Surface roughness and light transmission of biaxially oriented polypropylene films.
Optical properties of polyolefin films such as haze and transparency are of considerable importance in packaging applications. Factors affecting these optical properties have been the focus of numerous studies [1-5]. Haze, or the loss of transparency, is due to light scattering which can originate from both the interior of the film and from the film surface. The interior or bulk scattering is caused by refractive index fluctuations that occur on the length scale of the light wavelength (400-700 nm). Spherulitic morphologies are often the source of bulk scattering in semicrystalline polymers [6, 7]. Scattering at the air-film interface arises from surface roughness. For oriented polyolefin films, it is now generally recognized that the loss of transparency is primarily caused by surface scattering rather than by interior scattering [1-5].
Various studies that examined the effect of process and polymer variables established that the surface roughness of polyolefin films, and hence their transparency, depends on the frost line height [2, 3], the presence of nuclcation agents , the polymer melt elasticity , and the polymer molecular weight . Lowering the frost line height and adding nucleation agents are effective in reducing the crystalline haze by reducing the crystal size.
Recent studies of blown polyethylene films identified two fundamentally different sources of surface roughness [4, 5]. Very low melt elasticity facilitated the formation of spherulitic-like superstructures at or near the surface; whereas very high melt elasticity resulted in a fine-scale surface roughness due to elastic melt instabilities. By choosing a resin with the proper melt elasticity and molecular weight, surface haze can be minimized. The same studies demonstrated that surface height profiles obtained by atomic force microscopy (AFM) are a powerful and convenient means for characterizing the surface roughness. The relationship between surface roughness, as quantified by the root mean square (RMS) roughness, and the light transmission or haze was established.
Polypropylene (PP) is an attractive candidate material for packaging applications because of its low cost and improved thermal stability compared with polyethylene. However, poor melt strength and bubble instability historically prevented the use of PP in the blown film processes. Consequently, commercial PP films are typically oriented by a tentering process in which an extruded sheet is preheated and biaxially drawn into a film . In this case, the factors most affecting haze may be different from those affecting haze of polyethylene films obtained in the blown film process.
The optical transparency of biaxially oriented polypropylene (BOPP) films is the focus of this study. The BOPP films are drawn biaxially from compression molded sheets. Different crystalline morphologies are achieved in the molded sheets by varying the cooling conditions during compression molding. The sheets are oriented biaxially at temperatures and strain rates that compare with commercial conditions. The light transmission of the BOPP films is analyzed in terms of the surface roughness as characterized by AFM.
MATERIALS AND METHODS
The polymer used in this study was a polypropylene prepared with a postmetallocene catalyst by The Dow Chemical Company. It had weight average molecular weight [M.sub.w] of 3.42 x [10.sup.5] g [mol.sup.-1] and [M.sub.w]/[M.sub.n] of 2.9 as determined by gel permeation chromatography calibrated with polystyrene standards. The equivalent PP molecular weights were deduced using appropriate Mark-Houwink coefficients for PP and polystyrene. The data were provided by The Dow Chemical Company. The resin is substantially isotactic, and is characterized as having [.sup.13.C] NMR peaks of equal intensity at about 14.6 and about 15.7 ppm corresponding to a regio-error. It has at least 50% more of a regio-error than a comparable Ziegler-Natta PP, which results in shorter isotactic runs when compared with the Ziegler-Natta PP .
Unoriented sheets with thickness of ~0.6 mm were prepared by compression molding. Pellets were sandwiched between Mylar[R] sheets and preheated at 190[degrees]C under minimal pressure for 8 min and then compressed at 10 MPa for 5 min using a Model 3912 laboratory press, Carver (Wabash, IN). The procedure for cooling from the melt was varied in order to achieve three cooling rates: quenching, slow cooling, and very slow cooling. The quenched sheets were taken from the compression molder and dropped into ice water; slowly-cooled sheets were cooled in the compression molder with a low flow of cooling water; and very slowly-cooled specimens were cooled overnight in the press without cooling water. The cooling rates were estimated to be about 100, 30, and 3[degrees]C [min.sup.-1], respectively.
The sheets were microtomed through the thickness at -75[degrees]C to obtain thin cross-sectional slices. The slices were examined in the polarized microscope Olympus BH-2.
Thermal analysis was performed on the sheets with a Perkin-Elmer (Boston, MA) DSC-7 calorimeter under a nitrogen atmosphere. Samples weighing 5-10 mg were cut from the molded sheets, and thermograms were obtained at a heating rate of 10[degrees]C [min.sup.-1] from -60 to 190[degrees]C. The percent crystallinity was calculated using a value of [DELTA]H = 209 J [g.sup.-1] for the heat of fusion of 100% crystalline isotactic PP .
Square specimens measuring 85 mm x 85 mm were cut from the compression molded sheets, marked with a grid pattern, and stretched in the Bruckner (Greenville, SC) Karo IV Biaxial Stretcher. The sheets were simultaneously and equi-biaxially drawn at various temperatures at an engineering strain rate of 400% [s.sup.-1] based on the original specimen dimensions. The uniformity of the drawn specimen was determined from the even deformation of the grid pattern. The preheat time before drawing was fixed at 60 s.
An ultraviolet-visible spectrometer from Ocean Optics (Dunedin, FL) was employed to study the light transmission of the biaxially oriented films. The films were sandwiched between two glass slides to prevent curving, and then placed between the light source and light detector. In some experiments, mineral oil with refractive index of 1.500 was spread on one or both surfaces of the film. The diameters of the light source beam and the detector were both ~3 mm. The distance between the film and the detector was 6 mm unless indicated otherwise. The transmitted intensity at 633-nm wavelength was taken to calculate the light transmission as
T(%) = [[T.sub.633]/[T.sub.633.sup.o]] x 100 (1)
where [T.sub.633.sup.o] is the light transmission at 633 nm with only two glass slides as the reference and [T.sub.633] is the total light transmission at 633 nm of the two glass slides with the film in between them. The measurement was performed at five different locations on each film and the average and standard deviation are reported.
The roughness of the surfaces was characterized using a Nanoscope IIIa atomic force microscope, Digital Instruments (Santa Barbara, CA). Films were fixed to the sample stage with double-sided stick tape, and height images were obtained in the tapping mode.
RESULTS AND DISCUSSION
Biaxial Orientation of Molded PP Sheets
The polarized optical micrographs of cross-sections of quenched, slowly-cooled, and very slowly-cooled PP sheets are shown in Fig. 1. For the quenched sheet, no well-defined spherulitic structures were observed. With decreasing cooling rate, larger and better defined spherulites were produced.
The DSC melting curves of PP sheets are shown in Fig. 2. Crystallinity was calculated from the melting enthalpy using 209 J [g.sup.-1] as the heat of melting of the PP crystal . For quenched, slowly-cooled, and very slowly-cooled sheets, the weight percent crystallinities were 41, 48, and 50%, respectively. The DSC melting cndotherms for the three sheets had different shapes. The slowly-cooled sample had a sharper melting peak than the quenched sample. This was attributed to more uniform crystallization during the slower cooling rate. For the very slowly-cooled sample, there was a plateau region before the sharp melting peak.
The stretching characteristics of the three samples were tested in the temperature range between the onset of melting and the peak melting temperature. Experiments were terminated when the force exceeded the limit of the grips (~100 N) or when the displacement reached the limit of the instrument, 10 x 10. Although all the samples could be drawn over a fairly large temperature range, the results were not always considered successful. There were two criteria for successful drawing: achieving a draw ratio of at least 4 x 4 and achieving good uniformity of the drawn films as indicated by the deformation of a stamped-on rectangular grid pattern. Specimens that were judged by eye to be uniformly deformed also exhibited uniform birefringence over a circular area with a 25-cm diameter in the center of the film .
[FIGURE 1 OMITTED]
The temperatures at which successful drawing was achieved are indicated by open circles on each of the melting endotherms in Fig. 2. All the sheets had the same window, between 140 and 144[degrees]C, regardless of the thermal history. These temperatures lay 2-6[degrees]C below the peak melting temperature of quenched and slowly-cooled sheets, whereas for the very slowly-cooled sheet, they were in the plateau region about 4-8[degrees]C below the peak melting temperature. BOPP films were obtained by stretching the sheets to a draw ratio of 4 x 4. The BOPP films that were stretched from quenched, slowly-cooled, and very slowly-cooled sheet are referred to as Q, SC, and VSC, respectively, followed by the draw temperature (140, 142, or 144).
Light Transmission of BOPP Films
The see-through clarity of biaxially oriented PP films strongly depended on the cooling rate during sheet preparation. Qualitatively, objects were clearest when viewed through Q films, somewhat less clear through SC films, and much less clear through VSC films. This trend was also seen in the light transmission measurements. The percent light transmission decreased with decreasing cooling rate of the molded PP sheet that was used to prepare the BOPP film (Fig. 3a). It was also found that the light transmission decreased with increasing orientation temperature of BOPP films prepared from sheets with the same cooling rate.
Loss of light intensity can arise from both bulk and surface effects . The bulk contributes to the intensity loss mainly by light scattering from inhomogeneities with a mismatched refractive index, such as crystals dispersed in an amorphous matrix. Loss due to the surface is mainly caused by light reflection off the surface and by light scattering from surface roughness.
[FIGURE 2 OMITTED]
To distinguish the contributions of bulk and surface effects, refractive index matching oil was spread on the surfaces of the films. The oil acted to smooth the film surfaces and thereby to eliminate the contribution of surface roughness. With oil on both surfaces, the light transmission of all the films increased to almost 100%, which meant that the loss of light transmission was caused almost entirely by the surface roughness (Fig. 3b). When oil was spread independently on each of the surfaces, the results were about the same and moreover the transmission loss was reduced by about half compared with the loss when neither surface had oil, (Fig. 3c and d). It was concluded that the two surfaces contributed equally to the transmission loss.
[FIGURE 3 OMITTED]
Two surface effects can cause reduction in light transmission: reflection and scattering. Reflection occurs at the film-air interface. It depends upon the angle of incidence and upon the refractive indices. If the incident light is perpendicular to the surface, as in the transmission measurements performed in this study, the fraction of light reflected is given by Fresnel's equation 
R = [([n.sub.1] - [n.sub.2])[.sup.2]]/[([n.sub.1] + [n.sub.2])[.sup.2]] (2)
where [n.sub.1] and [n.sub.2] are the refractive indices of air and polymer. Using [n.sub.1] = 1.0 for air and [n.sub.2] = 1.5 for PP, the calculated loss due to reflection was ~4%. The remainder of the loss, which was most of the loss, was attributed to light scattering from surface roughness.
Surface Roughness of BOPP Films
The surface topology of Q, SC, and VSC films was characterized by AFM. Examination of the surfaces at various magnifications revealed roughness at two size scales: large scale roughness occurred at the size scale of the spherulites, and small scale roughness occurred at the size scale of the lamellae. The 200-[micro]m height images in Fig. 4 show the large-scale roughness at the spherulite size scale. The dashed lines identify the locations of the three height profiles shown below the image. Periodic surface features were observed for all specimens. Qualitatively, the amplitude and the period of the features increased substantially as the cooling rate decreased from Q to SC (not shown) to VSC films. The period P was estimated from the height profiles, Table 1, and was found to be about three times larger for VSC films than for Q films, 150 [micro]m compared with 50 [micro]m. In all cases, increasing the draw temperature tended to increase the amplitude of the surface features without affecting the period.
The large scale roughness of the drawn films was due to spherulite boundaries that were higher than the spherulite centers. The boundaries become higher and less sharp as the draw temperature increased. One explanation was that during the preheating process, polymer in the spherulite boundaries melted before polymer in the center, subsequent recrystallization during cooling produced the ridges. The effect was particularly pronounced in VSC films. For these films, it should be noted that the draw temperatures spanned the plateau region of the thermogram (see Fig. 2). It can be postulated that the melting temperature varied through the spherulite and the plateau arose from lower melting lamellae at the boundaries. The variation probably resulted from the very slow crystallization of VSC films. A gradual decrease in temperature as the spherulite slowly grew from the center outward could have resulted in secondary crystallization of thinner lamellae, with lower melting temperature.
[FIGURE 4 OMITTED]
The 2-[micro]m height images in Fig. 5 show the small-scale roughness at the lamellar or fibril size scale. Height profiles obtained at each of the three dashed lines in each AFM image are shown below the image. The amplitude and period of the small scale roughness were essentially indistinguishable among Q, SC (not shown), and VSC films. This reflected similarity in the lamellar structures of the various samples. The period of the small scale roughness was on the order of 0.1-0.2 [micro]m (Table 1).
The RMS roughness R was used to quantify the roughness amplitude as 
R = [square root of ([1/N][N.summation over (i=1)]([h.sub.i] - [bar.h])[.sup.2])] (3)
where [h.sub.i] is the height at position i, [bar.h] is the average height and N is the total number of positions averaged. The results in Table 1 were obtained by taking the height at 512 positions on each of the three AFM height profiles. For large scale roughness, R increased with the cooling rate of the molded sheet and with the draw temperature. For small scale roughness, R was about the same for all the films.
[FIGURE 5 OMITTED]
Light Scattering from Surface Roughness
When light passes through a polymer film, it is scattered from both surfaces. The backscattering is considered to be identical to the forward scattering. The primary effect of backscattering is to reduce the intensity of the transmitted beam. The scattered light intensity [I.sub.[theta]] at an angle [theta] is related to the amplitude and period of the surface roughness according to 
[I.sub.[theta]]/[I.sub.o] = [[[k.sup.4][R.sup.2][L.sup.2] [cos.sup.2] [theta]]/[[pi][r.sup.2]]][A.sub.M] exp (-[[[k.sup.2][L.sup.2] [sin.sup.2] [theta]]/4]) (4)
where [I.sub.o] is the incident intensity, k = 2[pi]/[lambda], [lambda] is the wavelength of the incident light, r is the distance from the scattering point, [A.sub.M] is the area of the scattering surface, R is the RMS roughness, and L is the correlation length which quantifies the periodicity P of the surface roughness. The magnitude of R affects the intensity of the scattered light envelope, whereas L affects both the intensity and the angular dependence of the scattered intensity.
Equation 4 was used to qualitatively illustrate how the differences in the period and amplitude of large and small scale roughness affected the light scattering. Based on the values of R and P in Table 1, Eq. 4 was plotted for a 100x difference in L and a 10x difference in R assuming an arbitrary value of [A.sub.M]/[r.sup.2] = [10.sup.-5]. When L = [lambda] and R = [lambda], representing large scale roughness, the scattered light was intense and concentrated at smaller angles (Fig. 6a). When L = [10.sup.-2][lambda] and R = [10.sup.-1][lambda], representing small scale roughness, the scattered intensity was reduced by six orders of magnitude and the light was scattered over wider angles (Fig. 6b). The effect of the [R.sup.2] dependence of the scattered intensity is included in the figure by comparing the scattering for a 2x difference in R.
Light scattering from the BOPP film surfaces combined the small angle scattering caused by large-scale roughness and the wide angle scattering caused by small-scale roughness. Although the calculations based on Eq. 4 were meant to be illustrative only it was apparent that the contribution of small scale roughness was insignificant and moreover did not vary from one film to another. The large scale roughness, which was the primary cause of light scattering, varied in both R and P depending on the thermal history of the molded sheet and the temperature of biaxial orientation. Figure 7 confirms that all the films conformed to a single relationship between R for large scale roughness and the measured light transmission.
Light Transmission Through BOPP Films
Clarity is usually discussed in terms of two effects, both caused by light scattering. One effect is aberration of the image (haze) which is associated with scattering at very low angles and in this case would be caused by large scale roughness. The other effect is loss of contrast which is associated with scattering at large angles and hence depends on small scale roughness . The difference between see-through clarity as determined by the human eye and the light transmission measurement is that the eye is sensitive to both haze and loss of contrast whereas the light transmission measurement detects only light scattering associated with haze.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Haze is conventionally defined as the percentage of the total transmitted light that is scattered from the incident beam by more than 2.5[degrees]. The light transmission experiment used in this study is slightly different. The light that is measured by the detector is the unscattered light plus a portion of scattered light that is determined by the detector diameter and the film-to-detector distance. If the range of the detector overlaps the small angle scattering region, the scattering profile from the large scale roughness will affect the measured light transmission.
Based on a detector diameter of 3 mm and a film-to-detector distance of 6 mm, it was estimated that light scattered at an angle less than 14[degrees] was measured by the detector. The film-to-detector distance was increased from 6 to 12 mm in order to decrease the angle over which the light was measured. If the scattering envelop extended beyond the angle of the detector, and the scattering intensity was strong enough, there would have been a decrease in the measured intensity. On the other hand, if the scattered intensity was not very high, this effect might not have been observed.
Equation 3 was used to qualitatively illustrate how the differences in P and R of large scale roughness for Q and VSC films affected the scattering profile assuming an arbitrary value of [A.sub.M]/[r.sup.2] = [10.sup.-5]. Representing the VSC film by L = [lambda] and R = 2 [lambda], and representing the Q film by L = 0.5 [lambda] and R = [lambda] (Table 1), the scattered light from the VSC film was an order of magnitude more intense, and the scattering profile was slightly narrower, compared with the Q film (Fig. 8). Therefore, decreasing the detector range from 14[degrees] to 7[degrees] was more likely to affect the measured light transmission through the VSC films than through the Q films.
The effect of decreasing the angle over which the scattered light was measured is shown in Fig. 9. Doubling the film-to-detector distance did not affect the light transmission measured for the Q films, it slightly decreased the intensity measured for SC films, and it substantially decreased the intensity measured for VSC films. Clearly, the scattered intensity from the VSC films was high enough to reveal the overlap between the scattering envelop from large scale roughness and the detector range. On the other hand, the scattered intensity from the Q films was low enough that the effect was not detected, even though the Q films had a somewhat broader scattering envelop than the VSC films due to the smaller P.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
BOPP films were prepared by simultaneous biaxial stretching at high strain rate and elevated temperature. The transparency of the films was found to depend on the thermal history of the sheet that was used for the oriented film and also on the temperature at which the orientation was performed. Thus, cooling the sheet more rapidly from the melt and orienting the sheet at a lower temperature resulted in a more transparent film. Surface roughness was demonstrated to cause the loss of transparency. Roughness on two size scales was characterized: the spherulitic texture produced large scale roughness of about 100 [micro]m, whereas the lamellar texture was responsible for small scale roughness of about 0.2 [micro]m. The loss in transparency depended on the magnitude of the large scale roughness. It appeared that partial melting of the spherulite boundaries at the orientation temperature, followed by recrystallization during cooling of the oriented film, produced the ridges and valleys that constituted the large scale roughness. It followed that the clearest films were obtained from sheets with the most homogeneous texture, such as obtained by quenching from the melt, and by orienting at the lowest temperature, which minimized the amount of melting.
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Y.J. Lin, (1) P. Dias, (1) S. Chum, (2) A. Hiltner, (1) E. Baer (1)
(1) Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
(2) Polyolefins and Elastomers R & D, The Dow Chemical Company, Freeport, Texas 77541
Correspondence to: A. Hiltner; e-mail: firstname.lastname@example.org
TABLE 1. Roughness characteristics of BOPP films. Large scale roughness Small scale roughness Period (P) (a) RMS (R) Period (P) (a) RMS (R) Film ([micro]m) (nm) ([micro]m) (nm) Q-140 50 82 [+ or -] 9 0.1 8 [+ or -] 1 Q-142 50 73 [+ or -] 11 0.2 11 [+ or -] 2 Q-144 50 103 [+ or -] 12 0.2 11 [+ or -] 1 SC-140 70 124 [+ or -] 31 0.1 7 [+ or -] 1 SC-142 70 170 [+ or -] 15 0.2 12 [+ or -] 1 SC-144 70 179 [+ or -] 16 0.2 10 [+ or -] 3 VSC-140 150 189 [+ or -] 12 0.1 8 [+ or -] 3 VSC-142 150 206 [+ or -] 16 0.2 9 [+ or -] 3 VSC-144 150 270 [+ or -] 52 0.2 7 [+ or -] 2 (a) Approximated from AFM images.