Laminated Safety Glass GEORGE B. WATKINS,Libbey-Owens-Ford Glass Company, Toledo, Ohio, of Chicago, Chicago, Ill.
T
IIE insnufacture of lairiinated safety glass may well he classed as oueof tlieoutstandingmoderniridustries, since its development has given what is probably the most important single contribution to safety in modern transportation, Evidence of public recognition of the merits of the product of this industry is best illustrated by its widespread use as standard equipment by the automobile manufacturers and the definite legislative steps that have been taken in sevcral states to the effect that all motor vehicles for public and private conveyance be equipped with laminated safety glass. fhSTORY
The principle of laminated glass as such is old, dating back to the latter part of the nineteenth century; but, like many otlier industries, during its early stages little money or well-directed scientific and engineering effortwas expended by those closely associated mith it, with the result t,hat,four or five years ago the industry was still iii its infancy and required corresponding treatment. The art of laminated glass dates back to 1885 when Fullicks of England obtained letters of patent (Z)for the manufacture of panes of glass for churcli and cat.hedral windows. Fullicks’ idea was to get the different coloring effects into one composite sheet of glass by carefully arranging pieces of differently colored glass in pattern form and cerncriting this pattern between two plates or sheets of clear glass. He proposed to use for his bonding materials transparent cements, such as varnishes, gelatin, or otlier materials that would stick glass articles together. For the idea of laniinatcd safety glass as we know it today, as far as public records are concerned, the honors go to an Englishman, Wood, who in 1905 obtained a British patent (3)
AND
WILLIAMD. HARKINS,University
which describes a method for safety glms nianufa,ctnre hy the use of Canada balsam for cementing a sheet of tranaparent celluloid between two sheets or plates of glass. The Safet.y Motor Screen Company, Ltd., inado samples of safety glass in this manner and exhibited them at the Spring Motor Show in England in 1906. Because of the high cost of materials, the general unsatisfactoriness of his product, and the small denrand, Wood’s venture was without success and the patent was allowed to lapse. The first man to capitaliqe on the idea of laininated safety glass wasa Frenchman, Benedictus (I), who obtained French and British patents in 1910. Benedictus named his product “Triplex” and employed the same general principle as Wood, except tbat he proposed gelatin instead of Canada balsam as tlie bonding adhesive for glass plates arid celluloid. Benedictus introduccd the manufacture of Triplex safety glass in 1912 in England whcre production started in 1913. The new indi~st~ry received an enormous impetus during the World War when laminated glms was used for tile manufacture of gas mask lenses and goggles, and for automobiles and airplanes. Although the manufacture of laminated safety glass was established as an industry during the war, for sonie years following it was a t a standstill if not in t,he waning class, because the producers of safety glass suddenly found that high-priced commodities and standards of quality acceptable daring the rage of battle failed to meet the approval of the close-range scrutinizing public in time of peace. Hovever, the merits of safety glass had been demonstrated beyond question, and Sar-sighted executives of sorrie of the glass companies, noting the trend in the motor industry from open to closed cars, realized the importance of and anticipated the future demands for a well-made safety glass which
1187
1188
I N D U S’r It I A I. A N D E N G 1 N E E K I N G C 11 E hl I S T R Y
would give satisfactory service for the average life of a motor car. It was also realized that such a product could not be made until considerable research and development work had been done, new processes worked out, improved glass and plastic developed, and new materials found for bonding together the glass and plastic layers. With this object in view, large sunis of money were invested in research facilities and personnel to speed development.
Vol. 25, No. 11
are made up in the desired colors, mottled designs, etc., and then laminated between two sheets or plates of transparent glass. LAMINATED SAFETY GLASS. Tile applications of the foregoing types of laminated glass are small compared to that of laminated safety glass as used in motor cars, windows, and windshields for speed boats and airplanes. This type is a three-ply lamination made by bonding in a unitary struct.ure two sheets or plates of glass with an interposed sheet of transparent plastic. Because of the greater demand and wider application the manufacturers have concentrated their research and development work on this type of glass-plastic structure, known to the public and to industry as laminated safety glass. GLASS The transparency of ordinary plate and sheet glass as manufactured today can, for all practical purposes, be considered ideal for visibility, since plates or sheets of glass approximately */s inch (0.3 em.) thick will transmit from 88 to 92 per cent of the visible light of wave lengths in the solar radiation (400 to 760 mp). I n addition to its oharacteristic of being transparent, glass is sufficiently hard to enable it to be ground and polished to a high luster finish x,ith a satisfactory wearing surface. This same glass, although ideal for visibility and wearing qualities, is so rigid and brittle at ordinary temperatures that i t readily breaks nto sharp, jagged pieces when subjected to shock or forces of impact or bending. This constitutes a dangerous hazard when used in automobiles. An ideal safety glass should have, in addition to the durability, transparency, and resistance to abrasion now possessed by ordinary glass, a certain malleability affording the glass resistance to the forces of impact and bending, much like a sheet of steel. Objections might justifiably he offered to this type of glass, for there are records of numerous cases where the breaking of a light or pane of glass with the hand or foot has opened the only avenue of escape from a burning building. The possibilities of development of a malleable glass &s described is undoubtedly far remote, and, although a vast SPECIFICATIONS FOR AN IDEAL S.4FETy
CZZE~KING PLASTICFOR T w i c ~ ~ e s s
The manufacturers up to this time had not been making any great amount of plate glass sufficiently thin to be used in safety glass. Accordingly, it was necessary to develop machinery for making a plate glass approximately one-half t u thick as the regular 3/,s-inch (0.5-em.) plate previously used in automobile glazing. Thin plate glass is now made on continuous lines so perfected that the quality of the glass with respect to flatness and surface defects far exceeds the fondest hopes of 1927.
PRINCIPLE OF LAMINATED GLASS The construction of laminated glass consists in bonding together two or more sheets or plates of glass with one or more interposed sheets of plastic or nonbrittle material to produce a composite structure. The problems involved in its manufacture are probably different in the main from those met in any other art, owing principally to the difficulties in satisfactorily bonding sucli unlike materials together to give a clear, transparent product. However, the characteristics of the finished article as to greater strength and more resistance to shock and penetration can be likened to other well-known structures which depend upon the principle of lamination for their desirable properties; examples are laminated wood strnctiires for airplane propellers, boat construction, etc., and laminated steel structures such as the common railroad rail and armor plate. BULLEFPROOFGLASS. Bullet-proof glass, capable of resisting 45-caliber army rifle and machine gun bullets, usually consists of three plates of glass and two layers of plastic. The central layer comprises a heavy core of plate glass reenforced on both sides with a layer of plastic and thin plate glass. This gives to the bullet-proof structure a total inch (2.9 cnt.). thickness of approximately l*/s m e n this type of glass is impacted with b u l k Erom higlipowered rifles, the first layer of thin glass and pia.& are punctured and the central glass core cracked or fractured, but the glass and plastic layer on the opposite side hold the thick central core in position and the force of the bullet is spent in pulverizing glass. DECORATIVE LAMINATED GLMB. There has been some demand in the past for decorative effectsin dresser and table tops and to some extent in glass tile for side walls in bath room and for many other similar articlcs. Plastic sheets
WWlTE A N D BLACK B o A l r D IVSPECTrO’V OF
I’LkSrIc
amount of resea,rch and experimental work has been done to develop a transparent plastic such as Cellophane, a cellulose ester plastic, or a syntbetic resin, sufficiently rigid and stable t,o take the place of glass, the nearest approach has been to furnish curved pieces of cellulose ester plastic for windshields in airplanes; however, even in this application, laminated safety glass is rapidly replacing the organic plastic. The development of an organic plastic material to take the place of ordinary glass is perhaps not as far distant in the future as the development of a mallcable glass. Eowever, because of the severe conditions imposed upon glass surfaces, particularly glass used in automobile windshields, by the abrasive action of windshield wipers in removing rain, snow,
November, 1933
I N D U S T R I A L A N D E N G 1 N E E R 1 N G C €I E ]\I I S T R Y
oil, sand, dust, and the like from its surface, many more years will undoubtedly lapse with the discovery of many new plastics and synthetic resins before the supremacy of glass is seriously threatened. An ideal safety glass is a t present limited to a composite structure in which glass as now made is re&nforced with plastic, and in turn the plastic is protected against abrasion and weathering action by the glass; inasmuch as the physical properties of glass as regards hardness and brittleness are fixed to a degree, any discussion of an ideal safety glass deals largely with the central layer of nonbrittle niaterial used to reenforce the glass layers. An ideal p l a s t i c laycr for laminated safety glass should be, Grst and foremost, as transparcnt a s t h e glass plates used in the composite structure. From tho s t a n d p o i n t of stability, the life of the plastic s h o u l d a p proach tlie life of the g l a s s . T h i s is a s e v e r e specification, as all suitable transparent plastics known today are strictly organic in nature, and organic materials are affeeted adversely to a greater or smaller degree by energy in the form of both light and heat. Inasmuch as laminated safety glass in motor cars is subjected to subzero weather of the extreme nortli as well as elevated temperatures in tropical climates, the plbstic should have a zero temperature coegcient of plasticity to give it equal resistance to shock in hot and cold climates. Toughness of the plastic, which is a major factor in determining the strength of the finished composite sheet, is a subject for debate. I n determining the optimum strength of laminated safety glass, it is necessary to consider all types of motor car accidents, ranging from head-on collisions at terrific speeds to merely rolling over in a ditch when turning around, and these various accidents occur in widely different climates.
TYPESOP PLASTIC MATERIALS One of the oldest plastics-namely, pyroxylin, invented and manufactured by the Hyatt brothers as early as 1860was the plastic suggested by Wood in 1905 for reenforcing glass, and it still has commercial significance in safety glass manufacture. Pyroxylin plastic is manufactured by colloidalizing cellulose nitrate or pyroxylin with suitable plasticizers, the most common of which is camphor. Since the days of the discovery of pyroxylin plastic, refinements have been made both in the nitration and purification of ceUulose nitrate, with the result that the present-day pyroxylin plastic is sufficiently clear for all practical purposes and has as low a temperature coefficient of plasticity as any material suitable and commercially available for safety glass manufaeture. Sheets or plates of glass reenforced with pyroxylin plastic 0.020 inch (0.508 nim.) thick produce a laminated safety glass sufficiently strong to decrease the hazard of Aging glass in motor car accidents many fold over the use of ordinary plate glass.
1189
From the standpoint of stability, pyroxylin or cellulose nitrate plastic, because of its higher energy levell, is not as stable to light and.heat as plastics made with other cellulose esters such as cellulose acetate, cellulose-propionate, etc., and cellulose ethers such as ethyl and benzyl cellulose. However, sheet pyroxylin or cellulose nitrate plastic has a much longer life when properly bonded between sheets or plates of glass than when exposed directly to solar radiation because all plate and window glass possesses the inherent eharacteristic of filtering out the major portion of the shorter wave len~tlisof the solar radiation that adversely affect thc layer of pyroxylin plastic. Also glass manufacturers have continually improved the stability of the glass itself, as well as its color, so that from a practical standpoint the g l a s s gives considerable protection to the olastic in this regard. The recent develow ment of ceUdose a&tate plastic and methods of bonding it to glass surfaces m&es available a laminated safety glass exceedingly stable to l i g h t and heat, and for all practical purposes as clear and transparent as ordinary glass. A suitable cellulose acetate plastic has about the same t e m p e r a ture c o e f f i c i e n t of plasticity as cellulose nitrate or pyroxylin plastic, and, although the acetate does not have quite as high a tensile strength as the nitrate, laminated glass made with a cellulose acetate plastic of slightly greater thickness produces a composite structure equally as strong as laminations made with the customary twenty-gage pyroxylin plastic. Plastics made from other derivatives of cellulose, including organic esters and ethers, offer promise for future development, as does the field of synthetic resins. Considerable experimental work has been done, and some safety glass has been made with syntlietic resins as the nonbrittle layer. Some of these resin-glass laminations are very satisfactory in appearance and stable to light and heat, but they all seem to have a hieh temuerature coefficient of nlasticitv. giving what is termed a “eold-short break” at a temperature of0”F. (-HOC.). “ I
TYPESOF LAMINATING OK BONDIXG PROCESSES Just as important as the plastic layer in safety glass is the bond or adhesion between the glass-plastic Imiinations for obviously the character of bond determines to a great degree the novel properties of laminated safety glass. A review of the patent literature in the United States and foreign countries serves to illustrate the various ideas of inventors for the manufacture of laminated safety g l s . A number of these patents have already lapsed and a great man,v more are nearing their expiration dates. The nature of the various adhesives and bonding agents proposed for joining together glass-plastic surfaces may well serve to group the laminating processes into two broad classes: (1) a process using as a bonding agent a material strictly
1190
INDUSTRIAL A N D ENG1NEERlNG CHEMISTRY
foreign in cheiiiiLal nature to both glass and piaitic and (2) a process using as a bonding agent a material similar in chemical natore to the plastie. Examples of the first class include tlie ad-kiioiril adhesives such as gelatin, glue, iainglass, casein, etc , and, in addition, various natural and synthetic resins Their application usually consists in either flowing, spra: ing, or otherwise coating one surface of each of t u o plates of glass xitti a thin film of the adhesive. After the adhesire 15 dried to ttic proper consistency, the g1a-s plates are nsseiiiikl witli a sheet of plastic in sandwich form and siibjcctcd to the application of heat and pressure to effect the bonding
capable of producing uniform and dependable adhesion which increase rather than decrease the stability of the finished laminations and have the cliaracteristics essential for continuous production. As a result of these improvements, laminated safety glass is made today on continuous lines which travel a t the rate of 60 to 70 feet (18.3 to 21.3 meters) per minute with little interruption.
I,AMINATINQ EQUIPMENT The laminating equipment for supplying the heat and pressure necessary for honding the glass-plastic sandwich consists broadly of tliree types: 1. Hydraniic presses equipped with nonyielding steel platens which are heated with circulating steam or hot water. 2. Flexible diaphragm presses or autoclaves. 3. Autoclaves in which true hydraulic pressures are obtained by immersing the glass-plastic sandwich, either protected from the autoclave fluid by means of rubber hags, etc., or in direct contact with the autoclave fluid. With the latter practice, it is necessary to close up the glass sandwich; this is ordinarily done by means of heat and pressure, either in a platen press or in a continuous manner, to prepare the sandwich for introduction into tho autoclave. The time, temperature, and pressure of the laminating cycles vary with the bonding agents and types of plastic used. The pressure varies from175 to 250 pounds per square inch (12.3 to 17.6 kg. per sq. em.) and the temperature from 230" to3OO'F. (l10'to 149OC.). The time cycleonmaximum temperature and pressure varies from 5 to 12 minutes.
EDQESEALIXQ PRE8SXNQ
GLAss-PLAsnc LAMINATIONS IN
A CONTINUOUS
MANNEH
Examples of the second class depend upon the adhesive forces of colloidalized cellulose derivatives, usually combined with compatible resins to promote better adhesion to the glass surfaces. Their application to the glass surfaces usually consists in applying a thin film or skin coat of a solution of the cellulose derivative which contains a small percentage of resins. After a thorough drying, tho surface of the skin coat, a5 well as the surface of the plastic, is peptized or softened by applying a small amount of solvent or plasticizer of high boiling point, and the sheets of glass and plastic are then immediately assembled in sandwich form and bonded by the application of heat and pressure. I n addition to the two general classes of adhesives or bonding agents, there are various combinations that may be included in both classes. To illustrate the development and progress made in bonding agents and their application, less than five years ago the process used by one of the safety glass manufacturers, whose product WRS considered to be quite satisfactory, included the following hteps: The glass plates, after cutting to pattern, were cleaned by washing, followed by coating one surface of each of the two lights of glass with a thin layer of a watersoluble adhesive. After this layer had thoroughly dried from 2 to 4 hours, a second layer was deposited by flowing over the first a nitrocellulose dope solution, resembling ordinary lacquer. After the second coat had thoroughly dried, the two glass plates were immersed in an aleolio1 bath and assembled in sandwich form with a sheet of pyroxylin plastic. Coming from the alcohol bath, the sandwich was assembled with the necessary padding and placed between the steel platens of a hydraulic press, where the necessary heat and pressure were applied to effectbonding. At present organized research and development have not only improved the quahty and stability of the glass and plastic used in safety glass, but bondmg agents have been developed
Vol. 25, No. 11
OB
LAUINATED SAFETY
GLASS
The plastic or nonbrittle layer of practically all laminated safety glass on the market is manufactured by collodialking or plasticizing cellulose derivatives. The cellulose derjvativeg (similar to their parent compound, cellulose) are hygroscopic, and in general all cellulose-derivative plastics take on or give off moisture, with subsequent expansion and contraction when subjected to atmospheres of varying humidities. As a result of the bonding operation of the glass-plastic sandwich, there is obtained a composite structure of safety glass with the marginal edge of the plastic exposed. If this is put in service in the more severe climates, such as the southern states and tropical climates, the alternate expansion and contractions of the edge of the plastic with changes in humidity, coupled with a slight shrinkage of the plastic due to the loss of plasticizer at the edge, results in a gradual breaking down of the bond at the marginal portion of the sheet. It is therefore desirable that the marginal edge of the plastic layer be protected against this type of weathering by hermetically sealing the composite structure. To accomplish the edge-sealing, the marginal portion of the plastic sheet, following the bonding operation, is removed to the approximate depth of '/s inch (0.3 em.), and this groove is filled with a thermoplastic sealing material capable of resisting atmospheric weathering and thereby sealing the composite structure. Another method for sealing pyroxylin plastic glass laminations consists in further plasticizing the exposed edge of the plastic to the approximate depth of I / m to I/,. inch (0.8 to 1.6 mm.). This method obviously eliminates the undercutting of the marginal portion of the plastic and may be accomplished by either immersing or otherwise treating the marginal portion of the finished laminations with a pyroxylin solvent of high boiling point and low vapor pressure; this further softens the exposed edge of the plastic, making it much less hygrhscopic and more resistant to atmospheric weathering.
November, 1933 L(OYI,ROL OF
I 1; D U S T 11 I A 1,
A ND
I3 N G I N E E R I N G C H E M I S T 13 Y
Haw hIa,reitiaLs AND T E ~ Tox S RFrsiatim FRODUOT
To the casual observer who views tlie factory operations in tlie manufacture of laminated safety glass, the problem appears simple, and rightfully so because every operation has been developed with the purpose not only of producing a dependa.ble product, but of handling mass production in a continuous rnanner. Rack of the scenes, bovever, are important control tests conducted on raw materials, such as the plastic sheet, adhesives, or bonding agents, together with a periodic testing of the fimished product. One of tlie important tests made on the plastic sheet is for residual solvents such as alcohol, acetone, ethyl acetate, eto., used by the plastic manufacturer to facilitate the solution or colloidaluation of the cellulose derivative and plasticizer. If any appreciable quantity of tllese low-b&,g solvents remains in tile finished lamination, the tendency for bubble formation in the comoosite structure during service in the warmer climates is increased. Also, the presence of excessive amounts of volatile solvents detracts from the general appearance of the finished product a t the time of manufacture. Another test made on the plastic sheet is the determination of nioisture content, since for any particular type of lamination there is a definite percentage of nioisture which gives the most stable form of lamination. The moisture content is carefully kept a t this percentage by holding the plastic in rooms which have conditioned atmospheres. This is done for a long period of time previous to its assembly in sandwich form between plates or sheets of glass which go to make up the coniposite structure. Other factors responsihle for uniformity of bond and stability of the laminated safety glass are the preparation and testing of all bonding adhesives in the research department before they are sent t o the laminating plant for commercial production. To determine the bond or adhesion between the glass-plastic laminations, hourly break tests are conducted on samples of the finished product. These are made in the laminating plant, and samples of each tested sheet are sent to the research department where they are checked for adhesion and testedforstabilitytowards heat byimmersing the samplesin boiling water for 3 hours. This gives a daily record of the quality of glass furnished to the various consumers.
1191
The relative hazards in driving a motur car behind the two types of glass is readily illustrated by comparing the size and amount of tho glass particles coming free from the laminated safety glass when subjected to the severe impact of the Iralfpound ball falling 16 feet with the size and shape of the sharp, jagged particles of plate glass produced by a less severe impact. The law of freely falling bodies indicat.es the velocity of the half-pound ball a t the time of impact, when allowed to fall froin rest through a distance of 2'/2 feet, to be approximately 8'/, miles (13.68 km.) per hour and through the distance of 16 feet to be approximately 22 miles (35.41 km.) per hour. RELATIONBETWEEN STRENGTH OF LAUINATED SAFETY GLASSAND THICKNESS OP PLASTIC Correlation between the thickness of pyroxylin plastic and the strength Of laminated safety glass has been made measuring the distance through which a 2-pound (0.91kg.) ball must in Order to rllpture X 12 inch into two Or more pieces.
~
CohlPARAT1vE
OF onDINARy
LAMINATED SAFETY GLASS
Am
The relative effectiveness with which ordinary plate glass and laminated safety glass resist impacts can best be illustrated by impacting both kinds of glass with a half-pound (227-gram) steel ball which is allowed to fall through any given d i s b c e . Results of such tests show that 12 X 12 inch (30.5 X 30.5 em.) samples of ordinary '/&ch (0.63-cm.) plate glass, which have a marginal support of approximately inch (1.27 cm.), will fail completely by breaking in numerous large, sharp pieces when impactcd in the approximate center with a half-pound steel ball falling through a distance of 2'/%feet (0.76 meter). When laminated safety glass is impactcd in a similar manner with the half-pound ball falling through a distance of 6 feet (1.53 meters), the glass plates will be cracked, but the sample will still be rigid and intact in size and shape. Close examination shows no glass lost by the composite structure. By increasing to 16 feet (4.88 meters) the distance through which the half-pound ball is allowed to fall before impacting the safety glass, a small amount of glass chips is caused to break conchoidally from the plate of glass opposite the point of impact, but the lamination still has rigidity and maintains its original dimensional form.
this test, 12 12 inah saIoples of la1uinated glass were made with which varied in thickuess from laminations were impacted o,020 to o,035 The at temperaturewith a 2-pound hall, and the distance required to produce rupture with more than 50 per cent of the samples tested was recorded. Results of the tests are RS follows: THICBNBB~ ow PLABTIC ~h.. inch Mm. 20 25
0.508
30 35
0.162
0.835
0.889
DIBTIIIO~ FOR FILLIW(~ B&&L TO Pnooooa R n m ~ n s Fee( 4.5 0.0 8.5 11.0
Mslsrs
1.37 1.83 2.59
3.35
These data show that the strength of laminated safety glass increases rapidly with the thickness of the plastic. I n special installations, such as aquaria, elevators, and panes of glass in large revolving doors, where greater strength is desired, instead of using a thicker plastic the requirements can he met by using a five-ply lamination consisting of three sheets or plates of thin glass with two sheets of plastic. Sueh a lamination, made from glass and plastic of the same thickncss as used in the ordinary three-ply safety glass, is exceedingly strong, and 12 X 12 inch samples resist impacts of a 2-pound steel hall falling through a distance of 14 feet (4.27 meters) without failure, and of a half-pound steel ball falling through a distance of 30 feet (9.14 meters) without causing any noticeable distortion of the laminated structure; however, a small amount of glass particles breaks conchoidally from the glass layer opposite the point of impact.
1192
INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED
(1) Benedictus, Eduard, French Patent 405,881 and British Patent 1790 (1910).
Vol. 25, No. 11
(2) Fullicks, A. T., British Patent 15,303 (Aug. 20, 1886). (3) Wood, J. C., British Patent 9972 (1905).
R E C E I V ~June D 19, 1933.
Cellulose Acetate Plastic Improves Laminated Safety Glass GEORGEB. WATKINSAND JOSEPH D. RYAN,Libbey-Owens-Ford Glass Company, Toledo, Ohio 'HEN i n 1927 g l a s s
Mi
art. This latter difficulty has The development of a satisfactory cellulose manufacturers w e r e been overcome by the developacetate plastic and methods of bonding this plastic confronted with an inment of bonding agents capable to glass surfaces has resulted in a marked imcreased demand for laminated of producing e q u a l l y as good provement in the quality of laminated safety safety glass, the only plastic then and more dependable adhesion glass. The history of the development of cellulose suitable and commercially availthan h i t h e r t o r e a l i z e d with able for such a composite strucpyroxylin plastic-glass laminaacetate plastic i n its application to safety glass ture was pyroxylin (plasticized tions. manufacture is briejly discussed. A comparison cellulose nitrate), substantially of cellulose acetate and cellulose of the properties RELATIVERESISTANCE TO IMthe same as the material used PACT OF LAMINATED nitrate plastics important to the safety glass for side curtains in the motor SAFETYGLASSES manufacturer, as well as the results of comcar, which glass had a t this time One of the chief objections j u s t a b o u t c o m p l e t e l y supparative tests conducted on laminated safety three or four years ago to the planted. glass made with both types of plastic, are outlined However, before a satisfacuse of c e l l u l o s e a c e t a t e for and illustrated. Data and photographs are laminated glass manufacture was tory safety g l a s s c o u l d b e presented to show the protection afforded by marketed in commercial quantithe relatively low resistance to cellulose nitrate and cellulose acetate laminated ties, raw materials, i n c l u d i n g impacts of the finished laminaglass, plastic, and bonding agents tion. This objection has been safety glass as compared with ordinary glass as well as the manufacturing o v e r c o m e by a careful study g laz ings. of the different types of cellutechnic, had to be i m p r o v e d . lose a c e t a t e a n d plasticizers. Unuuestionablv, the improveme& made i n ' a relativily short time were responsible for The result is a finished plastic which when properly bonded the increased demand for laminated safety glass. However, between two sheets or plates of glass produces a laminated far-sighted technical men who were familiar with the inherent safety glass comparable in strength with pyroxylin plastic unstable characteristics of cellulose nitrate-namely, its laminations as measured by standard impact tests. Tensile strength measurements of the two types of plastic high energy level-realized that eventually an improved type of plastic would be prerequisite to safety glass manufacture, used for laminated glass manufacture show that pyroxylin and research for the past three and a half years has been plastic or plasticized cellulose nitrate has a slightly higher directed towards obtaining a more suitable plastic than value than the cellulose acetate plastic. However, the strength of the finished lamination as regards its resistance to pyroxylin for safety glass manufacture. Obviously, the field for the development of such a plastic impact is only partially dependent upon the tensile strength is broad and includes cellulose derivatives, natural and syn- of the plastic used in the lamination; other factors, such as thetic resins, rubber, casein, and gelatin products. A careful shearing and adhesion forces, play an important part in examination resulted in the choice of a plasticized cellulose determining the strength of a laminated safety glass. The derivative, cellulose acetate; this choice was made, not with strength of laminated glass as measured by the impact test the idea that cellulose acetate constitutes the ultimate solu- increases materially with an increased thickness of plastic. tion to the problem, but because it was nearest commercial Hence, any slight difference in strength of the plastic which attainment and a t the same time offered marked improve- might affect the strength of the finished composite structure ment in stability over pyroxylin plastic. The fruits of the can be readily compensated for by a change in plastic thickresearch are now manifest in a cellulose acetate plastic made ness. At the present time, however, there is little difference with a special type of cellulose acetate and plasticized with a in the resistance to impact between laminations made with pyroxylin plastic and cellulose acetate plastic of equal thickspecial type and amount of plasticizer. There is little doubt that laminated safety glass made four ness. One hundred 12-inch (30.5-cm.) square samples of lamiyears ago with the cellulose acetate plastic then available was inferior to that made with pyroxylin plastic from the nated glass, approximately 0.25 inch (0.635 cm.) in thickness, standpoint of clarity and strength of the finished lamination. fifty made with acetate plastic and fifty with nitrate plastic, Other objections offered a t that time to cellulose acetate were impacted in accordance with the proposed Federal Speciplastic were its high cost and relative lack of plasticity at low fications' for laminated glass for automobile glazing-standard temperatures as compared to pyroxylin plastic. Undoubtedly, impact of a half-pound (0.23-kg.) steel ball falling from rest a factor of even greater importance which impeded the earlier through a distance of 16 feet (4.9 meters) a t 70" to 80" F. adoption of cellulose acetate plastic for use in laminated glass (21.1" to 26.7" (3.). A sample "failed" on impact if it broke into two or more was the failure to obtain proper and dependable adhesion be1 Proposed Federal Specification 12G5, Dec. 1, 1930. tween the glass-plastic surfaces by methods then known to the
