Multifunctional polymer nanocomposite based on magnetic nanoparticales

By

Sohail Nawaz

Supervisor Professor Ulf W. Gedde

Co-Supervisor Richard Olsson

Department of Fiber and Polymer Technology The Royal Institute of Technology (KTH)

Stockholm, Sweden, 2007

ABSTRACT Five batches Cobalt ferrite nanoparticles were made by ‘chemie-douce’ method. The molar ratio of iron to cobalt was 2.0. The synthesized nanoparticles were analyzed in TEM to see their shapes and whether they are agglomerated or not. The nanoparticles were then coated first with tetraethoxysilane to make them hydrophilic and then with methyl tri-methoxysilane to make them hydrophobic in nature. Hydrophobic nanoparticles were analyzed in TEM to see their thick coating. The coated nanoparticles were then ultrasonically etched to determine the thickness of the coating which was 8%. The hydrophobic nanoparticles were dried and well grinded to avoid from agglomerates. Well grinded particles were then blended with four different plastics i.e.-e polyethylene, polypropylene, polyamide6 and polycarbonate. Two of them are softer and two are harder plastics. The nanoparticles were added 8% by weight in the polymers. The nanocomposites of all four polymers were mechanically tested by tensile testing. They were kept in a room for 24 hours with 50% humidity. Polyethylene and polypropylene got good and uniform mixing and they were more stiff, ductile and good in strength than the pure polymers. Polyamide6 and polycarbonate were observed to have not good mixing, and because of this we did not get the good tensile testing results from them. The microwave test results showed that polyethylene, polypropylene showed more thermal resistance than the pure polymers. Polycarbonate also showed bit of thermal resistant, but in case of polyamide6 we got different results.

TABLE OF CONTENTES 1.  INTRODUCTION .............................................................................................................. 1 

1.1.  Background .................................................................................................................. 1 1.2.  Magnetic Nanocomposites .......................................................................................... 2 1.3.  Ferrites ......................................................................................................................... 3 

1.3.1.  Curie Temperatures .............................................................................................. 4 1.3.2.  Permeability ......................................................................................................... 4 1.3.3.  Saturation Magnetization ..................................................................................... 4 1.3.4.  Brittleness ............................................................................................................. 4 1.3.5.  Hardness ............................................................................................................... 4 

1.4.  Applications of magnetic nanocomposites .................................................................. 5 2.  EXPERIMENTAL ............................................................................................................. 5 

2.1.  Synthesis of cobalt ferrite nanoparticles ...................................................................... 5 2.1.1.  Materials ............................................................................................................... 5 2.1.2.  Batch synthesis ..................................................................................................... 5 

2.2.  Surface modification of nanoparticles ......................................................................... 6 2.2.1.  Materials ............................................................................................................... 6 2.2.2.  Silanization ........................................................................................................... 6 

2.3.  Ultrasonic etching ........................................................................................................ 7 2.4.  Grinding and mixing .................................................................................................... 7 

2.4.1.  Grinding of polymers ........................................................................................... 7 2.4.2.  Mixing of polymers with magnetic nanoparticles ................................................ 8 

2.5.  Hot pressing ................................................................................................................. 8 3.  RESULTS AND DISCUSSION ........................................................................................ 9 

3.1.  Particle synthesis ......................................................................................................... 9 3.2.  Surface modification of nanoparticles ....................................................................... 10 3.3.  Coating thickness by ultrasonic etching .................................................................... 12 3.4.  Visible colour differences between the nanoparticles ............................................... 12 3.5.  Water interaction difference of the particles after drying .......................................... 13 3.6.  Mechanical properties of nanocomposite systems .................................................... 13 

3.6.1.  Tensile testing .................................................................................................... 13 3.7.  Microwave testing ..................................................................................................... 23 

4.  CONCLUSIONS .............................................................................................................. 25 5.  FUTURE WORK ............................................................................................................. 26 6.  ACKNOWLEDGEMENTS ............................................................................................. 27 7.  REFERENCES ................................................................................................................. 28 

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1. INTRODUCTION

1.1. Background

The main classification of plastic materials is thermoplastics and thermosets. Thermoplastics are the materials that can be shaped and molded easily when they are hot. Thermoset materials are cross linked and do not melt. The glass transition temperature, Tg is different for each plastic. At room temperature some plastics are blow Tg and so they are hard. Other plastic are above Tg at room temperature and these are soft. Some polymers used as plastics are commonly polyethylene, polypropylene, polystyrene etc. Polymer nanocomposites are the very efficient and widely used materials in the world of materials technology. A polymer nanocomposite is the combination of two materials mainly. Nanometer scale materials are dispersed in polymer for the better performance. These nano sized materials are known as fillers and the purpose of fillers is to affect the properties of polymer. The mechanical, frictional and other properties are affected. There is constantly a need for stronger, lighter, less expensive and more versatile polymer composites to meet the demands of industrial consumers such as the automobile and aerospace industry. Polymer composites like carbon or glass fiber reinforced by thermoplastics and thermosets are very common. Other than fiber, polymer composites with inorganic and organic materials both synthetic and natural are commonly used. The polymer nanocomposites based on inorganic fillers are not that much commonly used .The inorganic nanoparticles such as CaCO3 have unique functions in concentration and uniform concentration distribution at almost molecular level in the reactive precipitation process and yield nanoparticles with size-controlled and uniform particle size distribution. The poor compatibility of inorganic nanoparticales with carbon-based synthetic polymers is largely because of hydrophilic character of the nanoparticles. The hydroxyl group causes agglomeration since this group strongly affects the interaction between the primary particles. The most recent nanocomposites are multifunctional based on magnetic nanoparticals, which will be our focus of work. The good thing with magnetic nancomposites is that the magnetic nanoparticals efficiently dispersed in solid polymer and have shown the different behavior from the magnetic solids and Ferro fluids. We will use ferrites instead of other magnetic materials. The ferrites are the oxide materials rather than the metals. Ferromagnetism is derived from the unpaired electron spins in only a few metal atoms, these being iron, cobalt, nickel, manganese, and some rare earth elements. The ferrites are preferred from other magnetic materials because they have good mechanical properties. They have less curie temperatures, resistivity and operating frequencies and moderately highly permeability which make them less expensive and hard and easy to handle.

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1.2. Magnetic Nanocomposites

Magnetic nanocomposite materials have their origins in the amorphous alloys that were brought to market in the 1970's. Amorphous materials have the characteristics that they lack of long range atomic order similar to that of liquid state. Production techniques include rapid quenching from the melt and physical vapor deposition is another. The lack of crystallinity causes amorphous materials to have a very low magnetic anisotropy. METGLAS 2605™ Fe78Si13B9 is a common amorphous magnetic alloy, in which B acts as a glass forming element. Because of anisotropic nature of the material we need to search for the material with isotropic magnetic properties. In magnetic materials the ferromagnetic exchange length expresses the characteristic distance over which a magnetic atom influences its environment, and has values on the order of 100 nm. The major problem for metallic materials is their low resistivity. Since it is impossible to dramatically increase their resistivity, metallic materials were excluded in high frequency applications and ferrites have been the only choice for five decades since World War II. Although efforts have been made extensively to improve the performance of the ferrites, very limited progress was obtained. Magnetic materials have been a key impediment for the miniaturization of electronic equipment. Some papers have been written on magnetic nanocomposites based on high temperatures and some on low temperatures. Magnetic nanocomposites consisting of iron oxide embedded in polymer matrixes are found to be behaving like transparent magnets with remarkable electrical and optical properties. The particle size effects dominate the magnetic properties of magnetic nanocomposites and the effect becomes more prominent when the particle size decreases. Nanophase materials and nanocomposites, characterized by an ultra fine grain size (< 50 nm) have created a great deal of interest in recent years by virtue of their unusual mechanical, electrical, optical and magnetic properties. Nanocomposite processing has also provided a new approach for fabricating soft magnetic materials. In a magnetic/ceramic or magnetic/polymeric nanocomposite, the resistivity can be drastically increased, leading to significantly reduced eddy current loss. In addition, the exchange coupling between neighboring magnetic nanoparticles can overcome the anisotropy and demagnetizing effect, resulting in much better soft magnetic properties than conventional bulk from materials The magnetic nanocomposites in which the magnetic nanoparticles are dispersed in nonmagnetic matrices have been found to possess entirely different magnetic characteristics with respect to their bulk counterparts. The following figure shows the hysteresis loop.

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Figure 1.2 Hysteresis loop illustrating coercivity (Hc), saturation magnetization (Ms), remnant magnetization (Mr), and permeability (µ).

1.3. Ferrites Ferrites are the nanoparticales used as fillers for the magnetic nanocomposites. They are magnetic in nature. There is clear difference between the properties of ferrites to the properties of other magnetic materials. Why we use ferrites instead of other magnetic materials because ferrites have better mechanical and other properties. Ferrites are the ceramic-like material with magnetic properties that are useful in many types of electronic devices. Ferrites are hard, brittle, iron-containing, and generally gray or black and are polycrystalline i.e., made up of a large number of small crystals. They are composed of iron oxide and one or more other metals in chemical combination. Ferromagnetism is derived from the unpaired electron spins in only a few metal atoms, these being iron, cobalt, nickel, manganese, and some rare earth elements. It is not surprising that the highest magnetic moments and the highest saturation magnetizations are to be found in the metals themselves or in alloys of these metals. The oxides serve a useful purpose in ferrites as they insulate the metal ions and therefore greatly increase the resistivity. This property of ferrites makes them useful at higher frequencies. Here are some of the magnetic and mechanical properties of ferrites that make it useful over other magnetic materials.

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1.3.1. Curie Temperatures All other magnetic materials have higher curie temperatures than the ferrites. All the magnetic materials lose their ferromagnetism at their curie temperature. The reason for considering the magnetic materials is that the curie point of the material be well above the proposed operating temperatures. The curie point depends on composition not on the geometry of the material. Even though some of the magnetic materials can be used at higher operating temperatures than others, very often the temperature limitations of the accessory items (wire insulation, potting or damping compound) can be more limiting; in this case, no practical advantage may be gained by the higher Curie point materials.

1.3.2. Permeability One good thing with the ferrites is that they can be made over wide range of permeability. The nickel-iron alloys have the higher permeability value in the range of 100,000. The powdered iron cores have low permeability in the range of 10-1000. As the operating frequency increases, ferrites with lower permeability are used because these have distinctly lower losses in these regions.

1.3.3. Saturation Magnetization Metals and alloys have higher saturation magnetization values. Thus, if high flux densities are required in high power applications, the bulk metals, iron, silicon-iron and cobalt-iron are unexcelled. Since the flux in Maxwell’s Ø = BA, where B = flux density in gausses and A = cross-sectional area in cm², obtaining high total flux in materials such as ferrites or permalloy powder cores can be accomplished only by increasing the cross-sectional area. Powdered iron has a fairly high saturation value, but exhibits low permeability.

1.3.4. Brittleness One drawback of ferrites over other magnetic materials is that they are more brittle. As they are ceramic in nature so a special care should be given in handling these cores. They are sensitive to mechanical shock and strain. To prevent from this affect tape wound cores are often used while damping compound. This prevents the transfer of shock or strain to the cores.

1.3.5. Hardness Ferrites are very hard materials when we compare them with other magnetic materials. This property is very useful in prevention to wear factor. Because of these properties ferrites materials are extensively used in magnetic recorder head applications.

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1.4. Applications of magnetic nanocomposites

Polymer nanocomposites have a lot of application in various fields like in electronics, automotives, biomedical etc. Inductive components are extensively used in high frequency (> 1 MHz) electronic devices from radar, satellite, telecommunication systems to home radio stereos. Nanocomposites materials also used in transformers. By increasing Ms and µ values will cause less magnetic material to be used for the transformer. Decreasing Hc will reduce the loss of AC applications and hence improving the efficiency. Nanocomposites are also used DC-DC power convertors. Nanocomposite particles made from encapsulation of magnetic nanoparticles in an inorganic matrix have a real interest in biomedicine due to their high resistance against biodegradation compared with nancomposites made with organic matrix. Carbon black filler rubbers (nanocomposites) have been used for tires applications for more than five decades. The clay containing composites are used in

• Automotive • Packaging • Health Care • Consumer Products • Flame Retardants • Manufacturers

Similarly they have many more uses in the field of materials technology.

2. EXPERIMENTAL

2.1. Synthesis of cobalt ferrite nanoparticles

2.1.1. Materials The materials that we used during the synthesis of cobalt ferrite nanoparticles (CoFe2O4) were cobalt chloride hexahydrate (CoCL2.6H2O), ferrous sulfate heptahydrate (FeSO4.7H2O), sodium hydroxide (NaOH), and potassium nitrate (KNO3). These chemicals were used the same as we received from the market. Highly purified water (Millipore MILLI-RO 4) with a resistivity of >10 was used to make the solutions of metals, sodium and potassium salts. An aqueous alkaline solution was used to clean all the reaction vessels from inside. After that they were cleaned with purified water.

2.1.2. Batch synthesis

The cobalt ferrite nanoparticles were prepared according to the ‘chemie-douce’ method. An aqueous solution of cobalt chloride and iron sulfate was made with measured amount of both the salts in miliQ water. The metal salt solution was made 5L in a reactor. The molar ratio of iron to cobalt was always 2.0. It was heated to 94 ºC approximately . The solution was stirred continously during heating to homogenise it. The other aqueous solution of sodium hydroxide

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and potassium nitrate (5L) was also heated to 90 ºC in another reactor. Then the metal salt solution at 94 ºC was mixed in the sodium and potassium solution. After mixing both the solutions, a black precipitate of nanoparticles was formed imidiately. The two concentration ratios – [Me²+]0/[OH-]0 and [Me²+]M/[OH-]M – are defined.

The reactor with both the solutions was kept on heating with continues stirring. The temperature was maitained between 90 ºC to 96 ºC. A cooling system was also attached to the reactor to avoid the solution to boil. This setup was kep for minimum 3 hours. After that this solution of nanoparticles was transferred to a clean vessel. This vessel was filled to 25 L of deionized water. The reaction vessel was put on the 2.3 T magnet to retain the nanoparticles over night. The vessel was emptied of water and filled again with deionized water and put on the magnet again. The synthesis was repeated for 4 to 5 times. Then the nanoparticles were taken out and poured in beaker to dry them.

Almost 5 batches were made. Two were made in the same way as the process is described above. The rest were made in slightly diffrent way. In this way two diffrent solutions of metal salts were made and heated separately. Then these two solutions were poured in the heated sodium hydroxide and potassium nitrate solution. All the process was repeated as in above case.

2.2. Surface modification of nanoparticles

2.2.1. Materials Tetraethoxysilane was used for the silanization of nanoparticles, and it was of reagent grade purity (98%) and used as received. The silanization reaction was performed in a solution of water and methanol. Concentrated acetic acid was used as catalyst for the hydrolysis reaction.

2.2.2. Silanization After particle synthesis the particles have been surface modified. This phenomenon is known as silanization. When particles have been dried prior to the surface modification step, then its called dry silanization Wet silanization is when the particles have been used directly after synthesis without drying them. The reactor was cleaned very carefully in the alkaline bath to make sure that reaction goes safely and accurately. The reactor was then silanized. It was done by filling the reactor with 90 % of deionized water and 10 % of methanol. 3 to 4 drops of acetic acid were added to make the solution acidic and to lower its PH to 4.7. The solution was homogenized by stirring it continuously. This process was kept for 45 minutes. Then the reactor was cleaned and dried in the oven for 1 hour. The reactor was silanized completely when we took it from the oven. The silanized reactor was taken and 75/25 vol. % water/methanol was used. In the reactor 6.5 L of water and 2.5 L of methanol were added. The solution was adjusted to the PH= 4.7 by adding 3 to 4 drops of acetic acid. The solution was vigorously stirred at 240 rpm for 20 min before adding the 1 L of nanoparticles from one of the beakers. After adding 1 L of nanoparticles in the water/methanol solution, it was stirred for 30 min. Then 100 ml of tetraethoxysilane (98 %) was added into the solution. The reactor was sealed with the aluminum foil completely and left for 2 days with stirring. A 100 ml sample of silanized

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nanoparticles was taken. The nanoparticles were cleaned with ethanol completely and then were analyzed in TEM. The next 100 ml of silane was added after taking first sample for testing in TEM. The solution was left for stirring for another 2 days and then 100 ml of sample was taken again and analyzed in TEM. The same procedure was repeated for 4 times and analyzed in TEM. The solution was then put on magnet for one day and nanoparticles were retained in the bottom. The clean water and methanol solution was disposed off. The particles were completely hydrophilic at this stage. Another solution of 75/25 vol. % of water and methanol was prepared in the reactor. The solution was stirred vigorously stirred for some time to make methanol dissolved completely in water. PH of the solution was again made 4.7. Hydrophilic nanoparticles were then added in the solution of water and methanol to make them hydrophobic. A 400 ml of Methyl-trimethoxysilane were added into the solution. The reactor was sealed and solution was kept on stirring over a weekend. The 100 ml sample of nanoparticles was taken from the solution for analyzing in TEM. All 5 samples that were taken for TEM analysis were kept for drying. The hydrophobic nanoparticles were retained and rest of the clean solution was poured off. The hydrophobic nanoparticles were kept in the oven for 2 to 3 days to dry them completely. The uncoated nanoparticles were also kept in the oven to dry them as well for the same period. Both hydrophobic and uncoated dried nanoparticles were then grinded 2 times to make them well separated from each other and less agglomerated. The particles were saved in two separate bottles for the later mixing with polymers.

2.3. Ultrasonic etching

For ultrasonic etching three samples of finally coated nanoparticles were taken. Each sample was containing 2 g of coated particles and each of them were put in three different beakers containing 150 ml of 2.5M NaOH solution each. The amplitude of ultrasonicator was kept on 40 %. The sample was processed in ultrasonicator for 40 minutes. Then it was cleaned 4 to 5 times with ethanol by putting magnet under the beaker. Similar process was repeated with all three samples. The cleaned samples were then dried and weighed again to find out the surface coating of the nanoparticles.

2.4. Grinding and mixing

2.4.1. Grinding of polymers

Four different Polymers which were used for grinding and then finally mixing with magnetic nanoparticles were.

• Polyethylene (MDPE)

• Polypropylene (PP)

• Polycarbonate (PC)

• Polyamide 6 (PA6)

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The polymers were grinded in small grinding mill. The polymers were first kept in liquid nitrogen for the period of 20 to 30 minutes until they were almost freezed in it. The polymers were then poured in the grinding mill very slowly so that they do not melt inside the grinder. MDPE and PP were the softer ones and they were grinded easily, but PC and PA6 were the harder polymers so they took some time to grind. All the powders of grinded polymers were preserved separately for further use.

2.4.2. Mixing of polymers with magnetic nanoparticles The grinded polymers which were in the form of powder were mixed with hydrophobic (coated) nanoparticles using twin extruder. The nanoparticles were mixed with 2% by weight with polymers. The magnetic nanoparticles were mixed with the ratio of 0.5 wt. % to polymers. MDPE and magnetic nanocomposite were mixed first in a bag and temperature for zone 1 , 2 and 3 of bra bender were set on 150 ºC, 160 ºC and 170 ºC respectively. The mixture of MDPE and magnetic nanoparticles was blended at the speed of 25. The blended material of MDPE and nanoparticles was then cut into pellets in the pelletizer. The same procedure was followed for mixing of Polypropylene. The temperatures for the three zones were set to 210 ºC, 230 ºC and 240 ºC. The speed was set to 40. The blended material was cut into pellets similarly. Since Polycarbonate is tough, dimensionally stable, transparent thermoplastic and it has higher Tg. So because of its unique properties we had to do some changes with the parameters. The temperature of the three zones was set to 280 ºC, 265 ºC and 250 ºC. The speed was maintained at 40 and the blended material was finally cut into pellets. Polyamide 6 is also harder thermoplastic so the temperature settings were made like 200 ºC, 210 ºC and 220 ºC for the three zones. Pellets of blended material were made with same method as made before. Summery of all the variables during blending can be seen in the table below; Polymer Zone 1 (ºC) Zone 2 (ºC) Zone 3 (ºC) Speed MDPE 150 160 170 25 PP 210 230 240 40 PC 280 265 250 40 PA6 200 210 220 30 Table 2.4.2 Different parameters during blending of different polymers and nanoparticles.

2.5. Hot pressing The pellets of blended materials of all four polymers were hot pressed using hot press. The thickness of the pressed material was maintained to 2 mm. Different parameters are mentioned below (See Table 2.3.3)

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Polymer Temperature (ºC) Pressure (tons) Time (min) MDPE 150 15 5-7 PP 210 20 5-7 PC 230 20 10 PA 6 220 20 10

Table 2.4.3 Different parameters during hot pressing of blended materials.

3. RESULTS AND DISCUSSION

3.1. Particle synthesis

A sample of cobalt ferrite nanoparticles was taken and analysed in the TEM. They were in good shape and somewere lying on each other becuase they were not coated yet (SeeFigure2.1.2(a),(b)). The minimum size of nanoparticles noted was 50 nm and maximum size was 200 nm. The shape of nanoparticles can be controlled by adjusting the particles growth, which is done by varying the temperature. Higher temperatures enhance the growth rates and it favours the formation of spherical particles as a result of less selective crystallographic growth direction.

(a) (b)

Figure 3.1.1(a), (b) The veiw of nanoparticles in TEM

The figure 3.1.1(a),(b) shows the samples of particles which were taken after 3 hours of digestion followed by washing and magnetic decanation. Most of the particales were spherical and round shaped cubic single particles. Half of the particles were decanated with magnet in a beaker to let the dry and the rest half of the particles were used for further use when they were surface modified.

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3.2. Surface modification of nanoparticles

In the above section (3.1) the particles have been synthesized prior to surface modification. In this section (3.2) the particles have been surface modified in the direct connection to synthesis which means the particles were not dried until after the surface coating with a surface modification agent. The coating agent is called silane and this process of coating with silane is known as silanization.

After four step process of silanization using tetraethoxysilane (98%) the particles became hydrophilic in nature. Tetraethoxysilane when added to methanol and water solution, it makes the nature of particles hydrophilic (water loving). Hydrophilic (water-loving) solutes tend to break down the hydrogen bonded structure. In fact, methanol exhibits both hydrophobic and hydrophilic character. It has an —OH radical which can bond readily with the surrounding water molecules. It also has a methyl radical which cannot bond and is hydrophobic. The methanol molecule polarises the local water arrangement, but in a manner which is consistent with preserving the water hydrogenbond network. It also allows the water to be fully hydrogen bonded with the —OH radical while forming the expected hydrogen-bonded net around the methyl radical.

When tetraethoxysilane is added to methanol water solution it disolves in the solution make the particles hydrophilic (water loving). The reason for using 4 steps was to build a thick coating that would work as a spacer between the particles. It is like if we take 2 magnets, the magnetic forces are strongest when the magnets are really close to each other, small spacers between the particles thus make it easier to disperse them because the particles are not magnetic surface to magnetic surface.

• Coating step 1

After each single step of coating with tetraethoxysilane we took a sample and analyzed in TEM. A considerable coating was observed after each coating step (See Figure 3.2 a, b, c,d,e).

Figure 3.2 a Nano particles view after first coating step in TEM

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• Coating step 2

Figure 3.2 b Nano particles view after second coating step in TEM

• Coating step 3

Figure 3.2 c Nano particles view after third coating step in TEM

• Coating step 4

Figure 3.2 d Nano particles view after fourth coating step in TEM

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• Final step coating

After the four step coating with tetraethoxysilane, Methyl-trimethoxysilane was used to make the nanparticles hydrophobic in nature. Traditionally, hydrophobic effects are associated with having low solubility in water. The methyl-trimethoxysilane reacts with water and hyrolysis process occur. The hydrolysed product then condenses by reacting with hydrolysed material resulting in polymerized product. The nanoparticles after this final silanization step were hydrophobic in nature. Actually plastics are carbon based and often more hydrophobic than hydrophilic. We coated the particles with the methyl functional silane to make them more like most plastics. After this coating step they became hydrophobic. These finally coated particles were then analyzed in TEM. They were reasonably well coated at that point (See figure 3.2 e)

Figure 3.2 e Nano particles view after final coating step in TEM

3.3. Coating thickness by ultrasonic etching

The ultrasonically etched and dried nanoparticles were weighed again. They lost some weight because of the ultrasonic etching. The thickness of the coating was found out by taking the mean of the coating thickness of all three samples (See Table 3.3)

Samples Weight before UE (g)

Weight after UE (g)

Weight Difference (g)

Wt.(%) of coating

01 2.002 1.860 0.142 7 % 02 2.001 1.832 0.169 8 % 03 2.006 1.825 0.181 9 %

Table 3.3 Coating thickness of different samples

From the table 3.3 it can be seen that the mean weight percentage of surface coating thickness was 8 %. The amount of ferrites present was 92 %.

3.4. Visible colour differences between the nanoparticles

The inspection of dried and well grinded nanoparticles showed that there was also a colour difference between coated and non coated nanoparticles. The uncoated nanoparticles were

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dark brown in colour while the coated particles were light brownish in colour. This colour difference was clearly note able after grinding them completely. This colour difference occurred because of the silane coating on the particles. It was observed in the TEM that the ferrite nanoparticles appeared dark and the surrounding layers of less darkness were the coatings.

3.5. Water interaction difference of the particles after drying

The effect of the surface coating was examined by spreading the particles on the surface of water. The uncoated particles sank immediately as agglomerates and did not spread horizontally on the water surface or at the bottom of the beaker. This happened because they have hydroxyl groups and they interact strongly with the water molecules. This was the result of particles being hydrophilic in nature.

The particles coated in the water and methanol solutions during silanization showed the distinct behaviour because of the hydrophobic character of the methyl-trimethoxysilane. Because of the presence of methyl function particles were repelled strongly by water even in case of putting a small magnet under the beaker. Even when the magnetic force was increased, the particles were first agglomerated and then submerged down to the bottom of the beaker.

The coupling agent reacts to the surface of the particles with hydroxyl groups with alkoxysilanes in following steps:

1. Hydrolysis of silane molecules to the silanols 2. Condensation of silanols to oligosilanols 3. Adsorption of oligosilanols onto the surface of the substrate 4. Condensation of the oligosilanols with the surface hydroxyl groups to form Si-O-M

bonds and water.

The hydrolysis step is energy driven and its equilibrium depends on the amount of water in the solution. In case of large excess of water, the hydrolysis of methoxy groups occur essentially irreversibly. The silanols are known to condense with each other and with other methoxy groups to produce oligomers. These reactions are normally catalyzed by acids or bases and proceed with minimum rate of pH 7. It has been suggested that one cause for the variation in deposition of the oligosilanols on the particles is related to the competing the polymerization and adsorption of the silanols, which vary with the amount of the alcohol in the solution. It is also suggested that there can be another reason for the variations in the deposition of the oligosilanols, which is colloid stability in alcohol/water solution.

3.6. Mechanical properties of nanocomposite systems

3.6.1. Tensile testing

Tensile testing on the mixed materials (Nanocomposites) was done on an instrument called Instron. Hot pressed samples of four different polymers mixed with nanoparticles were tested. Five samples of each polymer mixed with nanoparticles were tested and five sample of each

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pure polymer were tensile tested. The results of these were compared to see the improvement in the mechanical properties of nanocomposite materials. The humidity was kept 50% throughout the tensile testing. The last cell of 10 kN was used during all the tests. The length of each specimen was kept at 50 mm.

• MDPE

The mixing of nanoparticles with medium density polyethylene was quite uniform. The colour of mixed material was dark brownish. The tensile testing for MDPE nanocomposite was done at the strain rate of 15 mm/min.

The results of tensile testing for MDPE mixed material and MDPE pure from instron can be viewed and analyzed from the plots and the tables below;

3.6.1 a1 Plot of tensile stress Vs Extension for MDPE mixed material

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Table 3.6.1 a1 Tensile testing data for MDPE mixed materials obtained using instron

The tensile behaviour of pure MDPE can be seen below (See Figure 3.6.1a2)

Figure 3.6.1 a2 Plot of tensile stress Vs Extension for MDPE pure material

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Table 3.6.1 a2 Tensile testing data for MDPE pure obtained using instron

The plot of MDPE mixed material (See Figure 3.6.1a) shows that when nanoparticles were mixed in medium density polyethylene they become more ductile and the mixing of nanoparticles was quite uniform in these samples.

If we compare the extension behaviour of both MDPE mixed materials and pure MDPE, we can see that in the mixed materials extend uniformly. All five MDPE nanocomposite samples extend more than 1000% of their original length while the in the pure MDPE samples some of them extend to 1000% but some even break before that. The mean tensile stress at maximum load for nanocomposite was 20.58 MPa while in case of pure MDPE it was 19.83 MPa. The energy at break of MDPE nanocomposite was 141.82 J while in case of pure MDPE it was 140.40 J. This shows that the MDPE nanocomposites were stiffer and stronger than MDPE pure samples. The Mean modulus E for nanocomposite was 94 MPa. The standard deviation of modulus E was higher in case of nanocomposite than pure materials.

So overall the mechanical properties of MDPE improved after mixing cobalt nanoferrites with them. From the plot it can be seen that MDPE nanocomposite have good stiffness, more strength and improved ductility.

• Polypropylene

The tensile testing of polypropylene was done at strain rate of 15 mm/min. All other parameters were maintained the same as we used in MDPE. The colour of polypropylene mixed materials was dark brown.

The results of tensile testing for polypropylene mixed material and pure polypropylene can be seen and compared below;

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Figure 3.6.1 b1 Plot of tensile stress Vs Extension for Polypropylene mixed material

Table 3.6.1 b1 Tensile testing data for Polypropylene mixed material obtained using instron

The tensile behaviour of pure MDPE can be seen below (See Figure 3.6.1b2)

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Figure 3.6.1 b1 Plot of tensile stress Vs Extension for Polypropylene pure material

Table 3.6.1 b2 Tensile testing data for Polypropylene pure material obtained using instron

Mixing of nanoparticles with polypropylene worked quite well. The particles distribution was not as uniform as it was in polyethylene but still we got good results. From the plot (See Figure3.6.1b1) it can be seen that the polypropylene nanocomposite extent from 10% to 14 %, which shows that they have good ductility. They showed better stiffness and strength than the pure polypropylene materials. The mean maximum load for polypropylene mixed material was 570.68 N and for pure polypropylene it was 557.19 N, which shows that mixed material

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had better strength than the pure material. Similarly the mean tensile stress at maximum load of mixed material is higher than the pure material. Also the mixed polypropylene materials also have higher modulus which was 690.03 MPa than pure polypropylene which was noted 627.21 MPa.

• Polyamide 6

All the parameters were used the same as they were used in case of other two plastics. Since polyamide 6 is a harder and not very ductile plastic so it took very less time to the tensile testing as compared to other two plastics.

The results of tensile testing for polyamide 6 mixed material and pure polypropylene can be seen and compared below;

Figure 3.6.1 c1 Plot of tensile stress Vs Extension for Polyamide6 mixed material

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Table 3.6.1 c1 Tensile testing data for Polyamide6 mixed material obtained using instron

Figure 3.6.1 c2 Plot of tensile stress Vs Extension for Polyamide6 pure material

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Table 3.6.1 c2 Tensile testing data for Polyamide6 pure material obtained using instron

The mixing of nanoparticles with polyamide6 was not good. Strange results were obtained first during the mixing and then after tensile testing. After the mixing of nanoparticles with polyamide6 we got the mixed material with not uniform distribution of nanoparticles and we got some air gaps. The air gaps formed during hot pressing, and they can be formed because of air present between the pellets or can be because of the low temperature in the hot pressing.

The results obtained were strange because it didn’t improve their strength at all. The plot (See Figure3.6.1c1) of polyamide6 mixed materials shows that only their yielding strength was improved. They ductility increased if we compare them with the pure polyamide6 material behaviour. The mixed material extends from 15% to 20% of the original length of the samples, which was more than the pure PA6 samples. Stiffness of the nanocomposite did not increased neither their strength in comparison to pure PA6 material.

• Polycarbonate

All the parameters were used the same as they were used in case of all other plastics.

The results of tensile testing for polyamide 6 mixed material and pure polypropylene can be seen and compared below;

Figure 3.6.1 d1 Plot of tensile stress Vs Extension for Polycarbonate mixed material

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Table 3.6.1 d1 Tensile testing data for Polycarbonate mixed material obtained using instron

Figure 3.6.1 d2 Plot of tensile stress Vs Extension for Polycarbonate pure material

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Table 3.6.1 c2 Tensile testing data for Polycarbonate pure material obtained using instron

The blending of nanoparticles with polycarbonate was worked not as well as we expected and the results were not even as good as polyamide6. The nanoparticles distribution was not so uniform in polycarbonate. A lot of air gaps were present in the blended and pure material as well. The blended polycarbonate material extended from 2% to 5% which was even less than the pure polycarbonate which were 4% to 8%. The stiffness, strength and ductility did not increase in mixed polycarbonate materials. The modulus of mixed material was also lower than the pure material. In short the mixing of magnetic nanoparticles did not work well in the harder materials such as polyamide6 and polycarbonate.

3.7. Microwave testing

The nanocomposites were microwave tested to see melting behaviour of them as compared to pure polymers. All the nanocomposites were kept in microwave for certain period time and their temperature after this time was noted. Similar process was repeated for pure polymers and results if these were then compared. The microwave was set to 900 W. A ‘’K ‘’ type thermocouple was used to measure the temperature values. The temperature measurements for nanocomposites materials can be seen in the table below;

Nanocomposite Time (min.) Temperature (ºC) MDPE 4 43 Polypropylene 4 35 Polyamide6 2 52 Polycarbonate 2 35

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Table 3.7 a Temperature recoded of nanocomposites in microwave after specific time

The temperature measurements for pure polymers can be seen the table below;

Nanocomposite Time (min.) Temperature (ºC) MDPE 4 44 Polypropylene 4 39 Polyamide6 2 36 Polycarbonate 2 38

Table 3.7 b Temperature recoded of pure polymers in microwave after specific time

When MDPE were microwave tested, the MDPE nanocompsite material was heated less than the pure MDPE sample. This shows that the nanocomposite showed more thermal resistivity than the pure material. The same pattern was observed in case of polypropylene and polycarbonate. Their composites showed more thermal resistivity than the pure materials. In polyamide 6 we got the opposite behaviour in comparison to other polymers. The temperature polyamide6 composite material was more than the pure polyamide6 unlike to other polymers.

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4. CONCLUSIONS

The cobalt ferrite nanoparticles were prepared according to the ‘chemie-douce’ method. We have seen that most of them were round and square in shape. The nanoparticles were then first coated with tetraethoxysilane and finally with methyl tri-methoxysilane by silanization process. The particles were hydrophobic (hate water) in nature after final coating. The coating thickness was found out by ultrasonic etching which was 8 %. We coated them to make hydrophobic because later we have mixed them with plastics which are more hydrophobic than hydrophilic. The hydrophobic magnetic nanoparticles were then mixed 2% by weight with Polyethylene, Polypropylene, Polyamide6 and Polycarbonate. We got good mixing in softer plastic i.e. Polyethylene and Polypropylene as compared to harder plastics i.e. Polyamide6 and Polycarbonate. The mixed materials were then hot pressed. Tensile testing results of nanocomposites showed that mixing of magnetic nanoparticles was quite good and uniform in softer polymers as compared to harder polymers. MDPE nanocomposites extended almost 1000% to their original length and polypropylene nanocomposites 10% to 14%, which was more than the harder polymer composites. Mixing of nanoparticles with MDPE and polypropylene increased their stiffness, ductility and strength. Mechanical test results for polyamide6 and polycarbonate showed that mixing magnetic nanoparticles did not work so well with these plastics. With microwave testing of MDPE and polypropylene composites showed that their thermal resistant was improved as compared to their pure materials. Polycarbonate composite also showed more thermal resistant than its pure material. In case of polyamide6 we got opposite results.

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5. FUTURE WORK

1. The future work includes tailoring of the ferrite composition and magnetic character of

the nanoparticles to match with specific frequencies used for various electromagnetic equipments so that the composites can be efficient absorbers.

2. More work needs to be made with regards to how the coating thickness and functional

unites of the coatings affect the dispersion of the nanoparticles.

3. Smaller sized nanoparticles can be used for applications where optical transparency of the composites is needed.

4. New materials can be developed where the ferrite nanoparticle filler is actually used to generate heat inside polymer matrices when the matrices are irradiated. This heat can be used to trigger polymerization reactions with temperature sensitive curing agents. You can make glues that become hard when you irradiate them with microwave radiation.

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6. ACKNOWLEDGEMENTS

First of all, I would like to thank Richard Olsson for his continues support and guidance throughout this work. It has been great experience working with you, and always giving me your feedback about my work. I would also like to thank Bruska Azdhar and Johanna Möller for helping me to work with all the instruments that I had been using and giving me helpful ideas regarding mixing of nanoparticles with polymers and with tests. I would also thank here Thomas Blomfeldt for giving me good ideas for my work and specially helping me out with tensile testing. Finally I express my sincere gratitude to Professor Ulf W. Gedde for his guidance, support and discussions during my work. I am anxiously looking forward to work with you as graduate student.

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7. REFERENCES

1. Synthesis and Characterization of Cubic Cobalt ferrite Nanoparticles by R. T. Olsson, G. Salazar-Alvarez, M. S. Hedenqvist, S. J. Savage, M. Muhammed, U. W. Gedde

2. Controlled Synthesis of Near-Stoichiometric Cobalt Ferrite Nanoparticles by Richard T. Olsson, German Slazar-Alverez, Mikael S. Hedenqvist, Ulf W. Gedde, Fredrik Lindberg, Steven J. Savage

3. Polymer nanocomposites by Chai-jing Chou, A. E. Read, E. I. Garcia-Meitin, C. P. Bosnyak

4. Magnetic Nanocomposite Materials for High Temperature Applications by Frank Johnson, Amy Hsaio, Colin Ashe, David Laughlin, David Lambeth, Michael E. McHenry

5. A Critical Comparison of Ferrites with Other Magnetic Materials by Home office and factory

6. Mechanical properties of polymers, viscoelastic properties by Mikael Hedenqvist 7. Soft magnetic nanocomposites by L. K. Varga

8. Polymers by David A. Katz

http://www.chymist.com/Polymers.pdf

9. Thermoplastics http://www.pslc.ws/mactest/plastic.htm

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