Stationary Glass Replacement: Materials, Preparation and Installation

Purpose

If a vehicle sustains collision damage and stationary glass removal is necessary, it is extremely important that the glass opening receive proper metal preparation and refinishing prior to the installation of any stationary glass. Although the actual area of repair is not likely to be seen by the vehicle owner, the importance of superior paint adhesion and metal corrosion protection cannot be over-emphasized. If the primer and paint products are incompatible or do not adhere when applied, the urethane glass adhesives will not properly bind the glass to the vehicle. This could lead to an increased chance of occupant injury in the event of a vehicle impact or rollover.

Introduction

The United States Government has developed numerous safety standards which must be met by all vehicles sold in this country. The vehicle windshield is covered by FMVSS (Federal Motor Vehicle Safety Standard) 212 which requires that the windshield act as a safety net to keep the occupants of the vehicle within the confines of the passenger compartment during a crash. FMVSS 216, which also deals with the vehicle windshield, sets minimum strength requirements for a passenger car roof to reduce the likelihood of roof collapse in a rollover accident. Most automotive manufacturers have gone beyond the requirements of FMVSS 212 and consider all stationary glass to be structural components that contribute to the overall crash-worthiness of the vehicle. To achieve this level of glass retention, the automotive manufacturers use urethane adhesives which may have strength ratings as high as 600 psi (pounds per square inch).

While there are currently no FMVSS requirements set for bonding strength, most of the automotive manufacturer's windshield retention requirements exceed the level of performance established by FMVSS 212. The Society of Automotive Engineers (SAE) has published two Recommended Practice standards that deal with windshield replacement and automotive glass bonding. SAE J1556 (STATIONARY GLASS REPLACEMENT) provides guidance to automobile repair shops and personnel in the replacement of stationary glass when damaged in collisions. SAE J1836 (OVERLAP SHEAR TEST FOR SEALANT ADHESIVE BONDING OF AUTOMOTIVE GLASS ENCAPSULATING MATERIAL TO BODY OPENING) defines a procedure for the construction of a lap shear specimen for the purpose of testing the bondability of an automotive sealant adhesive to the elastomeric material used in automotive glass encapsulating.

Table 1 lists several adhesive types and their respective strengths.

Table 1
Relative Strengths of Windshield Sealants

1.	Butyl Tape 13 psi
2.	Liquid Adhesive 50 psi
3.	Silicone Sealant 115 psi
4.	Construction Urethane 185 psi
5.	Polysulfide 190 psi
6.	OEM Approved Urethane 600 psi

Windshield sealant data derived from GM Product Service Bulletin: Installation and Resealing of Bonding Windshield and Back Lights

While Butyl Tape (13 psi) has been used in the past to meet the requirements of FMVSS 212, most manufacturers currently use urethane adhesives with significantly higher bonding strengths. These adhesives are chosen by each manufacturer's engineers to satisfy the crash-worthiness requirements of the vehicle design. This normally takes into account the contributory affects of all stationary window glass.

Each automotive manufacturer has its own bonding strength requirements which can be difficult for the individual service shop to determine. Since there are no known adverse effects from using an adhesive that is too strong, the use of an adhesive/sealant with at least a rating of 600 P.S.I. is the safest method for installing all stationary glass, including windshields. However, it must be remembered that the strongest adhesive cannot provide adequate glass retention if the paint and primer do not adhere adequately to the metal surfaces.

Metal Preparation and Installation

If a vehicle has sustained damage requiring glass removal and/or replacement, all glass opening repairs must be performed correctly prior to installing the new glass. To begin this procedure, the window frame must first be returned to its original pre-accident condition via straightening or replacement. After all repairs are performed and welding completed, all areas on the pinch weld flange must be properly dressed. This will remove any protrusions which might contact the glass during or after installation.

During the refinishing process, both surface cleanliness and proper preparation are key to developing a proper glass to metal bond. (Surface preparation will include solvent cleaning, sanding or light grinding for corrosion removal, and chemical etching when necessary). Using a single manufacturer's line of products will insure product compatibility and maximize surface adhesion when the manufacturer's recommendations are followed. Proper surface cleanliness and preparation will also minimize the opportunity for corrosion to work its way under the adhesive and destroy the urethane bond.

Since primers, sealers and color coats separate the metal from the urethane sealants, they contribute significantly to the structural integrity of the vehicle by joining the glass adhesive to the vehicle metal. All bonding areas should be refinished with extreme care and precision prior to the actual bonded glass installation. The refinishing process should be of the latest technology and should include materials with the best adhesion properties applied as per the manufacturers instructions. (Materials will include epoxy, self-etching primers and urethane or catalyzed top coats).

Figure 1 shows the installation of the urethane between the vehicle pinchweld and the windshield glass.

In this figure the urethane is applied to a completely straightened and painted surface. The urethane adhesive should not be applied to unclean or improperly refinished surface areas as this could weaken the bond, resulting in a windshield that does not meet the manufacturer's safety requirements. All loose adhesive and paint materials must be removed and the underlying surface area refinished before preparing the surface for urethane bonding.

Air Bags

With the availability of passenger's-side air bags on many vehicles, the proper preparation of the windshield bonding surface takes on special importance. Unlike driver's-side air bags which are mounted in the steering wheel and use the steering column for support, the passenger's-side air bag is mounted in the dash-board and frequently uses the windshield for support while inflated. On some vehicles the windshield also acts as a reactionary surface to properly position the bag during deployment. Because of this additional stress on the windshield, the quality of the metal preparation, paint refinishing and glass installation must be performed with extreme care. Consult the automotive and adhesive manufacturers for specific product and application information.

The information provided in this bulletin is for educational purposes only. Although every effort has been made to ensure the accuracy of the information contained in this bulletin, Tech-Cor assumes no responsibility or liability for any repairs performed using information from any publication issued by Tech-Cor.

Any person performing repairs must determine whether any suggested or recommended procedures or repairs are suitable or appropriate for the particular vehicle being repaired. The repairer remains solely responsible for such determination, as well as for the proper completion of the repairs.

Reproduction of this bulletin is not permitted without the written approval of Tech-Cor.