Before the Lab: Read the hand-out's on preceramic polymers and their processing that were distributed in class last week.
Laboratory Location: 359 Mech E
Laboratory Supervisor: Merve Erdal (merdal@me.umn.edu)
Objective:
The objective of this lab is to familiarize the student with the processing of a relatively new family of polymers called preceramic polymers (PCP's). These materials are similar to conventional thermosetting polymers with the exception of conversion to ceramic with an additional high-temperature processing step. The students will not have as much hands-on work in this laboratory as they do in the other two. The session will include a lecture on PCP's and their processing, and the demonstration of a reimpregnation process stage, along with measurements for weight gain.
Session Itinerary:
Experimental Procedure and Calculations:
Wear gloves while handling any material component
In the first part, the lab supervisor will demonstrate vacuum reimpregnation of a PCP composite in a desiccator. Several parts that have been pyrolyzed previously will be reimpregnated with PCP. These parts are then to be cured in a cure oven and subsequently pyrolyzed.
Figure 1. Vacuum Desiccator for Part Reimpregnation
In the second part of the lab, the students will measure reimpregnation weight gains of parts that have been previously pyrolyzed. These parts have been processed in sequence, each having a different number of reimpregnation cycles. But the processing is the same in each cycle. The weight of each pyrolyzed sample is to be measured. The weights after cure (before pyrolysis) have been previously recorded and will be provided to the student.
Calculation of Weight Gain, Char Yield and Pore Formation after Each Pyrolysis during Reimpregnation Cycles:
In ceramic processing, char yield is defined as the percent mass that is retained at the end of the high temperature process stage. In PCP processing, mass is lost in the form of volatiles due to chemical reactions taking place during pyrolysis. Thus the char yield for a single part after each processing cycle can be determined by
Note that matrix is the PCP in the composite. Its mass can be calculated by subtracting the mass of the fiber fabric from the total mass of the composite. The fiber mass, mf, can be approximately calculated as:
where marea is the areal mass of a single fabric layer, Acomp the planar area of the composite and N, the number of fabric layers in the composite.
Due to mass loss and densification of the PCP in pyrolysis, local voids or pores will form in the composite, rendering it weak (hence, necessitating the reimpregnation cycles). The percent void production in the composite due to pyrolysis in each process cycle can be calculated as:
Note that the dimensions of each composite part need to be measured as well (for composite volume and area calculations). The cured and pyrolyzed PCP (matrix) volumes can be determined by dividing the mass by the respective density. Note that the densities of the cured and pyrolyzed PCP will be different.
Lastly, the percent weight gain after each process cycle is to be calculated. This is the gain in composite weight after pyrolysis during each cycle, and can e expressed as:
Tabulate all results (char yield, % pore formation and % weight gain) for process cycle. For a given cycle, average the values for all parts involved. Plot the results in the graphical form (versus cycle number - number the initial making as 0 and the reimpregnation cycles as 1,2,...etc.).
Reports:
Your report must be typed, and include the following sections: Objective, Calculations, Results and Discussion. The report for this lab session should not exceed 2 pages (excluding any data sheet handed out during the lab).
In the Calculations section, you will include the recorded experimental data and detailed calculations as outlined above.
In the Results sections, you will include the tabulated results and the graphical outputs.
In the Discussion section, you should mainly answer the following questions (but feel free to add any other observations, comments you might have):
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