In order to produce the highest number of quality parts quickly and economically, adhesive bonding in industrial applications will ideally require a minimum number of steps from start to finish. The simplest, fastest applications require just two steps—application of the recommended adhesive to the substrates, and mating of the parts. There are, however, a wide and growing variety of applications that require additional steps in the bonding process. Typically, these fall into one of the following two categories:
Adhesive Applications featuring difficult-to bond substratesHard-to-bond substrates have lately proliferated in the medical device and other markets, partly as a result of growing concern over the safety of certain plasticizers. For this and other reasons, many applications feature substrates that are difficult or impossible to bond without surface treatment. Examples of difficult-to-bond substrates include:
- PTFE (manufactured by DuPont as Teflon®)
Adhesive Applications requiring clean surfacesCleaning is the second major reason for surface treatment in an adhesive application. Clean surfaces may be required for a variety of reasons, including:
In a variety of different markets, surfaces must remain free of contaminants. Notably, medical device applications often require removal of all bio-contaminants to allow safe use on or within the human body.
- Adhesion promotion
Even on relatively high surface energy substrates, clean surfaces are preferred for adhesion. In addition, certain methods of surface preparation increase bond strength without the need for chemical primers, a critical requirement for select medical applications.
- Static and dust removal
Especially where aesthetics are of particular importance, or in electronic manufacturing applications.
Types of Surface Treatment for Adhesive ApplicationsMany different types of surface treatment are available, falling into four main categories:
- Mechanical methods, such as sanding and grit blasting
- Fluid application, like power-washing and solvent wiping
- Chemical means, e.g. caustics/acids & primers
- Physical treatments, including corona, flame and plasma
Because surface preparation methods differ so dramatically in how they interact with your substrates, it is sensible to investigate surface treatment methods criteria-by-criteria. A review of every different surface treatment and its attributes is beyond the scope of this overview, but we can focus on the most easy-to-quantify results to make basic comparisons. The graphs below contrast the ability of two different surface preparations—primer application and plasma treatment—to promote adhesion on a variety of substrates:
These results confirm the ability of both primer and plasma treatment to promote adhesion, dramatically at times. In particular, the application of primer radically increases bond strength on low-surface energy substrates, making it possible to use materials that would otherwise not merit consideration. Plasma treatment is also effective in promoting adhesion, to a lesser degree; but plasma’s most tangible benefit is to allow surface preparation to occur up to 24 hours in advance while still promoting adhesion, although the results measured after the delay vary unpredictably compared with those measured immediately after treatment. Interestingly, cold rolled steel, typically considered an easy-to-bond substrate due to its high surface energy, becomes more difficult to bond after surface treatment. This again underlines how carefully surface treatments must be chosen–not all substrates will react to surface treatments as desired, and many treatment types come with ancillary benefits or drawbacks. In order to aid your surface preparation decision making process, the major types of surface treatments are listed below, along with a brief outline of their advantages and disadvantages.
|Surface Prep Type||Advantages||Drawbacks|
|Mechanical Sanding Grit blasting||Straightforward application Promotes Adhesion Inexpensive||Damages the substrate Does not provide sterilization of cleaning|
|Fluid Power-washing Solvent wiping||Easy application Promotes adhesion||Can damage substrates Some methods may harm environment|
|Chemical Caustics/Acids Primers||Quick Application Promotes adhesion highly effectively||Can damage substrates Some methods may harm environment|
|Physcial Cornoa Flame Plasma||Promotes adhesion Some methods provide sterilization and/or cleaning||Requires special equipment Can be expensive Can be difficult to work into production line Some methods may harm environment|