Starch-Based Polymers for Plastic
Bags
ENGR 103 – Spring 2012
Engineering Design Lab III
Lab Section:
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009
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Date Submitted:
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May 24, 2012
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Group Number:
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02
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Advisors:
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Dr. Giuseppe Palmese
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An Du
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Group Members:
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Anjli
Patel
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Neal
Overbeck
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Hans
Guentert
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Dylan
Farrell
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Abstract
The goal of this design project was to research, create, and test bio-based polymer films that were economical and suitable for use as biodegradable trash bags. Nineteen composite polymers with varying ratios of starch and polylactic acid (PLA) were synthesized and tested for strength, biodegradability, and flexibility. The goal was to develop a sample that had a high ratio of starch to PLA and was strong enough to be used as a household trash bag. The starch/PLA polymers were formed by dissolving PLA and corn starch in chloroform and mixing the two solutions. The resulting solutions were then poured out into petri dishes and set in an oven to dry. The majority of the samples produced durable materials that could be used as trash bags. The samples went through three different tests for biodegradability, strength, and flexibility. The final deliverable of the project was a polymer sample a high starch/PLA ratio that was strong, did not lose structural integrity due to short-term exposure to moisture, and could biodegrade within a year.
Introduction
The goal of this design project was to research, create, and test bio-based polymer films that were economical and suitable for use as biodegradable trash bags. Nineteen composite polymers with varying ratios of starch and polylactic acid (PLA) were synthesized and tested for strength, biodegradability, and flexibility. The goal was to develop a sample that had a high ratio of starch to PLA and was strong enough to be used as a household trash bag. The starch/PLA polymers were formed by dissolving PLA and corn starch in chloroform and mixing the two solutions. The resulting solutions were then poured out into petri dishes and set in an oven to dry. The majority of the samples produced durable materials that could be used as trash bags. The samples went through three different tests for biodegradability, strength, and flexibility. The final deliverable of the project was a polymer sample a high starch/PLA ratio that was strong, did not lose structural integrity due to short-term exposure to moisture, and could biodegrade within a year.
Introduction
Problem Overview
Polymers play in integral role in
modern life by providing the basis for almost all plastics, films, and rubbers
used today. However, nearly 100 percent of the commercial synthetic polymers
used to create plastic products are fabricated from crude oil, natural gas, and
petroleum [2]. Prevalent examples of such products include plastic garbage bags
manufactured from polyethylene, a petroleum-based polymer. According to the New
York State Energy Research and Development Authority, about 10 percent of all
the crude oil and natural gas in the United States is used to develop polymers
[5]. These sources for polymer synthesis are neither renewable nor
environmentally sustainable. As a result, there has been an increased emphasis
on finding bio-based alternatives to non-renewable polymers. Bio-based polymers
would decrease reliance on non-renewable sources, lessen the negative effects
of natural gas and petroleum extraction, and provide a sustainable solution to
the issue of synthetic polymers.
Another issue related to synthetic
polymers from petroleum and natural gas is their inability to readily
biodegrade in landfills. It is a known fact that the earth’s landfills are
steadily increasing in volume, greatly due to the lack of biodegradable items
being collected in the landfills. In fact, the U.S. Environmental Protection
Agency estimates that 21 percent of all municipal waste in the United States is
due to plastics alone [2]. One major culprit is the plastic garbage bag. Unlike
grocery bags and other materials, garbage bags are not generally recycled and
contribute significantly to landfills on Earth. This problem may be addressed
by developing biodegradable polymers for plastic bags.
In light of these issues related to
plastic garbage bags from non-renewable sources, a bio-based and biodegradable
polymer is sought as a replacement. This project will focus on creating a
polymer film from renewable sources to replace the non-renewable polyethylene
polymer that is currently used to fabricate plastic garbage bags based on
durability, biodegradability, and cost factors.
Pre-Existing
Solutions
Several solutions have been proposed
to address the issue of non-renewable and non-biodegradable plastic bags.
Bio-based plastic bags have already been developed using polylactic acid (PLA),
a renewable polymer. The chemical composition of PLA is shown in Figure 1 below. PLA is a synthetic polymer that is fabricated from
bio-based monomers [6]. In the process of fabricating PLA, starch is first
extracted from agricultural products, such as corn. Then, the starch is
converted into lactic acid by microorganisms through fermentation. The lactic
acid is then treated in order to cause the lactic acid molecules to form long
chains of polymers. These polymer chains bond to form PLA. PLA is durable,
bio-based, and biodegradable in heat and moisture [1]. Despite the availability
of the materials used to synthesize PLA, the main drawback to its use is the
cost of fabrication. Therefore, PLA plastic bags would be considerably more
expensive than traditional polyethylene plastic bags.
Figure 1: PLA Chemical Composition
Another proposed solution to the
issue of non-biodegradable plastic garbage bags is incorporating starch into
polyethylene films used to create the bags. Starch is a natural polymer found
in plants, and it is composed of many glucose molecules bonded together. The
chemical structure of starch is shown in Figure 1 below. As seen in the
diagram, multiple hydroxyl groups of oxygen and hydrogen are seen throughout
the starch molecule [8]. The hydroxyl groups cause starch to break down in
moist conditions, allowing starch-based polymers to readily biodegrade. This
feature has been exploited to add biodegradable properties to polyethylene
films for plastic bags. For example, the company, Coloroll Ltd., has produced a
biodegradable polyethylene bag containing 7-10 percent starch [4]. While this
solution does offer a biodegradable alternative to non-renewable plastic bags,
it still utilizes mainly petroleum-based polyethylene.
In addition to integrating starch
into polyethylene bags, methods of producing polymer films from starch,
glycerin, and vinegar have also been developed. Glycerin would act as a
plasticizer to promote flexibility in the films, and vinegar would break down
the amylopectin structure of starch through acid hydrolysis to prevent
stiffness. See Figure 2 for the chemical structure of starch. These types of starch
films were initially synthesized in the project, but the resulting polymers were
not durable and prone to ripping and becoming deformed. See the appendix for
further details about starch film synthesis.
In order to effectively address the
issues of polyethylene bags, this project aimed to combine these two strategies
by developing a polymer film from a combination of plant starch and PLA. This
approach may offer a cost-effective and environmentally-friendly solution.
Figure 2: Starch
Chemical Structure
Project
Overview
Currently, garbage bags are
fabricated from petroleum-based polyethylene polymers, which are both
non-renewable and detrimental to the environment. The goal of this project was
to design an environmentally friendly polymer film that is comparable to
polyethylene used in standard garbage bags by primarily utilizing natural,
renewable polymers. This was done by combining corn starch with
PLA and other additives in order to develop a durable, cost-effective, and
biodegradable polymer film. As opposed to previous attempts to create plastic
bags by either adding a small quantity of starch to polyethylene or utilizing
only PLA, this project focused on combining PLA and starch-based materials in
order to develop a suitable and environmentally-friendly product for garbage
bags.
This product would potentially provide a competitive alternative to standard polyethylene plastic bags. To keep the bio-based polymer bags competitive with current petroleum-based bags, another major design goal was to limit the time and cost of producing the bags in order to keep the price of the product low and make it a more appealing option for consumers.
This product would potentially provide a competitive alternative to standard polyethylene plastic bags. To keep the bio-based polymer bags competitive with current petroleum-based bags, another major design goal was to limit the time and cost of producing the bags in order to keep the price of the product low and make it a more appealing option for consumers.
The deliverables for this project
included a bio-based and biodegradable polymer film that is suitable for
plastic garbage bag manufacturing. The film was produced using corn starch, PLA, and other additives necessary for fabrication. The desired
qualities of the polymer film included durability, low cost, and
biodegradability. To assess the performance of the polymer in these three
aspects, biodegradability, flexibility, and puncture tests were performed on the
desirable composite films. The data, findings, and results were
then presented in a detailed final report
and oral presentation. Observations and specific findings were also reported on a
weekly basis through blog postings.
Materials and Methods
Design Constraints
Several factors were considered in order to develop an effective biodegradable replacement for polyethylene garbage bags. To design a marketable product for consumers, the polymer film must have had equivalent tensile strength and durability when compared to the original petroleum-based garbage bags. In addition to strength and durability, the product needed to be manufactured in a cost-effective and timely manner, keeping the price relatively close to that of a standard polyethylene garbage bag.
In order to produce a viable test
product, issues such as common meeting times for lab research and lack of
experience with polymers were also considered during the overall development of
the project. Consequently, the development of the garbage bag material
for this project was limited to a time constraints, available materials and
equipment, and affordability.
Materials
To create PLA and starch composite films, the primary materials included pure corn starch, PLA pellets, and chloroform. Corn starch is water-soluble, and it may be used in polymer synthesis in the presence of plasticizers. PLA, or polylactic acid, is a bio-based polymer created from plant starch that is later fermented and processed to produce the polymer. Chloroform is a liquid with a density of 1.48 g/mL, and it was used as a solvent for starch and PLA. PLA is generally hydrophobic, and it dissolves well in chloroform. When mixed with chloroform, starch disperses throughout the liquid, providing a suitable mixture for polymer synthesis.
Other
equipment used for the project included small glass vials and stir bars in
order to form the solutions of starch and PLA. Glass petri dishes were used as
molds for the starch and PLA solutions, and an oven was also utilized to heat
the starch and PLA mixtures to cause any remaining chloroform to evaporate. Also, an Angstrom puncture test machine was utilized for testing purposes.
Film Synthesis
Before creating starch and PLA composites, pure PLA films were synthesized first in order to use as a control group for future composite films. The method of synthesizing the PLA films involved completely dissolving the PLA in chloroform and allowing the chloroform to evaporate in a petri dish to reveal a thin PLA film. First, the thickness of the film was chosen to be 0.05 mm, based on research of the thickness of commercial garbage bags. To calculate the amount of PLA necessary to form a 0.05 mm film, the surface area of the petri dish was measured and multiplied by the desired film thickness to obtain the necessary volume of PLA. The calculated volume was 0.3619 mL. Then, the density of PLA, 1.24 g/mL, was used to determine the required mass of PLA, which was 0.44885 g. A solution of 3% PLA to 97% chloroform by mass was favorable in order to create the films, so 14.96 g of chloroform was the proper quantity. This value for mass was converted to 10.11 mL using the density of chloroform, which was 1.48 g/mL.
After calculating and measuring the materials, the PLA and chloroform were thoroughly mixed for about 30 minutes using a magnetic stir bar, as shown in Figure 3. The resulting solution was poured into a petri dish in order to dry for 3 days. Then, the petri dish with the film was placed in an oven at 60 degrees Celsius for 24 hours. The final dried PLA film was then delicately extracted from the dish using tweezers. This process was repeated to create a total of three PLA films.
The next step was forming the starch/PLA composite films. Films
were created with 5, 10, 20, 30, 40, 50, 60, and 70 percent starch content by
mass. Two polymer films were synthesized at each starch concentration level, as shown
in Table 1. The method
of calculating the required amount of starch, PLA, and chloroform involved using
the desired film volume and the densities of starch and PLA to calculate the
required mass of each material. The equation for calculating the desired mass
of starch is shown below. As in the pure PLA films, the desired film volume
was 0.3619 mL for a thickness of 0.05 mm. The volumes of the starch and PLA were equated to the total film volume, according to the densities of the
materials and the desired mass ratio. As for the chloroform content, the necessary
volume of chloroform for the desired 97% chloroform solution was calculated separately
for the starch and the PLA according to the calculations described above.
Figure 3: Dissolving PLA in Chloroform
Equation 1:
Figure 3: Dissolving PLA in Chloroform
Equation 1:
Table 1: Starch/PLA Composite Films Data
After calculating and measuring the necessary quantities for starch, PLA, and chloroform, both starch and PLA were dissolved in chloroform in separate vials. Then, the resulting solutions were mixed together in a vial and poured into a petri dish. The petri dish was left to dry for two days and placed in an oven at 60 degrees Celsius for an additional 24 hours to ensure that the chloroform had completely evaporated. This process was repeated for each of the composite films. Images of the final films are shown below in Figure 4.
Figure 4: Completed 100% PLA film (left) and 60% Starch film (right)
Testing Procedures
The desirable physical characteristics for the garbage bag polymer films included durability, flexibility, and biodegradability. In order to test the performance of the films in these three aspects, a variety of tests were developed. The tests included dry and wet puncture tests, a flexibility test, and a biodegradability test.
A
flexibility test was conducted on each of the polymer films by wrapping the test specimens
around a uniform cylindrical pen with a diameter of 0.635 centimeters. This
test was essential in order to determine if the materials were flexible enough
to function as a garbage bag. While conducting the the flexibility test, observations were made to see if the films ripped, cracked, or ruptured while being wrapped around the pen.
For the biodegradability test, each of the test specimens was cut into a
small piece and submerged separately into small vials filled with water. The
vials were placed in an oven at 40 degrees Celsius in order to provide a
uniform temperature for the degradation test. After 3 days, the test specimens
would be removed from the oven and observed to determine the level of
biodegradability that each film exhibited.
After conducting the flexibility and biodegradability tests, one of the starch/PLA composite films was chosen to undergo two separate puncture tests. The chosen composite film was selected based on its ability to retain flexibility and relative strength while maximizing starch content and biodegradability. The purpose of the puncture tests was to determine the durability of the composite films when dry and wet. the puncture test was conducted using a home-made device of plastic cups, a funnel, tape, cardboard, a pen, and a large bottle of water. A diagram of this testing device is shown below in Figure 5. During the test, water was gradually poured into the large bottle of water at the top of the device, and the blunt end of the pen with a diameter of 0.643 cm was pushed down upon a film that was taped to the edges of a lidded plastic cup. The force required to puncture the films was calculated by multiplying the mass of the water and the large bottle by the acceleration due to gravity, 9.81 m/s^2. The displacement of the films were measured during the test. The test was repeated after submerging the specimen in water at room temperature. The puncture test was conducted on the composite film, as well as the 100% PLA film and a sample film from a polyethylene garbage bag for the purpose of comparison.
After conducting the flexibility and biodegradability tests, one of the starch/PLA composite films was chosen to undergo two separate puncture tests. The chosen composite film was selected based on its ability to retain flexibility and relative strength while maximizing starch content and biodegradability. The purpose of the puncture tests was to determine the durability of the composite films when dry and wet. the puncture test was conducted using a home-made device of plastic cups, a funnel, tape, cardboard, a pen, and a large bottle of water. A diagram of this testing device is shown below in Figure 5. During the test, water was gradually poured into the large bottle of water at the top of the device, and the blunt end of the pen with a diameter of 0.643 cm was pushed down upon a film that was taped to the edges of a lidded plastic cup. The force required to puncture the films was calculated by multiplying the mass of the water and the large bottle by the acceleration due to gravity, 9.81 m/s^2. The displacement of the films were measured during the test. The test was repeated after submerging the specimen in water at room temperature. The puncture test was conducted on the composite film, as well as the 100% PLA film and a sample film from a polyethylene garbage bag for the purpose of comparison.
Figure 5: Puncture Testing Device
Results
Observations
and Testing Results
By following the outlined procedure for starch and PLA films, composites were synthesized with up to 70 percent starch content by increments of approximately 10 percent. All of the films below the 70 percent starch content successfully formed uniform, flexible films. The 70 percent starch films, however, did not form desirable films due to their stiffness and likelihood to crack and rip. During polymer synthesis, it was observed that the transparency of the films gradually decreased as the starch content was increased. The films with higher concentrations of starch had an opaque white hue. However, this characteristic was not concerning because it would not interfere with the films' function as garbage bags. After undergoing the flexibility test described above, all of the polymers except for the 70 percent starch film successfully wrapped around the cylindrical pen. The 70 percent starch film cracked in multiple locations during the test.
The results of the biodegradability test clearly reflected a trend of increased biodegradability with increased starch concentration. As seen in Figure 6, the 100% PLA film exhibited little to no degradation after 8 days submerged in water at 50 degrees Celsius. The 60 percent starch film, however, clearly showed signs of degradation and structural breakdown in Figure 7. As a result of the flexibility and biodegradability testing, the 50 and 60 percent starch composite films were chosen for the puncture test and further analysis. The reason for choosing these films was their high starch content while retaining desired flexibility and relative strength.
Figure 6: PLA Biodegradability Test Results
Table
2: Puncture Test Data
Puncture Test Results
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Trial
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Puncture Force (N)
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Displacement (cm)
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Polyethylene Film
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5.16
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1.91
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100% PLA Film
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21.4
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1.11
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50% Starch Film
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7.51
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0.476
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60% Starch Film - Dry
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7.12
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0.635
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60% Starch Film - Wet
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6.63
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0.635
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Figure 8: Puncture Test Data Graph
Analysis
After completing the flexibility, biodegradability, and
puncture tests, it was concluded that the 60% starch polymer composites
were suitable alternatives for polyethylene garbage bags. The results were then analyzed according to cost and environmental factors. After estimating solely material costs for the average polyethylene garbage bag, the price per bag was determined to be $0.0137 per bag. Pure PLA bags cost $0.0368 per bag, which is nearly three times the cost of polyethylene bags. However, the 60% starch bags would cost $0.0280 per bag. This price is still higher than that of polyethylene bags, but it is significantly less than pure PLA bags.
In addition to cost analysis, environmental factors were also considered. Polyethylene bags are synthesized from petroleum, a non-renewable resource. Moreover, they are not biodegradable and therefore continue to fill up landfills. In contrast, the 60% starch/PLA composite bags would be completely synthesized from renewable, bio-based materials. Also, they would be readily biodegradable in landfills, as demonstrated in the biodegradability tests. Both of these factors indicate that the 60% starch films would have a significantly lower environmental toll. Although 60% starch bags would be more costly than polyethylene bags, the overall price on the environment would be significantly less.
Future Work
In addition to cost analysis, environmental factors were also considered. Polyethylene bags are synthesized from petroleum, a non-renewable resource. Moreover, they are not biodegradable and therefore continue to fill up landfills. In contrast, the 60% starch/PLA composite bags would be completely synthesized from renewable, bio-based materials. Also, they would be readily biodegradable in landfills, as demonstrated in the biodegradability tests. Both of these factors indicate that the 60% starch films would have a significantly lower environmental toll. Although 60% starch bags would be more costly than polyethylene bags, the overall price on the environment would be significantly less.
Future Work
The future of PLA-Starch composite garbage bags relies on time and
cost efficiency in the production process. The use of PLA raises the cost of
the garbage bag; however costs can be kept down through the manufacturing
process. Forming the garbage bags through a solution process using chloroform
is inefficient and unrealistic in a mass production scenario. In order to
quicken the process, as well as limit unnecessary costs for materials such as
chloroform, an extruder could be used instead. An extruder allows for
thermoplastic materials as well as other additives, in the form of liquids or
pellets, to be added together, melted, and formed into a film or other solid
object. The use of an extruder allows for the PLA pellets, as well as starch
additive, to be combined into a composite plastic film on a larger production
scale. This method will help to alleviate some of the cost for the
biodegradable garbage bags.
Research on biodegradable garbage bags has been
underway, including Coloroll Ltd. of England, who is producing a bag of 7-10%
starch and polyethylene. Another company creating biodegradable starch based
trash bags is GreenPAK, who has proposed that their bag degrades into carbon
dioxide and water in 90 days [10]. Furthering research on composite
thermoplastic and starch based biodegradable plastics will help to bring the
cost closer to standard petroleum based garbage bags, making it more appealing
to the consumer. Conclusion
After nine weeks of progress, many of the starch/PLA composite
films were satisfactory, but the 60% starch content film was most successful.
The 60% starch films were biodegradable, did not crack when flexed, and could withstand
more pressure than standard garbage bags. The important thing with this sample
was that it was so heavily starch based so the price per bag was more in line
with the price per bag of a standard low-density polyethylene bag. The 60%
starch bag cost about $0.0280 per bag, while the standard bag cost $0.0137 per
bag. The greater price of the starch bag was not so significant when regarding
the environmental costs. The starch films would have a significantly lower
environmental cost than the standard polyethylene bags.
Overall, the project was successful. The initial goal was to
compose a biodegradable plastic that could replace a polyethylene garbage bag.
The new plastic needed to be sufficiently durable, cost-effective, and
environmentally friendly in order to serve as an alternative for standard
plastic garbage bags. The 60% starch composite was successful in all of these
aspects.
References
[1] Bio-Polymers and New Materials: Polymers from Renewable
Resources [Online]. Available: https://docs.google.com/viewer?a=v&q=cache:GB7Xv9soLq8J:www.therenewablecorp.com/resources/white_papers/6.polymers%2520from%2520renewable%2520resources%25204-08.pdf+polymers+from+renewable+sources
[2] G. D. Bilby, Degradable Polymers [Online].
Available: http://www.angelfire.com/ne/mazin/degradable.html
[3] H. Catchpole (2012), The Indestructibles [Online].
Available: http://www.abc.net.au/science/articles/2005/01/27/2839596.htm
[4] H. L. Chum, Polymers from Biobased Materials. Noyes:
William Andrew Publishing, 1991, pp. 90-112.
[5] L. Greenemeier (2009, Dec. 15), How to Make Plastic with Less
Petroleum, Scientific American [Online]. Available: http://www.scientificamerican.com/article.cfm?id=bioplastic-with-less-petroleum
[6] L. Yu, Biodegradable Polymer Blends and Composites from
Renewable Resources. Hoboken: Wiley, 2009, ch. 1, pp. 2-13.
[7] Making Packaging Greener-Biodegradable Plastics. [Online]. Avaliable: http://www.science.org.au/nova/061/061print.htm
[8] M. Bishop (2010), An Introduction to Chemistry. [Online].
Available: http://preparatorychemistry.com/Bishop_Addition_Polymers.htm
[9] P.R. Gruber. (2001). Commodity Polymers from Renewable
Resources: Polylactic Acid. [Online]. Available: http://www.ncbi.nlm.nih.gov/books/NBK44131/
Appendix
Starch films were initially made using 15 mL of corn starch, 5 mL of glycerin, 60 mL of water, and 5 mL vinegar. In this process, glycerin acted as a plasticizer for the films, and vinegar was used to break down the amylopectin structure of starch through acid hydrolysis. These ingredients were heated in a beaker on top of a burner and constantly mixed until the mixture came to a clear gel-like consistency. This process took approximately 30 minutes. The polymer mixture was then spread out onto an aluminum sheet to dry for about 48 hours. The resulting dry polymer film was fairly rigid, which was undesirable.
Another starch film was formed using this same method. However, to prevent the previous cracked and rigid films, the
ingredient contents were altered. The proportion of glycerin was increased to
increase flexibility and plasticity.
The ingredient contents were as follows:
7 mL of corn starch
3 mL of glycerin
30 mL of water
2.5 mL vinegar
As seen in the ingredient list above, the original recipe was halved in order to provide a more manageable polymer quantity, and the glycerin/starch ratio was increased. The resulting film was much more flexible than the original starch film. Due to the use of starch, however, the film was not very durable. It was prone to ripping when removed from the aluminum foil.
The ingredient contents were as follows:
7 mL of corn starch
3 mL of glycerin
30 mL of water
2.5 mL vinegar
As seen in the ingredient list above, the original recipe was halved in order to provide a more manageable polymer quantity, and the glycerin/starch ratio was increased. The resulting film was much more flexible than the original starch film. Due to the use of starch, however, the film was not very durable. It was prone to ripping when removed from the aluminum foil.
