PA 12: on the wrist, in the abyss

An excellent selection of technical characteristics guarantees the watertightness for a line of underwater wrist computers

maresMan dreams of flying, and associates any improved performance in extreme sports with the idea of aiming high, facing challenges to lift the bar on expectations that little bit higher, with help from state-of-the-art equipment and materials. The same performance is also needed in the opposite direction, towards the marine abyss, where bodily conditioning and support from accessories can never fall short. It is a challenge that, inevitably, also involves equipment manufacturers, which over time have developed increasingly sophisticated solutions. Mares is one example. It has now released its third generation of underwater Dive Watches. More than just a watch, it is an underwater computer designed and made in Italy using polyamide 12 structural components, which are totally unaffected by water and can stand up to even the most extreme challenges. The case of the new Smart model consists of the case and inner supports made of Grilamid LVX SST (Super Stuff and Tough) a material supplied by EMS-Grivory, which has also collaborated on the design of components for the entire line.

High performance for structural applications
The properties of long-chain polyamides, such as PA 12, are today well known and consolidated, as is their use in various sectors. In the non-reinforced grades and grades for extrusion, toughness, flexibility and chemical resistance for uses in vehicles for the manufacture of fuel pipes in single and multi-layer solutions are appreciated. In the healthcare sector, as well as in the home appliance sector, grades reinforced with glass fibres – in food and water approved versions – challenge brass as components for submergence in hot water, which requires dimensional stability and rigidity under demanding operation conditions. In a sports context, on the other hand, carbon fibre reinforced PA 12 combines with the need for light weight, comfort, and resistance to weather agents.
All these intrinsic properties are combined and optimised in Grilamid LVX SST, compounds reinforced with variable percentages of glass fibre and a high elastic module, with improved isotropy and high toughness (figure 1). The materials pose an alternative to reinforced, semi crystalline and amorphous polymers, generally used for practical and structural applications that come into contact with water, because in addition to offering an excellent property profile, they guarantee simpler processing with greater stability in the injection moulding process.

Extreme performance
As the name suggests, Smart is the most intelligent solution for an underwater computer because it uses innovative functions and materials. Light, practical and ergonomic, its captivating design means it looks great on the wrist even when not taking part in sports. Its technical performance is advanced. In fact, its versatility makes it suitable for diving, as well as for harsh professional situations at depths of up to 150 metres, where it has demonstrated efficiency and reliability.
In order to ensure such performance, the manufacturer has tested the device in pressure conditions three times higher than the maximum operating conditions, namely at 45 bar, corresponding to a 450 metre column of water. At a similar depth, the architecture of Smart demonstrated it could stand up to extreme stresses while guaranteeing watertightness for the control buttons, and at the same time has demonstrated excellent flexibility in recovering elastically from any deformation. The LVX also demonstrates its toughness in the form of extreme resilience, which has been demonstrated as being up to four times higher than other polymers with the same deformation at break (figure 2).
Considering the extreme conditions under which it operates, Smart must also be fall and involuntary impact proof: this means that the polymer has to ensure very high energy absorption levels (J), simulated with repeated free fall trials, during which breakage must never occur (figure 3). The validation specifications are integrated with the extreme mechanical and functional resistance tests, where the Dive Watch is subjected to thousands of pushbutton actions from the pressure profile of immersion.

High dimensional stability
The watertightness and how the watch functions under operating conditions (pressure and immersion in water) depend on two fundamental requirements: dimensional precision and compliance of the case tolerances.
The differentiated linear shrinkage along the X and Y axes, in fact, can generate deformation at the centre or along the edges with undesired warping along the Z axis, which compromises the planarity of the item (figure 4). Grilamid LVX SST eliminates this post-moulding issue because, given the scarce hygroscopicity of PA 12, it does not show any significant volumetric variations typically caused by water absorption, and especially because of its reduced anisotropy, typical of semi-crystalline fibre-reinforced polymers. Tests have demonstrated the excellent dimensional stability of the case and other components – comparable to those achieved with an amorphous polymer -, with low linear shrinkage and reduced differentiation between length and cross wise shrinkage. The tolerance of planarity and the requirements for maintaining roundness, dictated by the nature of the project, have therefore been widely met.

Resistant and anti-corrosive
Validation tests were completed by saline fog corrosion tests in conformity with ISO 9227, accelerated exposure to UVA radiation, operation test after thermal cycles from -10 °C to 80 °C, and lastly tightness and correct operation at hydrostatic pressure. This latter test, similar to the creep tests that are carried out on thermoplastics, simulates the effect of pressure at more than 200 metres of depth for extended periods of time, therefore at typical operating conditions for Smart. The case, therefore, is required to ensure high resistance to creep (creep of the plastic under constant pressure), a requirement met by Grilamid LVX SST.
In the face of stress so severe it even challenges the resistance of some metal alloys (to the order of 50-60 Mpa), the deformation that occurs after hundreds of hours of prolonged stress persists in regard to Newtonian elasticity. The compound is therefore able to recover its original size after enduring conditions of stress, thereby ensuring the precision and dimensional stability of the case.

 

Figure 1 – mettere vicino al box

Comparison of the main features of Grilamid LVX-65H SST, PPS and PSU and reinforced with 40 and 30 per cent glass fibre respectively

 

Figure 2

Comparison of the shear deformation of the two grades from the Grilamid LVX SST family compared to PPS and PSU reinforced with 40 and 30 per cent glass fibre respectively

 

Figure 3

Comparison of the resistance to break from impact of Grilamid LVX-65H SST, of PA 6.6 and PA 12 reinforced with 50 and 65 per cent glass fibre respectively

 

Figure 4

Comparison of the deformation of the Z axis of Grilamid LVX-65H SST, PA 6.6 and PA 12 reinforced with 50 and 65 per cent glass fibre respectively

 

 

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