Automotive manufacturers and their Tier One suppliers spend endless engineering hours developing the PAB (passenger airbag) system, from the airbag and its propellant to the construction of the materials used in the composite instrument panel.
Most instrument panels are now produced from a multi-composite laminate structure consisting of a long glass-filled thermoplastic substrate retainer, with or without a PAB opening to accompany a bolted-on or dropped-in passenger airbag chute, foam and the coverstock, or “skin.”
The skin can be a compact sheet or a bi-laminate of skin and foam. Most skin materials used today are thermoplastic urethane (TPU), polyvinyl chloride (PVC), thermoplastic elastomer (TPE) or thermoplastic olefin (TPO).
The TPO surface material is often painted with a scratch-resistant layer to provide durability to abrasion.
Airbag deployment occurs at a very rapid pace after an automotive impact event, with the airbag opening through the instrument panel in the PAB area within 6–10 milliseconds. The instrument panel composite must not produce any fragments, during or after the event, that can act as projectiles.
As such, most skins and foams are scored via a mechanical cutting process to weaken and reduce the force translated into the composite layers of the instrument panel during the deployment.
The so-called “score line” is often executed in a “U-pattern,” allowing the airbag to open through the composite instrument panel around the mechanically weakened score-line. This affords a clean break.
The score-line is often less than 1 mm in thickness and is characterized in length by the amount of the skin remaining wall thickness (RWT) not scored.
For a 1.0 mm TPO skin, the RWT generally falls within a range of 0.4-0.6 mm after scoring. For 1.2-1.4 mm thick PVC skin and 0.7-1.2 mm thick TPU skin, the RWT is often in the range of 0.3-0.5 mm.
In the case of TPU skins, the score-line is often modified with a low-surface, free-energy wax that prevents the TPU material from self-healing upon heat aging. Thermoplastic elastomers (TPE), due to their low density and light-weighting capabilities, are now being more widely evaluated. TPE skins, with thickness ranges of 0.9-1.1 mm, are generally scored to 0.3-0.4 RWT.
After simulated heat aging (120oC for 500 hours), to determine the efficacy of the airbag weakening in the PAB composite material construction, it is often noted that the mechanical score-line becomes visible.
The visibility is generally due to the shrinkage of the instrument panel (IP) skin, either due to a change in crystallinity (olefins) or plasticizer migration. (In PVC, the plasticizer is responsible for the fog seen as a result of the plasticizer migrating out of the skin and condensing on the windshield.)
Score-line read-through after heat aging is not acceptable, and, if severe enough, it can result in a complete split of the top coverstock material.
In order to prevent this, automakers need to optimize the combination of skin thickness, foam thickness and foam density.
The skin is often produced at the thicknesses specified above, with the thickness of the bilaminate foam or foam produced through a foam-in-place operation (FIP), ranging from 2 to 8 mm.
To make the situation even more complex, the foam can be of different chemistries, with polyurethane thermoset material being used in foam-in-place operations, and crosslinked polyolefin material used in bilaminate constructions.
Both foams often perform best in the heat-age testing when produced at high densities.
The behavior of skins subjected to heat age score-line read-through varies, depending upon the density of the foam and foam type used.
But in general TPE and TPU based coverstock skins outperfrom PVC and TPO.