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\begin{document}

\title{Utilization of spent coffee grounds as fillers to prepare polypropylene
composites for food packaging applications}



\author[1]{Yurong Cai}%
\author[1]{Wenmin Song}%
\author[1]{Yuncong Yang}%
\author[1]{Xiuwen Cheng}%
\author[1]{Mingming Jiang}%
\author[2]{Rui zhang}%
\author[3]{Jiri Militky}%
\affil[1]{Zhejiang Sci-Tech University}%
\affil[2]{NMG Composites CoLtd}%
\affil[3]{Faculty of Textile, Technical University of Liberec, 46117 Liberec, Czech Republic}%


\vspace{-1em}



  
  \date{May 30, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Biomass-derived wastes as the additive of non-degradable plastics have
been paid more attention due to the ever-growing environmental pollution
and energy crisis. Herein, the spent coffee grounds (SCG) have been used
as fillers in polypropylene (PP) after the heat treatment to realize its
recycling utilization. The effect of the heat treatment atmosphere on
the properties of the obtained SCG and SCG/PP composites has been
investigated systematically. The results show that the residual coffee
oil can be removed more thoroughly under an air atmosphere than under a
nitrogen atmosphere at a relatively low cost and an eco-friendly
process. The lower residual oil rate of SCG is beneficial to improve the
comminution and further enhance the affinity with the PP matrix. The
obtained SCG/PP composite holds lower water absorption, higher
hydrophobicity, and better mechanical properties, implying its potential
applications in the field of food packaging.%
\end{abstract}%



\sloppy


\textbf{Utilization of spent coffee grounds as fillers to prepare
polypropylene composites for food packaging applications}

Wenmin Song\textsuperscript{1}, Yuncong Yang\textsuperscript{1}, Xiuwen
Cheng\textsuperscript{1}, Mingming Jiang\textsuperscript{1}, Rui
zhang\textsuperscript{2}, Jiri Militky\textsuperscript{3}, Yurong
Cai\textsuperscript{1,*}.

1. The Key Laboratory of Advanced Textile Materials and Manufacturing
Technology of Ministry of Education, National Engineering Lab for
Textile Fiber Materials and Processing Technology, School of Materials
Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018,
P R China

2. NMG Composites Co.Ltd., Chongfu, Tongxiang, Zhejiang, 314511, China

3. Faculty of Textile, Technical University of Liberec, 46117 Liberec,
Czech Republic

\textsuperscript{*}E-mail address:Caiyr@zstu.edu.cn (Cai Y);

\textbf{Abstract}

Biomass-derived wastes as the additive of non-degradable plastics have
been paid more attention due to the ever-growing environmental pollution
and energy crisis. Herein, the spent coffee grounds (SCG) have been used
as fillers in polypropylene (PP) after the heat treatment to realize its
recycling utilization. The effect of the heat treatment atmosphere on
the properties of the obtained SCG and SCG/PP composites has been
investigated systematically. The results show that the residual coffee
oil can be removed more thoroughly under an air atmosphere than under a
nitrogen atmosphere at a relatively low cost and an eco-friendly
process. The lower residual oil rate of SCG is beneficial to improve the
comminution and further enhance the affinity with the PP matrix. The
obtained SCG/PP composite holds lower water absorption, higher
hydrophobicity, and better mechanical properties, implying its potential
applications in the field of food packaging.

\textbf{Keywords}

Spent coffee grounds; heat treatment atmosphere; residual oil rate;
composites; mechanical properties

\textbf{Research highlights}

\begin{itemize}
\tightlist
\item
  Spent coffee grounds have been used as fillers in PP after the heat
  treatment.
\item
  The heat treatment in the air is more favorable for the removal of the
  coffer oil of SCG.
\item
  The low residual oil rate in SCG can improve its comminution and
  affinity with PP.
\item
  The SCG/PP composite holds excellent performances for food packaging
  applications.
\end{itemize}

\textbf{1 INTRODUCTION}

With the increasing awareness of green environmental protection, the
non-incineration utilization of biological waste is one of the subjects
that has been widely studied in many countries. Coffee is one of the
world's top three drinks, with huge consumption every year(Franca and
Oliveira, 2019). Spent coffee grounds (SCG), wastes from the coffee
production process, are rich in fiber, oil, and protein (Ballesteros et
al., 2014; Bomfim et al., 2022; McNutt and He, 2019) and have been
developed to recycle utilization in clothing (Kalebek, 2021), organic
fertilizer (Ragauskait\selectlanguage{polish}ė \selectlanguage{english}and \selectlanguage{polish}Š\selectlanguage{english}link\selectlanguage{polish}š\selectlanguage{english}ien\selectlanguage{polish}ė, \selectlanguage{english}2022; Santhanarajan et al.,
2021), medicine (Jamari et al., 2021; Nurman et al., 2021) and other
fields. In addition, SCG is contained in natural pigments to dodge or
reduce the use of artificial colors (Arya et al., 2022; Koh and Hong,
2019; Parra-Campos and Eduardo Ordonez-Santos, 2019). Biomass natural
fiber has the advantages of low cost, abundant sources, and better
biodegradability (Madyaratri et al., 2022).

It is worth noting that microplastic (Frias and Nash, 2019) produced by
the slight degradation of conventional plastic polypropylene (PP) can
directly harm human beings by polluting soil (Watteau et al., 2018;
Zhang et al., 2018), fresh water (Rodrigues et al., 2018), and air
(Zhang et al., 2020). Degradable plastics are conducive to solving these
problems, such as starch-based plastics, polylactic acid (PLA), etc. But
their application is always constrained due to the high development
costs, certain performance limitations, and immature manufacturing
technology.

A certain amount of biodegradable biomass waste is added to traditional
plastics as fillers to prepare biomass-plastic composite materials
(Bensalah et al., 2021; Qasim et al., 2020), which can effectively
utilize the waste biomass resources, reduce the use of plastics and
upgrade the degradability of the plastic. The characteristics of SCG,
particularly the residual oil content, are vital for the performance of
the subsequent SCG/PP composite plastics (Kalebek, 2021). The lower oil
content in SCG is believed to obtain composites with a denser fiber
structure and higher mechanical properties (Wu et al., 2016;
Zarrinbakhsh et al., 2016). The extraction methods of oil from biomass
include organic solvent extraction (Mueanmas et al., 2019; Unugul et
al., 2020), ultrasonic-assisted extraction (Le et al., 2020),
pressurized liquid extraction (de Almeida-Couto et al.,
2022)\textsuperscript{,} and pyrolysis (Kumar et al., 2020). Hesham
Moustafa et al. (Moustafa et al., 2017) heat-treated SCG at 270 \selectlanguage{ngerman}°C in
the nitrogen atmosphere. The obtained SCG was less oil and more
hydrophobic. The resulting SCG/PBAT composite had better mechanical
properties. Carmen Branca et al. (Branca et al., 2006, 2005) specified
that the non-inert atmosphere is also beneficial to the extraction of
bio-oil from cellulose and wood. Nonetheless, the effect of the
atmosphere on SCG and SCG plastic composites has not been studied
systematically.

In the present study, the effect of heat treatment parameters on the
properties of SCG and SCG/PP composite plastics is investigated from the
point view of the atmosphere. The results show that the SCG treated
under air atmosphere is less oil and higher abrasive than under nitrogen
atmosphere at the same temperature, which holds better affinity with PP
by increasing the hydrophobicity at a relatively low cost and an
eco-friendly process. The resulting SCG/PP composite is lower water
absorption, elevated hydrophobicity, and mechanical properties, which
have potential applications in the field of food packaging.

\textbf{2 MATERIALS AND METHODS}

\textbf{2.1 Materials}

The Polypropylene T30S (Homopolymer, the density is 0.905
g·cm\selectlanguage{english}\textsuperscript{-3}, the MFI index is 2.6
g*10min\textsuperscript{-1}, and the melting point is 170 degC) was
purchased from China Petroleum and Chemical Corporation Limited. SCG was
from a local coffee shop. Petroleum ether having a boiling range of
60-90 \selectlanguage{english} with analytical pure was purchased from Hangzhou Gaojing Fine
Chemical Co., LTD.

\textbf{2.2 Preparation of the SCG}

The SCG was dried in a 105 degC oven for 24 h, and then the dried SCG was
heat treated in a tube furnace under nitrogen and air atmosphere at 240
degC for 1.5 h with the heating speed of 5 degC*min\textsuperscript{-1},
respectively. The obtained SCG were named as NSCG and ASCG corresponding
to the heat treatment atmosphere of nitrogen and air separately.

\textbf{2.3 Preparation of the composite specimens}

The 25 wt.\% SCG, NSCG, and ASCG were mechanically mixed with PP,
respectively. Then they were processed at 190 \selectlanguage{english} 60 rpm for 15 minutes in
a torque rheometer with a reverse rotating head. Finally, the obtained
samples were pressed into thin sheets at 190 degC for 5 minutes with a
pressure of 10 MPa, and then taken out for use after cooling.

\textbf{2.4 Characterization}

2.4.1 Characterization of SCG

Mobile phone camera (Vivo S7), Scanning electron microscope (Vltra 55),
and optical microscope (Nikon eclipse ts100) were used to observe the
morphologies of SCG. Fourier transform infrared spectrometer was used to
analyze the functional groups of SCG before and after heat treatment.
Three kinds of SCG powder samples were ground with KBr at a mass ratio
of 1:100 and pressed into thin discs. KBr was used as a carrier, and the
spectral range is between 4000 and 500 cm\textsuperscript{-1} with 32
scans. The Water contact angle of SCG was measured by a video contact
angle instrument (JY-82A). The residual oil content of SCG, NSCG, and
ASCG was determined by using a Soxhlet extraction device and petroleum
ether was selected as the solvent. If the refractive index of the
petroleum ether after extraction is the same as that of the original
petroleum ether, it indicates that the oil extraction is complete. The
calculation formula of residual oil content is as follows:

\(\omega=\frac{m_{\text{SCG}}-m_{\text{Oil}}}{m_{\text{SCG}}}\times 100\%\)(1)

where m\textsubscript{SCG} is the mass of SCG and m\textsubscript{Oil}is
the mass of residual oil.

The calculation formula of relative weight loss of NSCG and ASCG is as
follows:

\(M=\frac{m_{1-m_{2}}}{m_{1}}\ \times 100\%\) (2)

where m\textsubscript{1} is the mass of SCG before heat treatment and
m\textsubscript{2} is the mass of SCG after heat treatment.

2.4.2 Characterization of SCG/PP composites

The morphologies of SCG/PP composites was observed by optical microscope
(Nikon eclipse ts100). The water contact angle was tested with a video
contact angle meter (JY-82A). The samples were dried at 50 \selectlanguage{english} for 24 h
according to GB-T 1034-2008 to test the water absorption rate. The
weight of the dried sample (m\textsubscript{0}) was measured with an
analytical balance with a precision of 0.01 g. The samples were immersed
in tap water at room temperature for 24 h and then removed to measure
their weight. The calculation formula of water absorption rate is: water
absorption rate = (m\textsubscript{t} - m\textsubscript{0})/
m\textsubscript{0}x100 \%, where m\textsubscript{0} represents the
initial weight and m\textsubscript{t} represents the weight of the
sample after soaking for 24 h. All tests were performed at room
temperature. A thermogravimetric analyzer (NETZSCH TG 209 F1) was used
to analyze the thermal decomposition process of samples. The temperature
was increased from 30 degC to 600 degC at a rate of 20 \selectlanguage{english}\textbf{*}
min\textsuperscript{-1} under the nitrogen atmosphere. Thermal analysis
of PP, SCG/PP, NSCG/PP, and ASCG/PP composites was performed using
differential scanning calorimetry (DSC 25, TA Instruments instrument)
equipped with a liquid nitrogen cooling system (REFRIGERATED COOLING
SYSTEM 90). 3-8 mg samples were taken respectively. First, the samples
were heated from room temperature to 200 degC at a rate of 10
degC*min\textsuperscript{-1} for 5 minutes, then cooled to room
temperature at a cooling rate of 10 degC*min\textsuperscript{-1}, and then
heated to 200 degC at the same rate of 10 degC*min\textsuperscript{-1} for 5
minutes. The minimum nitrogen flow is 50 mL*min\selectlanguage{english}\textsuperscript{-1}. The
melting enthalpy was obtained during the second heating process and
crystallization parameters were obtained through the cooling stage.
Crystallinity Xc = (\selectlanguage{greek}Δ\selectlanguage{english}H\textsubscript{f}/\selectlanguage{greek}Δ\selectlanguage{english}H\textsubscript{0}) \selectlanguage{ngerman}× 100\%,
where \selectlanguage{greek}Δ\selectlanguage{english}H\textsubscript{f} is the enthalpy of melting and
\selectlanguage{greek}Δ\selectlanguage{english}H\textsubscript{0}is the enthalpy of melting of crystalline PP with
crystals of 100\% (\selectlanguage{greek}Δ\selectlanguage{english}H\textsubscript{0}=209 J\selectlanguage{ngerman}·g\textsuperscript{-1}).
Undercooling degree \selectlanguage{greek}Δ\selectlanguage{english}T= T\textsubscript{m}-T\textsubscript{c}, where
T\textsubscript{m} is the equilibrium melting point and
T\textsubscript{c} is the crystallization temperature. The mechanical
properties of the dumbbell-shaped samples were tested by a universal
testing machine (INSTRON 5934) at a rate of 50 mm\selectlanguage{ngerman}\textbf{·}
min\textsuperscript{-1}according to ASTM D882. Each sample was tested in
parallel five times.

\textbf{3 RESULTS}

\textbf{3.1 Basis of the heat treatment process}

The TG curves of SCG in different atmospheres show in Figure 1. The mass
loss of the two atmospheres is roughly the same at 310 °C. There have
5\% of the mass loss in the first stage occurred at 200 °C, probably due
to the volatilization of free and bound water (Zarrinbakhsh et al.,
2016). The most significant mass loss begins at 240 \selectlanguage{english}. It is caused by
the breakdown of small molecular weight substances and hemicellulose.
After 310 \selectlanguage{english}, ASCG is oxidized and decomposed. To avoid excessive weight
loss of SCG, a heat treatment temperature of 240 degC was selected. Lignin
and cellulose decompose at lower temperatures.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{FIGURE 1} The TG curves of SCG under \textbf{(a)} N2
and\textbf{(b)} air atmosphere

\textbf{3.2 Characterization of SCG}

The influence of the heat treatment atmosphere on the color and
microstructure of SCG was investigated. The digital images of SCG, NSCG,
and ASCG are shown in Figure 2 (a-c). The colors of the SCG, NSCG, and
ASCG are light brown, dark brown, and light black, respectively. The
blackening degree of the ASCG is darker than that of the NSCG. The color
shifts indicate that SCG will be carbonized to some degree at the same
temperature, regardless of the atmosphere.~In addition, the SEM images
corresponding to the three samples are shown in Figure 2 (d-f). It can
be found that they are agglomerate. But after heat treatment, the
surface of particles becomes fluffy and the adhesion degree of particles
decreases markedly. It is the result of partial volatilization of small
molecular weight materials and low molecular weight hemicellulose (van
der Stelt et al., 2011).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{FIGURE 2} The digital images of \textbf{(a)} SCG,
\textbf{(b)}NSCG , and \textbf{(c)} ASCG; The SEM images of \textbf{(d)}
SCG,\textbf{(e)} NSCG, and \textbf{(f)} ASCG

Through the microscope, we observed the SCG under different atmospheres
and statistically analyzed its particle size distribution, as shown in
Figure 3. Figure 3 ( a-c ) shows that the original SCG is brown, and the
color after heat treatment in different atmospheres is black. This is
the same as the color reflected in the digital camera. Figure 3 ( d-f )
corresponds to the particle size distribution of SCG, NSCG , and ASCG,
respectively. From the distribution map, the particle size of the three
SCG showed a normal distribution, and the average particle size of the
SCG was 0.204 mm, while the average particle size of the SCG after heat
treatment in different atmospheres decreased to 0.188 and 0.186,
respectively. The decrease of particle size indicates that heat
treatment can effectively improve the grindability of SCG under the same
grinding time, especially in air atmosphere.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{FIGURE 3} The microscope images of \textbf{(a)} SCG,\textbf{(b)}
NSCG , and \textbf{(c)} ASCG; The distribution map of\textbf{(d)} SCG,
\textbf{(e)} NSCG, and \textbf{(f)} ASCG

To observe the influence of the atmosphere on the functional groups in
SCG, we obtained the FTIR spectra of SCG samples, as shown in Figure 4
(a). 3600-3100 cm\textsuperscript{-1} is ascribed to the vibration peak
of the O-H group. 3000-2700 cm\textsuperscript{-1} is attributed to the
tensile vibration region of C-H. The range of oxygen-containing
functional groups is between 1800 and 900 cm\textsuperscript{-1}, such
as carbonyl group, carboxyl group, an ether bond, etc. (Silverstein and
Bassler, 1962). The effect of heat treatment on the functional groups of
SCG is insignificant.~However, the peak intensity of ASCG diminished
significantly, because heat treatment reduced the content of O-H groups
and other oxygen-containing functional groups (Hussin et al., 2018).

The bar chart of SCG residual oil content and SCG relative weight loss
rate is shown in Figure 4 (b). The residual oil content of the SCG used
in this experiment is about 14\%, and that of NSCG is about 13\%, with
little change, but the ASCG is close to 10\%. The results showed that
air atmosphere heat treatment was more beneficial to the oxidative
decomposition of oil in SCG. In terms of relative weight loss, ASCG is
twice that of NSCG. The cause is the evaporation of oil and other
ingredients.

The results of the water contact angle of the three SCG samples are
shown in Figure 4 (c). The water contact angle of SCG is 93.86\selectlanguage{ngerman}°,
indicating that SCG showed weak hydrophobicity. The water contact angles
of NSCG and ASCG are 100.39° and 103.44°, respectively. The
hydrophobicity of SCG is enhanced, especially ASCG. The O/C ratio of SCG
decreased, resulting in enhanced hydrophobicity (van der Stelt et al.,
2011).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{FIGURE 4} \textbf{(a)} FTIR spectra, \textbf{(b)} Residual oil
content and weight loss of different SCG, \textbf{(c)} Water contact
angle of SCG samples

Through the characterization of SCG, a series of data show that
atmosphere has a certain influence on SCG. The most important
performance is the residual oil content, which affects the diversity and
comminution of SCG. In addition, hydrophobicity also changed
significantly. Then we explore the effect of SCG, NSCG, and ASCG as
fillers on PP performance.

\textbf{3.3 Characterization of SCG/PP composites}

3.3.1 Dispersion of SCG in PP matrix and Mechanical properties

The dispersion of SCG, NSCG, and ASCG in PP was observed by microscope
and the mechanical properties of the samples are shown in Figure 5. As
can be seen from the picture, SCG (a) has a large agglomeration and
uneven distribution in PP, and the dispersion of NSCG (b) has not been
significantly improved. However, ASCG (c) aggregates in PP became
smaller and dispersed evenly. Combined with the character analysis of
SCG, this observation can be attributed to the fact that the residual
oil content of SCG and NSCG is greater than that of ASCG. The presence
of oil causes agglomeration and affects the dispersity of SCG.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{FIGURE 5} Microscope images of composites: \textbf{(a)}
SCG/PP,\textbf{(b)} NSCG/PP, and \textbf{(c)} ASCG/PP

3.3.2 Water contact angle and Water absorption of SCG/PP composites

The water contact angle is one of the methods to characterize the
hydrophilicity of materials. Figure 6 (a) is the result of the water
contact angle. It shows the contact angle of PP is 84.37\selectlanguage{ngerman}°, exhibiting
weak hydrophilicity. The contact angle of the SCG/PP composite is 79.7°,
which is smaller than that of PP. The water contact angle of NSCG/PP and
ASCG/PP composites are 84.14° and 87.31°, respectively, which is close
to or greater than that of PP. The contact angle of SCG/PP is more
hydrophilic. It may be due to the poor dispersion of SCG in PP,
resulting in high surface roughness and surface energy of the material,
so the water wettability is stronger. After heat treatment, the
dispersion of SCG in PP is uniform, the interface compatibility is
better, and the relative probability of defects is small.

The water absorption capacity of plastic is closely related to its
service life, so we test the water absorption rate of the samples, as
shown in Figure 6 (b). It is worth noting that SCG/PP has the highest
water absorption rate of 1.41\%, followed by NSCG/PP with 1.2\%, and
ASCG/PP has the lowest water absorption rate of 0.9\%. The
hydrophobicity of the filler itself will affect the water absorption of
the composite. With the addition of filler, the water absorption of the
composites increased significantly. In addition, the water absorption
for composites is closely related to the interfacial gap and defect. The
material with better interface compatibility has less water absorption.
The above data prove that SCG with air atmosphere heat treatment is more
advantageous as a filler in solving the problem of interface
compatibility. Food packaging materials should have good ability to
block water. The stronger the hydrophobicity and the weaker the water
absorption capacity, the more valuable it is in the preparation of
finished products.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{FIGURE 6} \textbf{(a)} Water contact angle, \textbf{(b)} Water
absorption of PP and SCG/PP composites

3.3.3 Thermal stability

To study the influence of SCG under different heat treatment conditions
on the thermal stability of PP composite, the thermal decomposition
curve of the composite was tested, as shown in Figure 7. The initial
thermal decomposition temperature of PP is 385 \selectlanguage{ngerman}°C. After adding filler,
the thermal stability of PP decreases, which is manifested as the
advance of the initial thermal decomposition temperature. The thermal
decomposition trend was similar after the addition of fillers, and the
first stage of thermal decomposition was attributed to the partial
decomposition of SCG. Compared with the SCG/PP composite, the thermal
stability of the NSCG/PP and ASCG/PP composite are slightly improved,
and the initial thermal decomposition temperature is increased from 265
\selectlanguage{english} to 280 \selectlanguage{english}. After heat treatment, the O/C ratio of SCG decreases and has
stronger thermal stability (Leow et al., 2021). However, the thermal
stability of SCG/PP composites treated in different atmospheres is
similar, indicating that SCG fillers treated in different atmospheres
have little effect on the thermal stability of PP. The results of the
specific thermogravimetric values are summarized in Table 1.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{FIGURE 7} \textbf{(a)} TG, \textbf{(b)} DTG curve of PP and
SCG/PP composites

\textbf{TABLE 1} Mass loss temperature of PP and SCG/PP composites\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Sample & Initial mass loss / & Maximum mass loss / & Final mass loss
/\tabularnewline
PP & 385 & 468 & 500\tabularnewline
SCG/PP & 265 & 473 & 505\tabularnewline
NSCG/PP & 280 & 476 & 505\tabularnewline
ASCG/PP & 280 & 476 & 505\tabularnewline
\bottomrule
\end{longtable}

3.3.4 DSC

The melt-crystallization behavior of the polymer is determined by DSC.
Figure 8 is represented the DSC curves of PP and SCG/PP composites. It
can be seen from the melting curve (a) that the melting peak moves to
low temperatures at different degrees, which is the result of the
dilution effect caused by the addition of filler. In addition, the
crystallinity of PP decreased after the addition of SCG, because the
regularity of PP molecular chain was destroyed by the addition of filler
SCG. In addition, the cross-linking effect between the two restricts the
movement of polymer molecular chain, and the crystal growth of PP is
restricted, thus reducing the crystallization ability. The
crystallization curve (b) shows all the crystallization peaks are single
exothermic peaks, indicating that SCG does not affect the crystal type
of PP. During the cooling process, the crystallization peak moved
slightly to a high temperature after the addition of SCG, and the degree
of undercooling decreased, indicating that SCG played a role in
promoting heterogeneous nucleation and making PP crystallize at a higher
temperature. Table 2 is the data table of DSC test results.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{FIGURE 8} The DSC curves of PP and SCG/PP composites

\textbf{TABLE 2.} DSC results of pure PP and SCG/PP composites\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
Sample & T\textsubscript{m} / & \selectlanguage{greek}Δ\selectlanguage{english}H\textsubscript{m}
/J\selectlanguage{ngerman}·g\textsuperscript{-1} & T\textsubscript{c} / & \selectlanguage{greek}Δ\selectlanguage{english}H\textsubscript{c}
/J\selectlanguage{ngerman}·g\textsuperscript{-1} & \selectlanguage{greek}Δ\selectlanguage{english}T / & X\textsubscript{c} /\%\tabularnewline
PP & 144.2 & 130.6 & 103.5 & 109.7 & 40.7 & 62.49\tabularnewline
SCG/PP & 143.2 & 109.6 & 105.9 & 92.2 & 37.3 & 52.44\tabularnewline
NSCG/PP & 143 & 110.2 & 104 & 91.1 & 39 & 52.73\tabularnewline
ASCG/PP & 143.5 & 107.9 & 106.6 & 89.2 & 36.9 & 51.63\tabularnewline
\bottomrule
\end{longtable}

3.3.5 Mechanical properties

The mechanical properties of the samples are shown in Figure 9. Figure 9
(a) is the tensile strength of the sample. The tensile strength of the
composites are lower than that of pure PP. Compared with PP, the tensile
strength of SCG/PP and NSCG/PP composites decreased by 56.3 \% and 53.3
\%, respectively, while ASCG/PP only decreased by 34.4 \%. As shown in
Figure 9 (b), the elongation at break decreased by 61.9 \%, 67.2 \% and
43.8 \%, respectively. In addition, the elastic modulus of the composite
increases, as shown in Figure 9 (c). The larger the elastic modulus, the
harder the material. SCG acts as an inactive filler, which causes force
concentration inside pp, resulting in a decrease in the tensile strength
of pp. The different dispersion degree of the three fillers in PP leads
to the difference of mechanical properties. In contrast, ASCG / PP is
stronger and tougher. This can meet the most basic anti-cracking
requirements for food packaging materials.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{FIGURE 9} The mechanical properties of PP and SCG/PP composites

\textbf{4 CONCLUSION}

In summary, SCG was successfully used as a filler for PP to produce
biomass-plastic composites. The coffee oil in SCG is removed more
efficiently under air atmosphere than under nitrogen atmosphere at the
same temperature. The obtained SCG holds a better affinity with PP by
increasing the hydrophobicity, which further improves the water
absorption, hydrophobicity, and mechanical properties of the resulting
SCG/PP composite. The job offers a simple method for the preparation of
biomass-plastic composites at a relatively low cost and an eco-friendly
process. The composite shows potential applications in the field of food
packaging.

\textbf{Acknowledgement}

This research work was financially supported by bilateral joint R\&D
project of Zhejiang Province (2022C04027).

\textbf{CONFLICT OF INTEREST STATEMENT}

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to
influence the work reported in this paper.

\textbf{DATA AVAILABILITY STATEMENT}

All data generated or analyzed during this study are included in this
published article.

\textbf{ORCID:} \textbf{Yurong Cai.}

\emph{http://orcid.org/0000-0003-1942-8931}

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