1. B.Eng (hons) in Mechanical Engineering 2015/16
Supervisor: Dr. Marion McAfee
Polylactic Acid: Degradation rate based on size dependence.
By
Thomas McDonagh
This report is submitted in part fulfilment of the requirement
for the award of Bachelor of Engineering (Honours) in Mechanical Engineering
Institute of Technology Sligo June 2016

2. i
Declaration
This is to certify that I am responsible for the work in this report, that the original
work is my own except as specified in acknowledgements or references and that
neither the thesis nor original work contained herein has been submitted to this or
any other institution for the award of a degree or any other purpose.
______________________
Thomas McDonagh

3. ii
Abstract
Polylactic Acid, or PLA, is a polymer that can be derived from renewable resources.
It degrades in a relatively quick time period, breaking down into harmless
molecules like water and carbon dioxide. Examples of its many uses include
medical applications such as sutures and catheters, due to its biocompatibility. Its
ability to degrade “in vivo” negates unnecessary surgery to remove the medical
device after its job has been done. The degradation rate is very important when
designing these devices, to ensure they break down only when their role is
complete. There are many factors which influence degradation rate, like molecular
weight, processing conditions and environment; all which have been studied
extensively.
An area that has had very little attention is the size of device. It was found in a
previous study that larger samples of PLA degraded faster than smaller sizes, due to
an autocatalytic process. This study was carried out with the intention of proving or
disproving the hypothesis that degradation rate occurred in relation to sample size,
due to the nature of the degradation process. A mould was manufactured, samples
produced under identical conditions. The samples were then degraded in a
phosphate buffer solution for specific time periods. Changes in mass were observed,
to determine the degradation rate in each sample. Upon completion of the study, it
was found that the larger samples had lost more mass than smaller samples. It was
also found that pH of the buffer solution was more acidic with the larger samples.
Taking both of these indicators into consideration, this study confirmed the findings
of the original study; the larger samples were degrading at a faster rate due to an
autocatalytic degradation process.

4. iii
Acknowledgements
I would like to express thanks to the following people for their involvement in
writing this:
To my family for their support and encouragement over the last few months.
To my supervisor Dr. Marion McAfee, for her guidance, advice and supervision
over the project.
To Konrad Mulrennan and Darren Whitaker, for their help in the laboratory, advice
and information.
To Dr. David Mulligan, for his advice and help in the materials testing laboratory.
To Aidan Murtagh, for his help in the materials laboratory

5. iv
TABLE OF CONTENTS
TABLE OF FIGURES...........................................................................................VI
TABLE OF TABLES........................................................................................... VII
CHAPTER 1: POLY(LACTIC) ACID .................................................................. 1
1.1) BACKGROUND .................................................................................................. 1
1.2) AIM .................................................................................................................. 1
1.3) SPECIFIC OBJECTIVES....................................................................................... 2
CHAPTER 2: LITERATURE REVIEW............................................................... 2
2.1) WHAT IS PLA................................................................................................... 2
2.2) PROPERTIES OF PLA MEDICAL DEVICES ........................................................... 5
2.3) THE DEGRADATION PROCESS........................................................................... 6
2.4) DEGRADATION RATE INFLUENCING FACTORS.................................................. 8
2.4.1) Degree of Crystallinity:............................................................................ 8
2.4.2) Molecular Weight ..................................................................................... 8
2.4.3) pH Level.................................................................................................... 9
2.4.4) Purity........................................................................................................ 9
2.5) PREVIOUS WORK ON EFFECT OF SAMPLE GEOMETRY ON DEGRADATION RATE 10
CHAPTER 3: EXPERIMENTAL PROCEDURE.............................................. 12
3.1) SCOPE............................................................................................................. 12
3.1.1) Description ............................................................................................. 12
3.1.2) Assumptions............................................................................................ 12
3.2) MATERIAL...................................................................................................... 12
3.3) MOULD........................................................................................................... 13
3.4) METHOD DEVELOPMENT................................................................................ 14
3.5) SAMPLE PRODUCTION .................................................................................... 15
3.6) DEGRADATION TESTING................................................................................. 16
3.6.1) Visual Inspection: Samples @ 0 Hours: ................................................ 18
3.6.2) Visual Inspection: Samples @ 48 Hours: .............................................. 18
3.6.3) Visual Inspection: Samples @ 168 Hours: ............................................ 19
3.6.4) Visual Inspection: Samples @ 336 Hours: ............................................ 19
3.7) MECHANICAL TESTING................................................................................... 20
CHAPTER 4: RESULTS....................................................................................... 21
4.1) MASS CHANGE ............................................................................................... 21

6. v
4.2) INDIVIDUAL MASS CHANGES ......................................................................... 22
4.3) PH LEVEL....................................................................................................... 24
4.4) MECHANICAL TESTING................................................................................... 25
4.4.1) Loss in mechanical properties due to degradation ................................ 26
4.4.2) Thickness 1 (3.1mm)............................................................................... 26
4.4.3) Thickness 2 (3.8mm)............................................................................... 27
4.4.4) Thickness 3 (4.8mm)............................................................................... 28
CHAPTER 5: ANALYTICS ................................................................................. 30
5.1) ANALYSIS OF VARIANCE (ANOVA) .............................................................. 30
5.1.1) 48 Hours................................................................................................. 30
5.1.2) 168 Hours............................................................................................... 31
5.1.3) 336 Hours............................................................................................... 32
CHAPTER 6: CONCLUSIONS & RECOMMENDATIONS ........................... 33
6.1) CONCLUSIONS ................................................................................................ 33
6.2) DISCUSSION.................................................................................................... 33
6.3) RECOMMENDATIONS ...................................................................................... 34
REFERENCES....................................................................................................... 36
APPENDICES........................................................................................................ 38

7. vi
Table of Figures
Figure 2-1: Polymer Chain (Boundless, n.d.) ............................................................ 2
Figure 2-2: Polylactic Acid and its Stereoisomers (NPTEL, n.d.)............................. 4
Figure 2-3: PLA Hydrolysis (University of Washington, n.d.).................................. 7
Figure 2-4: Weight Loss of Degrading Samples (I. Grizzi, 1995)........................... 11
Figure 2-5: pH Changes in Degrading Samples (I. Grizzi, 1995)............................ 12
Figure 3-1: Mould Design in Solidworks, Partially Exploded View....................... 14
Figure 3-2: Finished Samples in Mould................................................................... 15
Figure 3-3: Samples in Vials, Prior to Degradation................................................. 17
Figure 3-4: Samples at 0 Hours Degradation........................................................... 18
Figure 3-5: Samples at 48 Hours Degradation......................................................... 18
Figure 3-6: Samples at 168 Hours Degradation....................................................... 19
Figure 3-7: Samples at 336 Hours Degradation....................................................... 19
Figure 3-8: 3-Point Bend Test.................................................................................. 20
Figure 4-1: Average Mass Changes at Time points................................................. 22
Figure 4-2: 48 Hour Mass Changes ......................................................................... 23
Figure 4-3: 168 Hour Mass Changes ....................................................................... 23
Figure 4-4: 336 Hour Mass Changes ....................................................................... 24
Figure 4-5: Average pH Levels at Time points........................................................ 25
Figure 4-6: Force Vs. Deflection, Thickness 2mm.................................................. 27
Figure 4-7: Force Vs. Deflection, Thickness 2 ........................................................ 28
Figure 4-8: Force Vs. Deflection, Thickness 3 ........................................................ 29
Figure 0-1: Solidworks Draft of Mould ................................................................... 41
Figure 0-2: ANOVA Calculations for 48 Hour Mass Changes ............................... 42
Figure 0-3: ANOVA Calculations for 168 Hour Mass Changes ............................. 42
Figure 0-4: ANOVA Calculations for 336 Hour Mass Changes ............................. 42

8. vii
Table of Tables
Table 2-1: Biopolymers: Sources & Applications (Vroman & Tighzert, 2009)........ 3
Table 2-2: PLA, Molecular Weight and Associated Properties (Auras, et al., 2010) 9
Table 3-1: PLA Material Properties (LLC, n.d.)...................................................... 13
Table 3-2: Amended Sample Mass and Thickness .................................................. 15
Table 4-1: Average Mass Changes at Time points .................................................. 22
Table 4-2: Individual & Average pH Levels ........................................................... 25
Table 4-3: Maximum Forces Prior To Failure ......................................................... 27
Table 4-4: Maximum Forces Prior to Failure .......................................................... 28
Table 4-5: Maximum Force Prior To Failure........................................................... 29
Table 5-1: Manual ANOVA at 48 Hours................................................................. 30
Table 5-2: Excel Generated ANOVA 48 Hours ...................................................... 31
Table 5-3: Manual ANOVA at 168 Hours............................................................... 31
Table 5-4: Excel Generated ANOVA 168 Hours .................................................... 32
Table 5-5: Manual ANOVA at 336 Hours............................................................... 32
Table 5-6: Excel Generated ANOVA 336 Hours .................................................... 33
Table 0-1: Glossary.................................................................................................. 38
Table 0-2: Sample Masses, Before and After Production........................................ 39
Table 0-3: Sample Masses, Before and After Degradation & pH Levels................ 40

9. 1
Chapter 1: Poly(Lactic) Acid
1.1) Background
Polylactic acid (PLA) is a bio-plastic, used extensively for commercial and medical
applications. It is used commercially for food packaging and for medical
applications such as catheters, bone fixation devices and drug delivery devices. Due
to its environmentally friendly nature and biocompatibility, it is a very interesting
material that is a viable alternative to crude oil derived polymers.
Currently, research work carried out in I.T. Sligo focuses on PLA and various
problems encountered during the production processes used to form it into useful
devices. Due to the nature of the material, it degrades through an internal
autocatalytic process. This means that it is a self-sustaining process; acidic
molecules are formed during the degradation process that aid further degradation of
the sample. By extension, this means that larger samples should degrade at a faster
rate than relatively smaller samples. It is necessary to consider this when designing
medical devices as the degradation rate affects the life of the device; this must
match the required life span of the device, to ensure it does its job before it breaks
down. While various studies have been carried out, the effect of size on the
degradation rate has not been studied in depth in the literature.
1.2) Aim
The aim of this report is to determine whether the size of a PLA sample has any
influence on its degradation rate and if so to quantify that effect.
The report will try to answer the following questions with regard to PLA devices:
 Is the size and geometry of a device a significant factor in:
o The rate of mass loss?
o The rate of loss of mechanical properties?
 Is it possible to predict the degradation rate, given the device geometry and
size, for a fixed material and processing method?

10. 2
1.3) Specific Objectives
The objectives of this study are to:
 Produce samples of identical shape, with varying thickness
 Carry out accelerated degradation testing on samples
 Test mechanical properties of the samples
 Record any mass loss and physical changes in samples during degradation
 Carry out statistical analysis of results
 Produce a mathematical model for the prediction of degradation rate
 Find a correlation between degradation rate and thickness/size of sample
Chapter 2: Literature Review
2.1) What is PLA
PLA is a biopolymer; a polymer is a substance that is made up of lots of repeated
molecular units called monomers, as in Figure 2-1. These monomers each consist of
identical atomic arrangement and usually feature hydrogen, oxygen and carbon
atoms. Depending on the atomic arrangement, they can have many different types
of properties that make them suitable for various uses. Most polymers are derived
from crude oil; a biopolymer is different because it is derived from a living (plant)
or renewable source, and is degradable or compostable as a result. It is possible to
synthesize a biopolymer from crude oil, which will break down quickly like a
naturally derived substance, but is still dependant on a finite resource for
production, so is not an ideal solution in the long term. (Avinc & Khoddami, 2009)
FIGURE 2-1: POLYMER CHAIN (BOUNDLESS, N.D.)

11. 3
Biopolymers can be categorized into 4 types, based on the source they are obtained
from. These are shown in Table 2-1 into the different types and some of their uses.
TABLE 2-1: BIOPOLYMERS: SOURCES & APPLICATIONS (VROMAN & TIGHZERT, 2009)
Type: Source: Industry: Applications:
Starch Potatoes, corn and wheat. Food
Agriculture
Medical
Food Packaging
Edible Food Film
Greenhouse Covering
Mulch Film
Bone Tissue Engineering
Sugar Sugar beet, potatoes, wheat, corn. Medical Drug Delivery
Stents
Cellulose Plants Various Cellophane Packaging
Synthetic Crude oil Various Packaging
Insulation
Manufacturing
Polymers consist of repeated monomer units; with PLA, the monomer is lactic acid.
Lactic acid has two stereoisomers1
: dextro (D) and levo (L). Natural fermentation
produces proportions of 99.5% (L) form lactic acid and 0.5% (D) forms. Both
isomers have identical physical properties; the only difference is the orientation
they take, see Figure 2-2 below; difference highlighted. In the human body, (L)
form lactic acid is produced naturally which is why (L) or (DL) lactides are
produced and researched, more-so than (D) forms. (Auras, et al., 2010)
1
An isomer is a molecule with an identical chemical formula as another molecule, but with a
different chemical structure; isomers contain the same number of atoms of each element, but have
different arrangements of their atoms. Stereoisomers are isomeric molecules that have the same
molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of
their atoms in space.

12. 4
Biopolymers, produced from sustainable sources, are very important materials as
they are a viable alternative to petroleum based polymers, and can be used in a
multitude in applications. They have excellent properties, most notably a relatively
fast decomposition rate; from six months to two years, as opposed to up to 1000
years for petroleum based polymers. And when they break down, non-toxic
elements are formed like water and carbon dioxide.
FIGURE 2-2: POLYLACTIC ACID AND ITS STEREOISOMERS (NPTEL, N.D.)
Polylactic Acid, or C3H4O2, is an aliphatic polyester (carbon atoms form open
chains) and is one of the most commonly used bioresorbable materials available.
When compared to other aliphatic polyesters, it has a higher strength, modulus and
melting temperature. In addition to this, it is non-toxic and biodegradable, both in-
vivo, and in the environment; biodegradability is slow when it is derived from only
one monomer. It has excellent bioresorption and biocompatibility properties which
make it an excellent material for use in various medical applications. (Oyama, et al.,
2009)
PLA is a biopolymer: a polymer that will naturally degrade during its intended
lifespan and can be produced from 100% renewable sources. It features molecule
chains with a helical structure. It is compostable and easily degrades by simple
hydrolysis; coming into contact with water or an aqueous solution. Because of this
attribute, it is an excellent material to use for packaging as it has a low
environmental impact. PLA is produced from sugars, via fermentation, to produce

13. 5
lactic acid. The sugars required for this are pentose’s (C5 sugars) or hexoses (C6
sugars) and are present in carbohydrates. They can also be produced from starch but
this starch has to be converted to a sugar via hydrolysis prior to fermentation.
(Auras, et al., 2010)
Biopolymers, such as PLA, are used in medical procedures due to the non-toxicity
and degradation properties they possess. They can be inserted into a patient, and
unlike metallic devices, do not have to be removed once their purpose has been
served; they decompose into harmless compounds naturally occur in the human
body, such as carbon dioxide and water. They also promote repair, where a metallic
implant would not; this is down to the degradation of the implant and the drug
delivered during its life, which encourages the damaged tissue to heal. A metallic
implant can discourage repair as it takes over the role of the tissue it is intended to
aid. (Sackett & Narasimhan, 2011)
2.2) Properties of PLA medical devices
PLA has unique and advantageous properties, such as good appearance, high
mechanical strength and low toxicity. It is also biocompatible; it is not harmful and
is non-toxic to human tissue. Another major advantage it possesses is its thermal
properties; primarily that it starts to degrade below its melting temperature. This
occurs in vivo, and is aided by conditions such as enzymes present, pH levels and
the aqueous solution it comes into contact with. As biopolymers can be tailored to
be used in a variety of roles as medical devices, different mechanical properties and
degradation times can be designed into the device. An ideal polymer should have, at
the very least, the following properties: (Middleton & Tipton, 2000)
 Does not cause an inflammatory/toxic response due to its insertion
 100% metabolism into the body after its purpose has been served
 Mechanical properties are maintained long enough to aid in healing
the adjacent area
 Easily sterilized
 Easily produced
 Has a good shelf life
However, its thermal properties can also be disadvantageous, as processing
conditions can cause degradation before the device or product has been produced.

14. 6
Another drawback it possesses is that the crystalline regions are harder to fully
break down in vivo; high-molecular weight PLA that is used in medical applications
contains a greater amount of crystalline regions. And coupled with this, the natural
degradation that occurs in the body can cause an increase in the crystalline regions
which can prolong the material fully breaking down into its constituents. Another
major downside is the difficulty in producing PLA for use in vitro and the
associated costs of purification. (Jamshidian, et al., 2010)
2.3) The Degradation Process
PLA is susceptible to hydrolysis (or hydrolytic degradation), which means that
coming into contact with an aqueous solution (like water) breaks the molecular
bonds in the polymer chains, shown in Figure 2-3. This consists of removing two
water molecules from two lactic acid molecules and this is the exact opposite of
how it is produced. A degrading polymer consists of three components; long
polymer chains, short chains and water molecules, where short chains are defined as
monomers and oligomers. All other chains can be considered as long. Short chains
can dissolve and diffuse in water, while long chains cannot. (Auras, et al., 2010)
The process can be broken down into the following:
Stage 1
 Relatively small water molecules are absorbed into the sample
 These water molecules attack the ester bonds in the amorphous region
polymer chains as in Figure 2-3
 Long chains are broken into short chains
 Molecular weight decreases without loss of physical properties
 Very small changes in pH level
 Samples increase in mass as aqueous solution is absorbed
Stage 2
 Acidic carboxyl groups are also created in this attack
 These acidic groups promote further ester bond attacks (autocatalysis)
 This sustains the breaking down of the long & short polymer chains
 The monomers can escape out of the sample due to their smaller size, with
rapid mass loss as a result
 Decrease in pH level as aqueous solution becomes more acidic

15. 7
When considering change in mass, the degradation process can be broken down into
two stages; an increase in mass as the water is absorbed into and preferentially
attacks the amorphous regions, and then a decrease in mass as the amorphous
regions start to degrade. This is due to the heterogeneous, or autocatalytic, internal
degradation of the material as the acidic molecules are trapped inside and create
further attacks on the ester linkages in the molecular chains. (Weir, et al., 2004)
The degradation rate is dependent on material factors such as molecular weight and
medium factors such as temperature and pH level. By altering any of these factors,
it is possible to manipulate the degradation rate. The “in vivo” hydrolytic
degradation rate is comparable to “in vitro” hydrolytic degradation; therefore the
degradation behaviour and rate, “in vivo”, can be predicted to a certain extent,
through “in vitro” testing. (Auras, et al., 2010)
FIGURE 2-3: PLA HYDROLYSIS (UNIVERSITY OF WASHINGTON, N.D.)

16. 8
2.4) Degradation Rate Influencing Factors
The following are factors that have a bearing on the degradation rate, and can be
adjusted through various methods to achieve a desired output.
2.4.1) Degree of Crystallinity:
Crystallinity is a major factor in the degradation process, as the amorphous regions
are degraded first via hydrolysis. So the greater the degree of crystallinity, the
longer it will take to degrade and vice-versa for a sample that comprises of a greater
amount of amorphous regions. It was found that the crystallinity actually increased
during degradation, in a study comparing samples of differing molecular weight and
crystallinity. This was thought to be due to rearrangement of amorphous regions in
crystalline domains, as the shortened molecular chains had room to move and settle
into a crystalline region. (Migliaresi, et al., 1994)
2.4.2) Molecular Weight
While molecular weight does not influence the degradation rate directly, it does
influence the amount of crystalline regions in the sample. It also influences
mechanical properties of the material, with a higher molecular weight resulting in
greater results for stress, strain and elastic modulus for samples tested, as shown in
Table 2-2. (Auras, et al., 2010) The greater the molecular weight, the longer it will
take for a sample to degrade. Processing conditions can cause a drop in molecular
weight, such as residence time, moisture presence and processing temperatures. The
residence time is the amount of time the materials is held in the processing
machinery, prior to production. To reduce this loss due to moisture presence,
measures such as drying the raw material prior to processing were found to be
successful. (Weir, et al., 2004)

17. 9
TABLE 2-2: PLA, MOLECULAR WEIGHT AND ASSOCIATED PROPERTIES (AURAS, ET AL., 2010)
Sample PDLLA 1 PDLLA 2 PDLLA 3
Molecular Weight (Mv, Da) 47,500 75,000 114,000
Tensile Properties
Yield Strength (MPa) 49 53 53
Tensile Strength (MPa) 40 44 44
Elongation at break (%) 7.5 4.8 5.4
Elastic Modulus (MPa) 3650 4050 3900
Flexural Properties
Flexural Strength (MPa) 84 86 88
Maximum Strain (%) 4.8 4.1 4.2
Elastic Modulus (MPa) 3500 3550 3600
Impact resistance
Izod, notched (kJ/m2
) 1.8 1.7 1.8
Izod, unnotched (kJ/m2
) 13.5 14.0 15.0
Hardness
Rockwell Hardness Scale (H) 78 72 76
2.4.3) pH Level
When placed in a pH neutral solution (about pH 7), degradation is slow as water
molecules are attacking ester bonds solely. Catalysis is affected by the pH level;
ester hydrolysis can be catalysed by an acidic or basic medium. Extremes of either
will result in the most rapid degradation of a sample. Accelerated degradation at
lowering pH levels is a result of the faster chain scission due to the increased acidity
of the solution. The monomers created from this scission are acidic (carboxylic
acids) which lower the pH level even further and promote an even faster
degradation, aiding the autocatalytic process. (Gopferich, 1996)
2.4.4) Purity
The degradation rate of material can be increased or decreased based on the
composition or material added during production. A study in 2007 showed that the
composition could be used to increase or decrease the degradation rate. In the case
of increasing the degradation rate of a scaffold, the added material acted as an

18. 10
impurity. This was also noted for the porosity of a sample; greater porosity resulted
in faster degradation rates. (Charles-Harris, et al., 2007)
2.5) Previous work on effect of sample geometry on degradation
rate
There is very little reported in the literature on size dependant degradation. All other
studies found, discussed varying compositions and producing copolymers to change
degradation rates. The most relevant study was carried out in 1995 and featured
differing size and geometry samples. (I. Grizzi, 1995) This study was carried out
with the hypothesis that larger samples would degrade faster than smaller samples
based on a diffusion-reaction model. This diffusion reaction model explains the
autocatalytic nature of the degrading sample; as molecular chains are broken down,
acidic molecules are released which aid further breaking down of molecular chains,
and so on.
For this study, the following samples were used:
 Compression moulded plates, 15 x 10 x 2mm
 Millimetric beads, 0.5-1.0mm diameter
 Submillimetric cast films, 3mm thick (other dimensions not shown)
 Submillimetric microspheres, 0.125 – 0.250mm diameter
The different geometries comprised of samples produced for the study; requiring
compression moulding for the large plates, casting for the films and a solvent
evaporation method for the beads. The samples were vacuum dried at 40°C for 48h
prior to processing. They were degraded in pH 7.4 buffer solution at 37° at a ratio of
100:1, by mass of solution to each sample.
Aspects monitored during the degradation testing were:
 Weight Loss
 Water Absorption
 Size-Exclusion Chromatography, to determine molecular weight
 pH level, to measure lactic acid formation

19. 11
Differences in mass were measured with the formulas:
Where: WL = Weight Loss
Wo = Original Weight
Wr = Residual Weight
Where: WA = Water Absorption
Ww = Wet Weight
FIGURE 2-4: WEIGHT LOSS OF DEGRADING SAMPLES (I. GRIZZI, 1995)
Legend: Sample
Film
Plate
After analysing the above areas with each sample it was found that the largest
samples, the plates, started degrading first and had a faster degradation rate than the
films and beads/spheres. When weight loss started to occur, it was rapid, as in
Figure 2-4. The pH levels were also monitored, and found to decrease as
degradation proceeded, indicating a rise in acidity, as in Figure 2-5.

20. 12
FIGURE 2-5: PH CHANGES IN DEGRADING SAMPLES (I. GRIZZI, 1995)
Film
Plate
Bead
Microbead
From the results, it could be observed that size did play an important role in the
degradation rate; the larger beads and plates were found to degrade at a much faster
rate than the films and microspheres. Because of this, it was concluded that small
devices would last much longer than large scale devices.
Chapter 3: Experimental Procedure
3.1) Scope
3.1.1) Description
The purpose of this study is to produce samples of PLA, of varying sizes, and carry
out an accelerated degradation process. Samples will also be tested for mechanical
properties before and during the degradation process. The shape of each sample will
be the same while the thickness of each sample will be changed, ranging in
thickness of approximately 3mm to 7mm.
3.1.2) Assumptions
As the samples will be produced using identical methods, it will be assumed that all
samples have the same molecular weight.
3.2) Material
The material used for this report was a PLA, called Ingeo Biopolymer 2003D, and
was obtained from NatureWorks LLC. It’s an industrial grade PLA and is intended

21. 13
use is for food packaging; it can be thermoformed or extruded. Below, Table 3-1
lists material properties. The material was supplied in pellet form; this was
cryogenically ground into a coarse powder prior to sample production.
TABLE 3-1: PLA MATERIAL PROPERTIES (LLC, N.D.)
3.3) Mould
When designing the mould, it was decided on an open cast design that would split
and allow easy removal of each sample, as in Figure 3-1. The mould was held
together with M10 threaded bars and nuts, not shown. An open cast mould was
required as extrusion or thermoforming were not available as production methods; it
was a relatively simple mould to design and manufacture within the timeframe of
the project.
Typical Material & Application Properties (Extruded/Thermoformed)
Physical Properties Ingeo 2003D ASTM Method
Specific Gravity 1.24 D792
MFR, g/10 min (210°C, 2.16kg) 6 D1238
Clarity Transparent
Mechanical Properties
Flexural Strength, psi (MPa) 12,000 (83) D790
Flexural Modulus, kpsi (GPa) 555 (3.8) D790
Tensile Strength @ Break, psi (MPa) 7,700 (53) D882
Tensile Yield Strength, psi (MPa) 8,700 (60) D882
Tensile Modulus, kpsi (GPa) 500 (3.5) D882
Tensile Elongation, % 6.0 D882
Notched Izod Impact, ft-lb/in (J/m) 0.3 (16) D256
Shrinkage is similar to PET (2)
Heat Distortion Temperature (°C) 55 E2092

22. 14
FIGURE 3-1: MOULD DESIGN IN SOLIDWORKS, PARTIALLY EXPLODED VIEW
3.4) Method Development
Initially, it was hoped to produce 5 different samples of varying thicknesses ranging
from approximately 2mm to 10mm. The mould had been designed to produce 5
different thicknesses but time constraints resulted in producing 4 thickness
variations. An initial trial run emulated a production process similar to extrusion.
(Weir, et al., 2004) This involved heating the material up to 200°C but the time was
extended to 2 hours, to ensure all of the PLA had melted sufficiently. After the
initial trial run, it was found that the PLA had stuck to the mould and was hard to
remove, resulting in damaged samples. To overcome this, baking paper was used to
line the mould; this worked satisfactorily and was easy to peel off each sample
when cooled down. A drawback of using baking paper was that it affected the
sample sizes, resulting in a reduction in the overall dimensions.
The size of the PLA granules was another factor that resulted in an adjustment of
the sample thicknesses. This was due to the granules being larger than anticipated; it
was not possible to fill each mould section with the correct mass of PLA as it
overflowed. To overcome this, each section was filled with the maximum amount of
PLA they could adequately hold – masses were recorded for each section and Table
3-2 was drawn up, to aid in sample production.

23. 15
TABLE 3-2: AMENDED SAMPLE MASS AND THICKNESS
Sample No. Average Sample Thickness (mm) Nominal Mass (g)
1 3.1 1.24
2 3.8 1.73
3 4.8 2.24
4 6.8 3.25
The nominal masses were then used when filling the mould sections; average
sample thicknesses were measured after production
3.5) Sample Production
The raw PLA was dried at 65°C for 4 hours in a standard oven, prior to producing
the samples. (Natureworks, n.d.) To produce the samples, an open cast mould was
used. An initial trial run producing samples found that the PLA material stuck to the
mould and fractured when trying to remove. To overcome this, baking paper was
used to line the mould, seen in Figure 3-2. The dried powder was then weighed to
ensure the correct amount was poured into each section of the mould. The mould
was placed in an oven at 200°C for 2 hours, removed and allowed at least a half
hour to cool. The samples were then removed from the mould and the baking paper
was removed from samples. The samples were weighed again, dimensions
measured and values recorded.
FIGURE 3-2: FINISHED SAMPLES IN MOULD

24. 16
It was decided upon to produce 48 samples in total, with 4 thickness variations. This
would require 12 samples for each of the four time periods: 48 in total. An initial
batch was produced to degrade, to get an idea of the extent of degradation after a
certain time. This initial time period was 144 hours but when complete, the samples
had not lost a significant amount of mass so it was decided to run the entire test
over a longer period, with degradation times of 0, 48, 168 and 336 hours.
To deal with the various sizes and degradation times, along with each sample being
tested in triplicate, a means of easily identifying samples was devised. This required
each sample being identified with a part number consisting of a number, letter and
number, for example:
0-A-1
Where:
“0” refers to the degradation time
0 – 0 Hours
48 – 48 Hours
168 – 168 Hours
336 – 336 Hours
“A” identifies which batch, out of the three, for testing purposes
A, B or C
“1” refers to the sample thickness
1 - 3.1mm
2 – 3.8mm
3 – 4.8mm
4 – 6.8mm
3.6) Degradation Testing
The degradation test required placing the 36 samples in vials of buffer solution,
with a pH of 7.3, at elevated temperatures. The samples were placed in 22ml vials,
as in Figure 3-3, with 20ml of Phosphate Buffer Solution, and placed in an oven at
70°C. (Weir, et al., 2004) The relevant samples were removed from the oven at their
specified time period, removed from the vials and dried at 30°C for 48 hours.
Masses were recorded for each sample; before the drying process (when wet) and

25. 17
then again after the drying process had been carried out. When each sample was
removed from its respective vial, the pH of the buffer solution was also measured
and recorded.
FIGURE 3-3: SAMPLES IN VIALS, PRIOR TO DEGRADATION

26. 18
3.6.1) Visual Inspection: Samples @ 0 Hours:
These samples were transparent, with some very small air bubbles that had been trapped
inside during the production process. Samples displayed smallest to largest, left to right.
FIGURE 3-4: SAMPLES AT 0 HOURS DEGRADATION
3.6.2) Visual Inspection: Samples @ 48 Hours:
After removing these samples, they had lost their clarity and become opaque. There was
very little change in pH levels, compared to 0 Hours.
FIGURE 3-5: SAMPLES AT 48 HOURS DEGRADATION

27. 19
3.6.3) Visual Inspection: Samples @ 168 Hours:
After removal, the samples had turned milky white and were covered in cracks. The
samples were very brittle, with edges breaking off with very little force. The pH levels had
dropped significantly.
FIGURE 3-6: SAMPLES AT 168 HOURS DEGRADATION
3.6.4) Visual Inspection: Samples @ 336 Hours:
These samples exhibited more cracks than the previous and resembled chalk. They were
very delicate and had to be handled carefully; there was visible evidence of volume loss
along the underside of the samples.
FIGURE 3-7: SAMPLES AT 336 HOURS DEGRADATION

28. 20
3.7) Mechanical Testing
When degradation testing was complete, samples from the 0 Hour and 48 Hour
degradation tests were subjected to a 3-point bend test on a Tinius Olsen H50KS
with a 2.5KN load cell. Initial testing of samples had shown this to register loads as
low as 2N so it was considered acceptable to carry out the testing with. Samples
from the 168 Hour and 336 Hour degradation tests were not tested mechanically;
they were too fragile and began falling apart when handled.
For the test, samples were placed on a holder with a span of ≈28mm, as shown in
Figure 3-8. The test was then carried out, placing a force on the centre of the sample
until failure occurred. Graphs and maximum load values were obtained for all
samples and recorded.
FIGURE 3-8: 3-POINT BEND TEST

29. 21
Chapter 4: Results
4.1) Mass Change
Mass levels were recorded at each stage of the samples production and testing stage
and recorded to determine any differences. Previous work had shown that samples
experienced mass gain during the initial portion of degradation and experienced
mass loss during the second and final portion. By measuring if the samples had
gained or lost mass, it could be determined which of the 2 distinct stages of
degradation they were undergoing. (I. Grizzi, 1995)
Both phases were observed for this study. After 48 hours degrading, 8 out of the 12
samples were showing masses greater than at 0 hours, 2 showed no change in mass
and 2 showed a mass loss. This ranged from approximately -0.7% to +4%. After
168 hours, mass loss was observed for all samples, ranging from approximately -
1.5% to -6%. And for the samples degraded for 336 hours, significant mass loss was
recorded for each sample, ranging from approximately -21% to -26%; the larger
samples showing greater mass loss than the smaller samples. This information is
displayed in Figure 4-1. These mass changes were recorded, averages calculated
and displayed in Table 4-1. (Please note; “+” represents mass gain and “-“
represents mass loss). Mass changes were calculated using:

30. 22
FIGURE 4-1: AVERAGE MASS CHANGES AT TIME POINTS
TABLE 4-1: AVERAGE MASS CHANGES AT TIME POINTS
Mass Difference, Averages
Time (H) Size 1 (%) Size 2 (%) Size 3 (%) Size 4 (%)
48 +1.34 +0.19 +2.37 +0.62
168 -2.68 -4.22 -3.41 -4.48
338 -21.50 -23.14 -24.12 -25.03
4.2) Individual Mass Changes
Individual mass changes are graphed in the following figures; they are colour coded
by size (going from smallest to largest, left to right) and the average of each sample
is displayed in black, with values. It can be seen that there was no relation between
mass change and sample thickness at 48 hours. Samples undergo a mass gain phase
initially before the mass loss phase. At 48 hours, it is shown that there is mass gain,
mass loss and no change at all; it is impossible to draw any conclusion from these
results; the test would need to be replicated with larger sample numbers. Carrying
out ANOVA confirmed that there was no relationship between mass change and
sample thickness at 48 Hours.
-30
-25
-20
-15
-10
-5
0
5
-14 36 86 136 186 236 286 336
MassChange(%)
Time (H)
Mass Change: Averages
Size 1 Size 2 Size 3 Size 4

31. 23
FIGURE 4-2: 48 HOUR MASS CHANGES
Mass changes at 168 hours showed mass loss in all samples, as in Figure 4-3. On
average, the largest thickness sample had degraded more than the smallest, (4.48%
and 2.68% respectively) Sample thicknesses 3 and 4 were not degrading in respect
to their sizes. (4.22% and 3.41% respectively)
FIGURE 4-3: 168 HOUR MASS CHANGES
1.34
0.19
2.37
0.62
-2
-1
0
1
2
3
4
5
MassChange(%)
Sample
Mass Change % : 48 Hours
A1 B1 C1 AVG 1 A2 B2 C2 AVG 2
A3 B3 C3 AVG 3 A4 B4 C4 AVG 4
-2.68
-4.22
-3.41
-4.48
-7
-6
-5
-4
-3
-2
-1
0
MassChange(%)
Sample
Mass Change % : 168 Hours
A1 B1 C1 AVG 1 A2 B2 C2 AVG 2
A3 B3 C3 AVG 3 A4 B4 C4 AVG 4

32. 24
For the final mass measurements, mass loss was shown to be greater with the larger
samples. Carrying out ANOVA on mass measurements for 336 hour samples had
shown that the sample thickness was significant in the rate of mass loss.
FIGURE 4-4: 336 HOUR MASS CHANGES
4.3) pH Level
The pH levels were recorded at each stage of the degradation process, to determine
the acidity of the buffer solution. A pH neutral solution (7.3) was used for the
degradation test and a rise in acidity, or drop in pH level, indicated a sample had
been undergoing degradation. This was due to the creation of acidic molecules
during the degradation process. As the samples were degraded, the pH level
dropped for each. The levels for each sample are shown in Figure 4-5: Average pH
Levels at and Table 4-2. The mean values of each of the size samples were used to
generate values for the graph.
-21.50
-23.14 -24.12 -25.03
-30
-25
-20
-15
-10
-5
0
MassChange(%)
Sample
Mass Change % : 336 Hours
A1 B1 C1 AVG 1 A2 B2 C2 AVG 2
A3 B3 C3 AVG 3 A4 B4 C4 AVG 4

33. 25
FIGURE 4-5: AVERAGE PH LEVELS AT TIME POINTS
TABLE 4-2: INDIVIDUAL & AVERAGE PH LEVELS
Time: Size 1 Size 2 Size 3 Size 4
48 A 7.1 7.1 7.1 7.1
B 7.2 7.2 7.2 7.2
C 7.2 7.2 7.2 7.2
Avg: 7.17 7.17 7.17 7.17
168 A 2.6 2.4 2.2 2.2
B 2.6 2.5 2.3 2.3
C 2.6 2.5 2.3 2.2
Avg: 2.60 2.47 2.27 2.23
336 A 2 1.9 1.8 1.8
B 2 1.9 1.8 1.7
C 2 1.9 1.8 1.8
Avg: 2.00 1.90 1.80 1.77
4.4) Mechanical Testing
Mechanical testing was carried out on samples that were subjected to 0 Hours and
48 Hours of degradation testing. This required 3 samples for each thickness from
both time points, which involved testing 24 samples in total. Samples from the 168
Hour and 336 Hour degradation testing were unsuitable to carry out mechanical
testing on, as they were too delicate and broke apart from gentle handling. It was
found in a previous study that the mechanical properties decreased for samples
degraded for 48 Hours, compared to those that were subjected to no degradation. A
0
1
2
3
4
5
6
7
8
0 50 100 150 200 250 300
pH Levels
Size 1 Size 2 Size 3 Size 4

34. 26
drop in molecular weight (due to degradation) is known to cause a decrease in yield
strength of PLA, which correlates with this study. (Auras, et al., 2010) It was found
that all of the largest thickness samples broke violently with very little force, less
than 5N. As a result of this, there were no graphs produced as the Tinius Olsen
subjected each sample to a 5N preload and the samples failed before this preload
had been reached. Because of this, data is only available for sizes 1, 2 & 3.
Samples are shown below after carrying out the 3-point bend test, displayed smallest to
largest from left to right:
IMAGE 4-1: 0 HOUR SAMPLES, AFTER MECHANICAL TESTING
IMAGE 4-2: 48 HOUR SAMPLES, AFTER MECHANICAL TESTING
4.4.1) Loss in mechanical properties due to degradation
To measure any loss in mechanical properties (δMP), the following formula was
used:
4.4.2) Thickness 1 (3.1mm)
Sample Thickness 1 was tested and the results shown in Figure 4-6 and Table
4-3below. 0 Hour samples produced a ductile curve, requiring a large force before
the samples fractured; it was not a clean break and further force was required to
cause further deflection. 48 Hour samples produced a brittle fracture; they endured
a steady amount of increased force before sudden failure. It can be seen from both
sets of curves that the 0 Hour sample took a much larger force before failing while
the 48 Hour samples were able to deflect up to three times as much as the 0 Hour
samples, before failing.

35. 27
FIGURE 4-6: FORCE VS. DEFLECTION, THICKNESS 2MM
TABLE 4-3: MAXIMUM FORCES PRIOR TO FAILURE
Mechanical Property difference due to degradation:
δMP = 100 – ( x 100 )
δMP = 45%
From this, it can be said that the degraded samples required on average 45% force
of non-degraded samples before failure.
4.4.3) Thickness 2 (3.8mm)
As with the smaller thicknesses, degraded samples required less force than non-degraded
samples and produced a brittle fracture when the sample failed. The non-degraded samples
also produced a more brittle curve, with sudden failure when the maximum load had been
reached. Samples from this batch required a lot more force to break than thickness 1;
approximately 50%. The samples that had been degraded for 48 Hours deflected less than
the 0 Hour samples.
0
20
40
60
80
100
120
140
160
180
200
0 2 4 6 8 10 12
Force(N)
Deflection (mm)
Sample Thickness 1
0-A 0-B 0-C 48-A 48-B 48-C
Sample 0-A 0-B 0-C 48-A 48-B 48-C
Max Force (N) 173.20 184.40 186.60 82.30 89.10 72.45

36. 28
FIGURE 4-7: FORCE VS. DEFLECTION, THICKNESS 2
TABLE 4-4: MAXIMUM FORCES PRIOR TO FAILURE
δMP = 100 - ( X 100)
δMP = 39%
From this, it can be said that the degraded samples required on average 39% force
of non-degraded samples before failure.
4.4.4) Thickness 3 (4.8mm)
With increasing size thickness, brittle fractures were experienced by nearly all of the
samples but like before the non-degraded samples required a lot more force. When Size 4
samples, the largest, were tested they all failed before any force had registered with the
tensile tester. They all suffered brittle fractures; it could be observed that ductility was a
feature of smaller, non-degraded samples while brittleness was a feature of larger, degraded
samples.
-50
0
50
100
150
200
250
300
350
0 1 2 3 4 5 6 7 8
Force(N)
Deflection (mm)
Sample Thickness 2
0-A 0-B 0-C 48-A 48-B 48-C
Sample 0-A 0-B 0-C 48-A 48-B 48-C
Max Force (N) 302.00 322.80 304.00 127.35 126.90 106.00

37. 29
FIGURE 4-8: FORCE VS. DEFLECTION, THICKNESS 3
TABLE 4-5: MAXIMUM FORCE PRIOR TO FAILURE
δMP = 100 - ( X 100)
δMP = 39%
From this, it can be said that the degraded samples required on average 39% force
of non-degraded samples before failure. After carrying out mechanical testing, it
could be concluded that larger samples lost more of their strength than the smaller
samples due to degradation.
0
100
200
300
400
500
600
0 0.5 1 1.5 2 2.5 3
Force(N)
Deflection (mm)
Sample Thickness 3
0-A 0-B 0-C 48-A 48-B 48-C
Sample 0-A 0-B 0-C 48-A 48-B 48-C
Max Force (N) 517.50 499.50 543.75 204.50 216.25 190.80

38. 30
Chapter 5: Analytics
5.1) Analysis of Variance (ANOVA)
To determine that rate of mass change was influenced by the samples size, an
ANOVA was carried out on the rate of mass change at time points 48, 168 and 336
Hours; the result of which would give an F Ratio, a ratio of expected results to
unexpected results. A critical F value is required, which can be obtained from an F
Distribution Table. An F Ratio value less than a critical F value would result in a
Null Hypothesis and a value greater would result in an Alternative Hypothesis. By
default, a Null Hypothesis (H0) means that there is no difference between the
groups, or sample sizes and their rate of degradation. Ending up with an Alternate
Hypothesis (H1) would mean that there is a difference between groups, or that size
did have an effect on the rate of mass change. (Donovan, n.d.)
After carrying out manual calculations, it was found that the degrees of freedom
were 3 “between samples” and 8 “within samples”. Checking an F-Distribution
table, with α = 0.05, a Critical value of 4.07 was gotten. An alpha level (α) of 0.05
means that there is a 5% chance that the findings are incorrect, a common level for
statistical testing. Any F Ratio calculated with AVOVA less than this would result
in a H0 while any value greater would result in a H1. The values below were
calculated in Excel, with the individual masses at their respective time points and
used in the following equation to calculate the F Ratio:
5.1.1) 48 Hours
TABLE 5-1: MANUAL ANOVA AT 48 HOURS
Variation Sum of Sq Df Mean Sq F Ratio
Between 0.000815 3 0.00027176
1.382258751Within 0.001573 8 0.00019660
Total 0.002388 11 0.00046836

39. 31
As 1.38 < 4.07, the Null Hypothesis must be accepted and therefore the test is
inconclusive. There is no level of significance between the thicknesses and the rate
of mass change, at this time point. This was evident without carrying out the
ANOVA, due to the large variation in results.
An ANOVA was also carried out using Excel’s Data Analysis function to back up
the manual calculations. The following is the result:
TABLE 5-2: EXCEL GENERATED ANOVA 48 HOURS
Anova: Single Factor
SUMMARY
Groups Count Sum Average Variance
Column 1 3 -0.040325 -0.013441 0.000340
Column 2 3 -0.005747 -0.001915 1.10E-05
Column 3 3 -0.070978 -0.023659 0.000236
Column 4 3 -0.018499 -0.006166 0.000198
ANOVA
Source of Variation SS df MS F P-value F crit
Between Groups 0.000815 3 0.000271 1.382258 0.316587 4.066180
Within Groups 0.001572 8 0.000196
Total 0.002388 11
5.1.2) 168 Hours
TABLE 5-3: MANUAL ANOVA AT 168 HOURS
Variation Sum of Sq Df Mean Sq F Ratio
Between 0.000603 3 0.00020097
0.740796688Within 0.002170 8 0.00027129
Total 0.002773 11 0.00047227
As 0.74 > 4.07 the Null Hypothesis must be accepted and therefore the test is
inconclusive. There is no level of significance between the thicknesses and the rate
of mass change, at this time point. This was not as evident prior to carrying out an
ANOVA; there was less variation than the 48 Hour results.

40. 32
An ANOVA was also carried out using Excel’s Data Analysis function to back up
the manual calculations. The following is the result:
TABLE 5-4: EXCEL GENERATED ANOVA 168 HOURS
Anova: Single Factor
SUMMARY
Groups Count Sum Average Variance
Column 1 3 0.080460 -0.026820 0.000159
Column 2 3 0.126742 -0.042247 0.000014
Column 3 3 0.102314 -0.034105 0.000910
Column 4 3 0.134512 -0.044837 0.000003
ANOVA
Source of Variation SS df MS F P-value F crit
Between Groups 0.000603 3 0.000201 0.740797 0.556947 4.066180
Within Groups 0.002170 8 0.000271
Total 0.002773 11
5.1.3) 336 Hours
TABLE 5-5: MANUAL ANOVA AT 336 HOURS
Variation Sum of Sq Df Mean Sq F Ratio
Between 0.000548 3 0.00018271
5.11812918Within 0.000286 8 0.00003570
Total 0.000834 11 0.00021841
As 5.11 > 4.07 Null Hypothesis must be rejected, therefore the test is conclusive.
There is a level of significance between the thicknesses and the rate of mass change,
at this time point. After carrying out ANOVA on the mass changes, it could be
confirmed that the thickness of the sample was significant in the rate of mass loss.
In addition to this, as the P-value shown in Table 5-6 is less than the Alpha value,
confirming that the Null Hypothesis must be rejected. (0.028 < 0.05)
An ANOVA was also carried out using Excel’s Data Analysis function to back up
the manual calculations. The following is the result:

41. 33
TABLE 5-6: EXCEL GENERATED ANOVA 336 HOURS
Anova: Single Factor
SUMMARY
Groups Count Sum Average Variance
Column 1 3 0.688534 0.229511 0.000027
Column 2 3 0.742775 0.247592 0.000054
Column 3 3 0.713293 0.237764 0.000060
Column 4 3 0.729904 0.243301 0.000001
ANOVA
Source of Variation SS df MS F P-value F crit
Between Groups 0.000548 3 0.000183 5.118129 0.028849 4.066180
Within Groups 0.000286 8 0.000036
Total 0.000834 11
Chapter 6: Conclusions & Recommendations
6.1) Conclusions
After completing this study, it can be concluded:
 Larger samples degraded faster than smaller samples
 Degradation had an effect on yield strength of the samples
 Degradation had an effect on the nature of failure; going from ductile to
brittle
 The degradation process causes a rise in acidity
6.2) Discussion
Completion of this study confirmed that larger samples had degraded faster than
smaller samples, when subjected to the same processing and degrading conditions.
This was not evident at 48 hours, where no pattern had emerged with mass
difference; some samples had taken on mass, some lost it and the remaining had
shown no mass change. At 168 hours, the largest and smallest samples were
degrading in respect to their size. It was at 336 hours where the all of the samples
were showing degradation in respect to their size, ranging from 21% to 25% for
smallest to largest.

42. 34
Mechanical properties were also affected; samples that were degraded for 48 hours
required a significantly smaller force, compared to 0 hours, to fracture when the 3-
point bend test was carried out on them. This thought to be due to a drop in
molecular weight; caused by degradation with a decrease in yield strength an effect.
(Auras, et al., 2010) The mechanical properties dropped at a greater rate as
thickness increased. The smallest thickness samples (3.1mm) retained 45% of their
yield strength after degradation where sample thicknesses 2 and 3 (3.8mm and
4.8mm thick respectively) required only 39% of the non-degraded force to break. It
was also observed that non-degraded samples were ductile during the mechanical
test while degraded samples were brittle.
The pH levels were monitored and recorded during the degradation process and
behaved as expected; initially there was very little change as the buffer solution was
absorbed into the sample but the levels dropped as the samples degraded, indicating
a rise in acidity in the buffer solution. (I. Grizzi, 1995) This rise in acidity is a key
factor in the degradation process that caused the larger samples to degrade faster.
6.3) Recommendations
After completing this study, the following recommendations could be made:
 Increase sample numbers
 Increase the number of time points
 Use larger vials
 Redesign mould
For this study, there were too little numbers produced and tested, to carry out any
meaningful statistical analysis. As samples were tested in triplicate, it was found in
many instances that 2 out of the 3 samples would produce identical results where
the 3rd
sample would deviate from the others. When checking mass at time points T
= 48 and T = 168, it was found that samples did not absorb the buffer solution or
degrade in respect to their size; it was only at the final time point, that this occurred.
Another thing to improve on this study would be to increase the time points, to
produce a better model for a degrading sample. It would also give a better
indication when the initial mass gain process had ended and the degradation process
had begun. When testing samples at T = 48 it was unclear if a number of the
samples had absorbed any buffer solution or if they were just starting to degrade, as

43. 35
they were showing no change in mass. As there was so much variation at 48 hours
and 168 hours, it was at 336 hours before desired results were obtained. It has to be
thought about, would the samples continue to degrade at the same rate, or would
they produce even more variation at the next time point.
When researching into the original study, it was found that samples were degraded
in buffer solution at a ratio of 100:1 (solution volume to sample mass) but this was
not feasible, given the size of the samples used for this study. For time points T =
168 and T = 336, there was very little change in the pH levels; it was unknown if
the buffer solution had become saturated and would not allow any further
degradation. And if so, had this saturation altered the degradation rate in any way.
Also, during the manufacturing of samples, it was found that the material stuck to
the mould and took a lot of force to be removed; damaging samples in the process.
Because of this, baking paper had to be used to ensure it was removable without
causing any damage but this decreased the sample sizes and caused irregular shapes
along the underside. This resulted in samples with a very irregular cross sectional
area that were hard to measure accurately with a micrometer. It would help if a
mould could be redesigned to allow the baking paper to fit into it easier, without
any creasing and thus not affecting the sample sizes.

44. 36
References
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46. 38
Appendices
TABLE 0-1: GLOSSARY
Term/Phrase Definition
Aliphatic Type of atomic bond, between carbon atoms
Autocatalytic A chemical reaction that produces catalysts to self-sustain the reaction
Biocompatible Not harmful or toxic to living tissue
Bioresorbable A material that can break down in the body and does not require
surgical removal
Bulk Erosion Internal degradation
Heterogeneous Non-uniform composition
Hydrolytic Breaking chemical bonds with addition of water
Hydrophobic Does not break down in water, e.g. oil
In Vitro Taking place outside of a living organism; in a petri dish, test tube, etc.
In Vivo Taking place in a living organism
Miscible Ability to form a homogenous material
Monomer A molecule that can be bonded to an identical molecule to form a
polymer
Oligomer A polymer whose molecules consist of few repeating units
Surface
Erosion
Exterior degradation

47. 39
TABLE 0-2: SAMPLE MASSES, BEFORE AND AFTER PRODUCTION
Test Batch Size P/N Pre Production Mass (g) Post Production Mass (g)
0
A
1 0A1 1.25 1.24
2 0A2 1.74 1.74
3 0A3 2.25 2.24
4 0A4 3.29 3.28
B
1 0B1 1.25 1.25
2 0B2 1.75 1.74
3 0B3 2.28 2.27
4 0B4 3.26 3.24
C
1 0C1 1.26 1.26
2 0C2 1.73 1.71
3 0C3 2.28 2.27
4 0C4 3.28 3.26
1
A
1 1A1 1.25 1.23
2 1A2 1.73 1.70
3 1A3 2.24 2.22
4 1A4 3.25 3.23
B
1 1B1 1.28 1.27
2 1B2 1.73 1.73
3 1B3 2.28 2.27
4 1B4 3.28 3.26
C
1 1C1 1.26 1.26
2 1C2 1.74 1.74
3 1C3 2.24 2.24
4 1C4 3.25 3.25
2
A
1 2A1 1.28 1.26
2 2A2 1.75 1.72
3 2A3 2.31 2.27
4 2A4 3.27 3.20
B
1 2B1 1.25 1.25
2 2B2 1.75 1.75
3 2B3 2.26 2.20
4 2B4 3.25 3.19
C
1 2C1 1.25 1.23
2 2C2 1.74 1.74
3 2C3 2.25 2.22
4 2C4 3.26 3.20
3
A
1 3A1 1.25 1.25
2 3A2 1.75 1.74
3 3A3 2.27 2.26
4 3A4 3.26 3.26
B
1 3B1 1.28 1.28
2 3B2 1.75 1.75
3 3B3 2.25 2.24
4 3B4 3.25 3.21
C
1 3C1 1.26 1.26
2 3C2 1.76 1.76
3 3C3 2.26 2.23
4 3C4 3.26 3.23

48. 40
TABLE 0-3: SAMPLE MASSES, BEFORE AND AFTER DEGRADATION & PH LEVELS
Sample 0Hr Mass (g) Wet Mass (g) Dry Mass (g) Buffer pH
48-A-1 1.23 1.25 1.26 7.1
48-A-2 1.70 1.72 1.70 7.1
48-A-3 2.22 2.27 2.24 7.1
48-A-4 3.23 3.27 3.24 7.1
48-B-1 1.27 1.30 1.26 7.2
48-B-2 1.73 1.75 1.73 7.2
48-B-3 2.27 2.40 2.36 7.2
48-B-4 3.26 3.31 3.24 7.2
48-C-1 1.26 1.31 1.29 7.2
48-C-2 1.74 1.77 1.75 7.2
48-C-3 2.24 2.40 2.29 7.2
48-C-4 3.25 3.33 3.32 7.2
168-A-1 1.26 1.23 1.23 2.6
168-A-2 1.72 1.69 1.64 2.4
168-A-3 2.27 2.24 2.14 2.2
168-A-4 3.20 3.17 3.06 2.2
168-B-1 1.25 1.30 1.23 2.6
168-B-2 1.75 1.80 1.68 2.5
168-B-3 2.20 2.32 2.20 2.3
168-B-4 3.19 3.25 3.05 2.3
168-C-1 1.23 1.24 1.18 2.6
168-C-2 1.74 1.76 1.67 2.5
168-C-3 2.22 2.25 2.12 2.3
168-C-4 3.20 3.23 3.05 2.2
336-A-1 1.24 1.18 0.97 2.0
336-A-2 1.74 1.62 1.32 1.9
336-A-3 2.26 2.13 1.73 1.8
336-A-4 3.26 3.01 2.47 1.8
336-B-1 1.28 1.23 0.98 2.0
336-B-2 1.75 1.66 1.32 1.9
336-B-3 2.24 2.16 1.72 1.8
336-B-4 3.21 3.02 2.43 1.7
336-C-1 1.26 1.22 0.97 2.0
336-C-2 1.76 1.66 1.31 1.9
336-C-3 2.23 2.10 1.68 1.8
336-C-4 3.23 3.06 2.44 1.8

49. 41
FIGURE 0-1: SOLIDWORKS DRAFT OF MOULD

50. 42
FIGURE 0-2: ANOVA CALCULATIONS FOR 48 HOUR MASS CHANGES
FIGURE 0-3: ANOVA CALCULATIONS FOR 168 HOUR MASS CHANGES
FIGURE 0-4: ANOVA CALCULATIONS FOR 336 HOUR MASS CHANGES