ANALYSIS OF LAMINATED DOCUMENTS USING SOLID-PHASE MICROEXTRACTION
The National Archives and Records Administration (NARA) was one of the first institutions to employ cellulose diacetate (CA) lamination on a large scale as a means of preserving documents (Stiber 1988). A heated hydraulic press was used to bond a thin film of CA to the document. This treatment was used from the 1930s to the 1980s, and during this time the laminating procedure evolved, including changes in the use of tissue reinforcement and deacidification. The formulation of the CA lamination film also varied. At least three different brands of films were used (Gear 1965), and the composition of these may have changed over time, depending on the manufacturer's formulation and quality control (Barrow 1965; Clements 1972).
The main difference between these films and others available was in the type and amount of plasticizers added to lower the softening temperature of CA (Gear 1957). Without these additives, the heat and pressure needed to melt the film for lamination would damage the paper (Wilson and Parks 1983). CA lamination film typically contains 20–30% plasticizers by weight (Stannett 1950). Table 1 lists phthalates and other additives commonly used with CA lamination film and some possible breakdown products.
The National Bureau of Standards (NBS, now the National Institute of Standards and Technology) conducted several research projects focusing on document lamination beginning in 1933 and culminating in 1959 with specifications for archival lamination films (Wilson and Forshee 1955, 1959a, 1959b). Research by NBS and others showed that some plasticizers are more stable than others. In fact, the plasticizer is typically more susceptible to degradation than the CA polymer (DeCroes and Tamblyn 1952; Shinagawa et al. 1992).
Therefore, to evaluate the condition of a laminated document, it is helpful to identify the plasticizers present and any breakdown products. This information may provide insight into the long-term stability of the item. For example, documents may be delaminated to repair damage, to remove degrading CA, or to prepare them for exhibit. An acetone bath is typically used to remove the CA, but occasionally more polar solvents are necessary to delaminate some documents (Page 2003). This change in solubility indicates that the CA has lost some degree of acetylation, which in turn may affect the compatibility of a plasticizer with the polymer (Stannett 1950). This deterioration could be due to a number of factors, including the composition of the film, the procedures used for lamination, and the subsequent exhibit and storage conditions. By studying the relationship between the composition of the CA and the present condition of the documents, it may be possible to better understand the deterioration process. This knowledge may help to more fully evaluate the current condition of documents and to anticipate potential problems with collections laminated with various types of film. For NARA, identifying the plasticizers can also suggest when a document was laminated, because the time periods during which different films were used are known. Since the lamination procedure evolved over time, knowing the treatment date may provide additional information about whether the document was deacidified before lamination and what other procedure may have been performed.
One commonly used method for identifying plasticizers is Fourier transform infrared (FTIR) spectroscopy (Haslam and Willis 1965; Ballany et al. 1998). Because CA has strong peaks in its FTIR spectrum that can overlap and obscure peaks from the plasticizers, it is often necessary to extract the plasticizers from the document. In addition, many of the commonly used plasticizers have similar spectra, so it can be difficult to positively identify a specific compound. The sample may also contain a mixture of plasticizers and other additives, further complicating the spectral interpretation.
Solid-phase microextraction (SPME) in combination with analysis by gas chromatography–mass spectrometry (GC-MS) is a convenient alternative method for identifying plasticizers. Invented in 1989 by Janus Pawliszyn and coworkers, SPME is a simple, sensitive, and economical technique that has become popular in a wide range of applications (Pawliszyn 1998, 1999; Supelco 2002). In the conservation field, it has been used to study pollutant gases and volatile degradation products (Rhyl-Svendsen and Glastrup 2002; Rhyl-Svendsen 2003; Lattuati-Derieux et al. 2004) as well as to screen materials to be used in exhibit and storage construction (Maines 2002).
This article describes the use of SPME–GC-MS for identifying plasticizers and their breakdown products in CA films and laminated documents. Various sampling methods were explored to refine the procedure. Laboratory samples prepared by the NBS as part of its earlier research were studied, as well as other materials in the collection of the NARA Research and Testing Laboratory. These techniques were applied to documents related to the Louisiana Purchase that had been laminated in the 1930s. These documents showed signs of CA degradation because they could be delaminated only by adding water to the acetone bath.