exural strength and color stability of
commonly used denture base materials.
Materials and methodology:
Materials tested were nylon based (Lucitone FRS, Valplast) and conventional heat polymerized
acrylic resin (DPI). For flexural strength, ten specimens from each test material were prepared.
Five specimens from each material were stored in dry condition for 24 hours and five others
were stored in water at room temperature for 7 days. Flexural strength (Mpa) was measured
with universal testing machine. For colour stability, twenty specimens from each type of
material were prepared. All specimens were stored in distilled water at 37°C for 24 hours.
While in water, the color (T0) of all specimens was spectrophotometrically measured. Then
five specimens of each material were immersed in coffee, Coca-Cola, curry solution and tea at
Comparative evaluation of flexural strength and
color stability of different nylon polymer denture
base materials with conventional heat polymerized
acrylic resin denture base material: an in vitro
Very much 12.0-+ Coffee Coca cola Curry Solution Tea DE NBS Unit* DE NBS Unit* DE NBS Unit* DE NBS Unit* Heat Cure Acrylic Resin 1 month 0.711 0.654 1.038 0.955 1.138 1.047 4.701 4.325 3 month 0.991 0.912 0.647 0.595 1.124 1.034 2.416 2.223 6 month 5.419 4.985 5.577 5.131 5.362 4.933 7.434 6.839 Agrawal P. et al: Flexural strength and color stability of nylon denture base materials: an in vitro study The JPDM I Vol 3 I Issue 1 I 2022 33 The colour changes for all materials after 6 months were mainly in the categories of “much” (Fig. 6). Scale: Y axis showing dE that is colour change Figure 6: Mean of colour change (DE) of three denture base materials after 1 month, 3 months and 6 months Valplast after 6 months of exposure in curry solution, showed colour changes in the category of ‘very much’. Lucitone FRS, after 6 months of exposure in all staining solutions showed colour changes in the category of ‘much’. Heat polymerized acrylic resin after 6 months of immersion in staining solutions, exhibited ‘appreciable’ colour changes except in tea where it was ‘much’ (Fig. 7). Lucitone FRS 1 month 1.503 1.383 1.162 1.069 5.838 5.371 1.237 1.138 3 month 1.643 1.511 0.826 0.760 4.457 4.101 1.949 1.793 6 month 7.831 7.205 7.726 7.108 12.547 11.538 8.812 8.107 Valplast 1 month 3.162 2.909 2.197 2.022 12.47 11.480 3.502 3.222 3 month 3.697 3.401 2.620 2.411 12.237 11.258 5.402 4.969 6 month 9.112 8.383 6.862 6.313 26.415 24.302 10.342 9.514 Agrawal P. et al: Flexural strength and color stability of nylon denture base materials: an in vitro study The JPDM I Vol 3 I Issue 1 I 2022 34 Scale: Y axis showing dE that is colour change Figure 7: Mean Colour changes (DE) with four staining solutions after 1 month, 3 months and 6 months DISCUSSION The present study compared the flexural strengths of two nylon denture base materials with control group heat polymerized acrylic resin. According to the ISO standard, Type 3 denture base materials require more than 65 MPa of flexural strength. Thus, the study indicates, PMMA correlates with standard values, but PA did not. The higher flexural strength of PMMA can be attributed to medium to high molecular weights and linear polymer molecules with monofunctional groups.On the other hand, reduced flexural strength of nylon denture base materials may be due to its low molecular weight, linear polyamide chains which exhibit decreased strength and rigidity.10 Such differences may exhibit plastic deformation and necking without fracture even after exceeding the proportional limits.11 It has been suggested that flexural strength and flexural deflection depend upon method of moulding, method of curing, and curing time.10 The present study results are consistent with previous studies.1,2,10,11 There are few studies which have obtained greater flexural strength of nylon polymer denture base materials than PMMA.17-19 The reason could be attributed to the difference in sample size (no. of specimens) and testing conditions. The two nylon denture base resins used in this study had the same base chemical structure of a polyamide. However, flexural strength of Valplast is significantly lower than Lucitone FRS in both dry and wet conditions which may be due to difference in filler content composition (not specified by respective manufacturers). Flexural strength of specimens, when stored in water was lower than those stored in dry condition. The findings are in agreement with literature.20-24 The reason for reduction in flexural strength can be attributed to water sorption. PMMA absorb water slowly over a period of time. A diffusion mechanism is primarily responsible for the ingress of water. Water molecules penetrate the PMMA mass and occupy positions between polymer chains interfering with entanglement of polymer chains.13 Nylon denture base materials are hygroscopic in nature. This is explained by the hydrophilicity of the numerous amide bonds forming the main chains of the polyamide resin. Amide groups are polar in nature, making nylon sensitive to water (water being polar solvent).25 When water molecule comes in contact with polyamide, a weak bond is formed between water and polyamide. The water molecule diffuses through the material, forcing polymer chains to apart. The separation of the polymer chains reduces the polar attraction between polyamide chains. This allows for increased chain mobility, resulting Agrawal P. et al: Flexural strength and color stability of nylon denture base materials: an in vitro study The JPDM I Vol 3 I Issue 1 I 2022 35 in diminished mechanical properties.26 The water absorption occurs among the molecular chains of amide bond. Thus, higher the amide group concentration, the greater the water sorption.11 In the present study, the decrease in flexural strength of denture base materials, stored in water was insignificant. Thus, the null hypothesis stating that the flexural strength of different denture base material is not affected by different storing conditions is not rejected. This finding is in accordance with previous studies where in PMMA resin specimens showed insignificant difference in flexural strength after 90 and 180 days of water storage.20,23,24 Colour Stability Colour stability is important for all dental restorative materials in terms of acceptable aesthetic appearance.6 It can be measured by Munsell or CIE l*a*b system Present study has reiterated the fact that, PMMA specimens have better colour stability than nylon denture base materials. The reason could be attributed to hydrophobicity of PMMA and hydrophilicity of nylon.9 The water sorption of PMMA is moderate compared to nylon polymer denture base materials because of PMMA’S greater contact angle with water.11 Lucitone FRS has better colour stability than Valplast. The reason could be attributed to difference in filler content composition.29 Valplast and Lucitone FRS specimens showed colour changes ranging from ‘appreciable’ to ‘very much’ with curry solution throughout the study. Turmeric powder (from curry solution) contains conjugated diarylhepnoids like curcumin which are responsible for orange colour and staining capacity of curry solution. 30 Staining occurs due to the physical penetration of pigments between the molecular lattices or the adsorption of pigments on the surface of specimens. Nylon polymer resin denture base materials tested contain chromophores (>C=O), and auxochromes (>N-). Polarized chromophores and auxochromes are responsible for staining.11 Coca cola stains because of caramel present in it, but it does not produce as much staining as others because it contains phosphoric acid. Due to phosphoric acid its pH is in the range of 2.5 – 4.2. Low pH damages the integrity of the materials; thus, continuous surface erosion of the object takes place. Hence, it results in reduced staining.9 Heat polymerized acrylic resin (DPI) showed maximum colour change in tea as compared to any other staining solution.31-33 Yellowish brown stains caused by tea are due to presence of high amounts of flavanoid and methylxanthine in tea leaves. 34 Staining property of the coffee changes with storage time, temperature and humidity. The caffeine and caffeic acid present in coffee are responsible for discoloration of polymeric materials.6,34 Lai et al., demonstrated that coffee produced greater colour changes than tea on denture base resins which may be partly attributable to different polar properties of the tested materials.33 Comparison amongst the staining solutions at all three intervals showed statistically significant difference in colour change (p<0.001). Thus, the null hypothesis that the colour stability of denture base material is not dependent upon the type of solution or time for which it is immersed was rejected. In this study, there was initial increase in colour change for 1 month reading, and then a decrease in the colour change values of the denture base materials at 3rd month. This was seen for Lucitone FRS in Coca cola and curry solution, Valplast in curry solution and heat polymerized acrylic resin in all solutions except coffee. This result was attributed to removal Agrawal P. et al: Flexural strength and color stability of nylon denture base materials: an in vitro study The JPDM I Vol 3 I Issue 1 I 2022 36 of the accumulated layer. As tea and curry layers reach a certain thickness (on the specimen), they break away from the surface of the specimens and return to the solution.6,34 Colour change of polymer could also be intrinsic discoloration due to degradation of various polymers, and oxidation of the amine accelerator. According to the present study, PMMA still remains material of choice for removable and complete denture prosthesis. But in compromised situations, where nylon will be choice of material, Lucitone FRS is preferred over Valplast. LIMITATIONS OF THE STUDY The actual oral environment is comparatively different because of the pellicles formed by the proteins and glucoproteins in saliva that led to the formation of plaque adhering to the dentures. So, in future studies, the staining solutions can be combined with artificial saliva. Also, the specimens can be cleaned with brush every day in order to simulate cleaning protocol of denture. The effect of thermocycling on colour change of denture bases can also be studied. In-vivo studies can also be carried out as they give more precise results. CONCLUSION Within the limitations of the study, the following conclusions were drawn: 1. Flexural Strength a. The flexural strength of nylon polymer denture base materials was lower than heat polymerized acrylic resin in both dry and wet storage conditions (p<0.001). b. The flexural strength of Lucitone FRS was greater than Valplast in both dry and wet storage conditions (p<0.0001). c. The dry and wet flexural strength of each tested material were comparable (p>0.05). 2. Colour Stability a. The greatest chromatic changes were found with immersion in the curry solution, followed by tea, coffee and Coca cola. b. Lucitone FRS exhibited colour stability similar to heat polymerized acrylic resin at 1 month and 3 month readings (p>0.05), but showed lower colour stability in 6th month reading (p<0.0001). c. Valplast specimens exhibited least colour stability at all 3 intervals (∆ET0T1, ∆ET0T2, ∆ET0T3) (p<0.0001). CONFLICT OF INTEREST – None REFERENCES 1. Yunnus N, Rashid AA, Azmi LL, Abu-Hassan MI. some flexural properties of a nylon denture base polymer. J Oral Rehabil 2005;32:65-71 2. 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exural strength and color stability of