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1 Karlstads universitet 651 88 Karlstad
Tfn 054-700 10 00 Fax 054-700 14 60
Faculty of Technology and Science
Department of Chemical Engineering
Muhammad Asif Javed
Novel Surface Modification Approaches for the
Production of Renewable Starch-based Barrier Coatings
Degree Project of 30 credit points
Master of Science in Engineering,
Degree Programme in Chemical Engineering
Date/Term: 2011-05-20
Supervisor: Caisa Johansson, KaU
Isabel Mira, YKI
Examiner: Lars Järnström, KaU
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Novel Surface Modification Approaches for the Production of Renewable Starch-based Barrier Coatings ABSTRACT
The purpose of this study was to improve the hydrophobicity of selected starch barrier coatings through surface modification approaches such as electrospinning and plasma coating. Firstly, electrospun fibers were produced form acetylated high amylose maize starch and deposited on a paper board pre-coated with hydroxypropylated starch, secondly a coating of plasma polymerized hexamethyl disiloxane was deposited on the electrospun fiber mat. The effects of the properties of the electrospinning starch solution (concentration, surface tension and conductivity), the substrate and some electrospinning process parameters on the resulting fiber mat coating was investigated. The morphology of the deposited fibers was investigated by means of scanning electron microscopy, the hydrophobicity of the coating was characterized in terms of water contact angle and the mechanical stability of the electrospun fiber coatings was assessed by means of tape tests. Fibers produced with high concentration (10%) were beaded and smaller in diameter than those produced from lower concentration solutions (5%). The initial water contact angle of the pre-coated board of ca. 37o increased to ca. 140o upon deposition of the electrospun coating. By plasma polymerizarion of hexamethyl disiloxane onto that electrospun fiber mat it was possible to produce a superhydrophobic surface, i.e. a surface having water contact angle greater than 150o. Failure occurred between the fibers and the substrate as well as between the fibers themselves when the coated specimens were subjected to a tape test. It is suggested that crosslinking approaches may be used to improve the mechanical stability of these coatings.
SAMMANFATTNING
Syftet med denna studie är att förbättra hydrofobicitet hos utvalda barriärbeläggningar baserade på stärkelse genom strategier för ytmodifiering såsom elektrospinning och plasmabeläggning. Först tillverkades elektrospunna fibrer av acetylerad högamylos majsstärkelse och deponerades på kartong förbestruken med hydroxypropylerad stärkelse, därefter belades den elektrospunna fibermattan med ett lager av plasmapolymeriserad hexametyldisiloxan. Effekter av egenskaper hos den stärkelselösning som användes vid elektrospinning (koncentration, ytspänning och konduktivitet), samt inverkan av substratet och några processparametrar på den resulterande fibermattan studerades. Morfologin hos de deponerade fibrerna undersöktes med hjälp av svepelektronmikroskopi. Hydrofobiciteten hos beläggningen karaktäriserades med avseende på kontaktvinkel med vatten och den mekaniska stabiliteten hos den elektrospunna fibermattan undersöktes med hjälp av vidhäftningsprov. Fibrer som tillverkats vid hög koncentration (10 %) uppvisade pärlformade strukturer och var mindre i diameter jämfört med dem som tillverkats från lösningar med lägre koncentration (5%). Den inledande kontaktvinkeln med vatten på den förbestrukna kartongen var ca. 37o vilken ökade till ca.140o genom beläggning med hjälp av elektrospinning. Plasmapolymerisering av hexametyldisiloxan på den elektrospunna fibermattan ledde till en superhydrofob yta med en kontaktvinkel större än 150o. Brott uppstod mellan fibrerna och underlaget samt mellan fibrer när de belagda proverna utsattes för vidhäftningsprov. Tvärbindning föreslås som en strategi för att öka den mekaniska stabiliteten hos de undersökta beläggningarna.
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EXECUTIVE SUMMARY
In the last decade, industry and academia joined hands in their efforts to replace environmentally unfriendly oil-based polymers with bio-based polymers in packaging considering the sustainability as major issue. Today´s materials in use for food packaging are metals, paper, board, petroleum-derived plastic polymers and glass. Mostly used materials for food packaging are oil based polymers, and from a sustainability point of view it is challenging to recycle or reuse food packaging material. Packaging materials which are recyclable and competing to meet the demand among consumers for a higher quality and safer food packaging are bio polymers or bio-based materials prepared from raw materials derived from marine or agricultural sources. These materials include starches, lipids and wheat gluten (WG), just to mention a few.
Bio-based materials like wheat gluten and starch have a great potential in packaging applications. They have good oxygen barrier properties but due to their hydrophilic nature they are poor barriers against water and water vapor. To improve the water vapor barrier properties, efforts have been made but development in water vapor resistance still need to be addressed.
In this study the focus lies on the improvement of the hydrophobicity (water repellency) of starch paper coatings through surface modification approaches. Starch is a highly abundant natural polysaccharide whose function is to store energy in plants.
Two modified starches that are of specific interest in this project are hydroxypropylated (HP) and Acetylated High Amylose Maize (AHAM) starches. In a hydroxypropylated starch hydroxypropyl groups are attached to the polymer backbone which changes several of the starch properties. These starches have higher solution stability, are more resistant to high pH, have better film forming properties and are used in coatings and construction products. Acetylated High Amylose Maize is modified by acetylation to a degree of substitution, DS, of 1.5 with vinyl acetate or acetic acid. It is soluble in non-polar solvents but also to some extent in water.
The electrospinning technique was used to deposit mats of spun starch fibers onto a pre-coated paper board leading to an improved hydrophobicity. Electrospinning is a simple process in which a charged polymer solution is drawn from a pipette by applying a voltage in the kilovolt range between two electrodes and fibers are collected on a grounded surface. The electrospun fiber mat was then plasma coated with hexamethyl disiloxane (HMDSO) to further improve the hydrophobicity.
Starch solutions were characterized in terms of rheology, conductivity and surface tension. The flow curves (viscosity vs. shear rate) of the different solutions were determined by means of a rheometer equipped with plate-plate geometry and low shear rate viscosity of the solutions was measured by a capillary based method. Surface tension determinations were performed by means of a KSV tensiometer using ring method while conductivity was determined by using digital laboratory conductivity meter.
The appearance of the different samples produced was investigated by means of a scanning electron microscope (SEM) and the hydrophobicity was characterized in terms of the water contact angle. Tape tests were performed to analyse the robustness of the electrospun fibers.
The paper board which was used as a base substrate in this study has a very low water contact angle (WCA) on the brown side (unbleached) i.e. ca 17o and on the white side (bleached) WCA is about 129o. After pre-coating with hydroxypropylated starch, the WCA on both sides was almost the same i.e. ca. 37o. After electrospinning an AHAM starch solution for 2 min the WCA was almost twice as high. For 15 min collecting time the WCA was increased to 140o.
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Fibers produced with 10% AHAM starch solution were thicker in diameter and had fewer beads as compared ES fibers from a 5% AHAM solution. Fibers had more or less similar morphology, independently of collecting time and substrate used. The WCA was not found to differ due to different fiber morphologies. Electrospun fibers with 5% AHAM starch on the bleached side of the paper board after 15 min collecting time have the same WCA i.e. 138o as with 10% AHAM starch although there was a remarkable morphology difference between these two. When electrospinning and plasma treatment with HMDSO are combined, the WCA was increased to ca. 170o.
When the electrospun fiber mat was subjected to tape test failure between the fibers and between fibers and the base substrate was found. Almost all the fibers were removed by the tape on sample for which the collecting time was 2 min during ES, indicating failure between fibers and the substrate (cohesive). For 15 min collecting time some of the fibers still remained on the substrate while some were removed by the tape. There is a strong need for improvement of the robustness of the ES fibers.
No fibers could be successfully spun from 10% HP starch solution at any of the process parameters used in this study. By adding either 10 % ethanol or 10 % ethanol together with 5 % NaCl to a 10% HP starch solution electrospinning was successful to some extent but SEM images showed that no fibers were formed.
In this study, 5% and 10% AHAM starch solution were successfully electrospun onto pre-coated paper board. By electrospinning coatings of AHAM starch solutions (5% and 10 %) on a HP starch pre-coated paper a remarkable increase in the water repellency (evaluated in terms of WCA) by a factor of ca. 4 was observed. The morphology of the electrospun fibers was changed significantly with the concentration of AHAM starch solution, whereas WCA was not changed to any great extent with the fiber morphology. Plasma polymerization of HMDSO onto an ES fiber mat of AHAM starch succeeded in rendering a superhydrophobic surface (from WCA of ca. 37o to 170o).
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Table of Contents ABBREVIATIONS 7
1. INTRODUCTION 8
1.1. Bio-based polymers in packaging 8
1.2. Poor resistance against water (vapor and liquid) of bio-based materials 8
1.3. Starches 8
1.4. Approaches to improve water repellency of starch coatings 9
1.4.1. Electrospinning as a means of producing surfaces with well defined roughness 9
1.4.2. Electrospinning of biopolymers 11
2. MATERIALS AND METHODS 12
2.1. Materials 12
2.1.1. Starches 12
2.1.2. Paper board 13
2.2. Methods 13
2.2.1. Preparation of starch solutions 13
2.2.2. Coating methods 13
• Electrospinning 13
• Plasma treatment/coatins 14 • Bench coating 14
2.2.3. Surface characterization methods 15
2.2.4. Solution Characterization methods 15
3. RESULTS AND DISCUSSION 16
3.1. Surface properties of base substrates and pre-coated board 16
3.2. Properties of coating solutions 17
3.3. ES of AHAM starch on HP starch pre-coated board 18
3.3.1. Effect of solution viscosity on ES process 18
3.3.2. Effect of solution viscosity on the morphology of fibers 20
• Different collecting times 22
• Different substrates 22
3.3.3. Effect solution viscosity on WCA 23
• Different morphologies 23
• Different collecting times 23
• Different substrates 24
3.4. Plasma coatings 25
3.5. Mechanical stability (robustness) 26
3.6. ES of HP vs. AHAM starch 29
6
3.6.1. Effect of solution properties on ES of aqueous solutions of HP starch 29
CONCLUSIONS 31
ACKNOWLEDGEMENTS 31
REFERENCES 32
7
ABBREVIATIONS
AHAM Acetylated High Amylose Maize
DS Degree of substitution
ES Elecstrospinning/Electrospun
FDA Food and Drug Administration
HMDSO Hexamethyl disiloxane
HP Hydroxylpropylated
kV Kilo volt
SEM Scanning electron microscope/micrograph
WCA Water contact angle
WG Wheat gluten
� Micro
(o) degree
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1. INTRODUCTION
1.1. Bio-based polymers in packaging
In the last decade, industry and academia joined hands in their efforts to replace environmentally unfriendly oil based polymers with bio-based polymers in packaging considering the sustainability as major issue. It is first priority on EU agenda to conserve resources for the upcoming generations.
Today´s materials in use for food packaging are metals, paper, board, petroleum-derived plastic polymers and glass. Mostly used materials for food packaging are oil based polymers. From a sustainability point of view it is challenging to recycle or reuse food packaging material. Bio-based materials are important as candidates for replacement of oil based polymers. Packaging materials which are recyclable and competing to meet the demand of consumers for a higher quality and safer food packaging are bio polymers or bio-based materials prepared from raw materials derived from marine or agricultural sources. These materials include starches, lipids and wheat gluten (WG)), to mention a few. Some materials having biodegradable properties can also be included in this category like edible oils and coatings or substances derived from marine prokaryotes [1].
1.2. Poor resistance against water (vapor and liquid) of bio-based materials
Transport of water vapor or liquid water through a packaging material may occur due to the diffusion, absorption or desorption when coming into contact with the polymer film [2]. Bio based materials like wheat gluten and starch have a great potential in packaging because of their good oxygen barrier and film forming properties.
WG has good oxygen barrier and film forming properties but poor water vapor repellency [3] due to the hydrophilic nature of the proteins. To improve the water vapor barrier properties, efforts have been made but development in water vapor resistance still need to be addressed.
Starches are also hydrophilic in nature and have poor moisture barrier properties; however starch films have excellent oxygen barrier properties [4].
1.3. Starches
Starch is a highly abundant natural polysaccharide with repeating anhydroglucose units; its function is to store energy in plants. Starches consist of two different polymers. One is amylose, which has a linear molecular structure and its molecular weight ranges from 105 to 106 g/mol. The other, amylopectin, is a branched chain polymer having molecular weight ranging from 107 to 108 g/mol (Figure 1). naturally starch polymers exist in the form of granules which have semi crystalline structure. Its crystalline structure is due to the amylopectin fraction, whereas amylose makes up the amorphous part [6].
Starch is hydrophilic in nature and starch films have poor moisture barrier properties. As compared to synthetic polymer films, starch films exhibit poorer mechanical properties. Starches exhibit thermoplastic behavior on adding plasticizers [5].
To obtain better functional properties for a particular application starch can be modified. Improvement of film forming properties, reduction in swelling and enhancement of hydrophobicity can be achieved by the modification of the starches [7][8][9] .
Two modified starches that are of specific interest in this project are hydroxypropylated (HP) and Acetylated High Amylose Maize (AHAM) starches.
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Amylose
Amylopectin
Figure 1. Molecular structure of amylose and amylopectin.
In a hydroxypropylated starch hydroxypropyl groups are attached to the starch which changes several properties. The starch gains higher solution stability, becomes more resistant to high pH and get better film forming properties. HP starch is used in coatings and construction products.
Acetylated High Amylose Maize is modified by acetylation to a degree of substitution (DS) 1.5 with vinyl acetate or acetic acid. It is soluble in non-polar solvents but also to some extent in water.
1.4. Approaches to improve water repellency of starch coatings
Water vapor repellency of the starch coatings can be increased. Permeation through the polymer film mainly takes place through defects like cracks and pinholes [10]. Film forming properties can be improved by adding plasticizers (e.g. glycerol) and ultimately results in better barrier properties. For instance barrier properties of the starches can be increased by electrospinning (ES), plasma coatings or by adding plasticizers [11]. The water repellency of the starches can be increased by cross linking [12] [13], electrospinning [14], plasma coatings [15] or by introducing hydrophobic substituents [16].
The water vapor repellency of oxidized and hydroxypropylated starches was shown to be even better than for hydrophobically modified starches [16]. By plasma treating thermoplastic corn starch films with sulfur hexafluoride (SF6) plasma treatment for 900 seconds a remarkable increase in hydrophobicity was observed [15].
1.4.1. Electrospinning as a means of producing surfaces with well defined roughness
The electrospinning (ES) technique is used to produce fibers using electrostatic forces. It is a simple process in which a charged polymer solution is drawn from a pipette by applying a voltage in kilo volt range between two electrodes and fibers are collected on a grounded surface. Process conditions as well as the solution properties have a great effect on the ES process. The ES process has been used in a number of fields having a wide range of applications like in drug delivery [17], tissue engineering [17], textile [18] and membrane technology [19].
The electrospinning process and the morphology of the resultant fibers are greatly affected by the polymer solution properties. In Table 1 some effects of the solution properties are summarized.
O
CH2OH
OH
OH
O
O
CH2OH
OH
OH
O
O
CH2OH
OH
OH
O
H H HH
H H H
H H H H H
H H H
O
CH2OH
OH
OH
H
H
HH
n
H, OH1
23
4
5
66
3 2
14
5
OH
O
CH2OH
OH
OH
OHO
O
CH2
OH
OH
O
O
O
CH2OH
OH
OH
O
O
CH2OH
OH
OHO
HH
HH
H H
HHH H
H H
H H
H H H H
H H
O
CH2OH
OH
OH
H
H
H
HH
O
b
1
23
4
5
6
1
23
4
6
O
CH2OH
OH
OH
OH
HH
H
H
H
a
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Table 1. Effect of solution properties on ES process and electrospun fibers
Solution property Range Effect
Viscosity
Low Beaded fibers[20]
Medium Spherical beads turn into spindle like[21]
High Smooth fibers[21], Pumping is difficult[12] , solution
may dry at needle tip[22]
Conductivity High Smooth, may be beaded and smaller in diameter[23]
Surface tension Low Uniform and smooth fibers[24]
Molecular weight (g/mol)
9000-10000 Beaded fibers, 250-1000 nm diameter
13000-23000 diameter 500-1250 nm
31000-50000 Flat fibers, 1-2 µm[40]
Process parameters also have a significant effect on the ES process and the electrospun fibers. These parameters include the applied voltage, distance between the needle tip and the collector, nature of collector, feed rate, temperature and humidity. Table 2 shows the effect of process parameters on the ES process in general.
Table 2. Effect of process parameters on electrospun fibers
Process parameter Effect
Supplied voltage At higher voltage better fiber crystallinity[25], more beads formation chances, smaller fiber diameter[26], with increase in voltage fiber shape changes from spindle like to spherical like[23].
Collector plate Very few fibers in case of non conducting plate but in case of conducing plate more and closely packed fibers[24]
Feed rate
Solvent has sufficient time to evaporate at low feed rate [28], thus the resulting fibers tend to be thinner.
At higher feed rate, due to greater volume there will be increase in fiber diameter and beads size[23]
Distance between needle tip and collector plate
Decrease in distance have similar impact as increase in voltage[23]
Needle diameter Smaller diameter favors to reduce the amount of beads [25]
Temperature At higher temperature fibers are uniform and smaller in size[21][29]
Humidity At very low humidity clogging of needle tip may occur[30]
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Surface energy and interactions are commonly described by the contact angle formed by a water drop on the solid surfaces [27]. Contact angle is an angle formed by the liquid at the junction of the solid liquid and vapor phases at the solid surface. In general surfaces having water contact angle greater than 150o and sliding angle less than 5o are considered as superhydrophobic surfaces [31].
Figure 2. Lotus leaves [32]
Superhydrophobic surfaces are involved in so-called self cleaning processes. Water droplets are not stable on such surfaces and can be rolled off with a slight shiver by which dust particles are simultaneously removed. This phenomenon is called self cleaning and occurs naturally in lotus leaves. The mechanisms controlling the self cleaning effect and water repellency of lotus leaves have been extensively studied by industry as well as academia. The secret behind the superhydrophobicity lies in the existence of a hierarchical nanostructure and very low surface energy [32].
The ES process has been used to produce superhydrophobic surfaces from different polymer solutions like polystyrene [33], polyvinylidene fluoride [34] and PHBV (Poly hydroxybutyrate-co-hydroxyvalerate) [35].
1.4.2. Electrospinning of biopolymers
In the past recent years efforts have been made to elctrospin biodegradable polymers and natural polymers. The purpose of these studies was to improve the surface properties of the polymer to be used in the field of medical, tissue engineering or in packaging. For instance, PHBV (Poly hydroxybutyrate-co-hydroxyvalerate [35], potato starch (amylose 20-25% and amylopectin 75-80%) , PVA (7 wt. %)[36], Gelatin type A[37] and Wheat gluten[38] have been successfully electrospun to produce surfaces with increased water repellency with respect to the neat polymer films.
The electrospinning technique was used in this study to deposit mats of spun starch fibers onto a pre-coated paper board leading to an improved hydrophobicity. The electrospun fiber mat was then plasma coated with hexamethyl disiloxane (HMDSO) to further improve the hydrophobicity.
Starch solutions were characterized in terms of rheology, conductivity and surface tension. The flow curves (viscosity vs. shear rate) of the different solutions were determined by means of a rheometer equipped with plate-plate geometry and low shear rate viscosity of the solutions was measured by a capillary based method. Surface tension determinations were performed by means of a KSV tensiometer using ring method while conductivity was determined by using digital laboratory conductivity meter.
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The appearance of the different samples produced was investigated by means of a scanning electron microscope (SEM) and the hydrophobicity was characterized in terms of the water contact angle. Tape tests were performed to analyse the robustness of the electrospun fibers.
Fibers produced with 10% AHAM starch solution were thicker in diameter and had fewer beads as compared ES fibers from a 5% AHAM solution. Fibers had more or less similar morphology, independently of collecting time and substrate used. The WCA was not found to differ due to different fiber morphologies.
When the electrospun fiber mat was subjected to tape test failure between the fibers and between fibers and the base substrate was found. Almost all the fibers were removed by the tape on sample for which the collecting time was 2 min during ES, indicating failure between fibers and the substrate (cohesive). For 15 min collecting time some of the fibers still remained on the substrate while some were removed by the tape. There is a strong need for improvement of the robustness of the ES fibers.
In this study, 5% and 10% AHAM starch solution were successfully electrospun onto pre-coated paper board. By electrospinning coatings of AHAM starch solutions (5% and 10 %) on a HP starch pre-coated paper a remarkable increase in the water repellency (evaluated in terms of WCA) by a factor of ca. 4 was observed. The morphology of the electrospun fibers was changed significantly with the concentration of AHAM starch solution, whereas WCA was not changed to any great extent with the fiber morphology. Plasma polymerization of HMDSO onto an ES fiber mat of AHAM starch succeeded in rendering a superhydrophobic surface (from WCA of ca. 140o to ca.170o).
2. MATERIALS AND METHODS
2.1. Materials
2.1.1. Starches
The hydroxypropylated(HP) starch used in this study was provided by Lyckeby, Sweden. This starch, which is available commercially under the trade name Perlcoat 155 is a FDA approved starch with a degree of substitution (DS) of 0.07, viscosity in solution = 180 cP at 20% and a cost of 10 SEK/kg. The basic raw material for the production of this type of starch is native potato starch. The hydroxypropylation modification is shown in the reaction scheme in Figure 3.
Figure 3. Schematic of the reaction between starch (StOH) and propylene oxide to give hydroxylpropylated starch.
Acetylated High Amylose Maize (AHAM) (not yet FDA approved) starch was supplied by Lyckeby, Sweden. The raw material for the production of this type of starch is maize starch. The AHAM starch is characterized by a high degree DS (close to 3) and a relatively high cost (45 SEK/ kg) relative to other starches. The acetylation reaction is schematically presented in Figure 4.
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Figure 4. Schematic of the acetylation reaction of starch (StOH) using acetic acid anhydride.
2.1.2. Paper board
The paper board substrate used in this study was Korsnäs Liquid Uncoated board (ca. 250 gsm) having an unbleached side (brown) and a bleached side (white), supplied by Korsnäs AB.
2.2. Methods
2.2.1. Preparation of starch solutions
For the preparation of aqueous solutions of HP starch the required amount of starch was pre-dispersed in cold water by means of a magnetic stirrer. The flask containing the dispersions was then immersed into a water bath @ 90-95oC for ca. 30 min while stirring with a propeller rotated at ca. 300-400 rpm. At the end of this process, the solution looked transparent and with no visible lumps. Afterwards the solutions were stored at room temperature (RT) and used within 1 week.
Solutions of AHAM in acetic acid were prepared by adding the starch to the acetic acid while stirring with a magnetic stirrer at room temperature. A homogeneous solution was typically obtained after stirring for ca. 1 hour.
2.2.2. Coating methods
In this study paper board (Korsnäs Liquid Uncoated) was first bench-coated with a starch solution to simulate a typical barrier coating. This pre-coating was meant to confer the board good barrier properties against O2 transmission and a smoother, more homogeneous surface to deposit the ES fibers onto. A mat of electrospun fibers of starch (a different one than that used in the pre-coating) were deposited on the pre-coated boards and the resulting coating was characterized in terms of hydrophobicity (WCA), morphology of the fibers/fiber mat (scanning electron microscopy imaging) and mechanical stability (tape test).
The sections 3.1 to 3.2 describe the general properties of the base substrates and different solutions used for ES, whereas sections 3.3 to 3.6 present and discuss the results in connection to the application of AHAM and HP-starch coatings by means of ES. The majority of the work performed in this study concerns the ES on AHAM starch on HP starch precoated board. Comparatively fewer studies were made on the ES of HP starch, where the work was concentrated on the optimization of the ES process more than on the properties of the resulting coatings.
• Electrospinning (ES)
For electrospinning, AHAM and HP starch solutions were taken in 10 ml syringe (B.Braun Medical Inc. Germany). The solutions were then pumped by a Sage Syringe Pump (Orion M362, Thermo electron corporation USA) at the desired flow rate through a needle (SWAGELOK-316RSL) of diameter 1.2 mm. To draw the solution from the tip of the needle a voltage (15 kV) was applied between the grounded collector plate (10 cm x 10 cm, and the needle tip. The distance between the collector plate and the needle tip was kept at 15 cm. During the optimization of the ES process, the electrospun fibers were normally collected on aluminum foil (which was wrapped around the collector plate). When depositing the fibers on paper board samples, a piece of paperboard (2.5 x 7.5 cm) was
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attached to the aluminum foil using double-sided sticky tape. Schematic diagram of the electrospinning process is shown in Figure 5.
Figure 5. Schematic diagram of the ES process[39]
• Plasma treatment/coatings
The plasma polymerization of HMDSO onto electrospun coated board was carried out on YKI’s plasma reactor which is shown in Figure 6. The sample was exposed to plasma coating for 2 minutes at 4.53 cc/min HMDSO vapor flow rate while applying 21 W plasma power at a controlled pressure of 25 mTorr. The base pressure was 8 mTorr and the pressure without HMDSO was 24 mTorr. The reactor was vented with air before removing the plasma treated samples.
Figure 6. Plasma reactors at YKI
• Bench Coating
The coating of the samples was performed by means of a bench coater (K-Control Coater. RK Print-Coat Instruments Ltd. UK) using a speed setting of 5 (ca. 5m/min). Different grooved rods (rod # 4 and 6) were used depending on the desired wet film thickness and the viscosity of the coating solutions. Coatings with coat weights of ca. 6.5 g/m2 or 13.0 g/m2 were deposited onto the substrates.
15
After coating, the samples were dried in a convection oven at 90oC for 2 min under restrained conditions and were air dried for about an hour. After drying the samples were stored in a climate room (25 oC Temprature and 50% RH).
2.2.3. Surface characterization methods
• Scanning Electron Microscope (SEM)
The appearance of the different samples produced was investigated by means of a scanning electron microscope (SEM) (XL30 ESEM TMP, FEI/Philips, The Netherlands). The samples were mounted onto double-sided carbon tape on aluminum stubs and coated with gold in a sputter coater (SCD 050, Balzers). Micrographs were then obtained in the SEM under low vacuum mode at 20 kV.
• Contact Angle determination
The steady state contact angle of coated surfaces (WCA after 30 s) to Milli-Q water (40 �S/cm) was measured in an OCA-40 micro instrument (Data Physics, Germany). The WCA measurements were carried out with SCA-software (SCA20) by using sessile drop method. The WCA value for each sample was obtained by averaging at least 3 runs. A 4 µL drop volume was used for the CA determination of samples of paper board, pre-coated board and electrospun coatings but for the samples produced in combination of electro spinning and plasma coatings a 8 µL drop volume was used. WCA was calculated using the ellipse fitting model for drop volume 4 µL while in case of large volume (8 µL) the Young-Laplace model was used.
• Robustness (tape tests)
The mechanical stability of the electrospun and plasma coated samples was assessed following a method based on ASTM standard method D3359-B for measuring adhesion by tape test. A piece of tape (1cm width and length was taken according to the length of the test specimen) was placed on the specimen surface and pressed with the finger for sufficient adhesion. After 2 minutes, the tape was pulled off avoiding any jerk at an angle close to 180o. After that the sample was analyzed visually as well as with SEM and light transmission microscopy.
2.2.4. Solution Characterization methods
• Rheology (rheometer and capillary rheometer)
The flow curves (viscosity vs. shear rate) of the different solutions was determined by means of a Kinexus rheometer (Malvern Instruments, UK) equipped with plate-plate geometry and typically using a 1000 �m gap. The measurements were typically performed within a shear rate interval of 1-1000 1/s and at room temperature (22°C).
The low shear rate viscosity of the solutions was sometimes measured and used as a simple indication of the flow behavior of the solution. This was done using an Ubbelohde or suspended-level viscometer, which uses a capillary based method of measuring viscosity.
• Surface tension
Surface tension determinations were performed by means of a KSV tensiometer (Bolin) of the De Nouy ring type. This is a method based on the use of a ring of perfectly known geometry which is suspended from a precision balance. To determine the surface tension the ring is immersed in the liquid of interest and the downward pull (i.e. the force) exerted by the liquid on the ring is measured.
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• Conductivity
The conductivity of the starch solutions were determined by using digital laboratory conductivity meter CG 855 (Schott-Geräte, Germany).
3. RESULTS AND DISCUSSION
3.1. Surface properties of base substrates and pre-coated board
For these studies the pre-coating was applied on either the bleached or unbleached side of the board and two different coat weights were tested. Table 3 shows the WCA of the uncoated and pre-coated paper boards used. As indicated by the CA values reported in Table 3, the base paper board is hydrophobic (WCA 129o) on the bleached side while the unbleached side is very hydrophilic (very low WCA). On the bleached side, which is initially hydrophobic (WCA 129o), pre-coating lowers the WCA, which was to be expected due the hydrophilic nature of the starch. The WCA of the HP pre-coated board is in agreement with that reported in the literature for oxidized hydroxypropylated starch free films (ca. 50o) [1]. On the other hand, the WCA of the unbleached side of the board increases after coating.
At the highest coat weight, the WCA of the pre-coated paper board is the same regardless of whether the pre-coating was applied on the bleached or the unbleached side of the board, indicating that the substrates are completely and homogeneously covered by the pre-coat. At lower coat weights, the small differences in WCA between the unbleached and bleached side of the board and the poorer reproducibility of the WCA (larger standard deviations) indicates that the coating may be less homogeneous and thus the properties of the base paper have some influence on the obtained WCA.
Note that the WCA of rod–coated AHAM starch on paper board (bleached side) is included in Table 3 for reference. The significantly higher WCA of AHAM-coated with respect to HP-coated board (68o vs. 36o), is consistent with the more hydrophobic nature of AHAM starch.
Table 3. Contact angle of base substrate and Pre-coated board with 20 % HP starch
Substrate
Coat weight (g/m2.) Contact Angle(o)
Bleached side Unbleached side Bleached side Unbleached
side
Base substrate - - 129±1 17±3
Board pre-coated with HP starch
6.5 6.6 43±3 37±5
13.1 13.1 36±2 36±2
Board pre-coated with AHAM starch ~ 6 -- 68 ± 2 --
17
3.2. Properties of coating solutions
Figure 7 shows the flow curves (viscosity as a function of shear rate) of solutions of HP and AHAM starch at different starch concentrations. All the starch solutions exhibit shear thinning behavior (i.e. the viscosity decreases with shear rate), the magnitude of the viscosity reduction with the increase in shear rate being greater the higher the starch concentration. This behavior is consistent with that of concentrated polymer solutions and normally attributed to the alignment of the polymer molecules in the direction of flow at high shear rates. At the same solids content (e.g. 10 %w), solutions of AHAM starch are much more viscous than solutions of HP starch, the difference being greater at low shear rates. It is interesting to note that the flow curves of the 10w% HP and the 5 w/v% AHAM starch solution are essentially identical and they overlap each other across the whole range of shear rates. The data in Figure 7 shows the response of the solutions during a shear rate ramp up and down. In all cases, essentially no hysteresis was observed between the up and down cycles.
Figure 7. Flow curves of HP and AHAM starch at different concentrations (HP starch w% and AHAM starch w/v %)
(Pa-
s)
18
Tables 4 and 5 show the properties of the HP and AHAM starch solutions which are important to the ES process, namely, surface tension, viscosity (low shear, Ubbelohde viscometer) and conductivity. As discussed in detail in section 1.4.1 of this report, the surface tension, viscosity and conductivity of the solution have a great influence on ES process and the characteristics of the resulting electrospun fibers. The relative influence of these parameters on the outcome of the ES process will be discussed in detail in the coming sections describing the ES of AHAM starch and HP-starch on different substrates.
Table 4. Properties of HP starch solutions used for ES
Starch type
Concentration (w%)
Solvent Surface Tension
(mN/m) Viscosity (mm2/sec)
Conductivity (µS/cm)
HP starch
20 Water not determined not determined not determined
10 Water 66.6 94.1 <20
10 water+10 w% ethanol 46.4 not determined 218
10 water+5 w% NaCl 65.1 not determined >20000
10 water+10 w%
ethanol+5w% NaCl 43.4 not determined >20000
Table 5 Properties of AHAM starch solutions used for ES
Starch type Concentration
(w/v %) Solvent
Surface Tension (mN/m)
Viscosity (mm2/sec)
Conductivity (µS/cm)
AHAM starch) 5 acetic acid 27.5 86.2 not determined
10 acetic acid 22.0 958.8 not determined
3.3. ES of AHAM starch on HP starch pre-coated board
3.3.1. Effect of solution viscosity on ES process
Solutions of AHAM starch could be successfully electrospun onto the collector by adjusting process parameters such as voltage and flow rate (see Table 6 for details). For a given set of conditions, it was possible to attain an uninterrupted spinning of fibers during the whole collecting time (no drying or clogging of the needle) which resulted in the deposition of a localized mat of well defined fibers.
When electrospinning fibers from a 10% AHAM solution the fibers were deposited in a much localized way, while the fibers become deposited over a larger area when spun from the lower concentration solutions (5% AHAM). This observation is consistent with trends reported in the literature [21] and has been explained in terms of the effect of the solution viscosity on the bending instability of the fibers. A high viscosity of the ES solution discourages the bending instability of the fibers to set in for a long distance. As a result, the jet path is reduced and the bending instability spreads over a smaller area [21].
19
Table 6. ES parameters for 5 and 10 % AHAM starch solutions
Solution Pre-coat (HP starch)
weight (g/m2) Collector
Distance (cm)
Voltage (kV)
Flow rate (mL/h)
5 % AHAM starch 6.6
Al foil
15.0 15.0 1.0 Al foil+bleached
board
Al foil+unbleached board
6.6
Al foil
10 % AHAM starch
Al foil+bleached board
15.0 15.0 0.5
Al foil+unbleached board
Table 7. Summary of electrospun and plasma coated samples
Starch, conc.
Flow rate
mL/min Substrate
Collecting time (min)
Plasma coated
No of samples CA(o)
5% AHAM starch
1
Al foil 2 1 96±4
Board-unbleached
2 1 94±3
15 3 138±2
HMDSO 1 173±2
Board-unbleached
2 1 66±2
15 3 87±3
HMDSO 3 171±0
10% AHAM starch
0.5
Al foil 15 1 141±1
Board-bleached
2 3 75±3
15 3 138±2
Board-unbleached
2 3 66±2
15 3 140±1
HMDSO 1 171±1
20
3.3.2. Effect of solution viscosity on the morphology of fibers
Solutions containing 5 % and 10 % of AHAM starch were electrospun for 2 and 15 minutes and deposited on the pre-coated board. Figure 8 shows SEM images of electrospun fibers on both the bleached and unbleached side of the pre-coated board. As shown in Table 6, ES conditions are the same except the flow rate which is 1 and 0.5 mL/h for the 5% and 10 % AHAM solutions, respectively. The morphology of the fibres was found to depend strongly on the viscosity (i.e. concentration) of the starch solutions (an increase in the polymer concentration in solution will result in greater polymer chain entanglements and ultimately higher viscosity).
• Different collecting time
The SEM micrographs in Figure 8 show that that fibers spun from a 5% AHAM solution are very thin (about 100 nm in diameter) while thicker fibers are spun from the higher concentration solution. This finding is in agreement with the trends reported by a number of authors [40] [41]. This effect has been explained in terms of the greater resistance of the solution to be stretched by the charges on the jet at higher viscosity [41].
Beads can be seen in the micrographs of fibers electrospun from both the 5 and 10w/v% AHAM starch solutions. It can be seen in Figure 8 that the beads on ES coatings produced from 5% AHAM solutions are present in larger amount as compared to ES coatings produced from 10% AHAM. Beads looked spherical in shape and most of them are collapsed with varying size (the diameter ranges from ca. 1µm to ca. 5µm).
ES coatings produced from the 10 % AHAM solution are characterized by fewer beads (bigger in size; 10 µm) and the presence of spindle like beaded fibers. This is in good agreement with the observation that by increasing the viscosity the shape of the beads changes from spherical to spindle like until smooth fibers are obtained. [21]
21
Figure 8. Scanning electron micrographs of AHAM 5 % (a,b) and AHAM 10 % (c,d) electrospun fibers on pre-coated board unbleached (a,c) and bleached (b,d) at different collecting time: 2 minutes (left column) and 15 minutes (right column). Magnification is 2000X and the scale bar is 10 µm
a
b
c
d
22
• Different substrates
For the ES process to work the collector plate needs to be conductive. The images in Figure 9 confirm, as expected, that there are no apparent differences in morphology between fibers spun onto different substrates (aluminum foil, bleached or unbleached side of the pre-coated board).
a
b
c
Figure 9. Scanning electron micrographs of AHAM 10% electrospun fibers on pre-coated board. Unbleached side (a), bleached side (b) and aluminum foil (c) at collecting time 15 minutes. Magnification is 500X (left column) 2000X (right column) and the scale bar is 50 µm (left column) 10 µm (right column).
23
There are reports in the literature showing that using a collecting surface with higher conductivity results in more fibers collected on it [27] and in fibers of smaller diameter [23]. In the present study, the size of the paper board was small (7.5cm x 2.5 cm) as compared to collector plate (10 cm x 10 cm) which was covered with aluminum foil. In consequence, it is possible that there was not much difference in conductivities of the different collecting surfaces.
3.3.3. Effect solution viscosity on WCA
• Different morphologies
WCA for samples prepared using 15 min collecting time with 5% and 10 % AHAM can be seen in Figure 11. In the right column it is shown that the WCA is same (ca. 138o) although there is a significant difference in morphology. In Figure 10 (left column) for 2 minute collecting time WCA is also the same (ca. 66o) in spite of different morphology. This finding is very intriguing as many researchers have reported that there is a direct connection between fiber morphology and WCA [26] [42]. It is not known why this is not the case here, but it could be due to the fact that despite the difference in morphology the degree of roughness is similar between the two different coatings (5 and 10 % AHAM). Further, small differences in surface properties (roughness) may not be captured by static contact angle measurements (as used in this study) but may only become apparent when studying hysteresis during wetting and rewetting processes, i.e. advancing and receding contact angles.
• Different collecting times
The resulting WCA of the fiber mat is not only a function of the morphology of the fibers but will greatly depend on the thickness of the coating (ES collecting time). Coatings produced using short collecting times (2 min) may not cover the surface completely and thus the resulting WCA will be influenced by the nature of the base substrate.
Figure 10 shows the WCA of mats of fibers spun from 5% and 10% AHAM solution deposited on pre-coated board (unbleached side) at different collecting times. It can be seen that there is an increase in contact angle with increase in collecting time.
At 2 minutes collecting time the contact angle on the substrate covered with fibers spun from a 5 % AHAM solution is 66o±2 while the initial WCA of the pre-coated board was 37o±5. In case of 15 minutes collecting time the contact angle is increased to 87o±3. There is not a significant change in fiber morphology for a longer collecting time but there is a higher surface coverage. The influence of the base substrate on the surface properties will then be less, which results in higher contact angle.
Figure 10. Scanning electron micrographs and electrospun fibers on pre-coated board (column) and 15 minutes (right column
• Different substrates
Figure 11 shows the WCA on mats of collecting time). Interestingly enough, the WCAbleached side of the pre-coated board are
On the unbleached side there is a significant difference in the contact angle for 5 % and 10 % AHAM electrospun fibers for the same collecting time.
a
b
Scanning electron micrographs and WCA of AHAM 5 % (a) and AHAM 10 coated board (unbleached side) at different collecting time
column) and 15 minutes (right column). Magnification is 2000X and the scale bar is 10 µm.
shows the WCA on mats of fibers spun from 5% and 10 % AHAM solutions (Interestingly enough, the WCA for the different sets of coatings applied
board are same although the morphology of the fibers is different.
there is a significant difference in the contact angle for 5 % and 10 % AHAM electrospun fibers for the same collecting time.
24
of AHAM 5 % (a) and AHAM 10 % (b) 2 minutes (left
and the scale bar is 10 µm.
(15 minutes applied onto the
is different.
there is a significant difference in the contact angle for 5 % and 10 % AHAM
Figure 11. Scanning electron electrospun fibers on pre-coated boardcolumn) at 15 minutes collecting
3.4. Plasma coatings
Plasma treatments/coatings can be used to improve the water repellence and barrier properties of starch films and coatings. In this plasma coated and the WCA was measured
Figure 12 shows WCA of the after plasma coating with HMDSO. WCA is increased ca. 3coatings on the electorspun fibers. superhydrophobic surfaces. The superhydrophobic nature of the sample makes it (sometimes impossible) to deposit a 4 determination (the water drop does not detach from the needle) and the shapedifficult to use standard fitting and analysis models for the determination of the W
a
b
Scanning electron micrographs and WCA of AHAM 5 % (a) and AHAM 10 % (b) coated board, unbleached side (left column) and on bleached
) at 15 minutes collecting time. Magnification is 2000X and the scale bar is 10 µm.
can be used to improve the water repellence and barrier properties of starch films and coatings. In this study selected electrospun samples at different condition
WCA was measured.
WCA of the fiber mat electrospun with 5% and 10% AHAM starch solutions th HMDSO. WCA is increased ca. 30o (ca.140o to ca.170
coatings on the electorspun fibers. In practice it is very difficult to determine the exThe superhydrophobic nature of the sample makes it is
(sometimes impossible) to deposit a 4 L drop of water onto the substrate during determination (the water drop does not detach from the needle) and the shape of the drop makes it difficult to use standard fitting and analysis models for the determination of the W
25
of AHAM 5 % (a) and AHAM 10 % (b) bleached side (right
and the scale bar is 10 µm.
can be used to improve the water repellence and barrier properties of electrospun samples at different conditions were
with 5% and 10% AHAM starch solutions 70o) after plasma
the exact WCA for is very difficult
L drop of water onto the substrate during WCA of the drop makes it
difficult to use standard fitting and analysis models for the determination of the WCA. This
26
problem was overcome using a larger volume of the drop (8 L) which in turn has the disadvantage of resulting in a slightly flattened (as opposed to perfectly spherical) drop (see Figure 12).
a b c
Figure 12. WCA of AHAM 5 % (a,b) and AHAM 10 % (c) electrospun fibers on unbleached pre-coated board at 15 minutes collecting after plasma coating with HMDSO
3.5. Mechanical stability (robustness)
For the ES coatings to be of real use in any packaging application, it is of outmost importance that the deposited fiber mat is mechanically robust, so that it can withstand handling during the converting and printing processes.
In Figure 13 it can be clearly observed that the fibers are detached after the tape test. Fibers on the tape were also observed by the naked eye. On a large magnification as shown in Figure 14 it can be seen that both adhesive and cohesive failure occurred. For a 2 minutes collecting time (Figure 14 b) almost all the fibers were removed from the surface indicating failure between fibers and the substrate. But for coatings produced using 15 min collecting time some of the fibers were removed, which means that cohesive failure occurred. It is quite possible that for 15 minutes collecting time, as the fiber mat is thicker and has good surface coverage, the tape might not adhere to the substrate itself but it has direct contact to the surface fibers only. But in case of 2 minutes collecting time the surface coverage seemed to be less than 50 % and a very thin layer of the fibers was obtained.
Hydrophobicity of the AHAM starch is increased through electospinning and plasma treatment but the fibers formed were found not to be mechanically robust. It is really important that the fiber mat is robust enough for use of ES in packaging applications. Mechanical properties [43] as well as water barrier properties [43][12] can be improved by cross-linking of the starches.
Starch can be cross-linked by means of multifunctional agents. Such reactions introduce molecular bridges or cross-links between the molecules, resulting in an increase in molecular weight. Most common cross-linking reactions with multifunctional agents involve reactions with free OH groups on the starch molecule. Due to its low DS, HP starch is susceptible to be subjected to crosslinking reactions while the same would not be possible with AHAM starch (as it has a high degree of substitution and thus limited amount of free OH groups). In this prospective it was decided to electrospin HP starch instead of AHAM starch so that the fiber mat could be cross-linked in order to improve the robustness. HP starch at different concentrations (20%, 15%, 10% and 5%) was attempted
173o±2 171o±0 171o±1
27
for electrospinning at varied process conditions (applied voltage, distance between collector plate and needle tip, feed flow rate) electrospun but no indication of a successful electrospinning was observed.
Since the flow behavior (see Figure 7) and the viscosity of 5 % AHAM (which was electrospun successfully) and 10 % HP starch solution was very similar, the 10 % HP starch solution was chosen for further studies of electrospinning by changing other solution properties like surface tension and conductivity.
after before after before
a b
Figure 13. Scanning electron micrographs of electrospun samples (a) with 10 % AHAM (b) 5% AHAM on the unbleached side for 15 minutes collecting time after tape test. Magnification is 100 X and the scale bar is 1 mm. Note the dashed lines, which marks the boundary between the surfaces before and after the tape test
Figure 14. Scanning electron micrographs aftertape test of AHAM 5 % (aunbleached (a) and bleached (a,c). Magnification is 2000
a
b
c
Scanning electron micrographs after (right column) and before tape test of AHAM 5 % (a) and AHAM 10 % (b,c) electrospun fibers on pre-
bleached (b,c) at different collecting time 2 minutes (b) and 15 minu). Magnification is 2000X and the scale bar is 10 µm.
28
(left column) -coated board:
collecting time 2 minutes (b) and 15 minutes
29
3.6. ES of HP vs. AHAM starch
3.6.1. Effect of solution properties on ES of aqueous solutions of HP starch
Electrospinning of aqueous solutions of HP starch proved to be elusive. No fibers could be successfully spun from 10% solutions or by means of tuning of the process parameters (applied voltage, distance between collector plate and needle tip, and flow rates). In an attempt to overcome this problem, the properties of the polymers solutions (surface tension and conductivity) were varied instead. Table 4 in section 3.2 summarizes the different solutions tested and their respective properties.
The original HP solution exhibits high surface tension (66.6 mN/m) as compared to 5 % AHAM (27.5 mN/m) and its conductivity is also low.
To improve the conductivity and to lower the surface tension NaCl and ethanol respectively were used. Table 4 shows the different HP starch solutions with added NaCl and ethanol and the resultant solution properties.
5 % NaCl was added to enhance the conductivity of the HP starch and a significant increase in conductivity was observed from <20 �S/cm to > 20000 �S/cm.
The surface tension was lowered to 46.4 mN/m from 66.6 mN/m by adding 10 % ethanol.
No fibers were observed on the aluminum foil after electro spinning of 10 % HP starch or with 10% HP starch containing 5 % NaCl. It might be due to the reason that because the surface tension is really high as compered to 10 % and 5 % AHAM (see table 4 and 5) which are successfully electrospun, so the electrostatic force applied is not enough to overcome the surface tension and to drag the drop from the tip of the nnedle.
With the 10% HP starch containing 10% ethanol small volumes of solution were drawn from the needle. A similar result was obtained when salt (5 % NaCl) was added to the solutions. The material collected on the aluminum foil was analyzed by SEM. As it can be seen in the micrographs presented in Figure 15, collapsed blobs in a broad range of sizes, and not fibers, are produced from the 10% HP starch containing 10 % ethanol. No fibers could either be observed in the sample produced from HP solution containing 5% NaCl and 10% ethanol. Fewer blobs were found in this sample compared to the one containing 10% ethanol. Micrographs of fibers deposited from 5 % AHAM starch solution are included in Figure 15 as a reference. Compared to the samples produced from HP starch, the ones produced from 5% AHAM starch contain fewer blobs (in these samples both blobs and beaded fibers are seen) The lower viscosity of HP solutions with respect to a AHAM solution at the same concentration (see Table 5), seems to indicate that the amount of the entanglement of the polymer chains in the solvent is not enough for the electrospinning jet to be stretched without breaking into small droplets. As in case of 5 % AHAM starch there are also some fibers along with the blobs but with 10 % HP starch containing 10 % ethanol or 10 % ethanol and 5% NaCl no fibers are observed. A possible explanation for this may be conformational changes of the polymer in the ethanol/water mixture, which may lead to a less extended conformation of the polymers which in turn would result in fewer entanglements and a reduction of the ability of the solution jet to be stretched.
Higher concentration of HP starch with 10% ethanol or even more could work to have enough polymer chain entanglements needed to stretch the electrospinning jet without breaking.
30
a b
c d
Figure15. Scanning electron micrographs of 10% HP (a,b) 5% AHAM (c) 10 % AHAM (d) electrospun fibers on aluminum foil by adding 10 % ethanol and 5% NaCl (a), by adding 10% ethanol (b) at collecting time 2 minutes (a,c,d) and 15 minutes (b). Magnification is 2000X and the scale bar is 10 µm.
31
CONCLUSIONS
• Solutions (5% and 10%) of AHAM starch in acetic acid were successfully electrospun onto pre-coated paper board. On the other hand, aqueous solutions of HP starch could not be successfully ES despite changes in process parameters and solution properties.
• By depositing electrospun coatings of AHAM starch solutions (5% and 10 %) on a HP starch pre-coated paper it was possible to increase the water repellency (WCA) by a factor of ca. 4 (from WCA of ca. 37o to 140o) .
• The morphology of the electrospun fibers changes dramatically with the concentration of the polymer in solution (5% and 10%). Remarkably, the resulting static WCA of the deposited fiber mat is not affected by the morphology of the fibers.
• Plasma polymerization of HMDSO onto an ES fiber mat of AHAM starch succeeds in rendering a superhydrophobic surface (from a WCA of ca. 37o to a WCA of 170o)
• None of the ES coatings was found to be mechanically robust. Failure occurred between the fibers (cohesive) and between fibers and substrate (adhesive) when subjected to tape test. Crosslinking of the ES starch coating is suggested as a possible means to improve the robustness of the ES coating.
ACKNOWLEDGEMENTS
Thanks to Isabel Mira, Caisa Johansson, for guidance and supervision and to Mikael Sundin, Rodrigo Robinson, and Annika Dahlman at YKI for assistance with instruments and experimental work during this study. Thanks also to Institute for Surface Chemistry, YKI, for the opportunity of performing this work.
32
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