Electric heating – Inductive heating – With heat exchange
Reexamination Certificate
1999-10-21
2001-11-06
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
With heat exchange
C219S603000, C219S634000, C156S272400, C156S274200
Reexamination Certificate
active
06313448
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for bonding substrates together, more particularly to a method which impacts the strength of the bond between a bonding agent and an electrically conductive substrate, and even more particularly to a method utilizing induction heating to heat an electrically conductive substrate before or upon application of the bonding agent to the substrate. The present invention also relates to a device for bonding two substrates which incorporates an induction heating system for improving the bond strength.
BACKGROUND OF THE INVENTION
In bonding processes, it is often important to provide a bonding agent that is capable of providing sufficient green strength in a relatively short period of time in order to hold objects together until the adhesive cures or solidifies. Green strength is equated with the shear strength of the bonding agent. The ability to achieve green strength is often dependent on the wetting of the bonding agent upon application and contact to a substrate. Certain substrates, or their respective physical properties, can adversely affect the wet out properties of the bonding agent. Additionally, processing conditions during application can affect the wet out properties of the bonding agent. The inability to obtain sufficient wet out will result in a weak bond between the bonding agent and the substrate.
SUMMARY OF THE INVENTION
The present invention involves a method for improving the bond strength of bonding agents applied onto electrically conductive substrates. The method of the present invention improves the wet out properties at the contact surface between the bonding agent and an electrically conductive substrate.
The present invention utilizes a non-conductive substrate having a bonding agent attached to at least a portion of the substrate. The bonding agent can be a pressure sensitive adhesive or a hot melt adhesive or it can include at least an exposed layer of a pressure sensitive adhesive or a hot melt adhesive. The utilization of a hot melt adhesive requires the heating of the adhesive to a bondable state. An electrically conductive substrate is then heated with an induction heating system. The electrically conductive substrate is suitable for receiving the bonding agent. The electrically conductive substrate has a greater thermal conductivity than the bonding agent. The contact surface of the substrate is heated to a temperature sufficient to enhance the wet out of the bonding agent upon application onto the substrate.
The pressure sensitive adhesive and hot melt adhesives are generally solid at room temperature and substantially stable in form and dimension. The hot melt adhesive can include thermoplastic hot melt adhesives or thermoset hot melt adhesives. Thermoset hot melt adhesives generally comprise heat-activated adhesives, moisture activated adhesives, radiation or light activated adhesives. The viscosity of the material at the contact surface of the pressure sensitive adhesive or the hot melt adhesive is affected by the elevated temperature of the contact surface of the electrically conductive substrate. The elevated temperature of the electrically conductive substrate enhances the ability of the pressure sensitive adhesive or the hot melt adhesive to wet out the substrate. With pressure sensitive adhesives, the method of the present invention beneficially affects the rate for building adhesive strength. The method results in the formation of a cohesive bond between the bonding agent and the electrically conductive substrate.
The present invention also includes a device for inductively heating a substrate to enhance the wetting properties of a bonding agent applied onto the substrate. The device includes a support for releasably holding a non-conductive substrate with a bonding agent affixed to at least a portion of a major surface of the non-conductive substrate. An induction heating system is attached to at least a portion of the support. The support is positioned near an electrically-conductive substrate. Upon activation of the induction heating system, the electrically conductive substrate is heated to beneficially affect the wet out of the bonding agent on the surface of the electrically conductive substrate.
It is an advantage to inductively heat an electrically conductive substrate in order to enhance the bond between the substrate and a bonding agent. The heated substrate enhances the wet out of the bonding agent at the surface or interfacial contact point between the electrically conductive substrate and the bonding agent.
For purposes of the present invention, the following terms used in this application are defined as follows:
“wet-out” means the ability of the adhesive to spread out over and bond to the contact surface prior to reaching a cured or solid state.
“electrically-conductive” means any ferromagnetic material or inductively responsive material capable of generating heat when subjected to an inductive field;
“non-conductive” means any material which exhibits a resistance to the transmittance of electricity or heat;
“bonding agent” means an adhesive material that is solid at room temperature and substantially stable in form and dimension;
“bondable state” means the adhesive, upon heating, posesses a measurable viscosity and is capable of wetting out upon application to a substrate;
“green strength” means initial holding power of the adhesive to a substrate either before or after initial curing or solidification;
“pressure sensitive adhesive” means adhesives that are normally tacky at room temperature and firmly adhere to a wide variety of dissimilar surfaces upon mere contact without the need for more than finger or hand pressure;
“hot melt adhesive” means a material that is substantially non-tacky at room temperature but is capable of being heated to a viscous state to form a bond to a substrate by wetting out the substrate and subsequently cooling to form a bond;
“tackification” means a state in which the adhesive reaches a temperature at which the adhesive is capable of wetting out and forming a bond with a substrate;
“thermal conductivity (k)” is defined as the time rate of transfer of heat by conduction, through unit thickness, across unit area for unit difference of temperature; and
“heat capacity (Cp)” is defined as the quantity of heat required to increase the temperature of a system or substance one degree of temperature;
Other features and advantages will be apparent from the following description of the embodiments thereof, and from the claims.
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3M Brochure entitled “Application Procedures for Pressure-Sensitive Body Side Moldings”, published more than one year before Oct. 29, 1999.
3M Innovative Properties Company
Knecht III Harold C.
Leung Philip H.
Szymanski Brian E.
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