Experimental Study of the Flame Retardancy of PMMA-Graphene Composite Materials

In this paper, Polymethyl methacrylate (PMMA)-graphene nano-composites were prepared and tested with the use of a cone calorimeter. Graphene was added to PMMA in limited weight percentages to improve the flame retardancy of PMMA. Two samples of PMMA-graphene, namely 1 and 3 wt%, were investigated. The combustion properties of the tested samples of PMMA-graphene composites, mass loss rate, heat release rate, and time to ignition were measured and calculated. It was found that the peak heat release rate of PMMA-graphene composites reduced by 17% when 3 wt% graphene was added to pure PMMA. Adding graphene to PMMA improves the thermal stability of PMMA by reducing the time of ignition . Also, the presence of graphene enhanced the formation of a continuous carbonized layer at the surface of the burned PMMA.


INTRODUCTION
Polymers are promising materials that can be widely used in a variety of applications and low prices [1,2].Polymethyl methacrylate (PMMA) is an example of polymer which has been used in various applications [3].The most desired properties of the PMMA are its low density, low thermal conductivity, low water absorption, high chemical corrosion resistance, and high flexibility, while its main disadvantages are its low melting temperature and decomposition to volatile combustible products when exposed to heat [4].In some fields, where flame resistance is required, PMMA is used with additives to enhance its flame retardant properties [5].The mechanical and thermal properties of PMMA can be enhanced by adding reinforcements [5].There are many additives used with PMMA to enhance its flame retardancy [6].However, the choice of flame retardants as additives depends on the specific applications and degree of retardants required [7,8].Graphene is often added as a nano-filler of PMMA matrices to enhance flame retardancy [9], since the addition of graphene even at very low loadings up to 0.7 wt.% enhances the flame retardancy of materials [10].PMMA-graphene as a nanocomposite has a widely range of applications [11,12].Graphene has a high surface area to volume ratio, and is considered as the thinnest material to date [13,14].The presence of graphene as nano-sheets in polymer composites enhances flame retardancy by producing intumescent char during their combustion [15].When polymer burns, graphene forms a dense carbonized layer, which halts the burning of polymer matrices [16].Also, incorporation of graphene reduces mass loss rate significantly by altering the diffusion path of pyrolysis products [10].Flame retardants are used to reduce the fire impact and fire growth and spreading [8].This study aims to reduce the heat release rate from the combustion of polymers like PMMA.Graphene is selected to be used as a nano filler to retard flame spreading during the combustion of PMMA.

II. PREPERATION OF SAMPLES
In this study, PMMA is used as a base material while graphene is used as nano-filler.Pure PMMA supplied by the SABIC factory was used as a row material.Figure 1 shows the pure form of used PMMA.Graphene is selected as nano-filler to PMMA matrices due to its thermal stability, Graphene will not be a combustion fuel, and can be found in the burned char.Brabender plastograph EC plus instrument was used to mix the graphene with the base material (PMMA) to get a uniformly homogenous mixture.A total of 30 g of PMMA with two different weight percentages (1 wt% and 3 wt%) of graphene were used as a reinforcement.Graphene and pure PMMA were added to the mixer and processed at 180 o C and mixing speed of 50 min -1 to prepare the nano-composites.The high mixing temperature and the suitable torque leads to good dispersion and homogeneous distribution of grapheme inside the PMMA matrix.Also, pure samples of 30 g weight were prepared with the Brabender plastograph EC plus mixer under the same parameters that used to prepare the PMMA-graphene nano-composites.After the samples got mixed by the Brabender plastograph EC plus mixer, they were put in a stainless steel mold and were pressed by a hot press machine.A stainless-steel mold with dimensions of 100×100×3 mm 3 has been cut and polished to meet the requirements of the sample holder used in the cone calorimeter.The prepared sample was introduced to the sample holder and was tested inside the cone calorimeter which is calibrated according to ISO 5660.The flat square 100×100 mm 2 sample with 3 mm thickness was exposed to heat flux of 50 kW/m 2 .The heat release rate was measured with the Clayton Huggett method which is in accordance with ISO 5660.Oxygen consumption was used to determine the heat release rate.For each sample, Peak Heat Release Rate (PHRR), Effective Heat Release Rate (EHRR), and Total Heat Release Rate (THRR), time to ignition t ig , and time to reach the PHRR, ti-peak, were measured and recorded.Heat release and mass of the sample were measured and recorded every 5 s.

III. RESULTS AND DISCUSSION
Many methods are used to enhance the flame retardancy of materials, one of these is by char formation [17].Some materials like graphene and its derivatives when exposed to heat or flame form a char.This leads to the formation of a layer of carbonized material that can act as a barrier between the flame and the underlying material, preventing or slowing further combustion [18].The addition of small load of graphite up to 5 wt% can significantly enhance the flame retardancy and mechanical properties [3].
Three samples of pure PMMA and PMMA-graphene composites were prepared and tested by the cone calorimeter.Figure 2 shows the prepared samples of pure PMMA and its composites.I.For PMMAgraphene composites, two weight percentages of graphene were considered (1 wt% and 3 wt%).Figure 4 shows the average values of heat release rate for pure PMMA and PMMA-graphene composites from the cone calorimeter test.Pure PMMA has larger PHRR (964 kw/m 2 ) than PMMA-graphene composites.However, the addition of 1 wt% of graphene to PMMA reduced the PHRR by 11%.With further increment of the addition of graphene to 3 wt%, the PHRR was further reduced to 17% of pure PMMA's.Authors in [19] found that addition of 1 wt% of Graphene Nano Plates (GNPs) leads to 12% reduction in peak release rate compared to pure PMMA.Also, authors in [5] found that the PHRR reduces from 950 to about 780 kW/m 2 , when 0.5 ml/mg of carbon nano tubes were added to PMMA [19].The mechanism of reducing the PHRR of PMMA in the presence of grapheme is attributed to the formation of a carbon layer when the

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composites are ignited.This low conductivity of carbon layers covered the surface of the matrix, reducing the heat release of the material [20].Authors in [21] found the same results when adding grapheme oxide to polypropylene where the PHRR reduced from 1140 to 936 kW/m 2 when 1 wt% of grapheme oxide was added.However, the time to reach the PHRR has the same importance as the PHRR itself.It was found that addition of 3 wt% of grapheme to PMMA reduces this time from 140 s for pure PMMA to 131 s for PMMA-3 wt% grapheme nanocomposite, as shown in Table I. Figure 5 shows the images of char residues of PMMA and PMMA -3 wt% grapheme composite.The char of pure PMMA contains white and black areas.However, PMMA-3 wt% grapheme has exterior black char as shown.This black char slows the combustion of interior layers and prevents the pyrolysis gases to release.Figure 6 shows the microstructure of pure PMMA char and PMMA-3 wt% grapheme char.The presence of graphene enhances the formation of a continuous carbonized layer at the surface of PMMA-3 wt% graphene.This compact continuous carbonized layer protects the sublayers of the nanocomposite in two ways: by hindering the pyrolysis gases to transfer to the upper layer and acts as an insulator reducing the amount of heat transferred from the surface toward the sublayers.It has been found that the presence of graphene in the epoxy matrix produces a continuous and compacted char layer which exhibits lower thermal conductivity from pure epoxy [22].Table I reveals that the induction of graphene in low percentages (1 wt% and 3 wt%) has less effect in reducing the THRR values of PMMA-graphene composites.This is expected, because the heat generated from the combustion of 1 g of composites will not change, so all the samples approximately have the same values of THRR.PHRR has more influence on the fire spreading.So, for the same THRR, a lower PHRR means lower fire hazard and flame spreading [20].
Graphene and its products in general improve the thermal stability of the polymers and reduce the time to ignition t ig as shown in Table I [21].Time to ignition is recorded when the HRR exceeds 10 kW/m 2 for the first time.Adding 1 wt% graphene reduces t ig from 41 s for pure PMMA to 23 s.Authors in [23] reported that the addition of 1 wt% graphene oxide to polypropylene decreases the t ig to 39 s from 49 s.
Figure 7 shows the mass loss rate for pure PMMA and PMMA-graphene composites materials recorded in the cone calorimeter tests.The heat release rate from the samples depends on the amount of gas volatiles released from the sample.So, the mass loss rate curves have identical trends as the heat released rate curves.PMMA has the largest Peak Mass Loss Rate (PMLR) of 33 g/m 2 s.The PMLR of PMMA-1 wt% graphene composite is reduced by 22% and even more PMLR reduction was obtained (30%) when graphene increased to 3 wt%.
The thermal conductivity of PMMA is reduced by the addition of graphene [24].The presence of graphene in PMMA suppresses the transfer of the heat generated by the cone calorimeter from the surface layer of the tested sample to the inner layers, reducing the pyrolysis of PMMA.As a result, the volatile gases are reduced and the mass loss rate is affected.In addition, the mass loss rate depends on the rate of volatile gases released from the surface of the sample.The presence of nano fillers in polymers reduces gas permeability [25].Authors in [26] found that the gas permeability of PMMA matrix reduced by 50% when 1 wt% oxide grapheme was inserted and with further addition to 10 wt%, the nancomposite became impermeable.
However, the gas permeability of nanocomposites depends on the nanofiller aspect ratio, volume fraction, and the distribution uniformity inside the polymer.Graphene as a single layer of graphite has a high aspect ratio of 2630 m 2 /g [27].The effect of the addition of graphene to PMMA to produce composite materials can be observed from the rate production of CO and CO 2 generated from the combustion of volatile gases.Figure 8 shows the production rate of CO and CO 2 from pure PMMA and PMMA-graphene composites.It seems that the production of CO and CO 2 shows the same trends of heat release rate.IV.CONCLUSIONS In this study, two levels of graphene particles were mixed with pure PMMA and pressed in a prepared mold, and the resulting composite materials were studied with regard to their flame retardancy.The main conclusions of the current study are:  The addition of graphene particles enhances the flame retardancy of PMMA  The peak heat release rate of composite materials (PMMAgraphene) is less than pure PMMA's, while there is no change between the total heat release rate.
 The time to ignition decreases from 40 s for pure PMMA to 20 s for 3 wt% added graphene to PMMA.
 Mass loss rate decreases when graphene is added to PMMA.
 The production of CO 2 and CO from the combustion of pure PMMA and PMMA-graphene nanocomposites have the same trends.

Figure 3
Figure3shows the results of heat release rate from the pure samples.The results of pure PMMA and PMMA-graphene composites indicate the consistency of the tests.Also, it can be concluded that the samples resulted from Brabender plastograph EC plus instrument are homogeneous and uniformly mixed, where the heat release rate is approximately the same for the three samples taken from the mixed batch.The average measured values are shown in TableI.For PMMAgraphene composites, two weight percentages of graphene were considered (1 wt% and 3 wt%).

Fig. 7 .
Fig. 7.Mass loss rate of PMMA and its composites.