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Surface roughness and foam morphology of cellulose acetate sheets foamed with 1,3,3,3-tetrafluoropropene.

INTRODUCTION

The use of renewable raw materials for the production of plastics helps saving limited fossil resources like crude oil. Recently, extensive research activities have focused on the production of bio-based plastics, their processing and application. In the field of packaging, poly(lactic acid) (PLA) is already used for applications such as cups, bottles, films or injection molded containers. However, it is usually not suitable for foam extrusion and hot-filling applications due to its low melt strength and low heat deflection temperature. Therefore, our research addresses cellulose acetate (CA), a bio-based polymer suitable for foam extrusion showing mechanical properties comparable to polystyrene (PS). Its high heat deflection temperature of 115[degrees]C makes it a potential substituent for PS in foamed trays for hot meals [1, 2].

CA is derived from cotton linters or wood pulp. Both are not in competition with food production [3]. Thermoplastic CA is produced by partial hydrolysis of primary cellulose triacetate to a degree of substitution (DS) of 2.4-2.5 [4, 5]. As the melt temperature of thermoplastic CA is close to its decomposition temperature, external plasticizers are needed to allow melt processing [6], The use of plasticizers lowers the glass transition temperature and the melting point without significant shift in the decomposition temperature [7]. Thus, the temperature window between sufficient thermoplasticity and decomposition of CA is enlarged, and melt processing is possible without degradation. Since conventional phthalate plasticizers exhibit certain toxicity [8], glycerol-based plasticizers for CA with US Food and Drug Administration (FDA) approval are state of the art [9].

Foam extrusion is a key enabling processing technique leading to significant material savings and unique product properties. One application are food trays, produced by thermoforming of foamed sheets [10]. In this context, the authors already studied the foam sheet extrusion of thermoplastic CA with different physical blowing agents (PBA) [11-13]. The use of environmentally noncritical 1,3,3,3-tetrafluoropropene (HFO-1234ze) as PBA and 0.5 wt% talc as nucleating agent resulted in a 10-fold expansion of the foam sheets [11].

Herein, we present our results on the effect of different cooling techniques on the surface roughness and foam morphology of CA foam sheets. Additionally, the deformation of nuclei in the die gap is described by use of the capillary number Ca and correlated with the cell density of the foam sheets.

EFFECT OF PROCESS PARAMETERS ON CELL DENSITY

The morphology of extruded polymer foams is dependent on a variety of process, material and formulation parameters. A multitude of studies has addressed this topic for various materials and blowing agents. The cell density of a foam is defined by the number of cells nucleated from the supersaturated polymer-gas mixture per unit volume [14]. In the foam extrusion process the pressure-induced supersaturation does not occur instantaneous but over a finite distance or period of time, respectively. Therefore, the pressure drop rate in the extrusion die is one of the most relevant process parameters controlling cell nucleation. The group of Park reported that the cell density increases by several orders of magnitude when increasing the pressure drop rate in a continuous foam extrusion process with PS and C[O.sub.2] [15, 16]. Additionally, the amount of blowing agent dissolved in the melt has a significant effect on the number of nucleated cells [17], In foam extrusion processes, the main nucleation mechanism is considered to be heterogeneous as the activation energy for homogeneous nucleation is too high [18, 19]. The nucleation is primarily controlled by the type and concentration of nucleating agents (e.g., talc) and the shear stress level; an effect extensively studied by many researchers [18, 20-22]. According to studies by Han et al., shear stress enhances cell nucleation in foam extrusion processes [20]. Leung et al. studied this effect in a foaming visualization system [22], Especially when using nucleating agents (e.g., talc), bubble-growth induced cell nucleation can be observed [22]. This behavior is explained by local shear stress and pressure fluctuations in the vicinity of a growing bubble. Chen et al. proposed a theory to calculate the shear forces in the extrusion process which lower the critical free energy [23]. As deformed cell nuclei have a larger surface and nonspherical shape, the equivalent critical radius [R.sub.c] reduced. The deformation of the nuclei is proportional to the capillary number Ca (Eq. 1).

Ca = [eta][??][R.sub.c]/[sigma], (1)

where [eta] is the viscosity in Pa s, [??] is the shear rate in [s.sup.-1], [sigma] is the surface tension in N [m.sup.-1] and [R.sub.c] in m is the critical radius of a nucleus for bubble growth. Experimental studies prove that the capillary number correlates with the cell density [23].

EXPERIMENTAL

Materials

Thermoplastic CA (Biograde V2844) was obtained as granules from FKuR Kunststoff GmbH (Willich. Germany). The density of the resin is 1,310 kg [m.sup.-3]; the melt flow rate is 24 g 10 [min.sup.-1] at 230[degrees]C under 5 kg load. HFO-1234ze was supplied by Honeywell Fluorine Products Europe B.V. (Amsterdam, The Netherlands) and used as PBA. HFO-1234ze has a zero Ozone Depletion Potential ODP and a Global Warming Potential GWP of 6 C[O.sub.2]-equivalents [24]. HFO-1234ze has a molecular weight of 114 g [mol.sup.-1], a boiling point of -19[degrees]C. and a vapor pressure of 490 kPa at 25[degrees]C. Ethanol 642 (99%) was obtained from Julius Hoesch GmbH & Co. KG (Diiren, Germany). A 20 wt% nucleating masterbatch based on CA was compounded on an intcrmeshing corotating twin-screw extruder (EMP 26-40, TSA Industrial, S.r.l., Cernobbio, Como, Italy) at Fraunhofer UMSICHT. The throughput was kept constant at 10 kg [h.sup.-1] with a screw speed of 250 [min.sup.-1]. The nucleating agent used was talc (Finntalc M05SL-AW, Mondo Minerals B.V., Amsterdam. Netherlands). It has platelet geometry with a specific surface area of 9.5 [m.sup.2] [g.sup.-1] and a median particle size d50 of 2 [micro]m.

Foam Extrusion Equipment

All trials for foam extrusion were performed on a 60 mm single screw extruder (Type 6E4. Barmag AG, Remscheid, Germany) with an L/D ratio of 40. The setup is shown in Fig. 1. A hot air dryer (Luxor A 80-E, Motan GmbH, Isny, Germany) was used to dry the CA compound and the masterbateh for approx. 10 h at 60[degrees]C to achieve a residual moisture below 400 ppm. CA compound and masterbateh were metered gravimetrically with a batch-blender (Gravimix 60, Motan Colortronic GmbH, Isny, Germany). The blowing agent was injected with a double piston pump (LEWA GmbH, Leonberg, Germany) at a barrel position of L = 19 D. In some experiments, ethanol was injected as coblowing agent at L = 15 D. The last 13 D of the extruder barrel are lluid-cooled to allow cooling of the mellblowing agent mixture. A melt filter (Type SF60, Gneuss Kunststofftechnik GmbH, Bad Oeynhausen, Germany) with a mesh size of 200 [micro]m was used to filter unmolten granules. An additional static mixer (SMB plus 40/3, Promix Solutions AG, Winterthur, Switzerland) ensures thermal homogeneity before the melt enters the die. Different extrusion dies were used in this study. The first experiments were performed with use of a Hat sheet die (width = 200 mm) with fluid cooled flex lips. The flat sheets were calibrated and drawn off by using a chill roll stack (CR 136/350, Dr. Collin GmbH, Ebersberg, Germany).

An additional adjustable contact calibration lip was used in some experiments with the flat sheet die (Fig. 2). The calibration lip allows contact cooling of the top surface of the foam sheet. Further experiments were performed with a fluid cooled annular die ([empty set] = 50 mm) with variable die gap (Fig. 3). The extruded tube was drawn over a cooling mandrel (stretching ratio 1:3), cut open with a rotating knife and wound up flat. An aluminum contact cooling ring was used to calibrate the outer surface of the foam sheet after the die exit. The support air of the cooling mandrel was used to adjust the contact between the contact ring and the sheet bubble.

FOAM EXTRUSION EXPERIMENTS

Contact Cooling With Chill Roll Slack (Test Series 1)

Foam sheets were extruded with the constant parameters summarized in Table 1. The temperature settings can be found in Table 2. The temperature of the chill roll stack and the temperature of the flex lip were varied according to Table 1.

Contact Cooling With Additional Calibration Lip (Test Series 2)

In test series 2, foam sheets were extruded with the formulation according to Table 1. The temperature settings can be found in Table 2. A temperature-controlled calibration lip was attached to the upper die lip to cool and calibrate the top surface of the foam sheet (Fig. 2). The temperature of the die lips, the calibration lip. and the haul-off speed were varied according to Table 1. In this test series, the chill roll temperature was kept constant. The setting of [T.sub.Roll] = 100[degrees]C was chosen since it had proven to be the best setting in test series 1.

Contact Cooling of Foam Sheets Extruded With Annular Die (Test Series 3)

In this series of tests, foam sheets were extruded with an annular die. A temperature-controlled contact cooling ring (Fig. 3) was attached to the die. Ethanol was used as coblowing agent to improve the cell structure and reduce decomposition of the compound, which can be caused by high shear forces [13, 25], Table 3 shows the constant parameters used in test series 3. The temperature settings can be found in Table 2. The temperature of the outer die lip as well as the temperature of the contact ring was varied as shown in Table 3.

Measurement of Foam Properties

Surface Roughness. The surface roughness of the foam samples [R.sub.a] was determined with a laser scanning microscope (VK-X200, Keyence Deutschland GmbH. Neu-Isenburg, Germany) and analysis software (VK-Analyzer 3.1. Keyence) according to ISO 4287. The roughness was measured in five test fields of 700 X 800 [micro][m.sup.2] within an area of 1.6 x 5.6 [mm.sup.2]. From these five values an average roughness and its standard deviation were derived.

Foam Density. The foam density of the flat sheet samples was measured with a helium pycnometer (AccuPyc 1330, Micromeritics Instruments Inc., Norcross). The density of the annular die sheet samples was determined by measuring average thickness and weight of circular cut outs with an area of 1,000 [mm.sup.2].

Cell Morphology. To analyze the foam morphology (cell density, cell size distribution, surface layer thickness), magnified images of thin cuts of the samples were processed by using digital imaging software (ImageJ, National Institute of Mental Health, Bethesda, Maryland). The volumetric cell density N can be calculated from the cell count n and the analyzed area A using Eq. 2.

N = [(n/(A).sup.3/2]. (2)

Surface Layer Thickness. The thickness of the surface layers was determined from normalized mean gray value distributions along the thickness axis of the samples (Fig. 4). The gray value was determined by applying the plot profile function of ImageJ on a rectangular area with a width of 3 mm. The function returns the mean gray value over the width of the area as a function of the thickness axis. These values were normalized, and a threshold for the surface layer was arbitrarily defined as 0.6, which correlates well with the visual impression of the morphology.

RESULTS AND DISCUSSION

Effect of Contact Cooling With Chill Roll Stack on the Surface Roughness of CA Foam Sheets (Test Series I)

The results of the surface roughness measurements and cell density analysis are summarized in Figs. 5-7 and Table 4. To explain the resulting surface roughness, the cell morphology of the sheets has to be considered. In general, it is observed that the roughness of the bottom surface is lower than that of the top surface (Fig. 5). This can be explained by the longer contact time of the sheet with the lower drum of the roll stack. At a roll temperature of 90[degrees]C, the surface roughness is higher due to cracks in the surface. Chill roll temperatures below 110[degrees]C are substantially lower than the glass transition of the compound (~115[degrees]C). Consequently, the surface solidifies rapidly, whereas the core of the foam sheet remains soft, and the expansion of the blowing agent inside the cells causes surface cracks. At [T.sub.Roll] = 90[degrees]C, the foam sheet has a pronounced surface layer with small cells and larger cells in the core (Fig. 6a). Due to the cracks in the surface, the roughness is comparably high. Increasing the temperature to 100[degrees]C reduces both the top and bottom surface roughness. The surface layer thickness is reduced, while there are still small cells in the outer layer and larger cells in the core (Table 4). For [T.sub.Roll] = 110[degrees]C the cells in the surface layer are destroyed and an almost solid surface layer of ~90 pm is observed. While the surface roughness of the bottom surface reaches the lowest value of ~7 [micro]m. the top surface roughness reaches a local maximum. The reason of the local maximum is probably the presence of small cells in the surface layer that continue to expand after passing the roll stack gap. For the bottom surface, the local maximum occurs at [T.sub.Roll] = 120[degrees]C (Fig. 5). The shift might be explained by the longer contact time of the lower drum of the stack, which prevents the cell expansion in the surface layer at [T.sub.Roll] = 110[degrees]C. For the settings of [T.sub.Roll] = 130 and 140[degrees]C, the surface roughness increases again due to further expansion of the whole foam sheet. The growth of cells from the core stretches the compact surface layer until it vanishes. Instead, the expanding cells from the core form a thicker surface layer with a larger cell diameter in the range of the core cells.

The influence of the flex lip temperature on the surface roughness and foam morphology is shown in Figs. 6b and 7. The lowest surface roughness is observed at a Ilex lip temperature of 180[degrees]C. Increasing the flex lip temperature to 185 and 190[degrees]C increases the surface roughness from 12 to 18 [micro]m for the top surface. The increased standard deviation at elevated flex lip temperatures might be caused by the lower cell density at the surface seen in Fig. 6b. Additionally, a higher flex lip temperature results in a lower overall cell density and larger cells in the surface region, which promotes roughness of the surface. The lower viscosity reduces the shear energy, results in a lower nucleation rate, and promotes cell growth as described by Lee [18].

Effect of Contact Cooling With Calibration Lip on the Sheet Properties (Test Series 2)

The analysis of the foam sheets produced with contact cooling by an additional calibration lip leads to the identification of various effects of the process settings on the sheet surface roughness and foam morphology. The analysis of mean average effects (Fig. 8) shows significant influences of the calibration lip temperature and the haul-off speed on the surface roughness. Lower calibration lip temperatures and slower haul-off speeds result in smoother sheets. At lower calibration lip temperatures, the cell growth at the surface is inhibited, leading to reduced roughness. Slower haul-off speeds result in a longer contact times between the sheet and the calibration lip. Due to the increased contact time, more heat is removed from the sheet, stabilizing the foam structure. Additionally, it can be assumed that the increased time of foam growth under constraint of the calibration lip reduces the surface roughness. The temperature of the flex lip has no clear effect on the surface roughness. However, the flex lip temperature affects the overall foam morphology and thickness of the surface layer (Fig. 9, Table 5). A flex lip temperature of 180[degrees]C results in a very thin sheet with open cells at the surface and a very low cell density. At low flex lip temperatures of 171-172[degrees]C, a fine-celled surface layer and larger cells in the core are observed. At [T.sub.flex lip] = 176[degrees]C, a foam sheet with a low density of 176 kg [m.sup.-3] and a fine and homogeneous cell morphology is produced. The surface layer shows a slightly higher cell density and a thickness of 0.34 mm. It can be assumed that the cell morphology and surface layer thickness are mainly influenced by the flex lip temperature and the resulting pressure drop rate and viscosity.

Effect of Contact Cooling on the Surface and Morphology of Foam Sheets Extruded With an Annular Die (Test Series 3)

The objective of test series 3 was to study the principle of contact cooling with an annular die as a setup of industrial relevance. The analysis of surface roughness, density, and cell morphology reveals the effect of the process parameters on the corresponding foam properties. As observed in the experiments with the flat sheet die, the surface roughness can be reduced by contact cooling al temperatures in the range of the glass transition temperature. Higher temperatures increase the surface roughness (Fig. 10). In comparison with the reference point (black bar in Fig. 10) without contact, the roughness reduction is small. During the trials, the die pressure, melt temperature, throughput, and gas concentration were kept constant. Nonetheless, small deviations were observed due to the influence of the contact ring temperature. The die pressure was kept in the range of 20-30 bars by changing the die gap. The deviations in the process parameters result in different cell densities of the foam sheets. To explain the results, it is necessary to compare the cell density with various process factors. In Fig. 11 (left), the cell density is plotted over the pressure drop rate in the die gap. The pressure drop rate was calculated from the measured die pressure. the volume flow, and the die gap width. The overall trend shows a decreasing cell density with higher pressure drop rates. This observation stands in contrast to the general theory [15, 16]. However. Baldwin et al. described a "post diffusion" case, where the residence time inside the die is too small compared to the required time for diffusion of the blowing agent into the cells [25]. This might also explain the results of this study. For the settings with a higher pressure drop rate, the die gap was smaller, leading to a shorter residence time in the die gap. For that reason, the saturation pressure is reached later, and time for nucleation and cell growth is reduced. This explanation is in agreement with the observation that the cell growth only starts after leaving the die in case of high pressure drop rates. However, some process settings with similar die gap and pressure drop rate resulted in considerably different cell densities (Fig. 11, left, triangular data points). This effect might be explained by slightly different concentrations of the co-blowing agent ethanol. which result from deviations of the overall mass throughput. Plotting the cell density at constant die gap against the concentration of ethanol (Fig. 11, right), shows a good correlation. Most probably, this effect is caused by different interaction parameters (e.g., surface tension) between ethanol and CA compared to HFO. This hypothesis can be supported with the influence of the capillary number Ca (Eq. 1), which was reported to be proportional to the cell density [23]. Since the pressure drop rate of the different settings was almost constant, the effect of the deviation of the blowing agent loading does not influence the viscosity and shear rate in the die. Therefore, the high increase of the cell density with higher concentration might be a result of a reduced surface tension, which was recently reported by Mahmood et al. [26]. The lower surface tension finally leads to a higher capillary number.

CONCLUSIONS

Our investigations revealed how the surface roughness of CA sheets foamed with HFO-1234ze can be controlled by contact cooling and other process parameters. Those results can also be useful for the foam extrusion of other plastic materials, especially polystyrene, to optimize the surface quality by systematic temperature control and die design. In experiments with a chill roll stack, the lowest surface roughness was achieved with contact temperatures in the range of the glass transition temperature. The roughness can also be lowered by using slower haul-off speeds. A variation of the flex lip temperature shows that the lowest surface roughness and highest cell density are achieved if the flex lip temperature is close to the die temperature. As a function of the flex lip temperature and contact temperature, a compact surface layer is formed with both the chill roll stack and the calibration lip. Finally, the process was transferred to an annular sheet die with a contact cooling ring. To explain the observed cell density of the foam sheets, the pressure drop rate in the die was calculated. High pressure drop rates unexpectedly reduced the cell density. This effect can be explained with different die gap settings and their influence on the resulting residence time in the die. In case the residence time is shorter than the time required for the diffusion of the blowing agent into the cells, the growth of the foam only starts outside the die, where the lack of pressure drop prevents further nucleation. Additionally, a correlation between the cell density and the concentration of the co-blowing agent ethanol was found. A higher concentration of ethanol increases the cell density, probably due to the reduced surface tension of the gas-loaded melt. This effect will be further explored in future studies.

ACKNOWLEDGMENTS

The investigations set out in this paper received financial support from the German Bundesministerium fur Bildung und Forschung (Project No. 03X2518B), to whom we extend our thanks. We thank our project partners FkuR Kunststofftechnik GmbH and Inde Plastik Betriebsgesellschaft mbH for their support. We would also like to extend our thanks to Honeywell Fluorine Products Europe B.V. for their support with supply of HFO-1234ze.

NOMENCLATURE

A Analyzed sample area, [mm.sup.2]

Ca Capillary number

N [mm.sup.-3] Cell density per unit volume

n Number of cells within a finite sample area

[R.sub.a] Average roughness of the foam sheet surface, [micro]m

[R.sub.c] Critical bubble radius, pm

[T.sub.Calibration lip] Temperature of the calibration lip, [degrees]C

[T.sub.Die] Temperature of the die body, [degrees]C

[T.sub.Flex lip] Temperature of the flex lips, [degrees]C

[T.sub.Ring] Temperature of the contact ring, [degrees]C

[T.sub.Roll] Temperature of the chill roll stack roll. [degrees]C

[v.sub.Haul-off] Haul-off speed of the foam sheet, m [min.sup.-1]

[??] Shear rate in the die gap. [s.sup.-1]

[eta] Viscosity of the melt. Pa s

[sigma] Surface tension of the gas-loaded melt, N [m.sup.-1]

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Christian Hopmann, (1) Sven Hendriks, (1) Claudia Spicker, (1) Stefan Zepnik, (2) Frank van Luck (3)

(1) Institute of Plastics Processing (IKV) in Industry and the Skilled Crafts at RWTH Aachen University, Aachen 52062, Germany

(2) Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT, Oberhausen 46047, Germany

(3) AlXtrusion Consultinq, Kaarst, 41564, Germany

Correspondence to: S. Hendriks; e-mail: sven.hendriks@ikv.rwlh-aachen.de

DOI 10.1002/pen.24440

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

Caption: FIG. 1. Experimental setup with fiat sheet die and chill roll stack. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. Sectional view of the flat sheet die with Ilex lips and additional calibration lip, cooling channels.

Caption: FIG. 3. Schematic drawing of the annular sheet die with contact cooling ring.

Caption: FIG. 4. Principle of surface layer determination. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Effect of the chill roll temperature on the surface roughness.

Caption: FIG. 6. Cell morphology of foam sheets calibrated at different chill roll temperature ([T.sub.Flex lip] = 180[degrees]C) and different flex lip temperatures ([T.sub.Chill roll] = 100[degrees]C).

Caption: FIG. 7. Effect of flex lip temperature on surface roughness (standard deviation at 180[degrees]C is too small to be visible in the plot).

Caption: FIG. 8. Mean average effects on the surface roughness.

Caption: FIG. 9. Morphology of foam sheets produced al different settings (see Table 5).

Caption: FIG. 10. Effect of the contact ring temperature on the surface roughness.

Caption: FIG. 11. Cell density plotted over the pressure drop rate at the die exit (left); triangular data points are plotted over ethanol concentration (right).
TABLE 1. Constant and variable parameters for test series 1 and 2.

                           Test series    Test series      Test
                                1a             1b        series 2

HFO-1234ze (wt%)               2.0            2.0           2.0
Ethanol (wt%)                   0              0             0
Talc (wt%)                     0.3            0.3           0.3
Screw speed                     6              6             6
  ([min.sup.-1])
Die temperature                174            174           174
  ([degrees]C)
Distance between die            75             75            75
  and chill roll (mm)
Chill roll temperature       90, 100,         100           100
([degrees]C)                110, 120,
                            130, 140
Flex lip temperature           180         180, 185,       170,
  ([degrees]C)                                           171, 172,
                                              190          176.18
Calibration lip                 --             --        130 150,
temperature ([degrees]C)                                 160, 170,
                                                            180
Haul-off speed                 1.3            1.3        0.8, 1.1.
  (m [min.sup.-1])                                       1.3, 1.4,
                                                          1.6, 1.9

TABLE 2. Temperature settings in test series.

Zone          Test series 1   Test series 2        Test series 3

                            Temperature ([degrees]C)

Feed zone        75                    75                53
Zone 1           175                  175               176
Zone 2           190                  190               205
Zone 3           205                  205               210
Zone 4           205                  205               210
Zone 5           197                  197               210
Zone 6           198                  198               200
Cooling          175                  178               184.5
  extension
Static           174                  174               184.5
  mixer
Die              174                  174               184

TABLE 3. Constant and variable parameters for lest series 3.

                                             Settings

HFO-1234ze (wt%)                             2.3-2.5
Ethanol (wt%)                                0.4-0.45
Talc (wt%)                                     0.3
Chain extender (wt%                            0.15
Screw speed ([min.sup.-1])                      11
Die temperature ([degrees]C)                   183
Cooling mandrel temperature                     30
  ([degrees]C)
Outer die lip ([degrees]C)                   183, 179
Contact ring temperature        100, 110, 121, 130, 140, 150, 170
  ([degrees]C)

TABLE 4. Surface layer thickness and cell diameters incl. standard
deviations of sheets calibrated al different chill roll temperatures.

[T.sub.roll]    Surface layer     Cell diameter     Cell diameter
([degrees]C)    thickness         surface           core
                ([micro]m)        ([micro]m)        ([micro]m)

90              147 [+ or -] 18    49 [+ or -] 5     89 [+ or -] 9
100             119 [+ or -] 14    66 [+ or -] 7    105 [+ or -] 12
110              91 [+ or -] 11    61 [+ or -] 7     98 [+ or -] 12
120             175 [+ or -] 21    58 [+ or -] 9    109 [+ or -] 16
130             189 [+ or -] 23    77 [+ or -] 11    84 [+ or -] 12
140             196 [+ or -] 24   129 [+ or -] 19   113 [+ or -] 17

TABLE 5. Parameter settings and results including standard
deviations for sheets calibrated with additional calibration lip.

                                  a                      b

Flex lip temperature             180                    176
  [T.sub.flex lip
  ([degrees]C)
Calibration lip                  180                    160
  temperature
  [T.sub.caiibration lip]
  ([degrees]C)
Haul-off speed                   1.9                    1.1
  (m [min.sup.-1])
Density (kg [m.sup.-3])    296 [+ or -] 11       175.9 [+ or -] 1.2
Cell density (core)       2.6 [+ or -] 1.25      19.7 [+ or -] 3.5
  ([mm.sup.-3])
Surface layer thickness          --              0.34 [+ or -] 0.03
  (mm)
Surface roughness          66 [+ or -] 22          33 [+ or -] 9
  [R.sub.a]([micro]m)

                                  c                    d

Flex lip temperature             171                  172
  [T.sub.flex lip
  ([degrees]C)
Calibration lip                  150                  130
  temperature
  [T.sub.caiibration lip]
  ([degrees]C)
Haul-off speed                   0.8                  0.8
  (m [min.sup.-1])
Density (kg [m.sup.-3])   278.5 [+ or -] 0.9   250.1 [+ or -] 15
Cell density (core)         4.5 [+ or -] 0.7    8.04 [+ or -] 1.3
  ([mm.sup.-3])
Surface layer thickness    0.43 [+ or -] 0.01   0.43 [+ or -] 0.01
  (mm)
Surface roughness           19 [+ or -] 5        7 [+ or -] 1.5
  [R.sub.a]([micro]m)
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Author:Hopmann, Christian; Hendriks, Sven; Spicker, Claudia; Zepnik, Stefan; van Luck, Frank
Publication:Polymer Engineering and Science
Article Type:Report
Date:Apr 1, 2017
Words:5218
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