nanoparticles as curing and adhesive aid for biobased and...

Monisha, Shukla S and Lochab B (2017)Nanoparticles as curing and adhesive aid forbiobased and petrobased polybenzoxazines.Green Materials,http://dx.doi.org/10.1680/jgrma.17.00004

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Green Materials

Research ArticleReceived 05/01/2017 Accepted 07/07/2017Keywords: green adhesives/renewable resources/sustainable chemistry

ICE Publishing: All rights reserved.

Nanoparticles as curing and adhesive aid forbiobased and petrobased polybenzoxazines
1 Monisha BSc, MSc
*Co

PhD student, Materials Chemistry Lab, Department of Chemistry, Schoolof Natural Sciences, Shiv Nadar University, Gautam Buddha Nagar, India

2 Swapnil Shukla MSc, PhD
Postdoctoral Researcher, Materials Chemistry Lab, Department ofChemistry, School of Natural Sciences, Shiv Nadar University, GautamBuddha Nagar, India
rresponding author e-mail address: [email protected]

3 Bimlesh Lochab MSc, MTech, DPhil*
Assistant Professor, Materials Chemistry Lab, Department ofChemistry, School of Natural Sciences, Shiv Nadar University, GautamBuddha Nagar, India
1 2 3

Alumina nanoparticles were incorporated in blends of benzoxazine monomers synthesized from bio- and petrobasedphenols, cardanol and bisphenol A, respectively, by way of a one-pot solventless methodology. The structures of themonomers were characterized by proton (1H) nuclear magnetic resonance and Fourier transform infraredspectroscopy. Thermal characterization was performed using differential scanning calorimetry and thermogravimetry.The size of bare alumina nanoparticles and those in the polymer nanocomposite was analyzed by powder X-raydiffraction. Blending of alumina particles resulted in lowering of curing temperature of benzoxazine monomer. Theeffect of nanoparticle incorporation was reflected in the curing behavior and adhesive properties. The adhesivestrength is found to be dependent on whether solution or solventless processing is used and on the weight percentof the nanoparticles in the monomer blend. The replacement of 75% bisphenol A content with cardanol in thebenzoxazine resulted in a copolymer which showed a better or comparable adhesive strength. The adhesion wasfound to improve further with the incorporation of alumina nanoparticles. The lower viscosity of cardanol-basedbenzoxazine allowed both solventless synthesis and application of monomers, which is a way forward towardgreener chemistry and low volatile organic compound processing strategies.

NotationTg glass transition temperatureT0 onset curing temperatureTP temperature of exothermic peakDH heat of curing reaction

1. IntroductionPhenol formaldehyde (PF) and epoxy resins are widely used foradhesive applications for a wide variety of substrates. Despitetheir good adhesive strength, ease of availability and low cost,they still have limited applicability toward high-temperatureusage.1,2 The ever-growing automotive and aerospace industriesalways search for adhesive materials that can withstand adverseenvironmental conditions, in particular high-temperature stability.

Recently, polybenzoxazines (PBzs) have appeared as attractivepolymeric materials with a proven higher performance-to-costratio compared with those of conventional resins. PBzs showedpotential in overcoming the limitations of PF and epoxy resinsby suitable design of the materials at the structural level.

Characteristic properties such as nearly zero shrinkage uponcuring, low water absorption, larger processing window, goodthermal stability and chemical resistance make them verygood candidates for adhesive applications. Like epoxy and PFresins, PBz resins possess phenolic and amine moieties that canadhere to the substrate by way of an extensive hydrogen(H)-bonding network with the additional benefit of high-temperature stability.3

However, sustainability is an important aspect that is beingencouraged at the industry level by governmental organizations toadvance ecological understanding with the current rise in higherstandards of living. There is a statistical increase in the price offossil-based raw materials, mainly due to the demand outstrippingsupply. Chemists worldwide are looking for alternative chemicalfeedstocks and developing methodologies for their incorporationin polymeric materials. The authors’ group values suchsustainable efforts and is also active in substituting phenolic rawmaterials in PBz synthesis with renewable phenols. India isconsidered as one of the leading producers of cashew nuts,

1

mailto:[email protected]

Green Materials Nanoparticles as curing and adhesive aidfor biobased and petrobasedpolybenzoxazinesMonisha, Shukla and Lochab

Offprint provided courtesy of www.icevirtuallibrary.comAuthor copy for personal use, not for distribution

sourced from the cashew nut tree, Anacardium occidentale L.Agricultural waste generated by industries processing cashew nutsfor consumption has previously been incinerated, but couldinstead be used for extraction of valuable chemical feedstocks.The extract of the cashew nut tree contains phenol richcompounds called the cashew nut shell liquid (CNSL), which isobtained by way of a simple, versatile heat extractionmethodology.4 CNSL contains nearly 60–70% cardanol, which iscurrently utilized in the authors’ group as a biobased feedstock forthe synthesis of the benzoxazine monomer (Ca).5 The bisphenolA-based benzoxazine monomer (BAa), based on petro-feedstock,is another one of the monomers most studied by the scientificcommunity working in the PBz thrust area. In the current work,the authors report varying the feed ratio (1:3 and 3:1) ofbiobased–petrobased phenols – that is, a cardanol–bisphenol A-based benzoxazine monomer was synthesized in a one-potsolventless methodology. Ratios with higher bisphenol A contentwere omitted to allow the solventless reactions to proceed and theeasy mechanical stirring of the reaction mixture to allowcompletion of the reaction.

The introduction of suitable fillers to reinforce the PBz network isa widely utilized strategy for enhancing the performance ofthermosetting resins by modulating properties such as Tg, charyield and flame and chemical resistance.6

The full realization of this approach is subject to the nature of thefiller, its dispersion and the ensuing interaction with the desiredpolymeric network. Aluminum is reported as one of the mostabundant metals in Earth’s crust with applications spanning a widespectrum of ceramics, cements, catalysts, absorbents and so on, toname a few.7 In the nanophase, a further leap in the physical andstructural properties has widened the scope of its applicabilityimmensely.8 Among several reported nanomaterials, nanoalumina(nano-Al2O3) has come up as a promising alternate owing to itsrelatively high abundance, cost viability and ease of surfacefunctionalization. Moreover, a previous study reported theintroduction of nanoalumina as a filler to reinforce the mechanicalstrength and adhesive strength of epoxy resins.9–11 Theincorporation of alumina in PBz led to an improvement inmechanical strength12 and thermal stability13 by loadings of 83 and7 wt%, respectively. Subsequently, nanoalumina can be expected toreinforce the polymeric structure and assist in ring-openingpolymerization (ROP). To the best of the authors’ knowledge, thepotential of alumina nanoparticles has not been explored yet inthe context of benzoxazines, which provides an impetus to carryout a detailed analysis about the said interaction. In the currentwork, the authors have utilized alumina nanoparticles to study theensuing effects on the curing behaviors of different partiallybiobased benzoxazine blends and the adhesive properties of thecured resins.

The two ratios 3:1 and 1:3 of the Ca–BAa monomer blends wereobtained by changing the feed ratio of the phenolic compound,

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aniline and paraformaldehyde. The monomers were structurallycharacterized by proton (1H) nuclear magnetic resonance (NMR)and Fourier transform infrared (FTIR) spectroscopy. The curingbehaviors of the various resin samples were studied usingdifferential scanning calorimetry (DSC). The thermal stability ofthe cured resins was evaluated using thermogravimetry analysis(TGA). The adhesive strength was measured by lap shear strength(LSS) measurements on stainless steel substrates.

2. Experimental

2.1 MaterialsCardanol was procured from Satya Cashew Chemicals Pvt. Ltd(India); paraformaldehyde and chloroform, from Rankem; aniline,from Merck; bisphenol A, from SRL Pvt. Ltd; and sodium sulfate,from CDH. Alumina nanoparticles (<100 nm, nearly spherical) wereprocured from Nanostructured & Amorphous Materials, Inc., USA.

2.2 Characterization techniques2.2.1 Structural characterizationAn FTIR spectrometer (Nicolet 200) was used for recording theinfrared spectra of the starting material and monomer as a thin filmon a potassium bromide (KBr) disk. A Bruker AC 400MHz FT-NMR spectrometer was used to record the proton NMR of thesamples. Proton NMR was recorded in deuterated chloroform(CDCl3) using tetramethylsilane as an internal standard. TheBrookfield viscosity was determined using a Brookfield digitalviscometer at 43°C with a BS29 spindle. The curing behavior of themonomer blends was evaluated by using a TA 2100 thermal analyzerwith a 910 DSC module. A heating rate of 10°C/min in static airatmosphere and a sample mass of 3–5mg were used. Thebenzoxazine monomers were dried under vacuum at 50°C for 1 hbefore DSC analysis. The isothermal curing of the 3:1 and 1:3(Ca–BAa) monomer blends was done by heating at temperatures of50, 150, 200 and 250°C for 1 h each sequentially in an air oven.However, the 3:1 Ca–BAa blend required further additional curing at250°C for an hour in an air oven. A powder X-ray diffraction(PXRD) analysis was performed on a Bruker D8-Discover usingcopper (Cu) Ka radiation (l = 0·154 nm) to characterize the nature ofthe nanoparticles and to determine their size. The adhesive strengthswere evaluated by determining the LSS of the bonded joints on steelplates with average roughness Ra = 0·9–1·1 µm in accordance withthe ASTM D 100214 standard. Tensile testing machine Instron 5582100 kN equipped with a personal computer for acquiring test data byusing the Merlin software and a cross-head speed of 1·3mm/minwas used. LSS samples were prepared by coating an adhesive (0·12± 0·02 g resin) on a 15 × 15mm2 area. The assembly was clampedwith paper clips (15mm) and cured in an air oven as per the curingcycle mentioned earlier. The values reported are the average of LSSsobtained for three specimens. LSS values for different species weredetermined as per the following relation

LSS ¼ maximum strength � thicknessð Þoverlap length1.



2.3 Synthesis of benzoxazineA synthesis methodology was adapted as reported previously inthe literature.15

2.3.1 Cardanol monomer (Ca)A mixture of cardanol (100 g, 0·33 mol), paraformaldehyde(19·8 g, 0·66 mol) and aniline (30·1 ml, 0·33 mol) was graduallyheated from 50 to 7°C over a period of 1 h and then at 80°C for1 h followed by heating at 90°C for 2 h. The reaction starts at70°C as indicated by the evolution of water, and the reactioncolor changes at 80°C from yellow to red brown. On cooling,water (500 ml) was added and the organic layer was extractedwith chloroform (2 × 100 ml). The organic layers were combinedand washed with water (3 × 100 ml), dried over sodium sulfateand filtered to obtain a red oil. The solvent was removed underreduced pressure, and the residue was dried at 70°C undervacuum to obtain cardanol benzoxazine (Ca) in quantitative yieldas a red-brown oil. nmax (film, potassium bromide disk)/cm−1

3008, 2926, 2853, 1623, 1601, 1579, 1257, 1241, 1031; protonNMR (300 MHz, deuterated chloroform, d parts per million(ppm)): 0·86–0·93 (m, CH3), 1·2–1·7 (m, aliphatic CH2 protons),1·92–2·20 (m, CH2CH=), 2·50 (t, CH2Ar), 2·80–2·90 (m, CH2

(CH=)2), 4·58 (s, ArCH2N–), 4·92–5·12, 5·20–5·50, 5·80–5·89(m, CH=, CH2=CH–, –OCH2N–, HC=CH2), 6·63–6·74 (ArH, m,2H), 7·05–7·30 (ArH, m, 4H).

2.3.2 Ca–BAa monomer blend (3:1)A mixture of cardanol (100 g, 0·33mol), paraformaldehyde (19·8 g,0·66mol) and aniline (30·1ml, 0·33mol) was gradually heated from50 to 70°C over a period of 1 h (indicated by the evolution of water)and then at 80°C (yellow to red brown) for 1 h followed by heatingat 90°C for 2 h (red-brown oil). At 90°C, bisphenol A (12·55 g,0·055mmol), paraformaldehyde (6·60 g, 0·22mol) and aniline

(10·0ml, 0·110mol) were added to the reaction mixture. Themixture was allowed to homogenize at 90°C for 0·5 h, and then thetemperature was raised slowly to 130°C at a heating rate of 20°C/hwith a temperature interval of 10°C. The contents were heated at130°C for 1 h. On cooling, water (500ml) was added, and theorganic layer was extracted with chloroform. The organic layer wascombined and washed with water (3 × 100ml), dried over sodiumsulfate and filtered to obtain a red solution. The solvent was removedunder vacuum, and the residue was dried at 70°C under vacuum toobtain the 3:1 blend of the benzoxazine monomers of cardanol andbisphenol A (Ca–BAa) in quantitative yield as a yellowish-red oil.The other monomer blend (1:3: Ca–BAa) was synthesized similarly.

The dispersion of abrasive filler nanoalumina (<100 nm) in theresin sample solution in acetone was achieved using a probesonicator for ~20 min.

3. Results and discussionAmong the 12 green chemistry principles, in the current paper, theauthors demonstrated monomer synthesis through utilization ofagrowaste as chemical feedstock, solventless reaction, one-pot atomeconomized reaction and no evolution of toxic waste, and the onlyby-product of the reaction is water. In addition, by virtue of the lowviscosity of the medium, solventless processing is also facilitated,enhancing the greener aspect of the whole process. Thecondensation reaction of the phenolic groups of cardanol andbisphenol A with aniline and paraformaldehyde resulted in theformation of monomers in quantitative yields. The solventlesssynthesis of the benzoxazine monomer BAa was found to beincomplete, with trapped unreacted reagents due to issues withstirring of the high-viscosity medium even with a mechanicalstirrer. However, the formation of the Ca–BAa monomer blends(1:3 and 3:1) in one pot was successful, as shown in Scheme 1.

High-viscositymechanical stirring is halted

Incomplete reaction

Ca–BAa blends

Low-viscositymechanical stirring is aided

Solventless synthesis

Solventless synthesis

N

N

N

O O

Sustainable

agrowaste

cardanol

Non-sustainablepetrobasedbisphenol A

–2H2O

CH2O–H2O

BAa

NH2Ca

R

n

O

Scheme 1. Synthesis of Ca–BAa-based blends through solventless methodology

3



The monomer blend formation is believed to be facilitated due tothe low viscosity of the presynthesized Ca monomer, whichallowed swift stirring of the reaction mixture for synthesis ofmonomer blends. Solventless synthesis of monomer blends withthe BAa percentage higher (>75%) than that of the Ca monomerwas not successful due to stirring issues. Therefore, in the currentwork, the monomer blend ratios studied were 1:3 and 3:1, withvariation in cardanol and bisphenol A, respectively.

4

The formation of the Ca monomer was further monitored by protonNMR spectroscopy before the addition of bisphenol A into thereaction mixture. The appearance of characteristic signals at 5·3 ppm(olefin, ArOCH2N) and 4·8 ppm (s, ArCH2N) and their ratio of 3:1suggested the formation of an oxazine structure and, hence, thesuccessful synthesis of Ca monomer.16 The ratio offset is 3:1 ratherthan the expected 1:1, which is attributed to the inherent alkyleneprotons present in cardanol in the region of 5·3 ppm. The protonNMR spectra of the Ca–BAa blends showed the characteristicoxazine signals, which confirmed the formation of the twomonomers. The proton NMR spectrum of 1:3 Ca–BAa is shown inFigure 1. In this blend, the proton NMR integral ratio in the region5·3–4·8 ppm due to (olefin, ArOCH2N)–(ArCH2N) was found to beoffset with a ratio of 1:0·8 rather than 3:1 due to the different feedratios of the phenol compounds and their degree of functionality.

In the FTIR spectra (Figure 2), the characteristic absorption band dueto phenolic O–H (3344 cm−1) was absent, suggesting the completeconversion of hydroxyl groups of cardanol to oxazine ring. Theformation of the oxazine ring due to Ar–C–O oxazine asymmetricand symmetric stretches at 1240 and 1030 cm−1, respectively, wasobserved. The absence of N–H stretch and bending vibrations at3360–3442 and 1619 cm−1 further suggests the absence of unreactedaniline in the monomer. The presence of unreacted phenol wasqualitatively determined using the ferric chloride test. Bisphenol Aand cardanol developed blue and green colors, respectively; however,

:

O

O

R

N

N

N

ArCH2NArH

(CH2)n

–CH3

–CH2(CH=)2

–CH2AR

–CH2CH=

CH=, CH2=CH–

ArOCH2N,

0 ppm1234567

0·12

0·23

1·47

0·28

1·01

0·38

0·20

0·19

0·17

0·76

0·01

0·01

0·02

0·04

0·02

1·00

0·04

0·15

0·39

0·10

0·26

0·83

0·82

0·59

8910

7·37

5

0·11

20·

968

0·99

01·

013

1·03

81·

402

1·49

91·

553

1·67

92·

130

2·58

02·

605

2·63

12·

891

4·29

94·

501

4·58

44·

633

4·66

94·

689

5·10

15·

179

5·41

75·

429

5·45

25·

525

6·74

16·

795

6·82

36·

888

6·93

96·

990

7·01

37·

036

7·06

47·

180

7·20

57·

304

7·33

07·

354

Figure 1. Proton NMR spectrum of 1:3 Ca–BAa blend in deuterated chloroform

Tran

smitt

ance

: %

Wave number: cm–1

Cardanol

10001500300035004000

990−9601260−1230

3335

Ca

1030

Figure 2. FTIR spectra for cardanol and cardanol-based benzoxazine



the monomer blend showed a brown precipitate, suggesting theabsence of unreacted phenol in the monomer.

To confirm the nature of the alumina nanoparticles and their size,PXRD was performed (Figure 3). The appearance and position ofthe observed diffraction peaks match well with those of thestandard peaks of a-alumina (Joint Committee on PowderDiffraction Standards card number 00-042-1468). From Scherrer’sequation,17 the average particle sizes of pristine nanoparticles andthose in the polymer composite were calculated to be ~27 and

31 nm, respectively. The broad peak at 2q of 14–30° in the X-raydiffraction pattern of the nanocomposite suggests the existence ofamorphous domains of the polymer network. The blending ofalumina nanoparticles with the monomer followed by thermalcuring led to a marginal increase in the average particle size,suggesting the minimal aggregation of the nanoparticles in thepolymer nanocomposite. Thermal curing of the benzoxazinemonomer led to the formation of the PBz network due to athermally activated ring-opening reaction that was facilitated dueto ring strain in oxazine functionality. The curing behaviors of themonomer blends with different loadings of nanoparticles werestudied by DSC. The curing transitions were characterized by theonset curing temperature (To), temperature of exothermic peak(Tp) and heat of curing reaction (DH) from the area under thecurve. Typical DSC thermograms of the 3:1 Ca–BAa blend withand without nanoparticle incorporation are shown in Figure 4, andthe curing characteristics (To, Tp and DH) of the monomer blendswith alumina nanoparticles are summarized in Table 1.

The addition of nanoparticles in the benzoxazine monomersshowed a catalyzing effect on the ROP reaction. It was found thatan increase in the weight percent of alumina from 1 to 5 wt% has avery pronounced effect on the To and heat of curing reaction andnearly no effect on Tp. The exploration of alumina particles inassisting the ROP of benzoxazine is novel in the present work. Themost acceptable mechanism so far for the polymerization ofbenzoxazine is the cationic mechanism, which proceeds in twosteps: (a) initially by the ring-opening reaction of the oxazine ringand followed by (b) electrophilic substitution reactions, whicheventually leads to the formation of a cross-linked network. Severalcatalysts ranging from acid catalysts to metal salts18 have beenfound to assist the polymerization reaction. More recently, animproved mechanism for the ROP of benzoxazines was reportedwith the catalyzed reaction of lithium iodide (LiI) salt.19 In theauthors’ case, the lowering of To also suggests the participation ofalumina nanoparticles in a similar manner to mediate the ring-opening reaction. A probable mechanism is proposed for theassistance provided by alumina nanoparticles for the ROP reactionof benzoxazine monomers (Figure 5). The nanoparticles are knownfor higher catalytic activities due to a higher surface area and theexposition of atoms on the surface. The aluminum ion coordinatesto the oxygen atom of the oxazine ring and mediates the formationof an iminium ion intermediate, which then reacts with another

2θ: º

Arb

itrar

y un

its

(012

)

(220

)(1

19)

(300

)(2

14)

(018

)

(116

)

(024

)

(113

)

(110

)(104

)

(a)

908070605040302010

(b)

Figure 3. PXRD patterns of the materials (a) nanoalumina and (b) 1:3poly(Ca–BAa) blend containing 5wt% nanoalumina. The particle sizeof nanoalumina in the composite was calculated using the 104 peakat 2q = 35·1° instead of the intense peak of nanoalumina at2q = 25·5° (012) as it is merged with the broader amorphouspolymer peak in the range of 14–30° in the polymer composite

Hea

t flo

w: m

W

Temperature: ºC

5% nanoalumina

Neat

3:1 Ca–BAa

3:1 Ca–BAa

350300250200150

ΔH = 40·12 J/g

ΔH = 94·96 J/g

Tp = 239ºC

To = 218ºC

To = 195ºC

Tp = 241ºC

Figure 4. DSC thermograms of the neat blend of 3:1 Ca–BAa andthe nanocomposite of the blend with 5 wt% nanoalumina

Table 1. Curing analysis of blends of benzoxazine monomers(Ca–BAa–nanoalumina, static air, heating rate 10°C/min)

Ca–BAa
Nanoalumina:% To: °C Tp: °C DH: J/g
3:1
0 218 241 40 1 212 244 46 3 207 242 77 5 195 239 95
1:3
0 211 231 223 1 196 231 110 3 194 229 114 5 191 228 165
5



oxazine ring (pathway a) or undergoes electrophilic aromaticsubstitution reaction (pathway b) to form an N,O acetal and aMannich bridge, respectively.20 The newly generated acidicphenolic hydroxyl groups, along with alumina, further allow anautocatalytic ROP reaction. It is also reported that the surfacehydroxyl groups of alumina possess both acidic and basicproperties.21 The curing temperature of benzoxazine is catalyzedand lowered by acidic functionalities, suggesting the simultaneousand substantial role played by the surface –OH groups in catalyzingthe ROP through the protonation of the oxazine ring.

The thermal stability of isothermally cured benzoxazines wasanalyzed by thermogravimetry (TG). The TG and derivativethermogravimetry (DTG) traces of the PBz network with and withoutalumina nanoparticle (5 wt%) incorporation are shown in Figure 6.The relative thermal stability of these samples was estimated bycomparing the temperatures required for mass loss (5 and 10%) andthe char yield at 700°C, and results are shown in Table 2.

The neat PBz blends containing a higher percentage of cardanol-based benzoxazine showed a much higher thermal stability than didthose with bisphenol A-based benzoxazine. It is expected that apolymer containing a higher Ca content should exhibit a lowerthermal stability due to the presence of a long alkylene chain.However, the observed T5% values of poly(Ca–BAa) containing themonomers in the ratios of 1:3 and 3:1 were 338 and 291°C,

6

respectively. Notably, the lower thermal stability of the resincontaining a higher content of petrobased against biobasedmonomer – that is, BAa against Ca – suggests the existence ofthermally labile linkages in the former thermally cured resin. Thiscould be explained on the basis that the alkylene bonds in Ca maybe involved in some additional cross-linking reactions or a muchlower viscosity of Ca may allow the steady growth and build-up ofa cross-linked polymer network accounting for the observeddifferential thermal stability of the cured resin. Thermalcharacterization studies indicate the presence of a nanofiller to bean important influence in directing PBz properties. The TGA ofthe PBz alumina nanocomposite also suggests the effect ofnanoparticle on thermal stability. The thermal stability of neat PBzblends is lower than that with the nanoparticle composite. Theaddition of the nanoparticle has improved both the initialtemperature of degradation and char yield in the blend containing ahigher weight percent of cardanol (75%). However, only the effecton char yield was noticed in the case of the blend with a higherbisphenol A content.

Surface functionalization of alumina nanoparticles in the form offree hydroxyl groups22,23 can be expected to catalyze the initiationof the benzoxazine polymerization reaction, simultaneouslyreinforcing the cross-linked PBz network due to hydrogen bonding.The same is reflected in the lowering of the curing temperature inDSC and enhanced thermal stability in TGA. Owing to its hydroxyl

RN

N N

O O

O

+Δ

Al 3+

OH OH

R

OH

N Nm n

No

Al 3+

R

NO

Al 3+

CH2+

Al 3+

R

N

O

R

N

OCH2

+

‡

NO

NO

n

Pathwaya

N

O

Pathwayc

OHPathwayb N

Mannich bridge

N,O acetal bridge

n

Figure 5. Probable mechanism for assistance provided by alumina nanoparticles in ROP reaction of benzoxazine monomers to form PBz resin



surface functionalization and higher surface area, nanoalumina canbe expected to interact intensively with the polymeric network in acompatible fashion. This in turn would ensure a synergisticmanifestation of the filler and polymeric properties.

Motivated by the positive turn of thermal studies, application of thesynthesized nanocomposites as stainless steel adhesives was furtherexplored and their properties were compared with those of therespective blends devoid of any fillers. To begin, the viscosityvalues of the neat blends were measured. The Brookfieldviscosities of the 1:3 and 3:1 Ca–BAa monomer blends were foundto be to be 8·1 × 104 and 500mPa s, respectively. The higherviscosity of the former could be attributed to the nature andfunctionality of the benzoxazine monomer. There is a higherpercentage of the structurally rigid bisphenol A-based bis-

benzoxazine monomer, while the former is a monobenzoxazine thatis flexible due to the longer alkyl/alkylene chain at the m-position.The benzoxazine monomers were applied in between stainless steelsubstrate as neat (without solvent) and in solution with a similarcontent followed by thermal curing to obtain the cross-linkedpolymer network. The LSS values were then measured tounderstand the effect of incorporation of different resin, aluminananoparticles content and processing technique on the adhesion ofthe steel plates. The LSS values obtained are shown in Figure 7.

At 50°C, 1:3 Ca–BAa exhibited a higher adhesive strength, but at ahigher temperature (~150°C), both monomer blends showed similaradhesive strength. This suggests that partial replacement of thepetrobased monomer with the sustainable alternate up to 25–75%did not affect the adhesive strength adversely. Moreover, owing toits low viscosity, Ca aids the application of the adhesive on thesubstrate without any dependence on organic solvents. A relativelyhigh viscosity of 1:3 Ca–BAa (1:3) (~105 mPA s) may account forthe poor wettability of the metal surface, resulting in voids whenapplied without any solvent. Subsequently, solution-assisted coatingled to a marked improvement in adhesive strength of 1:3 Ca–BAa.On the other hand, the monomer blend with the higher cardanolcontent showed the opposite behavior, which may be explained bythe inherently low viscosity (500mPa s) of the higher-Ca-content

Mas

s lo

ss: %

Mas

s lo

ss: %

Temperature: ºC

Temperature: ºC Temperature: ºC

Temperature: ºC

Char yield

Char yield

5% 11%

700 700600 600500 500400400300 300200 200

20

40

60

80

100

20

0

00

40

60

80

100

100 100

5% nanoalumina1:3 PCa−PBAa




Neat 1:3 PCa−PBAa Neat 3:1 PCa−PBAa

(b)(a)

(c)

Der

ivat

ive

mas

s lo

ss

Der

ivat

ive

mas

s lo

ss

(d)

700700600 600500 500400 400300 300200 200100 100

−60−120

−100

−80

−60

−40

−40−20

−20

Neat 1:3 PCa−PBAaNeat 3:1 PCa−PBAa

Figure 6. TGA (a–b) and DTG (c–d) traces of representative PBzs obtained after curing the Ca–BAa (1:3 and 3:1) blends and theirrespective nanocomposites with 5 wt% alumina nanoparticles. PCa, copolymers of cardanol; PBAa, bisphenol A benzoxazine

Table 2. TGA results of cure PBz resins with and without nanofiller

Poly(Ca–BAa)

Nanoalumina: %
T5% T10% Char yield at700°C: %
3:1
0 338 389 14 5 389 412 25
1:3
0 291 356 23 5 285 359 28
7



blend. It is also worth mentioning that BAa is a higher-functionalitymonomer (functionality = 2) compared to Ca (functionality = 1),and the latter monomer has a longer alkylene chain, which furtherdilutes the effective concentration of the polar groups’ phenolic–OH, and the >N–Ph functionality across the cross-linked networkaccounts for the higher adhesive strength of the former.

Incorporation of 5 wt% nanoalumina into 1:3 Ca–BAa led to atwofold improvement in the adhesive strength (from 44 to83 kg/cm2), while 3 wt% addition of 3:1 Ca–BAa contributedto the adhesive strength marginally by only 10 units (from 41 to48 kg/cm2) at 50°C. This could be attributed to the enhancement insurface area interaction, superior asperity interaction and betterwetting in the former sample. However, the limited enhancementin the LSS value of the latter resin may be due to the aggregationof nanofillers. The presence of surface hydroxyl functionalities onnanoalumina, the so-formed phenolic hydroxyl and secondaryamine functionalities in the polymer resin led to extensivehydrogen bonding, which may assist in bridging such interactionswith the metal specimens. The resultant hydrogen bonding networkformation and higher surface area provided by the nanoparticlescan easily cause a strong interaction, leading to better properties ofthe polymer–nanocomposite-reinforced adhesive material.

4. Summary and conclusionCardanol-based benzoxazine was successfully incorporated withpetrobased bisphenol A-based benzoxazine to yield blends indifferent ratios. Owing to the low viscosity of Ca, solventlesssynthesis of blends was achieved, which had not been a possibilityin the case of neat petroderived blends due to high viscositybuild-up. Apart from the incorporation of renewable content in themonomer, solventless synthesis and evolution of innocuous by-product water were achieved, adhering to the tenets of greenchemists. In order to enhance further the performance of theresulting polymers, incorporation of alumina as nanofiller was

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carried out. A steep rise in the thermal performance was observed,with a notable decrease in the onset of the curing temperature.Similarly, thermal stability was also enhanced, reflected by theincrements in char yield of D5% (1:3 PCa–PBAa blend) and D11%(3:1 PCa–PBAa blend). This indicates the pivotal role played bythe hydroxyl functionalization on the surface of the nanoparticles.Furthermore, both the blends and respective nanocomposites werestudied for their applicability as stainless steel adhesives. Here also,adhesion was found to improve markedly in the case ofnanocomposites. A comparison between solution-based andsolventless processing was also sought. In the case of solvent-mediated processing, blends with higher petrobased content werefound to be better adhesives, while solventless processing exhibitedenhancement in the performance of blends with a higher renewablecontent. In conclusion, a novel strategy based on exploiting thesurface functionalization of nanofiller alumina was found to beeffective in enhancing the properties of partially biobased, cardanol-derived benzoxazine blends, and the high potential of a sustainableroute leading to smooth adhesive processing was also analyzed.

AcknowledgementsThe authors would like to acknowledge the financial support andinfrastructure facilities from Shiv Nadar University. One of theauthors, Dr Bimlesh Lochab, is grateful to the Department ofScience and Technology, Delhi, India, for providing financialassistance, grant number SB/FTP/ETA-0069/2014. The authorsare thankful to Mr Satya Priye, Satya Cashew Chemicals Pvt. Ltd(SCCPL), for providing cardanol for research purpose.

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Neat 1:3 Ca–BAa

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Figure 7. LSS measurements for poly(Ca–BAa) blends (a) utilized as neat and solution-based adhesive and (b) effect of increasingnanoalumina on the adhesive property of the polymer



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