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STIFFNESS MEASUREMENT OF AEROSOL FOAMS Reproduced by permission from AEROSOL AGE Volume 8, No. 7, July 1963, page 33 By Paul Sanders "Freon" Products Division E. I. du Pont de Nemours & Company Wilmington 98, Delaware cn ro ID rH a r Stiffness Measurement of Aerosol Foams By Paul A. Sanders Technical Associate Freon* Products Laboratory E. I. du Pont de Nemours & Co. Wilmington. Del. O ooo A EROSOL foams have a number of qualities which "* make them desirable for many uses. They may be applied to any given place and they are attractive in appearance. Foam formulations are economical since they usually employ only a small quantity of propellant. Their physical properties, such as stability, stiffness, and wettability may be varied to meet the needs of different applications. suited for making comparative stiffness measurements on aerosol foams, they apparently have not yet been widely used in the aerosol industry. It is the purpose of this paper to illustrate the use of the Cherrv-Burrell curd tension meter for making comparative stiffness measure ments on aerosol foams and to investigate any possible relationship between the stiffness values and other proper ties of aerosol foams. A foam property such as density, can be measured accurately and easily if the foam has sufficient stability Apparatus and Procedure to permit a given volume of foam to be weighed. Other properties, such as the resistance of foam to deformations have also been used to characterize qualities of The Cherrv-Burrell Curd Tension Meter* is illustrated in Figure 1. It is automatic and gives direct readings. v foams, but often the deformations were of a rather in definite kind.1 One method, for example, was to deter mine the rate of descent of objects such as glass tubes into the foam. Another method of demonstrating the stiffness or strength of foam has involved the ability of the foam to maintain an object like a pencil in a verti cal position without allowing it to tilt to the side. In 1952. R. A. Foresman, Jr. reported that the three most important characteristics for aerosol shave creams were strength lor foam stiffness), stability, and wetta bility.- In discussing the foam strength of the product;, he disclosed a procedure for measuring the comparative strength or stiffness of aerosol foams which involved de termining the grams of force necessary to insert a stand ard knife through the foam at a uniform rate of speed. The instrument used by Foresman was an American Standard Dairy Curd-O-Meter, manufactured by the Sub marine Signal Co. in Salt Lake City. Following Mr. Foresman's original work, a similar procedure for measuring foam stiffness was adopted at the "Freon" Products Laboratory. Since the American Standard Dairy Curd-O-Meter was no longer available commercially, a Cherrv-Burrell curd tension meter was used. Although these curd tension meters are admirablv * Freon :s Du Pont , reentered trademark for its Huorocarbon propeilunts. Figure No. 1 In carrving out the test, the operator fills a suitable vessel with the aerosol foam and any excess foam is removed with a scraper. A crystallizing dish. 9.7 cm in diameter and 4.7 cm deep, has been found to be a satis factory container for the foam. Since the volume and weight of the dish can be determined, it was compara tively easv determine the foam density immediately EID11634 before the stiffness measurements were carried out. Foam densities are usuallv given in the tables along with stiffness results. The dish with the foam is placed on a scale platform which is directly underneath a specially designed curd knife and the rotating dial on the scales is adjusted so that the pointer is set at zero. The scale has a capacity of 500 grams and is graduated in 2 gram divisions so that measurements mav be recorded to the nearest grarn. After the starting button has been depressed, the curd knife is driven downwards at a constant, controlled rate of speed of 1 inch per 7.5 seconds by a synchronous motor. After the knife has contacted the foam, the resist ance of the foam to penetration by the knife causes the scale platform to depress. The extent of the foam re sistance land therefore the degree of depression of the scale platform i is measured by the number of grams recorded on the dial. The instrument is adjusted so that the knife will automatically reverse before it touches the bottom of the vessel containing the foam. The readings are obtained in grams and provide a valuable means for comparing the stiffness of different foams. They also may be used to detect changes in foams occurring during aging or variations that occur as the container is discharged. Stiffness measurements are also suitable for detecting changes in foam structure that occur with modifications in the aerosol formulation. The application of the instrument for some of these uses is illustrated by the data in the following sections. Experimental Results I. Effect of Time There are certain limitations that must be recognized in attempting to characterize foam properties by numeri cal values. Foams may change rapidly after discharge and in many cases if a numerical value that describes a phy sical property is to have any significance, the time interval between discharge and the measurement must be specified. If a foam is comparatively stable and expands very little after discharge, then measurements of stiffness and densitv mav be made within a reasonable time after discharge and the results will be reproducible. Some foams, however, expand immediately after discharge and continue to expand for a considerable length of time. This is particularly characteristic of foams formulated with "Freon-114" propellant. In reporting the density of foams of this type it is necessary to specify the time after discharge that the determination was made since the density changes rapidly with time. It was rather surprising that although the densitv changes rapidlv, the stiffness remains relatively constant for the periods up, to 20 minutes after discharge. On the other hand, foams prepared with "Freon12 "Freon-114" > 40 601 propellant expand verv little after discharge ~o that both the densitv and the stiffness remain reiativelv i-nnslant for some time. These effects were illustrated by measuring the stiffness and density of foams prepared with 10% of either "Freon-114" or "Freon-12"/"Freon-114" I 40/60 I propellant and 90% of the following aqueous concentrate A: Aqueous Concentrate A Stearic Acid .............................................. Coconut Fatty Acid ................................ Triethanolamine ........................................ Polvethylene Civcol 406 .......................... Mineral Oil ................................................ Water .......................................................... 6.31% 1.58% 5.16% 1.05% 0.11% 85.79% The variation of the stiffness and density of the foams as a function of time after discharge is illustrated by the data in Table I. TABLE I Effect of Time Propellant Time After Discharge Stiffness < grams i Density {x/cc' "Freon'114" 2 Minutes 48 0.142 "Freon-114" 6 Minutes 48 0.086 "Freon-114" 10 Minutes 46 0.081 "Freon-114" 20 Minutes 51 0.073 "Freon-12"/ 2 Minutes 58 0.063 "Freon-114" <40/60' "Freon-12"/ 10 Minutes 62 0.061 "Freon-114" (40/60) Thus, with "Freon-114" foams, the stiffness remained essentially constant while the density decreased rapidly during the first 20 minutes after discharge. The stiffness and density of foams prepared with "Freon-12'7"Freon114" (40/60) propellant changed very little during the first 10 minutes after discharge. In order to obtain reproducible stiffness measure ments it, therefore, is not as important to adhere to a rigid time interval after discharge as it is for repro ducible density values. However, in order to standardize the procedure as much as possible, both density and stiffness measurements were carried out immediately after discharge. Both measurements were completed within two minutes. II. Sample Variation In order to obtain some idea of the variation in read ings that might be expected from different samples of the same formulation, five samples of an aqueous foam product were prepared using 10% "Freon-12' `'Freon114" (40 60) propellant and 90% of aqueous concen trate A. The samples were packaged in six-ounce lacquerlined tinplated containers and allowed to age for 24 hours before testing. The samples were shaken by hand immediately before discharge. The readings obtained with the five samples are listed in Table II. The average for the five samples is a stiffness of 72 TABLE II Effect of Sample Variation Sample Stiffness* 1 2 3 * 5 \umber_____________________________________________________________ < grams i 1 .................................................................................. 74 2 h0 3 .................................................................................. "7 i ................................................................. '1 5 ................................................................... 08 EID11635 r grams and the difference between the low and high readings is 9 grams. The major reason for the variation is probably differences in the sampling technique in layer ing the foam into the crystallizing dish. III. Effect of Variation in Propellant Concentration The fact that increasing the propellant concentration in an aqueous foam product increases the stiffness was first reported by Reed4. Reed observed that with a sham poo concentrate higher propellant loadings resulted in stiff, dry elastic foams and lower propellant concentra tions gave soft and less resilient foams. This characteristic can be demonstrated very effectively by measuring the foam stiffness at various pro pellant concentrations. Thus, a series of samples was prepared using from 5% to 50% "Freon-12"/"Freon114" 140 60) propellant with the remainder consisting of aqueous concentrate A. Foam stiffness and densities of the various formulations are listed in Table III. These data show very conclusively how increasing the pro pellant concentration increases foam stiffness and de creases foam densitv. TABLE III Effect of Variation in Propellant Concentration Propellant Concentration i Ft. % i Stiffness y grams) Density Ig/cc) 5 43 0.138 10 65 0.078 15 94 0.053 25 114 0.045 50 126 0.036 IV. Effect of Aging the Emulsion The effect of aging the aerosol formulation in the con tainer for periods up to one month was found to be almost nezlible upon foam stiffness and density. Thus, at least with the present tormulation. it is not necessarv to define foam stiffness in terms of some specific time after the formulation has been prepared. This, of course, may not be true of other formulations. The present study was carried out by preparing a series of samples formulated with 10% "Freon-12/"Freon114" (40/60) propellant and 90% aqueous concentrate A. The samples were packaged in lacquer-lined tinplate containers, shaken vigorously, and then allowed to stand at room temperature for various periods up to one month. After each aging period, the samples were shaken again and stiffness and density determined. Individual samples were used for each measurement so that no sample was discharged twice. The results of the measurements are given in Table IV. These data indicate that aging periods up to one month have little effect upon foam stiffness. There ap pears to be a slight tendency for the density to increase, but the densities differ so little that the significance of the trend is questionable. TABLE IV Effect of Aging of the Formulation in Containers Aging Period Foam Stiffness < grams > Foam Density lg/ccl 0 24 hours 48 hours 1 week 1 month 68.5 71.5 68.0 67.0 66.0 0.066 0.069 0.070 0.072 V. Effect of Extent or Discharge Mr. Foresman pointed out in his discussion of foam properties2 that there is a drop in foam strength as in creasing amounts of material are dispensed. The reason for this is as follows: as an aerosol product is discharged from the container, the volume of the liquid phase de creases and the volume of the vapor phase increases correspondingly. In order to maintain a constant pres sure in the aerosol container during discharge, it is necessarv for some of the propellant in the liquid phase to vaporize and mizrate to the vapor phase. In sprav products containing a relatively hizh pro- EID11636 Propellant "Freon-12" "Freon-12"/" F reon-114" 40/60' "Freon-12"/**F:eon-'. 1 15/35.' "Freon-114" "Freon-l2"/`*Freon-li'' i50/50 "Freon l-,\/"Freon*ll`` <30/70 * Propellant 142b Propellant 152a TABLE VI Effect of Variation in Propellants ------ Foam Stiffness (grams! ---------- Retention oj Stiffness Initial After Aging 2 Hours Density (g/ccf --------- Aner Aging Initial 2 Hours 56 80% 0.053 0.049 76% 0.069 0.058 48 83% 0.090 0.061 46 70% 42 64% 0.057 0.056 36 50% 0.061 0.0.58 37 78% 0.039 0.037 42 74% 0.042 0.039 % Drainagi 0 1.5 7.0 15.0 21.0 33.0 0 22.5 portion of propellant, the loss of propellant from the liquid phase to the vapor phase has little noticeable ef fect upon the spray characteristics as the product is discharged, since the percentage loss of propellant is small. However, in foam products, which are usually formulated! with a minor proportion of propellant, the change in concentration of propellant in the liquid phase as the product is discharged is significant and will cause a noticeable change in foam properties. The data in Table III. which illustrate the change in foam stiffness and density as the propellant concentration changes, predict that as the concentration of propellant in the liquid phase decreases during use the foam stiff ness also will decrease and the density will increase. This effect of discharge upon foam properties was shown experimentally as follows: a 12-ounce container of the aerosol emulsion was prepared with 10% "Freon-12"/ "Freon-114" <40/60) propellant and 90% of the aqueous concentrate A. After the container had been rolled for 10 minutes, it was discharged into the crvstallizing dish for stiffness and density determinations. The container was then reweighed to obtain the quantity of product that had been discharged. After rolling for an additional 10 minutes, stiffness and density were again determined. This procedure was repeated until the amount of product remaining in the container was insufficient for a stiffness measurement. The relationship between the extent of discharge of the can contents and the foam stiffness and foam density is illustrated in Figure 2. Foam stiffness remains relatively constant until almost 60% of the can contents have been discharged. Foam density, however, appears to increase almost from the start. The extent to which these changes take place during discharge will vary, of course, with both the quantity of propellant initially present and the type of propellant that is used. VI. Effect of Variation in Propellant The tvpe of propellant used in formulating aqueous aerusoi foams mav have a considerable effect upon foam properties, -urn as stiffness. This is illustrated bv the 'lata :n the Pillowing sections: A. Effect Upon Stiffness The effect of varvinc the propellant upon stiffness was checked with 10% propellant and 90% of two different concentrates. One was concentrate A, which has a slight molar excess of triethanolamine relative to the fatty acids, and the other was concentrate B, which has a slight molar excess of fatty acids. Concentrate B is similar to many of the products now marketed as aqueous shaving-lathers5 and has the following composition: Aqueous Concentrate B Stearic Acid ............................................ 7.06% Coconut Fatty Acid ................................ 1.72% Triethanolamine ................................... 4.40% Glycerine .................................................. 2.00% Water .......................................................... 84.82% The stiffness resultsobtained with a variety of propel lants and the two concentrates are listed in Table V. f TABLE v Effect of Variation in Propellant Propellant Foam Stiffness <. grams) Concentrate A Concentrate B "Freon-12" "Freon-12"/"Freon*114"' 140/60' "Freon-12"/"f reon-il4'` (15/85) "Freon-114" "Freon- 12"/"Freon-l 1 " <50/501 "Freon-12"/" 1 reon-11" (30/70 Propellant 142b Propellant 152a 56 72 48 46 42 36 37 42 53 67 48 41 35 43 43 45 Maximum stiffness is achieved with "Freon-12" "Fre on-114" (40/60) propellant. It is possible that some other ratio of "Freon-12" to "Freon-114" would produce stiffer foams, but this has not been checked. B. Effect Upon Stability In an attempt to determine if there were any correla tion between stiffness values and foam stability, the samples listed in Table V that were prepared with con centrate A were treated for foam stability as follows: 1. Foams were dischanted onto paper towels and EID11637 checked tor foam collapse at intervals. 2. Foams were discharged into crystallizing dishes and the densities and stiffness values determined before and after aging for two hours. 3. Foams were also discharged into glass funnels in serted in graduates and the percent drainage de termined after aging 13 hours. Drainage rates are considered as one ot the methods for indicating tiie stabiiitv of foams.0 No conclusions regarding relative foam stability could be obtained from discharging the foams onto paper towels. The foams were all too much alike in their stability by this test to distinguish any real differences. The difficulty in judging foam stabiiitv bv this test has been mentioned bv Mr. F oresman.* The results of the aging tests upon foam stiffness and foam density and the results of the drainage tests are given in Table VI. There appears to be little correlation between the initial foam stiffness and either the percent retention of stiffness during aging or the percent drainage. Foams with the highest stiffness values were not necessarily the most stable. There appeared to he a trend for the foams which showed the highest retention oj stiffness during aging to also exhibit the least drainage. There very well may be a correlation between the two. but certainly the correlation was not as pronounced as could be desired. The change in density during aging of most of the TABLE VII Properties of Commercial Shaving-Lathers Product So. i o 3 4 5 6 7 8 9 Foam Stifiness i grams) in 106 8 89 87 37 32 "6 75 Foam Density <g/cc) 0.067 0.084 0.093 0.093 0.116 0.092 0.097 0.091 0.107 % Drainage (24 Hrs.) 6.5 16.0 22.0 9.5 44.0 2.5 32.0 18.0 16.0 foams was very slight. Since most of the foams decreased slightly in density, this indicated that the foams expanded very slightly. VIII. Properties of Commercial Aerosol Shaving-Lathers Nine different commercial aerosol shaving-lathers were evaluated in duplicate for foam stiffness, foam densitv. and drainage. The results were averaged and are listed in Table VII. The data show that foams may have the same stiffness with different densities and. conversely, foams may have the same densities with different stiffness values. The data in Table VII confirm the previous conclusion that the foams with the highest stiffness values were not necessarily the most stable, as shown by the comparison of the foam stiffness results with the percent drainage figures. Thus, two products both having an intermediate foam stiffness value of 87, had the highest and lowest drainage rates, respectively, of the entire series. IX. Summary The use of a Cherrv-Burrell curd tension meter as an instrument for determining the comparative foam stiffness of a variety of aerosol foams has been illustrated. The effects of varying such factors as the type of propellant, the proportion of the propellant, and the age of the sam ples, upon foam stiffness have been presented. A com parison of other foam properties, such as stability and density, with foam stiffness, indicates that there is no apparent correlation. References: 1. Bikerman. J. J.. '"Foams: Theory and Industrial Applications", Reinhold Publishing Corp., New York. 1,1953) p. 129. 2. Forestnan. R. A.. Chem. Specialties Manufacturers Association. Proc. 39th Annual Meeting, December 1952. p. 35. 3. Cherry-Burrell Corp.. Bulletin No. 1P5132-M, issued November 1955. 4. Reed. F. T.. Chem. Specialties Manufacturers Association, Proc. 39th Annual Meeting, December 1952, p. 32. 5. Carter, P. and Truax. H. M., Proceedings oj the Scientific Section o> the Toilet Goods Association, No. 35. May 1961. 6. Schwartz. A. M., Perry. J. W.. and Berch, J.. "Surtace Active Agents and Detergents", Volume II, Interscience Publishers, Inc.. New York (19581 p. 460. E ID 11638 E. I. DU PONT DE NEMOURS & CO. (INC.) "FREON" PRODUCTS DIVISION WILMINGTON, DELAWARE 19898 FOR FURTHER INFORMATION Please contact one of the following district offices: 60 Glenwood Avenue East Orange. New Jersey 07017 Phone: 6"6-1112 (Area Code 201) 7 South Dearborn Street Chicago, Illinois 60603 Phone: 222-5000 (Area Code 312) 701 Welch Road Palo Alto. California 94304 Phone: 326-2840 (Area Code 415) 712 Main Street Suite 324 Gulf Building Houston, Texas 77002 Phone: 222-2468 (Area Code 713) 3503 North Hoyt Street El Monte, California 91734 Phone: 283-0548 (Area Code 213) or: Wilmington, Delaware 19898 Phone: 999-3620 (Area Code 302) The information contained herein is based on technical data and tests which we believe to be reliable and is intended for use by persons having technical skill, at their own discretion and risk. Since conditions of use are outside of Du Pont's control, we can assume no liability for results obtained or damages incurred through the application of the data presented. Publication of the information in this bulletin should not be understood as permission or recommendation for the use of "Freon" compounds in violation of any patents. \ E ID 11639 **< VL ft. PAT O# Better things for better living.. .through chemistry UlNTtO 'N S *.