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Properties of self-reinforced ultra-high-molecular-weight polyethylene composites
Biomoterials 18 (1997) 645-655 0 1997 Elsevier Science Limited PII
ELSEVIER
SOl42-9612
(96)
Printed in Great Britain. All rights reserved 0142.9612/97/$17.00
00194-9
Properties of self-reinforced ultra-high-molecular-weight polyethylene composites Meng Deng” and Shalaby W. Shalaby+ ‘Department
Labs, Center
of Bioengineering, Clemson University, Clemson, South Carolina 29634, USA; +Po/y-Med, Inc., R&D for Applied Technology, 511 Westinghouse Road, Pendleton, South Carolina 29670, USA
The physical
properties
composites
have
resistance,
were
significantly
The longitudinal content,
of ultra-high-molecular-weight
been characterized. tensile
according
increased
strength
was no difference and tensile (SEM).
Received
impact
indicate
biomedical
applications.
1996; accepted
of Chemistry, 08855, USA.
were
polyethylene,
26 October
composites
Science
composite,
for fibre
UHMWPE.
Limited. mechanical
matrix.
with fibre
than plain
by scanning
matrix
and creep
into a UHMWPE
did not change
and plain
UHMWPE
fibre/UHMWPE
and modulus, increased
was higher
examined
$? 1997 Elsevier
content
UHMWPE.
of
There
The cross-section
electron
microscopy
may be good candidates All rights
reserved
properties,
SEM, DSC
1996
mechanical performance four categories:
Since its introduction in low-friction arthroplasty in ultra-high-molecular-weight polyethylene 1962’, (UHMWPE) has found wide application as a loadbearing material in th.e majority of joint endoprostheses in combination with metal or ceramic counterparts. This undoubtedly results from its combination of good physical and chemical properties and acceptable biocompatibility. Although the short-term function of most UHMWPE implants is satisfactory, their longterm performance has been a concern for many years. As the implants are expected to survive and function well beyond 10 years in the body, the long-term performance limitations of UHMWPE as an orthopaedic bearing surface begin to manifest as wear, creep and fatigue fracture. UHMWPE is a viscoelastic material, and the problems with the polymer originate from some of its inherent weaknesses (like creep and fatigue) compared to the metal stem and the cortical bone. Therefore, the wear and creep of such articulating materials must be reduced for improving long-term device performance, particularly for heavier, younger and more active patients’. Considering the existing problems associated with the use of UHMWPE implants, studies have been undertaken in attempts to understand the underlying mechanisms of polyethylene failure and thus extend the functional life of joint replacements. To date, the generally suggested approaches to increase the Correspondence to Dr S.W. Shalaby. *M. Deng presently at Department University, Piscataway, New Jersey
the composites
fibres
composite
strength
of the composites
between
strength
UHMWPE
self-reinforced
that the self-reinforced
Ultra-high-molecular-weight 27 August
incorporating
of the composites
(UHMWPE)
that the tensile
The transverse
strength
properties
surfaces
results
for load-bearing Keywords:
in wear
fracture
Overall
after
of the resulting
to the law of mixtures.
up to 7%. The double-notch
polyethylene
It was found
of UHMWPE have fallen into
1. processing control - raising processing temperature and/or pressure to facilitate the fusion of UHMWPE particles, and excluding oxygen during processing; 2. cross-linking and solid-state orientation techniques; 3. application of composite technology; 4. optimizing design of the final products. For the past 20 years, attempts to improve the UHMWPE biomaterials introduced in 1962 have been limited. Two such attempts worth mentioning are radiation-induced cross-linking and fibre reinforcement. The radiation cross-linking of UHMWPE was investigated in an attempt to improve the mechanical properties and thus the survival rate of its prostheses38, but without any significant success, since to achieve noteworthy improvement, high-dose irradiation has to be employed, which largely decreases the toughness of the polyethylene. Carbon fibre was used to reinforce UHMWPE to reduce its creep deformation’. This approach appears to be reasonable because a stiffer cup may improve the biomechanical situation, and a slow and less creep will extend the service life of the components. Early results showed that, compared to plain UHMWPE, significant increases in the compressive and tensile yield strength, elastic modulus, creep resistance and fatigue strength were achieved for the composites10-‘2. It was also claimed
Rutgers
645
Biomaterials
1997, Vol. 18 No. 9
646
Self-reinforced
that ‘wear is reduced’, the reduction being ascribed to ‘a lubricating effect of the abraded carbon particles’g-‘2. However, the late results were contradictory. It was found that carbon fibre-reinforced polyethylene articulating on various metal alloys showed frictional coefficients between 0.05 and O.lB*, but the wear rates were 2.6 to 10.3 times greater than those of plain UHMWPE. The addition of carbon fibres to the polyethylene does not improve the resistance of the material to surface damager3. Examination of carbon fibre-reinforced polyethylene surfaces revealed that the composites had no advantage over the plain polyethylene surfaces, since surface unevenness from the protruding carbon fibres was also observed in the new materials14. In surfaces that have been worn, the carbon fibres are torn due to poor bonding of the carbon fibres to the matrix material and difference in hardness of the two materialsr5. The fatigue crack growth resistance of the carbon fibre-reinforced UHMWPE was found to be significantly worse than that of the plain UHMWPE in the compact tension loading mode16,17. These undesirable results with carbon fibre-reinforced UHMWPE composites are possibly attributable to: 1. 2. 3.
4.
the brittle nature of carbon fibres; the difference in molecular structure and hence poor adhesion of the matrix to the reinforcement; the existence of residual stress in the composite due to the mismatch of thermal expansion coefficients of carbon fibre and UHMWPE; the ductile nature of the matrix itself.
The manufacturing method may also contribute greatly to the reported wear, considering that some carbon fibres with direction perpendicular to the wear surface will wear fast. For many years, the unsatisfactory mechanical performance has been one of the major factors in stimulating material scientists and bioengineers to seek the modification and improvement of UHMWPE used as orthopaedic implants. It is known that composite technology has been generally successful in achieving improved performance for many engineering materials. The addition of carbon fibres into UHMWPE has been shown to be unsatisfactory in improving the overall mechanical performance of the polymer due to the above-stated reasons. This led to the assumption that the physical compatibility of fibre and matrix is an important factor in the success of fibre-reinforced UHMWPE composites for orthopaedic implant application. Therefore, for improving the mechanical performance (e.g. creep resistance), it is logical to explore a different means of reinforcement. The availability of high or ultra-high strength and modulus UHMWPE fibres has created a new opportunity for composite assembly. These fibres have sufficient strength and stiffness for use as reinforcement, and at the same time they possess a ductile nature. In the past, UHMWPE fibres have been used in reinforcing epoxy 1s--21and low-molecular-weight polyethylene22-24. More recently, a self-reinforced UHMWPE composite matrix) been (UHMWPE fibre/UHMWPE has developedZ5. It is expected that the same molecular structure and the possibility of formation of transcrystals (as is the case in polyethylene fibre-reinforced low-molecular-weight polyethylene compositesz6) in Biomaterials
1997, Vol. 18
No. 9
UHMWPE
composites:
M. Deng and S.W. Shalaby
UHMWPE fibre-reinforced UHMWPE composites would offer physical compatibility and produce better interfacial bonding between the fibre and matrix phases. As a result, this study was conducted to characterize some physical properties of the selfreinforced UHMWPE composites.
EXPERIMENTAL Materials UHMWPE Medical grade UHMWPE (Hostalen GUR”) sample (GUR405) in fine powder form was provided by Hoechst Celanese Co., Texas, USA. This is a virgin polymer and is sometimes called reactor powder. The density of the melt-crystallized polymer is about 0.93 g mmp3. The molecular weight of GUR405 varies from 5 to 6 million.
UHMWPE fibres Gel-spun ultra-high strength and modulus UHMWPE fibre (Spectra 1000) was obtained from Allied Signal Inc., Virginia, USA. The fibres were delivered in the yarns of 650 deniers, each containing 120 filaments. The diameter of a single filament is about 30pm, as determined by scanning electron microscopy. Tensile strength of industrial fibres varies between 2 and 4GPa. Our tensile test on the fibres showed a strength of about 2 GPa. Specimen preparation All the specimens were made using compression moulding. For UHMWPE, the virgin UHMWPE resin was compression-moulded into sheets in a Carver Laboratory Press (Model C). The moulding temperature was around 180°C and a moderate compression force of about 7MPa was used. The compression-moulded UHMWPE sheets were used as the control for the selfreinforced UHMWPE composites. The processing of the composites is illustrated in the Results section.
Differential scanning calorimetry analysis Differential scanning calorimetry (DSC) was used to examine the thermal properties of UHMWPE and the fibres. A Du Pont TA Instrument 2000 thermal analyser with a computer data system was used for this purpose. The unit was calibrated using indium. The samples weighing approximately 4mg were placed in a twopart aluminium pan and heated at lO”Cmin-’ from room temperature to 300°C in air. The melting and oxidation behaviour of the materials was examined.
Scanning electron microscopy Scanning electron microscopy (SEM) analysis was used to examine the cross-section and the tensile fracture surface of the self-reinforced UHMWPE composites at the microscopic level. First, the composites were cut using a band-saw to make the cross-sections in such a way that the cutting direction was perpendicular to the fibre axis. Then the following two methods were used to prepare the SEM viewing surfaces of the composite cross-section. One cross-section was further polished using a microtome with a glass knife; the other was
Self-reinforced
UHMWPE
composites:
M.
Deng
and
S.W.
ground using silicon carbide papers. However, the tensile fracture surfaces of the composites were not further treated. Then. the samples were coated with gold for 2 x 600s in a Hummer X sputter coating instrument. The samples were viewed in JEOL JSM IC848 or JSM-35 scanning electron microscopes at a scanning mode of 10 or 15 kV.
Mechanical
647
Shalaby
characterization
Tensile test Tensile mechanical tests were performed on the UHMWPE and the composites using an Instron universal hydraulic testing machine (Model 1125). Dumb-bell-shaped tensile specimens were stamped from UHMWPE sheets and composite laminates using a metal cutting die. The composites were unidirectional laminate [O/O],and cross-ply laminates [O/90],. Figure 1 shows the tensile specimen geometry. In fact, the specimens were a little different from the Type III specimen defined in ASTM standard D63827. All the tensile tests were run under stroke control. The elongation velocity Iof 20 mmmin-’ and the gauge length of 20mm were chosen. Tensile strength and modulus were collecbed.
Impact test Impact tests on compression-moulded UHMWPE and its self-reinforced composites were conducted. The composite specimens were cross-ply laminates [O/ 9014,. The fibre weight fraction was 5%. Doublenotched specimens were cut directly from the laminate. Figure 2 shows the specimen geometry. The reading was recorded as ft-lbin~‘. Two kinds of notches were obtained, i.e. notched between fibres
(notched in y-z plane) and notched along fibre (notched in x-z plane). The samples were gammasterilized at 2.5 Mrad in air before testing.
Wear test Wear tests on compression-moulded UHMWPE and its self-reinforced composites were conducted. Figure 3 shows the wear test specimen geometry. The composite laminates were [O/90],,. The fibre weight fraction was 5%. In the case of composites, the wear surface was the fibre orientation plane (x-z plane). The wear test is similar to the one used in Ref. 28.
Creep test Creep test specimens were cut directly from the compression-moulded UHMWPE sheets and the selfreinforced UHMWPE composite laminates, using a band-saw. After cutting, the edges of the specimens were smoothed using abrasive paper. The composites had a fibre weight fraction of 5%. Figure 4 shows the creep specimen geometry. The creep measurement device includes a strain indicator (Model P-1500) from Measurements Group, Raleigh, North Carolina, USA, a loading system and a constant temperature chamber. The strain indicator was connected to the specimen via strain gauges that
f
a
Z
I/
Wear surface
59.82 *0.05
/r=17mm
Y X
1 29.21 + k1.27 Figure 3
Wear test specimen geometry (dimensions in mm). For composites, fibre orientation is in the x-z plane or parallel to the wear surface.
Figure 1
Tensile
test
specimen
geometry,
thickness
732 mm.
I/
2
k
Y
65mm
’ Figure 2
Strain gages symmetrically on both sides
Linear dimensions: kO.2 mm
-l-
Notch angle: &OS” Notch radius (razor notch): 0.2.5f0.05 mm Notch is symmetrically located in the center of the specimen.
12.7mm
Double-notched For the composites, fibre
X
l-l -
x
\
1_
Y
impact test specimen orientation is in the y-z
geometry. plane.
0-O ’ ‘12mm w
1Smm
Figure 4 Creep test specimen geometry; for composites, fibre orientation is in the z direction for the unidirectional laminate and in the x-z plane for the cross-ply laminate. Biomaterials
1997,
Vol.
18 No. 9
648
Self-reinforced
UHMWPE
composites:
M.
Deng
and
load. The other creep test procedures ASTM Standard D2990-77”.
S.W.
Shalaby
followed
the
RESULTS AND DISCUSSION
4
40
_Specimen
Loading
systems
4
Load
* Figure 5
Processing of self-reinforced UHMWPE composites
mm
for creep
tests
were glued to the centre of both surfaces of the two specimens using AE-10 adhesive of Measurements Group. One specimen was subjected to load and the other was used for temperature compensation. Thus, a full bridge was obtained for the measuring circuit. Strain gauges (CEA-13-125UN-350) from Measurements Group were used. Figure 5 shows the creep loading systems. Creep tests in tension were performed on both plain UHMWPE and the self-reinforced composite. The tests consisted of measuring the tensile strain as a function of time for a specimen subject to constant tensile load The environmental conditions. under specified applied load range for the creep test was chosen as 1 to 5MPa to represent the physiological compression load for normal knee and hip joints5. The temperatures were room temperature and 37°C. The test specimens were passively conditioned at the test temperature for 24 h to reach thermal equilibrium before the load was applied. Creep tests were conducted up to 24 h. In some cases samples were allowed to recover for 24 h to see the difference in permanent deformation for different samples. Loading a specimen was completed within 5 s and care was exercised to avoid any impact
Differential scanning calorimetry (DSC) analysis3’ of the melting behaviour of UHMWPE indicated that: (a) while the virgin GUR405 resin has a melting temperature (7’,) of around 145”C, the T, of the meltcrystallized CUR405 (remelting of the same sample) is below 135°C; (b) the original T,,, of 145°C cannot be regained for UHMWPE under normal processing conditionings. Therefore, a decrease in T, of about 10°C is experienced upon melt-processing the virgin polymer. The UHMWPE fibres (Spectra 1000) show two melting peaks with a higher T, of around 153°C and a lower T, of around 146°C. The differences in melting behaviour between melt-crystallized UHMWPE and polyethylene fibre are illustrated in Figure 6. The immediate conclusion following DSC analysis is that by carefully controlling processing parameters we can melt-process UHMWPE while the UHMWPE fibre’s integrity is not affected. Based on this principle, the self-reinforced UHMWPE composites are made in the Carver Laboratory Press in a similar way as for UHMWPE sheets. Both unidirectional and cross-ply laminates were made for the analysis of mechanical properties of this self-reinforced UHMWPE composite. A more detailed description of the processing method can be found elsewhere’“.
SEM view of composite cross-sections Figures 7 and 8 show the SEM micrographs of crosssections of the cross-ply laminates. Figure 7 is at a low magnification and the surface was prepared by microtome. The fibres and matrix can be easily distinguished in this picture. No manufacturing flaws, voids or debonding in the composites were visible. It
205.8”C
222.7-T
146.4”C
-6
! 0
50
100
I50
200
250
300
Temperature(“C) Figure 6 Biomaterials
Differential
scanning
1997, Vol. 18 No. 9
calorimetry
thermograms
of melt-crystallized
GUR405
and virgin
Spectra
1000.
Self-reinforced
UHMWPE
composites:
649
M. Deng and S. W. Shalaby
PlainUHMW-PE
Stress
Strain
Figure 7
Scanning electron micrograph of the crosssection of the cross-ply self-reinforced cofnposite. The surface was prepared using a microtome with a glass knife. Original magnific:ation x25.
Figure 8
Scanning electron micrograph of the crosssection of the cross-ply self-reinforced composite. The surface was polished using silicon carbide paper. Original magnification x 1500.
is to be noted that, due to the high toughness and flexibility of the UHMWPE fibres, the small cracks or separation within the fibre phase were created during the preparation of the samples. Figure 8 is at a high magnification and the surface was prepared by grinding with silicon carbide papers. In this picture, fibres and matrix phase can be seen on the surface. Again, no flaws, voids or debonding were detected even at a much higher magnification. These results have clearly shown that the manufacturing process for making the self-reinforced UHMWPE was quite satisfactory.
Tensile properties Longitudinal
tensile strength
When loaded along the fibre direction, the unidirectional laminates [O/O],of the self-reinforced UHMWPE composites showed different stress-strain behaviour compared to plain LJHMWPE, which is illustrated in
Figure 9. It is clear from this figure that for the composites
the
Figure 9
Schematic stress-strain curves reinforced UHMWPE fibre direction.
representation for olain composites
of typical tensile UHMWPE and the self[O/O], loaded along the
stress increases linearly with the strain up to the point where the fibres fracture. For the purpose of analysis, the stress at which the fibres fail was defined as maximum strength. By this definition, the maximum strength is the yielding strength at zero fibre weight fraction, i.e. plain UHMWPE. The load at fibre failure is higher than that at the composite failure for most of the fibre content used in the laminates for tensile testing. No yielding could be recorded for the composite before the failure of the fibres. After the fibres failed, the load suddenly dropped and obviously was carried completely by the UHMWPE matrix. Afterwards, the composites exhibit behaviour similar to plain UHMWPE. Finally, the composites fracture at a load slightly lower than the ultimate strength of plain UHMWPE. This drop in failure strength for the composites is due to the reduction in effective crosssection and maybe the stress concentration due to flaws introduced by fibre fracture. The failure strength at which the testing specimen fractured was lower than the maximum strength because the load was carried only by the UHMWPE matrix as soon as the fibres failed. Due to a relatively low fibre content, the differences in failure strength between plain UHMWPE and the composites were small. Figure 10 shows the experimental results of the longitudinal tensile maximum strength (al,,) as a function of fibre weight fraction ( W,) for the unidirectional laminates [O/O], loaded along the fibre orientation direction, which was also the maximum load that the composites could bear under the testing conditions. The results indicated that (1) CQ..increased significantly for the composites compared to plain UHMWPE; (2) CQ.. increased almost linearly with mf; and (3) due to the introduction of fibres into the UHMWPE the experimental data showed a larger scatter compared to plain UHMWPE. It can be seen that even for Wf less than 8%, the reinforcing efficiency of the polyethylene fibres is exceptionally high. At Wf = 5%, the increase in tensile strength was more than three times that of plain UHMWPE. Thus, in the case of longitudinal strength, most of the external load was carried by the fibres before the fibre fractured, Biomaterials 1997, Vol. 18 No. 9
Self-reinforced
650
3
z Gl B b VY i
.tr ;
120 P
90 60-
M. Deng and S.W.
0I{: = urn+ (gf - O,)Vf
Shalaby
(TIC= CT, + (CTf- 0,)Wf
30-3 0
I 2
(1)
or using Wf instead of Vf
T
J
I 4
I 6
8
Fiber WeightFraction(%) Figure 10 reinforced represents
composites:
interfacial bond for unidirectional tensile-loaded laminates in the fibre direction, and the fibre and matrix remain elastic until any failure takes place, then the stress on the composite can be expressed as
150-
0
UHMWPE
Longitudinal tensile maximum UHMWPE composite laminates a 95% confidence interval.
strength of self[O/O],. Error bar
since the polyethylene fibres were much stronger and stiffer than plain UHMWPE. Figure 1 I is the scanning electron micrograph of the fracture surface for typical unidirectional laminates loaded along the fibre direction, which showed a relative straight and smooth fracture surface within the matrix phase. A detailed examination of the picture indicates that the failure modes include fibre pull-out, matrix yielding, limited matrix plastic deformation, matrix fracture, fibrous fracture showing necking of fibres, and kink band formation within fibrils. It has been shown in the above that the longitudinal tensile maximum strength of unidirectional laminates increases linearly with the fibre weight fraction (wr). This linear increase in the strength may be described by the rule of mixtures. If there is a strong fibre-matrix
2
(2)
where olCis the stress on the composite, cr,,,the stress on the matrix, of the stress on the fibre, Vf the volume fraction of the fibre, pf the density of the fibre and pC the density of the composite. The above equation can be further changed into the following form if the fibre and the matrix exhibit linear elastic stress-strain relations until any failure occurs: Pr olc = E,,,E”, + (Efef - E,E,,,)W~ L Pf
(3)
where Em is the Young’s modulus of the matrix, Ef the Young’s modulus of the fibre, E, the strain of the matrix and af the strain of the fibre. If there is a perfect interfacial bond between fibre and matrix and no delamination occurs, then E, = Et =
(4)
E,
where E, is the strain of the composite. Inserting (4) into (3) gives ~1~= &,ef + (&ef - &,r:f)Wf ;
(5)
crlC= Emcf + (Ef - En,)cfwf 5
(6)
or Pf
The modulus EC of the composite may then be found as E, = En, + (Ef - E,,,)wf ;
(7)
Assuming the yielding strain of the matrix is greater than the ultimate strain of the fibres (note that polyethylene fibres have no yielding point), which is the case for the self-reinforced UHMWPE composites, then the longitudinal tensile maximum strength CQ:,! of the unidirectional self-reinforced composite laminates is found as olCu= E,sf, + (Ef - E,,,)q,lwf 2
Figure 11 Scanning electron micrograph of the fracture surface of the unidirectional laminate [O/O], loaded along the fibre orientation. Original magnification x100. Biomaterials
1997, Vol. 18 No. 9
(8)
where afu is the ultimate strain of the fibre which shows no yielding, macroscopically. Equation (8) clearly indicates a linear relationship between Q,, and wf, which was proved by the experimental data shown in Figure 10. Since Ef > E,,,, qcu is therefore determined mainly by the polyethylene fibres if Wf is not too small. Equation (8) can be used to provide failure criteria for longitudinal tensile strength of unidirectional selfreinforced UHMWPE composites, which is controlled by the ultimate strain of the polyethylene fibre. It can also be found that Equation (8) gives a yield strength of E,E~ for plain UHMWPE where ‘iicif= 0. This is obviously not true since the yield strength of plain UHMWPE is Em~mu,where a,,,, is the yielding strain of the matrix. It can be easily understood that E,,,c,,, is
Self-reinforced
UHMWIPE composites:
M. Deng and S.W. Shalaby
651
than Em&furconsidering that E,, > &h. Hence, Equation (8) is not applicable at very low wr and thus there is a critical mr below which fibres will not have a reinforcement effect. In this case, the function of the fibres in the composites may be considered to reduce the effective load-bearing cross-section of the materials. Therefore, the tensile strength of the composites is then determined by the matrix if a knee point at which fibres break is not taken as a yielding,
greater
It is clear that the tensile maximum strength of the composites is now the yielding strength of the matrix minus a percentage fibre weight component of Wr(p,/pr). In Equai.ion (9) the stress concentration caused by fibre l?a.cture has been neglected. From Figure 20,it is clear that the fibre weight fraction used in this research nlever reached the critical value. Letting Equations (811and (9)be equal gives the critical value Wf, of fibre weight fraction at which the composites will have the lowest strength,
I
I
2
4
I 6
8
Fiber Weight Fraction (%) Figure 12 Transverse tensile reinforced UHMWPE composite bar represents a 95% confidence
yield strength of selflaminates [90/90],. Error interval.
As an approximation, letting pf/pCx 1,E,, FS2~fuand 4 M lOOEm, then we find that mrC M 1%. Inserting (10) into (8) or (9)gives the lowest strength (brcu)minfor the composite: (%u)min
=
&FF In
Efef,
(11)
mu --
or
(12) where (~,r is the yield strength of the matrix, crf~the fracture strength of the fibres and u,f the stress on the matrix at the fibre fracture. It is clear from Equation (12) that since dmY:> umf, the lowest strength (crcu)min of the composites will be less than the yield strength cmY of the matrix. Using the above assumed data, it is found that (brcu)min2: O.99o,,. Transverse tensile strength When loaded in the direction perpendicular to fibre the unidirectional self-reinforced orientation, UHMWPE composite laminates 190/901, showed a stress-strain curve s.imilar to that of plain UHMWPE. The properties of the composites in this case are much more matrix-dominated, particularly for a low fibre content. The experimental results of the transverse tensile yield strength of the composites as a function of fibre weight fraction wr are shown in Figure 12. The results indicated that for Wf < 7% the tensile yield strength of unidirectional laminates in the transverse remained unchanged, although the direction composites did show a larger variation in data, compared to plain IJHMWPE. This kind of behaviour may suggest that the interfacial bonding of the composites does not affect the transverse yield strength of the composites due to the low percentage of fibres within the matrix. :It may also be due to the lower sensitivity of UHMWPE materials to cracks compared to a more brittle matrix like epoxy.
Figure 13 Scanning electron micrograph of the fracture surface of the unidirectional laminate loaded perpendicular to the fibre orientation. Original magnification x50.
Figure 13 shows the scanning electron micrograph of the fracture surface for typical unidirectional laminates loaded perpendicular to the fibre direction, which showed a very irregular surface. Compared to Figure 1 I, the massive matrix plastic deformation caused by stretching led to the fracture of the composites. A few stretching bands could be seen. However, no fibre fracture was visible in this case. To a first approximation the fibres can be regarded as cylindrical holes. If the fibres are considered as a simple square array, then the transverse yield strength cltc is given by the following formula, provided that the resin is not notch-sensitive31: ctc = E,E,,[~- 2(Vfn)0.5] Biomaterials
(13) 1997, Vol. 18 No. 9
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Self-reinforced
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M. Deng and S.W. Shalaby
(14)
For a fibre weight fraction wf = 5%, the strength reduction is about 80%, which indicates an important function of the interface. However, this result also indicates that the above equation is not a good estimation for the self-reinforced UHMWPE composites if the prediction is compared to the data in Figure 12. Cross-ply laminates Since very few composites are used in the form of unidirectional laminates, tensile tests were conducted on the cross-ply laminates [O/901,. The experimental results showed that the composites displayed a stressstrain curve similar to that of the unidirectional laminates loaded along the fibre direction. This indicates that in this case the strength of the laminates is mainly determined by the fibres. Figure 24 illustrates the experimental results of maximum tensile strength of cross-ply laminates as a function of wr. The results showed that the maximum tensile strength increased with increasing wr and doubled at mr = 5% compared with plain UHMWPE. It seemed that the strength increased linearly with mr. Figure 15 shows the scanning electron micrograph of the fracture surface for typical cross-ply laminates, which showed a relative curved and non-smooth compared to Figure 11. The fracture surface microscopic failure modes are similar to those of the laminates loaded along the fibre direction, which include fibre pull-out, matrix yielding, matrix peeling off, limited matrix plastic deformation, matrix fracture, fibrous fracture showing necking of fibres and kink band formation within fibrils. Tensile modulus To see the differences in stiffness, a comparison was made of the modulus of plain UHMWPE and the selfreinforced composites. Keeping wf at 5% for the composites, Figure 16 shows the tensile test results of modulus. It is clear from this figure that a large increase in modulus was achieved for both cross-ply laminates and unidirectional laminates loaded along the fibre direction, compared to plain UHMWPE. The modulus of laminates [O/90], is twice that of plain UHMWPE. These results were obviously due to the reinforcing effect of ultra-high strength and modulus polyethylene fibres. It is also seen that the transverse modulus was higher than for plain UHMWPE, which is possibly due to the fibre restriction effect on the transverse deformation. Creep results Figures 17 to 20 list the creep results, where the creep strain was plotted against creep time. Figure 17 clearly shows that, at room temperature and 5 MPa, both unidirectional and cross-ply laminates of the selfreinforced composites were much more creep-resistant than plain UHMWPE in a 24-h creep experiment; in fact, more than 100% reduction in creep deformation for UHMWPE was achieved after the introduction of polyethylene fibres. The figure also indicates that the Biomaterials
1997, Vol. 18 No. 9
3
6
9
12
Fiber Weight Fraction (%) Figure 14 Tensile maximum strength of self-reinforced UHMWPE composites, cross-ply laminates, [Oi90],. Error bar represents a 95% confidence interval.
Figure 15 Scanning electron micrograph of the fracture surface of the cross-ply laminate. Original magnification x100.
cross-ply laminate displayed less creep resistance than the unidirectional laminate because for the cross-ply laminate only half of 5% of the fibres was in the loading direction, which indicates the effect of fibre content on the creep performance for self-reinforced UHMWPE composites. Quantitatively, the cross-ply laminate showed about 30% more creep deformation than the unidirectional laminate. Figure 18 shows that, at 1 MPa and 3i’“C, the cross-ply laminate had a much higher creep resistance than plain UHMWPE. Again, more than 100% reduction in creep deformation for UHMWPE was obtained when it was reinforced by polyethylene fibres. The creep data for both plain UHMWPE and the composite indicated a full recovery of creep deformation 24 h after unloading (residual strain was 15 pe for UHMWPE and 60 p for the cross-
Self-reinforced
M.
UHMWIPE composites:
Deng
and
653
S. W. Shalaby
3.0 I
I
2.5 0.32
2.0 1.5
0.16 -)-
1.0 0.08
0.5 0.0 i
0 UHMW-PE
[0/9Ols
[90/9O]s
Crossply Unidirectional
10
20
30
40
50
Fiaure 19 Tensile creeo and recoverv results rekrforced composites at’37”C and 2.5 MPa.
for self-
[O/Ols
Time (hour)
Figure 16 Comparison of tensile modulus of plain UHMWPE composite UHMWPE and self-reinforced laminates. Error bar represents a 95% confidence interval.
1.6 -p-------a
1.4
A
1.2
-
UHMW-PE Cross-ply
1 .o
--u-
Unidirectional
0.8
unloading
*
0.6
0.2 -
q
0.4 0.2-
-
Crossply Unidirectional
,
1 10
0
Figure 17 Tensile creep self-reinforced composites
I 20
Time (hour)
results for plain UHMWPE and at room temperature and 5 MPa.
20
0
10
40
50
Figure 20 Tensile creep and recovery results reinforced composites at 37°C and 5 MPa.
for self-
30
UHMW-PE Crossply
40
50
Time (hour) Figure 16 Tensile weep and recovery results for plain UHMWPE and the self-reinforced cross-ply composite at 37°C and 1 MPa.
ply laminate). As the load was increased to 2.5 MPa, the creep deformation of the cross-ply laminate increased, compared to the result in Figure 18, as shown in Figure 19. Thus, when the load was increased from 1 to 2.5 MPa, the creep strain at the end of loading for a 24-h creep test was increased from 931 to 39oi’ps for the cross-ply laminate, more than 300% increase, and was 2673 p for the unidirectional laminate. The result also showed that the creep deformation did not fully recover 24 h after unloading for both cross-ply and unidirectional laminates, leaving a residual strain of 885 PC for the cross-ply laminate and 442~s for the unidirectional laminate. The residual strain of the
30
20 Tie
-
I0
I 30
(hour)
unidirectional laminate was 50% less than that of the cross-ply laminate at the end of loading, and the former recovered faster than the latter. Figure 20 shows that when the load was further increased to 5MPa, the 24-h creep deformation for both unidirectional and cross-ply laminates doubled and more unrecovered creep strain was seen 24 h after unloading, compared to the results in Figures 18 and 19. The creep strain at the end of loading was 8117 p for the cross-ply laminate and 5044~s for the unidirectional laminate, and the residual strain was 1872~~ for the cross-play laminate and 1161 PE for the unidirectional laminate after 24-h recovery. Comparing Figure 17 with Figure 20 indicates that when the temperature was increased from room temperature to 37”C, the increase in creep strain was less than 100% for the composites. It seems that the creep strain at 37°C for the composites increased proportionally to the load. Overall, the creep data obtained at the test conditions illustrated a significant improvement in creep resistance for plain UHMWPE after reinforcement by the UHMWPE fibres. The above results also illustrate a general trend for the creep of materials that at the initial stage of experiment the creep deformation developed rapidly, which includes an instant elastic response and the rapid viscoelastic response to the applied load. After this short period, creep developed into a stage where the creep strain propagated at a continuously reduced rate, which must be taken into account in trying to describe, mathematically, the creep behaviour of UHMWPE systems. Biomaterials
1997, Vol. 18 No. 9
654
Self-reinforced
UHMWPE
M. Deno and S.W. Shalabv
composites:
Wear and impact results The wear property of self-reinforced composites (crossply) was compared to the control. The control samples (Control-l) were machined from ram-extruded UHMWPE rods, compared to the control samples (Control-Z) which were compression-moulded directly from resin and then machined into wear specimens. Each data point represents an average of four samples. All the specimens were gamma-sterilized at 2.5 Mrad in an air environment before the wear test. Reciprocal sliding pin-on-disc wear tests were run for one million cycles. Figure 22 lists the results, where wear data (weight loss) are given as milligrams per million cycles. It is clear that the wear data showed a large variance. Statistical analysis of the data showed that there were no significant differences between the composites and the two types of control. The results are promising since the mechanical performance was improved as wear properties did not change for the self-reinforced composites as compared to plain UHMWPE. Figure 22 shows the impact results of double-notched specimens. Each data point is an average of at least three specimens. The impact results show an increase in impact strength for UHMWPE after introduction of Spectra 1000 fibres. The increase in impact strength due to fibre reinforcement was attributed to the higher energy absorption of polyethylene fibres (high toughness) and the energy dispersion occurring along the laminate interfaces, perpendicular to the direction of notching. Thus, the impact results showed an advantage for the self-reinforced UHMWPE composites.
Control 1 Figure 21 Wear test reinforced composites
I
I
100 200 Temperature(“C)
1
Figure 22 specimens.
23
Biomaterials
Comparison
of differential
1997, Vol. 18 No. 9
scanning
calorimetry
Impact
tl
t
results
Table 1 Differential scanning fibres taken from composites
and
self-
f uble-notched
for
calorimetry
data
Sample
of UHMWPE
Cryst. (%)
Virgin fibre Fibre from composite
147 146
153 152
202 205
80 66
135°C during the composite processing reduced the strength of the second melting peak, but the crystallinity may possibly increase. This may indicate some kind of change in the polyethylene crystal structure after heat treatment. This may also result from the effect of the compression force used to make the composites on the fibres during the composite processing.
1
300
m
300 Temperature (“0
a. Virgin fiber Figure
results for plain UHMWPE (cross-ply laminates).
-
Concerning the effects of composite processing conditions on UHMWPE fibres, differential scanning calorimetry (DSC) analysis was performed. The samples of UHMWPE fibres were taken from inside the self-reinforced UHMWPE composites. The numerical data of T,,,, T,, and crystallinity are given in Table 1, where T,, corresponds to the first melting peak and T,, the second melting peak, and To is the oxidation temperature. A comparison of their DSC thermograms is shown in Figure 23. Generally speaking, the shape of the DSC thermogram from the heat-treated fibre under the composite processing conditions showed a distinct difference compared to the as-received virgin fibre. Heat processing of the UHMWPE fibre around
-
Composite
3 b x u
Effects of processing on thermal properties of fibres
i 0
Control 2
b. Fiber from composite thermograms
of UHMWPE
fibres.
Self-reinforced
UHMWPE
composites:
M. Deng
655
and S. W. Shalaby
CONCLUSIONS It was shown that the tensile properties, creep resistance and impact strength of the self-reinforced UHMWPE composites are superior to plain UHMWPE. The wear properties of the composites were found to be similar to plain UHMWPE. A linear relationship exists between the longitudinal tensile strength and fibre content for the unidirectional laminates. The effect of heat processing on the thermal properties of UHMWPE fibres was small. SEM examination of the fracture surface revealed differences in failure mechanisms among i.he composites.
13.
14.
15.
16.
ACKNOWLEDGEMENTS The authors would like to thank Hoechst Celanese Co., Texas, for providing the UHMWPE GUI&05 resin, and Allied Signal Inc., Fiber Division, Virginia, for providing the SpectraH‘ fibres. We also thank Smith & Nephew Rechirds, Inc., Tennessee, for help in performing the wear and impact tests.
REFERENCES 1. 2.
3. 4. 5. 6.
7. 8. 9.
10. 11. 12.
Charnley, J., Low Friction Arthroplasty of the Hip. Springer-Verlag,Berlin 1979. Davidson,J. A. and Schwartz, G., J. Biomed. Mater. Res., 1987, 21-A3, 261. du Plessis, T. A., Grobbelaar, C. J. and Marais, F., Rad. Phys. Chem., 1977, 9, 647. Grobbelaar, C.J., ~duPlessis, T. A. and Marais, F., I. Bone Joint Surg., 1978, 60-B, 370. Shen, C. and Dumbleton, J. H., Wear, 1974, 30, 349. Streicher, R. M., In Ultra-High Molecular Weight Polyethylene as 13iomaterial in Orthopedic Surgery, ed. H.-G. Willert, G. H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, .1991, pp. 66-73. Dumbleton, J.H. and Shen, C., I. Appl. Polym. Sci., 1974,18,3493. McKellop, H., Clarke, I., Markolf, K. and Amstutz, H., 1. Biomed. Mater. Res., 1981, 15, 619. Farling, G. M. and Greer, K., In Mechanical Properties of Biomaterials, ed. G. W. Hastings and D. F. Williams. Wiley, 1980, pp. 53-64. Halcomb, F. J., and Bardos, D., Trans. Am. Artif. Intern. Organs, 1981, 27, 364. Wright, M., Fubayashi, T. and Burnstein, A.H., J. Biomed. Mater. Bes., 1981, 15, 719. Walker, P.S., Ben-Dou, M., Askew, M. and Pugh, J., Eng. Med., 1981, 10, 33.
17.
18.
19. 20. 21. 22. 23. 24. 25.
26. 27. 28.
29.
30. 31.
Wright, T.M. and Rimnac, C. M., In Ultra-High Molecular Weight Polyethylene as Biomaterial in Orthopedic Surgery, ed. H.-G. Willert, G.H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, 1991, pp. 202-207. Brill, W. and Mittelmerier, H., In Ultra-High Molecular Weight Polyethylene as Biomaterial in Orthopedic Surgery, ed. H.-G. Willert, G.H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, 1991, pp. 217-220. Stern, L.S., Manley, M.T., Parr, J.E., Stulberg, B.N., Price, H. and Ries, M., In Ultra-High Molecular Weight Polyethylene as Biomaterial in Orthopedic Surgery, ed. H.-G. Willert, G.H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, 1991, pp. 258-261. Connelly, G. M., Rimnac, C. M., Wright, T.M., Hertzberg, R. W. and Manson, J.A., I. Orthop. Res., 1984, 2, 119. Rimnac, C. M and Wright, T. M., In Ultra-High Molecular Weight Polyethylene as Biomaterial in Orthopedic Surgery, ed. H.-G. Willert, G. H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, 1991, pp. 28-31. Chang, H. W., Lin, L.C. and Bhatnager, A., 31st International SAMPE Symposium, SAMPE, Covina, California, 1986, pp. 859-865. Woods, D. W. and Ward, I.M., J. Appl. Polym. Sci., 1994, 51, 2572. Peijs, T., Rijsdijk, H. K., de Kok, J. M. and Lemstra, P. J., Compos. Sci. Tech., 1994, 52,449. Taboudoucht, A., Opalko, R. and Ishida, H., Polym. Compos., 1992, 13, 81. Harpell, G.A., Kavesh, S., Palley, I. and Prevorsek, D., European Patent Application No. 83101731.4, 1983. Marais, C. and Feillard, P., Compos. Sci. Tech., 1992, 45, 247. Teishev, A., Incordona, S., Migliaresi, C. and Marom, G., J. Appl. Polym. Sci., 1993, 59, 563. Shalaby, S. W. and Deng, M., Self-reinforced ultra-high molecular weight polyethylene composites. US Patent Application field, 1994. Ishida, H. and Bussi, P., Macromolecules, 1991, 24, 3569. Standard Test Method for Tensile Properties of Plastics, ASTM D-638,1989. Poggie, R. A., Wert, J. J., Mishra, A. J. and Davidson, J. A., In Wear and Friction of Elastomers, ASTM STP 1145, ed. R. Dentoo and M.K. Keshavan. American Society for Testing and Materials, Philadelphia, 1992, pp. 6581. Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics, ASTM D-2990, Annual Book of ASTM Standards, 1989. Deng, M. and Shalaby, S. W., Polym. Adv. Tech., 1993, 43. Hull, D., An Introduction to Composite Materials. Cambridge University Press, 1981.
Biomaterials 1997, Vol. 18 No. 9
ELSEVIER
SOl42-9612
(96)
Printed in Great Britain. All rights reserved 0142.9612/97/$17.00
00194-9
Properties of self-reinforced ultra-high-molecular-weight polyethylene composites Meng Deng” and Shalaby W. Shalaby+ ‘Department
Labs, Center
of Bioengineering, Clemson University, Clemson, South Carolina 29634, USA; +Po/y-Med, Inc., R&D for Applied Technology, 511 Westinghouse Road, Pendleton, South Carolina 29670, USA
The physical
properties
composites
have
resistance,
were
significantly
The longitudinal content,
of ultra-high-molecular-weight
been characterized. tensile
according
increased
strength
was no difference and tensile (SEM).
Received
impact
indicate
biomedical
applications.
1996; accepted
of Chemistry, 08855, USA.
were
polyethylene,
26 October
composites
Science
composite,
for fibre
UHMWPE.
Limited. mechanical
matrix.
with fibre
than plain
by scanning
matrix
and creep
into a UHMWPE
did not change
and plain
UHMWPE
fibre/UHMWPE
and modulus, increased
was higher
examined
$? 1997 Elsevier
content
UHMWPE.
of
There
The cross-section
electron
microscopy
may be good candidates All rights
reserved
properties,
SEM, DSC
1996
mechanical performance four categories:
Since its introduction in low-friction arthroplasty in ultra-high-molecular-weight polyethylene 1962’, (UHMWPE) has found wide application as a loadbearing material in th.e majority of joint endoprostheses in combination with metal or ceramic counterparts. This undoubtedly results from its combination of good physical and chemical properties and acceptable biocompatibility. Although the short-term function of most UHMWPE implants is satisfactory, their longterm performance has been a concern for many years. As the implants are expected to survive and function well beyond 10 years in the body, the long-term performance limitations of UHMWPE as an orthopaedic bearing surface begin to manifest as wear, creep and fatigue fracture. UHMWPE is a viscoelastic material, and the problems with the polymer originate from some of its inherent weaknesses (like creep and fatigue) compared to the metal stem and the cortical bone. Therefore, the wear and creep of such articulating materials must be reduced for improving long-term device performance, particularly for heavier, younger and more active patients’. Considering the existing problems associated with the use of UHMWPE implants, studies have been undertaken in attempts to understand the underlying mechanisms of polyethylene failure and thus extend the functional life of joint replacements. To date, the generally suggested approaches to increase the Correspondence to Dr S.W. Shalaby. *M. Deng presently at Department University, Piscataway, New Jersey
the composites
fibres
composite
strength
of the composites
between
strength
UHMWPE
self-reinforced
that the self-reinforced
Ultra-high-molecular-weight 27 August
incorporating
of the composites
(UHMWPE)
that the tensile
The transverse
strength
properties
surfaces
results
for load-bearing Keywords:
in wear
fracture
Overall
after
of the resulting
to the law of mixtures.
up to 7%. The double-notch
polyethylene
It was found
of UHMWPE have fallen into
1. processing control - raising processing temperature and/or pressure to facilitate the fusion of UHMWPE particles, and excluding oxygen during processing; 2. cross-linking and solid-state orientation techniques; 3. application of composite technology; 4. optimizing design of the final products. For the past 20 years, attempts to improve the UHMWPE biomaterials introduced in 1962 have been limited. Two such attempts worth mentioning are radiation-induced cross-linking and fibre reinforcement. The radiation cross-linking of UHMWPE was investigated in an attempt to improve the mechanical properties and thus the survival rate of its prostheses38, but without any significant success, since to achieve noteworthy improvement, high-dose irradiation has to be employed, which largely decreases the toughness of the polyethylene. Carbon fibre was used to reinforce UHMWPE to reduce its creep deformation’. This approach appears to be reasonable because a stiffer cup may improve the biomechanical situation, and a slow and less creep will extend the service life of the components. Early results showed that, compared to plain UHMWPE, significant increases in the compressive and tensile yield strength, elastic modulus, creep resistance and fatigue strength were achieved for the composites10-‘2. It was also claimed
Rutgers
645
Biomaterials
1997, Vol. 18 No. 9
646
Self-reinforced
that ‘wear is reduced’, the reduction being ascribed to ‘a lubricating effect of the abraded carbon particles’g-‘2. However, the late results were contradictory. It was found that carbon fibre-reinforced polyethylene articulating on various metal alloys showed frictional coefficients between 0.05 and O.lB*, but the wear rates were 2.6 to 10.3 times greater than those of plain UHMWPE. The addition of carbon fibres to the polyethylene does not improve the resistance of the material to surface damager3. Examination of carbon fibre-reinforced polyethylene surfaces revealed that the composites had no advantage over the plain polyethylene surfaces, since surface unevenness from the protruding carbon fibres was also observed in the new materials14. In surfaces that have been worn, the carbon fibres are torn due to poor bonding of the carbon fibres to the matrix material and difference in hardness of the two materialsr5. The fatigue crack growth resistance of the carbon fibre-reinforced UHMWPE was found to be significantly worse than that of the plain UHMWPE in the compact tension loading mode16,17. These undesirable results with carbon fibre-reinforced UHMWPE composites are possibly attributable to: 1. 2. 3.
4.
the brittle nature of carbon fibres; the difference in molecular structure and hence poor adhesion of the matrix to the reinforcement; the existence of residual stress in the composite due to the mismatch of thermal expansion coefficients of carbon fibre and UHMWPE; the ductile nature of the matrix itself.
The manufacturing method may also contribute greatly to the reported wear, considering that some carbon fibres with direction perpendicular to the wear surface will wear fast. For many years, the unsatisfactory mechanical performance has been one of the major factors in stimulating material scientists and bioengineers to seek the modification and improvement of UHMWPE used as orthopaedic implants. It is known that composite technology has been generally successful in achieving improved performance for many engineering materials. The addition of carbon fibres into UHMWPE has been shown to be unsatisfactory in improving the overall mechanical performance of the polymer due to the above-stated reasons. This led to the assumption that the physical compatibility of fibre and matrix is an important factor in the success of fibre-reinforced UHMWPE composites for orthopaedic implant application. Therefore, for improving the mechanical performance (e.g. creep resistance), it is logical to explore a different means of reinforcement. The availability of high or ultra-high strength and modulus UHMWPE fibres has created a new opportunity for composite assembly. These fibres have sufficient strength and stiffness for use as reinforcement, and at the same time they possess a ductile nature. In the past, UHMWPE fibres have been used in reinforcing epoxy 1s--21and low-molecular-weight polyethylene22-24. More recently, a self-reinforced UHMWPE composite matrix) been (UHMWPE fibre/UHMWPE has developedZ5. It is expected that the same molecular structure and the possibility of formation of transcrystals (as is the case in polyethylene fibre-reinforced low-molecular-weight polyethylene compositesz6) in Biomaterials
1997, Vol. 18
No. 9
UHMWPE
composites:
M. Deng and S.W. Shalaby
UHMWPE fibre-reinforced UHMWPE composites would offer physical compatibility and produce better interfacial bonding between the fibre and matrix phases. As a result, this study was conducted to characterize some physical properties of the selfreinforced UHMWPE composites.
EXPERIMENTAL Materials UHMWPE Medical grade UHMWPE (Hostalen GUR”) sample (GUR405) in fine powder form was provided by Hoechst Celanese Co., Texas, USA. This is a virgin polymer and is sometimes called reactor powder. The density of the melt-crystallized polymer is about 0.93 g mmp3. The molecular weight of GUR405 varies from 5 to 6 million.
UHMWPE fibres Gel-spun ultra-high strength and modulus UHMWPE fibre (Spectra 1000) was obtained from Allied Signal Inc., Virginia, USA. The fibres were delivered in the yarns of 650 deniers, each containing 120 filaments. The diameter of a single filament is about 30pm, as determined by scanning electron microscopy. Tensile strength of industrial fibres varies between 2 and 4GPa. Our tensile test on the fibres showed a strength of about 2 GPa. Specimen preparation All the specimens were made using compression moulding. For UHMWPE, the virgin UHMWPE resin was compression-moulded into sheets in a Carver Laboratory Press (Model C). The moulding temperature was around 180°C and a moderate compression force of about 7MPa was used. The compression-moulded UHMWPE sheets were used as the control for the selfreinforced UHMWPE composites. The processing of the composites is illustrated in the Results section.
Differential scanning calorimetry analysis Differential scanning calorimetry (DSC) was used to examine the thermal properties of UHMWPE and the fibres. A Du Pont TA Instrument 2000 thermal analyser with a computer data system was used for this purpose. The unit was calibrated using indium. The samples weighing approximately 4mg were placed in a twopart aluminium pan and heated at lO”Cmin-’ from room temperature to 300°C in air. The melting and oxidation behaviour of the materials was examined.
Scanning electron microscopy Scanning electron microscopy (SEM) analysis was used to examine the cross-section and the tensile fracture surface of the self-reinforced UHMWPE composites at the microscopic level. First, the composites were cut using a band-saw to make the cross-sections in such a way that the cutting direction was perpendicular to the fibre axis. Then the following two methods were used to prepare the SEM viewing surfaces of the composite cross-section. One cross-section was further polished using a microtome with a glass knife; the other was
Self-reinforced
UHMWPE
composites:
M.
Deng
and
S.W.
ground using silicon carbide papers. However, the tensile fracture surfaces of the composites were not further treated. Then. the samples were coated with gold for 2 x 600s in a Hummer X sputter coating instrument. The samples were viewed in JEOL JSM IC848 or JSM-35 scanning electron microscopes at a scanning mode of 10 or 15 kV.
Mechanical
647
Shalaby
characterization
Tensile test Tensile mechanical tests were performed on the UHMWPE and the composites using an Instron universal hydraulic testing machine (Model 1125). Dumb-bell-shaped tensile specimens were stamped from UHMWPE sheets and composite laminates using a metal cutting die. The composites were unidirectional laminate [O/O],and cross-ply laminates [O/90],. Figure 1 shows the tensile specimen geometry. In fact, the specimens were a little different from the Type III specimen defined in ASTM standard D63827. All the tensile tests were run under stroke control. The elongation velocity Iof 20 mmmin-’ and the gauge length of 20mm were chosen. Tensile strength and modulus were collecbed.
Impact test Impact tests on compression-moulded UHMWPE and its self-reinforced composites were conducted. The composite specimens were cross-ply laminates [O/ 9014,. The fibre weight fraction was 5%. Doublenotched specimens were cut directly from the laminate. Figure 2 shows the specimen geometry. The reading was recorded as ft-lbin~‘. Two kinds of notches were obtained, i.e. notched between fibres
(notched in y-z plane) and notched along fibre (notched in x-z plane). The samples were gammasterilized at 2.5 Mrad in air before testing.
Wear test Wear tests on compression-moulded UHMWPE and its self-reinforced composites were conducted. Figure 3 shows the wear test specimen geometry. The composite laminates were [O/90],,. The fibre weight fraction was 5%. In the case of composites, the wear surface was the fibre orientation plane (x-z plane). The wear test is similar to the one used in Ref. 28.
Creep test Creep test specimens were cut directly from the compression-moulded UHMWPE sheets and the selfreinforced UHMWPE composite laminates, using a band-saw. After cutting, the edges of the specimens were smoothed using abrasive paper. The composites had a fibre weight fraction of 5%. Figure 4 shows the creep specimen geometry. The creep measurement device includes a strain indicator (Model P-1500) from Measurements Group, Raleigh, North Carolina, USA, a loading system and a constant temperature chamber. The strain indicator was connected to the specimen via strain gauges that
f
a
Z
I/
Wear surface
59.82 *0.05
/r=17mm
Y X
1 29.21 + k1.27 Figure 3
Wear test specimen geometry (dimensions in mm). For composites, fibre orientation is in the x-z plane or parallel to the wear surface.
Figure 1
Tensile
test
specimen
geometry,
thickness
732 mm.
I/
2
k
Y
65mm
’ Figure 2
Strain gages symmetrically on both sides
Linear dimensions: kO.2 mm
-l-
Notch angle: &OS” Notch radius (razor notch): 0.2.5f0.05 mm Notch is symmetrically located in the center of the specimen.
12.7mm
Double-notched For the composites, fibre
X
l-l -
x
\
1_
Y
impact test specimen orientation is in the y-z
geometry. plane.
0-O ’ ‘12mm w
1Smm
Figure 4 Creep test specimen geometry; for composites, fibre orientation is in the z direction for the unidirectional laminate and in the x-z plane for the cross-ply laminate. Biomaterials
1997,
Vol.
18 No. 9
648
Self-reinforced
UHMWPE
composites:
M.
Deng
and
load. The other creep test procedures ASTM Standard D2990-77”.
S.W.
Shalaby
followed
the
RESULTS AND DISCUSSION
4
40
_Specimen
Loading
systems
4
Load
* Figure 5
Processing of self-reinforced UHMWPE composites
mm
for creep
tests
were glued to the centre of both surfaces of the two specimens using AE-10 adhesive of Measurements Group. One specimen was subjected to load and the other was used for temperature compensation. Thus, a full bridge was obtained for the measuring circuit. Strain gauges (CEA-13-125UN-350) from Measurements Group were used. Figure 5 shows the creep loading systems. Creep tests in tension were performed on both plain UHMWPE and the self-reinforced composite. The tests consisted of measuring the tensile strain as a function of time for a specimen subject to constant tensile load The environmental conditions. under specified applied load range for the creep test was chosen as 1 to 5MPa to represent the physiological compression load for normal knee and hip joints5. The temperatures were room temperature and 37°C. The test specimens were passively conditioned at the test temperature for 24 h to reach thermal equilibrium before the load was applied. Creep tests were conducted up to 24 h. In some cases samples were allowed to recover for 24 h to see the difference in permanent deformation for different samples. Loading a specimen was completed within 5 s and care was exercised to avoid any impact
Differential scanning calorimetry (DSC) analysis3’ of the melting behaviour of UHMWPE indicated that: (a) while the virgin GUR405 resin has a melting temperature (7’,) of around 145”C, the T, of the meltcrystallized CUR405 (remelting of the same sample) is below 135°C; (b) the original T,,, of 145°C cannot be regained for UHMWPE under normal processing conditionings. Therefore, a decrease in T, of about 10°C is experienced upon melt-processing the virgin polymer. The UHMWPE fibres (Spectra 1000) show two melting peaks with a higher T, of around 153°C and a lower T, of around 146°C. The differences in melting behaviour between melt-crystallized UHMWPE and polyethylene fibre are illustrated in Figure 6. The immediate conclusion following DSC analysis is that by carefully controlling processing parameters we can melt-process UHMWPE while the UHMWPE fibre’s integrity is not affected. Based on this principle, the self-reinforced UHMWPE composites are made in the Carver Laboratory Press in a similar way as for UHMWPE sheets. Both unidirectional and cross-ply laminates were made for the analysis of mechanical properties of this self-reinforced UHMWPE composite. A more detailed description of the processing method can be found elsewhere’“.
SEM view of composite cross-sections Figures 7 and 8 show the SEM micrographs of crosssections of the cross-ply laminates. Figure 7 is at a low magnification and the surface was prepared by microtome. The fibres and matrix can be easily distinguished in this picture. No manufacturing flaws, voids or debonding in the composites were visible. It
205.8”C
222.7-T
146.4”C
-6
! 0
50
100
I50
200
250
300
Temperature(“C) Figure 6 Biomaterials
Differential
scanning
1997, Vol. 18 No. 9
calorimetry
thermograms
of melt-crystallized
GUR405
and virgin
Spectra
1000.
Self-reinforced
UHMWPE
composites:
649
M. Deng and S. W. Shalaby
PlainUHMW-PE
Stress
Strain
Figure 7
Scanning electron micrograph of the crosssection of the cross-ply self-reinforced cofnposite. The surface was prepared using a microtome with a glass knife. Original magnific:ation x25.
Figure 8
Scanning electron micrograph of the crosssection of the cross-ply self-reinforced composite. The surface was polished using silicon carbide paper. Original magnification x 1500.
is to be noted that, due to the high toughness and flexibility of the UHMWPE fibres, the small cracks or separation within the fibre phase were created during the preparation of the samples. Figure 8 is at a high magnification and the surface was prepared by grinding with silicon carbide papers. In this picture, fibres and matrix phase can be seen on the surface. Again, no flaws, voids or debonding were detected even at a much higher magnification. These results have clearly shown that the manufacturing process for making the self-reinforced UHMWPE was quite satisfactory.
Tensile properties Longitudinal
tensile strength
When loaded along the fibre direction, the unidirectional laminates [O/O],of the self-reinforced UHMWPE composites showed different stress-strain behaviour compared to plain LJHMWPE, which is illustrated in
Figure 9. It is clear from this figure that for the composites
the
Figure 9
Schematic stress-strain curves reinforced UHMWPE fibre direction.
representation for olain composites
of typical tensile UHMWPE and the self[O/O], loaded along the
stress increases linearly with the strain up to the point where the fibres fracture. For the purpose of analysis, the stress at which the fibres fail was defined as maximum strength. By this definition, the maximum strength is the yielding strength at zero fibre weight fraction, i.e. plain UHMWPE. The load at fibre failure is higher than that at the composite failure for most of the fibre content used in the laminates for tensile testing. No yielding could be recorded for the composite before the failure of the fibres. After the fibres failed, the load suddenly dropped and obviously was carried completely by the UHMWPE matrix. Afterwards, the composites exhibit behaviour similar to plain UHMWPE. Finally, the composites fracture at a load slightly lower than the ultimate strength of plain UHMWPE. This drop in failure strength for the composites is due to the reduction in effective crosssection and maybe the stress concentration due to flaws introduced by fibre fracture. The failure strength at which the testing specimen fractured was lower than the maximum strength because the load was carried only by the UHMWPE matrix as soon as the fibres failed. Due to a relatively low fibre content, the differences in failure strength between plain UHMWPE and the composites were small. Figure 10 shows the experimental results of the longitudinal tensile maximum strength (al,,) as a function of fibre weight fraction ( W,) for the unidirectional laminates [O/O], loaded along the fibre orientation direction, which was also the maximum load that the composites could bear under the testing conditions. The results indicated that (1) CQ..increased significantly for the composites compared to plain UHMWPE; (2) CQ.. increased almost linearly with mf; and (3) due to the introduction of fibres into the UHMWPE the experimental data showed a larger scatter compared to plain UHMWPE. It can be seen that even for Wf less than 8%, the reinforcing efficiency of the polyethylene fibres is exceptionally high. At Wf = 5%, the increase in tensile strength was more than three times that of plain UHMWPE. Thus, in the case of longitudinal strength, most of the external load was carried by the fibres before the fibre fractured, Biomaterials 1997, Vol. 18 No. 9
Self-reinforced
650
3
z Gl B b VY i
.tr ;
120 P
90 60-
M. Deng and S.W.
0I{: = urn+ (gf - O,)Vf
Shalaby
(TIC= CT, + (CTf- 0,)Wf
30-3 0
I 2
(1)
or using Wf instead of Vf
T
J
I 4
I 6
8
Fiber WeightFraction(%) Figure 10 reinforced represents
composites:
interfacial bond for unidirectional tensile-loaded laminates in the fibre direction, and the fibre and matrix remain elastic until any failure takes place, then the stress on the composite can be expressed as
150-
0
UHMWPE
Longitudinal tensile maximum UHMWPE composite laminates a 95% confidence interval.
strength of self[O/O],. Error bar
since the polyethylene fibres were much stronger and stiffer than plain UHMWPE. Figure 1 I is the scanning electron micrograph of the fracture surface for typical unidirectional laminates loaded along the fibre direction, which showed a relative straight and smooth fracture surface within the matrix phase. A detailed examination of the picture indicates that the failure modes include fibre pull-out, matrix yielding, limited matrix plastic deformation, matrix fracture, fibrous fracture showing necking of fibres, and kink band formation within fibrils. It has been shown in the above that the longitudinal tensile maximum strength of unidirectional laminates increases linearly with the fibre weight fraction (wr). This linear increase in the strength may be described by the rule of mixtures. If there is a strong fibre-matrix
2
(2)
where olCis the stress on the composite, cr,,,the stress on the matrix, of the stress on the fibre, Vf the volume fraction of the fibre, pf the density of the fibre and pC the density of the composite. The above equation can be further changed into the following form if the fibre and the matrix exhibit linear elastic stress-strain relations until any failure occurs: Pr olc = E,,,E”, + (Efef - E,E,,,)W~ L Pf
(3)
where Em is the Young’s modulus of the matrix, Ef the Young’s modulus of the fibre, E, the strain of the matrix and af the strain of the fibre. If there is a perfect interfacial bond between fibre and matrix and no delamination occurs, then E, = Et =
(4)
E,
where E, is the strain of the composite. Inserting (4) into (3) gives ~1~= &,ef + (&ef - &,r:f)Wf ;
(5)
crlC= Emcf + (Ef - En,)cfwf 5
(6)
or Pf
The modulus EC of the composite may then be found as E, = En, + (Ef - E,,,)wf ;
(7)
Assuming the yielding strain of the matrix is greater than the ultimate strain of the fibres (note that polyethylene fibres have no yielding point), which is the case for the self-reinforced UHMWPE composites, then the longitudinal tensile maximum strength CQ:,! of the unidirectional self-reinforced composite laminates is found as olCu= E,sf, + (Ef - E,,,)q,lwf 2
Figure 11 Scanning electron micrograph of the fracture surface of the unidirectional laminate [O/O], loaded along the fibre orientation. Original magnification x100. Biomaterials
1997, Vol. 18 No. 9
(8)
where afu is the ultimate strain of the fibre which shows no yielding, macroscopically. Equation (8) clearly indicates a linear relationship between Q,, and wf, which was proved by the experimental data shown in Figure 10. Since Ef > E,,,, qcu is therefore determined mainly by the polyethylene fibres if Wf is not too small. Equation (8) can be used to provide failure criteria for longitudinal tensile strength of unidirectional selfreinforced UHMWPE composites, which is controlled by the ultimate strain of the polyethylene fibre. It can also be found that Equation (8) gives a yield strength of E,E~ for plain UHMWPE where ‘iicif= 0. This is obviously not true since the yield strength of plain UHMWPE is Em~mu,where a,,,, is the yielding strain of the matrix. It can be easily understood that E,,,c,,, is
Self-reinforced
UHMWIPE composites:
M. Deng and S.W. Shalaby
651
than Em&furconsidering that E,, > &h. Hence, Equation (8) is not applicable at very low wr and thus there is a critical mr below which fibres will not have a reinforcement effect. In this case, the function of the fibres in the composites may be considered to reduce the effective load-bearing cross-section of the materials. Therefore, the tensile strength of the composites is then determined by the matrix if a knee point at which fibres break is not taken as a yielding,
greater
It is clear that the tensile maximum strength of the composites is now the yielding strength of the matrix minus a percentage fibre weight component of Wr(p,/pr). In Equai.ion (9) the stress concentration caused by fibre l?a.cture has been neglected. From Figure 20,it is clear that the fibre weight fraction used in this research nlever reached the critical value. Letting Equations (811and (9)be equal gives the critical value Wf, of fibre weight fraction at which the composites will have the lowest strength,
I
I
2
4
I 6
8
Fiber Weight Fraction (%) Figure 12 Transverse tensile reinforced UHMWPE composite bar represents a 95% confidence
yield strength of selflaminates [90/90],. Error interval.
As an approximation, letting pf/pCx 1,E,, FS2~fuand 4 M lOOEm, then we find that mrC M 1%. Inserting (10) into (8) or (9)gives the lowest strength (brcu)minfor the composite: (%u)min
=
&FF In
Efef,
(11)
mu --
or
(12) where (~,r is the yield strength of the matrix, crf~the fracture strength of the fibres and u,f the stress on the matrix at the fibre fracture. It is clear from Equation (12) that since dmY:> umf, the lowest strength (crcu)min of the composites will be less than the yield strength cmY of the matrix. Using the above assumed data, it is found that (brcu)min2: O.99o,,. Transverse tensile strength When loaded in the direction perpendicular to fibre the unidirectional self-reinforced orientation, UHMWPE composite laminates 190/901, showed a stress-strain curve s.imilar to that of plain UHMWPE. The properties of the composites in this case are much more matrix-dominated, particularly for a low fibre content. The experimental results of the transverse tensile yield strength of the composites as a function of fibre weight fraction wr are shown in Figure 12. The results indicated that for Wf < 7% the tensile yield strength of unidirectional laminates in the transverse remained unchanged, although the direction composites did show a larger variation in data, compared to plain IJHMWPE. This kind of behaviour may suggest that the interfacial bonding of the composites does not affect the transverse yield strength of the composites due to the low percentage of fibres within the matrix. :It may also be due to the lower sensitivity of UHMWPE materials to cracks compared to a more brittle matrix like epoxy.
Figure 13 Scanning electron micrograph of the fracture surface of the unidirectional laminate loaded perpendicular to the fibre orientation. Original magnification x50.
Figure 13 shows the scanning electron micrograph of the fracture surface for typical unidirectional laminates loaded perpendicular to the fibre direction, which showed a very irregular surface. Compared to Figure 1 I, the massive matrix plastic deformation caused by stretching led to the fracture of the composites. A few stretching bands could be seen. However, no fibre fracture was visible in this case. To a first approximation the fibres can be regarded as cylindrical holes. If the fibres are considered as a simple square array, then the transverse yield strength cltc is given by the following formula, provided that the resin is not notch-sensitive31: ctc = E,E,,[~- 2(Vfn)0.5] Biomaterials
(13) 1997, Vol. 18 No. 9
652
Self-reinforced
UHMWPE composites:
M. Deng and S.W. Shalaby
(14)
For a fibre weight fraction wf = 5%, the strength reduction is about 80%, which indicates an important function of the interface. However, this result also indicates that the above equation is not a good estimation for the self-reinforced UHMWPE composites if the prediction is compared to the data in Figure 12. Cross-ply laminates Since very few composites are used in the form of unidirectional laminates, tensile tests were conducted on the cross-ply laminates [O/901,. The experimental results showed that the composites displayed a stressstrain curve similar to that of the unidirectional laminates loaded along the fibre direction. This indicates that in this case the strength of the laminates is mainly determined by the fibres. Figure 24 illustrates the experimental results of maximum tensile strength of cross-ply laminates as a function of wr. The results showed that the maximum tensile strength increased with increasing wr and doubled at mr = 5% compared with plain UHMWPE. It seemed that the strength increased linearly with mr. Figure 15 shows the scanning electron micrograph of the fracture surface for typical cross-ply laminates, which showed a relative curved and non-smooth compared to Figure 11. The fracture surface microscopic failure modes are similar to those of the laminates loaded along the fibre direction, which include fibre pull-out, matrix yielding, matrix peeling off, limited matrix plastic deformation, matrix fracture, fibrous fracture showing necking of fibres and kink band formation within fibrils. Tensile modulus To see the differences in stiffness, a comparison was made of the modulus of plain UHMWPE and the selfreinforced composites. Keeping wf at 5% for the composites, Figure 16 shows the tensile test results of modulus. It is clear from this figure that a large increase in modulus was achieved for both cross-ply laminates and unidirectional laminates loaded along the fibre direction, compared to plain UHMWPE. The modulus of laminates [O/90], is twice that of plain UHMWPE. These results were obviously due to the reinforcing effect of ultra-high strength and modulus polyethylene fibres. It is also seen that the transverse modulus was higher than for plain UHMWPE, which is possibly due to the fibre restriction effect on the transverse deformation. Creep results Figures 17 to 20 list the creep results, where the creep strain was plotted against creep time. Figure 17 clearly shows that, at room temperature and 5 MPa, both unidirectional and cross-ply laminates of the selfreinforced composites were much more creep-resistant than plain UHMWPE in a 24-h creep experiment; in fact, more than 100% reduction in creep deformation for UHMWPE was achieved after the introduction of polyethylene fibres. The figure also indicates that the Biomaterials
1997, Vol. 18 No. 9
3
6
9
12
Fiber Weight Fraction (%) Figure 14 Tensile maximum strength of self-reinforced UHMWPE composites, cross-ply laminates, [Oi90],. Error bar represents a 95% confidence interval.
Figure 15 Scanning electron micrograph of the fracture surface of the cross-ply laminate. Original magnification x100.
cross-ply laminate displayed less creep resistance than the unidirectional laminate because for the cross-ply laminate only half of 5% of the fibres was in the loading direction, which indicates the effect of fibre content on the creep performance for self-reinforced UHMWPE composites. Quantitatively, the cross-ply laminate showed about 30% more creep deformation than the unidirectional laminate. Figure 18 shows that, at 1 MPa and 3i’“C, the cross-ply laminate had a much higher creep resistance than plain UHMWPE. Again, more than 100% reduction in creep deformation for UHMWPE was obtained when it was reinforced by polyethylene fibres. The creep data for both plain UHMWPE and the composite indicated a full recovery of creep deformation 24 h after unloading (residual strain was 15 pe for UHMWPE and 60 p for the cross-
Self-reinforced
M.
UHMWIPE composites:
Deng
and
653
S. W. Shalaby
3.0 I
I
2.5 0.32
2.0 1.5
0.16 -)-
1.0 0.08
0.5 0.0 i
0 UHMW-PE
[0/9Ols
[90/9O]s
Crossply Unidirectional
10
20
30
40
50
Fiaure 19 Tensile creeo and recoverv results rekrforced composites at’37”C and 2.5 MPa.
for self-
[O/Ols
Time (hour)
Figure 16 Comparison of tensile modulus of plain UHMWPE composite UHMWPE and self-reinforced laminates. Error bar represents a 95% confidence interval.
1.6 -p-------a
1.4
A
1.2
-
UHMW-PE Cross-ply
1 .o
--u-
Unidirectional
0.8
unloading
*
0.6
0.2 -
q
0.4 0.2-
-
Crossply Unidirectional
,
1 10
0
Figure 17 Tensile creep self-reinforced composites
I 20
Time (hour)
results for plain UHMWPE and at room temperature and 5 MPa.
20
0
10
40
50
Figure 20 Tensile creep and recovery results reinforced composites at 37°C and 5 MPa.
for self-
30
UHMW-PE Crossply
40
50
Time (hour) Figure 16 Tensile weep and recovery results for plain UHMWPE and the self-reinforced cross-ply composite at 37°C and 1 MPa.
ply laminate). As the load was increased to 2.5 MPa, the creep deformation of the cross-ply laminate increased, compared to the result in Figure 18, as shown in Figure 19. Thus, when the load was increased from 1 to 2.5 MPa, the creep strain at the end of loading for a 24-h creep test was increased from 931 to 39oi’ps for the cross-ply laminate, more than 300% increase, and was 2673 p for the unidirectional laminate. The result also showed that the creep deformation did not fully recover 24 h after unloading for both cross-ply and unidirectional laminates, leaving a residual strain of 885 PC for the cross-ply laminate and 442~s for the unidirectional laminate. The residual strain of the
30
20 Tie
-
I0
I 30
(hour)
unidirectional laminate was 50% less than that of the cross-ply laminate at the end of loading, and the former recovered faster than the latter. Figure 20 shows that when the load was further increased to 5MPa, the 24-h creep deformation for both unidirectional and cross-ply laminates doubled and more unrecovered creep strain was seen 24 h after unloading, compared to the results in Figures 18 and 19. The creep strain at the end of loading was 8117 p for the cross-ply laminate and 5044~s for the unidirectional laminate, and the residual strain was 1872~~ for the cross-play laminate and 1161 PE for the unidirectional laminate after 24-h recovery. Comparing Figure 17 with Figure 20 indicates that when the temperature was increased from room temperature to 37”C, the increase in creep strain was less than 100% for the composites. It seems that the creep strain at 37°C for the composites increased proportionally to the load. Overall, the creep data obtained at the test conditions illustrated a significant improvement in creep resistance for plain UHMWPE after reinforcement by the UHMWPE fibres. The above results also illustrate a general trend for the creep of materials that at the initial stage of experiment the creep deformation developed rapidly, which includes an instant elastic response and the rapid viscoelastic response to the applied load. After this short period, creep developed into a stage where the creep strain propagated at a continuously reduced rate, which must be taken into account in trying to describe, mathematically, the creep behaviour of UHMWPE systems. Biomaterials
1997, Vol. 18 No. 9
654
Self-reinforced
UHMWPE
M. Deno and S.W. Shalabv
composites:
Wear and impact results The wear property of self-reinforced composites (crossply) was compared to the control. The control samples (Control-l) were machined from ram-extruded UHMWPE rods, compared to the control samples (Control-Z) which were compression-moulded directly from resin and then machined into wear specimens. Each data point represents an average of four samples. All the specimens were gamma-sterilized at 2.5 Mrad in an air environment before the wear test. Reciprocal sliding pin-on-disc wear tests were run for one million cycles. Figure 22 lists the results, where wear data (weight loss) are given as milligrams per million cycles. It is clear that the wear data showed a large variance. Statistical analysis of the data showed that there were no significant differences between the composites and the two types of control. The results are promising since the mechanical performance was improved as wear properties did not change for the self-reinforced composites as compared to plain UHMWPE. Figure 22 shows the impact results of double-notched specimens. Each data point is an average of at least three specimens. The impact results show an increase in impact strength for UHMWPE after introduction of Spectra 1000 fibres. The increase in impact strength due to fibre reinforcement was attributed to the higher energy absorption of polyethylene fibres (high toughness) and the energy dispersion occurring along the laminate interfaces, perpendicular to the direction of notching. Thus, the impact results showed an advantage for the self-reinforced UHMWPE composites.
Control 1 Figure 21 Wear test reinforced composites
I
I
100 200 Temperature(“C)
1
Figure 22 specimens.
23
Biomaterials
Comparison
of differential
1997, Vol. 18 No. 9
scanning
calorimetry
Impact
tl
t
results
Table 1 Differential scanning fibres taken from composites
and
self-
f uble-notched
for
calorimetry
data
Sample
of UHMWPE
Cryst. (%)
Virgin fibre Fibre from composite
147 146
153 152
202 205
80 66
135°C during the composite processing reduced the strength of the second melting peak, but the crystallinity may possibly increase. This may indicate some kind of change in the polyethylene crystal structure after heat treatment. This may also result from the effect of the compression force used to make the composites on the fibres during the composite processing.
1
300
m
300 Temperature (“0
a. Virgin fiber Figure
results for plain UHMWPE (cross-ply laminates).
-
Concerning the effects of composite processing conditions on UHMWPE fibres, differential scanning calorimetry (DSC) analysis was performed. The samples of UHMWPE fibres were taken from inside the self-reinforced UHMWPE composites. The numerical data of T,,,, T,, and crystallinity are given in Table 1, where T,, corresponds to the first melting peak and T,, the second melting peak, and To is the oxidation temperature. A comparison of their DSC thermograms is shown in Figure 23. Generally speaking, the shape of the DSC thermogram from the heat-treated fibre under the composite processing conditions showed a distinct difference compared to the as-received virgin fibre. Heat processing of the UHMWPE fibre around
-
Composite
3 b x u
Effects of processing on thermal properties of fibres
i 0
Control 2
b. Fiber from composite thermograms
of UHMWPE
fibres.
Self-reinforced
UHMWPE
composites:
M. Deng
655
and S. W. Shalaby
CONCLUSIONS It was shown that the tensile properties, creep resistance and impact strength of the self-reinforced UHMWPE composites are superior to plain UHMWPE. The wear properties of the composites were found to be similar to plain UHMWPE. A linear relationship exists between the longitudinal tensile strength and fibre content for the unidirectional laminates. The effect of heat processing on the thermal properties of UHMWPE fibres was small. SEM examination of the fracture surface revealed differences in failure mechanisms among i.he composites.
13.
14.
15.
16.
ACKNOWLEDGEMENTS The authors would like to thank Hoechst Celanese Co., Texas, for providing the UHMWPE GUI&05 resin, and Allied Signal Inc., Fiber Division, Virginia, for providing the SpectraH‘ fibres. We also thank Smith & Nephew Rechirds, Inc., Tennessee, for help in performing the wear and impact tests.
REFERENCES 1. 2.
3. 4. 5. 6.
7. 8. 9.
10. 11. 12.
Charnley, J., Low Friction Arthroplasty of the Hip. Springer-Verlag,Berlin 1979. Davidson,J. A. and Schwartz, G., J. Biomed. Mater. Res., 1987, 21-A3, 261. du Plessis, T. A., Grobbelaar, C. J. and Marais, F., Rad. Phys. Chem., 1977, 9, 647. Grobbelaar, C.J., ~duPlessis, T. A. and Marais, F., I. Bone Joint Surg., 1978, 60-B, 370. Shen, C. and Dumbleton, J. H., Wear, 1974, 30, 349. Streicher, R. M., In Ultra-High Molecular Weight Polyethylene as 13iomaterial in Orthopedic Surgery, ed. H.-G. Willert, G. H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, .1991, pp. 66-73. Dumbleton, J.H. and Shen, C., I. Appl. Polym. Sci., 1974,18,3493. McKellop, H., Clarke, I., Markolf, K. and Amstutz, H., 1. Biomed. Mater. Res., 1981, 15, 619. Farling, G. M. and Greer, K., In Mechanical Properties of Biomaterials, ed. G. W. Hastings and D. F. Williams. Wiley, 1980, pp. 53-64. Halcomb, F. J., and Bardos, D., Trans. Am. Artif. Intern. Organs, 1981, 27, 364. Wright, M., Fubayashi, T. and Burnstein, A.H., J. Biomed. Mater. Bes., 1981, 15, 719. Walker, P.S., Ben-Dou, M., Askew, M. and Pugh, J., Eng. Med., 1981, 10, 33.
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Wright, T.M. and Rimnac, C. M., In Ultra-High Molecular Weight Polyethylene as Biomaterial in Orthopedic Surgery, ed. H.-G. Willert, G.H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, 1991, pp. 202-207. Brill, W. and Mittelmerier, H., In Ultra-High Molecular Weight Polyethylene as Biomaterial in Orthopedic Surgery, ed. H.-G. Willert, G.H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, 1991, pp. 217-220. Stern, L.S., Manley, M.T., Parr, J.E., Stulberg, B.N., Price, H. and Ries, M., In Ultra-High Molecular Weight Polyethylene as Biomaterial in Orthopedic Surgery, ed. H.-G. Willert, G.H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, 1991, pp. 258-261. Connelly, G. M., Rimnac, C. M., Wright, T.M., Hertzberg, R. W. and Manson, J.A., I. Orthop. Res., 1984, 2, 119. Rimnac, C. M and Wright, T. M., In Ultra-High Molecular Weight Polyethylene as Biomaterial in Orthopedic Surgery, ed. H.-G. Willert, G. H. Buchhorn and P. Eyerer. Hogrefe & Huber, Toronto, 1991, pp. 28-31. Chang, H. W., Lin, L.C. and Bhatnager, A., 31st International SAMPE Symposium, SAMPE, Covina, California, 1986, pp. 859-865. Woods, D. W. and Ward, I.M., J. Appl. Polym. Sci., 1994, 51, 2572. Peijs, T., Rijsdijk, H. K., de Kok, J. M. and Lemstra, P. J., Compos. Sci. Tech., 1994, 52,449. Taboudoucht, A., Opalko, R. and Ishida, H., Polym. Compos., 1992, 13, 81. Harpell, G.A., Kavesh, S., Palley, I. and Prevorsek, D., European Patent Application No. 83101731.4, 1983. Marais, C. and Feillard, P., Compos. Sci. Tech., 1992, 45, 247. Teishev, A., Incordona, S., Migliaresi, C. and Marom, G., J. Appl. Polym. Sci., 1993, 59, 563. Shalaby, S. W. and Deng, M., Self-reinforced ultra-high molecular weight polyethylene composites. US Patent Application field, 1994. Ishida, H. and Bussi, P., Macromolecules, 1991, 24, 3569. Standard Test Method for Tensile Properties of Plastics, ASTM D-638,1989. Poggie, R. A., Wert, J. J., Mishra, A. J. and Davidson, J. A., In Wear and Friction of Elastomers, ASTM STP 1145, ed. R. Dentoo and M.K. Keshavan. American Society for Testing and Materials, Philadelphia, 1992, pp. 6581. Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics, ASTM D-2990, Annual Book of ASTM Standards, 1989. Deng, M. and Shalaby, S. W., Polym. Adv. Tech., 1993, 43. Hull, D., An Introduction to Composite Materials. Cambridge University Press, 1981.
Biomaterials 1997, Vol. 18 No. 9