Polymer International 45 (1998) 369È372

Polyurethane Elastomers: Synthesis andthe Effects of Dihydroquinone on

Properties

Rouhallah Bagheri* & Ali-Reza Jafarpour

Chemical Engineering Department, Polymer Group, Isfahan University of Technology, Isfahan, Iran

(Received 17 June 1997 ; accepted 29 October 1997)

Abstract : Poly(esterurethane) elastomers were synthesized by a pre-polymerprocess. The pre-polymer based on poly(caprolactone) (capa-225) and toluene-diisocyanate (TDI) was mixed with 1,4-butane diol (BDO) or dihydroquinone(HQ) in capa-225 (known as the quasi pre-polymer process) and then reacted in amould. Mechanical and thermal properties of the elastomers synthesized wereinvestigated using a tensometer, Shore hardness tester, di†erential scanning calo-rimeter and dynamic mechanical analysis. The chemical resistance of the sampleswas measured by a toluene immersion test. The properties of the samples con-taining HQ and BDO as chain extenders are compared. The ultimate tensilestrength, YoungÏs modulus, hardness and chemical resistance of the HQ samplesare higher than those of the BDO samples. Substitution of HQ for BDO resultsin a decrease in elongation at break and heat resistance of the samples. 1998(SCI.

Polym. Int. 45, 369È372 (1998)

Key words : polyurethane elastomer ; synthesis ; diisocyanate ; dihydroquinone ;polyol ; properties

INTRODUCTION

Two synthetic procedures leading to polyurethane elas-tomers (PUs) have been reported in the literature.1,2The Ðrst is the so called one-shot process, which isbased on mixing the precursors of the PU in a singlestage. The second procedure, known as the pre-polymerprocess, requires the formation of a pre-polymer, whichis then further chain extended. Elastomers made by thecasting method are characterized by high tensile andtear strengths. However, they are not known for goodresistance to elevated temperatures.1,3 The chemicalcomposition and relative size of the soft and hard seg-ments, along with hydrogen bonds between the hardsegments, determine the properties of PU elastomers.1,4We have recently reported the e†ect of hard segment

* To whom all correspondence should be addressed.

concentration on the thermal and mechanical propertiesof PU elastomers.5 It has also been shown that thethermal and mechanical properties of thermoplasticelastomers are increased by using diols with highermolecular weight as chain extenders.6

The aim of this paper was to study the e†ect of usingdihydroquinone as a chain extender on the properties ofPU elastomers and to produce PU elastomers havinghigh mechanical and chemical resistance.

EXPERIMENTAL

Materials

Poly(caprolactone) (capa 225) with a molecular weightof 2000 was supplied by Interox Chemicals. Toluenediisocyanate (TDI) comprising the mixed 2,4 and 2,6

3691998 SCI. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

370 R. Bagheri, A-R. Jafarpour

isomers in 80/20 ratio (Merck, Germany) was used.Dihydroquinone (HQ), 1,4 butanediol (BDO) and 1,4diazobicyclooctane (Dabco) were purchased fromAldrich Chem. Co. Ltd, UK.

Syntheses

The reactants were dried in a vacuum oven at 85¡C for8 h before being used for synthesis. The hydroxynumber of the polyol and isocyanate equivalent of TDIwere determined.4 A basic formula with a molar ratio of4É4 : 3 : 1 for TDI, HQ (or BDO) and capa-225 withconstant isocyanate index (1É1) was selected to preparethe samples.7

Pre-polymer preparation. The calculated amounts ofTDI and capa-225 were poured into a Ñask equippedwith a stirrer, after which the catalyst (Dabco, 1 wt%)was added. The Ñask was placed in an oil bath at105¡C. The contents of the Ñask were stirred at500 rev min~1 for 30 min.

Sample preparation. The pre-polymer was cooled to40¡C; the calculated amount of BDO, or HQ in capa-225 was added and the mixture stirred for 30 min. Themixture was then degassed under vacuum at 105¡C for2 min and cast in a polyethylene mould of dimensions150 mm] 150 mm] 2 mm. The Ðlled mould wasplaced in an oven at 115¡C, for 24 h. The sheets wereseparated from the mould and stored for at least 1 weekat ambient temperature.

Measurements

The tensile test was carried out on quadruplicatesamples of each material with an Instron universaltesting machine, model 1122 according to ASTM-D412.The dumb-bell shaped samples were 50 mm long, 10 mmwide and 2 mm thick. The machine was operated at100 mm min~1 at room temperature, and the initial gripseparation (or gauge length) was 50 mm. The ultimatetensile strength (UTS), YoungÏs modulus and elongationat break were determined. The Shore hardness(Eb%)values were measured at room temperature with cali-brated A scale indentors (Zorn, Germany) by placingthe 2 mm thick sheet on a Ñat surface. The average ofÐve readings was recorded for each sample.

Chemical resistance (oil and solvent) of the sampleswas measured according to ASTM-D471. The changesin weight of standard samples were determined. DSCmeasurements on the samples were performed with aDSC 1200 (Rheometric Co., UK) with a 10¡Cmin~1heating rate. The sample, weighing about 10 mg, was

placed in an aluminium pan and the cell was heatedfrom 25 to 400¡C under nitrogen. The peak tem-peratures of the transitions were recorded.

Dynamic mechanical testing was carried out on aDupont dynamic mechanical thermal analyser, model983 over a temperature range 25È250¡C at a heatingrate of 5¡Cmin~1 and frequency of 1 Hz. The dimensionof the samples were 40 mm ] 10 mm] 2 mm. The valueof tan d and storage modulus versus temperature wererecorded for each sample.

RESULTS AND DISCUSSION

Figure 1 shows the e†ect of HQ as a chain extender onof the samples. A signiÐcant decrease in isEb% Eb%

observed, which may be attributed to the presence ofbenzene rings in the hard segments causing higher rota-tional bonding barrier energy ; hence the rigidity of thehard segments is increased. The samples containingBDO as chain extender show high percentages of elon-gation, which is a characteristic of a good elastomer.The of the HQ samples is, however, in the rangeEb%for an acceptable rubber.

The decrease in of the HQ samples may also beEb%explained as follows. When the samples are deformedduring a tensile test, the hard segments disentangle, withsome phase mixing with soft segments ; both segmentsorient towards the direction of elongation, givingmaximum intermolecular interaction.8 This rearrange-ment is easier when the crystallinity content of the hardsegments and the rotational bonding barrier energy arelower. Figures 2 and 3 compare the ultimate tensilestrength, YoungÏs modulus and hardness of the HQsamples with those of BDO samples : substitution ofHQ for BDO increases all three parameters. This couldbe due to the high sti†ening e†ects of aromatic rings in

Fig. 1. Comparison of elongation at break of the BDOsamples (1) with the HQ samples (2).

POLYMER INTERNATIONAL VOL. 45, NO. 4, 1998

Synthesis of polyurethane elastomers 371

Fig. 2. Comparison of the ultimate tensile strength (UTS) andYoungÏs modulus of the samples synthesized with BDO (1)

and HQ (2) as chain extenders.

Fig. 3. Comparison of the chemical resistance and hardness ofthe samples containing BDO (1) and HQ (2) as chain

extenders.

Fig. 4. DSC thermograms of the samples based on BDO(curve [1]) and HQ (curve [2]) as chain extenders.

Fig. 5. Variation of dynamic elastic modulus versus tem-perature of the samples containing dihydroquinone (HQ) or

1,4 butanediol (BDO) as chain extenders.

the hard segment regions. Higher solvent resistance isalso observed for the HQ samples (Fig. 3). This isexplained by a lower di†usion rate of toluene in the HQsamples.

DSC thermograms of both types of sample are shownin Fig. 4. Comparison of the melting temperatures ofthe samples (i.e. 310¡C for the BDO sample and 265¡Cfor the HQ sample) shows a lower heat resistance of theHQ samples. This is conÐrmed by the results shown inFig. 5 of the variation of dynamic storage modulusversus temperature. Thus BDO samples show higherheat resistance compared with HQ samples. The pres-ence of benzene rings in the hard segment regionscauses a sti†ening e†ect which reduces the percentagecrystallinity and the amount of hydrogen bonding inthe hard segment regions. Because the mechanical andthermal properties of the samples are directly related topercentage crystallinity and the amount of hydrogenbonding between the urethane linkages of neighbouringmolecules, the heat resistance of the HQ samples shouldbe lower than that of the BDO samples.9

Simultaneous combination of the reactants duringsynthesis and the use of TDI with di†erent reactivity ofthe two isocyanate groups give a favourable distribu-tion of hard segment size,10 contributing to the highperformance of our samples. The excess of isocyanate(i.e. [NCO]/[OH]\ 1É1) allows a degree of cross-linking after the formation of linear polymer chains andduring cure in the mould, leading to a thermoplastic-thermosetting type polymer. The terminal isocyanategroups are believed to react with the internal urethanegroups of adjacent chains to produce allophanate crosslinks.1 Trimerization of the isocyanates, leading to iso-cyanurate rings, is another cross-linking reaction whichmight occur during the curing reaction1 at 115¡C for24 h. The properties of the samples are directly relatedto the degree of cross-linking.

POLYMER INTERNATIONAL VOL. 45, NO. 4, 1998

372 R. Bagheri, A-R. Jafarpour

CONCLUSIONS

From the results presented one can conclude thefollowing :

(1) Substituting dihydroquinone for 1,4-butanediol inthe PU elastomers increases ultimate tensilestrength, YoungÏs modulus and chemical resistanceof the samples.

(2) The samples containing HQ show lower heatresistance and elongation at break compared withthe BDO samples.

(3) Using a constant amount of excess isocyanate inthe synthesized samples leads to the same averagedegree of crosslinking in each sample type andimproves their properties.

(4) PU elastomer of high quality is synthesized by apre-polymer process using capa-225, TDI andBDO in a molar ratio of 1 : 4É4 : 3.

REFERENCES

1 Hepburn, C., Polyurethane Elastomers, 2nd edn., Elsevier, London,1992.

2 Dieterich, D., Angew. Macromol. Chem., 76 (1979) 79.3 Kirk-Othmer, Encyclopedia of Chemical T echnology, 3rd edn., Vol.

23, Wiley Interscience, New York, 1983.4 Doyle, E. N., T he Development and Use of Polyurethane Products,

McGraw Hill, New York, 1971.5 Bagheri, R. & Jafarpour, A. R., T hird International Rubber Con-

ference, Tehran, Iran, 28È30 Oct. 1996.6 Barikani, M. & Barmar, M., Iran. Polym. J., 5 (1996) 231.7 Karabanova, L., Gorbach, L. & Skiba, S., Composit. Polym.

Mater., 49 (1991) 35.8 Shibayama, M., Kawauch, T., Kotani, T., Nomura, S. & Matsuda,

T., Polym. J., 18 (1986) 719.9 Finelli, A., Marshall, R. & Chung, D., in Encyclopedia of Chemical

T echnology, 3rd Edn, Vol. 8, p. 626. (Ed : Kirk-Othmer). Wiley,New York, 1979.

10 Yu-Chin Lai, Edmond, T., Quinn & Paul L. Valint Jr., J. Polym.Sci. Part A, 33 (1995) 1767.

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