The solubility parameters of polymers are not greatly affected by changes in temperature, however the solubility parameters for liquids is affected by temperature. Polymers will generally be more soluble in solvents with similar solubility parameters to their own in a given state (liquid or solid). Mutual solubility between a polymer and a solvent may be determined using the Hildebrand solubility parameter. The table below provides a selection of solvents commonly used for bonding specific thermoplastics. The proper solvent choice for bonding is dependent on the solubility of the chosen thermoplastic in the solvent and the processing temperature. A dispenser is used to precisely control the amount of solvent applied on each surface. Once the part has been removed from the vat, a screen mesh or form pad is used to remove the excess solvent before the bonding surfaces are paired. 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. Application is generally performed using a hypodermic needle to allow for precise application in the joint gap. Commonly used with acrylic components, a consistent gap between the parts allows the solvent to flow along the surfaces to be joined via capillary action. 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. Some solvents available may not react with the thermoplastic at room temperature but will react at an elevated temperature resulting in a bond.
Liquid solvents are simpler and generally have lower manufacturing costs but are sensitive to surface imperfections that may cause inconsistent or unpredictable bonding. Solvent bonding can be performed using a liquid or gas solvent. 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 differs from adhesive bonding since the solvent does not become a permanent addition to the joined substrate. An advantage to solvent bonding versus other polymer joining methods is that bonding generally occurs below the glass transition temperature of the polymer (approximately 30☌ below T g). When the solvent evaporates this leaves a fully consolidated bond-line. 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. Solvent bonding is one of several methods of adhesive bonding for joining plastics.
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