Page 27 from: Recycling Technology 2020

27
2020

Research

produce a pellet used as a secondary raw ma-
terial. However, mechanical recycling has a
number of limitations, the main one being the
inherent inability to produce a virgin-com-
parable product. If we are to recycle more of
our packaging waste and develop new mar-
kets for recovered resources, the only way to
obtain truly virgin-quality recycled material
is through non-mechanical or chemical re-
cycling.

Primarily, three different types of technol-
ogy are being developed for recycling pack-
aging into virgin-quality polymers: pyrolysis,
solvent dissolution and chemical depolymer-
isation. Each uses a different process and is
suitable, or has been developed, for different
polymers.

Pyrolysis
Pyrolysis is the breakdown of material at

elevated temperatures in the absence of oxy-
gen. This has been used successfully for many
years on biomass to produce oil but has not
yet been widely used on plastic waste. Sig-
nificant work has been done on developing
pyrolysis processes for waste plastics, with
commercial installations already in place. For
example, a company in Spain is successfully
using pyrolysis to recycle agricultural film.

Pyrolysis is most suitable for polyole-
fins – polyethylene (PE) and polypropylene
(PP) – as they are simple hydrocarbons. It
produces a range of hydrocarbon products,
some of which can be used as a precursor to
new chemical products, including polymers.
Often light hydrocarbon fractions are burnt
on site to generate power/heat for the process.

Although researchers are trying to develop
technologies that accept all kinds of mixed
waste, it is likely these materials will have to be
sorted and cleaned before they can undergo
pyrolysis. Potentially, new virgin-quality pol-
ymer can be made from the resulting oil. The
process is particularly relevant for PE because
low-density polyethylene (LDPE) is so widely
used in film. However, mechanical recycling
of film is quite challenging and putting recy-
cled content back into film is very difficult.

Realistically, the only long-term solution
is to break it back down into an oil. While the
concept of taking the oil to a new virgin poly-
mer is quite new, large petrochemical compa-
nies such as BASF and SABIC are now inves-
tigating the possibility of producing polymers
from naphtha on a commercial scale in part-
nership with pyrolysis technology providers.

Engagement was low in the past but the
entire supply chain is beginning to work to-
gether. With a growing need to address flexi-
bles, pyrolysis may be the most feasible option
for maximum recovery.

It is essential that big companies with
money, know-how, equipment and invest-
ment power are engaged and part of the
search for solutions.

Solvent dissolution
Solvent dissolution involves selective

extraction of polymers using solvents. A
relatively pure feedstock, such as washed PP
from margarine tubs and other packaging, is
dissolved in a suitable solvent. Any colour-
ants, additives and non-target materials are
removed, and the resulting polymer can be re-
formed. This differs from pyrolysis because
the polymer is not broken down, making its
recovery for use in new products easier.

Solvent dissolution presents some key op-
portunities and can result in high yields of re-
covered polymer with only a 3-5% loss. There
are significantly fewer ‘by-products’ produced
compared to pyrolysis and the process re-
moves all additives, including pigments and
inks. A major benefit is that it can be used to
create food-grade recycled rPP – for example,
margarine tubs can be turned back into new
ones. It can also be used to recover PE or PP
from multi-layer, non-recyclable packaging
such as single-use sachet products, the col-
lection and recycling of which would help to
reduce pollution issues such as plastics enter-
ing the oceans.

Solvent recycling has its challenges: sol-
vents can be expensive and solvent recovery
is key.

The more non-target material present, the
higher the cost of recycling owing to potential
solvent loss. Feedstocks require high levels of
sorting and purity before dissolution and the
process is technically challenging to develop
and operate.

Companies actively involved in recycling
plastics through solvent dissolution include
ones based in the USA, the Netherlands and
Germany.

Chemical depolymerisation
Chemical depolymerisation is the break-

ing down of a polymer into either monomers
or intermediates using a chemical. It is being
developed primarily for polyethylene tere-
phthalate (PET), which can be broken down
using either water (hydrolysis) or monoethyl-
ene glycol (glycolysis). PE and PP cannot be
broken down in such a controlled way as the
polymer chain does not have any distinctive
‘cutting’ points.

Similar to solvent recycling, the process
removes additives and colourants while con-
tamination is filtered out. The monomers are
polymerised to produce virgin-quality PET.

As PET is used almost exclusively in fibre

Author:
Richard McKinlay graduated from
The University of Manchester with
a master’s degree in Chemical
Engineering and is currently Head
of Consulting at Axion. Richard has

worked on a range of research, development and
demonstration projects in the waste and resource
sector specialising in plastics packaging. He now
manages Axion’s consultancy activities focused
on helping other organisations transition to the
circular economy through innovative and practical
solutions.

and packaging, chemical depolymerisation
offers significant recycling advantages. It is
particularly effective for PET trays, which can
be difficult to recycle mechanically because of
their quality.

Challenges with chemical depolymerisa-
tion include the need to prepare the feedstock
to remove as much contamination as possible.
It is significantly more expensive than generic
mechanical processes because of adding the
chemical step to collection, sorting, washing
and shredding operations. Furthermore, the
environmental impact of chemical recycling
is not completely clear as only now are lab-
oratory-scale trials and pilot projects being
scaled up.

Large-scale plants capable of processing at
least 50 000 tonnes per year are needed for eco-
nomic viability. Although the cost of scaling-
up and the investment needed have previously
been prohibitive, companies are beginning to
see the potential of this exciting technology.
Virgin polymer producer Indorama has in-
vested in companies in the USA and the Neth-
erlands, while Plastipak has announced plans
for a processing operation in Italy.

The right direction
Public pressure is prompting the plastic

packaging sector into action and into invest-
ing in new recycling technologies to deal with
waste materials. Huge investment is needed,
as is further refinement of these processes,
but we are very much moving in the right
direction. If we want to recycle everything,
chemical recycling is the way we must go. Me-
chanical recycling techniques will only take
us so far.

Non-mechanical recycling can overcome
the limits of mechanical recycling to produce
high-quality end materials or recover value
from non-recyclable materials. However,
the increased cost and potential yield com-
promise of non-mechanical recycling means
mechanical recycling still plays a vital role.

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