3.1. Filler Testing Results
Hop residue after extraction was used to fill the PVC composites and characterized using ATR-FTIR (
Figure 1). A description of the characteristic bands in the spectrum of the hop residue is shown in
Table 1.
On the FTIR spectra, several ranges of bands from the vibration of groups that are characteristic for plant particles were visible (
Figure 1). The first band was associated with vibrations of OH groups in the 3030–3650 cm
–1 range; the second with vibrations of CH, and CH
2 groups; the third with vibrations of C=O groups; the fourth with vibrations of C=C groups in aromatic compounds; the fifth in the 900–1200 cm
–1 range resulting from vibrations of various groups in lignocellulosic materials. To describe the primary components in the hop residue, bands in the 900–1650 cm
–1 range were analyzed. In the work of Adapa [
51,
52], bands characteristic of these materials were described on the basis of studies of pure cellulose, hemicellulose and lignin and literature analyses. The results of this analysis were compared with the description of bands characteristic for primary components (cellulose, hemicellulose and lignin) in the spectra of the hop residue (
Table 2).
Most of the bands in the spectra of pure cellulose, hemicellulose and lignin were visible in the hop residue FTIR spectra. Thermal degradation was analyzed under an inert atmosphere and the results are shown in
Figure 2, while the results from the analysis of mass change (TG) and derived mass change (DTG) curves are shown in
Table 3.
TG curves were used to determine the temperature of 5% weight loss (T5%), weight loss at 180 °C and the residue after degradation at 750 °C. In turn, the DTG curves show the temperature of the maximum degradation rate at each stages of degradation (Tmax1, Tmax2, Tmax3, Tmax4, Tmax5 and Tmax6), maximum degradation rate at these temperatures (Vmax1, Vmax2, Vmax3, Vmax4, Vmax5 and Vmax6) and the mass change at each stage (Δm1, Δm2, Δm3, Δm4, Δm5 and Δm6), respectively.
In the hop residue about 180 °C, there was a mass loss of about 5% associated with the release of easily volatile substances (mostly water), other easily volatile low-molecular weight compounds, lipids and proteins. These products can affect the extrusion of the composites. After hop residue degradation, about 30 wt % of various forms of carbon may promote the formation of char during combustion. In the filler, 2.2% of the water absorbed by the hop residue and 1.4% of the water bound by hydrogen bonds (first degradation step) were lost through dehydration [
53,
54]. This step is associated with the vaporization of volatile components and non-polymeric constituents, i.e., lipids and proteins [
55], and during this stage, about 5 wt % was lost. The next degradation stage occurred from 215 to 280 °C, corresponding to a weight loss of 15%. Most of the degradation products formed in this stage probably came from the degradation of hemicellulose. The next degradation stage was completed at about 370 °C, during which about 29.5 wt % of the hop residue was lost, and most of the degradation products came from the pyrolysis of cellulose [
53]. However, it is important to note that for lignocellulosic components, a range between 220 and 380 °C occurred in the decomposition of cell-wall biopolymers: pectin, hemicellulose, cellulose and lignin [
56,
57]. In the next—fifth—stage of degradation, there was a mass loss of about 8.5%. The degradation rate in this step was four times lower than in the fourth, probably due to a lower (1319 kcal/mol) activation energy of thermal lignin degradation compared to that of cellulose [
57]. In the 380–620 °C range, the decomposition of lignin was observed although it could start as early as about 160 °C and last up to 900 °C [
58]. However, the highest rate of lignin degradation was achieved in the 380–460 °C range [
59]. The next degradation stage was completed at about 725 °C. This step can be attributed to the decomposition of inorganic compounds and the dehydrogenation and aromatization of char [
60,
61,
62]. Approximately 5 wt % was lost.
Figure 3 shows SEM images of hop residue after grinding and drying at different magnifications.
SEM images of hop residue show variation in the shape and size of their particles (
Figure 3a). Perpendicular shaped particles are visible, but also elongated fiber particles. This indicates that they may be the fragments of leaves, stems and, in the case of hop, cones. An analysis of the photograph at a higher magnification indicated that the particles had sizes ranging from several to 250 µm, and micropores were visible inside (
Figure 3b). Moreover, it was found that the filler surface was rough with numerous irregularities, which can positively influence the mechanical adhesion between polymer and filler.
3.2. Results of Composite Tests
Figure 4 shows the FTIR spectra of poly(vinyl chloride) and its composites with description bands characteristic of PVC and those visible in the spectra of the composites from the introduced hope residues.
Figure 4 shows the PVC spectrum with characteristic bands in the 2849–2961 cm
–1 range derived from C–H stretching vibrations in CH–Cl and CH
2 groups. The bands at 1433, 1324 cm
–1 corresponded to CH
2 deformation wagging, while 1244 cm
–1 corresponded to CH deformation rocking. On the other hand, the band at 1096 cm
−1 was associated with the C–C stretch bond on the PVC backbone chain. Other characteristic bands were 968 cm
–1, responsible for C–H wagging vibrations, and 830, 686 and 616 cm
–1 derived from C–Cl stretching [
17,
46]. There was also a distinct band at 1734 cm
–1 associated with the acrylic additives used in the PVC mixture.
The spectrum of the PVC composite containing 30% hop residue showed a band related to the presence of water introduced with the filler and intermolecular-bonded O–H stretching vibrations centered at 3273 cm
–1 (
Figure 4). There were also new characteristic bands associated with the presence of lignocellulosic filler. The band at 1022 cm
–1 originated from bond vibrations of C–O groups in cellulose, hemicellulose and methoxyl groups in lignin [
44,
51,
52]. There was also a clear increase in the intensity of the band at 1096 cm
–1 derived from C–O groups with aromatic backbone vibrations in lignin. In the 1500–1700 cm
–1 range, a 1604 cm
–1 band appeared derived from C=C and C=O groups associated with the aromatic lignin rings (
Table 1) [
51,
52].
A major problem in PVC processing is the significant thermal instability under melt processing conditions. A knowledge of PVC thermostability is particularly important for production using conventional techniques such as extrusion or injection molding. The addition of filler, including those of natural origin, may influence the thermal stability of PVC matrix blends. Therefore, in this study, the thermal stability of the produced materials was determined by Congo red and TGA. Based on a Congo red study of composites with hops, it was found that after the introduction of the filler, the thermal stability increased from 22 min for unfilled material to 100 min for composites with the highest filler content. Such a significant improvement of thermal stability after filler introduction is an extremely advantageous feature of these materials. The probable reason for the longer stability time was that the introduced material had a much lower thermal conductivity. The improvement of thermal stability may be related to the reaction between hydrogen chloride emitted from PVC chains and methyl groups from the methoxyl groups bound to the lignin phenolic rings [
63,
64]. A similar effect was found for the composites of rigid PVC with lignin treated with the copolyacrylate [
65]. The authors concluded that the improved thermal stability of PVC was related to the sterically hindered phenol structure of lignin. The influence of hop residue on the thermal stability of PVC was also characterized based on the TGA thermograms (
Figure 5a,b).
Table 4 summarizes the results of the analysis of curves of mass change (TG) and the derivative of mass change (DTG).
On the basis of the TG curves, the temperature of 2% mass loss (T
2%P), the mass loss at 180 °C (R
180P) and the degradation residue at 600 °C (R
600P) were determined. In turn, the DTG curves were used to determine the temperature of the maximum degradation rate at each stage (T
max1P, T
max2P), the maximum degradation rate at temperatures (V
max1P, V
max2P) and the mass change at each stage: Δm
1P = m
1P–m
2P, Δm
2P = m
2P–m
3P, respectively. The results of the analysis of the changes in these parameters are summarized in
Table 4.
In PVC processing, the temperature range associated with initial degradation is important; therefore, the temperature at which the 2% weight loss of the composites occurred was determined. The temperature of 2% weight loss decreased significantly with increasing the filler content in the PVC matrix (for a composite containing 30 wt % of filler T
2%P decreased by 89 °C), which was related to the decomposition of easily volatile substances contained in the hop residue, including water. This is, however, of minor importance since the T2%P values for composites containing 10 wt % and 20 wt % were, respectively, 40 °C and 10 °C higher than the temperature at which PVC is routinely processed that is below 200 °C. Due to the processing temperature parameters used in this study (maximum temperature value during injection molding 180 °C), the weight loss at 180 °C was also determined. It should be stressed that the weight loss at 180 °C was only 1.7 wt % for the composite with the highest filler content, i.e., 30 wt %. The degradation of PVC and the composites occurred in two stages [
66,
67,
68]. During the first stage (200–400 °C), weight loss on the level of 54–65 wt % was associated with PVC dehydrochlorination and the degradation of organic additives in the PVC blend and filler components: mainly hemicellulose, cellulose and lignin [
53,
56,
57,
58,
69]. Moreover, the weight loss decreased as the filler content in the matrix increased, which indicated the degradation of the matrix, especially the HCl release reaction, which dominated at this stage. For the composite containing 10% of the filler, the temperature at which the degradation rate reached its maximum value was identical to that of the unfilled PVC; an increase in the filler concentration caused a decrease in its value by 20–30 °C. The decrease in T
max1P temperature was likely due to the degradation of part of the hemicellulose and cellulose. At the same time, the introduction of hop residue into the PVC caused a more than two-fold decrease in the composite degradation rate in the first stage. This was probably the result of a hindered heat transfer caused by the introduction of larger amounts of filler.
In the second stage of PVC degradation at temperature 400–550 °C, cyclization and crosslinking reactions of conjugated polyene sequences occurred, leading to the formation of aromatic fractions [
70]. In addition, the further degradation of hop residue components, especially lignin, occurred in this temperature range [
58]. The degradation rate of PVC/H composites was slightly lower than that of PVC, and such as the temperature at which the rate was maximum, hardly at all depended on the filler concentration in the matrix. However, in each case, these values were slightly lower than unfilled PVC. In addition, the TGA results indicated that the residue at 600 °C increased significantly with hop residue loading and was almost three times higher for the composite with 30% hop residue compared to the char yield of unmodified PVC.
To evaluate the filler effect on the glass transition temperature (T
g), a thermal analysis of the PVC samples and PVC/H composites was carried out by the DSC method. The value of Tg determined from DSC thermograms (
Figure 6) decreased with an increasing filler concentration from 79 °C for unmodified polymer to 75.6 °C for the composite with a 30% hop content (
Table 5). A slight decrease in the temperature range of the glass transition of PVC may indicate a slight plasticizing effect of the residues of lipids, alpha and beta acids, fatty acids and others, introduced into the matrix with the filler.
An endotherm observed on thermograms in the 45–65 °C range was related to the melting of one of the PVC blend components described in [
17].
Figure 7 shows selected SEM images of the unmodified PVC and PVC/H composites. The layered character of the fracture surface shown in
Figure 7a is typical for the processed rigid PVC. A smooth surface, free from inclusions of residual granular elements attested to the properly chosen temperature and the shear parameters during gelation [
71]. The addition of filler to the PVC matrix caused a clear change in the morphology of the fracture, on the surface of which numerous grooves and striations were visible. It was also found from the SEM images of PVC/H composites that as the filler content in the matrix increased, the surface became rougher and more porous. This nature of the brittle fracture may have been related to the porous structure of the filler (
Figure 3b) and the possible release of residual moisture and volatile compounds in the hop residue. What is important is that a uniform distribution of filler particles in the matrix was observed without any visible agglomerates, which also confirmed the proper processing procedure for obtaining a homogeneous structure of the composites. Moreover, the processing by the extrusion method resulted in the reduction in the maximum filler particle size to about 150 µm, similar to that of other PVC composites, with the addition of natural fillers [
72,
73]. SEM photographs of the PVC/hop composites disclosed the presence of smooth cavities formed after pulling out the filler, in addition to gaps and voids around it, which indicated a weak interaction between the matrix and filler and may have contributed to the stress concentration in these areas, which would explain the reduction in some of the strength properties of the composites (see section below) [
74,
75].
Table 6 shows the results of density (D) measurements of PVC and PVC/H composites and the strength properties determined during static tensile testing. Based on the test, the Young’s modulus (E
t), stress at yield (σ
y), strain at yield (ε
y), stress at break( σ
b) and strain at break (ε
b) were determined.
Figure 8 shows a representative curve for each replicated test.
On the basis of the density test results, it was found that adding hop residue caused a slight increase in the density of the PVC composites. Its value increased only by 0.021 g/cm
3 for PVC/30H composite in comparison with the unfilled polymer. This small increase in composite density in spite of the porous structure of the filler found on SEM images (
Figure 3b and
Figure 7b–d) may have been caused by the filling of some pores by molten polymer during the injection molding. Similarly to other PVC composites with natural filler [
72,
73], the addition of hop residue to PVC caused a decrease in tensile strength such that the higher the filler concentration in the matrix, the lower the value (
Table 6). This deterioration of strength can be attributed to the incompatibility of the filler with the matrix, causing the formation of voids and gaps around the filler as observed on the SEM images of brittle fractures (
Figure 7b–d) which acted as sites of crack formation and stress concentration (
Figure 9a). Moreover, the tensile curves showed that the addition of hop residue clearly changed the stress–strain relationship (
Figure 8). In the case of PVC, a clear yield point was seen at about 56 MPa, followed by a decrease in stress and a further increase in strain, resulting in the ductile fracture of the sample. On the other hand, for the PVC/10H composite, no characteristic plateau was observed after the yield point, which indicated an increase in brittleness. In the composites with 20 and 30 wt % hop residue, however, no yield point was observed, and the curve acquired a shape typical of composites with brittleness failure when maximum stress was reached. This was accompanied by a significant decrease in the strain at break from 18.8 for PVC to 4.7, 2.6 and 2.2% for PVC containing 10, 20 and 30 wt % hop residue, respectively. At the same time, Young’s modulus increased by 70.8, 75.4 and 79.0% for PVC/10H, PVC/20H and PVC/30H, respectively, indicating a significant increase in the stiffness of the composites. Similar relationships were observed by Abdellah et al. [
75], who studied PVC–wood flour–calcium carbonate composites.
Table 7 shows the results of the Rockwell hardness (HR) and Charpy impact strength (KC) as well as the strength properties during the static bending test. Based on the tests, the following were determined: bending stress (σ
f), bending strain (ε
f) and elastic modulus (E
f).
Similar to the tensile strength, the addition of hop residue caused a decrease in flexural strength. However, the recorded changes were much smaller than for the maximum tensile stress, as the maximum flexural stress decreased by 1.7, 8.8 and 17.3% for PVC/10H, PVC/20H and PVC/30H, respectively. This may be related to the fact that three-point bending causes compression and tension in the material (
Figure 9b). Literature data indicate that WPC composites have an increased compressive strength than unfilled thermoplastics, even in the case of poor adhesion between filler and polymer [
76,
77]. As a result, the flexural strength of composites containing up to 20 wt % H was at the level of the reference sample. Increasing the H content to 30 wt % resulted in a marked decrease in flexural strength due to the significant limitation of the matrix to deformation. On the other hand, as expected, the addition of hop residue caused an increase in the modulus of elasticity, which changed linearly with an increasing filler content. It was also confirmed by the stress–strain curves (
Figure 10). In the case of the PVC/10H composite, the curve was similar to that of the unmodified PVC. As a result for these two materials, the measurement was carried out until a deflection arrow of 6 mm was obtained since no fracture of the specimen had occurred. In contrast, the addition of 20 and 30% hops caused the slope of the curve in the proportionality region to become greater, indicating an increase in stiffness, which was characterized by a brittleness failure before the arrow was obtained. This behavior of the materials also affected the strain values, which decreased by 17.1, 39.5 and 55% compared to the unfilled PVC. According to literature data, the increase in stiffness is accompanied by an increase in hardness [
78]. As seen in
Table 7, the addition of 10% hop residue did not change this value, while for PVC/20H and PVC/30H, there was an increase of 8.8 and 12.8%, respectively. The results confirmed that the addition of hop residue caused an increase in composite stiffness, which could be attributed to the reduced mobility of the polymer chains. However, the increase in the modulus of elasticity was less pronounced than Young’s modulus, which again can be explained by the bending behavior of the specimen, as this test combined compression and tension of the specimen. Furthermore, it has been found that natural fillers can deteriorate compressive stiffness [
76]. The reduction in chain flexibility can be explained by a reduction in impact strength, which agrees with the literature data [
79,
80,
81].
Poly(vinyl chloride) is frequently used in construction, and its suitability is often determined by its relatively high fire resistance [
82]. Its modification process often leads to a lower LOI [
22]. Therefore, the fire resistance of the fabricated composites was evaluated, and the results are summarized in
Table 8 and
Figure 11.
The value of the oxygen index for the unmodified PVC was 37.4%. The introduction of hop residue into this polymer reduced the LOI to 32.3% for a composite containing 30 wt % filler. The LOI values were due to the organic nature of the filler and correlated with the results presented by other researchers. Furthermore, as with flexural and tensile strengths, the reduction in LOI values may be due to the porous structure of the composite and filler which promotes the migration of oxygen and flame into the material. It is worth noting that, despite the reduction in LOI, all composites were characterized by flammability class V0, which still made them attractive for fire resistance. Flammability tests were also performed using a cone microcalorimeter. During the analysis, the characteristic parameters such as tti (time to ignition), pHRR (Peak Heat Release Rate), ttpHRR (time to Peak Heat Release Rate), EHC (Effective Heat of Combustion), THR (Total Heat Released) and PML (Percentage Mass Loss) were determined.
The results summarized in
Table 8 represent the arithmetic mean of the three measurements for each sample, while the graph shows the representative curves for each sample. The table shows that the hop content increased and the time to ignition of the sample decreased. As a result, the PVC/30H composite had a 44% lower tti than that of the unmodified PVC. One of the reasons was a significant decrease in temperature at the onset of the thermal degradation after the filler was introduced, which was observed during the thermogravimetric analysis. In addition, natural additives reduced the thermal conductivity which led to a rapid temperature rise at the sample surface and a faster ignition [
37]. This high temperature behavior of the composites correlates with the LOI results and, again, may be due to a poor compatibility between the filler and polymer and the porous structure. The analysis of pHRR indicated that the addition of hop residue reduced its value. Unmodified PVC was characterized by a pHRR of 105.7 kW/m
2, while the addition of 10, 20 and 30 wt % hop residue caused a 20.7, 28.0 and 34.4% decrease, respectively. The results were consistent with the literature data for thermoplastic composites with natural filler [
35]. On the other hand, EHC and THR did not change significantly for composites containing up to 20 wt % filler. However, increasing the content to 30 wt % resulted in an increase of 33.9 and 22.2%, respectively, compared to unmodified PVC. According to literature data, the EHC refers to the gas-phase combustion of the volatile gases released from the polymer, while PML refers to the carbonization effect in the condensed phase [
83]. These effects can be quantified by reducing EHC and PML, respectively. The increased char yield decreased the release of flammable volatiles; in turn, a reduction in EHC indicates flame inhibition or fuel dilution. The EHC of PVC/30H composite increased to 124.4% compared to the unmodified sample. The increase in residue (15.1% instead of 8.0%) slightly reduced the amount of released fuel to 92.3%. Therefore, increasing the THR value to 114.7% after adding 30 wt % of hop residue was related to the reduction in the chlorine content in the mass; as a result, its activity in the gas phase was limited, which was confirmed by the calculated value (124.4 × 92.3 = 114.8). Therefore, the reduction in pHRR was probably related to the protective barrier formed by the hop residue, since the addition of hop residue caused a reduction in PML which coincided with the results of the TGA and char yield. The relative protection effect was calculated to be 38.1% (124.4% × 92.3% × 61.9% = 65.6%) which compared to the charring effect (7.7%) and flame inhibition effect (−24.4%) confirmed that the barrier effect plays a major role in decreasing the pHRR. Similar dependencies were observed in the case of the remaining samples containing 10 and 20 wt % of hope residue. Probably, this ability of the composites to form a char caused them to obtain the V0 flammability class, despite their lower oxygen index and shorter time to ignition.
Based on an analysis using a cone calorimeter, it was possible to calculate the fire growth rate index (FIGRA), the maximum ratio of the HRR to the time it was achieved. The smaller the FIGRA index, the more slowly the fire spreads, which increases the time to evacuate and start extinguishing operations. The FIGRA values confirmed that the addition of hop residues did not significantly affect the rate of fire spread. However, a non-significant decrease in FIGRA was observed with increasing amounts of hop residue. The FIGRA for the tested materials was in the range of 0.96–1.10 (
Table 8), while for the reference sample it was 1.10. Such FIGRA and pHRR values in combination with the flammability class V0 indicate that the obtained materials still have a good flame resistance.