STATIC STRENGTH OF MODIFIED POLYETHYLENE FILMS AFTER PROLONGED

EXPOSURE IN DIFFERENT CLIMATIC ZONES

E. S. Umanskii, N. S. Shidlovskii, L. L. Stezhko, R. M. Kas'yan, V. S. Sychov, and B. N. Rybakov

UDC 539.3/5:678

In order to use polymer materials extensively in many branches of technology and the national economy it is important to have available data for the effect of various operation factors on their supporting capacity, in particular prolonged exposure in any climatic zones characterized by different degrees of solar radiation, changes in the surrounding tempera- ture, moisture, and other factors.

Extensive experimental data were given in [I] for the effect of prolonged exposure in different climatic zones of the country and in the tropical zone of the Atlantic on the sta- tic strength of polyethylene film.

In addition to film (All-Union State Standard (GOST) 10354-73), we use different modi- fications of polyethylene obtained by adding small amounts of alloying substances. The me- chanical properties of modified materials may be markedly altered in this way. As was noted in [2], addition to polyethylene of thermoelastoplastic (Technical Specification (TU) 3840 352- 74) markedly improves its deformation properties and it changes the rate of stress relaxa- tion.

Apart from alloying additions of the thermoelastoplastic type, carbon black has been added to polyethylene film, thus promoting blocking of ultraviolet rays. In view of this it is of interest to study the effect of thermoelastoplastic and carbon black on the stabil- ity of mechanical properties of modified polyethylene film during exposure in different cli- matic conditions.

These studies were carried out on the following materials: polyethylene with additions of 3 and 5% thermoelastoplastic and also polyethylene with additions of 3, 5, and 20% ther- moelastoplastic and carbon black. Specimens of modified polyethylene film were exposed in different climatic zones of the country. Climatic zone characteristics and the test proced- ure have been described in [i].

Apart from aging in the most severe conditions with direct access to solar rays, some of the specimens were exposed in locations equipped with special louvered screens. In this case specimens were free from direct access to solar radiation while retaining all of the rest of the climatic factors associated with the climatic zone being studied.

Tests were carried out on specimens cut in the form of two-sided blades with a gauge length of 30 ~m and a width of 5 mm. Test specimen thickness was 0.094-0.360 mr,. The lon- gitudinal axis of specimens coincided with the longitudinal axis of the material from which specimens were cut. Additional check for the effect of specimen cutting direction on the mechanical properties of the materials showed quite a high degree of isotropy.

The tensile test machine grip travel rate was i00 n~n/min. Testing was carried out at room temperature. Eight to ten specimens were tested in each batch.

A clear idea of the nature of deformability for original (untreated) specimens of modi- fied polyethylene film may be obtained from the tensile diagrams shown in Fig. 1 (curves 2- 6). Also given for comparison is a deformation diagram for unmodified polyethylene film (COST 10354-73) (curve I). In order to determine stresses, the tensile forces were referred to the initial cross-sectional area.

As can be seen, tensile diagrams for the test materials are represented by monotonic curves consisting of a rectilinear (Hookean) section and also a nonlinear section whose ex- tent depends on the content of alloying additions in the material. The nature of the defor-

Kiev Polytechnic Institute. Translated from Problemy Prochnosti, No. 2, pp. 47-52, February, 1985. Original article submitted March 22, 1984.

204 0039-2316/85/1702-0204509.50 �9 1985 Plenum Publishing Corporation

•, MPa

10

/'/f...z/. ~ ==.._.....=__ v '6

Ou R , MPa

1 2 4

10 a

ernox , ~

5

qO0 b

E, MP~

200 . ~

100 ,,,___

0 0 200 ~20 & % 10 2 IO 3 10 4 t, h

C

Fig. i Fig. 2

Fig. i. Deformation diagrams for the original polyethylene film (i), polyethylene with addition of 3% (2) and 5% (3) thermoelastoplastic, polyethylene with addition of 3% (4), 5% (5), and 20% (6) thermoelastoplastic and carbon black.

Fig. 2. Change in mechanical properties of polyethylene film with additions of 3% (i) and 5% (2) thermoelastoplastic, 3% (3), 5% (4), and 20% (5) thermoelastoplastlc and carbon black with the action of Moscow (city) climate.

TABLE i. Strength and Deformation Characteristics for Polyethylene Film and Modifi- cations of It in the Original Condition (before exposure in different climatic zones)

Material

)lyethylene ( ;OST I0354-q3)

)lyethylene with 3% termoelastop.lastic added he same with 5% thermo- asmplastic adde.d e same with 3% thermo-

tastoplastic and carbon ack added he same with 5% thermo- astoplastic and carbon ack added he same with 20% ,ermoelastoplastic nd carbon black dded

Ultimate strength Oult, M Pa

confidence average range value

20,7...22,9

18,4...21,I

II,8.. .27,7

15,5...17,4

16,4...19,3

I6,4...17,5

21,8

19,9

19,7

16,4

17,6

16,9

Maximum elongation Coeffi- % cient of ~max, varia- confidence tion, % range

517... 567

496...715

553... 656

557... 613

553...632

586._612

average value

542

595

604

585

593

599

Coeffi- cient of varia- tion, %0

Elasticity modulus E, MPa

confidence range

Coeffi- cient of

average varia- value tion, %

5,8

7,I

15,8

7,0

9,7

3,4

5,4

7,1

I0,2

5,6

6,3

2,5

129...145

148...195

143...210

98...I15

98...143

81...I03

137 7,9

171 16,2

177 7,5

I06 8,7

120 17,6

92 14,0

mation curves indicates that at room temperature the test materials are strain-hardening.

Results of determining the strength and deformation properties of these materials in the original condition, and also indices for scatter of experimental data calculated on the basis of the hypothesis of a normal rule for distribution with 95% probability, are given in Ta- ble i.

205

OuR, MPa 2

~0

t~CO

I0'3

E, MP~

I 5

1 b

ooo /

5

fO ~ t, h

~ MPa 1 2

,3 5 r 10

%o~,%

600

400

20G

E, MPa

2~

100

b

: ( -- J i

3

10 2 ;0 3 10 2 10 3 ,04t, h C C

Fig. 3 Fig. 4

Fig. 3. Change in mechanical properties of polyethylene film with additions of 3% (I) and 5% (2) thermoelastoplastic, 3% (3), 5% (4), and 20% (5) thermoelastoplastlc and carbon black with the action of Baltic Region climate.

Fig. 4. Change in mechanical properties of polyethylene film with additions of 3% (I) and 5% (2) thermoelastoplastlc, 3% (3), 5% (4), and 20% (5) thermoelastoplastlc and carbon black with the action of subtropical climate.

TABLE 2. Strength and Deformation Characteristics of Some Modifications of Polyethy- lene Film with Exposure in Different Climatic Zones

Climatic zone

Moscow city

Baltic region

Subtropics

Central Asia

Far North

8 " ~ E

I 6

12

12 18

12

1 6

12

1 6

12

Polyethylene with 5% thermo- Polyethylene with 5% thermo- elastoplastie added elastoplastic and carbon black

I add___ed . . . . . .

- ~ ~ ~ .~ :~

17.4 18.2 15.8

21.1 23.1 15.5 12.1 23.5 20.2 14.3 20.3 17.0 15.3

17.6 17.5 16.9

20.2 13.9 9.3

6.3 4.4

13.8 I0.2

14.9 8.0 7.2

4.7 8.0 7.6

8.3 2.3 6.8

478 I0.I 500 24.0 470 7.7

550 8.8 633 6.7 515 8.2 381 7.8

561 18.3 654 6.7 570 I 7.2

584 6.2 50C 6,2 451 7,3 583 8.1 543 5221 4,8 7,2

2061 19.6 269 ! 13.2 306 18.7

213 I I .3 275 25.6 283 15.7 282 l l . 9 254 5.8 268 29. I 196 18.2

221 13.5 351 25.6 359 20.8

151 16.0 164 12.8 204 12.2

[7.91 17.91 16.4

16., 17.9 15.6 14.9

18.9 17.8 17.3

17.5 16.8

18.0 17.7 15.9

3.4 [ 616 5,8 601 8.9 529

4.9 ] 550 9.2 1 599 2.01 573 2.41 497 6.3 591 3.6 607 1.4 618

9.7 584 8.6 573

6.4J 582 9.E 588 9.51 557

4.5 1 153 7.S l 148

I0, l [ 197 !

5,11 144 I0,31 199 5,9" 215 3,0 226 4.8 113 5.1 194 7.6 183

7.6 163 5. l 161

6,2 167 11,1 19C 7,6 162

Polyethylene with 20% thermo- ela~toplast/c and carbon black aQfleQ

17,8 12,8 1 6,5 26,0 t3,9t 5,1 17,0 14,31 5,9 9,1 16,1[ 2,6

24,5 11,7[ 6,6 21.6 13.4 7.5 20.9 14.8 9.7

16.6 15.1 3.7 19.8 13.5 4.1 16.9 13.6 5.2 21.8 13.4] 8.8 16.0 12.9 7.3 - - 12 ,6 5 , I

13.1 16.2 1.9 19.6 14.2 6.1 14.0 13.5 6.0

569 5.7 586 3.1 604 8.5

632 0.5 548 9.9 554 5.4 577 l . l

589 7.9 553 3. l 357 5,6 374 4,8 306 5,3 291 7.4

664 4,8 659 5 , 5 559 3 . 9

I05 11.7 141 27.7 150 22.6

l l3 4.6 144 21.3 171 23.4 187 25.9

l l4 15.9 140 I 1.4 219 I 17,7 #, 15,9

16,9 143 I 17,8

144 17,6 139 12,7 203 19,0

206

~ I MPa

1D

%o,,%

400

~00

E, MPa

'200

100

~ MPa

?0 : ~ I

t~ I

%o~,%

8OO

400

E, M P-"

,-.L 200

.. . . ~ 5 100

I

i L i

i I0 z 10 3 tO 4 t h 10 ~ :2 ~

C

Fig. 5

4

I i L

Fig. 6

Fig. 5. Change in mechanical properties of polyethylene film with additions of 3% (I) and 5% (2) thermoelastoplastic, 3% (3), 5% (4), and 20% (5) thermoelastoplastic and carbon black with the action of Central Asian climate.

Fig. 6. Change in mechanical properties of polyethylene film with additions of 3% (i) and 5% (2) thermoelastoplastic, 3% (3), 5% (4), and 20% (5) thermoelastoplastic and carbon black with the action of the Far North climate.

As can be seen, addition of 3-5% thermoelastoplastic leads to some reduction in ultimate strength and an increase in maximum elongation for specimens (-10% compared with unmodified polyethylene).

Addition of thermoelastoplastic has a very marked effect on the scatter of test results, and this leads to an increase in the coefficient of variation. Addition of 5% thermoelasto- plastic increases the coefficient of variation for ultimate strength by almost a factor of three.

Addition of carbon black markedly reduces the strength properties of polyethylene film. The least ultimate strength value (16.4 MPa) was noted for specimens of polyethylene with 3% thermoelastoplastic and carbon black added, and this is 75% of the ultimate strength of un- modified polyethylene in the original condition.

Strength and deformation characteristics for these modifications of polyethylene film after exposure in different climatic zones are given in Table 2. The dependences of ultimate strength, maximum elongation, and nominal instantaneous elasticity modulus on exposure time are given in Figs. 2-6.

It is desirable to describe mechanisms for the change in strength and deformation proper- ties with prolonged exposure of modified polyethylene for each modification.

Polyethylene with Addition of 3% Thermoelastoplastic. General tendencies for the change in strength characteristics of the material are almost indistinguishable from the tendencies for the change in these properties for polyethylene GOST 10354-73 [1], and in fact, a reduc-

207

tion in ultimate strength and maximum elongation and also an increase in elasticity modulus with increase in duration of climatic effects, are typical with almost all schedules.

Some increase in ultimate strength (during 5-30 days) is observed with holding under subtropical atmosphere conditions (curve 1 in Fig. 4a), and there is a very marked reduction in strength (to 46% of the original value) during 1 year of exposure.

At the same time, tests on specimens held for 2 years in this climatic zone, but with- out access to solar radiation, showed a total reduction in strength of 3%.

The effect of solar radiation on material strength during exposure in the Baltic region is less marked. The corresponding reduction in ultimate strength of specimens exposed in this zone for 18 months (in an open site) is 8% of the original value.

As already noted, a very marked reduction in deformability is observed with exposure of specimens in practically all of the zones considered. The greatest rate of change for max- imum elongation is observed in the range 6-12 months. This phenomenon is typical for the climatic zones of Moscow, the Baltic region, and the subtropics (curves 1 in Figs. 2b, 3b, 4b).

These features for the change in maximum elongation with aging of specimens in different climatic zones are typical, and residual elongation is about 70-80% of the maximum..

For all of the climatic zones considered, the greatest effect on elasticity modulus for polyethylene with addition of 3% thermoelastoplastic is shown by the atmosphere of Moscow (value ofE increased by a factor of 1.4 in lyear), the Baltic region (by a factor of 1.7 in 18 months), and Central Asia (by a factor of 1.5 in 9 months) (see curves 1 in Fig. 2c, 3c, 5c).

Aging of test material specimens without direct access to solar radiation in these cli- matic conditions also leads to an increase in elasticity modulus, although to a somewhat lesser degree (by a factor of 1.3 for the Baltic region, by a factor of 1.2 for the subtrop- ics, and by a factor of 1.5 for the Far North).

Polyethylene with Addition of 20% Thermoelastoplastic. Tests on specimens of this film material showed a somewhat higher sensitivity of strength characteristics towards exposure conditions. Ultimate strength decreased with exposure of material in all of the climatic zones, although its greatest reduction was noted in the time interval of 6-12 months with exposure in the subtropical and Baltic regions (by 40 and 60%, respectively, compared with the original condition).

Absence of direct solar radiation slowed down aging specimens of this material markedly. For example, over a period of 2 years of exposure in subtropical zone without access to so- lar radiation the ultimate strength was almost unchanged, and in the Baltic region the reduc- tion in aul t was 15% of the original value.

Exposure of specimens in all of the climatic zones leads to a marked reduction in their deformability.

The maximum elongation of specimens exposed in a open site in the Baltic region for about 18 months led to a reduction by a factor of about 1.5 compared with original values (curve 2 in Fig. 3b). At the same time, the elasticity modulus increased by a factor of 1.6 (curve 2 in Fig. 3c).

A similar tendency was detected for specimens exposed in the industrial atmosphere of Moscow (elasticity modulus increased by a factor of 1.7 during 12 months exposure) (curve 2 in Fig. 2b).

A somewhat greater increase in elasticity modulus was noted with exposure of specimens under Central Asian conditions. In this case, during 9 months the elasticity modulus is in- creased by a factor of two compared with the original value (curve 2 in Fig. 5b).

Polyethylene with Addition of 3% Thermoelastoplastic and Carbon Black. With exposure of specimens under Moscow industrial atmosphere conditions a reduction is observed in ultl- mate strength over a period of 1 year by 30% (curve 3 in Fig. 2a). Some increase in the val- ue of Oul t for specimens was noted with exposure in the Central Asian region, although subse- quently (after 20-30 days exposure) ultimate strength stabilizes at a level of 17-19 MPa (curve 3 in Fig. 5a).

Under the rest of the exposure conditions material strength characteristics were almost unchanged.

208

Exposure of specimens in all of the climatic zones considered for the time intervals in- dicated does not have a marked effect on deformation characteristics. However, introduction of carbon black as a filler promotes some increase in residual deformation, which in this case is 80-85% of the maximum deformation at the instants of Breaking.

It should also be noted that in all of the test climatic zones with exposure up to 6-9 months the material elasticity modulus changes very little. Stiffening only becomes marked after 9-18 months of exposure. Towards the end of 18 months of exposure on an open site in Baltic region elasticity modulus E increased by a factor of 2.2 compared with the original value.

Similar dependences are observed with exposure of these specimens in the Moscow atmo- sphere and in the subtropics.

Polyethylene with Addition of 5% Thermoelastoplastic and Carbon Black. For the material specimens considered high stability of strength properties with exposure in practically all of the climatic zones studied was established. The greatest change in ultimate strength did not exceed 15% of the original value.

Material deformability characteristics in all climatic zones during exposure are prac- tically unchanged, and only towards 18 months exposure is there a small reduction in Ema x and ere s (curves 4 in Figs. 3b and 6b).

The high stability of elasticity modulus values is also noted for exposure of specimens in practically all of the zones for periods up to 3 months. With further exposure some in- crease in elasticity modulus is observed. The greatest increase in this property is typical for exposure of specimens under subtropical conditions (by a factor of 1.5 in 9 months), in the Baltic region (by a factor of 1.9 in 18 months), and in Moscow (by a factor of 1.6 in 12 months).

Polyethylene with Addition of 20% Thermoelastoplastic and Carbon Black. A markedly greater content of thermoelastoplastic led to a marked reduction in strength characteristics both in the original condition and after exposure in the majority of climatic zones. With exposure of the material for 12-18 months in the Baltic region, the subtropics, and in the Far North, the ultimate strength decreased by 15, 24, and 25%, respectively, compared with the original value (curves 5 in Figs. 3a, 4a, and 6a).

The greatest aging was noted with exposure of specimens in Central Asia where the maxi- mum residual elongation of specimens decreased in 9 months on average by a factor of two (curve 5 in Fig. 5b). In the rest of the climatic zones the nature of change in deformabil- ity indices differs little from that described for the previous polyethylene film modifica- tions.

With an exposure time up to nine months there is a uniform increase in elasticity modu- lus under conditions in the Baltic region, the Far North, and the subtropic region. Subse- quently, a more marked increase in this index develops, particularly in the subtropical zone (curve 5 in Fig. 4c).

CONCLUSIONS

i. In the original condition, alloying of film polyethylene with thermoelastoplastlc and also with additions of carbon black leads to a reduction in ultimate strength and an In- crease in deformability.

2. Alloying with thermoelastoplastic has practically no effect on film polyethylene aging rate.

3. Alloying with thermoelastoplastic together with additions of carbon black reduces the aging rate for film polyethylene.

i.

.

LITERATURE CITED

E. S. Umanskii, N. S. Shldlovskil, V. V. Kryuchkov, et al., "Static strength of film polyethylene after prolonged exposure in different climatic zones," Probl. Prochn., No. 5, 82-86 (1984). N. N. Filippova, M. S. Akutin, E. D. Lebedeva, et al., "Films of alloyed polyethylene," Plast. Massy, No. i, 41-43 (1978).

209