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
pieces; a sample "supported" if after impact it retained it? original dimensional form. All of the lrundred samples break-tested showed on impact radial arid circumferential cracks with a small amount of glass breaking coiiclioidally from tlie glass layer opposite the point, of impact; thus, all of the samples supported (Figure 1). &9LATlVl2
CLAliITY OF LAXINAT'ED
SAFETY
GLAssEs
With the use oi cellulose acetate plastic in the manuiacture of laminated glass, it is not only possible but pract.iea1to have a finished laminatioil superior t u pyroxylin plastic Iaminations iroin the standpoint of clarity. While pyroxylin plastic
rnaniifactiirers liavc perfected their pvodui:t to tho poiiit. wlierc laniinationa rn:rdc with this siieetiirg are, for all prilctical purposes, ai; clear as ordinary plate glitss, the ccllolose scctiit.e plxstic iiow a\~ailablefor ltmiiriated materially h e r frmi tire i i t m or "s pyroxylin plastic lanrinatiiins. ft'reedonr imm Iiiize i n tlie froin differences in tlie case of a cellulose ester plastic iiri process of rnanufuct,ure of the est,er elf as m l l as diffcreiiccs in the process oi plastic miiriuiactine. Altimugli the lirrze or star dust i r e s e n t in a laminated safety glass is iiiiperceptililo to the eye, except 11 n d e r e x t rao r d i II a r y conditioiis of lighting, the safety glass mamifnetiirer strives to keep t h e a m o u n t of haze at a minimum in order t,a simulate plate glass. 1)iffcrences in haze can rcadily be detected in cr:llnIose acetate and celhili~senitrate lainiuation,~11y the customary i n s p e e t i o n i n front of the CoiqicrIIcaitt light; or, if a qiiantitiitiw iictenninatioii is desired, the two types of plastic can lie dissolved and tlicir solutions compared nopilelonretrioally. On the hasis oi such comparisiiiis, cellulose acetate plastic offers it distinct advantage over pyroxylin plastio; hence, cellulose aoetate laniinations would he more desirable when driving under unusual lighting coliditions, such as ag,ainst the sun in the late aftcriioon, bright headlights a t night, etc.
TexPm.wum COEFFICIESTY OF PIASTICITY OF CELLULOSE ACETAWC AND (hLLULOSE SITRATE
I,last,icity of the plastic Tlie tempcratare cocl material used in laminated glass manufacture has a marked bearing on the strength of the gl:lass-pl;istio structure at different temperatures. While it is possible to makc a direct measurement of the temperature coeificiont of plasticity on the plastic itself, the practical value in its application to
lamiiiat.al glass is rcadily determiiwd 1y , comparinx the strength of tlit: finished liminations at varying tenrperatores by means oi stnndard impact. test. Air ideal plastic for laminated glass ~iiamifacturewould be one that liad a zero temperaturc coefficient of plasticity, so that tlie resulting laminatioirs irould he equally strong in divergent clirnstes, such as tlie extreme irortli in midwinter or tropical climates in miclsmnrrrer. IIowcver, it is practically impissii)le to realize siicli ideal conditions with any plastic known t,odsy rmd many of the materials (iiot~ablyt.lic resins) which niiglit suggest their iise ior larriiiratcd safcty glass, liave a very high temperatiire coefiieicnt,
