Flame Retardants: Zinc Stannate Alternatives in Engineering Thermoplastics

Polyamides (PAs), including PA-66 and high-temperature PAs, are used in numerous engineering applications because of their superior mechanical and electrical properties at elevated temperatures, good chemical and wear resistance, and low friction levels. In electrical and electronic applications, the limits of these critical performance parameters are pushed by adding combinations of additives to provide flame retardancy in order to meet required standards. An ideal flame retardant (FR) additive mixture delivers the required FR performance while preserving desired mechanical properties as much as possible. The challenge of meeting increasingly demanding levels of desired polymer performance has become more complex with the miniaturization of electronic components and electrical devices to ever smaller dimensions, which requires filling thinner and more complex molds. This challenge is probably most acute in reinforced polymer systems, where interactions between additives and reinforcing materials have a major impact not only on component strength but also on flow properties into the mold. Regulations create an additional level of difficulty, as additives used to overcome technical and application-related problems must accomplish this without causing any harm to health or the environment.

Zinc Stannate FRs

William Blythe successfully launched zinc stannate and zinc hydroxystannate FRs from the Flamtard S and H series more than 20 years ago and has increasingly intensified research and development activities due to rising demand for non-hazardous, high-efficiency polymer additives. In accordance with regulatory directives, the company has comprehensively characterized the hazard properties of its tin-based FR product series to demonstrate that no harm to health or the environment is involved. Structurally, Flamtard S is an anhydrous bimetallic material composed of tin and zinc oxides and is thermally stable to temperatures of 400°C. When combined with a halogen source, both elements suppress flame and smoke due to their distinctive chemical structures. As shown in Figure 1, in the early stages of combustion, tin is released in the gas phase and combines there with halogen radicals released from the decomposition of halogenated FR. This prevents flame formation. The zinc component plays a role in the condensed phase, where it triggers the conversion of decomposing polymer to carbon char rather than smoke.

Experimental Evaluation

Taking into account the changing regulatory environment and the need to provide detailed application data, a test program was implemented to compare the performance of glass-filled PA flame retardants containing Flamtard S with those containing classical antimony trioxide (ATO) systems. A two-stage experimental program was implemented to examine interactions between Flamtard S and three brominated polymeric FRs: brominated polystyrene (ICL Grade FR 803P), brominated polyacrylate (ICL Grade FR 1025), and brominated epoxy (ICL Grade FR 2400), and thereby to identify the ideal combination. The selection of these three polymeric FRs was motivated by the fact that due to their high molecular weight, they are not subject to any hazard classification and are therefore ideal co-FRs for Flamtard S, particularly for applications where a non-hazardous system is specifically required. Five sets of glass-filled PA compounds were produced on a Leistritz MIC 27 GL/44 D twin-screw extruder operating at 270°C and a screw speed of 300 rpm. Test samples were injection molded under a Klöcker Ferromatik Desma FX-2F with a cycle time of 40–45 seconds, mold pressure of 120 bar, and injection temperature of 255°C. The compounds used varying amounts of Durethan A30 S PA from Lanxess, 30% CS 7928 [l = 4.5 mm] from Lanxess, a stabilizer package based on Irganox 1098 and Irgafos 168 from BASF, and either Caesit AV/PA from Baerlocher, FR-1025 brominated polyacrylate from ICL-IP, or Flamtard S plus ATO from Campine in the form of plastic raw material colorant (Table 1). It was found that Flamtard S at a level of 3.45% was most effective when used with 19.05% brominated polyacrylate. Accordingly, this combination was evaluated both against brominated polyacrylate alone and in combination with an alternative counterpart.

Three analytical methods were used to generate precise data sets for defining fire science in terms of the basic components of the classical fire triangle:

Oxygen via Limiting Oxygen Index (LOI) per ISO 4589-2:1999; energy release using cone calorimetry; and self-extinguishing capacity of the burnt component using UL 94. The results are shown in Table 2. This demonstrates that a counterpart such as Flamtard S (Application 3) or ATO (Application 4) is required to enhance the performance of brominated polyacrylate alone (Application 2) in order to achieve the highest V-0 standard for the thinnest sections. The inclusion of Flamtard S at a level of 3.45% has an effect that increases LOI, reduces energy release from the system, improves self-extinguishing properties, and does all this while also reducing smoke generation. Flamtard S is markedly superior to the classical ATO-based system and achieves the V-0 standard at significantly lower density with an additional smoke-suppression effect. As an engineering polymer, the compound must demonstrate high performance levels throughout its service life, making the mechanical properties of the compound of critical importance, and accordingly the effect of additives on polymer properties was also examined. The most valuable FR system is one that has no negative impact or has a beneficial effect on the mechanical properties of engineering PA materials. Table 3 shows the mechanical properties of various compounds. The reduction in tensile strength resulting from the inclusion of FR 1025 (Application 2) is largely recovered with Flamtard S (Application 3), accompanied by similar improvements in elasticity modulus and elongation properties. When Flamtard S is compared to the classical system (Application 4), it is recommended that it be considered a valuable component for engineering PA compounds due to having no effect or beneficial effects on mechanical properties. Kevin Hudson / Operations Manager / William Blythe Limited Haluk Yavuz / General Manager / Spefichem Kimyavi Maddeler ve Danışmanlık