Characterization of Conversion Products Formed during Degradation

24

Characterization of Conversion

Products Formed during Degradation

of Processing Antioxidants

1,2

3

4

4*

Downloaded by UNIV OF ARIZONA on July 31, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch024

J. Scheirs , Jan Pospíšil , M. J. O'Connor , and S. W. Bigger 1

Product

Technology

Section,

Kemcor

Australia

Ltd.,

228-238

Normanby

Rd., South Melbourne 3205, Australia 3

D e p a r t m e n t o f D e g r a d a t i o n a n d Stabilization o f P o l y m e r Materials,

A c a d e m y of Sciences of t h e

Czech

Republic,

H e y r o v s k y S q . 2., 162 0 6

Prague, Czech Republic 4

Department o f Environmental Management, Victoria University of

Technology,

PO

Box

14428,

Melbourne

City

Mail

Centre,

Melbourne

8001,

Australia

A range of analytical techniques was used to characterize

the

conver-

sion products of a common polyolefin processing stabilizer (Irgafos 168, a hindered

phosphite)

and a common polyolefin antioxidant

1076, a hindered phenol). Analysis by gas chromatography spectrometry

revealed

that the phosphite

phosphate under processing conditions. phosphite

can be quantified

troscopy. The applicability fluorimetry,

stabilizer

is oxidized 31

P NMR spec-

of other techniques such as IR

vestigated. The quantitation

to a

The extent of conversion of the

by using GC as well as

and supercritical

(Irganox (GC)-mass

fluid-liquid

chromatography

spectroscopy, are also in-

of Irganox 1076 in the presence of Irgafos

168 by GC is complicated because the phosphate co-elutes with Irganox 1076. The thermal degradation using both high-performance ible spectrophotometry The degradation

and by preparative

(HPLC)-UV-vis-

HPLC-NMR

of Irganox 1076 at 180 °C produced

dimeric oxidation products, at 250 °C produced

H I N D E R E D

of Irganox 1076 in air was studied by

liquid chromatography

whereas the degradation

dealkylation

PHOSPHITES

spectroscopy. cinnamate of Irganox

and 1076

products.

A N DH I N D E R E D

PHENOLS

are

w i d e l y u s e d as

sta

b i l i z e r s d u r i n g t h e p r o c e s s i n g o f p o l y o l e f i n s s u c h as p o l y e t h y l e n e ( P E ) a n d

2

Current address: ExcelPlas Australia L t d . , P O B o x 102, Moorabbin 3189, Australia.

*Corresponding author 0065-2393/96/0249-0359$12.00/0 © 1996 A m e r i c a n C h e m i c a l Society

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

360

POLYMER DURABILITY


polypropylene (PP) (1-4). Indeed, the processing of PP would be almost i m possible if processing stabilizers were not used. Phosphite processing stabiliz ers maintain the melt index and prevent polymer discoloration by decomposing hydroperoxides and by reacting with quinoidal conversion prod ucts of the hindered phenol (5, 6). A synergistic effect is obtained when hindered phosphites are used in combination with hindered phenols. Even though the precise nature of this synergism is not fully understood, aromatic phosphites preserve the concen tration of the hindered phenol during polymer processing (7). This observation led to the commercialization of the so-called "B-blends", which are mixtures of a hindered phosphite and a hindered phenol in a set ratio.

The Action of Organic Phosphites The principal reactions of hindered organic phosphites are shown in Scheme I. Phosphites decompose polymer hydroperoxides by reduction to alcohols, and in die process the phosphites are oxidized to phosphates (2, 8, 9). Phos phites may also react with • • •

peroxyl radicals to produce the phosphate and an alkoxyl radical an alkoxyl radical to produce a nonpropagating free phenoxyl radical that can, in turn, react with peroxyl radicals to terminate a chain an alkoxyl radical to produce the phosphate and a macroalkyl radical

In addition to decomposing hydroperoxides, aromatic phosphites can also • • • •

react with unsaturated (vinyl) groups in the polymer coordinate with transition metal residues help to preserve the hindered phenol prevent discoloration by reacting with quinoidal compounds

Furthermore, Schwetlick et al. (J, 8) showed that sterically hindered aryl phos phites may act as chain-terminating primary antioxidants. Hindered aromatic phosphites can function as chain-terminating antioxi dants by their interaction with alkoxyl radicals (8). A product of this reaction P(OAr) + ROOH —> 0=P(OAr) + ROH

(1)

P(OAr) + ROO

(2)

3

3

3

—> 0=P(OAr) + RO' 3

P(OAr) + RO' — » RO-P(OAr) + ArO'

(3)

P(OAr) + RO' —> 0=P(OAr) + R'

(4)

3

3

2

3

Scheme I.



24.

SCHEIRS ET AL.

Degradation of Processing Antioxidants

361

is the di-teri-butylphenoxyl radical that can react subsequently with a peroxyl radical to terminate the chain in the oxidation reaction (reaction 3 in Scheme I). Aliphatic phosphites, unlike their aromatic counterparts, are not chainbreaking antioxidants because they react with alkoxyl radicals to produce macroalkyl radicals, which in turn can react with oxygen to cause chain-propagation (8) (reaction 4 of Scheme I). At polymer processing tem peratures, the radical scavenging action of aromatic phosphites is comparable, if not better than, hindered phenolic antioxidants. However, phosphites pro vide no long-term stabilization at the lower temperatures experienced by poly mers during their service. Allen et al. (10) studied the effect of ^-irradiation on the oxidation of P P containing Irgafos 168 and found that the conversion of the phosphite to the phosphate reflects the role of Irgafos 168 in reacting with peroxyl radicals that are generated during irradiation. During the processing of high-density P E ( H D P E ) at 220 °C in the absence of a phosphite stabilizer, as much as 45% of the phenolic antioxidant is consumed after one extrusion pass (7). In con trast, the presence of a phosphite stabilizer reduces the consumption of phe nolic antioxidant to 20% after the first extrusion pass (7). Although the mechanism is not fully understood, one of the major roles of the phosphite is in the preservation of the hindered phenol. This function may be partly due to the phosphite altering the yield of specific radicals or the phosphite regen erating the phenol. In the case of H D P E , increased amounts of phosphite provide increased processing stability at a constant phenol concentration (7). Phosphites are known to form coordination complexes with metal ions, thereby changing the potential activity of the metal (11,12). In addition, some phosphite-metal complexes incorporated into polymers were shown to be ac tive stabilizers. One particular study found that in polyolefins containing high levels of catalyst residues, phosphites effectively improve the weathering re sistance of the polymer by interacting with the metal residues (13). In formulations of high-molecular-weight H D P E , where calcium or zinc stéarate (processing aid or acid scavenger) and high levels of phosphite exist, an interaction may occur between the phosphite and the free calcium or zinc ions. For instance, complex formation was observed in the form of precipitates when Irgafos 168 was added to calcium stéarate in solution (14). Furthermore, an early study on transition-metal coordination complexes found that aryl phosphates can coordinate with chromium ions (15). Phosphites are well known for preventing discoloration caused by the qui noidal conversion products of primary phenolic antioxidants (2). The phosphite disrupts the conjugated ir-electron systems of the quinone derivatives and renders the derivatives colorless. Scheme II shows the reaction of an aromatic phosphite with quinone and quinonemethide (both of which are highly col ored) to produce the colorless compounds aryl phosphate and aryl phosphonate, respectively. Phosphite stabilizers may also react with unsaturated atoms


362

POLYMER DURABILITY


ArO I ArO —P: I ArO

Ο +

ArO—d %

II

λ-0-P-OAr

(1)

ArO

phosphite

quinone (highly colored)

aryl phosphate (colorless)

phosphite

quinone methide (highly colored)

aryl phosphonate (colorless)

Scheme II. and thereby disrupt the polyene sequences in the backbone of oxidized P E to minimize coloration.

Dégradation Products of Processing Stabilizers The two basic conversion products of Irgafos 168 are the phosphate that forms as a result of oxidation and the phosphonate that forms as a result of hydrolysis (Figure 1). Both the phosphate and the phosphonate are pentavalent, and this valency renders them ineffective as processing stabilizers because the lone pair of electrons on the phosphorus is involved in the stabilization process (2). In practice, the fact that the phosphite decomposes is not of as much concern as is the extent to which it decomposes. Apparently, when Irgafos 168 is used in chromium-catalyzed H D P E , a significant amount of the phos phite is consumed in the initial compounding step (16). Factors that may be responsible for this excessive consumption are as follows: 1. 2.

The phosphite is depleted by its reaction with hydroperoxides formed when the aluminium alkyl cocatalysts are exposed to the air (17). The phosphite can complex with residual chromium sites (25), thereby reducing the effective concentration of Irgafos 168.

The oxidation of Irgafos 168 to its phosphate degradation product was stud ied previously by using the technique of multipass extrusion. This technique simulates processing and involves repeatedly passing the polymer through an extruder and collecting samples after each pass. The oxidation of Irgafos 168 by multiple extrusion was studied in various polymers including P P (18), linear



24.

SCHEIRS ET AL.


363

Figure 1. Structures of (a) Irgafos 168, (b) phosphate form of Irgafos 168 (oxidation product), and (c) phosphonate form of Irgafos 168 (hydrolysis product). low-density polyethylene ( L L D P E ) (4), and H D P E (6, 7, 12). In addition, the degradation products of Irgafos 168 resulting from 7-irradiation (JO) and elec tron-beam irradiation (19) were characterized. Although Irgafos 168 is quite resistant to hydrolysis it can be hydrolyzed under severe conditions. The hydrolysis of Irgafos 168 is shown in Scheme III. It follows a reaction sequence in which bis(2,4-di-ier£-butylphenyl)phosphonate is formed initially (reaction 1 in Scheme III) along with the free-phenol by-product 2,4-di-ferf-butylphenol (2,4-DTBP). The phos phonate formed in reaction 1 reacts further with water to produce the hydrophosphate (reaction 2). O n completion of the hydrolysis reaction (reaction 3), o-phosphorus acid is formed (20). In each reaction 2,4-DTBP is formed as the by-product. The slight antioxidant behavior of degraded Irgafos 168 at low temperature was attributed to the presence of the 2,4-DTBP by-product (21).


POLYMER DURABILITY

364

The oxidation of Irganox 1076 yields a primary quinonemethide and a cin namate, and these compounds may be transformed into the dimeric bis(quinonemethide) and the bis(cinnamate) species, respectively (22, 23). The structures of these species are shown in Figure 2. A recent publication by Klemchuk and Horng (24) discussed the various transformation products of Irganox 1076 that form as a consequence of its antioxidant activity.


Techniques Used To Study Antioxidant Degradation The experimental techniques used for the analysis of processing antioxidants are numerous and varied. Johnston and Slone (21) used Fourier transform IR (FTIR) spectroscopy to monitor the consumption of Irgafos 168 in L L D P E with progressive processing. Mass spectrometry (MS) was also used for de tecting the presence of the degradation products of Irgafos 168 (10). Phosphorus N M R spectroscopy is an ideal method for determining the extent of conversion of phosphite to phosphate because of the high degree of specificity of this technique. Allen et al. (JO) successfully used P N M R spec troscopy to study the decomposition of Irgafos 168 in P P during 7-irradiation. Klender et al. (20) also used this technique to examine the hydrolysis of organophosphorus stabilizers. Supercritical fluid extraction was used in various chromatographic studies (25-28) aimed at determining the concentration of organophosphites in polymeric materials. 3 1

Ο II

(ArO) P + H 0 — > (ArO) PH + ArOH Ο Ο

(1)

(ArO) PH + H 0 —> ArO-P-OH + ArOH Ο Ο

(2)

ArO-P-OH + H 0 —> H P-OH + ArOH

(3)

3

2

2

Il

2

II

2

Il

II

2

2

Scheme III.

CH

I R0C0CH

2

quinonemethide

cinnamate

bis-cinnamate

bis-quinonemethide

Figure 2. Transformation products of Irganox 1076.


24.

SCHEIRS ET AL.


365


The accurate quantitation of processing antioxidants in P E is an area that has gained more importance in recent years because of requirements placed on P E producers to provide their customers with certificates of analysis that verify the presence and levels of all additives in the formulation (16). The quantitation of polymer processing antioxidants such as hindered phosphites is of importance because it enables the polymer producer to determine the amount of antioxidant that is converted during polymer compounding. This chapter examines a range of techniques that can be used to characterize and quantify the commercial processing antioxidants Irganox 1076 and Irgafos 168 and their conversion products.

Materials and Methods The antioxidants were obtained from Ciba-Geigy (Australia) and were used as received. Irgafos 168 was incorporated at a level of 1300 ppm into samples of H D P E powder that had increasing degrees of unsaturation to produce the M o w i n g formulations: H D P E - 1 , H D P E - 2 , H D P E - 3 , and H D P E - 4 . The rel ative unsaturation levels in the virgin polymers were determined by the in tensity of the IR absorbance at 910 c m " , which is due to terminal vinyl groups. The samples were extruded up to five times in a twin-screw extruder at 180 °C and 60 rpm, and the concentration of phosphate was measured after each pass by gas chromatography (GC) (17). The G C analysis involved taking an accurately known mass of approxi mately 3 g of H D P E and extracting it with 20 m L of analytical-grade chlo roform. The samples were filtered and injected onto a chromatographic column. Analysis was performed by using a Hewlett Packard 3890 G C with flame ionization detection and a 10 m X 0.31 mm BP-5 capillary column with a stationary phase thickness of 0.52 μηι. The following temperature conditions were set in the chromatograph: inlet, 280 °C; oven, 280 °C; and detector, 300 °C. G C - M S analyses were performed by using a Hewlett Packard 5890 G C with a 5971 series mass selective detector. The chromatographic column was a J & W D B - 5 M S , 25 m X 0.2 mm, and stationary phase thickness of 0.33 μηι. The following conditions were set in the instrument: oven, 280 °C; inlet, 280 °C; transfer Une, 300 °C; detector, 190 °C; electron impact energy, 70 eV; and scan mode, 35-650 amu. Analysis of the samples by high-performance liquid chromatography ( H P L C ) was accomplished by using a Varian 9010 H P L C pump fitted with a Waters Resolve 12.5 cm X 5 μιτι C column connected to a Varian 9065 Polychrom diode array detector. A detection wavelength of 280 nm and a flow rate of 1 mL/min were used. The ethyl acetate-methanol mobile phase in the chromatograph was held constant at 10% ethyl acetate for 3 min and then increased linearly to 100% ethyl acetate over a 12-min period. 1

1 8


366

POLYMER DURABILITY

The P N M R experiments were performed in hexane on a Bruker M S L 300-MHz instrument. Fluorimetric studies were performed using a PerkinElmer model LS-50B spectrofluorimeter. Supercritical fluid chromatography of the various antioxidants was performed by Tony Tikuisis, Novacor Chem icals, Canada (25-27). 3 1


Results and Discussion Analysis of Irganox 1076 and Irgafos 168 by G C . During rou tine G C analyses of P E containing a 1:1 Irganox 1076-Irgafos 168 B-blend, the level of the Irganox 1076 analyte is seemingly higher than the amount originally added (16). Furthermore, the amount of Irgafos 168 is correspond ingly much lower than the amount originally added. Indeed, the relative amounts of these antioxidants remaining in the polymer as determined by G C is typically 2:1. This finding may suggest that during compounding the phos phite decomposes to form a phosphate product that co-elutes with the Irganox 1076 and causes falsely high Irganox 1076 readings (16). The phosphate peak overlaps with the Irganox 1076 peak during G C anal ysis as well as during H P L C analysis when using an RP8 column (29, 30). However, the use of an RP18 column and acetonitrile-dioxane (70:30, v/v) eluent purportedly (29) enables these compounds to be separated by H P L C . A recent publication by Nielson (31) described the use of an acetonitrilewater mobile phase that prevented Irgafos 168 from hydrolyzing and also prevented the hydrolysis products from interfering with the quantitation of Irganox 1076. Moreover, good separation can be achieved by H P L C using a newly developed method (30) that involves using an octadecylsulfonate col umn with a ethyl acetate-methanol-water (60:30:10, v/v/v) mixture as the mo bile phase. The elution order is Irgafos 168 (phosphate), Irganox 1076, and Irgafos 168 (phosphite) at 7.12, 12.2, and 13.45 min, respectively. Multipass Extrusion Experiments. Figure 3 is a plot of the phos phate concentration in H D P E as a function of the number of extruder passes at 230 °C. The plot shows that the phosphate concentration increases with increasing number of passes, which, in turn, reflects the decrease in the phos phite concentration. Two important observations are that most of the phos phite stabilizer is consumed during the first two extruder passes, and progressively less is consumed during subsequent passes; and as the initial vinyl content of the polymer becomes higher, the observed phosphite con sumption also increases. These results can be explained by the fact that in the postpolymerization stage of the polymer production process, the nascent, unstabilized polymer is exposed to air, and peroxidation of the allylic hydrogen atoms occurs. During the melt-compounding stages the hydroperoxides are reduced by the phos-


24.

SCHEIRS ET AL.


367


phosphate cone, (ppm)

no. of extruder passes at 2 3 0 ° C

Figure 3. Plot of phosphate concentration as a function of the number of extruder passes at 230 °C. phite: hence the higher the unsaturation, the greater the hydroperoxide con centration of the polymer and the higher the degree of phosphite consumption. This statement is consistent with the observation reported in the literature that the consumption of Irgafos 168 is higher when the material is processed in air compared with processing under nitrogen (6). Despite reports in the literature (12, 32) that the degree of conversion of phosphite to phosphate can be measured by FTIR, limitations of this method make it unsuitable for quantitative work. The major limitation is that the phos phate P - O stretching absorption at 968 c m is in the same region as the irans-vinylene group absorption in P E (11). The disappearance of the phosphite P - O absorption at 850 c m is indicative of complete oxidation of the phosphite. However, changes in the vinyl concentration with processing of L L D P E , which contains a partially degraded phosphite antioxidant, cannot be followed accurately by FTIR. This limitation is due to small absorbance contributions by the phosphite P - O stretch at 908 c m and the phosphate P - O stretch at 895 c m . Indeed the phosphate and phosphite absorbances produce a single peak whose intensity is a weighted average of the two ab sorbances. Johnston and Slone (21) found that only by studying samples of L L D P E in which the phosphite was completely oxidized could accurate de terminations of the vinyl group concentration be made. - 1

- 1

- 1

- 1

Analysis of Irganox 1076 by H P L C - U V - V i s Spectrophotometry. Figure 4 shows a chromatogram of a sample of Irganox 1076 that was de graded for 1 day at 180 °C in an air-circulating oven. The main product of thermal degradation at this temperature is the cinnamate, which elutes at 9.3 min and is slightly later than Irganox 1076 (8.1 min). The degradation of Irganox 1076 was also performed at 250 °C in air, and at this higher temper-


368

POLYMER DURABILITY

ature the cinnamate was not produced in appreciable quantities. Instead, both the mono- and didealkylated derivatives of Irganox 1076 were produced. These products can be readily identified by N M R spectroscopy following pre parative H P L C . At 250 °C, a retro-Friedel-Crafts reaction takes place, and isobutylene is liberated from the Irganox 1076 molecule (33). This dealkylation reaction (Scheme IV) can also occur in the presence of a Lewis acid, such as M g C l , which is a catalyst component in the polymerization system of some polyolefins (33). Therefore, it is important that polymers containing residual acid moieties


2

OH

CH

2

1

Roeocih

3.2

8.1

9.3

12.8

13.8

minutes

Figure 4. Chromatogram of conversion products of Irganox 1076. Peak identification: BET, 3.2 min, internal standard; Irganox 1076, 8.1 min; cinnamate, 9.3 min; bis(cinnamate), 12.8 min; conjugated bis(quinonemethide), 13.8 min. thermal

don

(>200°C)

Scheme TV.



24.

SCHEIRS ET AL.


369

be formulated with an aeid-acceptor such as calcium stéarate or hydrotalcite. This formulation prevents dealkylation of the phenolic antioxidants in the ad ditive package. The quinonemethide degradation product of Irganox 1076 (Figure 2) is unstable and can readily isomerize to the more stable cinnamate. Alternatively, it can dimerize to the bis(quinonemethide) (BQM), of which there are con jugated and unconjugated isomers. Similarly, the cinnamate can dimerize to the bis(cinnamate). Fortunately, all of these products are spectrally distinct (Table I) and have appreciable extinction coefficients, and therefore their de tection by UV-vis spectrophotometry is very convenient. B Q M has an inter rupted conjugated system that makes it a weaker chromophore than the strongly yellow-colored stilbene quinone of butylated hydroxytoluene (BHT). To cause similar discoloration of the polymer substrate, B Q M must be present at approximately 250 times the concentration of the B H T stilbene quinone. Analysis of Irgafos 168 by G C - M S . Figure 5 shows the mass spectra of Irgafos 168 and its phosphate degradation product. M S is a very specific technique for detecting the presence of the phosphate degradation product because an abundant ion peak occurred at m/z 316 in the mass spec trum of the phosphate (Figure 5b). This peak is totally absent in the spectrum of the phosphite (Figure 5a). The m/z 316 ion peak was also observed by Allen et al. (10) in extracts from irradiated P E containing Irgafos 168. The origin of this peak was assigned to a doubly charged molecular ion (m/z 662) minus two methyl groups. Also, an intense peak occurred at m/z 647 in the spectrum of the phosphate and was assigned to the quasimolecular ion formed by the loss of a methyl group from the molecular ion. The peak at m/z 207 i n the phosphate spectrum is due to the 2,4-di-feri-butylphenol species. The peak at m/z 57 is prominent in both the phosphite and phosphate spectra and is due to a terf-butyl fragment. This sequence suggests that some dealkylation has occurred. The m/z 91 ion peak present in both spectra is attributable to the tropylium ion. By using advanced versions of M S such as Fourier-transform cyclotronresonance M S and time-of-flight secondary-ion M S , Asamoto et al. (34) and Table I. Wavelengths of Maximum Absorbance of the Degradation Products of Irganox 1076 Degradation Product Quinonemethide Cinnamate Bis(cinnamate) Unconjugated bis(quinonemethide) Conjugated bis(quinonemethide)

(nm) 300 320 305 314 322



POLYMER DURABILITY

90irepunqi2


24.

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371


M awn et al. (35) observed an m/z 662 ion and attributed this ion to the phosphate degradation product of Irgafos 168. Analysis of Irgafos 168 by Supercritical Fluid Chromatography. Supercritical fluid chromatographic (SFC) analyses revealed that the phos phate degradation product of Irgafos 168 eluted slightly before the phosphite, whereas the free phenol hydrolysis product (2,4-DTBP) eluted 3 min earlier, near the elution time for B H T (25, 26). The similarity in elution times can be explained by the chemical similarity between 2,4-DTBP and B H T . Figure 6a shows the S F C chromatogram of an extract of H D P E film containing both Irgafos 168 and Irganox 1010. The peak at 5.5 min is the phosphate degradation product of Irgafos 168. The integrated area of the phosphate peak may be added to that of the phosphite peak so that the original concentration of Irgafos 168 can be calculated (26). Such a calculation is based on the premise that the response factor for the phosphate degradation product

(a) Oxidized Irgafos 168 START

5.970

(b) Hydrolized Irgafos 168 START

2.932

6.041

1-1010

Figure 6. Supercriticalfluidchromatograms of (a) mixture of Irganox 1010, Irgafos 168, and the phosphate degradation product of Irgafos 168; and (b) mixture of Irganox 1010, Irgafos 168, and 2,4-DTBP.


372

POLYMER DURABILITY


of Irgafos 168 is similar to that of pure Irgafos 168. In the second chromatogram (Figure 6b), the peak at 2.9 min is due to the hydrolysis by-product of Irgafos 168, namely 2,4-DTBP (25, 26). Quantitation of Irgafos 168 by S F C has been complicated by its degra dation and hydrolysis products (27, 28). For example, Raynor et al. (28) re ported a shoulder on the Irgafos 168 peak due to a degradation product of Irgafos 168 that forms during polymer processing. Similarly, Tikuisis and Cossar (27) showed that during S F C analysis of additives in H D P E , there is normally a secondary peak associated with the Irgafos 168 that is either due to degraded or hydrolyzed Irgafos 168 and also forms as a result of polymer processing. Analysis o f Irgafos 168 by P N M R Spectroscopy a n d Spectrofluorimetry. The P nuclear magnetic resonances of Irgafos 168 are shown in Figure 7. Irgafos 168 gave a discrete P N M R signal at δ = 128.3 ppm (relative to Η Ρ 0 ; δ = 0 ppm). The phosphonate signal (δ = - 0 . 1 ppm and δ = — 6.0 ppm) is split into a doublet because of the coupling of the phosphorus nucleus to the adjacent proton. The phosphate degradation product produces a single resonance at δ = —21 ppm. The presence of a weak signal at δ = - 2 1 ppm in the P N M R spectrum of "pure" Irgafos 168 standard is presumably due to some phosphate impurity. The fluorescence emission spectra of Irgafos 168 and its degradation product were recorded in hexane by using an excitation wavelength of 270 nm. Although the phosphite and phosphate are not spectrally distinct, the fluorescence quantum yield of the phosphate is much greater than that of the phosphite. This difference should, in principle, enable the quantitation of the phosphate concentration to be achieved using this technique. 3 1

3 1

3 1

3

4

3 1

phosphonate

phosphite

5

δ = 128.3

δ = -6.0

= -0.1

phosphate (impurity)

140

120

100

80

60

40

20

0

-20

ppm

Figure 7. P NMR spectrum of Irgafos 168. 3 1


24.

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373

Conclusions Both the literature and the G C - M S work demonstrate that the phosphate conversion product of Irgafos 168 co-elutes with the Irganox 1076 peak. This co-elution can result in erroneously high concentrations of Irganox 1076 ob tained during routine quantitation of P E containing a "B-blend" processing stabilizer package. Some recent H P L C methods overcome this problem by the judicious selection of mobile phases. The extent of conversion of the phosphite to phosphate can be quantified readily by using G C and P N M R spectroscopy. MS, fluorimetry, and S F C are also useful techniques for studying the degradation of Irgafos 168. During multipass extrusion of H D P E , a higher proportion of phosphite stabilizer is consumed on the first extruder pass, and as the initial vinyl content of H D P E is increased, the overall phosphite consumption during compounding is in creased. The thermal degradation of Irganox 1076 in air was studied by using both H P L C - U V - v i s spectrophotometry and preparative H P L C - N M R spectros copy. Degradation of Irganox 1076 at 180 °C produced the cinnamate and dimeric oxidation products, whereas the degradation of Irganox 1076 at 250 °C produced mainly dealkylation products.


3 1

Acknowledgments We are grateful to David Allen (Sheffield Hallum University, U K ) , Peter Klemchuk (Ciba-Geigy Corp.), Gerald Klender (Ethyl Corp.), Gilbert Ligner (Sandoz Huningue, France), Paul Roscoe (General Electric Corp.), Roberto Tedesco (Ciba-Geigy Corp., Switzerland), Tony Tikuisis (Novacor Chemicals, Canada), and the Kemcor laboratory staff. The help of Sandra Hopkins (Kemcor Ltd., Australia) during the preparation of the manuscript is also acknowl edged.

References 1. 2. 3. 4. 5. 6. 7. 8.

Schwetlick, K. Pure Appl. Chem. 1983, 55, 1629. Ray, W. C.; Isenhart, K. Polym. Eng. Sci. 1975, 15, 703. Lemmen, T.; Roscoe, P. J. Elast. Plast. 1992, 24, 115. Ligner, G.; Stoll, Κ. Η.; Wolf, R. Proceedings of the Annual Technical Conference of the Society of Plastics Engineers; Society of Plastics Engineers: Palatine, IL, 1991; p 1920. Pospisil, J. Polym. Degrad. Stab. 1993, 40, 217. Moss, S.; Zweifel, H . Polym. Degrad. Stab. 1989, 25, 217. Drake, W. O.; Pauquet, J. R.; Tedesco, R. V.; Zweifel, H . Angew. Makromol. Chem. 1990, 176/177, 215. Schwetlick, K.; Pionteck, J.; Koenig, T.; Habicher, W. D. Eur. Polym. J. 1987, 23, 383.



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RECEIVED

POLYMER DURABILITY

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ACCEPTED

revised manuscript February 21,


Characterization of Conversion Products Formed during Degradation

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