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FIRE RETARDANT ADHESIVES – 4 CRUCIAL PROPERTIES

The requirement for fire retardant adhesives is increasing and their use expands to a number of different industry sectors examples including aerospace, construction, electronics and public transport (trains in particular).

Regulations and approvals for each industry sector are different and also vary between countries. For this reason, as an adhesive manufacturer, it is vital we treat each customer application as a new product development to ensure the adhesive matches the customer’s performance and application requirements, as well as meets the particular industry / country regulations regarding fire retardancy.

The majority of approvals are application-based and the adhesive bonded joints or laminated parts need to be tested for fire retardancy in a typical assembly (so if bonding composite layers which have been sandwiched together with an adhesive, the whole “sandwich” is tested).  It is a good idea to start off with an adhesive that is confirmed as non-burning so you don’t waste time and money further down the track and have to start again. One of the most recognised testing standards is UL94 testing which is discussed in more detail later in this article.

The other added challenge with regulations and approvals is that they are constantly changing, becoming ever stricter (it is very rare to see a regulation relaxed!). Following a major fire disaster or transport incident where deaths have occurred due to fire or toxic smoke inhalation, industries review their standards and bring in new legislations.  After the London Kings Cross disaster, underground stations and trains were subject to important new rules to ensure any adhesives used in public areas were fire retardant. German rules were stepped up after the Dusseldorf airport fire. Aircraft cabins have to be flame resistant and not give off toxic smoke.  As a result, many traditional building materials such as wood have been replaced with non-burning composite materials to help inhibit the spread of fire; of course any adhesives used to bond these materials also need to be fire retardant.

So, one of the obvious key criteria is to be flame resistant / non-burning or better still, inhibit flames – properly fire retardant.

Criteria #2: the adhesive should not give off excessive or toxic smoke.

Criteria #3: The adhesive needs to maintain its structural integrity at high temperatures (have as good a temperature resistance as possible).

Criteria #4: Decomposed adhesive material should not contain toxic by-products.

Fire retardant adhesives

It looks like a tall order to come up with an adhesive that can match these requirements – and at this stage, viscosity, colour, cure speed and preferred cured method, gap fill, strength performance, thermal conductivity, and packaging haven’t even been considered. But the development chemists enjoy a good challenge so BRING IT ON!

Flame retardance

Adhesives can be developed to match a sliding scale of fire retardancy – here are details of the Underwriters Laboratory Testing classifications. As adhesive manufacturers, we are seeing requests mainly for the UL94 V-0 and occasionally for the HB.

  • HB: slow burning on a horizontal specimen. Burn rate <76mm/min for thickness <3mm or burning stops before 100mm
  • V-2: (vertical) burning stops in <30 seconds and any drips may be flaming
  • V-1: (vertical) burning stops in <30 seconds and drips allowed (but must not be burning)
  • V-0 (vertical) burning stops in <10 seconds and drips allowed (but must not be burning)
  • 5VB (vertical plaque specimen) burning stops in <60 seconds, no drips, specimen may develop a hole.
  • 5VA as above but not allowed to develop a hole.

The two latter classifications would pertain to a bonded panel rather than a specimen of adhesive.

The testing is pretty simple and doesn’t require sophisticated equipment, here is a basic test setup:

Fire retardant adhesives

It can be quite tricky to do this test on some adhesives alone, particularly for adhesives which won’t cure properly outside of a closed joint – in this case, tests would be limited to testing between bonded substrates. However, epoxy glue and UV adhesives can be cured in as a solid test specimen which can then be inserted into the jaws of the clamp stand.  Keep a sand bucket nearby and we strongly recommend doing this under extraction or in a fume cupboard and don’t set off any smoke alarms (especially when they are linked directly to the emergency services…). Catch the specimen on fire and time how long it takes for the flame to extinguish. Check for any drips underneath (hopefully you have a disposable tray in situ otherwise bye-bye nice worktop).

Adhesive chemists combine a number of additives to make fire retardant adhesives – and sometimes even to quench flames (although this feature is harder to achieve nowadays with many goods manufacturers now requesting halogen-free formulations).  Additives which can be included into fire resistant adhesives include:

  • Organic char-forming compounds which help to lower heat and smoke and protect the material underneath from further burning.
  • Heat absorbers, these are normal metal hydrates which help give the adhesive great thermal properties (often the fire retardant adhesives are selected for heat sink bonding applications where maximum thermal conductivity is required).

It is a careful balance as these additives will cause interference to other adhesive properties such as strength, rheology, cure speed, flexibility etc.

Is there a difference between fire resistant adhesives and fire retardant adhesives?

Yes! There is, both terms have been bandied about in the article but it is probably best to set the story straight.

Fire resistant adhesives – these are often products such as inorganic adhesive cements and sealants. They don’t burn and they withstand extreme temperatures. These types of products are often used for applications in blast furnaces, ovens etc. They don’t do anything to stop an assembly burning – but they do a great job of holding all the burning bits together.

Fire retardant adhesives – these help to extinguish the flames and slow the spread of fire. These types of adhesives are sought by many industries:

  • Electronics – for potting and encapsulating electronics, bonding heat sinks, circuit boards etc. An electronic short circuit can easily spark a fire but PCBs contain fire retardant compounds – it is often important that adhesives also have these properties.
  • Construction – cladding and flooring (particularly in public areas) often has to be non-burning and bonded with a fire retardant adhesive.
  • Public transport – train carriages, bus interiors, trams etc. Flame retardant adhesives are used for bonding composite panels, flooring, and other fixtures and fittings. Not only do the adhesives help stop the spread of fire, but they provide an aesthetic joint without the need for unsightly (and rattly) mechanical fasteners.
  • Aircraft – as mentioned earlier, cabin interior materials are under strict regulations and must be fire retardant and not fill the cabin with black smoke during a fire.

Permabond works closely with these industries and others requiring flame retardant adhesives and offers a bespoke product development service, small batch manufacture to help keep minimum order quantities down, and the capability to ramp up to large-scale manufacture as required.

For further help and advice, product recommendations and information, please feel free to contact Permabond and we will arrange for our chemists to assist you further.