Fuming / Smoking

We have created this article to explain in general terms a matter we are regularly contacted about – jackets/fabrics that start fuming or smoking when the application gets up to running temperature. This is often well below the stated maximum operating temperature of the fabric.

It could be the inner face of an insulation jacket, or the fabric component that is closest to, or touching a hot face.

Most* fabrics will fume or smoke on first heating, even the most basic uncoated loom-state fabrics will fume due to various weaving lubricants or ‘sizings’ applied to the yarns during the manufacturing process.

As virtually all fabrics sold are either treated or coated in some way then naturally this means more compounds have been applied during the manufacturing processes. This results in more compounds to thermally degrade as the finished items are run up to temperature for the first time.

Using glass fibre as the main example.  The glass itself can withstand up to 550’c.  Take an application running at 350’c continuous, where the jacket manufacturer has used a standard ‘plain’ or weavelocked glass cloth – a perfectly suitable choice.

The weavelock is a fire-retardant acrylic dip. Its purpose is to make fabrication, cutting and sewing easier by locking the fibres together and prevent fraying.  The finish is adding nothing to the overall performance of the fabric, and it will thermally degrade at low temperatures, creating some amount of smoke or fuming.  Here the organic acrylic compounds are reacting with oxygen to create Carbon Dioxide, Carbon Monoxide, and other sooty, longer chain hydrocarbon molecules.  There is no fire or burning though, just the gradual thermal decomposition of the compounds in question.

Another glass-based example would be our silica wash high temperature dip treatment, which will allow the fabric to be pushed all the way to 750’c.  Here there is a functional element to the finish allowing the glass to withstand the higher temperatures, when run at elevated temperatures. At 700’c for example then the functional compound is not thermally degrading, but we would still expect a certain amount of fuming or thermal degradation.  In these cases, it is the organic binders and softeners used, along with any sizings on the yarns, which ARE thermally degrading.  These binders and softeners are an integral part of the finished product and again make the product workable and easier to handle.

The amount of fuming and duration of initial fuming is dependent on several factors, but mainly it is about how hot it gets, and how quickly.  Something getting up to a very high temperature (700’c+) very quickly will produce more initial fuming, but the process should be completed significantly quicker than something slowly running up to a relatively low temperature (200’c).

During the initial heat cycles there may be some charring of certain fabrics, again this is to be expected, but please see our article about charring for more detailed information.

*There is one group of products we supply that do not smoke or fume, these being the Heat-Cleaned or Heat-Treated ranges.  These products are pre-fired or heated to remove all the weaving oils and impurities in a separate stage before shipment to the end user or fabric convertor.  These products are specified where it is critical there is no outgassing or fuming when the products are first run up to temperature.  The major drawback here is how difficult the products are to work with and convert, they are highly prone to fraying and are generally much less stable than the standard versions of the same fabrics (that would fume).

The process of thermal degradation/fuming/smoking is completely normal and to be expected, although it can be alarming to end users or those on site who may not be expecting it and can often be wrongly perceived as something being on fire. 

Making sure site owners and operators know what is going to happen when insulation jackets and other fabricated high temperature items are first run up to temperature is critical, and where possible plant rooms or other installation locations should be well ventilated during primary heat cycles.

We hope this article helps explain some of the common questions arising around fuming/smoking fabrics, but as ever please feel free to contact us with any technical queries.

Author 

Adam Charnock

Operations Director