Introduction
The heat cure denture base resins are extensively used for their properties such as ease of handling, durability and esthetics etc. The mechanical properties of denture base resins are very important for the clinical success of multiple type of prosthesis.1, 2 Water sorption is one of the important properties of acrylic resin. Poly-methyl methacrylate absorbs water in a relatively small amount, when placed in an aqueous medium. The introduction of water molecules within the polymerized mass reduces the strength of acrylic resins.3

Aims and Objectives
1. The aim of this study is to compare the effect of artificial saliva on flexural strength of three commercially available denture base acrylic resins. (TREVALON-HIGH STRENGTH, PYRAX ACRYL- HIGH STRENGTH, and TREVALON ). 2. The micro structural evaluation of fractured acrylic resin interface by scanning electron microscope.

Materials and Methods In this study three commercially available heat cure denture base resins namely Trevalon, Trevalon – Hi and PYRAX Acryl – Hi were taken. (Fig 1 & Fig 2) A total of 84 samples were made. 28 samples of each type of acrylic resin were prepared with the dimension of 50x25x2.5 mm. 11(Fig 3 & 4)

Dry strength

TREVALON (T) 4 Samples

TREVALON-HI(TH) 4 Samples

ACRYL-HI (AH) 4 Samples

Wet Strength ( 30 ,60 and 90 days immersion in artificial saliva)

T 30 - 8 Samples	TH 30 - 8 Samples	AH 30 -8 Samples
T  60- 8Samples	TH 60 - 8Samples	AH 60 -8Samples
T 90 - 8Samples	TH 90 - 8Samples	AH 90 - 8Samples

The artificial saliva used in this study was a carboxymethylcellulose (CMC) based saliva (SALIVEZE). In this study the artificial saliva was prepared in department of bio-chemistry lab.

The composition of the saliva is as follows5. (Fig 5 &6) 
(Sodium carboxymethylcellulose 10 g/l, Potassium chloride 0.62 g/l, Sodium chloride 0.87 g/l, Magnesium chloride 0.06 g/l,  Calcium chloride 0.17 g/l, Potassium di hydrogen orthophosphate 0.30 g/l, Sodium fluoride 0.0044 g/l, Sorbitol 29.95 g/l, Spirit of lemon 5ml., Deionized water 1000ml.) (Fig 5&6)

 

Flexural  Strength Test
The prepared dry and wet acrylic resin specimens were placed( one by one) on a jig having two parallel pins, 40 mm. apart. The load was applied on the sample and the sample shows flexion.
(Fig 7) After reaching the maximum flexion the sample broke under load and the reading was recorded.8

Flexural strength in MPa

Group

T

T 30

T 60

T 90

T H

T H 30

T H 60

T H 90

A H

A H 30

A H 60

A H 90

The composition of the saliva is as follows5. (Fig 5 &6) (Sodium carboxymethylcellulose 10 g/l, Potassium chloride 0.62 g/l, Sodium chloride 0.87 g/l, Magnesium chloride 0.06 g/l, Calcium chloride 0.17 g/l, Potassium di hydrogen orthophosphate 0.30 g/l, Sodium fluoride 0.0044 g/l, Sorbitol 29.95 g/l, Spirit of lemon 5ml., Deionized water 1000ml.) (Fig 5&6) Flexural Strength Test The prepared dry and wet acrylic resin specimens were placed( one by one) on a jig having two parallel pins, 40 mm. apart. The load was applied on the sample and the sample shows flexion. (Fig 7) After reaching the maximum flexion the sample broke under load and the reading was recorded.8

Group	T	T 30	T 60	T 90	T H	T H 30	T H 60	T H 90	A H	A H 30	A H 60	A H 90
1	91.2	94.39	84.10	84.33	119.04	102.80	101.53	100.11	104.45	95.73	81.29	98.88
2	90.0096	93.27	68.12	80.79	96.31	121.15	67.51	86.59	110.03	92.39	92.24	77.76
3	85.5936	84.17	91.78	79.68	99.76	87.21	88.78	84.13	108.86	113.97	98.69	99.23
4	88.1664	90.36	72.27	70.85	111.55	108.48	93.04	84.29	101.15	87.01	94.62	77.84
5		94.85	60.94	83.17		99.76	98.46	85.59		88.59	73.65	85.86
6		74.30	103.64	77.88		73.50	118.04	99.11		84.79	99.65	105.83
7		78.41	91.08	79.56		74.88	65.74	91.24		88.28	96.88	85.98
8		89.40	84.06	94.39		97.88	101.49	93.43		91.24	88.01	84.94
Mean	88.7424	87.39	82.00	81.33	106.67	95.71	91.82	90.56	106.12	92.75	90.63	89.54
SD	2.44	7.69	14.07	6.66	10.52	16.39	17.73	6.48	4.10	9.22	9.15	10.48
CV	2.75	8.80	17.16	8.19	9.86	17.13	19.31	7.16	3.68	9.94	10.10	11.71

One – way Analysis of Variance (ANOVA)T, TH & AH - Group
Source of Variation  Degrees of freedom   Sum of Squares Mean Square
-------------------------  -------------------------  ---------------------  ------------------
Treatment (between  2                         2.948                        1.474
Columns)
Residuals (within       9                      400.30

Comparison of Flexural Strength, all types of material with Time	Mean Flexural Strength of Dry and Wet   Samples of all the 12 groups
This graph shows the flexural strength of the                      samples decreases as the time period increases,                irrespective of the type of material T - 29% TH - 36%and AH35%	The order of flexural strength is as follows  TH >AH >T , T>T 30>T 60>T 90     AH>AH 30>AH60>AH90,  TH > TH30 > TH60 > TH90

44.478
Columns)
Total                           F = 3314017
The P value is 0.0001 considered extremely significant.
Variation among column means is significantly greater than expected by chance.

Results
1. The flexural strength of high impact resins are significantly higher than the conventional one. The high impact acrylic resins are the economically expensive options but in terms of flexural strength they are always better than the conventional one.8
The decreasing order of flexural strength is TH >AH >T.
According to the mean value, the flexural strength of Trevalon – Hi is the highest, followed by the Pyrax Acryl – Hi and the least strength is recorded with the Trevalon. One way analysis also shows a very significant result, regarding the type of material.
2.  The flexural strength for all three types T, TH, and AH decreases with time.
As the samples are placed in artificial saliva, it absorbs water and the flexural strength decreases.
The dry strength of T, TH, and AH group is always more than the wet strength.

Microstructural Analysis Of The Fractured Surface
After the fracture of the samples, 2 fractured segments were randomly taken from all the 12 groups.(T, TH, AH, T30, TH30, AH30,T60, TH60, AH60, T90, TH90, AH90).The SEM analysis of the fractured surface were carried out in the following manner.10,12 A slice of 1 cm. was cut from the  fractured surface. Cut section was placed on the stub. Gold – palladium coating was done.Fractured surface was analyzed  under scanning electron microscope
The SEM study was conducted in,Deparment of Microscopy, College of Veterinary Sciences , Pant nagar University, UTTARA KHAND.9,10 (Fig 9)

Summary
In this study the flexural strength of three commercially available denture base resins had been compared, out of which two are high impact and one is the conventional one.  The time dependent effect of artificial saliva on the Flexural Strength of denture base acrylic resin were also evaluated.
The high impact resins are a better option due to their higher strength properties, but they are  the expensive options.
Now the Indian brands (PYRAX Acryl- Hi) are available in the market which are equally good in
strength and esthetics.They are comparatively cheaper options as they are manufactured in India so they are free from the import – export charges. Hence are relatively economical and better treatment options as the results are concerned.The time dependent changes in the acrylic resin would need a further research for a longer duration as some samples show an  increased strength after the immersion in artificial saliva
Bibliography

1. • Machado C et al. Comparative study of the transverse strength of three denture base materials.  Journal of dentistry  2007; 35:930-33.
   • Arnold  A et al. Flexural and fatigue strengths of denture base resin.  J Prosthet Dent  2008; 100(1):47-51.
   • Anusavice k. Phillip’s Science of Dental Materials.W.B.Saunders Company; 10th edition: Elsevier; pp. 254-262.
   • Arundati R., Patil N.P.. An investigation into the transverse and impact strength of a new indigenous high-impact denture base resin, DPI – TUFF and its comparison with most commonly used two denture base resins. JIPS  2006; 6(3):133-38.
   • Preetha A and Banerjee R. Comparison of Artificial Saliva Substitutes. Trend Biomater. Artif. Organs 2005; 18(2):178-86.
   • Braden M. The absorption of  water by acrylic resins and other materials. J Prosthet   Dent. 1964; 14(2):307-16.
   • Faot F et al. Impact and flexural strength, and fracture morphology of acrylic resins with impact modifiers. Open Dent J.2009; 3:137-43.
   • Gurbuz O,Unalan F, Dikbas I. Comparison of transverse strength of six acrylic denture base resins. OHDMBSC 2010;IX(1):21-24.
   • Test M604: Scanning electron microscopy.
   • Parrington R, P.E. Fractography of Metals and Plastics.
   • Materials and Devices, American Dental Association, 211 E Chicago Ave, Chicago, 60611.) Council on dental material and devices. Revised American Dental association Specification No.12 for denture base polymers. Section 3.4 and 3.6.