Interface-reactive Two-Component Injection-Moulding
Multi- or 2-Component (2C) Injection Moulding as a special field of injection molding is receiving more and more significance because elaborate sticking, joining and mounting may be saved. The most important fields of application are rigid-flexible composites which are used in multi-functional devices. In those composites the high strength and modulus of the hard component is combined with the flexibility and vibration-absorbing properties, also as sealing element or as soft touch feeling part, of the soft component. Many of the hard and flexible polymers are bad compatible. Therefore one of the main problems, however, is the increasing of the adhesion of the two polymer materials at the interface. There are discussed several different mechanisms of adhesion in the scientific literature.
Many adhesion matrices for rigid-flexible combinations are reported in scientific papers and brochure of companies. Those matrices may be a useful basis in the decision-finding process for a specific problem. For one and the same rigid-flexible combination it can be found different adhesion numbers in the matrices, because both the two-component tensile test bar shape, and the test terms are not standardized and mostly not referred, too. For some applications special hard and soft material combinations are necessary. The bond strength of a composite is also affected by the specified technology (e.g. rotating mold or core back), the shape of the mold, especially the position of the sprue bushing, and the parameters of the injection molding machine.
The adhesion is supported by mutual interactions and interdiffusion of macromolecules for compatible polymers. In the case of less-compatible polymers, however, the lack of interactions results usually in poor adhesion and non-reliable 2C devices.
To improve the adhesion and consequently the bond strength level between those two components in the 2C-composite the following methods are known:
- The addition of specially tailormade compatibiliser, so called adhesion-modified polymer types were developed;
- Chemical reactions to form bonds in the interface, which can expected only for a few rigid-flexible combinations without modification beforehand, for example by ester and amide interchange reactions;
- Chemical reactions in the interface, after modification at least one material surface or at least one of the two bulk components.
In our group we deal mainly with the methods two and three.
We investigate extensively the following two-component rigid-flexible composites :
- PA / TPU;
- PA / TPU modified with isocyanate;
- POM / TPU;
- POM-blends / TPU;
- POM / TPU modified with isocyanate;
- POM-blends / TPU modified with isocyanate.
The bond strength is determined in a standardized tensile strength test with a two-component dumbbell tensile test bar, which has one or two interfaces, or a specially adapted roll peel test.