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Is Solvent Resistance Predictable ?

Is Solvent Resistance Predictable ?

To assess solvent resistance, first consider solvents spread out on a polarity scale. Non-polar solvents are at one end, such as paraffin, petrol and diesel. At the other ‘highly polar’ end we find alcohols and water.  In between are moderately polar solvents such as acetone, esters and chlorinated solvents.     On the same polarity scale we can place the families of thermoplastics. Polyethylene and polystyrene are at the non-polar end, polyamides at the highly polar end. PVC, acrylics and polyesters are somewhere in between in the ’moderately polar’ region.

Solvent Resistance pic

Where an amorphous thermoplastics and a solvent match up on the scale, we can expect total solubility, given a bit of time and heat to get the polymer to eventually dissolve.   However, semi-crystalline thermoplastics will not dissolve at ambient temperatures although the amorphous regions would mop up the matching solvent and cause swelling.  If a thermoplastic and solvent mismatch significantly on the scale then there would be little or no interaction, regardless of whether it is amorphous or semi-crystalline.

This explains why styrene polymers are seriously affected by hydrocarbon solvents. In contrast, polypropylene and polyethylene, being semi-crystalline, show much better resistance at ambient temperatures.  The polarity scale also explains why PVC is credited with ‘good chemical resistance’.   Although PVC, a moderately polar, amorphous thermoplastic, can be attacked and dissolved in moderately polar solvents such as tetrahydrofuran, it is resistant to the solvents at either end of the polarity scale — oil, fuel and water, which of course are the more generally encountered every day chemicals.

Solubility Parameter Scale

Non Polar                                                                                                Highly Polar

Hexane,Oil  Diesel    Toluene       Chloroform      Acetone       Ethanol             Water

——————————————————————————————————->

PP/PE                        PS           PMMA         PVC        PET   POM         PA 6.6


 

 

 

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