2.2 Catalysis
2.2.1 Catalytic Cracking of Polyethylene
The cracking process of polyethylene in the recycling process is one of the most important phases. The use of
chloroaluminate (III) in the cracking of polyethylene gives
rise to classes of compounds that differ from those of conventional methods (Adams et al. 2000). The cracking process reveals that ILs are a good substitute for conventional
ones. ILs are used as co-catalysts with an inorganic acid.
When a low-density polyethylene (LDPE) is suspended in an
ionic liquid (IL), it gives rise to a distribution of products
that are not dependent on the nature of the ILs.
3 Ionic Liquids in Plastics Recycling
3.1 Plastic Recycling
Recycling processes are the best way to cater to the waste
reduction we currently are plagued with. Researchers, for
long now, have been trying to ensure cost-effective strategies
and procedures to sustain the laudable ideas on recycling
plastic waste. Most virgin hydrocarbons where plastics’
monomer units are sourced are readily available and
cheap. For recycled monomers to do well, they must be
cleared of all contaminants and relatively cheap. In recycling
plastics, contaminants are classified into: (i) input contaminants (they are also known as plastics additives) (ii) chemicals used for the recycling process (detergents or solvents),
and (iii) products that arise from degradation during processing (mainly stabilizers).
A difficulty arises in the absolute classification of these
contaminants, and the development of an effective method
for treating these plastic wastes. Chemical companies may
overtime customize various use of additives for their unique
purposes; this poses more difficulty in general recycling.
Also, reuse of plastics (mainly bottles) for other unauthorized purposes—by end-users—may further lead to complication of processing. With these complications, it may be
sometimes difficult to proffer accurate solutions to the
varying degrees of the state of plastics. However, some
general tendencies and materials must be employed. For
example, most plastics additives are not chemically bonded
to the basic polymer chain, the application of ILs in separating these contaminants are becoming useful. More
research in the specific separation processes may reveal
useful information on the removal of contaminants such as
lubricants, release agents, fillers and reinforcement,
antifoaming agents, antioxidants, plasticizers, and stabilizers. As of 2007, the most efficient and effective recycling
method was chemical recycling (Woidasky 2018).
3.2 Chemical Recycling
Chemical recycling involves the transformation of polymer
chains—under recycling processes—into monomer units
suitable for re-polymerization reactions that reform recycled
plastics (Kamimura and Yamamoto 2007). Generally, there
are several types of recycling of plastics. A few—and by
chance commonest—include energy recycling, material
recycling, and monomer recycling (Al-sabagh et al. 2015).
Many studies on recycling processes revealed monomer
recycling as the best option in terms of optimum utilization
of carbon resources. This is because, in comparison to the
other recycling forms, there is higher conservation of the
non-renewable carbon resource (Kamimura et al. 2011). The
most critical part of the recycling process is the depolymerization phase, which is the leading process of recycling.
The conventional route has always involved thermolysis or
the use of supercritical fluids, which will require high temperature and increased working pressure. Organic solvents
have run their course due to the challenges like emissions of
VOCs, a requirement of apparatus that can withstand
extreme conditions. ILs, so far, have evaded all these challenges. The stability of ILs at high temperatures and their
non-volatility makes them suitable for chemical recycling.
More importantly, the possibility of achieving a near-zero
emission process makes ILs a great option.
3.3 Chemical Depolymerization
Depolymerization involves the transformation of polymer
chains into monomer units. It is one of the most promising
techniques (Hong et al. 2017). For an efficient depolymerization process, the extraction of the IL out of the reaction
must be straightforward, i.e., it must not result in a complicated side reaction. The type of IL used plays a very
important role as a total hydrophobic mixture will make it
difficult to extract the IL. To eradicate this challenge, a
hydrophilic IL can be used. The IL can be recovered easily
by a liquid–liquid extraction (Iannone et al. 2017). The
solubility of the IL can always be achieved through design
and redesign. A study carried out in 2017 revealed the use of
several ionic solvents for depolymerization of 6-nylon. The
conclusion drawn was simple: 300 °C is an optimum
Applications of Ionic Liquids in Plastic and Lignin Waste Recycling
333
2.2.1 Catalytic Cracking of Polyethylene
The cracking process of polyethylene in the recycling process is one of the most important phases. The use of
chloroaluminate (III) in the cracking of polyethylene gives
rise to classes of compounds that differ from those of conventional methods (Adams et al. 2000). The cracking process reveals that ILs are a good substitute for conventional
ones. ILs are used as co-catalysts with an inorganic acid.
When a low-density polyethylene (LDPE) is suspended in an
ionic liquid (IL), it gives rise to a distribution of products
that are not dependent on the nature of the ILs.
3 Ionic Liquids in Plastics Recycling
3.1 Plastic Recycling
Recycling processes are the best way to cater to the waste
reduction we currently are plagued with. Researchers, for
long now, have been trying to ensure cost-effective strategies
and procedures to sustain the laudable ideas on recycling
plastic waste. Most virgin hydrocarbons where plastics’
monomer units are sourced are readily available and
cheap. For recycled monomers to do well, they must be
cleared of all contaminants and relatively cheap. In recycling
plastics, contaminants are classified into: (i) input contaminants (they are also known as plastics additives) (ii) chemicals used for the recycling process (detergents or solvents),
and (iii) products that arise from degradation during processing (mainly stabilizers).
A difficulty arises in the absolute classification of these
contaminants, and the development of an effective method
for treating these plastic wastes. Chemical companies may
overtime customize various use of additives for their unique
purposes; this poses more difficulty in general recycling.
Also, reuse of plastics (mainly bottles) for other unauthorized purposes—by end-users—may further lead to complication of processing. With these complications, it may be
sometimes difficult to proffer accurate solutions to the
varying degrees of the state of plastics. However, some
general tendencies and materials must be employed. For
example, most plastics additives are not chemically bonded
to the basic polymer chain, the application of ILs in separating these contaminants are becoming useful. More
research in the specific separation processes may reveal
useful information on the removal of contaminants such as
lubricants, release agents, fillers and reinforcement,
antifoaming agents, antioxidants, plasticizers, and stabilizers. As of 2007, the most efficient and effective recycling
method was chemical recycling (Woidasky 2018).
3.2 Chemical Recycling
Chemical recycling involves the transformation of polymer
chains—under recycling processes—into monomer units
suitable for re-polymerization reactions that reform recycled
plastics (Kamimura and Yamamoto 2007). Generally, there
are several types of recycling of plastics. A few—and by
chance commonest—include energy recycling, material
recycling, and monomer recycling (Al-sabagh et al. 2015).
Many studies on recycling processes revealed monomer
recycling as the best option in terms of optimum utilization
of carbon resources. This is because, in comparison to the
other recycling forms, there is higher conservation of the
non-renewable carbon resource (Kamimura et al. 2011). The
most critical part of the recycling process is the depolymerization phase, which is the leading process of recycling.
The conventional route has always involved thermolysis or
the use of supercritical fluids, which will require high temperature and increased working pressure. Organic solvents
have run their course due to the challenges like emissions of
VOCs, a requirement of apparatus that can withstand
extreme conditions. ILs, so far, have evaded all these challenges. The stability of ILs at high temperatures and their
non-volatility makes them suitable for chemical recycling.
More importantly, the possibility of achieving a near-zero
emission process makes ILs a great option.
3.3 Chemical Depolymerization
Depolymerization involves the transformation of polymer
chains into monomer units. It is one of the most promising
techniques (Hong et al. 2017). For an efficient depolymerization process, the extraction of the IL out of the reaction
must be straightforward, i.e., it must not result in a complicated side reaction. The type of IL used plays a very
important role as a total hydrophobic mixture will make it
difficult to extract the IL. To eradicate this challenge, a
hydrophilic IL can be used. The IL can be recovered easily
by a liquid–liquid extraction (Iannone et al. 2017). The
solubility of the IL can always be achieved through design
and redesign. A study carried out in 2017 revealed the use of
several ionic solvents for depolymerization of 6-nylon. The
conclusion drawn was simple: 300 °C is an optimum
Applications of Ionic Liquids in Plastic and Lignin Waste Recycling
333