Compatibility and characterization of Bio-PE/PCL blends

In this work, blends based on environmentally friend polymers such as Biopolyethylene (Bio-PE), Polycaprolactone (PCL) and Polyethylene graft maleic anhydride (PEgMA) added as compatibilizer agent were produced by conventional extrusion, aiming to produce bio-blends with synergic properties at low processing cost, being at same time non-polluting and therefore contributing to the environment preservation. Differential scanning calorimetry (DSC) showed that blending does not significantly interfere on the melting and crystallization behaviors of neat polymers, suggesting being low miscibility compounds. Mechanical properties were observed changing with blend composition as the impact strength significantly increased reaching values higher than 130% when compared to neat Bio-PE. Scanning electron microscopy (SEM) images showed honeycomb morphology in Bio-PE/PCL blends, and the addition of PEgMA decreased the coalescence contributing to obtain more stable and synergic compounds. Bio-PE/PCL/PEgMA at 80/20/10 contents presented the best properties and may be used for packaging materials (food containers, film wrapping), and hygiene products.


Introduction
Currently petroleum-based polymer products are still dominant in the world market due to their excellent mechanical and thermal properties, as well as to their great versatility in several applications, providing an amount of approximately 300 million tons of plastic products produced by the end of this year. However, given the characteristic of nonbiodegradability and durability of some polymers as polyolefins, polyamides, polyesters and so on, a serious environmental problem follows the contemporary man with potential damage to nature, especially in the populous urban centers [1][2][3][4] .
Therefore, the society has been asking the industrial sector for adopting "ecologically acceptable" policies, such as the rational use of natural resources, mainly in the production of materials for the productive sectors. Focused on this subject polymer scientists have suggested as an alternative to the use of polymers derived from fossil sources the production of biopolymers (polymers produced from renewable sources) and biodegradable (polymers able to naturally degrade in the environment) ones [5][6][7] .
The use of biodegradable polymers appear as a possible and fast solution to reduce environmental pollution, they can be produced from renewable resources such as maize, sugar cane, cellulose and chitin, for instance, additionally they present shorter life cycle compared to the non-biodegradable ones (as polypropylene (PP), poly(ethylene terephthalate) (PET), nylons and so on) and when discarded they produce compounds not harmefull to the environment, as the case of poly(hydroxibutyrate) (PHB), PCL, poly(butylene adipate-co-terephthalate) (PBAT) for instance [7] .
Additionally, the use of "green" polymers, such as biopolyethylene (Bio-PE), produced from ethanol (from sugarcane), although not biodegradable, maintains the neutral balance of carbon dioxide (CO 2 ) in the natural environment. The CO 2 captured from the atmosphere by the biomass, when later released to the atmosphere by the combustion, is captured again by the sugarcane trough the photosynthesis process in the next harvest [7][8][9][10] .
Another alternative to this scenario would be the use of environmentally degradable polymers, which have the advantage of being stable over their useful life and being degraded in a short time after disposal in the environment; PCL is one of these polymers that has aroused interest in the substitution of conventional polymers since it is a fully biodegradable hydroxycarbonic acid based on polyester.
Research in polymer blends involving these two classes of polymers appears as a viable alternative to the process of developing ecologically friend materials. In addition, studies of polymer blends are an alternative to obtain materials with properties that, in general, are not found in a neat resin [18,19] .
Therefore, the objective of this work is to develop polymer blends based on environmentally friend polymers (Bio-PE and PCL) with different compositions; Bio-PE/PCL/PEgMA blends were also produced aiming the tenacification and compatibilization of Bio-PE upon addition of PEgMA, which has PE and MA segments,which are able to react with Bio-PE and PCL end groups. These blends were characterized by differential scanning calorimetry (DSC), heat deflection temperature (HDT), mechanical tensile and impact strength tests, scanning electron microscopy (SEM) and contact angle measurement.

Methods
Polymer blending carried out in a modular, interpenetrating, twin screw extruder with L/D ratio of 40, model ZSK 18 mm, Werner-Pfleiderer, Coperion (Wesseling, Rhein-Erft-Kreis, Germany). Prior to extrusion, the raw materials were manually mixed to promote further homogenization. For all blends, the following extrusion parameters were used: feed rate of 5 kg/h; screw speed of 250 rpm; temperature profile in the extruder zones 200°C in all zones. The output material was granulated and oven dried under vacuum at 40°C for 24h.
The compositions of the extruded blends and their codes are shown in Table 1. Figure 1 shows the screw used during the extrusion. The screw configuration has mixing sections with dispersive and distributive elements. The main feed zone of premixed materials is indicated in Figure 1 with the down arrow. The upward-facing arrows are degassing points (vents).
After extrusion, injected specimens were molded according to ASTM standards D 638, D256 and D648, for tensile, impact and HDT experiments, respectively. An Arburg Injector, Model Allrounder 270C Golden Edition (Loßburg, Baden-Württemberg, Germany), was used, operating at 180°C, with mold at 20°C. Blends, neat Bio-PE and PCL were subjected to the same injection parameters. An average of 10 specimens was used for each investigated composition.

Differential Scanning Calorimetry (DSC)
DSC analyzes were performed using a TA Instrument DSC-Q20 (New Castle, Delawere, EUA). The temperature program used was: heating from 20°C to 250°C, cooling to 10°C, reheating to 250°C, at a heating/cooling rate of 10°C/min, under inert environment with nitrogen flow of 50 mL/min. The samples tested weighed approximately 3.5 mg.

Heat Deflection Temperature (HDT)
HDT was determined according to ASTM D 648, in a Ceast equipment (Norwood, Massachusetts, EUA), model HDT 6 VICAT/N 6921.000, with a tension of 455 kPa, heating rate of 120°C/h (method A). The temperature was determined after the sample deflecting 0.25 mm. Series of five injected samples were tested and the HDT, with its respective standard deviation, is reported.

Mechanical test
The tensile tests were performed according to ASTM D 638. Properties as elastic modulus, tensile strength and elongation at break were measured. The tests were performed in a universal EMIC equipment (Curitiba, Paraná, Brazil), model DL10000, using a 100 kgf load cell, with deformation rate of 50 mm/min, operating at room temperature (~23°C). The results presented are an average of 10 specimens tested.

Mechanical impact strength test
The IZOD impact strength tests were performed on notched specimens, using a Resil 5.5 equipment from Ceast (Norwood, Massachusetts, EUA) and a pendulum of 2.75 J, according to ASTM D 256, at room temperature (~23 o C). The results reported were obtained from an average of 10 specimens.

Scanning Electron Microscopy (SEM)
SEM analyzes were obtained on the Tescan Vega 3 equipment (South Moravia, Brno, Czech Republic) with a voltage of 30 kV under high vacuum, images were captured on the fracture surface of the fractured impact specimen. The fracture surfaces of the samples were gold-covered (Shimadzu Metallic-IC-50, using a current of 4mA for a period of 3 minutes) in order to avoid negative charge accumulation. The average diameters of dispersed phases were computed using the Tesca See 3 software.

Contact angle measurement
The contact angle analysis to determine the hydrophilicity of the blends was performed by distilled water drop method through a Phoenix-i model of the Electro Optics -SEO Surface (Saneop-ro, Namwon, South Korea). This analysis was done on the surface of the injection molded specimens. An analysis was performed from 20 photos, using an interval of 10 seconds, totaling 200s.

Differential Scanning Calorimetry (DSC)
Understanding how the addition of PCL and PEgMA affect the morphology of Bio-PE is especially important because the resulting crystalline structure will influence the chemical as well as physical properties of the blends; to reach this aim DSC was employed, these scans are presented in Figure 2, and parameters determined from them are presented in Tables A1-A4 of Appendix 2 . DSC scans of Figure 2 (Top) present the exothermic peaks relative to melt crystallization of Bio-PE and PCL. The addition of PCL slightly changed the crystallization of Bio-PE, which has a crystallization range between 106.04°C and 119.19°C; the exothermic crystallization peak of PCL in the blends was observed between 32.91 and 42.48°C. Bio-PE has a degree of crystallinity ΔX c ~ 14.50% and PCL between 4.75-8.65%; these data are in the literature range as published by Fel et al. [20] for (high density polyethylene) HDPE and by Antunes & Felisberti [21] for PCL. The crystallization rates and τ 1/2 (time to reach 50% of crystallinity) of Bio-PE and PCL were subtly modified in the blends, as shown in Figures A4 and A5. These behaviors suggest the low miscibility of Bio-PE/PCL system, with respective crystalline phases, i.e. Bio-PE and PCL, crystallizing as separate phases, nevertheless phase segregation was not verified as further on presented in SEM images ( Figure 3) where Bio-PE is the matrix and PCL the dispersed phase, nevertheless  upon addition of PEgMA the particle sizes decreased as an indication of chemical interactions between Bio-PE/PCL and PEgMA, conducting to the blends compatibilization [22] .
DSC scans acquired during the second heating are presented in Figure 2 (bottom), two endothermic peaks are observed, in the lower temperature region 47.57-62.05°C and in higher temperatures 106.41-138.46°C, associate with the fusion of PCL and Bio-PE, respectively. Similarly to that observed during the melt crystallization, the melting behavior of Bio-PE was not altered in the blends, with the degree of crystallinity ΔX c : 13.27-18.28%. The parameters computed from these scans are found in Tables A1-A4 and in Figures A6 and A7 of Appendix 2 and 3.
The Molten Fraction plots presented a sigmoidal shape characteristic of phase transformation in polymers without discontinuities, behavior similar to that observed during crystallization from the melt (Relative Crystallinity); these curves are presented in Appendix 3, Figures A8-A11. The melting rates of PCL and Bio-PE increased in the blends with values between 30-50% higher than in the neat resins, which can be understood as a facilitated melting process, thus providing a processing with less energy consuming and possibly cheaper [23,24] . This decrease is most like due to the high flexibility, low melting temperature (≈ 60°C) and low glass transition (≈ -60°C) of PCL [25] . These results are in agreement with the data obtained by DSC.

Heat Deflection Temperature (HDT)
The addition of PEgMA to Bio-PE/PCL provided distinct results for the different concentrations of PCL. It is verified for Bio-PE/PCL/PEgMA (90/10/10 phr and 70/30/10 phr) a similar behavior to that presented by their respective binary blends. For the compound 80/20/10 phr, an increase in HDT compared to Bio-PE is observed, this increase being approximately 3.4%, results suggest in this concentration the effect of PEgMA is optimized in terms of higher heat deflexion temperature stability.
In general, the individual contribution of each component and the morphology generated by the phases in polymer blends are the most important characteristics concerned with its performance. Generally, the continuous phase provides greater contribution to the HDT of the blends, as also reported by Ferreira et al. [26] and Luna et al. [27] . Subsequently, the morphology of blends will be examined by SEM, where these results can be better elucidated. From the data shown in Table 3, it is possible to infer that Bio-PE and PCL have high elongation at break, that is, both are able of undergoing large deformations [28,29] .

Mechanical tests -tensile strength
Analyzing the effect of PCL addition on Bio-PE/PCL blends, it was observed that increasing PCL content did not promote a significant change in the Elastic Modulus nor in the Tensile Strength data. In general, the stiffness of immiscible blends may be related with the competitive effect between the performance of the interface and the stiff polymer content that presents higher stiffness (modulus), as reported by Machado et al. [30] , Rosa et al. [31] , Moura et al. [32] and Silva [33] . In the present work, despite the fact the Bio-PE/PCL blends are immiscible, their mechanical behavior was not negatively affected, by the contrary, the Elastic Modulus of Bio-PE/PCL was observed being ~5% higher than neat Bio-PE, producing a synergic performance.
In relation to the addition of PEgMA, it did not result in higher changes in the Elastic Modulus with observed decreases between 8-15%, on the other hand, the Elongation at Break of Bio-PE/PCL/PEgMA blends showed increases higher than 70% in relation to Bio-PE/PCL blends. These results are linked to the morphological effect among the phases, despite the immiscible character (as observed by DSC scans Figure 2, SEM images Figure 3, Table A1-A4 and Figures A6-A7 of Appendix 2 and 3), in the amorphous phases of both polymers, secondary interactions are possible to occur, additionally PEgMA contributes to better mechanical performance. SEM images captured with the aim of a better enlightenment, and shown further on [30,33,34] , suggest the Elongation at Break of Bio-PE/PCL blends, being the ternary systems with addition of the functionalized copolymer PEgMA improved (higher), which can be resulted from the reaction between maleic anhydride with the hydroxyl (OH)  end groups of PCL that may be taken place providing an interface with higher performance, behaviors which can be inferred from decreases in dispersed phase as showed in Figure 3 and Table 4 by dispersed phase's average diameter measurements [24,25,[35][36][37][38][39][40] . Table 5 shows the Impact Strength results of Bio-PE, PCL, Bio-PE/PCL and Bio-PE/PCL/PEgMA blends. It is verified that addition of 10% PCL did not promote a significant variation in the impact strength of Bio-PE. On the other hand, blends with 20% and 30% PCL showed higher impact strength with increases of 88.2% for Bio-PE/PCL (80/20) and 83.2% for Bio-PE/PCL (70/30). This increase may be related to the PCL effect that presents elastomeric characteristics, being able to act as a properly impact modifier, thus promoting a significant improvement in the energy absorption mechanisms of the produced blends in this work [33,38,41] . The addition of PEgMA also contributed to increase the impact strength, where increases of 133.2% for Bio-PE/PCL/PEgMA (80/20/10 phr) and 100.3% for Bio-PE/PCL/PEgMA (70/30/10 phr) were reached. This behavior can be attributed to the higher amount of linkages between Bio-PE/PCL phases promoted by the reaction trough maleic anhydride and hydroxyl groups of PCL, as well as the compatibility of PEgMA with Bio-PE, which efficiently drives the tension transfer mechanisms between the phases (Bio-PE matrix and PCL dispersed phase, see SEM images) [24,33,38] . Figure 3 presents SEM images of Bio-PE, PCL and Bio-PE/PCL and Bio-PE/PCL/PEgMA blends, these images were captured on the fractured surface of the specimens after impact experiments.

Scanning Electron Microscopy (SEM)
In Figure 3a-d is observed the surfaces of Bio-PE and PCL with characteristics of ductile fracture evidencing the elastic deformation followed by the plastic one, these images corroborate the previous results obtained with mechanical tests where deformations higher than 500% were reached for the neat polymers.
The increase of PCL content into Bio-PE/PCL blends conducted to an increase in the mean diameter of the dispersed phase, (see results in Table 4), leading to the coalescence between PCL domains, which are indicated by the red arrows in Figure 3. In addition, a larger number of PCL domains in Bio-PE/PCL 70/30 were pull out from Bio-PE matrix [43,44] , these results agree with those shown in the DSC analyzes and with mechanical properties, where low miscibility was observed.
The effect of PEgMA on the phase behavior of Bio-PE/PCL blends is also shown in Figure 3k-p. For Bio-PE/PCL/PEgMA 90/10/10 and 80/20/10 blends is verified a very similar morphology to that of Bio-PE. These images show a homogeneous morphology, and it is difficult to make distinction between the dispersed PCL phase from the Bio-PE matrix. This effect may occur due to the interaction and ability of the compatibillizer (PEgMA) to remain at the interface, promoting a reduction of the interfacial energy and avoiding the domain coalescence, this would be the driving force for the improvement in the impact strength as well as for the increase in the elongation at break as previously presented in mechanical results [45,46] .  Table 4 the dispersed phase's average dimater increases with PCL content in binary blends and decreases upon addition of PEgMA, trend observed for Bio-PE/PCL 90/10/10 and 80/20/10, for the blend 70/30/10 the trend change and coalescence increases, suggesting solubility limit barrier was reached.
Summing up, the incorporation of PEgMA provided a better adhesion between the phases, contributing to the homogeneity of the blends in relation to the non-compatibillized ones, i.e., PEgMA led to the morphology stabilization of the blends [32,44,46,47] . It is suggested the addition of PEgMA increases interfacial adhesion due to the chemical interaction between the hydroxyl group of PCL and the maleic anhydride groups, as previously reported by Bezerra et al. [24] .

Contact angle
The contact angle measurement allows evaluating the hydrophilicity and hydrophobicity of the polymer blend surfaces, where this means the interaction energy between the surface and the used liquid. The collected data for the contact angle demonstrates the increased degree of blend surface interaction with water, indicating an increase in its hydrophilic character with the increase of PCL content, which is expected since PCL is the most hydrophilic polymer [48][49][50] .   Figure 3 and Table 4 at this composition coalescence of PCL dispersed particles took place decreasing the contact area of PCL phase and possibly providing a lower contact angle as presented in Figure 4.
For Bio-PE/PCL/PEgMA blends, i.e., 90/10/10 and 80/20/10, it was observed that addition of PEgMA promoted stabilization of the contact angle; meanwhile an increase of this parameter was verified for the composition 70/30/10. As previously reported, this is probably due to the occurrence of reactions between the maleic anhydride group and hydroxyl groups of PCL, decreasing the disperse particle size and improving the the system compatilization [24] .

Conclusions
Processing of Bio-PE/PCL and Bio-PE/PCL/PEgMA blends does lightly interfere in the crystallization and melting events of neat polymers suggesting being mixtures with low miscibility. From HDT data reduced values were observed for the binary blends, meanwhile PEgMA provided subtle increase. Contact angle measurements indicate an increase in the blend's hydrophilic character increasing PCL content. Addition of PCL to Bio-PE reduced the elastic modulus, increased the elongation at break and impact strength, allowing a control of these properties by changing the blend composition. Impact Strength of compatibilized blends significantly increased when compared to neat Bio-PE being 113.2% higher for Bio-PE/PCL/PEgMA. Addition of PEgMA decreases the phase coalescence conducting to a more stable compounds as evidenced by SEM images. Summing up Bio-PE/PCL/PEgMA (80/20/10) is thermally stable presenting better homogeneity with higher HDT and Impact strength.