Stickiness prior to setting of some light cured resin-composites

Physico-Mechanical Properties and Bonding Performance of Graphene-Added Orthodontic Adhesives

This study aimed to assess the key physico-mechanical properties and bonding performance of orthodontic adhesives with graphene addition for bonding a fixed retainer. Transbond LR (3M) and Transbond LV (3M) with no graphene were set as the control groups. Graphene was added into LR and LV at concentrations of 0.01 wt%, 0.05 wt% and 0.1 wt%. The stickiness of the uncured samples (n = 5) and real-time degree of conversion (DC) of the samples (n = 3) were measured over a 24-h period using Fourier-transform infrared spectroscopy. The hardness and other mechanical parameters, including the Martens hardness (HM), indentation modulus (EIT), elastic index (ηIT) and creep (CIT), were measured (n = 5). To measure the shear bond strength (SBS), adhesive composites were applied using a mold to bond the retainer wire to the lingual surfaces of bovine incisors (n = 10). Fracture modes subsequent to the SBS test were examined under light microscopy. Statistical analysis was conducted using ANOVA and Tukey tests (α = 0.05). In the LR groups, the LR + 0.01 showed the highest SBS (12.6 ± 2.0 MPa) and HM (539.4 ± 17.9 N/mm2), while the LV + 0.05 (7.7 ± 1.1 MPa) had the highest SBS and the LV + 0.1 had the highest HM (312.4 ± 17.8 N/mm2) among the LV groups. The most frequent failure mode observed was adhesive fracture followed by mixed fracture. No statistical difference was found between the graphene-added groups and the control groups in terms of the EIT, ηIT and CIT, except that the CIT was significantly lower in the LR + 0.01 than in the control group. Graphene addition had no significant adverse effect on the stickiness and DC of both LR and LV.

Synthesis of Pore-Size-Tunable Porous Silica Particles and Their Effects on Dental Resin Composites

The filler/resin matrix interface interaction plays a vital role in the properties of dental resin composites (DRCs). Porous particles are promising fillers due to their potential in constructing micromechanical interlocking at filler/resin matrix interfaces, therefore improving the properties of the resulting DRCs, where the pore size is significantly important. However, how to control the pore size of porous particles via a simple synthesis method is still a challenge, and how their pore sizes affect the properties of resulting DRCs has not been studied. In this study, porous silica (DPS) with a dendritic structure and an adjustable pore size was synthesized by changing the amounts of catalyst in the initial microemulsion. These synthesized DPS particles were directly used as unimodal fillers and mixed with a resin matrix to formulate DRCs. The results showed that the DPS pore size affects the properties of DRCs, especially the mechanical property. Among various DPS particles with different pore sizes, DPS6 resulted in 19.5% and 31.4% improvement in flexural strength, and 24.4% and 30.7% enhancement in compression strength, respectively, compared to DPS1 and DPS9. These DPS particles could help to design novel dental restorative materials and have promising applications in biomedicine, catalysis, and adsorption.

Influence of Instrument Lubrication on Properties of Dental Composites

Resin composites are one of the most commonly used materials in restorative dentistry. To improve their handling and facilitate restoration sculpting, clinicians began to lubricate modeling instruments with various substances like alcohol, unfilled resins, or even bonding agents. Although the technique is commonly present in daily clinical practice, it has not been precisely described in the literature and both application methods and lubricating materials vary across the available studies. This study aims to summarize the currently available knowledge about influence of instrument lubrication on properties of dental composites. Literature selection was conducted within MEDLINE, SCOPUS, and EBSCO databases. Instrument lubrication seems not to be indifferent for composite mechanical and optical properties. Moreover, various lubricants can differently affect the composite material, so the choice of lubricating agent should be deliberate and cautious. Available in vitro studies suggest possible incorporation of lubricant into the composite structure. Unfilled resins based on bisphenol A-glycidyl methacrylate (Bis-GMA) seem to be the best choice for the lubricant, as bonding agents containing hydrophilic molecules and alcohols carry a bigger risk of altering the composite properties. Further research is necessary to evaluate lubricants' influence in clinical practice conditions.

36-Month Randomized Clinical Trial Evaluation of Preheated and Room Temperature Resin Composite

OBJECTIVE

This study evaluated the effect of preheating resin composites (RCs) on the clinical performance of class I restorations during a 36-month period using a split-mouth, double-blinded randomized design.

METHODS AND MATERIALS

A total of 35 patients were selected. Every patient received one pair of class I nanofilled resin composite (RC, Filtek Z350 XT) posterior restorations (n=70). One side of the mouth received preheated composites; on the other side, the composite was placed in a nonheated state following the manufacturer's instructions. These restorations were evaluated at 1-week (baseline), 12-months, 24-months, and 36-months using the FDI World Dental Federation criteria. The statistical analyses were also performed using the Wilcoxon and Friedman tests with the level of significance set at 0.05.

RESULTS

After 36 months, 33 patients attended the recall visits, and 66 restorations were evaluated. The Friedman and Wilcoxon signed-rank tests revealed insignificant differences between both groups (p>0.05) for all FDI parameters. However, a significant difference was detected for staining as a criterion at 36 months (p=0.01). Moreover, a significant difference in the staining was detected when the baseline and 36 months were compared in the nonheated RC group (p=0.001). For esthetic, functional, and biological properties, the nonheated composite exhibited 93.9%, 100%, and 100% of the clinically accepted scores, respectively, and the preheated group presented 100% for all properties. Four restorations had postoperative sensitivity at baseline for nonheated (11.4%) and five for preheated (14.2%), but the postoperative sensitivity scores were considered highly acceptable at 12-, 24-, and 36-months.

CONCLUSIONS

After 36 months, preheated nanofilled RCs showed an acceptable clinical performance similar to that of the nonheated ones in class I restorations, but with better resistance to marginal staining.

Rheological and Mechanical Properties of Resin-Based Materials Applied in Dental Restorations

Resin-based materials have been prevalent for dental restorations over the past few decades and have been widely used for a variety of direct and indirect procedures. Typically, resin-based dental materials are required to be flowable or moldable before setting and can provide adequate mechanical strength after setting. The setting method may include, but is not limited to, light-curing, self-curing or heating. In this review, based on different indications of resin-based dental materials (e.g., dental filling composite, dental bonding agent, resin luting cement), their rheological and mechanical properties were reviewed. Viscous and flexible properties were focused on for materials before setting, while elastic properties and mechanical strength were focused on for materials after setting. At the same time, the factors that may affect their rheological and mechanical properties were discussed. It is anticipated that the insightful information and prospections of this study will be useful to the future development and fabrication of resin-based dental restorative materials.

Handling properties and surface characteristics of universal resin composites

OBJECTIVE

This study investigated the handling and surface characteristics of universal resin composites and determined the interrelations among the composites' handling, mechanical, and physical properties.

METHODS

Five recently introduced universal resin composites were tested. Twelve specimens per material were used to measure the stiffness and stickiness (handling properties) of the resin composite pastes. Additionally, surface properties (Knoop hardness number [KHN], surface roughness [Sa], surface gloss [SG], water contact angle [CA], and surface free energy [SFE]) of cured resin composites were determined in 12 specimens per material immediately after preparation (baseline) and after subjection to thermal cycles (TCs).

RESULTS

Handling and surface properties of the resin composites were material dependent. All the resin composites showed significantly lower KHN in the post-TC subgroups than that in the baseline subgroups. However, the influence of TC on the other surface properties was dependent on the material used. Some resin composites did not indicate any significant differences in Sa, SG, or CA between the baseline and post-TC groups.

SIGNIFICANCE

Although the resin composites in the baseline groups presented with different handling and surface properties, the surface properties of most of the composites were significantly affected by TC. Extremely strong positive or negative correlations were observed between stiffness and stickiness, KHN and Sa, KHN and SG, Sa and SG, and CA and SFE. Most correlations between the handling and surface properties were weak. Therefore, the selection of resin composites in clinical situations should be based on comprehensive consideration of their properties.

Evaluation of rheological behaviour of flowable dental composites reinforced with low aspect ratio micro-sized glass fibres

OBJECTIVE

Experimental investigation is carried out to determine the flowability and stickiness of the developed composite material for dental restoration containing low aspect ratio (AR ≤ 100) surface treated micro-sized glass fibres.

METHODS

Specimens are manufactured by mixing low AR (50/70/100) micro-sized glass fibres with two different weight fractions (5%/10%) into UDMA/TEGDMA based resin. Particulate filler composite (PFC) containing 55% glass fillers is used as the control group. Dynamic oscillatory strain sweep tests are conducted to analyse the linear viscoelastic behaviour. Solid-to fluidic transition behaviour of dental composites is also calculated in terms of flow and yield stresses. Furthermore, the oscillatory frequency sweep tests are conducted at three different strains (0.5%, 5% and 50%) resembling the positioning of unset paste onto restorations for different real-life clinical situations. Additionally, stickiness of dental composites with handling instrument (steel) and dentine covered with bonding agent is also evaluated.

RESULTS

The results suggested the all the FRC groups exhibited non-Newtonian, shear-thinning behaviour. It is further established that inclusion of 5% of 50/70AR fibres into dental composites does not affect the flowability. Simultaneously, stickiness with dentine covered with bonding agent is more for these two compositions as compared to that of handling instrument (steel).

SIGNIFICANCE

This study suggest that visco-elastic properties of dental composites are greatly affected by the type of filler (spherical shaped particulate fillers or rod-shaped fibres) as well as fibre weight fraction/fibre AR. This phenomenon can be attributed to the varying interactions between micro-sized fibres of different AR/weight fraction, particulate fillers and monomers.

Influence of preheating on mechanical and surface properties of nanofilled resin composites

Background

Resin composite preheating is an innovative method that could be clinically beneficial by improving the handling properties, marginal adaptation, and surface properties of uncured nanofilled resin composite materials. There is conflict and unclear information regarding the effect of preheating on the microhardness, fracture toughness and surface roughness of nanofilled resin composites. Thus, it is important to assess whether dental clinicians can adopt preheating procedures without compromising composite mechanical strength. Objective: The purpose of this study was to evaluate the effect of preheating on microhardness, fracture toughness and surface roughness of nanofilled resin composite.

Material and Methods

In this study, one commercial nanofilled resin composite Filtek Z350 XT was used. A total of 28 disc-shaped specimens were fabricated in a Teflon mold (10 mm diameter x 2 mm thick) for Vickers microhardness indentation test and surface roughness test. The samples were divided into two groups of 14 samples each, one group of samples was light-cured at room temperature (24ºC) without preheating (non-heated group), and the other group was light-cured after preheating (preheated group). Vickers hardness measurements of 14 specimens (n=7) either preheated or non-heated of the top and bottom surfaces was measured by means of microhardness tester by applying 100 g load for 10 s. Surface Roughness measurements (Ra) were obtained from 14 specimens (n=7) either preheated or non-heated with the atomic force microscope. Fourteen single-edge-notched-beam specimens were prepared for fracture toughness test (n=7) either preheated or non-heated with measurements (2.5 x 5 x 25 mm3) and a crack 2.12 mm in length. The specimens were tested via three-point bending mode, using a universal testing machine at crosshead speed of 1.0 mm/min until failure occurred.

Results

Independent sample t- tests revealed no significant difference between non-heated and preheated groups for all tests (p>0.05). However, for Vickers hardness test, there were significant differences between top and bottom surfaces for non-heated and preheated groups (p<0.05). Moreover, surface roughness average Ra (nm) mean values of preheated group was higher than non-heated group but no significant difference between them was found (p>0.05).

Conclusions

Preheating procedure did not negatively affect microhardness, fracture toughness and surface roughness of nanofilled resin composites so preheating is recommended for the other potential clinical advantages.

Key words:Preheating, nanofilled composites, microhardness, fracture toughness, surface roughness.

Effect of pre-heating on the mechanical properties of silorane-based and methacrylate-based composites

BACKGROUND

The use of composites in dental restoration has been commonly criticized, due to their underwhelming mechanical properties. This problem may be solved partially by preheating. The present research aims to determine the effect of preheating on the mechanical properties of two different classes of composites.

MATERIAL AND METHODS

A Silorane-based (Silorane) and a Methacrylate-based (Z250) composite were preheated to different temperatures (25, 37, and 68 °C) and afterwards were tested with the appropriate devices for each testing protocol. The material's flexural strength, elastic modulus, and Vickers microhardness were evaluated. Two-way ANOVA, and Tukey's post hoc were used to analyze the data.

RESULTS

Microhardness and elastic modulus increased with preheating, while flexural strength values did not increase significantly with preheating. Furthermore the methacrylate-based composite (Z250) showed higher values compared to the Silorane-based composite (Silorane) in all the tested properties.

CONCLUSIONS

Preheating Silorane enhances the composite's microhardness and elastic modulus but does not affect its flexural strength. On the other hand, preheating Z250 increases its microhardness but does not change its flexural strength or elastic modulus. In addition, the Z250 composite shows higher microhardness and flexural strength than Silorane, but the elastic modulus values with preheating are similar. Therefore Z250 seems to have better mechanical properties making it the better choice in a clinical situation. Key words:Composite, elastic modulus, flexural strength, microhardness, preheating.

Microshear bond strength of preheated silorane- and methacrylate-based composite resins to dentin

The aim of this study was to investigate the effect of preheating on microshear bond strength (MSBS) of silorane and methacrylate-based composite resins to human dentin. The teeth were randomly divided into three main groups: (1) composite resins were heated upto 68 °C; (2) cooled to 4 °C; and (3) control [room temperature (RT)]. Each group was then randomly subdivided into four subgroups according to adhesive system used [Solobond M (Voco), All Bond SE (Bisco), Clearfil SE Bond (CSE) (Kuraray), Silorane adhesive system (SAS) (3M ESPE)]. Resin composite cylinders were formed (0.9 mm diameter × 0.7 mm length) and MSBS of each specimen was tested. The preheated groups exhibited the highest MSBS (p < 0.001) and the groups cooled to 4 °C exhibited the lowest MSBS (p < 0.001). The CSE showed higher MSBS than the other adhesives (p < 0.001). This study concludes that preheating of composite resins may be an alternative way to increase the MSBS of composites on dentin.

Mechanical properties of composites as functions of the syringe storage temperature and energy dose

Objective:

To investigate the mechanical properties of different classifications of composites indicated for posterior application as functions of the storage condition and of the energy dose.

Material and Methods:

Specimens (8x2x2 mm) were obtained according to the factors: I) Composites (3M ESPE): Filtek P60, Filtek Z350XT, and Filtek Silorane; II) Syringe storage conditions: room temperature, aged, oven, refrigerator, and freezer; and III) Energy dose: 24 J/cm² and 48 J/cm². After photoactivation, the specimens were stored at 37ºC for 24 h. After storage, a three-point bending test was carried out in a universal testing machine at 0.5 mm/min. Flexural strength (S) and flexural modulus (E) were calculated. Data were analyzed by three-way ANOVA and Tukey's test (α=0.05).

Results:

Different storage conditions significantly affected the silorane composite for S; conversely, no effects were noted in terms of E. The accelerated aging protocol significantly increased the S of Filtek P60 and Filtek Silorane, whereas storage in the oven significantly decreased the S for all of the composites tested. Filtek P60 was the only composite not affected by the lower storage temperatures tested for S, whereas for the silorane this parameter was impacted at the same conditions. The factor "dose" was not statistically significant.

Conclusions:

The syringe storage at different temperature conditions proved to influence mostly the flexural strength, a clinically important characteristic considering the posterior indication of the materials tested. The silorane composite should not be stored at lower temperatures.

Sealing performance of resin cements before and after thermal cycling: evaluation by optical coherence tomography

OBJECTIVES

Self-adhesive resin cements have been recently introduced; however, there is little data available on their long-term performance. In this in vitro study, swept-source optical coherence tomography (OCT) at 1310 nm center wavelength was used for monitoring adaptation of indirect resin restorations after thermal cycling.

METHODS

Resin inlays were luted to class-I cavities of extracted human teeth using three resin cements; Clearfil SA Luting (SA; Kuraray), Bistite II DC or Multibond II (Tokuyama Dental). Each cement was applied with or without pre-coating of dentin by a self-etch adhesive (Clearfil SE Bond) and a low-viscosity microfilled resin. OCT imaging was performed after 24 h, after 2000 and after 10,000 thermocycles (n=5). Selected samples were sectioned for interfacial observation by confocal laser scanning microscope (CLSM). Floor adaptation (percentage) was analyzed by software on 20 B-scans throughout each specimen, and subjected to statistical analysis by three-way ANOVA test at a significance level of 0.05.

RESULTS

Resin cement type, resin coating and thermal aging all significantly affected adaptation (p<0.05). Initially, SA showed the highest adaptation; however, thermal aging significantly affected its sealing. The best results for all the cements were consistently achieved when the resin coating technique was applied where no deterioration of interfacial integrity was observed in the coated groups. CLSM closely confirmed OCT findings in all groups.

SIGNIFICANCE

OCT could be used for monitoring of composite inlays with several interfacial resin layers. The application of a direct bonding agent in the resin-coating technique improved interfacial sealing and durability of all resin cements.

An indirect veneer technique for simple and esthetic treatment of anterior hypoplastic teeth

This study describes a technique for treating anterior hypoplastic teeth using indirect nanocomposite veneer restoration. The prime advantage of an indirect veneer technique is that it provides an esthetic and conservative result. One of the most frequent reasons that patients seek dental care is discolored anterior teeth. Although treatment options such as removal of surface stains, bleaching, microabrasion or macroabrasion, veneering, and placement of porcelain crowns are available, conservative approach such as veneer preserves the natural tooth as much as possible. Full veneers are recommended for the restoration of localized defects or areas of intrinsic discoloration, which are caused by deeper internal stains or enamel defects. Indirectly fabricated veneers are much less sensitive compared to a operator's technique and if multiple teeth are to be veneered, indirect veneers can be usually placed much more expeditiously. Indirect veneers last much longer than the direct veneers. Therefore, indirectly fabricated veneers are more advantageous than directly fabricated veneers in many cases.

Effect of filler size and temperature on packing stress and viscosity of resin-composites

The objective of this study was to investigate the effect of filler size on the packing stress and viscosity of uncured resin-composite at 23 °C and 37 °C. A precision instrument used was designed upon the penetrometer principle. Eight resin-composite materials were tested. Packing-stress ranged from 2.60 to 0.43 MPa and viscosity ranged from 2.88 to 0.02 MPa.s at 23 °C. Values for both properties were reduced significantly at 37 °C. Statistical analysis, by ANOVA and post hoc methods, were carried out to check any significant differences between materials tested (P < 0.05). Conclusions: Filler size and distribution will affect the viscosity and packing of resin-composites during cavity placement.

A method for assessing force/work parameters for stickiness of unset resin-composites

OBJECTIVES

To evaluate the stickiness of unset resin-composites, at different speeds and temperatures, in terms of maximum probe separation-force (F(max)) and work-of-separation (W(s)).

MATERIALS AND METHODS

Eight commercial light-cured resin-composites were selected. Each material was placed in a cylindrical mold (φ=7 mm × 5 mm depth) held at 25 °C or 37 °C. The maximum force (F(max), N) and work probe separation (W(s), N mm) were measured by using a texture analyzer to register force/displacement. A flat-ended stainless-steel probe (φ=6mm) was mechanically lowered onto and into the surface of the unset sample. When a 'trigger' compressive force of 0.05 N was registered, data-acquisition commenced. Descent continued until a compressive force of 1N was reached, which was held constant for 1s. Then the probe was moved vertically upward at constant speed. This was varied over the range 2, 4, 6 and 8mm/s. The tensile force produced on the probe by the sticky resin-composite was plotted against displacement and the maximum value was identified (F(max)). W(s) was obtained as the integrated area. Data was analyzed by multivariate ANOVA and multiple pair wise comparisons was done by using a Tukey post hoc test to establish homogenous subsets (at p=0.05).

RESULTS

F(max) and W(s) were taken as potential measures of stickiness. They ranged from 0.47 to 3.68 N and from 0.11 to 2.84 N mm, respectively. Multivariate ANOVA showed a strong interaction of withdrawal speed, temperature and materials on both F(max) and W(s) (p<0.001).

CONCLUSION

F(max) and W(s) are useful parameters for characterizing the handling-stickiness of resin-composite materials, additional to previously reported stickiness-strain or 'peak-height'. The resin-composites investigated could be differentiated, mostly showing increases in F(max) and W(s) stickiness with increased temperature and probe-withdrawal speed.

Clinical Evaluation of Resin-based Composites in Posterior Restorations: Two-year Results

Nanohybrid and low-shrinkage posterior resin composites, placed with self-etch adhesive systems in posterior teeth, showed satisfactory and similar results after two years.

Rheological properties of flowable resin composites and pit and fissure sealants

OBJECTIVES

The purpose of this research was to determine the viscoelastic properties of commercially available flowable resin composites and resin-based pit and fissure sealants. The weight percentage of filler particles and the morphology of the filler particles were also investigated.

METHODS

Eight flowable resin composites (Admira Flow, Filtek Flow, FlowLine, Grandio Flow, Point-4 Flowable, Revolution Formula 2, Tetric Flow and X-Flow) and four pit and fissure sealants (Clinpro, Delton FS+, Estiseal F and Guardian Seal) were tested. Rheological measurements were performed using a dynamic oscillation rheometer. The filler weight content was determined by thermogravimetric analysis (TGA) and the morphology of the particles was investigated by scanning-electron microscopy (SEM).

RESULTS

Flowable resin composites are non-Newtonian, shear-thinning materials. As the shear rate increased, the complex viscosity decreased drastically. They all showed elasticity even at the lowest frequencies. They also all showed thixotropy. Pit and fissure sealants are non-Newtonian, very low-viscosity fluids. No correlation was found between the rheological properties and the filler weight content or the particles' shape.

SIGNIFICANCE

Huge differences are observed in the viscosity and flow characteristics of flowable resin composites that can have a potential influence on their clinical behavior during handling and thus on their clinical indications. Pit and fissure sealants show very different rheological properties from one another.

Rheological characterization of composites using a vertical oscillation rheometer

OBJECTIVES

The purpose of this study was to investigate the viscoelastic properties related to the handling characteristics of composites.

METHODS

A custom-designed vertical oscillation rheometer (VOR) was used for the rheological measurements of composites. The VOR consists of three parts: (1) a measuring unit, (2) a deformation induction unit, and (3) a force-detecting unit. Two medium-viscous composites, Z100 and Z250, and two packable composites, P60 and SureFil, were tested. A dynamic oscillatory test was used to evaluate the storage modulus (E'), loss modulus (E''), and loss tangent (tan delta) of the composites as a function of frequency (omega) from 0.1 to 20Hz at 23 degrees C.

RESULTS

The E' and E'' increased with increasing frequency and showed differences in magnitude among brands. The complex moduli E* of the composites at omega=2 Hz, normalized to that of Z100, were 2.16 (Z250), 4.80 (P60), and 25.21 (SureFil). The magnitudes and frequency characteristic of loss tangent differed significantly among brands. The relationship among the complex modulus E*, the phase angle delta, and the frequency omega was represented by the frequency domain phasor form E*(omega)e(idelta)=E*(omega) angledelta.

SIGNIFICANCE

The viscoelasticities of composites, which influence handling characteristics, are significantly different among brands. The VOR is a relatively simple device for the dynamic rheological measurement of dental composites. The loci of the frequency domain phasor plots in a complex plane are a valuable method of representing the viscoelastic properties of composites.

Clinical evaluation of two packable posterior composites: 2-year follow-up

The clinical performance of two packable posterior composites, Alert (A)-Jeneric/Pentron and SureFil (S)-Dentsply, was evaluated in 33 patients. Each patient received one A and one S restoration, resulting in a total of 66 restorations. The restorations were placed by one operator according to the manufacturer's specifications and were finished and polished after 1 week. Photographs were taken at baseline and after 2 years. Two independent evaluators conducted the clinical evaluation by using modified United States Public Health Service criteria. After 2 years, 60 restorations (30 A and 30 S), 27 class I (16 A and 11 S) and 33 class II (14 A and 19 S) were evaluated in 30 patients. Criterion A for recurrent caries, vitality, and retention was applicable to all 60 restorations. Criterion B was distributed among 40 restorations as follows: surface texture (15 A; 2 S), color (5 A; 6 S), postoperative sensitivity (1 S), marginal discoloration (8 A), marginal adaptation (3 A), and wear resistance (2 A). Data were analyzed using the Exact Fisher and McNemar tests. After 2 years, S showed a significantly better performance than A with respect to surface texture and marginal discoloration. The clinical performance of both materials was considered acceptable over the 2-year period. Further evaluations are necessary for a more in-depth analysis.

Rheological properties of resin composites according to variations in monomer and filler composition

OBJECTIVES

The aim of this study was to investigate the effects of monomer and filler composition on the rheological properties related to the handling characteristics of composites.

METHODS

A resin matrix in which Bis-GMA was blended with TEGDMA at a ratio of 6:4, was mixed with silane-treated Barium glass (0.7 and 1.0 microm), 0.04 microm fumed silica or 0.5 microm round silica. The type and content of incorporated fillers were varied to achieve different viscosity levels of the experimental composites. Steady shear and dynamic oscillatory shear tests were performed with a rheometer. The viscosity (eta) of the resin matrix, the storage shear modulus (G'), loss modulus (G''), complex modulus (G*), loss tangent (tandelta), phase angle delta and complex viscosity (eta*) of the composites were evaluated as a function of frequency omega=0.1-100 rad/s. To investigate the effect of temperature on viscosity, a temperature sweep test was also performed.

RESULTS

Resin matrices were Newtonian fluids and all experimental composites exhibited pseudoplasticity. The viscosity exponentially increased as the percentage of filler volume was increased, but decreased with increasing temperature. For identical filler volumes, as the filler size decreased, viscosity increased. The effect of filler size on the viscosity was intensified with increasing filler content, and the increased filler content reduced the tandelta.

SIGNIFICANCE

The rheological properties of composites related to handling characteristics were greatly influenced by the formulation of the monomer and filler, shear rate and temperature. The locus of frequency domain phasor plots, G(*)(omega)e(idelta)=G(*)(omega) angledelta, in a complex plane was a valuable method for representing the viscoelastic properties of the composites.