Grip Solution for Testing Polymer Hydrogels

Due to the composition of polymer hydrogels, gripping and loading for the testing of mechanical properties can be difficult without the proper equipment. A manufacturer of polyacrylamide hydrogel was experiencing such challenges, so we conducted some tests to determine the best solution. We cut the specimens into rectangular strips measuring .5 inch width and 1.5 inch gauge length and ran the tests at a crosshead speed of 0.197 inches/minute (5 mm/minute). We evaluated both 100 N capacity pneumatic side-action and 100 N capacity screw action grips with a variety of serrated, smooth, and sandpaper surfaces of jaw faces to determine the optimal grip configuration.

Our results show several important findings. First, due to very low stress and strain values experienced at break, we recommend using an electromechanical test system with a 50 N load cell. We ruled out 100 N capacity pneumatic grips, as they supplied too great of a force – even at very low pressure values. When using the screw action grips, we found that the ability to adjust the grip force was critical to successful testing, as specimen thickness values varied greatly. We also found that serrated faces consistently led to breaking of the material at the jaw face; not enough tightening of them led to specimen slipping. Using sandpaper adhered to a smooth face was found to be the best solution for gripping the hydrogel without slipping or premature failure.

 

A final recommendation for the most accurate break results is to prepare the specimens with a "dogbone-shape" die in accordance with ASTM D412. This specimen shape has a reduced gauge length area and minimizes the chance of the specimen breaking at the jaw face or tearing due to spurs on the edges of the material.

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