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Materials and Design xxx (2008) xxx–xxx
Contents lists available at ScienceDirect
Materials and Design
journal homepage: www.elsevier.com/locate/matdes
Influence of nonmetals recycled from waste printed circuit boards on flexural
properties and fracture behavior of polypropylene composites
Yanhong Zheng, Zhigang Shen *, Chujiang Cai, Shulin Ma, Yushan Xing
Beijing Key Laboratory for Powder Technology Research and Development, Beijing University of Aeronautics and Astronautics, Beijing 100191, People’s Republic of China
a r t i c l e
i n f o
Article history:
Received 20 May 2008
Accepted 1 July 2008
Available online xxxx
Keywords:
Composites (A)
Fracture (E)
Scanning electron microscopy (G)
a b s t r a c t
Flexural strength and flexural modulus of the composites can be successfully improved by filling nonmetals recycled from waste printed circuit boards (PCBs) into polypropylene (PP). By using scanning electron
microscopy (SEM), the influence of nonmetals on fracture behavior of PP composites is investigated by in
situ flexural test. Observation results show that the particles can effectively lead to mass micro cracks
instead of the breaking crack. The process of the crack initiation, propagation and fiber breakage dissipate
a great amount of energy. As a result, the flexural properties of the composites can be reinforced significantly. Results of the in situ SEM observation and analysis to the dynamic flexural process supply effective test evidence for the reinforcing mechanism of the nonmetals/PP composites on the basis of the
energy dissipation theory.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Nonmetals, an industrial solid-waste byproduct, are produced
in large quantities during the recycling of waste printed circuit
boards (PCBs) by physical method. Thousands of millions of tons
of nonmetals are generated in the world each year. A huge source
of pollution, nonmetals can also be a huge resource. The storage
and disposal of nonmetals, designed to avoid environmental pollution, have become a worldwide problem, and the reuse of nonmetals is becoming increasingly important. Traditionally, the
nonmetals are landfilled or incinerated, which will cause resource
waste and potential environment problems. Recently, many
researchers have used nonmetals as fillers for paints, adhesives,
decorating agents and building materials, polyester composite
and phenolic molding compound [1–6]. But so far, according to
author’s knowledge little work has been reported on the use of
nonmetals as fillers for polypropylene (PP).
PP as one of the most important commodity polymers is widely
used in the packaging, textile and automobile industries because of
its good processibility and great recyclability [7–10]. Its application
as an engineering thermoplastic is somewhat limited because of its
poor fracture behavior. In fact if it is reinforced using filler or fiber
it can be used instead of other commodity thermoplastic and even
engineering thermoplastics [8–12]. Nonmetals recycled from
* Corresponding author. Tel./fax: +86 10 8231 7516.
E-mail address: [email protected] (Z. Shen).
waste PCBs contain 50–70% glass fibers having high length diameter ratio, high elastic modulus and low elongation. Therefore, nonmetals recycled from waste PCBs represent a potential substitute
for traditional mineral fillers or pure glass fibers and can highly improve the strength of plastic products. They have more advantages
than traditional fillers.
Although the nonmetals recycled from waste PCBs can be
successfully reused as reinforcing fillers in PP composites, influence of them on fracture behavior of the composites cannot be
neglected. Up to now, the research of particles reinforcing of
polymer was mostly going on at the end of the experiment
and was based on the results of the experiment, while the process of the particles reinforcing was unclear. Despite the importance of fracture behavior of polymer materials, there are limited
studies dealing with the fracture behavior in GF/PP, CaCO3/PP,
and GB/PPO composites [9,10,13–15]. These research works have
mostly been focused on the fracture behavior of composites via
tensile test or notched flexural test. But according to author’s
knowledge there is no evidence to show that investigation of
fracture behavior of composites via unnotched flexural test has
been performed.
In this article, the objective of the research is to study the dynamic process of nonmetals particles reinforcing of PP polymer.
In nonmetals/PP composites, the dynamic process of the crack initiation, propagation and fiber breakage are watched under scanning electron microscope (SEM) in situ unnotched flexural tests.
These changes caused by nonmetals supply effective test evidence
for the reinforcing mechanism of the nonmetals/PP composites on
the basis of the energy dissipation theory.
0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.matdes.2008.07.004
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Y. Zheng et al. / Materials and Design xxx (2008) xxx–xxx
2. Experimental
for each type of composite were tested at room temperature
(23 °C), and the mean values were reported.
2.1. Material and fabrication procedure of composites
2.3. Differential scanning calorimetry (DSC) measurement
Nonmetals, an industrial solid-waste byproduct, are produced
in large quantities during physical recycling waste PCBs. The waste
PCBs consist of a woven fiberglass (modified, 50–70%) mat impregnated with thermoset resins (epoxy resin or phenolic resin, etc.,
30–50%). By vibrating screen classification, large quantities of nonmetals with different particle size can be obtained. The nonmetals,
with particle size of less than 150 meshes, were selected for making composites. Microscopic observation shows that most of them
are single glass fibers and thermosetting resin powders (see Fig. 1).
They contain about 70 wt% single glass fibers. These glass fibers
possess many excellent characteristics, such as high length diameter ratio (L/D ratio), high elastic modulus, low elongation and low
thermal conductivity.
To improve the dispersion of nonmetals particles in PP matrix
and the compatibility between the nonmetals and matrix, all the
nonmetals are modified with 1.0 wt% content of silane coupling
agent KH-550 (c-Aminopropyltriethoxysilane, Nanjing Shuguang
Chemical Group Co., Ltd., China) through silanization for 30 min
at 80 °C with high speed mixer (SHR-5A, Zhangjiagang Qiangda
Plastics Machinery Co., Ltd., China) at 1800 rpm. Before the silanization, 40 vol% content of KH-550 is mixed and hydrolyzed in
the solvent (ethanol–water, volume ratio 7:3) for 30 min at room
temperature (23 °C) and 500 rpm in a stirrer. PP powder S1003
[Beijing Yanshan Petrochemical Co., Ltd., China, melt flow rate
3.6 g/10 min (ASTM D1238, 230 °C, and 2.16 kg)] is used as the matrix polymer. The PP powders and the modified nonmetals particles
are dried at 80 °C for 2 h. Then, the dried nonmetals particles and
PP powers are stirred and mixed by using high speed mixer. The
nonmetals/PP blends are extruded into thread with a screw extruder (TE-35, Coperion Keya (Nanjing) Machinery Co., Ltd., China) at
210 °C and 220 rpm. The extrudate is pelletized, dried for 2 h at
90 °C. Then standard flexural specimens were made through injection molding using an injection machine (CJ108M3V, Chen De Plastics Machinery Co., Ltd.) at 200 °C.
DSC measurements of PP composites with and without nonmetals recycled from waste PCBs were carried out with DSC-SP equipment (Rheometric Scientific, Ltd., USA) in a flowing nitrogen
atmosphere. The temperature range utilized was from room temperature to 300 °C at a heating rate of 10 °C/min. Endothermic
reactions as a function of the temperature were plotted as negative
heat flow.
2.4. In situ SEM experimental setup and observation in flexural test
To determine fracture behavior of pure PP and PP composites
with nonmetals, a specially designed small load frame (see Fig.
2) was built and utilized to apply three-point flexural loading.
The small load frame with a specimen is under a SEM (S-570, Hitachi, Ltd., Japan). Specimen dimension was 6 mm wide and 2 mm
thick in gauge section. One side surfaces of specimens were polished and coated with a thin layer of gold prior to microscopy to
avoid charge build up. The dynamic fracture process of pure PP
and PP composites was observed in the system of in situ SEM
unnotched flexural test when external load are imposed on the
composites.
3. Results and discussion
3.1. Flexural properties
Flexural test was carried out according to ISO 178:1993 standards using an electronic universal testing machine (DXLL-10000,
No. 4 Chemical Machinery Plant of Shanghai Chemical Equipment
Co., Ltd., China) at a cross head speed of 2 mm/min. Five specimens
Fig. 3 shows the flexural properties of pure PP and nonmetals/
PP composites by filling the nonmetals particles (0–30 wt%) at
room temperature. The content of the coupling agent was 1 wt%.
The particle sizes of the fillers are less than 150 meshes. The flexural strength of pure PP is 35.53 MPa, and the flexural strength of
PP composite is greatly increased to 46.81 MPa by filling 10 wt%
nonmetals. The flexural strength of PP composite increases quickly
as the nonmetals contents increase and are 56.08 MPa with addition of 20 wt% nonmetals. As the nonmetals contents increase to
30 wt%, the flexural strength is further improved and reaches
66.25 MPa in the experiment. The increases of the flexural moduli
of composites are greater than that from flexural strengths. The
flexural modulus of pure PP is 1.64 GPa, and the flexural modulus
of PP composite is greatly increased to 2.49 by filling 10 wt% non-
Fig. 1. SEM micrograph of nonmetals recycled from waste PCBs.
Fig. 2. A load frame built for SEM observation.
2.2. Flexural test
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the crystal form a because the melting temperature of a crystals
is 160–176 °C [16].
3.3. In situ SEM observation and analysis
Fig. 3. Flexural properties of the pure PP and nonmetals/PP composites.
metals. The flexural modulus of PP composite increases quickly as
the nonmetals contents increase and are 3.13 GPa with addition of
20 wt% nonmetals. As the nonmetals contents increase to 30 wt%,
the flexural modulus of the composite is further improved and
reach 3.82 GPa.
In other words, the flexural properties of the nonmetals/PP
composites are increased with increasing the nonmetals contents
from 10 to 30 wt%. The maximum increment of the flexural
strength and flexural modulus is 86.5% and 133.0%, respectively.
It is evident that the presence of nonmetals recycled from waste
PCBs is an important factor in influence on flexural properties of
the nonmetals/PP composites.
3.2. DSC analysis
DSC offers a convenient means of studying the reaction rates
and mechanisms under controlled conditions. This value is particularly important for conversion of the material into product for
their potential practical application.
Table 1 summarizes the results of the DSC tests of pure PP and
nonmetals/PP composites by filling the nonmetals particles (0–
30 wt%). The endothermic peak (Tm) of pure PP is 168.4 °C and
the heat of fusion (DHm) is 77.5 J/g. The Tm of the nonmetals/PP
composites are 165.1, 165.5, 166.8 °C and the DHm of the composites are 57.5, 52.3, 50.4 J/g with the addition of 10, 20, and 30 wt%
nonmetals, respectively. This indicates that addition of the filler
into PP changes little in the Tm of the composite, but that the
DHm decreases by 27.1 J/g with the addition of 30 wt% nonmetals.
Reduction of the Tm is caused because of no nucleating effect of the
nonmetals. And reduction of the DHm can be attributed to substitution of PP by nonmetals possessing low thermal conductivity.
In a word, the addition of the nonmetals into PP leads to a decrease
in the Tm and DHm of the composites.
Table 1 also shows that the Tm of pure PP and nonmetals/PP
composite range from 165.1 to 168.4 °C, and this indicated that
PP, both in the pure state and in the composite, exhibited only
Table 1
DSC data of the pure PP and nonmetals/PP composite
PP content (wt%)
Nonmetals content (wt%)
Tm (°C)
DHm (J/g)
100
90
80
70
0
10
20
30
168.43
165.14
165.51
166.82
77.52
57.51
52.33
50.36
The flexural properties results show that strength and rigidity
of the composites are significantly improved by filling the nonmetals into PP. With such millions of glass fibers and good compatibility between the nonmetals and matrix, there are mass
excellent supporting bodies, and appropriate interfacial adhesives
are formed between the particles and matrix. Every dispersed
particle triggers effective stress concentrations and lead to mass
crazes so that the big cracks cannot be formed in the nonmetals/PP composites. The process of the crack initiation, propagation
and fiber breakage dissipate a great amount of energy. Thus, the
matrix properties are improved with the addition of nonmetals
particles into PP. In this study, the effect of particles on fracture
behavior of PP composites is observed and analyzed during in
situ SEM observation in three-point flexural test. All results are
summarized as follows.
Fig. 4 shows the SEM micrographs for the in situ observation of
pure PP under the three-point flexural loading. The loading direction is horizontal. The surface of the specimen of the pure PP is
smooth without loading as shown in Fig. 4a. At the beginning of
the flexural loading, there is no change. When up to a certain loading it triggers many initial cracks, and the size of the crack is usually big, nearly parallel to the load direction as shown in Fig. 4b.
Subsequently, more cracks appear following crazes thickening
(Fig. 4c), and extend to the dominant crack rapidly. Then the pure
PP specimen gets flexural failure (Fig. 4d).
Fig. 5 shows the SEM micrographs for the in situ observation of
the PP composite filled with nonmetals recycled from waste PCBs
(30 wt%) under the three-point flexural loading. The loading direction is horizontal. For in situ SEM observation, the nonmetals/PP
composite get polishing treatment, and many single fibers are exposed on the surface of the composite (Fig. 5a). But the resin powders particle size is smaller than glass fibers and they are
intimately mixed in the PP matrix, which cannot be easily distinguished in the composite. Furthermore, the particles are dispersed
well in the matrix and the nonmetals are well wetted with PP
material because of good compatibility between nonmetals and
matrix (Fig. 5b). And the glass fibers in the matrix are nearly perpendicular to the load direction. That is mainly because the specimen was made through injection molding using an injection
machine. The glass fibers isotropy induced by fiber orientation is
beneficial to flexural properties of the nonmetals/PP composites.
Fig. 5a shows the initial condition of the composite specimen without the flexural loading. At the beginning of the flexural loading,
the SEM micrograph of the specimen shows no change, just as pure
PP. When up to a certain loading, it triggers an initial micro crack,
and craze is propagated by moving towards the interface of the
particle and matrix. Then the crack is either terminated when it
meets another particle or branched into mass finer crazing instead
of the breaking crack directly (Fig. 5b). Meanwhile, the partial
interfacial debonding between the fiber ends and matrix can be
seen at the bright place in Fig. 5b and c. As the flexural loading increases, it breaks the single glass fiber (Fig. 5c). When the loading
further increases, mass broken fibers appear (Fig. 5d). That is
mainly because the glass fibers possess high elastic modulus and
low elongation, they first undertake the loading when external
load is imposed on the composite. Meanwhile, the strength of
the loading is far greater than that of the single glass fiber, and
there is strong adhesion and good compatibility between the fiber
and the matrix, so the glass fibers are first broken compared with
the PP matrix. Subsequently, the PP composite specimen gets flexural failure.
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Fig. 4. SEM micrograph for the in situ observation of the pure PP. (a) Initial condition; (b) triggering initial crack; (c) appearance of more cracks; and (d) flexural failure. The
loading direction is horizontal.
Results show that the pure PP matrix can initiate big cracks in
the flexural test by in situ SEM observation. On increasing the loading, the bigger one of the cracks is extended into the dominant
crack rapidly and then the pure PP specimen gets flexural failure
rapidly. It means that the main energy absorption is in the crack
initiation. While nonmetals recycled from waste PCBs are filled
into PP matrix, mass micro cracks are triggered in the composite
specimen. The crack is either terminated when it meets another
particle or branched into mass finer crazing instead of the breaking
crack directly. Since glass fiber acts like a barrier, craze cannot pass
easily. In this condition, the crazes should turn glass fiber, move toward interface or break the glass fibers. The process of the crack
initiation, propagation and fiber breakage dissipate a great amount
of energy. Meanwhile, in the process of the flexural loading, partial
interfacial debonding can slowdown the propagation of the crack
and promote crack termination. These factors cause improvement
of the flexural properties of the nonmetals/PP composites by filling
the nonmetals particles recycled from waste PCBs evidently. The
results of the in situ observation and analysis to the dynamic process supply effective test evidence for the reinforcing mechanism
of the nonmetals/PP composites on the basis of the energy dissipation theory.
4. Conclusions
Flexural strength and flexural modulus of the composites can be
successfully improved by filling nonmetals recycled from waste
PCBs into PP. And the Tm and DHm of the composites decrease with
the addition of the nonmetals.
The dynamic flexural process of the pure PP and nonmetals/PP
composites are observed with SEM. Results show that the pure
PP matrix can initiate big cracks under the flexural loading, and
it extends to the dominant crack rapidly as the flexural loading increases. It means that the main energy absorption is in the crack
initiation. While nonmetals are filled into PP matrix, mass micro
cracks are triggered and consume tremendous energy. The glass fiber acts like a barrier, craze cannot pass easily. Therefore, the
crazes should turn glass fiber, move toward interface or break
the glass fibers. The process of the crack propagation and fiber
breakage dissipate a great amount of energy. Meanwhile, in the
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Fig. 5. SEM micrograph for the in situ observation of the nonmetals/PP composite (30 wt%). (a) Initial condition; (b) triggers an initial crack and interfacial debonding; (c)
fibers breakage; and (d) mass fibers breakage. The loading direction is horizontal.
process of the flexural loading, partial interfacial debonding can
slowdown the propagation of the crack and promote crack termination. All of these factors can prevent and delay the nonmetals/
PP composites getting flexural failure, and cause the improvement
of the flexural properties evidently.
In situ SEM observation and analysis experimental results show
that energy dissipation is the major factor of reinforcing mechanism. In situ observation and analysis to the dynamic process supply effective test evidence for the reinforcing mechanism of the
nonmetals/PP composites on the basis of the energy dissipation
theory.
Acknowledgements
The authors acknowledge with gratitude the financial support
of the National Natural Science Foundation of China (Grant No.
50774003), China Postdoctoral Science Foundation funded project
(Grant No. 20070420286) and the Joint Building Project of Beijing
Education Committee.
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