Improvement of Microstructure and Properties of Poly(methyl methacrylate)/Hydroxyapatite Composites Treated with Zirconate Coupling Agent

Abstract

Poly(methyl methacrylate) (PMMA)/hydroxyapatite (HA) composite has potential application in denture base materials. Surface treatment of HA using zirconate coupling agent (ZCA) has been carried out to improve the interfacial bonding between the PMMA matrix and HA filler. The effects of different concentration of ZCA (i.e., 2–8%) on the mechanical properties of PMMA/HA composites was investigated using tensile and three-point bending flexural tests. The morphological properties of the PMMA/HA composites was characterized using field emission scanning electron microscopy (FESEM). It was found that PMMA/5HA-2% ZCA composites exhibited higher flexural strength compared to that of untreated PMMA/HA composites. This is attributed to the enhancement of interfacial interaction between PMMA and HA, which can be evidenced by the FESEM and EDX technique. The kinetics of simulated body fluid (SBF) absorption of PMMA/HA composites were studied at 37°C for immersion duration of 2 months. The mathematical treatment used in analyzing the data was the Fickian diffusion and utilized Fick's second law of diffusion. It is worth mentioning that the kinetics of SBF absorption of the PMMA/HA composites conformed to Fickian law behavior, whereby the initial SBF absorption follows a linear relationship between the percentage gain at any time t and t^1/2, followed by saturation. It was found that the equilibrium SBF content of PMMA/HA composite was reduced by the ZCA treatment.

Keywords

poly(methyl methacrylate)hydroxyapatite composites denture base simulated body fluid

INTRODUCTION

Poly(methyl methacrylate) (PMMA) is widely used in prosthetic dentistry application, for example, denture bases, artificial teeth, and impression trays. The use of heat-polymerized, permanent, acrylic resin denture bases has certain advantages owing to their strong and rigid behavior, in which, they should provide the retention and stability of the final denture. When PMMA-powder and methyl methacrylate (MMA) monomer liquid are polymerized a semi-interpenetrating polymer network structure is formed [1–5]. Denture bases materials are responsible for artificial tooth fixation, stability, and distribution of masticatory forces over a large tissue-bearing area [6]. PMMA continues to be used in denture bases materials, which is attributed to its excellent appearance, ease of processing, ease in repair, accurate fit, stability in the oral environment, and superior esthetics [7]. However, the primary problem is its poor strength characteristics [8]. PMMA has been reinforced with additives (e.g., glass fibers, carbon fibers, barium particles, zirconium particles, silica, hydroxyapatite, etc.) to improve its mechanical, thermal, physical, and rheological properties.

Hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂, was used in various biomedical fields such as dental material, bone substitute, and hard tissue paste. HA can be produced either chemically or from natural resources, such as from corals via hydrothermal transformation, or from bones of humans or (preferably) animals, or from enamel (EHA) or dentine (DHA) of teeth. HA has been extensively used as a substitute material for damaged teeth or bone over the past three decades, and its compatibility with surrounding tissues has been experimentally proven [9–11]. HA-reinforced polymers have many potential clinical applications including use as a bone cement, as bioceramic-reinforced polymers for use as a coating of joint replacement prosthesis, and dental implants. Bio-active ceramics like sintered HA and glass-ceramic hybrids are known to experience good binding characteristics to bones in vivo and they are now applied even to a clinical extent. The addition of HA to a conventional PMMA-based bone cement can produce a bioceramic-reinforced polymer with improved mechanical and biological properties [12–14].

Denture base acrylic resins are subjected to many various types of intraoral and extraoral stresses. For example, repeated masticatory forces could lead to fatigue phenomena, while high-impact forces may occur as a result of dropping the prosthesis. As a result, stress concentration is generated, and denture base acrylic resin can initiate or propagate existent cracks, thereby influencing the failure rate. To compensate for these problems, the ability of the material to withstand the presence of notches and crack propagation is an important factor affecting denture performance. Thus, there is a need for improvement in the fracture resistance of PMMA denture base materials [15–17]. Approaches to strengthening and/or toughening the acrylic resin prosthesis have included modifying or reinforcing the resin by using filler and fiber, as well as chemical modification to produce graft copolymer high-impact resins. There are also some efforts to be done to increase the impact strength by the incorporation of a rubber-like substances in the acrylic resins [5,15,17]. A rubber phase may be introduced into the acrylic matrix as either co-polymer, rubber particles, or as core-shell particles. The most commercially successful method of reinforcement to date is rubber toughening; however, such materials have compromised flexural properties [18,19].

The mechanical properties of PMMA/HA composites could be limited by the incompatibility between the PMMA and HA. Thus, modification of PMMA/HA composites is required in order to achieve a high performance denture base materials with better mechanical properties. Polymeric compatibilizer and coupling agent can play a role to improve the interaction and adhesion between the organic PMMA matrix and inorganic HA particles [20,21]. Our previous works on the surface treatment of HA using silane coupling agent has demonstrated promising results in mechanical and thermal properties, as well as reduced water absorption and simulated body fluid (SBF) absorption of PMMA composites [22,23]. Water absorption of PMMA/HA is an important study for dentistry science. In oral environment, dental restorative materials are exposed to saliva which contain water. Sometimes, the dental materials are exposed to exogenous substances such as acids, base, salts, alcohols, and oxygen, during drinking or eating. SBF test is a method that is well recognized to characterize the in vitro bioactivity of ceramic materials consisting of their immersion in an aqueous SBF solution which simulates the properties of human plasma for certain period and verifies the formation of the HA layer on the surface of the samples [24]. The quantity of water molecules and SBF absorbed by the resin matrix of dental resin based composite and the rate of water sorption has been identified as being diffusion controlled.

In this article, we investigate the surface treatment of HA by zirconate coupling agent (ZCA), and its effects on the mechanical and morphological properties of PMMA/HA composites. The influence of the ZCA treated HA on the kinetics of SBF absorption for the PMMA/HA denture base composites have been discussed thoroughly.

MATERIALS AND EXPERIMENTAL

Preparation of PMMA/5HA Composites

Acrylic resins are prepared using prepolymerized PMMA powder with typical molecular weight 996,000 (Sigma Aldrich, USA), mixed with MMA stabilized with monomethyl ether hydroquinone as inhibitor (Sigma Aldrich, USA) and ethylene glycol dimethacrylate (EGDMA) crosslinker (Sigma Aldrich, USA). The ratio of powder to liquid was set as 10:4 according to the dental laboratory practice. The powder component was prepared by mixing of PMMA, 5 wt% HA (treated or untreated) and 0.5 wt% benzoyl peroxide (BPO) (Merck Chemical, Germany) using planetary ball milling. HA in powder form was supplied by Sigma Aldrich, USA with an average specific surface area (BET) 50 m²/g and average particle diameter of 18 µm. The liquid component was prepared by mixing MMA and EGDMA. The powder and liquid components were then mixed together. The mixture was then packed into a mold using compressor (MESTRA, Talleres Mestraitua, Spain) with the pressure of 14 MPa at room temperature for 25 min after achieving a dough stage. The polymerization was carried out using a water bath (Memmert, Germany) at 78°C for 90 min. Table 1 shows the materials’ chemical structure and their function. The materials designation and compositions are shown in Table 2.

Table 1.

Chemical structure and function of materials.

Table 2.

Materials designation and compositions.

	Materials composition
Materials designation	PMMA (wt%)	HA (wt%)	ZCA concentration based on HA (wt%)
PMMA/5HA	94.5	5	–
PMMA/5HA-2% ZCA	94.5	5	2
PMMA/5HA-4% ZCA	94.5	5	4
PMMA/5HA-6% ZCA	94.5	5	6
PMMA/5HA-8% ZCA	94.5	5	8

Surface Treatment on HA

ZCA (Neopentyl(diallyl)oxy trimethacryl zirconate) was supplied by Kenrich Petrochemical, Inc (USA). The boiling point and flash point of this ZCA are about 104°C and 60°C, respectively. The HA was surface treated using ZCA with different concentration (i.e., 2, 4, 6, and 8 wt% based on HA). The ZCA was added into toluene and was stirred for approximately 30 min in a vessel followed by addition of HA. Later, the HA filler was stirred together with solvent for approximately 1 h. HA filler was then filtered and washed to drive off excess ZCA and solvent. After this, HA filler was dried in oven at 80°C for 24 h.

Characterization of PMMA/5HA Composites

MECHANICAL TESTS

Tensile and three-point bending tests were performed according to ASTM D638 and ASTM D790-03 respectively, using an Instron 3366 machine (USA). The specimen geometry is 100 mm × 13 mm × 3 mm (length × width × thickness). For tensile tests, the gauge length and crosshead speed was set at 50 mm and 5 mm/min respectively. Tensile modulus, tensile strength, and elongation at break were evaluated from the stress-strain data. For the flexural tests, the support span length was set at 50 mm. The testing speed was set at 2 mm/min. At least five samples for each formulation were examined.

MORPHOLOGICAL CHARACTERIZATION

The morphological properties of the fractured surface of the PMMA composites were inspected in a field-emission scanning electron microscope (FESEM, Zeiss Supra 35VP). The sample surfaces were gold coated to avoid electrostatic charging during examination. Energy dispersive X-ray microanalysis system (EDX, EDAX Falcon System) was used to analyze the occurrence of elements in the specimens that were sputtered with gold.

SIMULATED BODY FLUID ABSORPTION

The specimens (in flexural bar geometry) were dried in a vacuum oven at 70°C until a constant weight was attained. The specimens were placed in a container of SBF. The containers were then put in a water bath at temperature of 37°C. The SBF was supplied by B-Braun Medical Industries (Penang, Malaysia). The weight change of the specimens was measured as a function of time after removing the SBF on their surfaces. The percentage change at time t, (M_t) as a result of SBF absorption was determined by Equation (1) and diffusion coefficient was calculated using Equation (2):

(1)

(2)

where W_d and W_w denote the initial weight of specimen prior to exposure to the SBF tests and weight of specimen after exposure to SBF tests, respectively. For Equation (2), the maximum moisture absorption (M_m) was calculated as an average value of several consecutive measurements that showed no appreciable additional absorption, h is the thickness of the specimens, and D is the diffusion coefficient.

RESULTS AND DISCUSSION

Mechanical Properties of PMMA/5HA Composites

Table 3 shows the flexural properties of PMMA/5HA composites with and without ZCA surface treatment. From our previous work [22], it was found that the flexural modulus of PMMA increased by the addition of HA filler. This is attributed to the rigidity and reinforcing-ability of HA. The rigidity nature of HA filler restrict the movement of the matrix phase in the vicinity of each particle which consequently contribute the enhancement in modulus and stiffness. In addition, mechanical interlocking and polymer adsorption between the PMMA and HA could also contribute to the increment of flexural modulus. Adsorption of polymer onto the filler restricts molecular motions, changes the density of packing of polymer chains, and consequently modified the conformation and orientation of chain segments in the neighborhood of the surface when load was applied [25]. Comparing the flexural modulus of PMMA/HA with and without ZCA treatment, it was found that the increment of the flexural modulus of ZCA treated HA filled PMMA is marginal. Although HA could contribute to the improvement of stiffness and modulus, the immiscibility between PMMA and HA leads to the reduction of flexural strength [22]. Table 3 clearly shows that the flexural strength of PMMA/5HA composites was increased significantly by the ZCA treatment, especially the 2% ZCA surface treatment of HA. This indicates that the interfacial contact between PMMA and HA can be improved by the ZCA. Similar observation had been reported by Carmen et al. [26] on the polyethylene/HA composites in the presence of ZCA.

Table 3.

Effect of different concentration of zirconate coupling agent on the flexural properties of PMMA/5HA composites.

Material designation	Flexural modulus (GPa)	Flexural strength (MPa)
PMMA/5HA	2.5 ± 0.07	50.5 ± 1.43
PMMA/5HA-2% ZCA	2.6 ± 0.04	71.0 ± 2.01
PMMA/5HA-4% ZCA	2.7 ± 0.04	64.3 ± 1.43
PMMA/5HA-6% ZCA	2.5 ± 0.03	65.7 ± 2.24
PMMA/5HA-8% ZCA	2.5 ± 0.03	61.9 ± 1.97

Table 4 shows the tensile properties of PMMA/5HA composites with and without ZCA treatment. It was found that the tensile modulus of PMMA/5HA composites decreased slightly by the treatment of ZCA. This is attributed to the plasticizing effect of the coupling agent, caused by formation of flexible interface layer in the composites. Similar finding was reported by Bose et al. [27] on the effects of coupling agent towards the properties of mica filled nylon-6 composites. From Table 4, it can be seen that the increment of tensile strength of PMMA/5HA composites is significant by the treatment of ZCA. It is believed that the ZCA could provide sufficient wettability to HA and thus contribute to better adhesion and interaction with PMMA. Strong interaction at the interface between the filler particles and the macromolecular chains of the polymer matrix through the coupling agent will lead to deep changes at the interfacial zone [28]. Moreover, the elongation at break of PMMA/5HA composites increased approximately 60%, 45%, 55%, and 55% with respect to 2-8 loading of ZCA. The enhancement in the elongation at break of the PMMA/ZCA treated HA composites could be attributed to the improved interfacial interaction and/or internal plasticization. Coupling agent induces lubricating/plasticizing action when it diffuses into the polymer matrix [29,30]. According to Zhu et al. [31], better dispersion of filler particles in matrix may give homogeneous debonding which reduces the effects of stress concentration and increase the elongation at break.

Table 4.

Effect of different concentration of zirconate coupling agent on the tensile properties of PMMA/5HA composites.

Material designation	Tensile modulus (GPa)	Tensile strength (MPa)	Elongation at break (%)
PMMA/5HA	2.24 ± 0.05	34.8 ± 0.51	2.0 ± 0.06
PMMA/5HA-2% ZCA	1.93 ± 0.06	46.3 ± 1.35	3.2 ± 0.31
PMMA/5HA-4% ZCA	1.95 ± 0.04	49.2 ± 0.29	2.9 ± 0.29
PMMA/5HA-6% ZCA	1.94 ± 0.03	42.1 ± 1.68	3.1 ± 0.11
PMMA/5HA-8% ZCA	1.94 ± 0.05	43.1 ± 1.91	3.1 ± 0.18

Morphological Properties

The FESEM micrographs of PMMA/5HA and PMMA/ZCA treated 5HA composites are shown in Figure 1. The smooth surface exhibited by PMMA indicates a typical fractographic feature of brittle fracture behavior. In general, filler can act as stress concentrators in polymer matrix and lead to initiation of failure deformation. In Figure 1(a), it was found that the fracture proceeds along the interface of PMMA and HA. There is noticeable gap between the PMMA and HA particle, which indicates the weak filler-matrix interface. It is interesting to note that, the ZCA treated HA was surrounded by ‘mono- and multi-layer track-like’ interfacial structure (cf. Figure 1(b) and (c)). It is believed that the ZCA can be absorbed onto the surface of HA and form the multilayer through hydrogen bonding. This indicates that the ZCA are able to provide interfacial bonding between HA filler and PMMA matrix. Nonetheless, Figure 1(c) shows the HA particles were pulled off from PMMA matrix. This is maybe due to the excessive concentration of ZCA which induce the plasticizing effects, rather than interfacial contact enhancement. Figure 2 shows the EDX spectra from PMMA/ZCA treated-5HA composite. Recall that the HA used for this study contains O, Na, P, and Ca. Note that the elements of O, Zr, Ca can be detected in the ‘multilayer track-like’ interfacial structure. This may prove that the interface multilayer is associated to ZCA, which contains the Zr element. These morphological observation evidenced the improved adhesion between PMMA and HA in the presence of ZCA. Figure 3 shows the possible interaction between HA and ZCA. It is hypothesized that the interaction between PMMA and HA can be controlled by the concentration of ZCA. One may find that plasticization predominated as the overloading of ZCA.

Figure 1

FESEM micrograph of (a) PMMA/5HA, (b) PMMA/5HA-2% ZCA, (c) PMMA/5HA-4% ZCA.

Figure 2

EDX spectra of PMMA/5HA treated with ZCA (Note: The EDX spectra was taken from the arrow pointed area).

Figure 3

Possible interaction between HA and zirconate coupling agent.

Kinetics of SBF Absorption

Figure 4 shows the effect of ZCA concentration on the SBF uptake of PMMA/5HA composites upon exposure to SBF immersion for 2 months. All specimens showed an SBF uptake process involving a rapid Fickian process which is linear to t^1/2 followed by a slower rate of uptake which is indicated as matrix saturation. The equilibrium of SBF absorption (M_m) was achieved after 2 months of immersion. The SBF uptake of polymer matrix is governed by either the free volume theory, in which water or aqueous solution diffuses through substrates or the interaction theory, in which it is controlled solely by the available hydrogen bond at the polar sites [32]. As can be seen, the M_m of PMMA/5HA treated ZCA composite is lower than untreated PMMA/5HA composite. This can be related to the enhancement in the interfacial bonding between PMMA and ZCA treated HA. Strong interfacial interaction leads to better aqueous solution barrier, and thus lower the SBF diffusion. In addition, it is believed that the reduction of M_m for PMMA/ZCA treated-5HA composite is due to the effect of the repulsive force between ion Zr⁴⁺ in ZCA and ion Na⁺, K⁺, Ca²⁺ present in SBF. Consequently, repulsive force of these ions from two substrates may restrict the moisture absorption to the composite and thus, reduce the SBF absorption (M_m).

Figure 4

The effects of zirconate coupling agent concentration on the SBF uptake of PMMA/HA composites.

Table 5 shows the maximum SBF absorption, diffusion coefficient, and diffusional exponent (n) of PMMA/5HA composites with and without ZCA surface treatment. In general, the water diffusion behavior in polymeric materials can be divided into three types, that is, Fickian diffusion, relaxation controlled, and non-Fickian [33]. Theoretically, the water diffusion behavior can be differentiated by the shape of the sorption curve (cf. Equations (3) and (4)):

(3)

(4)

where M_t is the moisture content at specific time (t), M_max is the maximum moisture content. The value of diffusional exponent (n) shows different behavior between the three cases of diffusion: Fickian diffusion (n = 0.5), relaxation (n ≤ 1) and anomalous transport (0.5<n<1.0). To confirm the water absorption behavior of PMMA/HA composites, the curves of log(M_t/M_max) versus log(t) of all the PMMA/HA composites have been plotted (cf. Figure 5), and the value of diffusional exponent (n) were calculated. From Table 5, it can be seen that the values of n show the SBF absorption in PMMA/HA composite (with and without ZCA coupling agent) and are close to the Fickian diffusion behavior. It is interesting to note that the ZCA treatment of HA reduced the diffusion coefficient for the PMMA/5HA composites. This can be related to the improved interfacial interaction between PMMA and HA, which in turn, reduce the transportation path of the water molecules into the PMMA composites. According to Yiu et al. [32], the polarity and hydrophilic groups in the composite may lead to trapping sites to stop the water for interaction which will subsequently hinder the water molecule diffusion.

Table 5.

Maximum SBF absoprtion (M_m), diffusion coefficient (D) and diffusional exponent (n) of PMMA/5HA composites and zirconate coupling agent treated PMMA/5HA composites after subjected to SBF tests.

Material designation	M_m (%)	D (m²/s)	n
PMMA/5HA	2.36	1.94 × 10⁻¹²	0.504
PMMA/5HA-2% ZCA	1.77	1.83 × 10⁻¹²	0.467
PMMA/5HA-4% ZCA	1.71	1.89 × 10⁻¹²	0.467
PMMA/5HA-6% ZCA	1.98	1.77 × 10⁻¹²	0.501
PMMA/5HA-8% ZCA	1.88	1.80 × 10⁻¹²	0.478

Figure 5

Plot log(M_t/M_max) vs log(t) for PMMA/HA composites.

CONCLUSIONS

Based on this work devoted to study the effect of ZCA on the microstructure, mechanical properties and SBF absorption behavior of PMMA/HA composites, the following conclusions can be drawn:

Surface treatment of HA using ZCA has resulted improvement in flexural modulus and strength of PMMA/5HA composites, attributed to the better interfacial interaction. The interfacial contact between PMMA and HA can be evidenced using SEM and EDX technique. The SBF absorption of PMMA/HA composites followed the kinetics of Fickian diffusion process. Moreover, the maximum SBF absorption and diffusion coefficient of PMMA/5HA was reduced by the ZCA treatment.

Footnotes

Figure 4 appears in color online

ACKNOWLEDGEMENT

This work was supported by the Science Fund [MOSTI, Malaysia, grant number 6013318]; Universiti Sains Malaysia [Incentive Grant, grant number 8021013]; Universiti Sains Malaysia [Research University Postgraduate Research Grant Scheme, grant number 8031015]; and USM Postgraduate Fellowship.

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