GENERAL ]\Iir.KOSCOPY 



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Olle Hallen 



PLASTICS 



In comparison with metals and fibers, mi- 

 croscopic investigation of plastic materials 

 is still in its infancy. It is true that more has 

 been done in this field in recent years, but 

 there is still no question of extensive re- 

 search. Microscopic methods are applied to 

 plastics for: (a) examining fillers with a view 

 to homogeneous dispersion; (b) examination 

 of the mixing of various plastics; (c) studying 

 fracturing of plastic materials; (d) stress in- 

 vestigation ; (e) penetration and distribution 

 of plasticizers. 



Investigation can largely be carried out 

 with intact materials, but study of fracture 

 and stress should be effected with sections or 

 films. All microscopic methods are suitable 

 for examining these materials. The normal 

 Hght microscope is used for observing dis- 

 persion of fillers. Sections or films are used 

 for this purpose. For very finely ground 

 fillers and for investigating the occurrence 

 of cracks under the influence of filler par- 

 ticles, a phase contrast microscope is used. 

 Differences in refractive index, occurring 

 particularly in mixing various plastics, can 

 be successfully investigated with a phase 

 contrast microscope for the purpose of quali- 

 tative examination. Quantitative examina- 



tion of these mixtui'es can best bt; clfected 

 with interference methods. 



For plasticizer penetration the interfer- 

 ence methods are excellent, and it is often 

 advisable to make use of so-called cine pho- 

 tographs. 



The polarizing microscope is mainly use- 

 ful for investigating stresses in plastic prod- 

 ucts and the occurrence of spherulites and 

 oriented structures. This method is especially 

 important for polyethylene and polysty- 

 rene investigation. The thickness of most 

 plastic objects makes it necessary to use 

 compensators which also compensate large 

 phase differences. Ehringhaus' rotary com- 

 pensator, of the Calcspar type, can be rec- 

 ommended. 



In examining a mixture of two plastics it 

 maj^ be advantageous to mcorporate a flu- 

 orescent dye, so that dispersion can be exam- 

 ined with a fluorescence microscope. 



In plastics research, surface techniques 

 have a fairly wide range of application. The 

 normal reflection microscope, both bright 

 and dark field, is used for appraising surface 

 irregularities and for examining surfaces of 

 breaks. In the latter case the purpose may 

 be to examine the breakage phenomena as 

 such, and in the case of highly filled plastics, 

 dispersion of the filler can also be easily ap- 

 praised. 



The interference methods, both double 

 and multiple beam, may be important aids 

 to surface and fracture examination. 



Electron microscope techniques are still 

 comparatively uncommon. Their range of 

 applications is in examining fine structures. 

 Fields where this method has already been 

 applied are the investigation of spherulite 

 formation in polyethylene and nylon films 

 and the examination of break surfaces by 

 means of replicas. 



The grain structure of suspension poly- 

 mers can successfully be investigated with 

 the electron microscope using ultra thin sec- 

 tions. 



390 



