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"The Etching of Fluoropolymers In Preparation for Bonding"
INTRODUCTION
In order to achieve some perspective, any discussion of the treatment of fluoropolymers for bonding should include at least a few comments on methods that have been tried in the past. The need to render the surface of these polymers wettable, and therefore bondable, arose almost as soon as their extremely low coefficient of friction was realized.
The most common fluoropolymer is polytetrafluoroethylene (PTFE) and its most easily identified property is its lubricity or its extremely low coefficient of friction - its almost total lack of surface energy. The universal recognition of this attribute is due, in large measure, to the early introduction of Teflon® coated frying pans to the mass market.
But fluoropolymers have come into use in numerous and diverse industries for many of their other properties like wear and chemical resistance, dielectric strength, temperature resistance and various combinations of these properties. Many subsequently developed fluoropolymers like FEP, PFA, PVDF and ETFE, along with the newest generation of modified PTFEs, have been introduced to meet the needs of an ever-increasing number of applications. Since PTFE is the prevalent fluoropolymer in industry today and many of these polymers are used in similar applications (with the difference being one of degree rather than of basic physical property), I will sometimes use "PTFE" to refer to fluoropolymers in general.
Its chemical resistance has made PTFE very useful in the chemical process industry as a lining for vessels and piping. The biomedical industry has found PTFE to be biocompatible and so has used it in the human body in the form of both implantable parts and devices with which to perform diagnostic and therapeutic procedures. In many applications, fluoropolymers have replaced asbestos and other high temperature materials. Wire jacketing is one such example. Automotive and aircraft bearings, seals, push-pull cables and fuel lines, among other components, are now commonly made with a virgin or filled PTFE component.
The dielectric property of this unique polymer has given rise to a whole new technology in printed circuit board design. This concept is responsible for the very latest in high-speed, high-frequency radar and communications found in our newest defense systems as well as in the next generation of ultra high-speed computers, the Global Positioning System, satellite TV and numerous other recently developed high-tech applications.
The list of such applications goes on and on. This sixty-some year old family of polymers is still behaving as if in its infancy with new uses being developed continuously. And the world's major resin manufacturers committed hundreds of millions of dollars in the 1990's to even further increase production capacity. These same companies as well as numerous fluoropolymer fabricators, end-use companies in almost every industry, and educational institutions worldwide devote additional millions to research and development every year.
In order to take advantage of any of the other remarkable properties of a fluoropolymer, it sometimes has to sacrifice its lubricity in order to be bonded to another material. The applications mentioned above all require these polymers to be etched to some degree to enable bonding.
Fluoropolymer film and sheet have to be etched on one side to enable bonding it to the inside of steel tanks and piping; the OD of small diameter, thin wall PTFE tubing is etched to bond to an over-extrusion resulting in a PTFE-lined guide catheter for medical use; fluoropolymer jacketed high-temperature wire is etched to allow the printing of a color stripe or other legend such as the gauge of the wire and/or the name of the manufacturer; PTFE based printed circuit boards require etching to permit the metallization of throughholes creating conductive vertical paths between both sides of a double sided circuit board or connecting several circuits in a multilayer configuration.
The first commercially viable processes were chemical in nature and involved the reaction between sodium and the fluorine of the polymer. In general, some quite toxic and very dangerous substances were involved such as Tetrahydrofuran (THF), anhydrous ammonia and of course the raw sodium itself.
In time, some of the chemistry was changed to make the process less potentially explosive and hazardous, but the essential ingredient -- sodium -- remains the most reliable, readily available chemical 'abrasive' for members of the fluoropolymer family.
WET PROCESSING OF PTFE
For many years, sodium was carried into the reaction by Tetrahydrofuran or liquid ammonia. 'Home brews' were concocted to achieve the desired wettability, hence bondability of these slippery surfaces.
An etchant made with sodium ammonia is not portable and is very difficult to store and handle because its boiling point is approximately -25°F). This, in conjunction with its odor and the hazardous nature of both the free sodium and the ammonia, is why it was never adopted as the etchant of choice with which to carry out this procedure.
On the other hand, THF being somewhat stable at room temperature (although highly flammable) came into prevalent use where PTFE etching needed to be done. It was usually conducted 'out in the alley' or in a shed well away from the normal process areas of the factory.
By the late 1950's, another group of solvents, the glycol diethers, was being developed and they would eventually be successful in replacing both liquid ammonia and THF as the carriers for sodium in most in-house etching operations. Also, what would prove to be a much safer way of handling sodium evolved -- that of mixing the sodium with naphthalene or 'moth balls'. These factors greatly furthered the premixed etchant business and enabled PTFE users to process their parts with much greater ease and safety.
THE "NEW" CHEMISTRIES
The development of the glycol diethers did much for many industries. Various members of this family of chemicals found their way into lithium battery production, pharmaceutical production as stabilizers, refrigeration, paint, pesticide, detergent and propellant production, just to name a few. Some other, quite diverse applications include gold purification, uranium extraction, and the production of colorfast and no-iron fabrics.
Three of these Glymes, as they are called, came in to use in the manufacture of etchants for Teflon:
- Ethylene Glycol Dimethyl Ether or MONOGLYME
- Diethylene Glycol Dimethyl Ether or DIGLYME
- Tetraethylene Glycol Dimethyl Ether or TETRAGLYME
MONOGLYME, being the simplest of the three, has the greatest affinity for sodium and, therefore, is the easiest solvent with which to complex sodium naphthalene. The solubility of sodium naphthalene in monoglyme is quite high so the resulting etchant is very sodium rich but also very viscous, usually needing to be thinned for particular applications.
The closed cup flash point of this solvent is approximately 30°F that makes it a very flammable material and a considerable hazard. A third factor regarding etchants made with monoglyme is their thermal stability. At temperatures above 32°F, these etchants begin a spontaneous reaction, consuming their active ingredient -- sodium -- in the process and giving off methyl vinyl ether. It is usually recommended to store these etchants under refrigeration in order to limit this deterioration.
DIGLYME, while it dissolves less sodium naphthalene in complex, has a much higher flash point (134°F which classifies it as a combustible) and makes an etchant with the viscosity of about that of water. Diglyme is very stable at room temperatures and above and does not exhibit the same spontaneous decomposition as monoglyme at temperatures up through its flash point. Also, the evolution of methyl vinyl ether is about one-tenth that of monoglyme.
TETRAGLYME, the most complex of the three solvents, has a very high flash point (around 230°F). However, an etchant made with this solvent yields poor bond strengths because the complexity of the solvent inhibits the release of the active ingredient -- sodium -- to the etching process.
ELEVATED TEMPERATURE ETCHING
So we have this relationship among these glymes, in the ascending order of their complexity, that bears directly on the efficacy of the etchants made with them.
Monoglyme, as the simplest solvent, dissolves sodium naphthalene the easiest and also gives it up the most readily in reactions with fluorine when etching PTFE. Diglyme, having a more complex molecule, dissolves sodium naphthalene less easily and is a little slower in giving in these reactions. And tetraglyme, being the most complex of the glymes, presents the least amount of active sodium to the etching process.
In 1987, in an attempt to release more active sodium from a diglyme-based etchant, the first elevated temperature experiments were done using a diglyme-sodium naphthalene etchant. The purpose was to determine the degree of the catalytic effect of heat on the reaction and the results were very favorable.
Several tests conducted at Lehigh University showed that etching at approximately 125°F increases the bond strengths between 50% and 75% over room temperature etching. This same elevated-temperature process was compared to parts etched with sodium ammonia and found to yield bond strengths over 25% higher than those etched by the sodium ammonia process.
The mechanism here seems to be simply that of the catalytic effect which is where molecules that are predisposed -- by a heat-induced higher level of molecular activity -- to react faster and more completely with each other. Corroborating this, it was found that, along with using a heated etchant, heating the parts to the same temperature further enhances the effect. This phenomenon proves helpful when re-etching must be done. It can often eliminate the need to bleach the defectively etched parts before re-etching. It is not recommended to heat monoglyme-based etchants because of their very low flash point.
Another distinct advantage to the elevated temperature process is the change in the viscosity of the etchant. Diglyme etchants, at room temperature, have approximately one half the viscosity of monoglyme and tetraglyme etchants. When heated, a diglyme etchant drops to about one half of its room temperature consistency (which is around that of water) rendering it in the range of 25% as viscous as etchants made with the other two solvents.
This is an important factor when trying to etch confined areas such as in small-diameter, high aspect ratio through-holes in printed circuit boards or tightly woven PTFE fabrics. Viscosity is a particularly important factor in the etching of fluoropolymer films where the fluid dynamics of a thinner etchant contribute considerably to a very uniform chemical reaction over large surface areas.
The recent development of the first wettable, bondable PTFE whisker -- Aclon® -- was greatly facilitated by the low viscosity of a diglyme-based etchant.
THE SHELF LIFE OF ETCHED SURFACES
There has been a great deal of concern over just how long an etched fluoropolymer surface will retain its original bond strengths. There are specifications that call for wrapping the etched parts in black plastic, storing them in humidity and temperature-controlled environments and to complete the bonding within eight hours of etching!
While it is true that temperature, humidity and UV light do have a detrimental effect on the etched surface, the deterioration of bondability is much slower than is commonly believed. Tests have shown that etched PTFE parts exposed to 250°F for 14 days exhibit bond strengths approximately 40% weaker than those done on the day they were etched.
Another interesting fact revealed in two other independent studies is that the bond strength actually increases, at room temperature, over the first 24 hours after etching.
We have not studied the impact of humidity and UV light on the etched surface in depth, but empirically we can say that those factors take a very long time to affect bondability. Depending upon the wavelength and intensity of the UV light source, the deterioration occurs over a period of months and perhaps years as opposed to hours or days which some believe.
All of this suggests that the sodium etching process creates a very active layer on the surface of fluoropolymers. The rate of that activity changes over time -- at first, fairly rapid as the fluorine is replaced by hydrogen and oxygen, forming hydroxyl groups with the carbon chain -- and then more slowly as the surface becomes more satiated and stable.
Another factor that is of concern in this process is that of the depth of the etched layer. The sodium reaction with fluorine is a self-limiting one and it has been shown to take place to a depth of only a few hundred to a few thousand Angstroms (not microns as had been believed). It is thought that, due to the somewhat amorphous nature of these polymers, a rotational migration occurs over time, accelerated by some ambient conditions -- especially heat -- that re-exposes more of the original
C2F4 molecule at the surface resulting in a lower coefficient of friction.
A much better understanding of these phenomena will be possible only when technology allows us to look at these surfaces at the atomic level. The scanning-tunneling and atomic force microscopes are doing just that for conductive materials today. The hope is that it will be only a matter of time before we will have a computer-generated photomicrograph of a fluoropolymer surface showing its topography, atom by atom, at which time we will be able to answer some of these lingering questions much more definitively.
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