REINFORCEMENT of SYNTACTIC FOAM with Sic NANOPARTICLES

REINFORCEMENT OF SYNTACTIC FOAM WITH SiC NANOPARTICLES

by Debdutta Das

A Thesis submitted to The Faculty of The College of Engineering and Computer Science In Partial Fulfillment of the Requirements for the Degree of Master of Science

Florida Atlantic University Boca Raton, Florida December 2009

ACKNOWLEDGEMENT

I would like to thank Dr. Hassan Mahfuz for his direction, assistance and guidance in the preparation of this thesis. I also wish to thank the members of the supervisory committee,

Dr. Palaniswamy Ananthakrishnan and Dr. Francisco Presuel-Moreno, for their valuable recommendations and suggestions.

Financial support in the form of a research assistantship from the Office of Naval

Research is gratefully acknowledged.

I would like to thank my nanolab members for their valuable support. I would sincerely thank my family for their advice and support.

Lastly, I would like to acknowledge a sincere appreciation of the above mentioned for the experience that is the foundation of future endeavors.

iii ABSTRACT

Author: Debdutta Das

Title: Reinforcement of Syntactic Foam with SiC nanoparticles

Institution: Florida Atlantic University

Thesis Advisor: Dr. Hassan Mahfuz

Degree: Master of Science

Year: 2009

In this investigation, polymer precursor of syntactic foam has been reinforced with SiC nanoparticles to enhance mechanical and fracture properties. Derakane 8084 vinyl ester resin was first dispersed with 1.0 wt% of SiC particles using a sonic cavitation technique.

In the next step, 30.0 wt% of microspheres (3M hollow glass borosilicate, S-series) were mechanically mixed with the nanophased vinyl ester resin, and cast into rectangular molds. A small amount of styrene was used as dilutant to facilitate mixing of microspheres. The size of microspheres and SiC nanoparticles were 20-30 μm and 30-50 nm, respectively. Tension, compression, and flexure tests were conducted following

ASTM standards and a consistent improvement in strength and modulus within 20-35%

range was observed. Fracture toughness parameters such as KIC and GIC were also determined using ASTM E-399. An improvement of about 11-15% was observed.

iv Samples were also subjected to various environmental conditions and degradation in material properties is reported.

v REINFORECEMENT OF SYNTACTIC FOAM WITH SiC NANOPARTICLES

LIST OF FIGURES…………………………………………………….…………….. viii

LIST OF TABLES……………………………………………………………………. xi

CHAPTER 1. INTRODUCTION…………………………………………………...... 1

1.1 Literature Review…………………………………………………………………… 5

1.2 Scope of Thesis…………………………………………………………………….. 9

CHAPTER 2. MATERIALS AND SYNTHESIS…………………………………… 11

2.1 DERAKANE 8084…………………………………………………………………. 11

2.2 Glass Microballons S series…………………………………………………………11

2.3 Styrene………………………………………………………………………………13

2.4 MEKP……………………………………………………………………………… 14

2.5 Cobalt Naphtenate…………………………………………………………………. 15

2.6 Silicon Carbide…………………………………………………………………….. 16

2.7 Synthesis of syntactic foam………………………………………………………... 18

2.71 Determination of the right ratio………………………………………………… 18

2.72 Procedure……………………………………………………………………….. 19

2.8 Density of syntactic foam………………………………………………………….. 21

CHAPTER 3. MICROSTRUCTURAL AND MECHANICAL

CHARECTERIZATION……………………………………………………………..23

vi 3.1 SEM Analysis…………………………………………………………………… 24

3.2 Mechanical characterization……………………………………………………….. 25

3.21 Compression Test………………………………………………………………. 25

3.22 Flexure Test…………………………………………………………………….. 27

3.23 Tension Test…………………………………………………………………….. 28

3.24 Fracture Toughness Test…………………………………………………………30

CHAPTER 4. DURABILITY OF SYNTACTIC FOAM UNDER

SALT WATER ENVIRONMENT ………………………….……………………….33

4.1 Moisture absorption of syntactic foam…………………………………………… 34

4.11 Environmental Exposure………………………………………………………. 35

4.2 Compression Test………………………………………………………………… 36

CHAPTER 5. RESULTS AND DISCUSSION……………………………………..37

5.1 Compression Test ASTM C-365…………………………………………………..37

5.2 Flexure Test………………………………………………………………………...40

5.3 Tension Test……………………………………………………………………….. 43

5.4 SENB Test………………………………………………………………………… 47

5.5 Durability Test……………………………………………………………………. 50

5.51 Compression test on wet specimens…………………………………………….55

CHAPTER 6. SUMMARY…………………………………………………………. 59

REFERENCES……………………………………………………………………… 61

vii LIST OF FIGURES

Figure 1: Pictographic representation of syntactic foam……………………………… 5

Figure2: SEM of syntactic foam……………………………………………………… 6

Figure3. Glass bubbles ………………………………………………………………. 12

Figure 4: SEM of S38 microspheres…………………………………………………..12

Figure 5: Molecular structure of styrene……………………………………………....14

Figure 6: Molecular alignment in MEKP……………………………………………. 15

Figure 7: Co-NAP……………………………………………………………………. 15

Figure 8: SiC crystals………………………………………………………………… 17

Figure 9: Flow Diagram……………………………………………………………… 19

Figure 10: Homogenization ………………………………………………………….. 20

Figure 11: Sonication…………………………………………………………………. 20

Figure 12: Mechanical mixing procedure ……………………………………………. 21

Figure 13: Casting in the mould………………………………………………………. 21

Figure 14: Two phased structure ……………………………………………………... 22

Figure 15: Three phased structure…………………………………………………….. 22

Figure 16: Scanning electron microscope (SEM)…………………………………….. 23

Figure 17: NEAT SPECIMEN ……………………………………………………… 24

Figure 18: NANO SPECIMEN………………………………………………………. 24

Figure 19: Orientation and size of specimen………………………………………….. 26

viii Figure 20: Dimension of flexure specimen……………………………………………. 27

Figure 21: Dimension of the tensile specimen……………………………………….... 29

Figure 22: Testing Procedure………………………………………………………….. 30

Figure 23: Dimension of the SENB specimen………………………………………… 31

Figure 24: Test Setup………………………………………………………………….. 32

Figure 25: Test Setup for compression…………………………………………………36

Figure 26: Stress –strain curve for compression test………………………………….. 38

Figure 27: Typical formation of crack during compression...... 39

Figure 28: Tested specimen…………………………………………………………… 40

Figure 29: Load displacement curve for flexure test………………………………….. 41

Figure 30: Tested Specimen…………………………………………………………... 42

Figure 31: Failure sequence of the syntactic foam……………………………………. 42

Figure 32: Graphical representation of Tension Test…………………………………. 44

Figure 33: Modulus of strength………………………………………………………...45

Figure 34: Tested Tensile Specimen………………………………………………….. 46

Figure 35: Load transfer on microspheres upon tensile loading……………………… 46

Figure 36: SEM picture of the fracture surface of the syntactic foam………………... 49

Figure 37: Graphical representation of SENB Test……………………………………47

Figure 38: Graphical analysis of water uptake……………………………………….. 53

Figure 39: Neat specimen in SWT……………………………………………………. 54

Figure 40: Comparison of Neat and Nano specimen in SW………………………….. 54

Figure 41: Comparison of Neat and Nano specimen in SWT………………………... 54

Figure 42: Comparison of Neat and Nano in HT…………………………………….. 55

ix Figure 42: Stress strain curves of compression test on wet specimens…….………. 57

x LIST OF TABLES

Table 1: Resin Properties…………………………………………………………. 11

Table 2: Microspheres Property…………………………………………………... 13

Table 3: Dilutant Property………………………………………………………… 14

Table 4: Nanoparticles Property…………………………………………………... 18

Table 5: Foam density…………………………………………………………….. 21

Table 6: Compression test results………………………………………………… 37

Table 7: Average of compressive test results…………………………………….. 38

Table 8: Determination of flexural test results…………………………………… 41

Table 9: Average of flexural test results………………………………………….. 41

Table 10: Representative of tension test results…………………………………... 43

Table 11: Average of tension test results………………………………………….. 43

Table 12: Fracture toughness test results…………………………………………. 48

Table 13: Average of SENB test results………………………………………….. 48

Table 14: Water uptake in week 1&2…………………………………………….. 51

Table 15: Water uptake in week 3&4…………………………………………….. 51

Table 16: Demonstration of moisture retention capacity………………………… 52

Table 17: Water uptake% in foams………………………………………………. 53

Table 18: Compression Test Results for wet neat specimens……………………. 56

Table 19: Compression Test Results for wet nano specimens…………………… 56

xi Table 20: Comparison under compressive loading……………………………57

xii CHAPTER 1: INTRODUCTION

The word syntactic is derived from the word “syntax” whose implied meaning is to piece together. Syntactic foams are examples of particulate composite materials.

According to American Society for Testing and Materials [1], syntactic foam is defined as a ―material consisting of hollow spherical fillers in a resin matrix. As per its definitions syntactic foam consists of hollow glass spheres known as microballoon that are coagulated in a binder phase, usually in a polymeric resin system. Syntactic foam are classified as composite materials which are synthesized by filling a metal, polymer or ceramic matrix with hollow particles called microballons.The hollow particles present in syntactic foams effectively results in lowering the density ,increasing the strength, lowering the thermal expansion coefficient and in special cases it also results in radar or sonar transparency. The matrix resin can be chosen from a wide range of thermoplastic and thermosetting resins depending on the service conditions. Similarly cenospheres of polymer, ceramic or metal can be chosen. There are other parameters which can be adjusted like the volume fraction of the matrix and cenospheres in the structure. These are two significant methods for changing the density of syntactic foams which directly influences their properties.

Composite material can be classified as a construction material that consists of two or more different materials; the objective here is to combine the various advantages of the single materials in the resulting material, and to eliminate their disadvantages [2] sign

1 work in recent years has resulted in specific parameters for materials of construction.

Sometimes it is not possible for an existing material to have all the required properties

specified which has led to development of new or improved materials. Synthetic

composite materials have been used to meet these needs in the past few decades.

Composite materials play a big role with regard to reducing weight by improving the

strength to weight ratio. An example is a fibrous composite material that has anisotropic

strength parameters. By optimizing the directional orientation of the reinforcement, the

part weight is usually minimized. Some other advantages composite materials may have

are high bending stiffness, corrosion resistance, ability to be molded to near finished part

dimensions, and good thermal insulation properties. Currently the primary areas of

application of composite materials are navy, aerospace, motor sports, automobile

industry, military, and sports equipment. The primary reason for these applications is the

requirement of high strength to weight ratio while having the manufacturer’s ability to

absorb the higher costs of the composite development and construction.

Syntactic foams were developed in the early 1960s as buoyancy aid materials for marine

applications. One of the most prominent features of syntactic foam is its tailorability.

Spectrum of engineering applications of syntactic foams is quite broad. Initial

applications of syntactic foams dates back to the 50s in the area of naval and marine engineering where they have been used for structural elements such as hulls, ribs and decks for components at deep depth (up to 3000 m) such as submarines, submerged buoys, deep sea platforms and pipe joints, and for shielding and repairing submerged apparatuses[3]. Applications of syntactic foams are often seen in civil and industrial engineering, in construction and as an imitation of wood and marble; often they are

2 employed as core materials due to good shear stiffness and strength, fatigue and impact

resistance. Glass or carbon bubble based syntactic foams are widely used in aeronautical

and aerospace engineering as rollers of alveolate structures and as protection shields for

space vehicles and missile heads. Syntactic foams have also found its applications in

motorcar industry, where they have been used for spoilers, dashboards and roofs. They

are also employed in electronics and telecommunication fields for shielding electronic

components or cables from vibrations, electromagnetic fields, radiations and high

temperatures.

There is a wide range of microballoon which includes cenospheres, glass microballons,

carbon and polymer microballons. The most widely used and studied foams are glass microballoon epoxy, glass microballons-aluminium and cenosphere-aluminium. Syntactic foams are renowned for their high specific compressive strength, low moisture absorption and excellent damping properties. They are also used for core materials in sandwich composites for weight sensitive structural applications. Due to their broad range of mechanical properties coupled with vibration damping characteristics and their ability to be fabricated in functionally graded configurations syntactic foam are known as multi- functional composite material.

Thermosetting polymers are normally preferred as a binder due to its simpler manufacturing technologies as opposed to thermoplastic binders which have better specific mechanical properties. The properties of the resulting mechanical syntactic foams are directly in relation to filler type and also filler binding interactions. In normal instance the preferred filler geometry is spherical which allows the best packing factor and hydrostatic compression strength. The diameter of the sphere generally varies

3 between 1μm to 100 µm. The microspheres which are a basic content in syntactic foam

have a diameter in range of (1-500) µm and a wall thickness of (1-4) µm. In rare

instances macrospheres are being used instead of microspheres with diameter larger than

500 µµm. Preferences are generally given to the glass microbubbles owing to their mechanical strength, smoothness and regularity of the surface. They are also renowned for their excellent wetting characteristics and their low cost. This leads to the formation of low viscous foam with well established production procedures and low cost. One of the most important properties of the syntactic foam is their extremely low degree of water absorption. This property is solely dependent on the chemical and physical properties of the filler and binder and their volume ratio. The thermal; properties of the syntactic foams are generally dominated by the matrix characteristics. The group polyimide shows the best mechanical properties at increasing temperature. Syntactic foams are also characterized by good thermal insulation and dielectric properties. At the best instance syntactic foams have displayed uniaxial compressive strength of 100 MPa, of, hydrostatic compression strengths around 150 MPa, uniaxial tension strengths of about 25-30 MPa, tensile Young's modulus around 2500-3000 MPa. Syntactic foam exhibits quite a ductile behavior when being tested for compression. DeRuntz and Hoffman [4] reported an experimental investigation on epoxy matrix/hollow glass microspheres foam with low degree of dispersed air bubbles, tested under biaxial and triaxial stress states. The failure locus is closed in the principal stress space so that the material exhibits a finite strength triaxial compression.

Syntactic foams are well known for their ability to be designed and fabricated according to the physical and mechanical property requirement of the application. The first method

4 involves in changing the volume fraction of the matrix and the cenospheres or the glass micro balloons in the structure. The second method provides with great design flexibility as any changes in the property of the syntactic foam can be influenced by one single

parameter i.e. the internal radius of the cenospheres.

Figure 1: Pictographic representation of syntactic foam

1.1. Literature Review

In the present work syntactic foams have been manufactured using vinyl ester resin and

hollow glass borosilicate microspheres with the infusion of nano particles in it. There

hasn’t been much work done in the process of manufacturing of syntactic foam with the

infusion of nano particles [5] in it.

The objective of this research is to improve the mechanical properties of syntactic foam

with infusion of nano particles in it at a proper ratio .The presence of nano particles in the resin matrix along with the micro balloons makes the syntactic foam in a three phase structure. The nano particle used for infusion is SiC (silicon carbide) crystals of size of 30 nm (nanometer). A detailed comparison of mechanical properties has been conducted in

this present research work to evaluate the effect of nano particles on the structure of

syntactic foam. The evaluations have been done on the basis of mechanical testing

satisfying the ASTM Standards. For analyzing the structure of syntactic foam scanning

electron microscope is used. A micrographic picture of syntactic foam is shown in figure

5 2. The micrograph is representation of a cut surface of a manufactured syntactic foam

specimen. Hollow glass micro balloons embedded in the matrix resin are visible in the structure. During the fabrication of syntactic foams some air is inevitably trapped in the structure and is present as open cell structured porosity. Presence of this entrapped air also commonly known as voids makes syntactic foams three phase materials.

Figure2: SEM of syntactic foam

It has been stated earlier Solid [6] and hollow [7] glass filled particulate composites are

used as core materials in composite structures for various weight sensitive applications.

Syntactic foams consisting of hollow glass particles (microballons) in an epoxy matrix have gained significant importance as core materials in sandwich composites due to their high damage tolerance [8], low moisture absorption [9] , high specific compressive strength [10]. In order to fabricate syntactic foams with minimum air entrapment in the matrix, different fabrication techniques [11-12] were studied by researchers. It was found

that the optimum resin-microballons interface can be achieved with minimum void

content.

Researchers have carried out several experiments on the mechanical behavior of syntactic

foams. Bunn et al. [13] fabricated syntactic foams using phenolic microballons and

6 studied the compressive properties with the variation in microballons volume fraction.

Experimental results revealed that the compressive strength of syntactic foams increased

with a decrease in the microballons volume fractions. Gupta et al. [14] fabricated

syntactic foams using glass microballons and studied the compressive properties with the

variation in microballons volume fraction. They found that the compressive strength of

syntactic foams fabricated with glass microballons was higher compared to syntactic

foams fabricated with phenolic microballons. Lin et al. [15] reported that the compressive

strength of syntactic foams decreased with an increase in the glass microballons volume

fraction. D‘Almeida [16] studied the effect of the microballons size on the compressive

properties of syntactic foams. D‘Almeida concluded that syntactic foams incorporated

with smaller size microballons show lower yield strength and modulus than syntactic foams incorporated with larger size microballons. Gupta et al. [17-18] studied the effect of density variations of microballons on the compressive properties of syntactic foams. In order to understand the effect of radius ratio (η) and aspect ratio on the compressive properties of syntactic foams, Gupta et al. conducted compressive tests on syntactic foams fabricated using S22, S32, S37, S38, and K46 microballons. Gupta et al. also found that the compressive strength and modulus of syntactic foams incorporated with lower radius ratio (η) microballons was higher compared to the syntactic foams with higher radius ratio (η) microballons. Lower density foams show a larger stress-plateau but lower strength compared to higher density foams. Larger stress plateau is a typical characteristic of high energy absorption materials. On contrary, higher density foams show high strength and lower plateau region. Kim et al. studied the compressive failure mechanism of syntactic foam having varying concentration of resin [19]. Kim et al.

7 concluded that longitudinal splitting and layer crushing of specimen takes place under compression. Effect of microballons radius ratio (η) and volume fraction on the tensile properties of syntactic foams was studied by Gupta and Nagony [20]. They found that syntactic foams fabricated with high density microballons exhibited high tensile strength and modulus. It was also found that the tensile strength and modulus values of syntactic foams decrease with an increase in volume fraction for similar density microballons.

Effect of microballons volume fraction on the tensile behavior of syntactic foams was studied by Kishore et al. [21]. Kishore et al. concluded that the tensile modulus and strength increase linearly with a decrease in the microballons volume fraction. As syntactic foams are used in naval applications, understanding the effect of moisture on compressive properties of syntactic foams is important. Gupta and Woldesenbet performed hygrothermal studies [22] on syntactic foams to understand the effect of moisture on compressive properties. They concluded that the modulus value decrease with an increase in the moisture content of the specimen. It was also observed that the compressive yield strength decrease with an increase in temperature.

Flexural properties of syntactic foam sandwich structures were studied by Gupta and

Woldesenbet. They have performed three and four point bending and short beam shear test on syntactic foam core sandwich structures [23]. From their studies, Gupta and

Woldesenbet concluded that the values of core shear stress and facing bending stress obtained in three-point and four-point tests were independent of wall thickness of microballons. In short beam shear test, the core shear stress and facing bending stress were found to decrease with an increase in the wall thickness of microballons. Flexural studies performed with microballons volume fraction variation [24] concluded that the

8 fracture toughness and flexural strength of syntactic foams sandwich structure decrease

with an increase in the volume fraction of microballons. Kim and Kemis [25] tested

syntactic foam sandwich structures for flexural properties and concluded that the flexural

modulus decreased with an increase in microballons volume fraction.

1.2 Scope of Thesis

The commercially available syntactic foams have proven to be very successful in

submersible applications however there is a continuous quest for further improvement of

mechanical and fracture properties of syntactic foams; so that as a weight saving and

specific strength can be enhanced more.

Nanotechnology is an efficient technique to upgrade the properties of materials in the fields of materials science. One additional advantage is that in order to achieve this it requires only a small amount of nanoparticles, typically 1–3%wt dispersed into the virgin materials and. When these nanoparticles are dispersed in a polymer they synergistically combine the properties of both the host polymer matrix and discrete nanoparticles therein

One of the ways to enhance the properties is to disperse nano particles in the matrix as discussed in the previous sections. We have chosen for example SiC nano particles as they have been successfully used in various composite materials to enhance mechanical and thermal properties. Our intent in this investigation is to employ nanoparticles inclusion as a mechanism to improve properties of syntactic foams.

In this thesis it is proposed to disperse a low weight loading (1% wt) of SiC nanoparticles into the vinyl ester resin. This vinyl ester resin will then be reinforced with microballons to manufacture the syntactic foam. Vinyl ester has been selected intentionally since this particular resin has minimum degradation under salt water

9 environment. Reason for selecting SiC nanoparticles is that these nanoparticles have

Reason for selecting SiC nanoparticles is that these nanoparticles have significantly improved mechanical and fracture properties of polyurethane foam in previous studies

[26]. Once the foam is made we intend to perform compression, flexure, and tension and

SENB fracture test to evaluate any improvement or deterioration in properties of the foam. The foam specimen will also be subjected to salt water environment to determine the moisture uptake in a fixed period of time. Selected specimens will also be tested for water absorption and compared with dry specimens. The overall goal of this investigation is to see if SiC nano particle inclusion can improve durability and mechanical properties of vinyl ester based syntactic foams.

Discussion of material synthesis, micro structural and mechanical characterization, durability under salt water environment, results and discussion are presented in the thesis.

10 CHAPTER 2. MATERIALS AND SYNTHESIS

In the present research work the materials which are being used for the manufacturing of

syntactic foam are DERAKANE 8084 (vinyl ester resin), 3M hollow glass borosilicate

microspheres namely S series, and STYRENE is being used as a dilutant. The detailed

description of the above specified materials is given in below.

2.1. DERAKANE 8084

DERAKANE 8084 Resin is an elastomeric-modified epoxy vinyl ester designed to offer

increased adhesive strength, superior resistance to abrasion and severe mechanical stress,

while giving greater toughness and elongation. This resin is chosen to use as a primer to

prepare a substrate surface (steel or concrete) for application of a corrosion resistant

lining. Typical liquid resin properties of DERAKANE 8084 are given below in table 1.

TABLE 1: Resin Properties

2.2. Glass Microballons S series

The glass microballons used in this study are non-porous in nature and are manufactured and supplied by 3M Company under the trade name of Scotchlite. The unique spherical

shape of Scotchlite glass bubbles offers a wide range of important benefits, including:

11 higher filler loading, lower viscosity/improved flow and reduced shrinkage and warpage.

It facilitates the blending of Scotchlite glass bubbles readily into compounds and makes

them adaptable to a variety of production processes including spraying, casting and

molding. 3M Glass Bubbles are engineered hollow glass microspheres that are

alternatives to conventional fillers and additives such as silica’s, calcium carbonate, talc, clay, etc., for many applications. These low-density particles are used in a wide range of industries for the reduction of part weight, lowering costs and enhancement of product properties. The chemically stable soda-lime-borosilicate glass composition of 3M glass bubbles provides excellent water resistance to create more stable emulsions. They are

also non-combustible and nonporous, so they do not absorb resin.

Figure 3. Glass bubbles Figure 4: SEM of S38 microspheres

The average diameter of S38 microspheres is 40µm; their low alkalinity gives 3M glass

bubbles compatibility with most resins, stable viscosity and long shelf life. Properties of

S series microspheres are given below.

12 TABLE 2: Microspheres Property

2.3 Styrene

Styrene can be extracted from the sap (benzoic resin) of styrax trees. Sufficiently low levels of styrene occur naturally in plants as well as a variety of foods such as in fruits, vegetables and meats. The production of styrene in the United States increased dramatically during the 1940s, when it was popularized as a feedstock for synthetic rubber. Vinyl group present in styrene allows it to polymerize. Commercially significant products include polystyrene, ABS, styrene-butadiene (SBR) rubber, styrene-butadiene latex, SIS (styrene-isoprene-styrene), S-EB-S (styrene-ethylene/butylenes-styrene), styrene-divinylbenzene (S-DVB), and unsaturated polyesters. These materials are used in rubber, plastic, insulation, fiberglass, pipes, automobile, boat parts and food containers.

For the current research work styrene is used as a dilutant to facilitate the proper gelling of the microspheres with the liquid resin DERAKANE 8084. The curing characteristics of vinyl ester resin changes only marginally on dilution with styrene. Figure 5 describes the chemical structure of styrene.

Figure 5: Molecular structure of styrene.

Properties of styrene are given in table 3.

TABLE 3: Dilutant Property

2.4 MEKP

MEKP commonly known as Methyl Ethyl Ketone Peroxides is organic peroxide and also a high explosive similar to acetone peroxide. MEKP is colorless, oily liquid while acetone peroxide is a white powder at STP (standard temperature and pressure). MEKP is mildly less sensitive to shock and temperature, and more stable in storage. MEKP is composed of methyl ethyl Ketone peroxide, a catalyst, added to polyester and vinyl

14 resins. When MEKP is mixed with the resin, the resulting chemical reaction causes heat to build up and cure or harden the resin.

Figure 6: Molecular structure of MEKP

2.5 COBALT NAPHTENATE

Cobalt naphthenate (Co NAP) is a mixture of cobalt derivatives of naphthenic acids.

These coordination complexes are being used as oil drying agents for the autoxidative cross linking of drying oils. Co NAP is vastly employed as catalysts because they are soluble in the nonpolar substrates, such as the alkyd resins or linseed oil. The fact that naphthenate are mixtures helps to confer high solubility. They are also very cost effective. A well-defined compound that exhibits many of the properties of cobalt naphthenate is the cobalt (II) complex of 2-ethylhexanoic acid. Often in technical literature, naphthenate are described as salts, but they are probably also non-ionic coordination complexes with structures similar to basic zinc acetate. In the current research work Co NAP is being used as a gel accelerator.

Figure 7: Co-NAP

15 2.6. Silicon Carbide

SiC is the only chemical compound of carbon and silicon. It was originally produced by a high temperature electro-chemical reaction of sand and carbon. Silicon carbide is an excellent abrasive and has been produced and made into grinding wheels and other abrasive products for over one hundred years. Today the material has been developed into a high quality technical grade ceramic with very good mechanical properties. It is used in abrasives, ceramics, and numerous high-performance applications. The material can also be made an electrical conductor and has applications in resistance heating, flame igniters and electronic components. Structural and wear applications are constantly developing.

Silicon carbide is composed of tetrahedral of carbon and silicon atoms with strong bonds in the crystal lattice. This produces a very hard and strong material. Silicon carbide is not attacked by any acids or alkalis or molten salts up to 800°C. In air, SiC forms a protective silicon oxide coating at 1200°C and is able to be used up to 1600°C. The high thermal conductivity coupled with low thermal expansion and high strength gives this material exceptional thermal shock resistant qualities. Silicon carbide ceramics with little or no grain boundary impurities maintain their strength to very high temperatures, approaching

1600°C with no strength loss. Chemical purity, resistance to chemical attack at temperature, and strength retention at high temperatures has made this material very popular as wafer tray supports and paddles in semiconductor furnaces. The electrical conduction of the material has lead to its use in resistance heating elements for electric furnaces, and as a key component in thermostats (temperature variable resistors) and in varistors (voltage variable resistors).

16 Silicon carbide was discovered by the American inventor Edward G. Acheson in 1891.

While attempting to produce artificial diamonds, Acheson heated a mixture of clay and powdered coke in an iron bowl, with the bowl and an ordinary carbon arc-light serving as the electrodes. He found bright green crystals attached to the carbon electrode and thought that he had prepared some new compound of carbon and alumina from the clay.

He called the new compound Carborundum because the natural mineral form of alumina is called corundum [27]. Finding that the crystals approximated the hardness of diamond and immediately realizing the significance of his discovery, Acheson applied for a U.S. patent. His early product initially was offered for the polishing of gems and sold at a price comparable with natural diamond dust. The new compound, which was obtainable from cheap raw materials and in good yields, soon became an important industrial abrasive.

Figure 8: SiC crystals

17 TABLE 4: Nanoparticles Property

Products Nanophase β-SiC powder

Particle Size ~ 40nm Mfg. method Laser Purity SiC > 98%

Impurity [O] ~ 1-1.5% [C] ~ 1-2% Other impurity is not detectable

Structure Crystalline

Specific Surface >= 109 2 Area (m /g)

2.7. Synthesis of Syntactic foam

Synthesis of syntactic foams is described in the following sections.

2.71 Determination of the right ratio

The foremost important thing for manufacturing is to properly determine the right ratio of the ingredients. After carrying work out proper calculation the ratios were determined as follows.

Syntactic foam composition

DERAKANE 8084 – 58% (Ashland Corp.)

3M Hollow Glass Micro spheres – 30% (3M Corp.)

Methyl Ethyl Ketone Peroxide (MEKP “Hardener”) – 1.2% (Sigma Aldrich)

Cobalt Naphtinate (Co Nap “Gel Accelerator”) - <1% (Sigma Aldrich)

Silicon Carbide (SiC) – 1%

Styrene – 10% (Sigma Aldrich)

18 All percentages are by weight

INGREDIENTSDISTRIBUTION

DERAKANE SiC MICROBALLONS MEKP CO-NAP STYRENE

Figure 9: Flow Diagram

2.72. Procedure

In the current research work we will concentrate on manufacturing of a nano infused syntactic foam. Firstly Styrene is being added to the resin DERAKANE 8084 and is being homogenized for 5 minutes. Liquid styrene acts as a dilutant which makes the resin less viscous.

Homogenization is the process of breaking down and blending components within a fluid. One familiar example is milk homogenization in which milk fat globules are broken-up and distributed into the bulk of the milk. Homogenization is also used to process other emulsions such as silicone oil and process dispersions such as pigments, antacids, and some paper coatings.

19 Since our work is concerned with the development of a nano infused syntactic foam so

we have concentrated on the nano particles. The SiC nano particles are in amorphous

form. So in order to infuse them properly in the prediluted resin, they are firstly added to

the resin and then they are rehomogenized for 5 more minutes. Once this process is

completed, then the nano infused resin was sonicated for 30 minutes. Sonication of the

nano infused resin is an very essential procedure for evenly dispersing the nano particles

in the liquid resin.

Figure 10: Homogenization Figure 11: Sonication

After Sonication the microspheres are properly mixed with this nano infused resin for a span of 3 minutes. Since the microspheres are very sensitive particles so it is best recommended having a mechanical mixing with a wooden spatula. When the mixing procedure is completed the mixture is cast into a rectangular mold and left to cure for 72 hours. The cured foam is then machined according the specifications of ASTM standards for mechanical testing. The following figures demonstrates the mechanical mixing of the microspheres with the nano infused resin and the mold where the solution is cast

Figure 12: Mechanical mixing procedure Figure 13: Casting in the mould

2.8 Density of Syntactic foam

After making the foam we carefully determined the density of each category of foam since density becomes very critical in the evaluation of mechanical properties. Our intent was to keep the identical densities for all categories of foam.

TABLE 5: Foam density

FOAM TYPE MICROBALLON CONCENTRATION DENSITY in g/cc

NEAT FOAM 30% 0.421

NANO FOAM 30% 0.464

As shown above in table 5 that the density of the nano infused syntactic foam is slightly higher, approximately 10%. The reason behind this increment is the introduction of nano particles in the resin matrix which is making the syntactic foam a three phase cellular structure as compared to the neat foam which is ideally a two phased cellular structure.

Figure 14: Two phased structure

Figure 15: Three phased structure

22 CHAPTER 3. MICROSTRUCTURAL AND MECHANICAL

CHARECTERIZATION

The scanning electron microscope (SEM) is a special type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The interactions of the electrons with the atoms make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. The types of signals produced by an SEM can be classified as secondary electrons, back-scattered electrons

(BSE), characteristic X-rays, light (cathodoluminescence), specimen current and transmitted electrons. Secondary electron detectors are common in all SEMs, but it is very less likely that a single machine would have detectors for all possible signals. The signal emerges as a result of interactions of the electron beam with atoms at or near the surface of the sample. In the most common or standard detection mode, secondary electron imaging or SEI, the SEM can produce very high- resolution images of a sample surface.

Figure 16: Scanning electron microscope (SEM)

23 3.1 SEM Analysis

Figure 17: NEAT SPECIMEN Figure 18: NANO SPECIMEN

Element Weight % Atomic % Element Weight Atomic % % C K 36.11 46.62 C K 24.45 35.29 O K 39.70 43.01 O K 44.10 42.75 Na K 1.14 0.86 Na K 0.81 0.55 Al K 3.83 2.46 Al K 1.95 1.12 Si K 13.80 7.62 Si K 27.68 16.68 K K 3.85 1.70 K K 1.95 0.77

Microstructures of the neat and nano-reinforced syntactic foams are shown in Fig. 14 and

15 and it is seen from these figures that the glass microballons are uniformly distributed similar to closely packed fashion in vinyl ester bed. Careful observations show that several air pockets (voids) are randomly distributed in the neat foams and a combination of voids, and nano clusters are present in the nano foams. Identification of microspheres

24 is easy as they are spherical shaped; however, identification of voids from matrix is

tedious if not impossible. It has been assumed that the voids are irregular and random size

and shape. First, the SEM image has been scaled using the scale bar and then a selected

portion of the image has been cropped discarding the unwanted sectors followed by

converting to a 16-bit grayscale for further improvement .As one can see, the image

quality has been improved drastically without altering the pixel values. The boundaries of

the spherical microballons are now very clear, and the voids can be easily separated from

the matrix by identifying zones with irregular hexagon as shown by the white areas. In the current research work the SEM analysis shows the presence of higher percentage silicon particles in the nano infused specimen. The nano particles infused are the Silicon carbide crystals in the resin matrix complex.

3.2 Mechanical characterization

Testing of the syntactic foam was performed in accordance to ASTM standards. In the current research work samples of neat and nano syntactic foam were tested for comparing

the improvement in mechanical properties of the developed foam. The tests performed

are described below.

3.21 Compression Test

Compression tests were carried out as per ASTM standard C365. The test specifications

are given as follows.

• Standards : ASTM C 365

• Machine : Zwick Z050 UTM

• Speed : 0.10 inch/min

• Dimensions : 25×25×12mm

Figure 19: Orientation and size of specimen

For the flat wise compression test, syntactic foam was machined to blocks of

25 × 25 × 12 mm3. The length and width were chosen according to ASTM C365-00. The tests were carried out at room temperature using a Zwick ZO50 UTM. The cross-head speed was 0.10 inch/min. The flat wise compressive yield strength of syntactic foam was calculated where σc is the compressive yield strength, P the load at yield, and A is the cross-sectional area. The flat wise compression modulus, Ec, was calculated by [28]

mt E = C A (1)

Where m is the slope of the initial linear region of the load–deflection curve, and t is the thickness of the syntactic foam. The results presented are an average of five tests. The error bars are derived from the standard deviation.

26 3.22 Flexure Test: ASTM D790

The testing was carries out as per ASTM standard D 790. The testing specifications are given as follows.

• Standards : ASTM D 790

• Machine : Zwick Z050 UTM

• Speed : 0.10 inch/min

• Dimensions : 203.2×25.4×12.7mm

Figure 20: Dimension of flexure specimen

Syntactic foams are particularly advantageous where flexural loads are encountered .Use of vinyl ester based syntactic foams, provides materials with high bending stiffness to weight ratio along with low density and high edgewise compressive strength. These properties make the material suitable for aerospace, transportation and marine applications. Syntactic foams provide relatively smoother surface compared to the open- cell structured foams as the porosity in syntactic foams is in closed form. Another disadvantage of open-cell structured foams is they contain depressions on the surface due to the presence of open porosity.

27 The present work deals with characterization of syntactic foams under flexural loading

conditions. FLEXURE or three point bending test are conducted on specimen with large

aspect ratio (span length /thickness) of 16:1. Load -Displacement plots were obtained for

each specimen and were used to determine the flexural properties. Ultimate flexural

stress was calculated as:

3PL =σ 2bd 2 (2)

Where σ is the flexural stress, P is the maximum load before failure; b and d are the

width and depth of the beam specimen. Tangent modulus from the Load-Displacement

curve was also calculated as:

3mL E = 4bd 3 (3)

Where L is the span length, m is the slope of the load displacement curve; b and d are the width and depth of the beam specimen. [29]

3.23 Tension Test

The testing was carries out as per ASTM standard D 638 [30]. The testing specifications are given as follows.

• Standards : ASTM D 638

• Machine : Zwick Z050 UTM

• Strain Rate : 0.04 inch/sec

• Dimensions : 165.1×11.73×6.7mm

Figure 21: Dimension of the tensile specimen

The present research work focuses on enhancing the tensile properties in syntactic foams through micro structural modification with nano particles. Previous experiments have shown that mechanical properties of syntactic foams can be enhanced by the addition of nano particles. Compressive properties, flexural strength and tensile strength of polymers can be enhanced through proper dispersion and exfoliation of nano particles. Such enhancement in mechanical properties occurs primarily due to the reinforcing effect of nano. Properly dispersed SiC nano crystals provide very high surface area of interaction with the surrounding matrix in a syntactic complex. Hence, they can be used at low volume fraction in order to achieve increase in strength without significantly affecting density of weight sensitive materials such as syntactic foams.

Tensile testing is carried out using a computer controlled Zwick Z050 machine. At least five specimens of each type of neat and nano infused syntactic foams are tested.

Specimens were subjected to tensile loading at a strain rate of 0.04 inch/sec. Load- displacement data obtained from the tests are used to calculate the strength and modulus of the specific foams.

Figure 22: Testing Procedure

3.24 Fracture Toughness (SENB) Test

Studies on the mechanical and fracture properties of syntactic foams are based on the maximum filler content of microspheres as this elicits the lowest possible weight of the composites. The syntactic foams for this research were produced by mechanical dispersion of hollow microspheres in vinyl ester resin. In the fracture toughness assessment under quasi-static loading, single-edge notched bend (SENB) specimens were loaded in three-point bend (3PB) geometry. S series microspheres were used for manufacturing the test specimens. The specimens were tested in accordance with ASTM

E 399. The specimen dimensions were 114.3×25.4×12.7 mm. This specimen geometry satisfies the requirement for plane strain conditions.

The test specifications are given as follows.

• Standards : ASTM E399

• Machine : Zwick Z050 UTM

• Speed : 10 mm/min

• Dimensions : 114.3×25.4×12.7mm

30 Figure 23: Dimension of the SENB specimen

The present work attempts to determine the fracture properties of syntactic foams through

micro structural modification by nano particles For the validity of fracture toughness test

the plain strain condition has to be satisfied. The plain strain condition is demonstrated in

Equation 4.

⎛ K ⎞ ⎜ IC ⎟ f .t 52 ⎜ ⎟ (4) ⎝ σ y ⎠

Where t is the sample thickness, KIc is the critical stress intensity factor, and σy is the yield strength.

For all the specimens, a constant crack-to-width ratio, a/W, of 0.5 was prepared by a vertical band saw. A sharp crack was introduced by tapping a fresh razor blade into a notch. The critical stress intensity KIC was calculated using Equation 5.

3 aPS IC = YK (5) 2tW 2 Where Y is a shape factor, P is the peak load at the onset of crack growth in a linear elastic fracture, and a is the crack length.

31 The shape factor Y can be determined using Equation 6.

2 3 4 ⎛ a ⎞ ⎛ a ⎞ ⎛ a ⎞ ⎛ a ⎞ −= ..Y 073931 ⎜ ⎟ + .5314 ⎜ ⎟ − .1125 ⎜ ⎟ + .8025 ⎜ ⎟ (6) ⎝ W ⎠ ⎝ W ⎠ ⎝ W ⎠ ⎝ W ⎠

SENB testing was also carried out in the Zwick Z050 machine. At least five specimens of

each type of neat and nano infused syntactic foams are tested. Load-displacement data

obtained from the tests is used to calculate KIC and GIC.

GIC was calculated using Equation 7. [31]

2 K IC 2 G 1 ν−= )( (7) IC E

Where KIC is the fracture toughness and E is the tensile modulus. Since the precise value

of ν was not measured we assumed the value of ν to be 0.325, as obtained from literature for identical density foam.

Figure 24: Test setup

32 CHAPTER 4. DURABILITY OF SYNTACTIC FOAM UNDER SALT

WATER ENVIRONMENT

The ever increasing demand for oil and optimistic estimation of reserves in deep water

sustain the development of offshore deepwater fields. Owing to these reasons, the ultra

deep water (3000m water depth) is becoming one of the next issues. The production of oil

in ultra deep water faces lots of challenges. One of the issues is flow assurance which

requires the use of thermally managed systems [32]. Due to multiphase flow in flow lines

and risers, and consequently possible wax and hydrates formation, thermal insulation of

subsea pipelines has become increasingly important. This thermal insulation is used to maintain fluid temperature from subsea completion to floating platforms above a given temperature around 40°C to facilitate the flow. One of the first approaches was to place insulation materials in the annulus of pipe-in-pipe systems. These systems are expensive and heavy to install. Alternative systems were developed, based on steel pipe surrounded by insulation materials without external steel pipe. In this case, the insulation, mainly based on polymeric material, must withstand the high pressure, water ingress, high internal and low external temperatures. Scientists developed modern technologies which essentially includes the specimen under study for this present research work: independent modules placed around the steel pipe the syntactic foam is then in contact

33 with water on its entire surface, on the one hand cold water on the outer surface on the

other hand hot water near the steel surface. This system was used among others for the

development of Girasol field multilayer coatings including syntactic foam layer for

insulation. In this case, only the external surface of the system is in contact with water.

This solution was used for example for the insulation of Bonga production steel catenary

riser. For the past decade, numerous studies were performed to better understand and

discuss degradation mechanisms of syntactic foams due to combined effects of pressure,

temperature gradient, and water ingress. The main parts of the studies were conducted on

small samples of material, often for convenience aspects. Some of them revealed

astonishing behaviors of syntactic foams, for instance, the effect of the nature of water

(sea water or natural water), the detrimental effect of combined solicitations including

pressure, temperature and water. All these generated results outlined the need to have

full-scale thermal testing protocols and facilities to study the behavior of thermal

insulation coating systems on lengths of pipes under simulated service conditions.

Experimental facilities were developed to directly measure thermal properties. More

recently, some full-scale prototype tests were modeled to address their global long term

behavior taken into account the water ingress and the highly non linear ageing behavior

of syntactic foams. Nevertheless, some behaviors need to be more accurately understood

to better describe and then model these systems.

4.1 Moisture absorption of syntactic foam

Researchers have performed a number of studies which explain the mechanism and effect

of moisture absorption in epoxy resins and unidirectional fiber reinforced composites.

However, results of moisture absorption in vinyl ester based syntactic foams cannot be

34 compared directly with that of matrix resin or unidirectional fiber reinforced composites

for many reasons. In case of syntactic foams tested in this investigation, weight fraction

of resin is only 0.58. The resin is present in the form of a thin film around the

microspheres, which are inert to the attack of moisture. Hence, moisture absorption

would be in specific channels in the random network of resin present in the structure.

Presence of air voids and some broken microballons in syntactic foams would lead to increase in moisture absorption in specific directions. The usefulness of UI or Ultrasonic

Imaging for syntactic foam type of materials is highlighted in some published studies

[33]. UI is known to be the most effective in obtaining the moisture absorption pattern within the specimen. Absorption of moisture changes the density and mechanical properties of the matrix material, which causes a change in its attenuation coefficient.

4.11 Environmental Exposure

The specimens tested in this study are compression test specimens where surface area of the side walls is nearly half of the total surface area of the specimens. Hence, moisture

intake is substantial from all surfaces [34]. The absorption from the side wall will lead to

the generation of difference in the attenuation coefficient across the cross section of the

specimen, giving the water absorption pattern. Moisture absorption trends for all the

specimens tested in this study are shown in Tables 14 and 15. The values shown in Table

16 are average values based on moisture absorption by five samples of neat and nano syntactic foam for a span of 4 weeks or 720 hours using ASTM D5229 standard. This

study evaluates the difference in the rate of water absorption between the neat and nano

syntactic foam specimen [35]. Large difference in the moisture absorption tendency is observed with the change in temperature. It is observed that for both types of syntactic

35 foams water absorption is less than 5% at room temperature. At high temperature, water

absorption increased to about 9 fold for nano specimens and about 5 to 7 fold for neat

specimens Comparison of results presented in following chapter which shows

considerable difference in the extent of moisture penetration. Dry region in high temperature immersed specimens is smaller compared to the low temperature immersed specimens as observed in these figures. Similar difference is observed in the salt water

immersed specimens also.

4.2 Compression Test after Environmental Exposure

The wet syntactic specimens were tested for compression test after environmental

exposure. The specifications of the testing are described in chapter 3 under section 3.21.

Three batches of syntactic specimens were tested to evaluate the compressive strength

and modulus of the neat and nano specimens [36]. The test results depict the

improvement or deterioration in the resin matrix complex upon inclusion of Sic

nanoparticles when exposed to salt water environment. The following chapter depicts the

results of the specific mechanical tests.

Figure 25: Test Setup for compression.

36 CHAPER 5. RESULTS AND DISCUSSION

The results obtained from various mechanical tests for the dry and environmentally exposed sample are given in the following sections.

5.1 COMPRESSION TEST ASTM C 365

The results are shown in table 6. The stress strain curve of the neat and nano sample are described in the following pages.

TABLE 6: TEST RESULTS

Neat Specimen Compressive Compressive Strength (MPa) Modulus (MPa)

1 50 490 2 54 510 3 51 505 4 49 487 5 56 508 AVERAGE 52 ± 2.6 500 ± 9.57 Nano Specimen 1 71 549 2 73 545 3 67 555 4 65 562 5 69 539 AVERAGE 69 ± 2.82 550 ± 17.77

37 TABLE 7: AVERAGE OF TEST RESULTS

SPECIMEN STRENGTH GAIN/LOSS MODULUS GAIN/LOSS (Average) (%) (Average) (%) (MPa) (MPa) NEAT FOAM 52 ± 2.6 - 500 ± 9.57 -

NANO FOAM 69 ± 2.82 32.69 550 ± 17.77 10

The test results show that there has been an improvement of 32.69% in compressive strength and 10% in compressive modulus of the nano infused syntactic foam in comparison to the neat specimen. These improvements confirm the success of our experiment in reinforcing SiC nano particles in the polymeric resin complex of the syntactic foam.

Stress vs Strain

40 Nano 1 Neat 30 Nano 2 Stress in MPa 20

0 0 0.05 0.1 0.15 0.2 0.25 Strain (mm/mm)

Figure 26: Stress–strain curves for compression test

From the comparison of stress-strain curves it can be noticed that the shape of the curves is the same for neat and nano reinforced samples for both types of syntactic foams. The trends shown by these curves are similar to the characteristics of elastic-perfectly plastic materials. After the elastic region, stress becomes nearly constant for considerable strain.

38 However, after the elastic region, stress becomes nearly constant during further

compression. This plateau region is referred as densification stage. Microballons fracture

under compressive stresses and the compressing material occupies the hollow space

exposed. These events lead to an overall increase in the density of the material. Hence,

this stage is referred as densification stage. The compression tests are stopped around 10-

15% strain for both neat and nano syntactic foams. At such strain level several cracks

originated in the materials and propagated to substantial length to cause failure. Figure

27, represents the failure process under compressive loading where substantial amount of

fragmentation can be observed in the sidewalls of the specimens. During compression

testing a crack propagation event yielded wedge-shaped fragments from the side walls in

syntactic foams.

Figure 27: Typical formation of crack during compression

From the generated results it can be concluded that infusion of nano particles in the

syntactic matrix have significantly improved the compressive properties of foam.

Syntactic foam matrix is composed of resin and microspheres. When these foams are tested for compression the adjacent microspheres tend to break inside the specimen which

results in propagation of the crack. Distinct microscopic feature of failure under

39 compression mode in syntactic foams is the appearance of debris of microballons on the

fracture plane [41]. Failure in shear mode does not give rise to debris. Appearance of

steps in such micrographs may be due to frequent change of crack path in order to grow

at an angle to the loading direction. Scanning electron microscopic examination

corresponding to specimens having different aspect ratios revealed features that aided the

recognition of fracture mechanisms

Figure 28: Tested specimen

5.2 Flexure Test

Load– displacement plots were obtained for each test specimen and were used for

calculation of flexural properties. For the flexural, large aspect ratio (length to thickness

ratio of (16:1) specimens were utilized respectively. The dimension and orientation of the specimen are shown in Figure 20.The specimen dimension were chosen in accordance with ASTM D 790. The average values of flexural strength were calculated by testing five specimens of both neat and nano samples of syntactic foams.

40 TABLE 8: FLEXURAL TEST RESULTS NEAT SPECIMEN FLEXURAL FLEXURAL STRESS (MPa) MODULUS (GPa) 1 22.89 2.39 2 20.56 2.35 3 19.65 2.45 4 21.07 2.49 5 24.63 2.42 AVERAGE 21.67 ± 1.78 2.42 ± 0.1077 NANO SPECIMEN 1 32.67 2.78 2 31.85 2.83 3 28.32 2.85 4 30.08 2.74 5 23.53 2.805 AVERAGE 29.29 ± 5.61 2.801 ± 0.0861

TABLE 9: AVERAGE OF THE FLEXURAL TESTS SPECIMEN FLEXURAL GAIN/LOSS FLEXURAL GAIN/LOS STRESS % MODULUS(GPa) S% (MPa) NEAT 21.67 - 2.42 ± 0.106 - SPECIMEN NANO 29.29 34.6 2.801 ± 0.37 15.74 SPECIMEN

NEAT vs NANO FLEX TEST

600

500

400

300 NEAT SAMPLE

NANO INFUSED 200 SAMPLE LOAD i n (N) 100

0 -202468 -100 DISPLACEMENT in (mm)

Figure 29: Load displacement curves for flexure test.

41 The curves in figure 29 show a sudden drop in the peak load in the load–displacement

curve. The nano infused syntactic foam specimen reaches to a higher peak load in region

2 in comparison to the neat specimen. This phenomenon clearly explains the increment of

34.6% in flexural strength in case of the nano specimen.

NEAT NANO

Figure 30: Tested Specimen

The results generated from the three point bending test shows a significant improvement

in flexural properties of the nano infused syntactic foam in comparison to the neat syntactic foam. There has been an improvement of 35% in flexural stress and 16% increase in the flexural modulus in comparison to the neat syntactic foam. These promising results prove that infusion of nano particles in the foam matrix visibly improves the performance of the syntactic foam. Figure 31 demonstrates the tested specimen of the flexure tests.

Figure 31: Failure sequence of the syntactic foam

42 Initially a crack originating on the foam surface in the direction of applied load initiates.

This crack grows towards the central plane of the foam construction and subsequently to the top (compressive side) surface of the specimen, leading to the complete fracture.

5.3 Tension Test

The following tables depicts the tensile test results

TABLE 10: REPRESENTATIVE OF TENSION TEST RESULTS

SPECIMEN TENSILE STRENGTH MPa TENSILE MODULUS GPa Neat 1 8.3 1.75 Neat 2 8.6 1.81 Neat 3 7.7 1.78 Neat 4 7.68 1.67 Neat 5 7.72 1.84 AVERAGE 8 ± 0.84 1.77 ± 0.13 Nano 1 9.8 1.39 Nano 2 10.3 1.47 Nano 3 10.5 1.49 Nano 4 9.23 1.37 Nano 5 10.17 1.48 AVERAGE 10 ± 1 1.44 ± 0.12

TABLE11: AVERAGE OF TENSION TEST RESULTS

SPECIMEN TENSILE GAIN/ TENSILE GAIN/ FRACTUR GAIN/ MODULUS OF GAIN/ STRENGT LOSS MODULUS LOSS E LOSS TOUGHNESS LOSS 3 H MPa % GPa % STRAIN% % μ in MJ/m %

NEAT 8 ± 0.84 1.77 - 5 0.02 FOAM

NANO 10 ± 1 25 1.44 22.9 6.2 24 0.03 50 IFOAM

Figure 32: Graphical representation of Tension Test

The above graph is a representative stress–strain curve of neat and nano infused syntactic

foams. Failure occurs in a brittle manner at the end of the elastic region on the curve.

Table 10 and 11 shows a comparison between results obtained from tensile testing of

nano and neat syntactic foams [37]. This increase in strength is attributed to the presence

of exfoliated/intercalated SiC nano particles. Dispersed nano particles restrict the

mobility of polymer chains during tensile loading. The crystalline structure of silicon

carbide nano particles provides very high surface area >= 109(m2/g), which reduces stress

concentration in the foam matrix. Upon the application of loading, bending and stretching

of polymer molecular chains occur, which results in the development and propagation of micro cracks. Hence, the material strength is highly dependent on the initiation of micro cracks and the constraints on their growth. An increase in strength of hybrid syntactic foams indicates that crack initiation and growth are delayed in the presence of nano

44 particles. Nanoparticles resist the initial crack growth at earlier stage of loading, which results in higher failure loads.

Figure 33: Modulus of strength

An important aspect of strength of materials is modulus of toughness. This quantity represents the entire area under the stress strain diagram. The modulus of toughness μ for the neat syntactic specimen is approximately 0.02 MJ/m3and for the nano infused syntactic specimen is 0.03 MJ/m3. This shows there has been almost 50% improvement in absorbed energy which depicts the success of the nano infusion in the syntactic specimens. The results generated from tension tests using neat and nano infused syntactic foam specimens revealed that there has been a improvement of 25% in tensile strength and 24% in fracture strain using nano infused syntactic foam in comparison to neat specimen. Although the tensile modulus of the nano reinforced specimen has decreased by 22.9% in comparison to the neat syntactic specimens. The test results prove that using a nano infused syntactic foam enhances the tensile properties of the specimen. Hence,

45 nano infused syntactic foams offer higher strength and higher fracture strain with a slight increase in the density of syntactic foams.

NANO NEAT

Figure 34: Tested specimens.

Upon tensile loading, significant load is transferred to the microspheres. High density and high strength microspheres are able to carry significant amount of the load, and tend to debond from the surrounding matrix without fracturing. Tensile properties of high density

foams are guided by the matrix strength, and the matrix–micro balloon interface. This has

been represented in figure 35.

FIGURE 35: Load transfer on microspheres upon tensile loading

46 Tensile properties of high density syntactic foams are guided by the matrix strength, and

the matrix–microballons interface.

5.4 SENB Test

The results of the fracture toughness tests are summarized in Table 12 and 13,

respectively. These results show that the absolute values of all the fracture properties are

significantly lower for neat foams than that of the nano reinforced syntactic materials,

particularly the fracture strain and the fracture toughness.

This indicates a strong influence of the nano infused material which shows an

improvement of 11.27% in fracture toughness and 48.782% in strain energy release rate

GIC. The fracture behavior of syntactic foam has been characterized through a SEM

pictograph to better understand the failure mechanism associated with the parameters

discussed earlier.

Figure 36 shows the SEM pictograph of the fracture surface of the syntactic foam which

has been removed from the specimen after conclusion of the testing.

Figure 36: SEM picture of the fracture surface of the syntactic foam.

47 TABLE 12: FRACTURE TOUGHNESS TEST RESULTS

NEAT FRACRTURE TOUGHNESS STRAIN ENERGY RELEASE 2 SPECIMEN KIC in MPa√m RATE GIC in J/m 1 1.32 671.06 2 1.36 668.62 3 1.19 594.9917 4 1.04 554.24 5 1.74 841.54 AVERAGE 1.33 ± 0.25 666.09 NANO SPECIMEN 1 1.35 864.38 2 1.39 890 3 1.43 854.15 4 1.51 980.78 5 1.72 1034.18 AVERAGE 1.48 ± 0.14 924.698

TABLE 13: AVERAGE OF SENB TEST RESULTS SPECIMEN FRACRTURE GAIN STRAIN ENERGY GAIN/ TOUGHNESS /LOSS RELEASE RATE GIC LOSS 2 KIC in MPa√m % in J/m % NEAT 1.33 - 1030.095 - SPECIMEN NANO 1.48 11.28 1532.6 38.824 SPECIMEN

Load vs Displacement

600

500

400

300 Neat Foam Nano Foam 200 Load in Newton in Load 100

0 -1012345 -100 Displacement in mm

Figure 37: Graphical representation of SENB Test

48 The load –displacement curve obtained from SENB tests are shown in figure 37.The

curve are almost linear up to fracture point and allow calculation of KIC and GIC as mentioned in section 3.24. This has peak load value for, P for nanophased foam is obvious in figure 37.Correspondingly there was an increase in 11% of KIC as shown in

table 13.The increase in KIC for 1% of SiC nano particles weight content suggests the

presence of a toughening mechanism which increases the fracture energy compared to

neat syntactic foam specimen. Again the nano infused foam outperform the neat foam

resulting in the highest value for KIC, suggesting that the infusion of nano particles

enhances the fracture toughness of the syntactic foam.

The present study proves that the type of major toughening mechanism has changed at

the transition point of 1 weight% of nano particle content. The fracture toughness of the

vinyl ester based syntactic foam is influenced by the microsphere content of the material.

The toughening due to the presence of 30 vol% microspheres is influenced by a

combination of the filler stiffening effect and crack bowing mechanisms in the presence

of round-shaped fillers. The microsphere content in polymeric resin complex decreases

the interparticle separation between the resin matrix complexes. Higher volume fractions

of microspheres allow more microspheres to debond from the matrix. Debonding is

accompanied by premature cracks. If the direction of these cracks is parallel to the crack

growth direction, the subcritical cracks act as precursors and facilitate crack propagation

which show that step structures prevail for the microstructures containing low volume

fractions of microspheres. These step structures are considered to be the evidence of the

existence of the crack front bowing mechanism. This mechanism was first observed

experimentally by Lange in 1970 [15]. Lange suggested that an approaching crack front

49 is pinned by a rigid particle. Secondary crack fronts will be formed and these secondary

crack fronts will bend/ bow between the rigid particles. Evidently there will be a point

when the crack front breaks away from the rigid particle. At this point, the arms of the

secondary crack fronts will come together and form a characteristic step structure as both

secondary crack fronts propagate at a different crack plane. We believe that these crack

branching are affected by the embedded nano-particle in the matrix.

5.5 Durability Test

The neat and nano syntactic specimens were exposed to salt water environment for 720

hours under different conditions. The water uptakes in these specimens were evaluated to

investigate the effect of SiC nanoparticles inclusion on the syntactic foam in order to

reduce the degradation under environmental exposure. The test results are depicted below

in a tabulated manner. The moisture uptake percentage has been depicted in a weekly

manner [38] . Readings were taken in a regular basis to study the day to day water uptake

in the neat and nano syntactic specimens. The experimental results compares the moisture

uptake in neat and nano syntactic specimens to effectively study the improvement or deterioration with the infusion of SiC nanoparticles in salt water environment and humidity chamber .

TABLE 14: WATER UPTAKE IN WEEKS 1&2

SAMPLE DRY SALT SALT HUMIDITY WEIGHT IN WATER WATER AT AT 50 oC GRAMS ROOM TEMP 40 oC NEAT 1 5.78 6 6.21 6.24 NEAT 2 5.65 5.75 6.09 6.26 NEAT 3 6.09 6.12 6.16 6.17 NEAT 4 6.06 6.14 6.21 6.23 NEAT 5 5.82 5.87 5.90 6.04 AVERAGE 5.88 5.976 6.114 6.188 NANO 1 5.93 6.24 6.19 6.36 NANO 2 6.07 6.13 6.21 6.34 NANO 3 6.11 6.23 6.24 6.42 NANO 4 6.15 6.23 6.21 6.34 NANO 5 5.92 6.24 6.3 6.89 AVERAGE 6.036 6.214 6.23 6.47

TABLE 15: WATER UPTAKE IN WEEKS 3&4

SAMPLE DRY WEIGHT SALT WATER SALT WATER HUMIDITY ROOM TEMP AT 40 oC AT 50 oC NEAT 1 5.78 6.24 6.32 6.26 NEAT 2 5.65 5.85 6.17 6.38 NEAT 3 6.09 6.34 6.25 6.34 NEAT 4 6.06 6.24 6.39 6.48 NEAT 5 5.82 5.95 5.99 6.21 AVERAGE 5.88 6.124 6.224 6.334 NANO 1 5.93 6.35 6.28 6.46 NANO 2 6.07 6.24 6.37 6.42 NANO 3 6.11 6.37 6.32 6.56 NANO 4 6.15 6.36 6.34 6.53 NANO 5 5.92 6.35 6.29 6.75 AVERAGE 6.036 6.334 6.32 6.544

Table 14 & 15 shows the water uptake for both neat and nano sample specimens of syntactic foams. It can be observed from the above tables that the specimen’s moisture absorption is varied at different conditions. In salt water at room temp [SW] the nano

51 specimens absorb higher percent than the neat specimens. The gain in the wt% absorption

for neat specimens of syntactic foams is 19.51%. , which proves the moisture retention capacity of neat specimens are lesser than nano specimens in salt water at room

temperature. When the specimens are subjected to a salt water environment at 40 oC

[SWT] the neat specimens absorbs more moisture in comparison to the nano infused

specimens. The weight gain percent of the neat specimen is 24.46 which shows the nano

specimen performs better in [SWT]. At humidity of 50 oC[HT] the nano infused syntactic

specimens retains more moisture than the neat specimens. The weight gain % in nano specimens is 8.9. Here the neat specimens outperform the nano specimens in water retention capacity. Table 16 demonstrates the gain/loss% in weight of the neat and nano

syntactic specimens when subjected to the entire three different environments.

TABLE 16: DEMONSTRATION OF THE MOISTURE RETENTION CAPACITY

SPECIMEN wt % in GAIN/ wt% in GAIN/ wt% GAIN/ [SW] LOSS% [SWT] LOSS% [HT] LOSS%

NEAT 4.1 5.85 -24.46 7.72 NANO 4.9 19.51 4.7 8.41 8.9

The observation from Table 16 shows that the neat syntactic foam absorbs more water

than the nano syntactic foams only in case of [SWT]. This foam needs very close attention as both types of syntactic foams have same matrix resin and microspheres- matrix volume fraction. The only difference between these foams is the 1wt% SiC nano

particle. However, borosilicate glass particles are inert to moisture attack, hence; such a

large difference in the moisture absorption is not expected. The following graphs

52 demonstrate the water uptake in neat and nano syntactic foams when subjected to different conditions.

TABLE 17: WATER UPTAKE% IN FOAMS

SAMPLE TIME IN ABSORPTION ABSORPTION ABSORPTION SPECIMEN HRS wt % in SALT wt% in SALT wt% in WATER ROOM WATER AT HUMIDITY TEMP[SW] 40 oC[SWT] AT 50 oC[HT] NEAT 168 0.82 1.86 2.6 NEAT 336 1.63 3.9 5.2 NEAT 504 2.08 4.5 6.5 NEAT 720 4.1 5.85 7.72 NANO 168 1.1 1.6 3.6 NANO 336 2.94 3.21 7.19 NANO 504 3.8 3.6 7.89 NANO 720 4.9 4.7 8.41

Figure 38: Graphical analysis of water uptake

Figure 39: Neat specimen in SWT

NEAT vs NANO in SW

5 4

NEAT in SW 3 NANO in SW

0 0 200 400 600 800

TIME in HRS

Figure 40: Comparison of Neat and Nano specimen in SW

NEAT vs NANO in SWT

4 NEAT in SWT

3 NANO in SWT

0 0 200 400 600 800

TIME in hrs

Figure 41: Comparison of Neat and Nano specimen in SWT

54 NEAT vs NANO in HT

9 8 7 6 5 NEAT in HT 4 NANO in HT 3 uptake% Water 2 1 0 0 200 400 600 800 TI ME in hr s

Figure 42: Comparison of Neat and Nano in HT

The use of polymer based syntactic foams reinforced with glass microballons for applications in deep water depths requires a better understanding of their behavior [39].

The above presented results demonstrate that the neat specimen absorbs more moisture in salt water at 40 oC in comparison to the nano specimens. While at salt water at room temperature and humidity at 50 oC the neat syntactic specimen outperforms the nano specimen by retaining lesser moisture in due course of time.

5.51 Compression Test on wet specimens

Results of compression test of neat and nano specimens of vinyl ester based syntactic foam are compared which were subjected to salt water exposure .Similarly, stress-strain curves for wet syntactic foam are shown in figure 43. The test results are depicted in

Table 18-20. The neat and nano specimens were tested in three batches to investigate the effect of the moisture uptake in their compressive property under all the three conditions

[SW, SWT, HT]

55 TABLE 18: Compression Test Results for wet neat specimens

Neat Specimen in SW Compressive Compressive Strength (MPa) Modulus (MPa) 1 47 490 2 49 505 3 51 516 AVERAGE 49 ± 2 505.34 ± 10.5 Neat Specimen in SWT 1 43 487 2 48 479 3 49 493 AVERAGE 46.67 ± 3.21 486 ± 7.02 Neat Specimen in HT 1 54 512 2 56 519 3 51 523 AVERAGE 53.67 ± 2.51 518 ± 5.56

TABLE 19: Compression Test Results for wet nano specimens

Nano Specimen in SW Compressive Compressive Strength (MPa) Modulus (MPa) 1 41 430 2 39 444 3 37 439 AVERAGE 39 ± 2 437.67 ± 7.09 Nano Specimen in SWT 1 31 450 2 33 439 3 35.5 443 AVERAGE 33.16 ± 2.25 444 ± 5.56 Nano Specimen in HT 1 45 438.7 2 42 442 3 39.4 446 42.13 42.13 ± 2.80 442.23 ± 6.65

56 TABLE 20: Comparison of neat and nano wet specimens under compressive loading

SPECIMEN COMPRESSIVE GAIN/L COMPRESSIVE GAIN/LO in SW STRESS (MPa) OSS % MODULUS(GPa) SS% NEAT 49± 2 -25.64 505.34 ± 10.5 -15.46 SPECIMEN NANO 39± 2 437.67 ± 7.09 SPECIMEN SPECIMEN in SWT NEAT 46.67 ± 3.21 -40.74 486 ± 7.02 -9.4 SPECIMEN NANO 33.16 ± 2.25 444 ± 5.56 SPECIMEN SPECIMEN in HT NEAT 53.67 ± 2.51 -27.39 518 ± 5.56 -17.13 SPECIMEN NANO 42.13± 2.80 442.23 ± 6.65 SPECIMEN

Neat vs Nano WET Specimen

40 NEAT SPECIMEN 30 NANO SPECIMEN NEAT SPECIMEN

Stress in MPA 20

0 0 0.05 0.1 0.15 0.2 0.25 Strain

Figure 43: Stress strain curves of compression test on wet specimens

57 From the comparison of trends observed in stress-strain curves it can be noticed that the

shape of the curves is the same for dry and moisture absorbed samples for both types of syntactic foams. These curves show trends that are similar to the characteristics of elastic- perfectly plastic materials..

In cases of wet syntactic specimens decrease in stress may be observed due to origination of crack near the corners of the specimens. This type of behavior is more significant for nano infused specimens for, which has much higher rigidity compared to the neat syntactic foam in dry conditions [40]. In salt water at room temperature the nano

specimens have shown a deterioration of 25.6% compared to that of the neat specimens

in compressive strength. At humidity chamber and salt water at 40 oC the nano specimens

have deteriorated by 27% and 40.64% in compared to the neat syntactic specimens.

Significant decreases in compressive modulus have also been observed in case of the

nano infused syntactic specimens as shown in Table 20. The generated result proves that

neat syntactic specimens are more suitable for salt water exposure compared to the SiC

infused specimens [41]. In the moisture absorption section it was expected that if nano

infused specimens were absorbing higher percentage of moisture the compressive

strength of high temperature wet nano specimens would show substantial decrease.

Hence, the experimental observation of marked decrease in compressive strength of the nano infused syntactic specimens validates this argument.

58 CHAPTER 6. SUMMARY

1. Vinyl ester based syntactic foam has been developed with the dispersion of SiC

nanoparticles into the resin. Nanoparticle loading was very low – at 1.0 wt%. of

the resin.

2. It was possible to increase the concentration of glass borosilicate microballons to

30.00 wt% through the use of a styrene dilutant. The density of the resulting

nanophased foam was around 0.462 g/cc which was very much comparable to

commercially available foams.

3. Manufacturing steps were simple, low cost, and highly efficient, that is

reproducible batch after batch with identical densities and properties. The steps

involved; sonication of SiC nanoparticles into the mixture of resin and dilutant,

mechanical mixing of microballons, and casting into a mold for cure.

4. SEM and EDS analyses conducted after manufacturing of foam revealed the

presence of silicon and excess carbon in the nanophased foam indicating the

success of SiC reinforcement. SEM micrographs also showed accumulation of

SiC particles appearing on the surface of microballons.

5. Compression tests have shown that the strength and modulus of syntactic foams

can be enhanced by 33% and 10%, respectively by SiC reinforcement.

6. Flexural tests also demonstrated significant improvement in both strength and

modulus by 35% and 16%, respectively.

59 7. Tensile tests revealed somewhat different results; there was substantial

improvement (25%) in strength but with a considerable loss in modulus (23%).

This was compensated by a 24% increase in fracture strain which resulted in 50%

improvement in modulus of toughness for the nanophased foam.

8. Stress intensity factor, KIC, and strain energy release rate, GIC were also

determined using SENB experiments. Results showed that there was an

improvement of 11-38 % in the fracture toughness parameters.

9. Foam specimens were subjected to various environmental exposures; salt water at

RT, salt water at 400C, and 85% humidity at 500C for a period of 6 weeks. Water

uptake measurement indicates that nanophased foams absorbed more water than

their neat counterpart by about 9-19%. Compression tests conducted after

environmental exposure reflected similar trend that is the compressive strength

and modulus of nanophased foams degraded at a faster rate than that of the neat

specimens.

10. Above experiments suggest that SiC nanoparticles can be successfully used to

improve mechanical and fracture properties of syntactic foam as long as they are

used under dry conditions. However, when exposed to salt water and

hygrothermal conditions the SiC nanoparticles are not good reinforcements. This

leads us to our future work that the surface of SiC nanoparticles can be modified

to reduce such degradation of properties.

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