PSV Glass Technical Information

Please find below information from our technical help database.

We hope this information assists you in using our glass products to maximum efficiency.

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If you require further information in using our products or services or require further technical assistance please don't hesitate to contact us.

Toughened Glass Manufacturing

Toughened glass is produced by heating a sheet of ordinary annealed glass to softening point (approx 620-640°C) then rapidly air quenching it. It is the speed of quenching that determines the toughened quality.

The air quenching solidifies the outer layers whilst the inner core continues to contract. The outer layers are put under compression and the inner core is put under tension. This builds a degree of stress in the glass which increases its strength to four times that of annealed glass. This gives it excellent resistance qualities.

It is in the heating and softening process that the forming and shaping of the glass occurs, including sag bending on a mould or pattern.

Toughened Glass Handling

Do not stand toughened glass on any hard surface, as the edges (which are the weakest point) are vulnerable and the glass could shatter.

Wet packaging material can give off alkalis which can attack the surface of the glass leaving a water mark.

Care must be taken over storage. Ensure that glass is evenly supported in a suitable glass storage system. As with all glass, toughened glass is susceptible to scratches and abrasions, therefore care must be undertaken when handling. It is advisable to wrap toughened glass if they are to be stored close together.

Toughened glass strength and flexibility is achieved by inducing precise predetermined stresses into annealed glass by an extremely accurate controlled thermal process. This negates the inherent weakness characteristics of annealed glass. The stresses form a ‘sandwich’ effect within the body of the toughened glass, which must not be disturbed as, in certain circumstances, this could lead to immediate failure (breakage) of the glass, or at least reduce its performance life cycle.

Therefore no attempt must be made to drill, cut or work on the edges. Surface treatment (face of the glass) is included, e.g. any etching, sandblasting or similar must be carried out prior to toughening.

Toughened Glass Breakage

Toughened glass is a safety glass and, if impacted, will shatter into small, blunt, dice-shaped particles which will continue to remain in position (unless broken by a high impact projectile) until vibrations through the body begin to dislodge particles radiating outwards from the primary impact area. The BS857 standard requires toughened glass to break into particles with a minimum count of 40 in a 5cm square.

Laminated Glass Manufacturing

Two pieces of matched glass are placed on a metal form or ring and transported into the furnace where they are heated to produce the degree of sag required for the flat glass to take the shape of the mould. The temperature of the glass is controlled so as to retain its annealed properties.

The two pieces of glass are then separated, inspected and cleaned and an interlayer of polyvinyl butyral (PVB) is placed in between (normally 2 x 0.38 mm sheets). The windscreen is then autoclaved at a temperature of over 120°C and the pressure causes a superheating of the PVB interlayer - the autoclave acts like a pressure cooker. This modifies the original opaque structure of the PVB and makes it transparent.

Laminated Glass Handling

In the installation and storage of laminated glass, careful attention needs to be paid to protecting the glass edges as these are particularly vulnerable to chipping, shelling and cracking. Ensure that they never come into contact with any sharp metal or hard surface. Even slight damage could propagate into a crack later on in its life.

Always store laminated glass vertically, loose packed and in a dry atmosphere so that the PVB interlayer is not degraded by absorbing water and de-laminating the unit. Wet packaging material can give off alkalis which can attack the surface of the glass leaving a water mark.

Laminated Glass Breakage

Laminated glass, as it is constructed from annealed glass, cracks when broken. Cracks are generally caused by impact damage from stones. To locate an impact point, run a ballpoint pen along a crack - the pen will stop when it reaches an anomaly in the fissure.

Improper installation, deformity of the aperture or defective glass could put excessive pressure through the glass resulting in a non-impact fracture, or ‘stress crack’.

Indirect Glazing

This is a well established method of securing a glass into an aperture using a lip type of gasket. This is usually manufactured from EPDM ( Ethylene Propylene Diene Monomer ) in a moulded section.

The glass, which is smaller than the aperture, is fitted into the EPDM and secured by pulling the lip over the aperture flange. A filler strip, or lace, is inserted to expand the EPDM into a waterproof tight joint between the glass and aperture.

Some methods require a non-setting standard sealant to be applied during installation to ensure a watertight seal.

The EPDM can lose its elasticity due to constant UV light exposure and needs to be replaced occasionally to ensure the glass is correctly installed.

Later indirect glazed rail vehicles use Neoprene for its fire retention properties. The glass is secured to the vehicle by compressing a moulded section of Neoprene between the glass and the frame flange.

Direct Glazing

Advantages over an indirect glazing system are:

  • Glass can absorb some of the torsional body loadings increasing rigidity and strength
  • Expansion of engineering and design freedom
  • Aerodynamic and aesthetically styled flush glazing possible
  • Improved water resistance

This is a method of securing the glass directly onto the painted metal / aluminium / glass reinforced plastic (GRP) aperture with the use of an adhesive sealant.

The integration of the glass onto the vehicle body increases the torsional strength and rigidity of the vehicle and the direct glazed glass, if of sufficient substance to take the stress load, becomes a semi-structural component.

Because of the transference of stress to the glass, the adhesives must be tough but flexible enough to allow for normal body movement.

The glass also has to be free from any edge deformation. For direct glazing to be effective, discipline and control is required to ensure cleanliness and correct application of materials (Process Control).

In addition, there must be particular emphasis placed on the condition of the aperture substrate ensuring it is free from corrosion.

Manufacturers’ instructions on usage and storage of sealant adhesives and primers must always be observed.

Twin pack systems are chemical curing adhesives (those that cure from inside to out) and must be protected from UV light either by a single pack system moisture curing adhesive (those that cure from outside to in) or a trim section.

Understanding Urethanes

High modulus urethanes

High modulus urethane is a sealant adhesive with high resistance to distortion or flexing, which substantially increases the torsional rigidity of the vehicle body when used in glass installation. High modulus adhesives have a higher G-modulus (shear resistance) than standard adhesives.

The improved torsional rigidity can contribute to the overall vehicle structure by distributing load forces with the direct glazed glass.

Nonconductive urethanes

Nonconductive urethanes are used in the installation of glass in aluminium bodied vehicles. The carbon content in conventional or standard urethane, combined with moisture, produces a galvanic element which can cause corrosion and adhesion failure. Using a nonconductive urethane can prevent this and provide a safe and durable installation. Nonconductive urethanes also allow ‘clean’ antenna reception.

Safe to manoeuvre time

This is the time the urethane has built up sufficient structural ‘green strength’ in the early curing process to enable the vehicle to be manoeuvred in the depot. This is dependent on the ambient temperature at the time of glazing. Lower temperatures result in longer safe to manoeuvre times. UV stable in-fill urethanes are moisture curing so are dependent on the relative humidity at the time of glazing. Lower humidity readings also result in longer safe to manoeuvre times.

Working time

Working time is the period of time between the start to finish of the urethane bead extrusion and the installation of the glass in the aperture, or the amount of time available to work with the urethane before it starts to skin over.

Cold weather conditions

Cold weather will slow down the curing process but there is another important factor to consider - condensation. In cold weather, at temperatures of below +5°C, condensation is likely to build up on a glass or metal surface. This can occur when the bus, coach, train or the glass component is brought from a cold environment into a warm depot, or when glazing is attempted in a cold depot.

Any condensation, water or ice present on the vehicle aperture or glass unit will have a detrimental effect on the adhesion qualities of the urethane, and must be completely eliminated before bead application. It will not be possible to achieve a good bonding adhesion at ambient temperatures below +5°C.

Hot weather conditions

Hot weather has the effect of speeding up the curing process to a level where the chemical cross-linking of the urethane components begins to ‘gas-off’. This can occur at ambient temperatures of above +25°C, which is equivalent to an aperture surface temperature of up to +40°C.

At these temperatures, the working time is considerably reduced to below that which is feasibly possible.

In these conditions, time must be allowed for the aperture surface temperature to cool down to below +25°C before bead application can be made.

Repainted surfaces

Where a body aperture has been repainted with an air drying paint, it is necessary that the freshly painted surfaces are allowed to dry for a minimum of 24 hours prior to bonding application. This is to allow the solvents in the paint to completely evaporate.

Low bake paints will need to dry for 4 hours minimum prior to glass installation.

WARNING – Never allow any silicone products to be used in the glazing vicinity. Airborne silicone particles will migrate onto the aperture / glass surfaces and cause a breakdown in adhesion of the primer coverage.

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General Requirements

Transport safety glazing must have an approval to be fitted to any passenger carrying vehicle:

BS857:1967, Specification for safety glass for land transport

 

ECER43, Uniform provisions concerning the approval of safety glazing materials 24.02.1988

GM/RT2456, Structural requirements for windscreens and windows on railway vehicles

BS5544:1978, Anti-bandit glazing (glazing resistant to manual attack)

All these regulations include calibrated tests which check the:

  • Mechanical strength (impact resistance) - determined by the resistance to a controlled height and weight drop ball.
  • Fragmentation - determined by breaking the glass and evaluating the fragmentation structure.
  • Optical quality - tested by light transmission measurements under perpendicular light incidence and measuring distortion and secondary image reflections.

Body sideglasses and windscreens can incorporate a tint to reduce infrared transmission and keep the vehicle interior cooler. The stronger the tint, the less the cooling system needs to work. However, windscreens must have a minimum visible light transmission of 75%. This solar control is usually achieved by adding a colourant to the glass melt process or adding splutter metallic coatings. Short laminated glass production runs may achieve solar control by using a tinted interlayer. Body sideglasses, from the windscreen back to the pillar behind the driver (B-post), including signalling assemblies and passenger entrance door glass, must have a minimum visible light transmission of 70%.

The obscuration band on the glass, sometimes referred to as ‘frit’, is a ceramic enamel ink fired on during processing. Since the ceramic enamel has to meet adhesion, colour, opacity, and gloss standards, it plays an important role in the glazing system. Aesthetic standards are achieved by using dot fade-out patterns.

The primary function of the obscuration band is to protect the adhesive sealant bond line by blocking UV light radiation (which causes chemical degradation of the adhesive, leading to potential bond separation). Its opacity must be below 0.1% light transmission to provide this necessary UV protection.

The ceramic coating is also a link in the chain that retains the glass in place. The coating material is required to generate a strong bond to the glass and to the adhesive system. Lap shear test methods and cataplasma testing (14-days at 100% humidity and at 70-80°C) are widely accepted tests for the adhesion stability of the ceramic coating.

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Quality Assurance

Because the specifications for each bus body manufacturer and toughened glass processor are almost always different, it is not surprising that a plethora of engineering specifications exist, but all to meet a common requirement and standard.

There are only two constituent elements in a toughened glass part that could contribute to a difference between two separately manufactured glass products manufactured by separate glass processors – taken that both parts are dimensionally within tolerance with regards to shape, height, width and thickness. One is the standard to which the glass has been manufactured to; the other is the opacity and adhesion stability of the ceramic band.

1. Manufacturing Standards:

All new and replacement glass shall be safety glass and must comply with The Road Vehicles (Construction and Use) Regulations 1986, clauses 30-32 and as defined in the VOSA Vehicle Inspectorate Operations Manual.

Transport safety glazing must have an approval to be fitted to any passenger carrying vehicle and must be optically correct, free from defects and distortion and must be indelibly and distinctly marked in accordance with:

BS857:1967, Specification for safety glass for land transport and/or

ECER43, Uniform provisions concerning the approval of safety glazing materials 24.02.1988

These regulations include calibrated tests which check the:

  • Mechanical strength (impact resistance) - determined by the resistance to a controlled height and weight drop ball.
  • Fragmentation - determined by breaking the glass and evaluating the fragmentation structure.

Optical quality - tested by light transmission measurements under perpendicular light incidence and measuring distortion and secondary image reflections.

PSV Glass assures that all toughened glass products supplied, will comply with the above standards and will be kite-marked accordingly.

2. Ceramic Obscuration Band:

The obscuration band on the glass is a ceramic enamel ink fired on during processing at a specified density. Since the ceramic enamel has to meet adhesion, colour, opacity, and gloss standards, it plays an important role in the body – primer – adhesive – primer - ceramic enamel – glass, glazing system. Aesthetic standards are achieved by using dot fade-out patterns on the print stencil.

The primary function of the obscuration band is to protect the adhesive sealant bond line by blocking UV light radiation (which causes chemical degradation of the adhesive, leading to potential bond separation). Opacity specifications for UV light range from 300-400 nm and for visible light 380 – 780 nm. Opacity must be below 0.1% ultraviolet and visible light transmission, to provide the necessary protection.

The topography of the ceramic coating is also a link in the chain that retains the glass in place. The coating material is required to generate a strong bond to the glass and to the adhesive system. To qualify in a variety of time, temperature, humidity and light energy sources, lap shear test methods and cataplasma testing (14-days at 100% humidity and at 70-80°C) are widely accepted tests for the adhesion stability of the ceramic coating.

PSV Glass assures that all glass products supplied with a ceramic band will comply with the above opacity and adhesion stability requirements.

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Windscreens

Windscreens often have high-tech performance specifications. New interlayers and windscreen glass tints keep bus interiors cooler by reducing the solar load. Laminated glass is also acoustically superior. The latest generation of polyvinyl butyral (PVB) interlayers in buses can even provide added sound reduction to keep bus interiors quieter. However, the surrounding structure and fixing system needs to be taken into account as sound reduction is measured across the whole area. In the future, more buses will have laminated glass in their side windows.

Bus windscreens are made up of a thin layer (0.76microns) of plasticised PVB film bonding two pieces of glass together. Early windscreens made of tempered glass might be less likely to break if hit by a stone but, if they did break, the glass fragments would either cover the driving cab or, if the fragments stayed within the frame, would prevent the driver from seeing through the windscreen.

PVB is supplied to windscreen manufacturers as a thin film of precise thickness and chemical composition. This film is manufactured by mixing PVB resin with a plasticiser and other additives that add performance characteristics to the windscreen, such as limiting the amount of ultraviolet light that can pass through the laminated glass. There are several different manufacturers of PVB sheets in the world and, although the films are used in the same application, each is chemically different from the others.

E43r references the Society of Automotive Engineers’ SAE Z26.1, a performance standard composed of 32 separate requirements. The most important requires windscreens to be resistant to penetration by a 6.82kg ball dropped from a height of 2 metres. E43r also specifies that, following an external impact, the glass must be retained on the interlayer.

The specification within E43r that requires the windscreen to hold occupants inside the vehicle is met by the use of PVB. It stretches on impact and spreads the impact force across a larger portion of the windscreen. This stretching characteristic of PVB interlayer controls the head-injury criteria laid down in the standard. The PVB layer is designed to tear when the impact forces are so great that the driver would suffer greater injury if the PVB were to remain in place. It is the complex chemical formulation of the PVB that ensures that the interlayer will work as designed over the life of the windscreen.

Specifically, it is the manner in which the PVB interlayer adheres to the glass that controls the performance of the system when impacted. Windscreen manufacturers make great efforts when fabricating windscreens to ensure that the glass is clean, the source is consistent and the orientation (the tin side versus air side during float-glass manufacturing) is as required. Most manufacturers even control, within very fine tolerances, the quality of the water they use to rinse the glass before assembly.

Other than dirt, water is another material that can prevent PVB from performing. To maintain the designed level of glass-to-PVB adhesion, the moisture content of the PVB must be closely controlled at manufactured levels. A variation by as little as 0.2% from the required moisture can result in less than optimal performance. PVB interlayers are packaged and shipped in moisture barrier containers because PVB film can absorb moisture from the air. Windscreens are produced in rooms with controlled humidity that prevent a change in the moisture content of the PVB prior to the moisture being locked in between two pieces of glass.

In the late 1990s, a new generation of PVB interlayer was introduced that reduced the effect that moisture has on the adhesion to glass. It is the development of PVB with reduced adhesion sensitivity to moisture that re-introduced laminated glass in vehicles’ side windows with exposed edges. Even with this improved product chemistry, however, PVB adhesion to glass will decline if the interlayer is allowed to absorb moisture.

If moisture is allowed to diffuse into the interlayer, whether at the time of manufacturing or through a crack in a broken windscreen, the adhesion between the interlayer and the glass will decline. Subsequent impact of the glass in the region of this increased moisture could result in spalling, which is characterized by glass fragments separating from the interlayer and potentially striking an occupant.

A dried-out interlayer can also change safety performance. At moisture levels below the design specification, the adhesion between the interlayer and the glass can become too high and the interlayer’s ability to resist penetration can be reduced. Impact to a windscreen with too low a moisture content can result in unexpected penetration of the interlayer.

Windscreen repair is accomplished by injecting an optically matched resin into the damaged area. The resin removes the air and fills the area, preventing further cracking. A windscreen repair carried out correctly should improve the optical clarity of the damaged area and restore the integrity of the glass. If vision remains impaired, the windscreen should be replaced. A code of conduct ( BS AU242a:1998) defines the permissible repair zones and size of repair.

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