Hot Embossing of Electron Beam Generated Structures in Polypropylene
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1002-N07-03
Hot Embossing of Electron Beam Generated Structures in Polypropylene Patrick W. Leech CSIRO, CMMT, Gate 5, Normanby Road, Clayton, Victoria, 3169, Australia ABSTRACT The hot embossing of electron beam generated structures with linewidths 0.35-3.25 µm has been examined in biaxially-oriented polypropylene (BOPP). The individual test elements consisted of pixels of 30 x 30 µm with a specific configuration of symbols and lettering. The embossing of these features has been performed as a function of temperature (80-140 ∫C) and applied pressure (2-20 kPa). Significant increases in both embossed depth and sidewall angle was evident over the temperature range 100 to 130 ∫C with a leveling off at higher temperatures attributed to the onset of a regime of viscous liquid flow. At temperatures within this regime, a critical level of pressure was required to fill the mold features. Accurate embossing of medium and coarse lettering (0.50-0.65 µm linewidth) and geometric symbols (1.25-3.25 µm linewidth) has been demonstrated at a temperature of 130 ∫C and an applied pressure of 13 kPa. The finest lettering (0.35 µm linewidth) was incompletely embossed under these conditions.
INTRODUCTION Hot embossing has become a key technique in the replication of nanostructures in polymers [1]. Large area arrays of nanostructures have been embossed into a range of thermoplastic polymers including PMMA [2] and polycarbonate [3]. The optimal embossing temperature of several polymers has been reported as 70-100 ∫C above the glass transition temperature, Tg [1, 4]. This range of temperature corresponds to a region of viscous liquid flow in which deformation of the polymer is irreversible [1, 4]. During deformation, the polymer behaves as a highly viscous liquid consistent with a non-compressible Newtonian liquid [5]. In the case of biaxially oriented polypropylene (BOPP), the value of Tg is ~ -10 ∫C and, in this polymer, a temperature stability of >110 ∫C has been used in forming processes. In terms of the structure of BOPP, the most common and stable form has been identified as a monoclinic α polymorph. The lamellae structure of α form has comprised isotactic sequences of higher molecular weight [6]. The α form was stable during reheating of polypropylene (PP) [7]. In addition, a small amount of a mesophase has frequently been identified in PP [7] which demonstrates a reversible transition to α structure during reheating [7]. While the forming of BOPP film has been studied in recent years, there has been very little published data on hot embossing of the film. Recently, Leech has reported the high quality embossing of grating structures in transparent BOPP film [8].The present paper extends this work to examine the effects of temperature and pressure on the accuracy of the embossing process. A range of feature sizes has been selected in order to determine the relationship between the feature dimensions and experimental parameters. The test structure consisted of an array of pixels of 30 x 30 µm, each comprising a configuration of sym
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