Identification and Characterization of Textile Fibers by Thermal Analysis

In the Laboratory

Identification and Characterization of Textile Fibers by Thermal Analysis Fiona M. Gray School of Chemistry, University of St. Andrews, St. Andrews, Fife, KY16 9ST, Scotland Michael J. Smith* and Magda B. Silva Departamento de Química, Universidade do Minho, 4710-057 Braga, Portugal *[email protected]

Textiles are essential components of clothes, household and vehicle upholstery, and floor and wall coverings. Any textile fiber may be classified as natural or manufactured on the basis of its origin. The oldest fabrics, as expected, are from natural-based sources and include animal (wool, mohair, and silk) and plant fibers (jute, flax, cotton, and hemp). Manufactured fibers only became available at the end of the 19th century with Chardonnet silk, viscose, and rayon developed as a response to the increasing commercial demand for new fabrics (1). Formally, manufactured fibers are defined as those produced by processing natural polymers into fibers. In contrast, synthetic fibers are formed during the chemical transformation of nonpolymeric substrates. From this description, we can understand that all synthetic fibers are manufactured but not all manufactured fibers are synthetic. The discovery of nylon-6,6 in 1935 marked the birth of a new era with chemistry at the epicenter of the rapidly expanding domain of synthetic fibers. As the full potential of the synthetic fiber industry became evident, an even greater chemical variety of fibers became available and new textiles were produced to provide fabrics for a broad range of applications. Although there are already a large number of different fibers available for textile manufacture, it is clear that the market continues to expand and that the chemist continues to assume a central role in the discovery of exciting new functional fibers (2) introduced in response to the demand for high performance materials. The physical and chemical properties of these new materials are tailored to the specific requirements of their market sectors. As the chemical constitution of composite fibers becomes more complex and fabric or finished-product manufacturers seek optimal combinations of operational properties, including mechanical strength, dye fastness, crease resistance, or perspiration wicking, the response has been to develop a wide range of chemical and physical procedures to study fiber composition and characterize textile fabric performance. Many of the methods developed may also provide criminal investigators with the ability to link suspects to a crime scene. Several authors have described the advantages of the crime scene approach to chemical analysis of unidentified residues in the classroom or teaching laboratory (3-6). This is certainly a direct consequence of the dramatization of the role of scientific method in crime scene investigation. The basic principle of forensic science, stated at the beginning of the 20th century by the French criminologist Edmond Locard, and known as “Locard's exchange principle” (7), is central to crime scene investigation: “Wherever he steps, whatever he touches, whatever 476

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he leaves, even unconsciously, will serve as a silent witness against him.” The methods used to analyze the samples must be reproducible and the evidence must be conclusive and not open to multiple interpretations. Thermal techniques, such as differential thermal analysis (also known as heat-flux differential scanning calorimetry, DSC) and, to a lesser extent, thermogravimetric analysis (TGA), are useful for characterizing polymers and fibers, as well as a wide range of other substances including fuels, lubricants, solvents, additives, minerals, adhesives, foodstuffs, pharmaceutical products, and cosmetics. The particular thermal characteristics from fiber specimens may provide fingerprinting information for forensic characterization purposes. A thermal analysis may successfully complement infrared (IR) microscopy, which is traditionally used for forensic purposes. The features of the sample that may be identified using these methods include the glass transition temperature; percentage crystallinity; melting temperature; temperature of cold crystallization; thermal processes associated with the release of water, additive, or solvent residues; phase transformations; and thermal degradation. Although these techniques can identify or eliminate possible textile sources, it has to be noted that, for the more complex fiber mixtures, some thermal observations may be difficult to resolve. In the experimental activity described, thermal analysis is evaluated as a simple and reproducible method of identifying particular textile fibers. This article also aims to demonstrate that DSC and TGA analytical techniques can be taught to undergraduate students in an interesting and informative way. The analysis of a range of textiles is presented as a laboratory experiment, using the forensic science scenario as backdrop. Experimental Procedure and Equipment Standard fiber samples of known composition may be obtained from textile suppliers (8-10) or a society of dyers (11). Samples for characterization by thermal analysis were prepared by using sharp, clean scissors to clip short (1 mm) lengths of textile fiber from trade proofs or swatches. The scissor blades and tweezers used to manipulate fibers were cleaned with acetonesoaked tissue between samples to eliminate transfer during cutting and handling. Clipped fibers were transferred to aluminum (DSC) or platinum (TGA) pans and subjected to heating rates of 2.5-15 °C min-1 and temperature limits between -60 and 450 °C (DSC) or room temperature and 600 °C (TGA). Sample masses were limited to about 3 mg and fibers were slightly

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Vol. 88 No. 4 April 2011 pubs.acs.org/jchemeduc r 2011 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed1004068 Published on Web 01/28/2011

In the Laboratory

Figure 1. Normalized thermograms of selected natural fibers: (A) polylactic acid; (B) linen; (C) wool; (D) rayon; and (E) silk.

Figure 3. TGA of selected natural and manufactured fibers: polylactic acid (O); silk (4); linen (f); rayon (0); and wool (-). Symbols are intended as a guide to the eye.

Figure 2. Normalized thermograms of selected synthetic fibers: (A) nylon-6,6; (B) vinyon; (C) olefin; (D) spandex; and (E) modacrylic.

compacted into the pans to achieve good thermal contact with the inner surface. One of the challenges in the analysis of fibers, particularly in a real-life forensic situation, is that the mass of sample available may be very small. The lack of sufficient sample can render analysis impossible even by extremely sensitive instrumental techniques. The instruments used were a Mettler DSC 831e with a LabPlant cooling probe and a Rheometric Scientific TGA 1000M. Selected thermograms obtained with a variety of natural, manufactured, and synthetic fibers, chosen as representative of the principal subclasses of textile fibers, are illustrated in Figures 1-4. Hazards None of the materials studied in this activity are considered to be harmful. Some care should be taken to avoid inhalation of dust or fiber fragments and the sharp edges of scissors during manipulation of samples. The application of normal rules of good laboratory practice should be sufficient to control any risk of ingestion, inhalation, or contact that might exist. Results Student access to instruments that are delicate and expensive is necessarily limited to well-controlled circumstances. Previous

r 2011 American Chemical Society and Division of Chemical Education, Inc.

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Figure 4. TGA of selected synthetic fibers: olefin (O); modacrylic (4); nylon-6,6 (f); spandex (0); and vinyon (-).Symbols are intended as a guide to the eye.

authors have presented thermal analysis as a dry-lab activity (12). Factors that contributed to this option were the number of students, the time involved in preparing samples and obtaining thermograms, and the difficulty in making the students familiar with the specific operational aspects of acquiring authentic experimental data. Our experience in laboratory classes confirms that simultaneous instruction with groups of up to 18 students does not provide an efficient introduction to specific instrumental techniques. Best results are obtained with small groups of two or three students. After observing the demonstration of sample preparation and instrument operation, students prepare two samples of the same fiber and use DSC and TGA equipment to analyze their materials. Their results are printed and the students are provided with supporting data and literature to complete their report, identify their unknown fiber sample, and answer specific questions regarding optimization of analytical conditions and the application of the techniques used in the analysis of samples collected from crime scenes. Normalized thermograms obtained for natural, manufactured, and synthetic fibers are presented in Figures 1 and 2. The most evident feature of the materials based on natural fibers is

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their capacity to adsorb water. This is one of the most critical aspects of fiber performance and explains why natural fibers feel softer, wick perspiration better than synthetic materials, and disperse electrostatic charge more efficiently. All the fibers made available to our students were stored in the same laboratory conditions and the differences in hydrophilic nature are manifest in the quantity of water liberated and the temperature range over which water is released. Thermograms of linen and wool are marked by exothermic degradation at temperatures of between 300 and 390 °C, whereas silk undergoes fusion before suffering endothermic degradation, at similar temperatures. On the basis of the results of thermal analysis, it is quite easy to distinguish between commercial samples of these fibers. Exposure of the organic polymers present in textile fibers to quite high temperatures under an inert atmosphere results in degradation through bond scission and loss of mass as a result of release of small molecular fragments. Bond scission may occur randomly at locations throughout the chain or as a chain-end process, often referred to as “un-zipping” of the polymer chain. Volatile fragments may be clipped from the end of the polymer chain, chain “back-biting” may occur, or chain branching may take place prior to or during degradation. In most cases, thermal degradation follows more than one mechanism and is accompanied by evaporative loss of fragments. Free radicals may be involved through main-chain cleavage, backbiting, and hydrogen transfer to contribute to small molecule elimination. When oxygen access is restricted by a flowing argon or nitrogen environment, degradation occurs with limited oxidation and the residual charred mass of fiber samples may be quite substantial. In our teaching laboratory, no attempt was made to characterize fibers in air (13). The information available from thermogravimetric analysis permits identification of characteristic degradation onset temperatures and the residual charred mass or ash yield to be evaluated (14). Although identification of fiber samples using unsupported data from thermogravimetric analysis (Figures 3 and 4) would be difficult, as a source of supplementary information this technique provides evidence that confirms the nature of DSC peaks. Thermal events that occur with loss of mass, for example, those associated with release of adsorbed water or thermal degradation, are observable with TGA. Other thermal events, such as glass transition, melting, or solid-solid phase transformations, are visible through analysis by DSC. If the solvent or water loss or degradation peaks can be identified by comparison with results from TGA, then the attribution of causes of the remaining thermal events visible in the DSC experiment is simplified. The thermal behavior of synthetic fiber properties is in sharp contrast with that of natural materials (compare the data in Figures 1 and 2), a consequence of their structural differences. Most synthetic fibers show characteristic fusion events, often with discernible multiple peaks or shoulders and endothermic degradation at higher temperatures (Figure 2). Spandex was the only fiber sample in which a glass transition temperature was visible. The stretching procedure applied to spun fibers as they stream from the spinneret encourages longitudinal alignment of polymer chains, leading to partial crystallization and a desirable increase in fiber strength. The distribution of the fiber molecules between amorphous and crystalline component phases is a consequence of the conditions applied by the manufacturer. The choice of a suitable heating rate may permit detailed 478


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characterization of specific features of the fiber melting profile and simplify identification. Metastable polymer crystals, present in semicrystalline synthetic fibers, tend to shown multiple melting endotherms as they suffer partial melting, crystal-perfecting, and recrystallization at temperatures close to their melting points (15). This aspect may provide useful evidence for forensic garment-fiber matching as these features are particularly sensitive to thermal and processing history. As with natural fibers, the information obtained from TGA (Figure 4) provides useful support in attributing thermal events to physical processes. Some additional DSC thermograms have been included in the supporting information. This information is intended to support issues such as the sensitivity of thermal techniques, the identification of fiber mixtures, and optimization of conditions used to analyze samples. Conclusions This experiment has several intended objectives. The simplest is for students to acquire a basic working knowledge and preliminary contact with thermal analysis instruments. Students are expected to interpret thermograms and recognize the usefulness of these techniques in providing additional information about certain properties of solid materials. The choice of materials for this study was motivated by the commercial significance of textiles and the wide range of applications based on fibers. This choice is also expected to provide an opportunity to introduce or revisit relevant topics in textile and polymer chemistry. The thermal analysis of polymers and natural fibers is complex and has been covered by many authors in specialized journals and textbooks (13-18). Although characterization in this exercise takes place in a flowing inert environment, many authors use static or flowing air, an atmosphere that is at the same time more realistic and more demanding as a result of the complexity of the reactions between organic substrates and oxygen. Exploration of the changes that this choice of purge gas introduces in fiber behavior might provide the basis of an interesting project for more advanced students. There are many aspects of forensic trace analysis that are relevant to normal practice in commercial or academic analytical laboratories. Our experience with the approach described in this article has been positive in that students show markedly more interest during the sample preparation, analysis, and postlaboratory research. This is reflected in an improvement in their discussion of the applicability of the method to specific forensic analysis. Often the most difficult task for the teacher is to capture the attention of the student and find the right motivation. The difficulty is how to present the first piece of evidence in an appropriate learning scenario. Once the challenge is accepted, the student frequently surpasses the teacher's expectations and follows the trail to a satisfactory academic end point. Acknowledgment The authors gratefully acknowledge the support provided through laboratory facilities in both host institutions and additional equipment and financial provision through the Centro de Química (UM-Fundac-~ao para a Ci^encia e Tecnologia).


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Literature Cited 1. Le Couteur, P.; Burreson, J., Napoleon's Buttons, How 17 Molecules Changed History; Tarcher/Putnam Inc.: New York, 2003; pp 105-122. 2. Shishoo, R. Textiles in Sport; CRC Press, Inc.: Boca Raton, FL, 2005. 3. Gaenssien, R. E.; Kubic, T. A.; Deslo, P. J.; Lee, H. C. J. Chem. Educ. 1985, 62, 1058–1060. 4. Kaplan, L. J. http://catalog.williams.edu/catalog.php?& strm=1053&subj=CHEM&cn=113&sctn=%20&crsid=010660 (accessed Jan 2011). 5. Zabzdyr, J. L.; Lillard, S. J. J. Chem. Educ. 2001, 78, 1225–1227. 6. Bender, S.; Lillard, S. J. J. Chem. Educ. 2003, 80, 437–440. 7. Chisum, W. J.; Turvey, B. Journal of Behavioral Profiling January 2000, Vol. 1, no. 1. 8. Textile Fabric Consultants, Inc., 521 Huntly Industrial Drive, Smyrna, TN 37167. 9. Testfabrics, Inc., 415 Delaware Avenue, P.O. Box # 26, West Pittston, PA 18643 or Westlairds Ltd., Patrixbourne, The Green, Datchet, Slough SL3 9JH, Berkshire, United Kingdom http:// www.testfabrics.com/index.html (accessed Jan 2011). 10. SDL Atlas, 3934 Airway Drive, Rock Hill, SC 29732-9200, USA or SDL Atlas, P.O. Box 162, Crown Royal, Shawcross St, Stockport


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11. 12. 13. 14. 15. 16. 17.

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SK1 3JW, United Kingdom http://www.sdlatlas.com (accessed Jan 2011). Society of Dyers and Colourists, P.O. Box 244, Perkin House, 82 Grattan Road, Bradford BD1 2JB, United Kingdom. D'Amico, T.; Donahue, C. J.; Rais, E. A J. Chem. Educ. 2008, 85, 404–407. Wiedemann, H. G.; McKarns, T.; Bayer, G. Thermochim. Acta 1990, 169, 1–13. Pielichowski, K.; Njuguna, J. Thermal Degradation of Polymeric Material; Rapra Technology Ltd: Shrewsbury, U.K., 2005. Jaffe, M. Fibers. In Thermal Characterization of Polymer Materials, 2nd ed.; Turi, E., Ed.; Academic Press: New York, 1996; Chapter 7. Popescu, C.; Oprea, C.; Segal, E. Thermochim. Acta 1985, 93, 397–400. Gaudette, B. D. Forensic Fiber Analysis. In Forensic Science Handbook, Vol. 3; Saferstein, R., Ed.; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1993. Lewin, M. Handbook of Fiber Chemistry, 3rd ed.; CRC Press: Boca Raton, FL, 2006.

Supporting Information Available Instructions for students; notes for instructors; additional DSC thermograms. This material is available via the Internet at http://pubs. acs.org.


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Identification and Characterization of Textile Fibers by Thermal Analysis

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