Tech service: non-carcinogenic tire extender oils providing good dynamic performance.

Non-carcinogenic mineral oil based plasticizers and extender oils are utilized more and more in rubber and tire compounds. The interest is driven by health and environmental consciousness, as well as by the coming European Union legislation prohibiting the use of high-aromatic tire extender oils. The chemistry of most discussed tire extender oil alternatives is sufficiently close to the utilized polymers, and they give fully acceptable mechanical and physical compound performance. The dynamic performance of a compound extended with a high-aromatic oil, however, is more difficult to match. Highly refined naphthenic oils comply with the set health and safety criteria and are, considering their chemistry, one of the best alternatives when it comes to compatibility with the common polymers. By taking into account certain characteristic features when choosing the plasticizer or extender oil, it is possible to find a safe product that performs in a similar manner as the high aromatic oils, not only when it comes to physical and mechanical characteristics, but also the dynamic properties.

The EU has, with directive 2005/69/EC, applied an amendment to directive 76/769/EEC, which prohibits the placing of polyaromatic hydrocarbon-rich tire extender oils or tires containing these oils on the EU market from January 1, 2010.

The directive defines that the extender oils allowed after the directive steps in force will have to meet two criteria. The content of eight defined individual polyaromatic hydrocarbons (PAH) cannot exceed 10 mg/kg, which is assumed to be met if the DMSO extractable amount in the oil is below 3% by method IP346.

Tires will be checked with method ISO 21461, which utilizes NMR and measures bay area protons in polycyclic aromatic compounds (PCA). It analyzes an extract from the tire compound. The result correlates to the amount of PCA in the extender oil. A maximum limit of 0.35% has to be met for the tire to be approved for marketing in the EU. The ISO method will be especially applicable to imported tires, as method IP346 and individual PAH analyses do not apply to extender oil extracted from a compound.

As the legislation will apply to all tires produced and marketed in the EU, the directive will also have effects on non-European tire manufacture. The work around finding alternatives has progressed quite far also outside the EU. As the types of alternatives readily available will vary to a certain extent with geographical location, solutions different from the treated distillate aromatic extract (TDAE) and mildly extracted solvate (MES), as recommended and preferred in the European market, may be the preferred ones.

Legislation does not seem to be discussed outside Europe, so even if non-European tire manufacturers choose new extender oils based on the IP346 and individual PAH criteria, it may be that the ISO method applied to compounds will be the most important to meet.

It is a known fact that to maintain all tire characteristics, a 1:1 replacement of the high-aromatic oils with the new types is not feasible. A difference is usually observed in the dynamic properties, which affect, e.g., the wet grip performance. Studies have been reported by a number of extender oil suppliers and polymer extenders.

Study

This study was performed based on the knowledge that the chemistry of naphthenic plasticizers and extender oils is perhaps the closest to the currently used high-aromatic oils when there is now a need to reduce the content of PCA. The naphthenic structure gives better compatibility with the elastomers used in tire compounds than, for instance, paraffinic alternatives do. The glass-transition temperature, Tg, of the oil influences the Tg of the compounds. It, in turn, influences the dynamic performance of the tire itself.

The oil glass-transition is steered by its molecular composition. When reducing the aromatic content, the Tg is also reduced. A high naphthenic content gives a higher Tg than a low one does. The Tg can also be balanced by the oil viscosity. A higher viscosity also means a higher Tg. The questions are:

* Does the oil Tg have the desired effect on the compound Tg?

* Are the physical and mechanical characteristics affected by altering the oil-elastomer compatibility (may change with the molecule size, which steers viscosity)?

Experimental

The oils included in this study were chosen to match the Tg of a traditionally used high-aromatic extender oil (DAE) and to get a better match than that obtained with an MES or a TDAE. The extender oil characteristics are listed in table 1. NAP1-NAP3 are naphthenic products marketed by Nynas (Nytex 8450, Nytex 5450 and Nytex 51150, respectively), while NAP 4 is a product from another supplier, also available on the market today.

The compound used in the tests was based on solution SBR and silica filler. The extender oil content was the same in all five compounds, and no adjustment was made to the other components to balance the exchange of oil. The formulation is seen in table 2.

The compounding and mixing procedure was performed according to ASTM D3182-89 (2001).

Results and discussion

Figures 1 and 2 display the vulcanization characteristics. The scorch times were quite equal. The DAE and NAP4 compounds displayed perhaps slightly shorter vulcanization times. As commonly assumed, this may be explained by the presence of the sulfur- and nitrogen-containing heterocyclic compounds that have a tendency to work as some kind of natural accelerators during the vulcanization reaction. This content is slightly higher in NAP4 than in the other naphthenic extender oils. The crosslink densities in all compounds reached the same level, as indicated by the torque data in figure 2.

[FIGURES 1-2 OMITTED]

The hardness levels were about equal, as displayed in figure 3. The tear strength, if the slight differences may even be considered significant, seems to be benefiting from higher viscosity, possibly in combination with a higher VGC, as seen in figure 4. A higher VGC (viscosity-gravity constant) means a more polar oil, which is beneficial to the oil-elastomer compatibility. Abrasion resistance has a tendency to increase with better compatibility. Here, however, the differences can barely be said to be significant (figure 5).

[FIGURES 3-5 OMITTED]

The mechanical characteristics seem to be benefiting from a better compatibility. The differences in tensile strength and modulus are not very marked (figure 6). A higher VGC possibly induces slightly higher values. The elongation values increase somewhat with increasing VGC values (figure 7).

[FIGURES 6-7 OMITTED]

The dynamic characteristics were measured at 1 Hz, 1% strain. These data are perhaps the most interesting in this study. The tan delta peaks all occurred within a few degrees temperature range (figure 8). The NAP1 and NAP2 compounds display exactly the same values as DAE, and the peaks of the higher viscosity naphthenics (NAP3 and NAP4) are positioned a couple of degrees higher. Consequently, the wet grip indicated by the tan delta at 0[degrees]C is perhaps even slightly better for the NAP3 and NAP4 compounds than the DAE compound. The rolling resistance, indicated by the tan delta at 60[degrees]C, shows the DAE and NAP1 - NAP3 compounds to be very similar, with perhaps a somewhat higher value for NAP 4 due to its slightly broader peak. The elastic moduli are seen in figure 9.

[FIGURES 8-9 OMITTED]

It is important to remember that any differences are possibly smaller in compounds based on solution SBR and silica. The combination of emulsion SBR and carbon black may show bigger differences. However, differences are also seen in silica filled compounds if we compare oils with significantly lower Tg values to high-aromatic types. Therefore, this study should still be a good indication of what may be expected if a safe oil with a Tg close to that of DAE can be chosen.

Conclusions

As previously reported by Nynas, as well as other oil suppliers and polymer extenders, some of the solutions considered as replacements for aromatic extracts work well when it comes to physical and mechanical characteristics. Their contribution to the dynamic characteristics is, however, not optimal.

This study indicates that, by balancing the properties gained from the naphthenic structures in the oil with a higher viscosity, it is possible to mimic the dynamic behavior of compounds extended with high-aromatic oils, without losing in mechanical and physical performance due to the differing chemistry of the oils. The availability of these high-viscosity naphthenic oils is, of course, not like that of, e.g., the MES, but there are a few actors offering these types of products on the market. Considering the availability of any other suggested alternative than MES, the availability of the high-viscosity naphthenics should not fall in a too negative light.

by Marika Joona ([e-mail]), Nynas Naphthenics AB

Table 1--oil properties
                               ASTM      DAE     NAP1

                                N 8450 N 5450 N 51150

Kinematic viscosity at         D445      840      450
40[degrees]C, cSt
Kinematic viscosity at         D445       21     16.4
100[degrees]C, cSt
Density at 15[degrees]C,      D4052    1,010      945
kg/[m.sup.3]
Flash point, [degrees]C         D93      230      220
Aromatic content, %           D2140       41       15
VGC                           D2501    0.989    0.850
Aniline point, [degrees]C      D611       22       81
Glass-transition             El 356      -36      -48
temperature, [degrees]C
PCA, (DMSO extr.             IP 346       >3       <3
content), wt%
                               NAP2     NAP3     NAP4

                                N 8450 N 5450 N 51150

Kinematic viscosity at          450    1,150      727
40[degrees]C, cSt
Kinematic viscosity at         13.8     22.6     19.3
100[degrees]C, cSt
Density at 15[degrees]C,        950      957      961
kg/[m.sup.3]
Flash point, [degrees]C         220      222      220
Aromatic content, %              17       17       25
VGC                           0.888    0.887    0.899
Aniline point, [degrees]C        74       77       66
Glass-transition                -46      -38      -44
temperature, [degrees]C
PCA, (DMSO extr.                 <3       <3        ?
content), wt%

Table 2--test compound formulations

SSBR (Buna VSN 5025-0)           100 phr
Silica (Ultrasil VN3)                 80
Silane (Si69)                        6.4
Carbon black N550                      5
Oil                                   40
Stearic acid                           2
Zinc oxide                             3
6PPD A.O.                              2
CBTS accelerator                    1.60
DPG accelerator                     2.00
Sulfur                              1.50