John Kerwin

Active pharmaceutical ingredients (APIs) have become more potent, therefore the requirements of good manufacturing practice (GMP) are making ever more stringent demands on valve design and sealing. An absence of dead space, ease of cleaning and flushing is the norm for valves where cross contamination must be avoided at all costs. Sealing valves to glass reaction vessels has lagged behind valve sealing for steel vessels. 

Aperture tolerances are far larger as a consequence of the glass vessel manufacture process. This article reports on new valve sealing techniques for glass reaction vessels offering improved sealing and productivity benefits. 

Glass reaction vessels are widely used in pharmaceutical manufacturing. Inherently resistant to many of the often aggressive ingredients, their use avoids metallic element contamination which, apart from being a contaminant in itself, can influence the rate of chemical reaction. Reactors can be operated at atmospheric pressure, elevated temperature or under vacuum. Besides hosting chemical reactions they can also act as mixers, heaters, holding tanks, crystallizing tanks and evaporators. In each case the vessel is emptied via a flush valve at the bottom. 

Typically glass and glass lined reaction vessels have capacities of between 1000 to 11000 L. For maximum utility the tanks are usually jacketed to permit temperature control, incorporate inlet and outlet valves, and various probes, injection ports and vents.  

Connecting valves and probes to the reaction vessel means overcoming the wide tolerances encountered in port apertures during the manufacture of the glass vessel. Elastomeric rubber seals are ideal for this purpose because their flexibility overcomes the imperfections of the glass aperture to create an effective seal.  

There is a wide range of elastomeric polymers, each designed to withstand a particular environment or combination of environments. These include resistance to high temperature, low temperature, oils and grease, fuels, solvents, oxygen and ozone, and a wide variety of chemicals and across the pH spectrum. Within these operating conditions the elastomer must retain the useful properties of strength, resilience and elasticity.  

The choice of rubber invariably involves a compromise. No rubber has all the specific chemical, temperature range and mechanical strength to meet the requirements of every API manufacturing process. The seal designer/manufacturer has two options: either select the rubber polymer type satisfying the primary property requirements and, by compounding, mitigate the other adverse properties, or take the polymer type with a good compromise of properties and, by careful compounding, enhance one or more of those properties.  

As the process demands increase, the choice of elastomer is further reduced. Compliance to international food, water and toxicological standards such as FDA, Food Contact Notification and United States Pharmacopeia (USP Class VI) limits the types of rubber that can be used further still. Meeting these standards has a direct bearing on the choice of elastomer ingredients and the levels of those ingredients available to the rubber formulator.  

Figure 1 shows that FFKM polymers perfluoroelastomer has the best combination of chemical and high temperature performance. The structure in Figure 2 reveals essentially why: the high level of fluorine in the polymer (70%) strongly attached to the polymer carbon backbone gives the polymer the inertness to chemicals and thermal oxidation resistance most desired in this application.  

 Figure 1 Performance comparisons by elastomer type.						

Other rubbers in Figure 1 with less resistance to thermal and chemical degradation have either less fluorine or a chemical structure from which fluorine is easily detached. Alternatively, the backbone of the polymer has regions of unsaturation (double bonds) that can be broken open in aggressive environments.  

Figure 2 Perfluoroelastomer structure.			

Because of inherently high raw material costs and current limited production capacity, FFKM elastomers can seem expensive compared with other polymer types. However, this has to be offset against the productivity benefits gained by using the glass reactor vessel for a broad range of processes and solvents without the need to keep changing the valve seal.  

FFKM elastomers do not all have the same crosslinking structure. Those that utilize the patented 'pseudo living polymerization' (PLP) technique produce a crosslink itself rich in fluorine giving significant improvement in chemical resistance while simultaneously maintaining good mechanical and sealing properties. 

An illustration of this can be seen in Figure 3 where such a polymer has been tested in an aggressive chemical test fluid compared with a standard FKM fluoroelastomer and other typical FFKM perfluoroelastomers. It shows how PLP FFKMs are more resistant to swelling; an important consideration when developing a valve seal.  

Figure 3 Immersion trials in standard aggressive fluid. FKM at 200 Â°C, FFKM and PLP at 250 Â°C.			

Figure 3 illustrates how FFKM and particularly the PLP FFKM have superior resistance to aggressive lubricants. For example the FKM at 200 °C swells by almost 60% after 1000 h, while the FFKM swells by 8% and the PLP FFKM by only 3%. 

Several grades of this type of perfluoroelastomer have been compounded to FDA formulation guidelines and have been extensively extraction tested to ensure compliance to FDA regulations for food contact, and pharmaceutical and biomedical applications. 

The glass reactor valve must be sealed into the large glass opening in the base of the vessel. 			

Polytetrafluoroethylene (PTFE) is often used within the valve and vessel assemblies because of its inherent chemical inertness; however, because it has no elasticity, it cannot be used solely in the sealing application.  

As noted previously, this opening is not necessarily exactly circular nor has it been manufactured to tight tolerances. The elastomer must, therefore, be manufactured into an effective seal despite these inconsistencies.  

Figure 4 A typical design of valve seal incorporating an o-ring around a PTFE valve connector.

Figure 4 shows a typical design of valve seal incorporating an o-ring around a PTFE valve connector. The advantages of such a design are that it is easy to manufacture (both seal and housing) and simple to install. However, a tight fit is required with good sealing compression and it is not always possible to develop sufficient uniform sealing forces across the uneven sealing face of the vessel's glass neck, this will result in premature failure and leaking. 						

Perhaps the most significant disadvantage of this design is that the seal can roll out on assembly, again destroying the required sealing properties and creating a leak path; a problem not always detected until the leak has occurred. 

Figure 5 shows another valve seal design that attempts to overcome the weaknesses of the seal design is Figure 4. It shows a cut-away section of a bellows arrangement with an inlet air port and an outlet port that will inflate the bellows once the valve has been assembled in place.  

Figure 5 A cut away section of a bellows arrangement with an inlet air port and the outlet port that will inflate the bellows once the valve has been assembled in place.			

The design improves the sealing against an uneven glass sealing face. The inflated bellows will conform to the contours of the glass surface. No rollout occurs, so the seal is effective and secure in this respect. However, it is expensive in terms of seal and valve manufacture, and failures occur as the dynamic flexing of the bellows at the corners where stresses increase at the valve edge cause the seal to come away from its valve fixing point. 

The new valve seal design is a broad band with several fins. It is comparatively easy to manufacture with conventional mould tools and easy to fit to the valve assembly. The design overcomes the difficulty of simple rings rolling out in application and also avoids the situation of a loading and flexing on a corner encountered in the Figure 5 design, where stresses increase over time making seal rupture more likely. 			

The design in Figure 6 facilitates ease of assembly. It offers several fail-safe options and plays to the elastomer material strengths by offering a seal that works in compression and tension over a wide area without producing stress points in critical areas of the seal design.  

Figure 6 Successful valve seal design.			

In addition, the use of broad fins enables sufficient flexibility to facilitate ease of fitting and provides an element of dynamic sealing beyond that of the static sealing generated from the material only within an o-ring design. 

				The valve seal design accommodates wide variations in circumferential dimension and will also permit the seal to work during any flexing, vibration and thermal expansion cycling that occurs during the reaction cycle time			

The valve seal uses a highly fluorinated FFKM elastomer. This makes it easier to bond the seal to the PTFE body of the valve assembly — a notoriously difficult process to complete successfully, requiring additional costly mechanical fixing. However, the high level of fluorine within the PSL FFKM provides a high level of compatibility and thus, a good bond can be achieved.  

The FFKM elastomers used were compounded to optimize the hardness and flexibility required by the valve seal design. Furthermore, a cure system was incorporated to produce the maximum compression set and high-temperature durability permitted by the elastomer. Trials of the new valve seal design have already indicated significant improvements in sealing performance.  

is an elastomer technologist at Precision Polymer Engineering, UK.

Articles

Active pharmaceutical ingredients (APIs) have become more potent, therefore the requirements of good manufacturing practice (GMP) are making ever more stringent demands on valve design and sealing. An absence of dead space, ease of cleaning and flushing is the norm for valves where cross contamination must be avoided at all costs. Sealing valves to glass reaction vessels has lagged behind valve sealing for steel vessels.