Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2000-07-06
2001-09-25
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C523S455000, C523S461000, C525S533000
Reexamination Certificate
active
06294259
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a polyimide hybrid adhesive comprised of an epoxy resin, an epoxy curing agent, and a polyimide oligomer of molecular weight up to about 8,000 grams/mole (M
n
) having a backbone that is unreactive with an epoxy resin. The polyimide oligomer may, optionally, be terminated by one or more functional groups that are reactive with an epoxy resin.
BACKGROUND OF THE INVENTION
Epoxy resins have found a wide range of uses in many industries since their commercial introduction. Industries in which epoxy resins are utilized as adhesives and coatings include the construction and electronics industries. Cured epoxy resins are noted for their low shrinkage, lasting adhesion, high dielectric strength, and chemical resistance. A major limitation of epoxy resins, particularly those intended for high temperature applications, is their inherent brittleness arising from their crosslinked structure. It is documented that development of approaches to toughening epoxy resins without sacrificing modulus and lowering the glass transition temperature (T
g
) would lead to an expansion of their use in areas such as primary aircraft structures, molding compounds and electrical and electronic components.
Methods for increasing toughness of epoxy adhesives are desired. In applications in the electronics industry, however, compositions that also maintain good thermal stability and adhesion to a wide variety of substrates are desirable.
SUMMARY OF THE INVENTION
One method for toughening epoxy adhesives involves the incorporation of thermoplastic toughening agents into epoxy resins. Polyimides are examples of thermoplastic resins that may be used for toughening epoxies. The resulting epoxy-polyimide adhesives are defined as polyimide “hybrids.” Polyimides are synthetic organic resins characterized by repeating imide linkages in the polymer chain. They are noted for their outstanding chemical and physical properties, particularly their high temperature oxidative stability and strength. However, most conventional polyimides, especially aromatic polyimides, are extremely difficult to process due to their high softening points, as well as their insolubility in organic solvents.
It is desirable to add these polyimide resins to epoxy resins in amounts great enough to maximize the toughness of the resulting polyimide hybrid adhesive composition. For many applications, however, it is desirable to also maintain low viscosity in the adhesive composition so that it may be hot-melt processed into useful adhesive films. Therefore, according to the present invention, polyimide is incorporated into an epoxy adhesive composition in the amount of preferably about 5% to about 60%, more preferably about 20% to about 40% by weight, based on total weight of the adhesive composition, to provide enhanced physical properties (such as a high glass transition temperature, T
g
) while maintaining a viscosity that is low enough to allow hot-melt processing of these adhesives.
Polyimide hybrid adhesives of the invention also have many advantages associated with cure thereof. Polyimide hybrid adhesives of the invention may be cured with a controlled rapid cure. “Controlled rapid cure” means being able to obtain rapid onset of cure utilizing thermal or photonic energy input. Controlled rapid cure allows rapid part assembly. Polyimide hybrid adhesives of the invention are also able to be cured at low temperatures. Low curing temperatures are also desirable because this allows these adhesive systems to be utilized in a wider variety of applications. In addition, minimizing thermal input helps save energy when these adhesives are used in assembly operations.
Polyimide hybrid adhesives of the invention are also able to be processed in an essentially solvent-free manner. Solvent-free polyimide hybrid adhesives are desirable as they minimize worker exposure to organic vapors and significantly reduce the risk of fire and explosion. These solvent-free polyimide hybrid systems also contribute significantly to compliance with stringent air quality standards.
Furthermore, when processing polyimide hybrid adhesives of the invention according to the preferred solvent-free manner (e.g., by hot-melt processing), residual solvent in the processed adhesive is eliminated, or at least reduced. Residual solvent in polyimide/epoxy adhesives may create bubbles in adhesive bonds (i.e., “popcorning”). Such popcorning may contribute to adhesive failure and/or corrosion of electronic components. Solvent-free processing according to the invention eliminates the need for resource-consuming solvent removal steps.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is comprised of a thermosettable blended polyimide hybrid adhesive composition comprising a polyimide oligomer, preferably a polyetherimide oligomer, having a molecular weight of less than about 8,000 grams/mole (M
n
) and containing a repeating backbone unit of the Formula (I):
where terminal end group Z is selected from:
and where terminal end group Z′ is selected from:
R
1
and R
2
are independently selected from hydrogen, halogens, alkyls having about 1-12 carbon atoms, alkoxys having about 1-12 carbon atoms, and aminos. As can be seen from the above chemical structures, the terminal end groups may, optionally, contain one or more functional groups that are reactive with an epoxy resin. An epoxy resin and an epoxy curing agent are also included in this polyimide hybrid adhesive composition.
The number average molecular weight (M
n
) of the polyimide oligomer is less than about 8,000 grams/mole. This requires that n in Formula I be controlled to a range of about 3 to about 6, on average, in the oligomer backbone.
The compositions are thermosettable. A “thermosettable” or “thermosetting” composition is one which can be cured (i.e., crosslinked), for example by exposure to, preferably, thermal radiation, actinic radiation, moisture, or other means, to yield a substantially infusible (i.e., thermoset) material. Combinations of various curing means may also be used (e.g., a combination of heat and actinic radiation).
This polyimide hybrid adhesive composition may be solvent-free and can be manufactured and applied in a hot-melt process utilizing melt processing of the adhesive components. The adhesive compositions of the present invention are particularly useful in applications that require heat resistance such as laminating adhesives, flip chip adhesives, anisotropic conductive adhesives, covercoats, and encapsulants for electronic components in the electronics industry.
The polyimide oligomer useful in the present invention may be synthesized by reacting 2,2′-bis[4-(2,3 dicarboxylphenoxy)phenyl]propane dianhydride with phenylenediamine. A solvent-free method for manufacturing these polyimide oligomers is disclosed in U.S. Pat. No. 4,585,852 to Lo et al., wherein the process involves (a) extruding a first mixture of an organic diamine and an aromatic bis(ether anhydride) under melt polymerization conditions to form a low molecular weight, anhydride-terminated polymer, wherein the first mixture contains a molar excess of the bis(ether anhydride).
The anhydride-terminated polymer of step (a) may be optionally reacted in a step (b) by extruding a second mixture of the low molecular weight anhydride-terminated polymer of step (a) with a stoichiometrically sufficient amount of aniline moiety, or functionalized aniline moiety, under melt polymerization conditions such that this anhydride-terminated polymer becomes terminated with functional groups, which functional groups may optionally be reactive with epoxy groups. For example, the reaction of step (b) may be done by reacting the terminal anhydride groups of the polymer formed in step (a) with 3-aminophenol, m-phenylenediamine, or a phenylamine moiety having two additional groups, R
1
and R
2
, substituted on the phenyl ring. R
1
and R
2
are independently selected from hydrogen, halogens, aminos, alkyls having about 1-12 carbon atoms, and alkoxys having abou
Anderson Gregory J.
Charles Scott B.
Kropp Michael A.
3M Innovative Properties Company
Aylward D.
Dawson Robert
Pribnow Scott R.
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