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Melt foamability of reactive extrusion-modified poly(ethylene terephthalate) with pyromellitic dianhydride using supercritical carbon dioxide as blowing agent.

INTRODUCTION

Supercritical carbon dioxide (scC[O.sub.2]), as a kind of environmentally friendly blowing agent for polymer foaming, has been receiving significant attention in recent decades. In addition, scC[O.sub.2] dissolved in polymer matrix causes many changes in physical properties, for example, polymer crystallization, interfacial tension between the polymer phase and the gas phase, which may provide many opportunities for the manipulation of foaming process [ l ].

Polyethylene terephthalate) (PET) is widely used as fibers, bottles, and engineering plastics. PET foams, which show good mechanical properties and high temperature dimensional stability, can be used in packaging, thermal insulating, and construction applications [2, 3]. Nevertheless, the semicrystalline PET usually has well-structured backbone and low molecular weight, and meanwhile demonstrates low melt elasticity and melt strength, which cannot resist the intense elongational deformations during cell growth in the foaming process.

It is well known that high molecular weight, broad molecular weight distribution, and long chain branch attached to the polymer backbone are required to improve the polymer melt strength through enhancing the possibility of entanglement in the polymeric melt, which are implemented by means of in-situ modification [4] or reactive extrusion [5-9]. And reactive extrusion is the most attractive method, which is less expensive and can be performed effectively using the single or twin-screw extruders operated at normal conditions.

Modifying agents with electrophilic functional groups such as cyclic anhydride [3, 10] and epoxide [11, 12] were developed to react with the nucleophilic end groups of PET within the residence time limits of the extruders. And pyromellitic dianhydride (PMDA) has been frequently used as PET modifier. The melting point of PMDA, which is close to the PET processing temperature, together with four functional groups ensure that the reactions between PMDA and PET are fast [13, 14]. PMDA-modified PET foam, prepared by extrusion foaming process using C[O.sub.2] as blowing agent, was with the foam expansion ratio between three and seven times and cell diameter between 270 and 530 pm [15]. And the chemical blowing agent was also used to produce PMDA-modified PET foam by extrusion foaming process and the cell diameter between 31 and 44 [micro]m was achieved as reported by Coccorullo [16].

In the melt foaming process of semicrystalline polymer, cell growth is hindered and the cell structure is solidified by the increasing stiffness of polymer matrix due to the crystallizing behavior during the cooling stage. Therefore, a high non-isothermal crystallization rate is desired to prevent the cell growth sooner and obtain fine cells, and the crystallization property is important to cell stabilization. It was reported that the crystallization rate of PET increased with the lower content of branching while showed a decline trend with higher branching degree [17-19].

In this study, our interest was focused on improving the melt strength of PET by reactive extrusion to obtain foamable PET. And PMDA was chosen as the modifier. The reactive extrusion process was evaluated on the basis of intrinsic viscosity at different extrusion temperatures and residence times. The storage and loss modulus as well as the complex viscosity of PETs modified with different contents of PMDA had been measured, and the changes of dynamic shear rheological properties directly relate to the structural evolution of PET. Differential scanning calorimetry (DSC) was used to investigate the crystallization properties of modified PETs. Besides, the changes of crystallization rate and melt crystallization temperature range under high pressure C[O.sub.2] were determined by the high pressure DSC. At last, the foaming temperature windows of different modified PET samples were explored.

EXPERIMENTAL

Materials

The virgin PET (VPET) particles, with density of 1.37 g/[cm.sup.3] intrinsic viscosity of 0.67 dL/g (corresponding number average molecular weight [M.sub.n] = 20,300 g/mol) and melt flow index of 29.3 g/10 min, were purchased from Sinopec Shanghai Petrochemical Co, Ltd, and the PMDA powders were commercially available from Shanghai Aoke Industrial Co, Ltd, both of which were dried in the vacuum oven at 110[degrees]C overnight to avoid the hydrolytic degradation during extrusion process and then well blended with a given mass ratio. C[O.sub.2] (purity: 99.9%, w/w) and [N.sub.2] (purity: 99.99%, w/w) were purchased from Shanghai Chenggong Gases Co, China. Phenol and tetrachloroethane were analytically pure and were purchased from Shanghai Lingfeng Chemical Reagent Co, Ltd. All gases and reagents were used without any further purification.

Reactive Extrusion Process

Reactive extrusion was conducted in a Nanjing Giant Machinery, 45 mm diameter single-screw extruder (SJ-45) of L/D ratio 40, which is attached to a feeder operated at a constant production speed of 12 kg/h. And the extruder is equipped with a 3-mm rod die. In order to investigate the effect of reaction temperature on the IV of extrudates, the temperature settings from the second to the fourth zones were all [T.sub.E]. [T.sub.E] was the reaction temperature. The extruded strand was cooled in a water bath and pelletized. All extruded samples were grinded into powders and then dried at 110[degrees]C in vacuum overnight before characterizing or foaming.

Intrinsic viscosity (IV) is an indication of the molecular weight of PET, and it was determined using the Ubbelohde viscometer with a mixture of phenol and tetrachloroethane (3/2, w/w) solvents at 25[degrees]C.

Dynamic shear rheological properties of PET samples were measured using a Haake Mars III Rheometer (Thermo Fisher Scientific, USA) in a parallel disk mode under nitrogen. All samples were compression molded at 260[degrees]C into 35 mm (diameter) by 1.8 mm (thickness) disks. Complex shear viscosity [[eta].sup.*], storage modulus G' and loss modulus G" as a function of angle frequency [omega] (=0.1-10 rad/s) were determined under a constant stress 10 Pa at 275[degrees]C.

DSC (NETZSCH 204 HP, Germany) was used to measure the crystallization properties of PET under atmospheric [N.sub.2] and compressed C[O.sub.2]. The DSC chamber was swept by [N.sub.2] three times after 10-15 mg sample was loaded in it. And the sample was heated to 280[degrees]C immediately at a rate of 10[degrees]C/min. After maintaining at this temperature for 10 minutes to eliminate all crystals, the system was cooled to 50[degrees]C at different rates (2, 3, 4, and 5[degrees]C/min). To study the influence of compressed C[O.sub.2] on the crystallization behaviors of modified PET, the DSC chamber was pressurized by C[O.sub.2] up to a desired level (1-5 MPa) after sample's loading, following which the temperature program was set the same as mentioned above. To minimize the effect of the noise induced by compressed C[O.sub.2] environment, at least two DSC measurements were conducted for each measurement to ensure the repeatability.

Foaming Process

A high-pressure autoclave with an internal volume of 115 mL was employed in the batch foaming process (Fig. 1). The pressure of autoclave was detected by a pressure transducer with an accuracy of [+ or -] 0.1 MPa and was maintained with the pressurization system. And two thermocouples were attached to the temperature control system, which could keep the temperature of autoclave with an accuracy of [+ or -] 0.5[degrees]C. In a batch foaming process, 0.5 g PET powder was loaded in a stainless sample cell sealed in the autoclave. The system was swept by low pressure C[O.sub.2] three times. Subsequently, the autoclave was heated to 280[degrees]C, to guarantee the completed melting of PET matrix and obtain a high diffusion rate of C[O.sub.2], pressurized to 20 MPa and kept at this saturation condition for 20 minutes. Afterward, the autoclave was cooled to a foaming temperature, [T.sub.f], re-pressurized to 20 MPa, and kept at the foaming condition for another 10 minutes. And the evaluated [T.sub.f] was between 210[degrees]C and 280[degrees]C as discussed later. Then the pressure was quenched to the ambient pressure via a maximum depressurization rate of 330 MPa/s. Finally, the autoclave was opened to take out the foam sample for subsequent analysis after cooling in an ambient-temperature water bath.

The expansion ratio of PET foams ([R.sub.v]) was defined as the ratio of the bulk density of the modified PET ([[rho].sub.0]) to that of PET foams ([[rho].sub.f]) and determined by the equation:

[R.sub.v] = [[rho].sub.0]/[p.sub.f] (1)

And the densities of PET foams ([p.sub.f]) were measured according to ASTM D792-00 by means of weighing PET foams in water with the help of a sinker. [p.sub.f] was calculated as follows:

[[rho].sub.f] a/[a + w - b] [[rho].sub.water] (2)

where a is the actual mass of specimen in air without a sinker, w is the mass of the sinker totally immersed in water, and b is the mass of specimen and sinker completely immersed in water.

A NOVA NanoSEM450 (FEI, USA) scanning electron microscopy (SEM) was used to characterize the cell morphologies of PET foams. The samples were immersed in liquid nitrogen for 10 minutes and then fractured. The SEM scanned the fractured surfaces with platinum coating. The average cell size and cell density ([N.sub.0]) were obtained through the analysis of SEM photos with the software of Image-Pro Plus (Media Cybernetics, Silver Spring, Maryland). The number average diameter of all the cells (D) in a SEM photo was calculated as below:

D = [summation][d.sub.i][n.sub.i]/[summation [n.sub.i] (3)

where [n.sub.i] is the number of cells with a perimeter-equivalent diameter of [d.sub.i] And [N.sub.0] was defined as the number of cells per cubic centimeter of unfoamed PET calculated by the following equation:

[N.sub.0] = [[n/A].sup.3/2] Rv (4)

where n is the number of cells in the SEM photo, A is the area of the photo ([cm.sup.2]), and [R.sub.v] is the volume expansion ratio.

RESULTS AND DISCUSSION

Reactive Extrusion Process

The anhydride groups of PMDA react with the hydroxyl end groups of PET, which forms two carboxyl groups for each PMDA. And then the esterification occurs between hydroxyl end groups of PET and the formed carboxyl groups of PMDA, which can involve all functional groups of PMDA to generate chain extended, branched, and/or cross-linked structure [20, 21]. In the meanwhile, thermal/mechanical degradation reactions cannot be avoided and compete with the modifying reactions during the reactive extrusion process [22].

Reaction temperature, [T.sub.E], is an important parameter to the reactive extrusion process, which determines not only the rates of the modifying and degradation reactions but also the states of reactants. As shown in Fig. 2, during the experimental range, extension/branching reactions prevailed over the degradation reactions and IV of extrudates were always higher than that of VPET ([IV.sub.0]). When [T.sub.E] was below the melting point of PMDA (~283[degrees]C-287[degrees]C), IV of modified PET showed an uptrend with the rise of [T.sub.E]. PMDA got high reactive activity and the IV of extrudates reached the maximum value as [T.sub.E] was close to the melting point of PMDA. When [T.sub.E] exceeded 290[degrees]C, IV of modified PET decreased with the continuous increasing of [T.sub.E], indicating that the degradation reaction started to play a dominative role. So 285[degrees]C was chosen as the rational value for [T.sub.E] in the following experiments.

The residence time in barrel is inversely proportional to the screw speed for a given extruder [23]. Under the stable operation condition of the used extruder, IV increased with the residence time monotonously with the contents of PMDA 0.5 and 0.8 wt%, respectively, and no obvious degradation was observed (Fig. 3).

Only degradation reactions occurred during the extrusion process when there was no PMDA reacted with PET, and the IV of extrudate was lower than [IV.sub.0] (Fig. 4). When the content of PMDA reached 0.2 wt%, the extension/branching reactions prevailed over the degradation reactions, and the IV of modified PETs were higher than [IV.sub.0]. As expected, IV increased with PMDA content. However, an opposite trend was observed with the content of PMDA higher than 0.8 wt%, since the main reaction between PET and PMDA converted to inefficient network formation, such as end-capping reaction. Therefore, higher melt strength of modified PET was not expected any more as PMDA content exceeded 0.8 wt%. The subsequent analyses were carried out for PETs modified with 0.2, 0.5, and 0.8 wt% PMDA (designated as MPET 2, MPET 5, and MPET 8, respectively).

Rheological Properties of Modified PET

Dynamic shear rheological properties are sensitive to the topological structure of polymers. As shown in Fig. 5, the Newtonian-zone of linear VPET was broad and no obvious shear thinning was observed. With the increase of PMDA content, the transition from Newtonian-plateau to shear-thinning regime shifted to lower frequency region. And furthermore shear thinning was observed during the whole experimental range for MPET 8 and MPET 5, which was ascribed to the long-time-relaxation mechanism, such as entanglement couplings between high molecular weight fractions and those associated with long chain branch. In addition, a broad relaxation time distribution was also used to explain the shear thinning [5, 11]. Complex viscosity in low frequency was proportional to the molecular weight, and increased with PMDA content, which was consistent with the IV results.

It was reported that Cole-Cole plots, in which the axes are represented as the logarithms of G' and G", are independent of temperature and molecular weight but strongly dependent on the long chain branch and molecular weight distribution [8, 11]. With the introduction of long chain branch and/or broader molecular weight distribution, G' increases at a given G". As demonstrated in Fig. 6, compared with VPET, modified samples got broader molecular weight distribution and long chain branch due to the reactive extrusion process. Figure 6 also contains a line indicating the data for which G' = G", and the position of the plots with respect to this line indicates the degree of melt elasticity. It could be concluded that with the rise of PMDA content, the melt elasticity of modified sample increased.

Storage modulus G' was associated with melt elasticity [14], and as shown in Fig. 7, increased with PMDA content. Another important information obtained from Fig. 7 was that G" was always higher than G' for VPET, MPET 2, and MPET 5 during the experimental range, while G' was higher than G" as angle frequency exceeded 1.3 rad/s for MPET 8. The crosspoint of G' and G" should shift to lower frequency region when the molecular weight increases and to lower modulus region when molecular weight distribution becomes broader [24]. It was inferred from Fig. 7 that the crosspoints would be at higher frequency and higher modulus for VPET, MPET 2, and MPET 5 if existed. Therefore, MPET 8 had the broadest molecular weight distribution and the highest molecular weight.

Non-Isothermal Crystallization Behaviors of Modified PET

The Avrami analysis was extended for non-isothermal crystallization of modified PETs [17], which is similar to the crystallization process during the cooling stage of melt foaming process.

X(t) = 1-exp(-[Z.sub.r]t") (5)

where [Z.sub.t] is the constant of crystallization rate, n is the Avrami exponent, and X(t) is the relative crystallinity developed at time t.

According to DSC curves, for non-isothermal crystallization at a given cooling rate, the relative crystallinity X([T.sub.c]) can be obtained as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

where [T.sub.c] is the crystallization temperature, [T.sub.0] and [T.sub.[infinity]] represent the onset and end temperature of crystallization, respectively, and [dH.sub.c] is the enthalpy of crystallization released within an infinitesimal temperature dT. And [T.sub.c] can be converted to crystallization time t as Eq. 7:

t = [absolute value of p[T.sub.0] - [T.sub.c]'/R] (7)

where R is the cooling rate.

Applying logarithmic properties to both sides of Eq. 5 twice, Eq. 8 can be acquired.

log [-ln (1 - X(t))] = log [Z.sub.t] + n log t (8)

By plotting log [- ln (1 - X(t))] vs. log t with the relative crystallinity range of 3%-95%, a straight line should be obtained, which allows the estimation of n and [Z.sub.t]. And [Z.sub.t], can be further deduced to [Z.sub.c], the crystallization rate constant of non-isothermal crystallization, by the following equation:

log [Z.sub.c] = log [Z.sub.t]/R (9)

The crystallization rate constant [Z.sub.c] and experimental half-crystallization time [t.sub.1/2] were applied to compare the rate of non-isothermal crystallization directly. With the increase of cooling rate, the crystallization rate increased for all samples as exhibited in Table 1. And with the introduction of long chain branch, [Z.sub.c] became higher in general while the half-crystallization time decreased, both of which indicated that the crystallization rate increased within the experimental range of branching degree. And the introduction of long chain branch brings dual effects on crystallization [17-19]. The long chain branch, as a kind of defect of polymer chain when crystallizing, results in longer nucleation time required to exclude the branching point from crystal nucleates, and decreases the nucleating rate as well as nucleation density. On the other hand, branching provides more crystal growing space due to the mutual repulsion of chains, which is beneficial to the growth of crystals owing to higher mobility of polymer chains. For the crystallization of modified PET, crystal growth was the rate controlling step. It should be noted that the crystallization enthalpies ([DELTA][H.sub.c]) of samples with long chain branch were slightly lower than those of linear samples, which was attributed to fewer crystal nucleates. And the value of Avrami exponent n of VPET and modified PETs, based on the modified Avrami analysis, were all between 3 and 4. Therefore, it was concluded that all PET samples crystallized according to three-dimensional growth of crystallization with either homogeneous or heterogeneous nucleation.

Takata et al. has ever reported that C[O.sub.2] dissolution increases the crystallization rate of linear PET [25]. And the modified Avrami analysis was applied to the DSC curves obtained under 5 MPa C[O.sub.2] for MPET 5 and MPET 8 as well. Although 5 MPa, the maximum reliable pressure in the high pressure DSC, was lower than the foaming pressure, the DSC results can illustrate the effect of C[O.sub.2] dissolution on the crystallization rate of branched PET qualitatively. It was easily deduced that higher crystallization rates were obtained with C[O.sub.2] dissolution since [Z.sub.c] increased while [t.sub.1/2] decreased compared with those gained under atmospheric [N.sub.2], as presented in Table 2. C[O.sub.2] dissolved into PET matrix increased the free volume and weakened the interchain interaction, leading to higher mobility of PET chains, which facilitated the retraction and fold of molecular chains and meanwhile decreased the crystallization enthalpies [26]. And during the cooling stage in melt foaming process, the residual C[O.sub.2] in cells and polymer matrix could accelerate the crystallization process further and help to gain ideal cell morphology. The Avranti exponent n decreased when polymer crystallized under high pressure C[O.sub.2], which was consistent with the results reported by Takata [25, 27].

Foaming of Modified PET with scC[O.sub.2]

It is known that the melt strength of polymer decreases with the rise of foaming temperature. The formed cells will collapse at high foaming temperature due to the low melt strength of polymer and then no foam would be gained. No foaming windows were found for MPET 2 and VPET because of the lower melt strength and crystallization rate. As showed in Fig. 8, the maximum foaming temperature of MPET 5 was 250[degrees]C, which was lower than that of MPET 8. For MPET 8, foaming temperatures higher than 280[degrees]C may be feasible but had not been explored in this study, for the reason that there is no significance of such high temperatures for PET processing due to the severe thermal degradation. And slight collapse still existed when the foaming temperature was closed to the maximum value, especially for the foam of MPET 5. The lowest foaming temperature of melt foaming process was the onset temperature of crystallization, which decreased evidently for isotactic polypropylene and poly(lactic acid) with the increasing C[O.sub.2] pressure [26, 28]. But for MPET 5 and MPET 8, nearly no declined trends of the crystallization onset temperature were observed with the increase of C[O.sub.2] pressure as presented in Fig. 9. And the crystallization onset temperature can be extrapolated to be slightly higher than 210[degrees]C for both MPET 5 and MPET 8. The crystals were found in PET foams obtained at the foaming temperature 210[degrees]C, below which bulk densities were measured as a result of extensive crystallization and no PET foam could be gained. It was noteworthy that the crystallization temperature decreased with increasing C[O.sub.2] pressure, especially for MPET 5, due to the increasing mobility of PET chains [26]. The crystallization temperatures of MPET 8 were lower than 200[degrees]C under different gas atmospheres, on which the enhancing chain mobility had little effect.

With the increase of foaming temperature, the amount of blowing agent available for cell nucleation decreased and the surface tension between gas phase and polymer melts, reducing with C[O.sub.2] dissolution [29], increased as a result of lower C[O.sub.2] solubility. Therefore, nucleation decelerated according to the classic nucleation theory and lower cell density were obtained as presented in Fig. 10 [30]. On the other hand, higher foaming temperature decreased the viscosity of the polymer matrix, resulting in the decrease of the retractive force restricting cell growth, increased the diffusivity of C[O.sub.2] within the matrix and extended cell growth period. As a consequence, the average cell diameter and the expansion ratio increased. As for MPET 5 foam, due to the lower melt strength, more cells collapsed or coalesced as presented in Fig. 8. Thereby lower apparent cell density, especially at higher foaming temperature, and bigger cell size along with higher expansion ratio were obtained compared with those of MPET 8 foam. The cell size distribution depended on the growth period [31]. At higher foaming temperature, longer time was required to reach the crystallization temperature and to solidify the cell structure. The cell size distribution became broader with the increase of foaming temperature as described in Fig. 11. Besides, compared with MPET 5 foam, more uniform distribution of cell size was gained for MPET 8 foam at the same foaming temperature, since that cell growth was limited more intensively due to the higher melt strength.

CONCLUSIONS

Foamable PET was prepared by reactive extrusion with PMDA, and broad foaming temperature windows of modified PETs were gained based on the batch foaming process. The intrinsic viscosity of PET was enhanced greatly by reacting with PMDA, and the optimum PMDA content was 0.8 wt%. The reactive extrusion process improved the melt elasticity and strength of PET through increasing molecular weight, broadening molecular weight distribution and introducing long chain branch to the backbone, which was characterized by the dynamic shear rheology. In addition, the long chain branch structure increased the non-isothermal crystallization rate under ambient pressure [N.sub.2] and high pressure C[O.sub.2], which was beneficial to stop the cell growth sooner. The foaming temperature windows of MPET 5 and MPET 8 were 40[degrees]C and 70[degrees]C, respectively, while no foaming windows were found for VPET and MPET 2. The PET foam with diameter between 15 and 37 [micro]m, the cell density between 6.2 X [10.sup.8] and 1.6 X [10.sup.9] cells/[cm.sup.3], and the expansion ratio between 10 and 50 times were controllably produced. Compared with MPET 8 foam, MPET 5 foam demonstrated bigger cells, higher expansion ratio as well as lower cell density due to the lower melt strength.

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Tian Xia, Zhenhao Xi, Tao Liu, Xun Pan, Chaoyang Fan, Ling Zhao

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, China

This article was published online on IB August 2014. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected on 22 August 2014.

Correspondence to: Ling Zhao; e-mail ID: zhaoling@ecust.edu.cn Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 21176070; Contract grant sponsor: National Programs for High Technology Research and Development of China (863 Project); contract grant number: 2012AA040211; Contract grant sponsor: the joint research project for Yangtze River Delta; contract grant sponsor: Doctoral Program of Higher Education of China; contract grant number: 20120074120019; Contract grant sponsor: Fundamental Reasarch Funds for the Central Universities; contract grant sponsor: 111 Project; contract grant number: B08021.

DOI 10.1002/pen.23995

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Parameters of non-isothermal crystallization
under atmospheric [N.sub.2] for VPET and modified PETs.

                                                               [DELTA]
         R ([degrees]     [Z.sub.c]      [t.sub.1/2]          [H.sub.c]
PET         C/min)      ([min.sup.-1])      (min)       n       (J/g)

VPET          2             0.024           8.34       3.33    -40.96
              3             0.084           6.31       3.25    -39.86
              4             0.177           5.65       3.78    -38.91
              5             0.349           4.21       3.40    -38.22

MPET 2        2             0.039           8.42       3.07    -39.46
              3             0.163           5.12       3.11    -38.17
              4             0.216           4.38       3.90    -37.44
              5             0.375           4.20       3.16    -36.46

MPET 5        2             0.044           6.45       3.16    -34.05
              3             0.181           4.58       3.13    -33.84
              4             0.285           4.15       3.27    -33.07
              5             0.384           3.54       3.50    -32.09

MPET 8        2             0.044           6.17       3.23    -36.05
              3             0.123           5.14       3.62    -35.31
              4             0.241           4.31       3.64    -33.96
              5             0.408           3.39       3.37    -33.39

TABLE 2. Parameters of non-isothermal crystallization
under 5 MPa C[O.sub.2] for MPET 5 and MPET 8.

                                                               [DELTA]
         R ([degrees]     [Z.sub.c]      [t.sub.1/2]          [H.sub.c]
PET         C/min)      ([min.sup.-1])      (min)       n       (J/g)

MPET 5        2             0.069           5.22       3.01    -32.45
              3             0.270           3.48       2.86    -32.29
              4             0.450           2.83       2.72    -32.01
              5             0.603           2.08       2.27    -30.73

MPET 8        2             0.103           4.46       2.80    -34.00
              3             0.294           3.26       2.80    -29.23
              4             0.546           2.40       2.94    -25.06
              5             0.679           2.02       2.23    -20.56
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Date:Jul 1, 2015
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