**2. Mechanical properties of acrylic resins**

## **2.1. Evaluation of water absorption and mechanical strength degradation caused by the exposure to saliva of classical heat-cured acrylic resins compared to alternative urethane-based light-cured resins**

It is well known that acrylates for dental use have poor resistance and these degrade in the wet environment of the mouth. Our studies involve evaluation of water absorption and mechanical strength degradation caused by the exposure to saliva of classical heat-cured acrylic resins compared to alternative urethane-based light-cured resins, which are also used for dentures manufacturing. Twenty samples (plates: 2 mm in thickness, 30 mm in length and 5 mm in width) of Meliodent (Heraeus-Kulzer) heat-curing acrylic resin and twenty samples of two urethane-based light-curing resins from the same system-Eclipse Resin System: Eclipse Base Plate and Eclipse Contour Resins (Dentsply-DeguDent) were analyzed, in saliva and dry environment. Ten samples were immersed in saliva with low microbial content and neutral pH, at 37°C for 30 days. The other ten samples were kept dry for 30 days. Saliva was collected from clinically healthy subjects and tested for germs with Vivacare line CRT bacteria 2 in one test kit. The test results showed level two of four possible contaminations and so the saliva was considered not severely contaminated. Its pH, determined with an indicator strip, was normal, with an average value of six, as shown in **Figure 2**. In order to determine the water percentage content, the samples were initially weighed and further weighed after 48, 144, 312 and 720 h.

**Figure 2.** (a) pH index and (b) quantitative evaluation of Streptococci and Lactobacilli microorganisms.

**Type Class (manufacturing) Group (presentation form)**

**Figure 1.** (a) Heat-curing acrylate powder and liquid and (b) mixing the acrylic paste.

Type 4 Light-cured resins Monocomponent system Type 5 Microwave-cured resins Bicomponent system

**Table 1.** The classification of resins according to DIN EN ISO 1567.

Type 1 Heat-cured resins (>65°C) Group 1: Bicomponent powder and liquid

diacrylic composite resins-type urethane polymers were also carried out at that time by Otto Bayer in the IG Farben Laboratories in Leverkusen. Acrylics, in fact poly(methyl methacrylate) (PMMA) mixed with methyl methacrylate, dominated denture technology for several decades. There were no competitors in manufacturing denture bases, artificial teeth, orthodontic appliances, single-tooth or provisional restorations or as veneering

The toxicity of the residual monomer, the complex wrapping system, difficult processing and poor resistance are some of the disadvantages of these materials. Many new classes of resins/macromolecular compounds which promise better quality came on to the market such as diacrylic, styrene, polycarbonate, epiminic, polyurethane, vinyl, polyamide, acetal and polyglass. Besides classic heat-curing, alternative technologies namely, casting and injection moulding are nowadays available in manufacturing acrylic resins for dental applications. In the case of alternative resins, light-curing or microwave polymerization techniques are also used [4, 5]. Light-curing, as a polymerization method for dental materials, appeared in the 1970s. Initially, ultraviolet light was used. Afterwards, it was replaced by visible radiation (visible spectrum wavelength/electromagnetic waves), the light source being either a halogen bulb or xenon stroboscopic lamps [6, 7]. The classification of resins according to DIN EN ISO

materials (**Figure 1**).

4 Acrylic Polymers in Healthcare

1567 is presented in **Table 1**.

Type 2 Self-cured resins (<65°C) Group 1: Bicomponent powder and liquid

Type 3 Thermoplastic resins Monocomponent system grains in cartridges

Group 2: Monocomponent

Group 2: Bicomponent powder and casting liquid

**Figure 3.** Correlation between specific weight and time for the tested polymers (specific weight = measured weight – initial weight).

Meliodent (Heraeus-Kulzer) was proven to have the highest water absorption capacity, followed by Eclipse Contour Resin and Eclipse Base Plate (Dentsply-DeguDent) (**Figure 3**).

The diagram showing correlation between the three types of resins and the percentage humidity content reveals that the heat-curing resin absorbs more water than the light-curing resins. The diagram indicates the maximal and minimal values for each material (**Figure 4**).

Zwick Roell extensometer (Zwick GmbH & Co.) was used to determine the moment of sample breaking or fracture point and its elongation. TestXpert software was used to standardize the applications (**Figure 5**).

**Figure 4.** The determination of percentage humidity content of polymer.

**Figure 5.** The determination of tensile mechanical resistance using the Zwick Roel extensometer.


**Table 2.** The mechanical properties of the studied materials kept in dry and in humid conditions for 1 month.

**Figure 6.** (a) Meliodent samples broken after stretching and (b) broken Eclipse samples.

Results showed that the humid samples (kept in saliva) had significant lower values than the dry samples, realized from the same material. The Young's elasticity modulus (*E*) and ultimate tensile strength (*R*m) were taken into consideration. The average values after the sample analysis are shown in **Table 2**.

There is an evident difference among the tested materials. The heat-curing resin has a lower value of the elasticity modulus than the urethane-based resins. The values decrease distinctly in humid environment, especially in the case of Eclipse Base resin. The mechanical strength also shows decreased values for the humid samples, for all three resins tested in the present work. Generally, the results indicate a higher decrease of mechanical strength values for urethane-based light-curing resins (Eclipse) when compared to heat-curing acrylate (Meliodent) resins. The heat-curing acrylate shows a higher decrease in elasticity than the light-curing resins (**Figure 6**). The differences noticed between humid and dry environmental conditions indicate that there is clear evident role of the saliva in the biodegradation of the denture base polymers [8].

#### **2.2. Fracture behaviour after elongation of two heat-curing acrylic resins**

**Figure 4.** The determination of percentage humidity content of polymer.

applications (**Figure 5**).

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**Figure 5.** The determination of tensile mechanical resistance using the Zwick Roel extensometer.

**Materials Normal conditions Humid conditions (1 month)**

Meliodent 1615.60 52.77 1168.50 48.51 Eclipse base 2527.60 94.46 1921.50 66.76 Eclipse contour 1955.00 40.53 1577.00 25.39

**Table 2.** The mechanical properties of the studied materials kept in dry and in humid conditions for 1 month.

*E* **[MPa]** *R***m [MPa]** *E* **[MPa]** *R***m [MPa]**

Meliodent (Heraeus-Kulzer) was proven to have the highest water absorption capacity, followed by Eclipse Contour Resin and Eclipse Base Plate (Dentsply-DeguDent) (**Figure 3**).

The diagram showing correlation between the three types of resins and the percentage humidity content reveals that the heat-curing resin absorbs more water than the light-curing resins. The diagram indicates the maximal and minimal values for each material (**Figure 4**). Zwick Roell extensometer (Zwick GmbH & Co.) was used to determine the moment of sample breaking or fracture point and its elongation. TestXpert software was used to standardize the

> The same type of samples (plates: 2 mm in thickness, 30 mm in length and 5 mm in width) made of Meliodent (Heraeus-Kulzer) and Royaldent Plus (Palatinal Kft.) heat-curing acrylic resins were tested to compare the fracture behaviour after elongation. Both longitudinal and transversal surfaces of the samples, after breaking, were analyzed using the Olympus type SZX7 stereomicroscope equipped with an image processing system QuickphotoMicro 2.2. soft (**Figure 7**).

> The two acrylic resins have different fracture behaviour. In the case of Meliodent resin, the elongation before fracture was lower compared to that of Royaldent resin, indicating a ductile fracture behaviour.

> The following data were obtained by testing the two resins using Zwick Roell extensometer (Zwick GmbH & Co.). Meliodent: Ultimate tensile strength (*R*m): 54–75 MPa, Young's elasticity modulus (*E*): 1383–1688 MPa and elongation: 1–3.5%. Royaldent: Ultimate tensile strength (*R*m): 69–90 MPa, Young's elasticity modulus (*E*): 1282–1937 MPa and elongation: 2–5%.

> In the longitudinal section, Meliodent samples do not show elongation. The final fracture being sudden compared to the Royaldent samples, which had a significant elongation before fracture. In the transversal section, one may remark that dark reinforcement fibres from Meliodent

**Figure 7.** Stereomicroscope Olympus type SZX7.

(**Figure 8a**) do not break together with its polymeric matrix, whereas both matrix and fibres are broken at the same time in the case of Royaldent sample (**Figure 8b**). The different fracture behaviour of the two acrylic resins may be explained by the differences between mechanical characteristics of reinforced fibres and the polymeric matrix. The mechanical characteristics of fibres are better than those of the matrix in the case of Meliodent samples, whereas Royaldent samples showed the similar characteristics for fibres and matrix. Therefore, Royaldent samples show a better behaviour to fracture than Meliodent samples as well as ductile behaviour.

Stereomicroscopic analyses of two acrylic resins showed that the entire sample has a brittle fracture having a quasi-crystalline aspect in the transversal section.

**Figure 8.** The stereo microstructural aspect of the samples: (a) Meliodent and (b) Royaldent.

#### **2.3. Fracture toughness evaluation**

Being long-term prosthetic pieces, complete dentures need a warranty regarding their mechanical resistance and lifetime. The fracture toughness, which reflects its resistance to fracture and represents the energy required for a crack to propagate through a material to its complete fracture, was evaluated in complete denture technology.

Samples without initial cracks were considered for testing so that the value of the stress intensity factor (*K*IC) depends only on sample's dimension and critical load value.

Two different heat-curing acrylics were selected. Meliodent (Heraeus Kulzer) and Royaldent Plus (Palatinal Kft.), and a light-curing urethane-based resin-Eclipse Resin System (Dentsply-DeguDent) were taken into consideration. The samples were disk-shaped with a circular hole in the centre. Five different samples with the following dimensions were taken for the test:

**Sample 1**: *R*out = 42 mm, *R*in = 4.2 mm, *R*in/*R*out = 0.1 and h = 2 mm. **Sample 2:** *R*out = 45 mm, *R*in = 6.75 mm; *R*in/*R*out = 0.15 and h=2 mm. **Sample 3:** *R*out = 48 mm, *R*in = 9.6 mm; *R*in/*R*out = 0.2 and h = 2 mm. **Sample 4:** *R*out = 50 mm, *R*in = 12.5 mm; *R*in/*R*out = 0.25 and h = 2 mm. **Sample 5:** *R*out = 53 mm, *R*in = 15.9 mm; *R*in/*R*out = 0.3 and h = 2 mm. where *R*in = hole radius, *R*out = disk radius and h = disk thickness.

Five samples were made of each material (**Figure 9**).

(**Figure 8a**) do not break together with its polymeric matrix, whereas both matrix and fibres are broken at the same time in the case of Royaldent sample (**Figure 8b**). The different fracture behaviour of the two acrylic resins may be explained by the differences between mechanical characteristics of reinforced fibres and the polymeric matrix. The mechanical characteristics of fibres are better than those of the matrix in the case of Meliodent samples, whereas Royaldent samples showed the similar characteristics for fibres and matrix. Therefore, Royaldent samples show a better behaviour to fracture than Meliodent samples as well as ductile behaviour. Stereomicroscopic analyses of two acrylic resins showed that the entire sample has a brittle

fracture having a quasi-crystalline aspect in the transversal section.

**Figure 7.** Stereomicroscope Olympus type SZX7.

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**Figure 8.** The stereo microstructural aspect of the samples: (a) Meliodent and (b) Royaldent.

The compression tests were performed with the same static loading machine (**Figure 6a**), model Zwick Roell of 5 kN (Zwick GmbH & Co.), connected to a computer (TestXpert specific soft.). The samples were compressed until breaking (**Figure 10**).

Each sample was loaded by a pair of point forces, which acted along the diameter. The distribution of the load across the thickness of the disk was uniform. When the force was applied, the micro-cracks situated in the proximity of the force line, at the edge of the inner hole, started to grow, and at a certain value of the force gave rise to a macro-crack. Other

**Figure 9.** (a) Meliodent samples, (b) Royaldent samples and (c) Eclipse samples.

**Figure 10.** Meliodent sample during compression experiment.

pre-existing cracks within the specimen did not grow. Here, the central hole, playing the role of the defect, initiated the fracture. As expected, in general the crack developed symmetrically, beginning from the inner hole as shown in **Figure 11**.

The fracture toughness (*K*IC) was calculated, depending only on the sample's dimensions and critical value of the load. As the brittle materials have high relative compression values and low tension values, the failure begins at the point of inner boundary. In general, the fracture direction was perpendicular to the loading direction. The following values for fracture toughness were obtained: *K*IC = 2.4 MPa√m for Meliodent, *K*IC = 2.65 MPa√m for Royaldent and *K*IC = 3.35 MPa√m for Eclipse (**Figure 12**).

**Figure 11.** Aspects of the samples after the compression experiment: (a) Meliodent, (b) Royaldent and (c) Eclipse.

**Figure 12.** Force-displacement diagrams resulted after caring out compression test: (a) Meliodent, (b) Royaldent and (c) Eclipse.

The results obtained for the three tested resins showed no significant differences, Eclipse resin shows the highest value for fracture toughness [9, 10].

Comparative studies were undertaken with the same materials using different methods, in order to get comparative results. Single-edge-notched beam (SENB) method and indentation strength (IS) method were used. In the case of a single-edge-notched beam (SENB) method, samples were prepared in the form of plates with dimensions of 50 × 50 × 2 mm from which rectangular beams with dimensions of 4 × 2 × 25 mm (width/thickness/length) were cut.

The bending tests were carried out on a Zwick-Roell 5 kN testing machine (Zwick GmbH & Co.) (**Figure 6a**). The two halves of the broken samples were used for the measurement of the notch depth c. The toughness and the experimental values for *K*ic, obtained using SENB method were 2.26 MPa√m for Meliodent and 3.18 MPa√m for Eclipse resins.

Indentation strength (IS) method uses a Vickers pyramid to determine the fracture toughness by analyzing the stress field at a crack tip. The indentations of the samples were made using a Vickers hardness tester, model HMO 10, in the middle of the tensile surface of the beams at a load of 98 N, for 15 s, magnitude which prevented radial cracks (**Figure 13**).

**Figure 13.** (a) The Vickers pyramid and (b) the samples.

pre-existing cracks within the specimen did not grow. Here, the central hole, playing the role of the defect, initiated the fracture. As expected, in general the crack developed sym-

The fracture toughness (*K*IC) was calculated, depending only on the sample's dimensions and critical value of the load. As the brittle materials have high relative compression values and low tension values, the failure begins at the point of inner boundary. In general, the fracture direction was perpendicular to the loading direction. The following values for fracture toughness were obtained: *K*IC = 2.4 MPa√m for Meliodent, *K*IC = 2.65 MPa√m for Royaldent

**Figure 11.** Aspects of the samples after the compression experiment: (a) Meliodent, (b) Royaldent and (c) Eclipse.

metrically, beginning from the inner hole as shown in **Figure 11**.

and *K*IC = 3.35 MPa√m for Eclipse (**Figure 12**).

**Figure 10.** Meliodent sample during compression experiment.

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**Table 3.** Measurement results for Meliodent and Eclipse.

The measurements of the fracture toughness (*K*IC) were found to be 2.31 MPa√m for Meliodent and 3.26 MPa√m for Eclipse resins.

Results obtained by the strength indentation (IS) method are comparable to those obtained by the SENB method at low loading rates (~0.05 mm/min), as shown in **Table 3**.
