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Compositional analysis of fluorinated and unfluorinated acrylic copolymers
J. Anal. Appl. Pyrolysis 85 (2009) 321–326
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Compositional analysis of fluorinated and unfluorinated acrylic copolymers Massimo Lazzari a,b,*, Dominique Scalarone a, Chiara Riedo a, Oscar Chiantore a a b
Dept. of Chemistry IPM and Nanostructured Interfaces and Surfaces – Centre of Excellence, University of Torino, Via P. Giuria 7, 10125 Torino, Italy Dept. of Physical Chemistry, Faculty of Chemistry and Institute of Technological Investigations, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 July 2008 Accepted 17 November 2008 Available online 27 November 2008
The monomeric components of a series of lattices suitable as protective coatings for the safeguard of cultural heritage, and containing unfluorinated and partially fluorinated acrylic units were determined by pyrolysis-gas chromatography/mass spectrometry. Their identification was possible on the basis of a good knowledge of the degradative behavior, also gained through the direct study of different reference fluoropolymers, which enabled to distinguish between monomers and monomer-related products of degradation. All the samples were identified as copolymers of an acrylic unit, either partially fluorinated or unfluorinated, with a methacrylic unit. Only in two cases different amounts of styrene as monomeric component were also identified. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Acrylic lattices Acrylic copolymers Fluoropolymers Polyfluoroacrylates Compositional analysis
1. Introduction Polymers based on (meth)acrylic esters are well-known for their good mechanical and film-forming properties, optical clarity, adhesion and overall chemical stability, and for such reasons have found important applications since the forties in the formulation of varnishes, paints and adhesives for different substrates [1]. On the other hand, the progressive availability of differently fluorinated (meth)acrylic monomers has resulted in an increasing interest for the polymers which may be produced from them, essentially because they exhibit a wide range of improved and peculiar characteristics [2,3]. The formal replacements of hydrogen atoms with fluorine atoms either in the main chain or in the alkyl side groups improves thermal- and photo-oxidative stability and modify drastically other properties, e.g. the ensuant reduction of surface energy results in a high water repellency [4]. Fluorinated coatings have been developed for diverse applications, such as anti-fouling agents, water-repellents and components for nonsticky surfaces [5], whereas the specific use of partially fluorinated acrylic-based products in the niche sector of conservation of the cultural heritage is still at the beginning [6,7]. Among the many different commercially available resins for the protection and consolidation of stone surfaces, wood, metallic
* Corresponding author at: Dept. of Physical Chemistry, Faculty of Chemistry and Institute of Technological Investigations, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain. Tel.: +34 981 563100x13011; fax: +34 981 595012. E-mail address: [email protected] (M. Lazzari). 0165-2370/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2008.11.021
objects, paper, textiles, etc. [8–10], copolymers and polymer mixtures either as solvent- or water-borne systems, often with undisclosed chemical structure and compositions, are the most commonly employed [1]. As many application-related properties strongly depend on the structural units of the polymer molecules, the chemical characterization of acrylic resins is therefore an important step of investigation, especially in the case of less studied fluorinated systems, which permits to correlate performance with composition. Examples of the influence of the fluorinated side-chain length or the overall fluorine content on both physico-chemical properties, and coatings performance as a function of composition of tailored-made materials have been presented and discussed in previous papers [11,12]. Within a comprehensive investigation on the durability of a series of acrylic products, both commercial resins [13,14] and newly synthesized polymers [15–17], potentially usable for the protection and consolidation of monuments and stone surfaces we have also carried out the necessary preliminary compositional analyses, whose most significant results are reported in this work. The monomeric components of a series of acrylic lattices containing fluorinated and unfluorinated monomeric units of different nature were determined by high temperature pyrolysis with direct analysis of volatile products of thermal degradation through a coupled system (pyrolysis-gas chromatography/mass spectrometry, Py-GC/MS), taking into account the pathways of thermal decomposition of acrylic and/or methacrylic homo- and copolymers [18]. The lattices were selected among those commonly used in different fields for diverse substrates and suitable as protective coatings for the safeguard of cultural heritage.
M. Lazzari et al. / J. Anal. Appl. Pyrolysis 85 (2009) 321–326
322
Table 1 Characteristics of commercial acrylic dispersions used as protectives. Trade name
Latex typea Solids contenta (wt%)pHa
Plextol B500 Acrylic 50 Primal E822k Acrylic 49.5–50.5 Repolem 2502Acrylic 46–48 Setalux AQ44 Acrylic 43–45 Foraperle 390 Fluoroacrylic 30 a b
MFFTb,a (8C)Application
9.5 5 8.5–9 16–20 8–9 3 8.6–9.2 <5 6–7 –
Buildings Concrete – Wood Textiles
Supplier’s data. Minimum film formation temperature.
2. Experimental 2.1. Materials The investigated lattices are produced by Rohm and Haas and traded with the names Plextol B500, Primal E822k, Repolem 2502, Setalux AQ44 and Foraperle 390. Their characteristics are reported in Table 1. The series of partially fluorinated (meth)acrylic homoand copolymers used as references (see below) were prepared by free radical polymerization in solution or in bulk using AIBN as initiator. Experimental details were already published elsewhere [15–17,19]. Around 100 mm thick films for infrared analysis or pyrolysis experiments were prepared by evaporation of the lattices at room temperature up to constant weight values. 2.2. Techniques Fourier transform infrared (FTIR) spectra were acquired with a 1710 PerkinElmer system, with DTGS detector and 4 cm 1 resolution. Pyrolysis experiments were carried out with an integrated system composed of a CDS Pyroprobe 1000 heated filament pyrolyser (CDS Analytical Inc.), a GC 5890A gas chromatograph (Hewlett Packard) equipped with capillary column HP-5MS crosslinked 5% Ph Me Silicone (30 m 0.25 mm 0.25 mm), and a Hewlett Packard GC 5970 mass spectrometer. Pyrolyses were performed at 600 8C for 10 s. The pyrolyser interface was set at 300 8C and the injector at 280 8C. The GC column temperature conditions were as follows: initial temperature 40 8C, hold for 2 min, increase at 8 8C min 1 to 250 8C, hold to the end of elution. Helium gas flow was set at 1 ml min 1. Mass spectra were recorded under electron impact ionization at 70 eV electron energy, in the range from m/z 40 to 600. Pyrolysis fragments were identified on the basis of their mass spectra and mass library searches (Wiley 138 and NBS75k). 3. Results and discussion FTIR spectra of dried films from the considered acrylic lattices did not permit to disclose the polymer structures. Many commercial dispersions, often copolymer formulations based on both acrylic and methacrylic monomers, have similar structural units and their full characterization by either infrared spectroscopy or other spectroscopic techniques, such as NMR in solution, is very difficult. In our case, spectroscopic analysis alone (see as examples FTIR spectra in Fig. 1) indicate the presence of aliphatic groups, showing the characteristic absorption in the C–H stretching region (3000–2800 cm 1), and of ester groups, through the typical peaks due to carbonyl stretching at around 1735 cm 1 and the C–O stretching in the region 1300–1150 cm 1. In principle, the multiplicity of the C–O-related peaks should allow the discrimination between acrylic and methacrylic units, as methacrylates present two peaks, whereas acrylates only shows a single band. Unfortunately, all the analyzed samples revealed a complex C–O
Fig. 1. FTIR spectra of films from Primal E822k (a) and Foraperle 390 (b).
region, eventually suggesting the presence of both types of structural units. In addition, one dispersion (i.e. Foraperle 390, whose spectrum is shown in Fig. 1b) showed more complex pattern at wavelength lower than 1400 cm 1, possibly related to its declared partially fluorinated nature: in such direction it is worth remembering that vibrations related to CF2 and CF3 groups absorb at around 1210, 1150, 660 and 560 cm 1. Finally, the presence of absorptions related to monosubstituted aromatic rings in the spectra from Repolem 2502 and Setalux AQ44 (figure not reported) reasonably identify styrene as one of their monomeric components. Monomer identification of commercial dispersions was carried out by Py-GC/MS on the basis of the relative intensity of monomers and monomer-related compounds produced under drastic conditions of thermal degradation, thus assuming a good knowledge of the degradation mechanism which enables to distinguish between primary and secondary pyrolysis products [18,20–23]. Figs. 2 and 3 show the pyrograms of the unfluorinated copolymers, Plextol B500 and Primal E822k, and Repolem 2502 and Setalux AQ44, respectively, grouped following the structural similarities pointed out by infrared spectroscopy. Peak assignments are listed in Table 2. The pyrograms in Fig. 2 effectively present some common features: (a) in both cases peak 4, identified as methyl methacrylate, is the main product of the pyrolytic degradation and possibly a monomeric component, as also suggested by the identification of a much smaller amount of the parent acrylic esters (peak 2, methyl acrylate) [18]; (b) other two important peaks are present, which were identified as ethyl acrylate (peak 3 in Fig. 2a) and butyl acrylate (peak 8 in Fig. 2b), together with the corresponding parent methacrylic ester (peaks 5 and 9, respectively). Moreover, the parent alcohols as well as dimeric and oligomeric fragments were also identified [24]. In general, it is assumed that all of the main
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Table 2 Product assignments for the chromatographic peaks of the acrylic copolymers.
Fig. 2. Pyrograms of Plextol B500 (a) and Primal E822k (b). Peak numbers refer to the assignments in Table 2.
Fig. 3. Pyrograms of Repolem 2502 (a) and Setalux AQ44 (b). Peak numbers refer to the assignments in Table 2.
No.
Compound name
Molecular weight
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Ethanol Methyl acrylate Ethyl acrylate Methyl methacrylate Ethyl methacrylate Butene Butanol Butyl acrylate Butyl methacrylate Carbon dioxide 2-Ethylhexene Styrene a-Methylstyrene 2-Ethylhexanol 2-Ethylhexyl acetate 2-Ethylhexyl acrylate 2-Ethylhexyl methacrylate
46 86 100 100 114 56 74 128 142 44 112 104 118 130 172 184 198
identified compounds are formed by direct pyrolytic decomposition of copolymers, in analogy with the thermal degradative behavior of reference homopolymers and commercial (meth)acrylic copolymers [18,22–24]. The formation of various minor products of pyrolysis resulting from structural defects and small impurities of the monomers, often present in commercial samples, as well as of products of secondary decomposition of primary pyrolysis products was considered as uninfluent in the process of determination of the qualitative monomeric composition. On the basis of such similarity between the mechanism of formation of pyrolysis products and that of thermal degradation, which mainly yields to the constituent monomers and smaller amounts of both the methacrylic ester parent to the acrylic units and the acrylic ester corresponding to the methacrylic units (Schemes 1 and 2), it was possible to identify the two copolymers as a combination of methyl methacrylate with an acrylic monomer, either ethyl acrylate or butyl acrylate. As reported in the simplified mechanism of decomposition in Scheme 1, for both acrylic and methacrylic units the predominant process consist in the depolymerization through unzipping reaction of the macroradicals resulting from homolysis of the main chain. In the case of the acrylic units, the formation of the parent esters may result from scission of primary macroradicals in g position or from main chain depolymerization processes towards the saturated chain end formed by hydrogen extraction, whereas the formation of lower amount of the parent acrylic ester from methacrylates possibly occurs through decomposition of saturated tertiary chain ends. The evolution of alcohols and olefins deriving from the ester side groups may be justified through the pathways reported in Scheme 2, which under the conditions of high temperature pyrolysis performed during these experiments appear unfavored with respect to the formation of compounds by scission on the main chain. Pyrograms of Repolem 2502 and Setalux AQ44 shown very similar patterns (Fig. 3), and almost the same peaks are present, although in different extent. In both cases the peaks 4 and 16, identified as methyl methacrylate and 2-ethylhexyl acrylate, sensibly allowed to pick out such compounds as the monomeric components. Also for these copolymers smaller amounts of the corresponding parent esters (i.e. peak 2: methyl acrylate, and peak 17: 2-ethylhexyl methacrylate) were identified, as well as remarkable amounts of products such as carbon dioxide (peak 10), 2-ethylhexene (peak 11), 2-ethylhexanol (peak 14) and 2ethylhexyl acetate, whose formation may be related to the decomposition of the higher ester. Moreover, the identification of styrene (peak 12) confirmed this compound as one of the
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Scheme 1. Simplified main mechanism of decomposition of either acrylic (R = H) or methacrylic (R = CH3) units. PH refers to the generic polymer chain.
Scheme 2. Main mechanisms of ester decomposition (R = H or CH3). PH refers to the generic polymer chain.
monomeric components, which is certainly present in bigger amount in Setalux AQ44. In the case of the fluoroacrylic dispersion, the more limited knowledge on the thermal degradation of fluoropolymers [25–27] made necessary the preparation and pyrolysis of a series of reference fluorinated polymers, as detailed below. At the same time, the interpretation of the EI mass spectra of the resulting fluorinated products of pyrolytic degradation resulted to be the best option for overcoming the limits of the electronic libraries in use. A series of partially fluorinated homo- and copolymers such as poly(2,2,2trifluoroethyl methacrylate) (PTFEMA), poly(1,1,1,3,3,3-hexafluoroisopropyl a-fluoroacrylate) (PHFIFA), poly(1,1,1,3,3,3-hexafluoroisopropyl methacrylate) (PHFIMA), poly(2,2,2-trifluoroethyl methacrylate-co-methyl acrylate) (TFEMA/MA) poly(1H,1H,2H, 2H-perfluorodecyl acrylate-co-butyl methacrylate (XFDA/BMA) poly(1H,1H,2H,2H-perfluorodecyl methacrylate-co-2-ethylhexyl
acrylate (XFDMA/EHA), poly(1H,1H,2H,2H-perfluorodecyl methacrylate-co-2-ethyl methacrylate-co-methyl acrylate) (XFDMA/EMA/ MA) were submitted to high temperature pyrolysis, and their products of degradation are reported in Table 3. As examples of the methodological approach, the pyrograms of two partially fluorinated copolymers, XFDA/BMA and XFDMA/EMA/MA, selected as the most representatives, are shown in Fig. 4 (the corresponding peak assignments are listed in Table 4). Similarly to unfluorinated (meth)acrylates, the pyrolysis of (meth)acrylates containing partially fluorinated units yields the corresponding monomeric components, whilst the extent of monomer-related product formation strictly depends on diverse structural features. For lower perfluoroalkyl methacrylates monomers are always the main products of degradation, together with small amounts of the parent acrylic ester and the olefin corresponding to the ester group. With higher methacrylic esters, the ester
Table 3 Products of pyrolysis of reference homo- and copolymers with partially fluorinated (meth)acrylic units. Fluoropolymer PTFEMA PHFIFA PHFIMA TFEMA/MA XFDA/BMA XFDMA/EHA XFDMA/EMA/MA a
Main products a
TFEMA HFIFAa HFIMAa TFEMAa, MA Butane, BMA, 1H,1H,2H-perfluorodecenea, XFDAa 1H,1H,2H-perfluorodecenea, 2-ethylhexene, 2-ethylhexanol, XFDMAa, EHA MA, 1H,1H,2H-perfluorodecenea, EMA, 1H,1H,2H,2H-perfluorodecanola, XFDMAa
Other important products Trifluoroethyl acrylatea
Trifluoroethyl acrylatea, methyl methacrylate XFDMAa, 1H,1H,2H,2H-perfluorodecanola Carbon dioxide, XFDAa, 2-ethylhexyl methacrylate, Carbon dioxide, methyl methacrylate, XFDAa
Identified through direct interpretation of mass spectra since the electronic libraries in use did not include such compounds.
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Fig. 5. Pyrogram of Foraperle 390. Peak numbers refer to the assignments in Table 4.
Table 5 Compositional analysis of unfluorinated and partially fluorinated (meth)acrylic copolymers. Trade name
Monomeric components
Plextol B500 Primal E822k Repolem 2502
Ethyl acrylate, methyl methacrylate Butyl acrylate, methyl methacrylate Methyl methacrylate, 2-ethylhexyl acrylate, styrene Methyl methacrylate, 2-ethylhexyl acrylate, styrene Butyl methacrylate, 2-ethylhexyl methacrylate, 1H,1H,2H,2H-perfluorodecyl acrylate
Setalux AQ44 Foraperle 390 Fig. 4. Pyrograms of the copolymers XFDA/BMA (a) and XFDMA/EMA/MA (b). Peak numbers refer to the assignments in Table 4.
decomposition becomes more important, leading to the formation of the corresponding perfluoroalcohols, higher amounts of olefins and carbon dioxide. Similarly, perfluoroalkyl acrylates give rise to the acrylic monomers and the parent methacrylic ester, although the ratio monomer to parent ester, as well as the amount of ester decomposition products, is usually lower than that observed for perfluoroalkyl methacrylates. Also on the basis of such findings, it was possible to disclose the qualitative composition of the fluoroacrylic dispersion Foraperle 390, whose pyrogram is shown in Fig. 5 (peak assignments refer to Table 4). The most abundant peaks are those of butyl methacrylate (peak 5) and 2-ethylhexyl methacrylate (peak 14). Other peaks such us those identified as butene (peak 2) and 2-ethylhexanol (peak 13) appear to be related to such methacrylic units. Finally, a unique fluorinated component, i.e. XFDA (peak 6), is possibly present, and the peaks 3, 11, 4 and 7, Table 4 Product assignments for the chromatographic peaks of the copolymers XFDA/BMA and XFDMA/EMA/MA, and of the fluoroacrylic copolymer Foraperle 390. No.
Compound name
Molecular weight
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Carbon dioxide Butene 1H,1H,2H-perfluorodecene 1H,1H,2H,2H-perfluorodecanol Butyl methacrylate 1H,1H,2H,2H-perfluorodecyl acrylate 1H,1H,2H,2H-perfluorodecyl methacrylate Methyl acrylate Methyl methacrylate Ethyl methacrylate 1H,2H,2H-perfluorodecanal 2-Ethylhexene 2-Ethylhexanol 2-Ethylhexyl methacrylate
44 56 444 462 142 518 532 86 100 114 460 112 130 198
corresponding to 1H,1H,2H-perfluorodecene, 1H,2H,2H-perfluorodecanal, 1H,1H,2H,2H-perfluorodecanol and XFDMA, respectively, can be considered as monomer-related products originated from this unit following the mechanisms speculated in Schemes 1 and 2 for unfluorinated units (Table 5). 4. Conclusions Py-GC/MS has proved to be an excellent and direct method for the identification of the monomeric components of commercial lattices with either unfluorinated and partially fluorinated (meth)acrylic units. Qualitative compositional analysis of a series of copolymers (results are resumed in Table 5) was made possible on the basis of a good knowledge of the pyrolitic behavior of different repeating units, also gained through the direct study of different reference fluoropolymers which allowed differentiating between monomers and monomer-related products. Acknowledgments This work has been realized with financial support from Ministero dell’Istruzione, dell’Universita` e della Ricerca. We also thank Mr. Stefano Carniccio for experimental support and Proff. Francesco Ciardelli, Valter Castelvetro and co-workers (University of Pisa) for the preparation of the reference polymers. References [1] Z.W. Wicks Jr., F.N. Jones, S.P. Pappas, D.A. Wicks, Organic Coatings: Science and Technology, 3rd ed., John Wiley & Sons, New York, 2007. [2] B. Guyot, B. Boutevin, B. Ameduri, R. Bongiovanni, V. Lombardi, A. Pollicino, A. Priola, A. Recca, Macromol. Chem. Phys. 199 (1998) 1879. [3] B. Ameduri, R. Bongiovanni, V. Lombardi, A. Pollicino, A. Priola, A. Recca, J. Polym. Sci. Part A: Polym. Chem. 39 (2001) 4227.
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[4] F. Ciardelli, M. Aglietto, L. Montagnini di Mirabello, E. Passaglia, S. Giancristoforo, V. Castelvetro, G. Ruggeri, Prog. Org. Coat. 32 (1997) 43. [5] D.G. Castner, D.W. Grainger (Eds.), Fluorinated Surfaces, Coatings and Films, ACS Symposium Series 787, American Chemical Society, Washington, 2001. [6] F. Piacenti, Sci. Total Environ. 143 (1994) 113. [7] L. Toniolo, T. Poli, V. Castelvetro, A. Manariti, O. Chiantore, M. Lazzari, J. Cult. Heritage 3 (2002) 309. [8] S.P. Koob, Stud. Conserv. 31 (1986) 7. [9] L.A. Kres, N.C. Lovell, J. Field Archaeol. 22 (1995) 508. [10] E. Carretti, L. Lei, Prog. Org. Coat. 49 (2004) 282. [11] V. Castelvetro, F. Ciardelli, O. Chiantore, M. Lazzari, J. Coat. Technol. 74 (2002) 57. [12] S. Vicini, E. Princi, E. Pedemonte, M. Lazzari, O. Chiantore, J. Appl. Polym. Sci. 91 (2004) 3202. [13] M. Lazzari, O. Chiantore, Polymer 41 (2000) 6447. [14] O. Chiantore, M. Lazzari, Polymer 42 (2001) 17.
[15] M. Lazzari, D. Scalarone, O. Chiantore, F. Signori, V. Castelvetro, Macromol. Rapid Commun. 26 (2005) 7. [16] F. Signori, M. Lazzari, V. Castelvetro, O. Chiantore, Macromolecules 39 (2006) 1749. [17] M. Lazzari, M. Aglietto, V. Castelvetro, O. Chiantore, Chem. Mater. 13 (2001) 2843. [18] O. Chiantore, M. Lazzari, Int. J. Polym. Anal. Charact. 2 (1996) 395. [19] O. Chiantore, M. Lazzari, M. Aglietto, V. Castelvetro, F. Ciardelli, Polym. Degrad. Stab. 67 (2000) 461. [20] F.C.-Y. Wang, J. Chromatogr. A 843 (1999) 413. [21] S.C. Mao, H. Ohtani, S. Tsuge, J. Anal. Appl. Pyrolysis 33 (1995) 181. [22] A. Asperger, W. Engewald, T. Wagner, J. Anal. Appl. Pyrolysis 49 (1999) 155. [23] M. Lazzari, D. Scalarone, G. Malucelli, O. Chiantore, Prog. Org. Coat., submitted. [24] D. Scalarone, O. Chiantore, J. Sep. Sci. 27 (2004) 263. [25] D.A. Ellis, S.A. Mabury, J.W. Martin, D.C.G. Muir, Nature 412 (2001) 321. [26] L. Shi, H. Saitoh, Y. Shibasaki, Polym. Sci. Part A: Polym. Chem. 38 (2000) 2794. [27] V.V. Zuev, F. Bertini, G. Audisio, Polym. Degrad. Stab. 91 (2006) 512.
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Compositional analysis of fluorinated and unfluorinated acrylic copolymers Massimo Lazzari a,b,*, Dominique Scalarone a, Chiara Riedo a, Oscar Chiantore a a b
Dept. of Chemistry IPM and Nanostructured Interfaces and Surfaces – Centre of Excellence, University of Torino, Via P. Giuria 7, 10125 Torino, Italy Dept. of Physical Chemistry, Faculty of Chemistry and Institute of Technological Investigations, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 July 2008 Accepted 17 November 2008 Available online 27 November 2008
The monomeric components of a series of lattices suitable as protective coatings for the safeguard of cultural heritage, and containing unfluorinated and partially fluorinated acrylic units were determined by pyrolysis-gas chromatography/mass spectrometry. Their identification was possible on the basis of a good knowledge of the degradative behavior, also gained through the direct study of different reference fluoropolymers, which enabled to distinguish between monomers and monomer-related products of degradation. All the samples were identified as copolymers of an acrylic unit, either partially fluorinated or unfluorinated, with a methacrylic unit. Only in two cases different amounts of styrene as monomeric component were also identified. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Acrylic lattices Acrylic copolymers Fluoropolymers Polyfluoroacrylates Compositional analysis
1. Introduction Polymers based on (meth)acrylic esters are well-known for their good mechanical and film-forming properties, optical clarity, adhesion and overall chemical stability, and for such reasons have found important applications since the forties in the formulation of varnishes, paints and adhesives for different substrates [1]. On the other hand, the progressive availability of differently fluorinated (meth)acrylic monomers has resulted in an increasing interest for the polymers which may be produced from them, essentially because they exhibit a wide range of improved and peculiar characteristics [2,3]. The formal replacements of hydrogen atoms with fluorine atoms either in the main chain or in the alkyl side groups improves thermal- and photo-oxidative stability and modify drastically other properties, e.g. the ensuant reduction of surface energy results in a high water repellency [4]. Fluorinated coatings have been developed for diverse applications, such as anti-fouling agents, water-repellents and components for nonsticky surfaces [5], whereas the specific use of partially fluorinated acrylic-based products in the niche sector of conservation of the cultural heritage is still at the beginning [6,7]. Among the many different commercially available resins for the protection and consolidation of stone surfaces, wood, metallic
* Corresponding author at: Dept. of Physical Chemistry, Faculty of Chemistry and Institute of Technological Investigations, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain. Tel.: +34 981 563100x13011; fax: +34 981 595012. E-mail address: [email protected] (M. Lazzari). 0165-2370/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2008.11.021
objects, paper, textiles, etc. [8–10], copolymers and polymer mixtures either as solvent- or water-borne systems, often with undisclosed chemical structure and compositions, are the most commonly employed [1]. As many application-related properties strongly depend on the structural units of the polymer molecules, the chemical characterization of acrylic resins is therefore an important step of investigation, especially in the case of less studied fluorinated systems, which permits to correlate performance with composition. Examples of the influence of the fluorinated side-chain length or the overall fluorine content on both physico-chemical properties, and coatings performance as a function of composition of tailored-made materials have been presented and discussed in previous papers [11,12]. Within a comprehensive investigation on the durability of a series of acrylic products, both commercial resins [13,14] and newly synthesized polymers [15–17], potentially usable for the protection and consolidation of monuments and stone surfaces we have also carried out the necessary preliminary compositional analyses, whose most significant results are reported in this work. The monomeric components of a series of acrylic lattices containing fluorinated and unfluorinated monomeric units of different nature were determined by high temperature pyrolysis with direct analysis of volatile products of thermal degradation through a coupled system (pyrolysis-gas chromatography/mass spectrometry, Py-GC/MS), taking into account the pathways of thermal decomposition of acrylic and/or methacrylic homo- and copolymers [18]. The lattices were selected among those commonly used in different fields for diverse substrates and suitable as protective coatings for the safeguard of cultural heritage.
M. Lazzari et al. / J. Anal. Appl. Pyrolysis 85 (2009) 321–326
322
Table 1 Characteristics of commercial acrylic dispersions used as protectives. Trade name
Latex typea Solids contenta (wt%)pHa
Plextol B500 Acrylic 50 Primal E822k Acrylic 49.5–50.5 Repolem 2502Acrylic 46–48 Setalux AQ44 Acrylic 43–45 Foraperle 390 Fluoroacrylic 30 a b
MFFTb,a (8C)Application
9.5 5 8.5–9 16–20 8–9 3 8.6–9.2 <5 6–7 –
Buildings Concrete – Wood Textiles
Supplier’s data. Minimum film formation temperature.
2. Experimental 2.1. Materials The investigated lattices are produced by Rohm and Haas and traded with the names Plextol B500, Primal E822k, Repolem 2502, Setalux AQ44 and Foraperle 390. Their characteristics are reported in Table 1. The series of partially fluorinated (meth)acrylic homoand copolymers used as references (see below) were prepared by free radical polymerization in solution or in bulk using AIBN as initiator. Experimental details were already published elsewhere [15–17,19]. Around 100 mm thick films for infrared analysis or pyrolysis experiments were prepared by evaporation of the lattices at room temperature up to constant weight values. 2.2. Techniques Fourier transform infrared (FTIR) spectra were acquired with a 1710 PerkinElmer system, with DTGS detector and 4 cm 1 resolution. Pyrolysis experiments were carried out with an integrated system composed of a CDS Pyroprobe 1000 heated filament pyrolyser (CDS Analytical Inc.), a GC 5890A gas chromatograph (Hewlett Packard) equipped with capillary column HP-5MS crosslinked 5% Ph Me Silicone (30 m 0.25 mm 0.25 mm), and a Hewlett Packard GC 5970 mass spectrometer. Pyrolyses were performed at 600 8C for 10 s. The pyrolyser interface was set at 300 8C and the injector at 280 8C. The GC column temperature conditions were as follows: initial temperature 40 8C, hold for 2 min, increase at 8 8C min 1 to 250 8C, hold to the end of elution. Helium gas flow was set at 1 ml min 1. Mass spectra were recorded under electron impact ionization at 70 eV electron energy, in the range from m/z 40 to 600. Pyrolysis fragments were identified on the basis of their mass spectra and mass library searches (Wiley 138 and NBS75k). 3. Results and discussion FTIR spectra of dried films from the considered acrylic lattices did not permit to disclose the polymer structures. Many commercial dispersions, often copolymer formulations based on both acrylic and methacrylic monomers, have similar structural units and their full characterization by either infrared spectroscopy or other spectroscopic techniques, such as NMR in solution, is very difficult. In our case, spectroscopic analysis alone (see as examples FTIR spectra in Fig. 1) indicate the presence of aliphatic groups, showing the characteristic absorption in the C–H stretching region (3000–2800 cm 1), and of ester groups, through the typical peaks due to carbonyl stretching at around 1735 cm 1 and the C–O stretching in the region 1300–1150 cm 1. In principle, the multiplicity of the C–O-related peaks should allow the discrimination between acrylic and methacrylic units, as methacrylates present two peaks, whereas acrylates only shows a single band. Unfortunately, all the analyzed samples revealed a complex C–O
Fig. 1. FTIR spectra of films from Primal E822k (a) and Foraperle 390 (b).
region, eventually suggesting the presence of both types of structural units. In addition, one dispersion (i.e. Foraperle 390, whose spectrum is shown in Fig. 1b) showed more complex pattern at wavelength lower than 1400 cm 1, possibly related to its declared partially fluorinated nature: in such direction it is worth remembering that vibrations related to CF2 and CF3 groups absorb at around 1210, 1150, 660 and 560 cm 1. Finally, the presence of absorptions related to monosubstituted aromatic rings in the spectra from Repolem 2502 and Setalux AQ44 (figure not reported) reasonably identify styrene as one of their monomeric components. Monomer identification of commercial dispersions was carried out by Py-GC/MS on the basis of the relative intensity of monomers and monomer-related compounds produced under drastic conditions of thermal degradation, thus assuming a good knowledge of the degradation mechanism which enables to distinguish between primary and secondary pyrolysis products [18,20–23]. Figs. 2 and 3 show the pyrograms of the unfluorinated copolymers, Plextol B500 and Primal E822k, and Repolem 2502 and Setalux AQ44, respectively, grouped following the structural similarities pointed out by infrared spectroscopy. Peak assignments are listed in Table 2. The pyrograms in Fig. 2 effectively present some common features: (a) in both cases peak 4, identified as methyl methacrylate, is the main product of the pyrolytic degradation and possibly a monomeric component, as also suggested by the identification of a much smaller amount of the parent acrylic esters (peak 2, methyl acrylate) [18]; (b) other two important peaks are present, which were identified as ethyl acrylate (peak 3 in Fig. 2a) and butyl acrylate (peak 8 in Fig. 2b), together with the corresponding parent methacrylic ester (peaks 5 and 9, respectively). Moreover, the parent alcohols as well as dimeric and oligomeric fragments were also identified [24]. In general, it is assumed that all of the main
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Table 2 Product assignments for the chromatographic peaks of the acrylic copolymers.
Fig. 2. Pyrograms of Plextol B500 (a) and Primal E822k (b). Peak numbers refer to the assignments in Table 2.
Fig. 3. Pyrograms of Repolem 2502 (a) and Setalux AQ44 (b). Peak numbers refer to the assignments in Table 2.
No.
Compound name
Molecular weight
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Ethanol Methyl acrylate Ethyl acrylate Methyl methacrylate Ethyl methacrylate Butene Butanol Butyl acrylate Butyl methacrylate Carbon dioxide 2-Ethylhexene Styrene a-Methylstyrene 2-Ethylhexanol 2-Ethylhexyl acetate 2-Ethylhexyl acrylate 2-Ethylhexyl methacrylate
46 86 100 100 114 56 74 128 142 44 112 104 118 130 172 184 198
identified compounds are formed by direct pyrolytic decomposition of copolymers, in analogy with the thermal degradative behavior of reference homopolymers and commercial (meth)acrylic copolymers [18,22–24]. The formation of various minor products of pyrolysis resulting from structural defects and small impurities of the monomers, often present in commercial samples, as well as of products of secondary decomposition of primary pyrolysis products was considered as uninfluent in the process of determination of the qualitative monomeric composition. On the basis of such similarity between the mechanism of formation of pyrolysis products and that of thermal degradation, which mainly yields to the constituent monomers and smaller amounts of both the methacrylic ester parent to the acrylic units and the acrylic ester corresponding to the methacrylic units (Schemes 1 and 2), it was possible to identify the two copolymers as a combination of methyl methacrylate with an acrylic monomer, either ethyl acrylate or butyl acrylate. As reported in the simplified mechanism of decomposition in Scheme 1, for both acrylic and methacrylic units the predominant process consist in the depolymerization through unzipping reaction of the macroradicals resulting from homolysis of the main chain. In the case of the acrylic units, the formation of the parent esters may result from scission of primary macroradicals in g position or from main chain depolymerization processes towards the saturated chain end formed by hydrogen extraction, whereas the formation of lower amount of the parent acrylic ester from methacrylates possibly occurs through decomposition of saturated tertiary chain ends. The evolution of alcohols and olefins deriving from the ester side groups may be justified through the pathways reported in Scheme 2, which under the conditions of high temperature pyrolysis performed during these experiments appear unfavored with respect to the formation of compounds by scission on the main chain. Pyrograms of Repolem 2502 and Setalux AQ44 shown very similar patterns (Fig. 3), and almost the same peaks are present, although in different extent. In both cases the peaks 4 and 16, identified as methyl methacrylate and 2-ethylhexyl acrylate, sensibly allowed to pick out such compounds as the monomeric components. Also for these copolymers smaller amounts of the corresponding parent esters (i.e. peak 2: methyl acrylate, and peak 17: 2-ethylhexyl methacrylate) were identified, as well as remarkable amounts of products such as carbon dioxide (peak 10), 2-ethylhexene (peak 11), 2-ethylhexanol (peak 14) and 2ethylhexyl acetate, whose formation may be related to the decomposition of the higher ester. Moreover, the identification of styrene (peak 12) confirmed this compound as one of the
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Scheme 1. Simplified main mechanism of decomposition of either acrylic (R = H) or methacrylic (R = CH3) units. PH refers to the generic polymer chain.
Scheme 2. Main mechanisms of ester decomposition (R = H or CH3). PH refers to the generic polymer chain.
monomeric components, which is certainly present in bigger amount in Setalux AQ44. In the case of the fluoroacrylic dispersion, the more limited knowledge on the thermal degradation of fluoropolymers [25–27] made necessary the preparation and pyrolysis of a series of reference fluorinated polymers, as detailed below. At the same time, the interpretation of the EI mass spectra of the resulting fluorinated products of pyrolytic degradation resulted to be the best option for overcoming the limits of the electronic libraries in use. A series of partially fluorinated homo- and copolymers such as poly(2,2,2trifluoroethyl methacrylate) (PTFEMA), poly(1,1,1,3,3,3-hexafluoroisopropyl a-fluoroacrylate) (PHFIFA), poly(1,1,1,3,3,3-hexafluoroisopropyl methacrylate) (PHFIMA), poly(2,2,2-trifluoroethyl methacrylate-co-methyl acrylate) (TFEMA/MA) poly(1H,1H,2H, 2H-perfluorodecyl acrylate-co-butyl methacrylate (XFDA/BMA) poly(1H,1H,2H,2H-perfluorodecyl methacrylate-co-2-ethylhexyl
acrylate (XFDMA/EHA), poly(1H,1H,2H,2H-perfluorodecyl methacrylate-co-2-ethyl methacrylate-co-methyl acrylate) (XFDMA/EMA/ MA) were submitted to high temperature pyrolysis, and their products of degradation are reported in Table 3. As examples of the methodological approach, the pyrograms of two partially fluorinated copolymers, XFDA/BMA and XFDMA/EMA/MA, selected as the most representatives, are shown in Fig. 4 (the corresponding peak assignments are listed in Table 4). Similarly to unfluorinated (meth)acrylates, the pyrolysis of (meth)acrylates containing partially fluorinated units yields the corresponding monomeric components, whilst the extent of monomer-related product formation strictly depends on diverse structural features. For lower perfluoroalkyl methacrylates monomers are always the main products of degradation, together with small amounts of the parent acrylic ester and the olefin corresponding to the ester group. With higher methacrylic esters, the ester
Table 3 Products of pyrolysis of reference homo- and copolymers with partially fluorinated (meth)acrylic units. Fluoropolymer PTFEMA PHFIFA PHFIMA TFEMA/MA XFDA/BMA XFDMA/EHA XFDMA/EMA/MA a
Main products a
TFEMA HFIFAa HFIMAa TFEMAa, MA Butane, BMA, 1H,1H,2H-perfluorodecenea, XFDAa 1H,1H,2H-perfluorodecenea, 2-ethylhexene, 2-ethylhexanol, XFDMAa, EHA MA, 1H,1H,2H-perfluorodecenea, EMA, 1H,1H,2H,2H-perfluorodecanola, XFDMAa
Other important products Trifluoroethyl acrylatea
Trifluoroethyl acrylatea, methyl methacrylate XFDMAa, 1H,1H,2H,2H-perfluorodecanola Carbon dioxide, XFDAa, 2-ethylhexyl methacrylate, Carbon dioxide, methyl methacrylate, XFDAa
Identified through direct interpretation of mass spectra since the electronic libraries in use did not include such compounds.
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Fig. 5. Pyrogram of Foraperle 390. Peak numbers refer to the assignments in Table 4.
Table 5 Compositional analysis of unfluorinated and partially fluorinated (meth)acrylic copolymers. Trade name
Monomeric components
Plextol B500 Primal E822k Repolem 2502
Ethyl acrylate, methyl methacrylate Butyl acrylate, methyl methacrylate Methyl methacrylate, 2-ethylhexyl acrylate, styrene Methyl methacrylate, 2-ethylhexyl acrylate, styrene Butyl methacrylate, 2-ethylhexyl methacrylate, 1H,1H,2H,2H-perfluorodecyl acrylate
Setalux AQ44 Foraperle 390 Fig. 4. Pyrograms of the copolymers XFDA/BMA (a) and XFDMA/EMA/MA (b). Peak numbers refer to the assignments in Table 4.
decomposition becomes more important, leading to the formation of the corresponding perfluoroalcohols, higher amounts of olefins and carbon dioxide. Similarly, perfluoroalkyl acrylates give rise to the acrylic monomers and the parent methacrylic ester, although the ratio monomer to parent ester, as well as the amount of ester decomposition products, is usually lower than that observed for perfluoroalkyl methacrylates. Also on the basis of such findings, it was possible to disclose the qualitative composition of the fluoroacrylic dispersion Foraperle 390, whose pyrogram is shown in Fig. 5 (peak assignments refer to Table 4). The most abundant peaks are those of butyl methacrylate (peak 5) and 2-ethylhexyl methacrylate (peak 14). Other peaks such us those identified as butene (peak 2) and 2-ethylhexanol (peak 13) appear to be related to such methacrylic units. Finally, a unique fluorinated component, i.e. XFDA (peak 6), is possibly present, and the peaks 3, 11, 4 and 7, Table 4 Product assignments for the chromatographic peaks of the copolymers XFDA/BMA and XFDMA/EMA/MA, and of the fluoroacrylic copolymer Foraperle 390. No.
Compound name
Molecular weight
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Carbon dioxide Butene 1H,1H,2H-perfluorodecene 1H,1H,2H,2H-perfluorodecanol Butyl methacrylate 1H,1H,2H,2H-perfluorodecyl acrylate 1H,1H,2H,2H-perfluorodecyl methacrylate Methyl acrylate Methyl methacrylate Ethyl methacrylate 1H,2H,2H-perfluorodecanal 2-Ethylhexene 2-Ethylhexanol 2-Ethylhexyl methacrylate
44 56 444 462 142 518 532 86 100 114 460 112 130 198
corresponding to 1H,1H,2H-perfluorodecene, 1H,2H,2H-perfluorodecanal, 1H,1H,2H,2H-perfluorodecanol and XFDMA, respectively, can be considered as monomer-related products originated from this unit following the mechanisms speculated in Schemes 1 and 2 for unfluorinated units (Table 5). 4. Conclusions Py-GC/MS has proved to be an excellent and direct method for the identification of the monomeric components of commercial lattices with either unfluorinated and partially fluorinated (meth)acrylic units. Qualitative compositional analysis of a series of copolymers (results are resumed in Table 5) was made possible on the basis of a good knowledge of the pyrolitic behavior of different repeating units, also gained through the direct study of different reference fluoropolymers which allowed differentiating between monomers and monomer-related products. Acknowledgments This work has been realized with financial support from Ministero dell’Istruzione, dell’Universita` e della Ricerca. We also thank Mr. Stefano Carniccio for experimental support and Proff. Francesco Ciardelli, Valter Castelvetro and co-workers (University of Pisa) for the preparation of the reference polymers. References [1] Z.W. Wicks Jr., F.N. Jones, S.P. Pappas, D.A. Wicks, Organic Coatings: Science and Technology, 3rd ed., John Wiley & Sons, New York, 2007. [2] B. Guyot, B. Boutevin, B. Ameduri, R. Bongiovanni, V. Lombardi, A. Pollicino, A. Priola, A. Recca, Macromol. Chem. Phys. 199 (1998) 1879. [3] B. Ameduri, R. Bongiovanni, V. Lombardi, A. Pollicino, A. Priola, A. Recca, J. Polym. Sci. Part A: Polym. Chem. 39 (2001) 4227.
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