Formation of multilayers on silica surfaces of a cationic polyelectrolyte and dissolved and colloidal substances originating from mechanical wood pulp-Adsorption and influence on adhesion

Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

Formation of multilayers on silica surfaces of a cationic polyelectrolyte and dissolved and colloidal substances originating from mechanical wood pulp-Adsorption and influence on adhesion Mats Rundlöf a,1 , Lars Wågberg b,∗ b

a AB Capisco Science & Art, Nygatan 13, SE 60234, Norrköping, Sweden Department of Fibre and Polymer Technology, Div. Fibre Technology, KTH, 10044 Stockholm, Sweden

Received 14 July 2003; accepted 28 January 2004

Abstract By measuring adsorption using a stagnation point reflectometer, it was shown that multilayers consisting of cationic polymers and dissolved and colloidal material released from wood fibres into process waters are formed on an anionic silica surface. The similarity of this experiment to the papermaking process was established. It was established using the “JKR-method” that this multilayer adsorption had a profound effect on the adhesion, in air, of an elastic polydimethyl siloxane (PDMS) probe to these surfaces. The adhesion of PDMS to a bare silica surface showed a large hysteresis, with a much stronger adhesion measured upon the separation of the two surfaces compared with the value obtained from gradually pressing them together. It was suggested that this hysteresis was due to specific interactions (hydrogen bonding) that develop over time. The first adsorbed layer, a cationic poly(dimethyldiallylammoniumchloride) (polyDMDAAC), decreased the magnitude of the hysteresis and gave a relatively low adhesion, which may be due to the fact that these specific interactions were blocked. The subsequent build up of layers of lipophilic wood extractives in the form of colloidal particles and cationic polymer increased the adhesion. This was interpreted as being due to the build up of a soft layer on the stiff mineral surface causing additional energy dissipation upon separation, even though the molecular adhesion was still decreased compared with the bare silica. This was supported by the fact that none of the individual components of the multilayer increased the adhesion when applied in a relatively thin layer on the silica, and by the fact that a thick deposition of cast coated wood extractives gave a very high adhesion. Stearic acid was chosen as a model substance for extractives and deposited onto silica from vapour phase. This resulted in a partial coverage of the surface where the stearic acid was present as patches of different size. This gave a significant reduction of the adhesion. The solid stearic acid melted and was redistributed when confined between the PDMS and the silica. This gave a thinner and more uniform layer. The application of a “monolayer” of C18 -tails covalently bound to the silica by siloxane links, gave a very significant reduction of the magnitude of the adhesion hystersis. It is concluded that the distribution of the contaminants over the surface is very significant for the adhesion properties. This is relevant to practical papermaking with respect to: (i) the build up of deposits on process equipment and its effect on adhesion to these surfaces; (ii) fibre–fibre adhesion and thereby paper strength, a phenomenon that can never be studied without directly measuring the adhesion, which makes specific solutions to the practical problems possible. © 2004 Elsevier B.V. All rights reserved. Keywords: Dissolved and colloidal substances; Mechanical pulps; Papermaking; Adsorption; Reflectometry; Adhesion

1. Introduction

∗

Corresponding author. Tel.: +46-87908294; fax: +46-87908101. E-mail address: [email protected] (L. Wågberg). 1 Formerly SCA Graphic Research, Sweden.

0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.01.029

Printing papers of low grammage are commonly based on mechanical pulp i.e. the wood raw material is separated into individual fibres and smaller particles, “fines”, by mechanical means rather than digested chemically [1]. Apart from fibres, the wood also contains lipophilic wood extractives which are used by the living tree for

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M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

protection against insects, fungae and other biological attacks [2]. The extensive mechanical action that is applied to separate the wood fibres from one another and make them suitable for papermaking also leads to “breakage” of the cells which contain the wood extractives. The lipophilic wood extractives are released from the pulp into process waters in the form of a colloidal dispersion during the subsequent papermaking together with wood polymers. The particle size of the colloidal wood extractives is in the micrometer range [3–6]. These substances are commonly divided into dissolved and colloidal substances (DCS). As mentioned above, the colloidal particles contain the lipophilic wood extractives, which may be divided into the following component groups: Fatty acids and their glyceride esters (fats), terpenes and terpenoids including resin acids, sterols and their fatty acid esters, and waxes. The dissolved fraction contains mainly wood polymers, such as polysaccharides, low molecular weight lignin fragments, lignans and pectins [2,6,7]. Many of the components among the dissolved substances (DC) and colloidal substances (CS) are of anionic nature in water [8]. It has been known for a long time that the dissolved and colloidal material may negatively influence both paper machine runnability, e.g. through deposits on paper making equipment, and the quality of the paper, e.g. the strength and the brightness may decrease [9,10]; because of this, the term “detrimental substances” is widely used. Both the decrease in strength and brightness have been linked to the adsorption of dissolved and colloidal substances onto the surface of the fibrous material [8], and it is a common belief that the wood extractives decrease the strength of the fibre contact points and thereby the paper strength. A number of different additives are used both to facilitate the papermaking process, e.g. in order to retain dissolved and colloidal substances in the paper, and to improve the properties of the paper sheet. These additives, of natural or synthetic origin, are commonly cationic polyelectrolytes since the fibres are of aninonic charge when dispersed in water. Consequently, the additives are able to adsorb onto the fibre surfaces, see e.g. [11]. In modern papermaking, the process water is re-circulated extensively within internal loops, which naturally leads to an enrichment of any substances that are released from the pulp into the water. This means that the concentration of DCS will continue to increase until some quasi-equilibrium is reached where the incoming material is balanced by the material that is carried out of the mill i.e. by retention in the paper or by following the process water sent to waste water treatment. Since the re-circulation of water also means that fibres and especially fragments of fibres, fines, will be re-circulated in the mill, the fibrous material will have the possibility to be exposed to cationic and anionic species several times until it is finally retained in the paper web.

This process closely resembles the experimental procedure for making polymeric multilayers, where many layers may be put “on top” of each other by consecutive adsorption [12]. A surface is exposed to anionic and cationic material alternately, with or without washing in between the stages. This process has been shown to alter the properties of the multilayer assembly by putting a new layer “on top”, so that both wetting properties and for example the net charge may change in each step. This process has been repeated more than 20 times in several cases without showing any tendency to stop the build up of the adsorbed layer, see e.g. [13–15]. The present work investigates the possibility of the formation of polymeric multilayers of cationic polyelectrolytes and anionic substances released from wood pulp, DCS, by using a SiO2 surface as a model for the anionic fibre surface and measuring the adsorption onto it. Furthermore, the adhesion properties of this model surface that has been covered with different numbers of multilayers is investigated. The implications of these results for papermaking are to give a more detailed description the adhesion between fibres in the fibre contact points and thereby the strength of the paper, as well as more insight into the formation of deposits on process equipment. This paper is based on results presented at the 219th ACS National Meeting in San Francisco [16], which have been complemented by measurements of more idealised systems to support the interpretation of the data.

2. Experimental 2.1. Materials A polyethyleneoxide (PEO) with a molecular mass of 1 × 105 , was delivered by Polysciences Inc.,Warrington, Pa, USA. PEO was used for calibrating the reflectometer. A poly(dimethyldiallylammoniumchloride) (polyDMDAAC) was obtained by fractionation of a commercial polyDMDAAC delivered by Allied Colloids, UK (Alcofix 130). The low molecular mass material was removed according to the method given by Aksberg and Ödberg [17]. The molecular mass was determined to 1.2 × 106 as described by Swerin and Wågberg [18]. A fully hydrolysed poly(vinylamine) (PVAm), was obtained from BASF GmbH, Germany (Catiofast PR8106). Octadecyltrichlorosilane and stearic acid were delivered from Sigma–Aldrich (The Netherlands). Both were of analytical grade (p.a.). The stearic acid was identical to the sample used in [19]. In one set of experiments an anionic polyacrylamide (A-PAM), was used. It consisted of a co-polymer of acrylamide and acrylic acid and the anionic molar content of the polymer was 16%. From the composition of the polymer, the charge was calculated to be 1.86 meq./g and according to the manufacturer the polymer had a molecular mass around 5 × 106 . This polymer was received from Allied

M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

Colloids, Bradford, England, as a powder and was used without further purification. The dissolved and colloidal substances were separated from an unbleached Thermo Mechanical Pulp (TMP) made from Scandinavian Spruce (Picea Abies). The pulp sample was taken at a consistency of about 30% from the blowline of one of the refiners at the Ortviken Mill, SCA Graphic Sundsvall, Sweden. Distilled water was used for dilution of the pulp when needed [8]. The silica wafers were supplied by Wacker Chemitronic GmbH, Germany. Polydimethyl siloxane (PDMS)-caps for the adhesion measurements were prepared from a standard two component system as described in Rundlöf et al. [20]. The caps were cured and then extracted in n-heptane (p.a.) to remove unreacted monomer. 2.2. Methods 2.2.1. Stagnation point adsorption reflectometry (SPAR) The adsorption onto a silica wafer was studied using stagnation point adsorption reflectometry (SPAR). The silica wafer was mounted in a liquid cell, where it could be exposed to different solutions. The geometry of the set-up was such that the flow of liquid came in normal to the surface, which creates a “stagnation point” close to the surface [21,22]. This geometry is important in order to ensure that the material is transferred onto the surface mainly by a diffusion mechanism [22]. Details on the experimental set-up are given by Wågberg and Nygren [11]. The adsorption onto the surface was monitored by recording the change in polarisation of a red laser beam reflected off the surface. The intensity of the perpendicularly polarised component of the reflected light, Is , and the parallel component, Ip , were used to define a quantity, S: S=

Is − Ip Is + I p

(1)

The ratio of the change in S-value upon adsorption, S, and the S-value of the bare silica surfaces, S0 , was used as a primary measure of the adsorbed amount. The S/S0 -values were recorded as a function of time and were then converted to adsorbed amounts, Γ (g/m2 ), using the approximate

35

relationship given by Wågberg and Nygren [11]: Γ ≈

S S0 0.129 dn/dc

(2)

Where dn/dc denotes the refractive index increment with concentration. These values for the materials used are listed in Table 1. The silica wafer was oxidised and cleaned according to Dijt et al. [21] prior to use. This gave a thin layer of silicone oxide on the surface, which made it anionic in water. The charge will be dependant on the pH [27]. The wafer was then cut into pieces, which were mounted in the SPAR equipment and exposed to water (MilliQ, Millipore). Pieces from the same wafer could be used for all the experiments, thus ensuring that the variations were kept as small as possible. A polyethyleneoxide with a molecular mass of 1 × 105 , was used for calibration. The adsorbed amount of PEO in deionised water onto the silica wafer was 0.61 mg/m2 , which is in good agreement with Dijt et al. [28] who reported 0.65 mg/m2 . In the subsequent experiments, the first solution was always a cationic polymer (polyDMDAAC) at a concentration of 30 mg/l and a pH of 8. The surface was subsequently exposed to either an anionic polymer or dissolved and colloidal substances, prepared from thermomechanical pulp according to Rundlöf et al. [8]. This adsorption sequence was repeated several times to investigate the formation of multilayers of the cationic polymer and the different anionic materials. 2.2.2. Fractionation of dissolved and colloidal substances from a TMP suspension Considerable efforts were undertaken to separate the dissolved and colloidal substances from a thermomechanical pulp to obtain a well-defined material. Furthermore, it was necessary to separate the DCS into a dissolved phase and a colloidal phase. An ultrafiltration approach was used to achieve this and extreme care had to be taken to avoid material losses in this equipment. Details of this preparation and results from the fractionation are given in [8]. 2.2.3. Preparation of multilayers on SiO2 Pieces of a single oxidised silicone wafer were exposed to different sequences of solutions by carefully submerging them in one vessel after the other. The silica surface was left for 10 min in each solution, then slowly withdrawn and

Table 1 Summary of the refractive index increment, dn/dc, of the materials used in the present investigation Material Poly-DMDAAC A-PAM Dissolved substances from TMP (DS) Colloidal substances from TMP (CS) Dissolved and colloidal substances from TMP (DCS) PEO

dn/dc (ml/g) 0.184 0.172 0.19 0.25 0.29 0.136

Conditions

Reference 23 ◦ C

1 M NaCl, 0.5 M NaCl, 23 ◦ C Deionised water 23 ◦ C Deionised water 23 ◦ C Deionised water 23 ◦ C Deionised water, 20–30 ◦ C

[23] [24] [25] [25] [25] [26]

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the excess solution was allowed to drip off. Finally, the silica surface bearing adsorbed multilayers was dried inside a laminar flow cabinet to reduce air-borne contamination. The surfaces were then stored in closed containers. 2.2.4. Preparation of a silica surface partly covered with stearic acid and with a C18 -monolyaer A silica surface was mounted face down, 6 mm above a layer of solid stearic acid in a glass container. The container was sealed and heated to 105 ◦ C for 16 h. The container was allowed to cool before opening. The C18 -monolayer was obtained using a similar set-up inside an evacuated dessicator. The silica surface was exposed to the vapour of octadecyltrichlorosilane for approximately 4 days (86 h). The chemical and the surfaces were handled under a dry nitrogen atmosphere. 2.2.5. Estimation of the surface coverage of stearic acid on SiO2 A crude estimation of the surface coverage was made based on the optical microscopy in the adhesion measurement. A mesh pattern was placed over the pictures and the pixels containing visible domains of stearic acid were counted. The ratio of these to the total number was used as a measure of the surface coverage. 2.2.6. Adhesion measurements (JKR-methodology) The adhesion between a comparatively soft elastic probe and the differently treated surfaces was measured in air at ambient conditions. A small spherical cap of cross-linked polydimethyl siloxane (radius 0.8–1.3 mm) was pressed against the flat surface and then pulled back again. Under these experimental conditions, the “JKR-theory” provides a relationship between the adhesion energy, W, the modulus, K, and the cube of the contact radius, a3 , at a given load, F [29,30].
R a3 = (F + 3πRW + 6πRW + (3πRW)2 ) (3) K The values of W and K were obtained by a fit of loading data to the above equation. R corresponds to the equivalent radius of the system. For a sphere on a flat substrate, as in the present case, R is equal to the radius of the elastic probe. The adhesion energy corresponding to the minimum load i.e. the instability point of the unloading curve, was determined according to the well-known relationship for the pull-off force for a load controlled JKR-experiment: Fmin

3 = − πRWmin 2

(4)

The cap was mounted on a glass slide which could be moved in the z-direction. The flat surface was mounted on an electronic analytical balance (Scientech SA 210, Scientech Inc., Boulder, USA) which is designed to keep the pan at a constant level and record the load. Since the data from the balance was recorded in mg it was chosen to keep

this dimension in the graphs presenting a3 as a function of applied load. The contact area between the cap and the flat surface was monitored as the optical contact area, as viewed by an optical microscope (Olympus BX 30 M, Olympus Optical Co., GmbH, Hamburg, Germany) in reflection mode. A 20× objective with a working distance of 12.0 mm was used. The whole experiment was controlled by a custom-made computer program i.e. the z-directional motion of the cap, the recording of load data and the collection of images showing the contact area by means of a CCD camera mounted to the microscope. The experimental protocol was similar to that applied by Falsafi [31]; the cap was slowly approached to the flat surface until initial contact occurred, often at a negative load. The cap was then pressed onto the flat sample with increasing pressure and the load and contact area were recorded after a pre-set equilibrium time. In this case, 10 min were found to be enough for the load and contact area to be stabilised. The loading continued until a maximum load of about 250 mg was reached, the surfaces were then left in contact for 30 min before the unloading of the system started. The surfaces were gradually pulled apart using the same equilibrium time and step length as above. The unloading continued until the surfaces finally came apart, and the minimum load at the point of instability was used as a measure of the pull-off force. We have previously applied this method to measure the adhesion to a model cellulose surface [20]. There are other techniques available to study surface interactions in paper related systems, as reported by e.g. Claesson [32] and Poptoshev et al. [33,34]. The “JKR-method” combines a comparatively simple experimental procedure and less stringent requirements on the samples which makes it attractive to use for this type of studies. 2.2.7. Scanning electron microscopy The scanning electron microscope was a Cambridge Stereoscan 360 (Cambridge Instruments, UK). 2.2.8. Atomic force microscopy The atomic force microscopy (AFM) images were generated in Tapping ModeTM using a Digital Instruments Dimension 3100 (Digital Instruments, Santa Barbara, CA, USA) equipped with a standard silicone nitride tip.

3. Results and discussion 3.1. Consecutive adsorption 3.1.1. PolyDMDAAC/DCS Fig. 1 shows the consecutive adsorption of colloidal substances and dissolved substances, and polyDMDAAC onto silica as a function of time. The dissolved fraction, DS, was free from colloidal particles and contained dissolved wood polymers mostly of anionic charge [8].

M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

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1, 08 S/S0 1, 06 1, 04 CS... 1, 02

CS

1 DS... PolyDMDAAC

0, 98

DS PolyDMDAAC

0, 96 0

500

1000

1500

2000

2500

3000

3500

4000

Time (s) Fig. 1. The adsorption onto a silica surface as a function of time, upon exposing it alternately to a water solution of cationic polymer and to water containing anionic CS or DS. The adsorption was monitored as the change in polarisation of a laser beam reflected off the surface.

Fig. 2. A SEM micrograph of a silica surface after exposing it to a cationic polymer and anionic DCS.

The figure primarily illustrates the build up of several “layers” upon changing from cationic polymer solution to anionic CS (or DS) to cationic polymer solution, etc. This is analogous to a typical polymeric multilayer experiment as described by e.g. Decher [12]. The exposure of the silica surface to the cationic polymer lead to an overcompensation of the charge. This gave a surface of net cationic charge able to interact with the anionic DS or CS, which also resulted in overcompensation leaving charged groups for the next adsorption step. In this way, the consecutive adsorption proceeds. The excess charge is localised in the outermost part of the multilayer assembly as shown by Schlenoff et al. [14,15]. It is obvious that the consecutive adsorption leads to a much higher adsorbed amount compared with the amount which is obtained in a single step, in both cases. The adsorbed amount of CS was the same regardless of the number of layers whereas the amount of adsorbed DS

was higher in the first layer compared to the following layers. This indicates that the colloid was significantly adsorbed onto the polymer layer on the silica surface, since the dissolved fraction is present in both waters. This is supported by an scanning electron microscopy (SEM) examination of the silica surface (Fig. 2). Particles below 1 ␮m in size are clearly visible on the surface, these particles are likely to be colloidal wood extractives since they are in the same size range as the particles normally found in DCS-containing water [6,10,35,36]. In this case, another cationic polymer was used and the micrograph is included for qualitative reasons. Details of this experiment are given by Wågberg and Nygren [11]. Upon closer examination of the curves in Fig. 1, an initial decrease in the adsorbed amount is seen upon changing solution. This can probably be related to desorption, presumably of loosely associated aggregates, through complexation with the cationic polyelectrolyte. This phenomenon was shown

S/S0 1,14 1,12 1,1 1,08 1,06 1,04 1,02 1 0,98

Water polyDMDAAC

0,96

DCS

0,94 0,92 0

500

1000

1500

2000 2500 3000 3500 4000 4500 5000 5500

Time (s) Fig. 3. The adsorption onto a silica surfaces as a function of time, upon exposure to several sequences of cationic polymer/water/DCS/water.

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Table 2 Summary of the adsorption data from the experiments with CS. The amounts of adsorbed charges were calculated based on the charge of the polymer and amount of charge needed to recharge the CS water fraction Layer

Adsorption of polyDMDAAC (mg/m2 )

Adsorption of CS (mg/m2 )

Adsorption of polyDMDAAC (␮eq./m2 )

Adsorption of CS (␮eq./m2 )

Charge ratio (polymer/CS)

1 2 3 4 5

0.18 0.10 0.14 0.14 0.15

0.50 0.43 0.46 0.52 0.62

1.1 0.61 0.86 0.86 0.92

0.075 0.067 0.068 0.075 0.088

0.068 0.11 0.079 0.087 0.096

to occur also when the surface was exposed to water before changing adsorbent i.e. when “a washing step” was included in the experiment (Fig. 3). An interesting consequence of the results shown in Figs. 1 and 3 is that multilayers of DCS may build up on the anionic surfaces of fibres and fines in a conventional papermaking process. This might totally change the surface properties of the fines and will decrease the ability of the fines material to contribute to paper strength [8,37]. In order to determine the adsorbed amounts from the results presented in Fig. 1 the procedure described by Wågberg and Nygren [11] was used, together with data in Table 1. Furthermore, by using the charge reversal data for the CS and the DS-waters, the amount of adsorbed charges of the different components could be compared as demonstrated in [38]. In order to make this latter comparison, the following values of charge reversal for the components were used: CS: 1.48 × 10−4 eq./g as determined with a polyDMDAAC treatment (charge reversal was evaluated from electrophoretic mobility measurements as described in [8]). DS: 1.35 × 10−4 eq./g. This value is representative for the unfractionated DCS sample, but the charge of this sample is dominated by the DS fraction, which justifies the use of this value also as representative for the DS. Tables 2 and 3, shows the adsorbed amounts and equivalent charge for the CS and DS fractions, respectively. The last column of Tables 2 and 3 shows that the charges from the adsorbed polyDMDAAC were only to a limited extent available for interaction with the dissolved and colloidal substances in the water. This was expected, since the adsorption of polyDMDAAC is mainly driven by a charge interaction between the polymer and the surface. This means that only a fraction of the charges of the adsorbed polyelectrolyte will be available for interaction after adsorption on an oppositely charged surface. It has been shown that the adsorption stoichiometry for a similar polyDMDAAC is around 90% i.e. 90% of the charges are neutralised upon

adsorption [39,40]. If this value is assumed to be valid for the present experiments, 10% of the charges would be available after adsorption, which means that there is an almost perfect match between the available charges of the polyDMDAAC and the charges of the material constituting the CS (Table 2). For the DS, a different behaviour is found; when the first DS layer was treated with a new portion of polyDMDAAC, no increase in the S/S0 value could be detected. Despite this, another layer of DS could be formed when the surface was again exposed to DS-water, indicating that the polyDMDAAC was present at the surface, exposing charges available for interaction. As suggested in case of the CS-water, one explanation might be that the surface was not treated with water between the treatments with the DS and PolyDMDAAC. DS components that were loosely associated to the preadsorbed polyDMDAAC, would stay on the surface and be rinsed away during the next treatment with polyDMDAAC. These loosely associated components would probably form complexes with the cationic polyelectrolyte and then desorb from the surface. The detected signal will then be affected by a combination of the desorption of DS and the adsorption of polyDMDAAC, which appear to balance one another in this case. This desorption from the surface due to a water “rinsing” step between the adsorbing solutions was also shown to occur for the CS-water (Fig. 3). 3.1.2. PolyDMDAAC/A-PAM To further demonstrate the applicability of the SPAR technique to multilayers, a similar experiment was performed using two well-defined polymer solutions, a cationic PolyDMDAAC and an anionic polyacrylamide. Fig. 4 shows the consecutive adsorption of these polymers onto a silica surface from a solution of deionised water and a solution containing 10 mM of NaCl. The treatment with several polyDMDAAC/A-PAM sequences in deionised water gave a similar increase in ad-

Table 3 Summary of the adsorption data from the experiments with DS. The amounts of adsorbed charges were calculated based on the charge of the polymer and amount of charge needed to recharge the DS water fraction Layer

Adsorption of polyDMDAAC (mg/m2 )

Adsorption of DS (mg/m2 )

Adsorption of polyDMDAAC (␮eq./m2 )

Adsorption of DS (␮eq./m2 )

Charge ratio (polymer/CS)

1 2 3

0.18 Not measurable Not measurable

0.42 0.20 0.21

1.1 – –

0.056 0.027 0.028

0.051 – –

M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

39

1,1

S/S0

1,08

10 mM NaCl

1,06 1,04 1,02 1

No salt added 0,98 0,96 PolyDMDAAC

0,94

A-PAM

0,92 Pure water

0,9 0

500

1000

1500

2000

2500

3000

3500

4000

Time (s)

Fig. 4. The adsorption onto a silica surfaces as a function of time, upon exposure to several sequences of cationic polyelectrolyte (polyDMDAAC) and an anionic polyelectrolyte (A-PAM).

sorption in each step. By calculating the adsorbed amount (cf. Tables 2 and 3) it was found that the adsorbed amount of polyDMDAAC was 0.173 mg/m2 and the adsorbed amount of A-PAM was 0.137 mg/m2 . These values correspond to an adsorbed charge of 1.1 ␮eq./m2 and 0.254 ␮eq./m2 respectively, which gives a ratio between cationic charges and anionic charges of 4.3. This is close to the value of 3.8 found by Hoogeven [38] for a system of polyvinylpyrrolidone and polystyrenesulphonate. The reason for this value is probably to be found in how the polyDMDAAC was initially adsorbed onto the silica wafer. If it is assumed that there is a 1:1 stoichiometry in the interaction between the free charges of the polyDMDAAC on the surface and the added A-PAM, about 23% of the adsorbed polyDMDAAC charges were available for interaction with the anionic polymer. This is in reasonable agreement with the charge ratio found for the CS and DS, considering the differences between a synthetic polymer and a more heterogenous mixture containing colloidal particles, the possible differences between the SiO2 -wafers from different batches and the uncertainties in the determinations. The results indicate that the charges on the surface are overcompensated by the charges of the cationic polymer and the free charges of this polymer are available for

interaction in the subsequent treatments. In this way, the initially adsorbed layer is important for the overall build-up of polymeric multilayers on these types of surfaces. Table 4 shows the data for the salt-containing polymer solution. As seen in the table, the number of adsorbed A-PAM charges is between 50 and 70% of the number of initially adsorbed polyDMDAAC charges. This indicates that the adsorption stoichiometry between the silica surface and the polyDMDAAC was drastically changed when the salt concentration was increased. This increase in adsorbed amount with increasing salt concentration is in agreement with the results reported by Schlenoff et al. [15]. The above interpretation can be extended to explain why the build up of the polymeric layers was so different for the polymer solution containing salt. As found by Hoogeven [38] for similar substrates and polymers, there is an increase in the adsorbed amount with increasing salt concentration. This is probably induced by the decreased repulsion between the adsorbed chains which makes an increased adsorption possible. This will also lead to a less perfect matching between the charges of the silica surface and the polyDMDAAC, which in turn means that there will be a larger amount of charges available for the A-PAM to

Table 4 Summary of adsorbed amounts and adsorbed charges of polyDMDAAC and A-PAM during the formation of multilayers of polyelectrolytes at salt concentration of 10 mM NaCl Layer

Adsorption of polyDMDAAC (mg/m2 )

Adsorption of polyDMDAAC (eq./m2 106 )

Adsorption of APAM (mg/m2 )

Adsorption of APAM (eq./m2 106 )

Ratio (cationic/anionic charge)

1 2 3

0.44 0.74 0.57

2.70 4.54 3.50

0.79 1.15 1.77

1.46 2.14 3.29

1.85 2.12 1.38

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M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

interact with (provided there is a good matching before the salt concentration is increased). This may be the reason why there is a larger amount of A-PAM adsorbed.

It was demonstrated above that a quite substantial adsorption of DCS may take place due to the build up of polymeric multilayers on silica surfaces (cf. Fig. 1). Adhesion on a molecular scale is commonly said to be governed by the outermost 5–10 Å of the adhering surfaces, this multilayer adsorption was therefore considered to most likely affect the adhesion properties of the silica. The silica surfaces used for the adsorption experiments are smooth on a nanometer level and therefore also suitable for measuring adhesion. In addition, these surfaces may be used as a reasonable model for a papermaking fibre [11]. To study the effect of the build up of multilayers on adhesion, silica surfaces bearing different numbers of layers were prepared and their adhesion properties were measured in air at ambient conditions. The adhesion of a comparatively soft elastic probe (PDMS) to the silica surfaces bearing different numbers of mulitlayers was measured. A small spherical cap of PDMS (equivalent radius ≈ 0.8–1.3 mm) was pressed against the flat silica surface and then pulled back again. Under these experimental conditions, the “JKR-theory” provides a relationship between the adhesion energy, W, and the contact area at a given load [29,30]. One of the benefits of this technique is that it provides both surface and interfacial information in one experiment. The adhesion energy obtained from loading data, when the probe is pressed towards the underlying surface, is the value closest to the thermodynamic work of adhesion and is mostly a function of the surface energies of the two materials [20,31]. The unloading data contain interfacial information i.e. contributions from specific interactions, interdiffusion, interdigitation, etc. that may have developed when the surfaces were in contact and, in addition, viscoelastic or plastic deformations that may occur upon separation. The adhesion curves are therefore often significantly hysteretic and the adhesion energy obtained from the unloading data is in that case always higher than the value obtained from loading data [41] (and references therein). As aptly put by Falsafi: “the unloading and loading part of the JKR-experiment may be said to be the counterparts of a contact angle measurement and a peel-test, respectively” [31]. 3.2.1. Adhesion between PDMS and a bare silica surface Fig. 5 shows the cube of the contact radius as a function of load for a PDMS cap pressed against a similar PDMS cap and for a PDMS cap adhering to a bare silica surface, both measured in air at ambient conditions. In the former case, the curves showed little hysteresis, as expected, and the work of adhesion obtained by fitting the data to the JKR-theory was WA = 41.5 mJ/m2 which is in good

0,003 0,0025 3

0,002

3

a (mm )

3.2. Adhesion to silica surfaces bearing multilayers

0,0035

0,0015

SiO2 loading

0,001

SiO2 unloading PDMS loading PDMS unloading

0,0005 0 -400

-200

0 Load (mg)

200

400

600

Fig. 5. The cube of the contact radius as a function of load for PDMS–SiO2 (open symbols) and PDMS-PDMS (solid symbols).

agreement with the literature [30]. The corresponding curves for PDMS against silica showed a very large hysteresis, which is in full agreement with the results reported by Kim et al. [42]. The loading data gave a value of WA = 53 mJ/m2 . It was not possible to fit the unloading data to the JKR-theory due to the shape of the curve, but the adhesion energy calculated from minimum load was Wpull-off = 884 mJ/m2 . The adhesion between PDMS and bare silica was found to be highly dependent on contact time, a power law fitted well to the data which is also the case for the adhesion of two silica surfaces as shown by Vigil et al. [43]. A contact time was chosen for the standard experimental protocol where the adhesion had reached its plateau and the changes with time were small. Different types of interactions have been suggested to account for the time dependent effects in adhesion to silica. Kim et al. [42] claim that the surface mobility of the PDMS can lead to sufficiently good proximity and alignment for hydrogen bonding to occur between the siloxane groups (Si–O–Si) in the PDMS network and silanol groups (Si–OH) on the opposing surface. The time dependence would then arise from the re-conformation of the PDMS network. Vigil et al. [43] suggest that chemical bonds can be formed across the interface when two silica surfaces come together, by sintering reactions. Further, we note the similarity between the present results and those obtained for PDMS/cellulose [20], the work of adhesion is WA = 49.5 mJ/m2 and a pronounced hysteresis gave a “pull-off” energy of Wpull-off = 201 mJ/m2 . 3.2.2. The first adsorbed layer, polyDMDAAC or PVAm Fig. 6 shows the adhesion of PDMS to a silica surface bearing a layer of polyDMDAAC, the hysteresis is now much less pronounced compared with the bare silica surface (cf. Fig. 5). Both the work of adhesion obtained from loading data and the “pull-off” energy were lower, 40.7 and 49.1 mJ/m2 , respectively. This indicates that the polymer adsorbed onto the surface, block the specific interactions which may develop between the PDMS and the silica with time. Both of the suggested mechanisms referred to above would indeed be blocked by an adsorbed polyDMDAAC layer, since the polymer is not capable of hydrogen bonding or participating in “sintering” reactions. The hydrogen bonding mechanism appears to be supported by a measurement

M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47 0,003 0,0025

3

a (mm )

0,002

3

0,0015 0,001

Loading Unloading

0,0005 0 -100

0

100

200

300

400

500

Load (mg)

Fig. 6. The cube of the contact radius as a function of load for PDMSpolyDMDAAC, SiO2 .

Fig. 7. A SEM micrograph showing adsorbed DCS on a silica surface. This surface was treated with polyDMDAAC followed by DCS three times.

of the adhesion of PDMS to a layer of poly(vinylamine), adsorbed onto silica. In this case, a pronounced hysteresis was found, giving a “pull-off” energy of 270 mJ/m2 . This observation is supported by several measurements involving this polymer, in which a strong adhesion hystersis was always observed [44]. 3.2.3. Multilayers of DCS and polyDMDAAC on silica SEM micrographs of the samples bearing multilayers of dissolved and colloidal substances revealed their irregular nature. This was also reflected in a large variation in the adhesion measurements (Fig. 7). This variation is believed to reflect properties of the system rather than being a consequence of a high variability in the experimental procedures. The measured differences in adhesion between the samples

41

were so significant that they were not overshadowed by the variations within the samples. Fig. 7 shows adsorbed colloidal particles and aggregates of colloidal particles on a silica surface exposed to three polyDMDAAC and DCS adsorption cycles. It is not surprising that such an inhomogeneous surface layer gave rise to different values of adhesion energy depending on which part of the surface that was sensed by the probe. We have chosen to report the values of work of adhesion obtained from loading data, even though the effects of irregularities in a surface on this value are not sufficiently well known. The minimum load is an unambiguous measure of the maximum force the system can withstand and represents the point of instability where pull-off will occur in a load controlled experiment. Since this value marks the onset of an instability it is difficult to measure exactly and repeated measurements generally show a larger variation than the values obtained from loading data [45]. The minimum load is well documented in the present investigation, since the measurement in this displacement controlled testing machine proceeds into the region of instability before the actual pull-off occurs. This means that data will be recorded at quasi-equilibrium both before and after the point of instability, which represent the “pull-off force” in a load controlled experiment. This value was used for the evaluation of all the silica surfaces bearing multilayers and the corresponding pull-off energies were calculated using the expression given by the JKR-theory. Table 5 shows the work of adhesion based on pull-off force measurements, for silica surfaces exposed to increasing numbers of polyDMDAAC (p)/DCS (D) adsorption cycles. Typical values have been selected to illustrate the observed trends. As shown above the adhesion energy decreased with the first polyDMDAAC layer. The build up of a multilayer assembly on the stiff silica surface resulted in increased adhesion energy, even slightly above the level of the adhesion of PDMS to a bare silica surface in another preparation (cf. Table 6). The multilayer assembly containing three “layers” was heated to 105 ◦ C in a closed container and allowed to cool to room temperature before the container was opened. This gave a drastic decrease in the adhesion, as shown in Table 6. For reference, wood extractives extracted from the same pulp as the DCS were applied to silica in two ways, by cast coating from acetone solution and by deposition from vapour phase (in a similar way as the heating experiment described

Table 5 The adhesion energies of PDMS to SiO2 bearing multilayers of PolyDMDAAC (p) and DCS (D) in different numbers. The values were obtained by a fit of loading data to Eq. (3) and Eq. (4)

(mJ/m2 )

Adhesion energy from loading data Adhesion energy from minimum load, pull-off energy (mJ/m2 )

SiO2

SiO2 /p

SiO2 /p/D

SiO2 /p/D/p/D/p/D

SiO2 /p/D/p/D/p/D/p/D/p/D

53 884

41 49

49 475

57 536

52 574

42

M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

Table 6 The adhesion energies of PDMS to SiO2 bearing multilayers of PolyDMDAAC (p) and DCS (D) in different numbers. Wood extractives deposited from acetone solutions and in vapour phase were included for reference. The values were calculated from Eq. (4)

Adhesion energy from minimum load, pull-off energy (mJ/m2 )

SiO2 /p/D/p/D/p/D

d:o heated to 105 ◦ C

Wood extractives on SiO2 applied by cast coating (thick layer)

Wood extractives on SiO2 deposited from vapour phase (thin layer)

1090

46

1270 to >2000

107

above). The former treatment resulted in a thick, rough, layer which showed the highest adhesion energies obtained in this investigation, whereas the vapour deposition resulted in a partial coverage of thin “domains”, as observed using SEM. These domains gave a low adhesion energy compared with the bare silica. The reason for the increase in adhesion with increasing number of layers may be that the multilayer assembly formed a soft phase on the stiff mineral surface, which gave rise to additional energy dissipation during the adhesion measurement due to deformations of this soft layer. This is known to affect the adhesion significantly, for example it has been demonstrated that the adhesion between PMBA surfaces is enhanced by more than two orders of magnitude if the temperature is increased above the glass transition temperature of the polymer. This increase in adhesion was associated with a deformation of the surfaces upon separation, causing the energy dissipation [41]. This proposed mechanism appears to be supported by the unusual shape of the loading curve, when the three layer assembly was measured. When the initial contact between the PDMS probe and this surface had been established, the contact area did not increase with increasing load during the first few compression steps. When the load had been increased by ca. 120 mg, the contact area began to increase in the same manner as is normally seen (cf. Fig. 5). This unusual behaviour indicates that the increase of the contact area was hindered, and when the load became high enough this resistance was overcome. This could mark the onset of a deformation of a soft phase which the cap penetrated through on its way down to the stiffer silica surface. In the measurement of cast coated wood extractives, it was obvious that the surface was covered by a thick layer of soft extractives, which the probe penetrated during the loading part of the experiment. The energy required to pull the probe out of this soft layer was large, resulting in pull-off energies between W = 1270 to > 2000 mJ/m2 . The probe left a crater shaped deformation in the soft layer. These observations support the suggestion that the multilayers formed a comparatively soft phase on the stiff silica surface, which enhanced the adhesion. The suggested mechanism is further supported by the fact that none of the components of the multilayers increased the adhesion by itself when applied in a thinner layer onto silica. They seemed to act as “contaminants” reducing the

molecular adhesion between PDMS–SiO2 . When a sufficiently thick layer was applied, the molecular adhesion was still weaker but an additional contribution to the adhesion became significant i.e. energy dissipation through irreversible deformation of the surface layer. This observation shows the complexity of the phenomenon of adhesion where not only the molecular interactions come into play but also the mechanism of breaking the joint. To get a detailed description of why adhesion is high or low in a given case “we need to study the adhesion on three different levels, from molecules, through mechanisms, to mechanics”, as stated by Kendall [45]. The possible application of the present results to adhesion phenomena relevant to papermaking is therefore discussed below. It seems likely that the dramatic decrease of the adhesion upon heating of the three layer assembly is due to rearrangement of the multilayer upon heating. This could occur by melting of the colloidal particles and by redistribution of some components in the vapour phase (as shown in Table 6) or a combination of the two. SEM micrographs seem to indicate that melting has occurred. This is not surprising considering that the melting point of “wood extractives” is probably well below 105 ◦ C, since some of the common fatty acids have a melting point in the region of approximately 30–70 ◦ C [46] and even below 0 ◦ C in the case of polyunsaturated carbon chains. A rearrangement where fatty acids are evaporated and then deposited on top of the layer would possibly influence adhesion to the multilayer, since fatty acids are known to decrease friction, [47], adhesion [48] and also paper strength [9]. The fatty acids have been pointed out as being the most “detrimental” component group among wood extractives [9,49]. Swanson and Cordingly [50] showed in an elegant experiment that a fatty acid representative of those present among extractives (stearic acid) is readily deposited onto paper by vapour phase transport at the temperature used in the present investigation (105 ◦ C). It is also possible that the layer of hemicelluloses covering the colloid [51,52] was influencing the adhesion properties of the multilayer assemblies and that these hemicelluloses were affected by the heating. The present results do not support any firm conclusions regarding which mechanism is the most important. The most interesting observation is that the distribution of the wood extractives seemed to have such a large influence on the adhesion.

M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

3.2.4. A model substance for “contamination” by wood extractives The complexity of the dissolved and colloidal substances shows the need for a more idealised set of measurements to support the interpretation of the above results. Stearic acid (n-octdecanoic acid) was chosen as a model substance since it is representative for a major component group among the extractives, the fatty acids. It is also a well-characterised substance used in many investigations for the purpose of surface modification, see e.g. [19,48,50]. The stearic acid was deposited onto silica from vapour phase [50] in a similar way as the wood extractives shown above. This procedure gave a partial coverage of stearic acid, which was present as patches of different size and number on the silica, essentially giving a gradient in the silica surface. Different areas of the sample contained patches of different size, ranging from approximately 10 ␮m down to below 1 ␮m. Adhesion measurements were made on several places on the samples and it was found that this difference in size of the deposits did not change the general effect of the stearic acid; to decrease the magnitude of the adhesion hysteresis and thereby the minimum load, the “pull-off energy” (Fig. 8). This decrease in adhesion is in agreement with measurements of monolayers of stearic acid on mica in the surface force apparatus [48]. The contact angle with water of the silica surface increased from 52 ±0.7 to 83±3◦ when the stearic acid was applied. An unusual observation was made during the measurement of the contact angles, the droplet of water “jumped-off” areas of the sample with a high degree of coverage and came to rest in an area, to a lesser extent, covered with stearic acid. This shows that the high hydrophobicity of parts of the sample was not fully reflected in the measured value of the contact angle. Chaudhury and Whitesides [53] have demonstrated that droplets move on a gradient surface and that it is even possible to make water run uphill. The adhesion energy WA from loading data was rather high as measured on first approach, 58.6 mJ/m2 . This is higher than what would be expected for a layer of C18 -tails covering the surface, a value of the surface energy of 0,0035 0,003

3

0,002

3

a (mm )

0,0025

0,0015

SiO

0,001

2 st

SiO + stearic acid 1 2

SiO + stearic acid 2nd

0,0005

2

0 -600

-400

-200

0

200

400

600

800

Load (mg)

Fig. 8. The cube of the contact radius as a function of load for PDMS against bare silica (open circles) and PDMS against silica partly covered with stearic acid (filled squares).

43

octadecane, C18 H38, of 28 mJ/m2 , is given by Israelachvili [54]. The higher value reported here could be due to the incomplete surface coverage of the stearic acid. It is also possible that the fatty acid molecules could rearrange to expose their polar head group to the surface upon contact with the PDMS. The adhesion energy calculated from the minimum load, the “pull-off” energy was 223 mJ/m2 , about 25% of the corresponding value for bare silica. The domains of stearic acid were clearly visible through the optical microscope during the initial part of the adhesion measurement. They gradually became invisible as the loading part of the experiment proceeded and the contact area did not contain any visible domains after unloading and separation of the surfaces. The stearic acid was at least partly transferred to the PDMS lens, as observed in the AFM, where it was deposited as domains similar to those seen on the silica. This redistribution gave a lower value of the work of adhesion as measured on a second approach, WA = 42.4 mJ/m2 , and it may be speculated that this was due to a more complete coverage of the silica. The unloading curves and values of the pull-off force were however quite similar both on first and second approaches. This is reasonable considering that the initial distribution of the stearic acid should affect the loading part of the experiment, and the redistribution during the compression probably gave a similar situation in the unolading part of both the first and the second measurements. It may be speculated that the redistribution of stearic acid was at least partly due to capillary melting in the narrow slit between the PDMS and the silica, which is the result of a depression of the melting point due to wetting of the surfaces of the liquid stearic acid in preference to the solid. This is demonstrated to occur for n-octadecane confined between mica surfaces in a surface force apparatus by Maeda and Christenson [55]. A trend was observed in the adhesion energies measured on different areas of the sample. It seemed like a smaller size of the domains of stearic acid and an increasing surface coverage both gave a decrease of the work of adhesion obtained from both loading data and from the pull-off force. Fig. 9 shows an illustration of this trend. This observation is consistent with the fact that the WA decreased upon second approach, when the stearic acid had been redistributed, and the strong indication that a thicker soft layer gives a stronger adhesion. Smaller domains also mean thinner deposits in this experiment, since the domains decrease in diameter and in height at the same time. 3.2.5. The effect of a covalently bound (non-perfect) monolayer of C18 -tails To further test this a thin layer C18 -tails were covalently bound to the silica. A treatment of the SiO2 with the vapour of octadecyltrichlorosilane gave a monolayer of C18 -tails covalently bound to the surface by siloxane links [30,56]. It is assumed that our sample was not covered by a perfect monolayer, since the art of making such layers is complicated and has become its own branch of natural science, see

44

M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

Fig. 9. Micrographs from the adhesion measurements showing domains of different size deposited on the silica surface. The in-sets show the corresponding values of the adhesion energy obtained from loading data, WA , and from the pull-off force, Wp , including a crude estimation of the surface coverage, Φ.

0,0035 0,003

3

0,002

3

a (mm )

0,0025

0,0015 SiO

0,001

2

Si-O-Si-C H

0,0005

18 37

0 -400

-200

0

200

400

600

Load (mg)

Fig. 10. The cube of the contact radius as a function of load for PDMS against bare silica and PDMS against silica covered with a layer of C18 -tails bound to the silica by siloxane links.

e.g. [57]. Our procedure was by no means optimised with respect to moisture, etc. This treatment did indeed decrease the adhesion (Fig. 10). The work of adhesion from loading data was reduced to 34.3–39.8 mJ/m2 depending on where the measurement was made. The magnitude of the hysteresis in the unloading curve and the “pull-off” energy was dramatically decreased by the C18 -tails, giving an adhesion energy of 59 mJ/m2 . The thinnest conceivable layer of a substance with an unfavourable interaction with the PDMS was sufficient to decrease the adhesion in the present system quite significantly.

4. Discussion The present work aims at studying the behaviour of substances which are common in process waters in paper mills and their influence on a surface, which may serve as a model of both a fibre surface and the surface of parts of a paper machine. The silica surface used here is, as most experimental methods, a model system, which does not imitate the real situation, but can provide information that may be useful in many practical applications since it gives some insight into the underlying casual connections. The most obvious implication is, that exposure of a surface to oppositely charged adsorbates led to a much higher

adsorbed amount compared with the adsorption of the individual components. This took place through a build up of a polymeric multilayer as demonstrated by many authors, see e.g. [12]. The modern papermaking process is a highly closed system, where the process waters are circulated in internal loops. The process waters contain fibres and especially fines which are circulated for a period of time, before they are retained in the paper web. This process closely resembles a typical multilayer experiment: The fibrous material is of anionic charge in water. Cationic polymers are added to the pulp suspension and adsorbed onto the surface of the fibres and fines, enabling adsorption of anionic dissolved and colloidal substances. From the point of view of an individual fibre it is thus possible to encounter cationic and anionic species consecutively in several “cycles” if the fibre is re-circulated in the process water. The present results strongly indicate that consecutive adsorption may take place onto fibre surfaces as well as onto the parts of the paper machine itself that are exposed to process water. Further, our results indicate that this adsorption may have a profound effect on the adhesion to these surfaces, which can have practical implications to e.g. deposits on process equipment and to fibre–fibre adhesion and thereby paper strength. 4.1. On the practical relevance The build up of a multilayer assembly on the silica appeared to give a soft layer which enhanced the adhesion. This was most probably due to the structure and softness of the layer, since none of the individual components enhanced adhesion by itself when applied in a thin layer onto silica. Indeed, a monolayer of covalently bound C18 -tails was enough to significantly reduce the adhesion between the PDMS probe and a silica surface. This adhesion behaviour is relevant to any interaction between a mineral surface and an elastic probe, the adhesion to a stiff mineral surface may be more or less directly applicable to metal surfaces in the process equipment. The discussion of the present results may also be extended to fibre–fibre adhesion;

M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

Firstly, to get a picture of the applicability of the adhesion measurement, we need to consider the complexity of the formation of a joint between papermaking fibres: I. The fibre surfaces are inhomogeneous, rough (i.e. not smooth on a nanometer scale) and swollen when dispersed in water. II. These wet surfaces may be subject to adsorption of both additives and contaminants, which can change their properties quite drastically as demonstrated above. III. The fibre-web is formed from a rather dilute water dispersion which is dewatered on a wire, this fibre web is then compressed by wet pressing. IV. The fibre surfaces are now rather close and the wet web begins to dry. The surfaces are pulled together by the disappearing water menisci and this is probably a prerequisite for enough contact area to be formed i.e. the area in molecular contact where molecular interactions can occur. The strength of these interactions will probably have a significant effect on the strength of the fibre–fibre joint. V. The elasticity of the wet fibre wall would probably be determining for the ability of the fibres to deform and create close proximity of fibre surfaces, which may then give molecular contact upon drying. VI. This process is probably assisted by interdiffusion of the outermost surface layers into one-another creating additional molecular contact [58]. VII. The mobility of polar groups in the interacting surface layers may also determine the extent to which specific interactions, such as hydrogen bonding, can be established once the surfaces are in contact. The adhesion measurement used here gives information on the interaction of surfaces that are in molecular contact or close to it. Information on irreversible processes such as deformation of surface layers upon separation and time dependent phenomena of surfaces in contact is also obtained. The elasticity of the fibre wall, its fibrillated nature, its swelling in water and behaviour during drying are not included (I, IV and V). It is probable that any specific interactions between fibre surfaces in molecular contact would be blocked by a monolayer of extractives in a similar way as in the case of PDMS silica. The negative effect of polyDMDAAC seen in our measurements appear to be valid also for fibre–fibre adhesion, which is demonstrated using rayon fibres [59] and also for mechanical pulp fines [8]. The increase in adhesion due to the presence of a soft multilayer assembly on a stiff mineral surface may not be seen in the case of fibres, since the fibre wall is much more elastic in itself [60]. The adhesion of an already soft substrate may not be increased as much by the build up of an additional soft layer. The observed decrease in adhesion by the introduction of wood extractives and (stearic acid) is very likely to be significant also for fibre–fibre adhesion. A thin layer of

45

extractives is likely to affect the adhesion in a similar way as demonstrated in the case of silica. A thicker layer may act as a weak boundary layer (weaker than the fibre–fibre joint), so that the fracture proceeds in the soft layer of extractives. The observed melting of the solid stearic acid is interesting since it means that colloidal wood extractives may well be smeared out to cover much more of the fibre surfaces than the size of the colloidal particle indicates. This melting will probably take place at normal papermaking temperatures. If the spreading over the surface does not occur spontaneously, it is likely to happen if a colloidal particle is trapped between two fibres when they are pulled together by the disappearing water meniscus during drying. Further, if the decrease in adhesion caused by only a very thin layer, even a monolayer, is valid also for the formation of a strong joint between two papermaking fibres, this melting and smearing out would be very significant for paper strength. A relation between the surface coverage of wood extractives on the surface of mechanical pulp fines and the strength of sheets formed of these fines has previously been shown [8]. In a real system, there seem to be a critical degree of surface coverage where the strength begins to decrease. Further, the presence of extractives on the surface layers may decrease the extent to which e.g. interdiffusion and other types of beneficial rearrangement of the surface layers can occur [58]. 4.2. A final remark The world production of paper was in the range of 300,000 kt during 1998 [61]. A crude attempt to estimate the addition of cationic polymers in paper making gave a figure in the range of 105 tonnes on a yearly basis. This points out the great potential in studying the possible, unintentional formation of polymeric multilayers, which may have a dramatic effect on the surface properties of the fibres and thereby most probably the properties of the paper.

5. Conclusions • Multilayers can be formed by consecutive adsorption of a cationic polyDMDAAC and anionic dissolved and colloidal substances released from mechanical pulp into process waters. This was shown using a reflectometry technique to measure adsorption onto a silica surface. • In the case where colloidal particles were present in the water, they were adsorbed onto the surface, which was reflected as a higher increase in the adsorbed amounts compared with the water containing only dissolved substances. • It was demonstrated that this consecutive adsorption is likely to occur onto fibrous material in the papermaking process, because of the high degree of re-circulation of process waters which contain fibres and fines as well as cationic polyelectrolytes and DCS.

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M. Rundlöf, L. Wågberg / Colloids and Surfaces A: Physicochem. Eng. Aspects 237 (2004) 33–47

• The adhesion of a PDMS-lens to the bare silica surface was highly hysteretic. The similarity to the adhesion of PDMS to cellulose was pointed out. • A cationic polyDMDAAC adsorbed onto the silica as the first layer in the adsorption sequence decreased the adhesion, by reducing the magnitude of the adhesion hysteresis and thereby the pull-off force. A similar experiment using a polymer which is capable of hydrogen bonding, poly(vinylamine), gave a much more pronounced adhesion hysteresis. • The build up of a multilayer assembly on the stiff silica surface resulted in an increased adhesion even above the level of the adhesion of PDMS to a bare silica surface. • The results furthermore indicate that the increased adhesion was due to the formation of a soft layer on the stiff mineral surface. The same substances did not contribute to the adhesion when deposited in thin layers onto the silica. • Smaller and thinner domains of stearic acid deposited onto silica gave a more pronounced decrease in adhesion, compared with larger and thicker domains. • The stearic acid was found to be redistributed when confined between the PDMS probe and the silica surface. • An octadecyltrichlorosilane (C18 ) covalently bound to the silica surface in a “monolayer” decreased adhesion quite drastically. • Consequently, the distribution of the “contaminant” over the surface seems to have a significant influence on the adhesion.

Acknowledgements The authors are deeply grateful to Professor Per Gradin, Mid Sweden University, for the discussion of the contact mechanics involved in the adhesion hysteresis. The skilful experimental work of Ms. Inger Nygren and Ms. Cecilia Engqvist is acknowledged. Techn Lic. N. Garoff, STFi, is thanked for providing the stearic acid already used in friction experiments and Dr. J. Kettle is thanked for the linguistic revision of the manuscript. Rundlöf is grateful to SCA for supporting this work and for allowing the publication of some of the data and to the Foundation for Strategic Research (SSF), Forest Products Industry Research College program for the financial support.

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