Effect of Temperature on Morphology and Mechanical Properties of Terephthalate Co-Polyesters Processed by Lamination Extrusion Coating and Film Casting

Aim of the Work
Nowadays polymers are materials of common use, both for commodities both for advanced specialized
applications, such in biomedical or nanocomposite elds. The great success of these materials is due
to dierent factors, such as availability of raw materials, ease of processing, lightness, low costs, a
great variety of properties, etc. Moreover, the polymer industries has developed in a signicant and
rapidly way new synthetic pathways and new techniques and methods of forming, depositing, coating,
processing these materials in advantageous ways. Today polymer processes, such as extrusion, injec-
tion moulding, thermoforming, and casting, provide parts and products with controlled and specic
shapes and sizes.
To obtain these important successes materials engineers must have a broad knowledge of fundamental
principles of polymer science and follow the requests of products that must be cheaper and higher
performing. Therefore, the development of high speed production processes and new machines allow
improvements in polymer characteristics and performances.
Film extrusion is one of the most popular polymer processing technique and lms are widely used
in our daily life. Recently, because of the competition with dierent materials, lm production have
moved to higher productivity, high speed lines and high quality. A few examples of this economically
driven technology are the production of very thin lms, dimensionally stable, and multi-layered lms
for applications in food packaging.
Extrusion coating is the most important process used to realize multilayer composites, where pa-
perboard is coated with plastics to combine the mechanical properties of the paperboard with the
barrier and sealing properties of plastics. In this way paperboard is suitable for a number of specially
demanding applications. The extrusion coating is used to produce materials for liquid and solid food
packaging, industrial packaging, building, medical and hygiene purposes.
The commonly used resin used in extrusion coating is polyethylene (LDPE). However, the recently
decreasing cost of terephthalate polyester resins, above all polyethylene terephthalate (PET), has at-
tracted a certain interest in manufacturing paper based structures that comprehend layers of extrusion
coated PET.
However, the extrusion coating process has grown and optimized for handling and processing PE
resins. The dierences in properties and processing conditions between polyolens and polyesters
make very hard the setting of machinery used for the process. A lot of parameters and congurations
have to be again evaluated and optimized in order to substitute PE with PET.
More specically, PET presents the drawback that it is obtained by extrusion coating in an amorphous
state and becomes brittle as a function of time spent at room temperature. The attempt to induce a
thermal crystallization in PET during the process is not eective because crystals not oriented have
poor mechanical performances. Only semicrystalline and bi-oriented lms have good properties, stable
over the time, but bi-orientation is not achievable by extrusion coating.
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In this context, the research work developed in this thesis has the aim to analyse the embrittlement
process of PET lms and to try to overcome the instability of the amorphous phase with developing
a strategy that can be upscaled in an industrial contest.
The nal aim is to provide a solution for the problems connected with the use of PET in
the extrusion coating process. This is a rst study on this subject and preliminary results will
be presented.
The originality of the work consists also in the fact that industrial samples, such as lms obtained
by lamination extrusion coating and lm casting, provided by Tetra Pak company are
analyzed and characterized.
The rst chapter is referred to the polymeric materials used in food packaging.
An introduction into the world of the polymers through their structure, their morphology, their phys-
ical and chemical properties, with an overview on the food packaging and the interactions between
food and polymeric materials used, is presented.
In particular polyethylene (PE) and polyethylene terephthalate (PET) are examined.
The second chapter is entirely dedicated to the multilayer materials used in food packaging, their
properties and the way to produce them. A particular importance to the Extrusion Coating pro-
cess is given, with a detailed description of all the parts of the process, including set-up for LDPE.
An overview on Extrusion Lamination, Co-extrusion, Cast lm, bi-axial orientation and co-injection
stretch blow moulding is presented.
The third chapter is based on the use of PET in the Extrusion Coating process with some remarks
about the comparison between the use of polyethylene resins and polyester resins. The latest studies
on the denition of the most correct process conditions for rightly handling polyester resins in the
Extrusion Coating are described.
The fourth chapter is a focus on the materials and methods used during the thesis work.
In this chapter all the examined samples are described. Moreover, the instruments and the methods
used for the characterization of the PET lms, provided by Tetra Pak, are described.
The fth chapter is dedicated to the experimental and to the results obtained. All the aspect of the
material properties are investigated, through the use of dierent characterization technique illustrated
in the fourth chapter.
The fth chapter starts with a preliminary analysis on the Extrusion Coating Industrial lms, to
investigate the eect of physical ageing on the embrittlement of the PET amorphous lms. A large
literature is exposed.
To dene an annealing temperature for the Industrial samples, a preliminary analysis of the Labora-
tory scale lms is executed.
The chapter continues with the experimental on the Industrial samples, dividing them in a rst set
composed by lms with the same composition but obtained with dierent production technique, and
in a second set with lms of dierent composition but obtained with the same production technique.
The sixth chapter is a summary of the full work, with a discussion on the results obtained and on
the possible next steps towards the optimization of the use of PET in the Extrusion Coating process.
Remarks on both the eect of the storage time on the embrittlement, and on the annealing experiments
to improve and stabilize the PET amorphous phase, are presented.
Chapter 1
Polymeric Materials and Food
Packaging
1.1 Molecular Structure of Polymers
A polymer is a sequence of repeating units, derived from small molecules called monomers, covalently
linked to form a long chain. When the polymer is formed by a unique kind of monomer, it is called
homopolymer. For example, Fig.1.1 shows the monomer ethylene.
With polymerisation thousands of ethylene molecules are joined to form polyethylene, that can have
a linear or a branched structure, as shown in Fig.1.2.
Moreover, it is also possible to built more complex structures, where the polymeric chains are inter-
connected,as shown in Fig.1.3
Figure 1.1: The ethylene monomer.
Figure 1.2: Linear (a) and branched (b) polyethylene.
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12 CHAPTER 1. POLYMERIC MATERIALS AND FOOD PACKAGING
Figure 1.3: Scheme of a cross-linked polymer.
When two or more monomers are present along the macromolecular chain, the material is called
copolymer. Fig.1.4 reports the possible sequences of monomers in a copolymer: the monomers are
alternated, randomly distributed or form blocks.
Figure 1.4: In this scheme: L and M are the monomers; (a) is an alternate copolymer; (b) is a random
copolymer and (c) is a block copolymer.
Polymeric materials are divided in two important classes:
- Thermoplastic polymers, that gradually soften with increasing temperature and melt. They
can be repeatedly molten and cooled.
- Thermosetting polymers, that presents a cross-linked structure. These materials can be
moulded once by heat and pressure. They cannot be re-softened and re-heating.
1.2. MOLECULAR WEIGHT 13
1.2 Molecular Weight
For a polymer is not possible to dene a molecular weight as for small molecules. Indeed, the polymeric
chains are not characterized by the same length. Some chains are shorter and others longer. For this
reason, it is correct to dene a distribution of molecular weights and to talk about an average molecular
weight.
The average molecular weight can be dened in dierent ways:
 Mn = number average molecular weight
It is dened by:
Mn =
P
i
N
i
M
i
P
i
N
i
(1.1)
where M
i
is the molecular weight of a chain and N
i
is the number of chains of that molecular
weight.
Mn can be predicted by polymerization mechanisms and is measured by methods that determine
the number of molecules in a sample of given weight; for example, colligative methods such as
end-group assay.
 Mw = weight average molecular weight
It is dened by:
Mw =
X
i
w
i
M
i
=
P
i
N
i
M
2
i
P
i
N
i
(1.2)
where w
i
is the weight fraction and M
i
is the molar mass of each chain. N
i
is the number of
chains with the M
i
molecular weight.
Mw takes into account the molecular weight of a chain determining contributions of the molecular
weight average. The more massive the chain, the more the chain contributes to Mw.
Mw is determined by methods that are sensitive to the molecular size rather than just their
number, such as light scattering techniques.
The ratio between Mw and Mn is dened as the molecular weight distribution
MWD =
Mw
Mn
(1.3)
The Fig.1.5 shows the molecular weight distribution in a polymer with an indication of Mn and Mw.
Figure 1.5: Curve of distribution of the molecular weight in a polymer.