Renewable Cardanol-Based Star-Shaped Prepolymer Containing a

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Renewable Cardanol Based Star-shaped Pre-polymer Containing Phosphazene Core as Potential Bio-based Green Fire-Retardant Coatings Hong-Xia Ma, Jun-jie Li, Jin-Jun Qiu, Yun Liu, and Cheng-Mei Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01714 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 27, 2016

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ACS Sustainable Chemistry & Engineering

RTICLE A Renewable Cardanol-based Star-shaped Pre-polymer Containing a Phosphazene Core as a Potential Bio-based Green Fire-retardant Coating Hong-Xia Maa,c, Jun- Jie Lia, Jin-Jun Qiua, Yun Liub, Cheng-Mei Liua* a

College of Chemistry and Chemical Engineering, Key Laboratory for Large-formal Battery Materials and Systems, Ministry of Education, Huazhong University of Science and Technology, Wuhan 430074, China. b Department of Chemical and Environmental Engineering, Jianghan University, Wuhan 434023, China c Department of Chemical and Environmental Engineering, Wuhan Institute of Bioengineering, Wuhan 430415, China *Corresponding author: [email protected]

ABSTRACT: A novel star-shaped cardanol oligomer (HCPP) derived from cardanol and hexachlorocyclotriphosphazene was prepared, and its structure was identified using proton nuclear magnetic resonance (1H NMR,

13

C NMR,

31

P NMR) and fourier transform infrared

spectroscopy (FTIR) techniques. The HCPP underwent a thermal-initiated curing reaction both with and without catalyst. Oxygen was essential to this curing reaction, and the film preparation process was very simple. The curing reaction of HCPP was monitored by differential scanning calorimeter (DSC) and FTIR methods. The catalyst (cobalt naphthenate) could remarkably improve the curing reaction speed. The cardanol-based cured films show excellent thermal stabilities. Interestingly, the char yield of the film cured at 800°C in nitrogen is above 29%, implying that the fire-retardancy of cardanol-based polymers can be notably increased by in traducing a hard phosphazene core

into

the

structure.

Meanwhile,

all

cured

films

are

highly

transparent

and

have

Tg values

above

50°C.

KEYWORDS: Cardanol, Phosphazene, Star-shaped precursor, Thermal curing reaction, Bio-based coatings, Green fire-retardant

■INTRODUCTION

cardanol with other vegetable oils (Figure 1), such as soybean oil

Currently, sustainable development is one of the most popular

and castor oil, it possesses a reactive phenolic hydroxyl group in

topics both in daily life and scientific fields. Unfortunately,

addition to an unsaturated C15alkyl chain, which makes it a reservoir

worldwide operations presently depend mainly on fossil resources,

for bio-based fine chemicals and value-added polymers by chemical

which are not inexhaustible. Considering the needs of future

modification of the benzene ring and double bond

generations, we should do our best to change from the current

declared cardanol derivatives include cardanol-based benzoxazine

fossil resources-dependent lifestyle to a renewable resources-

resins14-19, epoxy resin and its curing agents7,20-23, cardanol

dependent life model to meet the demands of sustainable

oligomers

, cardanol-based acrylates

development. One of these choices is utilizing renewable resources,

polyurethane

37-41

such as plant oils, instead of fossil resources to prepare all types of

properties for acting as surfactants

1, 3, 6-13

24-29

45-46

30-36

, and

. The

polyol

for

. Cardanol derivatives also show excellent 42-44

, modifiers for natural

, compatibilizers for biopolymers47, green nanocarriers

supplies to meet the present demands. To reach this goal, we

rubber

should develop cost-effective technologies for the conversion of

for chlorogenic acid

renewable natural resources into high value-added end products.

researchers have also paid attention to cardanol-based fire-

Cardanol, a natural alkyl-phenol mixture obtained from cashew nut

retardants

shell liquid (CNSL), is an agricultural by-product, and its highly

cardanol derivatives were possible choices for preparing fire-

efficient utilization has become increasingly urgent. Every year, a

resistant wood products. However, from the viewpoint of

large amount of cardanol (over 0.3 million tons per year) is

environmental protection, brominated-cardanol derivatives may

1-5

48

53-54

and matrices for biocomposites

49-52

. Some

and the results showed that the brominated-

produced in Asia, Africa and South America . Among the types of

negatively impact some side-effects for sustainable development.

inedible oils, cardanol can be regarded as one of best renewable

To overcome the shortcomings of brominated compounds, the

resources to be used as a feedstock for the preparation of fine

addition of a phosphorus atom into the chemical structures of bio-

chemicals and materials. Comparing the structural characteristics of

based polymers can remarkably improve their fire-retardancy as

ACS Paragon Plus Environment

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well as electronic conduction, metal adhesion and anti-corrosion 55

properties .

To

meet

the

demands

of

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Fourier transform infrared (FTIR) spectroscopy was used to monitor

sustainability,

the curing reaction of HCPP under different conditions, and all

phosphorylation of renewable resources has attracted much

experiments were run on a Bruker Vertex 70 FTIR spectrometer.

attention recently, and some phosphorus-containing cardanol

Elemental analysis was carried out on a German Vario Micro cube

56-60

.

micro-analyzer. A differential scanning calorimeter (DSC) was used

However, most phosphorus-containing cardanol derivatives only act

to analyze thermal transitions during the polymerization of HCPP,

as additives for other materials, and there are no reports

and all tests were carried out on a 204F1 DSC analyzer from Netzsch

concerning the preparation of phosphorus-containing cardanol

instruments (scanning rate: 10°C/min; nitrogen flow rate: 20

polymers. In this paper, a novel star-shaped cardanol derivative

mL/min; test temperature range: ambient temperature–200°C). The

with a hard phosphazene core (HCPP, Scheme 1, HCPP indicates six

thermal

(hexa) cardanol units connected to the phosphazene core) was

thermogravimetric analysis (TGA) on a Netzsch high resolution STA

prepared using a one-pot method. The hexa-armed cardanol

409PC thermogravimetric analyzer under nitrogen or air purge

oligomer can be cured by simple heating under air without any

conditions at a gas flow rate of 50 mL/min. The scanning

catalyst to obtain crosslinked polymers with improved properties.

temperature range was from ambient temperature to 850°C with a

Cobalt naphthenate can accelerate the curing reaction and reduce

heating

the curing time. All crosslinked films are highly transparent with a

measurements were conducted on an oxygen index instrument

yellow appearance and show high thermal stability as well as

(ZY6155A oxygen index display apparatus, China) according to

excellent fire-retardancy.

ASTM D 2863-97 (test specimen:100 mm × 6.5 mm × 0.2 mm)

derivatives have been prepared for different purposes

HC

O

H2C

O

of

crosslinked

20°C/min.

films

Limiting

were

oxygen

studied

index

by

(LOI)

thermal analysis (DMA) method was used to measure the storage

O O

rate

of

procedure for self-supported samples. The dynamic mechanical

OH

H2C

stabilities

R

modulus (E’) and tanδ of the cured films for estimating their

O R=

mechanical properties under a fixed testing frequency of 1Hz in a tensile model. A Perkin Elmer Diamond DMA was used for all tests,

O

Cardanol

Soybean oil

and specimens (50 × 10 × 0.2 mm) were studied in the range of

Figure1. Structures of triglycerides and cardanol.

ambient temperature to 200°C at a heating rate of 4°C/min. The

■EXPERIMENTAL SECTION Materials Cardanol was purchased from the Shanghai Sci. & Tech Co., Ltd.,

tensile properties of the cured films were measured on a microcomputer-controlled

electron

universal

tensile

testing

China. Hexachlorocyclotriphosphazene [N3P3Cl6, trimer] was purified

machine (CMT4104, Switzerland) at room temperature. The

by recrystallization from dried hexane followed by sublimation at

viscosities of the HCPP samples were measured with a digital

60°C under vacuum twice. Other chemicals and solvents, such as

viscosity meter (LDV-2+PRO, China). The thermal conductivities

cobalt(II) naphthenate(Co 8.0%), sodium hydroxide potassium

were tested using a solid thermal conductivity meter (Hotline

carbonate, magnesium sulfate, acetonitrile and ethyl acetate, were

method, XIATECH TC3000 instrument, China). The evaluation of the

obtained from the Sinopharm Chemical Reagent Co., Ltd., China.

scratch hardness of the cured films was carried out by pencil

Characterization and measurements

scratch hardness according to GBT6739-1996. The gloss was

1

13

31

P NMR) spectroscopy

determined using a Gloss Checker JKG gloss meter (Tianjin Jingke

was utilized for verifying the chemical structures of the synthesized

Materials Testing Co., China) according to ASTM D523. Transmission

compounds. The tests were run on a Bruker AV400NMR (Bremen,

electron microscopy (TEM) images were observed on an HI TACHI

Germany) spectrometer at a proton frequency of 400 MHz as well

JEM2100F microscope at an accelerating voltage of 200 kV. Gel

as the corresponding carbon and phosphorus frequencies using

permeation chromatography (GPC) was performed on a Waters 510

chloroform (CDCl3) as the solvent containing small amount of

pump with a Waters 410 differential RI detector. The polydispersity

tetramethylsilane (TMS) as an internal standard. Signals were

index was used to estimate the purity of HCPP. An Agilent 4294A

Nuclear magnetic resonance ( H,

1

C and

averaged from 256 transients for H NMR,

13

C NMR and

31

P NMR.

Precision Impedance Analyzer was used to measure the dielectric


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constants and dielectric losses of cured films by the two parallel

crosslinking of unsaturated alkyl chains and producing a

plate mode; all tests were carried out at ambient temperature in an

transparent, non-sticky and scratch-free film. With cobalt

air atmosphere in the frequency range of 100Hz-1MHz.

naphthenate as a catalyst, the curing reaction finished within

Synthesis of the HCPP

one hour.

In a 500mL three-necked round-bottom flask equipped with an argon inlet and magnetic stirring, dried acetonitrile(150mL), cardanol (70.00g, 0.231mol, 8eq.) activated K2CO3 (40.00g, 0.288mol, 10eq.), and hexachlorocyclotriphosphazene (10.00g, 0.0288mol,1eq.)

in

50mL

of

acetonitrile

were

added

sequentially. Then, the mixture reaction was heated to 85°C for one hour in an oil bath and maintained at this temperature for 36h. At the end of the reaction, the flask was cooled to ambient temperature, and the insoluble matter of the reaction mixture was filtered off (with K2CO3, produced KCl and possible crosslinking products remaining).The filtered precipitate was washed with a acetonitrile to remove the remaining cardanol. Residues were dissolved in ethyl acetate, and then, they were washed with distilled water, 5% NaOH to remove any water and impurities soluble in basic solution, and finally with distilled water, until the inorganic phase was neutral. The

Scheme 1. Synthetic route for the preparation of star-shaped HCPP.

■RESULTS AND DISCUSSION Preparation and characterization of HCPP

organic phase was dried (MgSO4) overnight, and ethyl acetate was removed by vacuum concentration, resulting in a brownish yellow transparent liquid

(46.6g, yield, 86%). IR -1

The six-armed cardanol derivative with a hard phosphazene core was prepared according to Scheme 1. Many researchers61 have proven that hexachlorocyclotriphosphazene is a highly active

(KBr): 3010, 2923, 2852,1644, 1583, 1210, and 983cm ; 1

intermediate that possesses six replaceable chlorine atoms linked HNMR (CDCl3, 400 MHz) δ: 7.1-6.8(24.00H), 5.9-5.8(2.69H),

5.5-5.3(20.65H),

5.1-5.0(5.16H),

2.8(13.99H),

2.6(12.34H),

2.0(19.35H), 1.5-1.3 (99.55H), and 1.0-0.9 (15.23H);13CNMR

to

a

phosphazene

ring.

Such

characteristics

endow

hexachlorocyclotriphosphazene a high degree of tailorability through appropriate choices of substituent groups in synthesis

(CDCl3, 400MHz) δ: 150.74, 144.36, 136.68, 130.38, 129.95, 128.9, 127.60, 126.84, 124.51, 120.81, 117.97, 114.65,35.76, 31.25, 29.77,29.31, 29.02,27.26, 25.60, 22.69, and 14.14;

31

procedures.

Because

the

environmental

friendliness

of

phosphazene derivatives was proven many years ago, introducing a P phosphazene component into renewable resources will not change

NMR (CDCl3, 400MHz) δ: 8.27.Elemental analysis Calcd.(%) for C126H210N3O6P3: C, 77.49; H,10.76; and N, 2.15. Found: C, 76.90;

62

the environmental friendliness of the final products . HCPP was prepared through a nucleophilic substitution reaction between

H, 9.65; and N, 2.20. cardanol

and

hexachlorocyclotriphosphazene

under

basic

Preparation of a crosslinked polymeric film from HCPP

conditions. K2CO3 was selected as the reaction reagent for its

A solution casting method was adopted to prepare a thin

abilities to form a phenolic oxygen anion (PhO ) and act as an

crosslinked film. A chloroform solution of HCPP was slowly cast

absorbent for the resulting water. Therefore, K2CO3 was provided in

on a cleaned glass plate and left at room temperature for an

excess. Acetonitrile is a suitable solvent for this preparation

appropriate time. Evaporation of all the solvent produced a

because its high polarity is favorable for PhO- formation and a

sticky thin film on the glass slide. Then, this sticky film was

gradual substitution reaction. Moreover, the solubilities of HCPP

exposed to air at ambient conditions for several days or

and cardanol in acetonitrile are quite different, and the unchanged

-

heating at 150°C in air for several hours, causing the oxidative



Page 4 of 12

cardanol can be easily washed away by acetonitrile after the

of HCPP, the peak at 5.6 (peak 11in Figure 3A (a)) corresponding to

reaction.

the proton of the –OH in cardanol disappears, indicating the

The FTIR technique was employed first to study the structures of

conversion of the phenolic hydroxyl group.

cardanol and HCPP. Comparing the FTIR spectrum of HCPP with that

Figure 3B shows the 13C NMR spectrum of HCPP. The chemical shift

of cardanol (shown in Figure 2), most of the characteristic peaks for

at 150.74 ppm results from the aromatic carbon of Ph-O-P (C1), and

cardanol also appear in the HCPP spectrum except for two distinct

the signal at 144.36 ppm originates from the aromatic carbon of Ph-

changes. First, the typical peaks for the phenolic hydroxyl group

R (C2). The signals at 136.68 ppm–114.65 ppm are due to the

−1

(3447 and 1347 cm ) are absent because of the conversion of the

unsaturated carbon atoms of other aromatic carbons and the side

HO–Ph bond into a P-O-Ph bond for a substitution reaction, and

chain, while the signals at 35.76 ppm–22.69ppm are attributed to

−1

such a change resulted in a new peak (located at 983cm ), which is

the CH2 carbon atom on the side chain. The chemical shift of the

attributed to the P–O–Ph deformation vibration. Second, the

-CH3 carbon is located at 14.14ppm. The peak located at 77.05 ppm

characteristic features of the phosphazene ring found at 1210 cm

−1

is assigned to deuterated chloroform (CDCl3).

indicate that the cardanol segments have been successfully introduced onto the phosphazene ring.

Figure 2. The FTIR spectra of cardanol and HCPP. To further verify the chemical structure of HCPP, NMR was also measured, and the spectra are shown in Figure 3.The peak pattern of HCPP was very similar to that of cardanol. In Figure 3A(a)the peaks(a,b and c) at approximately 6.1-6.3 ppm correspond to the protons on the benzene ring located next to the two phenolic hydroxyl groups29, but they disappear in Figure 3A(b), as these groups may be removed by the 5% NaOH solution. The characteristic peaks at 6.8-7.1 ppm correspond to the protons on the benzene ring. The ratio of the integral area was 24.00, corresponding to the number of hydrogen atoms of six benzene rings containing four hydrogen atoms each. The peaks at 5.5-5.3 (peak 5) are ascribed to the unsaturated double bonds of the side chain, while those at 5.9-5.8 (peak 8) and 5.1–5.0 (peak 9) are ascribed to the unsaturated double bonds of the end of the side chain. The ratio of these peaks was 2.69:5.16, while the theoretical value was 1:2. The signals at 2.8 (peaks 6 and 7), 2.6 (peak 1), 2.0 (peak 4), 1.3−1.5 (peak 2), and 0.9−1.0 (peak 10) are from the 21

cardanol alkyl chain . Comparing the results of cardanol with those

Figure 3. (A):1H NMR spectra of cardanol (a) and HCPP (b); (B):13C 31

NMR spectrum of HCPP; and (C): P NMR spectrum of HCPP.



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Figure


3C

displays

the

31

PNMR

spectra

of

HCPP

and

Curing reaction of HCPP determined using DSC

hexachlorocyclotriphosphazene. It is obvious that only one peak

The crosslinking reaction of HCPP was first studied by DSC in

appears at 8.27ppm, and the peak at 20.00ppm disappears. This

different environments to understand oxygen’s role in the curing

result is consistent with the expected structure of HCPP and implies

reaction. The DSC results are shown in Figure 5.When the test was

that the substitution reaction onto the cyclotriphosphazene ring

performed under aerobic conditions (in air), a wide exothermic

was nearly complete. The purity of purified HCPP was determined

peak was observed in the temperature range of 100–160°C with a

by GPC (Figure 4A). There is only one peak at 8.22 minutes, and the

peak maximum at 138°C. However, under anaerobic conditions

polydispersity index (PDI) was 1.12. The PDI value indicates that the

(nitrogen atmosphere), no exothermic peak appeared up to 160°C.

weight-average molecular weight of HCPP is very close to its

The absence of such a peak indeed suggests that the thermal curing

number-average molecular weight.

occurs through oxidative crosslinking and the crosslinking mechanism is the autoxidation of the unsaturated groups in the side chain (Scheme2). The free radicals are generated first by hydrogen abstraction on the methylene groups located between two double bonds, and then, additional free radicals are produced with the aid of oxygen, such as LOO·, LO·, and L· (L=cardanol lipid chain) during the autoxidation. These free radicals underwent a coupling reaction to form crosslinking films. The present results are consistent with those found in the literature13, 33, 58.

Figure 4. (A): The GPC process curve of HCPP and (B): The viscosity

Scheme 2. Curing mechanismof HCPP.

vs storage temperature and time plots of HCPP. The effect of the temperature on the fluidity of HCPP in air was observed by the viscosity behavior (Figure 4B), which indicates that the temperature affects the viscosity of HCPP significantly. With a temperature increase, the viscosity decreases rapidly from 293cp (25°C) to 43cp (78°C), which makes HCPP very convenient for processing

and

easily

mixable

with

other

components.

Simultaneously, in air, the viscosity of HCPP increases very slowly with the extension of storage time at ambient temperature because of a very slow autoxidative crosslinking reaction of the unsaturated side chain (inside picture in Figure 4B)

13, 25, 33, 40, 58

, indicating that

Figure 5. DSC thermograms of HCPP in different atmospheres. The curing reaction of HCPP monitored by FTIR

HCPP has the ability of self-curing with the aid of oxygen. However,

As mentioned above, the viscosity of HCPP increased gradually in air,

this tendency can be delayed by adding a free radical inhibitor or

indicating that the oxidative crosslinking reaction of double bonds

storage under an inert atmosphere.

occurred automatically in the presence of oxygen. The details of the



Page 6 of 12

slow curing reaction were monitored by FTIR, and the results are shown in Figure 6. The experimental results show that the disappearance time of the olefinic C–H of HCPP is over 14 days at room temperature (Figure 6A). The typical peak implying the -1

formation of carbonyl groups at 1722cm was also observed after -1

5days, and the wide peak at approximately 3450cm was due to carboxylic acid or a hydroxyl group. The crosslinking reaction can be accelerated remarkably by adding cobalt naphthenate as a catalyst. Figure 6A shows that the disappearance time of the characteristic

catalyst also accelerated the formation of carbonyl and hydroxyl

Figure 6.FTIR spectra for the curing of pure HCPP: (A) no cobalt naphthenate at room temperature in air; (B) with 5% cobalt naphthenate at room temperature in air; (C) no cobalt at 150°C in air; and (D) with 5% catalyst at 150°C in air.

groups. Extending the curing time to 72 hours (Figure 6B), no

The curing temperature is another key factor affecting the curing

remarkable difference was found in the spectrum when comparing

reaction. At 150°C without catalyst, the characteristic olefinic C–H

it with that of the film cured for 3 hours.

stretching of pure HCPP at 3008cm

-1

-1

olefinic C–H stretching at 3008 cm and 1635cm was reduced to less than 3 hours with a 5% catalyst solution. Meanwhile, the

-1

and 1635 cm

-1

did not

disappear completely after 7 hours of heating, and with extension of the curing time to 24 hours, no obvious changes were found (Figure 6C). In contrast, when a 5% cobalt naphthenate solution was used as a catalyst, the disappearance time of the characteristic -1

-1

olefinic C–H stretching at 3008 cm and 1635 cm was reduced to less than 30 minutes with heating at 150°C (Figure 6D). Meanwhile, with or without catalyst, the formation of carbonyl groups cannot -1

be avoided, as an intensive peak always appeared at 1722cm with -1

a weak peak at 3440-3450 cm . Thermal stabilities of the crosslinked polymers As is well known, all plant oils, including cardanol and its derivatives, are flammable, and their thermal stabilities do not always meet the demands for practical application. The design and preparation of cardanol-based biomass with high performance is a current topic. A great deal of research has certified that phosphazene derivatives 61

always show high thermal stability and fire-retardancy . Cured HCPP consists of a hard phosphazene core and six crosslinkable arms, and such a structure is favored to improve the thermal properties of the final polymer. The thermal stability of the crosslinked HCPP film was studied by TGA, and the results, such as the temperature of 5% weight loss (T5%), temperature of 10% weight loss (T10%), carbon residue rate at 800°C (YC), and calculated limited oxygen index (LOI), were obtained from TGA curves. The LOI 55

can be calculated according to the empirical formula : LOI=17.5+ 0.4YC



(1)

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where, Yc is the char yield of the cured sample at 800°C in a

TGA profiles of the cured HCPP films, and the corresponding data

nitrogen atmosphere. Figure 7A shows the TGA curves of cured

are collected in Table 1. The thermal stability of the cured film

films of pure HCPP in N2and air atmospheres. The two samples are

depends on the catalyst dosage and improved gradually as the

stable up to 300°C and start losing weight above this temperature.

catalyst loading increased from 0wt% to 5wt%. The T5%of P1-P3

In the N2 atmosphere, the degradation process of the cured film is

increases from 301°C to 350°C, and the char yield increases from

relatively simple; the rapid degradation process occurs in the range

29.7% to 37.9%, which is due to cobalt naphthenate acting as a

of 300-500°C. The T5% and T10% are observed at approximately 300°C

catalyst and speeding up the crosslinking reaction of the double

and 310°C, respectively, and the char yield at 800°C is

bond. The unexpectedly high char-formation ability is due to the

approximately 30%. In air, the cured film shows a similar

introduction of inorganic phosphazene. Meanwhile, the six-

degradation process as in N2, except the char yield decreases to

membered unsaturated side chain can form a crosslinked network

approximately 20%. The high thermal stability of the cured film is

to increase the char-forming ability. The calculated LOI values of the

derived from the phosphazene and crosslinking structure.

thermally cured films are over 29%, and the measured LOI values

Interestingly, the calculated LOI of the cured film is 29.5, which is

are 27-28, also indicating that cured HCPP is a novel bio-based fire-

very close to the measured LOI value of 27, implying high fire-

retardant polymer. Meanwhile, the thermal stability of the cured

retardancy of the cured film. In fact, the crosslinked film of HCPP is

film decreases gradually as the cobalt naphthenate loading

self-extinguishing in air. Fire-retardancy is very important in the

increases to above 5%, especially the initial decomposition

practical application of a renewable resource, but most vegetable

temperature. For example, when adding 10 wt% of catalyst, the T5%

oil derivatives are flammable63. This study provides a simple and

of P5 goes down to 233°C, where as the T5% of P3 is as high as 350°C.

economic

Simultaneously, the char yield also decreases slightly with increased

way

to

prepare

fire-retardant

cardanol-based

biopolymers.

catalyst loading, which is probably attributed to the earlier

The flame retardancy of cured HCPP is attributed to the presence of 64

the phosphazene moiety in its structure . According to elemental analysis results, the nitrogen content in cured HCPP

decomposition of the thermally unstable additive (more cobalt naphthenate).

is 2.20% and

the calculated phosphorus content is 5.00%. Phosphorus and nitrogen create a unique combination to improve the fireretardancy of common polymers, and such a combination has been regarded as eco-friendly. The presence of cyclotriphosphazene moieties on the backbone of cured HCPP can promote the formation of intumescent carbonaceous chars and thus enhance the flame-retardancy in the manner of a condensed phase (intumescent mechanism). Following this mechanism, an organic material can swell when exposed to fire or heat to form a foamed mass, usually carbonaceous, which acts in the condensed phase and promotes char formation on the surface as a barrier to inhibit gaseous products from diffusing to the flame and shielding the polymer surface from heat and air. Moreover, the crosslinked polymeric film prepared in this study is favorable for condensed phase flame-retardant action, which has been confirmed as a good carbonization agent. Therefore, the combustion of the polymer is retarded. The influence of the catalyst dosage on the thermal stability of the

Figure 7. TGA curves of films: (A) P1 in N2and air and (B) P1-P5 in N2.

cured HCPP film was also investigated by TGA. Figure 7B gives the



Page 8 of 12

Table 1. The thermal stabilities of cured HCPP films with different catalyst (cobalt naphthenate) loadings at 150°C in air. Sample Catalyst Curing T5% LOI T10% YC a b (wt%) Time(h) (°C) (°C) （%） P1 0 5 301 322 29.7 29.4 P2 3 2 310 337 31.4 30.1 P3 5 2 350 376 37.9 32.7 P4 8 2 326 387 29.6 29.3 P5 10 2 233 301 29.4 29.2 a: Calculated value of the LOI. b: Measured value of the LOI.

27 27 28 27 27

The mechanical properties of cured HCPP films The influence of the curing conditions on the dynamic mechanical properties of the films was investigated by DMA (Figure 8A), and the corresponding data are collected in Table2. The glass transition temperatures (Tg) of the three cured HCPP films (P6, P7, and P8) were 56°C, 54°C, and 60°C, respectively, according to the maximum of the tanδ peak. The storage modulus in the glassy state increased from 140 MPa for P6 to 1150 MPa for P8. At room temperature, the curing reaction is very slow, and the formation of the crosslinked network takes a relatively long time. At higher curing temperatures, the curing reaction was accelerated both by heating and catalyst, resulting in a highly cross-linked polymer film. The crosslinking density (υe) can be calculated from the storage modulus Eˊ,

Figure8. (A)DMA curves of P6-P8 and (B) the stress-strain curves of P6-P8.

measured at the rubbery plateau by the theory of rubbery elasticity, 65

which is given by :

A

B

C

（2））

υe = E2′/3RT

where R is the gas constant, T is the absolute temperature (rubbery plateau modulus at (Tg+ 50 K) and E2′ is the tensile storage modulus corresponding to the T.

Figure 9.Digital photographs of a cured HCPP film (A) and TEM

From Table 2, the crosslinking densities (υe) of P6, P7, and P8 are

images (B: 50nm, C: 100nm) of P8.

0.78, 2.95 and 3.23, respectively. The film cured at room temperature without catalyst shows the lowest crosslinking density

The cured HCPP is a highly transparent yellow film (Figure 9A) with

even after 30 days of reaction. Heating can remarkably accelerate

high gloss (60° gloss: above 106). From the TEM images (Figures 9B

the curing reaction and improve the crosslinking density. The strain-

and 9C) we can observe that the cured film is homogeneous, and

stress curves of cured films (Figure 8B) indicated that the tensile

the black area maybe the result of a pin hole. The scratch-free film

strength increases from 3.46MPa for P6 to 4.70MPa for P8, while

shows high resistance to organic solvents and is insoluble in any

the elongation at the break decreases from 9.8% for P6 to 4.3% for

common solvent due to its high crosslinking density. The pencil-

P8. The improvement in the mechanical properties of the films

scratch hardness of the samples is approximately 2H-3H (Table 2),

depending on the curing conditions can be attributed to the

which is good enough for some practical uses. The thermal

increased crosslinking density of the polymer networks.

conductivity of the cured HCPP film is approximately 0.1280 W/m·K (Table 2) at 25°C and it is possibly a good matrix for composites with high thermal conductivities.



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Table 2. The characteristics of the membrane of cured HCPP. Samples

P6

P7

P8

Curing conditions

R.T. 30days 2H 106 0.1276

150°C, 30min 3H 118 0.1280

150°C, 120min 3H 121 0.1270

140 56 379 7.33 0.78 3.5 9.8 61.5 4.5 0.046

840 54 377 27.80 2.95 4.4 6.9 111.4 3.8 0.037

1150 60 383 30.84 3.23 4.7 4.3 138.6 3.2 0.033

Pencil-Scratch hardness 60° Gloss

Thermal conductivity (W/m·K) Eˊ(MPa) at R.M. Tg(°C) Tg+50(K) E2 ˊ (MPa)at (Tg+50K) 3 -3 υe(10 mol. m ) Tensile strength (MPa) Elongation (%) Elastic modulus(MPa) Dielectric constant(10MHz) Dielectric loss(10MHz)

Dielectric properties of cured HCPP films The dielectric property of a bio-based polymer is one of the key characteristics for its application in microelectronic devices and 66

packaging . The dependencies of the dielectric constants and

Figure 10.The dielectric constant (a) and loss (b) of cured HCPP films (5% cobalt naphthenate).

dielectric losses of three samples (P6, P7, and P8) were studied in

■CONCLUSIONS

the frequency range of 1000 Hz-10MHz at room temperature

The thermal stabilities and fire-retardancies of plant-oil-based

(Figure 10), and the corresponding data are collected in Table 2. The

polymers are not high enough to meet the demands of practical

three samples showed relatively high dielectric constants (4.5, 3.8,

application. In this paper, we provide a cost-effective method to

and 3.2 at 10MHz, respectively), and the values depend on the

prepare a bio-based polymer with improved heat resistance and

crosslinking density. It is well-known that the dielectric constant is

lower flammability. Star-shaped cardanol oligomer with a hard

directly related to the polarizability of the material and could be

phosphazene core (HCPP) can be obtained with a high yield through

increased by increasing the polarizability. Therefore, the dielectric

a nucleophilic substitution reaction between cardanol and

constant and dielectric loss are strongly dependent on its chemical

hexachlorocyclotriphosphazene under basic conditions. HCPP

structure. The high dielectric constant of the cured film is possibly

undergoes a self-curing reaction in air by an autoxidative

due to the polar groups (carboxylic acid, hydroxyl and other

mechanism of the unsaturated double bond in the long alkyl chain.

impurities, such as catalyst) resulting from the curing reaction.

The use of a catalyst, such as cobalt (II) naphthenate, can

Meanwhile, the polar phosphazene ring also contributes to the

remarkably speed up the reaction. The cured film of HCPP shows a

dielectric constant. The polar groups (carboxylic acid and hydroxyl)

high thermal stability, the T5% values of nearly all samples are above

and impurities also affect the dielectric loss of the cured HCPP film.

300°C, and the char yield at 800°C is nearly 30%. However, the

The film cured at room temperature shows the highest loss (0.046)

excessive addition of catalyst will decrease the thermal stability of

due to it having the lowest crosslinking density.

the cured film. The estimated LOI values of all samples are up to 29, and such a result indicates that introducing an inorganic phosphazene ring into the biopolymer structure can largely increase the fire-retardancy of biopolymers. The Tg values of the cured films are over 50°C, and the tensile strengths are larger than 3.5 MPa. The dielectric properties of the cured film are dependent on the



crosslinking density, and increasing the crosslinking density will result in a low dielectric loss.

■ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No. 21274049), and the Natural Science Foundation of Hubei Province, China (Grant No. 2015CFB188).

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For Table of Contents Use Only A Renewable Cardanol-based Star-shaped Pre-polymer Containing a Phosphazene Core as a Potential Bio-based Green Fire-retardant Coating a,c

a

a

b

Hong-Xia Ma , Jun- Jie Li , Jin-Jun Qiu , Yun Liu , Cheng-Mei Liu

a*

a

College of Chemistry and Chemical Engineering, Key Laboratory for Large-formal Battery Materials and Systems, Ministry of Education, Huazhong University of Science and Technology, Wuhan 430074, China. b Department of Chemical and Environmental Engineering, Jianghan University, Wuhan 434023, China c Department of Chemical and Environmental Engineering, Wuhan Institute of Bioengineering, Wuhan 430415, China *Corresponding author: [email protected]

Brief synopsis: Renewable cardanol from Cashew is the key feedstocks and the resulting polymers containing phosphazene units show high thermal stability.



Renewable Cardanol-Based Star-Shaped Prepolymer Containing a

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