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Article

Preparation of Mullite/PU Nanocomposites by Double Waste Co-Recycling

by
Xiaohua Gu
1,2,*,
Yanwei Zhu
2,
Siwen Liu
3,
Shangwen Zhu
1 and
Yan Liu
1,*
1
School of Energy and Building Environment, Guilin University of Aerospace Technology, Guilin 541004, China
2
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China
3
College of Innovative Material & Energy, Hubei University, Wuhan 430062, China
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(21), 14310; https://doi.org/10.3390/su142114310
Submission received: 7 October 2022 / Revised: 27 October 2022 / Accepted: 31 October 2022 / Published: 2 November 2022
(This article belongs to the Special Issue Complex Solid Waste and Multipath Recycling)
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Abstract

:
The massive accumulation of industrial waste has become an environmental problem that is very difficult to deal with. In this paper, mullite whisker nanomaterials were developed independently using industrial waste residues, which were used to degrade polyurethane (PU) solid waste by alcoholysis with ethylene glycol (EG) and ethanolamine (ETA) bi-component, and mullite modified regenerated polyol materials were obtained by double waste synergistic recycling. Mullite/PU foam nanocomposites were prepared by one-step foaming. The analysis of the test results shows that, at EG/ETA = 2:1 and mullite whisker addition of 0.15%, the regenerated rigid PU foam obtained has low thermal conductivity and higher compressive strength, at which time the regenerated PU foam has the best performance. The FTIR test results show that the silanol of mullite reacts with isocyanate during foaming and is attached to the polyurethane chain, such that the compressive strength and thermal insulation properties are maximized. It provides a new way to create a “double waste synergy” for preparing high-value materials by comprehensively utilizing resources.

1. Introduction

Now, with the rapid development of economic construction, the production of industrial waste has increased sharply, generating more than 20 million tons of solid waste accumulation every year [1]. Industrial silica-alumina glue waste is one of the by-products of the oil refining catalyst industry, and its annual production is high and resource utilization rate is low. A large amount of silica-alumina rubber waste accumulation and landfill not only occupy arable land and cause pollution to the soil environment, but also may cause secondary disasters. How to make better use of limited resources and comprehensive utilization of industrial waste residue has become one of the current research hotspots in the field of resource utilization [2]. The main components for the preparation of mullite whiskers are silicon and aluminum, which are very common in industrial waste, so mullite whiskers can be prepared from silica-aluminum glue waste to achieve the reuse of resources, improve the utilization rate of resources, and improve the environment to some extent. The recycling of materials can reduce resources, energy consumption, and environmental emissions [3].
Mullite is a rare natural mineral in nature, but can be prepared artificially by chemical methods. Mullite chemical composition 3Al2O3-2SiO2 is the only compound in the silicon-aluminate system that is stable under atmospheric conditions and belongs to the rhombohedral crystal system, characterized by the presence of octahedral chains of [AlO6] at the apex and center of the unit cell, with the edges aligned parallel to each other along the “c” axis crystallographic direction [4,5]. Mullite has high mechanical strength, good creep resistance, and low coefficient of thermal expansion. Porous materials prepared from mullite whiskers are widely used in catalyst carriers, membrane carriers, and thermal insulation materials because of their excellent thermal shock resistance, high specific surface area, and low thermal conductivity [6,7,8].
Whiskers are generally micron-sized single-crystal fiber materials with perfect structure and almost no internal defects. The strength and modulus are close to the theoretical value of crystalline materials, which are a class of excellent mechanical properties of composite reinforcement toughening agents [9,10]. When a polymer-based whisker composite is subjected to an external force, the stress can be transferred from the matrix to the whisker through the interfacial layer and the whisker takes part of the stress so that the stress on the matrix can be distributed [11]. Whisker toughening is the process of changing the fracture mode of a polymer from brittle fracture to ductile fracture by preventing crack expansion through the combined action of whiskers in the stress field [12]. The whisker toughening mechanism mainly includes whisker bridging, pull-out effect, and crack deflection. The schematic diagram of the whisker toughening mechanism is shown in Scheme 1. Whisker bridging refers to the crack surface through the whisker to increase the closure stress, prevent crack expansion, and thus play a toughening effect. When the whisker reaches the shear yield strength of the substrate, the whisker is pulled out, and this process consumes fracture energy, which has a toughening effect. Crack deflection is when the crack extends to the high toughness of the whisker and must bypass the whisker in order to continue to develop; the whisker changes the direction of crack expansion, which means the crack expansion needs to absorb more fracture energy, thus having a toughening effect [13,14,15].
Mullite preparation methods can be divided into the molten salt growth method, reaction sintering method, solid state synthesis method, in situ production method, liquid state synthesis method, sol-gel method, and precipitation method. Among them, the molten salt method is the most common [4,16]. Fang J Y prepared mullite whiskers with an average diameter of 100 nm and an aspect ratio of 30 using (Al2SO4)3·18H2O, Sichuan Qionglai kaolin as precursors and Na2SO4 as the molten salt medium by holding at 900 °C for 2 h [17]. Zhang B prepared mullite whiskers of 3–4 μm in length and 0.5–1 μm in diameter using the molten salt method at 1200 °C by dosing SiO2 and Al2O3 in accordance with the mullite stoichiometric ratio and then adding K2SO4 molten salt [18]. Nie J H synthesized mullite whiskers of 50–200 nm in diameter and 5–10 µm in length using aluminum sulfate with silicon oxide in a molten salt of sodium sulfate at about 1000 °C [19].
The German chemist Otto Bayer invented polyurethane (PU) in the 1930s, which is a class of polymeric materials with -NHCOO- as a repeating group [20,21]. Polyurethane can be in the form of elastomers, adhesives, paints, or foams (PUF), of which polyurethane foam is the most used polyurethane product [22,23,24,25,26]. The PUF market is forecast to reach a total of $91.96 billion by 2024 [27]. Although PU products have unparalleled uses in upholstered furniture, walls, roofing, medical devices, footwear, coatings, adhesives, automotive interiors, flooring elastomers, and so on, statistics show that about 20–30% of waste PU products need to be disposed of each year. These wastes are thermosetting polymers that cannot simply be melted and reshaped and do not easily degrade in the natural environment, leading to global plastic pollution, and recycling of PU foam waste is a major issue that needs to be addressed urgently [28,29,30]. Polyurethane recycling can be divided into “mechanical (physical) recycling”, “chemical recycling”, and “energy recycling”. Mechanical recycling includes grinding, molding, gluing, and bonding of PU scrap. Chemical methods (also called “ feedstock recycling “) change the chemical properties of the material, which means breaking the target chemical bonds to recover valuable materials. Energy recovery refers to the incineration and decomposition (by cracking or hydrogenation) of PU waste. Enzymatic degradation is the hydrolysis of polyurethane bonds by the action of microbial enzymes [31,32]. Among all of the methods, alcoholysis is considered as the most convenient method for PU recovery [33].
Amundarain prepared rigid polyurethane foam from waste polyurethane foam by alcoholysis of ethylene glycol (EG), and sodium hydroxide (NaOH) as solvents and catalysts to obtain polyol; the addition of regenerated polyol resulted in a slight decrease in the compressive properties of the composites, while the tensile strength and modulus increased significantly [34]. D. Simon demonstrated the feasibility of the alcoholysis process for viscoelastic flexible PU foam scrap by alcoholysis of an industrial sample of viscoelastic flexible PU foam V-50 190 using diethylene glycol (DEG) as the alcoholysis agent and stannous octanoate as the catalyst, and the alcoholysis subphase obtained from the viscoelastic flexible PU foam scrap replaced up to 75% of the synthesis of virgin polyurethane rigid foam [35]. Zhu P degraded the polyurethane hard foam from waste refrigerators for two hours at 197.85 °C using EG as the alcohololytic agent and NaOH as the catalyst; the obtained mixed polyol can be used for re-production of PU, the excess EG can be reused, and the mixed polyol addition of regenerated polyurethane can meet the industrialization requirements at less than 10% [36].
Nanocomposites are usually two-phase materials with at least one of the phases having a size in the nanometer range (1–100 nm) [37]. The addition of nanoscale fillers to a polymer matrix results in the formation of nanocomposites, where the addition of only small amounts of nanomaterials can result in a significant increase in strength [38,39]. In 1987, Toyota researchers synthesized the first nylon 6/clay nanocomposite, pioneering the synthesis of polymer nanocomposites [40]. Nanomaterials have a small size effect and, after nanoparticles are uniformly dispersed in the polyurethane matrix because of their different properties of the microstructure, the system can obtain more comprehensive performance in terms of flame retardancy, oxidation resistance, physical stability, chemical stability, and structural properties [41,42]. At present, the types of inorganic nanomaterials applied to modified polyurethane synthetic composites focus on carbon nanomaterials, common small molecule nano-oxides, and layered silicate clays [43].
Ding X J used a twin-screw extruder to prepare polyphthalamide 6/polyphthalamide nanocomposites by blending intaglio with polyphthalamide, and found that the tensile strength and impact strength of the composites could be improved using polyphthalamide modified with needle-like intaglio [44]. Ding Z M used an in situ polymerization method to obtain high-quality monolayer montmorillonite (Mt) containing polyethylene glycol-polyethylene glycol ester (Exolit op550, Clariant, Shanghai, China) aggregation to assemble an environmentally friendly waterborne polyurethane coating, which formed an expanded carbon layer through the synergistic effect of montmorillonite and polyethylene glycol-polyethylene glycol ester, resulting in a reduction in the peak exothermic rate of the film compared with a pure aqueous polyurethane film by 36.8% [45]. Wang H M successfully synthesized Bi2WO6/boron-grafted polyurethane composite coatings composed of Bi2WO6 and boron-grafted polyurethane in the form of nanosheets, flowers, and microspheres to achieve efficient antifouling [46].
The mullite whisker preparation process has a high temperature, has a high energy consumption, is time-consuming, is expensive, and the large-scale preparation is somewhat hindered, which limits its application in high-end fields, so it is necessary to explore the low-cost and high-performance mullite whisker preparation method. This paper developed mullite whisker nanomaterials with industrial waste as a raw material. It is a green low-temperature method to prepare mullite whiskers. Using this material with two components of ethylene glycol (EG) and ethanolamine (ETA) through the alcoholysis method, the degradation of polyurethane solid waste was carried out by the alcoholysis method, which realized the reuse of two kinds of waste materials and achieved the purpose of double waste synergistic recycling, as well as the addition of mullite whisker in the process to obtain mullite whisker modified polyurethane rigid foam (PURF), which greatly improved the mechanical properties and thermal stability of polyurethane composites, and was more excellent than the general nanomaterials. The modified polyurethane foam synthesized from solid waste recycling can be processed and used in automobile roofs, building wall insulation, food refrigeration equipment, and other fields.

2. Materials and Methods

2.1. Materials

The materials used mainly include industrial waste silica-alumina glue (Shandong Zibo Company, Zibo, China, ≥99.0%), aluminum sulfate octadecahydrate (Tianjin Yaohua Chemical Reagent Co., Ltd., Tianjin, China, 98%), anhydrous sodium sulfate (Tianjin Tianli Chemical Reagent Co., Ltd., Tianjin, China, 98%), ethylene glycol (Jiangsu Yongfeng Chemical Reagent Co., Ltd.,Danyang, China, 98%), ethanolamine (Jinan Mingliang Chemical Co., Ltd., Jinan, China, 98%), KOH (Suzhou Haomao Chemical Co., Suzhou, China, 98%), and waste polyurethane foam (Daqing Company, Daqing, China, ≥99.0%)

2.2. Experimental Method

Firstly, the suspension obtained by mixing industrial waste silica-alumina gum suspension, aluminum sulfate octadecahydrate, and anhydrous sodium sulfate according to the molar ratio of Al/Si/Na of 3:1:0.2 was placed in the ball mill tank. Mixed in the ratio of ball/material/water = 4:2:1, using a ball mill running wet at 300 rpm for 6 h, pouring the suspension into a corundum crucible, drying it in an oven at 110 °C for 12 h, covering the corundum crucible with a lid, firing it in an electric furnace at 5 °C/min at 800 °C and holding it for 6 h, and cooling it naturally in the furnace to room temperature, and finally rinsing the sample with deionized water seven times, mullite whiskers were obtained.
The alcohol-degrading agent polyol was added to the reaction kettle in the ratio of ethylene glycol (EG) and ethanolamine (ETA) = 3:1, 3:2, 3:3, 2:3, and 1:3, and the ratio of alcohol-degrading agent to waste PU foam was 1:0.9. Moreover, 1 g of KOH and different masses of mullite whisker were added and warmed up to 120 °C and stirred until completely dissolved, the waste PU foam was broken up by the machine, and 100 g of PU waste was added and stirred. The degradation material was prepared by heating up to 180 °C. Then, the product was cooled down to 25 °C with a water separator cup, and the viscosity and hydroxyl value of the degradation material were tested and then analyzed by infrared spectroscopy. The foaming agent and catalyst were added to the product and placed into the mixer, then it was stirred until the mixture is hot, when stirring was stopped stirring and we waited foaming. After the foaming was completed, the newly prepared polyurethane foam was taken for testing and analysis.
The schematic diagram for preparation of the compound material is illustrated in Scheme 2 and the schematic scheme of “double waste synergy” resource utilization is shown in Scheme 3.

3. Results and Discussion

3.1. Study on the Degradation of Waste Polyurethane

Viscosity analysis was performed using NDJ-5S (Shanghai Hengping Scientific Instruments Co., Ltd., Shanghai, China). The appropriate amount of the recovered polyol was placed in a 100 mL Erlenmeyer flask and the hydroxyl value was measured according to GB/TQ 12008.3-2009 [47]. From Figure 1, it can be seen that the hydroxyl value of the regenerated polyol obtained by degradation decreased significantly with the decrease in the proportion of ETA, then the decrease became smaller; the viscosity gradually increased with the decrease in the proportion of EG. The hydroxyl value of degradation products is influenced by the degraded polyol, EG, and ETA as alcoholic solvents. When EG/ETA = 3:1, the hydroxyl value of degradation products is too high and there is an incomplete reaction of EG, while the hydroxyl value of subsequent groups is similar to that of commercially available polyether polyol 4110 (420), indicating complete degradation. Moreover, 418 mgKOH/g indicates that the degradation product performance is better at this time. Therefore, it can be concluded that the best ratio of alcoholic solvents is EG/ETA = 3:2 to achieve the best reaction effect.

3.2. Infrared Spectroscopic Analysis of Regenerated Polyols from Waste Polyurethane

The regenerated polyol and commercial polyether polyol 4110 obtained from the degradation of regenerated polyurethane by adding potassium hydroxide catalyst were taken for infrared spectroscopy testing and analysis using IR-960 (PE). From Figure 2, it can be seen that the regenerated polyol obtained from the degradation of potassium hydroxide catalyst is very similar to the commercial polyether polyol 4110, with a strong stretching vibration peak of the hydroxyl group at 3350 cm−1 and a stretching vibration peak of methyl group near 2900 cm−1; it has a characteristic absorption peak of carbonyl group near 1737 cm−1(weak shoulder peak owing to free C=O) [48]. Relative to commercial 4110, the prepared regenerated polyol contains a benzene pan-frequency peak near 1500 cm−1. There is an absorption peak near 1050 cm−1, which is a characteristic peak of the ether bond. This indicates that the urethane bond of the used polyurethane foam is broken and replaced by the alcohol hydroxyl group under the action of alcoholysis to produce a regenerated polyol mixture containing an ether bond, which is structurally similar to commercial polyether polyol 4110. It was demonstrated that the polyether polyol prepared by degradation can replace the commercial polyether polyol 4110.

3.3. Scanning Electron Microscope (SEM) Observations

Multiple portions of mullite whisker with different amounts are added into the regenerated polyurethane foam specimens accordingly. Using a scanning electron microscope, the results are observed. The captured images are shown in Figure 3. The SEM images of mullite whiskers are shown in (Figure 3A); it can be observed that the homogeneous needle-like mullite whiskers with anisotropic growth form a dense mesh structure; (Figure 3B,C) are pure-like foamed PU foams with a uniform and fine pore structure, and the pore structure has an important influence on the mechanical properties of polyurethane foam: the better the pore cell structure, the higher the level of strain resistance. From (Figure 3D–H), it can be seen that the pores of the regenerated polyurethane foams prepared with different mullite whisker additions are relatively intact without large-scale breakage. The pores become more uniform and the pore walls become thicker and the skeleton is more robust as the addition amount increases. (Figure 3F) shows that, when the mullite whisker addition is 0.25%, the pores of the regenerated polyurethane foam are relatively intact and close to hexagonal shape; when the mullite whisker addition reaches 0.5%, the pores occasionally rupture and the pore walls become thinner owing to the agglomeration of whiskers, and the pore closure is low and the strength of the foam is reduced at this time. (Figure 3H) shows that, when the mullite whisker addition reaches 0.75%, some micro-pores of different sizes appear on the pore wall and these micro-pores are independent of each other, making the pore wall relatively rough. This indicates that the right amount of mullite whisker can enhance the pore and pore wall structure of the foam, but when the amount is too much, the mullite whisker will agglomerate and affect the formation of pores, which will reduce the cross-linkage and strength of the foam, and the agglomeration of mullite whisker also affects the formation of pore film, which will break the pores and reduce the performance of the regenerated polyurethane foam.

3.4. Compressive Strength and Density Test Analysis

The compressive strength of the specimens was tested using a universal material testing machine. The regenerated polyurethane with different mullite whisker additions was made into a 50 × 50 × 50 mm specimen block and measured at a displacement speed of 20 mm/min. Multiple identical samples were repeated and the average value was taken. Apparent density was tested according to GB/T 6343-1986 [49]. The apparent density of five specimens of each sample was measured and the average value was calculated. The test results are shown in Figure 4. It can be seen from the graph that, with the addition of mullite whisker, the compressive strength and density of the regenerated polyurethane foam showed a trend of increasing, then decreasing, and then increasing. As the whiskers in the regenerated polyurethane foam can increase the support strength of the pores and enhance the compressive strength to a certain extent, it can be learned from the test that the compressive strength of the regenerated polyurethane foam with 0.15% mullite whiskers added is 0.231 MPa; the best compressive strength is achieved at this point, with a 65% increase in strength compared with recycled polyurethane foam without mullite whiskers and 60% stronger than carbon-nanotube-modified polyurethane composites [50]. When mullite whisker is added in excess, an agglomeration phenomenon will occur and the foam pore structure will be uneven, then the support performance of the foam pore wall will be reduced and the compressive strength will be reduced. Adding the right amount of mullite whisker compound in the regenerated polyurethane foam will make the compressive strength reach the maximum. For the whole polyurethane foam body, the density is not uniform and the foam needs to overcome the pressure from the abrasive tool, so the density around the foam body is larger than the density in the middle. The compressive strength and density of the regenerated polyurethane foam with the addition of 0.15% mullite whisker have increased, which may be due to the increase in the functional degree after the addition of mullite.

3.5. Thermal Conductivity Test Analysis

Polyurethane foam is commonly used as an insulation material for daily life, so thermal conductivity is an important indicator for polyurethane foam. Whisker toughening gives the prepared regenerated polyurethane foam better performance. The thermal conductivity of PURF was measured using a thermal conductivity tester according to GB/T 10295-2008 [51]. The thermal conductivity was measured for three specimens of each sample and the average value was calculated. The thermal conductivity of the regenerated polyurethane foam prepared by the mullite whisker compound is shown in Table 1. From the table, it can be seen that the thermal conductivity of the regenerated polyurethane foam with mullite whisker addition within 0.50% meets the national standard of less than 0.030 W/(m·K). Compared with the regenerated polyurethane foam without mullite whisker addition, adding a small amount of mullite whisker compound will lower the thermal conductivity of regenerated polyurethane foam. With the increase in mullite whisker compound addition, the thermal conductivity tends to decrease and then increase, indicating that a small amount of mullite whisker compound has an obvious optimization effect on the thermal conductivity of regenerated polyurethane foam, and the thermal insulation performance is improved, which can make better insulation materials. When the mullite whisker addition amount reaches 0.15%, the thermal conductivity is the lowest, which is due to the good dispersion effect of whiskers in polyurethane when the addition amount of nanowhiskers is 0.15%, which has a supporting and enhancing effect on the pore wall of polyurethane. The structure of the bubble pore skeleton and window becomes thicker, the permeability to gas is reduced, so the gas barrier is increased and the heat transfer effect is weakened, which reduces the thermal conductivity of the whisker-modified polyurethane composites. However, when the whisker is added to a certain degree, which is greater than 0.15%, the addition of the whisker reaches 0.25%, which exceeds the optimal addition of whisker dispersion of 0.15%. The whisker produces agglomeration in polyurethane, which makes the permeability of polyurethane become larger, and it cannot play the role of gas barrier and increases the thermal conductivity and reduces the thermal insulation performance. From the data in the table, we can conclude that, when the addition of whisker reaches 0.15%, the mullite whisker compound has an obvious optimization effect on the thermal conductivity of regenerated polyurethane foam, and the thermal insulation performance is improved, which can make better insulation materials. Therefore, when the mullite whisker addition reaches 0.15%, the thermal conductivity is the lowest and the thermal insulation is better, making 0.15% the best amount of whisker addition.

3.6. Infrared Spectral Analysis of Mullite Modified Polyurethane

The polyurethane samples with polyether 4110 blowing agent foamed and polyurethane materials with 0.00% (pure sample), 0.05%, 0.15%, 0.25%, 0.50%, and 0.75% mullite whisker samples were sampled and tested, respectively, and the test results are shown in Figure 5. The stretching vibration peak at 3325 cm−1 is related to the alcohol hydroxyl group; 1412 cm−1 corresponds to the bending vibration in the O-H bond plane of the alcohol; 2931 cm−1 is the stretching vibration peak of methylene (-CH2); 2868 cm−1 is the stretching vibration peak of -CH; there is a strong absorption band near 1708 cm−1, which belongs to the benzene type pan-frequency peak; 1450 cm−1 is the stretching vibration peak of the aromatic ring skeleton, which proves the presence of containing benzene ring; 1600 cm−1 corresponds to the stretching absorption peak of N-H in amide; 1515 cm−1 is the deformation vibration peak of -NH-CO-; 1318 cm−1 corresponds to the vibration peak of C-N bond; 1219 cm−1 corresponds to the stretching vibration peak of C-O-C in soft segment; and there is an obvious strong absorption band at 1054 cm−1, which belongs to polyurethane ether group absorption peak, which proves the presence of ether bonds [48]. The band near 751 cm−1 shows mullite characteristics, corresponding to the vibration of [AlO4] tetrahedro; the peak at 1080 cm−1 is usually attributed to the silicon-oxygen bond vibration and the presence of these peaks proves the successful binding of mullite to polyurethane.

3.7. Thermogravimetric (TG) Analysis of Mullite-Modified Polyurethane Materials

TG analysis was performed using Q5000IRS (TA) and polyurethane samples foamed with polyether 4110 blowing agent and polyurethane materials with 0.00% (pure sample), 0.05%, 0.15%, 0.25%, 0.50%, and 0.75% mullite whisker samples were sampled and tested, respectively; the thermal weight loss spectrum of recycled polyurethane foam under a nitrogen atmosphere is shown in Figure 6. It can be seen from the figure that the thermal weight loss of the regenerated PU foam consists of three parts, the first part being 0–150 °C. There is a slight decrease in the weight of the regenerated PU foam at this stage, mainly due to the evaporation of tiny residual moisture [52], and there is no significant difference at this point. The second part ranges from 150 to 300 °C and unmodified PU thermal stability is higher than the other five groups—this phenomenon can be attributed to the dehydration caused by the crystalline water carried by the crystals in mullite. The third part is 300–450 °C, and it can be seen from the enlarged figure that the thermal stability is significantly higher than that of unmodified PU after adding mullite whisker. The T50% of the pure sample is 347.01 °C and the T50% of adding 0.15% mullite whisker is 357.73 °C, which is 10.72 °C higher. The results showed that the addition of mullite whisker caused a significant increase in the thermal weight loss temperature and a significant decrease in the rate of thermal weight loss of the regenerated PU foam compared with the ordinary regenerated PU foam, indicating that the thermal stability of the regenerated PU foam modified with mullite whisker was improved. Therefore, mullite whiskers can enhance the thermal stability of the regenerated polyurethane foam. The best thermal stability of PURF modified by adding a 0.15% mullite whisker may be due to the better dispersion of mullite in polyurethane. As the addition of mullite whiskers increases, agglomeration may occur, leading to a decrease in thermal stability, so 0.15% is the best addition of mullite whiskers. The thermal stability of mullite-whisker-modified polyurethane foam was greatly improved compared with carbon-nanotube-modified polyurethane composites (T50% increased by 4.4 °C) [53].

4. Conclusions

In this study, EG, ETA, and mullite whiskers were used to degrade the used rigid polyurethane foam to obtain whisker-modified recycled polyol, and then the dual waste synergistic mullite/PU nanocomposites were successfully prepared. The results showed that the best performance of the recycled polyol was obtained when the mass ratio of EG/ETA was 3:2. When mullite whisker was added at 0.15%, the compressive strength of PURF prepared at EG/ETA = 3:2 was 0.231 MPa, which was about 61.5% higher than that of pure PURF without the addition of mullite whisker. The thermal conductivity of PURF modified with a 0.15% mullite whisker was 0.0179 W/(m·K) and the apparent density was 0.056 g/cm3. The addition of mullite whisker made the polyurethane foam pores more uniform and the skeleton more robust. The TG plot showed that the addition of 0.15% mullite whisker modified PURF increased the T50% by 10.72 °C, which greatly enhanced the thermal stability of the polymer. The mullite-whisker-modified recycled polyurethane composites obtained by recycling will give better play to the role of whisker in strengthening, toughening, and improving the thermal stability of polyurethane materials, which will empower the application of polyurethane composites in aviation, aerospace, military, and other fields and further expand the application of polyurethane foam materials in high-end new materials.

Author Contributions

Methodology, S.Z.; formal analysis, X.G.; data curation, S.L.; writing—original draft preparation, Y.Z.; project administration, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors gratefully acknowledge support from the Heilongjiang Provincial Department of Education Project (CLKFKT2021Z3, 145109301), China.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. The schematic diagram of the whisker toughening mechanism.
Scheme 1. The schematic diagram of the whisker toughening mechanism.
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Scheme 2. The schematic process for the preparation of mullite/PU nanocomposites.
Scheme 2. The schematic process for the preparation of mullite/PU nanocomposites.
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Scheme 3. “Double waste synergy” resource utilization.
Scheme 3. “Double waste synergy” resource utilization.
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Figure 1. Hydroxyl value and viscosity of regenerated polyols with different alcohol ratios.
Figure 1. Hydroxyl value and viscosity of regenerated polyols with different alcohol ratios.
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Figure 2. Infrared spectra of regenerated polyol and polyether 4110.
Figure 2. Infrared spectra of regenerated polyol and polyether 4110.
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Figure 3. (A) Mullite whisker. (B,C) Pure sample foam PU. (D) Whisker addition 0.05%. (E) Whisker addition 0.15%. (F) Whisker addition 0.25%. (G) Whisker addition 0.50%. (H) Whisker addition 0.75%.
Figure 3. (A) Mullite whisker. (B,C) Pure sample foam PU. (D) Whisker addition 0.05%. (E) Whisker addition 0.15%. (F) Whisker addition 0.25%. (G) Whisker addition 0.50%. (H) Whisker addition 0.75%.
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Figure 4. Compressive strength and density test analysis.
Figure 4. Compressive strength and density test analysis.
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Figure 5. Infrared spectra of mullite-modified polyurethane.
Figure 5. Infrared spectra of mullite-modified polyurethane.
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Figure 6. Thermal weight loss spectra of regenerated polyurethane foams with different mullite whisker additions.
Figure 6. Thermal weight loss spectra of regenerated polyurethane foams with different mullite whisker additions.
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Table
Table 1. Thermal conductivity of regenerated polyurethane foam prepared from whiskers.
Table 1. Thermal conductivity of regenerated polyurethane foam prepared from whiskers.
Mullite Whisker Addition Amount/%Thermal Conductivity/W/(m·K)
0.000.0297
0.050.0189
0.150.0179
0.250.0223
0.500.0310
0.750.0301
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Gu, X.; Zhu, Y.; Liu, S.; Zhu, S.; Liu, Y. Preparation of Mullite/PU Nanocomposites by Double Waste Co-Recycling. Sustainability 2022, 14, 14310. https://doi.org/10.3390/su142114310

AMA Style

Gu X, Zhu Y, Liu S, Zhu S, Liu Y. Preparation of Mullite/PU Nanocomposites by Double Waste Co-Recycling. Sustainability. 2022; 14(21):14310. https://doi.org/10.3390/su142114310

Chicago/Turabian Style

Gu, Xiaohua, Yanwei Zhu, Siwen Liu, Shangwen Zhu, and Yan Liu. 2022. "Preparation of Mullite/PU Nanocomposites by Double Waste Co-Recycling" Sustainability 14, no. 21: 14310. https://doi.org/10.3390/su142114310

APA Style

Gu, X., Zhu, Y., Liu, S., Zhu, S., & Liu, Y. (2022). Preparation of Mullite/PU Nanocomposites by Double Waste Co-Recycling. Sustainability, 14(21), 14310. https://doi.org/10.3390/su142114310

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