SEMs are being acquired increasingly by smaller organizations and laboratories especially those concerned with the manufacture of pulp and paper
Jouko Laamanen
JOUKO LAAMANEN is with the Finnish Pulp and Paper Research Institute in Helsinki.
The prototypes of the scanning electron microscope (SEM) were developed and constructed in England and the United States in the 1950s. However, it was not until 1965 that the first commercially available SEMS were introduced by two manufacturers in England and Japan. From the very beginning, the instrument became popular and proved its usefulness in the natural and engineering sciences. Today, SEMS are produced by ten or so manufacturers and several-hundred are sold annually throughout the world. The instruments are nowadays acquired by smaller and smaller organizations, laboratories and mills, which show s that the SEM must be among the most successful tools of research. It has several excellent practical properties. (1)
The list of SEM users shows that the main users are institutions working in the natural sciences, metallurgy, mineralogy, electronics and the chemical industry. Although one of the world's first prototypes from the 1950s was used in the Pulp and Paper Re search Institute of Canada and the Finnish Pulp and Paper Research Institute was the first SEM user in Finland (in 1968), the increase in the number of users in the pulp and paper industry has been slower than elsewhere. The reason for this is that, despite being very useful in research and even to some extent also in quality control, it has not become a common tool in product development.
Compared with the transmission electron microscope (TEM), the SEM is superior for its ease of specimen preparation, instrument operation and interpretation of images and results. All these functions can be learned as quickly as for the light microscope, but the SEM gives, among other things, a larger range of magnifications (from 10 to 100000 times), about 300 times greater depth of focus and several optional modes of image display, which also makes it a handy micro-analyser. The principle, construction and ,operation of the SEM are described in several books and articles. (2-4)
In the standard operation of the SEM, emitted secondary electrons or reflected primary electrons are used to obtain the topographical image of the specimen. In general, the only necessary pretreatment of the specimens is metal coating. This procedure is carried out in a vacuum evaporator and takes about 15 minutes. In the succeeding operation, the specimen can be inspected at different magnifications on the screen and photographed when necessary. The second important feature of the SEM specimen is its dryness, a result of the vacuum maintained in the specimen chamber. In most cases, the solid air-dried samples are suitable for study. In the case of wet samples, however, much more information about solid structures is revealed if the sample is either freeze-dried, liquid-exchange dried or critical-point dried instead of air-dried. (5)
Another relatively widespread application of the SEM uses either wavelength-dispersive (WDX) or energy-dispersive (EDX) X-ray analysis. The SEM is then equipped with a suitable spectrometer capable of identifying the elements from their X-ray spectra. The system is similar to an electron probe micro-analyser (EPMA). The SEM/EDX combination is more common and practical than the SEM/WDX. SEM/ EDX analysis is mostly restricted to the evaluation of inorganic materials (elements having Z ³ 10), and at its best it is a quantitative method. The areal distributions of elements in the specimen can also be determined.
SEM IN USE IN FINLAND discovering new uses for fibres
Before the SEM became available, the light and transmission electron microscopes were used for studying pulp fibres. Both suffered from a short depth of focus and could not clearly reveal the three-dimensional structure of the fibres. Starting from the raw material, the fibrous structures such as pores, wall layers, fibril orientations, lumen depositions, etc. have been studied with the SEM. (2) Some of the more interesting points from these studies concern the deformations of fibres caused by processes such as delignification, refining, beating, pressing, drying, swelling, and so on. (5, 6) The structure of the fibre surface, which has a great influence on the fibre bonding, can exhibit cellular or lignified wall layers, external fibrillation and various types of broken structures. They are often used to explain the fibre and paper properties measured by other tests. If the range of particle sizes is very wide, as is the case with mechanical pulps, the examination is often made from fractions. (6) This way, the specific potentials of the fibre material will be better understood.
The structure of paper is well adapted to SEM research. A lot of work has been done on the topography of the paper surface. When an X-ray analyser, especially an EDX, IS connected to a SEM, many types of material analysis from paper and board products become possible. Some examples of these applications are given below. (7, 8)
· The group of coated papers includes a great variety of papers with various quantities and qualities of coating pigments and different finishing. SEM micrographs, X-ray spectra and element distributions are used for the evaluation and identification of these objects.
· SEM/EDX analysis has proved extremely useful for research into speciality papers and converted products such as copy papers and plastic-paper combinations, to mention but a few.
· Because SEM can produce results almost as quickly as the light micro scope, the analysis of trouble-shooting can be performed easily. (4) This concerns the problems arising from the defects, spots and specks in products or agglomerations, and deposits and liming in process systems. However, we must remember that the SEM/EDX is only one of the useful analytical instruments for these purposes - light microscopes, spectrophotometers, chromatographs, thermo-analytical equipment etc. used together, especially for the samples of complex and multiphase systems, give the best results.
Practice in operating the SEM/EDX is given on several levels.
· International courses are arranged in many countries for SEM users in specific fields. These are intended either for beginners or for experienced workers. They are announced, for instance, in magazines like the Journal of the Royal Microscopical Society and the Microscope.
· In the field of pulp and paper, there are SEMS and qualified staff in most of the largest central institutes in paper-making countries. These institutes are often willing to give SEM training in their own field in the form of discussions, visitor contacts or shots courses. Available literature
The books intended for scanning electron microscopists can be divided into two parts. First, listed below are some books giving general information about the SEM technique.
· Hayat (10) has written a series of books, of which the first volume presents methods applicable to the scanning electron microscopy of botanical specimens, including some SEM operation and general preparation techniques.
· Although an operator with the minimum of training can obtain excellent micrographs with the modern SEM, he, nevertheless, has at his disposal a considerable number of variables. Oatley (11) provides the reader with a knowledge of the principles on which the construction of the instrument is based and an understanding of the facts relating to the interaction of the electron with solid specimens. · Reimer and Pfefferkorn (13) present the physical and technical principles of scanning electron microscopy, including some preparation methods and microanalytical approaches (in German).
· A relatively condensed book which serves as a guide to the main principles of instrument operation is that of Ohnsorge and Holm (14) (in German and English).
· A series of books by Glauert (15) contains one which covers the whole range of techniques for electron microscopy. This particular book is clearly one of the most comprehensive laboratory handbooks for people working with X-ray microanalysis.
· Wells (16) has written a book intended for both operators and research workers. He describes the basic principles of the SEM, its relationship with other microscopic methods and the results obtained from the SEM. Paper researchers will of course be especially interested in a chapter concerning paper-making applications. The second category of books gives illustrations of SEM micrographs of wood, fibres and papers.
· To make wood structure easier to understand, the three-dimensional perspective offered by SEM has been used in the preparation of a large number of photographs. Core, Côté and Day (18) and Butterfield and Meylan (19, 20) have treated the subject of wood structure and identification.
· The subject of fibre identification has also been illustrated in Côté,(21) but most of the micrographs are taken by light microscope, though a few are by SEM. 1. PAGE, T.F. 1980 Scanning electron microscopy - into the 1980's. Proc. Roy. Microsc. Soc., 15: 2, 123-126.
2. ILVESSALO-PFÄFFLI, M.-S. & LAAMANEN, I. 1970 Application of the scanning electron microscope in pulp and paper research. Finnish Paper and Timber, 11: 163-174.
3. WEIGL, J. & KÄSTNER, M. 1980 Anwendnungsbeispiele der Ràstermikroskopie und Röntgen-Mikroanalyse bei der Papier-herstellung und veredlung. Wochenblatt f. Papierfabr, 108: 18, 719-720, 722, 724, 726-728, 730.
4. PARHAM, R.A. 1973 X-ray analysis of materials: principles and applications to paper. Paperi ja Puu, 12.
5. TREIBER. E. & MARK, H.F. 1975 Die Anwendung des Rasterelektronenmikroskops in der Faserforschung. Lenzinger Ber.. 39: 12-18.
6. MOHLIN, U.B. 1977 Distinguishing character of TMP. Pulp Paper Can., 78: 12, 83-88.
7. PARHAN, R.A. 1975 On the use of SEM/ X-ray technology for identification of paper components. In Johari, O. (ed), Scanning electron microscopy (Part 11), p. 511-518, 528. Proc. of the Workshop on Scanning Electron Microscopy and the Law. IIT Res. Inst., Chicago, 111., April 1975.
8. CORVIN, K.K. & VAN LANDYUT, D.C. 1979 Micrographic analysis of the surface structure of electrographic papers. Proc. 1979 TAPPI Printing Reprography Testing Conf., Rochester, N.Y., 11-14 November 1979, p. 141-148.
9. GOLDSTEIN, J.l. & YAKOWITZ, H. 1975 (eds). Practical scanning electron microscopy - electron and ion microprobe analysis New York, Plenum Press. 582 p.
10. HAYAT, M.A. (ed). 1974 Principles and techniques of scanning electron microscopy. Vol. 1: Biological applications. New York, Van Nostrand Reinhold. 273 p.
11. OATLEY, C.W. 1972 The scanning electron microscope: Part 1: The instrument. Cambridge University Press. 194 p.
12. HEARLE, J.W.S. SPARROW, J.T. & CROSS, P.M. 1972 The use of the scanning electron microscope. Oxford, Pergamon Press Ltd. 278 p.
13. REIMER, L. & PFEFFERKORN, G. 1973 Rasterelektronenmikroskopie. Berlin, Springer Verlag. 263 p.
14. OHNSORGE, I. & HOLM, R. 1973 Scanning electron microscopy - - an introduction for physicians and biologists. Stuttgart, Georg Thieme. 121 p.
15. GLAUERT, A.M. (ed). 1977 Practical methods in electron microscopy, Vol. 5, Part 11: Chandler, J.A., X-ray microanalysis in the electron microscope. Amsterdam, North-Holland. 547 p.
16. WELLS, O.C. 1974 Scanning electron microscopy. United States, McGraw-Hill. 421 p.
17. PARHAM, R.A. & KAUSTINEN, H.M. 1974 Papermaking materials - an atlas of electron micrographs. Inst. of Paper Chem., Appleton, Wis. 55 p.
18. CORE, H.A., CÔTÉ, W.A. & DAY, A.C. 1979 Wood ultrastructure and identification. Syracuse University Press. 182 p.
19. MEYLAN, B.A. & BUTTERFIELD, B.G. 1971 Three-dimensional structure of wood. A scanning electron microscope study. London, Chapman and Hall. 80 p.
20. BUTTERFIBLD, B.G. & MEYLAN, B.A. 1980 Three-dimensional structure of wood. An ultrastructural approach. London, Chapman and Hall. 103 p.
21. CÔTÉ,. W.A. 1980 Papermaking fibres. Ä photomicrographic atlas. Syracuse University Press. 35 p., 79 plates.
WORLD FOOD DAY A message from the Director-General of FAO
Even the poorest farmer, fisherman or forest dweller belongs to a world community of food producers. It is to this community, which draws its members from both rich and poor countries, that World Food Day is dedicated. It is an important community, for nearly half of the world's economically active population works in the agricultural sector. Without the farmers, the fabric of human life would soon unravel.
Today, perhaps more than at any other time, we must face the need to inform people and governments that they must put food and agriculture high on their list of priorities. We must achieve world food security through intergovernmental agreement. We must assert the right of food producers to receive adequate returns for their heavy investment of labour and capital. Above all, we must strive to ensure for all people their basic human right to an adequate supply of food, of good nutritional quality and at prices that are fair both to them and to those who work the soil.
Edouard Saouma
· The surface of uncoated, super-calandered magazine paper, which contains mechanical pulp, chemical pulp, filler and size, can be examined under the SEM to compare two-sidedness, filler content of outermost surface layers, liming propensity (loosely bonded fibres) and roughness.
· All instrument manufacturers offer training courses of a few days to teach the operation and maintenance of their own instruments.
· Goldstein and Yakowitz (9) provide readers with a straightforward description of the concept of scanning electron microscopy and electron probe microanalysis. The book explains the fundamental operation of the instrument, including image production.
· The same topics are discussed in a simple and straightforward form in Hearle, Sparrow and Cross, (12) which also covers specimen preparation, interpretation of results and some fields of application like fibres, polymers and biological materials.
· An excellent atlas of SEM micrographs of paper-making materials has been published by Parham and Kaustinen. (17) Their book contains figures from wood, non-woody and woody fibres, pigments and paper surfaces.
References
16 October
Director-General
