Flexural Strength and E-GlassFRC

8/3/2019 Flexural Strength and E-GlassFRC

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Strojarstvo 50 (4) 221-230 (2008) D. VOJVODI et. al., Flexural Strength of E - glass Fiber... 221Flexural Strength of E - glass Fiber... 221221

CODEN STJSAO ISSN 0562-1887

ZX470/1347 UDK 616.314-77:678.6:539.41

Original scientic paper

Fractures of denture base polymer material are one of the most frequentreasons for repair of removable dentures. Therefore, there is a continuouseffort to strengthen them, and polymer materials of high resistance tofracture are being developed. The aim of this study was to determine theexural strength of denture base polymers (pressure-heat polymerizingand auto polymerizing) reinforced with E -glass bers and high impactstrength resin (injectional polymerization) material using the short beammethod. Specimens were tested after polymerization and after articialageing performed by storage at 37 C temperature during 28 days and

thermocycling. Microscopic examination was performed to determinethe quality of bonding between glass bers and matrix. The study showedsignicantly higher values of exural strength (130.1-163.88 MPa) of glassber reinforced specimens compared to the un-reinforced specimens (91.77 122.75 MPa) control group, matching those of high impact strength resin(145.67 MPa). Between the groups of samples tested after polymerizationand storage in water at 37 C during 28 days there was no statisticallysignicant difference in exural strength values while samples tested afterthermocycling unexpectedly revealed signicantly higher values. Fiberreinforced materials and high impact strength resin revealed similar resultsof exural strength both being acceptable for clinical use. Obtained resultssuggest that the increase of temperature (during thermocycling) had theeffect of prolonged polymerization which resulted in a decrease of residualmonomer volume, enhancing polymer mechanical properties.

Usporedba savojne vrstoe stomatolokih polimera ojaanihE-staklenim vlaknima i stomatolokih smola visoke otpornostina udarac

Izvornoznanstveni lanak

Lomovi polimernih materijala za izradu baza proteza jedan je od najeihrazloga za popravak pominih proteza. Stoga se ulau stalni napori kako bise ti materijali ovrsnuli, te su razvijeni stomatoloki polimerni materijalivisoke otpornosti na lom. Cilj ovog istraivanja bio je odrediti metodomkratke grede savojnu vrstou polimera za izradu baza proteza (kojitlano-toplinski polimeriziraju i auto-polimeriziraju) ojaanih uporabomE-staklenih vlakana, te stomatolokih smola visoke otpornosti na udarac(injekcijska polimerizacija) i to nakon polimerizacije i nakon skladitenjauzoraka u vodi temperature 37 C tijekom 28 dana, te termocikliranja.Izvrena je mikroskropska pretraga mikrostrukture kompozitnih uzoraka

kako bi se ocijenila kvaliteta sveze izmeu staklenih vlakana i matrice.Ispitivanje je pokazalo znaajano vie vrijednosti savojne vrstoevlaknima ojaanih kompozitnih uzoraka (130,1 163,88 MPa) u usporedbis neojaanim uzorcima samo materijal matrice (91,77 122,75 MPa)kontrolne skupine. Vrijednosti savojne vrstoe vlaknima ojaanihkompozitnih uzoraka podudarali su se s vrijednostima savojne vrstoestomatolokih smola visoke otpornosti na udarac (145,67 MPa). Izmeuskupina uzoraka ispitivanih nakon polimerizacije i skladitenja u voditemperature 37 C tijekom 28 dana nije bilo statistiki znaajne razlikeu vrijednosti savojne vrstoe, dok su termociklirani uzorci neoekivano pokazali znaajno vie vrijednosti. Vlaknima ojaani stomatoloki polimeri i stomatoloke smole visoke otpornosti na udarac ostvarili susline rezultate savojne vrstoe koji su prihvatljivi za kliniku uporabu.Dobiveni rezultati navode i na zakljuak kako je porast temperature

(tijekom termocikliranja) izazvao efekt produene polimerizacije koja jepak rezultirala sniavanjem volumena ostatnog monomera poboljavajuimehanike osobine polimernog materijala.

Denis VOJVODI1), Franjo MATEJIEK2),Zdravko SCHAUPERL3),,Ketij MEHULI1),Ivana BAgI-UKOVI1) andSanjaEgOVI1)

1) Stomatoloki fakultet, Sveuilite u Zagrebu,(School of Dental Medicine, University ofZagreb), Gundulieva 5, HR-10000 ZagrebRepublic of Croatia

2) Strojarski fakultet u Slavonskom Brodu,Sveuilite J. J. Strossmayera u Osijeku(School of Engineering in Slavonski Brod,J. J. Strossmayer University, Osijek),Trg Ivane Brli Maurani 2,HR - 35000 Slavonski BrodRepublic of Croatia

3) Fakultet strojarstva i brodogradnje u Zagrebu,Sveuilite u Zagrebu (School of Engineeringand Naval Constructions, University ofZagreb), Ivana Luia 5HR - 10000 ZagrebRepublic of Croatia

KeywordsDental materials

Flexural strength

Polymers

Kljune rijeiPolimeri

Savojna vrstoa

Stomatoloki materijali

Received (primljeno): 2007-12-20Accepted (prihvaeno): 2008-05-15

Flexural Strength of E - glass Fiber Reinforced Dental

Polymer and Dental High Impact Strength Resin

[email protected]


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222 D. VOJVODI et. al., Flexural Strength of E - glass Fiber... Strojarstvo 50 (4) 221-230 (2008)

1. Introduction

Since the beginning of the 1940s, when it wasused as denture base material for the rst time, methylmetacrylate has proved to be the most reliable material.

Despite many advantages, methyl metacrylate is proneto fracturing. Fractures of that denture base polymermaterial are one of the most frequent (64%) reasons fordenture repair [1-4]. Theoretically, an edentulous patientwould not be able to fracture a denture due to a reasonablehigh static rigidity of the denture construction and weakmasticatory forces that are developed during the use ofremovable dentures [5, 6]. But with the ever-increasinguse of implants, even for the anchorage of removabledentures, bite forces that are developing on the denturebase are growing [7, 8]. Also, the inuence of materialfatigue on exural strength of the material is decisive,which is one of the most signicant reasons for denturefractures [9, 10]. Polymer dentures may be strengthenedby modications of material itself, or by incorporatingvarious reinforcements into the polymer material thatenhance the exural strength and impact strength [11].

Initially, reinforcement of dentures was achieved byembedment of metal wires or nets, but that approachresulted only in partial improvement of exural andimpact strength. Subsequently, physical and mechanical properties of acrylic dentures were enhanced byintegration of different bers with different berarchitectures into the denture base polymer [12]. For

that reason graphite, glass, and organic bers, such as,aramide and polyethylene bers, were used to improvethe exural and impact strength [12-16]. Today, themost commonly used bers in dentistry are glass bers,because of their acceptable esthetics [14,17-21] and goodbonding with polymers via silane coupling agents [22-24].

Another approach to exural strength improvementis incorporation of rubber phase in polymer pearls thusproducing materials known as high impact strength resins[25-30].

Another great disadvantage of polymer materials is

a tendency to dimensional changes due to contractionof polymer material during polymerization. Thereforedifferent attempts were made in order to compensate polymerization contraction and achieve completereproduction of the modeled dental object in wax duringdentures production and in that way enable better contactof the denture and its bearing tissues [28-30].

As outlined by the manufacturer of one dentalhigh impact strength resin, their SR-IVOCAP (IvoclarVivadent, Schaan, Liechtenstein) procedure completelysolves the problem of dimensional changes, withimproved strength of the material itself [25]. Therefore,

this procedure should be especially applicable to differentdental appliances.

2. Aim of the study

Different strength and quality enhancers of polymermaterials have been previously described in literature, both dental and technical. But the results are often

contradictory, and instructions and explanations of thedental companies that produce denture base materials areusually biased and only rarely comply with the results ofthe objective investigations. Also, glass bers producedespecially for dental application by dental manufacturersare very expensive for wider clinical use especially incountries with lower living and health standards.

So, it is the aim of this study to assess the valuesof exural strength of polymer materials commonlyused for denture bases, but additionally reinforced withindustrial E - glass bers, and to compare them withthe exural strength values of dental high impact strength

resin. In order to simulate the ageing process that occurson the dentures placed in the mouth the samples weretested after: (I) material polymerization, (II) 28 days ofstorage in distilled water at 37 C, and (II) thermocycling procedure.The results should be statistically analyzedand compared in order to obtain statistical signicancethat would show which material is better for clinical useregarding investment/benet ratio.

3. Materials and methods

Two hundred and ten quadratic specimens with

smooth surfaces and dimensions of 18 x 10 x 3 mm,were made of Meliodent Heat Cure and MeliodentRapid (Heraeus Kulzer, Hanau, Germany) polymer,the aforementioned polymers reinforced with E - glassunidirectional bers (1200 tex, Kelteks, Duga Resa,Croatia) and net shaped bers (ST-250, Kelteks, DugaResa, Croatia), and high impact strength resin IvocapPlus High Impact (Ivoclar, Schaan, Liechtenstein).Specimens were split across seven different groups withthirty specimens each. To obtain uniform specimens withglass bers accurately placed, special metal cuvette, withtwo thick polished metal parts on the sides and two thin

metal parts in the middle, was constructed. The middlemetal parts had ten quadratic perforations of the size of aspecimen (18 x 10 mm). One thin metal part was 1 mmthick, whereas, the other thin metal part was 2 mm thick,and placed together (3 mm thick) they also served as aplaceholder for proper glass ber alignment (1 mm fromone side and 2 mm from the other side of a specimen).All metal parts of the cuvette were covered twice with athin layer of Ivoclar Separating Fluid (Ivoclar Vivadent,Schaan, Liechtenstein). Non-impregnated E glassbers were cleaned with 1.6 mol sulphuric acid for 30sec. They were rinsed in distilled water, and air dried

at room temperature for 24 hours. After that they weredipped into 98 % -metacryloxypropyl-trimethoxysilane


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(Sigma-Aldrich Co., St. Louis, MO, USA), and heatedin dental sterilizer (ISO 400, Aesculap, Tuttlingen,Germany) at temperature 100 oC for 2 hours in order tobe pre-impregnated. Afterwards they were impregnatedwith Meliodent Heat Cure polymer syrup (weight ratio

polymer/monomer 10:8) for eight minutes. Pressure-heat polymerization polymer (Meliodent Heat Cure) wasmixed according to the manufacturers instructions andplaced in both halves (one thick part + one thin middlepart) of the special metal cuvette. Impregnated E - glassbers, unidirectional or net shaped were then placed in-between. The unidirectional glass bers were laid alongthe specimens, so that they were orthogonal to the forceto be applied, whereas, the net shaped glass bers werepositioned at an angle of 45 o. The cuvette was closedand put in a hydraulic press (Zlatarne, Celje, Slovenia)under 200 bar. The cuvette was subsequently moved to a

manual bench vice and the polymerization was performedin a polymerizing apparatus (Type 5518, KaVo EWL,Biberach, Germany) according to the manufacturersinstructions. Firstly, the cuvette was placed in boilingwater and heating was stopped for 15 minutes, thenheating was again turned on and the cuvette was boiledfor 20 minutes. After boiling the cuvette was left to slowlycool down in water bath of polymerizing apparatus.A similar procedure was followed for the MeliodentRapid auto polymerizing material, the difference beingshorter impregnation time of glass bers - two minutesin Meliodent Rapid polymer syrup because of the auto

polymerizing character of the material. As suggested by the manufacturer, auto polymerizing material wasadditionally polymerized in a pressure pot (Polyclav,Dentaurum, Pforzheim, Germany) through 10 minutesunder 2 bar pressure at 45 oC temperature.

Given that the Ivocap Plus High Impact material(Ivoclar) required its own special ask and apparatus(SR-IVOCAP System, Ivoclar) these specimens weresomehow differently produced. Firstly, wax patternswith the aforementioned dimensions were modeledand mounted on a wax prole (3 mm thick) providingthe injection method of polymerization. One half of the

original Ivoclar ask was lled with Moldano plaster(Heraeus Kulzer, Hanau, Germany), and wax patternsmounted on wax proles were placed onto plaster surfaceat least 1 cm from the ask margin. After the plasterhad hardened and wax patterns were half imbedded, plaster surface was covered once with a thin layer ofIvoclar Separating Fluid. Then the other half of the askwas placed on the rst one, and lled completely withMoldano plaster. After hardening of that second portionof plaster the ask was opened and wax rinsed in therinsing machine (Type 5522, KaVo EWL, Biberach,Germany) leaving the impressions of future specimens.

The plaster surfaces were then isolated twice in a thinlayer again using Ivoclar Separating Fluid. Both halves

of the ask were joined together and tightened withIvoclar bench vice. The injection method was executedin a manner in which the capsule containing IvocapPlus High Impact material was initially prepared. Themonomer from the bottle was poured in the capsule with

polymer powder and then shaken for ve minutes in a Capvibrator (Ivoclar). The capsule with prepared polymerwas attached to the ask and the system of pressurized air(6 bar) was connected for ve minutes to inject polymermaterial into the ask. Later the ask was immersed inboiling water in a polymerization bath (Ivoclar) for 35minutes, after which it was held in cold water for 20minutes. Throughout this entire cooling process the askwith samples was still subjected to the 6 bar pressure.

After polymerization and cooling the cuvettes/asksused for both methods were opened and the specimenswere detached. Possible polymer excess on all the

specimens was removed with a carbide bur (Ivomill,Ivoclar Vivadent). The margins were nished usingsandpaper (Sianor 7/0B, Frauenfeld, Switzerland). Thespecimens of the stated dimensions were checked withcalipers (Dentarium 042-751, Dentarium, Ispringen,Germany), with the maximum allowed deviation of 0.05mm.

The specimens of all seven groups were furthersubdivided into three subgroups of ten specimens eachthat were further tested by the short beam method (Figure1) [31]. The moving speed of the blade was set to 1.5mm/min to determine the samples exural strength after

(I) polymerization of the specimens, (II) immersion indistilled water with temperature at 37 oC (thermostat Btuj,Poznan, Poland) for 28 days, and(III) thermocycling ofthe specimens according to Hanssons method [32]. Fiberreinforced specimens were placed in a testing holder, ina position wherein the ber reinforcements were closerto the posts (1mm away from the posts and 2 mm fromthe blade).The force causing breakdown was noted andthe exural strength was calculated according to theformula:

maxmax=

=

F l

b h

N

mm

MPa

4

62 2

(1)

Fmax measured force of the loader (N),

l distance between posts (here 15 mm),

b width of the specimen (here 10 mm),

h height of the specimen (here 3 mm).

Numerical results of the exural strength wereanalyzed with SPSS statistical package (SPSS Inc.,Chicago, USA). Statistical analysis was performed usingdescriptive statistics, one-way analysis, and univariateanalysis of variance. The statistical signicance of

difference between exural strength values of thespecimens was calculated using the Scheffe test.

,


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To determine the quality of bonding between bers andmatrix, glass ber reinforced specimens were randomlychosen, sealed in Durox material (Struers, Rodovre,Denmark), ground, and polished according to the routineprocedure [33], to obtain a smooth surface suitable for

microscopic examination, which was performed with alight microscope, Olympus BH2-UMA (Olympus optical,Tokyo, Japan). Characteristic images were photographedthrough the microscope ocular using a camera, OlympusC-5050 Ultra Zoom (Olympus optical, Tokyo, Japan).

Figure 1. Specimen loading scheme and dimensions

Slika 1. Optereenje uzoraka shema i dimenzije

Figure 2. Arithmetic means of exural strengthSlika 2. Aritmetike sredine savojne vrstoe


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4. Results

Heat-pressure polymerizing Meliodent Heat Cureand auto polymerizing Meliodent Rapid polymerspecimens (control groups) demonstrated the lowestexural strength, whereas, the specimens reinforced withglass bers showed higher exural strength values, inaddition to tested high impact strength resin (Figure 2).Scheffes test applied across seven investigated groups ofspecimens revealed a statistically signicant difference(p


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Table 2. Test of between subject effects on exural strength. Subjects: type of bers=unidirectional or net; polymer=MeliodentRapid, Maliodent Heat Cure or Ivocap; ageing procedure=after polymerization, after immersion for 28 days in distilled water 37oC, after thermocycling.

Tablica 2. Test izmeu imbenika koji utjeu na savojnu vrstou. imbenici: tip vlakana=jednosmjerna ili mreica;polimer=Meliodent Rapid, Meliodent Heat Cure ili Ivocap; umjetno ostarivanje=nakon polimerizacije, nakon pohranjivanja u

destiliranoj vodi kroz 28 dana pri 37 oC, nakon termocikliranja

Source / Izvor

Type III Sumof Squares /Tip III suma

kvadrata

df / stupslob

Mean Square/ Kvadratprosjeka

F Sig. / Znaaj.

Corrected Model / Korigirani model

Intercept / Presretanje

Ageing procedure / Umjetno ostarivanje

Type of bers / Tip vlakana

Polymer / Polymer

Ageing procedure* type of bers /Umjetno ostarivanje * tip vlakana

Ageing procedure* polymer / Umjetnoostarivanje * polimer

Type of bers* polymer / Tip vlakana *polimer

Ageing procedure* type of bers* polymer/ Umjetno starivanje * tip vlakana *polimer

Error / Pogrjeka

Total / Ukupno

Corrected Total / Korekcija ukupnog

33895.843

298542.885

7428.074

.252

9443.002

5377.329

1834.404

3105.919

1455.113

44418.850

3225253.375

78314.693

14

1

2

1

1

2

2

1

2

135

150

149

2421.132

2985842.885

3714.037

.252

9443.002

2688.665

917.202

3105.919

727.556

329.029

7.358

9074.724

11.288

.001

28.700

8.172

2.788

9.440

2.211

.000

.000

.000

.978

.000

.000

.065

.003

0.114

5. Discussion

Dental material investigations require differentprocedures of articial ageing, such as underwater storageand/or cyclic changes of temperature, in order to exposetheir inuence on mechanical properties of materials indemanding environment of oral cavity [34].

The authors use different periods of underwaterstorage, and different water temperatures (usuallyroom temperature or 37 C temperature). It should beemphasized that the important decrease of exuralstrength values occurs during, the rst four weeks ofimmersion, while the further period of storage does notpresent a statistically signicant decrease [35]. That isthe reason why four weeks immersion in water at 37 Ctemperature was used.

In this study most specimens made of autopolymerizing and heat-pressure polymerizing materialsrevealed only a slight decrease of exural strength values

after four weeks immersion in water at 37 C temperature.That was not observed in a group of Ivocap samples

which values of exural strength increased for evennearly 15%, but rather high values of standard deviations,usual for this type of experiment [34], have caused thelack of statistical conrmation. Decrease of exuralstrength could be explained with water absorption. Watermolecules penetrate into the areas between polymerchains, remain there and separate these chains. Water

entry is primarily caused by diffusion, and partly by thepolarity of polymer chains that is caused by unsaturatedmolecules and unbalanced intermolecular forces [36].

Absorbed water can act as poly(methyl metacrylate) plasticator, and may soften the polymer material ofdenture base, a fact that emanates from the interactionwith the polymer structure. It that way water diminishesthe mechanical properties of the material, resulting inlower exural strength and lower modulus of elasticity[36].

Ivocap samples were produced with injection methodof polymerization which results in lower polymerization

shrinkage [37]. Lower polymerization shrinkage meansless porosity and such polymer is more resistant to water


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absorption and all of its consequences. Also, immersionin water could cause relaxation of the stress in thematerial that occurred during polymerization shrinkage[38,39], which has been proven to be a possible cause foran increase of the exural strength values for the testedpolymer materials.

Ageing procedure that imitates ingestion of cold and

hot food/beverages, so called thermocycling, can alsohave a signicant impact on mechanical properties of

polymer materials [40,41], as well as on the color, surfacesmoothness and resistance to abrasion [41].

In this study thermocycling procedure did not causea decrease of exural strength values of samples. On thecontrary, it resulted in an increase of values, especiallyin subgroups of pressure-heat polymerized sampleswhich exural strength was up to 35% higher. It seems

that in this sample subgroups' increase of temperatureduring thermocycling resulted in the effect of prolonged

Table 3. Scheffe test for the signicance between different factors (polymer, type of bers, ageing procedure) inuencing bondstrength. * =The mean difference is signicant at the 0.05 level.

Tablica 3. Scheffe test za zaajnost razlike izmeu razliitih imbenika (polimera, tipa vlakana, umjetnog ostarivanja) koji utjeuna savojnu vrstou. *= Razlika aritmetikih sredina je znaajna na nivou 0,05.

(I) Factor /imbenik

(J) Factor /imbenik

MeanDifference /Razlika arit.

sred. (I-J)

Std. Error/ Stand.

pogrjekaSig. / Znaajno. Lower Bound

/ Donjagranica

Upper Bound/ Gornjagranica

MeliodentRapid

Meliodenti Heat Cure

Ivocap

-17.7417*

-9.9000

3.31174

4.05604

.000

.054

-25.9387

-19.9393

-9.5446

0.1393

MeliodentHeat Cure

Meliodent Rapid

Ivocap

17.7417*

7.8417

3.31174

4.05604

.000

.158

9.5446

-2.1977

25.9387

17.8810

IvocapMeliodent Rapid

Meliodent Heat Cure

9.9000

-7.8417

4.05604

4.05604

.054

.158

-0.1393

-17.8810

19.9393

2.1977

Unidirectionalbers /

Jednosmjernavlakna

Net / Mreica

Ivocap

-0.0917

-1.0750

3.31174

4.05604

1.000

.965

-8.2887

-11.1143

8.1054

8.9643

Net / MreicaUnidirectional bers /Jednosmjerna vlakna

Ivocap

0.0917

-0.9833

3.31174

4.05604

1.000

.971

-8.1054

-11.0227

8.2887

9.0560

IvocapUnidirectional bers /Jednosmjerna vlakna

Net / Mreica

1.0750

0.9833

4.05604

4.05604

.965

.971

-8.9643

-9.0560

11.1143

11.0227

Afterpoymerisation/ Nakon

polimerizacije

28 days in distilledwater / 28 dana udestiliranoj vodi

After thermocyclingprocedure / Nakon

termocikliranja

5.6100

-12.2800*

3.62783

3.62783

.306

.004

-3.3694

-21.2594

14.5894

-3.3006

28 days indistilled /28 dana u

destiliranojvodi

After polymerisation /Nakon polimerizacije

After thermocyclingprocedure / Nakon

termocikliranja

-5.6100

-17.8900*

3.62783

3.62783

.306

.000

-14.5894

-26.8694

3.3694

-8.9106

Afterthermocycling

procedure

/ Nakontermocikliranja

After polymerisation /Nakon polimerizacije

28 days in distilledwater / 28 dana udestiliranoj vodi

12.2800*

17.8900*

3.62783

3.62783

.004

.000

3.3006

8.9106

21.2594

26.8694

95% Condence Interval /

Interval pozdanosti


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polymerization, which can, in turn, result in the decreaseof residual monomer volume, enhancing mechanical properties of the material and increasing the exuralstrength values.

Flexural strength values in the Ivocap sample subgroup

(produced by injection procedure) only slightly increased(4%) when compared to the Ivocap samples immersedin water. This fact could be attributed to the already lowresidual monomer volume in these specimens, whichcould not be signicantly lowered with prolongedpolymerization during the thermocycling procedure. Itcan be stated that exural strength values remained stableduring articial ageing procedures. Archadian et al. [42]in their study also reported stable exural strength values,although they were somewhat lower (around 100 MPa)than in this study (130-156 MPa). The results of exuralstrength in our study are higher than in the study of

Karacaer et al. [43], which used Palajet injection methodfor the production of specimens.

Figure 3. Microscopic image of a specimen section good bonding between glass bers and polymer matrix(magnication 1000x).

Slika 3. Mikroskopska slika presjeka uzorka - dobra vezaizmeu staklenih vlakana i polimernog matriksa (poveanje1000x).

Control groups of heat-pressure (Meliodent HeatCure) and auto polymerizing (Meliodent Rapid) polymerrevealed the lowest values of exural strength, whereas,specimens made of the same polymers but reinforcedwith glass bers showed higher exural strength values(p


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the polymerization, in order to achieve better mechanicalproperties of the material.

Reinforcements of polymers using dental laboratorypre-impregnated industrial E - glass bers increasedtheir exural strength, which was then comparable to

that of the tested high impact strength resin IvocapPlus High Impact, and therefore can be recommendedfor clinical usage. Since these ber reinforcements arerelatively cheap, contrary to the special glass bers fordental use, but obviously with good reinforcement effect,their clinical use would also be more cost effective.

Acknowledgments

Presented results originate from the scientic projectInvestigation of materials and clinical procedures inprosthetic dentistry supported by the Ministry of Science,

Education, and Sports of the Republic of Croatia grantNo. 065-0650445-0413.

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Documents

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