مطالعه ساختاري و فرايند تخريب يك نمونه فيلم سينماتوگراف پايه نيترات سلولز

Technical Study and Mechanism of Deterioration in a Nitrocellulose based Film

فيلم هاي سينمايي بخش مهمي از ميراث فرهنگي- تاريخي و اسناد تصويري تاريخ ايران را تشكيل مي‌دهند. تغييرات ساختاري در فيلم ها، با توجه به عوامل مؤثر بر آن ها در طول زمان منجر به تخريب تصوير مي شود. مطالعات ساختاري و فرايند تخريب كمك شاياني به ارائه راهكارهاي حفاظتي و مرمتي در اين‌گونه آثار مي كند. در اين پژوهش يك نمونه فيلم سينماتوگراف موردمطالعه قرار گرفت. نتايج حاصل از آزمايش‌هاي ATR-FTIR، تصاوير ميكروسكوپ نوري و تصاوير SEM در نمونه فيلم نشان داد كه تكنيك ساخت نمونه ژلاتين و هاليد نقره بر روي پايه پلاستيك نيترات سلولز است و تخريب در لايه تصوير ناشي از آزاد شدن گاز NO2 در پايه فيلم است. گازهاي آرشيوي (پراكسيدها) ناشي از شرايط نامناسب محيطي و استفاده از مواد نامناسب در نگهداري نمونه موردپژوهش باعث اكسيد شدن نقره در لايه تصوير، مهاجرت آن به سطح فيلم و منجر به عارضه تاول هاي اكسايش- كاهش شده است. نتايج حاصل از آناليز SEM/EDS تاول هاي اكسايش-كاهش در سطح فيلم نشان‌دهنده تخريب يون‌هاي نقره در لايه تصوير است.

Nitrocellulose based film negatives make an important part of the cinematic work. In Iran, these films are also important since they are parts of cultural, historical and artistic heritages. Therefore, such works need to proper care and preservation. The decomposition process of these films occurs due to the inherent deterioration of nitrocellulose as well as the presence of environmental factors such as heat, light and humidity. Nitrocellulose film base decomposition can be very rapid and is generally categorized in five progressive stages; 1. The negatives begin to become yellowish-brown and NO2 gas is released; 2. The film becomes sticky and emits a strong noxious odor; 3. The film changes to an amber color material and the image begins to fade; 4. The negative becomes soft and can weld to adjacent negatives, enclosures and photographs; and 5. The negative decomposes into a brownish acid powder. Technical study and mechanism of deterioration of cinematograph negatives can help to provide better methods of conservation and restoration of this works. In this study, a cinematographic negative sample was investigated by Fourier Transform Infrared spectroscopy- Attenuated Total Reflectance (FTIR-ATR), spot tests (diphenylamine test and float test), Scanning Electron Microscopy/Energy-dispersive X-ray spectroscopy (SEM/EDS), optical microscope images and digital loop. The —ONO2, C—O, —OH, C=O groups in the base of the sample and —CH, —CN, —NH, —OH, amide I, and amide II groups in the image layer were identified by FTIR-ATR spectra. A solution of diphenylamine and sulfuric acid was used to identify the negative base, where it contains 90% of sulfuric acid. The color of the solution turned into a deep blue color upon the immersion of the sample. The sample was also sunk in trichlorethylene solution. Results of FTIR-ATR analysis, spot tests and cross section images by SEM indicated that the structure of the negative is gelatin and silver halide (image layer) on the nitrocellulose plastic base. The instability of nitrocellulose based cinematograph negative can be attributed mostly to the extremely thermal sensitivity of nitrocellulose. An examination of bond energy implies the oxygennitrogen (O—N 169 kJ; C—C 347 kJ; C—O 360 kJ; C—H 414 kJ) linkages are almost readily cleaved, leading to the formation of volatile nitrogen oxide decomposition products. The degradation in the image layer in the form of brown bobbles, due to release of NO2 gas in the negative base was observed by optical microscope and digital loop images. Redox blemishes also were observed by optical microscope and digital loop images in the shape of yellow-orange circles on the surface of the negative. Redox-blemishes were observed for the first time in 1960s on the surface of the microfilms. At first, the problem of redox blemishes was thought to be peculiar to microfilm. But, it was later found to occur on many types of silver photographic materials. Archival gases (peroxides) produced by improper environmental conditions and use of inappropriate materials in the maintenance of the sample cause the oxidation of silver in the image layer, its migration to the surface of the negative and led to the formation of Redox blemishes. The results of the SEM/EDS analysis of Redox blemishes on the negative surface indicate the deterioration of silver ions in the image layer. Humidity also accelerate Redox-blemishes.