931
Influence of cement layer thickness on the adhesive bond strength of polyalkenoate cements A.O. Akinmade and R.G. Hill School of Materials Science and Physics, Thames Polytechnic, Wellington Street, London, SE18 6PF, UK The fracture toughness and yield stress values of model zinc polycarboxylate and glass polyalkenoate cements have bean used to calculate plastic zone sizes. The size of the plastic zone at the crack tip in these materials has been used to predict whether cement layer thickness is likely to influence the adhesive bond strength. In the model zinc polycarboxylate cement studied, the plastic zone size was comparable to the cement layer thickness and had a pronounced influence on the shear bond strengths obtained. In contrast, the plastic zone sizes obtained for the glass polyalkenoate cements were much smaller and the shear bond strengths were found to be much less dependent on cement layer thickness. Keywords:
Dental materials,
polyalkenoate
cements,
adhesive
bond strength
Received 27 February 1992; revised 30 March 1992; accepted 24 April 1992
Glass polyalkenoate and zinc polycarboxylate cements are both widely used as adhesives in dentistry. These two cements based on poly(acrylic acid) are the only materials currently available that are capable of chemically bonding to dentine and enamel. Furthermore, unlike resin-based restorative materials they are hydrophilic and can wet dentine and enamel surfaces, which is an important prerequisite of any adhesive or dental material. Satisfactory adhesion is important in the placement of crowns, fixation of orthodontic brackets and retention of filling materials. Not only is adhesion important for the mechanical integrity of a joint or restoration but also adhesion to the tooth material ensures an hermetic seal. This helps prevent percolation of bacteria and fluids beneath a filling and thereby reduces the likelihood of secondary caries and pulp irritation. Numerous studies of the adhesion of glass polyalkenoate cements and zinc polycarboxylate cements to dentine, enamel and stainless steel alloys have been Despite these studies, our understanding undertaken’-33. of the adhesion of these cements is limited. Widely different values for the adhesive bond strengths of these materials have been obtained using a variety of test techniques by many groups of workers. Indeed, the experimental scatter with most test techniques is extremely large and many workers have failed to obtain meaningful results. The situation is even worse when it comes to the choice of dentine and enamel surface preparation techniques before bonding and controversy is rife in this important area18-33.
Of the previously published adhesion studies which used a thin film test configuration with polyalkenoate cements, only oneI has investigated adhesive bond strength as a function of the cement layer thickness. The present paper uses linear elastic fracture mechanics (LEFM) stress analysis to predict if the cement layer thickness is likely to influence the adhesive bond strength values obtained. The shear bond strengths of stainless steel lap joints bonded together with polyalkenoate cements are measured as a function of the adhesive layer thickness and compared with the predictions from the LEFM data. Stainless steel was chosen as the substrate for convenience. Providing cohesive failure occurs in the cement layer, similar results to those presented later should be obtained with dentine and enamel. The approach in this paper is identical to that used to characterize structural adhesives in the aerospace industry34.
Correspondence to Dr R.G. Hill,
where KI, is the fracture stress.
Materials
Science,
University
0 1992 Butterworth-Heinemann 0142-9612/92/130931-06
presently at Department of Limerick, Limerick, Ireland.
Ltd
of
THEORY When cohesive failure occurs in an adhesive joint, the adhesive material at the crack tip will deform plastically, as it would do in the bulk material. A plastic zone of radius R, will form as shown in Figure 2. The plastic zone radius for plane stress conditions3 is given by: R
=
,f,
1
K,c
2
(11
[ OY
toughness
Biomaterials
and cry is the yield
1992, Vol. 13 No. 13
932
Bond strenath
in Dolyalkenoate
I
Figure 2
0 +
Figure 1
*
2R
P
-
A plastic zone at a crack tip.
The size of the plastic zone present in the bulk materials in relation to the adhesive layer thickness will determine whether the material will fracture in a brittle or ductile manner. Brittle fracture will occur when the adhesive layer thickness is greater than zR,, while ductile fracture occurs when the adhesive layer thickness is less than Z?,. Clearly in the latter case the plastic zone size in the adhesive layer cannot be larger than the adhesive layer thickness. In such circumstances, the plastic zone will reduce to the size of the adhesive layer thickness, h,. A plastic zone is shown schematically in an adhesive layer in Figure 2. Since the toughness arises largely from the energy dissipated in forming the plastic zone, the adhesive fracture surface energy will decrease as the adhesive layer thickness (h,) is reduced past a certain critical value. A high-modulus substrate will impose a constraint on the adhesive and will tend to increase the stresses ahead of the crack tip compared with the homogeneous case as shown in Figure 3. This effect will tend to increase the length of the plastic zone (Z&J ahead of the crack tip. The degree of constraint will be greater with thinner adhesive layers and high-moduli substrates. These two factors combine to give a maximum in the adhesive fracture surface energy against adhesive layer thickness plot. The maximum adhesive fracture surface energy occurs when the adhesive layer thickness and plastic zone diameters are approximately equal:
The fracture toughness Ktc has been determined previously for some glass polyalkenoate3” and zinc carboxylate cements3?. However, values for the yield stress or for these materials are not available.
MATERIALS AND METHODS Cement powders An ionomer glass Chemfil was supplied by Dentsply (Weybridge, UK). This glass had the composition given in Biomaterials 1992, Vol. 13 No. 13
A.O. Akinmade and R.G. Hill
cements:
A plastic zone in an adhesive layer.
il ~ tl
Increasing
T
\
*
2 Rp
constraint
Adhesive
(bulk)-+
joints
Distance
r
Lt
2 Rp
(joints)
Figure 3 The influence of increasing the constraining stresses and plastic zone sizes ahead of crack tips.
on the
Table 3 and had a maximum
particle size below 45 ,em. The zinc oxide used for the zinc polycarboxylate cement, was PolyF base also supplied by Dentsply.
Pol~ac~lic
acid)s
The poly(acrylic acidjs were supplied by Allied Colloids (Bradford, UK) and are given the code letters E5, E7, E9 and El1 in order of increasing molecular weight. All the poly(acrylic acidfs were supplied as ~5% solutions, which were freeze-dried and then ground in a vibratory mill. The fractions that passed through a 45pm sieve were used in the preparation of the cements. Table 2 gives the polyethylene oxide equivalent values for the number average (a,,) and weight average (&&,,) molar masses of the poly(acrylic acid)s.
Preparation of the cements The base cement powders were mixed thoroughly with 1% by weight of inert glass microspheres obtained from Croxton and Carry (Dorking, UK). The glass spheres had previously been sieved into the six fractions given in Table 3. Incorporation of the glass microspheres into the cements enabled cement layers of defined thickness to be produced. The cement powders (5.00 g) containing the glass microspheres were then blended with the freezedried poly(acrylic acid) (0.70 g). This blend (1.10 g) was mixed with distilled water (0.30 g] to produce cements. In
Bond strength
Table 1
in polyalkenoate
A.O. Akinmade
cements:
and R.G. Hill
on polyalkenoate cements have not pursued the influence of joint geometry. The surfaces to be bonded were flat and were polished with 120 grit silicon carbide paper followed by cleaning in trichloroethylene and then acetone before bonding. The time at which the two halves of the lap joints were bonded together was found to be critical to obtaining successful adhesion. The bonding operation was carried out at 30 f 5 s after the start of mixing the cement. The two halves of the lap joints were squeezed together using a G clamp and excess cement eliminated. After setting and about 5 min from the start of mixing, the G clamps were removed and the specimen stored for 1 h at 37 +_ 1°C and 100% relative humidity, followed by a further period in a water bath before testing. The zinc polycarboxylate cements were tested at 19 IL 2°C after storage for 2 d at 37 f 1°C. The glass polyalkenoate cements were then tested at 19 f 2°C after 1 and 24 h in water at 37” + 2°C. The testing was performed in air immediately after removal of the specimens from the water bath, using a Monsanto Tensometer at a speed of 1 mm mine1 . The adhesive layer thickness was obtained by measuring the thickness of the joints in the absence and in the presence of the cement using a micrometer accurate to + l,~~rn. Test temperatures were chosen to enable previous fracture toughness values to be used. The results are unlikely to be influenced dramatically on changing the test temperature from 19 to 37’C.
Glass composition
Component
Weight
Si Al Ca F Na P
12.39 16.44 7.14 10.40 7.26 4.54
Table 2
Molar chromatography
mass details determined of poly(acrylic acid)s
Code
Batch number
it?,
E5 E7 E9 El1
159 AVS 228 220 415
7.30 1.13 4.04 1.12
Table 3
Glass layer thickness
sphere
fractions
percent
by gel permeation
Mw x x x x
1.15 2.27 1.14 3.83
lo3 lo4 lo4 lo5
x X x X
lo4 lo4 lo5 lo5
used to vary the adhesive
45,um 75pm 106pm 150pm 212pm 300pm
I:; $1 (5) (6)
> > > > > >
X > 38pm X > 45pm X > 75pm x > 106pm X > 150pm X > 212pm
the case of glass polyalkenoate cements, 10% m/m (+) tartaric acid was added to the water to retard setting.
Compression
tests
The compression specimens were produced in accordance with BS6039:19813’ and BS6814:19873g and tested in water at 0.1 mm min-l using a Mayes 30 KN testing machine (Windsor, UK]. A 0.1% offset yield stress was calculated from the load-deflection traces and used in conjunction with fracture toughness values to calculate plastic zone sizes. The zinc polycarboxylate cements were tested at 19 + 1°C after storage in water at 37 + 1°C for 48 h. The glass polyalkenoate cements were tested at 37 f 1°C after storage in water at 37 ? 2°C for 24 h.
Adhesive
bond strength
tests
Single lap joints consisting of 3.3 mm thick strips of 18/8 stainless steel with overlap length 40 mm and width 30 mm were used to measure the shear bond strengths meeting the criteria of ASTM D100240. The overlap length to adhered thickness ratio is very important in determining the stresses generated in single lap joints and the lap joint dimensions. Previous studies in the literature Table 4
The influence
Polyacrylic Code
nw
E5 E7 E9 El1
1.15 2.27 1.14 3.83
acid
x lo4 X lo4 x lo5 x lo5
of poly(acrylic
RESULTS Tables 4 and 5 show the fracture toughness (K,,), toughness [GIc), flexural strength (or), Young’s modulus (E) for zinc polycarboxylate and glass polyalkenoate cements as a function of the poly(acrylic acid) chain length or molar mass. The important conclusions previously drawn 36,37 from these data are: 1. The Young’s modulus
is independent of the poly(acrylic acid) chain length and remains approximately constant. This indicates that the chemistry of the setting reaction of the cement is unchanged and that only the physics of the failure process is being studied. The exception to this general conclusion is the lowest molar mass zinc polycarboxylate cement that exhibits a significantly lower Young’s modulus which is thought to be a result of a free volume effect. 2. The toughness, or strain energy release rate (G,) increases with the poly(acrylic acid) chain length for both types of cement, as expected qualitatively with a chain pull-out model for fracture. However, the magnitude of the increase in the toughness is not as large as that predicted by the reptation chain pull out
acid) molar mass on the fracture
K,c (MNm-3’z)
p=
0.21 0.46 0.61 0.80
0.02 0.08 0.17 0.12
5)
933
toughness
of zinc polycarboxylate
cements
GI, (Jmm2)
Of (MNm-‘)
SD (n = 5)
&Nmm2)
sR= 5)
10 31 67 115
4.65 7.13 10.74 11.08
0.28 1.26 0.36 1.14
4439 6611 5622 5580
372 675 429 520
Biomaterials 1992,
Vol. 13 No. 13
934
Bond strength
Table 5
The influence of poly(acrylic
Polyacrylic acid Code
mw
E5 E7 E9 El1
1.15 2.27 1.14 3.83
x X x X
lo4 lo4 lo5 lo5
K,, (MNmm3’*)
r=
0.13 0.16 0.23 0.26
0.01 0.01 0.02 0.02
5)
A.O. Akinmade
and R.G. Hill
cements
GI, (Jm-‘)
(T’ (MNm-*)
SD (n = 5)
E (MNmm2)
SD (n = 5)
10 15 30 39
7.06 8.05 9.99 10.30
1.05 0.61 0.85 1.66
1750 1754 1487 1242
314 374 173 259
The 0.1% offset yield stress values of 48 h old zinc polyalkenoate cements are shown in Table 6. The values for the three highest molar mass cements are approximately constant and significantly higher than the lowest molar mass cement. In the previous study36 the Young’s modulus for the lowest molar mass cement was significantly lower than for the higher molar mass cements and this was thought to be a result of a free volume effect reducing the glass transition temperature of the polysalt matrix. It is thought that a similar effect occurs with the yield stress values. The plastic zone diameters of these cements were calculated using Equation z and the fracture toughness data in Table 4. The calculated plastic zone size increases from approximately 40 pm with the lowest molar mass cement to approximately 290 pm with the highest molar mass cement. These plastic zone sizes are larger than or comparable with the adhesive layer thicknesses found with jacket crowns and are likely significantly to influence adhesive bond strengths. Clearly, as the temperature is raised from room temperature to mouth temperature, the plastic zone diameter would be expected to increase. The 0.1% offset yield stresses for the glass polyalkenoate cements are shown in Table 7. Note that in order to be able to use the previous fracture toughness values these tests have been carried out at 37’C. Here the yield stress increases appreciably with the poly(acrylic acid) chain length. The plastic zone diameters of these cements were again calculated using Equation z and the appropriate fracture toughness values from Table 5. The plastic zone diameters are also shown in Table 7. The plastic zone sizes are much smaller than for the zinc polycarboxylate cements and they increase slightly with the poly(acrylic acid] molecular weight. Because the calculated plastic zone sizes are comparable with the glass particle size present in these cements, the calculations are invalid but clearly the plastic zone sizes obtained here are much smaller than can be achieved in practical adhesive layers. 13 No. 13
cements:
acid) molar mass on the fracture toughness of glass polyalkenoate
Table6 Yield stress values and calculated diameters for zinc polycarboxylate cements
model. GIaN2 where N is the degree of polymerization, which is proportional to the chain length. 3. The rate of increase of toughness with poly(acrylic acid) chain length is significantly greater for the zinc polycarboxylate cements than the glass polyalkenoate cements. This is thought to be a result of the weaker ionic linkages between the poly(acrylic acid) chains in the zinc polycarboxylate cements giving rise to a more thermoplastic character. The thermoplastic character of glass polyalkenoate cements and zinc polycarboxylate cements has been established previously4* using dynamic mechanical and dielectric thermal analysis techniques.
Biomaterials 1992. Vol.
in polyalkenoate
0.1% Yield stress uy
1.15 2.27 1.14 3.83
x X x X
lo4 lo4 lo5 lo5
MPa
SD(n = 6)
18.93 30.99 28.54 26.58
0.95 1.94 0.75 0.77
39.9 70.3 145.7 290.9
0.1% Yield stress uy
x X x X
lo4 lo4 105 lo5
zone
Plastic zone diameter 2R, (pm)
Table 7 Yield stress values and calculated diameters for glass polyalkenoate cements
1.15 2.27 1.14 3.83
plastic
plastic
zone
MPa
SD(n = 8)
Plastic zone diameter 2R, (pm)
30.56 35.75 45.23 46.99
2.32 1.74 1.23 1.50
5.7 6.4 8.2 10.5
In the final part of the study the adhesive shear bond strengths of zinc polycarboxylate and glass polyalkenoate cement bonded stainless steel lap joints were measured. All the joints failed cohesively in the cement layer. The data for the zinc polycarboxylate cements are presented in Figure 4. There is clearly a significant influence of adhesive layer thickness on the shear bond strength of the joint. The most obvious feature of the plots in Figure 4 is the pronounced maxima in the shear bond strengths of the E9 molar mass cement. The lowest molar mass cement tested, with a weight average molar mass of 1.15 X lo4 shows a gradual increase in shear bond strength from 0.45 MPa at an adhesive layer thickness of approximately
T
3.2
4 5
2.4
F f ;;
1.6
-F 0"
0.8
?Y 6 0
60
120 Adhesive
180
240
layer thickness (urn)
Figure 4 The effect of adhesive layer thickness on the shear bond strengths of zinc polycarboxylate cements.
Bond strength
in polyalkenoate
cements:
A.O.
Akinmade
185
flrn whilst to 1.11 MPa where the thinnest adhesive layer obtainable was reached at 40,um. Figure 4 shows that the adhesive layer thickness has a dramatic effect on the observed shear bond strengths of the ES-based cement. These cements had a shear bond strength of 0.76 MPa for an adhesive layer thickness of 20pm. The shear bond strength rises sharply to a maximum of 2.43 MPa at a film thickness of approximately 62 pm. Indeed from adhesive layer thicknesses of 40 to 80 pm the shear bond strength is high and relatively constant. Above 80pm the shear bond strengths drops gradually to a value of 1.17 MPa at approximately 200 pm. The E9 cement has a molecular weight similar to that used commercially in zinc polycarboxylate cements and the proportions of polyacid and water content are also similar, thus this cement typifies commercially available materials. The adverse effects of not using the optimum adhesive layer thickness when working with this cement can be readily appreciated. A dentist using a cement layer thickness of say 180pm instead of 60pm, will obtain a joint, with only about half the optimum shear bond strength. The 48 h old El1 zinc polycarboxylate cement with an MW of 3.83 X lo5 again shows a pronounced adhesive layer thickness effect on shear bond strengths. A shear bond strength of 1.47 MPa was recorded for the thinnest adhesive layer tested of 20 pm. The shear bond strength rises to a maxima of 2.65 MPa for the largest adhesive layer tested of approximately 204 pm. It is envisaged that the shear bond strengths of the El1 and E5 cements will go through maxima in a similar fashion to that shown by the E9 cement: this is shown on Figure 4 by the dashed lines. The plastic zone sizes for the E5, E9 and El1 cements are also shown on Figure 4 by the solid lines. The approximate agreement between the plastic zone diameters and the maxima in the shear bond strengths is reassuring. In general, plastic zone sizes formed at crack tips in constrained layers between inflexible substrates are significantly smaller than in the bulk material and this is supported by the present data. Figure 5 shows the effect of adhesive layer thickness on shear bond strengths of an E7 glass polyalkenoate cement. The results shown are for cements tested after 1 h; similar results were found for cements tested after 24 h. It is observed that there is a gradual increase in the shear bond strength of the glass polyalkenoate cement
40 Adhesive
120
80 layer
thickness
(Pm)
Figure 5 The effect of adhesive layer thickness on the shear bond strengths of glass polyalkenoate cements.
and
R.G.
935
Hill
from 1.15 MPa at the thickest layers tested at between 110 and 130 pm to 1.50 MPa at 37 pm. In all probability the shear bond strength of the glass polyalkenoate cements would have increased further if a cement layer thinner than 37pm could have been obtained. It is envisaged that the shear bond strength would rise to a maximum of about 9 pm, which is the calculated plastic zone size of this cement. Studies were not performed with the higher molar mass cements because of the unlikelihood of being able to observe an adhesive layer effect on shear bond strength with layer thickness greater than 37 pm. DISCUSSION The adhesive bond strength of zinc polyalkenoate cements to stainless steel substrates is markedly influenced by the cement layer thickness as predicted by linear elastic fracture mechanics. At mouth temperatures the plastic zone size at the crack tip in zinc polycarboxylate cements would be expected to increase and an even more marked influence of cement layer thickness on adhesive bond strength would be expected. The adhesive bond strengths of the glass polyalkenoate cement bonded lap joints was not appreciably influenced by the adhesive layer thickness, which was also predicted by the fracture mechanics calculations of plastic zone sizes. However, the model glass polyalkenoate cements tested may well not be typical. Indeed for luting type glass polyalkenoate cements, the glass to water ratio and the poly(acrylic acid) concentration will be lower than for the cements tested here and both these factors will serve to promote greater plasticity at the crack tip and may well make the adhesive bond strength dependent on the adhesive layer thickness. Some evidence for this is present in the paper by Oilo and Euje17 who found that the bond strength of the luting type glass polyalkenoate cement to dentin was dependent on film thickness. A comprehensive study looking at both commercially available polyalkenoate cements and model cements is required in order to improve our understanding of this very important area.Nearly all the adhesive bond strength values published to date on polyalkenoate cements have neglected cement layer thickness and this may account for much of the experimental scatter and lack of consensus in this important field. Clearly, future studies on adhesion of glass polyalkenoate cements and zinc polycarboxylate cements should take account of cement layer thickness. This study indicates that a lot of the published literature on the adhesion of polyalkenoate cements may be of very limited value. A wide variety of techniques and test methods has been used to determine adhesion of these cements and most authors have failed to give any information on the thickness of the cement layers used in their tests. If we are to advance our understanding of this topic, it is important for research workers to develop and use a standard test method based on fundamental principles. The results presented here also have implications for the bonding of crowns, an application for which zinc polycarboxylate cements are widely used. The prevailing wisdom appears to be to get the crown to fit as tightly as possible on to the remaining tooth material. Clearly, from Biomaterials
1992, Vol. 13 No. 13
Bond strength
936
the results presented here, the retention of a good fitting crown could be worse than a poorer fitting crown, especially when a zinc polycarboxylate luting cement is used as the optimum film thickness appears to be much greater. A further point of interest is that the British and IS0 Standards for luting cements specify being able to obtain a film thickness of 25 ,nm, which if achieved in clinical practice with zinc polycarboxylate cements, would almost certainly result in a reduction in adhesive bond strength.
REFERENCES 1
2
3
4
5 6
Beech, D.R., Improvement in the adhesion of polyacrylate cements to human dentine, Br, Dent. J. 1973, 135, 442 Hotz, P., McLean, J.W., Seed, I. and Wilson, A.D., The bonding of glass ionomer cements to metal and tooth substrates, Br. Dent. J. 1977, 142, 41-47 Levine, R.S., Beech, D.R. and Garton, B., Improving the bond strength of polyacrylate cement to dentine. A rapid technique, Br. Dent. J. 1977, 143, 275-277 Negm, M.M., Combe, E.C. and Grant, A.A., Factors affecting the adhesion of polycarboxylate cement to enamel and dentine, J. Prosthet. Dent. 1982, 45, 405 Oilo, G., Bond strength of new ionomer cements to dentine, Stand. J. Dent. Res. 1981, 89, 344-347 Peddey, M. and Meech, D.R., The bond strength of polycarboxylic acid cements to dentine: effect of surface modification and time after extraction, Aust. Dent. J. 1981, 28, 178
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Vougiouklakis, G., Smith, D.C. and Lipton, S., Evaluation of the bonding of cervical restorative materials, J, Oral Rehabil. 1982, 9, 231 Aboush, Y.E.Y. and Jenkins, C.B.G., The effect of postextraction storage on the adhesion of glass ionomers to dentine, J. Dent. Res. 1983, 81, 441 Aboush, Y.E.Y. and Jenkins, C.B.G., Factors affecting the tensile bond strength of glass ionomer restoratives to dentine, J. Dent. Res. 1984, 83, 511 NcComb, D., Retention of castings with glass ionomer cement, J. Prosfhet. Dent. 1982, 48, 285-288 Finger, W., Evaluation of glass-ionomer luting cements, Stand. J. Dent. Res. 1983, 91,143-149 Oilo, G. and Jorgensen, K.D., Influence of surface roughness on the retentive ability of two dental luting cements, J. Oral Rehabil. 1978, 5, 377-389 Dahl, B.L. and Oilo, G., Retentive properties of luting cements: an in vitro investigation, Dent. Mater. 1986, 2,
in polyalkenoate
Thornton, J.B., Retief, D.H. and Bradley, E.L., Fluoride release from and tensile bond strength of Ketac Fil and Ketac Silver to enamel and dentine, Dent. Mater. 1986,2, Lizuka, H., Brauer, G.M., Rupp, N.B., Ohashi, M. and Paffenberger, G., Forces fracturing cements at die interfaces and their dependence on film thickness, Dent. Mater.
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Causton, B.E. and Johnson, N.W., The role of diffusable ionic species in the bonding of polycarboxylate cements to dentine. An in vitra study, J. Dent. Res. 1979, 58,
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Shalabi, H.S. and Amussen Jorgensen, K.D., Increased bonding of GIC to dentine by means of FeCl,, Stand. J. Dent. Res. 1981, 898,348-353 Powis, D.R., Folleras, T., Merson, S.A. and Wilson, A.D., Improved adhesion of glass-ionomer cements to dentine and enamel, J. Dent. Res. 1982, 81,1416-1422 Causton, B.E. and Johnson, N.W., Improvement of polycarboxylate adhesion to dentine by use of a calcifying solution. An in vitro study, Br. Dent. J. 1982, 52, 911
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Beech, D.R., Solomon, A. and Bernier, R., Bond strength of polycarboxylate acid cements to treated dentine, Dent. Mater. 1985, 1,154-157 Garcia-Godoy, F. and Malone, F.P., The effect of acid etching on two glass-ionomer lining cements, Quintess. Znt. 1986, 17, 617-619 Long, T.E., Duke, E. and Norling, B.K., Polyacrylic acid cleaning of dentine and glass-ionomer bond strength, J. Dent. Res. 1986, 65 Special Issue, AADR Abstracts, 345, No. 1583
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Andrews, S.B., Hermann, H.O., Liquid versus gel etchants on glass-ionomer effects on bond strengths, J, Dent. Res. 1986, 65 Special Issue, IADR Abstracts, No. 100 Aboush, Y.E.Y. and Jenkins, C.B.G., The effect of poly(acrylic acid) cleanser on the adhesion of a glass polyalkenoate cement to enamel and dentine, J. Dent. 1987, 15, 127-152
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White, G., Beech, D.R. and Tyas, M.N., Dentine pretreatment and the bond strength of glass-ionomer and gluma, J. Dent. Res. 1987, 88, 328, Abstract 91 McInness-Ledoux, P.M., Ledoux, W.R. and Weinberg, R., Bond treatment of dentinal bonding agents to chemomechanically prepared dentin, Dent. Mater. 1987, 3, 331-336
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McInness-Ledoux, P.M., Weinberg, R. and Grogono, A., Bonding glass-ionomer cements to chemomechanically prepared dentin, Dent. Mater. 1989, 5, 189-193 Prati, C., Nucci, C. and Montanan, G., Effect of acid and cleansing agents on shear bond strength and marginal leakage of glass-ionomer cements, Dent. Mater. 1989, 5, 260-265
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Kinloch, A.J., Adhesion and Adhesives, Chapman and Hall, London, UK, 1987 Knott, J.F., Fundamentals of Fracture Mechanics, 3rd Edn, Butterworths, London, UK, 1979 Hill, R.G., Warrens, C.P. and Wilson, A.D., The influence of poly(acrylic acid) molecular weight on the fracture of glass ionomer cements, J. Mater. Sci. 1989, 24, 363-371
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Cook, P.A. and Youngson, CC., An in vitro study of bond strength of GIC in the direct bonding of orthodontic brackets, Br. J, Orthod. 1988, 15, 247-253 Oilo, G. and Euje, D.M., A bond test for measuring cement-dentine bond, Dent. Mater. 1988, 4, 98-102 Maniatopoulos, C., Pilliar, R.M. and Smith, DC., Evaluation of shear strength at the cement endodontic post interface, Prosthet. Dent. 1988, 59, 662-668 Smith, D.C., Ruse, D.N. and Zuccolin, D., Some characteristics of glass-ionomer lining materials, Can. Dent.
and R.G. Hill
Causton, B.E., Samara-Wickrama, D.Y.D. and Johnson, N.W., Effect of calcifying fluid on the bonding of cement and composites to dentine in vitra, Br. Dent. J. 1976,140,
241-245
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cements:
38 39 40
41
Hill, R.G. and Labok, S., The influence of poly(acrylic acid] molecular weight on the fraction of polycarboxylate cements, J. Mater. Sci. 1991, 26, 67-74 BSI-BS6039: 1981, Specification for dental glass-ionomer cements BSI-BS6814: 1987 IS0 4104-1984, British Standard specification for dental zinc polycarboxylate cements ASTM Designation: D1002-72 (Reapproval1983), Standard test method for strength properties of adhesives in shear by tension loading [metal to metal) [Annual Book of ASTM Standards Vol. 03-01) Hill, R.G., Relaxation spectroscopy of polyalkenoate cements, J. Mater. Sci. Lett. 1989, 8, 1043-1047
Influence of cement layer thickness on the adhesive bond strength of polyalkenoate cements A.O. Akinmade and R.G. Hill School of Materials Science and Physics, Thames Polytechnic, Wellington Street, London, SE18 6PF, UK The fracture toughness and yield stress values of model zinc polycarboxylate and glass polyalkenoate cements have bean used to calculate plastic zone sizes. The size of the plastic zone at the crack tip in these materials has been used to predict whether cement layer thickness is likely to influence the adhesive bond strength. In the model zinc polycarboxylate cement studied, the plastic zone size was comparable to the cement layer thickness and had a pronounced influence on the shear bond strengths obtained. In contrast, the plastic zone sizes obtained for the glass polyalkenoate cements were much smaller and the shear bond strengths were found to be much less dependent on cement layer thickness. Keywords:
Dental materials,
polyalkenoate
cements,
adhesive
bond strength
Received 27 February 1992; revised 30 March 1992; accepted 24 April 1992
Glass polyalkenoate and zinc polycarboxylate cements are both widely used as adhesives in dentistry. These two cements based on poly(acrylic acid) are the only materials currently available that are capable of chemically bonding to dentine and enamel. Furthermore, unlike resin-based restorative materials they are hydrophilic and can wet dentine and enamel surfaces, which is an important prerequisite of any adhesive or dental material. Satisfactory adhesion is important in the placement of crowns, fixation of orthodontic brackets and retention of filling materials. Not only is adhesion important for the mechanical integrity of a joint or restoration but also adhesion to the tooth material ensures an hermetic seal. This helps prevent percolation of bacteria and fluids beneath a filling and thereby reduces the likelihood of secondary caries and pulp irritation. Numerous studies of the adhesion of glass polyalkenoate cements and zinc polycarboxylate cements to dentine, enamel and stainless steel alloys have been Despite these studies, our understanding undertaken’-33. of the adhesion of these cements is limited. Widely different values for the adhesive bond strengths of these materials have been obtained using a variety of test techniques by many groups of workers. Indeed, the experimental scatter with most test techniques is extremely large and many workers have failed to obtain meaningful results. The situation is even worse when it comes to the choice of dentine and enamel surface preparation techniques before bonding and controversy is rife in this important area18-33.
Of the previously published adhesion studies which used a thin film test configuration with polyalkenoate cements, only oneI has investigated adhesive bond strength as a function of the cement layer thickness. The present paper uses linear elastic fracture mechanics (LEFM) stress analysis to predict if the cement layer thickness is likely to influence the adhesive bond strength values obtained. The shear bond strengths of stainless steel lap joints bonded together with polyalkenoate cements are measured as a function of the adhesive layer thickness and compared with the predictions from the LEFM data. Stainless steel was chosen as the substrate for convenience. Providing cohesive failure occurs in the cement layer, similar results to those presented later should be obtained with dentine and enamel. The approach in this paper is identical to that used to characterize structural adhesives in the aerospace industry34.
Correspondence to Dr R.G. Hill,
where KI, is the fracture stress.
Materials
Science,
University
0 1992 Butterworth-Heinemann 0142-9612/92/130931-06
presently at Department of Limerick, Limerick, Ireland.
Ltd
of
THEORY When cohesive failure occurs in an adhesive joint, the adhesive material at the crack tip will deform plastically, as it would do in the bulk material. A plastic zone of radius R, will form as shown in Figure 2. The plastic zone radius for plane stress conditions3 is given by: R
=
,f,
1
K,c
2
(11
[ OY
toughness
Biomaterials
and cry is the yield
1992, Vol. 13 No. 13
932
Bond strenath
in Dolyalkenoate
I
Figure 2
0 +
Figure 1
*
2R
P
-
A plastic zone at a crack tip.
The size of the plastic zone present in the bulk materials in relation to the adhesive layer thickness will determine whether the material will fracture in a brittle or ductile manner. Brittle fracture will occur when the adhesive layer thickness is greater than zR,, while ductile fracture occurs when the adhesive layer thickness is less than Z?,. Clearly in the latter case the plastic zone size in the adhesive layer cannot be larger than the adhesive layer thickness. In such circumstances, the plastic zone will reduce to the size of the adhesive layer thickness, h,. A plastic zone is shown schematically in an adhesive layer in Figure 2. Since the toughness arises largely from the energy dissipated in forming the plastic zone, the adhesive fracture surface energy will decrease as the adhesive layer thickness (h,) is reduced past a certain critical value. A high-modulus substrate will impose a constraint on the adhesive and will tend to increase the stresses ahead of the crack tip compared with the homogeneous case as shown in Figure 3. This effect will tend to increase the length of the plastic zone (Z&J ahead of the crack tip. The degree of constraint will be greater with thinner adhesive layers and high-moduli substrates. These two factors combine to give a maximum in the adhesive fracture surface energy against adhesive layer thickness plot. The maximum adhesive fracture surface energy occurs when the adhesive layer thickness and plastic zone diameters are approximately equal:
The fracture toughness Ktc has been determined previously for some glass polyalkenoate3” and zinc carboxylate cements3?. However, values for the yield stress or for these materials are not available.
MATERIALS AND METHODS Cement powders An ionomer glass Chemfil was supplied by Dentsply (Weybridge, UK). This glass had the composition given in Biomaterials 1992, Vol. 13 No. 13
A.O. Akinmade and R.G. Hill
cements:
A plastic zone in an adhesive layer.
il ~ tl
Increasing
T
\
*
2 Rp
constraint
Adhesive
(bulk)-+
joints
Distance
r
Lt
2 Rp
(joints)
Figure 3 The influence of increasing the constraining stresses and plastic zone sizes ahead of crack tips.
on the
Table 3 and had a maximum
particle size below 45 ,em. The zinc oxide used for the zinc polycarboxylate cement, was PolyF base also supplied by Dentsply.
Pol~ac~lic
acid)s
The poly(acrylic acidjs were supplied by Allied Colloids (Bradford, UK) and are given the code letters E5, E7, E9 and El1 in order of increasing molecular weight. All the poly(acrylic acidfs were supplied as ~5% solutions, which were freeze-dried and then ground in a vibratory mill. The fractions that passed through a 45pm sieve were used in the preparation of the cements. Table 2 gives the polyethylene oxide equivalent values for the number average (a,,) and weight average (&&,,) molar masses of the poly(acrylic acid)s.
Preparation of the cements The base cement powders were mixed thoroughly with 1% by weight of inert glass microspheres obtained from Croxton and Carry (Dorking, UK). The glass spheres had previously been sieved into the six fractions given in Table 3. Incorporation of the glass microspheres into the cements enabled cement layers of defined thickness to be produced. The cement powders (5.00 g) containing the glass microspheres were then blended with the freezedried poly(acrylic acid) (0.70 g). This blend (1.10 g) was mixed with distilled water (0.30 g] to produce cements. In
Bond strength
Table 1
in polyalkenoate
A.O. Akinmade
cements:
and R.G. Hill
on polyalkenoate cements have not pursued the influence of joint geometry. The surfaces to be bonded were flat and were polished with 120 grit silicon carbide paper followed by cleaning in trichloroethylene and then acetone before bonding. The time at which the two halves of the lap joints were bonded together was found to be critical to obtaining successful adhesion. The bonding operation was carried out at 30 f 5 s after the start of mixing the cement. The two halves of the lap joints were squeezed together using a G clamp and excess cement eliminated. After setting and about 5 min from the start of mixing, the G clamps were removed and the specimen stored for 1 h at 37 +_ 1°C and 100% relative humidity, followed by a further period in a water bath before testing. The zinc polycarboxylate cements were tested at 19 IL 2°C after storage for 2 d at 37 f 1°C. The glass polyalkenoate cements were then tested at 19 f 2°C after 1 and 24 h in water at 37” + 2°C. The testing was performed in air immediately after removal of the specimens from the water bath, using a Monsanto Tensometer at a speed of 1 mm mine1 . The adhesive layer thickness was obtained by measuring the thickness of the joints in the absence and in the presence of the cement using a micrometer accurate to + l,~~rn. Test temperatures were chosen to enable previous fracture toughness values to be used. The results are unlikely to be influenced dramatically on changing the test temperature from 19 to 37’C.
Glass composition
Component
Weight
Si Al Ca F Na P
12.39 16.44 7.14 10.40 7.26 4.54
Table 2
Molar chromatography
mass details determined of poly(acrylic acid)s
Code
Batch number
it?,
E5 E7 E9 El1
159 AVS 228 220 415
7.30 1.13 4.04 1.12
Table 3
Glass layer thickness
sphere
fractions
percent
by gel permeation
Mw x x x x
1.15 2.27 1.14 3.83
lo3 lo4 lo4 lo5
x X x X
lo4 lo4 lo5 lo5
used to vary the adhesive
45,um 75pm 106pm 150pm 212pm 300pm
I:; $1 (5) (6)
> > > > > >
X > 38pm X > 45pm X > 75pm x > 106pm X > 150pm X > 212pm
the case of glass polyalkenoate cements, 10% m/m (+) tartaric acid was added to the water to retard setting.
Compression
tests
The compression specimens were produced in accordance with BS6039:19813’ and BS6814:19873g and tested in water at 0.1 mm min-l using a Mayes 30 KN testing machine (Windsor, UK]. A 0.1% offset yield stress was calculated from the load-deflection traces and used in conjunction with fracture toughness values to calculate plastic zone sizes. The zinc polycarboxylate cements were tested at 19 + 1°C after storage in water at 37 + 1°C for 48 h. The glass polyalkenoate cements were tested at 37 f 1°C after storage in water at 37 ? 2°C for 24 h.
Adhesive
bond strength
tests
Single lap joints consisting of 3.3 mm thick strips of 18/8 stainless steel with overlap length 40 mm and width 30 mm were used to measure the shear bond strengths meeting the criteria of ASTM D100240. The overlap length to adhered thickness ratio is very important in determining the stresses generated in single lap joints and the lap joint dimensions. Previous studies in the literature Table 4
The influence
Polyacrylic Code
nw
E5 E7 E9 El1
1.15 2.27 1.14 3.83
acid
x lo4 X lo4 x lo5 x lo5
of poly(acrylic
RESULTS Tables 4 and 5 show the fracture toughness (K,,), toughness [GIc), flexural strength (or), Young’s modulus (E) for zinc polycarboxylate and glass polyalkenoate cements as a function of the poly(acrylic acid) chain length or molar mass. The important conclusions previously drawn 36,37 from these data are: 1. The Young’s modulus
is independent of the poly(acrylic acid) chain length and remains approximately constant. This indicates that the chemistry of the setting reaction of the cement is unchanged and that only the physics of the failure process is being studied. The exception to this general conclusion is the lowest molar mass zinc polycarboxylate cement that exhibits a significantly lower Young’s modulus which is thought to be a result of a free volume effect. 2. The toughness, or strain energy release rate (G,) increases with the poly(acrylic acid) chain length for both types of cement, as expected qualitatively with a chain pull-out model for fracture. However, the magnitude of the increase in the toughness is not as large as that predicted by the reptation chain pull out
acid) molar mass on the fracture
K,c (MNm-3’z)
p=
0.21 0.46 0.61 0.80
0.02 0.08 0.17 0.12
5)
933
toughness
of zinc polycarboxylate
cements
GI, (Jmm2)
Of (MNm-‘)
SD (n = 5)
&Nmm2)
sR= 5)
10 31 67 115
4.65 7.13 10.74 11.08
0.28 1.26 0.36 1.14
4439 6611 5622 5580
372 675 429 520
Biomaterials 1992,
Vol. 13 No. 13
934
Bond strength
Table 5
The influence of poly(acrylic
Polyacrylic acid Code
mw
E5 E7 E9 El1
1.15 2.27 1.14 3.83
x X x X
lo4 lo4 lo5 lo5
K,, (MNmm3’*)
r=
0.13 0.16 0.23 0.26
0.01 0.01 0.02 0.02
5)
A.O. Akinmade
and R.G. Hill
cements
GI, (Jm-‘)
(T’ (MNm-*)
SD (n = 5)
E (MNmm2)
SD (n = 5)
10 15 30 39
7.06 8.05 9.99 10.30
1.05 0.61 0.85 1.66
1750 1754 1487 1242
314 374 173 259
The 0.1% offset yield stress values of 48 h old zinc polyalkenoate cements are shown in Table 6. The values for the three highest molar mass cements are approximately constant and significantly higher than the lowest molar mass cement. In the previous study36 the Young’s modulus for the lowest molar mass cement was significantly lower than for the higher molar mass cements and this was thought to be a result of a free volume effect reducing the glass transition temperature of the polysalt matrix. It is thought that a similar effect occurs with the yield stress values. The plastic zone diameters of these cements were calculated using Equation z and the fracture toughness data in Table 4. The calculated plastic zone size increases from approximately 40 pm with the lowest molar mass cement to approximately 290 pm with the highest molar mass cement. These plastic zone sizes are larger than or comparable with the adhesive layer thicknesses found with jacket crowns and are likely significantly to influence adhesive bond strengths. Clearly, as the temperature is raised from room temperature to mouth temperature, the plastic zone diameter would be expected to increase. The 0.1% offset yield stresses for the glass polyalkenoate cements are shown in Table 7. Note that in order to be able to use the previous fracture toughness values these tests have been carried out at 37’C. Here the yield stress increases appreciably with the poly(acrylic acid) chain length. The plastic zone diameters of these cements were again calculated using Equation z and the appropriate fracture toughness values from Table 5. The plastic zone diameters are also shown in Table 7. The plastic zone sizes are much smaller than for the zinc polycarboxylate cements and they increase slightly with the poly(acrylic acid] molecular weight. Because the calculated plastic zone sizes are comparable with the glass particle size present in these cements, the calculations are invalid but clearly the plastic zone sizes obtained here are much smaller than can be achieved in practical adhesive layers. 13 No. 13
cements:
acid) molar mass on the fracture toughness of glass polyalkenoate
Table6 Yield stress values and calculated diameters for zinc polycarboxylate cements
model. GIaN2 where N is the degree of polymerization, which is proportional to the chain length. 3. The rate of increase of toughness with poly(acrylic acid) chain length is significantly greater for the zinc polycarboxylate cements than the glass polyalkenoate cements. This is thought to be a result of the weaker ionic linkages between the poly(acrylic acid) chains in the zinc polycarboxylate cements giving rise to a more thermoplastic character. The thermoplastic character of glass polyalkenoate cements and zinc polycarboxylate cements has been established previously4* using dynamic mechanical and dielectric thermal analysis techniques.
Biomaterials 1992. Vol.
in polyalkenoate
0.1% Yield stress uy
1.15 2.27 1.14 3.83
x X x X
lo4 lo4 lo5 lo5
MPa
SD(n = 6)
18.93 30.99 28.54 26.58
0.95 1.94 0.75 0.77
39.9 70.3 145.7 290.9
0.1% Yield stress uy
x X x X
lo4 lo4 105 lo5
zone
Plastic zone diameter 2R, (pm)
Table 7 Yield stress values and calculated diameters for glass polyalkenoate cements
1.15 2.27 1.14 3.83
plastic
plastic
zone
MPa
SD(n = 8)
Plastic zone diameter 2R, (pm)
30.56 35.75 45.23 46.99
2.32 1.74 1.23 1.50
5.7 6.4 8.2 10.5
In the final part of the study the adhesive shear bond strengths of zinc polycarboxylate and glass polyalkenoate cement bonded stainless steel lap joints were measured. All the joints failed cohesively in the cement layer. The data for the zinc polycarboxylate cements are presented in Figure 4. There is clearly a significant influence of adhesive layer thickness on the shear bond strength of the joint. The most obvious feature of the plots in Figure 4 is the pronounced maxima in the shear bond strengths of the E9 molar mass cement. The lowest molar mass cement tested, with a weight average molar mass of 1.15 X lo4 shows a gradual increase in shear bond strength from 0.45 MPa at an adhesive layer thickness of approximately
T
3.2
4 5
2.4
F f ;;
1.6
-F 0"
0.8
?Y 6 0
60
120 Adhesive
180
240
layer thickness (urn)
Figure 4 The effect of adhesive layer thickness on the shear bond strengths of zinc polycarboxylate cements.
Bond strength
in polyalkenoate
cements:
A.O.
Akinmade
185
flrn whilst to 1.11 MPa where the thinnest adhesive layer obtainable was reached at 40,um. Figure 4 shows that the adhesive layer thickness has a dramatic effect on the observed shear bond strengths of the ES-based cement. These cements had a shear bond strength of 0.76 MPa for an adhesive layer thickness of 20pm. The shear bond strength rises sharply to a maximum of 2.43 MPa at a film thickness of approximately 62 pm. Indeed from adhesive layer thicknesses of 40 to 80 pm the shear bond strength is high and relatively constant. Above 80pm the shear bond strengths drops gradually to a value of 1.17 MPa at approximately 200 pm. The E9 cement has a molecular weight similar to that used commercially in zinc polycarboxylate cements and the proportions of polyacid and water content are also similar, thus this cement typifies commercially available materials. The adverse effects of not using the optimum adhesive layer thickness when working with this cement can be readily appreciated. A dentist using a cement layer thickness of say 180pm instead of 60pm, will obtain a joint, with only about half the optimum shear bond strength. The 48 h old El1 zinc polycarboxylate cement with an MW of 3.83 X lo5 again shows a pronounced adhesive layer thickness effect on shear bond strengths. A shear bond strength of 1.47 MPa was recorded for the thinnest adhesive layer tested of 20 pm. The shear bond strength rises to a maxima of 2.65 MPa for the largest adhesive layer tested of approximately 204 pm. It is envisaged that the shear bond strengths of the El1 and E5 cements will go through maxima in a similar fashion to that shown by the E9 cement: this is shown on Figure 4 by the dashed lines. The plastic zone sizes for the E5, E9 and El1 cements are also shown on Figure 4 by the solid lines. The approximate agreement between the plastic zone diameters and the maxima in the shear bond strengths is reassuring. In general, plastic zone sizes formed at crack tips in constrained layers between inflexible substrates are significantly smaller than in the bulk material and this is supported by the present data. Figure 5 shows the effect of adhesive layer thickness on shear bond strengths of an E7 glass polyalkenoate cement. The results shown are for cements tested after 1 h; similar results were found for cements tested after 24 h. It is observed that there is a gradual increase in the shear bond strength of the glass polyalkenoate cement
40 Adhesive
120
80 layer
thickness
(Pm)
Figure 5 The effect of adhesive layer thickness on the shear bond strengths of glass polyalkenoate cements.
and
R.G.
935
Hill
from 1.15 MPa at the thickest layers tested at between 110 and 130 pm to 1.50 MPa at 37 pm. In all probability the shear bond strength of the glass polyalkenoate cements would have increased further if a cement layer thinner than 37pm could have been obtained. It is envisaged that the shear bond strength would rise to a maximum of about 9 pm, which is the calculated plastic zone size of this cement. Studies were not performed with the higher molar mass cements because of the unlikelihood of being able to observe an adhesive layer effect on shear bond strength with layer thickness greater than 37 pm. DISCUSSION The adhesive bond strength of zinc polyalkenoate cements to stainless steel substrates is markedly influenced by the cement layer thickness as predicted by linear elastic fracture mechanics. At mouth temperatures the plastic zone size at the crack tip in zinc polycarboxylate cements would be expected to increase and an even more marked influence of cement layer thickness on adhesive bond strength would be expected. The adhesive bond strengths of the glass polyalkenoate cement bonded lap joints was not appreciably influenced by the adhesive layer thickness, which was also predicted by the fracture mechanics calculations of plastic zone sizes. However, the model glass polyalkenoate cements tested may well not be typical. Indeed for luting type glass polyalkenoate cements, the glass to water ratio and the poly(acrylic acid) concentration will be lower than for the cements tested here and both these factors will serve to promote greater plasticity at the crack tip and may well make the adhesive bond strength dependent on the adhesive layer thickness. Some evidence for this is present in the paper by Oilo and Euje17 who found that the bond strength of the luting type glass polyalkenoate cement to dentin was dependent on film thickness. A comprehensive study looking at both commercially available polyalkenoate cements and model cements is required in order to improve our understanding of this very important area.Nearly all the adhesive bond strength values published to date on polyalkenoate cements have neglected cement layer thickness and this may account for much of the experimental scatter and lack of consensus in this important field. Clearly, future studies on adhesion of glass polyalkenoate cements and zinc polycarboxylate cements should take account of cement layer thickness. This study indicates that a lot of the published literature on the adhesion of polyalkenoate cements may be of very limited value. A wide variety of techniques and test methods has been used to determine adhesion of these cements and most authors have failed to give any information on the thickness of the cement layers used in their tests. If we are to advance our understanding of this topic, it is important for research workers to develop and use a standard test method based on fundamental principles. The results presented here also have implications for the bonding of crowns, an application for which zinc polycarboxylate cements are widely used. The prevailing wisdom appears to be to get the crown to fit as tightly as possible on to the remaining tooth material. Clearly, from Biomaterials
1992, Vol. 13 No. 13
Bond strength
936
the results presented here, the retention of a good fitting crown could be worse than a poorer fitting crown, especially when a zinc polycarboxylate luting cement is used as the optimum film thickness appears to be much greater. A further point of interest is that the British and IS0 Standards for luting cements specify being able to obtain a film thickness of 25 ,nm, which if achieved in clinical practice with zinc polycarboxylate cements, would almost certainly result in a reduction in adhesive bond strength.
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2
3
4
5 6
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Kinloch, A.J., Adhesion and Adhesives, Chapman and Hall, London, UK, 1987 Knott, J.F., Fundamentals of Fracture Mechanics, 3rd Edn, Butterworths, London, UK, 1979 Hill, R.G., Warrens, C.P. and Wilson, A.D., The influence of poly(acrylic acid) molecular weight on the fracture of glass ionomer cements, J. Mater. Sci. 1989, 24, 363-371
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Hill, R.G. and Labok, S., The influence of poly(acrylic acid] molecular weight on the fraction of polycarboxylate cements, J. Mater. Sci. 1991, 26, 67-74 BSI-BS6039: 1981, Specification for dental glass-ionomer cements BSI-BS6814: 1987 IS0 4104-1984, British Standard specification for dental zinc polycarboxylate cements ASTM Designation: D1002-72 (Reapproval1983), Standard test method for strength properties of adhesives in shear by tension loading [metal to metal) [Annual Book of ASTM Standards Vol. 03-01) Hill, R.G., Relaxation spectroscopy of polyalkenoate cements, J. Mater. Sci. Lett. 1989, 8, 1043-1047
