By Jim Cook

What is glue? My web search reveals: “Around 1750, the first glue or adhesive patent was issued in Britain. The glue was made from fish.” If you search the records, patents have been issued for glue using natural rubber, animal bones, fish, starch, milk protein or casein. As a newly hatched engineer working his first job in the shoe machinery industry, occasionally I would refer to the sticky stuff as “glue”. My boss would have a fit and loudly proclaim, “Glue is made from horse’s hooves, and we are working with adhesives.”

What is Epoxy? In deference to my old boss, I will not refer to any epoxy as glue even though the the dictionary defines an adhesive as “a substance capable of holding materials together by surface attachment.” Epoxies used in clubmaking usually consist of two components (called an epoxy system) reacting with each other forming a hard, inert material. The two components are mixed in a certain ratio. Part A consists of an epoxy resin, and part B is the epoxy-curing agent, sometimes called a hardener. A chemical reaction occurs between the two parts generating heat (exothermic reaction) and hardening the mixture into an inert, hard “plastic”.

The major role of the hardener, other than the obvious hardening, determines many of the properties of the final cured epoxy. In our golf shop, the properties we are looking for include pot life, cure time and wetting ability.

Pot Life – After the two epoxy parts are combined, there is a working time (pot life) during which the epoxy can be applied or used. The pot life will be anywhere from minutes to one hour or longer. At the end of the pot life, the mixture becomes warm and quickly begins to harden.

Cure Time – The cure time is the time needed for your mixture to reach a major portion of its maximum strength. This time usually is associated with a temperature range. The cure time is lowest at a higher temperature. Temperature below a minimum cure temperature can mean the epoxy never totally cures, thus not achieving its maximum strength.

Wetting Ability – One of the most important characteristics on an epoxy system is its ability to wet the surfaces to be bonded. This wetting action is necessary for the chemical bonding. Most of the systems sold for club assembly have wetting agents in the formulation. There are mechanical procedures to help promote the wetting action.

How Does It Cure?

To help understand what happens during the curing operation, picture the epoxy part of the system to be long chains of molecules. The hardener causes those chains to be cross-linked together. The more complete the linking, the stronger the cured epoxy will be. Picture the long chains to be the uprights of a ladder and the cross-linking to be the rungs. The more rungs and the more secure the rungs, the stronger the ladder will be.
There must be a proper amount of hardener for a given amount of epoxy. If there weren’t enough rungs (hardener), the ladder would be weak. Too much hardener and there isn’t enough places on the uprights for joining. This curing is classified as a “thermo set” process. That is, once its cured, if it is broken by mechanical or thermal (over heating), it will not repair itself.

Sometimes someone will say he is waiting for the epoxy to “dry”. To a chemist or an engineer, drying occurs when a solvent evaporates and leaves the adhesive material behind. “Curing” is a chemical reaction and does not result from evaporation.

Types of Epoxy

There are hundreds of epoxy formulations. The epoxy, along with a given hardener, is referred to as a system. Each system is formulated to provide given properties for specific jobs. One property important in our clubs is toughness. The term “toughness” is a measure of the material’s resistance to failure, i.e. the total amount of energy required to cause failure. The bonding material used in our clubs must not be brittle; instead its toughness comes to light as it demonstrates its ability to absorb the tremendous shock of a hit. Deviating from the specific ration of hardener-to-resin could adversely affect the toughness of the cured material.

One of the important properties for clubmakers is the time which the bond takes place. Put yourself in a tour van where the pro just came off the practice range and wants to replace his shaft with the latest “Wiz Bang” model and wants to hit it after lunch. At this point, you reach past the 24-hour curing epoxy and grab the bottle of fast curing stuff. Thus, we have a tendancy to forget all the properties and label our adhesives as “fast” or “slow”. The speed of curing is the property of the system ans was considered during the original formulation. Adding more or less hardener will not result in a faster cure, only a weaker bond.

The fact no universal theory of adhesion exists confirms the science of adhesion is a complex and controversial subject. For our purposes, there are two general categories of bonding: mechanical and chemical.

Mechanical – During the machining of the inside of the hosel, unless it is polished, tool marks are left on the wall. In addition, as a shaft is prepared for bonding little irregularities in the surface are exposed or made. As the epoxy is applied and if there is enough time before the epoxy starts to thicken, it will flow into the irregularities on the surface of the shaft and the tool marks on the inside of the hosel. When the epoxy is hard it will lock the hosel to the shaft producing an interlocking mechanical action.

Chemical – The well known adhesion of epoxies is due to the strong polar bonds it forms with the surfaces it contacts. This chemical bonding takes place at a molecular level and requires very clean surfaces. Any amount of oil, grease and other agents will act to prevent this important chemical bonding.
Okay, enough of the preliminary stuff. Let’s see how this information applies to clubmaking.

Examination of Failure

Determine the location of the failure. When a customer shows up with the shaft in one hand and the club head in the other, it is a sign to start the examination. You need to understand where the failure actually is. In the area of the club we are concentrating on, there should be a bond between the epoxy and the inside of the hosel, a layer of epoxy, and finally a bond between the shaft and the epoxy.

Notice the three possible areas of failure and I mentioned two areas of bonding. Let’s concentrate on these first. If you see most of the epoxy on the shaft and very little epoxy on the inside of the hosel, it is a hosel-to-epoxy failure. If there is very little epoxy left on the shaft, classify it as an epoxy-to-shaft bond failure.
The third area of possible failure is within the epoxy itself. With this type of failure, you would expect to see epoxy on both the shaft and inside of the hosel.

If you have classified the failure as a bonding failure, there are two possible mechanisms that can contribute to a bonding type failure: mechanical or chemical. Above, I noted a degree of roughness to the surface to be bonded for a proper mechanical bond. If inside the hosel or the outside of the shaft is very smooth, this would reduce the mechanical lock we want. Typically, this happens when a club maker does not roughen up the outside surface of the chromed steel shaft prior to bonding. A close examination of the surface finish should indicate whether this item was the major contributor to the problem.

A bonding failure also could be caused by a reduction in the chemical bond. This has been a major cause of all the club failures I have examined. The proper chemical bond was reduced by the presence of oils and greases. It is surprising how little oil can cause total failure. It can be as thin as a layer only a molecule thick. It can come from fingerprints, oil left over from the original manufacturing steps, or just the lack of proper surface cleaning.

After you have prepared the surface to be bonded and within a short time when you will be applying the epoxy, all surfaces must be cleaned with alcohol. Using a clean, lint free cloth, wipe both surfaces. Do not use rubbing alcohol. Some rubbing alcohols have oils in them and many have a significant amount of water. Either ethanol or methanol can be used; one just significantly longer to evaporate. If you touch either cleaned surface after cleaning, the cleaning MUST be repeated. Nothing but the newly mixed epoxy will touch those surfaces after cleaning.

Contributing To Failure

One cause everyone seems to jump at is: “The epoxy is too old”. Epoxy systems do have a shelf life; from the time that they are made, they are degrading. Most epoxies formulated for club making have a shelf life marked on the bottle. If it is not marked, do so when you receive it. If your bottles have exceeded the shelf life, it is prudent to dispose of them properly. If you look at the hardener and notice crystals, dump it. Leaving the cap off either of the bottles allows moisture in and helps shorten the life. It is possible to lengthen its life by storing both bottles in the refrigerator (not frozen), but I cannot tell you how long the life has been prolonged.

The next “old wives tale” is: “You left them in the trunk of your car and they over heated.” All the epoxy systems I know of formulated for golf clubs fail at a temperature around 250 degrees Fahrenheit. Your black car parked in the desert all day gets hot, but not hot enough to get the epoxy in the bond line to its failure temperature.
Failure to mix properly is an easy thing to do and you might not realize it. When I put the proper amount of the two components in a small cup, I actually time myself for two minutes using the second hand on the shop clock. I make sure I scrape along the bottom, sides and corners of the cup and every so often scrape the sides of the tongue depressor or wooden coffee stirrer.

By the way, don’t use a wax coated paper cup to mix epoxy. Wax is a very good release agent; wax will not stick.

Another source of bonding failures occurred in the aerospace industry where I worked. Several years ago, a spectral analysis of the surfaces of a bonded structure that failed revealed silicone. Silicone is a dirty word to anyone trying to produce strong and repeatable bonds. Searching high and low in the factory and every step of the process did not reveal the source of the silicon, until our people found we were using masking tape with silicon in its adhesive. With tape protecting the surface, we would wipe the surface to be bonded with alcohol and the alcohol would spread the silicon from the exposed edge of the tape all over the surface we thought we were cleaning. After that, all tapes with silicon in their adhesive were banned from the bonding room.

Club Assembly

When I assemble a club I use the back end of a cheap Bic pen because it easily fits all hosels. With the pen, I spread a very thin coat of epoxy on the prepared shaft tip. This lubricates and allows the ferrule to be pushed to its proper position with the head. I have prepared the shaft from the very end of the shaft to half way up inside the ferrule. The epoxy bonds and the prepared shaft bonds well to the ferrule and prevent later movement. Using the pen, I then coat all the walls of the hosel and I coat the end of the shaft up into the flared portion of the ferrule. All this takes a minute or two and the mechanical action of spreading helps get the epoxy into the nooks and crannies. The shaft is inserted into the hosel and turned 90 degrees. This turning helps any air in the bond joint to escape. Clean all the epoxy that has oozed out and set the assembly aside in a secure, room temperature location to cure.

During the epoxy curing, it is important the parts not be disturbed. If the club is disturbed as only portions of the links have been made, the maximum strength of epoxy cannot be reached. As the shaft is twisted or moved, the linking is damaged. The damage does not repair itself; thus, your joint never reaches its full strength.

The ATTENTION TO DETAIL is the secret of making clubs that will never have the head slip off. To review:

• Surface cleanliness
• Proper surface preparation
• Surface cleanliness
• Correct amounts of epoxy and hardener and complete mixing
• Surface cleanliness

Not performing any of the above items will result in the club failing. The only question will be when will it fail?