Advanced viscoelastic seal for single screw extruder

Abstract

Embodiments of an extruder include an extruder housing having an inner wall and a single screw coaxially disposed within the extruder housing, the single screw including a shank, where the shank and / or an area of the inner wall proximate the shank includes a spiral channel. The extruder also includes a feed channel downstream of the shank, and a viscoelastic seal including the spiral channel and an annular gap between the shank and the inner wall, wherein the annular gap is variable over its length.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application Serial No. 63 / 396,250, filed on August 9, 2022, which is hereby incorporated by reference in its entirety. Technical Field

[0003] The present specification relates generally to single screw extruders used in polymer processing, and in particular to visco-seals used in these single screw extruders. Background Art

[0004] Viscoelastic seals are placed on the handle of the single screw extruder screw of the melt feed using a spiral channel that cuts into the screw handle or the extruder wall. These screws are generally used in large diameter extruders, which are used to granulate the resin produced by the polymer resin production plant. When most of the polymer resin is delivered to the feed channel of the extruder, a small portion of the resin is transferred to the annular gap generated between the handle of the screw and the wall between the starting point of the feed channel and the driving end of the screw. The spiral channel and annular gap in the main flow path of the resin pump back to the screw contain the viscoelastic seals of the extruder. In the absence of viscoelastic seals, resin will flow out of the extruder unexpectedly. Viscoelastic seals are also designed to keep resin in the extruder during high pressure events (that is, when the pressure is 40psig and greater). In some cases, a feed tank or a low pressure separator may also have a rupture disk, which fails when subjected to high pressure (such as 40psig and greater) and reduces pressure thereby.

[0005] Although conventional viscoelastic seals are able to retain polymer resins within the extruder even during high pressure events, viscoelastic seals often struggle as gas seals. Specifically, even if a separate environmental seal is included, the gas seal needs to prevent atmospheric oxygen from flowing into the extruder. Atmospheric oxygen can cause oxidative degradation of the resin, resulting in cross-linked material and black specks, which can render the resin unusable. In addition, without an effective gas seal, trace amounts of dissolved ethylene gas may escape the extruder through the annular gap.

[0006] Therefore, there is a need for a viscoelastic seal that is capable of retaining the polymer resin within the extruder even during high pressure events while also acting as a suitable gas seal. Summary of the invention

[0007] Embodiments of the present disclosure meet this need by utilizing a viscoelastic seal with a variable annular gap. Without being limited by theory, the variable annular gap adjusts the seal fill length, which synergistically holds the polymer resin within the extruder while also providing a gas seal for the extruder. In this case, the ability of the viscoelastic seal to act as a gas seal can include blocking atmospheric oxygen from entering the extruder, as well as blocking unreacted ethylene from leaving the extruder.

[0008] According to one embodiment, an extruder is provided. The extruder includes an extruder housing having an inner wall and a single screw coaxially disposed within the extruder housing, the single screw including a shank, wherein the shank and / or a region of the inner wall proximate the shank includes a spiral channel. The extruder also includes a feed channel downstream of the shank, and a viscoelastic seal including the spiral channel and also including an annular gap between the shank and the inner wall, wherein the annular gap is variable in length.

[0009] Additional features and advantages will be set forth in the following Detailed Description, and in part will become apparent to those skilled in the art from that description or may be learned by practicing the embodiments described herein, including the drawings, the following Detailed Description, and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a schematic view of a single screw extruder with a tapered inner wall according to one or more embodiments of the present disclosure;

[0011] Figure 2 According to one or more embodiments of the present disclosure Figure 1 A magnified view of a viscoelastic seal of a single screw extruder;

[0012] Figure 3 is a schematic view of a single screw extruder with a tapered screw shank according to one or more embodiments of the present disclosure; and

[0013] Figure 4 is a schematic view of a single screw extruder having a barrel wall with spiral flights according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0014] The specific embodiments of the application will now be described. However, the present disclosure can be implemented in different forms and should not be construed as being limited to the embodiments set forth in the present disclosure. On the contrary, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the subject matter to those skilled in the art.

[0015] definition

[0016] The term "polymer" refers to a polymeric compound prepared by polymerizing monomers (whether of the same or different types). Thus, the general term polymer encompasses the term "homopolymer," which is generally used to refer to polymers prepared from only one type of monomer, and "copolymer," which refers to polymers prepared from two or more different monomers. The term "interpolymer," as used herein, refers to a polymer prepared by the polymerization of at least two different types of monomers. Thus, the general term interpolymer includes copolymers, and polymers prepared from more than two different types of monomers, such as terpolymers.

[0017] "Polyethylene" or "ethylene-based polymer" shall mean a polymer comprising 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). Comonomers may include olefin 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, including both linear low density resins and substantially linear low density resins (m-LLDPE); medium density polyethylene (MDPE); and high density polyethylene (HDPE).

[0018] As used herein, "high pressure polymer resin" is a polymer produced at a pressure above 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" may also be referred to as "high pressure ethylene polymer" or "highly branched polyethylene" and is defined to mean a polymer that is partially or completely homopolymerized or copolymerized in an autoclave or tubular reactor at a pressure above 14,500 psi (100 MPa) by using a free radical initiator (such as a peroxide) (see, for example, US 4,599,392, which is incorporated herein by reference). LDPE resins typically have a molecular weight of 0.916 grams per cubic centimeter (g / cm 3 ) to 0.935g / cm 3 Density within the range.

[0019] The terms "comprising," "having," and derivatives thereof are not intended to exclude the presence of any additional components, steps, or procedures, whether or not specifically disclosed. For the avoidance of any doubt, all compositions claimed through use of the term "comprising" may include any additional additives, adjuvants, or compounds, whether polymeric or otherwise, unless stated to the contrary. In contrast, the term "consisting essentially of excludes from the scope of any subsequently recited component, step, or procedure, except those that are not essential to operability. The term "consisting of excludes any ingredient, step, or procedure not specifically recited or listed.

[0020] refer to Figure 1 , Figure 3 and Figure 4 , embodiments of the present disclosure relate to an extruder 5, which includes an extruder housing 10 having an inner 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 handle 20. Figures 1 to 3 As shown, the handle 20 may include a spiral channel 22. Figure 4 As shown, a region of the inner wall 30 proximate the shank 20 includes the spiral channel 22 .

[0021] Reference again Figure 1 and Figure 3 The extruder includes a feed channel 24 located downstream of the handle 20. In addition, the extruder 20 includes a viscoelastic seal including a spiral channel 22 and an annular gap 40 between the handle 20 and the inner wall 30. Figures 1 to 3 As shown, the viscoelastic seal surrounds the annular gap 40 and the spiral channel 22 of the shaft 20 .

[0022] Reference again Figure 1 and Figure 3 , the extruder 5 also includes a low pressure separator (LPS) 60 located upstream of the feed channel 24. The LPS 60 is used to remove a portion of the unreacted gas (e.g., unreacted ethylene gas) from the molten polymer resin. The LPS 60 is typically operated at between about 2 psig and 10 psig, from 4 psig to 8 psig, at least 5 psig, or 6 psig. The pressure from the LPS 60 forces most of the molten polymer resin into the feed channel 24 of the extruder 5; however, as discussed above, a small portion of the resin may be introduced into the annular gap 40.

[0023] In addition, 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 the annular gap 40 are disposed between the drive end 23 of the single screw 22 and the feed channel 24.

[0024] As mentioned above and Figures 1 to 3 As shown, the annular gap 40 is variable in its length. In a specific embodiment of variable length, 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 initial maximum thickness, or at least one-third of its initial maximum thickness.

[0025] refer to Figure 2 In the depicted embodiment, the narrow end c (i.e., the tapered end) of the annular gap can be 0.1% to 1.0% or 0.3% to 0.4% of the screw diameter. For a 12-inch diameter screw, the typical clearance at the narrow end c of the annular gap 40 will be 0.012 inches to 0.120 inches, 0.036 inches to 0.048 inches. At the wide gap a at the beginning of the viscoelastic seal taper, the clearance can be 2 to 5 times the clearance at the narrow end. For example, the typical clearance at the wide end a of the annular gap 40 will be 0.3% to 3.0% or 0.9% to 1.2% of the screw diameter. Without being limited by theory, in the case of a high pressure event (i.e., a pressure of at least 40 psig), the narrow end c ensures that there is significant pressure to keep pumping molten resin, while the wide end a and transition zone b ensure that the viscoelastic seal has sufficient seal fill length for gas seal performance.

[0026] Reference again Figure 1 and Figure 2 In the embodiment of the present invention, the inner wall 30 includes a variable diameter in the area close to the handle 20 (see area a, area b, area c) to form a variable annular gap 40. In this case, the inner wall 30 gradually becomes smaller while the diameter of the handle 20 remains substantially constant, thereby forming a variable annular gap 40.

[0027] exist Figure 2 In the depicted alternative embodiment, the handle 20 may include a variable diameter to form a variable annular gap. In this case, the handle 30 tapers while the diameter of the inner wall 30 remains substantially constant, thereby forming a variable annular gap 40.

[0028] Without being bound by theory, the ability of the variable annular gap viscoelastic seal of the present invention to act as a gas seal can be related 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 mm to 45 mm at a pressure of 5 psig to 6 psig and a diameter of about 18 inches (457.2 mm). In another embodiment, the seal fill length of the polymer resin can be 20 mm to 40 mm at a pressure of 5 psig to 6 psig.

[0029] In addition, the ability of the variable annular gap viscoelastic seal of the present invention to act as a gas seal can be related to the seal length, which is defined by the ratio of the seal fill length to the single screw diameter. In one or more embodiments, the ratio is 4% to 40%, 4% to 25%, 5% to 20%, or 5% to 15%. Without being bound by theory, values ​​below the 4% lower limit will not achieve the desired gas seal performance. As described above, an acceptable gas seal regulates the flow of gas between the inside and outside of the extruder so that atmospheric oxygen cannot intrude into the extruder and ethylene gas cannot escape the extruder through the handle.

[0030] In operation, the extruder 5 can be used to pelletize the polymer resin. Figure 1 and Figure 3 As shown, molten polymer resin 15 may be delivered from LPS 60 to feed channel 24 at a pressure of at least 5 psig. The viscoelastic seal may help convey any molten polymer resin within variable annular gap 40 back into feed channel 24. Pelletized polymer is produced and discharged from outlet 100 of extruder 5.

[0031] Test Method

[0032] density

[0033] The density of the samples was measured according to ASTM D792 Method B.

[0034] Melt index (I2)

[0035] Melt index or I2 (grams / 10 minutes or dg / min) is measured according to ASTM D 1238, Condition 190°C / 2.16 kg, Procedure B.

[0036] Seal length

[0037] The helical portion of the seal is set between 0.3 dia and 0.5 dia in axial length. The groove has 2 to 4 starting points, a lead length of 0.15 dia to 0.30 dia, a groove depth of 0.005 dia to 0.008 dia and a width of 0.2 dia to 0.4 dia. Calculations were performed on the Simcenter STAR-CCM+ software package as further described below.

[0038] Example

[0039] The embodiments will be further illustrated by the following examples.

[0040] The following commercial resins were used in the examples.

[0041] AGILITYTM 1000 has a thickness of 0.920 g / cm 3 A high pressure LDPE having a density of 0.0000 Å and a melt index (I2) of 0.15 g / 10 min, available from Dow Inc., Midland, MI.

[0042] DXM-445 has a strength of 0.920 g / cm 3 A high pressure LDPE with a density of 2 g / 10 min and an I2 of 2 g / 10 min, available from Dow Inc., Midland, MI.

[0043] DOW TM LDPE132i has a hardness of 0.921 g / cm 3 A high pressure LDPE with a density of 0.05 g / 10 min and an I2 of 0.25 g / 10 min, available from Dow Inc., Midland, MI.

[0044] Constant annular gap viscoelastic seal embodiment

[0045] Simcenter STAR-CCM+ computational fluid dynamics (CFD) software is used to carry out several numerical simulations.One such example is carried out to a viscoelastic seal with a constant annular gap of 0.035 inches.For a screw extruder with an 18-inch (457.2mm) diameter, the length of this molten seal is only about 2mm to 3mm.All three commercial resins are evaluated.The filling length of 2mm to 3mm is not enough to maintain a gas seal.As shown in Table 1, the seal filling length of each of the three resins is not enough to maintain a gas seal.For high pressure events (40psig), the filling length is in the range of 9.4mm to 14.55mm, which is just within the spiral geometry of the seal, and therefore the resin can be maintained in the extruder.

[0046] Additional numerical simulations were performed for a viscoelastic seal with a constant annular gap of 0.100 inches. The resin used was DOW TM LDPE132i. In this case, a fill length of 26.8 mm was achieved, thus providing an acceptable gas seal. However, a constant annular gap of 0.100 inches can only withstand a pressure of about 32 psig. Therefore, a high pressure event of 40 psig can cause the resin to flow out of the extruder.

[0047] Variable annular gap viscoelastic seal embodiment

[0048] As further shown in Table 1, a viscoelastic seal with a variable annular gap was also numerically simulated. At the cavity of the screw, the annular gap was 0.100 inches, allowing enough molten resin to provide an acceptable gas seal. The annular gap then tapers to a small gap of 0.035 inches over the next 160.5 mm, providing acceptable pumping during high pressure events. Figure 1 and Figure 2 As shown, the annular gap is formed by adjusting the diameter of the inner wall.

[0049] Table 1. Simulation Results - Seal Fill Length (mm) as a Function of LPS Pressure (psi)

[0050]

[0051]

[0052]

[0053] As shown in the above examples, the constant annular gap (0.035) viscoelastic seal embodiment exhibits a maximum seal fill length / screw diameter ratio of 1% at low pressure (5psig to 10psig). This is an insufficient seal length to achieve suitable gas seal performance. In contrast, at low pressure (5psig to 10psig), the variable annular gap viscoelastic seal embodiment produces a seal fill length / screw diameter ratio of greater than 4%. Therefore, the variable annular gap viscoelastic seal is a suitable gas seal.

[0054] It will be apparent to those skilled in the art that various modifications and variations may be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Therefore, this specification is intended to cover modifications and variations of the various embodiments described herein, provided that these modifications and variations fall within the scope of the appended claims and their equivalents.

Claims

1. An extruder, comprising: an extruder housing having an inner wall; a single screw coaxially disposed in the extruder housing, the single screw comprising a shank, wherein the shank and / or a region of the inner wall proximate to the shank comprises a spiral channel; a feed channel located downstream from the shank; and A viscoelastic seal includes the helical channel and further includes an annular gap between the stem and the inner wall, wherein the annular gap is variable in length.

2. The extruder according to claim 1, wherein the annular gap gradually becomes smaller away from the feed channel.

3. An extruder according to any preceding claim, wherein the annular gap tapers to at least half of its initial maximum thickness or to at least one third of its initial maximum thickness.

4. An extruder according to any preceding claim, wherein the inner wall comprises a variable diameter in the region adjacent the shank to form a variable annular gap.

5. The extruder of any one of claims 1 to 3, wherein the shank comprises a variable diameter to form the variable annular gap.

6. An extruder according to any preceding claim, wherein the shank comprises a spiral channel.

7. The extruder according to any one of claims 1 to 5, wherein the inner wall comprises a spiral channel.

8. An extruder according to any preceding claim, further comprising a low pressure separator located upstream of the feed channel.

9. The extruder according to any preceding claim, further comprising a gear box coupled to a drive end of the single screw, wherein the shank and the annular gap are disposed between the drive end of the single screw and the feed channel.

10. A method of pelletizing a polymer resin from an extruder according to any preceding claim, the method comprising: passing molten polymer resin to the feed channel at a pressure of at least 5 psig; conveying any molten polymer resin within the variable annular gap back to the feed channel using the viscoelastic seal; as well as A pelletized polymer resin is produced at the outlet of the extruder.

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

12. The method of claim 10 or 11, wherein the viscoelastic seal is defined by a ratio of seal fill length to the diameter of the single screw, wherein the ratio is 4% to 40% or 5% to 15%.

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