An ecological plasticizer with outstanding performance in NBR.

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 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 typically used; especially in the manufacture of soft articles that require higher loading levels of plasticizers (for example, roll covers of low durometer hardness).

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 makes Lanxess's 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 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 lead to excellent weathering resistance of the final product.

Mesamoll plasticizers are highly compatible with NBR and mix rapidly with the polymer. Comparison tests between DEHP and the Mesamoll plasticizer 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) and 100 pbw, respectively, of plasticizer were used. With the first formulation (20 pbw plasticizer), the Mooney viscosity of the compound and end product properties (after vulcanization) were measured. The mechanical properties, such as durometer 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[degrees]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. The 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.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Materials having 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 durometer A hardness of the plasticizer in an NBR vulcanized end product is slightly better compared to DEHP, while the mechanical properties such as tensile strength and elonga tion 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 seven days at 126.5[degrees]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 plasticizer.

In summary, 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.

(Note: This article was previously published in KGK Magazine (Heuthig Verlag) in May 2009.

Table 1--overview of the mechanical properties of DEHP/Mesamoll
plasticizes based NBR compound

Properties *                                  Unit         DEHP   Rank
Mooney viscosity        ML 1+4/               ME             35      5
  of the mixture        100[degrees]C
Properties of the
  vulcanizate
Hardness                                      Duro. A        56      4
Tensile strength                              MPa            18      5
Elongation at break                           %             620      5
Compression             70 h./100[degrees]C   %              15      5
  set *                 DIN 53517
Low temperature         DIN 53520             [degrees]C    -17      4
  index ([T.sub.R]) *

Volatility ***          7 d/126.5[degrees]C   (%)           -17      1

Properties *                                  Mesamoll   Rank
Mooney viscosity        ML 1+4/                39        4
  of the mixture        100[degrees]C
Properties of the
  vulcanizate
Hardness                                       53        5
Tensile strength                               20.5      5
Elongation at break                           640        5
Compression             70 h./100[degrees]C    17        5
  set *                 DIN 53517
Low temperature         DIN 53520             -15        4
  index ([T.sub.R]) *

Volatility ***          7 d/126.5[degrees]C    -7        4

* 25 % deformation

** Torsion vibration

*** 100 pbw plasticizes