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Nanjing, China) and cut into small granules with a pulverizer. These
particles were extruded into WF/PP composite boards (4 mm thick,
100 mm wide) through a single-screw extruder (SJ45, Nanjing Rubber
and Plastics Machinery Factory, China). Three ratios of WF to PP
were synthesized, as shown in Table 1.
2.3. Preparation of CPP Films. The CPP film preparation
process is shown in Figure 1. The CPP particles were evenly spread in
a square mold with an inside length of 160 mm and a thickness of 0.1
mm. The mold was hot-pressed at a temperature of 110 °C for 3 min,
under a pressure of 2 MPa. It was then cold-pressed at a temperature
of 20 °C for 5 min to shape the film.
2.4. Preparation of the Veneered WF/PP Composite Board.
The surface of the WF/PP composite board was covered with the
CPP film, onto which the poplar veneer was then placed, as shown in
Figure 1. The sandwich structure was pressed under a pressure of 5
MPa for 5 min at a temperature of 110 °C and then cold-pressed at a
temperature of 20 °C for 5 min.
As a control, the WF/PP composite board containing 60% WF was
covered with wood veneer by pressing at 180 °C without an
intermediate film or adhesive. PP accumulated on the surface of the
WF/PP composite acting as a “glue”.
impeded its application for industries. Guo et al. (2018) used
the MAPE and HDPE film as an intermediate layer,18 which
solved the problems of pollution and bonding strength, but the
high temperature required would melt the base biofiber-
reinforced board as well as degrade the surface decorating
materials.
In this study we explored a new technology for decorating
the WF/PP composite board by using an intermediate film as
an adhesive at a lower hot-pressing temperature. Chlorinated
polypropylene (CPP) was used to bond the wood veneer and
WF/PP composite base board. CPP is often used as a
modifying agent in coatings and adhesives. Hu et al. (2018)
synthesized a CPP emulsion by grafting methyl methacrylate,19
butyl acrylate, and acrylic acid onto it. The printability of the
PP film was improved after coating with the modified CPP
emulsion. Tomasetti et al. (2000) found that CPP solution
could diffuse into the PP/PE copolymer.20 On the other hand,
the grafting of chlorine (Cl) in the molecular chain increased
the polarity of CPP, thus improving its adhesion to the wooden
surface. CPP could, therefore, be used as an advanced
intermediate adhesive for the lamination of the PP board
with other materials.
The veneered WF/PP boards were then conditioned at a
temperature of 20 °C and 65% relative humidity for 1 week before
they were tested.
2.5. Preparation of CPP/PP Compound Films. In order to
detect the bonding mechanism between the CPP and PP accumulated
on the surface of the WF/PP composite, a CPP/PP compound film
was prepared. CPP and PP particles were hot-pressed into films at a
temperature of 110 and 180 °C, respectively, and then cold-pressed at
a temperature of 20 °C. The two films were then laminated together
at a temperature of 110 °C (same as the preparation of the veneered
WF/PP composite board, in Section 2.4). The CPP/PP compound
film had two surfaces, that is, one side was CPP and the other was PP.
The process for preparing and analyzing the CPP/PP compound film
is shown in Figure 2. The three surfaces of the compound film were
scanned with scanning electron microscopy (SEM) is shown in Figure
2.
2.6. Measuring the Penetration Depth of CPP and PP into
the Wood Veneer. To illustrate the bonding mechanism between
PP, CPP, and the wood veneer, we designed an experiment to
simulate the bonding between CPP, PP, and the wood board. The
CPP film was prepared and spread on the poplar wood veneer, then
pressed together at 110 °C and 2 MPa for 2 min. The board was
cooled to room temperature and cut into slices with a microtome.
The PP film-covered veneer was prepared in the same way but
pressed at a temperature of 180 °C. The cross-section of
microsections was scanned with scanning electron microscopy
(SEM).
2.7. Surface Bonding Strength Test. The surface bonding
strength between the veneer and WF/PP board was evaluated based
on the vertical drawing test as shown in Figure 3. The size of the
sample was 50 × 50 × 6 mm3, with a middle circle of 1000 mm2 area
curved on the surface. There were 12 specimens in each group. The
loading rate was 2 mm/s.
2.8. Water Resistance Test. The water resistance of the adhesive
layer and its interfaces between the wood veneer and WF/PP
composite board was evaluated. Samples (veneered WF/PP
composites, 75 × 75 × 6 mm3) were immersed in water at 63 °C
for 3 h and then dried in an oven at 63 °C for 3 h. The delaminated
length at four edges of the dried samples was measured. Six samples
were tested in each series.
The lamination process was conducted at a relatively low
temperature (90−110 °C), therefore avoiding the deformation
of the WF/PP base board (PP melting point was 168 °C in this
study) and degradation of the wood veneer surface. The
bonding strength and water resistance of the adhesive layer
were investigated. The bonding mechanism between the wood
veneer, CPP, and WF/PP was built based on analyzing the
penetration of CPP into wood conduits, surface roughness, and
chemical characteristics of the composite surface. A simulation
experiment of CPP and PP bonding was implemented to reveal
the real form of their interface. The proposed method is simple
and environmentally friendly.
2. EXPERIMENTAL SECTION
2.1. Materials. The rotary-cut Poplar wood (Populus tomentosa
̀
Carriere) veneers (thickness of 1.5 0.1 mm) were purchased from
Jinan Yuanfang Wood Trading Company. The moisture content of
these veneers was 28%. Some of the veneers were ground (40−80
mesh) to WFs for preparing the WF/PP composite. WFs and veneers
were dried at a temperature of 103 °C until the moisture content was
<3.0%. PP (T300, melting point 168 °C, density 0.91 g/cm3, melting
flow rate (MFR) = 2.5−3.5 g/10 min at 180 °C) was purchased from
Sinopec Daqing Petrochemical Company, Daqing, China. Maleic
anhydride grafted polypropylene (MAPP) (grafting percentage 1−
1.2%) was obtained from Shanghai Sunny New Technology
Development Co, Shanghai, China. CPP was supplied as pellets by
Shenzhen Jitian Chemical Products Limited Company, Shenzhen,
China, with a chlorination ratio of 32%, MFR = 16.6−20.1 g/10 min
at 110 °C, melting point 90 °C, and density 0.93 g/cm3.
2.2. Preparation of WF/PP Composites. PP, WF, and MAPP
were blended at different mass fraction ratios (Table 1) with a high-
speed mixer (SHR-10A, Zhangjiagang Tonghe Plastic Machinery Co,
Zhangjiagang, China). The mixtures were prilled using a corotating
twin-screw extruder (JSH30, Nanjing Rubber & Plastic Machinery,
2.9. Surface Roughness Test. The surface roughness of the WF/
PP base board was measured using a contact-type surface roughness
measuring instrument (SJ-210, Mitutoyo Japan Corporation,
Kawasaki, Japan). The maximum height of the profile (Rz), contour
arithmetic means deviation (Ra), surface profile height root mean
square (Rq), and the surface contour curve were measured and
calculated according to “ISO 4287:1997 geometrical product
specifications, surface texture: profile method, terms, definitions,
and surface texture parameters.” The testing length was 5 mm. The
Table 1. Component Mass Parts for Each WF/PP
Composite (Mass Part)
composite
WF
PP
MAPP
60 WF/40 PP
70 WF/30 PP
80 WF/20 PP
60
70
80
40
30
20
2
1.5
1
B
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