This issue of the SURFACE SCIENCE CORNER has the dual purpose of presenting a book review on a recently published volume while simultaneously having a look at the amazingly ubiquitous world of adhesives technology. The volume in question is:
ADHESION SCIENCE: PRINCIPLES AND PRACTICE
Steven Abbott, (DEStech Publications, Inc., 2015)
Though the title would give the impression of dealing with the broad topic of adhesion science the volume is in fact closely focused on the topic of adhesives technology. This is not only a good thing but a necessary one also since dealing with the general topic of adhesion would require an encyclopedia series as opposed to a relatively compact volume.
What struck me most upon reading the volume was the amazing ubiquity of adhesive applications which we encounter in our everyday lives and the broad range of thermal-mechanical requirements that these applications require. Have a look around your local supermarket if you are not convinced about pervasiveness of adhesives in our day to day lives. Every glass jar has an adhesive attaching the label, every cardboard box is held together with an adhesive, all the bubble pack packages containing everything from paperclips to cutlery are sealed with an adhesive. Even going over to the fresh produce tables with fruits and vegetables laid out completely bare of any packaging materials whatever one finds sneaky little labels adhered to nearly everything with the items PLU code printed on it. It is not an exaggeration to say that the entire contents of the store are held together with an adhesive of one kind or another.
Looking around my office I see the ever present Postit® notes plastered on nearly every vertical and horizontal surface as well as interleaving the pages of most of the volumes on my bookshelf. Thinking about it one realizes the that the Postit note is a rather remarkable technology in that the note must adhere quickly to a broad range of surfaces without any surface pretreatment whatever and must also be cleanly removable without damaging the surface in question. Thus the Postit note puts a stake in the ground at one end of the adhesive performance spectrum which requires rapid easily reversed adhesion to a wide range of surfaces without regard to any sort of surface treatment other than perhaps blowing off some dust.
Going to the other end of the spectrum we find the adhesives which are used to glue together high performance aircraft. The requirements for this application are totally the opposite from those of the Postit note. All surfaces are carefully cleaned and treated with special primers. Maximal joint strength is required over a wide range of temperature and environment conditions giving a bond that needs to be totally irreversible.
Sitting between the extremes of the Postit note and aircraft glue is a vast range of intermediate applications ranging from gluing together the common cereal box to gluing the windshield onto your automobile. Consider the cereal box. Here fairly strong adhesion is required since the container cannot fall apart too easily but the adhesion cannot be too strong since the consumer has to be able to open the package without excessive force. On top of these requirements the adhesive material must be inexpensive and easily applied without surface preparation since the manufacturing volumes involved are enormous.
The case of adhering to windshield glass presents an entirely different set of requirements. In case you are not aware, many windshields in the newer car models are glued to the frame. Not only that but you also find that the rearview mirror is also glued to the glass. I can attest directly to this fact since the rearview mirror on my car fell off recently and needed to be reattached. The requirements here are dramatically opposite to the cereal box. First the bond must be permanent. Second the bond must withstand a wide range of temperature cycling from say 10 degrees F below zero in Winter to over 100 F in summer for a vehicle parked in the sun. Third the adhesive must withstand extensive exposure to ultra violet radiation from the sun. Given these requirements I thought it best to go to the auto parts store and purchase an adhesive specially formulated for attaching rearview mirrors.
The procedure for attaching my mirror was more like using aircraft glue than bonding a cereal box. First the glass and the attachment button were cleaned with acetone to remove all residual adhesive. Second a primer layer had to be applied to the glass and allowed to set for a specific time. Finally I was directed to apply only a thin layer of the adhesive to the attachment button which was then pressed in place for a full minute to get initial attachment. An hour or so or curing time was needed to achieve full strength before the mirror could be attached to the button.
Getting back to the volume under review it is clear upon cursory reading that the author has spent considerable time in the adhesives formulation business. The table of contents gives an indication of the flavor of the topics covered:
1. Some basics: Reviews the rudiments of adhesion measurement and adhesion failure mechanisms.
2. The Myths around Surface Energy and Roughness: An entertainingly provocative review of the concepts of surface energy and surface roughness as applied to adhesives technology.
3. Intermingling and Entanglement: Brings into focus the critically important role of the adhesive bulk properties on its adhesion performance. In particular the crucial role of polymer molecular weight and chain segment mobility are discussed in regard to achieving high adhesion strength.
4. Time is the Same as Temperature: Discusses the critical importance of the concept of Time Temperature Superposition in determining the thermal-mechanical properties of all adhesive formulations.
5. Strong Adhesion with a Weak Interface: Review of the important topic of pressure sensitive adhesives.
6. Formulating for Compatibility: Discusses the importance of polymer solution thermodynamics in the development of adhesive formulations.
7. Measuring Adhesion: Perils and Pitfalls: Gives a review of the most common adhesion measurement methods used for evaluating adhesive formulations.
8. Putting Things into Practice: Gives a comprehensive summary of how all the above topics can work together in the “scientific” design of adhesive formulations.
Topics 2 and 3 above are of most interest to the subject of applying plasma technology to improving adhesion to polymer substrates. The essential point of chapter 2 is that the role of surface energy in promoting adhesion to crystalline polymers is basically misunderstood. It is well known that it is difficult to adhere anything to crystalline polymers such as poly(ethylene) PE, poly(ethylene-terepthalate) PET and poly(propylene) PP. It is also well known that plasma treatment greatly improves the adhesibility of all of these materials. The question is what is going on?
The standard answer is that plasma treatment is improving the surface energy of these materials thus allowing greater wetting as well as stronger interactions at the interface via the creation of functional groups. The author argues persuasively that this is unlikely to be the case. Looking at polyethylene for example, standard analysis reveals the surface energy to be roughly 32 mJ/m2 (milli joules per square meter). Plasma treatment will typically raise the surface energy to something like 42 mJ/m2 or about a 30% increase. This figure, however, does not square with the observed increase in peel test adhesion which can be in the range of 10 to 100 J/m2 (joules per square meter) or several orders of magnitude larger than the increase in surface energy.
Further suspicion is cast on the surface energy argument by the comparison of poly(ethylene terephthalate)PET to poly(vinyl chloride) PVC. The surface energies of these two polymers are close to 43 mJ/m2 but it is well known that it is difficult to adhere to PET compared to PVC. Again we have wonder what is going on?
A clue begins to emerge in the case of PE. This polymer comes in two basic forms one of low density LDPE and one of high density HDPE. The high density form is essentially one long completely linear chain of CH units and therefore tends to be highly crystalline. The low density form, however, is composed of linear strings of CH units broken up at intervals by short side chains of CH units terminating in a CH3 group. The many side chains of the LDPE prevent it from attaining the same level of crystallinity as its HDPE cousin thus resulting in lower density. It is also known that it is easier to adhere to low density PE than the high density form. If we combine this with the fact that the poorly adhesionable PET is a highly crystalline polymer and the easily adhesionable PVC is totally amorphous then we have to suspect that it is the level of crystallinity that is key in determining adhesability.
It is at this point that the mechanisms of chain intermingling and chain entanglement enter the picture. In essence amorphous polymers have a high degree of chain segment mobility which allows them to interpenetrate and form entanglements which give rise to high levels of energy dissipation when trying to pull them apart. Think of trying to pull part two lengths of string that have been randomly jumbled together. It is these strongly dissipative effects that give rise to the apparent strong adhesion observed in peeling apart these intermingled and entangled layers.
Now the question becomes what is the role of plasma treatment in improving the adhesionability of crystalline polymers? Aside from improving the wettability of the treated surface the most obvious mechanism is the disruption of the surface crystalline layers of the polymer. Plasma treatment always involves the making and breaking of chemical bonds and in the case of crystalline polymers we postulate that the plasma field breaks up a significant amount of surface crystallinity creating an amorphous layer with highly mobile chain segments capable of intermingling and entangling with applied surface layers. The existing level of surface energy must be sufficient to allow for a reasonable amount of wetting, but beyond that it does not add significantly to the overall peel removal energy which is dominated by the dissipative effects of chain interpenetration and entanglement.
Prof. Abbott points out that the adhesion mechanisms described above have a strong influence on how the typical adhesive is formulated. The redoubtable formulator is generally faced with the prospect of joining two poorly or wholly uncharacterized surfaces and his customer wants an inexpensive and easily applied glue that will hold these surfaces together with just the right amount of strength and durability called for. He has to assume that the surface energies will be in a reasonable range to give sufficient wetting without the need to perform any sort of involved surface analysis or surface preparation aside from a perfunctory cleaning. Plasma treatment will be a very handy tool when dealing with difficult surfaces such as the crystalline polymers but it must be remembered that all that is required is to sufficiently break up the crystallinity of just the top most surface layer in order to get chain interpenetration. Over treatment must be avoided so that one does not create a layer low molecular weight rubble that could act as a weak boundary layer and thus give even poorer adhesion than the original untreated surface.
As luck would have it not only did my rearview mirror fall off but also the soles of my cycling shoes delaminated. Thus I got to try out a totally different kind of adhesive from what I used to attach my mirror. Commonly called shoe goo it is apparently some kind of silicone polymer formulation. No surface preparation is required other than blowing out any loosely adhered debris. Contrary to the rearview mirror formulation there is no primer layer required and the glue is applied in a fairly thick layer and spread out as evenly as possible with a stick to give good coverage and penetration into all the nooks and crannies of the mating surfaces. After application of the glue I pressed together the mating surfaces under the weight of an inverted 10 pound sledge hammer in order to make maximal contact and then allowed everything to set and cure over a period of a day or two.
Thus the world of adhesives turns out not only to be very commonplace in our day to day lives but also rather subtle, deceptive and non-intuitive in terms of the science and technology required to create truly effective and useful formulations. The requirements from one application to another can be diametrically opposite and the formulator has but a limited number of theoretical tools available which must be carefully handled. The reader interested in further exploring this most engaging topic is encourage to refer to Prof. Abbott’s fine volume.
For my part I can confirm that both my rearview mirror and cycling shoes remain completely intact. So despite the subtle and intricate nature of adhesive technology good adhesives are available for nearly all practical purposes and Prof. Abbot’s volume is a most instructive guide for anyone faced with the problem of developing a working adhesive.
The author is happy to entertain any questions or comments concerning this topic and may be contacted at the coordinates below.
Dr. Robert H. Lacombe
Materials Science and Technology
3 Hammer Drive
Hopewell Junction, NY 12533-6124
Tel. 845-897-1654; 845-592-1963