Introduction

Distillers grains (DG) are a by-product of breweries. They are generally considered an industrial waste [1]. According to the Renewable Fuels Association, the production of ethanol in the USA has increased to 30 billion liters from 1980 to 2010, and DG as a by-product has amounted to 250 million metric tons [2]. In China, the amount of ethanol production increases yearly [3,4,5], which is associated with 25 million tons of DG. Balat et al. [6] indicated that the global production of bio-ethanol in 2000 increased from 170 to 460 l. Proskurina et al. [7] reported in 2019 that the development of bioenergy (from ethanol as biofuel) showed a steady growth trend. With the production of wine and biofuel, there is also a huge quantity of DG being produced, and this amount translates to a lot of waste. Hence, how to dispose of DG efficiently should be a top priority.

Traditional treatments for DG include composting and animal husbandry [8,9,10,11,12]. Leonardi et al. [13] reported that DG as a feedstock reduced the butterfat content of cow. Wu et al. [14] found that using DG as animal feeds was not good because animals may be fed with mycotoxins. Therefore, the traditional ways are not satisfactory enough for the efficient disposal of DG. We need other methods to deal with DG whose accumulation is constantly increasing.

Problems with environmental pollution have been aggravated because so much bio-based fillers are being generated: wood flour [15,16,17,18], DG [19], shell powder [20, 21], bagasse [22, 23], rice husk [24,25,26], eggshell [27], and tapioca [28]. Bio-fillers are renewable, resource-rich, cheap, and eco-friendly, in contrast to inorganic fillers that are non-renewable.

Toro et al. [29] contrasted the different types of fillers (inorganic fillers and bio-fillers), which were incorporated into eggshell/polypropylene composites through melt-blending. The mechanical properties of the resultant composites were more robust because of the reinforcement provided by the eggshell. Tsou et al. [30] used a twin-screw extruder to mix polylactide with rice husk, and added methylene diphenyl diisocyanate as an interfacial compatibilizer; the result was a renewable composite with enhanced tensile strength and biodegradation for 3D printing applications. Hence, biowaste can give improved characteristics to polymer matrices and boost the composite properties.

Polyethylene (PE) is one of most common polymers in the world and has been applied to many fields (e.g., in agriculture, packaging) [31,32,33]. PE was sold for $1.85–2.27/kg (0.91–1.12/lb) in 2012 [34]. If we added DG to PE to form semi-degradable DG/PE composites with a required total weight, the amount of PE that was otherwise required would be reduced, as DG was the other component that would contribute for the total weight; thus, it would lead to cost efficiency.

Studies on DG and polymer composites are few. Even fewer studies on DG/PE composites have been reported (i.e., the use of recycled DG in PE has not been investigated adequately). Luo et al. [35] examined DG as fillers for low density polyethylene (LDPE). They introduced 10–40% DG treated with 5% NaOH solution into the LDPE matrix, and found that DG had great potential application in thermoplastic composites. However, because LDPE lacked functional groups and had poor compatibility with DG, the mechanical properties (elongation at break and tensile strength) of the composites were considerably lower than those of pure HDPE.

To explore the potential application of DG as fillers in HDPE composites and to improve the properties of the composites, we modified HDPE (to give MHDPE) and added a content of DG as high as 50% to the MHDPE matrix. DG from waste disposal sites was recycled and blended with HDPE or MHDPE using a mixer to obtain DG/HDPE or DG/MHDPE composites. The mechanical property, thermostability, crystallinity, and compatibility were determined.

Experiment

Materials

HDPE (LH503) was produced from USI Co., Ltd. (Taipei, Taiwan), whereas HDPE-COOH is similar to that prepared by previous studies [36]. DG came from Sichuan Brewery (China).

Pretreatment of DG

Figure 1 shows the flow for preparing DG/HDPE and DG/MHDPE composites. DG was washed and dried at 80 °C. Afterward, it was ground to a powder form (200–300 mesh) using a vibration mill (Shanghai, China). After several times of crushing and grinding, DG flour (200–300 mesh) was produced. Finally, the DG flour was dried at 105 °C and kept for later use.

Fig. 1
figure 1

Preparation of DG/HDPE and DG/MHDPE composites

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Preparation of DG/HDPE and DG/MHDPE composites

Raw materials in varying proportions were weighed and combined to prepare DG-modified composites of HDPE and MHDPE. Table 1 shows the obtained composition. DG/HDPE and DG/MHDPE composites were prepared using a HAAKE mixer (Thermo Fisher Scientific, Germany), which was operated at a speed of 90 rpm and at 160 °C. The composites came out brown in color, and they were vacuum-dried at 80 °C for 8 h, then at 120 °C for 3 h prior to hot-pressing. Thermocompression was implemented at 160 °C and 15 MPa for 3 min. Afterward, the composites were cooled at room temperature. Ultimately, sheets of DG/HDPE and DG/MHDPE were produced.

Table 1 Composition of DG-modified HDPE and MHDPE composites
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Characterization

Mechanical tests were performed using a microcomputer-controlled electronic universal testing machine (Furbs, Xiamen Furbusi Testing Equipment Co., Ltd., Xiamen, China), according to the standard method of ASTM D638 Type IV. The test speed was 50 min/mm. Five samples were measured, and the results averaged. The tensile strength (σ, MPa), yield strength (σ, MPa), and elongation at break (ɛ, %) were calculated using the following equations:

$$ \sigma =\frac{P}{bd}\varepsilon =\frac{\mathrm{G}\hbox{-} {\mathrm{G}}_0}{{\mathrm{G}}_0}\times 100\% $$

where P was the maximum load (MPa); b and d were width and thickness (mm), respectively; G0 was the original gauge length of samples (mm); G was the pitch spacing (mm) when the sample broke.

Differential scanning calorimetry (DSC) of samples was conducted with a DSC-200 F3 calorimeter (Netzsch, Germany) under nitrogen. Samples were sealed hermetically in DSC pans and heated to 300 °C from room temperature at a rate of 10 min/°C, after which they were cooled to room temperature at a speed of 10 °C/min, then reheated to 300 °C with the same heating rate, to examine their melting and crystallization behavior, such as melting temperature (Tm) and crystallization temperature (Tc). The crystallinity (Xc) was evaluated from the following relationship:

$$ Xc\left(\%\right)=\frac{\varDelta {H}_m}{\left(1-\alpha \right)\varDelta {H}_m^0}\times 100\% $$

where α was the content of DG, ΔHm was the measured melting enthalpy, and ΔHm0 was the enthalpy of 100% HDPE crystals (293 J/g) [37].

Thermogravimetric analysis (TGA) was conducted using TGA instruments (Model STA 409PC, Netzsch, Germany); the heating was done under nitrogen environment from room temperature to 650 °C at a scan rate of 10 °C/min. The thermal decomposition temperature (Td) and DTG (derivative of TGA) were obtained.

The crystalline structure and crystallinity were evaluated using X-ray diffraction (XRD, Bruker-D2 HASER, Germany) equipped with Cu-Kα radiation; the data were recorded every 0.02 s at room temperature, and the test range was 5–90°.

Scanning electron microscopy (SEM, TESCAN, Czech) was used to observe the morphology of the fractured surface of samples subjected to tensile testing. Sample fragments were attached to a sample holder by using conductive adhesives, and were sputtered with a thin layer of gold, which was necessary to obtain distinct surface SEM images.

Fourier-transmission infrared spectroscopy (Nicolet 6700 FTIR spectrometer) was operated in transmission mode over a 4000–400 cm−1 wavelength range, with a resolution of 1 cm−1. Samples of pure HDPE and its composites, which were reduced in powder form, were mixed with KBr and subsequently pressed to shapes of disk.

Contact angles were measured using a contact angle meter (JC2000D, Shanghai, China). Samples were dried in an oven at 80 °C for 8 h before they were mounted on a platform, onto which 2 μL of DI water was dropped from a syringe. Contact angle data were recorded by a computer software.

Tsou et al. [30] described a method of water absorption measurement. All samples were dried in an oven at 80 °C for 8 h, then weighed before they were immersed in DI water for 24 h. After that period, the sample surface was gently dabbed with a tissue to remove the water droplets on the surface. Water absorption was calculated from the following equation:

$$ WA=\frac{W_0-W}{W_0}\times 100\% $$

where W0 was the initial weight and W was the measured weight.

Results and discussion

Mechanical property

Figure 2 presents data on yield strength, tensile strength, and elongation at break of HDPE, MHDPE, and DG/HDPE and DG/MHDPE composites. The tensile properties of MHDPE were shown to be lower than those of the neat HDPE; this result is similar to that of previous studies [36, 38]. After the polymer modification, the mechanical properties were slightly reduced. When MHDPE was blended with DG, the resultant composite exhibited higher yield strength than pure HDPE (Fig. 2a); it means that DG acted as a reinforcement agent in the HDPE matrix, because it increased the ability of the material to recover from permanent deformation. However, both the tensile strength (Fig. 2b) and elongation at break (Fig. 2c) showed a downward trend with increasing amounts of DG. Modified composites had better strength and toughness than pure HDPE. The strength and toughness of the composites decreased because of the weak interfacial binding force between DG as a biomaterial and the HDPE matrix [39, 40]. But the functional group in MHDPE improved the interfacial compatibility between the filler and the host polymer; therefore, DG/MHDPE composites had better mechanical properties than DG/HDPE composites.

Fig. 2
figure 2

Yield strength, tensile strength, and elongation at break of HDPE, MHDPE, and their composites

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Differential scanning calorimetry

Figure 3 depicts the second heating and cooling DSC curves. The melting temperatures of the composites were lower than those of the original HDPE (Table 2); this may be due to the addition of DG, which made the crystal size of HDPE and MHDPE smaller [34]. Therefore, when the composites were heated, the crystal was disintegrated early, which led to the decrease in melting temperature.

Fig. 3
figure 3

Differential scanning calorimetry for HDPE and its composites: a second heating curves; b second cooling curves

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Table 2 DSC data for HDPE and its composites
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Table 2 lists the DSC data on initial crystallization temperature (Tc), melting temperatures (Tm1 and Tm2), and peak values in cooling curves (Tp1 and Tp2) for HDPE and its composites. Tm1 and Tc1 (Fig. S1a) and Tp1 (Fig. S1b) were obtained from the first heating and cooling scans, respectively. Tc of composites was higher than that of HDPE. The value of (Tp1-Tc1) for HDPE dropped to 5.2 °C from 10.5 °C, and that of (Tp2-Tc2) to 5.1 °C from 9.5 °C. This drop indicated that DG acted as a nucleating agent, promoting the grain size of HDPE to refine and accelerate the crystallization rate and shorten the cycle of product molding [41,42,43]. On the other hand, the crystallinity of 50(DG)/MHDPE was considerably higher than that of HDPE or the other composites. The role of DG as a nucleating agent, as well as the interfacial adhesion between the bio-filler and the matrix, promoted a higher crystal density of the polymer. The good compatibility between DG and MHDPE promoted even dispersion of DG in MHDPE, and this, in turn, effectively increased the amount of nucleation at the initial stage; thus, the crystallization rate of the polymer was enhanced, and the crystal growth was facilitated. As a result, the crystallinity of DG/MHDPE was relatively high [44].

Thermogravimetric analysis

TGA and DTG curves for HDPE and its composites are represented in Fig. 4, and the data are summarized in Table 3. Because DG was added to the host polymer, the initial degradation decreased. According to Table 3, the initial degradation temperature of pure HDPE was 469.1 °C, but the lowest initial degradation temperature of the composites was 261.9 °C. The reason is because hemicellulose in DG was degraded first, leading to weight loss. DG contained a lot of cellulose and free water; hence, the DTG peak at 141.9–144.0 °C and 351 °C referred to water loss and cellulose degradation, respectively [45]. When the temperature reached 478.7–486 °C, DG, HDPE, and MHDPE began to degrade, causing maximal thermal degradation.

Fig. 4
figure 4

Thermogravimetric curves for HDPE and its nanocomposites: a thermogravimetric analysis; b derivative thermogravimetry

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Table 3 TGA and DTG data for HDPE and its composites
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Table 3 also indicates the temperatures at 10 wt% weight loss for HDPE and its composites. These results showed that as the DG was added, the thermal stability of the composites decreased with the increase in DG. Lomakin et al. [46] indicated that the thermal stability of LDPE/cellulose composites was lower than that of the pure sample. So, it is a normal phenomenon for the thermal stability of polymer composites with DG to be lower than that of the neat polymer matrix. In addition, the thermal stability of DG/MHDPE was higher than that of DG/HDPE composites. As reported previously, the adhesion between fillers and polymers had influence on the decomposing behavior of composites [20]. It might be attributed to the stronger adhesion and higher compatibility between DG and MHDPE, which led to better dispersion, and, in turn, better synergy between the two materials. In conclusion based on TGA, the thermal stability of all samples resulted in descending order: HDPE >25(DG)/MHDPE >50(DG)/MHDPE >25(DG)/HDPE >50(DG)/HDPE.

Although the results from TGA indicated that the thermal degradation temperature of composites was lower than that of the pure HDPE, the extreme temperature of HDPE was about 100 °C [47], and the initial cracking temperature in the first stage of all composites in this study was about 260 °C. Therefore, compared with the application of HDPE, the application of composite materials would not be too limited.

X-ray diffraction

Figure 5 shows the X-ray diffraction (XRD) patterns of HDPE and its composites filled with DG. The neat HDPE exhibited obvious reflection peaks at 21.3 and 23.7°, which corresponded to the typical orthorhombic unit cell structures for (110) and (200) reflection planes, respectively. These 2θ values agree with the reported values for HDPE [48, 49]. The two weak peaks at 2θ values of about 29.9 and 36.2° were ascribed to reflection planes (210) and (020), respectively [50]. The two crystalline characteristic peaks for (110) and (200) stayed the same after the addition of DG, demonstrating that the blending with DG did not influence the original crystal structure of the HDPE substrate.

Fig. 5
figure 5

XRD plots of HDPE and its composites

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The crystallinity data were 73, 48.8, 53.8, 50.7, and 76.7% (Table 4), which were calculated using a computer software; the trend was the same as that of the DSC data in terms of crystallinity. From Fig. 5, the intensity of 50(DG)/MHDPE was the strongest; DG improved the crystallinity of the polymer. The results showed that DG dispersed much better in MHDPE than in HDPE; more DG contributed to more efficient nucleation, which was beneficial for increasing the crystal density [44]. The DSC results also manifested that DG was a nucleating agent. Therefore, the crystallinity of DG/MHDPE composite was higher than that of HDPE or the other composites.

Table 4 Crystallinity of HDPE and its composites
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Scanning electron microscopy

SEM images of fractured surfaces of samples are shown in Fig. 6. The DG particles were not uniformly spread throughout the HDPE matrix, and were separated from the matrix (Fig. 6a, b) because the polar groups in DG repelled the nonpolar groups in HDPE [51, 52]; some defects were observed between DG and HDPE. For the DG/MHDPE composites, the DG particles were partially covered by MHDPE, which indicated that the interface bonding in DG/MHDPE composites was better because MHDPE contained partial polar groups. This morphology results are also similar to the results of previous research on rice husks [26, 30, 37], wood floor [18], tapioca [28], and oyster shell [20], which were blended with the polymer. The cross-section of DG/HDPE composites appeared relatively rough compared with that of the modified DG/MHDPE composites. This result confirmed that concentrations greater than 50 wt% DG in the MHDPE matrix led to higher yield strength, crystallinity, and thermal degradation temperature.

Fig. 6
figure 6

SEM images of DG/HDPE and DG/MHDPE composites: a 25(DG)/HDPE; b 50(DG)/HDPE; c 25(DG)/MHDPE; d 50(DG)/MHDPE

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Fourier-transform infrared spectroscopy

Figure 7 illustrates the FTIR spectra of HDPE, DG/HDPE, and DG/MHDPE. HDPE and MHDPE blended with DG exhibited strong peaks in the spectra at absorption bands of 3400 and 1050 cm−1, corresponding to –OH of free water and C–O groups [53, 54], respectively, because there were a lot of hydroxyl groups in DG. DG consists of multiple grains made up of many substances. Its main components are cellulose, hemicellulose, and lignin, so the main functional groups are derived from the –OH groups in cellulose and lignin. The increase in peak intensity at about 3400 cm−1 [55] when DG was added referred to the hydroxyl bond vibration (–OH), arising from the substantial contributions of cellulose in DG. The absorption bands at 1700–1850 cm−1 referred to C=O [56]. A new weak peak at about 1505–1510 cm−1 appeared, referring to C=C of the aromatic skeletal (lignin) resulting from the addition of DG to HDPE [57]. This new peak became strong and shifted to about 1513 cm −1 after MHDPE was blended with DG (Fig. 7b), which might correspond to asymmetric COO− stretching from MHDPE [58, 59]; it appeared when DG was mixed with MHDPE [60]. Therefore, an ester bond formed from the reaction between the carboxyl group in MHDPE and the OH group in lignin or the cellulose in DG. The aforementioned test results proved that DG and MHDPE had a better interfacial adhesion than DG and HDPE.

Fig. 7
figure 7

a FTIR spectra of HDPE, DG/HDPE, and DG/MHDPE composites; b FTIR spectra at 1400–1600 cm−1 range

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Contact angle

Figure 8 shows the results of contact angle tests on HDPE, DG/HDPE, and DG/MHDPE. It demonstrated that the values of contact angle for HDPE and MHDPE were similar; the value for MHDPE was slightly lower than that for HDPE. It could be attributed to the functional group from MHDPE, and it could be expected that COOH can enhance the hydrophilicity of the polymers [38, 56, 61]. The contact angle for pure HDPE was the largest. DG contained a large amount of –OH, which made the sample hydrophilic, so that the contact angle as a whole tended to decrease when DG was added. However, the contact angle for MHDPE was higher than that for the unmodified one, because the esterification reaction consumed the partial hydroxyl groups in DG and the carboxyl groups in MHDPE. The formation of ester bonds was confirmed by the FTIR results.

Fig. 8
figure 8

Contact angle data: (a) HDPE; (b) MHDPE; (c) 25(DG)/HDPE; (d) 25(DG)/MHDPE; (e) 50(DG)/HDPE; (f) 50(DG)/MHDPE

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Water absorption

Figure 9 displays the water absorption for HDPE and for DG/HDPE and DG/MHDPE composites. It indicates that the water absorption data for HDPE and MHDPE were almost the same. It also indicates a trend of increasing water absorption with the DG content. DG made the water absorption rate higher because DG as a bio-filler contained a large amount of hydroxyl groups. The water absorption rate for DG/MHDPE was slightly lower than that for DG/HDPE. This trend and the contact angle data were corroborative.

Fig. 9
figure 9

Water absorption data for HDPE, MHDPE, DG/HDPE, and DG/MHDPE

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Conclusion

In this present study, DG/HDPE and DG/MHDPE composites were prepared and characterized, and their properties were investigated. The results showed that the addition of DG increased the hydrophilicity of the composites and improved the water absorption. From the cross-sectional morphology, it was observed that MHDPE and DG had a better interfacial compatibility; the water absorption rate for DG/MHDPE was slightly lower than that for DG/HDPE, but the mechanical properties of DG/MHDPE were improved. When the content of DG was 50%, the yield strength, crystallization temperature, and crystallinity were higher for the modified composites than for the unmodified ones and pure samples. Therefore, DG can improve the performance of materials and reduce the amount of the polymer matrix. It is a bio-filler with great potential.