Solvent bonding (also called solvent welding) is one of several methods of adhesive bonding for joining plastics. Application of a solvent to a thermoplastic material softens the polymer, and with applied pressure this results in polymer chain interdiffusion at the bonding junction. When the solvent evaporates, this leaves a fully consolidated bond-line.[1] An advantage to solvent bonding versus other polymer joining methods is that bonding generally occurs below the glass transition temperature of the polymer.[2][3]
Solvent bonding differs from adhesive bonding, because the solvent does not become a permanent addition to the joined substrate.[4] Solvent bonding differs from other plastic welding processes in that heating energy is generated by the chemical reaction between the solvent and thermoplastic, and cooling occurs during evaporation of the solvent.[5]
Solvent bonding can be performed using a liquid or gaseous solvent. Liquid solvents are simpler and generally have lower manufacturing costs but are sensitive to surface imperfections that may cause inconsistent or unpredictable bonding.[6] Some solvents available may not react with the thermoplastic at room temperature but will react at an elevated temperature resulting in a bond.[2] Curing times are highly variable.
Four common application methods are:[5]
The proper solvent choice for bonding is dependent on the solubility of the chosen thermoplastic in the solvent and the processing temperature. The table below provides a selection of solvents commonly used for bonding specific thermoplastics.[5] Mutual solubility between a polymer and a solvent may be determined using the Hildebrand solubility parameter.[2][3] Polymers will generally be more soluble in solvents with similar solubility parameters to their own in a given state (liquid or solid). The solubility parameters of polymers are not greatly affected by changes in temperature, however the solubility parameters for liquids are affected by temperature. Increasing the temperature lowers the free energy of mixing, promoting dissolution at the interface and interdiffusion bonding.[2]
| Thermoplastic | Compatible Solvents |
|---|---|
| Acrylonitrile butadiene styrene (ABS) | Methyl ethyl ketone (MEK) |
| Methyl isobutyl ketone | |
| Methylene chloride | |
| Acrylic | Ethylene dichloride (EDC) |
| Methylene chloride | |
| Methyl ethyl ketone (MEK) | |
| Vinyl trichloride | |
| Polycarbonate (PC) | Ethylene dichloride (EDC) |
| Methylene chloride | |
| Methyl ethyl ketone (MEK) | |
| Polystyrene (PS) | Acetone |
| Ethylene dichloride (EDC) | |
| Methylene chloride | |
| Methyl ethyl ketone (MEK) | |
| Toluene | |
| Xylene | |
| Polyvinyl chloride (PVC) | Acetone |
| Cyclohexane | |
| Methyl ethyl ketone (MEK) | |
| Tetrahydrofuran | |
| Polyester | Cyclohexanone |
| Polybutadiene | Benzene |
| Cyclohexane | |
| Hexane | |
| Toluene | |
| Polysulfone | Methylene chloride |
There are three main mechanical testing methods for plastic bonding joints: tensile testing, tensile shear test, and peel test. Tensile testing using a butt joint configuration is not very conducive to polymers, particularly thin sheets, due to the challenges of mounting to the load frame. An epoxy may be used for mounting, but can lead to failure in the epoxy/polymer interface instead of in the bonded joint.[2] The most common method for testing solvent bonds is the tensile shear test using a lap joint configuration. Specimens are tested in shear to failure at a given overlap cross section via tensile loading. This testing method is particularly conducive to thin specimens due to distortion mitigation distortion in the test specimens due to the loading mechanism. Guidance for tensile shear testing may be found in ASTM D1002-05.[2]
There are several industries that utilize solvent bonding for their applications. A few of these include microchip manufacturing, medical, and potable and sanitary plumbing systems.[6][5][7]