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Rigid polyurethane foams from a soybean oil-based Polyol
Polymer 52 (2011) 2840e2846
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Polymer journal homepage: www.elsevier.com/locate/polymer
Rigid polyurethane foams from a soybean oil-based Polyol Suqin Tan a,1, Tim Abraham b, Don Ference b, Christopher W. Macosko a, * a b
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Cargill Bio-Based Polyurethanes, Plymouth, MN 55416, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2011 Received in revised form 10 April 2011 Accepted 16 April 2011 Available online 23 April 2011
Polyurethane (PU) rigid foams were synthesized by substituting a polypropylene-based polyol with soybean oil-based polyol (SBOP). All the soy-based foams maintained a regular cell structure and had even smaller average cell size than the control foams. The density of soy-based foams was within 5% of the controls, except that the density of foams from 100% SBOP was 17% higher. Soy-based foams also had comparable initial thermal conductivity (k value) and closed cell content, higher Tg and compressive strength. However, while foams from 50% SBOP showed similar increase in k value to the 0% SBOP foams, under accelerated aging conditions, the 100% SBOP foams aged faster. Gas permeation tests performed on PU thin films showed higher N2 permeation for PU thin films made from SBOP which is believed to be the cause of accelerated thermal aging. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Polyurethane foam Soybean oil-based polyol Thermal conductivity
1. Introduction Excellent insulation combined with good adhesion, high strength-to-weight ratio and durability make polyurethane (PU) rigid foam an indispensable material in the construction industry. PU rigid foam accounts for about 23% of all PU production. Like other polymers, PU rigid foams rely on petroleum feedstocks. Increasing concern over environmental impact and the supply of petroleum have motivated the development of PU from bio-renewable raw materials. According to a lifecycle comparison conducted by Omni Tech International Ltd., soy-based feedstocks showed 75% less total environmental impact than petroleum-based feedstocks due to significant reductions in fossil fuel depletion, global warming, smog formation, and ecological toxicity [1]. Natural oils have great potential to compete with petroleum in producing polyols used to make PU. However, except for a few oils like castor oil and lesquerella oil, most natural oils do not have hydroxyl groups which are needed to form urethane links with isocyanate [2,3]. The unsaturated sites in natural oils can be used to introduce hydroxyl groups, and a number of methods have been developed to synthesize natural oil-based polyols. As early as 1974, Lyon prepared PU rigid foams from hydroxymethylated castor oil, safflower oil, and polyol esters of castor
* Corresponding author. Tel.: þ1 612 625 0092; fax: þ1 612 626 1686. E-mail address: [email protected] (C.W. Macosko). 1 Present address: HB Fuller, 1200 Willow Lake Blvd., Vadnais Heights, MN 55110, USA. 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.04.040
acids with 20e65% cross-linkers added [4]. By adding a large amount of cross-linkers, these foams had satisfactory compressive strength and resistance to shrinkage on humid aging. Rapeseed oiland palm oil-based polyols have been used to make PU rigid foams as well [4,5]. In 1999, Guo and co-workers prepared HCFC- and pentane-blown PU rigid foams from soy-based polyols made by epoxidation followed by oxirane ring-opening [6]. However, these foams were found to have inferior mechanical and thermal insulating properties even with 10e25% glycerol added. Guo et al. [7] continued to investigate the effect of soy-based polyol structures on the PU rigid foams. They found that polyols with primary hydroxyls reacted faster and more completely than polyols which had secondary hydroxyls. The water-blown PU rigid foams prepared from soy-based polyols by Narine et al. [8] showed very low closed cell content. Tu et al. [9] prepared fifty vegetable oilbased polyols and replaced 50% of the petroleum-based polyols in water-blown PU rigid foams. However, most of the foams were inferior to 100% petroleum-based PU rigid foams. This previous work proved the feasibility and potential of preparing PU rigid foams from soy-based polyols. However, even with only partial substitution of petroleum-based polyols with soybased polyols, and with a large amount of cross-linkers added, soybased PU rigid foams had inferior properties, especially thermal conductivity (k value) and to a certain extent compressive strength. Furthermore, previous researchers have not studied the insulation properties in detail in order to determine the mechanism of k value aging. In an effort to understand how soy polyols affect foam properties, we started with a standard formulation used to make appliance insulation and partially or completely replaced the
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petroleum-based polyol with a soybean oil-based polyol (SBOP). The density, cell morphology, compressive strength and k value were compared. The loss of k value with aging was further investigated, the first study of long term insulation performance of soybased PU rigid foams. 2. Experimental 2.1. Materials A commercially available petroleum-derived polypropylenebased polyol, JeffolÒ FX31-240 (Huntsman International), was chosen as the control polyol. A commercial soybean oil-based polyol X-0210 (Cargill) was selected as the SBOP. Fig. 1 shows the structure of the control polyol and the general structure of X-0210. The properties of the two polyols are listed in Table 1. Note that the viscosity and water content of the SBOP are significantly higher than the control polyol. The isocyanate was polymeric methylenediphenyl diisocyanate, PMDI (Rubinate Ò M, Huntsman). Diethylene glycol (Huntsman International) was used as a chain extender to avoid foam shrinkage. N,N-Dimethylcyclohexylamine (PolycatÒ8, AirProducts, CAS 98-94-2) was used as the gelation catalyst. Pentamethyldiethylenetriamine (PolycatÒ5, AirProducts) was used as the blowing catalyst. Polyether-modified polysiloxane (TEGOSTAB B-8404, Goldschmidt Chemical Corporation) was the surfactant and denoted as surfactant A. TEGOSTAB B-8404 is a general-purpose silicone surfactant used in PU rigid foam formulations. It was found to work satisfactorily for the SBOP system compared to other surfactants [6]. In order to study the effect of surfactant hydrophobicity, BYK-LPX 7105 (BYK USA Inc.) which is much more hydrophobic than TEGOSTAB B-8404 was also used. This surfactant is denoted as surfactant B. The relative hydrophobicity was determined by a cloud point test of surfactant solutions. The physical blowing agent is n-pentane (Anhydrous, 99%, AldrichÒ). Distilled water was obtained in our laboratory. Water amount used was adjusted according to the water content in the polyols. All the chemicals were used as received. 2.2. Foam synthesis A low isocyanate index formulation which is similar to those typically used in appliance insulation for free rise foams was selected. Table 2 lists the formulation details. The amount of each ingredient was based on 100 parts by weight of total polyol. Polyol, surfactant, distilled water, n-pentane, and catalysts were weighed and placed in an 800 mL polypropylene cup. The mixture was homogenized using a 10-mm drill (Delta VSR Drill, D21008) equipped with a 2.8-in diameter mixing blade for 40e50 s, at a speed of 2500 rpm. The pre-weighed PMDI was then added to the mixture, and mixing was continued for another 10 s. The foam was then allowed to rise freely at room temperature. The foams were
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Table 1 Polyol properties comparison. Polyol
Control JeffolÒ FX31-240
SBOP X-0210
OH number (mg KOH/g) Molecular Weight (g/mol) Functionality Viscosity (mPa s) Acid Value (mg KOH/g) Water content (ppm) Manufacture/Resource
240 700 3.0 250 e e Huntsman
235 1100 4.4 8900 1.7 3000 Cargill
cut one week later and the properties of the foams measured, including density, cell morphology, k value and its aging, and dynamic mechanical properties. 2.3. Characterization 2.3.1. Foam reaction kinetics In order to understand the effect of adding SBOP, foam kinetics was studied. Adiabatic temperature rise is a simple and quick method [10,11]. Since PU rigid foams are good insulating materials, any container with a diameter over 5e10 cm is sufficient to maintain adiabatic conditions in the center of the reacting mixture [11]. Given the adiabatic conditions, isocyanate conversion can be calculated from the temperature profile using Equations (1) and (2) [10].
pðNCOÞ ¼
DTrxn
r DTm DTrxn
Q ¼ ¼ Cp mT
(1)
DHr;u
mw m þ DHr;r OH fn Mw MOH Cp m T
(2)
In the above equations, p is the isocyanate conversion; r is the stoichiometric ratio of functional groups (kept to 1 in all our experiments); ΔTm is the temperature rise during foaming; ΔTrxn is the maximum temperature rise; Q is the total amount of heat generated during foaming; Cp is the specific heat capacity of foam, which is taken as 1.5 J/(g∙ K) based on DSC measurements of the final foams; m is the reactant mass; ΔHr is the heat of reaction; M is the molecular weight of reactant; fn is the functionality of the polyol, subscripts u, r, w, T and OH represent urea, urethane, water, total and polyol respectively. ΔHr,u and ΔHr,r were taken as 125.5 kJ/mol and 93.9 kJ/mol [12,13]. An insulated plastic cup with a diameter of about 10 cm was used as the reaction vessel. The reaction mixture based on a total weight of about 25g was prepared using the same foam synthesis procedure as in section 2.2. Temperature profiles during foaming were monitored using a type J thermo-couple with a 0.25 mm diameter (OmegaÒ), which was connected to a digital-thermometer. An adapter was used to connect the digital-thermometer to a computer and the temperature was recorded. The physical
Fig. 1. Polyol structure: (left) control polyol, (right) major component in SBOP.
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Table 2 Foam and film formulation. Chemical Weight (pbw)
Foam
Polyol (petro/soy) Diethylene glycol (DEG) Glycerol Water PolycatÒ 8 PolycatÒ 5 Surfactanta n-pentane Acetone Isocyanate Isocyanate index
100 6.0 0 2.6 4.0 1.0 2.0 8.0 0 130 125
Film DEG added
Glycerol added
100 6.0 0 0 Vary 0 0 0 125 109.8 105
100 0 16 0 Vary 0 0 0 125 123.8 105
a Surfactant A is TEGOSTAB B-8404 (Goldschmidt Chemical Corporation) and surfactant B: BYK-LPX 7105 (BYK USA Inc.)
blowing agent n-pentane was not used and the isocyanate index was kept at 100 to make the reaction stoichiometrically balanced and thus isocyanate conversion depends only on the gelling and blowing reactions. An excess of isocyanate could lead to other reactions such as isocyanate trimerization and further reaction between isocyanate and urethane. 2.3.2. Foam density Foams from the core of the caps of the foam at the top of the cup were cut into 2.5 cm cubes and weighed to determine density. Three to four pieces were cut from three different foam batches and the average density was reported. 2.3.3. Scanning electron microscopy (SEM) Foams were cut with a razor blade into rectangular slices: 7 10 3 mm. Each sample slice was sputter coated with a layer of 50 Å platinum. A JEOL 6500 field emission scanning electron microscope was used to examine the cell morphology [14]. The sample was then attached to the stub using conductive carbon tape. The sample was imaged under an accelerating voltage of 10 kV and 37 magnification. The cellular structure of the foam was observed parallel to the free-rising direction. An average of 5 images was collected on each foam. In this study, apparent long axis, which is the maximum chord length of a cell [15], was measured using ImageJ. The average cell size and its standard deviation were based on 100e150 cells.
120 C and held for 10 min, and finally heated at 10 C/min to 220 C. The value of the heat capacity at room temperature was determined on the second heating cycle [16]. The specific heat capacity of foam was used to calculate thermal conductivity (k value) [17]. 2.3.7. Thermal conductivity, k, measurement Thermal conductivity or k value is the most important property for PU rigid foam [18e20]. A typical k value measurement is based on the rate of steady state heat transfer across a foam with known thickness, which is induced by two different known temperatures between two opposite surfaces of the foam [18,20]. However, the current steady state method requires a large sample size, typically 30.5 cm 30.5 cm 5 cm. This method is not suitable for lab scale samples. Recently, Harikrishnan et. al. have reported a simple and rapid technique based on a transient measurement using a needle probe [17]. This method is rapid with accuracy within 5%. It is widely used in the measurement of k value and thermal diffusivity of liquids and solids [17]. To evaluate the long term insulating performance of PU rigid foams, an accelerated k value aging test was done. Foam cubes with 12 cm on a side were cut from the top of the cup foam. They were placed in an oven at 70 C and the k value measured periodically. It has been determined that the air ingress is 6e12 times faster at 70 C compared to room temperature [18]. 2.3.8. Dynamic mechanical analysis (DMA) A disk (8 mm diameter 8 mm thickness) was cut from the foam and tested in sinusoidal oscillation between two 8-mm diameter serrated parallel plates (ARES, TA Instruments). Contact was maintained by loading the sample at the plate temperature of 160 C and applying a constant normal force of 16 8 g throughout the experiment. Strain sweep test was performed first to determine the linear viscoelastic region of the sample. Storage modulus (G0 ) and loss modulus (G00 ) were recorded at 1 Hz and 0.1% strain over a temperature range from 0 to 220 C at a temperature ramp rate of 3 C/min. 3. Results and discussion 3.1. Foam kinetics In Fig. 2 the isocyanate conversion profile showed that foaming of 0% and 100% SBOP had the same isocyanate conversion in the
2.3.4. Closed cell content Closed cell content was measured by gas displacement following the standard procedures described in ASTM D-6226 using Micromeritics AccuPyc 1340. 2.3.5. Compressive strength Compressive strength was measured using ASTM D-1621 on foam samples with dimensions of 5.0 cm 5.0 cm 2.5 cm. The samples were placed between two parallel plates with a larger area than the specimen and the force required to compress the foam at the rate of 0.25 cm/min was measured. The compressive strength was the value of the maximum applied force divided by the initial sample surface area when the maximum applied force occurred before the strain reached 10%. Otherwise, the compression applied force was taken when the strain was 10%. 2.3.6. Differential scanning calorimetry (DSC) DSC (Q1000, TA Instruments) was used to measure the heat capacity of the foams. About 7e10 mg of compressed foam sample was loaded into an aluminum standard inverted pan and sealed. The sample was first heated at 10 C/min e 150 C and equilibrated for 2 min, cooled to
Fig. 2. Isocyanate conversion from adiabatic temperature rise during foam formation for 0% and 100% SBOP.
S. Tan et al. / Polymer 52 (2011) 2840e2846
first 20 s. After the initial 20 s, SBOP reacted slightly slower than that of control, and reached a slightly higher plateau value of isocyanate conversion. The isocyanate can react with water, diethylene glycol and polyol. Based on their relative reactivity, the first 20 s was mainly due to the reactions of isocyanate with water and diethylene glycol. The slower reactivity afterward was mainly due to the lower reactivity of the secondary OH groups in SBOP. For the control, the reactions reached a plateau value by about 50 s, while for SBOP it reached a plateau only after about 100 s. The higher isocyanate conversion is probably due to the higher degree of phase mixing in the soy-based foaming system. 3.2. Foam density, cellular morphology and compressive strength The resulting foam properties are summarized in Table 3. The foam density increased with increasing SBOP substitution but only significantly at 100% substitution. With the same SBOP substitution, foam density did not change significantly between the two surfactants. Good insulating performance of PU rigid foam requires high closed cell content. All the soy-based PU foams had closed cell contents comparable to the control foam (see Table 3). SEM images (Fig. 3) showed that all the foams consisted of well-defined closed cells regardless of the polyol type. Foams with 0% SBOP had the largest average cell size. With SBOP substitution, PU rigid foams had a smaller average cell size. We suggest that the higher viscosity of SBOP (see Table 1) helped to reduce cell drainage by gravity at the initial foaming stage which led to a smaller average cell size. Using the more hydrophobic surfactant B, the average cell size became even smaller. This suggested that it acted more strongly in stabilizing cell structure. We don’t believe that this is due to the different compatibilities of B-side (polyol, diethylene glycol, catalyst, water, n-pentane and surfactant) with different surfactant hydrophobicities. Our compatibility test [21] has shown that there is no significant difference by using these two surfactants. Overall, the SEM study suggested that cell morphology was not significantly altered by substituting SBOP for the petroleum-based polyol in the foam formulation. Compressive strength also increased with soy substitution (Fig. 4) possibly due to the decrease in cell size. 3.3. Dynamic mechanical properties In Fig. 5, both G0 and tan (d) data were plotted for the foams from 0%, 25%, 50%, 75% and 100% SBOP. In the glassy state, all samples had comparable plateau G0 values of about 2 106 Pa. The higher G0 for the 100% SBOP sample was due to the higher foam density. As the SBOP substitution increased, glassy to rubbery transition shifted gradually to higher temperatures. In the rubbery state, all foams had comparable G0 of about 2 104 Pa. Tg was determined from the peak in tan (d) and increased from 98 C to 142 C with increasing SBOP substitution. With increasing SBOP substitution, the tan (d) peak broadened possibly due to the SBOP being more heterogeneous in structure compared to the petroleum-based polyol.
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3.4. Thermal conductivity Thermal conductivity is the key property that governs insulation applications for PU rigid foams. k value is closely related to foam density and cell morphology. Total k value consists of four parts: lgas, lradiation, lsolid and lconvection [20]. Foams with a high density have lower lradiation, but a higher non cellular PU portion leads to higher lsolid. Though the PU solid only accounts for a small percentage of the whole foam, it has a much higher k value (220 mW/(m$K)) than the physical blowing agent gas n-pentane (13.7 mW/(m$K)) and CO2 (15.3 mW/(m$K)). At a fixed foam density, lradiation will decrease with decreasing cell diameter. It has been reported that the k value increases substantially, nearly 50%, as cell size increases from 0.25 mm to 0.6 mm [22]. The level of closed cell content should also be kept as high as possible. Open cells allow more convection, and even air to enter the foam, which has much higher k value (24.9 mW/(m$K)) than that of n-pentane and CO2 [18]. Thus, a low k value results from a low foam density, a small average cell size and a high closed cell content. Thermal conductivity or k value measurement was done on samples from 0%, 50% and 100% SBOP. With surfactant A which was more hydrophilic, foams from 50% SBOP had comparable density and about a 10% smaller cell size, though 100% SBOP foams had much higher density and an even smaller cell size, which led to foam k value as low as control foams. The two surfactants of different hydrophobicities gave foams with thermal insulating properties comparable to the control foams. 3.5. Foam aging Since PU rigid foams are largely used in building and appliance insulation, the change in k value with time is also important. Long term insulating performance was investigated by an accelerated foam aging test. Fig. 6 showed the foam aging results for 0%, 50% and 100% SBOP for both surfactants. All these curves showed an initial rapid k value increase, followed by a more gradual increase after 50 days. Foams from 50% SBOP with both surfactants had very similar k value aging as the 0% SBOP foams; k value had only increased from 23 to 24 mW/(m$K) to about 26.5e27 mW/(m$K) after 150 days of aging. However, the foams from 100% SBOP aged faster. The k value increased from about 24 mW/(m$K) to as high as 28.5 mW/(m$K). With the more hydrophobic surfactant B, foams from 100% SBOP aged even faster, with the k value increasing to about 30.5 mW/(m$K). The foam aging tests for both surfactants revealed that soy-based foams from 100% SBOP had faster k value aging compared to control foams and soy-based foams from 50% SBOP. 3.6. PU film gas permeability study Over time, PU rigid foams will undergo k value changes due to the change in gas composition. For freshly made PU rigid foam, the gas trapped inside the cell is a mixture of mostly physical blowing
Table 3 Foam properties comparison. % SBOP substitution
0
Density (kg/m3) Tg ( C) K value (mW/(m K)) Closed cell content (%) Cell size (mm)
39.5 98 23.4 89.09 431
a
0.8 0.6 1.07 91
25
50
39.7 0.9 107 e e 392 84
39.8 123 24.0 88.76 390
Samples made with surfactant B, others with surfactant A.
1.1 0.6 0.44 102
75
100
41.3 1.3 134 e e 375 128
46.4 142 24.2 90.68 386
2.0 0.6 0.44 102
50a
100a
40.0 1.0 110 23.3 0.2 e 360 94
49.0 1.8 143 24.9 0.7 e 349 103
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Fig. 3. Foam SEM images: (a) 0%; (b) 50%; (c) 75% and (d) 100% SBOP, surfactant A.
agent (n-pentane in this study), some CO2, and a small amount of air. Driven by the partial pressure difference between foam cells and the atmosphere, air will gradually diffuse into the cells. Similarly, CO2 and pentane will diffuse out of the cells. Generally, CO2 diffuses out much faster than other gases [23]. The process of air diffusion is slower than CO2, but it’s concentration inside the cells will gradually increase with N2 building up faster than O2. For physical blowing agents like CFC and HFC, and pentane, the permeability is very low [23]. Since both N2 and O2 have higher k values than CO2 and n-pentane, the change in gas composition leads to the increase in k value over time. In order to determine whether the faster k aging was due to higher gas permeability with SBOP substitution, gas permeation
Fig. 4. Compressive strength of PU rigid foams.
through a PU thin film, which can represent the polymer in the PU rigid foam, was measured. This method eliminates the influence of foam cell morphology but requires high quality film samples. The film formulation is listed in Table 2. 3.6.1. Film preparation Since isocyanate can easily react with water and moisture in the air, the most difficult part was to avoid producing bubbles during the preparation procedure. Water was not added because urea linkages in the polymers only slightly affect the gas permeation [24]. Polyols were dried under high vacuum, high temperature (110 C), and high speed stirring for 2e3 h before use. Acetone was used to dilute the reaction mixture. This not only helped to make a thin and homogeneous film, but also helped to avoid trapping bubbles. The amount of catalyst was also adjusted to balance the rate of the gelling reaction and acetone evaporation. The gelling reaction should be fast enough to avoid the reaction between isocyanate and moisture as much as possible, but should be slow enough for most of the acetone to evaporate. PU thin films of 300e500 mm thickness were prepared in an open-faced film casting mold [21]. The mold was made from a 35.6 cm 35.6 cm high density polyethylene (HDPE) sheet with an aluminum picture frame with a height of 2.5 cm. Prior to casting the films, the mold was carefully leveled in order to obtain a uniform film thickness. Polyol, DEG, catalyst and isocyanate were blended with acetone in a 200 ml polypropylene plastic cup. The mixture was stirred gently until a homogeneous solution was formed. The solution was then poured into the mold carefully to avoid air entrainment. Another HDPE sheet was placed over the aluminum frame about 1 cm above the film to allow bubbles to escape and prevent direct contact with moist air. The film was kept at room temperature for 24 h, during which the solution gradually solidified and most of the acetone had evaporated. The film was removed from the HDPE
S. Tan et al. / Polymer 52 (2011) 2840e2846
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Fig. 7. N2 permeation of PU thin films. (- SBOP with DEG, , SBOP without DEG; B SBOP with glycerol) (Barrer ¼ 3.348 1019 kmol m/(m2 s Pa)).
Fig. 5. (a) Dynamic elastic modulus, G0 and (b) tan (d) vs. temperature.
sheet and cut into disks having a diameter of 4.2 cm. The disks were placed between MylarÒ sheets for further curing at room temperature for 120 h. A heavy piece of steel was placed on top to ensure film flatness. The disks were then placed in a vacuum oven for a final cure for 48 h.
3.6.2. Gas permeation measurement Gas permeation tests based on a constant volume-variable pressure method [25,26] were performed on a home-built apparatus [26]. The film was fit and sealed into the gas transmission cell to form a semi-barrier between the two chambers. The apparatus was evacuated overnight. N2 flow was fed to one side of the chamber and the pressure change in the opposite chamber which had been evacuated was monitored as a function of time. Gas permeation constants were obtained from the pressure change normalized with the pressure gradient across the sample, film area and thickness [27]. Fig. 7 illustrates the N2 permeation results of PU thin films from 0%, 50% and 100% SBOP. It was found that by substituting 50% SBOP, N2 permeation did not change significantly. However, films from 100% SBOP had a much higher N2 permeation, almost as twice that of the control films. The N2 permeation results showed good agreement with the foam k value aging results. The much higher N2 permeation of the 100% SBOP PU thin film led to faster k value aging than the control or 50% SBOP PU thin film. Another study was done to investigate the effect of a chain extender and a cross linker on N2 permeation of soy-based PU thin films. PU thin films were made from pure SBOP (i.e. without diethylene glycol), as well as SBOP with glycerol instead of diethylene glycol. Samples from pure SBOP had higher N2 permeation than the samples from SBOP with diethylene glycol. However, by adding glycerol instead of diethylene glycol, N2 permeation dramatically decreased. The gas permeation test has shown that the addition of a chain extender or a cross linker can decrease N2 permeation. 4. Conclusions
Fig. 6. Thermal conductivity increase with time at 70 C.
PU rigid foams were prepared from a soy-based polyol, SBOP. The experimental results demonstrated that by replacing a typical petroleum-based polyol with SBOP, Tg increased and foams had comparable foaming kinetics, density, cellular morphology, and initial thermal conductivity (k value). Compressive strength of the soy-based PU rigid foams were superior to those of petroleum-based foams possibly due to smaller cell size. However, while foams from 50% SBOP showed similar k value aging to control foams, foams from 100% SBOP aged much faster. The mechanism behind foam k value aging was studied by measuring the N2 permeation of PU thin films. It was found that PU thin films with 50% SBOP substitution had
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similar N2 permeation to the 0% SBOP films, but films from 100% SBOP had much higher N2 permeation, which was in good agreement with k value aging results. Adding glycerol to the films with 100% SBOP reduced N2 permeation significantly. Acknowledgments The authors would like to acknowledge Cargill Incorporated and IREE (Initiative for Renewable Energy and the Environment) for financial support. Throughout this work, we had many helpful discussions with Professor Tom Hoye, Professor Marc Hillmyer, Dr. Junghwan Shin and Dr. Harikrishnan G. of the University of Minnesota. We would also like to thank Darius Jaya and Jason Zhang of the University of Minnesota for their help with sample preparation, Professor Michael Tsapatsis for use of the permeability cell and Matthew Caldwell, Cargill Inc. for his assistance with closed cell content measurements. References [1] [2] [3] [4] [5]
Pollock JW. Abstr Pap Am Chem Soc 2004;227. 160eCELL. Petrovic ZS. Polym Rev 2008;48:109e55. Carlson KD, Chaudhry A, Bagby MO. J Am Oil Chem Soc 1990;67:438e42. Lyon CK, Garrett VH, Franker EN. J Am Oil Chem Soc 1974;51:331e4. Hu YH, Gao Y, Wang DN, Hu CP, Zu S, Vanoverloop L, et al. J Appl Polym Sci 2002;84:591e7.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
Guo A, Javni I, Petrovic Z. J Appl Polym Sci 2000;77:467e73. Guo A, Zhang W, Petrovic ZS. J Mater Sci 2006;41:4914e20. Narine SS, Kong X, Bouzidi L. J Am Oil Chem Soc 2007;84:65e72. Tu Y, Kiatsimkul P, Suppes G, Hsieh FJ. Appl Polym Sci 2006;105:453e9. Zhang L. Ph. D thesis, University of Minnesota, 2008. Macosko CW. RIM, Fundamentals of Reaction Injection Molding. New York: Hanser Publishers; 1989. Hager KF, Brandien JE. J Cell Plast 1968;4:298e303. Vespoli NP, Alberino LM. Polym Proc Eng 1985;3:127e47. Hafner B. Scanning Electron Microscopy Primer, Characterization Facility. University of Minnesota-Twin Cities; 2007. Rhodes MB. Low denisty cellular plastics. In: Hilyard NC, Cunningham A, editors. Physical Basis of Behaviour. New York: Chapman & Hall; 1994. Zhang L, Jeon HK, Malsam J, Herrington R, Macosko CW. Polymer 2007;48: 6656e67. Harikrishnan G, Macosko CW, Choi JH, Bischof JC, Singh SN. J Cell Plast 2008; 44:481e91. Randall D, Lee S, editors. The Polyurethanes Book. John Wiley & Sons, Ltd; 2002. Wirpsza Z, Kemp TJ, editors. Polyurethanes: Chemistry, Technology and Applications. Chichester: New York: E. Horwood; 1994. Szycher M. Szycher’s Handbook of Polyurethanes. Boca Raton: CRC Press; 1999. Tan Suqin M.S. thesis, University of Minnesota, 2010. Bogdan M, Hoerter J, Moore FO. J Cell Plast 2005;41:41e56. Modesti M, Lorenzetti A, Dallacoua C. Polym Eng Sci 2004;44:2229e39. Bartlett PL, Creazzo JA, Hammel HS. J Cell Plast 1994;30:126e42. Pye DG, Hoehn HH, Panar MJ. Appl Polym Sci 1976;20:1921e31. Jeong HK, Krych W, Ramanan H, Nair S, Marand E, Tsapatsis M. Chem Mater 2004;16:3838e45. Kim H. Ph. D thesis, University of Minnesota, 2009.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Rigid polyurethane foams from a soybean oil-based Polyol Suqin Tan a,1, Tim Abraham b, Don Ference b, Christopher W. Macosko a, * a b
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Cargill Bio-Based Polyurethanes, Plymouth, MN 55416, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2011 Received in revised form 10 April 2011 Accepted 16 April 2011 Available online 23 April 2011
Polyurethane (PU) rigid foams were synthesized by substituting a polypropylene-based polyol with soybean oil-based polyol (SBOP). All the soy-based foams maintained a regular cell structure and had even smaller average cell size than the control foams. The density of soy-based foams was within 5% of the controls, except that the density of foams from 100% SBOP was 17% higher. Soy-based foams also had comparable initial thermal conductivity (k value) and closed cell content, higher Tg and compressive strength. However, while foams from 50% SBOP showed similar increase in k value to the 0% SBOP foams, under accelerated aging conditions, the 100% SBOP foams aged faster. Gas permeation tests performed on PU thin films showed higher N2 permeation for PU thin films made from SBOP which is believed to be the cause of accelerated thermal aging. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Polyurethane foam Soybean oil-based polyol Thermal conductivity
1. Introduction Excellent insulation combined with good adhesion, high strength-to-weight ratio and durability make polyurethane (PU) rigid foam an indispensable material in the construction industry. PU rigid foam accounts for about 23% of all PU production. Like other polymers, PU rigid foams rely on petroleum feedstocks. Increasing concern over environmental impact and the supply of petroleum have motivated the development of PU from bio-renewable raw materials. According to a lifecycle comparison conducted by Omni Tech International Ltd., soy-based feedstocks showed 75% less total environmental impact than petroleum-based feedstocks due to significant reductions in fossil fuel depletion, global warming, smog formation, and ecological toxicity [1]. Natural oils have great potential to compete with petroleum in producing polyols used to make PU. However, except for a few oils like castor oil and lesquerella oil, most natural oils do not have hydroxyl groups which are needed to form urethane links with isocyanate [2,3]. The unsaturated sites in natural oils can be used to introduce hydroxyl groups, and a number of methods have been developed to synthesize natural oil-based polyols. As early as 1974, Lyon prepared PU rigid foams from hydroxymethylated castor oil, safflower oil, and polyol esters of castor
* Corresponding author. Tel.: þ1 612 625 0092; fax: þ1 612 626 1686. E-mail address: [email protected] (C.W. Macosko). 1 Present address: HB Fuller, 1200 Willow Lake Blvd., Vadnais Heights, MN 55110, USA. 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.04.040
acids with 20e65% cross-linkers added [4]. By adding a large amount of cross-linkers, these foams had satisfactory compressive strength and resistance to shrinkage on humid aging. Rapeseed oiland palm oil-based polyols have been used to make PU rigid foams as well [4,5]. In 1999, Guo and co-workers prepared HCFC- and pentane-blown PU rigid foams from soy-based polyols made by epoxidation followed by oxirane ring-opening [6]. However, these foams were found to have inferior mechanical and thermal insulating properties even with 10e25% glycerol added. Guo et al. [7] continued to investigate the effect of soy-based polyol structures on the PU rigid foams. They found that polyols with primary hydroxyls reacted faster and more completely than polyols which had secondary hydroxyls. The water-blown PU rigid foams prepared from soy-based polyols by Narine et al. [8] showed very low closed cell content. Tu et al. [9] prepared fifty vegetable oilbased polyols and replaced 50% of the petroleum-based polyols in water-blown PU rigid foams. However, most of the foams were inferior to 100% petroleum-based PU rigid foams. This previous work proved the feasibility and potential of preparing PU rigid foams from soy-based polyols. However, even with only partial substitution of petroleum-based polyols with soybased polyols, and with a large amount of cross-linkers added, soybased PU rigid foams had inferior properties, especially thermal conductivity (k value) and to a certain extent compressive strength. Furthermore, previous researchers have not studied the insulation properties in detail in order to determine the mechanism of k value aging. In an effort to understand how soy polyols affect foam properties, we started with a standard formulation used to make appliance insulation and partially or completely replaced the
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petroleum-based polyol with a soybean oil-based polyol (SBOP). The density, cell morphology, compressive strength and k value were compared. The loss of k value with aging was further investigated, the first study of long term insulation performance of soybased PU rigid foams. 2. Experimental 2.1. Materials A commercially available petroleum-derived polypropylenebased polyol, JeffolÒ FX31-240 (Huntsman International), was chosen as the control polyol. A commercial soybean oil-based polyol X-0210 (Cargill) was selected as the SBOP. Fig. 1 shows the structure of the control polyol and the general structure of X-0210. The properties of the two polyols are listed in Table 1. Note that the viscosity and water content of the SBOP are significantly higher than the control polyol. The isocyanate was polymeric methylenediphenyl diisocyanate, PMDI (Rubinate Ò M, Huntsman). Diethylene glycol (Huntsman International) was used as a chain extender to avoid foam shrinkage. N,N-Dimethylcyclohexylamine (PolycatÒ8, AirProducts, CAS 98-94-2) was used as the gelation catalyst. Pentamethyldiethylenetriamine (PolycatÒ5, AirProducts) was used as the blowing catalyst. Polyether-modified polysiloxane (TEGOSTAB B-8404, Goldschmidt Chemical Corporation) was the surfactant and denoted as surfactant A. TEGOSTAB B-8404 is a general-purpose silicone surfactant used in PU rigid foam formulations. It was found to work satisfactorily for the SBOP system compared to other surfactants [6]. In order to study the effect of surfactant hydrophobicity, BYK-LPX 7105 (BYK USA Inc.) which is much more hydrophobic than TEGOSTAB B-8404 was also used. This surfactant is denoted as surfactant B. The relative hydrophobicity was determined by a cloud point test of surfactant solutions. The physical blowing agent is n-pentane (Anhydrous, 99%, AldrichÒ). Distilled water was obtained in our laboratory. Water amount used was adjusted according to the water content in the polyols. All the chemicals were used as received. 2.2. Foam synthesis A low isocyanate index formulation which is similar to those typically used in appliance insulation for free rise foams was selected. Table 2 lists the formulation details. The amount of each ingredient was based on 100 parts by weight of total polyol. Polyol, surfactant, distilled water, n-pentane, and catalysts were weighed and placed in an 800 mL polypropylene cup. The mixture was homogenized using a 10-mm drill (Delta VSR Drill, D21008) equipped with a 2.8-in diameter mixing blade for 40e50 s, at a speed of 2500 rpm. The pre-weighed PMDI was then added to the mixture, and mixing was continued for another 10 s. The foam was then allowed to rise freely at room temperature. The foams were
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Table 1 Polyol properties comparison. Polyol
Control JeffolÒ FX31-240
SBOP X-0210
OH number (mg KOH/g) Molecular Weight (g/mol) Functionality Viscosity (mPa s) Acid Value (mg KOH/g) Water content (ppm) Manufacture/Resource
240 700 3.0 250 e e Huntsman
235 1100 4.4 8900 1.7 3000 Cargill
cut one week later and the properties of the foams measured, including density, cell morphology, k value and its aging, and dynamic mechanical properties. 2.3. Characterization 2.3.1. Foam reaction kinetics In order to understand the effect of adding SBOP, foam kinetics was studied. Adiabatic temperature rise is a simple and quick method [10,11]. Since PU rigid foams are good insulating materials, any container with a diameter over 5e10 cm is sufficient to maintain adiabatic conditions in the center of the reacting mixture [11]. Given the adiabatic conditions, isocyanate conversion can be calculated from the temperature profile using Equations (1) and (2) [10].
pðNCOÞ ¼
DTrxn
r DTm DTrxn
Q ¼ ¼ Cp mT
(1)
DHr;u
mw m þ DHr;r OH fn Mw MOH Cp m T
(2)
In the above equations, p is the isocyanate conversion; r is the stoichiometric ratio of functional groups (kept to 1 in all our experiments); ΔTm is the temperature rise during foaming; ΔTrxn is the maximum temperature rise; Q is the total amount of heat generated during foaming; Cp is the specific heat capacity of foam, which is taken as 1.5 J/(g∙ K) based on DSC measurements of the final foams; m is the reactant mass; ΔHr is the heat of reaction; M is the molecular weight of reactant; fn is the functionality of the polyol, subscripts u, r, w, T and OH represent urea, urethane, water, total and polyol respectively. ΔHr,u and ΔHr,r were taken as 125.5 kJ/mol and 93.9 kJ/mol [12,13]. An insulated plastic cup with a diameter of about 10 cm was used as the reaction vessel. The reaction mixture based on a total weight of about 25g was prepared using the same foam synthesis procedure as in section 2.2. Temperature profiles during foaming were monitored using a type J thermo-couple with a 0.25 mm diameter (OmegaÒ), which was connected to a digital-thermometer. An adapter was used to connect the digital-thermometer to a computer and the temperature was recorded. The physical
Fig. 1. Polyol structure: (left) control polyol, (right) major component in SBOP.
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Table 2 Foam and film formulation. Chemical Weight (pbw)
Foam
Polyol (petro/soy) Diethylene glycol (DEG) Glycerol Water PolycatÒ 8 PolycatÒ 5 Surfactanta n-pentane Acetone Isocyanate Isocyanate index
100 6.0 0 2.6 4.0 1.0 2.0 8.0 0 130 125
Film DEG added
Glycerol added
100 6.0 0 0 Vary 0 0 0 125 109.8 105
100 0 16 0 Vary 0 0 0 125 123.8 105
a Surfactant A is TEGOSTAB B-8404 (Goldschmidt Chemical Corporation) and surfactant B: BYK-LPX 7105 (BYK USA Inc.)
blowing agent n-pentane was not used and the isocyanate index was kept at 100 to make the reaction stoichiometrically balanced and thus isocyanate conversion depends only on the gelling and blowing reactions. An excess of isocyanate could lead to other reactions such as isocyanate trimerization and further reaction between isocyanate and urethane. 2.3.2. Foam density Foams from the core of the caps of the foam at the top of the cup were cut into 2.5 cm cubes and weighed to determine density. Three to four pieces were cut from three different foam batches and the average density was reported. 2.3.3. Scanning electron microscopy (SEM) Foams were cut with a razor blade into rectangular slices: 7 10 3 mm. Each sample slice was sputter coated with a layer of 50 Å platinum. A JEOL 6500 field emission scanning electron microscope was used to examine the cell morphology [14]. The sample was then attached to the stub using conductive carbon tape. The sample was imaged under an accelerating voltage of 10 kV and 37 magnification. The cellular structure of the foam was observed parallel to the free-rising direction. An average of 5 images was collected on each foam. In this study, apparent long axis, which is the maximum chord length of a cell [15], was measured using ImageJ. The average cell size and its standard deviation were based on 100e150 cells.
120 C and held for 10 min, and finally heated at 10 C/min to 220 C. The value of the heat capacity at room temperature was determined on the second heating cycle [16]. The specific heat capacity of foam was used to calculate thermal conductivity (k value) [17]. 2.3.7. Thermal conductivity, k, measurement Thermal conductivity or k value is the most important property for PU rigid foam [18e20]. A typical k value measurement is based on the rate of steady state heat transfer across a foam with known thickness, which is induced by two different known temperatures between two opposite surfaces of the foam [18,20]. However, the current steady state method requires a large sample size, typically 30.5 cm 30.5 cm 5 cm. This method is not suitable for lab scale samples. Recently, Harikrishnan et. al. have reported a simple and rapid technique based on a transient measurement using a needle probe [17]. This method is rapid with accuracy within 5%. It is widely used in the measurement of k value and thermal diffusivity of liquids and solids [17]. To evaluate the long term insulating performance of PU rigid foams, an accelerated k value aging test was done. Foam cubes with 12 cm on a side were cut from the top of the cup foam. They were placed in an oven at 70 C and the k value measured periodically. It has been determined that the air ingress is 6e12 times faster at 70 C compared to room temperature [18]. 2.3.8. Dynamic mechanical analysis (DMA) A disk (8 mm diameter 8 mm thickness) was cut from the foam and tested in sinusoidal oscillation between two 8-mm diameter serrated parallel plates (ARES, TA Instruments). Contact was maintained by loading the sample at the plate temperature of 160 C and applying a constant normal force of 16 8 g throughout the experiment. Strain sweep test was performed first to determine the linear viscoelastic region of the sample. Storage modulus (G0 ) and loss modulus (G00 ) were recorded at 1 Hz and 0.1% strain over a temperature range from 0 to 220 C at a temperature ramp rate of 3 C/min. 3. Results and discussion 3.1. Foam kinetics In Fig. 2 the isocyanate conversion profile showed that foaming of 0% and 100% SBOP had the same isocyanate conversion in the
2.3.4. Closed cell content Closed cell content was measured by gas displacement following the standard procedures described in ASTM D-6226 using Micromeritics AccuPyc 1340. 2.3.5. Compressive strength Compressive strength was measured using ASTM D-1621 on foam samples with dimensions of 5.0 cm 5.0 cm 2.5 cm. The samples were placed between two parallel plates with a larger area than the specimen and the force required to compress the foam at the rate of 0.25 cm/min was measured. The compressive strength was the value of the maximum applied force divided by the initial sample surface area when the maximum applied force occurred before the strain reached 10%. Otherwise, the compression applied force was taken when the strain was 10%. 2.3.6. Differential scanning calorimetry (DSC) DSC (Q1000, TA Instruments) was used to measure the heat capacity of the foams. About 7e10 mg of compressed foam sample was loaded into an aluminum standard inverted pan and sealed. The sample was first heated at 10 C/min e 150 C and equilibrated for 2 min, cooled to
Fig. 2. Isocyanate conversion from adiabatic temperature rise during foam formation for 0% and 100% SBOP.
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first 20 s. After the initial 20 s, SBOP reacted slightly slower than that of control, and reached a slightly higher plateau value of isocyanate conversion. The isocyanate can react with water, diethylene glycol and polyol. Based on their relative reactivity, the first 20 s was mainly due to the reactions of isocyanate with water and diethylene glycol. The slower reactivity afterward was mainly due to the lower reactivity of the secondary OH groups in SBOP. For the control, the reactions reached a plateau value by about 50 s, while for SBOP it reached a plateau only after about 100 s. The higher isocyanate conversion is probably due to the higher degree of phase mixing in the soy-based foaming system. 3.2. Foam density, cellular morphology and compressive strength The resulting foam properties are summarized in Table 3. The foam density increased with increasing SBOP substitution but only significantly at 100% substitution. With the same SBOP substitution, foam density did not change significantly between the two surfactants. Good insulating performance of PU rigid foam requires high closed cell content. All the soy-based PU foams had closed cell contents comparable to the control foam (see Table 3). SEM images (Fig. 3) showed that all the foams consisted of well-defined closed cells regardless of the polyol type. Foams with 0% SBOP had the largest average cell size. With SBOP substitution, PU rigid foams had a smaller average cell size. We suggest that the higher viscosity of SBOP (see Table 1) helped to reduce cell drainage by gravity at the initial foaming stage which led to a smaller average cell size. Using the more hydrophobic surfactant B, the average cell size became even smaller. This suggested that it acted more strongly in stabilizing cell structure. We don’t believe that this is due to the different compatibilities of B-side (polyol, diethylene glycol, catalyst, water, n-pentane and surfactant) with different surfactant hydrophobicities. Our compatibility test [21] has shown that there is no significant difference by using these two surfactants. Overall, the SEM study suggested that cell morphology was not significantly altered by substituting SBOP for the petroleum-based polyol in the foam formulation. Compressive strength also increased with soy substitution (Fig. 4) possibly due to the decrease in cell size. 3.3. Dynamic mechanical properties In Fig. 5, both G0 and tan (d) data were plotted for the foams from 0%, 25%, 50%, 75% and 100% SBOP. In the glassy state, all samples had comparable plateau G0 values of about 2 106 Pa. The higher G0 for the 100% SBOP sample was due to the higher foam density. As the SBOP substitution increased, glassy to rubbery transition shifted gradually to higher temperatures. In the rubbery state, all foams had comparable G0 of about 2 104 Pa. Tg was determined from the peak in tan (d) and increased from 98 C to 142 C with increasing SBOP substitution. With increasing SBOP substitution, the tan (d) peak broadened possibly due to the SBOP being more heterogeneous in structure compared to the petroleum-based polyol.
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3.4. Thermal conductivity Thermal conductivity is the key property that governs insulation applications for PU rigid foams. k value is closely related to foam density and cell morphology. Total k value consists of four parts: lgas, lradiation, lsolid and lconvection [20]. Foams with a high density have lower lradiation, but a higher non cellular PU portion leads to higher lsolid. Though the PU solid only accounts for a small percentage of the whole foam, it has a much higher k value (220 mW/(m$K)) than the physical blowing agent gas n-pentane (13.7 mW/(m$K)) and CO2 (15.3 mW/(m$K)). At a fixed foam density, lradiation will decrease with decreasing cell diameter. It has been reported that the k value increases substantially, nearly 50%, as cell size increases from 0.25 mm to 0.6 mm [22]. The level of closed cell content should also be kept as high as possible. Open cells allow more convection, and even air to enter the foam, which has much higher k value (24.9 mW/(m$K)) than that of n-pentane and CO2 [18]. Thus, a low k value results from a low foam density, a small average cell size and a high closed cell content. Thermal conductivity or k value measurement was done on samples from 0%, 50% and 100% SBOP. With surfactant A which was more hydrophilic, foams from 50% SBOP had comparable density and about a 10% smaller cell size, though 100% SBOP foams had much higher density and an even smaller cell size, which led to foam k value as low as control foams. The two surfactants of different hydrophobicities gave foams with thermal insulating properties comparable to the control foams. 3.5. Foam aging Since PU rigid foams are largely used in building and appliance insulation, the change in k value with time is also important. Long term insulating performance was investigated by an accelerated foam aging test. Fig. 6 showed the foam aging results for 0%, 50% and 100% SBOP for both surfactants. All these curves showed an initial rapid k value increase, followed by a more gradual increase after 50 days. Foams from 50% SBOP with both surfactants had very similar k value aging as the 0% SBOP foams; k value had only increased from 23 to 24 mW/(m$K) to about 26.5e27 mW/(m$K) after 150 days of aging. However, the foams from 100% SBOP aged faster. The k value increased from about 24 mW/(m$K) to as high as 28.5 mW/(m$K). With the more hydrophobic surfactant B, foams from 100% SBOP aged even faster, with the k value increasing to about 30.5 mW/(m$K). The foam aging tests for both surfactants revealed that soy-based foams from 100% SBOP had faster k value aging compared to control foams and soy-based foams from 50% SBOP. 3.6. PU film gas permeability study Over time, PU rigid foams will undergo k value changes due to the change in gas composition. For freshly made PU rigid foam, the gas trapped inside the cell is a mixture of mostly physical blowing
Table 3 Foam properties comparison. % SBOP substitution
0
Density (kg/m3) Tg ( C) K value (mW/(m K)) Closed cell content (%) Cell size (mm)
39.5 98 23.4 89.09 431
a
0.8 0.6 1.07 91
25
50
39.7 0.9 107 e e 392 84
39.8 123 24.0 88.76 390
Samples made with surfactant B, others with surfactant A.
1.1 0.6 0.44 102
75
100
41.3 1.3 134 e e 375 128
46.4 142 24.2 90.68 386
2.0 0.6 0.44 102
50a
100a
40.0 1.0 110 23.3 0.2 e 360 94
49.0 1.8 143 24.9 0.7 e 349 103
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Fig. 3. Foam SEM images: (a) 0%; (b) 50%; (c) 75% and (d) 100% SBOP, surfactant A.
agent (n-pentane in this study), some CO2, and a small amount of air. Driven by the partial pressure difference between foam cells and the atmosphere, air will gradually diffuse into the cells. Similarly, CO2 and pentane will diffuse out of the cells. Generally, CO2 diffuses out much faster than other gases [23]. The process of air diffusion is slower than CO2, but it’s concentration inside the cells will gradually increase with N2 building up faster than O2. For physical blowing agents like CFC and HFC, and pentane, the permeability is very low [23]. Since both N2 and O2 have higher k values than CO2 and n-pentane, the change in gas composition leads to the increase in k value over time. In order to determine whether the faster k aging was due to higher gas permeability with SBOP substitution, gas permeation
Fig. 4. Compressive strength of PU rigid foams.
through a PU thin film, which can represent the polymer in the PU rigid foam, was measured. This method eliminates the influence of foam cell morphology but requires high quality film samples. The film formulation is listed in Table 2. 3.6.1. Film preparation Since isocyanate can easily react with water and moisture in the air, the most difficult part was to avoid producing bubbles during the preparation procedure. Water was not added because urea linkages in the polymers only slightly affect the gas permeation [24]. Polyols were dried under high vacuum, high temperature (110 C), and high speed stirring for 2e3 h before use. Acetone was used to dilute the reaction mixture. This not only helped to make a thin and homogeneous film, but also helped to avoid trapping bubbles. The amount of catalyst was also adjusted to balance the rate of the gelling reaction and acetone evaporation. The gelling reaction should be fast enough to avoid the reaction between isocyanate and moisture as much as possible, but should be slow enough for most of the acetone to evaporate. PU thin films of 300e500 mm thickness were prepared in an open-faced film casting mold [21]. The mold was made from a 35.6 cm 35.6 cm high density polyethylene (HDPE) sheet with an aluminum picture frame with a height of 2.5 cm. Prior to casting the films, the mold was carefully leveled in order to obtain a uniform film thickness. Polyol, DEG, catalyst and isocyanate were blended with acetone in a 200 ml polypropylene plastic cup. The mixture was stirred gently until a homogeneous solution was formed. The solution was then poured into the mold carefully to avoid air entrainment. Another HDPE sheet was placed over the aluminum frame about 1 cm above the film to allow bubbles to escape and prevent direct contact with moist air. The film was kept at room temperature for 24 h, during which the solution gradually solidified and most of the acetone had evaporated. The film was removed from the HDPE
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Fig. 7. N2 permeation of PU thin films. (- SBOP with DEG, , SBOP without DEG; B SBOP with glycerol) (Barrer ¼ 3.348 1019 kmol m/(m2 s Pa)).
Fig. 5. (a) Dynamic elastic modulus, G0 and (b) tan (d) vs. temperature.
sheet and cut into disks having a diameter of 4.2 cm. The disks were placed between MylarÒ sheets for further curing at room temperature for 120 h. A heavy piece of steel was placed on top to ensure film flatness. The disks were then placed in a vacuum oven for a final cure for 48 h.
3.6.2. Gas permeation measurement Gas permeation tests based on a constant volume-variable pressure method [25,26] were performed on a home-built apparatus [26]. The film was fit and sealed into the gas transmission cell to form a semi-barrier between the two chambers. The apparatus was evacuated overnight. N2 flow was fed to one side of the chamber and the pressure change in the opposite chamber which had been evacuated was monitored as a function of time. Gas permeation constants were obtained from the pressure change normalized with the pressure gradient across the sample, film area and thickness [27]. Fig. 7 illustrates the N2 permeation results of PU thin films from 0%, 50% and 100% SBOP. It was found that by substituting 50% SBOP, N2 permeation did not change significantly. However, films from 100% SBOP had a much higher N2 permeation, almost as twice that of the control films. The N2 permeation results showed good agreement with the foam k value aging results. The much higher N2 permeation of the 100% SBOP PU thin film led to faster k value aging than the control or 50% SBOP PU thin film. Another study was done to investigate the effect of a chain extender and a cross linker on N2 permeation of soy-based PU thin films. PU thin films were made from pure SBOP (i.e. without diethylene glycol), as well as SBOP with glycerol instead of diethylene glycol. Samples from pure SBOP had higher N2 permeation than the samples from SBOP with diethylene glycol. However, by adding glycerol instead of diethylene glycol, N2 permeation dramatically decreased. The gas permeation test has shown that the addition of a chain extender or a cross linker can decrease N2 permeation. 4. Conclusions
Fig. 6. Thermal conductivity increase with time at 70 C.
PU rigid foams were prepared from a soy-based polyol, SBOP. The experimental results demonstrated that by replacing a typical petroleum-based polyol with SBOP, Tg increased and foams had comparable foaming kinetics, density, cellular morphology, and initial thermal conductivity (k value). Compressive strength of the soy-based PU rigid foams were superior to those of petroleum-based foams possibly due to smaller cell size. However, while foams from 50% SBOP showed similar k value aging to control foams, foams from 100% SBOP aged much faster. The mechanism behind foam k value aging was studied by measuring the N2 permeation of PU thin films. It was found that PU thin films with 50% SBOP substitution had
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similar N2 permeation to the 0% SBOP films, but films from 100% SBOP had much higher N2 permeation, which was in good agreement with k value aging results. Adding glycerol to the films with 100% SBOP reduced N2 permeation significantly. Acknowledgments The authors would like to acknowledge Cargill Incorporated and IREE (Initiative for Renewable Energy and the Environment) for financial support. Throughout this work, we had many helpful discussions with Professor Tom Hoye, Professor Marc Hillmyer, Dr. Junghwan Shin and Dr. Harikrishnan G. of the University of Minnesota. We would also like to thank Darius Jaya and Jason Zhang of the University of Minnesota for their help with sample preparation, Professor Michael Tsapatsis for use of the permeability cell and Matthew Caldwell, Cargill Inc. for his assistance with closed cell content measurements. References [1] [2] [3] [4] [5]
Pollock JW. Abstr Pap Am Chem Soc 2004;227. 160eCELL. Petrovic ZS. Polym Rev 2008;48:109e55. Carlson KD, Chaudhry A, Bagby MO. J Am Oil Chem Soc 1990;67:438e42. Lyon CK, Garrett VH, Franker EN. J Am Oil Chem Soc 1974;51:331e4. Hu YH, Gao Y, Wang DN, Hu CP, Zu S, Vanoverloop L, et al. J Appl Polym Sci 2002;84:591e7.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
Guo A, Javni I, Petrovic Z. J Appl Polym Sci 2000;77:467e73. Guo A, Zhang W, Petrovic ZS. J Mater Sci 2006;41:4914e20. Narine SS, Kong X, Bouzidi L. J Am Oil Chem Soc 2007;84:65e72. Tu Y, Kiatsimkul P, Suppes G, Hsieh FJ. Appl Polym Sci 2006;105:453e9. Zhang L. Ph. D thesis, University of Minnesota, 2008. Macosko CW. RIM, Fundamentals of Reaction Injection Molding. New York: Hanser Publishers; 1989. Hager KF, Brandien JE. J Cell Plast 1968;4:298e303. Vespoli NP, Alberino LM. Polym Proc Eng 1985;3:127e47. Hafner B. Scanning Electron Microscopy Primer, Characterization Facility. University of Minnesota-Twin Cities; 2007. Rhodes MB. Low denisty cellular plastics. In: Hilyard NC, Cunningham A, editors. Physical Basis of Behaviour. New York: Chapman & Hall; 1994. Zhang L, Jeon HK, Malsam J, Herrington R, Macosko CW. Polymer 2007;48: 6656e67. Harikrishnan G, Macosko CW, Choi JH, Bischof JC, Singh SN. J Cell Plast 2008; 44:481e91. Randall D, Lee S, editors. The Polyurethanes Book. John Wiley & Sons, Ltd; 2002. Wirpsza Z, Kemp TJ, editors. Polyurethanes: Chemistry, Technology and Applications. Chichester: New York: E. Horwood; 1994. Szycher M. Szycher’s Handbook of Polyurethanes. Boca Raton: CRC Press; 1999. Tan Suqin M.S. thesis, University of Minnesota, 2010. Bogdan M, Hoerter J, Moore FO. J Cell Plast 2005;41:41e56. Modesti M, Lorenzetti A, Dallacoua C. Polym Eng Sci 2004;44:2229e39. Bartlett PL, Creazzo JA, Hammel HS. J Cell Plast 1994;30:126e42. Pye DG, Hoehn HH, Panar MJ. Appl Polym Sci 1976;20:1921e31. Jeong HK, Krych W, Ramanan H, Nair S, Marand E, Tsapatsis M. Chem Mater 2004;16:3838e45. Kim H. Ph. D thesis, University of Minnesota, 2009.