Silicone surfactants are copolymers formed by a silicone base (apolar part) and polyethers (polar part), usually polyethylene oxide (OE) chains and polypropylene oxide (OP) chains. OE, OP and silicone content significantly affect the function of surfactants. Also the substituent at the terminal position of the polyether (R) can influence the properties of the surfactant. Therefore, you can also influence the final properties of PU foam by playing only with the surfactant, thus meeting the different needs for different applications that we can find on the market, such as flexible foams, HR foams, rigid foams, among others.
Purpose of these additives
Surfactants act as surface tension depressants by reducing the pressure differences between bubbles of different sizes formed and therefore a better stability of the bubbles and a small average cell size are obtained. Therefore, surfactants condition the formation of fine cells, thick cells, closed cells, and open cells, and these cellular structures influence the final properties of PU foam.
Types of surfactants
Thus, two main groups of surfactants can be distinguished, foam stabilizers and cell regulators.
Foam stabilizers are surfactants used in critical PU formulations, i.e. formulations that due to the type of chemical nature of the components of polyols and / or isocyanates or due to the amounts of catalysts or water, have very little stability. In these cases a surfactant with stabilizing capacity is required, which helps in the foaming process in the expansion of the gases. We find this type of surfactant in formulations where only TDI or TDI / MDI mixtures are used, but with a high percentage of TDI.
Secondly, there are cell regulators. These surfactants are primarily designed to regulate and unify the size and distribution of cells. These surfactants are aimed at PU formulations that by themselves already have a high stability, able to grow with a solid and consistent structure without collapsing, without the need for the help of a surfactant. We find this type of surfactant in formulations where only MDI or TDI / MDI mixtures are used, but with a high percentage of MDI. Cell openers are often requested on the market, which are cell regulatory surfactants that increase the size of the cell to prevent shrinkage problems.
Flexible HR PU foam
Concentrol surfactants in the STB PU-12XX range are driven by high resilience PU foam, HR. HR formulations have different needs for cell stabilization and regulatory efficiency, as we find TDI, MDI systems or mixtures of these. Therefore, a wide range of additives must be offered to meet all possible requirements. However, these systems are usually quite stable and a well-balanced foam opening is usually sought.
One of the most common requirements is low compressive strength. The optimal level of compression can be achieved with the proper selection of surfactant.
At the same time, due to the increasingly stringent legislation requirements of the automotive and mattress industry, the minimum contribution to volatile component emissions is sought.
Emissions are classified into VOC (volatile organic compounds) and FOG (fogging organic compounds). VOCs, which include VVOC (very volatile organic compounds) and VOCs, are easily released from PU foam in a closed car in the summer, when internal temperatures between 25 °C and 90 °C can be reached. FOGs are the sum of VOCs and SVOCs (semi-volatile organic compounds) that evaporate at temperatures above 90 °C.
Evolution of Concentrol’s range
Given the casuistry of the sector, in Concentrol the lines of surfactants have evolved in the following direction:
- STB PU-12XX PF: Second line designed, free of phthalates but high in aldehydes, more directed to the European market.
For the determination of VOCs and FOGs in car interior materials, the most widely used methodology in Europe is VDA278. The VDA (Verband der Automobilindustrie in German) is the German quality management system (QMS) for the automotive industry. Within these results, we must look for those volatile components that are produced by the surfactant.
- STB PU-12XX PFJ: Third line designed, free of phthalates with low content of aldehydes and aromatics, aimed more at the Asian market, especially China and Japan.
It should be noted that in other markets such as Asia, other methodologies are used for the analysis of VOCs. In addition, in this same market also seeks the minimum contribution of aldehyde and aromatic compounds. Thus, in recent years it has been investigated how to reduce the VOCs of silicone surfactants with a new generation of products that allow the obtaining of PU foams with emission values related to the stabilizer lower than 20ppm (for a dose of 1pphp).
For all these reasons, there are currently 3 types of products within the STB PU-12XX line, although there are no differences in performance between the PU foam.
Author: Adrià López & Josep Nadal
Technical Manager & Technical Director
