Kececi, Erkan

MOISTURE INGRESSION CHARACTERISTICS OF
LAMINATES COMPOSITES WITH VARIOUS BARRIER FILMS

FOR AEROSPACE APPLICATIONS
Erkan Kececi, Naif Alzahrani, Mohammed Alamir, and *Ramazan Asmatulu
Department of Mechanical Engineering

Wichita State University, 1845 Fairmount, Wichita, KS, 67260
*Email: ramazan.asmatulu@wichita.edu
ABSTRACT
Fiber reinforced polymeric composites are widely used in aerospace and wind turbine industries
because of their distinctive advantages compared to metals and alloys and other materials.
Polymeric composites used for aircraft components are usually subjected to wide ranges of
environmental conditions where they can absorb a significant amount of moisture or solvents
from the environment; thus, their chemical, thermal, and mechanical properties and service lives
are considerably reduced. Polymeric coatings are the most common moisture barriers for
aerospace composite components for environmental effects. However, due to the application,
service environment, weight increase, reliability, cost, and safety requirements, barrier film
coatings are chosen as alternative methods in addition to the surface coatings. In this paper,
moisture prevention and absorption characteristics of a variety of hydrophobic thin barrier films,
including polyvinyl fluoride (Tedlar), polyether ether ketones (PEEK), polyimide (Kapton), and
polytetrafluoroethylene (Teflon) were investigated in detail. These hydrophobic films were co-
bonded to pre-impregnated (pre-preg) glass, carbon and Kevlar fiber composite laminates during
the autoclave curing, and then moisture ingression tests were conducted on the composite
coupons to verify the moisture prevention characteristics in DI water. The experimental results
indicated that using the barrier films as the outermost ply on the composite laminates
significantly increased the moisture barrier properties, which can be a drastic improvement for
aerospace and wind turbine applications.
1. INTRODUCTION
1.1 General Background
For a couple of decades, laminate composites have been largely employed by the aerospace and
wind turbine energy companies because of the composites’ higher strength to weight ratio, and
other distinctive advantages, such as low coefficient of thermal expansion (CTE), corrosion
resistance, ease of manufacturing, part consolidation (e.g., ability to fabricate complex parts in
one step), design flexibility (be ability to fabricate different thickness dimensions within the
same structure), fatigue resistance, and high temperature resistance [1]. This trend will be
drastically increasing in the near future as the new applications of composites are extended in
different industries.
Two primary constituents that make up composites are matrix and reinforcement materials.
Copyright 2017. Used by CAMX – The Composites and Advanced Materials Expo.
CAMX Conference Proceedings. Orlando, FL, December 11-14, 2017. CAMX – The Composites and Advanced Materials Expo
Matrix materials are usually used to transfer stress between reinforcements, hold fibers together,
and protect the reinforcements from environmental effects. Most common matrix materials
employed in many industries are thermoset epoxy, polyester, polyimide, and phenolic [1]. On the
other hand, the reinforcements are generally employed to provide high strength and stiffness, and
improve the physical properties of the laminate composites. Most common reinforcement
materials include glass, carbon, aramid, silicon carbide and boron nitride fibers. Combinations of
these fibers and matrix materials can be utilized to create the hybrid composites for different
industrial applications [2].
Generally, laminate composites consisting of various matrix and fibers are subjected to a wide
ranges of environmental effects, such as moisture, high and low temperatures, aggressive
solvents and surfactants, acids and bases, pollutants, skydrol fluid, jet fuel, oxygen, ozone, etc.
Especially, with the absorption of liquid into the composite structures, the polymer matrix can go
through a plastification phase, or physical and chemical aging process as water molecules move
through the matrix. During the transport hydrolysis, water can get into interface between fiber
and matrix, as well as pores and cracks of the composites, and then matrix can be damaged or
degraded by osmotic fissuring [1-3].
1.2 Moisture Diffusion
Moisture diffusion is defined as a transport rate of a liquid through materials’ structure per unit
time (mm2/s) [1]. Moisture diffusivity is related to diffusion of liquid and gas into material which
can be quantified as a transport rate [2-5]. Usually, diffusion process take place faster when there
is enough voids, porosity, cracks, channels, and vacancies within matrix and reinforcements.
Temperature and pressure can accelerate water molecules to diffuse into the composite
structures. Diffusion rates are controlled by the amount of free volume in the laminate structure.
Water molecules can get into the structure in multipole directions (x, y and z) and stay there until
detrimental effects can take place on composite structures. In addition, moisture is attracted to
polar sites of the composites because of the electrostatic interaction and van der Waals attraction
forces for new bonds [6-10]. Figure 1 shows the environmental effects, defects in composite
structures, and damages on the composite laminate structures.
a) b) c)
Figure 1. Images showing a) the environmental effects on substrate, b) defects in composite
structures where water can accumulate, and c) damages on the composite laminate structures.
A number of factors, such as degree of cure, polarity, material properties, surface

hydrophobicity, homogeneity of material, environment and material conditions, pressure,
temperature, relative humidity, thickness, barrier coatings and films, and design factors mainly
affect the moisture absorption rates. For instance, high matrix volume laminates can absorb
more moisture when compared to fiber dominated laminates owing to the higher affinity of resin
to a liquid absorption [1]. Another factor is the composite surface morphology and defects, such
as holes, cracks, voids, and matrix defects that allow moisture to be absorbed easily, which
drastically reduce the physical and chemical properties of the laminate composites. Crosslink
rates can affect the solvent-polymer crazing process and reduce water solubility in the composite
structure. Water absorption is also affected by the bond strength between water molecules and
polymer molecules and geometry (concave and convex shapes) of the composites. It is reported
that unidirectional fiber composites absorb less moisture than woven composites although they
both have the same resin types and volume ratio [1,11-13].
2. EXPERIMENT
2.1 Materials
Polyvinyl fluoride (Tedlar), polyether ether ketone (PEEK) with 12.5 µm and 25 µm thickness,
polyimide (Kapton) and polytetrafluoroethylene (PTFE) Teflon film were purchased from
different suppliers. Pre-impregnated (pre-preg) glass, carbon and Kevlar (aramid) fiber
composites were provided from the local stores. These materials were used in these studies
without any modifications. Table 1 provides the materials used for fabrication of the laminate
composite test panels with and without barrier hydrophobic films [2].
Table 1. Materials used for fabrication of the laminate composite test panels.
Materials Specifications and Manufacturer
Carbon Prepreg Cytec E7K8 PW-3K-193 Epoxy Pre-preg
Aramid® Prepreg Hexcel F161 Epoxy with 285K Aramid Fiber
Glass Prepreg JD Lincoln L552FR Epoxy with 1581 Glass Fiber
Tedlar® DuPont™, 0.02-mm thick (both surfaces plasma treated)
APTIV film , by VICTREX® PEEK™, 0.025-mm thick
PEEK 1
(surface plasma treated)
APTIV film , by VICTREX® PEEK™, 0.0125-mm thick
PEEK 0.5
(surface plasma treated)
Teflon DuPont™, 0.025-mm thick (no surface treatment)
Kapton DuPont™, 0.025-mm thick (both surfaces plasma treated)
2.2 Methods
For comparative testing, three different types of epoxy pre-preg materials, such as carbon, glass,
and Kevlar fibers were used to fabricate composite test panels. Hand cut pre-preg carbon, glass,
and Kevlar composite plies were layed up on an Al tool separately without any vacuum
compaction during the lay-up process. Hydrophobic barrier films were layed-up as the last ply
on the bag side. Test panels were bagged and then cured at 176°C / 586 kPa for 2 hours to make
composite laminates in an autoclave. After the curing, the laminate coupons were cut to extract
test coupons. The edges of the coupons were polished by a finer polisher (32 RMS) to remove
all the cracks, dents and other loose fibers. The polished test coupons were also washed with
distilled water to eliminate all traces of polishing rouge. Tool side and the edges of the
specimens was coated with a thick layer of polyurethane paint to prevent water and ingress
through the unprotected surfaces of the composite panels. Figure 2 shows the test panel
configuration and bagging of a pre-preg laminate composite manufacturing in the autoclave
curing oven.
Figure 2. The test panel configuration and bagging of a pre-preg laminate composite
manufacturing in an autoclave.
In order to determine moisture barrier characteristics of the films, a series of tests were
conducted on test coupons that were subjected to moisture absorption through direct immersion
in DI water. The coupons were weighed to the nearest milligram on an analytical balance with
0.0001 sensitivity. Figure 3 shows the images of the composite panels layed up and covered with
PTFE release films prior to the autoclaving, and then the composite test coupons were extracted
from the cured composites, cut into 2.54x2.54 cm dimensions, and conditioned in DI water for
the moisture ingression tests. The composite coupons were dried in an air circulating oven at 82
± 5°C prior to water immersion tests. The drying process is continued until a constant weight is
achieved or weight difference within 2 consecutive measurements within 2 hr time elapse is less
than 0.01%. Note that the Teflon barrier films on the test coupons were disbanded from the
composite surface prior to the moisture ingression tests, so they were removed from the tests.
a) b)
Figure 3. Images showing a) the composite panels layed up and covered with PTFE release films
prior to the autoclave curing, and b) the composite test coupons were extracted from the cured
composites, cut into 2.54x2.54 cm dimensions, and conditioned in DI water for the moisture
ingression tests.
During the moisture ingression tests, the coupon weight was monitored for a maximum three day
interval until the equilibrium weight was reached. The equilibrium was reached when the
average moisture content of the traveler specimen changed by less than 0.05% for two
consecutive readings within a span of 3 ±0.5 days per Equation 1.
(W i -W i-1 )/W b < 0.0005 (1)
where W i is weight at current time, W i-1 is weight at previous time, and W b is baseline weight
prior to the DI water conditioning [1]. The test coupons were gently wiped with a tissue paper to
remove the surface free water prior to the weighing process. Moisture ingression characteristic
tests were conducted per ASTM D5229 by fully immersing coupons in DI water at 22 ±2°C for
14 and 29 days [11]. The duration of the DI water exposure was determined based on the
equilibrium time of the coupons which had no barrier film.
3. RESULTS AND DISCUSSION

Figure 4 shows the moisture gains of the pre-preg glass laminate composite coupons in DI water
as a function of the immersion time. The glass composite coupons without any barrier film
reached the moisture equilibrium in 27 days. The PEEK-1 applied coupons gained lesser weight
than the other alternatives. It is found that the PEEK-1 barrier film is able to prevent moisture
ingression as good as the Tedlar barrier applied coupons for the glass fiber laminate composites,
which is traditionally used barrier film for the aircraft industry [1].
Figure 4. The moisture gains of the pre-preg glass laminate composites in DI water as a function
of the immersion duration.
Figure 5 shows the moisture gains of the pre-preg carbon laminate composite coupons as a
function of the immersion time. The carbon fiber reinforced composite coupons without any
barrier film reached the moisture equilibrium in 20 days of the immersion. During the
immersion, the weight gain was considerably low. However, all the barrier films on the
composite laminates significantly reduced the moisture gains. Based on the experimental results,
it is concluded that the resin and reinforcement material types significantly affect the moisture
absorption (carbon vs. glass composites).
Figure 5. The moisture gains of the pre-preg carbon laminate composites in DI water as a
function of the immersion duration.
Figure 6 shows the moisture gains of the pre-preg Kevlar laminate composite. The Kevlar
moisture equilibrium time for the control coupons were about the half of the equilibrium time of
the glass and carbon fiber laminates - about 13 days. Although there was a peek after 7th days,
the weight gain went back to less than 0.05 %. The moisture gain was about 10 % less for the
coupon with the PEEK-05 barrier film compared to the control one.
Figure 6. The moisture gains of the pre-preg Kevlar laminate composites in DI water as a
function of the immersion duration.
Figure 7 shows the moisture gains of the pre-preg glass laminate composite covered with the
Kapton barrier films. In the absence of the Kapton barrier film, the glass composite laminates
gained about 400% moisture; however, in the presence of the Kapton film, the moisture gain was
significantly low (less than 50%). The test results clearly revealed that the Kapton barrier film is
as effective as PEEK and Tedlar barrier films for moisture prevention of the composite
structures. Thus, both PEEK and Kapton barrier films can be great options for the aircraft
composite protections along with the Tedlar barrier films.
Figure 7. The moisture gains of the pre-preg glass composites covered with the Kapton film in
DI water as a function of the immersion duration.
4. CONCLUSIONS
A number of hydrophobic barrier films (e.g., Tedlar, PEEK-0.5 and PEEK-1, and Kapton) were
co-bonded to the surfaces of glass, carbon and Kevlar pre-preg laminate composites, and then
moisture ingression tests were conducted on the prepared test coupons to verify their moisture
prevention characteristics in DI water. Coupons were immersed fully in water at 22 ±2°C until
moisture equilibrium reached, and then weight gain and moisture equilibrium durations were
measured and compared each other. Glass-PEEK-1 barrier coupons gained approximately 25%
less weight than the control specimens (no barrier films applied). Using a barrier film reduced
moisture gain about 400% for the glass-Kapton laminates. Carbon PEEK-1 laminates indicated
nearly 9% less moisture gain compare to bare carbon-epoxy composite coupons when it reached
moisture equilibrium at day 13. This is less than glass and Kevlar composite coupons. Moisture
absorption tests showed that the weight gain was roughly 1% for glass, 1.5% for carbon, and 2%
for Kevlar fiber composites when moisture equilibrium point is at the maximum. Overall, it is
concluded that PEEK and Kapton barrier films on laminate composites were as good as the
traditionally used barrier film (Tedlar) and can be good replacement materials for Tedlar films in
the future.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the National Institute for Aviation Research and Wichita
State University for technical and financial support of the present research studies.
REFERENCES
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Sandwich Structured Fiber Composites for Aerospace Applications,” International Journal
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Polymeric Laminate Composite Structures and Its Prevention by Hydrophobic Barrier
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MOISTURE INGRESSION CHARACTERISTICS OF

LAMINATES COMPOSITES WITH VARIOUS BARRIER FILMS

Department of Mechanical Engineering

1.2 Moisture Diffusion

A number of factors, such as degree of cure, polarity, material properties, surface

(W i -W i-1 )/W b < 0.0005 (1)

3. RESULTS AND DISCUSSION

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