Investigation of flame retardancy and physical-mechanical properties of mineral-based polyethylene composites

Abstract

Mineral-based flame retardants are widely used worldwide as they are both effective and inexpensive solutions. Especially alumina trihydrate (ATH) and magnesium hydroxide (MDH) are the first mineral additives that come to mind industrially due to their environmental friendliness and easy availability. In this study, the effectiveness of huntite-hydromagnesite mineral, which is not as widely used as the other two minerals, and its differences from its counterparts were investigated. For this purpose, composites with polyethylene carrier were produced by extruder method using binders at various ratios, and various physical, mechanical, and combustion properties of these samples were investigated. According to the data obtained, the samples containing huntite-hydromagnesite showed UL94 V0 class flame retardant performance with 5% less filler than their counterparts. The same samples showed superior mechanical properties in tensile strength between 23.4% and 70.8% and elongation at break between 60.7% and 73.1%.

Keywords

Flame retardant huntite-hydromagnesite alumina trihydrate magnesium hydroxide composite

Introduction

Polyethylene (PE) is widely used in our daily lives as it is a lightweight, flexible, high-strength polymer that is resistant to chemical interactions and abrasion. Although this type of polymer is preferred in many areas such as automotive, furniture, packaging, and textile sectors, it burns easily due to its carbon-rich and organic structure when in direct contact with flame. Therefore, in certain applications, it is necessary to limit the flammability of the material with various flame retardants aimed at preventing ignition and reducing the rate of combustion. Flame retardants are additives that do not ignite easily at high temperatures, self-extinguish if ignited, minimize the spread rate of the flame, and prevent the polymeric material from burning continuously. For the polyethylene structure to be flame retardant, its carbonization must be high even at low temperatures. This polymeric structure must be thermally stable above 400°C and its decomposition products must consist of non-combustible gases. Therefore, flammability and ignition times should be minimized with various flame-retardant additives.^1–8

There are flame retardants on the market that can be broadly classified as halogenated and non-halogenated. Since halogenated flame-retardant additives contain elements such as bromine and chloride, they produce toxic gases when heated and are therefore subject to various regulations and restrictions.^9–11 The use of halogen-free mineral-based additives is increasing with the aim of sustainability of flame retardants and prevention of toxic gas formation.¹² Mineral-based flame retardants are low-cost, environmentally friendly materials that have no side effects on the environment and living organisms. Among these, aluminum hydroxide (ATH) and magnesium hydroxide (MDH) are widely used mineral-based flame-retardant additives. Huntite/hydromagnesite (H/H), on the other hand, is not as common as the previous ones, but its use is increasing.

ATH and MDH decompose endothermically under the effect of temperature, releasing water and providing flame retardant properties through this factor. The decomposition temperature of ATH is between 180–200°C. Above this range, it releases water into the environment, making it unfavorable for use in applications requiring high temperatures for processing.¹³ Although MDH has a decomposition temperature above 300°C and good flame-retardant properties, it is not as preferred in industry as ATH due to its high cost. ATH starts to lose significant mass above 180–200°C and loses 34.6% of its mass at 350°C. Endothermic degradation of MDH occurs at 300°C.¹⁴

2 A l {(O H)}_{2} \to A l_{2} O_{3} + 3 H_{2} O

(1)

2 M_{g} {(O H)}_{2} \to 2 M_{g} O + 2 H_{2} O

(2)

H/H mineral is also used as a flame-retardant additive in plastics. Hydromagnesite (3MgCO₃.Mg(OH)₂.3H₂O) is a natural mineral that is a mixture of magnesium carbonate and magnesium hydroxide. Huntite (CaMg₃(CO₃)₄) is a calcium magnesium carbonate mineral. H/H is a mixture of the two and is an important additive due to its smoke suppressant, flame retardant, and environmentally friendly properties. It has a high potential as an alternative due to its low cost compared to other mineral additives. H/H provides its flame-retardant properties in three stages. Hydromagnesite decomposes in the temperature range of 220-550°C to form water molecules and carbon dioxide (Equation (3)). Huntite decomposes at 450-800°C to form calcium oxide and magnesium oxide compounds and release carbon dioxide gas (Equation (4)).^15–21

{M g}_{5} {({C O}_{3})}_{4} {(O H)}_{2} . {4 H}_{2} O \to 5 M g O + {4 C O}_{2} + {5 H}_{2} O

(3)

{M g}_{3} C a {({C O}_{3})}_{4} \to 3 M g O + C a O + {4 C O}_{2}

(4)

As a result, a charcoal-like layer forms on the material after combustion and suppresses the flame by cutting off the contact of the fire with air. ATH, MDH, and H/H have good flame-retardant properties if the polymer is loaded high enough. However, perhaps the biggest disadvantage of these minerals is that when used in high quantities, they cause a decrease in the mechanical properties of the composites.²²

In this study, the effect of halogen-free mineral-based flame-retardant additives such as ATH, MDH, and H/H on the flame retardancy and mechanical properties of polyethylene composites was investigated. For this purpose, the flame retardancy, mechanical and physical properties of mineral-based flame-retardant polyethylene composites prepared by extrusion method under laboratory conditions were compared and the results were evaluated.

Experimental

Materials

Linear low-density polyethylene (LLDPE) with a melt flow index of 20 g/10 min (190°C/2.16 kg) and a density of 0.913 g/cm³ was obtained from INEOS Group as a carrier polymer matrix. Three different flame-retardant minerals were supplied, each with a particle size of 3-4 μm. The Turkish huntite/hydromagnesite (H/H) mineral used to improve the flame-retardant properties of LLDPE was supplied by Likya Mineral Madencilik (Izmir, Turkey) under the trade name Ultracarb LH15C. The chemical formula of huntite and hydromagnesite are [CaMg₃(CO₃)₄] and [3MgCO₃.Mg(OH)₂.3H₂O)], respectively. Aluminum hydroxide (ATH) AluMILL MF132 and magnesium hydroxide (MDH) EcopireN 3.5 were supplied by Eggerding Industrial Minerals (Amsterdam, Netherlands) and Europiren B.V. (Rotterdam, Netherlands), respectively. Maleic anhydride grafted linear low-density polyethylene, LLDPE-g-MA, Bondyram 4108 (MAP), was supplied by Polyram Industries Ltd (Ram-On, Israel) to improve the mechanical properties of the high-fill mineral-containing polymer as well as to better bind the inorganic mineral with the polymeric matrix and improve the quality of dispersion. The density and the melt flow index are, respectively, 0.92 g/cm³ (ISO 1183) and 1 g/10 min (190°C/2.16 kg, ISO 1133) while the maleic anhydride (MA) content is 1 wt %.

Manufacturing

LLDPE-based composites containing flame retardant minerals were mixed in a benchtop high-speed mixer (Labtech Engineering, Thailand) at 2000 rpm for 10 min and prepared using a twin-screw extruder (Labtech Engineering, Thailand) with a screw diameter of 20 mm and barrel length of 880 mm (L/D = 44:1).

For the preparation of LLDPE composites containing H/H and MDH minerals, the main screw speed of the extruder was set to 200 rpm and the extruder zone temperatures were set between 150-180°C. For ATH with a degradation temperature of 180°C and above, the extruder zone temperature was set between 100–160°C. The different formulas applied for each mineral content are listed in Table 1. In the first stage of the formulation, the total flame-retardant mineral content was kept constant at 65 wt%. MAP was used in increasing proportions of 1 wt%, 3 wt%, and 5 wt%.

Table 1.

Formulas of flame-retardant samples in the first stage.

Sample	LLDPE (wt%)	H/H (wt%)	ATH (wt%)	MDH (wt%)	MAP (wt%)
65HH	35	65	—	—	—
65HH/1MAP	34	65	—	—	1
65HH/3MAP	32	65	—	—	3
65HH/5MAP	30	65	—	—	5
65ATH	35	—	65	—	—
65ATH/1MAP	34	—	65	—	1
65ATH/3MAP	32	—	65	—	3
65ATH/5MAP	30	—	65	—	5
65MDH	35	—	—	65	—
65MDH/1MAP	34	—	—	65	1
65MDH/3MAP	32	—	—	65	3
65MDH/5MAP	30	—	—	65	5

This stage was designed to see to what extent the use of LLDPE-g-MA in increasing proportions of 1-3-5 wt% improves the mechanical properties and dispersibility of high mineral content (65%) LLDPE composites.

In the second stage, 50-55-60% mineral-based flame retardants were produced keeping 3% LLDPE-g-MA constant. The formulations for this stage are shown in Table 2. The physical, flame retardant and mechanical properties of the composite materials obtained from this stage were tested.

Table 2.

Formulas of flame-retardant samples in the second stage.

Sample	LLDPE (wt%)	H/H (wt%)	ATH (wt%)	MDH (wt%)	MAP (wt%)
60HH/3MAP	37	60	—	—	3
55HH/3MAP	42	55	—	—	3
50HH/3MAP	47	50	—	—	3
60ATH/3MAP	37	—	60	—	3
55ATH/3MAP	42	—	55	—	3
50ATH/3MAP	47	—	50	—	3
60MDH/3MAP	37	—	—	60	3
55MDH/3MAP	42	—	—	55	3
50MDH/3MAP	47	—	—	50	3

A thermokinetic mixer (Gülnar, Turkey) and a hydraulic press machine (Gülnar, Turkey) were used to prepare the LLDPE composite plates. The mixer was operated at 2000 rpm with a mixing time of 20-25 s to prepare the composite plates. During this time, the pellets in the hopper of the device became molten under the action of the mixing blade and high-speed impact. The molten composite from the mixer was placed between two Teflon-coated plates and pressed in suitable molds as soon as possible to prevent heat loss. At this stage, the mixtures were first pressed in a hot press at 150°C for 20 s and then in a cold press for 1.5 min. Composite plates were prepared according to ISO 527-2 for mechanical tests and UL 94 vertical flammability standards for combustion tests.

Characterization

Density, ash content, melt flow index analysis, and filter pressure tests were performed for pellets processed in the extruder. Mechanical and combustion tests were performed on the plates obtained by melting and pressing the pellets. Each analysis was measured five times and average values were calculated.

The density of the pellets was measured according to the ASTM D792 standard. The samples were first weighed in air and then in a solvent of known density (Ethyl alcohol: 0.789 g/cm³) at 23°C.

Ash test was performed according to ISO 3451-1 standard to determine the ash content in the structure of mineral-containing flame-retardant composite pellets.

The melt flow index (MFI) of the composite pellets was measured with a melt flow device (Zwick/Roell, Germany) according to ASTM D1238. The temperature and load parameters applied to the composites were 190°C and 5 kg, respectively.

The filter pressure test was performed to determine the quality of dispersion of minerals and other additives in the composite pellets. A low filter pressure value (FPV) indicates a good homogeneous distribution. Filter pressure values were tested using a 25 µm filter according to ISO 23900-5 standard.

To determine the effects of mineral-based flame-retardant additives on the color of the polyethylene matrix, color analysis was performed with a spectrophotometer. Lightness (L*), redness (a*), and yellowness (b*) values were measured using an X-Rite Ci64 spectrophotometer according to the CIELab D65/10° color system and ASTM E308-18.

To measure and evaluate the flame resistance of the composite plates, a UL 94 vertical flammability test was performed according to ASTM D3801-10. For the test, specimens were prepared with dimensions of 125 mm length, 13 mm width, and 3 mm thickness. A piece of cotton 6 mm thick and 50 mm long was placed at the bottom of the test rig. A flame was applied to the specimens for 10 ± 0.5 s. The time required for the flame to go out was then recorded as t1. After the flame on the specimen was extinguished, the flame was applied for a further 10 ± 0.5 s and then the second flame time was recorded as t2.

The mechanical properties of the composites were tested using a 10 kN universal testing machine (Devotrans, Turkey). The tensile strength of the composites was tested according to ISO 527-2 at a test speed of 10 mm/min. Specimens for the tensile test were prepared in the form of a dog bone 170 mm long, 10 mm wide at the narrow part, and 4 mm thick. At the end of the test, elongation at break and tensile strength values were determined.

Results and discussion

Characterization tests of composites in the first stage

As expected, no significant difference in the density values of the composites was observed in the characterization tests performed in the first phase. In the ash tests, a mass loss of 56% was observed for H/H minerals, 37% for ATH, and 36% for MDH. The positive or negative effects of the proportionally higher mass loss in H/H were not observed in the first-stage combustion tests. Significant decreases in viscosity values were observed in all three types of composites as the MAP ratio increased. This result is particularly evident for H/H with a decrease of up to 98%. This can be interpreted as a good interface between the mineral and organic bonds in the polymer chain with the help of MAP and consequently, the flow becomes difficult. When maleic anhydride is grafted into a polymer chain, polar functional groups such as carboxylic acid groups appear, which can increase the intermolecular forces and reduce the flowability of the polymer. This has resulted in a decrease in the melt flow index, a measure of the fluidity of a polymer in the molten state, and this effect of grafted maleic anhydride on polyolefins has been widely reported in similar studies.^23–26 This result was also seen in improved dispersion in filter tests. When maleic anhydride is grafted into a polymer chain, it can improve compatibility between different polymers or between a polymer and a filler. This improved compatibility results in better dispersion of filler particles or other additives within the polymer matrix. In terms of filter pressure rating, maleic anhydride can affect the processability of a polymer and the behavior of a polymer melt during filtration. The addition of maleic anhydride can change the rheological properties of the polymer melt, such as viscosity and melt flow rate, which can affect the filtration process. As the melt flow rate decreases, the expected result is an increase in the shear stress in the extruder barrel and the associated energy transferred to the matrix. This can have a positive effect on particle distribution and dispersion. Regarding the FPV results, a general downward trend, i.e., better dispersion, is observed as the MAP ratio increases. However, dispersion decreased as the MAP ratio increased above 3%. This result is particularly evident in tests with MDH (Table 3).

Table 3.

Characterization tests of composites in the first stage.

Sample	Density (g/cm³)	Ash (wt%)	MFI (g/10 min)	FPV (bar/g)	L* (lightness)	a* (redness)	b* (yellowness)
65HH	1.47 ± 0.04	28.02 ± 2.12	33.64 ± 3.24	1.20 ± 0.02	84.04 ± 0.23	−1.44 ± 0.01	5.74 ± 0.16
65HH/1MAP	1.48 ± 0.10	28.94 ± 1.25	8.02 ± 1.25	0.18 ± 0.01	83.47 ± 0.31	−0.90 ± 0.01	6.47 ± 1.01
65HH/3MAP	1.47 ± 0.18	29.92 ± 2.14	0.60 ± 0.02	0.03 ± 0.01	81.42 ± 0.31	−0.71 ± 0.01	6.74 ± 0.28
65HH/5MAP	1.50 ± 0.11	29.89 ± 0.15	0.28 ± 0.10	0.00 ± 0.00	82.22 ± 0.65	−0.68 ± 0.02	6.60 ± 0.01
65ATH	1.49 ± 0.15	40.30 ± 0.19	22.00 ± 1.45	2.83 ± 0.19	80.80 ± 0.32	−1.10 ± 0.07	5.15 ± 0.32
65ATH/1MAP	1.44 ± 0.11	41.59 ± 2.21	14.82 ± 1.06	2.28 ± 0.18	80.08 ± 0.09	−1.13 ± 0.05	5.35 ± 0.54
65ATH/3MAP	1.50 ± 0.12	40.79 ± 0.11	10.86 ± 0.19	1.08 ± 0.20	79.57 ± 0.43	−0.79 ± 0.02	4.24 ± 0.87
65ATH/5MAP	1.51 ± 0.12	41.11 ± 1.89	6.34 ± 0.23	1.82 ± 0.21	77.05 ± 0.12	−1.12 ± 0.01	4.34 ± 0.45
65MDH	1.42 ± 0.05	42.20 ± 1.06	29.20 ± 2.41	2.18 ± 0.19	68.78 ± 0.29	−1.00 ± 0.03	2.64 ± 0.06
65MDH/1MAP	1.40 ± 0.10	42.02 ± 1.08	9.26 ± 1.37	1.51 ± 0.17	71.21 ± 0.54	−1.09 ± 0.14	3.05 ± 0.09
65MDH/3MAP	1.38 ± 0.07	42.93 ± 2.04	3.00 ± 0.24	1.05 ± 0.17	69.56 ± 0.07	−1.24 ± 0.12	3.85 ± 0.14
65MDH/5MAP	1.39 ± 0.09	42.32 ± 3.01	2.68 ± 0.26	3.38 ± 0.22	69.42 ± 0.02	−1.13 ± 0.09	4.29 ± 0.20

In terms of color values, it was observed that H/H mineral gave the highest value in terms of “lightness”, i.e., white tone, but remained high in “b” value, i.e., yellowish tone. Naked eye observations show that H/H and ATH have light-colored yellowish profiles. On the other hand, MDH gave the weakest result in the lightness value. It was observed that the composites have a dark color and a tone close to black at this point. (Table 3), (Figure 1).

Figure 1.

Images of the composites in the first stage before (a) and after (b) the burning test.

Combustion and mechanical testing of composites in the first stage

When the combustion test results of the composites are analyzed, all samples with 65% mineral additives achieved the UL94 V0 value. The other specimens were not superior to each other. Only the length of the extinction time of the sample coded 65ATH, which does not contain MAP additive and contains ATH, attracted attention. This sample was also one of the most unsuccessful samples in the filter pressure test.

A careful examination of the results shows that better-dispersed specimens have better combustion tests. (Table 3). Zhou et al., (2018) added epoxy prepolymer to the solution containing melamine polyphosphate to ensure fine and uniform dispersion of melamine polyphosphate particles. In this way, they achieved an excellent dispersion of the flame retardant in both the glue and the cured resin and showed that the improvement in the dispersion of melamine polyphosphate had a great impact on the final flame retardant.²⁷ Xue et al. (2020) found that resorcinol bis (diphenyl phosphate) acts as a compatibilizer to improve the dispersion of ammonium polyphosphate particles within the PLA matrix, resulting in improved flame-retardant efficiency. The better dispersion of ammonium polyphosphate resulted in a much faster self-extinguishing of the respective samples.²⁸ Huang et al. (2019) found that an improved polypropylene (PP)/intumescent flame retardant (IFR) dispersion results in greatly improved flame retardancy and mechanical properties compared to those with an irregular dispersion.²⁹

When the mechanical test results of the samples are examined, a general increasing trend is observed in the elastic modulus values as the MAP ratio increases, while no significant difference is obtained in the elongation at break values. (Table 4).

Table 4.

Combustion and mechanical testing of composites in the first stage.

Sample	1st Flame starting time (s)	2nd Flame growing time (s)	Cotton Indicator Ignited	UL 94 Vertical classification	Tensile strength (MPa)	Elongation at break (%)
65HH	4.4 ± 1.1	1.7 ± 0.5	No	V0	6.86 ± 1.13	2.29 ± 0.82
65HH/1MAP	0.5 ± 0.1	1.1 ± 0.7	No	V0	8.67 ± 0.47	3.09 ± 1.15
65HH/3MAP	0.4 ± 0.1	1.2 ± 0.7	No	V0	9.82 ± 0.91	2.06 ± 0.51
65HH/5MAP	0.8 ± 0.2	3.0 ± 1.1	No	V0	8.33 ± 1.29	1.93 ± 1.08
65ATH	2.3 ± 0.1	7.0 ± 2.4	No	V0	5.58 ± 0.82	3.67 ± 0.69
65ATH/1MAP	1.2 ± 0.2	4.5 ± 1.7	No	V0	4.69 ± 0.72	1.84 ± 0.43
65ATH/3MAP	0.8 ± 0.3	1.6 ± 0.2	No	V0	4.96 ± 0.89	2.66 ± 1.09
65ATH/5MAP	0.6 ± 0.2	1.8 ± 0.1	No	V0	5.89 ± 0.16	2.86 ± 0.61
65MDH	0.7 ± 0.1	1.4 ± 0.1	No	V0	6.39 ± 0.52	3.05 ± 0.56
65MDH/1MAP	0.4 ± 0.1	1.7 ± 0.1	No	V0	7.21 ± 0.55	3.58 ± 0.77
65MDH/3MAP	1.0 ± 0.1	1.0 ± 0.1	No	V0	8.05 ± 0.61	3.37 ± 0.71
65MDH/5MAP	0.6 ± 0.0	0.8 ± 0.1	No	V0	10.99 ± 0.44	4.05 ± 0.76

Characterization tests of composites in the second stage

In the second stage of testing the composites, 3% MAP was used where optimum results were obtained in the first stage. Since all samples showed superior combustion performance at 65% mineral filler, the mineral filler level was gradually reduced to 50%. This was done to determine which mineral type would perform better than the other. As the amount of high-density minerals in the composites decreased, the density values decreased as expected. In the ash tests, as in the previous table (Table 3), H/H lost 66%, ATH and MDH gradually lost about 36-37% mass. Filter pressure values showed worse results as the H/H mineral ratio decreased, while no change was observed for ATH and MDH. However, better results were obtained from these samples compared to the first-stage filter values. It was observed that the dispersion improved as the mineral content decreased. In the color tests, as in the previous tests, H/H and ATH gave a whiter and yellowish tone, while the clarity values were again low in the samples containing MDH, resulting in a dark gray color. In the naked eye tests of all samples, it was observed that the hues became lighter as the mineral content of the samples decreased (Table 5).

Table 5.

Characterization tests of composites in the second stage.

Sample	Density (g/cm³)	Ash (wt%)	MFI (g/10 min)	FPV (bar/g)	L* (Lightness)	a* (Redness)	b* (Yellowness)
60HH/3MAP	1.44 ± 0.01	26.05 ± 1.09	3.86 ± 0.69	0.12 ± 0.01	83.52 ± 0.12	−0.81 ± 0.01	7.50 ± 0.65
55HH/3MAP	1.37 ± 0.02	24.38 ± 2.69	8.98 ± 0.45	2.65 ± 0.12	83.55 ± 0.65	−1.01 ± 0.01	7.16 ± 0.21
50HH/3MAP	1.21 ± 0.01	19.68 ± 1.45	21.08 ± 2.14	2.12 ± 0.14	81.83 ± 0.55	−1.73 ± 0.02	6.09 ± 0.50
60ATH/3MAP	1.34 ± 0.03	38.36 ± 2.13	15.26 ± 1.65	0.27 ± 0.02	82.27 ± 0.98	0.43 ± 0.01	6.08 ± 0.22
55ATH/3MAP	1.28 ± 0.01	34.80 ± 3.13	19.52 ± 1.36	0.25 ± 0.02	83.13 ± 0.06	0.10 ± 0.01	6.78 ± 1.34
50ATH/3MAP	1.23 ± 0.11	31.92 ± 0.17	23.32 ± 3.98	0.32 ± 0.01	81.76 ± 0.54	−0.20 ± 0.01	6.09 ± 1.03
60MDH/3MAP	1.43 ± 0.02	38.61 ± 1.06	5.23 ± 0.08	1.33 ± 0.03	72.59 ± 0.48	−1.52 ± 0.03	4.64 ± 0.10
55MDH/3MAP	1.38 ± 0.09	34.26 ± 1.04	12.54 ± 2.04	1.49 ± 0.65	73.67 ± 2.91	−2.23 ± 0.02	6.42 ± 0.16
50MDH/3MAP	1.27 ± 0.05	32.35 ± 1.32	14.46 ± 2.06	1.52 ± 0.45	71.74 ± 1.04	−2.44 ± 0.08	7.65 ± 0.45

Combustion and mechanical testing of composites in the second stage

As for the combustion and mechanical tests of the composites, the first observation in the second stage was the change in the combustion behavior of the materials as soon as the loading dropped below 60%. No composite could be classified at 60% and below in the UL94 test (Table 6). The samples containing ATH exhibited dripping and burning on cotton. When the first flame was removed, extinction occurred in the 60% loaded samples of all three composites. However, when the second flame was applied, it was observed that ATH did not stop the extinction while MDH exhibited extinction behavior over 10 s. Since extinction was observed less than 30 s after the second flame, the product containing MDH, coded 60MDH/3MAP, was classified as UL94 V1. At this stage, the best-performing sample was the product containing H/H coded 60HH/3MAP. This sample had the fastest extinguishing characteristic after the first flame and an individual extinguishing characteristic of less than 10 s after the second flame. Accordingly, this sample was classified as UL94 V0. The samples with the highest weight loss during combustion are H/H minerals with 66%. The high amount of carbon dioxide and water released due to the decomposition reaction of this mineral caused it to show superior properties compared to other minerals. This effect can also be seen in the deformation of the post-combustion samples of the composites. In Figure 2, it can be seen that the specimens containing H/H lost more mass than the specimens containing other minerals.

Table 6.

Combustion and mechanical testing of composites in the second stage.

Sample	1st Flame starting time (s)	2nd Flame growing time (s)	Cotton indicator ignited	UL 94 vertical classification	Tensile strength (MPa)	Elongation at break (%)
60HH/3MAP	1.0 ± 0.1	2.0 ± 0.4	No	V0	9.43 ± 0.62	6.54 ± 0.37
55HH/3MAP	>30	—	No	NR	8.92 ± 0.44	7.37 ± 1.66
50HH/3MAP	>30	—	No	NR	7.50 ± 0.51	9.71 ± 4.32
60ATH/3MAP	1.4 ± 0.0	>30	Yes	NR	5.52 ± 0.57	3.78 ± 1.74
55ATH/3MAP	>30	>30	Yes	NR	5.49 ± 0.17	6.88 ± 1.26
50ATH/3MAP	>30	—	Yes	NR	5.47 ± 0.15	10.52 ± 3.61
60MDH/3MAP	4.3 ± 1.3	10.1 ± 2.2	No	V1	7.64 ± 0.39	4.07 ± 1.43
55MDH/3MAP	>30	—	No	NR	8.34 ± 0.33	9.18 ± 2.02
50MDH/3MAP	>30	—	No	NR	7.54 ± 0.15	10.64 ± 2.71

*NR: Not rated.

Figure 2.

Images of the composites before (a) and after (b) the second stage burn-in test.

In the mechanical tests, the performance of the material was measured for all three specimens as a function of decreasing mineral content. Similar to the previous stage, the H/H and MDH-containing specimens gave better tensile strength results than the ATH-containing specimens. However, no significant difference was observed between the specimens as the mineral content decreased and the polymer content increased. The best results in tensile tests were observed in the product coded 60HH/3MAP containing 60% H/H, and the best results were obtained in the combustion test. The samples containing ATH gave the worst results.

At this stage, a significant change was observed in the elongation at break values. In all samples, the flexibility of the material gradually improved as the polymer content in the formulation increased. However, at the lowest mineral content, no sample had a significant advantage over the others. The most notable result was again observed in the 60% filled specimens. Once again, the product coded 60HH/3MAP showed superior properties compared to the other 60% filled specimens containing ATH and MDH.

Conclusion

In this study, the flame-retardant performance of 3 different minerals in polyethylene-based composites was investigated. ATH and MDH minerals, which are the most widely used minerals on the market and the subject of numerous studies, were compared with the relatively less used H/H mineral. First, the optimum binder ratio of these minerals was determined and MAP was used as the binder in these studies. Then, the mineral ratios were changed and evaluated with various tests. According to the results, H/H gave the best results in combustion tests due to its high weight loss at the end of the combustion reaction compared to other minerals. At this point, the best flame-retardant performance was obtained with the product coded 60HH/3MAP containing 60% H/H. Mechanical tests also showed better results in tensile strength and elongation at break for similar loads.

Due to the superior performance of H/H in terms of flame retardancy and mechanical properties compared to its counterparts, it is predicted that this type of composite will be used more frequently in industrial and academic studies in the future. In future studies, MFI and FPV results showing the compatibility of binders between H/H and polymer can be verified by SEM analysis, and the research can be extended by performing LOI tests on film samples of composites produced at the micron scale.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Mert Yuceturk

References

Peacock

. Handbook of polyethylene: structures: properties, and applications. CRC Press, DOI: 10.1201/9781482295467

Aggarwal

Sweeting

. Polyethylene: preparation, structure, and properties. Chem Rev 1957; 57: 665–742, DOI: 10.1021/CR50016A004/ASSET/CR50016A004.FP.PNG_V03

Wang

. Synergistic effect of the charring agent on the thermal and flame retardant properties of polyethylene. Macromol Mater Eng 2004; 289: 208–212, DOI: 10.1002/MAME.200300189

Ghomi

Khosravi

Mossayebi

, et al. The flame retardancy of polyethylene composites: from fundamental concepts to nanocomposites. Molecules 2020;25:5157. DOI: 10.3390/MOLECULES25215157

Zhang

Horrocks

. A review of flame retardant polypropylene fibres. Prog Polym Sci 2003; 28: 1517–1538. DOI: 10.1016/J.PROGPOLYMSCI.2003.09.001

Santillo

Johnston

. Playing with fire: the global threat presented by brominated flame retardants justifies urgent substitution. Environ Int 2003; 29: 725–734. DOI: 10.1016/S0160-4120(03)00115-6

Moore

. Polypropylene handbook: polymerization, characterization, properties, processing, applications: Hanser Publishers, 1996, p. 419.

Liu

Wang

, et al. Modified expandable graphite as an effective flame retardant for LLDPE/EVA composites filled with Mg(OH)2/Al(OH)3. J Thermoplast Compos Mater 2020; 33: 938–955, DOI: 10.1177/0892705718815539

Yang

Sun

Xiang

, et al. Pyrolysis and dehalogenation of plastics from waste electrical and electronic equipment (WEEE): A review. Waste Manag 2013; 33: 462–473, DOI: 10.1016/J.WASMAN.2012.07.025

10.

Troitzsch

Antonatus

. Plastics flammability handbook: Carl Hanser Verlag GmbH & Company KG, 2021, DOI: 10.3139/9781569907634

11.

Bee

Hassan

Ratnam

, et al. Investigation of enhancing effect of nano-montmorillonite on fire-retardant added low-density polyethylene-ethylene vinyl acetate hybrid system. J Thermoplast Compos Mater 2014; 27: 1515–1529, DOI: 10.1177/0892705712474978

12.

Sauerwein

. Mineral filler flame retardants. Non‐Halogenated Flame Retardant Handbook 2021; 2: 101–168. Nd Edition DOI: 10.1002/9781119752240.CH3

13.

Hollingbery

Hull

. The thermal decomposition of huntite and hydromagnesite—A review. Thermochim Acta 2010; 509: 1–11, DOI: 10.1016/J.TCA.2010.06.012

14.

Weil

Levchik

. Flame retardants in commercial use or development for polyolefins. J Fire Sci 2008; 26: 5–43, DOI: 10.1177/0734904107083309

15.

Savas

Deniz

Tayfun

, et al. Effect of microcapsulated red phosphorus on flame retardant, thermal and mechanical properties of thermoplastic polyurethane composites filled with huntite&hydromagnesite mineral. Polym Degrad Stabil 2017; 135: 121–129, DOI: 10.1016/J.POLYMDEGRADSTAB.2016.12.001

16.

Dike

Tayfun

Dogan

. Influence of zinc borate on flame retardant and thermal properties of polyurethane elastomer composites containing huntite-hydromagnesite mineral. Fire Mater 2017; 41: 890–897. DOI: 10.1002/FAM.2428

17.

Atay

Çelik

. Use of Turkish huntite/hydromagnesite mineral in plastic materials as a flame retardant. Polym Compos 2010; 31: 1692–1700. DOI: 10.1002/PC.20959

18.

Hollingbery

Hull

. The thermal decomposition of natural mixtures of huntite and hydromagnesite. Thermochim Acta 2012; 528: 45–52. DOI: 10.1016/J.TCA.2011.11.002

19.

Yurddaskal

Celik

. Effect of halogen-free nanoparticles on the mechanical, structural, thermal and flame retardant properties of polymer matrix composite. Compos Struct 2018; 183: 381. DOI: 10.1016/J.COMPSTRUCT.2017.03.093

20.

Painter

Coleman

. Essentials of polymer science and engineering, 2008, p. 525.

21.

Hollingbery

Hull

. The fire retardant behaviour of huntite and hydromagnesite – A review. Polym Degrad Stabil 2010; 95: 2213–2225. DOI: 10.1016/J.POLYMDEGRADSTAB.2010.08.019

22.

Atay

Çelik

. Mechanical properties of flame-retardant huntite and hydromagnesite-reinforced polymer composites. Poly-Plas Tech Engi 2013; 52: 182. DOI: 10.1080/03602559.2012.735310

23.

Bettini

SHP

Agnelli

JAM

. Grafting of maleic anhydride onto polypropylene by reactive extrusion. J Appl Polym Sci 2002; 85: 2706–2717. DOI: 10.1002/app.10705

24.

Kim

. Modification of nano-kenaf surface with maleic anhydride grafted polypropylene upon improved mechanical properties of polypropylene composite. Compos Interfac 2015; 22: 433–445. DOI: 10.1080/09276440.2015.1048099

25.

Wang

Xue

Jia

, et al. Extrusion foaming behavior of wood plastic composites based on PP/POE blends. Mater Res Express 2019; 6: 115345. DOI: 10.1088/2053-1591/ab4f0c

26.

Altayan

Al Darouich

. Study of the properties of polyethylene/thermoplastic starch blends: determination of the critical compatibilizer concentration. Polym Sci B 2021; 63: 152–160. DOI: 10.1134/S1560090421020019

27.

Zhou

Chen

Duan

, et al. In situ synthesized and dispersed melamine polyphosphate flame retardant epoxy resin composites. J Appl Polym Sci 2019; 136. DOI: 10.1002/app.47194

28.

Xue

Zuo

Wang

, et al. Enhanced flame retardancy of poly(lactic acid) with ultra-low loading of ammonium polyphosphate. Compos B Eng 2020; 196: 108124. DOI: 10.1016/j.compositesb.2020.108124

29.

Huang

Pang

, et al. Enhanced dispersion, flame retardancy and mechanical properties of polypropylene/intumescent flame retardant composites via supercritical CO 2 foaming followed by defoaming. Compos Sci Technol 2019; 171: 282–290. DOI: 10.1016/j.compscitech.2018.12.029