Relationship between Microphase Separation Structure of Styrene-Butadiene Block Copolymer and Interlayer Adhesion for Multilayer Shrink Film

Abstract

A present commercialized shrink film for bottle package is mainly composed of series of polystyrene film due to superior tearing property. Series of polyester film is also adopted as shrink film due to superior shiny appearance and high chemical and mechanical resistance. To achieve functional properties of polystyrene and polyester film, multilayer film is expected as new functional shrink film. Generally different series of polymer are not compatible. The interlayer adhesive property between different series of polymer film is inferior to the same series of polymer film. To design multilayer film of polystyrene and polyester, hot melt adhesive layer is required and inserted to between the layers. For industrial application to reduce the production cost, the multilayer film excluding the hot melt adhesive layer is expected as a commercial product. In this study we developed multilayer shrink film composed of poly(styrene-co-butadiene) (SBC) block copolymer and poly(ethylene-co-cyclohexane terephthalate) (PETG) copolymer without hot melt adhesive layer. The multilayer film without the hot melt adhesive layer was processed by coextrusion. The coextruded multilayer film was extruded from T shaped die and continuously drawn in transverse direction (TD) against machine direction (MD) by tenter drawing. The draw ratio was 5. After drawing in TD the intercalation adhesive strength drastically decreased comparing to undrawn multilayer film. The intercalation adhesive strength between SBC and PETG layers decreased to less than 1 N/15mm. The target intercalation adhesive strength is currently more than 2 N/15mm. However, the mechanism to decrease the intercalation adhesive strength is not cleared. To investigate the cause of adhesive strength decrease we focused on microphase separation structure of SBC. The microphase separation structure, especially, near interface was observed by transmission electron microscope (TEM). The thin slice sample of film was sectioned to 30 to 40 nm by an ultra-microtome at room temperature. The sectioned thin slice sample was stained with osmium tetra oxide. Only butadiene component can be stained. The microphase separation structure of SBC was also analyzed in small angle x-ray scattering (SAXS). Variations of the microphase separation structure were observed in 3 directions, film surface through and edge views in MD and TD. The microphase separation structure of SBC and the variation in drawing process were studied comprehensively by TEM and SAXS studies. To study effect of drawing ratio, drawn multilayer film samples were processed in various drawing ratio. The drawing ratio was changed 1 to 5. The microphase separation structure of SBC in this study was lamella structure which was determined in SAXS study. For undrawn multilayer film sample, the lamella structure of SBC was oriented vertically to SBC/PETG interface. The lamella structure was confirmed from TEM and SAXS studies. The scattering peaks corresponding to lamella structure was observed in SAXS study. The multilayer film was drawn horizontally by tenter system. The lamella structure of SBC was bent and destroyed in TEM observation. The scattering peaks which observed in undrawn sample disappeared in film surface through view of SAXS study. Peel strength for undrawn multilayer film was 2.9 and 2.7 N/15mm in TD and MD, respectively. For drawn film peel strength dropped to 1.2 and 1.8 N/15mm in TD and MD, respectively. For TEM image of undrawn film, butadiene layer was observed near SBC/PETG interface. The butadiene layer performs as an adhesive component. However, the butadiene layer disappeared in the drawn sample. The butadiene layer presumed to thin in drawing process. The thinned butadiene layer cannot be observed in TEM image. We considered that thinning of the butadiene layer caused decrease of peel strength in SBC/PETG interface when the multilayer film was drawn horizontally. The microphase separation structure of SBC bent and destroyed gradually when drawing ratio increased. Bending and destruction behavior was observed in TEM images. The scattering peaks corresponding to lamella structure was observed in undrawn sample in film surface through view of SAXS study. When drawing ratio increased the secondary scattering peak disappeared at the drawing ratio of 1.5. Peel strength in SBC/PETG interface also decreased at the drawing ratio of 1.5. The butadiene layer which was observed near SBC/PETG interface also disappeared. Disappearance of the secondary scattering peak was corresponding to decrease of peel strength. The microphase separation structure of SBC closely related to intercalation adhesive property of SBC/PETG interface. We concluded that the microphase separation structure of SBC affected remarkably intercalation adhesive property of SBC/PETG interface. In horizontally drawing process the microphase separation structure bent and destroyed. The destruction of the microphase separation structure caused decrease of peel strength in SBC/PETG interface.