Anti-degradant dose optimization through TGA decomposition kinetics study.

A single property by which a material can be identified is its thermal behavior. Knowledge of this property is essential, not only for the selection of proper processing and fabrication conditions, but also for the full characterization of the material's physical and mechanical properties. This is also true for polymers. Thermal methods are widely employed to characterize polymers. In fact, as soon as a polymer is synthesized in a laboratory, one of the primary properties scientists are interested in is its thermal property, because of its relation to other fundamental properties and the practical implications. Differential scanning calorimetry (DSC), thermogravametric analysis (TGA) and differential thermal analysis (DTA) measurements are widely employed in day-to-day analysis of polymers (ref. 1). Among these, thermogravametric analysis is one of the most widely used techniques for composition analysis of polymer and rubber products (refs. 2-4). TGA methods are also used to predict the thermal and thermo-oxidative stability of polymeric materials (refs. 5-10). It offers different promising approaches in determining kinetic parameters of degradation, as well. Thermogravimetric studies of decomposition kinetics of different natural rubber have been reported (refs. 11 and 12). Mondal (ref. 13) studied the thermal decomposition of a multi-walled nano-tube reinforced polyurethane membrane. Adhikari et al (ref. 14) used TGA and DSC to study the thermo-oxidative stability of butyl rubber in the presence of a coupling agent.

In the present study, the authors have used TGA decomposition kinetic analysis to find the optimum dose of TMQ anti-degradant in a natural rubber compound. TMQ is a well known anti-degradant for rubber compounds. The basic mechanism of the anti-degrading action is described elsewhere (ref. 15). TMQ prevents the thermooxidative degradation of the rubber. However, dose optimization is essential for achieving the best property and for cost optimization. Unfortunately, there is no shortcut way to study the effect of anti-degradant dose variation on thermal stability (aging property) of the rubber compound. Observing the percentage retention of the physical property after accelerated aging of the compounds can only give an idea about the optimum dose. Accelerated aging is carried out at higher temperature (70[degrees]C or 105[degrees]C) for one to six weeks. This article describes a method to use the TGA decomposition kinetic data to determine the optimum TMQ dose. The results from TGA kinetic studies were verified from the retention of the physical properties of compounds after aging.

Experimental

The decomposition kinetic study was carried out using a Pyris-1 TGA (Perkin Elmer). Initially, the sample was heated under a nitrogen atmosphere up to 600[degrees]C, and then the gas was changed to oxygen and the heating was continued up to 800[degrees]C. Four different heating rates were applied (10, 15, 20 and 30[degrees]C/min., respectively) for this study. The kinetic analysis was done for the degradation of the rubber part only (first decomposition step in nitrogen atmosphere). The second decomposition step in oxygen at higher temperature was due to the burning of carbon black. The material remaining after burning off the carbon black is the ash (metal oxide residue). A representative TGA thermogram of compound IV (scanning rate 10[degrees]C/min.) is shown in figure 1.

[FIGURE 1 OMITTED]

The compound formulation, with varying doses of TMQ, is given in table 1. This type of formulation is generally used as a fire tread compound.

The mixing was carried out in two stages, master and final. The master compound was mixed for six minutes at a rotor speed of 60 rpm, and the temperature control unit (TCU) of the mixer was kept at 90[degrees]C. The final batch was mixed for four minutes at a rotor speed of 30 rpm, and the temperature control unit (TCU) of the mixer was kept at 60[degrees]C. The batch weight was taken as 70 grams, and the mixed batches were milled on a laboratory two-roll mill (Santosh Industries).

Curing of tensile slabs was done using a compression molding technique in an electrically heated curing press from Hind Hydraulics at 160[degrees]C for 60 minutes. The tensile samples were prepared in accordance with ASTM D 412C. The stress-strain properties of the aged and the unaged samples were determined using a universal testing machine (Zwick UTM 1445) in accordance with ASTM D 412. The aging of the compounds was carried out in a hot air oven at 70[degrees]C for four weeks and six weeks.

Theory

The TGA scanning kinetic software program is based on the observations of Ozawa, Flynn and Wall (refs. 11, 12 and 16). They noted that the activation energy of a thermal event can be directly determined from a series of TG runs performed at different rates. A solid polymeric material is considered to be undergoing thermal decomposition according to the reaction:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

Where A is the material before conversion, B is the material after conversion, K is the reaction rate and [DELTA]H is the heat of physical transition or heat of chemical reaction.

In the chemical kinetic calculations, we are interested in the rate of change of A with time and its relation with temperature. If the decomposition starts at a temperature To and it is carried out by thermogravimetry at a linearly increasing temperature T = To + [beta] t, where [beta] is the constant heating rate ([beta] = dT/dt), then the thermal decomposition can be expressed as:

d[alpha]/dt = [beta] d[alpha]/dT = [(1-[alpha]).sup.n] Z exp (-Ea/RT) (2)

Where Z is the pre-exponential constant, Ea is the activation energy of the reaction, R is the universal gas constant and T is the absolute temperature in degrees Kelvin.

The equation generates one dependent variable t, two dependent variables [alpha] and T, three unknown constants Z, Ea and n, and the universal gas constant.

After following a number of mathematical steps, equation 2 can be transformed to the following equation:

ln [beta] = In (ZEa/R)--5.3305--1.0516 Ea/RT--In F([alpha]) (3)

where F([alpha]) is a power series expansion from the integration of the equation (ref. 16).

For a constant degree of conversion ([alpha] constant), the plot of ln[beta] versus l/T, obtained from thermograms recorded at several constant heating rates, [beta], should result in a straight line whose slope is approximately -1.0516Ea/R. The quality of the statistical fit can be judged from the 95% confidence limits and from observing the ln[beta] versus 1/T plot. In an ideal case, the data points fall on parallel straight lines, the slope of which is used to calculate the activation energy. Other kinetic parameters can also be derived from the expanded form of the above equation 3.

Results and discussion

The beginning and the end parts of rubber degradation follow the zero or the pseudo zero order decomposition kinetics (ref. 12). The beginning and the end parts for a compounded rubber are even more complex. The early stage of degradation of a compounded rubber includes the volatilization and degradation of the low molecular weight processing ingredients, such as aromatic oil, wax, un-reacted sulfur, etc. To avoid such complicity, the 25% and 40% conversion (degradation) levels were taken as the representative points to find out the optimum activation energy. The activation energy of the different compounds at the 25% and 40% conversion levels is shown in table 2. A representative TGA thermogram of compound IV with different heating rates is shown in figure 2.

[FIGURE 2 OMITTED]

The un-aged and aged tensile strength and elongation at break of the compounds are given in table 3.

The relative drops in tensile strength (TS) and elongation at break (EB) after aging are plotted in figure 3 and figure 4 respectively. The relative retention of the physical properties (tensile strength and elongation at break) reached the optimum level in compounds IV and V.

The activation energies for 25% and 40% conversion (degradation) of different compounds are plotted in figure 5. From the activation energy plot, it is clear that the activation energy for 25% and 40% conversion (degradation) reached the optimum level in compounds IV and V.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

From the above observation, it can be concluded that the optimum level of TMQ for this particular compound is around 1 to 2 phr (parts per hundred parts of rubber).

Conclusion

This study shows that the TGA decomposition kinetics can be used to find out the optimum dose of anti-degradants. The time requirement for the decomposition kinetics study is less compared to the conventional aging study. However, to get accurate data, one should carry out the experiment at a low scanning rate. It is also required to complete the whole test in a single slot to minimize the error. However, a very minute dose variation of the anti-degradant may not be reflected in the TGA decomposition kinetic study.

[FIGURE 5 OMITTED]

References

(1.) Thermal Characterization of Polymeric Materials, 2nd Edition, Edith A. Turi, editor, Academic Press, San Diego, California, USA (1997).

(2.) S. Knappe and B. Fidler, "Rubber analyis made more accurate and informative by special TGA techniques," Rubber World, vol. 225, no. 4, pp. 33-34, January 2002.

(3.) B.K. Roy, J. Bhatt, A.K. Chandra and P.K. Mohamed, "Application of thermal analysis in rubber industry," Rubchem Review, May-June 2004, pp. 17-23.

(4.) I.A. Amraee, A.A. Katbab and Sh. Aghafarajollah, "Qualitative and quantitative analysis of SBR/BR blends by thermogravimetric analysis," Rubber Chem. & Tech., 69 (1), pp. 30-136 (1996).

(5.) Zhenguo Chi, Xiandong Yao, Yi Zhang and Jiarui Xu, "Thermal decomposition of thermotropic liquid crystalline polyesterimides," J. Appl. Poly. Sci., 98 (6), pp. 2,467-2,472 (2005).

(6.) L Ray, S. Roy, T.K. Chaki, D. Khastgir, "Studies on ther mal degradation behavior of EVA/LDPE blend," J. Elastomers & Plastics, 26 (2), pp. 168-182 (1994).

(7.) J.Y. Lee, M.J. Shim and S.W. Kim, "Thermal decomposition kinetics of an epoxy resin with rubber-modified curing agent," J. Appl. Poly. Sci., 81 (2), pp. 479-485 (2001).

(8.) Sunan Saikrasun, "Thermal decomposition behavior of in-situ reinforcing elastomer composite based on thermoplastic elastomer and thermotropic liquid crystalline polymer," J. Elastomers & Plastics, 39 (3), pp. 213-238 (2007).

(9.) Ante Agic, Emi Govorcin, Bajsic and Vesna Rek, "Kinetic parameters estimation for thermal degradation of polyurethane elastomers," J. Elastomers & Plastics, 38 (4), pp. 105-118 (2006).

(10.) S.C. Lin and E.M. Pearce, "Epoxy resins L The stability of the epoxy-trimethoxyboroxine system," J. Appl. Poly. Sci., 23 (11), pp. 3,355-3,374 (1979).

(11.) R.M. Moreno, E.S. de Medeiros, F.C. Ferrira, N. Alves, P.S. Goncalves and L.H.C. Mattoso, "Thermogravimetric studies of decomposition kinetics of six different IAC Hevea rubber clones using Flynn-Wall-Ozawa approach," Plastics, Rubber and Composites, 35 (1), pp. 15-21 (2006).

(12.) E.S. de Medeiros, R.B.M. Moreno, F.C. Ferrira, N. Alves, A.E. Job, P.S. Goncalves and L.H.C. Mattoso, "Thermogravimetric studies of the decomposition kinetics of four different Hevea rubber clones using Ozawa's approach," Progress in Rubber, Plastics & Recycling Technology, 19 (4), pp. 198-204 (2003).

(13.) S. Mondal, "Thermal degradation study of functionalized MWNT reinforced segmented polyurethane membrane," J. Elastomers and Plastics, 26 (2), pp 168-182 (1994).

(14.) A. Adhikari and R. Mukhopadhyay, "Effect of silane coupling agent (Si-69) on thermooxidative stability of butyl rubber compounds," presented in IRC '92, Beijing, China, Oct, 1992.

(15.) Encyclopedia of Polymer Science and Engineering, 2nd Edition, Vol. 2, pp. 73-91, John Wiley & Sons, New York, USA (1985).

(16.) J.H. Flynn, Aspects of Degradation and Stabilization of Polymers (ed. H.H.G. Jellinek), Ch. 12, pp. 573-615, Elsevier, New York, USA (1978).

by S. Chakraborty, M. Debnath, S. Dasgupta and R. Mukhopadhyay, HASETRI, and S. Bandyopadhyay, JK Tyre & Industries ([e-mail])

Table 1--compound formulation

Master:                                                        Master

Materials (parts per 100 parts of rubber, phr)
Natural rubber (NR)                                               100
Zinc oxide (ZnO)                                                  4.5
Stearic acid                                                      3.0
Carbon black                                                       50
Aromatic oil                                                       10

Final:
Materials (phr)          I     II    III     IV      V     VI     VII
Master               167.5  167.5  167.5  167.5  167.5  167.5   167.5
TMO                    --     0.1    0.5    1.0    2.0    4.0     7.0
Sulfur                2.25   2.25   2.25   2.25   2.25   2.25    2.25
Accelerator, N-oxy-    1.0    1.0    1.0    1.0    1.0    1.0     1.0
  diethylene benzo
  thiazyl
  sulfonamide
Pre-vulcanized        0.20   0.20   0.20   0.20   0.20   0.20    0.20
  inhibitor,
  cyclohexyl
  thiophthalamide

Table 2--activation energy at 25% and 40%
conversion (degradation) level

Compounds      Activation energy          Activation energy
                (KJ/mole) at 25%           (KJ/mole) at 40%
            conversion level, n = 1,   conversion level, n = 1,
              confidence level 95%       confidence level 95%

I                     108                       121.1
II                   129.4                      141.0
III                  143.1                      157.0
IV                   155.0                      175.0
V                    156.0                      177.0
VI                   157.0                      182.0
VII                  155.0                      175.0

Table 3--unaged and aged (two weeks) and
[four weeks] tensile strength and elongation
at break properties of the compound

Samples   Tensile strength (MPa)   Elongation at break (%)

I         22.75 (17.95) [12.15]     555.0 (421.5) [321.2]
II        23.55 (19.65) [16.20]     554.0 (446.5) [392.4]
III       22.80 (21.00) [18.20]     579.0 (488.0) [447.0]
IV        22.00 (20.70) [21.10]     582.0 (479.5) [462.4]
V         22.50 (21.25) [20.80]     542.5 (450.3) [439.4]
VI        23.05 (21.40) [20.14]     617.5 (529.0) [491.4]
VII       22.30 (20.30) [18.10]     646.0 (545.0) [495.5]