Eileen Harkin-Jones

Modeling of non-isothermal crystallization kinetics of isotactic polypropylene

Isothermal and non-isothermal melt crystallization kinetics of isotactic polypropylene (iPP) were investigated via differential scanning calorimetry (DSC). Isothermal melt crystallization kinetics were analyzed using the Avrami equation. An Avrami exponent close to three was obtained for iPP, which implies growth of three-dimensional spherulitic superstructures following heterogeneous nucleation. Non-isothermal crystallization kinetics data obtained from DSC in conjunction with a non-linear regression method were employed to estimate the kinetic parameters of mathematical models describing the non-isothermal crystallization of iPP. The results suggest that the available mathematical models are not successful in describing the non-isothermal crystallization of iPP over a wide range of cooling rates. It was found that the non-isothermal crystallization kinetics of iPP, over a wide range of cooling rates, could best be described by modifying the Ozawa model to include induction times.

Biaxial Characterisation of Materials for Thermoforming and Blow Moulding.

Abstract During free surface moulding processes such as thermoforming and blow moulding, heated polymer materials are subjected to rapid biaxial deformation as they are drawn into the shape of a mould. In the development of process simulations, it is therefore essential to be able to accurately measure and model this behaviour. Conventional uniaxial test methods are generally inadequate for this purpose and this has led to the development of specialised biaxial test rigs. In the present study, the results of several programmes of biaxial tests conducted at Queens University are presented and discussed. These have included tests on high impact polystyrene (HIPS), polypropylene (PP) and aPET, and the work has involved a wide variety of experimental conditions. In all cases, the results clearly demonstrate the unique characteristics of materials when subjected to biaxial deformation. PP draws the highest stresses and it is the most temperature-sensitive of the materials. aPET is initially easier to form but exhibits strain hardening at higher strains. This behaviour is increased with increasing strain rate but at very high strain rates, these effects are increasingly mollified by adiabatic heating. Both aPET and PP (to a lesser degree) draw much higher stresses in sequential stretching showing that this behaviour must be considered in process simulations. HIPS showed none of these effects and it is the easiest material to deform.

Surface modification of poly(e-caprolactone) using a dielectric barrier discharge in atmospheric pressure glow discharge mode

The role of roughening and functionalization processes involved in modifying the wettability of poly(epsilon-caprolactone) (PCL) after treatment by an atmospheric pressure glow discharge plasma is discussed. The change in the ratio of CO/C-O bonds is a significant factor influencing the wettability of PCL. As the contact angle decreases, the level of CO bonds tends to rise. Surface roughness alterations are the driving force for lasting increases in wettability, while the surface functional species are shorter lived. We can approximate from ageing that the increase in wettability for PCL after plasma treatment is 55-60% due to roughening and 40-45% due to surface functionalization for the plasma device investigated.

The Role of Tool/Sheet Contact in Plug-Assisted Thermoforming.

Abstract Plug-assisted thermoforming produces a wide range of polymer products through a combination of deformation by air pressure and contact with tool surfaces. In this paper the role of tool/sheet contact in determining the process output is investigated. A combination of thermoforming, friction and heat transfer tests were carried out on common tool and sheet materials. The results show that the typical friction coefficients for the material combinations are within the range 0.1 to 0.3, but the values rise sharply on approaching thermoforming temperatures. Thermal imaging tests demonstrate that all of the plug materials significantly cool the heated sheet on contact, even over very short periods of time. The temperature of the plug is very important. At low plug temperatures heat transfer effects predominate, whereas at high plug temperatures friction effects predominate. A plug temperature of approximately 100°C balances these effects and creates the most effective material distribution.

Electrical methods of controlling bacterial adhesion and biofilm on device surfaces.

This review will summarize the significant body of research within the field of electrical methods of controlling the growth of microorganisms. We examine the progress from early work using current to kill bacteria in static fluids to more realistic treatment scenarios such as flow-through systems designed to imitate the human urinary tract. Additionally, the electrical enhancement of biocide and antibiotic efficacy will be examined alongside recent innovations including the biological applications of acoustic energy systems to prevent bacterial surface adherence. Particular attention will be paid to the electrical engineering aspects of previous work, such as electrode composition, quantitative electrical parameters and the conductive medium used. Scrutiny of published systems from an electrical engineering perspective will help to facilitate improved understanding of the methods, devices and mechanisms that have been effective in controlling bacteria, as well as providing insights and strategies to improve the performance of such systems and develop the next generation of antimicrobial bioelectric materials.

Rotational molding of liquid plastic systems: An assessment of material moldability

This article describes the outcome of an investigation into the rotational molding of liquid plastics. Three liquid plastic systems were assessed: (1) a nylon block copolymer, Nyrim; (2) two grades of polyvinyl chloride plastisol, Acrol DS409A and Hydro PRC/65/0307/9270; and (3) two grades of polyurethane, Hyperlast 7850506 and Hyperlast 7853184. Initially, resin flow behavior was assessed in the uniaxial rotation mode. Characteristic flow regimes were identified for each liquid. The onset and duration of a particular regime depends on the initial viscosity of the liquid, its repooling behavior, and its curing profile. For a particular material, the finished part appearance (hydrocysts, bubbles, fault-free, etc.) depends on the mold rotation speed and material shot size, as well as mold geometry. A fault-free operating criterion is presented for the uniaxial rotation of liquid plastics. Uniaxial rotation tests are useful for predicting general trends in biaxial rotation of liquid plastics. The most important factors in determining the state of a finished part made by biaxial rotation are: (i) resin initial viscosity and curing profile; (ii) mold rotation speed; (iii) shot size; and (iv) mold shape. An ideal viscosity profile for a rotational molding liquid resin, based on information gained from both uniaxial and biaxial rotation tests, is proposed. The rheology of each material was also examined and the profiles observed were related to the molding behavior of each liquid.

Process modelling for control of product wall thickness in thermoforming

Abstract The present paper describes the results of an investigation into the modelling of plug assisted thermoforming. The objective of this work was to improve the finite element modelling of thermoforming through an enhanced understanding of the physical elements underlying the process. Experiments were carried out to measure the effects on output of changes in major parameters and simultaneously simple finite element models were constructed. The experimental results show that the process creates conflicting and interrelated contact friction and heat transfer effects that largely dictate the final wall thickness distribution. From the simulation work it was demonstrated that a high coefficient of friction and no heat transfer can give a good approximation of the actual wall thickness distribution. However, when conduction was added to the model the results for lower friction values were greatly improved. It was concluded that further work is necessary to provide realistic measurements and models for contact effects in thermoforming.

Finite element modelling of stretch-blow moulding of PET bottles using Buckley model: Plant tests and effects of process conditions and material parameters:

Abstract Plant tests and finite element (FE) analyses of the injection stretch-blow moulding (ISBM) process of polyethylene terephthalate (PET) bottles have been carried out in this study with a view to optimizing preform designs and process conditions. Plant tests were carefully conducted at first to make bottles in a 330 ml mould from four preform designs under different process conditions. Both a digital handheld thermometer and a FLIR ThermoCAM Imager system were used to measure the initial preform temperature distributions (IPTDs). Comprehensive FE analyses using ABAQUS were then carried out to model the ISBM of these bottles, using a physically based model (Buckley model) to model the complex constitutive behaviour of PET. It was found that the numerical simulations often resulted in free blowing or over-thinning of the bottle bottoms when the measured IPTDs and process conditions were modelled. Parametric studies of the IPTDs, the pre-blowing pressure and the material parameters of the Buckley model were carried out. It was demonstrated that all of them had considerable effects on the effectiveness of FE modelling. In particular, the stress-strain relations modelled by the Buckley model were very sensitive to two parameters used to model the strain-stiffening behaviour. By carefully adjusting the material parameters and process conditions, successful simulations with excellent bottle thickness predictions were then achieved. It is concluded that the model parameters must be obtained by accurately testing the bottle-grade PET with similar process conditions to those in industrial ISBM so that the Buckley model can be confidently used to model the ISBM process. It is also found that good predictions of bottle wall thickness alone do not necessarily justify the numerical modelling. Validation of the deformation process may be equally important.

Steady-State Rimming Flow of the Generalized Newtonian Fluid

Rimming flow of a liquid polymer on the inner surface of a horizontal rotating cylinder is investigated. Using a scale analysis, a theoretical description for steady-state non-Newtonian flow is obtained. Simple lubrication theory is applied since the Reynolds number is small and the liquid film thin. Since a steady-state viscometric flow is considered, the general constitutive law requires only a single function relating shear stress and shear rate that corresponds to a generalized Newtonian liquid. For this case the existence of a continuous steady-state solution is proved. The properties of the solution for the different flow regimes are discussed. Numerical results are carried out for the Carreau–Yasuda model, which exhibits the Newtonian behavior at low shear rates with transition to power-law shear thinning at moderate shear rates.

Processability, structural evolution and properties of melt processed biaxially stretched HDPE/MWCNT nanocomposites

Biaxial stretching of melt mixed high density polyethylene (HDPE)/multiwalled carbon nanotube (MWCNT) nanocomposites was conducted in the melt state at different stretching ratios (SRs). The addition of MWCNTs leads to significant strain hardening in the HDPE, greatly improving the stability and thus processability of the stretching process. Scanning electron microscopy shows that the MWCNTs in the polymer matrix are gradually disentangled and randomly oriented in the stretching plane with increasing SRs. All the stretched samples exhibit an increase in crystallinity (about 10%) due to strain induced crystallization and a broadened distribution of crystallite size according to the XRD and DSC results. The mechanical properties of the composites improve with increasing SRs, while they drop off after a SR of 2.5 for the neat HDPE which is likely to be due to the relaxation of polymer chains prior to solidification. The presence of the MWCNTs appears to inhibit this relaxation thus helping to maintain the orientation and mechanical properties at high SRs. The modulus, yield strength and breaking strength of stretched composites with 8 wt% MWCNTs increase by approximately 54%, 85% and 193% respectively compared with the neat HDPE at a SR of 3. The electrical percolation threshold for the unstretched material occurs at 1.9 wt% MWCNTs. As SR increases, the values of critical concentration increase from 1.9 wt% to 4.9 wt% implying the destruction of conductive networks due to an increased inter-particle distance. A loading of 6 wt% MWCNTs is sufficient to ensure that the sheet conductivity is robust to changes in the SR. Decreased values of critical exponent from 1.9 to 1.1 and morphological investigation reveal a transformation of the system structure from three dimensional to two dimensional as SR increases.