Effect of High Energy Electron Irradiation on HDPE and LDPE Studied by Differential Scanning Calorimetry (DSC)

Effect of High Energy Electron Irradiation on HDPE and LDPE Studied by Differential Scanning Calorimetry (DSC)

Subrata Mukherjee1,Sandip Pal Chowdhuri2,Arunava Mandal1,Sandip Pan1,Achintya K.Saha1

1Research Scholar, Physics Department, Visva-Bharati, West Bengal, India-731235

2 Research Scholar ,NIT ,Silchar, Assam, India

Asmita Sengupta*1

*1Professor, Physics Department, Visva-Bharati, West Bengal, India-731235 e-mail: asmita_sengupta@hotmail.com

Abstract

The effect of high energy electron irradiation on the thermal properties of high density polyethylene (HDPE) and low density polyethylene (LDPE) are studied using Differential Scanning Calorimetry (DSC). A systematic change in the thermal properties in respect of crystallinity and specific heat has been observed during thermal annealing for un-irradiated and irradiated samples. It is found that crystallinity and specific heat both vary with temperature. Specific heat goes on increasing upto the melting range indicating a lower crystallinity at this stage. Attempt has been made to correlate the crystallinity and specific heat of HDPE and LDPE with their structural changes occurring during heat treatment. The changes in crystallinity and specific heat, after the samples being subjected to annealing at 80, 90 and 100C respectively followed by the heat treatment, are found to be quite consistent. Probable explanations for the observed behaviour of both e-irradiated and un- irradiated samples are presented in this paper.

1. Introduction

   High density and low density polyethylene( HDPE & LDPE)are semi-crystalline, polymers made up of crystalline and amorphous regions. One of the fundamental properties which affect the physical properties of polymer is the degree of crystalinity. Differential Scanning Calorimetry (DSC) provides a rapid method for determining crystallinity based on the heat required to melt the polymer. The crystalline phase being a quasi stable state, generally the melting temperature of a polymer lies over a wide range and the degree of crystallinity decreases continuously with temperature within the melting range. Polymers having crystal polymorphism show transformation among different crystal modifications and melt-re crystallization during melting[1]. Crystallinity and peak melting temperature of poly ethylene undergo an increase after irradiation , depending upon the dose and the molecular weight of the material (2).

2. Experimental Details

   HDPE (crystallinity70%), LDPE (crystallinity 40%) samples from Buna AG Merseburg, Germany, are used for irradiation with 8MeV electron beam at a dose of 100 KGy in the as received sate. For DSC measurement the samples are cut into suitable sizes so that the weights of the samples remain in between 15 to 20 mg as specified for DSC.

   DSC measurements were carried out by NETZSCH DSC200 F3 Maia instrument under N2 atmosphere at a constant pressure of 0.3bar for preventing any sample oxidation and at a scanning rate of 10C/min. Annealing procedure consists of cooling the sample from 160c to the desired temperatures (80c,90c,100c respectively) and maintaining it for one hour before cooling to room temperature. Only for annealing at 100c, the cooling rate is 50c/min. Proteus Analysis software is used for finding out the specific heat and crystallinity of the samples.

3. Results and Discussions

   DSC monitors heat effects associated with phase transitions and chemical reactions as a function of temperature. The same temperature program is used for plotting the curve between heat flow and temperature for the standard sample (sapphire) and the sample under investigation. The Heat flow into the sample is given by, dH/dt = m * Cp * dT/dt

   Where dH/dt is the heatflow rate in calories/seconds, m is the mass of the sample in grams, Cp is the specific heat inCalories/gram/oK , dT/dt is the rate of change of temperature in oK/second. The final equation which determines the specific heat of the sample is given by,

   Cp/Cp = my/my

   Where Cp and m are the specific heat and mass of the standard, y and y are the ordinate deflections due to the sample and standard respectively in inches [5]. The variations of specific heat at different temperatures for reference as well as irradiated samples of HDPE and LDPE are shown in Figs.1and 2 below.

   25

   HDPE (ref)

   Specific Heat (J/(g*K))

   20 HDPE (irr)

   15

   However,difference is small indicating not much change in crystallinity for low irradiation dose[4].

   Table 1.Variation of crystallinity for the reference and irradiated samples

   10 Sample

   5

   Temperature condition

   Melting Area (J/g)

   Crystal linity (%)

   0

   0 20 40 60 80 100 120 140 160

   Temp (°C)

   HDPE

   (Ref)

   Unannealed 187.5 64.01

   Annealed at 800C 168.8 57.62

   Annealed at 90 0C 179.2 61.16

   Annealed at 1000C 187.5 63.98

   Figure 1.The variations of specific heat at different temperatures for reference and irradiate sample of HDPE

   10

   LDPE (irr)

   LDPE (ref)

   Specific Heat (J/(g*K)

   8

   6

   4

   2

   0

   0 20 40 60 80 100 120 140 160

   Temp (°C)

   HDPE

   (Irradiated)

   LDPE

   (Ref)

   LDPE

   (Irradiated)

   Unannealed 192.4 65.67

   Annealed at 800C 186.2 63.56

   Annealed at 900C 177.4 60.54

   Annealed at 1000C 181.9 62.09

   Unannealed 62.35 21.28

   Annealed at 800C 66.95 22.85

   Annealed at 900C 62.67 21.39

   Annealed at 1000C 75.82 25.88

   Unannealed 74.16 25.31

   Annealed at 800C 73.26 25

   Annealed at 900C 74.18 25.32

   Annealed at 1000C 73.69 25.15

   Figure 2.The variations of specific heat at different temperatures for reference and irradiate sample of LDPE

   It is found that the irradiated sample has a melting peak at a higher temperature signifying a larger temperature needed to dissociate the sample completely as there is a stronger bond due to cross-linking because of irradiation(6).As the temperature increases specific heat of both reference and irradiated samples goes on increasing till the melting peak beyond which specific heat decreases.With the rise of temperature, chains of polymer samples become unfolded and stretched. So free space for molecular vibration and thus internal energy become less and heat taken from outside becomes larger. So polymer samples now have some lower crystallinity and greater specific heat within the melting range(1).Further annealing beyond melting temperature causes changes in the configuration of the polymer crystal leading to an increase in the crystallinity and decrease in specific heat.

   m

   The degree of crystallinity is defined as Xc = Hf(Tm)/ Hf0(T 0),

   m

   m

   Where, Xc is the weight fraction of crystallinity. Hf(Tm) is the enthalpy of fusion measured at the melting point, Tm. Hf0(T 0) is the enthalpy of fusion of the totally crystalline polymer at the equilibrium melting point,T 0 [3]. Table1 presents the results of annealing at 800C, 900C and 1000 C. Both un-annealed reference and irradiated samples have the crystallinity higher than that of annealed samples. Variations in crystallinity for different annealing temperatures are more for reference samples compared to irradiated ones.

4. Conclusion

The temperature variation in specific heat in DSC measurement shows that after irradiation the crystallinity increases whereas the specific heat decreases till the melting peak beyond which it reverses due to configurational changes. nnealing at different temperatures results to structural changes causing variations in crystalinity depending upon morphological history of the sample

Acknowledgement

The work is sponsored by SERC Division, D.S.T., Govt. of India, project No. SR/S2/CMP-57/2007.

References

[1]. Hirohisa Yoshida, Dynamic analysis of the melting behavior of polymers showing polymorphism observed by simultaneous DSC/X-ray diffraction measurement Thermochimica Acta 267 (1995) 239-248

[2]. S K Bhateja, E H Andrews and S M Yarbrough Radiation Induced Crystallinity Changes in Linear Polyethylene Long Term Aging Effects Polymer Journal (1989) 21, 739-750; doi:10.1295/polymj.21.739

[3]. Y. Kong, J. N. Hay, The measurement of the crystallinity of polymers by DSC Polymer 43 (2002) 3873-3878

[4]. Quero, C.C. Puig, C. Albano, A. Karam, -Radiation (0-150 KGy) Effects on Slow and Fast Cooled HDPE/LDPE Blends Polymer Bulletin 59, 517526 (2007)

[5]. M. J. ONeill,Measurement of Specific Heat Functions by Differential Scanning Calorimetry,Analytical Chemistry,vol. 38(10).1966, pp. 1331.

[6]. I.Y.Al-Qaradawi,Electron irradiated low-density Polyethylene studied by Positron Annihilation Lifetime Spectroscopy, Radiation Physics and Chemistry 68(2003)467 4