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Solutions Current Issues > July.Aug.Sept_2007 >ON THE SURFACE

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TECH SESSION

ON THE SURFACE

Coatings and surface treatments can enhance the function and appearance of rubber, plastic and metal components.

Rick Hudson

WHERE THE BUCK STOPS: Rick Hudson is
founder and CEO of RL Hudson.

by Rick Hudson

We continually look for ways to add value and to give our customers more choices. One way to do this is to offer surface treatments and coatings that may be applied to rubber, plastic, and metal surfaces to change their properties, their appearance, or both.

Of particular interest are surface treatments and coatings that improve friction and wear properties, reduce permeability to gases and vapors, improve resistance to environmental conditions, or provide color differentiation. Some surface treatments or coatings can provide multiple benefits.

DEFINITIONS Before we go any further, let’s define what we mean by “surface treatments” and “coatings.” In general, a surface treatment chemically or physically changes the surface, while a coating does not. Surface treatments and coatings are usually applied to the article after it is manufactured, but it is also possible to apply one or more layers of a dissimilar material during the manufacturing process. Examples include fluoropolymer-encapsulated rubber O-rings and blow molded containers with one or more co-extruded inner barrier layers or an outer color layer.

DECIDING WHEN TO USE A SURFACE TREATMENT OR COATING While surface treatments and coatings confer many benefits, they also increase the cost of the article. With rubber, plastics, and metals, it may be more cost-effective to improve the properties of the base material. Further complicating the decision is that the best solution to a particular design problem may be to use a different material instead of attempting to improve a material’s properties by incorporating additives or applying a coating or surface treatment. For example, even when compounded with protective antioxidants, antiozonants, and waxes, SBR (Styrene Butadiene Rubber) cannot match the long-term outdoor weathering resistance of EPDM (Ethylene Propylene Diene Monomer). A surface treatment or coating is clearly called for when the desired combination of properties in the finished product cannot be obtained from the base material. An example is chrome-plated ABS (Acrylonitrile-Butadiene-Styrene) plastic; another (and very familiar) example is rust-resistant finishes on steel.

TREATMENTS AND COATINGS FOR RUBBER Here’s a quick overview of the various coatings and surface treatments available for rubber components:

For reduced friction and enhanced abrasion properties:

 

For reduced permeability to gases and vapors:

 

For enhanced resistance to environmental conditions (UV, ozone, oxygen, and water)

 

For color differentiation:

 

Rubber materials pose particularly difficult challenges to coatings, and to a lesser extent, surface treatments. This is because a rubber article may be elongated (stretched) a few percent to as much as three hundred percent or more, and in dynamic applications it may be repeatedly stretched and allowed to relax. Few coatings can tolerate such conditions. However, coatings can be very successful when the elongation is small.

The same holds true with compression and flexing, although the change in dimensions is smaller. Rubber O-rings have long been successfully coated with fluoropolymer, which is many times stiffer and less extensible than the rubber substrate.

A type of “coating” that can be applied to elastomers is a veneer of another elastomer. Although applying such a veneer at the time of molding adds complexity and cost to the process, the resulting composite is very reliable. In addition, the total cost can often be reduced by applying a thin layer of a more expensive material over a substrate of a less expensive material.

Veneering is far easier with extruded products, such as tubing, because an inner or outer layer (or both) can be co-extruded.

Because they become part of the surface, surface treatments can better withstand elongation, compression, and dynamic applications. However, certain desirable properties, such as color differentiation, are not available through the application of surface treatments.

TREATMENTS AND COATINGS FOR PLASTICS Plastics are often (but certainly not always) chosen over metal for their combination of light weight, low cost, and resistance to corrosion. Because it is easy to incorporate additives into plastics at the point of use, and to change the additives package when necessary, plastics are very often modified to suit the application instead of being surface treated or coated—as is also true of rubber.

However, when a coating is called for, certain plastics, called polyolefins, may have to be surface treated to permit adhesion of the coating. Two polyolefins in extremely wide use are polyethylene and polypropylene. To make the surface chemically active, one of three surface treatments may be used: flame, plasma and corona discharge.

In flame treatment, the surface is briefly exposed to a very hot oxidizing flame. The exposure isn’t long enough to melt the plastic, but it does break down the surface and provide active sites for the adhesion of the coating. In plasma treatment, a high-voltage electric arc ionizes the air in the vicinity of the plastic surface. The ions then attack the surface, breaking it down and enhancing adhesion.

In corona discharge treatment, a high-voltage electric arc directly impinges upon the plastic surface, again breaking it down chemically and structurally to promote adhesion.

Because these processes add cost and pose certain hazards, paints have been developed that will bond directly to olefinic plastics. They are, however, relatively expensive.

Surface treatments for reduced friction and enhanced abrasion in plastics:
• Surface fluorination
• Sulfonation
For reduced permeability to gases and vapors:
• Infusion of metal, metal oxide, or polymeric precursors
• Ethylene vinyl alcohol (EVOH) barrier layer
• Polyamide (nylon, PA) barrier layer
• Surface fluorination


For enhanced resistance to environmental conditions (UV, ozone, oxygen, and water):
• Metal plating
• Paint
• Vacuum metallization


For color differentiation:
• Paint
• Vacuum metallization
• In-mold color film application

TREATMENTS AND COATINGS FOR METALS Of the materials of interest to RL Hudson’s customers, metals probably enjoy the greatest variety of coatings and surface treatments. In part, this is because of the very long history and extremely wide use of metals. It’s also because most common metals are susceptible to corrosion, which plastics and rubber are not.

For reduced friction and enhanced abrasion properties:

 

For enhanced resistance to environmental conditions (primarily corrosion):

 

For color differentiation:

 

IN CLOSING I hope that this broad-brush introduction to the world of surface treatments and coatings will give you an idea of what’s possible, and encourage you to think of how your applications could benefit. If we can be of any assistance to you, please call us toll-free at 1-800-722-6766, or email us at info@rlhudson.com We look forward to putting our knowledge and experience to work for you