1. Introduction
Blending is one of the simplest and widespread methods to improve polymer properties. In fact, low-cost processing, typical of common blend manufacturing, allows obtaining the desired properties and a high variety of products [
1]. As a consequence, nowadays polymer blends represent around half of the total plastic production [
2]. The main limit of a polymer blend is the mutual miscibility of polymers. In fact, as established by the second law of thermodynamics, the variation of free energy ΔG is usually positive because of the high polymerization degree of polymers (affecting the variation of entropy ΔS) and the poor affinity between polymers (affecting the variation of enthalpy ΔH) [
3].
One of the main parameters affecting polymers miscibility is interfacial tension. In particular, higher interfacial tension leads to higher phase separation [
4]. In spite of this limit, a polymer blend can be compatibilized in order to increase dispersion and adhesion between polymers [
5]. Many strategies have been developed in this context, among which, the introduction of a compatibilizer is one of the most adopted [
6]. Recently, higher sensitivity to problems of oil-based polymer pollution has developed, stimulating research on bio-derived polymer production for application as potential substitutes of oil-based polymers [
7,
8,
9,
10]. Among bio-derived polymers, poly(lactic) acid (PLA) seems to be one of the most studied and applied, thanks to its properties which are comparable to or in some cases higher than traditional polyolefins [
11,
12,
13,
14]. PLA, in fact, has been frequently selected as a bio-derived polymer to obtain blends with a high amount of biodegradable polymer, reducing the amount of polyolefin [
15,
16,
17]. Moreover, depending on the compatibilizer added, an oil-based, bio-derived blend can be optimized and has specific properties. For example, a recent study compatibilized PLA/ high-density polyethylene (HDPE) polymer blends with cobalt stearate in view of a possible oxo-degradation process. This demonstrates the wide range of opportunities deriving from the addition of the right type and amount of compatibilizer [
18]. Several compatibilizing methods have been studied in order to improve oil-based–bio-derived polymeric blends, such as the use of functional molecules for reactive compatibilization during extrusion or the addition of a commercial modified polymer as a coupling agent [
19,
20,
21]. Usually, chemical compatibilization, obtained with the use of grafted polymers or random copolymers, is performed to reduce the size of the dispersed phase thanks to functional groups’ reactivity. In this way, a reduction of interfacial tension and of the coalescence impediment of the dispersed phase is possible [
22].
The development of an oil-based, bio-derived thermoplastic blend was therefore the preliminary goal of this work. In particular, an optimized high-density polyethylene and poly(lactic) acid blend could be produced in order to obtain oil-based–bio-derived thermoplastic blends with high amounts of bio-derived charge, while maintaining good mechanical properties.
Two kinds of compatibilizers were tested in order to improve the blend properties. Polybond 3029 and Lotader AX8840 seemed to be effective, thanks to the presence of maleic anhydride grafted on polyethylene chains for the former and polyethylene random copolymer with glycidyl methacrylate for the latter.
2. Materials and Methods
Eraclene MP90, commercial name of high-density polyethylene (HDPE) from ENI (Versalis, San Donato Milanese, Italy), was selected as the oil-based polymer. Among its properties are: a melt flow index (MFI) of 7 g/10 min (190 °C/2.16 kg), a nominal mass of 0.96 g/cm³, a tensile strength of 21 MPa, a tensile modulus of 1.2 GPa, and a Shore D hardness of 50. Poly(lactic acid) (PLA) Ingeo Biopolymer 3251D from Nature Works (Minnetonka, MN, USA) was selected as a bio-derived thermoplastic polymer, with an MFI of 35 g/10 min (190 °C/2.16 kg). This polymer is characterized by density 1.24, crystalline melting temperature in the range 155–170 °C, and a glass transition temperature in the range 55–60 °C. Polybond 3029 (Addivant, CT, USA) was selected as an additive, suitable for cellulosic fillers. In fact, Polybond 3029 is a maleated polyethylene with a melt flow index of 4 g/10 min (190 °C/2.16 kg), and a maleic anhydride (MA) content of 1.7 wt.% (high). Generally, it is sold as pellets of 3–4 mm diameter. Lotader AX8840 was selected with the same purpose. It is a random copolymer of ethylene and glycidyl-methacrylate (GMA), with a melt flow index of 5 g/10 min (190 °C/2.16 kg). The GMA content is about 8 wt.%. PLA was dried for one night at 80 °C in order to avoid possible bubble formation due to water evaporation during the production process.
A Micro 15 Twin-screw DSM research extruder (Xplore Instruments BV, 6160 MD Geleen, The Netherlands) was used in order to produce the samples. Temperature of 180 °C, screw speed of 75 rpm, nitrogen atmosphere, and a resident time of 4 min in the extruder were selected to avoid PLA degradation during the process [
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24].
Injection moulding was used to obtain dog-bone specimens, with a mould temperature of 55 °C and pressure parameters depending on polymer viscosity. For each family of samples, 10 specimens were produced.
Table 1 sums up the formulations produced.
2.1. Tensile Tests
Tensile tests were performed in accordance with the ASTM D638-14 standard, using Zwick/Roell Z010 (ZwickRoell GmbH & Co. KG, D-89079 Ulm, Germany), with a load cell of 10 kN and a 50 N preload. The crosshead speed was 5 mm/min. The tensile tests were performed on five dog-bone samples per series, with a gauge length section of 30 × 4 × 2 mm3 (L × W × T). For each family, five samples were tested.
2.2. Scanning Electron Microscopy (SEM)
The samples were observed with a Hitachi S2500 25 kV scanning electron microscope (Hitachi, Krefeld, Germany) in order to analyze blend morphology and interfaces. The samples were sputter-coated with gold particles before surface characterization.
2.3. Quartering
The samples produced, in the majority of cases, were characterized by some heterogeneity because of multiphase matrices. In order to obtain reliable results from the thermal analysis and analyze a representative number of samples, a cryogenic mill was adopted to obtain samples in the form of powders. A subsequent statistical approach, quartering, was used to select an exemplary number of samples used for chemical and thermal analysis. This method was based on the separation of the total amount of charge in four parts equal in weight. Then, two parts at the opposite side were mixed together, and the other two were separated.
2.4. Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry (DSC) tests were performed on a Q20 Thermal Analysis instrument (TA Instruments, New Castle, DE, USA) from 25 °C to 180 °C at 10 °C/min under a nitrogen flow of 50 mL/min−1. Two cycles were performed with a 4 min interval between them at 180 °C to eliminate trace of thermal history. The first cycle provided information about properties after injection moulding, while the second one gave material’s properties. Cold crystallization, melting, crystallization parameters (temperature and enthalpy), and glass transition temperatures were analyzed.
2.5. Thermogravimetric Analysis (TGA)
Thermogravimetric Analysis (TGA) tests were carried out on a Q500 Thermal Analysis instrument (TA Instruments, New Castle, DE, USA) up to 600 °C, with a scanning temperature of 10 °C/min under a nitrogen flow of 50 mL/min−1. From this analysis, we derived temperatures at which degradation started (Tonset), evaluated through the extrapolated onset temperature from the TGA curve and the ∆m, i.e., mass variation, during the test.
2.6. Attenuated Total Reflection–Fourier Transform Infrared (ATR–FTIR) Analysis
Attenuated Total Reflection–Fourier Transform Infrared (ATR-FTIR) tests were carried out to evaluate the interactions between HDPE, PLA, and the compatibilizers. The tests were performed with a thermo-scientific Nicolet IS10 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a spectral range 4000–400 cm–1 and 32 scans.