Dire Dawa Institute of Technology, School of Textile, Apparel, and Fashion Design. Textile finishing for 4th year BSc student

1. FLAME RETARDANT FINISHING
By: Hayelom Belay
DIRE DAWA UNIVERSITY
INSTITUTE OF TECHNOLOGY
July 24, 2021 Dire Dawa Ethiopia

2. Flame Retardant Finish

3. Application Area
• Back coatings for institutional Drapery,
• Upholstery, Carpets
• Aircraft /Automotive textiles
• Mattresses and bedding
• Racing suits
• Fire Fighters clothing
• Children’s nightwear
Factors affecting flame spreading
 Pure cellulosic and normal synthetic
fibres (Acrylics, acetates, nylons and
polypropylene fabrics not
recommended for drapery use.
 Light wt. fabrics (high aerial density).
 Raised-surface fabrics (highly
brushed

4. FLAME RETARDANT FINISHING
 Need to protect human life from fire has
been felt since ancient time
 Major causes of fire
 Textiles particularly cellulosic
 Wood
 Flammable organic solvents
 Short circuit in electric current
 Explosives
 Presence of silicon residues on PET
 Presence of certain pigments and dyes
Inherently FR Fibres
(Nomex, Twaron & Kevlar)
Or
FR Finish

5.  Very high add on (6-10%) (makes
fabric heavy)
 Stiffening of material
 Brittleness and hand loss
 Shade Change
 Cotton degradation by acid hydrolysis
 Generally Non durable
 Toxicity issues
Adverse effect of FR
1735
 First patent for flame resistance finish
of cotton based on alum sulphate and
borax
1882
 Gay-lussac published the first
systematic study of the use of flame
retardants.
 Since second world war (1945)
importance of flame retardant finishing
of textiles has been well established
HISTORICAL

6. What is burning ?
• Thermo-oxidative reaction
• It is exothermic
Heat
Burning Cycle

7. DEFINITIONS
o Fabric is seems to be flame retardant if it
does not Ignite and create a self-sustaining
flame when subjected to a heat source.
Pyrolysis:
 Temperature at which thermal
decomposition of solid starts
Combustion:
 Chemical process in which substances mix
with oxygen in air to produce heat and light.
Ignition:
 The process of something starting to burn
with the production of heat and light
Chemistry of FR
1. Inorganic - Metal hydroxides- aluminium
trihydroxide, magnesium, hydroxide, ammonium
polyphosphate and red phosphorus. (50% by
volume of the worldwide FR production) FR
synergists
eg. antimony trioxide.
2. Halogenated - based on chlorine and bromine.
(25%)
3. Organo phosphorus - phosphate esters (20%).
4. Nitrogen-based - used for some polymers.

8. Approaches to Flame retardancy
1. Chemical Action: FR agent dissociates into
radical species that compete with chain-propagating
steps in combustion. Eg. Halogens and some
phosphorus flame retardants.
2. Thermal quenching- Endothermic
decomposition of FR agent. Eg. Metal hydroxides,
metal salts and nitrogen compounds act to decrease
surface temperature and the rate of burning.
3. Protective coating -Glassy insulating film or
char barrier at fabric surface by low mp compounds.
Reduce heat transfer from flame to polymer. Eg.
Boric acid – borax, phosphorus compounds and
intumescent systems based on melamine
4. Inert Gas Dilution-Additives that produce large
volumes of noncombustible gases on decomposition
which dilute the oxygen supply and the fuel
concentration below the flammability limit. Eg.
Metal hydroxides, metal salts and some nitrogen
compounds
5. Inert fillers -Thermal sinks increase the heat
capacity or reduce fuel content of the polymer. Eg.
Glass fibres and minerals (talc).

9. MECHANISMOF BURNING
 The term flammability refers to the ease of ignition (process of starting to burn
with the production of heat and light)
 In order to understand the mechanism of ignition it is essential to understand the
mechanism of combustion of solid (chemical process in which substances mix
with oxygen in air to produce heat and light.)
EFFECT OF HEAT ON SOLID (FIBRE)
FIBRE
Heat
SOFTENING (Thermoplastic)
MELTING (Thermoplastic)
PYROLYSIS (Thermoplastic and non-ther)
COMBUSTION
IGNITION

10.  Glass Transition Temperature(Tg)
Thermoplastic polymers soften at the
glass transition temperature
 Melting Temperature Tm. Melt some
higher temperature
 (Tp), both thermoplastic and non-
thermoplastic solids will chemically
decompose (pyrolyze) into lower
molecular weight fragments.
 Chemical changes begin at Tp and as
temp. Rises combustion occurs (tc).
 These four temperatures are very
important when considering the flame
resistance of fibers.
Flammability Vs Temperature
Glass Transition Temperature = is defined as a
temperature at which amorphous polymer takes
on characteristic glassy-state properties like
brittleness, stiffness and rigidity
Pyrolysis temperature, TP At this temperature,
the fibre undergoes irreversible chemical
changes, producing non-flammable gases
(carbon dioxide, water vapour and the higher
oxides of nitrogen and sulfur).

11. LIMITING OXYGEN INDEX(L O I)
Another important factor in combustion is the limiting oxygen index (LOI).
LOI:-Amount of oxygen in the fuel mix needed to support combustion that is Minimum
quantity of oxygen a fibre needs in order to burn.
 Higher the number, the more difficult it is for combustion to occur.
 Percentage of oxygen in the air is 21%.
Fibres with an L.O.I. Lower than this level will burn easily
Those with a higher L.O.I. Will tend not to burn.
For effective flame retardancy L.O.I should be 26 and above

12. FLAMMABILITYPARAMETERSFOR FIBERS

13. BEHAVIOUR OF NATURALAND SYNTHETICFIBRES
 Natural fibers are not thermoplastic,
 When subjected to a heat source, pyrolysis and combustion
temperatures are encountered without softening or melting eventually
ignite.
 On the other hand, low melting thermoplastic fibers will melt and drip
away from the flame before pyrolysis and combustion temperatures
are reached. Therefore the burning of these fibres will be delayed.
 If the melt doesn't shrink away from the flame front, pyrolysis and
combustion temperatures are eventually reached and ignition (burning)
will occur.

14. COMBUSTION
 Combustion is an exothermic process
 Requires three components, heat, oxygen and a suitable fuel.
 When left unchecked, combustion becomes self-catalysing and will continue until the
oxygen, the fuel supply or the excess heat is depleted.

15.  When heat is applied, the fibre’s temperature increases until the pyrolysis
temperature, Tp, is reached.
 At this temperature, the fibre undergoes irreversible chemical changes, producing
non-flammable gases (carbon dioxide, water vapour and the higher oxides of
nitrogen and sulfur), carbonaceous char, tars (liquid condensates) and flammable
gases (carbon monoxide, hydrogen and many oxidisable organic molecules).
 As the temperature continues to rise, the tars also pyrolyse, producing more non-
flammable gases, char and flammable gases.
 Eventually, the combustion temperature, Tc, is achieved. At this point, the
flammable gases combine with oxygen in the process called combustion, which is
a series of gas phase free radical reactions
Some free radical
combustion reactions.
These reactions are highly exothermic

16.  These reactions are highly exothermic
 Produce large amounts of heat and light.
 Heat generated by the combustion process provides the additional thermal energy
needed to continue the pyrolysis of fibre
 Thereby supplying more flammable gases for combustion.
Contd.

17. Condensed phase
Mechanisms of Flame retatrdancy ( Combustion cycle Disruption)
Gas phase
Effective for fibre type Mainly cellulose, also
wool, catalysing their dehydration to char
All kinds of fibres, because their flame chemistry
is similar (radical transfer reactions)
Pyrolysis chemistry
Flame chemistry
Very effective because dehydration and
carbonization decrease the formation of
burnable volatiles
Fixation with binder changes textile properties such
as handle and drape, preferably for back coating,
With durable flame retardancy,
formaldehyde emission
Antimony oxide and organic halogen donators
(DBDPO and HCBC) are discussed as problems

18. 1. Heat Sink on / in the fibre
2. Insulating Layer
3. Condensed Phase: reaction to produce less
flammable volatiles and more residual char.
4. To interfere with the free radical reactions
Condensed phase

19. I. Prevent the access of oxygen to the flame or dilute the fuel gases in the flame to
concentration below which they will not support combustion.
Some phosphorous and borate flame retardant are thought to form glassy polymers on
the surface of the fibers, insulating the polymer from heat.
II. Increase the combustion temperature, Tc , of the fuels and/or interfere with their
flame chemistry. (Halogen based flame retardant).
GAS PHASE MECHANISM

20. CONDENSED PHASE MECHANISM
‘Condensed phase’ mechanism is based on removal of heat and the enhancement of the
decomposition temperature.
 Provide a heat sink on or in the fibre by use of materials that thermally decompose through
strongly endothermic reactions.
 If enough heat can be absorbed by these reactions, the pyrolysis temperature of the fibre is
not reached and no combustion takes place.
 Examples of this method are the use of aluminium hydroxide or ‘alumina trihydrate’ and calcium
carbonate as fillers in polymers and coatings
Endothermic decomposition
reactions.
1. Heat Sink on / in the fibre

21.  Treat fabric with material that forms an insulating layer around the fibre at
temperatures below the fibre pyrolysis temperature.
 Boric acid and its hydrated salts function in this capacity
 When heated, these low melting compounds release water vapour and produce a
foamed glassy surface on the fibre, insulating the fibre from the applied heat and
oxygen
Formation of foamed gases
2. Insulating Layer

22.  To influence the pyrolysis reaction to produce less flammable volatiles and more
residual char.
 This ‘condensed phase’ mechanism can be seen in the action of phosphorous-
containing flame retardants which, after having produced phosphoric acid through
thermal decomposition, crosslink with hydroxyl-containing polymers thereby altering
the pyrolysis to yield less flammable by-products
Cross-linking with phosphoric acid
Contd.
3. Condensed Phase

23. OTHER MECHANISM
o Phosphoric acid catalyse the dehydration and prevent the formation of undesired
levoglucosan , the precursor of flammable volatiles.
 Dehydration of
cellulose by
strong acid
 Thermal degradation of
cellulose.
 formation of flammable
levoglucosan

24. GAS PHASEOR VAPOUR PHASEMECHANISM
Interfere with the free radical reactions
 To interfere with the free radical reactions that provide the heat needed for the process to
continue.
 Materials that act in this ‘gas phase’ mechanism include halogen containing compounds which,
during combustion, yield hydrogen halides that form relatively long lived, less reactive free
radicals,
 Effectively reducing the heat available for perpetuating the combustion cycle, and which
decrease the oxygen content by flame gas dilution
 Chlorine and bromine operate in the vapor phase by forming free radicals that scavenge (to
remove) hydrogen and hydroxyl free radicals responsible for combustion.
 Hydrogen and hydroxyl radicals are major reaction species for combustion to proceed.

25.  The halogen radicals deactivate them, causing the chain reaction to break down.
Species that remove H. and or HO. will slow the combustion reaction.
Competing free radical reactions
during combustion of
Halogen (X)-containing material (M).
R is the organic residue.
Free Radical
Combustion Reaction
Contd.

26. FLAME RETARDANT FINISHES FOR CELLULOSICS
(cotton, rayon, linen etc.)
Levoglucosan: Levoglucosan and its volatile pyrolysis products are extremely
flammable materials and are the main contributors to cellulose combustion
 Compounds that are able to hinder levoglucosan formation are expected to function as flame retardants
for cellulose

27. Non-durable
 Applied to curtains, upholdtery, carpets etc.
 These items are occasionally washed.
 If washing is required the finish can be re-applied.

28. BORICACIDANDBORAX
• French chemist gay-lussac proposed a borax and ammonium sulphate flame retardant for
cotton in 1820.
• Presently, mixture of boric acid (H3BO3) and borax (Na2B4O7) (7:3) is effectively used as
Flame retardant for cotton at ~ 10 % solids add-on.
• Pad-dry application
• During drying the boric acid and borax dehydrate with the release of water vapour and then
melt and produce a foamed glassy surface on the fibre, insulating the fibre from the applied
heat and oxygen. Thus providing flame retardancy.
Formation of foamed
glass

29. AMMONIUMSALTS
 Chemicals that can yield phosphoric acid during the early stages of fibre pyrolysis form
the majority of successful flame retardants for cellulose.
 It is not sufficient to supply just phosphoric acid precursors. The presence of nitrogen
has been found to provide a synergistic effect with phosphorous.
 Minimum levels of of P and N add on estimated at ~ 2 % P and ~1 % N.
 Minimum levels can vary depending on fabric construction and test requirements.
 Ammonium salts of strong acids, especially phosphoric acid (P/N synergism) are
particularly useful as nondurable flame retardants for cellulose.
 Three commercially important products
 Diammonium phosphate, ammonium sulfamate , ammonium bromide.
 These salts form the corresponding strong acids at the pyrolysis temperature of
cellulose and prevent the formation of flammable levoglucosan during pyrolysis.

30. Diammonium phosphate
Ammonium sulphamate
Ammonium bromide

31. MECHANISM
DAP AND AMM. SULPHAMATE, AMM. BROMIDE
 At the pyrolysis temperature there will be formation of strong phosphoric acid from
diammonium phosphate and sulphamic acid from ammonium sulphamate .
 Thus there will be dehydration of cellulose. This will end up with
 preventing the formation of levoglucosan a flammable decomposition product of cellulose
during pyrolysis.
Thermal degradation of cellulose
Flammable

32. Cellulose Dehydrated cellulose
In extreme cases, possibility of crosslink formation between phosphoric or
sulphamic acid and cellulose.
Formation of these reaction products at the pyrolysis temperature
prevents the formation of flammable levoglucosan
Crosslinking with phosphoric acid
Water

33. AMMONIUMBROMIDE
 Ammonium bromide
 Formation of bromic acid (HBr) will also interfere in gas phase with the free
radicals .
 Prevent the formation of H. OH. free radicals
 Reduce heat energy so that pyrolysis temperature of cellulose is not reached.

34. DURABLEFLAMERETANDANTFINISHESFOR CELLULOSE
 Inorganic salts can provide excellent flame-retardant properties for cellulose,
 Most successful durable flame retardants for cellulose are based on phosphorous- and
nitrogen-containing chemical systems that can react with the fibre or form cross-linked
structures on the fibre.
 Most popular finish is tetrakis(hydroxymethyl)phosphonium chloride (THPC), made from
phosphine, formaldehyde and hydrochloric acid.
Synthesis of THPC

35. THPC-UreaProcess
THPC reacts with urea to form an insoluble structure on cellulose in a pad–
dry–cure process.
Reaction of THPC with urea

36.  Treatment of cured finish with hydrogen peroxide to convert the phosphorous atoms to their
highest oxidation state results in cellulosic goods with very durable flame retardancy.
 Applying 25 % THPC with 15 % urea yields a final phosphorous add-on of 3.5–4 %, which is
adequate for most fabrics.
 THPC–urea system gives highly effective and durable flame retardancy to cellulose,
Drawbacks
 Stiff feel
 Significant loss in tensile and tear strengths
 Releasing formaldehyde during curing. Environment and health hazard
 Use of softeners and mechanical finishing techniques are used to provide commercially
acceptable fabrics.
contd.

37. THPC-Urea-Ammonia Process
THPC–urea-ammonia system produce finishes with less stiffness and
fibre damage (Proban process).
precondensate is prepared by the careful reaction of THPC with urea.
Cotton fabric treated with precondensate pad-dry application
fabric is then exposed to ammonia vapours in a special reaction chamber,
followed by oxidation with hydrogen peroxide.
THPC-Urea_Ammonia reaction

38. VARIATION OF THPC PROCESS
use of the sulfate or hydroxy salts in place of phosphine chloride
THP-Sulphate to eliminate the possible formation of highly toxic
bis(chloromethyl) ether during processing, and
THP-OH to reduce acidic tendering of the goods.
Drawback THPC FINISHES
very few direct or fibre reactive dyes can withstand exposure to THP-
based finishes.
cellulosic goods that are to be flame retardant treated with a THP finish
should be dyed with vat dyes.

39. Treatment with N-methylol dimethylphosphonopropionamide
another durable phosphorous-containing finishes is the use of n-
methylol dimethylphosphonopropionamide in combination with
trimethylol melamine and phosphoric acid as catalyst
process. pad–dry–cure
Reaction of N-methylol dimethylphosphonopropionamide with
cellulose.
The required add-on is 20–30 % depending on the weight of the
fabric.
washing after curing is necessary to remove the phosphoric
acid,
finish may give rise to an unpleasant odour during the curing step. .

40. Flame-retardant finishse for fibre blends
 Imparting flame retardant finish to fibre blends is more difficult
 Blends of natural fibres with synthetic fibres, usually exhibit a flammability worst than that of either
component alone.
 Natural fibres develop a great deal of char during pyrolysis, whereas synthetic fibres often melt and drip
when heated.
 This combination of thermal properties in a fabric made from a fibre blend results in a situation where the
melted synthetic material is held in the contact with the heat source by the charred natural fibre. allowing
the blend to burn readily.
 This can be demonstrated by the LOI values of cotton (18–19), polyester (20–21) and a 50/50 blend of both
(LOI 18), indicating a higher flammability of the blend.
 But opposite behaviour is also known (modacrylic fibres with LOI 33 and cotton in blends from 40–60 %
can raise the LOI to 35).

41.  In order to flame retard natural/synthetic fibre blends, high levels of flame retardants are
required.
 One approach is to add the necessary amounts of retardant as a fabric coating.
decabromodiphenyl oxide (DBDPO) in combination with antimony trioxide
finish required 37 % add-on of the retardants in addition to a latex binder and softener.
colour and hand of the finished fabric are significantly altered and chemical costs are high.

42. EVALUATION OF FLAME RETARDANTS
Many factors influence the flammability of textiles, including the
fibre type, fabric weight and construction,
method of ignition,
extent of heat
pyrolysis products formed,
Differing performance requirements and government regulations have led
to the development of numerous test methods for evaluating the flame
retardancy of textiles.

43. Common flammability tests

44. LIMITING OXYGEN INDEX (L O I)
L O I is most important parameter for testing the efficiency of flame
retardant finish
LOI is defined as the content of oxygen in an oxygen/nitrogen mixture
that keeps the sample at the limit of burning:
oxygen content of air is 20-21 % corresponding to LOI = 20-21.
All textiles with lower LOI values will burn quite easily in air and those with
LOI values much higher than 20 will not burn.
Minimum L O I value for flame retardancy is 26

45. Limiting oxygen index (LOI) values of different types of fibres

46. Thank you