Polymer International 42 (1997) 117È120

Functionalisation of Polypropylene withMaleic Anhydride and Acrylic Acid for

Compatibilising Blends of Polypropylenewith Poly(ethylene terephthalate)

A. R. Oromehie, S. A. Hashemi*

Polymer Research Centre of Iran, PO Box 14185, 458 Tehran, Iran

I. G. Meldrum & D. N. Waters

Department of Chemistry, Brunel University, Uxbridge, Middlesex, UB8 3PH, UK

(Received 11 June 1996 ; revised version received 5 August 1996 ; accepted 4 September 1996)

Abstract : This paper reports an investigation of the functionalisation of poly-propylene with polar monomers as an interfacial stabiliser for mixtures of poly-oleÐns with polyesters. Polypropylene was functionalised with maleic anhydrideand acrylic acid in the presence of dicumyl peroxide. The reaction was carriedout in the molten state using a preheated batch Haake Rheometer mixer underconditions which were expected from literature studies to be optimal. The e†ectsof varying the concentrations of the monomers on the degree of functionalisationand properties of the products were investigated. An upper limit of only a fewper cent of functionalisation was observed. Slow degradation was observed atmelt processing temperatures.

Key words : polypropylene, functionalisation, maleic anhydride, acrylic acid

INTRODUCTION

This work is concerned with learning how to developinterfacial agents for controlling the properties of blendsof immiscible polymers and with improving the under-standing of how these agents function at the molecularlevel. The immiscible polymer pair being investigated ispoly(ethylene terephthalate) (PET) and polyoleÐns, par-ticularly polypropylene (PP). Although these are majorpolymers, surprisingly little research has been reportedon blending them and there would appear to be con-siderable scope for further development.

Investigations have been made of the processing con-ditions for blending PET with polyoleÐns with respectto the rheological characteristics of the blend com-

* To whom all correspondence should be addressed.

ponents and to their stability.1 These investigationshave shown that processing conditions can be foundwhich appear to meet requirements for matching therheological properties of the components withoutencountering stability problems. Although PP has beenconÐrmed to be subject to oxidation and to form car-bonyl groups, the residence time and processing tem-perature can be optimised to minimise degradation.Compatibilisers have been prepared for evaluation. Pre-viously reported studies have mainly made use of poly-oleÐns functionalised with maleic anhydride (MA) andacrylic acid (AA).2h5 Although these studies have shownsome signs of e†ectiveness at high concentrations of thefunctionalised polymers, it was thought that moredetailed information about their structure and proper-ties would be helpful in assessing their potential forPET/PP blends.

117Polymer International 0959-8103/97/$09.00 1997 SCI. Printed in Great Britain(

118 A. R. Oromehie et al.

Although the preparation of functionalised poly-propylenes is reported quite extensively in the literature,and they are available commercially, it was consideredto be important to explore the scope for increasing thedegree of functionalisation and to achieve a highenough level of polarity for interaction with PET. Inves-tigations of the functionalisation of preformed polymerswith monomers containing acidic or anhydride groupsin the melt3h5 have established that the efficiency andrate of functionalisation depend on the concentrationsof free radical initiators and functionalising monomerand on the temperature and residence time. Variousprocedures for the functionalisation of MA and AAhave been reported in the literature. These includeirradiation6 and the use of an organic peroxide in themelt,7 solid state8 or in solution.9 The approach takenin this work has been to use an organic peroxide in themelt. The same peroxide (dicumyl peroxide, DCP) hasbeen used throughout and the polypropylene grade, themelt temperature and the reaction time have all beenkept constant. Only the concentration of the functional-ising monomer was varied.

EXPERIMENTAL

Functionalisation reaction

The optimum conditions for processing in a HaakeRheomixer 90 were selected from published studies tobe a nitrogen blanket, temperature of 180¡C, mixingspeed of 60 rpm and residence time of 10 min. PP wassupplied in a granular form by ArakÏs PetrochemicalCo., Iran, and was introduced (40 g) into the mixer.Once the torque had stabilised (about 2 min), a mixtureof MA or AA monomer (Merck, 99%) and DCP(Aldrich, 98%) was added to the molten polymer. Theinitial monomer concentration was varied from 2 to10%. Since decarboxylation is extremely rapid andoccurs to a major extent when the monomer and freeradical initiator come into contact with the moltenpolymer, they were generally added in several portionsover a period of time. Failure to follow this procedurehas resulted in estimates of functionalisation exceedingthe maximum attainable value.8 After the last addition,the mixing was continued for an additional 2 min asdescribed by Gaylord.10

The extruded products were pressed into sheets about10 mm thick and cut into small strips, which wereextracted with boiling water for 4 h to remove unreactedMA and AA. The extracted Ðlms were dried in an ovenat 110¡C for 12 h and further puriÐed by dissolution inreÑuxing xylene and precipitation in acetone. Finallythey were dried in a vacuum oven for 4 days at 105¡C.This treatment also converted any maleic acid to theanhydride.11

Titration of functionalised polymers

The quantitative determination of MA and AA reactedwith PP was carried out as described by Callais andKazmierczak.11 A known weight of functionalisedpolymer was dissolved by reÑuxing in xylene, and thexylene-soluble fraction was titrated with alcoholicKOH, which had been standardised with potassiumhydrogen phthalate using 1% thymol blue in dimethyl-formamide as an indicator. The percentage of function-alised polymer (i.e. the weight of MA or AA per 100 gPP) was estimated using the following relationship :

% Functionalisation of PP

\ 100 ] vol. KOH] conc. KOH] EWof MA or AA/wt of sample

where EW\ equivalent weight (49 or 72 respectively).

Fourier transform infra-red spectroscopy

Samples of PP and PP/MA and PP/AA functionalisedpolymers were processed using the Haake rheometer at180¡C and compression moulded in a heated laboratorypress into approximately 0É1 mm thick Ðlms. Fouriertransform infra-red (FTIR) spectra were measured overa wavenumber range of 200È4000 cm~1 using a BrukerFTIR Spectrophotometer model IFS 48.

Rheological properties

The rheological properties of the components weremeasured with an Instron Capillary Rheometer model3211 with a capillary die of L /D\ 40, at barrel tem-peratures of 180, 200 and 260¡C and for residence timesof 5, 10, 15 and 20 min.

RESULTS AND DISCUSSION

In Figs 1 and 2, the degrees of functionalisation areplotted against the initial concentrations of MA andAA, respectively. Figure 1 shows that the degree of func-tionalisation of MA increases initially but reaches amaximum at 1É6% and then declines again, apparentlylevelling o† at about 1É2%. Figure 2 shows an increasein degree of functionalisation up to 2É3% of AA andthen a levelling o† at this value. Approximately twice asmuch AA was used to achieve this higher degree offunctionalisation compared with MA. Factors inÑu-encing the level of functionalisation include the pro-cessing conditions, type of functionalising monomer andtype of initiator. The maximum level of functional-isation evidently does not always correspond with thehighest level of initial monomer concentration.

The FTIR spectra for PP homopolymer and thePP/MA and PP/AA functionalised polymers are given

POLYMER INTERNATIONAL VOL. 42, NO. 1, 1997

Compatibilising blends of PP with PET 119

Fig. 1. Degree of functionalisation of MA versus initial MA at180¡C, 60 rpm and 10 min.

Fig. 2. Degree of functionalisation of AA versus initial AA at180¡C, 60 rpm and 10 min.

Fig. 3. FTIR spectra for pure PP and PP/MA with 2, 3 and4% MA.

Fig. 4. FTIR spectra for pure PP and PP/AA with 2, 3 and4% AA.

in Figs 3 and 4, respectively. Figure 3 shows two dis-tinct peaks at 1790 and 1865 cm~1 which correspond toa carbonyl group conjugated with methylene sequences,conÐrming the presence of anhydride groups in thePP/MA polymer. It is also noted that absorptionincreased when the MA concentration was increasedfrom 2É5 to 3% and decreased at 4%. Similar peaksoccur in Fig. 4 at wavenumbers of 1725 and 1700 cm~1corresponding to free acid (COOH), conÐrming thepresence of attached acrylic acid groups in the PP/AApolymer. Absorption increased as the initial concentra-tion of AA was increased. These peaks are not presentin pure PP.

Rheological properties are believed to have an impor-tant role in controlling degradation.12 Figure 5 showsplots of melt viscosity against shear rate for pure PP,PP] DCP and PP/MA and PP/AA functionalised

Fig. 5. Melt viscosity versus shear rate for PP, PP/MA,PP/AA and PP] DCP.

POLYMER INTERNATIONAL VOL. 42, NO. 1, 1997

120 A. R. Oromehie et al.

Fig. 6. Melt viscosity versus time for PP, PP/MA andPP/AA.

polymers at a temperature of 200¡C. In the absence ofDCP the viscosity of each polymer decreased withincreasing shear rate. In the case of PP ] DCP, the vis-cosity was lower than for PP and functionalised PP.This is attributable to polymer degradation during meltprocessing. The dependence of melt viscosity on timefor PP and PP/AA at 260¡C is shown in Fig. 6. Onincreasing the time from 5 to 20 min, the viscosity of PPdid not change but decreased slightly after 10 min forthe functionalised polymers. The dependence of meltviscosity on temperature at a shear rate of 100 s~1 isshown in Fig. 7 for PP, PP/MA and PP/AA polymers.The viscosities of PP and the functionalised polymers

Fig. 7. Melt viscosity versus temperature for PP, PP/MA andPP/AA.

decreased as the temperature increased from 225 to270¡C, but the functionalised polymers had a lower vis-cosity than PP.

CONCLUSIONS

Process conditions have been optimised for functional-ising PP with MA and AA. Although the level of func-tionalisation increased initially with increasing level offunctionalising monomer, there was an upper limit forboth MA and AA. The limit was higher for AA (2É4%)than for MA (1É6%). The low levels of these limits mayrestrict the potential of these functionalised polymersfor compatibilising a relatively polar polymer like PET.The basic reason for the limit appears to be competitionof the functionalising reaction with decarboxylation ofthe monomer on contact with molten polymer.However, even after taking steps to minimise this sidereaction, the degree of functionalisation is still restrictedto quite low levels. Stability measurements showed thatboth the functionalised polymers dropped in viscositywith time at 260¡C, indicating that they degrade slowlyat this temperature.

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