1. No category

How to make flexible packaging materials from natural
brittle plastics
Joakim Gauffin
Erik Hagel
Tobias Jansson
Fredrik Johansson
Abstract
This report is a study of plastic packaging materials made from renewable natural sources. It
provides an insight in the importance of replacing synthetic softeners in plastics with
equivalent natural counterparts.
A thorough description of plasticizers as an additive in natural polymeric materials is made.
The theories behind the plasticizing process are described. In experimental work, plastic films
consisting of amylopectin and glycerol where created to study the plasticizing effects. To
improve barrier properties of the films, nano clay was implemented in various amounts. The
performance of the films was then determined by tensile strength tests. The test results
showed that the clay improved fracture strength at the cost of strain strength. When making a
packaging material one must therefore consider the need of flexibility versus the need of
keeping reactive chemicals out of the container.
1
Index
Abstract..................................................................................................................................1
Index ......................................................................................................................................2
Introduction............................................................................................................................3
Exchange between packaging material and content .........................................................4
Environmental issues ......................................................................................................4
Theoretical approach on Plasticizers .......................................................................................6
General idea, why plasticizers are used ...........................................................................6
The Lubricant Theory, the plasticizing process ...............................................................6
Other theories .................................................................................................................7
Materials ................................................................................................................................8
Amylopectin ...................................................................................................................8
Glycerol..........................................................................................................................9
Cloisite ...........................................................................................................................9
Laborative works..................................................................................................................10
Material Creation ..........................................................................................................10
Film creation.................................................................................................................10
Film creation with introduction of cloisite na+ ..............................................................11
Deducing the melting temperature of Amylopectin .......................................................12
Test of tensile strength ..................................................................................................12
Results..................................................................................................................................14
Film creation.................................................................................................................14
Film creation with introduction of cloisite na+ ..............................................................14
Deducing the melting temperature of Amylopectin .......................................................14
Test of tensile strength ..................................................................................................14
Discussion ............................................................................................................................15
Conclusion ...........................................................................................................................16
References............................................................................................................................16
Appendix 1...........................................................................................................................17
Test1.............................................................................................................................17
Appendix 2...........................................................................................................................19
Test2.............................................................................................................................19
2
Introduction
Today’s packaging materials require extreme accuracy in choice of material.
Plastics become the natural choice due to its flexibility, low price and toughness.
A polymer combined with an additive becomes a plastic material. Variations of polymers and
additives give the material different abilities.
Even though the additives are a small part of the polymer, their effect is significant. An
additive can change the plastics flexibility, strength, color etc.
Plasticizers are an important additive in plastics. Its ability to change the plastic, to useful
properties is valuable. The most common plasticizer is phthalates. The production of
phthalates for food wrapping is slightly decreasing, due to its toxicities1. Therefore new
plasticizers are to be found. Likewise are synthetic polymers not always the given choice
when it comes to packaging materials.
A natural polymer such as starch is based on nature materials and is often more suited to the
environment. Therefore it’ll have an important impact on the development of future plastic
materials. Amylopectin is a branched polymer of glucose and is a part of the polysaccharides
category.
Amylopectin is insoluble in water and is not by itself categorized as a plastic.
Suitable plasticizers and additives for amylopectin, is required for the packaging industry.
How to retain strength and barrier abilities, but yet increase the flexibility is to be studied in
the project.
1
Handbook of plasticizers, Plasticizers types, Wypych, George ChemTec Publishing 2004
3
Exchange between packaging material and content
In packaging materials the exchange between the material and its content should be avoided.
When you use plasticizers in packaging materials there are three important issues to consider,
when it comes to changing the properties of the material. The three possible processes are
permeation, migration and scalping.
Permeation:
This is when liquids, like water and flavour can be transported from the packaging material to
the environment.
Scalping:
Scalping is when the packaging material absorbs some of the characteristic properties of its
content. For example, if a material absorbs the aroma of the material.
Migration:
Migration occurs when the packaging material, instead of letting the content out, diffuses in to
the product.
The process of permeation and scalping can be avoided by mutual diffusion. Doing this in real
life often means the packaging consists out of two different materials, so the diffusion can be
controlled.
Environmental issues
Plasticizers have recently become an environmental concern. In general it is the phthalates
who’s causing problems. Some of them behave like the hormones androgens and estrogens.
When the phthalates interferes with the hormones serious problems in the endocrine system
can emerge.
4
According to researchers at the University of Missouri 2, very low amounts of plasticizers
used in eyeglass lenses, food containers, nail polish and many other products caused
reproductive problems on mice.
At Tuft University1, a researcher found out plastic containers caused hormone changes on
cells. The changes were similar to that of estrogen. Many different health effects emerged.
Some of them were increased birth defects, breast cancer and decreased sperm counts.
In later years scientists have been searching for plasticizers in the environment. They have
been detected in air samples (collected in different kind of environments), in water samples
(both groundwater and drinking water) and the plasticizers have also been found in sediment
and soil samples. In general it is believed that most of them come from industrial sources.
They are also “leaking” from all kind of products. Two major mechanisms controls the loss of
plastizicers, those are biodegradation and adsorption.
Biodegradation is the name of the process where organic material converts back into H2O and
CO2. With plastic materials the process can also be called oxy-degradation. This process often
requires heavy-metals to commence. The US “Biodegradable Products Institute”3 has found
very high levels of heavy metals in some of the oxy-degradable plastics.
Adsorption is a process, where liquid or gas interferes with the surface of a material. This
process creates a molecular film, which reacts with the outer layer and releases the plasticizer
in the environment.
2
http://www.aerias.org/DesktopModules/ArticleDetail.aspx?articleId=60&spaceid=1&subid=7#plasticsplasticizers
3
http://www.bpiworld.org/BPI-Public/Documentary.html
5
Theoretical approach on Plasticizers
General idea, why plasticizers are used
The polymer structure consists of long chains of linked molecules. These chains are
interconnected via direct covalent bonds or weaker van der Waals forces. When a polymeric
material is subjected to a large enough force, it wants to lower its energy by deforming itself.
If the interconnections are to powerful, the only way for deformation to occur is to break a
bond and the deformation becomes permanent.
The addition of a plasticizer reduces chain to chain forces and permits an elastic stress
response whereas before plastic deformation would have occurred.
For a plasticizer to be successful the molecules will have to be smaller than those of the
polymer and it has to be a liquid.
The Lubricant Theory, the plasticizing process
The plasticizer molecules solves into the larger polymer chains. As it progresses, the
plasticizer arranges itself to form planes in between the chains. This creates a matrix of
polymer and plasticizer planes. Any irregularities in the polymer chains are compensated by
the filling of plasticizers. This allows different planes to smoothly glide about, thus reducing
any structural damage induced by an applied force.
Fig 1: Plasticizer molecules (small dots) acting as a lubricant and creating planes of low
friction between the polymer chains (larger dots).4
4
Handbook of plasticizers , Wypych, George ChemTec Publishing 2004
6
By solving a plasticizer into the material it will swell. The degree of swelling depends on the
polar relationship between the polymer and the solvent. If the attracting polar force is stronger
than the polymers chain to chain force, plasticizers will bond immediately near the polymer
surface. Otherwise the polymer and plasticizer will be repelled. As a result there are two types
of plane formation:
Case a, (Fig 1a): Polymer and plasticizer attracts and forms a layer of plasticizers at the
polymer surface, allowing for the remaining plasticizer molecules to form gliding fields in the
middle.
Case b, (Fig 1b): A gliding plane is provided by the low friction electro-magnetic repelation
field at the polymer surface.
Other theories
From the lubricant theory the gel theory arouses. It continues to build on the polar relationship
between plasticizer and polymer. Experimental work had shown that a plasticizer with a bulky
structure i.e. large side groups or aromatic rings in the chains, performed worse than straight
molecules.
It was deduced that plasticizers effectiveness depended on not only the polarity but also on the
mobility of the molecule.
7
Materials
Amylopectin
All different kinds of starches consist of both amylase and amylopectin. There are some
starches that consist of almost only amylopectin. Amylopectin is a big and branched
molecule. It’s made out of approximately 100 000 glucose monomers. Because of the
branched configuration it is able to cross link easily. The cross linking properties of
amylopectin locks it into a one shape structure without much flexibility. When used as a
packaging material it is important to loosen the bonds and make it more flexible.
2a
Fig2a: Amylopectin consists out of glucose monomers put together.5
2b
Fig2b: A branched network of amylopectin molecules.6
5
6
http://school.chem.umu.se/Experiment/pics/popcorn03.gif
http://www.gantschnigg.net/chemielexikon/gfx/AmylopektinStrukturB.gif
8
Glycerol
CH8O3
Fig3: Glycerol
Glycerol is a very common material and has a variety of usages incorporating food and
medicine. It is liquid at room temperature and the molecule is relatively small which makes it
an excellent plasticizer.
Cloisite
Na + Na0.2Ca0.1Al2Si4O10 (OH) 2(H2O) 10
Cloisite na+ consists of almost entirely montmorillonite (99.7%) and the rest is silicon.
Montmorillonite is a member of the clay group. It consists of microscopic or at least very
small crystals within the clay. Like all members of the clay group montmorillonite expands to
several times of its own volume when it get in contact with water.
The composition is mainly:
1. 43.77% Si1O2
2. 36.06% H2O
3. 18.57% Al2O3
The advantages of using nano clay are that the expected:
•
tensile and flexure strength will be greater
•
density of the material will be reduced
•
dimensional stability will be increased
•
gas barrier properties will be improved
•
chemical resistance will increase
•
flame retardant properties will be improved
9
Laborative works
Material Creation
Polymerisation of pure amylopectin produces a brittle and stiff material. To make a usable
material a plasticizer is required. For this experiment, glucose and glycerol was used. Mainly
because of their smaller molecular size compared to amylopectin and their low cost. Cloisite
Na+ is nano composite clay that improves mechanical properties when introduced in a
polymeric structure.
Materials:
S = solid L = liquid
Amylopectin (S)
Glucose (L)
Glycerol (L)
Distilled water (L)
Cloisite Na+ (S) nano clay
Equipment:
Heated oil bath
Magnetic stirrer
The goal was to create plastic films which could be used in endurance test to determine the
best composition.
Film creation
In the first sets of films amylopectin, glucose and distilled water was used. Amylopectin
powder and glucose was mixed separately with water. The mixtures was then poured into a
glass container and put in a heated oil bath with automatic temperature regulation and
magnetic stirring. The chosen temperature was 95o C. At this temperature amylopectin melts
rapidly and the mixture was given 14 minutes to mix. The specimen was stirred thoroughly
during this time (to encourage solving of the plasticizer).
10
After heating, the plasticized melt was poured out homogeneously on a large enclosed non
sticking surface and let to dry.
Five different compositions where used: (composition in mass %)
Film#
glucose
water
amylopectin
Film1
0.5
95.5
4.0
Film2
0.75
95.25
4.0
Film3
1.0
95.0
4.0
Film4
1.25
94.75
4.0
Film5
1.74
94.26
4.0
In the second sets of films, glycerol was used as plasticizer instead of glucose. The films
where produced in the same way as before. Again the same compositions where used, in
terms of mass percent.
Film creation with introduction of cloisite na+
Three films where created with different compositions of nano-clay and glycerol.
Amylopectin and glycerol was mixed with water in one container while clay and water was
mixed in another. This is enforced by the rather delicate solving procedure for nano-clay in
water. To be able to fully solve it, an ultrasonicator was used. Inside the sonicator, the sample
is bombarded with pulses of ultra high frequency sound waves. This causes vibrations among
the nano particles and forces them to separate from each other.
11
Film#
glycerol
water
amylopectin
cloisite Na+
Film1
1.67
94.33
4.0
0
Film2
1.79
93.91
4.0
0.3 (5%)
Film3
1.93
93.42
4.0
0.63 (10%)
Mass % (Mass % of dry mass)
Heating of Amylopectin
To improve the understanding of amylopectin as a base material a slow controlled heating
was performed. A specimen of pure polymerised amylopectin was heated from room
temperature to 100 oC at a rate of 10 oC per minute. The heating oven was linked to a digital
optic microscope and the progress documented by a camera. At 90 oC the amylopectin began
to gelatinise.
Test of tensile strength
Tensile tests are relatively simple to obtain and tells a lot about material properties. By
applying force to a sample until it breaks, you will gain information about the tensile profile.
Values of elongation, E-modulus and ultimate strength are obtained. These values are useful
for further examinations.
The samples must be properly prepared before testing. Geometry of the samples, air humidity
and temperature are measured. All these values are transferred to the software belonging to
the tensile testing equipment. To make sure the samples are properly dried, the tensile tests
are performed at two occasions.
The samples are mounted by hydraulic clamps in the testing equipment. Other operations are
managed in the computer software.
12
A conclusive factor is how the samples are mounted. If the sample doesn’t have the same
angel as the tension, a skewed elongation (Fig 4.b) is obtained. A correct tensile test is shown
in figure 4.a.
Fig. 4.a.
The figure shows a
correct mounting
of a sample
Fig. 4.b
The figure shows a
skewed mounting of a
tensile test
An additional fault factor is if the samples aren’t properly homogeneous. If a sample isn’t
homogeneous, the composition can vary and misleading results are obtained. Because of these
fault factors, it is recommended to repeat the tensile tests on many samples (with the same
composition). Through an average value of the tensile profile a more correct conclusion can
be reached.
13
Results
Film creation
Glucose/glycerol + amylopectin and water:
The thickness was inhomogeneous and the polymer and softener was not properly mixed. The
films were not durable enough to be removed from the containers without ruining them. The
material was non-transparent.
Film creation with introduction of cloisite na+
The plasticizer and the nano clay solved without problem and produced a fairly homogenous
film with a darker colour.
Heating of Amylopectin
Fig5: Captures of the heating process. As the temperature
rises, the grains move closer together and finally becomes
colloidal gel.
20°C
60°C
80°C
100°C
The attempt showed the crystallinity of pure amylopectin is indeed high, due to the melting
behaviour.
Test of tensile strength
Test 1 after two weeks drying (Appendix 1)
No substantial difference showed between 0 % MMT and 5 % MMT. Both films broke at
about 0.003 MPa. The films with 10 % MMT showed about twice the stress endurance and
half the strain capacity.
14
Test 2, after four weeks drying (Appendix 2)
The clayless film now showed roughly the same properties as the semi dried 10 % MMT film.
Higher stress limit but lower strain capability. The 5 % film more than doubled its maximum
stress capacity and went off the scale. The dried 10 % films where of poor quality and the
result displayed should be taken lightly.
Test 1:
Clay amount (dry)
0%
5%
10%
Ultimate strength (N)
0,63
0,46
1,35
Max strain
Ultimate strength (N)
1,28
1,29
1,33
Max strain
11%
10%
7%
Youngs modul (MPa)
0,05
0,05
0,29
Test 2:
Clay amount (dry)
0%
5%
10%
8%
6%
6%
Youngs modul (MPa)
0,23
0,25
0,32
Discussion
The result of the first laboration was most likely caused by a malfunctioned heating device.
The temperature became to low which resulted in poor mixing conditions. When the failed
samples were tested in the heating chamber melting temperature of amylopectin was
estimated to 90 oC. If the temperature was below 90, it would encourage the forming of nonsolved solid amylopectin. This leads to an inhomogeneous composition, were plasticizers
hardly can dissolve. This explains why the first films failed. The error was corrected in the
second attempt which produced films of good quality.
When comparing the tensile graphs showing different clay compositions, there where some
valuable information to acquire;
When it comes to choosing packaging materials, dimensional stability and flexibility are two
important factors. With an increasing level of nano clay the tensile strength improved.
Because the clay made the material stiffer there was also a loss of flexibility. Apart from the
increased mechanical properties it also contributes to a better barrier quality. This is very
useful in grocery packaging containers. Depending on the desired properties of the packaging
film the level of clay can be varied.
15
In the second attempt to test the tensile strength, the samples were dried in a room with lower
humidity. With these drier and stiffer samples the results showed a higher tensile strength.
Water has a strong plasticizing effect on the films. When this contributes as in the first
attempt, the results will show a higher elongation and lower strength.
Conclusion
It is very possible to plasticize amylopectin with glycerol and produce a softer flexible
packaging material. Nano clay can be added to improve structural integrity at the loss of
flexibility. When making a packaging material one must therefore consider the need of
flexibility versus the need of keeping reactive chemicals out of the container.
References
Chemistry: The Central Science, 9:th ed. T.L. Brown, H.E. LeMay, Jr, B.E. Bursten & J.R.
Burge, Pearson Education Inc. 2003
William D Callister, Jr: Fundamentals of materials science and engineering, John Wiley &
Sons.
http://www.wikipedia.org { glucose, glycerol, ftalater…}
Handbook of plasticizers, Wypych George ChemTec Publishing 2004
http://www.galleries.com/minerals/silicate/montmori/montmori.htm
https://epi.lyckeby.com/upload/Agri/Dokument/Staerkelse.pdf
http://webmineral.com/data/Montmorillonite.shtml
http://webmineral.com/specimens/picshow.php?id=2128
http://www.aerias.org/DesktopModules/ArticleDetail.aspx?articleId=60&spaceid=1&subid=7
#plastics-plasticizers
http://www.bpiworld.org/BPI-Public/Documentary.html
16
Appendix 1
Test1
0 Mass% MMT
0,007
0,006
Stress (MPa)
0,005
0,004
0,003
0,002
0,001
0
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0,16
Strain
17
5 Mass% MMT
Stress (MPa)
0,007
0,006
0,005
0,004
0,003
0,002
0,001
0
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0,16
Strain
10 Mass% MMT
Stress (MPa)
0,007
0,006
0,005
0,004
0,003
0,002
0,001
0
0
0,02
0,04
0,06
0,08
Strain
0,1
0,12
0,14
0,16
18
Appendix 2
Test2
0 Mass% MMT
0,007
0,006
Stress (Mpa)
0,005
0,004
0,003
0,002
0,001
0
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0,16
Strain
5 Mass% MMT
Stress (Mpa)
0,007
0,006
0,005
0,004
0,003
0,002
0,001
0
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0,16
Strain
19
10 Mass% MMT
Stress (Mpa)
0,007
0,006
0,005
0,004
0,003
0,002
0,001
0
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0,16
Strain
20