Adhesion promoters characteristics

Plasma processing technologies ate used for surface treatments and coatings for plastics, elastomers, glasses, metals, ceramics, etc. Such treatments provide better wear characteristics, thermal stability, color, controlled electrical properties, lubricity, abrasion resistance, barrier properties, adhesion promotion, wettability, blood compatibility, and controlled light transmissivity. 

A recent review [1] on polyimide adhesion to metal and ceramic surfaces shows the relevance of this topic to many different technological areas. Of all the polyimides studied thus far, it is evident that the most popular one is PMDA-ODA. It has very good mechanical, thermal, and electrical properties, but it suffers from poor adhesion characteristics. This problem is often overcome by the application of an adhesion promoter to the surface of interest. The most popular adhesion promoter appears to be APS. An excellent review concerning APS has been written by Ishida [2]. A wealth of information concerning silane coupling agents can also be found in the book by Plueddemann [3],... 

Adhesion of polyimides to inorganic substrates is of great importance to the microelectronics industry [1, 2]. The polyimide films are deposited most often by spin coating the polyamic acid (PAA) usually from a TV-methylpyrrolidone (NMP) solution onto the substrate surface followed by thermal imidization at temperatures up to 400<>C. The most studied polyimide is the pyromellitic dianhydride-oxydianiline (PMDA-ODA), which exhibits excellent mechanical and dielectric properties, but not so good adhesion characteristics. The latter has been generally overcome by application of an adhesion promoter, such as y-aminopropyltriethoxysilane [3-7]. The reactions of APS (coated from water solution) with the silicon dioxide surface as well as with polyamic acid have been well characterized by Linde and Gleason [4] however, we do not have such detailed information available on APS interaction with other ceramic surfaces. 

Primers and adhesion promoters work in a similar fashion to improve adhesion. They add a new, usually organic, layer at the interface. The new layer can be bifunctional and bond well to both the substrate and the adhesive or sealant. The new layer is very thin so that it provides improved interfacial bonding characteristics, yet it is not so thick that its bulk properties significantly affect the overall properties of the bond. 

Such migration requires only partial miscibility in the polymer matrix, silicone in the particular case studied by Stein et al. Owen and co-workers [9] have suggested that solubility parameter calculations can be useful in designing suitable compositions with partial miscibility. These ideas appear superficially to be at odds with compatibility/penetration and interpenetrating network concepts. However, these contrary solubility requirements are a consequence of the initial placement of the adhesion promoter. When used as a primer, it is already placed at the interface so needs no partial solubility characteristic to drive it there, as is needed when it is initially dispersed throughout the polymer matrix. In both cases it is the development of a strengthening interfacial phase that accounts for the enhancement in adhesion that is sought by the user. 

Zirconate coupling agents have a stracture very similar to that of titanates. Zirconium propionate is used as an adhesion promoter in printing ink formulations for polyolefins. Like the titanates, zirconate coupling agents are useful in improving the dispersion characteristics of fillers in polymer systems. 

While cellulose fiber reinforced polypropylene (PP) is already used by default for example in the automobile industry for interior parts (Karas and Kaup, 2005), the conventional use of cellulose fiber reinforced PLA is still at the beginning. But there are also some products such as biodegradable urns, mobile phone shells or prototypes of spare tyre covers made from natural fiber reinforced PLA at the market (Anonymous, 2007 Iji, 2008 Grashom, 2007). Maty studies deal with the use of natural fibers as reinforcements in PLA composites. An overview about the mechanical characteristics and apphcation areas of natural fiber-reinforced PLA can be foimd for example in Bhardwaj and Mohanty (2007), Avella et al. (2009), Ganster and Fink (2006), Jo-noobi et al. (2010), and Graupner et al. (2009). For the improvement of the composite characteristics it is still necessary to carry out optimization processes for fibers, PLA matrix and the interactions of both. Moreover the processing parameters, force elongation characteristics of fibers and matrix as well as the use of additives like plasticizers or adhesion promoters have decisive influences on the mechanical characteristics of the composites. 

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