JEC COMPOSITES MAGAZINE - Issue #110 - January/February 2017 - 52

TECHNOLOGY OPLC
"dry" flocked laminate reinforcing plies. It is remarkable too
that a toughness of nearly 2 kJ/m2 can be achieved with an
estimated flock density as low as 2 fibres/mm2 for a glass mat
laminate that was VAF processed. This effect is believed to
be caused by the added vacuum force that helps to drive the
flocked fibres more deeply into the fibrous reinforcement
ply. In the case of the glass fabric plies, in some instances, the
flock was so deeply embedded that some flock ends appeared
on the non-flocked side of the ply. It is believed that these
"dry flocked" composites have low flock densities because
many of the flock fibres do not get adequately entangled into
the fibrous ply's interstices. Consequently, loose flock fibres
are easily removed when the Z-flocked ply is lightly vacuumed or shook off before the flock treated plies are laid up
into an assembly and impregnated with an epoxy matrix resin.
Pre-flocked laminates
"Pre-flock" fabrication was achieved by first spraying a
high-volume/low-pressure spray gun to evenly coat fibreglass fabric layer/plies with a thin layer of aerospace-grade
epoxy varnish before flocking the resin-sized fabric samples
with nylon flock fibres. Figure 4 shows a photomicrograph
of a "pre-flocked" composite reinforcement ply composed of
3 denier, 1.2 mm long nylon flock fibre flocked onto a glass
fabric base ply fabric. While not shown here, the flock fibres
are "attached" to the fibrous surface and the fabric's structure
remains open and available for subsequent resin impregnation. This is a photograph of "pre-flocked" glass fabric plies
used to create the composite samples DCB tested to generate
the data presented in Table 4. In this experiment, flock fibres
were applied in three areal densities of sizing; a relatively low
density of 200 mg/mm2, an intermediate density of 290 mg/
mm2 and a high density of 430 mg/mm2.
Table 4 shows that the highest initial peak toughness of 4.14
kJ/m2 was achieved with the intermediate sizing density [19].
It is also noted that initial peak toughness decreased to 1.28
kJ/m2 when the sizing density was increased. This is probably due to the excess sizing resin blocking the interstices of
the glass fabric to the extent that the post-flock resin matrix
impregnation of the lay-up composite is frustrated. Here, a
thorough wetting of the glass fibres was not possible, leading
to a less than optimum final composite. It is obvious then that
the amount of "pre-flock" sizing-adhesive used to attach the
flock to the substrate is an important factor and must be controlled to achieve an optimum final composite.

Comparison of flock application methods
Figure 5 compares the maximum initial peak toughness values of a 12-ply epoxy matrix glass fabric laminate prepared
using the three described methods. Figure 5 also notes the
corresponding flock densities for these samples. All Figure 5
samples are composed of fibreglass fabric, nylon flock with
different denier and length; and are averages of five tested
samples. Figure 5 results clearly show that overall, the frac-

Fig. 5: Comparing DCB toughness and flock density of nylon-flocked glass fabric
laminates prepared using three Z-axis reinforcement flocking methods

ture toughness (interlaminar shear strength) of organic polymer laminar composites (OPLC) can be improved by applying Z-axis oriented flock to the interfacial zones of these
composites.
Mode-I fracture toughness enhancement of these Z-axis by
flock-reinforced composites seems to depend on the flock's
application methodology, flock density/length and type of
fibrous substrate being flock-modified. From an implementation standpoint, the most practical method of imparting
Z-axis flock into an OPLC is the "pre-flock" method. This
methodology provides the most convenient way for commercial fabricators of composite material structures to impart
higher interlaminar shear strength into their products.
Laminate thickness data in Tables 1 to 4 show differences
in a laminate's final thickness compared to the non-flocked
control laminates. This thickness difference depends on the
Z-axis flock application process used. Z-axis flock applied
by the "wet" and "pre-flock" methods shows a significant increase in laminate thickness. This would be expected since
the flock fibre's mass is added to each interface in the assembled laminate. Implanted flock fibres add "bulk" to each
interfacial zone and serve to form a (thin) second composite phase in the laminate; upright oriented monofilament
fibres in a resin matrix. Thickness changes in "dry" flocked
processed Z-axis fabricated composites were found to have
a lower thickness compared to the control. This could be
because as flock is applied to a "dry", as received, fibre mat
or fabric, the flock fibres are able to embed themselves more
deeply into the fibrous structure. There is no layer of "wet"
matrix resin blocking the penetration of the flock fibres into
the fibrous substrate's interstices.

Concluding remarks
This paper summarizes the highlights of UMD's research effort in the field of Z-axis inter-ply reinforcement of OPLCs
by flock fibres. Experimental results clearly show that Z-axis
oriented interfacial flock reinforcement fibres can improve
the Mode I fracture toughness of glass fabric, glass mat and
jute (natural fibre) fabric laminar composites. Other UMD
publications on this topic show that this Z-axis flock reinforcement effect applies to most other OPLC fibrous rein-

jec composites magazine / No110 January-February 2017

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