with more porosity and delaminations have been produced
in CP-3/4-E.
Table.1 indicates that the pressure application at highest peak produces low
porosity and low resin content. The pressure application at 3/4th
fall of peak produces
intense defects like voids and delaminations but the resin content is high. Therefore to
obtain high resin content with low porosity, the pressure should be applied at 1/3 fall of
M-phenol concentration on the curve. To obtain high resin content with considerable
porosity, the pressure should be applied at ½ fall of m-phenol concentration curve.
13
4. Conclusions
Cure monitoring of carbon-phenolic laminates was carried out by on-line gas
chromatography technique. The volatile evolution curves of M-phenol and water were
recorded with respect to the component temperature. M-phenol evolution curves are most
consistent and based on the falling trend of the curve the criterion for pressure application
was decided. Low porosity can be obtained in the component by applying pressure at
1/3rd
fall of m-phenol concentration on the curve; whereas, more resin content can be
obtained by applying pressure at ½ fall of M-phenol concentration.
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