The preparation of cross-sections of textile materials by grinding

Transcription

1 0 Journal of Microscopy, Vol. 92, Pt 1, August 1970, pp Received 16 March 1970 The preparation of cross-sections of textile materials by grinding by B. LOMAS and S. C. SIMMENS, Department of Textile Technology, University of Manchester Institute of Science and Technology, Sackville Street, Manchester, and the Shirley Institute, Cotton, Silk and Man-made Fibres Research Association, Didsbury, Manchester M20 8RX SUMMARY Difficulties encountered with existing methods of obtaining cross-sections of textile materials have been overcome by a grinding technique. This produces good, flat sections of large area and it can be used over a wide range of materials from fibres and cigarette filter tips to fabrics and metallic fibres. Little skill or experience is required and equipment costs are low. INTRODUCTION The preparation and examination of cross-sections of textile materials are an important part of textile microscopy, used extensively in fibre identification, fault analysis, quality control work and in the development of new yarns and fabrics. Cross-sections can be made by simple techniques, such as plate sectioning (Ford & Simmens, 1959), or the Hardy sectioning device (Heyn, 1954), or, where thin sections of uniform thickness are required, by recourse to a microtome technique. Experience shows that with these existing techniques difficulty is often encountered in producing sections of adequate quality, particularly where large areas are involved. The area of section produced by either plate sectioning or the Hardy device is fixed by the size of the hole or slot in the instrument, whereas with microtome sectioning, the cutting area is not limited except that a small cutting area will always give a better quality section. Microtome sectioning also has several disadvantages such as a high degree of operator skill, a high initial cost of equipment (E ) and frequent knife resharpening. In recent years considerable advances have been made in the manufacture of non-woven fabrics, laminates and plastic-coated fabrics. Examination of material of these types often requires sections of large area, and this fact plus the difficulties associated with the older techniques has necessitated a new approach to sectioning. Sections prepared by grinding, instead of cutting, provide an excellent alternative to the use of a microtome; very little skill is required for their preparation, large flat sections are obtained and equipment costs are low. Although simple 37

2 B. Lornas and S. C. Sirnrnens metallurgical grinding equipment is used it should be noted that polished sections are not the aim. The function of the grinding is only to reduce the thickness of the section to the required value; fine scratches that remain in the surface of the specimen are rendered relatively invisible by the use of a mounting medium. An important feature of the technique is the use of a casting resin that has good adhesion with the specimen, does not shrink appreciably during curing and is of a suitable type for grinding. MATERIALS AND METHODS Embedding resins The adhesive Araldite used in both sample preparation methods is readily obtainable in a handy twin-tube pack. Equal proportions of the Araldite resin and hardener are mixed together and then cured at a temperature of 80 C for 30 min. An alternative method uses an Araldite casting resin of low viscosity, CY212; the resin and hardener HY956 are mixed thoroughly in the proportions of 5: 1 respectively. CY212 is a cold setting resin requiring 24 h at room temperature to harden. This mixture is recommended by the manufacturers for its very low shrinkage and compression properties. Grinding instrument A suitable grinding instrument is the Lunn Minor sold by Vickers Instruments Ltd; it is relatively inexpensive (see Fig. 1). The instrument takes four Fig. 1. Lunn Minor grinding device with the silicon carbide papers in position. Also shown is a holder, made at the Institute, to hold the microscope slide during the grinding of the specimen. grades of silicon carbide paper and provision is made for continuous removal, by running water, of the debris during grinding; this also prevents heating up of the sample. The four grades of paper numbered 220, 320, 400 and 600 have particle sizes of 70, 35, 25 and 15 pm respectively. The two coarser grades are used to grind away most of the surplus resin and material; the finer grades are used 38

3 Preparation of cross-sections of textile materials to reduce the depth of the score lines in the surface of the specimen. The efficiency of the paper falls during use and it has to be changed fairly frequently, particularly the 220 grade. It is advisable to grind only in one direction of the paper to prevent particles becoming embedded in the sample. The grinding of a face can be completed within 5-10 min. Embedding of specimens Two methods of specimen preparation have been used: (1) a rapid method suitable for fibres and yarn sections and (2) a slower method used very successfully for fabrics and also for yarns. Method 1. Small holes are drilled into either a fibrous board material (e.g. vulcanized fibre board called Leatheroid), or flexible plastic or ebonite material about in. thick such as insulating material used in electrical work. The holes are about in. in diameter and 0.25 in. apart. A bundle of fibre or yarn is pulled into one of the holes, being packed sufficiently tightly to prevent the sample slipping out during trimming and embedding. The bundle is cut to a short tuft on one side of the board and on the other it is trimmed level to the board with a razor blade. The portion of the board containing the bundle is cut free and then stuck to a microscope slide with a thin film of adhesive with the (0) Bundle of fibres (b) Copper positioning wire,fabric strip - Polythene tubing Rubber b.lpq Fig. 2. (a), Method 1 for specimen preparation showing the specimen held in place in the plastic material which is then stuck to a microscope slide with Araldite adhesive. (b) Method 2 for specimen preparation. The specimen, a strip of fabric, is held in position in the polythene tubing prior to embedding. 39

4 B. Lomas and S. C. Simmens tuft of sample uppermost (Fig. 2a). The tuft of sample projecting from the board is coated with the same resin, the resin is cured in an oven and the slide is ready for grinding. Method 2. The sample is placed inside a short length ( in.), of thinwalled polythene tubing of 0.5 in. internal bore. The sample is held in position by placing the end into a slit cut in a rubber bung which fits snugly into the tubing, firmly anchoring the sample and also preventing any escape of casting resin from the tube during hardening. The sample is held vertically by passing it over a copper positioning wire that fits across the top of the tubing and then passes down the outside of the tubing (Fig. 2b), both sample and wire being held firmly in position by adhesive tape. The tubing is almost filled with resin, avoiding, if possible, the inclusion of air bubbles and the resin is then allowed to harden at room temperature. The tubing is then stripped from the block which is now ready for the next stage in grinding. If yarn samples are to be embedded, the polythene tubing can be replaced by polythene Beem capsules (LKB Products) which are used for embedding samples for electron microscopy; these have very thin walls and are easy to strip from the block. With this method of sample preparation two distinct phases of grinding are involved: the preparation of the lower ground face which is stuck to the microscope slide, and that of the upper face which is the one examined under the microscope. The reason for grinding the lower face is to make sure that the sample length is perpendicular to the microscope slide. This does not matter in the first method because the sample is held upright by the board material which is also flat and therefore only the upper surface requires grinding down to the required thickness. Grinding (a) Lower ground face. One end of the prepared block is ground flat, care being taken to ensure that this face is at right angles to the yarns or fabric. A thin disc in. thick, is cut from the block (a small hacksaw is recommended) and the ground face of this disc stuck to a glass microscope slide with a thin layer of adhesive Araldite. The prepared slide is placed in the oven to harden the Araldite adhesive after which the preparation is ready for the grinding of the upper face. (b) Upper ground face. This applies to slides prepared by either methods 1 or 2 and is the grinding of the face which is examined under the microscope. A problem associated with obtaining the final ground section of the required thickness of pm is the difficulty of holding the microscope slide during grinding. Whilst suitable but more complex slide holders can be devised (a prototype of such a holder made by the Institute is shown in Fig. l), a simple method is to stick a piece of wax (e.g. SIRA adhesive wax) on to the back of the slide while it is still hot from the oven: the wax is used to hold the slide during grinding. The main difficulties with this method are firstly that the wax can come away from the slide and secondly it is not easy to hold the slide parallel to the paper during grinding. The prototype holder made at the Institute has a recessed plate in which the slide is held with Twinstik, a double-sided adhesive tape; this recessed plate is held parallel to the grinding paper by a spring loaded back plate. The measurement of the thickness of the final section poses various difficulties and work is in progress to devise a precise method. As, however, it has been found that a wide range of thicknesses produce equally good results for much ofthe work, this factor is not critical and in any event it is a simple matter to reduce the 40

5 Preparation of cross-sections of textile materials thickness of the section if it is too thick. Indeed the ability to focus down into the section is often of considerable value in the examination of fabrics and yarns. Examination of the ground section The ground section is usually mounted in an inert liquid, e.g. cedar wood oil (refractive index 1.52) and examined under the microscope. The refractive index Figs Natural fibres. Fig. 3, cotton; Fig. 4, flax; Fig. 5, silk; Fig. 6, wool. 41

6 B. Lomas and S. C. Simmens Figs This group shows the cross-sectional shape of the main acryllic fibres. Fig. 7, Courtelle; Fig. 8, Acrilan; Fig. 9, Orlon 42; Fig. 10, Orlon

7 Preparation of cross-sections of textile materials Fig. 11. Continuous filaments of standard vicose rayon. Fig. 12. Continuous filaments of secondary cellulose acetate. Fig. 13. Distribution of cellulose acetate fibres in a cigarette filter tip. The cross-sectional shape of these fibres gives them a high surface-tovolume ratio which is required for filtering materials. Fig. 14. Cross-sectional shape of standard nylon filaments. 43

8 B. Lomas and S. C. Simmens 17 Fig. 15. The effect of bulking by the false-twist process and the crosssectional shape of standard nylon fibres. Fig. 16. Cross-sectional shape of trilobal nylon monofilament yarn. The shape alters the light-reflecting properties of the filament, giving it a soft, silk-like lustre (compare with Fig. 14). Fig. 17. Section through the warp yarns of a 2/1 twill cotton fabric, stained in Shirlastain A prior to embedding. of Araldite resins is similar to that of most fibres; this has the effect of reducing contrast between resin and undyed fibres. If the contrast is too low, as is generally the case, then the section can be stained in hot Shirlastain A solution for 10 min. This stains the Araldite (which is acid dyeing) a red colour, leaving the fibres either unstained or a different colour with the exception of certain fibres which are also acid dyeing, e.g. silk and wool; these fibres stain in a similar way to the resin. In these instances it is better to stain the sample prior to embedding. Attempts 44

9 Preparation of cross-sections of textile materials have been made to colour the Araldite resin prior to curing but these have not been a success; research is continuing into the preparation of coloured Araldite resins. Grinding marks can often be seen in the resin but provided the specimen has had sufficient preparation on the finer papers these marks do not effect the quality of the section, as the illustrations show. For photography, contrast between sample and resin can be achieved by colour filters and in fact the illustrations of fibre cross-sections included in this article have been made with a green filter. Figs. 5, 12 and 16 show cross-sections of silk yarns with the filaments gummed together, matt cellulose acetate filaments, and trilobal nylon; none of these sections has been stained in Shirlastain A. Sufficient contrast in Figs. 5 and 16 was obtained by stopping down the condenser diaphragm of the microscope. The other illustrations, except Fig. 17, have been stained with Shirlastain A and show the fibres appearing light on a dark background; in Fig. 6, the wool fibres have taken up the stain in preference to the resin and therefore appear dark on a light background. CONCLUSIONS The preparation of sections of any material by grinding is relatively simple and requires no great technical skill. A laboratory assistant given a little time to become acquainted with the technique can produce very good flat sections. This grinding technique is exceptionally useful for obtaining flat sections over large areas, e.g. fabric sections as in Fig. 17 and cigarette filter tips, Fig. 13. The fabric section illustrated of a 2/1 twill cloth was taken at the lowest limit of magnification of the microscope used and even so only 0.2 in. of a much longer section has been covered. The warp repeat and crimp in the weft yarn are clearly shown; the fabric sample was stained before embedding. The section of filter tip, was prepared by cutting a thin disc from the filter, sticking this down on a microscope slide with adhesive Araldite and then covering it with the same resin. The section after grinding was flat across the entire diameter of the filter (approximately 0.28 in.) and showed not only the shape of the fibres but also their spacing. This could not be achieved with the usual methods of sectioning. References Ford, J.E. & Simmens, S.C. (1959) Fibre section cutting by the plate method. JZ 50, 148. Heyn, A.N. 3. (1954) Fiber Microscopy. Interscience, New York. Text. Znst. 45