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Understanding Gel in Elastomer Samples: Causes, Effects, and Testing Methods

Gel in elastomer samples significantly impacts processing behavior and final product properties. By exploring crosslinking and advanced testing methods, we can better understand how gel content affects elastomers. Accurate gel content analysis is crucial for optimizing performance and enhancing product quality.

What is gel in an elastomer sample?

Gel in an elastomer sample is characterized as either:

  1. Inter-chain crosslinking between two macromolecules.
  2. Intra-chain crosslinking between two side chains.

Gel may also be characterized by extremely high molecular weight. Gel is not caused by branching or a curing agent.

How does gel affect elastomer performance?

The gel content in a elastomer has a significant impact on its viscoelastic properties, as well as the mechanical properties of the final product.

The gel content of a filled rubber compound also has tremendous influence on its processing behavior. This includes:

  • Mixing behavior
  • Black incorporation time
  • Die swell
  • Extrusion behavior
  • Surface appearance after extrusion

In most instances, excess gel is unwelcome. However, there are some contexts in which the presence of gel can be beneficial. For example, when a large amount of reinforcing filler, such as carbon black, is not properly dispersed, the compound tends to become crumbly and break into pieces rather than forming a shape. Gel can overcome this problem. Gel can also be an asset when striving for a mixed compound with green strength.

Understanding the gel content of an elastomer before mixing begins is critical to ensure that the desired outcome is achieved. This is especially important when the mixing takes place in a mixer with a closed chamber and the behavior of the rubber compound cannot be directly observed.

Testing for gel content

A common method for determining gel content is ASTM D3616: Standard Test Method for Rubber—Determination of Gel, Swelling Index, and Dilute Solution Viscosity. However, the analytical methods described in ASTM D3616 cannot determine the absolute value of gel content in the elastomer under test, since these methods depend on the choice of solvent, the time allowed for solubilizing, and of course on the temperature set in the defined procedure.

Rheological testing offers more effective methods for qualitative detection of gel in raw materials by measuring the materials’ viscoelastic properties. These methods include:

  • Time sweep
  • Frequency sweep
  • Stress relaxation testing

However, these methods are not advanced enough to quantify gel content or capture information on complex branching patterns, both of which are critical for fully understanding the processing behavior of elastomers and thermoplastics. Deeper analysis requires a more advanced analytical approach.

FT Rhealogy via LAOS

Describing the real response of material tested in the nonlinear region and quantifying the distortion requires FT rheology via large amplitude oscillatory shear (LAOS). Applying FT rheology via LAOS accounts for all odd higher harmonics in the torque signal during analysis.

The combination of aging and unsuitable stabilizers can result in a significant amount of gel in an elastomer. This may occur when raw materials are stored in a warehouse for long periods. Simulating these effects is the first step to quantifying the gel content in an elastomer material. There are two ways to simulate aging:

  1. Place the sample in a laboratory oven, set at 70-100°C, for several hours.
  2. Expose the sample to high temperature and high shear for several minutes inside an RPA testing die.

The second approach integrates seamlessly into a full test procedure using the Alpha Technologies Premier RPA to detect and quantify gel content in the sample:

  1. Before aging, the sample undergoes LAOS testing at 150°C to determine the LCB indexes.
  2. The sample is exposed to elevated temperatures and high shear.
  3. The sample undergoes a second round of LAOS testing at 150°C to determine LCB indexes.

The Lissajou figures and LCB indexes from the first and third steps are compared, to detect and quantify any gel present in the sample.

Case Studies

Alpha Technologies has validated this approach with several raw elastomers, including polybutadiene rubber (BR), emulsion-styrene butadiene rubber (e-SBR), and nitrile butadiene rubber (NBR). We have prepared three case studies detailing these experiments:

Case Study 1 tests the efficacy of a stabilizer agent in protecting an experimental grade of BR from oxidation by controlling molecular weight changes and gel formation.

Case Study 2 investigates the effects of gel on the compound processability—specifically extrusion behavior and filler disperson—of e-SBR.

Case Study 3 traces the root cause of inconsistent extrusion behavior in two NBR samples, sourced from different lots produced by the same manufacturer.

Download the Full White Paper

All three case studies and their accompanying charts, graphs, and images are available for download in this whitepaper.