Plastics Technology

DEC 2014

Plastics Technology - Dedicated to improving Plastics Processing.

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material is homogenous. The same TGA test as before is performed and the sample is heated from around 50 C to well past your material processing temperature at a heating rate of 10° C/min. The percent of CFA that turns into gas when combined with the formulation components can be determined using the calculations shown in Fig. 3. In this example the sample contained 0.625 mg of CFA and we measured a mass loss of 0.2563 mg. The resulting calculation is that the CFA lost 41% of its mass as gas. The measurement is shown in Fig. 2. The fnal step is to measure a fnished product to determine the amount of unreacted CFA (see Fig. 3). For this step it is important to take a representative sample of the part, as the amount of unre- acted CFA may vary from location to location in a larger part or in a part that has a complex geometry. You may want to test multiple locations on the part and get results from diferent areas. For this step it will be important to know the instrument's limit of detection. The instrument used for the example measurements shown has a limit of detection of 38 µg. The calculations for this example are shown adjacent to Fig. 3. The measurements show that 37% of the CFA remained inactivated in the fnished product. MODELING THE CFA REACTION The activation of CFAs is a chemical decomposition that pro- duces gases as the products of the reaction. The CFA needed to foam your part needs some combination of time and tempera- ture to react completely prior to depressurization and foaming. Kinetics is the term used to describe the reaction rates. By cre- ating kinetic models of the CFA reaction, limits on the produc- tion process can be calculated. It is important to understand how choices of CFA can afect limitations to processing temperatures, line speeds, or cycle times. To create the kinetic model, a series of measurements are made using either a diferential scanning calorimeter (DSC) or a TGA. For this example we will be showing data created from a DSC. Five separate tests were conducted with the CFA, heating it from room temperature well through its degradation point. Some CFAs have more than one degradation event; if this is the case, each event should be modeled separately. For this article we will use azodicarbonamide (azo), which has only a single decomposition step. The fve tests shown in Fig. 4 are identical except for the rate at which the temperature increases. The fve tests are cropped to exclude any data that isn't associated with the degradation. At this point the software is used to perform a series of very complicated calculations using algo- rithms that are protected by patents to determine the activation energy of the reaction as it progresses to completion. QUESTIONS ABOUT TESTING? For more articles on testing and quality control, visit short.ptonline.com/QC All CFAs have one thing in common: The molecule is designed to decompose at a specifc temperature and to produce a gas as part of that decomposition. FIG 2 FIG 3 Calculation for CFA Gas Generation with Formulation Components TGA Thermogram of a Finished Product Part Onset Temp = 150C Total Sample Mass = 25.8mg CFA Loaded at 2.5%, CFA Mass = 0.645mg CFA Mass Used = 0.625mg Mass Loss = 0.2563mg Calculations and Measurements: 1. Measure the activation temperature of the CFA. 2. Measure the amount of weight loss in the material formulation. 3. Calculate the maximum weight loss for 100% activation. 4. Measure the amount of weight loss due to decomposed CFA in the fnal product. 5. Calculate the amount of unreacted CFA. Onset Temp = 150C Mass Loss = 0.0978mg In a complete formulation, a sample contained 0.625 mg of CFA and the measured mass loss was 0.2563 mg. The resulting calculation is that the CFA lost 41% of its mass as gas. @plastechmag 59 Plastics Technology T E S T I N G C FA s

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