Clear, Radiation-Tolerant, Autoclavable Polypropylene

Originally published January 1997

TECHNICAL PAPER SERIES

Communication with the highest-volume consumers of medical grades of polypropylene indicates that the preferred polypropylene resin for injection-molded and radiation-sterilized medical devices would be a radiation-tolerant, clear, autoclavable formulation. Combined with the results of our own work in this field over a period of more than 20 years and the significant literature on the subject, this intelligence provides substantial justification for the development of the new material outlined in some detail below.

Normally stabilized polypropylenes are not suitable for sterilization by high-energy radiation because of the severe embrittlement and discoloration that occur in the plastic immediately after sterilization and then worsen with aging. While the embrittlement of the plastic after irradiation is an inherent property of the polymer, the discoloration is caused by reaction products of the phenolic antioxidants normally included in standard polypropylenes.^1­3

There are, however, several techniques in the design of propylene polymers and formulations that remedy these problems and yield resins suitable for irradiation at dosages up to 50 kGy. Early radiation-tolerant polypropylenes were homopolymers stabilized with small quantities of phenolic antioxidants and large amounts of sulfide diester secondary antioxidants. These materials did discolor slightly after irradiation.²

The modern resins that are most successful in withstanding irradiation exhibit reduced crystallinity and narrow molecular-weight distribution, are formulated with hindered-amine light stabilizers (HALS), and contain none of the discoloring phenolic antioxidants. Ethylene- containing random copolymers are useful substrates for building radiation-tolerant formulations, as are homopolymers with low isotacticity and homopolymers to which hydrocarbon oils or greases have been added. The HALS are, by themselves, noncoloring in polypropylene, but they interact with phenolic antioxidants to produce extremely deep yellow colors after irradiation. Therefore, when HALS are used in a polypropylene formulation, the phenolic antioxidants must be scrupulously avoided.^1­8

Although glasslike clarity is not essential for most applications, transparency is highly valued for both aesthetic and technical reasons. "See-through" clarity is very helpful in the filling, measuring, and dispensing operations for which many medical devices are routinely used. Clarity is commonly obtained in polypropylene in two ways. First, resins with lower total crystallinity will be clearer than those with higher, but a minimum level of crystallinity is necessary to provide the required strength, stiffness, and resistance to softening at elevated temperature expected of the polypropylene. Second, certain organic nucleating agents greatly improve the clarity of polypropylenes by producing only very small crystals in the polymer. These smaller crystals are below the size that scatters visible light and produces haze.⁹

Resistance to softening at elevated temperature allows a polypropylene to be sterilized by autoclaving, most commonly at 121°C or above. This property is obviously not required to ensure the sterility of a properly packaged, irradiated medical device as sold. Nevertheless, many of the major device manufacturers choose to offer this feature in their products as a bonus to the consumer. Autoclavability, for example, allows disposable syringes to be resterilized in the hospital as part of a procedure kit after having been removed from their original packaging and thus contaminated. Veterinarians often reuse disposable syringes and value the ability to sterilize them by autoclaving.

In order for a disposable medical device like a syringe to be autoclavable, the wall of the part must be able to withstand any applied stresses--such as the expansive stress of the compressed rubber piston--at the sterilization temperature. Although radiation-tolerant propylene random copolymers, with 3% or more ethylene comonomer, can sometimes withstand the sterilization conditions in a part that is not under significant stress, soft resins such as this are not generally autoclavable in a syringe or other similar configurations. Significantly higher crystallinity is required for a polypropylene to be autoclavable under such stresses.

Unfortunately, the three desired attributes in a premium polypropylene for medical devices--radiation tolerance, clarity, and autoclavability--were until recently thought to be incompatible. Any two of them are fairly easy to obtain in a single resin, however. For example, radiation tolerance and clarity are easily obtained in a properly stabilized, nucleated random copolymer.^10,11 Radiation tolerance and autoclavability have been available for years in properly stabilized homopolymers. Clarity and autoclavability, without radiation tolerance, are commonly achieved in nucleated homopolymers. There is at least one known method for producing a resin possessing all three of these attributes--a process that uses oils or greases as modifiers or "mobilizing agents" to stabilize the clear polypropylene homopolymer against the embrittlement caused by radiation without excessively reducing its resistance to softening at elevated temperature. This proprietary technology, however, is unavailable to the industry in general.^2,4­7

This paper describes research that has culminated in the development of a new type of polypropylene formulation displaying all three of the desired properties--clarity, radiation tolerance, and resistance to softening at elevated temperature. The work was engendered principally by the recent emergence, as commercial entities, of ultra-low-density, plastomeric, ethylene polymers produced from metallocene catalysts (plastomers).

It is our experience that the radiation resistance of propylene random copolymers is directly related to the ethylene content of the resins.¹⁰ We have also learned that bimodal mixtures of ethylene-lean and ethylene-rich propylene polymers are more radiation resistant than are random copolymers having a unimodal ethylene distribution, even though the average comonomer contents of the two materials are identical. For some time, we have had the goal of examining the radiation resistance of an extreme embodiment of this bimodal situation--namely, blends of minor amounts of polyethylene with polypropylene--but were reluctant to do so because of the adverse effect of the polyethylene upon the clarity of the mixtures. With the discovery of the ability of ultra-low-density ethylene polymers produced by metallocene catalysts to provide significant impact modification to blends with polypropylene with only small increases in haze, the goal of a premium polypropylene formulation seemed to be within reach. The extensive experimental program that was undertaken to establish that blends of ethylene plastomers with polypropylene could be devised with the three desired properties is described below.

EXPERIMENTAL

This paper documents three experiments. In the first experiment, we examined the effect of several levels of a plastomer previously found useful in polypropylene impact modification on the gamma-radiation tolerance of a clarified homopolymer that contained an effective radiation-protection additive package. The positive results from this experiment in the areas of resistance to radiation and to softening at elevated temperature led us to examine the effects of a broad range of plastomers on the haze of blends with nucleated propylene homopolymer. Our ability to identify a plastomer that had a negligible effect on haze then prompted us to formulate and test a blend designed to be clear, radiation resistant, and autoclavable.

All of the plastomers used in this work were ethylene-butene copolymers produced by metallocene catalysts and sold commercially by Exxon Chemical Co. under the name Exact. They were characterized by extremely narrow molecular-weight and comonomer distributions. The plastomers P1 and P5, which were used in experiments involving irradiation of blends with polypropylene, were "barefoot" polymers containing no additives whatsoever.

Experiment 1. Stabilization of Irradiated Homopolymer by a Plastomer Produced with Metallocene Catalyst. In this experiment, a nominal 1.0-melt-flow-rate (MFR) propylene homopolymer granule with moderately high crystallinity (as measured by a heptane insolubles level >95.5%) was treated with an organic peroxide in a melt extrusion­pelletization process to increase its MFR to 25 dg/min. The homopolymer was then repelletized with 0.08% calcium stearate; 0.05% Tinuvin 622 LD (Ciba Geigy Corp., Tarrytown, NY); 0.08% Ultranox 626 (General Electric Co., Parkersburg, WV); and 0.25% Millad 3940 (Milliken and Co., Spartanburg, SC); and with variable amounts of an additive-free, 4.0-melt-index ethylene-butene copolymer with a density of 0.885 g/cm³ (plastomer P1). The blends are described more fully in Table I.

Table I. Blends of propylene homopolymer with plastomer P1.

The blends and a sample of a performance standard--number 18277-054-008, consisting of PP 9074MED (a radiation-tolerant, clarified, 2.8%-ethylene, 24-dg/min-MFR random copolymer)--were then injection molded into ASTM test parts. The parts consisted of dog-bone-shaped tensile bars (165 * 12.7 * 3.18 mm), Gardner disks (88.9 mm diam * 3.18 mm), and flex bars (127 * 12.7 * 3.18 mm). Four sets of parts were established for each resin and then irradiated by Isomedix, Inc. (Morton Grove, IL), at nominal gamma dosages of 0, 25, 50, and 75 kGy using cobalt 60 at a rate of <10 kGy/hr. The parts were aged at 60°C for 21 days.

Testing of the properties of the samples was carried out as rapidly as possible following the aging regimen. The properties measured--all at 23°C--were tensile elongation at break (ASTM D 638-87b), Gardner impact strength (ASTM D 3029-84), deflection at peak flexural load, and Hunter-b color (Gardner Model PG 5500 photometric unit with C illuminant, 10° light source, port closed; Pacific Scientific, Gardner Laboratory Div., Bethesda, MD). In addition, we measured the heat-deflection temperature (HDT) at 455 kPa (ASTM D 648-82) of the range of samples irradiated at 50 kGy but not aged, as well as the secant flexural moduli (ASTM D 790-86) of unirradiated and unaged samples with all levels of plastomer.

Experiment 2. The Relationship between Plastomer Melt Index (MI), Density, and Haze of Blends with Polypropylene. In the second experiment, a 25-dg/min polypropylene base resin was first prepared by taking a nominal 1.0-dg/min-MFR propylene homopolymer granule with moderate crystallinity (as measured by a heptane insolubles level between 94 and 96%) and treating it with an organic peroxide while melt-blending and pelletizing it with 0.08% calcium stearate, 0.05% Tinuvin 622 LD, 0.08% Ultranox 626, and 0.25% Millad 3988. Ten pellet blends of this base stock were prepared with eight different plastomers selected according to a statistical design (2 variables, MI and density; 3 levels; full factorial with 1 missing combination; 3 replications of a center point). The weight ratio of polypropylene to plastomer was 90/10. The blends and the unblended homopolymer base stock (control) were injection molded into 1.0-mm haze plaques and tested for haze by the ASTM D 1003-92 method. The experimental runs are described in detail in Table II.

Table II. Experiment 2 results for blends of propylene homopolymer with various plastomers.

Experiment 3. Confirmation of Clarity, Radiation Tolerance, and Resistance to Softening at Elevated Temperature in a Single Blend of Propylene Homopolymer and Plastomer. In the final experiment, a 25-dg/min stock blending compound was first prepared by treating a nominal 1.3-dg/min-MFR propylene homopolymer granule with moderate crystallinity (as measured by a heptane insolubles level between 94 and 96%) with an organic peroxide while melt-blending and pelletizing it with 0.03% DHT4A, 0.06% GMS-11, 0.25% Millad 3988, 0.10% Tinuvin 622 LD, and 0.08% Ultranox 626. Three pellet blends were then prepared with different levels of P5, a metallocene-catalyzed, plastomeric ethylene-butene copolymer containing no additives. The neat PP formulation was used as a 0% plastomer sample to be compared with the blends. Table III shows in detail the characteristics of the four resins. Molding, irradiation, aging, and testing of these samples were carried out exactly as described in Experiments 1 and 2.

Table III. Polypropylene­P5 plastomer blends (experimental samples).

RESULTS

The data from Experiment 1 are reported in graphical form in Figures 1­6.

Figure 1. Deflection at peak flexural load of polypropylene-plastomer blends (Experiment 1).

Figure 2. Tensile elongation at break of polypropylene-plastomer blends (Experiment 1).

Figure 3. Gardner impact strength of polypropylene-plastomer blends (Experiment 1).

Figure 4. Color of polypropylene-plastomer blends (Experiment 1).

Figure 5. Heat-deflection temperature at 455 kPa of polypropylene-plastomer blends. Samples were irradiated at 50 kGy, with accelerated aging (Experiment 1).

Figure 6. The 1% secant flexural modulus of polypropylene-plastomer blends that are not irradiated or aged (Experiment 1).

The data from Experiment 2 are reported in Table II. These data were subjected to a multiple-regression procedure that provided an equation for the dependence of the haze of a polypropylene-plastomer blend upon the density and MI of the plastomer. The response surface plot of this equation is shown as Figure 7.

Figure 7. Response surface relating haze of sample blends to MI and density of plastomer component (Experiment 2).

The data from Experiment 3 relating to the physical properties and color of the samples are reported and graphical form in Figures 8­12. Data from the same experiment relating to the haze of the samples are reported in tabular form in Table III and graphically in Figure 13.

Figure 8. Haze of polypropylene-P5 plastomer blends (Experiment 3).

Figure 9. Deflection at peak flexural load of polypropyleneP5 plastomer blends (Experiment 3).

Figure 10. Tensile elongation at break of polypropylene-P5 plastomer blends (Experiment 3).

Figure 11. Gardner impact strength of polypropylene-P5 plastomer blends at 23° C (Experiment 3).

Figure 12. Heat-deflection temperature at 455 kPa of polypropylene-P5 plastomer blends that are not irradiated or aged (Experiment 3).

Figure 13. Color of polypropylene-P5 plastomer blends (Experiment 3).