9 Thermooxidative and Photooxidative Aging
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of Polypropylene; Separation of HeptaneSoluble and -Insoluble Fractions H . P . F R A N K and H . L E H N E R Chemie Linz A G , A-4021 Linz, Austria
Unstabilized PP was exposed to thermooxidative (150°C) and photooxidative (Xenotest 70°C) degradation. At various stages of oxidative degradation the material was sepa rated into heptane-insoluble and -soluble fractions; in the undegraded sample (before oxidation) the insoluble fraction is essentially isotactic whereas the soluble fraction is mainl atactic. The progress of oxidation in the fractions was followed by intrinsic viscosity measurements, GPC, ir, and DSC. The heptane-soluble fraction increases gradually during oxidation (from 7.0 to 46.4 wt % under extreme conditions) because of increasing formation of very low molecular weight heptane-soluble isotactic material. In the early stages of oxidative degradation there appear to be 5 to 10 times more C-C bond scissions in the soluble fraction than in the insoluble fraction. Tphermooxidative and photooxidative degradation of polypropylene (PP) -*· has been investigated intensively i n recent years (1,2). However, structural and morphological factors (tacticity, crystallinity) were taken into account comparatively rarely (3,4). I n this study unstabilized P P was degraded oxidatively, and at each stage the total P P sample was separated into heptane-soluble and -insoluble fractions to determine the progress of oxidative degradation in each fraction. Experimental Material. P P powder without additives (Petrochemie Schwechat G m b H ) was compression molded under m i l d conditions into thin sheets 0-8412-0381-4/78/33-169-109$05.00/l
© 1978 American Chemical Society
110
STABILIZATION
Table I.
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0
3.15
Tl T2 T3 T4
1 2 3 10
2.69 2.59 1.39 0.64
XI X2 X3 X4 X5
50 73 109 142 242
2.32 2.09 1.89 1.35 0.70
O F
P O L Y M E R S
Properties of Total Samples"
Oven TXenotest X(100 ml/g) Sample (hr) M hi 0
A N D D E G R A D A T I O N
cSoluble Fraction (wt %) 7
w
X
W
3
X
M
a
484
W
3
M /M w
n
68
7.1
7.0
6.9
7.7 8.0 13.5 46.4
6.3
8.3 8.5 9.4 13.5 24.2
— —
8.5
58 —
—
—
—
—
—
11
70
° T- and X-degradation series.
(0.3-0.4 m m ) . F o r properties of reference sheet 0 see Table I. Thermooxidation. Forced air circulation oven ( 1 5 0 ° C ) . Photooxidation. Xenotest 450 L F , original Hanau (black panel temp. 70°C). Heptane extraction. P P material was dissolved i n boiling xylene, was precipitated by adding excess methanol, and was extracted with n-heptane ( 5 ) . Intrinsic viscosity [ 9 7 ] , Decalin 135 °C. Gel permeation chromatography ( G P C ) . Waters Associates Inc., model 200, 1,2,4-trichlorobenzene 135°C. Infrared spectrometry (ir). P e r k i n - E l m e r 125; 30-80 μια molded P P films.
Ε Sample
hi
Μ
τ
X
W
3
M
Table II.
Properties
X
MJM
Fractions* w
X
W
3
M
n
W
3
a
7.2
0
0.58
50
65
Tl T2 T3 T4
0.45 0.43 0.32 0.22
37 35 25 16
(42) (39) (26) 16
(7.5) (7.5) (6.5) 6
(5.6) (5.3) (4.0) 2.7
XI X2 X3 X4 X5
0.43 0.40 0.34 0.26 0.17
35 32 26 20 12
(39) (35) (27) (20) 11
(7.5) (7) (6.5) (6) 5
(5.3) (5.0) (4.2) (3.3) 2.2
0 b
Data in brackets obtained by linear interpolation. E - C7-soluble extract.
9
9.
F R A N K
Thermooxidative and Photooxidative Aging 111
A N D L E H N E R
Differential scanning calorimetry (DSC). Dupont 900 thermal analyzer; heating to 200°C, cooling to 70 °C; second heating curve re corded (heating rate 10°C/min).
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Results The results of thermooxidative (oven, T-series) and photooxidative (Xenotest, X-series) degradation are summarized i n Tables I and II ( M was calculated from [η] data (6) ). The CO-bands ( 1700-1800 cm" ) of the ir spectra, using the 974 cm" absorption as a reference for film thickness, are shown i n Figure 1 for some highly oxidized fractions; absorption maxima and shoulders are observed i n a l l cases i n the range of 1700-1710 cm" (acids), 1710-1730 cm" (ketones), 1730-1740 c m " (aldehydes), 1740-1750 cm" (esters), ca. 1760 cm" (peresters), ca. 1780 cm" (γ-lactones) ( 7 , 8 ) . In view of the CO-band's complexity, no evaluation was attempted. (The Ε fractions are very difficult to manipulate, i.e., the intensity of the C O - b a n d is very sensitive to remolding of the films, etc.) However, the degree of oxidation of the Ε fractions is much higher than that of the corresponding R fractions. The D S C tracings of R fractions show a single melting peak ( 1 5 7 ° 165°C) whereas the Ε fractions display double peaks ( 1 4 8 ° - 1 5 4 ° C , 1 3 5 ° 1 4 4 ° C ) . The results are summarized i n Table III. T o clarify the nature of the double melting peak at least i n the u n degraded Ε fraction (sample 0 ) , P P was separated further on the basis of solubility i n heptane at 2 0 ° C : Fraction O / E / I soluble C / 2 0 ° C : 63 wt % , [ ] _ 0.76; Fraction O/E/II insoluble C / 2 0 ° C : 37 wt % , [η] — 0.30, D S C tracings for the original O / E fraction and for O / E / I and O/E/II v
1
1
1
1
1
1
1
1
7
η
7
of Fractions" R hi
9
¥
(
χ
io
3
Fractions
0
Iw X
io-
3
M
n
X IO'
3
MJM
n
3.31
455
508
115
4.4
3.87 2.78 1.56 1.00
380 360 180 100
(407) (385) (193) 102
(92) (87) (44) 26
(4.4) (4.4) (4.4) 3.9
2.49 2.24 2.05 1.52 0.87
320 280 250 170 84
(343) (300) (268) (182) 90
(78) (68) (61) (41) 19
(4.4) (4.4) (4.4) (4.4) 4.8
R- and C7-insoluble residue (T- and X-degradation series).
112
STABILIZATION
A N D D E G R A D A T I O N
O F
P O L Y M E R S
are shown i n Figure 2 (AH for O / E / I — 2.7 cal/g, for O/E/II = 19.1 cal/g). F r o m this data it is evident that only Fraction O / E / I is essen tially amorphous and atactic, but not O/E/II, even though it does con tain a very small and broad peak at ca. 105 °C with unknown significance. (O/E/II was thought to represent stereoblock P P , crystallizing i n triclinic y-modification ( 9 ) ; an x-ray powder diffraction pattern produced reflections corresponding to the following d spacings ( A ) : 6.32 vs, 5.21 s, Stabilization and Degradation of Polymers Downloaded from pubs.acs.org by NATL UNIV OF SINGAPORE on 05/06/18. For personal use only.
F
Figure 1. Carbonyl band (ir spectra) of highly oxidized fractions. Film thickness factor { = 1.0 (for X5/R and T4IR), 0.4 (for X5/E), 0.7 (for T4/E).
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9. F R A N K A N D L E H N E R
Figure 2.
Thermooxidative and Photooxidative Aging 113
DSC tracings for Fractions OIE, O/E/I, and O/E/II
114
STABILIZATION
A N DD E G R A D A T I O N
O F
P O L Y M E R S
4.79 s, 4.14 s, 4.02 vs; this agrees well with the normal monoclinic a-modi fication, and any indication of y-modification (4.42 vs) is conspicuously absent ( 9 , 1 0 , 1 1 ) ) . Discussion The C -soluble P P fraction obtained b y the conventional extraction method contains various components which have only the solubility i n heptane i n common—noncrystalline atactic P P , very low molecular weight crystalline isotactic P P (species with molecular weight below about 10,000), and perhaps low molecular weight stereoblock P P (although its presence could not be demonstrated here). A t increasing degrees of degradation the C -soluble fraction rises slightly at first (up to 13.5% at T3, T 4 ) , but then very dramatically (T4, X 5 ) owing to accumulation of soluble, low molecular weight, isotactic
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7
7
Table I V . Sample
Ε
C - C Bond Scissions"
Fractions
R Fractions
Kb*
Tl T2 T3 T4
0.5 0.5 0.9 1.2
0.05 O.06 0.29 0.63
10 8.3 3.1 1.9
XI X2 X3 X4 X5
0.5 0.7 1.0 1.1 1.9
0.09 0.13 0.16 0.33 0.92
5.6 5.4 6.2 3.3 2.1
° Τ- and X-degradation series; Ε and R fractions. a
f = IQQQ Q _ Q (number of scissions per 1000 C - C bonds)
P P . Evaluation of the number of C - C bond scissions (from M data) i n the C -soluble ( E ) and C -insoluble ( R ) fractions shows a predominance i n the soluble Ε fractions (Table I V : T l , T 2 ; X I , X 2 , X 3 ) that contain mainly atactic P P . W i t h increasing degradation, the data no longer com ply w i t h the original conditions because of the greatly increasing soluble, low molecular weight, isotactic species (T3, T4; X 4 , X 5 ) . The extent of oxidative bond scissions i n the insoluble R fractions ( i n analogy to polyethylene data (12)) may be concentrated i n the non crystalline isotactic P P chain segments (total initial crystallinity is about 50%, but increases presumably owing to secondary crystallization, par ticularly during thermooxidative degradation at 1 5 0 ° C ) . n
7
7
9.
FRANK AND LEHNER
Thermooxidative and Photooxidative Aging
115
A continuation of this study w i l l include stabilized P P which presents additional difficulties i n view of the nonuniform distribution of stabilizing additives (13,14).
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Literature Cited 1. Hawkins, W. L., "Polymer Stablization," Wiley-Interscience, New York, 1972. 2. Ranby, B., Rabek, J. F., "Photodegradation, Photooxidation, and Photostabilization of Polymers," Wiley-Interscience, New York, 1975. 3. Dulog, L., Radlmann, E., Kern, W., Makromol. Chem. (1963) 60, 1. 4. Kato, Y., Carlsson, D. J., Wiles, D. M., J. Appl. Polym. Sci. (1969) 13, 1447. 5. Fuchs, O., Makromol. Chem. (1962) 58, 247. 6. Chiang, R., J. Polym. Sci. (1958) 28, 235. 7. Carlsson, D. J., Wiles, D. M., Macromolecules (1969) 2, 587. 8. Adams, J. H., J. Polym. Sci.,A1(1970) 8, 1077. 9. Jones, A. T., Aizlewood, J. M., Beckett, D. R., Makromol. Chem. (1964) 75, 134. 10. Kardos, J. L., Christiansen, A. W., Baer, E., J. Polym. A2 (1966) 4, 777. 11. Samuels, R. J., J. Polym. Sci., A2 (1975) 13, 1417.
12. Hawkins, W. L., Matreyek, W., Winslow, F. H., J. Polym. Sci. (1959) 41, 1.
13. Frank, H. P., Lehner, H., J. Polym. Sci., C (1970) 31, 193. 14. Billingham, N.C.,Calvert, P. D., Prentice, P., Ryan, T. G., Am. Chem. Soc., Div. Polym. Chem., Preprint 18 (1), 476 (New Orleans, March, 1977).
RECEIVED May 12, 1977.
