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US-12617906-B2 - Methods of forming crosslinked polyolefin nanocomposites having high wear resistance

US12617906B2US 12617906 B2US12617906 B2US 12617906B2US-12617906-B2

Abstract

Methods for forming polyolefin nanocomposite precursor compositions are provided. In embodiments, such a method comprises mixing a polyolefin, unmodified graphite, and a peroxide crosslinker via solid-state shear pulverization under conditions to form a polyolefin nanocomposite precursor composition comprising the polyolefin; exfoliated, unmodified graphite dispersed throughout the polyolefin; and unreacted peroxide crosslinker dispersed throughout the polyolefin, wherein the polyolefin is polyethylene, a copolymer of polyethylene, or combinations thereof. Methods of forming crosslinked polyolefin nanocomposites, the polyolefin nanocomposite precursor compositions, and crosslinked polyolefin nanocomposites are also provided.

Inventors

  • John M. Torkelson
  • Tong Wei

Assignees

  • NORTHWESTERN UNIVERSITY

Dates

Publication Date
20260505
Application Date
20230328

Claims (19)

  1. 1 . A polyolefin nanocomposite precursor composition consisting of low-density polyethylene; exfoliated, unmodified graphite dispersed throughout the low-density polyethylene; unreacted peroxide crosslinker dispersed throughout the low-density polyethylene; and optionally, one or more of a dye, a preservative, and an antioxidant, wherein the exfoliated, unmodified graphite is present at an amount in a range of from 2 weight % to 5 weight %.
  2. 2 . The polyolefin nanocomposite precursor composition of claim 1 , having a diffraction peak at 26.5° having a normalized intensity that is within 5% of that of the low-density polyethylene without the exfoliated, unmodified graphite and without the unreacted peroxide crosslinker as measured by X-ray diffraction.
  3. 3 . The polyolefin nanocomposite precursor composition of claim 1 , wherein the low-density polyethylene has a density in a range of from 0.910 g/cm 3 to 0.940 g/cm 3 .
  4. 4 . The polyolefin nanocomposite precursor composition of claim 1 , wherein the unreacted peroxide crosslinker is dicumyl peroxide; cumene hydroperoxide; t-butyl peroxide; 2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane; 2,5-Bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne; Bis[1-(tert-butylperoxy)-1-methylethyl]benzene; or a combination thereof.
  5. 5 . The polyolefin nanocomposite precursor composition of claim 1 , wherein the unreacted peroxide crosslinker is dicumyl peroxide.
  6. 6 . The polyolefin nanocomposite precursor composition of claim 1 , wherein the unreacted peroxide crosslinker is present at an amount in a range of from 1 weight % to 7 weight %.
  7. 7 . The polyolefin nanocomposite precursor composition of claim 1 , wherein the unreacted peroxide crosslinker is present at an amount in a range of from 2 weight % to 5 weight %.
  8. 8 . The polyolefin nanocomposite precursor composition of claim 1 , wherein the low-density polyethylene has a density in a range of from 0.910 g/cm 3 to 0.940 g/cm 3 and wherein the unreacted peroxide crosslinker is dicumyl peroxide; cumene hydroperoxide; t-butyl peroxide; 2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane; 2,5-Bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne; Bis[1-(tert-butylperoxy)-1-methylethyl]benzene; or a combination thereof.
  9. 9 . The polyolefin nanocomposite precursor composition of claim 8 , wherein the unreacted peroxide crosslinker is present at an amount in a range of from 1 weight % to 7 weight %.
  10. 10 . A polyolefin nanocomposite precursor composition comprising low-density polyethylene; exfoliated, unmodified graphite dispersed throughout the low-density polyethylene; and unreacted peroxide crosslinker dispersed throughout the low-density polyethylene, wherein a crosslinked polyolefin nanocomposite, formed by subjecting the polyolefin nanocomposite precursor composition to a melt processing technique, exhibits a reduction in wear volume of at least 80% as compared to a comparative material formed by subjecting neat low-density polyethylene to the melt processing technique under identical conditions.
  11. 11 . The polyolefin nanocomposite precursor composition of claim 10 , having a diffraction peak at 26.5° having a normalized intensity that is within 5% of that of the low-density polyethylene without the exfoliated, unmodified graphite and without the unreacted peroxide crosslinker as measured by X-ray diffraction.
  12. 12 . The polyolefin nanocomposite precursor composition of claim 10 , wherein the low-density polyethylene has a density in a range of from 0.910 g/cm 3 to 0.940 g/cm 3 .
  13. 13 . The polyolefin nanocomposite precursor composition of claim 10 , wherein the unreacted peroxide crosslinker is dicumyl peroxide; cumene hydroperoxide; t-butyl peroxide; 2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane; 2,5-Bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne; Bis[1-(tert-butylperoxy)-1-methylethyl]benzene; or a combination thereof.
  14. 14 . The polyolefin nanocomposite precursor composition of claim 10 , wherein the unreacted peroxide crosslinker is dicumyl peroxide.
  15. 15 . The polyolefin nanocomposite precursor composition of claim 10 , wherein the exfoliated, unmodified graphite is present at an amount in a range of from 1 weight % to 7 weight % and the unreacted peroxide crosslinker is present at an amount in a range of from 1 weight % to 7 weight %.
  16. 16 . The polyolefin nanocomposite precursor composition of claim 10 , wherein the exfoliated, unmodified graphite is present at an amount in a range of from 2 weight % to 5 weight %.
  17. 17 . The polyolefin nanocomposite precursor composition of claim 16 , further wherein the unreacted peroxide crosslinker is present at an amount in a range of from 2 weight % to 5 weight %.
  18. 18 . The polyolefin nanocomposite precursor composition of claim 17 , consisting of the low-density polyethylene; the exfoliated, unmodified graphite dispersed throughout the low-density polyethylene; the unreacted peroxide crosslinker dispersed throughout the low-density polyethylene; and optionally, one or more of a dye, a preservative, and an antioxidant.
  19. 19 . The polyolefin nanocomposite precursor composition of claim 18 , wherein the low-density polyethylene has a density in a range of from 0.910 g/cm 3 to 0.940 g/cm 3 and wherein the unreacted peroxide crosslinker is dicumyl peroxide.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS The present application is a divisional application of U.S. patent application Ser. No. 17/378,857 filed Jul. 19, 2021, which claims priority to U.S. provisional patent application No. 63/053,997 that was filed Jul. 20, 2020, the entire contents of both of which are incorporated herein by reference. BACKGROUND Because of its many positive attributes, including flexibility, chemical resistance and low cost, low-density polyethylene (LDPE) is widely used for packaging and a range of industrial and medical applications, among others. However, due to its poor wear and abrasion resistance, the application of LDPE under continuous friction is limited. One possible solution is to incorporate rigid filler into neat LDPE matrix in order to improve the wear resistance. However, the efforts to enhance the wear resistance of LDPE are limited because the incorporation of nanofiller can only result in LDPE composites with moderate wear resistance. Consequently, most studies aimed at achieving good wear resistance in polyethylene (PE)-based materials have focused on another form of PE, ultra-high molecular weight polyethylene (UHMWPE), which exhibits superior wear resistance even in the neat state. Because of its low friction coefficient, chemical stability and biocompatibility, UHMWPE has been used for tribological contact pairs applications, such as artificial joints. However, due to the presence of extremely long chains and ultra-high melt viscosity, the melt processability of UHMWPE is severely limited. SUMMARY Provided are methods for forming polymer nanocomposite precursor compositions and crosslinked polymer nanocomposites formed therefrom. Methods for forming polyolefin nanocomposite precursor compositions are provided. In embodiments, such a method comprises mixing a polyolefin, unmodified graphite, and a peroxide crosslinker via solid-state shear pulverization under conditions to form a polyolefin nanocomposite precursor composition comprising the polyolefin; exfoliated, unmodified graphite dispersed throughout the polyolefin; and unreacted peroxide crosslinker dispersed throughout the polyolefin, wherein the polyolefin is polyethylene, a copolymer of polyethylene, or combinations thereof. Methods of forming crosslinked polyolefin nanocomposites are also provided. In embodiments, such a method comprises subjecting a polyolefin nanocomposite precursor composition comprising a polyolefin; exfoliated, unmodified graphite dispersed throughout the polyolefin; and unreacted peroxide crosslinker dispersed throughout the polyolefin, wherein the polyolefin is polyethylene, a copolymer of polyethylene, or combinations thereof, to a melt processing technique under conditions to induce chemical reactions to crosslink chains of polyolefin, thereby forming a crosslinked polyolefin nanocomposite. The polyolefin nanocomposite precursor compositions and crosslinked polyolefin nanocomposites are also provided. Other principal features and advantages of the disclosure will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Illustrative embodiments of the disclosure will hereafter be described with reference to the accompanying drawings. FIG. 1 shows a schematic representation of two-step process combining SSSP and compression molding to prepare well-mixed crosslinked LDPE/graphite nanocomposites. FIG. 2 shows X-ray diffraction data of as-received graphite, (top) neat LDPE, (middle) LDPE/3G hybrid prepared by melt mixing in a cup-and-rotor mixer and (bottom) LDPE/3G nanocomposites prepared SSSP. The intensities of neat LDPE and hybrids were normalized by making the area associated with PE crystal peaks equal. The highlighted area and inset are the area containing the characteristic diffraction peaks corresponding to unexfoliated graphite. FIG. 3 shows non-isothermal crystallization curves (obtained upon cooling) for neat LDPE and 1DCP-LDPE and 3DCP-LDPE prepared by SSSP. (Cooling ramp is 10° C./min.) FIGS. 4A-4C show wear track profiles of LDPE samples. FIG. 4A is a 2D top view of wear track on neat LDPE. FIG. 4B is a 3D view of wear track on neat LDPE obtained via a 3D laser confocal microscope. FIG. 4C is the depth profile of the wear tracks cross section (averaged over 300 μm in length along the wear direction). The curves have been arbitrarily shifted vertically for clarity. The hatched region in the profile is taken as cross section area for wear volume calculation. FIG. 5 shows wear volume of the wear tracks in LDPE, 1DCP-LDPE (lightly crosslinked) and 3DCP-LDPE (highly crosslinked) as a function of filler loading. The error bar is associated with the standard deviation of three measurements. FIGS. 6A-6B show the coefficient of friction in samples with various crosslinking density and filler loading. FIG. 6A shows the variation of coefficient of friction as a function of