Water-dispersed polymers for the conservation and restoration of Cultural Heritage: a molecular, thermal, structural and mechanical characterisation

Polymer Testing 20 (2001) 227–240 www.elsevier.nl/locate/polytest

Material Characterisation

Water-dispersed polymers for the conservation and restoration of Cultural Heritage: a molecular, thermal, structural and mechanical characterisation L. D’Orazio a

a,*

, G. Gentile a, C. Mancarella a, E. Martuscelli a, V. Massa

b

Istituto di Ricerca e Tecnologia delle Materie Plastiche del CNR, Via Toiano, 6 - 80072 Arco Felice, Napoli, Italy b Syremont, Bollate (MI), Italy Received 30 December 1999; accepted 15 March 2000

Abstract A molecular, thermal, structural and mechanical characterisation of two different commercial water-dispersed polymers, i.e. an unreactive aliphatic polyetherurethane (trade name Akeogard AT40) and a VDF/HFP/TFE terpolymer (trade name Fluorobase T300) has been performed by means of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA), Fourier transform infrared spectroscopy (FTIR) and Fourier transform nuclear magnetic resonance (FT NMR), wide and small angle X-ray scattering (WAXS and SAXS) techniques and an Instron machine. Such polymers were selected from among the available products used for conservation and restoration by Cultural Heritage, with the view of using water-based polyurethanes and fluorinated copolymers as coatings for artefacts belonging to Cultural Heritage, consisting of natural fibrous polymers, mainly textiles. The results obtained so far suggest that a comparatively higher potential for the conservation and restoration of textiles with a cultural value is shown by AT40 polyurethane. Notwithstanding this, the structural properties assessed in water-cast films for both the polymers indicate that desired properties could be conferred by suitably selecting the chemical and molecular structure of the starting monomeric units.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Polyetherurethanes; Fluorocopolymers; NMR; TGA; WAXS; SAXS

1. Introduction The application of polymer materials on art works is today a technique widely used for their conservation and restoration. The uniqueness of the treated items implies that these applications must be carried out with extreme caution in order to avoid further damage to works often already damaged by natural alterations and/or chemical attack by polluting agents. Notwithstanding that many classes of polymer materials are actually used in the conservation and restoration of Cultural Heritage items, with

* Corresponding author. Tel.: +39-081-8534-255; fax: +39081-8663-378. E-mail address: [email protected] (L. D’Orazio).

more or less satisfactory results, few scientific data are available on their chemical constitution and composition, structure and properties. The performances shown for different commercial products on technical data sheets deal mainly with application factors and safety handling, and cannot be compared appropriately in order to fulfil effectively the specific needs of conservation and restoration. In the present paper results of investigations aimed at achieving a molecular, thermal, structural and mechanical characterisation of two commercial polymers already used for preserving Cultural Heritage items consisting of leather and stone, are reported. The investigated polymers are an aliphatic polyetherurethane (trade name Akeogard AT40) and a partially fluorinated terpolymer (trade name Fluorobase T300), and have been selected with the

0142-9418/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 0 ) 0 0 0 2 7 - 1

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view of using water-based polyurethanes and fluorinated copolymers as coatings for artefacts belonging to Cultural Heritage, consisting of natural fibrous polymers, mainly textiles. The selection criteria have been the water dispersibility and photooxydative resistance [1]. Since textiles undergo chemical attack by organic solvents, especially if dyed, conservation interventions must be carried out without using such kinds of solvents — with clear advantages also in terms of eco-sustainability and safety handling. Good photooxydation resistance is essential in order to avoid radiation acceleration as well as degradation of the conservative agent. The aim of the work is to establish structure–property relationships which will be useful for comparing the potential for conservation of systems based on polyurethanes and fluorocopolymers, and subsequently to design conservation agents in terms of chemical and molecular structure. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA), Fourier transform infrared spectroscopy (FTIR) and Fourier transform nuclear magnetic resonance (FT NMR), wide and small angle X-ray scattering (WAXS and SAXS) techniques have therefore been applied to films obtained from AT40 and T300 commercial water dispersions by water casting at room temperature. The tensile mechanical behaviour shown by both polyurethane and fluorinated terpolymer has also been investigated by means of Instron equipment. This study has been undertaken within the framework of a research project whose final target is the development of a new polymer based formulation characterised by innovative performances tailored for the conservation and restoration of textiles.

is the following :

As regards durability, the intrinsic oxidability of the – CH2 groups bonded to the oxygen atoms in the polyether segments can lead to the formation of hydroperoxides and consequent chain scission [3]. 앫 A linear copolymer (trade name Fluorobase T300) constituted by the following three monomeric units:

Such kinds of fluorinated materials are usually prepared by free radical emulsion polymerisation [4]. In comparison to the AT40 polyurethane, the VDF/HFP/TFE terpolymer shows higher photo-oxydative stability and resistance to chemical agents, due to the properties of the fluorine atom and the carbon–fluorine bond [5,6]. Both of the investigated polymers are commercialised by Syremont Company as water dispersions; the dry contents in the dispersions being 35% and 30% (wt/wt) for AT40 and T300 respectively. Films of both the polymers were prepared from commercial water dispersions by casting at room temperature. Dumbbell shaped samples for mechanical analysis (UNI 882) were obtained by compression moulding of films at 70°C for 5 min and subsequent cooling at room temperature.

2. Experimental 2.2. Techniques 2.1. Materials and sample preparation The polymers characterised are: 앫 a thermoplastic aliphatic polyetherurethane (trade name Akeogard AT40) unreactive with the absence of free isocyanate groups. The most common commercial method of synthesising polyurethanes is by the reaction of di- or polyfunctional hydroxy compounds, such as hydroxyl-terminated polyethers or polyesters, with di- or polyfunctional isocyanate [2]. The general structure of a linear polyurethane derived from an ether compound (a) and a diisocyanate (b)

2.2.1. Differential scanning calorimetry (DSC) The thermal behaviour of the AT40 and T300 polymers was analysed by means of a differential scanning calorimeter Mettler DSC 30 equipped with a control and programming unit Mettler TC 11. Water cast films were heated under a nitrogen atmosphere from ⫺150 to 500°C with a scanning rate of 10°C/min. The heat evolved during the process was recorded as a function of temperature. 2.2.2. Thermogravimetric analysis (TGA) Thermogravimetric analysis was carried out on water cast samples using a Mettler microbalance equipped with a Mettler thermogravimetric analyser, model TG 50. The measurements were performed with a heating rate of 10°C/min from 50°C to 600°C in air and nitrogen atmos-

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Fig. 1.

229

Thermal behaviour of the AT40 polyurethane under nitrogen flow: (a) DSC trace; (b) TGA trace; (c) DMTA trace.

pheres to determine the polymers’ decomposition temperature. 2.2.3. Dynamic mechanic thermal analysis (DMTA) The storage modulus and the loss tangent d of water cast samples were measured by means of a dynamic mechanical thermal analyser (Rheometric Scientific MK

III). Test data were collected in bending and in tensile modes from ⫺100°C to 200°C using a scanning rate of 3°C/min and a frequency of 1 Hz. 2.2.4. Fourier transform infrared analysis (FTIR) FTIR spectra were obtained with a Perkin Elmer spectrometer (model Paragon 500) using 15 scans summation

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Table 1 Thermal transitions of AT40 polyurethane films water cast at room temperature Transition

Technique DSC

Glass transition of polyether segments Glass transition of 44°C urethanic segments Thermal 265°Ca– degradation in 335°Ca nitrogen flow Thermal degradation in air flow a b

cross-head speed of 20 mm/min at a temperature of 25°C and 50% relative humidity.

3. Results and discussion DMTA

TGA

⫺15°C 42°C 352°Cb– 461°Cb 343°Cb– 443°Cb

Temperatures for maximum heat absorption. Temperatures for maximum rate in weight loss.

and a nominal resolution of 4 cm⫺1. The analysis was carried out on thin films obtained by dissolving AT40 samples in chloroform and T300 samples in acetone and then casting these solutions directly on NaCl disks. 2.2.5. Fourier transform nuclear magnetic resonance (FT NMR) 1 H and 13C NMR spectra were recorded on a Bruker 300 MHz spectrometer. A few milligrams of AT40 and T300 were dissolved respectively in deuterated chloroform and acetone. 2.2.6. Wide angle X-ray scattering (WAXS) WAXS investigations were carried out on water cast films by means of a PW 3020/00 Philips diffractometer (Cukα Ni-filtered radiation) equipped with a sample holder for sample spinning. The high voltage was 40 kV and the tube current was 30 mA. A standard sample was employed to determine the instrumental broadening. 2.2.7. Small angle X-ray scattering (SAXS) SAXS studies were carried out on water cast films by means of a compact Kratky camera equipped with a Braun 1-dimensional positional sensitive detector. Nifiltered CuKα radiation generated from a Philips X-ray generator (PW 1730/10) operating at 40 kV and 30 mA, was used. The raw scattering data were corrected for parasitic scattering, absorption and slit smearing by using Vonk’s method [7]. The desmeared intensities were then Lorentz factor corrected by multiplying by s2 (s=2sinq/l) [8]. 2.2.8. Mechanical analysis Uniaxial tensile tests were carried out by means of an Instron 410L tensile testing machine operating with a

3.1. Thermal transition properties Most of the polymers used in conservation are characterised by a glass transition (Tg) value close to or slightly higher than room temperature [1]; i.e. by a glass-rubber transition occurring near to the temperature of use. To be taken into account is the following: a polymer coating with a Tg value considerably higher than room temperature is unable to respond to changes in the form of the item and could thus damage it. On the contrary, a polymer coating showing Tg value noticeably lower than room temperature is too soft to act as a consolidating agent, tending moreover to pick up dirt. In Fig. 1a the DSC trace of the polymer AT40 in the temperature range between ⫺150°C and 400°C is reported. As shown, three transitions can be observed: the first one is a clear Tg at 45°C, whereas the remaining transitions result in two involuted broad endothermic peaks centred at 265 and 335°C. A more careful evaluation of the AT40 Tg temperatures has been carried out by DMTA, revealing the presence of two glass transitions (see Fig. 1c): the first at ⫺15°C, the second at 45°C in agreement with the Tg value detected by DSC (compare Fig. 1a with Fig. 1b). Such Tg values are to be attributed to polymer sequences containing etheric and urethanic bonds respectively; i.e. to the soft and hard segments of the AT40 polyurethane. The endothermic DSC peaks can be attributed to the thermal degradation process as clearly shown by TGA (see Fig. 1b). Such degradation, starting at about 240°C, shows the maximum weight loss rate at 461°C. Moreover, by TGA it can be observed that the AT40 thermal degradation process is not significantly influenced by the oxygen presence: the temperatures corresponding to the maximum weight loss rates by heating in air or in nitrogen flow disagree for 20°C only. The AT40 thermal transition properties are summarised in Table 1. Fig. 2a shows the DSC trace of the T300 terpolymer in the range between ⫺150°C and 400°C. Three endothermic transitions can be detected: that centred at 475°C is attributed to the thermal degradation of the polymeric material. TGA shows in fact that the weight loss starts at about 480°C (Fig. 2b) and the maximum rate of the degradation is at about 516°C in air and at 534°C in nitrogen (see Table 2). The presence of a weak endothermic peak centred at 344°C, which is in the melting range of the polytetrafluoroethylene (PTFE) [9], is also found. The very low intensity of such an endothermic transition (⫺1.6 J/g compared with ⫺82 J/g for PTFE homopolymers [10] indicates that there are no long

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Fig. 2. Thermal behaviour of the T300 VDF/HFP/TFE terpolymer under nitrogen flow: (a) DSC trace; (b) TGA trace; (c) DMTA trace.

PTFE blocks along the T300 chain. Finally, the DSC thermogram shows a Tg at ⫺14°C (see Fig. 2a); by DMTA such a Tg is located at ⫺1°C (see Fig. 1c). In the DSC trace of the T300 terpolymer there is neither a Tg around ⫺73°C that is the PTFE Tg [11] nor an apparent melting temperature (T ⬘m) of the polyvinylidenefluoride (PVDF), expected between 154 and 184°C [12]. Such

experimental evidence indicates the absence of long tetrafluoroethylenic and vinylidenefluoride blocks. The thermal transition properties shown by the T300 terpolymer are summarised in Table 2. By comparing the data reported in Tables 1 and 2, it can be inferred that both the materials show a resistance to thermal degradation appropriate for use in the conser-

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Table 2 Thermal transitions for T300 terpolymer films water cast at room temperature Transition

Technique DSC

Glass transition Melting point for PTFE blocks Thermal degradation in nitrogen flow Thermal degradation in air flow a b

⫺14°C

DMTA

TGA

⫺1°C

344°C 475°Ca

534°Cb

516°Cb

Temperature for maximum heat absorption. Temperatures for maximum rate in weight loss.

vation and restoration of Cultural Heritage items. The other major factor to be taken into account is the Tg values exhibited by the two polymers. In fact, at the temperature of use AT40 films are expected to show comparatively better properties owing to the fact that along AT chains the amorphous part is in a glassy state. In contrast, the amorphous T300 chains are in a rubbery state.

Table 3 Absorption band assignments for AT40 polyurethane Frequency (cm⫺1)

Relative intensitya

3300–3500 2949 2859 1729 1710 1650

M S S VS S Sh

1546

M

1450 1365 1242 1194 1150 1114 991 751

M W S M S S Sh W

Main assignmentsb n(NH)bonded nas(CH2) nsym(CH2) n(C=O) free n(C=O) bonded n(C=O) amide d(NH)+n(CN) amide II band d(CH aliphatic) d(CH aliphatic) n(C=O)+n(O–CH2) n(C–O–C) n(C=O)+ν(O–CH2) n(C–O–C) n(C–C) b(CH2)

a The relative intensity is based on the whole infrared spectrum of a sample at room temperature: W=weak, M=medium, S=strong, VS=very strong, Sh=shoulder. b The main assignments are: n=stretching, d=bending, w=wagging, b=rocking, nas=asymmetric stretching, nsym=symmetric stretching.

3.2. FTIR analysis In Table 3 the frequencies of maximum absorption and the corresponding assignements are reported for AT40 polyurethane. The deconvolution of absorption bands in the region 1620–1750 cm⫺1 has been carried out by applying a Gaussian–Lorentz function. There are two bands characteristic for the stretching of the urethanic carbonyl [13], at 1731 cm⫺1 and 1710 cm⫺1. The peak at 1731 cm⫺1 is attributed to free urethanic carbonyls, whereas the peak at 1710 cm⫺1 is associated with the hydrogen bonded carbonyls. The degree of hydrogen bonded carbonyl represents the interaction among the urethanic groups in the polymer chains [13]. The fraction of urethanic groups with hydrogen bonds can be evaluated from the relative areas of the corresponding absorption by assuming that the extinction coefficients for both bonded and free carbonyl bands are the same. Such a method provides evidence that the fraction of bonded urethanes is equal to 40% approximately. In Table 4 the frequencies of maximum absorption and corresponding assignments for T300 terpolymer are reported [14,15]. Absorption bands in the range 1000– 1300 cm⫺1 are characteristic of C–F bonds.

Table 4 Absorption bands and assignments for T300 terpolymer Frequency (cm⫺1)

Relative intensitya

Main assignmentsb

3031 2950 2871 1428 1398 1353 1206 1157 999 926 889 820

W W W S VS S VS VS Sh Sh S W

n(CH2) nas(CH2) nsym(CH2) d(CH2) d(CH2) d(CF2 and CF3) d(CF2 and CF3) d(CF) n(C–C) n(C–C) n(C–C) and b(CH2) b(CH2)

a The relative intensity is based on the whole infrared spectrum of a sample at room temperature: W=weak, M=medium, S=strong, VS=very strong, Sh=shoulder. b The main assignments are: n=stretching, d=bending, w=wagging, b=rocking, nas=asymmetric stretching, nsym=symmetric stretching.

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Table 5 1 H NMR of T300 terpolymer: signals for octylphenol ethoxylate Chemical group

Relative intensity

Multiplicities dH (ppm)

CH3(h) CH3(f) CH2(g) OH(a) CH2(b) CH2(c) CH(e) CH(d)

9 6 2 — 2 2 2 2

1 1 1 — 3 3 2 2

0.9 1.5 1.9 2.5 3.7 4.1 6.9 7.4

3.3. NMR analysis The interpretation of the 1H and 13C NMR spectra of the polyetherurethane AT40, obtained by dissolving water cast film in deuterated chloroform, was prevented by the overlapping of signals of molecules with comparatively lower molecular mass, presumably dispersive substances, with those of the investigated polymer. Therefore, no clear information on the AT40 molecular structure can be obtained. As expected, taking into account the specific regioregularity of the hydrogen atoms along the T300 chain, the 1 H NMR spectrum allows identification of the dispersive agent used. In Table 5 we report chemical shifts, multiplicities, relative intensities and assignments of signals which, in the T300 1H NMR spectrum (see Fig. 3), are

Fig. 3.

1

233

ascribed to the dispersive agent. According to the data reported in the table, the dispersive agent used is an alkylphenol ethoxylate. It is known that such substances are used to obtain water dispersion of fluorinated elastomers [16]. In particular, the alkylphenol ethoxylate in question is an octylphenol ethoxylate, notwithstanding that by 1H NMR it is impossible to assign signals located in the range 3.5–2.8 ppm due to polyoxyethylenic segments. The structural formula of an octylphenol ethoxylate is as follows:

The 13C NMR spectrum exhibited by the T300 terpolymer is not easily interpretable (see Fig. 4) owing to the complex coupling of carbon atoms with fluorine atoms causing not only 1J 13C–19F couplings but also 2J 13C– 19 F couplings [17]. In the range 100–130 ppm there are in fact many complex signals of –CF3 and –CF2– groups (see Fig. 4a). The signal of C–F groups of the hexafluoropropene monomeric unit is a complex multiplet centred at 93 ppm (see Fig. 4b). 1J 13C–19F coupling is about 90 Hz. It should be noted that the C–F group seems to be mostly bonded to the CH2 group of the vinylidenfluoride monomeric unit. Assuming that the 2J 13C–19F coupling between the investigated carbon atoms and fluorine atoms of the CF3 and CF2 groups are the same, the complex multiplet can be interpreted as follows (see also Fig. 4b):

H NMR spectrum of the T300 VDF/HFP/TFE terpolymer.

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Fig. 4. 13C NMR spectrum of the T300 VDF/HFP/TFE terpolymer; (a) signals of CF3 and CF2 groups; (b) signals of CF groups; (c) signals of CH2 groups.

Useful information on the terpolymer molecular structure

is moreover provided by the signals of the CH2 groups in the range 20–46 ppm, notwithstanding the presence in the same ppm range of signals due to the acetone carbon atoms. In fact, in this region the spectrum shows signals with comparable intensity (see Fig. 4c) indicating that there is no preferential chemical neighbour of the vinylidene fluoride monomeric units.

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Fig. 5.

WAXS intensity profiles of water cast films of the investigated polymers.

235

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Table 6 X-ray reflection maxima with corresponding angle (2q), spacing (d) and relative intensity of AT40 polyurethane

Table 8 Uniaxial tensile properties for AT40 polyurethane and T300 terpolymera

Reflection

2q (°)

˚) d (A

Intensity

Tensile properties

AT40

T300

1 2 3

18.2 30.0 41.3

4.87 2.98 2.19

vs w w

Young’s modulus (MPa) Stress at break (sb) (MPa) Elongation at break (eb) (%)

139 4.1 174

2.25

a b

3.4. WAXS investigation The WAXS intensity profile of water cast films of AT40 is shown in Fig. 5; and in Table 6 the reflection maxima with the corresponding spacings (d) and their intensities are listed. As shown in Fig. 5a, the AT40 sample displays three convoluted diffuse scattering reflections from 2q=5° to 60°; the first (reflection 1 in Table 6) rather sharp and with very strong (vs) intensity is in the range 5°–25° of 2q. The last two (reflections 2 and 3 in Table 6), broad and weak in intensity, are in the range 20°–60° of 2q. Such results suggest an amorphous structure of both hard and soft segments, consistent with no melting transitions shown by the DSC thermograms of the AT films. It should be noted that the Bragg spacing calculated from the scattering maximum centred at 18.2 of 2q (see Table 6) is, within the experimental error, the same value reported in the literature for the dimension of the b axis of the triclinic unit cell of polyurethane elastomers in which the urethane blocks are formed from 4,4⬘-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO) crystallites [18,19]. The deconvolution of reflections 2 and 3 shows that the related scattering maxima are centred at 30.0 and 41.3 of 2q respectively. Such maxima do not match the literature values for MDI– BDO crystallinity [18–20]. It could therefore be hypothesised that the scattering at comparatively higher angles is due to additives. The WAXS intensity profile of water cast films of T300 is shown in Fig. 5, and in Table 7 the reflection maxima with corresponding spacings (d) and their intensities are listed. As shown in Fig. 5, this spectrum exhibits three reflections, the first two (reflections 1 and 2 in Table 7 X-ray reflection maxima with corresponding angle (2q), spacing (d) and relative intensity of T300 terpolymer Reflection

2q (°)

˚) d (A

Intensity

1 2 3 4 5

17 18.2 40.6 37.3 41.6

5.22 4.87 2.22 2.41 2.17

s vs w w vw

b

494

Values displayed represent an average of ten tests. Samples do not break.

Table 7) of strong (s) and very strong (vs) intensity respectively and highly convoluted are in the range between 10° and 30° of 2q. It is importtant to note that reflection 1 is rather sharp and represents a diffuse scattering with a maximum centred at 17° of 2q, whereas reflection 2 is a very sharp diffraction peak at 18.2 of 2q. The last reflection, broad and weak in intensity (reflection 4 in Table 7) is in the range 30°–60° of 2q. A more detailed analysis of such a WAXS intensity profile shows that the spectrum presents, in the range 30°–60° of 2q, two further reflections (reflections 3 and 5 in Table 7) which are rather sharp, but very weak (vw) in intensity, and which correspond to reticular distances (d) of ˚ and 2.17 A ˚ respectively. An interesting feature 2.41 A of such a spectrum is that reflections 2, 3 and 5 in Table 7 correspond to the main reflections exhibited by polytetrafluoroethylene (PTFE), whose WAXS intensity profile is also reported in Fig. 5 for the sake of comparison. Such findings show that the T300 copolymer is characterised by the presence of traces of tetrafluoroethylenic crystallinity. On the other hand, no diffraction reflections ascribed to polyvinylidene fluoride (PVDF) according to Hasegawa [21] are found, thus confirming that there are no long vinylidene fluoride sequences along the copolymer chain. The WAXS results show that both the materials are characterised by an amorphous structure, even though a comparatively higher certain regularity on a local scale is shown by the T300 copolymer. As far as conservation and restoration are concerned, it should be taken into account that crystallites act as sites for a combination of cross-linking and reinforcement, whereas the amorphous state allows for a degree of rubberlike elasticity. 3.5. SAXS investigation The Lorentz-corrected desmeared pattern of water cast film of AT40 is shown in Fig. 6. As shown, the SAXS profile exhibits a well defined maximum indicating that the polyurethane chains have an (HS)n-type structure consisting of alternating polydisperse hard (H) and soft (S) segments. Taking into account that the observed intensity of the small angle scattering (I(Q)) depends on

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Fig. 6.

237

SAXS Lorentz-corrected desmeared profiles of water cast films of the investigated polymers.

the difference in electron scattering density between the hard and soft segments (rh–rs), the position of the scattering maximum (Qmax) can be used to evaluate the average one-dimensional inter-domain spacing. By applying Bragg’s law, the “long period” calculated from peak pos-

itions can be obtained. According to the simplistic schematic model that is reported below, consisting of alternating parallel amorphous hard urethane and amorphous soft polyether domains, the average inter-domain spatial ˚ , indicatperiodicity of the AT40 polyurethane is 325 A

Fig. 7.

Typical stress–strain curves for AT40 and T300 water cast materials.

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L. D’Orazio et al. / Polymer Testing 20 (2001) 227–240

ing a significant degree of structuring at super reticular level.

The Lorentz-corrected desmeared pattern of water cast film of T300 is shown in Fig. 6. A continuous scattering profile characteristic of diluted solutions or liquid systems is obtained, revealing no degree of superreticular (“long-range”) order of the material. From the SAXS results it is clearly shown that, even though AT40 and T300 polymers exhibit an amorphous structure, they are characterised by different morphologies. AT40 material, in fact, shows heterogeneities due to micro-phase separation. 3.6. Tensile behaviour The tensile properties exhibited at room temperature by dumbbell shaped specimens of AT40 and T300 are given in Table 8; in Fig. 7 typical stress–strain curves for AT40 and T300 materials are shown. The initial slope and stress values at strains lower than 25% show that the AT40 material is much stiffer than T300 material and has a comparatively higher elastic modulus. Such a finding can be accounted for by considering that the AT40 hard phase is below its Tg at room temperature; i.e. the urethanes’ microdomains, being amorphous in the glassy state, act as reinforcing filler and thermally reversible crosslinks. The shape of the tensile curve indicates that for AT40 there is inelastic deformation at constant stress up to sample break. Such behaviour, to be related to the SAXS results showing a super-structural order in the material (see Figs. 6 and 7), suggests that the hard domains could impart shear stresses at the interface causing a permanent deformation and relatively low elongation values. No crystallisation process seems to be induced by the applied stress. Conversely, the behaviour of an uncrosslinked elastomer is shown by the T300 terpolymer (see Fig. 7), in spite of the presence of the trace of tetrafluoroethylenic crystallinity highlighted by the DSC and WAXS investigations. In fact, the shape of the curve probably indicates that because of the low strain rate the material relaxes faster than it is strained. It could be hypothesised that the phase structure is characterised by unconnected amorphous domains with few tie points constituted by crystalline tetrafluorethylenic domains. This would be in agreement with the desmeared SAXS profile exhibited by T300 terpolymer showing no long range order.

239

4. Concluding remarks A molecular, thermal, structural and mechanical characterisation of two different commercial water-dispersed polymers, i.e. an unreactive aliphatic polyetherurethane (trade name Akeogard AT40) and a VDF/HFP/TFE terpolymer (trade name Fluorobase T300), has been performed. Such polymers, already used in the field of conservation and restoration of Cultural Heritage, have been selected with a view to using water based polyurethanes and fluorinated copolymers as coatings for artefacts belonging to the textile heritage. It should be noted that Akeogard AT40 polymer is characterised by: 앫 two glass transition temperatures located at 45 and ⫺15°C, to be ascribed to hard and soft phases respectively, and thermal stability in air up to 200°C; 앫 a fraction of bonded urethanes groups equal to 40%; 앫 an amorphous structure of both hard and soft domains; 앫 a significant degree of structuring at superreticular level, i.e. “long range order”, with an average inter˚; domain spacial periodicity of 325 A 앫 a relatively high value of the elastic modulus and low values of elongation at break. For T300 terpolymer, it has been possible by FT NMR to identify the dispersive agent. An octylphenol ethoxylate has been used to obtain water dispersion of such a fluorinated copolymer characterised by: 앫 a single glass transition temperature ranging between ⫺14 and ⫺1°C and thermal stability in air up to 500°C; 앫 random configuration of the monomeric units; 앫 traces of tetrafluoroethylenic crystallinity highlighted by both DSC and WAXS investigation; 앫 no degree of superreticular order; 앫 mechanical behaviour in uniaxial tension of an uncrosslinked elastomer. The results obtained so far seem to suggest that a comparatively higher potential for the conservation and restoration of Cultural Heritage items constituting textiles and related artefacts is shown by AT40 polyurethane. Notwithstanding this, the structural properties assessed from water-cast films of both the polymers indicate that the desired properties could be conferred by suitably selecting the chemical and molecular structure of the starting monomeric units. Work is in progress to investigate the effects of applications of AT40 and T300 dispersions on cellulose and protein fibres mainly in terms of interfacial adhesion and aesthetic compatibility.

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