2.4 Results and Discussion
3.3.2 Tensile Behavior
The engineering stress–strain curves for the monolayer films of HDPE and PA-6 were showed in Figure 3-5. We see that the HDPE film deforms inhomogeneously at these three crosshead speeds, 4, 40 and 400 mm/min. A significant load drop can be seen to occur which is associated with stable neck formation at the central cross-section. The load drop is followed by the cold drawing, which is associated with neck propagation along the specimen. The true stress-strain curve was determined by measuring the local strain during neck region and applying Equations (3.3) and (3.4).
On the other hand, the PA-6 film shows the obviously different tensile behavior compared to HDPE films. The PA-6 film exhibits a higher level of engineering stress and deforms homogeneously at low level of crosshead speeds, i.e. 4 and 40 mm/min.
At higher level of strain, the engineering stress increases rapidly with the increase of engineering strain due to the molecular alignment [33]; this effect is known as strain hardening. The true stress-strain curve was calculated using Equation (3.1) and (3.2).
However, at high level of crosshead speed, i.e. 400 mm/min, the deformation mode changes from homogeneous to inhomogeneous and the true stress-strain curve was determined as HDPE. Figure 3-6 displays the true stress–strain curves of the PA-6 and
3-8
HDPE films.
To model the deformation behavior of film, we employed the following empirical constitutive equation [19,29]:
)
0 exp( T
T
σ γ ε
σ
= ⋅ (3.6)where σ0 and γ are the true yield stress and the strain hardening parameter, respectively. The constitutive equation can also be presented in the following form:
T
T
σ γ ε
σ
=ln 0 + ⋅ln (3.7)
From Equation (3.7), the relationship of the natural logarithm of true stress (ln
σ
) and true strain (ε
T ) should be linear in the region of plastic deformation. By treating theleast-squares approximation to the plot of the ln
σ
versusε
T , the slope and intercept correspond to the strain hardening parameter (γ) and the natural logarithm of the true yield stress (lnσ0), respectively. Figure 3-7 displays the fitted curves and experimental data of the lnσ
versusε
T for the HDPE and PA-6 films. A linear relationship for both films appears to exist over the range of true strains from 0.25 to 1.40 with the correlation coefficients all > 0.99 with respect to the approximation. The corresponding parameters of the constitutive equation for the HDPE and PA-6 films at various crosshead speeds are summarized in Table 3-4. For comparison, Figure 3-8 displays both the experimental true stress–strain data and the modeling curves. We could see that a good agreement exists between the modeling curves and experimental data for the range of plastic deformation of the HDPE and PA-6 films.Figure 3-9 presents a plot of the engineering and true stress–strain curves of three-layer films with various volume fractions of PA-6 at crosshead speed of 40 mm/min. It is clear that these curves all lie between those obtained for monolayers of HDPE and PA-6 films. The stress level increases upon increasing the volume fraction
of the PA-6 layer and, in addition, the strain hardening behavior due to the molecular alignment becomes more obvious. In the range of volume fractions of PA-6 that we investigated, all the three-layer films deform inhomogeneously at the crosshead speed.
As shown in Figure 3-10, a linear relationship also appears to exist for the three-layer films over the same range of true strains (0.25–1.40) as it did for the HDPE and PA-6 films at crosshead speed 40 mm/min. The correlation coefficients are also all > 0.99 with respect to the approximation. The corresponding parameters of the constitutive equation for the three-layer films at various crosshead speeds are also presented in Table 3-4. A comparison of the modeling curves and experimental true stress–strain data at crosshead speed of 40 mm/min was shown in Figure 3-11. There also exists a good agreement between the experimental data and the modeling curves over the range of plastic deformation for the three-layer films with various volume fractions of PA-6.
Similar to what we reported above for predicting permeability, we introduced a simple theoretical additive rule model [25] to predict the tensile behavior of a three-layer film from those of its individual component layers:
6 volume fractions of HDPE (including the tie layer) and PA-6 layers, respectively.
From Equations (3.6) and (3.8), the relationship between the true stress–strain of the three-layer and the individual component layer films is:
)
3-10
yield stresses of the HDPE and PA-6 layers, respectively, and γM,
γ
HDPE andγ
PA-6, are the strain hardening parameters of the three-layer film, HDPE, and PA-6 layers, respectively. It is reasonable to assume that the true yield stress of the three-layer film alone follows the additive rule:HDPE
and the strain hardening parameter of the three-layer film is defined as [30]:
[ ]
We calculated the parameters σ0M and γM for the additive rule by using the parameters of the individual component layers in Table 3-4. Figure 3-12 presents the dependence of these parameters obtained by additive rule and the experimental data (from Table 3-4) with respect to the compositions of films. We can see that a good agreement exists between the experimental data and the additive-rule model for both strain hardening parameter and true yield stress at low level of crosshead speeds, which suggests that this rule can be used to accurately predict the plastic deformation of the three-layer films. But a larger discrepancy existed between the model and experimental data at high crosshead speed for both these parameters of the three-layer films at high level of crosshead speed. This is due to the generation of heat during deformation [34]. The heat would cause a decrease in the stress to produce a given strain, which resulted in the discrepancy between experimental data and additive rule.
3.4 Conclusions
In this study, we have successfully fabricated HDPE/tie/PA-6 three-layer films,
typical multilayer structure A/B/C, by a coextrusion blown-film process. We have investigated the three gases, including N2, O2, and CO2, and water vapor permeabilities. We found that predicting both the gas and water vapor permeability of the three-layer films with respect to the volume fraction of PA-6 based upon those of the individual component layer films occurs in good agreement with the experimental data when using the series model. On the other hand, a constitutive equation was employed to describe the tensile behavior of the films over the range of plastic deformation. The tensile behavior of the component layer and multilayer films at various crosshead speeds can be precisely expressed by a constitutive equation having two parameters in the true stress–strain relationship, i.e., the true yield stress and the strain hardening parameter. We examined the relationships between the parameters of the monolayers of the component layers and those of the three-layer films by using an additive rule. By using the rule, we can also predict the tensile properties, including the true yield stress and strain hardening, of the three-layer film from those of the individual component layers with good agreements in the true stress–strain relationship. But there was a larger discrepancy between the model and experimental data at high crosshead speed due to the generation of heat during deformation.
In summary, we could design efficiently the compositions of the multilayer structure to achieve specific permeability and/or tensile property by employing these model predictions before processing.
3.5 References
1. J. Culter, J. Krohn and W. Todd, Pack. Tech. Eng., 8, 30 (1999).
2. J. Dooley, K. S. Hyun, and K. Hughes, Polym. Eng. Sci., 38, 1060 (1998).
3-12
3. W. J. Schrenk and S. A. Marcus, J. Plast. Film Sheet., 1, 30 (1985).
4. S. Hosoda, Y. Seki and H. Kihara, Polymer, 34, 4602 (1993).
5. S. J. Liu and C. H. Yang, Adv. Polym. Tech., 20, 108 (2001).
6. F. Hensen, Plastics Extrusion Technology, Hanser, New York,1997, Chap. 5.
7. I. I. Rubin, Handbook of Plastic Materials and Technology, Wiley, New York, 1990, Chap. 30.
8. M. B. Sabne, S. M. Thombre, A. S. Patil, S. D. Patil, S. B. Idage, and S. P.
Vernekar, J. Appl. Polym. Sci., 58, 1275 (1995).
9. T. S. Gill and M. Xanthos, Polym. Eng. Sci., 2, 248 (1996).
10. D. C. Climenhage, Packaging, 32, 39 (1987).
11. S. Eichler and J. Miltz, J. Appl. Polym. Sci., 50, 2095 (1993).
12. I. I. Rubin, Handbook of Plastic Materials and Technology, Wiley, New York, 1990, Chap. 16.
13. Y. P. Khanna, E. D. Day, M. L. Tsai and G. Vaidyanathan, J. Plast. Film Sheet., 13, 197 (1997).
14. G. W. Kamykowski, J. Plast. Film Sheet., 16, 237 (2000).
15. H. Tanaka, H. Shigemoto and H. Kawchi, J. Plast. Film Sheet., 12, 279 (1996).
16. S. S. Valdes, F. O. Villarreal, M. L. Quintanilla, I. Y. Flores, and L. F. Ramos de Valle, Polym. Eng. Sci., 38, 127 (1998).
17. J. V. Olmos, S. S. Valdes, and I. G. Yánez Flores, Polym. Eng. Sci., 39, 1597 (1999).
18. C. H. Huang, J. S. Wu, C. C. Huang, and L.S. Lin, Polym. J., 35, 978 (2003).
19. S.Bahadur, Polym. Eng. Sci., 13, 266 (1973).
20. Y. Wang, A. J. Easteal and X. D. Chen, Pack. Tech. Sci., 11, 169 (1998).
21. L. Lin and A. S. Argon, J. Mater. Sci., 29, 294 (1994).
22. S. Bianchi, S. Cantagallo, G. Consolati, M. Laporta, M. Pegoraro, G. Tieghi and L.
Zanderighi, J. Appl. Polym. Sci., 86, 559 (2002).
23. L. Lin and A. S. Argon, J. Mater. Sci., 29, 294 (1994).
24. J. B. Faisant, A. Aït-Kadi, M. Bousmina and L. Deschênes, Polymer, 39, 533 (1998).
25. W. J. Schrenk and T. Alfrey, Polym. Eng. Sci., 9, 393 (1969).
26. Automatic Manometric Gas Permeability Tester Operator Manual, Model L100-5000, Lyssy AG, 2001.
27. Automatic Manometric Water Vapor Permeability Tester Operator Manual, Model L80-5000, Lyssy AG, 2001.
28. M. Al-Hussein and G. Strobl, Macromolecules, 35, 8515 (2002).
29. V. Gaucher-Miri, G. K. Jones, R. Kaas, A. Hiltner and E. Baer, J. Mater. Sci., 37, 2635 (2002).
30. E. W. Hart, Acta Metall., 15, 351 (1967).
31. C. G’Sell, N. A. Aly-Helal and J. J. Jonas, J. Mater. Sci., 18, 1731 (1983).
32. Gěrard Buisson and K. Ravi-Chandar, Polymer, 31, 2071 (1990).
33. A. Peterlin, J. Mater. Sci., 6, 490 (1971).
34. I. H. Hall, J. Appl. Polym. Sci., 12, 739 (1968).
3-14
Table 3-1 Thickness and volume fraction of PA-6 layer in three-layer films
Thickness of PA-6 layer (µm) Volume fraction of PA-6 layer (%)
14 10 28 20 42 30 63 45
Table 3-2 Gas Permeabilities of monolayer PA-6, monolayer HDPE and three-layer films.
Permeability (ml-mm/m2-day-atm)
Volume fraction of PA-6 (%) N2 O2 CO2
0 (HDPE) 1.26 9.28 17.41
10 1.21 6.73 16.20
20 1.07 4.61 13.22
30 0.87 4.10 12.66
45 0.82 2.94 10.37
100 (PA-6) 0.55 1.73 7.50
3-16
Table 3-3 Water Vapor Permeabilities of monolayer PA-6, monolayer HDPE and three-layer films.
Volume fraction of PA-6 layer (%) Permeability
×
102 (g-mm/m2-day)0 (HDPE) 2.71
10 2.82 20 3.53 30 4.00 45 5.01
100 (PA-6) 987.52
Table 3-4 True yield stress (σ0) and strain hardening parameter (γ)of HDPE, PA-6 and three-layer films at various crosshead speeds.
Crosshead Speed (mm/min)
4 40 400 Volume fraction of
PA-6 layer (%) σ0 (MPa) γ σ0 (MPa) γ σ0 (MPa) γ 0 (HDPE) 21.54 0.80 23.57 0.78 26.20 0.75
10 22.20 0.87 25.79 0.86 27.66 0.78
20 24.78 0.91 27.66 0.89 30.88 0.81
30 25.66 0.97 30.88 0.92 34.47 0.84
45 29.67 1.02 33.78 0.97 38.09 0.86
100 (PA-6) 38.09 1.15 48.43 1.10 55.70 0.99
3-18
0 10 20 30 40 50 95100
0.0 0.5 1.0 1.5 2.0
series model experimental data
Volume fraction of PA-6 (%) N 2 Permeability (ml-mm/m2 -day-atm)
HDPE
PA-6
Figure 3-1 Nitrogen (N2) permeabilities of three-layer films with various content of PA-6 layer.
0 10 20 30 40 50 95100 0
2 4 6 8 10
HDPE
PA-6
series model experimental data
Volume fraction of PA-6 (%) O 2 Permeability (ml-mm/m2 -day-atm)
Figure 3-2 Oxygen (O2) permeabilities of three-layer films with various content of PA-6 layer.
3-20
0 10 20 30 40 50 95100
0 2 4 6 8 10 12 14 16 18 20
HDPE
PA-6
series model experimental data
Volume fraction of PA-6 (%) CO 2 Permeability (ml-mm/m2 -day-atm)
Figure 3-3 Carbon dioxide (CO2) permeabilities of three-layer films with various content of PA-6 layer.
0 10 20 30 40 50 95100 0
1 2 3 4 5 6 900 1000 1100
HDPE
PA-6
series model experimental data
Volume fraction of PA-6 (%)
Water vapor permeability x 102 (g-mm/m-day)
Figure 3-4 Water vapor permeabilities of three-layer films with various content of PA-6 layer.
3-22
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0 5 10 15 20 25 30
4 mm/min 40 mm/min 400 mm/min
Engineering Strain
Engineering Stress (MPa)
Figure 3-5(a) Engineering stress-strain curves of HDPE film at various crosshead speeds.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0
10 20 30 40 50 60 70 80 90
4 mm/min 40 mm/min 400 mm/min
Engineering Strain
Engineering Stress (MPa)
Figure 3-5(b) Engineering stress-strain curves of PA-6 films at various crosshead speeds.
3-24
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0 10 20 30 40 50 60 70 80 90 100
4 mm/min 40 mm/min 400 mm/min
True Strain
True Stress (MPa)
Figure 3-6(a) True stress-strain curves of HDPE films at various crosshead speeds.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0
50 100 150 200 250 300
400 mm/min 40 mm/min 4 mm/min
True Strain
True Stress (Mpa)
Figure 3-6(b) True stress-strain curves of PA-6 films at various crosshead speeds.
3-26
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0
4 mm/min 40 mm/min 400 mm/min Line:modeling curves Symbol:experimental data
True Strain
Ln True Stress
Figure 3-7(a) Modeling and experimental data of Ln true stress-strain of HDPE films at various crosshead speeds.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0
2.5 3.0 3.5 4.0 4.5 5.0 5.5
4 mm/min 40 mm/min 400 mm/min Line:modeling curves Symbol:experimental data
True Strain
Ln True Stress
Figure 3-7(b) Modeling and experimental data of Ln true stress-strain of PA-6 films at various crosshead speeds.
3-28
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0 10 20 30 40 50 60 70 80 90 100
4 mm/min 40 mm/min 400 mm/min Line:modeling curves Symbol:experimental data
True Strain
True Stress (MPa)
Figure 3-8(a) Comparison of true stress-strain curves between modeling curves and experimental data of HDPE films at various crosshead speeds.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0
20 40 60 80 100 120 140 160 180 200 220
240 400 mm/min
40 mm/min 4 mm/min Line:modeling curves Symbol:experimental data
True Strain
True Stress (MPa)
Figure 3-8(b) Comparison of true stress-strain curves between modeling curves and experimental data of PA-6 films at various crosshead speeds.
3-30
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0 20 40 60 80 100
HDPE PA-6 45%
30%
20%
10%
Engineering Strain
Engineering Stress (MPa)
Figure 3-9(a) Engineering stress-strain curves of three-layer films as a function of volume fraction of PA-6 layer at crosshead speed 40 mm/min. Thick solid lines represent the component layer of PA-6 and HDPE, respectively.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0
50 100 150 200 250 300
45%
30%
20%
10%
True Strain
True Stress (MPa)
HDPE PA-6
Figure 3-9(b) True stress-strain curves of three-layer films as a function of volume fraction of PA-6 layer at crosshead speed 40 mm/min. Thick solid lines represent the component layer of PA-6 and HDPE, respectively.
3-32
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0 2 3 4 5
45%
30%
20%
10%
Line:modeling curves Symbol:experimental data
True Strain
Ln True Stress
Figure 3-10 Modeling and experimental data of Ln true stress-strain of three-layer films as a function of volume fraction of PA-6 layer at crosshead speed 40 mm/min.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0
20 40 60 80 100 120 140 160
45%
30%
20%
10%
Line:modeling curves Symbol:experimental data
True Strain
True Stress (MPa)
Figure 3-11 Comparison of true stress-strain curves between modeling curves and experimental data of three-layer films as a function of volume fraction of PA-6 layer at crosshead speed 40 mm/min.
3-34
0 10 20 30 40 50 95100
0.0 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
HDPE
PA-6
additive rule experimental data
Volume fraction of PA-6 (%)
Strain hardening parameter
4 mm/min
Figure 3-12(a) Comparison of strain hardening parameter between additive rule and experimental data as a function of PA-6 content at crosshead speed of 4 m/min.
0 10 20 30 40 50 95100 0.0
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
HDPE
PA-6
additive rule experimental data
Volume fraction of PA-6 (%)
Strain hardening parameter
40 mm/min
Figure 3-12(b) Comparison of strain hardening parameter between additive rule and experimental data as a function of PA-6 content at crosshead speed of 40 mm/min.
3-36
0 10 20 30 40 50 95100
0.0 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
HDPE
PA-6
additive rule experimental data
Volume fraction of PA-6 (%)
Strain hardening parameter
400 mm/min
(c)
Figure 3-12(c) Comparison of strain hardening parameter between additive rule and experimental data as a function of PA-6 content at crosshead speed of 400 mm/min.
0 10 20 30 40 50 95100 0.0
15 20 25 30 35 40 45
HDPE
PA-6
additive rule experimental data
Volume fraction of PA-6 (%)
True yield stress (MPa)
4 mm/min
Figure 3-13(a) Comparison of true yield stress between additive rule and experimental data as a function of PA-6 content at crosshead speed of 4 mm/min.
3-38
0 10 20 30 40 50 95100
0.0 15 20 25 30 35 40 45 50
HDPE
PA-6
additive rule experimental data
Volume fraction of PA-6 (%)
True yield stress (MPa)
40 mm/min
Figure 3-13(b) Comparison of true yield stress between additive rule and
experimental data as a function of PA-6 content at crosshead speed of 40 mm/min.
0 10 20 30 40 50 95 100 0.0
15 20 25 30 35 40 45 50 55 60
HDPE
PA-6
additive rule experimental data
Volume fraction of PA-6 (%)
True yield stress (MPa)
400 mm/min
Figure 3-13(c) Comparison of true yield stress between additive rule and experimental data as a function of PA-6 content at crosshead speed of 400 mm/min.
4-1
Chapter 4
Adhesion, Permeability and Mechanical Properties of Multilayered Blown Films using Maleated Low-Density
Polyethylene Blends as Adhesion-Promoting Layers
4.1 Introduction
Coextrusion is a process in which two or more polymers are extruded simultaneously and joined together to form a single structure having different properties in each layer and to achieve a broad range of properties that are not available in any of the individual materials alone. In recent years, the packaging and container industries have paid increasing attention to the development of new or improved products formed by coextrusion, such as multilayer sheets, multilayer films, and multilayer containers [1–3].
The number of layers comprising these materials depends on the required end-use properties and the availability of polymer combinations suitable for specific applications.
Recently, it has become common [4,5] in food packaging technology to coextrude multilayer films consisting of distinct layers that are barriers for oxygen and moisture.
Polyethylene is an excellent moisture barrier for packaging, and its low cost, strength and ease of processing make it suitable for many applications. Its inability, however, to act as a barrier for oxygen, aromatics, and oils limits its potential applications. On the other hand, ethylene–vinyl alcohol copolymer (EVOH) possesses excellent barrier
properties to oxygen, aromatics, and oils [6–9]. Unfortunately, EVOH is highly sensitive to moisture, which alters its ability to acts as an oxygen barrier [10,11].
Therefore, using coextrusion to combine polyethylene and EVOH in a multilayer structure is very attractive for many demanding packaging applications, such as for food, drugs, and cosmetics. Typical commercial multilayer barrier films for food packaging contain EVOH as an oxygen barrier layer and polyethylene resins as the moisture barrier layer. This film possesses a multilayer structure in which outer PE layers protect an inner EVOH layer from continuous exposure to moisture. Because of the chemical dissimilarities between PE and EVOH, however, an extrudable adhesive polymer must be incorporated into the film as a tie layer that promotes adhesion. Graft copolymers are widely recognized as novel potential adhesive polymers for imparting improved adhesion. These copolymers are synthesized mainly by modifying polyolefin resins through the addition of functionality. This process is achieved by adding acid or anhydride units to polyolefins through grafting or by direct synthesis of copolymers.
Tanaka et al. [12] have successfully developed a new generation of tie layer adhesives, by combining graft and polymer blending, that maintain high adhesive strengths after thermoforming and orientation. Botros studied three-layer films, tie/EVOH/tie, using a coextrusion cast-film process [13] and found that the tie layers bind to EVOH through covalent and hydrogen bonding. The failure mechanisms of the three-layer films were of a mixed cohesive/adhesive type. Kim et al. [14] have investigated the mechanical and transport properties of various combinations of two-layer films, including LDPE/tie, Nylon 6/tie and LDPE/Nylon 6. The tensile strength and modulus of a coextruded film follows the additivity rule and its permeability follows the inverse additivity rule [15].
Kamykowski studied the adhesive properties of five-layer coextruded cast films [16]
and found that the adhesion properties generally improved upon increasing the overall
4-3
film thickness or the relative amount of the adhesive (maleic anhydride-grafted polypropylene). The molecular weight of the grafted resin had a small effect on adhesion. Homopolymer diluents outperform random copolymer diluents in their adhesion properties.
Having additional tie layers in a coextruded film makes the fabrication process more complex and expensive, because of the need in the coextrusion system for a specially designed die and additional extruders for the adhesive polymers. An alternative approach has been reported that overcomes this disadvantage by replacing the five-layer film coextrusion system with a three-layer film comprising EVOH as the central layer and LLDPE/LLDPE-g-MAH blends as the external layers [17,18].
In this paper, we report a method to eliminate the need for tie layers by using blends of low-density polyethylene (LDPE) and linear low-density polyethylene grafted with maleic anhydride (LDPE-g-MAH) that promote adhesion between LDPE and EVOH in coextruded three-layer blown films. We investigated the mechanical properties of the films, including their peel strengths, tensile properties, and tear strengths, and compared their oxygen and water vapor permeability to theoretical predictions [19]. These blown films could be a viable option for reducing the number of film layers in coextrusion processing.
4.2 Experimental
4.2.1 Materials
Commercial-grade, low-density polyethylene [LDPE, 6030F, M. I. (g/10 min, 190 °C, 2.16 kg) = 0.27, density = 0.922 g/cm3] was supplied in pellet form by Formosa Plastic Corp. (Taiwan). The ethylene–vinyl alcohol copolymer [EVOH, F101A, ethylene
content (mol %) = 32, M. I. (g/10 min, 190 °C, 2.16 kg) = 1.6, density = 1.19 g/cm3] was provided in pellet form by Kuraray Co. (Japan). The adhesive, Modic-AP L502, was obtained in pellet form from Mitsubishi Chemical Corp. (Japan). It is a low-density polyethylene-grafted maleic anhydride [LDPE-g-MAH, M. I. (g/10 min, 190 °C, 2.16 kg) = 1.0, density = 0.93g/cm3].