A new nitrosamine-free curing agent

Rubber World, May, 2007 by Pierre Lugez

The suppression of nitrosamine-generating products from rubber formulations is an ongoing concern for the rubber industry. Only a few accelerator families are available to achieve this task, including dithiophosphates, caprolactams, guanidines and thioureas. Some of them are expensive, while others show some different toxicity issues. This is why MLPC International is innovating to widen the range of possible solutions with Mixland TBP 75 GA, a new nitrosamine-free accelerator based on "Vultac" chemistry.

TBP 75 GA physical nature

TBP 75 GA is a predispersed polymer-bound chemical, with 75% active material. The binder is a thermoplastic acrylate polymer based on the patented Mixland technology.

This binder provides the following benefits:

* Very good stability in storage, no problem of caking up to 50[degrees]C, free-flowing pellets;

* very fast dispersion in main rubber compounds due to a low Mooney viscosity at mixing temperatures; and

* very good compatibility with either polar or non-polar rubbers.

The use of Mixland pellets enables manufacturers to avoid cross-contamination, reduce health hazards and to get a better consistency of rubber compounds.

Chemical nature

The TBP 75 GA is made from TBP: poly-tertiary-butyl-phenol disulfide (figure 1).

[FIGURE 1 OMITTED]

This chemical exhibits several properties, including:

* It is a sulfur donor, so it is able to prevent reversion;

* it contains no nitrogen in its formula, so it is fully nitrosamine-free;

* the phenolic function acts as a weak antioxidant; and

* due to its oligomeric nature, TBP exhibits some tackifying properties.

During test-runs, we demonstrated that TBP 75 GA is an efficient and versatile curing agent, a non-blooming accelerator and an odorless material.

Technical study

The reference formulation

We defined a standard recipe, which is able to correctly cure an EPDM compound for automotive profile (all rubber). The EPDM base recipe is named EPDM compound III (table 1).

The chosen curing system is based on a reference recipe containing TMTD (table 2). In earlier lab tests, we replaced TMTD with TBP 75 GA (equal weight active material), to switch to a nitrosamine-free system. We carried out rheometric tests at 190[degrees]C and obtained some very close curing curves (figure 2).

[FIGURE 2 OMITTED]

We observed that Mixland TBP 75 GA has the same efficiency as TMTD in the EPDM recipe. Moreover, the TBP 75 GA curve shows no reversion. So, we decided to carry out an experimental plan to describe the curing properties of this new chemical.

The experimental plan

To draw the maximum data from this new chemical, we chose a Taguchi L9 plan, which is able to manage four parameters at three different levels. The plan is described in table 3.

According to the Taguchi experimental procedure, we defined the nine curing recipes we had to produce to make the rheometric and mechanical measurements. From table 4, we calculated the weights of each ingredient needed for the nine lab compounds.

The rheometric results

We carried out rheometric tests with an ODR rheometer at 185[degrees]C during 30 minutes (according to ISO 3417), to show the effect of the temperature on the rubber reversion. The ten curves are together on figure 3.

[FIGURE 3 OMITTED]

We observed the reversion of the reference compound (containing TMTD), and no reversion with compounds 1 to 9, which contain TBP 75 GA.

We can highlight the fact that the delta torque increases with the quantity of TBP 75 GA in the recipe, as figure 4 shows.

[FIGURE 4 OMITTED]

Another effect is interesting: The anti-reversion effect is very suitable for high temperature curing conditions, like continuous extrusion and a UHF curing line for automotive door seals.

The mechanical properties

Test standards

The experimental work was carried out according to ISO or European standards of the rubber industry as shown in table 6.

[FIGURE 5 OMITTED]

We molded 2 mm thick rubber sheets and compression set parts at 185[degrees]C with a compression press, according to the rheometric curves (table 7).

Mechanical features of the ten compounds

The data show interesting improvements (table 8) of:

* Tensile strength according to the level of TBP 75 ( 10%);

* tear strength ( 15%); and

* compression set, as the compound EPDM number 8 shows.

The Taguchi plan makes it possible to calculate optimized recipes to reach the best compression set (CS) values, as we describe later.

We can exhibit the main interesting findings of the experimental plan, as shown in figures 6 and 7.

[FIGURES 6-7 OMITTED]

Ingredient influences

We could estimate the effects of each main curing agent on the properties of the rubber compounds. We sum up the effects in table 9.

Optimization of the compression set (CS)

The full optimization calculations show the possibility to improve the CS with the new TBP 75 GA-based curing system. For instance, the curing system noted as follows:

MBT 80 = 2 phr, CBS 80 = 0.5 phr, ZDTP 50 = 3 phr, TBP 75 = 1.1 phr

has a calculated CS = 42.6%, compared to 50% for the reference compound with TMTD (for data tables with all 81 calculated values from the Taguchi plan, click here (RW )).