Tannic Acid: A Bio-based Intumescent Char-forming Additive for Nylon 6
Weeradech Kiratitanavit1, Zhiyu Xia1, Ankita Singh1, Ravi Mosurkal2, Ramaswamy Nagarajan1,3
Departments of 1Plastics Engineering, 3Center for Advanced Materials,
University of Massachusetts, Lowell, MA 01854,
2
Bio-Science & Technology Team, Materials Science and Engineering Branch,
US Army Natick Soldier RDEC, Natick, MA 01760.
Abstract
Tannins are natural polyphenols, found in the barks
of trees and provide thermal and microbial
protection. Tannins can be classified into two
categories: hydrolysable tannin or tannic acid (as
shown the structure in Figure 1) and nonhydrolysable or condensed tannin [11]. The
condensed tannin has been reported to be an additive
that enhances thermal properties of plastics. Tannin
copolymers have been blended with polyesters such
as
polyethylene
terephthalate,
polybutylene
terephthalate and polycarbonate to improve thermal
stability while lowering dripping characteristics upon
ignition [12]. It has been reported that as low as 100
ppm of tannin copolymer (condensation product of
tannin with polyvinyl alcohol or polyethylene glycol)
reduces flame sustenance time by more than 50%.
However, hydrolysable tannin is often used only as a
binder [13]. From our earlier studies on thermal and
thermo-oxidative degradation of tannic acid it was
evident that this material exhibits intumescence [14].
Here we study the thermal characteristics of blends of
tannic acid with Nylon 6.
Intumescent and char forming additives are
typically blended into certain types of commercial
plastics to impart resistance to fire propagation.
Intumescent compounds such as ammonium
polyphosphate/ melamine/ pentaerythritol, silica
gel/potassium carbonate are already used as flame
retardant (FR) additives. In this work, a naturally
occurring polyphenol, namely tannic acid, is explored
as an intumescent and char forming additive for
polyamide - Nylon 6. The tannic acid was meltblended into Nylon 6 and the compounded plastic
was evaluated for thermal stability, total heat release
(THR) and heat release capacity (HRC). It was found
that HRC and THR of nylon blended with tannic acid
decreased by 50% and 20% respectively.
Introduction
Flame retardants are often compounded into
plastics to delay the initiation and propagation of fire.
The current global market for FR additives is over 2
million metric tons per year with a 4 % annual
growth rate. About 35% of commercial FRs are based
on halogenated compounds [1]. These additives are
becoming increasingly regulated or banned from
being used in polymers due to their toxicity and
environmental persistence [2]. Around 15% of FRs
used currently are based on organophosphorus
compounds. Inorganic compounds such as aluminum
hydroxide accounts for 40% of the FR market.
Development of non-toxic FR additives suitable for
incorporation in polymers such as nylon used in meltspun fiber applications is especially challenging.
Organophosphorus compounds are most commonly
used for fiber forming polymers such as Nylon 6.
More recently, the use of intumescent flame
retardants have also been reported in these
applications [3,4,5,6,7,8]. Incorporation of polyols
and melamine have been reported to facilitate
foaming and the formation of protective char layer on
the surface of the burning polymer during
combustion [9,10].
OH
OH
HO
O C
O
OH
O C
O
O
O C
O
O O
C
HO
OH
HO
OH
O C
O
OH
HO
OH
OH
O C
O
O
O
O C
O
HO
O
OH
C O
OH
C
HO
OH
OH
OH
O
O C
OH
OH
OH
OH
HO
OH
Figure 1. Chemical structure of tannic acid
In an attempt to move towards safer, bio-based
alternatives, here we explore the use of tannic acid as
an intumescent & char forming additive for Nylon 6.
1
SPE ANTEC™ Indianapolis 2016 / 1411
Experiments and Materials
and char yield for tannic acid under these conditions.
About 5% weight loss was observed below 200 °C
from tannic acid under nitrogen or air. This is
attributed to the release of acetic acid and water [14].
The major degradation of tannic acid occurred at
around 250 °C. Under nitrogen, tannic acid exhibited
high char yield of 27%, while under air, tannic acid
burnt completely leaving only 1.4% of char residue.
The char under nitrogen tannic acid exhibits
characteristic intumescence. The HRC and THR of
tannic acid as obtained by PCFC were 160.3 J/g-K
and 5.9 kJ/g respectively as shown in Table 1. Based
on HRC values, this material can be considered to be
moderately fire resistant [15].
Technical grade tannic acid (ACS reagent,
Formula: C76H52O46, Formula weight: 1701.20 g/mol,
mass loss on drying: <7.5%) was purchased from
Sigma-Aldrich and used as received. Nylon 6
(NYCOA 851, MFI = 12.8-13.0 g/ 10 min, ASTM
D1238: 235oC, 1 kg)) was provided by Nylon
Corporation of America (NYCOA).
Preparation of blends of tannic acid
and Nylon 6
Tannic acid was blended with Nylon 6 using a
micro twin-screw extruder (DACA Instruments).
Before blending, tannic acid and Nylon 6 were dried
at 76 oC overnight and dry-mixed samples with tannic
acid loadings of 5, 10, 15, 20 and 30% by weight
were prepared for this study. The dry-mixed samples
were added into the extruder and processed at 230 oC
for two and half minutes. The screw speed was kept
constant at 60 rpm. The extrudates were stored in
sealed plastic bags prior to further thermal
characterization.
100
Tannic Acid Under Air
Tannic Acid Under Nitrogen
Weight, %
80
Characterization
60
40
20
The thermal stability of all polymers and blends
was analyzed using Thermogravimetric Analyzer
(TGA, TA Q50). Approximately 10 mg of samples
were weighed in a ceramic pan and heated up to 750
o
C at the heating rate of 20 oC/min. All tests were run
under nitrogen or air with a constant gas purge of 30
ml/min. Pyrolysis Combustion Flow Calorimetry
(PCFC) was done using a FAA micro calorimeter
from Fire Testing Technology Limited. In PCFC, the
sample was heated at 1 oC/sec in the pyrolyzer from
90 to 750 oC under nitrogen. The volatile species
were then swept into the combustor and completely
oxidized under a simulated air atmosphere (a mixture
of nitrogen and oxygen gas= 80:20 v/v). The total
heat release (THR) and heat release capacity (HRC)
was obtained based on the oxygen consumption.
Typically, 3 mg of sample was used for the test. The
test was repeated two times in each case and the
results presented are the average values obtained
from these tests.
0
0
100
200
300
400
500
Temperature (°C)
600
700
Figure 2. TGA curves of tannic acid under air and
nitrogen
Thermal stability and heat release
properties of blends of tannic acid and
Nylon 6
Thermal decomposition (under nitrogen) profiles
of nylon blends were compared to pure nylon and
tannic acid in Figure 3. Nylon 6 used in this study has
a peak thermal decomposition temperature (Td) of
488 oC (from DTGA) while the pure tannic acid has a
Td of 333 oC. When tannic acid was blended into
nylon, there was a slight decrease in the Td and a
marginal increase in char forming (as shown in Table
2). At 30% tannic acid loading, the Td decreased to
470 oC with char yield of 13% as compared to 0%
char in the case of pure nylon. However the Td of this
blend is still much higher than that of pure tannic
acid. At higher tannic acid loading, a new peak in the
range of 350-370 oC can be observed from the
decomposition of tannic acid.
Results and Discussion
Thermal stability and heat release
characteristics of tannic acid
Figure 2 shows TGA curves of tannic acid
under air and nitrogen atmosphere. Table 1 also
summarizes observations of degradation temperature
2
SPE ANTEC™ Indianapolis 2016 / 1412
o
Deriv. Weight (%/ C)
Table 1: Thermal Degradation of tannic acid (TA),
Nylon 6 and Nylon 6 -tannic Acid Blends under
Nitrogen and Air
Degradation Parameters
T10% (oC)‡
T50% (oC)
Samples
Td (oC)†
Air
Air
N2
Air
N2
N2
TA
333 325 261 258 354 402
Nylon 6 488 487 446 436 481 479
10% TA* 483 480 425 418 477 475
20% TA 478 479 365 351 465 467
30% TA 470 468 330 318 454 454
*10% TA = 90% Nylon 6 + 10% Tannic acid
†
Td = Peak degradation temperature
‡
T10%= Degradation temperature at 10% weight
loss
Char under N2
550 oC 750 oC
31.5%
27 %
0%
0%
10.6 % 10.5 %
12.4 %
12 %
13.5 %
13 %
TA
Nylon 6
10% TA
20% TA
30% TA
Weight (%)
0
300
400
500
600
300
400
500
600
700
Comparative degradation study under nitrogen
versus air atmosphere can help assess the material’s
thermo-oxidative stability of the surface as opposed
to the bulk [16]. Figure 4 shows the Thermooxidative decomposition (in air) of nylon blends,
pure nylon and tannic acid. Under air, Nylon 6 has
two distinct thermo-oxidative degradation stages at
around 480 and 600 oC. On the contrary the blends
exhibit an extra degradation centered around 350 oC
mostly likely due to the degradation of tannic acid.
Pure nylon degrades completely without any
char while the blends exhibit some char at 550 oC.
However, the char produced disappears at 750 oC (as
shown in Table 2). It has been reported that other
natural polyphenols such as lignin can delay thermal
degradation of Nylon 6 in lignin-nylon blends. This
behavior is explained based on formation of
carbonaceous char [17]. In the case of tannin/nylon
blends we speculate that a similar intermolecular
network is formed between tannic acid and Nylon 6.
In blends of nylon 6 with red phosphorus it has been
reported that increasing the content of red phosphorus
increasing the char yield. More interestingly, the char
yield more closely correlated to the amount of solid
residue produced in TGA under inert atmosphere
rather than in air [18].
Tannic acid (TA)
Nylon
95% Nylon + 5% TA
90% Nylon + 10% TA
85% Nylon + 15% TA
80% Nylon + 20% TA
70% Nylon + 30% TA
200
200
Temperature ( C)
(a)
100
0.5
Figure 3(a) TGA and 3(b) DTGA curves of tannic
acid, Nylon 6 and Nylon 6 blended with tannic acid
under nitrogen
60
20
(b)
o
80
40
1.0
100
Char under Air
550 oC 750 oC
3.7 %
1.4 %
5.6 %
0%
7.9 %
0%
10.9 %
0%
16.1 %
0%
100
1.5
Tannic acid (TA)
Nylon
95% Nylon + 5% TA
90% Nylon + 10% TA
85% Nylon + 15% TA
80% Nylon + 20% TA
70% Nylon + 30% TA
0.0
Table 2. Char Remaining of tannic acid, Nylon 6 and
tannic Acid – Nylon 6 Blends at 550 oC and 750 oC
under Nitrogen and Air
Samples
2.0
700
Further, it is also very likely that the carbon
dioxide released from tannic acid dilutes the oxygen
therefore retards degradation propagation.
o
Temperature ( C)
3
SPE ANTEC™ Indianapolis 2016 / 1413
100
Table 3. HRC and THR Values of Tannic acid,
Nylon 6 and Nylon 6 -Tannic Acid Blends
(a)
Weight (%)
80
Materials
60
Tannic acid (TA)
Nylon 6
95 % Nylon 6 + 5 % TA
90 % Nylon 6 + 10 % TA
85 % Nylon 6 + 15 % TA
80 % Nylon 6 + 20 % TA
70 % Nylon 6 + 30 % TA
Tannic acid (TA)
Nylon
95% Nylon + 5% TA
90% Nylon + 10% TA
85% Nylon + 15% TA
80% Nylon + 20% TA
70% Nylon + 30% TA
40
20
0
100
200
300
400
500
600
o
Deriv. Weight (%/ C)
1.5
1.0
Tannic acid (TA)
Nylon
95% Nylon + 5% TA
90% Nylon + 10% TA
85% Nylon + 15% TA
80% Nylon + 20% TA
70% Nylon + 30% TA
Conclusion
We have demonstrated that tannic acid can be
used as a bio-based intumescent, char-forming
additive for Nylon 6. Tannic acid is highly
intumescent and can produce up to 27% char by
weight under nitrogen atmosphere at 750 oC. Melt
blending of tannic acid with Nylon 6 causes a slight
decrease in thermal stability. Good char forming
ability was observed in blends both under nitrogen
and air in temperature range of 500-600 oC. A 50%
reduction in HRC was observed in blends containing
30% by weight tannic acid. Considering the fact that
there are very few commercial flame retardant
additives available for Nylon 6, this study
demonstrate that reasonable reduction of HRC is
achievable even with these bio-based options.
(b)
0.5
0.0
100
200
300
400
500
600
THR
(KJ/g)
5.9
31.0
30.1
31.6
28.6
27.1
24.4
700
o
Temperature ( C)
HRC
(J/gK)
160
687
657
636
508
490
333
700
o
Temperature ( C)
Figure 4(a) TGA and 4(b) DTGA curves of tannic
acid, Nylon 6 and Nylon 6 blended with tannic acid
under air
Acknowledgement
Table 3 summarizes the results from PCFC for
the Nylon 6/tannic acid blends. The THR decreases
with increasing amount of tannic acid. HRC has been
used to evaluate the effect of intumescent
phosphorous-containing flame retardants in nylon. It
is reported that blends of Nylon 6 with 20% (by
weight) phosphorus-based flame retardants (Exolit®
OP1312) exhibit intumescent behavior and over 50%
reduction in HRC [19]. With the same loading of
tannic acid in Nylon 6 the reduction in HRC is
around 29%. Moreover our results show that HRC
values drops significantly to about 50% of that of
pure Nylon when tannic acid loading increases to
30%. At this loading there is also a 20% reduction in
THR of Nylon 6. This significant reduction in HRC
indicates the interaction between tannic acid and
nylon degradation products since HRC relies on the
rate of oxygen consumption during oxidative
degradation of pyrolyzed products. The evaluation of
mechanical properties of these blends is in progress. The authors would thank US Army Natick
Soldier Research Development and Engineering
Center (Grant Number: W911NF-11-D-0001/DO
0190/TCN 13017) for providing financial support for
this project.
4
SPE ANTEC™ Indianapolis 2016 / 1414
References
1. Townsend Solutions, Townsend's Eighth Report on
the Global Plastics Additives Market, Houston,
TX (2012).
2. O. Segev, A. Kushmaro, A. Brenner, Journal of
Environmental Research and Public Health, 6,
478 (2009).
3. Y.X. Ou, J. Wu, J. Wang, M., Feng, Z. Peng,
Zhongguo Suliao, 17, 61 (2003).
4. C. Ke, J. Li, K. Fang, Q. Zhu, J., Zhu, Q. Yan,
Polymers for Advanced Technologies 22, 2237
(2011).
5. H. Wu, M. Krita, K. Mourad, H. Joseph, Textile
Research Journal, 84, 1106 (2014).
6. S. V. Levchik, G. Camino, L. Costa, G. F.
Levchik, Fire and Materials, 19, 1 (1995).
7. S. V. Levchik, G. F. Levchik, A. F. Selevich, G.
Camino, L. Costa, Proc. Beijing Int.
Symp./Exhib. Flame Retard. 2nd, 197, (1993).
8. S. V. Levchik, L. Costa, G.Camino, Polymer
Degradation and Stability, 36, 229, (1992).
9. C. Wilkie, A. Morgan, Fire Retardancy of
Polymeric Materials, Second Edition, CRC
press, 130 (2009).
10. J. Gilman, S. Ritchic, T. Kashiwagi, SAMPE
Symposium and Exposition, 41st Proceedings.
41, 708 (1996).
11. K. Freudenberg, Die Chemie der Naturlichen
Gerbstoffe, Springer-Verlag (1920).
12. S. Nikkeshi, G. Kurokawa, US 6624258 (2003).
13.
E.T.N., Bisanda W.O. Ogola, J. V. Tesha,
Cement & Concrete Composites, 25, 593 (2003).
14. Z. Xia, A. Singh, W. Kiratitanavit, R. Mosurkal,
J. Kumar, R. Nagarajan, Thermochimica Acta
(submitted).
15. S. Ravichandran, R. Bouldin, J. Kumar, R.
Nagarajan, Journal of Cleaner Production, 19,
454 (2011).
16. C. Potter. AATCC Review, September/October,
45 (2012).
17. S. Bittencourt, M. Fernandes, F. Silva, K. Lima,
W. Hechenleitner G. Pineda, “Waste and
Biomass Valorization, 1, 323 (2010).
18. S.V. Levchik, G.F. Levchik, A.I. Balabanovich,
G. Camino, L. Costa, Polymer Degradation and
Stability, 54, 2-3 (1996).
19. H. Wu, Master thesis at the University of Texas at
Austin (2012).
5
SPE ANTEC™ Indianapolis 2016 / 1415