TPEs with low permeability, high damping.

TPEs with low permeability, high damping

A family of low permeability, high damping, dynamically vulcanized thermoplastic elastomers is now commercially available. Sold to the industry as Sarlink 2000, these materials exhibit moisture and gas permeability properties which are comparable to thermoset butyl rubber compounds. In addition, preliminary testing work indicates the material has excellent potential in applications requiring good vibration damping. The series is available in hardnesses ranging from 40 Shore A to 90 Shore A. All established grades can be both injection molded and extruded, with certain grades which can be processed using blow molding or blown film.

The 2000 series complements the two other product lines sold under the Sarlink trademark. The 1000 series demonstrates excellent oil and fuel resistance, abrasion resistance and bondability. The 300 series for multi-purpose use demonstrates a broad range of performance properties including high resiliency, low compression set, excellent weathering and moldability.

All series are available in a broad range of hardnesses and can be readily modified to meet specific end-use requirements. Figure 1 indicates how these materials are positioned against competitive thermoset materials.

Characteristics of the 2000 series

This family of TPEs may be characterized by four basic properties, namely: Morphology; rheology; thermo-mechanical properties and thermal properties.

Morphology

The morphology of the family has been studied by means of transmission electron microscopy. A continuous phase (white) appears which is a polyolefinic thermoplastic; dark stained particles appear which are saturated polyolefinic rubber. The rubber particles are in the order of 2.4 [mu]m while the staining effect shows the unusual structure of the dynamically vulcanized discontinuous elastomeric phase.

Rheology

The rheological properties, evaluated at 190 [degrees]C and at 220 [degrees]C on a Goettfert Rheograph 2001 capillary rheometer, are shown in figure 3. Significant deviation from Newtonian behavior is evident in the non-linearity of apparent shear rate and resultant shear viscosity.

At low shear rates (below 100 [sec.sup.-1]) the melt viscosity is high, while at high shear rates (above 1,000 [sec.sup.-1]), the apparent viscosity is lower by orders of magnitude. This phenomenon is generally known as |shear thinning' and results in improved processability and distinct economic benefits for the processor. In a fabrication process such as injection molding, the shear thinning effect enables the TPEs to be rapidly injected for fast mold filling and excellent definition and detailing of the mold cavity.

At the low (or zero) shear rates, the high viscosity results in melt integrity, shape retention and strength during cooling for early removal of the fabricated article for the cooled mold cavity.

The low (zero) shear rate/high melt viscosity effect is known generally as melt freezing, and when combined with shear thinning leads to short and economical injection molding cycle times.

Both phenomena are important in other plastic fabrication processes such as extrusion and blow molding. These lower shear activities are easier to control when the melt has higher viscosity or green strength.

It is also evident in figure 2 that shear rate is more significant in melt fabrication than temperature in the range of 190 [degrees]C to 220 [degrees]C. In controlling the melt fabrication of the TPEs, shear rate is the most important parameter in melt processing with the temperature effect relatively quite minor.

Thermo-mechanical properties

The thermo-mechanical properties of these TPEs are shown in figure 3 where the determinations were made at a frequency of 110 Hz. As can be seen in the trace over the testing temperature range, the level of tan [delta] (hysteresis loss) is both essentially constant and fairly high in the temperature range of approximately -50 [degrees]C to 140 [degrees]C. The level of tan [delta] at ambient temperature is indicative of a TPE having good damping.

The level and gradual fall-off of E' and E" indicates a reasonably soft material over the temperature range without significant hardening to -40 [degrees]C or modulus loss to + 140 [degrees]C.

From the Rheovibron data, it is concluded that the TPEs have significant low frequency (110Hz) damping properties, low modulus and a potential service range (dynamic) of about -40 [degrees]C to + 135 [degrees]C.

Thermal properties

The thermal behavior, as determined by differential scanning calorimetry (DSC), for the TPEs is displayed graphically in figure 4. As can be seen, the potential service range of the TPEs extends from about -50 [degrees]C to + 145 [degrees]C as indicated by thermal analysis using a Du Pont 9900 series thermal analyzer. This static service range is deduced from the [T.sub.g] and [T.sub.m] values obtained in the experimentation where the onset of melting of the continuous plastic phase occurs at 145.2 [degrees]C. The level of the heat of fusion (melting/ crystallization) at 11.03 J/g is fairly low but must be taken into account when melting the TPE during processing. It is also equally important when cooling from the melt stage during fabrication. Thus, heat transfer can be rapidly facilitated (e.g. mold cooling on injection molding) for rapid plastic fabrication techniques when forming.

Summary

As has been seen from the basic characterizing properties of this family of TPEs, the morphology of a discontinuous rubber phase in a continuous plastic matrix leads to several distinguishing properties:

* Melt is strongly non-Newtonian, displaying both melt freezing and shear thinning;

* Shear rate, not temperature, is prime parameter in melt behavior;

* Potential service ranges of -40 [degrees]C to + 135 [degrees]C (dynamic) and -50 [degrees]C to + 145 [degrees]C (static);

* High damping behavior seen over much of the service range.

Physical properties

Following the basic characterization of the new TPEs, testing has been undertaken using procedures more common to the rubber and plastic industries to yield |macro' property data. Comparisons have been made with a TPV of similar hardness as well as the standard butyl compound of ASTM D 3188, where the thermoset rubber compound was cured for 30 minutes at 160 [degrees]C when preparing test specimens.

The two especially useful properties which are apparent both from a knowledge of the heterogeneous blend and its characterizing basic properties of the TPEs are low rebound and permeability. The data from a comparative study with the materials are given in table 1.

Table : Table 1 - distinguishing properties

Goodyear Healey rebound (%) (ASTM procedure D 1054-87)

                   TPE 200   TPV-73A   ASTM D 3188 (butyl)
at 0 [degrees]C     31.5     42.0      16.9
at 23 [degrees]C    39.0     54.4      28.2
at 100 [degrees]C   56.4     69.6      64.2

Permeability

Product type                  Oxygen         Moisture (MVTR)
                              CC/100         GM/100 [IN.sup.2]/day
                              [IN.sup.2/day
Bromobutyl (pharmaceutical)     6.6               0.0196

(45 Shore A)

ASTM D3188 butyl compound       7.6               0.0370
2440 (40 Shore A)              50.5               0.0990
2450 (50 Shore A)              40.6               0.0875
2460 (60 Shore A)              24.8               0.0629
2170 (70 Shore A)               8.7               0.0296
2180 (80 Shore A)               8.8               0.0195
TPE * 3000 (60 Shore A)        41.0               0.0685
PP/EPDM TPV (73 Shore A)**    100.2               0.1530

Physical properties (general)

Many of the physical properties of the new family of thermoplastic elastomers are typical of those displayed by the TPV materials. In table 2 some of the general characterizing properties are given.

Table : Table 2 - general properties of the 2000 series

2440 2450 2460 2160 2170 2180

Stress strain

Unaged @ 23 [degrees]C hardness 40 50 60 65 75 85 (Shore A, 5 secs.)

100% Modulus MPa                   0.9    1.3    1.8    2.3    3.8    5.6
200% Modulus MPa                   1.5    2.1    2.7    3.8    4.6    6.4
300% Modulus MPa                   2.5    3.4    3.9      -      -      -
400% Modulus MPa                   4.0    5.2      -      -      -      -
Tensile strength, MPa              5.5    6.8    5.4    5.2    6.7    8.0
Elongation (%)                     490    470    385    280    268    272

The properties are retained at elevated temperatures (for easy mold removal in injection molding) and reflects the high viscosity of the TPE at high temperatures. A similar trend is seen in compression set data as shown in table 3.

Table : Table 3 - general properties

of series 2000 (ASTM procedure D 395-85)

Property Level

Compression set (%)

70 h/23 [degrees]C    20-25
70 h/70 [degrees]C    30-35
70 h/100 [degrees]C   35-40
70 h/125 [degrees]C   40-45

The air aging behavior of the new family of TPEs is shown in table 4, where changes in hardness and stress-strain properties are given. For easy comparison the unaged properties from table 2 are also shown. Rather surprisingly, the oil/fluid resistance of the TPEs are reasonable considering that they are composed of polyolefinic macromolecules. The data is given in table 5 for the volumetric changes after aging in the appropriate oils/fluids for the indicated time/temperature regime.

Table : Table - 5 oil/fluid resistance of series 2000

                      (ASTM D 471-79)
Oil/fluid     Test regime (h/[degree]C   Vol. change (%)
ASTM oil #1          70/100                   40-60
ASTM oil #3          70/100                  80-120
Water                24/100               0 [+ or -] 5

The TPE showed minimal change in hardness and stress/strain properties after accelerated aging for 2,000 hours in a Xenon arc weatherometer at an irradiance (at 340 nm) of 0.34 W/[m.sup.2] and including the wet cycle.

Conclusion

In conclusion, the unique combination of properties includes:

* Shear thinning/melt freezing on melt processing, e.g., short cycle times in injection molding;

* Broad service range - dynamic applications from -40 [degrees]C to + 135 [degrees]C. Static applications -50 [degrees]C to + 145 [degrees]C;

* Good air aging/weather resistance;

* Low compression set levels even at elevated temperatures;

* Reasonable resistance to both oils and aqueous fluids

* Good damping and low permeability

A diverse number of applications in the non-tire area are possible with the unusual combination of properties. These applications include automotive, construction and general rubber goods, where the economics of TPEs (thermoplastic processing techniques and material recycling) will broaden the spectrum of uses previously limited to butyl rubber vulcanizates, not the least being the possibility to color the new family of TPEs in the Shore A 50-80 range to any desired shade and hue.

Acknowledgements

"Low smoke, non-corrosive, fire retardant cable jackets based on HNBR and EVM," is based on a paper presented at the ACS Rubber Division meeting October 9-12, 1990. "A unique type of fluorocarbon elastomer," is based on a paper presented at the Energy Rubber Group meeting September 28, 1989. "TPEs with low permeability, high damping," is based on a paper presented at the SPE Antec meeting May 7-11, 1990. "Polyphosphazene elastomers in the oil field," is based on a paper presented at the Offshore Mechanics and Arctic Engineering conference February 18-22, 1990.

[Tabular Data Omitted]

PHOTO : Figure 1 - cost performance

PHOTO : Figure 2 - rheological properties

PHOTO : Figure 3 - thermo-mechanical properties at 110 Hz (Rheovibron)

PHOTO : Figure 4-DSC for TPE series 2000 (Du Pont 9900 series)