Chemical Engineering and Processing 47 (2008) 1451–1455

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Chemical Engineering and Processing:Process Intensification

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Ultrasonic degradation of low-density polyethylene

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Vaibhav Desaia, M.A. Shenoya, P.R. Gogateb,∗

a Department of Polymer Engineering and Technology, Institute of Chemical Technologyb Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumb

a r t i c l e i n f o

Article history:Received 18 May 2007Received in revised form 1 December 2007Accepted 17 February 2008Available online 10 March 2008

Keywords:Ultrasonic degradationLow-density polyethyleneIntrinsic viscosity

a b s t r a c t

Ultrasonic degradation ofhas been investigated usiconcentrations (1%, 1.2%, ato ultrasonic irradiation aof different operating parincreases with a decreasemajor part in enhancingdifference in the values ofurther ultrasonic irradiatnegligible. But 1.4% solutthat for 50 ml and 75 ml involume of operation.

1. Introduction

During the past few decades, new technologies have been devel-oped, each of which offers the hope of providing chemists withinnovative routes and chemical engineers with sophisticated meth-

ods of introducing energy to bring about chemical change. Use ofultrasonic irradiation or in general cavitation phenomena has alsobeen looked as a promising technology for physical as well as chem-ical processing applications [1,2]. The chemical effects of sonolysisare a direct result of the cavitation, which can produce localizedhigh temperatures and pressures (hot spots) along with genera-tion of highly reactive free radicals and liquid circulation currentsassociated with high shear [2,3]. In the field of polymer chemistryalso, application of ultrasound has been known to result in polymerdegradation which can be quantified either by molecular weightdistribution or by change in the intrinsic viscosity [4]. In classicalchemical usage, the term “degradation” means a breaking down ofchemical structure but in terms of polymer chemistry “degrada-tion” seems to imply a decrease in molecular weight or intrinsicviscosity of the solution.

Ultasonication has proved to be a highly advantageous methodfor depolymerizing macromolecules because it reduces theirmolecular weight simply by splitting the most susceptible chem-ical bond without causing any changes in the chemical nature ofthe polymer. In the degradation of polymers in solution, hot spots

∗ Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614.E-mail address: parag@udct.org (P.R. Gogate).

0255-2701/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.cep.2008.02.003

nga, Mumbai 40019, India19, India

density polyethylene (LDPE) polymer in o-dichlorobenzene as a solventcometry as a technique for monitoring the rate of degradation. Different

.4%, w/v) and different volumes (50 ml, 75 ml and 100 ml) were subjectedo different operating temperatures (60 ◦C and 80 ◦C) to study the effecters on the extent of degradation. It was found that extent of degradationction volume and concentration. Also lower operating temperature playsvitational effect and hence results in higher extent of degradation. The

iting viscosities (constant solution viscosity which does not decrease byfor 50 ml and 75 ml solutions for each of 1% and 1.2% concentration was

100 ml volume showed slightly higher value of limiting viscosity thanting lower efficacy of ultrasonic degradation at higher concentrations and

© 2008 Elsevier B.V. All rights reserved.

generated due to cavitation could be of minor importance as hotspots are highly localized and get quenched within a very shorttime (1 �s). According to several theories, motion of the wall ofviolently collapsing bubble causes movements of solvent moleculesaround these bubbles. These movements set up large shear fieldsthat are responsible for the degradation of polymer [4–8]. It isnow well established that prolonged exposure of solutions ofmacromolecules to high-energy ultrasonic sound waves producesa permanent reduction in viscosity [9]. Even when the irradiated

polymers are isolated and redissolved their viscosity remains lowin comparison with that of non-irradiated solutions [9].

Ultrasonic irradiation also results in degradation of the polymermelts when subjected to continuous irradiation, e.g. in processessuch as extrusion [10], which in turn affects their mechanicalproperties. Lin and Isayev [11] have investigated the effect of high-intensity ultrasound on the mechanical and rheological propertiesof polypropylene (PP), polyamide 6 (PA6), and their blends. It hasbeen reported [11] that the observed improvements in the mechan-ical properties of ultrasonically treated PA6 were attributed tocondensation reactions, which yield a higher molecular weight,a higher crystalline morphology, and a more uniform crystal sizedistribution. At high ultrasound amplitudes, for PP, the degrada-tion of polymer chains was observed with little deterioration of themechanical properties. There have been many other studies report-ing the similar effects of ultrasonic irradiation on polymer melts[12–14].

In general, it can be said that ultrasonic irradiation does have asignificant effect on polymers in terms of its mechanical, mechano-chemical and morphological properties. Understanding the effect

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The extent of ultrasonic degradation of polymer solutions has beenquantified by using a parameter, ϕ = ([�] − [�∞])/[�]) × 100, where[�] and [�∞] is initial intrinsic viscosity and limiting intrinsic vis-cosity, respectively.

3. Results and discussion

3.1. Effect of reaction volume

Effect of reaction volume on the extent of degradation has beeninvestigated at constant power dissipation of the ultrasonic reac-tor. Fig. 1 shows the effect of the reaction volume on the extent ofdegradation depicted in terms of the change in [�] at 1.4% polymerconcentration at 80 ◦C. It can be easily seen from the figure thatthe extent of degradation decreases with increase in the reactionvolume at same supplied ultrasonic power dissipation. To give aquantitative idea, in 180 s of irradiation time the extent of degra-dation for 50 ml of 1% concentration solution is 30% higher as

1452 V. Desai et al. / Chemical Engineeri

of different operating parameters in the case of ultrasonic irradia-tion on the extent of degradation of polymers, thus, becomes veryimportant. Earlier work on application of ultrasonic irradiation fordegradation of polymeric compounds includes studies with isotac-tic PP [15], polyvinyl alcohol [16,17], xylan [18,19], dextranes [20],carboxymethyl cellulose [21], polyvinyl acetate [22], polystyrene[23], etc. as the parent polymer compound. Though low-densitypolyethylene (LDPE) is important commercial polymer, there are nostudies on the ultrasonic degradation of this polymer. The objectiveof this study is to present new experimental data for the ultra-sonic degradation of LDPE and optimize the operating parametersfor maximization of the extent of degradation. The extent of degra-dation has been quantified in terms of the change in the intrinsicviscosity of the polymer solution, which is a simple method formonitoring the rate of degradation and used generally in moni-toring the rate of polymer degradation [14,16,17,21]. The extent ofpolymer degradation can also be monitored using the analysis ofmolecular weight distribution using gel permeation chromatogra-phy [12,13] but Li et al. [13] have shown that the type of analysistechnique does not affect the obtained trends in terms of the effectof operating parameters on the extent of polymer degradation.

2. Experimental

2.1. Materials

The material used was LDPE (Indothene, 20XL020), obtainedfrom IPCL, India. LDPE has the melt flow index of 2 g/10 min (ASTMD1238-90b). The solvent (reagent grade) used for this experimentwas o-dichlorobenzene (Loba Chemie, India).

2.2. Equipment and procedure

Solutions of LDPE with different concentrations (1%, 1.2%, and1.4%, w/v) were prepared with precision of ± 1 × 10−2 g (total vol-ume of solvent being 100 ml). The ultrasonic generator (DakshinIndia, 22.5 kHz, horn tip diameter of 2 cm) with the total suppliedpower input of 240 W was used. The actual power dissipated inthe system was estimated using calorimetric measurements [24]and was observed to be equal to 15.5 W giving an energy effi-ciency of approximately 7%. Calorimetric measurements for eachrun also indicated that the ultrasonic power dissipated in the solu-tion is same for each of the viscosities investigated in the present

work. The temperature of polymer solution and viscometer bathwas maintained within ± 1 ◦C using a thermo stated water bathand oil bath, respectively. The water bath temperature was con-trolled based on the periodic measurements of the temperature inthe reactor so that a near constant temperature can be achievedin the system. This was also possible due to the fact that substan-tially less amount of energy is being transferred into the reactordue to low energy efficiency of the ultrasonic equipment used inthe work. The degradation of solutions was carried out in the sameglass beaker for all the experimental runs. The top of the beaker waswrapped with aluminium foil to minimize solvent evaporation.

2.3. Measurement and characterization

Periodically samples of sonicated solutions were removed forviscosity measurement (at 88 ± 1 ◦C) by Ubbelohde viscometer(Technico, Number 1). The location of the sampling point was nearthe bottom of the reactor, i.e. away from the location of the ultra-sonic horn and same location was used in all the experiments for theremoval of the sample. Due to distribution of cavitational activityin the reactor (maximum very near to the horn surface and reduced

Processing 47 (2008) 1451–1455

activity away from the surface of the ultrasonic horn), it might hap-pen that maximum extent of degradation is obtained very nearto the ultrasonic horn and hence sampling point at this locationshould be avoided.

Relative and specific viscosities (�r and �sp, respectively) werecalculated using following formulae:

�r = t

t0(1)

�sp = �r − 1 (2)

where t and t0 are the efflux time for polymer solution and sol-vent, respectively. Reproducibility of the efflux time was within0.3 s. Experiments were repeated twice to check the reproducibil-ity of the obtained data for the variation of concentration againsttime for all the sets. It has been observed that experimental errorswere within ± 5%. The � values for the LDPE solution at differentconcentrations were calculated by the one-point intrinsic viscosityequation [25].

� = [2(�sp − ln �r)] 0.5

c(3)

The variation of either molecular weight or the intrinsic viscos-ity in the presence of ultrasonic irradiation reflects the ultrasonicdegradation of polymer. In this work, it was found that all degradedsamples were dissolved completely in o-dichlorobenzene, indicat-ing that no cross-linking took place under ultrasonic irradiation.

compared to degradation for 100 ml volume. This is attributed tothe fact that, increase in the reaction volume decreases the powerdensity of the system (power dissipation per unit volume) resulting

Fig. 1. Effect of reaction volume on the progress of ultrasonic degradation at 1.4%LDPE solution.

V. Desai et al. / Chemical Engineering and

Fig. 2. Effect of reaction volume on the progress of ultrasonic degradation at 1.2%LDPE solution.

in a corresponding decrease in the cavitational activity. Also in thecase of cavitational horn, the active cavitational volume is restrictedvery near to the transducer surface, resulting in non-uniform distri-

bution of the cavitational activity. With an increase in the operatingvolume, there exists enhanced number of dead zones where thecavitational activity is minimal resulting into detrimental effects.

Figs. 2 and 3 show the effect of reaction volume on the extent ofdegradation for two other concentrations (1.2% and 1.0%) studiedin the present paper. It can also be seen from Fig. 2 that for 1.2%concentration solution with 50 ml and 75 ml volume, percentagedecrease in intrinsic viscosity is nearly the same. Similar observa-tion has been found with 1.4% concentration. It can also be seenfrom Figs. 1–3 that, for volumes 50 ml and 75 ml, the extent ofdegradation for each concentration (1.0%, 1.2%, and 1.4%) is higheras compared to the 100 ml volume of reaction mixture.

The credence to the effect of power density of the system on theextent of polymer degradation can be obtained from the literatureillustrations. Groonroos et al. [6] have shown that the power den-sity needs to be always above the cavitation threshold required foronset of the cavitational effects and the extent of viscosity reduction(and hence the extent of polymer degradation) obtained is higher athigher levels of power density used in the system. Li et al. [13] havealso reported that the extent of degradation of ethylene propylenediene monomer (EPDM) increased with an increase in the power

Fig. 3. Effect of reaction volume on the progress of ultrasonic degradation at 1.0%LDPE solution.

Processing 47 (2008) 1451–1455 1453

Fig. 4. Effect of operating temperature on percentage degradation (ϕ) at differentconcentration and reaction volumes.

density and Harkal et al. [17] have reported similar results for thedegradation of polyvinyl alcohol.

3.2. Effect of operating temperature

Majority of chemical reactions are accelerated by an increaseof temperature. However, opposite effect is often seen for thechemical reaction induced by ultrasound. Indeed, this negative‘temperature coefficient’ has been cited as proof that a solutionprocess is mechanical in origin. Ultrasonic degradation of poly-mer solution often gives faster rate at lower temperature. Due tonon-soluble nature of low-density polyethylene molecules in theselected solvent at temperatures below 60 ◦C, 60 ◦C, and 80 ◦C hasbeen selected as the desired temperatures to investigate the effectof the operating temperature.

Fig. 4 shows the percentage degradation (ϕ) at 60 ◦C and 80 ◦C.At 60 ◦C, percentage degradation (ϕ) is much higher as comparedto that obtained at 80 ◦C for all the concentrations and reactionvolumes investigated in the work. The observed results can beattributed to the fact that, as the temperature of the solutionincreases, a large quantity of the solvent vapor enters the cavita-tion bubbles during their expansion and exerts a cushioning effect[4,26] during the collapse leading to diminishing of the intensity ofthe shock wave, leading to reduced degradation at higher temper-atures. The results reported in this study, are consistent with theresults reported for ultrasonic degradation of polymers like native

dextran [5], polystyrene [27], and polyacrylmethacrylates [28].

3.3. Effect of initial concentration of polymer

It can also be seen from Figs. 1–3 that at a constant reactionvolume of 50 ml for different concentrations of 1%, 1.2%, and 1.4%,respectively, the extent of degradation decreases with an increasein solution concentration at same supplied ultrasonic power. Thedegradation is much faster and extent of degradation is higher in theless concentrated solutions. Quantitatively speaking, the extent ofdegradation at 1.0% concentration is more than three times higheras compared to 1.4% concentration. Similar results in terms of lowerextent of degradation at higher concentration have been reportedearlier [29,30]. It should be also noted that the extent of decreasein the degradation observed in the present case for polymeric solu-tions is much higher as compared to other species reported in theearlier work [29,30]. This can be attributed to a strong dependenceof viscosity of the polymeric solution on the concentration whichseverely suppresses the degree of cavitation. It should be notedhere that the intensity of cavitation remarkably falls down withan increase in the viscosity of the medium. Independent bubble

ng and Processing 47 (2008) 1451–1455

1454 V. Desai et al. / Chemical Engineeri

Fig. 5. Variation of percentage degradation with power density for different con-centrations at 60 ◦C.

dynamics studies have clearly indicated that an increase in the vis-cosity results in decrease in the collapse pressure generated dueto cavitation [31]. Also an increase in viscosity with concentrationresults in the molecules to become less mobile in solution andthe velocity gradients around the collapsing bubbles to thereforebecome smaller, resulting in lower extents of degradation.

3.4. Relation of percentage degradation (ϕ), reaction volume (Vr),and actual power input (P)

The relationship of percentage degradation (ϕ), reaction vol-ume (Vr), and actual power input (P) has been established withan aim of developing design correlations for predicting the extentof degradation of LDPE. The variation of extent of degradation withpower density (P/Vr) at different concentrations has been shown inFig. 5. Following equations were obtained for 1.0%, 1.2%, and 1.4%concentration solutions, respectively

ϕ = 2.03(

P

Vr

)0.56(4)

ϕ = 2.09(

P

Vr

)0.49(5)

ϕ = 2.26(

P

Vr

)0.47(6)

Above equations shows that

(i) The proportionality index by which ϕ varies with respect to(P/Vr) is varying from 0.47 to 0.56 for all concentrations indi-cating a dependency of the intensity of cavitation on the extentof degradation.

(ii) The proportionality constant is nearly same for all the concen-trations studies in the present work.

3.5. Effect of reaction volume and concentration on limitingviscosity

Fig. 6 shows the effect of reaction volume on the limiting viscos-ity of LDPE solution when it is subjected to ultrasonic irradiation.Fig. 6 shows that, for each concentration investigated in the presentwork, the limiting viscosities at 75 ml and 50 ml reaction volume arenearly equal. The actual value of the limiting viscosity is howeverdependent on the concentration and increases with an increase inthe concentration. It can also be seen from figure that, for 100 mlsolution the limiting viscosity is higher than that of 50 ml and

[

Fig. 6. Effect of reaction volume and concentration on limiting viscosity.

75 ml reaction volume. This can be attributed to lower intensityof cavitation at higher volumes of operation and higher solutionconcentrations. The obtained results are consistent with the resultsreported for the ultrasonic degradation of poly(vinyl alcohol) [17].

4. Conclusions

The effect of reaction volume, concentration, and tempera-ture on the ultrasonic degradation of LDPE in o-dichlorobenzenehas been investigated. The results indicate that the percentagedegradation reduced with increasing reaction volume, increasingconcentration as well as reaction temperature. It has been observedthat major extent of degradation takes place in the initial period ofirradiation time indicating that continuous operation with ultra-sonic horn is indeed possible which is a must considering industrialscale operation and treatment of large volumes of the streams con-taining low concentrations LDPE. The limiting viscosity was alsodependent on the solution concentration and volume both affecting

the generated intensity of cavitation in the ultrasonic reactor.

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