JEC COMPOSITES MAGAZINE - Issue #95 - Special JEC Europe - March 2015 - 83

Tensile performance of the
non-reinforced resin

Tab.1: Origin and mechanical performance of epoxy matrices studied

Bioresource
origin

Young's
Modulus (GPa)

Breaking
stress (MPa)

Elongation
at break (%)

Crosslinking
conditions

Matrix

Supplier

Bioresource
content

EPOLAM 2020®

Axson

Petroleum-based

3.3±0.1

77±3

3.1±0.1

2h at 120°C

Greenpoxy 55®

Sicomin

ASTM D 6866:
55±2%

Biobased
epichlorhydrine

2.8±0.2

52±1

3.1±0.2

24h at Tamb + 24h
at 40°C

Epobiox
STR 100®

Sandtech

% by weight:
50-55%

Biobased BPA,
turpentine

1.7±0.2

31±1

3.2±0.1

2h at 80°C

groups in the resin. The fibre/matrix bond is quantified using the
microdroplet debonding test, which was chosen to determine the
interface strength of the different flax-fibre/epoxy matrix pairs at
the microscopic scale.
Droplets of liquid resin are deposited on single flax fibres (Figure
1), which are then placed in an oven under specific crosslinking
conditions for each resin under consideration. After full cure, the
geometry of the droplets is studied under an optical microscope.
For each sample, the length and height of each droplet are measured, as well as the fibre diameter at each end. The flax fibre with
the droplets is then placed between razor blades spaced at the
width of the fibre diameter.
The fibre was loaded at a rate of 0.1 mm/min. At least 20 validated debonding tests were performed for each matrix. Observation
of the debonded droplet showed that the failure mode did indeed
correspond to interfacial shear. The load-displacement curves
were recorded for each test so as to determine the debonding and
frictional forces.
The apparent interfacial shear stress τapp (Equation 1) was
computed using the value for the force required to debond (Fmax)
and Miller's method [5], which assumes that load is uniformly
distributed across the fibre/matrix interface:
τapp=Fmax/2πrL

(Equation 1)

where r is the radius of the fibre at the droplet extremities and L
is the length of the bonded droplet, assuming a circular cross-section of the fibre. The linear regression of the plot of debonding
force versus bonded area serves to determine the value for τapp.
Figure 2 shows the apparent interfacial stresses obtained for our
three studied systems compared with data in the literature [6], [7].
Interface strength depends strongly on the chemical bonds
created at the interface between fibre and matrix. The interface
strength of petroleum-based EPOLAM 2020® resin and partially
biobased Greenpoxy 55® resin is roughly equivalent because their
chemical nature is similar. The partially biobased Epobiox STR
100® resin contains a large number of hydroxyl groups. Moreover,
the high interface strength of the flax/Epobiox STR 100® pair (τ
= 28.5±6.5 MPa) is partly due to the hydrogen bonds created
between hydroxyl groups in the matrix and those in the flax fibre.

Fig. 2: Interfacial shear strength (IFSS) of flax/epoxy and glass/epoxy systems

The interfacial shear stress value for the flax/EPOLAM 2020®
pair is consistent with the literature data. The flax/epoxy bond
strength is slightly lower than that of the glass/epoxy bond (Figure 2), but we need to bear in mind that, unlike the glass fibres,
the flax fibres are devoid of any crosslinking or compatibility
agents.
The flax/petroleum-based epoxy resin and flax/partially biobased
resin pairs show satisfactory bond strength values at the microscopic scale. In a composite, however, other parameters also influence the interface properties, e.g. the form factor or state of the
reinforcement area and state of division of the fibres.

Micro-macro correlation
According to a number of studies ([7][8][9][10]), the stronger the
fibre/matrix bond (high shear stress), the better the macroscopic
properties of the composite. However, this improvement is rarely
proportional to the improvement of the microscopic properties.
The [±45°] composite laminates were cured under the conditions
specific to each resin, enabling a comparison of the results with
those from the microscopic analysis. The in-plane shear test on
[±45°] composite laminates is sensitive to both the fibre/matrix
interface and the properties of the matrix itself.
Figure 3 shows the shear stress measured at the macroscopic
scale versus interfacial stress measured at the microdroplet scale,
No95 March 2015 /

jec composites magazine 83

JEC COMPOSITES MAGAZINE - Issue #95 - Special JEC Europe - March 2015

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #95 - Special JEC Europe - March 2015