Surface preparation is the essential first-stage treatment of a substrate before any coating or plating is applied. The performance of a coating is considerably influenced by its ability to stick properly to the substrate material. It's usually well established that correct surface preparation is the most vital determinant of the success of any surface treatment.
The presence of even tiny amounts of surface contaminants, like oil, grease and oxides, will reduce coating adhesion to the substrate, and reduce the longevity of many materials. Surface preparation significantly boosts the durability of the adhesion bond, particularly once it's exposed to humidity. That's because invisible chemical contaminants, like chlorides and sulfates, attract moisture through coating systems, leading to premature failure. Here we'll take a look at the basics of surface preparation and how it can be used to avoid premature failure.
What Is Surface Preparation?
Surface preparation or treatment refers to a series of operations that includes cleansing, the removal of loose material, and physical and/or chemical modification of a surface to which an adhesive is applied. It's a way of treating a surface to increase its attraction to coatings. It may be mechanical or chemical and is crucial to the success of a coating. In plastics bonding, surface preparation is aimed at increasing the surface polarity, increasing surface wet-ability and making sites for adhesive bonding.
Reasons for Surface Preparation
There are many reasons for the surface preparation of materials. Surface preparation is meant to boost the bonding strength to metal surfaces, but the most vital reasons for applying surface treatments before bonding are:
- To eliminate or stop the formation of a weak layer on the surface of the substrate
- To maximize the degree of molecular interaction between the adhesive or primer and therefore the substrate surface
- To optimize the adhesion forces that develop across the interfaces to confirm adequate initial joint strength and joint strength through the service lifetime of the bond
- To produce a specific surface microstructure on the substrate
Surface preparation is employed in metals and plastics to alter their surface properties for:
- Decoration and reflectivity
- Improved hardness and wear resistance
- Corrosion protection
- A base to enhance the adhesion of different treatments like painting or sensitive coatings for printing
- Reducing cost
(See a case study in the article The Anatomy of a Coating Failure.)
Surface Treatment of Metals
Preparing the surface of a metallic object involves multiple steps. It's not possible to get a high-quality adhesive bond while not cleansing (and abrading) the metal surface. Metals have high-energy surfaces and absorb oils and different contaminants from the atmosphere.
The surface preparation steps required are listed below:
- Cleansing: Employing a solvent or different chemicals
- Elimination of loose materials
- Enhancement of corrosion prevention
- Surface hardening: This may be mechanical or chemical and is used to strengthen the surface
Metal surfaces are best cleansed by vapor degreasing with effective liquid systems. This treatment is followed by grit blasting to extend the adhesive contact by roughening the metal surface. Chemical etching removes feeble bonded oxides from the metal surface and forms an oxide that's powerfully bonded to the majority of the components.
Surface Treatment of Plastics
Polymers have an inherently lower surface energy than metals (assuming they're contaminant-free). As such, they have a tendency to make poor adhesion bonds. However, there are chemical, physical and bulk treatment strategies out there for adhesion improvement. These include:
- Chemical modification techniques, including those that sometimes require wet or chemical reactions. These include wet etching, grafting, acid-induced oxidation and plasma polymerization.
- Physical surface treatment strategies, including corona discharge, ion or electron beams, photon beams (laser, UV and X-ray), plasma discharge and flame oxidization.
- Bulk strategies involving additives, blending or recrystallization, all of which have an effect on the properties of plastics.
Some of the techniques are restricted within the scope of their use. For instance, chemical treatment (acid-induced oxidation) is the most often-used technique to impart adherability to plastic surfaces. Plasma treatment is restricted to smaller parts and components. Flame and corona treatments are effective for continuous films (often referred to as "webs") and thin sheets of plastic, typically operated at high speeds.
Methods of Surface Preparation
Any surface preparation needs the completion of one, two or all of the following operations:
- Degreasing or cleansing
- Physical treatment or abrasion
- Chemical treatment
Priming might also be administered in some cases to confirm superior durable bonds when a substrate is being used in a significantly adverse environment.
Degreasing or Cleansing
This involves the removal of all traces of oily contamination and grease from the substrate surfaces to be secured, and is important to the formation of sturdy adhesive bonds. Strategies such as vapor or ultrasonic degreasing, spraying, immersion and scrubbing may be used for degreasing, and should be used even if the substrate appears to be clean.
- Vapor Degreasing: In vapor degreasing, an object is submerged in trichloroethylene (TCE) or perchloroethylene vapors, although the vapors are pure uncontaminated solvent. Because the vapors condense on the components, contaminants are dissolved and drip off and dispense with the condensed solvent. Once a vapor degreasing unit isn't accessible, the joint surface is wiped with a material soaked with TCE, followed by complete evaporation from the joint surfaces. These solvents are harmful in each liquid and vapor form, requiring the work environment to be vented.
- Scrubbing: Consists of scouring the joint surfaces using a solution of a detergent. In scrubbing, the metal is immersed in or sprayed with a basic degreasing agent, followed by rinsing with clean, hot liquid and completely drying by hot air, steam or ambient air.
- Ultrasonic Degreasing: Ultrasonic degreasing provides glorious results for tiny elements. TCE, acetone and chloride, and tetrachloroethylene are among the solvents used for ultrasonic degreasing.
To verify the cleanliness of a surface that has been degreased, water or dyne liquids are used. If a drop of water forms a layer on the surface, then it's free from contamination. If a drop retains its form, degreasing of the surface should be continued.
Abraded, rough surfaces sometimes form stronger adhesive joints than extremely polished surfaces do, primarily owing to a larger contact area for adhesion. A properly abraded surface mustn't contain any sleek or polished areas. Abrasion treatment ought to be followed by a second degreasing treatment to confirm the removal of loose particles. Abrasion includes grit blasting (the most common technique), wire brushing, sanding and abrasive scrubbing. Grit blasting removes surface deposits like tarnish, rust or mill scale from metal surfaces. If grit blasting instrumentality isn't accessible, or the metal is just too skinny to resist blast treatment, clean the combined surfaces with a wire-brush, abrasive or sandpaper. Painted surfaces ought to be stripped all the way down to the substrate with a stripper before preparation, otherwise the adhesive joint won't be robust.
Sometimes, only a small amount of damage to the substrate is needed. Plastic grit, walnut shells or bicarbonate of soda will clean the substrate or take away the previous coating. Sometimes, the abrasive is propelled by pressurized water. (Discover the advantages of this technique in Understanding the Industry Shift To Wet Abrasive Blasting.)
Chemical treatment is a method of treating a clean surface by chemical means to boost its adhesion qualities. Solvent cleansing must always precede chemical treatment and, frequently, intermediate cleansing ought to be employed in between. Degreasing alone, or degreasing followed by abrasion and more degreasing, is adequate for several adhesive bonds. To get the most strength, reproducibility and resistance to deterioration, a chemical or electrolytic pretreatment is needed. Careful attention ought to be paid within the preparation of chemical solutions to ensure the correct proportioning of the elements needed for the formation of adequate bond strength. Exposure time within the solution application is vital. If the application is too short, it won't sufficiently activate the surfaces. Overexposure to the solution, on the other hand, builds up a layer of chemical reaction products that will interfere with the adhesion bond formation.
A quick and dirty test for adequate chemical treatment is to put a bead of water on the surface of the object and note if the water spreads. If so, the contact angle it forms with the surface should be extremely small. There are special fluids, referred to as "dyne liquids," which bracket the numeric worth of the surface energy of a material surface fairly precisely.
An adhesive primer is typically a dilute solution of an adhesive in an organic solvent. The solution is applied to the adherend (or adhesive film), creating a dried film with a thickness of 0.0015 to 0.05 mm. Priming has the following effects:
- It improves wetting
- It protects the adherend’s surface from oxidation when surface cleansing
- It helps inhibit corrosion
- It serves as a barrier coat to stop unfavorable reactions between the adhesive and adherend
- It holds adhesive films, or adherends, in situ throughout assembly
The use of primers gives additional versatile production planning, high dependability of joints, less rigorous cure conditions and a wider latitude in the selection of an adhesive system.
Atmospheric Plasma Technology
A newer surface preparation technology for cleansing surfaces for bonding is atmospheric plasma technology, a technology that's kind of like corona treatment. Plasma surface treatments allow nearly limitless surface modifications. Materials can come in any size, from nano-sized parts to endless sheets in continuous production. A large variety of materials may be treated with plasma surface technology, including glass, metals, metalloids, rubbers and polymers.
Factors that Affect Surface Preparation
In order to put surface preparation into perspective, the adherend-to-organic material (i.e. adhesive) interface should be considered based on factors ranging from style to fabrication. The joint design, adhesive choice and process should also be carefully considered. These factors are mutually beneficial. The use of an optimum surface preparation is of very little worth once an unsuitable adhesive is employed. In this case, the bond isn't properly processed, or the joint design involves peeling or cleavage stress. Correct surface preparation ensures that the weakest link in an adhesive joint exists among the adhesive or organic material layer and not at its interface inside the adherend. The fast depletion of active chemicals in an immersion bath, or the buildup of foreign materials within the bath that offers rise to weak boundary layers also warrant concern. Safety is also an important issue, as surface preparation uses many toxic chemicals that may pose a threat to workers if not handled carefully.