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BMI-689 is a unique low viscosity liquid bismaleimide based on a non-hydrogenated dimer diamine backbone. It can be homo-cured via UV or free radical initiators to form tough, hydrophobic, cross-linked polyimides. The material has excellent low pH hydrolytic resistance and thermal stability. The amorphous nature of this BMI allows it to be used in a variety of applications including mixed cure systems (e.g. epoxy and BMI), mixed BMI systems or where a low viscosity, free radical cured resin is required. It is soluble in most aromatic and aliphatic solvents such as toluene, xylene, NMP, etc.
BMI-689 is recommended for use as an additive or base resin in adhesives that are designed for high temperature resistance. It has excellent adhesion to a variety of substrates. When used as a base resin, it can produce films that are tough, flexible and demonstrate good peel strength.
The use of DMI BMI-BCI resins with epoxy can add a great deal of flexibility when optimizing a resin system to meet specific needs.
Almost linear adjustability is available for properties such as:
Another BMI-BCI/epoxy system feature is that most combinations turn black upon curing without the use of colorants or carbon.
The hydrophobic nature of the DMI BMI-BCI resins helps limit the moisture absorption observed after exposure to 168 hours of 85°C/85% RH. Significant reduction in moisture absorption was obtained with samples containing a high fraction of BMI resin. The more BMI in the system the better the resistance against humidity.
The BMI-689/epoxy system exhibits thermal property changes in a manner similar to that of the viscosity.
The Tg, α1 CTE and modulus can all be controlled by formulation selection.
DK/Df values of cured BMI/Epoxy mixtures were investigated and plotted below. The dielectric properties of the system can be controlled by varying the level of BMI to epoxy.
Blue= BMI-689/epoxy - Orange= BMI-1500/epoxy
It is important to point out the improved shear strength that formulations using BMI exhibit. As you can see, the amount of BMI enhances the compression shear strength on PC/PC substrates in the initial phase and on the long run at higher temperatures. As expected there is a drop in strength at medium temperatures since Bismaleimides are generally made to perform better at elevated temperature conditions.
Something interesting to note is that initial strength at Alu/Alu substrate testing is not performing well and drops from 25MPA to 5 MPa with 20% BMI content. It is possible that this will improve in different analogies and given enough time and temperature but this has not been tested.
You can see how some of the suggestions fare next to each other in the following chart. At the end of the day it seems like BMI 1400 and BMI 1500 are probably better alternatives for Glass and Aluminum substrates. It remains to be seen if the shear strength of 1400/1500 will continue rising once we add coupling agents and adhesions enhancing materials in these products.