Polymer International Polym Int 53:1936–1940 (2004)DOI: 10.1002/pi.1571

Effect of glycols on the properties of polyesterpolyols and of room-temperature-curablecasting polyurethanesChen Zhang and Shengyu Feng∗School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China

Abstract: A series of liquid polyester polyols (PEs) from adipic acid (AA), phthalic anhydride (PA) andtrihydroxymethylpropane (TMP), and such glycols as ethylene glycol (EG), diethylene glycol (DEG),triethylene glycol (TEG), butanediol (BD) and hexanediol (HD), were prepared. Polyurethanes (PUs)were obtained from the PEs and polyaryl polymethylene isocyanate (PAPI) at room temperature. Theeffects of the structures of the glycols on viscosity, glass transition temperature and crystallinity of the PEs,and the mechanical, thermal and boiling-water-resistant properties of PUs were studied. The experimentsshowed that the viscosities and glass transition temperatures of the PEs decreased as the length of theglycol chains increased. The polyester based on HD lost flowability because of crystallization. The tensilestrength and hardness of the PUs obtained decreased with increasing the length of the glycol chains,while the resistance to thermal deformation and boiling water increased. Thermogravimetric analysisdemonstrated that thermal degradation of the polyurethane based on DEG proceeded in one step and forthe others in two steps. The initial degradation temperature of the polyurethane based on EG was thelowest and that of the polyurethane based on BD was the highest. The residue of the former at 450 ◦C wasthe greatest, while that of the latter was the lowest. 2004 Society of Chemical Industry

Keywords: polyester polyol; polyurethane; glycol; viscosity; thermal deformation; thermal degradation

INTRODUCTIONStudies of structure–property relationships in polyure-thanes (PUs) has acquired importance due to thebroad range of applications of these polymers.1,2 Withrespect to the specific end-use, an elastic or rigid mate-rial can be designed by merely varying the chemicalconstituents of the polyurethane. Their properties canbe tailored simply by varying the components, such asrigid diols, flexible polyols and polyisocyanates. Fac-tors such as molecular weight, chemical nature of theunits and morphology in the solid state have importantinfluences on their properties.3 A numbers of attemptshave been made to establish correlations between the‘macroglycol’ structures, hard segment content, mor-phology, synthesis conditions and thermal treatmentand physical properties of Pus.4,5

The glass transition temperature (Tg) of themacroglycol segments is below room temperature(typically between −20.0 and −80.0 ◦C) and thesesegments are denoted as soft ones. The viscosity of themacroglycols is very important for the processability ofroom-temperature-curable PUs, which depends upontheir molecular weights, structures, side-chains, and soforth. The properties of PUs depend on the structures

of the soft segments to some extent. The effect ofthe chemical structures of polyether macroglycols onthe crystallization of the soft-segment phase and theordered region in the hard-segment phase has beenwell studied.6–10 Petrovic et al. have investigated theeffect of lengths and concentration of the soft segmentson the apparent activation energies of the thermaldegradation of PUs.11 Yoo et al. claimed that thedeformation and thermal properties of the PUs werestrongly affected by the molecular weight of the softsegments.12

However, the effect of soft segments on theproperties of PUs is not the same under differentconditions, and there have been few studies onthe resistance to thermal deformation of PUs,especially of room-temperature-curable PUs. Methodsfor improving the resistance to thermal deformation ofroom-temperature-curable PUs are often restricted byprocessability, which is mainly affected by the viscosityof the soft segments. Five types of glycols were usedto prepare polyester polyols (PEs), namely ethyleneglycol (EG), diethylene glycol (DEG), triethyleneglycol (TEG), butanediol (BD) and hexanediol (HD).A series of PEs were synthesized from adipic acid (AA),

∗ Correspondence to: Shengyu Feng, School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27, Jinan250100, ChinaE-mail: fsy@sdu.edn.cn(Received 18 September 2003; revised version received 5 January 2004; accepted 13 February 2004)Published online 12 October 2004

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Preparation of polyurethanes from various polyester polyols

phthalic anhydride (PA), trihydroxymethylpropane(TMP) and various glycols, with their compositionbeing shown below in Table 2. The PUs weresynthesized from polyaryl polymethylene isocyanate(PAPI) and the PEs in the presence of dibutyltin dilaurate (DBTDL) at room temperature. Theeffect of the glycols on the viscosities, glass transitiontemperatures and crystallization of the PEs, andthe mechanical, thermal and boiling-water-resistantproperties of the PUs, was discussed.

EXPERIMENTALMaterialsThe sources of the chemicals used for the synthesis ofthe various PEs and PUs are given in Table 1. PAPIwas used without any further treatment.

Preparation of polyester polyolsThe polyester polyols (PEs) were prepared from AA,PA and TMP at a fixed ratio with various glycols(Table 2). The sample of PEG represents the polyesterpolyol based on EG, and similarly, that of PDEGbased on DEG, PTEG based on TEG, PBD basedon BD and PHD based on HD. To a 250 ml round-bottomed flask, equipped with a stirrer, thermometer,nitrogen gas-inlet tube and distillation condenser,the reactants were added and then slowly heated to140.0 ◦C for about 1 h under N2, to 180.0 ◦C for about3 h, and subsequently to 200 ◦C over a period of 1 h.

Table 1. Chemicals used for the synthesis of PEs and PUs

MaterialDescription/Tradename Source

EG AR Shandong Chemical EngineeringCollege, China

BD AR Laiyang Agent Plant, ChinaHD CR Yantai Huada Chemical Company, ChinaDEG AR Tianjin Agent Plant, ChinaTEG AR Tianjin Agent Plant, ChinaAA CR Shanghai Agent Plant, ChinaPA CR Tianjin Agent Plant, ChinaTMP CR Yantai Huada Chemical Company, ChinaPAPI PM-200

(NCOcontent,3 %)

Yantai Wanhua Polyurethane Company,China

DBTDL CR Shanghai Agent Plant, China

The mixture was then kept at 200 ◦C for 3 h underreduced pressure for removal of the water formedin the reaction. The extents of the reactions werecontrolled by the product’s acid values. Reactionswere completed when the acid values were less than1.0 mg KOH g−1.

Preparation of polyurethanesThe PUs were prepared from PAPI and the dried PEsby a one-shot technique. The PEs, PAPI and DBTDLwere added together at a fixed NCO/OH ratio of 1.2and mixed thoroughly. The mixtures were poured intoa Teflon mold, defoamed under vacuum and cured atroom temperature. Samples were obtained after 7 d atroom temperature.

MeasurementsThe viscosities of the PEs were measured at 25 ◦Cwith a NDJ-79 rotational Viscometer (China). TheShore A hardness values of the PUs were measuredon a XHS rubber hardness tester (Yingkou Test-Machine Plant, China) at various temperatures andin boiling water. The tensile strengths were measuredon a XLD-A electric tensile tester (Changchun Test-Machine Plant, China). Water absorption data for thePUs were obtained after boiling for 3 h. The glasstransition temperatures of the PEs were examined bydifferential scanning calorimetry (DSC) (RheometricScientific DSC SP, USA), at a rate of 20 ◦C min−1.For measurements of the thermal stabilities of the PUs,thermogravimetric analysis (TGA) was carried outwith a SDTA 851 machine (Mettler, Im Langacher,Switzerland) at a rate of 20 ◦C min−1 in a dry nitrogenenvironment.

RESULTS AND DISCUSSIONProperties of polyester polyolsViscosityThe viscosities of the PEs are very importantparameters in the processability of room-temperature-curable casting polyurethanes. The viscosity of apolymer involves the function of many interactingvariables, such as the molar mass, structure andconformation of the polymer molecule.13 The effectof the structures of the glycols on the viscosities ofthe various PEs shown in Table 2. The viscositiesof the PEs changed in the following order: PEG

Table 2. Compositions of the PEs

Sample Glycol Composition (molar ratio)Hydroxyl value(mg KOH g−1)

Viscosity(mPa s)a

Tg

(◦C)

PEG EG AA/PA/TMP/EG (1/0.24/0.10/2.25) 218 800 −61PDEG DEG AA/PA/TMP/DEG (1/0.24/0.10/2.30) 216 300 −62PTEG TEG AA/PA/TMP/TEG (1/0.24/0.10/2.35) 223 190 −65PBD BD AA/PA/TMP/BD (1/0.24/0.10/2.30) 213 530 −68PHD HD AA/PA/TMP/HD (1/0.24/0.10/2.30) 215 — −69

a At 25 ◦C.

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< PDEG < PTEG < PBD. This was because,with increasing length of the glycols, the contentsof polar groups and the hydrogen bond number ofthe PEs decreased, and so the slip resistance amongthe molecules decreased, hence resulting in a lowerviscosity. PHD had no flowability at room temperaturebecause of its crystallization. Increasing the length ofthe glycol chains could decrease the viscosity of thepolyester, but too large a number of methylene unitsin the glycol chains probably caused crystallization ofthe product PEs.

Differential scanning calorimetry analysisThe glass transition temperatures (Tg) of the PEswere determined by differential scanning calorimetry(DSC) (Figure 1), with the results obtained given inTable 2. The chain flexibility of a polymer has animportant effect on its Tg. It has been suggested thatthe Tg of a polymer shifts to a lower temperature withan increase in the flexibility of the polymer chains.13

With changing the glycols from EG to DEG andTEG, and from EG to BD and HD, the length ofthe monomer units increased and the flexibility of theformed polymer chains also increased, and thereforethe glass transition temperatures of the formed PEsshift to a lower temperature.

The DSC curves of PBD and PHD showed thecrystalline melting peaks at about 17 and 24 ◦C,respectively. The crystallizing ability of the PEsincreased with increasing the number of joinedmethylene units in the glycols. This could beinterpreted as showing that the polyester polyol madefrom HD had no flowability at room temperature.PDEG and PTEG had no crystalline contents becausethere were ether linkages in the molecules and the‘regularity’ was destroyed. Therefore, these glycolshad more joined methylene units, and the polyesterpolyols had higher crystallinities. Such a feature isnot ‘good’ for the processability of room-temperature-cured casting PUs.

–200 –150 –100 –50 0 50 100–9

–8

–7

–6

–5

–4

–2

–1

0

1

PH

D

PB

DP

TE

G

PD

EG

PE

G

Hea

t flo

w (

mW

)

Temperature (°C)

–3

Figure 1. DSC curves of the various polyester polyols studied in thiswork.

Properties of polyurethaneMechanical propertiesTable 3 shows the tensile strength and hardness datafor the PUs. As before, the sample of PUEG representspolyurethane based on EG, and similarly of PUDEGbased on DEG, PUTEG based on TEG, PUBDbased on BD and PUHD based on HD. The tensilestrength decreased with changing the glycols from EGto DEG and TEG, and from EG to BD and HD.The same trend was also seen in the hardness data.This can be interpreted by the content of the C=Ounits for similar molecular weights of the PEs and thehydrogen bonds in the PUs between the N–H andC=O units decreasing with increasing lengths of theglycols, resulting in weaker interactions between thePU molecules, thus causing a lower tensile strengthand hardness.

Resistance to thermal deformationThe resistance to thermal deformation of the PUs wasshown by the change in hardness with temperature(Figure 2). This indicated that the hardness of allof the PUs apparently decreased with increasingtemperature. However, with changing of the glycolsfrom EG to DEG and TEG, and from EG to BDand HD, the ‘residual proportion’ of the hardness at100 ◦C increased. At this temperature, the hardnessvalues of all the PUs were nearly identical. This can be

Table 3. Properties of the PUs

Sample

Tensilestrength(MPa) Hardnessa Hardnessb

Waterabsorption

(%)

PUEG 7.8 94 69 5.3PUDEG 6.9 93 78 4.1PUTEG 6.1 90 78 4.0PUBD 7.0 90 78 2.9PUHD 5.2 85 77 2.3

a At 25 ◦C.b After boiling.

20 30 40 50 60 70 80 90 100 110

70

75

80

85

90

95

Sho

re A

har

dnes

s

PU

HD

PU

BD

PU

TE

GP

UD

EG

PU

EG

Temperature (°C)

Figure 2. Share A hardness values of the various polyurethanes as afunction of temperature.

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attributed to the fact that the hydrogen bonds betweenthe N–H and C=O groups in the PUs reduced withincreasing length of the glycol chains, and so therewere fewer hydrogen bonds to be destroyed at highertemperatures, hence resulting in a lesser reductionin hardness. Therefore, an increase in the length ofthe glycols was beneficial with respect to resistance tothermal deformation of the PUs.

Boiling-water-resistant propertiesThe hardness and weight increase in the sampleswere measured after keeping them in boiling water for3 h. The boiling-water-resistant properties showed thesame trend as the resistance to thermal deformation.The ‘residual proportion’ of hardness increased whilethe water absorption decreased upon changing ofthe glycols from EG to DEG and TEG and fromEG to BD and HD. The hardness values of thePUs after boiling-water treatment were, also nearlyidentical. The amounts of water absorption for PUBDand PUHD were lower than those of PUDEG andPUTEG, because there were no ether linkages in theBD and HD precursors.

Thermal stabilityThermogravimetry analysis was used to investigate thedegradation of the various PUs, with the date obtainedbeing shown in Figures 3 and 4, and Table 4. Theonset and maximum degradation rate temperaturesof the first step, T1on and T1Max, and for thesecond step, T2on and T2Max, illustrated a qualitativecharacterization of the degradation process. Thisshowed that PUDEG displayed only a one-stepdegradation, with a maximum degradation rate at344 ◦C, while the other PUs had two-step processes.In the initial stage of degradation, PUEG had thelowest stability while PUBD had the highest. Theorders of T1on and T1Max were as follows: PUEG <

PUDEG < PUTEG and PEG < PUBD < PUHD.In the second step of degradation, PUTEG had thehighest T2on and T2Max. In addition, PUEG had thelowest values, while T2on and T2Max for PUBD and

100 200 300 400 500 600–0.1

0.00.10.20.30.40.50.60.70.80.91.01.1

PUHD

PUTEG

PUBD

PUDEG

PUEG

Wei

ght

(%)

Temperature (°C)

Figure 3. TGA curves of the various polyurethanes.

100 200 300 400 500 600

–0.002

0.000

0.002

0.004

dW/d

t

PUBD

PUHD

PUTEG

PUDEG

PUEG

Temperature (°C)

–0.004

–0.006

–0.008

–0.010

–0.012

Figure 4. DTGA curves of the various polyurethanes.

Table 4. TGA data of the PUs

SampleT1on(◦C)

T1 max(◦C)

T2on(◦C)

T2 max(◦C)

Residue at450 ◦C (%)

PUEG 267 302 335 372 34PUDEG 292 344 — — 27PUTEG 300 355 390 402 31PUBD 308 365 380 392 24PUHD 294 358 380 390 30

PUHD were similar. PUEG had the highest residue at450 ◦C, while PUBD had the lowest amount.

CONCLUSIONSThese experiments showed that the viscosity and Tg

values of the PEs decreased with increasing length ofthe glycol chains, although the long joined methylenechains in the glycols could lead to crystallization ofthe formed PEs, for example, the PEs based onHD had no flowability at room temperature. Withincreasing length of the glycol, the tensile strength andhardness of the PUs decreased, while the resistance tothermal deformation and boiling water was improved.PUs based on BD and HD had better boiling-water-resistance properties than the PUs based on DEG andTEG, because these glycols contained ether linkages intheir molecules. PUs based on DEG displayed only aone-step thermal degradation process, while the otherPUs displayed two steps. The PUs based on EG hadthe lowest stabilities and the highest residues contentsat 450 ◦C. The polyurethanes based on BD had thehighest stabilities and the lowest residue contents at450 ◦C.

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