Background

All drivers will have had to wipe clean fogged windscreens, where a liquid film has condensed onto the windscreen, at one time or another. But can this problem be avoided at the source?
What causes Fogging?

The problem has been perceived as being due to chemical contamination from either the raw materials in use in the car interior or the external environment, i.e. petrol and diesel fumes being blown through air vents.

In a recent study, three fogged windscreens were subjected to detailed chemical analysis. On all three windscreens organic amines were found and for the most heavily fogged screen DABCO triethylene diamine was detected along with two other amines. DABCO is used in the production of polyurethane.
The Test

Raw material manufacturers have produced ‘low fogging’ variants of the principal ingredients in flexible PVC, namely PVC resins, plasticisers and thermal stabilisers. The fogging performance of these raw materials, and the leathercloth produced using them, is assessed in laboratory tests which are designed to simulate fogging. However, the situation is complicated because there are many variants of the test. Although a standard exists, it has been shown that, due to incompatibilities of surface tension, glass may not be an appropriate material with which to study the fogging of plasticisers. Additionally, looking at the saturated vapour pressures of plasticisers, it can be seen that they have very low volatility and so are unlikely to be the source of the problem.

It was clear, therefore, that real progress in the identification of species present on a fogged windscreen could only be made by detailed analysis of fogged windscreens. Table 1 summarises the findings of this study.

Table 1. Chemicals found on fogged windscreens.

WS


Degree of fogging


Sampling method


Chemicals identified


Analysis technique

A


Moderate


Dry swab


Organic amine


Laser induced mass analysis

B


Moderate


Glass scraping


Amine


Fourier transform infra red spectrophotometry

C


Heavy


Solvent swab


Triethylene diamine


Gas chromatography mass spectrometry










Tetramethyl N,N,N’,N’










N,N,N’,N’-1,6-










Hexane diamine










N-2-(dimethylamino)-










Ethyl –2propenamide

WS = Windscreen sample
Sampling

The windscreens were removed from cars for analysis. They came from various manufacturers and showed a variety of fogging levels. The windscreens were analysed in this work using different sampling techniques:

· taking dry swab samples and analysing them

· analysing glass scrapings which had been removed from the fogged surface using a scalpel blade

· solvent swabbing using analar dichloromethane to remove the fogging layer from the surface.
Findings

Each windscreen was analysed using the techniques shown in the table and they all had an organic amine present. For the third screen in which the quantity of fogging was greatest, it was possible to elucidate precisely the chemical structure of three contaminants because of the relatively high levels of contaminant on the screen. The mass spectrum corresponds to that of DABCO, a foaming catalyst commonly used in the polyurethane industry and a hexanediamine-based compound. This can also be used in polyurethane processing.

Tertiary amines are the catalysts most widely used in the production of polyurethane foams. Some waterblown flexible polyurethane foams are often catalysed with a synergistic mixture of one or more tertiary amines and an organo-tin catalyst. Triethylenediamine, in particular, finds use in most common types of polyurethane.
Conclusions

The presence of amines in three separate windscreen samples suggests that there is a link between these substances and the fogging problem. The failure to identify PVC plasticiser or any other material of PVC origin, coupled with results from models showing that phthalate emission in cars is highly unlikely clearly indicates that the causes of fogging are not linked to PVC. The presence of amines, in particular triethylene diamine, which are used it polyurethanes, suggests that this provides the link between a raw material and the phenomenon of fogging.