Organotin stabilizers are usually used for rigid foam boards. The majority of tin stabilizers used are tin mercaptides. They are cheaper than tin carboxylates and give excellent heat stability but poor weatherability. Also, they don`t act as kickers for ADC decomposition. Hence, formulations based on tin mercaptide, which are used for the majority of non-weathering foam sheet products, require kickers for ADC decomposition otherwise a low gas yield will result.
Whereas, tin carboxylates are more expensive than tin mercaptides but offer excellent weathering properties and work as kickers for ADC decomposition. These stabilizers are a good choice for free-foam sheets for outdoor applications
Endothermic CFAs are chemicals that take heat away from the chemical reaction. They are based on inorganic chemicals and generate CO2 gas. Sodium bicarbonate (SBC) is one of them. This produces foams with a much smaller cell structure, resulting in improved appearance and better physical properties. SBC decomposes to give CO2, H2O and Na2CO3. The reaction is endothermic and reversible and takes place over a wide temperature range, which includes the processing window of rigid PVC foam. The gas yield is only 125 cc/gm and compared with ADC the decomposition is slow and erratic, and a coarse cell structure is obtained. However, the advantages of SBC are that it is easier to handle than ADC (which is classed as an irritant), it produces a whiter foam and is less prone to give problems with cell collapse. Endothermic chemical foaming agents contain no hazardous materials so they are environmentally friendly, safe to process and grades are available for applications where contact with food occurs.
The best blowing agent to use for rigid foam profile is a mixture consisting of mainly SBC (90%) with a small amount of ADC. Combining SBC with ADC ensures a reasonably fine cell structure but avoids the problem of cell collapse.
To decipher the issue of cell collapse, foam regulators are used. These are usually high molecular weight, acrylic based processing aids. These serve the dual function of the improvement conferred in both melt elasticity and strength and also prevents collapse of the cellular structure, to get the PVC foaming products with uniform foaming. At the same time, it can also promote the plasticization of PVC to ensure that the melt has good fluidity. It can effectively reduce the plasticizing temperature, promote the plasticizing effect, improve the plasticizing efficiency, reduce the melt pulsation, prevent the melt from breaking, and significantly improve the surface smoothness of products. The foaming regulator with a fast plasticizing effect should be selected for the thin board and the foaming regulator with high strength for the thick board.
The concentration and molecular weight of the
Acrylic Processing Aid can give significant reductions in foam density and prevent the collapse of the cellular structure. Hence, the desired improvement in melt strength may be achieved either by the use of a high level or
High Molecular Weight Processing Aid.
It is essential to optimize the formulation in terms of the balance between the level of blowing agent and
Processing Aid to prevent the collapse of the cellular structure.
In addition to promoting material plasticization, lubricating processing aids have the function of metal peeling, preventing the adhesion between melt and metal surface, provide product surface quality.
