Enhancing the Performance of Polypropylene/High-Density Polyethylene Blends by the Use of a Compatibilizer and Montmorillonite Nanoparticles

Abstract

Nanocomposites composed of compatibilized polyolefin blends and organically modified montmorillonite (OMMT) nanoparticles were produced through melt mixing using a twin-screw extruder. High-density polyethylene (HDPE) and polypropylene (PP) blends were compatibilized with maleic anhydride-grafted PE compatibilizer (COMP). Blends with a 10/25 (w/w) HDPE/PP content were prepared and were reinforced with 1, 2, and 3 phr OMMT. Characterization of nanocomposites was performed using X-ray diffraction (XRD), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Tensile Testing, and Melt Flow Index (MFI) measurements. Preparation of polyolefin blend/OMMT nanocomposites with a twin-screw extruder was successful at low clay levels (1 phr). These nanocomposites presented increased onset temperature of thermal degradation, crystallinity, and stiffness, whereas their MFI values were lower than those of the pure matrix.

1. Introduction

Polypropylene (PP) and high-density polyethylene (HDPE) find many applications because of their unique properties, such as low density and good mechanical properties, accompanied by chemical and heat resistance. Mixtures of PP and HDPE display characteristics between those of the original polymers only when efficient compatibilization takes place during blending. Also, the addition of micro- and nano-fillers with the appropriate compatibilizers results in a significant improvement in the properties of these blends. Polymers reinforced with nanoclays have attracted significant attention because of their enhanced mechanical, thermal, and barrier properties in comparison with the neat polymer. To achieve optimal performance, however, effective dispersion of the organoclay within the polymer matrix and strong interfacial adhesion between the two phases are essential. Given that clay is highly organophilic, it is often difficult to achieve synergy, especially for hydrophobic polymers like polyolefins (POs). To address these difficulties, several strategies have been developed, including surface modification of the clay by organo-intercalants to prevent interaction between the clay platelets and improve compatibility with the polymer matrix, as well as polymer modification through the addition of compatibilizers. In some cases, both clay and polymer phases have been modified simultaneously to further enhance dispersion and interfacial bonding [1].

Gopakumar et al. [2] investigated the effect of clay exfoliation on the physical properties of montmorillonite/polyethylene (PE) composites, containing maleated polyethylene (PE-g-MA) as a compatibilizer. An increase in T_c was observed with both increasing and decreasing cooling rates, a behavior typical of crystallization processes controlled by nucleation. They also observed that while the degree of crystallinity of PE was unaffected by the addition of reinforcement, in the case of PE-g-MA, it decreased significantly upon the clay platelet exfoliation. This phenomenon was attributed to the larger interfacial area and stronger adhesive interactions between the PE-g-MA matrix and the exfoliated clay, which restrict the mobility of crystallizable polymer segments. The presence of exfoliated clay also influences the type of nucleation and the morphology of crystal growth of PE. Nanoparticles act as nucleating agents, facilitating a heterogeneous crystallization process. Effective dispersion of the particles increases the number of nucleation sites, thereby accelerating the crystallization rate and modifying both the kinetics and the geometry of crystal growth. However, the constrained mobility of polymer chains due to interactions with exfoliated platelets can significantly reduce the overall degree of crystallinity, potentially diminishing the reinforcing capability of the nanoclays.

A two-degree increase in T_c for 1 wt.% addition of organically modified montmorillonite (OMMT) in PE was reported by Zhai et al. [3], but additional amounts of clay did not show any significant effect. Avella et al. [4] studied the crystallization behavior of i-PP/organoclay nanocomposites and observed that by increasing the clay content, the nucleation density of PP increased. They concluded that the addition of clay always causes a decrease in the spherulite radial growth rate. This was ascribed to an increase in the energy of transportation of the polymeric chains in the melt when clay particles are present.

The effect of crystallization of clay/polyolefin nanocomposites was investigated by Ton-That et al. [5], who used low- (LCPP) and high-crystallinity (HCPP) polypropylene, compatibilized with PP-g-MA as matrices. They found that the addition of clay influenced not only the crystallization temperature and degree of crystallinity but also the crystalline structure. Even when the clays were effectively exfoliated during mixing, upon cooling and crystallization, the immiscibility between the PP chains and the intercalant on the clay surface caused the matrix to crystallize rapidly away from the organoclay platelets, leaving the clays embedded within the amorphous regions of PP and the compatibilizer. The above study confirmed that reducing the crystallinity of the PP matrix is crucial to achieving improved clay exfoliation.

PE and PP nanocomposites were prepared by Zhang et al. [6], with the addition of a novel organically modified clay, containing styrene, lauryl acrylate, and vinylbenzyl chloride. In both PE and PP composites, the mechanical properties were slightly lower than those of the pure polymers, indicating that the oligomerically modified clay had a plasticizing effect on the matrix. Additionally, notable reductions in the peak heat release rate were observed, reaching approximately 60% and 70% for organic clay loadings, of 5% and 8% respectively.

Gai and Li [7] studied the properties of ultrahigh-molecular-weight polyethylene/polypropylene nanocomposites reinforced with montmorillonite modified with a complex intercalator [2-methacryloyloxyethyldodecyldimethylammonium bromide/poly (ethylene glycol)]. The synergistic effect of the complex intercalator gave exfoliated and intercalated structures for montmorillonite. The presence of PMM resulted in a significant reduction in the melt viscosity with an improvement in the tensile strength and elongation at break of UHMWPE, except that the Izod-notched impact strength was without significant change.

Krump et al. [8] observed an increase in the melting temperature of PP nanocomposites as the clay content increased, which was attributed to the increase in the PP’s lamellar thickness. The slight decrease in the corresponding melting enthalpy was explained by a reduction in the overall amount of thicker lamellae within the polymer. Meanwhile, Modesti et al. [9] observed a significant rise in the crystallinity of PP nanocomposites, which was ascribed to the nanoclay platelets promoting heterogeneous nucleation. This effect enhanced the crystallization rate, resulting in a higher degree of crystalline structure.

Santos et al. [10] investigated how the type and amount of quaternary ammonium salts used as an organophilic modifier for montmorillonite (MMT), along with the choice of processing aid, influenced the intercalation and exfoliation processes of PP nanocomposites. They found that the addition of clay enhanced both the heat deflection temperature and thermal stability, with the most pronounced effect observed for clay modified with the smallest ammonium salt. Clays with larger d₀₀₁ values, when combined with poly (propylene glycol), led to a greater improvement in impact strength. Conversely, nanocomposites containing clays with smaller d₀₀₁ values exhibited a higher modulus compared with neat PP, attributed to the low amount of ammonium salt content, allowing the clay surface to interact more freely with the PP chains.

The effects of an adhesion promoter on the structure and properties of nano-filled polyolefins were extensively investigated by Eteläaho et al. [11], who used two different maleic anhydride-modified PP having different maleic anhydride contents and molecular weights. The low-molecular-weight adhesion promoter enhanced mechanical properties in both matrices, whereas the high-molecular-weight promoter was effective only in the PE matrix. More uniform intercalation and filler dispersion were observed with the low-molecular-weight, maleic anhydride-rich adhesion promoter, while the high-molecular-weight agent resulted in less uniform dispersion but a higher promotion of exfoliated clay particles. No substantial changes in melting behavior or crystallinity were noted for PP-based mixtures containing both clay and the adhesion promoter, although a slight increase in crystallinity occurred in samples with higher clay contents, likely due to nucleation by clay agglomerates. In HDPE compounds, the crystallinity decreases in the presence of compatibilizers because both adhesives are PP-based. The melting temperatures were similar for all compounds.

Nanocomposites of PP/HDPE blends with OMMT (Cloisite 15A) as a nano-filler and two maleated polyolefins (PE-MA and PP-MA) as compatibilizers were studied by Chiu et al. [12]. The degree of crystallinity of PP increased with OMMT, while that of the HDPE was only slightly affected. The thermal stability of the PP/HDPE mixture improved with nanoclays, and the improvement was more evident in air than in a N₂ environment. The stiffness of PP/HDPE blends increased slightly by adding Cloisite 15A and marginal changes upon further inclusion of PP-MA.

Mixing of PP/LDPE (80/20), maleated compatibilizers, and organoclays resulted in a homogeneous distribution of intercalated silicate layers in all the phases of the blend, which affected its thermal stability as well as its tensile and rheological properties. The elongation at break for PP increased from 28.1 to 155.6% when organoclay and maleated compatibilizers were present, while the thermal stability of PP increased from 269.8 to 303.38 °C in the same composite. However, the impact strength of the PP/LDPE blend decreased with the incorporation of organoclay [13].

Rigail-Cedeño et al. [14] studied the rheological and thermomechanical properties of recycled HDPE (rHDPE) and PP (rPP) modified by the incorporation of commercial organoclays (Cloisite 20A and Cloisite 92A), together with an olefin block copolymer (OBC) compatibilizer. The above authors reported that the organoclays and the OBC reduced the crystallinity of both rHDPE and rPP. Plate–plate rheometry measurements showed that the addition of organoclays increased the viscosity of blends at low shear rates, whereas the addition of OBC had the opposite effect at higher shear rates. The stiffness of the blends increased with the incorporation of organoclays, while the addition of OBC resulted in lower stiffness.

The effect of nanoclay (NC) and alumina on the thermo-mechanical characteristics of HDPE/PP blends, compatibilized with polyethylene-grafted maleic anhydride, was investigated by Jeevan and Krishna Prasad, employing various micromechanical models [15]. Their findings showed that the modulus ratio of the composite to the matrix rose proportionally with increasing the NC content.

Boufassa et al. [16] examined the influence of various compatibilizers (both apolar and polar) as well as surface-modified MMT particles on the structure and the properties of 80/20 iPP/HDPE blends. They found that incorporating 15% of any of the compatibilizers tested led to a reduction in the mechanical properties of the original blend. The impact of clay addition on the compatibilized blends was found to vary depending on the type of compatibilizer used.

A filler masterbatch containing nano-silicon dioxide, a compatibilizer, a lubricant, and an antioxidant, when incorporated in PP/LDPE blends, resulted in significant improvement in mechanical properties, related to the morphological structure [17].

Yang et al. [18] investigated the use of olefin-based compatibilizers in order to compatibilize immiscible HDPE/iPP blends and reported that these materials can effectively modify the interfacial morphology, leading to a reduction in the size of dispersed droplets. Specifically, the incorporation of a comb-like poly (propylene-co-high α-olefin) (PPO) compatibilizer in HDPE/iPP 70/30 blends induced a phase transition from droplet-in-matrix to a continuous morphology.

The compatibilization effect of an ethylene/octene elastomer copolymer on post-consumer recycled binary blends of PP, with either HDPE or LLDPE, was studied by Tal and Naveh [19]. They observed that the inclusion of compatibilizers contributed to a less apparent two-phased morphology, enabled by the introduction of largely amorphous ethylene/propylene copolymers, suggesting the improvement in compatibility and interfacial adhesion of the blend and more efficient stress transfer between phases.

Comingled plastic waste consisting of LDPE, HDPE, and PP was compatibilized by melt mixing using maleic anhydride (MAH) and bisphenol A (BPA) as coupling agents in the presence of benzoyl peroxide (BPO) as the initiator. The in situ reactive compatibilization enhanced the mechanical and thermal properties by increasing the coupling agent content, and beyond the optimum values, the properties decreased [20]. The mechanical performance of PP/PE blends can be improved using non-reactive, reactive, and bio-derived filler compatibilizers [21,22]. The influence of the blending sequence and screw speed on the properties of HDPE/PP blends filled with 1.25 wt.% graphene nanoplatelets (GNPs) was studied by Salehiyan et al. [23]. They observed that the degree of crystallinity and the crystalline structure were the most sensitive parameters to processing variations, whereas the tensile modulus remained unchanged. The elongation at break and tensile toughness were more responsive to these changes.

In a similar study, it was observed that the combined use of a compatibilizer (ABS-g-MAH) and OMMT enhanced the thermal stability of styrene-containing copolymer blends prepared in a twin-screw extruder [24].

In this work, the effect of different types of organically modified MMT on the thermal transitions, thermal stability, and rheological and mechanical properties of PP/HDPE blends compatibilized with polyethylene grafted with maleic anhydride (PE-g-MAH) and prepared by twin-screw extrusion was studied, in an effort to elucidate the mechanism of property enhancement in relation to the resulting nanocomposite structure. In addition, the effect of the twin-screw rotation speed and the content of the compatibilizer on the structure of the nanocomposites was evaluated.

2. Materials and Methods

2.1. Materials

The polymeric blends studied in this work were based on PP homopolymer (Midilena II-F401, Rompetrol Petrochemicals S.R.L., Navodari, Romania) with an MFI of 2.00–3.5 g/10 min at 230 °C/2.16 kg, and HDPE (Borealis GmbH, Vienna, Austria), with a density of 0.94 g/cm³, while maleic anhydride-modified PE, from PolyOne, with a grafting rate of 1 wt.%, was used as a compatibilizer. The polymers under study are used in extrusion-molding applications. The fillers for incorporation into the above blends were organically modified clays (OMMT (Cloisite 25A and Cloisite 30B)) (

Table 1. Main characteristics of nanoclays used in this work.

	Cloisite 30B	Cloisite 25A
Organic modifier	Methyl, tallow, bis-2-hydroxylethyl, and quaternary ammonium	Dimethyl, hydrogenated tallow, and 2-ethylhexyl quaternary ammonium
Modifier concentration	90 meq/100 g clay	95 meq/100 g clay
Weight loss in ignition	30%	34%

HT is hydrogenated tallow (~65% C₁₈, ~30% C₁₆, and ~5% C₁₄). T is tallow (~65% C₁₈, ~30% C₁₆, and ~5% C₁₄).

), supplied by BYK Additives, ALTANA Group, Wesel, Germany.

2.2. Methods

2.2.1. Preparation of Clay/Polyolefin Composites

Compatibilized polyolefin composites containing OMMT were prepared in a twin-screw extruder, with L/D-25 and a 16 mm diameter (Haake PTW 16, Thermo Fisher Scientific, Waltham, MA, USA), according to the conditions presented in

Table 2. Extrusion conditions for the production of compatibilized HDPE/PP blend nanocomposites reinforced with organoclay.

Temperature (°C)						Screw Rotation Speed (rpm)	Feeder Screw Rotation Speed (rpm)
In Extruder Barrel Zones
1st	2nd	3rd	4th	5th	Die
185	190	190	190	190	190	30–200	20–70

. The screw speed varied from 30 to 100 and 200 rpm, in an attempt to optimize the dispersion of the clay. This search revealed the speed of 200 rpm as optimal for all the experiments. The material was extruded through a circular die with an opening of 2 mm in diameter, thus producing a cylindrical strand, which was then pelletized in a Brabender pelletizer.

2.2.2. Injection Molding

An injection system (ARBURG 221K ALLROUNDER machine, ARBURG GmbH, Lossburg, Germany) equipped with a screw of a 25 mm diameter and a clapping unit force of 350 kN, was employed for molding, according to the conditions described in

Table 3. Injection-molding parameters for the nanocomposite specimens’ preparation.

Temperature (°C)					Injection Pressure (bar)		Packing Pressure (bar)	Cooling Time (s)
In Injection Barrel Zones
1st	2nd	3rd	4th	5th	(8 ccm)	(10 ccm)
190	195	195	195	200	850	800	270	5

. The mold contained two cavities, being suitable for dumbbell specimens for subsequent Tensile Testing (ASTM D-638, specimen type IV).

2.2.3. X-Ray Diffractometer (XRD)

The XRD patterns of the clays and nanocomposites were obtained in order to assess the appearance of the clay d₀₀₁ reflection. The tests were run in a Siemens 5000 (35 kV, 25 mA) using CuKα Χ-ray radiation with a wavelength of λ = 0.154 nm. The diffractograms were scanned in the 2θ range from 2 to 10°, with a rate of 2°/min. The samples for the above analysis were cut from compression-molded plaques in order to avoid any preferred orientation of the clay.

2.2.4. Melt Flow Index (MFI)

The MFI tests were performed at 230 °C/2.16 kg, following the ASTM D 1238 specification (procedure A), in a capillary rheometer (Kayeness Co. model 4004-Dynisco, Franklin, MA, USA).

2.2.5. Thermogravimetric Analysis (TGA)

Thermogravimetric analyses of the prepared nanocomposites were run in a Mettler Toledo (Schwerzenbach, Switzerland) thermogravimetric analyzer (model: TGA-DTA). All testing was carried out using samples of 10 mg, at a heating rate of 10 °C/min from 25 °C to 700 °C, in a nitrogen atmosphere.

2.2.6. Differential Scanning Calorimetry (DSC)

For the DSC test, a DSC 1 calorimeter (model: Mettler Toledo) (Schwerzenbach, Switzerland) was employed. Samples of about 10 mg were accurately weighed with an analytical balance and sealed in aluminum pans. All experiments were conducted under a nitrogen flow of 20 cm³/min to prevent thermo-oxidative degradation. The samples were cooled from 30 to −150 °C at a rate of −10 °C/min and stayed there for 5 min in order to erase their previous thermal history. After this cycle, they were heated from −150 to 30 °C at 5 °C/min. The values of the glass transition (T_g), crystallization (T_c), and melting (T_m) temperatures, as well as the heat of fusion (ΔH_m), were calculated from the thermographs corresponding to the heating cycle. The crystallinity of the specimens was calculated according to

$$X_c={\displaystyle \frac{{\Delta H}_m}{{\Delta H}_m^0}}\times 100$$

taking ${\Delta H}_m^0$ 292 [25] and 207 J/g [26] for HDPE and PP, respectively.

2.2.7. Tensile Tests

Tensile tests were carried out following the procedure described in ASTM D-638. Five specimens from each sample (tailored according to the type IV specimen) were tested in an Instron (model 4466) machine, equipped with a load cell of a maximum capacity of 10 kN, adjusted at a grip separation speed of 50 mm/min.

3. Results

3.1. XRD

Different parameters were adjusted in order to produce nanocomposites with intercalated and/or exfoliated structures. X-ray diffraction analysis was performed for the characterization of the obtained composite structures, and, more specifically, the following factors that affect the nanocomposite structure were examined: (i) the extruder rotation speed, (ii) the compatibilizer content, (iii) the HDPE/PP content, and (iv) the type of clay and (v) its content in the prepared nanocomposite. Two types of OMMT having different organic modifications were used: Cloisite 25A and Cloisite 30B.

Based on the above study, the screw rotation speed was set at 200 rpm because no obvious difference between the three alternative speeds was observed (Figure 1). A very small variation was observed in the extrusion at 200 rpm. In this case, the peak located at around 5° showed a lower intensity compared with the others, which indicated that with extrusion at 200 rpm, a larger proportion of the reinforcement was exfoliated. Furthermore, at an industrial level, it is important for the twin-screw extrusion system to operate at a high speed, as this allows the sample to be extruded in a shorter time. Therefore, all the other samples that were produced were extruded at 200 rpm.

The concentration of 10% HDPE in the polyolefin blend seemed to give better intercalated structures (Figure 2a,b), probably due to the lower viscosity of the mixture (identified by MFI measurements), which facilitated molecular penetration between the clay galleries.

Compatibilizer concentrations of 5, 10, and 20 phr were tested in the polyolefins blend, and the best performance was observed at 20 phr (Figure 3), where the characteristic peak attributed to MMT almost disappeared. Therefore, this compatibilizer content was selected for the continuation of this study.

Cloisite 25A and Cloisite 30B were shown to be the more effective reinforcements for preparation of intercalated and/or exfoliated structures, especially at low concentrations, as can be seen in Figure 4 and Figure 5, respectively, where both clays were incorporated in the optimum compatibilized HDPE/PP blend composition (10/90).

3.2. DSC

The presence of two peaks in the HDPE/PP systems in Figure 6a confirms the immiscibility of the prepared blends. In fact, the peak appearing at lower temperatures corresponded to the HDPE phase, while that at higher temperatures corresponded to the PP phase.

From the DSC results in

Table 4. DSC analysis data of OMMT/compatibilized polyolefin blend nanocomposites.

	Clay Type	Clay (phr)	Τm (°C)	ΔHm (J/g)			Τc (°C)
			1st Peak	2nd Peak	1st Peak	2nd Peak
HDPE	-	-	134.7 ± 0.15	-	209.06 ± 1.63	-	119.7 ± 1.25
PP	-	-	-	164.3 ± 0.11	-	90.29 ± 1.35	121.0 ± 1.22
COMP	-	-	111.0 ± 0.11	-	98.29 ± 1.45	-	99.1 ± 1.33
10/90 (w/w) HDPE/PP + 20 phr COMP		0	129.6 ± 0.25	162.7 ± 0.14	17.09 ± 1.17	59.30 ± 1.25	118.3 ± 1.55
	Cl 25A	1	130.5 ± 0.16	163.8 ± 0.36	17.76 ± 2.64	61.12 ± 2.28	117.2 ± 0.18
	Cl 25A	2	129.4 ± 0.18	163.4 ± 0.26	13.16 ± 2.33	62.54 ± 2.28	116.5 ± 0.44
	Cl 25A	3	129.6 ± 0.36	163.6 ± 0.28	14.05 ± 1.03	57.21 ± 0.59	116.3 ± 0.30
	Cl 30B	1	128.8 ± 0.39	163.2 ± 0.60	11.62 ± 0.40	59.85 ± 4.91	116.6 ± 0.38
	Cl 30B	2	128.3 ± 0.45	163.1 ± 0.32	11.66 ± 0.50	56.44 ± 0.38	117.2 ± 0.72
	Cl 30B	3	129.5 ± 0.64	163.2 ± 0.55	11.45 ± 0.95	57.04 ± 0.09	117.1 ± 0.46
25/75 (w/w) HDPE/PP + 20 phr COMP		0	131.1 ± 0.45	162.8 ± 0.28	33.10 ± 3.15	49.57 ± 1.09	117.9 ± 0.12
	Cl 25A	1	131.9 ± 0.11	162.5 ± 0.38	42.74 ± 6.31	47.09 ± 3.95	118.0 ± 0.57
	Cl 25A	2	131.6 ± 0.32	163.1 ± 0.22	38.05 ± 4.52	42.72 ± 2.04	117.7 ± 0.27
	Cl 25A	3	132.1 ± 0.23	163.4 ± 0.64	34.54 ± 0.94	41.86 ± 1.53	117.5 ± 0.01
	Cl 30B	1	131.8 ± 0.20	163.1 ± 0.64	41.37 ± 5.26	40.51 ± 2.68	117.7 ± 0.30
	Cl 30B	2	131.5 ± 0.52	163.0 ± 0.42	35.21 ± 4.91	44.50 ± 1.28	118.0 ± 0.69
	Cl 30B	3	132.0 ± 0.08	163.1 ± 0.06	41.10 ± 3.90	44.67 ± 1.95	118.9 ± 0.40

, it can be noticed that the melting temperatures of the HDPE and PP phases in the blends slightly decreased compared with those of the pristine materials due to the minor solubility of one phase in the other. This effect was further promoted by the presence of the PE-g-MAH compatibilizer. This behavior was expected, as the compatibilizer has a lower melting temperature than both HDPE and PP. Moreover, as the proportion of HDPE in the blend increased, while keeping the compatibilizer content constant, the melting temperature of PE increased, whereas that of PP remained almost unchanged.

It was found that adding OMMT did not significantly influence the melting or crystallization temperatures, which remained nearly constant. However, at a low OMMT concentration (1 phr), the melting enthalpy (ΔH_m) of PE increased compared with the unfilled blend (Figure 6b). This behavior was attributed to the exfoliated clay platelets functioning as nucleating sites that promote heterogeneous crystallization. At higher OMMT contents, though, the melting enthalpy of the PE phase decreased, likely due to the restricted mobility of polymer chains caused by the reinforcing filler. Avella et al. [4] examined isotactic polypropylene nanocomposites reinforced with clay and observed that the layers of mineral clay hindered the movement of the polymer chains, restricting their free volume during the crystallization process. As a result, smaller crystals were formed compared with the samples without reinforcement.

The incorporation of nanoparticles had a significant impact on the glass transition temperature (T_g). When strong interactions exist between the filler and the polymer matrix, the T_g of the amorphous polymer generally rises as the particle size decreases or as the filler concentration increases. This behavior is generally associated with the confinement effect, generating a reduction in chain mobility until the suppression of cooperative segmental motion of the confined macromolecules.

Based on the melting enthalpy, the crystallinity of each phase of the HDPE/PP blends was calculated, and the results are presented in

Table 5. Crystallinity percentage (X_c), based on DSC, of the HDPE and PP phases in OMMT/compatibilized polyolefin blend nanocomposites.

	Clay Type	Clay (phr)	Xc (%)
			Xc, HDPE	Xc, PP
HDPE	-	-	71.6 ± 0.56	-
PP	-	-	-	43.6 ± 0.65
COMP	-	-	33.7 ± 0.50	-
10/90 (w/w) HDPE/PP + 20 phr COMP		0	5.9 ± 0.40	28.6 ± 0.60
	Cl 25A	1	6.1 ± 0.90	29.5 ± 1.10
	Cl 25A	2	4.5 ± 0.80	30.2 ± 1.10
	Cl 25A	3	4.8 ± 0.35	27.6 ± 0.29
	Cl 30B	1	4.0 ± 0.14	28.9 ± 2.37
	Cl 30B	2	4.0 ± 0.17	27.3 ± 0.18
	Cl 30B	3	3.9 ± 0.33	27.6 ± 0.04
25/75 (w/w) HDPE/PP + 20 phr COMP		0	11.3 ± 1.08	23.9 ± 0.53
	Cl 25A	1	14.6 ± 2.16	22.7 ± 1.91
	Cl 25A	2	13.0 ± 1.55	20.6 ± 0.99
	Cl 25A	3	11.8 ± 0.32	20.2 ± 0.74
	Cl 30B	1	14.2 ± 1.80	19.6 ± 1.29
	Cl 30B	2	12.1 ± 1.68	21.5 ± 0.62
	Cl 30B	3	14.1 ± 1.34	21.6 ± 0.94

. It was observed that a higher degree of crystallinity was obtained at a lower OMMT reinforcement loading (1 phr), followed by a slight decrease at higher loadings. As indicated by the XRD results, better dispersion of the nano-filler, resulting in exfoliated nanocomposite structures, was achieved at low loadings, which effectively acted as nucleating agents and, consequently, increased the crystallinity of the HDPE/PP blends.

3.3. TGA

The thermal stability of polyolefin nanocomposites was assessed by means of TGA, and the relevant graphs are presented in Figure 7. From the results of TGA in

Table 6. TGA data of OMMT/compatibilized polyolefin nanocomposites.

HDPE/PP	Clay Type	Content (phr)	Τonset (°C)	Τpeak (°C)	Residue (%)
100/0 (w/w)	-	-	456.3 ± 0.82	476.4 ± 0.55	3.97 ± 0.46
0/100 (w/w)	-	-	392.0 ± 0.45	433.9 ± 0.35	4.27 ± 0.76
COMP	-	-	446.4 ± 0.64	472.4 ± 0.73	4.27 ± 0.75
10/90 (w/w) + 20 phr COMP	-	-	430.3 ± 2.95	457.9 ± 2.26	4.29 ± 0.74
	Cl 25A	1	434.2 ± 2.48	448.0 ± 1.04	4.40 ± 0.18
	Cl 25A	2	433.8 ± 2.17	446.4 ± 0.19	5.03 ± 0.71
	Cl 25A	3	436.2 ± 1.81	446.1 ± 0.80	5.90 ± 0.90
	Cl 30B	1	439.0 ± 0.98	453.5 ± 1.21	4.05 ± 0.34
	Cl 30B	2	431.6 ± 1.27	449.9 ± 3.20	4.85 ± 1.08
	Cl 30B	3	428.0 ± 3.01	447.2 ± 2.32	4.54 ± 0.46
25/75 (w/w) + 20 phr COMP	-	-	435.7 ± 4.57	453.7 ± 2.76	4.63 ± 0.74
	Cl 25A	1	434.7 ± 3.56	454.1 ± 0.82	4.29 ± 0.93
	Cl 25A	2	433.1 ± 3.90	448.2 ± 2.08	5.41 ± 0.30
	Cl 25A	3	434.8 ± 0.72	448.2 ± 1.48	6.31 ± 0.02
	Cl 30B	1	430.8 ± 5.83	452.9 ± 2.79	4.91 ± 0.93
	Cl 30B	2	430.8 ± 0.75	448.0 ± 2.47	4.57 ± 0.52
	Cl 30B	3	431.9 ± 1.10	447.6 ± 0.62	6.81 ± 1.45

, it was observed that, for the non-reinforced HDPE/PP blends, as the proportion of HDPE in the blend increased, which was characterized by higher thermal degradation temperatures, an improvement in the thermal stability of the material was observed. For all the nanocomposites studied, the inclusion of OMMT led to a reduction in the temperature corresponding to the maximum degradation rate (T_peak). This behavior was likely attributed to the thermal degradation of short molecular chains originating from the intercalating agents. The onset temperature (T_onset) was improved only for the nanocomposites with a 10/90 HDPE/PP matrix reinforced with OMMT, except for the one containing 3 phr of Cloisite 30B.

3.4. MFI

The decrease in MFI values (

Table 7. Tensile test results and MFI measurements of OMMT/compatibilized polyolefin nanocomposites.

HDPE/PP	Clay Type	Clay/COMP (phr)	Tensile Strength (MPa)	Modulus of Elasticity (MPa)	MFI (g/10 min)
HDPE	-	-	27.48 ± 0.35	764.64 ± 22.15	0.98 ± 0.09
PP	-	-	38.80 ± 0.24	1165.89 ± 33.14	2.69 ± 0.18
COMP	-	-	10.00 ± 0.46	207.90 ± 16.20	0.69 ± 0.07
10/90 (w/w)	-	0/20	34.36 ± 0.40	1058.16 ± 45.72	2.804 ± 0.15
	Cl 25A	1/20	34.06 ± 0.53	1039.75 ± 63.01	2.433 ± 0.09
	Cl 25A	2/20	35.33 ± 0.24	1120.67 ± 43.25	2.271 ± 0.18
	Cl 25A	3/20	33.99 ± 0.35	1020.22 ± 26.97	2.242 ± 0.11
	Cl 30B	1/20	35.03 ± 0.39	1100.38 ± 35.21	2.496 ± 0.11
	Cl 30B	2/20	34.51 ± 0.43	1073.87 ± 28.60	2.513 ± 0.25
	Cl 30B	3/20	34.34 ± 0.22	1084.02 ± 39.52	2.490 ± 0.30
25/75 (w/w)	-	0/20	34.21 ± 1.04	985.87 ± 36.67	2.058 ± 0.15
	Cl 25A	1/20	34.14 ± 0.68	1062.33 ± 35.96	1.987 ± 0.18
	Cl 25A	2/20	34.88 ± 0.60	1023.51 ± 29.03	1.829 ± 0.17
	Cl 25A	3/20	34.78 ± 0.37	1073.44 ± 37.41	1.836 ± 0.19
	Cl 30B	1/20	36.96 ± 0.59	1176.90 ± 32.78	1.871 ± 0.11
	Cl 30B	2/20	34.59 ± 0.32	1047.71 ± 51.21	2.000 ± 0.16
	Cl 30B	3/20	33.94 ± 0.51	1035.58 ± 23.13	1.928 ± 0.09

) shows that the addition of the compatibilizer provided more compact structures with higher resistance to flow. Moreover, the compatibilizer was characterized by a lower MFI value compared with PP and HDPE. Therefore, its incorporation into the blend was expected, according to the rule of mixtures, to result in a corresponding decrease. The addition of clay nanoparticles (for all the examined Cloisite types) led to a decrease in MFI values. This behavior was attributed to the restriction of polymer chain mobility induced by the presence of organoclay platelets and tactoids within the polyolefin matrix.

3.5. Tensile Tests

For the 10/90 HDPE/PP (w/w) blend with 20 phr of compatibilizer, reinforced with Cloisite 25A, an increase (of 5.9%) in the elastic modulus was observed only for the 2 phr content, while the tensile strength remained almost unchanged in the other samples (Table 7). In the nanocomposites of this blend reinforced with Cloisite 30B, the elastic modulus increased (up to 4%) for all the examined concentrations, while the tensile strength remained nearly unchanged.

For the 25/75 HDPE/PP (w/w) blend with 20 phr of compatibilizer, an increase in the elastic modulus was observed for both types of reinforcement—up to 9% for Cloisite 25A and 19% for Cloisite 30B—across all ratios. In these samples, the tensile strength showed only small variations compared with the unreinforced reference blend.

The highest tensile strength and elastic modulus were exhibited by the compatibilized 25/75 HDPE/PP (w/w) blend reinforced with 1 phr of Cloisite 30B. This result was consistent with the XRD findings, which revealed that these nanocomposites exhibited an almost exfoliated structure.

3.6. Micromechanical Modeling: Halpin–Tsai Model

The Halpin–Tsai model was applied to predict the longitudinal stiffness (E₁₁) of the polymer blends/OMMT nanocomposites as a function of the nanocomposite structure. This is a parametric model, and the results depend on several parameters, such as the number of layers (N), the d₀₀₁ space between the layers, etc. The equations of the Halpin–Tsai model are given in Equations (1) and (2), where f_p is the particle volume fraction, E_p/E_m is the particle/matrix stiffness ratio, and L/t is the particle aspect ratio.

$${\displaystyle \frac{E_{11}^{(H-T)}}{E_m}}={\displaystyle \frac{1+2(L/t)f_P\eta }{1-f_P\eta }}$$

(1)

$$\eta ={\displaystyle \frac{(E_P/E_m)-1}{(E_P/E_m)+2(L/t)}}$$

(2)

The nanocomposite structure can be described by a set of representative volume elements (RVEs). These elements contain 50–100 ‘effective particles’. The effective particle is the intercalated or exfoliated structure of the clay layers in the nanocomposite.

In this model, the particle volume fraction, the particle stiffness, and the particle aspect ratio are given in Equations (3)–(5), where ρ_m and ρ_silicate are the mass densities of the matrix and silicate sheet, respectively.

$$f_P\simeq \left({\displaystyle \frac{{\rho }_m}{{\rho }_{silicate}}}{\displaystyle \frac{1}{\chi }}\right)W_C$$

(3)

$${\displaystyle \frac{L}{t}}={\displaystyle \frac{L}{(N-1)d_{(001)}+d_S}}$$

(4)

$$E_P={\displaystyle \frac{N{d}_S}{t}}{E}_{silicate}$$

(5)

L and t are the length and the thickness, N is the number of layers in the particle, d₍₀₀₁₎ and d_s are the interlayer spacing and a single sheet thickness, and E_silicate is the silicate sheet stiffness.

The factor χ is given via Equation (6):

$$\chi ={\displaystyle \raisebox{1ex}{$N{d}_S$}\!\left/ \!\raisebox{-1ex}{$(N-1)d_{(001)}+d_S$}\right.}$$

(6)

From the literature, the values of d_s and E_silicate were found: 1.4 nm [27] and 400 GPa [28], respectively. The interlayer spacing was measured using the XRD technique. The matrix modulus was allocated experimentally for the two different blends. The particle aspect ratio was taken to be 50 [27,29].

A comparison between the elastic modulus’s experimental and theoretical values, derived from the micromechanical simulation models, was performed. More specifically, the Halpin–Tsai model was used, which interrelates Young’s modulus with the structure of the clay in the nanocomposite. This model gave a good approximation of the obtained experimental results, and it was confirmed that better exfoliated structures were obtained at low clay concentrations in the nanocomposite.

Figure 8 shows the fit of the experimental and theoretical values in the normalized modulus of the nanocomposites. The theoretical and experimental modulus of the nanocomposite were consistent only for the 10/90 HDPE/PP blend reinforced with 2 phr of Cloisite 25A. If the nanocomposites had a homogeneous structure, the 2 phr Cloisite 25A 10/90 HDPE/PP nanocomposite was expected to have intercalated particles with 20 sheets (Figure 8a). Figure 8b shows agreement for the 1 phr Cloisite 25A and 3 phr Cloisite 25A 25/75 HDPE/PP. The first nanocomposite had intercalated particles with 6 layers, while the second nanocomposite had intercalated particles with 20 layers.

Figure 9a shows that the 1 phr Cloisite 30B 10/90 HDPE/PP nanocomposite contained particles with nine sheets. Figure 9b shows that the 1 phr Cloisite 30B 25/75 HDPE/PP nanocomposite included particles with only 3 layers, which was the best result; the 2 phr Cloisite 30B 25/75 HDPE/PP nanocomposite contained particles with 14 layers; and the 3 phr Cloisite 30B 25/75 HDPE/PP nanocomposite contained particles with 25 layers. The decrease in the particle layers equaled a better macroscopic modulus, theoretically and experimentally.

4. Conclusions

In this work, it was observed that polyolefin blend/OMMT nanocomposites could be successfully prepared using a twin-screw extruder when the clay content was low. Nanocomposites with a low organoclay content (1 phr) exhibited an increase in crystallinity and in the onset temperature of thermal degradation. All examined compatibilized nanocomposites showed a decrease in MFI values compared with the respective unreinforced blends. The best mechanical performance, in terms of the tensile test, was exhibited by the 1 phr Cloisite 30B 25/75 HDPE/PP composite. This behavior was attributed to the enhanced interactions between the hydroxyl groups in the organic modification of montmorillonite and the carboxyl groups of the compatibilizer, which resulted in intercalated and exfoliated nanocomposite structures, as confirmed by XRD measurements.