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Durability of the Sizing Degree of AKD and ASA SizedPapers Investigated by Contact Angle Measurementsand ToF-SIMSRauni Seppänen aa YKI, Institute for Surface Chemistry , Stockholm, SwedenPublished online: 27 May 2009.

To cite this article: Rauni Seppänen (2009) Durability of the Sizing Degree of AKD and ASA Sized Papers Investigatedby Contact Angle Measurements and ToF-SIMS, Journal of Dispersion Science and Technology, 30:6, 937-948, DOI:10.1080/01932690802646330

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Durability of the Sizing Degree of AKD and ASASized Papers Investigated by Contact AngleMeasurements and ToF-SIMS

Rauni SeppanenYKI, Institute for Surface Chemistry, Stockholm, Sweden

The influence of storage conditions on the sizing degree of AKD and ASA sized pilot papers wasevaluated. A number of pilot papers sized with AKD or ASA were prepared from ECF bleached pulpfibers, unfilled and filled with 20% PCC, respectively and investigated in terms of sizing degree over aperiod of several months. The papers were stored at 23�C and 50% RH either wrapped in aluminiumfoil or as separate sheets exposed to open atmosphere. The unfilled papers stored protected fromambient atmosphere after papermaking showed only a marginal reduction in sizing during prolongedstorage. Only the paper having the lowest AKD-dosage suffered from reduced water-resistance, theCobb60-value changed from 34 to 79 g/m2. The PCC filled papers stored in the same conditions lostsome of their sizing, to a higher extent in the case of AKD than for ASA sized papers. This was attrib-uted to the further hydrolysis of the size catalyzed by PCC. In comparison, the sizing of the papersstored as separate sheets dropped significantly even after a few weeks in storage. In the end of thestorage the AKD papers, particularly the unfilled ones had lost their sizing efficiency to a clearlyhigher extent than the ASA papers. The reduction in the sizing level occurred mainly during the firstfive weeks for the unfilled ASA and AKD papers, after which the process continued at a slower rate.The ToF-SIMS analysis revealed that both AKD and hydrolyzed AKD, the latter being the majorportion, were present at the outermost surface of the unfilled AKD sized papers, but in significantlylower levels than in the case of the corresponding protected papers. In other words, a significant loss ofAKDmass had occurred for the papers exposed to an open atmosphere. This was attributed to migra-tion of AKD. The results demonstrated that ASA sized papers also suffered from size loss. TheToF-SIMS results showed no signal for active, nonbonded ASA and instead clear signals wereobserved for hydrolyzed ASA and for calcium and aluminium. As in the papers wrapped in aluminiumfoil, ASA was mainly in its hydrolyzed form. Although to a markedly lower extent, the reason forsizing loss in the case of ASA was the same as for AKD.

Keywords AKD, alkenyl succinic anhydride, alkyl ketene dimers, ASA, contact angle, ECFpulp, PCC, sizing, sizing degree, sizing loss, storage, ToF-SIMS, wettability, XPS

INTRODUCTION

Alkyl ketene dimers (AKD) and alkenyl succinicanhydrides (ASA) are widely used as internal sizing agentsto make cellulosic fibers hydropbobic and thus control the

wettability of paper and liquid packaging board byaqueous liquids and the runnability of papermachines.The sizing process comprises retention, distribution,spreading, and the anchoring of the sizing agent to fibers(for a recent review, see Hubbe[1]). Additionally, size rever-sion may occur. It has been stated that increasing the timeor temperature of heat treatment of laboratory papersheets caused a decrease in the amount of retained AKD,although the water-resistance of the sheets wasunchanged.[2] The authors explained the phenomenon bydiffusion and migration of AKD caused by physical redis-tribution. It has been reported that vapour-phase deposi-tion of alkyl ketene dimer on dry paper does not lead tosizing development since a preferential reaction with watertakes place.[3] In contrast, desorption and vapour phasetransfer leading to readsorption and thus to sizing has beenclaimed to be behind the sizing mechanism of AKD.[4,5]

Air-blowing treatment of laboratory paper sheets has been

Received 29 April 2008; accepted 5 May 2008.Part of the special issue, Surface and Colloid Chemistry

Without Borders: An International Festschrift for Professor PerStenius on the Occasion of His 70th Birthday.

The author would like to thank Hercules, Kemira Oyj, M-real,Stora Enso Oyj, Tetra Pak and UPM for financial support. Theauthor is very grateful to Hercules who supplied pilot papers.Karin Hallstensson and Mikael Sundin are thanked for the assis-tance with experimental work. Ulrika Johansson at SP TechnicalResearch Institute of Swedish is acknowledged for valuablediscussions and help with the ToF-SIMS analysis.

Address correspondence to Rauni Seppanen, YKI, Institutefor Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden.E-mail: rauni.seppanen@surfchem.kth.se

Journal of Dispersion Science and Technology, 30:937–948, 2009

Copyright # Taylor & Francis Group, LLC

ISSN: 0193-2691 print=1532-2351 online

DOI: 10.1080/01932690802646330

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observed to decrease sizing degree although the sizecontent was virtually unchanged.[6] This was explained byoxidation of AKD and formation of hydrolyzed AKD.In a previous article,[7] the authors suggested that AKDdiffusion due to evaporation of AKD occurs to a minorextent parallel to and=or after surface diffusion, thus par-tially explaining a desizing phenomenon, that is, decreasedhydrophobicity of paper after long-term storage. The studywas performed at elevated temperatures of 50�C and 90�C,respectively. Migration through surface diffusion occurredprobably as well. It has been demonstrated that desorptionand vapour phase transfer occur, but only to a marginalextent that did not result in sizing of laboratory paper heetby AKD.[8,9] The evaporated components from AKD weremainly fatty acids and hydrolyzed AKD. Using imagingXPS Shchukarev et al.[10] illustrated that after certainhydrodynamic spreading in the papermachine drier section,AKD can spread further via surface diffusion in the formof an autophobic monolayer precursor. Sizing loss ofAKD in the PCC-filled alkaline papers has been explainedby transformation of AKD to its hydrolyzed ketone formand calcium carbonate reaction products over time.[11]

The latter will decompose gradually to ketone as well. Incomparison, ASA sized handsheets may loose their sizingefficiency via auto-oxidation at a-carbons of double bondsin ASA molecules followed by introduction of somehydrophilic groups in the molecules by oxygen atoms.[12]

Additionally, it has been shown that ASA sizing can occurvia vapour phase diffusion.[5,9]

In recent work, change in the degree of sizing of pilotpapers over prolonged storage time was investigated interms of wettability using contact angle. From a thermody-namic viewpoint, the wettability of paper by a liquid isdetermined by the surface tensions of the paper and theliquid. The wetting dynamics may, however, depend on anumber of other properties such as the liquid viscosity[12]

and the heterogeneity, physical,[13] or chemical[14] proper-ties of the substrate, as well as sheet structure.[15] It is wellknown that the apparent contact angle on a rough surfaceis higher than the value on the corresponding smoothsurface.[13,15–17] The uneven structure creates barriers tospreading, which increase the contact angle compared tothe situation for a flat surface. However, there is arelatively good qualitative understanding of the factorsaffecting the determination of equilibrium contact angleon paper and the dynamics of wetting of sized paper.[15–19]

X-ray photoelectron spectroscopy (XPS) providesquantitative data on both the elemental composition anddifferent chemical states of atoms in the surface layer. Inthis work, the aliphatic C�C carbon peak of the high-resolution C1s signal was analyzed as an indicator of sizeamount. This approach was used earlier by Dorris andGray,[20] and Strom et al.[21] ToF-SIMS was used to studya detailed surface composition of the AKD and ASA sized

papers in addition to surface distribution in order to gaina deeper understanding of observed size reversion.ToF-SIMS has been used earlier to resolve AKD fromthe corresponding ketone on paper.[22–24] and to detectASA on paper.[25]

The objective of this work was to evaluate the influenceof storage time on the sizing degree of AKD and ASA sizedpilot papers made under the same conditions. The pilotpapers were prepared to represent fine paper or the fullybleached top-ply of cartonboard, but were not surface sizedor calendered. The papers were prepared without filler, andwith 20wt% precipitated calcium carbonate (PCC). Thepapers were stored either in a stack and wrapped inaluminium foil or as separate sheets exposed to the openatmosphere at 23�C and 50%RH. The sizing degree, as afunction of storage time was, followed by measuring thedynamic contact angle of two liquids having different sur-face tensions. The surface chemical composition of some ofthe samples was determined with XPS. The total amount ofsize in paper was analyzed using both liquid chromatogra-phy and mass spectrometry (LC-MS). To gain improvedunderstanding, the chemical structures and spatial distribu-tion of AKD and ASA on the surfaces of some of thepapers were studied using time of flight secondary ionmass spectroscopy (ToF-SIMS). The identification offactors contributing to decreased sizing over prolongedstorage time has not been reported in a systematic wayearlier.

EXPERIMENTAL

Materials

Pulp. Bleached ECF 70% hardwood and 30% softwoodpulp refined to 27�SR was obtained from Stora EnsoBerghuizer mill, the Netherlands.

Pilot papers. Pilot papers with a grammage of 80 g=m2

were prepared on a Fourdrinier pilot paper machineat the former Hercules European Research Center,Barneveld, the Netherlands. The machine speed was12m=min. Drying was performed using steam-heated dry-ing cylinders, at max. 105�C. Six different ASA or AKDaddition levels from 0.03 to 0.30wt%, based on active sizewere used. The papers were made at pH 8.0. After prepara-tion, the papers were sealed in aluminium foil and placed ina plastic bag and stored at 23�C and 50%RH for a weekbefore first measurements. From that time a half of thepapers were stored as separate sheets exposed to the openatmosphere at 23�C and 50%RH.

Sizing agents. AKD (melting point 55–60�C) emulsionand ASA-C18 emulsion were obtained from Hercules.

Filler. 20wt% calcitic scalenohedral PCC based onpaper from Sturge Lifford, UK was used. The specificsurface of the PCC was 6m2=g.

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Retention aid. 0.0125wt% cationic retention copolymerfrom BetzDearborn was used in the case of unfilled papers.0.5wt% cationic potato starch (N¼ 0.35%) fromRocquette Freres, France and 0.015% cationic retentionpolymer together with 0.2wt% refined bentonite fromAllied Colloids, UK were used for PCC filled papers.

Alum. 0.5wt% alum Al2(SO4)3 � 14H2O, based on theASA amount was added in the case of ASA sized papers.Alum was from Breusted Chemie Apeldoorn, theNetherlands.

Methods

Contact angle evaluation method. Apparent dynamiccontact angles of test liquid drops were determined fromside-images of the drop profile, which was monitored as afunction of time with a Dynamic Absorption Tester (Fibro1100 DAT, Fibro Systems AB, Sweden). The instrumentapplies a drop to the surface while a high-speed videocamera captures images as the drop spreads and=or isabsorbed. Liquid is automatically pumped out from a syr-inge and the drop is formed at the tip of a Teflon-coatedtube hence avoiding liquid sticking to the tip. Images ofthe drop are captured every 20 milliseconds. After the mea-surement, images are evaluated by image analysis in termsof drop volume, height, base diameter=area and contactangle. The test liquids were MilliQ Plus water and ethyleneglycol (Table 1). The drop volume of the test liquids used inthe measurements was 4 ml. Measurements were performedat five different locations on each sample. The cross-sectionof the drop was parallel to the machine direction. Themeasurements were performed at 23�C and 50%RH.

Total amount of size in paper. The analysis was carriedout with LC-MS instrumentation at StoraEnso ResearchCentre in Imatra according to a method reported byLaitinen.[26] The analysis was performed in the end of thestorage time on papers stored in a stack and well wrappedin aluminium foil at 23�C and 50%RH.

X-ray photoelectron spectrometer (XPS). The surfacechemical composition of pilot papers was determined witha Kratos AXIS HS x-ray photoelectron spectrometer(Kratos Analytical, UK) at a photoelectron take-off angle

of 90�. The samples were analyzed using a monochromaticAl Ka x-ray source for high-resolution carbon spectra andMg Ka x-ray source for wide and detailed spectra.Analyzes were made on an area less than 1.0mm2. Widespectra were run to detect elements present in the surfacelayer. The relative surface compositions were obtainedfrom quantification of detailed spectra run for eachelement.

The relative amounts of carbon species with differentbonds to oxygen were determined from high-resolutioncarbon C1s spectra using a Gaussian curve-fitting programfrom the spectrometer manufacturer. The chemical shiftsrelative to C�C (C1) used in the convolution were286.8 eV for C�O (C2), 288.2 eV for O�C�O or C¼O(C3), 289.4 eV for O¼C�O (C4), and 290.1 eV forcarbonate carbon (C5).

Time-of-flight secondary ion mass spectrometry(ToF-SIMS). This method is based on high-resolutionmass spectrometric analysis of secondary ions emitted fromthe surface during bombardment by an energetic ion beam.The secondary ion mass spectrum, negative or positive,represents fragments of the chemical compounds existingin the outermost layers of the analyzed material, and thusconstitutes a fingerprint of the surface chemical composi-tion. The technique is highly surface sensitive, with a typi-cal information depth of 1 nm. It should be noted that theintensity of signals for different secondary ions is not neces-sarily linearly dependent on concentration, which meansthat absolute quantifications cannot be made withoutcalibration against samples with known composition.Comparison of relative peak height can often be madebetween similar samples.

The samples were mounted in a sample holder andanalyzed in a ToF-SIMS instrument (ToF-SIMS IV,IonTof GmbH, Germany). Positive and negative secondaryion mass spectra were recorded from two different areas of500� 500 mm2 on ASA sized papers, whereas the AKDpapers were studied in the positive mode. 25 keV Bi3þ ionsat a beam current of 0.11 pA were used as primary ions.The time for recording each spectrum was 100 seconds.

RESULTS AND DISCUSSION

Degree of Sizing Over Storage Time

Papers Wrapped in Aluminium Foil

Figure 1 gives Cobb60 values as a function of theamount of size added to the stock during papermaking.This demonstrates that the papers prepared showed goodwater resistance at medium and high size dosages. A higherdosage of ASA was needed for the same sizing level withresult to AKD sized papers. A small difference in totalamount of ASA and AKD in the papers was observed, thatis, the sizes were not retained to the same extent. In this

TABLE 1Physical properties of test liquids

Liquid

Surfacetension mN=m(23� 2�C)

Density g=cm3

(24� 2�C)ViscositymPas

(25�C)

Water 72.0� 0.1a 0.998 0.89Ethyleneglycol

48.8a 1.110 16.1

aMeasured in our lab.

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work, the focus was however to evaluate how well theoriginal sizing degree of the papers is retained duringstorage and to investigate possible sizing reversion.

The results in Figure 1 agree well with those obtainedusing contact angle presented in Figure 2. Figure 2 showsapparent contact angles for water obtained at 10 secondsfor unfilled papers sized with AKD or ASA and storedsealed in aluminium foil at 23�C and 50% RH. The first

measurement was performed a week after paper prepara-tion, and the last one 68 weeks later. The initial contactangles display how an increased size addition improvesthe hydrophobicity of paper. Capillary penetration by aliquid is retarded resulting in reduced wettability of paperwhen the hydrophobicity of the paper is increased.Negative capillary pressures arise for contact angleslarger than 90�, which means an effective resistance againstliquid penetration according to the Laplace equation(Pc¼ 2c cos h=r where c is the surface tension of the liquid,h the contact angle, and r the pore radius). This is for anideal system or parallel cylindrical pores. Thus, the advan-cing contact angle of well-sized paper should be larger than90�. As illustrated in Figures 1 and 2, this is sufficient forhindering spontaneous uptake of water by the paper.

All the displayed contact angles are clearly over 90�,most likely enhanced somewhat by the surface roughnessof paper.[13] As seen in Figure 2, the contact angles of waterat high size addition levels reach a maximum close to 125�.It is difficult in practice to obtain contact angles thatexceeds 120�.[27] For the papers sized at high dosages(0.24% and 0.30%), almost no change is observed in theadvancing water contact angle with storage time. However,this does not mean that surface coverage of AKD isconstant, but simply that the water contact angle is nolonger sensitive to variations in AKD surface coverage.

The results in Figure 2 also illustrate how the sizingdegree (given here as contact angles with water) is nearlyunchanged over storage time for both ASA and AKD sizedpapers. No more than a slight decrease compared to the

FIG. 1. Cobb60 values vs. size dosage for unfilled pilot papers sized

with ASA and AKD.

FIG. 2. Effect of storage on the hydrophobicity of unfilled pilot papers sized with ASA (a) or AKD (b). The apparent contact angles for water

obtained at 10 seconds are average values of five measurements. The standard deviation was most often less than 3�. The papers were stored in a stack

and sealed in aluminium foil at 23�C and 50%RH.

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original values was observed. The sizing degree increasessomewhat in the ASA sized papers containing 0.06–0.12% added ASA over time. This may indicate thatunreacted ASA still present at fiber surfaces is consumedto form esters.[28] For the ASA paper with the lowest addi-tion level (0.03%) the first contact angle measured is low,approx. 37�. After one week’s storage water absorbed

immediately into this paper, and thus no contact anglecould be determined. It is well known that AKD sizingmay increase for several days, but Figure 2 does not showany improvement in the sizing degree with time. The AKDpaper with the lowest dosage shows nearly unchangedcontact angles of approximately 90� over the storage time.This means that if there are some changes in the AKD

FIG. 3. Apparent contact angles for ethylene glycol at 10 seconds for ASA (a) and AKD (b) sized unfilled pilot papers stored in a stack and sealed in

aluminium foil at 23�C and 50%RH.

FIG. 4. Effect of storage on the hydrophobicity of PCC filled pilot papers sized with ASA (a) and AKD (b) and stored sealed in Al foil at 23�C and

50%RH. The apparent contact angles for water obtained at 10 seconds are average values of five measurements.

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chemistry, they are relatively small. Moreover, ASAproduces papers with somewhat higher sizing degrees thanAKD at high addition levels.

It is useful to use a test liquid with a lower surfacetension to probe variations in the paper hydrophobicity,in particular amongst papers having high size coverage.Therefore, the sizing degrees and possible changes in them

were also probed by ethylene glycol. Ethylene glycolabsorbed rapidly into both ASA and AKD papers at low-est size addition level 0.03% in spite of higher viscosity thanwater. It is apparent from Figure 3 that ethylene glycolwets the papers better than water and thus reveals largerdifferences particularly at high degrees of sizing. The con-tact angles for both ASA and AKD sized papers with a size

FIG. 5. Apparent contact angles for water at 10 seconds for ASA (left) and AKD (right) sized unfilled pilot papers stored as separate sheets at 23�Cand 50%RH.

FIG. 6. Apparent contact angles for ethylene glycol at 10 seconds for ASA (left) and AKD (right) sized unfilled pilot papers stored as separate sheets

at 23�C and 50%RH.

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addition of 0.12% or higher were virtually unchanged overtime, whereas a slight decrease was observed for the otherpapers. The results also suggest that AKD papers, exclud-ing the two papers with high size additions, have highersizing degree than ASA at the same size addition level.

The results indicate that size reversion occurs on a minorscale, but this was observed only among the paperscontaining low size amounts. The papers with higher sizecontents have an excess size reservoir and therefore noclear decrease in the sizing degree over time was observed.

Figure 4 shows that sizing loss occurs to some extentin the PCC filled papers. A linear time-dependence is

observed. The AKD sized paper with the lowest size addi-tion displays a higher decrease in the hydrophobicity thanthe corresponding ASA paper. The reason most probablyis the catalytic effect of PCC[11,29] converting AKD to itshydrolyzed ketone form.

Papers Stored as Separate Sheets

The corresponding results shown in Figure 5 for thepapers stored as separate sheets open to the ambient atmo-sphere differ significantly from sheets stored in a stack. Aclear sizing loss seems to have occurred. For example, thepaper with 0.09% added size displays a significant decreasein its hydrophobicity already after 4 weeks’ storage. Onlythe papers containing high amounts of the added size, thatis, 0.24 – 0.30% show high contact angles against waterafter 68 weeks of storage. The AKD sized papers appearto suffer to a higher extent from the sizing loss than ASA.

FIG. 7. Initial dynamic contact angles for ethylene glycol at 0.1 seconds for ASA (a) and AKD (b) sized PCC filled pilot papers stored as separate

sheets at 23�C and 50%RH.

TABLE 2O=C ratios and relative amounts of C�C, that is,

normalized with respect to the total amount of carbondetermined with XPS for the unfilled and PCC-filled pilotpapers stored for 4 months. The original samples were

analyzed after 1 week

Sample and size addition O=C C�C in atomic%

Unfilled papersAKD 0.30wt%

Original 0.37 31Stored in Al foil 0.39 29Stored as separate sheet 0.44 26

ASA 0.30wt%Original 0.38 36Stored in Al foil 0.39 33Stored as separate sheet 0.51 23

TABLE 3Total amount of AKD in the unfilled papers

determined using LC-MS

Added amountof AKD (%)

Total amountof AKD in paper (%)

0.09, protected 0.0580.09, exposed to air 0.0290.12, protected 0.0740.12, exposed to air 0.038

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For instance, the 0.12% ASA paper shows only a smalldecrease in the water contact angles, whereas the contactangles of the corresponding AKD paper show cleartime-dependence.

The results in Figure 5 are confirmed by the data forethylene glycol displayed in Figure 6. A clear differenceis observed amongst the papers with the high size additionlevels. Interestingly, the reduction in the sizing degree forthe ASA sized papers mainly occurs during the first 4weeks, whereas the degradation kinetics for AKD appearsto be totally different. For instance, the original contactangle being slightly above 90� for the 0.12% paper was60� after 5 weeks’ storage. Different degradation regimeswere observed for the papers with high content ofAKD, that is, the 0.24% and 0.30% papers. The degrada-tion mainly occurs during the first 16 weeks, after that it

slows down. However, a significant decrease in the sizinglevel occurred already within a month. There is aonly small reduction in the sizing degree after 45 weeksof storage.

On the PCC filled papers, both test liquids absorbedrapidly into paper during the first seconds. To illustratethe hydrophobic character of these papers, contact anglesfor water measured at 0.1 seconds are shown in Figure 7.It is clear that the sizing loss is much more rapid in thepapers filled with PCC than for the unfilled grades above(Figure 5). The sizing degree of the ASA papers reducesrapidly during 16 weeks, after which the decrease continuesat a slower rate. Note that the decrease in the beginning wasnot as deep for the unfilled papers (Figure 6). The reason isagain most probably the catalytic effect of PCC,[11] wherethe original unbonded ASA has hydrolyzed to diacid.

FIG. 8. ToF-SIMS positive ion mass signals of 533.5 and 507.5 for the unfilled (two top images) and PCC-filled (two bottom images), 0.24wt%

AKD sized papers stored in aluminium foil. Two different areas per sample were analyzed.

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Surface Chemical Composition

The surface chemical composition of some of theunfilled and PCC-filled papers was analyzed with after 4months storage. Table 2 gives the ratios between oxygenand carbon, C=O, and relative amounts of C1 (C�C)carbon. The results confirm the sizing loss, especially forthe papers stored as separate sheets under ambient condi-tions. The O=C ratio increases indicating decreasedamounts of carbon from AKD or ASA. Additionally, thepapers stored in aluminium foil also show slightly loweramounts of C�C carbon than the original papers. Theresults are in good accord with the findings above.

There are several possible reasons for sizing loss or rever-sion in the papers stored as separate sheets. In the case ofAKD, the decay may be related to desorption and transferto the gas phase of AKD not bonded to fiber surfaces, aprocess which becomes dominant when the AKD reservoirs(in the form of retained particles=droplets) are emptied bysurface diffusive spreading. This was the conclusion in aprevious paper[7] where we reported decreased water contactangles with time for laboratory paper sheets stored unpro-tected either at 50�C or 90�C. However, the fact that thehydrophobicity significantly decreased with time indicatesthat nonextractable AKD has also reversed. In the currentwork, the storage temperature was 23�C, clearly below themelting point of the AKD (56–60�C). Water vapour diffu-sion (or transmission) through the fiber matrix in the papersover time may have caused hydrolysis of AKD, as claimedearlier by Akpabio and Roberts (1987), but also ruptureof possible ester bonds.

In comparison, the relatively rapid decline in the sizingdegree of the ASA papers may be due to hydrolysis,[28]

auto-oxidation,[30] or evaporation[5,9] or due to a combinedeffect of these processes. When there is active, unbondedASA at the paper surface, it most probably has beenhydrolyzed to alkenyl succinate acid by water vaportransmission through fibers during the time in storage.

One might also ‘‘speculate’’ that fluctuations inhumidity has loosened the bonds between fibers in thepapers stored as separate sheets resulting in larger poreradii. This in turn would influence the wettability.

Liquid chromatography mass spectrometry (LC-MS)analysis confirmed the loss of mass from the papers storedexposed to ambient conditions (see Table 3). Half of theAKD has probably migrated.

TABLE 4Relative signal intensities (%) from TOF-SIMS data ofpositive mass spectra for the reference paper and unfilled0.24% AKD paper stored either wrapped in aluminiumfoil or as a separate sheet at 23�C and 50% RH. Thegiven values are average values measured on two

different areas of 500� 500mm

IonMass(a.u.)

Sheets storedin aluminium

foil

Sheets stored asseparate sheetsin ambientconditions

UnfilledPCCfilled Unfilled

PCCfilled

AKD 533.5 43 6.4 8.9 2.9AKDketone

507.5 84 57 48 38

Totalcounts

10137277 10202095 8460664 10245967

FIG. 9. (a) ToF-SIMS imaging of unfilled (top) and PCC-filled

(bottom) 0.24wt% AKD sized papers stored in aluminium foil. Images of

total ions, AKD (533.5) and hydrolyzed AKD (507.5) were taken using ras-

ter size of 128� 128 and positive mode. Two different areas were analyzed.

(b) ToF-SIMS imaging of unfilled (top) and PCC-filled (bottom), 0.24wt%

AKD sized papers stored as separate sheets. Images of total ions, AKD

(533.5) and hydrolyzed AKD (507.5) were taken using raster size of

128� 128 and positive mode. Two different areas per sample were analyzed.

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To gain a deeper understanding of the mechanismbehind sizing loss, the papers were also studied for surfacechemical composition with ToF-SIMS. This was conductedafter prolonged storage of the papers. The AKD specificfragment ion masses in a SIMS spectrum are, for example533.5 for (MþH)þ where M is the AKD molecule, and507.5 (MþH�CO)þ for hydrolyzed AKD (ketone).Figure 8 shows these AKD specific fragment ions for theunfilled, 0.24% AKD sized paper stored in different ways.The relative signal intensity for peaks 533.5 and 507.5 ispresented in Table 3. The peaks are normalized againstthe total ion intensity for the respective samples. Theresults clearly show that there are both original, unbondedAKD wax and ketone in the outermost surface of the paperstored sealed in aluminium foil. However, AKD is presentto a higher extent in the ketone form. The correspondingpaper stored as a separate sheet shows a remarkably lowamount of original unbonded AKD (peak at 533.5). Addi-tionally, the amount of ketone is clearly lower than for thepaper being stored in foil. The reason for decreased hydro-phobicity in the PCC filled papers was assumed to be dueto hydrolysis of AKD catalyzed by PCC as suggested byBottorff (1994). As seen in Table 4 the main portion ofAKD is in the ketone form on the surface of the PCC filledpaper sized by 0.24% AKD.

Figure 9a top image shows that in the paper with arelatively high AKD dosage of 0.24% the originalunbonded AKD (at 533.5) is distributed evenly on thepaper surface and covers nearly the whole surface. The

surface roughness of the paper seems to be the reason forlack of signal in some small areas. The spectra shown herewere run in high mass spectra mode. There is clearly morehydrolyzed AKD (peak at 507.5) than AKD (peak at533.5) at the paper surface and that it is more in the formof bigger particles than for AKD.

Figure 9a bottom image confirms that there is less AKDat the surface of the PCC filled paper although the sameAKD dosage was applied during papermaking as in thecase of the unfilled paper in Figure 9a. The result wasexpected and most likely depends on the fact that thespecific surface area of PCC is higher than that of the fibersurface, but also on different retention and spreadingbehaviour of AKD on PCC compared to fiber surfaces.The major portion of AKD is in the hydrolyzed form, aswas assumed above.

It is clear from Figure 9b that the corresponding papersstored open to ambient conditions have both hydrolyzedAKD and original AKD on their surfaces. The amountsare, however significantly lower than for the papers storedcovered. The results strongly indicate that the mass loss ofAKD appears to be the reason for the reduced sizingdegree, that is the increased wettability presented inFigures 4 through 7.

Migration through desorption and vapor phase diffu-sion at the low storage temperature seems not possibledue to the relatively high molecular mass of AKD andketone (e.g., 532 for AKD-C18). Migration occurs prob-ably through surface diffusion, from a monolayer ofketone.

Since the sizing degree of the unfilled papers storedprotected was virtually unchanged during the prolongedstorage time according to the results above, it is possiblethat AKD has partly been in the hydrolyzed form at thefirst measurement, a week after papermaking. In view ofthis it was also interesting to examine the amount ofbonded AKD, that is, nonextractable by tetrahydrofuranat the end of the storage time. Figure 10 shows Cobb60-values versus amounts of bonded AKD. The papers

FIG. 10. Cobb60 values versus amounts of nonextractable AKD

for unfilled pilot papers sized using six different AKD dosages. The

measurements were performed in the end of the storage.

TABLE 5TOF-SIMS data from negative mass spectra of the unfilled0.24wt% ASA paper stored either wrapped in aluminiumfoil or as a separate sheet at 23�C and 50%RH. The givenvalues are average values measured on two different areas

of 500 m�500 mm

IonMass(a.u.)

Stored inAl-foil

Separatesheet

Hydrolyzed ASA 367.30 85 4.2ASA (M�Hþ) 349.29 6.4 6.3ASA(M�CO�Hþ) 321.28 6.6 6.8Total counts 6048903 5291435

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contained surprisingly small portions of nonextractableAKD as earlier demonstrated by other researchers[2,31,32]

and later by Laitinen.[26] The Cobb60-values of the papersmeasured in the beginning of this work was presented inFigure 1. It is noteworthy that only the paper with thelowest AKD dosage, that is, corresponding 0.002%nonextractable AKD demonstrates reduced water-resistance over the prolonged storage time.

Table 5 compiles specific peaks for ASA obtained fromthe negative ToF-SIMS mass spectra[25] of the ASA sizedpapers. The ASA unfilled paper stored in aluminium foil

shows peaks from both ASA and hydrolyzed ASA, thestrongest peak clearly being from the hydrolyzed ASA.The amounts are however relatively small. The same paperstored exposed to open air shows merely traces of ASA andhydrolyzed ASA on its surface.

The major signals in the ToF-SIMS analysis for theASA sized papers were found in the positive mass spectrafor calcium, Ca (40 a.u.) and aluminium, Al (35 a.u.) asshown in Figure 11. They most likely originate from thecomplexes of ASA with calcium and aluminium ions. Peaksoriginating from the expected ASA complexes were,however not found in the mass spectra as also reportedby Wasser and Brinen.[25] The results in this work supportthe earlier findings[26] that electrostatic bonding (ionicbonding and complex formation) is the main anchoringmechanism for ASA. Thus, the ASA-acid plays a key rolein ASA sizing.

CONCLUSIONS

The influence of storage on the sizing degree of pilotpapers sized with ASA and AKD and stored in differentways was investigated in terms of wettability over numer-ous months using contact angle.

The results clearly showed that the storage conditionsand PCC as a filler influence AKD and ASA sizing. Theunfilled, sized papers stored as separate sheets exposed toambient conditions significantly lost sizing after a fewweeks storage. It partly may depend on water vapour diffu-sion through the fiber matrix over time causing hydrolysisof AKD. The major portion was in the form of hydrolyzedAKD, that is, ketone as shown by ToF-SIMS. However,the XPS analysis showed sizing loss as lower C-C carbonamounts after only a few weeks storage. Furthermore,the ToF-SIMS spectra displayed decreased signals forAKD and ketone and the LC-MS analysis demonstrateda lower amount of total AKD including ketone. The lossof AKD mass occurred in spite of the fact that the paperswere stored at a low temperature of 23�C. PCC as a fillerincreased the breakdown of sizing. In the case of ASA sizedpapers, a typical feature was that only marginal amounts ofactive nonbonded ASA were found at the paper surface.Instead clear signals for hydrolyzed ASA and also forcalcium and aluminium were observed. This suggests thatASA was present at the paper surface mainly in the formof its hydrolyzed products, that is, ASA acid and ASAcomplexes with calcium and aluminium. ASA suffered lessfrom sizing loss.

Wrapping papers in aluminium foil hindered the sizingloss in the unfilled papers, indeed for more than a yearand after that the decrease in the sizing degree was quitesmall. It was clear that sufficient and durable sizing overtime is achieved at a low level of 0.005 percent of non-extractable AKD. The water-resistance reduced only for

FIG. 11. (a) ToF-SIMS imaging of unfilled, 0.24wt% ASA sized

paper stored in aluminium foil. Negative mode images (top) show ASA

(M�CO�H), ASA, hydrolyzed ASA and total ions. Positive mode images

(bottom) show aluminium, calcium and ASA and total ions. The images

were taken using a raster size of 128� 128; (b) ToF-SIMS imaging of

unfilled, 0.24wt% ASA sized paper stored as a separate sheet. Negative

mode images (top) show ASA (M�CO�H), ASA, hydrolyzed ASA and

total ions. Positive mode images (bottom) show aluminium, calcium and

ASA and total ions. The images were taken using a raster size of

128� 128.

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the paper having the lowest sizing degree in the beginningof this study. PCC as a filler reduced the sizing effect ofthe AKD papers most probably by catalyzing the hydro-lysis of AKD. The ToF-SIMS results confirmed that themajor portion was in the form of hydrolyzed AKD also inthe case of the unfilled papers. The degree of sizing wasreduced also in the PCC filled and ASA sized papers.

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[27] Wasser, R.B. and Brinen, J.S. (1998) Tappi J., 81(7): 139.[28] Laitinen, Risto. (2006) Development of LC-MS and extrac-

tion methods for the analyses of AKD, ASA, and rosin sizesin paper products, Ph.D. thesis, Lappeenranta University ofTechnology, Finland.

[29] deGennes, P.G., Brochard-Wyart, F., and Quere, D. (2004),Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls,Waves; New York: Springer-Verlag, pp. 1–291.

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[31] Lindstrom, T. and Glad-Nordmark, G. (2007) Nord. PulpPap. Res. J., 22(2): 161.

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