A new nitrosamine-free curing agent.

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+)).

The other predicted properties of this compound are shown in table 10:

Cost study

We can calculate the cost of the new curing system (3.1.1.1), which contains 1.1 phr of Mixland+ TBP 75 GA (table 11).

The conclusion is that the cost increase is minimal, especially when one considers the significant improvement of the mechanical properties and the suppression of nitrosamine-generating accelerator. To maintain the initial formulation cost, it could be possible to increase the level of fillers, for instance, and to maintain the initial mechanical properties.

Blooming 

We carried out blooming tests in an ozone atmosphere to simulate aging conditions in the engine compartment.

Test conditions were:

* 100 ppm (or 10,000 pphm) of ozone, during 24 hours at room temperature;

* 100 pphm of ozone during 24 hours at 40[degrees]C.

Under both conditions, we noticed no blooming effects on the surface of the tested cured sheets of compounds EPDM 1 to 9. So the addition of Mixland+ TBP 75 GA does not lead to blooming residues.

Rheological behavior of TBP 75 GA

Mixland+ TBP 75 GA was designed with the same features as the rest of the Mixland+ range, including a low Mooney viscosity from 50[degrees]C to 100[degrees]C. This is in order to achieve a very good and fast dispersion into various rubber compounds, even in very low viscosity compounds like sponge compounds. Figure 8 illustrates this key feature.

[FIGURE 8 OMITTED]

Conclusions

The new Mixland+ TBP 75 GA is a fully new nitrosamine-free curing agent. Moreover, it exhibits a package of interesting features and properties for the rubber formulator, including:

* An efficient and versatile vulcanization accelerator;

* a nitrosamine free accelerator; an odorless material;

* a sulfur donor (prevents reversion);

* a non-blooming accelerator; a light tackifier; and

* a light antioxidant.

Some mechanical properties are enhanced, including:

* Tear strength;

* tensile strength; and

* compression set.

which enables the formulator to improve mechanical features and to comply with new requirements from clients.

Pierre Lugez, MLPC International ([e-mail])

Table 1--EPDM compound

Fast cure EPDM (10.5% ENB)   100
Zinc oxide                     5
Lubricants                     4
Carbon black                 115
Mineral filler               111
Paraffinic oil                75
Total phr                    410

Table 2--reference curing system

Formulation          Reference with   Initial NA free lab test
                       TMTD 80 GA      with Mixland TBP 75 GA

EPDM compound III         410                   410
Mixland S 80 GA           0.5                   0.5
Mixland MBT 80 GA         1.5                   1.5
Mixland CBS 80 GA         1.0                   1.0
Mixland ZDTP 50           4.4                   4.4
Mixland TMTD 80 GA        1.9                    --
Mixland TBP 75 GA          --                   2.0

Table 3--the chosen parameters

Formulation             Accelerator        Taguchi    Taguchi
                         variations       parameter    level

EPDM compound III
Mixland S 80 GA      0.5 phr = constant
Mixland MBT 80 GA     1.0; 1.5; 2.0 phr      1st      1, 2 & 3
Mixland TMTD 80 GA                 None
Mixland CBS 80 GA     0.5; 1.0; 1.5 phr      2nd      1, 2 & 3
Mixland ZDTP 50       3.0; 4.0; 5.0 phr      3rd      1, 2 & 3
Mixland TBP 75 GA    1.1; 2.15; 3.2 phr      4th      1, 2 & 3

Table 4--Taguchi L9 experimental plan chart

Parameters   MBT     CBS     ZDTP    TBP
Number of    80 GA   80 GA   50 GA   75 GA
compounds    (phr)   (phr)   (phr)   (phr)

1                1     0.5     3       1.1
2                1       1     4      2.15
3                1     1.5     5       3.2
4              1.5     0.5     4       3.2
5              1.5       1     5       1.1
6              1.5     1.5     3      2.15
7                2     0.5     5      2.15
8                2       1     3       3.2
9                2     1.5     4       1.1

Table 5--metric data

Compounds           ML           MH   [DELTA]
            (lb.-inch)   (lb.-inch)   (MH-ML)

Reference          5.9         39.3      33.4
EPDM 1             8.5         42.5      34.0
EPDM 2             8.4         44.6      36.2
EPDM 3             8.5         47.8      39.3
EPDM 4             8.6         46.3      37.7
EPDM 5             8.9         42.7      33.8
EPDM 6             8.7         46.2      37.5
EPDM 7             9.5         47.1      37.6
EPDM 8             9.0         47.7      38.7
EPDM 9             8.8         40.9      32.1

Compounds         ts     t'c 50     t'c 90
            (min.:s)   (min.:s)   (min.:s)

Reference       1:06       1:51       2:53
EPDM 1          1:01       1:30       2:38
EPDM 2          1:02       1:33       3:39
EPDM 3          0:58       1:33       4:49
EPDM 4          0:54       1:29       5:39
EPDM 5          0:56       1:25       3:03
EPDM 6          1:03       1:42       5:52
EPDM 7          0:48       1:21       5:23
EPDM 8          0:56       1:35       6:35
EPDM 9          1:01       1:38       3:14

Table 6--the mechanical properties

Test              Conditions                     Standard

Rheometric test   ODR                            ISO 3417
Compression set   Small test piece               ISO 815
Stress-strain     Tensile, modulus, elongation   ISO 37
Tear resistance   Angular piece test             ISO 34-1

Table 7--molding conditions

Compounds        Time for    Time for CS
            sheets (min.)   parts (min.)

Reference          5 min.         7 min.
EPDM 1             5 min.         7 min.
EPDM 2             5 min.         7 min.
EPDM 3             7 min.         9 min.
EPDM 4             7 min.         9 min.
EPDM 5             5 min.         7 min.
EPDM 6             7 min.         9 min.
EPDM 7             7 min.         9 min.
EPDM 8            10 min.        14 min.
EPDM 9             5 min.         7 min.

Table 8--mechanical test results

Compounds    Tensile   Elongation    Mod.    Mod.    Mod.
            strength     at break    100%    200%    300%
               (MPa)          (%)   (MPa)   (MPa)   (MPa)

Reference        7.2          579    1.64    2.94    4.01
EPDM 1           7.6          666    1.67    2.90    3.94
EPDM 2           7.6          645    1.67    2.90    3.94
EPDM 3           8.0          619    1.88    3.22    4.26
EPDM 4           8.1          625    1.93    3.35    4.47
EPDM 5           7.6          616    1.70    3.01    4.07
EPDM 6           8.0          609    1.92    3.31    4.37
EPDM 7           8.1          600    1.87    3.30    4.40
EPDM 8           7.9          626    1.90    3.28    4.37
EPDM 9           7.4          637    1.63    2.87    3.91

Compounds      Hardness    Angular          CS 25%
            durometer A       tear          72h. @
                          strength   100[degrees]C
                            (kN/m)

            Inst.   15s

Reference      63    55       24.8             50%
EPDM 1         62    55       28.0             48%
EPDM 2         64    56       27.1             60%
EPDM 3         65    58       25.7             55%
EPDM 4         65    58       27.5             53%
EPDM 5         62    55       26.4             57%
EPDM 6         65    58       26.3             53%
EPDM 7         64    58       26.3             51%
EPDM 8         67    60       26.7             43%
EPDM 9         65    56       26.3             52%

Table 9--ingredient influence on properties

Feature                        MBT 80   CBS 80   ZDTP 50   TBP 75

Tensile strength                 0        0         0        +
Elongation at break              0        0         0        0
Modulus @ 100%                   0        0         0        +
Modulus @ 200%                   0        0         0        0
Hardness @ 15 sec.               0        0         0        0
CS 72 hours @ 100[degrees]C      +        0         +        +
Torque Mh-MI @ 185[degrees]C     0        0         0        ++
Time to scorch                   0        0         0        0
T 50% cure                       0        ++        ++       0
T 90% cure                       ++       0         +        ++

0 = no influence; + = weak influence;
++ = middle influence; +++ = strong influence

Table 10--other properties

                                Tear    Tensile   Mod. @
                            strength   strength      20%
                    Units       kN/m        MPa      MPa

TMTD system     Reference       24.7        7.2     0.74
Proposal with     3.1.1.1       28.9        7.6     0.92
  M+ TBP 75

Table 11--cost effect

In relative        Recipe   Curing        EPDM      Base +   Difference
values                      system        base   curatives
                              cost   (410 phr)

                                                      Cost            %
                                                 reference

TMTD system     Reference      3.5        96.5         100
Proposal          3.1.1.1      4.0        96.5       100.5          0.5
  with TBP 75