The relationship between the

EU test for isocyanates and the BgVV test for aromatic amines

 

A discussion document

 

Laurence Castle

Ministry of Agriculture Fisheries and Food

CSL Food Science Laboratory, Norwich NR4 7UQ (UK)

March 1998

 

0.         Background

There are possibilities for producers to test laminates for compliance with the EU restriction on isocyanates, using a simplified test based on the BgVV colour test for aromatic amines.  This would be quicker and cheaper than the full CEN method of analysis which is designed for enforcement but is long, complicated and involves specialised equipment.  This document gives the background to this proposal and indicates an intercomparison test that needs to be done to check if this approach will work in practice.

 

 

1.         Scope

Plastics laminates bonded with isocyanate adhesives fall within the scope of Commission Directive 90/128/EEC [1] which in Article 1 paragraph 2 states that ‘This directive shall apply to plastics materials and articles and parts thereof ’….. composed of two or more layers of materials, each consisting exclusively of plastics, which are bound together by means of adhesives or by any other means’.

Consequence.  EU directive takes priority over rules of individual member states.  Member states shall introduce extra restrictions only if there is a demonstrated risk to public health.

 

 

2.         Mode of control

Isocyanates are reactive monomers that decompose in foodstuffs or in aqueous food simulants.  There are 12 isocyanates and diisocyanates in the EU positive list of 90/128/EEC [1].  There is no migration limit on isocyanates or their decomposition products.  Rather, they are controlled indirectly by a group limit on the content (composition) of the plastic material or article.  The plastic should contain no more than 1 mg isocyanate (as NCO) per kg of plastic.  The restriction is expressed in the form QM(T)=1 mg/kg.  [QM = quantity in the material.  1 mg/kg = 1 part per million, 1 ppm].

Consequence.  The maximum permitted level of isocyanate depends on the thickness of the laminate when expressed on an area basis.

For a 25 g/m2 laminate then 0.025 mg/m2 free NCO is the limit

(1 m2 = 0.025 kg; 0.025 kg may contain up to (0.025/1) x 1mg = 0.025mg/m2)

In the same way;

25g/m2 laminate =         0.025 mg free NCO m2           (= 0.25 mg/dm2)

50g/m2 laminate =         0.050 mg free NCO m2           (= 0.50 mg/dm2)

75g/m2 laminate =         0.075 mg free NCO m2           (= 0.75 mg/dm2)

100g/m2 laminate =        0.100 mg free NCO m2           (= 1.00 mg/dm2)

 

 

3.         Chemical relationship between NCO and aromatic amine

3.1        Link between NCO and isocyanate

The NCO unit is only a small part of the total isocyanate molecule.  Therefore, a conversion factor is needed to calculate the amount of isocyanate that contains 1 mg of the NCO unit.  For example, there is 1 mg NCO unit contained within 2.1 mg of TDI (Table 1).  Therefore, the EU limit of 1 mg/kg NCO allows up to 2.1 mg/kg residual TDI to be present in a laminate.  The group limit of 1 mg/kg QM(T) then applies to the arithmetic sum of all isocyanates, expressed as NCO in this way.

 

3.2        Link between NCO and aromatic amine

The BgVV test [3] measures aromatic amines in a colour test.  The test result is expressed in aniline hydrochloride equivalents (AHCL).  Isocyanate NCO groups hydrolyse in water to amine groups but only aromatic isocyanates can form aromatic amines.  Aliphatic isocyanates cannot.  Each mg of aromatic NCO forms 3.1 mg of aniline hydrochloride equivalents.  Thus there is a 1:3 chemical relationship.

 

 

4.         Relationship between the EU test and the BgVV test

The EU test [2,4] measures the content of the plastic.  It measures all isocyanates, both aliphatic and aromatic because this is the law [1].

The BgVV test [3] measures the concentration of aromatic amines in solution.  These can be amines in the plastic that migrate to the test simulant or isocyanates in the plastic that migrate and then were hydrolysed.

A laminate can contain residual isocyanates but if they don’t migrate then the BgVV test will not detect them.

A laminate can contain aliphatic isocyanates but even if these migrate the BgVV test will not detect them.

The BgVV test will detect aromatic amines residues that migrate from the laminate but the EU test does not apply to these because they were not isocyanate residues.

 

It is for these reasons that the EU test for NCO is long, complicated and expensive [2,4].  For enforcement purposes where the test laboratory does not know which isocyanate(s) were used, the test must measure all 12 isocyanates and diisocyanates that are permitted by 90/128/EEC.  It must measure them individually because they have different conversion factors to NCO (Table 1).

 

 

5.         Practical possibilities

For manufactures there are other possibilities however.  If they know they do not use aliphatic amines then they can demonstrate compliance by performing the BgVV colour test for aromatic amines because there is the 1:3 relationship between NCO and AHCL (see 3.2).  There is still a problem because the EU test is a test for composition whereas the BgVV test measures only substances that have migrated.  So there are 2 possibilities.

 

5.1        Apply the EU extraction procedure

Hydrolyse the extract.  Use the BgVV colour test.  Convert the result to NCO (multiply by 3.1) and decide if pass/fail.

Advantages.  The EU extraction method followed by hydrolysis, is used in place of the BgVV migration test.  Avoids the complicated and expensive derivatisation and HPLC procedure.

Problem.  The effectiveness of the hydrolysis needs to be checked.  The test measures both free isocyanates and amines so it overestimates the NCO content (this is not a problem if the laminate passes).

 

5.2        Apply the BgVV migration  and colour test as normal

Convert the result to NCO (multiply by 3.1) and decide if pass/fail

Advantages.  Can continue to do the BgVV test that is done already for many years.

Problem.  Need to prove that migration from the laminate is 100%.  If not, then some residues of isocyanates will remain in the plastic and will not be measured.

 

 

6.         Proposal

From the above it is clear that there are possibilities for manufacturer to tests laminates for compliance with the EU restriction on isocyanates, using a simplified test based on the BgVV colour test.  It is proposed to compare 5 laminates using the EU NCO test and the BgVV colour test to check the points indicated at 5.1 and 5.2.

 

If these tests are satisfactory, then compliance with the NCO limit could be checked using the test for aromatic amines (AA).  For the standard BgVV pouch test of laminates where the ratio of 2 ml of test liquid in contact with 8 cm2 surface is used (100 ml for each 4 dm2) then the following relationship would apply to the extract.

 

 

Laminate wgt.è

 

 

g/m2

NCO limit   è

in film

 

mg/m2

Aromatic amine è

limit in film

 

mg/m2

[mg/dm2]

Aromatic amine limit in extract

 

mg/100 ml

 

 

 

 

25

0.025

0.078

0.195

 

 

[0.78]

 

 

 

 

 

50

0.050

0.155

0.388

 

 

[1.55]

 

 

 

 

 

75

0.075

0.233

0.583

 

 

[2.33]

 

 

 

 

 

 

 

 

 

100

0.100

0.310

0.775

 

 

[3.10]

 

 

 

 

 

 

 

7.         References

[1]        Commission Directive 90/128/EEC of 23 February 1990 relating to plastics materials and articles intended to come into contact with foodstuffs [1]Official Journal of the European Communities  L349, 26-47].

[2]        Liquid chromatographic determination of residual isocyanate monomers in plastics intended for food contact use.  A. P. Damant, S. M. Jickells and L. Castle.  J. AOAC Int., 1995, 78, 711-19.

[3]        Recommendation XXVIII of the BGA “Crosslinked polyurethanes as adhesive layers for packaging materials for foodstuffs”  June 1, 1981, as amended.

[4]        European Prestandard ENV XXXX-8.  July 1996. Materials and articles in contact with foodstuffs.  Plastic substances subject to limitation.  Part 8.  Isocyanates in plastics.

 

 

Table 1            Quantity of individual isocyanates equivalent to 1 mg NCO unit (remember, EU limit is 1 mg/kg as NCO)

 

 

Substance

 

      Abbreviation

  g   needed to give 1 mg

  of the NCO unit

       2,6-toluene diisocyanate

2,6-TDI

2.07

       2,4-toluene diisocyanate

2,4-TDI

2.07

       2,4-toluene diisocyanate dimer

DIMER

2.07

       hexamethylene diisocyanate

HDI

2.00

       cyclohexyl isocyanate

CHI

2.98

       diphenylmethane-4,4’-diisocyanate

4,4’-MDI

2.98

       diphenylmethane-2,4’-diisocyanate

2,4’-MDI

2.98

       diphenylether-4,4’-diisocyanate

DPDI

3.12

       1,5-naphthalene diisocyanate

1,5-NI

2.50

       phenyl isocyanate

PI

2.83

       octadecyl isocyanate

ODI

7.04

       3,3’-dimethyl-4,4’-diisocyanatobiphenyl

DIBP

3.14

       Isopherone diisocyanate(b)

IPDI

2.64

 

(b)  not included in Directive 90/128/EEC but known to be used.

 

 

 

 

 

 

[Back]