Advantages of oil-less dust stops for mixers. (Process Machinery).

In today's economic environment, companies are not only searching for ways to reduce costs, but also improve quality. Self-lubricating dust stops provide an opportunity for many companies to accomplish these goals. Today, oil-less dust stops are a reality and are used in all segments of the rubber industry. In some cases, cost savings amount to $40,000 to $70,000, and dramatic improvements have been made in quality. Historically, aluminum bronze dust stops have been used on most internal mixers. One of the problems with bronze dust stops is the high coefficient of friction, which requires large quantities of oil for lubrication. Without this oil, the bronze dust stops wear at a rapid rate. The cost of operating an internal mixer with bronze dust stops includes the cost of lubrication oil, the cost of oil collection and the disposal of spent oil. A summary of these costs for a #10MX mixer is shown in figure 1.

[FIGURE 1 OMITTED]

Product quality

Today, self-lubricating dust stops are used by companies in all segments of the rubber industry, as well as in plastic compounding. Oil-less dust stops are also used by automotive suppliers producing automotive seals, v-belts, hoses, tires and elastomeric valves, and electrical components such as spark plug wires. In plastic compounding, oil-less dust stops are used for compounding color concentrates and fillers with a variety of thermoplastics. Manufacturers of PVC tubing, wire insulation, flexible sheet and garden hoses are benefiting from the elimination of lubrication oil. The cost savings from the elimination of oil lubrication is significant; but, often times, the issue of quality is more important. Variations in elastomer properties caused by oil contamination result in costly scrap batches. The cost is magnified if expensive elastomers such as fluorinated or silicone polymers and/or expensive fiber reinforcements are used. Reinforcing fibers cause additional problems by plugging the lubrication holes of bronze dust stops.

Selecting a dust stop

An ideal oil-less dust stop would provide the mechanical properties and temperature performance of aluminum bronze with a low coefficient of friction. Dust stops are dynamic seals that perform at rotor speeds of 80+ rpm with face pressures of 80+ psi. A lower coefficient of friction reduces the buildup of heat. This is important since temperatures inside a rubber mixer can reach 350 [degrees] F and even higher in steam heated thermoplastic mixers. High performance engineered materials are required because temperatures at the dust stops can reach 300-500 [degrees] F.

Materials

Self-lubricating dust stops are chosen from two types of materials - reinforced thermosets and engineering thermoplastics. In general, thermoset resins are chemically cured and have no melting point, whereas thermoplastics will soften and eventually melt at elevated temperatures. Carbon fiber/polyimide thermosets provide the highest combination of mechanical strength, operating temperature capability and low coefficient of friction. They are cured in compression molding presses similar to rubber, but at temperatures of 630 [degrees] F for many hours. In the curing process, they become highly crosslinked. A typical thermoplastic polymer for use as a dust stop would be polyetheretherketone. Engineering thermoplastics have a lower performance capability in either elevated temperatures or mechanical strength and suffer from creep deformation at sustained loads. They require tradeoffs and choices in optimizing specific properties for the desired end use. To improve the mechanical strength, fiber reinforcement is added. Lubricity is enhanced by the addition of solid lubricants such as PTFE, which has an adverse effect on mechanical strength.

Strength vs. temperature

A comparison of tensile strength is shown in figure 2 for composites A (TS-A) and B (TS-B), which are thermoset polyimides, and composites C (TP-C) and D (TP-D), which are engineering thermoplastics. Composite C has a higher level of carbon fiber, whereas composite D has a higher level of solid lubricant. In figure 2, composite A (thermoset polyimide) experiences only a small change in tensile strength from 32,000 psi at room temperature to 28,000 psi at 550 [degrees] F. The engineering thermoplastic composite C has a tensile strength of 29,000 psi at room temperature, but dramatically falls off to 7,000 psi at 400 [degrees] F. The high mechanical strength of the polyimide composite allows it to be fabricated into a dust stop with the same dimensions of the bronze dust stops it replaces. In service, installation takes place on the same backing rings used with the bronze. The poor mechanical properties of the engineering thermoplastics at high temperatures require special backing rings for mechanical support.

[FIGURE 2 OMITTED]

Creep

Creep in polymers is the flow or deformation of the polymer under load versus time. The rate of deformation accelerates as temperature increases. Thermoset polyimides have negligible creep characteristics compared to thermoplastics and can absorb higher loads without deformation. Dust stops manufactured from thermoset polyimides can be machined to precise tolerances and can maintain these dimensions in service at high temperatures. Engineering thermoplastics creep under load and must be mechanically restrained.

Coefficient of friction

Because a higher coefficient of friction causes a greater buildup of unwanted heat, the coefficient of friction is the most critical property. A common test to screen prospective self-lubricating materials for seal applications is the thrust washer test (ASTM 3702). In this room temperature test, a rotating steel washer is pushed against a flat surface of the test material at constant speed and pressure. The dynamic coefficient of friction and wear rate of the material are derived from this test. A summary of the results from the tests is shown in figure 3. The thermoset polyimide samples have significantly lower coefficients of friction. This is attributed to the very low coefficient of friction of the neat thermoset resin and the high degree of crosslinking. The higher coefficients of friction of thermoplastics can be improved by the addition of solid lubricants. Not only was the coefficient of friction of composite D improved by the use of additional solid lubricants, but also by less fiber reinforcement at the expense of lower mechanical properties and dimensional stability. Figure 4 shows the low wear rates of the four composites measured in the laboratory at room temperature.

[FIGURES 3-4 OMITTED]

Performance

The laboratory provides important data for screening samples, but results obtained from mixers in field trials are invaluable. Thermoplastic materials have limited strength at higher temperatures and require special backing rings which restrain creep. The most significant difference in demanding applications is the imbedability of the abrasive carbon blacks and silica powders into the working face of the thermoplastic dust stops. Over time, these powders build up in the working face and can cause accelerated wear of the stellite wear ring.

Conclusions

The thermoset polyimides offer a combination of properties which make them ideal for use as oil-less self-lubricating dust stops. High mechanical strength at elevated temperatures combined with negligible creep allows fabrication of direct replacements for aluminum bronze dust stops. These materials have lower coefficients of friction and higher useable service temperatures than engineered thermoplastic materials. This advantage is even more significant in larger mixers with larger dust stops which reach higher linear speeds. Thermoset polyimide dust stops offer greater strength, improved dimensional stability, higher temperature and lower coefficient of friction than engineering thermoplastics. Polyimides can offer economic savings, improved quality and extended wear life over aluminum bronze dust stops.

Table 1 - summary of composite data

                          Composite A    Composite B
                           (WearComp)    (FiberComp)

Resin matrix              Thermoset PI   Thermoset PI
Carbon fiber                 Higher         Lower
Solid lubricant               Low           Higher
Tensile strength              High          Lower
  (400 [degrees] F)
Coefficient of friction       Low           Lower
Wear rate                     High           Low

                           Composite C     Composite D

Resin matrix              Thermoplastic   Thermoplastic
Carbon fiber                  Lower           Lower
Solid lubricant               Lower          Higher
Tensile strength             Lowest          Lowest
  (400 [degrees] F)
Coefficient of friction       High            High
Wear rate                     High           Medium
Table 2 - composite formulation data

                  Composite A    Composite B
                   (WearComp)    (FiberComp)

Resin matrix      Thermoset PI   Thermoset PI
Carbon fiber         Higher         Lower
Solid lubricant      Lower          Higher

                   Composite C     Composite D

Resin matrix      Thermoplastic   Thermoplastic
Carbon fiber          Lower          Higher
Solid lubricant       Lower          Higher