The effect of interface adhesion, water immersion and anatomical notches on the mechanical properties of denture base resins reinforced with continuous high performance polyethylene fibres N. H. Ladizesky* T. W. Chowt
Key words: Acrylic resins, dental materials, notches, polyethylene fibres. Abstract Previous work' has presented a study of the mechanical properties of denture base resins reinforced with a new type of high performance fibre. It is now shown that the substantial improvements demonstrated in those composites remain largely unaffected by a watery environment, anatomical notches, moulding pressure and other factors of denture construction. The understandingof the reinforced resins is here complementedwith a detailed study of the interface strength, taking into account the various couplings occurring within the system. (Received for publication March 1991. Accepted August 1991.)
Introduction Ladizesky, Braden et u Z . ' - ~ have explored the possibility of reinforcing denture base resins with highly drawn linear polyethylene (HDLPE) fibres. This recently developed material offers an array of properties of particular interest to dentistry, including high stiffness and strength, proven biocompatibility, white translucent appearance and
*Dental Materials Science Unit, Faculty of Dentistry, University of Hong Kong. ?Department of Prosthetic Dentistry, Faculty of Dentistry, University of Hong Kong. Australian Dental Journal 1992;37(4):277-89.
negligible water absorption. Incorporation of at least 30 volume per cent in longitudinally oriented fibres produced a substantial improvement in the mechanical properties of denture base resins.' It was shown that these properties were broadly insensitive to the interface adhesion as determined with single fibre pull-out experiment^.',^ The present paper reports further work in three main areas, namely, (a) study of the adhesion levels within the actual composites, (b) the effect of water immersion at 37OC on the mechanical properties of the reinforced resins and, (c) sensitivity of these properties to notches that mimic anatomical features. Previous publications4.' presented a method for the construction of woven HDLPE fibre reinforced acrylic denture bases, complemented with clinical trials. Another communication will report on the construction of lower and upper denture bases reinforced with parallel continuous HDLPE fibres,6 as used for the samples tested in this work. The mechanisms of failure taking place during these tests, revealed by microscopy techniques, are discussed elsewhere.'
General comments on the measurement of the interface adhesion of fibrous composites Information on the fibrehesin interface strength plays a fundamental role in the correct appreciation of the mechanical properties of composite materials. Coupling effects which mask the intrinsic shear properties of the interface may be removed by testing single filament^.^.' However, in a prac277
tical situation fibre interactions are always present and it is generally accepted that no study of mechanical properties is complete unless it includes the measurement of adhesion levels within the actual composite. There are various methods to measure the interface strength in fibre reinforced resins? of which the most widely used is the interlaminar shear strength (ILSS) test. This requires longitudinally oriented reinforcement and involves a 3-point bending mode of deformation using samples whose dimensions are chosen so that shear in a plane parallel to the fibre direction and in the middle of the thickness occurs before flexural failure. This condition implies a relatively small gauge length to thickness ratiolo which, for a number of commercial composites lies in the region of about five to one.
Experimental Materials Most of the details regarding materials and preparation of composite bars have been reported Therefore only a brief summary is given here, incorporating any changes applying to the present work. HDLPE multifilament fibrest were used throughout. The spun fibres were stretched to a draw ratio of 30: 1, resulting in a yarn of 280 denier with 180 filaments. Plasma treatment for enhanced adhesion was carried out with a Plasmaprep 300 unit.$ Bundles of fibres 290 mm long and 3.5 g mass were introduced into the reactor and treated with 120 W power for 2 min using a flow of 17 x lo3 mm3 min-I of O2gas as the plasma carrier. The pressure inside the reactor was 0.5 Torr. Five different resins were used, namely: (a) PMMA syrup, a non-proprietary type made with one part by mass of Rapid Repair powder11 with four parts by mass of Natural Coe-Lor 1iquid.l (b) RM-3,** a pour type resin diluted with 8 per cent by mass of methyl methacrylate. (c) Bis-GMA,tt mostly used as the matrix in composite restorative materials, diluted with 19 per cent by mass of methyl methacrylate.
(d) Crystic 272E.$$ (e) Araldite LY 1927/GB.§§ The first three are modified heat curing dental resins, while the last two are of the cold curing type (a polyester and an epoxy, respectively) specifically developed for fibre reinforcement. The composition and handling of all these resins are given elsewhere. 1,1
Sample production (a) Straight bars Straight composite bars of nominal dimensions 210 mm x 11 mm x 2.5 mm (L x W x T) were produced with the ‘leaky’ mould technique.’”’ The reinforcement was incorporated in the form of a longitudinally oriented bundle of fibres 290 mm long and 3.5 g mass. Two different loads were used to remove the excess liquid resin and obtain the predetermined thickness; (a) high pressure moulding, namely 25 kN slowly applied on the mould over several minutes, as used for the construction of dentures, (b) low pressure moulding with masses totalling 500 N left on top of the mould for about 30 min to 45 min. Unless otherwise stated all samples were produced with high pressure moulding and have standard nominal dimensions as stated above. In earlier work’ it was noted that reinforced bars made in ‘leaky’ moulds with PMMA syrup had patches of unwetted fibres, particularly when these were untreated. This problem was significantly improved in the present work by presoaking the bundles of fibres in the resin for about 20 hours, followed by squeezing out the excess liquid with gloved hands and four hours drying in a fume cupboard. For consistency, this procedure was also applied to the construction of bars made with RM-3 and Bis-GMA resins. Control tests were carried out with unreinforced acrylic bars of 210 mm x 12 mm x 2.5 mm (L x W x T) made with Trevalon C, 1 1 using a standard dental moulding technique. This included a two-part gypsum mould with one part having a flat smooth surface, while the other part had a groove made with a stainless steel blank, into which the resin was packed. (b) Notched bars The failure properties of acrylic dentures are highly sensitive to stress concentrations produced
tCelanese Research Company, Summit, NJ, USA. BNanotech Ltd, Manchester, UK. AD International Ltd, De Trey Division, England. (Coe Laboratories, Chicago, USA. **Ivoclar AG, Lichtenstein. ttNupol 46-4005. DMS Resin UK, South Wirral, England 270
StScott Bader, Wellingborough, England. §§Ciba-Geigy, Plastic Division, Cambridge, England 1 I(AD International Ltd, De Trey Division, UK. Australian Dental Journal 1992;37:4.
a
C
-
b
Fig. 1.-Dimensions (mm) of the notched regions of bars. a = 2; b = 8; c = 2; r=0.5; R = 1 . 5 .
Fig. 2.-Mould for processing an acrylic resin notch onto a straight reinforced bar. Similar moulds were also used to produce unreinforced PMMA bars.
by abrupt contour changes such as the frenal notch.I2 Testing of such effects requires the production of samples with very well defined geometry which should be reproducible to a high degree of accuracy. Also, the width of the bar at the base of the notch should be similar to the width of the sample outside the contoured region, namely the width of the straight bar with similar reinforcement. Visual examination of maxillary dentures suggested that frenal notches may be approximated by a symmetrical shape as indicated in Fig. 1. It was found that both unreinforced as well as reinforced bars can be shaped as desired with a milling machine, but careful inspection of the notches showed unsatisfactory reproducibility. ‘Leaky’ moulds’,” provided an alternative possiAustralian Dental Journal 1992;37:4.
bility for making notched reinforced bars. In this case the walls of the stainless steel mould were modified in order to produce samples with the desired shape. However, this technique was also unsuccessful because of poor contour duplication in the final samples, even if the soaked fibres were correctly positioned before curing. Also, the notched regions of the bars were prone to developed bubbles. Suitable contoured samples were finally obtained by processing an unreinforced resin notch (Trevalon C) onto a straight reinforced bar. The technique is similar to processing pink acrylic resin onto permanent denture bases and is summarized as follows: A stainless steel plate was constructed with 279
N
0 W
Table 1. Basic test data for ILSS, UFS and FM measurements Rod diameter
Sample length mm
Sample width mm
Sample thickness mm
Deflection rate mm min-’
20
11
2.5-2.0
2.0
50
65
11t
2.5
10
80
110
llt
2.5
20
2.5
10
Span mm
Test
Supporting mm
Loading mm
ILSS
6.0-2.5
6.0-4.0
UFS (ISO)*
3.2
3.2
UFS (RAE)
10
25
FM
10
25
160
200
10
3
*Loading for UFS and Charpy tests was applied on the broad face of the bars and, where applicable, at the centre of the notch. 712 mm for unreinforced bar.
Table 2.
Standard deviations of the mechanical measurements (per cent) Property
Resin
z-s ru 0
5
er
PMMA RM-3 Bis-GMA Crystic 272E Araldite 1927/GB
Reinforcement
No Yes Yes Yes Yes Yes
ILSS
13 8 8 4
4
FS
FM
6 13 4 4
-
Ab
Value
Decrease after three successive tests
Deflection at maximum load
10 10 10 10 -
30 30 30 -
16 8 8 8
-
1
straight bar was placed back in the groove followed by trial closure of the mould. The excess resin was removed prior to curing according to manufacturer’s recommendations. Notched unreinforced bars were produced with Trevalon C following standard dental technique in moulds as seen in Fig. 2. In this case the groove was made with a stainless steel plate of dimensions 130 m m x 12 mmx2.5 mm ( L x W x T ) to match the width of the unreinforced control bars. Figure 3 shows unreinforced and reinforced notched bars. Fibre loading The fibre content by volume in per cent, FV(Yo), of the straight bars was calculated with the relation:
F, (%)=
Fig. 3.-Untreated notched bars. (a) Unreinforced PMMA. All other bars reinforced with 48 vol% untreated HDLPE fibres. (b) PMMA. (c) RM-3. (d) Bis-GMA.
dimensions 130 mm x 10 mm X 2.5 mm (L x W X
T). One of the edges was shaped with a negative of the notch, placed in the middle of the long direction and produced with a numerically controlled electrical discharge machinel! to achieve the predetermined radius of curvature with less than 0.05 mm tolerance. A two part gypsum mould was made, one part having a smooth flat surface and the other, shown in Fig. 2, with the stainless steel plate permanently positioned adjacent to a groove. This was produced with the straight reinforced bar used as a master blank and previously polished to remove any trace of release agent left over from the leaky mould. Complete removal of the release agent was verified by the formation of a continuous water film after placing the bar under running tap water.*** The notched space was filled with Trevalon C and the surface wetted with MMA liquid. The ~
3.5 x 1, 290 x 0.97 x
XI00
(1)
x V,
where 1, and Vc are the length and volume of the bar, respectively, and 0.97 x g/mm3 is the density of the fibres.I3Measurements of 40 samples of various resins and fibre treatment showed the following fibre content, thereafter known as ‘standard’: Fv (Yo) = 48 vol (To) (SD = 2) (2). In some cases non-standard straight composite bars were also tested, namely having different fibre content from that stated above and/or 2.Q mm thickness instead of 2.5 mm. In all these cases the fibre volume fraction was calculated from relations (1) and (2). The fibre content of notched bars was taken as that of the corresponding straight bar. Testing All the mechanical testing was carried in 3-point bending mode of deformation, namely interlaminar shear strength (ILSS), flexural strength (FS), flexural modulus (FM) and absorption of energy at impact. The last is a Charpy type test with the load applied on the broad face of the samples and the values thus obtained are generally known as impact strength. Most of the experimental details related to FS, F M and Charpy tests have been described previously.’ There was however, an important change in that the FS test was modified to conform to I S 0 recommendations.ttt T o ensure continuity some tests were also performed with the previous geometry, arising from Standards established by the Royal Aircraft Establishment, UK*** for the
~
1lMakino EC-3040,Makino Milling Machine Co. Ltd, Tokyo, Japan. ‘“Sturgeon JB. Specimens and test methods for carbon fibre reinforced plastics. Royal Aircraft Establishment, UK 1971:IR71026. Australian Dental Journal 1992;37:4.
TttISO 1567-1978(E) 281
Table 3. ILSS of dental resins reinforced with longitudinally orientated HDLPE fibres Resin
Fibre treatment
Fibre content vol Q70
Nominal thickness mm
Conditioning environment
ILSS
Untreated
48
2.5
Treated
60 48
2.0 2.0 2.5
Untreated
48
2.0 2.5
Treated
48
2.0 2.5
Untreated
60 48
2.0 2.0 2.5
Treated
48
2.5
Untreated Treated Untreated
48 48 48 60 48 60
2.5 2.5 2.5 2.0 2.5 2.0
Dry Wet Dry Dry Dry Wet Dry Dry Wet Dry Dry Wet Dry Dry Dry Wet Dry Wet Dry Dry Dry Dry Dry Dry
16 13 13 11 22 20 22 26 24 24 28 19 25 21 20 19 36 26 19 23 18 15 26 23
PMMA
RM-3
Bis-GMA
Crystic 272E Araldite 1927iGB
Treated
MPa
Table 4. Flexural strength of straight dental resin bars reinforced with longitudinally orientated HDLPE fibres FS (ISO)
Resin
PMMA
Fibre treatment
Unreinforced (control) Untreated Treated
RM-3
Untreated Treated
Bis-GMA
Untreated Treated
Condirioning environment
Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wer
Value MPa 105 96 158 170 188 179 195 186 209 187 213 196 238 213
Decrease after Deflection three at maximum successive load tests (yo) mm Broke at 1st test 6 9 6 10 6 5 6 9 8 8 8 6
testing of carbon fibre composites. For convenience, these tests are here referred to as FS (ISO) and FS (RAE), respectively . The design data for ILSS, FS and FM measurements are shown in Table 1. The variables for the Charpy test remain as described in a previous paper.' 282
FS (RAE)
8 8 8 7 8 8 8 7 9 7 8 7 9 8
MPa
Decrease after Deflection three at maximum successive load tests ( 7 0 ) mm Unstable behaviour
17
-
4 6 4
-
15 14 15
174 168
5
15 15
89 -
155 154 151
-
178 178 182
-
-
-
-
4 5
-
-
-
-
16 16 16
-
Sturgeon*** recommended that the ILSS bending rig should be fitted with 6 mm diameter loading and supporting rods (smaller rollers could produce indentation of the samples during loading). It was found that these geometrical forms occasionally gave rise to transverse compression under the Australian Dental Journal 1992;37:4.
Table 5. Flexural modulus and impact properties of straight dental resin bars reinforced with longitudinally orientated HDLPE fibres Resin
Fibre treatment
Conditioning environment
FM GPa
Absorption of energy at impact First impact
PMMAlBroke Unreinforced (control) at first impact Untreated Treated RM-3
Untreated Treated
Bis-GMA
Untreated Treated
Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet
rollers before the onset of shear failure. In these cases different loadinglsupporting rods were used, namely 6 d 2 . 5 mm or 4 d 2 . 5 mm. Preliminary experiments showed that the ILSS of the samples was unaffected by these changes in the rod diameters. All tests were carried out at room temperature (23 "C f 2 "C). The effect of a wet environment was investigated by immersing the bars in water at 37 "C for three months, each in its own test tube. These were placed at room temperature about 20 hours before removing the samples for testing. Nonimmersed samples were kept at ambient condition between construction and testing, namely two to eight weeks. Immersed and non-immersed samples are referred to as 'wet' and 'dry', respectively. A number of wet samples were polished according to I S 0 recommendations. ttt Dry samples were left unpolished. Table 2 shows the standard deviations in per cent affecting the results. The number of nominally identical samples used for any specific test ranges from five to fifteen. Occasionally, fewer measurements were made when the property clearly matched the established trends. It may be noted that similar standard deviations were obtained from both notched samples and the corresponding straight bars, indicating the high notch reproducibility resulting from the production techniques used in the present work.
Results The mechanical properties of dental resins reinforced with longitudinally orientated continuous HDLPE fibres were measured as a function of Australian Dental Journal 1992;37:4.
3.2 2.9 17 16 22 22 21 22 25 26 20 22 26 27
10 10 117 118 109 134 99 99 90 102 117 118 100 96
(kJrn-I)
Second impact
20 33 8 35 10 10 10
-
12 19 7 6
several variables, namely type of resin, fibre treatment (adhesion level), water immersion, notch sensitivity and polishing before water immersion. Other variables were also studied for particular tests and these will be referred to in the appropriate sections. Straight bars The results may be seen in Table 3 for ILSS, Table 4 for FS (both I S 0 and RAE geonietries)and Table 5 for FM and Charpy tests. The averages for dry samples include the data presented irt a previous paper,' plus new measurements carried out to ensure continuity. The results are in general agreement with the values reported earlier' and any minor difference is due to the larger number of samples involved in the present work. Polishing of bars before immersion exposed some fibresSbut this had no effect on any of the properties measured. This variable was therefote excluded from the Tables and will not be considered further. It is of interest to compare the results given by the two FS tests, in order to assess the correlation between the previous' and present data. It may be seen that I S 0 geometry produced values about 10 per cent to 15 per cent higher than the RAE set up. Thus, the formula in Table 1 gave a moderately satisfactory correction for gauge length variations. Further comparisons between the two tests may be summarized as follows: (a) IS0 geometry (smaller span) gave a marginally larger decrease of the maximum load after three successive tests, that is, it is associated with higher damage to the system. 283
Table 6. Flexural strength and impact properties of notched dental resin bars reinforced with longitudinally orientated HDLPE fibres' Resin
Value MPa PMMA
Unreinforced Untreated Treated Untreated Untreated
RM-3 Bis-GMA
Absorption of energy at impact (kJm-')
FS (ISO)
Fibre treatment
Decrease after three Deflection at successive tests (70) maximum load (mm)
65 153 186 198 215
Broke at first test 9 6 5 6
3.4 8 8 9 8
First impact
Second impact
3.2 106 101 85 104
Broke at first impact 16 12 7 10
'All bars dry
Table 7. Characterization of the groups of ILSS test samples shown in Fig. 4' Group a b C
d e
Resin PMMA RM-3 PMMA PMMA Bis-GMA
Nominal Rods diameter thickness 'Onditioning (loadlsupport) environment mm mm 2 2.5 2.5 2.5 2.5
Dry Dry Dry Dry Wet
616-612.5 616 616 412.5 412.5
-
*All samples with 48 volume per cent fibre content.
(b) The deflection at maximum load is 100 per cent higher with the RAE set up, owing to the longer span used in this test. (c) The RAE geometry produced inconsistent behaviour of nominally identical unreinforced bars, namely some fractured at the first cycle while others did not.' With the I S 0 geometry all unreinforced bars broke at the first test, in agreement with the theoretical considerations made in the previous work.' Notched bars Only FS and impact strengths of dry samples have been tested and the results may be seen in Table 6 . Comparison with Tables 4 and 5 shows that notches drastically decreased the strength of the unreinforced resin, while the properties of the reinforced samples, namely the strength as well as the various characteristics of the deformation, remained virtually insensitive to the geometry. In this respect it should be noted that notches did not affect the conservation of the integrity of the tested reinforced samples, a characteristic already demonstrated for straight bars.'.'.1' Discussion Unless otherwise stated, the discussion refers to composites with 48 volume per cent fibre content. 284
Straight bars
m s The results are presented in Table 3 as a function of several variables. These may be divided into two main groups, (a) variables concerned with the validity and interpretation of the test and, (b) variables relevant to the clinical application of the composites.
(a) Considerations related to the validity of the test Smaller sample thickness and higher fibre content decreased the ILSS values. The effect was small when the variables were taken in isolation, but became significant when added together. The reduction of ILSS with decreasing thickness may be related to the transition from shear to flexural failure as the geometry changes." The results suggested that this occurred at about 2.0 mm thickness and, therefore, the standard 2.5 mm adopted for the bulk of these measurements appeared to be a satisfactory choice for the composites investigated in this study. The decrease of ILSS with increasing fibre content for 2.0 mm thick samples may be due to, (i) not enough resin to fully wet the surface of the reinforcement or, (ii) the thickness of the resin layer between the fibres may have been insufficient, leading to coupling effects and stress concentrations which produced early failure of the system. The first possibility appears to be ruled out by the excellent fibrehesin integration demonstrated in a previous study.'
(b) Consideration related to the performance of the composites Both plasma treatment and the resin used affected the ILSS values. Furthermore, there appeared to be an interaction between these two important variables. For convenience these matters will now be discussed for dry samples only. PMMA and Bis-GMA showed a substantial Australian Dental Journal 1992;37:4.
-
1 mm deflection
c _
1 mrn deflection
Fig. 4.-ILSS tests. Loadldeformation curves for PMMA composites. (a) Dry, untreated fibres. (b) Dry, treated fibres. (c) Wet, untreated fibres. (d) Wet, treated fibres.
increase of ILSS with plasma treatment of the reinforcement, while RM-3 was generally less sensitive to such treatment. Highest ILSS values were obtained with the Bis-GMAlplasma treated fibres system and this was, indeed, the highest interface strength observed with HDLPE fibre reinforced composites, even when using resins specifically designed for fibre reinf~rcement.'~A similar comparison showed that RM-3 was associated with the highest ILSS experienced with untreated HDLPE fibres. The values obtained with PMMA for both untreated and treated fibres were marginally below the respective levels obtained with resins manufactured for composite applications. The treatment of the fibres changed the nature of the load-deflection traces. Figures 4 and 5 show that for PMMA and Bis-GMA, untreated fibres were associated with smooth curves, while plasma treatment of the reinforcement introduced a distinctly jagged character to the traces. On the other hand, the load-deflection plots for RM-3 and the two industrial resins were fairly smooth in all cases, as seen in Fig. 6 for Crystic 272E composites. Nevertheless, a trace of roughness still appeared in the curves given by these resins reinforced with treated fibres. It was concluded that plasma treatment not only changed the intrinsic interface Australian Dental Journal 1992;37:4
Fig. 5. -1LSS tests. Loadldeformation curves for Bis-GMA composites: (a), (b), (c), (d) as per Fig. 4.
a
b
____
__. c
_
1 mm deflection
Fig. 6. -1LSS tests. Load/deformation curves for Crystic 272E composites: (a), (b) as per Fig. 4.
adhesion but, in addition, it affected the failure mechanisms operating during ILSS testing. The matter may be further elaborated as follows. Previous publication^^,^ reported pull-out experiments of monofilaments embedded in various resins. It was found that untreated fibres failed at the interface, by sliding of the filament. Plasma treatment produced a pitted surface topography into which the resin penetrated, resulting in fibre peeloff during pull-out. It is now proposed that similar mechanisms took place during ILSS testing. Untreated fibres would be associated with a 285
b
Fig. 7.-Samples after ILSS tests. The groups are characterized in Table 7. For each group the left and right sides correspond to untreated and treated fibres, respectively. Photograph taken with transmitted light.
continuously progressing failure, producing smooth curves as shown in Fig. 4a and 5a. Plasma treated fibres, on the other hand, gave rise to a stop-start mechanism and the failure propagated in stages (Fig. 4b, 5b). Each stage may be looked upon as releasing the stress concentrations, which had to build up again before the next stage became activated. The separation between successive stages may be affected by various factors, including the properties of the resin. Thus, some resins gave relatively smooth curves even if they were reinforced with treated fibres (Fig. 6b). Inspection of the actual samples (Fig. 7) supported the above considerations. For untreated fibres the shear was symmetrically distributed at each side of the loading area, and this may be regarded as a consequence of the continuous development of the failure occurring through slipping at the fibrehesin interface. For treated fibres, on the other hand, shear failure did not take place until fibre peel-off began. This mechanism was unlikely to be activated simultaneously at each side of the loading area but, instead, was probably initiated at one side owing to some minor unavoidable asymmetry of the samplehig set up. Once failure had been initiated the particular region was weakened and fbrther shear took place preferentially on the same side. Table 3 shows that the ILSS of the composites were generally insensitive to water immersion except for RM-3 and Bis-GMA reinforced with plasma treated fibres. In these cases water uptake was associated with significantly lower ILSS values, while the loaddeflection trace for the wet Bis-GMA samples had lost the jagged appearance which was so prominent when testing the respective dry bars (Fig. 5). It appeared that the reduction in ILSS and the change in the character of the trace were both 286
related to a modification of the properties of the resin by the wet environment, probably giving rise to a plasticing effect. PMMAheated fibres composites, on the other hand, produced similar jagged curves for both dry and wet samples (Fig. 4), as well as similar ILSS values. It follows that water sorption did not modify the properties of PMMA in a way that may be reflected in ILSS tests. For a given system, bars moulded under either high or low pressure had similar ILSS values. This applies for both dry and wet bars and indicated that the moulding conditions used for the construction of dentures have no significant effect on the fibrehesin interface. Moulding force has therefore been excluded as a variable in Table 3. Comparison of the results discussed above with those reported earlier3 indicated that pull-out experiments gave a broad prediction of the interface strength of the systems examined while, in a more detailed scale, failed to reflect several trends and relationships occurring in the actual composites. Thus, although pull-out tests were essentially suitable to follow the development of the fibrehesin adhesion, an understanding of composite behaviour required a study of ILSS properties. The most important performance aspects arising from Table 3 may now be summarized as follows. (1) Plasma treatment of the fibres generally increased the ILSS values, but the differences between treated and untreated reinforcement was not as large as suggested by pull-out experiment^.^ (2) The type of resin affected the interface strength of the composites. (3) The ILSS of denture base resinslHDLPE fibres were at least equal, and often superior to the values obtained with commercial resins reinforced with similar fibre^.'^ Australian Dental Journal 1992;374.
Fig. 8. -Loading area of PMMAlplasma treated fibre bars after FS test (1 cycle). (a) Notched. (b) Straight.
(4) Some variables of denture construction, such as polishing and moulding force, have been investigated and found to have no effect on the interface strength. (5) Water immersion had a modest deleterious effect on the ILSS of the composites, particularly for plasma treated reinforcement.
FS This section refers to IS0 tests only. Table 4 shows that fibre reinforcement produced 60 per cent to 100 per cent increase of FS, which was generAustralian Dental Journal 1992:37:4.
ally unaffected by the interface strength. Furthermore, the composite bars did not lose their integrity. These conclusions are in agreement with previously reported The flexural strength of the composites decreased by about 10 per cent after three cycles, indicating that the reinforcement is capable of retaining a significant proportion of the initial support after failure (maximum load) has occurred. It may be noted that the deflection at maximum load for the first cycle is between 7 mm to 9 mm, even for the unreinforced samples. Simple beam theoryI5 shows that this deflection 207
represents approximately 5 per cent of maximum tensile strain. As a trend, water immersion appeared to decrease the FS values of both the unreinforced and reinforced resins. The effect, however, was small and generally within the experimental scatter. All other aspects of the test remained unaffected by the conditioning environment. Table 4 shows that the FS of the composites had a small but noticeable dependence on the resin used, with Bis-GMA giving the highest values.
FM and impact strength For dry samples the results seen in Table 5 are in agreement with the conclusions presented earlier,' namely, the fibre reinforcement produces an increase of 700 per cent to 1200 per cent in both the FM and impact strength of the resin. For the latter property the composites did not disintegrate, but maintained their coherence over a large number of cycles (in the new experiments some reinforced PMMA samples were impacted forty times with no loss of coherence). However, a more careful control of fibre content, as well as the additional number of samples tested for the present work have revealed some minor trends and details not readily apparent before.' (a) For FM, there was a small increase of the values associated with plasma treatment of the reinforcement. The differences may be considered within the experimental error, but they were consistent throughout the range of resins. (b) For Charpy tests the decrease of absorption of energy with plasma treatment of the fibres (higher adhesion) was generally maintained, as reported and explained earlier.' RM-3, however, was an exception with a relatively low energy absorption for untreated reinforcement. This is likely to be associated with the high ILSS value obtained with the RM-3luntreated fibre system, as seen in Table 3. The effect of water immersion on Charpy tests was somewhat variable, but relatively small when compared with the drastic increase produced by the incorporation of HDLPE fibres in the resins. FM values, on the other hand, were insensitive to the conditioning environment. Table 5 shows that the absorption of energy at the second impact for reinforced samples is of the order, and often higher, than the energy absorbed by unreinforced resins at the first impact, when they fracture. Notched bars It may be argued that the large difference in notch sensitivity between the unreinforced and reinforced 208
samples (Tables 4, 5, 6) could be partly related to the different production methods (see section 3.2b). Therefore some unreinforced notched bars were also produced with the double moulding technique, namely a notch was added to an already made straight bar, and tested for FS and impact strength. The results were fully comparable with those obtained from equivalent bars produced in one stage. It is concluded that the notch insensitivity demonstrated for reinforced samples was solely due to the incorporation of the fibres. Nevertheless, the notches gave rise to stress concentrations in the reinforced samples, producing mechanisms of failure not necessarily analogous to those found in equivalent straight bars. For example, examination of two PMMA/plasma treated fibre bars after FS testing showed cracks and delamination associated with the notch (Fig. 8a), while these features were absent in tested straight bars (Fig. 8b). Both types of samples still produced similar FS results (Tables 4,6)because: (a) the final mechanical properties are controlled by the reinforcement, with little contribution from the resin other than keeping the array of fibres t ~ g e t h e rand, '~ (b) the fibres are more ductile than the resin and, thus, capable of relieving stress concentrations. " Kelly" discussed the large notch sensitivity of acrylic denture base materials and suggested modified procedures to eliminate abrupt changes in surface contour such as frenal notches. The present results indicate that HDLPE reinforced acrylic resins require no such precautions, providing a greater degree of freedom in the design of the prostheses. Conclusions Water immersion generally has little effect on the mechanical properties and interface strength of dental resins reinforced with HDLPE fibres. The large improvements previously reported' are fully maintained and the samples remain coherent even after very critical testing, such as forty successive impacts. Furthermore, the fibres virtually eliminate the high notch sensitivity of the unreinforced resin. ILSS tests complement the information obtained with pull-out measurements and give a fuller understanding of the mechanisms of adhesion and failure within the reinforced bars. In particular, it was shown that the type of resin has a significant effect on the interface strength, but this was not generally reflected in the performance of the composites. Dental resins reinforced with HDLPE fibres possessed an array of mechanical properties which were fully equal, and occasionally superior to equivalent composites produced with industrial Australian Dental Journal 1992;37:4.
resins. Some variables associated with denture construction have been studied and found to have no effect on the performance of reinforced denture base resins.
Acknowledgements The authors are grateful to Professor I. M. Ward and Dr D. W. Woods, of the Department of Physics, University of Leeds, UK, for supplying the HDLPE fibres. The work is part of a project supported by a research grant from the University of Hong Kong, No. 335.263.0003. References 1. Ladizesky NH, Chow TW, Ward IM. The effect of highly drawn polyethylene fibre on the mechanical properties of denture base resins. Clin Mater 1990;6:209-25. 2. Braden M, Davy KWM, Parker S, Ladizesky NH, Ward IM. Denture base poly(methylmethacry1ate)reinforced with ultra-high modulus polyethylene fibres. Br Dent J 1988;164:109-13. 3. Ladizesky NH. The integration of dental resins with highly drawn polyethylene fibre reinforcement. Clin Mater 1990;6: 181-92. 4. Clarke DA, Ladizesky NH, Chow TW. Acrylic resins reinforced with highly drawn linear polyethylene woven fibres: 1 - Construction of upper denture bases. Aust Dent J (in press). 5. Chow TW, Ladizesky NH, Clarke DA. Acrylic resins reinforced with highly drawn linear polyethylene woven fibres: 2 - Water sorption and clinical trials. Aust Dent J (in press). 6. Ladizesky NH, Ho CF, Chow TW. Reinforcement of complete denture bases with continuous high performance polyethylene fibres. J Prosthet Dent (in press).
Australian Dental Journal 1992;37:4
7. Ladizesky NH, Pang MKM. Mechanisms of failure of polymeric resins reinforced with high modulus polyethylene fibres - a microscopic investigation. Comp Sci Tech (in press). 8. Ladizesky NH, Ward IM. A study ofthe adhesion of drawn polyethylene fibrelpolymeric resin systems. J Mater Sci 1983; 18:533-44. 9. Chiao CC, Moore RL, Chiao TT. Measurement of shear properties of fibres composites: Part I - Evaluation of test methods. Composites 1977;8: 161-9. 10. Hull D. Introduction to composite materials. Cambridge: Cambridge University Press, 1981:Ch3. 11. Ladizesky NH, Ward IM. Ultra-high-modulus polyethylene fibre composites; I - The preparation and properties of conventional epoxy resin composites. Comp Sci Tech 1986;26: 129-64. 12. Kelly E. Fatigue failure in denture base polymers. J Pros Dent 1969;21:257-66. 13. Ward IM. The preparation, structure and properties of ultrahigh-modulus linear polyethylene. Adv Polym Sci 1985;70:1-70. 14. Ladizesky NH, Sitepu M, Ward IM. Ultra-high-modulus polyethylene composites: I1 - Effect of resin composition on properties. Comp Sci Tech 1986;26:169-83. 15. Williams JG. Stress analysis of polymers. London: Longman, 1973:Ch4.
Address for correspondenceheprints: N. H. Ladizesky, University of Hong Kong, Faculty of Dentistry, Dental Materials Science Unit, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong.
289
Key words: Acrylic resins, dental materials, notches, polyethylene fibres. Abstract Previous work' has presented a study of the mechanical properties of denture base resins reinforced with a new type of high performance fibre. It is now shown that the substantial improvements demonstrated in those composites remain largely unaffected by a watery environment, anatomical notches, moulding pressure and other factors of denture construction. The understandingof the reinforced resins is here complementedwith a detailed study of the interface strength, taking into account the various couplings occurring within the system. (Received for publication March 1991. Accepted August 1991.)
Introduction Ladizesky, Braden et u Z . ' - ~ have explored the possibility of reinforcing denture base resins with highly drawn linear polyethylene (HDLPE) fibres. This recently developed material offers an array of properties of particular interest to dentistry, including high stiffness and strength, proven biocompatibility, white translucent appearance and
*Dental Materials Science Unit, Faculty of Dentistry, University of Hong Kong. ?Department of Prosthetic Dentistry, Faculty of Dentistry, University of Hong Kong. Australian Dental Journal 1992;37(4):277-89.
negligible water absorption. Incorporation of at least 30 volume per cent in longitudinally oriented fibres produced a substantial improvement in the mechanical properties of denture base resins.' It was shown that these properties were broadly insensitive to the interface adhesion as determined with single fibre pull-out experiment^.',^ The present paper reports further work in three main areas, namely, (a) study of the adhesion levels within the actual composites, (b) the effect of water immersion at 37OC on the mechanical properties of the reinforced resins and, (c) sensitivity of these properties to notches that mimic anatomical features. Previous publications4.' presented a method for the construction of woven HDLPE fibre reinforced acrylic denture bases, complemented with clinical trials. Another communication will report on the construction of lower and upper denture bases reinforced with parallel continuous HDLPE fibres,6 as used for the samples tested in this work. The mechanisms of failure taking place during these tests, revealed by microscopy techniques, are discussed elsewhere.'
General comments on the measurement of the interface adhesion of fibrous composites Information on the fibrehesin interface strength plays a fundamental role in the correct appreciation of the mechanical properties of composite materials. Coupling effects which mask the intrinsic shear properties of the interface may be removed by testing single filament^.^.' However, in a prac277
tical situation fibre interactions are always present and it is generally accepted that no study of mechanical properties is complete unless it includes the measurement of adhesion levels within the actual composite. There are various methods to measure the interface strength in fibre reinforced resins? of which the most widely used is the interlaminar shear strength (ILSS) test. This requires longitudinally oriented reinforcement and involves a 3-point bending mode of deformation using samples whose dimensions are chosen so that shear in a plane parallel to the fibre direction and in the middle of the thickness occurs before flexural failure. This condition implies a relatively small gauge length to thickness ratiolo which, for a number of commercial composites lies in the region of about five to one.
Experimental Materials Most of the details regarding materials and preparation of composite bars have been reported Therefore only a brief summary is given here, incorporating any changes applying to the present work. HDLPE multifilament fibrest were used throughout. The spun fibres were stretched to a draw ratio of 30: 1, resulting in a yarn of 280 denier with 180 filaments. Plasma treatment for enhanced adhesion was carried out with a Plasmaprep 300 unit.$ Bundles of fibres 290 mm long and 3.5 g mass were introduced into the reactor and treated with 120 W power for 2 min using a flow of 17 x lo3 mm3 min-I of O2gas as the plasma carrier. The pressure inside the reactor was 0.5 Torr. Five different resins were used, namely: (a) PMMA syrup, a non-proprietary type made with one part by mass of Rapid Repair powder11 with four parts by mass of Natural Coe-Lor 1iquid.l (b) RM-3,** a pour type resin diluted with 8 per cent by mass of methyl methacrylate. (c) Bis-GMA,tt mostly used as the matrix in composite restorative materials, diluted with 19 per cent by mass of methyl methacrylate.
(d) Crystic 272E.$$ (e) Araldite LY 1927/GB.§§ The first three are modified heat curing dental resins, while the last two are of the cold curing type (a polyester and an epoxy, respectively) specifically developed for fibre reinforcement. The composition and handling of all these resins are given elsewhere. 1,1
Sample production (a) Straight bars Straight composite bars of nominal dimensions 210 mm x 11 mm x 2.5 mm (L x W x T) were produced with the ‘leaky’ mould technique.’”’ The reinforcement was incorporated in the form of a longitudinally oriented bundle of fibres 290 mm long and 3.5 g mass. Two different loads were used to remove the excess liquid resin and obtain the predetermined thickness; (a) high pressure moulding, namely 25 kN slowly applied on the mould over several minutes, as used for the construction of dentures, (b) low pressure moulding with masses totalling 500 N left on top of the mould for about 30 min to 45 min. Unless otherwise stated all samples were produced with high pressure moulding and have standard nominal dimensions as stated above. In earlier work’ it was noted that reinforced bars made in ‘leaky’ moulds with PMMA syrup had patches of unwetted fibres, particularly when these were untreated. This problem was significantly improved in the present work by presoaking the bundles of fibres in the resin for about 20 hours, followed by squeezing out the excess liquid with gloved hands and four hours drying in a fume cupboard. For consistency, this procedure was also applied to the construction of bars made with RM-3 and Bis-GMA resins. Control tests were carried out with unreinforced acrylic bars of 210 mm x 12 mm x 2.5 mm (L x W x T) made with Trevalon C, 1 1 using a standard dental moulding technique. This included a two-part gypsum mould with one part having a flat smooth surface, while the other part had a groove made with a stainless steel blank, into which the resin was packed. (b) Notched bars The failure properties of acrylic dentures are highly sensitive to stress concentrations produced
tCelanese Research Company, Summit, NJ, USA. BNanotech Ltd, Manchester, UK. AD International Ltd, De Trey Division, England. (Coe Laboratories, Chicago, USA. **Ivoclar AG, Lichtenstein. ttNupol 46-4005. DMS Resin UK, South Wirral, England 270
StScott Bader, Wellingborough, England. §§Ciba-Geigy, Plastic Division, Cambridge, England 1 I(AD International Ltd, De Trey Division, UK. Australian Dental Journal 1992;37:4.
a
C
-
b
Fig. 1.-Dimensions (mm) of the notched regions of bars. a = 2; b = 8; c = 2; r=0.5; R = 1 . 5 .
Fig. 2.-Mould for processing an acrylic resin notch onto a straight reinforced bar. Similar moulds were also used to produce unreinforced PMMA bars.
by abrupt contour changes such as the frenal notch.I2 Testing of such effects requires the production of samples with very well defined geometry which should be reproducible to a high degree of accuracy. Also, the width of the bar at the base of the notch should be similar to the width of the sample outside the contoured region, namely the width of the straight bar with similar reinforcement. Visual examination of maxillary dentures suggested that frenal notches may be approximated by a symmetrical shape as indicated in Fig. 1. It was found that both unreinforced as well as reinforced bars can be shaped as desired with a milling machine, but careful inspection of the notches showed unsatisfactory reproducibility. ‘Leaky’ moulds’,” provided an alternative possiAustralian Dental Journal 1992;37:4.
bility for making notched reinforced bars. In this case the walls of the stainless steel mould were modified in order to produce samples with the desired shape. However, this technique was also unsuccessful because of poor contour duplication in the final samples, even if the soaked fibres were correctly positioned before curing. Also, the notched regions of the bars were prone to developed bubbles. Suitable contoured samples were finally obtained by processing an unreinforced resin notch (Trevalon C) onto a straight reinforced bar. The technique is similar to processing pink acrylic resin onto permanent denture bases and is summarized as follows: A stainless steel plate was constructed with 279
N
0 W
Table 1. Basic test data for ILSS, UFS and FM measurements Rod diameter
Sample length mm
Sample width mm
Sample thickness mm
Deflection rate mm min-’
20
11
2.5-2.0
2.0
50
65
11t
2.5
10
80
110
llt
2.5
20
2.5
10
Span mm
Test
Supporting mm
Loading mm
ILSS
6.0-2.5
6.0-4.0
UFS (ISO)*
3.2
3.2
UFS (RAE)
10
25
FM
10
25
160
200
10
3
*Loading for UFS and Charpy tests was applied on the broad face of the bars and, where applicable, at the centre of the notch. 712 mm for unreinforced bar.
Table 2.
Standard deviations of the mechanical measurements (per cent) Property
Resin
z-s ru 0
5
er
PMMA RM-3 Bis-GMA Crystic 272E Araldite 1927/GB
Reinforcement
No Yes Yes Yes Yes Yes
ILSS
13 8 8 4
4
FS
FM
6 13 4 4
-
Ab
Value
Decrease after three successive tests
Deflection at maximum load
10 10 10 10 -
30 30 30 -
16 8 8 8
-
1
straight bar was placed back in the groove followed by trial closure of the mould. The excess resin was removed prior to curing according to manufacturer’s recommendations. Notched unreinforced bars were produced with Trevalon C following standard dental technique in moulds as seen in Fig. 2. In this case the groove was made with a stainless steel plate of dimensions 130 m m x 12 mmx2.5 mm ( L x W x T ) to match the width of the unreinforced control bars. Figure 3 shows unreinforced and reinforced notched bars. Fibre loading The fibre content by volume in per cent, FV(Yo), of the straight bars was calculated with the relation:
F, (%)=
Fig. 3.-Untreated notched bars. (a) Unreinforced PMMA. All other bars reinforced with 48 vol% untreated HDLPE fibres. (b) PMMA. (c) RM-3. (d) Bis-GMA.
dimensions 130 mm x 10 mm X 2.5 mm (L x W X
T). One of the edges was shaped with a negative of the notch, placed in the middle of the long direction and produced with a numerically controlled electrical discharge machinel! to achieve the predetermined radius of curvature with less than 0.05 mm tolerance. A two part gypsum mould was made, one part having a smooth flat surface and the other, shown in Fig. 2, with the stainless steel plate permanently positioned adjacent to a groove. This was produced with the straight reinforced bar used as a master blank and previously polished to remove any trace of release agent left over from the leaky mould. Complete removal of the release agent was verified by the formation of a continuous water film after placing the bar under running tap water.*** The notched space was filled with Trevalon C and the surface wetted with MMA liquid. The ~
3.5 x 1, 290 x 0.97 x
XI00
(1)
x V,
where 1, and Vc are the length and volume of the bar, respectively, and 0.97 x g/mm3 is the density of the fibres.I3Measurements of 40 samples of various resins and fibre treatment showed the following fibre content, thereafter known as ‘standard’: Fv (Yo) = 48 vol (To) (SD = 2) (2). In some cases non-standard straight composite bars were also tested, namely having different fibre content from that stated above and/or 2.Q mm thickness instead of 2.5 mm. In all these cases the fibre volume fraction was calculated from relations (1) and (2). The fibre content of notched bars was taken as that of the corresponding straight bar. Testing All the mechanical testing was carried in 3-point bending mode of deformation, namely interlaminar shear strength (ILSS), flexural strength (FS), flexural modulus (FM) and absorption of energy at impact. The last is a Charpy type test with the load applied on the broad face of the samples and the values thus obtained are generally known as impact strength. Most of the experimental details related to FS, F M and Charpy tests have been described previously.’ There was however, an important change in that the FS test was modified to conform to I S 0 recommendations.ttt T o ensure continuity some tests were also performed with the previous geometry, arising from Standards established by the Royal Aircraft Establishment, UK*** for the
~
1lMakino EC-3040,Makino Milling Machine Co. Ltd, Tokyo, Japan. ‘“Sturgeon JB. Specimens and test methods for carbon fibre reinforced plastics. Royal Aircraft Establishment, UK 1971:IR71026. Australian Dental Journal 1992;37:4.
TttISO 1567-1978(E) 281
Table 3. ILSS of dental resins reinforced with longitudinally orientated HDLPE fibres Resin
Fibre treatment
Fibre content vol Q70
Nominal thickness mm
Conditioning environment
ILSS
Untreated
48
2.5
Treated
60 48
2.0 2.0 2.5
Untreated
48
2.0 2.5
Treated
48
2.0 2.5
Untreated
60 48
2.0 2.0 2.5
Treated
48
2.5
Untreated Treated Untreated
48 48 48 60 48 60
2.5 2.5 2.5 2.0 2.5 2.0
Dry Wet Dry Dry Dry Wet Dry Dry Wet Dry Dry Wet Dry Dry Dry Wet Dry Wet Dry Dry Dry Dry Dry Dry
16 13 13 11 22 20 22 26 24 24 28 19 25 21 20 19 36 26 19 23 18 15 26 23
PMMA
RM-3
Bis-GMA
Crystic 272E Araldite 1927iGB
Treated
MPa
Table 4. Flexural strength of straight dental resin bars reinforced with longitudinally orientated HDLPE fibres FS (ISO)
Resin
PMMA
Fibre treatment
Unreinforced (control) Untreated Treated
RM-3
Untreated Treated
Bis-GMA
Untreated Treated
Condirioning environment
Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wer
Value MPa 105 96 158 170 188 179 195 186 209 187 213 196 238 213
Decrease after Deflection three at maximum successive load tests (yo) mm Broke at 1st test 6 9 6 10 6 5 6 9 8 8 8 6
testing of carbon fibre composites. For convenience, these tests are here referred to as FS (ISO) and FS (RAE), respectively . The design data for ILSS, FS and FM measurements are shown in Table 1. The variables for the Charpy test remain as described in a previous paper.' 282
FS (RAE)
8 8 8 7 8 8 8 7 9 7 8 7 9 8
MPa
Decrease after Deflection three at maximum successive load tests ( 7 0 ) mm Unstable behaviour
17
-
4 6 4
-
15 14 15
174 168
5
15 15
89 -
155 154 151
-
178 178 182
-
-
-
-
4 5
-
-
-
-
16 16 16
-
Sturgeon*** recommended that the ILSS bending rig should be fitted with 6 mm diameter loading and supporting rods (smaller rollers could produce indentation of the samples during loading). It was found that these geometrical forms occasionally gave rise to transverse compression under the Australian Dental Journal 1992;37:4.
Table 5. Flexural modulus and impact properties of straight dental resin bars reinforced with longitudinally orientated HDLPE fibres Resin
Fibre treatment
Conditioning environment
FM GPa
Absorption of energy at impact First impact
PMMAlBroke Unreinforced (control) at first impact Untreated Treated RM-3
Untreated Treated
Bis-GMA
Untreated Treated
Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet
rollers before the onset of shear failure. In these cases different loadinglsupporting rods were used, namely 6 d 2 . 5 mm or 4 d 2 . 5 mm. Preliminary experiments showed that the ILSS of the samples was unaffected by these changes in the rod diameters. All tests were carried out at room temperature (23 "C f 2 "C). The effect of a wet environment was investigated by immersing the bars in water at 37 "C for three months, each in its own test tube. These were placed at room temperature about 20 hours before removing the samples for testing. Nonimmersed samples were kept at ambient condition between construction and testing, namely two to eight weeks. Immersed and non-immersed samples are referred to as 'wet' and 'dry', respectively. A number of wet samples were polished according to I S 0 recommendations. ttt Dry samples were left unpolished. Table 2 shows the standard deviations in per cent affecting the results. The number of nominally identical samples used for any specific test ranges from five to fifteen. Occasionally, fewer measurements were made when the property clearly matched the established trends. It may be noted that similar standard deviations were obtained from both notched samples and the corresponding straight bars, indicating the high notch reproducibility resulting from the production techniques used in the present work.
Results The mechanical properties of dental resins reinforced with longitudinally orientated continuous HDLPE fibres were measured as a function of Australian Dental Journal 1992;37:4.
3.2 2.9 17 16 22 22 21 22 25 26 20 22 26 27
10 10 117 118 109 134 99 99 90 102 117 118 100 96
(kJrn-I)
Second impact
20 33 8 35 10 10 10
-
12 19 7 6
several variables, namely type of resin, fibre treatment (adhesion level), water immersion, notch sensitivity and polishing before water immersion. Other variables were also studied for particular tests and these will be referred to in the appropriate sections. Straight bars The results may be seen in Table 3 for ILSS, Table 4 for FS (both I S 0 and RAE geonietries)and Table 5 for FM and Charpy tests. The averages for dry samples include the data presented irt a previous paper,' plus new measurements carried out to ensure continuity. The results are in general agreement with the values reported earlier' and any minor difference is due to the larger number of samples involved in the present work. Polishing of bars before immersion exposed some fibresSbut this had no effect on any of the properties measured. This variable was therefote excluded from the Tables and will not be considered further. It is of interest to compare the results given by the two FS tests, in order to assess the correlation between the previous' and present data. It may be seen that I S 0 geometry produced values about 10 per cent to 15 per cent higher than the RAE set up. Thus, the formula in Table 1 gave a moderately satisfactory correction for gauge length variations. Further comparisons between the two tests may be summarized as follows: (a) IS0 geometry (smaller span) gave a marginally larger decrease of the maximum load after three successive tests, that is, it is associated with higher damage to the system. 283
Table 6. Flexural strength and impact properties of notched dental resin bars reinforced with longitudinally orientated HDLPE fibres' Resin
Value MPa PMMA
Unreinforced Untreated Treated Untreated Untreated
RM-3 Bis-GMA
Absorption of energy at impact (kJm-')
FS (ISO)
Fibre treatment
Decrease after three Deflection at successive tests (70) maximum load (mm)
65 153 186 198 215
Broke at first test 9 6 5 6
3.4 8 8 9 8
First impact
Second impact
3.2 106 101 85 104
Broke at first impact 16 12 7 10
'All bars dry
Table 7. Characterization of the groups of ILSS test samples shown in Fig. 4' Group a b C
d e
Resin PMMA RM-3 PMMA PMMA Bis-GMA
Nominal Rods diameter thickness 'Onditioning (loadlsupport) environment mm mm 2 2.5 2.5 2.5 2.5
Dry Dry Dry Dry Wet
616-612.5 616 616 412.5 412.5
-
*All samples with 48 volume per cent fibre content.
(b) The deflection at maximum load is 100 per cent higher with the RAE set up, owing to the longer span used in this test. (c) The RAE geometry produced inconsistent behaviour of nominally identical unreinforced bars, namely some fractured at the first cycle while others did not.' With the I S 0 geometry all unreinforced bars broke at the first test, in agreement with the theoretical considerations made in the previous work.' Notched bars Only FS and impact strengths of dry samples have been tested and the results may be seen in Table 6 . Comparison with Tables 4 and 5 shows that notches drastically decreased the strength of the unreinforced resin, while the properties of the reinforced samples, namely the strength as well as the various characteristics of the deformation, remained virtually insensitive to the geometry. In this respect it should be noted that notches did not affect the conservation of the integrity of the tested reinforced samples, a characteristic already demonstrated for straight bars.'.'.1' Discussion Unless otherwise stated, the discussion refers to composites with 48 volume per cent fibre content. 284
Straight bars
m s The results are presented in Table 3 as a function of several variables. These may be divided into two main groups, (a) variables concerned with the validity and interpretation of the test and, (b) variables relevant to the clinical application of the composites.
(a) Considerations related to the validity of the test Smaller sample thickness and higher fibre content decreased the ILSS values. The effect was small when the variables were taken in isolation, but became significant when added together. The reduction of ILSS with decreasing thickness may be related to the transition from shear to flexural failure as the geometry changes." The results suggested that this occurred at about 2.0 mm thickness and, therefore, the standard 2.5 mm adopted for the bulk of these measurements appeared to be a satisfactory choice for the composites investigated in this study. The decrease of ILSS with increasing fibre content for 2.0 mm thick samples may be due to, (i) not enough resin to fully wet the surface of the reinforcement or, (ii) the thickness of the resin layer between the fibres may have been insufficient, leading to coupling effects and stress concentrations which produced early failure of the system. The first possibility appears to be ruled out by the excellent fibrehesin integration demonstrated in a previous study.'
(b) Consideration related to the performance of the composites Both plasma treatment and the resin used affected the ILSS values. Furthermore, there appeared to be an interaction between these two important variables. For convenience these matters will now be discussed for dry samples only. PMMA and Bis-GMA showed a substantial Australian Dental Journal 1992;37:4.
-
1 mm deflection
c _
1 mrn deflection
Fig. 4.-ILSS tests. Loadldeformation curves for PMMA composites. (a) Dry, untreated fibres. (b) Dry, treated fibres. (c) Wet, untreated fibres. (d) Wet, treated fibres.
increase of ILSS with plasma treatment of the reinforcement, while RM-3 was generally less sensitive to such treatment. Highest ILSS values were obtained with the Bis-GMAlplasma treated fibres system and this was, indeed, the highest interface strength observed with HDLPE fibre reinforced composites, even when using resins specifically designed for fibre reinf~rcement.'~A similar comparison showed that RM-3 was associated with the highest ILSS experienced with untreated HDLPE fibres. The values obtained with PMMA for both untreated and treated fibres were marginally below the respective levels obtained with resins manufactured for composite applications. The treatment of the fibres changed the nature of the load-deflection traces. Figures 4 and 5 show that for PMMA and Bis-GMA, untreated fibres were associated with smooth curves, while plasma treatment of the reinforcement introduced a distinctly jagged character to the traces. On the other hand, the load-deflection plots for RM-3 and the two industrial resins were fairly smooth in all cases, as seen in Fig. 6 for Crystic 272E composites. Nevertheless, a trace of roughness still appeared in the curves given by these resins reinforced with treated fibres. It was concluded that plasma treatment not only changed the intrinsic interface Australian Dental Journal 1992;37:4
Fig. 5. -1LSS tests. Loadldeformation curves for Bis-GMA composites: (a), (b), (c), (d) as per Fig. 4.
a
b
____
__. c
_
1 mm deflection
Fig. 6. -1LSS tests. Load/deformation curves for Crystic 272E composites: (a), (b) as per Fig. 4.
adhesion but, in addition, it affected the failure mechanisms operating during ILSS testing. The matter may be further elaborated as follows. Previous publication^^,^ reported pull-out experiments of monofilaments embedded in various resins. It was found that untreated fibres failed at the interface, by sliding of the filament. Plasma treatment produced a pitted surface topography into which the resin penetrated, resulting in fibre peeloff during pull-out. It is now proposed that similar mechanisms took place during ILSS testing. Untreated fibres would be associated with a 285
b
Fig. 7.-Samples after ILSS tests. The groups are characterized in Table 7. For each group the left and right sides correspond to untreated and treated fibres, respectively. Photograph taken with transmitted light.
continuously progressing failure, producing smooth curves as shown in Fig. 4a and 5a. Plasma treated fibres, on the other hand, gave rise to a stop-start mechanism and the failure propagated in stages (Fig. 4b, 5b). Each stage may be looked upon as releasing the stress concentrations, which had to build up again before the next stage became activated. The separation between successive stages may be affected by various factors, including the properties of the resin. Thus, some resins gave relatively smooth curves even if they were reinforced with treated fibres (Fig. 6b). Inspection of the actual samples (Fig. 7) supported the above considerations. For untreated fibres the shear was symmetrically distributed at each side of the loading area, and this may be regarded as a consequence of the continuous development of the failure occurring through slipping at the fibrehesin interface. For treated fibres, on the other hand, shear failure did not take place until fibre peel-off began. This mechanism was unlikely to be activated simultaneously at each side of the loading area but, instead, was probably initiated at one side owing to some minor unavoidable asymmetry of the samplehig set up. Once failure had been initiated the particular region was weakened and fbrther shear took place preferentially on the same side. Table 3 shows that the ILSS of the composites were generally insensitive to water immersion except for RM-3 and Bis-GMA reinforced with plasma treated fibres. In these cases water uptake was associated with significantly lower ILSS values, while the loaddeflection trace for the wet Bis-GMA samples had lost the jagged appearance which was so prominent when testing the respective dry bars (Fig. 5). It appeared that the reduction in ILSS and the change in the character of the trace were both 286
related to a modification of the properties of the resin by the wet environment, probably giving rise to a plasticing effect. PMMAheated fibres composites, on the other hand, produced similar jagged curves for both dry and wet samples (Fig. 4), as well as similar ILSS values. It follows that water sorption did not modify the properties of PMMA in a way that may be reflected in ILSS tests. For a given system, bars moulded under either high or low pressure had similar ILSS values. This applies for both dry and wet bars and indicated that the moulding conditions used for the construction of dentures have no significant effect on the fibrehesin interface. Moulding force has therefore been excluded as a variable in Table 3. Comparison of the results discussed above with those reported earlier3 indicated that pull-out experiments gave a broad prediction of the interface strength of the systems examined while, in a more detailed scale, failed to reflect several trends and relationships occurring in the actual composites. Thus, although pull-out tests were essentially suitable to follow the development of the fibrehesin adhesion, an understanding of composite behaviour required a study of ILSS properties. The most important performance aspects arising from Table 3 may now be summarized as follows. (1) Plasma treatment of the fibres generally increased the ILSS values, but the differences between treated and untreated reinforcement was not as large as suggested by pull-out experiment^.^ (2) The type of resin affected the interface strength of the composites. (3) The ILSS of denture base resinslHDLPE fibres were at least equal, and often superior to the values obtained with commercial resins reinforced with similar fibre^.'^ Australian Dental Journal 1992;374.
Fig. 8. -Loading area of PMMAlplasma treated fibre bars after FS test (1 cycle). (a) Notched. (b) Straight.
(4) Some variables of denture construction, such as polishing and moulding force, have been investigated and found to have no effect on the interface strength. (5) Water immersion had a modest deleterious effect on the ILSS of the composites, particularly for plasma treated reinforcement.
FS This section refers to IS0 tests only. Table 4 shows that fibre reinforcement produced 60 per cent to 100 per cent increase of FS, which was generAustralian Dental Journal 1992:37:4.
ally unaffected by the interface strength. Furthermore, the composite bars did not lose their integrity. These conclusions are in agreement with previously reported The flexural strength of the composites decreased by about 10 per cent after three cycles, indicating that the reinforcement is capable of retaining a significant proportion of the initial support after failure (maximum load) has occurred. It may be noted that the deflection at maximum load for the first cycle is between 7 mm to 9 mm, even for the unreinforced samples. Simple beam theoryI5 shows that this deflection 207
represents approximately 5 per cent of maximum tensile strain. As a trend, water immersion appeared to decrease the FS values of both the unreinforced and reinforced resins. The effect, however, was small and generally within the experimental scatter. All other aspects of the test remained unaffected by the conditioning environment. Table 4 shows that the FS of the composites had a small but noticeable dependence on the resin used, with Bis-GMA giving the highest values.
FM and impact strength For dry samples the results seen in Table 5 are in agreement with the conclusions presented earlier,' namely, the fibre reinforcement produces an increase of 700 per cent to 1200 per cent in both the FM and impact strength of the resin. For the latter property the composites did not disintegrate, but maintained their coherence over a large number of cycles (in the new experiments some reinforced PMMA samples were impacted forty times with no loss of coherence). However, a more careful control of fibre content, as well as the additional number of samples tested for the present work have revealed some minor trends and details not readily apparent before.' (a) For FM, there was a small increase of the values associated with plasma treatment of the reinforcement. The differences may be considered within the experimental error, but they were consistent throughout the range of resins. (b) For Charpy tests the decrease of absorption of energy with plasma treatment of the fibres (higher adhesion) was generally maintained, as reported and explained earlier.' RM-3, however, was an exception with a relatively low energy absorption for untreated reinforcement. This is likely to be associated with the high ILSS value obtained with the RM-3luntreated fibre system, as seen in Table 3. The effect of water immersion on Charpy tests was somewhat variable, but relatively small when compared with the drastic increase produced by the incorporation of HDLPE fibres in the resins. FM values, on the other hand, were insensitive to the conditioning environment. Table 5 shows that the absorption of energy at the second impact for reinforced samples is of the order, and often higher, than the energy absorbed by unreinforced resins at the first impact, when they fracture. Notched bars It may be argued that the large difference in notch sensitivity between the unreinforced and reinforced 208
samples (Tables 4, 5, 6) could be partly related to the different production methods (see section 3.2b). Therefore some unreinforced notched bars were also produced with the double moulding technique, namely a notch was added to an already made straight bar, and tested for FS and impact strength. The results were fully comparable with those obtained from equivalent bars produced in one stage. It is concluded that the notch insensitivity demonstrated for reinforced samples was solely due to the incorporation of the fibres. Nevertheless, the notches gave rise to stress concentrations in the reinforced samples, producing mechanisms of failure not necessarily analogous to those found in equivalent straight bars. For example, examination of two PMMA/plasma treated fibre bars after FS testing showed cracks and delamination associated with the notch (Fig. 8a), while these features were absent in tested straight bars (Fig. 8b). Both types of samples still produced similar FS results (Tables 4,6)because: (a) the final mechanical properties are controlled by the reinforcement, with little contribution from the resin other than keeping the array of fibres t ~ g e t h e rand, '~ (b) the fibres are more ductile than the resin and, thus, capable of relieving stress concentrations. " Kelly" discussed the large notch sensitivity of acrylic denture base materials and suggested modified procedures to eliminate abrupt changes in surface contour such as frenal notches. The present results indicate that HDLPE reinforced acrylic resins require no such precautions, providing a greater degree of freedom in the design of the prostheses. Conclusions Water immersion generally has little effect on the mechanical properties and interface strength of dental resins reinforced with HDLPE fibres. The large improvements previously reported' are fully maintained and the samples remain coherent even after very critical testing, such as forty successive impacts. Furthermore, the fibres virtually eliminate the high notch sensitivity of the unreinforced resin. ILSS tests complement the information obtained with pull-out measurements and give a fuller understanding of the mechanisms of adhesion and failure within the reinforced bars. In particular, it was shown that the type of resin has a significant effect on the interface strength, but this was not generally reflected in the performance of the composites. Dental resins reinforced with HDLPE fibres possessed an array of mechanical properties which were fully equal, and occasionally superior to equivalent composites produced with industrial Australian Dental Journal 1992;37:4.
resins. Some variables associated with denture construction have been studied and found to have no effect on the performance of reinforced denture base resins.
Acknowledgements The authors are grateful to Professor I. M. Ward and Dr D. W. Woods, of the Department of Physics, University of Leeds, UK, for supplying the HDLPE fibres. The work is part of a project supported by a research grant from the University of Hong Kong, No. 335.263.0003. References 1. Ladizesky NH, Chow TW, Ward IM. The effect of highly drawn polyethylene fibre on the mechanical properties of denture base resins. Clin Mater 1990;6:209-25. 2. Braden M, Davy KWM, Parker S, Ladizesky NH, Ward IM. Denture base poly(methylmethacry1ate)reinforced with ultra-high modulus polyethylene fibres. Br Dent J 1988;164:109-13. 3. Ladizesky NH. The integration of dental resins with highly drawn polyethylene fibre reinforcement. Clin Mater 1990;6: 181-92. 4. Clarke DA, Ladizesky NH, Chow TW. Acrylic resins reinforced with highly drawn linear polyethylene woven fibres: 1 - Construction of upper denture bases. Aust Dent J (in press). 5. Chow TW, Ladizesky NH, Clarke DA. Acrylic resins reinforced with highly drawn linear polyethylene woven fibres: 2 - Water sorption and clinical trials. Aust Dent J (in press). 6. Ladizesky NH, Ho CF, Chow TW. Reinforcement of complete denture bases with continuous high performance polyethylene fibres. J Prosthet Dent (in press).
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7. Ladizesky NH, Pang MKM. Mechanisms of failure of polymeric resins reinforced with high modulus polyethylene fibres - a microscopic investigation. Comp Sci Tech (in press). 8. Ladizesky NH, Ward IM. A study ofthe adhesion of drawn polyethylene fibrelpolymeric resin systems. J Mater Sci 1983; 18:533-44. 9. Chiao CC, Moore RL, Chiao TT. Measurement of shear properties of fibres composites: Part I - Evaluation of test methods. Composites 1977;8: 161-9. 10. Hull D. Introduction to composite materials. Cambridge: Cambridge University Press, 1981:Ch3. 11. Ladizesky NH, Ward IM. Ultra-high-modulus polyethylene fibre composites; I - The preparation and properties of conventional epoxy resin composites. Comp Sci Tech 1986;26: 129-64. 12. Kelly E. Fatigue failure in denture base polymers. J Pros Dent 1969;21:257-66. 13. Ward IM. The preparation, structure and properties of ultrahigh-modulus linear polyethylene. Adv Polym Sci 1985;70:1-70. 14. Ladizesky NH, Sitepu M, Ward IM. Ultra-high-modulus polyethylene composites: I1 - Effect of resin composition on properties. Comp Sci Tech 1986;26:169-83. 15. Williams JG. Stress analysis of polymers. London: Longman, 1973:Ch4.
Address for correspondenceheprints: N. H. Ladizesky, University of Hong Kong, Faculty of Dentistry, Dental Materials Science Unit, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong.
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