1. Introduction
Recycling heterogeneous mixed plastics waste is a very difficult challenge because of the strong incompatibility of the different chemical and molecular structures of the polymers composing the mixture. This problem is usually overcome for two phase polymer blends by using a third component that reduces the interfacial tension between the two phases improving adhesion and decreasing the dimensions of the minor components in the matrix. This third components can be a copolymer partly compatible with both components or a functionalized polymer partly compatible with one of the two phases and that reacts with the second component giving rise to a copolymer compatible with both phases. This copolymer acts as a compatibilizer improving the adhesion between the two components. In this case the compatibilization is called reactive blending and many functionalized macromolecules or degraded polymers with oxygenated groups have been reported. Of course, this is not possible, or extremely difficult, in the presence of more polymers. Processing mixtures with different polymers such as polyolefins, polyesters or polyamides gives rise to a polymer system fragile with very poor mechanical properties, bad aesthetic appearance, etc.. A second and not negligible problem is connected with the different melting points of the different components. Mixing at the processing temperature of the highest melting point polymer could, indeed, imply a severe thermal or thermomechanical degradation of the other components. It should be then necessary to work at the lowest possible temperature for short times. These two conditions have been successfully used in two pieces of equipment for the production of multicomponent mixtures with good final properties.

In the paper, mixtures of strongly incompatible polymers such as polyolefins, polyvinylchloride and polyethylenetherephthalate are mixed in an unconventional mixer at low temperature, even below the melting temperature of some components and at high shear stress, producing a blend with good mechanical properties. It has been demonstrated that during the mixing in these extreme processing conditions it is possible to form copolymers that act as compatibilizers among the different phases, Similar conditions are used in the so called “solid shear stress pulverization”to obtain, in an ad-hoc designed twin screw extruder, blends with good mechanical properties starting from heterogeneous polymer mixtures.

Both processes are then based on three basic principles: (1) low temperature, even lower than the melting temperature of the highest melting temperature polymers; (2) short processing times; (3) high shear stresses. The first two conditions reduce the effects of thermal degradation, while the high shear stresses can break the macromolecules producing radicals that, reacting among them, can form copolymers that can compatibilizer the mixture. Of course, the limit of these processes is that they need special, ad hoc equipment.

Recently, various approaches based on the use of cryogenic temperatures and reactive extrusion in the presence of compatibilizers and nanofillers are emerging to simplify the waste recycling process.

The aim of this work is to evaluate if similar results can be obtained by using conventional apparatuses such as a laboratory mixer. Moreover, the process does not consider any use of compatibilizers or, in general, any other component. A heterogeneous mixture made by polyolefins, polyethylene, polypropylene and polystyrene, and polyethylenetherephthalate has been processed in a laboratory internal mixer in different processing conditions–temperature, rotational speed and time–to evaluate the effect of time, temperature and shear stress on the morphology, on the viscosity and on the mechanical properties of the final blend.

The experimental results put in evidence that with decreasing temperature and increasing the shear stress, the morphology of the blend improves mainly because the dimensions of the dispersed phases decrease and because the adhesion seems improved due to of the formation of copolymers by reactions between the macroradicals formed because of the mechanical stress applied to the melt. In particular, the processing time plays a very important role. A fragile-to-ductile transition is observed by decreasing the mixing time. Indeed, the better blend is obtained at a low temperature and mixing time, while morphology and properties become worse with an increasing temperature and mixing time. A possible competition between the formation of copolymers that can act as compatibilizers and thermomechanical degradation of the components and of the same copolymers can interpret this behavior.

2. Materials and Methods
2.1. Materials
The main characteristics of the materials used in this work are shown in Table 1. PET comes from bottles for water. High- and low-density polyethylene (HDPE and LDPE), polyethylenetherephthalate (PET), polypropylene (PP) and polystyrene (PS) are the more used polymers for the production of rigid and flexible packaging and are the more important polymers encountered in the urban plastic waste collection.

Table 1. Main characteristic of the polymers.

Materials	Supplier	Name	Density, g/cm3	MFI, g/10 min	Melting Point, °C
HDPE	Versalis	Eraclene DB506	0.939	0.26	127
LDPE	Versalis	Riblene FC 30	0.922	0.25	112
PP	Lyondellbasell	Moplen RP340H	0.9	1.8	164
PET	-	-	1.38	49	255
PS	IneosNova	Empera 251N	1.04	2.4	-

 

The values for HDPE, LDPE, PP and PS were taken from data sheet, while the value of the MFI of PET was measured at 270 °C under a weight of 325 g (condition K) .

2.2. Blends Preparation
The HDPE/LDPE/PP/PET/PS mixtures were prepared according to the composition given in Table 2 by melt mixing in a Brabender mixer (Brabender, model PLE 330, Duisburg, Germany)

Table 2. Composition of the mixture.

Materials	HDPE	LDPE	PP	PET	PS
Composition blend, %	30	30	15	15	10

The blends were prepared at different temperatures (see Table 3) and rotational speeds (see Table 4).

Table 3. Temperatures used for the preparation of the blends.

Temperature, °C	180	210	240	270
	A	B	C	D

 

Table 4. Mixing speeds used for the preparation of the blends.

Speed, rpm	60	120	250
	1	2	3

 

In the processing conditions A, B and C, the crystalline fraction of PET remains in solid state. Before blending, PET was dried in a vacuum oven at 120 °C overnight.

Table 5 shows all the blend combinations investigated. For example, D2 indicates a blend processed at 270 °C (D) and 120 rpm (2), while B3 is the code of a blend processed at 210 °C (B) and 250 rpm (3).

Table 5. Blends code for all the investigated blend.

Blends Code
-	-	A3
-	-	B3
-	C2	C3
D1	D2	D3

Figure 1 illustrates the production and characterization steps of heterogeneous mixtures.

Figure 1. Diagram illustrating the steps in the preparation and characterization of the heterogeneous mixtures.

In order to verify the presence of copolymers formed during the mixing, two binary blends were prepared: PS/PET and LDPE/PET at 180 °C, 250 rpm for 1 and 5 min. The two-blend composition was 40/60 for PS/PET and 65/35 for LDPE/PET. The same ratio between the two components is presented in the multiphase blend.

2.3. Characterizations
2.3.1. Rheological Analysis
Melt flow index (MFI) values of all the blends were measured with a CEAST extrusion plastometer (CEAST, model. 6542, Torino, Italy) at a temperature of 270 °C under a load of 2.16 Kg.

Complex viscosity curves were obtained using an ARES G2 rotational rheometer (TA Instruments, New Castle, DE, USA). The tests were performed in parallel plates mode with a diameter of 25 mm. Shear viscosity values of all the samples were measured at 270 °C from 100 to 0.1 rad/s.

2.3.2. Mechanical Analysis
Mechanical (tensile) tests were performed according to ASTM D638 -14 [22] using an Instron universal testing machine (Instron, mod. 3365, High Wycombe, U.K.). The elastic modulus was measured at the deformation rate of 1 mm/min until 3% deformation. Then, the crosshead speed was increased to 20 mm/min until the specimen failed. The reported results are an average of at least 7 measurements.

The specimens used to measure the mechanical properties were prepared by compression molding in a Carver laboratory hydraulic press (Carver, Wabash, IN, USA) at a temperature of 260 °C and a mold pressure of 300 psi for about 2 min.

2.3.3. Structural and Morphological Analysis
IR spectroscopic analysis was performed to study the interactions and to analyze the specific functional groups present in the blends. Fourier transform infrared (FT-IR) spectra were performed using a Perkin-Elmer FT-IR spectrometer (Perkin-Elmer, Norwalk, CT, USA). Spectra were collected in the range 4000–400 cm−1 with 32 scan numbers at 4 cm−1.

SEM images were obtained through a Phenom proX scanning electron microscope (Phenom World, Eindhoven, Netherlands). Before examination by SEM, specimens were fractured in liquid nitrogen. Image analysis was conducted using ImageJ software, which is freely available and in the public domain.

The numerical average diameter was calculated as follows:

The possible formation of copolymers has been monitored by dissolving the blends in a solvent of only one of the components. The presence of copolymers under the form of colloids gives some turbidity to the solutions. This test, known as the Molau test, has been used for blends of polyolefins and polyamides used as a solvent formic acid. In our case, the test has been used on two binary blends, PS/PET and LDPE/PET in order to verify if the polar components can form copolymers with matrix PE and with the dispensed polyolefin phase, PET. In the first case the solvent was tetrahydrofuran at room temperature, while for the LDPE the solvent was tetrahydronaphthalene at 80 °C. The suspensions obtained were analyzed visually and the turbidity was measured with a commercial portable turbidimeter (HANNA Instruments, mod. HI93102, Woonsocket, RI, USA).

 

Titone, V., Gulino, E. F., & La Mantia, F. P. Recycling of Heterogeneous Mixed Waste Polymers through Reactive Mixing. Polymers, 15(6), 1367. https://doi.org/10.3390/polym15061367
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