Elsevier

Ultrasonics Sonochemistry

Volume 12, Issue 3, February 2005, Pages 183-189
Ultrasonics Sonochemistry

Mechanochemical degradation kinetics of high-density polyethylene melt and its mechanism in the presence of ultrasonic irradiation

https://doi.org/10.1016/j.ultsonch.2003.10.011Get rights and content

Abstract

In this paper, the effect of ultrasonic intensity on the degradation of high-density polyethylene (HDPE) melt, degradation mechanism, ultrasonic degradation kinetics of HDPE melt as well as the development of molecular weight distribution of HDPE melt during ultrasonic degradation were studied. In the initial stage, the ultrasonic degradation of HDPE melt shows a random scission process, and the molecular weight distribution broadens. After that, the ultrasonic degradation of HDPE melt shows a nonrandom scission process, and the molecular weight distribution of HDPE melt narrows with ultrasonic irradiation time. The average molecular weight of HDPE decreases with the increase of ultrasonic intensity and increases and trends forward that of undegraded HDPE with the increase of distance from ultrasonic probe tip, indicating that attenuation of ultrasonic intensity in HDPE melt is very quick. Ultrasonic degradation kinetics of HDPE melt obeys the equation: Mt=M+Aekt. The theoretic calculation by this equation accords well with the experimental results. The plausible ultrasonic degradation mechanism of polymer melt based on molecular relaxation was also proposed in this paper.

Introduction

In recent decades, high-intensity ultrasound has been widely applied to assist chemical reactions in organic and organometallic synthesis, polymer degradation and polymerization due to cavitation generated by ultrasound in liquid [1]. Cavitation in the ultrasonic field implies nucleation growth and subsequent claps of bubbles or cavities, resulting in violent shock waves with a high temperature of ∼5000 K and a high pressure of ∼1000 bar [2], which is highly sufficient to the scission of chemical bonds. In the area of polymer chemistry, high-intensity ultrasonic irradiation has been widely applied to control the molecular weight and its distribution of polymers through ultrasonic degradation of polymers as well as to synthesize novel polymers and copolymers [3], [4], [5], [6], [7], [8], [9], [10]. Since Henglein [11] firstly reported the polymerization of acrylonitrile in the presence of polyacrylamide in aqueous solution under the influence of ultrasonic irradiation, a lot of block and graft copolymers have been synthesized by this advanced technique [10], [12], [13], [14]. The research work on the ultrasonic polymerization in solution mainly focused on following categories:

  • The copolymerization of polymers: the different macro-radicals formed through the chain scission of polymers can recombine into block/graft copolymers [15], [16].

  • The copolymerization of a solution containing a polymer and a monomer: the macro-radicals formed through the chain scission of polymer can initiate the polymerization of monomer to produce block/graft copolymers [9], [10], [12], [13].

  • Polymerization of monomer in the absence of initiator: the decomposition of water and surfactant in solution under high-intensity ultrasonic irradiation produces radicals to initiate polymerization of monomer [4], [17].

  • Polymerization of monomer in the presence of initiator: ultrasound is applied to decompose initiator to produce radicals that initiate the polymerization of monomer at room temperature [18], [19], [20].

  • Ultrasound enhanced modification of solid polymers in the liquid [13], [21], [22].


The cavitation generated by ultrasound in liquid is responsible for the polymerization. The effect factors beneficial to cavitation are favorable for the polymerization. The effects of ultrasonic intensity and frequency, nature of solvent and dissolved gases, temperature, concentration of polymer, molecular weight of polymer and pressure etc. on the ultrasonic degradation and polymerization of polymers have been widely investigated [1].

In other applications, Isayev et al. [23], [24] reported a novel ultrasonic technology for devulcanization of vulcanized elastomer. The high-intensity ultrasonic wave in the presence of pressure and heat can rapidly break up the three-dimensional network in crosslinked rubber through the scission of C–S, S–S, and C–C bonds. The devulcanized rubber can be reprocessed, reshaped, and revulcanized in the same way as virgin rubber. It was also found that the superposition of ultrasonic waves in extrusion could greatly decrease the viscosity of polymer melts and improve production rate. In our previous work [25], [26], [27], ultrasonic waves were introduced to polymer extrusion, and the experimental results show that ultrasonic oscillations can greatly improve the appearance and processability of polystyrene (PS), high density polyethylene (HDPE), linear low density polyethylene and metallocene catalyzed polyethylene as well as HDPE/PS blends. Ultrasonic oscillations can also greatly enhance the compatibility and mechanical properties of HDPE/PS blends due to in situ formation of inter-chain copolymer of PS–HDPE.

According to cavitation mechanism for the ultrasonic degradation of polymer solution, the degradation of polymer is hardly carried out when the viscosity of solution amounts to 2.0 Pa s due to the disappearance of cavitation. The melt viscosity of polymer is generally around 103–105 Pa s, so cavitation mechanism almost reject the possibility of ultrasonic degradation of polymer melt. However, previous work shows that the ultrasonic irradiation can cause the degradation of polymer melt. To the best of our knowledge, there are few reports on ultrasonic degradation kinetics of polymer melt and its mechanism. In this article, ultrasonic wave was introduced into HDPE melt. The kinetics and mechanisms of degradation of HDPE melt were studied. The effects of ultrasonic irradiation intensity and depth of ultrasonic irradiation on the mechanochemical degradation of HDPE were also discussed.

Section snippets

Materials and equipment

The material used is DGDA6098 HDPE with a number-average molecular weight of 3.18 × 104 and molecular weight distribution of 8.81 and melt flow index of 0.1 g/10 min and density of 0.948 g/cm3, supplied by Plastic Factory of Qilu Petrochemical Co, SINOPEC, China. The ultrasonic degradation of HDPE melt was conducted in a specially designed reactor described in Fig. 1. HDPE was filled into die and heated to a temperature of 200 °C. A probe of ultrasonic oscillations with a maximum power output of

Ultrasonic degradation of HDPE melt

In order to study the effects of ultrasonic irradiation time on the degradation of HDPE melt, molecular weight and its distribution of degraded HDPE samples on the cross section at 1 mm from the probe were first measured. The data were listed in Table 1. It can be clearly seen from the data listed in Table 1 that the weight average molecular weight and molecular weight distribution of HDPE depend on strongly the time of irradiation. In first 2 min of irradiation, HDPE melt undergoes great

Conclusions

  • 1.

    Ultrasonic degradation of HDPE melt occurs based on the mechanism of molecular relaxation of HDPE melt. In the initial stage, ultrasonic degradation of HDPE melt is a random process, and the molecular weight distribution broadens. After that, ultrasonic degradation of HDPE melt is a nonrandom process, and the molecular weight distribution becomes narrows. Finally, molecular weight trends toward a limiting value.

  • 2.

    The degradation kinetics of HDPE melt under ultrasonic irradiation obeys following

Acknowledgements

The authors are grateful to the Special Funds for Major State Basic Research Projects of China (G1999064800), National Natural Science Foundation of China (50233010, 20374037) and the State Education Ministry of China for financial support of this work.

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