Advanced viscoseals for single screw extruders.

Abstract

An embodiment of the extruder includes an extruder housing having an interior wall and a single screw coaxially disposed within the extruder housing, the single screw including a shank, the shank and / or a region of the interior wall adjacent to the shank including a helical channel. The extruder further includes a feed channel downstream of the shank and a viscoseal including an annular gap between the helical channel and the shank and the interior wall, the annular gap being variable over its length.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Application No. 63 / 396,250, filed August 9, 2022, which is incorporated by reference herein in its entirety.

[0002] This specification relates generally to single screw extruders used in polymer processing, and in particular to viscoseals used in these single screw extruders. [Background technology]

[0003] Viscoseals are placed on the shank of a melt-fed, single-screw extruder screw using a helical channel cut into either the screw shank or the extruder wall. These screws are typically used in large-diameter extruders used to pelletize resin produced from polymer resin manufacturing plants. While the majority of the polymer resin is delivered to the extruder's feed channel, a small amount of resin is transferred to an annular gap created between the shank and wall of the screw between the start of the feed channel and the drive end of the screw. The helical channel and the annular gap, which pump the resin back into the main flow path of the screw, constitute the extruder's viscoseal. Without the viscoseal, the resin would undesirably flow out of the extruder. Viscoseals are also designed to keep resin in the extruder during high-pressure events, i.e., when pressures are 40 psig or greater. In some cases, the feed tank or low-pressure separator may also have a rupture disk, which ruptures when exposed to high pressures, such as 40 psig or greater, thereby reducing pressure.

[0004] While traditional viscoseals may be able to keep the polymer resin inside the extruder even during high-pressure events, they often also serve as gas seals. Specifically, the gas seal must prevent atmospheric oxygen from entering the extruder, even when a separate environmental seal is included. Atmospheric oxygen can cause oxidative degradation of the resin, resulting in crosslinked material and black specks, potentially rendering the resin unusable. Furthermore, without an effective gas seal, traces of dissolved ethylene gas can escape from the extruder through the annular gap.

[0005] As a result, there is a need for a viscoseal that can maintain polymer resin within the extruder even during high pressure events while also acting as a suitable gas seal. Summary of the Invention

[0006] Embodiments of the present disclosure fulfill this need by utilizing a viscoseal having a variable annular gap. Without being limited by theory, the variable annular gap adjusts the seal fill length, which synergistically keeps the polymer resin within the extruder while also providing a gas seal for the extruder. In this case, the viscoseal's ability to act as a gas seal may include preventing atmospheric oxygen from entering the extruder and preventing unreacted ethylene from exiting the extruder.

[0007] According to one embodiment, an extruder is provided, the extruder comprising: an extruder housing having an interior wall; a single screw coaxially disposed within the extruder housing, the single screw including a shank, the shank and / or a region of the interior wall adjacent to the shank including a helical channel; a feed channel downstream of the shank; and a viscoseal including the helical channel and also including an annular gap between the shank and the interior wall, the annular gap being variable over its length.

[0008] Additional features and advantages are set forth in the following Detailed Description, and in part will be readily apparent to those skilled in the art from that description or will be recognized by practicing the embodiments described herein, including the drawings, the following Detailed Description, and the claims. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram of a single screw extruder having a tapered inner wall in accordance with one or more embodiments of the present disclosure. [Figure 2] FIG. 2 is an enlarged view of a viscoseal of the single screw extruder of FIG. 1 in accordance with one or more embodiments of the present disclosure. [Figure 3] FIG. 1 is a schematic diagram of a single screw extruder having a tapered screw shank in accordance with one or more embodiments of the present disclosure. [Figure 4] FIG. 1 is a schematic diagram of a single screw extruder having a barrel wall with helical flights in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] Specific embodiments of the present application will now be described. However, this disclosure may be embodied in different forms and should not be construed as limited to the embodiments set forth in this disclosure. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present subject matter to those skilled in the art.

[0011] definition The term "polymer" refers to a polymeric compound prepared by polymerizing monomers (whether of the same or different types). Thus, the generic term polymer encompasses the term "homopolymer," which is typically used to refer to a polymer prepared from only one type of monomer, as well as "copolymer," which refers to a polymer prepared from two or more different monomers. As used herein, the term "interpolymer" refers to a polymer prepared by polymerization of at least two different types of monomers. Thus, the generic term interpolymer includes copolymers and polymers prepared from more than two different types of monomers, such as terpolymers.

[0012] "Polyethylene" or "ethylene-based polymer" means a polymer containing greater than 50% by weight of units derived from ethylene monomers. This includes polyethylene homopolymers or copolymers (meaning units derived from two or more comonomers). The comonomers may include olefinic comonomers as well as polar comonomers. Common forms of polyethylene known in the art include low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultra low density polyethylene (ULDPE), very low density polyethylene (VLDPE), single-site catalyzed linear low density polyethylene (m-LLDPE), including both linear and substantially linear low density resins, medium density polyethylene (MDPE), and high density polyethylene (HDPE).

[0013] As used herein, a "high pressure polymer resin" is a polymer produced at pressures greater than 14,500 psi (100 MPa) and may include high pressure copolymers or homopolymers. This may include ethylene homopolymers, such as LDPE, or ethylene copolymers. The term "LDPE," sometimes referred to as "high pressure ethylene polymer" or "highly branched polyethylene," is defined to mean that the polymer is partially or fully homopolymerized or copolymerized in an autoclave or tubular reactor at pressures greater than 14,500 psi (100 MPa) using a free radical initiator such as peroxide (see, for example, U.S. Pat. No. 4,599,392, incorporated herein by reference). LDPE resins typically have a viscosity of 0.916 grams per cubic centimeter (g / cm). 3 )~0.935g / cm 3 The density is in the range of

[0014] The terms "comprising," "including," "having," and their derivatives are not intended to exclude the presence of any additional component, step, or procedure, whether or not it is specifically disclosed. For the avoidance of doubt, all compositions claimed through the use of the term "comprising" may include any additional additive, adjuvant, or compound, whether polymeric or not, unless stated to the contrary. In contrast, the term "consisting essentially of" excludes from the scope of any subsequent description any other component, step, or procedure, except those that are not essential to operability. The term "consisting of" excludes any component, step, or procedure not specifically delineated or listed.

[0015] 1, 3, and 4, embodiments of the present disclosure are directed to an extruder 5 including an extruder housing 10 having an interior wall 30 as shown. The extruder 5 includes a single screw 12 coaxially disposed within the extruder housing 10. The single screw 12 includes a shank 20. As shown in FIGS. 1-3, the shank 20 may include a helical channel 22. As shown in FIG. 4, a region of the interior wall 30 adjacent the shank 20 includes the helical channel 22.

[0016] 1 and 3, the extruder includes a feed channel 24 downstream of the shank 20. Additionally, the extruder 20 includes a viscoseal, which includes a helical channel 22 and an annular gap 40 between the shank 20 and the inner wall 30. As shown in FIGS. 1-3, the viscoseal surrounds the annular gap 40 and the helical channel 22 of the shank 20.

[0017] 1 and 3, extruder 5 further includes a low-pressure separator (LPS) 60 upstream of feed channel 24. LPS 60 is used to remove a portion of unreacted gas (e.g., unreacted ethylene gas) from the molten polymer resin. LPS 60 typically operates at about 2-10 psig, 4-8 psig, at least 5 psig, or 6 psig. The pressure from LPS 60 forces the majority of the molten polymer resin into feed channel 24 of extruder 5; however, as discussed above, a small amount of resin may be introduced into annular gap 40.

[0018] Additionally, the extruder 5 may include a gearbox 50 coupled to the drive end 23 of the single screw 12. As shown, the shank 20 and annular gap 40 are disposed between the drive end 23 of the single screw 22 and the feed channel 24.

[0019] As discussed above and shown in Figures 1-3, the annular gap 40 is variable across its length. In certain variable length embodiments, the annular gap 40 tapers in a direction away from the feed channel 24. In one or more embodiments, the annular gap 40 tapers to at least half of its original maximum thickness, or to at least one-third of its original maximum thickness.

[0020] Referring to the embodiment shown in FIG. 2, the narrow end c of the annular gap (i.e., the end of the taper) can be 0.1 to 1.0%, or 0.3 to 0.4%, of the screw diameter. For a 12-inch screw, typical clearances at the narrow end c of the annular gap 40 are 0.012 to 0.120 inches, or 0.036 to 0.048 inches. At the wide gap a, located at the beginning of the viscoseal taper, the clearance can be 2 to 5 times the clearance at the narrow end. For example, typical clearances at the wide end a of the annular gap 40 can be 0.3 to 3.0%, or 0.9 to 1.2% of the screw diameter. Without being limited by theory, the narrow end c ensures that significant pressure exists to continue pumping the molten resin in the event of a high-pressure event (i.e., pressures of at least 40 psig), while the wide end a and transition regime b ensure that the viscoseal has sufficient seal fill length for gas sealing performance.

[0021] 1 and 2, the inner wall 30 includes a variable diameter in the region adjacent the shank 20 (see regions a, b, c) to form a variable annular gap 40. In this case, the inner wall 30 tapers, while the diameter of the shank 20 remains substantially constant, thereby forming the variable annular gap 40.

[0022] In an alternative embodiment shown in Figure 2, the shank 20 may include a variable diameter to form a variable annular gap 40. In this case, the shank 30 is tapered, but the diameter of the inner wall 30 remains substantially constant, thereby forming the variable annular gap 40.

[0023] Without being bound by theory, the ability of the present variable annular gap viscoseal to act as a gas seal may be correlated to the seal fill length. In one or more exemplary embodiments, the seal fill length of the polymer resin in the annular gap is 15-45 mm at a pressure of 5-6 psig and a diameter of about 18 inches (457.2 mm). In further embodiments, the seal fill length of the polymer resin may be 20-40 mm at a pressure of 5-6 psig.

[0024] Furthermore, the ability of the present variable annular gap viscoseal to act as a gas seal can be correlated to the seal length, defined by the ratio of the seal fill length to the diameter of the single screw. In one or more embodiments, this ratio is 4-40%, 4-25%, 5-20%, or 5-15%. Without being bound by theory, values below the lower limit of 4% do not achieve the desired gas seal performance. As discussed above, an acceptable gas seal regulates gas flow between the inside and outside of the extruder such that atmospheric oxygen cannot enter the extruder and ethylene gas cannot escape the extruder through the shank.

[0025] In operation, the extruder 5 may be used for pelletizing polymer resin. As shown in Figures 1 and 3, molten polymer resin 15 may be delivered from the LPS 60 to the feed channel 24 at a pressure of at least 5 psig. A viscoseal may assist in transporting any molten polymer resin within the variable annular gap 40 back into the feed channel 24. Pelletized polymer is produced and discharged from the outlet 100 of the extruder 5.

[0026] Test Method density Samples for density are measured according to ASTM D792, Method B.

[0027] Melt Index (I2) Melt index, or I2, (grams / 10 minutes or dg / minute) is measured according to ASTM D 1238, Condition 190°C / 2.16 kg, Procedure B.

[0028] Seal Length The helical portion of the seal had an axial length of 0.3 to 0.5 diameters. The grooves had 2 to 4 threads, a lead length of 0.15 to 0.30 diameters, a groove depth of 0.005 to 0.008 diameters, and a width of 0.2 to 0.4 diameters. Calculations were performed with the Simcenter STAR-CCM+ software package, as further described below. [Example]

[0029] The embodiments will be further clarified by the following examples.

[0030] The following commercial resins were used in the examples:

[0031] AGILITY™ 1000 is a 0.920 g / cm 3 and a melt index (I2) of 0.15 g / 10 min.

[0032] DXM-445 is a 0.920 g / cm 3 and high pressure LDPE with an I2 of 2 g / 10 min.

[0033] DOW™ LDPE 132i is a 0.921 g / cm 3 copolymer available from Dow Inc., Midland, MI. 3 and high-pressure LDPE with an I2 of 0.25 g / 10 min.

[0034] Example of a viscoseal with a constant annular gap Several numerical simulations were performed using Simcenter STAR-CCM+ computational fluid dynamics (CFD) software. One such example was performed for a viscoseal with a constant annular gap of 0.035 inches. This fusion seal was only approximately 2-3 mm long in an 18-inch (457.2 mm) diameter screw extruder. All three commercial resins were evaluated. A fill length of 2-3 mm was not sufficient to maintain a gas seal. As shown in Table 1, the seal fill length for each of the three resins was insufficient to maintain a gas seal. For a high-pressure event (40 psig), the fill length ranged from 9.4 to 14.55 mm, a length well within the seal's helical shape and therefore capable of maintaining the resin within the extruder.

[0035] Additional numerical simulations were also performed for a viscoseal with a constant annular gap of 0.100 inches. The resin used was DOW™ LDPE132i. In this case, a fill length of 26.8 mm was achieved, thus providing an acceptable gas seal. However, the constant annular gap of 0.100 inches could only withstand a pressure of approximately 32 psig. Therefore, a high-pressure event of 40 psig could cause the resin to flow out of the extruder.

[0036] Example of a viscoseal with variable annular gap Numerical simulations were also performed on a ViscoSeal with a variable annular gap, as further shown in Table 1. At the screw pocket, the annular gap was 0.100 inches, resulting in sufficient molten resin to provide an acceptable gas seal. The annular gap then tapered over the next 160.5 mm to a smaller gap of 0.035 inches, thereby providing acceptable pumping during high-pressure events. The annular gap was created by adjusting the diameter of the inner wall, as shown in Figures 1 and 2.

[0037] [Table 1]

[0038] [Table 2]

[0039] [Table 3]

[0040] As shown in the examples above, the constant annular gap (0.035) Viscoseal example exhibits a maximum seal fill length / screw diameter ratio of 1% at low pressures (5-10 psig). This is an insufficient seal length to achieve suitable gas sealing properties. In contrast, the variable annular gap Viscoseal example yielded a seal fill length / screw diameter ratio of greater than 4% at low pressures (5-10 psig). As a result, the variable annular gap Viscoseal is a suitable gas seal.

[0041] It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Accordingly, this specification is intended to cover all such modifications and variations of the various embodiments described herein, provided they come within the scope of the appended claims and their equivalents.

Claims

1. An extruder housing having an inner wall, A single-screw coaxially disposed within the extruder housing, wherein the single-screw includes a shank, and the region of the inner wall adjacent to the shank includes a helical channel. The supply channel downstream of the shank, A viscoseal comprising the helical channel and an annular gap between the helical channel and the inner wall, wherein the annular gap is variable over its length, An extruder equipped with the following features.

2. The extruder according to claim 1, wherein the annular gap tapers as it moves away from the supply channel.

3. The extruder according to claim 1, wherein the annular gap tapers to at least half of its original maximum thickness, or to at least one-third of its original maximum thickness.

4. The extruder according to claim 1, wherein the inner wall includes a variable diameter in a region adjacent to the shank in order to form the variable annular gap.

5. The extruder according to claim 1, wherein the shank includes a variable diameter to form the variable annular gap.

6. The extruder according to claim 1, wherein the shank includes a helical channel.

7. The extruder according to claim 1, wherein the inner wall includes a helical channel.

8. The extruder according to claim 1, further comprising a low-pressure separator upstream of the supply channel.

9. The extruder according to claim 1, further comprising a gearbox coupled to the drive end of the single-screw, wherein the shank and annular gap are disposed between the drive end of the single-screw and the supply channel.

10. A method for pelletizing a polymer resin from an extruder according to any one of claims 1 to 9, The molten polymer resin is passed through the supply channel at a pressure of at least 5 psig, Using the Viscoseal, transport any molten polymer resin within the variable annular gap back to the supply channel, A method comprising producing the pelletized polymer resin at the outlet of the extruder.

11. The method according to claim 10, wherein the polymer comprises an ethylene homopolymer or an ethylene copolymer.

12. The method according to claim 10, wherein the Viscoseal is defined by the ratio of the seal filling length to the diameter of the single-screw, and the ratio is 4 to 40% or 5 to 15%.

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