X-Ray Microscopy of Paper 



J. IsiNGS, Ong Sin Poen, J. B. Le Poole and G. van Nederveen 



Central Laboratory, TNO, Delft 



1 HE examination of the structure of paper is on the 

 whole a laborious and time-consuming work. 



The normal light microscopical techniques are not 

 suited for the investigation of intact papers as most 

 types of paper do not transmit enough light of the 

 visible range of the spectrum. Moreover most papers 

 have a large thickness so that the depth of focus 

 of the objectives will be a limiting factor. Without 

 preliminary treatment it is only possible to investi- 

 gate the surface of the paper with the aid of a micro- 

 scope with vertical illumination. 



If it is necessary to get an insight into the structure 

 and the mutual connexion of the fibres in the paper, 

 cross sections are required. In practice they are 

 mostly made with a Hardy microtome, a little hand 

 instrument of simple device. By this method sections 

 having a thickness of 10-15 // can readily be made 

 and in some cases even thinner sections can be 

 obtained. It is, however, very difficult to adjust the 

 section thickness which in most cases is rather large. 

 Hence these sections can only be used for investiga- 

 tions at a low magnification. Even in that case the 

 section thickness is limiting the investigations to 

 such an extent that little can be seen of the mutual 

 connexion of the fibres in the three dimensional 

 network. 



For closer examination it is preferable to make 

 sections with the aid of the freezing microtome. After 

 embedding the paper in ice or frozen gelatin sections 

 3-5 // thick can be made which can be studied in the 

 phase contrast microscope (1). In order to recon- 

 struct the three-dimensional network of the fibres 

 in the paper a large number of successive sections 

 must be examined. 



We can, however, not prevent disturbance of the 

 paper structure by a fault of the microtome knife 

 or by the many manipulations required to fix the 

 sections on the slides. Even very small deflections 

 of the knife may have such a serious effect that it is 

 not possible to get a clear insight into the inner 

 structure of the paper in this way. 



Although with these methods many good results 

 have been obtained, it is clear that they are far from 

 ideal. As in both techniques the cutting of the paper 

 gives rise to disturbance of the structure, it is worth 

 while to think of a method which makes possible to 

 study the paper structure without any alteration of 

 the initial situation. 



Sometimes a suspension of fibres is dried on a 

 microscopical slide and the fibre conglomeration is 

 studied. With this method some results may be 

 obtained. It remains, however, doubtful as to how 

 far the pictures are representative for the original 



structure of the paper. The connexion of the fibres 

 in the suspension may differ from the structural 

 connexion of the fibres in the paper. The method 

 may be useful for the investigation of the fibrillation. 



Some years ago Pelgroms (2) published an article 

 on the microradiography of paper by the contact 

 method. In this way it is possible to get some im- 

 pressions of the paper but one needs a thin specimen 

 to get a good picture. If the specimen is a thick one 

 sectioning of the paper is still necessary. 



In our laboratory we studied suspensions of 

 beaten paper fibres in a permanent water containing 

 mountant (aqua mount or glycerogel). Just as is the 

 case with the previous method, only the fibrillation 

 of each individual fibre can be studied, but it is 

 impossible to get an impression as to how these 

 fibres are linked in the original paper. 



None of these methods giving satisfactory results 

 for our purpose, we proceeded to use the X-ray 

 projection microscope, and a large number of dif- 

 ferent papers was examined. 



We got interesting data on the structure of various 

 kinds of paper, viz. the felting of the fibres and 

 fibrils, the place of the fillers in a paper and the 

 structure of the upper layer of coated papers. In 

 the course of the investigation it proved necessary, 

 however, to make stereomicrographs in order to get 

 the most valuable results. When using non stereo- 

 scopic micrographs it was very difficult to get a clear 

 insight into the image, but by examining stereomicro- 

 graphs of the same object the three-dimensional 

 structure was clearly seen as well as the loading 

 material in it. 



If paper is studied in the X-ray microscope without 

 any pretreatment, little absorption of the X-ray takes 

 place. Consequently the cellulose fibres give a slight 

 contrast with the environment. Fillers on the con- 

 trary, in particular calcium carbonate and titanium 

 oxide, having more X-ray absorption, give a very 

 good contrast. Hence it was possible to study very 

 easily their position, size and shape in this way. 



To increase the contrast of the cellulose fibres 

 the paper was pretreated with an alcoholic iodine 

 solution. The heavy iodine atom of this solution was 

 adsorbed on the fibres and absorbed the X-rays to 

 such an extent that the cellulose fibres were clearly 

 visible in the pictures. The alcoholic iodine solution 

 does not swell the cellulose fibres, consequently the 

 structure of the paper is not influenced by this 

 treatment. 



It was, however, impossible to get a good impres- 

 sion of the fibrils brought about by the beating of the 

 fibres, unless the fibrils were stuck together forming 



