Growing single crystals of calcium gluconate monohydrate

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Introduction

Due to the great risk of fires to humans and their possessions, new methods are continuously being investigated and developed to prevent fires and reduce their effects. Some additives to polymers increase the materials resistance to ignition, the amount of fire stress it can withstand, retard the rate of combustion of the materials and prevent sustained burning (Gann, 1993). These additives incorporated in polymeric materials are called flame retardants. Intumescent flame retardants form a foamed carbon barrier layer on the polymer surface when it is exposed to heat (Vandersall, 1970; Camino, Costa, & Martinasso, 1989). Conventional organic systems are based on the acid catalysed dehydration of carbon rich polyols such as dipentaerythritol. It is known that metal oxides (Cullis & Hirschler, 1984) and other metal compounds (Gann; 1993, Green; 1996) have utility as catalytic char forming flame retardants. Both antimony and tin have been used to improve the flame resistance of cellulosics without the assistance from halogen containing compounds (Ramsbottom; 1947). Apparently they alter the condensed phase thermal degradation pathways in such a manner that more non-volatile char residues and less flammable gases are generated on the thermal decomposition of the polymeric material (Wills; 1952).

Thermal degradation, flammability and flame retardancy

Organic compounds degrade thermally, as is the case with polymers (plastics). If this thermal degradation of the combustible materials is oxidative and characterised by the generation and emitting of heat and light, the process is called a fire. The light emitted from the fire is the flame, which is a visual sign and is an indication of the heat generated. In the early 1990ís it was estimated that 29 000 injuries and 4 500 deaths are caused by fires each year in the United States alone and that the total global cost is over $100 billion annually (Gann, 1993). Today a fire in a structure occurs every 60 seconds, residential fires every 82 seconds, every 85 seconds in a vehicle and every 34 seconds in an outside property in the United States; 1.8 million fires a year. This resulted in $10 billion in property damage, 3 570 civil deaths and 21 875 injuries in 1999, down from the early 1990ís (Nelson & Wilkie, 2001). Combustion is usually a gas phase phenomenon. Volatile combustible species oxidise exothermically in the gas phase. Afterglow or glowing combustion is a form of non-gas phase combustion. Here the substrate is oxidised in the condensed phase to form both solid and gaseous products. This usually takes place at temperatures well below the ignition temperature of the material. For instance, the carbon residue in a carbon rich material is oxidised in the solid phase.

The history of flame retardants

The need for fireproofing polymers became important in the nineteenth century due to the commercialisation of cellulose nitrate plastics (Green, 1997). These materials are highly flammable and presented a major fire risk. The early large-volume polymers, including phenolics, melamine resins and rigid polyvinylchloride (PVC), possessed adequate intrinsic flame resistance. Unfortunately, the more recent volume plastics such as polyolefins (e.g. polyethylene, polypropylene), styrenics (e.g. polystyrene, acrylonitrile-butadiene-styrene copolymer (ABS)) and polyesters, are significantly more flammable than wood. Polyolefins have the same basic structure of fuels such as petrol, handy gas and flammable waxes (Sutker, 1988). Clearly, something needed to be done to reduce the fire risk presented by these polymers. This subsequently led to the development of different formulations that increased the fire resistance of polymers. By the early 1970’s, the flame retardant industry already produced ± 50 000 metric tons per year (Green, 1997). Unsaturated polyesters, PVC and cellulose photographic films, were the mayor groups of fire resistant polymers in use by 1970. Early flame retardant systems were mainly based on aluminium trihydroxide (Al(OH)3), also referred to as alumina trihydrate (Al2O3.3H2O) or ATH, halogenated compounds and antimony ñ if the price allowed it (Green, 1997; Gann, 1993). Some phosphate esters were also used.

Phosphorous flame retardants ñ Non intumescent

Phosphorous containing flame-retardants include phosphate esters and inorganic phosphates (Green, 1996). The mechanism whereby phosphorus flame retardants function varies depending on both the types of phosphorus compound and the specific polymer. Phosphorous-based flame retardants work in both the condensed and vapour phases (with the condensed phase being predominant), and interact according to both physical and chemical mechanisms (Weil, 1993; Weil, 1992a; Green, 2000; Lyons, 1987). Each of these mechanisms will be discussed separately. For example, they appear to function by promoting char formation, dripping of the burning polymer or by gas phase radical scavenging. Studies have however shown that condensed phase mechanisms are significantly more effective than the vapour phase mechanisms (Weil, 1992a). It is well known that both phosphorous and boron compounds act as inhibitors of carbon oxidation (Rakszawski, 1964; Green, 1997; Weil, 1993). Thus, phosphorous systems prevent this glowing combustion. Although the exact mechanism is unknown, it is believed that it involves the deactivation of the active centres on the carbon (Weil, 1993). Flame retardants based on phosphorous are widely used as synergists for halogen systems, inorganic compounds and organic systems. Organic flame retardants such as intumescent formulations are dependant on phosphorous compounds (Weil, 1993).

Oxygenated hydrocarbon flame retardants

Recently it was discovered that oxygenated hydrocarbons could be used as flame retardants on their own (Bisschoff, 2000; Bisschoff & Focke, 2001) in polyester fabrics. These compounds are food additives – containing only carbon, hydrogen and oxygen – and are thus environmentally friendly, non-toxic and relatively inexpensive. Compounds such as pentaerythritol, dipentaerythritol, benzophenone, 2-furoic acid etc. can be used. The main mechanism by which these systems operate is by enhancing the dripping of the polymer melt. Heat is removed from the polymer when molten polymer drips away from the flame zone. The rate of dripping for the treated polyester fabrics were up to 80% faster than the untreated fabrics, resulting in shorter self extinguishing times. These systems showed an optimum dosage level (Bisschoff, 2000; Bisschoff & Focke, 2001).

Planning

Studies showed that tailor-made substances incorporating two or more of the elements needed for intumescence in a single compound are more effective than mixtures of individual compounds (Camino et al., 1989). This is attributed to diffusion effects. In a single complex, the diffusion pathway of the carbonific and spumific to and from the intumescent catalyst (phosphate derivative) is shorter than in a mixture of compounds. Recent studies showed that organometallic compounds could be intumescent, char forming substrates (LabuschagnÈ, 1998). Potassium bitartrate and mixtures thereof with pentaerythritol showed considerable intumescence (Focke et al., 2000). The metal is believed to be the catalyst for the dehydration of a polyol to form an intumescent char. In this study, a number of organometallic complexes were prepared and investigated for their intumescent action.
The char yield calculated for all the samples during the tests only refers to carbon char and excludes the ash. The amount of ash in the char was calculated as the stable metal compound (e.g. metal oxide or carbonate) at the selected pyrolysis temperature for all the samples tested. The effects of both the metal cation and the organic portion of the molecular complex on performance were studied. The best performing intumescent complex was selected. Its thermal decomposition and foaming properties were characterised. Attempts were made at elucidating the mechanism of intumescence. The residues formed at high temperatures were found to be fine metal oxide and carbonate powders. Thus the thermal decomposition of these metal complexes also provides a potential preparation method for nano-sized oxide and carbonate particles. This was also investigated.

CONTENT :

• SYNOPSIS
• KEY WORDS:
• ACKNOWLEDGEMENTS
• CONTENT
• LIST OF FIGURES
• LIST OF TABLES
• NOMENCLATURE
• ABBREVIATIONS
• 1. Introduction
• 2. Background
  • 2.1. Thermal degradation, flammability and flame retardancy
  • 2.2. The history of flame retardants
  • 2.3. Flame retardants and synergism
  • 2.4. Inorganic flame retardants
  • 2.5. Halogenated flame retardants
  • 2.6. Phosphorous flame retardants ñ Non intumescent
    • 2.6.1. Condensed phase
    • 2.6.2. Vapour phase flame retardants
  • 2.7. Intumescent flame retardants
  • 2.8. Oxygenated hydrocarbon flame retardants
  • 2.9. Smoke suppressants
  • 2.10. Metals in flame retardants
  • 2.11. Fire testing
• 3. Experimental
  • 3.1. Planning
    • 3.1.1. The effect of the organic part of the organometallic complex
    • 3.1.2. The effect of the metal cation of the organometallic complex
    • 3.1.3. The intumescence of calcium gluconate monohydrate and ammonium gluconate hydrate
    • 3.1.4. The foaming properties of metal glucose (dextrose) derivatives
    • 3.1.5. The formation of fine metal carbonates and oxides
  • 3.2. Apparatus
    • 3.2.1. Laboratory furnace
    • 3.2.2. Pyrolysis glass tubes
    • 3.2.3. Open flame tests
    • 3.2.4. Inert atmosphere pyrolysis chamber and silica tube
    • 3.2.5. Thermal analysis
    • 3.2.6. X-ray diffraction analysis
    • 3.2.7. Infrared and Raman spectroscopy
    • 3.2.8. Matrix assisted laser desorption ionisation ñ time of flight ñ mass spectrometry
    • 3.2.9. Pyrolysis gas chromatography ñ mass spectrometry
    • 3.2.10. Thermal conductivity measurements
    • 3.2.11. Electric conductivity measurements
    • 3.2.12. Electron microscopy
    • 3.2.13. Surface area measurements
    • 3.2.14. Burn tests
    • 3.2.15. Growing single crystals of calcium gluconate monohydrate
  • 3.3. Method
    • 3.3.1. Preparation and study of the organic acidsí sodium salts
    • 3.3.2. Preparation and studies of the metal complexes of acetylacetone and gluconic acid
      • 3.3.2.1. Acetylacetone metal complexes
      • 3.3.2.2. Gluconic acid metal salts
    • 3.3.3. Characterisation and evaluation of the intumescence calcium gluconate monohydrate and ammonium gluconate hydrate
      • 3.3.3.1. The characterisation of the intumescent foams
      • 3.3.3.2. Thermal analysis of the gluconates
      • 3.3.3.3. Identification of the gaseous decomposition products
      • 3.3.3.4. Identification of the solid decomposition products
      • 3.3.3.5. Molecular mass determination of the carbon foam from the calcium gluconate
      • 3.3.3.6. Density calculations of the calcium gluconate monohydrate foam
      • 3.3.3.7. Thermal conductivity of the intumesced calcium gluconate
      • 3.3.3.8. Electric conductivity measurements
      • 3.3.3.9. Paint preparation and burn tests
      • 3.3.3.10. Growing single crystals of calcium gluconate monohydrate
    • 3.3.4. Preparation of the calcium and other metal dextrose derivatives and study of their intumescence
    • 3.3.5. Characterisation of the metal carbonates and oxides
    • 3.3.6. Understanding metal catalysed intumescence of polyols
• 4. Results and Discussion
  • 4.1. The organic acid sodium salts
  • 4.2. The metal complexes of acetylacetone and gluconic acid
  • 4.3. The intumescence of the calcium and ammonium gluconates
    • 4.3.1. The intumescent foam
    • 4.3.2. Thermal analysis
    • 4.3.3. The gaseous decomposition products
    • 4.3.4. The solid decomposition products
      • 4.3.4.1. XRD analysis
      • 4.3.4.2. IR spectroscopy
    • 4.3.5. The molecular mass of the carbon residue
    • 4.3.6. Foam density as a function of pyrolysis temperature and time
    • 4.3.7. The thermal conductivity of the foam
    • 4.3.8. The electric conductivity of the foam
    • 4.3.9. The fire retardancy of calcium gluconate monohydrate coatings
    • 4.3.10. The crystal data of calcium gluconate monohydrate
  • 4.4. The intumescence of the metal glucose (dextrose) derivatives
  • 4.5. Properties of the fine powder carbonates and oxides
  • 4.6. Understanding metal catalysed intumescence of polyols
• 5. Conclusion and recommendations
• 6. References
• 7. Appendices
  • 7.1. Appendix A
    • 7.1.1. Processing temperatures for commercial polymers
  • 7.2. Appendix B
    • 7.2.1. Limiting Oxygen Index for commercial polymers
  • 7.3. Appendix C
    • 7.3.1. List and structure of acids (and complexes) used
  • 7.4. Appendix D
    • 7.4.1. Acetylacetonate complexes used
  • 7.5. Appendix E
    • 7.5.1. The commercial preparation of gluconic acid and its derivatives
  • 7.6. Appendix F
    • 7.6.1. Vitamin supplement label
  • 7.7. Appendix G
  • 7.7.1. Pictures of the burn test setup
  • 7.8. Appendix H
  • 7.8.1. Screen grab of the data logging software, ìCaptureî
  • 7.9. Appendix I
  • 7.9.1. Photos of the cold finger used for the sublimation crystallisation
  • 7.10. Appendix J
  • 7.10.1. Elemental analysis of the leached SiO2 from Foskor Pty. Ltd
  • 7.11. Appendix K
  • 7.11.1. Preparation of CaDex (Venter, 2000)
  • 7.12. Appendix L
  • 7.12.1. Tabulated results for the pyrolysis of the sodium compounds and the synthesis and pyrolysis of the sodium salts
  • 7.13. Appendix M
  • 7.13.1. Summarised results for the gluconate synthesis
  • 7.13.2. Thermal analysis of pentaerythritol, the acetylacetonates and acetylacetonate/pentaerythritol mixtures
  • 7.13.3. Thermal analysis of the gluconates
  • 7.14. Appendix N
    • 7.14.1. SEM images of calcium gluconate monohydrate powder (crystals)
    • 7.14.2. SEM images of ammonium gluconate hydrate (crystals)
    • 7.14.3. SEM images of the plate like leached SiO
    • 7.14.4. SEM images of calcium gluconate pyrolysed in air at selected temperatures
    • 7.14.5. SEM images of calcium gluconate monohydrate pyrolysed in nitrogen at selected temperatures
    • 7.14.6. SEM images of calcium gluconate monohydrate and leached silica mixtures
    • pyrolysed in air
    • 7.14.7. SEM images of ammonium gluconate hydrate pyrolysed in air at selected temperatures
    • 7.14.8. SEM images of ammonium gluconate hydrate pyrolysed in nitrogen at selected temperatures
    • 7.14.9. SEM images of AP750 pyrolysed in air at 400 °C
    • 7.14.10. SEM images of PEN pyrolysed in air at 400 °C
  • 7.15. Appendix O
    • 7.15.1. Thermal decomposition analysis of gluconic acid
    • 7.15.2. Thermal decomposition analysis of calcium gluconate monohydrate
    • 7.15.3. Thermal decomposition analysis of ammonium gluconate hydrate
    • 7.15.4. Thermal decomposition analysis of the leached silica (ex Foskor)
    • 7.15.5. Thermal decomposition analysis of the expandable graphite (ex Fedmis)
  • 7.16. Appendix P
  • 7.16.1. XRD pattern of calcium gluconate monohydrate
    • 7.16.2. XRD pattern of ammonium gluconate hydrate
    • 7.16.3. XRD pattern of Leached silica from Foskor Pty. Ltd
    • 7.16.4. XRD pattern of calcium gluconate monohydrate pyrolised in air
    • 7.16.5. XRD pattern of calcium gluconate pyrolised in nitrogen
    • 7.16.6. XRD pattern of calcium gluconate monohydrate ñ leached silica mixtures pyrolised in air
    • 7.16.7. XRD pattern of ammonium gluconate hydrate pyrolised in air
    • 7.16.8. XRD pattern of ammonium gluconate pyrolised in nitrogen
    • 7.16.9. IR spectra of calcium gluconate monohydrate pyrolised in air
    • 7.16.10. IR spectra of calcium gluconate pyrolised in nitrogen
    • 7.16.11. IR spectra of ammonium gluconate hydrate pyrolised in air
    • 7.16.12. IR spectra of ammonium gluconate pyrolised in nitrogen
    • 7.16.13. Decomposition products of calcium gluconate monohydrate
    • 7.16.14. Thermal conductivity results from the SABS
  • 7.17. Appendix R
    • 7.17.1. Electric conductivity for the pyrolysed ammonium gluconate
  • 7.18. Appendix S
    • 7.18.1. Burn through tests for the painted balsa wood planks ñ Graphs
    • 7.18.2. Burn through tests for the painted balsa wood planks ñ Pictures
    • 7.18.3. Burn through tests for the painted aluminium plates ñ Graphs
    • 7.18.4. Burn through tests for the painted aluminium plates ñ Pictures
    • 7.18.5. Burn through tests for the painted cardboard sheets ñ Graphs
  • 7.19. Appendix T
    • 7.19.1. Light microscope and SEM images of calcium gluconate monohydrate crystals (powder)
    • 7.19.2. Light microscope and SEM images of calcium gluconate monohydrate crystals recrystallised through diffusion technique
  • 7.20. Appendix U
    • 7.20.1. Metal oxides and carbonates prepared from the metal dextrose solutions and calcium
    • gluconate monohydrate
  • 7.21. Appendix V
    • 7.21.1. Thermal analysis of selected calcium salts
    • 7.21.2. SEM images of glyceric acid hemicalcium salt monohydrate at selected
  • temperatures in air

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