Process for forming a multilayer, coinjected article

Details Process for forming a multilayer, coinjected article Process for forming a multilayer, coinjected article

2000-03-01

2003-05-13

Eashoo, Mark (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

Shaping against forming surface

Reexamination Certificate

active

06562276

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to a process comprising coinjecting or coextruding a structural polymer resin with one or more performance polymer resins to a form a multilayer article without melt flow defects.
BACKGROUND OF THE INVENTION
Poly(ethylene terephthalate) (PET) is an established bottle polymer that produces rigid bottles with excellent clarity and gloss. These containers are manufactured by a process that comprises drying the PET resin, injection molding a preform and, finally, stretch blow molding the finished bottle.
The injection molding of PET preforms requires the melting of polymer pellets and the injection of the molten, viscous PET material into a cavity, which also has a core rod. The molten PET forms a “skin” where it comes into contact with the cold cavity wall and core rod. This skin is composed of “frozen” PET and will remain fairly stationary throughout the remainder of the injection molding process.
At points extending radially inwardly away from the cavity wall and, outwardly from the core rod, or at the points at which the polymer does not directly contact the cavity wall or core rod, the polymer (which is still elevated in temperature) remains a viscous, flowing mass. This hot inner viscous material can still flow relative to the frozen skin layer although its viscosity increases as it continues to cool. Thus, a temperature transition region occurs in the radial direction as well as a corresponding melt viscosity transition (because of PET's viscosity dependence upon temperature). Regardless of the changes in melt viscosity as a function of radial distance from the skin, monolayer PET is, for the most part, unaffected by the shear that develops between the frozen skin of the PET and the molten polymer that pushes past it. After the entire cavity has been filled using this process, the polymer is held in the cavity until the preform has become sufficiently cool so that it can be blown immediately into a bottle or the preform is cool enough to be ejected. Cooled preforms that have been ejected are stored for later reheat blow molding into the final product.
Using this process, PET resin is used in a wide range of applications such as carbonated soft drink, hot-filled juice products and warm-filled foods. However, PET has insufficient barrier to meet the desired shelf lives of products with more demanding gas barrier needs.
In one particular application, in order to increase the gas barrier of a PET bottle, it is possible to inject a barrier layer into or onto a preform during the injection molding process. This barrier layer is injected into or onto the melt flow stream of the PET such that the barrier polymer resin flows past the skin of PET previously injected. This “coinjection” process allows two resins to be injected into a “multilayer” preform that can be blown to form the final bottle product.
Unfortunately, it has been found that the coinjection of a barrier polymer resin with PET can result in defects in the PET preform. A commonly observed melt flow defect is small “pulls,” frequently called chevrons because of their V shape. Chevrons are interfacial instabilities that occur between layers. Chevrons detract from the aesthetics of the finished article.
One barrier resin that may be used in a multilayer process is an ethylene-vinyl acetate copolymer (EVOH) modified with various levels of ethylene (“grades”). It is commonly known that these “grades” of barrier resins have different melt viscosities and melting points. Generally, it would be desirable to match both the melt viscosity of the barrier resin and the melt temperature of the barrier resin to the PET being used. Unfortunately, the commercially available EVOH (regardless of the grade) has a melt viscosity and degradation temperature far below that of commercially available PET. In addition, heat transfer from the hotter PET layer will further heat the EVOH above its desired processing temperature and result in even lower melt viscosity of the barrier resin during injection molding.
Most of the technology for coinjection is relatively new and is just becoming commercially viable for molding multilayer articles or preforms on a large scale. In addition, coinjection for most practical purposes is focused almost solely on the use of PET (or a copolymer thereof) as the structural resin for preform molding applications. In contrast, coextrusion is a well-established technique that is commonly applied to a wide variety of different polymers (e.g., PET, copolyesters, polyolefins, PVC, styrenics, nylons, etc.) and for a much wider range of applications.
In coextrusion, multilayer film or sheet is produced as opposed to a molded article. As with coinjection, there is one or more “structural” layers combined with one or more “performance” layers. The structural layers are usually (but not always) cheaper than the performance layers and are included to keep total cost down (since performance layers can often be expensive). Examples of coextrusion include the use of a barrier layer in packaging film, the use of a UV protecting layer on the outside layer of heavy gauge sheeting for outdoor weathering protection, the use of regrind in the center to reduce costs, the use of adhesive/sealing layers on the outside surface, and the use of glossy and/or pigmented layers to change the overall aesthetics of the film/sheet. Unlike the coinjection example cited above, the “performance” layer in coextrusion does not necessarily have to be on the inside of the multilayer structure.
In the process of coextrusion, the various resins are first melted in separate extruders and then brought together in a feedblock-a feedblock being nothing more than a series of flow channels which bring the layers together into a uniform stream. From this feedblock, this multilayer material then flows through an adapter and out a film die. The film die can be a traditional flat film/sheet die (e.g., a coathanger die) or it can be an annular die as is used in blown film. Coextrusion is also used making more complicated shapes like profiles. When we refer to coextrusion in this document, it is implied that all of these other coextrusion applications are also covered in addition to traditional film/sheet applications.
As with coinjection, coextrusion often suffers with the problem of chevrons and other visual defects. These defects in coextrusion and coinjection both result from high shear stresses developing at the layer interface during flow. These stresses are a function of the viscosities of the layers in addition to the relative position and thickness of the layers. In fact, knowledge gained from coextrusion can be used to help minimize the flow defects in coinjection.
In addition, coextrusion of flat film often suffers from the problem of poor layer distribution across the width of the sheet. For example, if one were to take a piece of coextruded film (for example, an A/B/A structure) and separate the layers, they might find that one of the A layers would be much thicker near the outer edges of the sheet, and very thin in the middle. The B layer would be just the opposite, that is, being thin near the edges and thick in the middle. Usually, it is desired that the layers be uniform in thickness across the full width of the sheet so that properties (e.g., barrier, color, stiffness, etc.) do not vary across the width.
Up until now, correcting these two coextrusion problems (poor layer distribution uniformity and flow defects) has really been more of an art than science. There have been some attempts to balance the viscosities of the resins (i.e., having a viscosity ratio close to one) to improve layer distribution, but this has met with only limited success. Thus, there exists a need for a process to properly select both the resin viscosity and elasticity parameters and the processing conditions in coextrusion such that both the interfacial instabilities (i.e., visual defects like chevrons) and poor layer distribution are eliminated.
In the coextrusion process according to this invention, therefore, t

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