Structures and physical properties of rigid polyurethane foams with water as the sole blowing agent

Science in China Series B: Chemistry 2006 Vol.49 No.4 363- -370363DOI: 10.1007/s11426-006-2007-8Structures and physical properties of rigid polyurethanefoams with water as the sole blowing agentLI Xiaobin12, CAO Hongbin' & ZHANG Yi11. Institute of Process Engineering, Chinese Academy of Sciences, Beiing 100080, China;2. Graduate School of the Chinese Academy of Sciences, Beijing 100039, ChinaCorrespondence should be addressed to Cao Hongbin (email: hbcao@home.ipe.ac.cn)Received March 22, 2005; accepted November 5, 2005Abstract Rigid polyurethane foams blown by varying water level were prepared in this study. Thestructures and physical properties of rigid polyurethane foams blown by water were measured withFT-IR, universal testing machine (Instron3365), scanning electron microscope (SEM) and differentialscanning calorimeter (DSC). The results show that polyurea and polybiuret were the typical charac-teristics, and the cream time and gel time were shorter for the fully water blown rigid foams than thatfor the fully cyclopentane blown foams. The density of foam samples decreased from 45.0 kg/m* to27.4 kg/m' with the increase of water level from 3 pph to 7 pph. Compressive strength exhibited thesimilar behavior with density. The average cell size of foam samples ranged from 241 um to 356 μumwith the increase of water level from 3 pph to 7 pph, respectively. At the same time, poorer dimen-sional stability was encountered with the increase of average cell size due to fast diffusion rate of CO2out of the foam. The results of thermal analysis show that the glass transition temperature (Tg) shiftedto a higher temperature with the increase of isocyanate level when more water was used as chemicalblowing agent.Keywords: polyether polyol, polyurethane rigid foam, compressive strength, dimensional stability, cellsize, glass transition temperature.Polyurethane rigid foams have been used for manyCFCs are newly proposed.applications such as pipelines insulation materials,Different from CFCs, water, a chemical blowingautomotive parts, solar water heater and construction agent, can be also used to make cellular rigid foam bymaterials2, due to their desirable physical properties.reaction with the MDI, which generates carbon diox-Traditional rigid foam is made by the reaction of a ide to form the bubbles. Since water is a non-ozonepolyol and 4,4'-diphenylmethane disocyanate (MDI)depleting，non-toxic and cheap blowing agent, somewith chlorofluorocarbons (CFCs), in particular tri-researches on water blown rigid foams and their ap-chlorofluoromethane (CFC-11) and/or HCFC-141b asplications have been performed-However, com-blowing agents. However, the CFCs blowing agentspared to the CFCs blown rigid foams, water blowncontain halogens, which may deplete the ozone layerrigid foams encounter some problems such as dimen-and cause the environmental issues such as globalsional stability, foam flow-ability, formulation viscos-warming'. So, new blowing agents substituting foity, friability, substrate adhesion, insulation perform-www.scichina.com www. springerlink.com中国煤化工MHCNMH G.364Science in China Series B: Chemistryance, K-factor aging and blowing agent solubility. A was held at 110°C and about 0.3 MPa for 5 h. Afterbroad study of different effects of polyol, blowingpolymerization, appropriate phosphoric acid andagent, MDI and catalyst on foam properties is verymagnesium silicate (8.0 g) were then added to the liq-necessary to address these issues. .uid to neutralize the catalyst for 1 h. After that, theTabor et al.!n investigated the effect of polyol func-yellow polyol product having a viscosity of 2000tionality on water blown rigid foams. In their work, itmPa-s (25°C) and hydroxyl value of 444.3 mgKOH/gwas shown that polyol functionality played an impor-was achieved after filtration.tant role in determining the adhesion to aluminum, cellsize, K-factor, dimensional stability and glass transi-1.3 Preparations of rigid foam samplestion temperature of foam. Nichol et al.8 reported theAll of rigid foam samples were synthesized with aeffects of equivalent weight and functionality o1one-shot method at room temperature (ca. 18C). Thepolyol on the properties of foams. Recently, Jung etmasterbatch (consisting of polyether polyol, surfactant,al.9.10 compared the properties of rigid polyurethanecatalyst, blowing agent) was weighed into a paper cup.foams blown by HCFC-141b with those by distilledThen, an appropriate MDI was added to the master-water. In their work, however, the structure character-batch aI vigorously stirred at 2500 r/min for 30 s.istics and dimensional stability of foams were not re-After mixing, the mixture was poured into a 200 mm xferred to, and all of data were only limited to the sys-200 mm x 80 mm mold with a detachable lid to pro-tem water level ranging from 0.5 pph to 3.0 pph. Induce free-rise foam. Cream time and gel time werethis work, a polyether polyol with sugar and glycerinmeasured in the course of blowing. The foam wasas the initiator was prepared. By varying the level ofcured in this mold for 24 h at room temperature beforewater, the structures and physical properties of foamsbeing removed, cut and tested.were discussed in detail.In addition， excess MDI (isocyanate index-NCO/- OH = 1.05) was added to assure the complete1 Experimentalreaction of the polyether polyol and water except forspecial interpretation (formulation for experiments,1.1 Materialssee Table 1).Propylene oxide was the product of Tianjin Dagu1.4 Analytical methodsChemicalPlant(Tianjin,China).MDI,4,4'-Diphenylmethane disocyanate was produced byFourier-Transform Infrared spectroscopyHuntsman Corporation (Bejing, China). Sugar was (FT-IR) spectra were obtained with a Perkin-Elmerfrom Nanning Sugar Factory (Guangxi, China). Glyc-Spectrometer“Spectrum-GX" (U.S.A) to identify theerin was purchased from Beijing Chemical Reagent molcular structures of rigid foam samples. A slice ofPlant (Bejjing， China). Triethylene diamine, aminefoam about 0.5 mm in thickness was prepared for IRcatalyst, dissolved in diethylene glycol to 33%, was measurement.from Air Products Corporation (Beijing， China).Mechanical properties of rigid foams were meas-B8423, B8462, the silicone surfactant, were supplied ured on an Instron3365 universal testing machineby Goldschmidt Corporation (Shanghai, China).(U.S.A) at room temperature. The size of the specimenDibutyltin dilaurate, tin catalyst, was from Air Prod-was50mmx50mmx50mm.Thespeedofcross-ucts Corporation (Beijing, China).head movement was 5 mm/min. Compressivestrengths of 5 specimens per sample were measured1.2 Preparations of polyether polyoland averaged.The mixture of sugar (79.0 g), glycerin (120 g) andDimensional stability of rigid foam samples waspotassium hydroxide (2.0 g) was put into a 1.2 L studied on a testing box with constant temperature andstainless steel reactor and heated until the temperaturehumidity according to ISO 2796- 1980. Three speci-of the mixture increased to over 90"C. Then appropri-menswiththesizeof100mmx100mmx25mmate propylene oxide was fed into the reactor, whichwere prepared中国煤化工MHCNMH G.Structures and physical properties of rigid polyurethane foams with water as the sole blowing agent65Cell size and morphology of rigid foam sampleschanging the blowing agent. The formulation data arewere measured on a JSM-6700F (Japan) scanninggiven in Table 1.electron microscope (SEM). A slice of foam about 1.0Table 1Formulation data and the physical properties of the rigidmm in thickness was prepared for SEM analysis byfoams blown by water and cyclopentane, respectivelycoating with gold before scanning. The slice was doneBlowing agentWater Cyclopentaneat an accelerating of 10 keV and a magnification levelPolyether polyol (pph*)100of 40X. The sample was observed in the free-rise di-Triethylene diamine (pph)0.2B8462 (pph)rection.Glycerin (pph)1.0Differential scanning calorimeter (DSC) analysisWater (pph)was made on a NETZSCH STA 449C (German) toDibutyltin dilaurate (pph)0.1measure the glass transition temperature (Tg) of rigidCyclopentane (pph)12foam samples. The samples were performed under aIsocyanate index (-0H/- NCO)051.05Cream time (8)2776nitrogen flow of 60 mL/min with a heating rate ofGel time (s)7310515 C/min from the room temperature to 300C.Density of frce rise foam (kg/m)33.731.7a) pph: Based on 100 parts polyether polyol by weight.2 Results and discussionFig.1 shows the FT-IR spectra of a fully water2.1 Characteristics of water blown rigid foamsblown foam and a fully cyclopentane one. As expected,A very interesting question is proposed that whatthe peaks at 1658一 1666 cm ' in the water blowndifferences on earth there are in molecular structuresfoam spectrum which are regarded as the typicalof foams blown by water and the physical blowing characteristics of hydrogen boned urea band"agent. A FT-IR analytical technique was employed towere much more obvious than that in the fullyaddress this problem. Here, foam samples were pre- cyclopentane blown foam spectrum. When water waspared with fully water and fully cyclopentane as theused as a blowing agent, it reacted with isocyanate toblowing agent, respectively. To evaluate their differ- generate polyurea, polybiuret and CO2 accompaniedences，all of parameters were kept the same juswith release of exothermic reaction heat:0.09.0-8.5MMjwwq7.57.0F5.0F4.5F3.31800I 70016001500140030012001100Wavenumber (cm)Fig. 1. FT-IR spectra of a fully water blown foam and a fully cyclopentane blown foam. 1, Sample blown by cyclopentane; 2, sample blown bywater.中国煤化工MYHCNMH G.366Science in China Series B: Chemistry0(1)Compressive strength exhibited the same trend asH,0+2RNC0- -RNHCNHR+CO2the density. The compressive strength of foam samplesincreased from 0.1469 MPa to 0.4005 MPa with the0 R"Odecrease of water level from 7 pph to 3 pph. It is wellR' NHCNHR +RNC0一+R'NHC -N- -CNHRknown that compressive strength of rigid foam has asignificant relationship with density. A power-law ex-So, urea was the main structure of water blown rigidpression can be used to show the relationship betweenfoam. This conclusion was also supported by the datacompressive strength and density' T0.in Table 1. In Table 1, the cream time and gel time ofE=A.p",(3)water blown foam are shorter than those of cyclopen-tane blown foam due to exothermic heat from eqs. (1)where E is the modulus or compressive strength ofand (2), which accelerated the reaction rate. Whenfoam samples, p is the foam density, n is the exponentcyclopentane was used as a blowing agent, more en-and A is a constant.Taking logarithms of eq. (3) yields:ergy was needed to vaporize it, which reduced thelnE=lnA+nInp.(4)temperature the foam reached and longer reaction timewas neededls!.A plot of lnE versus lnρ is shown in Fig. 2. FromThe peak at around 1695 cm-' was characteristic ofFig. 2, it was found that InE had a good linear rela-free urea carbonyl, but it was difficult to identify duetionship with Inp. This linear relationship can be ex-to being overlapped by other carbonyl peaks. Thepressed as follows:peaks at 1702- 1725 cm and 1730- 1740 cm-' wereLnE= -8.64397+2.03841Inp,(5)attributed to hydrogen bonded urethane carbonyl andwhere correlation coefficient R = 0.9971; significancefree urethane carbonyl, respectively. The peaks atP=1.9x 10+1597 cm-' and around 1500 cm were assigned toFrom the above discussion, water level has a sig-nificant effect on density and mechanical properties ofvibration of phenyl rings"" 14.rigid foams. So, foams with varying density and com-2.2 Effect of water level on density and mechanicalpressive strength can often be gotten by changing theproperties of rigid foamsamount of water when preparing water blown rigidA set of experiments was performed by varying thefoam in practical application.water level from 3 pph to 7 pph to investigate the ef-2.3 Effect of water level on dimensional stability offect on density and compressive strengths of rigidrigid foamsfoams. The amount of MDI was adjusted to react atTable 2 shows the results of dimensional stability ofthe same isocyanate index (1.05). The results for ex-foams at low temperature (- -25"C) and high tempera-periments are summarized in Table 2.As shown in Table 2, the core density of foam sam-ture (80"C), respectively. From Table 2, the dimen-ples decreased with the increasing water level, rangingsional variation at high temperature was much higherfrom 45.0 kg/m' (3 pph water) to 27.4 kg/m' (7 pphthan that at low temperature for the same foamwater). This result is due to more bubbles formed withsamples. At 80"C, dimensional variation of foam sam-the increase of blowing agent.ples blown with 6 pph water, for example, was aboutTable2 Effect of water level on physical properties of water blown rigid foamsWater level (pph)Overall density (kg/m2)28.929.835.739.346.9Core density (kg/m)27.429.033.737.645.0Shrink or notshrinkslightly shrinknc0.16950.22820.4005Dimensional stability (%)(- -25"C, 120 h)2.131.010.340.100.06Dimensional stability (% )(-25 C, one month)7.134.390.39 .0.380.19Dimensional stability (% )(80C, 96 h)6.97.02中国煤化工0.33HCNMHG.Srructures and physical properties of rigid polyurethane foams with water as the sole blowing agent670.8厂water level at the same temperature, similar to thedensity and mechanical properties. More importantly,1.0the dimensional variation of almost all of foam sam-ples came to a max before being aged for 36 h. Afterthat, a broad plateau region appeared, insignificantly旦-1.4-changing in dimensional stability with time.1.6-2.4 Effect of water level on cell size and morphologyof rigid foams-1.8-The cell size of rigid foam has an important effect2.0L333.4 3.5 3.6 3.7 3.8on thermal conductivity and mechanical properties,such as compressive strength and tensile strength. CellFig. 2. Power-law behavior between compressive strength and densitysize affects the mechanical properties by spreading theof foams.compressive/tensile stresses to the more numerous7 times higher than that at -25C. As we have known,structures available in small cell size foams to avoidCFC-11 blown foams trend to shrink at low tempera-concentrating the stresses onto fewer larger cell struc-ture due to blowing agent condensation and decreaseures. So, the mechanical properties are expected toof internal pressure. For a fully water blown foam,improve with the decrease of cell size7. In addition,however, CO2 diffusion is the key factor instead ofaccording to heat transfer theory of rigid foams, aboutblowing agent condensation. The diffusion rate of CO210%- 15% of the heat transfer of rigid foam can beout of the foam is about an order of magnitude fasterdirectly attributed to the radiation, which can bethan air into the foam for the same polymeric frame- minimized by reducing the cell size of the foaml!4!.work, especially at high temperature. Then foamFig. 4 shows the SEM photos of rigid foam samplesinternal pressure decreases. If the foam framework isblown by 3, 4, 5, 6 and 7 pph water, respectively. Thenot strong enough to balance the pressure diference results show that the average cell size of water blownbetween the inside and outside of the foam, a perma-foam samples ranged from 241 μm to 356 μum with thenent shrinkage of foam occurs.increase of water level from 3 pph to 7 pph. AccordingThe dimensional variation with time for foam sam-to the relationship between cell size and mechanical .ples aged at high temperature (80°C) is shown in Fig.properties, foam samples with smaller and more uni-form bubble structures should have higher compres-It was easily observed that the dimension stabilitysive/tensile strength, which was already confirmed byof foam samples became poorer with the increase ofdata in Table 2.Niyogi et al. developed a model to predict the bub-10Fble size distribution for the water blown foams. Theyreported that the average cell size decreased with the夏8◆7pptincrease of initial water concentration8! However,opposite conclusion was obtained in this study andJung's paper'. It may be a result from the effect ofmass transfer and more coalescence of bubbles in thepractical foaming process.2.5 Effect of water level on glass transition tem-perature of rigid foams80100120140 160Fig. 5(a) and (b) is curves of thermogravimetry (TG)t(h)and derivation of DSC (DDSC), as obtained by DSCFig. 3. Dimensional variation with time for foam samples blown with3,4,5,6,7pphwaterat80C.analysis for foa中国煤化工m Fig. 5, allYHCNMH G ..368Science in China Series B: Chemistry(a(b)c)Fig. 4. SEM photos of rigid foam samples blown by different water level. (a) Blown by 3 pph water; (b) blown by 4 pph water; (c) blown by 5 pphwater; (d) blown by 6 pph water; (e) blown by 7 pph water.DDSC (mW/mg/min)TG (%)00 {0.2008(a)>60.10)40)2 t90-0.1088-0.2086↑5001520025Temperature (C)(b2Fig. 5. Thermal analyses for foam samples blown by different water levels. (a) Effect of water level on Tg of foams; (b) part magnification of DDSCcurves. 1, 152.3C, blown by 3 pph water; 2, 158.6C, blown by 4 pph water; 3, 162.7C, blown by 5 pph water; 4, 166.3"C, blown by 6 pph water; 5,170.6C, blown by 7 pph water.中国煤化工YHCNMH G.Structures and physical properties of rigid polyurethane foams with water as the sole blowing agent369of the curves had the similar characteristics. No ap-dimensional stability due to fast diffusion rate of CO2parent weight loss for all of foam samples was foundout of the foams. Dimensional stability became poorerbefore being heated to 250"C, while DDSC curves with the increase of water level from 3 pph to 7 pph.showed strong endothermic characteristics. It sug-Dimensional variation at high temperature was muchgested that the endothermic peaks were not brought byhigher than that at low temperature for the same foamweight loss, such as water loss, but resulted fromsamples. Furthermore, almost all of foam samples hadmorphology transition of polymer (glass transition of the maximum in dimensional variation when beingrigid foams).aged for 36 h at 80°C. After that, lttle dimensionalAs expected, the glass transition temperature in-variation was observed.creased from 152.3C to 170.6°C with increasingAs seen in the results of morphology by SEM, thewater level from 3 pph to 7 pph. The reason for this isfoam cells had a spherical shape and the cell size in-that more isocyanate (about 14 g of MDI/1.0 g of wa-creased from 241 μm to 356 μm with the increasingter) is needed to react with polyether polyol and water water level from 3 pph to 7 pph. It may be a resultwith the increase of water level. According to eqs. (1)from mass transfer and more coalescence of bubblesand (2), more polyurea and polybiuret are generated,during practical foaming process.which have been considered more “rigid" than poly-With increase of water level from 3 pph to 7 pph,urethane. Additionally, the molecular structure of MDI the Tg of foam samples increased from 152.3C tocontains numerous aromatic rings that are also more170.6C due to numerous rigid polyurea and polybi-rigid than long carbon chains. AIll of these rigid struc-ruet generated as well as plenty of aromatic rings,tures cause the increase of Tg. Another important rea-which shifted Tg to higher temperature. In addition,son for this is increasing cross-link density with add-increasing cross-link density was also an importanting isocyanate level in formulation, which shifts Tg tofactor for Tg's increase.higher temperature according to network formationtheory.Acknowledgements This work was supported by the JointResearch Foundation of the Chinese Academy of Sciencesand Hebei Province, China (Grant No. 2004-015). The au-3 Conclusionsthors would like to thank kind Yuping Li for carrying outAs discussed in FT-IR analysis results, polyureaFT-IR measurements.and polybiuret were typical characteristics in molecu-Referenceslar structures of fully water blown foams. Compared1 Pottswith cyclopentane blowing process, shorter creamfoams for the insulation of district heating pipelines. J Cell Plast,time and gel time were obtained in water blown foams1985, 21(1):51-57due to exothermic reaction heat when water was used2 Demharter A. Polyurethane rigid foam, a proven thermal insulat-as the chemical blowing agent.ing material for applications between +130C and -196C. Cryo-With the increase of water level in formulation fromgenics, 1998, 38(1): 113-1173 pph to 7 pph, the density of foams decreased from3 Molina M J, Rowland F S. Stratospheric sink for chlorofluoro-45.0 kg/m’to 27.4 kg/m', respectively. Compressivemethanes: Chlorine atomic-catalysed destruction of ozone. 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