Research article Received: 30 December 2015
Revised: 18 May 2016
Accepted: 18 June 2016
Published online in Wiley Online Library: 27 July 2016
(wileyonlinelibrary.com) DOI 10.1002/jrs.4984
A new insight into polyurethane foam deterioration – the use of Raman microscopy for the evaluation of long-term storage conditions Susana França de Sá,a,b Joana Lia Ferreira,a,b* Ana Sofia Matos,d Rita Macedoa,c and Ana Maria Ramosa,b This research aims at bringing new contributions for lowering the polyether-based polyurethane (PUR) flexible foam high deterioration rate by studying selected storage conditions. With a life expectancy of 20–50 years, many PUR objects show severe deterioration signs that, so far, cannot be prevented. To establish a preservation strategy, a natural ageing experiment in the dark (12 months, 45–55% relative humidity) has been carried out for the study of four storage conditions: open air at room temperature (T) and sealed enclosures at room T (with/without oxygen) and low T (11–13 °C). For this research, unaged (model PUR) and naturally pre-aged (commercial PUR) references were included. The samples were analysed every 3 months by colourimetry, optical microscopy (visible and UV light), infrared and Raman microspectroscopies. Further statistical data treatment was carried out in order to identify significant spectral variations (ANOVA) and to uncover correlations between molecular spectral lines and physical aspects. Raman proved to be a powerful tool in the detection of early stage PUR molecular deterioration. For both model and commercial samples, open-air storage proved to be the most harmful condition, while anoxic storage showed the best results. In general, this experiment has enabled new contributions for lowering PUR high deterioration rate. Copyright © 2016 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: conservation; polyether-based polyurethane foam; long-term storage conditions; Raman microscopy
Introduction
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Polyurethane (PUR) foam is known as one of the most versatile polymers.[1] The potential to produce tailor-made PUR formulations is one of its most attractive qualities, and it is largely related to the unlimited design possibilities of PUR macroscopic cell structure and morphology. Moreover, it is one of the reasons why this polymer became a leading figure of the ‘Plastic Foam Age’.[2] In addition to the major application of PUR in industrial markets, this material has also met the artists’ desires, mostly since the 1960s.[3–5] In Museu do Design e da Moda (MUDE), Colecção Francisco Capelo (the design and fashion museum, Lisbon), as in other museums, many objects are the living proof of this fondness for PUR foams, fibres, synthetic leathers and rubbers. From furniture design to ready-to-wear fashion garments and accessories, MUDE’s collection shows how PUR has come into every home in the form of comfortable, stylish and luxury objects, especially since the 1960s. On the other hand, PUR is known for its ephemeral nature and is placed in the range of the most difficult polymers to preserve.[3–9] Like other aged PUR foam objects, several case studies from MUDE’s collection show a high level of instability clearly evident by surface brittleness, crumbling and intense foam discoloration (Fig. 1). Foams are particularly prone to deterioration because of their open cell and porous structure, showing a life expectancy of 20 to 50 years.[3–9] From information on labels and visual assessment, 20% of MUDE’s collection may include PUR-based elements, which means that it is vital to discuss preservation options for these historical objects.
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Plastics are known to deteriorate continuously, and according to Yvonne Shashoua, “prolonging the useful lifetime of plastics is possible today but is very limited in scope and effectiveness.”[10] In this context, it has been stated that ideally, plastics ‘should be stored in cold, dark, dry and oxygen-free conditions’,[11] but with the exception of one preliminary study for polyester-based PUR foams,[12], there is no systematic research for the definition of storage conditions for PUR foams.[4,13] For that purpose, we have started a conservation study focused on the assessment of storage conditions for this highly ephemeral material. This paper discusses the use of Raman microscopy for the evaluation of polyether-based PUR
* Correspondence to: Joana Lia Ferreira, Departamento de Conservação e Restauro, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal. E-mail:
[email protected] a Departamento de Conservação e Restauro, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516, Caparica, Portugal b LAQV-REQUIMTE, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516, Caparica, Portugal c Instituto de História da Arte, Faculdade de Ciências Sociais e Humanas, Universidade NOVA de Lisboa, Avenida de Berna, 26-C, 1069-061, Lisbon, Portugal d UNIDEMI, Departamento de Engenharia Mecânica e Industrial, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516, Caparica, Portugal
Copyright © 2016 John Wiley & Sons, Ltd.
A new insight into the preservation of PUR foams based on μ-Raman
Figure 1. Safari sofa, 1968, Poltronova production. Fibre-reinforced polyester, safari pattern fabric and polyurethane foam padding: (a) general view, (b) detail of the collapsed foam padding in 2012 (showing an intense brown colour) and (c, d) microscopy images of PUR foam cell buns collected from the Safari padding under darkfield light (showing different yellow shades). The arrows indicate the collapsed foam (b) and examples of microholes, pitting and cracks on the cell buns (c, d).
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synthetic paints.[19,20] The high sensitivity of Raman microscopy to polymer conformational structures and to C–C and C=C stretching vibrations are some of the reasons pointed for that use.[18,21] Regarding thermoplastic polyurethane elastomers (TPU), studies with important results have already been published. Namely, the characterisation of aromatic ring structures and polymer backbone groups is easily provided by Raman microscopy.[22] Also, ‘extra information about the domain structure and phase behaviour of copolyurethanes’ was achieved based on this technique.[23] Furthermore, different hydrogen-bonding interactions were confirmed for TPU films (prepared with and without solvents) based on the analysis of the amide I region.[24] This approach has also been applied by Weakley et al. for the study of hydrogen-bonding interactions on different hard-to-soft segment mole fractions for a TPU.[25] From these and previous studies concerning hydrogenbonding on proteins and amide solutions,[26,27] it can be concluded that Raman microscopy is highly sensitive to hydrogen-bonding interactions, which are particularly important in the design of PUR chemical and physical properties. Moreover, other authors have referred infrared spectroscopy as not as sensitive to amide I side chain environments as for example, Raman microscopy.[27] As mentioned in literature, ‘polyurethanes are strongly self-associated through intermolecular hydrogen bonding’,[28] and this subject has been a challenging question in chemical research.[29–42] It is now generally accepted that PUR foams are segregated structures comprising a two-phase morphology of the solid state (soft and hard segments, SS and HS respectively) and two types of crosslinking (chemical and physical).[32,33,43] The hydrogen-bonding interactions between HS chains consist on the physical crosslink of PUR flexible foams, also known as virtual crosslink.[33,35] According to Thomas et al., “the virtual cross-links are labile at elevated temperature but act like
Copyright © 2016 John Wiley & Sons, Ltd.
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foams’ long-term storage based on a natural ageing experiment for unaged and pre-aged references. Open-air (room T) and enclosed systems (room T, low T and anoxic) were tested. The potential use of Raman microscopy for the detection of PUR foams’ early deterioration stages is highlighted. Although the deterioration of PUR has been widely investigated,[6–9,14–16] to our knowledge, there are no publications concerning the use of Raman microscopy for its assessment. Instead, Fourier transform infrared spectroscopy in attenuated total reflection mode,[3–9,15] thermogravimetry and differential scanning calorimetry,[9,14] mechanical tests [5,6,9] and more recently pyrolysis-gas chromatography/mass spectrometry and headspace solid-phase microextraction coupled with gas chromatography and mass spectrometry[8] have already been used. Based on these studies, it is already accepted that polyether-based PUR foams are mainly susceptible to photo-oxidation. The extreme photochemical sensitivity of the soft segment has been referred to result in the formation of alcohols, carboxylic acids and formates (main photoproduct) and the presence of aromatic diisocyanates on the hard segment, to cause the formation of quinoid-type products (ex.: quinone) and peroxides that absorb in the range of 300– 400 nm (causing the yellowing of PUR foams). Because materials ‘that suffer from inherent chemical decay can be assumed to last approximately twice as long for each 5 °C drop in storage temperature’[17] and light and oxygen have been identified as the main deterioration causes of polyether-based PUR foams, anoxic and low T (≈12 °C) conditions were selected as promising preventive measures for the long-term storage of these foams [both in the dark and at 50–55% relative humidity (RH)]. Although Raman microscopy has not been commonly applied in conservation studies for these foams, it has proved to be a valuable technique for characterisation and ageing studies of plastics[18] and
S. França de Sá et al. ‘covalent crosslinks’ at room temperature.”[35] Because Raman microscopy has been proved to be sensitive to hydrogen-bonding interactions, this analytical approach was applied for the study of PUR foam deterioration in different storage conditions based on a natural ageing experiment in the dark. In addition, optical microscopy and infrared microspectroscopy were also used. From the Raman data, further statistical treatment was carried out based on the one-way analysis of variance (ANOVA), followed by the post hoc Tukey–Kramer multiple comparison test. Although the use of these statistical approaches has not been commonly applied in conservation studies,[44,45] their application in this study resulted in the identification of significant correlations between visual aspects (yellowing) and molecular fingerprints of the Raman spectra (changes in C=C stretching vibrations). For this research, two references were studied in order to include samples showing good and poor conditions: the model PUR (unaged reference) and the commercial sample (naturally pre-aged for 10 years), both composed of 2,6-toluene and 2,4-toluene diisocyanates (TDI) and polyetherbased polyols, and the first, following the typical specificities of slabstock PUR foams. From this research, storage conditions for polyether-based PUR foams are proposed; the key role of oxygen in its deterioration is stressed, and new contributions for lowering the PUR high deterioration rate are launched, mostly based on the monitoring of C=O stretching Raman bands assigned to hydrogen-bonding interactions on both urea and urethane linkages. Because these molecular changes were detected in samples stored in the dark, at room T and ≈50% RH, the high vulnerability of polyether-based PUR flexible foams’ segregated morphology is emphasised.
Materials and methods Samples and storage enclosures
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Two polyether-based PUR samples were selected for this research with the aim to include PUR samples showing different condition grades: an unaged reference (good condition grade) and a naturally pre-aged reference (poor condition grade). The production of the model PUR (unaged reference) was requested to Flexipol-Espumas Sintéticas S.A., following the typical specifications of water-blown polyether-based slabstock foams. The detailed information provided by the company was TDI-80 diisocyanate (2,6-toluene and 2,4-toluene diisocyanates), polyether-based polyols, 49.1 kg/m3 foam density (ISO845), CLD40% 4.5 kPa hardness (ISO3386/1) and 278 l/m2/s air permeability (ISO9237). A commercial PUR plate kept for 10 years in indoor conditions and without any special care was used as a naturally pre-aged reference. This plate, primarily acquired for upholstery purposes, was selected for this study because of the high probability of following the typical processes of slabstock production and therefore to serve as a comparison with the model PUR. Moreover, this reference shows an intense yellow colour and a fragile network clearly evident by the loss of flexibility and susceptibility to disintegration with handling. For both references, further infrared analysis confirmed the use of aromatic diisocyanates (TDI) and polyether-based polyols mainly based on poly(propylene glycol). These have been also reported in the literature as the most common raw materials for slabstock foams.[1,2,32,33] For the ageing experiment, both PUR references were cut into several specimens of 4 × 4 × 1 cm.
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For the enclosed systems, combined bags made of one side transparent ESCAL film and one side aluminium barrier film A 30T were used with the following specifications: ESCAL film PP/ceramic deposited PVA/PE, 112 μm thickness, oxygen and vapour permeation (20 °C) of 0.05 cm3/m2/day and 0.01 g/m2/day, respectively; aluminium barrier film PET/Al/LDPE, 120 μm thickness, oxygen permeation 0.01 cm3/m2day and vapour permeation 15), already after 1 month. On the other hand, the samples sealed in barrier film bags did not suffer colour changes, reaching total colour variations slightly above 1, even after 12 months. Concerning the naturally pre-aged commercial references, comparable results were obtained, although not as drastic because of their yellowing degree already at t = 0. The samples exposed in open air suffered total colour variations higher than three (ΔE* ≈ 3.5) after 12 months of ageing, and values below 2 were obtained for the other storage conditions. Although light has been commonly referred as the main cause of ether-based PUR discolouration, based on this first visual approach, it might be possible to attribute the discoloration to the presence and long-term availability of oxygen. Even though the anoxic storage was the only environment where this deterioration agent was lowered to
