A study has been made of the curing of epoxy resins from 2: 2‐bis‐(4‐hydroxyphenyl)propane with phthialic anhydride. It is shown that in all cases an equilibrium is set up in which, after complete reaction of the epoxy groups, there are present anhydride, monoester and hydroxyl groups. The effect of temperature on the content of these groups has been determined and shown to be considerable.
On heating cured resins, most of the monoester groups are split off and phthalic anhydride is evolved. A minimum point is obtained in the graph relating loss of weight and amount of anhydride used. The reason for this is discussed. The distribution of the various types of group has been determined in cured resins by chemical methods, which affords a good picture of the structure of resins cured under different conditions.
Bei der alkalischen Kondensation von Epichlorhydrin mit zweiwertigen Phenolen entstehen Hydroxyl‐ und endständige Oxydogruppen enthaltende aliphatisch‐aro‐matische Polyäther. Die Umsetzung derartiger Reaktionsprodukte mit Dikarbonsäureanhydriden führt zu hochvernetzten wertvollen Massen. Der Härtungsmechanismus wird in der Anfangsphase an einem technischen Produkt und über den ganzen Reaktionsverlauf an geeigneten Modellreaktionen analytisch verfolgt. Es zeigt sich, dass sowohl Diester‐ wie auch Ätherbrücken als Vernetzungselemente auftreten.
SynopsisDuring an investigation of various epoxide resin systems, some cases were found in which the room temperature density decreased with increasing curing temperature and increasing degree of cure. In other systems the density was found to be independent of the curing temperature. In these cases it is possible by deliberately stopping the reaction to measure density values which also decrease as the curing progresses. This unexpected behavior can be explained in a purely physical manner from the pattern of the density changes during an entire curing cycle. The density decrease of the noncured mixture, which is due to the increased curing temperature, outweighs all contraction effects consisting of isothermal chemical and cooling shrinkage, whereby the latter is dependent to a great extent on the glass temperature. In those cases where the glass temperature exceeds the curing temperature, the chemical reactions come to a standstill, when the temperature difference reaches a certain value, i.e. it "freezes chemically". By means of values that can be measured readily a t low temperatures, it is possible to construct diagrams from which the variation of the density a t higher temperatures of the curing cycle can be estimated.In a reactive system the density increase often serves to measure the course of a chemical conversion. Particularly in the field of macromolecular chemistry, this method is frequently used. In many studies on polymerization, the increase in amount of polymer formed is followed only by the change in density.While investigating the curing of epoxide resins, a case was found in which the density measured at room temperature decreased with increasing curing temperature.I n the present paper, nine different curing systems (A-J) have been investigated. It was found that the dependence of the room temperature density and the glass temperature on the curing temperature showed marked variations.
By reacting acid polyesters with di-epoxides and crosslinking them with anhydrides it was possible to increase the energy of deformation in the transition range by a factor of 10, compared with the conventional highly crosslinked epoxy resins.A further increase in toughness could be realised by introducing hydrogen bonds, e.g. urethane groups along the chains of the polymers. The syntheses of epoxy resins containing urethane groups are described. The physical properties of the crosslinked systems caused by different densities of the crosslinks are demonstrated and discussed.
SYNOPSISThe investigations in epoxy resins and other polymers have shown that by selecting the structure of the segments between the crosslinks it is possible to vary to a great extend the morphology and the physical properties. In a survey, a classification of 7 systems is given, each of them characterized by specific morphological and physical properties. It is demonstrated that conventional epoxide resins show a distinct maximum of the mechanical damping and elongation at rupture nearly at the same temperature. The toughness-temperature characteristic however shows a maximum 2O"-3O0C below the glass transition temperature. One system, the slightly crosslinked epoxy resins e.g. obtained by reacting long chain acid polyesters and diepoxides show a very high toughness in the transition range, compared to the conventional high crosslinked polymers.
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