RF active compositions for use in adhesion, bonding and coating

Details RF active compositions for use in adhesion, bonding and coating RF active compositions for use in adhesion, bonding and coating

2000-01-13

2002-02-19

Walberg, Teresa (Department: 3742)

Electric heating

Inductive heating

With heat exchange

C428S345000

Reexamination Certificate

active

06348679

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the use of media containing ionic compounds and/or nonionic compounds with high dipole moments as a radio frequency (RF) susceptors in RF activated systems.
2. Related Art
Radio frequency (RF) heating is a well established non-contact precision heating method that is used to generate heat directly within RF susceptors, and indirectly within materials that are in thermally conductive contact with RF susceptors. RF susceptors are materials that have the ability to couple and convert RF energy into heat energy within the material.
Conventional adhesives are not suitable RF susceptors that can be directly heated and activated by RF heating. Rather, these conventional adhesives are typically heated indirectly through thermally conductive contact with an RF susceptor material.
FIG. 1
illustrates two conventional methods that are currently used in industry for indirect RF heating of conventional adhesives: The first method is illustrated in
FIG. 1A
, where susceptor material
102
exists as a bulk macroscopic layer. RF susceptor material
102
is directly heated by RF energy, and adhesive layer
104
is indirectly heated through thermally conductive contact with RF susceptor material
102
. For example, adhesive layer
104
may be applied to a continuous surface of susceptor material
102
, such as steel or aluminum. The second method is illustrated in
FIG. 1B
, where susceptor material
112
consists of discrete macroscopic particles. Adhesive layer
114
is loaded with macroscopic particles of a RF susceptor material
112
, such as macroscopic particles or flakes of metal oxides, metallic alloys, or aluminum. With this conventional method, each RF susceptor particle
112
acts as a discrete RF susceptor, generating heat throughout adhesive layer
114
.
An example of a conventional RF energy activated composition, such as that shown in
FIG. 1B
, is described in U.S. Pat. No. 5,378,879, issued to Monovoukas (“Monovoukas”). Monovoukas utilizes macroscopic “loading particles” as discrete RF susceptors. The particles are heated by RF energy and in turn conduct heat to the surroundings. These macroscopic loading particles are thin flakes (i.e. in thin disk-like configuration) that are designed to be admixed to relatively thick extruded materials. However, these flakes are not well suited for use as susceptors in thin film bonding applications in which physical distortions, discolorations in the surface, or opacity of the bonded films would result from the flakes.
Another example of a conventional inductively activated adhesive is described in U.S. Pat. No. 3,574,031, issued to Heller et al. (“Heller”). Heller describes a method of heat welding thermoplastic bodies using an adhesive layer that contains uniformly dispersed macroscopic RF susceptors, typically iron oxide particles. These discrete RF susceptor particles are ferromagnetic in nature. A disadvantage of this type of method is that a tradeoff must be made between the size of the particle employed versus the power level and duration of the inductive heating process. For example, if susceptor particles are kept small in size, the mechanical strength of the bond tends to increase. However, as the size of these discrete susceptors is reduced, the power levels and dwell times required to heat the RF susceptor material and achieve acceptable bonds tend to increase. Another disadvantage of this type of method is the high levels of loading of the medium with RF susceptor particles that is required for efficient activation. Such high loading levels detract from the physical properties and rheology of the adhesive composition. Still another disadvantage is the dark color and opacity of the composition, which renders the composition undesirable for many applications.
An example of adhesive activated by a dielectric process is described in U.S. Pat. No. 5,661,201, issued to Degrand (“Degrand”). Degrand describes a thermoplastic film including at least one ethylene copolymer and a sufficient quantity of N,N-ethylene-bisstearamide that is capable of being sealed utilizing a current at a frequency of about 27.12 megahertz (MHz). A disadvantage of this type of film and sealing process is the inherent tendency to also heat the adherand.
U.S. Pat. No. 5,182,134, issued to Sato, discloses methods of curing a thermoset composition by applying an RF signal having a frequency of about 1 to 100 MHz to a composition comprising a major portion of a thermoset and a receptor. The receptor is described as being one of the alkali or alkaline earth metal sulfate salts (e.g. calcium sulfate), aluminum trihydrate, quaternary ammonium salts, phosphonate compounds, phosphate compounds, polystyrene sulfonate sodium salts or mixtures thereof. According to this patent, all of the exemplified compositions took longer than one second to heat.
U.S. Pat. No. 5,328,539, issued to Sato, discloses methods of heating thermoplastic susceptor compositions by applying an RF signal having a frequency of about 1 to 100 MHz. The susceptors are described as being one of the alkali or alkaline earth metal sulfate salts (e.g. calcium sulfate), aluminum trihydrate, quaternary ammonium salts, phosphonate compounds, phosphate compounds, polystyrene sulfonate sodium salts or mixtures thereof. According to this patent, all of the exemplified compositions took longer than one second to heat.
U.S. Pat. No. 4,360,607, issued to Thorsrud, discloses a composition suitable for sensitizing thermoplastic compositions to the heating effects of microwave energy comprising (1) an alcohol amine or derivative thereof, (2) a simple or polymeric alkylene glycol or derivative thereof, (3) silica and, optionally, (4) a plasticizer.
U.S. Pat. No. 5,098,962, issued to Bozich, discloses a water dispersible hot melt adhesive composition comprising:
(a) from about 40% to 95% by weight of a water dispersible ionically substituted polyester resin having a molecular weight from about 10,000 to about 20,000 daltons;
(b) from about 60% to about 5% by weight of one or more compatible plasticizers; and
(c) from about 0.1% to about 1.5% of one or more compatible stabilizers of the anti-oxidant type.
Examples of plasticizers that may be used according to this patent include one or more low molecular weight polyethylene glycols, one or more low molecular weight glycol ethers, glycerin, butyl benzyl phthalate and mixtures thereof.
U.S. Pat. No. 5,750,605, issued to Blumenthal et al., discloses a hot melt adhesive composition comprising:
(i) 10 to 90% by weight of a sulfonated polyester condensation polymer;
(ii) 0 to 80% by weight of a compatible tackifier;
(iii) 0 to 40% by weight of a compatible plasticizer;
(iv) 5 to 40% by weight of a compatible wax diluent with a molecular weight below 500 g/mole containing at least one polar functional group, said group being present at a concentration greater than 3×10
−3
equivalents per gram;
(v) 0 to 60% by weight of a compatible crystalline thermoplastic polymer; and
(vi) 0 to 3% by weight of a stabilizer.
What is needed is a composition (e.g. adhesive composition or coating) containing either dissolved or finely dispersed susceptor constituents that are preferably colorless or of low color. Further, the composition should be transparent or translucent throughout an adhesive matrix or plastic layer. This type of RF susceptor will result in more direct and uniform heating throughout an adhesive matrix or plastic layer. Further, it is desirable that such a composition will allow bonding with no physical distortion or discoloration in the bonded region of thin films. Still another desirable feature is activation of the RF susceptors at frequencies, e.g. of about 15 MHz or below, most preferably about 13.5 MHz, which are more economical to generate than higher frequencies and do not substantially heat dielectric substrates. A further desirable feature is that the composition can be activated or melted in less than one second and that it exhibit acceptable shear strength. It

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