Crystallization Behavior of PET Materials

Abstract

Polyethylene terephthalate, commonly coded as PET, PETE, is a thermoplastic polymer resin of the polyesters and is used in liquid containers, drinks, food and synthetic fibres. Depending on its processing and thermal conditions, PET may exist both as amorphous and as semi-crystalline. PET may appear opaque, white and transparent depending on its crystalline and amorphous structure. Its crystallinity and consequently its physical and mechanical properties are highly dependent on processing conditions like processing temperature, cooling rate, stretching process etc. In this study, it was tried to summarize all about PET crystallization by referring to all studies carried out before. Crystallization is very significant properties affecting all mechanical and physical properties of PET just as for all kind of polymers. As a result, this subject has taken in very good interest so far and it is believed that this interest will go on increasingly.

Full text

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Crystallization Behavior of PET Materials
Bilal DEMİREL1*, Ali YARAȘ2, Hüseyin ELÇİÇEK3
1* Erciyes University Faculty of Engineering, Department of Materials Science and Engineering, Kayseri.
2Bartin University Faculty of Engineering, Department of Metallurgy and Material Engineering, Bartin.
3Bartin University Faculty of Engineering, Department of Naval Architecture and Marine Engineering,
Bartin.
Abstract
Polyethylene terephthalate, commonly coded as PET, PETE, is a thermoplastic polymer
resin of the polyesters and is used in liquid containers, drinks, food and synthetic fibres.
Depending on its processing and thermal conditions, PET may exist both as amorphous
and as semi-crystalline. PET may appear opaque, white and transparent depending on
its crystalline and amorphous structure. Its crystallinity and consequently its physical
and mechanical properties are highly dependent on processing conditions like
processing temperature, cooling rate, stretching process etc. In this study, it was tried
to summarize all about PET crystallization by referring to all studies carried out before.
Crystallization is very significant properties affecting all mechanical and physical
properties of PET just as for all kind of polymers. As a result, this subject has taken in
very good interest so far and it is believed that this interest will go on increasingly.
Keywords: Crystallization, material properties, PET
PET Malzemelerin Kristalizasyon Davranıșı
Özet
Polyethylene terephthalate, PET veya PETE șeklinde kısaltması yapılan, sıvıların,
yiyecek ve içeceklerin saklanmasında ve tașınmasında, sentetik liflerin yapımında
kullanılan polyester sınıfından termoplastik polimer bir reçinedir. Termal ve proses
șartlarına bağlı olarak, PET amorf ya da semi-kristal yapıda olabilir. Bu özelliğinden
dolayı PET donuk, beyaz ya da camsı bir yapıda olabilir. PET’in kristal yapısı, buna
bağlı olarak da fiziksel ve mekaniksel özellikleri büyük oranda ișlem sıcaklığı, soğutma
hızı, gerdirme ișlemi gibi proses parametrelerine bağlıdır. Bu çalıșmada, PET’nin
kristalizasyonu ile ilgili daha önce yapılan bütün çalıșmalar özetlenmeye çalıșılmıștır.
Tıpkı bütün polimerlerde olduğu gibi kristalizasyon PET’nin bütün fiziksel ve
* Bilal DEMİREL, bilaldemirel@erciyes.edu.tr.

DEMİREL B., YARAȘ A., ELÇİÇEK H.
27
mekaniksel özelliklerini etkileyen çok önemli bir özelliktir. Sonuç olarak, bu konu
șimdiye kadar bir hayli ilgi çekmiștir ve bu ilginin daha da artacağına inanılmaktadır.
Anahtar kelimeler: Kristalizasyon, malzeme özellikleri, PET
1. Introduction
PET has the most application among plastics and is found most commonly in daily life.
It is used especially in containers produced for storing and carrying food and liquids; in
particular carbonated soft drinks (CSDs). However, some cracking problems have been
observed at the bottom of bottles; due to either the geometrical shape of the petaloid
base or the process parameters.
In this literature review the development of the PET bottle was reviewed, followed by a
discussion of physical and chemical properties of PET and the factors that affect these
properties. Then the problem of cracks occurring in the bottle base will be reviewed
and its causes investigated in our following review paper.
2. Development of the PET bottle
PET poly (ethylene terephthalate) was developed in the 1940’s and since then it has
played an important role in the food and beverage packaging industry [1]. Due to its
popularity the use of PET in carbonated soft drinks bottles has been studied extensively
[2]. Initially, PET bottles consisted of two pieces; the blown bottle section, and a
separate ‘cap’ section fitted over the over the hemispherical bottle base. The
polyethylene cap section made the bottle self-standing. In recent times, PET bottles
have been made in one piece with a self-standing petaloid-shaped base [3].
The desirable properties of PET (clear, lightweight, high strength, stiffness, favorable
creep characteristics, low flavor absorption, high chemical resistance, barrier properties
and low price) make it the material of choice for carbonated soft drinks containers,
fibers and films [1]. Due to low cost, better aesthetic appearance, and better handling,
PET is being preferred over polycarbonate (PC) polymers [4].
PET has been also known for many years as a textile fiber forming material. But lately,
it has started to be used in extrusion foam processing for textile fibers because of its
elastic nature [5]. PET is also used as a recyclable polymer, and the markets for
recycled PET (R-PET) are growing by the year.
3. Crystallization behavior of PET
‘Crystalline’ means that the polymer chains are parallel and closely packed, and
‘amorphous’ means that the polymer chains are disordered [8]. Most polymers exist as
complex structures made up of crystalline and amorphous regions. Crystallinity is
usually induced by heating above the glass transition temperature (Tg) and is often
accompanied by molecular orientation [6]. It is impossible to reach 100% crystallinity
with the lowest free energy because polymers do not have a uniform molecular weight.

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Instead, the polymers can only react to produce partly crystalline structures, usually
called "semicrystalline" [7].
The degree of polymer crystallinity depends on both intrinsic and extrinsic factors.
Narrow molecular weight, linear polymer chain structure, and high molecular weight
are very important pre-conditions in terms of obtaining high crystallinity [8].
Crystallinity is also affected by extrinsic factors, like stretch ratio, mode of extension
and crystallization temperature in the preparation of polymer films [9]. Below the glass
transition temperature, polymer chains are rigid; after reaching the glass transition
temperature, the chains become more flexible and are able to unfold under stress. If the
temperature is above Tg and stretching is carried out, the randomly coiled and entangled
chains begin to disentangle, unfold, and straighten and some of them even slide over
their nearest neighbor chains [10].
PET is a crystallizable polymer because of its regularity in chemical and geometric
structures. It is either in the semi-crystalline state or in the amorphous state. The levels
of crystallinity and morphology significantly affect the properties of the polymers
[11]. Even with limitations in its barrier properties and mechanical strength, crystalline
PET is still widely used. Polymers with high crystallinity have a higher glass transition
temperature Tg ( Tg is 67 °C for amorphous PET and 81 °C for crystalline PET ) and
have higher modulus, toughness, stiffness, tensile strength, hardness and more
resistance to solvents, but less impact strength [11,12].
Crystallinity in PET is usually induced by thermal crystallization and/or by stress or
strain induced crystallization. Thermally induced crystallization occurs when the
polymer is heated above Tg and not quenched rapidly enough. In this condition the
polymer turns opaque due to the spherulitic structure generated by thermal
crystallization aggregates of un-oriented polymers [13]. In stress-induced
crystallization, stretching or orientation is applied to heated polymer and the polymer
chains are rearranged in a parallel fashion and become closely packed [14]. The
crystallization process is composed of nucleation and spherulitic crystallization, and
may occur at temperatures above Tg and below the melting point Tm [15]. Quenching
the melt quickly results in a completely amorphous PET [12].
Crystalline polymers have a heterogeneous structure due to the interspersed amorphous
regions while amorphous polymers in all their forms (melts, rubbers, glasses, etc.) have
a homogeneous structure. Polymers are characterized by a glass transition temperature
Tg and a melting temperature Tm [16]. The glass transition behavior of semi-crystalline
polymers are greatly affected by the factors affecting degree of crystallinity such as
molecular weight, amount of crystalline phase and morphology [11, 15, 17]. The glass
transition temperature of semi-crystalline polymer is higher and broader than that of the
amorphous polymer [11].
Crystalline polymers are characterized by a Tm and amorphous polymers are
characterized by a Tg. At the melting point, polymers are like a rubber-liquid. For
crystalline polymers, the relationship between Tg and Tm has been described as follows
[8].

DEMİREL B., YARAȘ A., ELÇİÇEK H.
29
mg TT
3
2
→ (for unsymmetrical chains) (Equation 1)
and
mg TT
2
1
→ (for symmetrical chains) (Equation 2)
PET has a Tg between 340 to 353 K (67 to 80 °C) and a Tm of 540 K (267 °C).
The crystallization of PET has been widely investigated. The Avrami equation was
adopted by [18], with using the density balance method, where the amorphous fraction
was calculated from the final density at that condition, rather than the density of 100%
crystalline PET. X-ray analyses and polarizing microscopy were used to observe
crystalline structures. Different structures could be obtained by adjusting crystallization
temperature or previous melt conditions. The maximum rate of crystallization occurs at
180 °C. Further research in this subject has also been reported [19]. Studies have been
conducted on the kinetics of crystallization of different commercial PET materials in
terms of the Avrami equation with a Differential Scanning Calorimetry (DSC) method
and confirmed that the rate constant k is very sensitive to crystallization temperature
[20]. Different PET samples have different crystallization mechanisms. With
increasing crystallization temperature, spherulite diameter increases [21]. Ozawa
studied the kinetics of dynamic crystallization of PET. He obtained crystallization
curves through DSC at different cooling rates [22]. A modified Avrami equation was
applied to the primary crystallization in a non-isothermal situation. Jabarin compared
the crystallization rate parameters of both isothermal and dynamic processes, and found
that they are similar to each other in terms of mechanisms of crystallization. A method
was developed to predict the minimum cooling rate required to obtain non-crystalline
PET [23]. DSC spectrum of PET is shown in figure 1.
In addition to time and temperature, many other factors such as pressure, the degree of
molecular orientation and environment have influence on crystallization mechanism,
morphology, and final properties of PET [24, 25]. Nucleating agents also affect the
crystallization of PET. Some studies have investigated the effect of the additives on
crystallization behavior [11, 26].
Jiang et al have studied the effects of three kinds of nucleating agents, including talc,
sodium benzoate and an ionomer (Ion., Na+) on the crystallization kinetics of PET by
using DSC. They have used Avrami and Ozawa equations to obtain the parameters of
the isothermal and the nonisothermal crystallization kinetics, respectively. They
concluded that three nucleating agents can increase the crystallization rate of PET, and
sodium benzoate has the most excellent nucleating effect for the crystallization of PET
with the same content of nucleating agent. The crystallization mode of PET might shift
from three-dimensional growth to two dimensional growth by the addition of the
nucleating agents [42].

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Figure 1. DSC spectrum of PET.
The crystallization behavior of PET with and without catalysts has been compared by
[41]. They found that nucleation has a great influence on overall crystallization rate at
low temperatures near Tg. Moisture and molecular weight have a great effect on
crystallization [23, 27]. It is found that the kinetics of crystallization depends on
molecular weight and that with increasing percentage of moisture, the half-time
crystallization and induction time of crystallization decrease. Spherulite growth rate
was independent of water absorbed [20].
In a study with characterization methods for PET, different experimentals methods have
been investigated by Faraj at al. They have used different techniques such as X-ray
diffraction (XRD), energy dispersive X-ray (EDX), differential scanning calorimetry
(DSC), thermogravimetric analysis (TGA), atomic force microscopy (AFM) and UV
spectrophotometer for characterization the structural, thermal and optical properties of
the PET. The surface morphology and optical transmittance of PET substrate have been
reported. They recommend that high quality films on PET substrate give the possibility
to use as alternative substrates to the conventional glasses [43]. XRD patterns of PET,
EDX spectrum of the PET and AFM analysis of PET are shown in figure 2, figure 3 and
figure 4 respectively.
Stress is an important factor, affecting crystallization. The effect of stress-induced
crystallization of PET has been investigated with density measurements, wide-angle X-
ray diffraction and small-angle light scattering measurements [28]. Amorphous PET
films were stretched at constant strain rates below and above Tg. The stress-induced
crystallization has also been analyzed as a function of time and orientation level [29].
Marco et al. focused on the crystallinity induced by stretching PET at temperatures
above the glass transition and on the influence of stretch and blow molding parameters
on the properties of the final product [30].

DEMİREL B., YARAȘ A., ELÇİÇEK H.
31
Figure 2. XRD patterns of PET.
Figure 3. EDX spectrum of the PET.
Figure 4. AFM analysis of PET.

BAÜ Fen Bil. Enst. Dergisi Cilt 13(1) 26-35 (2011)
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A study has been conducted with PET material and found that reducing the shot size
(amount of material injected into the mould cavity) will minimize crystallinity while
hold time (length of time the gate remains open allowing more material to be pushed
into the mold cavity) has no effect at the lower shot size. However, with a larger shot
size, a low hold time is necessary to reduce crystallinity. The least crystallinity occurs
with minimum hold time and minimum shot size [31].
In a study conducted by Hanley et al., it was found that the extent of the orientation and
crystallinity depends upon the geometry of the bottle base, and that there is an abrupt
change from the amorphous region to the crystalline regions. The valley and the
transition region to the foot are the most biaxially oriented regions of the base. The
orientation in the middle of the foot is more circular and the crystallization is less. This
shows that the stretch in this region is more uniaxial (or less biaxial) but crystalline
lamellae are still observed [32].
Some experimental works has been conducted on the orientation and crystallization of
PET films subjected to uniaxial or biaxial drawing under industrial processing
conditions [38-40]. The changes in the degree of orientation and crystallinity have been
investigated using the wide-angle X-ray scattering (WAXS) technique [39]. By
analyzing the crystalline diffraction patterns, they found that the orientation of the
developing crystals depends on the relation of the draw rate and temperature to the
chain relaxation process and that the crystallization rate is highly temperature
dependent. Everall et al. used polarized attenuated reflection infrared spectroscopy to
quantify biaxial orientation in PET films and stretch blow molded bottles [41].
Crystallization may be due to many nuclei centres forming small spherulites at low
temperatures. Larger crystal structures may be obtained when the material is
crystallized at higher temperatures or by slow cooling from the melt but 100%
crystallinity is never possible in normal processing conditions [15, 17]. Usually the
percentage crystallinity is lower than 90% [17]. In general, polymeric materials are
semicrystalline with crystalline and amorphous phases co-existing [38]. Mixed
amorphous crystalline macromolecular polymer structure is shown in figure 5 [38].
Figure 5. Mixed amorphous crystalline macromolecular polymer structure [38].

DEMİREL B., YARAȘ A., ELÇİÇEK H.
33
The morphology is described by the spherulite radius, lamellar thickness and long
period; distance between two adjacent lamellae. Small angle light scattering
microscopy and X-ray analyses are usually applied to obtain these parameters [15].
Even at the same crystallinity content, samples crystallized at higher temperature are
more opaque and brittle [18]. Samples with smaller spherulite sizes have higher yield
stress, lower ultimate elongation and high brittleness temperature and higher impact
strength [40].
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