[ACS Symposium Series] Radiation Curing of Polymeric Materials Volume 417 || Photoinitiated Cross-Linking of Polyethylenes and Diene Copolymers

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  • Chapter 11

    Photoinitiated Cross-Linking of Polyethylenes and Diene Copolymers

    Bengt Rnby

    Department of Polymer Technology, The Royal Institute of Technology, S-100 44 Stockholm, Sweden

    During recent years we have studied the photoinitiated crosslinking of various synthetic polymers, e.g. poly-ethylenes (HD, LLD and LD), ethylene-propylene-diene elastomers (EPDM) and unsaturated polyesters, using a UV-absorbing initiator (mainly aromatic ketones) and a multifunctional crosslinker (e.g. an allyl ether) as additives. The crosslinking is largely related to hydrogen abstraction from polymer and crosslinker, giving free radicals which form crosslinks by combination. High degrees of cross-linking have been obtained (measured as gel content by extraction) after short irradiation times (10 to 60 sec). Kinetic studies of the crosslinking reaction indicate a second order reaction at the early stages and a lower order reaction with increasing gel content of the polymer. The mechanism of photocrosslinking of EPDM elastomers has been studied in more detail using model compounds for the dienes: ethylidenenorbornane (1), dihydrodicyclopentadiene (2) and 2-heptene (3). ESR studies of the spin trapped radicals formed has established that EPDM elastomers containing (1) and (2) dienes are crosslinked mainly by hydrogen abstraction to allyl radicals via combination. Only EPDM elastomers containing (3) crosslink by abstraction like (1) and (2) as well as free radical addition to the double bonds. The reported crosslinking reactions are rapid and convenient and require only inexpensive equipment. Therefore, photoinitiated crosslinking is promising for industrial applications both for polyethylenes and EPDM elastomers.

    During the last few years we have developed methods for rapid cross-linking of polyethylenes1, ethylene-propylene-diene elastomers ( E P D M ) 2 and linear unsaturated polyesters3 using U V light for initiation. Efficient crosslinking to high gel content has been obtained for these polymer systems, which contain small amounts of UV-absorbing initiator and a multifunctional monomer (a crosslinker),

    0097-6156/90/0417-0140$06.00A) o 1990 American Chemical Society

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    In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

  • 11. RNBY Photoinitiated Cross-Linking of Polymers 141

    with short UV irradiation times. These photochemical methods for crosslinking of important commercial polymers seem to offer an attractive alternative to crosslinking by chemical initiation and high-energy irradiation which now are used. A first full report on photocrosslinking of polyethylenes and polyesters was presented in 1987 (printed in 1988)4. The present paper reports data on the photo-crosslinking reactions of polyethylenes and EPDM elastomers.

    Experimental

    The polymer samples are mixed as melts with a UV-absorbing photo-initiator (about 1 wt% of benzophenone or a photofragmenting aromatic ketone) and a crosslinker (about 1 wt% of a multifunctional allyl ether or acrylate) in a Brabender plasticorder to a homogeneous compound. Samples for crosslinking are pressed to sheets of 0.3, 2, 5 and 10 mm thickness at temperatures above the melting point of the compound.

    The sheets are irradiated with U V light from a high pressure mercury lamp (1 or 2 kW H P M from Philips) in nitrogen atmosphere at a thermostated temperature (from room temperature for the elastomers to the melting range for the polyethylene) in a UV-CURE irradiator (Fig. 1). By moving the lamp L to different distances from the sample S, the intensity of the UV light can be varied about 50 times. The degree of crosslinking is measured as gel content by extraction with boiling xylene (for polyethylenes) or as swelling in cyclohexane (for diene copolymers). Crosslinking is also related to density for polyethylenes, measured in a density gradient column.

    Photocrosslinking of Polyethylenes

    Sheets of polyethylenes (LDPE, L L D P E and HDPE) compounded with benzophenone (BP), 4-chlorobenzophenone (4-CBP) or 4,4'-dichloro-benzophenone (4,4'-DCBP) and triallylcyanurate (TAC) as described were UV-irradiated. With 1 wt% 4-CBP and 1 wt% TAC,HDPE sheets are crosslinked most efficiently to the highest gel content (about 90%) while L L D P E and LDPE reach gel contents of 70 to 80%. The cross-linking reaction is a function of the irradiation temperature as shown in Fig. 2. One can see that accessibility and mobility of the polyethylene chains increase sharply at the melting point of HDPE (130 to 140C) and more gradually for LLDPE and LDPE (up to about 110C).

    The mechanism of crosslinking in this system is well established. The UV quanta (340-360 nm) are absorbed by the BP molecules and excited to a singlet state (S) which is short-lived and rapidly reverts to the triplet state (T) by intersystem crossing. The state is a rather long-lived biradical which abstracts hydrogen from surrounding molecules, e.g. polyethylene chains, which gives macroradicals on the chains (formula 1).

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    In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

  • 142 RADIATION CURING OF POLYMERIC MATERIALS

    Fig. 1 Schematic diagram of the UV-irradiation equipment U V CURE: shield, - lamp holder, I N2 inlet, L - - UV lamp, - stand, Q quartz plate, S sample, -- temperature detector, T i - track, V - ventilation and W - heating wire.

    Fig. 2 Effect of irradiation temperature on crosslinking of polyethylene with 1.5 % TAC, 1 % photoinitiator and 15 sec. U V irradiation: LDPE (A with 4-CBP and with 4,4'-DCBP) and HDPE (C with 4-CBP).

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    In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

  • 11. RANBY Photoinitiated Cross-Linking ofPolymers 143

    0

    1 hV. Ph Ph

    O S II 1

    c / \

    Ph Ph

    I S C |0 | HCH

    > + HCH Ph Ph HCH

    I

    OH I

    C + / ' \

    Ph Ph Ke t y l r a d i c a l

    I HCH C H

    HCH i

    (1)

    Two P E chain radicals may combine and form a crosslink. When TAC is present the BP T state may abstract hydrogen from the very reactive allyl groups. Allyl radicals are formed (formula 2) and take part in the crosslinking reaction by combining with chain radicals or other allyl radicals. Due to the high reactivity of the allyl groups, TAC has a pronounced effect on the rate of crosslinking. The ketyl radicals are rather unreactive, some combine to the dimer pinacol and some add to chain radicals and prevent crosslinks from forming. Allyl groups may also add a radical which initiates free radical polymerization.

    loi II

    OH I

    + R - O - C H - C H = CH R - O - C H - C H = C H 2 + C (2) Ph' Ph p h p h

    A l l y l r a d i c a l K e t y l r a d i c a l

    The rate of crosslinking is related to the chain length of the P E sample. This is shown in Fig. 3 where the rate of crosslinking is shown for a HDPE sample of low DP before and after adding 10 and 20 wt% respectively of a high DP sample of H D P E 5 . The added crosslinker has a strong effect on the crosslinking reaction, giving high gel content at short irradiation times (Fig. 4). Two HDPE sheets (5 mm thick) are compared, one containing 1 wt% 4-CBP (A) and one containing both 1 wt% 4-CBP abd 1 wt% TAC (Fig. 5). The sheets are irradiated for 20 sec. and then sliced by microtome to 0.25 mm thick sections for which the gel content is analysed by extraction. The added TAC is most effective in deeper layers. In deeper layers, the U V intensity is low due to absorption by BP in the top layers. Because of the high reactivity of the allyl hydrogens, the presence of T A C is increasing the rate of crosslinking also in deeper layers where only few triplet state BP molecules are formed. TAC as only additive gives insignificant crosslinking due to very weak UV absorption.

    Considering the reactions in formulas (1) and (2) and the foregoing discussion of the possible reaction mechanisms, it seems likely that the photoinitiated crosslinking is a second order reaction, i.e. the rate should be a linear function of the U V intensity in square. Assuming that the rate of crosslinking (which is difficult to measure) is proportional to the rate of gel formation (which is easy to measure), the data in Fig. 6 support this conclusion at low U V intensities, i.e. at

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    In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

  • 144 RADIATION CURING OF POLYMERIC MATERIALS

    90

    Gel

    70

    50 h

    Fig. 3 Effect of different HDPE blends on crosslinking. 4-CBP: 1 wt%, TAC: 1 wt%, T: 1 5 5 C . a: Pure Lupolen (M n = 2.7 104), b: Lupolen/Hostalen = 9/1 (Hostalen, M n = 9.8 104), c: Lupolen/Hostalen 8:2.

    80 U %

    Gel 60 h

    40 h

    20

    Fig. 4 Effect of TAC on crosslinking rate. 4-CBP: 1 wt%, T: 1 5 5 C , TAC: a_0 wt%, b 0.5 wt%, c 1 wt%, d 2 wt%.

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    In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

  • 11. RNBY Photoinitiated Cross-Linking of Polymers

    b

    1 I 1 2 3 Depth mm

    Fig. 5 Effect of TAC on homogeneity of crosslinking measured as gel content. 4-CBP: 1 %, T: 155C, TAC: a-0%, b-1%.

    Log 1 + 6 2

    (I in Einstein/cm *min) Fig. 6 A log-log plot of the rate of crosslinking measured as gel

    content versus light intensity for HD polyethylene.

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  • 146 RADIATION CURING OF POLYMERIC MATERIALS

    the early stages of crosslinking. At high intensities, the amounts of gel are larger and many crosslinks are formed inside the gel phase without increasing the amount of gel. This would explain why the slope of the curve decreases from 1.9 (~2) to 1.2 at increasing U V intensity. These results are further discussed and supported in a series of forthcoming papers5.

    Photocrosslinking of Diene Copolymers

    The photoinitiated crosslinking of three ethylene-propylene-diene copolymers (EPDM elastomers) has been investigated in a series of studies6*7*8*9. Commercial E P D M samples were used containing the following dienes; ethylidene-norbornene (ENB), dicyclopentadiene (DCPB) and hexadiene (HD). The main photoinitiators used were benzoyl-1-cyclohexanol (PI) from Ciba-Geigy and 2,4,6-trimethyl-benzoyl diphenylphosphineoxide (APO) from BASF, which both are photofragmenting (formulas 3 and 4):

    The PI initiator absorbs at 320-340 nm and the acylphosphineoxide with R = phenyl group (APO) absorbs at 350-400 nm. The four radicals formed initiate polymerization by addition to acrylic monomers or abstraction of hydrogen from allyl groups. To increase the rate of crosslinking, various crosslinking agents were added to the elastomer, e.g. trimethylolpropane triacetate (TMPTA), penta-erythritole tetraallylether (PETAE), triallylcyanurate (TAC), dilimonene-dimercaptane (DSH) and dodecyl-bismaleimide (MI).

    After mixing the E P D M elastomer with initiator and crosslinking agent in the Brabender plasticorder at room temperature, 2 mm thick sheets were pressed at 100C and embedded in an epoxy resin matrix with one side uncovered. The sample sheets were irradiated for 10 min. at 15 cm distance from the UV lamp in N2 atmosphere at room temperature in the UV-CURE (Fig. 1). After crosslinking, the rubber sheets were cut into 20 thin films by a Leitz microtome for IR

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  • 11. RANBY Photoinitiated Cross-Linking ofPolymers 147

    analysis. Sample sheets were also pressed to 0.5 mm thickness, UV-irradiated for 10 min., extracted and placed in cyclohexane for swelling for one week. The crosslinking density of the gel was calculated according to Flory-Rhener's theory.

    Because saturated ethylene-propylene copolymers are not crosslinked in this process, it is concluded that the crosslinking reaction is definitely related to the double bonds in the diene monomer units. However, the double bonds of the E P D M copolymers are not consumed in the crosslinking process, only decreased to some extent. The IR transmission spectra of the thin sections (20 ) of E P D M containing ENB units has absorption peaks at 1688 and 808 cm' 1 (assigned to C=C double bonds) both before and after crosslinking (Fig. 7). Solid state MAS NMR spectra do not give spectra sufficiently resolved to interpret the reaction mechanism.

    The use of model compounds, spin trapping and ESR spectroscopy has solved the problem. Three model compounds corresponding to the three dienes in the E P D M samples were used (formula 5):

    2-heptene dihydrodicyclopentadiene ethylidenenorbornane

    As spin trap in these experiments, pentamethylnitrosobenzene (PMNB) was used. The first problem was to find if the benzoyl radical as an initiator could add to the double bonds of the model compounds. Benzoyl radicals produced from benzil by a laser beam output of 308 nm added to 2-heptene (formula 6) but not to the other two model compounds. In addition allyl radicals are formed by hydrogen abstraction (formula 7).

    (5)

    C H 3

    (6)

    C H , C H = C H C 4 H 9 P h S . C H , C H = C H C 4 H 9

    P h - C = 0 C H 3 C H = C H C H J C S H T C H 3 C H = C H C H C , H T

    (7)

    American Chemical Society Library

    1155 16th St., N.W. Washington, D.C. 20036

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  • 148 RADIATION CURING OF POLYMERIC MATERIALS

    Spin trapping with...

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