Polymer Degradation and Stability 87 (2005) 555e561

www.elsevier.com/locate/polydegstab

FTIR analysis of thermally processed PU foam

David W. Hatchetta,*, Gayani Kodippilia, John M. Kinyanjuia,Flocerfida Benincasaa, Linda Sapochakb

aDepartment of Chemistry, University of Nevada, Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas, NV 89154-4003, USAbPacific Northwest National Laboratory, Richland, WA 99352, USA

Received 15 September 2004; received in revised form 25 October 2004; accepted 31 October 2004

Available online 15 December 2004

Abstract

The chemical composition, density, and modulus of PU foam was examined as a function of processing temperature (25 �C,45 �C, and 85 �C). Degradation of uretoneimine cross-links and the emergence of carbodiimide functional groups are monitoredand the remaining composition in the PU foam is determined using FTIR spectroscopy. The density and modulus of the PU foams

after thermal processing are measured and discussed with respect to the change in chemical composition of the materials. The dataindicates that the composition of chemical cross-links, the density, and the modulus all decrease as a function of increasingprocessing temperature. The modulus of the PU foam shows a linear dependence of the processing temperature decreasing by 20%

at the highest temperature. The results suggest that processing temperature may be used to target defined physical and mechanicalproperties of PU foams without changing the composition of reactants used.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Polyurethane; Uretoneimine; FTIR

1. Introduction

Polyurethane foams have found applications inindustry as general use, low-density, insulating materi-als. Applications include construction panel sandwichstructures, cushions and insulation panels for a varietyof appliances including refrigerators [1,2]. The materialis versatile and can be produced with open-cell structureto be more flexible or a close-cell structure to bemore rigid. The desired application dictates the degreeof rigidity required for the material to be useful. The useof rigid polyurethane foams to encapsulate electroniccomponents to mitigate harsh thermal and mechanicalshock and to provide electrical isolation is common. PU

* Corresponding author. Tel.: C1 702 895 3509; fax: C1 702 895

4072.

E-mail address: dahatchet@ccmail.nevada.edu (D.W. Hatchett).

0141-3910/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2004.10.012

foams are also used to encapsulate our nationalstockpile of nuclear weapons. The deterioration ofshock mitigation properties in the foams continues tobe a major concern in systems that undergo thermal andradiative exposure over time. Studies that characterizethe change in chemical properties of PU foams asa function of processing temperature, curing, and agingthat are non-destructive are important to distinguishshort term curing and long term aging characteristics.Moreover, correlating the resulting chemical changes ofthe polymer with the mechanical properties of thematerial will allow the stability of the materials to beexamined for extended time periods.

The formation of polyurethane foams follows thesame general reaction of isocyanate with polyol asshown in Scheme 1. The reaction demonstrates the basicstructural backbone of the polyurethane polymer.However, structural cross-links and microstructureswithin this system can provide additional structure

556 D.W. Hatchett et al. / Polymer Degradation and Stability 87 (2005) 555e561

within the polymer matrix. Typical R# groups of thepolyol include polyether and polyester units. Industri-ally important diisocyanates include 2,4- and 2,6-toluenediisocyanate (TDI), 4,4#-methylene diphenyl diisocya-nate (MDI), and 1,6-hexamethylene diisocyanate (HDI).The R groups can be aromatic rings (increase rigidity) oralkyl chains (increase flexibility). More flexible R and R#groups and low functionality of the isocyanate andpolyol favors the formation of more flexible foams. Incontrast, rigid R and R# groups and high functionalityfavors more rigid foams. Rigid foams are uniquebecause they have the highest insulating value of anyconventional insulation material commercially availabletoday. In addition, PU foams have very low thermalconductivity; gases trapped in the closed cell structurereduce heat transfer by conduction [3,4]. The complexchemistry of PU foam is based primarily on thereactivity of the isocyanate groups within the foammatrix.

The reaction of PU foams can be categorized by thesimultaneous reaction of multifunctional isocyanate andpolyols (gel reaction) coupled with a gas liberationreaction (foaming reaction). The foam can be waterblown through the formation and decomposition ofcarbamic acid groups to form a primary amine andcarbon dioxide or, blown with hydrocarbons [4e7]. Ineach case the process of gas evolution results in theexpansion of the polymer foam in the mold. In addition,the diisocyanate can further react forming additionalfunctional groups within the PU foams such as urea,allophonate, biuret, carbodiimide, uretidinedione, iso-cyanurate and uretoneimine structures [8,9]. We areparticularly interested in monitoring the formation ofuretoneimine structures and the effect of thermalprocessing on these structural units within the foammatrix [10].

A method for introducing the uretoneimine structureinvolves the use of liquid 4,4#-methylenediphenyldiisocyanate (MDI), a commonly used isocyanate inpolyurethane formation. MDI was developed as a re-placement material for toluene diisocyanate (TDI) dueto the hazardous nature of TDI. The modified pureMDI can be by reacted to form carbodiimide structureswith excess isocyanate in the system yielding a crosslinked uretoneimine structure with three un-reactedisocyanate groups, illustrated in Scheme 2. Thiscompound mixed with pure MDI results in a liquid

O C N R N C O

Diisocyanate

+HO R' OH

HN C OO

R'Rx

Polyol

Scheme 1. The general reaction of dissocyanate with polyol to form

polyurethane linkages.

carbodiimide modified MDI, which is stable and slightlyviscous, with a melting point below 20 �C. Theuretoneimine four member ring are incorporated intothe PU foam matrix as rigid structures that thermallydegrade at temperatures greater than 40 �C. Thistemperature dependence can be utilized adjust theconcentration of uretoneimine cross-link structurespresent in the PU foam.

Common physical and mechanical parameters havebeen the focus of previous studies of PU foams in ourlaboratory. We have focused on the density, modulus,and collapse strength of PU foams and how theseparameters are influenced by the reaction and processingconditions employed. The temperature dependence is inpart related to changes in the complex chemicalmicrostructure of the PU foam. The chemical reactionswithin the PU matrix can be influenced by parameterssuch as chemical constituents and composition, moldgeometry, and processing conditions. Processing con-ditions are unique because they influence the complexchemical reactions of the PU foam during the reactionand curing processes. There are two important questionsregarding the influence of processing temperature on PUfoams. First, do the chemical differences that result fromusing modified MDI and processing the PU foaminfluence the chemical microstructure? Finally, do thedifferences cause an observable change in the physicaland mechanical parameters such as the density andmodulus?

The purpose of this study is to examine the effect ofprocessing on the physical, chemical and mechanicalproperties of PU foam containing preformed uretonei-mine functional groups. The foam is processed at

CH2

N C N CH2

NCOOCN

OCN CH2

NCO

HeatP

O CH2OH

Carbodiimide Modified MDI

CH2

N C N CH2

NCOOCN

C N

OCN CH2

NCO

CH2

NCOO

Uretoneimine Modified MDI

Scheme 2. Reaction of 4,4#-methylenediphenyl diisocyanate (MDI) to

form the carbodiimide linkage between two MDI groups. Reaction to

form the uretoneimine cross-link structure with three un-reacted

isocyanate groups. This is the ‘rubinate’ form of MDI.

557D.W. Hatchett et al. / Polymer Degradation and Stability 87 (2005) 555e561

different temperatures (25 �C, 45 �C, and 85 �C) and thechemical composition is examined with respect to un-reacted isocyanate, the formation of carbodiimidefunctional groups and the corresponding change inuretoneimine and urethane linkages. The temperatureschosen encompass a temperature regime where ureto-neimine is stable (25 �C), 5 �C above the knowndegradation temperature for the uretoneimine linkages(45 �C), and a temperature well past the degradation ofuretoneimine (85 �C). Common physical and mechan-ical parameters such as density and modulus are alsoexamined to determine whether these properties can berelated to the chemistry changes in the foamed samplesafter processing at different temperatures. The primarymode of chemical analysis utilized is photoacoustic(PAS)/FTIR analysis, which is non-destructive so thatthe same sample can be examined over extended periodsof time.

2. Experimental section

2.1. Chemicals and solutions

The PU foam was prepared using Voranol 490 (DowChemical Company, Midland MI), DC 193 Surfactant(Dow Corning Corporation Taylor MI), Polycat 17amine catalyst (Air Products, New Milford CT),distilled water, and rubinate 1680 (Huntsman Polyur-ethanes, Geismar LA). The mass of each componentwas selected to produce a target foam density of 0.1 g/cc.

2.2. Polyurethane foam synthesis

A Model D700 polyurethane two-reactant mixingsystems by Decker Industries was used to automaticallymix and dispense the components used to formulate thePU foam. The Voranol, amine catalyst, surfactant andwater were pre-mixed at a ratio (by volume) of 86.96:2.03: 0.55: 1.00 respectively to make up a two-and-halfgallon batch. This mixture was then added to the ‘poly’compartment of the mixing system. An equal volume ofrubinate was added to the ‘iso’ compartment.

During the dispensing process, the components weremetered, mixed and injected into eight 28.7 mm innerdiameter cylindrical aluminum molds with a typical shottime of about 2 seconds. All eight of the cylindricalmolds were 143 mm tall. The molds were open on thetop so the foam could rise freely. A sufficient amount ofthe formulation was poured into each mold to producea small mushroom on the top of each cylinder. All sixcylinders bottoms were threaded with screw-on caps,fitted with o-rings and Teflon tape to prevent the foamfrom seeping out. Mold release (PTM and W Industries#PA0801) was applied to the inside of each cylindricalmold to help in extracting the foam once it was cured.

Three sets of samples were processed at controlledtemperatures of 25 �C, 45 �C, and 85 �C for a period of30 minutes. For these samples, the rising foamcompletely filled the cylindrical mold and mushroomshape was formed at the top. Post-curing of all foamsamples was conducted in a recirculating air oven at 66 �Cfor four hours, after which samples were extractedfrom the molds by unscrewing the tube cap andpushing lightly on the bottom end of the foam.

Using a band saw, the mushroom top was removedfrom each molded foam samples and then the remainderwas cut into four sections. To assure sample consistencyeach sample was cut and sanded to a height of 25.4 mmfor each section and a volume of 16.4 g/cm3. Thepolyurethane foam samples were characterized bydensity gradient measurements, photoacoustic FTIRspectroscopy, and to determine the modulus of thematerial.

2.3. Density analysis

The weight of the cylindrical sections was determinedusing a analytical balance (G0.0001 g). The weight andthe volume of each section were then used to calculatethe density. The average density for each processingtemperature was calculated by averaging the densities ofa minimum of 50 sections.

2.4. FTIR spectroscopy

All FTIR measurements were performed usinga BIO-RAD FTS-7000 spectrometer using a photo-acoustic detector (MTEC). Each sample was scanned 64times with a resolution setting of 4 cm�1 and averagedto produce each spectrum. To ensure consistency, allsamples used in the FTIR measurement were removedfrom the second section from top of the molded PUsamples. The data were treated using the techniquedescribed by Bhattacharjee et al. for the integration andquantitation of isocyanurate and isocyanate conversionin FTIR spectra [11].

2.5. Mechanical analysis

Compression test were performed to obtain themodulus in accordance with ASTM D1621 standardon the cylindrical sections using a United Testing(UTM) SSTM-1 frame with a 4.4 kN load cell. Tominimize variance, an equal number of sections fromeach vertical position in the PU cylinders were tested.

558 D.W. Hatchett et al. / Polymer Degradation and Stability 87 (2005) 555e561

3. Results and discussion

3.1. Photoacoustic spectrum (PAS/FTIR)of rubinate

FTIR/PAS spectroscopy is a useful in identifyingchemical species and functional groups in a variety ofcomplex samples. The main goal of the IR study is toidentify the fundamental groups in the PU foam anddetermine the effect of the processing temperature on thechemistry of the system [11e13]. Many functionalgroups give characteristic IR absorption at specific andnarrow frequency ranges and FTIR/PAS is useful in thequalitative and quantitative analysis and determinationof the composition and chemical functional groups inthe PU foam. In addition, PAS/FTIR spectra areobtained using pristine PU foam samples withoutphysical alteration of the samples (i.e. no grinding andmixing with KBr).

As stated, rubinate, voranol, silicone surfactant andamine catalyst are utilized as starting materials in thesynthesis of PU foam. Although the foam is a complexmixture of these components the rubinate and voranolare the key constituents in the reactivity and thefunctionality of the PU foam. The FTIR/PAS spectrumof rubinate is shown in Fig. 1a. The spectrum is a useful

Fig. 1. a. FTIR/PAS of rubinate at 25 �C. b. FTIR/PAS of rubinate at

85 �C. c. Difference spectrum between rubinate processed at 25 �C and

85 �C. Uretoneimine, carbodiimide and isocyanate are labeled for

reference.

in identifying the IR bands that arise from key chemicalgroups in the un-reacted rubinate. The functionalgroups identified in rubinate can also be used to identifycommon functional groups that appear in the PU foam.We will use this data as a guide in the identification ofbands in the IR spectrum of the PU foam. The rubinateis a mixture of partially reacted MDI with uretoneiminechemical cross-links and un-reacted MDI. The ureto-neimine uretoneimine ring structure (1379 cm�1) in thereacted MDI is shown in Fig. 1a. The un-reactedisocyanate (NCO) groups associated with the uretonei-mine structure, as well as un-reacted MDI appear in thespectrum at 2269 cm�1.

Uretoneimine exists as a thermally reversible four-member ring structure in MDI. Thermal decompositionof this bond occurs above 40 �C. The decomposition ofthe uretoneimine ring is signaled by a decrease in theuretoneimine carbonyl peak at 1726 cm�1, and CeNstretch peak at 1379 cm�1. The decrease of theuretoneimine carbonyl peak is minimized due overlapof other bands associated with urethane and esterstructures. At higher processing temperature, thethermally unstable uretoneimine degrades giving equalmolar quantities of isocyanate and carbodiimide (dou-blet at 2136 and 2100 cm�1) shown in Fig. 1b fora sample of rubinate heated to 85 �C. The carbodiimideformed from the uretoneimine degradation is a stablefunctional group that does not react appreciably athigher processing temperature. The result is a highintensity peak at higher processing temperatures thatdoes not diminish in comparison to isocyanate, whichcan continue to react over time.

To accentuate changes after heating the rubinate to85 �C the difference spectrum take between the rubinateat room temperature and 85 �C is provided in Fig. 1c.The spectrum shows that the carbodiimide peaks emergeas a direct result from the thermally initiated dissocia-tion of uretoneimine ring structures. The FTIR bandwith negative intensity at 1726 cm�1 and 1379 cm�1

indicate that the uretoneimine structure diminishes asa function of processing temperature. Integration of theuretoneimine band indicates that the ring structuredecreases by approximately 60%. In addition, theemergence of the carbodiimide functionality is observedas an increase in the band intensities at 2136 cm�1 and2100 cm�1. Finally small increases are observed in bandsassociated with the ring phenyl ring structures and CeNgroups in the rubinate. In contrast very little change isobserved in the spectral range between 800 and 1200cm�1 for the CeN, CeOeC and CeH bands.

3.2. Photoacoustic spectrum (PAS/FTIR)of polyurethane foam

Analysis of the FTIR spectrum of rubinate is useful indetermining the FTIR band assignments for the PU foam

559D.W. Hatchett et al. / Polymer Degradation and Stability 87 (2005) 555e561

[14,15]. In addition the thermally transient functionalgroups can be identified and monitored in the spectra asa function of processing temperature (25 �C, 45 �C, and85 �C). The PAS/FTIR spectra of PU foams processed at25 �C, 45 �C, and 85 �C, are presented in Fig. 2aec,respectively. The un-reacted isocyanate (2279 cm�1),carbodiimide (2136 and 2100 cm�1), and uretoneimine(1379 cm�1) are labeled in Fig. 2 for clarity.

The trends observed for rubinate in Fig. 1 can also beobserved for PU foams processed at elevated temper-atures. At 25 �C the amount of un-reacted isocyanateand uretoneimine functional groups is the largest inFig. 2a. The amount of un-reacted isocyanate and thecomposition of uretoneimine decrease as the tempera-ture reaches 45 �C, Fig. 2b. There is little difference interms of the FTIR response of the 45 �C and 85 �Cindicating that both contain comparable amounts of theun-reacted isocyanate and uretoneimine functionalgroups. The decomposition of the uretoneimine can beverified with increasing carbodiimide functional groupsin the foam. The amount of carbodiimide is fairly smallat 25 �C increasing rapidly for the 45 �C and 85 �Cprocessed PU foams. Comparison of the remaininguretoneimine and carbodiimide by integrating the FTIRbands in each spectrum provides a direct comparison ofthe samples in Fig. 3. If we assume that the integrated

Fig. 2. a. FTIR/PAS of PU foam at 25 �C. b. FTIR/PAS of PU foam

at 45 �C. c. FTIR/PAS of PU foam at 85 �C. Uretoneimine,

carbodiimide and isocyanate are labeled for reference.

area of the uretoneimine group in the PU foamprocessed at 25 �C represents 100% composition, wefind that there is a 45%G3% and 42%G4% decreasein the uretoneimine group at 45 �C and 85 �C,respectively. These values are the same within theexperimental error. Conversely the opposite trend isobserved for the carbodiimide functional group inFig. 3. The integrated area changes by 90% relative tothe initial integrated area for the carbodiimide speciesfor the PU foam processed at 25 �C.

3.3. Density of polyurethane foams processedat 25 �C, 45 �C, and 85 �C

The density of PU foams can be influenced bya number of parameters. Some of the parametersincluding experimental conditions and composition ofreactants can be controlled. However, the foamingprocess is primarily dependent on the amount ofblowing agent added to the system (water), thetemperature of the system, and the viscosity changeresulting from the gel reaction [1]. When the tempera-ture is sufficiently high to degrade thermally sensitivechemical cross-links such as uretoneimine the viscosityof the system is lower and the foam to rises morerapidly. Therefore the foam rise results in a material thatis less dense than the room temperature sample.

The densities of the PU foam samples with identicalvolumes, processed at 25 �C, 45 �C, and 85 �C arepresented in Fig. 4. The results indicate that the densityof the PU foam processed at 25 �C is the highest withvalues on the order of 0.147 g/cm3 with a relativestandard deviation of 2%. This value is higher than thetarget density of 0.100 g/cm3. In contrast the density forPU foam processed at 45 �C and 85 �C decrease to

0.15

0.2

0.25

0.3

0.5

1

1.5

2

2.5

30 40 50 60 70 80

Inte

grat

ed A

rea

(a.u

.)

Temperature (°C)

Uretoneimine

Carbodiamide

Fig. 3. Integrated FTIR/PAS bands from the PU foams presented in

Fig. 2aec. The arrows designate the corresponding y-axis for each

species.

560 D.W. Hatchett et al. / Polymer Degradation and Stability 87 (2005) 555e561

values of 0.124 g/cm3 and 0.110 g/cm3 with relativestandard deviations of 3% and 2%, respectively. Thepercent decrease in density for the PU foams processedat 45 �C and 85 �C is on the order of 16% and 25%,respectively. The results indicate that processing the PUfoam causes an increase in void volume in PU foamsamples. The increase can be attributed to the increasedthermal activity of the PU foam at higher temperaturesand a decrease in chemical cross-links in the material.More importantly, the data suggest that processingtemperature may be an effective way of producing PUfoams with pre-defined densities without altering thechemical composition of reactants. The range ofdensities shown here provides a method for decreasingthe density up to 25% without changing the amount ofchemical reactants in the system.

3.4. Modulus of polyurethane foams processed at 25 �C,45 �C, and 85 �C

The chemical and physical changes associated withthe decrease in uretoneimine structures and density inthe PU foam suggests that the materials might also havedifferent mechanical properties after processing atelevated temperatures. The modulus is a mechanicalparameter obtained from measuring the compression ofa sample as a stress is applied. The value for themodulus of the PU foam is obtained from the slope ofthe plot prior to the collapse of the PU foam. Previousstudies indicate that the mechanical properties of foamsare dependent on the rigidity of the foam, the density,and degree of chemical cross-links [1]. We have shownthat the degree of uretoneimine cross-links decrease asa function of increasing processing temperature. Wehave also shown that the density decreases as a functionof temperature. Therefore we expect a correspondingdecrease in the rigidity and modulus of the PU foam.

0.11

0.12

0.13

0.14

0.15

30 40 50 60 70 80

Temperature (°C)

Den

sity

(g/

cm3 )

Fig. 4. Density of PU foams plotted as a function of processing

temperatures of 25 �C, 45 �C, and 85 �C.

The modulus as a function of processing temperatureis plotted in Fig. 5. In contrast to the plots of densityand the change of chemical composition of uretoneimineas a function of processing temperature, we find that themodulus is linearly dependent on the temperature. Theregression coefficient for the plot is 0.998 with relativestandard deviations for each data point below 2%. Thelargest magnitude change is observed at 85 �C witha decrease in modulus by 20%. For comparison, thechange in modulus of 7% is observed when PU foam isprocessed at 45 �C. The difference between the PUfoams processed is related to differences in densityrather than chemical composition. We found littledifference between the relative chemical composition ofuretoneimine linkages in the samples processed at 45 �Cand 85 �C. However, there are a greater number ofuretonemine linkages in the more dense PU foamprocessed at 45 �C.

4. Conclusions

Thermal processing of PU foams at 25 �C, 45 �C, and85 �C results in materials with a lower number ofchemical cross-links, density, and modulus. The ther-mally initiated dissociation of uretoneimine structuresand formation of carbodiimide groups was monitoredand quantitated using FTIR spectroscopy. This conver-sion signifies the loss of chemical cross-links in thesystem. The results indicate that the density and moduluscan be reduced by a factor of 25% and 20%, respec-tively, for PU foams processed at 85 �C. The modulusshowed a linear dependence on the processing tem-perature with a regression coefficient of RZ0.998. More-over, the data indicates that processing temperature is aneffective parameter in the targeted formation of PU

11000

12000

13000

14000

15000

30 40 50 60 70 80

Mod

ulus

(ps

i)

Temperature (°C)

Fig. 5. Modulus of PU foams plotted as a function of processing

temperatures of 25 �C, 45 �C, and 85 �C.

561D.W. Hatchett et al. / Polymer Degradation and Stability 87 (2005) 555e561

foams with pre-determined chemical composition, den-sity and rigidity.

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