The In’s & Out’s of Primers

Are you gluing plastic or metal in Ireland? Do you need to prime these materials? Are primers important?  What is surface preparation?

These are questions which our technical department are asked on a regular basis!

Very often, the priming stage of any glue or for that matter sealant application, receives only scant attention. However, we must remember that some materials, in particular, certain types of plastics and elastomers, must be primed in advance of the application of any glue or sealant product. In addition, porous materials, such as wood and concrete, should be primed in advance of the application of sealants.

What is the function of a primer? Primers improve the performance of the bonding process, offering better adherence and better age resistance. Primers also create a barrier against plasticiser migration. Plasticiser migration is a very troublesome phenomenon during which certain chemicals in some materials, such as many flexible materials, leach into the adhesive film, effecting it in such a way that its mechanical properties are drastically effected. Finally, primers protect and stabilise certain types of surface, as an example metallic & coloured surfaces.

How do we select a primer? The selection of the most suitable primer, will depend on the surface properties & chemistry of the materials being glued. We must discover if the  surface is porous or non porous. Porous surfaces, such as some types of masonry and ceramic, are best primed. Typically, if an adhesive is applied to a porous surface, the adhesive may penetrate into the surface, and effectively starve the bonding surface. Priming may often be the only way to ensure a good bond!

Some surfaces, in particularly many plastics, rubbers and glass, are difficult to wet out. A simple test is composed of putting a drop of water on the surface. If the drop of water runs off the surface without leaving any residue, priming may be suggested.

We manufacture a range of primers which fulfil most applications. These primers have been formulated for ease of use, and offer an excellent base coat for the subsequent application of many adhesives & sealants.

  • X90. Typically used when bonding traditionally difficult to bond materials such as silicones, Santoprene®, EPDM, Suryn®, Teflon®, polypropylene, polyethylene, Neoprene® and Viton®.
  • X250. The choice primer for use on non-porous materials such as painted surfaces and ceramics. The choice product for use with our Hi-Bond® VST tapes.
  • X255. Designed for priming porous materials such as wood & concrete.
  • X260. A highly reactive primer designed for use on thermoplastics such as acrylic, PVC and ABS. Also suitable for composite materials such as vinyl ester and polyester.
  • X330. Our latest primer/bond activating product. Designed for use on glass, plastics and composites.

Before the application of any primer or other surface treatment, the bonding surfaces must be dry and scrupulously clean.The typical surface contaminants, such as oil, grease, lubricants , wax, dust, and rust, must be absent. Dirt or other heavy surface contaminants should be removed with soap and water, preferably using a scouring pad.

The surface should next be cleaned with a product such as our X35 or X320, which are solvent degreasers. X35 is a general purpose degreaser, fast drying and most suitable for small areas. X320 is a speciality degreaser, with a finely controlled evaporating rate. Ideally suited for degreasing larger plastic & composite parts.

It is good practice to change the cloth or towel after each wipe!

It is often suggested that very smooth plastic or composite surfaces should be roughened by abrading with P180 sandpaper.

Metal surfaces are best abraded with P120 sandpaper. Mill scale or surface rust must always be removed. It is important that all loose particles are removed prior to treatment. After the first solvent wiping step and abrasion, good practice suggests a second solvent wipe is undertaken. This will remove any loose materials remaining. All residues must be vacuumed or blown off the bonding surfaces with compressed air.

Once the surface has been solvent wiped, abraded, dusted off and solvent wiped again, the primer is ready for direct application. This can be achieved by immersion, dipping, spraying, brushing or wiping. Primer application using a lint free cloth or paper towel moistened with the primer, is a very effective and rapid wiping method.

Evaporation of the solvent within the primer can be performed at room or elevated temperatures. The formation of a consistent primer film, firmly adhering to the substrate, requires only a moderate atmospheric air flow, or depending on the chemistry of the primer, the relative humidity. As a guideline, a solvent flash off time of 4 to 9 minutes is usually sufficient. Ideally, the adhesive or sealant should be applied to the primed surface within 5 hours after priming. Should it be necessary, it is possible to reapply some primers, after an initial application.

Please note that several of our primers are  sensitive to humidity. In contact with moisture the liquid becomes turbid and the adhesive strength of the primers is impaired.

Of course, not all adhesives need primers. As an example, the Scigrip range of structural methacrylate’s are primerless adhesives!!

The selection of a suitable primer, both to suit the bonding surfaces, and the adhesive being used, is of critical importance to the longevity of the bonded assembly. We are experts in surface chemistry & our technical service will kindly assist you in all applications.