Solvent bonding
Solvent bonding is not a method of adhesive bonding, but rather a method of fusing two thermoplastic 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. An advantage to solvent bonding versus other polymer joining methods is that bonding generally occurs below the glass transition temperature of the polymer.
Solvent bonding differs from adhesive bonding, because the solvent does not become a permanent addition to the joined substrate. 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.
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. Some solvents available may not react with the thermoplastic at room temperature but will react at an elevated temperature resulting in a bond. Curing times are highly variable.
Applying solvent methods
Four common application methods are:- Brush-on method. The solvent is brushed onto the surfaces to be joined, with subsequent pressure being applied until full strength of the bond is formed after the solvent has fully evaporated.
- Capillary action method. Commonly used with acrylic components, a consistent narrow gap between the parts allows the solvent to flow along the surfaces to be joined, via capillary action. Application is generally performed using a hypodermic needle to allow for precise application in the joint gap.
- Dip-dab method. A surface to be joined is dipped into a vat of solvent, with the solvent depth being a controlled variable, for a set amount of time. Once the part has been removed from the vat, a screen mesh or foam pad is used to remove the excess solvent before the bonding surfaces are paired.
- Solvent dispenser method. A dispenser is used to precisely control the amount of solvent applied on each surface.
Thermoplastic and solvent compatibility
| Thermoplastic | Compatible Solvents |
| Acrylonitrile butadiene styrene | Methyl ethyl ketone |
| Acrylonitrile butadiene styrene | Methyl isobutyl ketone |
| Acrylonitrile butadiene styrene | Methylene chloride |
| Acrylic | Ethylene dichloride |
| Acrylic | Methylene chloride |
| Acrylic | Methyl ethyl ketone |
| Acrylic | Vinyl trichloride |
| Polycarbonate | Ethylene dichloride |
| Polycarbonate | Methylene chloride |
| Polycarbonate | Methyl ethyl ketone |
| Polystyrene | Acetone |
| Polystyrene | Ethylene dichloride |
| Polystyrene | Methylene chloride |
| Polystyrene | Methyl ethyl ketone |
| Polystyrene | Toluene |
| Polystyrene | Xylene |
| Polyvinyl chloride | Acetone |
| Polyvinyl chloride | Cyclohexane |
| Polyvinyl chloride | Methyl ethyl ketone |
| Polyvinyl chloride | Tetrahydrofuran |
| Polyester | Cyclohexanone |
| Polybutadiene | Benzene |
| Polybutadiene | Cyclohexane |
| Polybutadiene | Hexane |
| Polybutadiene | Toluene |
| Polysulfone | Methylene chloride |
| Polyethylene | p-xylene at 75°C for LDPE, at 100°C for HDPE |