Effect of adhesive layer thickness on the bond strength of a zinc polycarboxylate dental cement

BiomoterioJs

16 (1995)

149-154

Else&r Science Limited Printed in Great Britain. All rights reserved 014s9612/95/$10.00 0

1995

Effect of adhesive laver thickness on the bond strength ofJa zinc polycarboxylatk dental cement A.O. Akinmade and J.W. Nicholson* Materials Group, Laboratory of the Government Chemist, Teddington, Middlesex ials Unit, School of Medicine and Dentistry, King’s College, London, UK This study cement

reports

the effect

on its adhesion

a specially

designed

been done

previously.

failure

apparatus,

of the cement

these

results

layer.

They also show

confirm

The optimum Plus (De Trey, found

the cement failed

stresses Keywords:

previous

stress

and the lap joint

joints.

within

that there layer

was 205/*m

UK), for which the maximum

analyses

at adhesive

in bonded

is an optimum to be cohesive

analysis, These

shear

layer

joints

the adhesive

showed

layer

thickness

shear

thicknesses

because

failed

of below

of greater

cement

shear

layers.

and the mode joints.

of

Hence,

layers.

and tensile

that joints

in

as has

for the zinc polycarboxylate

for the particular the average

externally

spheres,

in the bonded

with thicker

with thin adhesive

layer

glass

bond strength

of the cement

dental

was controlled

of isodimensional

likely

Surrey,

zinc polycarboxylate

The thickness

that both the tensile

is more

Biomaterials

materials,

of a commercial lap joint.

by the thickness

Weybridge,

Using

cohesively

Dental

show

of the adhesive

concentrations

in these

single

than by the inclusion

findings

that failure

also been obtained.

the high stress joints

The results

Dentsply,

layers

the thickness

rather

are influenced

thickness

to be 4.03 MPa.

ullCmax), have

of varying

in a metal-to-metal

TW11 OLY, UK; *Dental Biomater-

studied,

PolyF

bond strength stresses,

z,~(,,,~~) and

at the interface 100pm.

between

This resulted

On the other

contributions

was

from

hand,

from

thicker

tensile

(1995) 16 (2), 149-154

zinc polycarboxylate,

Received 13 December 1993; accepted 24 January

adhesion,

optimum

thickness

1994

from 4Oprn at A4, of 1.15 x lo4 to 290.9pm at M, of In contrast, the glass-ionomer did not 3.83 x 105. exhibit an optimum thickness, but since its calculated plastic zone size varied only between 5.7 and 10.5pm over the same range of polymer molar masses, this was not surprising. The work of Akinmade and Hill not only established the existence of an optimum adhesive layer thickness for zinc polycarboxylate cement, but also established the large differences between the plastic zone size in it and the apparently closely related glass-ionomer. In order to control the thickness of the layers, Akinmade and Hill used glass ballotini incorporated into the cement to act as spacers when the sample was cured between two glass plates4. This technique can be criticized on the grounds that it introduced a foreign material into the cement, one that was likely to inhibit the setting reaction and in other ways alter the properties of the set cement’.“. The current work was therefore carried out in order to test the validity of the findings of Akinmade and Hill using an alternative method for varying the adhesive layer thickness of a commercial zinc polycarboxylate dental cement. A specially designed jig was built in which the thickness of the adhesive could be varied by altering the spacing of the stainless test pieces in a controlled way prior to

Zinc polycarboxylate cement has been used as a dental material since 1968l. The very first report highlighted the good adhesion to tooth materials and also the low irritancy. Since 1968, these properties have been exploited by using zinc polycarboxylates as cavity liners, as adhesives for the placement of crowns and for the fixation of orthodontic appliances2. There have been numerous studies on the adhesion of zinc polycarboxylate and related glass-polyalkenoate cements to dentine, enamel and stainless steel alloys, but with widely scattered experimental data3. This scatter reflects the sensitivity of adhesion measurements to both experimental conditions and procedures. One probable source of this scatter, that of variable adhesive layer thickness, has recently been studied by Akinmade and Hi114. In their paper, they reported the effect of varying the adhesive layer thickness for both a zinc polycarboxylate and a glassionomer cement, using a linear elastic fracture mechanics approach to analyse their data. They found that there was an optimum adhesive layer thickness for the zinc polycarboxylate, corresponding to a plastic zone of about the same size, and that the plastic zone size varied with the molar mass of the poly(acrylic acid) Correspondence to Mr Ademola Akinmade. 149

Biomaterials 1995, Vol. 16 No. 2

Adhesive

150

strength

of zinc polycarboxylate

dental

bonding. The use of this apparatus removed the need to include glass spacers within the cement itself. Test pieces fabricated in this way were in the form of lap joints that were then tested to failure in shear.

MATERIALS

AND METHODS

cement

with thickness:

G, is the shear modulus of the adhesive, while t, is the average applied shear stress acting along a joint of width b under the action of applied load F. The forces acting on the bonded joints can be resolved into the in-plane shear stress, r12, and the stress acting normal to the plane of the applied force, the transverse (out-of-plane) tensile stress, crI1: ~1~(max) =

All determinations were carried out on a commercial zinc polycarboxylate dental cement, PolyF Plus. This is a water-activated cement which was mixed at a power:water ratio of 5:1, the recommended ratio for luting cement consistency. Tensile shear bond strengths of single metal-tometal lap joints were determined as a function of adhesive layer thickness. These lap joints, made of stainless steel, were designed to conform to the appropriate ASTM standard7. They consisted of pairs of rectangular stainless steel plates of dimensions 126.0 x 25.4 x 1.6mm3. The lap joints were suspended during testing by a pair of holes which ensured self-alignment and thus allowed shear conditions to predominate. The overlap length, 1 of the lap joints was designed to be la;.5 i 0.25mm; in each case, the precise length for each specimen was measured using a low magnification travelling microscope. These lengths were determined from the expression: 1, = F,,dlT

where Fty is the yield point of lap joints (stainless steel) and d is the thickness of a lap joint. The value of z was arbitrarily set at 150% of the estimated shear strength in the bonded lap joint. The use of values of 1, derived from this expression ensured that the lap joints did not undergo strain, which would have complicated the adhesive strength measurements. The condition prevailing in the bonded lap joints, with the Young’s modulus of the substrate (Es) being much greater than that of the adhesive (E,), is such that 1,/d >> E,IE,. This represents the practical situation of a relatively plastic polymer-based material confined by rigid substrates. Under this condition, the bending stress factor of the bonded joints, k, is given by Equation 1, while the stress concentration factor in the adhesive is given by Equation 2: 1

k = 1 + 2(2)$

a=

and J.W. Nicholson

A.O. Akinmade

Cl+ Sk) 4

T’,

(za)+

coth(2a$

3

+ - (1 - k)

(3)

4

where h, is the adhesive layer thickness. &, bending stiffness, and M,, the bending moment unit width, for the substrate are given by: &

=

the per

Ed3

12(1 - ps2)b

and M = @(ha + d) e zb

The stainless steel components of the lap joints were pretreated by abrasion with a coarse waterproof silicon carbide paper (120 grit) (Metaserv, Metallurgical Services Ltd, Betchworth, Surrey, UK), followed by cleaning with acetone. The adhesive layer thickness was controlled by means of a stainless steel jig (Figure I) and a hydraulic pressing machine (Specac Ltd, St Mary Cray, Kent, UK) with an attached heating mechanism. The adhesive layer thickness was manipulated by attaching polyimide shims of various thicknesses to the base and/or arms of the stainless steel jig. The test assembly was then pressed at 37 f 2°C with the aid of the hydraulic press allowing excess cement to escape. This surplus material was cleaned off with a sharp edge to eliminate any possible effect of cement fillet on measured strength. The adhesive layer thicknesses of bonded lap joints were determined by subtracting the thicknesses of lap joint pairs from their cement-bonded values measured at the time of fracture. These measurements were carried out using a digital micrometer screw gauge (error of 0.05 mm) at predetermined points at the

tanh i (1 - psz)f$

(4)”coth(;)’

(21

where

and F

Figure1 thickness.

r0 =bl, Biomaterials

1995, Vol. 16 No. 2

Specially

designed

jig

to

control

cement

Adhesive Table 1

strength

of zinc polycarboxylate

The mechanical

properties

dental cement

with thickness:

of the bonded joints

Joint width (b) Bonded overlap length (/=) Adhesive shear modulus (G,) Substrate shear modulus (G,) Adhesive Young’s modulus (Ea)* Substrate Young’s modulus (ES)+ Substrate thickness (d) Adhesive thickness (h,) Substrate Poisson’s ratio (&

2.5 x 10W2m variable 2.0 x 10QmPa 7.7 x lOi mPa 5.4 x 10QmPa 1.9 x 10” mPa 1.6 x 10V3 m variable 2.5 x 10-I

‘Determined experimentally from three-point bend tests after 24 h storage of cements in water at 37 f 2°C. The fracture of six cement specimen bars was carried out in water, also at 37 * 2°C. ‘Literature value for stainless steel EN 58b (grade 321)“.

centre of the adhesive. Table I presents the mechanical properties of the bonded joints. Bonding was carried out for a controlled time from Table 2

Individual shear bond strengths for varying adhesive TlP(max)

a

(MPa) 11 15 17 19 21 24 28 30 39 41 47 48 48 54 58 61 63 69 73 79 88 88 93 93 94 98 99 105 107 112 114 117 119 129 129 133 140 146 150 152 158 161 161 162 166 169 179 180 187 189

2.02 1.78 2.61 2.60 2.65 2.72 2.41 2.62 2.71 2.73 3.62 2.19 2.42 2.86 3.21 3.11 3.29 3.30 3.41 3.67 3.67 3.53 3.55 3.74 3.39 3.55 3.52 3.98 3.83 3.78 3.53 3.57 3.82 3.54 4.01 3.52 3.51 3.97 3.82 3.64 4.11 3.53 3.70 3.83 3.63 3.27 3.64 4.02 3.55 3.99

12.05 9.11 14.28 12.58 11.44 11.58 9.68 10.60 9.05 8.83 11.27 7.61 7.63 8.90 10.55 9.56 10.08 9.64 9.16 10.09 8.72 9.53 8.69 9.26 8.65 8.36 8.70 9.35 8.90 8.86 8.16 8.50 8.55 7.64 8.86 7.88 7.65 8.22 8.20 7.56 8.37 7.21 7.53 7.61 7.83 6.99 7.27 8.08 7.09 8.09

9.74 8.20 6.82 7.57 6.66 6.51 6.08 6.10 4.85 4.64 4.39 4.48 4.50 4.38 4.03 4.29 4.24 4.00 3.60 3.68 3.21 3.38 3.17 3.20 3.34 3.00 3.20 2.98 2.94 2.98 2.92 3.03 2.79 2.65 2.73 2.79 2.68 2.51 2.62 2.51 2.43 2.45 2.44 2.36 2.63 2.60 2.46 2.39 2.36 2.40

A.O. Akinmade

and J.W. Nicholson

151

initial mixing (35 f 5 s), in order that the viscosity and wetting characteristics of the setting cement were consistent between specimens. This was followed by the application of a standard pressure on the bonded joint for a specified duration (100 f 10 s). The bonded lap joints were stored in an oven at 37 f 2°C and 100% relative humidity for 1 h prior to testing. Strength testing was carried out in an oven also set at 37 f PC, but at ambient humidity, using a universal testing machine (In&on 1185). Adhesive joints were tested under tension at a cross-head speed of 1 mm min-‘.

RESULTS Table 2 shows the shear bond strengths for the individual bonded joints, together with their thickness. Also layer thickness

ha

50

712(max)

(v)

(MPa)

(MPa)

193 195 203 203 206 212 213 222 230 236 238 242 247 250 256 260 261 263 267 270 273 281 287 292 297 303 310 315 316 333 333 340 340 340 341 346 354 357 361 366 372 378 390 395 400 400 421 429 453 460

3.82 3.53 4.14 3.60 4.03 3.47 3.72 3.56 3.95 3.60 3.71 3.77 3.27 3.33 3.50 3.39 3.43 3.97 3.56 3.77 3.91 2.99 3.88 3.53 3.67 3.50 3.29 3.88 2.41 3.32 3.17 2.78 2.54 2.54 2.87 2.78 2.68 2.59 2.54 3.20 2.40 2.43 2.50 2.73 2.99 2.99 2.36 2.59 2.31 2.35

7.15 6.88 7.55 7.16 7.70 6.61 7.09 6.96 6.85 6.62 6.67 6.87 5.82 6.01 6.46 6.27 6.40 7.03 6.22 6.60 6.74 5.02 6.79 6.34 6.69 6.27 5.76 6.02 4.05 5.61 4.93 4.56 4.19 4.15 4.78 4.58 4.38 4.25 4.26 5.19 3.86 4.05 4.13 4.79 4.83 4.76 3.65 4.96 3.50 3.55

Biomaterials 1995.

a

2.17 2.30 2.08 2.34 2.22 2.21 2.21 2.28 1.56 2.11 2.12 2.07 2.04 2.05 2.11 2.11 2.14 1.99 1.96 1.97 1.93 2.02 1.95 2.02 2.06 2.02 1.96 1.95 1.87 1.87 1.80 1.81 1.82 1.80 1.84 1.81 1.79 1.80 1.85 1.77 1.76 1.84 1.81 1.76 1.76 1.72 1.67 1.67 1.62 1.61

Vol. 16 No. 2

Adhesive

152

strength

of zinc polycarboxylate

included are values of ~~~~~~~ and all(,,) for each individual bonded joint. The data are plotted in Figure2, and show that shear conditions predominate during the testing of joints. Figure3 is a plot of maximum interfacial stress r,i(ma) = c(?’ and 512(max)against adhesive layer thickness. At low adhesive layer thicknesses, where the difference between these two terms is large, the mode of failure observed was interfacial. Where zla(max) and 7~i(max)coincided (h, > 250 pm), cohesive failure within the cement was clearly seen. At intermediate thicknesses (100 < h, < 250) pm, there was a mixed mode of failure. A reason for the failure of adhesive joints at low loads becomes clear from Figure 4. The figure plots 712(max) and 7. against adhesive layer thickness and shows that at low adhesive layer thickness the applied shear stress, z,, is greatly concentrated, a fact which leads to premature joint failure. Figure5 plots the results of the applied shear stress (adhesive strength) as a function of cement thickness. There is a clear maximum in this plot. From this figure the peak in the plot of tensile shear bond strength versus adhesive layer thickness for PolyF Plus was shown to lie in the region of 200pm. The equation fitting the curve of the effect of adhesive layer thicknesses on the strength of PolyF Plus was obtained, using quadratic data analysis of SuperCalc 5.5” (Computer Associates Int., Uxbridge, UK), as: y = -0.000

0311x’

+ 0.0127x

dental cement

A.O. Akinmade

9

14 12 h m

J.W.Nicholson

and

A Av. shear stress 0 Max. shear stress

0 ‘0 0

10

2

8

!:

6

111

4 2 0 0

I

I

I

I

1

I

1

1

I

50

100

150

200

250

300

350

400

450

Adhesive Figure4 thickness.

The

variation

layer of

thickness shear

20

A l

bond

Max. Max.

500

(microns) strength

with

tensile stress shear stress

15 m

-2

10

: : u-l

+ 2.44

with thickness:

5

J 0

100

50

150

Adhesive Figure5 The variation thickness.

1 0

I 50

I 100

y=-0.0000311x2 +0.0127x I I I I I I 150 200 250 300 350 400

Adhesive Figure2 The thickness.

layer

variation

of

thickness shear

+ 2.44 I 450 500

(microns)

bond

strength

with

200 layer

250

300

thickness

in shear

350

400

450

500

(microns)

and tensile

stresses

with

From this equation, the position at which shear bond strength is a maximum can be calculated, and is found to be at 205pm. This value is of the same order of magnitude as those found previously by Akinmade and Hi114. Their experimental technique, employing glass ballotini as spacers, has thus been shown to be acceptable, and not to alter the properties of the cement too severely.

DISCUSSION Interfacial l Max. shear

A

0

0

SWeSS

stress

I

I

1

1

I

1

I

I

I

50

100

150

200

250

300

350

400

450

Adhesive Figure3 Interfacial of cement.

layer

vs cohesive

Biomaterials 1995, Vol. 16 No. 2

thickness failure

500

(microns) in single

lap joints

Adhesive joints tend to fail by the initiation and propagation of crack?. Hence the technique of linear elastic fracture mechanics can be used to analyse the failure of such joints. This approach has been employed by Akinmade and Hill4 to study the effect of adhesive layer thickness on the bond strength of polyalkenoate cements. Briefly, they showed that the maximum obtainable adhesive strength of a bonded joint will not be obtained if the adhesive layer thickness of the joint is less than the maximum plastic zone that can be formed at the tip of a crack propagating in the bulk of the adhesive. It must be stressed that the plastic zone size is not a measure of the plasticity of a material. Rather, it is a combination of the yield at the tip of the crack (plasticity) and the resistance of the

Adhesive

strength of zinc polycarboxylate dental cement with thickness: A.O. Akinmade

material to crack initiation and propagation. This latter property determines the length of the crack, and hence relates to the size of the plastic zone, before catastrophic failure of the adhesive occurs. Therefore, a plastic material that fails prematurely (i.e. has low yield strength) will have a smaller plastic zone size than a less plastic but tougher material. There is a further factor which affects the size of the plastic zone, and hence the measured bond strength, namely, the increase in the level of the out-of-plane transverse stresses (oll) ahead of the crack tip9 caused by the action of high modulus substrates. For a dental cement, such as zinc polycarboxylate, the dentine of the teeth would act as just such a high modulus substrate. The effect of such substrates is to cause the length of the plastic zone ahead of the crack tip to increase, an effect which is greatest with thinner adhesive layers. Overall, these two factors combine to produce a maximum in the plot of bond strength versus thickness for any given adhesive4z8. The effect of adhesive layer thickness can be summarized as follows: In the case of a thin adhesive layer, the shear stress concentration factor, GI,increases and the value of tlz, becomes equal to the shear strength of the adhesive, and interfacial failure at the substrateadhesive interface occurs. In the case of a thick adhesive layer, both the adhesive and the substrate are deflected by a bending moment, and the maximum bending stress in the adhesive layer becomes equal to the strength of the adhesive. The joint then fails cohesively in the adhesive layer due to this bending moment. The effect of adhesive layer thickness on the stress concentration factor in bonded joints, containing identical substrate halves can be inferred from Equation

and J. W. Nicholson

153

of thickness, allows a mean value for shear strength of 3.27MPa to be calculated for all 100 data points, with a standard deviation of 0,56MPa, i.e. 17.5% of the mean strength. In contrast, considering results from the region 201~220pm only, gives a mean value of 3.79MPa, with a standard deviation of 0.08 MPa, i.e. a mere 2.1% of the mean strength. Controlling the adhesive layer thickness can thus be seen to reduce the scatter in measured shear strength. effect

CONCLUSION A test assembly has been designed that did not require the introduction of foreign matter, such as glass ballotini, into a cement in order to control its thickness as an adhesive layer. Using this apparatus, the existence thickness has been of an optimum demonstrated for a commercially available zinc polyalkenoate dental cement, PolyF Plus. This optimum lay at 205pm, a value that is in good agreement with the previous findings of Akinmade and Hill4 for a series of experimental zinc polycarboxylate cements. These results thus confirm those of Akinmade and Hill, and show that their technique of including glass ballotini to act as spacers in the cement did not adversely affect the properties of the resulting materials. The results also indicate that PolyF Plus has a plastic zone size comparable to those in the experimental cements, i.e. in the region of 200,um. In addition, this work has shown that thicker layers of zinc polycarboxylate cement are more likely to fail cohesively than thinner layers. ACKNOWLEDGEMENTS

2.

The results we have obtained with our experimental technique show in addition that the thickness of the adhesive layer influences the mode of failure, i.e. The clinically whether adhesive or cohesive. recommended use of zinc polycarboxylate cements for luting is as layers of the order of 25pm thick. We would therefore anticipate that failure under these circumstances would be predominantly adhesive at the tooth-cement interface, rather than cohesive within the cement. Despite the fact that the optimum in the plot of thickness against shear bond strength for these cements occurs at around 200pm, there remain reasons for using luting cements in thin layers. These include, inter alia, the requirement to reduce marginal leakage, and the need to minimize dimensional changes or stress between the bonded components in clinical service. In vivo studies might be appropriate to ascertain the trade-off between maximum in vitro adhesive strength of bonded joints and the clinical performance of such joints. Our results confirm a further finding of Akinmade and Hi114, namely, that the practice of ignoring the effect of thickness on shear bond strength has probably led to the extensive scatter in the data reported in the literature. Taking the current results, and ignoring the

The work reported in this paper was carried out as part ‘Materials Measurement Programme’, a of the programme of underpinning research financed by the UK Department of Trade and Industry. REFERENCES Smith DC. A new dental cement. Br Dent Il968;

125:

381-382.

Williams DF, Cunningham J. Materiols in Clinical Oxford University Press, 1979. Negm NN, Combe EC, Grant AA. Factors affecting the adhesion of polycarboxylate cement to enamel and dentine. JProsthef Dent 1982; 45: 405-410. Akinmade AO, Hill RG. The influence of cement layer thickness on the adhesive strength of polyalkenoate cements. Biomaterials 1992; 13:931-936. Nicholson JW, Hawkins SJ, Wasson EA. A study of the structure of zinc polycarboxylate dental cements. /

Dentistry.

Mater Sci, Mater Med 1993; 4: 32-35. Wilson AD, Nicholson JW. Acid-Base Cements: Their Biomedical and Industrial Applications. Cambridge:

Cambridge University Press, 1998.

D1002-72.In:1983 Annual

Book of ASTM Standards,

Vol. 08.61. Standard Test method for Strength properties of Adhesives in Shear by Tension Loading (Metalto-Metal). Biomaterials 1995,Vol. 16 No. 2

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strength of zinc polvcarboxvlate

Kinloch AJ. Adhesion and Adhesives (Science and Technology]. London: Chapman and Hall, 1987. Wang SS, Mandell JF, McGarry FJ. An analysis of the crack tip stress field in DCB adhesive tiacture specimens. Int J Fracture 1978; A2: 659-663. Matsui K. Size effect of adhesive thickness on the shear

Biomaterials 1995, Vol. 16 No. 2

dental cement with thickness: A.O. Akinmade

11

and J.W. Nicholson

and tensile strengths of adhesive-bonded rectangular or tubular lap joints under tension-shear. Int J Adhesion Adhesives 1990; 10:81-89. Woolman J, Mottram EU (eds). The Mechanical and Physical Properties of the British Standard for EN Steels. Vol. 3. Oxford: Pergamon Press, 1969.