Stabilization of Paper Through Sodium Borohydride Treatment

24 Stabilization of Paper Through Sodium Borohydride Treatment Lucia C. Tang Downloaded by UCSF LIB CKM RSCS MGMT on September 2, 2014 | http://pubs.acs.org Publication Date: February 1, 1986 | doi: 10.1021/ba-1986-0212.ch024

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Preservation Research and Testing Office, The Library of Congress, Washington, DC 20540 The treatment of bleached kraft pulp with sodium borohydride followed by incorporation of basic calcium salts increased the stability of paper made from this pulp by 1.5-2.6 times. When manufactured paper, rather than pulp, was subjected to sodium borohydride treatment followed by washing with an aqueous solution of calcium hydroxide, the stability of the paper increased by as much as 4 times for groundwood paper and 30 times for bleached kraft paper. THE DEGRADATION OF PAPER is a complex process. Besides acid hydrolysis, which appears to be the dominant degradative reaction under ambient conditions, oxidation contributes significantly to the deg radation of the cellulosic matrix. Hydroxyl groups in the cellulosic maeromolecule are oxidized to carbonyl and carboxyl groups (1,2); this oxi dation leads to the discoloration of paper (3) and a decline in its physical properties (4,5). Oxidative degradation can occur in the manufacture of paper during pulping and bleaching operations. Trace metals introduced through contact with process chemicals, natural water sources, or metal lic process units may further act as oxidation catalysts. Lignin residues, sunlight, U V light, air pollutants, heat, and humidity also facilitate the oxidation of cellulose in paper fibers. Reducing agents, such as sodium borohydride, have been used extensively to improve brightness and other physical properties of pulp and paper (6-17). Sodium borohydride is a moderate reducing agent that selectively reduces compounds such as aldehydes and ketones to the corresponding alcohols, without affecting carboxylic acid groups (6, 7, 12). Rapson (13) reported that chlorine dioxide and/or sodium borohy dride improved brightness stability during the processing of cellulosic bailing address: 9116 Bells Mill Rd., Potomac, MD 20854.

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society In Historic Textile and Paper Materials; Needles, H., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1986.

Downloaded by UCSF LIB CKM RSCS MGMT on September 2, 2014 | http://pubs.acs.org Publication Date: February 1, 1986 | doi: 10.1021/ba-1986-0212.ch024

428

HISTORIC T E X T I L E A N D PAPER MATERIALS

pulp. Wade (14) proposed a combination of treatments with borohy dride and ozone to bleach and brighten cellulosic fibers. Burr (15) dem onstrated that small amounts of sodium borohydride and sodium hydroxide in wash water increased the strength of cotton fabric and prevented the deposition of iron in the fabric. Sodium borohydride has been employed to bleach paper and paper artifacts (16,17). Bleaching of paper artifacts with dilute sodium borohydride solution has been suc cessfully, although infrequently, attempted by paper conservators for several years. In this work, the reducing action of sodium borohydride was com bined with the stabilizing effect of mildly basic compounds of calcium and magnesium (18,19) to enhance the permanence of paper. Washing of paper with dilute calcium hydroxide solution after treatment with sodium borohydride has been shown to greatly increase its stability (20-22). The effect of calcium hydroxide concentration in the wash solu tion has been investigated (18, 23). Jullander and Brune (8) showed that the presence of some metal salts, including those of calcium, increases the efficiency of borohydride reduction. This finding led me to investi gate the stability of paper made from a pulp slurry containing sodium borohydride and calcium hydroxide or carbonate. The reduction of copper species in paper by borohydride ions was observed. The inhibi tion of this reduction process in the presence of magnesium bicarbonate was studied qualitatively.

Experimental Materials. All test papers were obtained from a single roll or were made from pulp. The following test papers were used: (1) Foldur kraft paper, a bleached kraft paper made from 90% southern pine and 10% hardwood pulps, with 0.5% rosin size and 3% titanium filler; (2) newsprint paper made from 80% groundwood and 20% unbleached sulfite pulp; and (3) handsheets prepared from a 1:1 furnish of Weyerhaeuser New Bern softwood (NBS) and New Bern hard wood (NBH) bleached kraft pulps. In the preparation of handsheets, the NBS:NBH furnish was soaked for 2 h in 0.0005% Ca(OH)2 solution prior to the beating step. The pulp was then beaten in deionized water containing sodium borohydride (0.1% pulp weight) and 0.02% calcium hydroxide or 2.5% calcium carbonate (22). A Craftool Hollander labora tory beater was employed. Handsheets were prepared with a Noble and Wood brass sheet-making machine, which had been painted to prevent the contact of pulp with brass. The metal wire was overlaid with polyester fiber screening (75 mesh). Handsheets were also prepared from the same pulp furnish with the Noble and Wood brass sheet-forming machine before it had been painted. These papers had a copper content of 150 ppm as a result of contamination from the brass. Preparation of Solutions. To provide a constant flow of 0.002% Ca(OH)2 solution wash, a saturated calcium hydroxide solution was constantly diluted by using the chemical feeder technique (18). Other dilute calcium hydroxide solu-

In Historic Textile and Paper Materials; Needles, H., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1986.

24.

TANG

Stabilization of Paper by NaBHé Treatment

429


tions were also prepared by appropriate dilution of 0.2% (saturated) calcium hydroxide solution with deionized water. Magnesium bicarbonate solution was prepared by passing a current of carbon dioxide into an aqueous slurry of basic magnesium carbonate (19). Treatment of Papers. Foldur kraft and newsprint papers were prewet with 1:1 solution of denatured alcohol and distilled water before further treatment to facilitate their penetration. Test papers were treated with sodium borohydride by immersion in the appropriate solution for 30 min. The treated samples and untreated controls were then washed for 1 h in either a wash solution composed of dilute calcium hydroxide or in deionized water, or they were left unwashed. Some Foldur kraft samples were preaged before borohydride treatment and further aging. Paper samples were subjected to accelerated aging at 100 °C in a dry oven for 14 h or at 90 ° C and 50% rh for 7 h. Aging Procedures. All treated and/or washed samples were air dried and conditioned at 70 °F and 50% rh before being subjected to accelerated aging in humid (90 °C and 50% rh) or dry (100 ° C ) circulating-air ovens for 1, 2, 3, and 5 weeks. Prior to testing, the oven-aged samples were again conditioned at 70 °F and 50% rh. Determination of Physical Properties of Paper. B R I G H T N E S S . The bright ness was measured with a Photovolt model 670 reflectance meter. Measurements were made at 10 different places on both sides of the sheet, and the readings were averaged. FOLDING ENDURANCE. The Massachusetts Institute of Technology folding endurance test was run at 0.5 kg of tension in the machine direction; 10 specimens were tested per sample, according to the Technical Association of the Pulp and Paper Industry (TAPPI) T511-su-69.

pH V A L U E S . A variation of the TAPPI cold-extraction procedure for the determination of pH (T435-su-68) of paper samples was employed. Specimens weighing 2.5 g and 250 mL of deionized water were mixed for 45 s in a Waring blender. The pH was measured with a Fisher Accumet model 320 pH meter and glass electrode standardized against a pH 7.00 buffer. METAL CONTENT. Calcium, sodium, and boron contents of paper sam ples were determined by using a direct solid-sampling technique in conjunction with flameless atomic absorption spectroscopy (20,24). A Varian Techtron AA-6 spectrophotometer was employed with a model 90 carbon rod atomizer. In addition to the direct solid-sampling technique, calcium content was also deter mined by digesting samples with concentrated hydrochloric acid for 30 min.

Results and Discussion The effect of a reducing sodium borohydride treatment followed by a mildly alkaline wash on the physical properties and stability of paper was studied. Test papers were characterized by measuring their initial p H value, brightness, and folding endurance. Then they were subjected to borohydride treatment and washed with dilute calcium hydroxide solution or deionized water. The concentrations of sodium borohydride



430

HISTORIC T E X T I L E A N D PAPER MATERIALS

and calcium hydroxide solutions were varied. These test papers were subjected to accelerated aging under dry as well as humid environments. The progressive loss in folding endurance and brightness was monitored intermittently. The effect of treatment variables was compared b y computing relative permanence values. Plots of the logarithm of folding endurance versus aging time were employed to project the estimated lifetimes of samples and controls. Estimated lifetime is defined as the aging time required for the sample to retain only one double f o l d . The estimated lifetime of a sample d i v i d e d b y that of the corresponding control yields the relative permanence. These results are presented i n Tables I and II. Another measure of stability is the relative brightness retention. Variation of brightness was plotted against aging time. The initial slope of this line was computed b y a least-squares treatment. T h e value of the slope for the control d i v i d e d b y the slope for the treated sample is defined as the relative brightness retention. These results are presented in Tables III and I V . The results obtained for the treatment of Foldur kraft paper are shown in Table I. Generally, the folding endurance of the test papers increased as a result of the borohydride treatment. H o w e v e r , i n the case of papers treated w i t h 1.08! N a B H i without any subsequent washing, the folding endurance decreased markedly. The alkalinity i n these papers probably led to hydrolytic degradation of paper. The 0.002% calcium hydroxide wash was just sufficient to neutralize ( p H 7.0) the untreated F o l d u r kraft papers, although very little alkaline reserve was retained (0.183! C a C 0 or 702 p p m of C a ) . Because of the alkalinity of the borohydride solution ( p H 9.5-10.0), treated sheets were alkalized to a greater or lesser extent ( p H 6.2-9.5) depending upon the concentration of the solution employed. In treating F o l d u r kraft papers w i t h sodium borohydride, the relative stability of the treated paper was found to be related to its calcium and sodium content. Foldur kraft papers washed with 0.002% calcium hydroxide wash demonstrated about six to eight times the stability of samples left unwashed or washed with deionized water. Paper treated with 1.0% sodium borohydride, but left unwashed, degraded faster than the control sample. These samples had retained a large concentration of sodium (38.4%), whereas their calcium content had decreased f r o m 445 to 143 p p m . The washing efficiency of deionized water and water containing 0.002% calcium hydroxide differed markedly in the removal of residual sodium f r o m the 1.0% sodium borohydride-treated sheets. Treatment with 1.0% N a B U , f o l l o w e d b y efficient washing with dilute calcium hydroxide, increased the relative permanence of Foldur kraft paper b y about 27-33 times. This relative permanence was about four times that observed for paper subjected to calcium hydroxide washing 3



4

4

4

4

4

4

None None None 0.01% NaBH 0.01% NaBH 0.01% NaBH 0.1% NaBH 0.1% NaBH 0.1% NaBH 1.0% NaBH 1.0% NaBH 1.0% NaBH

4

4

4

Treatment

2

2

2

2

none deionized water 0.002% Ca(OH) none deionized water 0.002% Ca(OH) none deionized water 0.002% Ca(OH) none deionized water 0.002% Ca(OH)

Washing

Calcium Content (ppm) 445 292 702 361 465 684 418 425 713 143 228 779

Initial pH 5.1 5.7 7.0 6.4 6.2 7.2 9.0 7.5 7.6 9.5 7.9 7.7

0.73 0.14 0.11 1.06 0.64 0.10 5.65 1.61 0.19 38.40 2.6 0.72

4

Sodium Content (ppm Χ 10 ) 890 1045 1050 1255 1325 1285 960 1065 1345 20 1370 1030

Initial MIT Folds

Table I. Characterization and Aging of Foldur Kraft Paper

1.00 1.00 8.43 4.35 3.20 7.79 25.8 14.1 8.15 1.3 17.4 32.7

1.00 0.97 5.71 2.88 2.51 5.22 11.8 10.2 9.23 1.3 15.6 26.8

Humid Oven

Relative Permanence Dry Oven



4

4

4

4

4

4

None None None 0.01% NaBH 0.01% NaBH 0.01% NaBH 0.1% NaBH 0.1% NaBH 0.1% NaBH 1.0% NaBH 1.0% NaBH 1.0% NaBH

4

4

4

Treatment

2

2

2

2

none deionized water 0.002% Ca(OH) none deionized water 0.002% Ca(OH) none deionized water 0.002% Ca(OH) none deionized water 0.002% Ca(OH)

Washing 5.2 5.9 8.8 7.1 5.5 8.0 8.8 7.2 7.9 9.2 8.0 8.5

Initial pH 845 712 1883 1053 882 1581 580 665 1866 265 205 2135

Calcium Content (ppm) 340 400 450 460 540 480 560 595 525 10 600 600

1.4 0.3 0.2 5.50 1.82 0.2 18.1 6.8 0.3 60.5 11.7 1.5

4

Initial MIT Folds

Sodium Content (ppm Χ 10 )

Table II. Characterization and Aging of Newsprint Paper


1.00 1.00 2.67 1.97 1.54 2.57 2.53 3.64 3.48 0.88 4.07 3.87

Dry Oven

1.00 0.98 2.63 1.89 1.84 3.63 2.62 1.95 4.32 0.92 1.57 3.36

Humid Oven

Rehtive Permanence

24.

TANG

Stabilization of Paper by NaBHé Treatment


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Stabilization of Paper Through Sodium Borohydride Treatment

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