Trapping of volatile low molecular weight photoproducts in inert and enhanced degradable LDPE

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Polymer Degradation and Stability 61 (1998) 329-3420 1998 Elsevier Science Limited. All rights reserved

Printed in Great BritainPII: SOl41-3910(97)00217-6 0141-3910/98/$--see front matter

Trapping of volatile low molecular weightphotoproducts in inert and enhanced degradable

LDPE

Farideh Khabbaz, Ann-Christine Albertsson & Sigbritt Karlsson*Department of Poly mer Technology, The Roy al Inst itut e of Technology (KTH), S-100 44 Stockholm , Sw eden

(Received 17 July 1997; accepted 7 September 1997)

Degradation of enhanced degradable low-density polyethylene (LDPE) (withphotosensitizers and biodegradable filler) was carried out under accelerated UVexposure in sealed vials in order to capture the shortest and most volatile lowmolecular weight products. The photoproducts were identified by gas chromato-graphy-mass spectrometry (GGMS) and polymer-matrix degradation mon-itored by high temperature size exclusion chromatography (HTSEC), Fouriertransform infrared spectroscopy (FTIR) and differential scanning calorimetry(DSC). The components identified were series of ketones, linear and branchedalkanes, alkenes, carboxylic acids, lactones, alcohols and esters. The identifica-tion of 2,5_hexanedione has not been reported before. Degradation productsfrom the additives which were used to increase the degradation were found.Benzoic acid and benzaldehyde were formed in LDPE containing SBS pro-oxi-dant in master batch (MB) samples. 1,2,Ctrimethyl benzene and acetophenonewere identified only in samples containing carbon black. The degradation rate ofthe enhanced degradable LDPE differ and samples containing iron dimethyl-

dithiocarbamate (FeDMC) degraded faster than other materials, while LDPEmodified with both FeDMC and nickel dibutyldithiocarbamate (NiDBC) showedthe lowest degradation rate. The next most degradable material is LDPE con-taining starch and SBS pro-oxidant in MB. The high rate susceptibility of MBsamples to photodegradation is due to presence of both pro-oxidant system andcorn starch. 0 1998 Elsevier Science Limited. All rights reserved

1 INTRODUCTION

The excellent physical properties of polyolefinsmake them suitable as packaging and film materi-als. Photodegradable polyolefins have been pro-posed as one solution to decrease their longdegradation time and general low or even completeresistance to biodegradation.

Degradation in natural environments is influencedby a variety of factors such as heat, oxygen, moist-ure, stress, macro-organisms, ultraviolet light andmicro-organisms in the soil.’ To enhance the envir-onmental degradation of PE a number of differentapproaches are used, such as copolymerizationwith ketone containing materials or compounding

*To whom correspondence should be addressed.

329

with metal salts, starch and other additives.2-5These additives are designed to accelerate chemical,biological and/or photodegradation. An efficientway to increase photodegradability of PE is toincorporate additives, e.g. transition metal dithio-carbamates which work as photoinitiators. Ultra-violet (UV) energy absorbed by the metalcomplexes release the metal ions, which act as cat-alysts to break the polymer chains. Nickel, zinc,and cobalt complexes act as stabilizers, while ironand copper complexes act as sensitizers.6 In thecase of a combination of nickel dithiocarbamate(photo-antioxidant) and iron dithiocarbamate(photo pro-oxidant) a wide range of embrittlementtimes is obtained in which the induction period iscontrolled by the nickel complex and the post-

induction period rate by the iron complex.7 Irondithiocarbamate at low concentrations causes very



Trapping of vola tile low molecular w eight photo products 331

absorption range of the vials was between 200 and290nm and the samples were exposed to UVradiation between 290 and 350nm. During theirradiation the temperature increased to about47”C, and exposure time was 50 and 300 h.

2.3 Solid phase microextraction (SPME) procedure

The SPME assembly and fibers were purchasedfrom Supelco Inc. The SPME method developedinvolves a few steps. Two microextraction fiberswere selected for extraction of the degradationproducts. These were coated with poly-dimethylsiloxane (100 pm) and carbowax/divi-nylbenzene (65 pm) and useful for nonpolar andpolar compounds respectively. During headspaceSPME extraction, the 1 cm fiber contained in asyringe needle was inserted into the sealed samplevials for 30min at 60°C. The fiber was exposed tothe headspace above the polymer. The fiber was thenretracted back into the SPME needle and immedi-ately inserted into the GC injector for desorption.The fiber assembly was cleaned between injectionsby allowing the fiber to remain in the heated injec-tor of the GC for approximately 8 min.

2.4 Gas chromatography-mass spectrometry(GC-MS)

Analysis was performed with a Varian 3400 gaschromatograph coupled to a Finnigan SSQ7000mass spectrometer. The analysis columns were aDB-5MS (30 x 0.25mm I.D., film thickness0.25 pm) and a DB-Wax (30 x 0.25 mmI.D., filmthickness 0.25pm) capillary columns from J&WScientific.

Analysis with DB-5 MS column started by cool-ing the column to -178°C using liquid nitrogen inorder to trap the volatile components. Oven tem-perature was programmed from 30°C for 3 min to240°C at 6°C min-’ then held for 5 min at 240°C.

When using DB-Wax column, the oven tem-perature was programmed from 40°C for 3 min to220°C at 6”Cmin’ then held for 1Omin at 220°C.Helium was used as carrier gas. A split-splitlessinjector was used in the splitless mode. Injectortemperature was 250°C (DB-SMS) and 230°C (DB-Wax).

The identification of some of the degradationproducts was established by comparison of theirmass spectra with spectra recorded in the NST

mass spectra data base and checked by comparisonof retention times with that of a known sample.

Others were identified only by comparison with theNST data base.

2.5 Molecular weight determination

Changes in the molecular weights were measuredusing a Waters model 150-C high temperature SECapparatus equipped with two PLgel 10 p mixed-Bcolumns (30cm length, 7.8mm diameter) fromPolymer Laboratories. 1,2,3-trichlorobenzene con-taining Santonox R (a phenolic anti-oxidant) wasused as a mobile phase at 135°C and the flow ratewas 1.Oml min-‘. Polystyrene standards were usedfor calibration.

2.6 Fourier transform infrared spectroscopy (FAIR)

The FTIR spectra of the films were obtained in aPerkin-Elmer Model 1760 FTIR spectrometer,equipped with a micro holder and a KRS-5 prismhaving a incident angle of 45”, in the range of 800-4000cm-‘. Special interest was focused on thecarbonyl region. Carbonyl absorbance at1718 cm-’ was measured relative to the CH2 scis-soring peak at 1463 cm-‘.

2.7 Differential scanning calorimetry (DSC)

A Mettler-Toledo 820 DSC differential scanningcalorimeter was used. The heating rate was 10°Cmin-’ and the sample size, was 5-6mg. The runswere performed in a nitrogen atmosphere. TheDSC method for determining the percent crystal-linity of polyethylene samples is based on a heat offusion of 293.1 Jg-’ for 100% crystalline PE. Allmeasurements were performed with triple samples.

3 RESULTS AND DISCUSSION

Photo-oxidation of polyethylene leads to a sig-nificant formation of low molecular weight photo-products. As a consequence of their low molecularweight most of these products migrate very rapidlyfrom the polymeric matrix. In order to capturethese volatile components, a method to trap thesecompounds was developed. The samples wereplaced in sealed glass vials and exposed to UVradiation between 290 and 350nm in the weather-ometer. This is near the solar UV radiation thatreach the earth (from 295 nm in the summer to

about 3 10 nm in the winter). l2 The glass vials arethose which are used in direct head-space gas



332 F. Khabbaz et al.

chromatographic analysis of volatile compounds inliquids and solid materials.

3.1 Degradation products and pattern

The low molecular weight degradation products inpolyethylene films after 0, 50 and 300 h of UVradiation were extracted with SPME fiber from thegas phase above the samples. SPME is a fast, sim-ple, solvent free, and sensitive technique and hasbeen successfully applied to the analysis of bothpolar and non-polar analytes from solid, liquid, orgas phase.13 We have recently developed SPMEmethods for polymers and compared them withtraditional head-space GC technique.14 Severalvolatile and semi-volatile compounds from oxida-tion of the PE samples have been identified.

from the GS-1 material. The largest number andintensity of products were found in GSl materials.A wide variety of oxidation products wereobserved in the experiments, including oxygen-containing compounds and unoxidized hydro-

carbons. Table 1 presents the identified productsfrom the different materials. Similar degradationproducts at different concentrations were found inthe gas chromatogram of all the materials. Themain components found were ketones, linear andbranched alkanes, alkenes, carboxylic acids, lac-tones, alcohols and esters. The predominant prod-ucts were 2,5-hexanedione, carboxylic acids (C2,C4, C5), 4-oxopentanoic acid and a homologousseries of ketones (C2-C17). The concentration ofthese ketones in the gas phase decreased withincreasing molecular weight.

Figures 1 and 2 show the GC-MS chromato- In GS-1 samples, alcohols such as ethanol, l-grams of the volatile and semi-volatile products propanol, 1 -butanol and 1 -pentanol were identifiedextracted by dimethylsiloxane and carbowax fibers in low quantities only. Traces of tetradecanol and

P2 40/ !36

\

40

42

80 -

60 - 36

62

6!

Fig. 1. GC-MS chromatogram of the degradation products formed in GSl materials after 300 h of UV radiation, extracted withSPME and polydimethylsiloxane fiber and analyzed with a DB-5 column (nonpolar compounds).



Trapping of volat ile low molecular w eight photoproducts 333

100 1

40 -

20 -

I401

6 2

i,i-,,~_

IL 1000_____________________________________-------

Fig. 2. GC-MS chromatogram of the degradation products formed in GSl materials after 300 h UV radiation, extracted withSPME and carbowax fiber and analyzed with a DB-Wax column (polar compounds).

4 3

6 3

64

6 6

6 7

I6 8

77

hexadecanol were found only in the LDPE-starchsamples. The styrene part of the pro-oxidant(SBS) resulted in the formation of benzoic acidand benzaldehyde in the LDPE samples contain-ing MB. ls17 Degradation products of carbonblack were identified in GS2 samples only. Thesewere 1,2,4-trimethyl benzene and acetophenone.

Phthalates, identified, are either additives fromthe polymers or external contaminants from sam-ple preparation. Butylated hydroxy toluene (BHT),was identified only in GS3 samples, and is a pro-cess antioxidant. This is added to polymers toprotect them during processing, where heat expo-sure can occur during drying, compounding,extruding and molding. The lack of BHT in theother materials may be why their induction periodis much shorter which means that BHT has alreadydisappeared as a result of degradation. Evidence ofepoxides were found in the mass spectra. It was,however, not possible to establish the exact struc-

tures. Further studies on the formation of epoxideswill be undertaken in future.

Acetic acid, but no formic acid, and lowamounts of other carboxylic acids were extracted,separated and identified by SPME with dimethyl-siloxane fiber and a DB-5 GC column. Extractionperformed by SPME with carbowax fiber andanalyzed with DB-Wax GC column gave insteadcarboxylic acids (Cl-C14), 4-oxopentanoic acidand 5oxohexanoic acid as the predominant com-pounds. The nonpolar dimethylsiloxane fiberabsorbs very low amount of highly polar com-pounds.

3.2 Molecular weight changes

Figure 3(a)-(d) shows the changes in the weightaverage molecular weight (Mw), number averagemolecular weight (Mn), Mz-average molecularweight and polydispersity of LDPE containing theadditives compared to a control sample of pureLDPE.

All materials demonstrate a decrease in Mw.GSl films, which contained a photosensitizer,




Table 1. Volatile and semi-volatile prod ucts identified in the materials after 300 h of radiat ion

1

2345678910111213141516

171819202122232425262728293031

32333435

36373839404142434445464748495051525354555657585960

Compounds Gilead- Gilead- Gilead- Pure LDPE LDPE LDPEScott 1 Scott 2 Scott 3 LDPE + 20% MB t 2O%PO + 7.7% Starch

Hydrocarbons2-methyl, propaneb

butaneapentaneahexanea1,2-dimethylcyclopet~tane~heptanen3-methyl, heptaneb2-methyl, heptaneboctand3-methyl, octanebnonane”decane”2,2-dimethyl, nonanebundecane”dodecanentridecanea

2,2,7-trimethyl, undecanebbranched alkaneI-tetradeceneatetradecane”I-pentadecene”pentadecane3-methyl, pentadecanebI-hexadecene”hexadecane”3-methyl, hexadecanebI-heptadecene”heptadecane“3-methyl, heptadecanebI-octadeceneaoctadecanen

1-nonadecenebnonadecane”2-methyl, nonadecanebeicosanea

KetonesAcetone“2-butanone”2-pentanonea4-methyl, 2-pentanonea2-hexanone”3-heptanone”2-heptanone’2,5-hexanedione”2-methyl, 4-heptanoneb3-octanonea2-octanone”6-methyl, 2-octanoneb2-methyl, 4-octanoneb3-nonanoneb2-nonanonea5-decanoneb2-decanone”2-methyl, 4-decanoneb2-undecanone”2-dodecanone”2-tridecanone”2-tetradecanonen2-pentadecanonea2-hexadecanoneb2-heptadecanoneb

+

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+•k

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-t

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++++-c

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(continued)



Trapping ofvolatile low molecular w eight photop roducts

Tabk l-contd

Compounds Gilead- Gilead- Gilead- Pure LDPE LDPE LDPEScott 1 Scott 2 Scott 3 LDPE + 20% MB + 2O%PO + 7.7% Starch

- -Carboxylic acid

61 formic acid*,’

62 acetic acid”63 propanoic acid”64 butanoic acid=65 pentanoic acid”66 hexanoic acida67 heptanoic acid”68 octanoic acida69 nonanoic acid”70 decanoic acid“71 undecanoic acid”72 dodecanoic acid”73 tridecanoic acid”74 tetradecanoic acid”75 hexadecanoic acid”76 benzoic acid”

Ketoacids77 Coxopentanoic acidb*c78 5-oxohexanoic acidb,c

Lactones5-methyldihydro, 2(3H)-furanone”5-ethyldihydro, 2(3H)-furanoneb5-propyldihydro, 2(3H)-furanoneb5-butyldihydro, 2(3H)-furanoneb5pentyldihydro, 2(3H)-furanoneb5-hexyldihydro, 2(3H)furanoneb5-heptyldihydrom, 2(3H)-furanoneb5octyldihydro, 2(3H)-furanoneb5-nonyldihydro, 2(3H)furanoneb

798081828384858687

Alcohols

88 ethanol”89 I-propanol”90 I-butanol”91 I-pentanol”92 tetradecanoP93 hexadecanol”

Esters94 acetic acid, methylestefl95 acetic acid, ethylestef96 acetic acid, propylestefi97 butanoic acid, methylestefl98 acetic acid, butylestep99 propanoic acid, butylesteP

Miscellaneous100 Carbondioxide”101 diethylphthalate”102 bis(Zmethoxy ethyl) phthalateb103 acetophenone”104 1,2,4-trimethyl benzeneb105 2-methyl, hexanalb106 benzaldehyde”107 2-propyl-5-oxohexanalb108 butylated hydroxy toluene (BHT)

Epoxides109 *110 *111 *

+

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+ + + ++

+ + +++

+

+

+

+ + +

+ +

++

+

u Identified by mass spectrometry through comparison with the NST data base and standard retention indices.b Identified by mass spectrometry through comparison with the NST data bas.c Detected only when extraction was performed by carbowax/divinylbenzene fiber and DB-Wax was used as analysis column.* Some epoxides, that was not possible to establish the exact structures.



336 F. Kha bbaz et al.

(4 (b)n 50hUVi-lq 3OUhUV

60W0

T50000

(c)

EOOWO

Fig. 3. (a) The weight average molecular weight (Mw), (b) number-average molecular weight (Mn), (c) Mz average molecularweight and (d) polydispersity of different materials after 0, 50, 300 h of UV irradiation.

demonstrated a significant decrease of the Mw.After 300 h radiation the reduction in molecularweight were accompanied by the formation of alarge amount of low molecular weight degradationproducts.

GS2, LDPE-MB, LDPE-PO, LDPE-Starch andPure LDPE show also a decrease in Mw. Themolecular weight decrease was not monotonous

for all the materials. After 50 h of radiation therewas a slight increase in the Mw in the GSl and GS2

materials as opposed to the other materials whichshowed a slight decrease in Mw after the same timeof radiation. The initial increase in weight may becaused by the incorporation of oxygen into thesystem through the photo-oxidative mechanism. Itcan also be because hydroperoxides formed duringthe irradiation are very sensitive to UV light anddecompose quickly to radicals.18 When the amount

of radicals increase some of them probably reactwith each other and chain elongating reactions will



Trapping of vola tile low molecular w eight photop roducts 337

take place. 19,20 GS3 samples containing photo-stabiliser (GS3) demonstrated no significant chan-ges in Mw during the aging.

Changes in the Mn paralleled changes in Mw.GSl, GS2, LDPE-PO, LDPE-Starch demon-

strated a slight increase in Mn after 50 h radiationafter which Mn decreased. Reduction in Mn wasvery fast for GSl material. LDPE-MB and PureLDPE also showed a decrease in Mn. Mn for GS3increased slightly after 50 h radiation and remainedalmost unchanged after 300 h of UV exposure. Mnis influenced by the lower molecular weight frac-tions, while Mw is influenced by the higher molec-ular weight fractions. Reduction in both Mn andMw indicates that chain breaking reactions takeplace.

The Mz of GSl and GS2 increased slightly dur-ing the first 50 h after which it started decreasing.Mz of GSl reduced dramatically after 300 h oftreatment. LDPE-MB, LDPE-PO and LDPE-Starch also showed a decrease in Mz, but thechanges in Mz for LDPE-PO and LDPE-Starchwas small. GS3 showed no significant changes inMz. Opposite to the other materials Pure LDPEshowed a continuous increase in Mz. The reductionof Mz implies chain cleavage reaction for all sam-ples while the increase of Mz value for the PureLDPE indicates chain elongation.

The polydispersity index (Mw/Mn) which repre-sents the width of the Mw distribution, decreasedsignificantly for the GSl material from the initialvalue of 5.3 to 3.3. This indicates a narrowing inthe overall molecular weight distribution (MWD).GS2 and GS3 also showed a decrease in MWD butthe reductions are smaller than for GS 1. The poly-dispersity of LDPE-MB, LDPE-PO and LDPE-starch demonstrated a slight reduction after 50 h ofradiation, thereafter it increased again. For PureLDPE, polydispersity increased from 5.8-7.7 dur-ing the experiment which implies a broad MWD.

The results from SEC analysis show that the GSlmaterial with the highest reduction in molecularweight is more photodegradable than the othermaterials. The next more degradable material isLDPE-MB followed by LDPE-PO, GS2, LDPE-starch and pure LDPE samples. GS3 show thelowest reduction in molecular weight and therebythe lowest degradability. Main chain scission is thedominate degradation reaction in GSl, GS2, MB,PO and Starch materials, because both Mn andMw decrease. This process will result pre-

dominantly from the decomposition of polymerichydroperoxides, but also from peroxy and alkoxy

free radicals that lead to decrease in the averagemolecular weight of the polymer. Mn and Mw forpure LDPE become smaller while a broadening ofdistribution is observed. These indicate that mainchain scission in Pure LDPE is the dominant pro-

cess but some cross-linking also occurs.

3.3 Functional group change

The extent of oxygen uptake was followed bymeasuring the carbonyl absorption at 1710-1740 cm-* after 0, 50 and 300 h of UV radiation.Figure 4 shows the changes in the carbonyl absor-bance as a function of irradiation time (h) for thedifferent materials. The carbonyl index is almostconstant during the first 50 h. After 50 h exposure itincreases dramatically for the GSl samples (toalmost 66%). MB and PO show the second mostrapid increase in carbonyl index (to almost 29%).For GS2 carbonyl index increases to 23%. Changesin carbonyl index of LDPE-starch and Pure-LDPE are small, while GS3 samples containingnickel-iron dithiocarbamate show the longestinduction period and almost no changes in carbo-nyl index are detected.

Figure 5 shows the FTIR-spectra of all samplesafter 300 h irradiation. During the acceleratedexposure the apparent differences noted are the

carbonyl regions: 1780, 1740, 1718 and 1710 cm-’which are assigned, respectively, to y-lactone, ester,ketone and carboxylic acid species.

3.4 Variability in crystalliuity change

Polyethylene is a semicrystalline polymer. It can beconsidered to behave like a two phase system:alternating a well-ordered crystalline phase and aless rigid amorphous phase. Neighboring crystal-line lamellae are connected by tie molecules, pas-sing through the amorphous interlamellar regions.The crystalline phase of PE does not absorb gasmolecules to any detectable extent and oxygen isnot consumed for oxidative degradation reac-tions.21 As a consequence it is assumed that oxida-tion of polyethylene is restricted to the amorphousregions.

Figure 6 shows changes in crystallinity for dif-ferent materials. It was observed that the crystal-linity of all samples increased with the time ofexposure to photo-oxidation. GSl materials showthe largest increase in crystallinity. The increase in

crystallinity is due to oxidative crystallization andscission of constrained chains in the amorphous




0 50

Time (hours)300

Fig. 4. Carbonyl index as a function of irradiation time for different materials.

C,

b

woo 1wo 5zw nw 24000 2000 1wo 1 2 0 0 MO

Wave Number (cm-‘)

Fig. 5. FTIR spectrum of the materials after 300 h irradiation: (a) GSl; (b) MB; (c) GS2; (d) PO; (e) starch; (f) control; (g) GS3.



Trapping o f volat ile low molecular w eight photop roducts 339

2 5 J I

0 5 0 1 0 0 I S 0 2 0 0 2 5 0 3 0 0

Time (hours)

Fig. 6. Changes in the crystallinity as a function of photo-oxidation time for LDPE modified with different additives.

region that lead to relaxation of local stresses

which allows the resulting free segments to crystal-lize.**

While the crystallinity of all samples increasedduring the UV radiation the melting temperatureremained almost constant. Figure 7 shows thethermogram of unexposed, 50 and 300 h exposedfor GSl films. The melting peak becomes largerwith increasing time of exposure but the meltingtemperature remains almost steady. The broad-ening of the melting peak may occur due to cre-ation of new intermolecular polar bonds (carbonylgroups) which lead to secondary crystallization.23

3.5 Correlation between molecular weight,degradation products and carbonyl index

There is a good correlation between the molecularweight, carbonyl index and formation of degrada-tion products. GSl samples show the highestreduction in molecular weight (88%), have thehighest carbonyl index (66%) and have also shownthe largest number and highest concentration ofdegradation products. The increase in carbonyl

index during UV irradiation that is paralleled byreduction in molecular weight and forming of the

volatile degradation products is due to the photo

sensibilizing role of iron. Reduction of the trivalentiron complex by UV-radiation with the subsequentformation of initiating radicals leads to oxidativebreakdown of the polymer.24 LDPE containing

50 7 0 9 0 1 1 0 1 3 0

Temperature (“C)

Fig. 7. DSC termogram of GSl samples after 0, 50 and 300 hof UV irradiation.




iron dithiodimethylcarbamate and carbon black(GS2) shows moderate degradation. This behavioris expected since carbon black is a UV absorbingadditive and it prevents the photo excitation ofiron-dithiocarbamate. At the same time the energy

absorbed from UV by carbon black causes anincrease in the temperature in the material andthereby increases the possibility of degradation.Samples containing nickel-iron dithiocarbamate(GS3) show a negligible decrease in the molecularweight which is correlated to almost no changes incarbonyl index and the lowest concentration indegradation products.

LDPE containing masterbatch demonstrated thenext highest degradation after GSl. This was dueto the effect of both pro-oxidant system and starch.Samples consisting of LDPE and only pro-oxidantor starch showed lower rate of molecular weightdecrease than LDPE-MB. Both carbonyl indexand concentration of degradation products werehowever, almost similar in LDPE-MB and LDPE-PO, whereas they were lower for LDPE-starch.

Even LDPE-starch is more photo degradablethan Pure LDPE which is due to the rapid changein the degree of polymerization when poly-saccharides are exposed to UV light.25 Starch iscomposed of amylose and amylopectine which in

turn consist of glucose units linked by glycosidicbonds. The &C-O bond on the glucose units actsas a weak chromophore and absorbs UV light andcauses chain scission that affects also the LDPE-matrix. It is evident that the pro-oxidant is themain responsible part for the photodegradation ofLDPE-MB, but the starch may also contribute.

3.6 Mechanism for the formation of degradationproducts

The identified photodegradation products showthat the degradation follows the generally acceptedoxidation reaction scheme. The formation of freeradicals is followed by attack of free radicals on thepolymer chain, reaction of these with oxygen pro-ducing peroxy radicals, abstraction of hydrogen

-CH2-CH-(cH2)2-CH,-CH,-

B

_+ -CH>-CH-(CH&- CHi-CH2- -

-cH,-CH-(cH,)*- -CH,-CH, - % -CH,-CH-WH,),-CH -cH,- -.

b-0.I:

-CHZ-YH-(CH& -CH-CH2- % - CHI-CH-(CH2)z~~-~~7- &

OOH ' AOH 0-O.

R'+ -CH,-YH-(CHJ,-_FH-CH,

OOH OOH

-CH,-I- '(CH,),-&CH,- + 2H,O

mCw)

R 9 FI- CH2-CH2-C-(CH,)2-C-CH,-CH, - N=-CH,-CH,-C-(CH,),-c'CH,-CH2 \

a0, H

,CH-

FI /C&-C& \- -CH,-CH,-C-(CH,),-C;\ *CH-

O-H

0W

- -CH, -CH,-&(CH,)+? + CH,=CH -CH,-

I'OH

P +0

- CH,--CH,-c -_(CH,), -ii --CH, e0 0

CH;+CH,-CH&CH, + 2CH,=CH-CH,-

Scheme 1. A mechanism for the formation of 2,Shexanedione by intramolecular attack of the peroxy radical on the hydrogen atomin y-position.



Trapping of vol atile low molecular w eight photoproducts 341

producing hydroperoxides and more free radicals,cleavage of the O-O bond of the hydroperoxidesand formation of alcohols and carbonyl containinggroups such as acids, ketones, lactones, esters,etc.26

The formation of ketones results from the Nor-rish type I and type II mechanisms. The latter willgenerate vinyl unsaturation. Vinyl groups can notabsorb UV radiation above 300nm but they mayreact with the singlet oxygen generated photo-chemically in PE which makes them susceptible tofree radical attack.27

Rust2* has shown that compounds containingsuitably located tertiary carbon atoms may oxi-dized to give high yields of products of multipleattack. The most favored pattern of reaction

involves attack of the peroxy radical in the B-posi-tion. The next most favored is intramolecularattack by peroxy radical which is found when twoatoms intervene (y-attack). The formation of 2,5-hexanedione is due to intramolecular attack of theinitially formed secondary peroxy radical on thehydrogen atom in y-position, followed by forma-tion of a second peroxy group that abstracts ahydrogen from another polymer molecule to formthe second hydroperoxide. The decomposition of2,5-dihydroperoxides during UV-irradiation pro-duce the highly reactive hydroxyl radicals which

under certain conditions abstract the labile tertiaryhydrogen at the carbon atom. An intermediatebiradical is formed which subsequently gives twocarbonyl groups according to Scheme 1.

Ketoacids are formed by intramolecular hydro-gen abstraction via a cyclic transition state.29 Theformation of carboxylic acid is due to the Norrishtype I reaction. UV irradiation of carbonyl com-pounds yields carbonyl-bearing free radicals whichcan further oxidize to carboxylic acids. Carboxylicacid might also result from aldehydic inter-mediates.30 Carboxylic acids are not photosensitive;they do not undergo further reactions and aceumu-late in the system during photo-oxidation.31

The aliphatic hydrocarbons identified are formedas a result of chain scission while y-lactones areformed when reactions occur between carboxylicacid and hydroxyl group in the 1,4 position3’ orwhen 1,4 dihydroperoxide32 decomposes.

4 CONCLUSION

A method to trap the most volatile of the photo-products of enhanced degradable polyethylene has

been developed. Similar degradation products butat different concentrations were found in all thematerials. The main components identified wereketones, linear and branched alkanes, alkenes, car-boxylic acids, lactones, alcohols and esters. The2,5-hexanedione identified in all samples has to ourknowledge not previously been reported as aphotodegradation product. Its formation isexplained by the intramolecular attack of the initi-ally formed secondary peroxy radical on thehydrogen atom in the y-position. Benzoic acid andbenzaldehyde were formed in LDPE containingSBS pro-oxidant while 1,2,4-trimethyl benzene andacetophenone were identified only in the samplescontaining carbon black.

Samples containing FeDMC demonstrated a

larger degradation rate than all other samples.LDPE-MB showed the next highest degradationafter GSl while LDPE with FeDMC and carbonblack showed moderate rate of degradation. Theiron dithiocarbamate is more prone to absorbUV light early in the aging period while carbonblack which is also a UV absorbing additive alsoprevents the photo excitation of iron-dithio-carbamate. The energy absorbed from UV radia-tion by carbon black also causes an increase inthe temperature of the material which furtherpromotes the thermolysis. In the LDPE-MB

samples, the pro-oxidant system is the main partresponsible for degradation of that material.Samples containing FeDMC and NiDBC demon-strate negligible degradation during the investiga-tion period due to longer induction times at thepresent concentrations. Our results demonstratethat starch also has an influence on the photo-degradation of the LDPE-MB by the weak UVabsorption of the CO-C groups in the starchmolecule.

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Trapping of volatile low molecular weight photoproducts in inert and enhanced degradable LDPE

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