The Effect Of Food Simulating Solvents

Published Date: 02 Nov 2017

Abstract

Since the resin composites were first presented to dentistry more than half a century ago, the composition of resin composites has developed significantly. One of the most vital change that the reinforcing filler particles reduced in size to generate materials that show better physical and mechanical properties. Resin composites may absorb water and chemicals from the surrounding environment. In contrast, the resin composites may release elements to its surroundings. The physical/mechanical properties of a restorative material provide an indication of how the material will function under stress in the oral environment .The aims of this research were to examine the effects of water at 37°C on physical and mechanical properties, and food-simulating solvents on surface properties of a variety of experimental and contemporary resin composites. Eight representative resin composites were selected [Exp.VT, BL, NCB, TEC, GSO, XB, VDF and CXD]. Interestingly, due to produce the bulk fill material recently in the market during the course of this research the post-cure depth of cure of new bulk fill materials was also investigated. Five representative resin composites were selected [TBF, XB, FBF, VBF and SF].

Water sorption and solubility were investigated at 37°C for 150 days. Sorption and solubility affect by the degree of hydrophilicity of resin matrix. Bulk fill material shows the lowest water sorption and solubility. Laser scan micrometer (LSM) was used to investigate the hygroscopic expansion. The extend of hygroscopic expansion is correlated with the amount of water sorption positively.

Effect of water on fracture toughness was also examined. It was shown that the more hydrophilic resin matrix had a decrease in fracture toughness after 7 days storage at 37°C. In contrast the least water sorption material showed an increase in fracture toughness over time.

The effect of food-simulating solvents (distilled water, 75% ethanol/water and MEK) on surface micro-hardness, colour stability and gloss retention were investigated. The MEK solvent has caused the lowest micro-hardness and greatest colour change (ΔE) for most of the examined composites, while 75% ethanol/water solution caused the greatest loss in gloss for most of the examined composites. A highly filled nano-composite showed best result over time regardless condition storage.

Surface micro-hardness profile was used as an indirect method to assess the depth of cure of bulk fill resin composites. The examined bulk fill resin composites can be cured to an acceptable depth, according to the manufacturers’ claims.

Introduction

A range of both standard and new techniques were applied to meet the objectives of the current research. Standard techniques such as micro-hardness, colour stability and gloss of resin composite and mechanical properties for instance fracture toughness. In this chapter, some techniques, which conducted in the present research will be explained more, which include:

Hygroscopic expansion measurement using of a non-contact laser scan micrometer (LSM) system.

Fracture toughness (KIC) measurement using a Universal Testing Machine.

Colour measurement using a chroma meter

Gloss measurement using a gloss meter

The instruments used in the present study have been designed to embody and comply with the basic rules of engineering measurements. These rules include [1]:

Accuracy:

This is the closeness with which the measuring instrument can determine the true value of the property under specified conditions of use.

Sensitivity:

This is the linear or non-linear relationship between a change in the output reading for a given change of input. Sensitivity is often known as scale factor or instrument magnification and an instrument with a large sensitivity will specify a large movement of the indicator for a small input change.

Linearity:

Most instruments are specified to function over a particular range and the instruments can be assumed to be linear when additional changes in the input and output are constant over the specified range. The amount of non-linearity accepted is normally quoted as a percentage of the operating range.

Resolution:

This is defined as the smallest input increment change that provides some small but definite numerical change in the output.

Threshold:

If the instrument input is very slowly increased from zero there will be a minimum value needed to give a measurable output change. This minimum value defines the threshold of the instrument.

Repeatability:

This is the capability of an instrument to give same indications, or responses, for recurrent applications of the same value of the measure and under specified conditions of use.

Zero stability:

This is the measure of the capability of the instrument to return to zero reading after measure and variables have been removed for instance temperature, pressure and vibration.

Readability:

This is defied as the case with which readings may be taken with an instrument. The difficulties of readability may often happen owing to parallax mistakes when an observer is observing the position of a pointer on a calibrated scale.

The hygroscopic expansion measurement using laser scan micrometer (LSM)

The instrument consists of a laser scan micrometer (LSM) system (Measuring Unit LSM-503s and Display Unit LSM-6200, Mitutoyo Corporation, Japan), mounted on a heavy stainless steel base, 25 mm thick, with rubber feet. A disk specimen holder was rotated in a horizontal plane about a vertical axis by an electronic stepper-control unit. The LSM was interfaced, via the Display Unit, to a PC, with USB input, for further recording and data processing (Figure 3.1). The technical specifications of the laser micrometer are listed in Table 3.1.

The measuring LSM system (Figure 3.2) obtained the specimen dimensional data rapidly and accurately using a highly direction parallel-scanning laser beam. The laser beam, generated by a laser oscillator, was directed at a polygon mirror rotating at high speed and synchronized by clock pulses. Then the reflected laser beam passed through a collimator lens and maintained its constant direction through the beam window toward the disk specimen. The measurement light rays travelled as ‘parallel beams’ toward a photo-electric detector unit, but they were partially obstructed by the disk specimen. The extent of the beam-obstruction was directly proportional to the disk diameter. The resulting electrical output signal changed according to the duration over which the beam was obstructed. This was processed by the Display Unit CPU and the disk dimension was displayed digitally. The laser beam system was able to measure each disk specimen diameter to a resolution of 200 nm.

The stepper-control unit maintained stepwise rotation of the disk specimen (mounted on its holder) with a total of 800 steps per rotation. The speed of rotation was 28 steps per second. The laser scanning speed of the laser beam was 3200 scans / second and so 91,428 diametral measurements were taken per revolution and these were averaged, in sets of 1024, to give 89 recorded reading/revolution. In each sorption time-period, specimens were measured over five complete rotations. Therefore, the diametral values presented for each specimen at each time point were obtained as overall averages of 445data values, which were transferred to an Excel file. The grand mean for the 5 specimens per group was then obtained for each sorption time-period.

Calibration

The LSM system can be calibrated quite easily and with high accuracy. The LSM system was calibrated before each measurement using two reference gages according to the manufacturer’s instructions . For entering the calibration mode, a gage stand and 2 supported calibration gages were used. At the beginning, the power was turned on and wait at least 30 minutes for the system to thermally stabilize. Prior to use the gages, the gage and gage stand were wiped with a cloth soaked in alcohol from dust. Also, the plastic cylinder mount was unscrewed to get the room to stand and gags.

For setting the HIGH CAL stepped type gage (Figure 3.3), The HIGH CAL gage was mounted on the stand. The centre point of the gage which is indicated by an arrow (→) was the calibration point. The mark ( | ) on the end face of the gage comes to vertical. In the ready state, the H.CAL key was pressed. The previous set HIGH CAL value was displayed, and the HIGH CAL setup mode was entered.

For setting LOW CAL with-holder type gages (Figure 3.4). The LOW CAL gage on the stand was mounted. The centre point of the gage which is indicated by an arrow (→), was the calibration point. The laser beam was aligned with the vertical line ( | ) marked on the side of the gage. In the ready state, the L.CAL key was pressed. The previously set LOW CAL value was displayed, and the LOW CAL setup mode was entered. The gage stand and was removed and plastic cylinder mount was re-screwed in place. Thus, the LSM calibrated and ready to use.

Facture toughness (KIC) using a Universal Testing Machine

KIC testing is a challenging procedure, which need to quit expensive equipment as the appropriate specimen are also expensive and difficult to prepare . KIC measurements of dental materials are often used to investigate the properties of new or experimental materials. In this study a Universal Testing Machine (Zwick/Roell-2020) was used to conduct this experiment (Figure 3.5).

Both British Standard (BS) and American Society for Testing and Materials (ASTM) specify the requirements for any test. KIC for instance, the size of the specimen and shape should be determined to ensure the plane strain condition at the crack tip. Any size and shape can be used, as long as it is consistent with mode I crack displacement and the stress intensity calibration is known. The method used in this study was bending single edge-notched (SEN) specimen. The specimens were loaded until they fracture and then fracture loads are used to compute the KIC. The cast metal jig, which attached to Universal Testing Machine, was used to conduct this experiment (Figure 3.6).

Colour measurement using Chroma Meter

Chroma Meter CR-221 (Figure 3.7) is compact tristimulus colour analysers for measuring reflective colours of surfaces. It has a 3mm diameter measuring area and uses a 45° illumination angle and 0° viewing angle for measuring precise areas of printed matter and other glossy surfaces. CIE Illuminant D65 Iighting conditions were used for measurements. Absolute measurements were taken in L*a*b* Commission Internationale de I'Eclairage Standard (CIE 1976). Colour difference was measured as ΔE.

Calibration

The Chroma Meter has 20 different calibration channels numbered from 00 to 19. Channel 00 should be calibrated to standard white plate (Calibration Plate CR-A45 for CR-221); channels 01 to 19 may be calibrated to any user-selected surface. By providing 20 memory channels for storing calibration data, the Chroma Meter allows the appropriate calibration channel to be chosen based on the sample surface and measurement conditions. For best results, calibration and measurements should be performed under the same conditions.

Gloss measurement using glossmeter

The gloss is measured by directing a light beam at an angle of 60° to the test surface and monitoring the light reflected at the same angle. In this study a glossmeter was used to conduct this experiment (Figure 3.8).

Calibration

Instrument calibration should be checked regularly and then adjusting if required. Position the tile on the instrument, care should be taken when align the arrows with the white gridlines on the platen, and take a reading. The value displayed was compared with the value assigned to the tile for the matching measurement angle. If the values vary by more than ± 0.5 (GU or %) it will be necessary to adjust the calibration. This should be done with the instrument in place on the tile. Holding the CAL ↑ button will incrementally increase the value, whilst the CAL ↓ button will decrease it. Releasing the button will set calibration at the displayed value, e.g. if instrument reads too high.

Once the calibration has been adjusted to the appropriate level it can be checked by pressing the READ button. The calibration value is stored in memory when the unit switches off, so there is no need to recalibrate each time the instrument is switched on. The calibration configuration of your Elcometer 400 allows use of any standard

General discussion

Since the resin composites were first presented to dentistry more than half a century ago, the composition of resin composites has developed significantly. One of the most vital change that the reinforcing filler reduced in size to create materials that show better wear resistance and polished easily [1]. Due to advantages of resin composites for instance aesthetics, ease of handling and the ability to adhere to tooth structures, resin composites are widespread in restorative dentistry [2].

A continuous process of intra-oral degradation result from exposing resin composites to a number of factors include: chemical, thermal and mechanical. Surface properties for instance micro-hardness and wear resistance are affected primarily. In long-term, fracture strength also affected by intra-oral degradation, which ending in lower durability of the restoration. In addition, aesthetic properties include: surface texture, gloss and colour of resin composites are effected in consequences of the intra-oral softening [3]. Most of the mechanical property investigations of resin composite materials, including international standards, were achieved after storing the specimens for 24h in distilled water [4]. The influence of water on strength of most of resin composite is permanent . Resin composites exposing to water was shown to degrade owing to filler particle degradation, the matrix softening or the debonding of matrix/filler interfaces [4].

The aims of this research were to examine water sorption behaviour as well as hygroscopic expansion kinetics and mechanical properties at 37°C. Also, the effect the food-simulating solvents on surface properties at 37°C. The resin composites used in this research were experimental self adhesive, micro-hybrid, nano-hybrid, bulk fill and flowable. Interestingly, due to produce the bulk fill material recently in the market during the course of this research the post-cure depth of cure of new bulk fill materials was also investigated.

The investigation of these materials using different methodologies was focusing on whether solvent degradation affects the properties of resin composites. This research aimed to see the effect of different matrix composition and filler loading on physical, mechanical and surface properties of experimental and contemporary resin composites.

The resin composites may absorb water and chemicals from the environment. In sequence, soluble components may release in turn. Because of sorption and solubility process, deleterious effects may produce on the structure and function of the examined material. These effects for instance include dimensional changes (hygroscopic expansion), physical changes (matrix softening) and chemical changes (hydrolysis) [5]. All materials were stored in distilled water for the 150 days until equilibrium for water sorption and hygroscopic expansion experiments and the results were varied as shown in Chapter 4 and 5. The relationship between change in dimensions and water sorption in resin composites was established [6]. The relationship between water sorption and the percentage of volumetric hygroscopic expansion in this research suggested a positive linear relationship. This was confirmed by the positive linear correlations (r2 = 0.91) (Figure 10.1).

Water sorption by resin composites seems to be a diffusion-controlled process [7]. The results of water sorption of the examined materials were presented in Chapter 4. bulk fill composite (XB) exhibited the lowest water sorption followed by highly filled nano-hybrid composite (GSO) amongst the examined materials. It seems the amount of water sorption depend on the matrix composition. The choice of the monomers used in resin composites strongly affects the reactivity, viscosity and polymerization shrinkage, mechanical properties, water sorption and hygroscopic expansion [8]. It also seems that water sorption is affected by a poor filler/matrix interface bond [9]. Therefore, water sorption by the resin composite may be ascribed to the nature of the filler particle and the coupling agents. Because of the diffusion of water through the resin, resin composites are likely to accommodate more water at the filler/matrix interface [10]. Accordingly, when the matrix and filler are well coupled, the water sorption is reduced significantly [11]. Moreover, the highly cross-linked polymer result in a reduction in the free volume in the network [7], therefore reduce water sorption into polymer occur.

The solubility behaviour in resin composites exhibits the extraction of residual monomers and oligomers, filler particles and ions from its surfaces [5]. Self adhesive composite (Exp. VT) showed the highest solubility amongst the examined composites The high value may of solubility be due to a lower degree of conversion during the polymerisation [12]. The water sorption and solubility of resin composites is important clinically. Excessive water sorption and solubility of resin composites in addition to insufficient polymerisation may cause the monomer to be leached out, and this may affect the biological compatibility as well as result in inferior mechanical/physical properties of the material [13, 14]. Therefore, degradation may occur that may subsequently result in the failure of the restoration.

The dimensional stability of resin composite materials is affected by polymerization shrinkage, thermal contraction/expansion and interactions with an aqueous environment [15]. Hygroscopic expansion could relax the internal stresses of the resin matrix which compensate the shrinkage of resin composites [16-18]. The hygroscopic expansion occurs when the water diffuses into the polymer network and separates the chains [19]. micro-hybrid composite (BL), highly filled nano-hybrid composite (GSO and NCB) and bulk fill (XB) showed the lowest hygroscopic expansion among the examined materials. It has been shown that an expansion of the matrix for accommodating the absorbed water happens because water diffuses mostly into resin [20].

Dental restoration failure may occur owing to reduced load-bearing capacity caused by the degradation of the material in the oral environment [21]. The effect of water storage on fracture toughness has been studied in relation to a range of materials and specimen geometries [22, 23]. A single edge notch method was used in this study, which is one of the two common methods used to determine the fracture toughness of restorative materials [24]. Flowable resin composite (VDF) had the highest numerical KIC value (2.72 MNm-1.5) as well as highly filled nono-hybrid (GSO) and micro-hybrid (BL) after seven days’ storage in water. All materials that showed high KIC had the different matrix resin and filler loading, this may be owing to KIC is depend on the degree of adhesion of resin matrix to filler particles instead of the filler size [25, 26]. bulk fill composite (XB) showed increase in KIC after 7 days storage, which reflect a hydrophobicity and continuous polymerisation of resin matrix. On the other hand, Exp.VT (self adhesive composite) showed decreases in KIC due to its hydrophilic matrix and more degradation may occur.

75% ethanol/water solution and MEK were used beside distilled water for determining the surface properties (micro-hardness, colour and gloss). These solvents were selected according to solubility parameters (Table 7.4). Surface hardness is the resistance of a material to indentation. It is used as a predictor of the wear resistance of the material [27]. When resin composites absorb water between the filler/matrix interface, hydrolytic degradation can occur that may lead to displacement of filler particles [28]. Also, the conditioning of materials with the food-simulating solvents lead to the softening of composites, and consequently to a decrease in surface hardness [29]. A positive correlation between VHN and filler loading is confirmed (Figure 7.4). This positive correlation has been established between filler content and the hardness of resin composite [30]. The reduction in hardness values for some of the examined resin composites after storage in water indicates the extent of water sorption effect [31]. MEK caused the greatest softening (reduction in VHN) of most of the examined composites. highly filled nano-hybrid composite (GSO) was the least affected over time regardless of the storage conditions. Flowable resin composite (VDF) was more affected over time than those of the conventional hybrid resin-composites and bulk fill. It was shown that the surface layer was easy to degrade owing to a resin rich surface than the bulk material [32].

To achieve better colour stability and adequate smooth surface of restoration after polishing in long-term, several modifications have been made in the production of dental resin composites [33]. Results showed that all the examined materials exhibited some colour change (ΔE). Colour for most materials were affected by MEK solvent, while the gloss of most materials were affected by 75% ethanol/water solution. A quadratic regression function of log time showed a positive trend for ΔE and a negative trend for gloss for all materials (Figure 8.3 and 8.4). According to the established range of colour change perceptibility, ∆E values ranging from 0.0 to 1.1 were considered as not visible, between 1.1 and 3.3 as visually visible but clinically acceptable, whilst ∆E higher than 3.3 were considered as obviously visible and clinically not acceptable [34-37]. Resin composites stored in distilled water ΔE reached its maximum of 1.32 after 6 months of storage. After 6 months storage in 75% ethanol/water only GSO and VDF showed resistance to colour change (ΔE = 2.76 and 2.90, respectively). In addition, After 6 months storage in MEK Exp.VT and GSO showed a resistance to colour change of (ΔE = 2.55 and 2.04 respectively).

Gloss values in water showed less effect compared with 75% ethanol/water and MEK. In general, after 6 months, a 75% ethanol/water storage had an obvious effect on gloss for most of the examined resin composite except BL and NCB had no change in 75% ethanol/water and MEK. highly filled nano-hybrid composite (GSO) was the least affected, and was characterised with more colour stability and gloss retention over time, regardless of the storage conditions. It was suggested that the nano-filler size is smaller than the wavelength of visible light and it has no effect on the optical properties of the matrix [38].

During the last year, several bulk fill resin composites have been introduced in restorative dentistry. These materials can be placed in layers up to 4 mm. Due to the short period of time that these materials are commercially available, there isn’t adequate data to fully describe their behaviour. Bulk fill (XB) is one of the materials used to investigate previous studies in this research. It showed good results in terms of physical/mechanical properties.

A simple technique that has most commonly been used for the assessment of depth of cure is micro-hardness [39]. It has been shown that the insufficient polymerisation may lead to a decrease in the physical/mechanical and biological properties of resin composites [40, 41]. The examined bulk fill resin composites can be cured to an acceptable depth, according to the manufacturers’ claims. SF and TBF showed the greatest depth of cure among the examined composites. The refractive index matching between the resin/filler has an influence on the amount of transmission of visible light. A reduction in the refractive index difference between the resin/filler led to enhancing the polymerisation conversion, and thus an increased depth of cure [42, 43].

Conclusions

Within the limitations of this research it was concluded that:

The excessive water sorption and high solubility of the experimental self adhesive composite is significant amongst the examined composites which may also influence clinical performance.

The bulk fill composite showed the lowest water sorption value amongst the examined resin composites.

The bulk fill and highly filled nano-hybrid composites showed the lowest solubility values amongst the examined resin composite.

Non-contact laser scan micrometer (LSM) is a reliable method to detect the small change in dimension of resin composites.

The extensive hygroscopic expansion of the self adhering resin (Exp.VT) is a cause for concern, which may affect the mechanical stability.

The water storage of resin composites can modify fracture toughness of resin composites.

Bulk fill composite (XB) showed increases of fracture toughness value after 7 days’ storage in water, whereas self adhering composite decreased.

MEK caused the greatest softening (reduction in VHN) for most of the examined resin composites. A highly filled nano-hybrid composite (GSO) had the highest VHN over time, regardless of the storage conditions.

The MEK solvent has caused the greatest colour change (ΔE) for most of the examined composites.

75% ethanol/water solution caused the greatest loss in gloss for most of the examined composites.

A highly filled nano-hybrid composite GSO had the greatest colour stability and gloss retention over time, regardless of the storage conditions.

Bulk fill resin composites can be cured to an acceptable post-cure depth, according to the manufacturers’ claims.

Recommendation for future work

In order to complement the studies and future development of knowledge, the following areas of future work are suggested:

The effect of food-simulating solvents on the behaviour of sorption and solubility for a series of resin composites with different filler loading and matrix composition.

The effect of food-simulating solvents on the dimensional stability for a series of resin composites with different filler loading and matrix composition.

Study the bulk fill resin composites in terms of physical/mechanical properties.

Using different light cure units and curing protocols to determine the depth of cure of bulk fill resin composites.