CORK CONTRIBUTION IN THE FIRE PROPERTIES OF THE CORK-BASED TURF STRUCTURES

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Fire behaviour and fireproofing methods of floorings

Synthetic floorings include a wide range of textile floorings, generally called ‘carpets’, such as rugs, needle punched floorings, floorings made of plant fibres (sisal, sea rush, coir, etc.), etc. The structure of ‘carpets’ is similar to that of artificial turf as they are also composed of synthetic fibres inserted into a polymeric backing and held together by a back coating. Since the middle of the XXth century, the fire behaviour of carpet floorings was extensively studied.
The influence of the carpet’s constituents has been investigated by several authors [73], [74]. They highlighted that each component of the carpet contributes to the overall flammability performances but conventional latex back coatings, and the optional secondary backing, were the most influential factors in the fire behaviour of carpets, their presence leading to an ignition at lower radiant heat (0.78 W/cm² for the structure with a secondary backing against 1.25 W/cm² for the conventional structure). Other authors [75], [76] assessed the fire behaviour of carpets composed of different types of fibres or blend of fibres, especially using the radiant panel test. They demonstrated that wool and FR wool have shorter flame spread (only 8 cm within 10 min) than other synthetic fibres such as polyamide (PA), polyester (PET) or PP (up to 37 cm within 10 min). The density, the weight and the thickness of the pile also strongly influence heat dissipation. Nowadays, numerical simulations are even used to predict the fire behaviour and thermal properties of certain types of carpets, such as described in the study carried out by Diswat et al [77] who developed, and validated, a mathematical model to predict the thermal resistance of a cut pile hand tufted carpet. Based on these observations, many patents proposing solutions to improve the fire behaviour of carpets have been published [78]–[81]. Most authors have focused on the flame retardancy of carpet pile [82], [83] such as Erdem et al [84] who produced slow burning PP for carpet pile by incorporating silicon dioxide (SiO2) nanoparticles into the polymer at different rates. The Limiting Oxygen Index value (LOI) of the filaments increased progressively until 22 vol.% was achieved with 3 wt.% of SiO2 nanoparticles. Fireproofing of cotton and polyester carpet pile was also performed [85] by modifying the pile with polycarboxylic acid crosslinking systems by a spray, dry and cure process to impart flame-retardant properties, leading to improvements in LOI values up to 24.4 vol.% instead of 19.1 vol.% for untreated pile. In addition, Benisek [75] highlighted that using a FR latex back coating permitted to limit the spread of flame (9 cm of flame spread within 10 min for the FR latex back coating structure against 26 cm for the conventional one).
In order to find out whether these types of fireproofing methods are also relevant for artificial turf, a thorough study of its fire behaviour is first necessary. However, unlike carpets, fire hazards associated to the use of artificial turf are rarely discussed while the flammable organic materials composing these structures can greatly contribute to the development and spread of fire. In 2018, Kukfisz B [86] studied the degree of flammability of turf systems used for landscaping in residential and public buildings. The horizontal radiant panel test, in accordance with EN ISO 9239-1 standard, was used to characterize the structures by measuring the flame spread. The tested structures, mainly composed of polyethylene (PE) and polypropylene (PP), meet the requirements of class EFL, meaning that they are suitable only for household use (like outdoor decoration), and entail a considerable risk in roofed buildings. Regarding artificial turf, only very few structures currently meet the standards and articles or patents dealing with the development of flame retardant (FR) solutions to improve its fire behaviour are scarce. Reddick [2] patented a new infill composed of recycled waste glass or glass beads, giving the infill a non-flammable character. Rodgers [13] mentioned the use of inorganic salts encapsulated in a water-insoluble material as flame-retardant infill. Commercial FR solutions have also been developed. Recently, ethylene propylene diene monomer (EPDM) infill materials have been fire retarded with a high percentage of calcium carbonate (CaCO3) (up to 80 wt.%) [17]. Moreover, styrene butadiene rubber (SBR) infill material can be flame retarded with halogenated compounds. Studies have also been carried out on the development of FR solutions for turf fibres. Lehner [14] et al. have developed the use of 6H-dibenz[c,e][1,2] oxaphosphorin,6-[(1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-yl)methoxy]-, 6-oxide (DOPO-PEPA), as flame retardant for low density polyethylene (LLDPE). Mixed with commercial thermal stabilizers, the thermal stability as well as the fire retardancy of LLDPE/DOPO-PEPA films have been tested and promising results were obtained with 10 wt.% DOPO-PEPA and 2.5 wt.% stabilizer (increase of 26 vol.% in LOI value). The small amounts of flame retardants used are compatible with the processing of fibres and can therefore be considered as a solution to improve the fire behaviour of artificial turf. Some masterbatch manufacturers, such as Avient [18], also provide commercial halogenated-based solutions to enhance the flame retardancy of turf fibres. In addition to infills (SBR) and fibres, the latex used as back coating can also be flame retarded with halogenated compounds. However, as previously discussed, this approach is not considered in this project for reasons of health and environmental protection [72].

Goal and scope of the PhD thesis – scientific approaches and defined strategy

We have thus shown that the use of artificial turf for sports fields is becoming more and more popular and, that the uses of such materials are moving (not only sport but other activities such as concert…). This is explained because they have many advantages such as low maintenance and playability in all weather conditions. However, being mainly composed of highly flammable polymers, in case of fire, these structures can ignite and spread flames quickly representing of risk. Currently, there is very little information on the fire behaviour of such complex multi-layer materials, yet this information is essential to be able to define fire proofing strategies for artificial turf. Therefore, the first part of the study has been focused on a better understanding of the fire behaviour of artificial turf. These aspects are detailed in Chapter 1, PART III in which the impact on the artificial turf fire behaviour of each component, of the association of component and of the whole structure will be investigated. The results obtained in this chapter will permit to define the component with the highest impact and thus, a fireproofing method will be selected. Indeed, as explained in the previous section, the choice of fireproofing methods and of flame retardant additives will depend on the material to be fireproofed and of the expected properties. The selected method will be developed in Chapter 3, PART III. In this chapter, we will consider that the flame retardants used should not be toxic compounds (halogen-free, no nanoparticles …) to avoid being harmful to health and to the environment.
On the other hand, this literature review shows that artificial turf has to meet several specifications to be used either for indoor or outdoor applications. Regarding its fire properties, the EN ISO 9239-1 fire test has to be used. However, this test required large amount of materials and thus it could appear as a break considering development of new fireproofing methods and formulation. That is the reason why the design, optimization and validation of a laboratory scale test mimicking the EN ISO 9239-1 was considered. The results are presented in Chapter 2, PART III. This test as well as other technics will further be used in Chapter 4, PART III to validate the accuracy of the approach as well as to determine the main mechanisms of action of the fireproofing method that was selected. However, before presenting all the results of the study in PART III of this manuscript, the description of ‘materials and experimental methods’ used in the whole study is considered in the following part.

Fire testing: Mass Loss Cone calorimetry

The fire behaviour of materials has to be evaluated through fire tests. Mass Loss Calorimeter (MLC) is used because it is an efficient, fast and low material consumption way of determining the heat release rate of a material. This data is important because it can be a factor in the spread of a fire. In the case of flooring studies, it therefore provides additional information to that obtained from the standard radiant panel test (mainly providing information on the length burnt). MLC can be used to assess the burning behaviour of artificial turf structures or to investigate the fire behaviour of flame retarded solutions.

Artificial turf structures preparation

To perform the MLC tests, ‘backing + pile’ samples of 100 x 100 mm² size are cut with a cutter. Then, a metal grid (size of 110 x 110 mm²) is placed onto the backing, making sure that the pile remains straight. The grid prevents the sample from moving during the fire test. Sand is then distributed as evenly as possible over the backing and the grid, keeping the pile as straight as possible. Finally, the damping infill material is also uniformly spread above the sand layer. The specimens are weighed before and after fire testing in order to determine the weight loss. This procedure is used when the complete artificial turf structures are tested. However, specimens can also be tested:
– Without any damping infill material (‘backing + pile + sand’ sample)
– Without sand and infill (‘backing + pile’ sample)
– Without sand, infill and pile (‘backing’ sample). In that case, fibres emerging from the backing are removed using a razor blade. Infill materials can also be tested alone. In such case, no metal grid is used to perform the test. The next part is dedicated to the description of the MLC testing.

Mass Loss Calorimeter testing

To evaluate the fire behaviour of artificial turf structures, a MLC from Fire Testing Technology (FTT) (Figure 4 (a)) is used and tests are performed according to the EN ISO 13927 procedure [87], [88]. This bench-scale apparatus allows testing the reaction to fire of materials in a forced flaming combustion scenario. In MLC, the temperature of the degradation products during combustion is measured by a thermopile, consisting of four thermocouples located at the top of the chimney, and allows obtaining the heat release rate (HRR) according to a calibration curve obtained by burning methane at various flow. This is the main difference between the MLC and the cone calorimeter (ASTME-1354-90) [89], in which the oxygen consumption is measured during the test to determine  Figure 4: (a) Mass loss calorimeter fire testing and (b) an example of a recorded HRR vs time curve with the main determined parameters To perform a test, the assembly previously described or the materials (infill, backing, …) are placed onto a ceramic board and held by a frame covering the edges of the samples. This assembly is exposed horizontally to an external heat flux of 25 kW/m² corresponding to a developing fire scenario [90]. The distance between the upper infill surface and the radiant source is fixed at 35 mm. At the beginning of the test, an electric arc is placed between the sample and the heating resistance. During testing, air velocity is measured by an anemometer (supplied by OMEGA) and was fixed at around (1 ± 0,1) m.s-1.
MLC allows measuring the evolution of the HRR as a function of time (Figure 4 (b)). The main parameters considered for the comparison of the different results are: the peak of heat release rate (pHRR); the time to ignition (TTi); the total heat release (THR) corresponding to the area under the HRR vs time curve; and the time of flameout (TFO). All tests were performed at least three times to ensure repeatability and results are reproducible within a relative standard deviation of ±15 % for the TTi and of ±10 % for the pHRR and the THR. For the THR determination, the shortest TOF of a series of measurements is taken as the last measurement point to allow comparison of the THR data. To obtain additional information about the temperature evolution inside the samples during the fire tests, 6 thermocouples (0.5 mm diameter) are inserted from the bottom of the sample, bent and maintained at the backing surface, below the infill and sand layers as presented in Figure 5.

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Thermogravimetric analyses

Thermogravimetric analyses (TGA) were carried out using a TGA Discovery 5500 supplied by TA Instruments. This technique allows studying the thermal decomposition of samples by determining the weight loss versus temperature. Because of their small size and in order to obtain representative results, the granules of infills were not grinded and about 10 mg of sample were directly deposited in an open alumina crucible equipped with a gold foil. The thermograms were recorded in the 40–800°C temperature range, with a heating rate of 10°C/min under a 50 mL/min nitrogen flow. Then, data were analysed using the TRIOS software. The main parameters considered to have information on the thermal stability of the samples are: the temperature at the onset and at the maximum of the degradation, the final residual mass at 800°C and the mass loss rate at specific temperatures, thanks to the derivative curve (DTG).

Heat treatments

The thermal stability of infill materials was also evaluated by heating it in a benchtop furnace (Nabertherm, Controller P320) from room temperature to 600°C, the temperature at which cork is usually fully degraded. The purpose of these tests is to assess the degradation of material at high temperature and also to determine the percentage of residual mass in order to evaluate in a simple and rapid way its thermal stability. To perform the test, samples of around 250 mg were exposed to a temperature rise of 10 °C/min from room temperature to 600°C by going through 4 intermediate degradation temperatures, i. e. 200, 300, 400 and 500°C (no step at intermediate temperatures). When the required temperature was reached, i.e. 200, 300, 400, 500 or 600°C, residues were immediately taken out from the furnace and were cooled down until room temperature. Samples were then weighed after calcination to determine the final residual mass.

Stability of materials towards water

As artificial turf can be used outdoors, external aggressions linked to the weather (rain, snow, etc.) must be taken into account. In order to limit any potential soil contamination due to leaching, as well as to study the ageing due to the impact of water on the components of the artificial turf (in particular the infill), it is essential to study the stability of the infill materials in water.

Evaluation of leaching in water

Leaching refers to the extraction of soluble products by a solvent, and in particular by water circulating into the soil. The behaviour of infill regarding leaching in water is important to assess if toxic elements could be released into the soil over time. A first approach was proposed in this project but further experiments will be required for commercial applications. The preliminary test established by the lab is the following: 2.5 g of infill were immersed into 400 mL of demineralised water (pH = 7.9), under magnetic stirring (300 rpm) and maintained at 25 °C. For five days, three times a day at regular intervals, the pH value of the solution containing the infill was measured (MeterLab PHM210), and then a final value was measured 30 days after the first measurement. Beforehand, the pH meter was calibrated with two buffer solutions (pH 4 and 7).

Table of contents :

ACKNOWLEDGMENT
TABLE OF CONTENT
ABBREVIATIONS
GENERAL INTRODUCTION
PART I – LITERATURE REVIEW AND BACKGROUND
I. ARTIFICIAL TURF
A. EVOLUTION AND COMPOSITION OF TURF SURFACES
B. MANUFACTURING PROCESS
C. REQUIRED FUNCTIONAL PROPERTIES
II. FIREPROOFING METHODS
A. FIRE-RETARDANT APPROACHES
B. FIRE BEHAVIOUR AND FIREPROOFING METHODS OF FLOORINGS
III. GOAL AND SCOPE OF THE PHD THESIS – SCIENTIFIC APPROACHES AND DEFINED STRATEGY
PART II – MATERIALS & EXPERIMENTAL TECHNIQUES
SECTION 1 – COMPOSITION OF ARTIFICIAL TURF STRUCTURES
SECTION 2 – METHODS
I. FIRE TESTING: MASS LOSS CONE CALORIMETRY
A. ARTIFICIAL TURF STRUCTURES PREPARATION
B. MASS LOSS CALORIMETER TESTING
II. THERMAL STABILITY
A. THERMOGRAVIMETRIC ANALYSES
B. HEAT TREATMENTS
III. STABILITY OF MATERIALS TOWARDS WATER
A. EVALUATION OF LEACHING IN WATER
B. DYNAMIC VAPOUR SORPTION
IV. SPECTROSCOPIC ANALYSES
A. INFRARED SPECTROSCOPY
B. SOLID STATE NUCLEAR MAGNETIC RESONANCE (SS NMR)
PART III – EXPERIMENTAL RESULTS
CHAPTER 1 – COMPREHENSIVE STUDY OF THE FIRE BEHAVIOUR OF ARTIFICIAL TURF
I. FIRE BEHAVIOUR OF ARTIFICIAL TURF
A. FIRE BEHAVIOUR OF THE BACKING / PILE / SAND
B. FIRE BEHAVIOUR OF INFILL MATERIALS
C. FIRE PROPERTIES OF THE COMPLETE STRUCTURES
D. DISCUSSION AND CONCLUSION
II. CORK CONTRIBUTION IN THE FIRE PROPERTIES OF THE CORK-BASED TURF STRUCTURES
A. INFLUENCE OF THE CORK LAYER THICKNESS ON THE FIRE PROPERTIES
1. Lighter cork
2. Denser cork
3. Conclusion
B. PROTECTIVE BEHAVIOUR OF THE CORK LAYER
III. CONCLUSION OF CHAPTER 1
KEY POINTS & STRATEGIES OF CHAPTER 1
CHAPTER 2 – DEVELOPMENT OF THE EN ISO 9239-1 RADIANT PANEL TEST ON A LAB SCALE
I. OVERVIEW OF THE METHODS OF FIRE BEHAVIOUR ASSESSMENT
II. MATERIALS & METHODS
A. MATERIALS
B. METHODS
1. Turf structures preparation before testing
2. Large-scale fire testing according to the EN ISO 9239-1 standard
2.a. Test description
2.b. Test protocol
3. Small – scale test development
3.a. Test design
3.b. Test protocol
4. Design of experiments: theoretical part
III. RESULTS AND DISCUSSION
A. OPTIMIZATION OF THE SMALL-SCALE DEVICE SETTINGS THROUGH EXPERIMENTAL DESIGN METHODOLOGY
B. FIRE PROPERTIES: LARGE-SCALE TEST VS SMALL-SCALE TEST
1. Evaluation of artificial turf on the large-scale test
2. Evaluation of artificial turf on the small – scale test
3. Proof of concept: assessment of the radiant panel test downscaling reliability
4. Conclusion
C. EVALUATION OF CORK-BASED STRUCTURES AT THE RADIANT PANEL TEST
1. Evaluation of S – Cork at various infill thickness on the large-scale test
2. Evaluation of S – Cork at various infill thickness on the small-scale test
3. Discussion and conclusion
IV. CONCLUSION OF CHAPTER 2
KEY POINTS & STRATEGIES OF CHAPTER 2
CHAPTER 3 – CORK: BULK MODIFICATION BY PHOSPHORYLATION
I. CORK: A COMPLEX NATURAL MATERIAL
A. CORK CULTIVATION
B. CORK COMPOSITION
1. Suberin
2. Lignin
3. Polysaccharides
C. PROPERTIES OF CORK
D. APPLICATIONS OF CORK
II. CORK MODIFICATION
A. CORK PROCESSING METHODS
B. CORK FIREPROOFING METHODS
III. FLAME RETARDANCY OF NATURAL PRODUCTS
A. SURFACE TREATMENTS
B. BULK MODIFICATIONS
1. Phosphorus pentoxide
2. Phosphoric acid
2.a. Phosphoric acid combined with lithium chloride and urea
2.b. Phosphoric acid combined with phosphoric pentoxide
3. Phosphate salts
4. Phosphorus oxychlorides
5. Phosphonamide
6. Phosphonic acid
C. SELECTION OF METHODS FOR CORK PHOSPHORYLATION
IV. MATERIALS AND METHODS
A. PHOSPHORYLATION PROCESSES ADAPTED TO CORK MODIFICATION
1. Tetrahydrofuran and phosphorus pentoxide protocol
2. Phosphoric acid combined with phosphorus pentoxide protocol
3. Phosphate salts protocol
B. YIELD OF REACTION
C. PHOSPHORUS CONTENT DETERMINATION BY INDUCTIVELY COUPLED PLASMA
D. MICROSCOPY
1. Electron Probe Micro Analysis
2. Scanning electron microscopy
V. RESULTS AND DISCUSSIONS
A. CHOICE OF THE PHOSPHORYLATION PROTOCOL: PRELIMINARY CHARACTERIZATIONS
1. Characterization of P-corks
1.a. Mapping of the 31P phosphorus element by EPMA
1.b. Infrared spectroscopy analysis
1.c. Thermal stability
2. Discussion and conclusion
B. OPTIMIZATION OF THE PHOSPHORYLATION PROTOCOL
1. Design of experiments: theoretical part
2. Characterization of P-corks
2.a. Carbonization rates at 600°C
2.b. Yield of reaction
2.c. Mapping of the phosphorus element by EPMA
2.d. Infrared spectroscopy
3. Discussion and first screening of experimental conditions
C. UPSCALING OF THE PHOSPHORYLATION SYSTEM
1. Dimensional analysis modelling
1.a. Listing the independent physical variables
1.b. Writing the dimensional matrix
1.c. Selecting the repeated physical variables (basis) in the dimensional matrix
1.d. Identifying the residual matrix and the central matrix
1.e. Linearizing the central matrix and obtaining the modified residual matrix
1.f. Determining the dimensionless numbers from the modified residual matrix
1.g. Rearranging the dimensionless numbers
2. Optimal large-scale formulation
3. Characterizations
3.a. Reaction yield
3.b. Carbonization rates of LS P-cork
3.c. Discussion
4. Large scale formulation checking and conclusions
D. FULL CHARACTERIZATION OF LS P-CORK
1. Chemical characterization of LS P-cork
1.a. Microscopic analysis
1.b. Infrared spectroscopy analysis
1.c. Nuclear magnetic resonance spectroscopy analysis
1.d. Phosphorus content
1.e. Discussion
2. Evaluation of the other properties of cork
2.b. Ageing in water
2.c. Conclusion
VI. CONCLUSION OF CHAPTER 3
KEY POINTS & STRATEGIES OF CHAPTER 3
CHAPTER 4 – THERMAL DEGRADATION AND FIRE BEHAVIOUR OF PHOSPHORYLATED CORK
I. FIRE BEHAVIOUR OF LS P-CORK
A. FIRE BEHAVIOUR OF PHOSPHORYLATED CORK
B. FIRE BEHAVIOUR OF PHOSPHORYLATED CORK IN THE WHOLE ARTIFICIAL TURF STRUCTURE
1. Assessment of the fire behaviour of cork-based and LS P-cork-based structures using MLC test
2. Protective behaviour of the phosphorylated cork
3. Fire behaviour of LS P-cork based structure on the small-scale radiant panel test
C. CONCLUSION
II. INFLUENCE OF TEMPERATURE ON THE PROPERTIES OF LS P-CORK
A. THERMOGRAVIMETRIC ANALYSES OF CORK AND LS P-CORK
B. SOLID STATE NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY OF CORK AND LS P-CORK
1. Characterization of cork and LS P-cork versus temperature using solid-state 13C NMR
2. Characterization of cork and LS P-cork versus temperature using solid-state 31P NMR
C. PHOSPHORUS CONTENT OF CORK AND LS P-CORK AT 600°C
D. CONCLUSION
III. MECHANISMS OF ACTION OF P-CORK
A. MODES OF ACTION OF PHOSPHORUS FLAME RETARDANTS – STATE OF THE ART
1. Action in the condensed phase
2. Action in the gas phase
3. Mode of action in both phases
B. MECHANISMS OF ACTION OF PHOSPHORUS IN PHOSPHORYLATED CORK
IV. CONCLUSION OF CHAPTER 4
KEY POINTS OF CHAPTER 4
PART IV – GENERAL CONCLUSION AND OUTLOOK
REFERENCES

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