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Development and characterization of polyurethane foams with substitution of polyether polyol with soy-based polyol
Accepted Manuscript Development and Characterization of Polyurethane Foams with Substitution of Polyether Polyol with Soy-Based Polyol Gurjot S. Dhaliwal, Sudharshan Anandan, K. Chandrashekhara, J. Lees, Paul Nam PII: DOI: Reference:
S0014-3057(18)30734-1 https://doi.org/10.1016/j.eurpolymj.2018.08.001 EPJ 8514
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
19 April 2018 27 June 2018 2 August 2018
Please cite this article as: Dhaliwal, G.S., Anandan, S., Chandrashekhara, K., Lees, J., Nam, P., Development and Characterization of Polyurethane Foams with Substitution of Polyether Polyol with Soy-Based Polyol, European Polymer Journal (2018), doi: https://doi.org/10.1016/j.eurpolymj.2018.08.001
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Development and Characterization of Polyurethane Foams with Substitution of Polyether Polyol with Soy-Based Polyol Gurjot S. Dhaliwal1, Sudharshan Anandan1, K. Chandrashekhara*1, J. Lees2 and Paul Nam2 1
Department of Mechanical and Aerospace Engineering,2Department of Chemistry Missouri University of Science and Technology, Rolla MO, 65409, USA
ABSTRACT Bio-based polyols can replace petroleum-based polyols for producing a wide range of polyurethane (PU) products. Bio-based polyols are derived from renewable resources, however, their utilization is limited due to the complex molecular structure and relatively low primary hydroxyl content. This study aims to substitute the petroleum-based polyols with soy-based polyols for the fabrication of rigid PU foams with comparable or better physical properties required for thermal and structural applications. The effects of blending soy-based polyol, of hydroxyl number 230 mg KOH/g with high functionality soy-based cross-linker were investigated. The fabricated samples were tested for density, compressive strength, tensile strength, thermal resistivity, thermal stability and dimensional stability. The blended foam samples exhibited better thermal resistivity (8% higher), compressive properties (about 512% higher) and tensile strength (287% higher) when compared to control specimens. It was concluded that the polyols blended with soy-based cross-linker in 95:5 ratio by weight resulted in properties better than control samples, and comparable to commercial petroleum-based foams.
*Corresponding Author Email: [email protected]
1
INTRODUCTION
Global climate change and fluctuating prices of fossil fuels have incited a great interest of the polyurethane (PU) industry in the application of polyols made from renewable raw materials [1]. Traditionally used polyols are derived from petroleum, a non-renewable resource. The growing concern about environmental issues in material synthesis and crude oil price fluctuations have encouraged the scientific community to develop materials, including polyurethanes, based on renewable resources which do not depend on petroleum-based raw materials [2]. Soybean oil, palm oil, and rapeseed oil are the vegetable oils with the largest global production volumes and the most economical prices for large-scale use in commercial products [3]. These vegetable oils have been considered a favorable substitute as raw materials for polyol synthesis [4-6]. Soybeans account for approximately 60% of total oilseed production worldwide, and approximately 15% of soybean oil was used in industrial applications [7, 8]. The projected global production of soybeans in 2015 was over 317 million metric tons [8]. Most of the global soybean oil supply is produced in North and South America with the United States, Brazil, and Argentina as the major exporters of soybeans [9]. Life cycle assessments for vegetable oil-based polyols have demonstrated a significant reduction of petroleum-based raw materials consumption. Furthermore, lowering of greenhouse gas emissions could be achieved by using soy oil [10, 11]. Tan et al. synthesized PU foams by substituting a polypropylene-based polyol with soybean oil-based polyol [12]. The synthesized foams maintained a uniform cell structure but the cell size was smaller than the petroleum-based control foams. It was also observed that the density of 100% soy-based foams was 17% higher than the control foam. Soy-based foams also had comparable initial thermal conductivity (k2
value), comparable closed cell content, higher glass transition temperature (T g), and comparable compressive strength. It was also observed that foams made with 100% soy-based polyol degraded faster under accelerated aging conditions when compared to petroleum-based foams. Gas permeation tests of PU thin films showed higher nitrogen permeation for soy-based PU films, which is believed to be the cause of accelerated thermal aging. J. Merle et al. investigated the effect of additives such as hexamine, glyoxal, and tween 80, at different concentrations to have a better understanding of their impact on foam characteristics, aiming to improve foam fabricating process [13]. The study revealed that the viscosity of the mixture during foaming plays a crucial role in foam properties. The viscosity is reduced when solvents are added and results in lower mechanical properties and density of the foam. Using hexamine as the blowing agent increased the viscosity of the mixture, which leads to higher density and better mechanical properties. Various studies have shown that challenges remain for PUs produced with 100% vegetable oil-based polyols since their physical properties and performance were significantly lower than PUs fabricated using the blend of petroleum-based raw materials. PU foams based on vegetable oil polyols have exhibited poor dimensional stability and low strength, negating the potential savings of these cheaper renewable polyols [14, 15]. The preparation methods of vegetable-oil-based polyols often caused high viscosity, secondary functional groups (relatively low reactive) and low hydroxyl numbers, resulting in PU foams with inferior properties such as heterogeneous cell structure and difficulty in density control [16, 17]. Petroleum-based polyols exhibit primary functional groups (high reactivity), and high hydroxyl numbers, resulting in PU foams with superior mechanical and thermal properties [18]. 3
The motivation for this study is tailoring mechanical and thermal properties of PU foams. Two categories of blended PU foams were evaluated. The first category involves blending low functionality soy-based polyol HB230 (SBP) with petroleum-based polyol (PP). The second category involved blending of SBP with a high functionality soy-based cross-linker HB530 (SBC). The physical, thermal, and mechanical properties of the manufactured foams are evaluated and the effect of blending ratio on these properties was investigated.
2
MATERIALS AND MANUFACTURING
The study involves fabrication of samples divided into two categories. Category A refers to foams manufactured by blending soy-based polyol (HB230) with petroleum-based polyol. Category B involves the blend of soy-based polyol (HB230) with a soy-based cross-linker (HB530). 2.1
Materials The PU foam samples were manufactured by mixing two-part PU resin. The first
component is a blend of diphenylmethane diisocyanate (MDI) and polymeric methylene diphenyl diisocyanate (pMDI) procured from Covestro LLC (Pittsburgh PA) (NB#840859). The second component is a polyol blend. For Category A, it is a blend of SBP (procured from MCPU Polymer Engineering LLC, Pittsburg KS), and PP (RTM NB#840871, procured from Covestro LLC, Pittsburgh PA). For Category B, component B is a blend of SBP and SBC (procured from MCPU Polymer Engineering LLC, Pittsburg KS). The properties of SBC and SBP are mentioned in Table 1. The surfactant Xiameter OFX-0193 was obtained from Dow Corning, (Auburn MI) for the preparation of the foam. Dibutyltin dilaurate (DBTL) catalyst was purchased from SigmaAldrich (St. Louis, MO). Distilled water was used as the blowing agent.
4
Table 1. Properties of SBP and SBC obtained from datasheets [19] Polyol
HB 230
HB 530
Functionality
~2
~6
Hydroxyl Number (mg KOH/g)
220-240
510-550
Water (%)
≤0.1
≤0.1
Acid Number (mg KOH/g)
≤3.0
≤3.0
Viscosity at 25 ºC (cps)
375
54,000
Control samples were made using pure SBP (with no blending). Additional control samples were manufactured using a commercially available Baydur-683 foam formulation procured from Covestro LLC, Pittsburgh PA. 2.2
Preparation of foam samples The samples were manufactured by mixing appropriate blend of polyol, catalyst,
surfactant, and blowing agent using a low shear mixer at 1200 rpm for 10 seconds. The required amount of isocyanate was then added and further mixed for 10 seconds at 1200 rpm and poured into a rectangular mold of size 203.2 mm x 203.2 mm x 50.8 mm (8 in. x 8 in. x 1 in.). The compositions of PU foam chosen for this study are based on the findings of the previous study by the authors where effects of the amount of blowing agent and catalyst were analyzed [20]. The samples were fabricated using the formulations as shown in Table 2. Table 2. Formulation of foam samples Formulation
Polyol Blend
Blowing Agent
Surfactant
Catalyst
Isocyanate Index
I
100.00 g
0.75 g
5.00 g
0.08 g
1.14
II
100.00 g
1.50 g
5.00 g
0.08 g
1.14
5
Using the formulations mentioned in Table 2, various blends of SBP and PP were created with the percentage of PP varying from 25% to 75% for Category A samples. For Category B samples, SBP was blended with SBC, with the percentage of SBC varying from 5% to 20%. During initial run, it was observed that due to high viscosity of SBC, more than 20% substitution resulted in poor consistency of samples. The detailed formulations of the samples fabricated are mentioned in Table 3. Table 3. Formulation of SBP blends Category A -Formulation I Sample
SBP (g)
A-I-A A-I-B A-I-C A-I-D A-I-E A-I-F A-I-G
25.00 30.00 35.00 40.00 45.00 50.00 75.00
PP (g)
Category A -Formulation II
Water (g)
Sample
SBP (g)
PP (g)
Water (g)
0.75 0.75 0.75 0.75 0.75 0.75 0.75
A-II-A A-II-B A-II-C A-II-D A-II-E A-II-F A-II-G
25.00 30.00 35.00 40.00 45.00 50.00 75.00
75.00 70.00 65.00 60.00 55.00 50.00 25.00
1.50 1.50 1.50 1.50 1.50 1.50 1.50
75.00 70.00 65.00 60.00 55.00 50.00 25.00
Category B -Formulation I SBP (g) SBC (g) Sample Water (g) 95.00 5.00 0.75 B-I-A 90.00 10.00 0.75 B-I-B 85.00 15.00 0.75 B-I-C 80.00 20.00 0.75 B-I-D Control Samples SBC/PP SBP (g) Sample Water (g) (g) 100.00 0.00 0.75 Control-I
3
Category B -Formulation II Sample SBP (g) SBC (g) Water (g) 95.00 5.00 1.50 B-II-A 90.00 10.00 1.50 B-II-B 85.00 15.00 1.50 B-II-C 80.00 20.00 1.50 B-II-D Control Samples SBC/PP Sample SBP (g) Water (g) (g) 100.00 0.00 1.50 Control-II
EXPERIMENTS
The prepared samples were visually inspected for the consistency in the cell size and defects before further experimentation. The foam samples that were free of any defects were cut to the required size to analyze the thermal and mechanical properties. The prepared samples
6
were conditioned at 23 °C and 50% relative humidity for 48 hours prior to testing, in accordance with respective ASTM standards. 3.1
Evaluation of foam physical properties The effect of foam formulations on the density and microstructure of the manufactured
foam was evaluated. The density of the prepared PU foam sample was tested using ASTM D1622. For this test, the specimen size was 25.4 mm x 25.4 mm x 25.4 mm (1 in. x 1 in. x 1 in.). Three specimens were cut from the prepared samples and were weighed with a precision analytical balance. It had been observed in the previous study by the authors that density is a good indicator of the mechanical and thermal behavior of the prepared samples [20]. The microstructure of the fabricated foam samples was analyzed using S4700 (Hitachi) scanning electron microscope. Five samples of each category were sputter coated with Au/Pd to avoid electrostatic charging during the examination. Samples were analyzed using accelerating voltage of 10.0 kV. SEM images were used to determine the average cells size of the foam samples. Different cellular structure of the foam is the result of different compositions used, which led to the difference in the foam properties. 3.2
Thermal analyses The thermal conductivity is an important design factor for structural PU foams. In
addition, the thermal degradation mechanisms need to be identified. Thermal conductivity measurement is based on the temperature response of the foam samples to heat flow impulses. The heat flow is produced electrical heating of a resistive heater inserted in the probe, which is in direct contact with the tested sample [18]. The thermal conductivity of the prepared PU foam samples was measured according to ASTM C518, using QuickLine-30 Thermal property analyzer (Anter Corp., Pittsburgh, PA). 7
Thermal degradation of the foam samples was evaluated using Thermogravimetric analysis (TGA) using a TA Instruments Q50 equipment. During TGA, the weight change of a sample is measured as a function of increasing temperature. Samples were heated in air from room temperature to 600 ºC, at a heating rate of 10 ºC/min. The foam samples fabricated with soy-based polyol were compared with the Baydur-683 foam. 3.3
Fourier transform infrared spectroscopy The Fourier transform infrared spectroscopy (FTIR) is used to obtain an absorption
spectrum of the foam sample in the infrared range. The absorption spectra were recorded and analyzed within the range of 4000-400 cm-1 using Nicolet Nexus 470 (Thermo Scientific, USA) with Omnic software for data collection and to study the chemical changes in the samples. The effect of blending percentage on the organic groups in the foam was investigated. 3.4
Mechanical testing The mechanical properties of the foam were evaluated using compression force
deflection, constant deflection compression set, and tensile tests. Compression force deflection (CFD) test determines the ability of the foam to withstand compressive stresses and its agility after compression. This test was performed according to ASTM 3574-Test C, using Instron 5985 universal testing machine. Preconditioned samples were cut into 50.8 mm x 50.8 mm x 25.4 mm (2 in. x 2 in. x 1 in.) and pre-flexed twice by compressing to 50% of the thickness at the rate of 200 mm/min. After 6 minutes of final pre-flex, the specimens were compressed at the rate of 50 mm/min until 50% deflection (12.7 mm) was achieved. The load reading after 60 seconds of the compression was determined. The constant deflection compression set test consists of deflecting the foam specimen to a specified deflection, exposing it to specified conditions of time and temperature, and measuring 8
the change in the thickness of the specimen after a specified recovery period. This test was conducted according to ASTM D3574-Test D using Instron 5985 universal testing machine. The specimens were cut to 50.8 mm x 50.8 mm x 25.4 mm (2 in. x 2 in. x 1 in.), with parallel top and bottom surfaces and perpendicular sides. For each foam sample formulation, three specimens were tested. The initial thickness of the preconditioned specimen was noted and then compressed to 50% of the thickness. Within 15 minutes, the deflected specimen was placed in a convection oven at 23 ºC for 22 hours and 50% relative humidity. The sample thickness was measured after the recovery time. The constant deflection compression set (Ct) is expressed as a percentage of the original thickness, as follows:
Ct = (to – tf)/to x 100
(1)
where, to is the original thickness of the specimen and tf is the final thickness of the specimen. The tensile and tensile adhesion properties of foam were evaluated using tensile tests. This test was performed according to ASTM D1623-Type C, using an Instron 5985 universal testing machine. For this test, preconditioned samples were cut into 50.8 mm x 50.8 mm x 25.4 mm (2 in. x 2 in. x 1 in.) and were joined with 50.8 mm x 50.8 mm (2 in. x 2 in.) cross-section aluminum blocks with foam adhesive (Loctite Power Grab). The prepared samples were subjected to tensile loading at a rate of 1.3 mm/min. 3.5
Moisture absorption properties This test is performed to determine the water absorption by foam samples by immersing
the samples under 2 in. of water head. This test was performed according to ASTM D2842. For this test, the samples were cut into 50.8 mm x 50.8 mm x 25.4 mm (2 in. x 2 in. x 1 in.), weighed, and immersed in distilled water under 50.8 mm (2 in.) of head for 96 hours. After the 9
duration, the samples were weighed, and percent increase in weight of the samples was calculated.
4 4.1
RESULTS AND DISCUSSION
Physical properties of the foam The density of the samples was found to vary from 95.60 to 156.67 kg/m3 for Category
A-Formulation I, and 66.57 to126.10 kg/m3 for Category A-Formulation II samples. The comparatively higher density for Formulation I was observed due to the lower amount of blowing agent used. It was also observed that as the content of PP increased, the density of samples also increased (Figure 1).
Figure 1. Density of Category A-I and A-II samples w.r.t. PP content Category B samples exhibited lower density in the range of 46.33 to 62.40 kg/m3 for Formulation I, and 51.88 to 64.67 kg/m3 for Formulation II. It was observed that as the SBC content increased, the density of the samples also increased (Figure 2).
10
Figure 2. Density of Category B-I and B-II samples w.r.t. SBC content The control samples had density of 77.60±3.11 kg/m3, 55.95±2.33 kg/m3 and 79.75±0.59 kg/m3 for Control-I, Control-II, and Baydur-683 samples respectively. Overall, it was observed that Category B-I and Category B-II blended foam samples (except sample B-II-D) had lower density than Control-I and Control-II samples respectively. Whereas, for Category A-I and A-II samples, the density of blended samples was higher than Control-I and Control-II samples respectively. The detailed results are shown in Table 4.
11
Table 4. Formulations and density of the samples Category A-I
Category A-II 3
Sample
SBP:PP
Density (kg/m )
Sample
SBP:PP
Density (kg/m3)
A-I-A
25:75
156.67± 2.45
A-II-A
25:75
126.10± 4.30
A-I-B
30:70
129.73± 4.30
A-II-B
30:70
95.20± 1.37
A-I-C
35:65
126.20± 2.43
A-II-C
35:65
91.80± 4.90
A-I-D
40:60
108.40± 5.08
A-II-D
40:60
85.60± 2.12
A-I-E
45:55
105.30± 6.18
A-II-E
45:55
80.79± 3.76
A-I-F
50:50
102.63± 3.20
A-II-F
50:50
73.40± 1.61
A-I-G
75:25
95.60± 4.51
A-II-G
75:25
66.57± 3.09
Category B-I
Category B-II 3
Density (kg/m3)
Sample
SBP:SBC
Density (kg/m )
Sample
SBP:SBC
B-I-A
95:5
51.88± 0.57
B-II-A
95:5
46.33± 1.29
B-I-B
90:10
54.37± 0.87
B-IIB
90:10
50.46± 1.31
B-I-C
85:15
58.57± 1.51
B-II-C
85:15
54.50± 1.04
B-I-D
80:20
64.67± 0.41
B-II-D
80:20
62.40± 0.69
Control-I
100:0
77.60± 3.11
Control-II
100:0
55.95± 2.33
Baydur
Commercial
79.75± 1.02
The microstructure of the samples with highest and lowest densities were analyzed by Hitachi S4700 electron microscope. As observed from the SEM images of the cellular structure of sample A-I-A and A-I-G (Figure 3), and A-II-A and A-II-G (Figure 4) it was observed that the samples with lower density had bigger cell size as compared to samples with higher density. This can be explained by the fact that samples with lower density composed of higher blowing agent. Higher amount of blowing agent produced more CO2, resulting in bigger cells of the foam [26].
12
Figure 3 (a). Sample A-I-A
Figure 3 (b). Sample A-I-G
Figure 4 (a). Sample A-II-A
Figure 4 (b). Sample A-II-G
Figure 5 (a). Sample B-I-A
Figure 5 (b). Sample B-I-D
A similar trend was overserved for sample B-I-A and B-I-D (Figure 5), and B-II-A and B-II-D (Figure 6). Also, it was observed that the cellular structure of blended samples appears to be more regular as compared to Control-I and Control-II samples (Figure 7). The cell structure of Baydur 683 foam is shown in Figure 8. The details of cell sizes of foam samples are mentioned in Table 5. It was also observed that for Category B samples, as the amount of SBC increased, the cell size reduced. This can be explained by an increase in the viscosity of the formulation,
13
due to the high viscosity of SBC (Table 1). The effects of viscosity have been investigated by various researchers, and a decrease in cell size of the foam with an increase in viscosity has been reported [26, 27]. The cell sizes of blended foam samples were less than that of Baydur 683.
Figure 6 (a). Sample B-II-A
Figure 6 (b). Sample B-II-D
Figure 7 (a). Sample Control-I
Figure 7 (b). Sample Control-II
Figure 8. Sample Baydur 683
14
Table 5. Cell size of the foam samples
4.2
Sample
Cell Size (mm2)
Sample
A-I-A
0.09±0.01
B-I-A
Cell Size (mm2) 0.08±0.02
A-I-G
0.13±0.03
B-I-D
0.11±0.01
A-II-A
0.21±0.01
B-II-A
0.21±0.03
A-II-G
0.41±0.04
B-II-D
0.35±0.05
B-I-A
0.08±0.02
Control-I
0.34±0.07
B-I-D
0.11±0.01
Control-II
0.69±0.05
B-II-A
0.21±0.03
Baydur
0.63±0.08
Thermal analyses The thermal conductivity of the foam (λ) was measured (Table 6) and was used to
calculate thermal resistivity, also known as R-Value (R) using the following expression: R.
=
(2)
The relationship between blend ratio and thermal resistivity is shown in Figure 9 and Figure 10 for samples A and B, respectively. For Category A samples, the thermal resistivity increased with a decrease in the percentage of petroleum polyol, which also corresponds to decrease in foam density. Similar results have been reported by Tan et al. [12] and Guo et al. [24]. For Category B samples, the thermal resistivity decreased with an increase in SBC content. Blended polyol samples showed higher thermal resistivity as compared to control samples. In general, Formulation II exhibited higher thermal resistivity as compared to Formulation I, due to its lower density.
15
Figure 9. Relationship between R-Value and Density for Category A-I and A-II
Figure 10. Relationship between R-Value and Density for Category B-I and B-II
TGA test was performed to compare thermo-oxidative degradation behavior of the fabricated samples. The weight loss and weight loss derivative (DTG) curves are shown in Figure 11(a) and Figure 11(b) respectively. It was observed from DTG curves that Category A 16
samples followed two-step degradation process, whereas Category B samples followed threestep degradation. For Category B, the first step of degradation was at 180 ºC, which is due to dissociation of urethane bonds [28]. Category A samples along with Category B samples showed major degradation starting at 285 ºC till 315 ºC. This step is also associated with the degradation of urethane bonds [26]. The final degradation, above 520 ºC is associated with the final degradation of polyol backbone chain, due to oxidation of the foam samples. It can be concluded from the test that Category A samples had higher thermal stability as compared to Category B samples. Better thermal stability of Category A samples can be attributed to better cross-link density in Category A samples [27]. The char formation was negligible (2-4%) at end of the test. Also, Baydur foam exhibited better thermal stability as compared to all the fabricated samples.
Figure 14(a). Weight loss behavior of fabricated samples
17
Figure 14(b). Weight loss derivative curves for fabricated samples
4.3
Fourier transform infrared spectroscopy The FTIR spectra of the fabricated samples showed absorbance bands corresponding to
the chemical bonds in the foam samples. Collected spectra for the samples of different formulations were very similar, as shown in Figure 12. In the spectra, a band observed between 3250 cm-1 and 3400 cm-1 represents the stretching oh N-H bond in urethane and urea groups, and stretching vibrations in O-H groups [29]. The strong peaks at 2925 cm-1 and 2850 cm-1 are attributed to asymmetric and symmetric stretching vibrations of C-H bonds respectively [29]. The peaks at 1740 and 1710 cm-1 corresponds to stretching of C=O bond in urethane. The peak at 1470 is attributed to free C=O stretching, whereas peak at 1710 is due to hydrogen-bonded C=O stretching [25]. The peaks at 1599 cm-1 and 1512 cm-1 are attributed to stretching and bending vibrations of C-N and N-H bonds respectively in urethane. The peaks at 1215 cm-1 show the 18
stretching vibrations of C-O bond. The peak at 1070 cm-1 is attributed to the vibration of C-O bond. The chemical structure of the manufactured blended foam samples is similar across all formulations.
Figure 12. FTIR spectra of fabricated samples 4.4
Mechanical testing It was observed that CFD of the samples increased with the increase in the density. This
phenomenon is commonly observed in cellular foam systems [30]. Figure 13 shows the relation between thermal resistivity (R-Value) and compression force deflection (CFD) w.r.t. density of the fabricated samples. The CFD of Control-I and Control-II samples was 77.6 ±3.1 and 55.9±2.3 respectively. The fabricated samples had better CFD properties as compared to Control-I and Control-II samples. The possible explanation for this phenomenon can be the blended foam samples had regular cells as compared to control samples. The regular cell size
19
leads to high uniform load distribution, as compared to non-uniform cell structure, leading to high CFD [12, 22]. The detailed CFD values for all tested samples are mentioned in Table 6.
Figure 13 (a). CFD and R-Value of Category A-I, w.r.t. Density
Figure 13 (b). CFD and R-Value of Category A-II, w.r.t. Density 20
For Category A samples, an increase in PP content corresponds to an increase in CFD strength. Using PP content greater than 55% for Category A-I and 65% for Category A-II was required to produce foams with CFD strength greater than commercial Baydur 683 foam.
Figure 13 (c). CFD and R-Value of Category B-I, w.r.t. Density
Figure 13 (d). CFD and R-Value of Category B-II, w.r.t. Density
21
For Category B samples, an increase in SBC content corresponds to an increase in CFD strength. Using SBC content greater than 15% both Category B-I and 10% for Category B-II was required to produce foams with CFD strength greater than commercial Baydur 683 foam. Also, it was observed that all the blended foam samples had higher CFD as compared to Control-I and Control-II samples. Table 6. Mechanical and Thermal testing results of foam samples Density Thermal Insulation CFD Foam. # 3 kg/m S.D. R-Value S.D. kPa S.D. A-I-A 156.67 2.45 23.04 0.11 818.51 11.65 A-I-B 129.73 4.30 23.31 0.04 682.65 6.02 A-I-C 126.20 2.43 23.64 0.08 585.13 12.32 A-I-D 118.40 5.08 23.76 0.08 521.83 2.69 A-I-E 105.30 6.18 23.94 0.04 494.71 10.67 A-I-F 102.63 3.20 24.48 0.04 330.24 1.94 A-I-G 4.51 0.05 13.02 95.60 25.81 143.50 A-II-A 126.10 4.30 25.97 0.10 537.72 11.98 A-II-B 95.20 1.37 26.10 0.05 457.09 2.94 A-II-C 91.80 4.90 26.16 0.10 372.68 5.40 A-II-D 85.60 2.12 26.28 0.10 265.49 3.23 A-II-E 80.79 3.76 26.39 0.05 248.05 6.61 A-II-F 73.40 1.61 26.42 0.05 179.43 1.96 A-II-G 66.57 3.09 26.60 0.20 131.58 7.43 B-I-A 46.33 0.57 26.63 0.03 290.37 5.71 B-I-B 50.46 0.87 26.32 0.14 293.83 9.50 B-I-C 58.50 1.51 26.01 0.10 394.22 3.85 B-I-D 62.40 0.41 25.48 0.18 465.36 8.95 B-II-A 51.88 1.29 26.96 0.06 328.19 6.60 B-II-B 54.37 1.31 26.63 0.07 380.37 5.95 B-II-C 58.57 1.04 26.32 0.08 412.40 3.75 B-II-D 64.67 0.69 26.21 0.03 483.14 7.37 Control-I 77.60 3.11 25.06 0.05 111.70 1.53 Control-II 55.95 2.33 26.32 0.01 74.61 2.44 Baydur 79.75 1.02 28.44 0.03 366.90 4.70 The constant deflection compression set test was performed to determine mechanical and geometrical properties of the foam. It was observed that Category B samples had much 22
uniform Ct values (2.02-2.19 for B-I and 2.60-3.06 for B-II) as compared to Category A samples (1.34-8.82 for A-I and 1.40-6.88 for A-II). For Category A samples, it was observed that as the SBP content increased, Ct value decreased, giving more dimensional stability to the samples (Figure 14). However, for Category B samples, a small difference in the Ct values was observed with change in the content of SBC (Figure 15).
Figure 14. Ct for Category A samples Control samples I and II showed relatively superior Ct (0.82 and 1.49 for Control-I and Control-II, respectively) as compared to blended samples. Baydur 683 foam had Ct value of 3.360±0.053. The lower Ct values for blended samples is attributed to the elastomeric origin of the long chain polyol molecules [23].
23
Figure 15. Ct for Category B samples The tensile strength of the soy-based foam samples were found to be in the range of 105.11-654.71 kPa for Category A-I, 64.26-597.06 kPa for Category A-II, 215.66-279.21 for Category B-I, and 200.80-351.18 for Category B-II. The control samples had much lower tensile strengths of 85.99±2.83 kPa for Control-I and 52.53±2.34 kPa for Control-II. Also, commercial Baydur 683 sample had a tensile strength of 71.79±1.03 kPa. It was observed that amount of PP had a more significant effect on the tensile strength as compared the effect of SBC. Increasing PP content resulted in increasing tensile strength for A-I and A-II formulations. For B-I and B-II formulations, increasing SBC resulted in higher tensile strengths. Also, the samples with higher density had higher tensile strength (Figure 16).
24
Figure 16 (a). Tensile strength of Category A samples w.r.t. SBP and PP content
Figure 16 (b). Tensile strength of Category B samples w.r.t. SBP and SBC content
25
Table 7. Mechanical properties of the tested samples
4.5
Formulation
Ct Value
Foam. # A-I-A A-I-B A-I-C A-I-D A-I-E A-I-F A-I-G A-II-A A-II-B A-II-C A-II-D A-II-E A-II-F A-II-G B-I-A B-I-B B-I-C B-I-D B-II-A B-II-B B-II-C B-II-D Control-I Control-II Baydur
% 8.82±0.15 7.58±0.08 5.05±0.14 3.26±0.13 1.97±0.38 1.57±0.21 1.34±0.09 6.88±0.09 5.17±0.18 2.88±0.16 2.72±0.26 1.51±0.11 1.49±0.26 1.41±0.29 2.19±0.12 2.20±0.24 2.02±0.59 2.25±0.32 3.06±0.28 2.63±0.09 2.60±0.28 2.86±0.24 0.82±0.12 1.49±0.15 3.36±0.05
Tensile Strength kPa 654.71±4.46 637.88±2.51 506.04±9.61 435.87±6.14 385.9±5.91 211.17±5.34 105.11±4.26 597.06±5.03 501.46±2.55 456.65±6.14 372.65±5.34 308.19±3.29 262.07±5.93 64.26±2.33 215.66±13.31 231.30±15.48 259.52±10.42 279.21±11.23 200.80±14.05 283.51±13.41 317.90±18.14 351.18±12.16 85.99±2.83 52.53±2.34 71.79±7.03
Moisture Adsorption % 55.8±8.48 51.17±1.84 61.81±3.24 58.33±3.89 90.25±3.29 40.75±1.96 77.25±8.13 102.53±2.25 98.66±9.17 95.38±9.81 114.18±2.85 95.76±8.42 71.03±10.19 78.94±9.54 106.79±4.57 96.85±8.35 131.50±10.24 118.42±11.65 99.40±8.52 92.47±9.42 111.79±32.50 91.98±25.86 63.59±3.81 86.27±6.19 63.59±1.78
Moisture absorption properties The fabricated samples were cut to the required size and submerged in distilled water, as
mentioned in section 3.5. The samples were weighed before and after submerging in water, and the percent increase in weight was calculated. After the samples were immersed in distilled water for 96 hours at 2 in. head, the increase in weight was evaluated. The control samples, Control-I and Control-II showed 61.42±3.81% and 86.27±6.19% increase in weight, respectively, whereas 26
Baydur sample showed 63.59±1.78% increase in weight. The experiments did not show any direct correlation between PP or SBC percentage and moisture absorption in blended foams samples. Formulation A-I had moisture absorption lower than control samples and in the range of commercial Baydur-683 foams. In order to evaluate the ingress of moisture, the samples were submerged in dyed water for 96 hours and 2 in. of head. The samples were cut in half upon removal, and no penetration of water was observed inside the samples (Figure 17). Therefore, the increase in weight of samples can be explained by the presence of water in the cells on the outer surface of the foam. This argument is supported by the fact that the samples with large cell size showed more increase in weight after submerging in water.
Figure 17. Cross-section of foam samples showing no penetration of water
5
CONCLUSIONS
PU foam samples were fabricated by blending soy-based polyol HB230 (SBP) with polyether polyol (PP) and soy-based cross-linker HB530 (SBC) in varying amounts. The prepared foam samples were tested for various mechanical and thermal properties. It was observed that blended polyol samples resulted in better mechanical and thermal properties as compared to control samples fabricated by pure HB230 soy-based polyol and Baydur 683 PU 27
foam. It was observed that the blended foam samples had significantly higher CFD and tensile strength as compared with the control samples. Also, it was observed that foams blended with SBC had superior thermal resistivity, as compared to foams blended with PP and control samples. The density of the foam samples was dependent on the amount of PP and SBC, where a higher amount of PP and SBC resulted in a higher density of the samples. It was concluded that blend of SBP and PP in the ratio 75:25 by weight resulted in the samples with highest thermal insulation while having better CFD and tensile properties than control samples. It was observed that for 95:5 blend of SBP and SBC resulted in foam with 250% increase in tensile strength while having similar R-Value and dimensional stability as compared to Baydur 683 foam.
6
ACKNOWLEDGMENTS
This work is supported by the Missouri Soybean Merchandising Council (Jefferson City, MO). The authors would like to thank Mr. John Miller (MCPU Polymer Engineering LLC, Pittsburg, KS) for providing HB230 polyol and Dr. Steven Harasin (Covestro LLC, Pittsburgh, PA) for providing isocyanate and polyether polyol. Authors would also like to thank Mr. Stephan Johannesmeyer (Thermocore of Missouri, Jefferson City, MO) for his helpful suggestions.
7
REFERENCES
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11. L. Ugarte, A. Saralegi, R. Fernández, L. Martína, M. Corcuera, and A. Eceiza, “Flexible Polyurethane Foams Based on 100% Renewably Sourced Polyols”, Industrial Crops and Products, Vol. 62, pp. 545–551, 2014. 12. S. Tan, T. Abraham, D. Ference, and C. Macosko, “Rigid Polyurethane Foams from a Soybean Oil-Based Polyol”, Polymer, Vol. 52, pp. 2840-2846, 2011. 13. J. Merle, P. Trinsoutrot, and F. C. Bouhtoury, “Optimization of the Formulation for the Synthesis of Bio-Based Foams”, European Polymer Journal, Vol. 84, pp. 577–588, 2016. 14. H. Pawlik and A. Prociak, “Influence of Palm Oil-Based Polyol on the Properties of Flexible Polyurethane Foams”, Journal of Polymers and the Environment, Vol. 20, Issue 2, pp. 438– 445, 2011. 15. K. Badri, S. Ahmad, and S. Zakaria, “Production of a High-Functionality RBD Palm Kernel Oil-Based Polyester Polyol”, Journal of Applied Polymer Science, Vol. 81, pp. 384–389, 2001. 16. G. Woods, “Flexible Polyurethane Foams: Chemistry and Technology”, Applied Science Publishers, London, 1982. 17. M. Szycher, “Szycher’s Handbook of Polyurethanes”, Taylor & Francis, 2nd Edition, Boca Raton, FL, 2013. 18. Operating Service Manual, Quickline-30 Thermal Property Analyzer, Anter Corporation, March 2009. 19. Soy-based Polyol HB230 Technical Data Sheet, MCPU Polymer Engineering, LLC. 20. G. S. Dhaliwal, S. Anandan, K. Chandrashekhara, N. Dudenhoeffer, and P. Nam, “Fabrication and Testing of Soy-based Polyurethane Foam”, The Proceedings of CAMX Conference, Anaheim, CA, September 26-29, pp. 1-13, 2016.
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HIGHLIGHTS This study aims to substitute the petroleum-based polyols with soy-based polyols for the fabrication of rigid PU foams that have comparable or better physical properties required for thermal and structural applications. Properties of foams blended with soy-based cross linker were compared to 100% soy based foams, and a commercially available 100% petroleum based foam. The blended foam samples exhibited better thermal resistivity (8% higher) as compared to control foams. The blended foams also exhibited better compressive properties (about 512% higher) and tensile strength (287% higher) compared to control foam. Blended soy-PU foams with properties comparable to commercial petroleum-based foams were manufactured in the study.
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S0014-3057(18)30734-1 https://doi.org/10.1016/j.eurpolymj.2018.08.001 EPJ 8514
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
19 April 2018 27 June 2018 2 August 2018
Please cite this article as: Dhaliwal, G.S., Anandan, S., Chandrashekhara, K., Lees, J., Nam, P., Development and Characterization of Polyurethane Foams with Substitution of Polyether Polyol with Soy-Based Polyol, European Polymer Journal (2018), doi: https://doi.org/10.1016/j.eurpolymj.2018.08.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Development and Characterization of Polyurethane Foams with Substitution of Polyether Polyol with Soy-Based Polyol Gurjot S. Dhaliwal1, Sudharshan Anandan1, K. Chandrashekhara*1, J. Lees2 and Paul Nam2 1
Department of Mechanical and Aerospace Engineering,2Department of Chemistry Missouri University of Science and Technology, Rolla MO, 65409, USA
ABSTRACT Bio-based polyols can replace petroleum-based polyols for producing a wide range of polyurethane (PU) products. Bio-based polyols are derived from renewable resources, however, their utilization is limited due to the complex molecular structure and relatively low primary hydroxyl content. This study aims to substitute the petroleum-based polyols with soy-based polyols for the fabrication of rigid PU foams with comparable or better physical properties required for thermal and structural applications. The effects of blending soy-based polyol, of hydroxyl number 230 mg KOH/g with high functionality soy-based cross-linker were investigated. The fabricated samples were tested for density, compressive strength, tensile strength, thermal resistivity, thermal stability and dimensional stability. The blended foam samples exhibited better thermal resistivity (8% higher), compressive properties (about 512% higher) and tensile strength (287% higher) when compared to control specimens. It was concluded that the polyols blended with soy-based cross-linker in 95:5 ratio by weight resulted in properties better than control samples, and comparable to commercial petroleum-based foams.
*Corresponding Author Email: [email protected]
1
INTRODUCTION
Global climate change and fluctuating prices of fossil fuels have incited a great interest of the polyurethane (PU) industry in the application of polyols made from renewable raw materials [1]. Traditionally used polyols are derived from petroleum, a non-renewable resource. The growing concern about environmental issues in material synthesis and crude oil price fluctuations have encouraged the scientific community to develop materials, including polyurethanes, based on renewable resources which do not depend on petroleum-based raw materials [2]. Soybean oil, palm oil, and rapeseed oil are the vegetable oils with the largest global production volumes and the most economical prices for large-scale use in commercial products [3]. These vegetable oils have been considered a favorable substitute as raw materials for polyol synthesis [4-6]. Soybeans account for approximately 60% of total oilseed production worldwide, and approximately 15% of soybean oil was used in industrial applications [7, 8]. The projected global production of soybeans in 2015 was over 317 million metric tons [8]. Most of the global soybean oil supply is produced in North and South America with the United States, Brazil, and Argentina as the major exporters of soybeans [9]. Life cycle assessments for vegetable oil-based polyols have demonstrated a significant reduction of petroleum-based raw materials consumption. Furthermore, lowering of greenhouse gas emissions could be achieved by using soy oil [10, 11]. Tan et al. synthesized PU foams by substituting a polypropylene-based polyol with soybean oil-based polyol [12]. The synthesized foams maintained a uniform cell structure but the cell size was smaller than the petroleum-based control foams. It was also observed that the density of 100% soy-based foams was 17% higher than the control foam. Soy-based foams also had comparable initial thermal conductivity (k2
value), comparable closed cell content, higher glass transition temperature (T g), and comparable compressive strength. It was also observed that foams made with 100% soy-based polyol degraded faster under accelerated aging conditions when compared to petroleum-based foams. Gas permeation tests of PU thin films showed higher nitrogen permeation for soy-based PU films, which is believed to be the cause of accelerated thermal aging. J. Merle et al. investigated the effect of additives such as hexamine, glyoxal, and tween 80, at different concentrations to have a better understanding of their impact on foam characteristics, aiming to improve foam fabricating process [13]. The study revealed that the viscosity of the mixture during foaming plays a crucial role in foam properties. The viscosity is reduced when solvents are added and results in lower mechanical properties and density of the foam. Using hexamine as the blowing agent increased the viscosity of the mixture, which leads to higher density and better mechanical properties. Various studies have shown that challenges remain for PUs produced with 100% vegetable oil-based polyols since their physical properties and performance were significantly lower than PUs fabricated using the blend of petroleum-based raw materials. PU foams based on vegetable oil polyols have exhibited poor dimensional stability and low strength, negating the potential savings of these cheaper renewable polyols [14, 15]. The preparation methods of vegetable-oil-based polyols often caused high viscosity, secondary functional groups (relatively low reactive) and low hydroxyl numbers, resulting in PU foams with inferior properties such as heterogeneous cell structure and difficulty in density control [16, 17]. Petroleum-based polyols exhibit primary functional groups (high reactivity), and high hydroxyl numbers, resulting in PU foams with superior mechanical and thermal properties [18]. 3
The motivation for this study is tailoring mechanical and thermal properties of PU foams. Two categories of blended PU foams were evaluated. The first category involves blending low functionality soy-based polyol HB230 (SBP) with petroleum-based polyol (PP). The second category involved blending of SBP with a high functionality soy-based cross-linker HB530 (SBC). The physical, thermal, and mechanical properties of the manufactured foams are evaluated and the effect of blending ratio on these properties was investigated.
2
MATERIALS AND MANUFACTURING
The study involves fabrication of samples divided into two categories. Category A refers to foams manufactured by blending soy-based polyol (HB230) with petroleum-based polyol. Category B involves the blend of soy-based polyol (HB230) with a soy-based cross-linker (HB530). 2.1
Materials The PU foam samples were manufactured by mixing two-part PU resin. The first
component is a blend of diphenylmethane diisocyanate (MDI) and polymeric methylene diphenyl diisocyanate (pMDI) procured from Covestro LLC (Pittsburgh PA) (NB#840859). The second component is a polyol blend. For Category A, it is a blend of SBP (procured from MCPU Polymer Engineering LLC, Pittsburg KS), and PP (RTM NB#840871, procured from Covestro LLC, Pittsburgh PA). For Category B, component B is a blend of SBP and SBC (procured from MCPU Polymer Engineering LLC, Pittsburg KS). The properties of SBC and SBP are mentioned in Table 1. The surfactant Xiameter OFX-0193 was obtained from Dow Corning, (Auburn MI) for the preparation of the foam. Dibutyltin dilaurate (DBTL) catalyst was purchased from SigmaAldrich (St. Louis, MO). Distilled water was used as the blowing agent.
4
Table 1. Properties of SBP and SBC obtained from datasheets [19] Polyol
HB 230
HB 530
Functionality
~2
~6
Hydroxyl Number (mg KOH/g)
220-240
510-550
Water (%)
≤0.1
≤0.1
Acid Number (mg KOH/g)
≤3.0
≤3.0
Viscosity at 25 ºC (cps)
375
54,000
Control samples were made using pure SBP (with no blending). Additional control samples were manufactured using a commercially available Baydur-683 foam formulation procured from Covestro LLC, Pittsburgh PA. 2.2
Preparation of foam samples The samples were manufactured by mixing appropriate blend of polyol, catalyst,
surfactant, and blowing agent using a low shear mixer at 1200 rpm for 10 seconds. The required amount of isocyanate was then added and further mixed for 10 seconds at 1200 rpm and poured into a rectangular mold of size 203.2 mm x 203.2 mm x 50.8 mm (8 in. x 8 in. x 1 in.). The compositions of PU foam chosen for this study are based on the findings of the previous study by the authors where effects of the amount of blowing agent and catalyst were analyzed [20]. The samples were fabricated using the formulations as shown in Table 2. Table 2. Formulation of foam samples Formulation
Polyol Blend
Blowing Agent
Surfactant
Catalyst
Isocyanate Index
I
100.00 g
0.75 g
5.00 g
0.08 g
1.14
II
100.00 g
1.50 g
5.00 g
0.08 g
1.14
5
Using the formulations mentioned in Table 2, various blends of SBP and PP were created with the percentage of PP varying from 25% to 75% for Category A samples. For Category B samples, SBP was blended with SBC, with the percentage of SBC varying from 5% to 20%. During initial run, it was observed that due to high viscosity of SBC, more than 20% substitution resulted in poor consistency of samples. The detailed formulations of the samples fabricated are mentioned in Table 3. Table 3. Formulation of SBP blends Category A -Formulation I Sample
SBP (g)
A-I-A A-I-B A-I-C A-I-D A-I-E A-I-F A-I-G
25.00 30.00 35.00 40.00 45.00 50.00 75.00
PP (g)
Category A -Formulation II
Water (g)
Sample
SBP (g)
PP (g)
Water (g)
0.75 0.75 0.75 0.75 0.75 0.75 0.75
A-II-A A-II-B A-II-C A-II-D A-II-E A-II-F A-II-G
25.00 30.00 35.00 40.00 45.00 50.00 75.00
75.00 70.00 65.00 60.00 55.00 50.00 25.00
1.50 1.50 1.50 1.50 1.50 1.50 1.50
75.00 70.00 65.00 60.00 55.00 50.00 25.00
Category B -Formulation I SBP (g) SBC (g) Sample Water (g) 95.00 5.00 0.75 B-I-A 90.00 10.00 0.75 B-I-B 85.00 15.00 0.75 B-I-C 80.00 20.00 0.75 B-I-D Control Samples SBC/PP SBP (g) Sample Water (g) (g) 100.00 0.00 0.75 Control-I
3
Category B -Formulation II Sample SBP (g) SBC (g) Water (g) 95.00 5.00 1.50 B-II-A 90.00 10.00 1.50 B-II-B 85.00 15.00 1.50 B-II-C 80.00 20.00 1.50 B-II-D Control Samples SBC/PP Sample SBP (g) Water (g) (g) 100.00 0.00 1.50 Control-II
EXPERIMENTS
The prepared samples were visually inspected for the consistency in the cell size and defects before further experimentation. The foam samples that were free of any defects were cut to the required size to analyze the thermal and mechanical properties. The prepared samples
6
were conditioned at 23 °C and 50% relative humidity for 48 hours prior to testing, in accordance with respective ASTM standards. 3.1
Evaluation of foam physical properties The effect of foam formulations on the density and microstructure of the manufactured
foam was evaluated. The density of the prepared PU foam sample was tested using ASTM D1622. For this test, the specimen size was 25.4 mm x 25.4 mm x 25.4 mm (1 in. x 1 in. x 1 in.). Three specimens were cut from the prepared samples and were weighed with a precision analytical balance. It had been observed in the previous study by the authors that density is a good indicator of the mechanical and thermal behavior of the prepared samples [20]. The microstructure of the fabricated foam samples was analyzed using S4700 (Hitachi) scanning electron microscope. Five samples of each category were sputter coated with Au/Pd to avoid electrostatic charging during the examination. Samples were analyzed using accelerating voltage of 10.0 kV. SEM images were used to determine the average cells size of the foam samples. Different cellular structure of the foam is the result of different compositions used, which led to the difference in the foam properties. 3.2
Thermal analyses The thermal conductivity is an important design factor for structural PU foams. In
addition, the thermal degradation mechanisms need to be identified. Thermal conductivity measurement is based on the temperature response of the foam samples to heat flow impulses. The heat flow is produced electrical heating of a resistive heater inserted in the probe, which is in direct contact with the tested sample [18]. The thermal conductivity of the prepared PU foam samples was measured according to ASTM C518, using QuickLine-30 Thermal property analyzer (Anter Corp., Pittsburgh, PA). 7
Thermal degradation of the foam samples was evaluated using Thermogravimetric analysis (TGA) using a TA Instruments Q50 equipment. During TGA, the weight change of a sample is measured as a function of increasing temperature. Samples were heated in air from room temperature to 600 ºC, at a heating rate of 10 ºC/min. The foam samples fabricated with soy-based polyol were compared with the Baydur-683 foam. 3.3
Fourier transform infrared spectroscopy The Fourier transform infrared spectroscopy (FTIR) is used to obtain an absorption
spectrum of the foam sample in the infrared range. The absorption spectra were recorded and analyzed within the range of 4000-400 cm-1 using Nicolet Nexus 470 (Thermo Scientific, USA) with Omnic software for data collection and to study the chemical changes in the samples. The effect of blending percentage on the organic groups in the foam was investigated. 3.4
Mechanical testing The mechanical properties of the foam were evaluated using compression force
deflection, constant deflection compression set, and tensile tests. Compression force deflection (CFD) test determines the ability of the foam to withstand compressive stresses and its agility after compression. This test was performed according to ASTM 3574-Test C, using Instron 5985 universal testing machine. Preconditioned samples were cut into 50.8 mm x 50.8 mm x 25.4 mm (2 in. x 2 in. x 1 in.) and pre-flexed twice by compressing to 50% of the thickness at the rate of 200 mm/min. After 6 minutes of final pre-flex, the specimens were compressed at the rate of 50 mm/min until 50% deflection (12.7 mm) was achieved. The load reading after 60 seconds of the compression was determined. The constant deflection compression set test consists of deflecting the foam specimen to a specified deflection, exposing it to specified conditions of time and temperature, and measuring 8
the change in the thickness of the specimen after a specified recovery period. This test was conducted according to ASTM D3574-Test D using Instron 5985 universal testing machine. The specimens were cut to 50.8 mm x 50.8 mm x 25.4 mm (2 in. x 2 in. x 1 in.), with parallel top and bottom surfaces and perpendicular sides. For each foam sample formulation, three specimens were tested. The initial thickness of the preconditioned specimen was noted and then compressed to 50% of the thickness. Within 15 minutes, the deflected specimen was placed in a convection oven at 23 ºC for 22 hours and 50% relative humidity. The sample thickness was measured after the recovery time. The constant deflection compression set (Ct) is expressed as a percentage of the original thickness, as follows:
Ct = (to – tf)/to x 100
(1)
where, to is the original thickness of the specimen and tf is the final thickness of the specimen. The tensile and tensile adhesion properties of foam were evaluated using tensile tests. This test was performed according to ASTM D1623-Type C, using an Instron 5985 universal testing machine. For this test, preconditioned samples were cut into 50.8 mm x 50.8 mm x 25.4 mm (2 in. x 2 in. x 1 in.) and were joined with 50.8 mm x 50.8 mm (2 in. x 2 in.) cross-section aluminum blocks with foam adhesive (Loctite Power Grab). The prepared samples were subjected to tensile loading at a rate of 1.3 mm/min. 3.5
Moisture absorption properties This test is performed to determine the water absorption by foam samples by immersing
the samples under 2 in. of water head. This test was performed according to ASTM D2842. For this test, the samples were cut into 50.8 mm x 50.8 mm x 25.4 mm (2 in. x 2 in. x 1 in.), weighed, and immersed in distilled water under 50.8 mm (2 in.) of head for 96 hours. After the 9
duration, the samples were weighed, and percent increase in weight of the samples was calculated.
4 4.1
RESULTS AND DISCUSSION
Physical properties of the foam The density of the samples was found to vary from 95.60 to 156.67 kg/m3 for Category
A-Formulation I, and 66.57 to126.10 kg/m3 for Category A-Formulation II samples. The comparatively higher density for Formulation I was observed due to the lower amount of blowing agent used. It was also observed that as the content of PP increased, the density of samples also increased (Figure 1).
Figure 1. Density of Category A-I and A-II samples w.r.t. PP content Category B samples exhibited lower density in the range of 46.33 to 62.40 kg/m3 for Formulation I, and 51.88 to 64.67 kg/m3 for Formulation II. It was observed that as the SBC content increased, the density of the samples also increased (Figure 2).
10
Figure 2. Density of Category B-I and B-II samples w.r.t. SBC content The control samples had density of 77.60±3.11 kg/m3, 55.95±2.33 kg/m3 and 79.75±0.59 kg/m3 for Control-I, Control-II, and Baydur-683 samples respectively. Overall, it was observed that Category B-I and Category B-II blended foam samples (except sample B-II-D) had lower density than Control-I and Control-II samples respectively. Whereas, for Category A-I and A-II samples, the density of blended samples was higher than Control-I and Control-II samples respectively. The detailed results are shown in Table 4.
11
Table 4. Formulations and density of the samples Category A-I
Category A-II 3
Sample
SBP:PP
Density (kg/m )
Sample
SBP:PP
Density (kg/m3)
A-I-A
25:75
156.67± 2.45
A-II-A
25:75
126.10± 4.30
A-I-B
30:70
129.73± 4.30
A-II-B
30:70
95.20± 1.37
A-I-C
35:65
126.20± 2.43
A-II-C
35:65
91.80± 4.90
A-I-D
40:60
108.40± 5.08
A-II-D
40:60
85.60± 2.12
A-I-E
45:55
105.30± 6.18
A-II-E
45:55
80.79± 3.76
A-I-F
50:50
102.63± 3.20
A-II-F
50:50
73.40± 1.61
A-I-G
75:25
95.60± 4.51
A-II-G
75:25
66.57± 3.09
Category B-I
Category B-II 3
Density (kg/m3)
Sample
SBP:SBC
Density (kg/m )
Sample
SBP:SBC
B-I-A
95:5
51.88± 0.57
B-II-A
95:5
46.33± 1.29
B-I-B
90:10
54.37± 0.87
B-IIB
90:10
50.46± 1.31
B-I-C
85:15
58.57± 1.51
B-II-C
85:15
54.50± 1.04
B-I-D
80:20
64.67± 0.41
B-II-D
80:20
62.40± 0.69
Control-I
100:0
77.60± 3.11
Control-II
100:0
55.95± 2.33
Baydur
Commercial
79.75± 1.02
The microstructure of the samples with highest and lowest densities were analyzed by Hitachi S4700 electron microscope. As observed from the SEM images of the cellular structure of sample A-I-A and A-I-G (Figure 3), and A-II-A and A-II-G (Figure 4) it was observed that the samples with lower density had bigger cell size as compared to samples with higher density. This can be explained by the fact that samples with lower density composed of higher blowing agent. Higher amount of blowing agent produced more CO2, resulting in bigger cells of the foam [26].
12
Figure 3 (a). Sample A-I-A
Figure 3 (b). Sample A-I-G
Figure 4 (a). Sample A-II-A
Figure 4 (b). Sample A-II-G
Figure 5 (a). Sample B-I-A
Figure 5 (b). Sample B-I-D
A similar trend was overserved for sample B-I-A and B-I-D (Figure 5), and B-II-A and B-II-D (Figure 6). Also, it was observed that the cellular structure of blended samples appears to be more regular as compared to Control-I and Control-II samples (Figure 7). The cell structure of Baydur 683 foam is shown in Figure 8. The details of cell sizes of foam samples are mentioned in Table 5. It was also observed that for Category B samples, as the amount of SBC increased, the cell size reduced. This can be explained by an increase in the viscosity of the formulation,
13
due to the high viscosity of SBC (Table 1). The effects of viscosity have been investigated by various researchers, and a decrease in cell size of the foam with an increase in viscosity has been reported [26, 27]. The cell sizes of blended foam samples were less than that of Baydur 683.
Figure 6 (a). Sample B-II-A
Figure 6 (b). Sample B-II-D
Figure 7 (a). Sample Control-I
Figure 7 (b). Sample Control-II
Figure 8. Sample Baydur 683
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Table 5. Cell size of the foam samples
4.2
Sample
Cell Size (mm2)
Sample
A-I-A
0.09±0.01
B-I-A
Cell Size (mm2) 0.08±0.02
A-I-G
0.13±0.03
B-I-D
0.11±0.01
A-II-A
0.21±0.01
B-II-A
0.21±0.03
A-II-G
0.41±0.04
B-II-D
0.35±0.05
B-I-A
0.08±0.02
Control-I
0.34±0.07
B-I-D
0.11±0.01
Control-II
0.69±0.05
B-II-A
0.21±0.03
Baydur
0.63±0.08
Thermal analyses The thermal conductivity of the foam (λ) was measured (Table 6) and was used to
calculate thermal resistivity, also known as R-Value (R) using the following expression: R.
=
(2)
The relationship between blend ratio and thermal resistivity is shown in Figure 9 and Figure 10 for samples A and B, respectively. For Category A samples, the thermal resistivity increased with a decrease in the percentage of petroleum polyol, which also corresponds to decrease in foam density. Similar results have been reported by Tan et al. [12] and Guo et al. [24]. For Category B samples, the thermal resistivity decreased with an increase in SBC content. Blended polyol samples showed higher thermal resistivity as compared to control samples. In general, Formulation II exhibited higher thermal resistivity as compared to Formulation I, due to its lower density.
15
Figure 9. Relationship between R-Value and Density for Category A-I and A-II
Figure 10. Relationship between R-Value and Density for Category B-I and B-II
TGA test was performed to compare thermo-oxidative degradation behavior of the fabricated samples. The weight loss and weight loss derivative (DTG) curves are shown in Figure 11(a) and Figure 11(b) respectively. It was observed from DTG curves that Category A 16
samples followed two-step degradation process, whereas Category B samples followed threestep degradation. For Category B, the first step of degradation was at 180 ºC, which is due to dissociation of urethane bonds [28]. Category A samples along with Category B samples showed major degradation starting at 285 ºC till 315 ºC. This step is also associated with the degradation of urethane bonds [26]. The final degradation, above 520 ºC is associated with the final degradation of polyol backbone chain, due to oxidation of the foam samples. It can be concluded from the test that Category A samples had higher thermal stability as compared to Category B samples. Better thermal stability of Category A samples can be attributed to better cross-link density in Category A samples [27]. The char formation was negligible (2-4%) at end of the test. Also, Baydur foam exhibited better thermal stability as compared to all the fabricated samples.
Figure 14(a). Weight loss behavior of fabricated samples
17
Figure 14(b). Weight loss derivative curves for fabricated samples
4.3
Fourier transform infrared spectroscopy The FTIR spectra of the fabricated samples showed absorbance bands corresponding to
the chemical bonds in the foam samples. Collected spectra for the samples of different formulations were very similar, as shown in Figure 12. In the spectra, a band observed between 3250 cm-1 and 3400 cm-1 represents the stretching oh N-H bond in urethane and urea groups, and stretching vibrations in O-H groups [29]. The strong peaks at 2925 cm-1 and 2850 cm-1 are attributed to asymmetric and symmetric stretching vibrations of C-H bonds respectively [29]. The peaks at 1740 and 1710 cm-1 corresponds to stretching of C=O bond in urethane. The peak at 1470 is attributed to free C=O stretching, whereas peak at 1710 is due to hydrogen-bonded C=O stretching [25]. The peaks at 1599 cm-1 and 1512 cm-1 are attributed to stretching and bending vibrations of C-N and N-H bonds respectively in urethane. The peaks at 1215 cm-1 show the 18
stretching vibrations of C-O bond. The peak at 1070 cm-1 is attributed to the vibration of C-O bond. The chemical structure of the manufactured blended foam samples is similar across all formulations.
Figure 12. FTIR spectra of fabricated samples 4.4
Mechanical testing It was observed that CFD of the samples increased with the increase in the density. This
phenomenon is commonly observed in cellular foam systems [30]. Figure 13 shows the relation between thermal resistivity (R-Value) and compression force deflection (CFD) w.r.t. density of the fabricated samples. The CFD of Control-I and Control-II samples was 77.6 ±3.1 and 55.9±2.3 respectively. The fabricated samples had better CFD properties as compared to Control-I and Control-II samples. The possible explanation for this phenomenon can be the blended foam samples had regular cells as compared to control samples. The regular cell size
19
leads to high uniform load distribution, as compared to non-uniform cell structure, leading to high CFD [12, 22]. The detailed CFD values for all tested samples are mentioned in Table 6.
Figure 13 (a). CFD and R-Value of Category A-I, w.r.t. Density
Figure 13 (b). CFD and R-Value of Category A-II, w.r.t. Density 20
For Category A samples, an increase in PP content corresponds to an increase in CFD strength. Using PP content greater than 55% for Category A-I and 65% for Category A-II was required to produce foams with CFD strength greater than commercial Baydur 683 foam.
Figure 13 (c). CFD and R-Value of Category B-I, w.r.t. Density
Figure 13 (d). CFD and R-Value of Category B-II, w.r.t. Density
21
For Category B samples, an increase in SBC content corresponds to an increase in CFD strength. Using SBC content greater than 15% both Category B-I and 10% for Category B-II was required to produce foams with CFD strength greater than commercial Baydur 683 foam. Also, it was observed that all the blended foam samples had higher CFD as compared to Control-I and Control-II samples. Table 6. Mechanical and Thermal testing results of foam samples Density Thermal Insulation CFD Foam. # 3 kg/m S.D. R-Value S.D. kPa S.D. A-I-A 156.67 2.45 23.04 0.11 818.51 11.65 A-I-B 129.73 4.30 23.31 0.04 682.65 6.02 A-I-C 126.20 2.43 23.64 0.08 585.13 12.32 A-I-D 118.40 5.08 23.76 0.08 521.83 2.69 A-I-E 105.30 6.18 23.94 0.04 494.71 10.67 A-I-F 102.63 3.20 24.48 0.04 330.24 1.94 A-I-G 4.51 0.05 13.02 95.60 25.81 143.50 A-II-A 126.10 4.30 25.97 0.10 537.72 11.98 A-II-B 95.20 1.37 26.10 0.05 457.09 2.94 A-II-C 91.80 4.90 26.16 0.10 372.68 5.40 A-II-D 85.60 2.12 26.28 0.10 265.49 3.23 A-II-E 80.79 3.76 26.39 0.05 248.05 6.61 A-II-F 73.40 1.61 26.42 0.05 179.43 1.96 A-II-G 66.57 3.09 26.60 0.20 131.58 7.43 B-I-A 46.33 0.57 26.63 0.03 290.37 5.71 B-I-B 50.46 0.87 26.32 0.14 293.83 9.50 B-I-C 58.50 1.51 26.01 0.10 394.22 3.85 B-I-D 62.40 0.41 25.48 0.18 465.36 8.95 B-II-A 51.88 1.29 26.96 0.06 328.19 6.60 B-II-B 54.37 1.31 26.63 0.07 380.37 5.95 B-II-C 58.57 1.04 26.32 0.08 412.40 3.75 B-II-D 64.67 0.69 26.21 0.03 483.14 7.37 Control-I 77.60 3.11 25.06 0.05 111.70 1.53 Control-II 55.95 2.33 26.32 0.01 74.61 2.44 Baydur 79.75 1.02 28.44 0.03 366.90 4.70 The constant deflection compression set test was performed to determine mechanical and geometrical properties of the foam. It was observed that Category B samples had much 22
uniform Ct values (2.02-2.19 for B-I and 2.60-3.06 for B-II) as compared to Category A samples (1.34-8.82 for A-I and 1.40-6.88 for A-II). For Category A samples, it was observed that as the SBP content increased, Ct value decreased, giving more dimensional stability to the samples (Figure 14). However, for Category B samples, a small difference in the Ct values was observed with change in the content of SBC (Figure 15).
Figure 14. Ct for Category A samples Control samples I and II showed relatively superior Ct (0.82 and 1.49 for Control-I and Control-II, respectively) as compared to blended samples. Baydur 683 foam had Ct value of 3.360±0.053. The lower Ct values for blended samples is attributed to the elastomeric origin of the long chain polyol molecules [23].
23
Figure 15. Ct for Category B samples The tensile strength of the soy-based foam samples were found to be in the range of 105.11-654.71 kPa for Category A-I, 64.26-597.06 kPa for Category A-II, 215.66-279.21 for Category B-I, and 200.80-351.18 for Category B-II. The control samples had much lower tensile strengths of 85.99±2.83 kPa for Control-I and 52.53±2.34 kPa for Control-II. Also, commercial Baydur 683 sample had a tensile strength of 71.79±1.03 kPa. It was observed that amount of PP had a more significant effect on the tensile strength as compared the effect of SBC. Increasing PP content resulted in increasing tensile strength for A-I and A-II formulations. For B-I and B-II formulations, increasing SBC resulted in higher tensile strengths. Also, the samples with higher density had higher tensile strength (Figure 16).
24
Figure 16 (a). Tensile strength of Category A samples w.r.t. SBP and PP content
Figure 16 (b). Tensile strength of Category B samples w.r.t. SBP and SBC content
25
Table 7. Mechanical properties of the tested samples
4.5
Formulation
Ct Value
Foam. # A-I-A A-I-B A-I-C A-I-D A-I-E A-I-F A-I-G A-II-A A-II-B A-II-C A-II-D A-II-E A-II-F A-II-G B-I-A B-I-B B-I-C B-I-D B-II-A B-II-B B-II-C B-II-D Control-I Control-II Baydur
% 8.82±0.15 7.58±0.08 5.05±0.14 3.26±0.13 1.97±0.38 1.57±0.21 1.34±0.09 6.88±0.09 5.17±0.18 2.88±0.16 2.72±0.26 1.51±0.11 1.49±0.26 1.41±0.29 2.19±0.12 2.20±0.24 2.02±0.59 2.25±0.32 3.06±0.28 2.63±0.09 2.60±0.28 2.86±0.24 0.82±0.12 1.49±0.15 3.36±0.05
Tensile Strength kPa 654.71±4.46 637.88±2.51 506.04±9.61 435.87±6.14 385.9±5.91 211.17±5.34 105.11±4.26 597.06±5.03 501.46±2.55 456.65±6.14 372.65±5.34 308.19±3.29 262.07±5.93 64.26±2.33 215.66±13.31 231.30±15.48 259.52±10.42 279.21±11.23 200.80±14.05 283.51±13.41 317.90±18.14 351.18±12.16 85.99±2.83 52.53±2.34 71.79±7.03
Moisture Adsorption % 55.8±8.48 51.17±1.84 61.81±3.24 58.33±3.89 90.25±3.29 40.75±1.96 77.25±8.13 102.53±2.25 98.66±9.17 95.38±9.81 114.18±2.85 95.76±8.42 71.03±10.19 78.94±9.54 106.79±4.57 96.85±8.35 131.50±10.24 118.42±11.65 99.40±8.52 92.47±9.42 111.79±32.50 91.98±25.86 63.59±3.81 86.27±6.19 63.59±1.78
Moisture absorption properties The fabricated samples were cut to the required size and submerged in distilled water, as
mentioned in section 3.5. The samples were weighed before and after submerging in water, and the percent increase in weight was calculated. After the samples were immersed in distilled water for 96 hours at 2 in. head, the increase in weight was evaluated. The control samples, Control-I and Control-II showed 61.42±3.81% and 86.27±6.19% increase in weight, respectively, whereas 26
Baydur sample showed 63.59±1.78% increase in weight. The experiments did not show any direct correlation between PP or SBC percentage and moisture absorption in blended foams samples. Formulation A-I had moisture absorption lower than control samples and in the range of commercial Baydur-683 foams. In order to evaluate the ingress of moisture, the samples were submerged in dyed water for 96 hours and 2 in. of head. The samples were cut in half upon removal, and no penetration of water was observed inside the samples (Figure 17). Therefore, the increase in weight of samples can be explained by the presence of water in the cells on the outer surface of the foam. This argument is supported by the fact that the samples with large cell size showed more increase in weight after submerging in water.
Figure 17. Cross-section of foam samples showing no penetration of water
5
CONCLUSIONS
PU foam samples were fabricated by blending soy-based polyol HB230 (SBP) with polyether polyol (PP) and soy-based cross-linker HB530 (SBC) in varying amounts. The prepared foam samples were tested for various mechanical and thermal properties. It was observed that blended polyol samples resulted in better mechanical and thermal properties as compared to control samples fabricated by pure HB230 soy-based polyol and Baydur 683 PU 27
foam. It was observed that the blended foam samples had significantly higher CFD and tensile strength as compared with the control samples. Also, it was observed that foams blended with SBC had superior thermal resistivity, as compared to foams blended with PP and control samples. The density of the foam samples was dependent on the amount of PP and SBC, where a higher amount of PP and SBC resulted in a higher density of the samples. It was concluded that blend of SBP and PP in the ratio 75:25 by weight resulted in the samples with highest thermal insulation while having better CFD and tensile properties than control samples. It was observed that for 95:5 blend of SBP and SBC resulted in foam with 250% increase in tensile strength while having similar R-Value and dimensional stability as compared to Baydur 683 foam.
6
ACKNOWLEDGMENTS
This work is supported by the Missouri Soybean Merchandising Council (Jefferson City, MO). The authors would like to thank Mr. John Miller (MCPU Polymer Engineering LLC, Pittsburg, KS) for providing HB230 polyol and Dr. Steven Harasin (Covestro LLC, Pittsburgh, PA) for providing isocyanate and polyether polyol. Authors would also like to thank Mr. Stephan Johannesmeyer (Thermocore of Missouri, Jefferson City, MO) for his helpful suggestions.
7
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HIGHLIGHTS This study aims to substitute the petroleum-based polyols with soy-based polyols for the fabrication of rigid PU foams that have comparable or better physical properties required for thermal and structural applications. Properties of foams blended with soy-based cross linker were compared to 100% soy based foams, and a commercially available 100% petroleum based foam. The blended foam samples exhibited better thermal resistivity (8% higher) as compared to control foams. The blended foams also exhibited better compressive properties (about 512% higher) and tensile strength (287% higher) compared to control foam. Blended soy-PU foams with properties comparable to commercial petroleum-based foams were manufactured in the study.
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