Polyamide for the aviation industry — fire protection in sight

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The problem with components with aviation approval according to ABD and FAR is, on the one hand, the low flammability required, and on the other hand, no toxic fumes must be produced in the event of a fire. A polyamide rises to the challenge.

To date, polycar­bo­nate (PC) has often been used for aviation appli­ca­tions. However, this amorphous material causes problems because of its relatively poor resis­tance to stress cracking. There is a risk that cracks can be triggered by a super­po­si­tion of stresses in conjunc­tion with a chemical attack.

Stresses arise on plastic parts during the manufac­tu­ring process, for example during injec­tion molding or deep drawing — with good workman­ship and a plastic-friendly design of the compon­ents reducing the stress level. Depen­ding on the appli­ca­tion, the compon­ents can also experi­ence considerable stress in the later instal­la­tion situa­tion due to the use of thread-forming screws instead of thread-cutting screws or incor­rectly designed snap hooks.

The compon­ents required are technical functional parts of the seat mecha­nism. When installed, these are exposed to certain stresses and are conta­mi­nated with substances that trigger stress cracks, such as grease, but also food, hand creams or cleaning agents. After every long flight there is a cleaning, which is sometimes carried out in the most remote places in the world. This makes it diffi­cult to specify the substances with which plastic parts may be treated. However, the use of polycar­bo­nate, a material with high impact strength and the possi­bi­lity of flame retar­dancy, poses the risk of stress cracking with almost every commer­ci­ally available cleaning agent.

The property profile of the material that is approved for the aviation industry must meet certain requi­re­ments:

• Flamma­bi­lity on horizontal test (15s) accor­ding to App.F to part 25 part I § (a),(1),(v)
• Smoke density per FAR 25.853 (d)
• Smoke density accor­ding to ABD 0031
• Toxicity accor­ding to ABD 0031
• Chemical resis­tance
• Defined strength and elonga­tion values

After various tests, a dry impact-resistant PA6 GF 15 FR (Foramid GM 96/30–1) was created that was suitable for the aviation industry. The polymer matrix of this material consists of polyamide 6, a semi-crystal­line material with suffi­cient strength and elonga­tion values as well as high chemical resis­tance. To achieve the flame-retar­dant proper­ties, signi­fi­cant amounts of a phosphate- and heavy-metal-free flame retar­dant system were added. Further­more, around 15% glass fiber was compounded to increase strength and rigidity. With a propor­tion of 30%, the material was too brittle due to the high flame retar­dant content. An added rubber-based impact modifier helped to increase the dry impact strength.

Processing aspects

The material can be processed like a normal PA 6 – GF15. To ensure colora­bi­lity, the color must be adjusted exactly to the base material. However, it should be noted that colora­bi­lity is only possible to a limited extent due to the high flame retar­dant content.

The plasti­ci­zing and injec­tion condi­tions must be adjusted so that the blend does not separate. The parame­ters were deter­mined empiri­cally.

Despite the dry impact modifi­ca­tion, the finished injec­tion molded parts should still be condi­tioned with 2–3 % distilled water after comple­tion. This increases the impact strength of the relatively brittle polyamide compound due to the high flame retar­dant content.

The injec­tion mold is equipped with hard inserts made of hot-work steel (1.2344) and has optimal tempe­ra­ture control. Due to the relatively low volumes in the aviation industry, the injec­tion is carried out using a rod and distri­butor without hot runner techno­logy. Care was taken to ensure that the tunnel gate connec­tion was suffi­ci­ently thick. This is important for low shear and suffi­cient pressure supply to the semi-crystal­line material. The weld lines were placed in non-critical areas. Demoul­ding is carried out using flat and round ejectors and, in some cases, using complex slide techno­logy.

Compound production

Due to its versa­ti­lity, the polyamide polymer group has estab­lished itself as one of the most important and versa­tile thermo­pla­s­tics in the field of injec­tion molding granules. The macro­mole­cule is made up of one or two diffe­rent monomers. A distinc­tion is made between PA 6 (but also PA 11 and PA 12) and PA 6.6 (but also PA 4.6 and PA 6.9). The number indicates the number of carbon atoms in the molecule.

The combi­na­tion of halogen- and phosphate-free flame retar­dancy, glass fiber reinforce­ment and impact modifi­ca­tion required for the intended purpose repres­ents a parti­cular chall­enge for the compounder. The aspects of polymer compa­ti­bi­lity, coordi­na­tion of the mecha­nisms of action, safety of the process, environ­mental compa­ti­bi­lity and good econo­mics must be taken into account.

Based on many years of experi­ence, a flame retar­dant system based on melamine cyanurate ( melapur MC 25 from DSM ) proved to be suitable.

It is clear that in PA 6 the end groups NH2 and COOH form a bond with those of MCU, with the forma­tion reaction taking place through opening of the rings and attach­ment. Cyanuric acid is broken down, which at the same time leads to the break­down of Polymaid and thus to a lower smoke gas density. However, it has been shown that the dosage of melamine cyanurate in order to achieve the respec­tive fire class must be set much higher than speci­fied by the manufac­turer.

While 8 weight of % was enough for the MCU 25 product from Chemie Linz in 1992 to achieve V 0 accor­ding to UL 94 (1.6 mm) for unrein­forced PA 6, at least 12.5 weight of % is required with the afore­men­tioned product .

The fire behavior of PA 6 with glass fiber was there­fore only influenced by the flame retar­dant system to the extent that V 2 accor­ding to UL 94 (1.6 mm) was achieved. Since the compon­ents have a wall thick­ness of 3 mm, the flamma­bi­lity drops to V 1 accor­ding to UL 94 and thus meets the flame retar­dancy tests of the aviation industry, which differ from the tests of Under­wri­ters Labora­to­ries.

By adding the impact modifier, in this case an ethylene propy­lene rubber (EPM), to the already high flame retar­dant content, a compli­cated mixture is created that places high demands on the techno­lo­gical process of compoun­ding. Without three gravi­me­tric dosing units on the one hand and the correct choice of visco­sity as a quotient of shear rate and shear stress on the other hand, consis­tent quality cannot be guaran­teed.

Images: EP Kunst­stoff­technik

Authors

Dr. Gerhard Pohl, owner of Carl Pohl textile and thermo­pla­stic produc­tion Forst / Lausitz and managing director of Dr. Pohl-Textil- und Thermo­plast GmbH, Forst / Lausitz

Dipl.-Ing. Elmar Nachts­heim, Managing Director of Europlast, EP Kunst­stoff­technik GmbH, Ilsfeld