cure kinetic, the effect of functionalized ethylene

7/29/2019 Cure Kinetic, The Effect of Functionalized Ethylene

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The Effect of Functionalized EthylenePropylene Diene Rubber (EPDM) on the

Kinetics of Sulfur Vulcanization of NormalRubber/EPDM Blends

Alex S. Sirqueira, Bluma G. Soares*

Introduction

Blends of natural rubber (NR) and ethylene propylene

diene rubber (EPDM) were extensively studied in order to

achieve more suitable elastomer materials which have a

better ageing resistance. However, their thermodynamic

behavior associated to the cure rate incompatibility

normally results in poor mechanical properties.[1,2] Several

approaches have been reported in the literature to solve

this problem, including the addition of a low molar mass

third component, such as transoctylene rubber (TOR),[3] the

incorporation of an accelerator molecule on the EPDM

backbone,[4] and the addition of anhydride-functionalized

EPDM.[57]

Our group has also recently reported on the efficiency of

EPDM modified with mercapto group (EPDMSH) as a

compatibilizing agent for this NR/EPDM blends.[811] The

compatibilizer effect is based on the ability of mercapto

groups to react with the double bond of the unsaturated

rubber (natural rubber) which results in strong inter-

actions between the components. The mercapto group

when incorporated on ethylenevinyl acetate copoly-

Full Paper

The effect of mercapto- and anhydride-functionalized ethylene propylene diene rubber(EPDM) or ethylenevinyl acetate (EVA) copolymers on the vulcanization kinetics of naturalrubber/EPDM blends was investigated using the oscillatory disk rheometer. The mercaptogroups in both EPDM andEVA copolymers resulted ina significant decrease of thecuring time. The Coransmodel was applied to setthe kinetic constants withineach distinct step of the vul-

canization process. The high-est curing velocity was perceived in a blend containing 2.5 phr of mercapto-functionalizedEVA. The functionalized EVA, especially that which was functionalized with anhydride groups,also displayed a lower solvent uptake on blending, which would imply an increase of thecrosslink density as well a covulcanization phenomenon.

A. S. Sirqueira, B. G. Soares

Instituto de Macromoleculas, Universidade Federal do Rio de

Janeiro, Centro de Tecnologia, Bloco J, Ilha do Fundao, 21945-970,

Rio de Janeiro, RJ, Brasil P.O.Box 68525

E-mail: [email protected]

62

Macromol. Mater. Eng. 2007, 292, 6269

2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mame.200600332


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mers (EVASH) was also efficient as

an interfacial agent for several blends

such as: NR/ethylenevinyl acetate

(EVA),[12,13] styrene butadiene rub-

ber(SBR)/EVA,[14,15] nitrile rubber

(NBR)/EVA,[1618]

and NBR/EPDMblends.[11,19]

Besides the compatibilization, the

cure process in rubber is of large

importance from the technological

and economic point of view, since it

can also affect the physical properties

and ageing resistance of the vulcani-

zates. Both crosslink density and the

crosslinks on elastomer blends are

influenced by the accelerator nature,

the sulfur: accelerator ratio, the reac-

tion temperature and time, and may

also be influenced by the compatibili-

zing agent. In this way the mercapto-

functionalized copolymers can also

affect the vulcanization parameters.[10]

In the case of the NR/EPDM blend, the addition of a

small amount of EPDMSH decreases both scorch and

optimum cure times, and increases the maximum

torque.[10]

Considering the different vulcanization characteristics

of EPDMSH-modified NR/EPDM blends, we have decided to

investigate the vulcanization kinetic of this system as well

other modified blends. This work reports the effect of

anhydride- and mercapto-functionalized copolymers onthe vulcanization kinetics of accelerated sulfur NR/EPDM

blends. The functionalized copolymers used in this study

include EPDM and EVA copolymers containing mercapto

groups (EPDMSH and EVASH, respectively) or succinic

anhydride groups (EPDM-g-MA and EVA-g-MA, respec-

tively). Besides the vulcanization kinetics, the effect of

these copolymers on crosslink density was also evaluated

from solvent absorption experiments.

A Brief Description of Corans Kinetic Model

Popular techniques used to study the kinetics of rubber

vulcanization include differential scanning calorimetry

(DSC), and oscillating disk rheometry (ODR).[20] The

last one is based on the fact that crosslink density is

proportional to the stiffness of the rubber. Figure 1

illustrates a vulcanization curve for a typical accelerated

sulfur vulcanization process. The first region corresponds

to the scorch delay or induction period that provides a

safe processing time. It is believed that this period

involves mostly the accelerator chemistry. The second

region corresponds to the curing period and the third

region corresponds to the maturation of the network by

overcure.[21,22]

The period that corresponds to the curing process can be

described by a general equation, which relates the conversion

degree (degree of crosslink) (a) with the time. For isothermal

systems, this equation may be written as follows:[21]

@a

@t

T

kTfa: (1)

The function k(T) can be expressed as an Arrhenius type

relation:

kT k0 eE=RT; (2)

where k0 preexponential factor; E activation energy;

R gas constant; T temperature (K).

The second term of Equation (1) is a mathematic ex-

pression for the kinetic model as a function of the conver-

sion degree, described as:

fa 1 an; (3)

where n is the order of the vulcanization reaction.

Several mathematical models have been proposed,

based on the vulcanization parameters obtained from

ODR. Good reviews on this subject can be found in the

literature.[23,24] One of the most popular and simplified

models for accelerated sulfur vulcanization was proposed

by Coranmore than four decades ago.[25,26] This model was

based on a general mechanism, whose steps are summar-

ized in Figure 2.[20,23,2729]

The Effect of Functionalized Ethylene Propylene Diene Rubber (EPDM) . . .

Figure 1. Typical curve of torque versus time obtained by oscilatory disk rheometery.

Macromol. Mater. Eng. 2007, 292, 6269 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mme-journal.de 63


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The first step consists of the formation of an active

accelerator complex formed by the interaction of accelerator

and activator. This complex can react with molecular sulfur

to form the sulfuring agent.[24,30] The sulfuring agent reacts

with rubber chains to form a crosslink precursor, which was

experimentally evidenced as an accelerator-terminated poly-

sulfidic pendant group attached to the rubber chains.[31,32]

The precursor turns into an activated precursor, for example,

a polythiyl radical, which undergoes the formation of

polysulfidic crosslinks. The activated precursor can also react

with the active sulfuring agent, giving rise to a nonactivated

precursor or can even decompose into inactive side products.

In a subsequent step, there is a maturation of thepolysulfidic

crosslinks, during which desulfuration and decomposition ofthese crosslinks take place. All these steps are well

documented and discussed in the literature.[23,24,2728,31]

The simplified kinetic scheme proposed by Coran for

accelerated sulfur vulcanization takes into account the steps

described above,[25,26] and was employed in the studies

involved in this work. The scheme is illustrated as follows:

A !k1

B !k2

B !k3

aVu

A B !k4

bB

where A is the accelerator and/or its reaction products,

that is, the active sulfurating agent (see Figure 2); B is the

crosslinking precursor; B

is an activated form of B, forexample, a polythiyl radical; Vu is the crosslink; a and b are

adjustable stoichiometric parameters.

To determine the constant k2, the torque variation is

plotted against time, as follows:[25,26]]

ln 1 DMtDM

k2t; (4)

where DM corresponds to the difference between max-

imum (MH) and minimum torques (ML) and DMt corre-

sponds to the difference between the torque at a particular

time and the minimum torque. Figure 3 illustrates this

plot. The velocity constant k2 corresponds to the negative

slope of the straight line portion of the curve after the

induction period, that is, the conversion range that follows

the first order kinetics, assuming the formation ofVu to be

first order in B.[25,26] The first order nature of the crosslink

formation is not achieved immediately upon the onset of

crosslink formation. The time required for crosslinking to

become an unperturbed first order reaction is assumed to

be the time tdis required for the depletion of A and

corresponds to the time required for the curvature in the

log plot to disappear. The time ti corresponds to the

intersection of the two regions of the log plot and is

considered as the induction period.Coran has also proposed the determination of the cons-

tant, k1, from a mathematical treatment, taking into consi-

deration the rate of disappearance of species A.[25] The


Figure 2. General scheme for the accelerated sulfur vulcanization.

Figure 3. Typical curve of ln1 DMt=DM versus time, accordingto the Coran model.[25,26]

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following expression was proposed in the Coran model:

k1tdis ln k1 k2tdis ln k2: (5)

For the solution of this equation, Coran assumed that k1/

k2 Z,[25]

and

k2tdis ln Z=1 Z: (6)

Coran has calculated values ofk2tdis for various values ofZ

and plotted them against one another. From the k2tdisvalues obtained from rheometer traces, the Z values and

consequently k1 were obtained.[25] The constants k3 and k4

cannot be determined separately. However, the k4/k3 ratio

can be determined by the following equation:[26]

k4

k3

MAU

CA DM

DMt ln k2e

k1t k1ek2t

k2 k1 ; (7)

where k4/k3 is an adimensional ratio of velocity constant;

CA is the concentration of accelerator expressed as parts

per 100g of rubber; MA is the molar mass of accelerator;

U is the number of moles of double bonds per 100g of

rubber. In NR:EPDM blends used in this work, U

corresponds to 0.74 mol 100g1, which was calculated

using the nuclear magnetic resonance technique. This ratio

indicates the tendency of an accelerator or the product

formed in the first stages of the curing process to inhibit

the crosslink.

Experimental Part

Materials

Natural rubber (NR, Hervea Brasiliensis, from Brazil) (SMR-

CV60), Mooney viscosity (ML 1 4 at 100 8C) 60, was

kindly supplied by Michelin do Brasil S. A (Rio de Janeiro,

Brazil). EPDM rubber (Keltan 65) [diene content 9.11 wt.-%;

ethylene content 51.7 wt.-%; Mooney viscosity (ML 1 4 at

125 8C) 49.3] was kindly supplied by DSM Elastomeros Brasil

Ltda (Rio Grande do Sul, Brazil). EVA copolymer (containing

18 wt.-% of vinyl acetate (VA); mass flow

index 2.3 g min1 at 120 8C) was kindly

supplied by Petroqumica Triunfo, Rio

Grande do Sul, Brazil. EPDM functionalized

with maleic anhydride (anhydride con-

tent 5.1 mmol 100g1 of polymer) was

supplied by Uniroyal. EVA functionalized

with maleic anhydride (vinyl acetate con-

tent 28 wt.-%; anhydride content 8.1

mmol 100g1) was supplied by Du Pont

Inc. Zinc oxide, stearic acid, sulfur, irganox

245 and N-cyclohexyl-2-benzothiazol

sulfenamide (CBS) were of laboratory reagent grade and

kindly supplied by the local rubber industries. Thioacetic acid

(TAA) and thioglycolic acid (TGA), analytical grade, from

Sigma Aldrich Chemistry, were used as received and

2,20-azoisobutyronitrile (AIBN) from Merck Chemicals was recrys-

tallized from a methanol/water solution.

Preparation of the Functionalized Copolymer

The preparation of mercapto-functionalizedEPDM was carriedout

according to a previous report.[8] The functionalization was

performed in two steps, illustrated in Figure 4.

The first step was performed in toluene solution at 70 8C for 48

h. In order to avoid crosslink formation during synthesis, the

molar ratios of TAA/AIBN and diene/TAA were established as 10.0

and 1.0, respectively. The thioacetate-modified EPDM (EPDMTA)

was submitted to an alkaline methanolysis using 5 wt.-% NaOH

solution in order to obtain EPDMSH. At these conditions an

amount of thioacetate or mercapto groups in the functionalizedcopolymers corresponding to 2.5 mmol 100g1 was achieved.

EVASH was synthesized in our laboratory by a trans-

esterification reaction between EVA copolymer and mercaptoa-

cetic acid, according to the literature.[33] The mercapto content

was found to be 0.13 mmol 100g1.

Blend Preparation

The blends were prepared in a two roll mill operating at 80 8C and

at 20 rpm. The NR was masticated for 2 min and then EPDM and

the functionalized compatibilizing agents (2.5 phr) were subse-quently added. After the rubber blend homogenization (about 4

min), theother ingredients were added in thefollowing order: zinc

oxide (5.0 phr), stearic acid(2.0 phr), irganox245 (1.0 phr), sulfur(S)

(2.0 phr) and CBS (1.0 phr). The processing time after each

component addition was about 2 min.

Rheometric Measurements

The vulcanization parameters and the mixes cure rate were

determined from the torque versus time curves obtained using

ODR (Tecnologia Industrial, mod T100, Buenos Aires, Argentina)at


Figure 4. The reaction scheme involved in the functionalization of EPDM with mercaptogroups.



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1 deg and 160 8C, according to the ASTM D-2084-81 method. The

crosslink decomposition was evaluated from the reversion degree

(R), according to the Equation (8):

R% MH MH30

MH 100; (8)

whereMH is themaximum torqueandMH30 is thetorque after 30

min from the maximum torque.

Thesolvent uptake in these blendswas determinedaccordingto

the literature.[34] For these experiments, the weight, w, of

20 20 2 mm cured test samples was measured accurately

weighted andthe samples were immersed into toluene in air-tight

test bottles. At regular intervals, the test samples were removed

from the solvent and dried between filter papers to remove the

excess solvent on their surface. After that, the samples were

weighted immediately and reimmersed in the solvent to permit

the continuation of the kinetic sorption until saturation in excess

liquid was established. The results of sorption experiments wereexpressed as the weight percent of solvent sorbed by 100g of the

blend versus the square root of time.

Results and Discussion

Curing Kinetics

Figure 5 and 6 compare the torque versus time profiles

of nonmodified NR/EPDM blends, with those contain-

ing 2.5 phr of mercapto- and anhydride-functionalized

copolymers, respectively. Blends containing mercapto-

functionalized copolymers presented a significantdecrease of scorch and optimum curing times, indicating

an accelerating action of mercapto groups, as previously

reported.[10] The EVASH-modified blend displayed the

lowest curing time, in spite of the lower amount of SH in

this copolymer. These results suggest a more effective

interaction between SH groups of EVASH and the cura-

tives. The anhydride-modified copolymers (Figure 6) did

not exert any significant influence on the curing time but

did contribute to an increase in the maximum torque,

which is an indication of increased crosslink density.

On the basis of the different behavior of these modified

blends, we decided to study the influence of the functional-

ized copolymers on the kinetic involved in different steps

related to the curing process, by using the Coran simplified

model, previously described in the Introduction. Figure 7

and 8 illustrate the dependence of torque variation against

time, for NR/EPDM blends modified with mercapto- and

anhydride-functionalized copolymers, respectively. From

these curves the kinetic parameters were calculated, as

summarized in Table 1.The k1 values were calculated from a mathematical

relationship proposed by Coran,[25] as summarized in the

Introduction part of this work. As observed in Figure 7, the

time tdis required for crosslinking to become an unper-

turbed first order reaction was significantly lower in

the case of EPDMSH- and EVASH-modified blends. These

results corresponded to an increase of k1 values, which


Figure 5. Torque versus time curves of NR:EPDM (70:30 wt.-%)blends (a) without functionalized copolymer and in the presenceof (b) 2.5 phr of EPDMSH and (c) 2.5 phr of EVASH.

Figure 6. Torque versus time curves of NR:EPDM (70:30 wt.-%)blends (a) without functionalized copolymer and in the presenceof (b) 2.5 phr of EPDM-g-MA and (c) 2.5 phr of EVA-g-MA.

Figure 7. ln1 DMt=DM versus time for NR:EPDM (70:30 wt.-%)blends (a) without functionalized copolymer and in the presenceof (b) 2.5 phr of EPDMSH and (c) 2.5 phr of EVASH.

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were summarized in Table 1. The highest k1 value was

found in the EVASH-modified blend, indicating a shorter

induction time; that is, those blends with higher velocity

than that of the accelerator and/or their reaction products

with sulfur turn into B species, the crosslink precursor.

These results confirm the participation of the mercapto

groups in the first step of the curing process, which is

probably due to possible interactions between the mercapto

groups and the accelerator used in the vulcanizing system,

which increase the consumption velocity of the accel-

erator. This interaction was similar to the first step of the

scheme illustrated in Figure 2 for general accelerated

sulfur vulcanization. The EPDMSH also increased k1 but theeffect was not as pronounced as in the case of EVASH. It is

believed that EVASH is more dispersed inside the rubber

system, improving the interaction between the SH groups

and curatives.

The presence of anhydride groups in the functionalized

copolymers (EPDM-g-MA and EVA-g-MA) also increased

the velocity of accelerator consumption (Figure 8), but the

effect was not as high as in the case of EVASH. The

anhydride group present in both EVA-g-MA and EPDM-g-

MA can react with the ZnO, similarly to the stearic acid

used as the activator on the formation of the sulfuring

agent, in the first step of the scheme illustrated in Figure 2.

The EPDM-g-MA presented at a higher velocity probably

because the higher number of anhydride groups in this

copolymer.

After the induction period, the conversion of B speciesinto B (activated form of B) characterized by the constant

k2 was also affected by the presence of EVASH, where a

little increase of this value was observed. The functiona-

lized EPDM (EPDMSH and EPDM-g-MA) also resulted in a

little increase of this constant, whereas EVA-g-MA resulted

in a decrease of k2 related to the nonmodified blend.

The k4/k3 ratio was significantly affected by the

presence of the functionalized copolymer. This ratio was

related to the competition between the vulcanization

process (k3) and the reaction between B and the accele-

rators, which gives rise to the crosslinking precursor (k4).

The lower the k4/k3 ratio value, the higher the tendency of

the system towards crosslink formation. The presence of

mercapto-functionalized copolymers resulted in lower k4/

k3 values, indicating that the crosslink formation was

favored, reducing the probability of the reverse reaction.

This phenomenon was particularly important in the

EVASH-based blend.

It is interesting to emphasize that the presence of

anhydride-functionalized copolymers (EPDM-g-MA and

EVA-g-MA) have resulted in an increase of this ratio,

suggesting that the reverse reaction was favored, as

compared with nonmodified blend.

Regarding the decomposition process of the cross-

linking, determined from the reversion ratio (R), a decreas-ed tendency towards reversion in blends containing

EPDM-based functionalized copolymer was observed.

The lowest value was found in blend containing EPDMSH.

The reversion process reflects the crosslink degradation,

which occurs via a free radical mechanism. The mercapto

groups are able to react with the free radical species before

they attack the rubber network. The NR/EPDM blends

modified with EVA- based functionalized copolymers pre-

sented higher reversion degree as a consequence of the

crosslink degradation during the postcure process.


Figure 8. ln1 DMt=DM versus time for NR:EPDM (70:30 wt.-%)blends (a) without functionalized copolymer and in the presenceof (b) 2.5 phr of EPDM-g-MA and (c) 2.5 phr of EVA-g-MA.

Table 1. The effect of the functionalized copolymer on the kinetic parameters of NR/EPDM blends, calculated by the Coran model.

Functionalized

copolymer

tdis ti k1 k2 Reversion degree

s s %

none 3.8 3.7 1.13 0.917 3.67

EPDMSH 2.2 1.6 1.65 0.945 1.43

EVASH 1.7 1.2 2.8 1.03 3.82

EPDM-g-MA 3 2.8 1.65 0.948 2.40

EVA-g-MA 3.4 3.1 1.39 0.862 5.90



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Sorption Experiments

It is well established that the sorption and diffusion of

penetrants through elastomer materials is controlled by

the crosslink density and other parameters, such as tem-

perature, presence of fillers, nature and size of penetrants,and so on.[35,36] The effect of functionalized EPDM and

functionalized EVA copolymers on the liquid sorption

behavior of different NR:EPDM (70:30 wt.-%) blends is

shown in Figures 9 and 10, respectively. The penetrant

used was toluene. All compatibilized blends displayed

lower solvent uptake than the nonmodified blend, which it

is an indication of an increase of the crosslink density.

The presence of 2.5 phr of EPDMSH or EPDM-g-MA resulted

in a decrease of solvent uptake but the nature of the

functionalized group did not exert any additional influ-

ence on this behavior (Figure 9).

Blends containing 2.5 phr of EVA-functionalized copo-

lymers presented interesting results (Figure 10). The

EVASH resulted in a significant decrease of solvent uptake,

but the lowest sorption was achieved in the blend con-

taining 2.5 phr of EVA-g-MA. Sujith et al observed similar

results in natural rubber/EVA blends and attributed this to

the crystalline regions of EVA, which put up stiffer resis-

tance to the penetrant molecules, thus leading to a lower

solvent uptake.[35] Sujith et als studies were performed

with blends containing 20 wt.-% or more of EVA.

Our systems contained only 2.5 phr of EVA. In addition,

the EVA-g-MA sample used in our system was obtained

from EVA containing 28 wt.-% of vinyl acetate, whereas

the EVASH sample was obtained from EVA containing18 wt.-% of vinyl acetate. The former was considered less

crystalline than EVA with 18 wt.-% of vinyl acetate.

Therefore, the significant decrease of solvent uptake

behavior in EVA-g-MA could not be only attributed to

an increase of crystallinity of the medium, but also, and

more importantly, to an increase of the crosslink density of

the blend, promoted by a better dispersion of this copoly-

mer as a consequence of its lower viscosity. This lower

solvent uptake was also attributed to a phenomenon

known as covulcanization - when both components and

the interfacial agent take part on the network. According

to the literature, if interfacial bonds are formed during

covulcanization, the lightly swollen phase will restrict

swelling of the highly swollen phase.[36,37] This phenom-

enon could occur in EVASH-modified blends and especially

in EVA-g-MA modified blend.

Conclusion

From the results obtained in this work, it is possible to

conclude that:

EVA- and EPDM-functionalized copolymers with a low

amount of mercapto or anhydride groups were able to

accelerate the vulcanization process of NR:EPDM (70:30)

blends in the presence of sulfur and CBS. However, they

affected the distinct steps involved in the curing process

in different ways.

The presence of 2.5 phr of EVASH resulted in a substan-

tial increase in the velocity related to the first step of the

vulcanization process, which was related to the con-

sumption of the accelerator.

Both EPDMSH and EVASH resulted in a decrease of the

k4/k3 value, indicating an increase of the velocity of the

crosslink formation.

Anhydride-functionalized copolymers favored the reac-

tion between B and the accelerators, giving rise to the


Figure 10. The effect of functionalized EVA on the toluene uptakebehavior of NR:EPDM (70:30 wt.-%) blends, (a) without functio-nalized copolymer and in thepresence of (b) 2.5 phr of EVASH and(c) 2.5 phr of EVA-g-MA.

Figure 9. The effect of functionalized EPDM on the tolueneuptake behavior of NR:EPDM (70:30 wt.-%) blends (a) withoutfunctionalized copolymer and in the presence of (b) 2.5 phr ofEPDMSH and (c) 2.5 phr of EPDM-g-MA.

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crosslink precursor (k4), as indicated by the higher k4/k3value.

The reversion phenomenon during the overcure period

was more important in blends containing functionalized

EVA, whose highest value was found in the EVA-g-MA

based blend. The addition of functionalized copolymers resulted in a

decrease of solvent uptake behavior indicating higher

crosslink density and also a covulcanization phenom-

enon. This behavior was more pronounced in blends

containing EVA-g-MA.

Acknowledgements: We would like to acknowledge the ConselhoNacional de Desenvolvimento Cientfico e Tecnologico (CNPq),Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior(CAPES), Financiadora de Estudos e Projetos (FINEP), and Fundacaode Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ), for thefinancial support of this project.

Received: August 28, 2006; Revised: November 1, 2006; Accepted:November 2, 2006; DOI: 10.1002/mame.200600332

Keywords: curing kinetics; elastomer blends; reactive compati-bilization; solvent uptake

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cure kinetic, the effect of functionalized ethylene

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