- July 31, 2005, David J. Bentley, Jr., Contributing Editor
Providing practical information to the converting and packaging industries…
Calcium Carbonates For Polyolefin Extrusion Coating Applications
by Allen R. Guy, James E. Cara, Lane G. Shaw, and Michael D. Roussel, Omya, Inc.
The use of calcium carbonate is not new to the polyolefin industry. Calcium carbonate is finding significant use in LDPE, LLDPE, HDPE, and PP extrusion coating applications. The successful use of calcium carbonate requires selection of the proper calcium carbonate grade and carrier resin, correct preparation of the calcium carbonate concentrate, and the proper fabrication conditions. This paper will discuss critical raw material requirements, enhanced performance attributes, and processing parameters.
The selection of a calcium carbonate grade for use in extrusion coating is critical. A material with a median particle size of 2 microns and a top cut of 8–10 microns will yield the best performance. While these are typical measurement parameters for calcium carbonate, they by no means specify all the critical properties. A very narrow particle size distribution is also necessary. Of great importance, the calcium carbonate must have a very low retained amount on a 20-micron screen and no retains on a 45-micron screen due to the thin gauge of the extruded curtain and extruded layer. Larger particles can cause the curtain to tear or promote points of moisture permeation in the packaging.
The preparation of a properly compounded calcium carbonate concentrate is also an important parameter. The same resin used in the extrusion coating process is the best carrier for the concentrate. Typical concentrates will contain 75% calcium carbonate with 25% carrier resin.
The successful inclusion of calcium carbonate in products offers improved economics based on the increased output rates, improved properties, and raw material savings. Careful selection of calcium carbonate, concentrates, and processing parameters to achieve maximum performance is necessary. With additional experience, higher levels of calcium carbonate and many more applications will be possible.
Rheology of Glass-Fiber-Filled Polypropylene Melts
by Natti S. Rao, Plastics Solutions International, et al.
The advantages of glass-reinforced plastics in various branches of industry such as automotive and aircraft industries and in the manufacturing of furniture and sporting goods are well-known. In all these applications, knowledge of the melt flow of the composite material is necessary to design machinery for processing filled polymer. This paper is a contribution to the quantitative description of the rheology of glass-fiber-filled polypropylene melts. The effect of shear rate, melt temperature, and fiber concentration on the melt viscosity have undergone study by means of a high-pressure capillary rheometer. The proposed modeling can apply to any filled thermoplastic melt.
Owing to their ease of processing and availability, glass fibers are common reinforcing agents used as fillers for imparting strength to a polymer. Knowledge of the flow behavior of a melt is important to the design of processing equipment and process optimization. In this work, shear viscosities of polypropylene melts with glass fiber content by weight per cent of 0, 20, 25, and 30 were measured at temperatures of 200, 220, 240 and 260°C with a high pressure capillary rheometer. Based on these experimental results, a model for predicting the melt viscosity as a function of the fiber content was developed.
This work measured the shear viscosities of polypropylene melts with variable glass-fiber content. On the basis of these results, a single explicit relationship for the shift factor as a function of the fiber content and temperature was developed. Using the viscosity function at a reference temperature, the viscosity of the filled melt can be predicted with this equation for the shift factor at any temperature and shear rate. The method of calculation presented can apply to any thermoplastic material that incorporates a filler.
Non-Invasive Oxygen Measurement Using Fluorescence Technology And Its Implications For Active Packaging
by Devinder Saini and Nicole Millick, OxySense Inc.
Oxygen ingress into sealed food packages causes many problems within the food industry. Although most packages have oxygen barriers, oxygen can permeate into the package via micro-pores, holes, inconsistent sealing, etc. This leakage of oxygen into the packaging causes reduction of shelf life and may result in product spoilage. A need clearly exists for the measurement of oxygen within a package in the food industry to identify leaky packages and reject them before shipping to a consumer. Most current techniques for measuring oxygen within a package are invasive and therefore result in damage to the package. A new technology is necessary to measure the concentration of oxygen inside a package without causing any damage to the package.
A non-invasive sensor is now available that can measure oxygen concentrations in various types of packages used in the food industry. The oxygen measurement technique is based upon the fluorescence quenching of a metal organic fluorescent dye immobilized in a gas permeable hydrophobic polymer. The dye absorbs light in the blue region of the spectrum and fluoresces within the red region of the spectrum. The presence of oxygen quenches the fluorescent light from the dye causing a change in the emitted intensity and its lifetime as a function of oxygen concentration. There is a decrease in the amount of light being emitted from the fluorescent dye with a reduction in its lifetime with increasing oxygen concentration.
The oxygen concentration measurement within a sealed package occurs by placing a piece of the fluorescent dye polymer within the package in the form of a dot or a film and measuring the emission from outside the package. As long as the packaging material has transmission in the blue and red regions of the spectrum, non-invasive oxygen measurements are possible. Oxygen measurements are made using commercial equipment that can measure the fluorescence lifetime of emission from the oxygen sensitive dot.
Several experiments and tests have shown the effectiveness of this technique for oxygen measurements. The non-invasive oxygen measuring technology provides the ability to make repeated measurements inside sealed packages. One can observe the purging of oxygen from liquids or headspace before any shelf life or permeation studies begin. The same package can then undergo testing for oxygen permeation over time.
This non-invasive oxygen measurement technique has use for a number of applications where oxygen measurements are necessary. These include research and development, quality control, oxygen permeation studies, shelf-life studies, oxidation studies, oxygen scavenger studies, and in-line leak detection of sealed packages.
Using Simulations To Predict Coating Characteristics Of A Slot Die System
by Sam G. Iuliano, Extrusion Dies Industries, LLC
The slot die technique for applying coating fluids to a substrate provides some distinct advantages over other coating methods such as roll coating. Slot dies are premetered coating heads in which all the fluid fed to the die is applied to the substrate. This differs significantly from roll coating where the amount of fluid applied depends on factors such as fluid viscosity and roll speed. Without excess fluid requiring reclamation when using a slot die, volatile organic compound emissions from solvent-based materials are minimal. Slot die coating heads can generally achieve higher line speeds than roll coaters. This is because the slot die application geometry is less susceptible to “film-splitting” and “ribbing” defects that can develop at higher line speeds. A sharp final edge on the wiping lip provides a clean break-away point for the fluid so no fluid remains on the applicator.
To realize the advantages of the slot die method, an appropriately designed die and positioning stand are necessary. They require proper adjustment to provide defect-free coatings. Software tools are useful to model what occurs in the application area between the die lips and the substrate and can help to determine an ideal geometry.
The software in this study was accurate for predicting defect-free coating slot die positions. At larger lip offsets and at greater wet coating thicknesses, the software was accurate for predicting which defects an improperly positioned slot die would create.
When used with accurate rheological, surface tension, and die position data, simulations can help to determine a good starting position before beginning production trials. They can also have use to determine the size of the stable operating window for a specific coating fluid, slot die, and backing roll. A converter may be able to determine that a backing roll needs to be of larger diameter to work with an especially sensitive fluid for example before investing in costly production trials to make the same determination.
The slot die approach provides many benefits such as higher line speed capability and greater coating weight uniformity over other converting methods. To realize these benefits, one must be able to tune the geometry created in the area between the die lip faces and the substrate to achieve defect-free coatings at line speeds exceeding those achievable with the roll coating method. Any tool that can help reduce the time needed to achieve this target is desirable since it will greatly improve operational efficiency.
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