Polymer Degradation and Stability 91 (2006) 832e841

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The structureeactivity relationship of fire retardant phosphoruscompounds in wood

Ralph Stevens, Daan S. van Es*, Remko Bezemer, Aldo Kranenbarg

Agrotechnology and Food Innovations (A&F B.V.), Wageningen University and Research Centre, P.O. Box 17,

NL-6700 AA Wageningen, The Netherlands

Received 8 April 2005; received in revised form 7 June 2005; accepted 22 June 2005

Available online 18 August 2005

Abstract

Sawdust of Scots Pine sapwood was chemically modified with various alkyl- and phenylchlorophosphorus compounds. Theformation of covalent bonds was confirmed with solid state CP-MAS 13C NMR.

According to thermogravimetric analysis (TGA), all phosphorus compounds decreased the temperature for the maximum rate ofpyrolysis (from 350 �C to max. 240 �C) and increased the char formation (from 25% to max. 54%). Variation of the alkyl groups

(C2eC8) had no significant effect. Phenylphosphates decrease the temperature of pyrolysis more efficiently than the alkyl analogues,due to higher thermal stability. The order in which the phenylphosphorus compounds affect the pyrolysis of the modified sawdust isconsistent with their acidity order: organophosphateO organophosphonate[ organophosphinate.

All phosphorus compounds used in this study reduce the equilibrium moisture content (EMC). Whereas the results obtained withthe dialkyl phosphates are relatively poor, significant reductions in EMC can be achieved with the phenylphosphorus compounds.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Wood; Modification; Fire retardancy; Durability; Phosphorus; TGA; EMC

1. Introduction

Wood has long been a material of choice for theconstruction industry because of its durability, strengthand versatility. Wood, being a natural product, also hasseveral disadvantages when compared to materials likesteel, aluminium, concrete or plastics. The hydrophiliccharacter of wood gives rise to a high equilibriummoisture content (EMC), which can result in a poordimensional stability and a relatively high susceptibilityto microbiological decay. Furthermore, wood is in-herently flammable. In order to meet today’s incre-asingly stringent demands for fire safety as well as

* Corresponding author. Tel.: C31 317 477546; fax: C31 317

475347.

E-mail address: daan.vanes@wur.nl (D.S. van Es).

0141-3910/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2005.06.014

product durability, wood products have to be treatedwith additives. Although the additives currently in useare relatively efficient, they also have several technicaland ecological drawbacks. Wood can be renderedflame retarded by using additives like phosphate salts,ammonium polyphosphate being one of the morecommonly used. Whereas these salts impart effectiveflame retardancy on short term, due to their inherentsolubility in water they will be leached out during out-door exposure. As a result, the flame retardancy will bereduced through time, requiring frequent post-treatments.Furthermore, adding salts to the hydrophilic carbohy-drate matrix will further increase the EMC, leading topoorer dimensional stability and increased susceptibilityto microbiological attack. The latter can be dealt withby the use of so-called Wollman salts; efficient combi-nations of arsenic-, cupric- and chromate salts. However,with the increasing public disapproval of the use of toxic

833R. Stevens et al. / Polymer Degradation and Stability 91 (2006) 832e841

heavy metals, and increasingly stringent legislation withregard to the use of these substances, there is a need forviable alternatives.

The present study is part of a multi-year project withthe objective to develop new environmentally friendlyfire retardants for wood that furthermore improve thedimensional stability and durability of wood. The focusis on impregnation of reactive fire retardants with ahydrophobic character. The objective of the presentstudy was to investigate the relationship between thechemical structure of several phosphorus compoundsand their fire retardant and hydrophobing propertieswith regard to wood. Sawdust was used as a model forsolid wood to avoid problems with impregnation of thechemicals and to have a more homogeneous startingmaterial. The use of sawdust enables a fast screening ofthe properties like fire retardancy with thermogravimetricanalysis (TGA) and hydrophobicity by EMC. Themodification of sawdust is achieved by reaction of a fireretardant with the free hydroxyl groups of the woodcomponents (cellulose, hemi-cellulose and lignin). Thereactive fire retardants under investigation were chloro-phosphates (phosphonates or phosphinates) with one ortwo hydrophobic alkyl- or phenyl-groups. The fireresistance of the wood is improved by the introductionof the phosphorus compounds, while the alkyl- orphenyl-groups increase the hydrophobic character ofthe wood.

Previous research on fire retarded wood has shownthat most fire retardant systems for wood containphosphorus compounds [1e4]. The most common andoldest phosphorus fire retardant are phosphoric acidand mono- and diammonium (poly)phosphate salts.Other phosphorus containing systems are phosphatesalts of nitrogen containing organic compounds, likeguanidine, guanylurea, urea and melamine. Phosphoruscontaining chemicals are effective as fire retardants as aresult of their capability to change the thermaldegradation process of wood. During pyrolysis, phos-phorus compounds form acids that reduce the de-composition temperature [1]. This results in increaseddehydration of the wood components and char for-mation. The char forms a thermal insulation barrier aswell as a diffusion barrier for oxygen and volatilecombustible components. Furthermore, phosphoruscompounds are believed to inhibit the formationof combustible components (e.g. levoglucosan) byphosphorylation of cellulose [5,6]. Over 30 years agoArney and Kuryla already reported on the structureeactivity relationships of phosphorus containing flameretardants in cellulose [5]. In this study, the phosphoruscompounds were merely physisorbed onto cellulosetextile fabric. Ellis and co-workers have shown thatchlorophosphorus compounds react with wood inpyridine [2]. The esterification resulted in a massincrease and an improvement in fire retardancy. TGA

results showed that the temperature of pyrolysis haddecreased and the amount of char formed duringpyrolysis had increased considerably. Earlier, Katsuuraand Inagaki [7] and Schwenker and Pascu [8] had shownthat diethyl chlorophosphate and phenyl dichloro-phosphonate improved the fire retardancy of cellulosefibres. In our study, a comparable procedure was used tomodify sawdust of Scots Pine sapwood with variouschlorophosphorus compounds (Fig. 1).

The degree of modification was controlled bychanging the concentration of the chlorophosphoruscompounds, the reaction time and the reaction temper-ature. Table 1 lists the phosphorus compounds used inthis study.

The sawdust samples were washed extensively afterthe reaction to remove non-bonded chemicals. Afterdrying, the mass increase (weight percentage gain orWPG) was calculated based on the original dry weightof the samples. However, the WPG is indirect prooffor covalent bond formation. Solid state CP-MAS13C NMR was used to conclusively prove that covalentbonds were formed between the chlorophosphoruscompounds and the sawdust. Next, TGA was usedto investigate the influence of the modification ondecomposition temperature and the char formation.Subsequently, the effects on hydrophobicity of themodified sawdust samples were determined by measuringthe EMC’s.

2. Experimental

2.1. 13C and 31P nuclear magnetic resonance(NMR)

NMR spectra were recorded on a Bruker AvanceDPX300 spectrometer at 75 MHz (13C) and 120 MHz(31P). CDCl3 was used as solvent.

2.2. Solid state CP-MAS 13C nuclear magneticresonance (NMR)

CP-MAS 13CNMR spectra were recorded on aBrukerAMX400 spectrometer operating at 100.63 MHz.

2.3. Thermogravimetric analysis (TGA)

TGA measurements were performed on a PerkineElmer thermogravimetric analyser TGA-7. First, thesawdust sample (5e10 mg) was heated at 100 �C forapproximately 10 min to evaporate the moisture. Thesawdust was then pyrolysed in a flow of air by heating at20 �C per minute till 700 �C. During the heating andpyrolysis of the sample, the residual mass was monitoredcontinuously. The percentage char formation was de-termined from the TGA curve at 400 �C. The rate of

834 R. Stevens et al. / Polymer Degradation and Stability 91 (2006) 832e841

Fig. 1. The reaction of chlorophosphorus compounds with wood.

weight loss as a function of temperature was derivedfrom the TGA curve resulting in a derivative TGAcurve. The maximum of the first derivative of the TGAcurve was taken as the maximum rate of pyrolysis [2].

2.4. Synthesis of dialkyl chlorophosphates

Diethyl chlorophosphate (DEP-Cl), diphenyl chloro-phosphate (DPP-Cl), phenyl dichlorophosphate (PP-Cl2),phenyl dichlorophosphonate (PPO-Cl2), and diphenylchlorophosphinate (DPPI-Cl) were used as receivedfrom SigmaeAldrich.

Several dialkyl chlorophosphates were not commer-cially available and were therefore synthesised, accordingto the method of Modro [9].

2.4.1. Preparation of dialkyl chlorophosphates.General procedure

The synthesis was performed in a 3-necked round-bottom flask equipped with a dropping funnel, magneticstirrer and gas in- and outlet tube. The flask was filledwith POCl3 (1 mol equiv.) and cooled to 0 �C. Therequired alcohol (2 mol equiv.) was added drop-wise toPOCl3, at 0 �C, under stirring. The mixture was thenstirred vigorously for 1 h at 0 �C, while dry nitrogenwas passed through to remove the formed HCl (g). Toneutralise the formed HCl the nitrogen flow was passedthrough a flask containing aqueous NaOH solution.Additional stirring for 4 h at room temperature wasfollowed by work-up. The crude products of dibutyl-and dihexyl-chlorophosphate were purified by distilla-tion under reduced pressure (oil pump, 10�2 mbar). The31P NMR spectra indicated that only one phosphoruscontaining product was present after distillation and13C NMR spectra showed only the presence of signals

Table 1

Phosphorus compounds used in this study

Compound Formula Code

Alkyl chlorophosphates

Diethyl chlorophosphate (C2H5O)2POCl DEP-Cl

Di-n-butyl chlorophosphate (C4H9O)2POCl DBP-Cl

Di-n-hexyl chlorophosphate (C6H13O)2POCl DHP-Cl

Di-n-octyl chlorophosphate (C8H17O)2POCl DOP-Cl

Di-n-dodecyl chlorophosphate (C12H25O)2POCl DDP-Cl

Phenyl chlorophosphorus compounds

Diphenyl chlorophosphate (C6H5O)2POCl DPP-Cl

Phenyl dichlorophosphate (C6H5O)POCl2 PP-Cl2Phenyl dichlorophosphonate (C6H5)POCl2 PPO-Cl2Diphenyl chlorophosphinate (C6H5)2POCl DPPI-Cl2

expected for the dialkyl chlorophosphate products. Thedecomposition temperatures of dioctyl- and didodecyl-chlorophosphate are too low to allow for purification bydistillation under reduced pressure, as was also observedby Gamrath et al. [10]. The crude products were there-fore used for the modification of the sawdust withoutfurther purification. 31P NMR analysis of these mixturesshowed that the dialkyl chlorophosphate was the mainproduct and that some mono-alkyl and tri-alkyl productswere present.

2.4.2. Dibutyl chlorophosphate (DBP-Cl)Vacuum distillation: 80e95 �C/4! 10�2 mbar.

Isolated yield: 42% (clear, colourless liquid). 13CNMR (CDCl3): d 13.9 ppm (C4), d 19.1 ppm (C3),d 32.3 ppm (C2), d 69.9 ppm (C1).

31P NMR (CDCl3):d 5.9 ppm.

2.4.3. Dihexyl chlorophosphate (DHP-Cl)Vacuum distillation: 120e125 �C/1! 10�2 mbar.

Isolated yield: 36% (clear, colourless liquid). 13C NMR(CDCl3): d 14.6 ppm (C6), d 23.1 ppm (C5), d 30.4 ppm(C4), d 25.6 ppm (C3), d 32.0 ppm (C2), d 70.4 ppm (C1).31P NMR (CDCl3): d 5.9 ppm.

2.4.4. Dioctyl chlorophosphate (DOP-Cl)Crude product: clear, colourless, viscous liquid:

31P NMR (CDCl3): main (O80%) product dioctylchlorophosphate d 5.9; other octyl phosphate products:mono-octyl dichlorophosphate: d 7.8; tri-octyl phos-phate d 0.0.

2.4.5. Didodecyl chlorophosphate (DDP-Cl)Crude product: clear, colourless, very viscous liquid:

31P NMR (CDCl3): main (O80%) product didodecylchlorophosphate d 5.8; other dodecyl phosphate prod-ucts: mono-dodecyl dichlorophosphate: d 7.8; tri-dodecyl-phosphate d 0.0.

2.5. Modification of sawdust with chlorophosphoruscompounds

The sawdust used in this study was obtained bycarefully milling Scots Pine sapwood. Prior to treatmentwith the various chlorophosphorus compounds, thesawdust was extracted by the ASTM D1105 method andoven-dried for at least 24 h at 40 �C.

835R. Stevens et al. / Polymer Degradation and Stability 91 (2006) 832e841

2.5.1. General procedure for sawdust modificationIn a 3-necked round-bottom flask equipped with a

cooler, magnetic stirrer, gas inlet tube and thermometerthe desired amount of chlorophosphate was dissolved in50 ml pyridine dried on molecular sieves (3 A). Sawdust(2.5 g) was added to the solution under a dry nitrogenflow. The mixture was then heated under nitrogen to thedesired reaction temperature using an oil bath andstirred for 0.5e24 h to obtain the desired degree ofmodification of the wood. The reaction mixture wassubsequently cooled and the sawdust was then filteredoff over a glass filter and washed with pyridine. Toremove non-bonded chemicals and pyridinium chloride(formed during reaction) the modified sawdust waswashed with pyridine and water according to an in-house developed procedure. First, the sawdust waswashed with 75 ml pyridine during 2 h at roomtemperature in a flask equipped with a magnetic stirrer.Subsequently the sawdust was filtered over a glass filterand the procedure was repeated. The sawdust was air-dried overnight and then washed twice with demineral-ised water for 2 h at room temperature. Subsequentlythe sawdust was dried for at least 24 h in an oven at40 �C.

2.6. Weight percentage gain (WPG)/Molpercentage gain (MPG)

The WPG is the relative weight gain of the sawdustbased on the original weight of the untreated sawdust.Prior to treatment, the sawdust was oven-dried for at least24 h at 40 �C and then weighed. After the treatment andwashing steps, the re-dried sawdust was weighed again.The WPG of the modified sawdust was then calculatedbased on the original weight of the untreated sawdust:WPGZ (mass sawdust (modified)�mass sawdust (orig-inal))/mass sawdust (original).

To compare the properties of sawdust modifiedwith phosphorus compounds with different molecularweights, it is necessary to calculate the number of molesof the phosphorus compound bonded to the wood. Themol percentage gain (MPG) is calculated by dividingthe WPG by the molecular weight of the phosphoruscompound: MPGZWPG/MW.

2.7. Equilibrium moisture content (EMC)

Wood will absorb moisture from the surrounding airuntil its moisture content reaches an equilibrium value.Modified sawdust was exposed to 90% relative humidityat 20 �C until the maximum moisture content wasreached. The sawdust was then oven-dried for 16 h at105 �C and weighed again. The EMC of the modifiedsawdust was then calculated based on the weight of thedried sawdust: EMCZ (mass sawdust (90%RH)�masssawdust (0% RH))/mass sawdust (0% RH).

3. Results and discussion

3.1. Modification of sawdust with chlorophosphoruscompounds

A variety of chlorophosphorus compounds (see Table 1)was reacted with sawdust under identical conditions: 1 hat 115 �C. After extensive washing, the WPG of themodified sawdust was measured to determine if a co-valent bond formation had taken place (Table 2). In allcases the reaction resulted in a mass increase for themodified sawdust, which indicates that all the phosphoruscompounds reacted with the wood. Subsequently, solidstate CP-MAS 13C NMR analysis was used to conclu-sively prove the formation of covalent bonds betweenthe phosphorus compounds and the sawdust. Next, theCP-MAS 13C NMR analysis of the sawdust samplemodified with dibutyl-chlorophosphate (DBP-Cl) isdescribed and explained as example for all the othersawdust samples modified with the chlorophosphoruscompounds. CP-MAS 13C NMR spectra of untreatedsawdust, sawdust modified with DBP-Cl and a physicalmixture of sawdust and dibutyl phosphate (non-bonded)are shown in Fig. 2. The NMR spectrum of theuntreated sample (a) is typical for wood [11,12]. TheNMR spectrum of the sample modified with DBP-Cl (b)showed, apart from the characteristic signals of wood,also the 4 signals of the carbon atoms of dibutylphosphate at 14, 19, 33 and 65 ppm corresponding tothe signals of DBP-Cl. This showed that the modifiedand washed sample still contained the dibutyl phosphategroup. Unambiguous proof for covalent bond forma-tion was obtained with the NMR spectrum of thephysical mixture of sawdust and dibutyl phosphate.In this mixture dibutyl phosphate was used insteadof dibutyl chlorophosphate (DBP-Cl). If the modifiedsawdust samples contain non-bonded phosphorus com-pounds after the washing steps, it can only be dibutylphosphate, because DBP-Cl will be hydrolysed in theaqueous washing step. In the CP-MAS NMR spectrumof the physical mixture the signals of the butyl chains are

Table 2

Results of the modification of sawdust with chlorophosphorus

compounds under identical reaction conditions (100 mg/ml chloro-

phosphorus compound, 50 mg/ml sawdust, 115 �C, 1 h)

Chlorophosphorus compound WPG (%) MPG (mmol/g)

DEP-Cl 32 2.38

DBP-Cl 28 1.45

DHP-Cl 28 1.14

DOP-Cl 19 0.63

DDP-Cl 13 0.31

DPP-Cl 18 0.78

PP-Cl2 73 4.69

PPO-Cl2 35 2.51

DPPI-Cl 56 2.80

836 R. Stevens et al. / Polymer Degradation and Stability 91 (2006) 832e841

Fig. 2. CP-MAS 13C NMR spectra of untreated sawdust (a), sawdust modified with DBP-Cl (b) and a physical mixture of sawdust and dibutyl

phosphate (c); HP-DEC 13C NMR spectrum of a physical mixture of sawdust and dibutyl phosphate (d).

not detected because dibutyl phosphate itself is a liquidand hence cannot be detected by CP-MAS NMRtechnique (c). By using CP-MAS pulse sequences onlythe more crystalline material is observed in the NMRspectra, while when using high power (HP-DEC)decoupling pulse sequences the more flexible parts (e.g.liquids) can be observed [13]. The HP-DEC 13C NMRspectrum (d) proves that dibutyl phosphate is present inthe physical mixture. Thus, the presence of the signalsof dibutyl phosphate in the CP-MAS spectrum of themodified sawdust can only be explained by covalentbonding to the wood.

In order to compensate for differences in molecularweight of the chlorophosphorus compounds also thenumber of moles bonded per gram sawdust (MPG) wascalculated (Table 2). These MPG values were sub-sequently used to compare properties like fire retardancyand equilibrium moisture content.

According to the data in Table 2, modification ofsawdust with dialkyl chlorophosphates under identicalreaction conditions (115 �C, 1 h) leads to decreasingMPG values with increasing chain length of the alkylgroups. This can be attributed to increased sterichindrance as well as hydrophobic character with in-creasing chain length. The difference in MPG valuesand thus reactivity between diphenyl chlorophosphate(DPP-Cl) and phenyl dichlorophosphate (PP-Cl2) is dueto the higher reactivity of dichlorophosphate.

The MPG values for the various chlorophosphoruscompounds show large deviations. To objectively com-pare properties like fire retardancy and hydrophobicity,the TGA and EMC values of modified sawdust shouldbe compared at the same MPG value for differentchlorophosphorus compounds. Therefore, for eachchlorophosphorus compound a trend-line for theseproperties was determined by preparing two or more

sawdust sampleswithdifferentMPGvalues andmeasuringthe TGA and EMC values. In this study the MPG valueswere controlled by varying the solution concentrations,reaction temperatures and reaction times.

3.2. Thermogravimetric analysis (TGA)

During pyrolysis phosphorus compounds reduce thedecomposition temperature of wood by catalysingdehydration reactions, resulting in increased char for-mation. A decrease in temperature of pyrolysis andincreased char formation can be effectively determinedby TGA [2,14,15]. In Fig. 3, TGA curves are shown ofan untreated sawdust sample and a sawdust samplecontaining a fire retarding phosphorus compound. TheseTGA curves clearly show the effect of the phosphoruscompound on decomposition temperature and the charformation of the wood. The temperature for maximumrate of pyrolysis was lowered and the amount of residualchar at 400 �C was increased with respect to the blank.

Fig. 3. TGA curve of a sawdust sample containing a fire retarding

phosphorus compound and an untreated blank.

837R. Stevens et al. / Polymer Degradation and Stability 91 (2006) 832e841

The degree, at which the temperature of pyrolysis andthe residual char formation is influenced, is a measurefor the efficiency of the phosphorus compound. Accord-ing to the TGA results all chlorophosphorus compoundsdecrease the temperature of pyrolysis and increase thechar formation. In Fig. 4, the temperature for themaximum rate of pyrolysis of sawdust modified withdialkyl chlorophosphates is shown as function of MPG.

The temperature at the maximum rate of pyrolysisfor untreated sawdust was 350 �C. All dialkyl chloro-phosphates reduced the temperature of pyrolysis withthe same efficiency based on MPG. The size of alkylchain apparently has little influence on the temperatureof pyrolysis, with the exception of didodecyl phosphate.

Fig. 5 shows the residual char at 400 �C of thesawdust modified with dialkyl phosphates as a functionof MPG.

The residual char at 400 �C of untreated sawdust is25%. The effect of the dialkyl phosphates on the residualchar is comparable to their effect on the temperature forthe maximum rate of pyrolysis. All dialkyl phosphatesincrease the char formation with respect to untreatedsawdust. Again DDP-Cl was an exception; it increasedthe residual char about 10% more than the other dialkylphosphates at the same MPG value.

The anomalous behaviour of DPP-Cl can be ex-plained by the relatively low thermal stability of thedialkyl phosphates [10,16e19]. During attempted vacuumdistillations of dioctyl- and didodecyl chlorophosphatewe observed that these compounds decompose at 150e200 �C. NMR analysis of the distilled products showedthe presence of alkenes, formed by b-elimination. Thiswill also occur during the pyrolysis of the modifiedsawdust: the dialkyl phosphate groups will decomposeinto alkenes and phosphoric acid. The C2eC8 alkenesare highly volatile and evaporate from the wood, whilephosphoric acid remains in the wood, where it can actas fire retardant. Both we and Gamrath et al. [10]observed that with increasing size of the alkyl groups,

Fig. 4. Temperature for the maximum rate of pyrolysis of sawdust

samples reacted with dialkyl chlorophosphates.

the thermal stability of the alkylphosphates increases.The higher boiling point of dodecene (213 �C) comparedto e.g. octene (123 �C) could allow for an equilibrium toexist at the first stages of pyrolysis (Fig. 6). Yano et al.furthermore observed that the thermal stability of dido-decyl phosphate was 40e50 �C higher than that ofdodecyl phosphate leading to increasing formation ofphosphoric acid [19]. The latter can also be formedby disproportionation of two dodecyl phosphates todidodecyl phosphate and phosphoric acid.

Diphenyl chlorophosphate (DPP-Cl), phenyl di-chlorophosphate (PP-Cl2), phenyl dichlorophosphonate(PPO-Cl2) and diphenyl chlorophosphinate (DPPI-Cl)were used to study the effects of the phenyl-group and thedegree of substitution of the phosphorus compound onthe properties of sawdust. Also the effect of the oxidationstate of the phosphorus (phosphate, phosphonate orphosphinate) on the thermal properties of wood wasinvestigated. In Fig. 7, the temperature for the maximumrate of pyrolysis of the sawdust modified with phenyl-phosphorus compounds is shown as a function of MPG.

All phenylphosphorus compounds reduced the tem-perature for the maximum rate of pyrolysis. The resultsobtained with PP-Cl2 and DPP-Cl, are comparable,indicating that the number of phenyl-groups on thephosphate apparently has little influence. The perfor-mance of DPP-Cl is comparable to DDP-Cl, the bestperforming dialkyl phosphate.

The decrease in the temperature of pyrolysis stronglydepends on the phosphorus oxidation state, accordingto the order phosphateO phosphonate[ phosphinate.

During pyrolysis, the phenylphosphorus compoundswill eliminate from the wood at elevated temperatures.Unlike the alkyl analogues, the phenyl phosphoruscompounds cannot undergo further b-eliminations toyield phosphoric acid. DPP-Cl and PP-Cl2 will yielddiphenyl- and phenyl-phosphoric acid, respectively,while PPO-Cl2 will yield phenylphosphonic acid andDPPI-Cl diphenylphosphinic acid.

Fig. 5. Residual char at 400 �C of sawdust modified with dialkyl

chlorophosphates.

838 R. Stevens et al. / Polymer Degradation and Stability 91 (2006) 832e841

Fig. 6. Proposed b-elimination equilibrium mechanism for (di)alkylphosphates.

In Fig. 8 the percentage residual char at 400 �C ofsawdust modified with DPP-Cl, PP-Cl2, PPO-Cl2 andDPPI-Cl, is shown as function of MPG.

All phenyl phosphorus compounds increase the charformation during pyrolysis. PP-Cl2 and PPO-Cl2 contain-ing only one phenyl-group are just as effective inincreasing the char formation as the dialkyl phosphates(DAP).However,modificationwithDPP-Cl andDPPI-Clcontaining two phenyl-groups was less effective. Appar-ently, in contrast to the temperature of pyrolysis, the charformation does depend on the number of phenyl-groups.Again, the phosphinate has a dramatically reduced effectcompared to the phosphate and the phosphonate.

As phosphorus esters have been around for morethan 150 years, a wealth of data on these substances isavailable in the literature, albeit fragmented. Table 3is a compilation of available literature pKa values ofphosphorous compounds relevant to this study. Fromthe table it is apparent that the acidity order for theparent acids pKa (phosphoric acid)O pKa (phosphonicacid)O pKa (phosphinic acid) is reversed when thesubstituted analogues are considered. The series ofdialkyl phosphates clearly shows that with increasinglength of the alkyl group the acidity decreases. Alsosteric hindrance, especially from a-branching, reducesthe acidity further. Still, even the highly branched bis-2,2-dimethylhexylphosphate apparently has a lower pKa

value than the parent phosphoric acid in 75% ethanol.

Fig. 7. Temperature for the maximum rate of pyrolysis of sawdust

modified with phenylphosphorus compounds; a trend-line for the

dialkyl phosphates (DAP) is included for comparison.

When going from alkylphosphates to alkyl phosphi-nates, the influence of the size of the alkyl groups iscomparable. Striking however, is the difference in pKa

values between the phosphate and the phosphinate; forthe di-n-octyl derivatives approx. 2 orders of magnitude.As alkyl phosphates are relatively thermally unstable[10], it is to be expected that these compounds will beconverted to the less acidic parent acid as a result ofb-elimination. The exceptional performance of the morethermally stable didodecyl phosphate (DDP) underpyrolysis conditions can thus be explained.

Exchanging the alkyl substituents for phenyl-groupshas even more dramatic consequences. While phenyl-phosphate appears to be comparable to the linear alkylphosphates, diphenylphosphate has by far the highestacidity, almost two orders of magnitude higher than theparent acid. Molchanova and Dulova found comparabledifferences between diphenylphosphate and phosphoricacid over a wide range of solvents (alcohols, ketones,esters, and ethers) [20]. When the oxidation state of thesubstituted phenylphosphorus compounds is considered,the following acidity order can be observed phosphateOphosphonate[ phosphinate. The same order is ob-served in the TGA results with the phenylphosphoruscompounds.Whereas the dialkyl phosphates can undergo(double) b-elimination to the less acidic orthophosphoricacid, the phenylphosphorus cannot, as it lacks abstract-able b-hydrogen.

Fig. 8. Residual char at 400 �C of sawdust modified with phenyl-

phosphorus compounds.

839R. Stevens et al. / Polymer Degradation and Stability 91 (2006) 832e841

Table 3

Literature pKa values for phosphorus acids

Compound Formula pKa Solvent Ref

Orthophosphoric acid P(O)(OH)3 4.17 75% EtOH [21]

Phosphonic acid HP(O)(OH)2 3.15 75% EtOH [21]

Phosphinic acid H2P(O)(OH) 2.70 75% EtOH [21]

Dimethylphosphate (CH3O)2P(O)OH 3.01 80% EtOH [22]

Diethylphosphate (C2H5O)2P(O)OH 3.15 80% EtOH [22]

0.71 Water [23]

Di-n-butylphosphate (C4H9O)2P(O)OH 0.98 Water [23]

Di-n-hexylphosphate (C6H13O)2P(O)OH 1.07 Water [23]

Di-n-octylphosphate (C8H17O)2P(O)OH 3.30 75% EtOH [24]

Bis-2-ethylhexylphosphate (C8H17O)2P(O)OH 3.49 75% EtOH [24]

Bis-2,2-dimethylhexylphosphate (C8H17O)2P(O)OH 3.55 75% EtOH [24]

Di-n-decylphosphate (C10H21O)2P(O)OH 3.28 75% EtOH [25]

Diethylphosphinate (C2H5)2P(O)OH 3.15 Water [23]

Di-n-hexylphosphinate (C6H13)2P(O)OH 3.24 Water [23]

Di-n-octylphosphinate (C8H17)2P(O)OH 5.3 75% EtOH [24]

5.37 75% EtOH [25]

n-Octyl-phenyl-phosphinate (C8H17)(C6H5)P(O)OH 4.60 75% EtOH [25]

Bis-2-ethylhexylphosphinate (C8H17)2P(O)OH 5.88 75% EtOH [24]

Phenylphosphate (C6H5O)P(O)(OH)2 3.13 75% EtOH [21]

3.66 80% EtOH [22]

Diphenylphosphate (C6H5O)2P(O)OH 2.28 75% EtOH [21]

Phenylphosphonate (C6H5)P(O)(OH)2 3.96 75% EtOH [21]

Diphenylphosphonate (C6H5)(C6H5O)P(O)(OH) 2.85 75% EtOH [21]

Diphenylphosphinate (C6H5)2P(O)OH 4.10 75% EtOH [21]

4.24 80% EtOH [22]

So under pyrolysis conditions it is to be expected thatphenylphosphates will be the most acidic, and hencemost effective phosphorus compounds of the ones testedin this study. This is supported by the TGA results. Likealso observed by Lyons, other potential mechanismslike phosphorylation, phosphoric acid condensation,phosphate- and phosphonate ester hydrolysis and dispro-portionation reactions can (and probably do) alsocontribute to the flame retarding mechanism [6].

3.3. Equilibrium moisture content (EMC) analysis

The hydrophilic character of wood is a result of thelarge number of hydroxyl groups of the wood compo-nents (cellulose, hemi-cellulose and lignin). Duringmodification of the sawdust with phosphorus com-pounds, the hydroxyl groups are exchanged for morehydrophobic substituents. The hydrophilic character ofthe modified sawdust depends on the type of chloro-phosphorus compound used. To evaluate the hydro-phobicity of the modified sawdust, the EMC’s weredetermined. In Fig. 9, the EMC’s of sawdust modifiedwith dialkyl phosphates are shown as function of MPG.

The EMC of the blank at 90% relative humidityis 17.6%. All dialkyl phosphates used gave small tomedium reductions of the EMC. Whereas the effectof DEP was not significant, increasing the length ofthe alkyl chains to C6, C8, and C12 led to a moresubstantial decrease in EMC. From the data obtained

with DHP, DOP and DDP it would appear thata plateau is reached at approx. 15e16%.

Fig. 10 shows the EMC’s of the sawdust modifiedwith phenyl phosphorus compounds as function ofMPG. In contrast to the dialkyl phosphates, all phenylphosphorus compounds gave substantial reductions ofthe EMC. The diphenyl substituted compounds DPP-Cland DPPI-Cl were significantly more effective inlowering the EMC than the less hydrophobic mono-phenyl compounds PP-Cl2 and PPO-Cl2. DPP is clearlysuperior in reducing the EMC, and hence improving thedimensional stability and durability.

Fig. 9. Equilibrium moisture content of sawdust modified with dialkyl

chlorophosphates.

840 R. Stevens et al. / Polymer Degradation and Stability 91 (2006) 832e841

4. Conclusions

Modification of Scots Pine sapwood sawdust withvarious chlorophosphorus compounds resulted in co-valent bonding of organophosphorus groups to thesawdust. This was established by measuring the massincrease (WPG andMPG) after work-up, which includedextensive washing steps, and by solid state CP-MAS13C NMR. Detailed NMR analyses, exemplified in thecase of dibutyl chlorophosphate (DBP-Cl), gave un-ambiguous proof of covalent bond formation, which isnecessary for obtaining a non-leaching, sustainable woodtreatment.

The degree of modification of the sawdust dependson the type of phosphorus compound used, the concen-tration, reaction temperature and reaction time.

For dialkyl phosphates the degree of modificationdecreased with increasing size of the alkyl groups, due tosteric hindrance and increasing hydrophobicity. With thephenylphosphorus compounds the monophenyl com-pounds gave higher conversions than the correspondingdiphenyl substituted analogues. All modifications gavea decrease in the temperature for the maximum rate ofpyrolysis and an increase in the residual char comparedto the blank.

The C2eC8 dialkyl phosphates gave comparableTGA results. Due to the low thermal stability of thesecompounds they will undergo b-elimination to (the lessacidic) phosphoric acid under pyrolysis conditions. Themore thermally stable C12 analogue DDP has a higheracidity than phosphoric acid, which is reflected in themarkedly superior TGA results.

Phenylphosphates, -phosphonates and -phosphinatescannot undergo b-elimination due to the lack ofabstractable b-hydrogens, which results in a significantlyhigher thermal stability.

The superior TGA results obtained with the phenyl-phosphates can be rationalised based on their highthermal stability, combined with the fact that phenyl-phosphates are more acidic than the parent phosphoric

Fig. 10. Equilibrium moisture content of sawdust modified with

phenylphosphorus compounds.

acid. Whereas there is little difference in the TGA resultsobtained with phenylphosphate or phenylphosphonate,the difference in performance between the phosphateand the phosphinate is dramatic. The order in whichthe phenylphosphorus compounds affect the pyrolysisof the modified sawdust is consistent with the acidityorder of the compounds: phosphateO phosphonate[phosphinate. Thus, the TGA results can be rationalisedbased on the thermal stability and the acidity of thephosphorus compounds.

All phosphorus compounds used in this study reducethe EMC. Performance can be tuned by increasing thenumber, size and/or type of the substituents. Whereasthe results obtained with the dialkyl phosphates arerelatively poor, significant reductions in EMC can beachieved with the phenylphosphorus compounds, espe-cially the diphenyl substituted ones.

The objective of the project is to improve both thelong-term fire retardancy as well as the durability ofwood. The substances that best meet these two criteriaare didodecyl chlorophosphate, phenyl dichlorophos-phate and diphenyl chlorophosphate. With regard toavailability and reactivity the phenylphosphates arepreferred, especially diphenyl chlorophosphate.

Acknowledgements

This study is part of the long-term project ‘‘Sustain-able and fire retarded building materials’’. This project issupported with a grant of the Dutch Programme EET(Economy, Ecology, Technology) a joint initiative of theMinistries of Economic Affairs, Education, Culture andSciences and of Housing, Spatial Planning and theEnvironment. The programme is run by the EETProgramme Office, a partnership of Senter and Novem.The project is also financed by the Ministry ofAgriculture, Nature and Food Quality.

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