Compound based on post-modified EP(D)M with mineral fillers: a rheological study.

An experimental study was carried out on ethylene propylene copolymer based compounds for the fields in which extrusion based formulations of mineral fillers are used. The products obtained through extrusion reactive methodologies (Polimeri Europa patented) have been evaluated in comparison with traditional polymers.

These technologies, developed up to now on a laboratory scale, allow tailoring the rheology of the product for the specific application. Besides this, a recent technological evolution allows for the improvement of the economics and productivity of the process and the compatibilization between mineral fillers and elastomers.

EPDM or EPM based elastomeric compounds are used in various fields in which mineral filler based compounds are extruded. This is the case of compounds for wire insulation or for building profiles. The optimal rheology for these applications consists in obtaining on one hand the best fluidity in extrusion without affecting shape stability; on the other hand, the best shape stability without affecting fluidity in extrusion.

Systems to improve the relationship between shape stability and fluidity of the material operating mainly on the molecular weight distribution (mwd) and on chain branching are known. The mwd broadening improves the drop of viscosity in shear (shear thinning). Nevertheless, the broadening of the mwd in copolymerization processes (or terpolymerization processes), such as the Ziegler-Natta type, worsens the monomeric unit distribution and leads to fractions with low molecular weight and low ethylene content, which can give an unwanted stickiness and a slower vulcanization rate.

Chain branching can be obtained using radical initiators such as the well known peroxides used in EP(D)M vulcanization. The peroxide treatment can give partial crosslinking and increase the molecular weight.

This study, carried out on a laboratory scale, applies technologies of reactive extrusion to adapt standard polymers to the rheology required by these and other applications. In particular, a peculiar characteristic of the Polimeri Europa proprietary post-modification process is the contemporary improvement of the fluidity and the shape stability of the polymeric base submitted to treatment.

In order to balance the effect of the reactive extrusion cost, the company has developed a technological evolution of preparing a post-modified master containing polymer and filler, which improves the economics and productivity of the mixing process.

In the reactive extrusion process, the product rheology is modified without changing the compositional distribution, therefore avoiding the typical worsening of broad mwd products (stickiness and lower vulcanization rate) (refs. 1-4).

Experimental

All the polymers used were derived from the process of traditional slurry polymerization. They are commercial or experimental EPM and EPDM Polimeri Europa products. The post-modification processes have been carried out by a twin-screw extruder at high temperature, with an output of 5-8 kg/h.

The compounds were prepared according to well-known methodologies using an internal mixer (1,600 cc). The compounds containing 50 parts of filler were prepared by first inserting the polymer and then the other components (direct method). Those with more filler were prepared by first inserting the filler and the plasticizers, and then the polymer (upside down method).

The compound Mooney viscosity test and the Mooney stress relaxation test (parameters were calculated on the range of 1.6-5 seconds) have been carried out according to ISO 289 (ASTM D 1646) norms.

The dynamic mechanical tests were carried out with the RPA 2000 instrument (Alpha Technologies). The frequency sweeps were carried out with 14% strain, and the strain sweeps with a frequency of 1.2 rad/sec. For the cable insulation, tests were carried out at 110[degrees]C, while for the building profile they were carried out at 100[degrees]C.

The rheology tests under extrusion conditions have been conducted with the Extrusiometer instrument of Goettfert, with a screw temperature of 100[degrees]C and die temperature of 110[degrees]C. Under stable conditions, the extrusion output can be measured, and from this, the apparent shear rate is calculated.

The difference between the pressures measured along the slit die allows the shear stress to be calculated. With the shear stress and the shear rate corrected according to Rabinowitsch/Weilssenberg, the true viscosity is calculated.

Compounds for electrical wire insulation

The requested rheological properties for this application have to guarantee good fluidity, to allow for good extrusion output and good coating of the metallic cable. Moreover, they have to have good hot shape stability to prevent collapse at the exit of the extruder and avoid forming an undesirable oval shape. This aspect is more critical when the voltage is higher, and the wire coating is thicker.

Three different compounds have been prepared using the following polymers:

* EPDM 1--standard EPDM--high ethylene content (73%) with low molecular weight, having a ML(1+4) 100[degrees]C = 30;

* EPM B--standard EPM--high ethylene content (73%) with high molecular weight, having a ML(1+4) 125[degrees]C = 60; and

* PSM 1012--EPM--high ethylene content (73%) with low molecular weight, having a ML(1+4) 100[degrees]C = 26, obtained by post-modification of copolymer EPM B.

Compound 1021/FC/01 was prepared in an internal mixer by direct method using EPDM 1.

Compound 1021/FC/02 was prepared in an internal mixer by direct method employing the post-modified polymer PSM1012.

Compound 1021/FC/05, based on EPM B, was prepared applying the new evolution of Polimeri Europa post-modification technology, which reduces the mixing cost (ref. 3).

The composition for all three compounds is identical. The compounds contain, as main ingredients, 50 parts of calcined clay and 5 parts of paraffin wax per 100 parts of rubber.

The Mooney viscosity and Mooney stress relaxation parameters [slope and MLRA(area)/ML] are reported in table 1 (ref. 5). It is evident that, despite the lower Mooney viscosity, the 1021/FC/02 compound has a higher elasticity (lower slope, higher MLRA/ML) and better shape stability than the reference compound (1021/FC/01). According to the Mooney stress relaxation parameters, this better relationship between the Mooney viscosity and the shape stability was reached by applying both of the post-modification processes.

The dynamic mechanical tests (frequency sweep at 110[degrees]C and 14% strain) have been performed on these compounds using the RPA 2000 instrument (ref. 6). The results are reported in figure 1. As is known, the complex viscosity vs. frequency (rad/s) plot may give a similar behavior to the apparent viscosity vs. shear rate plot (ref. 7).

[FIGURE 1 OMITTED]

The Mooney stress relaxation results were well confirmed by the RPA 2000 test. The 1021/FC/02 and 1021/FC/05 compounds have lower viscosity at the frequencies related to the extrusion process (10-100rad/s). At low frequencies that relate to the shape stability, the post-modified compounds have higher viscosity.

The tan [delta] vs. frequency plot shown in figure 2 was derived from the same RPA 2000 frequency sweep experiment (110[degrees]C, 14% strain). It has been reported that the Mooney relaxation slope and MLRA/ML are related to the tan [delta] measured at about 0.1 rad/s (low tan [delta] = low slope = higher MLRA/ML) (refs. 8-11). This was confirmed by the plots in figure 2, where the 1021/FC/02 and 1021/FC/05 compounds have a much lower tan [delta] than 1021/FC/01 at low frequency. Similar to complex viscosity, the tan [delta] plot indicates a higher elasticity (better shape stability) at low frequency and a lower elasticity (better workability) at medium-high frequency for the post-modified compounds (1021/FC/02 and 1021/FC/05).

[FIGURE 2 OMITTED]

A processability and rheology test under extrusion conditions was performed using the Goettfert Extrusiometer MP 030 instrument (figure 3). This apparatus is essentially a laboratory scale single screw extruder with a 30 mm screw diameter and L/D = 20. It has four pressure transducers along the screw (P10-P20), one on the extruder head (PMR) and two along the rheometer slit die (PE and PA). Corresponding to each pressure transducer, there is a melt temperature sensor (TM10-TMA). The tests were carried out according with the ASTM D5099 method B.

[FIGURE 3 OMITTED]

The test data could be recorded and used to evaluate the extrusion process parameters. The output vs. screw speed (rpm) plot is reported in table 2 for all three compounds. The post-modified compounds (1021/FC/02 and 1021/FC/05) have better fluidity in extrusion conditions. It is also remarkable that the 1021/FC/05 compound (ML = 30) has a much higher output than the reference one (ML = 28).

However, the work done with a laboratory scale extruder may not be exactly representative of actual industrial conditions (e.g. too reduced scale, differences in die geometry, etc.). Therefore, a rheological evaluation of the material was carried out to overcome any doubts of the test validity and also to make better compound characterizations. The rheological evaluation allows the calculation of shear stresses, shear rates and viscosities from each test point. The real viscosity vs. shear rate plot is reported in figure 4. The good agreement to the RPA 2000 test is evident (ref. 7).

[FIGURE 4 OMITTED]

It is confirmed that both the compounds obtained through the Polimeri Europa proprietary processes (1021/FC/02 and 1021/FC/05) are characterized by a better rheology than the ones obtained with a standard polymer. As a matter of fact, both of the post-modification products have a better fluidity in extrusion conditions and better shape stability with whichever method is used to evaluate the rheology.

Table 3 lists the power law index, N, calculated on the basis of the well known power law model (ref. 12):

[tau] = K [([??]).sup.N]

There are considerable differences in shear thinning (index N) between the reference compound and the ones obtained through the post-modification technology developed by Polimeri Europa. There are no meaningful differences between the post-modified compounds.

To better define the characteristics of the compounds obtained through the different post-modification technologies, green strength tests have been carried out on the raw compounds (ref. 13). These compounds are characterized by the same polymeric basis and formulations. Therefore, any evident difference between these products is derived exclusively from the process used to produce them. The results are reported in table 4. According to these data, a large difference in mechanical properties is detected. It is quite surprising to find such a difference in compounds having identical composition. Therefore, the reason for such a difference should be due to the post-modification process. In other words, the recent technological evolution of the Polimeri Europa post-modification process produces compounds having better filler dispersion and elastomer-filler cohesion.

Compounds for building profile

The EP(D)M based compounds which are used in the building profile application field are mostly extruded and vulcanized with salt bath units (LCM). This vulcanizing technology allows the use of EPM copolymers, which have better stability and aging properties, but a worse rheology behavior. More specifically, this study is related to the flame retardant building profile field, where the use of large amounts of non-reinforcing inorganic filler (aluminium hydroxide) could affect the stickiness, the cohesion and the theology of the compound.

Two compounds were prepared having the following composition (for 100 parts of rubber):

* 40 parts of silica;

* 140 parts of aluminium hydroxide;

* 40 parts of paraffin oil; and

* minor amounts of other components such as coupling agent, zinc oxide and stearic acid (always in the same amount for both compounds).

Compound 1046/FP/02 is based on an EPM copolymer (EPM B) having high ethylene content (73% wt.) and ML(1+4) 125[degrees]C = 60, prepared applying traditional technology.

Compound 1046/FP/04 is based on the same copolymer as 1046/FP/02, but prepared applying the new Polimeri Europa post-modification technology.

The 1046/FP/02 and 1046/FP/04 Mooney viscosity, Mooney stress relaxation, raw and vulcanized compounds tensile tests are reported in table 5. The 1046/FP/04 raw compound has a much higher tensile modulus at 80% elongation (green strength) than 1046/FP/02. The post-modified compound has a lower elongation at break, probably a polymer-filler interaction effect giving higher short deformation strength, which is lost at higher elongation. The difference in reinforcing effect between the traditional (1046/FP/02) and post-modified (1046/FP/04) compounds remains evident after vulcanization.

Despite the lower Mooney viscosity, the 1046/FP/04 compound has higher elasticity than 1046/FP/02, as indicated from the Mooney relaxation parameters (slope, MLRA and MLRA/ML).

In order to confirm the rheological evaluation deriving from the Mooney relaxation test, the dynamic-mechanical tests were performed using a RPA 2000 instrument. The frequency sweep tests were carried out at 100[degrees]C and 14% strain. The complex viscosity, tan [delta] and G' curves vs. frequency are reported in figures 5-7.

[FIGURES 5-7 OMITTED]

The Mooney stress relaxation parameters were confirmed by the RPA 2000 tests. The 1046/FP/04 compound shows higher viscosity and elasticity than the reference compound at low frequency (better shape stability). At high frequency, 1046/FP/04 has lower elasticity than the reference compound, but, in contrast with the Mooney viscosity value (as reported in table 5), it still has a bit higher complex viscosity.

The former observation could be due to the high polymer-filler interaction, characterizing the post-modified compound, which could give a high reinforcing effect at low deformation not detectable when tested with a high deformation method (e.g., Mooney).

All the processing behaviors are related to high deformation phenomena, such as the flow in extrusion or in mixing or in injection. To deeply analyze the matter, an RPA 2000 strain sweep test series (1-1,200%) was carried out at 100[degrees]C and 1.2 rad/s frequency. The complex viscosity vs. strain plot is reported in figure 8. The tan [delta] and G' vs. strain plots are reported in figures 9 and 10.

[FIGURES 8-10 OMITTED]

It is now evident that, due to the polymer-filler interaction loss (ref. 14), the elasticity and viscosity drop is higher for the post-modified compound, 1046/FP/04. The 1046/FP/04 compound has, therefore, better workability (lower elasticity) at high strain (flow condition), but more consistency than the reference compound at low strain.

If the RPA 2000 frequency sweep is combined with the RPA 2000 strain sweep experiments, it is found that the 1046/FP/04 compound has an advantageous rheology for the extrusion process.

Hence, 1046/FP/04 is characterized by:

* Better filler reinforcing effect (higher polymer-filler interaction) and better cohesion of raw compound;

* better fluidity and workability in extrusion flow condition; and

* better shape stability.

Processability and theological evaluation tests were attempted using the Extrusiometer, but no significant data could be obtained at a Mooney viscosity higher than 60 at 100[degrees]C due to the die geometry.

In order to carry out processability tests, two compounds having lower Mooney viscosity were prepared, including:

* Compound 1047/FP/01 with standard technology using an EPM having a high ethylene content (73%) and ML(1+4) 125[degrees]C = 24 (EPM A); and

* compound 1047/FP/05 with the same technology and condition as 1046/FP/04, but using 55 parts of oil instead of 40.

Although the 1047/FP/05 compound has higher oil content than the reference compound, this comparison is still significant due to the better reinforcing effect (described above) for the post-modified compound, as confirmed by Mooney relaxation parameters and tensile tests (table 6).

The processability tests were performed using the Extrusiometer. The 1047/FP/05 compound evaluation was carried out starting from a screw speed of 6 rpm. According to the maximum pressure allowed for the die and head positioned transducers, a screw speed of 12 and 20 rpm was selected as further test points. Each test point was carried out without any problem in stable conditions and obtaining a smooth and regular extrudate.

The 1047/FP/01 compound evaluation was carried out starting from 4 rpm due to the high die pressure observed during a 6 rpm extruder filling up.

The 4 rpm test point was carried out in stable conditions, obtaining a smooth and regular extrudate. The screw speed was then pointed to 6 and 8 rpm, still obtaining die pressures near the maximum allowed for the instrument and deep elastic distortion phenomena (ripples) of surface extrudate (ref. 15), as shown in figure 11.

[FIGURE 11 OMITTED]

The extrudate ribbons picture of each of the six test points is shown in figure 11. There is an evident distortion in surface of the 1047/FP/01 compound extrudates, which was not detectable for the 1047/FP/05.

Since it was not possible to compare the 1047/FP/01 and 1047/FP/05 compounds at the same screw speed, the output vs. head pressure plot is reported in figure 12. In figure 13, the shear stress vs. apparent shear rate plot is reported.

[FIGURES 12-13 OMITTED]

The test points which are characterized by an evident surface extrudate distortion are over the "critical shear stress," as shown by the discontinuity on the flow curves. The 1047/FP/01 extrusion test flow conditions are not stable like 1047/FP/05, but are in an elastic turbulent state (ref. 15). Obviously, the shear rate correction according to Rabinowitsch/Weissenberg is not possible in this condition for 1047/FP/01. The rheological evaluation of 1047/FP/05 gave a power law index (N) of 0.25.

The critical rheological characteristics of compounds containing large amounts of non-reinforcing fillers are evident. However, using post-modification technologies, it is possible to overcome some of these problems.

Using the frequency and strain sweep RPA 2000 tests, it is possible to observe this residual elasticity in flow condition. The tan [delta] and G' vs. frequency plots are reported in figures 14 and 15. The G' vs. strain plot is reported in figure 16. The 1047/FP/01 residual elasticity in flow condition is easily detectable at high frequency and high strain conditions.

[FIGURES 14-16 OMITTED]

Conclusions

As a result of the laboratory scale activities, it has been possible to apply the Polimeri Europa proprietary post-modification technology in order to obtain compound processability advantages in the wire insulation and building profile applications.

The post-modified compounds have shown better shape stability and extrusion fluidity relationships (shear thinning) than the reference ones, whichever rheological evaluation test methods were used.

Although the results are only related to two well-described applications, they are obviously attainable in other fields in which the high shear thinning is required.

In addition, by applying a Polimeri Europa post-modification technology evolution, it is possible to reduce the mixing process cost. These post-modified compounds are characterized by an improvement in the filler-reinforcing effect. This improvement is evident and advantageous when a large amount of non-reinforcing filler is present in the formulation.

References

(1.) T. Tanaglia (to Polimeri Europa S.p.A); Eur. Patent 1 471 082; (October 27, 2004).

(2.) T. Tanaglia (to Polimeri Europa S.p.A); Eur. Patent 1 604 999; (December 12, 2005).

(3.) T. Tanaglia (to Polimeri Europa S.p.A); Eur. Patent 1 661 920; (May 31, 2006).

(4.) N.R. Dharmarajan, P.S. Ravishankar and CD. Burrage (to Exxon Chemical); U.S. Patent 5,952,427; (September 14, 1999).

(5. ISO 289-4, Rubber, unvulcanized--determination using a shearing disk viscometer--part 4: determination of the Mooney stress-relaxation rate.

(6.) H. Pawlowski and J. Dick; Rubber World; 206 (June 1992); p. 35.

(7.) W.P. Cox and E.H. Merz; J. Polym. Sci.; 28(1958); p. 619.

(8.) N. Vennemann, S. Lupfert; Kautschuk Gummi Kunststoffe; 44 (1991); p. 270.

(9.) H.C. Booij; Kautschuk Gummi Kunststoffe; 44 (1991); p. 128.

(10.) P.A.M. Steeman and J.H.M. Palmen; J. Appl. Polym. Sci.; 74 (1999); p. 1,220.

(11.) M. Demaio, S. Caldari and O. Chiantore; Int. J. Polym. Analyt. Charact.; 5 (1999); p. 85.

(12.) J.L. Leblanc; Rheologie des elastomeres; Edition Artel (1996).

(13.) ISO 9026, Raw rubber or unvulcanized compounds--determinations of green strength (1996.).

(14.) J.L. Leblanc; Prog. Polym. Sci.; 27 (2002); p. 627.

(15.) J.L. Leblanc; Rubber Chem, and Tech.; 54-5 (1981); p. 905.

by Tiziano Tanaglia, Polimeri Europa ([e-mail])

Table 1--Mooney viscosity and relaxation test

Compound                  1021/FC/01   1021/FC/02   1021/FC/05
ML (1+4) 100[degrees]C            28           24           30
Slope                           1.14         0.58         0.57
MLRA/ML                         0.80         5.70         5.60

Table 2--Extrusiometer processability test at 100[degrees]C

Compound                  1021/FC/01   1021/FC/02   1021/FC/05

Output at 4 RPM (kg/h)          0.44         0.50         0.49
Output at 6 RPM (kg/h)          0.65         0.79         0.74
Output at 12 RPM (kg/h)         1.21         1.60         1.51

Table 3--power law index evaluation

               N

1021/FC/01   0.43
1021/FC/02   0.31
1021/FC/05   0.32

Table 4--green strength test of raw compounds

               1021/FC/02   1021/FC/05

Ml00% (M Pa)          1.5          2.3
M200% (MPa)           1.4          2.4
T.S. (MPa)            1.3          2.3
E.B. (%)              245          345

Table 5--characterization of flame retardant building profile compounds

Compound                  1046/FP/02   1046/FP/04

ML (1+4) 100[degrees]C            76           63
Slope                           0.61         0.34
MLRA                           372.4        611.1
MLRA/ML                          4.9          9.7
Stickiness               Very sticky   Not sticky
M80 (MPa)                        0.7          1.3
E.B. (%)                       2,000           90

Cured compound
M100 (MPa)                       1.4          2.9
M200 (MPa)                       1.8          4.5
M300 (MPa)                         2          5.5
T.S. (MPa)                       5.3          6.7
E.B. (%)                         810          440

Table 6--characterization of flame retardant building profile compounds

Compound                   1047/FP/01        1047/FP/05
Base polymer                    EPM A             EPM B
Oil (phr)                          40                55
ML (1+4) 100[degrees] C            52                30
Slope                            0.80              0.62
MLRA                             93.6               114
MLRA/ML                           1.8               3.8
Stickiness                Very sticky   Slightly sticky
M100 (MPa)                        0.7               0.8
E.B. (%)                        1,500               120
Cured compound
M100 (MPa)                        1.5               1.4
M200 (MPa)                        1.9               2.1
M300 (MPa)                        2.1               2.8
T.S. (Mpa)                        6.3               5.1
E.B. (%)                          870               670