barry a.morris, john d. vansant and karlheinz hausmann more flexural ...€¦ · 3 special...

Barry A.Morris, John D. Vansant and Karlheinz Hausmann

More Flexural Strengthfor Thin Films

Multi-layer Packaging Films

5/2011Volume 101 www.kunststo�e-international.com

Magazine for Plastics

Rapid PrototypingPolypropylene

Powder for LaserSintering 45

PU SurfacesEnvironment-friendlyFoams and Functional

Coatings 58

SPECIAL Packaging with

Plastics from page 8

100YEARS1910-2010

DuPont de Nemours S.á.r.l.2, Chemin du Pavillon1216 Le Grand SaconnexGenevaSwitzerlandwww.packaging.dupont.com

© Carl Hanser Verlag, München. 2011. All rights including reprinting, photographic reproduction andtranslation reserved by the publishers.

Special reprint from Kunststo�e international 5/2011

SD DUPRONT_PE110757_PE5_FIN.qxp 05.05.2011 7:56 Uhr Seite 1

2 © Carl Hanser Verlag, Munich Kunststoffe international 5/2011

In designing moresustainable multi-layer packaging filmthe use of materialshas to be reducedwithout compromis-ing the processingand service proper-ties. Optimal posi-tioning of individuallayers with differentstiffnesses within thestructure can make asignificant contribu-tion to this (all photos

and sources: DuPont)

BARRY A. MORRISJOHN D. VANSANT

KARLHEINZ HAUSMANN

The stiffness of multi-layer packagingfilms (Title picture) influences proper-ties such as their haptic (softness),

ease of use on machinery and function(e.g. in stand up pouches). If the thick-ness of a multi-layer structure is to be re-duced in order to save material and sub-sequent disposal costs whilst a certainminimum stiffness is to be retained, two

things are needed: a good workingknowledge of how material mechanicalproperties affect stiffness as well as theavailability of suitable materials with ap-propriate stiffness.

Beam Bending at the Core ofthe Concept

Beam theory from traditional technicalmechanics is fundamentally suitable as amodel for mono-material films underbending load. In this bending resistanceis a simple function of the bending stiff-ness and the thickness of the material.Thebending stress built up in the beam is theproduct of the bending stiffness and the

local elongation resulting in a compres-sion stress on the top side of the beam anda tensile stress of the same magnitude onthe underside of the beam. Under thesimplified assumption (given below) that

More Flexural Strength forThin FilmsMulti-layer Packaging Films. With the help of a computer simulation it is possi-

ble 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.

Pressure stress

Neutral axis

Tensile stress

Fig. 1. Stress distribution within a mono-material film under bending load

© KunststoffeTranslated from Kunststoffe 5/2011, pp. 42–46Article as PDF-File at www.kunststoffe-international.com; Document Number: PE110757

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the compression stiffness is the same asthe tensile stiffness there is a plane pre-cisely in the middle of the beam where noforces act (the neutral axis, Fig. 1). A beam(a film) is stiffer than another if it deflectsless at the same stress.

The calculation of bending stiffness isclearly more complex if the beam – as isthe case for a multi-layer film – is com-posed of multiple layers with differentstiffnesses. In this case the film can behavein a similar manner to an I-beam (aka Hbeam) where the individual elements, the“web” and the two opposing “flanges”,make significantly different contributionsto the bending resistance.Applied to filmssoft layers act like webs and the stiffer lay-ers like flanges. Accordingly multi-layerstructures behave as if they have a se-quence of flanges and webs and theirbending resistance is a complex functionof the stiffnesses, thicknesses and posi-tions of the individual layers within thefilm. This function can be derived fromthe equations of technical mechanics [1]and then visualized in a computer mod-el developed by DuPont.

Broad Range of Stiffnesses

Table 1 shows the stiffnesses of commonfilm materials. The range of values givenresult from the different properties with-in a material family and can,amongst oth-er things, depend on the test and process-ing conditions as well as the level of mois-ture,particularly in the case of polyamide.As these values show, the stiffness of bar-rier materials such as aluminum foil,polyamide, EVOH, HDPE or PP general-ly lies above those of typical sealing me-dia such as LLDPE and ionomers. Whatalso stands out is that the ionomers (e.g.Surlyn from DuPont) in respect of theirstiffness cover a much wider range thanthose based on polyolefins.

It is generally the case that good seal-ing properties require a low meltingpoint. This decreases in the case of poly-olefinic sealing media with increasingcomonomer content and falling crys-tallinity. At the same time, however, thestiffness also declines. Thus while a typi-cal mPE at a comonomer content ofaround 10 to 12 % has good low temper-

ature sealing properties, it has a stiffnessof only around 70 MPa. The secant mod-ulus curve of this shown in Figure 2 corre-sponds to the one for flexural stiffness.

On the other hand with ionomers (acidcopolymers which have partially beenneutralized with sodium or zinc salts) it isthe hydrogen and ionic bondings that de-termine the stiffness which increases withrising comonomer content and degree ofneutralization. With the help of ter-monomers such as acrylates the stiffnesscan be reduced. In reality the stiffness canbe adjusted for particular applications be-tween wide limits to values between about48 and 480 MPa. However, ionomers re-tain their good low temperature sealingproperties (for high production speeds),high hot tack, sealing through contami-nation and transparency.

Computer Simulation DeliversValuable Insights

Data given in the literature was enteredinto the simulation program along withinformation about the thicknesses, stiff-nesses and position of each layer in a mul-ti-layer structure. Based on this variousfilm structures were qualitatively assessedin respect of their overall flexural stiff-ness. In order to validate the model theresults were then checked experimental-ly. To do this the force that is required topositively press each film into a narrowgap (Fig. 3) was determined as a measureof the flexural stiffness and comparedwith the calculated stiffness factor.

As can be seen in Figure 4 the numeri-cal model mirrors the relative differencebetween the flexural stiffnesses very pre-cisely indeed. The left part contains theresults for six 3-layer co-extruded blownfilms composed of HDPE and a sealant.The right parts shows the correspondingresults for the calculated and measuredvalues for six 5-layer films from externalsources. In both cases the measured flex-ural stiffness (arbitrary units) is com-pared to the calculated stiffness factor. Asthe results confirm the beam bendingmodel is very suitable for describing thebending behavior of multi-layer films.

Comonomer

700

MPa

500

400

300

200

100

00

1%-S

ecan

t m

odul

us

5 10 15 20 wt.-% 25

IonomersCGCT mPEsSSC mPEsZ-N LLDPEsEVA

Fig. 2. Influence ofthe comonomer con-tent on stiffness (rep-resented in the formof the 1 % secantmodulus) of variouspackaging materials(according to [2]),with Z-N: Ziegler-Natta catalyzed, SSC:Single Site catalyzed(Exxon technology),CGCT: ConstrainedGeometry CatalystTechnology (Dowtechnology)

© Kunststoffe

Table 1. Tensile stiff-nesses of films forpackaging structures(values collated fromvarious sources)

Film Stiffness [MPa]

Aluminum 69,000

PET-O 3,450– 3,800

PA6, cast film, not oriented 620 – 860

PA6, mono-oriented 1,380– 1,520

PA66, cast film, not oriented 620 – 830

PA66, mono-oriented 2,070– 2,420

EVOH, blown film, 32 mol-% 2,420–3,040

EVOH, blown film, 44 mol-% 1,450

PP-O 1,590

LLDPE, blown film 170 – 280

LDPE, blown film 170 – 210

Acid copolymers, blown film 110 – 160

Ionomers, blown film 48 – 480

DuPont de Nemours Deutschland) GmbHD-63263 Neu-Isenburg / GermanyTEL +49 6102 18-2638> www.dupont.com

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The absolute stiffness values cannot becalculated in this way, however, the pre-diction of the relative differences betweenvarious structures is very close to the re-al world.

Model Calculations Help to SaveDevelopment Time

Based on appropriate “what if” scenariosa very good understanding of how thematerial selection and arrangement of thelayers can influence the stiffness of a mul-ti-layer structure can be developed fromthe simulation. In general it can be seenthat: The larger the separation betweenthe cover layer and the neutral axis andthe stiffer the cover layer, the greater theflexural stiffness of the overall structure.As a simple example in Figure 5 two struc-tures are compared that are comprised offour layers, each 25 µm thick. If the soft-er mPE layers are placed to the outsideand the ten times stiffer HDPE layers inthe middle a relative stiffness factor of 1can be calculated. Simply by rearrangingthe layers (HDPE to the outside, mPE tothe inside) the calculated stiffness increas-es by a factor of four (whilst the measure-ment of stiffness in tensile tests wouldshow no differences!)

In practice structures for food packag-ing for instance often combine a stiff pro-tective or barrier layer (e.g. PA, PET-O,PP-O, aluminum foil or EVOH) with asealant in such a way that the stiff barri-er or protective layer lies in the neutral ax-

is. In this way the material with the high-est stiffness makes the least contributionto the flexural stiffness of the overallstructure. In such cases it is possible to re-duce the thickness and thus the use of ma-terial and costs just by using a stiffersealant without loss of stiffness.

The “I-beam effect” can be used par-ticularly efficiently in laminating film.Table 2 shows the comparison between ad-hesive and extrusion lamination for thebonding of a PP-O with a 25 µm thicksealant film (for example LLDPE or anionomer). In reference case A the sealantfilm was adhesive laminated with an

18 µm PP-O film. The adhesive layerthickness in this case was 2.5 µm. This re-sulted in a stiffness factor of 0.39. Switch-ing to extrusion lamination increased thissignificantly since through the greaterthickness of the extrusion lamination ad-hesive layer (12.7 µm in case B and25.4 µm in case C compared to 2.5 µm incase A) the separation of the PP-O andthe sealant film increased amplifying theI-beam effect. If the stiffness factor wereto be raised by a similar amount (0.39 to1.25) with adhesive laminating then thePP-O layer would have to be increased to38 µm (case D). This comparison showshow developers can use the model in or-der to achieve an optimum balance forthe factors of material use, sustainability,process costs as well as desired stiffness.

With this model the influence of vari-ous sealants on the flexural stiffness canalso be investigated. A 3-layer structure(PET-O/EVA/sealant) that for instance isused as a lid film for meat or snack pack-aging was chosen as an example (Table 3).

Case 1 looks at a commercial structure(PET-O/EVA/ionomer with thicknessesof 11.9 µm/22.9 µm/33 µm). Using typi-cal stiffness values a stiffness factor of 1.49can be calculated. In case 2 the ionomerwas replaced by a layer of much softermPE with the same thickness. This result-ed in a reduction in the flexural stiffnessof the overall structure of 67 %. Case 3

shows that the thickness of the mPE lay-er would have to be increased to 63.5 µmin order to achieve the same stiffness asin case 1. This option is, however, not rel-evant in practice since heating the thick-er sealant layer would require more timeand slow down production. By using astiffer ionomer (case 4) the thickness ofthe sealant medium could be reduced by7.6 µm.In case 5 the sealant layer was eventhinner, but at the same time the EVA lay-er thickness was increased so that thestiffer sealing layer was further from theneutral axis. As a result the stiffness re-mains at the level of case 1 whilst costscan be reduced. It should be noted that athinner sealing layer can change the seal-ing properties, which should be assessedin practical trials.

Force

Fig. 3. Determining a characteristic value forthe flexural stiffness of films

© Kunststoffe

3-layer structure

4

3

2

1

0a b c d e f

Stiff

ness

fac

tor

5-layer structure

8

6

4

2

0a b c d e f

Stiff

ness

fac

tor

CalculatedMeasured

CalculatedMeasured

Fig. 4. Correlation between the measured and calculated values for the flexural stiffness of six 3-layer (left) and six 5-layer films (right)

© Kunststoffe

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 690 2.5 0.39

B 18 LDPE 138 12.7 0.70

C 18 LDPE 138 25.4 1.25

D 38 Laminating adhesive 690 2.5 1.38

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 (stiff-ness 276 MPa)


Reducing Costs and MaterialUse, Increasing Sustainability

As these model calculations show thestiffness of a multi-layer film is a functionof the thickness, stiffness and arrange-ment of each layer. By using this factionomers, with their special combinationof good sealing properties and high stiff-ness, offer significantly more possibilitiesthan polyolefinic sealing media to opti-mize packaging structures, whilst retain-ing the haptic and processability, to re-duce the ecological footprint as well asmaterial and disposal costs. Often this fol-lows the path of positioning the stiffestlayer in the structure with the intentionof increasing the I-beam effect.

The concepts summarized here wererecently used in the redesign of a packag-ing for meat products such as smokedham [3]. Figure 6 shows the typical struc-ture of a conventional thermoformingsheet for the lower shell. The sealant lay-er placed to the inside of the sheet is56 µm thick, whilst a layer of only 24 µmof the significantly stiffer Surlyn (shownon the right) provides the same mechan-ical properties. As part of this optimiza-tion DuPont proposed additional changesto the structure in order to reduce thick-ness and costs. This includes shifting thesecond stiff component, a PA6 barrier lay-er, to the outside of the structure so thatthe I-beam effect can be used systemati-cally to further raise the flexural stiffness.

Additionally the concept envisages im-provements to the thermoformabilitythrough blending of less moisture ab-sorbing amorphous PA (Selar PA fromDuPont) into the PA6 layer. Because thismaterial increases the barrier effect thisstep also allows the barrier layer thicknessto be reduced. Lastly the new concept callsfor an inner layer of low cost LDPE, whichhas the effect that both of the stiff layers(Surlyn and PA) lie further from the neu-tral axis, which additionally optimizes theI-beam effect and overall helps to achievethe minimum thickness for the thermo-forming sheet. Nucrel from DuPont hasproved to be an ideal tie layer for suchstructures.

In total the new film concept is morethan 30 % thinner than the original struc-ture. The material costs are reduced by

4 %, and a further 4 % of cost saving re-sults from the reduced German packag-ing levy (DSD). Additional advantages inthe change to Surlyn are an around 25 %higher film puncture resistance, greaterpackaging integrity, because the high hottack can reduce the danger of leakers, andthe possibility of raising productivity dueto the low temperature at which sealingbegins.

In the interim upgraded versions of thecomputational model described here areavailable. They offer in particular addi-tional functions to include cost factors,greenhouse gas emissions and the use ofnon-renewable energy sources in the op-timization of multi-layer packaging.�

REFERENCES1 Jones, R. M.: Mechanics of Composite Materials,

McGraw-Hill Book Co., New York 19752 deGaravilla, J. R.: Ionomer, acid copolymer, and

metallocene polyethylene resins: A comparativeassessment of sealant performance. Tappi Journal78 (1995) 6, p. 191

3 Rioux, B.: Taking the Next Step to Reduce FlexiblePackaging Waste and Cost. White Paper, DuPont12/2010 - K-24651

THE AUTHORSBARRY A. MORRIS is a Senior Technology Associ-

ate at DuPont Packaging Resins, Wilmington, USA.JOHN D. VANSANT is a Senior Technical Special-

ist at DuPont Packaging Resins, Wilmington, USA.KARLHEINZ HAUSMANN is a Research Fellow at

DuPont Packaging Resins, Geneva, Switzerland.

Stiffness factor0Structure 1 2 3 4

HDPE

mPE

mPE

HDPE

mPE

HDPE

HDPE

mPE

Fig. 5. Influence ofthe arrangement ofstiff and less stiff filmlayers on the overallflexural stiffness of a structure (all layerthicknesses 25 µm,stiffness HDPE = 690 MPa, stiffnessmPE = 69 MPa)

© Kunststoffe

Standard structure Alternative based on Surlyn

LLDPE, 56 µm

LDPE, 40 µm

80 % PA +20% Selar PA, 23 µm

LLDPE, 38 µm

PA6, 46 µm

HV, 10 µm

HV, 10 µm

Surlyn, 24 µm

Nucrel, 10 µm

Nucrel, 10 µm

160

µm

120

100

80

60

40

20

0

Thic

knes

s

Fig. 6. In designing anew lower shellstructure for a ther-moformed packagingthe consideration ofthe I-beam effectmade a significantcontribution to reduc-ing the overall thick-ness from 160 µm to110 µm

© Kunststoffe

Case Sealant Stiffness of thesealant [MPa]

Thickness of thesealant [µm]

Thickness of theEVA layer [µm]

Overallthickness [µm]

Calculatedstiffness factor

Change instiffness [%]

1 Ionomer A 276 33 22.9 67.8 1.49 n/a

2 mPE 69 33 22.9 67.8 0.49 -67

3 mPE 69 63.5 22.9 98.3 1.48 -1

4 Ionomer B 483 25.4 22.9 60.2 1.49 0

5 Ionomer B 483 10.2 45.7 67.8 1.48 -1

Table 3. Calculated stiffness values for 3-layer structures made from PET-O (stiffness 3,450 MPa, thickness 11.9 µm), EVA (stiffness 69 MPa) and a sealant

© Carl Hanser Verlag, Munich Kunststoffe international 5/2011

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