April, - 1923
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Physical Properties of Dental Cements-I I’j2 By Paul Poetschke THEL. D. CAULKCo., MILFORD, DEL.
A knowledge of the physical properties of dental cements is of great importance to the dentist and the public because dental cement occupies a prominent place in m a n y restorative dental operations, such as cementation of crowns, bridges, inlays, orthodontic appliances, and fillings. Failures in operations inooloing the use of dental cement are due to inferiorities in the cement or to its incorrect mixing andapplication. A knowledge of the physical properties of dental cements enables the dentist to avoid the use of inferior products and to intelligently m i x and applg cements of suitable quality. Apparatus and methods are described for determining heat generation, texture, permeability, and setting time of dental cements. T h e results of tests of the more prominent brands of dental cement of the zinc oxyphosphate type are gioen, and the relationship of different properties discussed f r o m a dental oiewpoint. Heat generated in the setting of zinc oxyphosphate cements varies considerably in the six prominent brands which were tested. Under proper conditions of mixing none of the existing products would cause damage to a vital tooth. Unpleasant thermal e$ect would result more f r o m hurried mixing than f r o m di$erences in the cements. T h e heat generated m a y be controlled within wide limits by the rate of addition of powder to liquid and the time of mixing. Heat generation, crushing strength, texture, and permeability are all properties of dental cements which show a n intimate relationship. Crushing strength i s directly proportional to heat generation. Cements which generate the most heat in setting show the highest crushing
HEATGENERATION
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T IS well known that dental cements and even such industrial cements as magnesium oxychloride and portland cement generate heat in the process of setting. In dental cements the heat generation is much more pronounced and the setting time is correspondingly more rapid. Heat generation of dental cements is of importance in connection with dental practice, especially in crown and bridge work, because the bulk of cement used is in many cases large enough to give a decided thermal effect. This is especially true when the cement is hurriedly mixed by rapid and large additions of powder and limited spatulation on the slab, for under these conditions more heat is generated than when the cement is properly mixed. The degree of heat that may be applied t o the healthy tooth varies somewhat with individuals. Some will tolerate 135” F., whereas others are unpleasantly affected by a temperature of 120” F. The tooth is more susceptible to irritation by cold than by heat, and it will tolerate a temperature of 130” F. better than one of 40’ F. APPARATUS-Fig. 1 shows the apparatus designed and used in this laboratory for studies on the heat generation of dental cements. The most important part of the apparatus is the special thermometer A . Instead of the usual mercury bulb, the thermometer has a depressed mercury bulb with a cavity 7.5 mm. deep and 7.5 mm. in diameter. It is graduated from 40” t o 1 Received September 1, 1922. Presented before the Division of Industrial and Engineering Chemistry a t the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 t o 8, 1922 2 This paper is a continuation of studies on dental cements published in the previous papers “Germicidal Efficiency of Dental Cements,” THIS JOURNAL, 1 (1915), 196; “Physical Properties of Dental Cements,” Ibid., 8 (1916), j03.
strength. Cements showing the greatest heat generation and highest crushing strength, also show the finest texture. T h e texture of six prominent brands of zinc oxyphosphate cement varies f r o m a fine, vitreous to a coarse, chalky texture. Permeability of zinc oxyphosphate cemcnts to aqueousfluids is greater in cements haoing a coarse texture. Cements of fine vitreous texture are not penetrated by dye solutions. Setting time at room and body temperature was determined by several methods. Commercial designations such as “slow,” “medium,” and “rapid” setting are of little value, because no standard of setting time is adhered to by manufacturers. A cement which sets more rapidly at room temperature does not necessarily follow the same order at body temperature, and m a y actually be slower setting at body temperature than a cement which sets slower at room temperature. Slower setting at room temperature gives the dentist more time f o r mixing and application, and rapid setting at body temperature economizes his time. I t i s therefore desirable to have slower setting at room temperature with a degree of acceleration at body temperature which will resuIt in reasonably prompt setting in the mouth. Zinc oxyphosphate cements do not all show the same characteristics i n hardening. Some that show a f a i r or even good rate of hardening in I5 min. m a y not show a proportionate increase i n the next 24 hrs. Cements which show the highest heat elevation on setting, the greatest strength, the finest texture, and the least permeability to aqueous fluids also show the greatest increase in strength between the 15-min. and 24-hr. period.
200” F. in single degrees. The total length of the instrument is 12 in. B is a small thermometer of special form used to control the temperature of the apparatus. C is a Kolle culture flask F is a battery jar. E is a heating element, and G a stirrer The apparatus as described is virtually an artificial mouth
STANDARD PROCEDURE FOR ZISC OXYPHOSPHATE CEMENTS-Fill the jar F with water to the neck of the flask C. Turn on the current and heat up the water until the control thermometer B reaches 100”F., and maintain a t this temperature during the test. Weigh 1.5 g. of powder and transfer to a mixing slab previously regulated to a temperature of 68” F. Then transfer 0.5 cc. of the liquid to the slab and add the powder to the liquid in eight portions, completing the mix in 2 min. Collect the cement mix on the end of the mixing spatula, quickly remove the cover D and control thermometer R; raise the thermometer d and fill the cavity flush with the cement by running the spatula sidewise over the edge of the cavity. Immediately replace A, D, and B, and begin readings. This part of the process must be carried out quickly, and with a little experience the first reading may be taken 3/4 min. after completing the mix. A stopwatch is used for timing the readings. Readings are taken every min. for the first 4 rnin., then every min. for the next 4 min., and finally every minute until the temperature on thermometer A has dropped to 101” F. (The set cement is removed from the thermometer by immersing the bulb in dilute hydrochloric acid. A single test requires approximately l/z hr. Results may be duplicated with an accuracy of 0.5” F.) REsuLTs-Fig. 2 shows graphically the results obtained on six of the most prominent brands of zinc oxyphosphate
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cements. I t will be observed that the maximum temperature is reachcd in all cases in approximately 3 min. and that the temperature reached varies from 115" to 133.5" F. Fig. 3 shows graphically some results obtained on Cement M of Fig. 2 by varying the initial temperature and mixing conditions. If we start with the thermometer a t room temperature (75" F.) instead of 100" F. then the maximum temperature reached is only 98.75" F., but if the initial temperature is 100' F. then the maximum temperature reached is 133.5" F. This shows the importance of a definite initial temperature such as 100" F. When the powder is added to the liquid in fewer and therefore larger portions and the time of spatulation is reduced, the h e a t elevation is greater. Test B of Fig. 3 was made under average correct m i x i n g conditions, and the temperature reached was 133" F. By adding the powder twice as fast, as in A, a temperature of 143" F.is reached. By retarding the mixing time as in C, a temperature of only 128" F. is reached, which can be still further reduced by spatFio. 1 ulating the mass for a minute after all powder has been added, as in D where the maximum temperature rose to only 122" F. SIGNIFICANCE OF REsuLTs-The bulk of cement used in the test is considerably greater than would be employed in cementing a single crown or a bridge on a single tooth or abutment. The cement only fills the space between the crown or cap and the tooth structure. The thickness of cement will vary with the size and form of the tooth and the extent to which the enamel and dentin may have been removed by decay or design. Therefore, the intensity of the heat transmitted by the setting cement to the tooth structure is dependent upon bulk of cement, kind of cement, and conditions of mixing. However, the maximum temperature recorded for any of the properly mixed cements is a little less than 135" F., which is tolerated by the healthy tooth, and under no circumstances could we conceive of such a large bulk of cement being used on a single tooth. If the cement is not mixed too hurriedly, no unpleasant .sensation of pain or
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irritation to a vital pulp can occur to the patient under practical working conditions. Furthermore, it is an everyday practice among dentists to have the patient flow cold water over the cemented region should any undue heating take place. RELATION OF HEATGENERATION TO CRUSHING STRENGTH OF OXYPHOSPHATE CEMENTS Fig. 2 also shows the crushing strength of the various cements in relation to the heat elevation. The rather interesting and important discovery was made that higher heat generation was accompanied by increased strength and resistance to saliva. Reference to Fig. 4 will show that, notwithstanding the fact that six different brands of cement were tested, the crushing strength of all is directly prOportiona1 to the heat generated on setting. Therefore, the higher the degree of chemical reactivity, the higher the strength of the cement. This observation may be worth investigation in connection with other industrial cements.
RELATION OF HEATGENERATION TO TEXTURE METHOD-cylinders 5 mm. high and 5 mm. wide are allowed to set under paraffin oil at body temperature for 15 min., then rinsed in ether and dropped into an aqueous dye solution (as used for permeability tests) a t body temperature. After 24 hrs. storage a t body temperature, they are broken through the middle by placing a sharp knife blade against the cylinder and striking quickly with a hammer. This breaks the cylinder open with a clean cross cut. The surface is then examined under a magnifying glass.
Fig. 6 shows the texture of six commercial brands of dental cement a t a magnification of approximately six diameters. The texture varies from a h e vitreous, as in A, to a coarse chalky, as in F. Cement A is the same as M in Fig. 2, B the same as 0, C the same as P, D the same as Q, and E the same as R. These comparisons show that the higher the heat generation the finer and more vitreous the texture. Cements showing the lowest heat generation have a coarse, chalky texture.
PERM~ABILITY For a long time tests of dcntal cements have been made by placing the set cement in a solution of dye or red ink. These tests have been largely valueless and misleading, because no attention was given to consistency, conditions of mixing, time of initial setting a t body temperature, or time of storage. METHOD-In this laboratory test cylinders 5 x 5 mm. are prepared in the same manner as in tests for texture. In fact, the same cylinder serves for both tests. The cylinders are hardened under paraffin oil a t body temperature for 15 min., rinsed in ether, and placed in an aqueous solution of dyestuff and stored at body temperature for 24 hrs. Various vegetable and coal-car TIME in m&*S dyes may be emFJO. a ployed. A very satisfactory solution for the purpose is made by dissolving 2 g. of cotton scarlet extra (Badische) in 500 cc. of distilled water and adding thereto 0.1 cc. of 40 per cent formaldehyde solution. After 24 hrs. storage the
INDUSTRIAL A N D ENGINEERING CHEiWISTRY
April, 1923
341
TIME OF SETTIKG I9hTYLAILD OF POWDER
In a previous paper2 a method was given for determining the relative setting time of dental cements a t 68" F. This is the average room temperature, and the results obtained are of value in grading cements as to their relative working timethat is, the time which may be taken by the dentist to make the mix and apply the cement. However, when the cement is placed in position in the mouth, the setting is accelerated by the heat of the mouth. M It is, therefore, important to know to what extent the setting time of zinc oxyphosphate cements is accelerated by increased mass of cement and by the body temperature. Two methods were employed to determine the relation between setting time at r o o m temperature and body temperature. METHOD 1-The cement mix, using 0.25 cc. of liquid, was collected on the end of the spatula and allowed to HEAT ELEVATION OF remain there undisFro. 4 turbed for 1 min., and then quickly rolled int o a Dellet in the Dalm of th; hand and deposited upon a clean sheet of paper. The time required for the pellet t o set so that it is no longer indented by a pointed instrument, applied with moderate pressure while being held in the fingers as one would hold a pen, is taken as the pelletsetting time. This method shows the accelerating effect due to increased mass of cement at room temperature.
Test made ak roo
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i 6i 8I 10i 12i 1Q. t 16I 18I 20I 2% I 2+I 1 T I M E in minutes
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cylinders are rinsed in distilled water, dried with filter paper, split open in the same way as outlined for the texture tests, and the depth of penetration of the dye measured at four approximatelyiequidistant points along the edge of the cylinder.
The following results were obtained on six cements, the A = None B = None
D = E
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I/e t o 1/4 1/8mm.
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The significant thing about these results is that the cements having a fine vitreous texture are not penetrated by the dye, whereas those having a coarse, chalky texture are penetrated. Cement E", which has a very coarse texture, shows the greatest penetration. The presence of soluble constituents in the set cement mass probably accounts for the slight differences in permeability of Cements C, D, and E, but the general results indicate that texture plays an important part-the finer the texture the less the penetration.
METHOD2-Cylinders are prepared in the same manner as for crushing-strength tests,P and the time required for a cylinder to reach 325 to 350 lbs. crushing strength at body temperature is determined. At this point a cylinder of zinc oxyphosphate cement breaks sharp and does not squash under the. load. This method gives results which agree very closely with practical tests made by cementing facings and crowns at body temperature.
Fig. 5 shows the results obtained by the two methods as compared to the setting time a t 68" F. The figures in the circles show the time for the required cylinders to reach 325 to 350 lbs. crushing strength.
DISCUSSION OF REsuLTs-The setting range of six commercial brands of zinc oxyphosphate cement at 68" F. is from 12 to 22min. The pelletsetting range (Method 1) is 3 min. at 70" to 80' F. This shows the accelerating effect of increased mass of cement on the setting time due to the heat generated within
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the pellet, the setting range being narronred down from 12 to 3 min. The figures in t.he circles show a range of from 5 to 9 min. for the different cements. Table I gives a comparison of the three methods. Five of the cements are designated commcrcially &s “medium setting.” It. is evident that there is a wide difference in the setting t.ime of the fnre “medium-setting” cements, indicating that manufacturers have no definite standard and that the commercial designation “medium,” “rapid,” and “slow setting” may mean anything. T n e ~ sI
7 0 6 7.23 6.5 8
Froin a practical standpoint t I1 e s e results show some important characteristics. A cement which sets slowly at room temperature may show a more rapid setting at body temperature than one which sets faster at room teniperature. Cement M requires 22 min. at 68” F. and only 7 min. at body temperature, whereas Coment P, which sets in 16 min. at 68” F. requires 9 min. at body temperature. A dentist using zinc oxyphosphate cements is apt to assume that a cement which sets slowly on the slab will also set slowly at hody tempernture, and that a cement which sets faster on the slab will set more rapidly in the mouth. In actual praetice t.he dentist requires sufficient time for mixing and placing of the cement, and the more time available for mixing the better, provided setting will t a k e place promptly at body temperature. The majority of operators make the mis-
Time Required to Reach 326 to 350 Lbi. Commercial Mi,,. I)e.iznation 7 Rapid
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take of mixing cement too thin and too hurriedly, for the sake of gaining time. While this may bc nect’ssary with a cement that sets too rapidly on the slah, it is obviously not necessary with a cement that sets niore slo~vly on the slab. Where more time for mixing is available, more powder may be incorporated in the mix, and this makes for greater strength and resistance to oral fluids. Most failures in cementation are either due to mixing too thin and too hnrriedly, or to the use of inferior cements. There is little danger of mixing a zinc oxyphosphate cement too thick, because i t vould set before i t could be properly placed in position,’ and the operator need only remove it and make anot.her mix. If the cement is mixed too thin, then real damage is done, because the cement can be placed in position and the patient, dismissed with the defective cement in irlaw. h safe rule to follow for all cementing operations is to mix t,he e(:ment as thickas possible so that it,may still he properly placed. Another interesting feature of the resulk is that cements x-hieh set fairly rapidly on tlie slab and attain a fair crushing strength in 15 min. do not, shox a, corresponding increase in st,rengtli io 24 hrs. Fig. -I’ shows the result of crushing-strength tests after 15 min. in oil at body ten,peratnre. Coinparing the 15-min. nnd 24-hr. test it will be seen that Cement R shows an B increase of only 82 Ibs. in 21 hrs., show a heat elevation of 15OF., and a coarse, chalky texture (Cement E in Fig. 3 ) . Cement M shoivs an increase of 418 Ibs. in 24 lirs,, hasa heat elevation of 33.5” F., and shows a fine, vitreous teature (Cemcnt A in Fig. 3 ) . Ceinent hf has a 11igher strengtli than R in D 15 min. (I02.lbs. greater), and while M requircs 22 min. to set at 68” F., R requires only 13 min. Cement 0, which shows a high 15-miii. strength due t o undrr-caleination of the powder, increases only 268 Ihs. in 21 hrs. and sets in 15 min. at 68” F. Thisycement shows a heat elevation of 30.5‘ F. and a fine, vitreous ti texture.
