Analysis and Quantification of the Degree of Devulcanisation in Elastomers

Devulcanisation is a key process in the circular economy of elastomeric materials, as it enables the recovery of vulcanised rubber and its reincorporation into new applications, thereby reducing waste and dependence on virgin raw materials. However, devulcanising rubber presents significant challenges due to the complexity of breaking the chemical cross-links between the polymer chains of rubber without damaging its basic structure. This allows for the recovery of properties such as elasticity and facilitates its reuse, promoting sustainability. Furthermore, rubber formulations usually include several ingredients in addition to the main matrix, such as reinforcing fillers, plasticisers, antioxidants, etc. All these factors make the process complex, and various approaches are therefore used to evaluate the best solutions.

At present, various techniques are being used and developed to tackle this challenge, including chemical processes (using desulphurising agents), mechanical (through shear and heat), thermal (controlled pyrolysis) and biological methods (using enzymes or microorganisms). Each method has its advantages and limitations, and the ability to accurately characterise the degree of devulcanisation achieved is key to ensuring the quality of the recycled material and its suitability for future applications. This article focuses on the methods available to analyse such devulcanisation and their correlation with the properties of recycled rubber.

Figure 1. Schematic of the network structure in a vulcanisation and devulcanisation process.

Basic Evaluation Method: Gel Content

Gel content measures the insoluble fraction of a cross-linked rubber after extraction in a suitable solvent. This method indirectly evaluates the amount of cross-links present in the material by distinguishing between soluble and insoluble elements, the latter representing the remaining vulcanised rubber network structure. Ideally, if all cross-links were removed during the process, a fully soluble polymer in the appropriate solvent would be obtained (i.e. no gel content). However, an overly aggressive process could attack not only the cross-links but also the polymer backbone, limiting future material properties.

Applicable Standards:

• ISO 10147 and/or ASTM D2765: Used to determine gel content in cross-linked polymers. These standards are specific to cross-linked polyethylene (PE-X), but their method and results can be applied to other rubbers with modifications (appropriate solvent selection). In such cases, they can be used to determine the degree of devulcanisation if the initial gel fraction before devulcanisation is known.

Procedure:

1. The material undergoes an extraction process with a solvent such as xylene at a controlled temperature.

2. The amount of insoluble material (gel) remaining after extraction is measured.

3. This is compared with the initial gel content to determine the degree of devulcanisation.

Solvent Swelling: A More Accurate Assessment

This method is based on the material’s ability to swell in a solvent, a property directly related to the cross-link density. It is widely used in the rubber industry to correlate the degree of swelling with the cross-link density.

Figure 2. Schematic of the network structure in solid-state rubber and the network after swelling in solvent.

Prior to testing, a pretreatment with acetone is typically performed to remove extractables such as plasticisers or uncross-linked rubber already present in the original material.

Applicable Standards:

There is no universal specific standard for rubber swelling, but procedures derived from studies such as the Flory-Rehner theory are used.

• ASTM D6814 provides guidance for evaluating the degree of devulcanisation in ground vulcanised rubber by measuring the cross-link density through swelling in solvents.

Basic Procedure:

1. The sample is dried to determine its initial mass.

2. It is treated with acetone to remove uncross-linked materials (unvulcanised rubber, free sulphur, plasticisers, processing aids, natural oils or waxes, fatty acids, or antioxidants).

3. The sample is immersed in a suitable solvent (e.g. toluene), and the swollen mass is measured.

4. The solvent is evaporated to obtain the final dry mass.

5. The degree of swelling is calculated and correlated with the cross-link density using theoretical equations.

The Flory-Rehner equation is essential for describing the swelling of cross-linked polymers in the presence of a solvent and is used to calculate the cross-link density (ν). The basic equation is:

Donde:

Equation

Where:

• ν_s: molar volume of the solvent (cm³/mol)

• V_s: volume fraction of the swollen polymer at equilibrium
• χ: polymer-solvent interaction parameter (dimensionless)
• ν: cross-link density (mol/cm³)
• ρ: polymer density (g/cm³)

Main steps in the calculation:

1. Experimentally determine $Vs$ from the polymer volume before and after swelling.

2. Estimate the interaction parameter $\chi$ using tables or system-specific equations.

3. Solve the equation to obtain $\nu$, representing the cross-link density.

Lastly, there are other experimental approaches used to analyse devulcanisation of rubber networks. For example, cross-link density can be indirectly calculated from the elastic properties of a vulcanised rubber. According to rubber elasticity theory, the cross-link density (𝜈) is related to the elastic modulus (𝐺). Moreover, some researchers use techniques such as low-field Nuclear Magnetic Resonance (NMR) to obtain quantitative information on average cross-link density as well as network homogeneity or heterogeneity, offering a more detailed view of the structure. Low-field NMR is an advanced technique that can analyse polymer chain mobility and, consequently, infer the cross-link density through relaxation times.

Which Method to Choose?

Although there is no universal method, various approaches are available to characterise the degree of rubber devulcanisation after treatment. The choice of method depends on the level of detail required and the material to be assessed.
In industrial settings, gel content is ideal for quick, standard analyses, while solvent swelling, combined with Flory-Rehner theory, is preferable for deeper characterisation.
In an industrial environment, using both methods can provide a comprehensive view of the material’s condition and its recyclability for re-entry as a raw material.

If you’re unsure about the best solution, the Characterisation Laboratory at AIMPLAS can offer guidance and propose the most suitable option for your needs. Contact us.