January 2017

Multiple Steam Sterilization Cycles with Apec® 1745

Apec® 1745 high-heat polycarbonate from Covestro is a transparent engineering thermoplastic with heat-resistance beyond that of Makrolon® polycarbonate, yet is also tough and rigid. A comparison of some of the properties of Makrolon® and Apec® is given in the Table below:

Resin Vicat Softening Temperature (°C) Heat Distortion Temperature (°C) Charpy Impact (kJ/m2) Tensile Modulus (MPa)
ISO 306 ISO 75-1, -2 ISO179-1 eU ISO 527-1, -2
50N ; 120° C/h 1.80 MPa 0.45 MPa 25° C 25° C
Apec® 1745 170 148 160 N 2400
Makrolon® 3158 148 126 146 N 2400

Like medical-grade Makrolon® resins, Apec® 1745 is a medical-grade copolycarbonate that meets certain biocompatibility test requirements of USP Class VI and ISO 10993-1 requirements for up to 30-day patient contact. Its combination of properties make Apec® 1745 a clear choice for medical devices intended for sterilization by steam, especially where some autoclaves can reach temperatures as high as 135° C.

In this article, property retention of Apec® 1745 after multiple high-pressure steam sterilization cycles is shown by measuring physical properties after up to 100 cycles.

Experimental Details

Test specimens that were injection molded from Apec® 1745 551022 were used for this study. Recommended drying and processing parameters were followed. Covestro’s autoclave used deionized water and was set to maintain a pressure of +3 Bar (i.e. absolute) which, according to widely available “Steam Tables” corresponds to a temperature of approximately 136° C. Test specimens molded to either ISO or DIN standards were taken out of the autoclave for testing at intervals of 25 cycles, up to 100 cycles. Each sterilization cycle lasted at least 15 minutes, and the total duration of heating, sterilization and cooling totaled ca. 45 minutes.

The following tests were performed on the sterilized test specimens:

  • Charpy Impact Strength (ISO 179-1 eU) at room temperature
  • Multi-axial Impact (ISO 6603-2) at room temperature on 60x60x2mm plaques
  • Yellowness Index (DIN 5036-1) measured in transmission mode on 4mm-thick color chips

Results

The impact-strength properties measured at 25 sterilization cycle intervals are compared in the Figures below. The values shown reflect the average from all test values recorded, which were sometimes mixtures of broken and unbroken samples. For samples which yielded “No Break” after Charpy Impact strength testing, a value of 250 kJ/m2 was used for calculation purposes (i.e. the maximum value the instrument could record).

Although Covestro measured a decrease in impact properties with increasing sterilization cycles, it is important to know/observe/note ductility is still present over the course of the company’s experiments. For example, after 75 cycles, all nine of the impacted/sterilized specimens still showed excellent ductility. The image below shows the plaques that were impacted after 75 sterilization cycles, where Covestro observed yielding-type breaks from the dart impact. After 100 cycles, five of 10 specimens still showed excellent ductility.

Similarly with Charpy Impact Strength, although Covestro measured a slight decrease in average impact strength after steam sterilization, the majority of the test specimens at each interval were still “No Break.” Thus, while a slight decrease in impact strength can be measured after multiple steam sterilization cycles with Apec® 1745, Covestro still observed excellent ductility retention.

Apec® 1745 also shows outstanding color retention after multiple steam sterilization cycles, as shown by the extremely low changes in Yellowness Index recorded over the course of 100 steam sterilization cycles. (Figure below) With a change of 1.5 YI, a color change is extremely difficult to observe visually.

Covestro’s results suggest that with autoclave treatment with deionized water and careful molding parameters, Apec® 1745 is capable of showing excellent toughness and optical propertys retention after 100 steam sterilization cycles at +3 Bar/136° C.

Of course, retention of properties depends on many factors such as environment, design and processing. To help you determine whether Apec® 1745 from Covestro is suitable for your application, email plastics@covestro.com.

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