Radiolysis of composite polypropylene/hemp fibers

The radiolysis studies of PP/HF composites used both pellets and HF-containing moldings: 10, 20, 30, and 40 wt%. Pure PP and HF were also investigated. PP type Moplen HP 456J with MFR of 3.4 g/10 min (2.16 kg, 230°C) and density of 0.9 g/cm³ was supplied by Basell Orlen Polyolefins (Poland). Natural HFs, at 6 mm long, were purchased from the Institute of Natural Fibers and Medicinal Plants in Poznan (Poland).

PP granules and PP composites were obtained using a type BTSK 20/40D corotating twin-screw extruder (Bühler, Germany). The extrusion process was carried out at cylinder heating zone temperatures of 180–190°C and a die-head temperature of 190°C. The screw speed was maintained constant (150 rpm). Before the composite-extrusion process, the HFs were dried at 110°C for 24 h. PP and HF were dosed into the feed zone of the extruder in such a way so as to obtain PP composites with HF contents of 10, 20, 30, and 40 wt%, respectively. The extrudate was cooled in a stream of cold air and then pelletized. The obtained granules were then dried (70°C, 24 h) to get rid of the residual moisture. Part of the granule samples thus obtained was destined for fundamental testing (GC, MFR), and part of it used for obtaining standardized test shapes. Samples in the form of barrels and dumbbells were produced in accordance with PN-EN ISO 527-2:2012 using a Battenfeld Plus 35/75 screw injection-molding machine (Battenfeld GmbH, Germany). The injection-molding process was carried out at the same temperatures and screw speeds as the composite-extrusion process.

The radiative yield of hydrogen, the main gaseous product of the radiolysis of natural and artificial polymers, gives an estimate of the number of initially formed free radicals that initiate secondary chemical processes of oxidation, double-bond formation, or cross-linking. The study shows that this yield for PP is about twice as high as that for HPs (Fig. 2). The dependency of the hydrogen-radiation efficiency on the fiber content indicates a clear protective effect of the aromatic compounds contained in the HFs. Moreover, this effect is almost the same regardless of which type of radiation is used (EB or gamma). This is due to the fact that almost all the energy is transferred to the polymer by secondary electrons. A measure of the observed protective phenomenon is the deviation of the curves from additivity (Fig. 2). Even relatively small amounts of HFs (10%) were found to reduce the hydrogen evolution yield by 22%. This is a relatively large decrease, considering the content of aromatic compounds in HFs, which is estimated at about 6% (lignin and cannabidiol). The exponential trend of the changes in the radiative hydrogen yield as a function of the HF content in the composite clearly indicates the protective effect of HF.

Fig. 2.

Another aspect relates to the reaction of the free radicals, with oxygen being the reason for the postradical degradation processes of polymer plastics. In the case of the studied materials (PP, HF, and PP/HF), it was found – as expected – that the HFs, as natural raw materials, undergo oxidegradation.

However, this process is accelerated when HF is irradiated together with PP (PP/HF composite). This is due to the fact that in the composites studied, the main phase is the easily degradable PP. Therefore, the presence of PP accelerates the oxidation efficiency of the composite as a whole, which is indicated by a deviation from a linear relationship. It is also worth noting the high efficiency of postradical oxidation processes after 24 h (Fig. 3). A similar situation occurs in the analysis of the oxidation phenomenon during the aging process, where the oxidation is significantly more efficient in the presence of PP. Due to the oxidation process becoming highly efficient after a 40-day period (Fig. 4), it can be speculated that the free radicals regenerated in the chain oxidation of PP are partially transferred to cellulose or hemicellulose, which are easily degradable. This is especially likely considering that cellulose and hemicellulose contain heteroatoms in their cyclic rings.

Fig. 3.

Fig. 4.

Figure 5 shows the variation of the oxidation– –radiation efficiency as a function of the HF content and takes into consideration the two types of radiation used. The figure clearly shows the significant effects of irradiation time on the oxidation processes of the composites. The use of gamma radiation, which exposes the materials for longer compared to the EB radiation, results in significantly higher oxidation efficiency of the investigated composites. At a gamma radiation dose rate of 1.8 kGy/h, the oxidation efficiency is at least three times higher than for an electron beam with a dose rate of 14 000 kGy/h. Moreover, Fig. 5 clearly shows the protective effects of HF, manifested by the lack of linearity, in both gamma radiation and electron beam irradiation.

Fig. 5.

The confirmation of the protective effects of aromatic compounds in the processes of radiolysis of polymers, demonstrated by experimental results, makes it necessary to verify many previous works in which this aspect was completely disregarded. Comparison of the protective effects at the stage of the formation of macroradicals as a result of hydrogen atom detachment with the effects of reducing postradical oxidation suggests that it is the ionic phenomena (reduction in the number of macroradicals) that dominate over the secondary scavenging of free radicals.

Figure 6 shows the results of the MFR of PP and its composites before and after treatment with gamma radiation (30 kGy dose). The study shows that in the case of nonirradiated materials, an almost straightline decrease in MFR values is observed with an increase in HF content. This is a natural situation if an increasing amount of nonflow filler is added to the polymer matrix. A decrease in the value of MFR with an increase in the HF content is also observed in irradiated materials. However, in this case, the relationship is not rectilinear, but has an exponential form. In addition, all irradiated samples show higher MFR values compared to nonirradiated ones. First of all, note the very large (about 20-fold) increase in MFR values for pure PP after irradiation. Compared to the other samples, this increase is the largest and confirms the high susceptibility of PP to radiation degradation. As the mass fraction of HF increases, the ratio of MFR values between irradiated and nonirradiated samples decreases. This is mainly due to the observed protective effect of aromatic compounds present in hemp. Moreover, the results of MFR studies show that the protective phenomena occur mainly at the stage of ionic products of radiolysis.

Fig. 6.

The effects of the combined action of gamma radiation (with a dose of 30 kGy) and HF, including the protective effect of aromatics contained in HF, are also evident in some of the mechanical properties of the studied composites. This is most clearly seen in the change in the relative elongation at break (ε_B) (Fig. 7), and thus in the strain of the modified samples. The expected observation is that the value of ε_B decreases with the increase of the proportion of HF in the tested samples regardless of whether they were irradiated or not. This is a known change due to the use of long fibers limiting the deformation of the material. A more significant change in our observations is the difference (Δε_B) occurring between nonirradiated and irradiated samples. It is greatest in the case of pure PP. On the other hand, as the HF content increases, this difference decreases, indicating the presence of a protective effect of HF.

Fig. 7.

Figure 8 shows the results of the tensile strength tests (σ_M). In most cases, no significant changes are observed in the values of σ_M, which is mainly due to the large confidence intervals of the individual results. This refers to both pure PP as well as PP/HF composites. For the composite with the lowest HF content, a slight reduction in material strength is observed, regardless of whether that material was irradiated or not. This may be due to poor HF protection both at the extrusion stage of the composite (neutralizing free radicals formed by thermo-mechanical-oxidative degradation of PP) and after irradiation. The changes seen in Fig. 8 may also be due to other reasons, and include partial cross-linking, changes in the degree of polydispersity or the degree of crystallinity and crystallographic structure.

Fig. 8.

Irradiating composites with increasing HF content causes the difference (ΔE_t) between nonirradiated and irradiated samples to increase (Fig. 9). In the case of PP and the composite with the lowest HF content (10%), the differences are not yet significant. However, in irradiated composites containing at least 20 wt% HF, the E_t values are already higher than those for nonirradiated composites. This is partly consistent with the results of ε_B, since usually a reduction in strain is associated with an increase in material stiffness. Moreover, as it follows from Fig. 9, the E_t values of the irradiated samples increase faster with the rise in the HF content compared to the nonirradiated samples. This proves that the material that contains higher amounts of HF is subjected to less radiation degradation. Thus, the protective effect of HF is visible.

Fig. 9.

A slightly different situation is observed during the dynamic testing of mechanical properties. In contrast to static tensile testing, the protective effect is practically not visible in the impact test (Fig. 10). Here, the effects of radiation degradation, especially of HF-containing composites, occur primarily. Considering that the impact strength of PP after irradiation virtually does not change, and the greatest changes occur in irradiated composites containing HF, it can be assumed that this is due to the presence of the fibers themselves in the composite. The significant weakening of the impact strength may be due either to the radiation degradation of cellulose and/ or hemicellulose or, more likely, to deterioration of the adhesion at the interface by which the transfer of dynamic impact energy from the polymer matrix to the dispersed phase is impaired. This effect is particularly visible in dynamic investigation. It should be noted that the tests were performed immediately after irradiation. It can be assumed that some of the free radicals from PP were transferred to cellulose and aromatic compounds. Such an effect was observed in chromatographic studies. This would explain the relatively small change in PP impact strength after irradiation. Despite this, PP/HF composites after radiation still show good impact strength. The same can be said for the other mechanical parameters tested. Some of them have even improved (e.g., E_t).

Fig. 10.