EPDMs for automotive sponge products.

This article is concerned with the selection of EPDM polymers for producing continuous cured sponge extrusions for automotive type applications. The first part of the discussion will address identifying the key polymer properties that are necessary to select the polymer type for a given application. We will then contrast this with the polymer selection process according to a new technology.

The types of continuous curing methods available are microwave, hot air, LCM (salt bath) and fluidized bed. The curing times required are normally in the 1.5 to 2.5 minute range and the average product temperature achieved is 200[degrees]C.

EPDM polymer selection

Sponge compounds proceed through three stages of curing. The first is the precure where the temperature is controlled so that about 30% of the cure is established, but no blowing agent decomposition occurs. This establishes a modulus in the cell walls as they are formed which is strong enough to withstand the gas pressure from the decomposition of the blowing agent without rupturing, thereby forming a closed cell network. The second stage is the expansion and final cure where the temperature is increased so that the blowing agent completely decomposes, forming the cell structure, and the final state of cure is also reached. The third stage is normalization where the temperature is maintained at a high enough level so that the excess gas pressure in the cell structure is rapidly removed by diffusion. This is necessary to control the shrinkage of the final product and to insure the best compression set results.

With the short cure times involved to maintain the desired productivity, an ultra fast curing polymer is required for developing the specified physical properties, especially compression set, within this time constraint. The first polymer property requirement then is high unsaturation, which means that the third monomer of the EPDM needs to be ethylenenorbornene and the content has to be 7.5% minimum. The ethylenenorbornene type of unsaturation is more active in combining with the sulfur and accelerators than dicyclopentadiene (DCPD) or 1,4 hexadiene. The ultra fast cure rate will achieve a rapid development of physical properties, such as compression set, by establishing a higher crosslink density in a shorter time frame. It will also develop a smoother cured skin surface by providing a higher state of cure previous to the blowing agent decomposition. It will more effectively contain the gas pressure within the cell structure without rupturing, thereby providing lower expanded densities. The higher unsaturation level also creates less chance for surface bloom to occur because there are mote sites to accept the sulfur and accelerators. Higher productivity, with extrusion rates averaging 20 meters/minute, can be achieved.

The second most important polymer property to be considered is the ethylene/propylene ratio. If there are no low temperature requirements, higher ethylene/propylene ratios up to 75/25 can be used. The higher more crystalline type E/P ratios can provide the following benefits:

* Higher green strength;

* uncured cross-sectional shape stability at processing temperatures;

* accepts higher filler and plasticizer levels for lower cost compounding;

* excellent physical property development;

* fast mixing cycles--generates high shear rate for good dispersion properties;

* does not require oil extension; and

* less restricted for Mooney viscosity;

Precautionary measures include:

* Always use the friable bale form; and

* avoid polymer exposure to cold temperatures.

If low temperature properties are required, which is often the case for automotive sponge applications, the E/P ratio usually has a 60/40 maximum. However, this lower ethylene content creates problems concerning loss of green strength, higher compound cost (due to lower loading capabilities) and a reduction in physical properties such as tensile, tear strength and abrasion resistance. Mixing cycles are also usually longer. These problems can be overcome by going to a higher molecular weight (5.0 x [10.sup.5] to 7.5 x [10.sup.5]). The higher molecular weight will provide the following benefits without sacrificing the low temperature resistance:

* Provides the required green strength unrestricted by temperature;

* improves abrasion resistance;

* improves wrinkle resistance of cured skin surface;

* increases tensile and tear properties;

* allows more rapid mixing cycles;

* provides the ability to achieve super soft load deflections:

* improves compression set by developing stronger cell walls for faster recovery; and

* accepts higher filler and plasticizer levels for lower cost compounding.

Higher molecular weight polymers usually are oil extended to obtain more reasonable polymer Mooney viscosities for improving mixing and processing characteristics.

The third polymer property to be considered is molecular weight distribution (mwd), whether it's narrow, broad or medium. Narrow molecular weight distribution develops more cure rate efficiency from the amount of unsaturation available. It therefore provides fast cure rates and higher final states of cure, resulting in better overall physical properties--particularly compression set. The narrow molecular weight distribution polymers do not significantly decrease in viscosity during mixing and processing, thereby maintaining higher compound viscosities. The resulting compounds extrude with smoother surfaces and with less die swell for easier die tuning. The mixing cycles are also somewhat faster due to their higher shear rate. Narrow mwd polymers are not as good for green strength or maintaining bale friability during storage. They are also poorer for mill handling, with more of a tendency for mill roll tackiness and ragged edges on the sheet. A typical tangent delta for narrow mwd EPDM polymers is 1.4.

Broad mwd polymers tend toward a slower cure rate and lower final state of cure due to the presence of low molecular weight polymer chains. They also require more compound formula modification to achieve the required physical properties. Broad mwd polymers do decrease significantly in viscosity during mixing and processing, thereby causing a lower compound viscosity and longer mixing cycles. They generally produce slightly rougher extruded surfaces but, due to their flow characteristics, are better for eliminating ragged edges on cross-sections that have sharp corners or fins. The higher molecular weight portion of broad mwd type polymers provides good green strength and maintains bale friability longer in storage. Broad mwd polymers have improved milling characteristics, with less tendency to stick on the mill rolls, and have an overall smoother surface appearance. A typical tangent delta for broad mwd EPDM type polymers is 0.6.

A medium molecular weight distribution incorporates a compromise of the benefits of both narrow and broad mwd, providing an averaging effect and tending to tailor the distribution in the direction of the physical property requirements. A typical tangent delta for a medium mwd polymer is 1.0.

Taking into consideration all of the aspects of ENB content, E/P ratio, molecular weight and molecular weight distribution, an optimized EPDM polymer for automotive sponge weatherstrip extrusions is in table 1.

New technology

A new technology involving EPDM polymers represents a new generation of EPDM that provides optimized performance characteristics accomplished by controlled compositional distribution. This technology involves both the catalyst system and manufacturing process. It is not a simple cement or mechanical blend which produces an average of physical properties. Controlled compositional distribution produces a single EPDM polymer that provides the concept of multiple EPDM grades, each maintaining their own individual optimized physical properties. Examples of this are:

* Very soft load defections with low compression set, above average tensile, tear and abrasion resistance properties;

* extremely smooth extruded and cured skin surfaces with excellent green strength, shape stability and good low temperature properties; and

* excellent milling characteristics and die definition combined with fast mixing cycles, excellent dispersion and good friable bale integrity.

The examples above combine all of the most desired processing and physical properties for an EPDM sponge polymer. The ultimate new polymer to produce these properties would involve the combination of a very low molecular weight, amorphous, high ENB, broad distribution type polymer with a very high molecular weight, semi-crystalline, high ENB, narrow distribution polymer. This combination produces a very broad molecular weight distribution; however the polymers, by themselves, are narrow and broad so that you achieve the maximum benefits of both and not just an average.

This concept eliminates the need for using a very low Mooney viscosity EPDM (15 ML 1+4 @ 100[degrees]C) in a polymer blend for improving properties of extruded sponge compounds. A single polymer replaces two polymers for inventory reduction. The ML 1+4 @ 125[degrees]C of this type polymer averages 85. However, if you mix it in a typical sponge extrusion compound directly substituting it for 65 Mooney EPDM with a medium distribution, you will obtain a compound viscosity very much the same as that produced by the lower Mooney polymer and with excellent dispersion. In addition, greatly enhanced processing and physical properties in the resulting extruded sponge product are obtained. This is the effect of the low and high molecular weight portions of the new polymer working independently of each other to form the final sponge product.

In summary, the new polymer benefits which can be achieved are as follows:

* Ultra fast cure rate and high final state of cure;

* excellent green strength--shape stability;

* very smooth extruded surfaces with excellent die definition--no ragged edges;

* fast mixing cycles with excellent dispersion;

* compression set below 20% (with an expanded specific gravity of 0.35 g/cc or less);

* soft load deflection for low automotive door closing efforts;

* lower expanded densities;

* improved tensile and tear strength;

* lower cost compounds due to higher extendibility;

* very smooth extruded and cured skin surfaces;

* improved abrasion and wrinkle resistance;

* improved resistance to surface bloom;

* excellent low temperature properties;

* improved milling characteristics; and

* excellent bale friability retention.

Table 1

Mooney viscosity (ML 1+4@ 125[degrees]C)   50-60
ENB, weight %                              8.5
Ethylene/propylene ratio                   60/40
Molecular weight distribution              Medium/narrow
Tangent delta                              1.1
Oil extension, phr                         20
Bale form                                  Friable

Tredinnick, Donald W. Crompton/Uniroyal Chemical