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The evolution of polynorbornene.

The evolution of polynorbornene

Polynorbornene was introduced under the name Norsorex in 1976 in Europe, and in 1978 in the United States and Japan. This new original polymer, to be used as a rubber, but manufactured solely in powder form, was found to be different from the traditional polymers of the rubber world. One of the unique characteristics of this polymer is the oil absorption ability which allows formulations with up to 400 phr plasticizer and lowers the softness of vulcanizates far under the usual limits.

In spite of this high oil level, the oil resistance is acceptable for many applications. Furthermore, when we displayed two balls made with the same polymer, of the same hardness, but one bouncing and the other dead, polynorbornene became and continues to be known as a very unique rubber.

Why is polynorbornene different?

The main properties of this elastomer can be related to three basic characteristics:

* Chemical structure - polynorbornene is easy to cure and can be cured with conventional vulcanization systems. Specific curing systems depending on the customers application and/or process can be recommended. The compatability of polynorbornene is good with non polar plasticizers such as mineral oils. However, ester type plasticizers and paraffinics, while imparting good low temperature resistance, must be used in small quantities because of poor compatability.

Polynorbornene can be blended with many other elastomers depending upon the desired properties. The compatability is excellent with dienic types. Blends with SBR, natural rubber and chloroprene are common. Additionally, EPDM is commonly used as an antiozonant with polynorbornene.

* Powder form with expanded particles - polynorbornene with its high absorption capacity for platicizers can be processed as a dry-blend (for example, in Henschel type mixers). Also, as a powder, polynorbornene can be highly extended and cast at room temperature as a liquid rubber (Norsofluid process).

* Very high molecular weight (over 3,000,000) - because of its high molecular weight, polynorbornene has high binding power for large quantities of plasticizers and fillers. Compounds, even when highly extended, have outstanding mechanical properties. The ability of polynorbornene to accept high extension levels gives the compounder a great latitude in the properties which can be obtained. Another phenomenon of polynorbornene is the increase in green strength which is gained by adding small levels to compounds primarily based on other polymers.

Because of these characteristics, polynorbornene compounds usually contain only about 20% polymer (500 total RHC). This makes the proper choice of compounding ingredients (both type and amount) extremely important.

Selection rules for compounding ingredients

Influence of plasticizer type

Polynorbornene compounds can contain up to 60% plasticizer; so an accurate selection within the large range of available oils is essential. The following characteristics must be taken into account for proper oil section:

* Compatibility - the most compatible oils are aromatics and napthenics. Low compatibility will restrict the possible hardness range, increase the mixing cycles (figure 2) and can lead to oil migration.

* Low volatility - volatility of the plasticizer is the first factor affecting aging properties.

* Cold brittleness - plasticizer type and amount determines the cold resistance of the compound (table 1). Unfortunately the best cold resistance is obtained with oils which also have the highest volatility. Therefore, a compromise must be obtained.

* Damping properties - figure 3 indicated the large scale of possible transition temperatures that can be obtained by using plasticizer types. Here again, a compromise has to be found in accordance with the two previous characteristics.

* Compatibility with other rubbers - in case of blending, the plasticizer must be compatible with both polymers.

* Other physical properties like elongation (higher with aromatics) (table 1), fatigue resistance (related to damping), and/or staining.

Influence of filler type

The usual rules for carbon black selection are valid with polynorbornene compounds. Due to the possible high level of filler, the effect will be enhanced. The filler type acts especially on:

* Physical properties such as strength, abrasion, tear, and extrudability.

* Electrical conductivity - Norsorex compounds can be highly filled with conductive fillers.

* Damping properties - figure 4 shows the influence of different types of carbon black on the rebound resilience.

Examples of polynorbornene formulas are shown in tables 2 and 3. The curing system is adjusted according to the curing conditions.

Applications and developments

The commercial introduction of polynorbornene into the rubber industry required extensive development worldwide. This is primarily due to the unique handling characteristics of the material. Japan and Western Europe, particularly West Germany, have become the major users of polynorbornene based compounds.

In the United States, polynorbornene acceptance and usage is just starting after years of efforts. A key objection to this material was eliminated since we began working with well known competent American custom mixers able to supply top level quality compounds. Polynorbornene compounds are currently processed in the United States by compression, transfer and injection molding; extrusion; and calendering.

Major American usages are currently in automotive and appliance applications. These include body mounts, soft hood stops, grommets, diaphragms, trunk lock seals, and vibration and noise isolators. Appliance usages are primarily where noise reduction is of importance. A more complete worldwide usage is shown in figure 5.

At the present time, we observe a yearly progression of 30 to 40% worldwide. The 1989 consumption should be about 1,000 tons of polynorbornene powder (5,000 tons of compound). [Table 1-3 Omitted] [Figure 1-5 Omitted]
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Author:Hupp, Richard
Publication:Rubber World
Date:Jun 1, 1989
Words:880
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