Stabilization of plastics against UV-C radiation

For outdoor applications, plastics are routinely stabilized against UV-A and UV-B radiation using additives, but UV-C radiation does not occur in nature and has therefore not been considered so far in terms of developing additives. Consequently, it is unclear whether these additives also provide protection against UV-C radiation.

UV-C disinfection devices are expected to be used well beyond the Covid-19 pandemic, and some of the irradiated plastics – especially materials designed for indoor applications – will prematurely turn yellow, fade, crack and/or become permeable. The appearance and protective function of the plastic products are then jeopardized and complaints are foreseeable.

The objectives of the research project are therefore as follows:

• to test the effectiveness of the UV stabilizers that are available against UV-C radiation, and
• to identify (more) effective additive combinations based on existing additives.

Equipped with knowledge about the UV-C sensitivity of common polymers, companies may assess potential risks from UV-C disinfection for their materials available on the market. The aim is for material manufacturers, additive developers, plastics processors and manufacturers of plastic products and UV-C disinfection devices to be able to issue, based on the project results, specific recommended actions to their customers to avoid complaints. With the additive concepts emerging from the project, there may also be the chance to gain knowledge of promising additive combinations and concepts at an early stage and thus create the opportunity for all the companies involved to manufacture and market innovative UV-C-stable materials and products in a targeted manner.

Main focus and approach

The following approach has been devised in order to achieve the objectives. The project partners will jointly coordinate the details of the initiative at the beginning of the project.

The main focus of the project work will be (I) to examine the effectiveness of existing UV additives against UV-C radiation and (II) to devise powerful protection concepts based on existing additives. Due to their high prevalence, the Fraunhofer Institute for Structural Durability and System Reliability LBF proposes the examination of polyolefins such as PE and PP, as well as PA, PS, polyester, elastomers used for handrails or seals, and melamine and phenol resins, which are used in laminates, for instance. A parity adjustment of the substrates under consideration in the consortium is, of course, possible. Likewise, fillers, pigmentation and coloring may be taken into account.

With this in mind, test specimens are first produced from common, unstabilized polymers (e.g., PE, PP, PA, PS, polyester, PF/MF resins, elastomers). The materials are decided on jointly with the project partners and – if possible – are provided by them in the form used. The test specimens are produced at Fraunhofer LBF. The test specimens are subjected to UV-C radiation. At certain time intervals, color and gloss (C&G) and chemical degradation are examined using FTIR spectroscopy. In addition, notched bar impact tests are carried out to determine fracture mechanical characteristics and light microscopic images (LM) to detect near-surface changes. This allows the damage to the polymers caused by UV-C irradiation to be systematically recorded. The selection of measurement methods can also be adapted with the project partners to the selection of polymers and their applications.

I) Examination of the effectiveness of existing UV additives

Based on the interests of the partners and the knowledge acquired about the actual sensitivity to UV-C radiation, plastics are selected and have off-the-shelf UV stabilizers added to them. The purpose is to examine the potential of the different additive classes for UV-C stabilization. The impact caused by regular, very brief UV-C disinfection is comparable to the destructive effect of many years of outdoor weathering. Due to the higher energy of the photons, different degradation mechanisms than those involved in weathering are to be expected. The effectiveness of individual stabilizers or entire categories may therefore be reduced or completely prevented. UV absorbers, HALS, quenchers and antioxidants are also included in the studies with equal participation of the consortium to ensure proximity to application.

For this purpose, the test specimens containing additives are exposed to UV-C radiation and are physically and chemically characterized after certain intervals. By way of a reference, samples of the same composition are to be exposed to UV-A radiation according to standardized methods and characterized in order to assess the effectiveness of the additives in comparison to the known UV(-A) stresses.

II) Development of potent protection concepts based on existing additives

Based on these results, sample formulations with existing UV additives will be developed for selected polymers, which should produce the highest possible UV-C protection. The effectiveness of these formulations will be examined and evaluated using UV-C radiation and characterization based on color and gloss measurements, as well as by using notched bar impact tests, FTIR spectroscopy and light microscopy. The aim is to develop basic protection concepts that the project partners can use for stabilization against UV-C radiation in their existing formulations.

The results will be presented to the project partners in regular meetings as part of a lecture and the next work steps will be discussed. All results will be documented at the end of the project in the form of a detailed research report and made available to the project partners.