Flocking.

Flocking

Flocking. That is the term used for the process where short length (0.3 to 3.0 mm) monofilament fibers are coated onto a rubber surface perpendicular to the rubber surface. It is used to a great extent in the automotive industry, particularly in window channels, belt line seals and other places where there is a need for low coefficient of friction and good sealing of the surface.

Historically, normal thermoset elastomers such as EPDM, SBR, etc., have been the only rubber materials to which flocking has been applied. The technology for this is well established. While flocking of the surface is not inexpensive, it provides a number of distinct advantages over "bare" elastomers. These include:

* Lower coefficient of friction when moved over a hard surface such as glass, metal or hard plastics.

* Reduced coefficient of friction over hard surfaces - 0.2 to 0.3 versus 0.6 or more.

* Resistance to abrasion and wear.

* Provides compensation for dimensional tolerance problems.

* resistance to degradation in a variety of fluids.

* good appearance and "feel."

Recently, techniques have been developed to allow some of the newer elastomeric alloy (EA) types of thermoplastic elastomers to be flocked. Because of the fully saturated nature of the elastomer surface, achieving satisfactory adhesion and performance have been part of the challenge. However, the ease and economy of thermoplastic processing combined with the closer tolerances capable with TPEs have made the effort worth pursuing.

How is flocking done?

With normal rubber extrusions, the first part of the flocking line is devoted to normal extrusion and curing systems. Once cured, the surface to be flocked is coated with a suitable adhesive and the material run through the flocking unit.

Figure 1 shows basically how the flocking is applied. The coated extrusion is passed through an electrostatic chamber. The profile is run across a cathode screen while the fiber passes through an anode screen placed above the profile. The anode is kept at a positive voltage of 60 to 80 kilovolts greater than the cathode. In the electric field, the fibers (which are polar) align themselves with the field and accelerate toward the profile. They impact there at a speed of approximately 1-2 meters per second and at an angle nearly perpendicular to the surface. The fibers embed themselves into the adhesively coated surface and are there bonded in place.

The feeding chamber typically uses rotating brushes to supply the chopped fiber to the anode screen. Fiber that does no impact the rubber surface is recovered and recycled to the feeding chamber.

Important aspects of control for the flocking process are

* Use of proper adhesive (must bond to both the rubber and the fiber)

* Proper application of the adhesive

* Removal of contaminants from recycled fiber

* Humidity control in the chamber

* Temperature control in the chamber.

A "normal" production line will be approximately 300-350 feet in length. Processing speeds run in the range of 30-100 feet per minute.

What about TPEs?

Advanced Elastomer Systems, L.P. of Louvain La Neuve, Belgium, has worked out the technology of fabricating flocked profiles using elastomeric alloy TPEs.

The steps involved in the EA TPE flocking process are: profile extrusion; application of primer; cool to 120 [degrees] F; remove water; application of adhesive; electrostatic flocking; heat to 280 [degrees] F; cool to ambient; remove water and seven day aging (complete cure). As in the traditional process, the first step is extrusion of the profile. Immediately after leaving the extrusion die, the profile is sprayed with a reactive primer that is solvent diluted. At normal extrusion temperatures (370-390 [degrees] F), the solvent is evaporated rapidly and the primer is heat welded to the profile. The primer layer used is quite thin, 2-5 micrometers.

The coated profile is then passed through a water bath to cool it to about 130 [degrees] F. The surface is then dried using airknives and a one component urethane adhesive is applied. Normally, this adhesive is catalyzed to allow curing at lower temperatures. Application of the adhesive is by either spraying, brushing or other coating technique. By applying the adhesive at a temperature of 130 [degrees] F or so, solvents used in the adhesive are quickly removed and the adhesive itself is softened to allow better penetration of the fiber.

Application of the flock is essentially the same as with normal thermoset rubber. Flock is applied in an electrostatic chamber by passing the extruded profile through on the cathode screen while feeding in fiber from the anode side. In the work that has been done, the extrudate is about eight inches below the anode and the anode is kept at a positive voltage of 70 kv compared to the cathode. Faster line speeds can be accommodated by using two or more flocking units in series on the line.

The flocked profile is then passed through a heating oven to heat the adhesive to a temperature of approximately 280 [degrees] F. This allows the adhesive to partially cure. While the adhesive layer is being warmed, it is important to blow cool air on the underside of the profile so that the bulk temperature of the material does not exceed 212 [degrees] F. Oven time of 2-3 minutes is usually adequate.

This time does not fully complete the cure, but carries it to a point sufficient to hold the fibers properly in place. Further aging of one week at ambient temperature is required to complete the cure.

After this heating step, the profile is again run through a water bath to reduce the temperature to ambient. Airknives are again used to dry the profile.

Final curing of the adhesive takes place as the material ages at ambient temperature. Humidity during this aging should be kept at 50%. This final cure requires about one week.

How about performance?

Line speed on the flocked EA profiles that have been run has been slightly slower than the traditional rubber - 15 to 65 feet per minute. Factors affecting this include the profile shape and volume, the specific adhesive system used, the desired adhesion level, oven temperatures, length of oven and the number of flocking units being run.

As they have been run, typical flocking lines for EA materials are much shorter than the traditional lines: 125 to 210 ft. This includes the extruder and support equipment.

To date, a number of EA profiles with a variety of shapes have been run. These were done using available EA TPEs, fibers and adhesive systems. In these cases, bonding of the fibers to the EA have met the same requirements of traditional rubbers and passed. Tests run include the following:

* Fiber removal. Profile surface can show only minor fiber removal.

* Fiat wax peel test. The flocked profile is embedded fiber side down in a melted wax. Upon cooling, the rubber is peeled from the flocked surface in a tensile tester in a 180 [degrees] peel. Testing is done on aged and unaged samples. EA samples have easily exceeded minimum peel strength requirements. Values are limited by the tear strength of the rubber.

* Epoxy peel test. Similar to the Fiat wax peel test except that a two part epoxy is used. Also, care is taken that the epoxy is only allowed to coat the fiber, not the adhesive layer or the rubber. After curing the epoxy, peel tests are run on a tensile tester using peel angles of 90 [degrees] and 180 [degrees]. EA materials have again exceeded minimum required values and performed very similarly to traditional elastomers.

* Glass run channel test. The flocked surface of an automotive glass channel is cycled back and forth over an automotive glass surface under a specified load. After a given number of cycles (e.g. 30,000) the channel is inspected for wear and abrasion. EA samples have passed.

In the cases run, EA type thermoplastic elastomers have demonstrated performance and appearance equivalent to traditional rubber.

Why use EA materials?

Thermoplastic elastomers offer a number of advantages over conventional rubber in a number of areas.

* Ease of processing. Virtually everyone is aware of the relative ease and simplicity of processing plastic materials, particularly extrusions, compared to thermoset elastomers. While the price per pound is somewhat higher than most rubber materials, EAs offer a 15% to 40% advantage in density which offsets the higher cost per pound. Combining the density advantage with the lower cost of processing can result in overall savings of $.30 to $1.00 per pound of final product.

* Weight savings. As just noted, EAs have a significantly lower density than most traditional rubber materials.

* Dimensional control - thermoplastic elastomers, including the EA type, offer a significant advantage in dimensional control and holding close tolerances. Tolerance ranges can be reduced by as much as a factor of 3 compared to traditional rubbers. This can allow differences in design of parts.

* Color. Traditional rubber has a limited range of color available, unless you really like black. EAs offer the ability to color match to almost any desired color.

* Serviceability. EAs offer excellent resistance to aging and attack by both oxygen and ozone. In addition, they have outstanding resistance to fatigue and flex failure. The ability of the adhesive systems being used on the flocked material to match the performance of the EA is still uncertain.

* Co-extrusion. Use of EAs allows the possibility of co-extrusion with other EA elastomers or other plastics. This could allow for further process simplification and manufacture of composite parts that use a hard layer underneath a softer EA layer that can be flocked.

Summary

It appears that the capability of elastomeric alloy thermoplastic elastomers to perform in automotive applications has been demonstrated. All the equipment needed to manufacture this type of product is readily available. Flocking units that have been used for traditional rubber have been found suitable to run with the TPEs. And there are definite advantages to use of TPEs.

Feasibility has been proven. I suspect we'll see a shift in this business in the near future. [Figure 1 Omitted]