The compositions of the pristine PP, GF, cPP, and filters were verified by FTIR (Fig. 2 and SI Fig. S7.1) and TGA (Table 3 and SI S6). The FTIR spectrum of PP showed bands at 1170, 1000, 970, and 840 cm−1 which are characteristic of isotactic polypropylene [43]. The inorganic content, determined by TGA and inorganic content measurements, showed full decomposition of PP. For GF, the characteristic bands for silicate glass and bands from the applied sizing were observed by FTIR (Fig. 2). Thermally removing the sizing by heating (600 °C) and TGA showed a ≈1 wt% loss (Table 3 and SI S6), which is in agreement with typical sizing quantities of 0.2–1 wt% [44,45,46]. Further, the removal of sizing was also confirmed by FTIR from the elimination of the sp3 hybridised bands (Fig. 2). The decrease in fibre length (Fig. 3) could be as a consequence of the introduced thermal relaxation [47,48,49], as previous studies showed up to 70% strength degradation upon thermal treatment (600 °C) [48, 49]. However, the decrease in fibre length (Fig. 3) caused by thermal treatment does not challenge the filtration, as thermal treatment was only used post filtration for product analysis.
The FTIR spectrum of cPP (Fig. 2) showed repetitions of the bands found in PP and GF, except for GF’s sizing band at 1730 cm−1. The glass fibre content was determined to be 29–30 wt% (SI S5 and Table 3), which is in accordance with the glass fibre content declared by the producer (30 wt% ± 2 wt% [50]).
Finally, the FTIR spectra of the filters (SI Fig. S7.1) showed the characteristic bands of cellulose. The inorganic content of W- and L-filter were 0.0 and 0.2 wt% (SI Table S5.1), respectively. Thus, the mass contribution from filters upon measuring inorganic contents of filter cakes was considered negligible.
A way of increasing the profitability and sustainability of dissolution recycling is to work with as high concentrations as possible whilst maintaining processability. Results from Table 2 show that filtration using W-filter was possible for all PP/GF samples with concentrations below 54 g L−1, whereas increasing the concentration to 67 g L−1 clogged the filter. The ICC showed that PP/GF19W and PP/GF37W had higher inorganic contents in the filter cakes (≥ 98 wt%) than PP/GF54W (78 wt%). Furthermore, PP/GF54W had visible polypropylene residues on the fibres after drying (SI Fig. S4.1). These observations can be explained by the increased viscosity of the suspensions as concentrations increase [36]. This increasing viscosity reduces the filtrate flux and thus prolongs the filtration time. The filtration time is of essence, as the decrease in temperature results in increasing viscosity and eventually sets the suspension as a gel upon cooling. Hence, cooling dictates the processability upon increasing concentrations. A continuously heated filtration rig or heated (130 °C) environment could counteract such complications and potentially enable filtration at higher concentrations. However, for the given experimental setup, the conditions of PP/GF37W excels, as this concentration enabled high yields in addition to pure and well-separated polypropylene and GF fractions.
For the polypropylene composite with milled fibres (cPP), all concentrations clogged the W-filters. Comparing the concentrations shows that PP/GF37W filtered nicely while cPP13W clogged the W-filter. These observations are ascribed to the difference in fibre length. GF had an average length of 1.7 mm, whereas the fibres from cPP were on average 0.4 mm. As the fibre diameters are similar, the shorter fibre length results in a closer packing of the deposited fibres. The denser fibre packing reduces the filter cake porosity and thus increase the filter cake resistance. As with increased viscosity, increased filter cake resistance will prolong the filtration time and result in gelation and clogging. This is underpinned by the fact that reducing the filter paper resistance using the more porous L-filter resulted in that all cPP concentrations could be filtered. Despite coarser filter paper and smaller fibre lengths, the L-filter still captured 99–100 wt% of the fibres (Tables 2, 3, and Fig. 3) leaving a pure filtrate (0 wt% glass fibres in the polypropylene). In addition, it was demonstrated that filtering the suspension through the same filter cake again only resulted in loss of polypropylene due to processing without increased purity. Thus, the obtained purities of cPP13L and cPP27L were similar to PP/GF37W demonstrating removal of the most common fibre types (i.e. chopped and milled glass fibres) in thermoplastic composites.
FTIR was used to chemically characterise the dried filtrates and filter cakes of cPP27L and cPP27LD (Fig. 2 and SI Fig. S7.2). When comparing the filtrates to the spectrum of pristine cPP, the curvature arising from GF in the 1250–600 cm−1 range had disappeared, thus supporting the hypothesis of the removal of glass fibres. Furthermore, the spectra of the filtrates and PP were alike, with the only deviation being a weak band at 1730 cm−1. This band probably originates from sizing from the fibres transferred to the filtrate during the dissolution process, as organic solvents has been applied to extract GF sizing in previous studies [51, 52]. The FTIR spectrum of the filter cakes show the characteristic silica bands, like observed in the spectrum of GF, but with shifted sp3 C–H stretches, which could indicate differences in the chemical composition of the sizing used on the GF and cPP-fibres. When comparing the spectra of the filter cakes to cPP, no bands from polypropylene are observed, indicating that filter cakes are pure glass fibres. Heating (1 h at 600 °C) the dried filtrates (PP/GF37W and cPP27L) caused complete combustion (Table 2), similarly observed with pristine PP (SI Table S5.1). In contrast, the dried filter cakes showed a 1–2 wt% mass loss upon heating, similarly observed with pristine GF. This indicates that sizing remained on the fibres upon dissolution recycling. Likewise, the thermograms (SI S6) of cPP27L and cPP27LD showed similarities to those of GF and PP, validating the inorganic content measurements: the dried filter cakes and GF both remained around 99 wt% of their original masses. The 1 wt% mass loss were in accordance with the one observed from inorganic content measuring assigned to sizing removal (Table 2). In contrast, the dried filtrates and PP both fully decomposed in a single-step mass loss remaining 0 wt% of their original masses. The characteristic temperatures of the filtrates (270–305 °C) were close to that of PP (300 °C). SEM images of the cPP27LD filter cake (Fig. 3C) showed clean, recovered fibres with smooth surfaces similar to the surfaces of pristine GF (Fig. 3A and SI S9). Likewise, the SEM images of cPP27LD filtrate showed a homogenous polypropylene surface.
It is a strength of this study that both inorganic additives and polymeric matrix can be recovered for recycling. In addition, the usage of filtration to recover glass fibres can be applied to other common thermoplastic composites, once useful solvents and temperatures are identified. Although not included in this study, solvent recovery is paramount for this technique to become industrially relevant in terms of operating costs and sustainability. Fortunately, the chemical industry is familiar with handling and recovering solvents, and thus this challenge can be solved at an industrial scale. However, with proper solvents and temperatures, filtration might be useful for separation of fibres or inorganic fillers from other prevalent thermoplastic composites.
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