Radiation Effects on Polymers - American Chemical Society

Chapter 26

Stabilization of Polyolefins to Gamma Irradiation Role of the Initial Radicals 1

2

Downloaded by EAST CAROLINA UNIV on January 22, 2016 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch026

D. J. Carlsson, S. Chmela , and J. Lacoste

Division of Chemistry, National Research Council, Ottawa, Ontario K1A 0R9, Canada

The initial, atypical peroxyl radicals which start the free-radical chain oxidations have been identified in γ-irradiated polyolefins. These radicals were quantified in total by electron spin resonance and trapped by reaction with nitric oxide at -78°C to give nitrates which could be individually identified by infrared spectroscopy. Loss in physical properties was correlated with the accumulation of oxidation products formed during post-irradiation storage for polypropylene containing several stabilizer combinations. The best retention of impact strength corresponded to the minimum formation of oxidation products although none of the stabilizer combinations was able to completely suppress the oxidation processes.

The exposure of polyolefins to high energy radiations (γ, X-rays, electron beams, etc.) causes the formation of various free radicals (1,2). Some of these radicals combine with oxygen dissolved in the polymers to initiate chain reactions and form thermally unstable products. Degradation of polymers exposed to γ-rays results both from the direct radiation effects (C-C scission caused directly by radiation reactions) and from the much slower oxidative chain scissions (5). These continue after the end of irradiation, during the subsequent storage of the polymers. The oxidative process will involve peroxyl radicals derived directly from the radiation reactions (reaction 1) and also from the attack of these initial peroxyl radicals on the polymer (reactions 2 and 3). The peroxyl species in reaction 3 are "normal" in that their structure is determined by the selectivity of peroxyl radicals, as in any oxidation process (UV,

1 2

Current address: Polymer Institute, Slovak Academy of Science, Bratislava, Czechoslovakia Current address: Université' Blaise-Pascal, Clermont-Ferrand, France 0097-6156/91/0475-0432$06.00/0 Published 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, Roger L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

26. CARLSSON ET AL.

Stabilization of Polyolefins to Gamma Irradiation

thermal, mechano, or radiation initiated). The peroxyl r0 -fromreaction 1 is atypical in that its structure is determined by the cascade of post-irradiation events although its selectivity in reaction 2 will be identical to that of Ρ0 ·fromreaction 3 (4). Both types of hydroperoxide (rOOH and POOH) are believed to slowly breakdown at room temprature to initiate the post-irradiation degradation(2). 2

2

0

γ

2


Polyolefin —

r-

•

ιΌ · + Polyolefin (PH) P- + 0 2

• •

2

Ρ0 · + PH . 2

ιΌ ·, Ρ0 · 2

2

RT rOOH, POOH

•

•

ΓΌ ·

(1

2

rOOH + P-

PO. 2

POOH + Ρ

(2 (3a 0

2

• Ρ0 ·

(3b

2

Some chain scission products

(4

free radicals, rOH, ΡΟΗ

(5

Prevention of radiation induced oxidation will depend on the reduction of radical formation and/or the scavenging offree-radicalintermediates (r-, ιΌ ·, Ρ·, P0 ). An important necessity for medical equipment which is to be γ-sterilized is the complete absence of real or perceived contamination. This effectively eliminates all colorforming antioxidants such as phenols (yellow-brown products) or aromatic amines (red-brown products)fromplastic medical ware (5). Hindered amines such as these based on 2,2,6,6-tetramethylpiperidine operate as antioxidants at ambient temperatures in light-stabilization packages. Furthermore they and their products are colorless or only very weakly absorbing. These aliphatic amines have been previously shown to function as stabilizers to post-y-irradiation oxidation (2,6). Highly efficient stabilizer combinations might possibly suppress oxidation to the point where the atypical product(s) (rOOH) dominate. Post-irradiation oxidation is largely dependent upon initiation by the slow thermal decomposition of hydroperoxides (rOOH and/or POOH) (2,6). Hydroperoxide decomposition by an additive to give harmless products will also prevent this oxidative degradation. γ-Initiation of polymer oxidation is superficially a very clean method for studying polymer degradation mechanisms. The possible dominance of atypical species (r-, r0 -, rOOH) questions the universal nature of the resultsfromthis type of degradation and their application to other types such as those initiated by photo and thermal processes. In this study we have identified and quantified several of the key intermediates in γ-irradiated polyolefins, examined the effects of various stabilizers on the product yields and compared yields of productsfromefficiently stabilized systems with these of the initial, atypical products. 2

2

2

Experimental Polyethylene (high density, Union Carbide, 25 μπι) and isotactic polypropylene (Himont Profax, 25 μπι) were acetone extracted to remove antioxidants. Atactic In Radiation Effects on Polymers; Clough, Roger L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

433


434

RADIATION EFFECTS ON POLYMERS

polypropylene films (Hercules experimental resin, zero crystallinity and helical content) were castfromtoluene. Stabilized polypropylene films were prepared by melt compounding and hot pressing or melt extrusion. Samples were γ-irradiated in an AECL Gammacell 220, (dose rate ~1.0 Mrad/h). Products were measured by electron spin resonance (Vanan E4) and Fourier transform infrared (IR, Perkin Elmer 1500) spectroscopies. Peroxyl radicals were preserved at low temperature then reacted with gaseous NO for 2-3 hours at -78°C to give the corresponding nitrates which could be identified by IR. Hydroperoxides were observed as their corresponding nitrates after prolonged (>100h) reaction with NO at -20°C (7). Ketonic species were estimated by IR after reaction of all-OH species in oxidized films with SF (7). Impact properties of melt extruded plaques were measured by drop-weight impact, after exposure to the accelerated test conditions of 60°C storage in air. Stabilizers studied included bis 4-(2,2,6,6-tetramethylpiperidyl) decandioate (HALS-I, Ciba Geigy), bis(4-methylphenyl)carbinol (benzhydrol-I, Milliken XA-100), 2-hydroxy-4-octoxybenzophenone (2ΗΟφΟΟφ, American Cyanamid) and an alkane oil (oil-I, Witco 300) (6). 4

Results and Discussion Structure of the Initial Peroxyl Radical. The initial peroxyl radicals (r0 -) were generated by irradation of polymers at -78°C in air. These radicals were then studied at < -60°C when they neither propagate nor terminate in polyethylene and polypropylene under air or vacuum (5). The total peroxyl yield was measured by electron spin resonance (esr) together with the yield of any surviving macro alkyl radicals. However e.s.r. can give no information on the precise structure of the peroxyl radical in the solid state. More information in the structure of the initial peroxyl radicals may be obtained if they can be trapped to give products which can be easily identified. At Dry Ice temperature (-78°C), peroxyl radicals were found to react smoothly and quantitatively with gaseous nitric oxide to give organo-nitrates (reaction 6). After the complete removal of all oxygen, e.s.tf. showed that the peroxyl signal was lost within about 5 h. upon exposure to ~2(X) ton* of NO at -70°C. We have previously shown that organo-nitrates have sharp, intense IR absorptions which can be used to 2

r0 2

f rON0

]

2

(6 differentiate and quantify primary, secondary and tertiary nitrates (7). Sec.-nitrates formed from sec-hydroperoxides absorb at 1276 cm", tert.-nitrates absorb at 1294 cm' whereas primary nitrates absorb at 1276 cm" and -1641 cm" (Table I). The syn nitrate absorption at -1630 cm" is very intense and sensitive to nitrate structure but unfortunately sensitive to the polarity of its immediate environment (Table I). The IR spectrafromi-PP and HDPEfilmsafter γ-irradiation at -78°C in air followed by NO exposure at -78°C are shown in Figure 1. From Figure 1, negligible 1

1

1

1

1


26. CARLSSON ET AL.

Stabilization of Polyolefins to Gamma Irradiation 435 1

amounts of carbonyl species (at -1715 cm*) were detectable, consistent with the complete absence of peroxyl self reactions. For HDPE, the IR changes are consistent with the presence of sec. nitrate (from sec. peroxyl), whereas PP shows the presence of some tert. nitrate together with the dominant sec. nitrate. The twin peaks at 1631 and 1623 cm' in PP may imply the presence of peroxyl radicals in two different environments. With the reasonable assumption that the radicals formed at £-60°C are similar to these produced at ambient temperatures, the NO reaction leads to the initial peroxyl yields shown in Table Π. The peroxyl yields from polypropylene are markedly differentfromthe primaty: secondary: tertiary yields predicted from the selectivity of the oxidative propagation reactions (reaction 3) (primary: secondary: tertiary CH attack -1:20:200 for liquid model hydrocarbons) (4). Product yields from


1

Table I

Model Nitrate IR Absorption Maxima

Nitrate Structure

Absorption

Primary CH (CH ) -ONO CH -CH -CH-CH -CH 3

2

3

2

10

1641 1635

2

2

3

1

Maximum (cm' )

1278 1278

850 860

Environment

Hexane* f CH CN Hexane â

3

b

CH -0N0 2

2

Secondary CH (CH ) CH(-ON0 )CH CH CH.CCH^CHC-ONO^CH, CH CH(CT )CH(-ON0 )-CH(CH )CH CH CH(CH )CH -CH(-0N0 ) 2

2

4

2

3

2

3

3

2

3

] 1633

3

3

2

3

3

Tertiary CH -C(CH ) -CH -C(CH ) -0N0 CH -CH -C(CH )-CH -CH 3

2

2

2

3

1276

/

3

2

2

2

2

Hexane* Hexane C H C N * , or Tornac 3

c

2

2

3

867-860

b

I1623

CH -CH(CH )CH -CH(CH )

3

1276

3

2

1630

1294

1623

1305,1294

862

Hexane* C Hexane < CH CN

b

3

861

ΟΝΟ, 3

a Dilute solution, - 5 χ 10" mole/1 b Concentrated solution, - 2 mole/1 c Hydrogenated, random butadiene-acrylonitrile copolymer


a

3

RADIATION EFFECTS ON POLYMERS

436

atactic and isotactic polypropylene were identical within experimental error (Table II). Also shown in Table Π are macroalkyl radical yields for PP and HDPE measured after irradiation under vacuum at -196°C together with the persistent alkyl level found even in the presence of 0 for HDPE. The mixed peroxyl/alkyl radicals were quantified by recording spectra at low (0.5 mw) and high (10 mw) power levels. At 0.5 mw both radicals contribute fully to the signal, whereas at 10 mw only peroxyl radicals contribute fully and ( in our equipment) macroalkyl radicals contribute at 40% of their true level. The persistent macroalkyl signal in HDPE may comefromradicals produced in the 0 - impermeable crystalline phase. For PP, the more open (lower density) crystal is apparently 0 - permeable. The sec-nitratefromHDPE, detected as the dominant species implies the formation of only radical I (reaction 7a). The absence of peroxyl II from backbone cleavage implies that this reaction is rare or recombination of the macroalkyl pair 2


2

2

Table II Products from the v-irradiation of Polyolefins

3

Products (1Q mole/kg/Mrad) Polyolefin

a

b

[R-]

[R0 -] 2

Primary R0N0

c

2

sec.

tert.

R0N0

c

RON0

2

c 2

HDPE

2.8 ±0.5

2.1*0.5*

0

1.9 ± 0.2

i-PP

3.6 ± 0.5

3.5 ± 0.5

0.3 ± 0.1

1.7 ± 0.2

0.9 ± 0.2

a-PP

ND

3.2 ±0.5

0.4 ±0.1

1.6 ±0.2

0.7 ±0.2

a b

0

From esr, irradiation under vacuum at -196°C. From esr, irradiation under 0 or air at -78°C followed by ~15 h at -78°C under 0 or air. Identical yield when vacuum irradiated sample exposed to 0 or air for -15 h at -78°C. From FTIR, after R0 exposure to NO at -78°C. Also with (0.6 ± 0.2) χ 10" mole/kg of trapped macroalkyl radicals. 2

2

c d

2

2

3


26. CARLSSON ET AL.

Stabilization of Polyolefins to Gamma Irradiation 437

is rapid. A 1,6-migration reaction of the alkyl to give the radical precursor to III can occur, but is believed to be slow in comparison with 0 interception (8). For PP a more complex range of possibilities exist (reaction 8). The tert. nitrate probably results from IV above. The dominant sec. nitrate may come largelyfromreaction 8c as methane yields (reaction 8a) are very low from PP (9). 2

2

Û2

•

~CH -C~ + Η· 2

#

~CH -C~

(7a

2

k

(i)

ή

~CH -CH - -ΛΛΛΛ-* Downloaded by EAST CAROLINA UNIV on January 22, 2016 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch026

2

2

0, (7b

~CH * + *CH ~

.0 -CH ~

~C-(CH ) -CH Η

~Ç-(CH ) -CH H (III)

9

2

9

2

(II)

Radiation Effects on Polymers - American Chemical Society

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