STATIC STRENGTH OF POLYETHYLENE FILMS AFTER PROLONGED EXPOSURE

IN DIFFERENT CLIMATIC ZONES

r

E. S. Umanskii, N. S. Shidkovskii, V. V. Kryuchkov, R. M. Kas'yan, V. S. Sychov, and B. N. Rybakov

UDC 539.3/5:678

In recent years polymeric films have found extensive use in agriculture, construction, and a number of branches of industry where they are subjected to the direct action of solar radiation, alternating temperatures, moisture, different biological factors (mildew, bacteria, insects), and atmospheric pollution from industrial effluents. The economic loss as a result of damage to polymeric films in agriculture and construction alone is about four to five bil- lion rubles a year. Therefore, it is important to study the mechanical properties of polymer- ic films in view of their operation in different climatic conditions and to develop a series of physicochemical and technological measures leading to an increase in the supporting capacity of films.

Basic factors which significantly affect the supporting capacity of polymeric films are solar radiation, temperature of the surroundings and its variation (from --50 to +50~ moist- ure, active agents (acids, alkaline substances, oils, gases, etc.) and naturally the length of time under these conditions.

Results are given in the present communication of studying the mechanical properties of polyethylene films (GOST 10354-73) which were placed in different climatic zones under condi- tions of open and closed atmospheres. The term "open" atmosphere corresponds to conditions for film specimens in an open area without creating barriers for direct solar rays, moisture, etc., and a "closed" atmosphere is an area equipped with louvers where specimens were free from direct access to solar radiation with all of the rest of the climatic factors associated with this location.

Specimen exposure was carried out in test sites located in different climatic zones of the USSR and in the tropical zone of the Pacific ocean having atmospheres with markedly dif- ferent corrosiveness.

Factorscharacterizing the corrosiveness of an atmosphere are surface moistening and air pollution with corrosive agents. By surface moistening we assume wetting by a phase film and (or) adsorption of moisture films.

A phase film is assumed to be a film of moisture with surface wetting by liquids (rain, frost) or mixed (rain with snow, rain with hail) precipitates and dew. An adsorbed film is formed with relative humidity equal to 70% or more in the absence of dew precipitation.

Corrosiveness parameters are: overall duration of surface wetting; duration of surface wetting by a phase film of moisture; duration of surface wetting by an adsorbed film of moist ure; corrosive agent concentration. Values of corrosiveness parameters in exposure sites for polyethylene films are given in Table i.

In this work regular meteorological observations were carried out in accordance with pro cedures adopted with all climatic tests for materials at stations and proving grounds. Speci- mens under test conditions were exposed from several days to two years. On the basis of per- iodical measurements of temperature and relative humidity for the air histograms were plotted (Fig. i) which indicate over which period material aging occurred in the given ranges for rel ative humidity and temperature.

The set of mechanical properties for the test materials with short-term loading was de- termlned in a test unit for studying strength and deformability of film materials. This equipment provided reliable and accurate specimen fastening in the grips; high accuracy for

Kiev Polytechnic Institute. Institute of Materials Engineering Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 5, pp. 82~86, May, 1984. Original article submitted November 14, 1983.

706 0039-2316/84/1605-0706508.50 �9 1985 Plenum Publishing Corporation

TABLE i. Values of Parameters for Atmosphere Corrosivity at Test Sites [i]

Climatic zone

Baltic re~gion (Ekabpils) Far ~qortfi (Monchegorsk) Central Asia (Tashkent) Moscow Subtropics (Poti) Tropical zone of the Atlantic and

Indian oceans

Overall du- ration of surface wet- i~n~ h/yr

4060 3100 1440 3110 5230

6730

Overall surface I wetting by a phase [ilm, h/year

2190 1080 1380 2060 3940

6150

Duration of surface w ettin by adsorbed film, h / y r

1870 2020

60 1050 1290

580

Comment

Hig 2 chloride content

Industrial atmosphere High chloride content Content of CI" in the atmosphere

18-~0 m g / c m z

~oI .~o~ 30~ ~ # M

, o ~r.'e -,o o ,~r.oc -,o o. ,or.~ ~ ,o 2or,,c~o~ [~ #

~~176 ,.%r ~%r of,~',F _ aO~ _l~

1o - ~o fo Io 1o

~. 2or,~ -;o o so T, c -fo o to r,~ ~o 2o r.~ Io 2o T,~

, o ~a - 3

o -;o ; ,o r: o_~. o ,o u c [ ] _

II II[ IV

Fig. 1

Fig. i. Results of measuring temperature and relative humidity of the air in zones of specimen exposure: I) Central Asia (Tashkent); II) FarNorth (Monchegorsk); III) Baltic region (Ekabpils); IV) Subtropics (Poti); V) Atlantic ocean. (u = (Tr/T)o 100% [2], where Xr is specimen residence time in a given range of relative humidity (i -- H = 80...90%; 2 -- H = 70...80%; 3 -- H = 60...70%; 4 -- H = 30...60%) and temper- ature; �9 is overall specimen exposure time.)

af, tMPa J

~3 2 ?0 3 tO t~ t, h

Fig. 2

Fig. 2. Change in ultimate strength of film poly- ethylene with exposure under different conditions: i) Subtropical zone; 2) Baltic region; 3) Far North

707

TABLE 2. Strength and Deformability Characteristics for Film Polyethylene (GOST 10354- 73) with Exposure Under Different Climatic Conditions

Climatic zone

Original values Moscow

Balt ic reg ion

Subtropica l zone �9

Central Asia

Far North

Exposure t i m e

5 days 20 days

1 mor t 3 m o n . 6 mon .

12 mort . 5 days

2? days mono

3 mon . 6 mon .

12 mon . 18 mort .

5 days moll.

3 mon- 6 morn

12 mort-

days 1 days

m o n . 6 mon- 9 m o n .

~ days 2 days

m o n . 3 mort , 6 m o n .

12 mort,

av . v a l u e "

21,8 21,6 21.5 21,5 17,0 18,3 17,1

23,7 25,4 24,9 24,8 24,3 21,0 14,2

22,5 24,0 23,5 23,0 11,9

20,8 21,0 21,6 20,5 19,6

23,1 25,6 25,8 25,1 25,0 17,4

o f , MPa

c o n f i d e n c e range

20,7 ... 22,9 20,0 ... 23, I 20,5 ... 22,5 19,5 ... 23,5 16,2 ... 17,9 16,8 ... 19,8 15,6 ... 18,6

21,9 ... 25,4 23,3 ... 27,5 22,9 ... 26,8 22,9 ... 26,6 20,9 ... 27,7 18,6 ... 23,4 12,3 ... 16,1

21,4 ... 23,5 22,2 ... 25,6 21 ,l ... 25,8 21,1 ... 24,9

9,4 ... 14,5

19,7 ... 21,9 19,3 ... 22,7 20,0 ... 23,2 18,9 ... 22,l 18,3 ... 20,9

21,2 ... 25,0 23,1 ... 28,1 21,7 ... 29,9 22,5 ... 27,9 24,0 ... 26,0 16.2 ... 18,6

c o e f f i c i e n t of variation %

5,8 8,4 5,5

10,8 5,7

10,3 10,6

8,7 10,0 9,3 8,9

16,5 1o,8 15,7

4.4 3,4

11,8 I0,1 25,5

6,3 9,8 8,8 9,4 7,6

9,7 I 1,5 18,6 8,6 3,6 8,3

a v . v a l u e

542 556 573 551 546 532 406

533 550 574 530 517 487 492

514 455 431 415 410

452 469 471 459 434

569 505 507 469 462 546

Pmax, %

c o n f d e n c e range

517.. .567 518. . .594 545.. .599 499.. .602 510.. .562 505.. .560 382.. .430

485...581 524.. .575 549. . .600 513.. .546 379.. .653 433...541 418.. .566

483.. .545 400.. .510 395.. .467 378.. .452 372.. .448

411. .493 422.. .517 443.. .499 416.. .502 404.. .464

537...601 474, . .536 464. . .549 372.. .566 417.. .507 530.. .562

c o e f f i c i e n t ~/ var i a t ion ,

5,4 8,1 5,6

11,1 3,6 6,6 7,1

10,7 5,5 5,3 3,7

21,3 13,1 17,9

5,7 14,3 9,9

10,7 II,3

10,8 12,1 7,1

11,2 8,2

6,7 7.3 9,9

22,4 I 1,7 3,5

C l i m a t i c z o n e

Or ig ina l va lues Moscow

Balt ic reg ion

Subt ropica l zone

C e n t r a l Asia

Far North

Exposure t i m e av . va lue

: oe f f i c i en t . [ ~f va r i a t i on , [ yo I

2 5 days days

I mon. 3 mon. 6 mon. 12 mort.

i days 2 days mort.

3 mon. 6 mon. 12 mort . 18 mort .

5 days m o n .

3 mort. 6 m o n .

12 m o n .

days I days

m o n . 6 m o n . 9 m o n .

25 days days

l mon. 3 mon. 6 mon. 12 mon.

465 453 473 438 440 435 325

447 487 503 422 396 453 448

404 363 329 337 325

438 4OO 388 365 312

460 429 402 392 411 452

c o n f i d e n c e range

441.. .489 420.. .486 450.. .496 388. . .488 429...451 409.. .462 293.. .357

406.. .487 449.. .525 479.. .526 405.. .439 273.. .519 425...481 399. . .497

372.. .436 311.. .414 301.. .358 304.. .370 285.. .365

411.. .465 363.. .436 372...404 347.. .382 289.. .335

430...491 391.. .467 364...441 297.. .486 369.. .453 436.. .468

6,6 8,7 5,8

12,6 2,9 7,9

11,7

10,7 9,4 5,6 4,8

25,0 7,3

13,0

7,4 16,9 10,2 I 1,8 14,8

7,4 II , l 4,8 5,7 8,9

7,8 I0,6 11,4 26,0 12,3 4,2

av . va lue

137 145 143 127 165 165 260

141 156 151 169 214 232 239

147 143 137 144 172

121 148 158 163 175

122 148 162 148 199 272

E, MPa

con f idence range

129...145 123...167 121...166 111...142 137...192 151.. .178 185...336

I19. . .163 134...168 125...176 140...198 167...260 212.. .252 219. . .258

133...161 121...165 I12. . .163 I19. . .169 151...194

I08. . .134 131...165 153...163 142...184 147. . .203

I17. . .128 127...169 139...185 110.. .187 166...233 242.. .302

~! e f f i c i en t va r i a t i on ,

7,9 1 i,3 18,6 14,7 19,8 10,4 34,8

18,6 13,0 16,8 20,1 17,7 10,3 7,8

8,7 13,3 22,2 20,9 14,4

12,7 13,4 7,8 �9

15,6 19,0

5,0 16,5 17,1 21,6 18,0 13,2

708

TABLE 3. Strength and Deformability Characteristics for Film Polyethylene (GOST 10354-73) with Exposure in Different Climatic Conditions without Access to Solar Rays

C l i m a t i c zone

Tropica l zone

Subtropical zone

Cent ra l Asia

Far North

Baltic region

Exposure t im%mor t ,

6 12

6 24

6

6 12

6 12 18

av . va lue

23,4 22,9

18,3 18,1

22,0

23,0 21,7

24,9 22,2 20,9

o f , MPa %ax" %

conf idence range

19,6 ... 27,3 21, I ... 24,7

16,8 ... 19,9 16,8 _. t9,4

20,I ... 23,1

20,6 ... 25,4 20,1 ... 23,3

23,3 ... 26,5 20,9 ... 24,1 18,9 ... 22,9

coe f f i c i en t of var ia t ion . % I

19,8 8,5

6,9 8,3

6,1

12,6 9,0

7,7 8,5

I 1,4

av . va lue

560 507

513 523

566

534 522

494 471 469

conf idence range

468...651 477...538

463...562 463...583

529...602

486...583 480...555

465...523 416...526 423...515

coefficient o f var ia t ion ,

19,5 6,5

7,8 13,6

6,9

I0,I 7,3

6,9 13,8 I 1,6

C l i m a t i c zone

Tropica l zone

Subtropical zone

Cent ra l Asia

Far North

Baltic region

I xposure i m e , m o n .

6 12

6 24

6

6 12

6 12 18

av. value

475 430

400 376

464

434 420

405 394 371

E' t e , g �9

[confide.ace range

384...567 400._460

353...449 332_.420

433_.496

387...482 387...452

379...432 366...422 340...402

c o e f f i c i e m o f var ia t ion ?o

23,0 7,6

9,7 13,8

7,4

13,0 9,3

7,9 8,4 9,8

av . va lue

151 167

203 236

258

196 272

178 190 201

E, MPa

conf idence ['ange

134...167 136...199

190...215 200...272

217...299

176...217 223...321

149...207 161...219 174...228

:oe f f ic ient ~f var ia t ion , 7o

13,0 20,4

5,2 18,4

17,3

12,4 21,6

19,3 18,4 16,0

~ , % _ i - - - - - - - - - - - ' l r

500

I 400

300

200

Eres ~

500

#00

300

200

2

tO 2 103 I0 ~ t, h a

2

102 103 tO~t,h b

Fig. 3. Change in maximum (a) and residual (b) strain for film poly- ethylene with exposure under differ- ent conditions: i) Subtropical zone; 2) Moscow; 3) Central Asia.

709

specimen force and strain measurement during deformation; a wide range for the change in mo- bile grip displacement rate and maintenance of its stability during testing. Structural fea- tures of the equipment have been described in [3, 4].

Testing was carried out on specimens in the form of two-sided blades having the follow- ing dimensions: overall length 60 mm, gauge length 30 mm, blade width 8 mm, width of the specimen gauge length 5 • 0.15 mm, specimen thickness 0.094-0.130 mm. Mechanical properties were studied in specimens cut along and across the polyethylene film sheets.

Specimens from two series (8-10 pieces in each) were subjected to tension with a constant strain rate of i00 mm/min at 20 • I@C.

Directly after fracture (not later than after 30-40 see) overall failed specimen length was measured, and all of the rest of the mechanical parameters of the material were determine~ from the characteristic point recorded on strain diagrams. Elasticity modulus was found as the tangent of the slope angle for the initial part of the diagram to the abscissa axis (strain).

Statistical treatment of test results was carried out on the basis of a normal distribu- tion rule hypothesis. Confidence ranges for average values of mechanical parameters were determined with 95% probability. Scatter values for mechanical indices were also estimated by means of the coefficient of variation.

Results of experiments and corresponding statistical parameters for each selection (ser~ les of specimens having undergone synchronous exposurein a specific climatic zone) are pre- sented in Tables 2 and 3. It is noted that the properties of specimens cut across sheets are almost indistinguishable from those given in Tables 2and 3, i.e., this material may be consld. ered as isotroplc in the sheet plane.

It was established that a linear Hooke's law section occurs up to strains of about 4%. With strain increased to 200% there was an almost horizontal section. In view of the orienta, tlon of the macromolecules with further straining there is some material strengthening.

There is a marked reduction in ultimate strength ofpolyethylene filmafterexposure in a subtropical zone. With exposure under these conditions for twelve months specimen strength de. creased by about 45% (Fig. 2). Some increase in the ultimate strength of polyethylene is noted after 20-30 days exposure in Baltic region climatic conditions after which this parame- ter decreases almost to the original value and subsequently (18 months exposure) there is a reduction of 30-40% compared with the original value. Specimens exposed under these condi- tions but free from direct access to solar rays have almost unchanged strength properties for 12-24 months (Table 3).

Testing of polyethylene after exposure under the rest of the conditions indicated a re- duction in strength properties compared with the original values.

Exposure of film polyethylene specimens under different climatic conditions indicated a marked effect on their relative strain at the instant of fracture (maximum strain Emax).

f"

/IPa

250

200

~50

180

o - - I n--2 o--,3 c//t

102 103 I0 ~ t, h

Fig. 4. Change in elasticity modulus for film polyethylene during exposure under different conditions: i) Subtropical zone; 2) Moscow; 3) Far North.

710

Exposure of polyethylene specimens under subtropical conditions, Central Asia with an industrial atmosphere led to a reduction in maximum strain by 20-25%'comparedwith the orig- inal condition, whereas in the Far North and Baltic regions no marked effect on this parame- ter was detected (Fig. 3a, Table 2).

Residual elongation measured directly after fracture, eres, for polyethylene specimens is 80-90% of Cma x and it has a tendency to decrease with exposure in all climatic zones (Fig. 3b).

A significant reduction in residual strain was noted with exposure of polyethylene speci- mens under subtropical conditions. Over 12 months the value of eres decreased by 30% compared with the original value. A marked reduction in residual strain was also noted for specimens exposed under industrial atmosphere conditions in Moscow (325% for specimens exposed under these conditions for one year compared with 465% for original specimens).

In analyzing the dependence of elasticity modulus for film polyethylene on exposure time in different climatic zones no marked change was noted for this parameter with exposure over" 3 months. Subsequently, with an increase in exposure time a marked tendency develops toward an increase in elasticity modulus (Fig. 4). It should be noted that although this tendency was present in specimens of this material exposed in practically all of the climatic condi- tions considered, an increase in elasticity modulus is particularly marked with exposure in the open air.

LITERATURE CITED

i. GOST 9.039-74, Corrosivity of the Atmosphere, Introduced 1975. 2. Yu. N. Mikhailovskii and A. V. Skurikhin, "Procedure for treating meteorological inform-

ation applied to full-scale testing and modeling of atmospheric corrosion," Zashch. Met., No. 16, 550 (1980).

3. ~. S.Umanskii~ I. E. Debrivnyi, and V. V-Kryuchkov, "Studyof the strength and deformabil- ity of thin composite materials of the magnetic carrier type. Communication i. Strength and deformability at elevated temperature," Probl. Prochn., No. 5, 40-45 (1972).

4. ~. S. Umanskii, V. V. Kryuchkov, and N. S. Shidlovskii, "Supporting capacity of rolled polymeric films," Probl. Prochn., No. i0, 104-113 (1980).

INFLUENCE OF CRYSTALLOGRAPHIC ORIENTATION ON THE FAILURE

OF SINGLE CRYSTALS OF ZhS6F ALLOY

O. I. Marusii, N. G. Chausov, and L. V. Zaitseva

UDC 5 3 9 . 3 : 4

The possibility of practical use of single crystals of nickel alloys [i, 2] has made necessary a composite study of their mechanical properties and microstructural features of failure in relation to crystallographic orientation. The influence of the latter has been studied quite well on single crystals of pure metals [3-5]. For single crystals of solid- solution type and also coherent or noncoherent precipitate-type hardened alloys such informa- tion is limited. In addition, it has been presented primarily for alloys of aluminum and copper [3] and is either for the early stage or for the stage of uniform deformation (uni-" axial tension of flat samples). In connection with this, experiments conducted for the pur- pose of revealing the physical nature of the loss of the stability of deformation and the formation of the neck in single crystals of silicon iron oriented along the < II0> axis (steady loading with a rate of E ffi 3.10 -~ sec -I) [6] of niobium, copper, and aluminum [7] may be no- ted.

The dislocation mechanisms explaining the behavior of single crystals of Ni--AI--Cr alloys with <iii> and <ii0> axis orientations in static loading in the initial stage of deformation

Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Trans- lated from Problemy Prochnosti, No. 5, pp. 86-90, May, 1984. Original article submitted December 16, 1982.

0039-2316/84/1605-0711508.50 �9 1985 Plenum Publishing Corporation 711