506

MEKHANIKA

switch with an SD-2 motor. The repetition rate obtained in the first case is 60 ppm and in the second case, 2 ppm. The time-recording unit measures 1300 x 8000 x 170 m m and can be used to test 60 specimens simultaneously. The circuit diagram of the unit is shown in Fig. 2b. The microswitch with normally closed contacts is connected to points A and B of the circuit. The required counter, for example C1, is switched on by locking pushbutton PB1. Relay R1 is energized and counter C1 is connetted to the power supply. When the specimen fails, rke lever presses on the microswitch, relay R1 is deenergized, counter C1 is switched off, and lamp L1 lights up, showing that the first specimen has failed and must be replaced with another. The eIectric clock circuit is connected to terminals C-D of relay R61. Sockets Skl-60 monitor the operation of the counters by a shorting plug; they can also receive portable instruments. Normally, however, the instrument is connected with the static-Ioading units by a multicore cable soldered to terminals Pl-60. The time-recording unit is a completely self-contained instrument operating on a 220-V ac main supply. It can be widely employed for any tests in which it is necessary to register times from 1 minute to several days (in testing various materials for long-time strength, dynamic fatigue, etc, ), The time and temperature dependences of the adhesion of filled SKS-30 rubber to duralumin, obtained on the apparatus described, are

POLIMEROV

presented in Figs. 3 and 4, respectively. They are complicated in character and their detailed examination will be reserved for a subsequent communication. The instrument described makes it possible to investigate the temperature-time dependence of the adhesion of polymers to solids at various temperatures and loads with automatic registration of the time to faiIure. REFERENCES 1. M. S. DyI'kov, A. T. 8anzharovskii, and P. I. Zubov, DAN, 155, 389, 1964. 2. E. N. Andrade and B. Chalmers, Proc. Roy. Soc., A 138, 348,

1932. 3. A. G. Ward and R. R. Marriott, J. Sci. Instr., 28, 147, 1948. 4. S. N. Zhurkov and E. E. Tomashevskii, ZhTF, 28, 66, 1955. 5. Yu. S. Zuev, N. N. Bukhanova, and T. I. Dorfman, Kauchuk i rezina, 10, 44, 1980. 6. M. S. Dyl'kov and A. T. 6anzharovskii, Zav. lab., 6, 1965.

benin Moscow State Pedagogical Institute, Problem Laboratory of Polymer Physics

9 January 1967

THE EFFECT OF AGGRESSIVE MEDIA ON TIlE STRENGTH CHARACTERISTICS OF A G LASS-REINFORCED PLASTIC V. S. Gumenyuk and V. V. Lushchik Mekhanika Polimerov, VoI. 3, No. 4, pp. 757-760, 1967 U D C 6'/8:589.4.0t9.3 Various media tend to reduce the initial strength of both waterproofed and untreated glass-reinforced polyesters [1,2]. On the other

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$0

90

Fig. 1. Changes in the compression strength (zx) and specific impact viscosity (A) of PSP-t0E as a result of exposure to water. hand, the effects of media on high-initial-strength reinforced epoxy plastics have not been sufficiently investigated. We investigated changes in the mechanical characteristics of PSP-10E glass-reinforced plastic (EFB-4 epoxy binder, No. 10 spunglass fibers, No. 682 grease) as a result of its exposure to water, solar rue1, concentrated hydrochloric acid, boiling water, and varying weather conditions. We determined the compression strength and specific impact viscosity of the plastic, The tests were carried out in accordance with

GOST

winding parameters were as follows: tension per fiber s = (3-5%) PB, binder content 18-20% (by weight), winding speed 25 turns/rain, binder temperature in the impregnation bath 80 • 5° C. These parameters were maintained constant for ali plates. The degree of solidification of the glass-reinforced plastic was 93-96%. The plates were kept in a given medium the prescribed length of time and then cut up into prisms of dimensions (1 x 1 x 1.5)10 "2 and (1 x 1.5 x 12)10 " s m . The specimens were cut with a corundum wheel (2~0.2 m, t = 0,002) at a speed of 193 rad/sec, at minimum feed, and using a water-oil emulsion coolant. The cut specimens were wiped with filter paper and subjected to compression tests on an RS-2 universal hy&aulie testing machine; the straining rate was kept constant. The specific impact viscosity was measured with a drop hammer (power: 49 J). # ac°m •

108N/m~

~n . 10 5--,J/m2 0

!

3

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2

z

Fig. 2. The same changes as a result of exposure to solar fuel.

4661463 and 4647-62.

Test plates with a 1 : 1 layer ratio were prepared by winding them on a flat frame and then by pressing them (24.5 • l0 s N/m2). The

The effects of water. Moisture lowers the strength characteristics of glass-reinforced plastics by attacking the glass filler [3], the binder,

POLYMER

MECHANICS

507

and the adhesive bond between them [1]. The effect of moisture on glass-reinforced polyesters has already been theroughly investigated [1,2, 4]. The authors of these papers noted a 29-65% deterioration of the physicomechanical properties of reinforced plastics based on glass fibers not treated with an adhesive waterproofing agent and a 4 . 1 12.5% deterioration for waterproofed-fiber plastics. This deterioration of the mechanical properties of a glass-reinforced plastic is due to the fact that water penetrates very quickly into the plastic through small cracks perpendicular to its surface and a r e seeps in along the fibers. In addition, microscopic studies [5] have shown that the binder oniy partially fills the spaces between individual fibers in the formation of a laminar material. This means that there are capillaries between the glass fibers and the polyester resin through which water can seep. This excludes the formation of strong chemical bonds between the polyester resin and the glass surface; hence, bonding in rMs case is based on mechanical friction weakened by moisture.

3

[Ocom - 108 N/m2

!

z

3

~

$

6

Fig. 3. The same changes as a result of boiling in water. The authors of [6] put forward the hypothesis of two moisture absorption processes, the first of which is the penetration of water into various structural defects and the second its seepage into the material. They suggest that the first involves a slight iowering of the strength characteristics, but that the second results in m u c h more marked deterioration of these characteristics from chemical action on the fiber smfaces because of water in the microcapillaries. The reason for this seems to lie in the fact that the binder plays a greater rote in c o m pression than it does in tension. The effect of moisture on glass-reinforced epoxy plastics has been studied both abroad [q, 8] and in the USSR [9]. Some deterioration of strength characteristics has also been noted in these materials. Since epoxy resins adhere strongty to glass and are characterized by low shrinkage, their initially higher strength characteristics deteriorate less as a result of moisture. Moreover, wound glass-reinforced plastics have the m a x i m u m degree of homogeneity. Our tests (Fig. 1) showed that the compression strength of PSP-10E reinforced plastic decreased by only 16%; the specific impact viscosity loss was found to be 36%. This is because the specific impact viscosity is affected by the thickness of the filler: the thicker the filler, the higher the specific impact viscosity, even though the compression strength drops [10]. According to [11], pure epoxy resins are water-resistant, making it expedient to increase the amount of binder in the tayers that come into direct contact with water. The effects of solar fuel. PSP-10E specimens were kept immersed in solar fuel for g, 10, 30, 60, and 90 days. The specimens exposed for 80 days exhibited a 80% decrease in their compression strength and specific impact viscosity (Fig. 2). The process then leveled out, and the two characteristics exhibited no further deterioration. No similar studies have been described in the literature, so that we are unable to make any comparisons with data for glass-reinforced plastics using other binders. Boiling in water. One quick way to test the m e c h a n i c a l characteristics of glass-reinforced plastics is to boil t h e m in water. Several researchers [11,12] have suggested that two hours of boiling is equivalent to 30 days' immersion in water at normal temperature. We tested PSP-10E specimens that had been boiled from 1 to 6 hr. The curves in Fig. 3 show that the compression strength decreased by 80% and the specific impact viscosity by 60%. Such large decreases in

strength could have been due only to the complete absence of adhesion between filler and binder (which had a particularly marked effect on

4 °c°m" 108 N/m2

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7

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r-,-'-...

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Fig. 4. The same changes as a result of exposure to hydrochloric acid. the compression strength) and also the fact that 18-20% (by weight) of filler in the reinforced plastic was clearly too low for boiling tests to have meaning. Our tests failed to reveal an equivalence between a two-hour e x posure to boiling water and a 30-day immersion in normal-temperature water. Such tests can only serve as a means of comparison in the selection of new filler-binder systems. The effects of hydrochloric acid. According to [11], pure epoxy resins are resistant to concentrated hydrochloric acid. Our tests on PSP-10E epoxy glass-reinforced plastic immersed in 37% hydrochloric acid for 1, 3, 5, 1O, and 31q days (Fig. 4) showed that a 25-26% drop in the compression strength and specific impact viscosity occurred within the first 3 days. The characteristics first regained their initial values, but then began to drop again. After 817 days the compression strength had decreased by as much as 84% and the specific impact viscosity by up to 38%; about 86% of the transverse cross sections of the specimens had been corroded through almost complete dissolution of the binder. The effects of weather conditions. We also tested namraliy aged glass-reinforced epoxy specimens that had been exposed to the weather for 1500 hr (July and August 1966, Kiev). We see from Fig. 6 that the compression strength and the specific impact viscosity decreased by 3 0 - 3 6 9 , respectively. It has already been established [ t , 4 , 9] that 'acom'l.08N/m 21

800

I I I - - l - ~ - - [ an'lOSl/m2~.

800

f200

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Fig. 5. The same changes under the influence of climatic conditions. glass-reinforced plastics deteriorate with age, and epoxy-binder m a t e rials are no exception. REFERENCES I. P. M. Ogibalov and Yu. V. Suvorova, The Mechanics of Reinforced Plastics[in Russian], Moscow, 1965. '2. M. K. Smirnova, B. P. Sokolov, Ya. S, Sidorin, and A. P. Ivanov, The Strength of Fiberglas Vessel Hulls [in Russian], Leningrad, 1964. 3. M. G. Chernyak, e d . , Continuous Glass Fibers [in Russian], Moscow, 1965. 4. I, M. Al'shits, Polyester Plastics for Ship-building [in Russian], Leningrad, 1964. 8. F. Bartel, Schiff und Hafen, 3, 289, 1964. 8. K. V. Panferov and I. O. Romanenkov, collection: Studies on Structural Plastics and Structures Based on Them [in Russian], Moscow, 1962. 7. F. Morgan, Glass-Reinforced Plastics [Russian translation], Moscow, 1961. 8. Fiberglas Vessels (Design and Hull. Strength Calculations), Leningrad, 1964. 9. B. A. Kiselev, Glass-Reinforced Plastics [in Russian], Moscow, 1961.

508 10. B. A. Arkhangel'skii and I. M. Al'shits, Plastic Vessels [in Russian], Leningrad, 1968. 11. B. Doletel, The Corrosion of Plastic Materials and Rubbers [Russian translation], Moscow, 1964.

M~EKHANIKA P O L I M E R O V 12. W. Krolikowski, SPE J., 20, 9, 1031, I964. 25 February 1967

Institute of Mechanics, AS UkrSSR, Kiev