Ultrasonic depolymerization of aqueous polyvinyl alcohol

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Abstract

Ultrasonication has proved to be a highly advantageous method for depolymerizing macromolecules because it reduces their molecular weight simply by splitting the most susceptible chemical bond without causing any changes in the chemical nature of the polymer. Most of the effects involved in controlling molecular weight can be attributed to the large shear gradients and shock waves generated around collapsing cavitation bubbles. In general, for any polymer degradation process to become acceptable to industry, it is necessary to be able to specify the sonication conditions which lead to a particular relative molar mass distribution. This necessitates the identification of the appropriate irradiation power, temperature, concentration and irradiation time. According to the results of this study the reactors constructed worked well in depolymerization and it was possible to degrade aqueous polyvinyl alcohol (PVA) polymer with ultrasound. The most extensive degradation took place at the lowest frequency used in this study, i.e. 23 kHz, when the input power was above the cavitation threshold and at the lowest test concentration of PVA, i.e. 1% (w/w). Thus this study confirms the general assumption that the shear forces generated by the rapid motion of the solvent following cavitational collapse are responsible for the breakage of the chemical bonds within the polymer. The effect of polymer concentration can be interpreted in terms of the increase in viscosity with concentration, causing the molecules to become less mobile in solution and the velocity gradients around the collapsing bubbles to therefore become smaller.

Introduction

Most chemists regard sonochemistry as a relatively new branch of chemistry. However, applications of power ultrasound in polymer science date back to the 1930s when sonication of some natural polymers was discovered to reduce viscosity [1]. Schmidt and Rommel first observed the permanent reduction in the viscosity of a polymer solution and attributed it to the breakage of covalent bonds in the polymer chain. They also found that the initial depolymerization rate slowed and stopped completely when the minimum molecular mass was approached. The existence of this limiting degree of polymerization constitutes the basis of most degradation mechanisms [2].

Ultrasonication has subsequently proved to be a highly advantageous method for depolymerizing macromolecules because it reduces their molecular weight simply by splitting the most susceptible chemical bond without causing any changes in the chemical nature of the polymer. It is now well established that prolonged exposure of solutions of macromolecules to high-energy ultrasonic waves produces a permanent reduction in viscosity. Even when the irradiated polymers are isolated and redissolved their viscosity remains low in comparison with that of non-irradiated solutions [3].

Most effects in sonochemistry arise from cavitation. While some consequences of this, such as radical production, are used in the manufacture of polymers, the exact origin of the effects, whether from thermal ‘hot spots’ or electrical or coronal discharges, is relatively unimportant to the polymer chemist. Most of the effects involved in controlling molecular weight can be attributed to the large shear gradients and shock waves generated around collapsing cavitation bubbles [1].

Earlier studies have shown that the degradation is caused by

  • 1.

    the hydrodynamic forces of cavitation, i.e. the shock wave energy released on bubble implosion;

  • 2.

    the shear stresses at the interface of the pulsating bubbles;

  • 3.

    the associated thermal and pressure increases within the bubbles themselves [2].


In general, for any polymer degradation process to become acceptable to industry, it is important to be able to specify the sonication conditions which lead to a particular relative molar mass (RMM) distribution. This necessitates the identification of the appropriate irradiation power, temperature, concentration and irradiation time [4].

The aim of the present work was to study the effect of ultrasonic input power and ultrasonic frequency at three aqueous solution concentrations on the degradation of polyvinyl alcohol (PVA) with ultrasonic reactors constructed in this study (Fig. 1).

Section snippets

Reactors

Three ultrasonic reactors operating at 23, 40 and 900 kHz were constructed in this study. The volume of reactors was 10 l. The 23 and 40 kHz round-shaped reactors were made of steel. The rectangular-shaped 900 kHz reactor was made of plastic. The 23 kHz reactor had five and the 40 kHz reactor seven Langevin-type transducers installed on the bottom of the reactors. The surface of the 900 kHz transducer formed the entire bottom of the rectangular-shaped reactor.

Hydrophone measurements

There are many types of acoustic

Hydrophone measurements

Initially the effect of ultrasonic input power on ultrasonic intensity was studied from hydrophone measurements. Results showed the potential for measuring ultrasonic intensity by this method. Ultrasonic intensity increased linearly when the ultrasonic input power increased up to 500 W (Fig. 2).

The hydrophone measurements revealed several hot spots in the reactors, where ultrasonic intensity was greater than in the surrounding areas (Fig. 3). The hydrophone measurements therefore verified that

Conclusions

The reactors constructed in this study worked well in depolymerization and it was possible to degrade PVA polymer with ultrasound.

In earlier publications it has been reported that the higher the frequency of the ultrasonic wave, the more rapidly it is attenuated. However, it is important to recognize that when comparing the effects of frequency, intensity must be held constant [3]. In this study, when using an ultrasonic input power of 200 W, the highest intensity value at 23 kHz according to

Acknowledgements

The authors thank TEKES, the National Technology Agency of Finland, and VTT Energy, the Technical Research Centre of Finland, for their financial support.

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