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Carbon
Carbon is a unique element, in that it has a wide range of allotropes (polymorphs) which gives rise to a multitude of possible molecular structure, some of which are displayed in Figure 1-1. These allotropes consist of different forms of hybridised and unhybridised carbon atoms, vastly changing the material properties. Allotropes of carbon include carbon nanotubes, charcoal, graphite, diamonds and fullerenes (Gao & Zhang, 1996).
Intumescent flame retardants
An IFR can generally be described as a substance which expands when exposed to a flame to form a foam barrier. Commercially available IFRs include components such as ammonium polyphosphate, pentaerythritol, melamine and EDAP. When a material undergoes combustion the flame retardant will form a char foam layer at the surface of the material to protect it (Shen & Schilling, 2012).
This foam creates a barrier between a flame and the IFR substrate which causes a thermal shielding effect and decreases transfer of oxygen to the flame both of which decrease the ability of the flame to be sustained. This causes a flame to burn at lower temperatures, to burn more slowly and even results in self-extinguishing in some cases.
Materials
Two grades of EG, ES 250 B5 (onset temperature 220C) and ES170-300A (onset temperature 300C), were obtained from Qingdao Kropfmuehl Graphite (China). Milled natural Zimbabwean flake graphite was supplied by BEP Bestobell (South Africa). Exfoliated graphite forms were prepared at a temperature of 600 C by placing the EG powder samples in a Thermopower electric furnace.
Particle size, BET surface and density determination
The graphite particle size distributions were determined with a Mastersizer Hydrosizer 2000MY (Malvern Instruments, Malvern, UK). The specific surface areas of the graphite powders were determined. BET surface areas were measured on a Micromeritics Flowsorb II 2300 and a Nova 1000e BET instrument in N2 at 77 K. Densities were determined on a Micrometrics AccuPyc II 1340 helium gas pycnometer.
Preliminary synthesis of EDAP
EDAP is an IFR commonly used in industry. “It is self-intumescent and does not require a char-forming synergist” (Krems Chemie Chemical Services). The raw materials used when producing EDAP are very expensive and production of industrial quality EDAP requires extensive cooling and milling of the product. The chemical properties of EDAP are given in Table 3-1.
Materials
The chemicals 3,5-diamino benzoic acid [535-87-5], ammonium dihydrogen phosphate [7722-76-1] and hydrochloric acid [7647-01-0] were sourced from Sigma-Aldrich, Protea Chemicals and Merck respectively. Sasol Polymers supplied the LDPE in powder and pellet form. It was injection moulding grade LT019 with density 0.919 g.cm and MFI 20.5 g / 10 min @ 190 °C / 2.16 kg. Carbon black grade N660 was sourced from Ferro Industrial Products. The EG grade ES170-300A, with a high expansion onset temperature, was sourced from Qingdao Kropfmuehl, China. The d10, d50, and d90 particle sizes were 313 µm, 533 µm and 807 µm respectively (Mastersizer Hydroliser 2000, Malvern Instruments, Malvern, UK). The density was 2.23 0.01 g.cm and the surface area in the pre-expanded form was 0.66 m2. 1(Nova 1000e BET in N2 at 77K).
Preparation of the polyethylene compounds
Polyethylene compounds containing EG and / or IFRs were compounded on a 28 mm co-rotating intermeshing twin screw laboratory extruder (L / D = 16) at a screw speed of 140 – 220 rpm. The extruder screw design comprised intermeshing kneader elements with a forward transport action. The four extrusion processing stage temperatures, feed to die, were set at 120 °C, 175 °C, 175 °C and 180 °C respectively. The extruded strands were granulated and the pellets were air-dried. A polyethylene compound containing 5 wt.% carbon black (N660) was prepared in a similar way. This compound was used as the reference sample for cone calorimeter testing. The compounds containing 27 wt.% intumescent additives (EDAP or DABAP) also contained 3 wt.% carbon black. This maintained a consistent range of dark product sheets as delivered for all EG containing compounds. This was done to ensure comparable absorption of IR radiation during cone calorimeter testing.
Other materials
This section includes outlines other materials used during this study (excluding equipment). Carbon black grade N660 was sourced from Ferro Industrial Products and was used in compounding.Milled natural Zimbabwean flake graphit(henceforth referred to as flake graphite) was supplied by BEP Bestobell (South Africa).
Conclusions
Observations and analysis of the open flame and pyrolysis fire testing conducted showed that cohesive bonding of EG strings to cause uniform fire behaviour was achieved successfully for all binary systems. All binary systems delivered fire retardation exceeding any single flame retardant systems. Both binary flame retardant types delivered good resistance to ignition, with all EDAP containing binary systems preventing sample burn through completely while maintaining structural integrity of samples until eventual melting of the polymer media occurs. Analysis of IR camera data obtained for all open flame fire tests indicates that all binary flame retarded samples were able to withstand higher temperature before ignition, burn through or sag of the polymer is initiated. This may be attributed to a reduction of the temperatures at the polymer surface itself due to effective thermal barrier formation.
Chapter 1: Expandable graphite as flame retardant Foreword Executive summary
1-1. Introduction
1-2. Carbon
1-3. Graphite
1-3.1. Introduction to graphite
1-3.2 Flake graphite particle dimensions
1-4 Intercalation and graphite intercalation compounds
1-4.1 Intercalation
Chapter 2: Characterisation of commercial expandable graphite flame
retardants
Foreword
Executive summary Experimental Materials Particle size, BET surface and density determination
Thermogravimetry Composition of evolved gases
Chapter 3: Development and production of flame retardants
Foreword
Executive summary
Experimental
Materials
Preliminary synthesis of EDAP
Chapter 4: Cone calorimeter fire performance of low temperature ex pandable graphite and a novel flame retardant
Foreword
Executive summary
Experimental
Materials
Synthesis of DABAP
Preparation of the polyethylene compounds
Characterisation and analysis
Thermal analysis
Cone calorimeter flammability testing
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
Foreword
Chapter 6: Studying the thermal properties of polyethylene flame retarded
with intumescent flame retardant additives through conventional and novel
fire testing methods . Foreword Executive summary
Experimental
Materials
Polymeric materials
Flame retardants
Other materials
Chapter 7: Overall project conclusions and recommendations
References
Appendix
Ion Exchange
SEM
EDAP production
Production
DABAP production
Production
