The Course of Thermooxidative Degradation of LD- and HD

4 The Course of Thermooxidative Degradation of L D - and H D - Polyethylene under Accelerated Testing Conditions ARNE HOLMSTRÖM The Polymer Group, Chalmers University of Technology, Fack, 402 20 Gothenburg, Sweden Downloaded by CORNELL UNIV on September 6, 2016 | http://pubs.acs.org Publication Date: April 2, 1979 | doi: 10.1021/bk-1979-0095.ch004

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The search for accelerated testing procedures to predict the useful service life for polyethylene (PE) compositions has been the main incentive for numerous studies on the thermo-oxidative degradation of PE in oxygen-rich atmospheres below 200 °C (1). The basic requirement for reliable accelerated ageing is that the course of degradation is identical at the service and test conditions. Surprisingly enough, in spite of the many studies and the great technical and economical importance in being able to predict the useful service life for PE products, the information about the dependence of the most common accelerating factor on the structural changes during thermo-oxidative degradation is rather scarce. This is due to the fact that most publications concern measurements of only one structure parameter at a single temperature level. Furthermore, the PEs under investigation have mostly been sparsely characterized, especially with regard to additives and catalyst residues. In a series of investigations (2, 3, 4, 5, 6, 7) we have studied the temperature dependence of the structural changes occurring in well defined PEs and PE compositions when heated at temperatures between 70 °C and 180 °C in air. Our aim has been to cover the commonly employed accelerated testing conditions under which service life-time predictions of PE are made. The structural changes have been studied with a series of methods including gel chromatography (GPC), viscometry, IR, differential scanning calοrimetry (DSC) and gravimetric measurements. Experimental Further experimental details are given in references (2, 3, 4) Materials and Heat Treatment. The LDPE used, Unifos DFDS 6600, is a high pressure product produced in a tubular reactor. Unifos has reported the LDPE to be free from additives. It had Current address: Statens provningsanstalt, Laboratory for poly meric materials, Box 857, S-501 15 Boras, Sweden 1

0-8412-0485-3/79/47-095-045$05.00/0 © 1979 American Chemical Society Eby; Durability of Macromolecular Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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DURABILITY OF MACROMOLECULAR MATERIALS

3

3

3

MI = 0.31 g/10 min, density = 923 kg/m , M = 29·10 , 5^ = 159·10 and degree of long chain branching (LCB) = 1.5·10 . The HOPE re sin, Marlex 6001, was produced by N.V. Polyolefins, Belgium according to the P h i l i p s process. Since this resin contained un specified additives, i t was purified by dissolution i n p-xylene, f i l t r a t i o n , and precipitation i n methanol (8). After this treat ment no traces of antioxidants or other additives could be de tected by IR or UV. The sample contained ~ 1 ppm chromium both before and after the reprecipitation, however, as determined by atomic absorption ^spectrometry (8). I t had MI = 0.10 g/10 min. density = 960 kg/m , = 16-10 , = 150-10 and λ = 0.07-10 . Compounding LDPE with TiO^ ajjd carbon black (CB) was per formed i n a Brabender Plastograph for 10 min at 90 rpm under nitrogen. The mixing chamber volume was 60 ml and the o i l bath temperature 160 C. The antioxidants (AO) were incorporated by dry blending of 25 kg LDPE with AO and then extrusion and granulation 3-5 times on a L u i g i Bandera SRL extruder. 0.25 mm thick films were moulded at 130 °C for LDPE and at 150 C for HDPE between aluminium f o i l s or reinforced polytetrafluoroethylene films. The degradation experiments were performed on 20x40 mm strips cut from these films. The n-C^HQQ was obtained from Fluka AG, Switzerland as puriss quality. No irregular structures such as t e r t i a r y carbonatoms, o l e f i n i c or carbonyl groups could be detected by IR and NMR. The samples were placed on thoroughly cleaned microscope cover glasses and heated i n an a i r oven at temperatures from 70 to 180 C. The temperature constancy was better than -1 C. The heating times ranged from 3936 h at 70 °C to 4 h at 180 °C. n Π

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Structural characterization. The content of insoluble mate r i a l (gel) was determined by soxhlet extraction with p-xylene for 20 h under nitrogen. Details of the GPC-analysis have been given (9, 1£) . A Waters Associates GPC Model 200, operating at 135 C with 1,2,4-trichlorobenzene (TCB) as solvent, was used. The column combination con sisted of^five Styragel columns with permeabilities ranging from 10 to 10 Â, which gave good separation i n the MW range of i n t e rest. To calculate the molecular-weight d i s t r i b u t i o n (MWD), average MWs, and degree of long-chain branching (LCB) from GPC and visco s i t y measurements the computer program devised by Drott (11) was used; LCB i s given as λ = η /M, where η i s the weight-average number of t r i f u n c t i o n a l branch points per molecule. Because of the d i f f i c u l t i e s i n determining < 20·10 (by l i g h t scattering), the relation between i n t r i n s i c viscosity (I.V.) and fl^ i s hard to es t a b l i s h , which makes Drott s method ^inapplicable. Samples with H < 20 10 have been calculated as i f they contained only linear molecules. The calibration curve for linear PE was obtained v i a the universal calibration curve (9). 3

1

e

3

Eby; Durability of Macromolecular Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4.

HOLMSTRÔM

LD-

and

47

HD-Polyethylene

U

Determination of I.V. was carried out i n p-xylene at 105 C -0.01 C by using Ubbelohde viscometers with flow times for pure solvent greater than 100 sec. Like Drott (11) we had found (9) that p-xylene at 105 °C and TCB at 135 °C give the same I.V. f o r PE. Smooth curves were drawn to f i t the experimental points i n the plots of I.V. against heating time. In the GPC data treatment the curve values were then used. The samples (~0.2g) were weighed before and after hçat treat ment on a Mettier analytical balance S-6 to better than -0.01 mg. The melting behavior and determination of c r y s t a l l i n e content were studied with a Perkin-Elmer DSC-1 d i f f e r e n t i a l scanning calo rimeter. The heating rate was 8 °C/min, the sample size, 12.0 mg for LDPE and 3.0 mg for HDPE, and n-C^H^. The runs were per formed i n a nitrogen atmosphere. Before analysis a l l samples were given the same thermal treatment by heating to 150 C and cooling down to room temperature i n a controlled manner. In the determination of the c r y s t a l l i n e content i n PE dotriacontane ( " 32 56^ * 100 ^ c r y s t a l l i n e calibration material according to common practice (12). I t was found that this method resulted i n a cry s t a l l i n i t y of n-C^H^ higher than 100 %, i n spite of which the common method was retained for the PE samples, thus allowing for comparison with published results. T^ was the maximum temperature. IR-spectra were recorded on a Beckman IR-9 and a Perkin-Elmer IR-80 IR spectrophotometer with f i l m 0.20 mm thick. Bruker 270 MHz Fourier Transform NMR equipment was used to obtain NMR spectra.


+

n

C

H

w a s

u s e
R0 (A) R0 -+ RH -> ROOH + R- (B) ROOH + RO- + -OH (C) RO- + RH + ROH + R- (D) HO' + RH + H 0 + R' (E) R- + R+ R-R (F) Molecular enlargement R0 -+ R0 * ·* 2R0- + 0 (G) 2

2

2

9

2

2

2

At temperatures above 150 °C homolytic cleavage of the hydro peroxide i s l i k e l y to occur rapidly, reaction (C). The alkoxy and hydroxy radicals formed may then as one p o s s i b i l i t y undergo hydro gen abstraction, reactions (D) and (E). Hereby two neighbouring a l k y l radicals are created which i n the absence of "oxygen may com bine and result i n molecular enlargement, reaction (F). This reac tion sequence should be a p a r a l l e l to ordinary peroxide curing of PE and may account for the immediate molecular enlargement ob served above 150 C. The C-C-bonds formed should not be suscept i b l e to hydrolysis.


HOLMSTRÔM

LD- and

HD-Polyethylene


4.

Figure 5. DSC thermograms for additive- free LDPE heated at 105° and 123°C in air


55

56


The molecular diminishing observed, as well as the majority of the oxygen containing groups formed, (alcohols, acids, esters and ketones) i s due to a variety of reactions undergone by the alkoxy radicals. 9· R-C-R "

8 R-C-R" + Aldehyde Ketone Molecular

1


R

f

R(H) (R = H) (R = R) diminishing M

ff

Reaction (D) w i l l for example provide one possible route to alcoholic groups, while an alternative reaction, β-scission of a secondary alkoxy r a d i c a l , w i l l give aldehyde, reaction (Η), which rapidly oxidizes further to peracid. 3-scission of a t e r t i a r y a l koxy radical result i n ketones. Aldehydes and ketones may react further with peracid to give acid and ester groups. 0

0 tl

Π

R-C-OH

+

f

R OH

+

R-C-OR'+ H 0 (I) Molecular enlargement Because of the accumulation of acid and alcoholic groups, re action (I) becomes increasingly frequent. The formation of ester linkages provides another route to molecular enlargement besides the "peroxide curing". The ester bonds should be prone to hydro l y s i s , however. The mechanisms discussed above provide the basis for a rea sonable explanation to the observed differences i n the degrada tion course above and below 150 C. Above 150 C "peroxide curing" w i l l give immediate molecular enlargement. Later on, when degra dation products have accumulated, ester formation should also contribute to the extensive molecular enlargement observed. Mole cular diminishing should mainly be due to reaction (H). Below 150 C no "peroxide curing" seems to occur. Instead, molecular diminishing, reaction (Η), w i l l dominate for a long time u n t i l the necessary build-up of acid and alcoholic groups has been accomp lished to enable ester formation. The difference i n s e n s i t i v i t y to hydrolysis between the C-Cbonds formed by "peroxide curing" and the ester bonds made i t possible to test the suggested scheme. n-C^H^ was used as a mo del substance. The results from GPC-analysis are shown i n Fig. 6. Hydrolysis by KOH i n isopropanol at 50 C for 1 week of the mate r i a l heated at 123 C show that ester formation i s the only mole cular enlargement reaction operating at this temperature. At 180 C, on the other hand, i t i s evident that e s t e r i f i c a t i o n i s only partly responsible for the molecular enlargement, as there are significant amounts of enlarged material not affected by the KOH treatment. The loss of low MW material for the hydrolyzed samples i s due to i t s s o l u b i l i t y i n the isopropanol. 2



Figure 6.

Normalized GPC chromatograms for n-C H samples heated in air at 123° and 180°C before and after hydrolysis H

90



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There may be several reasons for the disappearance of the "peroxide curing" below 150 °C. One important factor i s the i n creasing a v a i l a b i l i t y of oxygen with decreasing temperature and reaction rates. This should make i t more d i f f i c u l t for two a l k y l radicals to survive long enough to undergo reaction (F). Instead they w i l l more probably form peroxy radicals according to reac tion (A) and end up i n alkoxy radicals according to reaction (G) or hydroperoxides, reaction (B). Another important factor may be that the probability for the formation of a pair of a l k y l r a d i cals w i l l decrease heavily because of the fact that reaction (C) becomes less favoured for the transformation of hydroperoxides, as recently emphasized by Hiatt (28) and Benson (29). Instead, assisted decomposition by oxygen containing structures or radical attack on the hydroperoxides are more l i k e l y to occur. Influence of additives on the degradation course Antioxidants. A number of AO-systems have been investigated at different temperature levels concerning their influence on the degradation course of LDPE, Table I. In a l l cases 0.1 % of each AO was used. Table I Antioxidant Systems Used Compound Temperature °C 2,4-tert.-butyl-6-methyl-phenol(ΒΗΤ)

105

123

150

180

χ

χ

χ

χ

X

X

1

4,4 -thiobis(3-methyl-6-tert.butylphenol), (Santonox R)

χ

1,1,3-tri(2-methyl-4-hydroxy-5-tert.butyl-phenyl) butane (Topanol CA)

χ

X

χ

X

X

X

χ

X

X

X

ΒΗΤ + DLTP

X

X

X

Topanol CA + DLTP

X

X

X

Irganox 1010 + DLTP

X

X

X

1

1

tetrakis [methylene-3(3 ,5 -di-tert.buty1-4 -hydroxylphenyl) prop ionate] methane, (Irganox 1010) 1

di-lauryl-di-thio-propionate (DLTP)

These AO-systems showed, as expected, considerable differen ces i n their s t a b i l i z i n g efficiency, but none of them showed any marked influence on either the rate or the course of degradation after the induction period (5). This result was somewhat surpris ing, as we observed marked effects on the degradation course by an unidentified AO-system i n an HDPE (Marlex 6001) at 150 and 160 °C previously (2).


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LD- and

59

HD-Polyethylene

Titanium Dioxide Pigments. The influence of a l l commonly used types of T i 0 pigments was tested at 150 °C, Table I I (_3) with 40 % loading, corresponding to a master-batch composition. 2

Table I I T i 0 Pigments Used Type Modified with 2

Designation

TiO content % Kronos rutile 99 1 RN45* A l coating 95 RN47I A l , S i coating, Zn 89 A l , S i , organic coating, Zn 92.5 RNCX A l , S i , s i l i c o n e , organic coating92.5 RNCX-P II If II II II ^2 5 CL220 Finntitan R|U Zn 98 anatase 99 Kronos AV A l , S i coating 96 1 T i 0 produced by Kronos Titan A/S, Norway 2 Kemira Oy, Finland The uncoated pigments promoted degradation while the coated q u a l i t i e s exerted a s t a b i l i z i n g action, increasing with the degree and complexity of the coating. Uncoated Ti0 -pigments also caused a pronounced yellowing of LDPE containing phenolic AO, already during the mixing operation. No effects on the degrada tion course were observed, however.


D 5

X

11

2

11

11

11

2

Carbon Black Pigments. A series of CB-pigments, Table I I I were also studied at 2 % load (7). They include the types re commended by the suppliers as UV-shielding pigments for polyolef i n s . The heating temperature was 105 C. Table I I I Carbon Blacks Used DBP absorp Av. Particle"*" Surface Volation Size Ajea tiles wt % Â m /g ml/100g 21 1.0 100 950 Flamruss 101" 102 1.0 120 Corax Ρ 87 190 -115 2.2 115 Printex 603 87 210 7.8 Printex V 110 83 120 250 6.6 Monarch 700^ 200 78 127 1.4 210 800^ 74 75 88θ1 8.4 74 115 220 900, 7.8 230 75 69 1100* 7.3 240 65 70 1 Values given by the producer as typical for the product. 2 Determined by heating at 950 C for 10 min. under nitrogen a Perkin-Elmer TGS-2 thermogravimetric equipment. 3 CB produced by Degussa, Frankfurt, W. Germany. 4 CB produced by Cabot Corporation, Boston, Mass., USA. Designation

Nigrometer Index


2

PH

7 9 9 4

in

1



60

At the concentration used, the different types of CB showed large differences i n s t a b i l i z i n g effect (_7 ). The extremes being Flamruss 101 at the low e f f i c i e n c y end and Printex V and Monarch 1100 on the other. Besides t h i s , CB also influences the degradation course. In the i n i t i a l stages after the induction period, no effects are ob served. At this stage, as i s the case for the undegraded material, the CB can be f i l t e r e d off from a 135 °C TCB-solution of the sample, without any detectable loss of PE. Later on the CBs cause the main part of the samples to be insoluble, however, presumably by forming a "carbon g e l " (30). This i s supported by the fact that i t was not possible to extract any further material from the "gel" by increasing the temperature of the TCB to 180 C and keep i t for 5 h or to treat the "gel" with KOH i n isopropanol which otherwise completely hydrolyze LDPE crosslinked by oxidation i n this temperature range, as discussed above. The presence of "car bon gel" also influences the mechanical properties of the degraded samples, making them considerably less b r i t t l e than comparable samples of LDPE free from additives or containing AOs or ΊίΟ^ pigments. Conclusions LDPE, HDPE and n - C H follow the same course of thermofrom additives and preoxidative degradation sent i n the l i q u i d state. The same course i s also followed by LDPE compositions containing several different AO-systems and T i 0 pigments. The CB-pigments tested caused the formation of significant amounts of insoluble and non-hydrolyzable material ("carbon gel"), however, which markedly influenced the structural changes and the mechanical properties of the degraded LDPE. Two large discontinuities i n the relation between degrada tion course and temperature were found. One at 150 C and the other at Τ for the PE. Extrapolation of accelerated testing results obtained at temperatures above Τ to predict service l i f e times under commonly employed conditions for PE therefore seems highly inaccurate. This i s also i n accordance with the findings of for example Howard and coworkers at B e l l (31), who have reached their conclusions on quite other evidence, however. A/

Qn

2

Acknowledgement Financial support from the Swedish Board for Technical Deve lopment i s gratefully acknowledged. The author also wish to thank Dr. E. Sorvik for the close co-operation and encouragement during the work.


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LD- and HD-Polyethylene

61


Literature Cited 1. See e.g. Reich L. and Stivala S.S. "Autoxidation of Hydrocarbons and Polyolefins" Marcel Dekker Inc., New York, 1969 and Hawkins W.L. (Ed.), "Polymer Stabilization", Wiley-Interscience, 1972. 2. Holmström A. and Sörvik E.M., J. Polym. Sci. Polym. Chem. Ed., In press. 3. Holmström Α., Andersson A. and Sörvik E.M. Europ. Polym. J . , (1977) 13 483. 4. Holmström A. and Sörvik E.M., Polym. Eng. Sci., (1977) 17 700. 5. Söderqvist G. and Ulfenborg B., Diploma works, Chalmers University of Technology, Gothenburg, Sweden (1976) and (1977). 6. Gustafsson I., Diploma work, Åbo Akademi, Åbo, Finland (1977) 7. Holmström Α., Unpublished results. 8. Holmström A. and Sörvik E.M., J. Polym. Sci. Polym. Symp. Ed. (1976) 57 33. 9. Holmström A. and Sörvik E.M., J. Chromatog., (1970) 53 95. 10. Holmström A. and Sörvik E.M., J. Appl. Polym. Sci., (1974) 18 761. 11. Drott E.E. and Mendelson R.A., J. Polym. Sci., A-2, (1970) 8 1361 and 1373. 12. Ke B., J. Polym. Sci., (1960) 42 15. 13. See e.g. Beachell H.C. and Tarbet G.W., J. Polym. Sci., (1960) 45 451. 14. Luongo J.P., J. Polym. Sci., (1960) 42 139. 15. Adams J.H., J. Polym. Sci., A-l., (1970) 8 1077 and 1269. 16. Melzer T.H. and Goldey R.N., SPE Trans., (1962) 2 (1) 11. 17. Melzer T.H. and Supnik R.S., J. Appl. Polym. Sci., (1964) 8 89. 18. Grafmüller F. and Husemann E., Makromol. Chem., (1960) 40 161 and 172. 19. Kavafian G., J. Polym. Sci., (1957) 24 499. 20. Hawkins W.L., Matreyek W. and Winslow F.H., J. Polym. Sci., (1959) 41 1. 21. Ballard D.G.H. and Dawkins J.V., Europ. Polym. J., (1974) 10 829. 22. Keller Α., Martuscelli E., Priest D.J. and Udagawa Υ., J. Polym. Sci., A-2, (1971) 9 1807. 23. Winslow F.H., Hellmann M.Y., Matreyek W. and Salovey R., Am. Chem. Soc. Polym Prep., (1967) 8 (1) 131. 24. Winslow F.H., Aloisio C.J., Hawkins W.L., Matreyek W. and Matsuoka S., Chem. Ind., (1963) 1465. 25. Winslow F.H., Aloisio C.J. and Hawkins W.L. Am. Chem. Soc. Polym. Prep., (1963) 4 (2) 706. 26. Winslow F.H. and Matreyek W., Am. Chem. Soc. Polym. Prep. (1966) 7 (2) 540. 27. Winslow F.H., Hellmann M.Y., Matreyek W. and Salovey R. Am. Chem. Soc. Polym. Prep., (1967) 8 (1) 131.


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28. Hiatt R. in Swern D. (Ed.) "Organic Peroxides Vol. 2" p.l., Wiley-Interscience, New York, 1971. 29. Benson S.W. and Shaw R. in Swern D. (Ed.) "Organic Peroxides Vol. 1". p. 105. Wiley-Interscience. New York, 1970. 30. See e.g. Donnet J.-B. and Voet A. "Carbon Black. Physics, Chemistry and Elastomer Reinforcement", p. 276, Marcel Dekker Inc., New York and Basel, 1976. 31. Howard J.B. and Gilroy H.M., Polym. Eng. Sci., (1975) 15 268. December

8, 1978.


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The Course of Thermooxidative Degradation of LD- and HD

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