introduction to adhesion and adhesive

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Chapter 1

Introduction to Adhesion and

Adhesives

Glu es have been ar ou nd for a long time; the ancien t Egyptians used them

in veneering the treasures of Tut an kh am un an d the ancient Greek w ord

for glue is Kohha, from w hich we get colloid. In all centuries up to a nd

including the 19th, glues originated from plan ts a nd animals; during the

20th century, however, synthetic chemicals have largely taken over, an d

the more respectable name of adhesive has been introduced. Animal

glues were mostly based on mammalian collagen, which is the mainprotein of skin, bone and sinew, and the plant kingdom provided

starches and dextrins from corn, wheat, potatoes and rice.

No wad ays adhesives are used in all types of manufac ture, an d in many

cases have displaced o th er mean s of joining . A range of adhesives (h ot

melt, vegetable glues and emulsions) are used in making cardboard

boxes, with rarely a staple t o be seen. Ap art from expensive han dm ad e

shoes, footwear is now adhesively bo nded using ho t melt adhesives for

the basic construction, na tura l ru bb er latex for linings, and solvent basedpolyurethanes o r polychloroprenes for sole atta ch m en t. Bookb inding is

by hot melt adhesives.

Adhesive bonding is used increasingly in the construction of aircraft.

Struc tural bonding began with the World W ar I1 De Haviland

Mosquito, which was made of plywood. Modern civil aircraft are

basically made of aluminium alloy, and rubber modified epoxide

adhesives are increasingly used.

Rubbe r-to-metal bonds are used for engine, transmission an d exh austmountings in automobiles and in railway bogie suspensions. Mass

produ ced ca r bodies are ma de of spot-welded mild steel; weight an d fuel

consumption can be reduced with aluminium bodies, which are more

difficult to spot-weld. The large-scale bon ding of car b odies is a prize th at

1



2 Chapter I

awaits the adhesives industry. A recent achievement was the bonding of

steel rails in the new Manchester tramway.

Hum an beings can be repaired by adhesives. This includes the use of

UV-curing cements in dentistry and acrylic bond cements in ortho-

paedic surgery. It has been said th at cyanoacry late adhesives were usedfor short term repairs during the Vietnam War.

Adhesives are not the only materials that must stick or adhere.

Adhesion is essential for printing inks, sealants, paints and other surface

coatings, and a t interfaces in composite materials such a s steel or textilefibres in rubber tyres and glass- or carbon-fibres in plastics. Mother

nature uses adhesion rather than mechanical fasteners (nuts and bolts,

nails, staples, etc.) in constructing plants a nd animals, and som e animals

are masters at the exploitation of adhesion. Here I am thinking ofbarnacles sticking to anything th at floats in the sea and the rem arkable

ability of many insects to walk on ceilings.A disadvantage of adhesives as a means of joining is that they are

generally weakened by water and its vapour. Also, their servicetemp erature ranges are less tha n for metal fasteners (nuts, bolts, welds,

staples, etc.), being limited by their glass transition temperature and

chemical deg rada tion . Advantages include their ability to join dissimilarmaterials and thin sheet materials, the spreading of load over a widerarea, the aesthetic and aerodynamic exteriors of joints, an d application

by machine or robot.

BASIC PROPERTIES

W hat is an adhesive and what a re its basic properties? A definition is a

material which when applied t o the surfaces of materials can join themtogether and resist separation. Th e terms adherend and substrate are used

for a body or material to be bonded by an adhesive. Other basic termsare shelf-life, for the time an adhesive can be stored before use, andpot-life, the maximum time between final mixing and application.

Basically an adhesive must do two things:

(i) It must wet the surfaces, th at is it must spread a nd ma ke a contac t

angle approaching zero, as is illustrated in Figure 1.1. Intimate

contact is required between the molecules of the adhesive and theatoms and molecules in the surface. When applied the adhesivewill be a liquid of relatively low viscosity.

(ii) Th e adhesive must then hard en to a cohesively stron g solid. Th is

can be by chemical reaction, loss of solvent or w ater, or by coo lingin the case of hot melt adhesives. Th ere is a n exception to this, an d



Introdu ction to Adhesion and Adhesives 3

Figure 1.1 To p: liquid droplets making n high and low contact angle on n j l a t ,

solid surfiice. Cen tre: high contact angle leading to 110 spreadiizg ona rough surface. B otto m: wetting on a rough surfiice.

that is pressure-sensitive adhesives which remain permanently

sticky. These are the adhesives used in sticky tapes and labels.

BASIC CHEMISTRY

All adhesives either contain polymers, or polym ers ar e formed w ithin t headhesive bond. Polymers give adhesives cohesive strength, and can be

thou ght of as strings of beads (identical chemical units joined by single

covalent b onds), which may be either linear, branched o r crosslinked as

illustrated in Figure 1.2.

Linear an d branched polymers have similar properties an d it is not

easy to distinguish them, an d they will flow a t higher tem pera ture s an d

dissolve in suitable solvents. These lat ter properties ar e essential in h ot

melt, and solvent-based adhesives, respectively.

Crosslinked polym ers will no t flow when heated, an d m ay swell, bu t

no t dissolve, in solvents. All st ruc tur al adhesives a re crosslinked because

this eliminates creep (deformation under constant load). Automotive

tyres ar e crosslinked na tur al o r synthetic rub be r, an d if they crept they

would permanently deform during parking, and a rough ride would

follow.



4 Chapter 1

b

Figure 1.2 Linear (top),branched (nziddle) and crosslinked (b ot to m) polymers.

Many adhesives contain additives that are not polymers are these

include stabilizers against degradation by oxygen and UV, plasticizerswhich increase flexibility and lower the glass transition temperature, andpowdered mineral fillers, which may reduce shrinkage on hardening,

lower cost, modify flow properties before hardening and modify final

mechanical properties. Other possible additives are tackifiers and silane

coupling agents.

THEORIES OF ADHESION

There are six theories of adhesion; physical adsorption, chemical

bonding, diffusion, electrostatic, mechanical interlocking and weak

boundary layer theories. As all adhesive bonds involve molecules inintimate contact, physical adsorption must always contribute.



Introduction to Adhesion and Adhesives 5

Physical Adsorption Theory

Physical ads orp tion involves van der W aals forces across the interface.

These involve attractions between permanent dipoles and induceddipoles, an d a re of three types. E,, is the potential energy, in a vacuum, of

a pair of permanent dipoles separa ted by distance r at their centres and is

given by equatio n 1.1, where p l and p2are the dipole moments, E~ is the

permittivity of a vacuum, k is Boltzmann's con stan t an d T the absolute

temperature.

If a no n-po lar molecule is close to a dipo le, then th e latter will induce a

dipole (pi) in the former. The induced-dipole moment is given by

equa tion 1.2, where a s the polarizibility of the n on- pola r molecule a nd

E is the electric field.

Th e potential energy for such an interaction is given by equ atio n 1.3,

where p 1 s the mom ent of the p erm anent dipole.

Instantaneous dipoles exist in non-polar molecules because of the

fluctuating distribution of electrons. These lead to attractive forcesbetween molecules, witho ut which n on-pol ar gases such as helium an d

argon would not be able to liquefy. The potential energy of a pair of

molecules is given by equation 1.4, where a, and a, are their

polarizabilities and I , and I , ar e their ionization potentials. Such forces

have the name of dispersion forces.

Th e results of som e calculations from th e abov e equations a re sho wn in

Figures 1.3-1.5, where th e molecules ar e in contac t at th e lowest points of

the curves, i.e.r = yo. Figure 1.3 is for a pa ir of water molecules a t 298 K;

the dipole mo ment of water is 1.85 D (1Debye = 3.336 x 10- 30 Cm) and

4 m 0 = 1.1126 x 10-'oJ- 'C2m-1. The radius of a water molecule,



6

rlnm

Chapter 1

Figure 1.3 Potential energy at 298 K fo r dipolar attraction between tw o water

molecules. T he moiecules m e in coritact at the point 0 .

tinm

0

7I-E" -507LU"

- 1 0 0

Figure 1.4 Potential eiteryy f o r dipole-induced dipole attractioii betweenwater and rnethnrie molecules. The molecules are in contact at thepoint .

calculated from its mo lar m ass an d density is 0.19 nm. W hen two water

molecules are in contact, E,, is - .12kJ m o l - '.Figure 1.4 is for the interaction of water with methane

(a 2.60 x 10-3 0m 3). Th e radius of the metha ne molecule is ab ou t

0.24nm, and when the two molecules are in contact E,, is - 5 J mo l- I ,

which is very much less than for a pair of water molecules.

The first ionization potential of methane is 1133 kJ mol- I. When in

contact , E ii is 909 J m ol- (Fig ure 1.5).

Th e potential energies of all these interactions are inversely propor-




rlnm

0 0.5 1.00

5 0 0

1 0 0 0

Figure 1.5 Poteiztinl eriergy fo r induced-dipole nttractiorz between two rnethnne

wolecu les . The rizolecirles are i n coritact nt the point a.

tional to the 6 th power of the distance of sep aratio n, meaning tha t the

values of - E,,, - ,, and - Eiifall off rapidly with distance. Doubling

the distance reduces values of - ,,, - ,, and - E,, to &th. Figures1.3-1.5 show th at the forces ar e only effective at less tha n two diame ters,

which mean s th at adhesion forces will only be felt by t he molecules tha t

are actually in the topmost surface layers.

Th e measuremen t of cont ac t angles, which is described in Ch ap te r 8, is

a means of investigating adh esion by physical a ds orp tio n. These a re the

weakest forces that con tribu te to adhesive bonds, bu t a re qui te sufficient

to ma ke s trong joints.

Chemical Bonding TheoryT he chemical bonding theory of adhesion invokes the formation of

covalent, ionic or hydrogen bon ds across the interface. Th ere is some

evidence tha t covalent b on ds ar e formed with silane coupling agents (see

C ha pte r 3), and it is possible tha t adhesives containing isocyanate grou ps

react with active hydrogen ato m s, such as hydroxyl groups, if wood o r

paper are the substrates. In these two examples, Si-0 bo nd s (strength

369 kJ mo l- ') and C -0 bonds (351 kJ mol - I ) would be formed. An othe r

possibility is the reaction of an epoxide adhesive with a surface containingamin e groups (see Chapter 4) to give C-N bon ds (291 kJ m ol- ').

Th e potential energy of two ions of char ge z l e an d z , e , separated by

distance r is given by equation 1.5.

z 1 z 2eL

4 1 1 ~ ~ ~ ~ ~E + - =-



8 Chapter 1

Here E, is the relative permittivity of the medium, which is 1 in th e case ofa vacuum or dry a ir. Ta king the following values of ionic radii (Na' =

0.95, A13+ = 0.50, Ti4 += 0.68, 0 2 - 1.40 and C1- = 1.81 nm)

strengths of ionic attractions are Na Cl5 03, A 1 3 + 0 2- 4290 a nd T i 4 + 0 2

5340 kJ mol- '. The two last energies ar e very high and may contribu teto adhesion between metals and epoxide adhesives, and can also accou nt

for the significant contribution th at carboxylic acid groups in adhesives

make to metal-adhesion.

A major problem with all adhesive joint s is their sensitivity to w ater,and possible explanations for this are its high permittivity, which by

equation 1.5 wou ld give a low value of E , -, nd a high surface tension.

These issues a re considered, respectively, in Chap ters 10 an d 8.Hydrogen bonds probably contribute to the attachment of postage

stamps to envelopes where the adhesive (polyvinyl alcohol) and paper

(cellulosefibres) bo th contain -O H groups. Wood is also rich in celluloseand the reactive adhesives based on formaldehyde contain hydroxyl o ramine groups capable of participating in hydrogen bonds. Th e strengthsof hydrogen bonds a re mostly in the range 8-42 kJ mol - ',with those in

water being at the t op of this range. Hydrog en bonds involving fluor ine

can be stronger than this, and the strongest of all is F - * . - H-F(243

The strengths of Lewis acids and bases in poorly solvating solvents

(usually hexane, cyclohexane or tetrachloromethane) can be obtainedfrom their heats of reaction ( -AH), which are related to EA and CA,

which are empirical parameters for the acid, and EB and C,, the

corresponding values for the base, by equation 1.6.

21 kJ m ol - I).

E , and E , are considered to be the susceptibilities of the acid a nd base toundergo electrostatic interactions, and CAand C, are their susceptibili-ties to form covalent bonds. The heats of reaction can be measured bydirect calorimetry or from shifts in IR spectra. An example of th e latter isthe shift in the O H stretched frequency of phenols (Av) when they reactwith amines in tetrachlorom ethane o r tetrachloroethene, w hich is givenby equation 1.7.

-AH(kca1mol - ') = 0.0103 Av(cm - ) + 3.08 (1.7)

Some values are given in Table 1.1. They are based on a large num berof measurements of -AH, with iodine EA = 1.00 and CA= 1.00 as thereference com pou nd , in the old units of (kcal mol - ) l I 2 .




Table 1.1 Values of E and C or some Lewis acids and bases.

Compound N o.of

cA ‘ EA EB

measurements

IodinePhenolBoron trifluoridePyridineDimethyl formamideDimethyl sulfoxideBenzeneEthyl acetate

39

34

5

21

4

14

5

14

1 oo

0.442

3.08

6.402.48

2.85

0.707

1.74

1oo4.33

7.96

1.17

1.23

1.34

0.486

0.975

Diffusion Theory

The difusion theory takes the view that polymers in contact mayinterdiffuse, so that the initial boundary is eventually removed (seeFigure 1.6). Such interdiffusion will occur only if the polymer chains ar emobile (i.e. he temperature m ust be above the glass transition tempera-

tures) and compatible. As most polymers, including those with verysimilar chemical structures such as polyethylene an d polypropylene areincompatible, the theory is generally only applicable in bonding like

rubbery polymers, as might occur when surfaces coated with contactadhesives are pressed together, and in the solvent-welding of thermo-plastics. An example of the latter is to swell two polystyrene surfaces withbutanone and then press them together. The solvent has the effect oflowering the glass transition temperature below ambient while inter-diffusion takes place; it later evaporates. This is the mechanism of

adhesion in mak ing plastic model kits. The kits ar e made of polystyreneand the adhesive is a solution of polystyrene in a n o rganic solvent, the

main purpose of the polymer being to thicken th e adhesive. The re are asmall number of polymer pairs made com patib le by specific interactions.

O ne pair is poly(methy1 methacrylate) and poly(viny1 chloride), which

permits the possibility of interdiffusion w hen stru ctural acrylic adhesivesare used to bond PVC.

Electrostatic Theory

The electrostatic theory originated in the prop osa l th at if two metals ar eplaced in contact, electrons will be transferred from one to the o ther so

forming an electrical doub le layer, which gives a force of attraction . As

polymers are insulators, it seems difficult to apply this theory toadhesives.



10 Chapter 1

Figure 1.6 Difusion theory of adhesion.

Mechanical Interlocking

If a subs tra te ha s an irregular surface, then th e adhesive may en ter th e

irregularities prio r t o hardening. T his simple idea gives the mechanical

interlocking theory, which contributes to adhesive bonds with porous

materials such a s wood an d textiles. An example is the use of iron-onpatches for clothing. Th e patches co ntain a ho t melt adhesive that, when

molten, invades the textile material.

Weak Boundary Layer

T he weak boundary layer theory proposes that clean surfaces can give

strong bonds to adhesives, but so me contam inants such a s rust an d oils

o r greases give a layer which is cohesively weak. N ot all co nta m ina nts

will form weak b ou nd ary layers, as in so m e circumstances they will be

dissolved by the adhesive. This is an area where acrylic structural

adhesives are superior to epo xides because of their ability to dissolve oils

an d greases.




POLYMERIZATION

Polym erization is a very impo rta nt m atte r in adhesion, in tha t adhesives

which harden by chemical reaction do so either by addition orcondensation polymerization, an d th e polymers, which are a vital an d

frequently major component of oth er adhesives, a re synthesized by th e

sam e processes.

Condensation Polymerization

In condensation polymerization, molecules react with one another

because they contain chemical groups which are mutually reactive.

There are at least two such groups per molecule, and if some

trifunctional molecules ar e present the n branching o r crosslinking will

result. Example of so m e functional g ro up s used in adhesives are show n

in Scheme 1 . 1 .

-O H + -NCO - NHCOO-

Alcohol Isocyanate Urethane

Arnine Epoxide

2CH20H - -CH20CHr + H20

Methylol Ether Water

Scheme 1.1

In some, but no t all, cases a sm all molecule such as water is formed. In

the case of phenolic adhesives, which cu re by the last reac tion in Scheme

1.1,at a temperature above 100°C,pressure m ust be applied to prevent

the unwanted formation of steam-filled voids.

Addition Polymerization

Com pounds containing double bonds o r rings can be polymerized by

addition polymerization, which at its most basic involves opening ofrings or double bonds, and joining them together to make a chain.

Addition polymerization is a chain reaction involving the sequential

steps of initiation, propagation and termination.



12 Chapter 1

The basic steps in free-radical addition polymerization are shown in

Scheme 1.2. Initiation involves the thermal or UV decom position of aninitiator (here benzoyl peroxide) and the radicals which are formed th en

attack a mono mer molecule. In p ropaga tion many monom er molecules

are now added to produce a long-chain radical. In termination tworadicals react either by recom bination or d ispropo rtionation. Radical

lifetimes ar e typically a few seconds and during this time thousands of

monomer units may be added.

o-coo*Initiation

+ CH2=CH -R

o C O O - C H 2 - : H I

-CHz-CHI

R

Propagation

wCH2-CH-CH-CH2-I I

R R

2 mCH2-CH / ecombination

disproportionation

Termination

Scheme 1.2

Alternatively, initiation can be by a redox reaction, of which a simple

example is the reaction of hydrogen peroxide w ith iron(11) ons (Fen ton'sreagent; Scheme 1.3).

In reactive acrylic adhesives, organic peroxides and transition metal

salts that are soluble in organic compounds may be used, but theprinciple is the same. As direct mixing of the reactants can cause




Fe2+ + H202 + Fe3+ + HO- + HO*

Scheme 1.3

explosions, at least on e com pon ent must be diluted in to the adhesive.

Redox initiation can be employed in emulsion polymerization using

water soluble peroxides such as ammonium or sodium persulfate.

Polymerization an d the hardening of adhesives can b e directly initiated

by ionizing radi ation , including electron beams.

In addition to free radicals, active centres can also be anions or

cations, an d the stability of the active cen tre is an im po rta nt facto r in

controlling the m echanism of polymerization. The m or e stable the activecentre, then the more likely is polymerization. It is well known that

'Superglue' (ethyl cyanoacrylate, see Cha pt er 4) polymerizes in seco nds

between finger ends. This is an anionic polymerization initiated by

hydroxide ions, the anionic active centres being stabilized by the

electron-withdrawing nitrile and ester groups.

Copolymerization

If the reaction m ixture in an addition polymerization c onta ins just on e

mo nom er, then all the repeat units in th e polymer will be the same, an dthe term homopolymer m ay be used t o describe it. If two m ono me rs A an d

B are used then both will be incorporated into the product, which is

termed a Copolymer,an d except in so me special cases the arran gem ent ofthe A and B units in the copolymer will be ran dom . Three mon om ers are

used to make terpolymers. The use of extra reactants in condensation

polymerization yields copolymers and terpolymers.Th e term copolym er is used no t only in th e specific sense jus t defined,

but also as a general term for polymers with m ore tha n o ne repeat unit.Copolymers ar e useful in adhesives in tha t materials with i nterm edia te

properties ca n be o btained , an d a s copolymers have lower structural

regularity than homopolymers, there is a tendency for them to be lesscrystalline and have lower melting points.

Crosslinking

Most addition polymerizations occurring in adhesives involve mono-

mers with one polymerizable C=C bond. The addit ion of a second

mo nom er with tw o such C=C bon ds leads to a crosslinked prod uct (seefor example acrylic adhesives in Chapter 4). Linear condensation

polymers ar e formed from difunctional m onom ers, and the addition of a



14 Chapter 1

monomer with three or more functional groups causes crosslinking.

Epoxides a re a good example; they a re described in Ch apter 4.

With b ot h types of polymerization, viscosity rises steadily a t first, bu t

at the gel-point w hen there is o n average one crosslink per m olecule, the

viscosity rises sharply. Th e whole polymer now becomes a single cross-linked molecule. Further crosslinking will occur beyond the gel-point.

GLASS TRANSITION TEMPERATURE

The mechanical properties of polymers radically change at the glass

transiton temperature ( T J ;molecular m otion is the underlying cause of

the change. Below Tg here is no translational o r rotational m otion of theatoms that make up the polymer backbone, but these motions are

present above T g .Below T g , olymers a re relatively ha rd, inflexible an d

brittle, whilst above it they a re soft and flexible. The terms glassy, andrubbery or leathery are used to describe properties in the twotemperature regions.

Both glassy and rubbery polymers are used as adhesives, examplesbeing the use of glassy adhesives for structural bonding in engineering

and bone cements in surgery, and rubbery ones as pressure-sensitiveadhesives and for bonding flexible substrates. I t is unacceptable for an

adhesive to pass through the glass transition during service.The theory most used to account for the glass transition is thefree

volume theory . T he basis is th at a polymer consists of occupied volumeplus free volume, with the latte r increasing on thermal expansion. Onc ethe fraction of free volume reaches a critical amount which is about2.5%, the chain segments become mobile and the polymer enters the

leathery state.The glass transition tem perature s of some polymers used in adhesivesare shown in Table 1.2.

Polar groups in polymers increase intermolecular forces and thusreduce free volume and increase T g .This is illustrated by the effect ofreplacing the C-CH, bond s in natural rub ber w ith C-Cl bonds, as in

polychloroprene, which is to increase T g y 25 "C. In contras t, non-polarside group s tend to hold chains apa rt and lower T,, as is shown by seriesof acrylic polymers from PM M A to PBMA, for which the repeat unitsare shown in structure 1.1.

The aromatic amine hardeners DAB and DDM for the diglycidylether of bisphenol-A (DG EB A) give higher glass transition temperaturesthan the aliphatic amines because the molecules are more rigid. DA PEE

is a particularly flexible molecule giving the lowest T, of the examplesshown. See structural formulae 1.2.




Table 1.2 Glass transition temperatures of some polymers.

Epoxides based on D GE BA and the following hardeners

4,4'-Diaminodiphen ylme thane (DD.M)Trieth ylene e tramine (TET A)Di-( 1-aminopropyl-3-ethoxy) ether (D A PE E)

1,3-Diaminobenzene (DAB)

Acrylic polymersPolym ethacrylic acidPoly(methy1 methacrylate)

Poly(ethy1 m ethacrylate)Poly(n-propyl methacrylate)Poly(n-butyl methacrylate)

Rubbers

Pol ychloroprenePolyisoprene ( natura l rubber)Poly(dimethy1 siloxane) (silicone rub be r)

(PMAA)( P M M A )

( P E M A )( P P M A )( P BM A )

1611199967

228105

653520

- 0- 5- 27

Polymethacrylic acid

Poly(methy1 methacrylate)

Poly(ethy1 methacrylate)

Poly-(n-pro$yl methacrylate)

y 3

-CH*-C- Poly-(n-butyl methacrylate)IC O ~ C H ~ C H ~ C H Z C H ~



16 Chapter 1

DAB DDM

H2NCH2CH2NHCH2CH2NHCH2CH2NH2

TETA

H~NCH~CH~CH~OCH~CH~OCHZCH~OCH$~H~CH~NH~

DAPEE

Liquids have a relatively high free-volume, so the effect of mixing a

liquid with a polymer is to lower T g .The plasticization of poly(viny1

chloride) (PV C) is a well kn own exam ple of this, where liquids such a s

ph tha late diesters a re used to conv ert rigid P V C, which is used for

window frames and gutterings, into the material used for flexiblehosepipes an d footwear. Plasticization of adhesives can be by ab sor bed

water or by additives such as tackifiers.

VISCOELASTIC PROPERTIES

Polymers are described as viscoelastic in tha t they show a com bin atio nof the properties of a spring, an d a da sh po t filled with a viscous liquid (a n

automotive shock-absorber). A spring will deform instantaneou sly whenloaded, and will recover fully and instantaneously when the load is

removed. Th e deform ation of a d ash po t will increase with time, an d it

will no t recover when t he lo ad is removed. A model which conta ins two

springs and two das hpo ts, an d which describes the qualitative behaviour

of polymers and adhesives is show n in F igu re 1.7. O n loading, spring B

will instantly deform a nd d as hp ot A will begin to flow interminably. T h e

response of spring C will be delayed by dashpot D. When the load is

removed, spring B will recover im mediately an d fully; th e recovery of

spring C will be total but delayed by dashpot D. The deformation of

dashpot D is irreversible.

Clearly the properties of an adhesive th at m ight be used in engineering

will be dominated by th e spring-like properties, an d such adhesives ar e

crosslinked to eliminate the viscous element. In contrast, pressure-

sensitive adhesives for tapes an d for sticky solids such as Blu-Tak'R hav e



Introduction to Adhesion and Adhesives

I unload

17

loadTIME

Figure 1.7 Four elernent model of viscoelnstic behaviour, arid its

time-dependent strain.

a large viscous component, and the transverse stripes which form onclear tapes at the point where they are left to dwell on the roll is due to

viscous flow.

introduction to adhesion and adhesive

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