Background

Generally, degradation of chemically unstable heritage materials leads to loss of mechanical strength and aesthetic quality of objects, diminishing their value – functional, aesthetic or social. When the degradation becomes unacceptable, objects are considered unfit for handling or display and therefore irreversibly damaged. The module uses expected lifetime, that is to say time until an object is expected to undergo unacceptable change, as a quantitative measure of climate-induced chemical damage risk to the object. Two fundamental characteristics of indoor climate – temperature and relative humidity (RH) – are linked in the formulae for the rate of chemical degradation of materials which allow the expected lifetime of objects in a particular indoor climate to be predicted. Most storage or display spaces have varying climates, therefore, instantaneous expected lifetime values are calculated for consecutive single readings of temperature and RH uploaded by the user and, then, the time averaged ‘long-term’ expected lifetime is calculated for the time interval analysed.

As users are usually interested in relative change in risk for various events or adjustments of temperature and relative humidity, the module calculates also relative expected lifetime that is to say ratio of the expected lifetime in the microclimate conditions provided by the user to the expected lifetime at stable room conditions (20°C, 50%rh).

Three currently available equations can be used by the user to calculate the expected lifetime and the relative expected lifetime for a range of low stability materials: the equation by Strlič et al. (2015b) applicable to wood pulp-based papers post-1850; the Preservation Index derived by the Image Permanence Institute from the study of chemical degradation of cellulose acetate, a polymer which is the base for most film materials (Reilly et al., 1995); Michalski’s equation, derived from a review of data on paper, film and dyes, considered applicable to a general category of low stability organic materials – the equation provides only for the relative expected lifetime (2000).

1. Chemical degradation of cellulose-based materials (Strlič et al., 2015b)

The paper production process changed during the centuries. Since its beginning in the medieval times until well into the nineteenth century, the paper making used linen, hemp and cotton rags as the source of the cellulosic material. To render the formed paper sheets water repellent and thus suitable for writing or printing, they were sized with gelatine extracted from animal tissues containing collagen. The gelatine-sized papers made of rags pre-nineteenth century have high chemical stability and their expected lifetimes are counted in millennia.

In the second half of the nineteenth century, wood fibres become dominant in the paper production, generally, either as mechanical or chemical pulp. Mechanical pulping involved mechanical separation of the cellulose fibres from one another leaving much of the lignin in the material. The mechanical pulp was used for low-cost products as newsprint or paperboards. In turn, chemical pulping involved chemical degradation of lignin and hemicellulose which could be removed from the cellulose. Alum-rosin size invented in 1807 gradually replaced the gelatine size due to its lower cost. The rosin-alum sized wood pulp-based papers post-1850 have low chemical stability and their degradation is considered in the module.

Chemical degradation leads to loss of mechanical strength in paper objects. The dominant mechanism in the process is the acid-catalysed hydrolysis of cellulose, leading to the cellulose chain scission, diminishing degree of polymerisation of cellulose DP and, consequently, increasing paper’s brittleness. The degradation rate k can be expressed as:

where DP₀ and DP are the degree of polymerisation of cellulose at time 0 and t.

The mechanical degradation rate depends on the acidity of paper, temperature and relative humidity. The proposed equation for k was based on 121 degradation experiments conducted on machine-made papers reported in the literature (Strlič et al., 2015b) :

where [H₂O] is moisture content in paper expressed as a fraction at a given RH and temperature, [H+] = 10^−pH (pH is determined in the cold water extract of paper in the standardised operation (Strlič et al., 2004)), and T is temperature given in K.

The current polymerization degree of cellulose DP₀ in a given paper object and its acidity – reflected in the cold water extract pH value - need to be known to calculate the degradation rate according to equation (1). For rosin-alum sized wood pulp-based papers, DP₀ between 500 and 1000 is characteristic of acidic papers and around 2500 for bleached pulp papers (Strlič et al., 2015b; Menart et al., 2014). pH values range from 4 to 6.5 (Zou et al., 1996; Menart et al., 2014; Świątkowska et al., 2018). In the module, DP₀ and pH can be selected by the user depending on the type of a paper collection.

In a study of mechanical degradation of historic papers post-1850, it was demonstrated that for objects with DP of cellulose in paper between 300 and 800 wear and tear may accumulate as result of daily interactions with objects, especially in reading rooms (Strlič et al, 2015a). Wear and tear accumulates only randomly for objects with DP > 800, while objects with DP < 300 are likely to accumulate significant damage - one large missing piece per 100 sheets - during each handling. This is accompanied by tearing and other, minor elements of mechanical deterioration.

In the module, a critical value of DP (DPcrit) can be selected by the user, depending on the type of access to a collection and intensity of use, as a suitable safe threshold value for handling, and the expected lifetime of objects is calculated as:

By way of example, Table 1 presents expected lifetimes of paper objects in years for DPcrit = 200 , several DP₀ and pH values; RH and temperature conditions over a year were typical of spaces housing collections accessed by staff and visitors.

Table 1 Expected lifetime of paper objects in years for DPcrit = 200 , several DP₀ and pH values.

2. Chemical degradation of cellulose acetate – the Preservation Index of the Image Permanence Institute (IPI) (Reilly et al., 1995)

Cellulose acetate or acetate esters of cellulose was produced by treating cellulose with acetic acid, acetic anhydride and sulphuric acid. The biopolymer obtained could be dissolved in acetone and further processed to bases of film materials. In the presence of moisture, the material undergoes hydrolytic decomposition, releasing volatile acetic acid giving rise to a characteristic vinegar odour. The degradation diminishes the degree of polymerisation of the material which leads to its shrinkage, embrittlement, and finally to separation of the emulsion layer, and crystallization of a plasticiser. The rate of hydrolytic degradation of cellulose acetate depends on temperature and relative humidity. The rate equation below fits the published experimental data for the cellulose acetate degradation (Tim Padfield, 2004):

in which k is expressed as the fraction of expected lifetime per year of the degradation and T is temperature given in K. The expected lifetime is therefore 1/k.

3. Chemical degradation of low stability organic materials (Michalski, 2000)

Michalski’s equation was derived from the research data on hydrolysis-dominated degradation of a range of low stability paper and photographic materials:

\begin{equation} \text{Relative expected lifetime} = (\frac{50\%}{RH\%})^{1.3} · exp[\frac{100000}{8.31} · (\frac{1}{T} – \frac{1}{293})] \label{eq:05} \end{equation}

in which T is temperature given in K.

References:

Menart, E., De Bruin, G., Strlič, M. 2014. Effects of NO2 and acetic acid on the stability of historic paper. Cellulose, 21:3701–13.

Michalski, S. 2000. Guidelines for Humidity and Temperature for Canadian Archives, CCI Technical Bulletin 23. Canadian Conservation Institute, Ottawa.

Padfield, T. 2004. The Preservation Index and The Time Weighted Preservation Index. https://www.conservationphysics.org/twpi/twpi_01.html

Reilly, J.M., Nishimura, D.W., Zin, E. 1995. New Tools for Preservation: Assessing Long-Term Environmental Effects on Library and Archives Collections. Commission on Preservation and Access, Washington, DC.

Strlič, M., Grossi, C. M., Dillon, C., Bell, N., Fouseki, K., Brimblecombe, P., Menart, E., Ntanos, K., Lindsay, W., Thicket, D., France, F., De Bruin, G. 2015a. Damage function for historic paper. Part II: Wear and tear. Heritage Science, 3:36.

Strlič, M., Grossi, C. M., Dillon, C., Bell, N., Fouseki, K., Brimblecombe, P., Menart, E., Ntanos, K., Lindsay, W., Thicket, D., France, F., De Bruin, G. 2015b. Damage function for historic paper. Part III: Isochrones and demography of collections. Heritage Science, 3:40.

Strlič, M., Kolar, J., Kočar, D., Drnovsek, T., Selih, V.S., Susic, R., Pihlar, B. 2004. What is the pH of alkaline paper? e-Preservation Science, 1:35–47.

Strlič, M., Kolar, J., Scholten, S. 2005. Paper and durability. In: Strlič, M., Kolar, J., Eds Ageing and stabilisation of paper. Narodna in univerzitetna knjižnica, Ljubljana.

Świątkowska, B., Skoczeń-Rąpała, Ł., Okrągła, D., Jędrychowski, M., Czop, J. 2018. Chemical degradation and physical failure: risk analysis for a paper collection. Studies in Conservation, 63:sup1, 251-256.

Zou, X., Uesaka, T., Gurnagul, N. 1996. Prediction of paper permanence by accelerated aging I. Kinetic analysis of the aging process. Cellulose, 3:243–67.