Styrene butadiene copolymers offer favourable economics over alternative transparent materials. New grades are being introduced for blister packaging and their features and properties are reported here.
Chevron Phillips Chemical Company LP, Bartlesville, Oklahoma, USA
Applications grow for styrenics
Transparent styrene butadiene copolymers (SBC) are used in applications where a combination of glass-like clarity, impact toughness, gloss and ease of processing are required. For example, disposable rigid packaging applications represent more than half of the K-resin SBC market by volume in the United States (US) and Europe. Furthermore, speciality SBC grades have now been developed for blisters, clamshell packaging and medical trays, which are the subject of this article.
There are two concepts of blister packaging. One design offers a preformed plastic blister and a printed paperboard card, which has a heat-seal coating; some packs use a foil, plastic or laminated material to bond to the preformed blister. The second format, clamshell packaging, has a formed plastic package with one or more hinges that are used to encapsulate and boldly display a product.
Blister packaging typically requires one or more of the following properties: transparency, impact resistance, tear resistance, puncture resistance, ease of thermoforming (adequate deep draw ratio), sterilisability, sealability, migration, barrier (humidity, ultraviolet light), chemical resistance and printability.
Medical trays are similar to blisters in that they protect their contents from impact, punctures and contamination. They must have excellent transparency and be easily manufactured, with good thermoforming and trimming characteristics and consistent sealability. The packaging for surgical operations kits requires a much greater wall thickness to hold heavier devices. Multiple openings in the trays are also needed, because the intricacies of these procedures call for numerous items such as needles, gauze, gloves and any number of additional specialised tools and supplies. After thermoforming and being filled with the medical supplies and instruments, these trays are normally covered with a peelable film layer in a form-fill-seal operation. The peelable film layer must maintain sterility in storage, but also provide visual evidence when opened that the sterile barrier has not been compromised through the seal.
Materials currently used for medical trays include high impact polystyrene (HIPS) and polyester copolymers such as glycol modified polyethylene terephthalate (PETG). HIPS is typically used for higher volume, lower cost, thinner trays requiring less structural integrity. PETG is used where clarity is essential, particularly for higher cost trays employed for surgical procedure kits. Use of poly(vinyl chloride) (PVC) for medical trays has been in decline because of concerns regarding waste disposal.
Styrenic copolymers for rigid packaging
SBCs have been used for many years in a variety of rigid packaging applications. SBCs when blended with crystal polystyrene offer high clarity and excellent practical toughness. A third component such as styrene butadiene styrene (SBS) or styrene isoprene (SIS) rubber can be added in low amounts to blends of crystal polystyrene and SBC to improve impact resistance without loss of visual quality. The advantage of these blends is that properties can be tailored in an unlimited number of combinations.
SBCs have a melt temperature of 175–220 °C and require no recrystallisation. They are also nonhygroscopic, which eliminates the need for drying. Another advantage provided by these blends is that they offer viable alternatives to the existing materials used for these applications: PETG, amorphous polyethylene terephthalate (APET) and PVC.
A new grade of SBC has been introduced for rigid packaging applications that is competitive with PVC, PETG and APET in terms of impact, clarity and toughness. This SBC resin blend can be blended with up to 25% crystal polystyrene to increase stiffness and reduce the cost of the finished package. The data in Table I show how the materials blended with crystal polystyrene compare with the other competitive polymers typically used in clear packaging.
Low temperature impact properties can also be improved with the addition of SBS, SIS or other more flexible SBC grades. An economic advantage over alternative clear polymers is offered in the density of SBCs (1.01 g/cc), which are 20–30% lighter than competitive clear polymers (Figure 1).
Gamma irradiation, exposure to ethylene oxide gas or electron beam (e-beam) are three typical methods of sterilising medical products. All three can, of course, potentially affect the medical device or packaging material being sterilised. Sheet samples produced from two primary blends utilising specific SBCs were exposed to these three types of sterilisation and then tested for physical properties. There is minimal to no effect on mechanical properties and impact toughness. Slight yellowing is noted at high exposure levels for gamma and e-beam. Again, at high exposure levels, a reduction in melt flow is also noted for gamma and e-beam. These changes are not anticipated to affect the functionality of the styrenic blends in medical packaging, considering the normal sterilisation levels utilised. Autoclave sterilisation is unacceptable for SBCs.
Heat sealing, solvent bonding, ultrasonic welding or adhesive bonding can all join and bond styrenic blends. There are also some proprietary technologies for radio frequency welding. Of course, other package closure options also exist. The performance characteristics of packaging produced using SBCs can be improved by coextrusion with other polymers to adjust specific properties such as chemical resistance, barrier or seal capability. Various coextrusion possibilities of packages produced with SBC are shown in Figure 2.
Most of the SBCs tested for biological performance meet the requirements of US Pharmacopoeia XXIII Class VI-50. They are compatible with blood, demonstrate no cytotoxic, mutagenic or irritant potential and are not sensitisers.
Innovation in materials
Certain types of SBC now offer several valuable features and new characteristics for the medical market. These innovative SBC blends can be optimised to meet the needs of many rigid medical packaging applications. They process easily, have multiple sterilisation options and provide a unique balance of clarity, toughness and value compared with competitive clear polymers.
Marc Heerbrant is Styrenic Polymers Technical Manager for Europe, Africa and the Middle East at Chevron Phillips Chemicals International NV, Brusselsesteenweg 355, B-3090, Overijse, Belgium, tel. +32 2 689 1211, e-mail: HEERBM@cpchem.com
Lee Carvell is Applications Development Director for Chevron Phillips Chemical Co. LP, Bartlesville, Oklahoma, USA. For information, contact Cliff Pettey
, Director of Global Market & Applications Development, Chevron Phillips Chemical Co. LP, The Woodlands, Texas 77380, USA, tel. +1 740 350 7761, e-mail: email@example.com www.cpchem.com
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