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Materials Chemistry and Physics 121 (2010) 295–301

Contents lists available at ScienceDirect

Materials Chemistry and Physics

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

eaction kinetics of functionalized carbon nanotubes reinforcedolymer composites

ingjing Qiua, Shiren Wangb,∗

Department of Engineering Technology, Texas Tech University, Lubbock, TX 79409, United StatesDepartment of Industrial Engineering, Texas Tech University, Lubbock, TX 79409, United States

r t i c l e i n f o

rticle history:eceived 18 February 2009eceived in revised form0 December 2009ccepted 21 January 2010

a b s t r a c t

Carbon nanotubes (CNTs) were modified by both chemical functionalization and mechanical shortening.The reaction kinetics of these CNTs-reinforced epoxy resins were examined according to the differentialscanning calorimetry (DSC) characterization. It was found that kinetics parameters and reaction conver-sion were significantly influenced by the undertaken functionalization, and the magnitudes of influence

eywords:arbon nanotubes (CNTs)eaction kineticsunctionalizationanocomposites

were dependent on specific functionalization methodologies. Mechanical shortening of CNTs elevatedthe size effect-induced interference and diminished the reaction enthalpy and conversion. Chemicalmodification showed a double-edge effect on the reaction kinetics and the final effect was dependenton the balance of size effect-induced interference and participation of surface functional groups intoreaction. Amino-grafted CNTs increased the activation energy of the curing reaction while epoxide-grafted CNTs slightly decreased the activation energy. These results will provide valuable guideline forthe nanocomposites design, fabrication, and applications.

. Introduction

Epoxy resin is a thermosetting polymer that widely applied fornsulating materials, coatings, adhesives and composite materi-ls [1]. Especially, epoxy resins satisfy the standard performanceequirement of advanced structural composite matrix, such as thetrength, stiffness and durability. Therefore, epoxy resin has beenhe norm in aircraft, aerospace and offshore boats for many years.

Enhancement of epoxy resins with carbon nanotubes (CNTs)as attracted a lot of attentions due to the unique structures andxceptional properties of CNTs as reinforcing fillers. The advantagef filling CNTs in polymer includes enhanced strength, improvedtiffness, better thermal stability and superior electrical conduc-ivity [2,3]. Therefore, CNTs-enhanced epoxy resin (CNT/epoxy)

ay offer more potential applications in micro-devices, sensorsnd electronic packaging [4]. In the recent years, chemical func-ionalization methods have been investigated to tailor the surfaceharacteristics of CNTs for improving the dispersion and enhanc-ng the interfacial bonding between CNTs and polymers [5,6]. Some

nteresting results have been observed for the thermo-mechanicalroperties of CNT incorporated polymer composites after function-lization [7–9]. Since the resulting nanocomposite properties aretrongly dependent on CNT modification and interactions between

∗ Corresponding author.E-mail address: Shiren.Wang@ttu.edu (S. Wang).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.01.039

© 2010 Elsevier B.V. All rights reserved.

CNTs and polymer matrix, it is crucial to study the influence ofchemical functionalization on the curing process of epoxy resinfor the nanocomposite design, characterization, and process opti-mization. It is also important to understand the effect of CNTfunctionalization on the curing kinetics in order to fully takethe advantages of CNT enhancement and to optimize materialsproperty.

During the curing reaction, epoxy molecules react with cur-ing agents to form a three-dimensional cross-linked network,which determines the thermomechanical properties of final parts.The degree and uniformity of curing reaction will significantlyaffect the bulk material properties. Many efforts have been madeto understand the curing kinetics and thermal properties ofepoxy/curing agent systems [10,11]. The curing kinetics of epoxyresin becomes different when various curing agents are employed[12,13]. Moreover, the cross-linking ratio between epoxy resins andcuring agents have also been investigated [14,15]. The variationof epoxy/curing agent is found to change the reaction activationenergy. Maximal curing rate also increases with the curing tem-perature and concentration of curing agent [16]. Lee et al. alsostudied the autocatalytic cure kinetics for an epoxy system byisothermal differential scanning calorimeter (DSC) analysis and the

autocatalytic expression can appropriately represent the curingkinetics [17]. Rou et al. studied the curing kinetics of diglycidylether of bisphenol A and diglycidyl ether of hydroquinone epoxyresin system and found that Sestak–Berggren equation is adequateto describe the cure kinetics of the selected epoxy resin system

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96 J. Qiu, S. Wang / Materials Chem

18]. Vyazovkin and Sbirrzaauoli studied the epoxy-amine sys-em, and found the dependence of activation energy on conversion19].

Surface modifications generate chemical groups on the CNTs,hich may participate in the curing reaction and affect the conver-

ion of curing reaction. Hence, the effect of CNT functionalization onuring kinetics will be an important topic for tailoring CNT/epoxyomposite properties. Some studies indicated that the additionf CNTs and other carbon fillers (such as carbon black, carbonbers or carbon nanofibers) influenced the curing reaction of epoxyesins, and the curing kinetics has been studied with Kamal’s model20,21]. It is well-known that functionalized CNTs demonstrate

wide range of surface characteristics due to the diversity ofodification methods, and the resulting chemical groups on CNT

urface may exhibit significant effect on the reaction kinetics ofNTs/epoxy. Fluorination and oxidization of CNTs were reportednd the effect of these functionalization methods on curing kineticsas studied [22–26]. However, different functionalization meth-

ds result in different chemistry of CNT surface, and their effectn epoxy curing kinetics may be different. For example, amino-rafting functionalization, will introduce amino-groups to CNTurface and their effect on their reaction kinetics are still unknown.

e investigated some unique chemical functionalization methodso tailor CNT surface [27–30], but the effect of these functional-zation methods on epoxy curing kinetics has not been studied.n this paper, CNTs are functionalized by both mechanical andhemical methods, including CNTs cutting, epoxy grafting andmino-grafting. The effect of CNTs functionalized by these uniqueethods on epoxy reaction kinetics is studied.

. Experimental

.1. Materials

The brand of epoxy resins is Epon862, purchased from Hexion Company. Single-alled carbon nanotubes (SWNTs) were purchased from Unidym Inc. produced byiPCo-process. Glycidyl methacrylate (GMA), stabilized 97%, was purchased fromisher Scientific Inc. Benzoyl peroxide (BPO, 97%, purchased from Sigma–Aldrichnc.) is used as a free radical initiator. Their structures are shown in Fig. 1.

Fig. 1. Structural formula of Epon

nd Physics 121 (2010) 295–301

2.2. Functionalization of CNT

2.2.1. Amino-functionalizationAt first, purified SWNT (p-SWNT) was ground to pasture with a little dimethyl

formamide(DMF) and then pasture was transferred to the beaker with DMF, sub-sequently BPO, diethyltoluenediamines (DETDA), Isoamyl nitrite and copper iodidewere added under stirring. The suspension was then subjected to ultrasonic pro-cessing at 40 W m−3 for 30 min (Ultra-sonication machine: Sonicator 3000 producedby Misonix Inc.). The dispersed SWNT mixture was transferred to a rotatable flaskequipped with a magnetic stir bar. The mixture was slowly heated to 80 ◦C androtated with the flask at the speed of 160 rpm as well as vigorous stirring. After12 h, suspension was filtered through 0.2 �m Teflon membranes. The filtered solidprecipitate was washed many times with DMF until filtration solution was clear.Subsequently, the precipitate was washed by water for several times. The resultantamino-functionalized CNT (N-CNT) membrane was dried in a vacuum oven (60 ◦C)for 10 h and collected for composite manufacturing. The degree of functionalizationis ∼4% by weight [27].

2.2.2. Epoxide-graftingIn the experiment, SWNTs was firstly ground with three drops of benzene, and

then transferred to a 250 ml beaker, which contained GMA. SWNTs were dispersedin GMA by ultrasonic processing for 15 min with a power of 30–40 W m−3. Subse-quently, the mixture was transferred to a rotated flask and BPO was added whenstirred. The mixture was slowly heated to 75 ◦C and kept for 24 h under strong stir-ring. Then the mixture after reaction was diluted with benzene and filtered through0.2 �m PTFE membrane, resulting in a thin film. Collected film was substantiallywashed with acetone, then ground and dispersed into benzene solvent for the infil-tration again. This washing processing was repeated for three times. Then the filmwas annealed at 60 ◦C for 10 h and weighed for record. The dried film was groundand dispersed into benzene for washing again. Then, the film was dried and weighedfor record. When the recorded weight was almost the same, the film was presumedto be clearly washed and we obtained epoxide-grafted CNTs (X-CNTs) [28,31]. Thegrafted epoxy group accounts for ∼54% by weight in the resulting functionalizedCNTs.

2.2.3. Mechanical cuttingThe pristine CNTs (P-CNTs) were dispersed into water and stabilized with surfac-

tant. Then the CNTs was aligned under a pressure about 150 kPa with a magnetic field

of 17.3 T [29]. After removing the surfactant, the aligned CNTs were dried and stackedup in the same direction under low temperature. A cryo-diamond knife (ultra-thin45◦ , Diatome Inc.) was employed in a Leica EM UC6/FC6 ultra-microtome machineto cut the caps of the aligned CNTs into 200 nm. It was reported that the open-endCNTs (S-CNTs) demonstrated a capability of delivery and enabled functionality ofextracellular agents [32].

862, EPI-W, BPO and GMA.

istry and Physics 121 (2010) 295–301 297

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the modifications also influence the curing enthalpy of the epoxycuring processing. Almost all the CNTs functionalization reducedthe curing enthalpy, but the reduction is dependent on the spe-cific functionalization methods, as shown in Table 1. For example,shortened CNTs show the largest magnitude of enthalpy reduc-

Table 1Comparison of non-isothermal DSC scan results.

Material name T0 (◦C) Tp (◦C) Q (J g−1)

J. Qiu, S. Wang / Materials Chem

.3. Preparation of CNT/epoxy composites

After the specific functionalization, SWNTs were ground to black paste withortar and pestle in a small amount of chloroform. Curing agent (based on the

equired curing ratio) was weighed and added to the SWNT paste. SubsequentlyWNTs and curing agent were mixed in the chloroform solvent under cup-hornltrasonic processing at 30–40 W for a half hour. Epoxy resins were diluted anddded to the SWNT suspension, resulting in a new mixture which was processedith cup-horn ultrasonic processing at the power of 12 W for 3 h. Finally, the mix-

ure was placed under high-speed shearing at 2000 rpm min−1 for 3 h. The resultantixture of SWNTs and epoxy resins was placed in the vacuum system for 12 h to

emove the residual solvent. The final mixture was sampled for DSC characterization.

.4. DSC characterization

Curing window is essential for the application of epoxy resins. The study ofhe curing kinetics is very useful in optimizing the curing conditions. In this study,he curing kinetics of the epoxy resin is studied by DSC Q-100 series from TAnstruments Inc. Curing kinetics of CNT/epoxy composite was investigated by bothon-isothermal dynamic scanning mode and isothermal mode of DSC tests. Bothhe non-isothermal scanning and isothermal scanning experiments were performedith a nitrogen flow of 40 ml min−1. The temperature range for non-isothermal

emperature test is from 25 ◦C to 350 ◦C. Isothermal study was performed at fiveifferent temperatures (180 ◦C, 190 ◦C, 200 ◦C, 210 ◦C and 220 ◦C). This calorimetricechnique allows for a quantitative measurement of the extent of the reaction andlass transition temperature (Tg). The reaction enthalpy and curing kinetics werelso tested.

. Model of cure kinetics

The kinetic analysis in this research is based on the assumptionhat the heat generated during the epoxy curing reaction is equalo the total area under the heat flow-time curve. Then the extent ofhe reaction can be determined from the heat of the reaction.

dQ

dt= Qtotal

d˛

dt= Qtotalk(T)f (˛) (3.1)

here dQ/dt is the heat flow, Qtotal is the total heat released whenuring, d˛/dt is the cure rate, ˛ is the extent of curing, k(T) is theate constant, T is the temperature and f(a) is the reaction model.or epoxy cures, f(˛) is usually considered in the form of (1 − ˛)n

reaction order kinetics) or ˛m(1 − ˛)n (autocatalytic cure) [33–38].he cure rate can be expressed by replacing k(T) with the Arrheniusquation:

d˛

dt= A exp

(− E

RT

)f (˛) (3.2)

here A is the pre-exponential factor, E is the activation energy,nd R is the gas constant. Especially, when a constant heating rates applied, the Eq. (3.2) can be transformed into:

d˛

dT= A

ˇexp

(− E

RT

)f (˛) (3.3)

here ˇ = dT/dt is the heating rate. Therefore, the extent of curing,, can be estimated by integrating DSC peaks for both isothermalnd non-isothermal conditions.

Usually, the curing kinetic parameters were obtained from fit-ing the data into the single-step reaction model, resulting in theverage curing activation energy for the overall curing process.owever, some studies also suggested that the curing process ofpoxy resins involve multiple steps and have different activationnergies in different stages. Therefore, more complex models suchs Kamal’s model [16–26] is:

d˛

dT= [k1(T) + k2(T)˛m](1 − ˛)n (3.4)

here m and n are the kinetic exponents of the cure reaction, k1(T)nd k2(T) are the rate constants with two different activation ener-ies E1, E2 and two different pre-exponential factors A1 and A2. Theonstant k1(T) can be graphically calculated as the initial reactionate at t = 0 [16–26].

Fig. 2. DSC test at a ramp rate 5 ◦C/min: (a) Pure Epoxy, (b) P-CNT/Epoxy, (c) N-CNT/Epoxy, (d) S-CNT/Epoxy, and (e) X-CNT/Epoxy.

The overall curing process of epoxy resin includes two steps. Theinitial acceleration is due to the autocatalysis in the first stage. How-ever, in the second stage, the reaction retards due to the gelatin. Thecrosslinking leads to an obvious increase in the exothermic peakof the curing matrix. When the curing temperature is above glasstransition temperature (Tg), the epoxy resin transforms from a rigidglassy state to an elastic rubbery state.

4. Results and discussions

In order to evaluate the effects of the CNTs on the curing kineticsof epoxy resin, a basic non-isothermal DSC dynamic scan was per-formed. The non-isothermal curing of various CNT/Epon862/EPI-Wcomposites at the same heating rate 5 ◦C min−1 are shown in Fig. 2.The mixture of Epon862 and EPI-W exhibited a broad exothermicpeak ranging from 100 to 300 ◦C. The onset curing temperature (To),the exothermic peak temperature (Tp) and the curing enthalpy (Q)values in Fig. 2 were summarized in Table 1.

We found that the exothermic peaks of the CNT/epoxy com-posites were smaller than that of the neat epoxy, implying thatthe addition of CNTs decreased the extent of conversion of theepoxy resins. It is possibly because that the existence of the CNTsphysically hindered the mobility of the epoxy monomers and alsoaffected the optimized curing ratio between Epon862 and EPI-W.It is evident that CNTs causes an acceleration effect on the epoxyreaction at the initial stage of curing and lowers the onset curingtemperature. Moreover, the functionalization of CNTs has a notice-able effect on the curing reaction kinetics of epoxy resin. Especiallywith the addition of epoxide-grafted CNTs (X-CNTs) into epoxy, theonset curing temperature shifted from 133.02 ◦C to 120.81 ◦C, thepeak temperature shifted from 178.08 ◦C to 173.92 ◦C. Moreover,

Pure Epoxy 133.02 178.08 361.9P-CNT/Epoxy 121.87 177.24 291.5N-CNT/Epoxy 127.64 179.95 273.9

S-CNT/Epoxy 122.22 178.73 256.7X-CNT/Epoxy 120.81 173.92 355.2

298 J. Qiu, S. Wang / Materials Chemistry and Physics 121 (2010) 295–301

tieigatasdpi

aC(ou(terCc

st

TC

Fig. 3. DSC scanning of the N-CNT/epoxy composite at different ramp rates.

ion. This may arise from the fact that shortening significantlymproves the dispersion and further enhance the size interfer-nce with the curing reaction. Amino-grafted CNTs also resultedn improved dispersion and further size interference, but amino-roups on the CNT surface also participated in the curing reactionnd partially compensate the effect of size interference. However,he low degree of functionalization (4%: 4 addends per 100 carbontoms) did not show noticeable contribution [27]. X-CNTs demon-trate the functionalization degree as high as 50% [28]. The highegree of epoxide grafting further compromises the size effect byarticipating the curing reaction and largely compensate the size

nterference-induced reduction.To achieve better understanding, the reaction was studied

t different ramping rates, and Fig. 3 shows DSC curves of N-NTs/epoxy/EPI-W system at ramping rate of 5, 10, 15 ◦C min−1

when these figures were overlapped, the software automaticallyptimized the layout of DSC curves for clarity by shifting curvesp or down). It indicates that Tp varies with different heating ratesˇ). Moreover, it demonstrates that Tp of the curing curves shiftso a higher temperature with an increasing heating rate for N-CNTnhanced epoxy composites. It was found that the initial curingate increased and the time to the maximum curing rate decreased.

ompared with neat epoxy, the activation energies of N-CNT/epoxyomposites are lower in the initial stage of curing reaction.

Similarly, DSC curves at various ramp rates for different curingystems were measured for the further kinetics analysis. In ordero determine the reaction kinetic parameters, the activation energy

able 2uring kinetic analysis of various CNT/epoxy composites at different ramp rates.

System Ramp rate (ˇ) (◦C min−1)

Epoxy/EPI-W5

1020

P-CNT/Epoxy/EPI-W5

1020

N-CNT/Epoxy/EPI-W5

1020

S-CNT/Epoxy/EPI-W5

1020

X-CNT/Epoxy/EPI-W5

1020

Fig. 4. Calculation of curing activation energy of N-CNT/Epoxy by regression anal-ysis.

was determined by fitting the data into the Kissinger’s equation[16–26]:

−ln

(ˇ

T2p

)= ln

(E

R

)− ln(An) − (n − 1) ln(1 − x)p + E

RTp(4.1)

where E is the activation energy, R is the gas constant(8.31 J K−1 mol−1), A is the pre-exponential factor, n is the orderof the reaction, and x is the extent of the reaction, respectively[16–26].

Taking N-CNTs/Epoxy/EPI-W as an example, the scatter graphbetween ln(ˇ/T2

p ) and 1/Tp is provided by Fig. 4.The activation energy can be determined by slope in the Fig. 4

and the calculation results are listed in Table 2. According to resultsin Table 2, dispersion of CNTs into epoxy system significantly affectsits curing X-CNTs reduced the activation energy. Shorting CNTsdemonstrate similar effect as P-CNTs does. This can be explainedby the shortening effect. Shortening CNTs just make CNTs shorterthan 300 nm while no chemical functional groups were covalentlyattached to the surface. Shortened CNTs have the size in the samescale as epoxy resin molecules, and may interference the diffusiv-

ity of epoxy and curing agent molecules, resulting in size effecton curing kinetics. Both chemical modifications demonstrate bothpositive and negative effect on the chemical reactions. Surfacemodifications-induced dispersion further enhance the size effectwhile covalently bonded groups (amino or epoxy groups) partic-

Tp (◦C) Q (J g−1) E (kJ mol−1)

178.1 361.951.6206.4 387.4

220.1 390.4

177.8 291.559.4198.5 296.7

217.3 310.4

179.9 273.956.0202.0 276.2

219.8 285.1

178.7 256.761.1199.2 278.7

215.4 307.7

173.9 355.247.5196.3 391.7

209.5 426.9

J. Qiu, S. Wang / Materials Chemistry and Physics 121 (2010) 295–301 299

C tests

iedmv

taats

vr[

˛

f

Fig. 5. Heat flow of isothermal DS

pate the curing reactions and partially compensate for the sizeffect [27]. Since X-CNTs are highly functionalized and show theegree of functionalization as high as 50%, they eventually showsore positive effect on the reaction, resulting in reduction of acti-

ation energy.In addition of ramping test of various CNT/Epon862/EPI-W sys-

em, isothermal DSC scanning at the different temperatures werelso carried out. Specifically, the isothermal tests were performedt 180 ◦C, 190 ◦C, 200 ◦C, 210 ◦C and 220 ◦C, respectively. The evolu-ions of heat flow with curing time at the specific temperature arehown in Fig. 5.

It was reported that the extent of reaction (a, or called the con-ersion percentage) at different times can be determined from theatio between the heat evolved up to a time t and the total heat24–26].

= �HT

�Htotal× 100% (4.2)

The reaction rate, d˛/dt as a function of time t was calculatedrom the measured heat flow (dH/dt):

d˛

dt= dH/dt

�Htotal(4.3)

at several specific temperatures.

where dH/dt is the heat flow as a function of the curing timeobtained from isothermal DSC scan curves and �Htotal is the reac-tion heat evolved up to a certain time at temperature of T for anisothermal experiment. The evolutions of reaction rate were calcu-lated according to the corresponding heat flow, and are shown inFig. 6.

Based on the curves in Fig. 5, and then a complex model as shownin Eq. (3.4) is assumed in this paper. Subsequently, the kineticsparameters were calculated by regression analysis and the resultsare shown in Table 3.

When Eq. (2.4) is used for epoxy/amine system, the overall reac-tion order is known as 2. In this case, the m + n value was about 2. Itis found that the cure temperature shows the positive effect on them value and negative effect on the n value. Considering the results,m is thought to be related with the maximum cure rate, and n isconsidered to be related with the post-cure reaction. The k1 valuesincreased with increasing cure temperature. Surface treatment of

carbon nanotube also slightly increased the k1 value in compar-ing to pure CNTs. The conversion of curing reaction is significantlyinfluenced by the CNTs and specific surface modification.

Since CNTs are in the same size scale of epoxy molecules andinterference with curing reactions by influence the molecular diffu-

300 J. Qiu, S. Wang / Materials Chemistry and Physics 121 (2010) 295–301

SC te

stte

TC

Fig. 6. Reaction rate of isothermal D

ivity, pure CNTs shows negative on the curing reaction and reducehe conversion of curing reaction. Some high-degree chemical func-ionalizations, such as epoxy-grafting, partially compensate for thisffect and improve the conversion.

able 3uring kinetic analysis of various CNT/epoxy composites at different temperatures.

System Isothermal Temperature (◦C) k1 min−1

Epoxy/EPI-W180 0.046190 0.056200 0.063

P-CNT/Epoxy/EPI-W180 0.042190 0.051200 0.062

N-CNT/Epoxy/EPI-W180 0.041190 0.054200 0.064

S-CNT/Epoxy/EPI-W180 0.04190 0.053200 0.061

X-CNT/Epoxy/EPI-W180 0.048190 0.055200 0.066

sts at several specific temperatures.

In addition, it is interesting to find that there is a dependenceof activation energy on the conversion in the CNT/epoxy/curingagent system. For X-CNT/epoxy curing system, the conversion isthe lowest, but the reaction energy is highest. Short-CNT/epoxy

k2 min−1 m n �f (%) At 40 min

0.63 0.87 1.13 740.67 0.92 1.08 770.71 0.96 1.01 81

0.61 0.86 1.12 630.65 0.88 1.11 660.72 0.91 1.05 69

0.62 0.85 1.14 580.69 0.86 1.12 630.75 0.92 1.02 68

0.62 0.86 1.12 600.68 0.84 1.09 650.73 0.90 1.03 71

0.62 0.87 1.13 710.69 0.91 1.03 740.75 0.98 1.01 77

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J. Qiu, S. Wang / Materials Chem

ystem, N-CNT/epoxy and P-CNT/epoxy systems showed similarxten of conversion and also similar activation energy. These exper-mental results verified the theoretical predictions in the reportediteratures [19]. This may stem from the conversion-dependent

olecular diffusivity.

. Conclusions

This study examines the reaction kinetics of functionalized CNTsnd polymer matrix. It is found that kinetics parameters and reac-ion conversion are significantly influenced by the undertakenunctionalization, and the magnitudes are dependent on specificunctionalization methodologies. Chemically modification shows aouble-edge effect on the reaction kinetics and the final effect isependent on the balance of size-effect interference and graftedhemical group participation into reaction. P-CNTs, N-CNTs and S-NTs increased the activation energy of the curing reaction while-CNTs decreased the activation energy. All the above function-lization methods lead to the low conversion of reaction. It waslso found that there is a dependence of activation energy on theonversion in the CNT/epoxy/curing agent system.

cknowledgement

Authors acknowledge the support from Texas Tech Universitytartup funding.

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