PLASTICS 



Sectioning of Plastic Materials 



Hard plastics, such as polystyrene and 

 methacrylates can most suitably be cut with- 

 out further preparation, by means of a 

 stable sledge microtome. The specimen 

 holder must be as vibration-free as possible; 

 the spiH'imen may protrude only slightly 

 outside the holder and the knife must be 

 secured in a heavy knife block. Moreover, 

 only that part of the knife which is close to 

 the point where it is secured should be used. 

 Sometimes vibration cannot be entirely 

 avoided. Especially in phase contrast or 

 interference microscopic examination, care 

 should be taken not to appraise specimens 

 too quickly. As a rule, vibration becomes 

 troublesome only when a number of succes- 

 sive specimens is to be used for spatial 

 reconstruction. With such objects the blade 

 of the knife soon gets damaged and grooves 

 occur in the cut surface. Every slice must 

 therefore be judged immediately with the 

 aid of a dissecting microscope, so that a 

 different part of the knife can be used at 

 once if any damage is found to have been 

 started by the knife. 



The knife must be of higher quahty than 

 is required for cutting biological material. 

 The blade must be very accurately gromid. 

 All slices tend to curl up. As soon as the sec- 

 tions are obtained they should be tem- 

 porarily unrolled with a couple of small 

 artists' brushes. They can be fully extended 

 by placing them on a drop of xylene. This is 

 usually sufficient to obtain a completely flat 

 specimen. 



The specimen can be mounted in Aqua- 

 mount or Canada balsam. 



Elastic plastic materials, such as poly- 

 ethylene, are cut with a freezing microtome; 

 here again the sledge type is used. The re- 

 quirements for cutting hard materials apply 

 also to elastic materials. Vibration resulting 

 from cutting is noticeably less than with 

 hard material. Ai'tifacts due to cutting with 

 a damaged knife blade are found, however. 



Regular inspection of the slices immediately 

 after cutting reduces the adverse effects of 

 this. Here again the sections can be stretched 

 on xylene after the slices have first dried. 

 This is a successful method especially for 

 polyethylene and polypropylene. 



The production of thin sections for inves- 

 tigating filler dispersion is preferable to the 

 melting method (see below). Cutting with a 

 microtome does not alter the distribution of 

 the fillers. Large aggregates do not disinte- 

 grate, nor are new ones formed. It is also 

 possible to examine a spatial arrangement 

 by means of a series of sections. Moreover, 

 the influence of spherulite structures on filler 

 dispersion can be appraised only from sec- 

 tions. 



Examination of Films Obtained by 

 Melting 



Small pieces of plastic are heated until 

 they have just melted. They are then pressed 

 between two microscope slides until they are 

 the desired thickness. These films, after 

 mounting in Aquamount or Canada balsam, 

 can be used for microscopic examination. 

 This method is fairly widely used for exam- 

 ining filler dispersion in plastics. Its draw- 

 backs are: 



(1) Spatial distribution of the fillers can 

 no longer be determined. There are fairly 

 strong lines of flow in the specimens and no 

 opinion can thus be formed regarding the 

 uniformity of dispersion. 



(2) Fillers often form a very labile system 

 with the plastic. The big risk in melting is 

 that the dispersion found in the films may 

 not be the same as it was originally. Large 

 aggregates may disintegrate and smaller 

 granules may cake together. 



(3) Orientation effects are no longer 

 found. The filler granules are often oriented 

 in the plastic. This orientation is lost. 



(4) The influence of spherulite structures 

 on filler dispersion can no longer be ascer- 

 tained. The spherulites are either destroyed 

 or in any case deformed. These structures 



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