Influence of the compatibilizer on the properties of composites based on low density polyethylene and polybutylene terephthalate

. Composite materials based on a polymer mixture - low density polyethylene and polybutylene terephthalate - have been obtained and investigated. To improve the compatibility of two thermodynamically incompatible polymers, a compatibilizer was used, which is polyethylene modified with maleic anhydride. It was found that the compatibilization of the polymer mixture low density polyethylene/polybutylene terephthalate, leads to a significant change in the basic physical and mechanical characteristics of the composite. It is shown that the introduction of compatibilizer leads to an increase in the viscosity of the polymer mixture, which is caused by a change in the density of molecular entanglements. In turn, compatibilized composites are inferior in hardness to noncompatibilized composites. It was found that changes in the morphology of composites upon compatibilization lead to an improvement in the strength of the material. It was shown that the plasticity of the polymer mixture increases upon compatibilization, which makes it possible to improve the dissipative capabilities of the material.


Introduction
A directed change in the basic physical and chemical properties of basic industrial polymers by combining them with other polymers is an important direction in the creation of polymeric materials with the required performance characteristics [1][2][3]. The idea that amorphous polymers are thermodynamically incompatible due to the closeness of the entropy of mixing to zero due to high values of molecular weight characteristics, and therefore the impossibility of significant changes in the properties of one polymer when mixed with another, dominated for a long time [4,5]. In such a situation, we were not talking about crystallizing polymers due to phase separation during crystallization. However, later, numerous scientific and applied works revealed the possibility of limited thermodynamic compatibility of amorphous and amorphous-crystalline polymers [6][7][8]. As a result of these works, it was found that in order to achieve the goals of a directed change in operational characteristics, some thermodynamic compatibility in polymer-polymer mixtures is desirable. As a rule, a positive result was achieved in the case of compatible polymer-polymer systems, the properties of which are a multifactorial sum of parameters, such as the rate of polymer crystallization, thermodynamic affinity, molecular weight characteristics, chemical and physical structures of the mixture components [9][10][11].
The aim of this work was to study the effect of a compatibilizer on the properties of composites based on high density polyethylene and polybutylene terephthalate.

Experimental
In this work, in the preparation of polymer composites, industrial polymers were used: low density polyethylene (LDPE) grade 15813-020 (GOST 16337-77) and polybutylene terephthalate (PBT) grade B-0 (TU BY 700117487.170-2006), which is a modified polyester thermoplastic elastomer "Belast". At the same time, for their better combination, Compoline, grade CO/LL 05, which is polyethylene modified with maleic anhydride, manufactured by Auserpolimeri (Italy), was used as a compatibilizer, it`s structural formula shown on fig. 1. Polymer mixtures were obtained by mixing in a melt on a single-screw extruder-granulator with a ratio of L/D = 20. The process of extrusion of the mixtures was carried out with three-zone heating with a temperature distribution over the zones of 180, 230 and 210 °C in the Ι, ΙΙ and ΙΙΙ zones, respectively. Then, standard samples were made to study the basic physical and mechanical properties.
Samples for testing were made by injection molding on an RR/TSMP 2 injection molding machine from RAY-RAN (England). In this case, the casting modes for PBT, LDPE and composites based on them: cylinder temperature -220-240 °С, mold temperature -50-70 °С, casting pressure -8 atm. Before casting, the composites were dried in a vacuum oven at 130 °C for 4 h.
The melt flow index (MFR) of the obtained composites was determined on an IIRT-M capillary viscometer at the corresponding temperatures and a load of 2.16 kg (for LDPE it is 190 °C, for composites based on LDPE and PBT -230 °C), the capillary diameter is 2 mm.
The determination of Vica heat resistance was carried out in accordance with GOST 15088-83 (ST SEV 3760-82). The tests were carried out in a liquid medium. For testing, samples were used in the form of a plate with a thickness of 3.0 to 6.4 mm with a test surface size of at least 10 mm along the length of the side of the rectangle or the diameter of the circle.
The strength, modulus of elasticity and deformation of the composites were determined using a universal testing machine GT-2000, corresponding to GOST 4648-71. Samples were used for testing according to GOST 11262-80. Before testing, the samples are conditioned in a standard atmosphere in accordance with GOST 12423-66 for at least 16 hours. Before testing, the thickness and width of the samples were measured in accordance with GOST 11262-80.
Izod impact strength was determined on a Gotech Testin Machin GT-7016-A3 installation, according to GOST 19109-84 (ISO 180-82). Samples of two types were used for testing: with a notch and without a notch. Linear dimensions of samples: 80 × 10 × 4 mm (sample type 4 according to GOST 19109-84). In the case of using a sample with a notch, notch type A was used (according to GOST 19109-84), i.e. a wedge-shaped incision with an angle of 45 degrees, a depth of 0.85 ± 0.05 mm. The number of samples for each tested material was at least 10. Before testing, the samples were conditioned in accordance with GOST 12423-66.
The hardness of the composites was determined using an OS-2 hardness tester. The tip of the indenter was at a distance of at least 12 mm from the edge of the sample. Sufficient pressure is applied to the supporting surface to ensure reliable contact with the sample. The readings of the indicator device are taken after 15±1 sec. If it is necessary to make an instant measurement, then the reading is taken within 1 second after pressing the supporting surface against the sample. In this case, the maximum value that the durometer indicator will show is recorded.
Five measurements of hardness are carried out in different places on the surface of the sample, but at a distance of at least 6 mm from the point of the previous measurement, and the average value is determined.

Results and discussion
The study of changes in the behavior of melts and the heat resistance of polymer-polymer mixtures of LDPE/PBT showed that the values of the MFR and heat resistance of the LDPE/PBT composites are lower than the values of the initial polymers (Table 1).
It should be noted that the MFI decreases with a decrease in the content in the LDPE/PBT composites. When using a compatibilizer, the MFI of composites increases with decreasing PBT content in composites. This behavior of the rheological properties of LDPE/PBT and LDPE/PBT/compatibilizer composites is obviously due to the fact that, at minimum MFI values, the composites are characterized by higher levels of interpenetrating networks, i.e. high density of the network of molecular links [12]. In turn, this will lead to a change in the supramolecular structure, rheological, strength characteristics, and deformation of the composite [13].  (Table 1), it can be seen that polymer mixtures containing a compatibilizer combine better and form, thanks to the latter, a more homogeneous mixture [14]. In turn, this leads to a change in the hardness of the material ( Table 2). As can be seen from Table 2, the content in LDPE up to 20 % by weight of PBT leads to an increase in the hardness of the resulting composite in comparison with the original LDPE. However, the introduction of a compatibilizer into an LDPE/PBT mixture containing up to 10 % PBT by weight leads to a decrease in the hardness of the composite in comparison with the original polymers. Obviously, this is due to the effect of the compatibilizer on the supramolecular structure of the LDPE / PBT mixture, i.e. the formation of a looser structure [15], which is reflected in the density of composites ( Table 2). Studies of the mechanical properties of LDPE/PBT/Compatibilizer composites showed significant changes in their deformation and strength characteristics, and their improvement was observed for PBT/LDPE/ Compatibilizer mixtures with a PBT content of 5-15 wt %. Comparative characteristics of the performance properties of polymer blends PBT/LDPE/Compatibilizer are shown in Table 3.
Studies of the mechanical properties of LDPE/PBT/Compatibilizer composites showed significant changes in their deformation and strength characteristics, and their improvement was observed for PBT/LDPE/ Compatibilizer mixtures with a PBT content of 5-15 wt %.
Comparative characteristics of the performance properties of polymer blends PBT/LDPE/Compatibilizer are shown in Table 3. As can be seen from Table 3, LDPE/PBT composites without a compatibilizer are inferior in their physical and mechanical characteristics to composites containing a compatibilizer. This can be explained by the difference in the supramolecular structures of the composites, i.e. the formation of a different interfacial layer (IFS) at the interface of thermodynamically incompatible polymers [16]. Thus, during the crystallization of LDPE and PBT, which have different melting points and crystallization rates, the free volume in the MFS, where the molecules of both components can be located, increases significantly. The introduction of a compatibilizer into the mixture obviously leads to a decrease in the free volume in the MFS [17]. Consequently, structural changes in the LDPE/PBT/Compatibilizer composites occurring both within each phase and in the MFS, in general, are the cause of the change in the density of the composites and the better deformability of these mixtures, in contrast to LDPE/PBT ( Table 3).
The increase in the plasticity of LDPE/PBT mixtures upon the introduction of a compatibilizer is also confirmed by the obtained values of the Izod impact toughness (Table  3) and the hardness of the composites (Table 2). So, from table 3 it can be seen that the original LDPE and the samples containing the compatibilizer are not destroyed. Such a change in the impact toughness can be explained by the structure of the mixtures, i.e. the introduction of a compatibilizer into the LDPE/PBT mixture leads to a decrease in the volume and proportion of MFS. The latter, in turn, depend on the compatibility of the components, the better the compatibility of the components, the less these indicators, i.e. the volume and proportion of IFS, which increase the deformability of composites, are the dissipative capabilities of the material [18] (Table 3).
These results indirectly confirm the change in the hardness of the composites (Table 2). In particular, the introduction of a compatibilizer into a polymer-polymer mixture of LDPE/PBT leads to a decrease in the hardness of the composite as compared to the initial LDPE and polymer-polymer mixture of LDPE / PBT. Table 3 also shows that the introduction of a compatibilizer into the LDPE/PBT mixture also leads to an increase in the values of the elastic moduli in tension and bending in comparison with the values of the initial polymer.
The increase in modulus is associated with the arrangement of compatibilizer molecules in the LDPE/PBT mixture, leading to an increase in intermolecular interactions between polymers, forming a denser packing of macromolecular linkages [19]. Obviously, these structural changes lead to a decrease in the time to failure. In turn, the shorter the time to failure, the shorter the time until the relaxation processes occur and the lower the degree of their completion, which leads to an increase in the elastic modulus [20]. In this case, the material will undergo less deformation.

Conclusions
Thus, the results of a study of the rheological and deformation-strength properties of LDPE/PBT and LDPE/ PBT/compatibilizer blends show that the introduction of a compatibilizer into a PBT/LDPE polymer blend leads to an improvement in the compatibility of two thermodynamically incompatible polymers -LDPE and PBT. The result of improved compatibility of two polymers, which belong to different classes, are high physical, mechanical and performance characteristics of the material.