Crater Formation Mechanism on the Surface of a Biaxially ... · Crater Formation Mechanism on the Surface of a Biaxially Oriented Polypropylene Film Satoshi Tamura,1 Koichi Takino,2

Crater Formation Mechanism on the Surface of a BiaxiallyOriented Polypropylene Film

Satoshi Tamura,1 Koichi Takino,2 Toshiro Yamada,2 Toshitaka Kanai3

1Prime Polymer Company, Limited, 580-30, Nagaura, Sodegaura-City, Chiba 299-0265, Japan2Kanazawa University, Kakuma-Machi, Kanazawa, Ishikawa 920-1192, Japan3Idemitsu Kosan Company, Limited, 1-1, Anesaki-Kaigan, Ichihara-City, Chiba, 299-0193, Japan

Received 29 August 2011; accepted 12 January 2012DOI 10.1002/app.36803Published online 1 June 2012 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: Biaxially oriented polypropylene (BOPP)films are used in a variety of areas around the world.They are especially well suited for food packaging andindustrial usages because of their high productivity. Manystudies on the stretchability with regard to crystal struc-ture changes have been reported by various researchersbecause the machine speed has been increasing and thedemand to produce thinner films has been becoming moreimportant. Furthermore, a number of studies on the sur-face structure of BOPP films with craterlike roughnesseshave been reported since the 1980s. Although a craterlikesurface roughness was formed under specific film processconditions, the formation mechanism and the controllingmethod of the craterlike film surface are yet to be clarified.In this report, we demonstrated a new hypothesis for thecraterlike film surface roughness formation mechanism by

analyzing the morphology of the surface layer of polypro-pylene (PP) sheets and by investigating the relationshipbetween the surface structural changes and the changes inthe entire structure. As a result, we found that an overcrit-ical crystallization time was needed to form the crater onthe surface of the BOPP film, and the crater formationmechanism was closely related not only to the surfacestructure changes but also to the deformation phenomenonof the spherulite in the PP sheet during stretching. Fur-thermore, this report shows the controlling factors in theformation of the crater structure from the viewpoint of theproduction conditions. VC 2012 Wiley Periodicals, Inc. J. Appl.Polym. Sci. 126: E501–E512, 2012

Key words: films; orientation; poly(propylene); (PP);surfaces

INTRODUCTION

Polypropylene (PP) is one of the most popular ther-moplastic resins and is produced at a rate of about42 million tons per year around the world.1 It is awell-known fact that there are some crystal types ofPP with different crystal systems, and the surfaceroughnesses of biaxially oriented polypropylene(BOPP) films have been investigated in associationwith crystal types.2–8 Many basic studies have beenreported on the formation of a and b crystals.9–14

Natta and Corradini9 reported in their paper in 1960that the unit lattice of the a crystal is the monoclinicsystem, and now, it is known to be the most domi-nant crystal in PP. The b crystal was discovered byKeith et al. in 1959, and the unit lattice is a hexago-nal system.10 It was reported that b crystals areformed easily by the addition of a b nucleator intoraw PP11 under a controlled temperature gradient12

and under a strong shear stress in an extruder, suchas at a low extruding temperature.13 In addition to

these studies, the characteristics of b crystals werereported, and it was found that they have lowerdensities and melting temperatures (Tm’s) than acrystals.9

It was also reported that the formation of a crater-like surface roughness in BOPP films is due to thecrystal dislocation system. Because b crystals withlower densities change into a crystals with higherdensities after the sheets, which contain both a andb crystals, are stretched, craters form where the bcrystals existed. To obtain a craterlike surface rough-ness in BOPP films, Fujiyama et al.3 reported thatthe stretching temperature in the machine direction(MD) process should be controlled between the Tm’sof both the b and a crystals. It was also reportedthat the stretching temperature in the transversedirection (TD) process should be controlled between150 and 155�C because the craters became unclear inthe roughened BOPP film at a temperature lowerthan 150�C, and the craters disappear when it ismelted at a temperature higher than 155�C.4 On theother hand, although Fujiyama et al.5 reported thedifference in the surface roughness between a BOPPfilm produced by the sheet with an a nucleator andthat with a b nucleator, the relationship between thecrater shape and b crystals with regard to parame-ters such as amount and shape has not yet been

Correspondence to: S. Tamura ([email protected]).

Journal of Applied Polymer Science, Vol. 126, E501–E512 (2012)VC 2012 Wiley Periodicals, Inc.

clarified.5 It was reported that there was a good rela-tionship between the chill-roll temperature and theaverage roughness of the BOPP film surface, whichis a parameter of crater depth, but the relationshipbetween the chill-roll temperature and the crater di-ameter is yet not clear.6 From these reports, the cra-ter formation mechanism of BOPP films has not yetbeen clarified because these reports have onlyfocused on the b crystals.

Various studies have reported changes in the crys-tal structure during the stretching process.15–23

Although Koike and Cakmak15 reported a change inthe PP crystal structure during uniaxial stretching,observed by the birefringence technique, and Phillipsand Nguyen16 made a report on the relationshipbetween the stretching stress and the change in mor-phology of the PP sheet,16 no information related tosurface structure has been reported. Kanai and co-workers17–20 reported on PP spherulite deformationand crystalline orientation during stretching in termsof the stretching stress pattern and on the analyticalresults of the crystal structural changes observed bybirefringence, light scattering, and small-angle X-rayscattering. Kanai and coworkers21–23 also reportedthat there was a strong relationship between the ob-servation results of crystal structures in PP sheets andthe simulation results obtained by cooling calcula-tions conducted with regard to the thermal transfercharacteristics of PP sheets with the chill-roll coolingsystem. Although it was concluded in the report thatthe cooling simulation is an efficient method for con-trolling the crystallization of a PP sheet, which is animportant factor in the stretchability of BOPP, therewas no information on the surface structure of BOPP.

It was found in our previous study24 that the cra-terlike structures on the BOPP film surface were

related to the morphology of the surface layer of thePP sheets. However, even though it is an importantfactor in the formation of the craterlike structure ofBOPP, the method for controlling the morphology ofthe PP sheet has not yet been discussed. The rela-tionship between the changes in the surface struc-ture and the whole structure has not yet been dis-cussed either. Therefore, in this report, we givesome perspective on the formation mechanism ofthe craters with regard to controlling the morphol-ogy of the surface layer of the PP sheet and also onits connection to changes in the entire structure.

EXPERIMENTAL

Samples

To clarify the mechanism of the formation of surfacecraters, PP-A polymerized by Ziegler-Natta catalystwith a high tactility and a meso pentad mmmmvalue, which is a parameter of the isotactic indexmeasured by 13C-NMR,25 of 96 mol % was prepared.The thermal properties of Tm, melting enthalpy(DHm), and crystallization temperature (Tc) weremeasured by differential scanning calorimetry (DSC;PerkinElmer, Waltham Massachusetts, USA, B014-3018/B014-3003), and the molecular weight distribu-tion parameter [weight-average molecular weight/number-average molecular weight (Mw/Mn)] wasmeasured by gel permeation chromatography. Theproperties of PP-A are shown in Table I.PP sheets were produced with a sheet-forming

machine (GM Engineering, Yokohama, Japan) withan extruder diameter of 35 mm (Fig. 1). The PP resinwas extruded from a T-die with a width of 200 mmat 250�C. It was cast by a chill-roll with a diameterof 250 mm at a speed of 0.8 m/min, and PP sheetswith various thicknesses were produced at variouschill-roll temperatures to investigate the relationshipbetween the morphology of the PP sheets and thesheet-forming conditions. In this study, the sampleswere named with respect to their resin type, chill-roll temperature, and thickness. For example, sampleA80–500 was the sheet made by PP-A at a chill-rolltemperature of 80�C with a thickness of 500 lm. TheBOPP film names are defined with an f at the end ofthe PP sheet names, for example, A80–500f.

TABLE IProperties of the PP Resin

Properties Unit PP-A

Melt flow rate g/10 min 3.5mmmm mol % 96Tm

�C 164DHm J/g 105Tc

�C 115Mw/Mn — 5.1

Figure 1 Schematic diagram of the PP sheet-formingmachine.

Figure 2 Diagram production and evaluation methods ofeach sample.

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Journal of Applied Polymer Science DOI 10.1002/app

Figure 2 shows the schematic diagram for the pro-duction and evaluation methods of the samples. Afterthe PP sheets were cut into square shapes 85 � 85mm2 in size, they were stretched to BOPP films by atable tenter (Bruckner, Tokyo, Germany, KARO IV) toanalyze the surface structure. After they were pre-heated at a definite temperature for 1 min, they werestretched at maximum stretching ratio of five times tothe MD at first and then seven times to the TD. ThePP sheets were also stretched by an opt rheometer(Orc Manufacturing Co., Ltd., Tokyo, Japan) to ana-lyze the crystal structural changes. PP sheets with arectangular shape 25 � 10 mm2 were stretched to theMD at a stretching ratio of five times at a strain speedof 28.6%/s after they were preheated at 159�C for 5min. The stretching force was measured by a loadcell, which was equipped on a chuck of the opt rhe-ometer with maximum load of 100 N.

Evaluation of the PP sheets and the BOPP films

The analysis methods of the PP sheets and BOPPfilms and the properties obtained from these meas-urements are shown in Table II. The sectional struc-tures of the PP sheets were observed by an opticalmicroscope (Nikon, Tokyo, Japan, ECLIPSE-LV100POL). When we observed the morphologies ofthe PP sheet with the sectional view of an opticalmicroscope, the crystal sizes were measured by vis-ual inspection from pictures taken with a polarizinglens. The thermal properties, Tm and DHm, of the PPsheets were measured by DSC (PerkinElmer, Wal-tham Massachusetts, USA, B014-3018/B014-3003) ata heating rate of 10�C/min.

X-ray diffraction was conducted on the PP sheetswith a Rigaku Denki, Tokyo, Japan, RINT-2500 dif-fractometer with Ni-filtered Cu Ka radiation. Thecrystallinity (vc) and b-crystal content (K) values ofthe PP sheets were calculated from the X-ray diffrac-tion curves.26,27 The change in the crystal structureduring stretching was analyzed by light-scatteringdata obtained by the opt rheometer with a high-power He–Ne laser. The spherulite size (Umax) of thePP sheet was calculated from eq. (1), obtained by theHv scattering pattern, which reflected the scatteringof the polymer superstructure:

Umax ¼4:09

ð4p=kÞ sin hmax(1)

where k is the wavelength of the He–Ne laser (632.8nm) and ymax is the scattering angle, which indicatesthe yield point of the scattering intensity to 45�

against a plane of polarization.The surface structures of the BOPP films were

observed by scanning electron microscopy (SEM;JEOL, Tokyo, Japan, JSM5600LV). Samples weresputtered with gold in a vacuumed atmosphere, andthe crater diameters were measured by visualinspection from photographs. In addition, the shapesof the craters were measured by a highly preciseshape-measuring machine (Kosaka Laboratory, To-kyo, Japan, SURFCODER ET4000A) with a diamondhead at a head pressure of 70 lN. The ten-point av-erage roughness (Rz) defined by JIS B0601 (1994)was used as the parameter of depth of the crater onthe BOPP film surface.Moreover, to investigate the crater formation

mechanism at the surface layer of PP sheet, theinfluence of cooling speed was studied at the sheet-forming condition described at section 2.1). The tem-perature of PP resin starts to drop after PP sheet isextruded from the die and touches the chill roll. PPcrystallization starts when the sheet temperaturereaches Tc at 115�C in the case of PP-A. After sheettemperature is kept at Tc until the latent heat is con-sumed, the sheet temperature starts to drop again.Crystallization time is defined as the time duringwhich PP sheet temperature is kept at Tc, and PPcrystallizes during the crystallization time. In thisreport, it is assumed that heat is conducted by pri-mary thermal conduction expressed in eq. (2) and istransferred at the boundary condition expressed ineq. (3):

@T

@t¼ k

cpq� @2T

@x2(2)

k@T

@x¼ hðTw � T1Þ (3)

where T is the sheet temperature (K), t is time (s), kis the thermal conductivity (0.28 W m�1 K�1), Cp isthe heat capacity (1.93 J kg�1 K�1), q is the density

TABLE IIAnalysis Methods for the PP Sheets and Films

Sample Analysis method Properties obtained from the measurements

Sheets Optical microscopy Sectional structures (size of the crystal grain and spherulite)DSC Thermal properties (Tm, DHm)WAXD Crystalline structures (vc, K)Light scattering Crystalline structures (Umax)

Films SEM Crater shape (diameter)Shape-measuring machine Crater shape (depth, Rz)

BIAXIALLY ORIENTED POLYPROPYLENE FILM E503


of PP, x is the sheet position from the chill roll (m),h is the heat-transfer coefficient (756 W m�2 K�1), Tw

is the surface temperature of the sheet (K), and T1is the chill-roll temperature (K). Although the den-sity of PP depends on the temperature, the fixedvalue 890 kg/m3 was used in this study to make thecalculation simple. The heat-transfer coefficient wasdetermined by measurement of the real tempera-tures of the chill roll and PP sheet with a touch-typethermometer.

RESULTS AND DISCUSSION

Influence of the sheet-forming conditions on thecrater formation of the BOPP film

First, the influence of the thickness of the PP sheetson the crater structure was studied. Figure 3 showsthe polarized micrographs of the cross-sectional

views of the PP sheets in the MD and the normaldirection. As for the A80–500 sheet, a number ofwhite grains were observed only in the surface layerof the opposite side of the chill roll. Because theamorphous parts are not portrayed in white on apolarizing microscope, these white grains were con-sidered to be b crystals of PP4 and also transcrys-tals.28,29 Although these white crystal grains wereobserved even in the A80–400 and A80–300 sheets,with thicknesses of 400 and 300 lm, respectively,they were not observed in the A80–200 and A80–100sheets, with thicknesses of 200 and 100 lm, respec-tively. Figure 4 shows the relationship between theshapes of the crystal structure and the thickness ofthe PP sheet. As a result, although the crystal grainswere not observed in the surface layer of the chill-roll side, the crystal grain size (depth and diameter)in the surface layer of the opposite side of the chill

Figure 3 Optical microscopy images of PP sheets with different thicknesses.

Figure 4 Crystal structure of PP sheets with different thicknesses: (*) crystal grain on the surface layer of the oppositeside of the chill roll, (~) crystal grain on the surface layer of the chill-roll side, and (n) spherulite in the central part ofthe sheet.

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Figure 5 Properties of PP sheets with different thicknesses as measured by (a) DSC and (b) WAXD.

Figure 6 SEM images of the BOPP films on the opposite side of the chill roll obtained from PP sheets with differentthicknesses.

Figure 7 Relationship between the PP sheet thickness and the crater shape.



roll became larger as the PP sheet became thickerthan 200 lm, whereas the size of the spherulite inthe center of the PP sheet got larger as the PP sheetbecame thicker from 100 lm.

To examine this more deeply, the PP sheet wasanalyzed by DSC and wide-angle X-ray diffraction(WAXD). Tm gradually increased from 164.5 to167.7�C as the PP sheet thickened [Fig. 5(a)]. BecauseTm is closely related to the thickness of lamella,29 theresult indicates that the lamella became thicker withthe increase in PP sheet thickness. DHm increasedslightly as the thickness of the PP sheet increasedfrom 100 to 300 lm and was saturated from 300 to500 lm [Fig. 5(a)]; this was in accordance with thebehavior of vc measured by WAXD [Fig. 5(b)]. Fur-thermore, b crystals were detected at thicknesses ofthe PP sheet of 400 lm or greater.

Next, the surface structures of the BOPP filmsobtained by the stretching of PP sheets with differ-ent thicknesses were observed (Fig. 6). Clear craterstructures were observed on the opposite side of thechill roll of the BOPP films obtained from the PPsheets with thicknesses from 500 to 300 lm. Craterstructures were not observed on the BOPP filmsobtained from the A80–200 and A80–100 sheets oron the chill-roll sides of all of the samples. Theseresults show an adequate relationship between theexistences of the crystal grains on the PP sheets andthe formation of craters on the BOPP films. Figure 7shows the relationship between the crater shape andthe thickness of PP sheets. The crater shape indi-cated an adequate relationship with the thickness ofthe PP sheet. Figure 8 shows the relationshipbetween the crystal grain structure of the PP sheetsand the crater structure of the BOPP films. Becausethere was also a good relationship between the crys-tal grain structure and the crater structure, weassumed that the formation mechanism of the craterwas related to the crystal deformation.

These results indicate that crater shape could becontrolled by the white crystal grain size, which wasinfluenced by the thickness of the PP sheet. Toexamine this more deeply, the sheet temperatures atseveral distances from the surface of the oppositeside of the chill roll were simulated by eqs. (2) and(3). Figure 9(a) shows the calculated results of A80–500 at intervals of 100 lm, where 0 lm indicates thesurface of the opposite side of the chill roll and 500lm indicates the surface of the chill-roll side. Fromthese results, we observed that the temperature ofthe chill-roll side dropped more quickly than that ofthe opposite side of the chill roll. To investigate theinfluence of the thickness of PP sheet on the sheetsurface temperature, the calculated results of the pri-mary sheet temperatures of the opposite side of thechill roll and the chill-roll side are shown in Figure9(b,c), respectively. A flat area at a Tc of 115�C wasobserved on the opposite side of the chill roll ofA80–500, but no flat area was observed on eitherside of A80–100. These flat areas, from 106 to 124�C,were defined as the crystallization time, and therelationship between the calculated results of thecrystallization time and the crystal grain sizes areshown in Figure 9(d). As a result, we found thatthere was an adequate relationship between them.Furthermore, the relationship between the crystalli-zation time and the crater formation was investi-gated [Fig. 9(e)]. From these results, we found that acrystallization time of greater than 1.5 s wasrequired to create the crystal grain and crater on theBOPP films on the opposite side of the chill roll inour experimental method, and this study, withregard to the thickness of the PP sheet, was not dis-cussed in previous reports.Next, the influence of the chill-roll temperature on

the crater structure was studied. Figure 10 shows thepolarized micrographs of the cross-sectional views ofthe PP sheets in the MD and the normal direction.

Figure 8 Relationship between the crystal structure of the PP sheets and the crater structure of the BOPP films.

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Crystal grains were not observed in the surface layerof either side of the chill roll with chill-roll tempera-tures ranging from 30 to 65�C. On the other hand,many crystal grains were formed in the surface layerof the opposite side of the chill roll from A80–300 toA95–300 sheets with chill-roll temperatures from 80to 95�C, and they became larger with increasingchill-roll temperature (Fig. 11). Furthermore, crystalgrains in the surface layer of the chill-roll side wereobserved with chill-roll temperatures at and above

90�C, although they were not observed in the surfacelayer of the A80 sheets cast at 80�C with severalthicknesses, from 500 to 100 lm.Figure 12 shows the melting properties and crystal

structures measured by DSC and WAXD. BecauseTm and DHm increased as the chill-roll temperatureincreased, we assumed that the thickness of thelamella and the amount of the crystal part increasedwith increasing chill-roll temperature or, in otherwords, with slow cooling [Fig. 12(a)]. As proof, vc

Figure 9 Calculated results of the temperature of the PP sheets with different thicknesses (conditions: die temperature¼ 250�C, chill-roll temperature ¼ 80�C, stretching temperature ¼ 159�C, MD/TD stretching ratio ¼ 5/7).



Figure 10 Optical microscopy images of the PP sheets with different chill-roll temperatures.

Figure 11 Crystal structure of PP sheets with different chill-roll temperatures: (*) crystal grain on the surface layer ofthe opposite side of the chill roll, (~) crystal grain on the surface layer of the chill-roll side, and (n) spherulite in the cen-tral part of the sheet.

Figure 12 Properties of PP sheets with different chill-roll temperatures measured by (a) DSC and (b) XRD.

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increased with increasing chill-roll temperature [Fig.12(b)]. Furthermore, b crystals were detected at andabove the chill-roll temperature of 90�C.

Figure 13 shows the SEM images of the surfacesof the BOPP films obtained from PP sheets with dif-ferent chill-roll temperatures. Although crater struc-tures were not observed on the surface of the oppo-site side of the chill roll of the A30–300f to A65–300ffilms, they were observed in the A80–300f to A95–300f films. Additionally, they were observed on thesurface of the chill-roll side, too, from A90–300f toA95–300f, in which crystal grains in the surface layerof the chill-roll side were observed in the PP sheets.From these results, a higher chill-roll temperature isrequired to produce craters on the BOPP film sur-face of the chill-roll side. Furthermore, crystal grainsin the PP sheet are needed to produce craters on theBOPP film.

To investigate the influence of the chill-roll tem-perature on the sheet surface temperature, the calcu-lated results of the primary sheet temperatures ofthe opposite side of the chill roll and chill-roll sideare shown in Figure 14(a,b), respectively. Thecrystallization time became longer as the chill-roll

temperature became higher. This graph shows that acrystallization time of more than 1.5 s was needed tocreate the crater on the surface of the BOPP filmwith our experimental method. This results obtainedfrom the PP sheet with the different chill-roll tem-peratures were similar to those of PP sheets of dif-ferent thicknesses. It was confirmed that crater for-mation could be controlled by the crystallizationtime, which was a function of the thickness of thePP sheet and the chill-roll temperature. In addition,craters could be formed on the BOPP film surface ofthe chill-roll side when we controlled the crystalliza-tion time suitably, whereas previous reports dis-cussed only the cases of the crater on the surface ofthe opposite side of the chill roll.

Relationship between the surface structure and thewhole structure of the BOPP film

To study the crater formation mechanism, PP sheetscast at different chill-roll temperatures of A80–300with a high vc of 65% and A30–300 with a low vc of34% measured by WAXD were stretched by the optrheometer, which gave information on the

Figure 13 SEM images of BOPP film surfaces obtained from PP sheets with different chill-roll temperatures.



superstructure during stretching. Figure 15 showsthe change of the surface roughness parameter (Rz)value with the stretching ratio. Although the Rz ofA80–300 was larger than that of A30–300, the Rz val-ues of both samples had a yield point at a stretchingratio of 2 times, and Rz gradually decreased as thestretching ratio increased. The reason for this wasthat the deep dent that was obtained in the earlystage of the stretching process changed to a shallowcrater with cutting through a thick wall at stretchingratios of 2 times or higher, as mentioned in our pre-vious report.24 In other words, despite the fact thatthe crystal grains were formed in the surface layeruniformly, the deformation of the surface structurestarted partially as the weak parts were stretchedfirst. These results coincide with the results obtainedby the stretching test with the table tenter and wereassumed to be caused by the difference in crystalgrain size between the sheets.

Figure 16 shows the change in the light-scatteringpattern with the stretching ratio. A80–100 was usedinstead of A80–300 because the light-scattering peakof A80–300 was too week for data analysis. The

Figure 14 Calculated results of the temperature of the PP sheets with different chill-roll temperatures (conditions: die tem-perature ¼ 250�C, thickness of the PP sheets ¼ 300 lm, stretching temperature ¼ 159�C, MD/TD stretching ratio ¼ 5/7).

Figure 15 Rz values of the BOPP films stretched by theopt rheometer obtained from PP sheets with differentchill-roll temperatures.

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light-scattering patterns of the unstretched sheets ofboth samples showed clear clover patterns, and thisdemonstrated the existence of spherulites. Therewere no considerable changes in the light-scatteringpattern at a stretching ratio of 2 times, and thespherulites still remained in the PP sheet (Table III).The spherulites were destroyed at higher stretchingratios because the clover pattern changed to a streakpattern in the equatorial direction. This result indi-cates that a partial deformation of the spherulite wasobserved at the stretching ratio lower than 2 times,and the crystal structure changed totally at higherstretching ratios. Although the sizes of the crystalgrains of these two PP sheets were different, the de-formation patterns of the spherulites were similar inthese two PP sheets. Because the stretching ratiowith the maximum Rz value and that of the exis-tence limit of the spherulite was the same, we con-cluded that the crater formation on the BOPP filmsurface was closely related to the spherulite defor-mation of the entire BOPP film.

CONCLUSIONS

The formation mechanism of the craterlike surfaceroughness of BOPP films was investigated. It wasfound that the thickness of the PP sheet had a greateffect on the crater shape of the BOPP film becausethe crystal grains in the surface layer of the PP sheetchanged according to the thickness of the PP sheets.The formation mechanism of the craterlike surface

roughness of the BOPP film was investigated. Theinvestigation showed that a crystallization time overa critical value was required to produce crystalgrains in the surface layer of the PP sheet with b-type transcrystals.Furthermore, the effect of the chill-roll temperature

in the preceding PP sheet was investigated. It wasfound that crystal grains in the surface layer wereeasily created at higher chill-roll temperatures. Fromthe analysis of the cooling calculation, it was con-firmed that more than 1.5 s of crystallization timewas required to produce crystal grains in the surfaceregions of the PP sheet. It was also found that a cratercould be formed on both surfaces of the BOPP films,regardless of the sides of the chill roll, when the crys-tallization time was controlled for more than 1.5 s. Itmay be said that the technology of crater formationcould be settled by the simulation of PP sheets pro-duced under different conditions.In addition, the crater formation mechanism was

researched in terms of the stretching behavior of theentire PP sheet with an opt rheometer. The changein the surface roughness with the stretching ratioshowed that the deformation of the surface structurepartially started before the craters were formed onthe BOPP film surface, and Rz indicated a maximumvalue at a stretching ratio of 2 times. In addition, itwas found that the spherulites began to decay at astretching ratio higher than 2 times through analysisof the light-scattering method. As a result, we con-cluded that the crater formation on the BOPP filmsurface was closely related to spherulite deformationand collapse of the entire BOPP film.

References

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Figure 16 Light-scattering patterns stretched by the opt rheometer obtained from PP sheets with different chill-roll tem-peratures (P ¼ polarizer, A ¼ analyzer).

TABLE IIIChanges in Umax (lm) During Stretching in the MD

Sheet

Stretching ratio

1 2

A80–100 4.9 5.6A30–300 6.5 9.8



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Crater Formation Mechanism on the Surface of a Biaxially ... · Crater Formation Mechanism on the Surface of a Biaxially Oriented Polypropylene Film Satoshi Tamura,1 Koichi Takino,2