The PX-300 series of adhesives is based on bismaleimide resins. After the curing process they have a similar molecular structure to polyimides with whom they share the characteristic imide structure and high temperature resistance. Polyimides have therefore been widely used as a base material for flexible printed circuit boards in the electrical and electronics industries. Major differences between polyimides and bismaleimides become apparent if the creation of their chemical structure is compared and how that reflects to their properties.
Polyimides are obtained by polycondensation of di-anhydrides with di-amines resulting in high-molecular (but still soluble) precursor polymers. These linear polymers need to be treated by an imidization process in order to create the typical imide structures. This process produces the final linear, non-soluble polymers with high thermal resistance. Polyimides cannot be melted – their shape must be defined prior to the imidization process.
On the other hand, bismaleimides are low molecular substances (dry powders) containing imide structures already in this momomer form. These monomers can conduct various polyaddition reactions with themselves as well as with other co-monomers. In contrast to polyimides, the polyaddition reactions on bismaleimides do not produce linear polymers but three-dimensionally crosslinked, thermoset structures that also exhibit high temperature resistance. Moreover, the polyaddition reactions do not produce any volatile side products so that bismaleimides can be cured without forming volatiles.
Bismaleimides are hardly used in their pure, solid form. In most applications they come in conjunction with reactive co-monomers (e.g. vinyl and allyl compounds, allyl phenols, aromatic amines etc.). A great advantage of these mixtures over the pure powder is better processability. With the help of co-monomers bismaleimide powders can be turned into paste-like liquids and cast to the desired shape. However, the viscosity of such systems is often very high so that solvents are added in order to make processing easier or even to make processing possible at all.
A typical field of application of such bismaleimide systems is their use as matrix resins for high-performance fibre-reinforced composite materials in the aviation and space industries. Very often these matrix resins require high curing temperatures (more than 200 °C) and long curing times.
By special modification Polymerics GmbH has developed novel adhesives based on bismaleimide resins. Important aims of this development were:
As a result of this development the PX-300 series of adhesives evolved for applications with high demands on long-term temperature resistance and media resistance. These adhesives are suitable for joining metals (aluminium, stainless steel, steel etc.) glass, ceramics and selected plastics. They exhibit high thermal resistance, very high solvent resistance, hydrolysis resistance and many other interesting properties.
Another interesting development in the field of bismaleimides are our new bismaleimide adhesive films PX-306 and PX-307. These films come as a dry layer of bismaleimide adhesive cast onto a polyester carrier film. Processing of film adhesives is in many cases much simpler than that of adhesive pastes, since the adhesive can be cut to the desired shape and then applied to the bond area. Very clean and precise adhesive application is possible with these films.
After cutting the shape, the adhesive film is placed on the bond area and the polyester carrier film is peeled off (fig. 3, left). Then the second bond area is pressed against the adhesive (fig. 3, center) and the ahesive is subseqently cured by heating to 150 °C. During the curing the substrates must be fixed since the adhesive film does not have any pressure sensitive adhesion properties.
Adhesive strength of the film adhesives is comparable to the paste-like BMI adhesives – in some cases the films perform even better. Film adhesives are preferebly used on flat bond areas. During the curing the film softens somewhat so that small irregularities of the bond surface are equalized and the area is completely wetted by the adhesive. However, this viscosity change is not as strong as in the case of the adhesive pastes which can be used to wet even the smallest shapes.
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