PROGRESS
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THROUGH COOPERATION EDWARD R. WEIDLEIN Mellon Institute, Pittsburgh, Penna.
History and Development o f Laminated Safety Glass
The Franklin Institute, on March 31, honored the Carbide & Carbon Chemicals Corporation, E. I. du Pont de Nemours & Company, Inc., the LibbeyOwens-Ford Glass Company, the Pittsburgh Plate Glass Company, and the Monsanto Chemical Company for their progressive attitude in carrying on the Benjamin Franklin ideal of the application of science to industry for the benefit of mankind. The work of these companies has made available a n improved and high-test safety glass. The results reveal laudable cooperation; and following the address which we print here, Nicol H. Smith, director in charge of chemistry of the Franklin Institute, gave an impressive demonstration with tests of this new material.
HAT science in industry has helped to make living safer and more comfortable is due to the coordinated efforts of a large number of capable men. Technologic progress is in fact rarely the result of one man’s labors; it is usually an integration of effort to which several persons or groups of workers have contributed. It has often been said that “ ~ T T T O heads are better than one.” This maxim is recognized especially in industrial research, not because one man would necessarily be incompetent to carry out the development of something new to a successful conclusion, but simply because in many cases any one mind is too lacking in scope, one lifetime is too short to go into the many ramifications that a new development frequently involves. There is no successful substitute for teamwork, without which a contest is seldom won and a research is always slowed down. If some new development is under way that will make life safer or more pleasant, it is for the common good that it be made available to the public as soon as possible. This reason is sufficient in itself for utilizing the coordinated research efforts of many specialists. MAY, 1939--Page 563
SAFETY GLASS, THE RESULT OF COORDINATED EFFORT
Many examples show how well the system of coordinated effort functions. Fulton would probably be more astonished at seeing the Queen Mary than were his contemporaries when they first glimpsed a steamboat, and Wohler would be more fascinated by today’s big production of urea than were his friends when they first gazed upon a few crystals of a natural product prepared synthetically. Fulton’s and Wohler’s contributions to the art of building steamships and of making synthetic organic chemicals should by no means be undervalued, but equal importance should be attached to the accomplishments of a host of other technicians who have carried these developments to their present scale of magnitude. Laminated safety glass is also the result of the efforts of many workers, coordinated through sound research management. The problems that have arisen in bringing this product to its present state of perfection have been shared and solved by many specialists : ceramists who worked out suitable formulas for both glass and refractories; engineers who devised processes and equipment for casting, grinding, and polishing plate glass as well as methods for constantly reducing the thicknesses of the glass so made; chemists who developed the plastics that have been used in the interlayer and who evolved the plasticizers that go to render this interlayer flexible; chemists and engineers who invented compositions to make some of the plastics used adhere to the glass and solved many problems involving the fabrication of the laminated glass itself so that it could be produced economically in usable quality. All these scientists contributed valuably to the art as we know it today. As is quite usual in such cases, the perfection of the product has not taken place in a uniform manner. It was sufficiently useful in its early forms of development to fill a definite need and so it was given to the public as the best available. Research continued, however, and suddenly discoveries came that markedly improved the product. In consequence, a new and better laminated safety glass THE AMERICAN WAY
is now available through the efforts of the Pittsburgh Plate Glass Company, the Libbey-Owens-Ford Glass Company, the Carbide & Carbon Chemicals Corporation, E. I. du Pont de Nemours & Company, and the Monsanto Chemical Company. Nothing has yet been found which is BASICFEATURES. comparable to glass in hardness and resistance to abrasion and which, at the same time, possesses its clarity. Hence
glass. This invention was certainly laminated glass, but it was not laminated safety glass. The credit for inventing laminated safety glass goes to John Wood, who obtained a British patent on his product in 1905. This patent discloses a method of producing laminated safety glass by cementing a sheet of transparent cellulose nitrate between two plates of glass, using Canada balsam as the adhesive. Actual manufacture under the Wood patent was started and safety glass made by this process was exhibited at a motor show in England in the spring of 1906; but since automobile production was small at that time, there was little demand for the product. This fact, coupled with technical difficulties with the product, caused Wood’s venture to be a financial failure. A Frenchman, Benedictus, was granted French and British patents in 1910 on laminated glass, in which gelatin rather than Canada balsam was used as the adhesive to hold the cellulose nitrate plastic to the glass. The product, “Triplex”, was made in England in 1913. With the outbreak of the World War, a demand arose for its use in the manufacture of airplane air screens or visors, automobile windshields, goggle and gas mask lenses, and other appliances essential to military combat. The commercial production of laminated glass thus got its start. It was a product that would not now be acceptable if measured either by performance or cost, but it did have sufficient merit to prove that the principle of laminating glass with a plastic sheet was workable and that the product had a definite field of usefulness. After the war and before the production of closed automobiles reached its present high figure, there was little demand for laminated glass and the industry languished. When, however, the swing reached closed cars and the subsequent almost universal adoption of laminated glass in them, the manufacture of laminated safety glass became one of the country’s important industries.
THE STRENGTH AND STRETCH OF THE RECEA-TLY PERFECTED PLASTIC FOR HIGH-TESTSAFETY GLASSGIVE ADDED PROPROBLEMS IN EVOLUTION OF SAFETY GLASS TECTION TO MOTORISTS One of the first problerrs that confronted engineers and chemists entrusted with the development of laminated glass manufacture was to find some means of making a product that glass is ideally suited for innumerable purposes where a would remain clear and colorless during the period of its product with such properties is desirable. In most uses glass useful life. The plastic interlayer employed in the original windows, or “lights” as they are called in the industry, are “Triplex” was cellulose nitrate. This material has the unseldom subject to sufficient shock to shatter or break them. desirable characteristic of decomposing under the influence But, with the advent of the motor car and other vehicles of the shorter wave-length portion of solar radiation, so that operating a t high speeds, the picture changed and accidents it turns brown and loses its transparency. In addition to frequently resulted in the destruction of glass with accomthis defect, the decomposed plastic loses its adhesion to the panying danger to anyone who might come in contact with the glass and separation occurs. With these changes the glass fragments. This hazard became even more apparent with the is no longer nonshatterable. This difficultywas successfully markedly increased popularity of the closed car during the overcome by two methods: One was to change the composipast few years and with the higher speeds a t which modern tion of the glass itself so that it no longer transmitted the automobiles operate. Various transparent materials are particular light rays that caused decomposition of the plastic; available that are tougher and much more resistant to shatanother successful solution of the problem was obtained by tering than is glass, but they do not possess its abrasion substituting cellulose acetate for cellulose nitrate. Cellulose resistance and hardness. They would make less durable acetate, unlike the nitrate, is not appreciably affected by windows or “lights” by themselves and are employed only in sunlight. special instances as substitutes for glass where durability is Many million square feet of laminated glass have been not essential. If, however, these transparent, shock-resisting produced in the past ten years using these two plastics, and plastics are firm-ly adhered between two sheets of glass, the this safety glass has been made available to every motorist resulting lamination or “sandwich” has the desirable properirrespective of whether he drives a cheap or an expensive ties of both the glass and the plastic. So long as the light car. The safety glass manufactured during this period has thus made is not broken, it is just as clear, just as hard, and been entirely satisfactory with regard to its ability to remain just as resistant to abrasion as glass. And if it is broken, the clear and colorless over a period of years; it has unquestionfragments remain anchored to the plastic interlayer and yield ably reduced the possibility that flying glass may be a major thereto so that the possibility of accident from flying glass is cause of injury in automobile accidents. greatly diminished. Both cellulose nitrate and cellulose acetate, however, are HISTORY.Laminated glass was first worked out from a relatively stiff or inelastic plastics. If, therefore, some object decorative, rather than a safety, point of view. In 1885 that is traveling fast strikes a piece of laminated glass made Fullicks, living in England, obtained a British patent on the with these plastics, i t first breaks the glass portion of the idea of obtaining a decorated pane of glass by cementing sandwich, with the absorption of a certain amount of the pieces of variously colored glass between two sheets of clear 554
INDUSTRIAL AND ENGINEERING CHEMISTRY
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kinetic energy of the missile; subsequently the plastic interlayer has to ahsorh the balance of the kinetic energy or tlie missile will go on through the glass. We are all familiar with the fact that a baseball catcher pulls his hand hack with the ball when he is catclringa "fast one." This practiceis todeaccelerate the baseball over a certain displacement so that all its kinetic energy is ahsorhed over a greater period of time, with the result that there is less sting in the ball. The same plienomenon makes i t less painful to land in a firemen's net than on a coucrete sidexl.alk. Similar conditions hold for safety glass. If the kinetic energy of a fast-flying ohject that strikes a piece of laminated glass can he distributed over a greater displacement, there is much less chance that the pla.stic interlayer will fail; moreover, less damage is done to tlre object t.hat hits the glass. This latter eonsideration is imgortitnt if the ohject happens to be the driver or passenger in a ear. Theoretically, then, from the standpoint of safety alone, the ideal m a . t e d for the construction of windshieiris ami sidelights in vehicles vould be a very elastic suhstanee that \~-oulilact more like a fireman's net than a piece of boiler plate in case of accident. Because such an elastic material has not hccn found that would he acceptable Sor iise by itself, advantage has to he takon of the clarity and harclness of glass i o pre\Tent the plastic becoming useleis 11y ahrasion. Laminated glass, liowercr, can tie rcnilered safer Iiy decreasing the thickness of thc glass and so decreasing t,lre amount of energy iiecessa.ry to hreak it before the Aexiliility of tlre plastic comes into play. In R*lditioo, thinner glass a.llo\rs the making of a thinner sandvich, so that a lamination of two pieces of glass and the plast,ic interlaycr will not lie any thicker than a single pitw of glass of sufficient thickness to he used alone. The production of thin plate glass, some\\-hat less than imili thick, is an accomplisl~edfact and is a triumph of engineering skill. The glass manufacturers had developed improved grinding aiid polishing rnachinery which manuiaotured thin plate glass of higlr quality. That in.. augurated the successful production Sor automotive use of a laminated safety plate glass which does not have the vaviriess and distort.ion of sheet glass. It is advisable to point out here that anothcr type of safety glass is available which is not a laminated product. I t consists of a single sheet of glass, so heat-treated that the outer siirfacc is in a st.ate of compression. As a result of these forces, t.he glass is much inore resistant to Iireaking than rmtreated glass. It does break, howe.r,er, if some sharp ohject perforates the outer skin of glass that is under compressioii. When this puncture occurs, t.he Sorces released cause the glass to break into a multitude of mall pieces. As t.he glass is very strong and rigid, it absorbs the kinetic energy of an ohject striking i t over a very small displacement. For this reason it is apt to bc more destructive to an ohject striking i t than a more elastic material. The ideal laminated glass rvould eliminate all danger of accident owing to flying glass and a t the same time would minimize tlie danger of concussion or other damage t o the person striking it. Automobiles are operated over a wide range of temperature, and it is desirable that laminated safety glass be effcctive throughout this compass. If the plastic interlayer becomes much stiffer or more brittle at lower temperatures, it. is unable to vithstand so great an impact. On the other hand, it should not become too soSt at elevated temperatures or it is likely to Fail through lack OS strength. Collulose xcet.ate and nitrate have t.he tendency of becoming more rigid a t lower temperatures, so that. laminated safety glass manufactured from thein is not as effective as i t is at higher temperatures, As a result of this falling off in effectiveness at lower temperatures, the industry was encouraged to seek new plastic interlayers that would give a more shatterproof product. MAY, 1939-Page 565
A great amount of experimentation was undertaken and many plastics were examined and evaluated. The requirements of a suitable safety glass plastic are varied and difficult. The material has to be transparent and colorless under all conditions that will he encountered in service and must remain so over a long period. It has to adhere to glass and must be unaffected by atmospheres of high or low humidity. These requirements, Tyhieh are the
inost important "static" specifications to he met, may %e summarized by saying that the plastic interlayer has to remain invisible and unnoticed between the layers of glass to wlrich i t remains adhered, waiting Sor the time when it may be called oii to function. TTlien an accident happens, tlie properties of tlie plast.ic that may he called "dynamic" come into play. I t is the contribution of science in noticeably improving t.he "dynamic" pronerties of lanrinatod dass that is being stressed here. I\1ANJFACTUKE
Although most consumers of a product are more conccrncd with its performance than with the details of its manufacture, let lis glance at the steps necessary in the making of laminated safety glass. The glass and the plastic are first rigidly inspected to make sure there are no defects t o impair the quality of the product; then both are accurately cut to the required shape. The plastic sheet and the glass are carefully washed, and an adhesive is applied to the glass, if the plastic being used requires such a step. The sandlvich is assembled and pressed together under moderate pressure and heat in order to seal the edges to some extent; then the glass is given a final pressing at higher temperature and pressure so that the \Thole area of the glass is brought into intimate contact with the plastic. 111 order to obtain uniform pressure over the surfaces, this final pressing operat,ion is generally conducted in ail aut,oclave rather than in a platen machine. The hydrostatic pressure of a liquid in the autoclave is used to exert the necessary pressure, and the necessary heat is supplied by the liquid. The pressing operation may be considered to meld the plastic
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and glass together so that they are one unit. After pressing, the lamination is cleaned of liquid and is then ready for the finishing operations that depend to some extent on the type of plastic employed. As moisture penetration into the edge of laminated glass made from some plastics results in a loss of adhesion of the plastic, it is necessary to edgeseal laminated glass in certain cases. This operation is carried out by removing a small amount of plastic from the edge of the glass and filling the void so made with an edge-sealing compound of low moisture transmission. Edge sealing also prevents the loss of the somewhat more volatile plasticizers used with some of thedplastics. The two features that most impress the average visitor when he sees laminated glass being manufactured are the rigid inspection system in use and the cleanliness of the operations. A product results that is as free of imperfections as is humanly possible. Because of this care and precision in manufacturing laminated safety glass, the available product has had excellent “static” properties and has adequately met such requirements as permanent clarity and adhesion. I n other words, the laminated glass produced for some years has been very satisfactory as regards its durability; the only remaining improvements are along the line of increasing its effectiveness under impact. A lamination that would be more elastic when broken would be desirable to accomplish this improvement in dynamic properties. Because such properties are more a function of the plastic than of the laminated glass, the solution to the problem lay in the direction of finding a new plastic with the required characteristics. The cellulose esters that had given such a good account of themselves in making a n adequately durable laminated glass are somewhat too stiff to yield laminated glass capable of withstanding relatively high impacts or even moderate impacts a t lower temperatures. The need for softer or more elastic interlayers mias accordingly recognized, and the first commercial advance in this direction was the introduction of laminated glass with a polymerized acrylic acid ester as the interlayer. This resin allows the production of laminated glass possessing somewhat improved impact or break characteristics a t ordinary temperatures, an advantage over cellulose acetate that becomes less apparent, however, a t low and high temperatures. POLYVINYL ACETAL RESIN
The use of acrylate resin as the plastic interlayer gave a laminated glass that behaved quite differently under impact from glass made from cellulose acetate plastic, owing to the fact that the latter is much harder. The laminated glass made with acrylate resin was found to bulge under impact rather than to give the tight “drumhead” break that was characteristic of the cellulose plastics. This property, together with the improved impact resistance of the acrylate laminated glass at ordinary temperatures, established the product as a definite contribution to the industry. But there was still a demand for something better, a n urgent need for a laminated glass that would have better impact resistance than anything yet available, especially a t the lower temperatures; consequently, after extensive research a new resin was developed that appears to be satisfactory. This novel plastic is a polyvinyl acetal resin. We will not go into the details of its creation and use, but some insight into the nature of this product may show how it functions when the demonstrations to follow are carried out. PLASTICIZERS. Many resins are somewhat too stiff or rigid in themselves to give flexible plastics. For this reason, if flexibility is required, a material is added to the resin to impart the desired flexibility. This material may be a liquid or a solid and is usually so high boiling that it does not readily leave the plastic when once incorporated in it. For example, 566
camphor is added to nitrocellulose for the purpose of plasticizing the latter, and some of the phthalate esters are added to cellulose acetate to accomplish the same result. With other resins various high-boiling solvents are used as plasticizers. The addition of such materials to resins increases their softness in almost direct proportion to the amount of plasticizer added, while the tensile strength of the mixture drops off a t a corresponding rate. It follows, therefore, that if a soft plastic is prepared by the addition of relatively high proportions of plasticizer, it has little tensile strength. This principle applies to plasticizers that are solvents for the resins they are to soften. On the other hand, if the plasticizers are nonsolvents for the resins, it is usually impossible to incorporate sufficient plasticizer to soften them sufficiently. VINYL ACETALPLASTIC. Organic compounds can arbitrarily be divided into water-soluble and water-insoluble compounds. The compounds containing a considerable number of hydroxyl groups are generally water soluble, whereas the substances with a preponderance of hydrocarbon groups are water insoluble. Polyvinyl alcohol is water soluble because of the large number of hydroxyl groups in it. If this compound is reacted with an aldehyde so that some of its hydroxyl groups are no longer present as such, it becomes 8. compound known as a partial polyvinyl acetal that is partly alcoholic in nature, owing to its residual hydroxyl groups, and partly hydrocarbon, because of the presence of the hydrocarbon portion of the aldehyde. Accordingly it contains both hydrophylic and hydrophobic radicals. Under such conditions the residual hydroxyl groups tend to keep the resin from becoming completely soluble in certain plasticizers; the hydrocarbon groups impart to it the ability to absorb a sufficient quantity of plasticizer to make the plastic elastic without markedly decreasing its tensile strength. When softened with the proper proportion of plasticizer, the new vinyl acetal plastic therefore has the remarkable property of stretching under relatively low stresses until its elastic limit is reached, a t which point considerable additional stress is necessary to make it fail. Such physical properties render the material ideally suited to resist impact when used as a safety glass interlayer. When some object strikes laminated glass made with this plastic with sufficient force to break the glass, the plastic stretches to a considerable extent, and a t the same time it is absorbing the kinetic energy of the blow. When the plastic has traveled through a displacement of several inches, it reaches a point where it no longer “gives ground,” and a t this point it has sufficient tensile strength to prevent the flying object from going through the glass. For this reason its resistance to impact is much higher than that of any other material proposed for safety glass. On the other hand, a plastic possessing such unusual mechanical properties might not necessarily be suitable as a laminated glass interlayer unless it fulfilled the other specifications required for such a product. It must be adequately clear and colorless and must remain so under all conditions that it will meet. It must have and must retain adequate adhesion to the glass. This vinyl acetal plastic appears to meet all these specifications. It is unaffected by sunlig stays clear and colorless. It has , and no adhesive is necessary in excellent adhesion t making laminated g m it. Its adhesion is not destroyed under condi high humidity, so that edge sealing of the glass is unnecessary. These unique properties simplify the process of making laminated glass from this plastic. The remarkable properties of the laminated glass manufactured with this new resin are a tribute to science and industry, and another example of the progress that can be effected through concerted research effort. T o humanity the benefits of such a development are enormous, for it will prevent uncountable tragedies.
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