How Does Rubber Bond to Metal?

rubber bushings

Like most problems in today’s engineering world, there are many different solutions that will work for rubber to metal bonding. Perhaps the first, and most simple, you may think of is just adhering your vulcanized rubber piece to your metal substrate using a glue. Companies like Masterbond provide one and two-part adhesives that are capable of this type of bonding. While this may be adequate for some applications, others require a bond much more durable and sound.

In the 1950’s, great advancements were made in rubber to metal bonding technology when a primer and adhesive two-coat system was made available to bond a wide variety of elastomer types to metal. These advancements included adhesives that were heat-activated in order to bond and only adhered to the substrate during the rubber molding process. Today, companies like Parker LORD and Dow Dupont have become leaders in this heat-activated type of rubber to metal bonding and have developed a large array of products to fit certain markets and applications specifically. Products include one or two coat systems, low-HAP adhesives, and water based formulations.

When selecting one of these adhesives to use for an application, there are a multitude of factors to consider:

Elastomer Type

In order to select an adhesive, it is important to think about the rubber polymer type that will be used in the application as most of these products are categorized by rubber types. Some elastomers are harder to bond due to their polarity or lack of polarity. Some elastomers with high degrees of fluorination (FKM or Viton rubbers) can be difficult to bond so some products have special formulations to target these rubber types specifically. Rubber compounding can also play a factor as types of fillers, plasticizers, and curing agents can all affect bonding with an adhesive.

Parker LORD’s variety of Chemlok rubber to metal bonding products can be found here.

Substrate Preparation and Activation

Surface preparation is key when it comes to rubber to metal bonding.

It is not to be overlooked how important surface preparation is for your substrate material. Many metal parts can be supplied with small amount of cutting oil residue and other contaminants left behind which can severely diminish a bond’s strength. It is best to use a solvent wash (IPA or MEK) to remove anything left behind on the surface of your metal substrate. Then, a next step to further improve the surface area capable of bonding, it is recommended to mechanically abrade or grit blast the surface. Once the substrate surface has been mechanically treated, is it best to minimize the amount of time that occurs before applying the primer or adhesive to prevent further contamination or metal oxidation.

Primer and Adhesive Application

A spray gun commonly used for rubber to metal bonding adhesives.

Several methods of applying primer and adhesive to substrates are typically used. These include dipping, brushing, spraying, and rolling. The most important thing to remember when applying is that an optimal dry film thickness if achieved. Most adhesive and primers have a technical data sheet that will list the optimal dry film thickness however many primers need to be around 5 – 10 microns and many adhesives need to be around 15 – 20 microns. For applications like spraying and dipping, careful consideration must be given to the viscosity of adhesive required to give an optimal film thickness. It is common for these primers and adhesives to be diluted with a solvent in order to become workable enough to apply appropriately. Between a primer and adhesive coat and as well as before being over-molded, the coated part must be dry and stored in a manner that prevents contamination.

Molding

Rubber parts that are being bonded to metal substrates are typically molded using compression, transfer, or injection molding. The molding process must be given careful consideration so that the rubber vulcanizes at the same time the adhesive cured to it. Adequate pressure must be maintained within the mold cavity so that the rubber inside and the adhesive are in close contact with each other. Metal parts that are coated with adhesive can be prebaked before going into the rubber mold so in order to aid the molding process. Proper care must be taken so that prebaked parts do not get too hot and prematurely activate and cure the adhesive coated on the part’s surface. It is important to know the activation temperature of your adhesive so that you can account for this in your molding process parameters. Most bond failures are a result of inadequate activation of the adhesive or contamination of the adhesive-coated surface prior to rubber molding.

Contact Wayne Rubber to explore how we can provide you with quality rubber to metal bonded parts in any volume!

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