Rubber smells: Odour profile of natural rubber
Rubber smells
Natural rubber stinks. Once it has been bled from the rubber tree and begins to coagulate, degradation processes kick in to produce a range of malodorous compounds which pollute the air within processing plants and the local neighbourhood, as well as water released by the plant.
The smell is produced whether the latex is allowed to coagulate naturally or is treated to accelerate coagulation. However, the strength of the bad odour depends on the particular drying process used and the quality of the rubber product, those of poor quality producing stronger odours. The guilty compounds are produced by a combination of enzymatic and microbial degradation, resulting from the presence of enzymes and bacteria in the latex, as well as thermal degradation during processing.
In spite of the acknowledged smell, there have not been many published studies on its composition. Some of the guilty compounds are low-molecular-weight fatty acids like acetic, butyric and isovaleric acids, longer-chain acids like stearic acid, and aromatic compounds like p-xylene and phenol. A better understanding of the compounds involved is needed in order to be able to devise appropriate odour treatment systems for rubber processing plants.
To this end, French scientists have undertaken a comprehensive study of the odour emitted from two types of natural rubber. One was prepared by the natural coagulation of latex and the other by controlled coagulation using acetic acid. The latter process was carried out in the presence of hydroxylamine sulphate to react with aldehydes in the latex and act as a radical scavenger.
Sniffing latex emissions
Pascale Chalier from the University of Montpellier, and colleagues, collected the volatile compounds from the rubbers by headspace SPME before characterising them by GC/MS and GC with combined olfactometric and flame ionisation detection. The SPME fibre was coated with layers of divinylbenzene and carboxen, which trapped larger and smaller molecules, respectively, widening the range of compounds that could be analysed.
After sampling the headspace, the SPME fibre was inserted into the injector of the GC/MS system for thermal desorption of the volatile compounds. They were separated on a a high-polarity poly(ethylene glycol) column and detected following electron ionisation in full-scan mode. The compounds were identified from their relative retention times and by matching their mass spectra against a commercial library.
For GC-O-FID, the same column was used and the eluting compounds were spilt for sniffing and flame ionisation detection. A panel of seven sniffers was recruited from the lab. One person described the odour and estimated its strength while another recorded the retention times.
New odorous rubber volatiles
This comprehensive approach paid off. A total of 43 volatile compounds were identified and 16 of these had never been reported before from natural rubber. They covered several compound classes but there was only a little variation between the profiles of the two rubbers, with 31 compounds common to both. Other compounds were detected but they were only emitted in trace amounts, making identification difficult.
By carrying out SPME at different temperatures, a wider range of volatiles was trapped. Lower temperatures favoured the low-molecular-weight compounds like short-chain acids, esters and aldehydes and the extremely volatile benzene, toluene and xylene. Higher temperatures helped to extract medium-to-long chain fatty acids and esters and benzyl compounds.
Carboxylic acids from C2-C18 were found in both rubbers. Those of low molecular weight like acetic, propionic, butanoic and pentanoic acid contributed most to the unpleasant smell, accounting for more than 50% of the odorant active compounds. The acids were produced from the action of microorganisms on lipids present in the rubber. The acids were also degraded further to produce aldehydes, ketones and alcohols.
Some fatty acid esters were also detected, with methyl and ethyl pentanoate, ethyl hexanoate, methyl myristate and methyl palmitate found in rubbers for the first time. They could have been formed by microbial action or by enzymatic attack on the corresponding acids by an esterase during coagulation.
Trimethylamine, which has a strong fishy smell, was found in both rubbers for the first time. It probably originates from the anaerobic reduction of trimethylamine N-oxide and is another strong contributor to the overall odour. This compound was only detected after SPME at the highest temperature for the longest time, which might explain how it had not been detected before.
The aromatic compounds like benzaldehyde, benzoic acid and benzaldehyde were also identified for the first time in natural rubber. Phenylacetic acid and phenylpropionic acid were only found in the naturally coagulated rubber and, along with hexanal, were strong odorants. The odours of the two phenyl acids were representative of the overall smell of this rubber. In contrast, 2,6-dimethoxyphenol, with its tarry smoky odour was found only on the rubber that had undergone controlled coagulation.
The broad range of aromatic compounds was identified in rubber by a combination of SPME under different conditions with GC/MS and GC-O-FID and should help in building odour control systems in rubber processing plants to protect workers in-house as well as people living and working nearby.