Plasticizers

Plasticizers are a central component of many rubber compounds. They are specifically added to the rubber to adjust the processability, low-temperature flexibility, hardness, and damping behavior of the resulting elastomer. Without suitable plasticization, many seals, profiles, and molded parts would be too hard at low temperatures or would be difficult to extrude or injection-mold during production.

The Role of Plasticizers in Rubber Compounds

In elastomer compounds, plasticizers primarily serve two functions:

1. In the compound (unvulcanized):
   • They reduce viscosity and improve flow behavior during mixing, extrusion, and injection molding.
   • They facilitate the filling of fine contours in molded parts and sealing lips.

1. In the finished elastomer (vulcanized):
   • They lower the glass transition temperature (Tg) and shift the thermoelastic range toward lower temperatures.
   • They increase low-temperature flexibility: the elastomer remains elastic even at low temperatures instead of becoming glassy and hard.
   • They influence hardness (Shore A), damping behavior, and resilience. Depending on the type, a material can be made softer, more flexible, or more "rubbery."

Thus, plasticizers play a decisive role in ensuring that a rubber component retains its required elasticity, sealing effect, and functionality within the planned operating temperature range.

Typical Types of Plasticizers in Rubber Compounds

The following plasticizers are primarily used in rubber compounds:

• Plasticizing oils (mineral oils, e.g., aromatic, naphthenic, or paraffinic oils)
• Synthetic ester plasticizers, e.g., phthalates, adipates, sebacates (depending on the system and regulations)
• Polymer or resin plasticizers, e.g., low-molecular-weight resins or polymers that are highly compatible with the rubber

The selection depends on:

• The type of elastomer (NBR, EPDM, CR, NR, etc.)
• The desired low-temperature flexibility and hardness
• Requirements for media resistance, fogging, emissions, and regulations (e.g., phthalate-free systems)

Influence on Thermoelastic Behavior

Plasticizers embed themselves between the polymer chains of the rubber and increase their mobility. The result:

• The transition from a glassy to a rubber-elastic state shifts to lower temperatures.
• The elastomer remains in the elastic range longer within its subsequent application window.

For example, a seal can be adjusted to remain soft and functional at –20 °C or –30 °C instead of becoming brittle and forming cracks.

Migration and Aging

In many systems, plasticizers are not covalently bonded, but are physically dissolved within the rubber network. As a result, over time they can:

• Diffuse within the material,
• Migrate to the surface and appear there as a slight sheen or sticky film,
• Partially transfer into adjacent media (e.g., plastics, oils).

The extent of this migration depends on:

• The type of plasticizer and its molecular size,
• Compatibility with the elastomer,
• Temperature, media contact, and component geometry.

As migration increases, an elastomer can become harder and more brittle, while low-temperature flexibility and damping decrease. Modern formulations therefore often rely on low-migration or polymer-bound plasticizers when long-term stability, low emissions, or media-critical applications are required.

Significance in Sealing Technology

In sealing and elastomer technology, plasticizers influence, among other things:

• Assembly capability of O-rings, profiles, and sealing lips (elongation paths, sensitivity to kinking)
• Low-temperature flexibility of construction and facade profiles, automotive seals, or building technology components
• Damping and vibration behavior of bearings, buffers, and elastomer springs
• Long-term tightness, when migration and aging are taken into account

The targeted selection of plasticizer type and dosage is therefore a key lever for designing a rubber compound that is application-optimized – from soft, low-temperature flexible sealing elements to damping molded parts with defined recovery behavior.