SAMPE Journal - November/December 2021 - 73

a loading rate of 10 mm/min. The climbing drum
peel test (CDP) was employed to evaluate the
stripping resistance moment between the frontpanel
and the bottom core referring the standard
of ASTM D1781-98 on a universal testing machine
(UTM5105, SUNS, China) at a displacement rate of
20 mm/min. The samples were prepared with a size
of 300 mm × 60 mm (length/width). The designed
peeling interfacial layers were about 30 mm longer
than the corresponding core sections at both
ends. The flame retardancy of samples (length/
width/thickness: 70 mm × 6 mm × 2.75 mm) was
evaluated by testing their limit oxygen indexes
(LOIs) according to the standard of GB/T 5454.
RESULTS AND DISCUSSION
Prepreg Properties
The basic properties of the PPF prepregs produced
from the new and used filters are shown in Table
1. When the gauge spaces varied from 1.65 to 1.75
mm, the resin contain of the new PPF prepreg
slightly shifted from 69.1 (N-1.65) to 70.3 % (N1.75).
During fabrication of composites, more resin
was squeezed out from the sample N-1.75 and the
resin loss rate increased from 28.77 to 40.15 %. The
resin contents in the used PPF prepreg elevated
more obviously from 68.7 (U-1.65) to 72.1 % (U1.75),
but the resin loss rates merely changed from
27.55 to 27.56 %. These results revealed that the
used PPF could absorb more resin during profusion
at a large roller space and hold more resin during
hot pressing. For industrial production, the used
PPF processed at a large roller space is preferred.
With a small roller space, the resin profusion and
holding amounts were similar in the new and used
PPF prepregs. To reduce the differences in the resin
contents of composites, the roller space was set at
1.65 mm for all samples in the following studies.
Morphology of PPF Composites With and
Without Honeycomb Cores
Figure 1 displays the images of filters and PPF
composites with and without honeycomb cores.
Compared to the cream color of the new filter
(Figure 1a), the used filters exhibit brown color
with dark brown stripes (Figure 1b). These stripes
are the locations where the filters contacted with
the metal frames of the filter set. The color depth
of the used filters varies from piece to piece due to
different extents of aging. The composites made
from the prepregs of the new filters show a maizeyellow
color (Figure 1c), while the composites
produced with the prepregs of the used filters
exhibit dark brown color with almost black stripes
(Figure 1e). The PPF composites with honeycomb
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NOVEMBER/DECEMBER 2021
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SAMPE JOURNAL |
73
Table 1. The properties of prepregs.
Sample
index
N-1.65
N-1.75
U-1.65
U-1.75
Roller
Space
(mm)
1.65
1.75
1.65
1.75
Weight of
Prepreg
(g)
84.52
85.92
83.07
88.81
Resin
content
(%)
69.1
70.3
68.7
72.1
Squeeze
out amount
of resin (%)
24.32
34.50
22.89
24.48
cores show similar color with the filter fabrics. The
honeycomb pattern also exhibits on the composite
surfaces (Figure 1d, f).
Mechanical Properties of PPF Composites
With and Without Honeycomb Cores
The flexural strengths of the PPF composites with
and without the GF or CF skin layers are shown
in Figure 2a. The composites made from only two
layers of used PPF filters possess a flexural strength
of 27.85 MPa and this value is 42.5% higher than
the strength of the corresponding composites
fabricated from new filters (19.54 MPa). The
possible reason might be that, since the used
filters subjected to a long-term filtration process
under an acidic condition at high temperatures, an
active surface could be generated leading to better
binding between the PPF reinforce fabrics and the
epoxy resin. The combination of the GF and CF
skin layers can substantially enhance the flexural
strengths of PPS composites. Compared to the
sample UPPF and NPPF, the flexural strengths of the
sample GF-UPPF and GF-NPPF increased 495 and
527.6% and the values of the sample CF-UPPF and
CF-NPPF raised 552.8 and 692.7%. These results
also reveal that skin layers of high-performance
Figure 1. The photographs of the new and used PPF (a, b) and the PPF
composites and the honeycomb core PPF composites made from the new
(c, d) and used filters (e, f).
Resin loss
Rate (%)
28.77
40.15
27.55
27.56
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