User:Benatar.1/WE7406 Article 2

= Solvent Bonding of Plastics =

Solvent Bonding is a subset of adhesive bonding used to join plastics. The use of a solvent on a thermoplastic material softens the polymer and with applied pressure results in polymer chain interdiffusion at the bonding substrate until the solvent evaporates leaving a fully consolidated bondline. In order for a bond to form, parts need to be pressed together for 10 to 30 seconds before being handled. A curing time of several hours at room temperature may be necessary for the bond to fully form and allow the solvent to completely evaporate from the joint. This time may be reduced by using a slightly elevated temperature. An advantage to solvent bonding versus other polymer bonding methods is that bonding generally occurs below the glass transition temperature of the polymer (approximately 30°C below Tg). Solvent bonding can be performed using a liquid or gas solvent depending on the application. Solvents used in the gas phase present additional safety concerns and challenges. Due to the vaporization involved in gas solvents a vapor chamber or vacuum equipment should be used to decrease the exposure to the vapors. This can lead to complex logistics and higher cost during production. 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. The proper solvent choice for bonding is dependent on the solubility of the chosen thermoplastic in the solvent and the processing temperature. Mutual solubility between a polymer and a solvent may be determined using the Hildebrand solubility parameter, (δ). Polymers will generally be more soluble in solvents that have 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 is affected by temperature. Increasing the temperature lowers the free energy mix, thus promoting dissolution at the interface and interdiffusion bonding. Using the Gibbs free energy of mixing, equal to or less than zero the solubility of a polymer can be determined.

Testing Solvent Bond Joints
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 and can lead to failure in the epoxy / polymer interface instead of in the bonded joint. The most common for testing solvent bonds is the tensile shear test using a lap joint. Specimens are tested to failure at a given overlap cross section.