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Article

Comprehensive Enhancement of Prepolymer-Based Flexible Polyurethane Foams’ Performance by Introduction of Cost-Effective Waste-Based Ground Tire Rubber Particles

by
Wiktoria Żukowska
1,
Paulina Kosmela
1,
Paweł Wojtasz
1,
Mariusz Szczepański
1,
Adam Piasecki
2,
Roman Barczewski
3,
Mateusz Barczewski
4 and
Aleksander Hejna
1,*
1
Department of Polymer Technology, Gdańsk University of Technology, Narutowicza 11/12, 80-233 Gdańsk, Poland
2
Institute of Materials Engineering, Poznan University of Technology, Jana Pawła II 24, 60-965 Poznan, Poland
3
Institute of Applied Mechanics, Poznan University of Technology, Jana Pawła II 24, 60-965 Poznan, Poland
4
Institute of Materials Technology, Poznan University of Technology, Piotrowo 3, 61-138 Poznan, Poland
*
Author to whom correspondence should be addressed.
Materials 2022, 15(16), 5728; https://doi.org/10.3390/ma15165728
Submission received: 22 July 2022 / Revised: 13 August 2022 / Accepted: 18 August 2022 / Published: 19 August 2022
(This article belongs to the Special Issue Current Developments in Polyurethane Materials for Different Applications)
Browse Figures
Versions Notes

Abstract

:
Material innovations in polyurethane (PU) foams should ideally combine performance enhancement, environmental impact limitation, and cost reduction. These goals can be achieved by applying recycled or waste-based materials without broader industrial applications, implicating their low price. Herein, from 5 to 20 parts by weight of ground tire rubber (GTR) particles originated from the recycling of postconsumer car tires were incorporated into a flexible foamed PU matrix as a cost-effective waste-based filler. A two-step prepolymer method of foams manufacturing was applied to maximize the potential of applied formulation changes. The impact of the GTR content on the foams’ processing, chemical, and cellular structure, as well as static and dynamic mechanical properties, thermal stability, sound suppression ability, and thermal insulation performance, was investigated. The introduction of GTR caused a beneficial reduction in the average cell diameter, from 263.1 µm to 144.8–188.5 µm, implicating a 1.0–4.3% decrease in the thermal conductivity coefficient. Moreover, due to the excellent mechanical performance of the car tires—the primary application of GTR—the tensile performance of the foams was enhanced despite the disruption of the cellular structure resulting from the competitiveness between the hydroxyl groups of the applied polyols and on the surface of the GTR particles. The tensile strength and elongation at break were increased by 10 and 8% for 20 parts by weight GTR addition. Generally, the presented work indicates that GTR can be efficiently applied as a filler for flexible PU foams, which could simultaneously enhance their performance, reduce costs, and limit environmental impacts due to the application of waste-based material.

1. Introduction

Polyurethane (PU) foams are widely applied in multiple branches of industry, including bedding, furniture, building, construction, and automotive sectors [1]. They account for over half of the total PU market, so their development aimed at performance enhancement, price reduction, and limiting the environmental impacts is essential in a broader perspective [2,3]. The perfect innovations should combine these aspects to maximize the application potential and benefits for future users and the environment. In the case of PU foams, the novel solutions could include modifications of currently applied production and processing techniques or substituting conventionally applied raw materials with new alternatives, either from renewable or waste-based resources [4]. Significantly more affordable and innovation absorbing is the second direction related to the substation of conventional petroleum-based raw materials for PUs.
Over the years, the main direction has been the application of plant oils or their derivatives in biopolyols manufacturing [5]. Biopolyols are usually obtained from plant oils, which are available in a particular region: for Europe, rapeseed and sunflower oils [6,7]; for Asia, palm and coconut oil [8]; for North America, soybean oil [9]. Multiple processes have been developed to convert plant oils into biopolyols, but they all have one disadvantage. They are mainly using edible vegetable oils, whose broader application in PU manufacturing may adversely affect their price and cause an increase in the prices of various food products [10,11]. Moreover, the application of virgin vegetable oils in biopolyols manufacturing, despite the benefits related to the reduced global warming potential compared to their petroleum analogs, implicates relatively high environmental impacts related to the inputs of crop production, including seeds, fertilizers, plant protection products, and land and water use [12,13].
Therefore, lately, more emphasis has been placed on using waste-based materials as PU components, either as raw materials for biopolyols’ production or as fillers, which could be introduced into the PU matrix and reduce the use of virgin material [14,15,16,17,18]. Ideally, the application of waste-based material should not only provide an effective method for its utilization but also enhance the performance of the unfilled matrix. Superior selected properties should characterize such material compared to the polymer matrix, e.g., higher strength or thermal stability, reduced flammability or smoke generation, and higher conductivity. In the case of solid materials, mechanical enhancement may be achieved by incorporating various types of waste fibers, including natural ones [19,20,21]. Thermal stability enhancement or reduced flammability may be induced by the application of various waste-based inorganic fillers, e.g., fly ash [22], red mud [23], copper slag [24] or steel slag [25], as well as organic ones like chicken feather [26], nut shells [27,28], or leather [29]. Conductivity can be boosted by the incorporation of waste-based metal particles [30,31] or char obtained from plant-based wastes [32,33].
Besides the above-mentioned conventional waste fillers based on lignocellulose or inorganic mineral materials, plastic wastes can be recycled and introduced into virgin polymer matrices. Such an approach creates the utilization methods and provides new features to the prepared composites resulting from the properties of primary materials, such as reduced specific gravity due to the incorporation of foamed plastics [34], reduced flammability related to the use of phenolic foam or poly(vinyl chloride) residues [35,36], or enhanced toughness attributed to the incorporation of rubber particles [37,38]. Considering waste rubber, the market offers an excellent opportunity for using postconsumer car tires. They are generated in massive amounts irrespectively of the location and are considered problematic waste. Due to their composition and related toxic effects, they should not be combusted without proper complex and expensive apparatus. Moreover, such an approach enables only 40% recovery of the energy used for their production [39]. Material recycling yielding ground tire rubber (GTR) poses a significantly more beneficial alternative. To improve its profitability and popularize it, there is a need for the potential industrial applications of GTR, so it is justified to investigate its incorporation into polymer composites. Until now, GTR has been investigated as filler in the construction of tunnels, base layers of road surfaces, and sports surfaces, as well as for manufacturing car mats, windshield wipers, cattle mats, or asphalt modifiers [40]. However, these applications still do not guarantee sufficient demand for processed rubber waste. Therefore, a great deal of research is currently underway to find potential applications for GTR in the plastics industry. Incorporating GTR into PU matrices has an undeniable economic advantage because its price is at least ten times lower than PU systems [41].
In our previous works, we applied GTR as a filler for foamed PU composites [42,43,44,45,46,47]. Foams contained unmodified GTR, thermomechanically reclaimed with or without the addition of the oils and chemically oxidized. Nevertheless, in most cases, despite the selected beneficial effects, the disruption of the cellular structure was noted irrespectively in the applied GTR treatments, which limited the overall composite performance. Similar effects were noted by other researchers [48,49,50]. However, in all of the works mentioned above, the foams were obtained using the single-step method of PU preparation. None of the research works reported the preparation of foamed PU/GTR composites using the second method of PU preparation—the prepolymer method, which is applied and beneficial in the case of the increased viscosity of polyol mixture [51], which was observed in previous works after GTR incorporation [44,46].
Therefore, the presented work was aimed at a comprehensive investigation of the performance of foamed flexible PU/GTR composites obtained by the two-step prepolymer method. The impact of the introduced rubber particles on foams’ processing, chemical, and cellular structure, as well as static and dynamic mechanical properties, thermal stability, sound suppression ability, and thermal insulation performance, was analyzed.

2. Materials and Methods

2.1. Materials

Table 1 provides the details on the materials applied in the presented study. Moreover, Figure 1 presents the FTIR spectra of the applied GTR. Most of the observed signals are associated with the different vibrations of the C-H bonds included in various chemical groups. Peaks at 2848 and 2916 cm−1 are characteristic for the symmetric and asymmetric stretching vibrations of the C-H bonds in the methylene groups present in the rubber macromolecules. The band at 1542 cm−1 points to the stretching of C=C unsaturated bonds present in the benzene rings included in the structure of styrene–butadiene rubber [52]. Signals at 1432 and 1373 cm−1 can be attributed to the deformation vibrations of the C-H bonds in the C=CH2 and CH3 groups, respectively [53]. The noticeable peak at 1063 cm−1 points to the vibrations of the C-O-C bonds. Smaller shoulder bands at 995 and 901 cm−1, as well as minor peaks at 801, 733, and 701 cm−1, are characteristic for C-O, C-C, C-H, S=O, and C-S bonds [52].

2.2. Preparation of Flexible Polyurethane Foams

PU foams were prepared on a laboratory scale by a two-step method. The first step was the preparation of the prepolymer. Poly(tetramethylene ether)glycol (PTMG) was dehydrated at 90 °C under vacuum for 90 min. Subsequently, the calculated amount of toluene diisocyanate (TDI) was introduced, and synthesis was carried out at 60 °C under a vacuum for 120 min. The reaction was carried out in a 1 L reactor with a three-necked lid and a connected Rocker Tanker 230 vacuum pump. The content of the unreacted isocyanate groups in the prepolymer was 18.20%, measured using method for the determination of free isocyanate group content by titration with dibutylamine, according to ASTM D-2572. Next, the blend, including PTMG, glycerol, catalysts, foam stabilizer, blowing agent, and, in the case of composite foams, GTR, was prepared. In the case of composite foams, GTR in the amount of 5–20 parts by weight (pbw) of the PU system was homogenized with PTMG and glycerol for 30 s at 500 rpm prior to the incorporation of other components. When the complete blend was prepared, it was mechanically mixed with a previously prepared prepolymer for 10 s at 1000 rpm and poured into a closed aluminum mold heated up to 60 °C. After demolding, samples were held at 60 °C for 24 h and conditioned at room temperature (21–23 °C) for another 24 h. Moreover, for the analysis of foaming kinetics, additional samples were prepared without molding and left for the free rise. Table 2 contains the details of foam formulations.

2.3. Measurements

The following processing times of the prepared PU-based composites were measured: rise time (time of the end of volumetric expansion) and the tack-free time (time until the surface stopped being tacky to the touch). Moreover, during polymerization, the temperature of the foam core was measured using a thermocouple. The temperature was checked every 2 s.
After conditioning, foamed PU composites were analyzed following the standard procedures.
The color of the unfilled PU foam and foamed PU/GTR composites was evaluated following the recommendations of International Commission on Illumination (CIE) [54]. Applied system consists of three color components: L*—lightness (L* = 0 for black and L* = 100 for white), a*—the green(−)/red(+) component, b*—the blue(−)/yellow(+) component. For each sample, 15 specimens were analyzed. The color was determined by optical spectroscopy using Hunter Associates Laboratory, Inc. (Reston, VA, USA) Miniscan MS/S-4000S spectrophotometer, placed in a specially designed light trap chamber to eliminate the impact of the external light sources. The following color parameters were determined:
  • total color difference parameter (ΔE*), calculated according to the Equation (1) [55]:
ΔE* = [(ΔL*)2 + (Δa*)2 + (ΔL*)2]0.5
  • chroma (C*ab), calculated according to the Equation (2):
C*ab = [(a*)2 + (b*)2]0.5
  • hue (hab), calculated according to the Equation (3):
hab = tan−1 (b*/a*)
Determined color parameters were also converted to the commonly used Adobe RGB color space [56].
The apparent density of the prepared composites was determined according to PN-EN ISO 845 standard. The cube-shaped samples were measured with a slide caliper with an accuracy of 0.1 mm and weighed using an analytical balance with an accuracy of 0.0001 g.
The chemical structures of the prepared samples were determined using Fourier transform infrared (FTIR) Nicolet Spectrometer IR200 from Thermo Scientific (Waltham, MA, USA). The device was equipped with an ATR attachment with a diamond crystal. Measurements were performed with 1 cm−1 resolution in the range from 4000 to 400 cm−1 and 64 scans.
The cellular structure of the prepared PU/GTR composites was investigated with a scanning electron microscope (SEM) MIRA3 from Tescan (Brno, Czech Republic). Analyzed samples were carbon-coated with a Jeol JEE 4B vacuum evaporator from Jeol USA (Peabody, MA, USA). The thickness of the coating was approximately 20 nm. The accelerating voltage of 5 kV and secondary electron detector were used.
The images obtained with SEM microscopy were analyzed with ImageJ software. The following shape descriptors of cells were determined:
Circularity (C), calculated according to the Equation (4):
R = (4 ∙ π ∙ A)/P2
Aspect ratio (AR), calculated according to the Equation (5):
AR = LL/LS
Roundness (R), calculated according to the Equation (6):
R = (4 ∙ A)/(π ∙ LL2)
where P is the perimeter, LL and LS are the lengths of the longer and shorter axis of the fitted ellipse, and A is the area of the fitted ellipse.
The content of open cells in manufactured materials was determined with Ultrapyc 5000 Foam gas pycnometer from Anton Paar (Graz, Austria). Following measurement settings were applied: gas—nitrogen; gas pressure—3.0 psi; measurement type—corrected; flow direction—sample first; target temperature—20.0 °C; flow mode—monolith; cell size—medium, 45 cm3; preparation mode—flow, 0.5 min.
The thermal conductivity coefficient (λ) of the obtained materials was determined using the heat flow meter HFM 446 from Netzsch (Selb, Germany). Samples with a thickness of 4 cm were tested in the temperature range from 1 to 19 °C using the average temperature of 10 °C.
The sound absorption coefficients of the material samples were determined following the ISO 10534-2 [57] and ASTM E1050-8 [58] standards. The following equipment was used to carry out the tests: two BSWA impedance tubes (SW422 and SW477), MC 3242 data acquisition hardware, PA50 power amplifier, BSWA VA LAB4 software (produced by BSWA-Technology Co., Ltd., Beijing, China), and two MI 19 microphones—1/4 inch IEPE standard (produced by Roga Instruments, Nentershausen, Germany). The measuring system was calibrated with a CA114 acoustic calibrator (BSWA Technologies Co., Ltd., Beijing, China). The LB−575 climate meter (produced by LAB-EL, Reguły, Poland) monitored atmospheric pressure, temperature, and air humidity. The preparation of the samples for testing included cutting out from the base material (approximately 22 mm thick) and discs with a diameter of 30 and 100 mm. The samples were tested, removing about 6 mm of the uneven upper layer. This treatment was also aimed at exposing the internal cellular structure of the material (foam). After cutting off the top layer, the samples were 16 mm thick.
The composites’ tensile strength was evaluated following ISO 1798 standard. The beam-shaped samples with 10 × 10 × 100 mm3 dimensions subjected to static tensile tests, which were conducted using Zwick/Roell tensile tester (Ulm, Germany) at a constant speed of 500 mm/min.
Dynamical mechanical analysis (DMA) was performed using a Q800 DMA instrument from TA Instruments (New Castle, DE, USA) at a heating rate of 4 °C/min and the temperature range from −100 to 150 °C. Samples were cylindrical-shaped, with dimensions of 10 × 12 mm.
The thermogravimetric (TGA) analysis was performed using the TG 209 F3 apparatus from Netzsch (Selb, Germany). Samples weighing approx. 10 mg were placed in a ceramic dish. The study was conducted in a nitrogen atmosphere from 30 to 800 °C with a temperature increase rate of 10 °C/min.

3. Results and Discussion

Figure 2 presents the influence of the GTR content on the foaming kinetics of the prepared composite foams. Significant differences were noted in processing times, which can be attributed to the introduction of solid particles into the polyol mixture. For the unfilled foam, the rise time was 32 s, while the addition of 20 pbw of GTR resulted in its elongation to 45 s. Such a phenomenon was attributed to the increase in the polyol mixture’s viscosity induced by solid particles. This effect is commonly observed in the case of composite PU foams [1]. Moreover, the PU structure has been weakened because of the disrupted balance between hydroxyl and isocyanate groups present in the PU system. Applied GTR particles, particularly hydroxyl groups present on their surfaces, as proven in our previous works [47,59], partially attracted isocyanate groups and reduced their amount taking part in PU polymerization. A similar effect was noted for the tack-free time, which was noticeably elongated for the highest GTR contents due to the competitivity between hydroxyl groups present in the reacting mixture [46].
Surprisingly, for the lowest contents of GTR, foaming was accelerated compared to neat PU. Such an effect could be associated with the nucleating activity of solid rubber particles. During PU foaming, before beginning the volumetric expansion, the reacting mixture of the polyols and isocyanates must reach supersaturation with blowing gas. Afterward, the nucleation begins and foam rises [60], so the GTR nucleating activity leads to more rapid foaming. Apparently, for the lower contents of GTR, the additional nucleating activity effect overpowered the viscosity increase, which contradicts and limits the foaming rate.
Considering the maximum temperature reached by the foams’ core during polymerization, it was slightly higher for composite foams (101–104 °C) than for the reference sample (97 °C). The highest value was noted for the P5 foam, related to the fastest polymerization and foaming. Therefore, the heat dissipation by convection to the surrounding environment was hindered due to more rapid heat build-up caused by exothermic chemical reactions.
Figure 3 presents the FTIR spectra of unfilled foamed PU matrix and prepared PU/GTR composites. Qualitatively, all spectra show a very similar appearance, indicating a lack of significant changes in the chemical structure induced by GTR incorporation. Signals marked with the numbers 1–5 indicate the presence of bonds characteristic of PU materials. Signals (1) at 3230–3370 cm−1 were attributed to stretching vibrations of N-H bonds, while peaks (3) around 1598 cm−1 and (4) at 1515–1530 cm−1 were related to the bending N-H vibrations [61,62]. Carbonyl bonds C=O can be recognized by the presence of peaks characteristic of their stretching vibrations at 1700–1735 cm−1 [63]. Signals (5) at 1220–1225 cm−1 are typical for stretching vibrations of C-N bonds [64]. The signals mentioned above confirm the efficient generation of urethane bonds during PU foams’ preparation. Other notable absorption bands were noted at 2860–2950 cm−1 and 1010–1120 cm−1 and were attributed to the presence of C-H and C-O bonds, respectively. Their presence was associated with the chemical structure of components used for manufacturing PU matrix and introduced GTR particles.
Table 3 presents the impact of the introduced filler on the appearance of the foams in quantitative terms. Due to the introduction of dark, black rubber particles, the parameter affected most significantly was foams’ lightness. It decreased proportionally to the content of the GTR in the composite foams. A significant decrease, even for 5 wt% content of GTR, was noted. Other color parameters were noticeably less influenced by the filler incorporation, which is attributed to the fact that, irrespectively of composition, the color of all samples could be described as gray. Gray colors are characterized by the low values of a*, b*, and chroma [65,66]. For such low chroma values, the influence of hue angle on the actual color of the material is negligible [67]. Moreover, the numerical data presented in Table 3 are related to the average color of the whole material. Actual foam images presented in Table 3 show that the appearance of the material is not homogenous, which is associated with the micrometric size of applied GTR particles. Nevertheless, the appearance of composite foams is in line with the appearance of PU/GTR foams presented in previous work [45].
Table 4 shows the parameters describing the cellular structure of prepared foams, whose images made with scanning electron microscopy are presented in Figure 4. It can be seen that the introduction of GTR particles into foamed PU matrix caused noticeable changes in cellular structure. The most crucial change was associated with the reduction in foams’ average cell diameter, one of the main parameters describing the foams’ structure, critical for their performance, and essential applications like thermal and acoustic insulation [68,69]. It can be observed in composite PU foams when filler particles act as nucleating agents [70,71]. Such an effect was noted in our previous work of flexible PU foamed composites filled with GTR [47]. Nevertheless, GTR particles may disrupt the cellular structure for higher loadings, which can be associated with the changes in the viscosity of the reacting mixture during polymerization and the insufficient surface area of the rubber particles [46]. Disruption of cellular structure can also be associated with changes in cells’ shape. The increasing filler content caused an increase in cells’ aspect ratio, indicating they were more ellipsoidal than unfilled foam. It can be attributed to the presence of filler particles, which increase the viscosity of the reaction mixture and increases the structure heterogeneity [72]. Similar effects have been noted by other researchers [73]. Roundness, as the antagonist of aspect ratio according to Equations (2) and (3), decreased with the GTR content, meaning the cells in the composite foams were less similar to a perfect circle compared to the unfilled matrix. Another parameter describing cells’ shape is circularity, which, compared to roundness, includes the aspect ratio between perpendicular diameters and the perfection of cells’ perimeter [74]. Therefore, its values are lower than roundness, decreasing significantly with the loading of GTR particles. Such an effect is related to the imperfections of cells’ perimeters at the interface with filler particles.
Besides the parameters describing the shape of cells, another important parameter describing the cellular structure of PU foams is the open cell content, which is critical for the many applications of these materials [75]. The opening of the cells due to the GTR incorporation could be attributed to the reduced strength of cells, which were not strong enough to keep foaming gas inside closed cells [76]. Again, such an effect can be attributed to the partial attraction of isocyanate groups by hydroxyl functionalities present on the surface of the GTR particles, which reduced the strength of the PU phase.
Structural changes in the PU foams induced by the GTR incorporation also affected the apparent density of the composites. For lower contents, a slight drop in this parameter was noted despite the presence of solid particles due to their nucleating activity [77]. Similar to changes in foaming kinetics, the additional nucleating activity of the GTR overpowered the increase of polyols’ mixture viscosity, so the enhanced nucleation reduced composites’ apparent density. Despite the nucleating activity for higher contents of GTR, the viscosity of the reacting mixture was apparently too high, which, combined with the above-mentioned reduced strength of the PU phase, limited the volumetric expansion and increased the apparent density.
Table 4 presents the sol fraction content values determined during swelling of the foams in toluene. It can be seen that they are increasing with the GTR content in the composites. For the unfilled PU foam sol fraction content, related to the presence of a noncrosslinked portion of the material, which can be extracted during swelling, it equaled 1.35 wt%, indicating slight imperfections in the balance between the isocyanate and hydroxyl groups present in the system. For the PU/GTR composites, the value of this parameter increased to 1.91–2.74 wt%, which can be attributed to the partial decomposition of the rubber network during the shredding of the postconsumer car tires aimed at the production of GTR. Similar effects were noted in our previous works on PU composites filled with GTR particles [46,47].
The above-mentioned parameters of the cellular structure have a significant influence on the thermal conductivity of the cellular materials, as thermal insulation is one of their primary applications. The precise, quantitative impact of the structural parameters on the thermal conductivity coefficient (λ) is very complex due to the dual nature of foamed materials and different heat transfer mechanisms. The most critical parameter is the apparent density, which quantifies the share of solid and gas phases in foamed materials due to the significant differences in λ values between solid PU, 200–250 mW/(m·K) reported by most studies [78,79], and gases present inside closed or open cells, from ~10 mW/(m·K) for hydrofluorocarbons to 12–15 mW/(m·K) for C5 hydrocarbons and carbon dioxide, to 25 mW/(m·K) for air [80,81].
Other structural parameters, cell size, and open cell content are more critical for heat exchange via radiation and convection. Increasing the cell size significantly enhances the radiative heat transfer and area of heat convection, so thermal insulation materials should be characterized by possibly small cells [82,83]. The high content of open cells facilitates temperature gradient-induced gas flow, simultaneously enhancing the convective heat transfer [84].
Considering the incorporation of solid GTR particles into prepared foams, they should replace the solid PU part in the material. Therefore, assuming a lower thermal conductivity coefficient of GTR compared to 200–250 mW/(m·K) reported by most studies for solid PU [78,79], the addition of GTR seems beneficial for insulation performance [85].
The above-mentioned theoretical considerations have been somehow confirmed for prepared PU/GTR composite materials. It can be seen that the incorporation of GTR resulted in a slight decrease in thermal conductivity coefficient compared to the unfilled foam. Such an effect should be considered beneficial because multiple works pointed to the disruption of the cellular structure due to GTR incorporation, which unfavorably affects the insulation performance [43,44,45,46,86]. The slight reduction of λ values could be attributed to the reduction of average cell diameters resulting from the nucleating activity of solid GTR particles and the increased polyol mixture viscosity. Nevertheless, increasing the content of open cells and the higher apparent density provide contradictory effects. Therefore, despite the significantly finer cellular structure, only minor changes in the thermal conductivity coefficients were noted.
The results of the sound absorption test are presented in Figure 5 in the form of characteristics containing the values of sound absorption coefficients in 1/3 octave bands (100–6300 Hz). The characteristics were created based on partial results obtained in the bands 63–500 Hz and 250–1600 Hz (using an impedance tube SW422 with a diameter of 100 mm and different spacing of microphones) and in the band 1000–6300 Hz (using a tube SW 477 with a diameter of 30 mm). The final result for each sample is the result of averaging three measurements. In addition, for each type of material (samples), the average value of the sound absorption coefficient αavg and the weighted sound absorption coefficient αw were determined (Figure 6). The value of αavg was calculated according to the Equation (7):
αavg = 1/n · ∑(i = 1)n · αf(i)
where αf(i) are the sound absorption coefficients (at the center frequencies f(i) from 100 Hz to 6.3 kHz) and n is the number of 1/3 octave bands.
Weighted sound absorption coefficients αw were determined following the ISO 11654 standard. The basis for determining αw are the practical sound absorption coefficients αp(i) and the reference curve (specified in the standard), which are placed on one diagram. Values of αp(i) are calculated in octave bands from 125 Hz to 4 kHz by averaging sound absorption coefficients in the 1/3 octave bands (arithmetic mean value of the three 1/3 octave sound absorption coefficients within the octave). Next, the reference curve is shifted in steps of 0.05 towards the measured values αp(i) until the sum of the unfavorable deviations is less than or equal to 0.10. An unfavorable deviation occurs when the measured value is less than the value of the reference curve. Only these deviations are taken into account in the calculations. The weighted sound absorption αw is defined as the value of the shifted reference curve at 500 Hz.
Considering sound absorption performance of cellular materials, the most important are the parameters quantitatively describing the structure, such as the average cell diameter and open cell content. The prepared foams presented behavior typical for such materials, with the sound absorption coefficient increasing with the sound frequency [87]. Such an effect is associated with the decreasing wavelength and facilitates the penetration of the PU cellular structure by the sound waves. Compared to the unfilled PU matrix, introducing the GTR particles slightly enhanced the αavg values, indicating the increased damping ability. Other researchers noted similar effects due to the excellent damping performance of rubber [48,49]. Among the prepared composites, sample P10 is characterized by the lowest value of sound absorption coefficient, which can be attributed to the smallest cell size and open cell content limiting the sound attenuation effect [88]. The closed-cell structure has a limited ability to absorb sound waves, which has been reported in the literature [89,90]. The sound absorption performance of foams is also affected by their mechanical properties. The results of the dynamic mechanical analysis presented in the following sections point to the reduced damping ability of the cellular structure, limiting the conversion of the sound waves into kinetic energy.
Nevertheless, the prepared foams are characterized by a relatively high apparent density for sound-absorbing materials, which noticeably affects the portion of pores inside the material, where the sound suppression effect occurs. Therefore, the impact of the structural parameters on the sound absorption performance is not very significant. Generally, the sound absorption performance of the prepared foamed composites does not allow for classifying them as a sound-absorbing material dedicated to the acoustic insulation of buildings [91].
Figure 7 presents the results of performed static analysis of the prepared PU/GTR composite foams. Incorporating the GTR particles caused a slight enhancement of foams’ tensile performance. It can be associated with the increase in apparent density but also points to the satisfactory interfacial adhesion between PU matrix and GTR particles. Moreover, Table 4 indicates the reduction in the average cell size, which caused stress dispersion and decreased stress concentration, enhancing foams’ strength [92,93]. Such an effect could be one of the components contributing to the slight enhancement of tensile strength. A similar phenomenon was responsible for slightly increasing elongation at break with GTR content. Moreover, the above-mentioned attraction of the isocyanate groups by the hydroxyls present on the surface of the introduced rubber particles yielded a reduced crosslinking of the PU structure, which beneficially impacted its ductility.
The results of the dynamic mechanical analysis of the prepared PU/GTR composites are presented in Figure 8. It can be seen that the incorporation of the rubber particles reduced the storage modulus of the unfilled foam, especially for the lowest contents of filler. Such an effect could be associated with the disruptions of the PU structures caused by the presence of hydroxyl functionalities on the surface of the GTR particles [59]. However, increasing the filler content led to the enhancement of the foams’ modulus, which can be attributed to the superior performance of the car tires (the primary product from which GTR was produced) compared to the flexible PU foam. For higher GTR loadings, incorporated particles were increasingly more “responsible” for the material’s mechanical performance, so their superior performance overcame the unfavorable effect of the disrupted isocyanate:hydroxyl balance.
Figure 8b, showing the temperature plot of loss modulus, indicated that the damping performance of PU foam was reduced after the introduction of GTR particles. Such an effect indicated that the incorporation of the GTR particles slightly limited the foams’ ability to dissipate mechanical energy, which could be associated with the potential chemical interactions at the interface. A similar effect was noted in previous works dealing with foamed flexible PU/GTR composites [44], as well as in the other works on flexible foamed PU-based composites containing fillers that could chemically interact with isocyanates during foaming [15]. Moreover, the temperature positions of the E” peak can be used to determine the glass transition temperature, which, for all analyzed materials, was in the range from −64.3 °C to −63.7 °C, so the changes were hardly noticeable.
Figure 9 presents the results of the thermogravimetric analysis of the prepared PU/GTR composites. It can be seen that the incorporation of waste rubber particles did not change the course of the PU thermal decomposition, which was mirrored in the relatively similar appearance of differential thermogravimetric curves. Minor changes in the position of the individual peaks and their magnitude can be attributed to the differences in the chemical composition between the PU and GTR phases. The first peak, around 270 °C, can be attributed to the decomposition of the hard segments, including the urethane bonds. The magnitude of this peak is relatively weak due to the low value of the isocyanate index applied during foams’ preparation. Second, a more potent peak around 422 °C was associated with the decomposition of the soft PU segments, mainly hydrocarbon chains of polyols and styrene–butadiene rubber present in the structure of the GTR [94]. Moreover, the mass loss rate was slightly increased between 340 and 390 °C, which was related to the decomposition of the natural rubber phase of the GTR [46]. Quantitatively, the enhancement of the PU foam thermal stability was noted, which was expressed by the shift of the thermal decomposition onset from 232.5 to 243.6 °C for 20 wt% GTR content. A similar effect was noted in previous work [42] and was attributed to the notably higher stability of the GTR compared to the unfilled foam. Despite the oxidation and partial scission of sulfur bridges during the shredding of the car tires resulting in the generation of GTR, the majority of the crosslinked rubber structure prevailed, maintaining the high thermal stability of the applied particles [95]. The onset of degradation for the GTR applied in the presented work was 256.6 °C, as reported in previous work [42]. Moreover, the char residue after thermal degradation was noticeably higher, mainly due to the high content of carbon black [96].

4. Conclusions

The goal of the presented study was the investigation of the impact of applying GTR particles originated from the material recycling of used car tires as a cost-effective waste-based filler for flexible PU foams. Foams were obtained by the two-step prepolymer method instead of the conventional single-step method for better control of the polymerization and volumetric expansion. The introduction of GTR slightly extended the rise time of the foam from the initial 32 s to 33–45 s because of the higher viscosity of the polyol mixture and the attraction of the isocyanate groups by the hydroxyls present on the surface of the rubber particles. The higher viscosity during the volumetric expansion inhibited the dissipation of the heat generated in the exothermic reactions and slightly increased the temperature reached by the foams’ core from 97 °C to 101–104 °C. Nevertheless, for the lower GTR loadings, changes in the foaming kinetics were insignificant due to the contradictory nucleating activity of the solid rubber particles.
Changes in foams’ polymerization induced by GTR were mirrored in their cellular structure. The average cell diameter was reduced from 263.1 µm to 144.8–188.5 µm, which was associated with the above-mentioned nucleating activity of the GTR. The most significant and beneficial reduction was noted for the lowest filler content when the viscosity of the polyol mixture was only slightly elevated. For the higher loadings, the above-mentioned attraction of the isocyanate groups by the hydroxyls present on the surface of rubber particles caused the weakening of the PU matrix, increasing the content of the open cells from 49.0% to 51.9% for 20 pbw of GTR. Cumulatively, the changes in the cellular structure caused a 1.0–4.3% drop in thermal conductivity coefficient, enhancing the thermal insulation performance of the composites compared to the unfilled PU foam. On the other hand, the sound absorption performance of the investigated materials was limited due to the relatively low content of the open cells. It did not allow for classifying them as sound-absorbing materials dedicated to the acoustic insulation of buildings.
Due to the excellent mechanical performance of car tires, the primary application of GTR, the tensile performance of the foams was enhanced despite the disruption of the cellular structure resulting from competitiveness between the hydroxyl groups present in the structure of polyols and on the surface of the GTR particles. Tensile strength and elongation at break were increased by 10 and 8% for 20 parts by weight GTR addition, pointing to the satisfactory interfacial adhesion between the PU matrix and the GTR particles. Moreover, the average cell size reduction caused stress dispersion and decreased stress concentration, enhancing foams’ strength. On the other hand, the dynamic mechanical performance and damping ability of the unfilled foam were reduced after the GTR incorporation due to the weakening of the PU structure.
The application of GTR as filler also enhanced the thermal stability of the foam, shifting the thermal decomposition onset from 232.5 °C to 243.6 °C for 20 pbw GTR content. Such an effect was attributed to the higher stability of the rubber filler compared to the unfilled PU foam, exceeding 256 °C.
Generally, the presented work indicates that GTR can be efficiently applied as a filler for flexible PU foams, which could simultaneously enhance their performance, reduce costs, and limit the environmental impacts due to the application of waste-based material.

Author Contributions

Conceptualization, P.K. and A.H.; methodology, A.P., M.B., R.B. and A.H.; validation, P.K., M.B. and A.H.; formal analysis, A.H.; investigation, A.P., W.Ż., M.S., P.W. and R.B.; resources, A.H.; data curation, A.H.; writing—original draft preparation, W.Ż., M.B. and A.H.; writing—review and editing, P.K., M.B. and A.H.; visualization, M.B. and A.H.; supervision, A.H.; project administration, P.K. and A.H.; funding acquisition, A.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by The National Centre for Research and Development (NCBR, Poland) in the frame of LIDER/3/0013/L-10/18/NCBR/2019 project—Development of technology for the manufacturing of foamed polyurethane-rubber composites for the use as damping materials. The investigations of sound absorption performance were carried out as part of the research activities described in the 0612/SBAD/3576 task allocated in Poznan University of Technology.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in Comprehensive enhancement of prepolymer-based flexible polyurethane foams’ performance by introduction of cost-effective waste-based ground tire rubber particles.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Hejna, A.; Kopczyńska, M.; Kozłowska, U.; Klein, M.; Kosmela, P.; Piszczyk, Ł. Foamed Polyurethane Composites with Different Types of Ash-Morphological, Mechanical and Thermal Behavior Assessments. Cell. Polym. 2016, 35, 287–308. [Google Scholar] [CrossRef]
  2. Bukowczan, A.; Hebda, E.; Michałowski, S.; Pielichowski, K. Modification of Polyurethane Viscoelastic Foams by Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS). Compos. Theory Pract. 2018, 18, 77–81. [Google Scholar]
  3. Fei, Y.; Chen, F.; Fang, W.; Hejna, A.; Xu, L.; Liu, T.; Zhong, M.; Yang, J.; Kuang, T. Conductive Thermoplastic Polyurethane Nanocomposite Foams Derived from a Cellulose/MWCNTs Aerogel Framework: Simultaneous Enhancement of Piezoresistance, Strength, and Endurance. J. Mater. Chem. C 2021, 9, 13103–13114. [Google Scholar] [CrossRef]
  4. Das, A.; Mahanwar, P. A Brief Discussion on Advances in Polyurethane Applications. Adv. Ind. Eng. Polym. Res. 2020, 3, 93–101. [Google Scholar] [CrossRef]
  5. Sawpan, M.A. Polyurethanes from Vegetable Oils and Applications: A Review. J. Polym. Res. 2018, 25, 184. [Google Scholar] [CrossRef]
  6. Petrovic, Z. Polyurethanes from Vegetable Oils. Polym. Rev. 2008, 48, 109–155. [Google Scholar] [CrossRef]
  7. Desroches, M.; Escouvois, M.; Auvergne, R.; Caillol, S.; Boutevin, B. From Vegetable Oils to Polyurethanes: Synthetic Routes to Polyols and Main Industrial Products. Polym. Rev. 2012, 52, 38–79. [Google Scholar] [CrossRef]
  8. Marcovich, N.E.; Kurańska, M.; Prociak, A.; Malewska, E.; Kulpa, K. Open Cell Semi-Rigid Polyurethane Foams Synthesized Using Palm Oil-Based Bio-Polyol. Ind. Crops Prod. 2017, 102, 88–96. [Google Scholar] [CrossRef]
  9. Feng, Y.; Liang, H.; Yang, Z.; Yuan, T.; Luo, Y.; Li, P.; Yang, Z.; Zhang, C. A Solvent-Free and Scalable Method to Prepare Soybean-Oil-Based Polyols by Thiol–Ene Photo-Click Reaction and Biobased Polyurethanes Therefrom. ACS Sustain. Chem. Eng. 2017, 5, 7365–7373. [Google Scholar] [CrossRef]
  10. Chu, M.M.; Maguire, G. Global Edible Oil Prices near Their Peak, but Retreat May Be Slow—Analysts. Available online: https://www.reuters.com/article/us-global-palmoil-prices-analysts-idUSKBN2BG2CJ2021 (accessed on 20 July 2022).
  11. George, B. Oilseeds: World Markets and Trade; Foreign Agricultural Service, U.S. Department of Agriculture: Washington, DC, USA, 12 August 2022. Available online: https://www.fas.usda.gov/data/oilseeds-world-markets-and-trade (accessed on 20 July 2022).
  12. Fridrihsone, A.; Abolins, A.; Kirpluks, M. Screening Life Cycle Assessment of Tall Oil-Based Polyols Suitable for Rigid Polyurethane Foams. Energies 2020, 13, 5249. [Google Scholar] [CrossRef]
  13. Fridrihsone, A.; Romagnoli, F.; Kirsanovs, V.; Cabulis, U. Life Cycle Assessment of Vegetable Oil Based Polyols for Polyurethane Production. J. Clean. Prod. 2020, 266, 121403. [Google Scholar] [CrossRef]
  14. Hejna, A.; Kosmela, P.; Klein, M.; Formela, K.; Kopczyńska, M.; Haponiuk, J.; Piszczyk, Ł. Two-Step Conversion of Crude Glycerol Generated by Biodiesel Production into Biopolyols: Synthesis, Structural and Physical Chemical Characterization. J. Polym. Environ. 2018, 26, 3334–3344. [Google Scholar] [CrossRef]
  15. Członka, S.; Bertino, M.F.; Strzelec, K.; Strąkowska, A.; Masłowski, M. Rigid Polyurethane Foams Reinforced with Solid Waste Generated in Leather Industry. Polym. Test. 2018, 69, 225–237. [Google Scholar] [CrossRef]
  16. Augaitis, N.; Vaitkus, S.; Członka, S.; Kairytė, A. Research of Wood Waste as a Potential Filler for Loose-Fill Building Insulation: Appropriate Selection and Incorporation into Polyurethane Biocomposite Foams. Materials 2020, 13, 5336. [Google Scholar] [CrossRef]
  17. Kurańska, M.; Barczewski, M.; Uram, K.; Lewandowski, K.; Prociak, A.; Michałowski, S. Basalt Waste Management in the Production of Highly Effective Porous Polyurethane Composites for Thermal Insulating Applications. Polym. Test. 2019, 76, 90–100. [Google Scholar] [CrossRef]
  18. Kurańska, M.; Leszczyńska, M.; Malewska, E.; Prociak, A.; Ryszkowska, J. Implementation of Circular Economy Principles in the Synthesis of Polyurethane Foams. Polymers 2020, 12, 2068. [Google Scholar] [CrossRef] [PubMed]
  19. Salasinska, K.; Barczewski, M.; Aniśko, J.; Hejna, A.; Celiński, M. Comparative Study of the Reinforcement Type Effect on the Thermomechanical Properties and Burning of Epoxy-Based Composites. J. Compos. Sci. 2021, 5, 89. [Google Scholar] [CrossRef]
  20. Matykiewicz, D.; Barczewski, M. On the Impact of Flax Fibers as an Internal Layer on the Properties of Basalt-Epoxy Composites Modified with Silanized Basalt Powder. Compos. Commun. 2020, 20, 100360. [Google Scholar] [CrossRef]
  21. Otto, G.P.; Moisés, M.P.; Carvalho, G.; Rinaldi, A.W.; Garcia, J.C.; Radovanovic, E.; Fávaro, S.L. Mechanical Properties of a Polyurethane Hybrid Composite with Natural Lignocellulosic Fibers. Compos. Part B Eng. 2017, 110, 459–465. [Google Scholar] [CrossRef]
  22. Zanoletti, A.; Ciacci, L. The Reuse of Municipal Solid Waste Fly Ash as Flame Retardant Filler: A Preliminary Study. Sustainability 2022, 14, 2038. [Google Scholar] [CrossRef]
  23. Kusakli, S.; Kocaman, S.; Ceyhan, A.A.; Ahmetli, G. Improving the Flame Retardancy and Mechanical Properties of Epoxy Composites Using Flame Retardants with Red Mud Waste. J. Appl. Polym. Sci. 2021, 138, 50106. [Google Scholar] [CrossRef]
  24. Barczewski, M.; Hejna, A.; Sałasińska, K.; Aniśko, J.; Piasecki, A.; Skórczewska, K.; Andrzejewski, J. Thermomechanical and Fire Properties of Polyethylene-Composite-Filled Ammonium Polyphosphate and Inorganic Fillers: An Evaluation of Their Modification Efficiency. Polymers 2022, 14, 2501. [Google Scholar] [CrossRef]
  25. Tang, G.; Liu, X.; Zhou, L.; Zhang, P.; Deng, D.; Jiang, H. Steel Slag Waste Combined with Melamine Pyrophosphate as a Flame Retardant for Rigid Polyurethane Foams. Adv. Powder Technol. 2020, 31, 279–286. [Google Scholar] [CrossRef]
  26. Kuru, D.; Akpinar Borazan, A.; Guru, M. Effect of Chicken Feather and Boron Compounds as Filler on Mechanical and Flame Retardancy Properties of Polymer Composite Materials. Waste Manag. Res. J. Sustain. Circ. Econ. 2018, 36, 1029–1036. [Google Scholar] [CrossRef]
  27. Salasinska, K.; Celiński, M.; Mizera, K.; Kozikowski, P.; Leszczyński, M.K.; Gajek, A. Synergistic Effect between Histidine Phosphate Complex and Hazelnut Shell for Flammability Reduction of Low-Smoke Emission Epoxy Resin. Polym. Degrad. Stab. 2020, 181, 109292. [Google Scholar] [CrossRef]
  28. Zabihi, O.; Ahmadi, M.; Yadav, R.; Mahmoodi, R.; Naderi Kalali, E.; Nikafshar, S.; Ghandehari Ferdowsi, M.R.; Wang, D.Y.; Naebe, M. Novel Phosphorous-Based Deep Eutectic Solvents for the Production of Recyclable Macadamia Nutshell–Polymer Biocomposites with Improved Mechanical and Fire Safety Performances. ACS Sustain. Chem. Eng. 2021, 9, 4463–4476. [Google Scholar] [CrossRef]
  29. Battig, A.; Sanchez-Olivares, G.; Rockel, D.; Maldonado-Santoyo, M.; Schartel, B. Waste not, Want not: The Use of Leather Waste in Flame Retarded EVA. Mater. Des. 2021, 210, 110100. [Google Scholar] [CrossRef]
  30. Abdulkareem, S.A.; Adeniyi, A.G. Recycling Copper and Polystyrene from Solid Waste Stream in Developing Conductive Composites. J. Solid Waste Technol. Manag. 2019, 45, 39–44. [Google Scholar] [CrossRef]
  31. Adeniyi, A.G.; Ighalo, J.O.; Abdulkareem, S.A. Al, Fe and Cu Waste Metallic Particles in Conductive Polystyrene Composites. Int. J. Sustain. Eng. 2021, 14, 893–898. [Google Scholar] [CrossRef]
  32. Kim, W.Y.; Jang, J.; Lee, H.S.; Kim, S.Y. Electrical Conductivity of Polymer Composites Based on Carbonized Wood Flour via Plasma Post-treatment as an Effective and Economical Filler. Polym. Compos. 2021, 42, 4814–4821. [Google Scholar] [CrossRef]
  33. Giorcelli, M.; Bartoli, M. Development of Coffee Biochar Filler for the Production of Electrical Conductive Reinforced Plastic. Polymers 2019, 11, 1916. [Google Scholar] [CrossRef] [PubMed]
  34. Gama, N.; Ferreira, A.; Barros-Timmons, A. 3D Printed Thermoplastic Polyurethane Filled with Polyurethane Foams Residues. J. Polym. Environ. 2020, 28, 1560–1570. [Google Scholar] [CrossRef]
  35. Zhang, L.; Liang, S.; Chen, Z. Influence of Particle Size and Addition of Recycling Phenolic Foam on Mechanical and Flame Retardant Properties of Wood-Phenolic Composites. Constr. Build. Mater. 2018, 168, 1–10. [Google Scholar] [CrossRef]
  36. Matykiewicz, D.; Sałasińska, K.; Barczewski, M. The Effect of Poly(Vinyl Chloride) Powder Addition on the Thermomechanical Properties of Epoxy Composites Reinforced with Basalt Fiber. Materials 2020, 13, 3611. [Google Scholar] [CrossRef]
  37. Ratna, D.; Banthia, A.K. Rubber Toughened Epoxy. Macromol. Res. 2004, 12, 11–21. [Google Scholar] [CrossRef]
  38. Aoudia, K.; Azem, S.; Aït Hocine, N.; Gratton, M.; Pettarin, V.; Seghar, S. Recycling of Waste Tire Rubber: Microwave Devulcanization and Incorporation in a Thermoset Resin. Waste Manag. 2017, 60, 471–481. [Google Scholar] [CrossRef]
  39. Kandasamy, J.; Gökalp, I. Pyrolysis, Combustion, and Steam Gasification of Various Types of Scrap Tires for Energy Recovery. Energy Fuels 2015, 29, 346–354. [Google Scholar] [CrossRef]
  40. Formela, K. Sustainable Development of Waste Tires Recycling Technologies—Recent Advances, Challenges and Future Trends. Adv. Ind. Eng. Polym. Res. 2021, 4, 209–222. [Google Scholar] [CrossRef]
  41. Hejna, A.; Korol, J.; Przybysz-Romatowska, M.; Zedler, Ł.; Chmielnicki, B.; Formela, K. Waste Tire Rubber as Low-Cost and Environmentally-Friendly Modifier in Thermoset Polymers—A Review. Waste Manag. 2020, 108, 106–118. [Google Scholar] [CrossRef]
  42. Kosmela, P.; Olszewski, A.; Zedler, Ł.; Hejna, A.; Burger, P.; Formela, K. Structural Changes and Their Implications in Foamed Flexible Polyurethane Composites Filled with Rapeseed Oil-Treated Ground Tire Rubber. J. Compos. Sci. 2021, 5, 90. [Google Scholar] [CrossRef]
  43. Piszczyk, L.; Hejna, A.; Formela, K.; Danowska, M.; Strankowski, M. Rigid Polyurethane Foams Modified with Ground Tire Rubber—Mechanical, Morphological and Thermal Studies. Cell. Polym. 2015, 34, 45–62. [Google Scholar] [CrossRef]
  44. Piszczyk, Ł.; Hejna, A.; Formela, K.; Danowska, M.; Strankowski, M. Effect of Ground Tire Rubber on Structural, Mechanical and Thermal Properties of Flexible Polyurethane Foams. Iran. Polym. J. 2015, 24, 75–84. [Google Scholar] [CrossRef]
  45. Piszczyk, L.; Hejna, A.; Danowska, M.; Strankowski, M.; Formela, K. Polyurethane/Ground Tire Rubber Composite Foams Based on Polyglycerol: Processing, Mechanical and Thermal Properties. J. Reinf. Plast. Compos. 2015, 34, 708–717. [Google Scholar] [CrossRef]
  46. Hejna, A.; Olszewski, A.; Zedler, Ł.; Kosmela, P.; Formela, K. The Impact of Ground Tire Rubber Oxidation with H2O2 and KMnO4 on the Structure and Performance of Flexible Polyurethane/Ground Tire Rubber Composite Foams. Materials 2021, 14, 499. [Google Scholar] [CrossRef]
  47. Kosmela, P.; Olszewski, A.; Zedler, Ł.; Burger, P.; Piasecki, A.; Formela, K.; Hejna, A. Ground Tire Rubber Filled Flexible Polyurethane Foam—Effect of Waste Rubber Treatment on Composite Performance. Materials 2021, 14, 3807. [Google Scholar] [CrossRef]
  48. Gayathri, R.; Vasanthakumari, R.; Padmanabhan, C. Thermal and Mechanical Behavior of Polyurethane Foam Modified with Nano Silica, Nano Clay and Crumb Rubber Fillers. Int. J. Sci. Eng. Res. 2013, 4, 301–308. [Google Scholar]
  49. Zhang, X.; Lu, Z.; Tian, D.; Li, H.; Lu, C. Mechanochemical Devulcanization of Ground Tire Rubber and Its Application in Acoustic Absorbent Polyurethane Foamed Composites. J. Appl. Polym. Sci. 2013, 127, 4006–4014. [Google Scholar] [CrossRef]
  50. Shan, C.W.; Idris, M.I.; Ghazali, M.I. Study of Flexible Polyurethane Foams Reinforced with Coir Fibres and Tyre Particles. Int. J. Appl. Phys. Math. 2012, 2, 123. [Google Scholar] [CrossRef]
  51. Tian, Z.; Zhang, Z.; Zhang, K.; Huang, S.; Luo, Y. Preparation and Properties of High Viscosity and Elasticity Asphalt by Styrene–Butadiene–Styrene/Polyurethane Prepolymer Composite Modification. J. Appl. Polym. Sci. 2020, 137, 49123. [Google Scholar] [CrossRef]
  52. Zedler, Ł.; Kowalkowska-Zedler, D.; Vahabi, H.; Saeb, M.R.; Colom, X.; Cañavate, J.; Wang, S.; Formela, K. Preliminary Investigation on Auto-Thermal Extrusion of Ground Tire Rubber. Materials 2019, 12, 2090. [Google Scholar] [CrossRef]
  53. Zedler, Ł.; Kowalkowska-Zedler, D.; Colom, X.; Cañavate, J.; Saeb, M.R.; Formela, K. Reactive Sintering of Ground Tire Rubber (GTR) Modified by a Trans-Polyoctenamer Rubber and Curing Additives. Polymers 2020, 12, 3018. [Google Scholar] [CrossRef] [PubMed]
  54. International Commission on Illumination. CIE Recommendations on Uniform Color Spaces, Color-Difference Equations, and Metric Color Terms. Color Res. Appl. 1977, 2, 5–6. [Google Scholar] [CrossRef]
  55. Hejna, A.; Marć, M.; Kowalkowska-Zedler, D.; Pladzyk, A.; Barczewski, M. Insights into the Thermo-Mechanical Treatment of Brewers’ Spent Grain as a Potential Filler for Polymer Composites. Polymers 2021, 13, 879. [Google Scholar] [CrossRef]
  56. Hejna, A.; Barczewski, M.; Skórczewska, K.; Szulc, J.; Chmielnicki, B.; Korol, J.; Formela, K. Sustainable Upcycling of Brewers’ Spent Grain by Thermo-Mechanical Treatment in Twin-Screw Extruder. J. Clean. Prod. 2021, 285, 124839. [Google Scholar] [CrossRef]
  57. ISO 10534-2; Acoustics-Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes—Part 2: Transfer-Function Method 2003. International Organization for Standardization: Geneva, Switzerland, 2007.
  58. ASTM E1050-08; Standard Test Method for Impedance and Absorption of Acoustical Material Using a Tube, Two Microphones and a Digital Frequency Analysis System 2008. ASTM International: West Conshohocken, PA, USA, 2008.
  59. Zedler, Ł.; Kosmela, P.; Olszewski, A.; Burger, P.; Formela, K.; Hejna, A. Recycling of Waste Rubber by Thermo-Mechanical Treatment in a Twin-Screw Extruder. Proceedings 2021, 69, 10. [Google Scholar]
  60. Rao, R.R.; Mondy, L.A.; Long, K.N.; Celina, M.C.; Wyatt, N.; Roberts, C.C.; Soehnel, M.M.; Brunini, V.E. The Kinetics of Polyurethane Structural Foam Formation: Foaming and Polymerization. AIChE J. 2017, 63, 2945–2957. [Google Scholar] [CrossRef]
  61. Kosmela, P.; Hejna, A.; Formela, K.; Haponiuk, J.; Piszczyk, Ł. The Study on Application of Biopolyols Obtained by Cellulose Biomass Liquefaction Performed with Crude Glycerol for the Synthesis of Rigid Polyurethane Foams. J. Polym. Environ. 2018, 26, 2546–2554. [Google Scholar] [CrossRef]
  62. De Sousa, T.P.T.; da Costa, M.S.T.; Guilherme, R.; Orcini, W.; Holgado, L.D.A.; Silveira, E.M.V.; Tavano, O.; Magdalena, A.G.; Catanzaro-Guimarães, S.A.; Kinoshita, A. Polyurethane Derived from Ricinus Communis as Graft for Bone Defect Treatments. Polímeros 2018, 28, 246–255. [Google Scholar] [CrossRef]
  63. Kosmela, P.; Hejna, A.; Suchorzewski, J.; Piszczyk, Ł.; Haponiuk, J.T. Study on the Structure-Property Dependences of Rigid PUR-PIR Foams Obtained from Marine Biomass-Based Biopolyol. Materials 2020, 13, 1257. [Google Scholar] [CrossRef]
  64. Hejna, A.; Haponiuk, J.; Piszczyk, Ł.; Klein, M.; Formela, K. Performance Properties of Rigid Polyurethane-Polyisocyanurate/Brewers’ Spent Grain Foamed Composites as Function of Isocyanate Index. e-Polymers 2017, 17, 427–437. [Google Scholar] [CrossRef]
  65. Faria, P.; Nogueira, T.; Ferreira, A.; Carlos, C.; Rosado, L. AI-Powered Mobile Image Acquisition of Vineyard Insect Traps with Automatic Quality and Adequacy Assessment. Agronomy 2021, 11, 731. [Google Scholar] [CrossRef]
  66. Hejna, A.; Barczewski, M.; Kosmela, P.; Aniśko, J.; Mysiukiewicz, O.; Marć, M. Mandarin peel as an auspicious functional filler for polymer composites. Maced. J. Chem. Chem. Eng. 2021, 40, 1–18. [Google Scholar] [CrossRef]
  67. Onur Cinko, U.; Behcet Becerir, B. Dependence of Colour Difference Formulae on Regular Changes of Colour Coordinates in CIELAB Colour Space. Ind. Text. 2019, 70, 248–254. [Google Scholar] [CrossRef]
  68. Mester, E.; Pecsmány, D.; Jálics, K.; Filep, Á.; Varga, M.; Gráczer, K.; Viskolcz, B.; Fiser, B. Exploring the Potential to Repurpose Flexible Moulded Polyurethane Foams as Acoustic Insulators. Polymers 2021, 14, 163. [Google Scholar] [CrossRef]
  69. Valipour, F.; Dehghan, S.F.; Hajizadeh, R. The Effect of Nano- and Microfillers on Thermal Properties of Polyurethane Foam. Int. J. Environ. Sci. Technol. 2022, 19, 541–552. [Google Scholar] [CrossRef]
  70. Piszczyk, Ł.; Hejna, A.; Formela, K.; Danowska, M.; Strankowski, M. Morphology, Mechanical and Thermal Properties of Flexible Polyurethane Foams Modified with Layered Aluminosilicates. Polimery/Polymers 2014, 59, 783–792. [Google Scholar] [CrossRef]
  71. Stachak, P.; Hebda, E.; Pielichowski, K. Foaming Extrusion of Thermoplastic Polyurethane Modified by POSS Nanofillers. Compos. Theory Pract. 2019, 19, 23–29. [Google Scholar]
  72. Song, Z.L.; Ma, L.Q.; Wu, Z.J.; He, D.P. Effects of Viscosity on Cellular Structure of Foamed Aluminum in Foaming Process. J. Mater. Sci. 2000, 35, 15–20. [Google Scholar] [CrossRef]
  73. Sung, G.; Kim, J.H. Influence of Filler Surface Characteristics on Morphological, Physical, Acoustic Properties of Polyurethane Composite Foams Filled with Inorganic Fillers. Compos. Sci. Technol. 2017, 146, 147–154. [Google Scholar] [CrossRef]
  74. Takashimizu, Y.; Iiyoshi, M. New Parameter of Roundness R: Circularity Corrected by Aspect Ratio. Prog. Earth Planet. Sci. 2016, 3, 2. [Google Scholar] [CrossRef]
  75. Kurańska, M.; Malewska, E.; Polaczek, K.; Prociak, A.; Kubacka, J. A Pathway toward a New Era of Open-Cell Polyurethane Foams—Influence of Bio-Polyols Derived from Used Cooking Oil on Foams Properties. Materials 2020, 13, 5161. [Google Scholar] [CrossRef] [PubMed]
  76. Fan, H.; Tekeei, A.; Suppes, G.J.; Hsieh, F.H. Rigid Polyurethane Foams Made from High Viscosity Soy-Polyols. J. Appl. Polym. Sci. 2013, 127, 1623–1629. [Google Scholar] [CrossRef]
  77. Smoleń, J.; Olszowska, K.; Godzierz, M. Composites of Rigid Polyurethane Foam and Shredded Car Window Glass Particles—Structure and Mechanical Properties. Compos. Theory Pract. 2021, 21, 135–140. [Google Scholar]
  78. Somdee, P.; Lassú-Kuknyó, T.; Kónya, C.; Szabó, T.; Marossy, K. Thermal Analysis of Polyurethane Elastomers Matrix with Different Chain Extender Contents for Thermal Conductive Application. J. Therm. Anal. Calorim. 2019, 138, 1003–1010. [Google Scholar] [CrossRef]
  79. Li, R.; Shan, Z. Research on Structural Features and Thermal Conductivity of Waterborne Polyurethane. Prog. Org. Coat. 2017, 104, 271–279. [Google Scholar] [CrossRef]
  80. Zhang, H.; Fang, W.Z.; Li, Y.M.; Tao, W.Q. Experimental Study of the Thermal Conductivity of Polyurethane Foams. Appl. Therm. Eng. 2017, 115, 528–538. [Google Scholar] [CrossRef]
  81. Mukhopadhyaya, P.; Kumaran, K.; Normandin, N.; van Reenen, D.; Lackey, J. High-Performance Vacuum Insulation Panel: Development of Alternative Core Materials. J. Cold Reg. Eng. 2008, 22, 103–123. [Google Scholar] [CrossRef]
  82. Wu, J.W.; Sung, W.F.; Chu, H.S. Thermal Conductivity of Polyurethane Foams. Int. J. Heat Mass Transf. 1999, 42, 2211–2217. [Google Scholar] [CrossRef]
  83. Kurańska, M.; Prociak, A.; Kirpluks, M.; Cabulis, U. Polyurethane–Polyisocyanurate Foams Modified with Hydroxyl Derivatives of Rapeseed Oil. Ind. Crops Prod. 2015, 74, 849–857. [Google Scholar] [CrossRef]
  84. Bogdan, M.; Hoerter, J.; Moore, F.O. Meeting the Insulation Requirements of the Building Envelope with Polyurethane and Polyisocyanurate Foam. J. Cell. Plast. 2005, 41, 41–56. [Google Scholar] [CrossRef]
  85. Benazzouk, A.; Douzane, O.; Mezreb, K.; Laidoudi, B.; Quéneudec, M. Thermal Conductivity of Cement Composites Containing Rubber Waste Particles: Experimental Study and Modelling. Constr. Build. Mater. 2008, 22, 573–579. [Google Scholar] [CrossRef]
  86. Formela, K.; Hejna, A.; Zedler, Ł.; Przybysz, M.; Ryl, J.; Saeb, M.R.; Piszczyk, Ł. Structural, Thermal and Physico-Mechanical Properties of Polyurethane/Brewers’ Spent Grain Composite Foams Modified with Ground Tire Rubber. Ind. Crops Prod. 2017, 108, 844–852. [Google Scholar] [CrossRef]
  87. Małysa, T.; Nowacki, K.; Wieczorek, J. Assessment of Sound Absorbing Properties of Polyurethane Sandwich System. Compos. Theory Pract. 2016, 16, 244–248. [Google Scholar]
  88. Kurańska, M.; Barczewski, R.; Barczewski, M.; Prociak, A.; Polaczek, K. Thermal Insulation and Sound Absorption Properties of Open-Cell Polyurethane Foams Modified with Bio-Polyol Based on Used Cooking Oil. Materials 2020, 13, 5673. [Google Scholar] [CrossRef] [PubMed]
  89. Zhang, C.; Li, J.; Hu, Z.; Zhu, F.; Huang, Y. Correlation between the Acoustic and Porous Cell Morphology of Polyurethane Foam: Effect of Interconnected Porosity. Mater. Des. 2012, 41, 319–325. [Google Scholar] [CrossRef]
  90. Abdollahi Baghban, S.; Khorasani, M.; Sadeghi, G.M.M. Soundproofing Flexible Polyurethane Foams: The Impact of Polyester Chemical Structure on the Microphase Separation and Acoustic Damping. J. Appl. Polym. Sci. 2018, 135, 46744. [Google Scholar] [CrossRef]
  91. Lind-Nordgren, E.; Göransson, P. Optimising Open Porous Foam for Acoustical and Vibrational Performance. J. Sound Vib. 2010, 329, 753–767. [Google Scholar] [CrossRef]
  92. Gong, W.; Jiang, T.H.; Zeng, X.B.; He, L.; Zhang, C. Experimental-Numerical Studies of the Effect of Cell Structure on the Mechanical Properties of Polypropylene Foams. e-Polymers 2020, 20, 713–723. [Google Scholar] [CrossRef]
  93. Zhang, R.; Chen, J.; Zhu, Y.; Zhang, J.; Luo, G.; Cao, P.; Shen, Q.; Zhang, L. Correlation Between the Structure and Compressive Property of PMMA Microcellular Foams Fabricated by Supercritical CO2 Foaming Method. Polymers 2020, 12, 315. [Google Scholar] [CrossRef]
  94. Nadal Gisbert, A.; Crespo Amorós, J.E.; López Martínez, J.; Garcia, A.M. Study of Thermal Degradation Kinetics of Elastomeric Powder (Ground Tire Rubber). Polym.-Plast. Technol. Eng. 2007, 47, 36–39. [Google Scholar] [CrossRef]
  95. Zedler, Ł.; Przybysz, M.; Klein, M.; Saeb, M.R.; Formela, K. Processing, Physico-Mechanical and Thermal Properties of Reclaimed GTR and NBR/Reclaimed GTR Blends as Function of Various Additives. Polym. Degrad. Stab. 2017, 143, 186–195. [Google Scholar] [CrossRef]
  96. Formela, K.; Klein, M.; Colom, X.; Saeb, M.R. Investigating the Combined Impact of Plasticizer and Shear Force on the Efficiency of Low Temperature Reclaiming of Ground Tire Rubber (GTR). Polym. Degrad. Stab. 2016, 125, 1–11. [Google Scholar] [CrossRef]
Materials 15 05728 g001 550
Figure 1. FTIR spectra of GTR applied as filler for investigated PU-based composites.
Figure 1. FTIR spectra of GTR applied as filler for investigated PU-based composites.
Materials 15 05728 g001
Materials 15 05728 g002 550
Figure 2. The impact of GTR addition on the processing times (shaded areas) and temperature (curves) reached by the core of foam during volumetric expansion.
Figure 2. The impact of GTR addition on the processing times (shaded areas) and temperature (curves) reached by the core of foam during volumetric expansion.
Materials 15 05728 g002
Materials 15 05728 g003 550
Figure 3. FTIR spectra of prepared PU/GTR composite foams and the unmodified PU foam.
Figure 3. FTIR spectra of prepared PU/GTR composite foams and the unmodified PU foam.
Materials 15 05728 g003
Materials 15 05728 g004 550
Figure 4. Images showing the cellular structure of prepared samples obtained with scanning electron microscopy.
Figure 4. Images showing the cellular structure of prepared samples obtained with scanning electron microscopy.
Materials 15 05728 g004
Materials 15 05728 g005 550
Figure 5. Sound absorption coefficient characteristics prepared samples in 1/3 octave bands (100–6300 Hz).
Figure 5. Sound absorption coefficient characteristics prepared samples in 1/3 octave bands (100–6300 Hz).
Materials 15 05728 g005
Materials 15 05728 g006 550
Figure 6. The average value of the sound absorption coefficient αavg and the weighted sound absorption coefficients αw of PU foams depending on GTR content.
Figure 6. The average value of the sound absorption coefficient αavg and the weighted sound absorption coefficients αw of PU foams depending on GTR content.
Materials 15 05728 g006
Materials 15 05728 g007 550
Figure 7. Results of static tensile tests performed for prepared samples.
Figure 7. Results of static tensile tests performed for prepared samples.
Materials 15 05728 g007
Materials 15 05728 g008 550
Figure 8. Temperature plots of (a) storage modulus and (b) loss modulus for prepared foams.
Figure 8. Temperature plots of (a) storage modulus and (b) loss modulus for prepared foams.
Materials 15 05728 g008
Materials 15 05728 g009 550
Figure 9. Results of thermogravimetric analysis performed for PU/GTR composite foams and the unfilled PU foam: (a) mass loss curves and (b) differential thermogravimetric curves.
Figure 9. Results of thermogravimetric analysis performed for PU/GTR composite foams and the unfilled PU foam: (a) mass loss curves and (b) differential thermogravimetric curves.
Materials 15 05728 g009
Table
Table 1. Materials used to prepare flexible PU foams in the presented study.
Table 1. Materials used to prepare flexible PU foams in the presented study.
MaterialProducerProperties/Additional Information
PTMG 2000DuPont (Wilmington, NC, USA)Poly(tetramethylene ether)glycol, hydroxyl value—56 mg KOH/g, molar mass—2000 g/mol
GlycerolSigma Aldrich (Poznań, Poland)Hydroxyl value—1800 mg KOH/g
Toluene diisocyanate (TDI)Sigma Aldrich (Poznań, Poland)Mixture of 2,4-TDI and 2,6-TDI in the 80/20 ratio
Dabco33LVAir Products (Allentown, PA, USA)Catalyst, 33 wt% solution of 1,4-diazabicyclo[2.2.2]octane in dipropylene glycol
Dibutyltin dilaurate (DBTL)Sigma Aldrich (Poznań, Poland)Organic tin catalyst
Tegostab B8460Evonik Industries AG (Essen, Germany)Foam stabilizer, polyether polydimethylsiloxane copolymer
Distilled water-Chemical blowing agent
Ground tire rubberGrupa Recykl S.A. (Śrem, Poland)Average particle size—0.6 mm
Table
Table 2. Formulations of PU foams analyzed in the presented study.
Table 2. Formulations of PU foams analyzed in the presented study.
ComponentFoam Symbol
P0P5P10P15P20
Content, pbw
1st stepPTMG27.03
TDI30.09
Total prepolymer57.12
2nd stepPTMG36.80
Glycerol5.11
33LV0.31
DBTL0.27
B84600.20
Water0.20
GTR05101520
Table
Table 3. Composites’ appearance and parameters describing it in quantitative terms.
Table 3. Composites’ appearance and parameters describing it in quantitative terms.
SampleColor ParametersDigital Color ReproductionFoam Skin ImageFoam Core Image
L*a*b*ΔEChromaHue
P086.25−0.273.460.003.47−85.54 Materials 15 05728 i001 Materials 15 05728 i002
P569.80−0.082.7116.472.71−88.31 Materials 15 05728 i003 Materials 15 05728 i004
P1065.40−0.393.3620.853.38−83.38 Materials 15 05728 i005 Materials 15 05728 i006
P1561.97−0.513.3324.283.37−81.29 Materials 15 05728 i007 Materials 15 05728 i008
P2058.470.085.4127.855.4189.15 Materials 15 05728 i009 Materials 15 05728 i010
Table
Table 4. Parameters describing cellular structure and thermal insulation performance of prepared foamed PU/GTR composites.
Table 4. Parameters describing cellular structure and thermal insulation performance of prepared foamed PU/GTR composites.
ParameterP0P5P10P15P20
Average cell diameter, µm263.1 ± 64.9188.5 ± 49.7144.8 ± 87.8170.2 ± 78.6178.3 ± 109.0
Circularity0.48 ± 0.060.45 ± 0.170.43 ± 0.220.39 ± 0.210.35 ± 0.20
Aspect ratio1.32 ± 0.281.35 ± 0.301.38 ± 0.441.42 ± 0.441.48 ± 0.41
Roundness0.79 ± 0.140.77 ± 0.140.78 ± 0.170.75 ± 0.170.72 ± 0.16
Open cell content, %49.00 ± 2.5353.23 ± 1.9749.09 ± 2.1150.14 ± 2.0151.92 ± 2.66
Apparent density, kg/m3311.8 ± 4.3297.0 ± 1.4309.5 ± 1.9313.0 ± 7.2322.9 ± 3.4
Sol fraction content, wt%1.35 ± 0.021.91 ± 0.082.18 ± 0.322.35 ± 0.302.74 ± 0.21
Thermal conductivity coefficient, mW/(m·K)62.90 ± 1.8961.22 ± 1.8462.23 ± 1.5662.27 ± 1.7160.20 ± 1.51
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Żukowska, W.; Kosmela, P.; Wojtasz, P.; Szczepański, M.; Piasecki, A.; Barczewski, R.; Barczewski, M.; Hejna, A. Comprehensive Enhancement of Prepolymer-Based Flexible Polyurethane Foams’ Performance by Introduction of Cost-Effective Waste-Based Ground Tire Rubber Particles. Materials 2022, 15, 5728. https://doi.org/10.3390/ma15165728

AMA Style

Żukowska W, Kosmela P, Wojtasz P, Szczepański M, Piasecki A, Barczewski R, Barczewski M, Hejna A. Comprehensive Enhancement of Prepolymer-Based Flexible Polyurethane Foams’ Performance by Introduction of Cost-Effective Waste-Based Ground Tire Rubber Particles. Materials. 2022; 15(16):5728. https://doi.org/10.3390/ma15165728

Chicago/Turabian Style

Żukowska, Wiktoria, Paulina Kosmela, Paweł Wojtasz, Mariusz Szczepański, Adam Piasecki, Roman Barczewski, Mateusz Barczewski, and Aleksander Hejna. 2022. "Comprehensive Enhancement of Prepolymer-Based Flexible Polyurethane Foams’ Performance by Introduction of Cost-Effective Waste-Based Ground Tire Rubber Particles" Materials 15, no. 16: 5728. https://doi.org/10.3390/ma15165728

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