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Plastics & Elastomers

CET Resins, A Novel Alternative for Medical Applications

SpecialChem / Mar 13, 2013

Gamma Sterilization


The growth of thermoplastic materials in medical markets is a proof of the suitability of these materials to meet the demands in the manufacture of medical devices. Historically, there has been a wide gap between the low cost resins, such as polystyrene, polyethylene and polypropylene, which are either clear or tough, but not both, and the high cost resins, such as polycarbonate and cellulosic, which are both clear and tough. Moreover there are certain applications with intermediate specifications, where the properties of low cost resins are inadequate, and the high cost ones are over specified.

Resirene's CETs, clear styrene-acrylics copolymers, are a good compromise for medical applications for numerous reasons. They are a low cost alternative to acrylic, polycarbonate, clear ABS, PETG and SAN that meet the transparency, mechanical properties, dimensional stability, scratch resistance and flexibility requirements.

As examples, packages composed of CET116/CET123 resin are tough enough to protect their contents and can be designed to open easily. Medical devices made of CET116 are clear enough to determine the nature, amount or condition of their contents during use, and tough enough to resist accidental breakage. Components made of CET240 provide impact strength, dimensional stability and a good transparency.

CETs are easily and economically processed using conventional processing techniques. Recommended processing temperatures are lower than polycarbonate and clear ABS ones while handling, storage and processing conditions are comparatively similar. CETs are adequate for recycling, since they are thermally stable.

From the medical point of view, CET copolymers are less costly than acrylic and provide the same alcohol, blood, and lipid resistance. CETs meet USP Class VI specifications, withstand gamma sterilization and are compatible with tooling designed for many other resins such as acrylic, vinyl, and polycarbonate.

There are two basic types of CET copolymers: Series 100 and Series 200 (See table 1). Series 100 are stiff materials recommended for injection molding (CET116/CET123, CET116). Series 200 are rubber-modified, impact- resistant materials intended for injection molded (CET240) and blow molded (CET270) applications. All of them meet the specifications of section 177.1640, 177.1010 and 177.1830 of Title 21 of the FDA for food contact.

SI Units CET-116 CET-240 CET-270
Specific Gravity 1.08 1.05 1.03
Melt Flow Rate (G - 200°C/5 kg) g/10 min 2.8 6.5 10
Tensile Strength, Break Mpa 52 22 19
Break Elongation % 5 45 220
Modulus Mpa 3300 2000 1200
Impact Resistance
Notched Izod Impact J/m 16 21 534
HDT @ 264 psi (annealed) °C 86 70 60
Vicat Softening Temperature °C 96 72 70
Light Transmission % 90 88 86
Haze % 0.3 1 1.5

Table 1: CET Properties

Gamma Sterilization

CET polymers, including impact modified grades, are transparent. Then they can be used in many medical applications such as syringes, spikes, connectors and luers, suction devices, urine meters, blood plasma separators, drip chambers, cuvettes, dialyzer casings, chest drainage units, bottles for fluids, vaginal speculums, flow valves, aspirators, and containers for operating instruments among others.

Prior to their use or re-use, these medical articles require sterilization which is commonly accomplished, for example, by exposing the article to low levels of gamma radiation. Doing so, however, induces yellowing and loss of light transmission in the article which alters the appearance in an aesthetically unfavorable way and therefore, it is important to study the effects sterilization over CET resins.

Gamma radiation has become a very popular choice for sterilization, but physical properties and color changes occur in thermoplastics after sterilization with gamma radiation. These changes are well documented by Sturdevant (1) and Hermanson (2). For example, PMMA (polymethyl methacrylate), although far less prone to discolor and lose properties on exposure to sunlight than other plastics, is subject to yellowing when irradiated with gamma radiation. This yellowing reduces the clarity of PMMA and alters its appearance in an esthetically unfavorable way. Much of the yellow color will be lost on aging, especially if the sterilized sample is maintained at an elevated temperature, such as about 60°C, but the level of residual color is still unattractive when compared to the non-irradiated sample.

On the other hand, aromatic polycarbonate resins are frequently used in medical products due to their high impact resistance and heat resistance, good clarity and safety. However, polycarbonate resin, suffer yellowing when exposed to gamma radiation, due to the decomposition of the resin. This tendency becomes very strong where the radiation sterilization is conducted under a substantially oxygen-free atmosphere.

To test CET performance on gamma sterilization, different CET resins were injection molded into standard ASTM D638 Type I tensile bars and 10.16 cm diameter impact discs (0.25 cm thick). For sterilization purposes it's recognized that a dose of 25-60 kGy of gamma radiation is very effective. The molded parts were exposed to low dosages (10, 20 and 40 kilogray) of Cobalt 60 gamma radiation as well as to high dosages (60, 80 and 100 kGy) of radiation to simulate an extreme case of multiple exposures. The following standard tests were performed: tensile strength (ASTM D638), instrumented dart impact (ASTM D3763), yellowness index (YI) (ASTM E313).


The results of the standard tests (tensile and instrumented dart impact) are summarized in Tables 2 and 3. Each table lists the selected physical properties of the control samples and the properties after exposure to gamma sterilization. Gamma sterilization does not significantly affect the tested physical properties of CETs, but PMMA tensile strength decreases about 10% when exposed to 40 kGy, and a 40% reduction in tensile strength is seen after 100 kGy of radiation. CET116 and CET240 shows minimal reduction on instrumented dart impact after gamma sterilization up to 80 kGy, and about 40-45% loss when exposed to 100 kGy of radiation. CET270 is relatively unaffected by high energy sterilization and PMMA did show a loss about 50% in dart impact after high doses of radiation.

kGy CET116 CET240 CET270 PMMA
0 59.1 31.2 20.3 80.2
10 60.1 31.4 20.3 78.2
20 59.2 31.9 21.0 80.1
40 60.2 32.2 21.3 72.2
60 60.3 29.4 20.5 70.3
80 58.2 29.6 21.2 55.9
100 59.0 30.4 21.6 48.3

Table 2: Tensile Strength, MPa (ASTM D638)

kGy CET116 CET240 CET270 PMMA
0 755.8 671.0 1722.5 628.8
10 675.8 588.2 1608.3 621.3
20 513.0 614.4 1930.2 768.1
40 704.1 541.3 1582.1 597.3
60 533.8 630.4 1735.9 407.3
80 533.8 617.1 1556.0 306.4
100 397.7 397.7 1572.5 289.3

Table 3: NTT Total Energy, J/m (Instrumented Dart Impact, ASTM D3763)

All of the CET resins presented in this paper show some color change after irradiation. Figure 1 shows the color changes observed in the CET resins after gamma sterilization. After exposure to 10 kGy sterilization, all CET116, CET240 and CET270 resins developed some discoloration, but lower than PMMA. Both CET240 and CET270 have excellent stability when exposed to gamma irradiation, and CET116 is also very color stable; at 10 kGy some discoloration is seen, but no further discoloration was observed on higher radiation dosages, up to 100 kGy, when some additional yellowness was observed. PMMA shows a large amount of color change after gamma sterilization. The color decreases with time, but significant discoloration remains.

Discoloration of Samples After Gamma Radiation Exposure

Fig: 1 Discoloration of Samples After Gamma Radiation Exposure


CET resins are suitable for use in a variety of medical applications. They provide a good balance of properties when compared with PMMA and other competitive resins commonly used on medical applications, as it can be seen of Figure 2. CET resins can be especially used for applications that are exposed to gamma radiation sterilization.

Multi-Attribute Analysis: Resins for Medical Applications

Fig: 2 Multi-Attribute Analysis: Resins for Medical Applications

Of course, using the sterilization data from standard tests is an acceptable method for the preliminary screening of potential applications for medical devices. This information can be used to narrow the material choices, but CET performance in the final application must be tested.


The authors thank the ININ (Instituto Nacional de Investigaciones Nucleares) for gamma irradiation of the samples. Additional thanks are extended to Oscar Ricardo García-González for his help.


  1. M. Sturdevant, "Sterilization Compatibility of Rigid Thermoplastic Materials", Society of Plastics Engineers/Society of Plastics Industry Medical Regional Technical Conference Proceedings, October 1988.
  2. N. J. Hermanson and J. Steffen, "The Physical and Visual Property Changes in Thermoplastic Resins After Exposure to High Energy Sterilization — Gamma Versus Electron Beam", Society of Plastics Engineers Annual Technical Conference, May 1993.

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