Viscoelastic characterization of different catalyst BRs.

Polybutadiene rubber is a homopolymer of 1,3-butadiene. It can be manufactured either by an emulsion or solution process, with the latter predominating. In the solution process, polybutadiene rubber (BR) is produced by both anionic lithium initiators and Ziegler Natta catalyst types. It is well known that there is a wide range of high cis polybutadiene rubbers, polymerized with alternative catalysts, offering different balances of properties. This is due to different manufacturing technologies developed by the different suppliers. The polybutadiene rubbers are in general characterized by microstructure (cis content) and macrostructure (molecular weight, molecular weight distribution and degree of branching). The aim of the high cis-BR producers is to attain the goal of 100% cis content, and thereby to achieve an almost perfect structure that enables self-reinforcing strain-induced crystallization like natural rubber (ref. 1).

The different technologies give rise to different levels of macrostructure and microstructure. It is reported that uranium catalyst was found to achieve the highest level of cis-l,4 linkages, but this development was not pursued because of possible radiation hazards. Polybutadiene rubber with neodymium and nickel catalyst gives linear macrostructure, and cobalt catalyzed polybutadiene rubber has a wide molecular weight distribution (MWD), along with a high level of branching (ref. 2).

Raw rubber is neither completely viscous nor elastic in nature, but rather viscoelastic in its properties. The term "characterization" means a different subject to different people. The most commonly used characterization method for gum rubbers is the Mooney viscosity. It has been for many years, and still is, almost the only processing parameter provided by the supplier in the certificate of analysis with each polymer lot. Both suppliers and users are utilizing the Mooney viscosity parameter as a specification tool. But, even identical Mooney values cannot guarantee the same type of microstructure and macrostructure of polymer, and also the same type of processing behavior (ref. 3).

To overcome the above problem, the RPA 2000 instrument is able to detect changes in structural features of polymers in a very short time. The changes made in the manufacturing technology are not normally part of the delivered specification and are measured only by lengthy and difficult laboratory analytical procedures with highly sophisticated tools like gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) (refs. 4 and 5).

In general, dynamic mechanical properties refer to responses to periodically varying strains or stresses. It has been reported that performing a short frequency sweep on the raw rubber in the RPA 2000 provides data that indicate long chain branching (LCB) and/or molecular weight distribution (MWD).

In this study, it was emphasized to differentiate one grade of polybutadiene rubber from another in terms of molecular architecture by understanding the viscoelastic profile. Also, the supplier's claim of using new technology in the product was verified.

Experimental

Materials

All the samples from different suppliers and with different catalysts were tested as received and are listed in table 1.

Equipment

The test equipment used included a Mooney viscometer and a rubber process analyzer, from Alpha Technologies.

Test conditions

Mooney viscosity, ML (1+4), was measured at 100[degrees]C. The frequency sweep test in the RPA was performed at 100[degrees]C temperature, keeping 14% strain level and 0.05 to 32 Hz frequency in logarithmic progression having 10 intervals.

Results and discussion

The Mooney viscosity values of polybutadiene rubbers with different catalyst systems were measured and are given in table 2. The tangent delta values of polybutadiene rubbers with different catalyst systems were measured in frequency sweep with the RPA and are given in table 3. Refer to figures 1-6 for frequency sweep tangent delta and tan delta difference for polybutadiene rubbers based on different catalyst systems.

[FIGURES 1-6 OMITTED]

The difference between tangent delta at low frequency and tangent delta at high frequency indicates variation in long chain branching (LCB) and/or molecular weight distribution (MWD).

* Low values indicate high LCB or broad MWD;

* High values indicate low LCB (linear polymer) or narrow MWD.

Variation in LCB and/or MWD of neodymium catalyst BR

From table 3 and figure 2, it is clear that among four Nd catalyst BRs, the difference in tangent delta in frequency sweep is in the following descending order: BR-2 > BR-4 > BR-3 > BR-1. BR-2 has the lowest LCB or narrow MWD (highest tan 8 difference), and BR-1 has the highest LCB or broad MWD (lowest tan [delta] difference).

Nd catalyst robbers are relatively linear, poor in processing and better in performance properties. However, BR-1 shows exceptionally high branching and/or broad molecular weight distribution to facilitate the easy processing and also to get the advantage of a linear polymer. The supplier of this grade has claimed that the rubber was developed with a Mooney jump reaction technology (ref. 6). The coupling effect as reported by the supplier due to the Mooney jump reaction was verified and can be seen in figure 2 and table 3.

Variation in LCB and/or MWD of nickel catalyst BR

From table 3 and figure 4, it is clear that among the three nickel catalyst BRs, the difference in tangent delta in frequency sweep was in the following descending order: BR-7 > BR-5 > BR-6.

BR-7 has the lowest LCB or narrow MWD (highest tan [delta] difference) and BR-6 has the highest LCB or broad MWD (lowest tan [delta] difference).

BR-6 indicates exceptionally broad molecular weight distribution (lowest tan [delta] difference) compared to the other two nickel catalyst BRs. This was supported by Castner's process (ref. 7). According to this process, the macro-structure of nickel-HF catalyzed BR can be shifted from linear to a branched one without altering the desirable high-cis microstructure.

Variation in LCB and/or MWD of cobalt catalyst BR

From table 3 and figure 6, it is clear that among the four cobalt catalyst BRs, the difference in tan 8 in frequency sweep for BR-11 and BR-10 was similar and higher than that of BR-8. The difference in tan 6 was found to be the lowest for BR-9.

All types

The LCB and/or MWD of all the types of BR with respect to tan [delta] difference was found to be as follows:

Low LCB or narrow MWD [right arrow] (increasing) [right arrow] high LCB or broad MWD;

BR-2 > BR-4 > BR-3 > BR-11 > BR-10 > BR-7 > BR-5 > BR-6 > BR-8 > BR-9 > BR-1.

BR-2 was found to have the lowest long chain branching or narrow molecular weight distribution (highest tan [delta] difference), and BR-1 had the highest branching or broad molecular weight distribution (lowest tan [delta] difference).

It is well known that a broad molecular weight distribution has a favorable influence on the processing properties of the polymer, and linear molecular weight distribution has a favorable influence in performance (ref. 8).

Conclusion

By separating BR-1 from other neodymium-BR, BR-6 from other nickel-BR and BR-9 from other cobalt-BR in terms of indication about LCB or MWD, RPA measurement enabled rapid characterization of raw polymers with little sample preparation time and also showed a high level of discrimination existing among polymers varying in the key elements of polymer structure. These polymer structure variations also have an important influence on processing and performance properties that are normally not exhibited from Mooney viscosity testing. By utilizing this indication, one can select the type of BR that satisfies the required processing and performance targets.

Table 1--all the samples from different suppliers and
with different catalysts were tested as received

                         Type of catalyst
SI. no.     Sample      used to manufacture

      1       BR-1              Nd
      2       BR-2              Nd
      3       BR-3              Nd
      4       BR-4              Nd
      5       BR-5              Ni
      6       BR-6              Ni
      7       BR-7              Ni
      8       BR-8              Co
      9       BR-9              Co
     10      BR-10              Co
     11      BR-11              Co

Table 2--Mooney viscosity, ML (1+4) @ 100[degrees]C, measured by MV
2000E

Neodymium catalyst type

 BR-1      BR-2      BR-3      BR-4
 45.0      44.2      41.3      42.4

Nickel catalyst type

 BR-5      BR-6      BR-7
 46.4      45.0      43.0

Cobalt catalyst type

 BR-8      BR-9     BR-10     BR-11
 42.3      49.1      45.8      42.4

Table 3--tangent delta at different frequencies measured by RPA 2000

Frequency,               Neodymium catalyst type
Hz                BR-1      BR-2      BR-3      BR-4

0.05             0.564     1.107     0.912     0.897
0.10             0.496     1.023     0.838     0.875
0.21             0.483     0.954     0.813     0.861
0.43             0.500     0.899     0.772     0.805
0.88             0.513     0.804     0.723     0.750
1.81             0.517     0.712     0.671     0.683
3.71             0.508     0.614     0.598     0.605
7.61             0.484     0.521     0.532     0.531
15.61            0.450     0.442     0.465     0.456
32.00            0.405     0.376     0.403     0.392
Tan [delta]      0.078     0.578     0.410     0.469
Difference        (L)       (H)

Frequency,           Nickel catalyst type
Hz                BR-5      BR-6      BR-7

0.05             0.744     0.653     0.851
0.10             0.690     0.602     0.773
0.21             0.647     0.571     0.731
0.43             0.618     0.538     0.692
0.88             0.581     0.512     0.649
1.81             0.546     0.489     0.600
3.71             0.510     0.456     0.552
7.61             0.456     0.424     0.498
15.61            0.419     0.390     0.449
32.00            0.366     0.360     0.399
Tan [delta]      0.281     0.211     0.332
Difference                            (H)

Frequency,               Cobalt catalyst type
Hz                BR-8      BR-9     BR-10     BR-11

0.05             0.625     0.553     0.750     0.784
0.10             0.598     0.530     0.743     0.760
0.21             0.605     0.512     0.736     0.750
0.43             0.615     0.519     0.732     0.733
0.88             0.609     0.515     0.688     0.692
1.81             0.591     0.517     0.638     0.644
3.71             0.555     0.504     0.574     0.577
7.61             0.512     0.475     0.506     0.516
15.61            0.463     0.445     0.441     0.451
32.00            0.413     0.406     0.383     0.392
Tan [delta]      0.192     0.106     0.353     0.358
Difference                            (H)       (H)

References

(1.) A.J.M. Sumner, G. Marwede and S.A. Kelbch, "Neodymium BR--the environmentally friendly future for polymer blends, "published in ITEC 96 Select.

(2.) A.J.M. Sumner, G. Marwede and S.A. Kelbch, "Neodymium BR advantages aid in butadiene rubber applications," Tire Technology International 1997.

(3.) N. Nakajima, "Viscoelastic characterization of gum rubbers," Kautschuk Gummi Kunstoffe 50. Jahrgang, Nr. 2/97.

(4.) H. Pawlowski and J. Dick, "Viscoelastic characterization of rubber with a new dynamic mechanical tester," Rubber World, June 1992.

(5.) Rubber process analyzer application notes on polymer characterization, by Alpha Technologies, 1999.

(6.) Bayer technical information: KA-FK-0258E/02/04.97.

(7.) Goodyear Chemical technical information: "The improved processing polybutadiene: Budene 1280."

(8.) S. Reissinger, "Properties and processing of polybutadiene," published in Leverkusen, den 24, March 1964.