MARTIN M. GROVER
Polymer Adhesives -1970 Rapid growth and diversification of adhesives are reviewed and discussed
he new decade augurs continued uncommon growth in adT hesives and diversification of their uses. Their role has changed from one in which they were adopted with reluctance because "nothing else would work," to one where their desirability is acknowledged and accommodations are made for their use. This is signaled by the increased use of plastic adherends, the advent of lightweight, composite, structural forms, and the recognition that bonding is reliable and offers inherent economy. T h e textile and packaging industries will rely more heavily on adhesives, and their uses will expand in building and vehicle construction. The current market for adhesives and sealants is estimated a t 3.5 billion pounds annually, with predicted growth more rapid than the general economy (74). T h e literature during the past year has amply reflected this burgeoning trend with the appearance of new polymers, resins, and intermediates primarily intended for adhesive compositions. Moreover, the intention to adapt adhesives to the assembly line
TABLE I. TENSILE SHEAR ON STAINLESS STEEL O F MONOETHER POLYQU I N O X A L I N E
Test condztions
Tensale shear, psi
Room temperature 1 hr/316'C 200 hr/3 16"C 1 hr/37loC 50 hr/371 "C
3350 2930
2280 1870 2540 1325
10 min/538'C
is clear as one notes the emphasis on 100% solids systems in various forms, quick-cure and self-cure compositions, and the continued upgrading of pressure-sensitive adhesives. Struclural Adhesives
C H N~
.Figure 1.
( C H ~ COH-N~HI C-NH& II
New polymer structures and catalysts f o r structural adhesives
Faster and higher flight has spurred the investigation into polymers that retain their properties a t extreme high and low temperatures. Polyquinoxaline polymers (Figure 1 ) have been developed that age well a t high temperatures (34). After a n elevated temperature, three-step cure in nitrogen, the test d a t a indicate strength retention after heat aging (Table I). An organometallic polymer has been disclosed that is formed by reacting a metallocene, such as osmocene or ruthenocene, with a n aldehyde in the presence of a Lewis acid catalyst (56); see Figure 1. A ceramic adhesive for metal bonding has been proposed consisting of borosilicate glass mixed with a metal additive. T h e glass provides a medium for reaction; a t the refractory temperature, it flows out of the interface leaving a metallic bond (43). Epoxies and modified phenolics having high cross-link density, showed poor strength a t - 320'F. Polybenzimidazoles, which are structural and attain high-temperature strength because of a high T g (glass transition temperature), rather than through crosslinking, are superior a t low temperature. T h e surprising low-temperature behavior for PBI is explained by its higher polymer density, strong hydrogen bonding, and stress-release mechanisms much below its T g (28). Polyether-polyurethanes, particularly based on tetrahydrofuran, are outstanding at cryogenic temperatures, but admittedly suffer a t high temperatures (28, 64). I t is suggested that balanced low- and high-temperature performance in a n adhesive c a n be obtained with a block polymer containing carbamate linkages, and both urethane and aromatic (or heterocyclic) segments (76). A polymeric adhesive comprising these elements showed a reasonVOL. 6 2
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able balance of cryogenic and high-tcmperature strength. I t was produced by the reaction of a bisphenol epoxy, a n isocyanateterminated polyether polymer based o n tetrahydrofuran, 4,4 'diaminodiphenylmethane, and tetramethyl ammonium iodide
degradation of a n epoxidc-aromatic amine cure was observed (49). I t is shown by tga (thermogravimetric analysis) that increased cure temperature produces a more thermally stable cure. This is consistent with the view that higher curing temperatures result in a higher degree of cross-linking. TVeight loss a t a constant rate of heating for various cure schedules is shown in Figure 2. Examination of the thermal degradation of a high-tcmperaturc resistant polyquinoxaline showed that breakdown began in the temperature range of 500 "-640°C with main chain degradation (82). Various aspects of structural bonding were interestingly re\+wed in the International Conference, ,\dhesion; Uiiivcrsity of Kottingham, 1966. 11 text of these proceedings was rccently published and is recommended (26).
(67). T h e ubiquitous epoxides continue under study, with emphasis o n attainment of "quick-cures," and flexibility with retained heat resistance. Xylene formaldehyde resins biere found useful as plasticizing diluents for an epoxy adhesive (76). .\n internally plasticized epoxy resin with good heat resistance is disclosed as the reaction product of a n epoxy rcsiii with tu-o polycarboxylic anhydrides, one of which contains a n ether oxygen. T h e recovered resin is then cured in use through its remaining epoxy functionality (20). .\ flexible epoxy resin obtained through chain extension with a high molecular weight polyisocyanate has also been developed (2). T h e use of a n acrylic polymer as a flexibilizer in a n epoxy adhesive composition is claimed. I n a particular example, a copolymer of 87Yc ethyl aci*ylate/3~
