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Development of new BOPP Barrier Films by Coextrusion and SimultaneousBiaxial Orientation

Brueckner Maschinenbau GmbH, Siegsdorf, Germany,Dr. M. Wolf, Dr. J. Breil, Dipl.-Ing. R. Lund:

Abstract: Semiindustrial production of transparent, biaxial oriented polypropylene(BOPP) barrier films with very thin EVOH layers and excellent barrier properties wassuccessful. Also EVOH based display shrink barrier films as well as opaque, densityreduced barrier films were produced. Comparisons with commercial film types will bepresented, PVDC based barrier films could be replaced.Metalized UHB BOPP films with improved OTR barrier values of about 0.2 cm³/m² d bar(0,01 cc/ 100in2 d bar) (23°C (73,4°F)/ 75% r.h.) will be discussed. Aluminium andmetalized BOPET-films could be substituted.First successful application tests of the new EVOH based packaging barrier films havebeen performed in the framework of a funded network project, mainly as snack pack-ages.Further cost reductions could be achieved by producing a multifunctional, biaxialoriented EVOH based barrier film in one step, saving converting steps.

1) Introduction:

Biaxial stretching of thermoplastic polymer films improves mechanical, optical as well asbarrier film properties. This refinement by a biaxial stretching process is applied for manythermoplastics, the worldwide capacity for biaxially oriented films amounts to about 12.5Million t/ a in 2008. The biggest share with 65% comes from BOPP, followed by BOPET with26%.As in recent years the profit margins for standard biaxial oriented films, e.g. BOPP coex orBOPP tape decreased mainly due to high resin prices and overcapacity, many BOPPproducers are looking for added-value specialty BOPP films. Additionally, as summarized infigure 1, current market trends are demanding new packaging solutions. Main general trendsin our opinion are cost reduction and convenience.

Figure 1: General market needs/ trends and derived possible film developments.

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2) Processes for the production of biaxial oriented films:

There are three different techniques to produce biaxial oriented films: The sequential tenterframe process, the simultaneous tenter frame orientation as well as the double-bubbleprocess, which is also a simultaneous stretching method. Thereby for different film typescertain processes have prevailed: For example BOPP and BOPET are mainly produced bythe sequential process, whereas for BOPA all three techniques are used at about the sameshares.

Figure 2: Sequential and simultaneous biaxial stretching techniques for film production.

With the sequential stretching process the first step is a stretching of an extruded cast sheetin machine direction between roll pairs at very high stretching speeds. As a second step intransverse direction the film is gripped by a transfixed clip system and via a track railstretched over the width. Due to reasons of product quality and processing stability thestretching ratios in MD as well as in TD, MD resp. TD, can be varied only between rathernarrow limits, typical values for e.g. BOPP are MD=5 and TD=9.

By contrast, with the simultaneous tenter frame process the MD- and TD stretching is doneat moderate stretching speeds (max. 300 %/s ) but at the same time. This is possible due toa continuous extension of the distances between clips in machine direction during thesimultaneous transverse track rail stretching. Due to a low flexibility in stretching ratios aswell as high mechanical efforts and low line speeds the long well known simultaneouspantograph- or spindel-systems are seldom used.These disadvantages were overcome by the simultaneous tenter frame LISIM® technology,an abbreviation for “Linear Motor Simultaneous Stretching Technology” as shown in figure 3.With this new technology all clips can be separately driven by linear motors, thus, incomparison to the sequential process, significantly increasing the utilizable range of MD- aswell as TD-stretching ratios, as shown in figure 4. Even MD stretching ratios up to 10 arepossible, resulting in a significant enhancement of mechanical film properties, e.g. E-modulus, in machine resp. processing direction as shown in table 1: The higher the MDstretching ratio the higher the mechanical value.

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Figure 3: Principle - Linearmotor Simultaneous Stretching Technology

Figure 4: Comparison of simultaneous and sequential stretching process regarding utilizablerange of stretching ratios in MD- and TD direction.

Additional selected advantages of the simultaneous linear motor process are a broadaccessible range of shrinkage film properties, processability of high barrier EVOH grades aswell as contact free stretching of very low SIT materials as shown in figure 31-5).

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Table 1: Comparison of simultaneous and sequential stretching process regardingmechanical film properties in MD- and TD direction.

The double-bubble-process results in balanced film properties by simultaneous stretching ofa cooled and reheated bubble. Disadvantageous regarding film product quality and repro-ducibility is – in contrast to tenter frame processes - the difficult control of temperatures andother process parameters of this open double-bubble process. Additionally film output israther low, typically 300 kg/h.

3) Simultaneous biaxial orientation of different EVOH grades

Among the transparent high barrier films the fastest growing barrier film materials are EVOHand glass coated film structures as shown in table 26). Despite the efforts to make EVOHmore useful in oriented films to our knowledge only about 5% of the total EVOH film con-sumption are used in oriented film applications, including also monoaxially oriented films.

Table 2 : Amount of high-barrier materials worldwide.

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For our experiments we used EVOH grades with varying ethylene contents of the companyKuraray, the chemical formula is given in figure 5. The ethylene contents have significantinfluences on several processing parameters as well as endfilm properties. For example ahigher ethylene content gives better orientability and a better moisture barrier, but on theother hand results in lower barrier properties. Orientation of EVOH should result in higherbarrier and mechanical properties.

At the starting point of our investigations we found, that the crystallisation of EVOH typeswith a certain ethylene content causes a deterioration of the stretchability, resulting in “netstructures” and therefore bad optical film properties, as shown in figure 6 on the left side.This means, that obviously the crystallisation temperature of a certain EVOH type, especiallywith lower ethylene contents, e.g. 24%, overlaps the optimum orientation temperature rangeof polypropylene, the base material of our barrier films. Especially the sequential orientationprocess of EVOH is difficult due to MD orientation crystallisation and formation of microfibrilstructures, which result upon transverse stretching in voids and other optical defects.

Figure 5: Influence of the ethylene content of different EVOH grades on their processabilityand film properties

Figure 6:Comparison of optical film properties under polarized light for sequential (left) andsimultaneous (right) tenter film samples.

By using the simultaneous tenter frame stretching technology we found a way to overcomethis problem: A simultaneous orientation at low stretching temperatures and high stretching

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speeds subdues crystallization and enables the defect free stretching of EVOH coex layers,resulting in films as shown in figure 6 on the right side.

Starting point of the high barrier evaluations were laboratory scale experiments on adiscontinuous laboratory stretching frame, developing the principles for the production ofoptical defect free 7-layer barrier films with thin EVOH layers.

The upscaling experiments on a continuous simultaneous linear motor pilot line could beperformed successfully. It was first time possible to produce biaxially oriented barrier filmrolls with thin EVOH layers (1,3 - 1,8 µm) and total thicknesses between 20 and 30 µm andEVOH grades containing 24% and 27% of ethylene without optical defects. A typical layerdistribution is shown in figure 7. OTR values of about 1 cm³/m² d bar (23°C (73,4°F)/ 75%r.h.) could challenge PVDC coated BOPP films as summarized in table 3.

Figure 7: Structure of a 7-layer transparent BOPP/ EVOH barrier film and key characteristics

Table 3: Comparison between novel biaxial oriented 7-layer barrier films and commercialproducts

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By using suitable polypropylene based copolymers in the thick intermediate layers it was alsopossible to produce display shrink barrier films with shrinkage values – depending on appliedtemperature – up to 50% in MD- and TD-direction, as shown in figure 8.

Figure 8: Shrinkage data of a 7-layer transparent display BOPP/ EVOH barrier film .

Besides transparent and shrink barrier films also density reduced white opaque films couldbe produced successfully. By adding inorganic fillers into the intermediate PP-layer theformation of vacuoles occurred upon biaxial stretching. The resulting white opaque barrierfilms with an total density of 0,78 g/ cm3 (Solid PP: 0.91 g/ cm3 yield increase for 30µm:16%) display excellent UV protection of the filling goods. OTR- and WVTR-values are onlyslightly higher than for transparent barrier films, typical values are OTR=3,5 cm³/m² d bar(0,23 cc/ 100in2 d bar) (23°C (73,4°F)/ 75% r.h.) and WVTR=1.4 g/ m2 d (0,09 g/ 100in2 d)(23°C (73,4°F), 85% r.h.).

It is known, that barrier properties of EVOH containing barrier films deteriorate withincreasing humidity7). Figure 9 summarizes for different EVOH types at an EVOH layerthickness of about 1,5 µm the dependence of the OTR values from the applied relativehumidity.

Figure 9: Influence of relative humidity on OTR values of BOPP based EVOH barrier films atEVOH thickness of 1,5 µm

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Figure 9 also clearly shows, that - compared to PVDV coated films and even SiOx coatedBOPP films - the biaxial stretched EVOH barrier films show improved OTR values over avery broad range of relative humidity, at least up to 85%-90% r.h.. In our opinion even for wetpackaging goods, due to the layer of PP between good and EVOH, relative humiditiesbetween 80 and 85% are realistic conditions and in this range the biaxial oriented barrierfilms show only minor barrier deviations.

The EVOH stretching trials can be summarized as follows:

Sequential stretching is possible for EVOH grades with ethylene content of 44%and higher, but

simultaneous stretching by contrast is possible with all available types, even downto a content of 24% ethylene

high stretching speeds and/ or low stretching temperatures suppresscrystallization and thus „net structures, enabeling the production of films withoutoptical defects and good OTR values.

Due to the choice of base material and machine settings there´s a high flexibilityin end film properties, examples are shrink barrier and density reduced opaquewhite films.

Barrier measurements have shown, that chain orientation effects due to thebiaxial stretching process improve the barrier properties of EVOH by an factor ofabout 2.38)

4) Ultra-High Barrier Metalized and SiOx coated BOPP UHB Film

Besides EVOH based, predominantly transparent barrier films also metalized, simultaneouslybiaxial oriented „ultra-high-barrier“-films, abbreviated as UHB-films, with exceptional OTRvalues have been developed. Also transparent UHB films with SiOx coating could beproduced. Their OTR values of 0.2 cm³/ m² d bar (0,013 cc/ 100in2 d bar (23°C (73,4°F)/ 75%r.h.) approach those of aluminium foil and are about 250 times lower than that of standardBOPP metalized films, which are in the range of 40-70 cm³/ m² d bar.Aluminium foil is essentially impenetrable by moisture and oxygen. However, when packagesmade with foil are subjected to the repeated manipulation and handling that are common withmanufacturing, cartoning, shelf stocking and consumer handling in the retail environment,they can develop pinholes and become permeable10). But an even more important reason toreplace thin aluminium foil with metalized BOPP ultra high barrier films are permanentlyincreasing aluminium prices and the fact, that thin foils have a thickness of 7 µm, whereasthe aluminium coatings on BOPP films are in a thickness range of 40-60 nm.

Figure 10: Structure of a metalized 5-layer UHB-barrier film and main characteristics

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Figure 10 summarizes the detailed structure and main characteristics of the UHB-film: A verythin 0.5-0.8 µm surface layer of a so called “high surface energy polymer“ results in a verystrong bonding of the evaporated aluminium to this film surface, among others due to a veryhigh surface energy in the range 52-56 dyn. No surface treatment by e.g. corona or flame isnecessary to reach these surface energy values. By using standard optical densities of about2.3 exceptional OTR values down to 0,15 cm3/ m2 d bar (0,01 cc/ 100in2 d bar) have beenobtained, a typical WVTR value is 0.3 g/ m2 d (0,02 g/ 100in2 d). As the high surface energypolymer layer can be applied in thicknesses clearly below 1 µm a cost-efficient production ispossible. Alternatively a transparent SiOx coated UHB film with similar barrier values hasbeen manufactured. At the moment a UHB film with the given polymer layer composition canbe obtained without optical defects only by the simultaneous biaxial tenter frame process.

5) Joint Project along the Added Value Chain: “Thin Films”

In the framework of a funded network project called “Thin Films”8,9,11-13) the above discussedEVOH based biaxial stretched barrier films as well as the UHB film have been included intothin packaging film laminates for commercial nut snack filling goods to possibly substitutecurrent snack packagings.

In the course of this project different packaging film systems have been analyzed along theentire value-added chain, starting from products of film producer´s like Kopafilm or in thiscase Brueckner, followed by metalization and lamination steps, e.g. on Applied Films andWipak equipment, up to food packaging trials with Bosch VFFS-machines at food companieslike Kraft and Lorenz. Main packaging goods were nut and chips snacks. The projectpartners are summarized in figure 11. The overall project coordination was with theFraunhofer IVV (Institute for Process Engineering and Packaging) in Freising, Germany.Financial fundings were granted by the German BMBF (German Federal Ministry forEducation and Research).

Figure 11: Joint Project Along the Added Value Chain: “Thin Films”

The target of this project was to demonstrate with selected film systems along the entireadded value chain up to the final application, that a 50% reduction in material is possiblewithout significant limitations of the technical functionality. Interesting questions among

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others were the minimum thickness values to obtain the required film properties, e.g.sufficient dimension stability to prevent damaging of coating layers by elongation of the basefilm or sufficient E-modulus to produce packages without wrinkles.Main motivations for material reductions are of course cost savings, but also the Europeanpackaging law, which makes avoidance and material reduction a priority. The demonstrationof conformity with the European packaging directive is possible according to DIN EN 13 427ff, reduction of material input at retained functionality is in this norm an explicitly termedcriteria 8,9,10-12).

One reference packaging system we investigated is a three layer packaging for peanutsnacks, meaning it is a triplex compound with two required laminating steps. One of thelayers, the middle one is metalized, either 15µm BOPP or 12 µm BOPET, inner layer is a50µm PE blown film, outer layer usually is a reverse printed BOPET film. The detailed struc-ture is given In figure 12 on the left side.

During first trials we successfully reduced the thickness values of single layers and switchedfrom triplex to duplex laminates, using e.g. 7µm strong BOPET films against 30-40µm strongBOPE/ PP sealing layers, the latter biaxial oriented to compensate for losses in mechanicalstability.

Additionally for the example of peanut snacks we substituted the commercial film laminate fora duplex laminate of a transparent EVOH based barrier film and a 30 µm biaxial orientedsealing layer. For UV protection also the white opaque EVOH based barrier film has beentested. This approach also resulted in an overall thinner laminate structure as well aspossible cost savings – regarding material and production costs – up to 25%, as illustrated infigure 12.

In the following selected results from the network project „thin films“ will be presented anddiscussed. Figure 13 gives an overview of OTR and WVTR values for commercial andexperimental high barrier laminates. For PET based film systems the thinner 7 µm films showafter metalization and lamination comparable, partly even slightly improved barrier values incomparison to thicker film systems.Also the investigated EVOH based barrier film laminates with biaxial oriented sealing filmlayer for peanut snack packaging display OTR values of about 0,3 m3/ m2 day bar (0,02cc/100in2 d bar) and WVTR values below 0.8 g/ m2 d (0,051 g/ 100in2 d), thus challengingmetalized systems BOPETmet/ PE. The same is true for the barrier values of the UHB filmbased barrier laminate.

Figure 12: Cost comparison between commercial triplex packagings for peanut snacksagainst experimental novel biaxial oriented EVOH based duplex laminates

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Figure 13: Comparison of OTR and WVTR values for commercial and experimentalpackaging film laminates

As summarized in figure 14 for BOPP/ BOPP-systems the thin films are comparable to thethicker reference films also regarding aroma permeation. The required breakthrough time forexcellent aroma barrier is t 30 days (720 h)11). In the case of BOPET- and EVOH-basedbarrier film laminates no breakthrough times could be determined as the measuring valueswere clearly below the detection limit.

Figure 14: Aroma breakthrough data for selected packaging barrier film laminates.

Additionally we investigated, how possible elongation processes during the further pro-cessing chain, e.g. during lamination, influence the barrier values of different film laminates,

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especially coated ones8,9). The most delicate laminates were those based on very thinBOPET/ PE films. In contrast for EVOH based systems a slight elongation initially resulted ina reproducible barrier value improvement.Assuming that a 2% film elongation has no negative impact on barrier values, figure 15shows, that the arising forces during the lamination step are clearly smaller than thosenecessary for a 2% elongation of the film systems, therefore no deteriorated barrier valueshave to be expected.

Figure 15: Overview of forces for a 2% elongation of different film systems in comparison toarising forces during a lamination step.

Comparable results have been obtained during the packaging process for VFFS packagingmachines at the film form shoulder and at the film propulsion unit, but only when continuousworking VFFS machines are used.

Packaging trials with selected barrier laminates and different snack filling goods with fol-lowing sensory tests have been successfully performed. For the system biaxial orientedBOPP EVOH// BOPP7 BOPE a shelf life of 6 month has been granted. Several trials with“thin films” with 100% tight packages have been performed11).

The findings of the funded network project „thin films“ can be summarized as follows:

With biaxial orientation the thickness of film laminates based on vacuum coatedfilm structures can be reduced down to 50% compared to standard film laminates.

Resulting barrier properties are comparable or even better than for standard filmlaminates.

Barrier properties of thin film laminates are not impaired by following convertingprocesses, as the resulting elongations are clearly below critical values, whenusing an continuous working, vertical packaging machine.

Intact sealed seams down too 27µm for BOPP/ BOPP ando 37µm for BOPET/ BOPP-PE

Handling and acceptance by end users has to be evaluated.

For more information please visit www.duenne-folien.de 8,9,11-13)

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6) Path forward (“Integrated Production Process”) and Conclusion

A next possible future step for further cost reductions could be the total elimination oflamination steps. Nowadays the production of a barrier film laminate based on vacuumcoating technology (e.g. aluminium, SiOx, AlOx) usually takes four processing steps:

Extrusion of the carrier film, e.g. BOPP, BOPET, BOPA Extrusion of the sealing film, e.g. c-PP, c-PE Barrier coating of the carrier film Lamination of sealing layer to carrier layer

As the main driving factor in the film supply chain are costs, it would be advantageous notonly to save material by reduced thicknesses, but to produce a barrier film with all requiredfunctionalities, including barrier as well as e.g. sealing properties by a one-step extrusionprocess and a following biaxial orientation with the simultaneous tenter frame process. Interms of an “integrated production process” all necessary functionalities – depending on therequirements of the packaged good – should be implemented in one processing step.By developing, testing – also in long-term storage trials with packaged goods - and producingone-step high barrier films with barrier values challenging laminated structures based on e.g.PET-Al or PET-SiOx, the replacement of these now existing, clearly more expensive barrierlaminates is our long-term target. Figure 16 proposes cost savings of up to 45% for replacinga triple laminate BOPET/ BOPETmet/ PE for peanut snack packagings by a biaxially oriented7-layer EVOH containing one-step BOPP barrier film structure with reduced sealing layerthickness (estimated costs). The technical feasibility regarding required barrier and sealingproperties has been proven in the funded project “Thin Films”.

Figure 16: Assumed cost savings of about 45% for replacing a triple laminate for peanutsnack packs by a biaxially oriented 7-layer EVOH-containing one-step BOPPbarrier film (estimated costs).

Besides the narrow application range of the funded network project there are much moreapplications for the discussed barrier/ high barrier films like lidding films in MAP-, CAP- andvacuum packages as well as inner bag applications with barrier protection, in the last casepartly substituting paper laminates. A typical film laminate for lidding film applications asshown in figure 17 consists of a 45 µm blown film sealing layer with barrier function and abiaxial oriented film for mechanical strength, e.g. a 20 µm BOPP film14).

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Figure 17: Typical film laminate for barrier lidding film applications.

For inner bag applications also PVDC coated systems are in use as shown in figure 19 onthe left side.

Regarding barrier and mechanical properties these laminated barrier film structures could bereplaced with a 50 µm simultaneous biaxial oriented, frontal printed one-step film, themechanical properties in machine and therefore processing direction of the latter due to thelinearmotor stretching process even being higher, compensating for a loss in bendingstiffness. Figures 18 and 19 propose cost savings of up to 40 % resp. 32%.

Figure 18: Proposed cost reduction range by one-step 7-layer barrier films for lidding filmapplications.

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Figure 19: Proposed cost reduction range by one-step 7-layer barrier films for inner bag filmapplications.

Next steps for us should be sample production for converting and packaging trials as well asdiscussions with film converters and end users. Additionally we will try to further optimize theproperties of our films, e.g. sealing properties, hot-tack, shrink, UV protection of transparentfilm systems, mechanical properties and test alternative raw materials e. g. UV-Stabilizer,humidity absorbers or O2-scavengers.We think that in the future it could be possible in selected cases to substitute PVDV coatedfilms, shrink barrier films or certain film laminates with our barrier films, additionally we hope,at least in some cases, also to penetrate into the domain of metalized films!

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Literature:

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[9] H.-C. Langowski: Barrier mechanisms of vacuum coated polymers: Where are thelimits for downgauging of substrates and for barrier properties of coated films?Technical Conference at the International Converting Exhibition ICE 2005, November22nd, 2005, Munich.

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[13] M. Wolf, J. Breil, R. Lund – Neuartige Materialkombinationen in Barrierefolien durchCoextrusion und simultane biaxiale Orientierung, VDI-Fachtagung Extrusion 2007,13. & 14.Juni 2007, Neu-Ulm, Germany.

[14] M. Wolf, J. Breil, R. Lund – Neuartige Barrierefolien durch Coextrusion und biaxialeOrientierung, Innoform Coaching - PE und PP-Folien für flexible Verpackungen27.+28. November 2007, Germany.