Durability of Macromolecular Materials - American Chemical Society

2 Physical Factors in Polymer Degradation and

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 13, 2016 | http://pubs.acs.org Publication Date: April 2, 1979 | doi: 10.1021/bk-1979-0095.ch002

Stabilization F. H. WINSLOW Bell Laboratories, Murray Hill, NJ 07974

As a rule polymers that contain hydrogen are vulnerable to oxidative degradation at elevated temperatures and during outdoor exposure. A few also f a i l in ozone and most natural polymers are susceptible to moisture and microorganisms as well. In order to prevent these and other forms of deterioration, nearly all polymers must be protected during processing, and especially during weathering. Chemical breakdown usually involves oxidative chain reactions that cause embrittlement of semicrystalline polymers and discoloration of poly(vinyl chloride) and polymers with aromatic groups. The reactions are complicated by the presence of transient intermediates and by rates that depend on minute concentrations of molecular defects, impurities and additives. They also depend on several important physical factors outlined in this brief overview of polyolefin degradation. Two of these factors, the transfer of excitation energy and the transport of products and protectants, play a major role in stabilization processes. Morphology and Reactivity Early studies of cellulose degradation revealed for the first time that hydrolytic agents selectively attacked the amorphous fraction (1) of the polymer, breaking and reordering accessible chain segments (2). Later work on both poly(ethylene terephthalate) (3,4) and polyethylene (5) confirmed that localized reactions were characteristic of all polymers with impervious crystalline regions. Oxidative chain scission processes in polyethylene have been described in detail (6). During the course of exhaustive oxidation, the reaction rate subsides as accessible

0-8412-0485-3/79/47-095-011$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


chains are consumed. As might be expected, ultimate oxygen uptake increases i n polyethylene as unstable c r y s t a l l i t e s melt with r i s i n g temperature, as shown i n F i g . 1. A smaller temperature dependence of cumulative oxygen uptake is observed i n rubbers and other polymers f u l l y accessible to oxygen. The consumption of oxygen also varies with morphology at a given reaction temperature. During prolonged degrada tion at 100 C, the solution - c r y s t a l l i z e d polyethylene povxuer i n F i g . 2 reacted with only half as much oxygen as the molded f i l m . Viscosity and gel permeation chromato graphic measurements demonstrated that 90% of the chain scissions occurred during an i n i t i a l oxygen uptake of 10 ml/g. Yet, the M of the f u l l y oxidized powder was s t i l l 20% of the o r i g i n a l value (6) and chain fragments with lengths equal to or double the lamellar thickness accounted for less than l u % by weight of the degraded polymer. The results correspond closely to those reported by Peterlin (7) on single crystals of linear polyethylene etched with n i t r i c acid. He concluded that the lamellae had amorphous surface layers. But unlike oxidation i n solution, the gaseous reac tion forms nonvolatile products which accumulate to form a protective layer over the lamellar surface leaving at least 90% of the chain folds intact. w

Evidently the c r y s t a l l i t e s i n poly (4-methylpent-l-ene) are permeable to oxygen. The c r y s t a l l i n e and amorphous forms of the polymer have nearly the same densities and o x i dation patterns at 100°C (see F i g . 2) and consume tenfold more oxygen than the linear polyethylene. The ultrahigh molecular weight linear polyethylene i n Figs. 3 and 4 has a large weight fraction of t i e chains l i n k i n g adjacent lamellae. When the t i e chains break during oxidation the dangling segments realign into denser struc tures largely inaccessible to oxygen. Though their i n i t i a l densities and melting points were comparable, the linear polymer i n F i g . 3 ultimately reacted with one f i f t h as much oxygen as the branched sample. The abrupt i n i t i a l r i s e i n the density of the linear polymer i n F i g . 4 resulted more from reordering of t i e chains than from an increase i n oxy gen content. The accessible f r a c t i o n , remaining after reordering was complete, was tenfold greater i n the branched polymer than i n the linear polymer. Oxidation raised the y i e l d stress, increased elongation and reduced the stress at break. Figure 5 shows that the linear polyethylene was b r i t t l e at room temperature after an

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

wiNSLOw

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400

OXIDATION T I M E

Figure 1.

(HOURS)

Oxygen consumption by linear polyethylene (M > 10 ) during pro longed reaction at various temperatures 6

w

500 ο»

400

0

100

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OXIDATION

Figure 2.

300

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T I M E !N HOURS

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600

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AT 100 C E

Exhaustive oxidation of molded (O) and solution-crystallized ( Q ) linear polyethylene and poly(4-methylpent-l-ene)


DURABILITY OF MACROMOLECULAR MATERIALS 240 200 160 120


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χ

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40 0

Figure 3.

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200

RANCI- ED

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400 600 800 1000 1200 1400 1600 TIME IN HOURS AT 100°C

Oxidation patterns of molded linear polyethylene (M > 10 ) and an ethylene-butene block copolymer 6

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Figure 4.

40 80 120 160 200 CUMULATIVE OXYGEN UPTAKE (ml/g)

240

Rise in density with oxygen uptake of the polymers in Figure 3


Polymer

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8000,

Figure 5.

Stress-strain behavior of the linear polyethylene in Figure 3. The yield stress increases with oxidation.




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Figure 6. Variation in ultimate elongation with oxygen uptake by original and remolded films of the linear polyethylene in Figure 3. Draw rate was two in./min.


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oxygen uptake of about 4 rol/g. When the sample was melted and remolded the mechanical strength was restored as Kafavian (8) had reported e a r l i e r . In fact, the remolded specimen i n F i g . 6 consumed an even greater volume of oxygen before becoming b r i t t l e once again. In the absence of any protection, b r i t t l e f a i l u r e occurs after much l e s s oxidation in linear polyethylene than i n the branched polymer. Nevertheless, i t has been reported (9) that s t a b i l i z e d linear polymers are more durable because they retain s t a b i l izers better than branched types. Antioxidant A c t i v i t y and Mobility For f u l l protection against oxidation, polyolefins require a combination of radical chain terminators, peroxide decomposers and metal deactivators. Maximum effectiveness depends on retention, a prime problem with polyolefins i n which v o l a t i l e protectants are v i r t u a l l y insoluble. During cooling from the melt, the mobile s t a b i l i z e r s concentrate i n amorphous regions where they are needed most. Unfortunately their s o l u b i l i t y i s usually so low that the more v o l a t i l e compounds migrate rapidly out of the polymer and are l e s t . As a result superior antioxidants for long-term use gen e r a l l y have molecular weights higher than 5UU. Their vola t i l i t y and d i f f u s i o n rates are low but s t i l l s u f f i c i e n t for the s t a b i l i z e r to readily reach reactive s i t e s . I t i s also possible to incorporate s t a b i l i z i n g groups into a polymer chain but the resulting bound antioxidants are inherently mere expensive and les? effective. Seme carbon blacks and other pigments act as immobile antioxidants (10). The antioxidant action of carbon black varies inversely with p a r t i c l e size and d i r e c t l y with con centration, degree of dispersion, and quantity of bound oxy gen or sulfur as indicated i n F i g . 7. The sulfurized blacks are more e f f e c t i v e , most l i k e l y because they form sulfur dioxide which i s free to migrate through the polymer decom posing hydroperoxides. On the other hand, the oxidative chain reaction and i t s v o l a t i l e products must move over an average distance of 20 nm or more to reach the surface of an oxidized carbon black p a r t i c l e and be terminated. As a result the mobile aromatic amines and phenols are more e f f i cient antioxidants than carbon blacks. Eut, unlike the v o l a t i l e types, the immobile carbon blacks are more effec tive i n s o l i d than i n molten polyethylene (see F i g . b). Carbon blacks interact s y n e r g i s t i c a l l y with some antioxidants and antagonistically with others. Ihe reduc tion in a c t i v i t y of aromatic amines as inhibitors i n the




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50

100

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TIME IN HOURS ( 140 C ) E

Figure 7. Stabilization of branched polyethylene by activated carbons (3 wt % ) with an average particle size of 20 nm. Numbers indicate oxygen contents. S corresponds to a carbon black with a sulfur content of 9%.

INDUCTION PERIOD (HRS) Figure 8. Relationship of morphology to the effectiveness of carbon black as a stabilizer for branched polyethylene: ( ), indicates the melting temperature of the polymer. Effectiveness of molecular antioxidants does not depend on morphology.



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presence of carbon has been attributed both to induced decomposition of the amine (11) and to i t s adsorption and immobilization on the carbon surface (12). The adverse action of carbon black on most aromatic amines and phenols i s i l l u s t r a t e d i n F i g . 9. Thiobisphenols are noteworthy exceptions since they form synergistic combinations with carbon which behave as radical scavengers and peroxide decomposers. Also, oxidized blacks greatly improve the retention of thiobisphenols i n polyethylene (13) without impairing their a c t i v i t y as antioxidants. Radiation Effects During oxidative degradation, a concentration gradient always develops a t a f i l m surface. Inasmuch as the depth p r o f i l e depends on permeabilities and reaction rates, the effect i s more noticeable i n photooxidations than i n thermal oxidations. An unusually marked skin effect observed i n photooxidized polypropylene has been ascribed (14) to the action of chromophores located at or near the surface. Deterioration outdoors i s i n i t i a t e d by u l t r a v i o l e t radiation of wavelengths greater than 29Unm. In addition to chemical structure and impurities, the reaction rate depends on temperature, u l t r a v i o l e t energy and f i l m thickness. I t varies more than tenfold during the year, reaching a maximum about one month after the summer s o l s t i c e i n the northern temperate zone (6). Since pure hydrocarbons only absorb at wavelengths well below 290nm, i t has been suggested (15) that various contam inates and adventitious chromophores act as photosensitizers for polyethylene weathering. Yet, absorption by the polymer above 300nm i s weak even after the extensive oxidation shown in F i g . 10. Chromophores such as carbonyl groups and Hydro peroxides formed during processing or outdoor reactions with ozone are p r a c t i c a l l y unavoidable. Besides functioning as i n i t i a t o r s , the groups are responsible for two types of chain scission reactions (16) i n polyethylene.




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Figure 9.

Reduction in effectiveness of N,N'-diphenyl-p-phenylenediamine in the presence of carbon black

200

300

HA

400

WAVELENGTH (nm) Figure 10.

UV absorbance of polyethylene before and after photooxidation


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R'CCH CH CH R

hv ,

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R A*

2.

Emission A* -> A + l i g h t or heat

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Degradation A* -> decomposition products

4.

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and return to the ground state, A, without undergoing degra dation. However, i f A* has a r e l a t i v e l y long l i f e t i m e , i t has a greater probability of either decomposing (process 3) or of transferring i t s excess energy to a deactivating molecule, D. S t a b i l i z a t i o n , of course, occurs when the rate of deactivation i s faster than the degradation process. Photostabilizers suppress the degradation rate by absorbing, quenching or screening out most of the damaging excitation energy. Surface coatings are seldom used f o r



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protecting polymers other than wood. Instead, fine aispersions of pigments such as carbon black are o r d i n a r i l y pre ferred. The blacks are highly effective. Well-dispersed p a r t i c l e s with an average size of 2Unm i n concentrations as low as 1% have prevented any s i g n i f i c a n t change i n the mechanical properties of a polyethylene exposed to weather ing i n F l o r i d a for nearly 50 years. The results i n Fig 11 imply that carbon acts primarily as a l i g h t screen since the black f i l m oxidized faster than the underlying polyethylene f i l m containing no additive. However, the carbon black altered the course of oxidation i n the polymer. After the same oxygen uptake the polymer containing carbon had fewer v i n y l groups ( i . e . , had undergone l e s s Type I I chain s c i s sion) and a s i g n i f i c a n t l y higher hydroperoxide content. Also, the d i s t r i b u t i o n of carbonyl products formai i n the black polymer resembled that found i n thermal oxidations (17). However, a thiobisphenol reduced the rate of hydro peroxide buildup i n both clear and black films and i n combi nation with carbon black showed synergistic behavior i n i n h i b i t i n g the photooxidation of polyethylene (17). In contrast, the o-hydroxybenzophenone had no notice able e f f e c t on the decomposition chemistry. Though i t has been commonly referred to as an u l t r a v i o l e t absorber, i t actually behaves more l i k e a deactivator since the screen f i l m containing i t had a lower oxidation rate i n F i g . 11 than the underlying clear f i l m devoid of additives. Absorp tion can hardly account for the protective action of the s t a b i l i z e r i n thin films which are almost completely tran sparent to 300nm radiation. Neither can the hydroxybenzophenone be an important terminator of radical chain reac tions because i t i s much l e s s e f f e c t i v e than other hindered phenols i n i n h i b i t i n g oxidation i n the dark. U n t i l recently the nickel dialkyldithiocarbamates and piperidene derivatives were regarded as deactivators of singlet oxygen and other excited species. But Scott (19) has reported that the nickel compound acts mainly as a l i g h t screen, radical chain terminator and perc

Durability of Macromolecular Materials - American Chemical Society

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