Mankind, since time immemorial has been using polymers. Before
you read any further, lift up your head and look around. Almost everything that
catches your eye is either a polymer or made up of polymers. Such is the
influence of these macro molecules on one of nature’s macro creations, the Humans.
Most polymers that we use on a daily basis are petroleum based i.e. they are
made up of long chain of carbon, making them highly inflammable. A tiny spark
can cause a massive burn down of everything around us. But this seldom occurs
because of materials that are added to polymers called Flame Retardants.
Flame Retardants are compounds added added to polymers to
check their ability to catch fire and there by prevent the scale of damage
caused in a fire accident. Flame Retardants have been used in the production of
dyes, varnishes, adhesives, resins, plastics, textiles, mattresses, couches, carpet padding, curtains, flooring,
composite materials, packaging, insulating materials, glues, surface coatings,
electronic components and many more. In order to understand the working of Flame
Retardants we must first understand how a polymer catches fire and burns out
rapidly. When a polymer comes in contact with thermal stress like fire, it
first degrades. As a result, small molecules are liberated, which mix with air.
If these particles are at a temperature above their ignition temperature, they
react with the atmospheric oxygen and combustion occurs. This further increases
the temperature of the system. A closed loop system with heat-feedback is generated.
The heat feedback increases the intensity of fire in multitude. In such a case,
Flame Retardants come as a saviour to avoid any massive destruction.
Different Flame Retardants act in different ways based on
their structure. Some form a char layer on the polymer surface. This layer insulates
the polymer and cuts off the heat feedback. Another type of Flame Retardants
are materials which degrade at decomposition temperature of the polymer to form
particles which scavenge the combustion propagation radicals.
Flame Retardants can be broadly classified into three
categories based on their chemical composition: organohalogen compound, organo
phosphates and minerals. They work by either cooling, diluting or insulating
and thereby slowing down or preventing the spread of fire. Flame Retardants
don’t make
the polymer resistant to fire but give people the time to escape and
put off the fire in case of an accident. Typically, 1-30% of Flame Retardants
are present in a polymer depending upon the nature of the material. The biggest
challenge about choosing the right Flame Retardant is that it should not alter
the properties of the host polymer. Hence, they are usually made up of volatile
compounds and added such that there is no bond formed with the polymer.
Like every invention till date, Flame Retardants have also
been a two-edged knife. Mineral Flame Retardants like Alumina trihydrate
requires open cast mining. This affects the natural habitat of several species,
causes erosion and disrupts hydrology. Though an excellent flame retardant,
because of these reasons, it cannot be used in a large scale. The organo
halogen flame retardants like organochlorides and brominated compounds are very
good flame retardants in gas phase. But they are soluble in organic solvents.
They enter the food chain and traces of them being accumulated in human tissues
have raised severe concerns. The toxicology studies further led to ban on these
compounds. The organophosphates, though expensive than organohalogen compounds
are very effective flame retardants and active in solid state. In the presence
of excess of oxygen, there is char formation and thus insulation. But when the
availability of oxygen is very less, they form volatile radicals and become as
hazardous as the organohalogen compounds. These volatile compounds are called Persistent
Organic Pollutants. They break down very slowly and persist in the environment
as well as in human tissues. In the presence of UV light, they break down to
give toxic substances like dioxin and furan.
These concerns gave rise to the need for development of a non-toxic,
Eco friendly and biodegradable Flame Retardant. One such material can be
synthesised using plant precursors like Gallic acid and 3,5-dihydroxy benzoic
acid which are abundantly present in grass and buckwheat. Conversion of these
hydroxyls to esters using phosphites gives the eco-friendly flame retardants. Phosphorous can be recovered from the the
waste water treatment plants. It is a very
important element in all organisms, present in food and hence not toxic. About
5-20% of this flame retardant was
incorporated into the polymer and tested for its properties. It is not
petroleum based, but plant based. This reduced the huge pressure on non-renewable
sources. Also, the compound can be easily synthesised on a large scale due to abundance
of starting materials in nature. Upon decomposition, the degradation fragments
can be consumed by microbes thus making it a more sustainable and safe material
to guard us against fire accidents.
