BY ALT Technologies | 15th July 2020
Airbag inflator seals are a crucial component in airbag development. But without a solid and finely-tuned adhesive, the inflators can turn into a volatile occupant safety part.
Airbag inflator seal tapes are custom-made metal-foil products with adhesive. The most widely used metals are aluminium, stainless steel and copper. These foils protect the inflator’s blowing agent and gas from moisture contamination, grease and dust.
Because inflators are incredibly volatile, they must be sealed delicately to go off at precisely the right moment. If the adhesive is not performing accordingly, for example, it is applied too thinly, is not pressed well, or there is moisture or humidity leakage, the airbag could go off when it isn’t necessary. If sealed too tightly, the inflator might be too slow to go off, and the airbag won’t do its job correctly.
The adhesives used to seal inflators are either pressure-sensitive or heat-activated adhesives. The pressure-sensitive adhesive is Europe’s most widely used inflator seal adhesive, but heat-activated is now becoming more widely accepted. This article will discuss the differences between the two bonds concerning inflator seals.
A PSA binds two surfaces when pressure is applied. They don’t require a solvent, water, or heat activation. However, in the case of inflator seals, we use the PSA tape to a material with another lamination material and then ship it to the point where it is fixed. The liner can be removed and affixed to another material. The strength of the bond is determined by the amount of pressure applied during installation. In their dry form, they are aggressive and permanently tacky at room temperature. This ensures greater manufacturing flexibility and supply chain smoothness.
A fast application is possible with PSAs and removes the need for mechanical bolts and fasteners. They also allow for widespread thickness and gap-filling, which is crucial in inflator seals due to the threat of moisture and dust creeping into the propellant.
Depending on the application and substrates, PSA comes in different forms: rubber, acrylic and silicone. For example, you would need a rubber or modified acrylic PSA to bond well to polypropylene. PSAs are a viscoelastic material, meaning they have properties relating to both liquid and solid states; liquids concern flow and energy absorption and solid conditions concern internal strength and resist deformation.
When you look at the adhesion of PSAs, contact and wetting are essential factors to consider when choosing this adhesive. The bond strength at the interface of the materials and how long this can hold must be regarded. The cohesion, or when testing the material’s shear mode and internal strength, is another critical factor in seeing how long the material can last in sealing the inflator. The balance of these two properties (adhesion and cohesion) is essential. There is no ideal balance; it simply depends on the application.
Surface energy measures the attraction of the material’s molecules to itself and other materials. It describes wetting or close contact. The types of bonds you get with PSA are physical. Good wetting and low distances between the bonds are a must. If there is high surface energy, it is easy to adhere and has suitable adhesive “wet out”. How a PSA is coated must also be considered: a thin and uniform layer and viscosity must be at the right level.
PSA tapes require up to 24 hours (max 72 hours) to reach final condition hours of curing after application installation.
Heat-activated adhesive is dry at room temperature (RT), so you have to increase the temperature, melt it and make it tacky. Because of this, it does not stick to itself, making it easy to work into different shapes and porous areas. This gives the manufacturer time to appropriately place the substrate precisely before heating the application to make it adhere firmly. Only then will it have a suitable viscosity. When it cools down, it freezes again and locks the polymer into place. HA adhesive is more physical; it is used on fibrous felts and non-wovens. You must increase the temperature to make it into a liquid, which flows into the highly porous fibres. It cools down and locks into place.
HA adhesives follow two phases. Bonding happens between the substrate and the adhesive when heat activation begins. The adhesive tape quickly cools after it is heated. Phase two occurs after the adhesive cools down. The adhesive’s chemicals start to crystalise, increasing the bonds’ strength.
Depending on the different applications, it should suit the type of bonding solution that we provide them. For example, a different HA adhesive works by heating it to have the proper viscosity. You have a second heat cycle and secondary crosslinking during the heat process, for example, half an hour at 150C. You then achieve a powerful bond.
Once these tapes are fully cured, the bonds are permanent. They also keep out moisture during dynamic sealing, making them a good choice for moisture and dust resistance in inflator seals. They readily adhere to various materials such as rubber, PU foam and PVC but will not stick to silicone, unlike PSA tapes.
Knowing the suitable materials to use is one thing, but putting these into practice over the years adds to a strong solutions portfolio. ALT has extensive experience with comprehensive material databases and knowledge of curing processes. A lot of our applications today involve multi-purpose transfer tapes. As well as sourcing suitable adhesives, we can provide in-depth information on the substrates and their properties to ensure you build a long-lasting inflator seal solution.
Through creating customised solutions for over 50 years, ALT has built a vast portfolio when thinking outside the box and providing solutions for complex applications. If you need a unique solution today, please contact us; our team will gladly help you.
This article originally appeared on ALT Technologies LinkedIn.
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