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One-Part, Primerless Windshield Adhesive
ISSN: 0148-7191, e-ISSN: 2688-3627
Published February 01, 1990 by SAE International in United States
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A one-part, primerless, fast curing, adhesive sealant designed for structurally bonding the stationary glass to automotive frames has been developed. The hybrid polyurethane adhesive is autocatalytic, cures at room temperature, reaches the handling strength in a matter of minutes, and is suitable for applications requiring deep section vulcanization. By eliminating primers and reducing the number of interfaces and by bonding directly over electro deposited surfaces, chemical bonding and a superior adhesion is obtained. The system should increase the production rates in the assembly plants and cut down the application costs.
Significant advances have been made in the area of primerless one-component fast curing adhesive sealants. The purpose of this paper is to describe a new system and a simplified bonding method which shows enhanced adhesion and excellent durability. Conventional one-part moisture cure urethanes are slow cure and fail to cure altogether if large bond lines are involved. Two-part systems tend to require fairly complex meter-mix and dispense equipment and need continuous maintenance.
Ordinarily in order to obtain good bonding strength, joint surfaces must be prepared and primed. The primer adds another interface to the existing layers, making the subtrate surface and structure more complex and therefore the adhesive bonds less reliable. In addition the assembly is more complicated with a primer and requires more time than a one-step primerless application.
Automotive adhesives must meet a number of requirements which are more or less independent of joint performance. They must be usable under conditions which include: high production rates with short, unvarying times for each operation; low tolerance for health and safety hazards; and factors such as cure times, pressures, and temperatures which are somewhat variable. Once these factors and requirements have been met and the adhesive bonding is made, the joint must perform for the life of the car under very severe conditions. No other mass produced product, approaching the complexity of a modern automobile, is expected to function under wide ranges of temperatures, salt water, fuel, oil, high humidity, vibration, impact and dust.
Over the past few years, there have been significant changes in the industry that have necessitated major improvements in the structural adhesives used. The requirement for higher productivity has led to the development of room temperature fast curing adhesive sealants.
In developing a one-part fast curing adhesive, parameters such as shelf life, open time, cure rate and handling strength of the bonded joints must be considered.
Conventionally, in order to obtain a good bonding strength, joint surfaces must be prepared. Such a surface preparation particularly in bonding glass to the body of the car is accomplished by priming the painted surface. When primers are required the assembly is most complicated and needs more time than does the one-step application of an adhesive which does not require a primer.
The need for a primerless sealant system is a result of the following: Motor vehicles today are coated with a wide variety of paints. Each paint has its own unique formulation and characteristics. Minor changes in the paint formulations may lead to a totally new surface property and therefore, adhesive failure in the glass bonding system. Paint formulations undergo changes on a frequent basis. These changes include, but are not limited to, changes in paint reducing solvents, changes in minor additives, yearly color changes, and other changes dictated by the user location and application parameters.
Today's typical automotive paint system is composed of electrocoat paint, primer surfacer, base coat (metallic and non-metallic) and clear coat. Over this paint system, a special sealant primer is needed to insure the proper bonding of the adhesive sealant to the painted metal. Each quality of automotive paint may require a unique adhesive primer to insure proper bonding. Figure 1, illustrates a typical bonding system.
Neglecting the electrocoat steel interface, there are six interfacial regions present in the coating system.
Sealant to sealant primer interface.
Sealant to primer to clear coat interface.
Clear coat to base coat interface.
Base coat to aluminum or mica interface.
Base coat to primer surfacer interface.
Primer surfacer to electrocoat interface.
By applying the sealant directly over the electrocoat, the number of interfaces is reduced to one. This is illustrated in Figure 2. The electrocoat steel interface is so strong that it is virtually impossible to remove. By eliminating five interfacial regions, the potential failure of the bonding system has been greatly reduced. In addition, it is possible to realize the cost savings from this technique.
Extensive research efforts have led to the development of a one-component, primerless, fast curing adhesive which requires no surface preparation when used on electrocoat surfaces. Furthermore, this adhesive system has excellent physical and performance properties.
The one-part, primerless, fast curing adhesive combines the properties of both silicone RTV sealants and polyurethane adhesives in a single formulation. The hybrid polyurethane sealant contains urethane segments for strength, toughness, elasticity, and organofunctional silane moieties for faster cure properties. In addition, the presence of the polyurethane network in the cured sealant will lend typical polyurethane properties of toughness and elasticity to the cured adhesive system; whereas, the crosslinks and cures derived from the silane groups, present in the uncrosslinked polymer, will promote tenacious adhesion of the cured polymer system to non-porous surfaces, such as glass or metals, without the use of primer system. The cure is initiated by a catalytic amount of water found on any surface or work environment, the reaction is autocatalytic, and the adhesive continues to cure, both from inside out as well as outside in. Therefore, the sealant cures rapidly in applications requiring deep section vulcanization. The bond reaches the handling strength depending on the formulation from a matter of minutes to in excess of one hour with ultimate cure at about 24 hours.
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