Shrinkage behavior after the heat setting of biaxially stretched poly(ethylene 2,6‐naphthalate) films and bottles

Amorphous preforms of poly(ethylene 2,6‐naphthalate) (PEN) were biaxially drawn into bottles up to the desired volume under industrial conditions. These bottles were used to characterize the shrinkage behavior of the drawn bottles with or without heat treatment and to study structural variations during heat setting. During drawing, a rigid phase structure was induced, and the amount of the induced rigid phase structure was linearly related to the square root of the extra first strain invariant under equilibrium conditions. During the production of these bottles, this equilibrium was not attained because of high stretching conditions and rapid cooling after stretching. The structure after orientation contained a rigid amorphous phase and an oriented amorphous phase. The shrinkage behavior was a function of the temperature and time of heat setting. Long heat‐setting times, around 30 min, were used to characterize the possible structural variations of the oriented PEN after heat setting at equilibrium. Under the equilibrium conditions of heat setting, the start temperature of the shrinkage was directly related to the heat‐setting temperature and moved from 60°C without heat treatment up to a temperature of 255°C by a heat‐setting temperature of 255°C; this contrasted with poly(ethylene terephthalate) (PET), for which the start temperature of shrinkage was always around 80°C. For heat‐setting temperatures higher than 220°C, the structural variations changed rapidly as a function of the heat‐setting time, and the corresponding shrinkage of the heat‐set samples sank below 1% in a timescale of 30–60 s for a film thickness of 500 μm. The heat treatment of the oriented films taken out of the bottle walls with fixed ends stabilized the induced structures, and the shrinkage of these heat‐set films was zero for temperatures up to the heat‐setting temperature, between 220 and 265°C, if the heat‐setting time was sufficient. According to the results obtained, a heat‐setting time of 30 s, for a film thickness of 500 μm, was sufficient at a heat‐setting temperature of 255°C to stabilize the produced biaxially oriented PEN bottles and to take them out the mold without further shrinkage. During the drawing of PEN, two different types of rigid amorphous phases seemed to be induced, one with a mean shrinkage temperature of 151°C and another rigid amorphous phase, more temperature‐stable than the first one, that shrank in the temperature range of 200–310°C. During heat setting at high temperatures, a continuous transformation of the less stable phase into the very stable phase took place. The heat‐set method after blow molding is industrially possible with PEN, without the complicated process of subsequent cooling before the molds are opened, in contrast to PET. This constitutes a big advantage for the blow molding of PEN bottles and the production of oriented PEN films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1462–1473, 2003     

A comprehensive study of the bias and variance of frequency-response-function measurements: Optimal window selection and overlapping strategies

This paper investigates the measurement errors involved in measuring frequency response functions from weighted-overlapped segment averaging, a technique that has become a standard in modern spectral analysers due to its computational advantages. A particular attention is paid to leakage errors, for which this procedure has been frequently criticised. Exact and asymptotic expressions for the bias and variance are provided, whose minimisation enables the derivation of the optimal settings to be used with this procedure. Our main finding is that a Half-sine or Diff window with overlap achieves the best compromise to reduce leakage errors, and this is independently of the system frequency response function. This conclusion is to be contrasted with the customary habit of using a Hanning window with overlap.     

Thermal and spectroscopic analysis of worn polyoxymethylene surfaces and wear debris explaining degradation and polymerisation mechanisms

Degradation and polymerization of polyoxymethylene homopolymer (POM-H) surfaces after sliding at 8 to 150 MPa and 0.005 m/s over a total sliding distance of 3000 m is investigated by using thermal analysis (DSC, TGA, DTA) and Raman spectroscopy of worn surfaces or wear debris. There is mainly mechanical interaction and slight softening at 8 MPa (relatively high friction, low wear), softening at 16 to 55 MPa (decreasing friction and high wear) and finally melting at 150 MPa (very low friction, overload wear). At low contact pressures, wear debris remains amorphous and degradation of noncrystallised material during sliding manifests in broadening of the melting peak below the melting temperature. Degradation of C–O–C due to chain scission and radical reactions into CH₃ end groups are illustrated by Raman spectra. It is confirmed that the debris has long resident times and the maximum polymer surface temperature (T* = 93°C) is below the crystallisation temperature. At intermediate contact pressures, crystallisation results in a polymer fraction with higher thermal resistance. From the calculated temperatures T* = 120 to 150°C, crystallisation is beneficial for coherent transfer with larger particle sizes. At high contact pressures, the wear debris is immediately removed from the contact interface due to melting (T* = 200°C) and has thermal properties similar to the bulk material. There is no reaction between the debris in the interface, resulting in a thick polymer transfer film.     

Time series analysis in the frequency domain

This correspondence presents a parametric frequency domain identification algorithm for autoregressive moving average (ARMA) processes that does not suffer from spectral leakage errors. It is based on an extended transfer function model that takes into account the begin and end effect of the finite data record. The relationship with the one-step-ahead prediction error method is established. The advantages of the proposed method are easy prefiltering and leakage-free spectral representation of the raw data     

Identification of linear systems in the presence of nonlineardistortions

This paper treats the identification of linear systems in the presence of nonlinear distortions. It extends the theory developed for measurement setups where the input is exactly known and the output is observed with errors (output error framework) to measurement setups where both the input and output are observed with errors (errors-in-variables framework). An appropriate measurement strategy and identification algorithm are presented     

Role of internal additives in the friction and wear of carbon‐fiber‐reinforced polyimide

Polyimide composites should function in sliding contacts under high temperatures, but the interference of carbon fibers with sliding mechanisms is difficult to predict: they often increase the coefficients of friction and act abrasively but show lubricating properties under other conditions. The friction and wear behavior of thermoplastic polyimides reinforced with short carbon fibers and filled with solid internal lubricant (polytetrafluoroethylene) or silicon oil was investigated in this study with a reciprocating cylinder‐on‐plate tester under 50 N at 0.3 m/s with steel counterfaces that were heated at 23–260°C. We concluded that polytetrafluoroethylene additives effectively reduced the coefficients of friction over the entire temperature range, especially under thermally controlled sliding conditions at 120°C, whereas the internal silicon oil increased the coefficients of friction. The wear rates of the fiber‐reinforced polyimide significantly decreased with respect to those of the thermoplastic polyimide, whereas additional fillers slightly increased the wear rates. We further analyzed the role of internal additives by considering the deformation and maximum polymer surface temperature during sliding. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Application of polymer nanoparticle coating for tuning the hydrophobicity of cellulosic substrates

Nanoparticles of partially imidized poly(styrene–maleic anhydride) were applied from an aqueous dispersion as a one- or two-layer coating onto paper substrates, for controlling the paper surface hydrophobicity and improving the water barrier resistance. The effect of deposition conditions and thermal treatments on the topography and properties of the coating was studied by scanning electron microscopy, atomic force microscopy (AFM), contact angle measurements, and friction measurements. The wettability of paper surfaces with adsorbed nanoparticles can be controlled by tuning the chemical and topographical surface parameters: the water contact angles were found to increase at higher imide content as determined by Raman spectroscopy (depending on synthesis and thermal treatment), and higher average surface roughness determined by AFM (depending on the deposition method). The present technique may serve as a unique replacement for chemical treatments hydrophobizing fibrous substrates.     

Box-Jenkins identification revisited—Part III : Multivariable systems

Part I of this series of three papers handles the identification of single input single output Box-Jenkins models on arbitrary frequency grids in an open and closed loop setting. Part II discusses the computational aspects and illustrates the theory on simulations and a real life problem. This paper extends the results of Parts I and II to multiple input multiple output systems. Contrary to the classical time domain approach, the presented technique does not require symbolic calculus for multiple output polynomial Box-Jenkins models.     

Fully automated spectral analysis of periodic signals

In this paper, we propose a method that allows to make a fully automated spectral analysis of a periodic signal, including a noise analysis, without any user interaction. The only action required from the user is to provide a data record that contains more than two periods of the signal (no integer number of periods is required). No synchronization between the generator and the data acquisition is needed, and different sampling rates are allowed (no integer number of samples/period is required).     

Design of narrowband, high-resolution multisines

A method is presented to minimize the crest factor of narrowband high-resolution multisines. An adequate use of the envelope of the signal allows a significant reduction of both the required memory space and the calculation time as compared with a direct approach. A lower bound for the attainable crest factor is derived, and it is demonstrated on some experiments that the proposed algorithm comes very close to this bound