A METHOD FOR PET MECHANICAL PROPERTIES
ENHANCEMENT
Raffaella Aversa
University of Naples, Italy
E-mail: raffaella.aversa@unina2.it
Relly Victoria Virgil Petrescu
IFToMM, Romania
E-mail: rvvpetrescu@gmail.com
Antonio Apicella
University of Naples, Italy
E-mail: antonio.apicella@unina2.it
Florian Ion Tiberiu Petrescu
IFToMM, Romania
E-mail: fitpetrescu@gmail.com
Submission: 12/9/2018
Accept: 1/5/2019
ABSTRACT
A method for PET mechanical properties enhancement by reactive
blending with HBA/HNA Liquid Crystalline Polymers
for in situ highly fibrillar composites preparation is presented. LCP/PET blends were reactively extruded
in presence of Pyromellitic Di-Anhydride (PMDA) and then characterized by Differential Scanning Calorimetry, Thermally Stimulated Currents and tensile mechanical properties. Moderate amounts of LCP in the PET (0.5 and 5%) and small amounts
of thermo-active and reactive compatibilizer in the blend (0.3%) were found to significantly improve LCP melt dispersion, melts shear transfer
and LCP fibril formation and adhesion. An unexpected improvement was probably due to the presence
of two distinct
phases’ supra-molecular
structures involving PET-LCP and PMDA.
Keywords: Biotechnology; Bioengineering; PET; Reactive
Compatibilizer; Situ Composites; Differential Scanning
Calorimetry; LCP melt
dispersion; Recycle.
1. INTRODUCTION
After a slight decline
as a result of the 2008 financial crisis, plastics
production is increasing and in 2010 it has reached 265 million tonnes worldwide and 57 million in Europe (PLASTICS
EUROPE, 2011).
In the same year, plastic converters in Europe
processed 46.4 million tonnes of products, of which approximately 40% were short- term applications, especially for packaging
purposes, which resulted
in 24.7 million
tonnes of plastic waste from post-consumable Europe (2011). It is not surprising that environmental concerns have also increased
over the last decades,
reinforcing efforts to reduce the impact of polymer materials on the environment. In 2009, for the
first time in Europe,
the amount of plastic waste used exceeded
the quantity entering the landfill.
This favorable trend continued
the following year and 6 million tons were recycled for new products, 8.3 million
tons of energy and 10.4 million tons of waste (PLASTICS EUROPE, 2011).
However, for the treatment
of certain waste streams,
composting has proven to be the most advantageous method (Kale et al., 2007), so that biodegradable and compostable polymers have found applications in various
fields.
Although the magnitude of the process is often controversial, a consensus
has been reached
on the potential
depletion of petrochemical raw materials (HOEL; KVERNDOKK, 1996; SHAFIEE;
TOPAL, 2009).
As in other areas, the plastics
industry has begun to seek alternative sources of raw materials over the past decades, and its special interest is natural and renewable
solutions. Polymers based on polymers, ie polymers
produced from renewable raw materials, biomass in general,
could replace fossil sources and also have considerable environmental benefits such as low carbon emissions.
Although the term "biopolymer" is used in several different ways, depending on the scope, the generally accepted
definition covers polymers belonging to the abovementioned categories, ie they are either renewable, biodegradable or both. The global production capacity of these materials is dynamic
(SHEN et al., 2009).
Both environmental concerns and market trends are behind this trend, as in the case of oil prices,
conventional polymers will become more and more expensive. Consumers' expectations cannot be neglected, as many customers consider the ecological effect of the products they buy. The proportion of biodegradable polymers compared to non-degradable biodegradable types has also recently increased.
One of the reasons
that have led to this trend could be considerable changes to legislation on compostable products
in recent years. Long-term
projections, however, predict the dominance of non-degradable biopolymers.
The relative importance of biological and biodegradable classes in polymer production could further
increase as production technology improves and becomes
more cost-effective. According to different
estimates, only 4% of the world's biomass is used by mankind,
mainly for food, being only a fraction
for chemical production and production of plastics
(SHEN et al., 2009) capacity
increase.
Biopolymers have great potential and many advantages, but they also have some disadvantages. Despite the increase
in production capacity,
they are still quite expensive compared to commodity
polymers, and their properties are also often inferior or at least not in line with the expectations of converters or users. Although natural polymers are available
in large quantities and are also cheap, their properties are even further away from those of the base plastics.
Consequently, biopolymers must often be modified
to meet market expectations.
In order to use its potential and penetrate
new markets, the performance of biopolymers must increase
considerably. Consequently, the modification of these materials is at the heart of scientific
research. Unlike the development of new polymeric
materials and new
polymerization paths, blending is a relatively inexpensive and rapid method of adapting
the properties of plastics. As a result, this approach
can play a crucial role in increasing the biopolymer's competitiveness. In our present paper, we try to provide a summary
of recent trends and achievements in the field of biopolymer blends, with a special
focus on compatibility-compatibility- ownership relationships (AVERSA et al., 2018a;
AVERSA et al., 2018b;
AVERSA et al., 2017a; AVERSA et al., 2017b;
AVERSA et al., 2016a;
AVERSA et al., 2016b;
AVERSA et al., 2016c;
AVERSA et al., 2016d;
AVERSA et al., 2016e;
AVERSA et al., 2016f;
AVERSA et al., 2016g;
AVERSA et al., 2016h;
AVERSA et al., 2016i;
AVERSA et al., 2016j;
AVERSA et al., 2016k;
AVERSA et al., 2016l;
AVERSA et al., 2016m;
AVERSA et al., 2016n; ANNUNZIATA et al., 2006; APICELLA et al., 2018a; APICELLA et al., 2018b; APICELLA et al., 2018c; ARMAH, 2018; MARQUETTI; DESAI, 2018; TAMBURRINO et al., 2018; WILK et al.,
2017).
As
mentioned in the introductory part, the term "biopolymer" refers to polymers
that are biodegradable, biodegradable or both. Before discussing the different
aspects of biopolymer blends, we define these categories in this section
to help understand subsequent discussions.
1.1.
Biopolymers
Replacement of fossil raw materials with renewable raw materials is one of the main actions of the modern plastics industry.
Natural polymers are a specific class of
materials among polymers based on natural
resources. They appear in nature
as macromolecules and include natural or physically modified polymers
in this class. Typical
examples are cellulose, hemicellulose,
lignin, silk and starch.
Another class of materials consists
of synthetic polymers based on natural
or biological bases, whose monomers come from renewable resources. This category
includes poly (lactic acid) (PLA) as well as conventional biological polymers such as polyethylene (PE), poly (ethylene terephthalate) (PET) and polyamide
(PA), because the polymer
is produced by the bacterial
fermentation process. Biodegradability, on the other hand, is independent of the above mentioned categories, so that biodegradable polymers are not necessarily of
natural origin.
The conditions for determining the content of bio-based polymer materials are described in European
Standard CEN / TS 16295: 2012. The approach
is based on the carbon content
of biofuels as a fraction
of the total organic carbon content. However, precise
legislative details
and precise protocols for determination should be developed in the future.
1.2.
Degradation,
biodegradation
Each polymer degrades to a certain
degree over a period of time, depending
on environmental conditions. However, a precise
definition of this feature
is necessary to obtain a useful
description of the degradation of the polymer. According to the related
standard (CEN / TR 15932: 2010), these polymers
can be called degradable, where degradation results in molecular
weight loss by dividing the chain into the backbone, and degradation is complete,
ie the final products are low biosynthesis. It does not consider either the chain failure mechanism or the environmental effect of the final products.
In biodegradable polymers,
on the other hand, chain cleavage
is caused by cellular activity
(human, animal, fungus,
etc.), so it is an enzymatic process, although it is usually accompanied and promoted by physicochemical phenomena. The two types of processes, ie physical
and enzymatic processes, can not be distinguished and/or separated in general,
their combined effect results in complete polymer degradation
(CEN / TR 15932: 2010).
The assessment and laboratory testing of the biodegradability of the polymeric materials are well defined in the European standards (EN ISO 14851: 2004, EN ISO 14852: 2004, EN ISO 17556: 2004, EN ISO 14855-1: 2007/2: 2009). The tests are
based on the measurement of the oxygen demand or the amount of carbon dioxide that has evolved in the process.
It is important to note that most polymers containing
various degradation agents (oxo-biodegradable polymers) (SCOTT,
2000; CHIELLINI et al., 2006) cannot be considered biodegradable according to the standards mentioned above. Although fragmentation and disintegration may occur, degradation is never complete
under test conditions that simulate natural environments (soil, water,
and compost).
The environmental effect of the high molecular weight residual fractions is not satisfactorily described and is therefore
of serious concern. Consequently, these agents could help solve pollution problems, but not the fundamental problem arising
from the slow degradation of synthetic polymers.
The use of such plastics has been widespread and encouraged in the past by imperfect
legislation. Early standards (ASTM D3826-98
(2008)) have determined a certain decrease in traction resistance as a condition of degradability, which can easily be achieved by the use of pro-oxidants, for example
without a real environmental
advantage (Imre and Pukánszky, 2013).
2. METHODS AND MATERIALS
A
method for PET mechanical properties enhancement by reactive
blending with HBA/HNA Liquid Crystalline Polymers for in situ highly fibrillar
composites preparation is presented. LCP/PET blends were reactively extruded
in presence of Pyromellitic Di-Anhydride (PMDA) and then characterized by Differential Scanning Calorimetry,
Thermally Stimulated Currents
and tensile mechanical properties.
The formation of specific macromolecular structures, where the PET, the LCP, and the reactive
additive are involved,
has been hypothesized in the reactively extruded
blends from TSC analysis
evidence.
The use of a reactive
additive improved
the matrix LCP compatibilization and adhesion
as indicated by the SEM analysis and mechanical testing. Moderate amounts of LCP in the PET (0.5 and 5%) and small amounts of thermo-active and reactive compatibilizer in the blend (0.3%) were found to significantly improve LCP melt dispersion, melts shear
transfer and LCP fibril formation and adhesion.
Blends of PET and LCP containing the compatibilizer favored the formation of a well dispersed and homogeneous fibrillar phase whose particle size distribution did not show great coarsening and coalescence leading to significant elastic properties improvements from 0.8 for not compatibilized to 3.1 GPa for compatibilized 0.5% LCP loaded PET blends
that were even higher than those expected
from ordinary theoretical calculation. This unexpected improvement was probably due to the presence of two distinct phases’ supra-molecular structures involving PET-LCP and PMDA.
The post-consumer PET is mainly derived from injection
molded bottles prepared
using high intrinsic
viscosity and a high
molecular weight PET fiber. Intrinsic viscosity
(IV) is a measure
of polymeric molecular weight (MW) and therefore reflects the melting
point of the material, crystallinity and tensile
strength. The length
of the polymer chain in PET determines the molecular weight of the material
and the mechanical properties that determine
large IV PET useful for high strength
applications (The higher the traction,
impact, or operating temperature, the higher the intrinsic
viscosity of the polymer).
Applications of this
high molecular weight PET and intrinsic
viscosity include carbonaceous bottles, solder bands, films, and photographic bands, while low- density
IV molds are used in
the fiber and textile industry.
However, high molecular weight PET suffers a significant reduction during the recycling process
due to the reduction in molecular weight due to spontaneous melting
processes which greatly reduce the intrinsic viscosity and the final mechanical properties of the material.
These recycled materials can only be used for smaller
structural value applications such as carpet fibers, geotextile or extruded
into a forming sheet by thermoforming and by blowing
non-food containers.
Mixing with glass or carbon fiber could help improve
the mechanical and thermal
stability of these materials.
However, the processing of these composites does not have always had immediate results in improving mechanical properties
(APICELLA et al., 1980; NICOLAIS et al.,
1981).
Breaking fibers and high viscosity
of composite material during the process severely limit
their application. Liquid
crystalline polymers can have great potential as high strength
modifiers which can be easily processed
because they tend to dissolve in the matrix
at high processing temperatures, reducing
the melt viscosity, and facilitating extrusion
and injection processes.
Pure LCP (not in a mixture
with a polymer
the matrix) can be potentially used in many industrial fields such as electronics, composites, and packaging, where high dimensional stability, low environmental sensitivity is required.
However, once oriented,
their very anisotropic structure, while increasing
their power in the direction
of orientation, greatly
reduces their resistance in the transverse direction.
Therefore, it is necessary
to mix with other engineering thermoplastic materials. However, the morphology of these mixtures
is unstable; the distribution and size of the dispersed phase, in fact, tend to increase when they are melted reduced shear conditions, for example,
as may occur during certain
injection molding conditions, resulting in deleterious effects
on the final properties
of the
blend.
The shear viscosity of the high-temperature isotropic phase is, in fact, higher than the viscosity
of the mosquito
phase at lower temperatures. During the flow, the eye leaks easily between them when it is the anisotropic phase is oriented
along the flow due to the fact that LCPs have four times greater
relaxation times than the regular polymers, the elongational flow of an anisotropic melt containing local orientation domains induces stretching
and alignment which is kept cool
in a solid state.
Some research (DUTTA et al., 1996) also showed that the potential
level or LCP order
close to the unit (0.90)
that can be reached for a specific
stretch on the pure LCP is never approached when mixed with a matrix host; the current level of control
in the same degree is 0.4/0.5. This may be related
to the weak interactions between the two phases present in the melt; However, interactive interaction/interaction is better through chemical changes, the greater will be the improvement (CRUZ; FIU, 2015; ZHANG et al., 2000).
So-called in-situ composites formed from a polymeric curing polymer matrix have been extensively investigated in the past (APICELLA et al., 1989, NICODEMO et al., 1981, SKOVBY et al., 1991, ZHANG et al., 2000, GOH; TAN, 2012, CRUZ; SON, 2015).
Even in such cases, however, the properties of the resulting materials
were lower than expected.
This was primarily
due to poor control of interaction and adherence between oriented LCP and array host. This lack of adhesion
is related to Van der Waals's weak forces, which have links between
the LCP segments
and the host matrix.
In our investigation, the melt phase interactions and solid phase adhesion
between the two components were improved and controlled by reactive
alloy processes in which LC polymer molecules containing reactive groups (such as OH,
NH and the like) are mixed with PET-based
host polymers in the presence of an anhydride-specific reaction promoter.
Commercial commercial HBA / HNA extrusions (VECTRA 950) were made with recycled PET to produce composite composites in situ which, after thermal expansion at an appropriate temperature improving adhesion
of LCP / host polymer fibers and
superior mechanical properties (AVERSA;
APICELLA, 2016).
3. MATERIALS, APPARATUS AND PROCEDURES MATERIALS
LCP fillers and reactive
additives: Vectra A
950
(Hoechst) prepared from HBA and HNA
and Pyromellitic Anhydride (PMDA) has been used as a
reactive additive.
Matrix: Recycled PET’s of IV-0.60 dl/g have been used. Intrinsic viscosity has been measured
at 25°C in phenolortodichlorobenzene (60/40 weight fraction)
solutions.
3.1.
Apparatus
and Procedures
3.1.1. Blending
and Reactive Extrusion
The PET matrix, the dispersed LCP, and the reactive
additive (PMDA) compounds
were plasticized after
previous drying (24 h at 140°C under vacuum) in a Haake counter-rotating intermeshing twin screw extruder.
Mixtures containing 0.5 and 5.0% of LCP with and without
the reactive additive
(0.30%) were extruded
at 290°C, quenched
in a water bath and then
pelletized.
3.1.2. Filming
and Orientation
Base polymer and mixtures in the form of pellets were dried in a vacuum oven at 140°C for 16 h and then extruder in a single screw extruder Haake equipped
with a flat die. The strand of the blends and neat resin were extruded
into a water bath and then calendered without stretching. General purpose
tensile tester (Instron model 4500) equipped with a thermostatic chamber set at 190°C has been used to stretch under nitrogen the extruded
strands up to two times the initial
length at the crosshead
speed of 1 mm/min.
3.1.3. Mechanical
Testing
The tensile modulus of the stretched
samples was measured
on the same Instron 4500 mechanical tester with a constant crosshead speed of 10 mm/min on 3 cm samples.
3.1.4. Differential
Scanning Calorimetry
An ADSC Mettler Differential Scanning Calorimeter has been used for the thermo-
calorimetric characterization. Samples films of 0.5 mm containing 0.5 and 5% of LCP before
and after stretching and annealing
were tested. Thermal
scans from 0 to 300°C
were carried out at 5°C/min.
3.1.5. Thermally
Stimulated Depolarization Currents (TSDC)
A
Solomat model 41000 spectrometer equipped with liquid nitrogen cooling system has been used on samples films of 0.5 mm containing 0.5% of LCP before and after stretching and annealing was tested.
Thermal scans from 0 to 200°C were carried out were carried out at 5°C/min
on samples polarized for 5 min at 40, 60 and 90°C
in a polar field of
200 V/mm and
quenched to -100°C.
3.1.6. Scanning
Electron Microscopy (SEM)
A
Hitachi electron microscope has been used for surface morphology characterization. Namely, samples
containing 0.5 and 5% of LCP were both fractured in brittle
mode under nitrogen and in the plastic mode in tensile tests. Fracture
surfaces of the samples of extruded and extruded/stretched films were observed
at 400
to 6000
X magnification (AVERSA; APICELLA,
2016).
4. RESULTS AND DISCUSSION
The reactive reaction was carried out on polymer blends of PET and LCP previously mixed in a double extruder by the addition
of 0.3% of poly-anhydride (PMDA). The resulting material was extruded and films with different
LCP content were
obtained.
A
preliminary thermo-calorimetric investigation on those samples
indicated that for the investigated compositions this technique
was not able to detect
any difference between
the samples with and without
the reagent additive.
The thermogram against the PET / LCP (Vectra)
blend with and without the reagent
additive was compared. In both situations, the resulting
curves exhibit an inflection of about 60 ° C which (for a directed
film) is representative of the glass transition temperature range of the polymer.
The
intense tip at 250 °
C is relative to the melting of the crystalline phase. The test materials
exhibit a crystallinity of about 50%, taking them as reference crystallization heat for 100% crystalline PET 28.1 cal / g or 117 J / g (GROENINCKX et al., 1980).
To verify that the LCP and the host polymer have undergone some chemical interaction during reactive extrusion, a highly sensitive
analytical technique (TEYSSEDRE; LACABANNE, 1995), heat-stimulated sample depolarization currents were used. Under an external
electric field, the polymer (which is a dielectric medium) accumulates electrical charges in a form that can be retained for a long time, where increasing internal
segmental mobility
can release these loads that produce a
depolarization current.
The peaks observed in the diagram that report the intensity
of such depolarization currents as a function of
temperature during a thermal
scan are related to the release of electrical charges in the samples in the physical events occurring in the (molecular) and phase (super-molecular).
Characteristics of TSC diagrams of mixtures containing 0.5% of LCP (with and without
PMDA) and pure LCP were also compared. This method offers
high sensitivities even with
currents in the order of
10-13 amps and decreases.
Diagram of the mixture in the absence of the reagent (PMDA) is characterized by a peak of about
30° C above the polarization temperature that characterizes the PET transition glass (about 60° C), and that the LCP (VECTRA) which, measured
by the same technique, given a higher peak at 71.5° C.
The test carried out under the same experimental conditions but on the same admixture with the reagent
(0.3%) is compared and analyzed. Surprisingly, the new chart shows no peak in the glass transition region of the two component
mixtures, in addition,
a second thermal event takes place around 110-140
° C (which is obviously
widening the tip).
Therefore, TSC analysis indicates
that the presence of the reagent additive changes the internal
structure (molecular structure)
of the mixture.
Even if in a very small amount, only 0.5%, LCP was still detected by TSDC analysis, the tip being probably related to the presence
of LCP while the presence
of the peak at 140 ° C in the thermogram over the additive
the mixture is representative of a structural macromolecular feature where PET, LCP, and the additive
is involved.
This second thermal event, which may be characteristic of this particular segment rearrangement of the PET / LCP reaction
mixture was further confirmed
in a TSDC test performed on an additive polarized mixture at a higher temperature of 90° C. At this elevated temperature, electrical charges are accumulating and associated with events
that have place at temperatures
above 90° C.
Thermal tests (DSC and TSDC) associated with mechanical analysis and electronic
scanning microscopy (SEM).
SEM analyzes were performed on PET containing 5% LCP fractured in liquid
nitrogen.
Figure 1 shows the liquid crystalline polymer in (this hypothesis is validated
by the presence
of spherical voids on fracture
surfaces that eventually contain decoded LCP particles). Extrusions and shooting procedures in the absence
of the reagent
additive did not lead to significant LCP orientation, as indicated
by the almost spherical
inclusion model.
Figure 1: SEM image of PET/5.0%LCP
blends without PMDA reactive
additive
The surface of the ET / 5.0% LCP sample containing 0.3% of the reactive
additive is shown
in Figure 2.
Figure 2: SEM image of PET/5.0%LCP blends with 0.3% of PMDA reactive
additive
The same extrusion and extrusion processes
used for the sample described above, homogeneously obtained
in this case and elongated
LCP fibers. Extrusion
by reactive extrusion
promotes better
dispersion by improving
phase interactions between the two
molten components.
SEM analysis was performed on the surfaces
of the same PET/LCP
mixtures with and without
the reagent additive which was broken
in traction tests at room temperature. As shown in Figure 3, situ does not contain the additive, it is characterized by the presence
of ellipsoidal particles (approximately 8 × 4 microns)
from the dramatic
phase of the liquid crystal yields and does not increase
plastic around
LCP inclusions.
The presence of non-adherent ellipsoidal inclusions that remain undeformed after the matrix breaks lead to intense
stress concentration and to a plastic
flow at the matrix
/ PET filter interface. Due to the lack of interaction during processing and the lack of
a solid state, the LCP filler
does not act as a
curing agent.
Figure 3: SEM
image of PET/5.0%LCP blends without PMDA reactive additive. Room
temperature fracture is accompanied
by
plastic flow around
poor adhering LCP fillers
In the presence of PMDA, the better it is dispersed (about 2 microns in diameter) and well directed
LCP phases, as shown in Figures
4 and 5, improving
the mechanical properties of the matrix.
This occurrence was probably
due to the fact that improved physical dispersion in the presence of the reactive
additive leads to higher surface/volume ratios, playing a significant role in fracture tests performed at room
temperature.
Figure 5 is a top elevation
of the previous
image showing as an LCP fiber that adheres
to matrix and plastic.
This is
an indication of the active role of the additive
in improving both the mechanical processing properties and the final mechanical properties of the PET/LCP blends.
This emergence further confirms the macromolecular nature of hypothetical structural change by analyzing TSC diagrams.
Figure 4: SEM image of PET/5.0%LCP
blends with PMDA reactive additive.
Good adhering and plastically deforming LCP
fillers
Figure
5: Higher magnification of the previous SEM image showing the plastic deformation
of the LCP filler