Thermal stability Polyimide adhesives

More recently, St. Clair and co-workers176) reported the use of aromatic amine terminated polydimethylsiloxane oligomers of varying molecular weights in an effort to optimize the properties of LARC-13 polyimides. They observed the formation of two phase morphologies with low (—119 to —113 °C) and high (293 to 318 °C) temperature Tg s due to siloxane and polyimide phases respectively. The copolymers were reported to have improved adhesive strengths and better thermal stabilities due to the incorporation of siloxanes. 

Metal ion modified polyimide films have been prepared to obtain materials having mechanical, electrical, optical, adhesive, and surface chemical properties different from nonmodified polyimide films. For example, the tensile modulus of metal ion modified polyimide films was increased (both at room temperature and 200 0 whereas elongation was reduced compared with the nonmodif ied polyimide (i). Although certain polyimides are )cnown to be excellent adhesives 2) lap shear strength (between titanium adherends) at elevated temperature (275 0 was increased by incorporation of tris(acetylacetonato)aluminum(III) (2). Highly conductive, reflective polyimide films containing a palladium metal surface were prepared and characterized ( ). The thermal stability of these films was reduced about 200 C, but they were useful as novel metal-filled electrodes ( ). 

Following this initial period of polyimide development, interest reached a steady-state and remained there until the late 1970s. During this time a major impetus to the polyimide area was provided by the aerospace industry. The need for composite matrix resins as well as structural adhesives with excellent oxidative and thermal stability appeared to be at least partially met by polyimide type resins. Ultimately, requirements of high flow and low void content in relatively thick parts directed these efforts into different directions. Another upswing occurred in the early 1980s with the potential application of... 

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

Epoxy Coreactants. One of the most successful epoxy coreactant systems developed thus far is an epoxy-phenolic alloy. The excellent thermal stability of the phenolic resins is coupled with the valuable adhesion properties of epoxies to provide an adhesive capable of 371°C short-term operation and continuous use at 175°C. The heat resistance and thermal-aging properties of an epoxy phenolic adhesive are compared with those of other high-temperature adhesives in Fig. 15.5. Epoxy-phenolic adhesives are generally preferred over other high-temperature adhesives, such as the polyimides and polybenzimidazoles, because of their lower cost and ease of processing. 

Kapton polyimide has been widely used in the electronic industry because of its low dielectric constant, good mechanical properties and high thermal stability. Many applications require good adhesion between Kapton polyimide film and metal. Various processes to improve adhesion of metal to Kapton polyimide have been reported in the literature. DeAngelo et al., (D describe a process to form metal oxides on the surface of polyimide to improve adhesion. Other efforts to improve adhesion of a metal layer involve roughening of the surface of polyimide substrate by methods such as cathodic sputtering (2), chemical attack (2., 1), and reactive ion etching (1,4). 

Aromatic polyimides have found extensive use in electronic packaging due to their high thermal stability, low dielectric constant, and high electrical resistivity. Polyimides have been used as passivation coatings, (1) interlayer dielectrics, (2) die attach adhesives, (3) flexible circuitry substrates, (4) and more recently as the interlevel dielectric in high speed IC interconnections. (5) High speed applications require materials with a combination of low dielectric constant, flat dielectric response versus frequency and low water absorption. 

Because of the improved thermal stability for PADS containing polyimides vis a vis other available siloxane polyimides, a screening program to correlate properties such as Tg, solubility, thermal stability, adhesion properties, and water absorption characteristics to structure was undertaken. Several copolymers were prepared from diamines and co-dianhydrides. An ODAN/ ODA copolymer in which 30 mole % PADS was substituted for ODAN, was prepared and TGA analysis at 450°C indicated that the material was the first siloxane containing polyimide identified that exceeded the established thermal stability criteria for interlevel dielectric applications. Stability and solubility of these materials as a function of PADS concentration is illustrated in Table VI. 

Thermal treatment of the metal-polder structures serves at least two purposes to test their thermal stability and to enhance the adhesion if possible. One certainly would like to have both the stability and enhanced adhesion after such a treatment. Oftentimes, however, thermal treatment only leads to degraded adhesion between the metal and polymer, as noted for polyimide The thermal treatment may also generate useful information regarding the interactions between the metals and polymers. The Cu/PTFE system was heated between 350 and 400°C, with a mild enhancement in adhesion, as reported earlier Here, we describe some of the more recent results using polymer films in both mixed and unmixed forms. For the former, dispersions of twD,polymers are mixed in a 1 1 ratio before spin-coating the supporting substrate and sintering, details of which have been describe elsewhere... 

LARC-13 polyimide adhesive was selected as a candidate based on preliminary data obtained by Boeing which demonstrated good elevated temperature thermal stability and desirable failure modes. The base resin synthesis and cure chemical reactions are illustrated below ... 

A serious limitation to the use of organic polymers in general and of adhesives, in particular, is their poor resistance to thermal degradation. Considerable effort has been put into the development of High-temperature adhesives and examples of the materials that have been produced are described in articles on Polybenzimidazoles, Polyether ether ketone, Polyimide adhesives and Polyphenylquinoxalines. Some of the general principles used in the search for enhanced thermal stability are discussed in this article. 

In 1908, aromatic polyimides were first reported by Bogert and Renstiaw [1]. Aromatic polyimides became well-known in 1950, after successful development of two-step polyimide synthesis by DuPont [2]. This class of polymers possesses a number of outstanding properties such as excellent thermal stability, mechanical strength, and electrical properties that have led to application in several fields from engineering thermoplastics to the aerospace and electronics industries, as well as for fibers and adhesives and in matrices for composite materials [3-5]. In addition, polyimides have high thermo-oxidative stability and chemical- and solvent-resistive properties, leading to many membrane-based applications... 

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