Laminated glass

Automobile windshield with "spider web" cracking typical of laminated safety glass.

Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral (PVB), ethylene-vinyl acetate (EVA), or thermoplastic polyurethane (TPU), between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic "spider web" cracking pattern when the impact is not enough to completely pierce the glass. The thermoset EVA offers a complete bonding (cross-linking) with the material whether it is glass, polycarbonate, PET, or other types of products.

Laminated glass is used when there is a possibility of human impact or where the glass could fall if shattered, and for architectural applications. Skylight glazing and automobile windshields typically use laminated glass. In geographical areas requiring hurricane-resistant construction, laminated glass is often used in exterior storefronts, curtain walls, and windows.

Laminated glass is also used to increase the sound insulation rating of a window, because it significantly improves sound attenuation compared to monolithic glass panes of the same thickness. For this purpose a special "acoustic PVB" compound is used for the interlayer. In the case of the EVA material, no additional acoustic material is required, since the EVA provides sound insulation.^[1][2] TPU is an elastic material, so sound absorption is intrinsic to its nature. An additional property of laminated glass for windows is that an adequate TPU, PVB or EVA interlayer can block nearly all ultraviolet radiation. A thermoset EVA, for example, can block up to 99.9% of all UV rays.^{[citation needed]}

History[]

Firefighters breaking through a laminated windshield

In 1902, the French Le Carbone corporation obtained a patent for coating glass objects with celluloid to render them less susceptible to cracking or breaking.^[3]

Laminated glass was invented in 1903 by the French chemist (1878–1930), inspired by a laboratory accident: a glass flask had become coated with the plastic cellulose nitrate, and when dropped it shattered but did not break into pieces.^[4] In 1909 Bénédictus filed a patent, after hearing about a car accident where two women were severely injured by glass debris.^[5][6] In 1911, he formed the Société du Verre Triplex, which fabricated a glass-plastic composite to reduce injuries in car accidents.^[7] Production of Triplex glass was slow and painstaking, so it was expensive; it was not immediately widely adopted by automobile manufacturers, but laminated glass was widely used in the eyepieces of gas masks during World War I. In 1912, the process was licensed to the English Triplex Safety Glass Company. Subsequently, in the United States, both Libbey-Owens-Ford and Du Pont with produced Triplex glass.^[8]

Meanwhile, in 1905, John Crewe Wood, a solicitor in Swindon, Wiltshire, England, patented a laminated glass for use in windshields. The layers of glass were bonded together by Canada balsam.^[9] In 1906, he founded the Safety Motor Screen Company to produce and sell his product.^[10]

In 1927, the Canadian chemists Howard W. Matheson and Frederick W. Skirrow invented the plastic polyvinyl butyral (PVB).^[11] By 1936, United States companies had discovered that laminated "safety glass" consisting of a layer of polyvinyl butyral between two layers of glass would not discolor and was not easily penetrated during accidents. Within five years, the new safety glass had substantially replaced its predecessor.^[12][13]

In the Road Traffic Act of 1930, the British Parliament required new cars to have safety-glass windshields.^[14] However, that regulation did not specifically require laminated glass.

By 1939, some 600,000 square feet (56,000 m²) of "Indestructo" safety glass was being used every year in vehicles produced at the Ford Motor Company works in Dagenham, England.^[15] "Indestructo" safety glass was manufactured by British Indestructo Glass, Ltd. of London.^[15] This was the laminated glass used by the Ford Motor Company in 1939, chosen because "it gives the most complete protection. In addition to being splinter-proof, it is crystal clear and permanently non-discolourable."^[15] This quote hints at some of the technical issues, problems and concerns that stopped laminated glass from being widely used in automobiles immediately after it was invented.

Specifications[]

A typical laminated makeup is 2.5 mm glass, 0.38 mm interlayer, and 2.5 mm glass. This gives a final product that would be referred to as 5.38 laminated glass.^{[citation needed]}

Strength can be increased with multiple laminates and thicker glass. Bullet-resistant glass is usually constructed using polycarbonate, thermoplastic materials, thermoset EVA, and layers of laminated glass.^{[citation needed]} A similar glass is often used in airliners on the front and side cockpit windows, often three plies of 4 mm toughened glass with 2.6 mm thick PVB between them.^{[citation needed]} This is one of the makeups used for the Boeing 747 cockpit side windows.^{[citation needed]} The BAC/SAF Concorde forward pressure windshields had 7 plies, 4 glass and 3 PVB total thickness 38 mm.^{[citation needed]}

Production[]

Modern laminated glass is produced by bonding two or more layers of ordinary annealed or tempered glass together with a plastic interlayer, usually polyvinyl butyral, Thermoplastic Polyurethane (TPU) or ethylene-vinyl acetate (EVA). The plastic interlayer is sandwiched by the glass, which is then passed through a series of rollers or vacuum bagging systems to expel any air pockets. Then the assembly is heated for the initial melt. These assemblies are then heated under pressure in an autoclave (oven) to achieve the final bonded product (fully crosslinked in the case of the thermoset EVA). The tint at the top of some car windshields is in the PVB. Also, colored PET films can be combined with the thermoset EVA material, during the laminating process, in order to obtain a colored glass. Digital printing is now available for architectural applications by either printing directly to the glass and then laminating or printing directly to the PVB as is the case with the trademarked Dupont SentryGlas Expressions process.^{[citation needed]} Full CMYK images can be printed to the interlayer prior to the autoclave process, and present vivid translucent representations. This process has become popular in architectural, Interior design and signage industries.^{[citation needed]}

Once a thermoset EVA is properly laminated during the process, the glass can be exposed frameless and there should be no water/moisture infiltration, very little discoloration, and no delamination, due to the high level of bonding (crosslinking).^[16]

Newer developments have increased the thermoplastic family for the lamination of glass. Beside PVB, important thermoplastic glass lamination materials today are ethylene-vinyl acetate (EVA),^[17] thermoset EVA ethylene-vinyl acetate (EVA)^[18] and thermoplastic polyurethane (TPU).^[19] The adhesion of TPU is not only high to glass, but also to polymeric interlayers. Since 2004, metallised and electroconductive polyethylene terephthalate (PET) interlayers are used as substrate for light emitting diodes and laminated to or between glass. Coloured interlayers can be added to provide a permanent transparent colour for a laminated glass panel. A switchable interlayer can also be added to create a panel which can be clear when a small electric current is passed through the interlayer and opaque when the current is switched off.

• Top layer: Glass
• Interlayer: Transparent thermoplastic materials (TPU or PVB, EVA) or transparent thermoset material (EVA)
• Interlayer: LED (light emitting diodes) on transparent conductive Polymer
• Interlayer: Transparent thermoplastic materials (TPU or PVB, EVA) or transparent thermoset material (EVA)
• Bottom layer: Glass

Laminated glass is also sometimes used in glass sculptures and is widely utilised in architectural applications.

Cutting[]

Plastic interlayers in laminated glass make its cutting difficult. There is an unsafe practice of cutting both sides separately, pouring a flammable liquid such as denatured alcohol into the crack, and igniting it to melt the interlayer to separate the pieces.^{[citation needed]} The following safer methods were recommended by the UK Government's Health and Safety Executive in 2005:^[20]

• Special purpose laminated cutting tables
• Vertically-inclined saw frames
• A blowlamp or hot air blower.
• High pressure abrasive waterjet.

Repair[]

According to the United States National Windshield Repair Association, laminated glass repair is possible for minor impact damage using a process that involves drilling into the fractured glass to reach the lamination layer. Special clear adhesive resin is injected under pressure and then cured with ultraviolet light. When done properly, the strength and clarity is sufficiently restored for most safety related purposes. The process is widely used to repair large industrial automotive windshields where the damage does not interfere with the view of the driver.^[21]

Disposal[]

Waste disposal of laminated glass is no longer permitted in landfill in most European countries as the End of Life Vehicles Directive (ELV) is implemented. A study by University of Surrey and Pilkington Glass proposes that waste laminated glass be placed into a separating device such as a rolling mill where the glass is fragmented and the larger cullet is mechanically detached from the inner film. The application of heat then melts the laminating plastic, usually polyvinyl butyral (PVB), enabling both the glass and the interior film to be recycled. The PVB recycling process is a simple procedure of melting and reshaping it.^[22] Also TPU is easy to recycle as all non crosslinked plastics.

See also[]

• Toughened glass
• BS 857
• LED Film
• Composite glass

References[]

External links[]

• UNECE Reg. 43 UN Regulation on automobile safety glazing materials
• BS 857:1967 British specification for safety glass for land transport