Surface Peparation

Introduction.
 

When it comes to coatings, there is one single factor that influences the success of an application more than any other; adhesion. Without a strong bond to the substrate, any coating will be fundamentally flawed and the longevity of the solution may be jeopardised. Happily, Belzona is on hand to deliver the support required to ensure that every application can be successful.

Firstly, we have spent many years researching adhesion and developing our products with this in mind – so most of the work is already done! All that is left to do is to pay attention to a few key points during surface preparation and application; but before we get onto that, let us talk a little about adhesion theory.

Four Modes of Adhesion

Before any Belzona product is applied, it is important that the surface onto which it is being applied must be prepared, otherwise, the adhesive bond between Belzona and substrate may not be sufficient. There are four modes of adhesion:

  • Mechanical Adhesion

This varies with the penetration of the adhesive into the substrate, and is the major reason for preparing the substrate surface prior to bonding.

  • Electrical Theory

This theory states that two surfaces should be considered as two plates of an electrical condenser, which have opposing charges and, therefore, have mutual attraction. It is not certain whether metal to polymer laminates are charged prior to separation.

  • Diffusion Theory

This theory is only applicable for materials bonded together which are of a similar nature. Adhesion is due to mutual diffusion of molecules across an interface. This mode of adhesion occurs when one Belzona product is applied to another within the overcoating time as stated on the Instructions For Use leaflet.

  • Adsorption Theory

This states that the forces that hold two surfaces in contact are the same as those that cause the cohesion of all materials i.e. Van der Waals forces, hydrogen bonding and covalent bonding. Since the force of attraction between two atoms is proportional to the distance between them, strong adhesion is only obtained when the adhesive is in intimate contact with the substrate.

 

 

Theory For Successful Applications.

 

A key mode of adhesion utilised by Belzona products is mechanical adhesion, which contributes significantly to the overall bond strength. It may not be obvious, but on the micro-scale most surfaces are very rough, with steep peaks and troughs across the entire area. Mechanical adhesion relies on this irregularity to form a bond. The product is applied in a liquid or paste form, allowing it to seep into all the small gaps and creating what is known as mechanical interlocking. When the product solidifies it is then locked into place, which over the whole area creates a very strong bond.

 

 

Adhesion can be greatly improved and optimised by the correct surface preparation techniques. In order to achieve this, it is important to remove any corroded or loose material, as well as contamination such as moisture, oil, salts and general dirt. Such contamination will prevent the product from making contact with the surface, reducing the overall adhesion. In addition, trapped contamination can cause damage to the substrate, even if a protective coating is placed over the top. The second point to note is that by using abrasive preparation techniques, such as grit blasting, the surface roughness is increased, which in turn amplifies the effect of the mechanical interlocking and leads to greater adhesion.

The roughness or “profile” of a surface is expressed as a measurement of distance, typically given in microns or mils. This value represents the distance between the highest peak and lowest trough of the surface. For optimum bond strength, a minimum average surface profile of 75 microns (3 mils) is recommended.

 

Profile height

 

Surface preparation.

 

Now that we have established what we are looking for in surface preparation, we can move on to talk about how these criteria can be achieved. Listed below are the methods used to clean and roughen a surface for improved bond strength, ranked in order of effectiveness:

  • Abrasive blasting

Removes oil, grease, mill scale, corroded metal, general dirt and most other types of contamination, whilst also creating a rough surface profile. Angular type abrasive grits are recommended for the best results. This is the only method of surface preparation that Belzona recommends, prior to application of coatings destined to operate in immersed conditions.

  • Machining

Overall, a rough, irregular finish is desired, which removes the top layer of material whilst simultaneously taking away most types of contamination. This method is ideal for shaft applications, as these can be easily achieved on a lathe.

  • Mechanical grinding

Allows for a rough, irregular surface, which removes contamination, as with machining. This provides a good result, however abrasive blasting gives a better surface profile. Ideal for scenarios where abrasive blasting equipment cannot be used, such as where access to the area is limited.

  • Hand tools

Good for removing general contamination and corroded material, however it can prove difficult to remove mill scale, so hand tools are not recommended for new metal. Furthermore, the mechanical bonding potential is reduced with hand tools because it creates a less than ideal surface profile. This method is preferred for scenarios where short term/emergency repairs are being made, or in softer metals, such as lead, where the profile can be created more easily.

  • Emery paper

This will remove some contamination and roughen softer surfaces such as lead. Also, it can be used to clean and roughen some existing paints or coatings before over-coating.

 

Always contact Belzona for advice on applying over existing paints and coatings.

 

Where possible Belzona also recommends using an industrial degreaser to clean the surface prior to and post surface preparation. Degreasing the surface before surface preparation will remove most of the grease and oil, preventing this from embedding into the surface during preparation. Degreasing after surface preparation removes any final contamination that may have been left on the surface during preparation. This is necessary as the tool used to prepare surfaces can often pick up and hold contaminants during preparation, hence it may still leave small amounts on the surface. If salt contaminants are detected on the surface, it is recommended that high-pressure water jetting is used. This effectively removes any soluble salts present in the substrate, creating a preferred surface for improved performance. Furthermore, salt removal aids, such as Saltaway, are also available to improve and speed salt removal.