Feature

Ecological Plasticizer for Outstanding Performance in NBR

Posted on 29 January, 2010 | Tags: Green Additives

Phthalates have been identified as one of the Substances of Very High Concern (SVHC) that may have serious effects on the environment. Inclusion of substances in this list may result in legal obligations for raw material suppliers. Substitution of phthalate-based plasticizers with environment friendly products is of great interest to the manufacturers.

Nitrile butadiene rubber (NBR) is used in articles that come into contact with oils, fuels and solvents because of its good chemical and physical resistance with such materials.  Typical applications of NBR include hoses, gaskets, diaphragms, seals and conveyor belts. With the help of plasticizers, the viscosity, tackiness, cold flex behavior and extrusion properties of NBR compounds are optimized so as to achieve the desired properties. Phthalates have historically been the plasticizers of choice.  However, new environmental and ecological legislation has now restricted the use of certain types of plasticizers in targeted applications in Europe. In some NBR compounds, the phthalate based plasticizer DEHP (Di-2-ethylhexylphthalate; formerly known as DOP) is used typically; especially to  manufacture soft articles that require higher loading levels of plasticizers (for example, roll covers of low Shore hardness).

Difficulties with Phthalates 
In August, 2008, the European Chemicals Agency (ECHA) published a list of Substances of Very High Concern (SVHC), which includes the plasticizer DEHP. The substances identified on this list may have very serious effects on humans and the environment. Substances on this list may subsequently become subject to authorization by the European Commission, and inclusion of a substance on this list may result in legal obligations for the raw material suppliers as well as compounders and in end use applications.  As a result, the substitution of DEHP with a suitable plasticizer is of great interest to manufacturers.
  One of the major challenges to replace DEHP is its outstanding vulcanization process. Growing legislative pressure on DEHP make  Mesamoll plasticizer, a phthalate free alkane sulfonic acid ester, which is not SVHC-listed and has very low volatility, the ideal replacement for DEHP. The MESAMOLL® plasticizer has a proven track record in the PVC industry. Given that it has faster gelling characteristics than standard plasticizers, manufacturing rates can be increased thus improving efficiencies and reducing unit production costs.  The product's good saponification characteristics further leads to excellent weathering resistance of the final product.

Environment Friendly Plasticizers
Mesamoll plasticizers are highly compatible with NBR and mix rapidly with the polymer.    Comparison tests between DEHP and the Mesamoll plasticizer by Lanxess were carried out to determine processability, vulcanization and mechanical properties in an NBR compound.  In the trials, NBR compounds loaded with either 20 pbw (parts by weight) or 100 pbw, respectively, of plasticizer were used. With the first formulation, (20 pbw plasticizer) the Mooney Viscosity of the compound and of the end product (after vulcanization) was measured. The mechanical properties such as Shore A hardness, tensile strength, elongation at break, compression set and the cold flex temperature were recorded. With the second formulation, the volatility after seven days at 126.5°C was determined.
The quantitative results are listed in table 1 and represented in a spider diagram in figure 1, with the value of 0 representing undesirable properties and 6 indicating excellent behavior in an NBR compound.
The spider diagram clearly shows that the Mesamoll plasticizer performs as well, as if not better, than DEHP in most areas. This plasticizer's impact on the viscosity of the compound is slightly less effective than DEHP under equivalent loading conditions, resulting in a slightly higher Mooney Viscosity value.

Applications of Mesamoll
Materials with lower Mooney Viscosity (30 to 50) can be used in injection molding or calendering processes, while higher Mooney products (60 to 80) can be more highly extended and used in extrusion and compression molding. The Shore A hardness of the LANXESS plasticizer in an NBR vulcanized end product is slightly better compared to DEHP, while the mechanical properties such as tensile strength and elongation at break point are comparable with both plasticizers.  The compression set testing measures the ability of NBR to return to its original thickness after prolonged compressive stresses at a given temperature and deflection. As nitrile rubber is compressed over time, it loses its ability to return to its original thickness. This loss of resiliency (memory) may reduce the capability of an elastomeric gasket, seal or cushioning pad to perform over time. In these trials both plasticizers show a similar behavior in an NBR compound under compression.
Volatility plays an important role during the manufacturing process. A compound with a lower volatility is desired. As an example, a plasticized NBR vulcanized product (extreme loading 100 parts by weight of plasticizer) containing Mesamoll plasticizer or DEHP was used in order to determine the difference in volatility after 7 days at 126.5°C. The DEHP-based NBR compound showed a significantly higher volatility than the one based on MESAMOLL® plasticizer illustrating one of this plasticizer's advantageous features. This is attributed to the low vapor pressure of the Mesamoll plasticizer.

Conclusion
All key findings show that the Mesamoll plasticizer is the ideal alternative to DEHP in NBR applications.  Naturally, this is only one example of the kind of performance that can be expected from a phthalate free alternative plasticizer.  Its overall performance makes it the plasticizer of choice in a multitude of applications.

   

FEF= Fast Extrusion Furnace Black
SRF= Semi Reinforcing Furnace Black
EV-System= Accelerator system for heat resistant compounds (so called Efficient Vulcanization System)
pbw = Parts by Weight

Melanie Wiedemeier, LANXESS Deutschland GmbH, Functional Chemicals Business Unit
Email: amlanabha.das@lanxess.com