Foaming of Low Density Polyethylene with Carbon Dioxide Based on

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Foaming of Low Density Polyethylene with Carbon Dioxide based on Its insitu Crystallization Behavior Characterized by High-Pressure Rheometer Chen Wan, Yiquan Lu, Tao Liu, Ling Zhao, and Weikang Yuan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02842 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

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Industrial & Engineering Chemistry Research

Foaming of Low Density Polyethylene with Carbon Dioxide based on Its in-situ Crystallization Behavior Characterized by High-Pressure Rheometer Chen Wan, Yiquan Lu, Tao Liu*, Ling Zhao and Weikang Yuan Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China. *

Corresponding author. Tel.: +86 21 64253470; fax: +86 21 64253528. E-mail: [email protected] (T. Liu)

1

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Abstract The crystallization behaviors of low density polyethylene (LDPE) under ambient N2 and compressed CO2 were characterized using high-pressure differential scanning calorimetry (CO2 pressure ranging from 0.1 to 8 MPa) and high-pressure rheometer (CO2 pressure ranging from 0.1 to 28 MPa), respectively. The modulus variation during the crystallization characterized by HP-Rheometer indicated that there were three typical regions, where the modulus was nearly invariable with time corresponded to the crystallization beginning, the modulus increased dramatically corresponded to the crystallization ongoing and the modulus stabilized at a higher value corresponded to the crystallization ending. The foaming was conducted at different storage modulus while other operation factors such as saturation pressure and temperature were maintained consistent. The results suggested that the crystal would affect the bubble size distribution even constrained the bubble growth when the relative crystallinity was more than 50%. Below this value, the crystal could facilitate cell nucleation and improve the melt strength. Keywords: Foaming, High-Pressure Rheometer, Crystallization

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1. Introduction Depending on the polymer matrix, plastic foams can be generally categorized into two major groups, amorphous polymer foams and semi-crystalline ones. As the representatives for the former, polyurethane (PU) and polystyrene (PS) foams occupy the majority of foams market shares1-5. The semi-crystalline plastic foams include low-density polyethylene (LDPE), polypropylene (PP) and polyethylene terephthalate (PET) foams providing improved mechanical properties and thermal properties due to their existed crystals. Especially, the polyethylene (PE) foams have been widely used in the applications such as cushioning, packaging and wire & cable materials due to the relative low density, good chemical resistance, excellent electrical properties and low cost 6. The rheological properties play an extremely important role on the polymer foaming. Although the rheology of polymeric materials can be manipulated by reactive modification, blending with rheological modifiers and crystallization, it is still unknown how to get control on the foam process quantitatively by rheology

7, 8

.

In general, polymers with a pronounced shear-shinning behavior were more suitable for melt foaming than those appearing the obvious first Newtonian-region in the low frequency range

9-11

. The loss factor (the ratio of loss modulus to storage modulus)

kept at a lower value was in favor of the bubble growth control, indicating that increase of the storage modulus was beneficial to the foaming process

7, 8, 12, 13

. Thus,

long-chain branch was usually introduced into the linear-structure polymers (e.g., PP, PET and poly(lactic acid) (PLA)) to adjusting the melt foaming results feasibly 3


14-16

.


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Moreover, the polymers foamability, determined by their rheological properties, was also influenced by the processing conditions. A higher saturation pressure and lower temperature often resulted in larger viscoelasticity, which was beneficial for the polymer foaming

10, 11

. For a multi-phase system like crystalline polymers, a small

amount of crystal could also enhance the storage modulus dramatically

17

. From the

perspective of rheology, the crystal formation might be helpful for the foaming. Crystals can also act as a heterogeneous agent to improve cell nucleation and maintain foam structure due to the increased storage modulus during the foam stabilization process

18-20

. The interface between crystal lamellar and amorphous

domains is a high energy region leading to the preferential nucleation of cells 21, 22. On the other hand, an excessive amount of crystals resulted in an improvement of the viscoelastic properties of polymer-gas mixture significantly, which hindered the expansion of foams. Furthermore, gas cannot dissolve into the crystalline regions effectively, ultimately leading to non-uniform bubble distribution in the regions with different crystallinity and structures. Park et al. investigated the influences of shearing and dissolved-CO2 on the crystallization of branched poly-lactide (PLA) and found that the crystallization allowed high-expansion-ratio microcellular foams to be stably produced over a wide temperature window in a foaming extrusion process 23. Li et al found that the presence of compressed CO2 postponed the crystallization peak to a lower temperature region while effectively reduced the half-crystallization time and enhanced the crystallinity of PLA and PP using high-pressure differential scanning calorimeter (HP-DSC). On the basis of these results, a two-step quenching process 4


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was proposed to fabricate the foams using ordinary PLA and PP

24, 25

. The crystals

were also introduced into the PP matrix as the nucleating agent for a better foaming behavior by matching the crystallization and foaming

26

. All the work indicated that

crystallization played important roles on the polymer foaming, favorable or unfavorable. However, in these work, only could the ex-situ crystallization behavior be investigated because the foaming pressure was much higher than the pressure that HP-DSC could realize. In addition, the crystallinity degree was merely qualitatively characterized and correlated in the foaming behavior since control of the crystallinity and facilitation of the cell nucleation and bubble growth were challenging during the foaming process. There are various factors affecting polymer crystallization. Apart from the temperature, cooling rate and CO2 pressure, the flow fields including the shear and/or extensional flow are also correlative with the crystallization process

27

. Several

methods have been conducted to characterize the crystallinity under high pressure but far from the foaming conditions. The crystallization behaviors of polymer/gas solution were mainly characterized by using HP-DSC microscopy (POM)

28-31

, polarized optical

32, 33

, magnetic suspension balance (MSB)

34, 35

, in-situ

high-pressure Fourier transform infrared spectroscopy (FTIR) and wide/small angle X-ray scattering (WAXD/SAXS)

36, 37

. The maximum allowable pressure of the

HP-DSC for polymer/CO2 system was restricted below 10 MPa because higher pressure gas introduced inacceptable noise/signal ratio

24, 25, 35

. High-pressure FTIR

and MSB were used to characterize those polymers with extremely low crystallization 5



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kinetics such as PET and polycarbonate (PC) by considering the change of the molecular conformation or CO2 solubility variation on account of the arising crystals. In fact, dynamic mechanical measurement has been applied to follow the crystallization behaviors via monitoring the storage modulus and/or viscosity dependence of time and/or temperature by using rotational rheometry under atmosphere conditions

37-40

. The experimental technique can be employed to

investigate the shear-induced crystallization which matched the practical processing conditions. Furthermore, the shear and subsequent measurement of the mechanical spectrum are combined to detect even small microstructure changes in the polymer materials. It could be available for those systems other methods did not work, for example, the colored system and/or the composite filled system

38, 41

. However, this

method was rarely applied at high pressure due to the difficulty in characterizing the rheology behavior under high pressure. Raps et al have attempted to investigate the onset crystallization temperature of high melt strength polypropylene under high pressure and gas-loading using high-pressure rheometer without involving in the crystallization kinetics 42. In this work, a modified high-pressure rheometer was used to characterize the crystallization behaviors of LDPE under compressed CO2 at the pressures ranging from 0.1 to 28 MPa through observation of the storage modulus variation. The isothermal crystallization behaviors were investigated in detail and batch foaming was simultaneously conducted on the LDPE with different crystallinity according to the storage modulus variation. The foaming results were quantitatively correlated with the 6


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crystallinity, which advanced the understanding of the role of crystallinity on the foaming process. 2. Experimental Materials. Low-density polyethylene (LDPE, DFDA-1253NT) pellets with melting flow index of 1.8 g/10 min were supplied by the DOW Chemical Company. The crystallization and melting temperature of the LDPE were 97.1 °C and 115.3 °C, determined by DSC (NETZSCH DSC 204 HP, Germany) under atmospheric N2. The samples obtained were compression molded at 160 °C into circular geometry with outer and inner diameters of 30 mm and 14 mm, respectively, and thickness of 2.0 mm for the rheology characterization and foaming. CO2 (purity: 99.99%, w/w%) was purchased from Air Products Co., Shanghai, China. Characterization of crystallization kinetics. The crystallization kinetics of LDPE under N2 and compressed CO2 was characterized by using HP-DSC and high-pressure rheometer. For HP-DSC measurement, the sample was saturated at 40 °C held for 15 min and different pressures of gas, heated to 160 °C at the rate of 10 °C/min, held for 10 min to eliminate all crystals and thermal history, and then cooled to 50 °C at a rate of 2 °C/min. During the non-isothermal crystallization process, the relative crystallinity (Xt)) for DSC can be obtained as below:

Xt

∫ = ∫

Tc

T0 T∞

T0

(dHc / dT )dT (1)

(dHc / dT )dT

where Tc is the crystallization temperature, T0 and T∞ represent the onset and ending crystallization temperature, respectively. dHc is the enthalpy of crystallization released 7



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within an infinitesimal temperature dT. The rheology characterization of LDPE was conducted by using the HAAKE MARS III high pressure Rheometer. As shown in Figure 1, compared with the original configuration, a support block was fixed below the rotor with a gap of 2 mm. The thermocouple contact with the barrel of the block for a precise test on the polymer/gas temperature. On the basis of the modification, the diffusion distance for CO2 into LDPE was reduced from 20 mm to 4 mm corresponding to the reduction of the saturation time from 12 hr to 2 hr approximately. In addition, the quantity of the sample needed was reduced significantly which avoided the foaming samples filling the chamber thoroughly. The procedure was described as follows for characterization of the isothermal crystallization kinetics. The sample was loaded on the gap between the rotor and the support block at the temperature of 150 °C and different CO2 pressures, and held for 2 hours for thoroughly melting of the crystallization region and the generation of LDPE/CO2 homogeneous solution. Thereafter, the LDPE/CO2 solution was quickly cooled to the desired crystallization temperature. An angular frequency of 0.5 rad/s was preformed to trace the evolution of storage modulus (G’) with time until the crystallization completed. Figure 1 For characterization of the crystallization process by using HP-Rheometer, the relative crystallinity was estimated by applying a logarithmic normalization of the G’ data in dynamic mechanical measurements, as Pogodina et al 43 proposed,

X (t) =

' log G ' (t) − log Gmin ' ' log Gmax − log Gmin

8


(2)

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where G’(t) was the storage modulus at time t, G’min and G’max the starting storage modulus and ending plateau storage modulus, respectively. It should be noted that the obtained rheological results were not the materials’ actual value because the contact surface was just the bottom area without the side and upper surface. Even so, the relative value was still suitable for characterization of the relative crystallinity by analyzing the evolution of storage modulus (G’) dependence of time

39

. The crystallization process was characterized through the response of the

sample to a low enough oscillatory strain at the relatively lower fixed frequency or a low enough shear rate. It should be emphasized that the imposed strain or shear rate must be low enough not to influence the crystallization kinetics and damage the sample 38. In this work, a controlled stress of 40 Pa corresponding strain of 0.2%-1% was adapted to avoid the crystallization kinetic to be affected as small as possible. The Avrami equation was employed to analyze the crystallization kinetics of LDPE samples, lg  − ln (1 − X ( t ) )  = n lg t + lg k

(3)

where Xt was the relative crystallinity of the sample at time t, the crystallization kinetics constant, k, for nucleation and growth rate, and the Avrami exponent, n, that reflect the mechanism of crystal nucleation and growth with value between 1 and 4. n and the logarithm of kinetics constant, ln(k), can be determined by plotting ln[-ln(1Xt)] versus ln(t). For the non-isothermal crystallization process, the Jeziorny method based on the Avrami model was used to determine the crystallization kinetics constant as below, 9



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lg k ' = lg k / R

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(4)

where k’ is the non-isothermal crystallization kinetics constant, and R the cooling rate. The crystallization half-time, t1/2, defined as the time at which the relative crystallinity is 50%, can be determined as below: 1n

 ln 2  t1 2 =    k 

(5)

Foaming Process. As shown in Figure 1, a pressure-quench process was used to fabricate LDPE foams in the high-pressure chamber. Cooling water was cycled around the barrel for a precision control of the electric heating process. CO2 is delivered into the cell via a TELIDE S-486-JN-60 pump with the maximum output pressure up to 42 MPa. A valve of type Swagelok SS-1RS8MM was mounted on the cover plate to release CO2 with a depressurization rate of 300-400 MPa/s in the chamber. The cell morphologies of the foamed samples were observed by using a JSM-6360LV (JEOL Ltd. Tokyo, Japan) scanning electron microscopy (SEM). The samples were immersed in liquid nitrogen for 10 min and then fractured. The average cell size was obtained through the analysis of the SEM photographs by the software Image-Pro Plus. Details of the analysis can be seen from literatures 32, 44. The volume expansion ratio, Rv, defined as the ratio of the bulk density before and after foaming, is estimated using Eq. (6): Rv =

ρ0

ρf

(6)

where ρ0 and ρf are the mass densities before and after foaming; ρf, is measured 10


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according to ASTM D792-00 by weighing a sample of the polymer foam in water using a sinker. The cell density, N0, the number of cells per cubic centimeter of solid polymer is determined from Eq. (7):

 nM 2  N0 =    A 

3/2

Rv

(7)

where n is the number of cells in the SEM micrograph, M the magnification factor, A the area of the micrograph (cm2), and Rv the expansion ratio.

XRD analysis. Wide-angle X-ray diffraction (WXRD) patterns of LDPE foams were obtained with an X-ray diffractometer (D8 Advance, Bruker) to characterize their crystallinity. The samples were exposed to an X-ray beam generated by a Cu-Kα radiation at 40 kV and 40 mA. The foamed samples were first compressed between two PP films to form a flat surface using plate vulcanizing machine for 30 min at room temperature. They were then stacked and scanned in the angular region of 3-50° with step size of 0.02°.

3. Results and Discussion 3.1 Non-isothermal crystallization behavior of LDPE under compressed CO2 Figure 2 shows the cooling curves of LDPE under atmospheric N2 and compressed CO2 at different pressures. Table 1 lists the corresponding crystallization parameters derived from the Avrami equation modified by the Jeziorny’s model. As expected, the crystallization onset temperature, Tc,onset, and crystallization temperature, Tc, shifted to a lower value with increasing CO2 pressure due to the strong plasticization effect of gas on the polymer matrix 33. In fact, the plasticization mainly attributed to two aspects, i.e., static pressure and dissolved gas, which was analogous 11



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to the rheology properties under high pressure CO2

44, 45

. The results showed that the

plasticization effect resulted from the dissolved CO2 was dominant to the thickening effect on account of the static pressure. The dissolved CO2 induced LDPE swelling so as to increase the free volume, which must facilitate the chains mobility to depress the crystallization temperature.

Figure 2. Table 1. In fact, the crystallization onset temperature was closely related to the foaming process. Xia et al proposed that the lowest temperature available for the melt foaming process was the Tc, onset for the semi-crystalline polymer, which was associated with the high-pressure CO2 atmosphere. The highest foaming temperature should be controlled by the melt elasticity and strength

46, 47

. As shown in Table 1, the

depression magnitude of Tc, onset (△Tc, onset = Tc, onset N2 - Tc, onset CO2), measured by using HP-DSC, as a function of CO2 pressure is: Tc, onset = 101.4 - 0.718×P (MPa). However, the △Tc was just 4 °C even at the saturation pressure of 28 MPa when using HP-Rheometer. It was far below that HP-DSC measured as shown in Figure 3. Moreover, the start modulus decreased with increasing the saturation pressure. Initially, the storage modulus increased nearly linear with decreasing the temperature indicating no crystal formed. The modulus increase was due to the temperature decrease that constrained the chain mobility. When crystallization took place, the storage modulus increased dramatically until the crystallization completed as shown

12


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by the plateau region at the end of the cooling process. The modulus increase in the crystallization region was attributed to two aspects, i.e., the temperature decrease that impeded chains mobility and the crystal formation. The formed crystal nucleus acted as solid particles to improve the storage modulus. Obviously, the crystal formation played a dominant role in the modulus variation. In fact, the magnitude of depression of melting and crystallization temperature was associated with the dissolved CO2 in the matrix and the stress including shear and extension flow that put on the polymer

48, 49

. The high quantity of dissolved CO2 and

low stress would strengthen the depression magnitude. In this work, the minor shear when using HP-Rheometer would facilitate the crystallization though the stress and strain were low.

Figure 3 3.2 Isothermal crystallization behavior of LDPE under compressed CO2 The isothermal crystallization behavior of LDPE was investigated using HP-rheometer under atmospheric N2 and different compressed CO2. Figure 4 shows the evolutions of storage modulus, G’, measured during the isothermal crystallization at the temperature of 110 °C. The growth of G’ was attributed to the filler effects of the crystals. The G’ exhibited a constant value before starting crystallization, increased rapidly and then approached the plateau values at the ending of the crystallization stage. As the CO2 pressure increased, the time at the intersection defined as the induction time for the crystallization, shifted to the longer time.

13



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Figure 4. At each CO2 pressure, Avrami exponent n, k’, t1/2 and standard deviation, derived from the Avrami plots, are listed in Table 2. The n is lying between 1 and 2, indicating the one-dimensional heterogeneous crystal growth dominated the process which was consistent with that under HP-DSC as shown in Table 1. In addition, the crystallization rate decreased with increasing CO2 pressure, and the corresponding t1/2 also increased obviously. The effect of CO2 on the crystallization rate of polymers has been investigated previously. Takada et al have thoroughly investigated the effect of dissolved CO2 on the isothermal crystallization behaviors of PET, PP and PLLA 28-30. Their results suggested that the gas dissolved in the polymers increased their free volume, hence the mobility of the polymeric chains by reducing the inter-chain interactions. The crystallization rate was accelerated by CO2 at the temperature in the crystal-growth rate controlled region and depressed in the nucleation controlled region. The boundary of the crystal nucleation and growth region was defined based on the equilibrium melting temperature (Tm0) and glass transition temperature (Tg)

28-30

. The

temperature of the overall maximum temperature (Tmaxc), which was nearly equal to the mean of the Tm0 and Tg: Tmaxc = (Tg + Tm0)/2, should be nearly 35 °C. In this work, both of the non-isothermal and isothermal crystallization process were located on the nucleation-controlled region. Thus, the dissolved CO2 obstructed the crystal nucleation and as well as the crystallization rate.

Table 2.

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3.3 Foaming behaviors of LDPE with different crystallinity From the non-isothermal crystallization process, the crystallization onset temperature was known to be postponed to a lower temperature by the dissolved CO2. On the basis of the isothermal crystallization process under compressed CO2, there were three distinct regions as the crystallization onset, ongoing and ending. The foaming strategy was conducted at different storage modulus during the small amplitude oscillatory sweep process. The other foaming parameters such as the foaming temperature, CO2 pressure (15 MPa) and depression rate (300~400 MPa/s) were maintained constant.

Figure 5 shows the overall morphology of LDPE foams prepared with different relative crystallinity at 110 °C. When the temperature was reduced to the desired temperature, the oscillatory sweep test proceeded 5 min at which the corresponding Xt was zero. It can be seen that the cell walls of the obtained foams are quite thin, and the bubble coalescence and collapse is visible as shown in Figure 5(a). Foaming at this stage, the expansion ratio is quite high as well as the bubble size. When the Xt increased to 25% as the time proceeded 12 min, the cell walls become relatively thick and the shape of most cells is polygonal due to the increased melt strength as shown in Figure 5(b). In addition, the expansion ratio increased from 28 to 32 with average bubble diameter decreased from 155 µm to 125 µm. When the Xt increased to 37%, the expansion ratio and average bubble size both reduced obviously as shown in

Figure 5(c). It should be noted that no crystal region could be observed in the above mentioned three SEM images since the nucleation might dominate the process. The 15



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relative crystallinity improvement mainly was derived from the crystal grain which could also serve as foaming nucleation agent.

Figure 5. However, when the Xt reached 49%, the red arrows pointed in Figure 5(d) show the distinct crystal region occurred gradually. The bubble diameter at the interface between crystal region and amorphous area is larger than other area due to two aspects. One was the nucleation, as the interface between the crystal region and the amorphous area was favorable to the cell nucleation. Bubble tended to form earlier at these interface, leaving the bubble to have longer time to grow during foaming. The other was the nucleated bubbles in the surrounding amorphous region would also be collapsed due to cell coarsening/ripening. These CO2 would diffuse to the larger bubbles and make them larger. Further increase of the Xt led to smaller bubbles paralleled with several larger bubbles, as shown in Figure 5 (e, f and g). This phenomenon was analogous to the PP foaming process using two-step pressure quenching method

25

. During the crystallization process, the gas would be excluded

from the crystal region resulted in infant cells. During the crystallization process, CO2 inclined to diffuse into these cells forming larger bubbles instead of generating cell nuclei

50

. In addition, the cell density increased dramatically indicating that the

formed nuclei facilitated the cell nucleation indeed. When the crystallization completed, as shown in Figure 5(h), the matrix possessed too much higher “melt strength” so that no obvious bubbles appeared. The bubble size and density even cannot be counted since no shaped bubbles could be considered. Figure 6 shows the 16


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evolution of expansion ratio, mean cell diameter and cell density against relative crystallinity. The results indicated that the expansion ratio peaked at the Xt of 25%, and the bubble size decreased with increasing crystallinity.

Figure 6. For a better understanding on the effect of crystallinity on the foaming behavior, the isothermal crystallization and foaming behaviors were also conducted at 105 °C.

Figure 7 shows the evolution of storage modulus against shear time. Each ending modulus corresponded to the conditions at which the foaming process was performed. The morphology of the generated foams was shown in Figure 8. Both the expansion ratio and bubble size decreased with increasing crystallinity. There was no obvious crystal region and large bubbles even at higher relative crystallinity. The lower crystallization temperature was beneficial to the crystal nucleation while inhibited the growth in comparison with the foaming temperature at 110 °C. Thus, it could be assumed that the density of nuclei increased while the crystal size did not change at 105 °C. The foaming was just conducted at 110 °C and 105 °C because the crystallization rate would be too slow at high foaming temperature and too fast at low foaming temperature. If the crystal dominated the polymer matrix corresponding to a relative lower foaming temperature, the gas escape would become serious resulting in low expansion ratio and irregular bubble structure. If the crystallinity was lower, the crystal might be beneficial for the cell nucleation and bubble growth control. In addition, the crystallization resulted in not only the gas release from crystal, heterogeneous nucleation, and stiffness, but also the strain/stress-induced cell 17



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nucleation around expanding bubbles (especially those around heterogeneous nucleating site)

18-19

, which affected the foaming behaviors. It was important to

determine the threshold value for crystallinity to relate the crystallization behaviors with foaming which would also be instructive for other semi-crystalline polymers. However, due to the HP-Rheometer itself especially the torque limitation, it was not easy to adjust the temperature and pressure in a wide range.

Figure 7 Figure 8 Figure 9 shows the normalized XRD patterns of the foamed samples assigned as crystalline and amorphous peaks. The crystalline regions are defined at approximately 21.4°, 23.9° and 36.4° in 2θ. The typical amorphous regions were located at 19.3° and 30.0° nearly. The crystallinity was calculated by dividing the total area of crystalline peaks by the total area under diffraction curve. The crystallinity increased with consistent of the relative crystallinity. The crystallinity increased from 22% to 43% while the corresponding Xt from 0 to 100%. The crystal would affect the bubble size distribution even constrained the bubble growth when the crystallinity was more than 32%. Below this value, the crystal could act as an effective nucleation agent or improve the melt strength.

Figure 9. 4. Conclusions The non-isothermal and isothermal crystallization behaviors of LDPE were 18


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carefully characterized under ambient N2 and high-pressure CO2 by using HP-DSC and HP-Rheometer, respectively. HP-Rheometer investigated the crystallization kinetics at the CO2 pressures of as high as 30 MPa which was much higher than that HP-DSC could reach. Avrami equation was chosen to characterize the crystallization kinetics at 110 °C under various pressures. The results suggested that the crystallization rate decreased obviously with increasing CO2 pressure because the gas swollen the free volume which impeded the crystal nucleation rate. In addition, the nucleation onset temperature was postponed to a lower temperature under CO2 atmosphere. The modulus variation characterized by HP-Rheometer indicated that there were three typical regions, as modulus was nearly invariable with time corresponding to the crystallization beginning, modulus increased dramatically corresponding to crystallization ongoing and modulus stabilized at a higher value corresponding to crystallization ending. On the basis of these results, a batch foaming process was established to investigate the effect of the relative crystallinity on the LDPE foaming behavior quantitatively. The gas was being released varied with the storage modulus to fabricate the LDPE foams. The foaming results indicated that the bubble size decreased and expansion ratio increased as the Xt increased from 0 to 25%. It was because the formed crystal nuclei be beneficial to the cell nucleation as well as the melt strength improvement. Further increase of the Xt led to smaller bubbles and lower expansion ratio. When the Xt increased to 80% or more, too much crystal would hold up the bubble growth. As a result, the shaped bubbles could merely be observed.

Acknowledgements 19



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This work was supported by the National Key Research and Development Program of China (2016YFB0302204) and the National Science Foundation of China (21376087).

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23



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Captions Figure 1. Schematic of the experiment setup for the rheology characterization as well as foaming process.

Figure 2. Representative DSC cooling curves of LDPE melt at cooling rates of 2 °C/min under N2 and compressed CO2 during non-isothermal crystallization.

Figure 3. Representative cooling curves of LDPE melt at cooling rates of 2 °C/min under N2 and compressed CO2 when using HP-Rheometer.

Figure 4. Changes of storage modulus, G’, with time during crystallization at 110 °C under different saturation gas pressures.

Figure 5. Evolution of foaming morphology against different relative crystallinity at the temperature of 110 °C and CO2 pressure of 15 MPa. (a) Xt =0, (b) Xt =25%, (c) Xt =37%, (d) Xt =49%, (e) Xt =67%, (f) Xt =80%, (g) Xt =91%, (h) Xt =100%.

Figure 6. Characterization of LDPE foaming samples obtained at different relative crystallinity. (a) expansion ratio, (b) mean cell diameter, (c) cell density. The lines only indicated the trend.

Figure 7. Evolution of storage modulus against shear time at 105 °C and 15 MPa. Each ending modulus corresponded to the conditions when foaming performed.

Figure 8. Evolution of foaming morphology against different relative crystallinity at temperature 105 °C, pressure 15 MPa. (a) Xt =0, (b) Xt =21%, (c) Xt =33%, (d) Xt =47%, (e) Xt =69%, (f) Xt =82%, (g) Xt =93%, (h) Xt =100%.

13761648037

Figure 9. XRD patterns of foamed LDPE at 110 °C with different Xt. Table 1. Kinetics parameters and standard deviation (R2) based on the Avrami model of non-isothermal crystallization for LDPE under ambient N2 and compressed CO2.

Table 2. Kinetics parameters and standard deviation based on the Avrami model of isothermal crystallization for LDPE under ambient N2 and compressed CO2 at 110 °C using HP-Rheometer. 24


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Figures and Tables

Figure 1. Schematic of the experiment setup for the rheology characterization as well as foaming process.

25


8.0 MPa CO2 6.0 MPa CO2 4.0 MPa CO2

DSC, mw/mg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Endo


2.0 MPa CO2 0.1 MPa N2

80

85

90

95 100 105 110 Temperature (oC)

115

120

Figure 2. DSC cooling curves of LDPE melt at cooling rates of 2 °C/min under N2 and compressed CO2 during non-isothermal crystallization.

26


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16000 0.1 MPa 6.2 MPa 12.5 MPa 22.4 MPa 28.1 MPa

14000 12000

G' (Pa)

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10000 8000 6000 4000 2000 0 90

100 110 Temperature (oC)

120

Figure 3. Representative cooling curves of LDPE melt at cooling rates of 2 °C/min under N2 and compressed CO2 using HP-Rheometer.

27



14000 0.1 MPa 6.0 MPa 12.0 MPa 20.0 MPa 30.0 MPa

12000 10000

G' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8000 6000 4000 Tc, induction

2000 10

100

1000 time (s)

10000

Figure 4. Changes of storage modulus, G’, with time during crystallization at 110 °C under different saturation gas pressures.

28


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Figure 5. Evolution of foaming morphology against different relative crystallinity at the temperature of 110 °C and CO2 pressure of 15 MPa. (a) Xt =0, (b) Xt =25%, (c) Xt =37%, (d) Xt =49%, (e) Xt =67%, (f) Xt =80%, (g) Xt =91%, (h) Xt =100%.

29



180

35

(b) Mean Diameter (µm)

105 oC 110 oC

(a)

30 Expansion Ratio

25 20 15 10

150

105 oC 110 oC

120 90 60 30

5

0

0 0.0

0.2 0.4 0.6 0.8 Relative Crystallinity Xt

Cell Density×10-6 (cells/cm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12

0.0

1.0

0.2 0.4 0.6 0.8 Relative Crystallinity Xt

1.0

(c)

105 oC 110 oC

9 6

3 0 0.0

0.2 0.4 0.6 Relative Crystallinity Xt

0.8

1.0

Figure 6. Characterization of LDPE foaming samples obtained at different relative crystallinity. (a) expansion ratio, (b) mean cell diameter, (c) cell density. The lines only indicated the trend.

30


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14000

Storage Modulus G' (Pa)

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12000 10000 8000

Xt = 0 Xt = 21% Xt = 33% Xt = 52% Xt = 69% Xt = 82% Xt = 93% Xt = 100%

6000 4000

time

Figure 7. Evolution of storage modulus against shear time at 105 °C and 15 MPa. Each ending modulus corresponded to the conditions when foaming performed.

31



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Figure 8. Evolution of foaming morphology against different relative crystallinity at temperature 105 °C, pressure 15 MPa. (a) Xt =0, (b) Xt =21%, (c) Xt =33%, (d) Xt =47%, (e) Xt =69%, (f) Xt =82%, (g) Xt =93%, (h) Xt =100%.

32


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Normalized Intensity

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Xc=43% Xt=100% Xc=42% Xt=91% Xc=41% Xt=80% Xc=39% Xt=67% Xc=34% Xt=49% Xc=32% Xt=37% Xc=27% Xt=25% Xc=22% Xt=0%

10

20

30

40

2θ

Figure 9. XRD patterns of foamed LDPE at 110 °C with different Xt

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Table 1. Kinetics parameters and standard deviation (R2) based on the Avrami model of non-isothermal crystallization for LDPE under ambient N2 and compressed CO2. Pressure

n

k’

t1/2 (min)

0.1 MPa N2 2 MPa CO2 4 MPa CO2 6 MPa CO2 8 MPa CO2

1.35 1.55 1.41 1.36 1.45

0.078 0.045 0.057 0.063 0.063

5.04 5.84 5.24 5.23 5.23

Tc onset (°C) 101.4 100.2 98.7 97.1 95.5

34


Tc (°C) 97.0 95.7 94.5 92.6 92.0

R2 0.980 0.965 0.973 0.968 0.976

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Table 2. Kinetics parameters and standard deviation based on the Avrami model of isothermal crystallization for LDPE under ambient N2 and compressed CO2 at 110 °C using HP-Rheometer. Pressure

n

k’

t1/2 (min)

R2

0.1 MPa N2

1.45

0.005

30.0

0.991

5.8 MPa CO2 11.5 MPa CO2 20.2 MPa CO2 26.3 MPa CO2

1.23 1.42 1.34 1.40

0.009 0.004 0.004 0.003

34.2 37.7 46.8 48.8

0.980 0.977 0.990 0.978

35



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For Table of Contents Only

s u l u d o m e g a r o t S LDPE, 15 MPa CO2, 110 oC

time

36


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Foaming of Low Density Polyethylene with Carbon Dioxide Based on

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