Bend-Brittle and Shatter Points GLENN E. KING The Johnson Rubber Company, Middlefield, Ohio i
W
’ITHIN the past two projection on the frame to give Many rubberlike materials possess two years the performance a bend test. The rate of bendrelative potnts of change in physical properof rubberlike composiing was not stated; however, a ties corresponding to the bend-brittle tions at low temperatures has high rate of operation would temperature and the shatter point. Since become important. A number probably simulate a shatter test. the bend-brittle point varies and apof tests have been devised to Recently (8) this apparatus was measure the loss of rubberlike improved to control the rate as proaches the shatter point, the shatter properties of many of these well as the magnitude of point seems t o be the true brittle temcompositions. stress. perature. These points change although As the temperature is lowered, Garvey (3, 4) attempted to they hold 1 heir relative positions in respecf the first indication of the loss differentiate between a freezing to each other f o r various p1astieize.s and of rubberlike properties is temperature, a stiffness by bendhardening of the material. As ing under a load, and a brittle for time of freezing. The two points the temperature is Iowered point. The brittle point was approach each other as hardness increases farther, thematerial continues to tested by placing a rod on the through loading. Although there is a stiffen and harden to a point opposite end of a securely held minimum distance f r o m which a weight where it becomes quite inflexible; sample and bending the sample must be dropped to shatter a given sample, bending or striking with a solid by striking the rod with a object then causes breaking or hammer. The temperature of a greater distance does not raise the shatter shattering due to the brittleness breaking was the brittle point. point. of the material. Up to this time A sample of plasticized polythe temDerature a t which breakvinyl chloride was tested a t ing occ& has been called the “brittle point” of the material, low temperature by Russell (12) for a break after cooling for with no distinction made between the temperature a t which 5 minutes a t each temperature. The point of breaking ocbreaking occurs upon bending and the temperature at which curred with no observable bending. breaking occurs upon striking with a sharp blow from a In making a bend test by cooling a sample and bending solid object. It would be easy to assume that the two points through an angle, it was noticed that the breaking temperaoccur a t the same temperature. ture did not correspond to the shatter point when struck a Koch (9) tested the brittle point of elastic materials by sharp blow with a hammer, and that in the former case the measuring the bend stress with a micrometer gage placed in break was usually a straight fracture while in the latter it was the center of the specimen that was supported at each end. a shattering into a number of pieces. A recheck of the tests By observing the distortion caused by weights, the elastic showed that shattering occurred at a higher temperature than modulus could be measured to the breaking point which octhe bend-breaking point. It was considered advisable to curred at a definite temperature. This bend-brittle temperadetermine whether this condition existed in a large number of ture was called the “high elasticity” temperature by Houwink elastomers. (6) and the “elasticity” temperature by Whitby (14). An DETERMINATION OF POINTS OF CHANGE apparatus was developed by the Thiokol Corporation (If) for determining the flexibility of rubberlike materials at low BEND-BRITTLE TEST. An adaptation of the Hycar brittle test was run by dieing samples from a sheet 0.0625 inch thick to temperatures. The samples are mounted between two plates make the samples 3.0 inches long and 0.25 inch wide. Each end and bent by turning a crank. The apparatus was designed of the sample was placed in holes drilled in the test block. The so that the distance between the plates can be varied. The holes were 0.25 inch in diameter and 2 . 0 inches from center t o temperature of breaking was called the “brittle point”. center. This caused the sample t o form a n arch approximately 1.25 inches high. The samples are bent by placing a rod on the Kohman and Peek (IO)tested the brittle point by bending sample and pushing firmly. Thc rate is sufficient to bend each zt strip through 90” with a hammer blow. They found that sample in 1-1.5 seconds. the brittle temperature was independent of the dimensions SHATTER POINT.The apparatus (Figure 1) consisted of a steel and the bending angle; however, a high rate of deformation bar mounted vertically in a sleeve fitted with ball bearings for free sliding. The bar weighed 5 pounds and had a cutting edge at its was necessary for the duplication of results. Kemp (7) lower end of a 90” wedge, 0.010inch wide and 1 . 0 inch long. For applied this test to rubber samples and likewise made no very soft compounds the bar was dropped from a height of 3.0 differentiation between a bend-brittle point and a shatter inches. This distance was sufficientt o shatter the sample withpoint. A similar method (6) was reported from Germany out cutting it. For samples above 50 Shore hardness, a height of 5.0 inches was used. The base upon which the sample was except that, in place of a hammer blow, a falling weight was placed was hardwood. used to break the sample which was bent back upon itself. HAND BENDTEST. The sample was grasped by gloves kept in Selker and co-workers (IS) mounted their samples on a the cold box and bent with a continuous motion through 90’ at a wheel and, by rotation, caused the samples to strike against a rate from 1-1.5 seconds. It was found that the hand bend test 949
INDUSTRIAL AND ENGINEERING CHEMISTRY
9so
-
sile, elongation, snd hardness, were selected for the teet. Table I shows the resultsof the bend-brittle and shatter points corresponding to the plasticizers used. In all these compounds two points were noted that had differencefrom 518" F. There is SII indication that, while some plasticizers set &s freeze depressants,others, in spite of their low freezing points, do not depress the shatter p i n t as would be expected. Some plasticizers act a8 freeze depressants although their freezing points nre higher than the brittle or shatter point of the compound. Usually increasing the amount of plasticizer will lower the bend-brittle and shatter noints. Hardness is
T n s ~ r :I. E ~ . r . e cOF ~ PLASTICIZERR ON UEND-URITTLE AND S t r n ~ r i wPOINT IN A KBOPRENB FR COMPOUND' Plsaticirar ( 2 0 . 5 Parte by Wt.)
Band-Brittl= Point.
F.
nutri eeiiosoive scetaLe
Shatter Point. 0
Diisohutyl adipata
-90
Dibutyl sebacate phthsl&te
--m YV -82 - 82
Dismyfi phtli8iaW Diespry1 phthslste
F.
--6269
-S2
-76 -E7 -62 - E7 - 69 -E2 -62
-
Plrs**l;i.ei SC 90 n a t o m ethv. *it No. phosphate 10 -YO -s2 -74 -44 TP-IC plaatioirer a Base formuin: Neoprene P R 100.0.stea~ioacid 1.0, Flrctol H (anti. oxidant) 2.0.*"lfur 1.0, lithsrge 5 . 0 . white fsotice 50.0, Tbsrmax 1uO.U. weramn 1.0.Eenltcv.5.
p p
Yynthetir;
Bteario acid
zinc O I i d B suliur P a 3 blmk Circo oil
Whiting ParasjD
Altar
Thioner DPO Permd".
cantax Tvida Bemom soid
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Shatter
Point Apparatus
waz pmaetieally identical with the bend-brittio test; therefox, the latter W M used in place 01 the hand bend test in the following TUOS.
The cold box used wyaj described bv the du Pont Comonnv ( 1 ) .
box. EFFECT OF VARIABLES
EFFP~T OF PLASTICIZERS. To a Neoprene FR base compound equal amounts of various plasticizers were Bdded and the compounds cured. The penk cures, as indicated by ten-
EFFECPOF TIMEOF FREEZING.Rubber compounds exposed to low temperature have a higher brittle temperature due to progressive crystallization. Selected types of synthetics representing the butadiene-styrene, hutadieneacrylonitrile,chloroprene, and polysulfide rubbers were frozen for 24 hours at each temperature (Table IV). A l l of the samples tested broke at a higher temperature after 24 hours of freezing, and the shatter point was higher than the hendbrittle point. Since the samples were 0.0625 inch thick, it
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INDUSTRIAL AND ENGINEERINQ CHEMISTRY
Sptember, 1943
EFFECTOF DROPPINO WEIOIFTON SHATTER POINT.As TABLEIV. SHATTERAND BEND-BRITTLE POINTSOF ELASTOMERS -Frosen Shatter point, F. 5s Qumrabbcr (light load) -72 Buna S -31 Butaprene NM - 3 rene NXM 49 Buty 8-1.5 - 3 Hyoar OR-IS - 6 Hyaar 0 6 1 0 -72 Hyonr O S 2 0 40 Neoprene ON - 3 Neoprene IL8 Nore 01 P-346-D Thiogol FA Thiokol RD 20 Plasticized polyvinyl 58 chloride-acetate Plwtioised polyvinyl butypte - 4 Plastioiied ethyloellulose 22
-
-
+2:
-
-
(.
b
In good oondition at -76’ In good condition at -50’
15 Min.Bendbrittle point, F. 67
-
a
-49 - 6
- 6 13
--4913 - 13 0
+-4923
-67 -49 -58
-Frosen Shatter point, * F.
..... -60 -11 ..... -42 ..... -49
-35 ..... ..... -a5 .....
.....
..... .....
24 HourBendbrittle point, F.
’
..... a
-35 ..... 6 ..... ..... -55
-45 ..... ..... -42 ..... ..... ..... .....
F. F.
mentioned, Kohman and Peek (10) observed that the dimensions of the samples and the bending angle did not affect the brittle temperature, although a high rate of deformation wm necessary to duplicate his results. The size of the sample did not seem to affect the bend-brittle or shatter points except that the dropping weight must be dropped from a height sufficient to produce a sharp blow without cutting the unfrozen sample. This, by experiment, determined the distance through which the weight fell. There is a definite minimum height for various stocks and hardnesses. Trial and error showed that, under 40 hardness in a Shore Type A durometer, a distance of 3.0 inches met these conditions. If the sample gave a shatter affect a t a minimum height, dropping the weight through a greater distance did not raise the shatter point. The relation between the thickness of sample and height of dropping was not determined. OF LOADING t)N HARDNESS IN BBNDTABLB VI. EFFECT BRITTLE AND SHATTER POINTTESTS
was felt that the shorter time gave a complete quick freeze without complete crystallization, and that the longer interval indicated a progressive condition if not crystallization. The greatest change, possibly due to crystallization, was found in the Hycar OS-20 compound. It is well known that the butadiene synthetics do not crystallize easily. However, there are some indications that at very low temperatures crystallization does take place to some extent. EFFECT OF RATEOF BENDINO. For the past two years this laboratory has recognized that the rate of bending varies the bend-brittle temperature. An instantaneous bend point was obtained in the same manner as the bend-brittle point except that the rod was given a hard, sharp push in an attempt t o duplicate as rapid a blow as possible; however, the rod was always in contact with the sample, Table V indicates that the instantaneous bend gives a temperature that approaches the shatter point. If the rate was truly instantaneous, the bend test would be the same as the shatter point. A similar condition was noticed in the hand bend test when the samplewas quickly bent back upon itself in contrast to a slow bend over a 1-1.5 second period. The same conclusions were made by Kemp and co-workers (8) in their studies of the brittle temperature of rubber under variable stress. The shatter point appears to be the true brittle temperature as the bend test varies with the rate, and approaches the shatter point as the rate of bending increases.
Neoprene FR
Hardness, Shore A 40 50 62 70
Hyoar OR-15
85 40
Neoprene QN
70 40
50 60
50
60
70 80
Band Brittle, F. -80 -7s -78 -77 -75 38 30 -25
--
-22
--60 --4245
48
-36
Shatpr Point, F. -75
--75 75 -73 -72 -31 - 23 -23 24
-
-
89
---3834 36
-33
There is probably a relation between bend-brittle and shatter points. I n most rubberlike materials a large number of variables would affect the results, such as type of materid, loading, softeners, curing agents, and time and temperature of curing or processing. Fuoss (9) found that, in a sample of plasticized polyvinyl compound, there was a change in electrical properties several degrees higher than the brittle point, This electrical point may correspond to the shatter point, which is higher than the bend-brittle point. I n designing specifications for low-temperature testing, it is desirable to state the property or properties to be tested, the time and temperatures of test, the size of specimens, the procedure and apparatus to be used, and the rates of operation of the test. LITERATURE CITED
AND BEND-BRITTLE POINT TABLEV. RATNOF BENDING
Butaprene NM Hyoar 0 6 1 0 Neoprene ILS Plaetioized polyvinyl ohlorideaoetate Plasticized polyvinyl butyrate
Bend-Brittle Point, F. One-sea. Imtantaneoun bend bend -49 -83 13 - 8 -13 - 5
--4967
-62’ - 8
Spatter point, F. -31 - 6
- 3
-- 58 4
EFFBCT OF HARDNESS TaROUOH FILLERS. Harder Stocks, as measured with the Shore durometer, appeared to have a more narrow range between the bend-brittle and shatter points, possibly due to lower percentage of rubberlike material in the compounds, and also to the dilution beyond the reinforcing range which decreases the cohesive bonds. Table VI shows this for three types of materials. Each compound of each type varied only in the semireinforcing carbon black content.
(1) D u Pont Co., Rubber Chem. Div., Rept. 421, 17-18 (1942). (2) Fuoss, R. M., J. Am. Chem. Soa., 63,369,374(1941). (3) Garvey, B.S.,Jr., IND. ENQ.C H ~ M34, . , 1321 (1942) (4) Qarvey, B. S., Jr., Juve, A. E., and Sauser, D. E., Ibid., 34, 167 (1942). (6) Houwink, R., “Elasticity, Plasticity, and the Structure of M a t ter”, pp. 45-8, 65, 186-9, London, Cambridge Univ. Presa, 1937. (8) I. G. Farbenindustrie, Kunststoffe-RohstoffLaboratmiurn,28, 171 (1928). (7) Kemp, A. R., J . FrankZinInst.,211,37 (1931). (8) Kemp, A. R., Malm, F. S., and Winspear, G . 0..IND.ENO, CHEM.,35,488 (1943). (9) Kooh, E.A.,Kautschuk,16,161 (1940). . , 81 (10) Kohman, G.T.,and Peek, R. L., Jr., IND. ENQ.C ~ M 20, (1928). (11) Martin, S. M., Jr., Rubber A g e ( N . Y.), 52,227 (1942). (12) Russell, J. J., IND. ENO.CHEM.,32,509(1940). (13) Selker. M.L.,Winspear, G . G., and Kemp, A. R . , Ibicd., 34,15.7 (1942). (14) Whitby, G.S., Tram. Inst. Rubber Ind., 6,46 (19SOg. PBBIBBNTB~ before the Division of Rubber Chemistry at trha ZWth h4eetiag of the AMERICAN CHEMICAL SOCIETI, Detroit, Mioh.
