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Polymer Degradation and Stability 94 (2009) 527–532

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Influence of melt behaviour on the flame retardant properties of ethylenecopolymers modified with calcium carbonate and silicone elastomer

Linus Karlsson a,*, Anna Lundgren b, Jonas Jungqvist c, Thomas Hjertberg a

a Chalmers University of Technology, Department of Chemical and Biological Engineering, SE-412 96 Gothenburg, Swedenb Chalmers Industriteknik, Chalmers Science Park, SE-412 88 Gothenburg, Swedenc Borealis AB, SE-444 86 Stenungsund, Sweden

a r t i c l e i n f o

Article history:Received 4 November 2008Received in revised form22 January 2009Accepted 30 January 2009Available online 8 February 2009

Keywords:Melt behaviourViscosityFlame retardantEBAEMAACalcium carbonate

* Corresponding author. Tel.: þ46 31 772 3404; faxE-mail address: linus.karlsson@chalmers.se (L. Kar

0141-3910/$ – see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.polymdegradstab.2009.01.025

a b s t r a c t

This work describes a halogen-free flame retardant material consisting of a polar ethylene copolymer,calcium carbonate and silicone elastomer. The flame retardant properties when using poly(ethylene-co-methacrylic acid) (EMAA) and poly(ethylene-co-butyl acrylate) (EBA) as the copolymer have beencompared. Rheological measurements showed an increase in complex viscosity above 250 �C due toionomer formation between acidic groups in the polymer and calcium ions. The increase in viscosityoccurs at lower temperatures with the EMAA material and the increase is stronger. This has great impacton the fire performance, as shown with cone calorimetry and dripping test. In order to further elucidatethe flame retardant mechanism in detail, thermogravimetric analysis and infrared spectroscopy havebeen used. The influence of the amount of comonomer and melt flow ratio of the polymer is alsocommented.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Flame retardancy is of great importance in many areas. For cableapplications halogen-free materials have gained much interest inrecent years due to their low environmental impact and lowtoxicity of combustion products. Much focus lies in findinga formulation that combines good mechanical and processingproperties together with high fire retardancy. Mineral additives canact as flame retardants in polyolefins by diluting and by shieldingthe surface. Hydrates such as aluminium trihydrate (ATH) andmagnesium dehydrate (MDH) form water when decomposed andact by cooling. The flame retardant mechanism of a halogen-freematerial based on calcium carbonate, silicone elastomer and anethylene acrylate copolymer was described by Hermansson et al.[1]. The flame retardant mechanism was shown to take place in thecondensed phase. Degradation products from the polymer formintumescent gases and calcium ions interact with acidic groups onthe polymer and form ionomer cross-linking leading to anincreased melt viscosity and reduced transportation rate of gases inand out of the material. The type of copolymer used has beenshown to be important. Most work on this material has been

: þ46 31 772 3418.lsson).

ll rights reserved.

performed with ethylene butyl acrylate (EBA) as the copolymer butother acrylate copolymers like ethylene ethyl acrylate (EEA),ethylene methyl acrylate (EMA) and ethylene vinyl acetate (EVA)have also been investigated. Among these copolymers, EBA hasshown the best flame retardant performance. Previous investiga-tions of the flame retardant properties of systems with calciumcarbonate, silicone elastomer and different copolymers state threekey properties of the polymer [2];

� melt viscosity (MFR)� type of acrylate side-group� amount of acrylate comonomer

A high melt viscosity is desired for several reasons. Firstly itreduces the transportation rate of volatiles originating from thedegradation of the polymer and secondly it prevents drippingwhich otherwise can contribute to flame spread. Acrylate copoly-mers work well in a system with calcium carbonate mainly due tothe ability to form carboxylic acid that reacts with calcium to formionomeric cross-links. At temperatures when the polymer starts todegrade, degradation products cause the material to swell, leadingto the formation of a protective intumescent structure [3]. Theintumescent behaviour of the system based on EBA, EBA withpolypropylene and EMAA respectively was investigated by Krameret al. [4]. EMAA based Casico did not swell to as high extent as EBA

L. Karlsson et al. / Polymer Degradation and Stability 94 (2009) 527–532528

and the heat release rate was significantly lower for the EMAAmaterial.

This study is a comparison of the fire performance of formula-tions based on EBA and EMAA. Grades with different comonomercontent and melt flow ratio (MFR) are compared.

2. Experimental

2.1. Materials

Poly(ethylene-co-butyl acrylate), EBA, and poly(ethylene-co-methacrylic acid), EMAA, of varying acrylate content and MFR wereprovided by Borealis AB, Sweden. The formulations investigatedcontain copolymer (52.3 wt.%), calcium carbonate (35.0 wt.%), sili-cone elastomer master batch of low density polyethylene andpolydimethylsiloxane (giving 5 wt.% polydimethylsiloxane) andantioxidant (0.2 wt.%). The polydimethylsiloxane (Silastic Q4-2735)was provided by Dow Corning, the calcium carbonate, a limestonewith d50¼ 0.7 mm was provided by Reverte S.A. and the antioxidant(Irganox 1010) was provided by Chemtura Corporation.

The flame retardant formulations are referred to as CaSi (Cafrom calcium carbonate and Si from silicone elastomer) followed bythe abbreviation of the copolymer used, i.e. CaSiEBA and CaSiEMAA.For clarity these formulations are followed by their acrylate content(mol%) and MFR (190 �C and 2.16 kg load). Hence, CaSiEBA thatcontains 4.3 mol% butyl acrylate and has a MFR of 1.1 g/10 min iscalled CaSiEBA-4.3/1.1. The MFR and comonomer content areproduct specifications measured by Borealis AB quality control inStenungsund.

2.2. Sample preparation

The flame retardant formulations were produced on a labora-tory two-roll mill at 160 �C. The total mixing time was 20 min.Polymer and antioxidant were first added, followed by calciumcarbonate (after 5 min) and silicone elastomer (after 12 min). Thefoils were pelletised using a Rapid granulator, which providedcoarse granules. A Schwabenthan Polystat 400 S plate press wasused to manufacture plates with a thickness of 3 mm. The sampleswere pressed 150 �C at a pressure of 20 bar for 1 min followed by200 bar during 5 min. The pressure was kept at 200 bar duringcooling at 15 �C/min.

2.3. Fire testing

A Stanton Redcroft cone calorimeter was used to obtain heatrelease data. Test specimens (100�100� 3 mm) taken froma compression-moulded plate were exposed to a heat flux of35 kW/m2 and an airflow of 24 l/s according to ISO 5660 [5]. Thedistance to the cone heater was 60 mm. The data presentedrepresent an average of triplicate cone calorimeter results.

Dripping test was done on compression-moulded plates tomeasure the ability to withstand dripping while subjected to heat.

Scheme 1. Ester pyrolysis of EBA followed by io

A test specimen (65� 65� 3 mm) was placed on an 8 mesh gridwith a 1 kW burner set at a 45� angle from below (the tip of theouter flame was centred under the test specimen). The burner wasnot removed during the test. Flaming drops were quenched withwater and collected and weighed. The time when dripping startedand the total burning time were noted. The results are presented asdripped fraction of initial sample weight based on the average ofthree tests.

2.4. Thermal treatments

Samples of CaSiEBA and CaSiEMAA were treated isothermally atdifferent temperatures in a tubular oven in order to follow theformation of ionomers. Test specimens with the dimensions10� 30 mm and 1 mm thickness were placed on a microscope glassslide and treated at temperatures between 200 and 350 �C for20 min in a nitrogen atmosphere. The thermally treated CaSi-formulations were analysed with infrared spectroscopy (trans-mission FTIR) on a Perkin Elmer 2000. The samples were grinded tofine particles and pressed to KBr tablets. The development of theester peak at 1735 cm�1 and the carboxylic acid peak at 1698 cm�1

were compared. The spectra presented have been normalisedagainst the –CH2– absorption peak at 2010 cm�1.

Thermogravimetric analysis (TGA) was performed using a Per-kin Elmer TGA 7 instrument. Samples of 5 mg were decomposed intechnical air (20 ml/min) under a heating rate of 10 �C/min (30–600 �C).

TGA/FTIR is a useful technique for studying the degradation ofpolymeric materials [6,7]. In this study, a TA Instruments SDT2960TGA was used to analyse the volatile decomposition products up to900 �C. The samples were decomposed in technical air (100 ml/min) under a heating rate of 10 �C/min (30–900 �C). The evolvedgases from the TGA experiment were analysed with a NicoletMagna IR Spectrometer 550. The connecting line had a temperatureof 220 �C and the detector was running at 280 �C.

Complex viscosities were obtained using an Anton Paar PhysicaMCR 300 in parallel plate mode. The test specimens (25 mmdiameter, 2 mm thick) were heated from 150 to 350 �C in nitrogenatmosphere with a heating rate of 1.7 �C/min. The angularfrequency was 10 rad/s and the strain was 1%.

3. Results and discussion

When a cable based on the investigated system is exposed tofire, several mechanisms contribute to the resistance to ignitionand flame spread. Swelling, increased viscosity and the formationof a silica enriched surface layer are the most important mecha-nisms and the synergistic effect is important. The fire intensity canbe observed by heat release measurements, and viscosity effects areinvestigated by dripping and rheological measurements. At hightemperatures, the polymer degrades and combustible gases areevolved that cause the material to swell. A high melt viscosity isdesired since it reduces both the rate of volatiles transported

nomer formation with calcium carbonate.

Table 1Data from cone calorimeter test at 35 kW/m2. The data are presented as averagevalues from triplicate tests.

Materials HRR (max) [kW/m2] Ignition time [s] EHC [MJ/kg]

CaSiEBA-1.7/0.25 299 109 36.2CaSiEBA-1.7/0.45 308 122 37.6

CaSiEBA-4.3/1.1 337 95 38.0CaSiEBA-4.3/8.0 331 94 36.2

CaSiEMAA-1.3/3.0 369 106 38.1CaSiEMAA-1.3/7.0 400 112 40.8

CaSiEMAA-3.1/1.5 174 120 33.1CaSiEMAA-3.1/2.5 168 118 34.0

CaSiEMAA-3.1/10 156 105 38.7CaSiEMAA-4.1/1.5 177 114 36.6

Table 2Time when dripping occurred and relative amount of dripping for Casico formula-tions. The data are presented as average values from triplicate tests.

Materials Tstart dripping [s] Tburning [s] Dripping [%]

CaSiEBA-1.7/0.25 56 179 23.5CaSiEBA-1.7/0.45 67 173 27.9CaSiEBA-3.2/0.4 68 179 21.5CaSiEBA-4.3/1.1 69 165 15.6CaSiEBA-4.3/8.0 52 151 32.5

CaSiEMAA-1.3/3.0 50 152 38.6CaSiEMAA-1.3/7.0 48 149 39.1CaSiEMAA-3.1/1.5 87 174 10.0CaSiEMAA-3.1/2.5 85 186 11.2CaSiEMAA-3.1/10 70 190 12.1CaSiEMAA-4.1/1.5 99 187 3.1

L. Karlsson et al. / Polymer Degradation and Stability 94 (2009) 527–532 529

through the material and the probability of dripping. However,a too high viscosity reduces the ability of the material to swell. Themost optimal situation would therefore be to have a relatively lowviscosity at temperatures when the material is able to swell andhigher viscosity after the swelling, resulting in a more stable,foamed structure and reduced diffusion rate.

EBA contains butyl acrylate groups that undergo ester pyrolysisaround 300 �C to form acrylic acid and butene [8]. CaSiEBA swellwhen volatiles are formed inside the material. When ionomers areformed, the cross-links stabilise the foamed structure. EMAAcontains acrylic acid initially and is hence, able to react with thecalcium ions at lower temperatures than EBA. This results in lessswelling and a lower transportation rate of volatile gases. Theadvantages and disadvantages of using either polymer will here bediscussed. The ester pyrolysis and ionomer formation from EBA andcalcium carbonate are shown in Scheme 1.

3.1. Fire behaviour

Data from cone calorimeter measurements on CaSiEBA andCaSiEMAA formulations are presented in Table 1. The CaSiEBAmaterials have similar values in peak heat release rate (PHRR) butdiffer in ignition time. Materials with higher butyl acrylate contentignite sooner and seem less stable than the materials with lowerbutyl acrylate content. CaSiEBA-1.7/0.45 and CaSiEBA-4.3/1.1 breakup in a cross-like pattern over the whole surface at ignition.CaSiEBA-1.7/0.45 retains the initial formed cracks and the burningcontinues at those cracks while CaSiEBA-4.3/1.1 is very unstable

Fig. 1. Heat release rate from cone calorimeter measurements at 35 kW/m2.

after ignition and new cracks are formed continuously. The longerignition time of CaSiEBA-1.7/0.45 confirms earlier results [2] andshows that a high acrylic content is crucial for the initial fireperformance.

Low acrylic acid CaSiEMAA materials perform very poorly in thetest and have the highest PHRR of all materials and a short, intenseburning. A large reduction in heat release rate is seen for CaSiEMAAmaterials with acrylic acid content above 3 mol%. The test behav-iour is similar for these materials with a slow burning at one ora few cracks in the surface and long burning times. The ignitiontime is similar for all CaSiEMAA materials with high acrylic acidcontent with values within a 15 s interval. Fig. 1 shows the heatrelease rate curves of CaSiEBA-1.7/0.45, CaSiEBA-4.3/1.1 andCaSiEMAA-4.1/1.5.

For a material to pass as good flame retardant cable material itdoes not only need to have a low heat release but also needs not tosustain dripping since dripping is a possible cause for flame spread.Dripping of material can also expose underlying material to theflame and intensify the fire. Intumescent systems can collapse andlose its protecting surface if the material is not strong enough towithstand dripping. In Casico, a protective glassy layer is formed ontop of the swelled material when cyclic siloxanes react with oxygento form silicone dioxide [1]. If dripping occurs, this barrier layerloses its effect and combustible polymer is exposed to the flame.Table 2 shows the relative amount of dripping, time to dripping andtotal burning time for different CaSiEBA and CaSiEMAAformulations.

Formation of a protective layer occurs both on the top andbottom of the specimen since the flame is applied from beneath thesurface. The viscosity of the melt is crucial for the behaviour in thetest, both for the resistance to drip and deformation aspects.CaSiEBA showed dripping tendencies after approximately 1 min.When comparing CaSiEBA formulations with similar acrylatecontent but different MFR it is seen that low MFR materials drippedless than materials with higher MFR. This confirms earlier obser-vations that a high melt viscosity of the base resin is important fordripping resistance. CaSiEBA-4.3/1.1 showed best resistance todripping of the CaSiEBA materials. CaSiEMAA with low MAAcontent started to drip at an early stage and showed severe drip-ping. CaSiEMAA materials generally dripped less than CaSiEBA withexception of the formulations with lowest MAA content. Fig. 2shows the amount of dripping of CaSiEMAA against mol% MAA. It isclear that the amount of dripping depends of the amount of MAAand not to the MFR of the polymer. No similar trend with BAcontent is observed for CaSiEBA. A possible explanation to thisdifference is that ionomers are present at the temperature whenthe CaSiEMAA material starts to drip while in CaSiEBA, the drippinghas already started when ionomers are formed and the stabilisingeffect is not as pronounced.

Fig. 4. IR absorbance spectra of volatile decomposition products from TGA at 461 �Cfor CaSiEBA-4.3/1.1 and 480 �C for CaSiEMAA-4.1/1.5 (when maximum intensity wasobtained in the detector).

Fig. 2. Average dripping of CaSiEMAA formulations with varying comonomer content.

L. Karlsson et al. / Polymer Degradation and Stability 94 (2009) 527–532530

3.2. Thermal treatments

The flame retardant performance of a material is much depen-dent on the behaviour in the initial stage of the fire where delayedignition time and slow fire propagation are desired. The formationof ionomers, foaming and formation of a protective surface layer areimportant factors for improved flame resistance. In the followingsection, changes in weight, chemical structure and rheologicalproperties during heating are discussed. In common for themethods used are that they investigate the behaviour of thematerial prior to ignition and the heating rate is much lower than ina real fire.

The mass loss rate depends much on transport rate of volatilesto the surface. Ionomer formation leads to increased melt viscosityand, hence reduced release rate of gaseous products. Fig. 3 showsthe thermogravimetric curves up to 600 �C for CaSiEBA-4.3/1.1 andCaSiEMAA-4.1/1.5. These two materials have been chosen due totheir similarity in molar content of comonomer and MFR. However,all materials of same chemical composition (independent of acry-late content or MFR) had similar mass loss curves. The main massloss of CaSiEMAA takes place at approximately 75 �C highertemperature than for CaSiEBA.

Fig. 4 shows the IR absorbance spectra of volatiles fromCaSiEBA-4.3/1.1 and CaSiEMAA-4.1/1.5 when the intensity in the

Fig. 3. Weight loss curves of CaSiEBA-4.3/1.1 and CaSiEMAA-4.1/1.5 decomposed in air.

detector was highest (461 and 480 �C respectively). In Table 3, themost significant peaks are listed.

In Figs. 5 and 6, thermograms for CaSiEBA-4.3/1.1 and CaSiE-MAA-4.1/1.5 are presented together with the development of themost abundant peaks in the IR absorbance spectra of the evolvedgases. In both cases, the highest absorbance is seen for carbondioxide, originating mainly from degradation of the polymerbetween 300 and 500 �C and conversion of calcium carbonate tocalcium oxide at 700 �C. The amount of carbon dioxide formed fromthe ionomer formation reaction is very small compared to theamount formed from the degradation of the polymer. In CaSiEBA,the relative height of the CO2 peak compared to other peaks orig-inating from alkanes and carbonyl groups from the polymerdegradation together with Si–O peaks from siloxanes degradationis larger than for CaSiEMAA. A possible explanation to this could bethat the narrower temperature range for the main mass loss inCaSiEMAA makes the gas phase combustion reactions less exten-sive and more of the initial degradation products reach thedetector. In the spectra from CaSiEMAA, a peak is present at950 cm�1 probably originating from ethylene.

Fig. 7 shows the absorbance spectra of CaSiEBA-4.3/1.1 ther-mally treated at different temperatures in tubular oven. The peakat 1735 cm�1 originating from the ester group in EBA is presentup to 300 �C. The large peak at 1300–1500 cm�1 originating fromthe calcium carbonate is broadened at higher temperatures andat 300 �C a shoulder at approximately 1560 cm�1 is present. Thisis due to interactions between formed acrylic acid and calciumions from the calcium carbonate leading to ionomer formation. At325 �C, all butyl acrylate groups are converted to acrylic acidgroups. The FTIR spectrum from CaSiEMAA-4.1/1.5, Fig. 8, showsa peak at 1698 cm�1 originating from the carboxylic acid group in

Table 3Peaks identified in IR absorbance spectra of volatile decomposition products fromCaSiEBA and CaSiEMAA.

Wavenumber [cm�1] Origin Present in

669 CO2 CaSiEBA and CaSiEMAA950 Ethylene CaSiEMAA1026 Si–O stretching vibration CaSiEBA and CaSiEMAA1745 C]O stretch CaSiEBA and CaSiEMAA2359 CO2 CaSiEBA and CaSiEMAA2938 –CH2– stretch CaSiEBA and CaSiEMAA

Fig. 5. Weight loss curve (dotted line) of CaSiEBA-4.3/1.1 together with the develop-ment of the most abundant absorbance peaks in IR the spectra. Fig. 7. IR absorbance spectra of residues from CaSiEBA-4.3/1.1 treated in nitrogen

atmosphere at 200–350 �C. The ester peak at 1735 cm�1 is fully diminished at 325 �C.

Fig. 8. IR absorbance spectra of residues from CaSiEMAA-4.1/1.5 treated in nitrogenatmosphere at 200–350 �C. The carboxylic acid peak at 1698 cm�1 is fully diminishedat 325 �C.

L. Karlsson et al. / Polymer Degradation and Stability 94 (2009) 527–532 531

the polymer. Since acid is already present in the polymer, ion-omers are formed with the calcium ions at lower temperaturesthan in CaSiEBA. The carboxylic acid peak is absent up to 250 �C,after that all acidic groups are believed to interact with thecalcium ions.

Melt viscosity is a crucial parameter for the flame retardantproperties as a high melt viscosity prevents dripping and facilitatesfor the material to retain its shape during fire. The complexviscosity of CaSiEBA-4.3/1.1 and CaSiEMAA-4.1/1.5 are given inFig. 9. The virgin polymers have similar MFR (1.1 and 1.5 respec-tively). However, a somewhat higher melt viscosity was observedfor CaSiEMAA at 150 �C, most likely due to interactions with thecalcium carbonate and possible anhydride formation. As thetemperature increases, the viscosity decreases for both formula-tions until the acidic groups start to interact with the calcium ionsto form ionomers. For CaSiEMAA-4.1/1.5 this occurs around 250 �C.After this, the increase in viscosity is rapid up to 350 �C. Theviscosity of CaSiEBA-4.3/1.1 continues to decrease to 325 �C andthereafter increases before the test ends at 350 �C.

The EBA polymers used in this study generally have lower MFRthan the EMAA grades giving a higher initial melt viscosity.Although the initial melt behaviour is important since a low MFRprevents dripping, it is not sufficient for the whole fire process andother strengthening reactions need to take place.

Fig. 6. Weight loss curve (dotted line) of CaSiEMAA-4.1/1.5 together with the devel-opment of the most abundant absorbance peaks in IR the spectra.

Fig. 9. Complex viscosity of CaSiEBA-4.3/1.1 and CaSiEMAA-4.1/1.5 measured inparallel plate mode between 150 and 350 �C.

L. Karlsson et al. / Polymer Degradation and Stability 94 (2009) 527–532532

4. Conclusions

The effect of using a methacrylic acid containing copolymertogether with calcium carbonate and silicone elastomer comparedto a butyl acrylate containing copolymer has been shown in termsof flame retardant structural and rheological properties. Comparedto CaSiEBA, the CaSiEBA formulation obtains a 50% decrease in peakheat release. The methacrylic acid initially provides the materialwith carboxylic acid groups that interact with calcium ions fromthe calcium carbonate starting at a temperature as low as 250 �C.The earlier formation of interactions in CaSiEMAA results in highermelt viscosity and less swelling i.e. the overall flame retardantperformance is improved. At the stage when ionomers are formedin CaSiEMAA, the fire is not fully developed and hence a good fireretardant effect is obtained. In the EBA case, butene has to bereleased by ester pyrolysis before acid groups are available forcross-linking. This occurs above 300 �C and postpones the fireretardant effect compared to EMAA. The intumescent tendency issomewhat lowered due to the high viscosity.

Acknowledgements

BOREALIS AB, Business Region Gothenburg and Region VastraGotaland are gratefully acknowledged for their financial supportthrough PLUS Competence Centre at Chalmers University of

Technology. The authors would also like to thank Professor RichardHull and Dr. Anna Stec for help with TGA/FTIR measurements.

References

[1] Hermansson A, Hjertberg T, Sultan B-Å. The flame retardant mechanism ofpolyolefins modified with chalk and silicone elastomer. Fire and Materials2003;27(2):51–70.

[2] Lundgren A, Hjertberg T, Sultan B-A. Influence of the structure of acrylategroups on the flame retardant behavior of ethylene acrylate copolymersmodified with chalk and silicone elastomer. Journal of Fire Sciences2007;25(4):287–319.

[3] Hermansson A, Hjertberg T, Sultan B-Å. Linking the flame-retardant mecha-nisms of an ethylene-acrylate copolymer, chalk and silicone elastomer systemwith its intumescent behaviour. Fire and Materials 2005;29(6):407–23.

[4] Kramer RH, Blomqvist P, Hees PV, Gedde UW. On the intumescence ofethylene–acrylate copolymers blended with chalk and silicone. PolymerDegradation and Stability 2007;92(10):1899–910.

[5] Reaction to fire tests – heat release, smoke production and mass loss rate – Part1: Heat release rate (cone calorimeter method), ISO 5660-1:2002. Geneva:International Organization for Standardization; 2002.

[6] Suzuki M, Wilkie CA. The thermal degradation of acrylonitrile–butadiene–styrene terpolymer as studied by TGA/FTIR. Polymer Degradation and Stability1995;47(2):217–21.

[7] Wilkie CA. TGA/FTIR: an extremely useful technique for studying polymerdegradation. Polymer Degradation and Stability 1999;66(3):301–6.

[8] Sultan B-Å, Sorvik E. Thermal degradation of EVA and EBA – a comparison. I.Volatile decomposition products. Journal of Applied Polymer Science 1991;43(9):1737–45.