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\begin{frontmatter}

\title{Title of the paper}

% One \author per person. The letter in brackets matches an \affiliation.
\author[a]{Your Name\corref{cor1}}
\ead{you@example.com}

\author[b]{Second Author}
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\affiliation[a]{organization={Department of Something, University of Somewhere},
                city={Somewhere},
                postcode={12345},
                country={Country}}

\affiliation[b]{organization={Institute of Something Else, Other University},
                city={Othertown},
                postcode={67890},
                country={Country}}

\cortext[cor1]{Corresponding author}

\begin{abstract}
% One paragraph: the problem, what you did, what you found.
Recycled polyethylene is blended with virgin resin to keep processing windows
wide enough for extrusion, but the blend degrades sooner than either component
alone. We measured the onset of thermal degradation for blends containing 0 to
60 percent recycled content, using thermogravimetric analysis under nitrogen at
three heating rates. Onset temperature falls by 19~K across that range, and the
apparent activation energy falls by 24 percent, with most of the change
occurring below 30 percent recycled content. The residual catalyst carried in
the recycled fraction accounts for the shape of that curve, and a short washing
step recovers about half of the lost stability.
\end{abstract}

\begin{keyword}
polyethylene \sep recycling \sep thermal degradation \sep thermogravimetry
\end{keyword}

\end{frontmatter}

\section{Introduction}
\label{sec:introduction}
% What the problem is, why it matters, what this paper adds.
Mechanical recycling returns polyethylene to the melt at a fraction of the
energy cost of new resin, and the fraction of recycled content in commodity
film has risen steadily for a decade. The limit on that fraction is not
mechanical: blends well above 60 percent recycled content meet the tensile
requirements of most film applications. The limit is thermal. Recycled resin
carries residue from its first life, and the blend that results begins to
degrade at a lower temperature than virgin resin, narrowing the window between
the temperature at which the material flows and the temperature at which it
breaks down.

That window is what a process engineer actually sets a line by, and it is
poorly documented for blends. Published degradation data covers virgin grades
in detail \cite{peterson2018} and post consumer waste streams in bulk
\cite{ferreira2020}, but the intermediate compositions that industry runs are
usually characterized in house and not reported. Where blends have been
studied, the heating rate is often fixed, which makes the activation energy
impossible to separate from the measurement \cite{tanaka2019}.

This work measures the onset of degradation across the full blend range at
three heating rates, so that the activation energy can be estimated rather than
assumed. Section~\ref{sec:methods} describes the materials and the procedure,
and Section~\ref{sec:results} reports the measurements and what they imply for
processing.

\section{Materials and methods}
\label{sec:methods}
Virgin low density polyethylene of melt flow index 0.9~g per 10~min was dry
blended with post consumer recyclate from a single sorting facility, at
recycled contents of 0, 10, 20, 30, 45 and 60 percent by mass. Each blend was
compounded in a twin screw extruder at 190~degrees Celsius and pelletized.

Thermogravimetric analysis ran on samples of 10~mg under nitrogen at 40~mL per
minute, at heating rates of 5, 10 and 20~K per minute, from 40 to 700~degrees
Celsius. Onset temperature was taken as the intersection of the tangents before
and after the first mass loss step. Every composition was measured three times
and the reported value is the mean.

The apparent activation energy $E_a$ follows from the shift in onset
temperature with heating rate $\beta$, using the Kissinger relation,
\begin{equation}
  \label{eq:kissinger}
  \ln \frac{\beta}{T_p^2} = -\frac{E_a}{R T_p} + C,
\end{equation}
where $T_p$ is the temperature of maximum mass loss rate, $R$ is the gas
constant, and $C$ collects the terms that do not depend on the heating rate. A
straight line fitted to the three heating rates gives $E_a$ from its slope.

\section{Results and discussion}
\label{sec:results}
Table~\ref{tab:onset} reports onset temperature at 10~K per minute and the
activation energy fitted across all three rates. Both fall with recycled
content, and both fall fastest at the low end: the first 30 percent of recycled
material costs 14~K of the total 19~K.

\begin{table}[t]
  \centering
  \caption{Onset of degradation at 10~K per minute, and apparent activation
  energy fitted over three heating rates.}
  \label{tab:onset}
  \begin{tabular}{rrr}
    \toprule
    Recycled content (\%) & Onset (degrees Celsius) & $E_a$ (kJ per mol) \\
    \midrule
     0 & 421 & 243 \\
    10 & 416 & 232 \\
    20 & 411 & 221 \\
    30 & 407 & 205 \\
    45 & 404 & 192 \\
    60 & 402 & 185 \\
    \bottomrule
  \end{tabular}
\end{table}

A curve of that shape points to a contaminant that saturates rather than to a
property of the recycled polymer itself. Washing the recyclate in dilute
detergent before compounding recovered 8~K of the 19~K at 60 percent content,
which is consistent with residual catalyst and metal fines on the particle
surface \cite{ferreira2020}. It is not consistent with chain scission during
the first processing life, which would scale with the recycled fraction rather
than saturate.

For a film line, the practical reading of Equation~\eqref{eq:kissinger} and
Table~\ref{tab:onset} is that the safe processing window narrows by about 14~K
as soon as any meaningful recycled content is used, and then barely narrows
further. A line already qualified for 30 percent recycled content can usually
take 60 percent without a further change to the temperature profile.

\section{Conclusions}
Onset of thermal degradation in recycled polyethylene blends falls sharply over
the first 30 percent of recycled content and then levels off, and the apparent
activation energy follows the same shape. A washing step before compounding
recovers about half of the loss, which supports surface contamination rather
than polymer damage as the cause.

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