Speciality chemicals and adhesives supplier Henkel South Africa, which is also the manufacturer of the Loctite brand of sealants, maintains that liquid gaskets produce seals that create better results than traditional materials.
To ensure the integrity of a seal between two bolted flanges, it is necessary to have some form of gasket between the mating surfaces that will remain intact and leak-free in often extreme and varying conditions over a prolonged time.
In a test where a sealed steel casing was subjected to torsional stresses, a number of significant differences between various sealing methods were noted, reports Henkel.
In particular, conventional solid gaskets showed leaks after 150 000 cycles, whereas seals made with an anaerobic product con- tinued to show reliability beyond 250 000 cycles.
Further, in the conventional gasket, the tension of the bolt dropped by 25% within just a few thousand cycles.
There are three typical types of gaskets: conventional precut compression gaskets, formed-in-place (FIP) liquid gaskets and cured-in-place (CIP) gaskets.
Conventional precut compression gaskets are made from paper, rubber, cork, metal or other materials.
Although a well-used and traditional method of sealing joints, these gaskets do present some significant and practical disadvantages, says Henkel.
For example, conventional gaskets require an initial compressive load to deform it into the irregularities of the mating surfaces, which, over time, is a major cause of gasket failure.
This occurs when the gasket loses its elastic properties and becomes less resilient. The load on the gasket and relative motion causes a general decrease in the gasket thickness, resulting in a bolt clamp load loss that can cause it to leak.
FIP Liquid Gaskets
FIP liquid gaskets are applied to one of the mating surfaces before part assembly. Once joined, the sealant spreads between the surfaces, filling gaps, voids, scratches and surface irregularities, after which the gasket cures and forms a durable seal.
Two types of sealants are available for FIP gasketing jobs: anaerobics for rigid joints, such as Loctite 510 and Loctite 518; and special silicone products for applications requiring a flexible seal, such as Loctite 5699.
Rigid flanges, which are designed to achieve the greatest stiffness between two mating parts, to reduce movement between the components and to transmit forces from one part to another, will require the use of a liquid adhesive to enable the surfaces to come together with metal-to-metal contact, ensuring the correct clamp load is maintained indefinitely.
Further, no retightening of the bolts is required and no allowance is needed for gasket thickness, reports Henkel.
Meanwhile, anaerobic sealants provide high shear strength that stops movement to eliminate bolt loosening and fretting and to increase the assembly’s structural strength.
Anaerobic sealants also allow relaxation of surface finish and flatness tolerances that enable scratches and scored surfaces to be sealed without rectification.
Anaerobic gasket materials only cure in the absence of air when trapped between metal surfaces. This enables it to offer extensive on-part life when exposed to the atmosphere, making multiple application methods possible and reducing the problems associated with the use of volatile or moisture-cured materials on production lines, notes Henkel.
Further, the anaerobic technology enables excess material to be wiped from exterior surfaces or flushed away from interior faces maintaining free, unblocked internal passages and channels.
An advantage of cured anaerobic gaskets is its resistance to petroleum-based fuels, lubricating oils, water-glycol mixtures and most other industrial chemicals.U
nlike precut compression gaskets that are difficult to automatically position on the parts, anaerobic gaskets can be automatically dispensed by robotic dispensers, screen printing or stencil printing systems.
Further, anaerobic gasket products reduce inventory costs, as one container of an anaerobic can be used for flanges of varying sizes and shapes. Precut gaskets can only be used on specific flanges.
CIP compression gaskets are a replacement for die-cut rubber, die-cut foam rubber or moulded gaskets, reports Henkel.
They are applied as a fluid in precise beads to one of the surfaces and are cured before use, either by ultraviolet light, heat cure, two-part mix or moisture curing.
Once the bead is cured, an elastomeric material is formed that adheres to the surface. Sealing is achieved through compression of the cured gasket during assembly of the parts.
The advantages of these gaskets include automatic application, serviceability, labour savings, improved quality, reduced inventory and flexible manufacturing.
Meanwhile, Henkel states that flanges, such as pressed steel parts and thin castings with widely spaced bolts, can be considered as flexible, have inconsistent clamp pressure and may move under pressure or dynamic loading.
Sealant manufacturers offer special silicones, formulated to meet all of the requirements of a gasket for flexible joints. Such products provide high elongation to compensate for any micromovement, as well as long-term adhesion to most substrates.
Other benefits of gasketing silicones include the ability to seal large gaps – up to several millimetres – and a wide operating temperature range of between –70 °C and 315 °C, with exposure to up to 350 °C for some grades.
It is important to consider the nature of the surfaces involved in a gasketed assembly.
For example, the original substrate used for the component may be metal – but for operational reasons the part has been painted. It is not a metal-to-metal contact that needs to be sealed but rather a metal-to-paint or paint-to-paint configuration.
In those circumstances, an anaerobic that relies on a metal surface in the absence of air to initiate the cure is inappropriate. However, a silicone – even if a rigid joint is involved – is the correct choice.