Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2003-06-11
2004-10-26
Eashoo, Mark (Department: 1732)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S543000
Reexamination Certificate
active
06808805
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to profile extrusion of thermoplastic polymers to form shaped articles commonly referred to as profiles. More particularly, this invention relates to processes of profile extrusion utilizing certain polyester compositions.
BACKGROUND OF THE INVENTION
Thermoplastic polymers are commonly used to manufacture various shaped articles which may be utilized in applications such as automotive parts, food containers, signs, packaging materials and the like. Profile extrusion is a common, cost-effective method for producing these shaped articles. Polymers such as polyvinyl chloride (PVC), acrylics and polycarbonates are typically used in profile extrusion. Each of these polymers suffers from one or more disadvantages. PVC is undesirable from an environmental standpoint since PVC produces toxic gases during melt extrusion and is difficult to dispose of after use. Acrylic articles are brittle and shatter when dropped or struck against another object. Polycarbonate is difficult to work with from a processor's perspective and is too expensive for many applications. Polyesters, being notoriously difficult to process compared to many other polymers, have not been utilized as often in profile extrusion. As compared to polymers typically used in profile extrusion, polyesters have lower melt strengths and insufficient shear thinning resulting in a greater propensity for melt fracture if extruded at high output rates or low temperatures. Both melt strength and shear thinning are extremely important from the standpoint of profile extrusion.
Profiles are defined herein by a combination of two factors: shape and process of manufacture. The shape of a profile has a particular two-dimensional cross-section and an infinite length. The process of manufacture is known as profile extrusion. The cross-section lies in the x-y plane and the length lies along the z-axis. The x-y plane usually corresponds to the face of the die, whereas the z-axis corresponds to the extrusion or “take-off” direction. Profiles can take on a wide variety of cross-sections varying in size, shape and complexity. Common “simple” profile shapes include hollow tubes, solid round stock, square cross-section stock, etc. More complex shapes such as those used for pricing channels, corner guards, and house siding can also be made.
By this use of shape as part of the profile definition, fiber, film and sheet might also be considered as special classes of profiles. Fibers have very small circular cross-sections and are extruded continuously in one direction. Film and sheet have rectangular cross-sections and are extruded continuously. However, in the industry as a whole and as defined herein by the additional definition factor of process of manufacture, film, sheet and fiber are not profiles because of how they are manufactured. Film or sheet, while infinite in length, are manufactured by processes that include the use of calendering or chill rolls. Fiber processes involve very high drawdowns, along with spinning cabinets and godet rolls. Profiles, in the industrial vernacular, represent constant cross-section, axially extruded structures, which have axial rigidity and are not wound. Profiles are usually cut to length and bundled, stacked or otherwise bound for transport. This axial rigidity obviously has important implications for what kind of “haul-off” equipment is used to convey the extruded product. Furthermore, the issues of melt strength and melt fracture are not important factors in fiber, film and sheet due to the nature of the take-up/winding equipment and the fact that shape definition is already trivial. Thus, as defined in the industry and herein, “profile” shall not include fiber, film and sheet.
Profiles are fabricated by melt extrusion processes that begin by extruding a thermoplastic melt through an orifice of a die forming an extrudate capable of maintaining a desired shape. The extrudate is typically drawn into its final dimensions (along the z-axis) while maintaining the desired shape (in the x-y plane) and then quenched in air or a water bath to set the shape, thereby producing a profile. In the formation of simple profiles, the extrudate preferably maintains shape without any structural assistance. With extremely complex shapes, support means are often used to assist in shape retention. In either case, the type of thermoplastic resins utilized and its melt strength during formation is critical. Melt strength is defined as the ability of a polymer to support its weight in the molten state. For example, when extruded vertically from a die, a polymer with low melt strength will quickly sag and hit the floor; whereas, a polymer with high melt strength will maintain its shape for a much longer amount of time.
There are a number of quantitative and qualitative means for measuring melt strength. One standard test is disclosed in U.S. Pat. No. 4,398,022 wherein melt strengths for a polyester used in extrusion blow molding processes were measured at values between −10 and 10 percent. This same test is utilized herein and involves vertically extruding the polymer from a 0.1 inch (0.25 cm) diameter capillary die that is 0.25 inches (0.64 cm) long at a shear rate of 20 s
−1
up to a total length of 19 inches (49 cm). At this point the strand is cut near the die face and allowed to cool at room temperature. The diameter 6 inches (15 cm) from the end of the extrudate is then measured and expressed as a percentage change relative to the capillary diameter to give the melt strength. For example, if the strand diameter at a point 6 inches (15 cm) from the bottom was 0.12 inches (30 cm), then the polymer melt strength at that given melt temperature would be 20 percent (i.e. MS=(0.12−0.1)/0.1 *100 percent). Similarly, the “die swell” is obtained by measuring the diameter ½ inches (1.3 cm) from the bottom of the extrudate and expressing it as a percentage change relative to the capillary diameter.
Polyesters due to their poor melt strength may have a negative value for the melt strength since the 6 inches (15 cm) point diameter could be less than the nominal diameter. For example, linear poly(ethylene terephthalate) modified with 1,4-cyclohexanedimethanol (PETG) having an inherent viscosity (IV) of 0.76 dl/g has been observed to have a melt strength of −4 percent at 200° C. and −24 percent at 220° C. This means that the diameter of the extrudate measured 6 inches (15 cm) from the end of the strand was 4 percent smaller (200° C. sample) than the die opening. Typical melt strengths for PVC under standard processing conditions (160 to 200° C. processing temperature) are in the order of 20 to 30 percent. To achieve this melt strength with linear PETG would require an IV of around 0.95 dl/g. Thus, for applications in which melt strength is critical, polyesters will often not supplant these competitive polymers.
Another common melt strength test involves measuring the time period that an extrudate takes to reach a predetermined length below a die for a given flowrate/shear rate. While not standardized, this test provides an easy method for material comparison on a typical processing line and is used in some of the examples cited herein. Other non-standard melt strength tests such as measuring the degree of drooping in a horizontal profile extrusion line can also be applied giving a more application specific measure of melt strength.
Profile extrusions are usually run horizontally, and thus melt strength is important to minimize the amount of “drawdown” and gravity-induced sagging the polymer experiences upon exiting the die. Drawdown is defined in profile extrusion as the amount of thickness reduction between the die and the take-up system and is expressed as the nominal thickness or width dimension at the die divided by the same dimension in the final part). For example, a typical polyester drawdown is about two. This means that the width of the final part is ½ that of the width at the die exit. Similarly, the final thickness is &f
Cobb Daniel Christopher
Jensen Julian Harold Scott
Shelby Marcus David
Strand Marc Alan
Boshears B. J.
Eashoo Mark
Eastman Chemical Company
Groves, Jr. Bernard J.
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