Characterisation of expandable graphite and its flame retardant abilities in flame retardant systems for polyethylene by Hermanus Joachim Kruger A thesis submitted in partial fulfilment of the requirements for the degree Doctor of Philosophy in Engineering in the Department of Chemical Engineering Faculty of Engineeri ng, the Built Environment and Information Technology University of Pretoria Pretoria February 2017 © University of Pretoria Characterisation of expandable graphite and its flame retardant abilities in flame retardant systems for polyethylene Student : Hermanus Joachim Kruger Supervisor : Professor Walter W. Focke Department : Chemical Engineering University : University of Pretoria Degree : Doctor of Philosophy in Engineering Synopsis In the pursuit of lower cost intumescent flame retardant (IFR) systems, the compound expandable graphite (EG) was identified. This compound delivers high flame retardant performance but provides non-uniform thermal shielding when exposed to open flame from below due to negative gravitational effects. It was theorised that this may be remedied either through ion exchange of the interstratified ions with low glass transition ions or through use in binary systems with other compounds. Two classes of commercial EG were identified, namely a low and a high expansion onset temperature EG compound. Extensive characterisation of each EG compound was undertaken to assess its composition, expansion mechanisms and onset temperatures in order to identify compatible compounds for binary use. The susceptibility of each compound to ion exchange was also assessed. An industrial IFR ethylenediamine phosphate (EDAP) and a novel flame retardant were synthesised for assessment in binary use with EG. Coupled with the above study, this project developed two novel fire testing techniques as low cost alternatives to well-established fire testing methods such as cone calorimetry. The first technique involved an open flame fire testing method which allowed vertical or horizontal testing. Digital and infrared (IR) video recording during operation facilitated comparison of multiple performance indicators further strengthening this method. The second technique allowed assessment of the mass loss resistance of each compound during laser pyrolysis. i © University of Pretoria Characterisation of the EG compounds allowed development of structural models to describe each compound and explain the mechanisms of their expansion and gaseous release. Exhaustive ion exchange testing did not deliver favourable results, necessitating the pursuit of compounds for binary use with EG. A novel IFR was synthesised by neutralising 3,5-diaminobenzoic acid hydrochloride salt with ammonium dihydrogen phosphate. This compound, which melts at 257 C, decomposes concurrently to release carbon dioxide gas which promotes intumescent charring. The flame retardant performance of this compound and EDAP as primary flame retardants and in combination with expandable graphite was evaluated. As a proof of concept, the novel compound was tested as a primary flame retardant using cone calorimetry after which its utility in binary systems with low temperature expandable graphite was tested. Substantial decreases in peak heat release rate (pHRR) and flame out time were achieved for all binary systems. This success led to testing of a number of combinations of low and high expansion onset EG and the other IFRs to identify the highest performing combination, which proved to be the 10-10 EDAP-EG system. Combinations of EG and the novel compound also showed excellent results. The novel fire testing techniques proved effective in identifying high performance combinations and showed comparable trends to those measured in cone calorimetry, at a greatly reduced cost and material requirement. IR analysis of open flame fire testing indicated increases in the temperatures required for ignition and burn through of the substrate. Observations, corroborated by optical video, showed that cohesive and uniform thermal shielding was achieved in all binary systems tested. This study illustrates that systems of 10% EG combined with either 10% DABAP or 10% EDAP are both the most economical binary systems tested but are extremely high performance systems as well. Both of these systems delivered excellent results while being more economic than the widely used industrial system with a 25-30% EDAP loading. It is recommended that these compounds be considered for industrial use. Furthermore, the effective fire testing techniques developed in this study may be utilised in future fire testing to identify high performance compounds at a lower cost prior to further assessment through methods such as cone calorimetry. KEYWORDS Expandable graphite; exfoliation; intumescent flame retardant; thermal analysis; cone calorimeter; characterisation; graphite oxide; graphite intercalation compound ii © University of Pretoria Acknowledgements Firstly, and most importantly I would like to thank my wife Danielle for her understanding and love and unwavering support during the many long hours and late nights. Many a late evening and weekend was spent checking that the machines were still running and grabbing the newest results and she always stayed by my side. I’d like to thank her for all her advice and input on my writing and thank her for always being willing to look over what I wrote. Without Danielle’s mentorship as a highly skilled technical editor I would never have attained the writing and technical editing proficiency that I today possess. As with all large projects a lot of hands go into the many facets of the project. Many people have supported me to achieve the goals I set out to achieve. I have had the privilege of working with a lot of great individuals during this project and had the privilege of supervising three sharp minds as well and feel honoured to have had this opportunity. I would like to thank my supervisor for accepting me into the research group. Thanks for the many hours of technical advice and for all the off the cuff advice on McGuyvering my way through setting up some of the harder setups of the research. I would also like to thank Walter for his support on a personal level and for always making me feel like a valuable part of the team. I’d also like to thank the following people: Suzette, Isbe and Rainer who helped me organise things, learn how to use machines and helped keep everything running smoothly. All of the staff at the various University of Pretoria analytical departments for their time and help during the use of their equipment. Hendrik and Hendrik, Washington, Stix and Hein for their friendship and for their advice on all the intricacies of how postgrad and the research group work. Dewan, Albert and Albertus for their great work. I had a great time supervising you and am glad to have seen you all go from student to graduate engineers. © University of Pretoria All my other friends for being there for me. In particular Michael for his friendship at Tuksdorp postgraduate residence and my best friend Colin for being the man he is. For the years of fun together growing up and pushing each other to succeed and aim to become engineers. It has always, and continues to be, a privilege having you in my life. I am glad that we could encourage each other in our respective postgrad projects. My parents for raising me into a passionate individual and instilling the strength I needed to get through school and undergrad to get to where I am now. I’d also like to thank them for supporting me in my studies at university. Lastly I’d like to thank my bursars and financial supporters who made this project possible: This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Foundation (NRF). Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and therefore the NRF ad DST do not accept any liability with regard thereto. © University of Pretoria Table of Contents Synopsis .................................................................................................... i Acknowledgements ................................................................................... 1 List of Tables .......................................................................................... iii List of Figures .......................................................................................... v Nomenclature .......................................................................................... xi List of acronyms .................................................................................... xiii Introduction .............................................................................................. 1 Document overview ................................................................................... 3 Chapter 1: Expandable graphite as flame retardant ...................................... 4 Foreword ............................................................................................4 Executive summary .............................................................................4 1-1. Introduction ................................................................................5 1-2. Carbon .......................................................................................6 1-3. Graphite .....................................................................................7 1-3.1. Introduction to graphite .........................................................7 1-3.2 Flake graphite particle dimensions ............................................9 1-4 Intercalation and graphite intercalation compounds ....................... 11 1-4.1 Intercalation ......................................................................... 11 1-4.2 Graphite intercalation compounds ........................................... 12 1-4.3 Crystal structure of graphite and GICs .................................... 14 1-5 Flame retardants .......................................................................... 16 1-5.1 Ignition and combustion ......................................................... 16 1-5.2 Introduction to flame retardants .............................................. 16 1-5.3 Mechanisms of flame retardant action ..................................... 17 1-5.4 Intumescent flame retardants .................................................. 17 1-6 Expandable Graphite ................................................................... 18 1-7 Characterisation methods for carbon materials .............................. 20 © University of Pretoria 1-7.1 Overview of characterisation and testing techniques ................ 20 1-7.2 Particle analysis techniques.................................................... 21 1-7.2.1 Particle density measurement .................................... 21 1-7.2.2 Surface area measurement ......................................... 22 1-7.2.3 Particle size distribution ........................................... 23 1-7.3 Visual characterisation .......................................................... 26 1-7.3.1 Optical microscopy ................................................... 26 1-7.3.2 Scanning electron microscopy ................................... 27 1-7.3.3 Field emission scanning electron microscopy ............. 27 1-7.3.4 Transmission electron microscopy ............................. 28 1-7.4 Atomic and crystal structure characterisation .......................... 28 1-7.4.1 X-ray diffraction ...................................................... 28 1-7.4.2 Raman spectroscopy ................................................. 30 1-7.4.3 Fourier transform infrared spectroscopy ..................... 31 1-7.5 Compositional analysis .......................................................... 32 1-7.5.1 Inductively coupled plasma – mass spectrometry ........ 32 1-7.5.2 X-ray fluorescence ................................................... 32 1-7.5.3 Elemental analysis - gas chromatography coupled with conductivity measurement ......................................... 33 1-7.5.4 Energy dispersive x-ray spectroscopy ........................ 33 1-7.6 Thermal analysis techniques ................................................... 35 1-7.6.1 Thermomechanical analysis ....................................... 35 1-7.6.2 Thermogravimetric analysis ...................................... 36 Chapter 2: Characterisation of commercial expandable graphite flame retardants ............................................................................................... 37 Foreword .......................................................................................... 37 Executive summary ........................................................................... 37 2-1. Introduction .............................................................................. 38 © University of Pretoria 2-2. Experimental ............................................................................ 41 2-2.1 Materials .............................................................................. 41 2-2.2 Particle size, BET surface and density determination ............... 41 2-2.3 Thermogravimetry ................................................................. 41 2-2.4 Composition of evolved gases ................................................ 41 2-2.5 Graphite composition determinations ...................................... 42 2-2.6 Ion exchange ......................................................................... 42 2-2.7 Thermomechanical analysis .................................................... 42 2-2.8 X-Ray diffraction .................................................................. 43 2-2.9 Raman spectroscopy .............................................................. 43 2-2.10 Scanning electron microscopy ............................................... 43 2-3. Results ..................................................................................... 43 2-3.1 Graphite particle characteristics ............................................. 43 2-3.2 Thermogravimetry ................................................................. 46 2-3.3 Composition of evolved gases ................................................ 47 2-3.4 Graphite composition determinations ...................................... 49 2-3.5 Thermomechanical analysis .................................................... 51 2-3.6 X-Ray Diffraction (XRD) ....................................................... 51 2-3.7 Raman spectroscopy .............................................................. 53 2-4. Discussion ................................................................................ 54 2-5. Conclusions .............................................................................. 57 Chapter 3: Development and production of flame retardants ....................... 58 Foreword .......................................................................................... 58 Executive summary ........................................................................... 58 3-1. Introduction .............................................................................. 59 3-2. Experimental ............................................................................ 59 3-2.1 Materials .............................................................................. 59 3-2.1.1 Preliminary synthesis of EDAP.................................. 59 © University of Pretoria 3-2.1.2 Preliminary synthesis of DABAP ............................... 61 3-3. Results ..................................................................................... 63 3-3.1 Quantification of synthesised EDAP quality and bonding ability63 3-3.1.1 Raman and FTIR ....................................................... 63 3-3.1.2 Scanning electron microscopy ................................... 64 3.3.2 Quantification of synthesised DABAP quality and bonding ability ............................................................................................ 66 3-4. Conclusions .............................................................................. 67 Chapter 4: Cone calorimeter fire performance of low temperature expandable graphite and a novel flame retardant ......................................................... 68 Foreword .......................................................................................... 68 Executive summary ........................................................................... 68 4-1. Introduction .............................................................................. 69 4-2. Experimental ............................................................................ 70 4-2.1. Materials ............................................................................ 70 4-2.2. Synthesis of DABAP ............................................................ 70 4-2.3. Preparation of the polyethylene compounds ........................... 71 4-2.4. Characterisation and analysis ............................................... 71 4-2.5. Thermal analysis ................................................................. 72 4-2.6. Cone calorimeter flammability testing .................................. 73 4-3. Results and discussion ............................................................... 73 4-3.1. Characterisation .................................................................. 73 4-3.2. Thermal analysis ................................................................. 75 4-3.3. Flammability ....................................................................... 78 4-4. Conclusions .............................................................................. 87 Chapter 5: Cone calorimeter fire performance of low and high temperature expandable graphite in binary systems with ethylenediamine phosphate and 3,5-diaminobenzoic acid phosphate to determine optimal synergistic combinations .......................................................................................... 88 Foreword .......................................................................................... 88 © University of Pretoria Executive summary ........................................................................... 88 5-1. Introduction .............................................................................. 90 5-2. Experimental ............................................................................ 91 5-2.1. Materials ............................................................................ 91 5-2.2. Preparation of the polyethylene compounds ........................... 92 5-2.3. Characterisation and analysis ............................................... 92 5-2.4. Thermal analysis ................................................................. 93 5-2.5. Cone calorimeter flammability testing .................................. 93 5-3. Results and discussion ............................................................... 94 5-3.1. Characterisation .................................................................. 94 5-4. Thermal analysis ....................................................................... 96 5-5. Cone calorimeter fire testing ...................................................... 98 5-6. Conclusions ............................................................................ 113 Chapter 6: Studying the thermal properties of polyethylene flame retarded with intumescent flame retardant additives through conventional and novel fire testing methods ............................................................................... 114 Foreword ........................................................................................ 114 Executive summary ......................................................................... 114 6-1. Introduction ............................................................................ 116 6-2. Experimental .......................................................................... 118 6-2.1. Materials .......................................................................... 118 6-2.1.1 Polymeric materials .......................................................... 118 6-2.1.2 Flame retardants ............................................................... 118 6-2.1.3 Other materials ................................................................. 119 6-2.2. Preparation of the polyethylene compounds ......................... 119 6-3. Characterisation and analysis ................................................... 121 6-3.1. Scanning electron microscopy ............................................ 121 6-3.2. Thermogravimetry ............................................................. 121 6-3.3. Thermomechanical analysis ................................................ 121 © University of Pretoria