Photoinitiated Crosslinking of Low Density Polyethylene. 111: Degradation and Stabilization of Photocrosslinked

Polyethylene* YAN QING and XU WENYING

Department of Material Science and Engineering University of Science and Technology of China

Hefei, Anhui 230026, P.R. of China

and

BENGT &BY**

Department of Polymer Technology The Royal Institute of Technology

S-1 00 44 Stockholm, Sweden

Electron spectroscopy (ESCA) and reflectance infrared spectroscopy (ATR) were used to measure surface oxidation of photocrosslinked polyethylene. The deterio ration of bulk properties during an artificial weathering test was also measured. It was found that the UV-irradiation during crosslinking process decreases the stability of the material considerably. Three kinds of antioxidants and photostabi- lizers (hindered phenols, hindered amines, and organic sulfides) which have no absorption in the UV region of 300 to 400 nm were added before crosslinking to improve the stability of the crosslinked material. Neither the rate nor the degree of photocrosslinking at different depths of the sample were affected significantly by these additives. It was found possible to prevent the decrease in stability due to the photocrosslinking by using small amounts of a stabilizer, e.g. 0.05% Tinuvin 770 (hindered amine).

INTRODUCTION stability, because UV light, one of the harmful factors leading to degradation during practical use of the material, is applied to initiate the crosslinking pro cess. The following two basic questions should be answered before the photocrosslinking method is used more widely in industry:

he useful lifetime of polymeric materials is an important concern for many applications (1).

Crosslinked polyethylene (XLPE) is usually oxidized more readily than the normally molded un- crosslinked polymer (2). That is because the

T

crosslinking in most cases is a radical reaction which introduces extraneous groups on the macromolecu- lar chains. The degradation of a polymer is also in- duced by radical reactions. Moreover, most effective antioxidants and photostabilizers operate as free rad- ical scavengers. Their presence may therefore inter- fere with the crosslinking process, and may also con- sume an extra amount of the initiator. It is important to study crosslinking with stabilizers present.

For photocrosslinked polyethylene, it is even more interesting to study its stability, especially the photo - *This is Part 111 of a series of papers on ”Photoinitiated Cross-linking of Low Density Polyethylene”. For Part 1 and 11. see Refs. 3 and 4. **To whom correspondence should be addressed

1. How much does the photocrosslinking affect the

2. How much does an added stabilizer affect the pho stability of the material?

tocrosslinking process?

To answer these questions, we have analyzed the changes of the surface and bulk structures and prop erties of the crosslinked and uncrosslinked samples when exposed to simulated sunlight and elevated temperatures. We have selected stabilizers transpar- ent in the UV region of 300 to 400 nm, which is the irradiation used to initiate the crosslinking. In this work, we attempt to interpret the mechanism of degradation and stabilization, and try to find a simple

446 POLYMER ENGINEERING AND SCIENCE, MID-MARCH 1994, VOI. 34, NO. 5

Photoinitiated Crosslinking of LDPE. III

Table 1. Stabilization Composltions and OIT Stability.

Samples #O #1 #2 #3 #4 #5 #6 #7 #8 #9 #10

Crosslinking - + + + +- + + + + + + lrganox 1076 0 0 0.1% 0.2% 0.5% 0 0 0 0.2% 0.2% 0.2%

0 0 0 0.05% 0.10% 0.25% 0 0.10% 0.10% Tinuvin 770 0 0 DLTP 0 0 0 0 0 0 0 0 0.4% 0 0.4% O.I.T. (mid 5.6 2.8 2.8 2.6 3.0 2.8 3.6 3.2 4.4 3.2 2.4

method to improve the stability of the pho- tocrosslinked polyethylene.

EXPERIMENTAL.

Materiab

Low density polyethylene resin LDPE 1IA-1 from YanShan Petrochemical Co. (China). d = 0.92 1, M. I. = 2; M , = 1 036 000, and MJM, = 36 (results from GPC analysis).

Initiator: benzophenone (BPI Crosslinker: triallyl isocyanurate (TAIC) Antioxidants and Photostabilizers:

1.

2.

3.

4.

Hindered phenol, Irganox 1076 (Ciba-Geigy) t-Bu 0

)----\ I1 H O V C H , C H , C -0C 18H37

t-Bu’ n-Octadecyl-P-(3,5-tert-butyl-4-hydroxyphenyD- propionate Hindered amine, Tinuvin 770 (Ciba-Geigy)

Bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate

Thioester (organic sulfides), DLTP (Lisheng, Tian- jin, China)

0 11

,CH,CH ,C - OC 12 H,, S .

‘CH,CH,C - OC 12 H 25 II 0

Dilauryl thiodipropionate Tri( 2,2,6,6 - tetramethyl - piperadyl(oxide) phos- phite, GW-544 (Beijing Chemical Plant) is a free radical which acts as a stabilizer like a hindered amine.

The sample preparation and UV irradiation are the same as described in the first papers of this series (3, 4). All samples contain 1% photoinitiator (BP) and 1% crosslinker (TAIC). The different stabiliza- tion systems are listed in Table 1.

Weathering Crosslinked sheets of the polymer with a thickness

of 0.5 mm are exposed in a WEL-6XL-HC weatherom- eter, which has a 6 kW Xenon lamp as the source of simulated sunlight. The temperature is kept at 45 f 2°C. the relative humidity at 75&5%, and the “rain”/dry frequency is 18/120 min.

Aging tests of the crosslinked samples are carried out in air in an oven at 135 f 1°C.

Measurements Gel content is measured by extraction with xylene

in a Soxhlet extractor for at least 24 h. Photo-electron spectra (ESCA) are obtained on a

VG Scientific ESCALAB MK-I1 spectrometer. Attenuated total reflectance (ATR) infrared absorp

tion is recorded on a Nicolet 170SX FT-IR spectrome- ter with KRS-5 ATR attachment (TlI-TlBr). The inci- dent angle is 45” and the scan number is 600.

Oxidation induction time (0.I.T.) is measured with a Shimadzu DT 30B differential thermal analyzer. A sample of about 5 mg is heated to 200°C in nitrogen atmosphere for 10 min. Then oxygen is passed through at a rate of about 100 ml/min until the exothermic peak of oxidation is observed. The time between the entrance of 0, and the start of oxidation is the O.I.T. (5).

Mechanical properties are measured on a Shi- madzu instrument DCS-5000. The strain rate of the cross-head is 200 mm/min.

RESULTS AND DISCUSSION

Pre-Oxidation

Weathering of polymeric materials usually starts from the surface and proceeds to the bulk. Therefore. surface analyses are appropriate for the study of stability of the materials. Such measurements make it possible to predict long-range durability in a rela- tively short experimental period ( 1).

Figure 1 is the ESCA spectra of the samples. The peak of 0, increases with the aging time; which implies that the aging is an accumulative oxidation. The appearance of the high binding energy c o m p nents in C, band is due to the formation of oxygen- containing functional groups, such as C - 0, C = 0 and -COO-.

If the stoichiometric O/C is used to characterize the degree of oxidation (Rg. 21, it is surprising to And that the crosslinked sample (Sample #1) is oxidized “more slowly” than the uncrosslinked one (Sample #Oh

POLYMER ENolNEERlNQ AND SCIENCE, MID-MARCH 1994, Vol. 34, NO. 5 447

Yan Qing, X u Wenying, and Bengt RBnby

I . , . , . , . # , , . , . u I 281 283 285 287 289' 530 532 534

Bond Energy (ev)

Fg. 1. ESCA spectra of the samples before and after weatk ering aging.

0 120 240 360 480

Exposure lime (hr)

Fig. 2. Surface oxidation measured by ESCA as ratio O / C . The oxygen content of the original samples has been studied.

For further details, ATR infrared spectra have been recorded, as shown in Figs. 3A and B , the crosslinked and uncrosslinked samples are quite different in the progress of weathering aging. The C=O peak at 17 15 cm- ' increases much faster in the crosslinked sample than that in uncrosslinked one. In the un- crosslinked sample, there is another rapid increase at 1020 cm-' which corresponds to C-0 bonds. This band exists originally in the crosslinked sample but weakens after aging.

Although it is difficult to obtain the oxygen content quantitatively from infrared spectroscopy, it is esti- mated from the spectra in Q. 3 that the crosslinked sample contains more oxidized groups than the un- crosslinked one. This result is just contrary to that from the ESCA analysis.

To interpret these results, it should be observed that the ESCA spectra refer to the composition in a very thin surface layer only about 10 nm, whereas the penetration depth of the reflected infrared in ATR is a few micrometers (determined by the ATR crystal, the incident angle, and the wavelength of infrared) (6). This means that ATR-IR spectra give structural

0 cu

2000 1600 1200 800 400

Wavenumber (cm A-1)

Fg. 3A. Am-IR spectra of the uncrosslinked samples before and after weathering aging.

Ohr

2000 1600 1200 800 4 0 0

Wavenumber (cmA-1)

Fig. 3B. Am-IR spectra of the crosslinked samples before and after weathering aging.

information of a more than 100 times deeper surface layer than ESCA spectra.

Therefore, the difference between the results from the two experiments indicates that the photo-oxida- tion of the uncrosslinked sample is concentrated to a relatively thin layer at the surface during the first stage of aging, while the crosslinked sample is oxi- dized more deeply and at a faster rate.

The reason for this deterioration of the pho- tocrosslinked samples is supposed to be due to the proposed " pre-oxidation" reactions when the material is being crosslinked. The primary surface reaction is the formation of free radicals which add the adsorbed oxygen and form hydroperoxide groups as the origi- nally existing C-0 band in the infrared spectra. The hydroperoxides will act as chromophores when the sample is being heated or further UV-irradiated afterwards, and decompose to ketone groups and water:

A/ hu -CH2CH-- - -CHZCH- +'OH-

I I OOH 0.

448 POLYMER ENGINEERING AND SCIENCE, MID-MARCH 1994, YO/. 34, NO. 5

Photoinitiated Crosslinking of LDPE. Ill

-CH2C- + H 2 0 I I 0

This mechanism explains the decrease of the C-0 band and the rapid increase of the C = O band in ATR spectra of crosslinked samples. I t also accounts for the ESCA result, in which the crosslinked sample, after aging, contains less oxygen than the un- crosslinked. The break-down of peroxide groups in the surface layer results in the loss of oxygen as small molecules such as H,O which are produced and vaporized.

The crosslinking reaction in deeper layers of the sample means introduction of new bonds on the poly- mer chains at positions, such as tertiary carbon bonded hydrogens, which react at a faster rate and give decreased stability after crosslinking. This is an- other reason for the fact that the bulk properties of the photocrosslinked polyethylene are destroyed more easily than those of the uncrosslinked, as evidenced by their tensile behavior (Figs. 4 and 5).

Antirad Effect

The stabilizers chosen in this work have no absorp tion in the region of 300 to 400 nm. I t is reasonable to assume that their presence does not prevent the initiator from absorbing photons emitted by the W lamp during the crosslinking. But as scavengers of free radicals, hindered amines, and hindered phe- nols, they may interfere with the secondary reactions in photocrosslinking and retard the process. That is the so-called “antirad” effect as described in radiation chemistry.

From the experimental results shown in Figs. 6A and B, the two stabilizers (hindered phenol and hin- dered amine) have no appreciable effect on the crosslinking reaction. These stabilizers apparently do not react with the primary polymer radicals formed in the photochemical process. The reason may be steric hindrance due to the bulky molecules of the stabiliz- ers, which still are effective enough to break the oxidation chain reactions.

The added GW-544 is a very active photostabilizer. It has a quenching effect on the crosslinking which appears as an induction period in the gel formation (Fig. 6C).

In crosslinking on a thicker sample (i.e. 3.5 mm), it is noticed that the presence of DLTP, an organic sulfide peroxide decomposer, retards the rate of crosshking to a certain extent (Flg. 7). This is in agreement with previous work (7). Organic phos- phites are reported to be better in this case. The homogeneity of crosslinking, however, is not affected very much (Fig. 8). This confirms the assumption that there is no “screen effect” induced by the “W- transparent” stabilizers studied.

Our results show that the normal commercially available stabilizers can be effectively used during photocrosslinking. The antirad effects are minimal. Even for the stabilization system containing an or-

- d r f;

t B

CD

a

a c

- -

I I

0 120 240 360 480

Exposure Tlme (hr)

Fig. 4. Tensile strength of uncrosslinked (#O) and crosslinked (#1) samples us. weathering time.

1 ow 1

1 I I I 0 120 240 360 480

Exposure Tlme (hr)

0. 5. Elongation at break of uncrosslinked (#O) and crosslinked (#1) samples us. weathering time.

6 0 -

40 - 0 SampleX1

SampleX2 SampleX3 Sample14

I

0 60 120 180

Irrrdlrtlon Tlme (SOC)

Flg. 6A. Effect of hindered pheml on the photocrosslinking process (Lamp: 50OW. 1 0 em).

ganic sulfide (DLTP), the gel content can still reach a level as high as > 70%.

Stabilization The measurement of oxidation induction time

(O.I.T.) with differential thermal analysis (DTA) has

POLYMER ENGINEERING AND SCIENCE, MID-MARCH 1994, Vol. 34, NO. 5 449

Yan Qing, Xu Wenying, and Bengt R&nby

80

60-

40

z -

-

I E

C 0 0

c

- d

loo I f

Tinuvin 770

0 Sampler1

I + SampleXfJ I SampleX6

Sample(l7

0 6 0 120 180

Irradlatlon Time (roc)

Fg. 6B. Effect of hindered m i n e on the photocrosslinking process (Lamp: 500W. 10 em).

z

- d

80 t I

40 t 2[

C: GW-544

0 Sampler1

SampleX9

0.1% 'GW-544' )

L ._ 0 6 0 120 1 8 0

Irradiation Time (see)

Fig. 6C. Eflect of nitroxyl radical on the photocrosslinking process (Lamp: 500W, 10 cm). GW-544 added to sample # 1 .

c C a

0 0

c

- d

0 SampleX1

SarnpleX10

bl SampleX8

06 . I t I 0 60 120 180

Irradiation Time (aec)

Fig. 7. Effect of DLTP and stabilizers on the photocrosslink ing of thick samples (3.5 4. Lamp: 1OOOW. 10 crn

become a routine method to measure the stability of the materials (5). It is based on the assumption that the oxidation of a polymer is a series of self-accel- erated reactions. The time before the beginning of auto-oxidation, i.e. rapid oxygen consumption, can be

z c E c

0 0 - d

t

0 SampleX1

0 0 1 2 3 4

Depth from Surface (mm)

Fig. 8. Effect of DLTP and stabilizers on the homogeneity of photocrosslinked samples (3.5 4.

used as an index of the antioxidation ability of the materials.

For this purpose, the O.I.T. values of all samples with different composition of stabilization system are measured and the results are listed in Table 1. All the O.I.T. values of the crosslinked samples are rather low with only small differences between them. They are all lower than that of the blank (uncrosslinked) sam- ple which contains no stabilizers.

These results confirm that the photocrosslinking also causes a " preoxidation". The induction period, during which a certain amount of active groups are introduced as the first stage of aging, has been com- pleted by the pre-oxidation during the crosslinking process. For the crosslinked samples, the oxidation reaction can start without induction time. In this case, the measurement of O.I.T. is no longer a useful method to estimate the stability of the material and the quality of a stabilization system. Traditional artifi- cial accelerated weathering and oven tests have to be applied.

Figures 9 and 10 show the changes of tensile prop erties of the differently stabilized samples after the weathering exposures. The photostabilizer Tinuvin 770 is so effective that only a small amount (0.05 wt%) can retain the mechanical properties for a long period of time. The antioxidants Irganox 1076 and Irganox 1076 + DLTP (which is good in oven aging tests) are not so effective, perhaps because of photol- ysis of the phenol stabilizer itself during the crosslinking (8).

Figure 1 1 shows the changes of gel contents during aging. The trend in structural change is similar to that in mechanical properties.

Synergism and Antagonism

Stabilization of polymeric materials by a combina- tion of antioxidant and photostabilizer is an impor- tant method employed in industry. The interaction between the components should be studied carefully to avoid antagonistic effects and take advantage of any synergistic effect.

450 POLYMER ENGINEERINGAND SCIENCE, MID-MARCH 1994, Vol. 34, NO. 5

Photoinitiated Crosslinkmg of LDPE. III

In this work, a hghly detrimental antagonistic ef- fect is found when Tinuvin 770 is used together with the thioester DLTP. This is associated with the reac- tion of nitroxyl radicals with sulfenyl radicals to give inactive sulfonamides (9). The decomposition of hy-

f m

$! i5

5 0 120 24 0 360 480

Exposure Time (hr)

Fig. 9. Tensile strength of stabilized samples us. weathering time.

z

1MM

0 SampleX5 800 ,r 0 rn SampleX9

I II

400 - . 0 Samplew3

0 SampleXB L

200 - b

0 - I I

0 120 240 360 480

Exposure Tlmo (hr)

Fig. 10. Elongation at break of stabilized samples us. weattt ering time.

z

droperoxides by DLTP also prevents the formation of nitroxyl radicals. Since the nitroxyl radicals are b e lieved to be the main effective species in the course of stabilization by piperidine derivatives (101, these r e actions will reduce the efficiency of hindered amine stabilizers SigniAcantly as shown in Figs. 9 to 1 1 (Sample # 10).

Phenolic antioxidants may also have antagonistic effects on the photostabilization of hindered amine by the following reactions, although this effect has not been found in this work (Sample #9).

\ / N - 0 ’ + HO mQRa

tl3U mu.

\ ,N-OH+ 0

mu . mu.

The reported synergistic effect between DLTP and phenolic stabilizers is unfortunately inefficient for protection against light in the weathering aging test (Sample #8). On the other hand, the photostabilizer Tinuvin 770 is not so efficient in the protection against heat in our oven tests at 135°C in air.

Therefore, the search for a suitable system to stabi- lize the photocrosslinked polyethylene against both light and heat should be continued using a wider selection of stabilizers than those applied here.

CONCLUSION

The results of surface analyses show that there is a “ pre-oxidation” during the photo-irradiation when a sample is crosslinked. The preoxidation causes a shortening of the oxidation induction time of the

iM ,

0 120 240 360 480

Exposure Tlmo (hr)

Fig. 1 1. Gel content us. weathering time.

Sample #O

Sample X1

Sample #3

Sample #8

Sample 15

Sample #6

Sample X 7

Sample #9

Sample #lo

POLYMER ENQWEERING AND SCIENCE, MID-MARCH 1994, Vol. 34, N o . 5 451

Yan Qing, X u Wenying, and Bengt RBnby

photocrosslinked polyethylene. For the same reason, the crosslinked sample will be oxidized more deeply and destroyed more quickly in a weathering test than the uncrosslinked sample.

Tinuvin 770 (hindered amine), Irganox 1076 (hindered phenol), and DLTP (thioester), which have no absorption in the UV region of 300 to 400 nm, were used as stabilizers. I t was found that they have no significant “antirad” effects on the photocrosslink- ing process. It was demonstrated that Tinuvin 770 is the most efficient stabilizer for photocrosslinked polyethylene against photooxidation in weathering tests. Its addition makes the samples retain their gel content and tensile properties for quite a long weath- ering period. On the other hand, Irganox 1076 + DLTP shows a strong synergism on antioxidation at an elevated temperature in oven tests. The efforts to find a stabilization system efficient against both thermal and photochemical degradation have so far not been successful.

REFERENCES

1. B. &by, and J . F. Rabek, Photodegradation Photo- Oxidation and Photostabilization of Polymers, John Wiley & Sons, London (1975).

2. F. H. Winslow, C. J. Aloisio, W. Matreyek, and S . Matsuoka, Chem & Ind (London) 533 (1 963).

3. Qing Yan, Wenying Xu, and B. &by, Polym Eng. Sci., 31, 1561 (1991).

4. Qing Yan, Wenying Xu, and B. Rhby , Polym Eng. Sci., 31, 1567 (1991).

5. D. I. Marshall, E. I . George, and J. M. Tumipseed, Polym Eng. Sci., 13, 415 (1973).

6. J. F. Rabek, Experimental Methods in Polymer Chem istry, Physical Principles and Applications, John Wiley & Sons, New York, (1980).

7. Y. L. Chen, and B. %by, J. Polym Sci. Polym Chem Ed.. 28, 1847 (1990).

8. Yizhi, Gui, Organic Additives Used in Polymers, People’s Education Press, Beijing (1981).

9. G . Scott, in New Trends in the Photochemistry of Poly mers, p. 227, N. S . Allen, and J. F. Rabek, eds., Elsevier, London (1985).

10. N. S. Allen, Ref. 6, p. 209.

452 POLYMER ENGINEERING AND SCIENCE, MID-MARCH 1994, VOI. 34, NO. 5