For threaded closures, torque has many implications for the integrity of a container and closure system and an understanding of its behavior is critical for package development and validation purposes. The applied torque (Application) is the rotational force in which a closure is married to a container. Applied torque effects the seal integrity of the bottle and the closure. Back-off torque (Removal) is the rotational force necessary to open, loosen, or remove the closure. Torque degradation is the relationship between applied and back-off torque given certain variables such as storage conditions and time, and influences the sealing characteristics of the bottle and closure assembly.
There are a number of factors that might be considered when evaluating the torque profile of a specific product-package system. Development of a comprehensive qualification testing program considers and directly evaluates these critical factors which may risk package performance. CS Analytical can develop and validate a Torque Study specific to your product-package system that accounts for all of the potential impacts the system may be subjected to.
The amount of force applied will have a major impact upon any container and closure combination. While it is assumed that a higher application torque will result in improved (tighter) seal, this is usually not the case. Overexertion of stress on the threads or sealing surfaces could distort physical characteristics and impact removal torque requirements. Changes to physical characteristic can also lead to leak pathways.
Changes or inconsistencies in bottle or closure dimensions can result in differences in the critical contact surface areas. Out of tolerance dimensions at critical areas can create binding that will affect removal torque or open potential leak pathways. The dimensions and tolerances of both the container and the closure must be carefully designed and produced to assure the performance of the package.
Relaxation of the stress induced by applying a closure begins as soon as it is applied. Extensive testing has been performed on this phenomenon which has proved that a significant proportion of the torque decay occurs immediately. It is generally accepted that “immediate” removal torque tests should occur within five to ten minutes after application, to allow for relaxation. The continuation of this decay is dependent upon the closure and container being tested.
Drug product that comes into contact with sealing surfaces can create the unwanted effect of either increasing or decreasing the torque depending on the lubricating qualities of the product. It will impact not only the application by changing the frictional characteristics, but also the removal by possibly drying to a sticky consistency.
Material of Construction
The type and variety of materials being used for the bottle and closure system are significant. Plastics often are molded with the addition of release agents, fillers, and colorants which could affect the frictional characteristics of the sealing surfaces. Glass, and some plastics, are frequently coated with slip agents to reduce damage to the container or to make handling on a production line easier. Any additive that will change the materials’ physical properties may have an impact on the application and retention of torque.
A product is exposed to vibrations and physical shocks during transportation. It is possible for these events to relieve the stress that the torque has developed between the bottle and cap threads.
Compression is often encountered in the warehousing of drug package containers. A top load is applied to the package it may affect the torque. Compression of the cap and bottle can crush and deform the liner or sealing mechanism and result in a lower removal torque when the weight is removed.
Changes in temperature and relative humidity will result in different expansion and contraction coefficients for the bottle and for the cap. These changes can relate to dimensional differences that may alter the torque remaining in the system. It is possible for humidity to expand the fibers of some closure lining materials and result in an increased removal torque or cause damage to the seal integrity.
The force exerted by a pressurized headspace, either from a gas flush filling operation or from a temperature extreme, will change the sealing force. It is possible for a product to scalp gasses from a headspace and end up with a vacuum in the package that makes it very difficult to open. A change in the sealing force will result in a change to the removal torque.