Flexible Thermoplastic Composite of Polyvinyl Butyral (PVB) and Polyvinyl Chloride (PVC) with Rigid Polyurethane Foam (PUr)

Rigid polyurethane foam (PUr) is a common residue from the footwear industry that seeks a sustainable disposal alternative. A flexible composite incorporating PUr that could be used in this same industry is an exciting solution. Poly (vinyl chloride) (PVC) and polyvinyl butyral (PVB) were tested as possible matrixes for flexible composites containing 20%, 35%, and 50% of PUr produced by two different processes, extrusion and injection molding. PVB/PUr and PVC/PUr were compared considering the morphology observed with scanning electron microscopy (SEM), density, degradation during processing, and dynamical mechanical properties. PVC was severely degraded when processed with PUr and showed a low adhesion to PUr particles resulting in decreased storage modules and increased glass transition temperature. PVB is a better matrix for PUr flexible composites since it showed no sign of degradation and formed a stronger interface with PUr particles resulting in PVB/PUr composites with higher storage modulus.


Introduction
Footwear production worldwide reached 24.2 billion pairs in 2018, which means a 2.7% growth compared with 2017 1,2 .This way, Brazil is currently the 3 rd most significant footwear revenue, reaching 43.28 billion USD (US$) in 2022, and has a perspective to rise year after year, carrying an expectation to reach 63.85 billion USD (US$) in 2025 3 .Whereas footwear production shall increase, synthetic polymers from the footwear industry come up as a waste disposal problem that needs to be fixed.
Polyurethanes are highly versatile materials utilized as flexible thermoplastics or rigid foams.They possess low density and low thermal conductivity, making them suitable for various industries such as automotive, construction, medical, footwear, adhesives, and elastomers [4][5][6] .However, rigid polyurethane foams (PUr) are highly polluting to the environment 5,[7][8][9][10] .Firstly, PUr has a low density, and therefore, production and the post-consumer disposal of PUr products generate significant volumes of solid residues 6,8 .In this context, the chemical recycling of PUr is hampered by the presence of crosslinking bonds in the rigid polyurethane structure.Therefore, large-scale industrialized chemical recycling of PUr is rare.However, the mechanical recycling method (i.e., only changing PUr physical form with a simple operation) effectively improve its recyclability in the short term 7,11 .
Moreover, utilizing PUr as a filler in thermoplastic composites 12 allows applying these recycled products within the same industry that generates the waste.
Poly (vinyl chloride) (PVC) is widely utilized in the footwear industry and is the second most consumed thermoplastic globally.Its popularity stems from its remarkable versatility.PVC can be employed as rigid tubes for construction purposes or as flexible laminates in the footwear industry.The different properties and applications depend on the specific additives incorporated into PVC formulations, such as plasticizers or fillers 13 .Despite being widely used, PVC is easily thermal and photo degraded, which requires special machinery, several additives, and many precautions during processing 14,15 .
Polyvinyl butyral (PVB) is an exciting option to substitute flexible PVC.PVB is an amorphous random copolymer of vinyl butyral, vinyl alcohol, and vinyl acetate, mainly used in laminated safety glass, automotive, aerospace, and architectural glass sections [16][17][18] .The vinyl butyral unit is hydrophobic and promotes good processability, toughness, elasticity, and compatibility with many polymers and plasticizers.The hydrophilic vinyl alcohol and vinyl acetate units are responsible for the high adhesion to inorganic materials such as glass 19 .Commercial PVB contains approximately 17-22 wt% of vinyl alcohol, 1-3 wt% of vinyl acetate and 75-82 wt% of vinyl butyral unit.
PVB used in windshields is highly plasticized, and different plasticizers, such as alkyl phthalate, dibutyl sebacate, and di-2-ethyl hexanoate of trimethylene glycol, could be present to different extents 20 .
Grigoryeva et al. 21attempted to recycle PUr waste to produce PVC/PU blends.In this process, PUr is initially subjected to chemical recycling to dismantle its crosslinks, resulting in a thermoplastic blend with PVC.These PVC/PU blends are intended for applications such as soundproofing materials, pipe lining, the shoe industry, medicine, housing, and plasticizer or polymeric modifiers for PVC 21 .The available literature on rigid and crosslinked PUr in association with PVB or PVC is limited.Previous works evaluated the mechanical properties of PVB/PUr 12 and PVC/PUr 22 composites.Although pure PVB and PVC were plasticized polymers with similar mechanical behavior, adding PUr caused more severe changes in PVC/PUr composites.The addition of PUr increased the elastic modulus (E) of both composites, reaching a maximum of 91MPa and 34 MPa for PVC/PUr (50/50) and PVB/PUr (50/50), respectively.However, this tendency is more intense for PVC/PUr, with an increase of 771% when compared to pure PVC.Differently, the elastic modulus of PVB/PUr just increased 130%, compared to the pure matrix.The tensile strength of PVC/PUr composites is approximately 6 MPa for all PUr content tested, while it reaches a maximum of 16 MPa for PVB/PUr (80/20).As expected, the tensile strain at break decreased with PUr addition, reaching the minimum values of 97% for PVB/PUr (50/50) and 20% for PVC/PUr (50/50).A deeper investigation regarding degradation and dynamic mechanical properties is needed to explain how PUr content affects each composite.
The primary objective of this study was to create flexible thermoplastic composites by incorporating rigid polyurethane (PUr) foam through extrusion and injection molding processes.PUr foam, a byproduct of the footwear industry, was targeted for sustainable reuse, particularly within the same industry.Additionally, the performance of PVB and PVC as flexible matrices for composites with PUr was compared.

Materials and Methods
The PVB (density of 1.20 g/cm 3 ) is a recycled material from the laminate glass of the automotive industry and contains 16% of dibutyl sebacate as the plasticizer 23 .
Plasticized PVC (K= 65 ±1), provided by Braskem, was mixed with additives in a mixer using a velocity of 1760 rpm.The composition adopted was 100 phr of PVC, 60 phr of dioctyl phthalate (DOP) plasticizer, 2 phr of Ba/Zn stabilizer, 0.5 phr of stearic acid and 3 phr of epoxidized soybean oil.
PUr is a residue from the footwear industry donated by the Union of the Footwear Industry of Jaú-SP, Brazil.It was milled in a conventional granulator (Wittmann Battenfeld MAS1) and sieved (12 mesh).

Processing
Firstly, the matrixes and PUr were mechanically mixed in the solid state adopting the formulations shown in Table 1.
The melt mixture of the composites was performed in a single screw extruder equipped with a Maillefer screw with a 45-mm diameter, L/D = 25, from Miotto Brazil.A mixer block type pineapple was added to the screw in the metering zone.The Maillefer screw and the pineapple mixer increase the shear rate of the mixture, improving the incorporation of several additives and fillers into the thermoplastic matrix.A flat die measuring 150.0 mm in width and 3.0 mm in thickness was adopted.The temperature profile was 155 °C /170 °C / 175 °C in the extrusion barrel and 180°C in the extrusion die, and the screw speed was 20 rpm.After extrusion, the composite plates were granulated in a conventional granulator (Wittmann Battenfeld MAS1).
The melt flow index (MFI) of pure PVB and the composites was measured in a Gottfert 011043 instrument, using a weight of 21,6 kg, at the temperature of 175 °C based on standard ISO 1133.The MFI for each formulation is shown in Table 2.The addition of PUr led to a notable decrease in the MFI of the composites, suggesting a substantial rise in composite viscosity due to the presence of PUr.
This alteration in rheology demanded distinct injection molding parameters for each formulation.Various injection pressures (Table 2) were established for each formulation to ensure a comparable injection velocity or shear rate during the injection molding process of the composites.The specimens were injection molded in a Battenfeld 25/75 Unilog B2 injector, using the temperature profile of 160 °C / 170 °C / 180 °C and the mold temperature of 60 °C.

Morphological characterization
The morphology of the cryogenically fractured composites was examined using a scanning electron microscope (FEI QUANTA 400) at an accelerating voltage of 30 kV.The polymer samples were metalized by sputter-coating with a thin layer of gold prior to SEM analysis.
The pore diameter was investigated by analyzing SEM images with ImageJ software 24 .

Degradation analysis
Ethanol and tetrahydrofuran (THF) were used as solvents to extract PUr from PVB and PVC composites, respectively.Both extractions were performed at 60°C for 12 hours.The PUr extracted from composites and the original PUr were analyzed by Fourier transform infrared spectroscopy (FTIR), using a Nicolet spectrometer, model 4700.The FTIR spectra were obtained from the Attenuated total reflectance (FTIR-ATR) method at room temperature and wave numbers ranging from 700 to 4000 cm -1 .

Density
The real density of the PUr and nanocomposites were measured according to the Method B of ISO -1183 25 .
The tests were performed in a gas displacement pycnometer brand Micromeritics Instruments Corp, model AccuPyc 1330.
The apparent density of the PUr was estimated by dividing the mass by the volume of three replicates of each material.The mass was obtained through a weighing-machine brand Mettler-Toledo, model XS205.The volume was obtained with a caliper ruler, brand Mitutoyo model CD-6"CX-B, through a ten-time measure in each different region (Length, Width, Height) for each specimen.

Dynamical mechanical analysis
The dynamical mechanical analysis (DMA) was performed using a dynamic mechanical analyzerfrom TA Instruments, model DMA Q800.Dual cantilever claws were used in flexural mode at a frequency of 1 Hz with an amplitude of 25 µm and a temperature range from -100°C to 70°C at a rate of 3°C/min.The analysis of pure PUr used a temperature range from -100°C to 100°C, with the same rate.The tan δ versus temperature curves were deconvoluted using OriginPro Learning Edition.Two peaks were marked and fitted with a Gaussian function, and iterations were performed until the cumulative peak fit curve was converted.

Morphological characterization
Figure 1 shows the micrographs of the cryogenically fractured surface of the PVB/PUr and PVC/PUr composites.In the PVB composites, PUr particles had a lighter shade than the matrix, as indicated in Figure 1a. Figure 1d and 1e shows PVC-PUr composites where PUr particles assumed a darker shade than the PVC matrix.During processing, the melt PVB and PVC matrixes penetrated the foam pores leading to the spherical shapes (inclusions) observed in Figure 1.
PUr particle size decreases with the increase in PUr content as if PUr particle were broken or torn during processing.
Figure 2 shows PUr foam as received and after incorporating in PVB and PVC matrices, where the porosity change after processing is evident.The PUr particles (Figure 2b and 2c) are distinct from the as-received PUr (Figure 1a), as if processing caused thermo-mechanical degradation of the PUr crosslinked structure leading to a softer material with different morphology and porosity.
When particles are not broken or torn, the pores present in the PUr foam are filled with matrix, forming inclusions inside the particles, highlighted by red arrows in Figure 2b and 2c.The PUr pore penetration with matrix was less frequent in composites with higher PUr content, like PVB/PUr (50/50) and PVC/PUr (50/50), shown in Figure 1e and 1f, respectively.
The mean diameter of the PVB and PVC inclusions in PUr particles in Figure 2b and 2c is 75 ±13 µm.This diameter is consistent with the pore diameter distribution observed in the as-received PUr, as shown in Figure 2d.
Figure 3 shows the fractured surface of the PVB and PVC composites with 20% of PUr, where the poor adhesion between matrix and particulate is evident (red arrows).When the material was cryogenically fractured, the crack propagated through matrixes inclusions (Figure 3 -region A) which work as a mechanical anchoring between particles/matrix.The fracture also occurred through the PUr particle (Figure 3-region B) in a brittle manner.Moreover, particle/matrix debonding was also observed (Figure 3 -region C), creating fractured surfaces with empty pores and PVB molded with the pore form.Few PUr particles have signs of plastic deformation (blue arrows in Figure 3), which are more consistent with the break/tear of PUr particles during processing than the cryogenic fracture.
As shown in Figure 2, the PUr porosity significantly changed during processing as if PUr particles were softened by temperature and shear, acquiring a new morphology.Figure 3 (blue arrows) shows PUr particles with signs of plastic deformation and particle tearing which agrees with the PUr softening during processing.Such change would only be possible if PUr crosslinked structure was not so intense to prevent particle softening.
The filling of PUr pores with matrix (Figure 2) is an interesting feature in the composites.When the matrix fills the foam pores, it creates a mechanical interlocking between these components, which results in crack propagation through the filled pore (Figure 3).Such mechanical interlocking between PUr and matrix can partially compensate for their weak interface and the PUr debonding (Figure 3) [26][27][28] .

Degradation
Figure 4 shows the pure PVC and PVC-PUr composites where the color change is evident.While the pure PVC has a pale-yellow color, the composites are dark brown, a known sign of plasticized PVC degradation.Differently, no sign of degradation was observed in the PVB/PUr composites.
PUr degradation during the composites processing was investigated with FTIR after the foam particles of the composites were extracted by dissolving the matrix.The spectra obtained was compared to the spectra of the as-received PUr.This comparison is shown in Figure 5 where a very similar spectra for all specimens is displayed.The bands in the region of 3300 to 3400 cm -1 are the overlapping of OH, and NH stretching frequencies, the band around 2954 cm -1 is attributed to symmetric and non-symmetric stretching of -CH 2 bond with the carbonyl [29][30][31] .Polymerized urethanes are characterized by peaks at 1720 cm -1 , 1600 cm -1 and 1540 cm -1 , typical of stretching (C=) and NH 30 .
The thermo-mechanical degradation of the PUr crosslinked structure, which was investigated with FTIR (Figure 5), would also explain the morphology change in PUr.FTIR studies of thermal degradation polyurethane coatings (PU) 32,33 and rigid foams (PUr) 29,34 under air atmosphere evidenced the presence of an isocyanate peak (~2280 cm -1 ) and weakening of peaks at 1730 cm -1 ,1604 cm -1 , and 3389 cm -1 .Further degradation would cause the decline of peaks around 1537 cm -1 (N-H), 2900 cm -1 (C-H), and 330 cm -1 (N-H).[28,29] As shown in Figure 5, the spectra of PUr extracted from the composites are very similar to the spectra of the as-received PUr and show no indication of degradation.Therefore, there is no evidence that PUr has degraded during processing.Differently, the color change of PVC-PUr composites (Figure 4) is a notorious sign of the PVC degradation, which was also observed by Bowden et al. 33 when studying thermal degradation of PVC sheets in contact with polyurethane (PU) foam.Their Microline focus spectrometry (MiFS) and Raman spectrometry results suggest that the contact interface between PVC and PU shows a greater extent of degradation than the PVC-air interface due to the presence of more dehydrochlorination initiation sites formed in the PVC-PU interface.One possible explanation to this fact is that the amine catalyst residues from the PU foam may create more dehydrochlorination initiation sites 35 .The decomposition of DOP plasticizer can also contribute to the color change observed in the PVC/PUr composites 36 .Thermograms of the plasticized PVC with DOP show that its degradation occurs in the temperature range of 180 to 385 °C, corresponding to the onset of plasticizer decomposition with PVC dehydrochlorination 37 .
The absence of visible signs of degradation in PVB/PUr composites is a point in favor of adopting PVB instead of PVC as a flexible matrix.

Density
The real and the apparent densities of the composites and the pure materials -PVB, PVC and PUr -are shown in Figure 6.PUr foam shows the lowest apparent density among the materials tested, as expected.However, its real density, measured by a pycnometer that ignores the foam porosity, is like that found for PVC and PVB matrices.
Remarkably, the addition of PUr foam, even at a substantial content of 50%, had negligible effects on the real and apparent densities of the composites (Figure 6).The consistent real density observed across different compositions can be attributed to the inherent similarity in real density displayed by each individual component.
Interestingly, it is evident that the apparent density of the composites deviates from the expected mixture rule.Mounanga et al. 38 investigated mixtures of concrete and rigid polyurethane foam wastes, which exhibited higher density compared to pure concrete.According to the authors 38 , concrete exerted high hydrostatic pressure on PUr aggregates and was absorbed into the open pores of the PUr foam.Here, a comparable phenomenon can be attributed to the high hydrostatic pressures exerted during injection molding and matrix penetration on the open pores of the PUr foam, as depicted in Figure 2.

Dynamical mechanical analysis
The dynamical mechanical analysis of PVB, PVC, PUr, PVB/PUr and PVC/PUr composites are shown in Figure 7.The PUr storage modulus is considerably lower than that measured for all other specimens, as shown in Figure 7a and 7b.As expected, the PVC/PUr composites storage modulus is smaller than the modulus of pure PVC (Figure 7b).Curiously, the PVB/PUr composites show a synergetic effect between filler and matrix since the addition of PUr caused a considerable increase in storage modulus compared to the pure PVB (Figure 7a).However, the storage modulus level observed here is still compatible with a flexible material.Moreover, Figure 7a shows that the PUr content of 50% resulted in a storage modulus smaller than the measured for the other formulations.
Figure 7c and 7d show how the tan δ of the pure constituents and the PVB/PUr and PVC/PUr composites varies with temperature.Figure 7c shows that the tan δ peaks of PVB/PUr composites are thinner and smaller than the tan δ peak of pure PVB.Otherwise, the tan δ peaks of PVC/PUr composites present an intermediate behavior between PVC and pure PUr.Such differences can be better observed when tan δ peaks were deconvoluted in peaks related to each matrix and filler, as shown in the supplementary information (Figure S1 and S2) 39,40 .Table 3 shows the glass transition temperatures (Tg) of the materials measured at the tan δ highest values from experimental data and from deconvoluted tan δ curves.From the deconvoluted tan δ curve, the Tg of the PVB matrix is approximately the same as the Tg measured for pure PVB, while the Tg of the PUr filler considerably decreased from 51.6 °C (as-received PUr) to 37.9°C (PUr in PVB/PUr (50/50)).Interestingly, the opposite tendency was observed in PVC composites.PUr filler in PVC showed no significant change in Tg, while the PVC matrixes showed a considerable increase in Tg compared to the pure PVC.
Many aspects can explain this tan δ behavior, such as particle/matrix interface, particle size, inclusion interface and mechanical interlocking, degradation, and plasticizing migration or decomposition.
The SEM images showed a poor interface between PUr and PVB.However, the tan δ curves (Figure 7c) of PVB/PUr composites are sharper and less intense than the pure PVB curve, which indicates a strong interface between PUr particles and PVB matrix, hampering the energy dissipation during dynamic analysis.There are interfaces in the PUr particle surface and the inclusion surface inside the particle.Additionally, the mechanical interlocking promoted by the inclusions also contributes to this effect.These factors can explain the increased storage modulus observed in PVB/PUr composites.At a PUr content of 50%, particles were more frequently broken/torn, resulting in smaller particle sizes and fewer inclusions.Therefore, PVB/PUr (50/50) has more interfaces at the particle surface but fewer inclusion interfaces and mechanical interlocking, which can lead to lower storage modulus among the PVB/PUr composites (Figure 7a).The Tg (Table 3) measured with the deconvoluted tan δ curves provided more information regarding the phenomena happening.The Tg of PUr in the PVB/PUr composite decreased while the Tg of the PVB matrix was approximately the same as the pure PVB.Considering that no intense degradation occurred in PUr (Figure 5), the migration of plasticizer (dibutyl sebacate) 23 from PVB to PUr can explain the Tg decrease of PUr and the increase in storage modulus of the composites with a less plasticized PVB matrix.
The tan δ curves (Figure 7d) of PVC/PUr composites indicate a weaker interface between particles and matrix, contributing to the decrease in the observed storage modulus (Figure 7e).The Tg measured from the deconvoluted tan δ curves of PVC/PUr (Table 3) shows no significant change in the Tg of PUr, indicating no plasticizer (dioctyl phthalate (DOP)) migration from PVC to PUr.The DOP plasticizer has longer chains than dibutyl sebacate (PVB plasticizer), which can hamper migration to the crosslinked molecular structure of PUr.Additionally, Tg of the PVC matrix considerably increases with PUr content, consistent with the plasticized PVC degradation process intensified by PUr presence 41 .
The dynamical mechanical analysis evidenced complex effects in PVB/PUr and PVC/PUr composites, such as degradation, morphology changes during processing and plasticizer migration.

Conclusion
Extrusion and injection molding produced flexible thermoplastic composites filled with recycled rigid polyurethane foam (PUr).PUr is a residue from the footwear industry; therefore, a sustainable alternative to reuse this material is exciting to this sector.Composites adopting two different flexible matrices, PVB and PVC, were compared.
In both composites, the PUr foam particles exhibited pore filling by the matrix, resulting in mechanical interlocking between the two phases.This also contributed to the absence of a decrease in the apparent density of the composites, as anticipated.
Higher filler concentrations induced greater shear stresses on the particles, leading to particle break/tear/softening and subsequent reduction in porosity.
PVB demonstrated superior performance as a matrix for the flexible composite due to improved interfacial adhesion between the two phases, with no evidence of degradation.However, the composite exhibited indications of plasticizer migration from the matrix to the PUr, requiring optimization.
Composites with PVC as the matrix displayed signs of degradation, such as color alteration and increased Tg.It is believed that the presence of PUr may have exacerbated the degradation process, which poses a limiting factor in composite development.
Concluding, PVB is a better choice as a matrix for flexible composites using the waste of rigid polyurethane foam that could be applied in the footwear industry.These results exemplify how complex composite can be and how challenging is the mechanical recycling of polymeric residues.

Figure 3 .
Figure 3. Fracture surface of (a) PVB/PUr (80/20) and (b) PVC/PUr (80/20), highlighting the different crack propagations.Where A points out the crack through the filled pore, B indicates crack through PUr particle, and C shows particle/matrix debonding.Red arrows highlight the poor particle/matrix adhesion, and blue arrows indicate signs of plastic deformation of PUr particles.

Figure 5 .
Figure 5. FTIR spectra of original PUr and PUr extracted from PVB and PVC composites.

Figure 6 .
Figure 6.Real and Apparent density of PVB, PVC, PUr and the PVB/PUr and PVC/PUr composites.

Figure 7 .
Figure 7. Dynamical mechanical analysis of the composites and their constituents: Storage modulus of (a) PVB and (b) PVC composites; Tan δ of (c) PVB and (d) PVC composites.

Table 1 .
Formulations of the PVB/PUr and PVC/PUr composites.

Table 2 .
Melt 12ow index (MFI -175°C/21,6Kg) of the composites and injection pressure adopted during injection molding12.*Test of PVC pure did not follow the standard due to the low viscosity of the polymer.

Table 3 .
Glass transition temperatures (Tg) of PVB/PUr and PVC/PUr composites and their modelled isolated components.