18
G. Martínez-Barrera et al.
2 Methods for Recycling or Reusing Polyester Resin
As it is known, the most current recycling processes are identified in three groups:
mechanical, chemical, and thermal recycling. In the case of mechanical recycling,
this in a first stage, follows the shredding and grinding processes; then a method for
separation of each component, and finally its application depends on the final use.
Such recycling process has been used for to obtain thermostable polymers, which
are used as fillers in different materials.
In the thermal recycling, chemical structure and composition are taking into
account. Polyester has a disordered macroscopic network, but in some case, this
can be partially crystalline. The disordered state is related to the vitreous performance. Moreover, in a molten state, it can have liquid-viscous characteristics. In
the case of the recycling of polyester resin, the thermal process requires of temperatures ranging from 300 to 1000 °C. After applying such heat, thermal energy is
produced. The processes involved in such treatment involves pyrolysis, gasification
or combustion.
Chemical recycling, unlike the mechanical one, is considered like a process that
improves the properties of waste materials [3]. Polyester resin recycling are related
to degradation processes; which are made with chemicals (organic or inorganic solvents), or aggressive mixtures. The processes involve depolymerization, removal of
monomers, and decomposition of the polymer chains [8, 9].
Thermoset polyester can be recovered by subcritical hydrolysis using sodium
hydroxide (NaOH) and potassium hydroxide (KOH) as catalyst. A high performance
styrene-fumaric acid (SFC) copolymer can be recovered by this process; which has
advantages over the thermolysis method; because glass fibers and the inorganic materials are recovered after pyrolysis, however the thermoset resin is not recovered. In
this chemical treatment, styrene chains are produced during the cross-linking with
polyester molecules. If all ester bonds trapped by styrene chains are hydrolyzed,
styrene-fumaric acid copolymer (SFC) is obtained; whose molecular structure is
similar to that of styrene maleic acid copolymer, which is a functional polymer. The
conditions of the recycling process, include to use 0.38 mol/L of KOH, and two
hours of reaction at 230 °C; or use 0.72 mol/L and one hour of reaction at 230 °C.
Such conditions generate a resin conversion rate of 82% using NaOH, and 92% by
KOH. These values are due to the significant reactivity of subcritical water, which
is promoted by its higher ion and low dielectric constant, equivalent to those of
organic solvents. The SFC copolymer could be used as raw compound or additive
for manufacturing novel resins and functional polymers [6].
Other recycling method consist in the mixture of waste polyester-based materials
and phenolic resins, in order to obtain high-performance flame-retardant panels,
which are used in the construction and electrical industries. In such composites,
particle size of the designer polyester materials must be reduced, for their use as
reinforcements. Then, they are mixed with phenolic resins (that act as binders). The
resulting composites have low density, high fire resistance, high electrical and thermal
isolation; moreover, smoke toxicity can be obtained in case of ignition [10].
G. Martínez-Barrera et al.
2 Methods for Recycling or Reusing Polyester Resin
As it is known, the most current recycling processes are identified in three groups:
mechanical, chemical, and thermal recycling. In the case of mechanical recycling,
this in a first stage, follows the shredding and grinding processes; then a method for
separation of each component, and finally its application depends on the final use.
Such recycling process has been used for to obtain thermostable polymers, which
are used as fillers in different materials.
In the thermal recycling, chemical structure and composition are taking into
account. Polyester has a disordered macroscopic network, but in some case, this
can be partially crystalline. The disordered state is related to the vitreous performance. Moreover, in a molten state, it can have liquid-viscous characteristics. In
the case of the recycling of polyester resin, the thermal process requires of temperatures ranging from 300 to 1000 °C. After applying such heat, thermal energy is
produced. The processes involved in such treatment involves pyrolysis, gasification
or combustion.
Chemical recycling, unlike the mechanical one, is considered like a process that
improves the properties of waste materials [3]. Polyester resin recycling are related
to degradation processes; which are made with chemicals (organic or inorganic solvents), or aggressive mixtures. The processes involve depolymerization, removal of
monomers, and decomposition of the polymer chains [8, 9].
Thermoset polyester can be recovered by subcritical hydrolysis using sodium
hydroxide (NaOH) and potassium hydroxide (KOH) as catalyst. A high performance
styrene-fumaric acid (SFC) copolymer can be recovered by this process; which has
advantages over the thermolysis method; because glass fibers and the inorganic materials are recovered after pyrolysis, however the thermoset resin is not recovered. In
this chemical treatment, styrene chains are produced during the cross-linking with
polyester molecules. If all ester bonds trapped by styrene chains are hydrolyzed,
styrene-fumaric acid copolymer (SFC) is obtained; whose molecular structure is
similar to that of styrene maleic acid copolymer, which is a functional polymer. The
conditions of the recycling process, include to use 0.38 mol/L of KOH, and two
hours of reaction at 230 °C; or use 0.72 mol/L and one hour of reaction at 230 °C.
Such conditions generate a resin conversion rate of 82% using NaOH, and 92% by
KOH. These values are due to the significant reactivity of subcritical water, which
is promoted by its higher ion and low dielectric constant, equivalent to those of
organic solvents. The SFC copolymer could be used as raw compound or additive
for manufacturing novel resins and functional polymers [6].
Other recycling method consist in the mixture of waste polyester-based materials
and phenolic resins, in order to obtain high-performance flame-retardant panels,
which are used in the construction and electrical industries. In such composites,
particle size of the designer polyester materials must be reduced, for their use as
reinforcements. Then, they are mixed with phenolic resins (that act as binders). The
resulting composites have low density, high fire resistance, high electrical and thermal
isolation; moreover, smoke toxicity can be obtained in case of ignition [10].