structure and properties of polyurethanes based on halogenated and nonhalogenated soy–polyols

Structure and Properties of Polyurethanes Based onHalogenated and Nonhalogenated Soy–Polyols

ZORAN S. PETROVIC, ANDREW GUO, WEI ZHANG

Kansas Polymer Research Center, Pittsburg State University, 1501 S. Joplin, Pittsburg, Kansas 66762

Received 13 July 2000; accepted 21 August 2000

ABSTRACT: Four polyols were prepared by a ring opening of epoxidized soybean oil withHCl, HBr, methanol, and by hydrogenation. Two series of polyurethanes were preparedby reacting the polyols with two commercial isocyanates: PAPI and Isonate 2143L.Generally, the properties of the two series were similar. The crosslinking density of thepolyurethane networks was analyzed by swelling in toluene. Brominated polyols andtheir corresponding polyurethanes had the highest densities, followed by the chlori-nated, methoxylated, and hydrogenated samples. The polyurethanes with brominatedand chlorinated polyols had comparable glass transition and strength, somewhathigher than the polyurethane from methoxy containing polyol, while the polyurethanefrom the hydrogenated polyol had lower glass-transition and mechanical properties.© 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4062–4069, 2000Keywords: polyurethanes; soybean oil; halogenated; polyols

INTRODUCTION

Polyurethanes prepared from vegetable oils havea number of excellent properties derived from thehydrophobic nature of triglycerides.1–14 However,polyols prepared from vegetable oils intrinsicallyhave a very heterogeneous structure arising fromthe variation in structure of vegetable oils. Thestructure of soybean oil varies depending on thetype of soybean, weather conditions, type of soil,and so forth. The structure of triglyceride mole-cules, that is, the type of fatty acids in the trig-lycerides of the same oil, differs from molecule tomolecule. This causes a variation in the chainlength between crosslinks in polyurethanes ob-tained from oil-based polyols or a different den-sity of crosslinking from one to the other section ofthe polymer network. It has been found that de-spite such variations, the glass transition of poly-urethanes prepared from different oils varies

fairly consistently with the hydroxyl content.4,15

Although triglycerides in soybean oil have on av-erage 4.5 double bonds, not all are converted tohydroxyl groups. Our polyols are typically tetrolswith four OH groups in the bromine containingpolyol and about 3.8 groups in the others threepolyols. The hydroxyl groups are in the middle ofthe fatty-acid chains of the triglycerides. In thiswork we have varied the structure of soybeanoil–based polyols by varying the nature of theside group on the C atom adjacent to the hydroxylgroup. These groups were Br—, Cl—, CH3O—,and H—. The typical structure of the polyols isgiven in Figure 1.

When these polyols are crosslinked with iso-cyanates, part of the chain will be pendant.Crosslinking sites in linoleic and linolenic acidsare 1–3 carbon atoms apart (Fig. 2).

In the case of saturated acids the whole acidbecomes a pendant chain in the crosslinked net-work. Pendant chains do not support stress whenthe sample is under load and may act as plasti-cizers. Large chlorine and bromine atoms mayintroduce additional polarity in the crosslinkednetworks, which could result in their increased

Correspondence to: Z. Petrovic. (E-mail: [email protected]).Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 4062–4069 (2000)© 2000 John Wiley & Sons, Inc.

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density and modulus. Our objective was to exam-ine the effect of structure of four types of soy-based polyols on their properties as well as on theproperties of the resulting polyurethanes. Thestructure and properties of the polyols used inthis work were described in a previous article.16

Polyols were crosslinked with diphenylmethanediisocyanate (MDI) to obtain polyurethanes. Twomodified MDI–based liquid isocyanates wereused in this work. The structure of polyurethaneswas examined by Fourier transform infrared(FTIR)spectroscopy and solid-state NMR. Differ-ential scanning calorimetry (DSC), thermo-gravimetry (TGA), thermomechanical analysis(TMA), dielectric analysis (DEA), dynamic me-chanical analysis (DMTA), and mechanical meth-ods were used to learn the physical and mechan-ical characterization of the polyurethanes.

EXPERIMENTAL

Materials

Two isocyanates were used in this study: Isonate2143L (a liquid MDI prepolymer containing car-

bodiimide bonds) and PAPI 2901 (a crude MDI).Isonate 2143L has NCO content of 29%, an iso-cyanate equivalent weight of 144.5, a molecularweight of 303, a functionality of 2.1, a viscosity of33 mPa s, and density of 1.214 g/cm3 at 25 °C. It’sa clear, light yellow liquid that when heatedabove 90 °C reverts to MDI.

PAPI 2901 is a crude polymeric MDI, with anNCO content of 31.6%. It is a dark brown liquid,having a functionality of 2.3, an isocyanate equiv-alent weight of 133, a viscosity of 55 mPa s, and adensity of 1.22 g/cm3. Both isocyanates were sup-plied by Dow Chemical. The syntheses of the poly-ols used in this work were described earlier in thisarticle. Functionality and OH numbers are givenin Table III.

Synthesis of Polyurethanes

Polyurethanes were synthesized using a 5% ex-cess of isocyanate (index 5 1.05). Cocure55 cata-lyst (0.2% of the polyol), supplied by CasChemIncorporated, was used in all cases. The polyolswere preheated to 60 °C, mixed with isocyanatesand the catalyst, and poured to a mold. Curingwas carried out at 60 °C for 1–2 h and postcuredat 110 °C overnight.

RESULTS AND DISCUSSION

Polyols were conveniently crosslinked with twoliquid isocyanates: PAPI and Isonate. The densi-ties of the polyurethanes followed the same pat-tern as with the polyols, that is, the brominatedpolyurethanes had the highest density with 1.26g/cm3, then the chlorinated polyurethanes with1.15 g/cm3, the polyurethanes with methoxylatedpolyol with 1.116 g/cm3, and finally the hydroge-nated polyol-based polyurethane with 1.09 g/cm3.

Figure 1. Representative structure of soybean oil-based polyols: X 5 Br, Cl, CH3O, or H.

Figure 2. Schematic representation of crosslinking sites in different fatty acids.

SOY–POLYOL-BASED POLYURETHANES 4063

It was found that the density of a polyurethane inour study depended not only on the chemical com-position but also on the crosslinking density,which in the ideal case should be about the samein all our systems. However, polyurethane forma-tion from halogenated polyols was found to pro-ceed at slower rates and to require longer curing.Some loss of the chlorine (about 3%) occurredduring curing of the soy–HCl/PAPI. Also, a some-what different degree of crosslinking may haveresulted from the different functionalities of thepolyols (Table I).

Crosslinking density can be assessed from theswelling in good solvents. Higher swelling is theindication not only of a lower crosslinking densitybut also a higher solubility of the network chainsin a given solvent. Table I shows the swelling datafor crosslinked polyurethanes in toluene. The sol-

vent polymer interaction parameter, x, necessaryfor the calculation of Mc, was evaluated from thesolubility parameters of the solvent, d1, and thenetworks, d2:

x 5~d1 2 d2!

2V1

RT

where R is the gas constant, T is the absolutetemperature, and V1 is the molar volume of thesolvent. The solubility parameter for the polyure-thanes was calculated using Hoy values17 for themolar attraction constant, F, of the groupspresent in the polymer. Calculated solubility pa-rameters for the four polyurethanes were fairlyclose, at about 22 J1/2/cm3/2 (Table II). The toluenesolubility parameter was taken to be 18.2 J1/2/cm3/2. Calculated x values at 25 °C in toluenewere 0.733 for the soy–HCl/MDI system, 0.7016for soy–HBr/MDI, 0.5218 for soy–H2/MDI, and0.5293 for soy–Met/MDI.

The molecular weights of the network chains,Mc, were obtained from the Flory–Rehner rela-tionship:

Mc 5 @2r2V1~f21/3 2 2f2/f!#/

@ln~1 2 f2! 1 f2 1 x12f22#

where r2 is density of the network, V1 is the molarvolume of the solvent, and f2 is the volume frac-tion of the polymer in the swollen network. Thefunctionality of the network, f, was taken to be 4or 3.8. The results shown in Table II presume thatend-linked networks were prepared, which is notthe case here. This can be corrected by taking into

Table I. Degree of Swelling of Soy-BasedPolyurethane Networks in Toluene

Sample ID

Percent Swollen (%)

Density(g/cm3)Uncorrected

Correctedfor

InactivePart

Soy–H2/Isonate 51.3 77 1.090Soy–H2/PAPI 46.7 62 1.093Soy–Met/Isonate 48.1 54 1.116Soy–Met/PAPI 44.3 60 1.113Soy–HCl/Isonate 44.4 59 1.152Soy–HCl/PAPI 48.3 64 1.152Soy–HBr/Isonate 51.4 70 1.270Soy–HBr/PAPI 45.7 62 1.262

Table II. Solubility Characteristics of Soy-Based Polyurethane Networks in Toluene

Sample Name

SolubilityParameter, d2

(J1/2/cm3/2)x

(Polymer in Toluene) Mc

Mc Correctedfor Inactive Part

Soy–H2/Isonate 21.7 0.5218 356 (478) 728 (365)Soy–H2/PAPI 21.7 0.5218 308 (478) 494 (365)Soy–Met/Isonate 21.7 0.5293 340 (518) 412 (374)Soy–Met/PAPI 21.7 0.5293 297 (518) 492 (374)Soy–HCl/Isonate 22.3 0.7332 648 (524) 1539 (376)Soy–HCl/PAPI 22.3 0.7332 827 (524) 2103 (376)Soy–HBr/Isonate 22.3 0.7016 1073 (478) 3096 (391)Soy–HBr/PAPI 22.3 0.7016 775 (478) 1884 (391)

The values in brackets are calculated from the structure of the components assuming complete conversion and the use of pureMDI.

4064 PETROVIC, GUO, AND ZHANG

account the position of crosslinking sites (Fig. 2)and the molecular weight of the pending chains.The inactive part (pending chains) in the bromi-nated polyol constitutes 27% of the total mass,25.3% in the soy–Met polyol, 25.8% in the chlori-nated polyol, and 24.9% in the hydrogenatedpolyol. If the inactive part of the network is sub-tracted from the unswollen and swollen sampleweights, a new, corrected degree of swelling isobtained. In the case of complete reaction, thepredicted molecular weight of network chains,which include pendant side groups, was calcu-lated to be: Mc (Cl) 5 524; Mc (Br) 5 583, Mc (Met)5 518, and Mc (H2) 5 478. If, however, pendantchains are excluded, then the molecular weightsbecome: Mc (Cl) 5 376; Mc (Br) 5 391; Mc (Met)5 374, and Mc (H2) 5 365. The network chainshave two branches from the fatty acids and oneMDI unit. These values may vary slightly depend-ing on whether OH substitution was taking placeat position 1 or 2 of the double bond.

In the case of incomplete conversion, higherexperimental Mc values compared to the calcu-lated ones (in brackets) were expected. This wasthe case with both the soy–HCl- and soy–HBr-based networks. The other two networks ap-peared to have a lower Mc than theoretical value,even when correction for the inactive part wastaken into account. Taking the inactive part of thechains into account increases the experimentalMc and decreases the calculated values. There areseveral potential causes of the lower Mc values—the isocyanate has a higher functionality than thecalculated one, the calculated x parameter wasunderestimated, and, most important, the theorywas applied to relatively highly crosslinked sys-

tems consisting of short non-Gaussian chains,which may not be obeying Flory–Rehner theory.For “ideal” crosslinking–that is, the calculated Mcvalues from Table II— with the given experimen-tal degree of swelling, the x parameter in tolueneshould be 0.513 for the soy–HBr/MDI system,0.6843 for the soy–HCl/MDI, 0.6573 for the soy–Met/MDI, and 0.6154 for the soy–H2/MDI. Atthese small molecular weights a small variationin the interaction parameter causes a large rela-tive error in Mc. Therefore, the results show, inspite of all the problems, relatively good agree-ment between the theory and the experiment inall cases.

DSC curves of the polyurethanes are shown inFigure 3. They display a single feature: a glasstransition in the region between 30 °C and 80 °C.

The thermomechanical method (dilatometry)displayed typical volume relaxation during the

Figure 3. DSC curves of soy-based polyurethanescured with PAPI.

Figure 4. Thermomechanical curves of soy-basedpolyurethanes cured with PAPI.

Figure 5. Storage moduli of soy-based polyurethanescured with Isonate.


glass transition, expressed as a break on thecurves, even during repeated runs (Fig. 4). Thereported glass transition was taken as the onsetof the break on the curves. Coefficients of linearthermal expansion were measured 25 °C aboveand 25 °C below the glass transition for all net-works.

The storage moduli and the loss moduli of thefour polyurethane networks crosslinked withPAPI, calculated from dynamic mechanical ther-mal analysis (DMTA), are displayed in Figures 5and 6, respectively. The lowest glass transitionwas observed in the polyurethane with the hydro-genated polyol, and the highest was seen in thechlorinated polyol.

Crosslinking density affects glass transition ofthe networks. Glass transitions measured by sev-eral methods are given in Table III, which showthat irrespective of the Tg measurement method,hydrogenated polyol produces polyurethanes withthe lowest Tg, at 30–40 °C, while chlorinatedpolyol produces polyurethanes with the highestglass transition, about 77 °C, followed closely bythe brominated polyurethanes and a somewhat

lower Tg for the methoxy-containing polyure-thanes. Assuming an analogous situation to vinylpolymers, in which PVC has a glass transitionabout 20 °C lower than that of poly(vinyl bro-mide), a higher Tg would be expected in our bro-minated polyurethanes at the same degree ofcrosslinking.

It should be pointed out that the reaction ofhalogenated polyols with isocyanates is slowerand does not continue to completion, as shown bythe presence of a small isocyanate peak in thespectra of cured polyurethanes. This offsets asomewhat higher functionality of the brominatedand chlorinated polyols. The molar volume of bro-mine is larger than that of chlorine, which issomewhat larger than that of the methoxy group.It appears that these groups introduce strain inthe crosslinked networks and perhaps reduce thefree volume of the chains. The lower values for theTg of the brominated polyurethanes, as comparedto the chlorinated ones, may be partially relatedto the loss of some bromine during curing. As willbe shown later, the thermal stability of the bro-minated polyurethanes is significantly lower thanthat of other soy-based polyurethanes.

The linear coefficients of thermal expansion(LCTE) follow the changes in density closely—that is, the lowest LCTE both below and above theglass transition were found in brominated poly-urethanes, followed by chlorinated polyure-thanes, and then methoxy-containing polymers(Fig. 7). The highest LCTE was found in polyure-thanes with hydrogenated polyols. A typicalLCTE for glassy polymers is around 60 3 1026

K21, while our networks are generally above thatvalue, except for the brominated PU. On the otherhand, a typical rubbery LCTE is about 250 3 1026

K21, while our polymers displayed somewhatlower values in the case of the soy–Met/PAPI andsoy–H2/PAPI systems and considerably lower val-ues in the case of halogenated polyurethanes.

Figure 6. Loss moduli of soy oil-based polyurethanescured with Isonate.

Table III. Glass-Transition Temperatures of Soy-Based Polyurethanes Cured with PAPI and Isonate from DSC,TMA, and DMTA

PolyolOH Number(mg KOH/g) Functionality

DSC Tg (°C) TMA Tg (°C) DMTA Tg (°C)

Isonate PAPI Isonate PAPI Isonate PAPI

Soy–H2 212 3.5 34 31 46 46 38 39Soy–Met 199 3.7 70 72 57 65 66 71Soy–HCl 197 3.8 73 77 75 76 80 88Soy–HBr 182 4.1 68 75 80 68 76 77


Lower values may result from increasedcrosslinking density, but the variance among dif-ferent polyurethanes comes primarily from differ-ences in free volumes of substituent groups.

Mechanical Properties of Polyurethane Networks

Tensile and flexural properties of polyurethane net-works are given in Table IV. Tensile strengths of allpolyurethane samples fall in the same range, be-tween 40 and 50 MPa, except for the polyurethanesbased on hydrogenated polyol, which display abouthalf that strength. Young’s modulus was deter-mined from the initial slope of the stress–straincurve. The stress–strain curves of PAPI-curedpolyurethanes are given in Figure 8.

The strength of different samples dependsstrongly on the curing time and sample preparationtechnique. In some instances, for example, the ten-sile strength of soy–HBr/PAPI samples reached avalue of 55 MPa. The samples tested immediatelyafter preparation displayed somewhat lower values.

The elongation at break of hydrogenated poly-urethanes was about 20% versus less than 10%for all other networks. Thus, hydrogenated polyolproduced softer polyurethanes (of a lower modu-lus) of lower strength but higher elongation thanother polyols. Although the hydrogenated polyolhad a somewhat lower functionality (3.5 vs. 3.8and 4.1), it appears that lower intermolecularattraction in these networks, which was respon-sible for the lower density, also had a negativeeffect on strength. The principal reason for thelower mechanical strength of the polyurethanesthat were based on the hydrogenated polyol istheir low glass transition, which is close to thetesting temperature of 25 °C. The hydrogenatedpolyurethanes were tested in their leathery re-gion, while all other samples were tested in theglassy region. Glassy polyurethanes havestrengths similar to amine-cured bisphenol A ep-oxy resins, but they have a somewhat lower mod-ulus and higher elongation at break, reflecting aslightly lower crosslinking density. It is interest-

Figure 7. Linear thermal expansion coefficients ofsoy-based polyurethanes cured with PAPI measured at50 °C below and above their glass transitions.

Table IV. Tensile Strength of Soy-Based Polyurethanes

Sample IDDSC Glass

Transition (°C)Tensile Strength

(MPa)Elongation at

Break (%)Young’s Modulus

(MPa)

Soy–HBr/PAPI 75 44 7.7 1102Soy–HBr/Isonate 68 40 7.3 955Soy–HCl/PAPI 77 48 7.5 1204Soy–HCl/Isonate 73 46 8.9 1190Soy–Met/PAPI 72 45 8.4 986Soy–Met/Isonate 70 46 9.0 979Soy–H2/PAPI 31 19 29 383Soy–H2/Isonate 34 16 15.4 362

Figure 8. Stress–strain curves for soy-based polyure-thanes cured with PAPI.


ing that the comparative tests on glass-reinforcedcomposites with these polyurethanes compared toepoxy resins showed that the polyurethane com-posites had somewhat higher strengths than didthe epoxy composites.

Thermal Stability of Oil-Based Polyurethanes

Polyurethanes prepared from soybean oil gener-ally have better thermal stability in air thanpoly(oxypropylene)-based polyurethanes.4,15 Fig-ures 9 and 10 show, respectively, thermogravi-metric (TGA) curves in nitrogen and in air ofPAPI-cured polyurethanes obtained at a heatingrate of 10 °C/min. Figure 9 shows that soy–Met–PAPI and soy–H2–-PAPI have comparable ther-mal stability in nitrogen, but both are better thanthe soy–HCl–PAPI sample. Soy–HBr–PAPI, whichstarted losing weight well below 200 °C, alreadyhad lost 30% of its weight by 280 °C, versus 14% forthe chlorinated polyurethane and 2–3% for the

other two at the same temperature. The first kneeon the curves occurred at about 200 °C for thebrominated sample, at about 250 °C for the chlo-rinated sample, and at about 300 °C for the othertwo samples. The initial stages of degradation inair were not very different from those in nitrogen,and the order of thermal stabilities was preserved(Fig. 10). Polyurethane degradation usuallystarts with the dissociation of the urethane bondand carbon dioxide and isocyanate evaporation.In the case of the brominated and chlorinatedpolyurethanes it is clear that the initial weightloss is the result of bromine and chlorine dissoci-ation, similar to the loss of chlorine in PVC.

Dielectric Properties Polyurethanes ofSoy–Polyurethanes

The permittivity, «9, and the loss factor, «0, wererecorded as functions of temperature and fre-quency. Figures 11 and 12 show the change of per-mittivity and loss factor for soy–H2/PAPI. The per-mittivity of all samples in the unrelaxed (glassy)state was typically between 2.5 and 3, somewhatlower at higher frequencies. In Soy–H2-based poly-urethanes permittivity in the relaxed state (aboveTg) would reach a plateau value of 6 at 1 Hz andsomewhat lower values at higher frequencies. Theactivation energy of the glass transition was calcu-lated from the shift in the temperature of the max-ima on the loss factor–temperature curves (Tg) withfrequency, using the relationship:

lnf 5 lnf0 2 E/RT

where f is frequency, R is the gas constant, and T isabsolute temperature. Values of 181 and 182 kJ/mol

Figure 9. TGA curves in nitrogen of soy-based poly-urethanes cured with PAPI.

Figure 10. TGA curves in air of soy-based polyure-thanes cured with PAPI.

Figure 11. Change of permittivity of soy–H2/PAPIpolyurethane with temperature and frequency.


were obtained, respectively, for the soy–H2/PAPIand the soy–H2/Isonate polyurethanes. These val-ues are about 40 kJ/mol higher than that found forthe glass transition of the polypropylene oxide softsegment with an Mn of 2000 in segmented polyure-thanes with 70% hard segment concentration. Thisreflects both a lower flexibility of the hydrocarbonchain in oils and a higher crosslinking density (low-er Mc) in oil-based networks. Two soy–Met polyolshaving OH numbers 180 and 206 mg KOH/g werecrosslinked with PAPI 2901, producing polyure-thanes of a different crosslinking density. The acti-vation energy of the lower crosslinked polyurethane(with soy–Met 180) gave an activation energy of 183kJ/mol, while the other, based on a polyol with anOH number of 206, gave an activation energy of253 kJ/mol, indicating that crosslinking density isthe determining factor in the mobility of networkchains.

Polyurethanes based on halogenated polyolsdisplayed color changes when heated at elevatedtemperatures, presumably due to loss of halogen,which affected the electrodes and may have con-tributed to increased ionic conductivity. Permit-tivity in soy–HCl- and soy–HBr-based polyure-thanes does not reach a plateau value above theglass transition but tends to increase continu-ously, especially at lower frequencies (1–100 Hz).Loss factor in these polyurethanes also showed alarge increase above the glass transition, withoutevident peaks. Such behavior was attributed toionic conduction as a result of impurities.

CONCLUSIONS

Four series of polyurethanes were prepared fromthe brominated, chlorinated, hydrogenated, and

methoxylated epoxidized soybean oil and two di-isocyanates. Both isocyanates produced polyure-thanes with comparable properties. The thermalstability of the brominated polyurethane was thelowest, followed by that of the chlorinated poly-urethanes, while the methoxy-containing and hy-drogenated polyols had the highest stability.

Polyurethanes with brominated and chlori-nated polyols had comparable glass transitionand strength, somewhat higher than in the me-thoxy-containing polyol-based polyurethane. Atroom temperature polyurethanes from the hydro-genated polyol had a lower glass transition andmechanical properties.

REFERENCES AND NOTES

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Figure 12. Change of the loss factor of soy–H2/PAPIpolyurethane with temperature and frequency.


structure and properties of polyurethanes based on halogenated and nonhalogenated soy–polyols

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