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Heritage Science

, 7:49 | Cite as

Analysing for 4,4′-diaminodiphenylmethane in heritage collections containing solid and medium density flexible linear polyester polyurethanes using liquid chromatography/mass spectrometry

  • Robert Tillotson
  • Timothy E. CrumplinEmail author
  • Graham J. Jones
  • George MarshallEmail author
  • Joe DawsonEmail author
Open Access
Communication
  • 63 Downloads

Abstract

Polyurethane (PUR) shoe soles from collections held by the Alfred Gillett Trust, stored for up to 50 years, were analysed for the presence of 4,4′-diaminodiphenylmethane (4,4′-MDA)—a substance of very high concern in Europe and classified as a carcinogen in USA. A review of the literature revealed no papers on long term room-temperature hydrolysis of urethane or urea linkages leading to the spontaneous formation of 4,4′-MDA in polyester or polyether polyurethanes made from 4,4′-diisocyanatodiphenylmethane (MDI). However, evidence emerged of its potential formation and a possible handling hazard was consequently identified in the heritage collection. By chemical analysis, shortcomings in the current literature could be addressed. Calibration of liquid chromatography–mass spectrometry equipment showed analysis of 4,4′-MDA was possible down to 1 ppm with an error of 2.5 ppm. No 4,4′-MDA was found in the PUR analyte solutions at a concentration > 1 ppm. Under these experimental parameters the samples were shown to comply with the industrially accepted CertiPUR 2017 standard for commercial slab-stock foams. Furthermore, no 4,4′-MDA was found in solution after an ‘accelerated anaerobic hydrolysis test’ on the sole materials. This test was designed and developed to assess the likelihood of future formation of the aromatic amine via a hydrolysis-only mechanism/s. Finally, 4,4′-MDA itself was heated in air at 70 °C under ‘humid’ conditions to examine its stability. In this experiment the 4,4′-MDA altered in appearance and was reduced to ca 30% of its original weight. Subject to more work, it is conceivable 4,4′-MDA could be formed by hydrolysis, but degraded over time, and not detected in these tests.

Keywords

4,4′-Diaminodiphenylmethane MDI Polyester Polyurethane Hydrolysis Liquid chromatography/mass spectrometry CertiPUR 2017 

Abbreviations

AGT

Alfred Gillett Trust

EIC

extracted ion chromatogram

ES

ester

ET

ether

GC

gas chromatography

HSE

Health and Safety Executive

LCMS

Liquid chromatography–mass spectrometry

MDI

4,4′-diisocyanatodiphenylmethane

ppm

parts per million

PUR

polyurethane

TDA

2,4-diaminotoluene

TDI

2,4-toluenediisocyanate

UoN

University of Nottingham

SVHC

substance of very high concern

4,4′-MDA, MDA

4,4′-diaminodiphenylmethane, 4,4′-methylenedianiline

The evidence for potential production of 4,4′-diaminodiphenylmethane

Degradation of PUR artefacts is emerging as a salient theme for collections professionals in heritage institutions [1]. Problems with degraded PUR soled shoes have been reported [2] and commercially available PUR shoe soling from diverse and largely unknown manufacturers and PUR material suppliers was chosen by the Alfred Gillett Trust (AGT) as a model substance for this investigation. Historically, PUR soles have been manufactured using either polyester (ES) or polyether (ET) polyols with “Pure MDI” [3, 4]. Spontaneous hydrolysis of PUR(ES) by atmospheric moisture and/or microorganisms has been reviewed [5]. Ether groups in PUR(ET) are shown to be stable to hydrolysis [6]. Literature on hydrolysis of PUR(ES) concentrates on degradation of the base polyol components and loss of physical performance. Little information exists on the potential production of 4,4′-MDA by scission of the urethane and urea linkages by natural hydrolysis. A priori, the influence of pigment and dye colourants might affect the stability of the urethane or urea linkages.

In the USA 4,4′-MDA appears in the NIOSH Occupational Cancer List [7] (synonyms: 4,4′-methylenedianiline, MDA). It is a substance of very high concern (SVHC) in Europe [9]. In the UK, the HSE [8, 15] imposes limits on employee exposure to 4,4′-MDA in the spraying of rigid PUR foams, because it is a known cause of contact dermatitis [10, 11]. 4,4′-MDA is a product in the recovery processes from scrap PUR [12]. Stability to hydrolysis of the urethane link in polyether polyurethanes at elevated temperature has been reported [13]. Production of small amounts of 4,4′-MDA by the hydrolysis of PUR made from polycarbonate-based polyols was studied for in vivo applications [16]. For furniture applications using MDI in the production of slab stock foam, a voluntary manufacturers specification of ≤ 5 ppm for 4,4′-MDA within the foam structure is given in CertiPUR 2017 [17] and the analytical method published therein was adapted to use samples of 0.5 to 5 g.

Pellizzi et al. [14] reported the formation of TDA (2,4-diaminotoluene) in naturally degraded PUR objects made with TDI formulations, and 4,4′-MDA formation could be expected in chemically analogous MDI formulations.

LCMS analysis

A commercial sample of 4,4′-MDA (Sigma Aldrich, > 97%, GC) was obtained to construct a calibration curve in the range 0–100 ppm using LCMS methods. A 4,4′-MDA solution in 1% aqueous acetic acid was used in this process, made by serial dilution of a 1000 ppm stock solution using volumetric methods. The extracted ion chromatogram (EIC) obtained by mass spectrometry was then used to construct new calibration graphs in the range 0 ppm to 10 ppm. The limit of detection of this method was determined to be < 1 ppm. To account for variances in the LCMS system over time, this calibration process was repeated for each of 4 batches of PUR samples supplied over several months (Fig. 1).
Fig. 1

Four independent LCMS calibrations of commercial 4,4′-MDA at 0–10 ppm

Thirty PUR soles were selected from a range of manufacturers spanning 1970 to 1999. The analysis samples were sourced from the interior of the sole portion to avoid contamination by surface impurities which could distort the analysis results. The analysis liquors were prepared by mixing each cut sample independently with aqueous 1% acetic acid at a 1:5 weight/weight ratio in an ultrasonic bath for 2 h. To eliminate possible error at this very low-level detection (< 1 ppm), after each sample LCMS run a supplementary LCMS quality control run was performed. The original shoe sample liquor was ‘spiked’ with an additional 5 ppm of 4,4′-MDA by adding a 10 ppm 4,4′-MDA in 1% acetic acid solution in 50/50 ratio to the original shoe sample liquor. The 4,4′-MDA level in the sample can be determined by subtracting 5 ppm from the measured LCMS value in the spiked sample. This spiking methodology allows the performance of the analytical method to be validated by ensuring that the amount of 4,4′-MDA added is equal to that analytically observed. Importantly, correlation between the un-spiked and ‘corrected value’ for the spiked samples validates our data (Table 1).
Table 1

Normal and spiked LCMS results obtained from naturally hydrolysed polyurethane shoe soles stored for up to 50 years

Sample number

Approx. date of manufacture

AGT Sample identification code

PUR sample construction

PUR sample colour

Normal LCMS solution results

Spiked LCMS solution results

Chromatogram peak area

ppm in PUR sample

Chromatogram peak area

ppm in PUR sample

1

1970

M19+SD118

Single density

Black

0

0

256,000

4.8

2

1974

1234/2926

Single density

Dark brown

0

0

217,000

3.9

3

1972

M19+SD100

Single density

Tan

0

0

200,000

3.6

4

1970

M19+SD122

Single density.

Black

0

0

162,000

2.9

5

1971

1234/1918A

Single density

Black

0

0

201,000

3.7

6

1970

1974/770

Single density

Black

0

0

206,000

3.8

7

1977

1234/3115

Single density solid elastomer

Tan-orange

0

0

211,000

3.9

8

1970

M19+SD123

Single density

Black

0

0

203,000

3.8

9

1975

1234/2934

Single density

Beige-mid-brown

0

0

169,000

3.1

10

1975

1234/2933

Single density

Mid-brown

0

0

183,000

3.4

11

1974

1234/2889

Single density

Black

0

0

194,000

3.5

12

1973

1234/2864

Single density

Tan-orange

0

0

181,000

3.3

13

1975

W19/SD865

Single density

Beige-mid-brown

0

0

197,000

3.6

14

1973

1234/2865B

Single density

Mid-brown

0

0

190,000

3.5

15

1981

M19/SD339

Dual density Outsole

Tan- orange

0

0

105,000

2.6

16

1981

M19/SD339

Dual density. Mid-sole

Black

0

0

119,000

3.1

17

1989

C19+SD614

Dual density. Midsole

Off-white

10,200

0.3

98,700

2.5

18

1985

C19+SD458LA

Single density

Light grey

0

0

119,000

3.0

19

1997

Sundapple

Single density

White

0

0

102,000

2.6

20

1996

0161

Dual density. Outsole

Black

11,800

0.3

189,000

4.5

21

1996

0161

Dual density. Midsole

Grey

0

0

138,000

3.5

22

1982

W19+SD132

Single density

Tan

0

0

135,000

3.4

23

1980–1999

SHO/5/23/3

Single density

Cream

0

0.00

778,000

2.7

24

1980–1999

SHO/5/8/1

Single density

Cream

0

0.00

776,000

2.7

25

1980–1999

CR/361/17

Single density

Brown

24,700

0.02

342,000

1.2

26

1980–1999

SHO/5/37/1

Single density

Cream

27,800

0.02

419,000

1.4

27

1980–1999

SHO/5/5/2

Single density

Cream

0

0.00

768,000

2.7

28

1980–1999

SHO/5/155

Single density

Cream-brown

23,100

0.02

333,000

1.1

29

1982

W19/SD1322

Single density. High blow (hydrolysed 4 weeks)

Tan

0

0.00

149,000

3.0

30

1982

W19/SD1322

Single density. High blow (hydrolysed 8 weeks)

Tan

0

0.00

757,000

2.6

All samples were coloured by pigments or dyestuffs and showed appreciable physical degradation. Results for accelerated hydrolysis tests are shown in Sample numbers 29 and 30

The results of the LCMS analysis of the shoe liquors of naturally hydrolysed and spiked samples are shown in Table 1. No. 4,4′ MDA was detected above 0.3 ppm (within an error of ca. 2.5 ppm) under these experimental conditions.

Accelerated hydrolysis tests

It is conceivable 4,4′-MDA would be produced by spontaneous hydrolysis in future years [13]. To intentionally accelerate hydrolysis of the urethane and urea links in a PUR foam, ca 0.4 g to 0.5 g of two highly blown PUR samples were suspended over deionised water in sealed pressure tubes and held at 70 °C for 4 weeks and 8 weeks respectively. The highly blown PUR formulations contain larger proportions of MDI and could be more likely to produce 4,4′-MDA. The residues were analysed by LCMS in a similar way to that described in the LCMS analysis section above (Table 1, Sample numbers 29 and 30).

Accelerated atmospheric ageing effects on commercial grade 4,4′-MDA chemical (97% purity)

A vial containing 22.1 mg of commercial 4,4′-MDA was suspended in a flask over a deionised water reservoir. The flask was open to the atmosphere and held at 70°C for 4 weeks with a condenser attached to prevent loss of water. After analysis of the resultant sample by LCMS, only ca. 30% of the original 4,4′-MDA remained in the “aged” acetic acid solution compared to a recently prepared 5 ppm solution (Table 2). The aged 4,4′-MDA left a yellow insoluble deposit indicating decomposition.
Table 2

Normal and spiked LCMS results obtained from accelerated ageing of commercial grade 4,4′-MDA

 

Normal LCMS solution results

Spiked LCMS solution results

Chromato-gram peak area

ppm in PUR sample

Chromato-gram peak area

ppm in PUR sample

Aged 4,4′-MDA

469,000

1.6

1,370,000

4.9

Conclusions

In naturally hydrolysed and accelerated hydrolysed PUR(ES) no 4,4′-MDA was detected in the resultant liquor under these experimental conditions, even in physically degraded sole samples. The maximum 4,4′-MDA concentration recorded was 0.3 ppm, indicating the level of 4,4′-MDA in PUR(ES) is ≤ 5 ppm under these experimental conditions and below the CertiPUR 2017 [16] limit for PUR foams. The colour of the PUR had no observable influence on hydrolysis under these conditions. The method used was developed in the absence of a procedure to analyse heritage objects containing polyurethane for the existence of 4,4′-MDA, and confirmed that heritage collection items in this instance are safe to handle for the foreseeable future. In moist aerobic conditions, 4,4′-MDA degrades at 70 °C through an as-yet undetermined mechanism. Conceivably 4,4′-MDA might be formed by hydrolysis of the urethane or urea linkages degrading over time but not able to be detected in this work, using these experimental conditions. Further investigation is required for confirmation, or otherwise, to monitor long-term levels of 4,4-MDA produced by polyurethane held in heritage collections.

Notes

Acknowledgements

The Alfred Gillett Trust initiated this research.

Authors’ contributions

RT drafted the manuscript and designed the testing programme. TEC provided samples from the heritage collection at AGT. GM and JD developed the LCMS analytical technique, oversaw sample preparation, analysis work and interpreted the results. GJJ undertook the LCMS analytical work. All authors read and approved the final manuscript.

Funding

Not applicable.

Competing interests

This research has been carried out without any commercial or financial relationships that might knowingly involve any competing interests.

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Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors and Affiliations

  1. 1.Alfred Gillett TrustStreetUK
  2. 2.The Business Partnership Unit, School of ChemistryUniversity of NottinghamNottinghamUK

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