Project Report(Complete)

7/31/2019 Project Report(Complete)

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Biodegradable polymer/graphene oxide

composite films

Ashutosh Kumar

Department of Mechanical Engineering

Indian Institute of Technology, PatnaIndia

Supervised by

Prof. Debes Bhattacharyya

Dr.Dongyan Liu

Centre of Advanced Composite Materials(CACM)

University of Auckland, New Zealand

2012


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Abstract

In the current age of growing environmental awareness and energy crisis

situation, biodegradable composites have gained wide acceptance in various

facets of engineering. Poly(lactic acid) (PLA) has several applications in various

areas such as in woven and non-woven fabrics, paper coatings, food and

medicine packaging, and biomedicine (sutures, scaffolds and implants).

This aliphatic polyester is prepared from lactic acid (therefore derived from

100% renewable sources, e.g. corn or sugarcane), and is biodegradable and

biocompatible.In order to make this material more attractive for someapplications, as a strong alternative to petrochemical plastics, some properties

should be improved, namely mechanical properties and gas barrier properties.

Graphene due its remarkable properties is centre of attraction for most of the

researchers these days. Our objective is to use graphene in composite as

reinforcement,a single atomic layer of carbon whose existence had beenknown for a long time but which was produced and identified only as recently

as 2004.Andre K. Geim and Konstantin S. Novoselov of the University ofManchester, UK, were awarded the 2010 Nobel Prize in Physics for their ability

to isolate this single sheet of carbon atoms.

The present work is to manufacture biodegradable polymer (PLA)/Grapheneoxide(GO) composites by twin extrusion and compression moulding methods

and characterize their mechanical and gas barrier properties. Five types of

PLA/GO films of different compositions were prepared and used in this study.

Theoretical modelling of gas barrier properties is done to compare the

experimental results with the prediction by various models proposed by

scientists. Effects of introducing the GO flakes into the matrix have been tried

to understand.


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Acknowledgement

I sincerely thank The Faculty of Engineering and CACM, The University of

Auckland, for providing me with this wonderful opportunity to work on aproject that was exciting and enthralling. I thank Prof. Anil K. Bhowmik, my

reverend Director, for giving me this opportunity to visit New Zealand and

work on a wonderful project.

This project has only been possible because of special contribution and great

assistance of many people.

Prof. Debes Bhattacharyya, your dedication, guidance and feedback, not to

forget your extensive manufacturing and composites knowledge, have beeninvaluable in assisting me throughout my project under your supervision.Its a

pleasure to thank Dr. Dongyan Liu for her constant inspirationand valuableinputs in manufacturing and experiments which helped to develop my

knowledge base in experimental research and made it possible to complete my

work well within time. Without her this never would have been possible.

I am grateful to all the technicians for their kind attention and help. Jos, forpatiently explaining the safety measures vital for working in the lab and

helping me out in doing the tensile test of films . Steve for providing essential

equipments required at all the stages of manufacturing. Jimmy, Shane and

Callum, for their practical inputs and assistance in various stages of

manufacturing and processing.

My sincere appreciations to my colleagues Kalyan and Vijay for all their

support to make this project possible.

Last but not the least, I thank my parents and my little sister for their love andsupport, and for standing by me at all times.


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Contents

Abstract.2

Chapter 1

Introduction.5

1.1 Composites.....5

1.2 Film Composites and its applications....5

1.3 Manufacturing techniques and instruments used6

1.3.1 Twin Extruder6

1.3.2 Compression moulding....71.3.3Scanning Electron Microscope..71.3.4 Differential Scanning Calorimetry..8

1.3.5 Optical Microscope91.4 Permeability10

1.4.1 Factors affecting permeability.13

1.4.2 Applications of permeability to industry..14

1.4.3 Theoretical Modelling of Gas Barrier Properties.14

Chapter 2

Materials19

2.1 PLA............19

2.2 Graphene Oxide..20

Chapter 3

Manufacturing Procedures and its Description...21

3.1 Preparation of Graphene Oxide Film..21

3.2 Preparation of Composite Films.24

Chapter 4

Testing Methods...28

4.1 SEM of Fracture Surface of Films..28

4.2 Films under Optical Microscope....31

4.3 Tensile test...32

4.4 DSC of the films....35

4.5 Permeability test and Theoretical Model....36

Chapter 5

5.1 Results and Discussions45

References..48

Appendices.......50


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Chapter 1: Introduction1.1 CompositesA composite is a material which is a mixture of two or more distinct materialswhere each material has different physical and chemical properties. Moreover,

a composite usually possesses properties superior to its constituents. It

consists of a matrix(dispersion phase) and a reinforcement(dispersed

phase).Matrix and reinforcement offer different properties.

Matrix

Transfers load to the reinforcement Holds the dispersed phase Provides chemical and temperature resistance

Reinforcement

Provides strength and stiffness Impact resistance Enhances gas barrier properties

1.2 Film Composites and its applicationsA thin film is a layer of material ranging from fractions of a nanometer(monolayer) to several micrometers in thickness. Films have been used in

industry for manifold purposes be it packaging industry,microelectronic

integrated circuits, magnetic information storage systems, optical coatings or

wear resistant coatings. However, the mechanical performance of these

materials tends to depend on fabrication and post-processing parameters.

With the purpose of improving the mechanical and gas barrier properties of

films a relatively novel idea of mixing GO in PLA matrix is used in this research

project.

PLA/GO composites can be prepared by melt-blending, solvent-casting or in

situ polymerization. In this work twin-extrusion and compression moulding are

used to obtain thin films and characterise them.


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1.3 Manufacturing techniques and instruments used1.3.1 Twin ExtruderIn the twin extruder, raw material in the form of small beads is gravity fed from

a top mounted hopper into the barrel of the extruder [11].The material enters

through the feed throat and comes into contact with the screw. The rotating

screw pushes the beads forward into the barrel which is heated to the desired

melting temperature of the polymer fed. A heating profile is set for the barrel

in which three or more independent PID controlled heater zones gradually

increase the temperature of the barrel from the rear.Extra heat is contributed

by the intense pressure and friction taking place inside the barrel.

At the front of the barrel, the molten plastic leaves the screw and travels

through a screen pack to remove any contaminants in the melt.The screens

are reinforced by a breaker plate (a thick metal puck with many holes drilledthrough it).After passing through the breaker plate molten plastic enters the

die. The die is what gives the final product its profile and must be designed so

that the molten plastic evenly flows from a cylindrical profile, to the product's

profile shape. Long continuous strands of polymer are obtained from the

extruder that was used.

Twin screw extruders are usually run starve fed. There is an independent

control of both the feed rate and the screw speed.

Fig.1.1- Twin extruder, CACM, University of Auckland.


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1.3.2 Compression mouldingIt is a manufacturing technique in which the desired object moulds are

compressed under high pressure and temperature. The moulds are inserted

between the two metallic plates which are connected to hydraulic pumps to

apply pressure. The advantage of compression moulding is its ability to mould

large, fairly intricate parts. Also, it is one of the lowest cost moulding methods

compared to other methods such as transfer moulding and injection moulding;

moreover it wastes relatively less material, giving it an advantage when

working with expensive compounds. However, compression moulding often

provides poor product consistency and difficulty in controlling flashing, and it is

not suitable for some types of parts. Fig.1.3 shows hydraulic press for

compression moulding.

Fig.1.2- Hydraulic Press (for compression moulding), CACM , University of Auckland

1.3.3 Scanning Electron MicroscopeSEM is a type of microscope which enables us to observe morphology of

materials at micro and nano-level. In this microscope an electron beam is

emitted over the desired region and the sample response is sensed that

reflects its topography. The response is due the conductivity of the sample

being observed. The signals produced by SEM contain data about its

composition, topography and other properties. Wide range of magnification is

possible i.e. from 10 to 500,000. Its magnification doesnt depend on the

power of the objective lenses.


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Glass Transitions Melting and Boiling Points Crystallization time and temperature Percent Crystallinity Heats of Fusion and Reactions Specific Heat Oxidative/Thermal Stability Rate and Degree of Cure Reaction Kinetics Purity

Fig.1.5- Differential scanning Calorimetry setup, courtesy: CACM University of

Auckland

1.3.5 Optical Microscope

Optical microscope is an instrument that uses visible light and a system of

lenses to magnify images of small samples.All modern optical microscopes

designed for viewing samples by transmitted light share the same basic

components of the light path, listed here in the order the light travels through

them:

In addition the vast majority of microscopes have the same 'structural'

components:

Ocular lens (eyepiece) Objective turret or Revolver or Revolving nose piece (to hold multiple

objective lenses)


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Objective Focus wheel to move the stage ( coarse adjustment, fine adjustment) Frame Light source Diaphragm and condenser lens Stage (to hold the sample)

Fig.1.6- Optical Microscope , Plastic Centre, University of Auckland

1.4 Perm eabi l i t y

To quantify and characterize the barrier properties of a polymer film or

membrane, the most frequently measured and reported quantity is the

permeability P.

Permeability P is a measure of the amount of gas that passes through a film of

thickness l and area A within a finite amount of time t.

= ()()()()() (1-1)

Permeability or transmission rate is dependent upon two factors: the solubility

of a gas or vapour and the rate of diffusion through the barrier. In order for


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permeation to occur, these two mechanisms one thermodynamic (solubility)

and the other kinetic (diffusion) must both occur.

= (1-2)where, D and S represent the diffusion and solubility coefficient respectively

and P is the permeability.

Diffusion through a polymer occurs by small molecules passing through voids

and other gaps between the polymer molecules (free volume) [48].If the

speed at which a molecule diffuses through a polymer obeys Ficks first and

second laws, as is the case for oxygen permeating through an MFC under

standard conditions, it is termed Fickian diffusion[7,8,9]:

Ficks first law:

= (1-3)

whereJ is the steady-state flux per unit area, D is the diffusion coefficient and

Cthe gas concentration.

It is well known that Ficks first law is also analogous to Darcys law, which can

be used to predict the permeability of a homogeneous system to gases or

liquids.

= (1-4)

Solubility is determined by the enthalpy change on dissolution of the molecule

in the polymer matrix and the volume available for occupation. The solubility is

in particular influenced by the state of the polymer; if it is in the rubbery state

then most common gases in polymers follow Henrys law behaviour.

However in glassy polymers, Henrys law is observed for the more non-

condensable gases (O2, helium, H2, N2, argon) while condensable gases such as

CO2 more accurately follow the dual-sorption model [7].


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Four stages are involved in the permeation of a gas through a film or polymer

matrix ,and they are [50]:

1. Absorption into the surface of the polymer

2. Solution of the gas or vapour into the polymer matrix

3. Diffusion through the wall along a concentration gradient

4. Desorption from the other surface

There are no universally accepted units for gas transmission through polymer

films or sheets; however a few common terminologies are defined in the ASTM

Standard D3985-05 [51] to report permeability.

Oxygen transmission rate (OTR) is the quantity of oxygen gas passing through

a unit area of the parallel surfaces of a plastic film per unit time under the

conditions of test. The SI unit for OTR is mol/m2 s, however it is usually

recorded as cm3. (STP)/m2 day[12, 15].

Oxygen permeance (PO2) takes into account the pressure difference between

the two sides of the film as shown in Equation (2-4) . The SI units of Permeance

are mol/(m2.s.Pa).

PO2 = (1-5)

Permeance does not take into account the thickness of the material and hence

is only useful when comparing specimens of similar thickness.

Oxygen permeability coefficient(P) is the product of permeance andthickness.

P = PO2x t (1-6)

While the SI unit is mol/m2s Pa, authors in literature usually report

permeability as cm3 (STP).mm/m2 day.atm which is the same as saying a cubic


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centimeters of gas that passes through a square meter of film in a day when

the gas pressure differential on one side of the film, at a specified temperate,

is one atmosphere greater than that on the other side[13,14].

1.4.1Factors influencing permeability

There are a number of factors which influence diffusion and solubility and

hence permeability. These include:

1. Crystallinity

2. Filler particle3. Molecular orientation

4. Temperature

5. Pressure

6. Humidity

Crystallinity affects permeability as the chains are highly ordered in crystalline

regions compared to amorphous regions and hence there should be very little

free volume and the path should be extremely tortuous. The amount of freevolume depends on density and polymer characteristics. In crystalline regions

this provides less free volume while amorphous regions will depend upon the

direction of the penetrant [20].


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Filler material also affects the diffusion behaviour because if the particle is

impenetrable as it creates a more tortuous path for the diffusing molecule.

The reason for this is for every 5C increase in temperature, a 30 to 50% rise in

permeability occurs. From the mass transport equation, the flow of a gas andpartial pressure difference is affected by temperature changes. At the

molecular level, increasing temperature leads to a rise in the mobility of the

molecular chains and thermal expansion leads to a reduction in density. This

results in more free volume and thus higher solubility since free volume is

directly proportional to free volume [19].

1.4.2Applications of permeability to industry

Gas barrier properties are most important to the packaging industries. One of

the requirements of the packaging material is to prevent passage of gases like

oxygen to prevent degradation of stored material. There is also water and

other gases such as carbon dioxide and nitrogen which are important factors to

consider in packaging materials.

Preventing oxygen from entering a package is an important requirement for

most food products. If oxygen is allowed in the package, this will break down

organic materials initiating or accelerating the decay process which is themechanism for staleness and loss of nutritive value. On the other hand, to

maintain the bright red colour in meat, a high rate of oxygen transmission is

required while a low water transmission rate is required to prevent

drying the meat .Oxygen permeability plays an important role in maintaining

the quality of milk. High oxygen permeability of package will accelerate the

oxidation reaction of inside milk and in turn causes quality deterioration.

1.4.3 Theoretical modelling of Gas Barrier Properties

Factors under consideration during modelling would include the dispersed

phases aspect ratio, orientation, dispersion, shape and volume fraction, as

well as the density and crystallinity of the matrix and the affinity between the

constituent polymers and diffusing species [16, 17, 18] .


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The majority of models found in literature only take into account a couple of

the variables mentioned above.

Nielsens model for filled polymer systems

Nielsens model is based on the tortuosity factor where the filler particles are

impenetrable to a diffusing gas or liquid molecule,resulting in the diffusing

molecules following a tortuous path through the polymer. The relation is as

follows:

=

(1-7)

where, Pand Pm are the permeability of the composite and pure polymer,is the volume fraction of the matrix polymer and is the tortuosity factorwhich is the ratio of the distance a molecule must travel to get through the

film to the shortest route. If the filler particles are circular or rectangular the

tortuosity factor is represented by the following:

= 1 + ( 2) (1-8)

where, L the length of the filler, Wis the filler thickness and is the volumefraction of the filler or reinforcement. This model represents the ideal casewhere the particles are completely exfoliated and uniformly dispersed along

the preferred orientation in the polymer matrix.

Series and parallel model

The series and parallel models represent the upper and lower bounds for

permeability modelling. The upper bound is represented by the parallel model

where the reinforcing phase is orientated parallel to the direction of

permeation.The series model is the lower bound case where the reinforcing

phase is orientated across the direction of permeation [25].

Parallel model: = + (1-9)


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Series model:

= + (1-10)where, Pis the permeability, is the volume fraction and the subscripts mand f denote the matrix and reinforcing polymers.Geometric Mean Model

If one assumes a random distribution of phases, the film permeability can be

estimated using the weighted geometric mean of the polymer permeabilities

via a model known as the Geometric Mean Model[26]:

= + (1-11)

Generalised Maxwell-Rayleigh relationship

There also exists the Rayleigh relation for cylinders. This differs from Maxwells

equation[26] (also known as Maxwells relation for spheres) only by the value

of the shape factor defined in the following generalised equation as f, which is

equal to 1 for Rayleighs relation and 2 for Maxwells relation.

= 1 + (1+)( 1)( +) ( 1) (1-12)where,fis the shape factor in this equation. Whenfapproaches infinity

Equation (1-11) reduces down to the parallel model (1-9). Forf = 0 the

equation becomes the Series model (1-10). Whenf = 2, the equation

represents Maxwells equation for spheres. Lastly iff = 1, we get Rayleighs

relation [66] for long transverse cylinders.

Lewis and Nielsen equation for two-phase systems

A theoretical model developed by Nielsen [24, 26] to predict the elastic

modulus of two phase systems has also been applied by others to the

prediction of electrical and thermal conductivity. Likewise, permeability can be

predicted from component values using this model:


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= 1+1 (1-13)where

A = ke - 1, where ke is the Einstein coefficient and is equal to 1.5 for

fibres and 2.5 for spheres.

Also:

= 1

+(1-14)

= 1 + 1 (1-15)where represents the maximum fibre packing fraction. This value is 0.785for square packing, 0.82 for random packing and 0.907 for hexagonal packing.

Bttcher formula

Bttcher gave a formula that was originally applicable for random dispersion of

spherical particles which was later modified to a more general form that could

be applied to ellipsoidal shaped particles. This equation is [21]:

()+() +

()+() = 0 (1-16)

whereA is related to the shape of the reinforcement,A=1/3 for spheres and

A=0.5 for rods.


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Bruggeman formula

The differential effective medium (DEM) theory was introduced by Bruggeman

to estimate the effective thermal conductivity of composites at high volume

fractions [27]. Bruggeman formula for spherical particles is given by:

1 3

= (1 ) 1 (1-17)The permeability for random-oriented laminated flat particles is:

+2+2 = 1 (1-18)

Higuchi Model

Higuchi demonstrated when particle-particle interactions were neglected, the

model led to the well known Rayleigh-Clausius-Mosotti equation (labelled

Maxwell equation in this report). In another paper by Higuchi et al. [23] the

same principles from an earlier paper were used to derive a model to predict

the permeability of two-phase mixtures. His model is represented by:

= 1 + 3[1(1)] (1-19)The quantity Kinvolves the distribution function for random spheres and is a

function of the volume fraction of the reinforced polymer. Higuchi found

K = 0.78 provided a good fit between the experimental data and the

predicted model. When K = 0, this model reduces down to the Maxwell

equation for random spheres. is a measure of the permeability differencebetween the two phases and is given by:

= +2 (1-20)


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Chapt er 2 : Mater ia ls

2.1 PLA(Poly-lac t ic Ac id)

PLA [22] stands for poly-lactic acid and is a thermoplastic aliphatic polyester

derived from renewable resources, such as corn starch, tapioca products

(roots, chips or starch) or sugarcanes. It can biodegraded under certain

conditions, such as the presence of oxygen, and is difficult to recycle.Bacterial

fermentation is used to produce lactic acid from corn starch or cane sugr.

PLA polymer 2002D[28] , that we are using , is a clear sheet grade and

processeseasily on conventional extrusion and thermoforming equipment.Its

specific gravity is 1.24.Its glass transition temperature is in between 60-65 C

.Its melting temperature is 210 C. Its tensile yield strength at 60MPa.PLA is

PLA is used to make clear compostable containers and PLA lining is used in

cups and containers as an impermeable liner. PLA is biodegradable, and fully

compostable. It uses 65 percent less energy to produce than conventional oil-

based plastics and generates 68 percent fewer greenhouse gasses and contains

no toxins.


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Fig. 2.1- PLA Pellets, CACM, University of Auckland

2.2 Grap hene Oxi de (GO)

Graphene oxide is oxidised form of Graphene , the two dimensional sheet of

sp2

hybridised carbon atoms which has evolved as a material with remarkable

mechanical, electrical and thermal properties. Heaps of research has been

carried out and still being carried out to derive various applications of

graphene and its derivatives in the field of nano electronic devices, composite

materials and gas sensors,biomedical applications and energy storage devices.

Graphene oxide sheets have been used to prepare a strong paper-like material.

Graphene oxide is prepared by oxidation with strong oxidizers. It typically

preserves the layer structure of the parent graphite, but the layers are buckled

and the interlayer spacing is about two times larger (~0.7 nm) than that of

graphite. Besides oxygen, epoxide groups (bridging oxygen atoms), other

functional groups experimentally found are: carbonyl (=CO), hydroxyl (-OH),

phenol groups [2,3,4] attached to both sides.


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Fig.2.2 Graphene Oxide structure

Highly oxidized Graphene oxide is insulating where as graphene is

exceptionally conducting this is mainly due to the extensive presence sp3

carbon atoms of oxygen (highly electronegative) containing functional groups

which do not allow free movement of electrons.[29]It is almost asemiconductor, with differential conductivity between 1 and 510

-2S/cm at a

bias voltage of 10 V.[31]Its conductivity can be varied by varying the level of

oxidation, temperature and other environmental factors.[30]

Graphene oxide sheets have tensile modulus of 32 GPa [32] whereas of

graphene its 130GPa.[33]Its spring constant is also very high. Such chemically

and structurally tuned graphene sheets hold significant promise for novel

sensors, membrane based NEMS devices, transparent conductors for

optoelectronic applications, smart composite materials, and others.

Chapter 3 : Manufac t ur ing Procedures and i t s

Descr ip t ion

3.1 Preparation of Graphene Oxide film

Modified Hummers Method

Procedure:

1. First of all, 2g. of powdered Graphite and 1.5g. of sodiumnitrate are taken and poured into a flask containing

H2SO4(concentrated 66%).

2. Then, the mixture is stirred for 30min.at 600 rpm usingmagnetic stirrer.

3. Then the flask is transferred to an icebath which had beencooled down to 0o C.


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4. 6 g. of KMnO4 is slowly added to the flask at room temparatue.5. Then, the flask is transferred to a water bath to maintain a

temperature of 35 3o

C for 60 min.

6. Then the suspension is diluted with 100 ml. of water veryslowly causing violent effervescence and increase in

temperature.

7. The temperature is maintained at 80oC in hot waterbath for 15min.

8. The solution is again diluted to 300ml. with warm water9. Followed by, addition of 10 ml. of H2O2(30%) which results in

the colour change of the suspension to yellow.

10.Then, 200 ml. of HCl (20%) is added to the suspension.11.The suspension is left overnight.

Fig.3.1-GO suspension, Chemistry Lab. , CACM, University of Auckland.

12.Filtration and of the suspension is done till the pH of thesuspension turns neutral.

Fig.3.2 Filtration by suction

13.After filtration centrifugation of the GO is done.


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Fig.3.3-Centrifugation at Chemistry Lab., University of Auckland, Tamaki

Campus.

14.Then, ultrasonification of GO particles is done to break theminto finer particles.

Fig.3.4 Ultrasonification at Chemisty Lab. ,University of Auckland, Tamaki Campus.

Fig.3.5 Ultrasonifier

15.The Graphitic oxide is heated in the oven for drying.


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Fig. 3.6 Oven Dryer @ Chemistry Lab.,University of Auckland, Tamaki Campus.

16.The film that we get is as shown below.

Fig.3.7 GO film

3.2Preparation of Composite Films

Five different types of films have been prepared.

1. Pure PLA2. PLA + 0.5 % GO (Master Batch)3. PLA + 1% GO (Master Batch)4. PLA + 0.5 % GO (Non-master Batch)5. PLA + 1% GO (Non-master Batch)


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Preparation of Master Batch

Master Batch is prepared by mixing 0.5 g. and 1 g. of GO and 19.5 g. and 19 g.

of PLA respectively in solvents DMF+THF in the ratio 1:1 . GO and PLA are

dispersed in the solvent and they are dried in an oven under vacuumconditions. We get solid stuff as show in the figure.

Fig.3.8 Master Batch

Procedure :

1. First PLA pellets are ground into smaller granules (


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Fig.3.10-Grinder

3. GO is mixed with PLA in 1 and 0.5 weight percent for the non masterbatch.

4. For master batch we mix 80 gm of PLA granules to each concentration(0.5 and 1 wt. per cent).

5. The mixtures are again grinded in a grinder for proper mixing.6. Pure PLA and the four mixtures are first extruded into long strands using

twin extruder (see figure).

Fig.3.11- Long strands of PLA/GO being enrolled.

7. Then, the strands are cut into smaller size pieces using palletizer.


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Fig.3.13- Palletized PLA, PLA+GO(0.5%)& PLA+GO(1%)

8. After pelletisation keep the pelletised pieces are kept in vaccum oven for3 hours .

Fig.3.14-Vacuum Drier, CACM, University of Auckland.

9. 2.5 gm of each concentration (including pure PLA pallets) are weighedand hot compression moulded into thin sheets using the hydraulic press.

Fig.3.15 -Films after compression moulding.


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Chapter 4: Testing Methods

4.1 SEM of Fractured surface of Films

SEM samples were prepared by cutting films into thin strips. These strips weredipped into Liquid nitrogen and were fractured inside the liquid nitrogen.

These fractured surfaces were observed under Scanning Electron

Microscope(SEM) and following images were obtained. GO samples were also

observed under SEM.

PurePLA

PLA + 0.5 %

GO(MB)


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PLA + 1 %

GO(MB)

PLA + 0.5 %

GO(NMB)

PLA + 1 %

GO(NMB)


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GO film

GO

suspension

Reduced

GO


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4.2Films under Optical Microscope

Films were reviewed under optical microscope, since after SEM test the two

phases (matrix and reinforcement) we not separately visible. So, under 100x

and 400x magnification films were observed and picture of the morphology

were capture which as shown below.

PLA + 1 %

GO

PLA + 0.5 %

GO

Pure PLA


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4.3Tensile Test

The films were cut into thin strips of 13mm width using the click press

machine(fig.4.1) , which were then again cut into dumbbell shaped strips using

the dumbbell shaped blades and a press machine.These strips are then tested

in the INSTRON 5660 where the gauge length fixed is 25 mm , full scale load is

1000N.

Fig.4.1Click press

Fig.4.2 Stress v/s Strain Graph

obtained for PLA


obtained for PLA/GO(0.5%)


obtained for PLA/GO(1%)


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To summarize the results obtained in the tensile test, we got the best set of

properties for PLA+0.5%GO .The mechanical properties are almost identical for

both master batch and non master batch films. Not much improvement is

observed by introducing GO in the matrix by twin extrusion and hot

compression moulding method, this is owed to the fact that above 150o

C

there is some change in the properties of GO .The table shows the results

obtained from the test.

Table4.1 Tensile Test Results

Material Modulus(GPa)

YieldStrength

(MPa)

UltimateTensile

Strength

(MPa)

MaximumLoad

(N)

Pure PLA 3.16 52.92 53.03 30.60

PLA+0.5%GO

(MB)

3.83 57.83 57.83 32.43

PLA+ 1%GO

(MB)

3.50 53.30 53.30 31.96

PLA + 0.5%

GO (NMB)

3.52 56.70 56.70 32.10

PLA + 1% GO

(NMB)

3.39 54.04 54.07 31.95


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4.4 DSC Test for the Films

Differential scanning calorimetry (DSC) was done on the twin-extruder and

compression moulded films of PLA and GO in order to determine the thermal

behaviour of the films.52 mg. of each of the three films(PLA,PLA+0.5% GO

MB,PLa+1% GO MB ) was cut and sealed to be tested and loaded in the

machine. The results are presented in Figure below.

Fig.4.7.DSC Test graphs obtained after analysis


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Table4.2 DSC Test Results Summary

S. No. Material

Glass

Transition

Temperature

Onset of

Crystallization

Temperature

Degree of

Crystallinity

(%)

1. Pure PLA 56.40oC 108.32oC 0.27

2. PLA+0.5%GO 56.77oC 105.74o C 0.86

3. PLA + 1% GO 55.48o C 97.80o C 4.12

DSC tests reveal that the glass transition temperature of all the films remain

very close to 56oC with not much variation. From this result it is inferred that

there is not much of a strong interaction or bonding between the interface of

the materials. Though the increase in concentration of GO in the matrix has led

to a decrease in the onset of crystallisation temperature i.e. presence of GO

induces nucleation in the matrix. Also, crystallinity of the films has increased

with increase in the concentration of GO in the films.

4.3 Permeability Modelling and oxygen gas barrier results:

Permeability testing was successfully done using the MOCON OX-TRAN 2/10

machine.The films were cut from the edges to fit in the fixture for testing and

the process took around 10 hours for completion. The films under exposure to

the oxygen were 50 cm2

and thickness varied with each film. Ambienttemperature fixed was 23

oC and the standard procedure of testing was

followed which involved 10 cycles of 30 minutes each and a conditioning

period of 3 hours. The results obtained are shown in Table

In section 1.4.3 a brief description of the various models found in literature

was provided. Most of the models are conductivity or elasticity models for two

phase materials modified because of their close analogy to permeability. To

check the applicability of the models, the models were plotted againstexperimental data found in table 4.3. All these models use the permeability of


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matrix. The reinforcement is highly impermeable as found in literature so the

permeability value for GO is taken to be zero.

Table4.3 Permeability Test Results

Sample Transmission rate

cc/[m2-day]

Permeability

cc-mm/[m2-day]

Permeability

cc-mil/[m2-day]

Pure PLA 74.95 35.41 548.96

PLA+0.5%GO 69.14 13.27 522.64

PLA+1 % GO 70.10 12.75 502.30

These models provide a good starting point for predicting the permeability in

MFC. Each of them follows the same downward trend seen in the experimental

data and with some modification to the gradient; these models should provide

a more accurate representation of the data.

As earlier mentioned in section 4.1.3 the parallel and series model give the

upper and lower bound of the gas permeability value of composites. Fig.4.8

shows the plot of oxygen permeability v/s GO volume fraction. It can be seen

from the figure that the experimental data falls well within the upper and

lower bounds predicted by the models.

Fig.4.8 Permeability v/s GO volume fraction

-100

0

100

200

300

400

500

600

-0.002 0 0.002 0.004 0.006 0.008 0.01

OxygenPermea

bility(cc-mil/[m2-day)

Reinforcement(GO) concentartion(vol%)

Experimental

Parallel

Series


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Fig.4.9 shows the comparison of experimental and predicted oxygen gas

permeability values by the Maxwell-Rayleigh model and geometric mean

model for the composite. Though all the models overestimate the permeabilityvalues for the composite, the geometric mean model is the closest of the

existing models.


Four other models were used for prediction of oxygen gas permeability of thecomposite films (Lewis-Nielson, Bottcher, Bruggeman and Higuchi) shown in

fig. 4.10. All these models over predict the permeability for low contents of

PET. The permeability values predicted by these models are within the same

vicinity of each other. The predicted values of oxygen gas permeability by

Bottcher and Bruggeman are almost coinciding and the hence the graphs of

the two models are overlapping.

490

500

510

520

530

540

550

560

0 0.002 0.004 0.006 0.008 0.01

OxygenPermeability(cc-mil/[m2-da

y])

Reinforcement(GO) concentration (vol %)

Experimental

Geometric mean

Maxwell

Rayleigh


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39

Fig.4.10Permeability v/s GO volume fraction

Modification of models

In this section each of the models listed in section 4.1.3 have been modified to

make them more applicable to the composite film. This is done by fitting the

models to the experimental results by changing the various factors present in

the models. The equations or formula listed in table 4.4 have been modified to

fit the experimental results.

Table 4.4

S.No. Model Name Equation /Formula Involved

1. Maxwell-Rayleigh

equation = 1 + (1 +)( 1)

( +) ( 1)

2. Lewis and Nielsenequation for two-phase

systems

= 1 +1

490

500

510

520

530

540

550

560

0 0.002 0.004 0.006 0.008 0.01

OxygenPermeability(ccmil/[m2-day])


Experimental

Lewis-Nielsen

Bottcher

Bruggeman

Higuchi


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40

3. Bottcher formula ( ) + ( ) +( ) + ( ) = 0

4. Bruggeman Formula

1 3= (1 ) 1

5. Higuchi model = 1 + 3[1 (1 )2]

Maxwell-Rayleigh equation

The generalised Maxwell-Rayleigh equation takes into account the models

derived by Maxwell and Rayleigh for spheres and long transverse cylinders

respectively, through a shape factor that also incorporates the parallel and

series models. A recap of which model corresponds to the relevant shapefactor is given in Table 4-4.

Table 4.4

Model Shape factor (f)

Maxwell 2Rayleigh 1

Parallel Series 0

Using the shape factors predicted by Maxwell and Rayleigh the value of

permeability found out exceeds that of the experimental results.

The shape factor is determined to fit the experimental results and is found out

to be f = 0.094.


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Lewis and Nielsen equation for two-phase systems

The Lewis and Nielsen equation is based on the model derived by Halpin and

Tsai. They extended the Halpin-Tsai equation to include maximum packing

fraction of the filler which is considered to be important for viscosity of

suspensions and they pointed out the relation between the shape factor

constant (A) and the generalised Einstein coefficient.

The shape factor constant A = Ke1 and the value of Ke is found out to be1.095 that best fits with the experimental data.

490

500

510

520

530

540

550

560

0 0.002 0.004 0.006 0.008 0.01

OxygenPermeability(ccmil/[m2-day])


Experimental

f = 0.0944

Maxwell(f = 2)

Rayleigh( f = 1)


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Bottcher formula

Bttcher derived a formula to correlate the dielectric behaviour of powder

with bulk based on the earlier derivation from Bruggeman. Figure 4.13 shows

the plot of permeability v/s GO volume fraction in which the shape factor A

assumed was 1/3 and which is modified to 0.905 to match the experimental

results so that the best fit is obtained.

Table 4.5

Shape of dispersedphase

Shape factorA

Sphere 1/3

Rods 1/2

Experimental

fitting 0.905

490

500

510

520

530

540

550

560

0 0.002 0.004 0.006 0.008 0.01

O

xygenPermeability(cc-mil/[m2-day])


Experimental

Lewis-Nielson

Modified Lewis-Nielson


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43


Bruggeman Formula

Bruggemans formula was originally developed to predict the conductivity in

spherical particles. The formula proposed is shown below.

= (1 ) 1 Fig. shows that the above formula over predicts the value of oxygen gas

permeability. Modification made in the above formula so that it can be

applicable for flaky GO particles in the PLA matrix is that the index of = 13is changed to 0.54 to match with the experimental results obtained.

490

500

510

520

530

540

550

560

0 0.002 0.004 0.006 0.008 0.01

OxygenPermeability(cc-mil/[m2-da

y])


Experimental

Bottcher

Modified Bottcher


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Higuchi model

Higuchi derived his model from the theory used to develop his new

relationship for dielectric properties of two-phase mixtures. The quantity K is

considered to be the shape factor in this model and the value used in 0.78 wasbased on dielectric constant data for powders and suspensions. Using the

results from experiment, the constant K was modified to fit the experimental

data and was found to be 3.49.

490

500

510

520

530

540

550

560

0 0.002 0.004 0.006 0.008 0.01

Ox

ygenPermeability(cc-mil/[m2-

day]

)

Reinforcement(GO) concentration (vol%)

Experimental

Bruggeman

Modified Bruggeman


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45


5.1 Results and Discussion:

Films of PLA(Poly-Lactic Acid) and Graphene oxide were successfully

manufactured and their mechanical, thermal and oxygen gas barrier properties

were characterised. The results can be summarised as follows:

1. Tensile Test results show that the modulus of elasticity , yield strengthand ultimate tensile strength improved with the addition of 0.5(wt%)

GO in the PLA matrix. Though the improvement was not significant in

the light of the fact that the tensile strength of Graphene is

remarkable ~1GPa, nevertheless, the properties did not deteriorate

on addition of GO.

It was also observed that the increase in the concentration of GO in

the PLA matrix led to a decline in the mechanical strength. 1% GO inPLA showed lower values of modulus of elasticity, yield strength,

490

500

510

520

530

540

550

560

0 0.002 0.004 0.006 0.008 0.01

Oxyge

nPermeabilty(cc-mil/[m2-day])

Reinforcement(GO) concentration (vol%)

Experimental

Higuchi

Modifiied Higuchi


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46

ultimate tensile strength and maximum load. This behaviour of the

PLA/GO film is owed to the fact that the increase in GO content led to

non-uniform dispersion, agglomeration and formation of lumps

increasing the non-homogeneity of the samples.

2. To gain more insight of the trends in mechanical strength of thesamples, the SEM test for the fractured surface of the films was done.

The pure PLA fracture surface showed a more regular pattern of scaly

surface. The roughness of surface increases with increase in GO

content. This, in a way, testifies for the formation of lumps and non-

homogeneity in the films with the increase in GO content. But, the

main objective of this test was to observe the GO particles inside the

PLA matrix, which could not be done as the two phases (matrix andreinforcement) were not distinguishable, may be due to less

resolution. Difference in the surfaces of Master Batch and non-

master batch is due to the process of manufacture, one being more

dispersed due to use of solvent and the other being directly

incorporated.

3. Due to our inability to observe the GO particles dispersed in PLAdistinctly, we took resort to optical microscope to explore more.

The two phases of films were, now, clearly distinguishable and the GO

matter dispersed in the PLA matrix could be easily viewed. The GO

was randomly distributed in the PLA matrix and looked like dark

patchy flat paper or flaky structured. The large GO particles were

easily visible while there were many smaller particles dispersed but

could not be seen properly.

4. After the morphology of the films became clear, the thermalproperties of the films were tested by DSC technique. The resultsobtained were quite satisfactory and in congruence with the trends

observed in the mechanical strength of the films. The glass transition

temperature of all the three films was around 56oC, which shows that

the interaction at the interface of the two materials is not strong

enough to bring about much improvement in mechanical properties.

Another interesting feature observed was the decrease in the onset

of crystallisation temperature of the films with the increase in the GO

content in the matrix. This shows that the presence of GO induces the


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nucleation process to occur faster. The %crystallisation of films was

calculated using the data obtained in the test and it was found that

with increase in GO concentration it increased.

5. The oxygen gas barrier properties testing was the last test conductedon the films, as the material under consideration was perceived to be

a potential material to be used in packaging industry. The

permeability of pure PLA film was found to be 548.96 cc-mil/[m2-day]

which is quite high very close to the value in the other reference

texts. The addition of 0.5% GO (MB) showed around 10% decrease in

permeability which as a matter of fact is astounding looking the

amount of GO used. The value of permeability found was 489.08 cc-

mil/[m2-day]. But, further increase in GO content led to a decrease in

barrier properties and hence, rise in the permeability value for 1% GO(MB) in PLA matrix. The actual value that was observed for 1% GO film

was 500.3271 cc-mil/[m2-day]. This was not expected in light of the

permeability theories given by researchers (Lewis-Neilson, Maxwell-

Rayleigh, Higuchi). The oxygen gas barrier test for the non- master

batch samples was also done. The permeability value for GO(0.5%) in

PLA was found out to be 522.64 cc-mil/[m2-day] and that of GO(1%) in

PLA was found to be 502.30 cc-mil/[m2-day], which was in accordance

of the theoretical gas barrier models which predict that with increase

in the reinforcement percent the gas barrier properties increases.6. In Section 6.2, some of the models found in literature were used to

predict the oxygen gas permeability value of the composite films and

was matched with the experimental data of non-master batch

PLA/GO films. Each of the models were modified to make them more

applicable to the experimental data for PLA/GO(NMB) composite

films. None of the modified shape factors provided an ideal fit to the

data in particular for models like Lewis-Nielsen, Maxwell-Rayleigh,

Bottcher and Bruggeman. Shape factors or indices were changed to fit

the formulae to match with the experimental data.


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References

[1]Wikipedia.

[2]Lipatov YS. Interfacial effects in polymer blends. Review. Polymer ScienceUSSR (English Translation of Vysokomolekulyarnye Soyedineniya Series A)1978;20(1):1-18.

[3]Jeong H-K, Lee YP, Lahaye RJWE, Park M-H, An KH, Kim IJ, et al.Evidence of graphitic AB stacking order of graphite oxides. J Am Chem Soc2008;130:13626.

[4]Szab T, Berkesi O, Forg P, Josepovits K, Sanakis Y, Petridis D, et al.

Evolution of surface func-tional groups in a series of progressively oxidizedgraphite oxides. Chem Mater 2006;18:27409.

[5] Lerf A, He H, Forster M, Klinowski J. Structure of graphite oxide revisited.J Phys Chem B 1998;102:447782.[6]Massey, L K, "Permeability Properties of Plastics and Elastomers", 2003,Andrew Publishing.[7]W.F. Smith, Foundations of Materials Science and Engineering 3rd ed.,McGraw-Hill (2004)[8]H.C. Berg,Random Walks in Biology, Princeton (1977)

[9] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, JohnWiley & sons, (1976)[10] Skoog, Douglas A., F. James Holler and Timothy Nieman (1998).Principles of Instrumental Analysis (5 ed.). New York. pp. 805808.[11]Rauwendaal, Chris (2001), Polymer Extrusion, 4th ed, Hanser.[12]Hanne Larsen, Achim Kohlr and Ellen Merethe Magnus, "Ambient oxygeningress rate method", John Wilew & Sons, Packaging Technology and Science,Volume 13 Issue 6, Pages 233 241.

[13]F2622 Standard Test Method for Oxygen Gas Transmission Rate ThroughPlastic Film and Sheeting Using Various Sensors.[14]ASTM. Standard Test Method for Oxygen Gas Transmission RateThrough Plastic Film and Sheeting Using a Coulometric Sensor. D3985-02. p.458-463.[15]Yam, K. L., "Encyclopedia of Packaging Technology", John Wiley & Sons,2009.[16]Shields,R.J, Bhattacharyya, D., Fakirov, S., Oxygen permeabilityanalysis of micro-fibril reinforced composites from PE/PET blends, Composites

Part A: Applied Science and Manufacturing, Accepted, 2008;39:940-949.[17]Gas barrier properties of PP/EPDM blend nanocomposites


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Masoud Frounchi, Susan Dadbin , Zahra Salehpour , Mohsen Noferesti.[18]Gas transport properties of polyacrylate/clay nanocomposites preparedvia emulsion polymerization Jose M. Herrera-Alonso, Zdenka Sedlakova, EvaMaranda.

[19]Gas Permeability and Free Volume of Highly Branched SubstitutedAcetylene Polymers byYu. P. Yampolskii, A. P. Korikov, V. P. Shantarovich,K. Nagai, B. D. Freeman, T. Masuda, M. Teraguchi and G. Kwak.[20]Moisture Permeability of Polymers. I. Role ofCrystallinity and Orientation by S. W. LASOSKI, JR., and W. H. COBBS, JR.,

E.I.duPont de Nemours and Company, Film Department,Buffalo, New York.[21]Effective Medium Theories for Artificial MaterialsComposed of MultipleSizes of Spherical Inclusions in a Host Continuum William M. Merrill, Student

Member, IEEE, Rodolfo E. Diaz, Michael M. LoRe, Mark C. Squires, andNicolaos G. Alexopoulos, Fellow, IEEE[22]Sdergrd, Anders; Mikael Stolt (February 2002). "Properties of lactic acidbased polymers and their correlation with composition". Progress in PolymerScience27[23]Physical models of diffusion for polymer solutions, gels and solids byL. Masaro, X.X. Zhu.[24]Models for the Permeability of Filled Polymer Systems by Lawrence E.Nielsen at CENTRAL RESEARCH DEPARTMENT, MONSANTOCOMPANY ST., LOUIS, MISSOURI.[25] Polymer blends, Lloyd M. Robinson , Hanser.

[26]Characterisation of the Mechanical and Oxygen Barrier Properties ofMicrofibril Reinforced Composites by Ryan John Shields.[27]Generalized Bruggeman Formula for the Effective Thermal Conductivityof Particulate Composites with an Interface Layer by J. Ordez-Miranda J. J.Alvarado-Gil , R. Medina-Ezquivel.[28]Techical Data shee_2002D, by NatureWorks.[29] Boukhvalov, D. W.; Katsnelson, M. I. J. Am. Chem. Soc. 2008, 130,10697.[30] Tunable Electrical Conductivity of Individual Graphene Oxide Sheets

Reduced at Low Temperatures Inhwa Jung, Dmitriy A. Dikin,, Richard D.Piner, and Rodney S. Ruoff.[31] C. Gomez-Navarro et al. (2007). Nano Letters, volume 7, issue 11, page3499 doi: 10.1021/ nl072090c[32] "Graphene Oxide Paper". Northwestern University. Retrieved 2011-02-28.[33] Lee, C. et al. (2008). "Measurement of the Elastic Properties and IntrinsicStrength of Monolayer Graphene".Science 321 (5887): 385. Bibcode2008Sci...321..385L


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Appendix

Calculation of volume fraction:

Volume fraction of GO in the matrix is calculated from mass fraction of GO

used and density of PLA and GO.

= + (1 )Where, = volume fraction of GO

w = mass fraction of GO

= density of PLA matrix = 1.24 g/cc = density of GO = 1.48 g/cc.

Calculation of percent crystallinity:

= 100Where, = per cent crystallinity

= enthalpy of melting

=enthalpy of cold crystallization

= enthalpy of 100% crystalline sample of the polymer = 93 J/g. = mass fraction of polymer in the matrix.

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