Stretchability and Properties of LLDPE Blends for Biaxially Oriented Film

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1 FILM H. Uehara 1 *, K. Sakauchi 1, T. Kanai 2, T. Yamada 3 1 Okura Industrial Co., Ltd., Kagawa-ken, Japan 2 Idemitsu Petrochemical Co., Ltd., Chiba, Japan 3 Kanazawa University, Ishikawa, Japan Stretchability and Properties of LLDPE Blends for Biaxially Oriented Film In order to improve the stretchability of linear low density polyethylene (LLDPE) for double bubble tubular film (DBTF), the blending of LLDPEs have been investigated. Three LLDPEs of varying densities were blended, and several films having different densities, molecular-weight distributions and composition distributions were produced. The stretchability and properties of these films were investigated and the relationship between the stretchability and the material properties were also discussed. The material properties of LLDPE which showed better stretchability were reported. Furthermore, the stretchability of LLDPE by using the laboratory tenter stretched film (LTSF) and the DBTF were compared. The prediction of the stretchability for the DBTF by using the LTSF was reported. 1 Introduction The double bubble tubular polypropylene film is used to package stationary, groceries, foods and so on because of it s comparatively easier processability [1 to 6] and relatively lower resin cost. But recently, shrinkage film for this usage is required to have a superior appearance and shrinkage strength. The DBTF of LLDPE has both good shrinkage and high physical properties. For this reason the demand for DBTF of LLDPE has been increasing [7 to 9]. The deformation behavior, processability, and physical properties for the DBTF of LLDPE have been reported in the previous report [10]. LLDPE has a narrow stretchability range for the DBTF compared with polypropylene, hence blending of LLDPEs have been done in order to improve the stretchability and some patents [11 to 13] for blending of LLDPEs have been applied for. Basic research such as phase behavior of blends with LLDPEs has been investigated by M. J. Hill and P. J. Barton [14, 15]. But the biaxial stretchability changes in LLDPE blends and the reasons for such, have not been reported upon systematically. This paper will report on the relationship between the material properties and the biaxial stretchability of blends of three different density LLDPEs. * Mail address: H. Uehara, Functional Products Dept., Okura Industrial Co., Ltd., 1515 Nakatsu-cho, Marugame-shi, Kagawa-ken , Japan h-uehara@okr-ind.co.jp 2 Experimental 2.1 Three different density LLDPEs were used. Some characteristics which were measured using pellets of these materials are given in Table 1. These materials were blended according to weight percentage by a mixer before extrusion. The blend ratios of materials and some characteristics of the blended films which are the primary films of DBTF, are given in Table Laboratory Tenter For the off-line biaxial stretching of films, a laboratory tenter type BIX-703 made by Iwamoto Seisakusho Co. was used. This is the same machine as used in the previous report [10]. 2.3 Double Bubble Tubular Film Machine LL-A LL-B LL-C Density (g/cm 3 ) Melt index (g/10 min) Melting point ( C) *1 M W *2 M N *3 M W /M N *1 weight average molecular weight, *2 number average molecular weight, *3 molecular weight distribution Table 1. The characteristics of the materials The double bubble tubular film machine which is shown in Fig. 1, was also the same as in the previous report. The DBTF line with a 65 mm extruder from Modern Machinery Company and a 180 mm annular die from Tomi Machinery Manufacturing Corporation has been constructed by our company. A torque measurement instrument SS201 made by Ono Sokki Co. in Japan was set between the take-up nip roll and driving motor, to evaluate the stretching stress [16, 17] from the stretching torque. Intern. Polymer Processing XIX (2004) 2 Hanser Publishers, Munich 163

2 *1 2.4 Film Preparation No Laboratory Tenter Stretched Film (LTSF) The test piece film for the laboratory tenter was the primary film from the DBTF process. A slip agent and anti-blocking agents were not used in order to prevent slippage from the clip. The stretch ratios were set at 5 in the machine direction (MD) and 5 in the transverse direction (TD). The thickness of this primary film was 300 lm, and the stretched film thickness was 12 lm. Film size was a 95 mm square, but with allowances for clipping, the effective stretching film size became a 70 mm square. The heating time was 2 minutes, and the stretching speed was 30 mm/s. The stretching temperature range was measured in 2 degree intervals, and also measured was the relationship between the stretch ratio and the stretching force under each stretching temperature. Further stretched film properties such as shrinkage, Young s modulus and haze were measured Double Bubble Tubular Film The MD and TD stretch ratios were 5. The thickness of the primary film was 375 lm, and the film width was 235 mm, while the final stretched film thickness was 15 lm and the film width was 1180 mm. The output rate was 47 kg/h. The stretching ranges of the DBTFs were measured by using stretching torques. The stretching torques were changed by the stretching temperature which was controled by the pre-heater and stretching heater in Fig. 1. The stretching stresses were calculated from the stretching torques. The calculation theory of stretching stress was explained in detail in the previous report [10]. The definition of the stretching range was from the stress at the onset of bubble stability to the stress just before bubble burst. With more than 5 Nm of stretching torque fluctuations, the bubble visibly moved, and as such was regarded as bubble instability. Erucic acid amide (0.2 wt%) and silica (0.2 wt%) were used as slip agents. The film properties such as shrinkage, tear strength, haze, and Young's modulus at the conditions of maximum stretching stress and minimum stretching stress, were measured. LL-A LL-B LL-C Density (g/cm 3 ) Melt index (g/10min) M W *2 M N *3 M W /M N * *1 these materials were blended according to weight percentage by mixer before extrusion, *2 weight average molecular weight, *3 number average molecular weight, *4 molecular weight distribution Table 2. The blend ratios and the characteristics of the blended films Fig. 1. Schematic drawing of double bubble tubular film blowing process Evaluation Methods of Material and Film Properties The material and film densities were measured by Accupyc 1330 made by Micromeritics Instrument Corporation. This equipment can measure the density faster than the density gradient tube method, while maintaining the same measurement accuracy. The melt index was evaluated by the measurement method of ASTM D 1238, and the melting point was measured by a DSC (Seiko Instruments Inc. EXSTAR DSC6200R). The composition distribution was measured by the temperature rising elution fractionation (TREF) method using equipment made by Idemitsu Petrochemical Co., Ltd [18]. The measuring conditions were as follows. The solvent was ortho-dichlorobenzen, the flow speed was 1.0 ml/min, with the cooling temperature rate being 10 C/h from 135 to 0 C and the heating temperature rate, 40 C/h from 0 to 135 C. An infrared detector was used and the TREF column size was U 4.2 mm 150 mm, Chromosorb P was used as the fillers and the pour quantity was 0.5 ml with a concentration of 4 g/l. TREF relies on the crystallization and re-dissolution process to separate polymers having different degrees of branching. 164 Intern. Polymer Processing XIX (2004) 2

3 (wt%) No The shrinkage, tear strength, haze and Young s modulus were evaluated by the measurement method of ASTM D 2732, ASTM D 1922, ASTM D 1003 and ASTM D 882, respectively. 3 Results and Discussion 3.1 Laboratory Tenter Stretched Film (LTSF) Stretchability LL-A LL-B LL-C Item Haze Stress Haze Stress Haze Stress Haze Stress Haze Stress Haze Stress Haze Stress % MPa % MPa % MPa % MPa % MPa % MPa % MPa 106 broken broken broken broken Stretching broken broken temperature broken ( C) Melt Melt Melt Melt Melt Melt Melt Stretching stress range (MPa) Stretching stress range in hatched area (MPa) The film was broken at too low a stretching temperature, and melted at too high a stretching temperature in the tenter. The stretching temperature ranges for each blended LLDPE film is shown in Table 3. The relationships between stretch ratio and stretching force within each stretching temperature range are shown in Fig. 2. The relationships between stretch ratio and stretching force in machine direction (MD) and transverse direction (TD) of LTSF have already been examined in the previous report. The data of MD and TD were almost the same, so only the MD data is shown. The stretching stresses were calculated from the stretching force measured when the film was stretched to 5 times it s original size. The stretching stress and haze of the stretched films are also shown in Table 3. The stretching ranges of the DBTF were predicted from the haze and stretching stress of LTSF as described in the previous report [10]. In general, haze increases with an increase in the stretching temperature. Therefore, when the increase of haze exceeded 50 % of the haze of the lowest temperature stretched film, it was deemed that the stretching temperature was too high and was beyond the stable stretching range for DBTF. In some cases it was difficult to judge only by haze, so the stretching stress below 7.5 MPa was judged as an unstable range for * Hatched area is the prediction of the stable bubble stretching range of DBTF, when considering the deterioration of film transparency and stretching stress. Table 3. The relationship between stretching temperature and stretching stress and haze of the films stretched by laboratory tenter the DBTF. In the previous report, the stable stretching stress range for the DBTF was from about 9 to 18 MPa, and the stretching stress of the LTSF that corresponds with it, was from about 7.5 to 15.3 MPa (MD) which was about 85 % stretching stress of the DBTF. Hence, the stretching stress of 7.5 MPa was decided upon as the benchmark. As a result, the prediction of the stable bubble stretching range of DBTF is marked by hatching in Table 3. Table 3 shows that when the very low density LL-B was blended to LL-A at a ratio of 15 % to 85 %, the stretching temperature range was shifted 2 degrees lower than LL-A. Furthermore, when the LL-B was blended to LL-A at a ratio of 30 % to 70 %, the stretching temperature range was shifted 4 degrees lower than for LL-A. This means that the film density decreases by blending low density LLDPE, which results in a better stretchability at lower temperatures. However accompanying this was a slight decrease in the higher stretching temperature stretchability. When the LL-C was blended to LL-A at a ratio of 15 % to 85 %, the stretching temperature range was shifted 2 degrees higher than for LL-A. But when the LL-C was blended to LL-A at a ratio of 30 % to 70 %, the stretching temperature range was narrower than for LL-A. Hence it is considered that if the high density composition is too high, stretchability is suppressed. The blended film of LL-A 70 %, LL-B 15 %, and LL-C 15 % (No. 3) had the widest stretching temperature range. Density of the No. 3 film was the same as LL-A. The No. 3 film s stretching force was lower than LL-A at the same stretching temperature as shown in Fig. 2, suggesting that a very low density of LLDPE works to reduce the stretching force. The No. 3 film showed the best stretchability. The reason will be discussed in the next section. Increasing the LL-C content in the blend of LL-A increased the yield load and stretching force as shown in Fig. 2. As a re- Intern. Polymer Processing XIX (2004) 2 165

4 sult, film breakage occurred at the lower stretching temperature and the stretching temperature range narrowed Relationship between Stretchability and TREF, and DSC The material properties such as composition distribution by TREF and the heat of fusion by DSC have been measured in order to analyze the stretchability. Fig. 3 shows the composition distribution of the blended films. When LL-B was blended to LL-A (Fig. 3A, B), the composition distribution became wider at lower elution temperatures which resulted Fig. 2. Relationship between stretch ratio and stretching force at each stretching temperature (A) No. 1 LL-A/B/C = 70/30/0; (B) No. 2 LL-A/B/C = 85/15/0; (C) No. 3 LL-A/ B/C = 70/15/15; (D) No. 4 LL-A/B/C = 100/0/0; (E) No. 5 LL-A/B/C = 85/0/ 15; (F) No. 6 LL-A/B/C = 70/0/30; (G) No. 7 LL-A/B/C = 40/30/30 in a lowering of the high density composition peak which was shown at about 96 C. When LL-C was blended to LL-A (Fig. 3E, F), the composition distribution became narrower and raised the high density composition peak. When LL-B 15 % and LL-C 15 % were blended to LL-A (Fig. 3C), the composition distribution became wider at lower temperatures and at the same time raised the peak of high density composition. Further more when LL-B 30 % and LL-C 30 % were blended to LL-A (Fig. 3G), two distinct large peaks more than 20 K apart emerged. From this it can be concluded that the high density composition is associated with stretchability at a higher stretching tem- 166 Intern. Polymer Processing XIX (2004) 2

5 perature and the stretching force, while the composition at the lower elution temperature is associated with stretchability at the lower temperature. The composition distribution of material is very important for stretchability. The relationship between the elution temperature and integral of the melting composition up to the elution temperature analyzed from the TREF results are shown in Fig. 4. The relationship between the temperature range corresponding to a certain range of the integral of the melting composition and the stretchability were investigated. The gradients (%/K) of the total melting composition from 40 to 70 % (Region 40 to 70), almost all data represented by straight lines, were correlated with the stretchability. The blended film density was also considered Fig. 3. The measurement results of temperature rising elution fractionation (TREF) of each blended film (A) No. 1 LL-A/B/C = 70/30/0; (B) No. 2 LL-A/B/C = 85/15/0; (C) No. 3 LL-A/ B/C= 70/15/15; (D) No. 4 LL-A/B/C = 100/0/0; (E) No. 5 LL-A/B/C = 85/0/15; (F) No. 6 LL-A/B/C = 70/0/30; (G) No. 7 LL-A/B/C = 40/30/30 to be an important parameter to evaluate the stretchability of LLDPE. Fig. 5 shows the relationship among film density, gradient (%/K) of the Region 40 to 70 in TREF and stretchability which is represented by the stretching stress range shown in Table 3. Fig. 6 shows the relationship among the film density, gradient (%/K) of the Region 40 to 70 in TREF and the stretchable stress range, which corresponds to the temperature range where films with acceptable haze were obtained. The deterioration of the film transparency may correspond to the phenomena in that the film surface once melted and roughened during cooling, resulted in bubble instability for the DBTF. Furthermore, the deterioration of the film transparency lowers the value of products. Hence it is necessary to consider the deterioration of Intern. Polymer Processing XIX (2004) 2 167

6 Fig. 4. Integrated ratio of melting composition of TREF Fig. 5. Relationship among film density, gradient of TREF and stretching stress range of LTSF No. 6 LL-A/B/C = 70/0/30, No. 7 LL-A/B/C = 40/30/30 Fig. 6. Relationship among film density, gradient of TREF, and the stretching stress range for obtaining films with acceptable haze No. 6 LL-A/B/C = 70/0/30, No. 7LL-A/B/C = 40/30/30 the film transparency of the LTSF, in evaluating the processing window of the DBTF. Fig. 6 shows that a better stretchability range is from about 2.0 to 3.0 (%/K) of the gradient in TREF. It is considered that the wide temperature range of Region 40 to 70 of the integral melting component allows for easier stretching, but too large temperature range is detrimental to the stretching. The reason is as follows. Because of the moderately wide composition distribution, the temperature range of the melting component necessary for the stretching is also wide, and hence the stretching temperature range is also wide. However when there is too wide a composition distribution (shown in Fig. 3) at low stretching temperature, the high density composition does not melt. When the temperature is raised to the level necessary for stretching of high density composition, the lower density composition will melt completely, creating an unstable bubble condition. Figure 6 also shows that a film density around g/cm 3 is better for stretchability. It is considered that a density lower than g/cm 3 makes low stretching stress which can not support the bubble, while a density higher than g/cm 3 has too much high density composition preventing good stretchability. Fig. 7 shows the relationship between the temperature and integral of melting components of DSC. The same analysis Fig. 7. Integrated ratio of melting components of DSC Fig. 8. Relationship among film density, gradient of DSC, and stretching stress range in consideration of the transparency deterioration of LTSF No. 6 LL-A/B/C = 70/0/30, No. 7LL-A/B/C = 40/30/ Intern. Polymer Processing XIX (2004) 2

7 No techniques of TREF were used. The result is shown in Fig. 8, but the relationship between stretchability and gradient is not clear. The reason is considered as follows. The heat of fusion measured in DSC only represents the crystalline part in the film. The amorphous part in the film fulfils the important role for stretching. Hence, the DSC is not good enough to evaluate the stretchability of LLDPE. LL-A LL-B LL-C Thickness (lm) 12 Stretching temperature ( C) Young s modulus (MPa) MD TD Haze (%) C 9/10 5/5 8/9 6/7 6/7 5/5 4/4 Shrinkage 100 C 21/22 16/16 15/16 14/15 12/14 9/10 9/10 (%) 110 C 49/51 44/45 42/43 32/33 30/31 23/25 28/29 MD/TD 120 C 73/71 70/69 73/70 72/70 74/71 64/64 65/ Table 4. Properties of films stretched at the lowest stretching temperature by laboratory tenter Fig. 9. Relationship between film density and shrinkage of LTSF stretched at the lowest temperature No. 6 LL-A/B/C = 70/0/ Film Properties The film properties of each stretched film at the lowest temperature are shown in Table 4. Figs. 9, 10 and 11 show the relationships between film density and properties such as shrinkage, haze and Young s modulus. Fig. 9 shows that the shrinkability is better in the film that LL-B is blended when compared to the film that LL-B is not blended, especially at 110 C. The density of the blended film No. 3 and LL-A are the same, but the shrinkability of No. 3 is better than LL-A, because No. 3 has a lower temperature stretchability than LL-A. Fig. 10. Relationship between film density and haze of LTSF stretched at the lowest temperature No. 6 LL-A/B/C = 70/0/30 No Density (g/cm 3 ) Gradient of TREF (%/K) LL-A LL-B LL-C C 5/5 5/5 7/7 5/5 5/5 5/5 3/3 Shrinkage 100 C 11/11 11/11 12/13 10/11 9/10 9/10 9/10 (%) 110 C 28/28 28/28 29/30 27/28 24/24 23/25 19/20 MD/TD 120 C 70/64 70/65 70/68 73/67 70/67 64/64 59/59 Table 5. Shrinkage of LSTFs stretched at 116 C Intern. Polymer Processing XIX (2004) 2 169

8 Fig. 11. Relationship between film density and Young s modulus of LTSF stretched at the lowest temperature No. 6 LL-A/B/C = 70/0/30 Fig. 10 shows the relationship between the film density and haze. All films have excellent transparency. In a higher density film, the haze becomes slightly worse, because of an increase in the crystalinity. The relationship between the film density and Young s modulus is shown in Fig. 11. The Young s modulus was strongly influenced by the film density. The shrinkage of films stretched at 116 C are shown in Table 5. This table shows that the shrinkability was influenced by not only film density but also composition distribution. Especially, No. 7 film had worse shrinkability than No. 3 and No. 4 films which both were of the same film density. This reason can be explained by TREF (Fig. 3). The elution temperature was about 30 C lower than the stretching temperature, because of the use of solvent. Hence, the difference is shown at about 86 C elution temperature in Fig. 3C, D, G. No. 7 film s concentration at this temperature is lower than that of No. 3 and No Double Bubble Tubular Film Stretchability The stretching stress range of the DBTFs are shown in Table 6. The stretching stress range, i. e. difference between maximum and minimum stress, decreased with the blending of LL-B. Fig. 12. Relationship among film density, gradient of TREF and stretching stress range of DBTF No. 1 LL-A/B/C = 70/30/0, No. 3 LL-A/B/C = 70/15/15, No. 4 LL-A/B/ C = 100/0/0, No. 6 LL-A/B/C = 70/0/30, No. 7 LL-A/B/C = 40/30/30 This result corresponds to the lowering of stretching force recorded in the LTSF study (see Fig. 2). Furthermore the stretching stress range became narrower when LL-B was blended because of the decrease in the high density composition. As a consequence, the No. 1 film in which 30 % of LL-B was blended to LL-A, has a narrower stretching stress range than LL-A 100 % film. The stretching stress increased with the blending of LL-C, however the stretching stress range became narrower. Hence No. 3 is considered to have a good composition distribution, which leads to the widest stretching stress range. These results follow a similar pattern regarding the stretchability order of the LTSF. Fig. 12 shows the relationship among film density, gradient (%/K) of the Region 40 to 70 in the TREF and stretchability, i. e. stretching stress range, for the DBTF. This data shows that a better stretchability range is around 2.5 %/K of the gradient, and around g/cm 3 of the film density. The sample No. 7 (LL-A/B/C = 40/30/30 wt%) has a very narrow stretching stress range compared to other samples. ATREF curve displaying two distinct large peaks more than 20 K apart, composition distribution as shown in Fig. 3 ( in other words under 2.0 %/K gradient of the Region 40 to 70 melting composition of the total in TREF ), resulted in a deterioration of the bubble stability for the DBTF. The stretching stress at the lowest stretching temperature of the DBTF is greater than that of the LTSF be- No LL-A LL-B LL-C Stretching torque min (Nm) max Stretching stress min r MD max (MPa) R* * R : stretching stress range Table 6. Stretching stress range of the double bubble tubular films 170 Intern. Polymer Processing XIX (2004) 2

9 cause of non-isothermal stretching. This has been discussed in the previous report [10]. The Mw/Mn is a useful indicator of the processability of blown film. It was found that a film with a Mw/Mn less than 3.0, did not display good stretchability for DBTF (see Tables 2 and 6) Film Properties The properties of films prepared at the maximum and minimum stretching stresses are shown in Table 7. When comparing the properties of a higher stretching stress films with a lower stretching stress ones, they follow the same trends that have been reported previously [10]. Those are A higher stretching stress film had better shrinkability and stronger physical properties than a lower stretching stress film. A higher stretching stress film had a better shrinkage balance and strength balance of MD and TD than lower stretching stress film. This is due to the higher stretching stress film being stretched less in MD in the pre-heater oven, and being stretched more simultaneously than the lower stretching stress film [10]. There were a slight changes in the haze in the DBTF, because the bubble became unstable when the temperature was too high, which caused the haze to be worsen by the melting of the bubble surface. Comparison of the blend films suggested that lower density film displays better shrinkability, and that the Young s modulus was influenced by the film density, i. e. a higher film density resulted in greater Young s modulus. 4 Conclusions No. 1 a 1 b 3 a 3 b 4 a 4 b 6 a 6 b 7 a 7 b LL-A LL-B LL-C Thickness (lm) 15 Film density (g/cm 3 ) Stretching stress r MD (MPa) Haze (%) Young s modulus MD (MPa) Tear strength MD/TD (mn) 150/90 160/ /70 180/ /90 180/140 70/50 150/110 90/70 120/100 Shrinkage 90 C 12/19 17/22 10/15 13/17 7/13 10/15 7/11 8/13 10/16 12/18 (%) 100 C 23/31 30/36 16/22 24/30 14/21 20/26 11/18 14/20 21/29 23/29 MD/TD 110 C 52/58 58/63 36/43 48/53 35/47 48/52 24/31 31/35 41/48 46/ C 78/75 78/79 72/74 73/75 71/74 73/75 68/69 68/70 73/75 74/76 a: the lowest stretching stress film, b: the highest stretching stress film Table 7. Properties of the double bubble tubular films blending, the composition distribution expands and the stretchable temperature range becomes wider. The lower density material reduces the stretching stress and extends the lower temperature range for stretching. The higher density material maintains a high stretching stress. However there are two important points to improve the stretchability of LLDPE blends. In the first instance, the gradient (%/K) from 40 to 70 % melting composition of the total in TREF needs to be around 2.5 %/K, and secondly the blended film density is achieved at about g/cm 3. Predicting the stretchability of LLDPE for the DBTF using the data of the LTSF, taking into account the haze and the stretching stress, has become more reliable. References 1 U. S. Patent (1961), Goldman, M. 2 U. S. Patent (1970), Tsubosita, K., Kano, T. 3 U. S. Patent (1966), Lindstrom, C. 4 U. S. Patent (1967), Bild, F., Robinson, W. 5 U. S. Patent (1978), Nash, J. L., Polish, S. J., Carrico, P. H. 6 U. S. Patent (1979), Kazuo, K., Toyoki, W. 7 U. S. Patent (1986), Ralph, G. 8 U. S. Patent (1999), Douglas, J. 9 U. S. Patent (1994), Schirmer, H. 10 Uehara, H., Sakauchi, K., Kanai, T., Yamada, T.: Intern. Polym. Process. 19, p. 155 (2004) 11 U. S. Patent (1989), Mizutani, T., Isozaki, H. 12 U. S. Patent (1972), Ostapchenko, J., Williamsville 13 U. S. Patent (1996), Barry, R., Pellereau, B. 14 Hill, M. J., Barton, P. J.: Polymer 9, p (1994) 15 Hill, M. J., Barton, P. J.: Polymer 8, p (1997) 16 Person, J. R. A., Petrie, C. J. S., J. Fluid: Mech. 42, p. 609 (1970) 17 Kanai, T., Campbell, G. A. (Eds.): Film Processing. Hanser Publishers, Munich (1999) 18 Housaki, T.: Bunseki 9, p. 518 (2000) It is effective to blend the different density of LLDPEs for the improvement of the stretchability of LLDPE for DBTF. By Date received: December 18, 2002 Date accepted: December 11, 2003 Intern. Polymer Processing XIX (2004) 2 171