Miljøprojekt, 1087 – Metoder og procedurer til reduktion af uønskede stoffer i gummi

Metoder og procedurer til reduktion af uønskede stoffer i gummi

Summary

New possibilities of developing environmentally friendly rubber products

In future the Danish rubber industry will be able to facilitate the development and documentation of environmentally friendly and safe products. The analytical technique that makes this possible is called Thin Layer Chromatography (TLC). The method is based on the separation of low-volatility rubber chemicals on an adsorbent material using a fluid and capillary effect. The method was developed in connection with the project entitled "Methods for the reduction of unwanted substances in rubber products". The TLC method is simple and inexpensive enough to allow the industry itself to carry out screening for environmentally problematic substances. The method has been compared to other analytical techniques, and advantages as well as limitations have been examined.

Background and purpose

Unlike plastics, rubber materials must be vulcanized in order to become dimensionally stable. Vulcanization can be compared to a baking process. During vulcanization the molecular chains of the rubber are bonded together to achieve elasticity.

Therefore, vulcanization agents and promoters are added to rubber mixtures. The range of promoters is very wide. Some are complex sulphur compounds, others are amines or condensation products containing amines, and the last group consists of peroxides.

Rubber for food contact is required not to contain any residual peroxide, and for other products, such as dummies for babies, there may be a requirement that no nitrosamines must be formed. In addition, there may be limitations regarding certain accelerators suspected of being carcinogenic.

Other rubber chemicals are added to the rubber as well, for instance antioxidants and antiozonants the purpose of which is to delay degradation of the rubber.

Finally, softening agents are often added in order to control the hardness of the rubber.

As regards substances used to prevent degradation of the rubber or to soften it, there are chemical substances that may be unwanted from a health perspective, such as phthalates.

The purpose of this project was to examine whether the rubber industry could use a simple analytical method based on Thin Layer Chromatography (TLC) to prevent unwanted substances in its rubber mixtures and ensure uniform rubber quality. The study included both non-vulcanized rubber mixtures, so-called compounds, and vulcanized rubber in order to see whether any conversion takes place during the process.

The study has involved various rubber types and vulcanization systems.

Developing and testing a simple method to establish the chemical properties of rubber

The project was carried out jointly by the Danish Technological Institute and two representatives of the rubber industry. The industry supplied the samples for the project, whereas the Technological Institute headed the study programme and trained the companies in TLC analysis and the principles governing the substitution of chemicals.

The study included check analyses. The following types of analysis have been employed: X-ray analysis, infrared analysis and gas chromatography based on mass spectroscopy detection. The X-ray analysis provides complementary information about the chemical elements in the sample, such as heavy metals. The infrared analysis gives a "fingerprint" of the sum of the organic substances added to the rubber. And, finally, the gas chromatography method is particularly suitable for the determination of very small quantities of chemical substances in the rubber.

TLC analysis can provide useful information about the chemical properties of rubber

Following a literature search, four promising TLC analysis methods were selected. The methods were applied to extracts of both non-vulcanized and vulcanized rubber samples. In addition, various methods aimed at visualising the chemical substances after separation were tested.

On the basis of the TLC screenings and the results from the check analyses, we can draw the following conclusions:

The best visualisation method is the so-called Gibb's reagent, which causes characteristic colours to appear for the different rubber chemicals. In addition, visualisation using copper sulphate or iodine contributes to determining the type of rubber chemical that you are dealing with.

The method is useful when determining whether the recipe for a certain rubber is the same for various batches as regards the content of organic chemical substances.

The method is suitable when assessing the accelerator system used in the recipes in question when dealing with sulphur-vulcanized rubber.

The method is particularly suitable for screening for added antiozonants and antioxidants.

The method is not suitable for the analysis of peroxide-vulcanized rubber samples to detect peroxide residues or conversion products from the peroxide formed during reaction, such as acetophenone (with an unpleasant smell) from dicumyl peroxide.

The method is not sufficiently sensitive to assess organic substances at tracing level, such as PAH or decomposition products from the accelerators, such as nitrosamines.

The method has limited applicability when determining the type of ester softeners. In this case the infrared analysis or the gas chromatographic mass spectroscopy (GC/MS) analysis is definitely the preferred method. GC/MS can differentiate different, closely related phthalate softeners from each other, unlike the infrared method.

The method is not suitable for the determination of elements in the samples, such as heavy metals.

When comparing the TLC screenings and the check analyses for rubber samples with a known recipe, no discrepancies have been found in relation to the information provided by the supplier.

As regards silicone rubber and fluorine rubber, the amount of extract for analysis was very limited, and we did not obtain any useful TLC chromatograms for the characterisation of these rubber types.

Project results

Selection of TLC methods

TLC is used to detect substances as they travel at different speeds on a glass plate covered with an adsorptive material. In this study we used silica gel, which is a polar material made from sand. In the process a fluid (solvent) travels up to the sample application point and 10-15 cm onwards due to the capillary effect.

Through literature searches the Danish Technological Institute has found five relevant references for TLC analysis of rubber. Following our initial studies we selected four TLC systems for the screenings. The selection was based on analyses of a number of rubber chemicals and the use of actual rubber mixtures for comparison.

The reason why it is necessary to use several different TLC systems is that rubber chemicals may be very different as regards chemical structure. Consequently, they may vary a lot as regards polarity. Polarity reflects the properties of a molecule in relation to water and fatty substances. The more polar the rubber chemical is, the more polarity is required in the solvent used to draw the chemical as it travels through the plate. Conversely, we use less polar solvents to make more fatty substances travel; this applies to a number of softening agents and antioxidants.

Visualisation

A number of chemical substances are colourless, so it may be necessary to induce a colour reaction in order to see these substances. A simple method consists of lighting up the TLC plate using ultra-violet light, making some of the substances visible. Other substances require a chemical reaction, e.g. oxidation or a reaction that gives the substance a particular colour through complexing. Several methods have been employed in the project. The most universal method is based on the so-called Gibb's reagent. It induces characteristic colours to a number of accelerators and antioxidants. But the use of iodine and copper sulphate can also provide characteristic colours that facilitate substance identification.

TLC analyses

We carried out TLC analyses on a number of different rubber types supplied by the participating companies. The samples included both non-vulcanized and vulcanized rubber. The results show very little difference between the chromatograms for non-vulcanized and vulcanized samples. Comparisons with reference substances for rubber, the recipe of which is known, are consistent with the specifications in the recipe.

The study shows that sulphur-vulcanized rubber types (in particular) and rubber types with antioxidants or antiozonants added are suitable for characterisation using TLC. This applies to EPDM, SBR and nitrile rubber (NBR). But important information can also be found for chloroprene rubber (Neoprene, CR). We have determined that the chloroprene rubber types examined do not contain the vulcanization agent ETU, which is suspected of being carcinogenic. It is possible to ascertain whether accelerator types have been used which can form nitrosamines. Several of the more volatile nitrosamines are carcinogenic. Consequently, they must not be found in rubber nipples used in dummies for babies.

It has been found that the TLC methods are not suitable for the characterisation of softening agents and peroxides or conversion products from peroxides. Other analytical methods are required for this purpose. The same applies to the constituent chemical elements.

Check analyses

We have carried out select check analyses using other analytical techniques in order to obtain complementary information about the rubber mixtures examined.

The use of alternative analytical methods is required when you want information about:

Constituent chemical elements
Softening agents
Organic substances present at tracing level.

The analysis of the constituent chemical elements can be carried out by means of X-ray spectroscopy. This allows the detection of any heavy metals (arsenic, tin, nickel, cadmium, lead). The two chloroprene rubber samples examined contained no heavy metals.

The types of softening agents can be determined through the infrared analysis. The reason for this is that the softening agents have different resonance frequencies in the infrared spectral region. Thus, it is possible to distinguish between the following softening agent types: mineral oils, ester-based agents and alkylsulphonate acid esters of phenol, which are all used in rubber. In order to distinguish between isomeric types of softening agents, such as DNOP and DEHP, a GC/MS analysis is required.

Organic substances at trace level that can be brought to vapour form can be detected and quantified by means of GC/MS analysis. For instance, this applies to PAHs, aromatic amines, e.g. aniline and degradation products from accelerator, such as carbon disulphide.

The PAH content levels measured in select samples are very low compared to the amount found in an average car tyre. The highest amount of benzopyrene is thus 0.3 ppm. In a tyre the content is typically 1-2 ppm.