Rigid Polyurethane Foam Fabrication Using Medium Chain Glycerides

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Research Article pubs.acs.org/journal/ascecg

Rigid Polyurethane Foam Fabrication Using Medium Chain Glycerides of Coconut Oil and Plastics from End-of-Life Vehicles Aleksandra Paruzel,† Sławomir Michałowski,‡ Jiří Hodan,† Pavel Horák,† Aleksander Prociak,*,‡ and Hynek Beneš*,† †

Institute of Macromolecular Chemistry AS CR, v.v.i., Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic Cracow University of Technology, Department of Chemistry and Technology of Polymers, Warszawska 24, 31-155 Cracow, Poland

‡

S Supporting Information *

ABSTRACT: Polycarbonate and polyurethane scraps from end-oflife vehicles were converted into liquid recycled polyols with hydroxyl number around 300 mgKOH·g−1 by using medium chain glycerides of coconut oil. The obtained polyols were used for preparation of low-density rigid polyurethane foams. It was found that up to 50 wt % of the virgin petrochemical polyol can be replaced by the recycled polyols without any negative effect on the foaming process. The obtained foams exhibited the apparent density of 40−44 kg·m−3, the homogeneous cellular structure with a high content of closed cells (>91 vol %) and the beneficially low value of lambda coefficient (∼23 mW·m−1·K−1). The exceptionally high compressive strength (>350 kPa in parallel to foam rise direction) of the rigid PUR foams with 50 wt % of recycled polyol derived from polycarbonate scrap resulted probably from the unique structure of recycled polyol combining rigid aromatic segments together with flexible coconut oil glyceride units. In conclusion, this approach utilizing the renewable coconut oil-derived reagent provides a sustainable recycling solution for two major plastics from automotive waste. KEYWORDS: Polyurethane, Polycarbonate, Rigid foam, Vegetable oil, Chemical recycling, Degradation, Renewable resource, Microwave irradiation

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INTRODUCTION

from other thermoplastics (PE, PP, etc.), which enables their recovery via chemical recycling. Various methods of chemical recycling for PUR wastes have been broadly explored during the last three decades and were the subject of several reviews.3−5 Despite the numerous chemical recycling techniques that have been developed for PUR, only a few studies investigated potential applications of recovered products (recycled polyols) to prepare novel PUR materials,6−9 especially rigid PUR foams.10−13 The rigid PUR foams, because of their excellent properties such as low density and thermal conductivity, high strength-to-weight ratio, low moisture absorption, etc., are widely used as thermal insulations in building construction and refrigeration, cushioning and insulating materials in automotive industry, or in packaging.14 The addition of recycled polyols into rigid PUR foam formulation must be well balanced since it often increases the viscosity of the polyol stream and leads to failure of foam properties. Prociak et al. found that the glycolysis product might replace a virgin polyol of rigid PUR foam formulation in the amount not exceeding 30 wt % due to processing limits connected to too high a viscosity of the recycled polyols.15 The

The global automotive plastics consumption is expected to grow from 7.1 million tons in 2012 to 11.3 million tons by 2018.1 Polypropylene (PP) leads consumption by 36% followed by polyurethanes (PUR) (17%), acrylonitrile− butadiene−styrene copolymer (ABS) (12%), composites (11%), high-density polyethylene (HDPE) (10%), polycarbonates (PC) (7%), and poly(methyl methacrylate) (PMMA) (7%).1 End-of-life vehicles (ELVs) thus present one of the main sources of different plastic wastes. Some of them such as polyolefins (including HDPE, PP) can be relatively easily reprocessed, while the others, such as PUR, must be subjected to chemical recycling (e.g., glycolysis, hydrolysis, etc.) in order to be converted into low molecular weight products with functional groups capable of further reacting.2 However, the chemical recycling methods might be also useful for thermoplastic waste treatment, especially when additives, admixtures, or other impurities initiate undesirable polymer degradation during reprocessing, which deteriorates mechanical properties of recycled products. A typical example might be a PC scrap of automotive headlamps containing siloxane coatings and/or metallic paints making its reprocessing without unwanted PC degradation difficult. On the other hand, both PUR and PC wastes might be easily dismantled from ELVs and sorted out © 2017 American Chemical Society

Received: April 18, 2017 Revised: May 15, 2017 Published: May 19, 2017 6237

DOI: 10.1021/acssuschemeng.7b01197 ACS Sustainable Chem. Eng. 2017, 5, 6237−6246

Research Article

ACS Sustainable Chemistry & Engineering

Preparation of Recycled Polyol from Polyurethane Waste. The postconsumer waste of semirigid PUR foam from ELVs was first pressed and cut to small pieces (ca. 1 × 1 × 1 cm3). Next, the PUR waste (150 g) and TCCO (165 g) were placed into a 2500 mL threeneck flask equipped with nitrogen inlet, reflux, and mechanical stirrer. The reaction mixture was heated in a multimode microwave reactor (Romill, Ltd., Czech Republic, f = 2.45 GHz, maximum power of 1000 W). The temperature was controlled using a shielded Pt100thermometer inserted directly into the reaction medium. After ca. 30 min of microwave heating with the constant power of 400 W, the temperature of reaction mixture reached 210 °C. Then, the temperature was kept constant at 210 °C for further ca. 15 min. The reaction was complete when no PUR oligomers were found in the mixture, which was checked using size exclusion chromatography (SEC). The product was cooled, and the solid impurities were filtered off. The obtained product, recycled PUR polyol (RPUP), was a brown liquid. Preparation of recycled polyol from polycarbonate waste. The granulated postconsumer PC waste from ELV (800 g) and TCCO (800 g) were placed into a 2500 mL three-neck flask equipped with nitrogen inlet, reflux and mechanical stirrer. The reaction mixture was heated in the same microwave reactor with the same temperature control as described above for the PUR waste decomposition. After ca. 40 min of microwave heating with the constant power of 500 W, the temperature of such reaction mixture reached 215 °C. Then, the temperature was kept constant at 215 °C, and within ca. 40 min the reaction was complete since no PC oligomers were found in the mixture (verified by SEC). The product was cooled, and the solid impurities were filtered off. The obtained product, recycled PC polyol (RPCP) was a pale yellow liquid. Characterization of the Recycled Polyols. The SEC of the recycled polyols was performed on a Modular GPC system equipped with a refractive index detector RIDK-102 (Laboratorni pristroje Praha, Czech Republic) and an UV−vis photometric detector LCD 2084 (ECOM, Czech Republic) operated at λ= 254 nm, and a set of two columns PLgel 10E3 and 50 Å, 10 μm particle size, 300 mm × 7.5 mm (Polymer Laboratories, UK) was used. Chromatographic data were collected and treated using Clarity software (Data-Apex, Czech Republic). Tetrahydrofuran and toluene were used as a mobile phase and a flow marker (retention time of toluene was 17.72 min) at a flow rate of 1 mL/min, respectively. Polystyrene standards with weightaverage molecular weights (Mw) of 500, 1000, 3000, and 10 000 were used for calibration. FTIR spectra were measured using the attenuated total reflectance (ATR) technique on a Spectrum 100T FTIR spectrometer (PerkinElmer, USA) with a DTSG detector fitted with a Universal ATR accessory with a diamond/ZnSe crystal. All spectra were recorded in the range 650−4000 cm−1 at 16 scans per spectrum at 4 cm−1 resolution. The viscosity of recycled polyols was measured using a Bohlin Gemini HR nano- rheometer (Malvern Instruments) with cone/plate geometry (40 mm diameter, 4°angle, 0.15 mm gap) at a temperature of 25 °C in the range of shear rates 0.001−100 s−1. The hydroxyl number and acid number of recycled polyols were determined according to ISO 2554:1974 and ASTM D 4662-93, respectively, using a titrator SCHOTT TITRONIC universal (SCHOTT-GERÄ TE GmbH). The water content in recycled polyols was determined using the Karl Fischer titration method according to ISO 760. Number-average functionality of recycled polyols ( f n, OH) was calculated from the determined hydroxyl number (OH) and numberaverage molar mass (Mn) using the following eq (eq 1):

glycolysis product addition often causes an undesirable increase of foam density, which can be slightly adjusted using density modifiers such as starch.16 Zhu et al. reported the preparation of rigid PUR foams meeting industrial requirements only when the content of glycolysis product in the polyol mixture was below 10 wt %.17 Lee et al. used the reaction of butyl glycidyl ether with amines to convert aromatic amines presented in the glycolysis product to polyols.18 This modification allowed increasing the content of recycled polyol (up to 30 wt % in a polyol mixture) in rigid PUR foams with no deterioration of their physical properties. The novel types of recycled polyols derived from oleochemical (rapeseed oil and fish oil based) polyols enabled 100% replacement of virgin polyols, but only high-density rigid PUR foams19 or semirigid PUR foams20 were obtained. Contrary to widely explored chemical recycling methods of PUR producing recycled polyols, only a few examples of PC glycolysis yielding polyol products, namely bis-hydroxylalkyl ethers of bisphenol A (BPA), have been mentioned in the literature.21−23 Lin et al. applied the produced bis-hydroxyalkyl ethers of BPA for preparation of PUR elastomers.21 No further application for polyols produced during PC chemolysis has been described yet in the literature. Generally, the higher content of recycled polyols, prepared by the conventional glycolysis, in rigid PUR foams resulted in a too brittle foam structure due to a high content of aromatic segments in the PUR network interconnected via short glycolderived linkages. It was found that this drawback might be eliminated when rapeseed, castor, and fish oil-based polyols, bearing hydroxyl groups on glyceride units, were used for polymer decomposition instead of short glycols.19,20,24−26 However, these polyols contained mainly long chain glycerides acting as soft segments of PUR networks producing semirigid PUR foams. In one of a last study, Beneš et al. used a waste fraction from the soap industry composed uniquely of medium chain triglycerides of coconut oil (CCO) for synthesis of a biobased reagent for PC degradation.26 The reagent was prepared by transesterification of CCO with glycerol from biodiesel production. The obtained transesterified CCO (TCCO) is thus 100% renewable-based, prepared only from byproducts and was already proven to be an efficient reagent for solvolysis of PC technological scraps.26 In this paper, TCCO was used for microwave-accelerated decompositions of two postconsumer wastes (PUR and PC) coming from ELVs. The PUR and PC wastes were converted by TCCO into two types of recycled polyols, both composed uniquely of renewable and recycled components. The obtained polyols were analyzed and tested for preparation of low-density rigid PUR foams applicable as thermal insulations. The influence of recycled polyols’ additions on the foaming process as well as on the morphology and properties of final PUR foams were investigated in details.

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EXPERIMENTAL SECTION

Transesterification of Medium Chain Glycerides of Coconut Oil. CCO, the byproduct from soap industry composed of >99% triglycerides of caprylic (C8:0) and capric (C10:0) acids, was kindly received from Environ Ltd. (Czech Republic) and transesterified using glycerol (99.5%, Preol, Czech Republic), the byproduct from biodiesel production, according to the procedure previously described.26 The prepared transesterified product (TCCO) exhibited the hydroxyl number of 560 mgKOH·g−1 and the number-average functionality of 2.15. TCCO was used as the reagent for conversions of PUR and PC wastes into recycled polyols.

fn,OH = OH·M n /56100

(1)

Mn was determined from SEC calibrated on polystyrene standards. Foam Formulation. Rigid polyurethane foams were prepared using a commercial virgin polyol Rokopol 551 (oxypropylenated sorbitol, PCC Rokita, Poland), which was partially replaced by the recycled polyols (RPUP or RPCP). Polymeric diphenylmethane-4,4′6238


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ACS Sustainable Chemistry & Engineering Table 1. Optimized Rigid PUR Foam Formulations (Parts by Weight) component

0%

virgin polyol RPUP RPCP water catalyst surfactant n-pentane PMDI

100

1 1.5 1.5 12 130

10% RPUP 90 10 1 1.15 1.5 12 127

10% RPCP

30% RPUP

30% RPCP

70 30

70

90 10 1 1.15 1.5 12 126

1 0.8 1.5 12 123

30 1 0.8 1.5 12 118

50% RPUP 50 50 1 0.45 1.5 12 118

50% RPCP 50 50 1 0.45 1.5 12 110

Scheme 1. Reaction of TCCO with PUR and PC Wastes

diisocyanate (PMDI, Ekopur B) with 31% content of NCO groups was kindly provided by Minova Ekochem, Poland. The NCO index of 110 was applied in the case of all prepared foams. N,N,N′,N″,N″Pentamethyldiethylenetriamine (Polycat 9, Air Products) as the catalyst and Niax Silicone L-6900 (Momentive Performance Materials) as the silicone surfactant were used. Typically, all components (except PMDI) were well homogenized in a 0.5 dm3 plastic cup and then PMDI was quickly added and mixed for 10 s at 2000 rpm. The first series of water-blown rigid PUR foams were prepared in 0.5 dm3 cups in order to optimize the content of catalyst. The next series of rigid PUR foams using the optimized formulations with n-pentane (Lach-Ner, Czech Republic) as the physical blowing agent were prepared in 5 dm3 open mold. The optimized rigid PUR foam formulations are given in Table 1. All foams were conditioned for 24 h at room temperature. Foaming Parameters. During the PUR foam preparation, the following parameters were observed and characterized: cream time, the time when the polyol and isocyanate mixture begins to change from the liquid state to a creamy and starts expansion subsequently; gel time, the time the foam start to stiffen; free rise time, the time at which a freely rising foam stopped expanding; and tack-free time, when the surface of foam stopped being sticky. Additionally, the foaming process was analyzed using a FOAMAT device (Messtechnik, Germany) that allows the determination of characteristic parameters, such as foam rise height, reaction temperature, and dielectric polarization during foaming. Foam Properties. Free rise (apparent) density was determined according to ISO 845. Closed cells content (%) was determined according to ISO 4590. The thermal conductivity factors were determined using a Laser Comp heat flow instrument Fox 200. The measurements were made at an average temperature of 10 °C (temperature of the cold plate was 0 °C and that of the warm plate was 20 °C). Water absorption was measured according to ISO 2896. Dimensional stability was measured according to ISO 2796 in dry conditions at 70 °C. The samples were measured to an accuracy of 0.01 mm as it is shown in Figure S1.

Scanning electron microscope (SEM) Vega Plus TS 5135 (Tescan, Czech Republic) was used for observing the morphology of PUR foams. The micrographs were done using a secondary electron imaging (SEM/SE) at 30 kV. The samples were broken in liquid nitrogen. The samples were fixed on a metallic support with carbon adhesive tape and sputtered with Pt (vacuum sputter coater, SCD 050, Balzers, Lichtenstein). A compression test was performed according to ISO 844. The specimens (50 mm-side cubes) were cut from blocks of PUR foam. The tests were performed at room temperature in parallel or perpendicularly to foam rise at the speed of 5 mm·min−1 using an Instron 6025/5800R tester (Instron, UK). The compressive stress at 10% deformation or the compressive strength reached at lower deformations was evaluated. Dynamic mechanical and thermal analysis (DMTA) was conducted on an ARES-G2 rheometer (TA Instruments, USA). The temperature dependence of the complex shear modulus of rectangular samples (dimension: 20 × 10 × 3 mm3) was measured by oscillatory shear deformation in the temperature range of 25−250 °C at a temperature ramp rate of 3 °C·min−1 with 0.2% strain and 1 Hz frequency. The temperature of main (alfa) transition (Tα) was evaluated as the maximum of tan δ peak. The typical precision of the measurements was Tα ± 2 °C. Thermogravimetric analysis (TGA) was performed on a Pyris 1 TGA thermogravimetric analyzer (PerkinElmer, USA). The samples were heated at a rate of 10 °C·min−1 from 30 to 800 °C under nitrogen atmosphere.

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RESULTS AND DISCUSSION Characteristics of Prepared Recycled Polyols. The decomposition of PUR and PC wastes took place according to Scheme 1. The PUR waste of the semirigid foam used in car interiors was based on aromatic isocyanate (PMDI) and polyether polyol(s). The PC waste of car headlights was BPAbased PC containing siloxane top coating (0.2 wt %), which was removed from the recycled PC polyol by filtration. 6239


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carbonate group or BPA end-capped with one monoglyceride and one glycerol units through carbonate groups) considering monoglycerides and glycerol as the main components of TCCO.26 FTIR spectrum of RPCP (Figure S2) revealed the presence of aromatic and aliphatic structures, phenolic and alcoholic OH groups as well as glyceride and carbonate linkages. The detailed analysis of RPCP components was previously performed26 and therefore is not mentioned in this paper. The determined average number functionality of RPCP ( f n, OH = 1.8, Table 2) was lowered due to the presence of OHmonofunctional (e.g., BPA end-capped with one diglyceride unit through carbonate group) and OH-free (e.g., BPA endcapped with two diglyceride units through carbonate groups) compounds in RPCP. The low functionality together with the presence of mostly linear aromatic structures caused the narrower molecular weight distribution of RPCP as well as its lower average molecular weight and viscosity compared to RPUP (Table 2). Renewable and recycled contents in RPUP and RPCP were calculated (Table 2). Both polyols were composed uniquely of the renewable (derived from TCCO) and the recycled (originated from PUR or PC wastes) raw materials with approximately equal proportion. Foaming Parameters of Rigid Polyurethane Foams. Rigid PUR foams were produced under the reaction of a polyol with a slight excess of PMDI (molar NCO/OH ratio of 1.10) in the presence of several additives (catalyst, blowing agent, stabilizer, and surfactant). The amount of PMDI was varied for each PUR foam formulation as a function of polyol hydroxyl number, keeping the NCO/OH ratio constant. The first series of PUR foams were prepared with the constant amount of catalyst (Polycat 9) and different content of the recycled polyols (RPUP and RPCP) using only water as a chemical blowing agent (Figure 1). The additions of both recycled polyols significantly accelerated the foaming process. Cream time was only slightly influenced by the content of recycled polyols. Gel, rise, and tack-free times became apparently shorter with the increasing content of RPUP (Figure 1a) due the presence of aromatic amines, which accelerated foaming process.11,19,27 The accelerated foaming was also observed for the formulations with RPCP (Figure 1b) due to the presence of low molecular weight components (BPA and free glycerol), which enhanced the reaction with isocyanates.28−30 Moreover, both recycled polyols contained traces of DBTL catalyst coming from the synthesis of TCCO,26 which might also accelerate the PUR foaming process.

SEC records of both recycled polyols (not shown here) displayed no presence of high molecular weight oligomers giving thus evidence of completeness of polymer degradation. The physical-chemical properties of the recycled polyols are summarized in Table 2. Table 2. Properties of RPUP and RPCP Recycled Polyols Prepared from PUR and PC Waste, Respectively name

RPUP

RPCP

hydroxyl number [mgKOH·g−1] viscosity at 25 °C [mPa·s] water content [wt %] acid number [mgKOH·g−1] Mw [g·mol−1] Mw/Mn f n, OH renewable/recycled content [wt %]

333 9500 0.1 1.1 1322 2.22 3.5 52/48

270 3500 0.01 1.2 438 1.16 1.8 50/50

The higher TCCO/polymer weight ratio used for the preparation of RPUP resulted in its slightly higher hydroxyl number compared to the respective value of RPCP. The different viscosities of both recycled polyols were primary connected to their different molecular weights. The average molecular weight (Mw) of the more viscous RPUP was 3 times higher than the Mw of RPCP (Table 2). Moreover, the broad molecular weight distribution (the high Mw/Mn ratio) of RPUP originated from the presence of a large variety of branched and linear compounds with rigid aromatic as well as flexible aliphatic segments. During the decomposition of chemically cross-linked PUR waste by means of TCCO, an original (aliphatic) polyol with a functionality of ≥3 (an unknown structure, here expressed as HO−R1(OH)−OH, see Scheme 1) and a mixture of aromatic (carbamate) polyols and amines were formed. The FTIR spectrum of RPUP (Figure S2) confirmed the presence of NH and OH groups, carbamate, and ester linkages as well as aromatic structures and aliphatic chains. The resulting RPUP was thus composed of various compounds differing in chemical structure (aliphatic vs aromatic), functional group (aliphatic OH and aromatic NH2), functionality, and supramolecular structure (branched vs linear). Therefore, RPUP had broad molecular weight distribution and its determined average number functionality ( f n, OH = 3.5) was quite high (Table 2). Contrary to that, the linear PC was predominantly converted by the reaction with TCCO into diols (BPA and BPA endcapped with one monoglyceride unit through carbonate bond), triols (BPA end-capped with one glycerol unit through

Figure 1. Effect of (a) RPUP and (b) RPCP addition on PUR foaming parameters. 6240


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Figure 2. Temperature and dielectric polarization changes of PUR systems modified with (a,c) RPUP and (b,d) RPCP during the foaming process.

Figure 3. Microphotographs of cell structure of the prepared rigid PUR foams containing no recycled polyol (a, e), 50 wt % of RPUP (b, f), 50 wt % of RPCP (c, g), and 75 wt % of RPCP (d, h); in parallel (a−d) and perpendicularly (e−h) to foam rise directions.

Table 3. Average Cell Diameter (d), Apparent Density (ρ), Closed Cell Content, Water Absorption, and Thermal Conductivity (λ coefficient) of Rigid PUR Foams Containing Various Amounts of RPUP and RPCP Recycled Polyols recycled polyol content [wt %] 0 RPUP: 10 30 50 RPCP: 10 30 50 75 a

da [μm]

db [μm]

ρ [kg·m−3]

closed cell content [vol %]

water absorption [vol %]

λ coefficient [mW·m−1·K−1]

430 ± 118

387 ± 85

41.1 ± 1.3

88.8 ± 1.2

0.43 ± 0.01

24.3 ± 0.2

322 ± 102 254 ± 84 222 ± 59

269 ± 77 258 ± 71 243 ± 62

41.5 ± 1.5 41.8 ± 0.4 42.4 ± 1.1

90.1 ± 0.1 91.6 ± 1.1 91.3 ± 1.0

0.43 ± 0.00 0.34 ± 0.01 0.35 ± 0.02

23.6 ± 0.1 23.3 ± 0.1 23.4 ± 0.3

± ± ± ±

329 ± 60 280 ± 65 260 ± 80 73 ± 23

39.8 42.5 43.9 48.0

± ± ± ±

89.9 ± 0.6 91.1 ± 0.4 90.6 ± 0.5 n.d.

0.40 ± 0.03 0.34 ± 0.01 0.35 ± 0.01 n.d.

23.9 ± 0.2 23.0 ± 0.2 22.8 ± 0.1 n.d.

347 387 216 261

100 109 61 127

0.9 0.7 2.0 1.6

In parallel to foam rise direction. bPerpendicularly to foam rise direction, n.d. = not determined.

Consequently, the PUR formulations had to be optimized by decreasing the content of catalyst (Table 1). The foaming parameters of optimized PUR formulations containing various amounts of the RPUP and RPCP and n-pentane as a physical co-blowing agent were investigated using the foam qualification

system FOAMAT. The effect of type and amount of recycled polyols in the PUR formulations on temperature and dielectric polarization during foaming are shown in Figure 2. The increase of both recycled polyols content caused a decrease of reaction mixture temperature (Figure 2a,b). The 6241


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Figure 4. Compressive strength (normalized to the density of 40 kg·m−3) of rigid PUR foams containing (a) RPUP and (b) RPCP.

wt % was applied.16 Generally, the higher contents (≥50 wt %) of recycled polyols from PUR waste caused the substantial increase in density of rigid PUR foams.10,19 Contrary to that, recycled polyols prepared from polyethylene terephthalate (PET) waste, the linear and aromatic structure of which is similar to PC, might be applied in rigid PUR foams at higher amounts. For example Ghaderian et al. used uniquely recycled polyols from PET glycolysis for preparation of PUR foams with a density of 22−41 kg·m−3 and cell diameters of 300−480 μm;36 giving slightly larger cells than in our case. Kacperski et al. prepared PUR foams with similar densities (26−35 kg·m−3) but with a much lower amount of the recycled product applied (10−30 wt %).37 Paberza et al. described the synthesis of combined bio/recycled polyol from rapeseed oil and glycolyzed PET, which enabled its application at high loadings (up to 75 wt % virgin polyol replacement) in low-density (40−45 kg· m−3) rigid PUR foams without unfavorable density increase.33 Thermal Conductivity of Rigid Polyurethane Foams. The influence of recycled polyol addition on thermal conductivity (λ) of the prepared PUR foams is seen from Table 3. The same trend is evident for both types of recycled polyols; λ was decreased (i.e., heat insulating properties of PUR foams were improved) with the increasing recycled polyols’ contents. Generally, the thermal conductivity of plastic foams depends on thermal conductivities of the polymer (solid) phase, gas trapped in cells plus convective and radiative components.38 In this study, all prepared materials were lowdensity PUR foams, in which the solid polymer phase represented a negligible fraction and therefore its contribution to overall foam thermal conductivity might be omitted. The thermal conductivity of the gas phase was assumed to be the same for all prepared PUR foams, since the same blowing agents (combination of carbon dioxide and n-pentane) were always used. Then, the overall thermal conductivity performance of rigid PUR foam had to be influenced by the cell size and the fraction of closed cells.34 The mean cell diameters in parallel and perpendicular to foam rise determined from the SEM images are given in Table 3. The results showed that the progressive addition of recycled polyols decreased the cell size and narrowed the cell diameter distribution up to a recycled content of 50 wt %. Only the rigid PUR foam containing 75 wt % of RPCP exhibited significant cell anisotropy (see SEM image in Figure 3d). All the other prepared PUR foams exhibited very fine and homogeneous cellular structure with isotropic cells. It is well-known that a large number of small cells resist gas exchange better than a small number of large cells (the contribution of λ radiative component is reduced),38 which is in agreement with our thermal conductivity results (Table 3). Moreover, the addition of the recycled polyols’

higher content of recycled polyols (30 and 50 wt %) caused a fast temperature increase in the beginning of foaming (up to ca. 100 s) indicating the larger reaction heat release and the higher extent of isocyanate−polyol reaction. The temperature profiles corresponded well to the dielectric polarization evolution, which reflected the progress in chemical reactions between OH and NCO groups.31 The PUR cross-linking reaction ultimately suppressed all dipole mobility during foaming.32 The decrease of dielectric polarization thus reflected the PUR cross-linking reaction progress. The faster decrease of dielectric polarization was observed at the highest content of recycled polyols (50 wt % replacement of petrochemical polyol) indicating the faster foaming kinetics (Figure 2c,d). However, the incorporation of both recycled polyols did not bring any important changes of PUR system reactivity. From this point of view, all PUR formulations were successfully optimized by decreasing of the catalyst content. Structure of Rigid Polyurethane Foams. The PUR formulations were adjusted by the amount of the chemical (water) and physical (n-pentane) blowing agents in order to receive the rigid PUR foams with unified apparent density of ca. 40 kg·m−3, as common for PUR thermal insulation materials in building industry.33 SEM images of the prepared rigid PUR foams displayed homogeneous cell structure even at high loadings (50 wt %) of both recycled polyols (Figure 3). The average cell diameter decreased with increasing levels (up to 50 wt %) of recycled polyols (Table 3). The cell size decrease might indicate good recycled polyol compatibility with blowing agents and (analogous to surfactant addition) the recycled polyol addition decreased surface tension in the polyol blend.34 The faster foaming and gelling due to recycled polyol addition (see Figure 3) might also yield PUR foams with smaller cell size.35 Visible anisotropy of cellular structure was only observed when 75 wt % of RPCP was applied. The same replacement of virgin polyol by RPUP led to cell-opening during foaming, which resulted in collapse of cellular structure. RPUP might be thus added into low-density rigid PUR foam system in lower amount compared to RPCP. However, the highest replacement of virgin polyol by RPCP (75 wt %) produced the rigid PUR foam with the higher apparent density (48 kg·m−3) than was desired. From this point of view, the addition of both recycled polyols was limited to 50 wt % in order to prepare the rigid PUR foams with sufficient low apparent density in the range of 39.8−43.9 kg·m−3 (Table 3). In literature, only few studies reported a successful preparation of low density rigid PUR foams with a high content of recycled polyols. Nikje et al. successfully prepared rigid PUR foams with densities around 40 kg·m−3 only when the content of recycled polyols from PUR glycolysis below 30 6242


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Figure 5. Thermo-mechanical properties of rigid PUR foams containing (a) RPUP and (b) RPCP; storage modulus (G′) and loss factor (tan δ) curves.

Table 4. Results of DMTA (Tα, G′G ), TGA (T5%, char yield at 800°C) and Dimensional Stability of Rigid PUR Foams with Various Amounts of RPCP and RPUP dimensional stability [%] recycled polyol content [wt %] 0 RPUP: 10 30 50 RPCP: 10 30 50 75 a

char yield at 800 °C[wt %]

l

b

δ

161

2.9

288

14.8

0.09 ± 0.00

0.13 ± 0.05

0.40 ± 0.09

158 159 142

3.2 4.2 4.4

273 275 264

15.5 17.6 17.9

0.19 ± 0.02 0.06 ± 0.01 0.15 ± 0.04

0.12 ± 0.01 0.14 ± 0.09 0.22 ± 0.03

0.40 ± 0.04 0.43 ± 0.03 0.24 ± 0.04

156 145 138 n.d.

3.0 4.1 5.1 n.d.

285 236 235 231

16.8 18.5 22.1 15.1

0.05 ± 0.03 0.05 ± 0.02 0.04 ± 0.03 n.d.

0.06 ± 0.01 0.07 ± 0.02 0.11 ± 0.04 n.d.

0.31 ± 0.05 0.34 ± 0.01 0.31 ± 0.07 n.d.

Tαa

[°C]

GG′ [MPa] b

T5%c

[°C]

Maximum of tan δ peak. bStorage shear modulus at glassy state at 50 °C. cTemperature of 5% weight loss.

of reference PUR foam without recycled polyols. The best mechanical properties had the PUR foam with 50 wt % of RPCP, which exhibited the compressive strength values parallel and perpendicular to foam rise directions, 38% and 29% higher, respectively, compared to the reference foam. The further increase of RPCP content (75 wt %) in the rigid PUR foam caused a slight increase (+10%) and decrease (−15%) in foam compressive strengths parallel and perpendicular to foam rise directions, respectively, compared to the reference foam, which originated from foam structural anisotropy attributed to cell elongation in the direction of foam rise (Figure 3d). It is worth mentioning that the compressive strength above 350 kPa (in parallel to foam rise direction) of the rigid PUR foams with 50 wt % of RPCP was exceptionally high with respect to the low value (40 kg·m−3) of foam apparent density. These anomalous increased mechanical properties of PUR foams probably originated from the high content of aromatic segments in the RPCP providing stiffness of PUR structure and from the optimal hard block segregation enabling physical bonding and thus compensating the lower cross-linking density, that is an effect of the low functionality of RPCP.30,41 Moreover, the aromatic segments in PUR foam were interconnected via the flexible glyceride chains of TCCO avoiding excessive rigidity and brittleness of a PUR network. Low-density foams with similar mechanical properties and containing such a high amount of recycled polyols have not been described yet in the literature. Ivdre et al. prepared the rigid PUR foam based on the recycled polyol from PET waste with an apparent density of 45 kg·m−3 and a maximal

slightly increased the content of closed cells (∼91 vol %), which beneficially decreased both the thermal conductivity (∼23 mW·m−1·K−1) and the water absorption (∼0.34 vol %) of PUR foams (Table 3). The negligible water absorption of PUR foams based on the recycled polyols was also connected to a hydrophobic character of RPUP and RPCP. Both recycled polyols were composed of the aromatic and the CCO glycerides’ structures exhibiting thus “water-repellent” character. Mechanical Properties of Rigid Polyurethane Foams. It is well-known that the mechanical properties of rigid PUR foams are closely related to the foam density.38 Therefore, the measured values of compressive strength in parallel and perpendicular to foam rise directions at 10% compression were normalized to apparent density of 40 kg·m−3 according to (eq 2):39,40 σnorm = σexp(40/ρ)2.1

(2)

where σnorm is the normalized compressive strength [kPa], σexp is the measured compressive strength [kPa] and ρ is the apparent density [kg·m−3] of PUR foam. The calculated values of normalized compressive strength are plotted in Figure 4. The compressive strength of rigid PUR foams, both parallel and perpendicular to the directions of foam rise, increased with the increasing content of both types of recycled polyols in the range of 0−50 wt % virgin polyol replacement. The compressive strength parallel and perpendicular to the directions of foam rise for the sample with 50 wt % of RPUP content was 28% and 16% higher, respectively, than in the case 6243


Research Article

ACS Sustainable Chemistry & Engineering compressive strength of 310 kPa.42 Prociak et al. prepared the rigid PUR foam with 40 wt % of the recycled PUR polyol, which exhibited an apparent density of 38 kg·m−3 and a compressive strength of 220 kPa.15 Thermomechanical Properties of Rigid Polyurethane Foams. Figure 5 shows the DMTA results (storage modulus, G′ and tan δ) of the prepared rigid PUR foams. The different content of recycled polyols influenced both G′ and Tα values of rigid PUR foams. The G′ values in the glassy region (GG′ in Table 4) slightly increased at the higher recycled polyols’ contents (30 and 50 wt %) contrary to the rubbery region above Tα where the G′ values slightly decreased with the increasing recycled polyols’ amounts (Figure 5). This behavior indicated that the addition of recycled polyol promoted physical bonding (probably via π−π interactions of aromatic rings and H-bonding of urethane and urea structures) and hard segment formation (similar to thermoplastic PUR elastomers) in the PUR cross-linked structure. The highest G′G value exhibited the PUR foam with 50 wt % of RPCP, which corresponded well to its highest aromatic content. The physical cross-linking disappeared as segmental mobility of PUR chains was released at the main transition temperature region. The Tα shift of rigid PUR foams (Table 4) depended on the mobility of chain segments driven by both the cross-linking density and the weight fraction of aromatic structures in PUR foams. As expected, the reference PUR foam (without recycled polyol) exhibited the highest cross-link density and therefore the highest Tα. However, the PUR foams with 10 and 30 wt % of RPUP exhibited only a small shift in Tα to lower temperature (Figure 5a and Table 4). The more significant Tα decrease was observed in the case of PUR foam with the highest (50 wt %) content of RPUP. In this case, the broad molecular weight distribution of RPUP led to the formation of a heterogeneous PUR network containing highly cross-linked (hard) and more linear less-cross-linked (soft) segments as evidenced from the substantial broadening of the tan δ peak (Figure 5a.)44 In contrast, the increasing RPCP content led to the gradual Tα decrease of rigid PUR foams (Figure 5b and Table 4). The low average functionality of RPCP (see Table 2) significantly reduced the degree of chemical cross-linking of rigid PUR foams. Thus, a less chemically cross-linked structure provided more free volume for network chain segment relaxation, shifting Tα of the rigid PUR foams to lower temperatures. The excellent thermo-mechanical properties of PUR foams modified with RPCP and RPUP were confirmed by their very good dimensional stability at 70 °C (Table 4). The extent of dimensional changes after the test was lower than 1% for all tested PUR foams. Thermal Degradation of Rigid Polyurethane Foams. The thermal stability of the prepared rigid PUR foams was evaluated using TGA under nitrogen atmosphere (Figure S3 and Table 4). The reference PUR foam degraded in one broad weight loss in the temperature range of 220−600 °C (with the maximum DTG peak at 360 °C). The series of PUR foams based on RPUP exhibited no mass loss below 220 °C analogous to the reference PUR foam (Figure S3a). The PUR foam with 10 wt % of RPUP exhibited the same TGA curve as the reference PUR foam. The DTG curves of PUR foams with 30 and 50 wt % of RPUP displayed two shoulders on the DTG peak at low (220−320 °C) and high (410−600 °C) temperature regions. The first shoulder slightly shifting the beginning of thermal degradation (expressed as the temperature of 5% weight loss, T5%, in Table 4) to lower

temperature as the result of slightly decreased cross-link density of PUR network43 and the presence of thermally labile dangling acylester chains;20 the latter shoulder indicated the improved thermal stability of the PUR foams at higher temperatures (above ca. 400 °C) due to the higher aromatic content in the PUR network.44 The RPCP content of 10 wt % did not influence the thermal behavior of rigid PUR foam (Figure S3b). The higher contents (30−75 wt %) of RPCP decreased significantly the initial mass loss of rigid PUR foam to lower temperatures (to ca. 150 °C). This first mass loss shift to lower temperature was attributed to more diluted PUR networks containing thermally less stable urethane groups.20,34 The novel DTG peak (at 292 °C) was formed in the case of PUR foam with the highest RPCP content (75 wt %). The increased char formation during TGA of PUR foams based on recycled polyols was due to the increased content of aromatic structures originating from RPUP and RPCP33,36 confirming the highest aromatic content in the PUR foam with 50 wt % of RPCP. Structure of Rigid Polyurethane Foams Based on Recycled Polyols. The two recycled polyols exhibited several differences. RPUP had a much higher functionality, the broader molecular weight distribution and contained beside OH groups also more reactive NH2 groups. Contrary to that, RPCP had a higher aromatic content, lower molecular weight, and much narrower molecular weight distribution. Surprisingly, these differences in recycled polyol structure did not have a significant effect on foaming (only the slightly higher reactivity of RPUP compared to RPCP was adjusted lowering the catalyst amount) and the properties of rigid PUR. The PUR foams with 50 wt % of RPCP exhibited the best mechanical properties at room temperature, producing a more diluted PUR network compared to that with RPUP. On the other hand, both recycled polyols had similar features which played a crucial role in formation of the structure of the PUR foam and had an impact on foam properties. The structure of both recycled polyols (RPUP and RPCP) combined the rigid aromatic segments (derived from PUR/ PC segments) and the flexible pendant chains derived from CCO glycerides. The former reinforced the cross-linked structure of PUR via π−π interactions of aromatic rings and H-bonding of urethanes and disubstituted ureas, whereas the latter avoided brittleness of the PUR network creating flexible glyceride covalent connections between the aromatic units. The final network of PUR foams at the glassy state (i.e., at temperatures below Tα) is thus composed of physical and covalent cross-links providing excellent stiffness as well as toughness. On the basis of the above-mentioned findings, the idealized structure of rigid PUR foam based on the recycled polyol is proposed (Scheme 2). The schematic structure is derived from RPCP but we suppose similar structure features for the RPUPbased PUR foam. The shown foam structure contains the hard aromatic segments derived from PMDI and BPA, bonded through urethane (U) or disubstituted urea (UA) groups, and the flexible glyceride chains derived from CCO. The presence of diglycerides leads to creation of pendant chains.

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CONCLUSIONS Polyurethane (PUR) and polycarbonate (PC) scraps from endof-life vehicles were converted into two types of recycled polyols, RPUP and RPCP, respectively, using medium chain 6244


Research Article

ACS Sustainable Chemistry & Engineering Scheme 2. Schematic Structure of Rigid Polyurethane Foam Containing Recycled Polyol from Polycarbonate Waste

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Method of foam size measurements during dimensional stability test, FTIR spectra of TCCO, RPUP, and RPCP, and TGA curves of the prepared rigid PUR foams (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Phone: +420 296 809 313. Fax: +420 296 809 410. E-mail: [email protected]. *Phone: +48 12 628 30 16. Fax: +48 12 628 29 47. E-mail: [email protected]. ORCID

Hynek Beneš: 0000-0002-6861-1997 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Ministry of Education, Youth and Sports and Ministry of Science and Higher Education in the frame of Polish-Czech bilateral project (ID/CZ 7AMB14PL021 and ID/PL 9023/R14/R15) and by Ministry of Education, Youth and Sports, National Sustainability Program INPU I, Project POLYMAT LO1507.

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glycerides of coconut oil under microwave irradiation. Thus, prepared polyols, composed uniquely of renewable and recycled components, were characterized in detail, having a hydroxyl number of ca. 300 mg KOH·g−1, which made them suitable for rigid PUR foam applications. Despite the higher recycled polyols reactivity, the PUR formulations were successfully optimized by the decreasing catalyst content. Then the replacement of a virgin petrochemical polyol up to 50 wt % by the recycled polyols did not bring any important changes the reactivity of the PUR system, which enabled preparation of low-density PUR foams with apparent density of 40−44 kg·m−3. The prepared PUR foams exhibited with the increasing recycled polyols’ contents (up to 50 wt %) a finer homogeneous cellular structure, a higher content of closed cells (>91 vol %), and beneficially a lower lambda coefficient (∼23 mW·m−1·K−1) and water absorption (∼0.35 vol %) compared to the reference PUR foam without recycled polyol. Moreover, the PUR foams with high content of recycled polyols (50 wt %) exhibited the best mechanical properties; the compressive strength above 350 kPa (in parallel to foam rise direction) of the rigid PUR foams with 50 wt % of RPCP was exceptionally high with respect to the low value (40 kg·m−3) of foam apparent density. The unique mechanical properties of PUR foams with high recycled polyols’ contents probably originated from the presence of rigid aromatic segments (from PUR and PC) and flexible connections derived from coconut oil glycerides, which shifted the main (glass) transition of PUR foams to lower temperatures. However, all the prepared PUR foams were thermally stable up to 220 °C. In conclusion, both recycled polyols can be applied up to 50 wt % virgin polyol replacement in low-density rigid PUR foams with potential application as a thermal insulating material in the building industry.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01197. 6245


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Rigid Polyurethane Foam Fabrication Using Medium Chain Glycerides

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