Analysis of Selected Properties of Biocomposites Based on Polyethylene with a Natural Origin Filler

The study investigates the effect of the content and size of wheat bran grains on selected properties of a lignocellulosic biocomposite on a polyethylene matrix. The biocomposite samples were made by injection method of low-density polyethylene with 5%, 10% and 15% by weight of wheat bran. Three bran fractions with grain sizes <0.4 mm, 0.4–0.6 mm and 0.6–0.8 mm were used. The properties of the mouldings (after primary shrinkage) were examined after their 2.5-year natural aging period. Processing properties, such as MFR (mass flow rate) and processing shrinkage, were determined. Selected physical, mechanical and structural properties of the produced biocomposite samples were tested. The results allowed the determination of the influence of both the content of bran and the size of its grains on such properties of the biocomposite as: color, gloss, processing shrinkage, tensile strength, MFR mass flow rate, chemical structure (FTIR), thermal properties (DSC, TG), p-v-T relationship. The tests did not show any deterioration of the mechanical characteristics of the tested composites after natural aging.


Introduction
The use of composite polymer materials is now a common practice that allows adaptation of material properties to even most demanding requirements of the automotive, electrical, aviation and household industries [1,2]. Modifications of properties are most often made by mixing polymers with various types of fillers or fibers [3]. The most popular in mass applications are mineral fillers such as mica, talc, kaolin, wollastonite, calcium carbonate, silica and montmorillonite [4][5][6]. There are many reasons for the high popularity of mineral fillers. These include relatively low price, availability, a large range of sizes from micro to nano and a predictable and reproducible effect on the properties of the final product. The main advantages of their use are higher stiffness and the reduction in processing shrinkage and flammability-the properties that were a significant limitation for the use of unmodified thermoplastic polymers [1,7]. However, due to growing ecological awareness and the increasing level of environmental pollution, an increased interest in biocomposites based on biodegradable polymers and containing fillers of natural origin has been observed for several years [8][9][10]. The practical use of biodegradable polymers, such as polylactide or thermoplastic starch, is still limited. The reason is the rigorous and technically difficult process of their production and processing, which results in a much higher price than that of petrochemical plastics [11][12][13]. Therefore, an intensively developed trend in the field of polymer processing is the use of natural fillers, which, in addition to their unique properties,

Test Stand
Measurement samples in the form of spatulas were made using a CS-88/63 screw injection moulding machine (Vihorlat Snina n.p., Snina, Słowacja), equipped with a mould with two cavities, shaped and sized according to the EN ISO 294-1: 2017-07 [76] standard with pinpoint gates. The injection process was carried out in accordance with the technological parameters presented in Table 1 by introducing a premixed polyethylene (PE) powder with bran without adding any proadhesive. Due to the risk of thermal decomposition of lignocellulose components, resulting in intense gas release, low temperatures were used during the process.

Materials
The test samples were made according to a low-density polyethylene injection method with a wheat bran filler from a local mill near the city of Lublin (Poland). The polyethylene powder Dowlex 2631. 10UE [77], manufactured by The DOW Chemical Company (Schkopau, Germany), was used. This material is intended for the injection of elements which require high dimensional accuracy and for the rotational casting of thin-walled elements.
The polyethylene was filled with wheat bran obtained from a local mill i.e., wheat grain shells, which are separated as waste when the grains are ground into white wheat flour. They consist mainly of crude fiber, which includes fibrous substances such as cellulose, lignin and hemicellulose. Other ingredients of wheat bran are phytic acid, oligosaccharides, nonstarch polysaccharides as well as fats and proteins [70,78].

Research Programme and Methodology
Before use, the bran was ground into a fine powder in a grinding mill, and then subjected to a drying procedure in a laboratory drier for 24 h at 50 • C. Then, using a shaker equipped with a column of sieves with a mesh size of 0.8, 0.6 and 0.4 mm, three fractions with grain sizes < 0.4 mm, 0.4-0.6 mm and 0.6-0.8 mm were separated.
The experimental studies were carried out according to the complete plan, in which the following variable factors were adopted: (1) Content by weight of bran: 0%, 5%, 10%, 15%; (2) The grain size of the bran fraction used <0.4mm, 0.4-0.6 mm and 0.6-0.8 mm.
In order to produce the measurement samples, polyethylene powder and individual fractions of ground wheat bran in the amount of 5%, 10% and 15% by weight were mechanically mixed in a planetary mixer and then injected under the conditions given in Table 1. As a result, nine measurement series of biocomposite samples were obtained and one control series of unfilled polyethylene. Some of the samples were tested 24 h after their production, while the rest only after the natural aging process. The samples were subjected to natural aging as a result of storage for 2.5 years in a dark room at a temperature of 20-25 • C and a humidity of about 55%, where they were not exposed to UV radiation or any chemicals.
The conducted experimental studies included: • Observation of the sample surface with the optical microscope (Nikon, Tokyo, Japan), model Eclipse LV100ND equipped with a DS-U3 camera using the NIS-Elements AR 4.20.00 software (Nikon, Tokyo, Japan). The transmitted light method was used for observation. Observations were made at three points of the sample, marked in Figure 1. All samples were observed in identical magnification and illumination parameters.
Materials 2020, 13, x FOR PEER REVIEW 5 of 27 room at a temperature of 20-25 °C and a humidity of about 55%, where they were not exposed to UV radiation or any chemicals. The conducted experimental studies included: • Observation of the sample surface with the optical microscope (Nikon, Tokyo, Japan), model Eclipse LV100ND equipped with a DS-U3 camera using the NIS-Elements AR 4.20.00 software (Nikon, Tokyo, Japan). The transmitted light method was used for observation. Observations were made at three points of the sample, marked in Figure 1. All samples were observed in identical magnification and illumination parameters. • Measurement of the color of samples in accordance with ASTM E308 [79], for which the Ci4200 spectrophotometer (X-Rite, Grand Rapids, MI, USA) was used. The color is described in the CIELab system, where it is defined in the L*, a*, b* area. Parameter a* describes the color from green (negative values) to red (positive values); parameter b*, the color from blue (negative values) to yellow (positive values); and parameter L* is luminance-brightness, representing the gray scale from black to white (0 is black and 100 is white). The difference between the two colors-two points in the three-dimensional space L*, a*, b* are described by the relationship: where: ΔL, Δa and Δb, respectively, represent the difference in color parameters between the compared samples. The measurements of the color of the injection mouldings were taken at points A and B and marked in Figure 1; • Measurement of the gloss of the surface of samples using the X -Rite Ci4200 spectrophotometer, according to ISO 2813: 2001 [80] at an angle of 60° of the aperture of the image of the light source and the receiver. Gloss of injection moulded parts was measured at points A and B, as marked in Figure 1;  (20,40,60,80,100 and 120 MPa). Then, the set pressure was kept constant and the samples were cooled at a rate of 5 °C /min to a temperature of 35 °C while the changes in specific volume were measured, and the process was repeated for the next higher compression pressure value. • Measurement of the color of samples in accordance with ASTM E308 [79], for which the Ci4200 spectrophotometer (X-Rite, Grand Rapids, MI, USA) was used. The color is described in the CIELab system, where it is defined in the L*, a*, b* area. Parameter a* describes the color from green (negative values) to red (positive values); parameter b*, the color from blue (negative values) to yellow (positive values); and parameter L* is luminance-brightness, representing the gray scale from black to white (0 is black and 100 is white). The difference between the two colors-two points in the three-dimensional space L*, a*, b* are described by the relationship: where: ∆L, ∆a and ∆b, respectively, represent the difference in color parameters between the compared samples. The measurements of the color of the injection mouldings were taken at points A and B and marked in Figure 1; • Measurement of the gloss of the surface of samples using the X -Rite Ci4200 spectrophotometer, according to ISO 2813: 2001 [80] at an angle of 60 • of the aperture of the image of the light source and the receiver. Gloss of injection moulded parts was measured at points A and B, as marked in Figure 1;  (20,40,60,80,100 and 120 MPa). Then, the set pressure was kept constant and the samples were cooled at a rate of 5 • C/min to a temperature of 35 • C while the changes in specific volume were measured, and the process was repeated for the next higher compression pressure value. • FTIR analysis of samples using a FTIR TENSOR 27 spectrophotometer (Bruker, Germany) equipped with an attenuated total reflection (ATR) attachment with diamond crystal. Spectra were collected in the range of 600-4000 cm −1 with 16 scans per sample with a resolution of 4 cm −1 . • Differential scanning calorimetry (DSC) tests of the obtained injection mouldings using the DSC 204 F1 Phoenix differential scanning calorimeter (NETZSCH, Günzbung, Germany) and the NETZSCH Proteus data processing software (NETZSCH, Günzbung, Germany), according to ISO 11357-1: 2016 [85]. The DSC curves were recorded in the heating (I) cycle from −150 to 150 • C (at a rate of 10 K/min), cooling from 150 to −150 • C (at a rate of 10 K/min) and heating (II) −150 to 150 • C (at a rate of 10 K/min). The samples were tested in aluminium crucibles with a pierced lid. On the basis of the DSC curves, the degree of crystallinity X c , the enthalpy of melting ∆H m , the melting point T m , the crystallization temperature and the glass transition temperature T g of the injection mouldings were determined. The degree of crystallinity was calculated from the relationship assuming that for PE ∆H 100% = 293 J/g [86]. The inflection point of the DSC curve in the area of the glass transition was taken as the glass transition temperature.
• Thermogravimetric analysis (TG), carried out using the STA 449 F1 Jupiter thermal analyzer (NETZSCH, Günzbung, Germany). The tests were carried out at temperatures of 30-700 • C in an atmosphere of synthetic air with a gas flow rate of 20 mL/min and the heating rate 10 • C/min. The samples weighing about 10 mg were tested in crucibles made of Al 2 O 3 .

Results
The test results were statistically analyzed in the STATISTICA 13 program (StatSoft, Tulsa, OK, USA). In order to determine whether there were significant differences between the compared results, the ANOVA analysis of variance was used. Before that, the required assumptions, such as the normality of the distribution of variables, were checked (W. Shapiro-Wilk test), and the homogeneity of variance (Levene's or Brown Forsythe's tests). The Welch test was used in the few cases where heterogeneity of variance was found. However, when the variables did not meet the condition of normal distribution, the nonparametric Kruskal-Wallis test was used. When the above-mentioned analyses confirmed the presence of statistically significant differences, the Tukey's multiple comparison test was performed. This test, called the posthoc test, enables the grouping of means and isolating of homogeneous groups. The significance level p = 0.05 was adopted in the analyses. The obtained results are presented in the form of graphs, in which the mean values and their standard deviations were presented.

Visual Characteristics of Injection Mouldings
The results of the color and gloss measurements are shown in  Color parameters L, a, b of the injection mouldings are shown in Figures 2-4, grouped into the mass content and the grain size of the introduced bran fraction. For comparative purposes, Figure 2 also shows the results obtained for mouldings made of polyethylene alone. Measurements were carried out in two parts of the mouldings marked at the measurement points (Figure 1), A-on the expansion of the mouldings on the side of the plastic supply through the pinpoint gate and B-on the expansion of the mouldings on the side opposite to the pinpoint gate, at the end of the moulding cavity. It was observed that introducing 5% bran content into polyethylene caused a distinct color change ∆E = 15.9-19.2. The color of the samples changed towards the shades of yellow and red, and at the same time, the samples darken, as evidenced by the decrease in the luminance L value. Differences in the color of the compacts related to the grain size of the introduced bran fraction were also observed. With the increase in the size of the grains while maintaining the same mass content, the differences in color increased, starting from the means 2 < ∆E < 3.5, recognizable by the inexperienced observer, through the clear differences 3.5 < ∆E < 5, and in some cases even large differences ∆E > 5. They are mainly caused by the darkening of the samples, which is reflected in the decrease in luminance L. Within the individual bran fractions, increasing their mass content from 5% to 10% caused a color change towards blue shades, and the b parameter decreased. These changes can be classified from average ∆E = 2.2 for the smallest bran fraction < 0.4 mm and ∆E = 3.6 for the 0.4-0.6 mm fraction, to large color changes ∆E = 6.9 at the largest fraction 0.6-0.8 mm. A further increase in the content of bran from 10% to 15% had a much smaller effect on the color change of the samples. In most cases, they fell within the range 1 < ∆E < 2, which is recognizable only by an experienced observer. experienced observer.
Large changes in color were found along the length of the mouldings-along the flow path of the material (Figures 2-4), which was manifested by different values of the color parameters determined for points A and B. The color of the mouldings became darker on the flow path, as evidenced by the reduction in the luminance L, and changes towards shades of yellow, and parameter b increased. The described differences were most pronounced in the case of the smallest fraction of bran with grain size <0.4mm, where ΔE = 6 with 5% content and, respectively, ΔE = 8.9 and ΔE = 9.1 with 10% and 15% of bran content. In the case of the fractions with larger grains, the observed color differentiation along the length of the mouldings was similar and amounted to approximately ΔE = 5.5-large color differences. Most likely, it is related to the different position of the filler grains along the flow path of the material. It is clearly seen in Figures 6-11 that with a growing distance from the injection point, more and more grains were located at the surface of the sample. Due to their structure, the bran can scatter light rays, which is perceived as a decrease in the brightness at point B of the injection mouldings. At this point, it should be noted that the influence of wheat bran on the color of biocomposites based on polyethylene differs depending on their mass share. It is shown above that when filling with bran up to 15% by mass, the color darkened with an increase in the bran content, which may be associated with a gradual loss of transparency. On the other hand, with a much higher mass fraction of bran up to 50%, with the complete lack of transparency, increasing the mass fraction caused an increase in the brightness of the color of biocomposites based on polyethylene [70].       Figure 1).
Large changes in gloss were also observed along the length of the mouldings ( Figure 5). The addition of bran caused a significant reduction in the gloss of the mouldings on the side of the plastic supply through the gate-point A. The gloss in this area from the value of 46 obtained for PE alone decreased to an average of five for all tested fractions and bran content. This is largely due to the presence of clear traces of the stream front of the material perpendicular to the direction of its flow (photo A in Figures 6-11). These types of traces are most often caused by the pulsating flow of the material, which occurs especially when processing multiphase mixtures of plastics. In this case, the reason is the bran grains flowing through the pinpoint gate. For comparison, Figure 12 shows the appearance of the moulding in point A, made of unfilled polyethylene. In the central part of the    Figure 1). Large changes in color were found along the length of the mouldings-along the flow path of the material (Figures 2-4), which was manifested by different values of the color parameters determined for points A and B. The color of the mouldings became darker on the flow path, as evidenced by the reduction in the luminance L, and changes towards shades of yellow, and parameter b increased. The described differences were most pronounced in the case of the smallest fraction of bran with grain size < 0.4 mm, where ∆E = 6 with 5% content and, respectively, ∆E = 8.9 and ∆E = 9.1 with 10% and 15% of bran content. In the case of the fractions with larger grains, the observed color differentiation along the length of the mouldings was similar and amounted to approximately ∆E = 5.5-large color differences. Most likely, it is related to the different position of the filler grains along the flow path of the material. It is clearly seen in Figures 6-11 that with a growing distance from the injection point, more and more grains were located at the surface of the sample. Due to their structure, the bran can scatter light rays, which is perceived as a decrease in the brightness at point B of the injection mouldings. At this point, it should be noted that the influence of wheat bran on the color of biocomposites based on polyethylene differs depending on their mass share. It is shown above that when filling with bran up to 15% by mass, the color darkened with an increase in the bran content, which may be associated with a gradual loss of transparency. On the other hand, with a much higher mass fraction of bran up to 50%, with the complete lack of transparency, increasing the mass fraction caused an increase in the brightness of the color of biocomposites based on polyethylene [70].       Figure 1). . Figure 8. The appearance of individual parts of the injection moulding obtained with a 5% content of bran fraction with a grain size of 0.4-0.6 mm at measuring points A, M, B (the position of the points is marked in Figure 1).  Figure 1).  . Figure 8. The appearance of individual parts of the injection moulding obtained with a 5% content of bran fraction with a grain size of 0.4-0.6 mm at measuring points A, M, B (the position of the points is marked in Figure 1).  Figure 1).   Figure 1).

Figure 12.
Appearance of the unfilled polyethylene moulding at measuring point A (the position of the point is marked in Figure 1).

Mass Melt Flow Rate
The results of the research on the mass flow rate MFR of samples taken from injection mouldings are shown in Figure 13. Compared to polyethylene alone, for which MFR = 3.3 ± 0.1 g/10 min, a statistically significant decrease in the index value occurred from 10% in the case of fractions with grain size 0.4 mm and 0.4-0.6 mm, while for the 0.6-0.8 mm fraction from the content of 5%. The decrease in the value of the mass melt flow rate with increasing the filler content is the obvious effect of the increase Large changes in gloss were also observed along the length of the mouldings ( Figure 5). The addition of bran caused a significant reduction in the gloss of the mouldings on the side of the plastic supply through the gate-point A. The gloss in this area from the value of 46 obtained for PE alone decreased to an average of five for all tested fractions and bran content. This is largely due to the presence of clear traces of the stream front of the material perpendicular to the direction of its flow (photo A in Figures 6-11). These types of traces are most often caused by the pulsating flow of the material, which occurs especially when processing multiphase mixtures of plastics. In this case, the reason is the bran grains flowing through the pinpoint gate. For comparison, Figure 12 shows the appearance of the moulding in point A, made of unfilled polyethylene. In the central part of the mouldings (photo M in Figures 6-11), these marks take the form of longitudinal lines. At point B of the part, the decrease in gloss value was much smaller. In this area (photo B in Figures 6-11), the aforementioned traces were absent or barely visible. Posthoc analyses of the results of measurements carried out in this area of mouldings showed that the smallest comparable decrease in gloss was caused by a 5% addition of bran fractions with grain sizes of 0.4-0.6 mm and 0.6-0.8 mm. The largest decrease in gloss was observed for the fraction with the smallest grains < 0.4 mm in the range of 5%-15%. Statistically comparable values were obtained for the highest 15% content of the bran fractions with larger grains and 10% of the 0.4-0.6 mm fraction. Figure 11. The appearance of individual parts of the injection moulding obtained with a 15% content of the bran fraction with a grain size of 0.6-0.8 mm at measuring points A, M, B (the positions of the points are marked in Figure 1).

Figure 12.
Appearance of the unfilled polyethylene moulding at measuring point A (the position of the point is marked in Figure 1).

Mass Melt Flow Rate
The results of the research on the mass flow rate MFR of samples taken from injection mouldings are shown in Figure 13. Compared to polyethylene alone, for which MFR = 3.3 ± 0.1 g/10 min, a statistically significant decrease in the index value occurred from 10% in the case of fractions with grain size 0.4 mm and 0.4-0.6 mm, while for the 0.6-0.8 mm fraction from the content of 5%. The decrease in the value of the mass melt flow rate with increasing the filler content is the obvious effect of the increase in the viscosity of the composition due to the presence of fine particles dispersed in the polymer matrix [50,73,[87][88][89]. A clear proportional decrease in the MFR value was observed, on average by 0.16 g/10 min with a 5% increase in the content of the two bran fractions 0.4 mm and 0.4-0.6 mm. Statistical analysis showed that the results obtained with both of these fractions are comparable. A greater decrease in the MFR value was observed for the fraction with a grain size of 0.6-0.8 mm, which is 0.35 g/10 min with an increase in the content by 5%. With the highest content of 15% of this fraction, the lowest MFR value was obtained and equalled 2.4 ± 0.07 g/10 min. Compared to the remaining fractions with smaller grains with the same content, it was a decrease by 0.48 g/10 min, while in relation to the lowest tested content of other fractions, it was a decrease by as much as 0.8 g/10 min (25%). This proves a significantly limited composition flow by large bran particles (0.6-0.8 mm) present in the fraction.  Figure 1).

Mass Melt Flow Rate
The results of the research on the mass flow rate MFR of samples taken from injection mouldings are shown in Figure 13. Compared to polyethylene alone, for which MFR = 3.3 ± 0.1 g/10 min, a statistically significant decrease in the index value occurred from 10% in the case of fractions with grain size 0.4 mm and 0.4-0.6 mm, while for the 0.6-0.8 mm fraction from the content of 5%. The decrease in the value of the mass melt flow rate with increasing the filler content is the obvious effect of the increase in the viscosity of the composition due to the presence of fine particles dispersed in the polymer matrix [50,73,[87][88][89]. A clear proportional decrease in the MFR value was observed, on average by 0.16 g/10 min with a 5% increase in the content of the two bran fractions 0.4 mm and 0.4-0.6 mm. Statistical analysis showed that the results obtained with both of these fractions are comparable. A greater decrease in the MFR value was observed for the fraction with a grain size of 0.6-0.8 mm, which is 0.35 g/10 min with an increase in the content by 5%. With the highest content of 15% of this fraction, the lowest MFR value was obtained and equalled 2.4 ± 0.07 g/10 min. Compared to the remaining fractions with smaller grains with the same content, it was a decrease by 0.48 g/10 min, while in relation to the lowest tested content of other fractions, it was a decrease by as much as 0.8 g/10 min (25%). This proves a significantly limited composition flow by large bran particles (0.6-0.8 mm) present in the fraction.

p-v-T Diagrams
The results of the research on the relationship between pressure p specific volume  and temperature T during isobaric cooling of the tested samples are shown in Figures 14-16.

p-v-T Diagrams
The results of the research on the relationship between pressure p specific volume ν and temperature T during isobaric cooling of the tested samples are shown in Figures 14-16. Figure 14 shows the results for PE alone and with 5% bran content in the form of p-ν-T graphs, while Figure 15 for polyethylene with 10% and 15% bran content.       The course of the curves obtained was similar, but it can be noticed that with increasing pressure, a distinct shift towards higher temperature values underwent a phase change-crystallization temperature, the curves of which corresponded to a rapid decrease in proper volume. The course of the curves differed more for higher pressure values. In the case of polyethylene alone, the reduction in the specific volume in the liquid state was from 0.002 cm 3 /g at the lowest pressure to 0.006 cm 3 /g at the highest pressure for each 10 • C during cooling. In the solid state, it was from 0.006 cm 3 /g at the highest pressure to 0.011 cm 3 /g at the lowest pressure, for each 10 • C. For polyethylene with the highest content of 15% bran, the reduction in the liquid specific volume was from 0.002 cm 3 /g at the lowest pressure to 0.0055 cm 3 /g at the highest pressure for each 10 • C reduction in temperature. However, in the solid state, it was from 0.006 cm 3 /g at the highest pressure to 0.01 cm 3 /g at the lowest pressure, for each 10 • C. Therefore, it can be concluded that the rate of changes in the specific volume along with the decrease in the temperature of PE itself and the examined content of bran is very similar. The specific volume of all tested samples in the solid state decreased with the temperature decrease slightly faster than in the liquid state.

Chemical Structure
In order to verify the impact of the injection process and the addition of a different amount of biofiller with different granulation on the chemical structure of polyethylene extrudate, an ATR-FTIR analysis was performed. FTIR spectra made for brans ( Figure 17) with different granulation were identical. The presence of characteristic absorption bands on the spectra confirms that, regardless of their granulation, bran consists of polysaccharides, phenolic and lipid compounds and proteins, as well as absorbed water [78]. The −OH groups present in polysaccharides, polyphenols and water gave an absorption spectrum at 3311 cm −1 . Vibrations of −OH groups from water absorbed by starch appeared on the spectrum at 1648 cm −1 [90,91]; however, these broad absorption spectra may also come from vibrations of carboxylate groups (present in the pectin structure), vibrations of phenyl rings (present in polyphenols and lignin) and the amide C-N group present in proteins [91]. The absorption spectra at 1546 cm −1 resulting from the vibrations of the N-H amide group also indicate the presence of protein in the bran composition [92]. The absorption spectra at 1743 cm −1 came from the vibration of the C=O groups present in the carbonyl compounds building the structure of pectins and hemicellulose [91,93]. The presence of starch, cellulose, hemicellulose, pectin and lignin in the bran is confirmed by the absorption spectra visible at 1150, 1078, 1017 and 999 cm −1 , which came from the vibrations of the C−O−C and C−O groups.
On the other hand, the FTIR spectrum of polyethylene ( Figure 17) showed characteristic absorption spectra at 2916, 2849, 1471 and 1377 cm −1 , resulting from stretching and deformation vibrations of methylene groups. Additionally, the presence of the absorption spectra at 1377 cm −1 proves the presence of branches in the structure of the linear PE. In the FTIR spectra of PE extrudate with bran ( Figure 17), absorption bands are observed in the spectra of both components. However, the absorption bands were due to the vibrations of the C−O−C and C−O groups and were shifted slightly towards higher wavenumbers. Due to the content of bran, the decrease in specific volume in the area of phase transformation was clearly smaller, and due to the phase transformation taking place in a wider temperature range, it became decreasingly clear. These changes increase with increasing pressure. The reduction in specific volume as a result of cooling from 155 to 35 • C was at a pressure of 20 MPa for polyethylene 0.177 cm 3 /g (decrease by 16.5%), and for polyethylene with 15%, bran 0.159 cm 3 /g (decrease by 15.4%), while at a pressure of 120 MPa, 0.148 cm 3 /g (decrease by 14.2%) and 0.133 cm 3 /g (decrease by 13.3%).
The lower specific volume was that of polyethylene samples with the highest 15% bran content of 0.996 cm 3 /g (at a temperature of 35 • C and a pressure of 120 MPa). Under the same conditions, the specific volume of the samples with a content of 10% bran was 1.010 cm 3 /g, with a 5% hemline content of 1.021 cm 3 /g, and polyethylene alone, 1.040 cm 3 /g.
There was no effect of the applied bran fraction in the studied range on the determined p-v-T relationships. The obtained curves (Figure 16) with the highest 15% content of both fractions with grain sizes < 0.4 mm and 0.6-0.8 mm had almost the same course, and small differences were included in the measurement errors.

Chemical Structure
In order to verify the impact of the injection process and the addition of a different amount of biofiller with different granulation on the chemical structure of polyethylene extrudate, an ATR-FTIR analysis was performed. FTIR spectra made for brans ( Figure 17) with different granulation were identical. The presence of characteristic absorption bands on the spectra confirms that, regardless of their granulation, bran consists of polysaccharides, phenolic and lipid compounds and proteins, as well as absorbed water [78]. The −OH groups present in polysaccharides, polyphenols and water gave an absorption spectrum at 3311 cm −1 . Vibrations of −OH groups from water absorbed by starch appeared on the spectrum at 1648 cm −1 [90,91]; however, these broad absorption spectra may also come from vibrations of carboxylate groups (present in the pectin structure), vibrations of phenyl rings (present in polyphenols and lignin) and the amide C-N group present in proteins [91]. The absorption spectra at 1546 cm −1 resulting from the vibrations of the N-H amide group also indicate the presence of protein in the bran composition [92]. The absorption spectra at 1743 cm −1 came from the vibration of the C=O groups present in the carbonyl compounds building the structure of pectins and hemicellulose [91,93]. The presence of starch, cellulose, hemicellulose, pectin and lignin in the bran is confirmed by the absorption spectra visible at 1150, 1078, 1017 and 999 cm −1 , which came from the vibrations of the C−O−C and C−O groups.

Thermal Properties
In order to determine the effect of the amount of bran of various sizes on the glass transition temperature, crystallization temperature, melting point and crystallinity of injection mouldings, tests were carried out using differential scanning calorimetry. Table 2 presents numerical data characterizing selected thermal properties and the level of crystallinity, calculated on the basis of DSC curves ( Figure  18). The glass transition temperatures of the samples determined from the first heating cycle were noticeably higher than for pure PE. However, the Tg values determined for the samples from the second heating cycle, after removing their thermal history, were almost identical for PE and for samples filled with bran with the smallest granulation. Higher Tg values were maintained for samples obtained from bran with higher granulation.
Compared to pure polyethylene, the melting point of the composites, determined from both the first and second heating cycles, was slightly higher, whereas the crystallization temperature was lower. More noticeable differences in Tm values were visible in the case of the first heating cycle. This increase may indicate that the structure of the mouldings contains larger PE crystallites than in pure PE. On the other hand, after cooling the samples and reheating, the internal structure was probably rearranged and smaller PE crystallites formed, and thus the Tm of the extrudate is comparable to the Tm of pure PE. On the other hand, the observed degree of crystallinity was clearly higher than for pure PE, and the Xc values determined from the second heating cycle were greater than those from the first heating cycle. In a previous work [70], the effect of bran addition on Xc of PE expeller was presented, and we noticed that the Xc values were lower for the extrudate than for the initial PE. However, it should be taken into account that the amount of biofiller used was much higher. Previously, it was as much as 50% of bran, whereas in this case, 5%-15% by weight was used. The hydrophilic structure of the bran, mainly due to the presence of hydroxyl groups in their chemical structure, is not fully compatible with the structure On the other hand, the FTIR spectrum of polyethylene ( Figure 17) showed characteristic absorption spectra at 2916, 2849, 1471 and 1377 cm −1 , resulting from stretching and deformation vibrations of methylene groups. Additionally, the presence of the absorption spectra at 1377 cm −1 proves the presence of branches in the structure of the linear PE. In the FTIR spectra of PE extrudate with bran ( Figure 17), absorption bands are observed in the spectra of both components. However, the absorption bands were due to the vibrations of the C−O−C and C−O groups and were shifted slightly towards higher wavenumbers.

Thermal Properties
In order to determine the effect of the amount of bran of various sizes on the glass transition temperature, crystallization temperature, melting point and crystallinity of injection mouldings, tests were carried out using differential scanning calorimetry. Table 2 presents numerical data characterizing selected thermal properties and the level of crystallinity, calculated on the basis of DSC curves ( Figure 18). The glass transition temperatures of the samples determined from the first heating cycle were noticeably higher than for pure PE. However, the T g values determined for the samples from the second heating cycle, after removing their thermal history, were almost identical for PE and for samples filled with bran with the smallest granulation. Higher T g values were maintained for samples obtained from bran with higher granulation. Table 2. Glass transition, crystallization and melting temperatures and degree of crystallinity of polyethylene and its composites with bran obtained on the basis of DSC tests.

Sample
Heating I Cooling Heating II lamella areas, as postulated by the authors of the study [95]. A similar effect was also observed for PE composites with halloysite nanotubes [96].  The thermal stability of the obtained biocomposites was determined on the basis of a thermogravimetric analysis carried out in an atmosphere of synthetic air. Figure 19 shows the TG and DTG curves of bran with different granulation and PE and the samples obtained with 5% and 15% by mass of filler. On the basis of the TG curves, it can be concluded that the thermal resistance of the bran is not influenced by its particle size. For all bran fractions, a 6% weight loss to the temperature of about 150 °C was observed on the TG curves. This is related to water evaporation [70]. At the temperature of about 170 °C, the TG curves of bran showed a loss in mass related to the processes of their degradation. Under measurement conditions, polyethylene presented thermal stability up to 270 °C ; therefore, when selecting the processing temperature, the thermal resistance of the biofiller was of prime importance. Considering the TG curves of biocomposites containing bran with different particle sizes, it can be concluded that both samples containing 5% by weight and 15% by weight of bran are thermally stable up to about 250 °C. The first weight loss, whose maximum rate was observed at about 290 °C, was related to the degradation of the biofiller. It was 5% for samples containing 5% bran and 16% for samples containing 15% bran. Such a loss of mass was not observed on the TG curve of pure PE, which was thermally more resistant than the tested composites. However, the course of TG curves of pure PE and its composites is surprising. The rate of thermal decomposition of PE was higher than for composites. An expression of this difference may be the temperature at which 50% of the sample decomposed (T50%). For PE, T50% was 406 °C, for composites containing 15% bran with increasing particle size: 416, 421 and 424 °C, and for composites containing 5 bran: 421, 412 and 400 °C , respectively. Only the extrudate containing 5% bran with the largest grain diameter had a lower T50% value than PE. The remaining extrudates decomposed slower than PE, which was already explained by the slower diffusion Compared to pure polyethylene, the melting point of the composites, determined from both the first and second heating cycles, was slightly higher, whereas the crystallization temperature was lower. More noticeable differences in T m values were visible in the case of the first heating cycle. This increase may indicate that the structure of the mouldings contains larger PE crystallites than in pure PE. On the other hand, after cooling the samples and reheating, the internal structure was probably rearranged and smaller PE crystallites formed, and thus the T m of the extrudate is comparable to the T m of pure PE. On the other hand, the observed degree of crystallinity was clearly higher than for pure PE, and the X c values determined from the second heating cycle were greater than those from the first heating cycle. In a previous work [70], the effect of bran addition on X c of PE expeller was presented, and we noticed that the X c values were lower for the extrudate than for the initial PE. However, it should be taken into account that the amount of biofiller used was much higher. Previously, it was as much as 50% of bran, whereas in this case, 5%-15% by weight was used. The hydrophilic structure of the bran, mainly due to the presence of hydroxyl groups in their chemical structure, is not fully compatible with the structure of PE chains, which probably hinders the formation of PE crystallites on the bran surface [94]. However, when using up to 20% of bran, they act as a kind of separator. They separate low-density polyethylene chains that have a significant amount of short-chain branching hindering crystallization, allowing the formation of crystallites from long-chain PE fragments.
In the DSC curves (Figure 18), there was one more transition region in the temperature range 30-50 • C from the first heating cycle. The most visible phase transition in this temperature range was shown by the extrudates containing the bran of the highest granulation. The lack of this transition in the DSC curves from the second heating cycle may suggest that it is related to the local melting of the smallest lamella areas, as postulated by the authors of the study [95]. A similar effect was also observed for PE composites with halloysite nanotubes [96].
The thermal stability of the obtained biocomposites was determined on the basis of a thermogravimetric analysis carried out in an atmosphere of synthetic air. Figure 19 shows the TG and DTG curves of bran with different granulation and PE and the samples obtained with 5% and 15% by mass of filler. On the basis of the TG curves, it can be concluded that the thermal resistance of the bran is not influenced by its particle size. For all bran fractions, a 6% weight loss to the temperature of about 150 • C was observed on the TG curves. This is related to water evaporation [70]. At the temperature of about 170 • C, the TG curves of bran showed a loss in mass related to the processes of their degradation. Under measurement conditions, polyethylene presented thermal stability up to 270 • C; therefore, when selecting the processing temperature, the thermal resistance of the biofiller was of prime importance. Considering the TG curves of biocomposites containing bran with different particle sizes, it can be concluded that both samples containing 5% by weight and 15% by weight of bran are thermally stable up to about 250 • C. The first weight loss, whose maximum rate was observed at about 290 • C, was related to the degradation of the biofiller. It was 5% for samples containing 5% bran and 16% for samples containing 15% bran. Such a loss of mass was not observed on the TG curve of pure PE, which was thermally more resistant than the tested composites. However, the course of TG curves of pure PE and its composites is surprising. The rate of thermal decomposition of PE was higher than for composites. An expression of this difference may be the temperature at which 50% of the sample decomposed (T50%). For PE, T50% was 406 • C, for composites containing 15% bran with increasing particle size: 416, 421 and 424 • C, and for composites containing 5 bran: 421, 412 and 400 • C, respectively. Only the extrudate containing 5% bran with the largest grain diameter had a lower T50% value than PE. The remaining extrudates decomposed slower than PE, which was already explained by the slower diffusion of heat into the interior of the sample, especially at higher temperatures, and on the other hand, by hindered diffusion from the interior of the sample by products of gaseous decomposition [70].

Longitudinal Contraction
The influence of the mass content and grain size of the introduced bran fraction on the longitudinal shrinkage of the mouldings is shown in Figure 20. The nonparametric statistical tests performed showed that there were significant differences between the shrinkage values of the compared samples. Subsequent posthoc tests showed that the longitudinal shrinkage of samples made of polyethylene alone before aging was not significantly different from that occurring after aging, and that the introduction of bran within the tested range did not significantly affect the shrinkage of the samples before aging. However, there were significant differences in shrinkage between samples before aging with the content of 5%-15% of the smallest bran fraction <0.4 mm and samples after aging with the highest content of 15% of the fraction 0.4-0.6 mm and the fraction 0.6-0.8 mm in the entire range of 5%-15%. The shrinkage of samples before aging containing 15% bran of all types of fractions also differed significantly from that occurring after aging in samples containing 5%-15% of the largest fraction 0.6-0.8 mm and the highest 15% fraction content of 0.4-0.6 mm. The lowest shrinkage value of 2.15 ± 0.05% was observed in samples before aging with 15% bran content of the smallest fraction < 0.4 mm, while the highest value of 2.53% ± 0.01% in the samples after aging with 15% content of the largest fraction of bran, 0.6-0.8 mm.
If the filler used is not subject to the processing shrinkage to the same extent as the polymer matrix, it leads to a shrinkage of the material on the filler grains by compressive stress, while the matrix by tensile stresses. Long-term exposure to compressive stresses could lead to the collapse of the bran particles, which at least partially explains the obtained results of the shrinkage of the mouldings [97]. of heat into the interior of the sample, especially at higher temperatures, and on the other hand, by hindered diffusion from the interior of the sample by products of gaseous decomposition [70].

Longitudinal Contraction
The influence of the mass content and grain size of the introduced bran fraction on the longitudinal shrinkage of the mouldings is shown in Figure 20. The nonparametric statistical tests performed showed that there were significant differences between the shrinkage values of the compared samples. Subsequent posthoc tests showed that the longitudinal shrinkage of samples made of polyethylene alone before aging was not significantly different from that occurring after aging, and that the introduction of bran within the tested range did not significantly affect the shrinkage of the samples before aging. However, there were significant differences in shrinkage between samples before aging with the content of 5%-15% of the smallest bran fraction <0.4 mm and samples after aging with the highest content of 15% of the fraction 0.4-0.6 mm and the fraction 0.6-0.8 mm in the entire range of 5%-15%. The shrinkage of samples before aging containing 15% bran of all types of fractions also differed significantly from that occurring after aging in samples containing 5%-15% of the largest fraction 0.6-0.8 mm and the highest 15% fraction content of 0.4-0.6 mm. The lowest shrinkage value of 2.15 ± 0.05% was observed in samples before aging with 15% bran content of the smallest fraction <0.4 mm, while the highest value of 2.53% ± 0.01% in the samples after aging with 15% content of the largest fraction of bran, 0.6-0.8 mm.
If the filler used is not subject to the processing shrinkage to the same extent as the polymer matrix, it leads to a shrinkage of the material on the filler grains by compressive stress, while the matrix by tensile stresses. Long-term exposure to compressive stresses could lead to the collapse of the bran particles, which at least partially explains the obtained results of the shrinkage of the mouldings [97].

Static Tensile Test
The obtained results of the Young's modulus measurements for the tested samples are shown in Figure 21. The tests showed that a significant increase in the modulus of the tested samples, compared to those made of PE alone, occurred after introducing at least 10% of bran. Before aging, a significant

Static Tensile Test
The obtained results of the Young's modulus measurements for the tested samples are shown in Figure 21. The tests showed that a significant increase in the modulus of the tested samples, compared to those made of PE alone, occurred after introducing at least 10% of bran. Before aging, a significant increase in the modulus for all wheat bran particles sizes by an average of 34 MPa occurred between the extreme contents of 5% and 15% of bran. Comparing the obtained results after the aging, it was found that from 10% of the bran content, there was a significant increase in the Young's modulus in comparison to the samples before the aging. Aging caused a marked increase in the modulus along with the content of all examined bran fractions. The maximum difference between the 5% and 15% samples was 71 MPa on average. On the other hand, no effect of the size of the bran grains on Young's modulus before and after aging was observed. The values of Young's modulus of samples from polyethylene alone before and after aging also showed no significant differences. The conducted tests showed a statistically significant reduction in tensile strength of the samples with the highest content of 15% with all the tested bran fractions compared to those made of polyethylene alone (Figure 22), both before and after aging. This reduction was on average 1.46 MPa for samples before aging and 1.11 MPa for samples after aging. However, the posthoc tests did not show any significant differences in the strength of the samples made of polyethylene alone before and after aging, or the aging effect on the strength of the compared samples with the same content and size of bran grains. On the other hand, there were differences between the samples before aging with the lowest 5% and the highest 15% content of all examined bran fractions. These samples, after aging, showed no significant differences in strength. However, an increase in the dispersion of the obtained measurement results was observed in their case.
A similar effect of the tested factors was observed on the elongation at break of the tested specimens ( Figure 23). The elongation values of polyethylene mouldings alone before and after aging did not differ. A statistically significant reduction in the elongation at break of the samples as compared to PE alone was observed after the introduction of 10% and 15% bran from all tested fractions. As the content of the bran fraction with the smallest particles <0.4 mm increased, the elongation at break decreased to the greatest extent from 360% to 52%. For the 0.4-0.6 mm fraction, the maximum reduction in elongation before aging was 168%, while for the 0.6-0.8 mm fraction, it was only 49%. In most cases, there was no statistically significant effect of aging on the elongation at break of the compared samples with the same content and size of the bran fraction.
Lignocellulose is characterized by better stiffness than most of the polymeric materials commonly used for the production of biocomposites with natural fillers [98,99]. Therefore, increasing the filler content increases the value of the modulus of elasticity. In the absence of modification of the adhesion The conducted tests showed a statistically significant reduction in tensile strength of the samples with the highest content of 15% with all the tested bran fractions compared to those made of polyethylene alone (Figure 22), both before and after aging. This reduction was on average 1.46 MPa for samples before aging and 1.11 MPa for samples after aging. However, the posthoc tests did not show any significant differences in the strength of the samples made of polyethylene alone before and after aging, or the aging effect on the strength of the compared samples with the same content and size of bran grains. On the other hand, there were differences between the samples before aging with the lowest 5% and the highest 15% content of all examined bran fractions. These samples, after aging, showed no significant differences in strength. However, an increase in the dispersion of the obtained measurement results was observed in their case.
A similar effect of the tested factors was observed on the elongation at break of the tested specimens ( Figure 23). The elongation values of polyethylene mouldings alone before and after aging did not differ. A statistically significant reduction in the elongation at break of the samples as compared to PE alone was observed after the introduction of 10% and 15% bran from all tested fractions. As the content of the bran fraction with the smallest particles <0.4 mm increased, the elongation at break decreased to the greatest extent from 360% to 52%. For the 0.4-0.6 mm fraction, the maximum reduction in elongation before aging was 168%, while for the 0.6-0.8 mm fraction, it was only 49%. In most cases, there was no statistically significant effect of aging on the elongation at break of the compared samples with the same content and size of the bran fraction.

Conclusions
The conducted research showed a significant influence of wheat bran on the appearance of the obtained injection mouldings. There were both clear changes in the color towards shades of yellow and red with an increase in the content of bran, and significant differences in the color and gloss of injection mouldings along with the distance from the injection point. In the immediate vicinity of the gate, clear traces of pulsating flow of the stream of the material were observed, affecting the visual quality, which blurred with distance, affecting the gloss gradient along the length of the samples. The intensity of the observed pulsating flow lines depended on both the content and the size of the filler grains. Moreover, the farther away from the gate, the more filler grains accumulate at the surface of the forming cavity, thus affecting the color perception. The obtained results of the research on visual properties suggest that

Conclusions
The conducted research showed a significant influence of wheat bran on the appearance of the obtained injection mouldings. There were both clear changes in the color towards shades of yellow and red with an increase in the content of bran, and significant differences in the color and gloss of injection mouldings along with the distance from the injection point. In the immediate vicinity of the gate, clear traces of pulsating flow of the stream of the material were observed, affecting the visual quality, which blurred with distance, affecting the gloss gradient along the length of the samples. The intensity of the observed pulsating flow lines depended on both the content and the size of the filler grains. Moreover, the farther away from the gate, the more filler grains accumulate at the surface of the forming cavity, thus affecting the color perception. The obtained results of the research on visual properties suggest that Lignocellulose is characterized by better stiffness than most of the polymeric materials commonly used for the production of biocomposites with natural fillers [98,99]. Therefore, increasing the filler content increases the value of the modulus of elasticity. In the absence of modification of the adhesion between the polymer matrix and the filler, despite the increase in stiffness, the brittleness increases, which is manifested in a reduction in tensile strength and often a significant reduction in deformability. The nature of the fracture itself also changes from plastic to brittle [32,46,55,67,99,100]. Significant differences in the values of Young's modulus before and after aging may be related to the observed changes in the longitudinal contraction of mouldings in the analyzed period of time. The potential collapse of the filler grains during the aging period could lead to an increase in internal stresses [90], which translates to the observed increase in stiffness.

Conclusions
The conducted research showed a significant influence of wheat bran on the appearance of the obtained injection mouldings. There were both clear changes in the color towards shades of yellow and red with an increase in the content of bran, and significant differences in the color and gloss of injection mouldings along with the distance from the injection point. In the immediate vicinity of the gate, clear traces of pulsating flow of the stream of the material were observed, affecting the visual quality, which blurred with distance, affecting the gloss gradient along the length of the samples. The intensity of the observed pulsating flow lines depended on both the content and the size of the filler grains. Moreover, the farther away from the gate, the more filler grains accumulate at the surface of the forming cavity, thus affecting the color perception. The obtained results of the research on visual properties suggest that when designing moulds intended for injection of biocomposites with a natural filler, it will be most advantageous to locate point gates at the bottom of the manufactured elements. The lines formed during the pulsating flow of the material will not adversely affect the visual perception of the finished products. Due to this, the dull and rougher area around the gate will be located in a less visible place, and the flow lines will be blurred before reaching the places visible during standard use. However, the color in the most visible places will be more uniform. The melt flow rate, which is the primary index of processability, decreased with increasing bran content, due to an obvious increase in the viscosity of the composition. Nevertheless, the composition can be easily processed by injection moulding, and the obtained MFR values do not constitute a prerequisite for the use of processing aids with the analyzed filler contents.
The p-v-T tests showed that the content of bran reduces the decrease in specific volume during crystallization, which translates into lower processing shrinkage during injection. Measurements of processing shrinkage after a long storage period show that shrinkage progresses over time, which in turn may be the result of compressive stresses around the filler grains during the cooling of the compact.
The analysis of the chemical structure by means of FTIR spectroscopy shows the presence of components typical of lignocellulosic materials, but also the presence of water in the filler structure and branches in the polyethylene structure.
The obtained DSC results suggest the presence of changes in the polyethylene structure caused by the presence of bran, as evidenced by the differences in the melting point and the degree of crystallinity of the composites and unfilled PE during the 1st heating cycle, the absence of these differences during the 2nd heating cycle and the reorganization of the composite structure. Considering the thermal resistance of biocomposites under air atmosphere, it can be concluded that they are thermally stable up to about 250 • C. The first weight loss observed at about 290 • C on TG curves relates to the degradation of the biofiller; the second one at about 400 • C is connected with PE degradation.
The obtained values of tensile strength properties are typical for natural fillers, i.e., an increase in stiffness and brittleness as well as a decrease in tensile strength and deformability. The tests showed that for the tested contents, the deterioration of the tensile strength is on average about 1.4 MPa, i.e., 10% of the value for the content of 15wt.% of filler. The use of such a wheat bran content will allow the significant reduction in the production costs and polyethylene consumption while maintaining comparable strength.
It is important, however, that no deterioration of the mechanical characteristics of the tested composites after aging was observed. The tested materials are, therefore, suitable for use in mass production and do not lose their properties during long-term storage. As a result, the tested biocompositions may be suitable for the production of elements with seasonal use, which often require several months of storage. Funding: The project/research was financed in the framework of the project Lublin University of Technology-Regional Excellence Initiative, funded by the Polish Ministry of Science and Higher Education (contract no. 030/RID/2018/19).