adhesion of polymers in paper products from the … · · 2017-10-27journal of adhesion science...

Journal of Adhesion Science and Technology 25 (2011) 557ndash579brillnljast

Adhesion of Polymers in Paper Products from theMacroscopic to Molecular Level mdash An Overview

Boxin Zhao alowast and Hyock Ju Kwon b

a Department of Chemical Engineering and Waterloo Institute for Nanotechnology200 University Avenue West Waterloo Ontario Canada N2L 3G1

b Department of Mechanical and Mechatronics Engineering University of Waterloo200 University Avenue West Waterloo Ontario Canada N2L 3G1

AbstractPolymers are widely used in the manufacture converting and end-use of paper products where the qualityof polymer adhesion is important at both paper and fiber levels This article reviews a number of relevanttheories of polymer adhesion and such adhesion phenomena as revealed in recent investigations of theadhesion between polymer thin films and paper surfaces the adhesion of polymer molecules to cellulosefibers and surface forces measurements between model cellulosic surfaces It is concluded that molecularadhesion and viscoelasticity of cellulosendashpolymerndashcellulose joints play primary roles in the paper strengthwhich can be tailored by a rational design of polymer additives either as strength enhancers for strong paperproducts or debonding agents for soft paper tissuescopy Koninklijke Brill NV Leiden 2011

KeywordsPolymer adhesion paper adhesion adhesion phenomena paper strength interfiber bond cellulosepolymerinteraction adhesion and failure mechanisms

1 Introduction

Polymer adhesion is an important consideration in many processes of papermak-ing converting and end-use where the quality of polymer adhesion is fine-tunedto ensure the success of processing and deliver the desired performance of paperproducts to consumers In papermaking an aqueous suspension of cellulose fibersis filtered through wire webs where the adhesion between wet fibers is minimizedto prevent the fibers from agglomerating As the cellulose fibers lie on each otherlike ribbons on the plane of the paper sheet and dry the adhesion forces are estab-lished in overlapping zones of fibers giving integrity to paper This self-adhesionof cellulose fibers is the physical basis in the formation of the paper sheet Of-ten water-borne polymers are added in papermaking to enhance the strength of

To whom correspondence should be addressed E-mail zhaobuwaterlooca

copy Koninklijke Brill NV Leiden 2011 DOI101163016942410X525821

558 B Zhao H J Kwon J Adhes Sci Technol 25 (2011) 557ndash579

Figure 1 Adhesion phenomena in paper from the macroscopic to molecular level (a) Typical exampleof adhesion phenomenon at the macroscale mdash paperadhesive laminate (b) adhesion phenomenon atthe microscale mdash interfiber bonds (c) asperity contacts in the overlapping area of an interfiber bond(d) an individual asperity contact containing many microfibrils entanglements (e) long-chain structureof cellulose giving rise to molecular adhesion and (f) structure of one nanometer-sized β-glucose thebasic unit of cellulose molecule

interfiber bonds In such paper converting and application processes as xerographylamination of plastic film to paperboard corrugated box construction address-labelapplication polymer adhesion to paper and paper board plays a dominant role Xe-rographic toners and lamination films are heated during contact with paper so thatthe thermoplastic polymers can spread on the paper and form intimate contact toestablish strong molecular adhesion forces By contrast box construction papersplicing and address-label applications employ adhesives which promote adhesionbetween paper surfaces

Figure 1 illustrates various adhesion phenomena from the macroscopic to themicroscopic nanoscopic and molecular levels At the macroscopic level (sim10 cm)(Fig 1(a)) the paper is a common adhesive carrier because of its rough surfacerelatively high surface energy and low cost and is used in such adhesive productsas Post-itreg notes and adhesive tapes On the other hand polymeric adhesives suchas splicing tapes are employed in paper converting processes to joint individual pa-per rolls for continuous processing where a robust and strong paperadhesivepaperbond is required upon instant contact in the process at a speed of sim60 kmh [1]At the microscopic levels (10 micromndash1 mm) paper is virtually a network of cellulosefibers held by adhesion forces in overlapping areas called interfiber bonds A typ-ical softwood pulp fiber is about 1 mm long and 20 microm wide and 4 microm thick Itis flexible and it intertwines with neighboring fibers through the interfiber bonds(Fig 1(b)) The apparently simple interfiber bonds are complex from the perspec-

B Zhao H J Kwon J Adhes Sci Technol 25 (2011) 557ndash579 559

tive of adhesion The fiber surface is rough and contains port holes called pits and iscovered with microfibrils The interfiber bonding area is far smaller than the lsquorealrsquosurface area consisting of a number of contacting asperities of about 1 microm in diam-eter (Fig 1(c)) On each asperity contact the extended submicrometer-size fibrilsentangle with those on the opposing surfaces The fiber surface is also composi-tionally heterogeneous The major components are cellulose hemicelluloses anda small amount of lignin which are overall hydrophilic and can absorb water andswell (Fig 1(d)) At the nanoscopic levels (01ndash100 nm) the cellulose is a long-chain polysaccharide (Fig 1(e)) consisting of a number of one-nanometer glucoseunits (Fig 1(f)) The hydroxyl groups on the glucose units are capable of forminghydrogen bonds at a distance of 01 nm between the polymer chains These hydro-gen bonds assemble the cellulose molecules into a crystalline structure (nanofibrilsor elementary fibrils sim4 nm) so that the bulk material is strong having an elasticmodulus above 10 GPa [2] Its surface is often covered with amorphous celluloseand hemicelluloses chain ends which are able to diffuse entangle and interact at theinterface of two contacting cellulose fibers [3]

The adhesion phenomena associated with the manufacture and end-use of paperproducts have long been observed and utilizing polymers in and with paper ma-terials has been a common practice to improve and add value to paper productsHowever only a few research studies have focused on polymer adhesion in papersin contrast to the huge body of literature on both polymer adhesion and paper tech-nology most of these studies having been conducted from the perspective of paperphysics (or paper materials science) [4] The current understanding of the poly-mer adhesion in paper is limited particularly at small scales This article providesan overview on the adhesion of polymers in paper products mainly from the per-spective of adhesion science We will first review the recent advances of polymeradhesion theories relevant to paper and cellulose materials and then discuss typi-cal adhesion phenomena occurred in paper products at different length scales andimplications to the way to improve paper properties

2 Polymer Adhesion Theories

There is a large body of research literature on the mechanics of adhesion Fivecommon theories in the adhesion science proposed to explain various adhesionphenomena are (i) mechanical interlocking theory (ii) electrostatic theory (iii) dif-fusion theory (iv) adsorption theory and (v) weak-boundary layer theory Detailsconcerning these theories can be found in classic adhesion textbooks such as thatby Kinloch [5] In the following we briefly review the polymer adhesion theoriesin the framework of the classic thermodynamics and contact mechanics theories

Thermodynamically the propensity of polymer adhesion is described by the con-cepts of surface or interface energy and the work of adhesion and cohesion The


work of adhesion W is defined as the energy change per unit area due to the elim-ination of two bare surfaces and the creation of an interface

W = γ1 + γ2 minus γ12 (1)

where γ1 and γ2 are the surface energies of the two bare surfaces γ12 is the in-terfacial energy If the two surfaces are the same γ1 = γ2 and γ12 = 0 and thenW = 2γ1 called the work of cohesion The work of adhesion is a useful quantitybecause it distinguishes two states contact and separation It is worthwhile to de-scribe the relationship between the work of adhesion and contact angles that areoften used to describe energetic interactions between a solid surface and a liquidprobe A balance of equilibrium forces at the contact line relates the surface andinterfacial energies to one another and to the thermodynamic work of adhesionthrough the YoungndashDupre equation

W = γl(1 + cos θ) (2)

where γl is the surface energy of the liquid and θ is the equilibrium contact angleof the liquid on the solid surface

The molecular origin of the work of adhesion are the intermolecular attractiveinteractions [6] When two smooth polymer surfaces approach each other within adistance of a few nanometers they jump into contact because of such intermolecularinteractions as the universal van der Waals interactions and other types of specificmolecular interactions such as polar interactions hydrogen bonding and acidndashbaseinteractions The work of adhesion can be estimated from the van der Waals in-teraction in terms of equilibrium separation distance (D0 asymp 02 nm) and Hamakerconstant A12 [7] whose value depends on the surface chemistry of materials incontact

W = A12

12πD20

(3)

The classical JKR (Johnson Kendall and Roberts) theory [8] describes the ad-hesion behavior of polymeric elastomers which relates a loading force to substratedeformation elastic modulus and surface energy For an elastic sphere of radius R

when pressed by a load L against a flat surface of the same material of effective (orcombined) elastic modulus K and the work of adhesion W there is a flat contactarea of radius a given by

a3 = R

K[L + 3πRW +

radic6πRWL + (3πRW)2] (4)

in which the surface energy γ is related to the thermodynamic work of adhesionW by equation (1) and W = 2γ for the same materials in contact The first right-hand side term of equation (4) is the Hertz result for non-adhesive surfaces iethe radius of contact area is given by a3

H = (LR)K The remaining terms give the


effect of the work of adhesion on the contact area The adhesion (or the maximumseparation or the lsquopull-offrsquo force) is given by

Fad = minusLpull-off = 3

2πRW (5)

or

Fad = minusLpull-off = 3πRγ (6)

if the two surfaces are same It is worth noting that the separation occurs not ata = 0 but at a finite contact radius as = (3R2W2K)13 Recent experimental stud-ies of polymer adhesion have made extensive use of the JKR theory as reviewed byShull [9] This theory has been found to work well for lsquoidealrsquo (clean smooth elas-tic) surfaces showing no hysteresis viz the loadingndashunloading paths described byequation (4) are reversible and predict the correct (thermodynamic) values for γ tobetter than 10 [8 10 11]

The concepts of the work of adhesion and surface energy and the JKR-adhesiontheory have provided a theoretical framework to understand polymer adhesionbut they are too simplistic for most polymer systems where nonequilibrium time-dependent molecular changes occur at the contacts and where bulk viscoelasticdeformations occur in debonding These nonequilibrium processes have been re-ferred to as lsquoadhesion dynamicsrsquo [12] for which there are currently no applicabletheories except for a few practical or phenomenological descriptions For instancethe JKR equations have been extended to non-equilibrium adhesion experiments toobtain the lsquoeffectiversquo surface energy γeff by relating the maximum pull-off forceto the effective surface energy using equation (6) Lpull-off = minus3πRγeff [12 13]The difference γeff minus γ is the adhesion hysteresis Many factors including contacttime temperature and separation velocity have been identified as influencing theadhesion hysteresis [12ndash14] Two extreme cases are the behaviors of pure solidsand liquids There is little hysteresis between two rigid solid surfaces or at veryrapid loadingndashunloading rates because no molecular arrangements occur duringthe time scale of the measurement On the other hand liquid-like surfaces ex-hibit little hysteresis since the molecular relaxations occur much faster than theexperimental time scale ie the system is always at equilibrium An amorphousviscoelastic surface is the intermediate situation where the hysteresis is at its high-est

The viscoelastic behavior is often characterized by the Deborah number De(De = τtf) and the Weissenberg number Wi (Wi = τ λ) where τ is the charac-teristic relaxation time of the material at the temperature of the measurement tf isthe characteristic time of the flow and λ is the shear rate (sometimes defined asthe inverse of the experimental lsquoobservationrsquo time λ = 1tf) The Deborah andWeissenberg numbers are traditionally used in rheology at a high Deborah number(De gt 1) the flow is fast compared with the fluidrsquos ability to relax and the fluidthen responds more like a solid while for De lt 1 the fluid is more liquid-like Shulland Creton [15] proposed that the Deborah number can also be used as a quanti-


tative parameter to describe the transition from liquid-like to solid-like behavior ofthin polymer films In the Surface Force Apparatus experiments Israelachvili andcoworkers [12 13] used the Deborah number to describe the adhesion dynamicsof polymers from viscous to glassy states and found that the largest hysteresis oc-curred at about De = 1 Although the solidndashliquid transition is determined by morethan just the temperature in the field of polymer science the concept of glass tran-sition temperature (Tg) is commonly used to describe the transition of a polymerbetween a liquid-like viscous state and solid-like glassy state At a high tempera-ture (T gt Tg) a material is a liquid with a very fast rate of structural relaxationie small value of the relaxation time τ this corresponds to a small De at a fixedexperimental lsquoobservationrsquo time In adhesion studies the concept of Tg has beenconveniently used as a starting point for studying the viscoelastic behavior of amaterial [12 13]

In complex polymeric systems more than one molecular relaxation process canoccur at an interface so that a number of maxima of adhesion hysteresis appearat different temperatures and measuring rates Furthermore bulk deformation andshape changes often occur on contact and separation involving sintering or coa-lescence stringing crazing and cavitation These bulk deformations are known tobe the major adhesion mechanisms for pressure-sensitive adhesives [16ndash18] Re-cent experimental work on adhesion of viscoelastic polymers has shown that themechanical energy required to separate two bonded surfaces commonly referred toas lsquopracticalrsquo adhesion in literature [19 20] to be distinct from the lsquoidealrsquo adhesionor thermodynamic work of adhesion can be expressed as a function of the rate ofseparation v by the following empirical formula [9 21]

G = G0

(1 +

(v

vlowast

)n) (7)

where G is the practical adhesion also named as critical strain energy release ratein the field of fracture mechanics G0 is a threshold separation energy when v rarr 0vlowast is a characteristic separation rate which is controlled by the relaxation timesof the polymer and n (lt1) is a parameter that describes the relationship betweenG and v The difference between the practical adhesion and the thermodynamicwork of adhesion G minus W is the adhesion hysteresis which is always positive andcould be thousands of times larger than the work of adhesion

The adhesion hysteresis arises from both the bulk viscoelastic deformations andthe molecular interactions at interfaces The energy dissipation processes (both bulkand interfacial) have been found to be strongly dependent on the separation rate v

[9 11 12] The ratio vvlowast in equation (7) might be considered approximately as thereciprocal of Weissenberg or Deborah number Thus the prediction of equation (7)that the practical adhesion increases monotonically with separation rate appears tobe contrary to the rheological prediction of the maximum adhesion hysteresis atabout De = 1 To the authorsrsquo knowledge there is no research report to addressthis discrepancy perhaps most polymeric systems studied in the context of equa-


tion (7) have only been characterized within the low Deborah number (De lt 1) orthe liquid-like regime

At the molecular level when two pieces of polymers are brought into con-tact their surface functional groups often rearrange themselves by adapting to thechange of conditions from exposure to air or other medium to exposure to anotherpolymer surface [22] Such specific molecular interactions as acidndashbase interac-tions and hydrogen bonding may be established leading to stronger adhesion [23]The improvement of adhesion via the establishment of specific interactions hasbeen long realized and utilized for strength enhancement particularly by surfacechemists [24] Furthermore driven by the interfacial interactions and the tendencyof system entropy to increase polymer chains from the two sides tend to diffuseacross the interface by the process of reptation [25] forming a mixed region re-ferred to as interphase The polymer diffusion has been a well-studied subject asreviewed by Kausch and Tirrell [26] Generally chain interdifussion of amorphouspolymer across the interface is a slow process but it occurs rapidly when the temper-ature is above the glass transition temperature Tg of the polymer For self-adhesionthis interdifussion causes the strength of the interface to increase with time until itreaches the cohesive strength of the material For adhesion between different poly-mers a finite interfacial width or interphase is established upon equilibration Thisinterfacial width is controlled by the balance between attractive entropy of mix-ing of the two polymers and a repulsive enthalpy of mixing (often described bythe FloryndashHuggins interaction parameter χ ) Helfand and Sapse [27] showed thatthe interfacial width w is given by w = 2a

radic6χ where a is the size of a statis-

tical polymer segment The interphase (interfacial width and density profiles) hasbeen probed using neutron reflection techniques revealing that the density profilesacross the interphase follow a hyperbolic tangent profile [28] Since the interfaceof polymers gains adhesion strength from intermolecular interactions and chainsentanglement the adhesion strength relates directly to the nature and structure ofthis interfacial region or interphase For a pure surface contact without molecularre-configuration and chain diffusion the G0 = W

Attempting to correlate measured adhesion strength (or energy) to van der Waalsforces or to the thermodynamic work of adhesion or more recently to the interfacialwidth has been a major theme in studies of polymer adhesion aimed at predictingadhesion forces from the knowledge of polymer chemistry Many correlations arereported in the literature [20] but a general relationship allowing for the predictionof mechanical strength of adhesion bond from the molecular interaction parametershas not yet been identified partly because of complex micromechanical processes(chain pull-out or scission mechanisms) involved in debonding and partly becauseat the more macroscopic level surface deformations that accompany the separationof the surfaces also contribute to the measured adhesion strength Neverthelessthe recent advances in polymer adhesion science have provided paper scientistsengineers and technologists new perspectives and insights into investigating andunderstanding adhesion phenomena in papermaking and devising proper strategies


to tailor the adhesion properties for better paper properties and performance Fromthis context we will examine the adhesion aspects in paper products at differentlength scales

3 Polymer Adhesion Phenomena in Paper

There is a very broad set of topics related to adhesion to and in paper as we can seefrom the articles published in this Special Issue Here we focus on a few typicaladhesion aspects investigated recently from the authorsrsquo own research and relatedwork in literature including the adhesion between polymer thin films and papersurfaces the adhesion of polymer molecules to cellulose fibers and surface forcesmeasurements on model cellulosic surfaces It may be useful to briefly describecommon adhesion measurements employed to paper and cellulose materials beforediscussion of the adhesion phenomena

The quality of polymer adhesion bonds is mainly characterized by their resis-tance to mechanical stresses and strains in either the normal direction (Type 1 mdashthe open mode) or the lateral direction (Type II mdash the shear mode) However a me-chanical force cannot be set directly at the interface but is positioned some distanceaway so this force is separated from the interface by a mechanical mechanism (egpulling peeling or shearing) which can be quite complex As the force is raised toa point at which a crack runs along the interface the adhesion bond is separatedthe maximum force is the adhesion force Figure 2 illustrates three typical types ofadhesion testing (a) JKR-type contact adhesion testing (b) single-lap shear testing

Figure 2 Three typical adhesion tests (a) JKR-adhesion testing (b) single-lap shear testing and(c) peel adhesion testing and their force curves (d) JKR-type plot of contact radius a vs load L(e) shear force vs displacement curve and (f) peel force vs displacement (or peel distance) curve


and (c) peel testing All three types of tests measure adhesion in forms of the re-lations between the applied force and characteristic length (contact radius in (a) orseparation distance in (b) and (c)) but provide different adhesion information dueto differences in their test geometries

In the JKR-type contact adhesion testing (Fig 2(a)) smooth curved samples areused to allow for fundamental interpolation of the data in the framework of contactmechanics theories eg JKR and DMT (Derjaguin Muller and Toporov) theories[29] Typically the radius of the contact area is measured at a constant (loadingand unloading) force rate L [12 30] or displacement rate D [31] depending onthe rigidity of the test system and the radius of contact area a (or its cube a3)is plotted as a function of compressive load L (Fig 2(d)) By fitting the data toequation (4) it is possible to determine the effective elastic constant K and thework of adhesion W The maximum pull-off force in the unloading branch is theadhesion force which is often normalized by the radius of curvature of the surfacesgiving an energy unit of Nm or Jm2 A deviation of the unloading curve from theloading curve indicates the occurrence of adhesion hysteresis

The shear (Fig 2(b)) and peel adhesion (Fig 2(c)) tests are destructive and areoften used to assess the bonding strength of laminated materials when at least one ofthe layers is flexible The shear test requires only a small amount of polymer samplein the overlapping area of two adhering films The force is plotted as a function ofdisplacement and is often normalized by the laminated area Bonding strength P isestimated by dividing the highest force value by the bonding area ie P = FmaxA

(Nm2) In some cases it is expressed as the bonding energy per unit area ie thearea under the forcendashdisplacement curve shown in Fig 2(e) divided by the bondingarea In such cases the unit becomes Jm2 = Nm Since the shearing often causescohesive failure of such viscoelastic polymers as pressure-sensitive adhesives it isoften employed to measure the cohesive strength of polymer adhesives [32]

In contrast to shear testing peel testing gives relatively low separation force valueand leads mostly to interfacial failure and involves a continuous separation [32] Theforce vs displacement peel trace contains useful information about both the polymeradhesive and substrate and is sensitive to the variation of substrate surface qualityFor example low adhesion spots could be easily detected from the force variationsin the peel trace (Fig 2(f)) However because of the asymmetric geometry peeltests involve complex mechanics such as bending moments and plastic deformationand the measured peel force strongly depends on the peel angle [32] For a generalpeel test where an incremental peeling occurs by dl (Fig 2(c)) the force can bederived from the energy balance approach as

G = F

b(1 minus cos θ) (8)

where θ is the peel angel G is the energy release rate in equation (7) and b isthe width of the bonded area Therefore the peel force is usually normalizedby the width of bonded area and G is frequently used as bonding strength ina peel test The bonding strength of a 90 peel (Fig 2(c)) can be expressed as


G = Fb = W + ψ where W is work of adhesion and ψ represents other energylosses caused by plastic deformation viscoelastic deformation and bending Notethat the failure mode of a 90 peel test is Type-I as opposed to Type-II in a sheartest In practice a 90 peel test and a shear test are commonly used together to char-acterize a polymer adhesive in terms of interfacial (adhesion) strength and cohesivestrength respectively

31 Macroscopic Adhesion Phenomena mdash Polymer FilmPaper Laminates

At the macroscopic level paper adhesion is conventionally described in terms ofits surface energy and such surface chemistry characteristics as contact angles sur-face composition and acidndashbase functional groups The thermodynamic work ofadhesion W between a paper surface and a layer of polymer adhesive can be de-termined by equation (1) as a function of the surface energy of the paper γP thesurface energy of the polymer adhesive layer γA and the adhesivepaper interfa-cial energy γPA W = γP + γA minus γPA Although it is unlikely that W will be usefulin predicting practical adhesion it is important because it illustrates the relation-ship between ideal adhesion and surface energy which in turn is dependent uponsurface chemistry Paper surface components such as hydrophobic sizes wood ex-tractives and fillers (CaCO3 clay and talc) lower the surface energy [33ndash35] Thenegative effects of these surface components can be offset by plasma treatmentwhich introduces polar surface groups For coated papers adhesion depends on thenature of the coating material Welander [36] showed that the type of binder in thecoating has a marked influence on the adhesion of polyethylene to coated paper Forinstance polyethylene displayed a stronger adhesion to a paper coating containingstyrenendashbutadiene and CaCO3 pigment than to a coating containing poly(vinyl ac-etate) binder and clay pigment

Many publications describe the determination of paper surface energy for ex-ample from contact angle measurements and inverse gas chromatography [24 3738] Paper surface energy can be characterized using the concept of critical surfacetension γc determined by the classic Zisman approach [39] of plotting cosine ofcontact angle θ as a function of surface tension for a series of liquids and extrapo-lating to θ = 0 The critical surface tension of paper is important for the wetting ofa polymer melt onto paper surfaces which is a prerequisite for an intimate contactand good adhesion [19] To characterize more effectively the paper surface energyit is often desirable to separate surface energetics into polar and non-polar (or dis-persion) components Using contact angle analysis Luner and Sandell [40] foundthat 30 of dry cellulose surface energy would originate from dispersion forcesHowever the contact angle analysis on a paper surface is limited by paper surfaceroughness and porosity Paper surface energy analysis via inverse gas chromatog-raphy relies on the thermodynamics of the adsorption process of hydrocarbons onpaper fibers Using non-polar adsorbates the dispersion energy component can bedetermined Using polar adsorbates the polar component of paper surface energyor the acidndashbase properties can be determined The use of the acidndashbase concept for


cellulose substrates may provide more insight into the thermodynamic characteris-tics of paper surfaces but the analysis method and the interpretation of adsorptiondata are complex limiting the application to paper The surface energy values forpapers vary from 25 to 60 mJm2 For more complete lists of surface energies referto Borchrsquos review [41] and van Oss book [42]

Borch [43] showed that the adhesion of thermoplastic toners to paper increasedwith the surface energy of paper estimated from contact angle measurements andinverse gas chromatography Other examples in which the role of thermodynamicswas commonly considered include the corona treatment of films and paper andchemical modification of sizing agents However attempts to link paper surfaceenergy to adhesion forces were scarce mdash these are summarized in Borchrsquos review[41] Swanson and Becher [44] reported poor adhesion between polyethylene andpaper when the critical surface tension of paper was lower than that of polyethylene(γc = 31 mNm) Similar conclusions were reached by Gervason and coworkers[45] who showed that the delamination force for polyethylenendashpaper laminatesincreased with paper surface energy for a series of sized papers

The quality of polymer adhesion to paper is eventually determined by the resis-tance of the adhesion bond to mechanical stresses in term of either failure stressreferred as the bond strength or the mechanical energy applied to separate thebonded system referred as practical adhesion At this point it may be helpfulto make distinctions between adhesion (making a bond) and the adhesion bondstrength or practical adhesion (separating a bond) Once an adhesion bond has beenformed such interfacial forces as van der Waals forces and acidndashbase interactionsinvolved in making adhesion bond are no longer a primary concern since the in-terfacial separation is not necessarily the only failure condition This distinction isparticularly helpful for understanding the behavior of polymerpaper laminates Theformation of adhesion bond with paper is a fairly easy process because of its rela-tively high surface energy and porous structure But compared with the glass andplastics paper is a weak substrate since paper has a layered network structure and issusceptible to the actions of tearing and delamination Therefore the paperpolymerlaminates do not often separate at the interface

Since paper is film-like and flexible and often subject to the action of peelingin applications the failure mechanism and adhesion bond strength are frequentlystudied using peel tests Bikerman [46 47] was one of the first authors to discuss thepeeling behavior of polymer adhesive tapes from paper noticing that a maximumforce occurred as paper delaminated from surfaces More than twenty years afterBikermanrsquos work Yamauchi et al [48 49] reported the first systematic peel studiesof adhesive tapes from paper Recently Zhao and coworkers [50ndash53] conducted aseries of peel experiments to investigate the influence of paper properties on theperformance of adhesive tapes from which a detailed relationship between paperproperties and adhesion performance was established and the peeling-induced paperdelamination process was clarified


Figure 3 (a) Typical failure modes and force vs distance curves of paperadhesive laminate [51] and(b) schematic illustration of a generalized peeling map in the form of log peak peel force vs log peelrate relation consisting of two linear domains rate-dependent interfacial failure and rate-independentpaper failure and three characteristic parameters the interfacial peel force (Fin) at a low peel rate of1 mmmin the maximum peel force (Fc) and the slope (Sp) of the interfacial peel force and a criticalpeel rate Vc

Figure 3(a) shows two typical examples of peel force vs distance curves whenpeeling a piece of adhesive tape from newsprint surfaces The lower peel rate causedinterfacial failure whereas the higher peel rate caused paper failure In the case ofinterfacial failure the peel curve is noisy but approximately constant resemblingthe peeling from stainless steel and glass Although the tape surface after peelinglooked clean to the unaided eye microscopic examination of the tape surface afterpeeling revealed small debris on the tape surface In the paper technology literaturethis situation is called lsquopickingrsquo and is a source of contamination during someprinting operations The peel curve at the higher peel rate is more complicated Thepeel force initially rises to a maximum point (the peak force Fp) and then drops to alow steady-state value corresponding to catastrophic delamination (paper failure)There is at least one layer of fibers embedded in the tape after peeling The paperfailure often starts at the weaker spots in the contact line and then the spots broadenand merge until a whole layer of fibers is peeled to cover the tape The term lsquomixedfailurersquo denotes this transition region between interfacial failure and paper failureNote that most engineering polymers applied to paper are designed to have strongbulk strength so the cohesive failure of polymer is rare

Many peel experiments revealed that the peak force Fp (ie the maximum forcein the force vs distance peel curve) is the most important for studying the poly-merpaper interactions [50] Based on this finding a novel peel data analysis methodhad been developed by which the overall peel behavior of a polymerpaper combi-nation is conveniently summarized by plotting the log peak peel force as a functionof log peel rate This analysis yielded a generalized peel curve consisting of a rate-dependent interfacial failure domain and a rate-independent paper failure domain(Fig 3(b)) [51] The influence of polymer adhesive properties and peel angle on the


generalized peel curve was further studied It was found that the peel angle shiftedthe generalized peel curves vertically whereas the polymer adhesive properties in-fluenced the slope of the interfacial failure segment but had no significant effecton the paper failure segment The rate-dependent interfacial failure region is con-sistent with the prediction from equation (7) typical behavior of pressure-sensitiveadhesives Furthermore although it has been a long-known phenomenon that thetendency of a paper to delaminate in peeling is sensitive to peeling direction mdashpeeling against the fiber orientation direction leads to paper delamination more eas-ily than peeling along the fiber orientation direction mdash the peak forces for the twopeeling directions were found to be same suggesting the maximum peel force is adirection-independent parameter [54]

Three independent parameters were extracted from the generalized peel curvethe interfacial peel force (Fin) at a low peel rate of 1 mmmin the maximum peelforce (Fc) and the slope (Sp) of the interfacial peel force mdash see Fig 3(b) There-fore the interactions between paper and polymer adhesives can be analyzed bymonitoring these three parameters In addition the transition from interfacial fail-ure to paper failure can be quantified by a critical peel rate Vc whose value dependson both the paper and the polymer adhesive properties By the use of advancedstatistical analysis and the newly developed approach for analyzing polymer ad-hesivepaper peel curves Zhao and coworkers [53] identified links between paperproperties and the performance of adhesive tapes The paper properties influencingpeel force in interfacial failure domain were found to be primarily the paper surfacechemistry characterized by oxygencarbon ratio (determined by XPS) and secon-darily surface roughness the peel force increased both with oxygencarbon ratioand with the surface roughness The logndashlog slope in the interfacial failure domainwas found to be independent of paper properties it is determined by the polymeradhesive rheology The governing paper property in the paper failure domain wasfound to be the paper internal bond strength as measured by a paper internal (Scott)bond tester Therefore the maximum strength a polymer filmpaper laminate canreach is determined by the cohesive strength of paper or the paper internal bondstrength

32 Microscopic Adhesion Phenomena in Paper mdash Interfiber Bonding

It is the interfiber bonds that hold the fibers together to form a macroscopically con-tinuous material Two basic structural elements determining the strength of paperare the strength of fiber and that of interfiber bond The modern understanding ofpaper strength at the fiber level started with Pagersquos semi-empirical theory [55] Pageproposed that the reciprocal of the tensile strength is proportional to the sum of thereciprocals of the interfiber bond strength and the fiber strength In the simplestform the relationship was expressed by the following equation

1

T= 1

F+ 1

B (9)


where T is the tensile strength F is the fiber strength and B is the interfiber bondstrength The Page theory provides a simple picture of the relationship betweenthe tensile strength of paper and that of a fiber and interfiber bond It predicts thefact that the strength of weakly-bonded paper is controlled by the interfiber bondstrength while the fiber strength becomes important for well-bonded paper Therehave been extensive studies of interfiber bonding and its relationship to the me-chanical strength of paper which had been summarized in some excellent reviewseg by Dodson [56] Uesaka [57] and Clark [58] and more recently in Niskanenrsquosbook [4] Now it is a common practice in papermaking processes to improve paperproperties by enhancing interfiber bond strength through such treatments as pulpbeating wet-pressing and adding chemical additives

As for the adhesion aspect of interfiber bonding most paper material scientistshave focused on the phenomenon at the microscopic or fiber level generally as-suming the fiberndashfiber bonds as planes of zero thickness [59] Hence the interfiberbond strength B can be assumed as the product of the bonded area A and the specificbond strength σ that usually refers to the shear strength ie B = A middotσ Fundamen-tal studies on adhesion and friction by Berman and Israelachvili have revealed thatthe shear strength is different but is related to adhesion energy or force in the nor-mal direction [60] Assuming that the fiber surface is microscopically homogeneousand σ is proportional to the work of adhesion W (=2γ ) the bond strength is pro-portional to the product of the bonded area and the surface energy ie B prop A middot γ Based on this assumption we may be able to gain some insights from the per-spective of the classical JKR adhesion theory into the effects of such commonpulp treatments as pulp beating wet pressing and adding chemical agents on in-terfiber bond strength during sheet formation in terms of three parameters bulkelastic modulus K surface energy γ and external loading force L For instancethe pulp beating delaminates fibers walls to reduce the bulk elastic modulus K thewet-pressing treatment gives an external loading force L according to equation (4)both of these lead to a larger contact area and then greater bond strength As for thechemical treatments the adsorbed polymers or other additives alter the fiber surfaceenergy γ which subsequently changes both the contact area A and the interfacialshear strength σ and the bond strength B However for most practical papermak-ing fibers their surfaces are rough and have crevices and valleys (pores) over awide range of distance scales Pulp beating may also cause fiber wall fibrillationto increase fiber surface areas and contacts during sheet formation In addition thecrystalline cellulose surface is commonly coated with amorphous hydrophilic poly-mers including lignin hemicelluloses and polymer additives The amorphous layersbetween fibers diffuse and entangle with the opposing surfaces to form a bondedarea with a finite thickness making the assumption of zero-thickness bonded areainvalid These structural and compositional heterogeneities of cellulose fibers makeit almost impossible to apply the well-developed adhesion theories directly to inter-fiber bonds


33 Nanoscopic and Molecular Adhesion in Paper mdash Model Cellulosic Surfaces

The inherent complexity of a wood fiber surface has made it difficult to study themolecular processes in the formation of interfiber bonds Therefore we have seenan increasing interest in the development of well-characterized model cellulose sur-faces The commercially available cellophane and regenerated-cellulose membranehave been traditionally used as model cellulose surfaces [61] Recently a numberof methods have been developed to fabricate molecularly-smooth model cellulosesurfaces for molecular and surface forces studies including (i) LangmuirndashBlodgettdeposition of precursor cellulose derivatives such as trimethylsilyl cellulose re-sulting in mostly amorphous cellulose films [62 63] (ii) spin-coating of extractedcellulose nanocrystals directly from wood pulp mdash using this method the crystalstructure of the native cellulose crystalline cellulose I can be preserved [64] and(iii) spin-coating and re-crystallizing extracted cellulose nanocrystals into thermo-dynamically more stable crystalline cellulose II [65]

The use of molecularly smooth model cellulosic systems has made possible themeasurement of surface forces which can provide detailed information on the mole-cular adhesion and offer insights into the act of interfiber bonding Both the surfaceforces apparatus and colloidal probe technique have been used to study these sys-tems at varied surface and solution conditions in terms of force vs distance relationsor force laws It has been realized that the characteristics of the forcendashdistancecurves depend strongly on the method used to prepare the cellulose surfaces forcecurves characteristic of van der Waals electrostatic and steric forces have beenobserved for the different surfaces [65] The measured forces on amorphous spin-coated cellulose surfaces have typically been dominated by a steric interactioncaused by a lsquodangling tailrsquo where a few chains from the highly swollen surfaceextend significantly away up to 100 nm from the surface [62 66 67] Rutland andcoworkers have shown that outside of this steric region the measured interactionprofile can be reasonably fitted by using DLVO theory and furthermore that thesteric interaction is dependent on the ionic strength of the solution [68] The mea-sured forces between two cellulose I surfaces are monotonically repulsive becauseof the surface charge introduced through the acid hydrolysis procedure [69] Themeasured interaction when using crystalline cellulose II films could be tuned to beeither attractive or repulsive in line with predictions from DLVO theory At highpH the interaction was well described between the constant charge and constantpotential limits [70 71] At low pH the cellulose surfaces were not charged andso there was little double-layer repulsion and the Hamaker constant of cellulosendashwaterndashcelloluse had been determined to be about 8times10minus21 J [62 70 72] Note thatthe Hamaker constant for cellulosendashairndashcellulose is about 8 times 10minus20 J suggestingthe van der Waals attractive force in water is about ten times lower than that in airaccording to equation (3)

Based on the fundamental studies on model cellulosic surfaces Fig 4 illus-trates a simplified process of the formation of interfiber bonds assuming smoothfiber surfaces covered by an amorphous nanometer-thick layer of cellulose chains


Figure 4 Simplified processes in the formation of an interfiber bond (andashc) Cellulose self-adhesion(d) a schematic force L mdash distance D relation showing the repulsion forces (electrostatic double-layersteric and hydration) in water and adhesion forces (van der Waals hydrogen bond and chain entangle-ment) in air (endashg) polymer-mediated adhesion between cellulose fiber surfaces

(Fig 4(a)ndash(c)) Because of their hydrophilic nature the amorphous cellulose sur-faces swell in water to form gel-like thin layers which are negatively charged inwater (Fig 4(a)) As they approach each other in water the surfaces may experi-ence long-range double-layer and steric repulsion forces because of the protrud-ing charged cellulose chains and a short-range (sim1 nm) hydration repulsion forcebecause of the hydroxyl groups of glucose units The universal van der Waals at-traction force in water is relatively weak in comparison to repulsion forces Asthe water is removed during drying the capillary force overcomes the repulsionforces and brings the two gel-like surfaces together At the onset of gelndashgel contactopposing gel polymers including polymer loops and tails may interpenetrate and in-termix to form an lsquointerphasersquo at nanometer scales as proposed by McKenzie [3] ina process described by the diffusion theory of adhesion [73] Interfiber bonds are es-tablished upon the dehydration of hydrogel layer in contact (Fig 4(c)) Figure 4(d)is a schematic force law (ie forcendashdistance relationship) showing the repulsionforces (double-layer steric and hydration) in water and adhesion forces (van derWaals forces hydrogen bonds and chain entanglement) in air It may be instructiveto notice that the interfacial structure and its properties essentially determine thestrength of interfiber bonds although the fiber properties and bondingdebondingconditions (eg the level of moisture contact time separation speed) have of-ten been observed to influence the bond strength The acknowledgement of thisfact has had significant practical implications as evidenced by wide application


of water-borne polymeric strength additives to alter the interfiber bond strengthFigure 4(e)ndash(g) illustrates the polymer-mediated adhesion mechanism Positively-charged polymers are often employed for this purpose because they can readilyadsorb onto the negatively-charged fiber surfaces via electrostatic attraction Thestrength of polymer-mediated bond will depend on the nature of polymer additiveand its interaction with the cellulose surface

A number of research groups have focused attention on understanding the rela-tionship between the structure of polymer additives and paper mechanical strengthaimed at developing a predictive approach for the enhancement of paper strengthThis effort has proceeded in parallel with the attempts in adhesion science to relatepractical adhesion to adhesive properties [20] Pelton [74] reviewed the design ofpolymers for increased paper dry strength and emphasized the predictive role of thediffusion theory of adhesion A key concept in the diffusion mechanism is that thepolymers at the interface must be compatible to mix Thus compatible polymerswill give strong adhesion whereas incompatible polymers will not Peltonrsquos groupdemonstrated the role of fiber surface mdash polymer compatibility by preparing andtesting papers from mixtures of two types of cellulose fibers [75] The compatibilitybetween polymer and cellulose fibers was qualitatively ranked in terms of the hy-drophobicity of polymers leading to the suggestion that polymers should be morehydrophilic than poly(ethylene oxide) to increase paper dry strength and less todecrease it More quantitative approaches have also been established for character-izing the thermodynamic adhesion between polymer and cellulose using solubilityparameter and acidndashbase interaction parameter [76 77] Based on the hypothesisthat the more negative the minimum free energy of mixing the more compatiblethe polymer and the cellulose an assumption which in turn suggests strongeradhesion Zhao and coworkers [76] calculated the free energy of mixing of vari-ous polymers with cellulose using UNIFAC to rank the effectiveness of polymericadhesives for cellulose A comparison of the calculated free energies of mixingwith the measured shear strengths of regenerated cellulose films laminated withpolymers shows a reasonable correlation The calculations did not anticipate theexceptional strength-enhancing properties of carboxymethyl cellulose nor did theypredict molecular-weight effects Nevertheless the approach may have utility as ageneral tool to relate polymer chemistry to adhesion performance

Further to the importance of the thermodynamic compatibility in the formation ofinterfiber bonds as revealed in recent studies the viscoelasticity and polymer layerthickness are also important for determining the bond strength Thicker polymerlayers or films can consume more energy because of various viscoelastic deforma-tions a fact proven in many polymeric systems of thickness varying from a fewnanometers to a few hundred micrometers However considering the possible con-finement effect on the state of polymer chains at one extreme and the effect of bulkdefects of polymer adhesives at the other extreme perhaps the effect of polymerlayer thickness on the measure of adhesion energy is not linear In the last decadewe have seen increasing interest in understanding the behaviors of surface-confined


polymer thin films in the field of polymer physics tribology and adhesion science[12 13 78ndash80] Such rheological concepts as Deborah number relaxation timeand polymer chain dynamics chain disentanglement or pull-off in separation andconfinement effects on the transition from fluid to glassy states particularly at thenanoscopic and molecular levels have been introduced in the studies of polymeradhesion

The formation of interfiber bonds involves the change of cellulose fibers from awet semi-fluid state to a dry glassy state There is little adhesion between wet fibersadhesion bonds are established only after dehydration It has long been a challengeto understand the adhesion and failure mechanisms as a material changes from liq-uid to solid state and vice versa published research on this subject is scarce Thesurface force studies of LangmuirndashBlodgett cellulose films by Holmberg et al [62]showed different adhesion mechanisms in dry glassy and wet liquid-like sates theadhesion between the cellulose surfaces in dry air is strong but at increased hu-midity (up to 100 RH) the adhesion is weak dominated by the negative Laplacepressure of the capillary condensate formed around the contact area while the cel-lulose film swells considerably due to the absorption of water to become moreliquid-like Recently Zhao and coworkers [30] used the Surface Force Apparatusto study the way two thin glucose (the basic structural unit of cellulose) films ad-here to and detach from adhesive contacts The use of glucose is unique because theamorphous glucose has a glass transition temperature Tg = 39C and its viscosityvaries continuously by sim10 orders of magnitude (from about 102ndash1012 Pa s) overa temperature range of 50C around its glass transition temperature Sugar glucoseis therefore an ideal model material for studying the failure mechanism of materi-als as they change from the solid to the liquid state This study was conducted anddiscussed from the perspective of the contact mechanics and emphasized the role ofrheological characteristics in understanding the adhesion and failure mechanisms

Figure 5 shows two typical sets of the contact dynamics experiments on glu-cose surfaces in a viscous state (a)ndash(c) at T = 75C and a glassy state (d)ndash(f) atT = 23C When the two smooth glucose surfaces approach each other within afew nanometers they jump into contact because of such intermolecular attractionsas van der Waals forces In the viscous state the two surfaces coalesce immedi-ately upon contact with an outwardly growing meniscus due to the capillary forcesat the boundary (Fig 5(a) and 5(c)) Figure 5(b) shows a top view of the surfacedeformations at the early stage of separation revealing micrometer-sized SaffmanndashTaylor-type ripples or waves growing into the fluid [81] and much finer secondarystructures forming at the external edge of the contact neck In the glassy state atT = 23C the two surfaces jump into contact and the contact area grows (crackhealing) only under external loading (Fig 5(d) and 5(f)) It is clear from the contactdiameter vs compressive load relations in Fig 5(f) that the loading and unloadingprocesses were not reversible there was a strong adhesion hysteresis The loadingcurves were JKR-like a fitting curve by JKR equation predicts the surface energy ofglucose of 45 mJm2 which is close to the experimental value [42] The unloading


Figure 5 Two typical sets of the contact dynamics experiments on amorphous glucose-coated surfacesin a dry N2 environment (a) coalescence of two viscous glucose surfaces at 75C forming a capillarybridge (b) optical image of finger-like viscous deformations during separation (c) contact diametervs time plot in coalescence and separation (d) schematic of the contact area changes (or crack healingor propagation) during loading and unloading at 23C (e) SEM image of cracked surface at 23C(f) JKR-plot of contact diameter as a function of compressive load (Adapted from [30])

trace significantly deviated from the loading trace and the jump-off or separationforce was found to increase with the contact time and unloading rate similar to thenon-equilibrium (hysteretic) adhesion processes previously observed for for exam-ple surfactant and polymer surfaces [12 13] However unlike the polymer surfacesthe unloading trace was a straight almost horizontal path until the failure pointThe failure of two glassy glucose surfaces appeared to be a brittle material-likeabrupt fracture this was further evidenced by the SEM image of detached surface


(Fig 5(e)) many sharp microcracks nucleated at the external boundary and rapidlypropagated along the original contact interface

Zhao et alrsquos findings [30] revealed many interesting features during the transi-tion from liquid to solid behavior and provided insights into the adhesion and failuremechanisms of materials at the microscale and nanoscale levels which are relevantto the action of interfiber bonding The glassy glucose surfaces adhered sponta-neously due to the van der Waals interactions while the hydrogen bonds establishedwith time leading to a larger adhesion hysteresis with a higher separation forceSimilar adhesion behavior had been reported in the contact between spin-coatedcellulose and poly(dimethylsiloxane) surface [82] very likely these adhesion phe-nomena occur in the contact of two cellulose surfaces Furthermore the separationof semi-fluid nanometer-thick film resulted in viscous deformations (ripples) simi-lar to the viscous fingers observed at the peel front during the peeling of a piece ofadhesive tape from a glass surface It is worthwhile to note that the film is only about50 nm thick while most adhesive tapes are thousand times thicker about 50 microm Theviscous fingers have also been reported in various dynamic studies of confined vis-coelastic polymers [12 13] Since the viscous fingering can consume considerableamount of energy this observation suggested a possible way to enhance the inter-fiber bond strength in wet conditions by adding polymer to adjust the rheologicalproperties of the interfacial zone between fibers We expect to see further researchon this topic in the near future

4 Summary

In this article we reviewed a number of theories of polymer adhesion and suchadhesion phenomena as revealed in recent investigations of the adhesion betweenpolymer thin films and paper surfaces and the adhesion of polymer moleculesto cellulose fibers At paper level the paper surface is a good adhesion substratebecause of its relatively high surface energy and porous structure providing highcontact area Paper surface chemistry and thermodynamic adhesion are importantconcepts for an in-depth understanding of the bonding process However because ofits layered structure paper is often the weakest layer in polymerpaper laminatestherefore the performance of polymer adhesion to paper is limited by the cohe-sive strength of paper At fiber level fiber surfaces are rough and are covered withamorphous hemicelluloses polymer chains which swell in water and diffuse intothe opposing fiber surfaces Our understanding of events at the fiber level is mostlyempirical due to the surface compositional and topological heterogeneities of cellu-lose fibers Cellulose and hemicelluloses chain diffusion is believed to be essentialto the formation of interfiber bonds Although the understanding of the real for-mation process of interfiber bonds is limited adding charged polymers to improvefiberndashfiber bonding capacity has long been a practice in which thermodynamiccompatibilities seem to offer a guideline to predict the qualitative performance ofpolymers At the molecular level recent surface force measurements on model


regenerated cellulose surfaces (amorphous and crystalline regenerated cellulosesand amorphous glucose surface) revealed intermolecular interactions between cel-lulosic surfaces including the electrostatic double-layer repulsion polymer stericrepulsion in water and van der Waals attractions hydrogen bonding and chain en-tanglement as adhesion interactions in air Recent understanding of the adhesionand failure mechanism of glucose (structural unit of cellulose) surfaces has sug-gested that the physical state (glassy vs viscous) and the rheological properties ofthe interfacial layers could be important issues in understanding the interfiber bondstrength A rational design of water-borne polymer additives which can adsorb onthe fiber surfaces and alter molecular adhesion and its viscoelasticity may be a wayto tailor the interfiber bond strength for desired paper properties either as strengthenhancers for strong paper products or as debonding agents for soft paper tissues

Acknowledgements

We would like to acknowledge the support from the University of Waterloo Start-up grants during the writing of this manuscript Zhao would like to express hisappreciation to Drs Robert Pelton Hongbo Zeng Yu Tian and Jacob Israelachviliwith whom he had worked on a number of experiments and had numerous insightfuldiscussions

References

1 B Zhao The interactions of pressure sensitive adhesive with paper surfaces PhD Thesis Depart-ment of Chemical Engineering McMaster University Hamilton Ontario Canada (2004)

2 E Retulainen K Niskanen and N Nilsen in Paper Physics K Niskanen (Ed) pp 55ndash83 FapetOy Helsinki Finland (1998)

3 A W McKenzie Appita 37 580 (1984)4 K Niskanen (Ed) Paper Physics Fapet Oy Helsinki Finland (1998)5 A J Kinloch Adhesion and Adhesives Chapman and Hall (1987)6 K Kendall Molecular Adhesion and Its Application p 46 Kluwer AcademicPlenum Publishers

(2001)7 J N Israelachvili Intermolecular and Surface Forces pp 201ndash204 Elsevier London (1992)8 K L Johnson K Kendall and A D Roberts Proc R Soc London Ser A 324 301 (1971)9 K R Shull Mater Sci Eng R 36 1 (2002)

10 R G Horn J N Israelachvili and F Pribac J Collloid Interface Sci 115 480 (1987)11 M Tirrell Langmuir 12 4548 (1996)12 G Luengo J Pan M Heuberger and J N Israelachvili Langmuir 14 3873 (1998)13 H B Zeng N Maeda N Chen M Tirrell and J N Israelachvili Macromolecules 39 2350

(2006)14 Y L Chen C A Helm and J N Israelachvili J Phys Chem 95 10736 (1991)15 K R Shull and C Creton J Polym Sci B 42 4023 (2004)16 A Zosel J Adhesion 30 135 (1989)17 A Zosel Int J Adhesion Adhesives 18 265 (1998)18 C Creton J Hooker and K R Shull Langmuir 17 4948 (2001)


19 K L Mittal Polym Eng Sci 17 467 (1977)20 J C Berg in Adhesion Science and Engineering M Chaudhury and A V Pocius (Eds) Vol 2

pp 1ndash75 Elsevier Amsterdam (2002)21 L Li M Tirrell G Korba and A Pocius J Adhesion 76 307 (2001)22 T P Russell Science 297 964 (2002)23 K L Mittal (Ed) AcidndashBase Interactions Relevance to Adhesion Science and Technology Vol 2

VSP Utrecht (2000)24 J C Berg Nordic Pulp Paper Res J 8 75 (1993)25 P G de Gennes J Chem Phys 55 572 (1971)26 H H Kausch and M Tirrell Annu Rev Mater Sci 19 341 (1989)27 E Helfand and A M Sapse J Chem Phys 62 1327 (1975)28 T P Russell Mater Sci Rep 5 171 (1990)29 K L Johnson Contact Mechanics Cambridge University Press Cambridge UK (1985)30 B Zhao H Zeng Y Tian and J Israelachvili Proc Natl Acad Sci USA 103 19624 (2006)31 M Deruelle L Leger and M Tirrell Macromolecules 28 7419 (1995)32 D Satas in Handbook of Pressure Sensitive Adhesive Technology D Satas (Ed) pp 61ndash96

Van Nostrand Reinhold New York NY (1989)33 B Fredholm and L Westfelt Svensk Papperstldnlng 7 201 (1979)34 A Kempi Paper and Timber 79 330 (1997)35 A Kempi Paper and Timber 80 296 (1998)36 M Welander Nordic Pulp Paper Res J 2 61 (1987)37 W Shen Y J Sheng and I H Parker J Adhesion Sci Technol 13 887 (1999)38 F M Etzler in Contact Angle Wettability and Adhesion K L Mittal (Ed) Vol 3 pp 219ndash264

VSP Ultrecht (2003)39 W A Zisman in Advances in Chemistry Series No 43 pp 1ndash51 American Chemical Society

Washington DC (1964)40 P Luner and M Sandell J Polym Sci Part C 28 115 (1972)41 J Borch J Adhesion Sci Technol 5 523 (1991)42 C J van Oss Interfacial Forces in Aqueous Media p 181 Marcel Dekker New York NY (1994)43 J Borch Tappi J 65 72 (1982)44 J W Swanson and J J Becher Tappi J 49 198 (1966)45 G Gervanson J Ducom and H Cheradame Br Polym J 21 53 (1989)46 J J Bikerman Tappi J 44 568 (1961)47 J J Bikerman and W Whitney Tappi J 46 420 (1963)48 T Yamauchi T Cho R Imarnura and K Murakarmi Nordic Pulp Paper Res J 3 128 (1988)49 T Yamauchi T Cho R Imarnura and K Murakarmi Nordic Pulp Paper Res J 4 43 (1989)50 B Zhao and R Pelton J Mater Sci Lett 22 265 (2003)51 B Zhao and R Pelton J Adhesion Sci Technol 17 815 (2003)52 B Zhao and R Pelton J Pulp Paper Sci 31 33 (2005)53 B Zhao L Anderson A Banks and R Pelton J Adhesion Sci Technol 18 1625 (2004)54 B Zhao and R Pelton Tappi J 3 3 (2004)55 D H Page Tappi J 52 674 (1969)56 D Dodson Rep Prog Phys 33 1 (1970)57 T Uesaka Handbook of Physical and Mechanical Testing of Paper and Paperboard p 379 Mar-

cel Dekker New York NY (1983)58 J drsquoA Clark Pulp Technology and Treatment for Paper 2nd edn Miller Freeman San Francisco

(1985)


59 A Torgnysdotter A Kulachenko P Gradin and L Wagberg J Composite Mater 41 1619 (2007)60 A D Berman and J N Israelachvili in Modern Tribology Handbook B Bhushan (Ed) p 567

CRC Press Boca Raton FL (2001)61 A D McLaren J Polym Sci 3 652 (1948)62 M Holmberg J Berg S Stemme L Oedberg J Rasmusson and P Claesson J Colloid Interface

Sci 186 369 (1997)63 F Rehfeldt and M Tanaka Langmuir 19 1467 (2003)64 C D Edgar and D G Gray Cellulose 10 299 (2003)65 S M Notley M Eriksson L Wagberg S Beck and D G Gray Langmuir 22 3154 (2006)66 R D Neumann J M Berg and P M Claesson Nord Pulp Paper Res J 8 96 (1993)67 A Carambassis and M Rutland Langmuir 15 5584 (1999)68 M W Rutland A Carambassis G A Willing and R D Neumann Colloids Surfaces A 123 369

(1997)69 J Lefebvre and D G Gray Cellulose 12 127 (2005)70 S M Notley B Petterson and L Waringgberg J Am Chem Soc 126 13930 (2004)71 S M Notley and L Waringgberg Biomacromolecules 6 1586 (2005)72 L Bergsrom S Stemme T Dahlfors H Arwin and L Odberg Cellulose 6 1 (1999)73 L H Sharpe J Adhesion 67 277 (1998)74 R Pelton Appita 57 181 (2004)75 R Pelton J Zhang L Wagberg and M Rundlof Nordic Pulp Paper Res J 15 400 (2000)76 B Zhao L Bursztyn and R Pelton J Adhesion 82 121 (2006)77 W T Y Tze D J Gardner C P Tripp and S C OrsquoNeill J Adhesion Sci Technol 20 1649

(2006)78 G Luengo J N Israelachvili and S Granick Wear 200 328 (1996)79 G Luengo F Schmitt R Hill and J N Israelachvili Macromolecules 30 2483 (1997)80 L Leger E Raphael and H Hervet Adv Polym Sci 138 185 (1999)81 P G Saffman and G Taylor Proc R Soc London Ser A 245 312 (1958)82 M Eriksson S M Notley and L Wagberg Biomacromolecules 8 912 (2007)


Figure 1 Adhesion phenomena in paper from the macroscopic to molecular level (a) Typical exampleof adhesion phenomenon at the macroscale mdash paperadhesive laminate (b) adhesion phenomenon atthe microscale mdash interfiber bonds (c) asperity contacts in the overlapping area of an interfiber bond(d) an individual asperity contact containing many microfibrils entanglements (e) long-chain structureof cellulose giving rise to molecular adhesion and (f) structure of one nanometer-sized β-glucose thebasic unit of cellulose molecule

interfiber bonds In such paper converting and application processes as xerographylamination of plastic film to paperboard corrugated box construction address-labelapplication polymer adhesion to paper and paper board plays a dominant role Xe-rographic toners and lamination films are heated during contact with paper so thatthe thermoplastic polymers can spread on the paper and form intimate contact toestablish strong molecular adhesion forces By contrast box construction papersplicing and address-label applications employ adhesives which promote adhesionbetween paper surfaces

Figure 1 illustrates various adhesion phenomena from the macroscopic to themicroscopic nanoscopic and molecular levels At the macroscopic level (sim10 cm)(Fig 1(a)) the paper is a common adhesive carrier because of its rough surfacerelatively high surface energy and low cost and is used in such adhesive productsas Post-itreg notes and adhesive tapes On the other hand polymeric adhesives suchas splicing tapes are employed in paper converting processes to joint individual pa-per rolls for continuous processing where a robust and strong paperadhesivepaperbond is required upon instant contact in the process at a speed of sim60 kmh [1]At the microscopic levels (10 micromndash1 mm) paper is virtually a network of cellulosefibers held by adhesion forces in overlapping areas called interfiber bonds A typ-ical softwood pulp fiber is about 1 mm long and 20 microm wide and 4 microm thick Itis flexible and it intertwines with neighboring fibers through the interfiber bonds(Fig 1(b)) The apparently simple interfiber bonds are complex from the perspec-









W = γ1 + γ2 minus γ12 (1)


W = γl(1 + cos θ) (2)



W = A12

12πD20

(3)



a3 = R

K[L + 3πRW +







2πRW (5)

or








G = G0

(1 +

(v

vlowast

)n) (7)






















G = F






















1

T= 1

F+ 1

B (9)


























4 Summary




Acknowledgements


References















(1985)















W = γ1 + γ2 minus γ12 (1)


W = γl(1 + cos θ) (2)



W = A12

12πD20

(3)



a3 = R

K[L + 3πRW +







2πRW (5)

or








G = G0

(1 +

(v

vlowast

)n) (7)






















G = F






















1

T= 1

F+ 1

B (9)


























4 Summary




Acknowledgements


References















(1985)










2πRW (5)

or








G = G0

(1 +

(v

vlowast

)n) (7)






















G = F






















1

T= 1

F+ 1

B (9)


























4 Summary




Acknowledgements


References















(1985)










G = G0

(1 +

(v

vlowast

)n) (7)






















G = F






















1

T= 1

F+ 1

B (9)


























4 Summary




Acknowledgements


References















(1985)

























G = F






















1

T= 1

F+ 1

B (9)


























4 Summary




Acknowledgements


References















(1985)


























1

T= 1

F+ 1

B (9)


























4 Summary




Acknowledgements


References















(1985)
































4 Summary




Acknowledgements


References















(1985)









Acknowledgements


References















(1985)















(1985)







adhesion of polymers in paper products from the … · · 2017-10-27journal of adhesion science...

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