Fourier Transform Infrared Spectroscopy - A Tool for Understanding the Deterioration of Plastics

Topics Covered

Background

Early Plastics

Fourier Transform Infrared Spectroscopy

FTIR as a Conservation Tool

Cellulose Nitrate

Cellulose Acetate

Steering Wheels

Plastic Packaging Materials

Summary

Background

If your old car steering wheel is oozing fluid or furring up and your prized collection of dolls is discolouring, then be warned. You are probably the victim of plastic degradation. Such strange changes - along with many others - are caused by chemical reactions of plastics and their additives with impurities or external substance. These problems were little expected when the materials were first made more than a century ago, and the widespread growth of plastic degradation has left many mystified.

Early Plastics

The earliest known plastics are over 130 years old, and until recently museums and private collectors could do little more than watch as some exhibits made of them cracked, flaked or even turned to liquid. However, such rampant breakdown of these materials has inspired researchers to seek out the roots of the problems in the hope of stopping the rot as well as identifying and protecting other items potentially at risk.

Fourier Transform Infrared Spectroscopy

One of the most potent analytical techniques being employed to search for chemical clues is Fourier transform infrared (MR) spectroscopy, which allows researchers to measure accurately the proportions of chemical groups in the affected plastics and so track the degradation. Warwick-based company Nicolet Instruments was among the pioneers of FTIR spectroscopy in the early 1970’s and its technology is now used by both industry and museum conservators in the battle against plastic breakdown.

FTIR as a Conservation Tool

One of the key conservation experts making use of Nicolet’s MR technology is Dr Anita Quye, an analytical research chemist with the National Museums of Scotland (NMS). The NMS first noticed a degradation problem with plastics in its collections in the early 1990’s when problems were also beginning to emerge at other museums around the world. Quye collaborated with Professors Littlejohn and Pethrick at the University of Strathclyde and together they have attracted funding from the NMS, the Scottish Conservation Bureau of Historic Scotland and the Engineering and Physical Sciences Research Council for two postgraduate projects. Initial work concentrated mainly on the degradation of cellulose nitrate, which was originally developed to simulate expensive ivory and tortoiseshell. Degradation is characterised by cracks and crazing on the surface of objects made out of the plastic. Quye and her students use a Nicolet 510 MR spectrometer and a NIC-Plan microscope. ‘Although you can get more than one absorbing species in the IR region, FTIR gives you a good indication of bond changes in the degrading polymer,’ she says.

Cellulose Nitrate

In the case of cellulose nitrate, their investigations have revealed that degradation increases in the presence of the sulphates left behind as a result of the old manufacturing process. In-house doping experiments have shown that 5mg of sulphate per gram of plastic will cause degradation. The plan now is to investigate the conditions which trigger degradation and devise a simple test to show which items are at risk.

Cellulose Acetate

The MR technology is also being used in the second three year project to study degradation of cellulose acetate, the plastic originally used in dolls. By artificially ageing new cellulose acetate and comparing it to samples, the researchers hope to build up quantitative data on how and why the plastic’s spectrum changes over time. At the moment tests involve artificial ageing of samples at 35, 50 and 70°C, each at relative humidities of 12, 55 and 70%. The project has another 18 months to run, but its results are already being shared with worldwide groups concerned with plastics degradation, including Quye’s own Historical Plastics Research Scientists’ Group.

Steering Wheels

Problems are not confined to objects made from very old plastics such as cellulose nitrate and cellulose acetate - more modern objects are affected too, and again, MR can help identify the source of the problem. One case involved the steering wheel from a late 1950s Morris Minor. ‘It was dripping an orangey fluid which turned out to be plasticiser from the cellulose di-acetate which the wheel was made of,’ says Quye. A member of the Riley Club has also contacted her recently after noticing similar problems with their steering wheels.

A subtly different challenge came in the form of the steering wheel off a 1920’s tractor. ‘At a distance it looked like fur, but was really a thick, white, crystalline-like coating,’ says Quye. FTIR spectroscopy showed that the crystals were in fact triphenyl phosphate.’ This information merely enhanced the mystery, though. ‘Tri-phenyl phosphate was used as a fire retardant in rubbers, but it wasn’t introduced until after the 1920’s,’ says Quye. ‘Either this is not the original wheel, or it has been sprayed with some sort of flame retardant. The prospect of materials being affected by later chemical treatments is another kettle of fish altogether.’

Plastic Packaging Materials

The plasticisers used in some plastic packaging materials are another recent source of concern, with paper documents and photographic negatives all at risk from the damage they can cause. While inert materials such as polythene and polyester are not a risk, archivers and conservators often have little or no idea in which plastics their materials have been stored. Quye uses MR spectroscopy to find out and, if appropriate, offer advice on alternative packaging strategies. `The fundamental problem is that we still sometimes have very little idea which polymers are present,’ she says. ‘People often say ‘plastic is plastic’, but some weep, some crack and others give off something nasty.’

Summary

Modern plastics manufacturers take a great deal of care to prevent their products meeting the same fate as those made of older materials, but Quye still sees major challenges ahead. ‘There are hundreds of new plastics on the market,’ she says, ‘and although I have only come across one or two problems so far there are almost certainly going to be more in the future.’

 

Source: Materials World, Vol. 6 no. 5, pp. 274-75 May 1998

 

For more information on this source please visit The Institute of Materials, Minerals and Mining

 

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