UDC 678:589. 4. 0. 19+541.013. 5 INFLUENCE
OF C E R T A I N
MEDIA
ON P R O P E R T I E S
OF POLYCAPROLACTAM
S. M. Perlin, A. V. Ermolina, L. N. Obishchenko, and B, S. Petrovskii Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 4, No, 8, pp. 288-255, 1968 The kinetics of absorption of working m e d i a (water, benzene, kerosene) and the variation in the elasticity modulus in relation to the degree of absorption and degree of crystallinity of p o l y e a p r o l a c t a m were studied. It was shown that water produces a considerable reduction in the elasticity modulus of caprone, the effect b e c o m i n g more pronounced with decreasing degree of crystallinity, in t i m e the e l a s t i c i t y modulus becomes stable even though water continues to be absorbed. Benzine and kerosene appear to be inert with respect to caprone. It /s a known fact that m e c h a n i c a l properties of polymers often deteriorate under the influence of working media. The magnitude of this effect depends on the c h e m i c a l nature of the m e d i a and on the structure of a given polymer and its c h e m i c a l m a k e up. Polyamides, to which caprone belongs, containing polar hydroxyl groups are resistant to nonpolar substances (benzene, etc. ), but swell or dissolve in polar substances (water, phenol, etc. ). Absorbed polar substances p a r t i a l l y weaken intermolecular bonds [1] and thereby induce the movement of segments that do not form a part of crystals. As a result of this process there is a partial plasticizing of polyamides. The morphology of the structure of polyamides has also a n o t i c e a b l e effect on the absorption of water and sorption of miners1 acids from their aqueous solutions [2]. And so, t h e absorption of water by poIyamides is reduced after a heat treatment, this effect being attributed to an increase in the degree of crystallinity [8], By varying the ratio of the amorphous and crystalline phases and by changing the character and the fine structure of supermolecular formations in polymers, it is possible to obtain parts with different m e c h a n i c a l properties [4]. For instance, if higher stiffness, good resistance to wear, and a low friction coefficient are required, a given polymer should be characterized by a high degree o f crystallinity. In cases when elasticity and high impact strength are of primary importance, one should aim at the minimum degree of crystallinity. In most published articles on the influence of the structure of polyamides on their m e c h a n i c a l properties no account is taken of the effect of working m e d i a , especially moisture. This led to unsuccessful attempts to use caprone as a structural m a t e r i a l in the fabrication of equipment for gasoline industry. At present there are several known methods of investigating the influence of working m e d i a on m e c h a n i c a l properties of polyamides. In particular, the behavior of hard (stiff) thermoplastics (block polystyrol, organic glass, e t c . ) is characterized in terms of their u l t i m a t e strength properties [5]. In the case of more resilient thermoplastics (including caprone), it is recommended to describe their properties in terms of the m a x i m u m permissible strains. We studied the creep of specimens (in working media) in uniaxial static tension, Our investigation was concerned with the influence of the structure and working m e d i a on properties of polycaprol a c t a m (caprone) which is used as a m a t e r i a l of various parts in the gasoline equipment. The tests were carried out on standard specimens, i . e . , rectangular cross section bars measuring 4 x 6 x 55 ram. Two batches of specimens with different structures were prepared: Group I comprised specimens with a finely spherulitic structure and a high degree of crystallinity and Group II with a coarsely sphemlitic structure and a low degree of crystallinity. Materials with different structures were obtained by varying the temperature conditions during their preparation and by applying an appropriate heat treatment. To evaluate the advisability of t i m e - c o n s u m i n g creep tests in working m e d i a and to elucidate the mechanism of the action of these m e d i a a series of tests was carried out in which the kinetics of absorption of working m e d i a and variation in the e l a s t i c i t y modulus in bending were determined. Before these tests the specimens were held in vacuum until their weight reached a constant level. Experimental caprone specimens were stabilized by holding them for 160 days at 25 ~ C in air with a relative h u m i d ity of 65 %. It was found (by density measurements) that group II specimens have a t e n d e n c y to increase their degree of
185
crystallinity during this treatment initial level (59-61~
(from 43 to 50~
while the degree of crystallinity of group I specimens remains at the
The structure of specimens was determined from data on the degree of molecular order estimated by X-ray diffraction measurements (Fig. 1) and on the character of their supermolecular structure, and by microscope examination. The
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Fig. 1. X-ray diffraction intensity of curves of caprone specimens of group I (solid line) and II (dashed line).
Io 2o 8o 4o Time, days
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Fig. 2. Kinetics of water absorption X and variation in the elasticity modulus E of captone specimens of group I (solid lines and II (dashed lines).
X-ray diffraction data for stabilized specimens are shown in Fig. 1 in the form of curves representing the distribution of X-ray diffraction intensity in the range of scattering angles 2~ = 10-84 ~ The curves were obtained with the aid of a type URS-SOI diffractometer with filtered copper radiation. The X-ray diffraction intensity curves of each group of specimens are characterized by the presence of three reflections, two of which (100 and 010) are indexed with respect to a monoclinic lattice and one (110) in respect to a hexagonal lattice. The reflection (110) of group I specimens is quite intense which indicates a relatively higher degree of crystallinity of these specimens (by 13%). The degree of crystallinity was calculated from the ratio of intensities of the corresponding reflections, The experimental working media included water, which is always present (as vapor or condensed moisture) in a gas-conveying system, diethyleneglycol and a liquid fraction present in the gas and representing a mixture of benzine and kerosene. The results are reproduced in Fig, 2 which shows that in normal climatic conditions the equilibrium absorption of water is reached in 49 days, the saturation absorption of group I and II specimens being 9 and 10~ respectively. After the first 14 days, during which the absorption of water by group I and II specimens had reached 5 and 5.7~ respectively, a sharp reduction in the elasticity modulus was observed: From 20 000 to 5000 kg/cm z (a 4-fold reduction for group I specimens) and from 19 000 to 3000 kg/cm 2 (a 6-fold reduction for group II specimens). Further exposure to water produces very small reductions in E which after 20 days' exposure remains constant, though the absorption of water continues. No changes in the weight and elasticity modulus were observed in the case of specimens similarly tested in a mixture of benzine and kerosene. REFERENCES 1. 2. g. 4. 5.
B. B. C. A. A.
Dolezhel, Corrosion of Plastic Materials and Resins [in Russian], Izd. Khimiya, Moscow, 1964. Maxwell, I. Polymer Sci., no. 9, 43-60, 1965. D, Weiske, Kunststoffe, 54, no. 10, 626-634, 1964. V. Ermolina, L, A. Igonin, and V. A. Kargin, Vysokomolekulyamia soedineniya, no. 9, 1962. N. Tynnyi and A. I. Soshko, FKhMM [Soviet Materials Science], no. 1, 1967.
2 October 1967
186
Gubkin Moscow Institute of Oil Refining and Gas Industry