Indian Journal of Textile Researcb
Vol. 2, December1971,pp. 119-121
Infrared Studies on the Mechanism of Reactions between
THPC-based Fire Retardant and Cellulose Powder*
JAGDISH CHANDRA GUPTA, KRISHAN LAL & HARI L~BHATNAGAR
Departmentof Chemistry,KurukshetraUniversity, Kurukshetra132119
Received
6 July1977; accepted15 November1977
The reactionbetweencellulosepowderand tetrakis (hydroxymetbyl)pbospbonium
chloridehas been studied
underdifferentconditionsof pH and thermalcuringusingthe infraredtechnique. The data obtainedhavebeenanalysedand comparedwith thosefor the reactions betweencellobiose,THPC, thioureaand ammoniumdihydrogen
orthophosphate.The data have beenused for understandingthe mechanismwhereby THPC-based[formulations
imp~t flame-resistantcharacteristicsto cellulose.
C
ONSIDERABLE
research is currently
being
carried out on the imparting of flame retardancy to cotton through chemical treatment.
A number of flame retardants developed for cotton
in recent ':tears are ~ased on tetr~~is (hydr?xymethyl)
phosphonIum chlorIde ~PC)
.!n
VIew of the
Importance of the pro~l~l!llt was cons~dered desir~ble
to explore the posslbIII.ty of formlt;1g a chemIcal
bond between THPC or Its adducts Wl~ other co~pounds and the anhydroglucose UnIt. For this
purpose, a few probable reactions between a model
Cellobiose was reacted with THPC thiourea and
ammonium dihydrogen orthophosp~te
in the same
stoichiometric ratios as those used in the case of
cellulose.
Infrared spectros~opy -~nfrared
spectra of the
~ples
were obtaIned USIn~ a Beckman spectrophotometer; model I~-20
(U.S.A.) on potassium
bromide; pe~lets containing the samples. Some liquids
or se,mlsoltds were used directly on potassiwn
bromIde plates.
cellul?sic compound
(ce~lobio~e) and THPC a~d other
chemlcal~
have been Investigated
by the Infrared
technique.
Results
Materials and Methods
THPC was prepared by treating phosphine with
~ a~ueo~s solution of fo!maldehyde and hydrochlorI~ ac,ld8. THPC solutIon (45% vol./vol.) prepared In dIstilled water was neutralized to the required
pH (solution A). A 22.5% (wt/vol.) aqueous solution
of thiourea (solution B) was prepared
Equal volumes of solutions
(A)
and (B) were
mixed and ammonium dihydrogen orthophosphate
was added. It was ensured tbat the proportion of
ammonium
dihydrogen
orthophosphate in the
resultant formulation did not exceed 0.5 %. The
lose, either singly or in combination, it was found
desirable to study the IR spectra of single c,°mpo~
nents separa~ely and compare th~ ~ame wIth t~e
spe?tra obtaIned on systems. consIsting of combln!i;tlons of the compounds. S~ncecellulo~e IS a compl1cated molec,ule spectroscopIcally,
a model compound, cellobIose, ,,:,asused ~o stud~ the IR ~pectral
changes ~~e to chemIcal reactions which may sImu1ate
the condItIons analogous to the presence of cellulose
a~,one of the react!i;nts.
The IR spectra of cellob!os~, cellulose, thIourea,
THPC,
ammonium
dlhydrogen orthophosphate separate~y and of various
final a4jus~ent
of pH.was made with sodium hydroxideto obtaIn the requIred pH (sol"tion C).
THPC-based
formulatIons
were recorded.
and Discussion
To understand
the mechanism
by which THPC
and other chemicals impart fire retardancy to cellu-
of cellobIose
and cellulose
About 6 g cellulose powder (p. Mer9k) was treated
with 100 m1 of solution (C) for about 10 min and
filt;ered through a G-3 sintered glass funnel. The
residue was dried at 100°C in an oven for 10 min and
cured at 160°C for approximately 10 min.
After cooling, the sampJe was washed for about
30 min with distilled water and dried again at 100°C.
The dipping and curing cycle was repeated and the
residue washed and dried. Final drying of the samples
prepared at various pH values ranging from 4.0 to
6.5 was done in a vacuum drier column using toluene
as the heating liquid.
The samples were stored in
a desiccator over sodium hydroxide.
The ~ spectra of these compounds/formulation~
~howed ~ app~rance of a nC(~.,band ~.t 1530 cm-1
In. the case of (I) -.rHPG t. thIourea, (1.1)~HPC +
thiourea + cellobIose,
(111)THPC + thIourea +
c,ellobiose + ~onium
dihydrogen orthophosphate,
(I':) THPC + thIourea + cellu1os.e,!;lnd (v) .THPC +
thIourea + 'cellulose -:1-. ammOnIum dIhydrogen
~rthophosphate~. In additIon, another ba,nd at 1350
cm-1 was seen In .the. cas~ ~f
THPC-thiourea
systems. A band In thIs regIon IS also observed for
both cellobiose and cellulose. Mino~ shifts in the
1350 cm-1 band ha~e ~~n <:>bservedat dif!ere~t pH
values for systems (II), (I~l), (IV) and (v). It...~ dIfficult
to say whether these shlft~ are due to the I~uence
.Supportedby the U.S.D.A.-A.R.S., U.S.A. under Project of pH on. cellul~se or due to the intluenGe of pH on
No. UR-A(7)219.
THPC-thiourea adduct.
119
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INDIAN J. TEXT. RES.,YOLo2, DECEMBER1977
It appears reasonable to assume that the new
band at 1530 cm-1 is due to the reaction between
T~PC and thiourea Vfhich results i~ a co:polymer(~).
This band can be assIgnedto the thioamIde II which
is genera~lyobserved in the range 1~70-~510 cm-1
in the solId state and 1550-1500cm-1 In dIlute solutions cQntaining a compound with thioamide II
structure~ll. It has been shown that N-methylacetamide shows an amide II band12-15at 1565, 1534
and 1490 cm-l in liquid, solution and vapour phases
respectively. Schiedf6 has observed that amide ~I
band does not exist in lactams (secondary cyclIc
amides) containing less than nine member~ in the
ring. Thus, the band at 1530 cm-1 (approxImately)
is sensitive to the change of state.
The new band at 1530cm-1 can be assignedto the
structure (A) resulting from the reaction between
THPC and thiourea.
5 H
0
II
II
I
H~N-C-N-CH~-P-CH~OH
tH~OH
(A)
To study the effect of curing an,d laundering ~n the
reaction products of cellulose wIth THPC, thiourea
and ammonium dihydrogen orth~phoshate, the .-,~
IR spectra of these products (obtaIned at pH 4.0,
1.5,.5.0, 5.5, 6.0 and 6.5). were record~d after su~Jectmgthem to thermal curing at 160:!:: Z for 10mIn
once or twice. No appreciable effect of}aundering
on the IR data was observed. Though thermal
curings gave quantitative differences in absorption
at var!ous wave numbers, no t;lewband was observed;
nor dId any o.bservedban,d dIsappear.
The follo~mg .conclusIons may be drawn from
the above dISCUSSIO~.
.
.(a) THPC an~ ~hiourea form a copolymer Vfhich
eIther gets precIpItated on the cellulose matrIx or
forms a hydrogen bond with cellulose. The latter
is destroyed on repeated washing.
.-.d
(b) The reaction product of THPC and thiourea
may have either of the following two structures:
I
CH
5
I ~
II,}
7H~
t
<il Clep~CHzOH),+C=S
-CH2-P-CH2-NH-C-NH
~H2
tH2
To confirm that the band at 1530 cm-1 is due to
thioamide
II, JR data
THPC and monophenyl
..
120
5
II ~
(ii)
j,
X
I
or
on reaction products
of (a)
thiourea and (b) THPC and
diphenylthiourea were obtained.
(a) THPC +
3200, 2880, 2580, 2040, 1635,
mon?phenyl- 1510, 1410, 1290, 1260, 1160,
thIourea
1100, 1030, 900 cm-1
(b) T~PC +.
3200, 2880, 2580, 2040, 1630,
dIphenylthio- 1415, 1300, 1250, 1180, 1160,
urea
1100, 1040, 910 cm-1.
It can be seenthat no absorP.tionban~ is observed
at 1530 cm-1 for THPC +. dIphenylthiourea, .but
for THPC +. monophenylthio~ea, a co.mparatively
weak band IS observed. ~s
conclusIvely proves
that the band at 1530 cm-1 IS due to -N-C=S
I
..".
H.
(thio~de
11), the mIxed. VIbr~1!ton.N-H In-plane
bendIng ~nd C-N stretchIng VibratIon modes.
The exIstence?f another new band at 1~50 cm-1
for the THPC-thiourea system can be assIgned to
P=O stretching vibration17. It can also be assigned
to CNH group18. Secondaryamides show a characteris1;icban,d in. the 1310-1200 cm-1.region due to
a mI.xed vIbra,tion of C-N stretchi~g and N-H
be~dIng. ThIS band has been ~sIgnated as the
amIde III band19,2O.Any mechanism suggestedfor
the reaction between THPC and thiourea must take
both the possibilities into account, i.e. the existence
of both P=O and CNH groups.
Another interesting feature observed is the shift
of 3360 cm-1 in cellulose to 3280 cm-1 when treated
with
unwashed THPC-ammonium
dihydrogen
orthophosphate-thiourea
system. However, on
washing the sample, the original band with midpoint
at 3360 cm-1 was reproduced. This indicates that
some kind of hydrogen bonding exists between the
copolymer of THPC and thiourea with cellulose,
hi h
h
..
w c IS eI~ er partla11yor fWly repIaced on washing
or laundering.
-
0
II
H~-C-N-CH2- ~-CHzOH
CHOH
(A) 2 .*'
The mechanism of formation of the copolymer by
the reaction between THPC and thiourea has been
discussed elsewhere21.
Quaternary phosphonium structure was first
assumed to be converted to the phosphine oxide
during fabric processing, but later results22showed
that the phosphonium structure is retained partially.
This view was further strengthened by the fact that
when the APO-THPC system23was used as a fire
retardant finish, THPC was found to act as a tetrafunctional group and a large percentage of chlorine
was retained by the copolymer.
.It is .not possible to e;stablishconclusi~ely whether
a chemIcalbond does eXIstbetweencellobIose or cellulose and the copolymer of THPC and thiourea, as
the bond CNH observed at 1350 cm-1 could either
be due to the precipitated ~polymer and held. by
hydrogen bonds or to a reaction between cellob!ose
or cellulose and the copolymer, as both cellobIose
and cellulose show bands in the vicinity of 1350 cm-1.
.).,
Acknowledgement
.
The authors are. grateful to the United. States
Department of AgrIculture, Far Eastern Agriculture
ResearchServIce for supportIng t~e researchthrough
the grant of a research fellowship to one of them
(J. C. G.).
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121