Polypropylene: establishment of a rapid extraction test for overall migration limit compliance testing

Polypropylene: Establishment of a RapidExtraction Test for Overall Migration LimitCompliance Testing

By Anthony O'Brien, Allison Leach and Ian CooperPIRA International, Randalls Road, Leatherhead, Surrey KT22 7RU, UK

In order to demonstrate compliance with the overall migration limit given in ECDirective 90/128/EEC, overall migration tests are usually performed on the®nished food packaging with food simulants. As a consequence, the packagingindustry is confronted by a signi®cant compliance testing burden to demonstratethat their products are suitable for food use applications. However, the Directivealso allows compliance with migration limits to be demonstrated by use of a`more severe test'. In this study a `more severe test' has been developed forpolypropylene materials and articles involving extraction with the solvent,diethyl ether. The solvent was removed from the samples, evaporated to drynessand the mass of the non-volatile residue determined. This extracted residuerepresents the overall migration value. This procedure has been optimized so thatit is simple and rapid to perform and, in general, gives slightly higher resultscompared to overall migration tests conducted with fatty and aqueous foodsimulants. Copyright O 2000 John Wiley & Sons, Ltd.

Received December 1999; Accepted 12 January 2000

KEY WORDS: overall migration; polypropylene; food packaging; rapid extraction

INTRODUCTION

EC Directive 90/128/EEC1, and amendmentscovering plastics materials and articles intendedto come into contact with foods, contains anoverall migration (OM) limit of 10 mg/dm2 or60 mg/kg, which has the primary function ofpreventing unacceptable contamination of pack-aged foods by the packaging. In order to demon-strate compliance with this restriction, overallmigration tests are normally performed on the®nished plastics packaging, using the appropriateEC food simulants. For `general purpose' use,covering all classes of foodstuffs, overall migrationtests would be performed using the food simulants

3% (w/v) aqueous acetic acid (simulant B), 10%(v/v) aqueous ethanol (simulant C) and olive oil(simulant D), using test conditions given in the 2ndamendment to EC Directive 82/711/EEC, ECDirective 97/48/EEC.2 However, Annex 1 ofDirective 90/128/EEC also makes the followingprovision Ìt shall in all cases be permissible todemonstrate compliance with migration limits bythe use of a more severe test'. In addition, Directive97/48/EC2 also recognizes àlternative tests' andèxtraction tests' which can be used instead ofmigration tests with fat simulant, provided theyhave been shown to give higher or equivalentresults.

Polypropylene is widely used for manufacture

PACKAGING TECHNOLOGY AND SCIENCEPackag. Technol. Sci. 2000; 13: 13±18

Copyright 2000 John Wiley & Sons, Ltd.

* Correspondence to: A. O'Brien, PIRA International, Randalls Road, Leatherhead, Surrey KT22 7RU, U.K.E-mail: [email protected]

of containers for packaging of foodstuffs byinjection moulding, blow moulding and manufac-ture of packaging ®lms. Overall migration tests,particularly using olive oil,3 require skilled analy-tical work and are costly and time-consuming toperform, with the exposure conditions commonlyspeci®ed as 10 days contact time between thesimulant and the test specimens.The objective of this study was to devise a

simpler, quicker procedure for compliance testingof polypropylene material which would be bettersuited as a quality control test, yet still provideassurance that the overall migration limit was notexceeded.One option considered was to use alternative

tests with volatile media, such as iso-octane.4 Areview of reported uses of volatile alternatives hasbeen published,5 however, only limited data existon the use of alternative simulants and the testwould still not be ideal as a QC test requiring 10days/40°C for 95% ethanol or 2 days/20°C for iso-octane. Another option considered was use of anaggressive extraction solvent to totally remove allpotential migrants,6 but this approach is onlyapplicable to thin ®lms. The approach pursued inthis study was to devise a `more severe test' thanoverall migration tests, as provided for in the

Directive 90/128/EEC, employing a rapid solventextraction. Ideally, one rapid test would:

. Cover migration tests with all food simulants.

. Produce results within one working day.

. Be applicable to all commercial polypropyleneformulations used for food packaging.

. Be reliable and simple, without requiringspecial equipment.

This approach has already been applied to themeasurement of overall migration from rigidPVC.7 This paper describes its application topolypropylene.

EXPERIMENTAL

Samples tested

To ensure that the rapid extraction methoddeveloped in this study had as wide an applicationas possible, a broad range of plastics were selectedto establish and test the procedure. The propertiesof the plastics, where available, are given in Table1.In total, 17 plastics have been studied in this

Table 1. Speci®cation of plastic samples

Sample Melt ¯ow index (g/10 min)Molecular weight

distribution Plastic type Monomers used

1 2 M HPP C3

2 3 M HPP C3

3 7 M HPP C3

4 35 N HPP C3

5 12 M HPP C3

6 35 M/N RCP C3 and C2

7 44 N ICP C3 and C2

8 3 M HPP C3

9 35 M/N HPP C3

10 6 M ICP C3 and C2

11 8 M HPP C3

12 3 M HPP C3

13 12 M/N ICP C3 and C2

14 8 M RCP C3 and C2

15 5 M ICP C3 and C2

16 (Clear ®nished product. No data available)17 (Coloured ®nished product. No data available)

HPP = homopolymer;RCP = random co-polymer;ICP = impact co-polymer; M = medium; N = narrow.

Copyright O 2000 John Wiley & Sons, Ltd. 14 Packag. Technol. Sci. 2000; 13: 13±18

A. O'BRIEN ET AL.

investigation. Of these, 15 were prepared as 2 mmthick plaques from polypropylene materials usedin food contact applications and comprised ninehomopolymers, four impact co-polymers (blockco-polymers) and two random co-polymers. Themelt ¯ow indices for the plastics ranged from 2 to44. The samples contained a number of additives,which included antioxidants, antistatic materials,slip agents, DGMS and aids to polymerization. Thetotal concentration of additives in the plasticstested ranged from approximately 0.2% to 0.6%. Inall cases the density of the plastics has beenreported as being 905 kg/m3.The remaining two samples were ®nished

products that could be found in the market placeand intended for contact with fatty foods. One wasa clear plastic (sample 16) whilst the other (sample17) was coloured with addition of a masterbatch atthe 2% level. No further information on thephysical and chemicals properties was availablefor these two samples.

Solvents evaluated

A number of different solvents were evaluated inthis study and were selected to cover a range of

polarities and chemical types. The solvents usedare listed in Table 2 and included ketones, ethersand alkanes.

Procedure to measure overallmigration

The migration tests were performed in triplicate,by total immersion, using the procedures given inCEN ENV 1186 Prestandard.8 The test conditionsselected to measure overall migration with simu-lants B, C and D were 10 days at 40°C.

Procedure used for rapid extractiontesting

Apparatus.

. Analytical balance capable of weighing to0.1 mg.

. Water bath and steam bath.

. Pyrex tubes 250� 35 mm in diameter withB34/35 ground glass socket.

. Condenser, water-cooled, with B34/35 cone.

. Stainless steel dishes.

. Measuring cylinder, 250 ml.

. Oven adjusted to 105� 2°C.

Procedure. Cut the sample into suitable testspecimens. A total sample area of 1 dm2 is usedin all measurements. For samples with a thicknessless than 0.5 mm, the surface area of only one sideof the test specimen is used in calculating the testresults. Where the sample has a thickness of0.5 mm or greater, the surface areas of both sidesare used (i.e. for a 1 dm2 test specimen the surfaceused to calculate the results is 2 dm2).

Measure 100 ml of the extraction solvent into®ve Pyrex tubes, two of these acting as blanks.Place the test pieces in the tube, ®t the condensersand heat in a water bath set at a suitabletemperature; for example, when using diethylether a water bath temperature of 50°C was foundto be satisfactory. When the solvent is re¯uxing(usually after 1±2 min), start timing the re¯uxperiod.

After re¯uxing for 1 h, remove the condensersand the test pieces from the solvent. Pour thesolvent extract, and the two blanks, into ®ve taredstainless steel dishes. Evaporate the solvent to

Table 2. Rapid extraction for sample 6, solventselection. Extraction with various solventscompared with olive oil overall migration

Extraction (mg/dm2)

Replicates

Solvent 1 2 3 Mean

Acetone 2.6 3.3 3.0 3.0Methanol 1.2 1.8 1.5 1.5Ether 11.3 10.4 9.7 10.5MTBE* 20.4 19.7 18.3 19.5Pentane 11.4 12.3 12.2 12.0

Overall migration (mg/dm2) 10 days/40°C

Replicates

Simulant 1 2 3 Mean

Olive oil 7.6 7.6 7.1 7.4

* MTBE = methyl tertiary butyl ether.


OVERALL MIGRATION TESTS

dryness on a steam bath and transfer the dishes toan oven at 105°C for 15 min. Transfer the dishes toa desiccator and, after allowing to cool for 1 h,weigh the dishes plus residue to 0.1 mg using ananalytical balance. Repeat the drying procedureuntil constant mass is achieved by placing thedishes in the oven for 15 min and allowing them tocool in a desiccator for 1 h. Constant mass isassumed to have been achieved when two con-secutive weightings are within 0.5 mg. The quan-tity of substance extracted from the test specimenis determined by subtracting the mean valuecalculated for the mass of the duplicate solventblank residues from the mass of the dried residuefrom the test specimen. The replicate resultsshould agree to within 1 mg/dm2.All extraction chemicals used in this study were

super-purity solvents with a non-volatile residueof �0.0001%

RESULTS AND DISCUSSION

Selection of solvent

A number of simple volatile solvents were initiallyinvestigated as possible alternatives for foodsimulants. These included alcohols, ketones, ethers

and alkanes. Sample 6 (properties given in Table 1)was selected for this screening process, as itexhibited the highest overall migration value usingolive oil, which is usually quoted as giving thehighest test results of all the simulants.All solvent extraction tests were carried out

under re¯ux, which should make the proceduremore straightforward. Minor changes in solventboiling point owing to atmospheric pressurevariations are not expected to in¯uence theextraction result signi®cantly. The results of thesolvent extraction tests for polypropylene sample 6are compared to olive oil overall migration valuesin Table 2.Diethyl ether and pentane under re¯ux for 1 h

were found to give the most promising extractionvalues. To evaluate these solvents further, fourmore samples of plastic were tested by the rapidextraction procedure, using pentane and diethylether. The results are listed in Table 3.The mass of material extracted by the two

solvents from the plastics was comparable. How-ever, diethyl ether gave values marginally closer to

Table 3. Rapid extraction of polypropylenesamples. Comparison between diethyl ether

and pentane

Extraction (mg/dm2)

Olive oilReplicates overall

migrationSample 1 2 3 Mean (mg/dm2)

1Diethyl ether 1.5 1.8 1.8 1.7 1.1Pentane 1.7 1.4 1.3 1.4



13Diethyl ether 3.3 3.2 2.4 3.0 0Pentane 4.3 4.2 4.0 4.2

Table 4. Rapid extraction of polypropylenesamples. Diethyl ether extraction values

Extraction (mg/dm2)

Replicates

Sample 1 2 3 Mean

1 1.5 1.8 1.8 1.72 1.6 0.8 1.0 1.13 1.3 1.3 1.2 1.34 1.8 1.9 1.8 1.85 1.0 0.8 0.9 0.96 11.3 10.4 9.7 10.57 7.1 7.1 7.0 7.18 0.7 0.5 0.7 0.79 2.0 2.4 1.9 2.3

10 3.6 3.2 3.8 3.511 2.8 2.6 2.7 2.712 1.8 1.7 1.8 1.813 3.3 3.2 2.4 3.014 5.6 5.6 5.4 5.515 1.7 1.7 1.7 1.716 9.7 9.5 10 9.717 11.0 11.0 10.5 10.8

Combined relative standard deviation = 7%.


A. O'BRIEN ET AL.

the olive oil overall migration values than pentaneand was used to extract the remaining plasticsamples.

Evaluation of rapid extraction test

The results for rapid extraction for all 17 samplesare given in Table 4. Also given in this table is thecombined relative standard deviation for the rapidextraction test, which was found to be 7% for thesamples evaluated in this study. This precision fora sample with a rapid extraction value of 10 mg/dm2 would equate to an error of �0.7 mg/dm2.In Table 5 the rapid extraction values are

compared to overall migration data with simulantsB, C and D. The requirements in Directive 97/48/EC are that the extraction values are equal to orgreater than those obtained by overall migration.The results show that for the majority of theplastics tested this was the case. There were twoexceptions to this:

1. Sample 9, using olive oil food simulant.2. Sample 17, using 3% acetic acid food simu-

lant.

These two results will be discussed in detail below.

Rapid extraction test compared to olive oiloverall migration for sample 9. The mass ofmaterial extracted from sample 9 using the rapidextraction test was 2.3 mg/dm2. This compares toan olive oil overall migration value of 2.7 mg/dm2.

Clearly, the olive oil overall migration value isthe higher of the two values, but both results areclose to each other and well within the analyticaltolerance of �3 mg/dm2 allowed in Directive 90/128/EEC for the olive oil overall migration test andthe precision for the rapid extraction data set of�0.7 mg/dm2. The values for olive oil overallmigration for this sample ranged from 2.2 mg/dm2 to 3.1 mg/dm2 and covers the rapid extractionvalue. It was concluded that the difference was notsigni®cant.

Both values for this sample are well belowthe overall migration limit of 10 mg/dm2; how-ever, as a safeguard it is suggested that whenthe rapid extraction test gives values that are closeto the overall migration limit, tests using theEC conventional food simulants should be carriedout.

Rapid extraction test compared to 3% aceticacid overall migration for sample 17. The rapidextraction and the 3% acetic acid overall migrationtests for sample 17 gave results of 10.8 and26.2 mg/dm2, respectively. Clearly, these are verydifferent. However, it should be emphasized thatthis product was intended for fatty food contactonly and would not have been subject to aceticacid overall migration testing. This discrepancywas investigated further. Sample 17 was the onlyplastic tested that was coloured and contained 2%of a masterbatch. The residue from the aceticmigration test was observed to be white andbrittle, suggesting the possibility that the residuewas a calcium salt, which was con®rmed byatomic absorption analysis (probably calciumacetate, as the residue was not observed in thesimulant C migration test). This residue couldhave been derived by the reaction of acetic acidwith calcium carbonate in the plastic, which canbe used as an extender in the production ofmasterbatches.

Inorganic salts that may be found in a plasticwill not be extracted by organic solvents such as

Table 5. Comparison of rapid extraction withoverall migration for polypropylene

Rapidextraction

Overall migration (mg/dm2)

Sample (mg/dm2) Olive oil 3% HAC 15% EtOH

1 1.7 1.1 0.5 0.72 1.1 0.9 0.9 0.83 1.3 0.7 0.7 0.84 1.8 1.2 0.6 1.05 0.9 0.3 0.5 0.36 10.5 7.3 0.3 0.27 7.1 2.3 0.3 0.28 0.7 0.2 0.5 0.29 2.3 2.7 0.3 0.1

10 3.5 1.2 0.1 0.111 2.7 1.0 0.1 0.112 1.8 1.1 0 0.113 3.0 0 0.3 0.314 5.5 3.0 0.5 0.415 1.7 0 0.5 0.516 9.7 1.8 1.2 0.817 10.8 3.2 26.2 1.7


OVERALL MIGRATION TESTS

diethyl ether. Therefore, where the plastic samplecontains an acid-soluble calcium compound therapid extraction test cannot be used as a replace-ment for the overall migration test with the aceticacid simulant.

CONCLUSIONS

A rapid extraction test has been developed, usingdiethyl ether under re¯ux, that can be used todemonstrate compliance of polypropylene foodpackaging with the overall migration limit, as areplacement for overall migration tests with EUfood simulants C and D and test conditions of 10days at 40°C. These test conditions cover condi-tions of use up to 40°C for any time of exposure.A wide range of polypropylene materials has

been evaluated in the development of this testmethod, which should be directly applicable to allmaterials with similar properties.The requirements in Directive 97/48/EC are

that the extraction values are equal to or greaterthan those obtained by overall migration. A total of17 different plastics have been examined. The®ndings from this study indicate that for all butone of the test results using simulants C and D, thiswas the case. In the one exception, the solventextraction value was lower than the olive oiloverall migration result. The difference was small,0.4 mg/dm2, and was well within the analyticaltolerance of �3 mg/dm2 allowed by Directive90/128/EEC for the olive oil overall migrationtest. As a result, this difference was not consideredsigni®cant.It is recommended that if the rapid extraction

test described in this paper gives values thatapproach the overall migration limit, e.g. 7 mg/dm2 or greater, the plastic should be evaluatedusing standard overall migration procedures.Care must exercised when using the rapid

extraction test to replace overall migration testingwith 3% acetic acid. This study has demonstratedthat if calcium salts are present within the

polymers these can dissolve in the acetic acidsimulant. These salts will not be soluble in thesolvent diethyl ether, and the rapid extraction testwill give a lower value when compared to theoverall migration measurement and must not beused. If the speci®cation of the plastic shows thatcalcium salts are not present, the rapid extractiontest is an alternative to overall migration withsimulant B. Otherwise a rapid pre-screening testfor the presence of a calcium salt, for example ashort re¯ux of the polymer with 3% acetic acidfollowed by an examination of the residue byinfra-red spectroscopy, will be required beforesolvent extraction procedures can be used.

REFERENCES

1. Corrigendum to Commission Directive 90/128/EECof 23 February 1990 relating to plastics materials andarticles intended to come into contact with foodstuffs.Of®cial J. Eur. Commun. 1990; L 349/26, 13 December.

2. Commission Directive 97/48/EC amending for thesecond time Council Directive 82/711/EEC layingdown the basic rules necessary for testing migrationof constituents of plastic materials and articlesintended to come into contact with foodstuffs. Of®cialJ. Eur. Commun. 1997; L 222/10, 12 August.

3. European Committee for Standardisation (CEN).ENV 1186, Part 8: Test methods for overall migrationinto olive oil by article ®lling. May 1993.

4. De Kruijf N, Rijk MAH. Iso-octane as fatty foodsimulant: possibilities and limitations. Food Addit.Contam. 1988; 5: 467±483.

5. Baner AL, Beiber W, Figge K, Franz R, Piringer O.Alternative fatty food simulants for migration testingof polymeric food contact materials. Food Addit.Contam. 1992; 9: 137±266.

6. Baner AL, Franz R, Piringer O. Alternative methodsfor the determination and evaluation of migrationpotential from polymeric food contact materials.Deutsche Lebensmittel-Rundschau 1994; 90 (6):181±185.

7. Cooper I, Goodson A, Tice PA. PVC thermoformedcontainers for food packaging: establishment of arapid extraction test for overall migration limitcompliance testing. Packag. Technol. Sci. 1997; 10:169±176.

8. European Committee for Standardisation (CEN).ENV 1186, Parts 2 and 3. May 1993.


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Polypropylene: establishment of a rapid extraction test for overall migration limit compliance testing