SD-V12N5-May2013 Layout 1...rods of GC Nature-Cryl Hi Impact acrylic tested, four of the rods did...

Acrylic: The “Hard” TruthJim Collis, CDT

48 Spectrum dialogue – Vol. 12 No. 5 – May 2013

SD-V12N5-May2013_Layout 1 13-05-03 1:03 PM Page 48

It turns out to be a true statement that theacrylic you choose can literally “make orbreak” your denture case. At my removable

dental laboratory, I have frequently had dentistsrequest that I process cases in “hard” acrylic or the“hardest” acrylic available. What the practitionersreally wanted was not “hard” in terms of beingbrittle or rigid, but rather, an acrylic that would bethe most impact resistant. The acrylic needed tobe able to absorb strong bite force stress withoutbreakage, particularly in attachment and implantcases. It would also be helpful if the acrylic couldwithstand the occasional mishap of a patientaccidentally dropping the appliance.

GC America recently introduced a new line ofacrylic known as Nature-Cryl Super Hi Impactacrylic. In the company’s literature, they showedresults of independently conducted Charpy teststhat measured impact strength of Nature-CrylSuper Hi Impact acrylic against leadingadvertised competitors. The independent testresults showed GC’s Nature-Cryl Super HiImpact acrylic to have significantly higherimpact strength than the other brands tested.Impact strength equates directly with breakage

resistance. The higher the impact strength of anacrylic, the better it is able to absorb impact andresist breakage.

I decided to conduct my own test to seewhether I could replicate the company’sadvertising claims of how well the Nature-CrylSuper Hi Impact acrylic could absorb impactversus Diamond D’s acrylic, as Diamond D wasone of the competitors named in GC’sadvertising. I chose to conduct a different, yetreliable, impact strength test, the Kravitz test. Iselected the Kravitz test because I had access tothe equipment necessary to conduct the test.

The first step was to prepare test samples (Fig. 2). I made two 64mm x 68mm x 6.3mmwax ups, invested them in flasks and then boiledthem out. I used the original shade in both theNature-Cryl Super Hi Impact acrylic and theDiamond D acrylic. The next step was to mix,press, pack and overnight cure (163°F for 8 hrs.)both acrylic specimens following exact companyspecifications (Fig. 3). The processed sampleswere then bench cooled in the flasks to roomtemperature (Fig. 4). The specimens wereremoved from the flasks and each was cutlengthwise into twelve equal rods. The twelverods of each specimen were then milled to thewidth and depth prescribed for the Kravitz test,3.25mm x 2.75mm + .02 mm (Fig. 5). Once

Fig. 1 Fig. 2

Fig. 3 Fig. 4

Spectrum dialogue – Vol. 12 No. 5 – May 2013 49


milled, each of those rods was cut across theirlength into two equal pieces. The end result was24 milled rods of each specimen (Fig. 6).

Each rod was then measured with a digitalcaliper to ensure that it was within the + .02 mmvariance allowed. Two of the Nature-Cryl HiImpact acrylic rods and three of the Diamond Dacrylic rods were eliminated because they werenot within acceptable variance after milling(Figs. 7 and 8). Each company’s remainingacrylic rods were soaked in two separate flasks ofwater for 24 hours (Fig. 9). The specimens werethen towel-dried in preparation for the Kravitztest.

The next step was to prepare the Kravitzmachine for testing. The machine consists of aheavily weighted metal foot with screws at eitherend with which to level the base. From the

center of the base rises a post topped with a semi-circular plate marked in joules (aninternational unit of energy equal to the workdone when a force of one newton acts through adistance of one meter) (Figs. 1 and 10). Behindthe semi-circular plate, and attached to an axle,is a swing arm to which a weight is fastened.

The swing arm has a hook release mechanismso that the person performing the test will notinfluence (increase or decrease) the force withwhich the arm swings. When the arm is releasedto swing, it moves a small notched knob in frontof the semi-circular plate. The notched knob isattached to the same axle as the swing arm.When the arm swings, it is the movement of thenotch on the knob that registers the impact ofthe swing. The impact reading in joules isobtained by moving the needle on the semi-circular plate to line up with the notch.

Fig. 5 Fig. 6

Fig. 7 Fig. 8

Fig. 9 Fig. 10



Photo courtesy of Jim Collis, CDT

STRENGTH, DURABILITY AND BEAUTY... ALL IN ONE!NATURE-CRYL SUPER HI IMPACT has superior impact strength that exceeds all other denture based resins on the market today.

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In the center of the base is an angled opening inwhich to insert a specimen. The specimen is thentightly fastened with the specimen clamp to hold itin place during the test (Fig. 11). The Kravitz testis designed so that the swing arm, once released,will hit the specimen. The opening for thespecimen is intentionally angled so that theweighted arm will strike an edge of the specimenrather than impact the specimen across its surface.This provides a more accurate impact strengthreading.

The Kravitz test was performed on 20 rods of theNature-Cryl Hi Impact acrylic and 20 rods of theDiamond D acrylic. Before each rod was tested, thenotch in the knob on the Kravitz machine was setto zero joules and, without a specimen inserted,the weighted arm was released to swing once toverify that it registered at zero joules force. Then aspecimen to be tested was inserted and fastened inplace and the weighted swing arm was released and

allowed to impact on the acrylic rod being tested.As each rod was tested, I recorded a physicalobservation of whether or not the weight brokethe acrylic sample. I also recorded the impactresistance in joules (as determined by the liningup the needle to match the resulting position ofthe notch on the knob attached to the axle).After each five rods were tested, the base of themachine was checked to make sure it was still level.

Fig. 14

Fig. 15

Fig. 11

Fig. 12 Fig. 17

Fig. 16

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In terms of physical observations, out of the 20rods of GC Nature-Cryl Hi Impact acrylic tested,four of the rods did not break but only bent whenimpacted (Figs. 12 and 13, GC results). Of the 20rods of Diamond D acrylic tested, all 20 rods brokewhen impacted (Figs. 14 and 15, Diamond Dresults). When I compared the impact resistancemeasurements recorded in joules for the 20samples of Nature-Cryl Hi Impact acrylic againstthe measurements recorded for the 20 samples ofDiamond D, I found that in 17 of 20 times, thejoule readings for the Nature-Cryl Hi Impactacrylic were considerably higher than the impactreadings obtained from the Diamond D acrylicsamples. In 3 of 20 times, the joule readings forthe Diamond D acrylic were slightly higher thanthe impact readings obtained from the Nature-Cryl Hi Impact acrylic. Though I have to allow forsome margin of error in my testing technique, GCNature-Cryl Hi Impact acrylic outperformed

Diamond D acrylic in terms of impact resistancebased on the results of my Kravitz tests.

Ultimately, why is impact strength so importantwhen it comes to acrylic? When breaking out adenture case, an acrylic with high impact strengthwill allow the technician to slip it past undercutswithout breaking flanges. Also newer compositeteeth are being manufactured to resist wear. Theseteeth perform well with a more durable acrylicthat will extend denture longevity. Additionally,as much as denturists or dentists advise patients totake precautions when handling dentures, all toofrequently appliances are dropped into porcelainbathroom sinks or onto tiled floors. Impactresistant acrylic reduces the occurrence ofbreakage. In terms of attachment cases, eventhough technicians do their best to block outcopings to allow tissueward movement, there canstill be fulcrum point issues with overdentures

Fig. 19


snapped onto Locator or ERA attachments (Figs. 16 and 17). These attachments do haveprocessing pieces to allow tissueward movement ifproperly used by the technician during theprocessing stage or by the clinician during a clinicalcure in the mouth. If a fulcrum point is still present,then cracking may subsequently occur over thispoint unless the overdenture has been fabricatedwith a high impact strength acrylic. In the case ofimplant-retained dentures which are fixed in themouth, the denture base which is wrapped around amilled titanium bar or other cast metal bar willbetter withstand fracturing in the mouth if it isfabricated with a high impact strength acrylic (Fig. 18).

When it comes to selecting an acrylic, “hard”should be defined in terms of impact resistance.Impact strength is the quality that makes dentures,attachment overdentures, and implant-retaineddentures break resistant and durable. Although I donot purport to be a scientist, nor do I claim theresults of my testing to be scientific, the results ofmy Kravitz testing do coincide with, and furthersupport, the conclusions of the independently

conducted Charpy tests described in GC’s literaturewhich find Nature-Cryl Hi Impact acrylic to havehigher impact strength than other brands tested(Fig. 19).

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About the author

Jim Collis, CDT, has owned and operated CollisProsthodontic Laboratory since 1980. The

laboratory specializes in a wide range ofremovable prosthodontic appliances including:full and partial dentures, attachment cases,designed and scanned milled titanium hybrid

bars, and removable/crown and bridgecombination cases. The lab primarily serves a select

clientele of dental offices in the western suburbs of Chicago.

Jim has presented numerous lectures and hands-on clinics atvenues throughout the United States, Canada and Germany.He is an Advisory Board Member for the publicationSpectrum Dialogue, The Voice of Techno-Clinical Dentistry,Palmeri Publishing Inc. and has published numeroustechnical articles. He is also a consultant for severalcompanies in the industry. Jim previously served atNorthwestern University Dental School as an instructor in theJunior/Senior laboratory for ten years. He possesses a degreein Dental Technology from Triton College, and has been aCertified Dental Technician for over thirty years.


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