Classroom Demonstrations of Polymer Principles Part V: Polymer Fabrication Ferdinand Rodriguez School of Chemical Engineering,Olin Hall, Cornell University, Ithaca, NY 14853
In past articles, demonstrations of polymer principles have been described, including molecular structure and molecular mass ( l ) ,polymer formation (21, physical states and transitions (31, and mechanical properties (4). The concepts introduced in those articles are brought together in the process of fabrication, that is, in the production of recognizable and mostly useful objects. Fabrication Fabrication involves taking polymers or polymer precursors and changing their physical, and sometimes their chemical, form. Often, the product of fabrication is a consumer item, such a s a molded toy or extruded tubing. Sometimes the product is the raw material for yet another fabrication prociss, as in the conversion of polymer pellets tosheet by extrusion. The sheet may then serve as the starting point for a vacuum-forming operation to produce a contour map or a desk drawer.
Fabrication Techniques Because there are so many fabrication techniques, it is convenient to group them roughly into three categories according to the number of dimensions (distances)being cantrolled One-dimensionalprocesses-in which only the thickness is usually considered, such as the application of coatings and adhesives !&o-dimensional processes-that produce standard profiles, such as fibers, tubing, and gaskets, using extrusion and and laminating operations other free-film-forminz .processes . Three-dimensional processes-typified by compression and injection molding, sheet forming, casting, and foaming (5) Although none of these operations is hard to visualize, we will use the production of a birthday cake to illustrate all three categories. The cake itself is produced by a casting process (three-dimensional molding) using a pan as a mold. Once baked (dimensionally stabilized), the cake can be coated with a n icing (a one-dimensional process). Finally, decorative fluting made of icing can be added by extrusion of a mixture through a star-shaped nozzle (a twodimensional process). Descriptions of the various processes are contained in many textbooks and handbooks. The Modern Plastics Encvclooedia (6) contains brief articles on fabrication pro&is and plastic materials. The lJlastic,sPrinm' by Curley is a particularly readable synopsisof typical operations 171.
Most of the demonstrations described here use readily available household or s h o ~ items. Eauioment for hieh .* temperatures is less common. Industrial or shop heat guns are oreferred over domestic hair dmers. which often do not pro&ce air that is hot enough to de"fo& plastics. However, there are occassions when simole household items are preferable. To get a surface with contfolled temperature, certain kitchen hot plates can be less expensive and more easily regulated than laboratory-grade hot plates. For example, one electric fry pan that was tested maintained a set temperature to within 1T (in the range 6% 220 OC). as measured bv a thermocouple in an aluminum the pan surfa&. A warm-up period of 8 min was block reauired to reach a steady temperature. However, the measured temperature was aboui 10 OC higher than the indicated dial setting, so calibration is recommended before using such an appliance in the classroom or laboratory. Sources are cited for the specific materials used, but alternate materials exist for most. A plastics fabrication shop should supply many of the matesals, such as silicone spray mold release and representative plastics. (If such a shop is nearby, it would be a good place to take the students for a field trip.)
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One-Dimensional Processes Linseed Oil Film Formation
Ordinary paints form films by the successive processes of solvent evaporation and chemical reaction. Film formation from a polyker solution is easily demonstrated by the drying of nail polish or any one of the many spray lacquers that are available commercially. Linseed oil is a typical drying oil (unsaturated triglyceride), which cross-links on reaction with atmospheric oxygen. The reaction is rather slow at room temperature even whencatalyzed by cobalt metal. I n this demonstration, the oil is held a t 65-70 'C on a hot plate. Even then, the oxida-
Equipment and Materials Needed Caution: Excessive beating of any of the polymen in air can liberate potentially harmful gaseous products of oxidation and deco&position. Most of the materials used in these experiments &e consumer products, not industrial materials. Manufacturer's warnings should be taken seriously.
Presented in part at the ACS Meeting, Washington, DC, August 2&31,1990.
Figure 1. Linseed oil films. Three strips of cellophane tape A make two lanes: one foroil 6, and one for oil with drier, C. Microscope slide D used as a draw-down blade. Volume 69 Number 11 November 1992
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tion is scarcely detectable for the first 15 min, so it is best to start the emeriment a t the beeinnine of the oeriod. Then start thehiscussion on coati&s or adhesives, stoppinc at intervals to monitor the promess of the reaction. - ~Ihreestrips of cellophane tape are placed on a projectorslide cover elass (3-1/4 x 4 in.). (See Fie. 1.)In one lane. between celiophane strips, place a dropUofraw linseed oii (Montgomery Ward, Chicago, IL). In the other lane, place a drop of linseed oil containing cobalt metal (0.1% by weight), added as the naphthenate (cobalt naphthenate 6%, Pfaltz and Bauer, Waterbury, CT). A microscope slide can serve as a draw-down blade to convert each drop of oil to a fdm. If each sample is first diluted with an equal volume of volatile solvent, such as petroleum ether, the films will be thinner. The slide is then carefully placed on a hot plate held at 65-70 'C. To show the class the progress of reaction, the slide can be placed temporarily on the table of the overhead projector. However, it should be returned to the hot plate quickly aiter each examination. After about 15 min, a lead pencil drawn through the films will pick up an oxidized skin from the cobalt-containine film. but not the other film. After about 1h the cobaltfrle oil &ill still be liquid and will wet a tissue, whereas the cobalt-containine film will have skinned over wm~letelv and will be dry & the touch of a tissue. If the fdms are set aside at room temperature overnight, the cobalt-free film usually will not have skinned over even a h r 16 h. Comparing Adhesives
A good illustration of a permanently liquid adhesive is "two-sided" cellophane tape, which is a strip of cellophane with an amorphous, plasticized polymer on both sides. Even two lavers of oolvethvlene. whose surfaces are usuto ilue, ;an be held together by a ally considered dig& strip of this cellophane tape. With this system there is no progressive increase in bond strength with time because no ohvsical or chemical chances occur aRer the initial has been applied. Tho Popular Cements Two popular household cements can be compared. Both are in the general class of cements that contain volatile diluents and set without chemical reaction. One should be a solvent-based cement, such as Duco cement (Devcon, Wood Dale. IL). The other should be an aoueous-based emulsion, such as Elmer's Glue-All (Borden, Columbus, OH). As a first comparison, the class can be asked to examine the cements and to note both the odor and the flammability of the diluent. The class should be told that due to the well-publicized dangers of "glue-sniffing",solvent-based cements now contain an irritant, such as oil of mustard. For a second comparison, a drop of each adhesive is spread on a class plate. After about 10 min, the solventbssed cemenCformha peelable film. Although the aqueousbased adhesive skins over, it does not readily dry through. When usine " the emulsion. the class should note the convenience and advantage in using a water-adsorbing substrate. The adhesion to glass should be compared. Finally, both adhesives can be used to attach wood tongue-depressors to a wood block (Fig. 2). A comparison of the bond strength is made after about 30 min, so this should be done at the beginning of the period. The strength is measured by liiting a mass at the end of the tongue-depressor. In one such test, the solvent-based cement canied a 300-g mass but failed with a 500-g mass. The emulsionbased adhesive carried 1200 e but failed at 1500 e. This test illustrates how adhesivescan be compared. ~tshould be emphasized that there is no such thing as the "best" 916
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Figure 2. Adhesion. a) Wood pieces are attached to a wood block with various adhesives (under some clamoina stress). b) Bond strengtn is estmatea oy adoing masses (alwaysat distance dj to the cant lever beam formeoby the test piece.
adhesive; the surface condition and adhesive application can change the apparent order of usefulness. Adhesiues That Undergo Chemical Reaction Adhesives that set by undergoing a chemical reaction are common, but they usually require more time. Exceptions are some new fast epoxy systems and the cyanoaaylate adhesives in general. In the average lecture hall it would be hard to duplicate the television ads in which massive objects, such as automobiles, are suspended by cables that were glued toeether. As an approxim&on to s;ch displays, two 1/2-in. baits can be glued together head-to-head after both heads have been carefully smoothed and made perfectly flat. Within a minute of gluing the two together with one ofthe various "miracle" cements (e.g., Duro Quick Gel, Loctite, Cleveland, OH), the bond can be tested by bolting each end of the assembly to a 1-in. angle iron that is about 5-ft long. The nuts are threaded on so that the two angles will be parallel when separated by a short piece of I-in. pipe or rod (Fig. 3). The strength is easily measured. Ask one person to stand on the end of the angles away from the bolted end and move the lever-point (the piece of pipe) in stages towards the bolted end, until the bond fails. This simple procedure can also be used to easily disprove the common misconceotion that this strength is duito avacuum that forms wh& the smooth surfaces are pushed toeether. Simplv - substitute any other liquid for &e adhesice. Fluidized-Bed Coating
Relatively thick layers of thermoplastics can be applied to rigid substrates with this method. Thermosets are also used. In either case, the absence of a volatile solvent is a major economical and ecological advantage over conven-
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Polyethylene powder
20 cm
Frif ted gloss plofe,
Figure 4. Fluidized bed wating
Figure 3. Heavy-duty test rack. a) Metal angles with adjustable lever arm. b) Bolted end undertest. The two bolts (a)are held in place with nuts (b). tional coatings. I n one application, glass bottles are heated to 200 or 350 'C and lowered into a fluidized bed of powdered polyethylene to apply a shatter-retaining coating. The efficiency of the operation can be increased by electrostatic charging of the object to be coated or by charging of a sprayable powder. In the following experiment, an ordinary uncharged system is used. Experimental Apparatus A straight tube containing a coarse, fitted-glassdiffuser (Ace Glass, Vineland, NJ; Catalog item 7305-22) (At its lower end, the tube is fitted with a ~ b b e stopper r and another tube.) Source of air under pressure (Even an ordinary hair dryer (unheated)will suffice.) Heat gun or bunsen burner Materials 'Powdered polyethylene (Micmthenepolyethylene, U.S.I. Chemicals, New York, NYI Metal abject to be coated (e.g., 114-in.wing nut) Procedure Enough powder is put in the upper half of the tube to give a bed that is about 5-cm deep. Air is admitted at sufficient pressure to expand the bed to a height of 6 em. The object to be coated is held on the end of a thin wire while it is heated in a flame or hot air stream (Fig. 4). Then, still on the wire, i t is dipped in the fluidized bed and moved from side to side. Although the coating can be fused from just the latent heat stored in the metal, to get a smooth wating you may need to reheat the part. Once the surface is smooth, the part can be immersed in water for about 30 s to harden the coating. Two-Dimensional Processes A large nut and bolt can illustrate the conveying action of a helical screw. Holding the nut in one hand, the bolt is
turned by the other. If the nut is visualized as the polymer melt, it becomes apparent that effective conveying will occur only if the melt adheres to the wall of the barrel, which is represented by the hand with the nut. Maximum conveying &curs if themelt slips on the screw. In the practical case, the melt adheres to both the wall of the barrel and the skew, so the conveying action resembles the movement of a parallel-plate viscometer in which the two plates are helixes. Amodel extruder can be built around some readily available screw, such as a 112-in. augur bit used for drilling holes in wood. A respectable screw conveyer can be made by grinding off the front end and cutting off the squared end of the shaft. It can be fitted into a transparent plastic tube (or a drilled-out plastic rod) (Fig. 5). This model is a good illustration even when there is no material to move. In lieu of a better material, toothpaste can be extruded. Ram Extrusion of Solutions and Melts Polymer Solution Ram extrusion of a polymer solution into a coagulating medium is feasible with a variety of systems. For instance, a 5% solution of poly(viny1alcohol), such as Elvanol 50-42 (E. I. duPont de Nemours, Wilmington, DE), in water can be extruded slowly from a 10-mL plastic syringe through a short No. 20 hypodermic needle (preferably with a squared end) into a Petri dish of acetone on the overhead projector. A2O.m-long fiber can be removed after a few seconds with
Figure 5. Model extruder. a) Auger bit is trimmed off at a and b, and collar c is added. b) The barrel is made complete by addingcap-piece d, with a small hole and feed-port e, to the plastic tube. Volume 69 Number 11 November 1992
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discount stores. The "polyester" consists of unsaturated polyester (about 213 of the liquid mixture) and styrene monomer (the other 113). Fairly strong sheets can be made by following the suppliers instructions. Afree-radical initiator (usually methyl ethyl ketone peroxide) is activated by a second minor component, often cobalt naphthenate (cobalt naphthenate 6%. Pfaltz and Bauer, Waterbury, CTT). Carrying out the reaction on a warm surface (hot plate) should allow quick production of a glass-reinforced panel. Although silicone-snraved aluminum foil can be used on either size of the l a k n a t e , a fluorcarbon sheet is even better. (You can also use a fluorocarbon-coated cookie sheet that has been cut to provide molding surfaces). A sandwich is made: fluorocarbon sheet, polyester-glass suspension, and fluorocarbon sheet. Three-Dimensional Processes Compression Molding Extrudate
Figure 6. Batch melt extruder. a) The sholt bottom bolt has a hole drilled through. b) The apparatus is tightened with wrenches to extrude the heated contents. a pair of tweezers and placed in a beaker of acetone to harden for a few minutes. After this, the fiber can be dried on a paper towel. When dry, the fiber is rigid. With slight moistening, the fiber becomes elastic as the plasticizing action of the water lowers the glass transition temperature T,. Poly(viny1 alcohol) has a T,of about 85 'C when dry. Polymer Melt Ram extrusion of a polymer melt requires pressures that are higher than those obtained in a syringe. A 518-in. bolt and nut arrangement is easily constructed (Fig. 6). The short bolt has a small hole (0.1-cm diameter) to serve as the extrusion die. The three pieces are preheated by placing them on a hot plate set for 220 OC. Holding the metal pieces with pliers (locking pliers are esueciallv recommended). vou can thread in the bottom bok comGletely. Then add i g of polyethylene cut from a milk bottle or plastic bag, and thread in the long bolt. Screwing in the top bolt eventually starts a thread of extruded oolvmer about twice the diameter of the die. With some mk&ials, melt fracture (melt flow instability) can be seen a t high rates of flow. The whole experiment must be completed in about 2 or 3 min because the apparatus cools in air. After the apparatus is assembled with the polymer, the entire arrangement can be reheated bv contact with the hot plate. There is little danger of overheating. I n addition to melt instabilitv, the students should notice die swell (diameter of extrud& divided by diameter of die). Also, the amount of material extruded before a change in opacity indicates crystallization. In film extrusion this is oRen called the "frost line."
This process is the grandfather of the polymer fabrication family. Shellac, rubber, and guttapercha were compression molded even before the advent of plasticized cellulose nitrate in 1868. I n addition to illustrating a n important contemporary molding process for thermosets, this experiment also introduces a discussion of the merits of transfer and injection molding. Experimental
Apparatus T w o 518-in.bolts and one long nut (Fig. 7) T w o rod sections and a close-fittingtube (Fig. 8) Variable-temperature hot plate Materials Phenolic molding powder (walnut molding compound, Plenco 411, Plastics Engineering, Sheboygan, WI) Polyethylene molding pellets or powder or pieces cut from a plastic milk bottle The smooth-ended bolt and the nut are assembled and placed on the hot-plate (200 'C) with the indented bolt. Putting an inverted 1-L breaker over the parts will block cooling air currents. To carry out the molding, about 2 g of phenolic powder is poured into the mold, and the indented bolt is inserted immediately. By using a box wrench or a vise on the nut and a ratcheewrench on the bolt, the pressure can be auulied to the heating oowder within 15 s of filling the maid: After the assembl;6as tightened, it can be reulaced on the hot d a t e for a minute or so to comulete the reaction.
Mold ossembly
Polyester-Glass Composites Caution: Although styrene monomer is not considered to be especially dangerous, as with most aromatic compounda, there remains the question of possible carcinogenic activity. The odor is objectionable, and proper ventilation must be used. Caution: mastic gloves should he used when handling the glass mat or cloth and the polymer mixture.
A convenient source of polymer and glass cloth is the auto body repair kit found in most auto parts stores and 918
Journal of Chemical Education
Figure 7. Compression molding.
:
5/27" bolts
Pressure
Figure 8. Plug and flash molds. a) Aluminum plug mold. b) C-clamp press. c) Extractor. d) Pliers and coins for flash molding.
To impress students with the dimensional stability of the thermoset at the molding temperature, the part should be removed while still hot. With a pair ofheavy mill gloves as added Drotedion. the assembly can be handled on a wood board 60protect the table surfke). First the smooth bolt is removed. Then the indented bolt is unscrewed, carrying with it the threaded, phenolic plug (Fig. 7). In a variation in the molding process, two pieces of rod and a close-fitting tube are used. (See Fig. 8 . ) Then the thermoplastic pellets in the preheated mold can be compressed by applying pressure in a C-clamp. Once under pressure, the assembly is cooled by immersion in a bucket of water. Extraction of the molded part is simplified if a plug and ring arrangement is used in the C-clamp. As a fast illustration (Fig. 8), a thermoplastic can be held between two coins in a pair of pliers. (For example, poly(viny1 acetate) with a T, of about 28 'C works well.) The plastic can be softened and "flash-molded" between the coins by heating it with an air stream or by dipping it in boiling water. Immersing the assembly in cold water while still under pressure completes the process. Extrusion-Blow Molding Billions of plastic bottles are used annually in this country. Almost all the bottles are produced by some variation of blow molding. The most common process is to form a tube of the plastic, and then blow it up while in the molten (rubbery) state, like a balloon inside a hollow (usually split) mold. On touching the walls of the mold, the plastic cools and bewmes dimensionally stable. The tube can be formed by extrusion (see Two-Dimensional Processes). Injedion-blow and stretch-blow molding are separate variations of the process. Experimental Apparatus Heat gun or bunsen burner Glass or metal battle for use as a mold (A 1-02,widemouthed pill bottle is suitable.) Pliers Material .Commercial extruded polyethelene tubing, 318411. 0.d. and 114in. ad. Procedure Heat the bottom 114-in. of a 6411. long piece of tubing to the molten state over the burner or gun.Then crimp it with a pair of pliers to seal offthe end. Next, bring the lower end of the tube to the molten state. The entire heated length should fit into the mold (Fig. 9). Then blow into the cold
Figure 9. Blow molding. a) The heated tube is inserted into the bottle. b) The tube is expanded.
end of the tubing to expand the hot end. Leave the expanded polyethylene in the mold under pressure for about 1min to attain dimensional stability Because the polyethylene is relatively flexible, the cooled object can be removed easily without a split mold. t v the ~olvmerin this a ~ ~ l i c a t i o n A desirable ~ r o ~ e rfor would be abroad range for b o d temperature and-time for rubbew behavior above T, (melti~lg.temperature). A low molec;lar weight material usuall; becomes a liquid on melting. Then it will not sustain the pressure of blowing but will flow or fail by blowing out a hole in the wall. On the other hand, a polymer with a molecular weight that is too high will not flow rapidly enough to seal the end of the tube. A rapid rate of crystallization is desirable because it gives dimensional stability once the object has been molded. It is useful to pass amund commercial bottles for the students to examine. &
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Vacuum-Forming Although there are many varieties of sheet-formingprocesses, the simplest to illustrate in the classroom is straightforward vacuum-forming. The process is related to blow molding in that a heated, thin-walled preform is conformed to the shape of the mold under rather low pressures. Due to its limited heat capacity, the thin material rapidly reaches dimensional stability as it comes in contact with the wld surface of the mold. Familiar applications include blister-style packages for hardware, contour maps, and individual serving packages for jams and cream. Experimental Appamtus Heat gun Buchner funnel (ceramic, about 6-cm i.dJ with a common laboratory filter flask Retainer ring held with rubber bands or springs Source of vacuum (water aspirator, mechanical pump, or band-operated pump) Materials Disposable polystyrene Petri dishes (Nalgene polystyrene dish, N370-5009, Nalge, Rochester, NY 78910) Heavy-gauge polyethylene fdm (10 mil) Procedure A polystyrene Petri dish (or a cover plate) is held on top of the Buchner funnel with the retainer r i m - .(Fie. 10). The plastic is heated for 5-10 s with a hot air stream from a 12-A heat gun. A vacuum, is applied immediately when
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........
.
Lid
/,
Heat Figure 11. Rotational molding.
Figure 10. Sheet forming. a) Apparatus based on Buchner funnel. b) Plastic memory. heating stops. The vacuum can be broken and the part removed after a few seconds. Many other materials can be used for the demonstration. Polyethylene film, about 10 mils thick, is a good com~anion to the polystyrene because polyethylene Kecomes dimensionally stable by cooling below T,, and polystyrene by cooling below T,. In either case the original sheet must be quite flat so that the vacuum will be effective. Before the apparatus is assembled, it is useful to draw grid lines on the plastic sheet using a (permanent) marking pen with oil-soluble ink.The distortion of the grid lines in the final product emphasizes the nature of the drawing process. The ring for holding down the plastic sheet can be made bv cuttina out the center of a iar lid or bv usine the lid of a-replaceable-top canning jar. A sealing"surfa& on one side can be made in place using a room-temperaturevulcanizing silicone sealant butted against a polyethylene or paraffi-coated surface (Fig. 10a). Examples of commercial packages that use sheetformed, transparent plastics can be exhibited. As in blow molding, a wide range of rubbery behavior is required for successful sheet forming. Even cross-linked materials can be used. The "plastic memory" of fabricated objects is instructive in this regard. When an iniection-molded obiect is heated above its distortion temperature, the relaxation of oriented polymer chains gives a somewhat random deformation to the object. A sheeeformed drinking cup or cream dispenser can be used to demonstrate memory. When they are heated, the deformation is not random; they return directly to a flat sheet (Fig. lob). For example, when subjected to a hot air stream for about 1 0 s, fast-food plastic cups revert to a shallow dish with almost perfect retention of the circular shape (3). RotationalMolding
Large hollow objects can be fabricated in inexpensive molds by fusing a thermoplastic resin powder and then cooling to obtain dimensional stability. Since little pressure is applied, the integrity and uniformity of the walls depends greatly on many factors: the original particle size, the even flow of particles, a controlled heating rate, and
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Journal of Chemical Education
constant rotation of the assembly around more than one axis. At one time, most applications of this molding method involved toys or store displays where uniformity of wall thickness was not critical. Later, other items were made that call for a high degree of control: gasoline tanks, h i ture parts, chemical vats. Experimental
Apparatus Glass Petri dish, 5.Metal lid for dish Heat gun or bunsen burner Lab tongs Materials .Powdered polyethylene (microthene polyethylene, U.S.I. Chemicals, New York, NY) Silicone spray and mold release (Ordinary silicone spray lubricant will do.) Procedure The inside of the Petri dish and the metal lid are sprayed with mold release. The metal lid should be thiek enough to remain flatly in contact with the glass when grasped with the tongs (Fig. 11).The lid should have several "ears" to reeister it on the dish. but the rim should run all the wav ~" around because this idterferes with disassembly. r loaded into the dish. The covered About 6 a of ~ o w d eis dish, held by pair of tongs, is moved about and rotated rapidly in a hot air stream or over a burner. The fusion of the resin can be observed in the glass dish. Once all the resin appears to be transparent (usually several minutes), the assembly should be cooled in an unheated air stream for about 1min, then immersed in water. To extract the sample, it may be necessary to slip a spa& ula under the metal and around the sides of the dish (like extracting a cake from a baking pan). The final object is a hollow container about the size of a pill box.
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Literature Cited 1. R~~~ez.F;Mathiae,L.J.;Kmsch~tz.J.:Camaher,C.E.,J~J.Chen.Edue. IS87, M , 72. 2. Rodriguez,':Mathia*,L.J.; Kmschwitz,J.; Camsher, C. E., IJ J. Chon. Educ 1981, 64.886.
3. Rodriguez,F.;Mathias,L.J.;Kmach~tite. J.;Camaher,C.E., Jr.J.Ckm.Edur 19@, 65, 352. 4. Radrignez, F J. Chem Educ. 19W, 67,784. 5. Rodriyez, €. Prlneiplee of Pdymer System, 3rd ed; Hemisphere: New Yo& NY, 19R9, Chapter 12. 6. ModernPlosflrs:McGraw-HiU,NwYork;The Oetoberiasueis always themcydopediaiasve. 7. Carley, J. F."APlastic. Primer";MMhiemPlasfles 1880,67(111,4.
