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Adhesion in Dentistry
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Adhesion in Dentistry
SAA and BSDR Dental Materials Group
One Day Meeting
11 t" May 2000
Society of Adhesion and Adhesives Board
Chairman
ViceChairman
Secretary
John Bishopp
Tony Kinloch
Malcolm Bowditch
John Comyn
Keith Allen
Cathy Pearcey
Steve Abbott
Bob Adams
Robin Chivers
Dave Dixon
Tim Jessop
Graham Lake
Jim Palmer
Steve Shaw
David Tod
Hexcel Composites
Imperial College
Consultant
Consultant
Consultant
TOM
SATRA
University of Bristol
Smith & Nephew
BAE Systems
Welding & Joining Society
University of East London
Evode Ltd
DERA
DERA
Future Events
Sealants
7/12/00
SCI, London
SAE VI
4-6/7/01
Bristol
Adhesion '02/Euradh '02 9-1319102
University of Strathclyde, Glasgow
name
affiliation
MCLEAN J
NICHOLSON J
ALLEN K
BURKE F
BREWIS D
BOWDITCH M
VAN NOORT R
WATSON T
IRELAND A
PEARSON G
KINLOCH A
COMYN J
ARMSTRONG K
PALMER J
CHIVERS R
GRAHAM A
BROWN D
CHADWICK R
ROBINSON P
ADUSEI G
WINFIELD P
DATE R
WILLMER P
ELLAKWA A
SHORTALL A
FLEMING G
YOUNG A
SHERPA A
NEUSER F
KONKEL C
EMMERSON G
SILIKAS M
WYLIE P
EASTMAN DENTAL
KINGS COLLEGE
OXFORD BROOKES
UNIV. OF GLASGOW
UNIV. OF LOUGHBOROUGH
CONSULTANT
UNIV. OF SHEFFIELD
GUYS, KINGS &ST THOMAS DENTAL INST
ROYAL UNIT.HOSP.
QUEEN MARY
IMPERIAL COLLEGE
LOUGHBOROUGH
CONSULTANT
EVODE
SMITH & NEPHEW
S&M PRODUCTS
GKT DENTAL INST.
DUNDEE UNIV.
KINGS COLLEGE
GKT DENTAL INST.
OXFORD BROOKES
PROCTOR & GAMBLE
IVOCLAR-VIVADENT
UNIV. OF BIRMINGHAM
UNIV. OF BIRMINGHAM
UNIV. OF BIRMINGHAM
SCHOTTANDERIQUEEN MARY
SCHOTTANDERIQUEEN MARY
PROCTOR & GAMBLE
ICI STG
ICI STG
UNIV OF MANCHESTER
DERA FORT HALSTEAD
PRESS RELEASE (14 APRIL 2000) THE SAA: A FOCUS FOR ADHESION AND ADHESIVES SCIENCE AND
TECHNOLOGY
The Society for Adhesion and Adhesives [SAA] was launched today during the
Adhesives technical session at Materials Congress 2000. The SAA has been formed to
promote the advancement of the science and technology of adhesion and adhesives
via seminars, conferences, and by co-operation with and between learned societies,
for whom adhesion and adhesives are an enabling.
Inaugural Chairman, John Bishopp of Hexcel Composites, said, "With
today's launch, there is now a clear focus in the UK for all matters concerning
adhesion and adhesives. This association between the old IoM Adhesives Section and
the Welding and Joining Society [WJS] and the Macro Group brings together
important national bodies having a common interest in the science of adhesion and the
underlying chemistry and technology of adhesives". He continued, "A consolidation
of existing strong links with other national groups within Europe, and an agreed cooperation with the Adhesion Societies in the USA and China, means that the SAA
will be an effective force on a truly international basis".
Launching the new Society, Steve Abbott of SATRA encouraged all
attendees to "register their interest by joining the SAA, which is free of charge until
April 2001. SAA members will be kept abreast of current adhesion issues via
notification of conferences - many of which will offer an advantageous rate to SAA
members, a biannual newsletter, and access to the SAA website which will include
links to other like minded bodies "
Programme
10.00
10.15
10.20
10.30
11.00
11.30
12.00
12.30
14.00
14.30
15.00
15.30
16.00
16.15
Registration and coffee
Chairman - R A Chivers (Smith and Nephew)
Welcome
Introduction
J W McLean (Eastman Dental Institute)
An overview of some relevant aspects of adhesion
science.
K W Allen (Oxford Brookes University)
Adhesion and adhesive in dentistry
J W Nicholson (Guy's, Kings and St Thomas Dental
Institute)
Clinical uses of adhesive materials in dentistry.
F J T Burke (University of Glasgow)
Preparation of surfaces for bonding.
D M Brewis (University of Loughborough)
Lunch
Chairman - G J Pearson (Queen Mary and Westfield
College)
Adhesion in harsh environments.
M R Bowditch (Consultant)
Problems of bonding to dentine.
R van Noort (University of Sheffield)
Imaging of failures.
T F Watson (Guys, Kings and St Thomas Dental
Institute)
Adhesive bonding in Orthodontics.
A J Ireland (Royal United Hospitals, Bath)
Conclusions
G J Pearson (Queen Mary and Westfield College)
Tea
"AN OVERVIEW of SOME RELEVANT ASPECTS of ADHESION SCIENCE"
Joint Seminar "Adhesion in Dentistry"
Society for Adhesion and Adhesives
and
BSDR Dental Materials Group
Thursday May 11 2000
K.W " A11en
Joining Technology Research Centre,
Oxford Brookes University.
Ultimately no other forces than the valence bonds of chemical
structure to provide the strength of any solid material. There
are no others available. So if something is broken then some of
these chemical bonds have to be themselves broken. Moreover,
this is true whether a single coherent material is involved or
some sort of joint between two different materials.
Thus if we are interested in adhesive bonds we are inevitably
concerned with the characteristics of these bonds and with
achieving both strength and durability of them.
One of the most relevant properties is that all these bonds are
of very short range, at any distance greater than 10- 1 metre
Thus in the initial stage any
they are totally ineffective.
adhesive has to wet and spread over the surfaces which it to
bond. Subsequently it has to solidify so that it becomes strong
enough to withstand the working stresses necessary for useful
service.
The whole business of wetting and spreading depends upon surface
energies and the fundamental thermodynamic law that any system
will change towards its lowest energy state. A major part of our
concern is to control surfaces so that this minimum energy state
is attained to our advantage, and this will be discussed in some
detail.
Formation and failure of adhesive bonds
KWA11en
Joining Technology Research Centre, Oxford Brookes University, England
Introduction
The use of adhesives dates back more than six millenia. One
of the earliest references being in the Book of Genesis'
where the builders of the tower of Babel were said to use
`slime' or `bitumen'.
Summaries
Formation and failure of adhesive bonds
Two questions concern and intrigue many of the people occupied in various applications:
Why do things stick together? Is it possible to predict which materials will stick? Any
attempt to answer them leads into several apparently disparate fundamental topics of the
physics and chemistry of materials. First there is a consideration of the forces that are
available to confer strength, both cohesive and adhesive, and their characteristics. Then on
investigation of the nature and properties of surfaces and the consequences for obtaining
effective bonding.
Thereafter comes o consideration of adhesives and their properties, both before,
during and after 'cure'. This leads on to the thermodynamics of surfaces and interfaces.
A fresh approach considers the theoretical strengths of materials and bonds, in
contrast to the observed strengths, and sources of these differences can he considered.
Finally, it is possible to begin to predict adhesive properties and potential.
Bildung and Fehlschlag der Klebbaftungen.
Nei Frogen hondeln sich and faszinieren viele Leyte in verschiedenen Gebieten. Warim
kleben sich Dingo aneinondern ? 1st es mtglich welche Stolle sich eneinondern kleben
warden vorauszusogen? lode Versuch diese Fragen zu antworten geht in Fuhrung Rath
vielen onscheinend disparalen Grundlagenthemen im Bereich des Physiks and Chemie von
Steffen. luesrt gibt es eine Uberlegen der Kri:ifte sowohl Kohnsions -- and Adhusionskthfte
and deren Chorokterisliko. Donn kommt eine Untersuchung der Art and Oberflicheneigenschoftea and folglich fur dos eekommen einer wirksamen Verbindung.
Danach kommt vine Uberlegeug der Klestoffe and demo Figenschatten vet, wtihrend
and nosh der Hartung. Dies geht 'on der Thermodynamik der Obeiflichen and
Grenzfltichen. Ein neues Ansoti uberlegt die theoretisehen Krofte der Stoffe and
Verbindungen im Vergleicli den bestimmten Krofte and kann die Ursprungen dieser
Unterschiedene iibedegen. Letzlkh ist es miiglich d Voraussagen dens
.
Ahnsionseigenchaften and deren Mdglichkeiten zu beginnen.
Certainly bitumen was used as an adhesive in the buildings in Babylon in about 1500 BC. 2 These were followed by
practical men in many applications in an empirical fashion
for many centuries in various civilisations. However even
this practical understanding seems to have been lost for a
considerable period. Studies of furniture show that gluing
had fallen into disuse between the fall of the Roman empire
(about 400 AD) and the sixteenth century when gluing reappeared as a method of constr uction. The first report of a
commercial plant for the manufacture of glue is in 1690 in
Holland 3
In spite of Sir Isaac Newton's challenge:" There are therefore agents in nature able to make the particles of bodies
stick together by very strong attractions. And it is the business
of eaperinreutal philosophy to find then out, it was not until
well into this century that the search for any explanation of
the fundamental causes and origins of adhesion was recognised and under'taken. 5
However, Michael Faraday" had begun to understand the
significance of the cohesion of ultimate particles as the
source of the strength of materials.
Available forces and their characteristics
As, during the earlier part of the present century, knowledge
of the structure of atoms and of the interatomic forces of
valence developed, so the origins of the properties of molecules and of the cohesive strength of materials evol ved.
Then, as the science of adhesion (as distinct from the
technology of the use of adhesives) began, so the close similarities between adhesion and cohesion were recognised.
The forces involved were the same; indeed there were no
others available. So studies of the forces of adhesion
between different materials can begin from the simpler
Figure I: Forces of ollradian and repulsion, together with the resultant 'Morse'
curve
1
La formation et leffondrement des liaisons adhesives
Deux questions occupent at intriguent plusieurs des gens ayant un intent en divers "
tfomaines. Pourquoi des (hoses collent-ils run contre l'ature? Estil possible de predire
quelles substances vont se collet -- Toute tentative Ies repondre conduite ii divers at
opporemment disparates !piques fondameniaux dons be domaine de to physique at et la
chimie des materiaux. D'abord it y o une consideration des faeces disponibles pour occorder
la puissance et cohesive et adhesive at egolement de lours coracteristiques.. Puis une
investigation de la nature at les properties de surfaces et aussi les consequences n regard
de I'obtention de liaisons efficoces.
Ensuite it y a une consideration des adhesifs et lours caracterisliques a la his avant,
pendant at apres la 'Caisson'. Ceti conduite o la thermodynamique des surfaces et
interfaces.
line nouvelle tentative considere Ins puissances theoretiques de moterioux e1 de
liaisons, contre les puissances determines, on sourait considerer les origines at ces
divergences. Enfin, it est possible, de commencer 6 predire des carocteristiques d'odhesion
et les possibilites qui en resultent.
Energy
Internuclear
distance
Resultant
Attraction force
Table 2 Roughness values (expressed as the average deviation item the mean
line; half through-peak values) MI in pm
figure 2: Bond energies and lengths
Metals:
l00
Milled
1.6-6.3
Bored or turned
0.4-6.3
Ground
0.1-1.6
Lopped at polished
0.05-0.4
Glass:
E
Optical 'flats'
0.005-0.05
Mica Cleaved
<0.0005
(<51‘1
0.0001-0.0006
(1--6A)
01
Interatomic force range
0
(50--5000A)
CO
600
0
2
3
4
5
Distance x 10 -10/m
problem of the source of cohesion within single materials,
and the valence theory as developed for the understanding
of the chemistry and hence the strength of materials. The
energy relationship between two atoms is represented by a
diagram in which the potential energy of the combination is
plotted against the particle separation (Figure 1) to give a
curve of general shape first described empirically by Morse'
and known by his name. This curve is the resultant of the
forces of repulsion and attraction. Whilst those of repulsion
are broadly common to all situations, those of attraction are
more diverse. Thus these forces of attraction control the
conditions of equilibrium and the properties of the bonds
formed eventually.
For a number of years the forces aril their resultant
bonds have been well described and generally have been
divided into two main groups: the Primary Valence forces
and the Secondary or van der Nuts forces, and then into
sub-groups. These are shown in Figure land the range of
principal characteristics are given in Table 1.
Table 1: Principle characteristics of various types of inter-atomic bonds
Bonding type
Directional?
Real surfaces
When real surfaces are considered it is important to appreciate their topography. While for many of their normal uses
they may be considered as smooth, on the scale of distances involved with these atomic forces they are very
rough. Even the best polished metal surfaces have irregularities greater by two orders of magnitude than the range of
interatomic forces, as illustrated in Table 2. Thus if two of
these surfaces are brought together, only a very small fraction of the surface areas of two surfaces can ever approach
near enough to interact. The situation has been described as
similar to inverting the Alps over the Himalayas! It is only
by using the surface of freshly cleaved mica in a cont rolled
atmosphere that anything like a molecularly flat surface can
be obtained.
Energy
Length
kJ/mole
nm
600-1200
0.2-0.4
No
Table 3: Values of cohesive strength calculated on the basis of primary and
secondary valence farces separately, and compared with experimental values.
Primary
Ionic
More recently it had been recognised that there are some
further types of interaction that are appended to Table 1 as
'intermediate' forces.
It is important to realise that the ionic (elect rostatic)
forces are proportional to r ' while the van der \Vauls forces
are proportional to r-', and the forces of repulsion are
approximately proportional to r- ''
However, one must always remember that these are idealised models and, that even relatively simple molecules
have to be described as having characteristics which can be
described only as partially another eg. 1-IF is 60% ionic, 40%
covalent; while HCI is 17% ionic, 83% covalent.''
One of the most important points which must be recog nised about all these forces are that they are only effective
over extremely short distances (less than 1nm, a few
Angstroms); distances comparable with inter-atomic distances or less. The significant implication for creating adhesive bonds is that the two components, adherencl and adhesive, must be brought into intimate contact so that the forces
can be effective.
Covalent
60-800
0.08-0.3
Yes
Metallic
100-350
0.2-0.6
No
4-20
0.2-0.4
Yes
Secondary
Calculated
Dipole-dipole
<2
Dipole-induced dipole
Dispersion(l.ondon)
0.8-40
Primary
forces
alone
GPa
Secondary
forces
alone
GPa
MPa
P/F rosin
42
0.39
76
C/F resin
38
4
0.32
38
0.196
5.9
Yes
0.4-0.8
No
Intermediate
Hydrogen bonds
<40
-0.3
Dour-acceptor
<30
-0.4
All
ihese are guile specific except
the London
Yes
dispersions faeces which are universal.
Experimental
NaCl
Liquid to solid phase change
The inevitable consequence of the roughness of surfaces is
that if any of the available forces are to be exploited a fluid
must be used to achieve the necessary very close contact.
This fluid has to flow into the irregularities of the surface of
the adherend and achieve intimate contact with it. Then the
forces already described may act.
However the characteristic property of a fluid of yielding
under the slightest stress means that it cannot then fulfil the
ultimate purpose of an adhesive. It cannot transmit any
stress. To he useful the adhesive must now change phase
from liquid to solid.
A great deal of the complex chemistry involved in the formulation of adhesives is concerned with accomplishing and
controlling this phase change.
Adhesive interactions
Now, with some understanding of the available forces and
of the surfaces which are involved, the actual processes
involv ed and the explanations concerned with generating
adhesive bonds may be considered.
From fairly pragmatic considerations, five distinct broad
theor ies have been advanced; diffusion, elect rostatic, pressure sensitive, mechanical, and adsorption. The fast three
are of only specialised significance and can be dealt with
briefly. The last two are of more general application and
have received a great deal more attention in the recent past,
and in the present context need more extensive consideration.
Diffusion theory
This was advanced and developed by Voyutskii' and
Vasenin for the autohesion of two similar polymers well
above their Glass Transition Temperature. In simple terms
polymer chain-ends diffuse across the interface in both
directions and eventually no interface can be discerned. This
is now well accepted as the explanation for the autohesion
of masticated rubber in the manufacture of motor tyres, and
for some instances of both heat sealing and sol vent welding.
Vasenin m has provided a firm quantitative theoretical basis
for this explanation within its explicit limits, although this
has been challenged.
Electrostatic theory
The principal proponent of this theory is the Russian
Deryaguin. 1e It considers the adhesion of a pressure sensitive tape to a smooth surface and treats these two components as the two plates of an electrostatic condenser and
relates the energy of this condenser to the work of adhesion.
Other workers have found difficulty in reproducing the
work of Deryaguin and his co-workers, and certainly indicate much smaller contributions from this electrostatic mechanism to the adhesion. t= However the explanation of the
glow discharge which can be seen when a pressure sensitive
tape is peeled rapidly from a substrate must lie in this area.
volatility, normally coated on to a flexible tape. During its
useful life this liquid never solidifies. The bond strength
needed to detach the tape from a substrate depends upon
the energy which is required to cause this liquid to flow as
the two adherends are separated. It can lie shown that this
force for separation is proportional to the third power of the
pressure used to bring the two surfaces together.'- Within
certain fields of non-critical application (eg fixing number
plates to cars), this type of adhesive is exceedingly important and surprisingly durable.
Mechanical adhesion (or interlocking)
There is a long-standing intuitive belief that roughening a
surface will improve any bond strength because the adhesive will interlock with the irregularities.
On a relatively macro scale (>l.tm) for fibrous materials,
such as paper, cloth, leather and to some extent wood, this
concept of interlocking provides an explanation which is
demonstrably satisfactory. Wake and I3orroff''f in a piece of
elegant work on textile reinforcement for tyres and similar
products demonstrated that the adhesion depended mainly
on the extent of the embedding of fibre ends of the staple
yarn in the rubber. Similarly for electroless plated plastics a
mechanical interlocking of the metal with the substrate provides a significant proportion of the bond strength.
For timber, some of the experimental datn ,i appears to
suggest that strength decreases with increasing roughness.
However, a critical examination suggests that this discrepancy is clue to damage and weakening of the surface by the
roughening treatment.
For hard structural materials, mechanical interlocking is
more difficult to envisage. It is difficult to conceive an adhesive interlocking with a smooth metal or ceramic surface.
1-However it has become apparent more recently that interlocking is important in at least sonic of these examples; but
the size of the significant topography is much smaller
(<lnm).
For some considerable period (twenty five years or
more), methods of pre-treatment of surfaces for satisfactory
adhesive bonding even in critical applications (eg aeroplane
structures), have been established on an empirical basis. As
techniques of ever increasing degrees of sophistication for
the study of surfaces have been developed, so the structure
of these surfaces has been revealed. Particularly the work of
Venables and his colleagues[ ; on aluminium and titanium for
aerospace applications showed that the strongest, most
durable bonds are obtained when the oxide surface of the
metal is both porous and has protruding whiskers as shown
in Figures 3 and 4. Thus the cured adhesive penetrates into
the oxide and the oxide whiskers are emhedded in the
adhesive.
Generally the optimum surface is now understood to
have the maximum roughness on a micro-scale, provided
that the features of this roughness (whiskers, pores, crystalites) are themsel ves both strong and are firmly attached to
their substrate, as well as being stable and resistant to
hydrolytic degradation.
`Adsorption` theory
Pressure sensitive adhesion
This is quite different from any other type of adhesion, so
different that it is often omitted from this sort of discussion.
The adhesive is an extremely viscous liquid of very low
Originally discussions of bonding clue mainly to simple secondary valence forces was called 'adsorption' theory
because of the analogies with classical physical chemistry of
physi-sorption of gases on solids. As the significance of the
Figure 4: (Top) Ulira-high resolution stereo SEM micrograph and (bottom) isometric
drawing of the oxide morphology on a PAA aluminium surface
Figure 3: (Top) Ultra-high resolution stereo SEM micrograph and (bosom) isometric
drawing of the oxide morphology on a FPL-treated aluminium surface
1*114''
ITW
lit,
-1000 A
-40000 A
AP
-400 A
-400 A
Oxide film
A
/0
-400 A
r-^ Oxide
- Af
various other valence forces has been recognised and the
distinction between them blurred, so the name became less
felicitous but no alternative has yet come into common use.
Secondary forces
Whenever particles approach sufficiently closely London
dispersion forces, because of their universal nature, will
become effective. This will be true in addition to any other
interactions that may also be present.
It is quite clear that dispersion forces are sufficient to
account for far greater bond strengths than are ever
observed, 17 as is illustrated later for the cohesive strength of
materials. However these interactions may be disrupted relatively easily, particularly by the presence of moisture.
For the autohesion of oxidised polyethylene, hydrogen
bonds are believed to exist either between the enol tautomer of one carbonyl group and a second carbonyl group,
or with a water molecule between two carbonyl groups.
More recently Lewis acid./base interactions have been
introduced into the discussions.
Fowkes'ti suggested that the total adhesion situation is the
result of a summation of contributions from a number of
types of interaction thus:
WA =Wei +\VX+Wi +\V : ' i '+ wit
where the superscripts indicate:
d
x
ab
h
London dispersion forces
dipole-dipole interactions
induced dipole interactions
acid/base interactions
hydrogen bonding
Following the Lewis approach (in which an acid is a proton
acceptor and a base a proton donor) he then went on to
suggest that hydrogen bond attractions are a sub-set of
acid/base interactions and that the dipole interactions are
negligible, so that this relationship reduced
WA \' 1I
Intermediate forces
The first of these intermediate forces to be recognised were
hydrogen bonds. They have been shown to be of significance in many situations, for example between the carbonyl
groups of a cyanoacrylate or the amino group of a silane
end the hydroxyl groups at the surface of aluminium oxide.
+
r
Thereafter, Fowles developed a treatment of \V i' based on
recent theories of acid/base interactions due initially to
Drago.'" This requires that the work of adhesion arising from
acid-based interactions is given by:
WA = k(C:'C1t + F.al.is)n.."
where CA, E'' are two constants characterising the acid and
similarly C'', E" the base; k is a constant close to unity and
nAI is the number of acid/base pairs per unit area.
While this has had some success, it is limited by relatively few values for the constants C and E being available.
Moreover the whole treatment has been seriously questioned n
Primary forces
So far all this has ignored any involvement of the Primary
valence forces in bonding, partly because of the comparative difficulty in studying them.
However, Andrews and Kinloch" and Gent and
Kinloch'; measured the intrinsic Adhesive Fracture Energy
G. for a number of interfaces, including some where covalent bonds might be predicted. Such energies were far higher than the thermodynamic work of adhesion WA (which
later assumes only secondary bonding). Although the
Fracture Energy includes some energy of deformation, this
clearly indicates the existence of primary bonding forces.
Further Koenign and his co-workers''-} have demonstrated
the existence of Si-O-Si bonding between aminoproplyltriethoxy silane and glass surfaces by Raman and FTIR spectroscopy; and Gettings and Kinloch'' used SIMS with another silane and a mild steel surface to show Fe-O-Si bonds.
It is important to recognise the analogies with chemisorption, and particularly the high energy required and the
difficulty encountered to reverse them, and hence their
advantages in some situations. Where a particularly durable
bond is needed, then clearly at least some degree of primary bonding is necessary.
fractions of lower molecular weight. As well as these additives, the surface is frequently contaminated with extraneous
matter; grease, mould release agents, and plain clirt. All of
these will tend to concentrate at the surface and prevent the
formation of strong bonds.
Moreover, any surface which is exposed to the atmosphere will very rapidly acquire an adsorbed layer from the
atmosphere of oxygen, nitrogen, carbon dioxide and water.
Generally this is very tenaciously held and only displaced
with some difficulty.
Surface thermodynamics
Because all adhesion involv es interaction between a solid
adherend and a liquid adhesive, the energy relationships at
interfaces between two phases are important; so some study
of surface thermodynamics is necessary. This study involves
not only physico-chemical aspects but also mathematical
manipulations to bring the various functions into the most
useful and informative forms.
The first consideration of this aspect is due to Thomas
Young's who in 1805 related the forces acting at the edge of
a drop of liquid resting on a solid surface and in an atmosphere of its own vapour, as shown in Figure 5.
Figure5: Forces {from surface tensions} otling al the three-phase point between a
liquid, its vapour, and a solid surface, and the contact angle 0
Failure of adhesive bonds
Beginning with single materials and their cohesive properties; de Boer's calculated, at least approximately, the tensile
strengths that might be expected from primary and secondary valence forces separately and compared these with
experimental values. The results of these calculations are
shown in Table 3. It immediately become apparent that the
strengths which were actually obtained were far less than
the calculated values. Pure materials are never as strong as
theories might indicate.
The first studies of this weakness were made by Griffith'for glass fibres and metals. He showed that in these materials it was due to poor cohesion at crystal boundaries, irregularities in the crystal structures, bubbles of gas in the adhesive or at the interface, and particularly surface cracks
(Griffith Cracks) which provide points of stress concentration.
All these factors will exist within the adhesive in joints
causing weakness. Further by the geometry of joints there
may be areas of stress concentration which provide points
of weakness and incipient failure.
A further group of reasons can be and have been
advanced for this weakness. Particularly it has been attributed to the formation of 'weak boundary layers' and this
applies not only to pure materials but also to adhesive
bonds. It involves the recognition that the outermost surface
layers of any materials are often different from the bulk.
Metals may be covered with an oxide film which has a lower
cohesive strength than the metal itself and/or may be poorly
attached to the underlying metal. Plastics, as encountered in
useful form, contain a range of additives, antioxidants, plasticising agents, and fillers. Also they will include polymer
Y5v
Yst
If this was in equilibrium, then the surface tensions are related to give the Young equation thus:
Ysv = Its,. + Yrv cos0
The next advance in the discussion was clue to Dupre'" who
considered the work required to separate two surfaces
which had been in contact. This work is the 'Work of
Adhesion' \V,,. Two fresh surfaces have to be formed when
an interface is destroyed, so the work required is:
WA = Yr + Yz- Yrz
However it is important to recognise that this implies a totally clean separation with the two resulting surfaces in equilibrium with their own vapour, unlike the condition considered by Young. To combine these two relationships needs
the introduction of a correcting factor it, the 'spreading pressure' defined hy:
n= Yso -Ysv
Then, by combining the two, we have the Young-Dupre
equation
Wa = YL.v(I + cosO) + it
Later it was recognised that surface tension (the force acting
across a line in a surface) and surface free energy (the work
or energy to create fresh surface) are equivalent. They arc
Thus: y = y' + yP for each separate y.
Hence, for the interfacial free energy, Fowkes gives:
Figure 6: Forces ailing on molecules ai the interface between Iwo liquids
Yu = Ya ±
Y2 - 2(Y1d-72d)l
2 -
2( Yi r Y ^')'.
which can be combined with Youngs equation to give
Y27(Ycos9) = (y'd) , ,
+ (Y1')1 2 + (yi1`s)i '
and this enables us, moderately easily, to separate the two
components of the surface f r ee energy by plotting the data
from observations of contact angles for several liquids of
known parameters.
However, it must be added that the use of the geometric
mean expression for the polar component has been criticised on theoretical grounds as being unsound and without
justification32 although it provides some useful (empirical)
results.
Predictions
measured in Newtons per metre or in Joules per square
metre or Pascals; but more commonly in milli-Joules per
square metre which is both convenient and numerically
equal to the older dyne/cm or erg/cm'
The behaviour of two liquids (1 and 2) in contact is represented in Figure 6, where the force by which a molecule
of 1 at the interface is attracted to the bulk of its own kind is
y1, the surface free energy of liquid 1; and similarly for a
molecule of liquid 2 is 72.
For a pair of non-polar liquids where the only forces
involved are London dispersion forces; the force by which
the molecule of 1 is attracted to liquid 2 has been considered by Fowkesi ° to be the geometric mean of the two sm .
31 considers it to be better rep--facerngis,whlWu
resented by the harmonic mean of the two surface free energies.
Thus: for interfacial attraction Z
either:
Z = (Y,.y,)'"
Fowkes
or:
1/Z = 0.5(1/y, + 1/y,) Wu
Z = 2 71 .Y2/(7, + Y2)
Thus, using the Fowkes geometric mean, the total force acting on a molcule of liquid 1 is y, - (y,.Y,) t /2
and similarly for a molecule of liquid 2 y, - (y 1 .y2) 1"2
The total across the interface y,, = y, + y_ - 2(y 1 .y,) a" 2
While this is more clearly discussed for two liquids, the
principle is unaltered if one is considering a liquid and a solid.
Each of the surface free energies (y,;Y,) is separately
composed of at least two components. These are from dispersion forces and from polar forces. It is assumed that
these are additive but can be treated independently and that
interactions between them are negligible.
Predictions of adhesive behaviour are always hazardous but
some useful progress is possible. However frequently they
have to be considered in two stages, whether or not a particular formulation will form a bond of adequate strength to
the substrate involved, and as a second, almost separate,
issue what are the serv ice conditions and what is the durability of this bond.
The first requirements are that the formulation must flow
and must wet the substrate. So the viscosity and the surface
free energy relative to that of the substrate give a valuable
indication of ability to give initial bonding.
The commonest service requirements which are particularly demanding involve the presence of water - which is
about the most destructive agent for adhesive bonds that
exists. If this is the situation [hen a deeper exploration of the
thermodynamic relations may be helpful. Certainly one has
to seek some degree of a primary type of bonding to withstand displacement of the adhesive layer from the substrate.
References
1. Genesis 11, 3. King James Bible: 'they had bricks for
stone and slime had they for mortar'. Revised English
Bible: 'they used bricks for stone and bitumen for mortar'
2. Alsalim HS, 'Adhesion 5' Ed.
Pufhlishewx pp.151-156, 1981
K W Allen, Applied Science
3. Stombo DA, Historical Table in 'Adhesion and Adhesit'e&'
2nd edition Ed. R Houwink & G Salomon Elsevier
Publishers pp. 534-536, 1965
4. Newton Sir Isaac 'Optiks' 3rd edition p.363, 1721
5. McBain 'Reports of the Adhesives Research Committee'
HMSO 1922, 1926, 1932
6. Faraday Michael 'On the various forces of Nature'
2
2
7. Morse
PM, Phis. Review, 32, 335, 1929
8. Pauling Linus 'The nature of the chemical bond' Cornell
Univ. Press, p.46, 1942
9. Voyutskii SS, 'Autohesion and Adhesion of High
Polymers' English translation Interscience 1963
10. Vasenin
1961
12M, RAPRA Translations 1005, 1006, 1010 1960,
11. Deayaguin BV, 'Adhesion: Fundamentals and Practice'
McLaren 1969
12. Wake \VC, 'Adhesion and the Formulation of Adhesives'
Applied Science Publishers 2nd Edition, 87, 1982
13. Wake WC, 'Adhesion and the Formulation of Adhesives'
Applied Science Publishers 2nd Edition, 98, 1982
14. l3oroff EM and Wake \VC, Trans. Inst. Rubber Ind., 25,
210, 1949
15. Maxwell JW, Mtns. Am. Soc..Ilech. Eng., 67, 261, 1945
16. Venables JD, i. 'Adhesion 7' Ed. KW Allen Applied
Science Publishers, 87, 1983. ii. J.Adhesion, 39, 79, 1992
17. l3untsbergerJR, 'Treatise on Adhesion and Adhesives
Volume 1' Edward Arnold & Marcel Dekker, p.146, 1967
18. Fowkes FM, J. Adhesion, 4, 155. 1972
19. Fowkes FM, 'Physicochemical aspects of polymer surfaces' Ed: K L Mittal Plenum Press Vol.2 pp.583, 1983
20. Drago RS et a], J.Am. Chem Soc., 93, 6014, 1983 and, 99,
3203. 1977
21. Shanahan MRS, J. Colloid !Hied Sci. 215, 170, 1999
22. Andrews EH and Kinloch Aj, Proc. Roy. Soc., A332, 385
& 401, 1973
23. Gent AN and Kinloch
J. Polymer Sci., A2, 659, 1971
24. Keonig Jl, and Shih PTK, J. Colloid Interf Sci., 36, 247,
1971
25. Gettings M and Kin]och AJ, J. Mat. Sci., 12, 2049, 1977
26. Allen KW, 'Aspects of Adhesion I' Ed. DJ Ah3er, Univ.
London Press 1965 pp.11-22 after de Boer Trans Faraday
Soc., 32, 10, 1936
27. Griffith AA, Phil. Trans. R; Soc, A221, 163, 1920
28. Young T, Phil. Trans. R. Soc., 95. 65, 1805
29. Dupre A, Theorie mechanique de la chaleur' Paris,
p.369, 1869
30. Fowkes FM, Ind. Eng. Chem., 56. 12, 40, 1964
31. Wu SJ, Adhesion, 5, 39, 1973
32. Allen KW, Int. J. Adhesion and Adhesives, 13, 67, 1993
Wake WC, Polraner, 19, 291, 1978
■
Adhesion and adhesives in dentistry
John Nicholson
Guy's King's and St Thomas' Dental Institute,
King's college London
Modern restorative dentistry makes considerable use of adhesive materials and of the
phenomenon of adhesion. This may either be through the deployment of special
bonding agents for composite resins and compomers, or through the use of glassionomers and related cements, which are inherently adhesive to the tooth surface. It
follows that a proper understanding of adhesion is essential if these materials are to be
used effectively. This paper seeks to do that, by considering the nature of the tooth
surface and how it is altered by the various preparation processes, and also by going
onto consider what happens as the material is placed onto the finished surface and
develops its adhesive bond. The tooth surface itself is hydrophilic. However, despite
imaginative claims to the contrary in the literature, almost certainly does not consist
of exposed calcium and phosphate ions. Instead, it probably consists mainly of a
tightly bound water layer, possibly admixed with proteins and other important
biomolecules. To bond composite resins, this surface must be changed to have an
essentially hydrophobic character, and this is achieved through the application of
bonding agents in a number of layers. By contrast, glass-ionomers can bond directly
to the tooth surface, a phenomenon which probably arises from the strong interaction
between the carboxylic acidlcarboxylate groups on the polymer and the tightly bound
water layer. The paper concludes with a brief consideration of the effectiveness of
adhesion bonding in dentistry, noting that a few materials are reported as failing by
loss of adhesion, and that other material properties, notably wear resistance, are what
actually determine the longevity of dental restorations.
ADHESION IN DENTISTRY
Clinical Uses of Adhesive Materials
The development and use of adhesive materials in dentistry represents a
major breakthrough in the restoration of teeth. The advantages of using
adhesive techniques include revised need for tooth preparation to achieve
mechanical retention and reduced risk of post-operative sensitivity.
Furthermore, many adhesive techniques involve the use of aesthetic, toothcoloured materials.
Glass-ionomer materials developed in the 1970s provide reliable chemical
adhesion to tooth substance. These materials are most frequently used in the
restoration of Class V (cervical) cavities, but also have applications in Class III
cavities and in restoration of cavities in deciduous teeth. Recently developed,
more heavily filled materials provide better wear resistance, and improved
physical properties. These may be used as core build-up materials.
Resin-based composite (RBC) materials are bonded to tooth substance using
dentine and enamel bonding systems. These tooth materials are appropriate
to restorations in anterior teeth, but are finding an increasing application in
restorations in load-bearing situations in posterior teeth, early problems of
poor wear resistance having largely been overcome. However, these RBC
materials are technique sensitive to place.
Porcelain veneers and resin-retained bridges are two adhesive, minimalintervention techniques which are providing high rates of success in clinical
evaluations.
Enamel and dentine bonding agents may also be used in the repair of
fractured teeth, but the results of such treatment are not well documented.
These bonding agents may also be used to bond ceramic crowns and inlays
to teeth, and these applications are finding increasing favour with clinicians
and patients, with documented success rates proving to be satisfactory.
Recently-introduced applications of bonding techniques include the
cementation of metal, glass-fibre, carbon and ceramic posts but there is little
documented evidence of success, due to the comparatively recent
introduction of these. For the future, bonding techniques may be used to
place enamel/dentine inlays.
Preparation of surfaces for bonding
Derek M Brewis, Loughborough University, Loughborough, Leics, LE11 3TU,
UK
e-mail: d.i^,brewis(a,lboro.ac.uk
The quality of a substrate surface plays a key role in its adhesion performance. The
factors relating to a surface are: a. surface chemistry, b. topography or surface
geometry and c. the presence of any cohesively weak layers on the substrate. If the
quality of a surface is unsatisfactory with regard to these factors, it will be necessary
to carry out a pretreatment. A wide range of treatments exists for different types of
substrates. Some examples for polymers and metals are:
Polymers:
solvents, grit blasting, flame, corona discharge, plasma, active gases,
and etching solutions such as potassium permanganate.
Metals
solvents, grit blasting, chromic acid etching and anodising.
In some cases, it is sufficient to remove cohesively weak layers from a substrate, but
in other cases it is necessary to modify the topography and/or chemistry of a surface.
Many techniques are available to study changes caused by pretreatments.
Topography may be studied by electron microscopy or scanning probe techniques,
whereas the best methods to study surface chemistry are X-ray photoelectron
spectroscopy XPS and static secondary ion mass spectrometry SSIMS.
ADHESION IN HARSH ENVIRONMENTS
M R Bowditch (Consultant) and 7 M Lane (DERA, Farnborough)
BACKGROUND AND INTRODUCTION
This paper addresses problems associated with the use of adhesives and adhesive-like
materials to make structural bonds and seals to underwater structures. In particular it
is concerned with sub-sea repair procedures developed in response to problems posed
by customers including the MoD and the offshore oil and gas industries.
Water poses special threats to the integrity of adhesive joints due largely to changes in
mechanical properties and loss of adhesion at bonded interfaces resulting from the
physical absorption of water. In addition, the presence of water at the joint making
stage can also prejudice the formation of satisfactory adhesion interfaces, as will be
discussed later.
Work within DERA, concerned with underwater adhesion, commenced in the early
seventies when a requirement arose for adhesives to be used underwater on HM
submarines. Since that time, repayment work has been undertaken for the Offshore
Supplies Office of the Department of Energy who were interested in the concept of
using adhesive-assisted repairs to steel offshore structures. More recently, repair
techniques involving the use of adhesives and adhesive-like materials have been
developed in response to a wide range of problems. Three of these repair methods
will be discussed in this presentation at the end of the initial section addressing basic
theoretical requirements for successful adhesive and sealant systems for use
underwater.
BASIC REQUIREMENTS
a) Adhesive bonding
A primary requirement is for an adhesive system capable of converting to a
cohesively strong solid in the underwater environment and subsequently for it to
remain substantially unaffected by the presence of water. Insofar as commonly used
adhesives are organic-based materials, there will always be some degradation of
mechanical properties as a result of resin plasticisation resulting from absorption of
water. However, such effects need not be serious and can allowed for. Equally, it is
comparatively simple to design adhesive materials which are hydrolysis resistant in
the sub-sea environment. Of greater concern are potential degradation problems
arising as a result of hydrolytic attack on interfaces between the matrix resin material
and inorganic fillers used in the formulation of the adhesive system.
A second critical requirement is that the substrate surface shall be sound and free of
unplanned contaminants. A commonly used and effective cleaning process for steel
and other metallic substrates involves grit blasting, a process readily adapted to
underwater use. Although the use of abrasion techniques can be effective for the
surface preparation of composite materials, such an approach when used alone is
often ineffective for use on metals. Careful examination of abraded surfaces shows
the presence of `smearing' with the overlaying of sound base metal with flakes of
the presence of `smearing' with the overlaying of sound base metal with flakes of
metal which appear to be responsible for the creation of weak boundary layers in
adhesive joints made to such surfaces.
Having acquired a suitable adhesive and prepared a satisfactory substrate surface, a
further essential requirement is that the resinous system shall be capable of `wetting'
the substrate in the presence of water.
The `wetting' process involves the
achievement of intimate contact between adhesive and substrate which is essential to
the development of adhesion forces dependent upon physical, short range,
intermolecular attractive forces. Equally, such wetting is also required if subsequent
adhesion is to result from the generation of chemical bonds across the adhesivesubstrate interface. This requirement presents a major challenge where high surface
free energy substrates are involved. The problem derives from the fact that it is
energetically favourable for water, with its relatively high surface tension, to be
preferentially adsorbed onto the surfaces of such substrates (see Figure I).
Unfortunately, most structural materials, including metals and ceramics, do have
highly energetic surfaces, with only composites employing organic matrices being
more amenable to wetting by adhesives in the presence of water. The solution of this
problem is fundamental to the successful use of adhesives for underwater structural
applications. Two approaches have been used by DERA to overcome these problems.
The first depends upon the use of a so-called sacrificial pre-treatment (SPT) where an
ephemeral hydrophobic - but resin compatible - film is applied to the energetic
surface prior to the application of the adhesive material which then either absorbs or
displaces it prior to the formation of a strong adhesive bond to the underlying
substrate. A second approach employs a rotating abrasive brush which cleans the
substrate surface whilst, at the same time, deposits a thin layer of hydrophobic
material on the surface. This simultaneous cleaning and pre-treatment process
(SCAP) depends upon the presence of abrasive particles, often silicon carbide, within
the (usually) nylon bristles of the brush. The nature of the bristle material is critical to
the amount of material deposited and also to the ability of the adhesive to
absorb/displace it. Solubility parameter considerations show that polyamide-based
bristle material is often compatible with epoxy-based adhesive formulations.
Adequate durability in the service environment is a prerequisite. The formation of an
initially strong adhesive joint where failure on testing to destruction occurs within the
adhesive layer is a challenging but achievable objective, even when making joints
underwater. The difficulty is in making joints which resist the ravages of water when
thermodynamic considerations predict that water shall preferentially occupy the
metallic adherend surface and that joint weakening shall therefore result. Answers are
to be found in the careful design of adhesives (to include low diffusion coefficients)
and of joint geometry such that time taken for water to access hydrolytically sensitive
interfaces is long compared to required operational lifetime. A second approach can
involve the generation of water-stable interfaces through the use of chemical coupling
agents, such as silanes, where formal covalent bonds may be formed across the
interface. In practice, it is common for both approaches to be used wherever possible.
In any event, the production of adhesive joints with good durability in the presence of
moisture and under demanding operational conditions continues to challenge
adhesives formulators and applications engineers regardless of whether the joints are
made underwater or in the atmosphere.
b) Leak sealing
Requirements for effective leak sealing parallel those indicated above for underwater
adhesives. Certainly effective wetting is essential for both adhesion and sealing
operations. The wetting process, involving, as it does, the spontaneous spreading of
resin over the substrate surface, is best and most rapidly achieved via the use of a low
viscosity liquid adhesive/sealant material with sufficient pot life to ensure that the
wetting process is complete before cure. However, mobile liquids are not readily
applied to vertical or overhanging surfaces and do not lend themselves to application
in the thicker section often needed for sealant applications. The DERA solution to
this problem is to immobilise sealant/adhesive resin within the pores of reticulated,
flexible polyurethane foam prior to application. In this way, the desirable qualities of
the resin system are retained whilst application problems are effectively overcome.
A second and most valuable benefit to be derived from this approach is that, when the
resin-impregnated foam is compressed over a leak, both pore dimensions and resin
flow rates are reduced with the effect that an immediate seal may be achieved (see
Figure 2). The degree of resistance to resin displacement required, is determined by
the leakage pressure which, in turn, determines the degree of resin/foam composite
compression. Within limits therefore, the resin/foam composite is automatically
compressed to meet the needs of the particular application.
PRACTICAL APPLICATIONS
a) Leak sealing in hulls and within ships and submarines.
A requirement was for adhesive-assisted repairs to pressurised systems such as hulls,
tanks and pipes in a sub-sea environment. Instant seals and restoration of structural
integrity were to be achieved. In the absence of suitable commercially available
products, cold-curing, epoxide-based resins were formulated and appropriate
technologies were developed, as detailed above. Leaking GRP panels, simulating the
hulls of mine countermeasure vessels, were effectively sealed and repaired
underwater at depths appropriate to the hulls. The strength of the adhesive bonds
made to the GRP were shown to be greater than the interlaminar cohesive strength
(-7MPa) of the GRP used.
b) Subsea Riser Repair
As a result of damage sustained to the nylon sheathing of a flexible riser in the West
of Shetlands field (Foinaven), owned by BP AMOCO, it was feared that there was
potential for the corrosion of inner, carbon steel armour wires. A proposed solution,
involving the use of DERA adhesive/sealant technology together with SP Offshore
composite clips was funded by the company. The requirements were for the repair
procedure to be suitable for ROV application, to bond to nylon and to cure at -2°C
whilst ensuring a seal against the ingress of oxygenated sea water with a pressure
differential of up to 50 psi for a required service life of 10 years.
A suitable sealant resin was formulated and, after successful trials at the National
Hyperbaric Centre at Aberdeen, all sixteen damaged areas on the riser have been
repaired.
c) Emergency Repairs to Damaged Pipelines
A joint industry project has just been completed in which techniques for the
emergency repair of damaged pipelines either underwater or in the splash zone have
been developed. Solutions were developed for both leaking pipes and those which
have sustained structural damage as a result of corrosion or fatigue. The work was
carried out by DERA (the lead partner) and DML whilst other participants included
BP Amoco, Elf Exploration, Enterprise Oil, the HSE, Marathon Oil, Rockwater and
Shell UK Exploration and Production. The project was monitored by DNV and
Lloyds Register.
ACKNOWLEDGEMENTS
The agreement of co-workers (DML) and sponsors BP Amoco, Elf Exploration,
Enterprise Oil, HSE, Marathon Oil, Rockwater and Shell UK Exploration and
Production for this presentation to be published is gratefully acknowledged.
© British Crown Copyright 1996 / DRA. Published with the permission of the
Controller of Her Brittanic Majesty's Stationery Office.
Figure 1
BONDING TO STEEL UNDERWATER:
THE PROBLEM AND SOLUTION
Steel
Adhesive will not "wet" surface
Adhesive absorbs sacrificial pretreatment
and "wets" underlying steel
Figure 2
Dependence of resin flow rate on progressive compression
of foam and increasing resin viscosity as cure proceeds
Pore
Size
0
Flow rate
Time
0
Adhesive Bonding in Orthodontics
Tony Ireland
Orthodontic Department
Royal United Hospital
Combe Park
Bath
BA1 4DP
Introduction
In order to fulfil the idealised orthodontic treatment aims of improved aesthetics,
function and health it is necessary to be able to move teeth accurately in all three
planes of space. In modern day orthodontics this is accomplished using fixed
appliances. The most commonly used of which is the preadjusted edgewise system
based on the work of Andrews (1976). Using such a system, each tooth has its own
individually prescripted bracket, which must be accurately placed on the labial/ buccal
surface of its respective tooth. The centre of the bracket should overlie the centre of
the clinical crown, with the mesial and distal surfaces of the bracket being parallel and
equidistant to the long axis of the tooth.
The tooth surface and orthodontic bracket as the adherends, with the adhesive
sandwiched between can be considered to comprise the orthodontic adhesive joint.
This is perhaps an oversimplification, since the tooth surface may comprise enamel,
composite resin, amalgam alloy, porcelain, precious or non-precious metal, and in
some instances two or more such substrates may be found on the one surface.
Similarly the orthodontic bracket base can be metallic, plastic or ceramic. At first
glance the joint may appear a simple butt joint, but on closer inspection this too is an
oversimplification since loading in shear could lead it to be considered a lap joint
whilst at the microscopic level other joint configurations, such as a double scarf joint,
might be operating.
Whatever the materials comprising the tooth surface and however it might be loaded
in service the four main clinical requirements of the joint and its component parts are:
1. It should be easy to use, both in placement and removal, 2. It should last for
approximately two years without failure, 3. There should be sustained fluoride release
throughout the two year treatment period, 4. The tooth surface should be unaffected at
the completion of treatment. Within these four basic ideals there are obviously more
specific physical and chemical ideals for the component parts of the joint.
History
Orthodontics has been performed since the times of the Greeks and the Etruscans
(Proffit 1986), but in order to achieve the three-dimensional tooth movements alluded
1
to earlier, fixed appliances are required. The earliest of these consisted of a metal arch
tied to the teeth using fibrous ligatures (Fauchard 1728). However, the accuracy of the
tooth movements achieved using such an appliance are little better than that seen with
removable appliances, namely tipping movements, but with the added disadvantage of
possible gingival stripping due to the action of the ligatures around the necks of the
teeth. Schange (1841) developed the metal clamp band that could be tightened around
the tooth using an adjusting screw, but here the problem is that plaque growth beneath
the band could lead to caries. It was not until Magill (1871), with the introduction of
"oxychlorid of zinc cement," that plain metal bands, which encircled the crown of the
tooth, could be successfully used in orthodontics. Zinc phosphate cement was first
used for band cementation in 1894 (Chupein 1894) and was in common use in this
country up until 10 years ago for band cementation. The first recorded use of stainless
steel bands was by Ohara in 1937 and such bands are still used on molar teeth today,
especially when headgear is to be worn. Polycarboxylate cement was tried as a band
cement with the reported advantage being adhesion to both the enamel and the band.
However, their high solubility lead to bands becoming uncemented during treatment
and consequently caries forming beneath the loose band (Mizrahi and Smith 1971).
Glass poly(alkenoate) cements have more recently become the band cement of choice
due not only to their adhesive properties, but also their relatively low solubility once
set and their ability to leach fluoride (Fricker and McLachlan 1985)
Although orthodontic bands provide a suitable attachment of the orthodontic bracket
to the tooth, there are problems with their use. These include the need to separate the
teeth a week before band placement, the time required to select and fit the bands, poor
aesthetics during treatment and the presence of interdental band spaces at the time of
appliance removal. Early attempts were made to bond brackets directly to the teeth
(Mitchell 1967) following the observation that zinc phosphate cement, used with
bands, frequently remained attached to the enamel surface at band removal (Berkson
1950). The adhesion observed was thought to be due to etching of the enamel surface
by the free acid within the cement prior to it setting and the subsequent formation of a
micro mechanical interlock between the set cement and the enamel surface (Wisth
1970). Both zinc and copper phosphate cements were found to be unsuitable as
orthodontic bonding agents due to their poor solubility and relatively weak tensile
strength. At around the same time, Buonocore (1955) investigated the direct bonding
of acrylic blobs to the surface of the teeth of volunteers following acid etching of the
enamel using 85% o-phosphoric acid. Subsequent to this Newman (1964) described
the first direct bonding of metal orthodontic brackets to the enamel using epoxy resin.
Bonding agents
Since the introduction of the acid etch technique a number of different bonding agents
have been used to bond brackets to enamel. The original epoxy resins did not find
favour due to poor aesthetics and their prolonged setting time, which in some cases
was as long as 30 minutes (Retief et al. 1970). Poly(methyl methacrylate) based
adhesives were also tested as potential bonding agents but had the disadvantages of a
high polymerisation shrinkage and a high coefficient of thermal expansion (Newman
et a1.1968), both of which will lead to the formation of internal stresses within the
2
joint and an increased likelihood of in service bond failure. Crabb and Wilson (1971)
suggested the use of a cyanoacrylate adhesive for bonding orthodontic brackets. Such
a material should be ideal since it can undergo anionic addition polymerisation, being
initiated by weak bases such as water, which is found on most surfaces. The enamel
would not require acid etching prior to use and the second bonding substrate, the
bracket base, could be of a simplified design. Also the adhesive is single component
and does not require mixing or the application of an external source of light or heat to
initiate polymerisation. However, it was found to be unsuitable for clinical use due to
the deleterious effects of moisture, which has subsequently been confirmed by
Howells and Jones (1989). Moisture is thought to ingress along the interface between
the adhesive and any metal substrate (Drain et al. 1984), which in turn may lead to
hydrolytic degradation of the cyanoacrylate polymer (Leonard et al. 1966).
Cyanoacrylates have recently been marketed again for orthodontic bonding, and it has
been suggested the enamel is acid etched prior to bonding. This may serve two
purposes, namely improving the bond at the enamel adhesive interface and secondly
slowing the polymerisation process so that the bracket can be accurately positioned on
the tooth surface before polymerisation takes place.
The mainstay of orthodontic bonding in combination with the acid etch technique
have been the diacrylate resins based on Bis-GMA as developed by Bowen (1962)
and which undergo free radical addition polymerisation. The large structure of the
Bis-GMA molecule and the ability to crosslink means a greatly reduced
polymerisation shrinkage and a reduced coefficient of thermal expansion compared
with the acrylates. They are available for use in various guises for clinical use. The
chemical cure adhesives can be either twin paste materials, where there are two filled
resin pastes, one containing initiator and the other the activator, or they may be
supplied as so called "no-mix" adhesives. In the case of the latter, the operator paints
the initiator peroxide, which is within a fluid carrier, onto both the etched enamel
surface and the bracket base. Following this, filled resin containing activator is placed
onto the bracket base. Once the bracket is applied to the tooth surface rapid
polymerisation takes place as the thin sandwich of "initiator/ activator containing
resin/ initiator" is subsequently created. Light cured diacrylate adhesives are also
commonly used in orthodontic bonding having been first described by Tavas and
Watts (1979) and in more recent years they are also available already in place on the
bracket base as so called Adhesive Pre-Coat or APC brackets (3M Unitek, USA).
Glass poly(alkenoate) cements are not only used as orthodontic band cements but
have also been used for direct bonding of brackets to enamel. Cooke (1990) first
described the use of glass poly(alkenoate) cement for the direct bonding of brackets to
anterior teeth. Stated advantages of their use for this purpose include no need for prior
acid etching of the enamel, fluoride release and consequently less decalcification in
treatment (Ashcraft et at. 1997). However, conventional glass poly(alkenoate)
cements have been found to have an unacceptably high rate of bond failure in clinical
practise, ranging from 12.4% (Marcusson et at. 1997) up to as high as 50 % (Miguel
et at. 1995). Resin modified glass poly(alkenoate) cements have more recently been
introduced for orthodontic bonding. Comprising the conventional acid - base reaction
between the acidic polymer and basic glass there is the additional presence of a
polymerisable resin, usually HEMA (hydroxyethyl methacrylate). Silverman et at.
(1995) describe the use of such a material for direct bonding and found a relatively
low bond failure rate of only 3.2% after 8 months. This is particularly impressive
3
when teeth as far back as the second permanent molars were bonded. More recently a
12 month study involving a cross mouth control on incisor and cuspid teeth found a
5% bond failure rate for the resin modified glass poly(alkenoate) compared to 8.3 %
for a conventional composite resin (Fricker 1998).
Enamel surface preparation prior to bonding
A huge amount of literature is available on the variables involved in enamel surface
preparation prior to bonding with both the diacrylates and the resin modified glass
poly(alkenoate) cements. The enamel surface is naturally covered by pellicle, a
proteinaceous coat and prior to acid etching it was usual to use a slurry of pumice in
water and a rubber cup in a slow speed handpiece in order to remove it. However,
work on pumicing as a surface pre-treatment prior to the use of conventional
composite resin and the acid etch technique, has shown it to have no effect on
observed bond failure rates (Barry 1995, Lindauer et al. 1997).
Following the work of Silverstone (1971) it became standard practise to etch the
enamel prior to bonding with a diacrylate bonding agent, using 30 - 40% ophosphoric acid for 60 seconds followed by washing with copious amounts of water
and air drying using an oil free air supply until the enamel assumed a frosty white
appearance. The aim is to provide a roughened surface capable of providing a
mechanical interlock with the bonding agent. Alternative acids such as 10% maleic
acid (Olsen et al. 1997) and nitric acid (Blight and Lynch 1995) have been used in an
attempt to reduce enamel loss on etching or to alter the locus of bond failure in order
to reduce the risk of enamel failure when using ceramic brackets. 0-phosphoric acid
is still the mainstay acid for enamel etching but in order to reduce enamel loss
Cartensen (1993) has suggested that a lower concentration of 2 - 5% may be equally
as effective as the conventional 37%. Shorter etch times of 15 seconds have also been
found to equally as effective as a 60 second etch, but with less overall enamel loss
(Sheen et al. 1995, Barkmeier 1995). Shorter etch times than this are not practical in
the clinical setting. Whether the acid is in the form of a solution or gel, there is no
difference in the etch pattern produced. (Brannstrom et al.1978). However, washing
after etching with a gel should be for twice as long as that for a solution. Wash times
of 15 seconds per quadrant have been recommended for acid solutions (Gwinnett
1982) implying 30 seconds per quadrant for a gel, both of which seem somewhat
excessive in the clinical setting. Five types of etch pattern may be seen on the enamel
surface following the use of o-phosphoric acid, namely at the prism core, prism
boundary, combinations of these two, a pitted surface unrelated to the prisms and
finally a flat smooth surface (Silverstone et al. 1975, Galil and Wright 1979). Once
etching washing and drying of the enamel has taken place it is important the enamel is
not contaminated with saliva. However, recently a moisture insensitive primer has
been introduced (3M Unitek, USA) which can be painted onto slightly moist or saliva
contaminated etched enamel surface and yet still enable an effect bond to be produced
when using a diacrylate bonding agent. As well as containing Bis-GMA it also
contains HEMA and poly(alkenoic acid).
Whenever the acid etch technique is used prior to bonding there will inevitably some
enamel loss. This can be as much as 50 - 60µm. In a farther attempt to reduce this loss
enamel surface crystal growth has been investigated using poly(acrylic acid) and
4
sulphate solution ( Maijer and Smith 1986). However, the technique is time
consuming and there is an increased bond failure rate and so it is not used clinically.
With the resin modified glass poly(alkenoate) cements there is no overall consensus
of opinion as to the most appropriate enamel pre-treatment regimen. The role of
pumicing prior to their use is currently under investigation. If their adhesive mode of
action is by diffusion of poly(alkenoic acid) into the enamel surface (Akinmade and
Nicholson 1993), or the formation of ionic bonds (Wilson et al. 1983, Yoshida et al.
2000) it might be supposed that removal of the enamel pellicle would be important.
The need to acid etch the enamel surface prior to the use of these bonding agents is
not convincing. Silverman et al. (1995) reported a 3.2% bond failure rate when the
enamel was polished but not etched prior to bonding whilst Fricker (1998) also
reported a low bond failure rate of only 5% when the enamel was polished and also
etched using 10% poly(acrylic acid). What does seem important is that the enamel
remains moist prior to placement of the bonding agent otherwise there is likely to be a
very high bond failure rate.
Bonding to tooth surfaces other than enamel.
Adhesion to restorations can be a challenge in orthodontic bonding. Silane coupling
agents can be used with diacrylate bonding agents to bond brackets to porcelain
restorations but the necessity to rebond at some time during treatment is not unusual.
9.6% hydrofluoric acid has been suggested as an etchant treatment for porcelain in
order to provide mechanical rather than the chemical adhesion seen with the silane
(Zachrisson and Buyukyilmaz 1993). But in the same way that toxic organic tin
compounds which have been suggested to promote bonding to gold crowns its use
should be confined to the dental laboratory. Such a highly corrosive acid cannot be
recommended for intraoral use. It would be more sensible to accept the need to band
the tooth in such situations where the silane coupling agent was found to be
unsatisfactory. The intraoral sandblaster has also been suggested as a means of
providing a micro mechanical interlock with gold and amalgam restorations with
some success. Once again a simple orthodontic band will often be more effective.
Orthodontic Brackets
Orthodontic brackets can be metallic, plastic or ceramic. Metallic brackets can be
made from various grades of stainless steel, titanium, cobalt chromium and even gold.
Whatever the metal and whether they are cast or milled the adhesion with the bonding
agent is micro mechanical in nature. This may be in the form of a system of slots or
grooves, or via gauze mesh which has been brazed or welded to the bracket base.
Metal brackets are available with a polymeric coating already in place on the bracket
base, which is supposed to afford chemical bonding with the bonding agent.
Diacrylate adhesives containing adhesion promoters such as 4-META and phosphate
esters (10-MDP) have been tested with metal brackets and with limited success
(Ireland and Sherriff 1994). Following water immersion the effect of the adhesion
promoter was lost and mechanical adhesion was still the principal mechanism.
5
Chemical adhesion can be seen between some diacrylate bonding agents and plastic
brackets. Such brackets are usually made from polycarbonate or filled polyurethane.
However, such brackets are not poplar due to poor mechanical properties, particularly
their inability to effectively apply torqueing forces to teeth.
Ceramic brackets are popular particularly amongst adult patients as well as some
orthodontists. Certainly they do not suffer from the mechanical failings seen with
plastic brackets. The first ceramic brackets, introduced some 15 years ago, were
smooth based brackets that relied on silane coating to provide a chemical union
between the bracket base and a diacrylate bonding agent. However, it was noted by
several operators that bracket removal at completion of treatment was difficult and
occasionally lead to enamel failure (Winchester 1991, Gibbs 1992). Such failure is
more likely to occur with silane coated bases than ceramic bracket that rely on
mechanical adhesion (Redd and Shivapuja 1991). Failure is thought to occur due to
the high fracture toughness of the ceramic, the thin uniform film thickness of the
bonding agent between the bracket base and enamel surface (Winchester 1991) and
the likely presence of infraction lines within the enamel surface from where crack
propagation may proceed. In order to overcome this risk of enamel failure many of
today's ceramic brackets rely on mechanical interlock with the bonding agent. In
addition, both a polycarbonate based ceramic bracket (Fox and McCabe 1992) and
latterly an epoxy mesh based ceramic bracket have been marketed in order to
facilitate safe debonding. Other manufacturers have provided brackets with a notched
base, which will act as a stress raiser and promote failure at the bracket base rather
than at the enamel surface. The role of the resin modified glass poly(alkenoate)
cement with ceramic brackets as a means of reducing the risk of enamel failure at
debond has not been fully explored.
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