Thermal degradation of urethane modified polyisocyanurate foamsbased on aliphatic and aromatic polyester polyol

E. Dominguez-Rosadoa,*,1, J.J. Liggata, C.E. Snapeb, B. Elingc, J. Pichteld

aDepartment of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UKbSchool of Chemical, Environmental and Mining Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK

cHuntsman Polyurethanes, Everslaan 45, B-3078 Everberg, BelgiumdDepartment of Natural Resources and Environmental Management, Ball State University, Muncie, IN 47306, USA

Received 23 August 2001; received in revised form 11 February 2002; accepted 5 March 2002

Abstract

Combustion of polyurethane foams releases toxic gaseous products. Therefore, decreasing the flammability of polyurethane

foams is of practical significance to public health and the environment. The reported study investigated the thermal stability ofurethane modified polyisocyanurate foams based on the presence of aromatic, aliphatic polyester polyol and polyether polyol moieties.Thermogravimetric analysis and differential scanning calorimetry demonstrated that the foam containing the lowest isocyanate

index (220) and the lowest molecular mass of polyether polyol (200) was the most flammable (35% of char residue). Furthermore,the foams which contained a high molecular mass of polyether polyol (2000) and high isocyanate index (460) experienced fire per-formance (45% of char residue) similar to those foams containing aliphatic and aromatic polyester polyol (41 and 44% of char

residue respectively). # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Flammability; Thermal degradation; Polyurethane; Polyisocyanurate; Polyester polyol

1. Introduction

Polyurethane (PU) foams are used widely for insu-lation in construction, transportation and industrialapplications, for example, refrigerators, freezers, cavitywalls, floor panels and roofing materials [1]. Theflammability of these products affects both public healthand the environment. Since the mid-1970s the urethaneindustry has been subject to increasingly stringentflammability standards for foams used in construction[2]. Safety standards, such as the Japanese BuildingCode, require not only flame retardancy but also alowering of smoke generation and associated emissionsof toxic gases. Similar regulations have been imple-mented in the US and Europe [3].

During polymer combustion, there is a relationshipbetween char formation and flammability. The fireperformance of insulating foams can be improved bypromoting the formation of a carbonaceous layer, i.e.,char, during combustion. The char can serve as a solidinsulating layer which may protect the residue from theheat of combustion, limit the access of oxygen to the poly-mer, retard degradation, and reduce smoke production [4].Char formation is enhanced by modifying the struc-

ture of reactive groups on the polymer. Halogenatedflame retardants such as chlorinated or phosphorus-containing polyols used as reactive hydroxyl-containingcompounds successfully reduce the flammability of PUfoams. However, during combustion, smoke andenvironmentally unfriendly toxic gases are produced,the most significant being carbon monoxide. Otherproducts are carbon dioxide, nitrogen oxides, ammonia,benzene, toluene, acetaldehyde, alkenes and tracehydrogen cyanide [5].Pure polyisocyanurate foams offer a means to

improve flammability resistance of PU foams. The iso-cyanurate linkage possesses an inherently higher ther-mal stability than that of the urethane linkage (the latter

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00086-1

Polymer Degradation and Stability 78 (2002) 1–5

www.elsevier.com/locate/polydegstab

* Corresponding author. Fax: +1-765-285-2606.

E-mail address: elendominguez@yahoo.com

(E. Dominguez-Rosado).1 Current address: Department of Natural Resources and Envir-

onmental Management, Ball State University, Muncie, IN 47306,

USA.

dissociates at approx. 200 �C as opposed to 350 �C forthe polyisocyanurates) [6]. However, in spite of thehigher flame resistance, pure polyisocyanurate foamssuffer from excessive friability, thus limiting their prac-tical utility. Combining urethane and isocyanurate lin-kages in the foam composition addresses both problemssimultaneously—the urethane component provides thedesired physical characteristics while the isocyanuratethe needed flame retardancy.The molar ratio of isocyanate/polyol (NCO/OH) also

plays an important role in determining the structure andthermal properties of these foams [7], which can be for-mulated with polyether polyol or polyester polyol. Gaoet al. [8] study the fire performance of rigid poly-urethane foams based on polyether polyol (average ofmolecular mass=425) with different proportions of iso-cyanate. The polyether polyol component was the pri-mary site of the thermal degradation process. Severalstudies have focused on gaseous products from com-bustion of PU products [9–11]; however, because of thehighly cross-linked structure of the char [3,12] formedeven at low temperatures, detailed chemical analysis ofthis residue proves difficult.Flammability of urethane modified polyisocyanurate

foams based on low and high molecular mass of polyetherpolyol with a fixed proportion of isocyanate was studiedby Cunningham et al. [13]. Foams with the highest poly-ether polyol content experienced the highest values oflimiting oxygen index (LOI) and, therefore, the lowestflammability. These results are confirmed by Dick et al.[14] who investigated the flammability of the same seriesof foams and the properties of the resultant carbonaceouschar. They found that foams based on polyether polyoldid not exhibit different behavior under oxidative condi-tions. However, significant differences were determinedwhen experiments were carried out under nitrogen.Polyester-based urethane foams require less flame retard-ing additive than polyether foams to achieve the samedegree of fire retardancy [15]. The polyether foams areless thermally resistant because of the oxidative cleavageof the polyether structure at high temperature [16].The objective of the current study is to determine the

flammability of the rigid urethane modified poly-isocyanurate foams (UMPIRs) based on aliphatic andaromatic polyester polyol and compare their flamm-ability behavior with those based on polyether polyol.

2. Experimental methods

Imperial Chemical Industries (ICI), Polyurethanes(now Huntsman Polyurethanes2) supplied four urethanemodified polyisocyanurate foams (UMPIR 1–4). The

UMPIR1 and UMPIR2 are formulated with a back-bone of 4, 40-diisocyanato diphenylenemethane (MDI)and polyester polyol. The UMPIR1 contains aliphaticpolyester polyol [poly (ethylene adipate)] and UMPIR2contains aromatic polyester polyol [poly (ethylenetherephtalate)]. UMPIR3 and UMPIR4 are formulatedbased on MDI and polypropylene glycol (PPG) with anaverage molecular mass of 200 and 2000, respectively.The general structure of the UMPIR foams is representedin Fig. 1. Substituting R’ by 1 and 2 in the structureproduces UMPIR foams based on aromatic polyesterand aliphatic polyester polyol. Substituting R’ by 3produces UMPIR foam based on aliphatic polyetherpolyol. Selected chemical characteristics of the foamsare shown in Table 1.The isocyanate index indicates the proportion of iso-

cyanate incorporated into the polymer—high values ofthe index indicate a high content of the trimeric cyclicisocyanurate component. Isocyanurate linkages possesshigher thermal stability (breakdown occurs at 350 �C)than that of the urethane linkage (which dissociates at200 �C) and flammability decreases as the proportion ofisocyanurate in the foam increases.LOI values, a measure of the flammability of polymer,

were provided by ICI Technology Center, UK. Highvalues of LOI indicate strong flammability resistance[17]. UMPIR formulated with a high molecular mass ofpolyether polyol results in high thermal resistance.UMPIR 3 and 4, both formulated with polyether polyol,were included to compare flammability characteristicswith foams based on polyester polyol (Table 1).

2.1. Thermogravimetric analysis

During combustion of polymers, temperatures of over600 �C may be attained within a few milliseconds. Theseconditions are not readily simulated by many analyticaltechniques. However, thermogravimetric analysis anddifferential scanning calorimetry are able to show thethermal behavior of the polymers.A Stanton Redcroft TG-750 Thermogravimetric

Analyser was used to study the thermal performance ofUMPIR foams. The apparatus was calibrated with pel-lets composed of standardized UMPIR. About 0.4–0.6mg of foam was used to obtain the TGA profiles. Theheating rate was set to 10 �C min�1 under a flow of 10ml min�1 N2. The temperature range under analysis wasfrom 0 to 610 �C. The results were obtained as weightloss versus temperature (�C).

2.2. Differential scanning calorimetry

DSC measurements were conducted using a DuPontModel 9900 instrument calibrated with Indium. A sampleweight of 6 mg was compacted into an aluminum pan soas to provide good thermal contact with the sample2 An international business of Huntsman International LLC.

2 E. Dominguez-Rosado et al. / Polymer Degradation and Stability 78 (2002) 1–5

vessel and to optimize the heat flow between heat sourceand sample. Pans were sealed and the lids pierced sothat the sweep gas contacted the sample and the out-gassing products could be released.All analyses were conducted under N2 at a flow rate

of 80 ml min�1, with a heating rate of 10 �C min�1 from100 to 550 �C. The oven was cleaned between runs byheating to 400 �C and holding for 15 min.

3. Results and discussion

3.1. Thermogravimetric analysis

From 0 to 107 �C, all UMPIR foams exhibited similarprofiles (Fig. 2). From 0 to 250 �C, UMPIR 1, 2 and 3experienced similar weight loss. Between 250 and 360 �Cthe UMPIR2 profile overlaps that for UMPIR3. From

Fig. 1. General structure of UMPIR foams.

Table 1

Characteristics of UMPIR foams based on PPG and polyester polyol

Polyol type UMPIR

foam

Polyol molecular

mass

Polyol weight

fraction

Isocyanate weight

fraction

LOI Isocyanate

index

Aliphatic polyester polyol 1 – 0.25 0.75 22.6 340

Aromatic polyester polyol 2 – 0.25 0.75 22.4 490

Polyether polyol 3 200 0.25 0.75 20.4 220

Polyether polyol 4 2000 0.25 0.75 22.0 2078

E. Dominguez-Rosado et al. / Polymer Degradation and Stability 78 (2002) 1–5 3

360 to 610 �CUMPIR2 appears to be more stable, havinga higher content of char residue (44%) than UMPIR3(35%) and UMPIR1 (41%) (Table 2). The UMPIR3apparently possesses the lowest thermal stability.Weight loss for both UMPIR1 and UMPIR2 foams

under nitrogen follows similar trends (Fig. 2). Weightloss occurs in one step—the onset of degradation beginsat 258 �C, followed by an increase in degradation rate,which levels above 456 �C. The aromatic polyester con-tent in UMPIR2 resulted in a slightly greater weight lossbelow 420 �C. Above this temperature, however, theUMPIR2 showed greater stability.From 107 to 280 �C, UMPIR4 shows greater stability

than UMPIR1 and 2. From 280 to 370 �C weight lossfrom UMPIR4 coincides with that of UMPIR1. In thisregion, UMPIR2 is the least stable of the three UMPIRfoams. Conversely, from 440 to 610 �C, UMPIR2 and 4show similar trends, with a char residue of 44 and 45%,respectively.Table 2 summarizes the residue content at 610 �C

from TGA analysis. The foam based on the lowest mol-ecular mass of polyether polyol, UMPIR3, was the mostflammable of the four foams. This result is consistent withLOI data. UMPIR1, 2 and 4 show similar flammability,despite UMPIR2 and 4 having almost the same percen-tage of weight residue at 610 �C. The foam based on high

molecular mass of polyether polyol, UMPIR4, exhibited awide range of stability (from 107 to 440 �C).

3.2. Differential scanning calorimetry

In DSC analysis more differences in flammabilitybehavior are evident between UMPIR1 and 2 (Fig. 3).The UMPIR2 foam experienced a greater endothermicdip at 320 �C, indicating lower thermal stability than forUMPIR1, which had no significant endothermic dips.The exothermic effect begins at 330 �C for bothUMPIR1 and 2, reaching the exothermic peak at 380 �Cfor UMPIR1 and 350 �C for UMPIR2. In UMPIR1 theexothermic effect increases at a slower rate than inUMPIR2. However, both tend to stabilize at about thesame value, i.e. 450 �C. The foam containing aliphaticpolyester polyol (UMPIR1) apparently imparts lessflammability than the foam containing aromatic poly-ester polyol (UMPIR2).UMPIR2 experiences a sharp endothermic dip at

320 �C, approximating the temperature at whichUMPIR3 experiences a dip (314 �C). The exothermiceffect of UMPIR3 starts at 330 �C and reaches its peak

Fig. 2. TGA profiles of the four UMPIR foams.

Table 2

LOI values and percentage of char residue left at 610 �C from TGA analysis

Polyol type UMPIR

foams

LOI TGA (% weight residue

at 610 �C)

Aliphatic polyester polyol 1 22.6 41

Aromatic polyester polyol 2 22.4 44

Polyether polyol 3 20.4 35

Polyether polyol 4 22.0 45

Fig. 3. Comparison of DSC trends for the four UMPIR foams.

4 E. Dominguez-Rosado et al. / Polymer Degradation and Stability 78 (2002) 1–5

at 440 �C. According to LOI data and TGA results,UMPIR2 possesses good thermal stability (Table 2).However, DSC results indicate its general instability dueto its endothermic dip. UMPIR4 experiences no sig-nificant endothermic dip. However, it experiences anexothermic peak at 440 �C. The UMPIR2 and 3 appar-ently have less stability than UMPIR1 and 4, whichhave no significant endothermic dips (Table 3). In TGA,the main degradation process for UMPIR2 and 3 takesplace from 258 to 440 �C. The endothermic dips fromDSC occur in this temperature range (314 and 320 �Crespectively). The exothermic peak of UMPIR4 occursat higher temperatures (440 �C) than that for UMPIR1(380 �C). Hence, UMPIR4 possesses greater stabilitythan UMPIR1. This result is consistent with TGAresults but contrary to LOI data, UMPIR4 does notposses the highest LOI value, which represents the betterflame resistance. In contrast, UMPIR1 possess the high-est LOI value, 22.6. However, the differences betweenLOI values for UMPIR1 and 4 is slight and may not beuseful as a flammability index. The least stable of the fourUMPIR foams is UMPIR3, which has an endothermicdip at low temperatures, 314 �C (Table 3). This is con-sistent with LOI values and TGA results (Table 2).

4. Summary and conclusions

Thermal stability of foams based on aliphatic poly-ester polyol UMPIR1 and aromatic polyester polyol,UMPIR2 was investigated. Their behavior was com-pared to those foams based on polyether polyol withmolecular mass 200 and 2000 (UMPIR3 and 4, respec-tively). LOI values, TGA and DSC results show thatUMPIR2 and 3 have lower stability than UMPIR1 and4. The LOI values, DSC and TGA results show thatUMPIR3 is the least stable and most flammable of thefour foams. According to TGA and DSC results,UMPIR4 (which also has a high LOI value) is the leastflammable of the four foams, although it does not ex-hibit the highest LOI value. The current study showsthat foams formulated with a high molecular mass ofpolyether polyol such as UMPIR4, can attain the sameor better thermal stability than those foams based onpolyester polyol (UMPIR1 and 2).

Acknowledgements

The authors would like to thank C. Lindsay forsupplying UMPIR foams and G. Seeley for providingLOI data.

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Table 3

Temperature of endothermic and exothermic peaks of UMPIR foams

Polyol type UMPIR

foams

Endothermic

dips (�C)

Exothermic

peaks (�C)

Aliphatic polyester polyol 1 – 380, 440

Aromatic polyester polyol 2 320 350

Polyether polyol 3 314 440

Polyether polyol 4 – 440

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