Polymer Wear and Its Control - American Chemical Society

26

Downloaded by UNIV OF NEW SOUTH WALES on September 19, 2017 | http://pubs.acs.org Publication Date: September 12, 1985 | doi: 10.1021/bk-1985-0287.ch026

Quantitation of Nonvolatile Components from the Photochemical Decomposition of Poly(ethylene terephthalate) 1

S.Krishnan ,S. B. Mitra, P. M. Russell, and G. Benz Central Research Process Technologies Laboratory, 3M Center, 3M Company, St. Paul, MN 55144

The photochemical d e g r a d a t i o n of poly(ethyleneterep h t h a l a t e ) was s t u d i e d u s i n g an analytical s p e c t r o s c o p i c procedure t o q u a n t i t a t e the water s o l u b l e n o n - v o l a t i l e products. Irradiations of f i l m samples were c a r r i e d out i n air, under ambient humidity using l i g h t i n the range of wavelengths from 280 t o 400 nm. Under these c o n d i t i o n s o n l y c h a i n s c i s s i o n s occur l e a d i n g t o the f o r m a t i o n o f water extractable phthalate residues. The extent o f photodegradation was measured as the amount of water extractable phthalate residues. This information was used t o study the e f f i c i e n c y o f an a d d i t i v e used t o retard photodegradation of poly(ethyleneterephthalate). Photochemical degradation of poly(ethyleneterephthalate) (abbr. PET) has been e x h a u s t i v e l y d e s c r i b e d by r e s e a r c h e r s at the N a t i o n a l Research Council of Canada (1-5). These reports describe the effect o f various l i g h t wavelength regions on the polymer, the v a r i a t i o n s i n the y i e l d of v o l a t i l e decomposition products and chemical changes i n both the s u r f a c e and b u l k r e s u l t i n g from the v a r i a t i o n s i n the pressure and the composition of the gaseous environment around the sample b e i n g i r r a d i a t e d . I t has a l s o been p o i n t e d out t h a t i r r a d i a t i o n s i n the presence o f a i r or oxygen do not induce c r o s s l i n k i n g i n the polymer ( ί , 6 - 8 ) . In t h i s study we observe t h a t the photochemical chain s c i s s i o n i n i r r a d i a t e d PET can be monitored by a procedure i n v o l v i n g aqueous e x t r a c t i o n o f a sample f o l l o w e d by u l t r a v i o l e t s p e c t r a l a n a l y s i s o f the e x t r a c t . Conventional u l t r a v i o l e t l i g h t absorbers i n c o r p o r a t e d i n PET f i l m s do not i n t e r f e r e i n the a n a l y s i s . We observe t h a t the amount o f water e x t r a c t a b l e m a t e r i a l i n c r e a s e s w i t h the time o f exposure t o radiation.

1

Author to whom correspondence should be directed. 0097-6156/85/0287-0389S06.00/0 © 1985 American Chemical Society

Lee; Polymer Wear and Its Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER WEAR AND ITS CONTROL

390


Experimental M a t e r i a l s . F i l m samples o f PET were o b t a i n e d from the S p e c i a l t y Films D i v i s i o n of 3M Company, St.Paul, Minnesota 551*A» U.S.A. Two samples of thicknesses 6.3 u and 12.5 u contained no a d d i t i v e s . A l l of the other samples contained varying amounts of a substituted 2,kdihydroxybenzophenone type (9»10) u l t r a v i o l e t l i g h t absorber. The d e n s i t i e s o f the f i l m samples were i n the range 1.31 t o 1.32 g / m l . R e l a t i v e v i s c o s i t i e s of 1:1 phenol:tetrachloroethylene solutions of PET were determined u s i n g a Ubbelohde v i s c o m e t e r at 2 5 + . 0 . 1 ° C . I n t r i n s i c v i s c o s i t i e s were c a l c u l a t e d using the equation of Solomon ( l l ) . Average m o l e c u l a r weights were c a l c u l a t e d u s i n g the MarkHouwink-Sakurada r e l a t i o n s h i p ( l 2 ) . I r r a d i a t i o n s . A Q U V - a c c e l e r a t e d weathering t e s t e r (Q-Panel C o . , C l e v e l a n d , Ohio) was used i n the experiments. T h i s chamber was provided with a t o t a l of eight QFS-^0 lamps set i n two arrays. The f i l m samples were mounted on aluminum p a n e l s (30 cm X 10 cm) and arranged i n the chamber so that the surface of the samples were 2.5 cm away from the s u r f a c e o f the lamps. F i g u r e 1 d e p i c t s the range of wavelengths and the r e l a t i v e energy outputs for the lamp measured at 2.5 cm away from the surface of the lamp. A constant temperature of 60°C was set i n the chamber. A l l i r r a d i a t i o n s were done at ambient humidity and the lamps were switched o f f o n l y f o r b r i e f periods while a sample was being withdrawn for analyses. Successive samples were taken out of the same l o c a t i o n i n the chamber and the remaining samples moved i n a queue t o advance through each o f the possible exposure positions to e v e n t u a l l y take up the exit s l o t for the l a s t a l i q u o t o f exposure. T h i s scheme e n a b l e d the samples t o navigate through a l l p o s s i b l e l o c a l d i f f e r e n c e s i n the r a d i a t i o n dosage within the chamber. The 50 u samples were cut i n t o 2.5 cm X 20 cm s t r i p s and two s t r i p s of d i f f e r e n t f i l m s were mounted on the same p a n e l s i d e by side. Each panel contained a s t r i p of f i l m #6 (Table I) and one of another 50 u sample. After i r r a d i a t i o n for a s p e c i f i c period of Table I . Film#

1 2 3 k 5 6

C h a r a c t e r i s t i c s o f PET F i l m s

Film Thickness

6.3 u 12À u 25.0 u 50.0 u 50.0 u 50.0 u

UV-Absorber Content(a) (w/w I ) 0.0 0.0 2.5 2.0 3.0 k.O

(a) 2-hydroxybenzophenone derivative ( r e f . 9 ,

Density (g/ml)

1.318 1.320 1.315

Molecular Weight 19,500 22,100 21,201+

10).

time, samples for analysis were cut out of the same p a n e l - l a t i t u d e on b o t h f i l m s ( F i g u r e 2). T h i s p r o c e d u r e a s s u r e d t h e same i r r a d i a t i o n temperature and dosage for a d i r e c t comparison as w e l l as enabled the comparison of a l l films normalized with respect to f i l m #6. Lee; Polymer Wear and Its Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Decomposition of Polyethylene terephthalate)


KRISHNAN ET AL.

350

WAVELENGTH

450

400

(nm)

Figure 1. I r r a d i a t i o n Source: QFS-40 Lamp.

ADHESIVE

TAPE

FILM

ΓΖΖΙ

SAMPLE FOR ANALYSIS

CZZI

Figure 2. Mounting Film Samples.



392


S p e c t r a . A P e r k i n - E l m e r H i t a c h i model 200 spectrophotometer was used to record a l l u l t r a v i o l e t spectra. The infrared spectra of the f i l m s were o b t a i n e d u s i n g a N i c o l e t FTIR-7199. The t r a n s m i s s i o n s p e c t r a o f f i l m s were o b t a i n e d f r o m s a m p l e s m o i s t e n e d i n tetrachloroethylene (2) and mounted between N a C l p l a t e s . Attennuated t o t a l r e f l e c t i o n (ATR) spectra were taken by p l a c i n g the exposed s i d e o f the f i l m i n contact w i t h a germanium c r y s t a l at a k3 a n g l e o f i n c i d e n c e . F l u o r e s c e n c e from f i l m s u r f a c e s were measured u s i n g a P e r k i n - E l m e r fluorescence spectrometer. The excitation beam (3^0 nm, s l i t , knm) was incident on the f i l m at h5° and the emission (1*00-500 nm) was measured at 90° to the excitation beam. Extraction of N o n - v o l a t i l e Decomposition Products. A sample of PET f i l m (5-10 mg) was weighed using a Cahn 25 Automatic electrobalance and placed i n a 10 ml volumetric f l a s k . Deionized water was added t o the sample and made up t o the v o l u m e t r i c mark. The f l a s k was then l o o s e l y stoppered and placed i n a support frame immersed i n the water bath o f a Bronson B321 u l t r a s o n i c a t o r . The water bath temperature was between 55 and 60°C. A f t e r h a l f an hour o f s o n i c a t i o n , the v o l u m e t r i c f l a s k was removed and e q u i l i b r a t e d t o room temperature. More d e i o n i z e d water was added ( u s u a l l y a few drops, i f needed) to the volumetric f l a s k to make up for any loss of solvent due to evaporation. The absorbance of the aqueous extract was measured at 2^2 nm by t a k i n g 2 ml o f the s o l u t i o n i n a q u a r t z c u v e t t e (path l e n g t h , 1 cm). The sample from the c u v e t t e was t r a n s f e r r e d back t o the v o l u m e t r i c f l a s k and the s o n i c a t i o n was repeated for another h a l f hour period followed by a measurement of absorbance o f the s o l u t i o n at 2^2 nm. The e x t r a c t i o n was deemed complete and exhaustive when the absorbance of the extract at 2^2 nm reached a maximum constant value. At l e a s t 3 samples were cut from different p a n e l - l a t i t u d e s and an average value of the absorbance of the e x t r a c t s was used i n c a l c u l a t i o n s o f the amount o f s o l u b l e components extracted from the sample. A preparatory extraction was performed by u s i n g 20 g o f 12.5 u PET t h a t had been i r r a d i a t e d to b r i t t l e n e s s ( c i r c a 80 hours). Chromatography. The material extracted from i r r a d i a t e d PET (12.5 u) was used i n the chromatographic analyses. Liquid chromatography was performed using a Hewlett-Packard 108^+8 instrument with a v a r i a b l e wavelength detector. The sample s o l u t i o n concentration was 1 mg/ml i n methanol. The chromatogram was o b t a i n e d by i n j e c t i n g 50 u l o f the s o l u t i o n v i a a loop i n j e c t o r on to a 10 u ES Chromegabond MC-18 (30 cm, h.6 cm, i . d . ) column. The s o l v e n t system (MeOH-water, both c o n t a i n i n g 0.2$ ( v / v ) HOAc) was programmed from 10$ MeOH t o 100? MeOH i n 30 minutes. The e l u t i o n o f peaks were r e c o r d e d by c o n t i n u o u s l y m o n i t o r i n g the absorbance at 2^-0 nm. The f l o w o f solvent was stopped at the maxima of peaks and a continuous s p e c t r a l scan of the s o l u t i o n i n the detector c e l l was recorded i n the range 200-350 nm. N o n - v o l a t i l e m a t e r i a l (ca. 5 mg) from i r r a d i a t e d PET was d e r i v a t i z e d u s i n g b o r o n t r i f l u o r i d e - m e t h a n o l (13) reagent. The


26. KRISHNAN ET AL.

Decomposition of Poly (ethylene terephthalate)

393


p r o d u c t was d i s s o l v e d i n 0.5 m l o f e t h e r and u s e d f o r gas chromatographic a n a l y s i s . A 0V-101 (5# on Chrom-GP, 2m X 2mm i.d.) column i n s t a l l e d i n a H e w l e t t - P a c k a r d 58^0 gas chromatograph i n t e r f a c e d through a g l a s s j e t s e p a r a t o r t o a duPont 21-U91B mass spectrometer was used for the a n a l y s i s . A 2 u l sample was used for each i n j e c t i o n and h e l i u m c a r r i e r gas f l o w was 20 m l / m i n . The e l u t i o n was temperature programmed from 100 t o 3 3 0 ° C at 2 0 ° C / m i n . and the e f f l u e n t was s p l i t i n two p a r t s and channeled t o "both a flame i o n i z a t i o n d e t e c t o r and the mass spectrometer. The mass spectrometer was operated i n the electron impact mode. Results The v i s c o s i t y average molecular weights of the PET samples used i n t h i s study are presented i n Table I. The u l t r a v i o l e t spectra of the u n s t a b i l i z e d f i l m samples were s i m i l a r t o those r e p o r t e d i n the l i t e r a t u r e (2,8). The samples t h a t c o n t a i n the u l t r a v i o l e t l i g h t absorber show an absorbance o f > 2 f o r wavelengths under 365 nm. U n s t a b i l i z e d f i l m s became b r i t t l e a f t e r a b o u t 80 h o u r s o f irradiation. The u l t r a v i o l e t spectrum of the aqueous extract from i r r a d i a t e d PET showed peaks at 195, 2^2 and 290 nm. In the exhaustive extraction procedure, the absorbance of the extract measured at 2k2 nm r e a c h e d a maximum v a l u e i n a b o u t 3 h o u r s ( F i g u r e 3) o f sonication. The time required for the complete extraction of water s o l u b l e decomposition products from i r r a d i a t e d PET films was found t o be the same f o r a l l f i l m t h i c k n e s s e s used i n t h i s study. The amount of water s o l u b l e decomposition products that absorb l i g h t at 2^2 nm was formed at a greater rate i n the u n s t a b i l i z e d films than i n the films containing the u l t r a v i o l e t l i g h t absorber. Extraction of c o n t r o l samples, oven aged i n the dark for the same duration as the i r r a d i a t e d f i l m s y i e l d e d s o l u t i o n s t h a t have n e g l i g i b l e absorbance at 2^2 nm. F l u o r e s c e n c e from the i r r a d i a t e d s u r f a c e o f 12.5 u PET f i l m s i n c r e a s e d w i t h exposure time. The emission spectrum c o n t a i n e d a s i n g l e peak at k60 nm (lk). For the u n s t a b i l i z e d 12.5 u PET f i l m a number of broad bands i n the 3600-2500 cm region were observed i n both t r a n s m i s s i o n and ATR i n f r a r e d s p e c t r a . F i g u r e s ^,5, and 6 depict the trends with increasing exposure periods. The v i s c o s i t y average molecular weights decreased r a p i d l y i n the i n i t i a l stages of i r r a d i a t i o n ( F i g u r e 7). L i q u i d chromatography o f the MeOH s o l u t i o n o f the m a t e r i a l i s o l a t e d from i r r a d i a t e d PET required the solvent system be spiked w i t h a c i d (HOAc) t o get c l o s e t o i d e a l peak shapes on the e l u t i o n profile. There were four major components i n the extracted mixture. The u l t r a v i o l e t spectra of a l l these compounds obtained through the "stop-scan" procedure contained peaks around 2h0 and 290 nm (Figure 8). Gas chromatographic mass s p e c t r a l analysis of the methyl esters showed three major compounds. Table II l i s t s the retention times, mass spectra of compounds and the structures assigned to the parent compound or the l a r g e s t fragment observed on the mass spectrum. Compounds e l u t i n g from the column at temperatures above 300°C y i e l d e d poor mass s p e c t r a as the t r a n s f e r l i n e s i n c l u d i n g the j e t



394

56 HR. EXPOSURE


Ο

Ο

40 HR. EXPOSURE

16 HR. EXPOSURE Λ 0

50

MINUTES OF

Τ 100

Ο

Ο

"Τ 150

Ι 200

ULTRASONICATION

Figure 3. Exhaustive Ultrasonic Extraction.

Figure 4. FT-IR Subtraction Spectra.



KRISHNAN ET AL.

Decomposition of Poly(ethylene terephthalate)

120

IRRADIATION TIME

(hours)

Figure 5. Changes i n Infrared Transmission.

Figure 6. Changes i n Attennuated Total Reflection (Infrared).



25000

Ί

20000


12.5 u PET 15000E3

ιοοοοΗ

5000 Η 0

Η

1 20

\ 40

1 60

IRRADIATION TIME

Η

g, Η 13

1 80

(hours)

1 100

Figure 7. Molecular Weight vs. Time.

240 nm

RETENTION TIME ( m i n u t e s ) Figure 8. Liquid Chromatographic Analysis.


26. KRISHNAN ET AL.


397

separator had been maintained at 300°C. The highest mass fragment observed on the mass spectrum o f the l a s t major peak on the chromatogram had an m/e value of 577.


Discussion The major n o n - v o l a t i l e materials i s o l a t e d from photooxidized PET are a c i d i c compounds. This necessitated the use of acid spiked solvent system f o r the l i q u i d chromatographic s e p a r a t i o n o f t h e mixture. A l l the major compounds separated on the l i q u i d chromatogram showed t y p i c a l phthalate u l t r a v i o l e t absorption spectra with peaks at 2^0 and 290 nm (inset, figure 8). Three major compounds are observed on the GC-MS chromatogram ( T a b l e I I ) . The l a r g e s t mass s p e c t r a l fragment (m/e=577) can be accounted f o r on the b a s i s o f a parent Table II. Gas Chromatographic Mass Spectrometry Retention Time (minutes)

Structure (Compound or Fragment)

Mass Spectrum m/e ( r e l . i n t . )

19M17) 163(100)

5.92

H C00C-C H^-C00CH 3

6

3

135(19)

103(12) 76(7) 252(3)

H C00C-C H -C00CH C00CH

577(1.5) 385(3) 359(3.5)

H C(OOC-C H -COOCH CH )^| _ H COOC-C H -COO-J

221(2) 163(100) 11+9(10)

8.21+

13 AO

3fcl(26) 221(25) 193(100)

3

3

6

1|

6

3

2

4

2

6

3

2

4

compound with a t l e a s t t h r e e aromatic n u c l e i . T h i s i s a d i r e c t evidence for B i a i s c a l c u l a t e d value of 3 repeat units per polymer c h a i n on the i r r a d i a t e d (555 hours, Xenon a r c ) s u r f a c e o f PET (5.). The s t r u c t u r e s deduced f o r the methyl e s t e r s ( T a b l e I I ) show t h a t p h o t o o x i d a t i o n o f PET r e s u l t s i n both aromatic and a l i p h a t i c carboxylic acid groups. The procedure used t o q u a n t i t a t e t h e amount o f e x t r a c t e d p h t h a l a t e i n v o l v e d an estimate o f a g r a v i m e t r i c e x t i n c t i o n coefficient for the phthalate peak around 2h0 nm i n the absorption s p e c t r a (Table I I I ) . An average e x t i n c t i o n c o e f f i c i e n t o f 106.8 ml/mg.cm was used t o convert the absorbance values into amounts o f phthalate residues extracted from 1 mg of i r r a d i a t e d f i l m (soluble fraction). 1



398 Table III.

E x t i n c t i o n C o e f f i c i e n t s of Phthalates

Compound

Abs.Max. (nm)


Terephthalic a c i d Dimethylterephthalate PET(a)

2kl 21+1 2kh

(a) Cheung, P-S.R.; Roberts, 1809.

M •''cm

x

17,500 20,630 23,880

mlmg cm

105.h 106.3 108.6

C.W. J . A p p l . Polym. S c i . 1979, 2l+,

P l o t s o f t h i s s o l u b l e f r a c t i o n from i r r a d i a t e d PET v e r s u s exposure p e r i o d s are shown i n F i g u r e s 9 and 10. The amounts o f extractable n o n - v o l a t i l e products i n i t i a l l y showed a l i n e a r increase with i r r a d i a t i o n time and then decreased t o almost a constant value at longer exposure times. The y-va lues at these plateaux (Figures 9 and 10) can be r e c o g n i z e d as i n v e r s e l y p r o p o r t i o n a l t o the f i l m thicknesses. When t h e d e n s i t i e s o f t h e s a m p l e s a r e n o t s i g n i f i c a n t l y d i f f e r e n t from one another, the same weight o f 6.3 u f i l m has about four times the surface area exposed t o r a d i a t i o n as t h e 25 u f i l m and t w i c e as t h e 12.5 u f i l m . The 25 u f i l m c o n t a i n i n g the s t a b i l i z e r undergoes d e g r a d a t i o n at a much s l o w e r rate and takes a much longer period o f exposure t o produce the same amount o f photodegradation products from the same unit surface. F l u o r e s c e n c e from the s u r f a c e o f i r r a d i a t e d PET has been ascribed t o the product from the hydroxylation o f the aromatic r i n g i n the polymer (lip. The increase i n the fluorescent emission (1+60 nm) from the i r r a d i a t e d s u r f a c e o f the 12.5 u f i l m ( F i g u r e 11 ) i s s i m i l a r to the data a v a i l a b l e from the l i t e r a t u r e (3). Hydroxy l a t i o n of the aromatic r i n g during the photooxidation o f PET introduces a c o n t i n u a l l y i n c r e a s i n g s u r f a c e c o n c e n t r a t i o n o f t h e 2hydroxyphthalate function. The 2-hydroxyphthalate group can act as a u l t r a v i o l e t screen by f i l t e r i n g l i g h t a t the surface. This group may a l s o a c t as a u l t r a v i o l e t l i g h t absorber through mechanisms i n v o l v i n g the d i s s i p a t i o n o f energy i n K e t o - E n o l i n t e r c o n v e r s i o n (15). These factors operate t o produce a steep, l i n e a r increase i n f l u o r e s c e n c e a t f i r s t and then a decrease i n emission a t l o n g e r exposures (Figure l l ) . Comparative FT-IR spectroscopy o f the 12.5 u samples i s i n agreement with the r e s u l t s o f the extraction study on photoxidised PET. Representative subtraction spectra o f c o n t r o l PET f i l m s from i r r a d i a t e d ones are shown i n F i g u r e h. A number o f broad bands i n the 3600-2500 cm" r e g i o n a r e t y p i c a l h y d r o x y l and carboxy 0-H stretching v i b r a t i o n s (l6). S i m i l a r bands have been reported i n the l i t e r a t u r e f o r PET f i l m s i r r a d i a t e d w i t h Xenon and carbon a r c radiation(£). The p r o g r e s s i v e i n c r e a s e i n h y d r o x y l and c a r b o x y l groups upon exposure i s q u a l i t a t i v e l y evident from Figure h. The band centered a t 3270 cm*- was assigned t o carboxyl 0-H s t r e t c h i n g mode(l6) and i t s i n c r e a s e was p l o t t e d a g a i n s t i r r a d i a t i o n time i n F i g u r e 5. The band centered around 3500 cm was a s s i g n e d (17) t o h y d r o x y l group and was a l s o s i m i l a r l y p l o t t e d ( F i g u r e 5 ) · Both curves show p r o f i l e s analogous to that observed for the increase i n 3


KRISHNAN ET AL.


0.015-1


0.010H

V Ζ)

0,

6 . 3 u FILM

Δ

Δ

V

V

005 Η

ν

12.5 V

Δ 0.000-

0

Α

A

Δ

u FILM

Δ

Δ

-Ε

I

I—

"20" 40 40 60 IRRADIATION TIME

100

80 80

(hours)

F i g u r e 9. S o l u b l e F r a c t i o n v s . Time.

4.On 25 u PET,

UVA STABILIZED

ο

3.OH ο

ο

ο

o^oo 2.OH

ο 1.0H

ο •

0.0-

ο

Λ

1

1

1

1

50

100

150

200

IRRADIATION TIME

1

250

(hours)

F i g u r e 10. S o l u b l e F r a c t i o n v s . Time.



400


fluorescence and the amount o f extractable material. The r e s u l t s o f the ATR subtraction spectra (Figure 6) a l s o show comparable p l o t s . The decrease i n molecular weight (Figure 7) indicates the absence o f cross - l i n k i n g . The procedure o f exhaustive extraction followed by a n a l y t i c a l s p e c t r o s c o p y o f t h e aqueous e x t r a c t was a p p l i e d t o determine t h e i n c r e a s e i n p h o t o s t a b i l i t y when t h e u l t r a v i o l e t l i g h t absorber content i n t h e 50 u PET f i l m i s i n c r e a s e d from 2 t o k % (w/w). F i g u r e 12 shows t h e v a r i a t i o n o f t h e s o l u b l e f r a c t i o n with t h e 12.5 •

u PET •

•

•

Ι9ΠΠ J

< Œ

\— »H CD . cr toooH

Polymer Wear and Its Control - American Chemical Society

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