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Employing Azodicarbonamide as a Nucleating Agent
in Thermoplastic Foams

Everyone is well aware of the use of exothermic foaming agents to reduce the density of plastic parts and lower cost by using less resin. Exothermic foaming agents are also used to eliminate splay marks in injection-molded parts and to improve transmission and electrical properties in wire and cable insulation. However, did you know that even those who utilize physical blowing methods often use exothermic foam concentrates, such as azodicarbonamide, (ADC) as nucleating agents?

Nucleation taking place at an interface between the polymer and another phase is called heterogeneous nucleation. This may be contrasted to homogeneous nucleation where thermodynamic instabilities are used to generate the nuclei of the gas cells. In heterogeneous nucleation, nucleating agents are critical for controlling the morphology of the cells being formed during the foaming process. The presence of an efficient nucleating agent will facilitate the production of a larger number of smaller cells. That is, a higher cell density (cells/cm3) and thereby a lower density foamed product. Typically, particulate solids such as talc or silica are used as nucleating agents. The blowing gas is adsorbed on the surface of the particulate during phase separation. At this point, the accumulated gas is in an unstable intermediate state known as an embryo. If the embryo at the surface/resin interface exceeds the critical size to survive (it is then termed a nucleus) a cell will be formed. This results in a finer cell structure than simply blowing the part without any nucleating agent. However, these types of nucleating agents are constrained from producing very fine cell structure and especially microcellular structure by their size.

Figure 1. Percent Density Reduction Versus Nucleator Type.

Another, extremely effective, method of nucleation exists using finely dispersed ADC in the melt. Being strongly exothermic, the ADC reacts to create "hot spots" in the melt, reducing the melt viscosity and surface tension locally. Figure 1 shows the density reduction of an injection-molded part using an endothermic blowing agent (Ampacet 703061-H), lab control, 703061-H with a talc nucleator, and with an ADC nucleator. In Figure 1 it can be seen that the blowing agent's efficiency is greatly enhanced by the addition of a nucleating agent, whether it is talc or ADC. In this work, the ADC outperformed talc, of comparable particle size and same concentration, in terms of density reduction.
This in part can be explained by a contribution from the gas generated by the ADC, but also by a significant contribution from the effectiveness of the ADC as a nucleator. Cross-sections of the injection-molded parts reveal a finer cell structure (smaller cells, higher density of cells) for the part foamed employing ADC as a nucleator versus that of a talc nucleator.
A. ADC Nucleation

B. Talc Nucleation

Figure 2. Comparison of effect of nucleation by ADC versus talc in an injection molded part.

 

Some possible explanations for the differences include: the number of embryos produced, the number of embryos that reach the critical size to become active nuclei, and the density distribution of active nuclei. These factors and others conceivably control the number of cells created and the rate at which they will coalesce and thereby determine the cell structure of the foamed product. Whatever factors dominate, it is clear the ADC produces a finer cell structure resulting in smaller and a greater number of cells

When using ADC as a nucleating agent it is important to carefully control the processing temperatures so the ADC activates at just the right time. Optimally, the ADC is activated immediately before foaming. Premature activation can result in a loss of effectiveness of the ADC nucleating agent. Fortunately Ampacet offers a variety of ADCs that activate at various temperatures so you can choose the one that best fits your current process.


Figure 3. Percent Nucleating Agent Versus Average Cell Size.

The concentration of the ADC in the part determines the number of cells and therefore the cell size. The relationship is demonstrated in Figure 3, where increasing nucleating agent concentration results in a decrease in average particle size. For nucleation purposes, a concentration of one to five percent of Ampacet's 701540-W is added; most typically about 4% is used to give a 0.3% ADC concentration. You may opt to use a more concentrated product, such as 701709-H, at a lower dosage. Nucleation by ADC is a valuable method of generating a foamed plastic part or insulation with a fine cell structure. Ampacet offers ADC foam concentrates in a wide range of concentrations, activation temperatures, and a variety of resin carriers. Let us help you select the one that best fits your needs.
For more information, please contact Ampacet’s technical support team at 888-822-7546 or 812-466-9828.
Disclaimer:  The information and recommendations contained in this document are based upon data collected by Ampacet and believed to be correct.  However, no warranty of fitness for use or any other guarantees or warranty of any kind, expressed or implied, is made to the information contained herein, and Ampacet assumes no responsibility for the result of the use of the products and processes described herein.  This is an uncontrolled document and information may be out of date.