Volatile organic compound (VOC) emissions from plastic materials used for storing and displaying heritage objects

Volatile organic compound (VOC) emissions from plastic materials used in storage and display (e.g. Plastazote or Tyvek) were analysed. Polymer types of 42 material samples provided by UK heritage institutions were identified using attenuated total reflectance Fourier transform-infrared spectroscopy. These samples were also analysed for VOC emissions using headspace solidphase micro-extraction gas chromatography/mass spectrometry. Acetic acid was detected from 28 of the samples, including Moistop and Plastazote. A calibration was developed to estimate the concentrations of acetic acid emitted, which were found to be between 222 – 346 ppb. Additional detected VOCs include other polymer oxidation products such as aldehydes, and limonene, which is likely absorbed from the museum environment. These results indicate that plastic materials can oxidise in a museum environment to emit acidic VOCs and the way in which they are used in heritage institutions needs consideration. Introduction The damaging impact of volatile organic compounds (VOCs) on historic materials is well known. VOCs such as carboxylic acids and aldehydes can cause metal corrosion, degradation of calcareous natural history specimens and the embrittlement of organic materials (Grzywacz 2006). The sources of these VOCs are often the materials in which objects are stored or displayed and tests such as the Oddy test have been developed to identify potentially damaging materials (Oddy 1973). Previous work in this area includes analysis of VOC emissions from showcases, wood and wood products, resins, foams, adhesives and insulation materials (Schieweck and Salthammer 2011; Hatchfield and Carpenter 1986; Thickett 1998; Baer & Banks 1985). Plastics are a well-known source of damaging VOCs. For example objects composed of both cellulose acetate (CA) and cellulose nitrate (CN) are known to emit acetic and nitric acid respectively, which can damage other materials in their vicinity (Allen et al. 1987; Shashoua 2009). In the case of CA, this is known as the vinegar syndrome. Carboxylic acids and aldehydes have been detected from a range of other polymeric materials, including poly(ethylene) (PE), poly(propylene) (PP), poly(styrene) (PS) and polyurethane (PUR) (Hakkarainen et al. 1997; Larkin et al. 2000; Gurman et al. 1987; Thiebaut et al. 2007). Many specialised plastic materials are used in the storage and display of historic objects (e.g. Tyvek, Plastazote and Melinex). Previous work by this author showed acidic emissions from some such materials had a degrading impact on historic paper (Curran et al. 2014). However, only a small number of materials were studied. This paper expands on that work, with a particular focus on the detection and quantification of acetic acid (AA) emissions. Method Source and characterisation of samples Plastic samples were provided by the National Records of Scotland, the Museum of London and The National Archives. Samples were analysed by attenuated total reflectance Fourier transform-infrared spectroscopy (ATR-FTIR) using a Bruker Alpha FTIR Spectrometer with an ATR Platinum Diamond single-reflection module #CFBFA32D. 24 scans were collected over the wavenumber range 4000 to 375 cm with a resolution of 4 cm. Analysis of VOC emissions VOC analysis was performed according to a previously published method (Curran et al. 2016) using headspace solidphase micro-extraction gas chromatography/mass spectrometry (HS-SPME-GC/MS). 50 ± 5 mg of each material cut into small pieces was analysed. 1 ml aliquots of aqueous AA solutions were analysed by the same method after equilibration for 24 hours at room temperature. Vapour phase AA concentrations were calculated using experimental conditions and Maple 14.01 from Maplesoft. AA peak areas were weighted using an external standard (MISA Group 17 Non-Halogenated Organic Mix 2000 mg/ml in methanol; 48133 Supelco, diluted 1/50 in methanol). 1 ml aliquots of the diluted standard were analysed according to Curran et al. (2016). The environmental conditions (temperature and relative humidity) of the laboratory were recorded using an Onset Hobo data logger (U12-011) placed beside the GC/MS. Results Polymer identification Using ATR-FTIR spectroscopy, a range of different polymer types were identified including PE, PS and poly(butyl terephthalate) (PBT). The most common polymer type was PE. The samples and polymer identifications are shown in Table 1. Table 1. Samples used in this research and polymer types identified by FTIR Sample name Source Commercial source Polymer type Vacuum bags NRS Protective Packaging Poly(ethylene) Extruded LD45 grey NRS Preservation Equipment Poly(ethylene) Extruded LD45 white NRS Preservation Equipment Poly(ethylene) LD45 black perforated NRS Polyformes Poly(ethylene) LD45 grey NRS Paulamar Company Ltd Poly(ethylene) LD45 black NRS Paulamar Company Ltd Poly(ethylene) Tyvek 1 NRS Preservation Equipment Poly(ethylene) Tyvek 2 MOL N/A Poly(ethylene) Tyvek 3 TNA Preservation Equipment Poly(ethylene) Biodegradeable Bag NRS Ferrari Packaging Ltd Poly(ethylene) Ethafoam MOL N/A Poly(ethylene) Reflective Mylar MOL N/A Poly(ethylene) Marvelseal MOL N/A Poly(ethylene) Plastazote 1 black MOL N/A Poly(ethylene) Plastazote 2 grey MOL N/A Poly(ethylene) Plastazote 3 white MOL N/A Poly(ethylene) Plastazote 4 blue MOL N/A Poly(ethylene) Plastazote 5 black TNA c Kewell Converters Ltd Poly(ethylene) Jiffy foam MOL N/A Poly(ethylene) Coroplast with UV inhibitor NRS N/A Poly(propylene) Correx MOL N/A Poly(propylene) Charcoal cloth MOL N/A Poly(ethylene terephthalate) Bondina 30 gsm NRS Conservation by Design Poly(ethylene terephthalate) Bondina 100 gsm NRS Conservation by Design Poly(ethylene terephthalate) Reemay NRS Conservation by Design Poly(ethylene terephthalate) Vivak TNA c Bayer Poly(ethylene terephthalate) glycol modified Base for medals, coins, finds MOL N/A Poly(styrene) White Gatorfoam (inside) MOL Alcan Composites Poly(styrene) Standard Foamboard 5mm (inside) TNA c Conservation by Design Poly(styrene) Bump ons MOL N/A Polyurethane UV filter material MOL N/A Poly(methylmethacrylate) Moistop MOL N/A Poly(butylene terephthalate) Coloured film backing plastic MOL MACtacA Bemis Company Poly(butylene terephthalate) Melinex 75 micron TNA c Preservation Equipment Poly(butylene terephthalate) Coloured film 798-01 frosted MOL MACtacA Bemis Company Poly(vinyl chloride) Coloured film 798-02 dusted MOL MACtacA Bemis Company Poly(vinyl chloride) Coloured film 738-00 MOL MACtacA Bemis Poly(vinyl chloride) offshore blue Company Coloured film 748-00 refreshing mint MOL MACtacA Bemis Company Poly(vinyl chloride) Coloured film 708-00 sparkling yellow MOL MACtacA Bemis Company Poly(vinyl chloride) Coloured film 758-00 romantic rose MOL MACtacA Bemis Company Poly(vinyl chloride) Coloured film 778-00 luxurious Gold MOL MACtacA Bemis Company Poly(vinyl chloride) Lexan 9030 TNA c theplasticshop.co.uk Polycarbonate National Records of Scotland b Museum of London c The National Archives The FTIR spectra of three samples, showing common polymer types among the materials studied are shown in Figure 1. The Tyvek sample, composed of PE shows characteristic peaks at 2947 and 2914 cm (CH stretch), at 1472 and 1462 cm (CH deformation) and at 730 and 716 cm (CH rocking). Coroplast, composed of PP shows peaks at 2949, 2917, 2867 and 2838 cm (CH stretch), at 1456 and 1375 cm (CH deformation) and peaks at 1167, 998 and 973 cm (C-C skeletal). Charcoal cloth, composed of poly(ethylene terephthalate) (PET) shows characteristic peaks at 2959, 2916, 2848 cm (CH stretch), a strong peak at 1728 cm (C=O) and peaks at 1240 and 1159 cm (C-O-C groups) (Socrates 2001).