Thermochimica Acta 555 (2013) 46 52

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Thermochimica Acta

jo u r n al hom ep age: www.elsev ier .c

Therm fofrom b

Yong Zhaa Faculty of Forb FPInnovation

a r t i c l

Article history:Received 15 OReceived in re18 December Accepted 20 DAvailable onlin

Keywords:BarkBark extractivPhenol-liqueBark-derived PThermal stabilityThermal degradation

F) reseetlestabilvimemal dy of ttly fr

differin maed a h

1. Introdu

Phenoladhesives, compoundstance [1]. Gand condencross-linkedlinear thermpolymer de

Thermalon its strucproperty ofthe thermabeen suggean auto-oxithe rst thformed betwcured PF reof cross-linsuch as meand water.

CorresponE-mail add

0040-6031/$ http://dx.doi.oction

formaldehyde (PF) resins have been widely used ascoatings, thermal insulation materials, and molding

due to their good mechanical properties and heat resis-enerally, phenol and formaldehyde go through additionsation reactions under either alkaline condition to form

thermoset polymers or under acidic condition to formoplastic polymers. The structure and properties of the

pend highly on the reaction conditions. stability of a thermoset polymer is highly dependentture and cross-linking density [2,3]. It is an important

polymer durability. Various studies have investigatedl degradation mechanisms for PF resins [48]. It hassted that the thermal degradation of PF resins followsdization process that mainly includes three stages. Inermal degradation stage, additional cross-linking was

een the remaining un-reacted functional groups in thesins. In the second thermal degradation stage, the levelking was reduced with some degradation by-products,thane, hydrogen, carbon monoxide, small oligomers,In the third thermal degradation stage, the cured resin

ding author. Tel.: +1 4169468070; fax: +1 4169783834.ress: ning.yan@utoronto.ca (N. Yan).

network would collapse followed by the carbonization and graphi-tization processes [4,5].

Thermogravimetric analysis that measures a samples mass lossas a function of temperature and/or time is a common technique forcharacterizing polymer thermal degradation behavior [68]. Kinet-ics during the thermal degradation process has been studied toacquire fundamental understanding of the structural changes inphenolic resins [4]. The technique used to obtain the kinetic param-eters is termed either as a differential or an integral method. Bothdifferential and integral method can be further classied based oneither a single heating rate or multiple heating rates method is used[9,10]. It is believed that multiple heating rates method gives morereliable results than the single heating rate method with a smallerexperimental error [10]. The model-free isoconversional meth-ods are widely adopted techniques for deriving relevant kineticparameters using thermogravimetric (TG) and differential ther-mogravimetric (DTG) curves measured at different heating rates,which make no presumption about the reaction function and reac-tion order [915].

Previous studies [16,17,19] have also shown that thermalstability and thermal degradation kinetics of the PF resins are sig-nicantly affected by the resin synthesis conditions. For example, itwas found that the activation energies of the resol resins with var-ious molar ratios of formaldehyde to phenol decreased sharply atrst and then remained almost constant during the thermal decom-position process. However, the activation energies of the novolac

see front matter 2012 Elsevier B.V. All rights reserved.rg/10.1016/j.tca.2012.12.002al degradation characteristics of phenoleetle infested pine barks

oa, Ning Yana,, Martin W. Fengb

estry, University of Toronto, 33 Willcocks Street, Toronto, ON, Canada M5S 3B3s, Wood Products Division, 2665 East Mall, Vancouver, BC, Canada V6T 1W5

e i n f o

ctober 2012vised form2012ecember 2012e 3 January 2013

esed barkF resin

a b s t r a c t

In this study, phenolformaldehyde (Pliqueed barks from mountain pine b(Pinus contorta Dougl.). The thermal resins were investigated by thermograPF resins at different stages of the therSpectroscopy (FTIR). Thermal stabilitthe lab PF resin but differed signicanPF resin made from bark extractives kinetics from the bark-derived PF resmade from the bark extractives showom/ locate / tca

rmaldehyde resins derived

ins were synthesized using both bark extractives and phenol- (Dendroctonus ponderosae Hopkins) infested lodgepole pineity and thermal degradation kinetics of the bark-derived PFtric analysis (TGA). The structural changes in the bark-derivedegradation were studied using the Fourier Transform Infraredhe post-cured bark-derived PF resins were similar to that ofom that of the post-cured commercial PF resin. Bark-deriveded signicantly in thermal stability and thermal degradationde from the phenol-liqueed bark. The bark-derived PF resinigher thermal stability.

2012 Elsevier B.V. All rights reserved.

Y. Zhao et al. / Thermochimica Acta 555 (2013) 46 52 47

resins with some formaldehyde to phenol molar ratios were almostunchanged in the whole degradation process, while the activationenergies of the novolac resins with other formaldehyde to phenolmolar ratios gradually decreased [19].

The incllose bers, thermal sta[18,20,21,2found to haconditions t[18]. Meanwble at lowerto PF resinsbers to PF resol resins

Recentlysubstitute psis [22,23]. either liquea solvent. Tand extractbonding strenergy andin curing ding structustability of certain appbark compoof the resulthermal deinvestigated

2. Experim

2.1. Resin s

Liqueeextractive-paccording tomade PF resby followinderived-PF oratory Checommerciaas a compar

2.2. Measur

The LBPFan oven andground intomesh screenQ500, TA, Uplatinum paheating rate

2.3. Kinetic

The kinethe bark-deanalysis emof thermal d

d

dt= f ()k

Fi

is trepre

A ex

A is tempingeWallsed ves ation) forere uation

the s:

i

,i

)=

i isper

ith te the K)

=

Tp isted r

IR

resi, 600 C, and 800 C were collected and analyzed using Fourierorm Infrared Spectroscopy (FTIR). The FTIR measurementsarried out using a FT-IR TENSOR 27 spectrometer withtachment (Bruker Optics, USA) having a frequency range of400 cm1. Elemental analysis was carried out using a 2400II Carbon Hydrogen Nitrogen (CHNS) elemental analyzer.

ults and discussion

ermal stability of the bark-derived PF resins

thermal degradation curves of the different resins are shown1. When the temperature increased from room temperatureC, the weight loss of the cured resins was mainly causedusion of other materials, such as lignin and cellu-in the PF resin synthesis has been shown to modifybility and degradation kinetics of the resulting resins4]. Ligninphenolformaldehyde (LPF) resol resins wereve higher thermal stability under both nitrogen and airhan PF resins with different degradation characteristicshile, the LPF novolac resins were found to be less sta-

temperatures than PF resins, but they were comparable at higher temperatures [20]. The addition of celluloseresol resins reduced thermal resistance of the resulting

[21]., phenolic compounds from bark were used to partiallyetroleum-derived phenol in the PF resol resin synthe-In these studies, bark components were obtained byfaction of bark in phenol or extraction of bark usinghe bark-derived PF resins, with both liqueed barked bark components, were able to have comparableengths to a commercial PF resin. But the activation

pre-exponential factors of the bark-derived PF resinsiffered from those of commercial PF resins, suggest-ral differences in these cured adhesives. Even thoughthe cured adhesives is an important consideration inlications, no previous study has reported the impacts ofnents on the thermal stability and degradation kineticsting PF resins. In this study, the thermal stability andgradation kinetics of the bark-derived PF resins were.

ental

ynthesis

d bark-phenolformaldehyde (LBPF) resin and barkhenolformaldehyde (BEPF) resin were synthesized

the methods reported previously [22,23]. A laboratoryin (lab PF) without any bark components was preparedg the same procedures used in the synthesis of barkresins. All chemicals were purchased from Caledon Lab-micals, Canada, and used without further purication. Al PF resin for oriented strandboard face layers was usedison.

ements of thermal stability

, BEPF, lab PF and commercial PF resins were placed in cured at 80 C for 48 h. The cured resin samples were

ne powders of sizes that had passed through a 100- and then tested in a thermogravimetric analyzer (TGA,SA). About 10 mg of each cured sample was added to an and heated from room temperature to 800 C at the

of 5, 10, 15 and 20 C/min under N2 atmosphere.

analysis

tic parameters for the thermal degradation process ofrived PF resins were obtained by the thermogravimetricploying multiple heating rates. It is known that the rateegradation can be expressed as:

(T) (1)

wherek(T) is

k(T) =

whereis the t

KissFlynnwere uadhesiInterna(ICTACdata, wdegrad

Forgiven a

ln

(

T2

wherethe temunder

For

ln

(

T2p

wherethe tes

2.4. FT

The400 CTransfwere cATR at4000Series

3. Res

3.1. Th

Thein Fig. to 200g. 1. Thermal degradation curves of the phenolic resins.

he degree of conversion, f() is the reaction model, andsented by an Arrhenius equation shown as:

p(

ERT

)(2)

he pre-exponential factor; E is the activation energy; Terature; R is the ideal gas constant.rAkahiraSunose (a more accurate version ofOzawa) and Kissinger methods [9,11,12], whichpreviously to study thermal degradation kinetics of PFnd recommended by the Kinetics Committee of theal Confederation for Thermal Analysis and Calorimetry

performing kinetics computation on thermal analysissed to determine the kinetic parameters of the thermal

process for the bark-derived PF resins.KissingerAkahiraSunose method, the expression is

const ERT

(3)

the heating rate under ith temperature program. T,i isature at which the degree of conversion, , is reachedmperature program.issinger method, the expression is:

ln(

AR

E

) E

RTp(4)

the peak temperature obtained from the DTG curves ofesins at different heating rates.

n samples after degradation at temperatures of 200 C,

48 Y. Zhao et al. / Thermochimica Acta 555 (2013) 46 52

Fig. 2. Der

by the evapration of coof water cothe residuaination mig

From 20dation. Theand water the temperdecompositdegradationand volatilenol, and theincreased tlike structumonoxide acarbonizati

Fig. 2 clthe commelab PF and ture reachedegradationtemperaturreason for twas presensuch a tempgen contenand it decretemperatur

The weidegradationof the commhigher thanAfter the enPF resin shoresin, lique

The inclufor its high

ctivat the K

ed baempraturprod

PF a losst res

weispec

lossith p

F res to d

diffend li

chetives

strue liqulene igheThe lic rie bark exlt for

erma

depation

theen lnKissi

Table 1Weight losses

Resin type

CommercialLiqueed baBark extractLab-PF ivative thermal degradation (DTG) curves of the phenolic resins.

oration of moisture, dehydration, as well as the evapo-mpounds with small molecular weights. The evolutionuld be attributed to the condensation reaction betweenl methylol groups and phenolic OH groups. Water elim-ht lead to the formation of new crosslinks [4,5].0 C to 400 C, the resins underwent further degra-

release of free phenol, aldehyde, short oligomerswere main contributors to the weight loss. Whenature increased from 400 C to 600 C, major polymerion took place. The weight loss was the results of the

of the polymeric molecules and the formation of small molecules, such as CO, CO2, benzaldehyde and phe-

initial formation of char [68]. After the temperatureo above 600 C, the degradation continued and a char-re of carbon was gradually formed, generating carbons by product [48]. It was also considered to be the

on process.early shows that the thermal degradation process ofrcial PF resin differed signicantly from those of thebark-derived PF resins, especially when the tempera-d higher than 200 C. Two distinct derivative thermal

(DTG) peaks were observed (shown in Fig. 2) in thee range of 200400 C for the commercial PF resin. Thehis was due to the fact that a signicant amount of ureat in the commercial PF resin. Urea is known to degrade aterature range. Elemental analysis revealed that a nitro-t of 13% was present in the cured commercial PF resinased to 4% when the temperature increased from roome to 400 C.ght losses of all the tested resins during the thermal

process are shown in Table 1. The total weight lossercial PF resin from room temperature to 800 C was

those of the lab PF resin and bark-derived PF resins.

Fig. 3. Alated by

liquein the ttemperesins the labweightthe tessimilar8.7%, reweightnents wbark-Pstarted

Theresin aence inextracrecentthat thmethyand a hresin. phenothan thand badifcu

3.2. Th

Thedegradaffectsbetwe1/T (tire thermal degradation process, the bark extractive-wed the highest residual weight, followed by the lab PFed bark-PF resin and commercial PF resin.sion of urea in the commercial PF was the main reason

er weight loss (35.3%) than those of the lab PF (20.7%),

not shownat differenKissingerAFig. 3. Evengiven the fo

of different phenolic resins during the thermal degradation process.

Weight loss at different temperature range (%)

RT 200 C 200400 C 400600 PF 13.2 22.1 14.9 rk-PF 9.2 13.5 12.8 ive-PF 7.5 11.2 14.0

10.3 10.4 14.8 ion energies of the tested resins at different conversion levels calcu-issingerAkahiraSunose method.

rk-PF resin (22.7%) and bark extractive-PF resin (18.7%)erature range of room temperature to 400 C. When thee increased from 400 C to 600 C, the bark-derived PFuced slightly lower weight losses in comparison withnd the commercial PF resin. From 600 C to 800 C, the

of the commercial PF resin was the lowest among allins, at 5.0%. The lab PF and bark-extractive PF resin hadght losses during this temperature range, at 8.1% andtively; while the liqueed bark-PF resin had the highest, i.e. 12.1%. It is believed that the liqueed bark compo-henolic side chains or branch structures in the liqueedin were thermally stable at lower temperatures butegrade at higher temperatures.rence of the weight loss between the bark extractive-PFqueed bark-PF resin could be attributed to the differ-mical composition between the liqueed bark and bark

as well as the difference in the resin structures. In thectural study of the bark-derived PF resins, it was foundeed bark-PF had a higher ratio of para-para/ortho-paralink, a higher unsubstituted/substituted hydrogen ratior methylol/methylene ratio than the bark extractive-PFphenolated products in the liqueed bark had multi-ngs and had more reactive sites toward formaldehyderk extractives. The large molecules of the liqueed barktractives with lower molecular mobility could make it

the resulting resins to form cross-links [27,28].

l degradation kinetics

endence of the activation energy on the extent of provides information on how the polymer structure

degradation kinetics [9,10]. The linear relationship(/T2p ) and 1/Tp (Kissinger method), ln(/T2,i) andngerAkahiraSunose method) were obtained (gures). The activation energies (E) of the tested resinst stages of thermal degradation calculated by the

kahiraSunose method from the slope is shown in

though the pre-exponential factors were also obtained,cus of the study was on the thermal stability as indicated

Total weight loss (%)

C 600800 C RT 800 C5.0 55.2

12.1 47.68.7 41.48.1 43.6

Y. Zhao et al. / Thermochimica Acta 555 (2013) 46 52 49

Fig. 4. FTIR spatures.

by the actireaction rat

In the eahad the lowthat the comwhich easilresins, the aversion reathe gradual

The highlab PF resinversion ratethe late theorderly andsition mechthan 70% [1occurred at

The barkenergy comand commedegradationresin was mPF. This is the bark exbefore. The their post-ccal composiliqueed bastability an

The actithe Kissingfrom their energy of t

TIR sp

ignme

umbe

3500 2914 2227 1650,

Benzene ringCH2 scissor bendingPhenolic OH in-plane deformation

1253, 12031253, 1170 Alkyl-phenol C O stretchAromatic CH in-plane deformationAromatic linked CH3 rock and/or aromatic CHPolysubstituted aromatic ringCH2 out-of-plane ring deformationCH2 out-of-plane ring deformation

esins in most of the cases. The same was observed by theerAkahiraSunose method. For the commercial PF resin,ivation energy of the second peak was lower than the other

which could be attributed to urea. The results and trendsted by the Kissinger method were also consistent with the

calculated by the KissingerAkahiraSunose method.

Table 2Activation ene

Bark extractLiqueed baCommercialLab PF

Values in the bectra of the bark extractive-PF resin at different degradation temper-

vation energy parameters instead of the degradatione, they were not included in the discussions.rly stage (conversion

50 Y. Zhao et al. / Thermochimica Acta 555 (2013) 46 52

Fig. 6. FTIR spectra of the commercial PF resin at different degradation tempera-tures.

groups of the tested resins changed with the increase of thermaldegradation temperature.

The intensity of the OH peak at around 33503500 cm1 forall the tested resins decreased when temperature was increasedfrom room temperature to 400 C. This was due to the conden-sation process involving either residual methylol groups or thephenolic hydroxyl groups and the formation of the new cross-links. Possible reactions are shown in Fig. 8 [4,5]. The OH peakintensity at 1353 cm1 also decreased with the increase of tem-perature due to the same mechanisms. When the temperaturewas furthepeak at 3351353 cm1

to poly-aro2914 cm1

Fig. 7. FTIR spectra of the lab PF resin at different degradation temperatures.

and aliphatic CH2 symmetric stretch, respectively, decreased withthe increase of thermal degradation temperature.

The peak at 1740 cm1 due to the carboxylic groups wasobserved in the bark extractive-PF resin, liqueed bark-PF resin andlab PF resin when the thermal degradation temperature reachedto 400 C. It also appeared in all the tested resins when the ther-mal degradation increased to 800 C. The intensity of the peakwas weak, but its appearance was considered as an evidence foroxidation during the resin thermal degradation under the inertatmosphere

bony0 cmationecreak ofr increased to above 600 C, the intensity of the OH03500 cm1 decreased signicantly and the peak atdisappeared. The main structures of the resins changedmatic structures [4]. The intensity of the peaks atand 2848 cm1 due to aliphatic CH2 asymmetric stretch

Carto 160degradpeak dthe peFig. 8. Possible condensation reactions during the therm [4,5,18]. The possible reactions are shown in Fig. 9.l groups appearing at 1650 cm1 as a shoulder peak next1 were be observed in all the resins when the thermal

temperature reached to 200 C. The intensity of thisased when the temperature increased. The intensity of

benzene rings at 1467 cm1 increased for all the resinsal degradation process.

Y. Zhao et al. / Thermochimica Acta 555 (2013) 46 52 51

OH

CH2

OH OH

+OH

OH

HOH2CH2C

+OH

OH

OHC+OH

OH

HOOC

OH

therm

during the t879 cm1 awith the inthe occurrepeak assignwhen the teresin netwo

The pea1170 cm1

temperatur600 C for laalkyl-phenostill present

For thedecreased aof ether linresins, the but appeare400 C. It inthermal deg

When th400 C, the tially intact600 C, dramresins due taromatics, [4,18].

It is also tra of the coAccording t2167 cm1

groups werwas 200 C.icantly whil

perrma

the cme

are s

clus

postundigniive-Pffererk exral cerive

the is. Thterishen

. TheOH

CH2

OH

OH

CH

O

Fig. 9. Possible oxidation reactions during the resin

hermal degradation process. The intensity of the peak atssigned to the poly-substituted aromatic ring increasedcreasing thermal degradation temperature, indicatingnce of the carbonization process. The intensity of theed to the methylene bridges at 1435 cm1 decreasedmperature increased, reecting the occurrence of therk decomposition.k of alkyl-phenol C O stretching at 1213 cm1 anddecreased with the increasing thermal degradatione. It disappeared when the temperature was aboveb PF, commercial PF and bark extractive-PF resins. Thel C O stretching peak of the liqueed bark-PF resin was

even when the temperature reached 800 C. bark extractive-PF resin, the peak at 1249 cm1

t 200 C but increased at 400 C, indicating formationkage. While for the lab PF and the commercial PFpeak at 1249 cm1 was absent in the initial spectrad when the thermal degradation temperature reacheddicated that the oxidation occurred during the resinradation process.e thermal degradation temperature was lower than

polymer network of the phenolic resins remains essen-

the temthe theson forthe combefore

4. Con

Thewas fofered sextractited diThe bastructubark-dduringanalyscharacresin w400 C. When the thermal degradation temperature was aboveatic changes were seen in the FTIR spectra of all the

o the collapse of the polymeric network to form poly-which was consistent with past studies on PF resins

noteworthy that the differences between the FTIR spec-mmercial PF resins after heating at 200 C and 400 C.o a previous study [25], the peaks at 2227 cm1 andattributed to CN triple bond in cyanamide and the NCOe observed when the thermal degradation temperature

The intensity of the peak at 2167 cm1 decreased signif-e the intensity of the peak at 2227 cm1 increased when

Fig. 10. Possible reactions for urea decomposition.

was observ

Acknowled

FinanciaExcellence:ated. Authothe bark samful to Dr. Symeasureme

References

[1] A. Pizzi, New York

[2] V.A. Er, A(1976) 46

[3] T.R. Man135313

[4] Y.F. Chenof pheno-H2O

OH

+ CO2

OH

C

OH

O

al degradation process.

ature increased to 400 C. Both peaks disappeared whenl degradation temperature increased to 600 C. The rea-hange may be due to the thermal degradation of urea inrcial PF resin. The possible reactions that were reportedhown in Fig. 10.

ions

-curing thermal stability of the bark-derived PF resins to be similar to that of the lab PF resin but dif-cantly from that of the commercial PF resin. The barkF resin and the phenol-liqueed bark-PF resin exhib-

nt thermal stability and thermal degradation kinetics.tractive-PF resin showed a better thermal stability. Thehanges and the differences in structures among thed PF resins, the lab PF resin and the commercial PF resinthermal degradation process were revealed by the FTIRe commercial PF resin showed different FTIR spectraltics from those of the bark-derived PF resins and lab PF

the thermal degradation temperature was at 200 C and thermal degradation mechanism involving oxidations

ed for the bark-derived PF resins.

gements

l support from the Ontario Research Fund-Research Bark Biorenery program partners is greatly appreci-rs would also like to thank FPInnovations for providingples and the commercial PF resin. We are deeply grate-ed Abthagir Pitchai Mydeen for helping with the TGAnts.

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Thermal degradation characteristics of phenolformaldehyde resins derived from beetle infested pine barks1 Introduction2 Experimental2.1 Resin synthesis2.2 Measurements of thermal stability2.3 Kinetic analysis2.4 FTIR

3 Results and discussion3.1 Thermal stability of the bark-derived PF resins3.2 Thermal degradation kinetics3.3 Structural changes of the bark-derived PF resins during thermal degradation

4 ConclusionsAcknowledgementsReferences