PLASTIC WELDING: Splicing of Thermoplastic Materials
A long time before there was welding, bolting or soldering, there was splicing. Splicing is the oldest and one of the simplest ways of joining two pieces together. People in the Stone Age were the first users of adhesives. They used a mixture of bees-wax and birch sap to fasten arrowheads by using a hot stone wedge as a soldering iron. Over the centuries, the use of adhesive materials has advanced considerably. The following will provide you with an overview of the six basic types of adhesives that can be used when hot-air or extrusion welding is not an option for the joining of thermoplastic materials.

The concept of splicing involves the connection of pieces by using the surface adhesion between the parts and the adhesives’ firmness or cohesiveness. If you are aware of some of the features splicing exhibits, it will save unnecessary surprises later. One of the best features is an even distribution of stress. Additionally, totally different materials can be brought together, large splicing areas can be made quickly, and the adhesive itself fills uneven surfaces. On the other hand there are some features that are not as desirable. You will find that the strength is limited, not all materials are suitable for splicing (Polyolefines for example), and proper storage of the chemicals is important. Most adhesives will exhibit problems when temperature changes are involved. Also, a different expansion between the plastic and the adhesive will cause stress and loss of strength in the splicing.

Let’s go back for a moment to the polyolefine plastics. These plastics (PP, PE etc.) show a very low surface energy. The surface energy can be raised by using a primer or an electrical or flame treatment. This will help to produce a decent joint, but it is only temporary. These polyolefines are much better to weld.

As I mentioned in the beginning, adhesives have been used for quite some time. Today there are approximately 250,000 different adhesives for all kinds of applications. To choose the right one can be challenging. Therefore proper research is necessary before using a new plastic or adhesive substance. It is also important to know how the plastic will react to the adhesive as well as the application and the environment where the finished part will be put into service.

There are basically six adhesive groups. Because of the differences in the chemical structures, they all have certain technological properties. A large number of adhesives or glues can be found in each of the following groups:

Melting Glue (Hot glue) is a one-component, thermoplastic adhesive that requires heating-up to the melting stage. The advantages are the fast curing time, easy storage and handling, they are solvent free and efficient. The disadvantage is the low temperature resistance and not being capable of handling large splicing areas.

Polycondensate (Phenolics) is a (typically) two-component, thermoset adhesive. The components react to each other when combined. The advantage is very strong splicing, a wide range of use, and enough time to work with it as well as good heat resistance. The disadvantage is the mixture - it has to be exact. The right storage is critical and this can be quite expensive.

Solvent Adhesive (Cement) is a one-component, thermoplastic adhesive, which is dissolved in a solvent. After the solvent disappears, the adhesive hardens. The advantage is its use in large areas and uneven surfaces. The disadvantage is the storage issue and the harmful gases.

Polymerisate (Cyanacrylates) is a one-component, thermoplastic adhesive, which needs a catalyst to start the reaction. In this case the moisture from the surrounding environment will do. The advantages are a fast curing time and strong bond. The disadvantage is the brittle splicing result, as well as the parts need to have good matching surfaces.

Adhesive Dispersion (Wood glue) is not common in plastic work-up. It belongs in the group of adhesives, but the use is more for woodworking.

Polyaddition Adhesive (Epoxies) is a (typically) two-component, thermoset adhesive. The two components react to each other. The advantages are the very strong splicing, a wide range of use, and enough time to work with as well as good heat resistance. The disadvantage is the mixture. It has to be exact. Proper storage is critical and can be quite expensive.

The design of the connection depends, like always, on the application. If an overlap, a T-connection or any other kind of connection is chosen the following will apply with all splicing. Make sure that the adhesive is able to handle the medium the glue will come in contact with later.

The preparation of the splicing surface is a crucial step in assuring a successful bond. Surfaces never start out clean. Therefore, you should first remove any heavy dirt, dust or grease. Next, use sanding or grinding paper (grit 100 - 150) to roughen the splicing area. Remove the grinding dust by using oil-free compressed air. If this is not available, use a degrease-able solvent to remove any residue. Consider that some plastics are not resistant to some solvents. If you are not sure, try the cleaner first on a sample. Alcohol, gasoline etc. are not recommended, because they leave residue.

After these steps are done, avoid touching the prepared surface with bare hands. Every fingerprint and any soil left on the surface weakens the strength of the bond. In any case, the splicing should be done immediately after the areas have been prepared.

Before starting the process, the following should be done; clean the area where the bonding will occur, make sure you have enough room for the pieces, tools, and personal protection, etc. Lastly, prepare the equipment you will need, such as:

Ask co-workers to help with large or complicated parts before you begin splicing. You don’t want to run around looking for some to help when the glue is already on the material and you realize "I need more hands, now". As soon as the mixture is prepared, the time clock is running and advanced preparation will assure that everything goes smoothly.

It is also a good idea to have enough adhesive prepared so that you can use the leftover to monitor the curing time of the mixture. This ensures that it really is hardened and the mixture was correct. Leave it overnight in a safe place and check on it the next day.

Some adhesives will cure faster by using UV-light rays or heat, consult the instructions.

The storage of adhesives influences their quality and shelf life. Again, read the instructions before beginning to work with unfamiliar adhesives. Basically, a cool, dark, dry space with good ventilation is ideal.

Some other safety considerations should be observed as well. Eating, drinking and especially smoking in the splicing area should be prohibited at all times. There are fumes and dust particles floating in the surrounding air and you may swallow or inhale these particles with your food and cigarettes. Besides, open flames are not allowed in close proximity because of the flammability of most adhesive’s fumes. Avoid skin contact because some solvents will find their way directly into the blood stream, contact could also cause allergic reactions. Good ventilation is important whenever adhesives or solvents are used and read and follow the MSDS’s (Material Safety Data Sheet) strictly. Lastly, all adhesives are basically special waste and should be disposed of in the proper way.

The above provides some basics. Additional questions will occur after you have started to work with different adhesives. Experience is often the best teacher and will allow you to find the right methods that work for you. Next issue we will discuss "Repairs on Plastic Parts".

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