2003 - Effects of Carboxymethyl Cellulose and Ethyl(Hydroxyethyl) Cellulose on Surface Structure of Coatings Drawn Down From Polystyrene Suspensions

7/28/2019 2003 - Effects of Carboxymethyl Cellulose and Ethyl(Hydroxyethyl) Cellulose on Surface Structure of Coatings Dra

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2003 Spring Advanced Coating Fundamentals Symposium

EFFECTS OF CARBOXYMETHYL CELLULOSE AND ETHYL(HYDROXYETHYL) CELLULOSE ON

SURFACE STRUCTURE OF COATINGS DRAWN DOWN FROM POLYSTYRENE SUSPENSIONS.

Alexandra Wallstrm Lars Jrnstrm Peter Hansen Jouko PeltonenDept. of Chem. Eng. Dept. of Chem. Eng. STFI Dept. of Phys. ChemistryKarlstad University Karlstad University Box 5604 bo Akademi University

SE-651 88 Karlstad SE-651 88 Karlstad SE-114 86 Stockholm FIN-20500 boSweden Sweden Sweden Finland

ABSTRACT

The surface structure of coating layers drawn down from polystyrene suspensions was investigated. The effects oftwo different soluble cellulose ethers, ethyl(hydroxyethyl) cellulose and carboxymethyl cellulose, were compared.Each of the suspensions was investigated at three different concentration levels of the soluble cellulose ethers. AnAtomic Force Microscope was used to investigate the surface structure of coating layers drawn down on polyestersubstrates from aqueous polystyrene (PS) suspensions. The surface structure was interpreted by calculating the pairdistribution function ( )rg , where r is the radius of a ring centered at a particle.

The colloidal stability of the polystyrene suspensions was also investigated by means of rheology. The relativeviscosity was calculated in order to elucidate the colloidal stability and degree of flocculation.

Ethyl(hydroxyethyl) cellulose possesses temperature responsive properties, and the suspensions were investigated at

two temperatures, 23 o C and 55 o C. The effects of the concentration of cellulose ether on the relative viscosityindicated that ethyl(hydroxyethyl) cellulose, at least at the higher temperature, destabilized the PS suspensions by acapillary induced phase separation mechanism

INTRODUCTION

Characterization of the surface structure of coating layers is important since the particle orientation and the degree oforder of the surface determine the gloss and affect the penetration of fluids, e.g. printing inks.

The micro-roughness associated with the particle alignment on the surface of the final coating plays a major role indetermining the gloss of the coated paper [1] and the gloss is one of the most important surface factors andinfluences the quality of the printing. An enhanced micro-smoothness has been shown to improve the missing dot

performance and the print gloss. Therefore the importance to characterize the surface structure and the pigmentparticle alignment, especially since investigation has shown that there is a correlation between runnability, papergloss and particle alignment that goes down to the micro-roughness [2].

The current state-of-the-art analysis of surface structures, i.e. the distribution of pigment particles in the uppermostlayer, is restricted to quantitative or-semi-quantitative methods such as comparison of micrographs or to indirectmethods such as roughness and porosity measurements. However, by pair distribution function, ( )rg analyses,quantitative information about the surface structure of the coatings, the size of the ordered domains and the degreeof order of the pigments can be provided. The pair distribution function is here used to describe how added water-soluble polymers affect the surface structure in the dry coatings, i.e. after the completed consolidation process.

The pair distribution function method makes it possible to quantify the concept of surface structure. In this study,simple model coating colors consisting of spherical polystyrene pigment particles and water-soluble cellulose ethers(polymers) were chosen. Two different water-soluble polymers were studied: one non-temperature responsive

polymer (carboxymethyl cellulose, CMC) and one temperature responsive polymer (ethyl(hydroxyethyl) cellulose,EHEC). The pair distribution function method can readily be used for evaluation of more complex model colors,such as systems also containing dispersions binders (polymer latexes) or for monodisperse spherical mineral

pigments. However, the method is primarily developed as a laboratory method for examination of the effects ofdifferent soluble polymers and binders on the distribution of pigment particles in the dry coating layer.


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that diverged, with the respect to next neighbor, more than a particle radius from the average local surface wereexcluded. I.e., particles located far down were treated as holes. The remaining surface particles were projected in theaverage surface plane. The surface structure was then interpreted by calculating the pair distribution function ( )rg ,where r is the radius of a ring centered at a particle i [9]:

( ) ( )=

+

= N

1i 2rr2r

rin0A

N

1exprg [1]

where Nis the number of particles, 0A is the average area per particle (i.e. /1 where is the average number

density of particles) and ( )rni is the number of particle centers contained in a ring of radius r and thickness r centered at particle i . In the pair distribution function [Eq. 1], the distribution of particles around particle i isnormalized with respect to a homogeneous (random) distribution of particles given by the function ( )rngas

( ) ( )( )22

gas rrrrn += [2]

which equals the number of homogeneously distributed particle centers contained in the ring at a particle density of . The effect of the normalization is that ( )rg approaches the limiting value 1 with increasing degree of disorder.The radial distance r was represented in a discrete form of ( )...,2,1== mrmr with maxr = 16 particle diameters.

A theoretical pair distribution function [Eq. 3] can be fitted to the experimentally obtained pair distribution function[Eq. 1]. This enables additional information about the degree of order and the size of the ordered domains for thedifferent systems. The theoretical pair distribution function [Eq. 3] is based on the structure of a triangular(hexagonally packed) order, ( )triangrg . The peaks in ( )triangrg were broadened by a normal distribution, with a

standard deviation ( )r [Eq. 4] to account for the statistical fluctuations of the particle positions around thetriangular lattice points. The calculated pair distribution function is

( ) ( )( ) ( ) ( )

1

rexp1dx

r2

xexp

r2

rxrgrg

2

2

1/2triang+

= [3]

where x is the distance from the position the particle would have in a lattice of perfect triangular order. Thestandard deviation ( )r is given by

( )

+=2/3

0b

r1r [4]

The empirical fit to ( )exprg can be optimized by using the three parameters 0 , and b . This optimization is done

with respect to the positions and amplitudes of the peaks in the experimental ( )rg curve. The parameter correlates to the size of the ordered domains.


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The viscosity of the suspensions and the corresponding aqueous phases were measured by a rotational rheometer(MCR 300, Physica Messtechnik GmbH, Germany). The relative viscosity (

r ) was taken as a measure of the

degree of flocculation and was calculated as

o

r=

[5]

where are the viscosity of the suspension and o the viscosity of the aqueous phase. All viscosity measurements

were performed at the same temperature as the suspensions had been stored at (23 o C or 55 o C). The geometry used

was double gap for the suspensions measured at 23 o C and for the aqueous phases at both temperatures, andconcentric cylinders for the suspensions measured at the higher temperature.

RESULTS AND DISCUSSION

The distribution of PS particles was strongly affected by the concentration of the water-soluble polymer. This could

be exemplified by AFM images of dry layers of PS-CMC blends at 23o

C (Figure 1).

Figure 1. AFM images of PS layers containing 0.2 pph CMC (left) and 2.0 pph CMC (right) at 23 o C.

In order to facilitate comparison between AFM images and to estimate the degree of order and the size of theordered domains for the different systems, the pair correlation function ( )rg was calculated from information aboutthe particle positions. The particle positions were determined from the topographic AFM images and imagesanalysis.

The degree of order could be estimated from the experimental ( )rg curve as the value of the dimensionless length( 2ar ) until the systematic variation in ( )rg vanish, i.e. the distance until peaks no longer can be observed in the

( )rg graphs. The peaks relate to how many particles away the ordering structure of the hexagonal lattice remains.The width of the peaks also reflects the degree of order and the height of the peaks reflects the short-range degree oforder.It was observed that the added cellulose ethers decreased the degree of order of the dry coating layers. Thiswas valid for both ethyl(hydroxyethyl) cellulose and carboxymethyl cellulose. This was reflected in the pairdistribution function as a fast decay of the number of peaks with increasing concentration of the cellulose ethers.This is shown for the CMC-containing suspensions at two temperatures in Figures 2 and 3.


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Figure 2. The ( )rg curves for CMC-containing PS layers at 23 o C.

Figure 3. The ( )rg curves for CMC-containing PS layers at 55 o C.

This order-disorder behavior was approximately independent of the temperature, even if a slightly higher degree oforder was observed for layers containing 0.2 pph CMC when dried at the lower temperature compared to the

corresponding layers dried at 55 o C. The doublet observed around 22ar is evidence for hexagonal close packing

in two dimensions, and the feature at 22ar can be taken as an indication of the resemblance to or deviation from

the hexagonal packing.

For the EHEC-containing suspensions, the temperature significantly affected the distribution of particles in the dry

layer (Figure 4-5). At the higher temperature (55 o C), the structure of the layers prepared from EHEC-containing PS


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suspensions was less affected by the concentration of the cellulose ether than at the lower temperature (23 o C). Thefunction ( )rg at the higher temperature showed a low degree of order, also at low concentration of cellulose ether(0.2 pph EHEC).

Figure 4. The ( )rg curves for EHEC-containing PS layers at 23 o C.

Figure 5. The ( )rg curves for EHEC-containing PS layers at 55 o C.

The ( )rg curve for 0.2 pph EHEC at 23 o C was quite similar to ( )rg curve for 0.2 pph CMC at 23 o C, even if aslightly higher degree of ordering was observed for the CMC-containing system (Figure 6), this is shown by the

more pronounced double peaks at 22ar and 32ar for the CMC-containing system. However, at 55 o C a


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substantial difference between the two cellulose ethers was observed (Figure 7). The ( )rg curve for 0.2 pph EHEC

at 55 o C showed significantly less surface order than the corresponding CMC-containing layer. This corresponds

well to the AFM images of the dry coated layer at 55 o C (Figure 8).

Figure 6. The ( )rg curves for PS layers containing 0.2 pph cellulose ether at 23 o C.

Figure 7. The ( )rg curves for PS layers containing 0.2 pph cellulose ether at 55 o C.


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Figure 8. AFM images of PS layers containing 0.2 pph CMC (left) and 0.2 pph EHEC (right) at 55 o C.

Theoretical ( )rg curves were fitted to the experimental ( )rg curves. The three parameters ( 0 , and b ) wereoptimized with respect to the positions and amplitudes of the peaks in the ( )rg curve.

Since the two parameters 0 and b are dependent on each other and determines the standard deviation, the

parameter that estimates the size of the ordered domains gives most information about the degree of order on the

surface. A low value of corresponds with a smaller size of the ordered domains.

The values of the parameter obtained by the fitting of the theoretically obtained ( )rg curves to the experimental( )rg illustrates that the degree of order and the size of the ordered domains decrease with an increasing

concentration of cellulose ether (Table 1). The value of for the higher concentrations of cellulose ether (2.0 and

5.0 pph) was half of the value of at the low concentration (0.2 pph).

Polymer pph Temperatureo

C

*/2a Temperatureo

C

*/2a

CMC 0.2 23 2.9 55 2.5

CMC 2.0 23 1.5 55 1.6

CMC 5.0 23 1.6 55 1.6

EHEC 0.2 23 2.8 55 1.6

EHEC 2.0 23 1.4 55 1.3

EHEC 5.0 23 1.4 55 1.3

* The correlation length, , was normalized with respect to the particles diameter, 2a.

Table 1. The Correlation length .

The values of also supported the observation that the degree of order decreased at the higher temperature (55 o C)

for the EHEC containing systems (Table 1).


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The fact that the degree of ordering observed at low content (0.2 pph) of EHEC at 23 o C was slightly less comparedto the corresponding CMC-containing layer can be an effect of differences in adsorptive properties and thecorresponding differences in flocculation behavior between the two water-soluble polymers. It has been reportedelsewhere that EHEC adsorbs on latex surfaces to which CMC does not adsorb [10].

The aqueous phase of the suspensions was separated by centrifugation, and the viscosity was measured (Figure 9).

The viscosity measurements at 23o

C showed that the viscosity of the aqueous phase of the 0.2 pph EHECsuspension was almost identical to that of water, even if EHEC was more efficient as a thickener of the aqueous

phase at high concentration than CMC. This indicated that at the low concentration (0.2 pph) almost all EHEC wasadsorbed. The viscosity of the aqueous phase of the CMC-containing system at the low concentration (0.2 pph)differed slightly from that of water, which indicated that CMC was not absorbed.

Figure 9. Viscosity of the aqueous phase of the PS-suspensions at 23 o C. Measured viscosity of water at 23 o C isadded as a reference.

As a reference to the influence of concentration on the aqueous phase viscosity as depicted in Figure 9, viscosityvalues of pure solution of the cellulose ethers has been measured (Figure 10). The pure cellulose ether solutions

where adjusted to pH=8 before the rheology of the was measured at 23 o C. This gives additional information aboutthe systems and more evidence that EHEC is adsorbed at the polystyrene particles since the viscosity for the EHECsolution was higher than the viscosity of the CMC solution at the same concentration.


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Figure 10. The viscosity-concentration relation for the two cellulose ethers at 23 o C.

The relative viscosity ( r ) of the polymer-containing suspensions was calculated and taken as a measure of thedegree of flocculation (Figure 11). The relative viscosity of the CMC-containing suspensions indicated that thesuspensions were flocculated at high concentrations of CMC, probably by a depletion mechanism. No flocculation atall was observed at 0.2 pph CMC. Depletion flocculation needs a certain concentration to be activated, i.e. aconcentration threshold is required for the depletion mechanism to overcome the electrostatic stabilization. For thesystems with a higher amount of CMC, a depletion flocculation mechanism was likely to be operative. Depletionflocculation is a particle destabilization induced by the addition of a free polymer. A depletion layer is formed nextto the particle surface in the presence of a free non-adsorbing polymer. Upon close approach of the particles,essentially no soluble polymer will remain between the particles. This creates a reservoir of pure solvent that in turnwill create an osmotic pressure gradient that leads to an attraction between the particles and flocculation will occur.


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Figure 11. The relative viscosity for the CMC/EHEC-containing suspensions at 23 o C.

An important feature of depletion flocculation is that it exist a certain concentration region for flocculation, belowand above this region no depletion flocculation will be observed. At low concentrations of free polymer, thedepletion layer is at its maximum thickness, which should promote attraction, however the osmotic pressuredifferences is so small at low polymer concentration that flocculation will not occur. As the polymer concentration isincreased above the overlap concentration, the depletion layer thickness decreases while the osmotic pressureincreases [11, 12, 13]. Thus the degree of flocculation is not expected to increase at very high concentrations of non-adsorbing soluble polymer (due to reduction of the depleted volume at high concentrations), in good agreement withFigure 11. The corresponding pair distribution functions (Figures 2-5) are in accordance to the flocculation behavior.

For the EHEC-containing PS suspensions, the flocculation started already at a very low addition level and a furtherincrease in concentration level did not substantially affect the value of the relative viscosity (Figure 11). Thus a

bridging flocculation mechanism is very likely. Bridging flocculation is expected to occur already at lowconcentrations, in good agreement with Figure 11.

Bridging is considered to be a consequence of the adsorption of segments of individual polymer molecules on thesurface of more than one particle. Only a portion of the polymer chain is in direct contact with the solid surface.Each polymer chain is attached in sequences separated by non-adsorbing segments, which extend away from thesurface into the solution together with tails at each end of the polymer. Since segments and tails extending from one

particle can absorb onto another particle, flocculation of suspensions can be modeled by a bridging mechanism inwhich polymer chain binds two or more particles together [14, 15]. Bridging flocculation occurs when the polymerchain is long enough and the surface coverage by adsorbed polymers is low [16, 17]. The flocculation has often anoptimum at some concentration. Too low polymer concentration makes the flocculation less effective. At very high


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concentration of adsorbing polymer only a few free surface sites are available for bridging and the limit of stericstabilization is reached at very high concentrations [16].

A weak maximum in flocculation may be observed at some low concentration of adsorbing polymer in the case of abridging mechanism, which in fact was observed for the EHEC-containing suspensions in Figure 11 at high shearrates. The corresponding pair distribution functions (Figure 4-5) differ from the expected behavior, since the highrelative viscosity at low levels of EHEC did not resulted in the expected low degree of order. The 0.2 pph EHECsuspension may be an example of a suspension where the flocculated structure does not resist the capillary forcesthat are present during drying.

Measurements of relative viscosity at 55 o C (Figure 12) revealed a different behavior of the EHEC-containing

suspensions compared to those observed at 23 o C. At 55 o C, the relative viscosity was dramatically affected by theconcentration of EHEC, and the relative viscosity increased with increasing concentration of EHEC. On the other

hand, the relative viscosity of the CMC-containing suspensions at 55 o C closely resembled the behavior observed at

23 o C.

Figure 12. The relative viscosity for the CMC/EHEC-containing suspensions at 55 o C.

The differences in relative viscosity of the EHEC-containing suspensions between 23 o C and 55 o C can be explainedby the fact that EHEC is a temperature responsive polymer. CMC is not a temperature responsive polymer, andconsequently, the temperature did not substantially affect the behavior of the CMC-containing suspensions. The

influence of EHEC concentration on the relative viscosity at 55 o C (Figure 9) did not showed the expected behaviorfor bridging flocculation, since the relative viscosity started to increase with increasing concentration of EHEC.


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However, this behavior can be fully understood if capillary induced forces are assumed. Polymer adsorption is aprerequisite for bridging flocculation as well as for capillary induced phase separation (CIPS). Flocculation bycapillary induced phase separation has been reported to occur in EHEC-containing suspensions at temperatures close

to the cloud point of EHEC [5, 7]. The cloud point for the grade of EHEC used here was about 65 o C. The capillaryinduced forces act over long distances and the flocculation mechanism involves a considerable mass transport of thesoluble polymer to the region between the particles. Due to the mass transport involved, the degree of flocculation is

expected to increase with increased concentration of the soluble polymer. Another effect would be that the kineticsof flocculation would be relatively slow, which may influence both the formation and the breaking-up of the particleaggregates. A capillary induced phase separation mechanism may explain the less ordered structure observed at

55 o C for the layers containing 0.2 pph EHEC (Figures 3-4). The pair distribution function at 0.2 pph EHEC

resembled those at 2 and 5 pph, if the suspensions were held at 55 o C (Figure 5). Besides capillary induced phaseseparation, adsorption of soluble polymers may also lead to bridging flocculation. Thus it is likely that the observed

effects for the EHEC-containing suspensions at 55 o C was a combination of bridging flocculation and capillaryinduced phase separation (CIPS). The temperature responsiveness of EHEC and differences between EHEC andCMC are illustrated inFigure 13.

Figure 13. The relative viscosity versus the amount of water-soluble polymer added, at a shear rate of 100 s-1.

CONCLUSIONS

A novel method for analyzing the particle ordering in the surface layer of the coatings has been developed. Thismethod is based on pair distribution function analysis. This method was used in order to establish the relation

between flocculation and consolidation for two model colors containing two different water-soluble cellulose ethers.


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AFM supported by pair distribution analysis is a powerful tool in the description of surface structures and order-disorder phenomena that occur during consolidation of coating layers. The degree of order in coatings drawn down

at 23 o C decreased at increasing concentration of cellulose ether. This was valid both for the CMC and EHECcontaining systems. For the EHEC-PS systems, coating at elevated temperature increased the degree of disorder ofPS particles at low concentrations of EHEC. For the CMC-PS systems, the temperature did not substantially affectthe distribution of PS particles in the dried layers.

The relative viscosity of the systems indicated that different flocculation mechanisms occurred for the EHEC- andthe CMC-containing PS suspensions. In the CMC-containing PS systems the flocculation was induced by depletionflocculation and in the EHEC-containing systems the flocculation seemed to be induced by bridging mechanism and

by capillary induced phase separation. (CIPS).

ACKNOWLEDGEMENTS

The financial support from the Swedish Pulp and Paper Research Foundation, the Knowledge Foundation and theSwedish Agency for Innovation Systems (VINNOVA) is gratefully acknowledged.

References

[1] P.A.C. Gane and L. Coggon, Tappi Journal 70(12), 87-96 (1987).

[2] P.A.C. Gane, A. Grunwald and J.J. Hooper, Tappi 1995 coating conference proceedings, (1995).

[3] P. Dahlvik, G. Strm, M. Karathanasis, E. Hermansson and D. Eklund, J. Pulp Paper Sci., 25(7), 229-234 (1999).

[4] P. Dahlvik, M. Rigdahl and E. Hermansson, Tappi J., 84(1), 104 (2001)

[5] E. Freyssingeas, K. Thuresson, T. Nylander, F. Joabsson and B. Lindman, Langmuir 14(20), 5877-5889 (1998).

[6] H. Wennerstrm. K. Thuresson, P. Linse and E. Freyssingeas, Langmuir 14(20), 5664-5666 (1998).

[7] F. Joabsson and P. Linse, J. Phys. Chem. B., 106, 3827-3834 (2002).

[8] G. Binnig, C.F. Quate and Ch. Gerber, Phys3. Rev. Lett. 56, 930 (1986).

[9] P.H.F. Hansen, S. Rdner and L. Bergstrm, Langmuir 17(16), 4867-4875 (2001).

[10] K. Backfolk, Adsorption of polymers onto mineral and latex particles in coating dispersions, Thesis, boAkademi University, Finland (2002).

[11] J.L. Burns, Y.D. Yan, G.J. Jameson and S. Biggs, Colloids and Surfaces A. 162, 265-277 (2000).

[12] A.J. Milling, J. Phys. Chem. 100, 8986-8993 (1996).

[13] P.M. McGenity, J.C Husband, P.A.C Gane and M.S Engley, Tappi Coating conference proceedings, Orlando1992, 133-146 (1992).

[14] T.W. Healy, J. Colloid Science 16, 609-617 (1961).

[15] T.W. Healy, V.K. La Mer, J. Colloid Science 19, 323-332 (1964).

[16] G.J. Fleer, J. Lyklema, J. Colloid and Interface Science 46, 1-12 (1974).

[17] R.K. Iler, J. Colloid and Interface Science 37, 364-373 (1971).

Documents

2003 - Effects of Carboxymethyl Cellulose and Ethyl(Hydroxyethyl) Cellulose on Surface Structure of Coatings Drawn Down From Polystyrene Suspensions