The effect of structure-modifying additives on the temperature-time dependence of the strength of polyethylene

THE EFFECT OF STRUCTURE-MODIFYING ADDITIVES ON THE TEMPERATURE-TIME DEPENDENCE OF THE STRENGTH OF POLYETHYLENE* M. A. NATOV a n d S. V. VASILEVA Higher Institute of Chemical Technology, Sofia

(Received 12 July 1968) THE modern theory of the strength of polymers regards the process of breakdown as a time-dependent process [1]. The kinetic theories are based on Griffith's concept that rupture of a solid body can be regarded as a process involving the growth of micro-fissures [2]. The time dependence of the strength of polymers is expressed b y various formulae that take account of the effect of temperature [3-5]. ~Ve showed in reference [6] that the time dependence of the strength of polyethylene (PE) is depicted b y an S-shaped curve. Mechanical stresses of constant magnitude bring about consider chemical degradation of the polymer [6]. Experimental studies in recent years have shown that supermolecular structure has a substantial, and in some instances decisive effect on the physicomechanical properties of a polymer [7-9]. For that reason we decided to study the effect of some structure modifying additives on the temperature-time dependence of a polymer, taking P E as an example. EXPERIMENTAL

For this work we used samples of high density P E of different molecular weight. The characteristics of the samples are shown in the Table. Grade GN polyvinylehloride with a heat resistance of 21 min at 170 ° and M=80,000, without stabilizer b u t coloured with a dye, fl-naphthol of m.p. 120.5 ° and 4,4'-thio-bis-(6-tert. butylmethacresol) of m.p. 157 ° were compounded with the P E in various quantities. Each sample of P E was homogenized b y milling for 6 hr in a porcelain ball mill. After this treatment the samples were moulded to the test-piece form at 140 ° in two stages, first under a pressure of 10 kg/cm 2 for 3 rain then 50 kg/cm ~ for 5 min. The mould was cooled without release of pressure at the rate of 20°/rain. The test pieces were of the double blade type with circular apertures in the middle of both wide sections to facilitate suspension of the load. The specimens were placed in a half-darkened chamber in air at 60% relative humidity, and loads * Vysokomol. soyed. A l l : No. 9, 1906-1912, 1969. 2171

2172

M. A. NATov and S. V. VASILEVA CHARACTERISTICS OF POLYETHYLENE

Sample code

[t/] at 105° in xylene

M

Melt index, g/10 rain

63,000 69,000 91,000 104,000

1-68 1"82 2"34 2"58

PE-1 PE-2 PE-3 PE-4

SAMPLES

m

1820 0"954 0.780

H e a t resist-

ance, ~ (20 rain, 150°) 0.030 0.057 0.062 0.071

Specific gravity 0"935 0"932 0.950 0.958

calculated to give the desired stresses in the specimens were a t t a c h e d . The durability of the specimens was expressed as the time (in seconds) from the application o f the load to r u p t u r e of the specimen. RESULTS AND DISCUSSION

The effect of annealing of the test specimens on the t e m p e r a t u r e - t i m e dependence o f the s t r e n g t h of P E was determined. F o r this purpose specimens of PE-2, p r e p a r e d as described above, were annealed a t 110 ° for times from 1 to ~n"E~88c'

8

I

r

I

I

I

70

90

fro

130

lEO

~, kg/cm 2

FIo. 1. Effect of the time of annealing at 110° on the durability of PE-2- 1--0, 2--2, 3--3, 4--12 hr. 12 hr. T h e d e p e n d e n c e of the t i m e - t o - b r e a k (z) on the applied load at 20 ° is shown in :Fig. 1. E a c h p o i n t on the graphs represents the average o f t e n tests. I t was f o u n d t h a t specimens annealed a t 110 ° for 3 hr h a v e the longest durability. The annealing conditions h a v e the greatest effect on the recrystallization

Temperature-time dependence of strength of polyethylene

2173

stress of P E (arecryst) (the middle section of the curves in Fig. 2). As the time of annealing is increased arecryst increases from 104 to 113 kg/em ~. The general shape of the curve is the same as t h a t of the curve of the dependence of the tensile strength at break (ab) on the time of annealing (Fig. 2). Similar curves

b,kg/cm2

O'PecP~,S~.~k~/cm 2 I#5

250

2~ 230

~30 I

220

-bo

//× fO5 ~ 0

J

I

I

2

I 8 Time , hn

I #

-1200 5

FIG. 2. Effect of the time of annealing at ll0 ° on the reerystallizations s t r e s s (O'recryst) and the tensile strength at break: 1, 3--tTrecrystfor PE-4 and PE-2; 2, 4--a b for PE-4 and PE-2 respectively. are obtained for the other samples of PE also, for example for PE-4, which has a higher molecular weight than PE-2 (Fig. 2). For PE-4 however the stress range is higher, from 129-147 kg/cm 2. In view of the fact t h a t the highest, and subsequently constant values of the characteristcs of the polyethylene samples were obtained after an annealing period of 3 hr at 110 ° all subsequent tests were made on specimens of both modified and unmodified polyethylenes t h a t had been treated in t h a t way. Specimens of PE-1 were placed in the chamber at different temperatures (20, 50 and 80 ° ) and under different stretching stresses produced by attachment of different loads (Fig. 3). I t was found t h a t the temperature-time dependence of the strength of PE can be expressed by a family of S-shaped curves, extrapolating to approximately the same origin and diverging fanwise with change in the test temperature. The value of the time-to-break under zero

M. A. NATOV and S. V. VASZT,~,VA

2174

stress can obviously be taken as a criterion of the life of unstressed polyethylene specimens. Articles made from well purified polyethylene free from stabilizers do in fact begin to break down after a certain time as a result of chemical change and recrystallization. This is shown very clearly b y P E subjected to external /n"

, ,.see

i

10 -

5"-

z~

I

50

150

250 o',k,.q//cm2

FzG. 3. Temperat~ure-time relationship for PE-I: 1--20, 9--50, 8--80% weathering. However, one cannot exclude the possibility that under very small stresses the temperature-r relationship might undergo some change though it is true this would not be very large. /n T~ $8C

20 ~ . . .

10

0

:

I

50

I

I

150

I

x"l

250 d~ k,.q/cm2

Fze. 4. Effeot of molecular weight on the time-to-break for polyethylene a~ 20°: -/--PE-2, 21~=69,000; 2--PE-3, 21~=91,000; 3--PE-4, M = 104,000.

Temperature-time dependence of strength of polyethylene

2175

The effect of molecular weight on the temperature-~ dependence is particularly interesting. Under constant stress P E of high molecular weight gives higher values of z than P E of low molecular weight. This difference is most marked

20 ~ \ \\\f 75

2

3

\i

5

50

f50

250 6, kg/crn z

FIO. 5. Effect of various additives on the time-to-break for PE-2 at 20°:/--without additive, 2-- 0.05 % of 4,4'-thio-bis-(6-tert.butylmetacresol), 3-- 0. 1% of fl-naphthol, 4-- 5 ~ of polyvinylchloridc. at comparatively low stresses, as is seen from Fig. 4, and shows up most clearly in the region of neck-formation. The latter is regarded as a recrystallization phenomenon, which does not involve change in the molecular weight of the polymer. To explain this contradiction it may be supposed that the ends of the polymer molecules constitute defects in the crystal lattice, from which recrystallization begins. Both breakdown of the specimens and neck-formation are dependent on the presence of lattice defects, and the larger the number of these present the greater is the probability of the occurrence of both these processes. This assumption also explains the relationship between the annealing conditions and the recrystallization stress for PE. The more rapidly the specimens are cooled the larger is the number of defects of the crystal structure formed, as a result of which a neck is formed at a lower recrystallization stress. The stabilizers used up to the present time for P E are usually chosen for their effect as inhibitors of thermal and photochemical degradation. We have shown previously that they do not protect the polymer from the mechano-chemical degradation brought about by the action of mechanical stress [6]. In a study

2176

M . A . NATOV and S. V. VASILEVA

of the effect of various stabilizers it was found that the molecular weight of stressed and unstressed PE undergoes little change when the polymer contains from 0.5 to 1.5% of 4,4'dhio-bis(6-tert.butylmetaeresol). At the same time this 320 -.

2#0

160

E 8O

J

0

•

60

'

I

100

i

I

1#0 T,°C

FiG. 6. Thermogram of PE-2 (rate of heating ih DTA apparatus 0.2°/min).

stabilizer does not significantly alter the temperature-~ relationship of PE (Fig. 5). The effect of fl-naphthol and polyvinylchloride which have not been used before as additives for PE, was still more interesting. Figure 5 shows that an extremely small amount of fl-naphthol increases the durability of PE conside-

%

FIG. 7. Photomicrographs (×800) of the structure of: 1--PE-1,. 2--PE-1 with 0.1~ of fl-naphthol.

Temperature-time dependence of strength of polyethylene

2177

rably, and no further change in its effect is found when the concentration of fl-naphthol is increased from 0.1 to 1.0 %. The addition of 4-5 ~o of polyvinylch]oride has a still greater effect on the durability of PE. I t is seen from Fig. 5 t h a t

t,p/c, dl/g 2"8 6

'7

2"#

2.0

1"8 I

I I I

I

12 [

0

J

0"2

~ 0'#

i

0"8 c, g/lO0 ml

i

0"8

i ¢'0

FIG. 8. Variation in the specific viscosity of solutions of P E in xylene at 105°: / - - o r i g i n a l

PE, 2-4--after ageing for 75 days at 20° with 0-1 Yo of B-naphthol (2); with 5 ~ polyvinylchloride (3); without additives (4); 5 - 6 - - P E with 0.1~ •-naphthol added (5) and 5~o polyvinylchloride (6) under stress of 104 kg/cm = at 20° for 75 days; 7--without additives under stress 104 kg/cm ~ at 20° for 15 days. the steep rise in the curve of the variation of the durability of modified PE under a static load of 100 kg/cm 2 indicates the occurrence of a safe stress zone, or at least a marked increase in durability. I t is difficult at present to explain these facts. The P E we used gives a maximum in the thermogram, recorded in an apparatus of high sensitivity [10], corresponding to a phase transition of i 18-119 ° (Fig. 6). I t must be borne in mind t h a t both substances (polymer and stabilizer) have similar crystallization temperatures (fl-naphthol at 120.5°). This gives grounds for the suggestion that fl-naphthol forms crystallization nuclei in the polymer melt. In fact photomicrographs of the fracture surface of a highly cooled sample of PE containing fl-naphthol show a more freely grained structure than pure PE (Fig. 7). The specific gravity of the modified PE was 0.928, i.e. significantly lower than t h a t of pure PE (0.935). This change in structure (see Fig. 7) is associated with a sharp fall in the area of free crystalline surface, as determined by the BET method. I t is these changes in structure t h a t are responsible for the high durability of PE. Simultaneously with this the molecular weight of PE

2178

M.A. NATOVand S. V. VASrLEVA

changes only slowly when fl-naphthol is present (Fig. 8). This indicates that it has some stabilizing effect on PE. When polyvinylchloride is added to P E a different change in structure occurs. These changes are seen very clearly in films containing 5 % of polyvinylchloride, obtained by slow cooling from the melt. Large, well-formed, circular spherulites are formed in such films (Fig. 9) and the specific gravity is higher (0.980)

FIG. 9. Photomicrographs ( × 800), taken through a polarizing microscope, of the structure of: a--PE-1; b--PE-1 with 5~ of polyvinylchloride. than that required by the additivity principle. The occurrence of larger spherulites in P E containing polyvinylchloride results in increase in the recrystallization stress and to decrease in the deformation of the specimens. I t must be assumed that fibrillar aggregates (bundles of polyvinylchloride) reinforce the t)E, and as a result of this the service life of the polymer increases. P E containing polyvinylchloride undergoes a smaller change in molecular weight under load, as is shown in Fig. 8. CONCLUSIONS

(1) A study of the time-to-break for polyethylene of different molecular weight has shown that increase in molecular weight increases its durability under the range of stresses studied, and its effect of increasing the recrystallization stress is very marked. This is explained by decrease in the proportion of defects in the crystalline structure of the polymer. (2) Study of the temperature-v relationship for polyethylene containing structure-forming additives has shown that in polyethylene containing 0.1% of fl-naphthol or 5% of polyvinylchloride structural changes occur, and these bring about a marked increase in the service life of polyethylene modified in this way. Translated by E. O. PHILLIPS REFERENCES

l. G. M. BARTENEV, Dokl. Akad. Nauk SSSR 133: 314, 1960; Mekhanika polimerov, 74, 1966 2. A. GRIFFITH, Phil. Trans. Roy. Soe. A221: 163, 1921 3. S. N. ZHUItKOV and B. N. NARZULAYEV, Zh. tekh, fiz. 23: 1647, 1953 4. G. M. BARTENEV and V. Ye. GUL', ZhVKhO ira. D. I. Mendeleyeva 6: 394, 1961

2179

Dipole polarization in copolymers

5. W. E. GLOOR, Modern Plastics, No. ll, l l l , 1960 6. M. A. NATOV, S. V. VASILEVA and V. KABAIVANOV, Dokl. Akad. Nauk SSSR 180: 646, 1968 7. V. A. KARGIN and T. I. SOGOLOVA, Dokl. Akad. Nauk SSSR 88: 867, 1953 8. V. A. KARGIN, V. A. KARANOV and I. Yu. MARCHENKO, Vysokomol. soyed. 1: 94, 1959 (Translated in Polymer Sci. U.S.S.R. 1: 1, 41, 1960) 9. L. E. NIELSEN, J. Appl. Polymer Sci. 2: 351, 1959 10. M. NATOV, I. PEEWA and B. SERAFIMOV, Makromol. Chem. 117: 231, 1968

DIPOLE POLARIZATION IN COPOLYMERS OF ~-METHYLSTYRENE WITH fl-CYANOETHYLMETHACRYLATE* G. P. MIKHAILOV (dec.) and T. I. BORISOVA Institute for High Molecular Weight Compounds, U.S.S.R. Academy of Sciences (Received

12 J u l y 1968)

STUDY o f t h e dielectric p r o p e r t i e s of fl-derivatives of p o l y e t h y l m e t h a c r y l a t e h a s s h o w n the occurrence of considerable dipole p o l a r i z a t i o n o f t h e e n d g r o u p s o f t h e side chains of t h e p o l y m e r molecule, in which t h e m a i n chain does n o t t a k e p a r t [1-3]. T h i s p o l a r i z a t i o n h a s a c h a r a c t e r i s t i c a l l y s h o r t r e l a x a t i o n t i m e , which increases as t h e dipole m o m e n t o f the g r o u p a t the end of t h e side chain increases. I n t h e w o r k r e p o r t e d in this c o m m u n i c a t i o n a s t u d y was m a d e o f t h e relationship b e t w e e n c o p o l y m e r c o m p o s i t i o n a n d r e l a x a t i o n of dipole p o l a r i z a t i o n of t h e e n d g r o u p s o f t h e side chains in c o p o l y m e r s o f f l - c y a n o e t h y l m e t h a e r y l a t e (fl-CNEMA) a n d a - m e t h y l s t y r e n e .

EXPERIMENTAL Copolymers of fl-CNEMA with ~-methylstyrene were prepared by radical polymerization in Koton's laboratory. The Table gives some characteristics of the copolymers used. DENSITY

Tg

p, REFRACTIVE I N D E X nD AND GLASS T E M P E R A T U R E

OF POLY-fi-CYAlqOETHYLI~ETHACRYLATE

(P-fl-CNEMA) AND

ITS COPOLYMER W I T H ~-lV[ETHYLSTYRENE

Sample code P-B-CNEMA C-75 C-55 C-41

Molar content of fl-CNEMA, % 100 75 55 41

g/cm3 p,

1.237 1.184 1.163 1.139

* Tg defined from electrical measurements.

* Vysokomol. soyed. All: No. 9, 1913-1918, 1969.

nD

Tg,°C *

1.507 1.524 1.548 1.559

96 109