Improving Liquid Silicone Rubber Productivity Without Post-Curing

Medical Design Briefs INSIDER

Thanks to their purity and good skin compatibility, silicones increasingly find use in medical-technology applications. ELASTOSIL® LR 5040 liquid silicone features excellent tear strength after curing, not requiring a thermal post treatment. (Credit: WACKER)

To fulfill regulatory requirements, articles made from liquid silicone rubber must be post-cured, particularly if they are intended for sensitive applications. With a new liquid silicone rubber, this time-consuming step can cost-effectively be eliminated.

To achieve the properties required for sensitive applications, vulcanizates made from conventional liquid silicone rubber must generally be post-cured (thermal post-treatment) for several hours. This treatment improves the mechanical properties and reduces the volatile content to the required level. This extra step costs time and energy as well as interrupting the highly automated process, since the post-curing ovens are generally manually loaded and emptied. Therefore, fabricators should highly appreciate avoiding this complicated and cost-intensive procedure.

Fig. 1 – The process steps shown in grey, are obsolete when ELASTOSIL® LR 5040 is used.

It is precisely for this reason that Wacker developed the ELASTOSIL® LR 5040 series liquid silicone rubber. Even with post-curing, rubber goods manufactured from these new grades show a property profile that already satisfies the requirements for baby-care, food-contact, and medical applications (see Figure 1).

Fig. 2 – In a non-post-cured state, ELASTOSIL® LR 5040 (blue curve) features significantly higher tear resistance than standard liquid silicone rubber (violet curve).

ELASTOSIL® LR 5040 LSRs are fast-curing, liquid silicone rubber grades (see Table 1). After curing, they have excellent properties even without post-curing:

Very low volatile content
Meets the requirements of BfR, FDA, EN 1400, and EN 14350
Compliant with core requirements of ISO 10993 and USP Class VI
Excellent tear resistance
Narrow tolerance of ±3 Shore A durometer

The ELASTOSIL® 5040 product series is available in hardness levels between 30 and 70 Shore A, including a 45 Shore A grade best suited for applications in the baby care sector. These hardness levels are achieved even without post-curing, with a particularly narrow tolerance of ±3 Shore A points. Rubber parts from this series are translucent with a delicate blue shimmer. In addition, the cured rubber products exhibit tear resistances of up to 45 N/mm2 in their non-post-cured state —measured as per ASTM D 624 B (see Figure 2). This is particularly important for manufacturing of baby-care articles such as pacifiers and teething rings.

Silicone parts intended for use in the baby-care sector and the food industry in Europe must contain no more than 0.5 percent volatile substances. For sensitive applications, weight loss is often used as a measure of the volatile content. Typical examples include the European Standard EN 14350-2 for baby and infant drinking equipment and EN 1400 for pacifiers, recommendation “XV. Silicones” of the German Federal Institute for Risk Assessment (BfR), and legal requirements in other European countries. On heat treatment (typically 4 hours at 200 °C), the silicone article must show a weight loss of no more than 0.5 percent. Standard tests show that non-post-cured parts made of ELASTOSIL® LR 5040 are well below that limit.

Table 1. Characteristics of ELASTOSIL® LR 5040.

Wacker is represented in the Americas Region by Wacker Chemical Corporation, and is home to sales and administrative offices, technical center, logistics, distribution, and silicone production facilities.

Source