Crystallinity of Halogen-Free Flame-Retardant Polyolefin Compounds Loaded with Natural Magnesium Hydroxide

Abstract

A typical halogen-free flame-retardant (HFFR) formulation for electric cables may contain polymers, various additives, and fire-retardant fillers. In this study, composites are prepared by mixing natural magnesium hydroxide (n-MDH) with linear low-density polyethylene (LLDPE) and a few types of ethylene–octene copolymers (C₈-POE). Depending on the content of LLDPE and C₈-POE, we obtained composites with different crystallinities that affected the final mechanical properties. The nucleation effect of the n-MDH and the variations in crystallinity caused by the blending of C₈-POE/LLDPE/n-MDH were investigated. Notably, in the C₈-POE/LLDPE blend, we found a decrease in the crystallization temperature of LLPDE compared to pure LLDPE and an increase in the crystallization temperature of C₈-POE compared to pure C₈-POE. On the contrary, the addition of n-MDH led to an increase in the crystallization temperature of LLDPE. As expected, the increase in the crystallinity of the polyolefin matrix of composites led to higher elastic modulus, higher tensile strength, and lower elongation at break. It has been observed that crystallinity also influences fire performance. Overall, these results show how to obtain the required mechanical features for halogen-free flame-retardant compounds for electric cable applications, depending on the quantities of the two miscible components in the final blend.

1. Introduction

With the development of the polymer industry, polymeric materials have found a wide range of applications, such as medicine, construction, automotive, electronics, etc. Nevertheless, most polymeric materials have the disadvantage of being flammable. Smoke and even toxic gases released during combustion are the main cause of the loss of human lives in the event of a fire. Combustion is a radical exothermic oxidation reaction that leads to the development of heat, flames, smoke, and gas. The combustion of any material requires three components: heat, oxygen, and the combusting material or fuel [1].

Flame retardants are introduced to limit the material’s flammability and the flame spread of polymers [2]. The main applications for flame-retardant polymers are in the construction and electrical cable industries and the transportation sector, where electronics are expanding. Halogen-based flame retardants are the most representative flame retardants in the polymer industry because they are highly cost- and performance-effective, despite the related environmental and toxicity issues [3]. In response to the negative impacts of halogen-based flame retardants, the industry has moved towards halogen-free flame retardants (HFFR), which are considered safer alternatives. Halogen-free flame-retardant (HFFR) polyolefin compounds, also named LZSH or LS0H (Low Smoke Zero Halogen), or even NHFR (Non-halogen flame retardant), have gained great popularity in the last two decades as the preferred materials in safe electrical cable applications, especially in Europe, the Middle East, and Asia. Halogen-free flame retardants (HFFRs) are available in several forms, including metal hydroxides, phosphorus-based compounds, nitrogen-based compounds, and nanocomposites. Among these, phosphorus-based compounds work by creating a protective barrier on the cable surface, insulating the underlying material from heat and oxygen, thus limiting combustion. These include organic and inorganic phosphates, phosphonates, and phosphinates, as well as red phosphorus [4].

Recent advances in phosphorus-based fire retardants, especially phosphates, have seen significant progress in their application within cable insulation to enhance fire safety. Researchers have been focusing on developing more effective and environmentally benign phosphorus-containing compounds [5]. For instance, recent studies have explored the use of nano-sized phosphorus-based materials, which not only improve the dispersion of the fire retardant within the polymer matrix but also enhance the overall flame retardancy and thermal stability of the material. These nano-phosphates create a more stable char layer, which is crucial for preventing the spread of fire.

Additionally, bio-based phosphorus fire retardants are gaining attention due to their reduced environmental impact compared to traditional halogenated fire retardants. For instance, a study demonstrated that the incorporation of bio-based phosphate into polypropylene cables not only improved flame retardancy but also maintained the mechanical properties of the cables, making it a promising approach for future applications in the electrical and electronics industries [6]. Another recent study focused on the development of novel phosphorus–nitrogen synergistic flame retardants, which showed remarkable efficiency in enhancing the fire resistance of polyolefin cables [7].

Metal hydroxides, on the other hand, function through an endothermic reaction that releases nonflammable molecules, such as water (H₂O), and provides charring through the decomposition by-products [8]. The majority of current halogen-free flame retardant compounds for wire and cable are based on metal hydroxides due to their combination of low cost, environmentally friendly nature, low smoke generation, and flame retardancy. However, the main disadvantage of these FRs is the high dosage required to achieve an appreciable improvement in flame retardancy (at least 40–50% by weight, but in some applications up to 65–70% by weight), which often leads to processing issues and deterioration in the mechanical properties of the compound [9,10].

The most commonly used are aluminum trihydroxide (ATH), Al(OH)₃, magnesium hydroxide (MDH), and Mg(OH)₂ [11,12]. ATH and MDH endothermically decompose at T > 200 °C and T > 300 °C, respectively, with the release of water vapors, thus resulting in a fire retardancy effect due to cooling and dilution of the oxygen near the burning areas [13].

In this work, a natural magnesium hydroxide obtained by mining, selecting, and milling natural brucite was used as a flame retardant. During thermal decomposition, MDH releases MgO and water vapors (Equation (1)), with the first forming protective layers on the burning polymer surface, preventing heat and oxygen from reaching the polymer [14,15].

Mg(OH)₂ (s) + Heat → MgO (s) + H₂O (g)

(1)

Polyolefins, with their many desirable properties such as excellent electrical properties, easy processability, and a wide range of mechanical properties, are among the most interesting polymer matrices for highly filled composites for insulation and sheathing of electrical cables. Polyolefin elastomers (POE), such as copolymers of ethylene with 1-octene (C₈), were selected as the preferred polymeric matrix obtained by metallocene catalysis. The physical properties of C₈-POE, such as crystallinity, melting point, and density, depend upon comonomer content [16,17], whose increase leads to a decrease in the degree of crystallinity. This is because the short-chain branches interfere with the ability of the polymer backbone to organize into regular crystals. The advantages of C₈-POEs include good impact resistance, high flexibility, and recyclability [18]. They can also withstand a large percentage of filler (up to over 65% by weight) thanks to their low crystallinity [19]. In addition, to comply with all the performance requirements of a thermoplastic HFFR compound for cable applications (i.e., elongation at break > 150% and tensile strength > 10–12.5 MPa or 12.5 MPa for sheathing and insulating applications, respectively [10]), C₈-POE is generally used in synergistic combination with linear low-density polyethylene (LLDPE). The LLDPE has the aim of increasing the tensile strength and thermal stability up to 90 °C [19] due to its high flexural modulus and melting point > 110 °C.

Moreover, the incorporation of a high amount of inorganic fillers leads to a substantial increase in nucleating sites, which influences the crystallization level and the internal morphology of the composite and mechanical properties [20,21,22]. This work aims to study the influence of the introduction of LLDPE and MDH fillers on the crystallization behavior of C₈-POE. The variation in mechanical properties caused by the different compositions was also investigated.

2. Materials and Methods

Four grades of poly(ethylene-co-octene), supplied by Dow Chemical (Dow Europe GmbH, Horgen, Switzerland) under the trade name ENGAGE™ [23,24] and characterized by different contents of comonomer, were selected. As LLDPE, Dowlex 2045G, was selected, a C₈-LLDPE ethylene–octene copolymer from Dow Chemical was produced with metallocene catalysis in the solution process. The characteristics of the polyolefins are summarized in

Table 1. Characteristics of C₈-POE samples.

	Comonomer Content (wt.%) 1	Density (g/cm³) 2	XC (%)	Tm (°C) 3	MFI (g/10 min) 4
Engage 8180	37.6	0.863	15.1	47.0	0.5
Engage 8100	34.2	0.870	16.9	51.3	1.0
Engage 8003	25.1	0.885	24.5	71.1	1.0
Engage 8480	16.8	0.902	31.3	93.1	1.0
Dowlex 2045G	2.7 [25]	0.920	45.5	114.8	1.0

¹ By ¹³C-NMR/FTIR, ² ASTM D792/ASTM D1505, ³ T_m = melting temperature, ⁴ MFI = Melt Flow Index, measured at 2.16 kg@190 °C as per ASTM D1238/ISO 1133 [26].

, and the second DSC heating scan is shown in Figure 1.

The used MDH was Ecopiren 3.5 (micronized brucite, n-MDH), supplied by Europiren (Rotterdam Science Tower, Rotterdam, The Netherlands) at 92% purity, D₅₀ = 3.5–4 μm, and a surface area of 11 m²/g. Figure 2 compares the needle-like morphological structure of natural magnesium hydroxide in comparison to the hexagonal structure of synthetic magnesium hydroxide (s-MDH).

Fusabond N525™ (Dow Europe GmbH, Horgen, Switzerland) was used as a coupling agent (maleic anhydride modified LLDPE), supplied by Dow Chemical, with a density of 0.880 g/cm³, MFI = 3.7 g/10 min 2.16 kg@190 °C and a grafting level in the range of 0.5–1 wt.%.

All blends and composites were prepared using a twin-roll mixer to obtain a good dispersion of MDH in the polymer at a temperature of 150–160 °C for 10 min.

The crystallinity of blends and composites was measured with a DSC 4000 Perkin Elmer (PerkinElmer, U.S. LLC., Shelton, CT, USA) at a heating rate of 10 °C/min under a dry nitrogen atmosphere. About 5 mg of the sample was sealed in an aluminum pan for each measurement. A heat–cool–heat cycle was used, heating up from −65 to 160 °C, then cooling to −65 °C, followed by a second heating ramp to 160 °C. The heat flow versus temperature was recorded, and the peak melting temperatures of PEO and LLDPE were obtained from the heating scans. The degree of crystallinity (X_C, %) was finally calculated using Equation (2).

$$X_C={\displaystyle \frac{\mathsf{\Delta }{H}_{fus}-x_{graft}\mathsf{\Delta }{H}_{graft}}{\mathsf{\Delta }{H}^0{x}_{mix}}}100$$

(2)

where ΔH_fus is the melting enthalpy of the sample, ΔH⁰ is the melting enthalpy of the 100% crystalline polyethylene (292 J/g) [12], ΔH_graft is the melting enthalpy of the Fusabond N525, x_mix is the mass fraction of polymer in the composites [27], and x_graft is the mass fraction of Fusabond N525 in the composites. The equation’s correction is because adding filler reduces the measured crystallinity.

Mechanical properties were measured with a Lloyd Instruments LS 500 dynamometer using an elongation speed of 250 mm/min at 23 °C. The width and thickness of the specimens were 4.0 mm and 2.0 mm ± 0.2 mm, respectively, and the stretched length was 20 mm ± 0.5 mm, according to the standard ISO 37 [28].

Fire properties were verified with the vertical burning test, an internal laboratory-scale test referring to the modified DIN 4102 [29] B2 test. The measurements were carried out on samples with a dimension of 200 × 140 × 1.4 mm and a graduation line of 150 mm, as shown in Figure 3. The specimen was fixed to a specimen holder, and a Bunsen burner (Fisher Scientific Italia, Segrate, Italy) flame was applied to the bottom of the sample for 2 min.

3. Results

3.1. Crystallization Behavior of C₈-POE/LLDPE Blends

Each grade of Engage™ was blended in a 3:1 (POE:LLDPE) ratio to produce four blends (

Table 2. Blends of C₈-POE and LLDPE in a 3/1 ratio with each Engage™.

	M1	M2	M3	M4
Engage 8180	75
Engage 8100		75
Engage 8003			75
Engage 8480				75
Dowlex 2045G	25	25	25	25

).

This is considered the optimum concentration in agreement with what is commonly used as the polymeric base in flame-retardant compounds for electrical cable applications. Furthermore, it provides an adequately diluted concentration to investigate the impact of a predominantly amorphous matrix (C₈-POE) on LLDPE crystallization while still maintaining a high sensitivity to detect the signal of LLDPE crystallization.

Figure 4 shows the cooling DSC curves of C₈-POE, LLDPE, and their 75:25 blend, whereas

Table 3. Temperature of crystallization peaks of C₈-POE/LLDPE blends.

Base Polymer	Blend	TC C8-POE (°C)	ΔTPOE (°C)	TC LLDPE (°C)	ΔTLLDPE (°C)
Engage 8180	M1	26.7	+0.8	88.0	−7.1
	Calculated M1	25.9		95.1
Engage 8100	M2	33.5	+1.4	87.8	−7.3
	Calculated M1	32.1		95.1
Engage 8003	M3	55.8	+6.6	88.4	−6.7
	Calculated M1	49.2		95.1
Engage 8480	M4	77.6	+7.6	88.0	−7.1
	Calculated M1	70.0		95.1

reports the crystallization temperatures (T_C) of the POE and LLDPE fractions in the blend. These T_C values are compared with those obtained by a linear combination of the pure polymer curves, assuming no interaction.

In all figures, it is evident that the crystallization temperature (T_C) of the LLDPE fraction blended with C₈-POE (peak at the higher temperature of blue line b.) shifts toward lower values compared to the T_C of pure LLDPE (red line c.), as evidenced by the red arrow. The shift at a lower crystallization temperature is explained as a delay in LLDPE crystallization because the concentration of LLDPE in the PEO-rich matrix is too low to form nuclei [30]. No broadening of the LLDPE crystallization peak is observed.

On the contrary, for the C₈-POE fraction within the blend, the crystallization temperature (peak at the lower temperature of line b.) is shifted to higher values than the T_C of pure C₈-POE (green line a.). In this case, the crystallites of the already solidified LLDPE act as nucleating agents for the C₈-POE (less crystalline and with a lower Tc), causing it to crystallize at a higher temperature [31]. Furthermore, the C₈-POE signal in the blend is significantly broader than the calculated signal, indicating a strong influence of the intermixing with the LLDPE and the co-crystallization with the C₈-POE.

Additionally, it can be observed that the advance of the C₈-POE crystallization temperature (ΔT_POE = T_C (M_X) − T_C (M_X calculated)) increases as the comonomer content decreases: for C₈-POE copolymers with a lower comonomer concentration (=higher ethylene content), co-crystallization with LLDPE is the favorite, and the Tc is higher than for more amorphous C₈-POE.

Similarly,

Table 4. Enthalpy of the crystallization peaks of C₈-POE/LLDPE blends.

Base Polymer	Blend	ΔHC POE (J/g)	ΔHM −ΔHCALC POE (J/g)	ΔHC LLDPE (J/g)	ΔHM −ΔHCALC LLDPE (J/g)
Engage 8180	M1	24.1	−4.8	24.1	−4.7
	calculated M1	28.9		28.8
Engage 8100	M2	30.4	−2.2	25.9	−2.9
	calculated M2	32.6		28.8
Engage 8003	M3	48.9	−3.6	22.5	−6.3
	calculated M3	52.5		28.8
Engage 8480	M4	68.3	−3.8	22.6	−6.2
	calculated M4	72.0		28.8

shows the crystallization enthalpies for all C₈-POE/LLDPE blends.

In this case, it can be observed that the crystallization enthalpies of POE and LLPDE are slightly lower than those calculated as a linear combination of the two pure polymers, and there is no trend as the comonomer content changes.

3.2. Crystallization Behavior of C₈-POE/LLDPE/n-MDH Composites

Mineral particles may trigger the nucleation and growth of C₈-POE crystals. This could also be the case of n-MDH, which is obtained by micronizing brucite and used as a flame retardant in HFFR polyolefin compounds for electric cables.

Table 5. Compounds based on neat C₈-POE (series A), C₈-POE/LLDPE mix 3/1 ratio (series B), and C₈-POE/LLDPE mix 1/1 ratio (series C), with the relative characteristics (density and MFI).

	A1 (%)	A2 (%)	A3 (%)	A4 (%)	B1 (%)	B2 (%)	B3 (%)	B4 (%)	C1 (%)	C2 (%)	C3 (%)	C4 (%)
Engage 8180	36.8				27.6				18.4
Engage 8100		36.8				27.6				18.4
Engage 8003			36.8				27.6				18.4
Engage 8480				36.8				27.6				18.4
Dowlex 2045G	-	-	-	-	9.2	9.2	9.2	9.2	18.4	18.4	18.4	18.4
Fusabond N525	3	3	3	3	3	3	3	3	3	3	3	3
Ecopiren 3.5	60	60	60	60	60	60	60	60	60	60	60	60
Irganox 1010	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2
Properties
Density (g/cm3)	1.402	1.404	1.414	1.435	1.413	1.414	1.424	1.436	1.419	1.426	1.433	1.443
MFI (g/10 min) 1	2.7	4.1	3.7	4.3	2.8	4.1	3.4	4.8	2.7	3.7	3.5	4.1

¹ measured at 21.6 Kg@ 190 °C.

shows the twelve formulations prepared, containing 36.8 wt.% C₈-POE+LLDPE, 60 wt.% MDH, 3 wt.% coupling agent (Fusabond N525), and 0.2 wt.% phenolic antioxidant stabilizer (Irganox 1010 (BASF Corporation, Dispersions and Pigments, Charlotte, NC, USA)). Among these, the A series is based on neat C₈-POE, and the B and C series have a POE/LLDPE ratio of 3:1 and 1:1, respectively. The latter ratio was investigated because a higher concentration of LLDPE gives both economic and performance benefits. The economic advantages are due to the lower cost of the polymer, while the performance benefits are associated with a higher tensile strength. These formulations were studied in previous work [32], but no studies about the contribution of the C₈-POE and LLDPE contents to the ultimate properties of the composites were investigated.

DSC thermograms were recorded for each C₈-POE/LLDPE/n-MDH composite and compared with related C₈-POE/LLDPE blends with the same C₈-POE/LLDPE ratio and pure polymers.

Figure 5 shows cooling DSC curves of the type “A” formulations based on C₈-POE versus the respective pure polymers.

As expected, an increase in crystallization temperature can be observed in the presence of MDH particles, which may be attributed to the heterogeneous nucleation caused by the fillers [33].

Table 6. T_C and ΔH_C of “A” formulations and pure polymers and crystallinity (X_C) of “A” formulations.

Properties	Engage 8180	A1	Engage 8100	A2	Engage 8003	A3	Engage 8480	A4
TC (°C)	26.0	28.1	32.1	34.7	50.0	58.3	69.9	78.7
ΔT (°C)	+2.1		+2.6		+8.3		+8.8
ΔH (J/g) 1	38.6	32.3	43.4	38.3	70.0	59.7	96.1	81.6
ΔHA-ΔHPOE	−6.3		−5.1		−10.3		−14.5
XC (%)		10.6		12.7		20.0		27.5

¹ ΔH of compounds calculated considering only the mass relative to the polymer, without the weight of Mg(OH)₂.

shows the T_C and ΔH_C values recorded for all C₈-POE+n-MDH composites compared to pure C₈-POE. It can be observed that the addition of MDH increases the peak crystallization temperatures by 2.1 °C for the low crystallinity formulation (A1) and by 8.8 °C for the high crystallinity formulation (A4), following a regular trend by moving from amorphous to more crystalline C₈-POE grades. Similarly, the ΔH_C of formulation “A” is influenced by the comonomer content. In particular, there is a greater deviation from the enthalpy of the pure polymer as the comonomer content decreases. This greater sensitivity of highly crystalline polymers to filler could be due to the fact that filler disturbs the alignment of the molecular chains more significantly. However, in all cases, lower enthalpy values are obtained compared to pure polymers due to the high concentration of filler that disturbs crystallization; this is in line with the study of Chen et al. [34].

Figure 6 shows cooling DSC curves of the type “B” formulations (filled C₈-POE/LLDPE blends) and the respective pure polymers and polymer blends.

In the presence of n-MDH particles, only the LLDPE fraction of the C₈-POE/LLDPE/n-MDH composite shows an increase in crystallization temperature. There is no significant effect on the T_C of the C₈-POE. The nucleating role of the additives is, therefore, evident only for the LLDPE, that is, the polyolefin with the highest crystallinity [35]. There is also a broadening of the crystallization peaks of the composites compared to pure polymers: the presence of the mineral particles leads to the formation of crystals of several different sizes, which is reflected in their different thermal stability. These results possibly indicate a certain interaction between the polymeric matrix and the mineral triggered by the presence of the LLDPE-g-MAH coupling agent.

Table 7. Crystallization temperature of C₈-POEs, LLDPE, C₈-POE+LLDPE blends, and C₈-POE+LLDPE+n-MDH composites.

Properties	Engage 8180	M1	B1	Engage 8100	M2	B2	Engage 8003	M3	B3	Engage 8480	M4	B4	LLDPE
TC (POE) (°C)	25.9	26.7	27.0	32.1	33.5	34.0	49.2	55.8	55.9	70.0	77.6	79.8	-
TM/B-TPOE	-	+0.8	+1.1	-	+1.4	+1.9	-	+6.6	+6.7	-	+7.6	+9.8	-
TC (LLDPE) (°C)	-	88.0	97.0	-	87.8	96.9	-	88.4	97.1	-	88.0	98.7	95.1
TM/B-TLLDPE	-	−7.1	+1.9	-	−7.3	+1.8	-	−6.7	+2.0	-	−7.1	+3.6	-

and

Table 8. Crystallization enthalpy of C₈-POEs, LLDPE, C₈-POE+LLDPE blends, and C₈-POE+LLDPE+n-MDH composites and crystallinity (X_C) of “B” formulations.

Properties	Engage 8180	M1	B1	Engage 8100	M2	B2	Engage 8003	M3	B3	Engage 8480	M4	B4	LLDPE
ΔHC (POE) (J/g)	28.9	24.1	23.4	32.6	30.4	29.7	52.5	48.9	48.6	72.0	68.3	70.5	-
ΔHM/B-ΔHPOE	-	−4.8	−5.5	-	−2.2	−2.9	-	−3.6	−3.9	-	−3.8	−1.5	-
ΔHC (LLDPE) (J/g)	-	24.1	23.7	-	25.9	24.8	-	22.5	22.0	-	22.6	21.0	28.8
ΔHM/B-ΔHLLDPE	-	−4.7	−5.2	-	−2.9	−4.0	-	−6.3	−6.8	-	−6.2	−7.8	-
XC (%)			15.8			18.2			23.7			30.9

show the T_C and ΔH_C values of C₈-POE/LLDPE/n-MDH composites in comparison with pure C₈-POE and C₈-POE/LLDPE blends (M). The overlapping DSC peaks were deconvoluted using peakfit software v4.12 (systat) in order to calculate the enthalpy of crystallization provided by each of the two polymers present. The integration ranges for formulations B1/M1 and B2/M2 were between 115 °C and −15 °C, those for formulations B3/M3 were between 115 °C and −5 °C, and those for formulations B4/M4 were between 115 °C and 10 °C.

The T_C of the C₈-POE fraction in C₈-POE+LLDPE+n-MDH composites and C₈-POE+LLDPE blends (peaks at lower temperatures of the red and blue lines, respectively) are similar and higher than that of pure C₈-POE (upper black line). Therefore, the main evident nucleating effect is addressed to the already crystallized LLDPE rather than to mineral particles. Regarding the LLDPE fraction, it can be observed that the T_C of C₈-POE+LLDPE polymer blends (peak at a higher temperature of the blue line) is significantly lower than that of pure LLDPE (lower black line). However, the opposite behavior occurs for C₈-POE+LLDPE+n-MDH composites (peak at a higher temperature in the red line), with a T_C higher than that of pure LLDPE (lower black line). This means that the nucleating effect of mineral particles of n-MDH (that is, T_C increasing) is more than enough to compensate for the “dilution” effect of amorphous C₈-POE (that is, T_C reducing), as demonstrated by that measured for C₈-POE+LLDPE polymer blends. Furthermore, in all formulations (C₈-POE+LLDPE+n-MDH composites and C₈-POE+LLDPE blends), the crystallinity enthalpies of both C₈-POE and LLDPE fractions are lower than those of the pure polymer.

3.3. Influence of n-MDH Content on Composites T_C

Two new formulations, reported in

Table 9. Formulations of C₈-POE+LLDPE+n-MDH composites with C₈-POE:LLDPE = 3:1 and different n-MDH contents.

	B3-20 (%)	B3-40 (%)	B3-60 (%)	M3 (%)
Engage 8003	57.6	42.6	36.8	75
Dowlex 2045G	19.2	14.2	9.2	25
Fusabond N525	3	3	3	-
Ecopiren 3.5	20	40	60	-
Irganox 1010	0.2	0.2	0.2	-

, were produced and analyzed by DSC. The values of T_C and crystallization enthalpy are shown in

Table 10. Crystallization temperature and enthalpy of Engage 8003+LLDPE blends and Engage 8003+LLDPE+n-MDH composites with different MDH content (20%, 40%, and 60%).

Properties		B3-20 (%)	B3-40 (%)	B3-60 (%)	M3 (%)
TC (°C)	POE fraction	54.2	53.9	55.9	55.8
TB-TM		−1.6	−1.9	+0.1	-
ΔHC (J/g)		46.2	47.1	48.6	48.9
ΔHB-ΔHM		−2.7	−1.8	−0.3	-
TC (°C)	LLDPE fraction	89.8	94.3	97.1	88.4
TB-TM		+1.4	+5.9	+8.7	-
ΔHC (J/g)		20.2	19.8	22.0	22.5
ΔHB-ΔHM		−2.3	−2.7	−0.5	-

.

The cooling thermograms of the B3-60, B3-20, and B3-40 formulations were reported in Figure 7.

It can be observed that the T_C of the LLDPE fraction increases in agreement with the quantity of filler (T_C (B3-60) > T_C (B3-40) > T_C (B3-20)). Also, the broadening of the crystallization peak of the LLDPE fraction is apparent as the filler fraction increases. This phenomenon may be ascribed to different heterogeneous nucleation mechanisms [33] leading to the formation of LLDPE crystals of different sizes and, as already noted, possibly addressed to the effective interaction between the polymeric matrix and the mineral provided by the LLDPE-g-MAH coupling agent. On the other hand, with increasing mineral content, there is no significant effect on the T_C of the C₈-POE fraction or on the crystallization enthalpy of the C₈-POE and LLDPE fractions.

3.4. Tensile Properties of C₈-POE+LLDPE+n-MDH Composites

Depending on the degree of crystallinity, polymers have different mechanical properties. In the C₈-POE+LLDPE+n-MDH composites, the presence of the maleated LLDPE-g-MAH coupling agent influences the final mechanical behavior.

The C₈-POE/LLDPE/n-MDH composites were tested via stress–strain experiments, and the curves are shown in Figure 8.

Table 11. Physical properties (DSC crystallinity, tensile strength (TS), elongation at break (E@B), Young’s modulus) of the C₈-POE+LLDPE+n-MDH composites.

Base Polymers	Composite	XC (%)	Tensile Strength (MPa)	Elong. @break (%)	Young’s mod. (MPa)
Engage 8180	A1 (only C8-POE)	10.6	9.2 ± 0.3	447 ± 36	30
	B1 (C8-POE:LLDPE 3:1)	15.8	10.7 ± 0.5	319 ± 15	90
	C1 (C8-POE:LLDPE 1:1)	24.5	12.6 ± 0.3	214 ± 29	300
Engage 8100	A2 (only C8-POE)	12.7	9.9 ± 0.1	408 ± 16	50
	B2 (C8-POE:LLDPE 3:1)	18.2	11.2 ± 0.5	308 ± 26	130
	C2 (C8-POE:LLDPE 1:1)	27.3	12.8 ± 0.2	175 ± 7	450
Engage 8003	A3 (only C8-POE)	20.2	12.4 ± 0.6	380 ± 43	100
	B3 (C8-POE:LLDPE 3:1)	23.7	13.2 ± 0.3	258 ± 26	390
	C3 (C8-POE:LLDPE 1:1)	32.1	14.1 ± 0.4	150 ± 35	650
Engage 8480	A4 (only C8-POE)	26.3	13.5 ± 0.3	201 ± 31	370
	B4 (C8-POE:LLDPE 3:1)	30.8	14.7 ± 0.3	150 ± 28	510
	C4 (C8-POE:LLDPE 1:1)	37.8	15.3 ± 0.4	101 ± 22	740

reports the values of elongation at break, Young’s modulus, and tensile strength in relation to crystallinity values.

As expected, the composites without LLDPE and based only on C₈-POE (A1, A2, A3, and A4) show higher elongation at break in comparison to composites containing LLDPE, thanks to the more amorphous polymeric matrix. For the same reason, by comparing the A1 vs. A2 vs. A3 vs. A4 composites, we observe a clear increase in tensile properties and a decrease in elongation [36]. As expected, the introduction of LLDPE leads to a decrease in E@B and an increase in tensile properties due to the higher crystallinity of the composite.

Figure 9 shows the mechanical properties of all the formulations as a function of the crystallinity.

An almost linear trend with increasing crystallinity can be observed for all the investigated mechanical properties. Notably, there is an increase in tensile strength and modulus and a decrease in elongation at break. The desired mechanical properties can be achieved by controlling the total crystallinity of the polymer matrix. This is independent of the method employed to achieve crystallinity: it is possible to combine the more crystalline POE with a smaller amount of LLDPE or the more amorphous POE with a larger amount of LLDPE.

3.5. Fire Performance of the Highly Filled POE Compounds

In the field of electrical cables, vertical burning tests are widely used as a screening method to assess their relative flammability and performance relatively quickly.

The results of the vertical burning test are shown in

Table 12. Experimental results obtained from the vertical burning test of all formulations.

Sample	t1 1 (s)	t2 2 (s)	Self Extinguish	tburning drops 3 (s)	Flame Height (cm)	Flame Width (cm)
A1	80 ± 4	128.5 ± 0.7	NO	54 ± 1	28.5	8.0
A2	80 ± 9	131 ± 5	NO	58 ± 6	28.0	8.0
A3	83 ± 3	164 ± 3	NO	79 ± 5	28.0	7.5
A4	98 ± 7	175 ± 9	NO	99 ± 8	27.0	7.0
B1	86 ± 3	144.5 ± 0.7	NO	79 ± 4	27.7	7.5
B2	94.5 ± 5	148 ± 1	NO	86 ± 8	28.2	7.0
B3	86 ± 3	150 ± 7	NO	87 ± 2	27.5	7.0
B4	101 ± 11	169 ± 1	NO	103 ± 9	26.7	7.5
C1	101.5 ± 2	164 ± 8	NO	84 ± 7	26.5	7.5
C2	108 ± 6	166 ± 7	NO	101 ± 4	26.5	7.0
C3	99.5 ± 8	169 ± 2	NO	54 ± 1	28.7	8.0
C4	105 ± 7	200 ± 20	NO	58 ± 6	27.5	7.5

¹ t₁ (time 1): top of the flame reaches the graduation line, ² t₂ (time 2): flame base reaches the graduation line, ³ t_{burning drops}: time to dripping start.

.

A delay in flame propagation (higher t₂) and dripping can be observed as crystallinity increases. This may be due to a combination of higher thermal resistance and a more compact structure than more amorphous polymers, which limits dripping. A decrease in t₁ is also observed, but this value is influenced by various factors, such as the height of the flame. The same value can be obtained for a composite that burns slowly but with a long flame or for a composite that burns quickly but with a short flame. On the other hand, there are no significant differences in the height and width of the flame.

Figure 10 shows the photos of the fire test to better appreciate the marked difference between the lower crystallinity formulation (A1) and the higher crystallinity formulation (C4).

It can be observed that after 150 s, the flame base of formulation A1 has already reached the graduation line, while the flame base of formulation C4 is still at the beginning of the specimen.

4. Conclusions

Electrical cable compounds require strong mechanical properties and flame resistance. This performance is achieved using POE with different comonomer (1-octene) content combined with LLDPE and through the incorporation of flame-resistant metal hydroxides into the polymer matrix.

Firstly, the effect of blending LLDPE and various POEs with different 1-octene contents was investigated. Our studies suggested that there is a delay in the crystallization of the LLDPE fraction. This delay is independent of the type of POE used and is merely due to the dilution effect of the POE fraction on the LLDPE fraction. An advance in the crystallization of the POE fraction was also observed. This advance correlated with comonomer content: for POE with a lower 1-octene content, the advance was more significant than for POE with a higher 1-octene content. This phenomenon is due to the already-formed LLDPE crystals acting as nucleating agents for the POE fraction.

For the C₈-POE/n-MDH composites, the influence of filler addition on the crystallization of each POE with different 1-octene contents was investigated. Our studies demonstrated that, in general, the introduction of n-MDH provided a strong nucleating effect, significantly accelerating the crystallization process of the polyolefin matrices examined in this work. Specifically, it was observed that the lower the comonomer content in the POE, the higher the increase in the crystallization temperature of C₈-POE.

For C₈-POE+LLDPE+n-MDH composites, the combined effect of filler addition and blending LLDPE with each POE at different 1-octene contents was studied. Among the two opposing effects of C₈-POE (reducing T_C) and n-MDH (increasing T_C) on the T_C of LLDPE, the nucleating effect provided by the inorganic particles was predominant. On the other hand, there is no effect of the filler on the crystallization of the POE fraction. The T_C of LLDPE was also influenced by the amount of n-MDH: the lower the amount of n-MDH, the lower the measured T_C.

In terms of crystallization enthalpy, both polymer blending and the introduction of n-MDH reduced the crystallinities of the pure polymers.

As expected, the higher the crystallinity of the polymer matrix, the higher the tensile strength and Young’s modulus, and the lower the elongation at break. The increase in crystallinity also results in a delay in the propagation of the flame and in the drip time.

These compounds are suitable for designing cables complying with the EN 50399 [37] fire test as per CPR (Construction Production Regulation) in Europe for building power cables. The tests for electrical cables in Europe are conducted under highly demanding conditions, typically lasting around 40 mi, with powerful burners. This extended application of fire allows the magnesium hydroxide to reach the necessary temperature to decompose, releasing water and acting as an effective flame retardant. In a future study, we will use cone calorimeters for the evaluation of the flame retardancy of this kind of compound, as we did in the past [38].