Mass Deacidification of Papers and Books

Mass Deacidification of Papers and Books

III: Study of a Paper Strengthening and Deacidification Process with Amino Alkyl Alkoxy Silanes

by S. IPERT, E. ROUSSET & H. CHERADAME

Introduction

This series of papers deals with the mass deacidification of books and documents. In the preceding paper, a process was described using amino alkyl alkoxy silanes as a solution in ethanol, and it was shown that the use of 3-amino propyl trimethoxy silane and other similar silanes could be simultaneously both an efficient deacidification treatment and provide an alkaline reserve necessary for permanence. The degraded state of the books and documents requiring preservation means that something must be done to maintain their mechanical properties and to allow further handling. The Library of Congress evaluated that the cost of microfilming a badly degraded book was 500 times more than the same operation on a book which can be easily handled1. The loss of mechanical properties of acidic papers is the major reason for the destruction of books printed on paper containing high yield pulp. It seems that the association of lignin with acidic conditions is one of the main starting points for poor permanence of paper-based documents.

Previous processes simultaneously deacidifying and strengthening

It is not the first time that a process able to deacidify is also able to improve mechanical properties. The gas phase process with a mixture of ammonia and ethylene oxide is one example. Ethanol amine is formed in situ in the paper and the hydrogen bonding given by ethanol amine seems to be responsible for the increased interfiber bonding energy. However, the use of a mixture of these two dangerous gases is not trivial.

As for the liquid phase processes, two different variations have to be considered, i.e. whether to choose aqueous solutions or organic solvents. Of these, aqueous calcium hydroxide must be mentioned, since it gives a pH of around pH 8 to the treated paper, with an alkaline reserve of more than 100 meq/kg. An improvement of the mechanical properties is observed as shown by an increase in the folding resistance2. In order to deacidify, a calcium hydroxide concentration of 10-2 mole/L is apparently satisfactory. However, these solutions are not stable in open air, and give a precipitate of insoluble calcium carbonate. It must also be noted that many documents cannot be exposed to contact with water without the danger of damage. Similarly, treatment with aqueous barium hydroxide has been reported to give good results3. Mechanical strength of samples after treatment has been shown to be higher than that of the controls, whatever the paper composition4. It has also been reported that aqueous solutions

In the case of deacidification with organic solvent solutions it has been shown that the diethyl zinc (DEZ) treatment did not cause any decrease in the tensile strength of acidic mechanical paper. After thermal ageing, there was in most of the cases, no significant decrease or containing magnesium salts such as magnesium acetate, carbonate or bicarbonate have improved the mechanical properties of paper, with the pH of the paper reaching the range pH 9-9.5, and producing an alkaline reserve of around 0.8% w/w magnesium carbonate after treatment. Folding endurance was seen to have improved by a factor of 2 or more, according to measurements carried out at the Institut Royal du Patrimoine Artistique of Bruxelles (Belgium)5. This improvement was obtained, whatever the paper (wood pulp, chemical pulp, etc.), with a Mg(HCO3)2 concentration of 0.04 mole/L. However, a yellowing effect was sometimes observed, increase of the tensile strength of the DEZ treated papers6. It was mentioned that after the DEZ treatment no significant decrease in folding endurance was found, but after thermal ageing the folding endurance decreased in all cases.

Three deacidification methods have been tested recently, namely DEZ (Akzo), magnesium butoxy glycolate (FMC) and methoxy ethoxy methyl magnesium (Sablé process)7. In tests for these three methods it has been shown that the de-acidified papers had a general tendency to be slightly weaker than the references, especially those treated by the FMC process.

It is also possible to find, in the literature, processes in solvent phase which do not seem to be detrimental to the mechanical properties of paper. This is the case with a solution of barium hydroxide in methanol at a concentration of 1% (w/w) whereby deacidification, an alkaline reserve and better folding endurance are simultaneously observed. However, this treatment has the drawback of using a toxic heavy metal of the alkaline earth family. The process based on methyl magnesium carbonate would seem to be more interesting8. The use of this one is reported to have improved folding endurance when applied in pure methanol or in a mixture of methanol-chlorofluoro carbon or in trichloro trifluoro ethane (Freon 113). However, it was mentioned that papers containing mechanical pulp suffered a loss of folding resistance8. It is clear that processes involving fluorine containing volatile compounds cannot be recommended. In contrast, some reports have mentioned that the FMC process based on magnesium glycolate dissolved in tri-chloro trifluoro ethane does not improve mechanical properties, measured as the resistance to elongation stress (breaking length), though it does not have any detrimental effect.

It is worth mentioning that in some instances a process able to improve mechanical properties is absolutely necessary, the state of degradation of some items being extreme. A comparison between the effect of the two processes based on alkoxy magnesium glycolate having different behavior demonstrates that the mechanism of improvement is not completely understood. However, besides fluorine-containing hydrocarbons, it can also be considered desirable to give up reactants based on alkali cations for reasons which have been mentioned in the preceding paper: alkaline earth cations such as magnesium cations can constitute a source of problems on long term aging9,10. Consequently, new deacidification processes have had to be investigated.

Another process that has been described is impregnating the cellulosic material with a suitable monomer, such as ethyl acrylate and ethyl methacrylate as a mixture, which is further polymerized by a radical mechanism on exposure to a suitable irradiation. This process has been developed by the British Library, and sometimes a dramatic increase of a paper's mechanical properties has been observed. However, its general use cannot be envisaged because of the necessity to employ y irradiation under nitrogen and because it is not a deacidification process.

It is not enough to bring some protection to the paper after a deacidification treatment, but its behavior upon ageing must also be considered. Of course, accelerated ageing techniques have to be used to appraise this aspect. For instance, it is sometimes mentioned that three weeks in an oven at 90°C and 50% RH are equivalent to 1200 years at room temperature (20°C). For the FMC process using magnesium glycolate it was reported that this ageing treatment did not modify the breaking length: a rather satisfactory result, and as the ageing condition is quite aggressive, the reliability of the assumed equivalent to 1200 years is not necessarily brought into question.

The various aspects of the mechanical properties are not of equal value. Among them, tensile strength and folding endurance are of utmost importance, since they give an indication of the resistance to mechanical degradation upon handling. The problem is complicated by the fact that these two parameters often vary in the opposite direction: the more rigid the paper, the lower the folding endurance and the higher the tensile strength.

This article is devoted to the study and understanding of the effects on paper and its mechanical properties when treated with amino alkyl alkoxy silanes.

Experimental

Chemicals used for paper reinforcement

In order to examine the general behavior of amino alkyl alkoxy silanes, the effect of some of the silanes used as agents for mass deacidification treatment in the preceding paper was investigated:

• ATMS: 3-amino propyl trimethoxy silane - NH2-(-CH2-)3-Si(O-CH3)3 (Al-drich);

• AMTMS: 3-amino-2,2-dimethyl propyl trimethoxy silane - NH2-CH2-C(CH3) 2-CH2-Si(O-CH3)3 (Crompton-Witco Corp.);

• DMATMS: 3-(N,N-dimethyl amino) propyl trimethoxy silane - (CH3)2 N(-CH2-)3-Si(O-CH3)3(Gelest-ABCR);

• AMDES: 3-amino propyl methyl diethoxy silane - NH2-(-CH2-)3-Si(O-CH2-CH3)2-CH3 (Gelest-ABCR).

• A new bifunctional amino derivative was also examined:

• AATMS: 3-(2-amino ethyl) amino propyl trimethoxy silane - NH2-CH2-CH2-

NH-(-CH2-)3-Si(O-CH3)3 (Gelest-ABCR). It contains two amine functions.

The silanes were used as received.

Ethanol was used in the absolute state unless otherwise stated. Sometimes a pure grade at 95% was also used without noticeable change. These solvents were used as received.

Treatment of papers, documents and books

It was decided to study the new deacidification process on naturally aged books already having some acidic charge in the paper. This procedure has the advantage of imitating a real situation, but also has some problems such as a limited page space without printing ink, etc. Several books printed at the beginning of the 20th century were used, and some characteristics are described on Table 1.

The treatment solutions of amino alkyl alkoxy silanes were prepared at different concentrations in absolute ethanol in a glove box under nitrogen. After being impregnated for 10 min at room temperature the books and documents were pressed in order to remove excess solution. Drying was effected in an oven at 45°-50°C under slight vacuum (water pump). The duration of drying depended on the size of the document: 20 min. for a block of 8 pages, 5 hours for a book. It was observed that this last duration depended on the oven used and on the heat

Table 1 : Some characteristics of the various acidic books treated in this research

exchange rate between the book and the heater in the oven. For complete books, the procedure was similar to that dealing with samples of paper, but the equipment was adapted to the size of the document, and drying was carried out under primary vacuum with a mechanical pump.

The pH of the books listed in Table 1 was between pH 3.2 and pH 4.6. In addition to them a modern paper sample was submitted to the tests.

Previously, it had been observed that the treatment of old papers with pure ethanol gave a breaking length11 increase in the amount of ca.10%. It is to be assigned to the reorganization of the fibrils upon drying. Also, a slight decrease of yellow index and acidity was observed. This demonstrated that an ethanol treatment acted as a kind of paper cleaning, but that the effects were too modest to make it attractive in itself. It will be seen below that the introduction of amino silanes has a beneficial effect justifying their uses for the treatment of damaged, acidic papers.

Physico-mechanical determinations

The characterization of the effects of the introduction of reactants in the paper web was done measuring the surface pH, the initial acidity or alkaline reserve and the breaking length. pH was measured as described in part 1 of this series12, i.e. using a flat electrode for surface measuring. The reason for using this method was discussed in the second part of this series11. It must be recalled that the surface pH before treatment is a good indicator of the acidity state of the paper, but after treatment by an amino silane the surface pH can decrease from a high value, in the pH 8.5 - pH 9 range, to the pH 6 - pH 7 range while the paper still keeps an alkaline reserve.

Tensile strength, given under the usual form as breaking length, was tested on samples measuring 50 mm in length and 15 mm width in the machine direction on conditioned (20°C, 50% RH) samples using an Instron instrument, working at a rate of 10 mm/min, according to the usual standard13. For each book a block of 8 pages was kept untreated, representing the initial state. The folding resistance was measured according to the usual standard14.

Results and discussion

Reinforcement effect of ATMS

A preliminary study was necessary to establish the general effect of the process on the paper properties. Amino propyl trimethoxy silane (ATMS) was one of the silanes used the most in this study. Its effect was investigated on different acidic papers from damaged old books. The paper acidity being neutralized, as already described in the second paper of this series, the variations of mechanical properties were examined using mainly the breaking length (MD). Results are shown in Table 2.

It is to be noticed that the incorporation of ATMS induced a serious increase of tensile strength. This increase varied with the nature of the book; it was about 50% of the initial breaking length for the treating solution containing 10-15% w/w ATMS. The absorption depended on the internal porosity. As a general indication, papers of the type treated above absorbed around 80% (w/w) of the solution. It is worth recalling here that the fact that the concentration of the ethanolic solution was, e.g. 12% did not mean that the paper absorbed 0.8x12 = 10% in weight of the silane. Indeed, it has been observed that the weight gain (around 6%) is generally lower than that expected from the absorption of the treating solution. This can be easily determined by the weight of the solution uptake. It seems that a Chromatographic effect partly prevents the penetration of the silane into the paper web, at least under the conditions used for the experiment, i.e. ca. 10 min contact with the treating solution. This point will certainly deserve further investigation.