The cross-infection effect of 105 polymer samples was studied, using cellulose as a reference test material. In total 14 polymer types were studied, comprising “modern materials” commonly found in historic and artistic collections including: cellulose acetate (CA), cellulose nitrate (CN), poly(vinyl chloride) (PVC), polyurethane (PUR) and a selection of specialised packaging materials used in art and heritage conservation. Polymer samples were placed in glass vials containing a piece of the cellulose reference and vials were sealed before being heated to 80 C for 14 days. The cross-infection effect on the reference cellulose was measured using viscometry to calculate the degree of polymerisation relative to that of a control reference and a classification system of the cross-infection or preservation effect is proposed. Solid phase micro-extraction (SPME)-GC/MS was used to detect and identify the emitted volatile organic compounds (VOCs) from a select number of polymer samples. CN was identified as the polymer with the most severe cross-infection effect while others e.g. polycarbonate (PC) had no effect or even a beneficial effect. Acetic acid was found to be the most characteristic emission detected from the most severely cross-infecting materials
Analytical methods have been developed for the analysis of VOC emissions from historic plastic and rubber materials using SPME-GC/MS. Parameters such as analysis temperature, sampling time and choice of SPME fibre coating were investigated and sampling preparation strategies explored, including headspace sampling in vials and in gas sampling bags. The repeatability of the method was evaluated. It was found that a 7 d accumulation time at room temperature, followed by sampling using a DVB/CAR/PDMS fibre, with a sampling time of 60 min at room temperature was a suitable strategy for the detection of VOC emissions from a wide range of historic plastic and rubber artefacts. For 20 mL vials, a sample size of 50 mg was found to be appropriate and grinding the samples improved the repeatability of the analysis and yielded higher levels of emissions. A nondestructive adaptation of the method that could be used directly on historic objects in a museum environment is also presented. The detected emissions improve understanding of ongoing degradation processes within historic plastic and rubber materials, in addition to providing information on material composition.
KeywordsVolatile Organic Compounds; SPME-GC/MS; Heritage; Plastic; Modern Materials;
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Figure CaptionsFigure 1. Selected VOC emissions detected from 3 different plastic objects depicted as the peak areas detected using the CAR/PDMS fibre relative to those detected using the DVB/CAR/PDMS fibre. The error bars show the average range between maximum and minimum values. Hydrocarbons 1 and 2 are two hydrocarbons detected in the VOC emissions of polyethylene samples which could not be identified based on their mass spectra.
The use of VOC analysis to diagnose degradation in modern polymeric museum artefacts is reported. Volatile organic compound (VOC) analysis is a successful method for diagnosing medical conditions but to date has found little application in museums. Modern polymers are increasingly found in museum collections but pose serious conservation difficulties owing to unstable and widely varying formulations. Solid‐phase microextraction gas chromatography/mass spectrometry and linear discriminant analysis were used to classify samples according to the length of time they had been artificially degraded. Accuracies in classification of 50–83 % were obtained after validation with separate test sets. The method was applied to three artefacts from collections at Tate to detect evidence of degradation. This approach could be used for any material in heritage collections and more widely in the field of polymer degradation.
In the presence of several Pd(II) catalysts, cis-bicyclo[4.2.0]oct-7-ene (4a) was found to undergo olefin isomerization ("ring walking") and oligomerization, resulting in the formation of cis-bicyclo[4.2.0]oct-2-ene (4d) and the low-molecular-mass cycloaliphatic oligomers 5a−d, respectively. The catalysts studied are [Pd(NCEt) 6), and [(2,9-dimethyl-1,10-phenanthroline)Pd(CH 3 )(NC(CH 2 ) 6 CH 3 )][SbF 6 ] (7). Isomerization included the formation of both 4d and the olefinic end groups of 5a−d and ranged from 94% using catalyst 7 to 29% employing catalyst 2. Ab initio and DFT calculations at the LMP2/6-31G** and B3LYP/6-31G** levels show that the thermodynamic stabilities of the bicyclo[4.2.0]octene isomers increase in the order 7-ene 4a < 1(8)-ene 4b < 1-ene 4c ≈ 1(6)-ene 4e ≈ 3-ene 4f < 2-ene 4d. A mechanism of isomerization via subsequent β-hydride eliminations and olefin reinsertions is proposed. These results are in contrast to the reactions of bicyclo[3.2.0]hept-6-ene (3a) catalyzed by 1, 2, and 7 and the reaction of 4a catalyzed by Cp 2 ZrCl 2 / MAO (Cp = η 5 -C 5 H 5 ), all of which produced polymers in good yields (73−99%).
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