Load-Versus-Compression Characteristics of Gelatins, Fibers, and Other Materials ERWIS J . S i X L , Industrial Trust Bldg., Proridence, R . I .
The qualities of gelatins, jellies, and similar materials are physically determined by gel strength, gel factor, yield point, elastic hysteresis. etc. The author has developed a precision apparatus with which it is possible to determine these characteristics with an exactness of four significant figures under reproducible conditions in a simple manner. The method
and the instrument lend themselves also to measurements of food products, greases, canned and cooked fruits and vegetables, and other plastic and semiplastic materials. In many instances the universal gelometer effectively supplements findings obtained by plastometers, compressometers, elastometers, and stiffness testers.
G
ELATIXS and glues are purciiaaed substantially on the basis of their Bloom number-that is, the number of grams necessary to push a cylindrical plunger 12.75 mm. in diameter 4 mm. deep into a standard sample of gelatin (1) Gelatins of high gel strength require more weight (applied by shot loading) for that purpose, while softer gelatins require less shot. While the penetration of 4 mm. was chosen because in most instances, a t the concentrations described in the original paper, no yielding occurs a t penetrations as low as 4 mm., we have to go beyond that depth to investigate the yield point, the gel factor, and elastic hysteresis. Furthermore, because it has been found necessary not only to load the specimens but to observe their behavior a t a systematic reduction of the load, shot loading has been found impractical. Instruments of this type should be designed to give not only data as to penetrating properties, but such factors as elastic recovery, hysteresis phenomena, gel factor, and yield point, which cannot be practically determined with the Bloom gelometer. The gelometer described here, which has been used successfully in research and practice for several years,
makes possible the precise evaluation of all these factors in a reproducible manner, and does not conflict with patents that restrict the use of the Bloom gelometer. It is manufactured commercially.
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The Apparatus The general physical principles of the apparatus are rather broad (1, 8). The apparatus consists substantially of a sensitive balance with a movable tare weight (Figure 1, right). A sample prepared under standard conditions is placed upon the pan of this scale and a plunger with a hardrubber tip is depressed into it. The piston, with rack and pinion drive, vernier, and attached reading glass, is shown on the left side of Figure 1. A load-versus-depression characteristic can be determined by depressing the plunger into the sample a given distance and moving the weights on the balance arm to bring the pointer back t o zero. Stiffness as well as elastic recovery may be tested in this manner. Additional agents such as glycerol, sodium benzoate, zinc sulfate, etc., i n h e n c e the elastic recovery and elastic hysteresis.
Determination of Gel Strength The method is applicable not only to glues and gelatins, but also to rubber, biological matter, cheese, fruits, jellies, m e a t , g e l a t i n desserts, textile fibers, canned food, ice cream, bread, and other baked products, mayonnaise, hard and soft gelatin capsules, and marshmallows. The recovely characteristics and their change with age, moisture content, and heat are particularly important for marshmallows. The drier, older, and harder the marshmallows are, the less good is their recovery. Because the elastic recovery in the wet state and in the dry state has a definite relation, measurements of elastic recovery plastic deformation and elastic hysteresis are of considerable scientific and industrial importance. A shot-loading instrument does not readily permit the deduction of load and, therefore, the customary types of gelometers are useless for this purpose. There is no such t h i n g a s a n i d e a l texture, because in one instance a high stiffness is d e s i r a b l e w h e r e a s i n another plastic
FIGURE1 82
defoi ination and a high grade of elastic recovery are preferable. Specification3 must be adapted to the requirements of the manufacturers of candy, niarshmallows, and other food products. Factors such ab clarity in color, freedom from obnoxiou~ odors, viqcosity and gluing pon-er (tackiness), surface frictioii and smoothness (5’), swelling, hydrogen-ion concentration (IO), a h , isoelectric point. etc , are albo of importance i n cert:iiii applications of gelatin. I
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1
L
E I
/
I
9 FIGL-RE 2
THE S.4MPLE. 111 all instances, the samples muat be prep u e d analogously and properly in order to get con~parablr conditions. With the great inultitude of possible materials ranging from tooth paste t o jello, it is not practical to give .specifications for the preparation of each sample. Instead, q)ecifications are presented as they apply to the measurement of gelatins, in order t o get the 8ame results as Kith the standa d Bloom test, (1). COSTAINERFOR JELLY. The container recommended is ari extra widemouthed bottle of the following specification*: capacity, 150 cc. ; inside diameter of body, 59 mm.; outside diameter, titi mm.; height over-all, 85 mm.; t’o take rubber stopper S o . 9. Bottles should show a variation of not more than 1 mm. iri Average internal diamet,er from these dimensions. The tapered ,topper is cut in half and the upper portion is perforated by plunging a red-hot, 2.5-cm. (1-inch) rod through it at the center The upper half of the stopper is used to obtain a snug fit in the neck of the bottle and the air vent prevents the stopper from being blown out during the melting and heating of the sample. The containers and stoppers must be clean and dry. To cut the rubber stopper in half, a tool has been developed -1, Miner D. Given, of the Eastman Gelatine Corporation (Figure 2) : The rubber stopper, A , is inserted between two holding bars, B and C, and depressed by a bar, D, with the aid of the pliers, E and F . The rubber stopper, held rigid in this manner, i:, cut easily and straight Tvith a knife that has been wet with caustic soda. Instead of making a 2.5-cm. (1-inch) hole, a small wire rod or knitting needle, 2 mm. in diameter, may be heated and plunged red hot through the center of the st’opper. This provides a small air vent and facilitates heating and lifting the stopper after heating the sample. This smaller hole is a better protection against impurities contaminating the surface of the gelatin, and guardagainst undue evaporation of the u-ater in the sample while heating. Moreover, it can be covered easily with the finger and the liquid hot gelatin tilted until there are no air bubbles on t,op of the gelatin. Violent agitation of the hot solution is t o be avoided because it causes damage in the viscosity, and in some cases persistent ioams are formed which may interfere with the subsequent jell test. It is important that t,he plunger rest upon a clean gelatin surface, not upon jelled and frozen foam. SAMPLING AND PREPARATION OF SAMPLE.. I grab sample of ground glue or equivalent in the rat’io of 28 grams per 45 kg. (1 ounce per 100 pounds) is taken a t random from not less than 20 per cent of the containers. The total number of samples so taken should not be less than ten; u-hen the number of containers in the shipment is less than 10, a sample should be taken from each container . A Sam le of sheet, flake glue, etc., is taken as for ground glue, except, t l a t in case of large lots the portion taken from each container is reduced proportionately so that the total sample does not exceed 4.5 kg. (10 pounds). The entire sample is then ground to at least 4-mesh, or finer if it appears necessary, so as to minimize weighing errors and to shorten the soaking period. The entire ground sample is thoroughly mixed and quartered down until reduced to two 454gram (1-pound) samples, which are placed in air-tight containers. One of these is used for test and the other held as a reserve sample.
COKCESTRATIOX. For 105 cc. of distilled water 7.5 grams of gelatin are used-a concentration of 1 to 14. This is called the gelatin scale. For glues, which have a lower gel strength, 15 grams of glue are dissolved in 105 ce. of water-concentration of I to 7 . This is called the glue scale. An automatic pipet with a three-way stopcock may be used to deliver the water correctly in routine work. The dissolved sample in the tight,ly stoppered container is allowed to cool to 45” e.,preferably in a \vater bath. The fingei is then placed over the perforation in the stopper and the container is inverted several times to mix in the water that has condensed on the iralls of the bottle and the under side of the stopper. The containers are then placed in a constant-temperature chill bat>hfor not less than 16 nor more than 18 hours at 10” * 0.1 C.! taking care not to insert too many samples to maintain the temperature capacity of the equipment. The temperature variation and reasonable a itation of the chill bath are important. Unless the batch contrjs the temperature within the limits prescribed, it will be necessary to use “standards” and thus make relative, not absolute, tests. Thermostatic recording and regulating equipment may be used. TECHWC OF MEASUREMEST. After the sample has been made a beaker of cold water, which has as near as possible a temperature of 10’ C., is placed upon the pan of the balance. The plunger is inserted into this chill bath and left there for 2 minutes, so as t o be reasonably close to the temperature of the testing specimen.
PENETRATION
mm
FIGURE 3 The tip of the plunger is made of hard rubber in order to reduce the thermal conductivity. After t’his tip has been chilled for 2 minutes, if the samples are brought in reasonably fast, the temperature of the plunger surface is the same as that of the sample to be tested. The plunger is now dried with a towel. (Between samples the plunger is cleaned.) The arm on the rack and pinion drive is lifted so that the reading on the vernier scale is exactly zero, and the plunger is lowered first with the acme screw only (at the right side of the arm). By turning the knob on top of the screw, the plunger can be lifted or lowered until it just touches the surface of the test sample in the bottle. This can be asccrtained with great precision by watching the pointer a t the right side of the scale, which shows the motion of the pan about four times enlarged. Before lowering the plunger, a sample contained in the water bath is calibrated by t,he tare weight (the metal cylinder running upon a screw in back of the scale). By calibrating with the scale pointing at zero and all weights in zero position, readings taken will be direct load-versus-penetration readings, and no subtraction has to be made for the tare. After the sample has been brought into position, the acme screw is clamped tightly n.ith a clamp in front of its bearing. All
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the motion applied to the plunger must noT3- come from the rack and pinion drive. The drive is now lowered until the vernier reads exactly 0.40 em. Because the plunger depresses the sample, t,he pointer at the right side of the scale points upward. The weight in grams necessary to bring the pointer back to zero if the standard plunger is depressed 4 mm. into the gelatin sample prepared as above is called the Bloom number. This standard depression can be read with the precision of four significant figures. The maintenance of this constant depth is more precise than a gap with contact points that may arc under t.he influence of electric current and burn out,, as in some apparatus. Furthermore, the weight can be read t o 0.1 gram, which is one more significant figure than the commercial types of gelometers give. The new gelonieter is independent of electric batteries and clean and dry shot. This instrument, therefore, gives a t least one decimal fraction more than the standard testing instrument, both in the precision of the load and in depth readings. Several important characteristics can be determined readily with the new gelometer which cannot be ascertained in a practical manner rrith prelious instruments (3, 6).
Elastic Hysteresis
increased to 30 grams, n-hich is practically maintained until zero load.
Measurements on Staple Fiber While the methods described above apply primarily to the evaluation of plastic and semiplastic materials, the apparatus lends itself also to the investigation of fibrous mat'erials such as wool, and staple fiber in bulk, and fabrics (4, 6). For making tests for resiliency and elastic recovery, loadversus-compression characterist'ics were t'aken on a standard sample (Figure 4). First a plunger 1.27 em. (0.5 inch) in diameter is attached on top of a standard sample first of spun rayon and then of synthetic wool, weighing 16.9 grams each. As we are mrking within one test jar and are dealing ITith identical quantities, the only variable in the load-versus-compression characteristic beheen the two is the elasticity and elastic recovery. After the jar with contents is tared out, the circular plunger 1.27 em. (0.5 inch) in diameter is lowered with the acme screw until it touches the surface of the sample. Weight is then moved out upon the balance arm and the plunger is lowered until the pointer of the scale again indicates zero. By this means we arriye at the T-ieight necessary t o produce a given compression, as indicated by Table I.
Weight cannot only be applied with the gelometer tlescribed above, but can be reduced after load application has taken place. Then, by lifting the plunger point by point, the elastic recovery of the product investigated can be measured. Some plastic deformation does take place, but the total amount varies for different' materials, and is not necessarily proportionate to gel strength. In other words, in determining only the penetrat,ion for 4 mm., we have no nieans of predicting whether the jellying substance will recover elastically or will remain plastically deformed. Figure 3 shorn a hysteresis loop determined by first increasing the weight and measuring the penetration which brought the balance back to zero, and thereafter decreasing the weight again. The area of this loop is proportionate to the internal distributed forces that counteract a recovery of the sample to zero position. Here a perinanent deforniation of 0.03 em. has been effected. Moreover, the difference between load applied and load removed per unit of penetration is m a l l at the beginning and increases thereafter, At 3.2-mni. depression a load difference of 20 grams exist,s between the ascending and the descending branch of the characteristic. At a depression of 0.60 em., however, this weight difference has PENETRATION
mm
FIGURE 5
COMPRESSION
FIGFRE4
cm
If, instead of the staple niateiial, n e had had a 100 pel cent elahtic hpring in the jar, this spring would have returned along the curve through which it Tyas depressed first. On the other hand, if we were dealing with 100 per cent plastic body, the elastic recovery would have been zero 01' a permanent plastic deformation would have been achiered that would be identical to the maximum depression made. Between the extremes of 100 per cent elastic recovery and 100 per cent plastic deformation, there is the relative elastic recoreiy of the materials compared (Figure 4). Materials are th? more elastic, the narrower the area of the loop. The relative characteristics of elasticity are inversely proportionate to the area of the hysteresis loops. The elastic recovery of the synthetic aool is between two and three times as good as that of the original spun rayon used in comparison, particularly in those areas in which characteristic strebses during actual operation occurred. At 200 grams compression the diameter of the synthetic wool loop is 1.6 divisions, while the spun rayon loop is 3.0 divi>ions.
FEBRUARl 15, 1938 TABLE I.
ASALYTIC.41, EDITIOl.
LO.4D-~ERSUS-COMPRESSION
Filled with Spun Rayon Grsms Compression 0
10
20 40 60 80 190 290 390 490 590 490 390 290 190 90 60 40 20 10
n
Cm. 0.0 0.79 1.1 1.24 1.35 1.47 1.82 2.09 2.28 2.53 2,il 2.67 2.61 2.52 2.42 2.22 2.07 1.83
1.61 1.45 0.94
CHARACTERISTICS
Filled with a Kew Type of Sgnthetic Wool Grams Compression Cm. 0 10 20 60 80
190 290 390 490 590
490 390 290 190 90 60 40 20 10 0
0 0 0.44 0.68 0.90 1.00
1.28 1.42 1.62 1.74 1.89 1.86 1.83 1.73 1.59 1.43 1.33 1.2G
1.24
1.03 0.55 j
The same principle of nieasureIneIlt applies to 1rially other. substances. Because of its inherent siniplicity, accuracy, and ryide range, the neiv gelonieter is applicable in practically 211 instances where various forma of plastometers have been used heretofore, particularly in measurements 011 rubber and rubber compounds, synthetic rePins in combination with textiles such as crush-proofing tests for urea-formaldehyde-treated velveteens and rugs (instead of conipressometers), and in taking complete load-versus-compression characteristics on folded or rolled sheetlike niaterials such as paper, cloth, and metal foils, I n this latter inst'ance the universal gelometer nicely supplements the results obtained with tlie stiffnew tester. T h e apparatus can also be adapted readily for tests heretofore taken with the elastometer.
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the plunger penetrates into the material. A t this point the elastic limit has been trespassed. The yield point and subsequent slope of the load-penetration characteristic are influenced by the mixt'ure of compounds ordinarily called gelatin. The elastic liniit is not necessarily influenced by the Blooln number of the gelatin. Everything else being equal, the yield point a t a Comparatively low st'iffness number may lie higher. I n other words, the maximum stress to n-hich such a jelly may be subjected and still recover completely is not necessarily dependent upon the Bloom number. Figure 5 will make t h k clear. I n this instance curve b has a higher Bloom number as shown by a load of 142 grams a t 4-mm. penetration (point A ) , yet the maximum load which this material can sustain lies at 190 grams (point P ) . On the ot,her hand, the comparatively softer material represented by curve (I, the Bloom number of which is 46 granis lower (point B ) ,yields a t a substantially higher load-namely, 246 grams (point R ) . Khile b will give suitable data for gelatins where stiffness only is desired, curve A is preferable for jellies where the ilesirable characteristic is resistance against filial tleforination. -4 certain amount of elastic giv? is possible ivliich can raise the yieltl point, substantially. The question of yield point is also important from a different T-iejT-point. Comparative meaSurements on Tvhich & report is to be made SliOrtly sho~vthat, t,he yield point stays in a certain relation t o the maximum tensile strengt,h to \-hi& the gelatill films or materials covered Tvith gelatin films can he subjected. This is of particular interest, in the paper, textile, leatller, and photographic industries. 320 300
80
Elastic Recovery Instead of taking complete hysteresis curves, it is possible merely to determine t h e elastic recovery which a gelatin or other material is capable of sust'aining after a certain load has been applied during a given time. I n many instances this simplified measurement xi11 give the desired first-hand information about the forces of elastic recovery. Care should be taken t o work with the same total amount of time in all instances of comparative measurements; otherwise the time factor influences the readings. Deformation and recovery are not instantaneous; a small load applied over a considerable lengt'h of time may produce more permanent deformation than a comparatively larger load applied for a rather short time. Questions of rheological nature (such as plastic flow) enter here. The technic for such determinations is to make the standard s ecimen and calibrat'e it with the tare weight upon the scale, d e n lower the plunger until it touches t'he surface of the gelatin. A given weight-for instance, 100 grams-is applied for one minute and the plunger is lowered 7%-iththe rack and pinion drive until the pointer at, the right hand of the scale again indicates zero. The depression is now read on the vernier. The weight of the balance is returned to zero and the plunger is lifted on the rack and pinion drive until the hand of the scale again points to zero. Cnder these conditions the plunger will not return to zero but a cert'ain amount of impression will remain permanent. It is read directly on the vernier. The recovery of jellies to the origi,lal form in Jyliicll tliev were cast svill be the more colnple,e the better the of gelatin test specimens after removal of the load.
reco3-eri.
Yield Point The yield point appears at the point of the load-versuspenetration curve where, without further addition of loatl,
60
40 20
200 0
9
80 60
40
{
20 100
80
60 40
20 '0
I
2'
3
4
5
6
PENETRATION
7 8 8
9
10
mm.
FIGURE6
It is imperative that means for eraluating the physical characteristics of jellies shall have a range wide enough to continue with the measurements until tlie final break. CFor this purpose facilities for penetration in excess of the 4 mm. t o which the standard test is limited must be used, as the yield as a rule occurs at a higher penetration than 4 mm. For most practical instances 4 mm. lies below t>heyield point. In viev of this fact, care has been taken in the apparatus clevelopetl byithe m i t e r to provide for ample transport of the
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standard plunger and for sufficient total weight to continue experiments as far as the yield point and beyond.
The Gel Factor The importance of a higher expression for gel strength waa first suggested by Sheppard in terms of the torsional elasticity of the gel (9). The dope of the load-versus-penetration characteribtic alone, as indicated by one point on this slope (the Blooiii figure) is not indicatire alone of the gel strength of gelatin. What is indicative of gel strength is the total work nece>saiy before the final break in the jelly occurs. It is proper. therefore, to determine this gel factor in teiiny of the area of the triangle included between the load-versubpenetration curve and the abscissa. This total area O.IK (Figure 6) is then
A
= ‘/z
the breaking load X penetration (gram per cm.)
I n this specific instance the gel factor, J , would equal x 294 X 0.75 = 110.6 (gram per cm.). This result, niultiplied by a numerical constant, C, can be compared with the expression for proof resiliency, as suggested by Sheppard iii terms of the torsional elasticity of a standard jelly cast. The term “gel factor” was suggested by the writer in 1935 in order to avoid confusion with the term “jelly strength” which in comnion use today is assumed to be proportionate to the Bloom number only (6). For industrial control tests as well as for research ~\~Orli, it has been found more accurate to determine the value of
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gelatin in terms of the gel factor. It is entirely possible ttJ have a high gel factor despite a low Bloom number and vice versa. \$‘hat seems to be most characteristic for gelatin ic. the amount of total work necessary before the final break in the jelly occurs, and not the amount of load to produce one haphazardly taken penetration depth. Tests should represent ronditions that are actually of primary importance for the product, and not necessarily certain points that are conI enient for a limited number of producers. It is, of course, easily possible to attach a different plungei Iiead of conical, needle-shaped, hall, 01 otliei desirable form. In this manner the apparatus lends itself also to penetrometer iiieawrements, such as those deicrihed by T d z , Culpepper, Jloon, and hleyers ( 2 ) ,
Literature Cited DeBeaukelaer, F. L., Powell, J. R., and Rahlmann, E. F., Isi). EXG.CHEM., Anal. Ed., 2, 348 (1930). Liita, J. hl., Culpepper. C. W., Moon, H. H., and Meyers, D. T., Canning A g e , 14,404, 414, 428 (1933) : Canner, 77, 14 (19331. Saxl. I. J., Electronics (January, 1937). Saxl, I. J., Melliand Teztilber., 19, 47-8 (1938). Saxl, I. J., Physics, 7, 62-6 (1936). Saxl, I. J., Te.rtile Research, 8, 5-14 (1937). Sheppard, S. E., “Gelatin in Photography,” p . 217, New York, D. Van Nostrand Co., 1923. Sheppard, S. E., and Elliott, F. A . . J . Optical SOC.4 m . , 9, 181-4 (1924). Sheppard, S . E., and Sweet, ,S. d . , IND.ENQ.CHEM.,15, 571 (1923). Sheppard, S.E., Sweet, S. P., and Henedict, A . J., J . -47n. f ‘ h e t n . Soc., 44, 1857 (1922).
Discontinuous Fractional Extraction Apparatus Utilizing Reflux H . E. HERSH, K. A. VARTERESSIGV. R . A . RUSK, AND &I. R . FEUSKE The Pennsylbania State College, State College, Pa.
Solvent-extraction processes are very useful in effecting the separation of complex liquid mixtures by physical means. This is especially true if reflux conditions and countercurrent contacting of the phases are employed. In this way, the sharpness of the separation is better and the segregation of the components more complete. This paper describes a suitable small-scale
F
OR several years this laboratory has devoted some efforts
to the development of the fundamentals of estraction, to the design of efficient estraction equipment, and to the application of these in the field of solvent refining of petroleum products (3,5,10-13). This paper describes a discontinuous fractional extraction apparatus, developed in this laboratory, that is novel in design and satisfactory in operation. It is particularly valuable in the analysis of lubricating oils where a given batch of material must be resolved into its component parts. The results of such analyses will be given in subsequent papers.
Principles of Design Aside from auxiliary equipment for the control of temperatures and of rates of flow, the apparatus consists of three
batrh-extraction proress ulilizing reflux for conditions where the solvent is either lighter or heavier than the liquid undergoing treatment. The apparatus is shown to be reproducible, efficient, and practically selfoperating. It permits the separation of a liquid mixture by solvents into as many fractions as desired. iliain sections: a leaching section, a countercurrent cotitacting section, and a reflux-producing section. The solvent is continuously introduced to the leaching section where the batch of oil to be solvent-fractionated is charged. There, a portion of the oil goes into solution, forming a solvent phase which separates because of density diffpi ence and flows into the contacting section. I n general, the dissolved portion of the oil will contain the constituents of the charge in proportions that are in the order of their solubilities in the solvent and of their amounts in the charge. From the contacting section the solvent phase passes into a still where i t is stripped of part or all the solvent The resulting oil phase is returned to the contacting section where it flows countercurrent to the solvent phase; in this section the oil and solvent phases interact, with the result that the solvent phase becomes I
