Joining different plastic composites is a common step in product fabrication. But when products are structurally complex or have more demanding performance specifications, choosing the right sealing process can sometimes be a challenge.
Many industrial containment products, for example, are constructed of layers of different types of plastic sheets. Radio frequency welding is a solid possibility with polyurethane to acrylic coated materials and solvent bonding is a good process to employ to seal the edges if needed.
Disposable medical products also present special welding challenges. Rigid and flexible components are often welded together in precise configurations, such as flexible tubing joined to IV ports or connectors – fittings which could be easily distorted or damaged by hot welding techniques. In addition, the finished joints of medical products must be aesthetically pleasing as well as durable enough to withstand subsequent treatments such as gamma radiation.
So which sealing solutions are a good match for more complicated plastic part assemblies? Adhesive bonding is one approach. It’s a fast, versatile, and cost-effective method that works well for sealing many products. But where the use of adhesives is either impractical or undesirable, another excellent choice is solvent bonding.
What is solvent bonding?
Solvent bonding (also called solvent welding) is a direct joining technique that allows polymer materials to be welded at room temperature with the use of a solvent. The solvent is applied to both pieces being joined, temporarily softening and dissolving the surfaces of the materials. As the components or layers are fitted together, usually under pressure, the surface molecules form entangled polymer chains. These chains will then form a solid mass once the solvent permeates through the materials and evaporates into the environment.
The resulting plastic-to-plastic bonds are clean, strong, and fluid tight, with less sensitivity to thermal cycling than adhesive-bonded joints.
What kinds of solvents are used in solvent bonding?
The first and most important consideration in choosing a solvent is compatibility. If the polymers and solvents used in the bonding process are not compatible, there will not be sufficient intermolecular movement across the joint interface to create a strong bond.
Compatibility can be achieved in a variety of ways. The most straightforward method is using parts made from the same polymer (such as bonding PVC tubing to a PVC port with cyclohexanone). However, similar (not identical) polymers can also be combined when paired with a common solvent. For example, acrylic and ABS are dissimilar materials that can be successfully bonded because they are both compatible with the solvent methyl ethyl ketone (MEK).
Solvent formulas. In addition, solvents can be applied in different forms that change the evaporation rate or viscosity of the solvent solution. Creating a solvent slurry, consisting of the pure solvent plus up to 25% of the base (parent) plastic, can make the solvent easier to use, especially when filling gaps between parts. The primary solvent can also be combined with other compatible solvents to produce a solvent mixture optimized for a specific combination of materials or parts.
Common solvents. The most common solvents used in solvent bonding – in both pure and combined forms – include:
- Methyl ethyl ketone (MEK)
- Methylene chloride
- Ethyl acetate
Other solvent considerations. Some polymers, including amorphous materials, are more susceptible to stress cracking than others. In many cases, these risks can be minimized by careful selection and formulation of the solvent. Using appropriate surface preparation and ventilation techniques during the fabrication process will also help to minimize cracking and crazing.
What other factors are important in solvent bonding?
While solvent bonding is a relatively simple and inexpensive technique, making consistently good solvent welded joints requires a clear understanding of several key process steps.
Surface preparation. Surfaces typically have some residual contamination from the molding process, operator handling, or other sources. Reducing contamination increases the wettability and surface activation of the material and makes it easier to bond. A simple way to reduce contamination is to clean the part surfaces with isopropyl alcohol.
More advanced techniques include plasma treatment, a process that increases surface energy by bombarding part surfaces with gas ions, and corona treatment, which improves adhesive strength on plastic surfaces by exposing them to an electrical discharge in the presence of air.
Solvent application. Solvent should be applied to cover the entire joint without leaving excess that can affect the integrity of the bond or cause the joint to haze. Any residual solvent should be cleaned off immediately. Assembly of the parts should be made while the surfaces are still wet and fluid from the solvent application.
Ventilation and curing. Proper aeration and venting throughout the curing period must be provided to allow solvents to completely evaporate from the materials. It is also important to remember that complete solvent evaporation can take at least 24 hours. Shorter periods can result in stress cracking. Completed joints should remain untouched until the bonds have cured enough to be handled.
Annealing. In some instances, annealing of plastic components prior to bonding and assembly can help to reduce the development of residual stress cracks. Optimizing the geometry of the joints being bonded also helps to reduce the risks of stress cracks and other potential bonding flaws.
Moisture control. Solvent bonded products should be stored in environments that minimize excess moisture. Moisture can affect the integrity of bonded joints.
Are there disadvantages to solvent bonding?
Like all welding methods, solvent bonding has both advantages and disadvantages. Some of the more common drawbacks include the potential for stress cracking, as described earlier, and the extended drying times required for curing.
Another challenge is the risk of solvent becoming trapped in the joint and compromising the integrity of the part. Avoiding the application of excessive solvent during fitting and assembly can largely eliminate this problem.
The need for adequate ventilation during the bonding and drying stage is also a consideration. Vented protectors are often recommended to allow the safe evaporation of the solvent vapors.
What products are suitable for solvent bonding?
The uses for solvent bonding in product fabrication span a wide range of industries and products – from military applications to disposable medical devices, food containers, and industrial containment structures for air or fluid containment.
Whether you need to join surfaces made from dissimilar materials, attach fittings to sealed containment bags, or solve other complex bonding challenges, SealWerks can help you explore all the advantages of solvent bonding for your product.