More Flexural Strength for Thin Films

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1 SD DUPRONT_PE11757_PE5_FIN.qxp :56 Uhr Seite 1 Special reprint from Kunststoffe international 5/211 Barry A.Morris, John D. Vansant and Karlheinz Hausmann More Flexural Strength for Thin Films Multi-layer Packaging Films Volume YEARS /211 Magazine for Plastics PU Surfaces Environment-friendly Foams and Functional Coatings 58 Rapid Prototyping Polypropylene Powder for Laser Sintering 45 SPECIAL Packaging with Plastics from page 8 DuPont de Nemours S.á.r.l. 2, Chemin du Pavillon 1216 Le Grand Saconnex Geneva Switzerland Carl Hanser Verlag, München All rights including reprinting, photographic reproduction and translation reserved by the publishers.

2 Packaging In designing more sustainable multilayer packaging film the use of materials has to be reduced without compromising the processing and service properties. Optimal positioning of individual layers with different stiffnesses within the structure can make a significant contribution to this (all photos and sources: DuPont) More Flexural Strength for Thin Films Multi-layer Packaging Films. With the help of a computer simulation it is possible to visualize the effects of the targeted positioning of functional materials and different sealing media on film flexural strength. Results for specimen systems show that ionomers, which have stiffnesses that can be adjusted within broad limits as well as very good sealing properties, offer a wide range of opportunities to reduce the thickness of such multi-layer films without at the same time having to accept a loss in flexural strength. BARRY A. MORRIS JOHN D. VANSANT KARLHEINZ HAUSMANN T he stiffness of multi-layer packaging films (Title picture) influences properties such as their haptic (softness), ease of use on machinery and function (e.g. in stand up pouches). If the thickness of a multi-layer structure is to be reduced in order to save material and subsequent disposal costs whilst a certain minimum stiffness is to be retained, two Translated from Kunststoffe 5/211, pp Article as PDF-File at Document Number: PE11757 things are needed: a good working knowledge of how material mechanical properties affect stiffness as well as the availability of suitable materials with appropriate stiffness. Beam Bending at the Core of the Concept Beam theory from traditional technical mechanics is fundamentally suitable as a model for mono-material films under bending load. In this bending resistance is a simple function of the bending stiffness and the thickness of the material. The bending stress built up in the beam is the product of the bending stiffness and the local elongation resulting in a compression stress on the top side of the beam and a tensile stress of the same magnitude on the underside of the beam. Under the simplified assumption (given below) that Pressure stress Neutral axis Tensile stress Fig. 1. Stress distribution within a monomaterial film under bending load 2 Carl Hanser Verlag, Munich Kunststoffe international 5/211

3 1%-Secant modulus 7 MPa Ionomers CGCT s SSC s Z-N LLDPEs EVA wt.-% 25 Comonomer the compression stiffness is the same as the tensile stiffness there is a plane precisely in the middle of the beam where no forces act (the neutral axis, Fig. 1). A beam (a film) is stiffer than another if it deflects less at the same stress. The calculation of bending stiffness is clearly more complex if the beam as is the case for a multi-layer film is composed of multiple layers with different stiffnesses. In this case the film can behave in a similar manner to an I-beam (aka H beam) where the individual elements, the web and the two opposing flanges, make significantly different contributions to the bending resistance. Applied to films soft layers act like webs and the stiffer layers like flanges. Accordingly multi-layer structures behave as if they have a sequence of flanges and webs and their bending resistance is a complex function of the stiffnesses, thicknesses and positions of the individual layers within the film. This function can be derived from the equations of technical mechanics [1] and then visualized in a computer model developed by DuPont. Film Stiffness [MPa] Aluminum 69, PET-O 3,45 3,8 PA6, cast film, not oriented PA6, mono-oriented 1,38 1,52 PA66, cast film, not oriented PA66, mono-oriented 2,7 2,42 EVOH, blown film, 32 mol-% 2,42 3,4 EVOH, blown film, 44 mol-% 1,45 PP-O 1,59 LLDPE, blown film LDPE, blown film Acid copolymers, blown film Ionomers, blown film Fig. 2. Influence of the comonomer content on stiffness (represented in the form of the 1 % secant modulus) of various packaging materials (according to [2]), with Z-N: Ziegler- Natta catalyzed, SSC: Single Site catalyzed (Exxon technology), CGCT: Constrained Geometry Catalyst Technology (Dow technology) Broad Range of Stiffnesses Table 1. Tensile stiffnesses of films for packaging structures (values collated from various sources) Table 1 shows the stiffnesses of common film materials. The range of values given result from the different properties within a material family and can,amongst other things, depend on the test and processing conditions as well as the level of moisture, particularly in the case of polyamide. As these values show, the stiffness of barrier materials such as aluminum foil, polyamide, EVOH, or PP generally lies above those of typical sealing media such as LLDPE and ionomers. What also stands out is that the ionomers (e.g. Surlyn from DuPont) in respect of their stiffness cover a much wider range than those based on polyolefins. It is generally the case that good sealing properties require a low melting point. This decreases in the case of polyolefinic sealing media with increasing comonomer content and falling crystallinity. At the same time, however, the stiffness also declines. Thus while a typical at a comonomer content of around 1 to 12 % has good low temperature sealing properties, it has a stiffness of only around 7 MPa. The secant modulus curve of this shown in Figure 2 corresponds to the one for flexural stiffness. On the other hand with ionomers (acid copolymers which have partially been neutralized with sodium or zinc salts) it is the hydrogen and ionic bondings that determine the stiffness which increases with rising comonomer content and degree of neutralization. With the help of termonomers such as acrylates the stiffness can be reduced. In reality the stiffness can be adjusted for particular applications between wide limits to values between about 48 and 48 MPa. However, ionomers retain their good low temperature sealing properties (for high production speeds), high hot tack, sealing through contamination and transparency. i Contact DuPont de Nemours Deutschland) GmbH D Neu-Isenburg / Germany TEL > Computer Simulation Delivers Valuable Insights Data given in the literature was entered into the simulation program along with information about the thicknesses, stiffnesses and position of each layer in a multi-layer structure. Based on this various film structures were qualitatively assessed in respect of their overall flexural stiffness. In order to validate the model the results were then checked experimentally. To do this the force that is required to positively press each film into a narrow gap (Fig. 3) was determined as a measure of the flexural stiffness and compared with the calculated stiffness factor. As can be seen in Figure 4 the numerical model mirrors the relative difference between the flexural stiffnesses very precisely indeed. The left part contains the results for six 3-layer co-extruded blown films composed of and a sealant. The right parts shows the corresponding results for the calculated and measured values for six 5-layer films from external sources. In both cases the measured flexural stiffness (arbitrary units) is compared to the calculated stiffness factor. As the results confirm the beam bending model is very suitable for describing the bending behavior of multi-layer films. Kunststoffe international 5/211 3

4 Force Fig. 3. Determining a characteristic value for the flexural stiffness of films The absolute stiffness values cannot be calculated in this way, however, the prediction of the relative differences between various structures is very close to the real world. Model Calculations Help to Save Development Time Based on appropriate what if scenarios a very good understanding of how the material selection and arrangement of the layers can influence the stiffness of a multi-layer structure can be developed from the simulation. In general it can be seen that: The larger the separation between the cover layer and the neutral axis and the stiffer the cover layer, the greater the flexural stiffness of the overall structure. As a simple example in Figure 5 two structures are compared that are comprised of four layers, each 25 µm thick. If the softer layers are placed to the outside and the ten times stiffer layers in the middle a relative stiffness factor of 1 can be calculated. Simply by rearranging the layers ( to the outside, to the inside) the calculated stiffness increases by a factor of four (whilst the measurement of stiffness in tensile tests would show no differences!) In practice structures for food packaging for instance often combine a stiff protective or barrier layer (e.g. PA, PET-O, PP-O, aluminum foil or EVOH) with a sealant in such a way that the stiff barrier or protective layer lies in the neutral axis. In this way the material with the highest stiffness makes the least contribution to the flexural stiffness of the overall structure. In such cases it is possible to reduce the thickness and thus the use of material and costs just by using a stiffer sealant without loss of stiffness. The I-beam effect can be used particularly efficiently in laminating film. Table 2 shows the comparison between adhesive and extrusion lamination for the bonding of a PP-O with a 25 µm thick sealant film (for example LLDPE or an ionomer). In reference case A the sealant film was adhesive laminated with an Stiffness factor Calculated Measured a b c d e f 3-layer structure 18 µm PP-O film. The adhesive layer thickness in this case was 2.5 µm. This resulted in a stiffness factor of.39. Switching to extrusion lamination increased this significantly since through the greater thickness of the extrusion lamination adhesive layer (12.7 µm in case B and 25.4 µm in case C compared to 2.5 µm in case A) the separation of the PP-O and the sealant film increased amplifying the I-beam effect. If the stiffness factor were to be raised by a similar amount (.39 to 1.25) with adhesive laminating then the PP-O layer would have to be increased to 38 µm (case D). This comparison shows how developers can use the model in order to achieve an optimum balance for the factors of material use, sustainability, process costs as well as desired stiffness. With this model the influence of various sealants on the flexural stiffness can also be investigated. A 3-layer structure (PET-O/EVA/sealant) that for instance is used as a lid film for meat or snack packaging was chosen as an example (Table 3). Case 1 looks at a commercial structure (PET-O/EVA/ionomer with thicknesses of 11.9 µm/22.9 µm/33 µm). Using typical stiffness values a stiffness factor of 1.49 can be calculated. In case 2 the ionomer was replaced by a layer of much softer with the same thickness. This resulted in a reduction in the flexural stiffness of the overall structure of 67 %. Case a b c d e f 5-layer structure Fig. 4. Correlation between the measured and calculated values for the flexural stiffness of six 3-layer (left) and six 5-layer films (right) Stiffness factor Calculated Measured shows that the thickness of the layer would have to be increased to 63.5 µm in order to achieve the same stiffness as in case 1. This option is, however, not relevant in practice since heating the thicker sealant layer would require more time and slow down production. By using a stiffer ionomer (case 4) the thickness of the sealant medium could be reduced by 7.6 µm. In case 5 the sealant layer was even thinner, but at the same time the EVA layer thickness was increased so that the stiffer sealing layer was further from the neutral axis. As a result the stiffness remains at the level of case 1 whilst costs can be reduced. It should be noted that a thinner sealing layer can change the sealing properties, which should be assessed in practical trials. Case Thickness of the PP-O film [µm] Adhesive Stiffness of the adhesive [MPa] Thickness of the adhesive [µm] Calculated stiffness factor A 18 Laminating adhesive B 18 LDPE C 18 LDPE D 38 Laminating adhesive Table 2. Relative stiffness values for adhesive and extrusion laminated structures from a PP-O film (stiffness 1,586 MPa) and a 25 µm sealing film (stiffness 276 MPa) Kunststoffe international 5/211 4

5 Packaging Case Sealant Stiffness of the sealant [MPa] Thickness of the sealant [µm] Thickness of the EVA layer [µm] Overall thickness [µm] Calculated stiffness factor Change in stiffness [%] 1 Ionomer A n/a Ionomer B Ionomer B Table 3. Calculated stiffness values for 3-layer structures made from PET-O (stiffness 3,45 MPa, thickness 11.9 µm), EVA (stiffness 69 MPa) and a sealant Reducing Costs and Material Use, Increasing Sustainability As these model calculations show the stiffness of a multi-layer film is a function of the thickness, stiffness and arrangement of each layer. By using this fact ionomers, with their special combination of good sealing properties and high stiffness, offer significantly more possibilities than polyolefinic sealing media to optimize packaging structures, whilst retaining the haptic and processability, to reduce the ecological footprint as well as material and disposal costs. Often this follows the path of positioning the stiffest layer in the structure with the intention of increasing the I-beam effect. The concepts summarized here were recently used in the redesign of a packaging for meat products such as smoked ham [3]. Figure 6 shows the typical structure of a conventional thermoforming sheet for the lower shell. The sealant layer placed to the inside of the sheet is 56 µm thick, whilst a layer of only 24 µm of the significantly stiffer Surlyn (shown on the right) provides the same mechanical properties. As part of this optimization DuPont proposed additional changes to the structure in order to reduce thickness and costs. This includes shifting the second stiff component, a PA6 barrier layer, to the outside of the structure so that the I-beam effect can be used systematically to further raise the flexural stiffness. Thickness 16 µm LLDPE, 56 µm HV, 1 µm PA6, 46 µm HV, 1 µm LLDPE, 38 µm Standard structure Structure Stiffness factor Additionally the concept envisages improvements to the thermoformability through blending of less moisture absorbing amorphous PA (Selar PA from DuPont) into the PA6 layer. Because this material increases the barrier effect this step also allows the barrier layer thickness to be reduced. Lastly the new concept calls for an inner layer of low cost LDPE, which has the effect that both of the stiff layers (Surlyn and PA) lie further from the neutral axis, which additionally optimizes the I-beam effect and overall helps to achieve the minimum thickness for the thermoforming sheet. Nucrel from DuPont has proved to be an ideal tie layer for such structures. In total the new film concept is more than 3 % thinner than the original structure. The material costs are reduced by Surlyn, 24 µm Nucrel, 1 µm LDPE, 4 µm Nucrel, 1 µm 8 % PA + 2% Selar PA, 23 µm Alternative based on Surlyn Fig. 6. In designing a new lower shell structure for a thermoformed packaging the consideration of the I-beam effect made a significant contribution to reducing the overall thickness from 16 µm to 11 µm Fig. 5. Influence of the arrangement of stiff and less stiff film layers on the overall flexural stiffness of a structure (all layer thicknesses 25 µm, stiffness = 69 MPa, stiffness = 69 MPa) 4 %, and a further 4 % of cost saving results from the reduced German packaging levy (DSD). Additional advantages in the change to Surlyn are an around 25 % higher film puncture resistance, greater packaging integrity, because the high hot tack can reduce the danger of leakers, and the possibility of raising productivity due to the low temperature at which sealing begins. In the interim upgraded versions of the computational model described here are available. They offer in particular additional functions to include cost factors, greenhouse gas emissions and the use of non-renewable energy sources in the optimization of multi-layer packaging. REFERENCES 1 Jones, R. M.: Mechanics of Composite Materials, McGraw-Hill Book Co., New York degaravilla, J. R.: Ionomer, acid copolymer, and metallocene polyethylene resins: A comparative assessment of sealant performance. Tappi Journal 78 (1995) 6, p Rioux, B.: Taking the Next Step to Reduce Flexible Packaging Waste and Cost. White Paper, DuPont 12/21 - K THE AUTHORS BARRY A. MORRIS is a Senior Technology Associate at DuPont Packaging Resins, Wilmington, USA. JOHN D. VANSANT is a Senior Technical Specialist at DuPont Packaging Resins, Wilmington, USA. KARLHEINZ HAUSMANN is a Research Fellow at DuPont Packaging Resins, Geneva, Switzerland. 5 Carl Hanser Verlag, Munich Kunststoffe international 5/211

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