This foaming process is sometimes called hand mixing. It is used as a simple method of evaluating foaming systems for comparison of raw materials and resulting foam properties. Based on the optimum formulation obtained by cup foaming, scale-up foaming processes, that is, box foaming followed by machine foaming, are employed. The process requires the following:
Equipment and supplies:
- High speed electric mixer
- Paper/plastic cups
- Top-loading balance
- Micro syringes (0.001 g accuracy)
- Solvent for cleaning mixing blade (e.g., methylene chloride)
An example of foaming procedures is as follows:
Step-1. Weigh 100g of a polyol blend into a 0.5-liter paper or plastic cup.
Step-2. Successively add blowing agents into the polyol component with gentle stirring with a spatula. Vaporization of the physical blowing agent must be avoided in the blending procedure. When the blowing agent is added and stirred, a portion of it evaporates, so its loss should be adjusted by adding more blowing agent. The addition and stirring are repeated at least three times to reach an exact amount. After the physical blowing agent is dissolved completely in the blend, the blowing agent does not evaporate within a short period of time.
Step-3. Weigh the isocyanate component in a separate cup. The total amount is the theoretical amount plus the remaining amount in the cup after pouring. The remaining amount can be determined separately in advance.
Step-4. Then pour the isocyanate component into the polyol-containing blend and mix it for about 5 seconds.
Step-5. Allow the foaming mixture to expand in the same cup or pour it into a separate cup. Measure cream, rise, and gel times.
Step-6. Clean the stirrer immediately after mixing by using methylene chloride or other solvents.
Step-7. Cure the foam at room temperature or by accelerated curing at 80 to 90°C for one hour. Cut samples for foam testing from the upper parts, not the lower parts, in the cup.
Cream Time: The time interval between the final mixing of all the foam ingredients and the time at which the clear mixture turns creamy and starts to expand.
Gel Time: The viscosity of the foaming liquid increases with reaction time due to the cross-linking reactions. To test for gel, touch the foam with a spatula and pull away; a thread will form between the spatula and the foam.
Rise Time: The time interval between the start of the final mixing and the time of complete expansion of the foaming mass. In machine mixing it is the time interval between pouring the foamable mass into the vessel and the time of complete of expansion of the foaming mass.
Tack-free time: The time interval between pouring the liquid mixture and the time when the surface of the foam does not show stickiness when touched with a spatula. This means that chemical reactions have been completed. In some cases the tack-free time of rigid foams occurs before rise time.
Measuring Free Rise Density (FRD): To calculate free rise density of foams, divide the mass or weight of cut foam by the weight of foam in water (volume of the foam). This method called “Archimedes method”.