New foamed EAs and other technologies.
Thermoplastic elastomers have become an important rapid growth area for the rubber and plastics industry. Much attention has been paid to the replacement of a wide variety of rubber parts by the more economical thermoplastic elastomers (TPEs). An important class of TPE that has been developed is the elastomeric alloys (EAs). The EAs have penetrated the general cross-section of rubber market applications, and developments have continued to be made in these specialized applications. Foaming of TPEs has seen significant advancement and only recently the technology has been developed to allow the preparation of very low density rubber foams with an elastomeric alloy, Santoprene rubber. In another application area, rubber covered rolls, the properties of EAs have made them desirable for use. However, fabrication technology has not been fully developed until recently to allow their use in a wide variety of rubber covered rolls. Large rollers covered with thermoset materials require vulcanization time from several hours to a day or two because of the large size of these roll coverings and the relatively low heat transfer property of rubber. However, rollers covered with elastomeric alloys do not require any vulcanization. For those thermoset rubber rolls with quality flaws, the rubber has to be stripped off and the process repeated. The thermoplastic elastomer cover can be repaired in some cases, and, at worst, can be removed, reground and reprocessed. Similarly, the use of EAs as a sound deadening acoustical barrier has been limited because of the relatively low density of these materials. The need for sound and noise control in automotive and industrial applications prompted the development of several new elastomeric alloy grades that would exhibit improved sound attenuation capabilities. This article reports the progress made in advancing the penetration of EAs into these three areas. The new developments reported here will support the continued rapid growth of TPEs and EAs in particular.
Elastomers, and including conventionally vulcanized foamed rubber, have found use in a variety of sealing applications. The low force required to detect and compress a foamed rubber seal makes it an ideal material for gaskets in automotive and mechanical rubber goods applications. The process to make these conventionally vulcanized foamed rubber products is susceptible to high scrap rates which nullifies the advantage of the low cost per volume of foamed thermoset rubber products. A TPE that can be reprocessed has a significant cost advantage over conventional rubber use in foamed rubber products.
Thermoset elastomers have been adopted as roll covering materials in both small and large rollers across a wide variety of industries and applications. Thermoplastic elastomers have made relatively little penetration into this area because of the lack of application technology to resolve the practical function and fabrication issues. Rubber use in roll coverings represents a significant volume of material used in non-tire applications. Development of roll covering and adhesion technology for a TPE opens the market to these materials and allows them to compete with conventional rubber. Roll coverings are used in many industries, including paper, food and office equipment. We will report three different processes used to prepare EA rubber covered rolls.
Another product application area for elastomers that requires specialized attention is noise control. Noise control has become an increasingly important issue in all industries because sound deadening products are regarded as performance and high quality products. TPE products with enhanced noise control characteristics that provide the improved quality and consistency versus conventional rubbers have been developed. The products reported here will provide competitive economics and high sound barrier properties compared to conventional rubber products.
Foam technology has been extended to elastomeric alloys to allow their use in foamed rubber applications. Foaming of a TPE has been accomplished for an extruded rubber part via two processes. The first uses a chemical foaming agent. The second is a mechanical foam process using a foaming agent. A comparison of the density reductions achievable by the two processes is shown in figure 1.
Chemical foaming is a logical extension of the technology often used in conventional thermoset rubber and to some extent in thermoplastic processing. A chemical foaming agent is an additive which decomposes thermally to evolve a gas. Several which have worked are Kempore 60/14 (Olin Chemicals), Expancell 0-113 and 0-157 (Thefa Corp.), Celogen AZ-130 (Uniroyal Co.), and Nortech XMF 1307 (USI Chemicals). Also used are citric acid and bicarbonate compounds such as Hydrocerol-CLM 70 (Boehinger Ingelheim). The azodicarbonamide and modified azodicarbonamide compounds form a closed cell structure which is maintained as the molten elastomeric alloy cools. The chemical foaming process is typically limited to a density reduction of 15 to 25%. Attempts made to inject nitrogen or air directly into the melt to achieve density reductions below about 20 percent were unsuccessful. The limiting factor is the ability to solubilize the gases generated by the blowing agents.
A lower density elastomeric alloy foam is achieved using the mechanical foaming process using ozone safe blowing agents, such as C[F.sub.3]CH[Cl.sub.2]. The ozone safe fluorocarbon is metered directly into a vacuum port of a 30:1 L/D extruder. Careful control of the temperature in the last barrel section is required to obtain a smooth skin and development of a closed cell foam structure. Density reductions up to 80% can be achieved by the mechanical foaming of elastomeric alloys.
Foamed elastomeric alloys exhibit excellent mechanical properties. Tensile properties achieved in several foamed elastomeric alloy grades are shown in figure 2, where the specific gravity is from 0.72 of 0.84. The primary design feature in many foamed rubber products is the load deflection force. Figure 3 shows the load deflection forces achieved for several elastomeric alloys. Foamed elastomeric alloys also exhibit excellent resistance to taking a set while under compression, as seen in figure 4. For many foamed rubber elastomer applications, it is critical that the foam cell structure consist of closed cells to prevent absorption of water or other materials. A water absorption test characterizes the amount of closed cell structure, as shown in figure 5.
A comparison of foamed elastomeric alloy 67 Shore A hardness grade to conventional thermoset foamed EPDM is shown in table 1. At equivalent load deflection and similar density, the foamed elastomeric alloy shows a tensile strength 58% higher than foamed EPDM. The foamed elastomeric alloy compression set is 30% vs. 65% for foamed EPDM.
A mechanically foamed elastomeric alloy was tested versus three commercially foamed EPDM samples. The foamed EPDM had similar load deflection characteristics when compared to elastomeric alloy 67 Shore A grade. The tensile strength, tear strength and compression set of each were determined. The results are summarized graphically in figure 6. These data show that the low density elastomeric alloy has superior tear and tensile strength vs. the comparable density EPDM. The tear strength exceeds that of the 0.55 specific gravity EPDM, and the compression set is better than any of the foamed EPDM, especially the 0.39 specific gravity EPDM. As the density of foamed EPDM is reduced it has lower tensile and tear strength. The elastomeric alloy foam has superior mechanical strength when these low densities are achieved.
Elastomeric alloy foaming technology furnishes the mean to prepare a low density elastomeric product using lower cost thermoplastic processing. The properties of the foamed elastomeric alloy are similar to those of conventionally foamed thermoset rubber.
Application of this foaming technology opens the use of elastomeric alloys in sealing applications to a wide cross-section section of industries. Foamed elastomeric alloy seals in appliances, automotive, office equipment, and architectural and residential door and window seals is now commercial. The performance of the elastomeric alloy foam makes it an excellent candidate for this large variety of applications. Economics of the process are based on the relatively low cost of thermoplastic processing, and make it commercially important to fabricators and users.
Roll cover technology
The technology developed to cover rolls can be broken into two application areas based on the size of the rolls. Small rolls are readily covered with a thermoplastic elastomer and conventional technology can be applied. Very large rolls have special handling problems that make them more difficult to cover.
Smaller rollers, from a few centimeters to nearly a meter in length, have been covered with elastomeric alloys using insert injection molding and then machining the roll cover to tightly toleranced dimensions. In some cases a mechanical interlock to the roll is an adequate bond for performance, but an adhesive can be applied to the roll to achieve a higher level of bonding between the elastomeric alloy and the roll surface. Another technology used to cover small rolls successfully is to extrude a tube shaped preform. The preform can be press fitted over a roll using a lubricant or an air assist technique. For rolls requiring less stringent dimensional tolerances, the latter technique is an attractively low cost means for preparing rubber covered rolls using an elastomeric alloy.
Two fabrication methods have been developed to cover large rolls that are 1 meter or longer and 30 cm diameter with an elastomeric alloy. The first is a molten ply technique and the second is a cold ply process.
The molten ply technique is to preheat the roll to 175 [degree] C in a hot air oven for about an hour. After mounting the hot roll in wind-up system, a powdered adhesive of modified polypropylene (Polybond 1016, BP Chemicals) is sprinkled onto the outside surface. The roll is then returned to the hot air oven for an additional 15 minutes to melt the adhesive. The adhesive covered roll is then mounted into a sheet extrusion line system. The elastomeric alloy is extruded through a standard sheet extrusion die at a thickness of .5 to 3.8 mm at 205-215 [degree] C. The sheet is tightly wound onto the roll to assure good interlayer adhesion by pulling slightly to a draw ratio of 1.0 to 1.25. The roll is rotated until the elastomeric alloy melt is layered to the thickness desired. Once covered, the roll is dismounted and cooled in water (|70 [degree C] for approximately an hour. The roll cover is then machined on a lathe to the desired dimensions. The molten ply process has been successfully demonstrated in a range of elastomeric alloy grades from 73 Shore A to 53 Shore D hardness. The covers were found to have good adhesion and to be free of any voids and no delamination was found.
An alternate cold-ply method has also been developed as a two step process. First a trapezoidal strip of elastomeric alloy is extruded and wound up. A coating of an epoxy based adhesive (Metallon 2108) is applied to the roll. The trapezoidal shaped strip is coated with an adhesive primer (Reno Primer 360), then spirally wound on the roll to the desired thickness. A fabric tape is applied tightly to the cold elastomeric alloy roll to apply pressure, and the wound roll is placed in an oven/autoclave at 150-160 [degree] C for several hours. During this time the pressure fuses the layers together This technique provides a solid rubber coating free of voids. The roll is removed and cooled, then machined to the finished dimensions as in the molten ply technique.
The performance of elastomeric alloys in these applications has been demonstrated in a variety of commercial applications ranging from office machines for paper handling, to industrial rolls in the food, textile and paper processing industries. The properties which elastomeric alloys offer to applications in these areas are rubber-like friction characteristics, good abrasion resistance moderate fluid resistance, excellent heat aging resistance, and excellent compression set. Economics of the described technology are better than normally achieved with conventional thermoset rubber, because the elastomeric alloy does not require any vulcanization process. So the driving force is better cost/performance for the roll covered with an elastomeric alloy.
Elastomeric alloy technology has been developed for use in applications where noise and sound attenuation are needed. This has been accomplished by increasing the density of the elastomeric alloy to take advantage of the mass law effect. A greater mass per unit volume reduces acoustical power transmission logarithmically.
Several grades of the elastomeric alloys are compared to thermoset EPDM rubber properties in table 2.
The data show that high density elastomeric alloys compare favorably to high density EPDM, as well as to the lower density, more conventional, EPDM rubbers. These high density elastomeric alloys provide the physical properties to meet the hot air aging and fluid resistance demands of automotive and industrial applications while providing improved sound reduction.
Accoustical chamber tests of sound transmission characteristics of the EPDM rubber and the sound attenuating elastomeric alloy grades have been conducted using an anechoic chamber. The apparatus consisted of a white noise source connected to a tube shaped sample of the elastomer. The total sound power transmitted through the sample was measured in the chamber. Samples of each material were measured at two wall thicknesses. The total sound power transmitted through a 75 mm diameter by 2.5 mm wall tube for each material is shown in figure 7. Results of these tests demonstrate the reduced sound transmission characteristics of the high density elastomeric alloys. Total sound power reductions versus standard elastomeric alloys are achieved in the 2 to 2.5 dBA range. These reduction levels are very significant and would often require major mechanical revisions to accomplish by engineering or design means. In figure 8 the frequent breakdown of the sound transmission is shown for 1.5 gravity EPDM and a 1.5 gravity elastomeric alloy. The frequency spectrum of the sound shows that the elastomeric alloy has better attenuation at the lower frequencies, which is often preferred, e.g. due to reduced propagation of the sound into an automobile passenger compartment. Attenuation of high frequency sound is comparable to thermoset EPDM.
An improved sound attenuating elastomeric alloy will provide significant benefits in applications that include sound barriers, seals and covers in industries such as automotive, office equipment, appliances and industrial equipment. The competitive advantages of using a lower cost option like elastomeric alloys while obtaining significant sound and noise attenuation is important is such applications where consumer quality is critical.
Summary and conclusions
Use of elastomeric alloys in the special market applications of foamed rubber, roll covers and sound attenuation has been made possible by the development of application technology for these areas. These developments will make it possible for the processing advantages of elastomeric alloys to be realized in such applications.
Elastomeric alloys are well known for good fluid resistance, excellent heat aging, superior dynamic fatigue, excellent compression set performance and good mechanical properties. The ability to fabricate products as discussed here will bring these advantages to a variety of industries including commercial equipment, automotive, office equipment, appliances and architectural construction. Users in these industries will be able to obtain better cost/performance with elastomeric alloys versus thermoset rubbers such as EPDM, polychloroprene and chlorosulfonated polyethylene.
PHOTO : Figure 1 - foamed EAs - density comparisons
PHOTO : Figure 2 - foamed EAs - physical properties
PHOTO : Figure 3 - foamed EA - load deflection
PHOTO : Figure 4 - chemical foamed EAs - compression set
PHOTO : Figure 5 - chemical foamed EAs - water absorption
PHOTO : Figure 6 - mechanical foam EA - property comparison to foam EPDM
PHOTO : Figure 7 - density effect on sound transmission
PHOTO : Figure 8 - sound power transmission spectrum
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|Title Annotation:||elastomeric alloys|
|Date:||Mar 1, 1990|
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