Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
pubs.acs.org/jchemeduc
Differential Scanning Calorimetry for Art Conservation Graduate Students: A Practical Laboratory Exercise Using Polymer Blends Rebecca Ploeger* Patricia H. and Richard E. Garman Art Conservation Department, SUNY Buffalo State, Buffalo, New York 14222, United States S Supporting Information *
ABSTRACT: Polymer chemistry is an integral part of art conservation graduate training programs. The list of materials used by artists and artisans and by conservators is vast, with the majority being polymeric in nature. Conservators use an array of polymers as coatings, adhesives, fills, supports, and structures. Aside from the chemistry of these materials, all students need to have a good understanding of their thermal behaviors. This includes mechanical and physical changes due to thermal transitions, such as glass transition and melting. Additionally, students need to have some understanding of blending polymeric materials and miscibility, especially if they are tailoring a material to behave in a particular way. Differential Scanning Calorimetry (DSC) lends itself well to investigating the thermal transitions of polymer blends and their compatibility. This paper will present a practical laboratory exercise that explores both polymer miscibility and thermal transitions in the context of art conservation materials. KEYWORDS: Graduate Education/Research, Polymer Chemistry, Thermal Analysis, Hands-On Learning/Manipulatives
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INTRODUCTION The laboratory experiment presented in this paper was taught for the past four years1 as part of the first year “Polymers in Art and Conservation” curriculum in the art conservation graduate program at SUNY Buffalo State, Buffalo, NY. This one academic term course serves as an introduction to the nomenclature, chemistry, physics, degradation, identification, and characterization of polymers used to create and treat works of art. It is an ambitious and fast-paced class and is designed to complement concurrent practical conservation treatment classes (paper, paintings, and objects), as well as, to serve as a science foundation to build upon throughout the program. The laboratory itself is designed with the visual and tactile learning nature2 of art conservation students in mind and uses differential scanning calorimetry (DSC) to investigate a range of miscibilities of common oligomeric and polymeric materials used in art conservation. It also serves as a practical exercise, where not only solubility, handling, and optical properties of the polymeric blends are observed, but also demonstrates that not all substances mix together as one may expect. In general, DSC is a very practical and important analytical technique used in the field of art conservation. It is most commonly used for the investigation of thermal transitions, such as glass transition (Tg) and melting point (Tm). However, it has also been applied to investigate the curing of alkyd paints,3−6 for following mechanical/handling changes of polymeric materials (linked to thermal transition changes) from aging reactions,7,8 and investigating miscibility.9 DSC is also used for the study of metal alloys and ceramics.10 Thermal transitions and processes of polymers is an important subject in modern art conservation studies. Conservators often choose treatment materials based on availability, working/storage environment, and working/aging properties. Polymers have very different properties above and below their glass transition temperature (Tg), so understanding how plasticizers, pigments, environmental conditions, aging, and solvent exposure affect © XXXX American Chemical Society and Division of Chemical Education, Inc.
the glass transition temperature is important, especially when making treatment decisions. Often, conservators are looking for a robust, yet reversible treatment option where there is minimized risk of repair failure, such as failure of a bond or slumping in warm climates where the adhesive is at or above its Tg.11 The materials chosen for this lab were based on their common use in the art conservation field, their use in adhesives, and previous research to confirm the results.9,12−16 Poly(ethylene vinyl acetate) (EVA) is present in many adhesives − both in aqueous dispersions and hot-melts. The acrylic copolymer (Paraloid B-72) is one of the most popular acrylic copolymers used in conservation as a solvent-based adhesive, varnish, and in-painting medium. Poly(vinyl acetate) (PVA) is also common in conservation, in particular, as aqueous dispersion adhesives, and as an in-painting medium. The low molecular weight resins (oligomers), urea-aldehyde resin (Laropal A 81), ketone resin17 (Laropal K 8018), and hydrocarbon resin (Regalrez 109419), are all used for varnishes, in-painting media, and/or as a tackifier for conservation adhesives. A paraffin wax with a melting temperature of approximately 65 °C was used to create ternary blends; the wax served multiple purposes, that are described in the Discussion section. This exercise does not explore the complex theory and thermodynamics of polymer miscibility, as they are out of the scope of the course. Crystallinity and enthalpy of melting calculations were also not considered. Building the concept of the laboratory around adhesive design was done to compliment lecture material but also because they are commonly used in conservation and for day-to-day personal use. Most conservation materials are borrowed from industry and may be Received: December 20, 2017 Revised: April 29, 2018
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DOI: 10.1021/acs.jchemed.7b00976 J. Chem. Educ. XXXX, XXX, XXX−XXX
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used either in their “pure” form (i.e., polymer pellets from the manufacturer, subsequently dissolved in a solvent for application) or tailored to suit a particular application. Often, the tailoring process involves blending materials, sometimes with some undesirable or unexpected outcomes.11 The materials chosen are, as mentioned, commonly used in conservation, though they are also common in the commercial manufacturing of adhesives. It is expected that other educators in chemistry and engineering can relate to this as well. The primary outcomes of this exercise are 1) to relate visual observations and tactile properties to DSC measured data 2) to demonstrate the thermomechanical consequences of solvent retention in samples 3) to develop a greater understanding of thermal transitions and how they are measured 4) to gain some practical knowledge of blending polymers 5) to show that not all substances mix or behave as expected. This laboratory exercise stands out, in that it investigates blends of materials with known variable outcomes; the result of an immiscible, or partially miscible, blend is just as important and successful as a miscible one. There are very limited teaching strategies and materials for DSC analysis in art conservation, and all of the DSC laboratory articles for other disciplines (useful and important in their own right) that the author was able to find used either a single homopolymer, or a copolymer, including those found in this Journal.20−22 It is hoped that this practical exercise will help other educators not only in cultural heritage but also in cross-disciplinary fields in developing DSC laboratory exercises. With some preliminary testing, other blends, perhaps more relevant to other disciplines, could be adapted and developed to fit the general concept of this laboratory.
our purposes, and something the students can work with using only a calculator, is the Fox equation (eq 1)9,24 w w 1 = 1 + 2 Tg Tg1 Tg2 (1) where w1 and w2 are the weight fractions of the polymer components, and Tg1 and Tg2 are the glass transition temperatures of the components (in Kelvin). Changes in the melting transition (Tm) can also be observed. A change in this transition can be linked to interactions and/or cocrystallization between the crystalline phase of the main polymer with another crystalline phase, in the case of this exercise, a paraffin wax.25 Though not part of the laboratory exercise, an example of cocrystallization between phases is shown, along with how it changes based on DSC cooling rates26,27 as part of a theoretical exercise in the course lectures. The DSC laboratory exercise is performed in student pairs (10 students per academic year) and requires three full lab sessions to prepare and execute (3 h periods, once a week for 3 weeks), as well as a solid basis of knowledge of polymer molecular weight, chemistry, solubility, crystallinity/Tm, and amorphous phases/Tg. For this reason, it is scheduled as the final lab exercise of the course. A laboratory handout detailing the full procedure can be found with the Supporting Information. The instrument used was a TA Instruments Q1000 MDSC with an autosampler and a TA Instruments DSC Refrigerated Cooling System. TA Instruments Advantage and Universal Analysis software were used for data collection and analysis. The DSC program was a “catch-all” program and consisted of two cycles28 starting at −70 °C with a 20 °C/min29 ramp to 100 °C. A full description of the procedure and instrumentation can be found with the Supporting Information. Week one
This lab period is dedicated to the preparation of the binary and ternary blends in toluene (20 wt % solids in solvent) see Table 1 for blend information and the Supporting Information
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LABORATORY CONCEPT AND PROCEDURE Glass transition and melting are two properties that can easily be taught as polymer theory, and the values are readily available in product data sheets. Why teach DSC in a conservation training program then? As suggested in the previous section, in order to achieve a unique balance of final properties in a conservation material, sometimes conservators blend polymers.11 This raises a number of other questions − the principle two being 1) are the polymers compatible − will they phase separate or form a usable blend [This is a topic that is of particular importance for the development of new adhesive formulations for conservation9,13,14,23 but is also relevant for any situation when polymeric materials are mixed, i.e. varnishes, paints, etc.] and 2) what effects may solvents have on film properties. This has direct consequences for the performance of a blend over time. An ideal way to study polymer compatibility is by performing DSC analysis. Though the chemistry of polymer miscibility can be very complex, a relatively simple way to evaluate component blending is through the shift in the thermal transitions compared to the pure materials; for example, in the case of Tg, one may observe an “average” of the two components (a single T g rather than two T g ’s corresponding to the components). There are a number of complex mathematical prediction tools for determining the final Tg of a miscible blend of polymers (none were attempted for this laboratory, as they are out of the scope of the course); the easiest tool available for
Table 1. Polymer Blend Formulations Polymer 1a
Blend 1 2 3 4 5
a b a b a b a b a b
c
EVA EVAc EVAc EVAc Acrylicd Acrylicd PVAce PVAce PVAce PVAce
Polymer 2b
Wax
Urea-aldehyde resin Urea-aldehyde resin Ketone resin Ketone resin Urea-aldehyde resin Urea-aldehyde resin Hydrocarbon resin Hydrocarbon resin Ketone resin Ketone resin
No wax Paraffin No wax Paraffin No wax Paraffin No wax Paraffin No wax Paraffin
a
Polymer 1 are polymers. bPolymer 2 are oligomers (low molecular weight resins). cEVA poly(ethylene vinyl acetate). dAcrylic methyl acrylate/ethyl methacrylate (MA/EMA) copolymer. ePVAc poly(vinyl acetate).
for full blend descriptions. These blends, and all individual components (in as-supplied pellet form), were cast onto glass microscope slides. The slides were stored in a fume hood for at least 1 week to allow the solvent to evaporate before DSC analysis.30 The students were encouraged to record all relevant observations of the solvent blends and film formation properties, such as viscosity, heat required to dissolve B
DOI: 10.1021/acs.jchemed.7b00976 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Table 2. Visual and Tactile Observations of Blends 2a and b, As Well As the EVA and Ketone Resin, Cast onto Glass Slides and Allowed to Dry in Ambient Laboratory Conditions for a Week Observations of the Polymers in Two States Sample EVA Ketone Resin Blend 2a Blend 2b a
Polymer Solution
Polymer Film
Clumps at bottom; slowly dissolves with continuous agitation and slight heat (60 °C); becomes viscous; transparent Quickly dissolves; no stirring; low viscosity; transparent/slightly yellowa
Transparent film; flexible and stretchy
Ketone resin dissolved first; required continuous agitation and heat (65 °C); medium viscosity; transparent/slightly yellow Required continuous agitation and heat (65 °C); high viscosity; almost transparent/slightly yellow
Transparent; flexible and stretchy; smooth; malleable Milky/hazy; cracking/wrinkling; waxy; malleable
Transparent, glassy/brittle/flakes; thin film
The ketone resin is slightly yellow as a pure pellet.
wax are also opaque, again a visual clue that there is some degree of either phase separation and/or a degree of crystallinity in the wax. Other important observations are that 1) most of the ternary blends are transparent in the solvent blend and, as they dry, become opaque; 2) the setting time is faster with a wax; and 3) the viscosity may be different with a wax. As conservators are often visual and tactile learners, being able to compare and connect their observations with an instrumental measurement can be a critical link in understanding the results. Another discussion point that is important for conservators is Since we are casting f ilms in solvent we may see solvent ef fects in our data, mainly a decrease in the expected Tg. If there is a solvent retention effect, the Tg of the pure material cast on the glass slide will be slightly shifted to a lower temperature than that of the theoretical/literature value. This is important for DSC data interpretation. A second, more practical point as a material dries (i.e., solvent evaporating from an adhesive) its thermomechanical properties will gradually shift over time. This may be important for the storage and handling of certain cultural materials. The acrylic polymer (MA/EMA copolymer, Table 1) is a material that can retain solvent, particularly aromatic solvents, for a long period of time. A common observation that practicing conservators make is that these films can be initially flexible at ambient room temperature, and over the course of months or years start to become stiff.15 This is likely a plasticizing effect due to solvent retention and the gradual shifting and increase of the glass transition temperature as the solvent evaporates. With these points in mind, as well as those listed in the laboratory exercise, the students have a great start for DSC data interpretation. All the different mixtures yield different miscibility behaviors. Alongside all the blends, the pure materials that were cast out on glass slides were also analyzed to determine their Tg’s and Tm’s, which may have shifted due to solvent retention.
components, optical changes, wettability of the glass slide, setting time, etc. They were also encouraged to take photographs of the solutions and the final films. Weeks Two and Three
Due to the time required for each DSC analysis, week two was dedicated to observations of the dried films and DSC sample preparation.31 The students were encouraged to examine and evaluate the properties of the film, including optical and physical/handling properties, as they prepared their samples for DSC analysis. The samples were analyzed in the days following, and the data was ready for interpretation by week three. At this point, the students were given a list of important considerations and reminders to aid in their observations and for compiling their final laboratory reports (the method of formal assessment); these study tips can be found in the Supporting Information in the student laboratory handout. Common questions from students during DSC interpretation are with the Supporting Information.
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CHEMICAL AND LABORATORY HAZARDS All chemical laboratory safety protocols should be implemented during this exercise, including the use of lab safety goggles/ glasses, appropriate gloves, close-toed shoes, and a lab coat. The primary hazard of this laboratory is the use of toluene as a solvent for the blends. All students should be aware of the safety data sheets (SDS) and associated information about the chemical properties (i.e., boiling point, density, etc.), handling and safety/health hazards (i.e., ventilation/fume hood, gloves, etc.), use (i.e., decanting, measuring, heating, etc.), and disposal of toluene. Other hazards arise from cuts due to broken glassware or from a scalpel while removing the films from the slides. The film casting method in this exercise requires breaking the tips off glass pipettes; this is done in the fume hood (sash lowered) with gloved hands and by wrapping the pipette in a paper towel. All hazards are discussed with the students prior to starting the laboratory.
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RESULTS This lab was repeated four times with success. There is always some manageable variation between years, because of extrinsic factors, such as drying time, film thickness, laboratory environment (temperature and relative humidity), and minor differences in weights of components (experimental and human error); however, the general trends of the results are the same. As an example,32 the typical results from blends 2a and b are described, where a) is an EVA and ketone resin blend, and b) is an EVA, ketone resin, and paraffin wax blend (Table 1). In this case, the EVA copolymer is a semicrystalline material (Tg and Tm), the ketone oligomeric resin is amorphous (Tg), and the
DISCUSSION An ideal place for students to start is this point on the discussion list (see the Supporting Information): When your blends are dry, record any interesting optical features are they clear, hazy, can you see dif ferent phases, are there dif ferences in the surface morphology. All the oligomeric and polymeric materials listed in Table 1 are amorphous or semicrystalline in nature and, as pure films, are transparent (the exception is the paraffin wax which is opaque). If a blend is opaque, this could be a visual clue that there is some phase separation, as phase boundaries will scatter light. All the ternary blends containing C
DOI: 10.1021/acs.jchemed.7b00976 J. Chem. Educ. XXXX, XXX, XXX−XXX
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paraffin wax is mostly crystalline (Tm). The visual and tactile observations of the blends are reported in Table 2. These initial clues are helpful in interpreting the DSC results. The transparency of the binary blend (2a) suggests that the materials are miscible, and the opaque nature of the ternary blend (2b) is due to the crystalline nature of the wax but may also suggest some cocrystallization between the crystalline portion of the EVA copolymer and the wax. The flexibility of the blends suggests that the Tg (or Tg’s) of the system is around or below ambient room temperature. The DSC analysis of the EVA pure film showed a Tg33 of approximately −28 °C, and the Tm is around 67 °C. These values are similar to those from the manufacturer34 and reference values collected previously on the EVA polymer pellets,9 suggesting little to no solvent retention effects. The result for the ketone resin film showed a Tg of approximately 32 °C, which indicates a degree of solvent retention, as the literature Tg value is reported around 50 °C.35 The DSC results of the blends complement the visual observation and characterization of tactile properties. There are two new Tg’s in the binary blend (2a): a prominent one at approximately −17 °C and a very subtle one around 20 °C. This is indicating some degree of compatibility between the components, and that the film should be flexible at ambient room conditions. This double Tg phenomenon in EVA and ketone resin blends has been observed in the literature.9 It is suspected that the solvent may be playing a role; however, there may also be a larger EVA (vinyl acetate) rich phase and a smaller ketone resin rich phase coexisting together. The theoretical Tg according to the Fox equation should be around −3 °C (using literature values). If the Fox equation Tg is calculated using the Tg’s from our DSC measurements (taking into consideration solvent retention), it is around −7 °C, though not exact, it is within reason for the −17 °C Tg that was measured as the most prominent one. The Tm36 of the blend was also slightly shifted from 67 °C in the pure EVA to around 62 °C in the blend; the melting region also broadened. Though the change was only a few degrees (and within product specs), the students explored ideas of the ketone resin slightly altering the Tm of the crystalline phase (polyethylene) due to the different environment caused by the new vinyl acetate-ketone resin interactions. The DSC data for the binary blend (2a) is in Figure 1. The ternary blend (2b) showed similar Tg’s as the binary blend, again a prominent Tg around −18 °C and a very weak glass transition around 28 °C (difficult to calculate with confidence, as the tail end of the transition occurs at the beginning of the melting transition). The Tm transition occurs around 66 °C, which now is dominated by the paraffin wax component, which has a melting transition around 65 °C. As a conclusion to these results, the students were able to compare the DSC results with their observations, to discuss the overall miscibility of the blends, and comment on why they observed particular properties in the final films. These included notations such as toluene (solvent) effects and phase separation. The students also commented on the validity/ success of the Fox equation, relevancy of the results to their conservation practice, issues they encountered, and how they can make better materials and testing decisions to fit a specific need. Please refer to the Supporting Information for tables comparing the DSC results between academic years.
Figure 1. Relevant DSC data (second heating cycle) of the binary blend (2a) containing 60 wt % of EVA and 40 wt % ketone resin. The calculated Tg temperatures are shown for both Tg transitions, and the Tm is reported. The midpoint Tg for Tg1 is −17 °C, the midpoint Tg for Tg2 is 20 °C, and the Tm is ∼62 °C.
Additional Remarks on the Other Blends That Were Not Featured
In the case of blends 1 and 2 and all the ternary wax blends (Table 1), a complication that may be encountered is the difficulty calculating the Tg’s of blends because of the melting transition. In the case of the ternary blends, this is not as much of an issue, since the Tg or Tg’s, depending on miscibility, can be easily calculated using the binary blends. Academic articles and references addressing these concerns are provided to the students (please see the Supporting Information). The choice of EVA copolymers was based on their use and relevance in the field of art conservation − understanding the potential complication and difficulty of determining the Tg of a blend because of the melting transition is an important lesson for this group of students. The students have the unique benefit of a close working relationship with all the educators in the art conservation program, where they have ample time to work one-on-one with their professors. This permits the study of more complex blends. The author recognizes that this may not be the case for other programs. Educators may select another polymer, such as PVAc, which does not show this complication. One can present a similar argument for the ternary blends with wax, though important in the field of art conservation and thus included in the blends; other educators may select to not investigate ternary blends or to use another type of wax with a different melting point (due to a potential overlap of thermal events and additional difficulty in data interpretation). It should be noted that the Fox equation is a simple average tool to “predict” Tg; it is not as accurate as other more sophisticated computational tools. It is a manageable tool for this purpose and gives the students a calculated Tg value they can use to compare with their DSC data. These problems are all used as valuable teaching points. For example, the discrepancy between the calculated Tg and the theoretical Tg (i.e., the Fox equation Tg), described previously, proves that the Fox equation is not perfect and is also an ideal opportunity to discuss how the Tg can be a range and can vary based on drying time, instrumentation, analysis, and temperature programs. D
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(5) Ouldmetidji, Y.; Gonon, L.; Commereuc, S.; Verney, V. A differential scanning calorimetry method to study polymer photoperoxidation. Polym. Test. 2001, 20 (7), 765−768. (6) Ploeger, R.; Scalarone, D.; Chiantore, O. Thermal analytical study of the oxidative stability of artists’ alkyd paints. Polym. Degrad. Stab. 2009, 94 (11), 2036−2041. (7) Lavédrine, B., Fournier, A., Martin, G. Preservation of Plastic Artefacts in Museum Collections; Éditions du Comité des travaux historiques et scientifiques: Paris, 2012. (8) Ciliberto, E.; Spoto, G. Modern analytical methods in art and archaeology; John Wiley & Sons, Inc.: New York, 2000. (9) Cimino, D.; Chiantore, O.; de La Rie, E. R.; McGlinchey, C. W.; Ploeger, R.; Poli, T.; Poulis, J. A. Binary mixtures of ethylene containing copolymers and low molecular weight resins: A new approach towards specifically tuned art conservation products. Int. J. Adhes. Adhes. 2016, 67, 54−62. (10) Giordana, A.; Peacock, E.; McCarthy, M.; Guilbeau, K.; Jacobs, P.; Seger, J.; Ramsey, W. Estimation of Firing Temperature and Compositional Variability of Archaeological Pottery by Differential Scanning Calorimetry. In MRS Proceedings Materials Issues in Art and Archaeology VII Boston; Vandiver, P., Mass, J., Murray, A., Eds.; Materials Research Society: Boston, 2004; pp 311−317, DOI: 10.1557/PROC-852-OO8.6. (11) Schmidt, P. L.; Shugar, A.; Ploeger, R. Analytical Observations Regarding Butvar B-98 and Paraloid B-72 Blends as a Suitable Adhesive in Hot Climates. MRS Advances 2017, 2 (35−36), 1927− 1941. (12) Arslanoglu, J.; Learner, T. The evaluation of Laropal A81: Paraloid B-72 polymer blend varnishes for painted and decorative surfaces−appearance and practical considerations. Conservator 2001, 25 (1), 62−72. (13) McGlinchey, C.; Ploeger, R.; Colombo, A.; Simonutti, R.; Palmer, M.; Chiantore, O.; Proctor, R.; Lavedrine, B.; de la Rie, E. R. Lining and consolidating adhesives: some new developments and areas of future research, In Proceedings of Symposium 2011: Adhesives and Consolidants for Conservation: Research and Applications, October 17−21, Ottawa, 2011. (14) Ploeger, R.; Del Grosso, C.; Poulis, J.; Cimino, D.; Poli, T.; de la Rie, E.; McGlinchey, C. Consolidating Adhesive Project. MRS Advances 2017, 2 (33−34), 1731−1741. (15) Koob, S. P. The use of Paraloid B-72 as an adhesive: its application for archaeological ceramics and other materials. Stud. Conserv. 1986, 31 (1), 7−14. (16) Down, J. L. Adhesive compendium for conservation; Canadian Conservation Institute: Ottawa, 2015. (17) In the academic year 2015/16, the ketone resin was substituted with Foral 85-E (Eastman); this is an ester of hydrogenated rosin (marketed as a tackifier resin for adhesives). The substitution was curiosity driven; there were no underlying issues with the Laropal K 80 blends. (18) Laropal K 80 has been thoroughly studied in the conservation literature. It was discontinued by BASF around 2010 and is being phased-out of conservation. It is still available and used in conservation spaces, hence why it was selected for this laboratory exercise; however, other ketone resins are available that may be suitable for this exercise. The author has not tested any alternative ketone resins for this laboratory exercise. Laropal K 80 was also the original ketone resin used in a popular conservation adhesive called BEVA 371; it has been studied thoroughly, and there is information available in the literature. (19) In the academic year 2014/15, Regalrez 1085 was used; it is a similar product to Regalrez 1094, with the principle difference being the molecular weight (Regalrez 1085 Mw 1000; Reglarez 1094 Mw 850). There is a minimal change in the glass transition temperature. (20) Badrinarayanan, P.; Kessler, M. R. A Laboratory To Demonstrate the Effect of Thermal History on Semicrystalline Polymers Using Rapid Scanning Rate Differential Scanning Calorimetry. J. Chem. Educ. 2010, 87 (12), 1396−1398. (21) D’Amico, T.; Donahue, C. J.; Rais, E. A. Thermal analysis of plastics. J. Chem. Educ. 2008, 85 (3), 404.
CONCLUSIONS This practical laboratory exercise was successfully employed for the past four years for teaching DSC analysis of polymer blends. It uses visual and tactile observations to support instrumental data and vice versa. The laboratory is also designed to help the students improve their understanding of complementary lecture topics, such as the design and function of adhesives, and the solubility of polymers. Though the laboratory exercise spans 3 weeks, every week is a valuable opportunity to learn and discuss the practical aspects of blending polymeric materials, how to investigate miscibility, and the importance of DSC analysis. The final valuable lesson for art conservation students, in particular, is that they will always have access to a visual tool − their eyes, a tactile tool − their hands, and a simple mathematical tool − the Fox equation to help them predict and understand the final behavior of a polymer blend. For the average practicing art conservator, access to a DSC may be nearly impossible, especially in the field, so this is an excellent starting point for them. It is hoped that other educators interested in this laboratory exercise can develop blends and variations suitable for their lessons and be able to draw upon these important practical aspects as well.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00976. Student laboratory handout (truncated to omit all the DSC background theory) (PDF, DOCX) Protocol to obtain the DSC traces (PDF, DOCX) Additional information, common questions from students (PDF, DOCX) Comparisons of all the Tg data collected from 2014 to 2017 (PDF, DOCX) Reading and reference list (PDF, DOCX)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ploeger@buffalostate.edu. ORCID
Rebecca Ploeger: 0000-0001-6392-4532 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS The author extends a special thank you to the art conservation graduate student pair who prepared and analyzed the samples used as an example in this paper. Selected data and conclusion notes were reproduced with their permission.
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REFERENCES
(1) Academic years: 2014/15; 2015/16; 2016/17; and 2017/18. (2) The average art conservation student is a hands-on learner. (3) Duce, C.; Bernazzani, L.; Bramanti, E.; Spepi, A.; Colombini, M.; Tiné, M. Alkyd artists’ paints: Do pigments affect the stability of the resin? A TG and DSC study on fast-drying oil colours. Polym. Degrad. Stab. 2014, 105, 48−58. (4) Mallégol, J.; Gonon, L.; Commereuc, S.; Verney, V. Thermal (DSC) and chemical (iodometric titration) methods for peroxides measurements in order to monitor drying extent of alkyd resins. Prog. Org. Coat. 2001, 41 (1), 171−176. E
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(22) Folmer, J.; Franzen, S. Study of polymer glasses by modulated differential scanning calorimetry in the undergraduate physical chemistry laboratory. J. Chem. Educ. 2003, 80 (7), 813. (23) Ploeger, R.; McGlinchey, C. W.; de la Rie, E. R. Original and reformulated BEVA® 371: Composition and assessment as a consolidant for painted surfaces. Stud. Conserv. 2015, 60 (4), 217−226. (24) Brostow, W.; Chiu, R.; Kalogeras, I. M.; Vassilikou-Dova, A. Prediction of glass transition temperatures: binary blends and copolymers. Mater. Lett. 2008, 62 (17), 3152−3155. (25) Li, C.; Kong, Q.; Zhao, J.; Zhao, D.; Fan, Q.; Xia, Y. Crystallization of partially miscible linear low-density polyethylene/ poly (ethylene-co-vinylacetate) blends. Mater. Lett. 2004, 58 (27), 3613−3617. (26) Personally collected data are used for this purpose − one could also collect their own data using 2, 3, 10, and 20 °C/min temperature ramps in the program supplied in the Supporting Information. (27) Altering the scan rates may aid in event separation. (28) The Tg and Tm were calculated from the second DSC heating ramp to account for memory and history effects that may be present in the first heating ramp. (29) This rate was selected for time constraint reasons; other educators may choose to use a different scan rate to aid in event separation. (30) Toluene is retained in some of these samples for a long period of time. This leads to solvent retention effects in the DSC results; this is a key part of this laboratory exercise. (31) Students were shown how to prepare DSC samples, and the importance of sample contact with the bottom of the pan was emphasized. A template of required mass values was created to help the students record relevant values for analysis, such as pan mass, sample mass, etc. (32) The DSC data is from the laboratory performed in November of 2016 (third time the lab was performed, academic year 2016/17). (33) All Tg values reported are the midpoint Tg calculation. (34) DuPont DuPont Packaging & Industrial Polymers: Elvax 150. http://www.dupont.com//content/dam/dupont/products-andservices/packaging-materials-and-solutions/packaging-materials-andsolutions-landing/documents/elvax_150.pdf (accessed April 2018). (35) BASF Laropal grades. http://www.conquimica.com/wpcontent/uploads/2015/06/ft_laropal_a_81.pdf (accessed April 2018). (36) Tm was defined as the lowest (or average lowest) point in the endothermic melting transition, not the onset; this was done for simpliciy. A thorough explanation of different ways of calculating the Tg and Tm and the fact that these transitions occur over a temperature range is discussed during the lecture component of the course.
F
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