Block copolymer moisture effect

Moisture and oxygen must be rigorously excluded from the system if the synthesis is to be successful. Inadvertant introduction of trace protic impurities of metal oxides during the synthesis leads to the formation of polydimethylsilioxane homopolymer. This material, present in even small amounts, is detrimental to the mechanical properties of the final block copolymer. This effect can be illustrated by deliberately including known amounts of polydimethylsiloxane in a block copolymer of demonstrated mechanical strength. These effects are summarized in Table I. 

The effect of moisture on nylon 6 resins is well-known and well-documented. (15) Because of this, the susceptibility of nylon block copolymers to moisture absorption was expected. Evaluation of requirements for exterior automotive body panels and other applications requiring dimensional stability indicated that early nylon block copolymer formulations would not meet specifications... 

Different types of water-based emulsions are used in EPI adhesives. The most common are poly(vinyl acetate) (PVAc) emulsion, ethylene vinyl acetate (EVAc) emulsion, vinyl acetate-acrylate copolymerized (VAAC) emulsion, acrylic-styrene (AcSt) emulsion or styrene-butadiene rubber (SBR) latex or modified versions of these emulsion types [1, 8, 9], It has also been reported that tri- or ter-polymer emulsions like vinyl acetate-butyl acrylate-hydroxypropyl methacrylate or emulsions with different combinations of block copolymers can be used [4], Emulsion polymers containing cross-linking functional groups are especially well suited [4,6, 9]. The choice of emulsion(s) will, to a large extent, influence the adhesive properties such as setting time, bond quality, heat resistance, and moisture resistance. EPI adhesive systems are, however, very complex and the total composition (including the choice of cross-linker) and the interaction between the different components will determine the properties of the adhesive. Due to this it is difficult to describe in detail the effect of choosing one type of emulsion over the other. 

Permanent antistats do not depend on the relative humidity and they do not lose their effectiveness in a short time. One type is exemplified by the use of polyether-polyamide block copolymers combined with an intrinsically conducting substance, and another class consists of neoalkoxytitanates or zirconates. These compounds form non-blooming, bipolar layers, producing a surface and volume electron-transfer circuit, which produces a permanent antistatic effect. They are independent of atmospheric moisture and compatible with a wide range of polymers, including polyolefins, polyesters, polystyrene and PVC. Inherently conducting polymer additives such as sulfonated polyanilines are also used. They are discussed further in Chapter 5. 

The use of polyol rubber blocks In NBC gave unexpected Increase In moisture absorption over that of nylon 6. The effect of absorption on linear expansion Is shown In Figure 7 for three types of polyol blocks. Polyol level was 20% of copolymer. Polyols A and B not only Increased the total moisture absorption, but greatly Increased the rate of absorption as well. Reinforcement with glass fiber will give further improvement in dimensional stability, as illustrated below, but the degree of improvement is Insufficient to meet specifications for critical applications. 

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