Adhesives for Hard-to-bond Substrates & Clean Surfaces Require Surface Treatment

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 substrates

Hard-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:

A common feature of these substrates is their low surface energy. When substrates with low surface energy come into contact with liquid adhesives of a higher surface energy, the adhesive is denied the crucial ability to “wet out,” or cover the surface effectively. Instead, low surface energy substrates cause liquid adhesives to bead up, limiting their surface coverage and significantly lowering bond strength. The picture below illustrates the difference between proper wetting (at left) and undesirable beading (at right):

Adhesive Applications requiring clean surfaces

Cleaning is the second major reason for surface treatment in an adhesive application. Clean surfaces may be required for a variety of reasons, including:

Bio-compatibility

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 Applications

Many different types of surface treatment are available, falling into four main categories:

Methods from every category can be effective, though each has advantages and drawbacks. Selecting an appropriate surface treatment is a matter of balancing the requirements of the application against what is achievable in the production environment. For example, mechanical treatments may often provide a cheap and extremely effective means of promoting adhesion; but they are not generally appropriate for use in medical applications, which require sterilization and typically cannot tolerate weakened or damaged parts.

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 TypeAdvantagesDrawbacks

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