- August 01, 2006, David J. Bentley, Jr., Editor
A New Family of sHDPE Polymers For Enhanced Moisture Barrier Performance
by Norman Aubee, Patrick Lam, and Sarah Marshall, NOVA Chemicals Corp.
APPLICATION: New sHDPE resins offer superior moisture barrier properties, exceeding the barrier performance of multilayer, multiple HDPE component films without compromising processability.
Conventional Ziegler-Natta catalyzed high density polyethylene homopolymer has use as a moisture barrier for the production of multilayer films for packaging dry foodstuffs and other moisture sensitive products. Barrier properties for these materials improve by narrowing the molecular weight distribution and decreasing the molecular weight at the expense of melt strength and processability. A new class of sHDPE resins for moisture barrier applications using advanced technology and a single site catalyst is now available.
The new sHDPE grade offers excellent moisture barrier performance over a wide range of film orientation and process time conditions. Films constructed with the new sHDPE material offer moisture barrier properties equivalent to or better than existing high barrier film constructions using high melt index skin layer HDPE materials and may offer bubble stability improvements over those structures. This allows for structure simplification and offers reduced material handling opportunities to the film converter. The new sHDPE product offers comparable melt strength without long chain branching to conventional MMW HDPE of similar melt index. It offers enhanced melt strength vs. high melt index skin layer HDPE materials that should result in better bubble stability. The new sHDPE processes at comparable pressure and amperage compared with Zeigler Natta catalyzed MMW HDPE with only a slight reduction in specific output rate. This resin runs easily on existing MMW HDPE film lines.
Dual reactor solution polyethylene manufacturing technology provides a profound method for controlling and changing the properties of sHDPE over a wide range. Polymer molecular weight distributions and compositional architectures can be changed almost independently.
Discoloration Resistant Polyolefins, Part IV: Gamma Irradiated LLDPE Films
by R. E. King III, Ciba Specialty Chemicals
APPLICATION: Gamma irradiated polyolefin films are finding a variety of end-use applications in areas such as medical packaging and hygiene related articles where sterilization plays a kety role in the life cycle of the plastic product.
The preservation of physical properties and aesthetics are two important aspects of the film industry. Forethought and care are therefore necessary regarding the type of polymer and stabilization systems that have use to derive robust and attractive film products. For most applications, a combination of a phenolic antioxidant and a phosphate melt processing stabilizer can provide the base stabilization that is necessary to provide the retention of physical properties, good processability, and long thermal stability without compromising the aesthetic appearance of the final film product.
In selected end-use applications, it is particularly desirable to have film products that do not discolor during initial processing or while the product undergoes storage. Under a selected set of circumstances, certain types of phenolics are susceptible to discoloration due to inadequate stabilization, harsh processing, film storage with oxides of nitrogen (pollution), and various types of ionizing radiation.
Significant potential exists with food packaging. While this sterilization technique is valuable to end users, the stress of the plastic article is very significant in terms of not only maintaining the original physical properties of the product but also the aesthetic appearance. The study in this paper used a film grade linear low density polyethylene to compare phenol and phenol free stabilization systems for their ability to maintain physical and aesthetic properties. The results of this study demonstrate that phenol free systems were better than the phenol based systems. While the concept of going phenol free in linear low density polyethylene is still more of a challenge than going phenol free in polypropylene or high density polyethylene, hopes remain high for developing multiple niche markets especially for color critical applications.
PVDC Barrier Concepts For Demanding Coextrusion And Laminating Applications
by Valerie Renard, The Dow Chemical Co.
APPLICATION: PVDC resins remain a premier barrier packaging material although 60 years have passed since their original introduction.
Copolymers of polyvinylidene chloride (PVDC) are-well known for their use in demanding barrier packaging applications to minimize the permeation of oxygen, water vapor, and odors. New die designs incorporating patented early encapsulation technology coupled with temperature isolation technology developed by equipment suppliers have eliminated degradation problems existing in the typical spiral mandrel dies used in the blown film process. These new die designs coupled with new resin offerings enable processing of PVDC resins on large multilayer blown film dies. Multilayer films incorporating PVDC resins with higher temperature polymers such as Nylon, PP, and LLDPE in the skins are now possible.
The advantage of coextruded multilayer films incorporating PVDC are that they offer in a one-step process a combination of outstanding barrier performance, resistance to in-use conditions such as humidity and flex cracking, and specific physical properties given by the skin polymers. They represent the possibility of increasing product shelf life or down-gauging the barrier material and reducing processing steps compared with laminated composite products. Its barrier performance being unaffected by humidity makes PVDC very suitable for retort applications. In China, PVDC has wide use in retort applications as a monolayer film. Interest in incorporating PVDC in multilayer films for retort is growing.
Although PVDC resins were first commercialized in 1939, they have undergone many changes. Improvements to the resin and formulation have resulted in less thermal degradation. Improvements to the fabrication techniques have provided new application areas. PVDC coextruded multilayer films can be effective when demanding end-use packaging conditions occur and when cost-effective solutions are necessary.
New potential application areas such as extrusion coating and blow molding may be possible in the near future to provide even more opportunities. As a result, PVDC resins remain a premier barrier packaging material over 60 years after their original introduction. The use of PVDC resins should therefore continue to grow for many years in the future.
An example of an opportunity is retort packaging. PVDC blown coextruded films are an excellent option to replace foils or metallized films when a microwaveable or transparent package is desirable with resistance to flex cracking and moisture. A new tie layer technology under development allows combining polypropylene as a skin layer and PVDC polymer in a multilayer film that can withstand the retort process. Such a film will provide in one step the combination of sealability with oxygen and aroma barrier even in high humidity conditions with resistance to pin holing with a transparent solution.
Improving The Gas-Fading Performance Of Polyethylene Film Resisn By Addressing The Over-Oxidation Of Phenolic Antioxidants
by T. Tikuisis, L. Ellis, and S. Chisholm, NOVA Chemicals Corp.
APPLICATION: This paper demonstrates the processing color and gas-fade resistance performance of a new, commercially available 2.3 melt index (MI) film resin and provides experimental data for a 1.0 MI film resin presently under development.
Discoloration of polyethylene due to “gas-fading” is an ongoing nuisance in the film packaging industry and can result in significant product claims and potential loss of business. It is well known that gas-fading can be attributed to the over-oxidation of phenolic antioxidants. There is a correlation between the changes in color that can occur during processing and the subsequent discoloration due to gas-fading. New gas-fade resistant polyethylene film resins are now available. This paper presents data comparing the melt flow and color stability, physical property performance, long-term thermal stability, and improved gas-fading resistance of these resins to conventionally stabilized formulations. The new resins have been designed to tolerate the addition of moderate levels of phenolic antioxidant (from blending with other film resins) without a significant effect on color performance.
The processing discoloration of two conventional polyethylene film resins containing phenolic antioxidants was directly correlated with the propensity of these resins to gas-fade. Before removing the phenolic antioxidant from a stabilization package for a film resin, various factors should be considered. These include the impact on the melt flow stability of the resin during processing and the impact on the long-term thermal stability of the extruded film. Other work has shown that both concerns can be addressed by incorporating lactone or hydroxylamine-based processing stabilizers and by using a hindered amine light stabilizer.
It is possible to reduce both types of discoloration through the use of phenol-free stabilizer systems. The use of a hindered amine light stabilizer was not necessary to provide long-term thermal stability for the film stabilization systems evaluated since the properties of the films did not deteriorate during oven-aging studies. In addition, the presence of the stabilizer in the film systems evaluated did have a detrimental impact on color performance.
The design of phenol-free stabilizer systems should consider many factors including the molecular structure of the polyethylene resin, the processing conditions and blending operations used by the film converters, and the intended end-use of the film. Phenol-free stabilizer systems which are sufficiently robust to accommodate the challenge resulting from typical polymer blends and conversion operations can be effective for film resins.
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