New ultra-low viscosity EP(D)M.

Recent advances in polymerization, processing and recovery have allowed DSM Copolymer to broaden its product line in the range of ultra-low viscosity (ULV). DSM has proven the technical viability of manufacturing ULV EP(D)Ms in a patented process (ref. 1). The new commercial products are Keltan 1446A and DE291, which are unique EP(D)M grades because of their form, composition and viscosity. A new developmental ULV EPM, Keltan DE298, will also be introduced. All these products have Mooney viscosities, ML 1+4 at 125'C, in the range of 6-14 and are pelletized free flowing products that exhibit exceptional storage stability.

K-1446A has ENB unsaturation of 6.6 weight percent and an ethylene content of 58 weight percent. This product can function as a curable plasticizer, minimizing the conventional plasticizer to give better performance under severe conditions. K-DE291 is a true EPM with an ethylene content of 61 weight percent. It can function as a non-curable, polymeric plasticizer and can be used in many other applications that will be discussed. Both products were designed for high quality and value added applications, but are finding uses in many others as well. When blended with conventional elastomers or used as the sole elastomer component, ULV products improve processing by lowering compound viscosity, improving flow and surface properties in calendered, extruded and molded products. The narrow MWDs combined with ULVs give these products optimum flow properties as is well known with conventional elastomers (refs. 2 and 3). In addition to the properties mentioned above, the ULV polymers possess desirable characteristics of conventional EPDM: excellent heat, weather and ozone resistance and low temperature elasticity.

Traditional thermoset applications in which ULV products are being used include super high hardness EPDM, high heat resistant injection and compression molded EPDM and ultra high flowing TPEs. Newly developing applications for ULV products include viscosity modifiers and dispersants for lubricants, low profile additives in thermoset resins, curable and non-curable sealants, adhesives and mastics and as an additive for other elastomers to improve ozone resistance.

In the following sections, the uses and advantages of ULV Keltans in traditional thermoset, thermoplastic and non-traditional applications as denoted above will be discussed.

Ultra-low viscosity EPDM compositional ranges

The production of these ULV polymers is a patented process, the details of which are proprietary. The finishing operations differ from traditional EPDM in that it does not result in solid baled products, but lends itself to the production of free flowing, pelletized products. The process is truly unique in that by careful control and manipulation of crystallinity and crystalline domain distribution, highly amorphous products, i.e., ones containing 56-62% ethylene such as K1446A or DE291, can be recovered as stable pellets. The latter achievement represents a significant advance as traditional, higher Mooney viscosity EP products with the same ethylene content would, if produced in pellet form, quickly reagglomerate to a solid mass.

GPC molecular weights for the nominal 6-14 Mooney ULV products have Mn's ranging from 30-40 thousand and Mw's ranging from 80-120 thousand. Polydispersity indices range from 2.3-3.1, indicating very narrow MWD. Other compositional variables such as diene content and ethylene propylene ratio can be maintained within ranges typical of traditional EPDM. Figure 1 summarizes these compositional ranges for ULV products in schematic fashion, while table I provides a listing of the key polymer properties for the ULV products. Figure 2 gives the DSC thermograms for ULV products, which show the crystalline transformations responsible for the unusual behavior that allows stable pelletization (ref. 4).

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Application development with ULVs

Molded brake cups

Automotive brake cups tend to be rubber rich, peroxide cured EPDM formulations that are designed to withstand the high heat buildup caused by rapid braking. A serious problem with these formulations is that brake fluid is an excellent extractor of monomeric plasticizer, i.e., paraffinic oil. The plasticizers are used primarily to impart the necessary processing characteristics for a system based on higher molecular weight rubber. The extraction of oil from the compound can cause part shrinkage, leading to brake fluid leakage and potential system failure.

Use of ultra-low viscosity K-1446A allows compounds to be formulated with less hydrocarbon oil, giving improved service and reliability, but with acceptable or improved processing performance. The high diene content of K-1446A further allows for greater crosslink density and higher states of cure. This leads to improved physicals such as modulus and compression set, particularly if the oil level is reduced.

K-1446A was evaluated as a curable plasticizer in a standard brake cup formulation. The control was based on K-4506, a conventional low viscosity EPDM with broad MWD. K-1446A was varied from 0-20 phr in this evaluation as was the conventional plasticizer. Table 2 gives the formulation, experimental design and experimental results.

Results on compound viscosity showed a strong effect of K-1446A. Compound viscosity increased when conventional plasticizer was removed from the control, i.e., based on K-4506. The addition of K-1446A lowered viscosity to nearly the level of the control with oil. Even lower viscosities were achieved with K-1446A and conventional plasticizer.

Injection mold flow was simulated using a spider mold. For all screw pressures tested, the addition of K-1446A gave improved compound flow relative to compounds based on the control EPDM and no oil. However, the effect of oil on flow was somewhat stronger than that of ULV. As would be expected. polymeric plasticizers do not impart quite the flow enhancing effects as would be achieved by monomeric plasticizer. Otherwise, spider mold flow was closely correlated to compound Mooney viscosity.

Table 2 gives rheometer cure characteristics for the comparison. Compounds based on K-1446A and no oil yielded higher maximum torque values than did the control compounds with conventional plasticizer, indicating a higher crosslink density and greater state of cure. Unaged physical properties confirmed this finding, where K-1446A yielded increased tensile, modulus and hardness, but decreased elongation. The low tensile value for compound 0202 with 10 phr K-1446A was somewhat anomalous based on its rheometer and modulus values and further study will be required for explanation. The high rheometer torques and normal physicals are attributed to the high ENB content in Keltan 1-446A which is known to contribute to the increased crosslink density and higher state of cure (ref. 5).

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Compression set is another important test with brake cups as it indicates sealability. K- 1446A based compounds yielded lower compression set values than the controls with plasticizer. This effect was largely due to the higher rubber level although the higher unsaturation of K-1446A undoubtedly contributed. Higher compression set was observed when oil was added. Use of K1446A allowed the removal of oil which improved compression set and cure state properties while maintaining acceptable processing characteristics.

Increased demand for higher temperature performance has been a continuing trend in automotive applications. Brake cup compounds were heat aged for 70 hours at 1250C. Overall compounds based on K1446A and reduced oil performed better than the control formulations with oil. This observation was due to the reduced oil level that the ULV allowed.

Of critical importance to brake cups are the various fluid exposure tests. Brake cup compounds were aged in brake fluids DOT 3, 4 and 5 and ASTM #1 oil, with an aging condition of 70 hours at 125[degrees]C. Vulcanizates based on K1446A than did the corresponding controls with conventional plasticizer. This finding would allow brake parts to swell instead of shrink when exposed to brake fluids. Vulcanizates based on K1446A yielded lower volume swells after exposure in #1 oil than did the corresponding controls with conventional plasticizer. No advantage was seen when blending K1446A with free oil. While brake fluid swell is desirable, minimizing volume swell after oil exposure is always preferred. Overall the addition of Keltan 1446A produced a vulcanizate that was more resistant to extraction.

Surface characteristics were measured in two ways. A Surfanalyzer was used to measure surface roughness and 85[degrees] gloss measurements were taken. Compression molded vulcanizates with K1446A at 10 and 20 phr gave higher gloss and smoother surfaces than did the control formulations with equal free oil. However the combination of K1446A and free oil resulted in increased surface roughness, perhaps indicating poor dispersion from an over plasticized compound.

Injection molded comers

Injection molded products normally utilize medium viscosity, fast curing EPDM polymers to obtain acceptable flow properties and short cure times. A narrow range of compound viscosity is used to obtain an efficient processing compound, i.e., too low or too high viscosity can be difficult to process. A narrow range for the cure rate also exists, if a compound cures too fast it will scorch during plastication, and too slow curing compound will give long cycle times.

K-1446A was evaluated in a sulfur cured, injection molded comer formulation as the sole polymer component, in a blend with conventional EPDM, and as a curable plasticizer, where it was used to replace oil. The control was based on K-314, a conventional viscosity ultra fast curing amorphous EPDM. K-1446A level was varied from 20-100 phr and the conventional plasticizer level va, phr. Direct comparisons of the K-1446A recipes to the control formulations that are designated; C-A phr oil, C-B had 10 phr oil, and C-C had 20 phr oil. Tables 3-5 give the formulations, experimental designs and experimental results.

As seen in the brake cup evaluations, K-1446A significantly affected compound viscosity results. The addition of K-1446A lowered viscosity and it was lowered even further with the addition of conventional plasticizer.

Again the spider mold was used to simulate injection mold flow. For all screw pressures tested, the addition of K-1446A, whether as the sole polymer, in a polymer blend or in place of conventional plasticizer, improved flow relative to compounds based on the controls. As expected, the addition of free oil gave increased flow compared to that of K-1446A with no oil. Extru-sion rate was also examined. An improvement in the extrusion rate was achieved in each evaluation with K-1446A and conventional plasticizer.

Rheometer cure characteristics and stress strain properties for the comparison are given in tables 3-5. Predictably, the high diene level of K314 influenced the maximum torque properties. Higher values were obtained with the K-314 controls except the formulations where K-1446A was evaluated as the sole polymer. This finding was confirmed with the unaged physical properties and compression set, where K-314 yielded increased tensile, modulus and hardness with decreased elongation and compression set. The maximum torque values for the K-1446A polymer study were somewhat unusual based on its stress strain and compression set values. The addition of oil diluted the effect of K-314 on the properties.

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Surfanalyzer and 85[degrees] gloss tests were used to measure surface characteristics. As demonstrated in the brake cup formulations, the addition of K-1446A improved the gloss of a compound as well as the surface smoothness. The same phenomenon was seen in the injection molded compounds, no matter if K-1446A was evaluated as a curable plasticizer, polymer or in a polymer blend. Roughness did not decrease in the compounds in which K-1446A was evaluated as a curable plasticizer.

Super high hardness

The combination of high diene content and extremely low Mooney viscosity makes K-1446A well suited for high hardness applications such as extruded profiles and molded goods. A high diene content allows for greater crosslink density and a higher state of cure. Effect of ULV on hardness is somewhat indirect: It allows a compound to be formulated with comparable processing characteristics, but with a significantly reduced oil content. This feature allows for increased vulcanizate hardness and gloss, and improvements in other physical and aging properties related to oil content such as extraction, volatility and migration staining.

To illustrate these effects, K-1446A was examined in a proprietary carbon black filled, sulfur cured, extruded window formulation that yielded a nominal 50 Shore D hardness. An experimental design was employed comparing K-1446A in blends with a conventional low Mooney viscosity high crystalline EPDM, K-2308, with levels of plasticizer varying from 0-40 phr. Table 6 gives the experimental design variables and levels, while figures 3-7 give contour plots of some of the more significant property responses.

A serious limitation of the conventional 25 Mooney viscosity EPDM is clearly evident from the high compound viscosity that resulted even at the higher oil loadings. This finding suggested a very difficult processing regime would likely ensue. Alternatively, use of the ULV polymer either in blends with K-2308 or as the sole polymer allowed significant reductions of oil content and improvements in hardness while maintaining a favorable processing window. The vulcanizates based on ULV and reduced oil were also more stable in regard to their volatility and extraction performance, while otherwise maintaining equal or improved physical properties. A possibility for a further hardness increase exists if a high crystalline analog of amorphous K-1446A were available. While technically feasible, the commercial viability of this product is being determined.

High heat resistant EPDM

Optimum high temperature performance of EPDM compounds is generally achieved using EPMs or EPDMs with low diene content. This minimizes the potential for thermal degradation reactions such as chain scission or crosslinking which often occur at residual double bonds. Peroxide cure systems are also preferred over sulfur as a means of producing more thermally stable crosslinks. Of the important compound variables, the presence of minimal levels of paraffinic plasticizer and carbon black is also of primary importance to prevent die further loss of physical properties during aging or severe service attributable to plasticizer or black filler instability.

High temperature performance of EPDM compounds is often limited by the amount of paraffinic plasticizer needed to achieve adequate processing characteristics (ref. 6). When one reduces or eliminates the amount of conventional plasticizer sufficiently to improve aging performance, a poor processing compound is often the result. A good way to limit plasticizer, while maintaining processing performance and other key physical properties such as low temperature flexibility, is to employ an ultralow viscosity product.

Eight such heat resistant, peroxide cured EPDM formulations were tested, which comprise a nominal 60 Shore A molding compound (formulations are available from the authors). The control formulation was based on a traditional 50 Mooney viscosity amorphous low diene EPDM, K-5206, with 30 phr paraffinic plasticizer. The remaining formulations displayed the effects of both oil level and K-DE291, a 10 Mooney viscosity amorphous ULV EPM. For the latter effect, the ULV material, DE291, was employed in a 30/70 blend with high viscosity K-5206, these blends then being compared, at various oil levels, to formulations in which K-5206 the sole polymer.

The use of the ULV product allowed for a 30 phr reduction of plasticizer, while maintaining acceptable level of compound Mooney viscosity, Mooney scorch and extrusion properties, indicating an acceptable processing window for these formulations. Other normal vulcanizate properties were either equal to or superior to the formulations based on ULV and reduced plasticizer.

A big advantage of ULV was evident in the aging and exposure properties. At comparable levels of compound viscosity, the formulations based on the 30/70 blend (and less oil) gave significantly improved hot air aging at 160[degrees]C. In addition, ETA extractability and volatility were reduced, indicating more stable vulcanizates with regard to severe long term ex sure. These findings were reduction in plasticizer content that the ULV allowed, while maintaining essentially the same processing characteristics.

Ultra high flow TPEs

EPDM and EPM are widely used to make thermoplastic elastomers via mechanical blending of the base elastomers, resins and other components. TPO in particular is experiencing rapid growth in automotive bumper and fascia applications. One important reason for this success has been due to the ability of TPO to be readily shaped via injection molding into large, consolidated parts, which typify the modern automobile. Consequently, materials with high melt flows are considered a definite advantage to the processor. In a typical commercially compounded TPO consisting of EPDM blended with polypropylene, it is the elastomer component that historically tended to be flow limiting because of the medium-to-high viscosities of the traditional EPDM. In this regard, ultra-low-viscosity EPM offers the attractive prospect of yielding exceptionally high flowing TPO compounds, which take full advantage of new high flow commercial polypropylene resins now being offered.

Figure 8 illustrates the effects of ULV on TPO compound melt flow for TPOs based on a 30% elastomer modified polypropylene. In this example, the elastomer component was K-DE291, a 10 Mooney viscosity amorphous type, which was compared with three conventional EPMs in the Mooney viscosity range of 30-70. Melt flow responses are given for four polypropylenes with base melt indexes ranging from 12 to 60. The effect on TPO melt flow rate due to ultra-low viscosity is clearly evident, giving a rise to a set of TPO compounds in the 20-40 melt flow range. TPOs based on ULV products exhibited ductile failures, improved gloss and acceptable stress-strain performance. Their property responses were directionally equivalent to those observed in previous investigations (ref. 7).

A high ethylene ULV copolymer product, K-DE298, is also very effective as a PP impact modifier and in TPOs. The primary difference between DE298 and its amorphous analog, DE29 1, is the balance of low temperature and room temperature impact and stress strain properties. K-DE298 is more crystalline and would tend to impart higher room temperature hardness and stress strain properties. Being more amorphous, K-DE291 would exhibit superior low temperature ductility, elasticity and impact performance. The presence of these two versatile ULV EPMs suggests a potential development of new classes of high flowing TPO materials, with melt indexes well above those of current commercial high flow TPOs, which typically fall into a flow range of 15-25.

Non-traditional applications

K-1446A and DE291 were also designed for use in non-traditional rubber applications such as adhesives, curable and non-curable sealants, mastics and high quality thermoset and thermoplastic applications where improved processing, flow and surface qualities are desired. Applications will be presented below where ULV products were evaluated by the customer in proprietary formulations, and only the attributes of Keltan will be discussed.

* Industrial adhesive tape: In recent years, the use of EPDM based adhesive tape systems has increased, replacing cement type adhesives. This change was largely due to handling advantages and environmental concerns. K-1446A was evaluated in a tape system where conventional EPDM was being used. K-1446A was blended with conventional EPDM and yielded improved incorporation of compounding ingredients, particularly the tackifying resins. Improved processing and smoother surface appearance were observed on extruded parts. Lower compound Mooney viscosity values and improved peel adhesion were also observed when K- 1446A was added.

* Automotive sealants and mastics: The automotive industry requires numerous mastic compounds for various barriers, sealers and dampeners. These products are based on either curable or non-curable polymers. K-1446A and DE291 were evaluated as the elastomeric component of these compounds. K-DE291 gave improvements in processing, quicker polymer incorporation, faster mixing, improved flow properties and faster extrusions. Its use had a plasticizing effect to reduce nerve and improve adhesion where there was reduction of oil. K-1446A gave many of the same attributes as DE291 and has shown other effects such as improved long term adhesion and weatherability.

* Molded/extruded sealing system: K-1446A was evaluated in both dense and sponge comer moldings and extruded weatherstrips, where it was used primarily in blends with conventional EPDM. Improvements in processing, higher cure rates, improved flow, a smoother and glossier surface and improved tear strength were observed. One customer reported that the faster cure enabled them to reduce the amount of curatives in their compound which lowered cost. Another customer reported that they could obtain superior skin on their molded comer sponge when K-1446A was used. K- 1446A has enabled customers to reduce extender oil levels and improve mold flow. In one example a reduced oil level helped resolve adhesion problems in the manufacture of urethane coated window channels.

Conclusions

Throughout these investigations, the application of Keltan ULV products offered significant improvements in processing and performance. This was evident whether the ULV product was used to replace the conventional elastomers, or as a curable plasticizer to replace oil. These findings are summarized as follows:

* lowered compound Mooney viscosity;

* improved processing and flow behavior;

* improved surface characteristics;

* allowed a reduction in process oil that improved properties, i.e., heat aging, extractability and volatility;

* use of ULV products to broaden compounders "window," i.e., super high hardness;

* pellet form of ULV products offers better handling. Overall, Keltan ULV polymers offer many unique features in both traditional rubber and non-traditional applications.

References

[1.] E.J. Olivier, R.T. Patterson and P.N. Nugara, United States Patent,- serial no. 5,391,617 assigned to DSM Copolymer Inc., February 21, 1995. [2.] R.F. Karg, "Injection molding of elastomers," presented at the Grupo Hulero Mexicans Meeting, Mexico City, Mexico, September, 1984. [3.] M.T. Gallagher, "EPDM microstructure effects in polypropylene," Antec `91, 2389, 1991. [4.] A.L. Gay, DSM Copolymer Inc. internal investigation, performed in June, 1993 [5.] R.F. Karg, "Compounding of EPDM elastomers," presented at The Energy Rubber Group meeting, Arlington, Texas, September 25, 1985. [6.] J.R. Dunn, "Compounding ethylene-propylene elastomers for high temperature aging resistance," International Rubber Conference, Harrogate, England, June, 1987. [7.] S.D. Brignac, "EP(D)M structural and thermal property effects in polypropylene compounds," SPE technical papers Volume XXXX, 3, 3433, 1994.