Laminated glass consists of two or more lites of glass bonded together by one or more layers of plastic interlayer under heat and pressure in an autoclave. The interlayer holds the glass fragments in place if the glass is broken, maintaining a degree of structural integrity that simple monolithic glass cannot match. This property makes laminated glass the material of choice for automotive windshields, overhead glazing, hurricane zones, security applications, and acoustic barriers.
PVB: The Original Laminated Glass Interlayer
Polyvinyl butyral, known as PVB, was first developed in the 1930s and became the standard interlayer for automotive windshields. The motivation was safety: in the early days of motoring, drivers and passengers were frequently ejected through the windshield in accidents. By the 1960s, laminated windshields were mandatory in most developed countries.
PVB is manufactured by reacting polyvinyl alcohol with butyraldehyde. The resulting resin is plasticised to give it the necessary flexibility and adhesion. A sheet of PVB, typically 0.38 millimetres thick or a multiple thereof, is placed between the glass lites, and the assembly is passed through nip rollers to expel air, then heated in an autoclave to approximately 140 degrees Celsius under about 12 bar of pressure. The heat and pressure cause the PVB to flow and bond chemically to the glass.
The standard PVB thickness for automotive windshields is 0.76 millimetres, made of two layers of 0.38 millimetre PVB. For architectural applications, thicker interlayers of 1.52 millimetres are common, providing greater stiffness, post-breakage strength, and sound insulation.
SGP: High-Strength Ionoplast Interlayer
SentryGlas Plus, abbreviated SGP, is an ionoplast interlayer developed by DuPont and now manufactured by Kuraray. SGP was introduced in 1998 and offers roughly five times the tear strength and 100 times the stiffness of PVB at elevated temperatures. This dramatically improved mechanical performance allows SGP-laminated glass to behave more like monolithic glass in terms of deflection under load while retaining post-breakage integrity.
SGP is particularly valuable in applications where glass must carry structural loads after breakage, such as frameless glass balustrades, glass floors, glass stairs, and overhead canopies. In hurricane-prone regions, SGP interlayers help glazing resist windborne debris impacts and repeated pressure cycling. SGP also has much lower moisture sensitivity than PVB, making it suitable for exposed edge conditions that would cause PVB to delaminate over time.
EVA: Versatile and UV-Stable
Ethylene-vinyl acetate (EVA) is a copolymer interlayer that has grown in popularity for architectural and decorative applications. Unlike PVB, which requires controlled humidity during processing due to its hygroscopic nature, EVA is far less sensitive to moisture. EVA can be processed at lower temperatures and is compatible with embedded materials such as fabrics, metal mesh, paper, and LED films, making it the preferred interlayer for decorative laminated glass panels. EVA also exhibits better resistance to UV yellowing and moisture ingress than PVB, though its mechanical properties are generally lower than those of SGP.
Bullet-Resistant and Security Glass
Laminated glass can be engineered to resist ballistic attack by stacking multiple layers of glass and interlayer. The UL 752 standard defines levels of bullet resistance. Level 1 resists three shots from a 9mm handgun. Level 3 resists three shots from a .44 Magnum. Level 8 resists five shots from a 7.62mm rifle, a military-grade requirement. Bullet-resistant glass is typically manufactured by alternating thicker glass plies, often 10 or 12 millimetres, with PVB or SGP interlayers to create a laminate 20 to 75 millimetres thick overall. The interlayer absorbs energy through stretching and delamination, while the glass layers erode and slow the projectile.
In blast-resistant design, laminated glass with SGP interlayers is used because it can absorb blast energy while remaining attached to the frame, preventing glass shards from becoming lethal airborne projectiles - historically the greatest cause of injury in bombing incidents.
Hurricane and Impact Glass
Hurricane-resistant glazing must pass testing protocols such as ASTM E1996 and Miami-Dade County protocols in the United States. The large missile impact test fires a 2.7 kilogram timber projectile at a speed of approximately 15 metres per second into the glass, followed by 9,000 positive and negative pressure cycles. Laminated glass, typically using 1.52 to 2.28 millimetre interlayers, is the only glazing type that consistently passes these demanding tests.
Sound Reduction
Acoustic laminated glass uses specially formulated PVB interlayers with enhanced sound-damping properties. These interlayers can reduce sound transmission by 2 to 5 decibels compared to standard PVB of the same thickness, which translates to a perceptible reduction in traffic, rail, and aircraft noise. The weighted sound reduction index Rw for an acoustic laminated glass with a monolithic glass of the same mass can reach 39 to 42 decibels, compared to 35 to 37 decibels for standard laminated glass.