Characterisation of expandable graphite and its flame retardant abilities in flame retardant systems

226 Pages·2017·9.58 MB·English

by  Kruger, Herman

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.

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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

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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

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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

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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

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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

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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

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