ABSTRACT
The methodology of the experiment involves three main stages. The first part describes the synthesis of the PF (novolac) resins. Novolac (resins) were prepared with an excess of phenol over formaldehyde under acidic conditions. PF molar ratio were varied from 1P:2F to 1P:8F. The structure of the PF was determined using FT-IR techniques. The preparation and characterization was reported. The physicochemical properties tests carried out on the (novolac) resins include pH, viscosity, water-tolerance, density, resin solid content, gel time, cure time and yield%. Emission tests such as free formaldehyde content and free phenol content were determined . The second part involves synthesis of chitosan from commercial chitin with different degree of substitutions determined by potentiometric titration. Characterization using FT-IR of the fresh chitin and the deacetylated chitosan was carried out and functional groups were observed and compared with the standard values. The following analysis were carried out on the chitin: loss on drying, moisture content, % solubility, solubility in chemicals, density, pH value, and ash content. The solubility of the chitosan were tested in various solutions like distilled water, acetone, ethanol, acetic acid and lactic acid. The third part was the production of the composites with different P: F molar ratio 1P:2F to 1P:8F and filler loading were varied 10% to 50%. A total of thirty five composites were produced, fabricated and subjected to mechanical properties.from the physiochemical properties of density, water absorption and tensile strength/modulus of composites at different filler loading of 10% to 50% using 5%w/v Heaxemethylenetetramine (HMTA) as the hardener, loading of 40% with 1P:4F and 1P:2F gave excellent results in terms of mechanical properties. The results of the mechanical properties revealed that the tensile strength and the tensile modulus increases with the increase of the filler loading. Further analysis was carried out to determine the optimum from the composites at different filler loading. To prove the results, optimization of 1P:2F to 1P:4F were carried out with the filler at different degree of deacetylation (DD) % values. In this case, the DD% values were increased by increasing the concentration of the alkali (NaOH) at 30%, 40% and 50%w/v at a temperature of 120oC. The approximate calculated values of 70%, 81.47% and 90.92% DD of the chitosan were obtained and composites using P: F molar ratio of 1P:2F and 1P:4F were prepared. The composites were then subjected to various analysis to obtain the optimization of the composites. In order to characterize the chitosan phenol formaldehyde composites, several analysis were carried out on the following sample, neat polymer (100%), untreated, 70%, 81.47% and 90.92% DD were carried out. Swelling behaviors tested using the following solvents water, carbon
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tetrachloride, ethanol and acetone, and the results obtained were reasonable. Chemical resistance on the 40% of 1P:4F and 1P:2F were carried out in 1N HCl and 1N NaOH at 72 hours. A very good results were obtained for samples 70%, 81.47% and 90.92% with an average result of around 7%. But the basicity behaviors was poor in 1N NaOH solution with the highest value in the untreated 12.5% were observed. Other tests carried out were density and water absorption, the densities were in the range of 1.8g/cm3 for 60% to 0.82g/cm3 for untreated.The water absorption measurement was found to be excellent in all the samples, lowest value was observed in 91% and highest for 70% DD. Neat matrix showed complete resistance to water absorption. Mechanical properties tests of: tensile strength/modulus (38% improvement), flexural strength/modulus (27% improvement), impact (65% improvement) and hardness (25% improvement) of filler loading of 40% at DD of 70% gave an excellent results. Formaldehyde emission test using flask method were carried out on the composites. The effect of DD on emission was found to be highest in 70% DD and lowest in 91% DD. The effect of chitosan in reduction of formaldehyde release in the production of chitosan/P:F composite mechanism of reaction between chitosan and novolac resin were drawn. The use of chitosan in the production of composite have significantly reduced the emission. The SEM studies of the samples were investigated and. morphological results clearly showed that when the polymer resin matrix was reinforced with the different loadings of chitosan 70% to 90.92% DD surface modification took place. Wide angle x-ray analysis (XRD) was carried out to investigate the effectiveness of the resin interaction Thermal analysis of the reinforced chitosan phenol formaldehyde composites as compared to the virgin polymer had improved from 330oC to 615oC. Improved stability was manifested throughout the whole range of temperature studied. The overall results of the research show that composites which were deacetylated, reinforced and cured with hardner (HMTA) gave excellent results than those without deacetylation and curing.
TABLE OF CONTENTS
Cover page
Fly leaf
Title page…………………………………………………………………………………………
Declaration ……………………………………………………………………………………………………………………….. i
Certification …………………………………………………………………………………………………………………….. ii
Acknowledgements …………………………………………………………………………………………………………. iii
Table of Contents ……………………………………………………………………………………………………………. vii
List of Tables ……………………………………………………………………………………………………………… xviii
List of Figures ………………………………………………………………………………………………………………… xx
List of Plates ………………………………………………………………………………………………………………… xxv
List of Schemes ……………………………………………………………………………………………………………. xxvi
List of Appendices ………………………………………………………………………………………………………. xxvii
List of Abbreviations …………………………………………………………………………………………………. xxviii
CHAPTER ONE
1.0 INTRODUCTION ………………………………………………………………………………………………………… 1
1.1 Resins……………………………………………………………………………..………1
1.1.1 Polymers ………………………………………………………………………………………………………………………. 2
1.1.2 Polymer Gel……………………………………………………………………… ….. . 3
1.2 Polymer Composite ………………………………………………………………………………………………………. 5
1.3 Composite Concepts ……………………………………………………………………………………………………… 5
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1.3.1 Types of Composites Materials ………………………………………………………………………………………. 7
1.3.1.1 Metal Matrix Composites……………………………………………………………………………………………… 7
1.3.1.2 Ceramic Matrix Composites …………………………………………………………………………………………. 8
1.3.1.3. Polymer Matrix Composites …………………………………………………………………………………………. 8
1.3.2.1 Fibre ………………………………………………………………………………………………………………………….. 8
1.3.2.2 Matrix ………………………………………………………………………………………………………………………… 9
1.3.2.3 Reinforcements …………………………………………………………………………………………………………… 9
1.3.3 Composite Fabrication …………………………………………………………………………………………………. 9
1.3.3.1 Filament Winding ……………………………………………………………………………………………………… 10
1.3.3.2 Pultration……………………………………………….…………………….………..10
1.4.1 Physical and Chemical Properties of Formaldehyde ………………………………………………………. 10
1.4.2 Usage and Application of Formaldehyde ………………………………………………………………………. 12
1.4.3 Formaldehyde and Health Effects ………………………………………………………………………………… 13
1.4.4 Formaldehyde as Indoor Pollutant ……………………………………………………………………………….. 14
1.5 Phenol …………………………………………………………………………..……15
1.5.1 Physical and Chemical Properties of Phenol …………………………………………………………………. 16
1.5.2 Phenol Usage and Production ……………………………………………………………………………………… 17
1.5.3 Phenol and Health Effect ……………………………………………………………………………………………. 19
1.6.1 Physical and Chemical Properties of Phenolic Resins …………………………………………………….. 22
1.6.2 Formation and Structure of Phenol Formaldehyde Resin ………………………………………………… 24
1.6.2.1 Resole Type (Base Catalysed) …………………………………………………………………………………….. 25
1.6.2.2 Novalac Type (Acid catalysed) …………………………………………………………………………………….26
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1.7 Chitosan ………………………………………………………………………………………………………. 27
1.8.1 Statement of the Research Problems ………………………………………………………………………….. 28
1.8.2 Research Questions …………………………………………………………………………………………………. 24
1.8.3 Justification of the Present Study ………………………………………………………………………………. 29
1.8.4 Aims of the Research ………………………………………………………………………………………………. 29
1.8.5 Objectives of the Present Works ……………………………………………………………………………….. 30
1.8.6 Scope and Limitation of the Research ……………………………………………………………………….. 30
1.8.7 Expected Results ……………………………………………………………………………………………………..30
CHAPTER TWO
2.0 LITERATURE REVIEW …………………………………………………………………………….. 32
2.1 Physical, Chemical and Toxicology Properties of Formaldehyde ……………………………….. 32
2.2 Physical and Chemical Properties of Phenol Toxicity and Health Effects …………………… 33
2.3 Phenolic Degradation / Pyrolysis ……………………………………………………………………………….35
2.4 Chemistry of Phenolic Resin ……………………………………………………………………………….. 39
2.4.1 Resole Chemistry ……………………………………………………………………………………………………….39
2.4.2 Novolac Chemistry——————————————————————————————– — 41
2.5 Reaction of Phenol-Formaldehyde Novolac Resin and Hexamethylenetetramine …………. 44
2.5.1 Synthesis of Hexamethylenetetramine (HMTA) ……………………………………………………………..45
2.6 Synthesis of Novolac resin ………………………………………………………………………………….. 45
2.6.1 Mechanism of Synthesis of Novolac Resin ……………………………………………………………………..46
2.6.2 Curing Behavior in the Synthesis of Novolac ………………………………………………………………….47
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2.7.1 Synthesis of Phenol (Novolac/Resole) Phenolic Oligomers ……………………………………………….51
2.7.2 Analytical Methods for Phenol Determination ………………………………………………………………..52
2.7.2.1 Derivatization Methods ……………………………………………………………………………………………….53
2.7.2.1 Spectrophotometric Methods ………………………………………………………………………………………54
2.7.2.1.1 4 – AAP [4 – aminoantypyrine] Spectrometry method …………………………………………………….54
2.7.2.1.2 Berthelot’s Reaction Spectrophotometry Tao [1983] Method ………………………………………….55
2.7.2.1.3 MBTH [3 – methyl – 2 – benzothiazolylzone] Method …………………………………………………….56
2.7.2.1.3 UV – Derivative Spectrophotometry [UVDS] Method ……………………………………………………..56
2.7.3 Determination of Free Phenol in Polymer Composite …………………………………………………….57
2.8. Synthesis of Formaldehyde ………………………………………………………………………………………..57
2.9.1 Analytical Methods for Formaldehyde Determination ……………………………………………………58
2.9.1.1 In Situ Methods …………………………………………………………………………………………………………59
2.9.1.2 Derivatization Methods ……………………………………………………………………………………………..60
2.9.1.2.1 [2, 4- DNPH] Method …………………………………………………………………………………………………61
2.9.1.2.2 Chromotrophic Acid (CA) Method NIOSH (1994) …………………………………………………………..56
2.9.1.2.3 The Acetylacetone (acac) (Hantzsch Synthesis) Method Nash (1953) ………………………………63
2.9.1.2.4 Pararosahiline Method Miksch et al (1981) [Malachite Green Method] …………………………..64
2.9.1.2.5 AHMT (4- amino- 3- hydrazino- 5- mercapto- 4H- 1, 2, 4- triazole) ………………………………….65
2.9.1.2.6 MBTH Method ………………………………………………………………………………………………………….65
2.10 Test Methods Used for Measuring Formaldehyde Emissions from Composite Products…….66
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2.10.1 Large/Small Chamber Method……………………………………………………………………………………….67
2.10.2 Perforator Method ………………………………………………………………………………………………………68
2.10.3 Flask Method ………………………………………………………………………………………………………………68
2.10.4 Desiccator Method ………………………………………………………………………………………………………69
2.10.5 Gas Analysis ………………………………………………………………………………………………………………..69
2.10.6 DMC (Dynamic Micro Chamber) Method ………………………………………………………………………..70
2.10.7 Formaldehyde Emission from Composites ……………………………………………………………………..71
2.10.8 Mechanism of Formaldehyde Emission from Composites ………………………………………………..73
2.10.9 Reduction of Formaldehyde Emission from Composites Panel ………………………………………….75
2.10.10 Regulations and Testing of Formaldehyde …………………………………………………………………….76
2.11 Chitin/Chitosan ………………………………………………………………………………………………………….78
2.11.1 Chemical and Physical Properties of Chitosan ………………………………………………………………..80
2.11.2 Crystallinity ……………………………………………………………………………………………………………….81
2.11.3 Degree of Acetylation (DA) ………………………………………………………………………………………….83
2.11.4 Solubility and Molecular Weight …………………………………………………………………………………..84
2.11.5 Chemical Reactivity …………………………………………………………………………………………………….85
2.11.6 Crosslinking ……………………………………………………………………………………………………………….86
5.11.7 The Effect of Acetyl Group on Mechanical Properties of Chitin/Chitosan ………………………….87
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CHAPTER THREE
3.0 MATERIALS AND METHOD …………………………………………………………………………. 89
3.1 Materials………………………………………………………………………………..89
3.1.1 Reagents ……………………………………………………………………………………………………………………….89
3.1.2 Equipment/Instruments ………………………………………………………………………………………………….89
3.2 Methods …………………………………………………………………………………………………………….. 90
3.2.2 Analysis of the Synthesized Novolac Resins ……………………………………………………………………..90
3.2.2.1 pH Measurement …………………………………………………………………………………………………………..90
3.2.2.4 Viscometric Measurement ………………………………………………………………………………………………91
3..2.2.2 Water Tolerance …………………………………………………………………………………………………………….91
3.2.2.3 Specific Gravity Measurement …………………………………………………………………………………………91
3.2.2.5 Resin Solids Content Measurement ………………………………………………………………………………….92
3.2.2.6 Gel – time Measurement ………………………………………………………………………………………………..92
3.2.2.7 Cure – time Measurement ……………………………………………………………………………………………..93
3.2.2.8 Estimation of synthesized P:F Yield and Yield%————————————————————-93
3.2.2.9 Determination of Free Formaldehyde ………………………………………………………………………………93
3.2.2.10 Determination of Free Phenol …………………………………………………………………………………………94
3.2.2.11 Infrared Measurement …………………………………………………………………………………………………..95
3.3 Chitosan Preparation …………………………………………………………………………………………………….95
3.3.1 Deacetylation (DD) of the Chitosan …………………………………………………………………………………95
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3.3.3 Determination of Percentage Degree of Deacetylation (DD %)………………………………………..95
3.3.4 ANALYSIS OF THE CHITIN ………………………………………………………………………………………….……….96
3.3.4.1 Loss on Drying ……………………………………………………………………………………………………………96
3.3.4.2 pH Value ………………………………………………………………………………………………………………….96
3.3.4.3 Solubility in Chemicals ………………………………………………………………………………………………..97
3.3.4.4 Solubility Test …………………………………………………………………………………………………………….97
3.3.4.5 Ash Content ………………………………………………………………………………………………………………97
3.3.4.6 Moisture Content ……………………………………………………………………………………………………….98
3.3.4.7 Density of the Chitosan ……………………………………………………………………………………………….98
3.3.4.8 Viscometric Measurement of Chitin/Chitosan ……………………………………………………………….98
3.3.4.9 Characterization of Chitin/Chitosan ……………………………………………………………………………..99
3.4 Fabrication of Composites ………………………………………………………………………………………….99
3.4.1.0 Analysis of Polymer Composites …………………………………………………………………………………100
3.4.1.1 Water Absorption ……………………………………………………………………………………………………………100
3.4.1.2 Density ……………………………………………………………………………………………………………………100
3.4.1.3 Chemical Resistance …………………………………………………………………………………………………101
3.4.1.4 Swelling Test ……………………………………………………………………………………………………………101
3.5 Determination of Optimum Filler Load on P: F Mole Ratio (1P:2F to 1P:8F) ……………….. 102
3.5.1 Tensile Properties……………………………………………………………………………………………………..102
3.5.1.1 Tensile Strength Test ………………………………………………………………………………………………..102
3.5.2 Flexural Test ……………………………………………………………………………………………………………103
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3.5.3 Impact Strength ………………………………………………………………………………………………………..103
3.5.4 Hardness Testing ………………………………………………………………………………………………………104
3.6 Determination of Formaldehyde Released from the Fabricated Composites …………….. 105
3.7 X-ray Diffraction (XRD) ………………………………………………………………………………………… 105
3.8 Scanning Electron Microscophy( SEM) ………………………………………………………………….. 106
3.9 Thermal Analysis ……………………………………………………………………………………………………107
CHAPTER FOUR
4.0 RESULTS ……………………………………………………………………………………………………….. 108
4.1 Results Obtained from Synthesized Phenol Formaldehyde (Novolac) Resins …………………… 108
4.2.1 FT-IR Analysis of Synthesized P:4F, 1P:5F and 1P:6F P:F Novolac Resins——————————108
4.2.2 characterization of Synthsized Phenol Formaldehyde Novolac Resins …………………………………..110
4.3 Results Obtained from Chitosan Preparation ……………………………………………………………. 110
4.3.1 Characterization of Chitin / Chitosan ………………………………………………………………………………..110
4.4 Fabrication of Composite ………………………………………………………………………………………. 112
4.4.1 Results of the Synthesized Resins Reinforced With Chitosan at Different Loading . ……………..112
4.4.2 Optimization of Chitosan/Phenol Formaldehyde Composite ……………………………………………….113
4.4.2.1 Results obtained from Characterization of the Optimum Composites of 1P:2F and 1P:4F ………114
4.4.2.2 XRD, SEM, and TGA/DTA Composites of 1P:2F and 1P:4F ……………………………………………………114
4.4.2.2.1 XRD of Neat Matrix and the Deacetylated (DD) 40% of 1P:4F of 70%, 81% and 91% . …………114
4.4.2.2.2 SEM Images of fractured surface of neat matrix & composites of 70% and 91% DD. ………….118
4.4.2.2.3 TGA/DTA of neat matrix and the deacetylated (DD) of 1P:4F of 70%, 81% and 91% . …………122
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CHAPTER FIVE
5.0 DISCUSSION ………………………………………………………………………………………………….. 126
5.1 Physiochemical Properties of Synthesized 1P: 2F to 1P:8F Novolac Resins. …………………….. 126
5.1.1 Viscosity and pH of Synthesized Novolac Resins ………………………………………………………………..126
5.1.2 Water Tolerance ……………………………………………………………………………………………………………128
5.1.3. Density …………………………………………………………………………………………………………………………131
5.1.4 Resin Solid Content ………………………………………………………………………………………………………..132
5.1.5. Gel-time and Cure- time …………………………………………………………………………………………………134
5.1.6 Cure-time and Yield % ……………………………………………………………………………………………………136
5.1.7 Free Formaldehyde / Free Phenol and pH ………………………………………………………………………..137
5.1.8 FT – IR ………………………………………………………………………………………………………………………….146
5.2 Physicochemical Properties of Chitin / Chitosan ………………………………………………………… 148
5.2.1 Degree of Deacetylation (DD) ………………………………………………………………………………………….148
5.2.2 Loss on Drying —————————————————————————————————— 149
5.2.3 pH Value ………………………………………………………………………………………………………………………150
5.2.4 Solution Properties ………………………………………………………………………………………………………..151
5.2.4.1 Solubility in Different Solvents ………………………………………………………………………………………..151
5.2.4.2 Solubility ………………………………………………………………………………………………………………………152
5.2.5 Ash Value ……………………………………………………………………………………………………………………..153
5.2.6 Moisture Content ………………………………………………………………………………………………………….153
5.2.7 Density …………………………………………………………………………………………………………………………154
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5.2.8 Viscosity ………………………………………………………………………………………………………………………..155
5.2.9 FT-IR ……………………………………………………………………………………………………………………………..156
5.2.10 X-ray Diffraction (XRD) ……………………………………………………………………………………………………………157
5.3 COMPOSITE FABRICATION.. …………………………………………………….…………………………………………… 157
5.3.1 Density..………………………………………………………………………………………………………………………………, 157
5.3.2 Water Absorption …………………………………………………………………………………………………………..160
5.3.3 Mechanical Properties of Phenol Formaldehyde at Different Filler Loading . ………………………..162
5.3.3.1 Tensile Strength /Modulus ……………………………………………………………………………………………..164
5.3.4 Optimization of Chitosan Phenol Formaldehyde Composite ………………………………………………168
5.3.5 Analysis of the Optimum Composites of 1P:2F and 1P:4F ………………………………………………….169
5.3.5.1 Water Absorption …………………………………………………………………………………………………………169
5.3.5.2 Density..………………………………………………………………………………………………………………………………. 171
5.3.5.3 Chemical Resistance ……………………………………………………………………………………………………..173
5.3.5.4 Swelling Behaviors ……………………………………………………………………………………………………….176
5.3.5.5 Tensile Strength and Tensile Modulus ……………………………………………………………………………178
5.3.5.6 Flexural Test ……………………………………………………………………………………………………………….181
5.3.5.7 Impact Strength …………………………………………………………………………………………………………..183
5.3.5.8 Hardness Testing ……………………………………………………………………………………………………….. 186
5.3.5.9 Effects of Degree of Deacetylation (DD) on the Emission of Formaldhyd ………………………… 187
5.3.5.10 Effect of Chitosan in the Reduction of Formaldehyde Release i …………………………………………199
5.3.5.10.1 Mechanism Reaction of Chitosan and Novolac Resin ………………………………………………………200
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5.3.6 X-ray Diffraction ( xrd) ……………………………………………………………………………………………….218
5.3.6.1 Degree of deacetylation (DD) and X-ray diffraction (XRD) ………………………………………………218
5.3.6.2 Effect of DD on Crystallinity (Cr) ………………………………………………………………………………….208
5.3.7 Scanning Electron Microscophy (SEM) …………………………………………………………………………211
5.3.7.1 Changes in Morphology during Deacetylation ………………………………………………………………211
5.3.7.2 Morphological Study ………………………………………………………………………………………………….212
5.3.8 THERMAL ANALYSIS ………………………………………………………………………………………………….214
5.4.8.1 Thermal (TGA/DTA) …………………………………………………………………………………………………..214
CHAPTER SIX
6.0 SUMMARY, CONCLUSION AND RECOMMENDATIONS ……………………… 217
6.1 Summary……….………………………………………………………………………………………………………………….217
6.2 Conclusion ……………………………………………………………………………………………………. 222
6.3 Recommendations …………………………………………………………………………………………………..227
6.4 Contribution to Knowledge………………………………………………………………………………………229
REFERENCES …………………………………………………………………………………………………… 230
APPENDICES ……………………………………………………………………………………………………………………………..312
CHAPTER ONE
1.0 INTRODUCTION
1.1 Resins
Resins are low molecular weight amorphous polymers. They normally have a melting or softening range, and are brittle at the solid state. Their main applications are in adhesives, inks, and chewing gum. Mostly, they are used together with larger polymers, which form the backbone of the adhesive and thus generate cohesion. Formulators use resins to create the best balance between adhesion and cohesion. Resin can be divided as synthetic and natural resins (Chretien , 1999).
Synthetic resins systems that are commonly used in advanced fibre-reinforced composites are namely, thermosets and thermoplastics. The difference between the two categories is that a thermosetting material undergoes an irreversible reaction during processing, where by it is permanently hardened due to the formation of cross-links between the monomer units. Thermosetting resins generally degrade if heating is continued . Major thermosetting resins for adhesive are phenol-formaldehyde (PF) resins, and melamine formaldehyde. Others are epoxy and polyester resins. Thermoplastic resins are polymers that soften and flow at elevated temperature and solidity at low temperatures. They are also called hot melt adhesives. Adhesives strengths with these thermoplastics resins are mainly from mechanical interlocking. Polypropylene and polyethene are examples of thermoplastics ( Knop and Pilato 1985; Martin, 2013).
Rosin, a natural resin, is one of the oldest raw materials for the adhesives industry. There are three types of resin manufactured, gum rosin, wood rosin and tall oil rosin, all generated from the pine tree. Rosin resins, unlike hydrocarbon resins are not polymers. In fact, they are a blend of different molecules. They contain abietic acid and its double bond isomers as main
2
components. Their molecules have poor stability caused by unsaturation. Stability can be improved by either disproportionation or hydrogenation. Phenolic resins are therefore often classified as “water-soluble”, “alcohol-soluble”, “oil-soluble”, etc ((Chretien, 1999; Pizzi, 2003).
1.1.1 Polymers
Phenolic resins (PF) are one of the most widely used thermosetting polymers. They are polycondensation products of phenol and formaldehyde. These are widely used because of their thermal stability, water resistance, binding strength, chemical resistance, electric insulation and dimensional stability. These have attracted special interest because of their large range of industrial applications, in adhesives, casting, molding and structural parts. PF are synthesized by reacting phenol with formaldehyde in the presence of an acid or alkali. Depending on the ratio of phenol (P) to formaldehyde (F) and the type of catalyst used, these are classified as resoles and novolacs. Novolacs are prepared with a formaldehyde to phenol (F/P) molar ratio <1.0 in presence of an acid catalyst where as resoles usually have F/P molar ratio >1.0 and employ an alkali catalyst (Christy, 2000).
Of these two types, resoles are reactive type resins and one – step process that are either liquid or brittle, soluble, fusible solids. They can be cross-linked by application of heat, without any catalyst (Knop and Scheih, 1979). Novolacs do not contain any reactive groups and hence require the addition of a crosslinking agent and heat to acheive cure. The novolacs are referred to as two-step process resins. The literature indicates that PF resin is a major adhesive used in exterior grade wood-based panels (Knop and Pilato, 1985; Christy, 2000). However, the marked disadvantages of the PF resin are relatively longer cure and press time, dark colour bond line, environmental issues of toxicity, and non-biodegradation. And also, the relationship between the synthesis conditions, the structure and the mechanical properties has not yet been completely
3
clarified. One of the reasons for this is that the polymezation of the resins has a complicated reaction mechanism.
Chitosan is a modified carbohydrate polymer derived from chitin, one of the most abundant polysaccharides in nature, by deacetylation of chitin to different degrees. Chitin is present in crustacean shells, insect exoskeletons, fungal cell walls, microfauna, and plankton. It is formed by β-(1→4)-linked 2-amino-2-deoxy-D-glucopyranose and 2-acetamido-2-deoxy-D-glucopyranose units (Moiteiro and Airoidi, 1999). The presence of of amino groups in chitosan is responsible for its polycationic nature in acidic solutions. An important advantage of chitosan is the possibility of agents that perform chemical modifications on its structure by binding to amino and hydroxyl groups.
It is quite clear from the chemistry of chitosan and PF that crosslinking is possible between chitosan molecules and PF by methylene linkages. It was planned to utilize this possibility of cross-linking for the modification of PF resin by incorporating natural polymer like chitosan for the preparation of this composites.
1.1.2 Polymer Gel
Polymer gels are cross-linked networks of polymers which behave as viscoelastic solids. Because the polymer network is cross-linked, the gel network consists of a very large branched polymer which spans the entire gel. While gel can be soft and deformable, they also hold their shape like solid. Depending on the physical structure of the polymer network, gels can be strong, weak, or pseudo gels (Ross-Murphy, 1995). Chemically cross-linked gels are considered strong gels. Soft polymer gels can have excellent adhesion properties due to both the elastic and viscous properties (Zosel, 1991; Lenhart, 2006 ; Andrews and Jones, 2006).
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A gel is dispersion of a three dimensional network that entraps a solvent medium. Classification of gels is based on the type of links that creates the three dimensional structures, the size and shape of the gel configuration and the type of solvent. Gels can form by covalent bonding, called chemical gels or by physical interactions at colloidal level, called physical gels. From the technical point of view, the determination of the gel time is important because it gives an indication of the curing process of the resin and the phase transition from the liquid to the solid state. Chemical cure of P: F resins as thermosetting polymers is associated with significant changes in their physical properties such as gelation and vitrification. As the condensation of resin commences, molar mass advancement occurs, leading to a gel state which is no longer soluble in water. While the water is evaporated during heating, the flexible phenolic intermediate becomes rigid and cross-linked (Gardziella, 2000).
The main drawback preventing wider use of P: F resins in the manufacture of composites is its relatively slow cure rate. Many attempts have been made to cure P: F resins faster for the composite to be bonded at higher moisture contents and at lower press temperatures. Faster curing would also result in lower emission of volatile organic compounds during manufacturing of composites (Sііmer and Kaljuvee, 2008). Different catalysis, additives, or modified resin formulations have been proposed as P: F resins cure accelerators (Astarloa-Aierbe, 1998; Lorenz and Conner, 2000). The fast curing process of PF resins may lead to changes in micro cross-linking within the cured resin system, which has implication with respect to the thermal properties, thermal stability, durability, and service life of the composites (Yuzhu-Chen et al., 2014).
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1.2 Polymer Composite
The first composite materials may have been bricks fashioned by the ancient Egyptians from mud and straw (Julien, 2011). Commercialization of composites can be traced to earlier this century when cellulose fibres were used to reinforce phenolic and later urea and melamine resins. Probably the most familiar composite materials is fiberglass, which is widely used to form large, lightweight reinforced structures.
Polymer matrix composites are composites made from polymers or polymer along with other kinds of materials such as fibres (carbon, glass, cellulose, metal etc.) as reinforcement. The principal constituents of polymer matrix composites, which influence the strength and stiffness of the modern advanced polymer composites, are the reinforcing fibres, the matrix and interface. Each of these individual phases has to perform certain essential functions based on their mechanical properties so that a system containing them may perform satisfactorily as a composite.
Composites are used in a wide range of applications, wherever high strength – to – weight ratios are important. Principal uses are found in the automobile, marine and construction industries. In the majority of cases, especially those requiring high performance in the automotive and aerospace industries, the discontinuous phase or filler is in the form of a fibre. Typical fibres for composites applications include carbon or graphite, glass and aromatic polyamide (Knop and Pllato, 1985).
1.3 Composite Concepts
The desire for specified material properties has led to the unusual combination of materials. This is especially true for materials needed for many of our modern day technologies. This desire has led to the production of certain special materials known as composites. A composite
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material is made by combining two or more materials to give unique combination of properties. This definition is found to be more general and can include metal alloys, plastic co-polymers, minerals and wood. One of the most unique composites is fibre-reinforced. They differ from the earlier mentioned materials in that their constituent materials are different at molecular level and are mechanically separable. The constituent materials together provide the desired properties but remain in their original forms. The final properties of the composite materials are better than that of the constituent materials.
The main concept of a composite is that it is made up of matrix materials. Most composites are formed by reinforcing fibres in a certain matrix resin. The reinforcements can be fibres, particulates or whiskers and the matrix material can be metals, plastic or ceramics. The scheme 1.1 below shows a schematic diagram illustrating the formation of a composite.
Fibres Matrix Composite
Scheme 1.1: Formation of a composite material
In a composite, the fibres can be continuous, discontinues and particulate. The fibre carries the load and its strength is greatest along the axis of the fibre. The scheme 1.2 shows the fibre forms in a composite.
+
=
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Continuous fibre composite discontinues fibre composite
Scheme 1.2: Fibre forms in a composite
Long continuous fibre in the direction of the load results in composite with properties far better than the plain matrix. The same is obtainable with the shorter fibres except that it provides properties less than that offered by the continuous fibre composites ( Knop and Pilato, 1985). The fibre form is selected depending on the application and manufacture method.
1.3.1.0 Types of Composites Materials
Broadly, composite materials can be classified into three groups on the basis of matrix material. They are, metal matrix composites (MMC), ceramic matrix composites (CMC), and polymer matrix composites (PMC).
1.3.1.1 Metal Matrix Composites
Higher specific modulus, higher specific strength, better properties at elevated temperatures and lower coefficient of thermal expansion are the advantages of metal matrix composites over monolithic metals. Because of these attributes metal matrix composites are under consideration for wide range of applications viz combustion chamber nozzle (in rocket, space shutter), housings, tubing, cables, heat exchangers, structural members etc.
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1.3.1.2 Ceramic Matrix Composites
One of the main objectives in producing ceramic matrix composites is to increase the toughness, naturally it is hoped and indeed often found that there is a concomitant improvement in strength and stiffness in ceramic matrix composites.
1.3.1.3 Polymer Matrix Composites
Polymeric matrix composites are the most commonly used matrix materials. The reasons for this are two-fold. In general the mechanical properties of polymers are inadequate for many structural purposes. In particular their strength and stiffness are low compared to metals and ceramics. By reinforcing other material with polymers, these difficulties can be overcome. Secondly high pressure and high temperature are not required in the processing of polymer matrix composites. For these reasons polymer composites developed rapidly and became popular for structural applications in no time. Polymer composites are used because overall properties of the composites are superior to those of the individual polymers. Modern composites are usually made of two components a fiber and a matrix ( Ashish and Blbir, 2013).
1.3.2.1 Fibre
Fibres are polymers that have very high resistance to deformation-they undergo only low elongations(<10-50%) and have very high moduli (>35.000Ncm2) and tensile strengths (>35.000Ncm-2) (Kurita and Sannan, 1992). According to (Alves and Mano, 2008), fibre is a material made into a filament, a single fibre usually has a diameter up to 15μm. Bigger diameters generally increase the probability of surface defects. The main fuctions of fibres are to carry the load and provide stiffness, strength, thermal stability and other structional properties to the fibre reinforced polymer (Maron, 1998). To perform these functions, the fibres in fibre reinforced polymer composite must have high modulus of elasticity, high ultimate strength, and low
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variation of strength among fibres, high stability of their strength during handling and high uniformity of diameter and surface dimension among fibres (Warner and Rezau, 1997).
1.3.2.2 Matrix
The matrix materials is a polymer composed of molecules made of many simpler and smaller units called monomers (Bledzki et al., 1993). The matrix must have a lower modulus and greater elongation than those of fibres, so that fibres can carry maximum load. The important functions of matrix material in fibre reinforced polymer composite include some of the following; binding the fibres together and fixing them in the desire geometrical arrangement; transferring the load to the fibres by adhesion and/or friction; provide rigidity and shape to the structural member; isolate the fibres so that they can act separately, resulting in slow or no crack propagation; provide protection to the fibres against chemical and mechanical damages; influence performance characteristic such as ductility, impact strength; and provide final colour and surface finish for connections. The fibre is embedded in the matrix in order to make the matrix stronger.
1.3.2.3 Reinforcements
Fillers with high aspect ratio (I/d), are classified as reinforcements. Where I: fibre length and d is the diameter of the fibre. The amount of fibre: the strength and stiffness of the composites increase with increasing the volume fraction: Orientation of the fibre has a great role in the strength of the composites. The strength and stiffness of polymers or composites are improved by adding fibres of glass, carbon, boron etc, as reinforcements for polymer composites.
1.3.3.0 Composite Fabrication
Composites are processed by a variety of methods, including compression and resin- transfer molding. Specialized processing operations for composite fabrication are filament winding and pultrusion (Rahman, 2008).
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1.3.3.1 Filament Winding
In this process fibres are pulled from bobbing through a bath containing the composite resin, such as epoxy or polyester formulation, and then the impregnated fibres are wound on to a form (the mandrel) in some predetermined arrangement. Once the mandrel is uniformly covered to the desired thickness and fibre orientation, the composite is cured at elevated temperature and the mandrel may be removed or left as an integral part of the composite. Filament winding may be used to prepare corrosion-resistant (fibreglass) tanks and pipes. Employing advanced resin materials, filament winding is also being used to prepare high-performance composites for structural and other applications. The continuous reinforcement and controlled fibre orientation that can be achieved by filament winding to provide a higher level of reinforcement than discontinuous reinforcement using individual fibres (Knop and Pilato, 1985; Julien, 2011).
1.3.3.2 Pultrusion
A simple pultrusion operation is a completely continuous process since the cure is on-line. This makes pultrusion a suitable process for commercial production lines producing a variety of composite shapes or profiles. A roving of continuous fibres (e.g E.glass) and a continuous-strand met (typically glass/polyester) are combined and immersed in a resin both before passing, through a forming guide and the curing oven. The majority of composites that are pultruded are the fibre-glass variety prepared from unsaturated polyester resin and E – glass. Fibre loading in pultrusion may range from 20% to 80% (Kaith and Ashish, 2008).
1.4.1 Physical and Chemical Properties of Formaldehyde
Formaldehyde was first reported by Russion chemist Aleksandr Butlerov (1828-86) and was conclusively identified by August Von Hofmann, it is a colourless gas that is highly reactive. The compound is soluble in water, ethanol diethyl ether and acetone. In aqueous solution,
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methyleneglycol and polymethylene are formed. Formaldehyde is commonly purchased with a stabilizer. In a recent review an estimated output of 32 million tons of formaldehyde in 2006 with the highest producers being china (34%), the United States (14%), and Germany (8%). More than 65% of the total formaldehyde is used to synthesise- resins. Formaldehyde is more complicated than many single carbon compounds because it adopts different forms. Formaldehyde is a gas at room temperature, but the gas readily converts to a variety of derivatives, which are often used in place of the gas. One important derivative is the cyclic trimer formaldehydryde or trioxane (Julien et al., 2011).
Formaldehyde when dissolved in water combines with water to form methanediol or methylene glycol. The diol also exists in equilibrium with a series of didomers, depending on the concentration and temperature. A saturated water solution, that contains about 40% formaldehyde by volume or 37% by mass, is called Neat matrix formalin .usual a small amount of stabilization such as methanol, is added to limit oxidation and polymerization. A typical commercial grade formalin may contain 10 to 12% methanol in addition to various metallic impurities (Ko, 1976).
Formaldehyde is naturally occurring substantialy in the environment and in the upper atmosphere. Formaldehyde is an intermediate in the oxidation (or combustion) of methane as well as other carbon compound e.g. forest fires, in automobile exhaust, and in tobacco smoke. When produced in the atmospheric methane and other hydrocarbon it becomes part of smog. Formaldehyde has also been detected in outer space, as well as its oligomers and organisms. Formaldehyde is the primary cause of methanol’s toxicity, since methanol is metabolized into toxic formaldehyde by methanol dehydrogenation. Formaldehyde does not accumulate in the environment, because it is broken down within a few hours by sunlight or by bacteria present in
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soil or water. Human’s metabolism destroys formaldehyde quickly, so it does not accumulate, and is converted to dermic aid in the body (Knop and Pilato, 1985; Julien, 2011).
Small amounts of formaldehyde are produced in case of incomplete combustion of methane gas. Since formaldehyde is a colourless pungent irritating gas it is generally marketed as a mixture of oligomers of polymethylene glycols. Paraformal is a white solid containing mostly polymethylene glycols with 100 repeat units. It is prepared by distilling aqueous formaldehyde solution and generally contains 1-7 wt % water. Structure with 8- 100 formaldehyde units per molecule. The name paraformaldehyde describes polymeric structure with 8-100 formaldehyde units per molecule. The cyclic trimer of formaldehyde (C3H8) is called 1, 3, 5-trioxane. Formaldehyde has a dipolar resonance, which makes the molecule a typical electrophone (Julien et al., 2011)..
1.4.2 Usage and Application of Formaldehyde
Formaldehyde is a chemical feedstock for numerous industrial processes. It is also used as a preservative, disinfectant, and biocide. As far as the indoor environment is concerned, its use as a component of thermosetting plastics is of particular significance (Knop and Pilato, 1985).
Urea-formaldehyde (UF) adhesives (so-called aminoplasts) are still the most commonly used products in the manufacturing of wood-based materials and furniture due to their rapid curing, their compatibility with additives, and their low price. In the first step, mono-, di-, and trimethylolurea are formed from formaldehyde and urea in a Mannich reaction. This is followed by condensation reactions to build up the polymer. UF adhesives have poor water resistance: the presence of water results in a hydrolysis of the C-N bond and, as a consequence, the release of formaldehyde ((Julien et al., 2011).
Melamine-urea-formaldehyde (MUF) adhesives are similar to UF adhesives. They are produced by mixing portions of UF and melamine-formaldehyde (MF) or by condensation of
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all monomers in one batch. Melamine-formaldehyde resins are excellent exterior wood adhesives because of their water resistance (Mano, 1992).
Phenol-formaldehyde (PF) adhesives (so-called phenoplasts) are made by electrophilic substitution to methylol phenol in the first step. In alkaline solution, the reaction results in highly viscous resins of low molecular weight, called resoles. A novolac with a high degree of crosslinking is formed in acidic solution. PF adhesives are very stable and water-resistant and have a high adherence to wood. In the past, plastics made of PF resins were also known as Bakelite and were, among other things, used as casings for telephones, radios, etc (Knop and
Melamine-urea-phenol-formaldehyde (MUPF) adhesives are used for the production of moisture-proofed wood based products and for construction materials. Like MUF adhesives, they are produced by the addition of small amounts of phenol (Mano , 1992).
1.4.3 Formaldehyde and Health Effects
Formaldehyde (FD) is naturally occurring substance which is widely met both in human and plant. It is also used in the production of other chemical products and material (i.e. dyes fabrics, medicines). Indoors formaldehyde can be emitted from the furniture that contain composite wood products, by various sheds, in sulfating materials, wall papers, paints etc. Formaldehyde is a colourless, but strong smelling gas and thus can be easily detected. When present in the air at levels above 0.I ppm, it can cause watery eyes burning sensations in the eyes, nose and throat nausea, coughing, chest tightness, whetting and headache, can affect people differently, some people are very sensitive to formaldehyde while others may not have any noticeable reaction to same levels. Most of these symptoms disappear via exposure in clean air. Some people may also show skin rashes, and allergic reactions. It is pointed out that FD is neither accumulated in the environment nor in the human organism because it is quickly oxidized and biodegraded (Mari, 1983; Julien et al., 2011).
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It is argued that there are more severe effects at long – term exposure. Scientific studies in the early 1980s have demonstrated that formaldehyde is a strong nasal carcinogen (Richardson, 1999). Recently, FD was classified as a probable” human carcinogen (category 2A) by the international Agency for Research on cancer (IARC), under the world health organizations (WHO). In June 2004, ARC decided to reclassify formaldehyde to known” human carcinogens. This was based on studies of reducing carcinogenicity in industrial or exposed to high FD comes 30 to 60 years ago. In the European Union, FD is classified in category 3, and it varies from one country to another ( Mari, 1983; Julien et al., 2011).
1.4.4 Formaldehyde as Indoor Pollutant
Discussion about formaldehyde as a possible carcinogen started in 1980 when the carcinogenicity of formaldehyde in rats and mice after long-term inhalation exposure was reported. These publications and the results of studies of human exposure assessment for formaldehyde triggered an avalanche of scientific work as well as stories in the yellow press. Although electronic databases and powerful search engines are now available, it is still difficult to survey all papers in the technical and medical literature. Notwithstanding this, formaldehyde is definitely the most common and the best-known indoor air pollutant (Salthammer et al., 2010).
Over the years, the release of formaldehyde from building products has been decreasing. On the other hand, formaldehyde concentrations in ambient air are increasing continuously, especially in the urban environment. The number of articles on indoor pollution with formaldehyde is more as compared to outdoor. This high number of articles is because of the various resources of formaldehyde and also low air exchange rates in the indoor environment (Salthammer et al., 2010). A relation between wood based products and formaldehyde emission was recognized by
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scientists during 1960 to 1970. Particle board, could release high concentration of formaldehyde into the homes and offices because of reversibility of reaction between urea and formaldehyde.
Sevaral European standard committee for formaldehyde and other regulated dangerous substances have laid down permissible expore levels of formaldehyde by inhalation. And these include the formaldehyde in the Indoor Environment Chemical Reviews, (2010), formaldehyde emission from furniture coatings in Germany, 1992 and high formaldehyde concentrations in mobile homes in the United States 2006.
However, in 2004, formaldehyde discussions were generally taken up again when formaldehyde was considered as carcinogenic for humans. As a consequence, various authorities and institutions have proposed new indoor air guidelines, giving values that are nearly ubiquitous. Although a prioritized ranking of chemicals and exposures that cause concern is difficult and uncertain, the Scientific Committee on Health and Environmental Risks (SCHER) states that formaldehyde (like carbon monoxide, nitrogen dioxide, benzene, naphthalene, environmental tobacco smoke (ETS), radon, lead, and organophosphate pesticides) is a compound of concern in the indoor environment (Salthammer et al., 2010).
1.5.0 Phenol
Phenol compounds are some of the most important contaminants present in the environment as a result of various processes, such as the production of plastics, dyes, pesticides, paper and petrochemical product (Connnzalez-Toledom, 2001 ; Penalver, 2002). They are often found in waters ( LJompart, 2002), soils Baciocchi (2001), and sediments, because of their toxicity, phenols are included in the list of proxity pollutants in many countries and are required to be determined. Therefore phenols are monitored as important organic pollutants in the world (Deutsche, 1984; APHA, 1985). There are many kinds of phenols (e.g. mono-, binary-, polyhydric – phenols, polysubstituents and polysubstituted phenols and their isomer etc.
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1.5.1 Physical and Chemical Properties of Phenol
Phenol is a member of a homologous series of compounds containing a hydroxyl group bound directly to the aromatic ring. Phenol, or PhOH, belongs to the family of alcohol due to the presence of the OH group. The hydroxyl group of phenol determines its acidity whereas the benzene ring characterizes its basicity. Thus, it is formally the enol form of the carbonyl group. Phenol has a low melting point, it crystallizes in colourless prisms and has a characteristic, slightly pungent odour. In the molten state, it is clear, colourless, mobile liquid. In the temperature range T<68.4oC, its miscibility with water is limited: above this temperature it is completely miscible. The melting and solidification points of phenol are quite substantially lowered by water (Knop and Pilato, 1985; Mo et al., 2014) .
A mixture of phenol and 10% water is called phenolum liquefactum, because it is actually a liquid at room temperature. Phenol is readily soluble in most organic solvents (aromatic, hydrocarbons, alcohols, ketones, ethers, acids, halo-generated hydrocarbons etc.) and somewhat less soluble in aliphatic hydrocarbons. Phenols form azeotropic mixtures with water and other substances. Phenol can be considered as the enol of cyclohexadienone. While the tautometric keto-enol equilibrium lies far to the ketone side in the case of aliphatic ketones, for phenol it is shifted almost completely to the enol side. The reason of such stabilization is the formation of the aromatic system. The resonance stabilization is very high due to the contribution of the ortho- and para-quinonoid resonance structures which stabilize the negative charge (Mo,. et al, 2014).
In contrast to aliphatic alcohols, which are mostly less acidic than phenol, phenol forms salts with aqueous alkali hydroxide solutions. The contribution of ortho- and para-quinonoid resonance structures allows electrophilic substitution reactions such as chlorination, sulphonation, nitration, nitrosation and merceration. The reaction in the presence of acid catalyst
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is used to remove impurities from synthetic phenol. Phenol readily couples with diazonium salts to yield coloured compounds. Alkylation and acylation of phenols can be carried out with aluminium chloride as catalyst: methyl groups can also be introduced by the Mannich reaction. Diaryl ethers can be produced under extreme conditions (Knop and Pilato, 1985; Julien et al., 2011).
1.5.2 Phenol Usage and Production
Phenol is one of the most versatile and important industrial organic chemicals. Until World War II, phenol was essentially a natural coal-tar product. Eventually, synthetic methods replaced extraction from natural sources because its consumption had risen significantly. For instance, as a metabolic product, phenol is normally excreted in quantities of up to 40 mg/ L in human urine. Currently, small amounts of phenol are obtained from coal tar. Higher quantities are formed in coking or low-temperature carbonization of wood, brown coal or hard coal and in oil cracking. The earlier methods of synthesis (via benzenesulphone acid and chlorobenzene) have been replaced by modern processes, mainly by the Hock process starting from cumene, via the Raschig–Dow process and by sulphonation. Phenol is also formed during petroleum cracking ( Moder and Frank, 1997; Julien et al., 2011)..
Phenol has achieved considerable importance as the starting material for numerous intermediates and final products. Phenol occurs as a component or as an addition product in natural products and organisms. For example, it is a component of lignin, from which it can be liberated by hydrolysis. Lignin is a complex biopolymer that accounts for 20 to 30% of the dry weight of wood. It is formed by a free-radical polymerization of substituted phenyl propane units to give an amorphous polymer with a number of different functional groups including aryl ether linkages, phenols and benzyl alcohols. Most pulp-processing methods involve oxidative degradation of lignin, since its presence is a limitation to the utilization of wood pulps for high
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end uses such as print and magazine grade paper. Such limitation is due to the photo induced yellowing of lignin-rich, high-yield mechanical pulps and, as a result, the photooxidative yellowing has been extensively studied in the hope of understanding its mechanism and ultimately preventing its occurrence. Phenoxyl radicals are produced during the photo oxidation of lignin and their subsequent oxidation ultimately leads to quinones, which are actually responsible for the yellow colour (Knop and Sheih, 1979 ; Rebecca, 2003).
Phenol was first used as a disinfectant in 1865 by the British surgeon Joseph Lister at Glasgow University, Scotland, for sterilizing wounds, surgical dressings and instruments. He showed that if phenol was used in operating theatres to sterilize equipment and dressings, there was less infection of wounds and, moreover, the patients stood a much better chance of survival. By the time of his death, 47 years later, Lister’s method of antiseptic surgery (Lister spray) was accepted worldwide. Its dilute solutions are useful antiseptics and, as a result of Lister’s success, phenol became a popular household antiseptic. Phenol was put as an additive in a so-called carbolic soap. Despite its benefits at that time, this soap is now banned. In (Salthammer et al, 2010), Dangerous Properties of Industrial Materials, one finds frightening phrases like ‘kidney damage’, ‘toxic fumes’ and ‘co-carcinogen’. Clearly, phenol is totally unsuitable for general use, but the benefits 130 years ago plainly outweighed the disadvantages. However, because of its protein-degenerating effect, it often had a severely corrosive effect on the skin and mucous membranes ( Christy, 2000; Julien et al., 2011).
In the US, phenols are ranked in the top 50 major chemicals. In 1995 the total annual production of phenols was estimated at 4 to 5 billion pounds. In Japan the production of phenols in the late nineties was estimated approximately on the same level -1,200,000 tons per year. In 1995, 95% of US phenol production was based on oxidation of cumene, the exception being one company that used toluene oxidation and some companies that distilled phenol from petroleum. Two
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major uses of phenols in 1995 were the production of Bisphenol-A [4, 4-isopropylidenediphenol] (35%) and the production of phenolic resins (34%). Other uses include production of caprolactam (15%), aniline, (5%), alkylphenols (5%), xylenols (5%) and other miscellaneous compounds (1%). Bisphenol A is one of the raw materials widely used in the production of epoxy resins. Being inert, strong and adhesive with high insulator properties these polymers found their application in construction, coatings and bonding. In addition, Bisphenol A is used for the production of polycarbonate plastics, found in such products as baby food bottles, food cans, dental sealants, food packing and coatings. In the US alone, 1.65 billion pounds of this polymeric compounds are produced each year, and in Japan its production is estimated nowadays at over 200,000 tons per year (Moder and Frank, 1977).
Another group of very widely used compounds is phenols with long aliphatic chains R like octyl or nonyl as shown below. These compounds are important intermediates in the production of polyethoxylate surfactants, which are compounds consisting of alkyl chains attached to a phenol ring and combined with a variable number of ethylene oxides. In 1994 their production in EC countries reached 110,000 tons, mainly for industrial, agricultural and household uses. Moreover, the annual production in all developed countries has been estimated at 0.35 Mton. Environmental effects of substituted phenols owing to their poor ultimate biodegradability and the possible environmental hazard of their metabolites, alkylphenol ethoxylates have been replaced in household applications, mainly by alcohol ethoxylates. However, for industrial applications, this replacement has not been carried out yet due to the excellent performance of alkylphenol ethoxylates and their low production costs (Julien et al., 2011).
1.5.3 Phenol and Health Effect
Wide use of phenol and its derivatives led to studies of its occupational exposure and toxicity. Phenol toxicity in humans is not a big surprise, as this compound is toxic to most microorganisms, which explains its common use as a general disinfectant. This fact complicates
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treatment of phenol-containing wastewater by conventional biological processes. Phenol genotoxicity was determined using Syrian hamster embryo cells. Phenol induced morphological transformation, gene mutation, chromosomal aberrations, sister chromatid exchanges and unscheduled DNA synthesis. 2, 4, 6-Trichlorophenol induced mononuclear cell leukemia in male rats and liver tumors in mice (Gonzales-Cosado et al., 1998). In another study, genotoxicity of this compound was established in V79 Chinese hamster cells. Conversely, 2, 4-dichlorophenol did not cause any increase in tumors in rats or mice in the 2-year study. In fact, mononuclear cell leukemia in rats and lymphomas in mice were decreased in these studies (Stich, 1991).
The damaging effect of long-term exposures (6+ months) to pentachlorophenol (PCP) on the immune system was studied in 190 patients. The distribution of PCP levels in blood was: 0–10 μg l−1 (69%), 11–20 μg l−1 (20%), and >20 μg l−1 (11%). The patients had various clinical symptoms and complained of the following: general fatigue (64%), rapid exhaustion (59%), sleeplessness (53%), headache (44%), mucous membrane, throat and noise irritation (39%), frequent common diseases (36%), bronchitis (30%) and nausea (13%). Analogous symptoms were described in previous studies. Blood levels of PCP were associated negatively with total lymphocyte counts and several other blood immune parameters (Colosia et al., 1993). These data provide clear evidence that immunological abnormalities are associated with high levels of PCP in plasma of individuals with long-term exposure. PCP also induces chromosomal aberrations in mammalian cells in vitro and in lymphocytes of exposed persons in vivo (Gonzalez et al.,1998).
Several case-control studies have shown significant associations of polychlorophenols with several types of cancer, with the most consistent findings being non-Hodgkin lymphoma and soft-tissue sarcoma. Occupational exposure to chlorophenols can be a risk factor for nasal and nasopharyngeal cancer.
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In the studies of polychlorophenols, great importance is attached to a compound’s purity as its contaminants can include very toxic and carcinogenic dioxins. One has to be sure that the observed toxicity effect is connected with the main compound and not with the impurity. Polychlorophenols are also known to uncouple oxidative phosphorylation, alter the electrical conductivity of membranes and inhibit cellular enzymes, such as ATPase, β-galactosidase etc. The genotoxicity of the rodent carcinogen 2, 4, 6-trichlorophenol was studied in V79 Chinese hamster cells (Richardson et al., 1999). This compound did not induce mutation or structural chromosome aberrations; however, it did produce dose-related increases in hyperdiploidy and micronuclei. It appears that it causes chromosome malsegregation as a major mode of genotoxic action. As mentioned above, pentachlorophenol has different solubility at acid and neutral pH. It was shown that this compounds toxicity also depends on pH. Studies of wastewater from a Baikalsk pulp and paper mill allowed one to evaluate a ‘pure’ cellulose bleaching process pollution at Lake Baikal, where it is located, has no agriculture and only little municipal pollution. Although mutagenic activity was effectively decreased during 1960 Victor Glezer biological and chemical treatment, even modern wastewater purification systems do not totally abolish potential toxicity and mutagenity of the effluents (Cristina et al., 2009).
Chlorinated phenols can degrade with formation of highly carcinogenic dibenzo-pdioxin and dibenzofuran derivatives. It can occur by thermolysis, slow combustion, photocatalytically, by photochemical degradation and by photolysis. Even in the presence of TiO2, which in many cases leads to the total degradation of organic compounds, photocatalytic degradation includes formation of polychlorinated dibenzo-p-dioxins and dibenzofurans. It was shown that the level of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans in commercial animal products, raised near incinerators, are elevated compared to products from areas with no such industrial sources. It is related primarily to meat, milk from cows and eggs from chicken. Some
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phenol derivatives can act like hormones (e.g. estrogens) and interact with the human hormonal system (Knop and Pilato, 1985; Cristina et al., 2009).
Two main phenol groups must be mentioned here—Bisphenol A and octyl- and nonylphenols. Bisphenol A might be a factor in decreasing sperm count in males and increasing rates of breast cancer in women. It was also shown that increased sensitivity to Bisphenol A during the perinatal period causes an increase in body weight soon after birth and in adulthood and a decrease of plasma luteinizing hormone level in adulthood. Octyl- and nonylphenols are formed during anaerobic biodegradation of the corresponding alkylphenol ethoxylates. These compounds are known to cause proliferation of breast cancer cells by acting as estrogenic mimic. They also cause endocrine-disrupting effects and ‘feminization’ of male species. In the context of health effects with an emphasis on cancer, phenols, as an independent class of organic compounds, are generally not genotoxic. This means that they cannot modify genes and therefore are not considered to be a direct cancer risk (Richardson et al., 1999).
Laboratory studies have demonstrated that while not genotoxic, phenols can be co-carcinogens or promoters, increasing the effect of environmental genotoxic carcinogens. This promoting effect is highly dependent on the dosage and chronicity of exposure. Recent studies have demonstrated that some phenols found in fruits and vegetables, as well as synthetic phenolic antioxidants, exert protective effects against cancer, demonstrating antimutagenic, anticarcinogenic properties, and can also antagonize the effect of promoters. However, in a high dose range some of them can cause cancer in animals through mechanisms like cytotoxicity, regenerative cell duplication and hydroxyl radical generation. Generally, the neoplastic effects of phenolic antioxidants can be observed at high dietary levels and occur only after effective biological defense mechanisms are overloaded. Therefore, the public needs to be much more aware of the importance of dosage and exposure time (Lee, 2005; Liu et al., 2008; LWPF, 2009).
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1.6.1 Physical and Chemical Properties of Phenolic Resins
Phenolic resins are yellow to brown in colour, and the colouration can be very intense. Pale phenols resins become coloured immediately after production during storage or processing. The colouration is less intense only in the case of phenolic resins from para-alkyl-substituted phenolics they are typically brown in colour, and novolacs are lighter in colour than resoles. Resoles are dark yellow, orange reddish, or brownish even when made with pure raw materials. However, if the alkali is neutralized resoles become almost colourless. Characteristic UV absorption maxima lie at 254nm and 280nm (Lin et al., 2005).
Phenolic resins which are not crosslinked are commercially available as solids or solutions. For particular application e.g. in themoset, the polycondensation can be driven so far that the resins are no longer soluble but can only be swelled by organic solvents (Lee, 2005).
The softening point of solid resins can be determined by the capillary melting point according to DIN53244, by the ring and ball methods or similar procedures. Simple PF resins are readily soluble in alcohol,, esters, ketones, phenols, and some ethers, and insoluble in hydrocarbons and oils. As a class, resoles tend to be more soluble in alcohols and water, and novolacs tend to be more soluble in hydrocarbons. In the early stages of condensation resoles are often soluble in water, owing to the presence of methylol phenols, especially polyalcohol (Gardziiella et al., 2000).
The miscibility with solvents, usually described as ‘’ solubility’’ depends on the structure of the resin ranging from solubility in water to that in naphtha. Resins often have limited miscibility with certain solvents. Soluble phenolic resins have a broad molecular mass distribution; determination usually by gel permeation chromatography and have values of more than 50,000 depending on the type of phenol monomer. The viscosity of phenolic resins or their solutions is measured at high concentrations in 30 to 80% solution.
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Cross-linked phenolic resins are hard substances which only have a small fracture strain and cannot be melted. Decomposition reactions begin at 120 to 250oC, depending on the molecular structure, although oxidative degradation takes the form of attack at the methylene bridges to produce substituted, dihydroxy benzophenones. Above this temperature, they begin to char slowly, and at higher temperatures charring is more rapid at about 400oC decomposition is rapid, yielding phenols and aldehydes, and leaving a coke like residue There are, however also some type of phenolic resin (ether resins) which are stable for some time up to 300oC. Hardened PF resin have a specific gravity of approximately 1.2 to 1.3, a refractive index of 1.6, and a specific heat of 0.5. The phenolic resins can be plasticized, their compatibility with plasticizers can be adjusted by introduction of hydrophilic or hydrophobic groups (Moder and Frank,1997 ; Gardziiella et al., 2000 ; Pistin, 2002)..
1.6.2 Formation and Structure of Phenol Formaldehyde Resin
Phenol-formaldehyde resins, as a group are formed by a step-growth polymerization reaction that can be either acid or base catalyzed by condensing phenol and formaldehyde. Since formaldehyde exists predominantly in solution as a dynamic equilibrium of methylene glycol oligomers, the concentration of the reactive form of formaldehyde depends on temperature and pH. Phenol is reactive towards formaldehyde at the ortho and para sites (sites 2, 4 and 6) allowing up to 3 units of formaldehyde to attach to the ring. The initial reaction in all cases involves the formation of a hydroxymethyl phenol;
HOC6H5 + CH2O HOC6H4 CH2OH
The hydroxymethyl group is capable of reacting with either another free ortho or Para site, or with another hydroxymethyl bridge and the second forms an ether bridge (Knop and Pilato, 1985) (Scheme 1.3).
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OH
Phenol
OH
CH2OH
OH
CH2OH
CH2OH
OH
HOH2C CH2OH
CH2OH
Resole Type Novolac Type
+
+
H H
O
+
Formaldehyde
CH2-O-CH2
OH OH
OH OH
CH2
Ether linkage Methylene linkage
Scheme1.3 : ll lustration of the formation of the Resole and Novolac type structure respectively.
1.6.2.1 Resole Type (Base Catalysed)
Base-catalysed phenol-formaldehyde resins are made with formaldehyde to phenol ratio of
greater than one (usually around 1.5). These resins are called resoles. Phenol, formaldehyde,
water and catalyst are mixed in the desired amount, depending on the resin to be formed, and
are then heated. The first part of the reaction at round 700C, forms a thick reddish-brown tacky
material, which is rich in hydroxymethyl and benzylic ether groups. The rate of base catalysed
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reaction initially increases with pH, and reaches a maximum at about pH 10. The reactive species is the phenoxide (C6H5O-) formed by de protonation of phenol. The negative charge is delocalized over the aromatic ring activating sites 2, 4 and 6, which then react with the formaldehyde. Being thermosets, hydroxymethyl phenols will crosslink on heating to around 1200C to form methylene and methyl ether bridges. At this point the resin is a 3 – dimensional network which is typical of polymerized phenolic resins. The high cross linking gives this type of phenolic resin its hardness, good thermal stability, and chemical imperviousness (Young and lovell, 1991).
1.6.2.2 Novalac Type (Acid catalysed)
Novalac are phenol-formaldehydes resins made where the molar ratio of formaldehyde to phenol is less than one. The most common precursors for preparing novolac oligomers and resins are phenol, formaldehyde sources and to a lesser extent, cresols. Three reactive sites for electrophilic aromatic substitution are available on phenol which give rise to three types of linkages between aromatic rings i.e. ortho-ortho, ortho-para, and para-para. The polymerization is brought to completion using acid-catalysis. Typically, 1 to 6wt% catalyst is used. Approximately 4 to 6wt% phenol can typically be recovered following novolac reactions. Free phenol will be removed by washing with water repeatedly. The importance of an acid catalyst is attributed to facilitated decomposition of any dibenzyl ether groups formed in the process. The phenol units are mainly linked by methylene groups. Novalacs are commonly used as photo resists. The molecular weights are in the low thousands, corresponding to about 10 to 20 phenol units. Hexamethylene tetramine or “hexamine” is a hardener that is added to crosslink novalac. At > 1800C, the hexamine forms cross links to form methylene and dimethylene amino bridges. Oxalic acid is preferred since resins with low colour can be obtained. Oxalic acid also
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decomposes at high temperature [>180oC] to CO3, CO and water, which facilitates the removal of this catalyst thermally ( Megson, 1972 ; Nabi and Jog, 2004 ) .
1.7.0 Chitosan
Chitosan is typically obtained by deacetylation of chitin under alkaline condition. It is one of the most abundant organic materials being second only to cellulose in the amount produced annually by biosynthesis. Chitin is an important constituent of the exoskeleton of insects and marine animals, such as crustaceans, mollusks etc. It is also the principal fibrillar polymer in the cell wall of certain fungi. Chitosan is a linear polysaccharide, composed of glucosamine and N- acetyl glucosamine .Chitosan displays interesting properties such as biocompatibility biogradability, and its degradation products are non-carcinogenic. Therefore chitosan has prospective applications in many fields such as biomedicine, waste water treatment, functional membrane, and flocculation. However, chitosan is only soluble in few dilute acid solutions which limit its applications. Recently there has been a growing interest in the chemical modification of chitosan in order to improve its solubility and widen its application Derivatization by introducing small functional groups to the chitosan structure such as alkyl or Chelat carboxymethyl groups can drastically reduce the solubility of chitosan at neutral and alkaline pH values without affecting its cationic character( Kurita, 1988; Kumar, 2000).
1.8.1 Statement of the Research Problems
Many of our modern technologies require composite materials, because of their unusual combination of properties, which are not found in conventional materials (metals, alloys, ceramics, and polymeric materials). In line with these demands, the need for continuous research in composite materials is paramount towards improving the existing and developing new materials.
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In fact, free formaldehyde in any product is undesirable for health reasons. The main drawback of the Phenol formaldehyde (PF) resins is its slow release of formaldehyde in finished product to the environment, contributing to worsening the indoor air quality, hence there is great desire of producing PF resins with low free formaldehyde content. The marked disadvantages of synthesized PF resins are relatively longer cure , dark colour , toxicity, and non-biodegradation. The composite industry continues to look for eco-friendly processes that substitute for toxic chemicals. In this point of view, chitosan is an excellent candidate of an eco-friendly materials. PF is suitable for use in a wide number of applications due to the abundant hydroxyl groups on the phenolic resins leading to strong intra- and intermolecular hydrogen bonds with a number of materials containing electron donors such as carbonyl and amide groups. The presence of free amino groups in chitosan is responsible for its polycationic nature in acidic solutions. The degree of deacetylation is an important property in chitosan production as it affects the physiochemical properties, hence determining appropriate applications
1.8.2 Research Questions
This research work asked the following questions:
i. Is there any need for the research work on the existing chitosan polymer composites?
ii. How to synthesize different novalac/prepolymer resins with different phenol/formaldehyde (P/F) mole ratios (1P:2F to 1P:8F).
iii. How to analyze and characterize the chitosan and synthesized novolac resins.
iv. What percentage of deacetylation (%DD) of Chitosan is the best for synthesizing Chitosan composite?
v. What method can be used to fabricate the different polymer composites?
vi. How to analyse the fabricated polymer composites ?
vii. How to determine the optimum composite from different filler loading.
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viii. How to determine the mechanical properties of chitosan reinforced phenolic composites.
ix. How to characterize the chitosan phenol formaldehyde composites.
x. What are the areas of application of the composite produced?
xi. How can these researches bring development in the area ?
1.8.3 Justification of the Present Study
1. The Phenol formaldehyde (PF) polymers are the oldest commercial synthetic polymers over 100 years ago. PF is a synthetic adhesive which, when its two components are combined, produces a thermosetting polymer with properties completely distinct from the phenol or formaldehyde.
2. They possesses, many advantages, such as readily available raw materials, simple production processes, highly durable, more stable and good mechanical and hygroscopic strength. The versatility that makes phenol formaldehyde suitable for use in wide number of factories applications was soon recognized (Knop and Pilato 1985; Sellers 1985).
3. This is due to the abundant hydroxyl groups on phenolic resins causes these materials to form strong intra- and intermolecular hydrogen bonds with a number of materials containing electron donors such as carbonyl and amide groups.
4. An important advantage of chitosan is the possibility to perform chemical modifications on its structure by binding to amino and hydroxyl groups.
5. It is quite clear from the chemistry of chitosan and Phenol formaldehyde (PF) that crosslinking is possible between chitosan molecules and PF by methylene linkages. It was planned to utilize this possibility for the preparation of this composites with low emission of formaldehyde.
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1.8.4 Aim of the Research Work
This research work is aimed at synthesizing phenol formaldehyde resin with low emission and then reinforced with Chitosan and determine its characterization. The physical and chemical characterization of the composite will suitable end use of application?
1.8.5 Objectives of the Present Works
The objectives of this research will include the following:
1. To synthesize and characterize novalac/prepolymer resins with different phenol/formaldehyde (P/F) mole ratios (1P:2F to 1P:8F) to optimize production.
2. To prepare chitosan with different degrees of deacetylation 70%, 81% and 91% [DD] and to determine its physical and chemical properties. FT-IR, SEM, and XRD of the chitosan.
3. To fabricate and characterize PF/chitosan composite to optimize the production.
4. To analyze and study Infrared spectroscopy (FT-IR), X-ray diffraction study Scanning Electron Microscopy (SEM) (XRD), and thermo gravimetric analysis (TGA/DTA) of the composite .
1.8.6 Scope and Limitation of the Research
The research work is limited only to the synthesis of PF (novolac) as well as the physical and chemical characterization of chitosan phenol formaldehyde composite.Production of composite using PF and chitosan, optimization of composite and its characterization Area of utilizations and applications of the polymer composite will be reported. Seven prepolymer resins of different molar ratios of novalac phenol formaldehyde (P: F) will be prepared. These P: F resins will be reinforced with Chitosan to form the composites.
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1.8.7 Expected Results
The Standard values specified for phenol formaldehyde properties are tensile strength 76 MN/m2, flexuaral modulus 8.3 GN/m2, Impact strength 1.1 J/cm, resin solid content 49.6%, formaldehyde content 30mg/kg or 2.5mg/L, denisity 0.671g/cm3 and water absorption 1.5%. The application of PF is in the production of plastics. It is expected that the phenolic resins when reinforced with chitosan will give excellent results in terms of mechanical properties enhancement and better physical and chemical properties than those without chitosan. The results of the optimum PF/composites are to be compared with standard values and works of the other researchers in the subject or closely related areas. The results obtained will form the basis for further research work in this area and also to assess the possibility of using these polymers composite in a number of applications.
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