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Ready for the Future

PTS

Testing & Developmentof Migration Barriers

Source: G.F Smith

I hies_f ihm i i

TECHNOLOGY

Papiertechnische Stiftung PTS

Testing and Developmentof Migration BarriersBy Markus Kleebauer, Antje Harling and Max Schneider, Pirnaer Straße 37, 01809 Heidenau

This article was presented at the PTS Seminar"Paper and Board in Food Contact" 2018

The present publication includes at first the basicprocesses for the application of migration barriers andtheir general requirements. Subsequently testing andassessment of migration barriers are introduced. Theinfluence of temperature, contact time and criticalareas as cutting edges, inner flaps and creasing linesare discussed. In the second part, layer and formula-tion concepts for water-based barrier coatings arepresented. Polymer coating agents are listed andeffects by addition of pigments are illustrated. Thepublication is completed by a survey on potential costscenarios and a short summary.

• Suitable for food contact (Recommendation XXXVI, FDA)

• Applicable with conventional coating units

• Barrier effect for vaporised hydrocarbons (mineral oil constituents)

• As few changes as possible in steam permeability

• No disruptive effects on the recyclability of paper and board

• No blocking

• No invisible set-off

• As few changes in the barrier properties as possibleafter creasing and folding operations

• The surface can be printed, glued and coated

• As few changes as possible in adhesive frictionand slip values

Fig. 1: Requirements profi le based on folding boxboard

Developed and published by BASF AG

Gravimetrical determination of the amount of hexane, which evaporates through a (coated)paper sample (diameter 50 mm).

In the case of MOSH/MOAH a correlation with the results of the tenax method is possible.

'A 0.2

95 g/(m2d) z";

51 g/(m2d)

-7- a-1111'rn2r111'

D 0 1.0 2 0lln,oldj

Semple Ia Sarno,.2

Sengs 3—LInear (Samplo 1)—Cheer (Semple 2)—Lin.), (Sample 3)

Fig. 2: Hexane Vapour Transmission Rate HVTR

Barrier functions can be applied to paper andboard by three basic processes: Impregnation, coat-ing and lamination. In the case of coating, it canbe differentiated between the application of moltenpolymers by extrusion and water-based formulations. Inpractice, the applied process is determined by the salesquantity of the functionalized paper. Extrusion coatingand lamination are chosen when rather small quantitiesare needed and are often used by paper converters andupgraders whereas water-based coatings are used by thepaper manufacturer.

Before the development of a paper or board grade withan integrated migration barrier starts, the requirementprofile should be determined in detail. As the case of amigration barrier on the reverse side of a carton boardshows, the requirement profile could be rather complexincluding a lot of single points (Fig. 1).

Normally, a great number of coated samples mustbe tested and assessed during development work. Toreduce the expenditure of time, pre-tests are extremelyuseful. A well-known and often used pre-test for pin-holes in barrier coatings is based on dye solutions. In thecase of migration barriers against mineral oil hydrocar-bons (MOSH, MOAH), development time can be reducedby performing the HVTR-test (Fig. 2). The test methodis based on the gravimetrical analysis of the amount ofhexane evaporating through a paper sample and wasintroduced by BASF AG.

In the last years, extensive work was carried out todevelop methods for the assessment of barrier coat-ings against migrating compounds. Up to now, theTenax-migration method is most common (Fig. 3). Thecompounds migrating through the barrier layer areadsorbed to Tenax, afterwards extracted with an organicsolvent and analyzed usually by chromatography (GC,LC). The use of migration cells made of stainless steelallows a space-saving storage in the oven for e. g. 10days at 40°C. The result is given as migrated substancein mg per dm2 and needs recalculation for comparisonwith a limit in food (mg/kg) with the expected surface to

volume ratio under real food contact conditions.

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TECHNOLOGY

Tenax migration experiments can be carried out withcontaminated paper and board samples respectively. Butalso a donor paper (filter paper) doped with surrogatesubstances can be used in order to get independentfrom appropriate contaminated paper substrates. In themigration cell the doped paper is placed beyond the bar-rier coated paper or board sample (Fig. 4).

Within research work, PTS identified surrogate sub-stances for mineral oil hydrocarbons (MOSH: n-tride-cane, 2,2,4,4,6,8,8-heptamethylnonane, heneicosane;MOAH: 1-phenyldodecane, biphenyl, 2,6-diethylnaph-thalene), plasticizers (diisobutylphthalate, dibutylphtha-late, diethylhexylphthalate) and photo initiators (ben-zophenone). The substances are commercially availableand can be dissolved in n-hexane to give a surrogatesolution (each substance with 1000 pg/l).

Different contact times and temperatures can be usedin contact with Tenax (max. temperature is 175°C). Thevalues for temperature and time should be adequatefor future contact conditions with food. As expected,the adsorbed amount of substances increased withrising contact time, but after very long contact timesthe amounts of substances in the donor paper aremore and more exhausted and a leveling of the valuesoccurs. Increasing temperature by decreasing contacttime allows a ranking in the barrier effect of differentcoatings, but the accelerant effects are not equal in anycase (Fig. 5).

In practice, creasing lines, inner flaps and cutting edgesare critical zones for an additional mass transfer ofmigrating compounds. Up to now, there is no standardtest method to determine the influence of cutting edges,inner flaps, creasing and folding lines. Practical consid-erations resulted again in using the tenax method but in amodified way. The modifications are mainly related to thesamples (Fig. 6). The data gained from the measurementscan be used for the estimation of the mass transfers ofmigrating compounds (e. g. mineral oil hydrocarbons) inpackaging products like folding boxes (Fig. 7).

For the implementation of migration barrier coatings, ahigh number of water-based raw dispersions and aque-ous polymer solution are available. They can be mixedup with further additives to optimize barrier effects andapplication properties. Besides migration barriers, addi-tional barrier functions (against water vapor, oxygen,flavor compounds, fat and oil) can be added. In the caseof food contact papers, compliance with the actual legis-lation has to be ensured. Inorganic pigments can be animportant part of the whole barrier concept (cost opti-mization, precoatings, tortuosity effects). Besides thebarrier properties, also converting and processing prop-erties as well as recyclability are very important factors.

Homogenous films without any defects are necessaryto reduce the amount of migrating compounds sig-

ee,4

10 days / 40° C

Dissolutionof adsorbedcompounds

ILAnalytics

• _ _ _ _ _ _ Tenax ® (Food simulant), 4g/dm2 (DIN 14386)

Board

Stainless steel Tenax ®porous polymeric substanceChemical name: Poly(2,6-Diphenylenoxide)- very strong adsorber- low affinity to water- Simulant for dry food stuffs

Migration cell of stainless steel

•

Fig. 3: Investigation of Migration using Tenax method

independence from paper substrate

Mineral oil HC

0

7 - 20

paper or cardboard samplefor testing

Filter Paper doped withSurrogate Substances

Tenax Powder(2,8 g, equaly distributed)

Phthalates

2.6 - 3,0

7 - 10

Benzophenone

2.9

6.5

Fig. 4: W orking with Surrogate Substances

_100 -cu

,7, • 90> ,w 80ca I25 70 -

" 6 0

• 50T.,•

10 d / 40°C

-se.c 30

ö 20

02 10

O.

5 d/ 60°C

f

TridecaneNonaneBiphenyl

a DENBenzophenoneDiBPDodecylbenzeneDBPEicosane

• DEHP

Substrate: Single sided machine calendered kraft paper (120 g/m2)Coating: each with 15 g/m2 of Vinylacetate-PVOH-Copolymer (Vac-PVOH),Hydroxyethylcellulose (HEC), Polyvinylalcohole, fully saponified (PVOH)

Fig. 5: Effect of temperature

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1.111 TECHNOLOGY

Example: White lined chipboard WLC (GD2), 320 g/m2, mineral oil barrier on thereverse side

Migration from an area of 0.71 drn2 without creasing line / cutting edge:45 pg MOH

Migration from an area of 0.71 dm2 with 3 creasing lines9-9.5 cm length, 1 cm distance between, 28 cm length in total:130 pg MOH

Migration from an area of 0.71 drn, with 4 cutting edgesof 18 cm length in total :132 pg MOH

Fig. 6: Migration testing on creasing lines and cutting edges

63 pg/dm2

6.8 pg/cm

4.3 pg/cm

In the folding b0x(1)

15 drn2 945 pg

116 cm 789 pg

In the food stuff(2)

0.95 mg/kg

0.79 mg/kg

204 cm 877 pg 0.88 mg/kg

2.6 mg/kg

2.5 mg/kg

(1) Dimension 20 cm x 5 cm x 26 cm (A x B x1-1, ECMA A1010)(2) 1 kg Food stuff, e.g. noodles(3) Sum of MOSH und MOAH

Fig. 7: Estimation of the amount of mineral oil hydrocarbons

migrating from a folding box of common size

• For financial reasons, the quantity of barrier coating applied should be minimised if possible

• Defect-free, uniform films with a grammage of 5-15 g/m2 would frequently be adequate toproduce the necessary barrier action.

• The surface to be coated is usually porous, rough and absorbent.

Scenarios illustrating a coating with a barrier effect for mineral oils:

Scenario ABlade coatingSingle layer

20 g/m2

Scenario BBlade coating

Precoat + barrier12 + 8 g/m2

Scenario CCurtain coating

Single layer12 g/m2

Fig. 8: Strategies for applying barrier coatings

nificantly. The surfaces for coating (e.g. reverse side ofcarton boards, testliners) are normally rather rough andhave a high water uptake. To achieve a sufficient barrieragainst mineral oil hydrocarbons, several possibilitiescan be taken into account depending on the coatingaggregates available. The options lead to strongly dif-fering material and investment costs (Fig. 8).

A reduction of the coating weight of an expensive coat-ing material, e. g. polyvinyl alcohol, can be achieved viaa pre-coating. However in this case two coating aggre-gates are necessary. With a pre-coating made up withcheaper raw materials significant cost advantages canbe obtained (Fig. 9).

During research work at PTS the barrier properties ofdifferent coating formulations against the already men-tioned surrogate substances were determined via tenaxmethod at 40°C for 10 days (Fig. 10). Good barrier effectswere achieved with polyvinyl alcohol, ethylen vinyl ace-tate-copolymers and cellulose derivatives (hydroxy ethylcellulose and methyl hydroxyl propyl cellulose) althoughdisadvantages in the practical application properties(especially low solids contents) are suspected.

Inorganic pigments can influence barrier properties indifferent ways (Fig. 11). The so called tortuosity effectleads to an increase in barrier properties at equalcoating thickness when pigments are added. The effectshould be more signif icant when the shape of theapplied pigments is flaky. The theoretical predictionsare based on strongly simplified assumptions so thatan exact agreement in pract ical cases cannot beexpected.

Within research work at PTS, the effect of flaky pigmentsin water-based coating formulations on migration bar-rier properties were studied. Equal amounts of pigmentswere mixed up with a biobased barrier polymer. Theseformulations were used in equal application weightson a paper substrate (Fig. 12; P1, P2: Kaolins; P3, P4:Talcs; P5 to P7: Bentonites; P8: Mite; P9: Mica; P10:PCC). The determination of the migration by the tenaxmethod (60 °C15 days) shows a reduction of mass transferin all cases.

Based on an exclusively selected SEM picture a verifica-tion of the tortuosity effect and its theoretical magni-tude was conducted (Fig. 13). For the preparation of thesample best possible free of artefacts an ion beam cross-section polisher was used. The SEM picture was analyzedby measuring the layer thickness and the length andthicknesses of all visually recognizable particles. Thepractice shows somewhat bigger but in the same orderof magnitude lying layer reduction factors than in SEMpicture analysis. The deviation between theory andpractice can be explained by the rather crude theoreti-cal model. The orientation of the pigments is in practicemainly parallel to the surface, but exceptions are visible

28 bio-fibremAGADNE ipw 7-8/2018 www.ipwonline.de

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SB latex (40 pph)

Precoat 2 Kaolin (100 pph)PVOH (40 pph)

Coating PVOH 5

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Substrate: Single sided machine calendered kraft paper (120 g/m2);

Coatings: each with 15 g/m2

12

12

• Tridecane

• HM-Nonane

• Biphenyl

• 2,6-DEN

13eneophenone

DiBP

.Dodecy lbeneene

a D V

Heneicosane

DEHP

(1) Immobilisation of permeating compounds via adsorption on the surface

(2) Filling a part of the layer volume

(3) Reduction of the layer thickness via tortuosity effect

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• High density- High crosslinking• High cristallinity

d: layer thicknessL171 cr: Path extension

cl-1 d =1+ _ _ _ _ _ L: mean length of pigment particles2 W W: mean thickness of pigment particles

L/W: Aspect ratioVf: Volumen portion of pigment particles

Fig. 9: Examples of the

effects of precoats

Fig. 10: Comparison of

di fferent migration

barrier coatings

Fig. 11: Effects of

pigments

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TECHNOLOGY

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3 50 —

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3 TridecaneNonaneBiphenyl

a DEN

BenzophenoneIDIBP

DodecylbenzeneDBPEicosaneDEHP

Substrate: Single sided machine calendered kraft paper (120 g/m2)Coatings: each with 15 g/m2 ofHydroxyethylcellulose (80 pph) + Pigment (P1-P10, each with 20 pph)

Fig. 12: Comparison of the effects of different flaky pigments

Evaluation of layer thickness,particle length and thickness:

Layer thickness (8 ± 1) prn.

Mean particle length 1,48 pm,Mean particle thickness 0,26 pm.Aspect Ratio 5,7.

Volumene portion of the pigment Vf 0,31.

d-/d = 1,9 (factor of about 2).

Experimental factors of 2-5!

SEM picture of the cross section of a coatedSample at a magnification factor of 5000(80% B10 / 20 % P2 (Kaolin), 15 g/m, on craft paper)

Fig. 13: Theory and reality of the tortuosity effect

in the SEM picture. Additionally, an adsorption effectmay also be present.

Based on the layer, reduction factor's potential cost sav-ings were estimated for two different coating systems ofhigh and middle price range (Fig. 14). The used quantityof pigments was fixed to 20 %, an amount secured bythe experimental work. It is evident that with the addi-tion of pigments significant cost savings can be achieved.

As summary for the practical development work the fol-lowing points can be recorded:

Besides an analysis of the target markets, the develop-ment of paper and board grades with a migrationbarrier requires an exact specification of all necessaryproperties at the beginning (definition of require-ment profile).

•›> Based on that, a production concept can be devel-oped, which includes the production processes and allneeded raw materials as fundamental elements.In the case of aqueously applied migration barri-ers, the use of pre-coatings and pigments can be animportant step to an improvement of the barrier func-tions in relation to costs.For the assessment of migration barriers, the tenaxmethod is suitable, whereas the contact time and thecontact temperature need adjusting to the demands(long and short time storage, relative and absolutecomparison).The use of surrogate substances is a feasible tool toevaluate the barrier effect against single compoundsand groups of compounds.With some restrictions, the Tenax method can bemodified in such a manner that the influence of cut-ting edges, flaps, creasing and folding lines can beestimated.

Scenario AWithout Pigment

15 g/m2

Scenario BWith Pigment

15 g/m2

Scenar i o A - w i thout p i gm ent

Scenar i o B - with pigment

Cost reduction in %

15 g/m2 15 g/m2

8.00 4.00

0.12 0.06

8.00 4.00

0.40 0.40

80 / 20 80 / 20

60 50

9.00 7.5

0.06 0.03

50 50

4f3 i3C7Z-Z: ..7.2 3Z- 7

Acknowledgement

Most of the results are taken from theresearch project IGF 18656 BG. Theresearch project IGF 18656 BG of theresearch association PTS was fund-ed wi thin the program of promot-ing "pre-competitive joint research(IGF)" by the German Federal Ministryfor Economic Affairs and Energy fol-lowing a resolution of the GermanBundestag and carried out under theumbrella of the German Federation ofIndustrial Co-operative Research Asso-ciations (AiF). We would like to expressour warm gratitude for this support.We would also like to thank the paperand supply sector companies involvedin the project for supporting the work.

Fig. 14: Cost scenarios

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