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\DOI{10.5802/crchim.258}
\datereceived{2023-06-09}
\daterevised{2023-07-18}
\dateaccepted{2023-08-29}
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\dateposted{2024-01-17}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Materials and Clean Processes for Sustainable Energy and Environmental Applications}
%%\def\cdr@specialtitle@french{Mat\'eriaux et proc\'ed\'es propres pour des applications \'energ\'etiques et environnementales}
%\makeatother

\title{Development of a reliable and efficient combined grinding and
separation process for PVC-coated fabric waste}

\alttitle{D\'{e}veloppement d'un processus combin\'{e} fiable et
efficace pour le broyage et la s\'{e}paration des d\'{e}chets de tissus
enduits de PVC}

\author{\firstname{Walid} \lastname{Chaouch}\CDRorcid{0000-0002-7441-0884}\IsCorresp}
\address{Textile Engineering Laboratory - LGTex, University of
Monastir, Monastir, Tunisia}
\email[W. Chaouch]{walid.chaouch@isetkh.rnu.tn}

\author{\firstname{Slah} \lastname{Msahli}}
\addressSameAs{1}{Textile Engineering Laboratory - LGTex, University of
Monastir, Monastir, Tunisia}

\keywords{\kwd{PVC-coated polyethylene terephthalate
fabric}\kwd{Chemical separation}\kwd{Grinding
process}\kwd{Sieving}\kwd{Characterization}}

\altkeywords{\kwd{Tissu en poly\'{e}thyl\`{e}ne t\'{e}r\'{e}phtalate
enduit de PVC}\kwd{S\'{e}paration chimique}\kwd{Processus de
broyage}\kwd{Tamisage}\kwd{Caract\'{e}risation}}

\begin{abstract} 
This study aims to highlight a mechanical recycling process
specifically tailored for PVC-coated fabric waste. While ample
information exists on the recycling of pure PVC, a noticeable lack of
information regarding PVC-coated fabrics has been identified in the
literature.

Therefore, the primary challenge lies in accomplishing a thorough
separation between PET fibers and PVC powder while simultaneously
ensuring the optimal recovery of small PVC particles. This separation
is crucial for maintaining the quality and purity of the resulting
materials.

The effectiveness and efficiency of this process were evaluated through
a comprehensive analysis that incorporated qualitative and quantitative
studies. 

For the qualitative study, an optical microscope was utilized to
provide a rapid and non-destructive approach for evaluating the overall
properties of the samples. This method provided valuable insights into
the shapes, interactions, and cohesive bonds among the different
components. In contrast, the quantitative study involved a chemical
method that resulted in a destructive process. Using this technique,
precise values were obtained, including the exact percentage of each
constituent.

By integrating the findings from both qualitative and quantitative
studies, informed decisions can be made regarding the selection and
profitability of the method for achieving a complete separation between
PVC powder and PET fibers.

The results obtained demonstrated a successful recovery rate of 83.92\%
for PVC compared with the initial amount. The remaining 16.07\% of PVC
was found to be either entangled with PET fibers or lost during the
grinding process, particularly in the pulverizer machines.
\end{abstract} 

\begin{altabstract} 
L'article vise \`{a} mettre en \'{e}vidence un processus de recyclage
m\'{e}canique sp\'{e}cifiquement adapt\'{e} aux d\'{e}chets de tissus
enduits de PVC. Bien que de nombreuses informations existent sur le
recyclage du PVC pur, un manque notable d'informations concernant les
tissus enduits de PVC a \'{e}t\'{e} identifi\'{e} dans la
litt\'{e}rature.

Par cons\'{e}quent, le d\'{e}fi principal r\'{e}side dans la
r\'{e}alisation d'une s\'{e}paration compl\`{e}te entre les fibres de
PET et la poudre de PVC, tout en assurant simultan\'{e}ment la
r\'{e}cup\'{e}ration optimale des petites particules de PVC. Cette
s\'{e}paration est cruciale pour pr\'{e}server la qualit\'{e} et la
puret\'{e} des mat\'{e}riaux obtenus.

L'efficacit\'{e} et l'efficience de ce processus, ont \'{e}t\'{e}
\'{e}valu\'{e}es par une analyse approfondie int\'{e}grant des
\'{e}tudes qualitatives et quantitatives.

Pour l'\'{e}tude qualitative, un microscope optique a \'{e}t\'{e}
utilis\'{e} pour fournir une approche rapide et non-destructrice
permettant d'\'{e}valuer les propri\'{e}t\'{e}s globales des
\'{e}chantillons. Cette m\'{e}thode a fourni des informations
pr\'{e}cieuses sur les formes, les interactions et les liaisons
coh\'{e}sives entre les diff\'{e}rents composants obtenus. En revanche,
l'\'{e}tude quantitative a utilis\'{e} une m\'{e}thode chimique
impliquant un processus destructif. Cette technique a permis d'obtenir
des valeurs pr\'{e}cises, y compris le pourcentage exact de chaque
constituant.

En int\'{e}grant les r\'{e}sultats des \'{e}tudes qualitatives et
quantitatives, des d\'{e}cisions \'{e}clair\'{e}es peuvent \^{e}tre
prises concernant la s\'{e}lection et la rentabilit\'{e} de la
m\'{e}thode permettant d'obtenir une s\'{e}paration compl\`{e}te entre
la poudre de PVC et les fibres de PET.

Les r\'{e}sultats obtenus ont montr\'{e} un taux de
r\'{e}cup\'{e}ration de PVC r\'{e}ussi de 83,92~\%, par rapport \`{a}
la quantit\'{e} initiale. Les 16,07~\% restants de PVC ont \'{e}t\'{e}
retrouv\'{e}s soit entrelac\'{e}s avec les fibres de PET, soit perdus
lors du processus de broyage, notamment dans la machine de
pulv\'{e}risation.
\end{altabstract} 

\maketitle
\vspace*{25pt}

\twocolumngrid

\end{noXML}

\section{Introduction}

Artificial leather is a type of coated textile substrate created by
applying a soft layer of polyurethane (PU) or polyvinyl chloride (PVC)
onto a textile fabric base. Polyvinyl chloride (PVC) is widely used in
the \mbox{coating} industry because of its affordability, low density,
excellent insulation properties, and high mechanical and thermal
characteristics. However, PVC is inherently rigid and brittle,
necessitating the use of plasticizers to enhance its flexibility when
applied as a coating~\cite{1,2,3,4}. To formulate plasticized PVC, also
known as plastisol, polyvinyl chloride powder (resin), plasticizers
(such as diisononyl phthalate DINP),\linebreak calcium carbonate filler,
heat stabilizers, and pigments are combined~\cite{4,5}. The
plasticizers and other additives are gradually added to the dry PVC
resin and homogenized using a mechanical stirrer. Vacuum treatment is
employed to eliminate air bubbles from the mixture, and when producing
expanded layers, a puffing agent is incorporated along with the
\mbox{additives}~\cite{6,7}.

In applications where a thicker coating is required, PVC-coated
polyethylene terephthalate (PET) fabric can be created by applying
multiple successive coating layers~\cite{7,8,9}. This process typically
involves three layers, as depicted in Figure~\ref{fig1}: (1)~a thin PVC
superficial layer that serves for esthetic or technical enhancements,
(2)~a PVC expanded layer that functions as a sponge-like middle layer,
and (3)~a textile substrate layer composed of PET fabric, which imparts
flexibility and strength to the material.

\begin{figure*}
\includegraphics{fig01}
{\vspace*{-.3pc}}
\caption{\label{fig1}Different layers of PVC-coated fabrics.}
{\vspace*{-.3pc}}
\end{figure*}

The production and consumption of PVC-coated fabrics generate
significant amounts of waste. During the manufacturing process, scrap
is discarded from the production lines. In industrial applications,
most PVC-coated fabric-based parts are designed for single use,
resulting in their disposal as waste once they become non-functional.

Similar to other composite materials, PVC-coated fabrics pose specific
challenges in terms of waste disposal. The primary difficulty lies in
the irreversibility of the PVC coating. Once a PVC part undergoes
complete fusion and curing, it becomes impossible to recover the
plastisol from the material~\cite{10}.

Indeed, the issue of waste is exacerbated, particularly in the case of
PVC-coated PET fabrics, as they possess inherent resistance to
degradation, making it challenging for microorganisms to break them
down~\cite{11,12}. When these PVC-coated fabrics are discarded in the
environment, complete destruction may take several years. In addition,
the presence of numerous additives in plastic-derived materials
\mbox{further} contributes to aquatic pollution~\cite{13,14}.
Therefore, the urgent need to identify an efficient approach for the
treatment and recovery of PVC-coated fabrics has emerged as a critical
environmental \mbox{concern}.

In the realm of waste management and recycling, PVC-coated fabric waste
poses unique challenges that set it apart from pure PVC. These
challenges stem from the distinct properties of PET fabric and
plasticizer PVC, such as differing melting temperatures, densities, and
hardness. Therefore, \mbox{conventional} methods that effectively
handle pure PVC cannot be directly applied to PVC-coated fabric waste.
The mixing of these compounds, PET fabric and plasticizer PVC, during
the recycling process compromises their original properties and
practical functionality. Hence, the key hurdle in recycling PVC-coated
fabric waste lies in effectively separating the PET fibers from the
plasticizer PVC during the recycling process~\cite{14}. It is crucial
to identify and implement the most appropriate combination of recycling
and separation methods to address the environmental concerns resulting
from the accumulation of PVC-coated fabric waste. By recognizing the
optimal methods, a powerful strategy can be developed to mitigate the
impact of PVC-coated fabric waste \mbox{pollutants}.\looseness=-1

Extensive research has been conducted in the scientific literature to
explore a wide range of recycling and separation methods specifically
for pure PVC
waste~\cite{11,13,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33}. 

Each method possesses its own set of advantages and disadvantages,
depending on the specific requirements of the user.

One commonly employed approach for disposing of PVC waste, especially
when it contains a notable amount of impurities, is
incineration~\cite{39}. Incineration, also referred to as quaternary
recycling, remains a popular method for waste volume reduction and
energy recovery~\cite{34,40}. However, incinerating PVC presents
additional challenges because of its high chlorine content, which
results in the release of substantial quantities of hydrochloric acid
(HCl) during thermal decomposition~\cite{37,38}. The emission of
hydrochloric acid gas contributes to the formation of acid
rain~\cite{12,23,26,27,28}.

An alternative recycling method for PVC-coated fabrics involves the
chemical separation of the PET fabric from the plasticized PVC. This
process begins by dissolving the PVC and then proceeds by filtering the
fibers and distilling the solvent to reclaim the plasticized
PVC~\cite{36}. Once successfully chemically isolated, PET fibers and
PVC can be transformed into valuable raw materials suitable for diverse
production processes~\cite{24,25,41,42}.

The literature~\cite{10} emphasizes that PVC dissolves effectively in
various solvents, some of which have known carcinogenic properties.
However, tetrahydrofuran (THF) is regarded as the most suitable
\mbox{because} of its exceptional dissolution ratio, which can be
achieved within just 10~min at 40~{\textdegree}C~\cite{33,43,44}.

Chemical recycling through solvent processes is generally not widely
accepted in the plastic industry for several reasons. These include the
lengthy and costly nature of the process, the need for significant
amounts of solvents, the potential alteration of the chemical structure
and physical properties of the components, and its limited suitability
for large quantities of waste~\cite{10}. To ensure the recovery of PVC
with consistent quality and minimize the release of hazardous
substances into the environment, it is crucial to explore appropriate
methods. As a result, when the source of PVC waste is known, mechanical
recycling is preferred~\cite{33,43,44}. Mechanical recycling is an
efficient technique that upholds the inherent value of plastic by
recycling waste materials into secondary raw materials while
maintaining the material's chemical structure and composition
intact~\cite{11,35}. For pure PVC, this approach provides accessible
and cost-effective grinding facilities, environmentally friendly
processes, and significant potential for profitability, particularly
when compared with chemical or physical recycling methods.

However, despite the advantages of mechanical recycling, there is a
current lack of comprehensive data available in the literature that can
be readily adopted and implemented on a large scale within the
PVC-coated fabrics recycling industry.

Therefore, an effective strategy for reducing the accumulation of
environmental pollutants caused by PVC waste, particularly in
industrial applications, would involve integrating recycling and
PVC-coated fabric waste separation technologies.

This study aims to contribute to existing knowledge by developing a
tailored process specifically for the recycling and separation of
PVC-coated fabric waste. By combining insights from both qualitative
and quantitative studies, informed decisions can be made regarding the
selection and profitability of a method that achieves complete
separation between PVC powder and PET fibers. The qualitative study
provides an overall understanding of the sample's characteristics,
whereas the quantitative study offers precise data, enabling a more
comprehensive evaluation of the separation method's effectiveness and
\mbox{efficiency}.

Ultimately, the findings of this study will provide valuable insights
into the recycling of PVC-coated fabric waste and contribute to the
development of a cleaner and greener future.

\section{Materials and methods} \label{sec2}
\subsection{Raw materials} \label{sec2.1}

The PVC-coated fabric scraps used in this recycling study were
generously provided by Plastics Company (Sayada, Monastir, Tunisia), a
specialized manufacturer of transfer coatings. The relevant parameters
of the raw material are presented in Table~\ref{tabI}.

\begin{table*} %tab1
\caption{\label{tabI}Characteristics of PVC-coated fabric scraps}
\begin{tabular}{cccc}
\thead
& Composition & Grammage g/$\text{m}^{2}$ & Percentage\\
\endthead
Coating layers & PVC & 420 & 84\%\\
Textile substrate & PET (Circular jersey, gauge~28) & \080 & 16\%
\botline
\end{tabular}
\vspace*{.15pc}
\end{table*}

\subsection{Plastic scrap grinder machine} \label{sec2.2}

The CMB Grinder machine was specifically designed for the primary
grinding of thermosensitive PVC and other plastics to reduce them into
smaller pieces. Equipped with powerful blades, this machine efficiently
and swiftly grinds plastic scrap, facilitating the recycling process.
The technical parameters employed for this machine are provided in
Table~\ref{tabII}.

\begin{table} %tab2
\caption{\label{tabII}Main technical parameters of the CMB grinder
machine}
\begin{tabular}{cc}
\thead
Model & MS 36\\
\endthead
Rotating blades & 3 blades per 2 rotors\\
Fixed blades & 2 blades per 2 rotors\\
Capacity & 500~kg/h
\botline
\end{tabular}
\vspace*{.4pc}
\end{table}

\subsection{SMF plastic pulverizer} \label{sec2.3}

The Carthage Plastic Company's pulverizer (Kaala Kbira, Soussa,
Tunisia) was used in this study.

The plastic pulverizer machine is chiefly designed to grind PVC and
fibers in micronized form. 

This pulverizer uses a high-speed rotor. This rotor, which consists of
several sections of disks and several blades, is responsible for
grinding PVC waste into a fine powder. Its output can reach
70--350~kg/h (20--120~mesh). The technical parameters used are listed
in Table~\ref{tabIII}.

\begin{table} %tab3
\caption{\label{tabIII}Main technical parameters of the SMF plastic
pulverizer}
\begin{tabular}{cc}
\thead
Model & SMF 600\\
\endthead
\morerows{1}{Milling fineness} & 20--120 (mesh)\\
& 841--125 ($\upmu$m)\\
Rotor diameter & 600 (mm)\\
Capacity & 70--350~kg/h
\botline
\end{tabular}
\vspace*{-.4pc}
\end{table}

\looseness=-1
In this process, the use of a vibrating sieve is crucial to ensure the
desired output size. The vibrating sieve comprises two outlets: the
fine powder outlet~(7) and the coarse powder outlet~(6) (refer to
Figure~\ref{fig2}). The fine particles that successfully pass through
the sieve mesh are collected at the fine powder outlet~(7). On the
other hand, the coarse powders retained by the sieve are directed to
the milling chamber for further grinding using the coarse powder
outlet~(6).

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Plastic pulverizer.}
{\vspace*{.85pc}}
\end{figure*}

Regarding the PVC-coated fabric scraps, the fine powder particles are
directed through the vibrating sieve with the assistance of air flow
and collected in the product collector located at the fine powder
outlet~(7), designated as outlet~P. However, the PET fibers, which are
retained by the sieve, are vacuumed and subsequently recovered in a
cloth bag connected to the coarse powder outlet~(6), identified as
outlet~F (see Figure~\ref{fig3}).

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Pulverizer outlets: (a)~recovered PVC powders
(outlet~P). (b)~Recovered fibers (outlet~F).}
\end{figure*}

\subsection{Optical microscopy} \label{sec2.4}

The morphology and structure of the expanded PVC layers were examined
using a Leica DM 500 \mbox{optical} microscope. The microscope was
equipped with various objectives connected to a color view camera and
controlled by analysis software. The observations were conducted in the
reflection mode at different magnifications of the objective.

To ensure accurate characterization of the particle images, it was
necessary to utilize magnification and numerical aperture settings that
provided sufficient resolution. The magnification factors employed
ranged from 6.3$\times$ to 40$\times$, with the commonly used eyepieces
typically set at 6.3$\times$--25$\times$.

\subsection{Test sieve shaker} \label{sec2.5}

Machine sieving is conducted by stacking the sieves in ascending order
of aperture size and placing the powder on the top sieve
(Figure~\ref{fig4}).

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Machine sieving.}
\end{figure*}

A closed pan, acting as a receiver, is positioned at the bottom of the
stack to collect the fines. A lid is placed at the top to prevent the
loss of powder. This ``nest'' of sieves is vibrated until the residue
on each sieve contains particles that can pass through the upper sieve
but not through the lower sieve. 

Typically, the results obtained from the sieving process are expressed
as a cumulative percentage based on the nominal sieve aperture. For the
analyses, RETSCH standard sieves were used.

\subsection{Chemical separation} \label{sec2.6}

In the chemical separation procedure, tetrahydrofuran (THF) was used as
the solvent to accurately isolate PET from PVC. A piece of the coated
fabric was subjected to conditioning in an oven at 105~{\textdegree}C
for over 5~h, followed by cooling to room temperature in a desiccator.
The conditioned sample was then weighed and immersed in an Erlenmeyer
flask containing THF solvent while being stirred.{\break}
The~\mbox{solubility} of PVC in THF depends on various factors such as
the liquor ratio, temperature, and time.

To establish the effectiveness of the chemical method, several
preliminary tests were conducted at different temperatures (20, 40,
60~{\textdegree}C) and durations (5--40~min). These tests aimed to
validate the test protocol for chemical separation. It was observed
that the solubility of PVC increased with higher{\break} temperatures,
which were maintained using a silicone bath.

\subsection{Distillation} \label{sec2.7}

In this study, distillation served as the second step in the separation
process, primarily aimed at extracting the nonvolatile PVC from the THF
solvent (Figure~\ref{fig5}).

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}Distillation.}
\end{figure*}

The homogeneous mixture comprising PVC and THF solvent was heated in a
closed distillation flask. Because of its volatile nature, the THF
solvent evaporated, forming a vapor. This vapor was subsequently cooled
by passing it through the condenser,{\break} resulting in the
condensation of pure THF solvent in a collection vessel. The recovered
THF solvent can be reused for subsequent tests. The evaporation of the
solvent facilitated the retrieval of the PVC component.
\vspace*{-.15pc}

\subsection{Sampling for analyses} \label{sec2.8}

The sampling procedure in this study aims to ensure that the collected
sample accurately represents the complete physical, chemical, and
morphological characteristics of the system under investigation. There
are two key rules that should be followed during the sampling process:
\begin{enumerate}[(1)]
\item Sampling of the powder should be conducted while it is in motion
to prevent agglomerate formation. This ensures that the sample obtained
is a true representation of the dispersed powder particles.
\item The entire sample stream should be collected over multiple short
time increments, rather than selecting only a portion of the stream for
the entire duration. This approach helps to capture variations in the
powder composition and properties that may occur over time.
\end{enumerate}

Therefore, in this study, the sampling procedure involved extracting
small portions of powder from the bulk sample during the pulverization
process, ensuring that the samples were taken from a moving powder
stream. These samples were collected over several short time increments
and later combined to form a representative sample, minimizing any
potential sampling issues that may arise due to powder
\mbox{segregation}.
\vspace*{-.15pc}

\section{Results and discussion} \label{sec3}
\subsection{Grinding process of PVC-coated fabric scraps}
\label{sec3.1}

\looseness=-1
The process of obtaining PVC powder from PVC-coated fabric scraps
involves several stages of cutting and grinding. Initially, the waste
PVC-coated PET fabric is cut into small pieces using a band saw,
ensuring that their dimensions are suitable for the subsequent steps
and equipment, with a maximum dimension of $1~\text{m}^{2}$.

Subsequently, a first grinding process is conducted using a plastic
waste grinder machine to crush the PVC-coated fabric component and
\mbox{generate} a bulk mixture, typically ranging between 10 and 15~mm
in size. During this stage, the PVC remains combined with the PET knit,
and they are not yet separated from each other.

Next, a second grinding process is employed using a pulverizer. This
process further shreds the plasticized PVC component, resulting in the
production of fine PVC powders that are effectively separated from the
PET fabric (see Figure~\ref{fig3}). An additional advantage of this
grinding process, particularly with the involvement of a pulverizer, is
its capability to not only grind the coated fabric scraps into PVC
powder but also separate them from PET fibers. This characteristic
provides an additional benefit to the overall recycling process. For
visual representation, Figure~\ref{fig6} illustrates the considered
grinding process map.

\begin{figure*}
\includegraphics{fig06}
{\vspace*{-.2pc}}
\caption{\label{fig6}Grinding process map.}
{\vspace*{-.4pc}}
\end{figure*}

The following section presents a comprehensive qualitative and
quantitative analysis conducted to investigate the behavior and
effectiveness of the separation between PET fibers and PVC powder.

\subsection{Qualitative and quantitative analyses} \label{sec3.2}

\looseness=-1
By integrating qualitative and quantitative approaches, a comprehensive
understanding of the behavior and efficiency of the separation between
PET fibers and PVC powder can be achieved. This synergy combines the
advantages of rapid observations with the precision of constituent
percentage measurements, resulting in a comprehensive analysis.

\subsubsection{Qualitative analyses using microscopic{\hfill\break}
observations} \label{sec3.2.1}

The qualitative study relies on the use of an optical microscope, which
enables rapid and non-destructive assessment of the shapes and
characteristics of the obtained components. This approach provides an
insightful overview and facilitates a deeper understanding of the
behavior exhibited by the samples, while ensuring that no damage is
caused. Such observations play a crucial role in making initial
decisions related to the process.

Separate quantities of PVC powder and PET fibers obtained from outlets
(P and F) after pulverization were examined using image processing
techniques. An optical microscope equipped with a high-resolution
FEIQ250 Thermo-Fisher was used.

According to Figure~\ref{fig7}, the PET fibers obtained from
``outlet~F'' are observed to be covered with \mbox{residual} PVC.
Microscopic observations reveal that these fibers are intricately
entangled with the PVC particles, making their mechanical separation
\mbox{challenging}.

\begin{figure*}
\includegraphics{fig07}
{\vspace*{-.3pc}}
\caption{\label{fig7}PET fibers recovered at ``outlet~F''.
(a)~Magnification 6.3$\times$. (b)~Magnification 16$\times$.}
\end{figure*}

Fibers were also observed in the case of output~``P'' (see
Figure~\ref{fig8}), indicating that the grinding process did not
achieve complete separation of PVC from the PET fabric. To obtain more
meaningful conclusions regarding the separation efficiency between PVC
powder and PET fibers, a quantitative study was conducted.

\begin{figure*}
\includegraphics{fig08}
{\vspace*{-.3pc}}
\caption{\label{fig8}PVC powder recovered at ``outlet~P''.
(a)~Magnification 6.3$\times$. (b)~Magnification 16$\times$.}
\end{figure*}

\subsubsection{Quantitative analyses using chemical{\hfill\break}
separation} \label{sec3.2.2}

This quantitative study relies on a chemical method that involves the
solubilization of PVC in THF. Despite the destructive and
time-consuming nature of this method, it offers the advantage of
yielding precise values, including the exact percentage of each
constituent, after the grinding process.

To ensure reproducible results, it is essential to perform reliable
sampling (see Section~\ref{sec2.8}) for all analytical procedures prior
to subjecting the samples to any analysis.

\paragraph{Chemical separation} \label{sec3.2.2.1}

The use of THF solvent revealed that the optimal performance for PVC
removal was achieved at 40~{\textdegree}C within a 10-min timeframe,
with a liquor ratio of 1:6 (Figure~\ref{fig9}a). This outcome aligns
with findings in the existing literature~\cite{45}. Following the
treatment, the solution was filtered under an oil-pump vacuum, and the
resulting insoluble PET fibers were thoroughly washed with water.
\mbox{Subsequently}, the fibers were dried at 105~{\textdegree}C for
24~h (Figure~\ref{fig9}b).

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}(a)~Homogeneous mixture containing a PVC and THF
solvent. (b)~Recovery of insoluble PET fibers.}
{\vspace*{-.3pc}}
\end{figure*}

In this study, distillation was employed as the second step of the
separation process to extract the THF solvent from the nonvolatile PVC
(Figure~\ref{fig5}).

As depicted in Figure~\ref{fig10}, the distillation process resulted in
the recovery of highly pure plasticized PVC, which was devoid of any
PET fibers or residual THF solvent. The recovered THF solvent can be
reused in the next process cycle.

\begin{figure*}
\includegraphics{fig10}
\caption{\label{fig10}Plasticized PVC. (a)~Magnification 6.3$\times$.
(b)~Magnification 16$\times$.}
\end{figure*}

\paragraph{Chemical separation efficiency} \label{sec3.2.2.2}

Before quantifying the percentages of PET fibers and PVC powder, it is
crucial to assess the effectiveness of the chemical separation method.

Microscopic examination (Figure~\ref{fig11}) following THF treatment
reveals that the knitted structure, composed of intermeshed loops, is
visible. These results confirm that the THF solvent effectively
dissolves the PVC without causing degradation of the PET fibers.

\begin{figure*}
\includegraphics{fig11}
\caption{\label{fig11}Insoluble knitted textile substrate of PET
fibers. (a)~Magnification 6.3$\times$. (b)~Magnification 16$\times$.}
\end{figure*}

Furthermore, to validate the complete removal of the coated PVC and the
absence of any residual THF solvent from the surfaces of the textile
substrate, analysis using Fourier Transform Infrared Spectroscopy
(FTIR) is recommended (Figure~\ref{fig12}).

\begin{figure*}
\includegraphics{fig12}
\caption{\label{fig12}Infrared spectrum of poly(ethylene
terephthalate).}
\end{figure*}

The band assignments for PET have already been reported in the
literature~\cite{46,47,48} and are listed in Tables~\ref{tabIV}.

\begin{table*} %tab4
\caption{\label{tabIV}Band assignments for the infrared spectrum of
PET}
\begin{tabular}{cc}
\thead
Description of vibration (Assignment) & Wavenumbers
($\text{cm}^{-1}$)\\
\endthead
Absorbed moisture & 3535\\
O--H stretching of diethylene glycol end-group & 3440\\
Aromatic C--H stretching & 3060\\
Aliphatic C--H stretching & 2960, 2880\\
Aromatic summation band & 1950\\
Carbonyl C=O stretching & 1720\\
Aromatic skeletal stretching bands & 1615, 1450, 1430,\\
- $\text{CH}_2$ - deformation band & 1465\\
C(O)--O stretching of ester group & 1270\\
Bands in the skeletal ring region indicate 1,4-substitution & 1175,
1120, and 1020\\
O--$\text{CH}_2$ stretching of ethylene glycol segment in PET & 980\\
C--H deformation of two adjacent coupled hydrogens on an aromatic ring
& 850\\
\parbox[t]{12cm}{\raggedright\hangindent 2pc Associated with the
out-of-plane deformation of the two carbonyl substituents on the
aromatic ring} & 730\vspace*{2pt}
\botline
\end{tabular}
{\vspace*{6pt}}
\end{table*}

The observed results align with the spectrum of pure PET as reported by
Ramesh {\etal}~\cite{47}.

Infrared spectroscopy further confirms that the fibers recovered after
THF dissolution are composed of pure PET, devoid of any residual PVC or
traces\linebreak of solvent. This finding complements the results obtained by
optical microscopy, providing a comprehensive understanding of the
composition and purity of the recovered fibers.

\paragraph{Determination of the different amounts of PET fibers and PVC
powder using chemical separation} \label{sec3.2.2.3}

In this case study, a 50-g sample of ground PVC-coated fabric scraps
was dissolved in THF, following the previously described method. The
PVC-coated fabric scraps used in this study exhibit the characteristics
listed in Table~\ref{tabI}.

Table~\ref{tabV} provides a comprehensive summary of the PET fibers and
plasticized PVC proportions present in the PVC-coated fabric scraps. It
should be noted that all experiments were conducted three times to
ensure accuracy and reliability.

\begin{table} %tab5
\caption{\label{tabV}PET fibers and plasticized PVC proportions present
in the PVC-coated fabric scraps}
\tabcolsep 2.1pt
\begin{tabular}{cccc}
\thead
& Average (g) & Percentage (\%) & CV (\%)\\
\endthead
PET fibers & \07.89 & 15.77 & 3.34\\
Plasticized PVC & 41.88 & 83.75 & 1.35
\botline
\end{tabular}
\end{table}

According to Table~\ref{tabV}, the analysis reveals that the PVC-coated
fabric scraps contain approximately 15.77\% PET fibers and 83.75\%
plasticized PVC.{\break} These findings closely align with the
characteristics presented in Table~\ref{tabI}, which indicated 16\% PET
fibers and 84\% plasticized PVC. The remarkable similarity between
these two datasets serves as compelling evidence that affirms the
reliability of the chemical method employed in this study.

Furthermore, Table~\ref{tabV} highlights a notable reduction in the
total mass recovered through the implementation of the chemical method
compared with the mass of the initial sample. To gain a comprehensive
understanding of the underlying mechanisms causing this decrease in
initial mass, it is essential to consider the thorough characteristic
study already undertaken, which involved meticulous microscopic
observation and Fourier Transform Infrared Spectroscopy (FTIR).

The experimental results following chemical separation demonstrate the
successful recovery of the PET knitted fabric. However, it is crucial
to \mbox{acknowledge} that despite confirming the retrieval of highly
pure plasticized PVC without any PET fibers or residual THF solvent
through distillation, several studies~\cite{10,43} suggest a potential
loss of a fraction of the plasticizer during the THF solvent extraction
process via distillation.

This loss can be attributed to the relatively volatile nature of the
plasticizer compound, which makes it prone to evaporation. As a result,
when the THF solvent is distilled off, the plasticizer also evaporates,
leading to a decrease in the overall mass after the chemical method.

For the purpose of our study, we can primarily focus on the initial
phase of PVC dissolution in the THF solvent because the successful
recovery of the PET textile substrate and determination of its
percentage allow us to infer the corresponding percentage of
plasticized PVC.

To determine the average weights of PET fibers and plasticized PVC in
each outlet (PVC powder outlet (P) and fiber outlet (F)) after
pulverization, the chemical separation method was employed. To ensure
accuracy, the experiments were conducted three times.

\begin{table*} %tab6
\tabcolsep=10pt
\caption{\label{tabVI}Different amounts of PET fibers and PVC powder
after pulverization}
\begin{tabular}{ccccc}
\thead
&& Average (g) & Percentage (\%) & CV (\%)\\
\endthead
\morerows{1}{PVC powder outlet (P) $=$ 50~g} & Fibers & \03.07 & \06.15
& 18.51\\
& PVC & 46.19 & 92.39 & \02.62\unskip\vspace*{3pt}\\
\morerows{1}{Fiber outlet (F) $=$ 50~g} & Fibers & 36.09 & 72.17 &
\04.32\\
& PVC & 12.54 & 25.08 & \05.80
\botline
\end{tabular}
{\vspace*{-.3pc}}
\end{table*}

According to the chemical separation results
(\mbox{Table}~\ref{tabVI}), the quantity recovered from outlet P
primarily comprises PVC powder, accounting for approximately 92.39\%.
However, a small amount of PET fibers, not exceeding 6.15\%, was also
detected. On the other hand, outlet F contains a higher percentage of
residual PVC, reaching 25.08\%, whereas the quantity of PET fibers
amounts to approximately 72.17\%.

These findings confirm previous observations and indicate that the
mechanical grinding process fails to achieve complete separation
between the powders and PET fibers. As mentioned earlier, the challenge
of separation remains largely unresolved, especially when it comes to
achieving complete isolation of textile fibers from plasticized PVC.

Consequently, our primary focus is to enhance the separation process
between PET fibers and PVC powder to identify the most efficient
method.{\vspace*{-.7pc}}

\subsection{Sieving separation} \label{sec3.3}

The mechanical grinding process described earlier allows the
transformation of PVC-coated fabric scraps into powders. However, it is
limited by the challenge of obtaining a clean PVC powder. To optimize
the separation efficiency between the PVC powder and fibers and to
maintain the PVC in powder form, an additional sieving step was
incorporated in conjunction with the mechanical grinding process.

The sieving process encompassed preparatory steps, including sampling
and selection of an appropriate sieve~\cite{49,50,51,52,53,54}. The
samples used for mechanical sieving were obtained in accordance with
the previously outlined sampling procedure.

\subsubsection{Selection of an appropriate sieve aperture}
\label{sec3.3.1}

To achieve optimal separation, selecting an appropriate sieve with the
correct aperture is crucial to ensure complete detachment of fibers
from the powder. Accurate prediction of this aperture using theoretical
methods or approaches is challenging because of a limited understanding
of the fundamental particle motion and screen penetration
rate~\cite{52,53}. \mbox{Therefore}, a series of sieves with various
apertures were tested to assess their separation efficiencies in terms
of PET fibers and PVC powder. 

The sieving process (Figure~\ref{fig4}) involved placing a 500~g sample
of powder onto sieves with fixed aperture sizes (2~mm, 1~mm, 0.8~mm,
0.6~mm, 0.4~mm) and agitating the sieves with an amplitude of 1.2~mm. A
receiving pan was positioned underneath all the sieves to collect the
samples.

Subsequently, the efficacy of this sieving method in separating PET
fibers from PVC powder was tested for different durations (2, 5, and
10~min). Initial observations for each sieving time (2, 5, and 10~min)
revealed that the top sieve with the largest aperture size (2~mm)
retained only the fiber clumps, allowing all other particles to pass
through the sieve apertures (Figure~\ref{fig13}). However, the
subsequent sieve with an aperture size of 1~mm retained only a small
amount of PVC powder and fiber clumps (Figure~\ref{fig14}).

\begin{figure*}
\includegraphics{fig13}
\caption{\label{fig13}Fiber clumps collected on a sieve (aperture $=$
2~mm).}
\end{figure*}

\begin{figure*}
\includegraphics{fig14}
\caption{\label{fig14}Fiber clumps with powder collected on a sieve
(aperture $=$ 1~mm).}
{\vspace*{-.4pc}}
\end{figure*}

The experimental observations can be attributed to the phenomenon of
entangled fibers forming agglomerates through interlocking, which
facilitates their separation via vibratory sieving.

During vibratory sieving, particles experience a three-dimensional
movement that combines circular motion with vertical throwing motion.
These combined motions result in increased interactions and cohesive
bonds between PET fibers, primarily because of their unique
shapes~\cite{55}. According to Blancoa {\etal}~\cite{56},
this cohesion leads to
the formation of Van der Waals forces (dipole--dipole interaction) and
frictional effects caused by electrostatic charges or forces.

As mentioned earlier, due to the cohesive forces and interlocking
nature of the fiber clumps, several of them cannot pass through the
uppermost sieve (2~mm) because their sizes exceed the mesh size of this
particular sieve.

Regarding the remaining sieves (Figure~\ref{fig15}) with aperture sizes
of 0.8, 0.6 and 0.4~mm, as well as the \mbox{receiving} pan,
microscopic examinations revealed the presence of PVC powders alone,
without any fibers. These findings were further substantiated using the
chemical separation method. Specifically, after subjecting the samples
to treatment with tetrahydrofuran (THF) solvent, no trace of fibers was
recovered, and all of the powder demonstrated solubility in the
solvent.

\begin{figure*}
\includegraphics{fig15}
\caption{\label{fig15}Retained powder with different aperture sizes:
(a)~0.8~mm, (b)~0.6~mm, (c)~0.4~mm, (d)~pan.}
\end{figure*}

Therefore, a single sieve with a 1-mm aperture size can effectively
separate the ``oversized'' fiber amount from the ``undersized'' PVC
powder.

\subsubsection{Verification of the complete separation{\hfill\break}
procedure} \label{sec3.3.2}

To achieve comprehensive separation and ensure reliable
reproducibility, an additional step was incorporated alongside the
mechanical grinding process. A simple sieving in motion, utilizing a
sieve with a \mbox{1-mm} aperture, was conducted for 5~min. The sieve
was placed on top of a collecting pan to gather the separated materials
(Figure~\ref{fig16}).

\begin{figure*}
\includegraphics{fig16}
\caption{\label{fig16}Simple sieving (aperture $=$ 1~mm).}
{\vspace*{.5pc}}
\end{figure*}

To evaluate the efficiency of the separation process, a representative
sample weighing 50~g from the powder outlet (P) was sieved using a
standard sieve with a 1-mm aperture. Subsequently, the oversize and
undersize portions obtained from the sieving process were subjected to
chemical testing. The corresponding results are presented in
Table~\ref{tabVII}. It is worth noting that all experiments were
conducted in triplicate to ensure reliability and consistency.

\begin{table*} %tab7
\tabcolsep=10pt
\caption{\label{tabVII}Different amounts of PET fibers and PVC powder
after sieving}
\begin{tabular}{ccccc}
\thead
&& Average (g) & Percentage (\%) & CV (\%)\\
\endthead
\morerows{2}{Oversize} & Total oversize weight & \03.68 & 100 & \08.94\\
& PVC & \00.25 & \06.98 & 18.41\\
& Fibers & \03.42 & 93.02 & \08.49\vspace*{5pt}\\
\morerows{2}{Undersize} & Total undersize weight & 45.92 & 100 & \00.49\\
& PVC & 45.92 & 100 & \00.49\\
& Fibers & 0 & 0 &
\botline
\end{tabular}
\end{table*}

The experimental findings presented in Table~\ref{tabVII} demonstrate
that, following the sieving process, approximately 45.92~g (equivalent
to around 91.84\%) of PVC powder was successfully recovered in the
collecting pan (undersize), while all fibers were retained (3.42~g,
approximately 6.84\%) by the 1~mm mesh sieve, indicating that a
complete separation between PET fibers and PVC powder was achieved.

These results align with those obtained from the chemical separation
analysis (Table~\ref{tabVI}), confirming that the quantity of residual
fibers in the recovered PVC powder at the outlet (P) of the pulverizer
was approximately~6\%.

The combination of the mechanical process (cutting, grinding,
pulverization) and sieving (Figure~\ref{fig17}) proves to be a
straightforward, cost-effective, environmentally friendly, and
potentially profitable approach. Moreover, this method effectively
addresses both objectives: (1) separating fibers from PVC, and
(2)~maximizing the recovery of small PVC powders.

\begin{figure*}
\includegraphics{fig17}
\caption{\label{fig17}Process map of grinding supplemented by sieving.}
\end{figure*}

The employed methods, namely microscopic observation and chemical
separation, serve the initial purpose of validating the efficacy of the
preceding process in successfully obtaining finely separated PVC
particles from the fibers. The subsequent crucial aspect involves
evaluating the yield of this process, specifically determining the
percentage of recovered PVC powder in relation to the initial quantity
of PVC introduced into the process.

\subsection{Process yield} \label{sec3.4}

Process yield, which is a widely accepted practice, provides a valuable
metric for assessing process performance, typically quantified using
Equation~(\ref{eq1}).
{\begin{equation}\label{eq1}
\text{Process Yield} = \frac{\text{Q(PVC)out}}{\text{Q(PVC)in}}
\end{equation}}\unskip\noindent
Q(PVC) out: Quantity of PVC recovered at the end of the process. 

\noindent
Q(PVC) in: Initial quantity of PVC entering the \mbox{process}.

This metric ensures that the recovered PVC quantity is neither
entangled with the fibers nor ``lost'' within the machinery
(pulverizer, sieve).

For this particular case study, the PVC-coated PET scrap (10~kg)
possesses the characteristics outlined in Table~\ref{tabI}. To
demonstrate the overall effectiveness of the process,
Table~\ref{tabVIII} presents the total mass of recovered PVC powder
after pulverization and subsequent sieving. To ensure statistical
validity, the process is repeated three times, yielding reliable and
conclusive results.

To adhere to the maximum sieved material limit of 3~kg for these test
sieves, three passes are required for each test to ensure the complete
sieving of all PVC powder derived from pulverizer outlet~P. To provide
a clearer visualization of the results, the following process map has
been included.

The results demonstrate that approximately 7.05~kg of PVC powder is
successfully recovered at the end of the process, following the sieving
step. This amount corresponds to a percentage of 83.92\% in comparison
with the initial PVC quantity (approximately 8.4~kg). Such high
efficiency (0.839) is particularly promising and encourages further
development of this process on an industrial scale.

Regarding the overall process, there is a difference of approximately
1.35~kg ($8.4-7.05$), representing approximately 16.07\%. This
discrepancy results from both agglomeration with the fibers and
significant retention within the pulverizer.

In addition, it should be noted that the quantity of PET fibers
recovered at outlet 6 (as depicted in Figure~\ref{fig18}) is combined
with the amount retrieved at outlet~4.

\begin{figure*}
\includegraphics{fig18}
{\vspace*{-.2pc}}
\caption{\label{fig18}Schematic of the recovery process.}
\end{figure*}

\begin{table*} %tab8
\caption{\label{tabVIII}Different amounts of PET fibers and PVC powder
during complete mechanical separation}
\begin{tabular}{cccccc}
\thead
&&& Average (kg) & Percentage (\%) & CV (\%)\\
\endthead
\morerows{2}{Pulverizer} & Input weight && 10 & 100 &\\
& \morerows{1}{Output weight} & Outlet F (4) & 2.06 & 20.63 & 12.8\\
&& Outlet P (5) &&&\vspace*{8pt}\\
\morerows{2}{Sieve (aperture $=$ 1~mm)} & Input weight && 7.79 & 77.87 & \03.41\\
& \morerows{1}{Output weight} & Oversize (6) & 0.73 & \09.34 & \04.91\\
&& Undersize (7) & 7.05 & 90.58 & \03.57
\botline
\end{tabular}
\end{table*}

The aforementioned experimental findings will be further explored in
subsequent publications to leverage the potential of pulverized
PVC-coated fabric scraps. Specifically:
\begin{itemize}
\item Recovered PVC powder (outlet~(7), Figure~\ref{fig18}): The
successful separation of PVC powder enables its reuse as a substitute
for PVC resin in plastisol, particularly for use in floor coverings.
Detailed results pertaining to the process of PVC-coated fabrics
derived from PVC powder waste will be presented in a forthcoming study.
\item Recovered fibers (outlets (4 $+$ 6),
Figure~\ref{fig18}):\break
These PET fibers, along with residual traces of PVC, can be
repurposed as a binder under high-temperature conditions to produce
nonwoven materials through a melt process. These initial findings are
currently undergoing laboratory testing and will be further elucidated
in subsequent studies that explore diverse applications for these
materials.
\end{itemize}

\section{Conclusion} \label{sec4}

This investigation focuses on the mechanical recycling process of
PVC-coated fabric scraps. The technique employed involves a grinding
process consisting of three main steps: cutting, first grinding, and
second grinding. These steps enable the transformation of PVC-coated
fabric scraps into PVC powder while partially separating them from the
PET fibers.

The effectiveness and yield of this process are discussed through
qualitative and quantitative analyses. 

The qualitative analysis, conducted using microscopic observation,
reveals that the PET fibers from ``outlet~F'' are entangled with the
PVC particles, posing challenges for their mechanical separation. In
contrast, the quantitative analysis utilizes tetrahydrofuran (THF) to
dissolve the plasticized PVC, providing precise values such as the
exact percentage of each constituent following the grinding process.

Combining the qualitative and quantitative analysis methods, it can be
concluded that the separation between PET fibers and the PVC powder
recovered from both pulverized outlets (outlet~P and outlet~F) is not
perfect, even though the percentage of PET fibers in the powder does
not exceed 6.15\% for outlet~P. The presence of PVC alongside PET
fibers significantly compromises the quality of the recycled material
because of the distinct properties of these two components.
Consequently, the grinding process alone is insufficient for the reuse
of PVC-coated fabric scraps.

To achieve complete separation between the PVC powder and PET fibers,
supplementary mechanical sieving using a 1-mm aperture sieve was
\mbox{implemented}. This efficient separation was corroborated by
chemical analysis, which demonstrated that approximately 90.58\% of the
PVC powders were successfully recovered through sieving, whereas all
the PET fibers were retained by the sieve. These results provide strong
evidence of the achievement of complete separation of the PVC powder.

Finally, the study reveals that while there is an overall loss of PVC
in the process, approximately 16.07\% is either lost through
agglomeration with the fibers or retained within the pulverizer.
However, it is noteworthy that 83.92\% of the initial PVC was
effectively recovered as powder by the end of the process, providing a
valuable starting point for the development of new products. This
substantial recovery of PVC powder confirms the economic feasibility
and viability of the proposed method.

Future research should also consider understanding the morphology and
size distribution of powder particles, as these factors are crucial
prerequisites for various manufacturing operations. Morphology and size
distribution have a significant impact on the functional properties of
the powder, such as density and flowability. Therefore, studying these
characteristics is essential for optimizing the manufacturing processes
and ensuring desirable functional properties of the powder.
{\vspace*{-.4pc}}

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

{\vspace*{-.4pc}}

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