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As the demand for silicone polymers continues to grow across various industries, the need for effective recycling methods has become increasingly important, because recycling silicone products reduces landfill waste, conserves resources, and uses less energy. Chemical recycling involves the depolymerization of silicone waste into oligomers, which can then be used to produce virgin-grade silicone. While this sector of the recycling industry is still in its infancy—we estimate that 35,000 to 45,000 metric tons of silicone waste will be chemically recycled worldwide in 2024—an increasing number of companies are beginning to explore the implementation of closed-loop systems to recycle silicones. This article examines the technical options and challenges for recycling silicone polymers, the major degradation chemistries available for depolymerizing silicones, and the current industrial reality of chemical recycling of silicones.

The paper discusses the work carried out over the past 7 years within RILEM Technical Committee 190-SBJ Service Life Prediction of Sealed Building and Construction Joints towards the development of RILEM Technical Recommendations (RTR) on durability test methods for wet-applied (gun-grade) curtain-wall sealants. This paper is published along with the two draft RTR methods to provide some insight into the discussions within the committee during the development and early evaluation of both methods. The two draft RTR methods define test and evaluation protocols for accelerated artificial weathering and natural outdoor weathering. Both draft RTR methods are based on a novel test specimen configuration that allows simultaneous exposure of the sealant to compression and extension during movement cycles. Because of the special test joint configuration, the sealant is exposed to variable extension and compression amplitudes along the joint axis, allowing an evaluation of the effect of different movement amplitudes on the sealant when taking weatherability considerations into account. The results of the initial evaluations indicate that the test methods are capable of differentiating between products with regard to their resistance to accelerated or outdoor weathering and mechanical cycling. The type of failure and the changes in surface appearance observed during the test regimes are similar to those observed in actual service conditions. It is hoped that exposure studies based on these two new RTR methods will provide the underpinnings for the development of future ISO and ASTM durability test standards for building sealants.

European and International Standardization Organization (ISO) standards are important for US sealant manufacturers. Manufacturers that are interested in self-certifying a product for sale to the European Community (EC) must apply European standards, which are those of the ISO. It is highly likely that the CEN memberstates will adopt ISO 11600 as an European requirement standard once it is published in its final form. As such, sealant manufacturers in the US should be prepared to comply with ISO 11600 if they plan to offer their products for sale in the EC. ISO/DIS 11600 applies to glazing sealants (type G) and construction sealants for use in building joints other than glazing (type F). The standard limits the maximum flow of all sealants, independent of their type and class, to 3 mm. It is important to note that the convention used in ISO to describe sealant extension differs from that used in most national standards: ISO test and classification standards for sealants systematically refer to extensions as the ratio between the extended and the original joint width, expressed as percent.

In the MAC project, for aesthetic reasons, the SSG bonding was partially required to be on enamelled glass. Therefore, the adhesion of the structural glazing sealant on the enamelled glass substrates had to be proven to meet the requirements according to ETAG 002. A working group of experts from different industries, composed of producers of glass enamels, independent institutes, producers of enamelled glass and a supplier of structural glazing solutions was established.

To assess their ranking in the future classification system to be mandated by ISO/DIS 11600, the following commercially available silicone construction sealants were evaluated: 1. two acetoxy cure (clear, silica filled), 2. five alcoxy cure (pigmented, calcium carbonate or calcium carbonate-silica filled, 3. one benzamide cure (pigmented, calcium carbonate-silica filled), and 4. two oxime cure (one clear, silica filled and one pigmented, calcium carbonate filled). The resulting classification is similar to the one obtained when tested against national European standards with the exemption of the non-silicone plasticized, acetoxy cure silicone sealant ACE2, which does not qualify as a glazing sealant according to ISO/DIS 11600. Although cumbersome and labor intensive, this standard can be regarded as a step towards an international harmonization of requirement standards within the sealant industry.