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Thermal and optical properties of electron beam irradiated cellulose triacetate

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Thermal and optical properties of electron beam irradiated cellulose triacetate
   Indian J. Phys.   83  ( 6 ) 813-819 (2009)  © 2009 IACS *Corresponding Author Thermal and optical properties of electron beam irradiatedcellulose triacetate                                         Abstract : Samples from Cellulose triacetate (CTA) sheets were irradiated with electron beam in the doserange 10–200 kGy. Non-isothermal studies were carried out using thermogravimetric analysis (TGA) to obtainthe activation energy of thermal decomposition for CTA polymer. The CTA samples decompose in one main breakdown stage. The results indicate that the irradiation by electron beam in the dose range 80–200 kGy increasesthe thermal stability of the polymer samples. Also, the variation of melting temperatures with the electron dosehas been determined using differential thermal analysis (DTA). The CTA polymer is characterized by the appearanceof one endothermic peak due to melting. It is found that the irradiation in the dose range 10–80 kGy causesdefects generation that splits the crystals depressing the melting temperature, while at higher doses (80–200kGy), the thickness of crystalline structure (lamellae) is increased, thus the melting temperature increases. Inaddition, the transmission of these samples in the wavelength range 200–2500 nm, as well as any colorchanges, were studied. The color intensity  E  * was greatly increased on increasing the electron beam dose,and accompanied by a significant increase in the blue color component. Keywords : Electron beam irradiation, thermal properties, color response, polymers. PACS No. : 78.20.-e 1. Introduction Radiation modification of polymers is one of the modern methods, which leads to newpolymeric materials with specific properties. Also, electron-beam irradiation can beconsidered as one of the most popular and well established processes for severalapplications [1]. The action of electron beam rays on polymers leads to severalchanges in the polymer properties due to the induced chain scissions and cross-links.The degradation induced by electron beam irradiation is a prompt way to simulate theaging of polymeric materials and to study their radiation stability or the changes inphysical properties in view of their industrial applications [2,3]. Also, polymers can beeasily affected by the variations of temperature. In fact, such effects would inducemodifications in the chain segment mobility of polymers, indicated by changes in              transition temperatures [4,5]. Cellulose triacetate is one of the polymers that his   beenusefully employed in a number of different fields of science and technology [6]. Severalinvestigations have been introduced to study the changes in physical properties ofpolymers due to irradiation [7  –  14]. The aim of the present study is to obtain informationconcerning the interaction of electron beam rays with cellulose triacetate polymer toimprove its performance in several industrial applications. 2. Experimental 2.1. Samples :  Cellulose triacetate polymer used in this study is 0.25 mm thick sheet manufacturedby Eastman Kodak Company, Rochester, New York. 2.2.Irradiation facilities :  The irradiation process was performed in air, at room temperature 25 ° C, using 1.5 MeVelectron beam accelerator of the ICT-type. It operates with insulating core transformer,with a beam current of 25 mA. The conveyer was attached with a cooling system toavoid heating of the samples. The method of irradiating polymer films comprises inmultiple irradiations at 10 kGy per pass. The dose was adjusted frequently usingFWT ’ 60-00 dosimeter that was calibrated by irradiation in gamma facility againstCeric  Cerous dosimeter supplied by Nordion, Canada. It is recognized that transfer ofthe calibration from gamma to 1.5 MeV electron beam irradiation involves an addeduncertainty. We estimate this uncertainty to be less than 5%. 2.3.Experimental apparatus :  The thermal behavior was investigated using differential thermal analysis (DTA) andthermogravimetric analysis (TGA) with a type Shimadzu-50 instrument.  -AI 2 O 3  powderwas used as a reference for DTA measurements. Thermal experiments were carried outat a heating rate of 10 ° C  min with N 2  as a carrier gas at a flow rate of 30 cm 3  min.The transmission measurements were carried out using a Shimadzu UV-Vis-Nirscanning spectrophotometer, type 3101 PC. This unit measures in the wavelength range200  –  3000 nm. The CIE (Commission International De E ’  Claire units x, y, and z)approach was used in the present work for the description of colored samples. The L *, a  *, b  * intercepts used in this system are based on the CIE color triangle. In thissystem, the L * value specifies the dark-white axis, a  * the green-red axis, and b  * theblue-yellow axis. The L *, a  *, b  * intercepts of CTA films were measured and taken asa reference. The color difference (  E  *) between the non irradiated CTA sample andthose irradiated with different doses was calculated according to the CIELAB color-difference equation [15,16] :  E  * = [(  L *) 2  + (  a  *) 2  + (  b  *) 2 ] 1/2              3. Results and discussion 3.1. Thermal properties : 3.1.1. Thermogravimetric analysis (TGA) :  Thermogravimetric analysis (TGA) is a technique that measures the change in weightof a sample during heating. It provides information on the initiation and termination ofweight change and the amount of change. TGA was performed for irradiated and nonirradiated CTA samples in the temperature range from room temperature up to 600 ° C,at a heating rate of 10 ° C  min. It is found that the CTA polymer decomposes in onemain weight loss stage. Using these TGA thermograms, the values of onset temperatureof decomposition T  0  (the temperature at which the thermal decomposition starts) werecalculated and are given in Table 1. Table 1.  Values of the onset temperature ofdecomposition T  0 , activation energy of thermaldecomposition E  a   and melting temperature T  m   for CTAsamples as a function of the electron dose.Electron dose T  0  ( ° C) E  a (eV) T  m ( ° C)(kGy)01771.86323101661.62319201621.31322401561.05314801490.883061001591.093101301791.183141601891.613172001921.74319 Figure 1 shows the variation of T  0  with the electron dose. The figure shows that T  0  decreases until a minimum value around the 80 kGy irradiated sample indicating a Figure 1.  Variation of onset temperature of decomposition T  0  with the electron dose.050100150200Electron dose (kGy)200190180170160150140    T   o    (             °    C   )              decrease in thermal stability of the polymer samples due to degradation ( i.e.  preferentiallychain scission), then increases on increasing the electron dose up to 200 kGy due tocross-linking process. 3.1.2. Activation energy of thermal decomposition :  Not only TGA gives the ability to find out the temperature at which the thermaldecomposition starts T  0  but also allows the measurement of the activation energy ofthermal decomposition E  a  , which is useful for studying the thermal stability of thematerials. The method proposed by Horowitz and Metzger [17] has been used in thepresent study for the measurements of the thermal activation energies. In this methodTG curves obtained at a heating rate of 10 ° C  min are required where the followingequation is valid :In {In [( W  0    –    W  f  )  ( W     –    W  f  )]} = E  a    RT  s  2 where R   is the general gas constant, W  0  and W  f   are the initial and final weights ofthe stage, W   is the remaining weight at a given temperature T  ,   is the temperaturedifference between T   and T  s  .According to the above equation, a plot of In {In [( W  0    –    W  f  )  ( W     –    W  f  )]} against   leads to a straight-line relationship in the range where the decomposed ratios areequal. Hence, the activation energy of thermal decomposition E  a   can be evaluated fromthe slope of the line. T  s   is the temperature which satisfies the equation :[( W     –    W  f  )  ( W  0    –    W  f  )] = (1  e  ) = 0.3679Using the TGA curves, values of activation energy of thermal decomposition E  a   werecalculated and are given in Table 1. Figure 2 shows the variation of E  a   with theelectron dose. From the figure it is clear that E  a   decreases until a minimum valuearound the 80 kGy irradiated sample due to chain scission, followed by an increaseon increasing the electron dose up to 200 kGy due to crosslinking mechanism. Figure 2.  Variation of activation energy of thermal decomposition E  a   with the electron dose.050100150200Electron dose (kGy)       E     a      (     e      V      ) 3.1.3. Differential thermal analysis (DTA) :  Differential thermal analysis DTA was performed, in the temperature range from room              temperature up to 400 ° C, at a heating rate of 10 ° C  min on the CTA samples. All thethermograms were characterized by the appearance of one endothermic peak at themelting temperature. On heating, the samples pass through a range of badly specifiedsoftening temperatures. This can be attributed to the fact that any polymeric chain hasmore degree of freedom than non-polymeric matter, and there is variability in chainlength. Approximate indicative values of these melting temperatures were calculated andare given in Table 1. The values obtained indicate that the melting temperature T  m  almost decreases until a minimum value around the 80 kGy irradiated sample, thenincreases with increasing the electron beam dose up to 200 kGy. The meltingtemperature T  m   is sensing the crystalline domains of the polymer. It is possible tospeculate that at low doses (10  –  80 kGy), defects generation splits the crystalsdepressing the melting temperature. For such doses, the decrease of the polymerlength contributes also to the shift of T  m    towards lower temperatures. At higher doses(80  –  200 kGy), the thickness of crystalline structures (lamellae) is increased. 3.2. Color changes :  The transmission spectra of CTA samples in the wavelength range 200  –  2500 nm havebeen investigated. The spectra appeared, for all CTA samples, as a band with differentintensities. The color intercepts ( L *, a  *, and b  *) before and after exposure are shownin Table 2. The accuracy in measuring L * is ±0.05 and ±0.01 for a  * and b  *. It canbe seen that the color parameters were totally changed after exposure to electronbeam irradiation. The red (+ a  *) color component of the non-irradiated film was changedto green (  –  a  *) after exposure to electrons in the dose range (130  –  200 kGy).The color intensity  E  *, differences between the non-irradiated and irradiatedsamples was calculated, given in Table 2 and plotted in Figure 3 as a function ofelectron dose. Table 2.  The color intercepts ( L *,  a  *, and b  *) andcolor intensity  E  * of CTA samples as a functionof the electron dose.DoseColor intercepts   E  *(kGy) L * a  * b  *034.40.583.80.001033.50.463.31.652034.20.373.873.324033.  –  0.327.264.4216033.4  –  0.247.564.5420033.0  –
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