Tensile Tests of Glass Powder Reinforced Epoxy Composites: Pilot
Study
H Ku
, 1, a
, P Wong
2,b
,
J Huang
3,c
,H Fung
4,d
and T Mohan
5,e
1, 2, 3, 4, 5
Faculty of Engineering and Surveying, University of Southern Queensland, West Street,
Toowoomba, 4350, Australia
a
ku@usq.edu.au,
b
kywong@vtc.edu.hk,
c
jolbert99@yahoo.com.hk
d
j07239155d@polyu.edu.hk
e
tradam@usq.edu.au
Keywords: Yield strength, tensile strength, Young’s modulus, epoxy resin, and glass powder.
Abstract. Epoxy resin was filled with glass powder to optimize the strength of the composite for
structural applications by a research centre in the University of Southern Queensland (USQ). In order
to reduce costs, the centre wishes to fill as much glass microspheres as possible subject to maintaining
sufficient strength and fracture toughness of the composites in structural applications. This project
varies the percentage by weight of the glass powder in the composites. After casting the composites
to the moulds, they were cured at ambient conditions for 24 hours. They were then post-cured in a
conventional oven and subjected to tensile tests. It was found that the best percentage of glass
powder by weight that can be added to the epoxy resin to give an optimum yield and tensile strengths
as well as Young modulus and cost was five percent. It was also found that the fractured surfaces
examined under scanning electron microscope were correlated with the fracture toughness. The
contribution of the study was that if tensile properties were the most important factors to be
considered in the applications of the composites, glass powder is not a suitable filler. It is also hoped
that the discussion and results in this work would not only contribute towards the development of
glass powder reinforced epoxy composites with better material properties, but also useful for the
investigations of tensile properties in other composites.
Introduction
Organic-inorganic hybrid materials consisting of inorganic materials and organic polymers are a new
class of materials, which have received much attention in recent years. The composite materials
exhibit characteristics of both inorganic materials and organic polymers. It has been established in
recent years that polymer reinforced with a small percentages of strong fillers can significantly
improve the mechanical and thermal properties [1].
The most widely used and least expensive polymer resins are the polyesters and vinyl esters; these
matrix materials are used primarily for glass fiber-reinforced composites. The epoxies are more
expensive and, in addition to commercial applications, are also utilized extensively in polymer matrix
composites for aerospace applications; they have better mechanical properties and resistance to
moisture than the polyesters and vinyl resins [2]. This research project is to investigate the yield
strength, tensile strength and Young’s modulus of epoxy composites reinforced with varying
percentage by weight of glass powder, the filler, with a view to finding out the optimum percentage by
weight of the glass powder that can be added to the composites.
The epoxy resin used in this study is Kinetix R246TX Thixotropic Laminating Resin, an opaque
liquid, and the hardener used is kinetic H160 medium hardener which has a pot life of 120 minutes.
Other hardeners like H126, H128, H161 and H162 can also be used [3]. The glass powder was first
mixed with epoxy resin, after this the hardener, kinetic H160 medium was added. The by weight ratio
of resin to hardener used was 4:1 [3]. The composite was then cast to moulds of tensile test pieces and
left to cure under ambient conditions for 24 hours. The tensile test specimens were taken out of the
moulds and then post-cured in an oven at 40
o
C for 16 hours, and then at 50
o
C for 16 hours and finally
at 60
o
C for 8 hours. This is to ensure the heat distortion temperature (HDT) is above 63
o
C. To bring
the ultimate HDT to 68
o
C, another 15 hours of post-curing will be required [3]. The specimens were
then subjected to tensile tests.
Epoxy resin
The family of polymers gets its name from the epoxy functional group that terminates molecules or
that is internal to the structure. Epoxies are really polyethers, because the monomer units have an
ether type of structure with oxygen bonds, R—O—R [4]. Whereas the building blocks and chemical
reactions involved in producing and crosslinking of unsaturated polyesters are similar for different
polyester types, the situation is rather complex with epoxies. A wide range of building blocks are
used in polymerization and numerous different compounds in crosslinking. In the following, only one
epoxy configuration is considered and will have to serve as a representative for the entire epoxy
family. An epoxide, or oxirane, group consists of one oxygen and two carbon atoms arranged in a
ring. Often the epoxide group contains yet another carbon atom and is then referred to as a glycidyl
group. The most common epoxy is based on condensation polymerization of epichlorohydrin and
bisphenol A creating diglycidylether of bisphenol A (DGEBPA or DGEBA) [3]. The number of
repeating units in an epoxy molecule is much lower as compared with that of polyesters, typically on
the order of 10; at an average n value of 2 DGEBPA is solid at room temperature. Although epoxies
are normally referred to as polymers, they are therefore strictly oligomers (few mers).
Glass powder
The glass powder used is SPHERICEL® 60P18 (spherical) hollow glass spheres. They are used to
enhance performance and reduce viscosity in paints and coatings and as lightweight additives in
plastic parts. They are chemically inert, non-porous, and have very low oil absorption. Typical
properties of the spheres are shown in Table 1 [4]. SPHERICEL® 60P18 hollow spheres products
offer formulators flexibility in polymer composites. The addition of hollow spheres to fiberglass
reinforced plastics (FRP), epoxy, compounds, and urethane castings can provide weight reduction
cost, savings and improved impact resistance. Insulating features of hollow spheres also work to the
chemists’ advantage in thermal shock and heat transfer areas. Two densities available are 0.6 to 1.1
g/cc; it provides choices to best fit mixing and target weight requirements [5]. The density of the
hollow glass powder used in this research is 0.6 g/cc because the other filler, ceramic hollow spheres
or SLG used in similar study is 0.7 g/cc; this will give a better basis for comparison of results obtained
in the future. When used in polymer concrete, hollow spheres provide a cost effective alternative
without degrading physical properties.
Table1: Typical properties of hollow glass spheres Table 2: Tensile strength of alumina trihydrate filled epoxy
composites.
Shape
Spherical
Colour
White
Composition
Proprietary Glass
Density
1.1 g/cc and 0.6 g/cc
Particle Size
Mean Diameter 11 and 18 microns
Hardness
6 (Moh’s Scale)
Chemical
Resistance
Low alkali leach/insoluble in water
Crush Strength
>10,000 psi
The Composite Samples
The reinforcer was glass powder (glass hollow sphere) particulates and they were made 0 % to 35% by
weight in the cured epoxy composite, EP/GP (X %), where X is the percentage by weight of the filler.
Above 35% by weight of filler, the slurry would be too sticky to be cast into moulds. As the raw
materials of the composites are liquid and glass hollow spheres, the tensile test specimens were cast to
shape. The resin is an opaque liquid and is first mixed with the catalyst. After that the glass powder is
Particle size
(microns)
Volume fraction
(%)
Tensile
strength
(MPa)
Unfilled
0
75.9 ± 8.8
1
10
58.0 ± 3.4
8
10
29.9 ± 1.7
12
10
27.2 ± 2.4
added to the mixture, they are then mixed to give the uncured composite. The mixture of glass
powder, resin and accelerator was blended with mechanical blender to ensure a more homogenous
mixture. The uncured composite was then cast into the moulds and cured in ambient conditions.
After initial 24-hour curing when the test pieces were removed from the mould, they were post-cured
for 40 hours. This was achieved by curing the pieces in an oven.The test pieces were then
tensile-tested in accordance with an Australian standard [6].
Results and Discussion
Figure 1 illustrates the yield strengths of varying percentage by weight of glass hollow spheres
reinforced epoxy matrix composites. The yield strength of the neat resin was 17.95 MPa, which was
higher than those of the composites with any percentage by weight of glass powder other than 5 w/t %
(18.24 MPa) of glass powder. After dropping to 14.64 MPa at 10% by weight of filler, it remained
stable up to 20 % percent by weight of glass powder. After this, it dropped further to 13.62 MPa at 25
% by weight of filler and remained so up to 35 % percent by weight of glass powder. In general, the
higher the percentage by weight of glass powder, the lower was the yield strength.
Yield strength of epoxy composites filled with
varying glass powder by weight
12
14
16
18
20
0 5 10 15 20 25 30 35
Percentage of glass powder
Yield strength
(MPa)
Yield strength (MPa)
Tensile strength of epoxy composite filled with
varying glass powder by weight
10
15
20
25
30
0 5 10 15 20 25 30 35
Percentage by weight of glass powder
Tensile Strength(MPa)
Tensile strength
(MPa)
Fig. 1: Yield strength of epoxy composite reinforced Fig. 2: Tensile strength of epoxy composite reinforced
with varying glass powder by weight with varying glass powder by weight
Figure 2 shows the tensile strengths of epoxy composites with varying percentage of glass powder
by weight. The tensile strength of the neat resin was 24.80 MPa, which was only lower than that
(25.14 MPa) of composite with 5 % by weight of filler, but higher than those of the composites with
any percentage by weight of glass powder. At 10 percent by weight of filler, the tensile strength
dropped to 17.79 MPa; it then remained up to 20 % percent by weight of glass powder; after this glass
powder reinforcement dragged the values of tensile strength further down; it dropped to 14.72 MPa
when the percentage by weight of filler was 25% remained so up to 35 % percent by weight of glass
powder. The variation of tensile strength with respect to percentage by weight of glass powder is the
same as that of yield strength. If cost and tensile strength were considered at the same time, composite
with 5 % by weight of filler is the best. It can be found that the trend for the graphs of yield and tensile
strengths are the same and it can be argued that the results were correct in trend.
The tensile strength of neat resin used in the study (24.8 MPa) is much lower than that used by the
studies of Nakamura et al. (77.3 MPa) and Radford (75.9 MPa). The former did not mention the
epoxy resin used and the latter used anhydride-cured epoxy resin [7, 8]. In this study, the pot life of
the hardener is 120 minutes; therefore, the epoxy resin used must be amine-cured as well. Effects of
particle size on the tensile properties of cured epoxy resins, filled with spherical silica particles
prepared by hydrolysis of silicon tetrachloride, were studied by Nakamura et al. [7]. Particles were
sorted into five kinds of different mean sizes in the range from 6-42 microns. Static tensile tests were
carried out. Tensile strengths were found to increase with a decrease in the particle size but with
increase particle contents [7]. This trend is supported by the tensile strength results of epoxy/alumina
trihydrate particulate composites in Table 2 [7]. In this study, the tensile strengths were found to
decrease with increase particulate loading and it can be argued that this happened because the glass
powder particles had not been treated by hydrolysis of silicon tetrachloride.
Figure 3 shows the Young’s modulus of varying by weight of glass hollow spheres reinforced
phenol formaldehyde matrix composite. The Young’s modulus of the neat resin was 2.91 GPa and it
dropped to 2.63 GPa when the percentage by weight of glass powder was 15%. It remained stable up
until 25 % by weight of glass powder. It then bounced back to 3.02 GPa at 35 % by weight of filler.
Table 3 shows the values of Young’s modulus mentioned above with their standard deviations in
brackets. From neat resin to 20 % by weight of glass powder, the yield strength, tensile strength and
Young’s modulus behaviors of the composites were more or less the same. From 20+ % to 35 % by
weight of the filler, values of yield and tensile strength decreased further with increasing particulate
loading, while those of Young’s modulus moved in the opposite direction.
Young's modulus of epoxy composite filled with
varying glass powder by weight
2.6
2.7
2.8
2.9
3
3.1
0 5 10 15 20 25 30 35
Percentage by weight of glass powder
Young's modulus(GPa)
Young's modulus (GPa)
Fig. 3: Young’s modulus of epoxy composite reinforced with varying glass powder by weight
It was found that a reduction in cost by one percent is followed by 1.5 % increase in tensile
strength. For other percentages by weight of filler, the loss in tensile strength will not be
compensated by the reduction in cost. It can be argued that 5 % by weight of filler is the best.
Figure 4 shows the scanning electron microscope image of neat epoxy resin post-cured for a total
of 40 hours at 40
o
C, 50
o
C and 60
o
C respectively at a magnification of 200 times. Faint striations
followed by a ‘turbulent flow’ can be found in the fractured surface of the neat resin. This shows that
plastic deformation had taken place in the resin. Figure 5 illustrates the scanning electron microscope
image of epoxy reinforced by 25 % by weight of glass powder and post-cured for the same number of
hours and temperatures at a magnification of 200 times. Holes were spotted and this explained why
the tensile strength (24.80 MPa) of neat epoxy resin was stronger than that (14.72 MPa) of epoxy
composite with 25 % by weight of glass powder. The holes were formed during the mixing process
and the higher the percentage by weight of glass powder, the more holes would be expected.
Fig. 4: SEM image of fractured neat epoxy resin, 200X Fig. 5: SEM image of fractured 25 % glass powder filled epoxy
composite, 200X
Flow
Striations
Hole due to dislodged glass powder
Holes
Glass powder
Conclusions
This study has evaluated the yield strength, tensile strength and Young’s modulus of varying
percentage by weight of glass powder reinforced epoxy resin; in all cases, the fluidity of the slurry
composite was high and could be cast easily into moulds. The values with no filler had also been
compared with those found by other studies but the tensile properties of some cases did not agree with
this study and some did. Since the sizes of porosities of the composites found in this study were very
small, it can be argued that the values of tensile properties obtained were very good and reliable as
their standard deviations were low. Some air bubbles were found due to imperfect manufacturing of
the samples. It can also be argued that the interfacial adhesion between epoxy resin (matrix) and glass
powder (reinforcer) would be improved by treating or coating the glass powder and the properties of
the composites would be improved.
References
[1] S. Qi, C. Li, C and Y. Huang, AIAA 57th International Astronautical Congress, IAC 2006, v 8,
pp. 5237-5240
[2] W.D. Callister, Materials Science and Engineering: An Introduction, Wiley, 2003, pp. 505, 550.
[3] ATL composites Pty Ltd, Kinetix Thixotropic Laminating Resin, undated, Australia, pp. 1-3.
[4] Potters Industries, undated,
http://www.pottersbeads.com/markets/polySphericel.asp <viewed on 14 August 2009>
[5] Potters Industries, undated,
http://www.pottersbeads.com/markets/polycomposites.asp <viewed on 14 August 2009>
[6] Australian Standard 1145.2 (2001). ‘Determination of tensile properties of plastic materials –
Test conditions for moulding and extrusion plastics’.
[7] Y. Nakamura, M. Yamaguchi, M. Okubo, and T. Matsumoto, T, Journal of Applied Polymer
Science, Vol. 45, Issue 7, pp. 1281-1289.