Nanocomposite From Depolymerized PET Waste For Food packaging Application 5 Chapter-2 LITERATURE REVIEW Polyethyleneterephthalate (PET) PET is considered as one of the most important engineering polymers in the past two decades due to rapid growth in its use. It is regarded as an excellent material for many applications and is widely used for making liquid containers (bottles). It has excellent tensile and impact strength, chemical resistance, clarity, process ability, color ability and reasonable thermal stability. 38 Many companies produce virgin PET globally giving it different trade names. 39-40 Commercial PET has a wide range of intrinsic viscosity [η] that varies from 0.45 to 1.2 dl g -1 with a polydispersity index generally equal to 2. The PET chain is considered to be stiff above the glass transition temperature (T g ) unlike many other polymers. The low flexibility of the PET chain is a result of the nature of short ethylene group and the presence of p-phenylene group. This chain inflexibility significantly affects PET structure-related properties such as thermal transitions. 40-41 PET production process involves two different starting reactions. The first starting reaction is an esterification reaction where terephthalic acid (TPA) reacts with ethylene glycol (EG) at a temperature of between 240 o C and 260 o C and a pressure between 300 and 500 kPa. The second reaction is trans-esterification reaction, where dimethyl terephthalate (DMT) is reacted with EG at 150 o C 42 , 180–210 o C 39 , 140–220 o C at 100 kPa 43 shown in Scheme 2.1. Trans-esterification is the much preferred process due to easier purification. The output of both these processes is BHET. The pre-polymerization step follows in which BHET is polymerized to a degree of polymerization (DP) of up to 30. Pre-polymerization reactions conditions are 250–280 o C at 2–3 kPa. 39-40 The third stage is the polycondensation
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Nanocomposite From Depolymerized PET Waste For Food packaging Application
5
Chapter-2
LITERATURE REVIEW
Polyethyleneterephthalate (PET)
PET is considered as one of the most important engineering polymers in the past two
decades due to rapid growth in its use. It is regarded as an excellent material for many
applications and is widely used for making liquid containers (bottles). It has excellent tensile
and impact strength, chemical resistance, clarity, process ability, color ability and reasonable
thermal stability.38
Many companies produce virgin PET globally giving it different trade
names.39-40
Commercial PET has a wide range of intrinsic viscosity [η] that varies from 0.45
to 1.2 dl g-1
with a polydispersity index generally equal to 2. The PET chain is considered to
be stiff above the glass transition temperature (Tg) unlike many other polymers. The low
flexibility of the PET chain is a result of the nature of short ethylene group and the presence
of p-phenylene group. This chain inflexibility significantly affects PET structure-related
properties such as thermal transitions.40-41
PET production process involves two different starting reactions. The first starting
reaction is an esterification reaction where terephthalic acid (TPA) reacts with ethylene
glycol (EG) at a temperature of between 240 oC and 260
oC and a pressure between 300 and
500 kPa. The second reaction is trans-esterification reaction, where dimethyl terephthalate
(DMT) is reacted with EG at 150 oC
42, 180–210
oC
39, 140–220
oC at 100 kPa
43shown in
Scheme 2.1. Trans-esterification is the much preferred process due to easier purification.
The output of both these processes is BHET. The pre-polymerization step follows in which
BHET is polymerized to a degree of polymerization (DP) of up to 30. Pre-polymerization
reactions conditions are 250–280 oC at 2–3 kPa.
39-40 The third stage is the polycondensation
Nanocomposite From Depolymerized PET Waste For Food packaging Application
6
process, where the DP is further increased to 100. Up to this stage, PET is suitable for
applications that do not require high molecular weight (MW) or [η] such as fibers and sheets.
Solid state polymerization (SSP) might be required to produce a high MW PET. SSP is used
to increase the DP to 150, and also increasing MW. SSP operating conditions are 200–240
oC at 100 kPa for 5–25 h.
43 Bottle grade PET that has an [η] of 0.7–0.81 dl g
-1 is normally
produced by SSP at 210 oC for around 15–20 h.
44-45
COOH
COOH
TPA
CH3COO
CH3COO
DMT
COOCH2CH2OH
COOCH2CH2OH
BHET
HOCH2CH
2OH
Direct esterification-H
20
HOCH2CH
2OH
Transesterification catalyst
-CH3OH
-HOCH2CH
2OH
Polycondensation
Catalyst
HOH2CH2COOC COOCH2CH2OH
130-150
PET
Scheme 2.1: PET polymerization process
Virgin PET manufacturers have tended in recent years to produce PET co-polymer;
such as isophthalic acid modified PET, rather than homopolymer PET. PET bottles are
normally made from co-polymer PET because of its lower crystallinity, improved ductility,
better process ability and better clarity.46-47
Nanocomposite From Depolymerized PET Waste For Food packaging Application
7
Virgin PET Thermal Transitions and Crystallization
Commercial PET has a melting temperature (Tm) of between 255 and 265 oC and for
more crystalline PET is 265 oC.
42 Tg of virgin PET varies between 67 and 140
oC. The
thermal transitions and crystallization of virgin PET with a focus on reversing crystallization
and melting have been analyzed by several researchers.48-49
An interesting phenomenon was
reported in which the virgin PET experiences multiple endothermic transition during thermal
analysis.48-50
It was reported that this phenomenon is attributable to morphological and
structural re-organization. As the temperature increases, better crystal structures are
achieved because of the re-organization of the less perfect crystals. Virgin PET is well
known for having very slow crystallization rate. The highest crystallization rate takes place
at 170 oC
42, or 190
oC.
41 Cooling PET rapidly from the melt to a temperature below Tg can
produce an amorphous, transparent PET. Semi-crystalline PET can be obtained by heating
the solid amorphous PET to a temperature above Tg where 30 % crystallinity can be
achieved.41
The rate of crystallization of virgin PET depends greatly on temperature and
reaches its maximum at a temperature of 150–180 oC. The rate of crystallization also
depends on other factors such as MW, the presence of nucleating agents, the degree of chain
orientation, the nature of the polymerization catalyst used in the original production of PET
and the thermal history.39
PET Applications and Processing
PET is used broadly in products such as bottles, electrical and electronic instruments,
automobile products, house-wares, lighting products, power tools, material handling
equipment and sporting goods.39
PET films and fibers are the oldest applications of PET.
Films are produced by biaxial orientation through heat and drawing. PET films are used in
Nanocomposite From Depolymerized PET Waste For Food packaging Application
8
photographic applications, X rays sheets and in food packaging.40
PET films are also
reported to be used in electrical and for recording tapes.42
PET is also used as an electrical
insulator. PET’s insulating properties are regarded as good due to the severe restriction of
the dipole orientation at room temperature that is well below the transition temperature.42
PET fibers are another important application of PET and are produced by forcing molten
PET through small holes in a die. Fiber strength is achieved by applying tension to align the
chains through uniaxial stretching. Virgin PET is produced at different specifications
because different application requires different properties.51
PET granules can be processed
in many ways depending on the application and the final product requirements. The main
PET processes are extrusion, injection moulding and blow moulding.
PET/Montmorillonite Clay Nanocomposites
Polymer/clay nanocomposites are synthesized via melt-intercalation, common solvent
mixing, or in situ polymerization.52
In the process of melt mixing, the layered silicate is
mixed with a molten polymer matrix. If the silicate surfaces are sufficiently compatible with
the chosen polymer, then the polymer can enter the interlayer space and form either an
intercalated or an exfoliated nanocomposite.53
Exfoliation or a high level of intercalation, is
important in producing a polymer/clay nanocomposite, with such separation of individual
clay sheets the high aspect ratios are obtained with the inorganic reinforcing materials. The
synthesis of PET clay nanocomposites has not been as successful as compared with other
polymers. Takekoshi et-al54
prepared polyester clay nanocomposites via in situ
polymerization with quaternary ammonium salt modified clay and cyclic PET oligomers.
They observed good nanoparticle dispersion and improved physical properties, such as
improved impact strength and elastic modulus. A more commercially viable approach with
Nanocomposite From Depolymerized PET Waste For Food packaging Application
9
conventional polymer processing techniques is melt-mixing of the polyester with an
organicallymodified-clay.55
However, as Matayabas et al56
found, this approach has been far
less successful usually leading to poorly dispersed clay particles. This may be attributed to
the low decomposition temperature (250 °C) of the organic modifier bound to the clay
surface.
Need for PET Recycling
PET is saturated polyester of terephthalic acid and ethylene glycol. The growing
interest in PET recycling is due its widespread use mainly as jars and bottles. Since the
middle of the 1970s, first in the USA and Canada and subsequently in Western Europe,
increased quantities of PET are used for the production of soft drink bottles, and a further
increase in its application in this area is predicted. The overall world consumption of PET
currently amounts to about 13 million tons, of which 9.5 million tons is processed by the
textiles industry, 2 million tons is used in the manufacture of audio and video tapes, and 1.5
million tons is used in the manufacture of various types of packaging mainly bottles and jars.
PET is seen as a noxious material due to. its high resistance to the atmospheric and biological
agents. Ecological as well as economic considerations advocate the introduction of wide-
scale PET recycling, similar to the recycling of traditional materials such as glass, paper, or
metals.57
Classification of Polymer Recycle
Polymer recycling can be classified into four categories e.g. primary, secondary, tertiary and
quaternary recycling.
Nanocomposite From Depolymerized PET Waste For Food packaging Application