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Applied Surface Science 257 (2011) 7600–7608

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

urface modification of ultra high molecular weight polyethylene fibers via theequential photoinduced graft polymerization

hi Li, Wei Zhang, Xinwei Wang, Yongyi Mai ∗, Yumei Zhangesearch & Development Center of Shanghai Research Institute of Chemical Industry, 345 YunLing Road (East), Shanghai 200062, PR China

r t i c l e i n f o

rticle history:eceived 19 August 2010eceived in revised form 13 February 2011ccepted 24 March 2011vailable online 2 April 2011

a b s t r a c t

In this study, a sequential photoinduced graft polymerization process was proposed to improve the poorinterfacial bonding property of ultra high molecular weight polyethylene (UHMWPE) fibers. The polymer-ization was initiated by dormant semipinacol (SP) groups and carried out in a thin liquid layer. Methacrylicacid (MAA) and acryl amide (AM) were grafted stepwise onto the surface of UHMWPE fibers. Attenuated

eywords:hotoinduced graft polymerizationHMWPE fibers

nterfacial bonding property

total reflectance infrared spectroscopy (ATR-IR) and thermo gravimetric analysis (TGA) confirmed thegrafting. The analysis result of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) indicatedthe structure of grafted chains. Scanning electron microscopy (SEM) images and atomic force microscopy(AFM) images revealed the apparent morphology changing, and the grafted layers were observed. Inter-facial shear stress (IFSS) test of the modified fibers showed an extensively improved interfacial bondingproperty. The active groups grafted onto the fibers would supply enough anchor points for the chemicalbonding with various resins or further reactions.

. Introduction

UHMWPE fiber as an organic fiber has played more and moremportant roles in various industrial fields due to its superior

echanical properties and effective cosmic shielding properties1]. However, the poor interfacial adhesion between UHMWPEbers and polymer matrix [2] leads to show weak composite prop-rties, which limits its application in the aspect of composites. Toxpand its application for the polymer composites, fiber surfaceodification is necessary. Therefore, surface modification tech-

iques are commonly used to introduce appropriate functionalroups onto the fiber surface.

There are many methods [3,4] to modify the surface of UHMWPEbers, such as plasma treatments, chemical etching and UV-

nitiated graft copolymerization. Among them, UV-initiated graftopolymerization is endowed with promised future for its powerfulffectiveness, low cost of operation and relatively small influencen the bulk polymer. While traditional UV-initiated methods inerms of surface modification of UHMWPE fibers had some suc-

ess, but there are disadvantages [5] such as long reaction time, UVcreening effect, etc. Generally, the traditional ways often needs0 min to modify the UHMWPE substrates [5–8], and make anxcessively waste of reactant for the homopolymerization.

∗ Corresponding author. Tel.: +86 021 52813860; fax: +86 021 52808504.E-mail address: SRICIshanghai@163.com (Y. Mai).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.03.134

© 2011 Elsevier B.V. All rights reserved.

In our work, to overcome the shortcomings of the traditionalmethod, the novel method has been used to improve the interfacialbonding property of UHMWPE fibers.

The sequential photoinduced graft polymerization is depictedin Scheme 1. The reaction was carried out in a thin liquid layersurrounded with a monomer gaseous phase, which would have abetter chemical efficiency and avoid UV screening effect efficiently[9]. The detail strategy is described as following:

(1) 1st step: firstly, UHMWPE fibers coated by the initiator solutionwere irradiated under UV lamps, a large amounts of dor-mant semi-pinacol groups would be obtained after the initiatorabstracted hydrogen of the PE chains [7,10–14];

(2) 2nd step: the second (with monomer A), far-UV irradiation wasused to cleave this bond again, and a “quasi-living” polymeriza-tion via the recombination and photo-cleavage of semipinacolwas carried out and the radical polymerization of the monomerwould occur [15];

(3) 3rd step: the last, according to the second step described inprocedure (2), monomer B could be grafted onto the surface ofUHMWPE fibers stepwise.

By this method, inert polymer surface was easily functionalizedin this work, and a better improved IFSS was achieved compared tothe traditional ways [8].

Z. Li et al. / Applied Surface Science 257 (2011) 7600–7608 7601

equen

2

2

gwsP

2

wfid

m

Fm

Scheme 1. The schematic procedure and chemistry for the s

. Experimental

.1. Equipment

The experimental schematic diagram for surface photoinducedraft polymerization is shown in Fig. 1. The grafting temperatureas kept at about 30 ◦C, the radiation source were four high pres-

ure mercury lamps (two GGZ300/two GGZ125, � = 200–600 nm,hilips).

.2. Materials

Commercial UHMWPE fibers (Tensile strength 3.5 GPa, 103D)ere received from Super fiber Co. in China. To eliminate impurity,

bers were Soxhlet-extracted in acetone for at least 20 h [16] andried in a desiccator before grafting.

UHMWPE films were produced in our lab using a hot-pressachine (from Lab TECH).

ig. 1. The experimental schematic diagram for surface photoinduced graft poly-erization.

tial photoinduced graft polymerization of UHMWPE fibers.

Benzophenone (BP) from J&K chemical, USA, analytical regentgrade, was used as initiator.

Methacrylic acid (MAA, 99.5%) and acryl amide (AM, 99.5%) fromJ&K chemical, USA, were used as received without any purification.

Acetone (99.5%), ethanol (99.5%) and heptane (99.5%) fromShanghai Chemical Reagents Co. Ltd., China and the deionized waterwith a conductance (0.52 �S cm−1) were used as solvents.

Epoxy resin E-51 and the curing agent 593# were both receivedfrom Shanghai Xinhua resin plant, China.

Nitrogen from Shanghai Leinuo Gas Co. in China was of highpurity (99.999%).

2.3. Modification

2.3.1. 1st step: coupling of BP onto the surface of fibersFibers were first presoaked for 3 h in 30 ml benzophenone of

5 wt% in heptane solution. After that, the fibers were placed ina quartz tube which had been purged with nitrogen for 10 min.The temperature was kept at 30 ◦C by the heated nitrogen bub-bled through the solution (Fig. 1). UV irradiation was carried outfor 10 min with an intensity of 2.5×107 �W/cm2. The nitrogenleaving the reactor was absorbed in a gas washing bottle filledwith 0.1 mol/l NaOH. Thereafter, the samples were taken out andSoxhlet-extracted by refluxing hot acetone and then dried to a con-

◦

stant weight in a vacuum oven at 60 C.

2.3.2. 2nd step: photografting of MAAAfter the first step, fibers were soaked in 30 ml methacrylic acid

(MAA) solution (MAA dissolved in the mixed solution containing

7 Science 257 (2011) 7600–7608

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0% deionized water and 80% acetone) for 3 h, and then irradi-ted by UV irradiation for a given time. Thereafter the fibers wereoxhlet-extracted by refluxing hot acetone and washed with hotater several times, and then dried.

.3.3. 3rd step: further photografting of AMSimilar to 2nd step, acryl amide (AM) could also be grafted onto

he above resulting fibers.

.4. Characterization

Grafting degree (GD) was calculated according to the followingquation:

D (wt%) = W1 − W0

W0× 100 (1)

here W0 and W1 denotes the weight of fibers before and afterurface modification, respectively.

Attenuated total reflectance-infrared spectroscopy (ATR-IR)pectra of the UHMWPE fiber samples were recorded on a Nicoletexus 670 spectrometer which has an ATR accessory (PIKE ATR Max

I) utilized with ZnSe (n = 2.43) as the internal reflection elementafer.

The surface morphology of the grafted layer was observed by aEOL JSM-6360LV scanning electron microscopy (SEM).

Atomic force microscopy (AFM) observations were carried out inir at atmospheric pressure with a microscope PICO LE (Molecularmaging, Tempe, AZ). AFM images were acquired exclusively in theapping mode, with a silicon cantilever. For the analysis, one fibers fixed on a mica support by gluing each extremity of the fiber withouble-coated tape.

For a line containing N data points, the root-mean-squared (rms)oughness is given by the average deviation of the data, determinedy the standard definition:

ms =

√∑Ni=1(Zi=1 − Z̄)

2

N − 1

here Z̄ = mean z height. The rms has been calculated on the totalmage sample after a second-order flatness treatment of the rawata to take into account the curvature of the fiber surface. Imagesave been recorded on different zones in order to be representativef the total sample surface state.

Thermo gravimetric analysis (TGA) was performed using a NET-SCH TG209 F3 thermo gravimetric analyzer. The system wasperated in the dynamic mode in the temperature range 30–600 ◦C,t a heating rate of 20 ◦C/min. All the experiments were carried outnder a nitrogen atmosphere and the flow rate was 20 ml/min.

The Py-GC/MS experiments were carried out by a 2020is Pyro-robe pyrolyzer (Japanese Frontier Laboratories Ltd.) coupled to aCMS-QP2010 gas chromatograph & mass spectrometer (Japaneserontier Analysis Instrument Co.). Sample aliquots (about 1.00 mg)ere set in the platinum cup and dropped to the pyrolysis quartz

apillary. The pyrolysis was carried out under helium carrier gas atflow rate of 50 ml/min, and the temperature is 600 ◦C. The GC col-mn was a DB-5MS (30 m × 0.25 mm × 0.25 �m), and the GC ovenemperature was initially held at 40 ◦C for 3 min, was programmedo 250 ◦C at 10 ◦C/min and held for 10 min. The temperature of the

C/MS interface was set at 300 ◦C. Mass spectra were recordednder electron impact ionization energy of 70 eV, and the flow rateas kept constant. The MS detector was scanned from 29 to 600 m/z

t a scan rate of 1.8 scan/s. The data were searched in the NISTibrary 107.

Fig. 2. UV–vis spectra of pristine UHMWPE fibers and UHMWPE-SP fibers.

3. Results and discussion

3.1. The coupling of BP with the fibers

The UHMWPE fibers were firstly coupled with BP. To confirm theexistence of semi-pinacol (SP) groups, UV–vis spectra, Py-GC/MSand ATR-IR spectra have been used.

UV–vis spectra for the pristine fibers and UHMWPE-SP fibers aregiven in Fig. 2. For pristine UHMWPE fibers, the absorption peak at230 nm was strong which was attributed to the addition of antiox-idant 1010, but the absorption peak at 280 nm was extremely low.On the other hand, for UHMWPE-SP fibers, the absorption peaks atabout 280 nm and 230 nm were both strong, which correspondedto the absorption of benzene ring and hydroxyl group in the SPgroups.

The Py-GC/MS spectra of UHMWPE-SP fibers also illustrates thatthe benzene rings had been grafted onto UHMWPE fibers. Fig. 3ashows the total ions current (TIC) of UHMWPE-SP fibers, and thecharacteristic fragments of benzene and toluene were detected asshown in Fig. 3b and c.

To further prove the existence of SP groups, the UHMWPE filmswere also used as the substrate to be coupled with BP. Fig. 4 showsthe ATR-IR spectra of UHMWPE-SP films. In Fig. 4, the peaks at 1654,1586, 1544, 751.2 cm−1 are attributed to the absorption of benzenerings, which indicates the existence of SP groups.

3.2. Surface functionalization with MAA and AM

Fig. 5a shows the effect of UV irradiation time on the graftingdegree (GD) and grafting rate (GR) of MAA. During the first 2 minthe lamps was unstable and the irradiation density was low, sothe GD increased slowly. The grafting rate increased linearly andreached the peak at the 5th min. After 10 min of irradiation, theliquid layer was exhausted and the monomer in the nitrogen gaswould attribute little to the GD, and the grafting rate also declinedto zero. The effect of monomer concentration on the GD of MAAis shown in Fig. 5b. In this process, there were enough SP groupsto initiate the polymerization of MAA, so the GD increased linearlywith the increase of monomer.

Fig. 6a presents the effect of UV irradiation time on the graftingof AM in the 3rd step, which had the same tendency with the 2ndstep, but the optimum reaction time was more short than that of

the2nd step, because AM has the higher reactivity than MAA. Fromthe curves of GR, it also can be seen that the interzone of graftingtime of AM was narrower than that of MAA. The effect of monomerconcentration on the grafting degree of AM was shown in Fig. 6b.This can be attributed to that in this process AM had the higher

Z. Li et al. / Applied Surface Science 257 (2011) 7600–7608 7603

Fig. 3. The pyrolysis gas chromatograph (p

rtid

wtfiab1

F

Fig. 4. ATR-IR spectra of UHMWPE-SP films.

eactivity than MAA and there were no enough SP groups to initiatehe polymerization of AM. With the increase of the concentration,t was easier to form the homopolymers for AM also resulted in theecline of the grafting degree of AM.

The graft polymerization of MAA and AM onto the fibers surfaceere confirmed by FT-IR/ATR analysis. Typical spectra for the pris-

ine and modified fibers are depicted in Fig. 7. For pristine UHMWPEbers, the peaks at 1500–1700 cm−1 can be observed, which can bettributed to the absorption of the residual additives [17]. This cane confirmed by the disappearance of the peaks at 1500-1700 cm-in the extracted UHMWPE fiber (Fig. 7b). For UHMWPE-pMAA

ig. 5. Effect of the UV irradiation (MAA 40 wt%) and monomer concentration (irradiation

yrolysis-GC) of UHMWPE-SP fibers.

fibers, the absorption bands at 1696, 1389 and 1257 cm−1 in Fig. 7care assigned to C O stretching, –CH3 and C–O wagging vibration,respectively, which indicates the existence of grafted MAA. ForUHMWPE-pMAA-pAM fibers, the absorption bands at 1600 and1532 cm−1 in Fig. 7d are assigned to amide band I and II, and bandsat 3285 and 1370 cm−1 are assigned to –NH and –C–N stretchingof AM [18], which indicates the copolymerization of MAA and AMwas achieved.

The graft polymerization of MAA and AM onto the fibers surfacewere also confirmed by Py-GC/MS spectra of the grafted fibers areshown in Fig. 8.

The characteristic ions of MAA monomers (m/z 41 and m/z 69)shown in Fig. 8a (c and d) were detected to testify the pyrolysis ofpMAA. What is more, several new peaks (14.270 min, 20.005 min,and 24.615 min) in Fig. 8a (b) appeared comparing to the TIC of pri-mal UHMWPE fibers (Fig. 8a (a)). The detected fragment at retentiontime 14.265 min was attributed to 2-(2-methyl-1-cyclohexen-1-yl) propanal, which was due to the recomposition and Diels–Alderreaction of the pyrolysis fragments in pMAA chains (Scheme 2(2)).The detected fragment at retention time 20.005 min and 24.615 minwas attributed to some alkanes which maybe due to the decarboxy-lation of grafted chains (Scheme 2(3)).

The TIC of UHMWPE-pMAA-pAM fibers is shown in Fig. 8b, thecharacteristic ions of AM (m/z 26, m/z 52 and m/z 53) were detectedto approve the pyrolysis of PAM. The fragment at retention time

6.395 min approved the existence of PAM (Scheme 2(4)), whichmaybe due to the backbifing-chain transfer reactions [19,20].

In Fig. 8, the signal of MAA monomer is stronger than that ofAM, so it can be seen that pMAA was easy to break up and formthe MAA monomers, and pAM preferred to be cracked into alkyl

time 10 min) on the grafting of MAA; 1st step: BP 5 wt% irradiation time 10 min.

7604 Z. Li et al. / Applied Surface Science 257 (2011) 7600–7608

Fig. 6. Effect of the UV irradiation time (AM 25%) and monomer concentration (irradiatio40 wt% 10 min.

F(

cl

3

atpoUt(S

cg

ig. 7. ATR-IR spectra of (a) pristine UHMWPE fibers, (b) extracted UHMWPE fibers,c) UHMWPE-PMAA fibers and (d) UHMWPE-pMAA-pAM fibers.

yanides. Several main pyrolysis courses of the grafted chains areisted in Scheme 2.

.3. Thermal property of modified UHMWPE fibers

Figs. 9 and 10 show the TGA and DTG curves of pristine fibersnd the modified fibers. In Fig. 9, fibers coupled with BP had a higheremperature peak and initial degradation temperature than theristine fibers. This was assigned to the excellence thermal stabilityf benzene rings on the surface of UHMWPE fibers. Contrary to theHMWPE-SP fibers, there has been a sharp decline in the tempera-

ure peak and initial degradation temperature of the grafted fibers

Fig. 10). This was due to the break up of the chemical bonds of theP groups on the grafted fiber surface.

An increase of the thermal degradation residue (%) of the fibersan be observed in Fig. 10, which was attributed to the increasingrafting rate. From the initial degradation temperature, UHMWPE-

Fig. 8. Py-GC/MS spectrometry of UHMWPE-pMA

n time 10 min) on the grafting of AM; 1st step: BP 5 wt% 10 min and 2nd step: MAA

pMAA fibers and UHMWPE-pMAA-pAM fibers had a decreasedthermal stability than the pristine fibers. This was due to thedestruction of the PE chains owing to the grafting, which can beseen partly in the rms (Table 2). Compared to the UHMWPE-pMAAfibers, UHMWPE-pMAA-pAM fibers showed a higher initial degra-dation temperature and degradation temperature peak, this wasdue to the following factors:

The molecule weight of grafted chains of UHMWPE-pMAA-pAMfibers was larger than that of UHMWPE-pMAA fibers, and the chainswere more difficult to degradation, what is more, a cyclization reac-tion [6] shown in Scheme 3 involving the PAM and PMAA moietieswould take place during the course of heating and could absorbmore energy.

In the DTG curves of the grafted fibers, a gradually formednew thermo-gravimetric peak was observed at 795.5 K, which wasassigned to the degradation of hyperbranched PMAA-PAM chainsand polyethylene chains cross-linked [21].

3.4. Morphology of the modified UHMWPE fibers

Fig. 11 shows the SEM micrographs of the pristine fibers andmodified fibers. The extremely smooth surface of the pristine fiberscan be observed, which can also be observed in the AFM images inFig. 12a. After coupled with BP, there are no significant changes [22]between the pristine and UHMWPE-SP fibers (Fig. 11b).

In the SEM images and AFM images of UHMWPE-pMAA fibersmany ‘mushrooms’ grafted [23,24] can be observed, this was due tothe hyperbranched MAA grafted chains. The size of the mushroomsgrafted was about several hundred nm (Figs. 11j and 12e). In orderto verify the successful grafting, we used only one UV light sourceso that only semi surface [25] of the fibers having been exposed to

UV light and only this semi surface had been modified which canbe observed in Fig. 11c and d. In other words, Janus morphologyhad indeed been formed after the modification.

After the further grafting of PAM, the ‘mushrooms’ was encom-passed by polyacrylamide, an obvious coating layer was achieved,

A fibers and UHMWPE-pMAA-pAM fibers.

Z. Li et al. / Applied Surface Science 257 (2011) 7600–7608 7605

sis co

ato

3U

a

Scheme 2. The main pyroly

s shown in Figs. 11g–i and 12f. Thus, the results indicated thathe semi-pinacol groups successfully initiated the polymerizationf PMAA and PAM on the surface of UHMWPE fibers.

.5. Properties of the micro-composite reinforced by modifiedHMWPE fibers

Fig. 13 and Table 1 show the single fiber pull-out strengthnd IFSS of the micro-composite based on Epoxy resin and fibers

Fig. 9. TGA and DTG curves of pristine UH

urses of the grafted chains.

separately. The data show that grafted fibers had good adhe-sion because of polar groups such as carboxyl groups and amidegroups were connected to the surface of UHMWPE fibers afterbeing grafted [8]. As is shown in Fig. 13, the single fiber pull-out

strength of the composites based on the UHMWPE-pMAA-pAMfibers was 271.25% times of that of the composites untreated fibers,which was larger than that of the traditional ways [8]. This couldbe attributed to the introduction of the polar groups. When thelength of the fibers immersed in the resin reached 7 mm, the fibers

MWPE fibers, UHMWPE-SP fibers.

7606 Z. Li et al. / Applied Surface Science 257 (2011) 7600–7608

Fig. 10. The TGA and DTG curves of (a) pristine UHMWPE fibers, (b) UHMWPE-PMAA fibers and (c) UHMWPE-PMAA-PAM fibers.

Scheme 3. Thermal cyclization of grafted polyacrylamide on UHMWPE fibers.

Fig. 11. SEM micrographs at high magnification of (a) the pristine fibers/magnification 1000×; (b)UHMWPE-SP fibers/magnification 1000×; (c–f, j) UHMWPE-pMAAfibers/magnification 1000×, 2000×, 5000×, 10,000× and (g–i) UHMWPE-pMAA-pAM fibers/magnification 1000×, 2000×, 5000×.

Z. Li et al. / Applied Surface Science 257 (2011) 7600–7608 7607

Fig. 12. AFM images at high magnification of (a) the pristine fibers; (b) extracted UHMWPE fibers; (c–e) UHMWPE-pMAA fibers and (f) UHMWPE-pMAA-pAM fibers.

F immew

crefinTafi

TI

TT

ig. 13. The curves of pull-out load versus displacement (a) the length of the fibersas 7 mm.

ould not be pulled out and were broken when the pull out loadeached 22 N shown in Fig. 13b. What is more, the extracted fibersxpressed a slightly improved adhesion than that of untreated

bers. The reason may be due to the increased surface rough-ess [26] after extracting by acetone, which can be achieved inable 1. In Table 1 it can be seen that the extracted fibers hadhigher rugosity than pristine fibers. Compared to the extractedbers, the fibers grafted with MAA and AM had a lower rugos-

able 1FSS of the original UHMWPE fibers and grafted UHMWPE fibers.

GD (%) Original gaug

Original UHMWPE fibers 0.000 500.0Extracted UHMWE fibers 0.000 500.0UHMWPE-pMAA fibers 1.250 500.0UHMWPE-co-pMAA-pAM fibers 2.100 500.0

able 2he roughness analysis of pristine UHMWE fibers and modified UHMWPE fibers.

Image statistics Pristine UHMWPE fibers Extracted UHMWPE

Img. Z range (nm) 786.99 1558Img. Rms (Rq) (nm) 189.52 351.82Img. Ra (nm) 159.32 294.51

rsed in the epoxy was 3 mm and (b) the length of the fibers immersed in the epoxy

ity, this maybe due to the grafting of MAA and AM shown inFig. 12 (Table 2).

As is known, UV-induced grafting do extremely little harm to the

initial property of the fibers, and this was also achieved in Table 3. InTable 3, it can be seen that the excellent mechanical property of thefibers had no apparent change at all. This due to that polyethyleneis less sensitive to UV degradation and the grafting mainly occurredon the surface of the fibers.

e length (mm) Linear density (dtex) IFSS (MPa)

114.4 0.4715114.4 0.5118114.4 1.768114.4 1.910

fibers UHMWPE-co-PMAA fibers UHMWPE-co-PAM fibers

934.42 75.954154.47 12.613118.15 10.286

7608 Z. Li et al. / Applied Surface Scien

Table 3The mechanical properties of the original UHMWPE fibers and grafted fibers.

Initial modulus(GPa)

Breaking tenacity/(cn/dtex)

Breakingelongation (%)

Original UHMWPE fibers 143.8 32.28 3.424

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Extracted UHMWPE fibers 139.4 31.80 3.419UHMWPE-SP fibers 144.6 31.32 3.414UHMWPE-co-MAA fibers 141.8 31.08 3.442UHMWPE-co-PAA fibers 142.6 31.66 3.437

. Conclusions

In summary, the present work has demonstrated a versatilepproach for preparing the well modified UHMWPE fibers by aoutine UV-induced sequential surface grafting polymerization.ormant semipinacol groups were coupled onto the surface ofHMWPE fibers, and initiated the stepwise grafting of MAA andM. A variety of functional groups were grafted onto the surfacef UHMWPE fibers. The extensively improved interfacial bond-ng strength of UHMWPE fibers was achieved. In the followingesearch, varying monomers can be grafted onto the surface ofHMWPE fibers versus varying resins, furthermore, the activeroups grafted onto the fibers can supply enough anchor pointsor further reactions.

cknowledgements

We acknowledge the financial support of Shanghai Economicsnd Information Committee and Ministry of Finance People’sepublic of China. Special thanks to Mr. Yiping Tang from the Indus-rial catalytic laboratory for use of equipment and discussions. Morehanks to Mr. Jincheng Xia and Miss Xianting Shen from the Polymer

aterial laboratory for supplying the UHMWPE films.

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