Q: We measure gel time on all incoming samples of PUNB and sometimes we notice a pooling of solvents in the gel. Will this lead to increased smoke and core gas? What is the best use for gel-time data?
A: Testing the "gel time" of incoming Phenolic Urethane No-Bake (PUNB) binders is a good quality-control practice, but the results of gel-time testing are prone to over-interpretation.
A gel test is a relatively simple way to analyze binder reactivity: Mix all the components together and observe how long the chemical reaction takes. Perhaps because of the simplicity, and the visible results, the test may influence some conclusions about the “quality” of the binder system. Different observations are thought to be indicative of mold curing speed, tensile-strength performance, and especially gas-related casting defects.
Sometimes you may observe a gel that exhibits a partial phase-separation, where some of the binder’s solvents pool, exude, or evaporate from the polymer matrix. This may seem abnormal but it can be an effect of the testing conditions, and can be normal. Often, this behavior is misinterpreted to predict the binder’s likelihood for developing gas-related casting defects.
Some large assumptions would have to be true for this to be accurate. Gel testing would have to be indicative of performance on sand, and solvents generated during curing would stay on the sand. However, gel testing completely ignores the impact of sand.
Sand plays an important role as a heat sink for both the molten metal and reacting binder. Without sand the heat generated in exothermic gelling reaction is localized, which leads to higher temperatures and can cause solvents to evaporate and expel from the polymer. This is the primary reason that gels exude solvent.
Curing PUNB binders on sand always results in some solvents being distributed on the sand and some entrapped in the resin matrix. In fact, any solvents that evaporate during tensile development, and are not retained within the polymer matrix, result in lower total gas and smoke during pouring. These types of easily evaporating solvents are the same solvents that can pool during a gel test. When measuring total gas generated on sand under pouring conditions, gas evolution is the preferred test. Therefore, solvents generated in a gel test have no connection to gas-defects.
In fact, some testing variables can be manipulated to cause a gel to generate solvents regardless of binder. Modifying the testing conditions can affect solvent generation.
Increasing gel sample size, increasing catalyst, temperature of components, and changing the ratio of components are all ways to affect solvent generation. Hence, observations of strange gels should not be indicative of binder quality. A product can have excellent tensile and gas evolution but still may exhibit solvent exudation during a gel test.
So, what is the best application of gel testing? It should be used as a reactivity or quality-control test. To get consistent and meaningful results, make sure the binder is at room temperature and gel size is less than 50 grams, and pick a catalyst level that will result in a gel time greater than 60 seconds. Measured gel times are correlated to expected strip times, although shorter than measured strip time on sand.
As a quality control test, a fast or slow gel test may mean something is wrong with one of the binder system’s components. If the gel test is normal, but there are problems on sand, then focus your inquiry on the sand.
Please use caution against over interpreting gel testing, and use it for its intended purpose — evaluating reactivity.
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