I. 16. Coloration of Polyethylene Terephthalate (PET) Film by 3MeV Proton Beams

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1 CYRIC Annual Report 2001 I. 16. Coloration of Polyethylene Terephthalate (PET) Film by 3MeV Proton Beams Matsuyama S., Ishii K., Yamazaki H., Endoh H., Yuki H., Satoh T., Sugihara S., Amartaivan Ts., Tanaka A., Komori H., and Orihara H.* Department of Quantum Science and Energy Engineering, Tohoku University, Cyclotron and Radioisotope Center, Tohoku University* Introduction Though the PIXE analysis is well known as a nondestructive method, discoloration of samples occurs by beam irradiation, which is a serious problem for very important archaeological samples. The discoloration of glass, glazed ceramics, china and paper samples in PIXE analysis has been reported. We previously investigated discoloration of samples such as paper, Japanese vessels and japan bowls. The degree of discoloration was different for each sample. In the case of glazed ceramics, discoloration increased with the dose and gradually disappeared after irradiation. In recent years, discoloration of plastic items has become a problem in commercial applications. Coloration of glassy materials using γ-rays has been extensively studied and the colored materials are commercially produced. Coloration of polymeric materials by ion beams has not been studied until quite recently and has been treated as an unwanted phenomenon. In this study, we consider proton beam irradiation as a coloring technique. Because ion beams can be controlled two-dimensionally using magnetic or electrostatic fields and beam energy is concentrated in the Bragg peak at the end of range, the coloration by proton beam irradiation can be carried out in 3 dimensions by varying beam energy. Experiments Experiments were performed by the use of a Dynamitron accelerator at Tohoku University. Thin films were uniformly scanned horizontally and vertically by means of a submilli-pixe camera with 3 MeV proton beams. The scanning area was 10 x 10 mm 2. Beam irradiation was carried out in air with average beam currents of ~1.6nA. Proton 85

2 beams were extracted from the beam duct through a Kapton window. Irradiation effects were investigated by means of absorption spectroscopy, electron spin resonance (ESR) spectroscopy and Fourier transform infrared absorption (FT-IR) spectroscopy. The time dependence of the absorbance and the ESR signal was measured. Results and Discussion PET films of 100 µm thickness were irradiated in-air with doses of µc/cm 2. The PET films used in this study were obtained from a commercial source (Toray, Lumirror, T60). The films discolored to light brown. Since the coloration faded as a function of time, the absorption spectra were measured within 10 min. after irradiation. The absorption spectra are shown in Fig. 1 for irradiation doses of 0 30 µc/cm 2. The absorbance strongly increased with the dose in particular in the ultraviolet region (UV) and had a broad peak in the green region (500 nm). The absorbance at 400 nm and 500 nm as a function of dose is shown in Fig. 2, where the absorbance measured one year after irradiation is also plotted. Figure 3 shows the changes in absorbance with time after the irradiation with a dose of 30 µc/cm 2. The absorbance decreased gradually after irradiation. The absorbance at long wavelength > 500 nm recovered after 3 months. However, the absorbance at short wavelength < 500 nm still shows higher values than those of unirradiated film. Figure 4 shows the absorption spectra in the ultraviolet (UV) region. A red shift was observed in the irradiated films. Similar phenomena are reported for samples irradiated by γ-rays and explained by changes in the molecular structure. The absorbance at 400 nm as a function of time after irradiation is shown in Fig. 5. The absorbance decreases exponentially except during the first 100 minutes. The decay constant of the absorbance curve decreases with the dose. These results show that at least three types of color centers, permanent, long-lived and short-lived, are formed by irradiation. The absorbance by permanent color centers as a function of dose is shown in Fig. 2. The degree of absorbance by permanent color centers is very feeble at wavelength of 500 nm. Since annealable color centers decay during the irradiation, absorbance at 500 nm is nearly saturated as exponential build up of defects compete with their exponential decay. The ion beam irradiation causes chain scission, creation of free radicals and cross linking and forms color centers 8). The annealable color centers are formed by reactive species such as free radical. Radical ions are very reactive and unstable at room temperature. 86

3 Radicals produced by beam irradiation were measured by using electron spin resonance (ESR) spectrometer. The intensity of ESR signals decreased with time and finally vanished. The time variation of ESR intensity for the dose of 10 µc/cm 2 is shown in Fig. 6. While the decay time is three times shorter than that of absorbance, the decay curve shows a similar tendency. This result suggests that the annealable color centers arise from the formation of radicals. Chemical changes of the irradiated films were studied by Fourier-transform infrared spectroscopy (FT-IR). Figure 7 shows the FT-IR spectra before and after irradiation for the dose of 2.5µC/cm 2 and 25 µc/cm 2. We could not measure the spectra for wavenumber below 1600 cm -1 which is typical of the vibration modes of aromatic rings, because the absorbance in 100 µm film was too large. The C-H and O-H stretching vibration appears in the region of 3600 cm -1 ~2500 cm -1. In the case of high dose, the degradation by beam irradiation is evident and a new band appeared at 2349 cm -1, which disappeared after few days. This bond can be assigned to the vibration of CO 2 molecules. Since CO 2 molecules slowly diffuse out through the surface, the peak disappears after a few days. We also measured the time variation of absorbance of the samples preserved under atmosphere of nitrogen, oxygen, air and in vacuum. Irradiation dose was around 10 µc/cm 2. Figure 8 shows the time dependence of absorbance for oxygen gas, nitrogen gas and vacuum. Annealing proceeded rapidly in oxygen, but not in vacuum. It is thus apparent that the color centers react with oxygen gas which permeates into the sample. Figure 9 shows the time dependence of absorbance for temperatures of -10 o C, +20 o C and +60 o C in air. The annealing time shows a strong temperature dependence and becomes short at high temperature. Since the permeability of oxygen and nitrogen increases with temperature, radicals are consumed through reactions with these gases. Though a first order process and a second order (bimolecular) process have been proposed for the annealing process of irradiated polymers, the present results do not support this explanation. We propose the following sequential process: R1 (1-ε)/τ1 R2 1/τ2 Ma ε/τ1 Mb Primary radicals (R 1 ) are formed by beam irradiation and transform into secondary radicals (R 2 ) and inert molecules M b with decay constants (1-ε)/ τ 1 and ε / τ 2, respectively. The 87

4 radicals (R 2 ) are forming the absorption centers. The molecules M b do not absorb light. Subsequently, the radicals (R 2 ) decay into inert molecules M a with decay time 1/ τ 2 and M a do not absorb light. The time variation of radical concentrations N 1, N 2 for R 1, R 2 can be written as follows. dn dt dn dt 1 =, τ 1 N = ( ε N1 N 2. τ τ 2 1 ) 1 2 The absorbance A λ at wavelength λ can be written as A + λ = k1n1 k2 N 2, where k 1, k 2 are extinction coefficients for wavelength λ. Using the above formalism, the calculations are shown by solid lines in Fig. 8. The values of ε, τ 1, τ 2 are varied corresponding to ambient conditions. The values of k 1,, k 2 are determined from the experimental results under vacuum condition. The values of ε, τ 1, τ 2 for in-air preservation are derived from those of N 2 and O 2 preservation considering their partial pressures. The calculations reproduce the experimental data. This suggests that the coloration occurs by the proposed sequential reaction process. While the value of 1/ τ 1 is almost constant, the values of 1/τ 2 and ε increase with the O 2 partial pressure. Figure 9 shows the calculated results at different preservation temperatures. The calculations reproduce the experimental data well. The coloration of PET films under proton beam irradiation was examined, but faded within a few days. However, this fading phenomenon could be controlled by the beam dose and by the preservation technique, therefore, it can be used commercially. Figure 10 shows a demonstration of proton beam writing. The Chinese character of [ 和 ] was written on the PET film. Acknowledgements The authors acknowledge the help of Mr. R.Sakamoto and Mr. M.Fujisawa during operation of the Dynamitron accelerator. The authors express their gratitude to Dr M.Watanabe and Dr Y.Hakuta for help and advice with measurements of the ESR spectra. References 88

5 1) Swann C.P. and.fleming S.J, Nucl.Instr. and Meth., 181, 205 (1981). 2) Zeng X., et al., Nucl.Instr. and Meth., B47, 143 (1990). 3) Matsuyama S., et al., International Journal of PIXE, 9 (1&2),57-61 (1999). 4)) Matsuyama, S., et al., International Journal of PIXE, 9 (1&2),47-50 (1999). 5) R.L.Clough, et al., Polymer degradation and Stability, 49, (1995). 6) Clough R.L., et al., Radiat. Phys. Chem, 48 (5), (1996). 7) Matsuyama, S., et al., International Journal of PIXE, 8 (2&3), (1998). 8) Steckenreiter T., et al., Nucl. Instrum. and Meth., B131, (1997). 9) Harrah H.A., Organic solid State Chemistry, 197 (Gordon and Breach Science Publishers, New York). 10) Michaels A.S., Vieth W.R. and Barrie J.A., J. App. Phy., Vol.34, No1 13 (1963). 11) Wulkop B., et al., Nucl. Phys. B44, 542 (1995). Fig. 1. Absorbance from 400 to 600 nm for irradiation dose of 0 30 µc/cm 2. Fig. 2. Absorbance at 400 nm and 500nm as a function of the irradiation dose. 89

6 Fig. 3. Absorption spectra after irradiation. Fig. 4. Absorption spectra in the ultra-violet region. Fig. 5. Absorbance at 400 nm as a function of elapsed time from irradiation. 90

7 Fig. 6. ESR signal intensity as a function of time after irradiation. Fig. 7. FT-IR Spectra for irradiation dose of 2.5 and 25 µc. Fig. 8. Absorbance variation for preservation under atmospheric pressure of nitrogen, oxygen, air and in vacuum. 91

8 Fig. 9. Absorbance changes for preservation temperature. Fig. 10. Proton beam writing. 92