Document Type : Research Paper

Authors

• Mostafa Khaje-mahmouid ¹
• Mehrnaz Azadi boyaghchi ²
• Mohsen Mohammadi achachlui ³
• majid ghahramanafshar ⁴

¹ PhD in conservation and restoration of cultural and historical objects, Faculty of Conservation and Restoration, Isfahan Art University, Isfahan, Iran

² Associate Professor, Faculty of Conservation and Restoration, Isfahan University of Arts, Isfahan, Iran

³ Assistant Professor, Faculty of Conservation and Restoration, Isfahan University of Arts, Isfahan, Iran

⁴ Assistant Professor, Chemistry and Process Research Department, Niroo Research Institute (NRI), Tehran, Iran

Abstract

Background and objectives: Paper bleaching is a concern of many conservationists. Research has been conducted with the aim of evaluating some traditional bleaching materials used in the preservation of documents and books, such as hydrogen peroxide and sodium borohydride, which shows that these materials do not significantly improve the mechanical properties of historical papers. Also, investigating the effect of reducing agents on oxidized papers has shown the effectiveness of tert-butylamine borane complex. In another study, five dyeing processes with oxidizers (calcium hypochlorite, hydrogen peroxide in two concentrations, light bleaching and potassium permanganate) and two dyeing processes with reducing agents (sodium borohydride and tert-butylamine borane) were compared. And their effect on pure cellulose (Whatman filter paper) and two different historical papers have been investigated. The results indicate that all the tested dye removal methods have led to a clear increase. Due to the differences of opinion, in this research, the effectiveness of two reducing agents sodium borohydride and sodium dithionite on the appearance and structural characteristics of paper is investigated.
Methodology: In this research, Whatman Filter Paper No. 1 (Whatman Filter Paper No. 1) made in England with a thickness of 180 microns, a diameter of 11 cm, a base weight of 87 grams per square meter and an ash percentage of 0.06% was used. Chemical reagents including potassium persulfate, sodium borohydride and sodium dithionite in the highest purity were prepared in a laboratory from Merck, Germany.
Three solutions including potassium persulfate (2% by weight/volume), sodium borohydride (1% by weight/volume), sodium dithionite (1% by weight/volume) in distilled water (distilled water, pH; 7), to perform tests on zinc. Paper samples have been used by immersion.
Whatman filter paper samples were subjected to oxidation in 2% peta persulfate in distilled water for 2 days (48 hours) and then the samples were immersed. Next, the identified samples were modified by two solutions of sodium borohydride and sodium dithionite 1% in distilled water for 1 to 5 hours and then purified in holy water. Also, the samples were subjected to accelerated aging according to ASTM standard number D4714-96 at a temperature of 90±2 centigrade and a relative humidity of 50±2% for 384 hours. Test methods include potentiostat, colorimetry, tensile strength, pH measurement, scanning electron microscope and infrared examination.
Results: The results show that potassium persulfate is the most neutralizing solution among the three solutions used on the first day, and it also indicates the reducing power of two solutions of sodium dithionite and sodium borohydride, which according to the test results, sodium borohydride is the reducing power. Compared to sodium dithionite. The sample prepared with pH 92.3, after accelerated aging, was accompanied by an increase of several units of pH after modification with two reducing substances. Examining the data obtained from the tensile strength test shows that in comparison with the tested samples with a tensile index of 1.06, the samples modified with sodium borohydride and sodium dithionite increased the index in all five modified periods. Tensile of paper samples. The infrared spectrometry investigation of the samples modified with sodium borohydride and sodium dithione shows that this old material improves the structure of cellulose after aging by reviving the C=O bonds to C-O and reducing the area of 1640 ^cm-1. Accelerated. The reduction of the absorption band in the region of 1640 ^cm-1 has been opened in almost all time cases with this material compared to the modified sample. Based on the data, samples modified with sodium borohydride and sodium dithionite had a significant increase (approximately 25 units) in the L-factor (light-darkness) after aging, indicating that two substances have increased the brightness of the sample by reviving the color compounds. In the microscopic examination of the surface of the fibers, in the samples identified with potassium persulfate, local separation of the fibers is observed. In the samples modified with sodium borohydride and sodium dithionite, fibers with higher consistency and entanglement are observed.
Conclusion: Double bonds and carbon groups in paper samples formed with potassium persulfate have been the main responsible for the yellowness and darkness of the paper, the reducing agents sodium borohydride and sodium dithionite reduce this aldehyde and ketone to hydrocarbon and as a result. With the loss of multiple conjugates, the light absorption of the paper is reduced and this causes the paper to lighten. The double bonds and their regeneration also cause rearrangement of the chain cells, which has resulted in increasing the mechanical resistance and strength of the paper samples.

Keywords

• Yellowing
• Paper oxidation
• Whitening
• Reducing agents

Main Subjects

• Pulp and paper