Investigations of Flame Retardant Properties of Zinc Borate Accompanying with Huntite and Hydromagnesite in Polymer Composites

Fires have been a danger to human beings for the centuries. As people lost their lives and property in fires, they tried to fight the fire and their efforts in this area continued increasingly. Unfortunately, it is still not possible to completely reset the risk of starting the fire. But it seems likely to extinguish immediately after the fire has started, and this is very important to save people’s lives. Scientists have been working in this field in recent years; they are concentrated on producing fire-resistant materials using different materials. This can be provided by different ways; either fire-resistant material can be produced new, or the fire resistivity can be provided by incorporating the additive material into a flammable material. In our previous studies, we used huntite and hydromagnesite minerals to give fire resistance property to polymer materials, very successful results were obtained. In this study, huntite and hydromagnesite minerals were used for accompanying with zinc borate in polypropylene composites in order to increase the flame retardant property of a polymeric materials. Different content of minerals were blended with polypropylene, and composites were produced by twin-screw extruder for observing synergistic effect. Scanning electron microscopy (SEM) analyses were conducted to determine the structural and morphological properties of the composites. Thermal properties were determined according to thermogravimetric analysis (TGA). Tensile and three point bending tests were carried out to obtain mechanical properties. Flame retardant performance was evaluated according to UL 94 vertically flammability test. It was concluded that very good synergistic effects were obtained that zinc borate was significantly influential with huntite/hydromagnesite in the flammability characteristics of composites because higher char formation is observed with zinc borate addtion. Moreover, the zinc borate reduced the smoke generated during combustion.


Introduction
Polypropylene (PP) is the most widely used commodity polymer in different commercial applications due to its good impact strength, good chemical resistance, good electrical insulation, low cost and easy processing. However, despite these wonderful features, PP is also known with its flammable property. Aliphatic hydrocarbon structure of this polymer gives rise to its severe flammability [1]. As pointedly, it should be investigated in order to enhance the usage of the polymer against fire to reduce losses of life and property [2][3][4][5]. With this regard, tendency of the addition of the flame retardant and the smoke suppressant additives has increased rapidly. Their effect is to drop off the chances of a fire starting with providing resistance to ignition. Even if ignition does occur, flame retardants can act to delay the spread of flame, provide extra time in the early stages so the fire can be extinguished or an escape can be made [6,7].
Flame retardant additives are generally classified into two groups; halogenated and halogen free materials. Some halogenated ones are tris(2-chloroethyl) phosphate, tris(1,3-dichloroisopropyl) phosphate, pentabromobenzyl acrylate and tris(1-chloro-2-propyl) phosphate. Indeed, they are not very preferred in the recent times because they release toxic gas to the environment and they can affect the endocrine, immune, reproductive and nervous systems. Some animal studies have shown that long-term exposure to those flame retardants can lead to cancer [8]. Halogen free flame retardants, which involve phosphorus, nitrogen, silicone, boron, zinc, iron, and aluminum-containing materials are considered as non toxic and environmentally harmless. In order to increase flame resistivity of polymers, environmental friendly systems have been also favored lately [9][10][11].
Huntite [Mg₃Ca(CO₃)₄] and hydromagnesite [Mg₅(CO₃)₄(OH)₂ · 4H₂O] minerals are defined as a natural mixture and classified as salt type carbonate minerals. This naturally formed mineral is commercially mined and processed as an alternative to the commonly used flame retardants because of its attractive properties, such as low smoke generation, environmentally safe, halogen free, recyclable, noncorrosive, and low combustion. At present, economically important reserves of huntite and hydromagnesite are found in Turkey and Greece. Production facilities in Turkey have lowered the cost, improved quality control, and should further the use of these minerals. [12][13][14][15][16]. The thermal degradation of huntite and hydromagnesite gives off both water and carbon dioxide when heated, with a minor loss at about 230°C, and major losses at about 420 and 550°C [17]. On the other hand, Zinc borate (ZnB) that is an effective inorgan-Submission date: 11-01-2020 | Review date: 02-04-2020 ic flame retardant additive is widely used both solely and in combination with other materials. Its influence as corrosion inhibitor, smoke suppressant, afterglow suppresant, synergistic agent improves elongation properties of polyoefins reported and emphasized by many authors [18][19][20].
To observe the synergistic effect with huntite and hydromagnesite and to increase the flame retardant property of the polymer composite, ZnB is used as a collateral additive in this study. Obtained samples were characterized by scanning electron microscopy (SEM) analysis to determine the structural and morphological properties of the composites. Tensile and three point bending tests were carried out to obtain mechanical properties. Thermal properties were determined according to thermogravimetric analysis (TGA). Finally, flame retardant performance of samples were observed according to UL 94 vertically flammability tests.

Materials
Polypropylene (PP) pellets were used as a main polymer matrix, were supplied by Petkim Petrochemical Co. (Izmir, Turkey) with the melt flow index of 4.7 g/10 min (MFI; 230°C/2.16 kg, ASTM D1238), melting point of 163°C and density of 0.905 g/cm3. All the samples of natural huntite/hydromagnesite, which were used to improve flame retardancy properties of PP, were supplied from Tirtar region of Isparta. Commercial zinc borate was supplied by MELOS A.S. to increase flame retardancy properties of PP with huntite and hydromagnesite and its particle size range from 5 to 45 µm.
The samples for thermal, mechanical and flammability tests were produced by using laboratory scale press (Hydraulic Laboratory Press, Labtech) which was heating and cooling platens at 200°C for 9 min. according to ISO R 527 and ISO 178, respectively.

Characterization of Composites
The surface morphology and microstructure of ZnB, huntite/hydromagnesite and composite samples were characterized by a scanning electron microscope (SEM, Zeiss Sigma 300 VP-FESEM). Thermal properties of the all composites were investigated by TGA analyzer (Perkin Elmer, STA 8000) by heating from room temperature to 600°C at a heating rate of 10°C/ min under nitrogen atmosphere. Tensile testing samples were obtained by using a mold which cut the plate with blades according to ASTM D638 standard. Tensile properties were tested by using tensile test machine (SHIMADZU, AGS-X 5kN) with a crosshead speed of 50 mm/min. The flexural modulus and flexural strength of samples were tested by test machine (SHI-MADZU AGS-X 5kN) which involves a three-point bending tests at a crosshead speed of 1 mm/min according to DIN EN ISO 178. Vertical burning test (UL 94) were conducted to investigate the flammability properties of composites on the test bars of 125 x 13 x 3.2 mm3 angular radius 1.3 mm, according to American National Standard UL-94 standard.

Results and Discussion
Tab. 1. Designation and composition of PP and PP composites  Fig.1 shows the SEM images of ZnB and huntite/hydomagnesite powder. As can be seen from the Fig.1-a that the particle size of ZnB is in between 5-10 micron; however, there seems also some aglomeration. Huntite/hydromagnesite particles change from 1 to 10 micron. To prevent aglomeration, some deflocculating materials can be used. Fig. 2 shows the detected TGA curves of the samples. The initial degradation temperature (T 1% ), the maximum degradation temperature (T max ) and the char residue values of PP and PP composites at 600°C are shown in Table 2. Degredation temperature for polypropylene matrix starts at 416.80°C and ends at 451.06°C. Initial degradation temperature of PP matrix was retarded approximately 15°C by adding maximum amount of huntite/hydromagnesite (50%) and 10% Zinc Borate. Moreover, it is seen in Fig. 2 and Table 2, increasing percentage of additives increases char residue. Maximum degradation temperature of the composites alters at the higher temperatures compared with pure PP, because of char layer formed by flame retardants. Therefore, PP matrix is protected from heat and combustion [21]. PP70H30 and PP60H30ZnB10 have 435.14°C and 438.46°C initial temperature of the sample's decomposition process. (T 1% ), respectively. The difference of the initial degradation temperature between PP70H30 and PP60H30ZnB10 shows that synergistic effect of zinc borate on huntite/hydromagnesite reinforced PP.
Tensile strength, Young's modulus, Flexural strength and modulus values of the samples are listed in Fig. 3 and Table  3. It can be seen that the tensile strength and strain properties were deteriorated by increasing huntite/hydromagnesite mineral quantity. borate addition, respectively. This decreasing is related to the poor compatibility between filler and polymer matrix which can be improved by the use of filler surface treatment [26]. In the tensile elongation results, it was obtained that increasing amount of mineral powder decreased the tensile elongation of PP by 91%; strain value of pure PP is 20.76% and 1.10% for PP40H50ZnB10 [26,27]. The plastic deformation of the polymer matrix and isolated filler particles are related to poor adhesion, which is directly related with the decrease of elongation for the samples with higher filler content [22]. In addition, agglomeration of the additives is another reason for deterioraiton of the mechanical properties due to the weakened interaction between the agglomerated filler and polymer matrix [23]. This interaction is hindered at higher amount additive contents, resulting in lower strength of the composites [24]. Elongation is inversely proportional to the modulus of a material [28,29]. Elastic modulus of pure PP is 0.996 GPa, while PP40H50ZnB10 is 2.257, thus it is increased by 126%. It is markedly improved by adding fillers that have higher stiffness than the matrix and are able to improve the modulus of composite to a polymer matrix [30].
If  Fig. 4 represents the SEM micrographs of tensile fractured surfaces of PP based on the composites which are PP70H20ZnB10, PP60H30ZnB10, PP50H40ZnB10 and PP60H50ZnB10 composites. Morphology of samples was investigated by 1000 times magnification. Pure PP shows ductile behaviour. It is seen that the fractured morphologies of huntite/hydromagnesite and ZnB reinforced composites show resembling brittle failure by increasing amount of huntite. If SEM images of the samples of PP70H20ZnB10 and PP40H50ZnB10 are compared, it can be found out that 20% reinforced composite shows the ductile fracture surface which is seperated as fibers.
Flame retardant properties of specimens have been investigated by UL 94 vertical test method. Flame starting time, flame growing time, UL 94 rating and cotton ignition results of the samples are indicated in Table 4. Fig. 5 shows that photographs of the samples after UL 94 testing. Flaming time of all composite samples are higher than neat PP. The flame starting and growing times were delayed by 5 seconds with the addition of %50 huntite/hydromagnesite in composites. It can be seen that UL 94 rating is changed from V2 to V1 by increasing the amount of huntite/hydromagnesite from 20% to 30% in the composite. This result is consistent with the results found in the Ref [31] by %50 huntite/hydromagnesite addition to thermoplastic polyurethane, thus UL 94 rating was achieved as V1. The work principle of the additives is related with the dilution effect in gas phase by the help of water and CO₂ release, endothermic decomposition and protective char layer [12,21,31,32]. On the other hand, the assistive effect of ZnB was observed, and this effect is considered that it is based on increasing barrier effect of char layer in the condensed phase and formation of incombustible gasses due to the acid base interactions. As mentioned Table 2, the char layer was 25.32% for the sample of PP70ZnB30 while it is 15.23% for PP70H30. Dike et al. [20] obtained the similar result of contribution of ZnB as synergistic additive for flame retardant in polymer.

Conclusion
Synergistic effect of ZnB on the flame retardant, mechanical and thermal properties of huntite/hydromagnesite reinforced PP composites were investigated. Composites were produced by using twin-screw extruder. SEM observations showed that there is no interaction between the flame retardant additives and polypropylene, except to mechan-  The addition of ZnB caused significant synergistic effect with huntite/hydromagnesite in the flammability characteristics of composites. Higher char formation is observed by zinc ZnB additon. In addition, it reduced smoke that generated during combustion.