UDC8~8:539.4.019+841.013.5 ESTIMATION OF T H ~ MEDIA
DURABILITY OF POLYMER MATERIALS IN LIQUID
A, I, $oshko, G, I. Sarmr, N, O, Kaltnin, and A. N, Tynnyi Flziko-~himicheskaya Mokhanlka Materi#,lov, Vol. 8, No. ~, pp, ~81=~88, 196~ Mechanical properties and swelling of polytRfluorochloroethylene (teflon) in water, oti, kerosene and acerune were studied, Tim ~esuRs showed tha~ w a ~ and oll have no effec~ on the mechanical properties of teflon whose nmngth ts subs~amially reduced in acetone and kerosene, and that the actual working conditions mus~ be taken into &ccoun~when the resi~ance of polymers ~o ~he action of working media is estimated, (
A~ l}msen~, the main c~Re~ion of the abilRy of polymer pans ~o work in liquid media is the degree of swellingof a material in an unmeued s~&~e ~I, ~], A~ the same ~Ime, experience shows that ~he strengthof polymers which do not swell and are regarded as s~able in a given medium is subs~amlally reduced under working condRions, The resultsof recent sludies[8, 4] indic&Is th&~ :he marked reduction in the service lifeof polymer parts may be a~l',ribu~ed~o a slmuRanooul acllon of mess~ (appliedto residual), working media and temperature) the combined effec~ of ~hem factorsis noc taken into considerationin the current methods of de,straining the durabllRy of ~lyme~s. ,I ~r o8 ,! Medium In this &r~tcle a method is described which makes it possible I kg/mm i kg/mm I enlmate ~he durabllRy of polym~ m&terlals In various llquld medla and to de,ermine the llmlting conditions of the application Ai~ I,~ wa~ 8,0 I.~ ofa given material under mrvioe conditions, The method was a,O ell ~ested on poly~rifluorochloroe~hylene(teflon-~) which was chosen Kelosel~o 0,6~ LO because R ts widely known as a material resistant to the action Acetone hl 0~ Q,~8 of various li~uldmedia, We prepared I-~ m m thick specimens in accordance wl~h GOST-ll~09=08~ they wee s~re~sedin unl&xlal ~emion under a eons:an~load, The expe~lmenr&l liquid medl& included water, oi1 (VM-4), acetone and kerosene. Du~in~ e&ch ~es~, in which a specimen was sub,issuedm a simultaneous ac~ion of s~mss~nd a working medium, the following cha~ac~e~Is~le~were meaeu~od, degree of ewoll~g, elonga~Ion and sbor~-~imetonsilss~rength.
N~: l: ~ e w ~ ifll~ta|w
l~Ig, 9, The sur[aceof a s p e N m ~ af= car sa:ur&tlonin e.o~cone,
Of e spe~ImeN in of ~h~ ae~io[~of me~s
[email protected] i~ ~h~ initialw~i~i: of ~i~ sp~Gim~i a~id 0 ~h~ w~ijh~ of swollen m&~e~ial, The results~re ~eprodLicedin :he
the first cracks. Changes in the specimen weight and the elongation during the formation and growth of cracks are negligible. As the time of the polymer-medium interaction is increased, the size and number of cracks increase to certain definite limits after which elongation of the specimen and changes in its weight take place, this being accompanied by the polymer turning white. At the time time, wedge-like cracks in planes normal to the direction of the tensile stress
Fig. 3. Kinetics of the absorption of kerosene by teflon-3 specimens tested at the following stress levels (kg/mm 2) : 1) 0.65; 2) 0.7fi; 3) 0.9; 4) 1.0.
Fig. 4. Kinetics of the absorption of acetone by teflon-3 specimens tested at the following stress levels (kg/mm2): 1) o. 5; 2) o. 55; 3) o. 65; 4) 0.70; 5) 0.80; 6) 0.85; 7) 1.10.
(Fig. 1) are replaced by heavily deformed zones (Fig. 2). After a certain time, the system reaches a state of equilibrium, The degree of swelling and elongation of the material are directly related to the number of cracks formed. Qualitatively, the time dependence of elongation of t e f o n in kerosene and acetone is the same as the time dependence of the degree of swelling (Figs. 3-5). The quantity of kerosene absorbed at high stresses is larger than the quantity of absorbed acetone, whilst at low stress the rate of acetone absorption is faster. In addition, Oct and Os* recorded in these media are different.
*00 10o I
*O ~ days
Fig. 5. Relative elongation of teflon-3 specimens tested in acetone plotted against time at the following stress levels (kg/mm~-): 1) 0.5; 2) 0.55; 3) 0.70; 4) 0.80; 5) 1.10.
a+ aS o.a
Fig. 6. Stress dependence of relative elongation of teflon-3 in 1) acetone and 2) kerosene.
*The stress at which a specimen becomes saturated without fracture.
The maximum elongation and the degree of swelling in both these media depend on the applied stress level. The results of this investigation showed that such media as kerosene and acetone (and most likely many other media) lead to a marked deterioration in the physico-mechanical properties of teflon-S subjected even to very low stresses. And yet, teflon-3 is cited in reference books as a material resistant to the action of these media, which means that tests currently applied to polymers do not give the full information about their stability in liquid media. Consequently, the determination of the stability of a given polymer material in liquid media must involve measuring its susceptibility to swelling on stressed specimens. Tests of this kind should be carried out at stresses depending on the service loads applicable to a given polymer; the following properties should be determined at each stress level in a given medium: the degree of swelling, time elapsed to the appearance of cracks and elongation. From this data it is possible to determine the service life of polymer parts. REFERENCES
I. 2. 3. 4.
G O S T 10315-62. G O S T 421-59. A. I. Soshko and A. N. Tynnyi, F K h M M [Soviet Materials Science], no. 5, 1965. N. A. Bokshitskaya and I. Ya. Klinov, Plasticheskie massy, no. 12, 1962.
20 April 1967
Institute of Physics and Mechanics AS UkrSSSR, L'vov