I. Introduction
Polyethylene oxide resin is a high molecular weight homopolymer synthesized by multiphase catalytic ring-opening polymerization of ethylene oxide, abbreviated as PEO. Its molecular formula is HOCH2CH2O[CH2CH2O]nH.
Due to its water solubility, low toxicity, and ease of processing, polyethylene oxide resin is widely used as a drag reducer, dispersant, flocculant, thickener, temporary adhesive, fabric sizing agent, and water-soluble packaging material. In the paper industry, its dispersing and filtering properties can improve paper uniformity and strength. In the oil extraction industry, its drag-reducing and thickening effects can improve oil recovery rates.
II. Main Properties of Polyethylene Oxide
1. Physical Properties
Polyethylene oxide resin is a white granular and powdered polymer with high crystallinity; high molecular weight polymers exhibit a spherulitic structure. The softening point is 65–67℃, the embrittlement point is -50℃, the density is 1.2 g/cm³, the apparent density is 0.2–0.3 g/cm³, and the residue on ignition is less than 2%.
Polyethylene oxide resin is completely soluble in water, and its aqueous solution is neutral or weakly alkaline. High molecular weight polyethylene oxide resin can be miscible with water in any proportion at room temperature. At low PEO concentrations, it is a viscous solution; as the PEO concentration increases, the solution gradually changes from a gel-like state to a rubbery elastomer.
Polyethylene oxide is soluble in acetonitrile, chloroform, dichloromethane, dichloroethane, trichloroethane, trichloroethylene, benzene, etc., at room temperature. When heated to 30–60℃, it is soluble in toluene, xylene, acetone, 1,4-dioxane, etc. The viscosity of polyethylene oxide aqueous solutions is related not only to the molecular weight of the polymer and the solution concentration, but also to the solution temperature, shear rate, and the concentration of added inorganic salts. The viscosity of the solution decreases with increasing temperature. For polymers with a molecular weight of (1–50) × 10⁵, the viscosity of the solution can decrease by an order of magnitude when the solution temperature increases from 10°C to 90°C. Due to the non-Newtonian nature of aqueous solutions, the viscosity decreases with increasing shear rate. Adding inorganic salts will decrease the dissolution temperature of polyethylene oxide and the viscosity of the solution. The magnitude of the decrease depends on the type and concentration of the salt.
High molecular weight polyethylene oxide exhibits significant fiber-dragging properties even in aqueous solutions with concentrations below 0.1%, and it also has agglomerative effect on suspensions containing various fine particles. The higher the molecular weight of the polymer, the greater this agglomerative effect. In papermaking, polyethylene oxide is used as a dispersant for pulp, exhibiting agglomerative effect even in small amounts.
Polyethylene oxide aqueous solutions are relatively stable in neutral or alkaline conditions, but less stable in acidic conditions, especially at pH values of 3–5. The presence of metal ions and oxidants in the aqueous solution will promote the degradation of polyethylene oxide, leading to a decrease in the viscosity of the aqueous solution. Although there are many reasons for the decrease in viscosity of polyethylene oxide (PE) aqueous solutions, as long as there is no oxidant present and it is used in neutral or weakly alkaline conditions, its aqueous solution is relatively stable, and the viscosity of the solution remains essentially unchanged.
2. Chemical Properties
Although PE has good chemical stability, due to the unshared electron pairs on the ether oxygen atoms in the long polymer chain, it has a strong hydrogen bond affinity and can form complexes with some electron acceptor monomers or polymers. Compounds that form associations with PE include maleic acid, acrylic acid, tannic acid, polyacrylic acid, polymethacrylic acid, and copolymers of urea and thiourea.
High molecular weight PE, whether stored in solid form, processed in thermoplastics, or in aqueous solution, is sensitive to oxidative degradation. In thermoplastic processing, the melt viscosity decreases rapidly with increasing temperature and time; the viscosity of aqueous solutions at room temperature decreases with increasing storage time; these are all due to oxidative degradation. The presence of trace amounts of chloride peroxides, permanganates, persulfates, and certain transition metal ions (such as Cu+, Cu2+, Fe3+, and Ni2+) accelerates oxidative degradation. To mitigate oxidative degradation, stabilizers are typically added during thermoplastic processing or in aqueous solutions. For example, adding 0.01–0.5% (by weight) of phenothiazine, butylated hydroxytoluene, or butylated anisole; or adding 5–10% (by weight) of anhydrous isopropanol, ethanol, ethylene glycol, or propylene glycol to aqueous solutions can effectively reduce the rate of oxidative degradation.
III. Synthesis of High Molecular Weight Polyethylene Oxide
Ethylene oxide undergoes ring-opening polymerization to form high molecular weight polyethylene oxide under the action of a heterogeneous catalyst. The polymerization mechanism belongs to the coordination anionic polymerization mechanism. Effective catalysts often contain a "metal-oxygen-metal" structure, indicating that two metal atoms are involved in chain growth. Catalysts for coordination polymerization include hydroxyl groups and amines of alkaline earth metals such as calcium and barium, and hydroxyl groups of aluminum, magnesium, and zinc. A series of white granular polyethylene oxide resin products with molecular weights ranging from 5 x 10⁵ to 4 x 10⁸ were prepared using an organometallic compound-based catalyst. The experimental conditions and results are briefly described below.
Experimental Section
(1) Main Raw Materials and Specifications
Catalyst (Cat), self-made; Ethylene oxide (EO), aldehyde content <30 ppm, water content <100 ppm; 120# gasoline (Solv), distillation range 80–120℃, iodine value 0.1–0.3, water content <30 ppm.
(2) Determination of Polymer Molecular Weight
A 0.05% (by weight) aqueous solution was prepared from a sample. The intrinsic viscosity [η] of the aqueous solution was measured, and the average molecular weight of polyethylene oxide was calculated using the Mark-Houwink formula.
(3) Experimental Method
The catalyst was prepared in a four-necked glass flask equipped with a stirrer, dropping funnel, reflux condenser, and thermometer. 2. Experimental Results and Discussion
(1) Effect of Catalyst Concentration
The catalyst concentration was expressed as the molar ratio of catalyst to ethylene oxide (Cat/EO). The results of changing the catalyst concentration (expressed as polymer molecular weight and polymerization yield, the same below) are shown in Figure 1.
Figure 1 Effect of Catalyst Concentration
As can be seen from Figure 1, the polymerization yield increases with increasing Cat/EO. The polymer molecular weight initially increases with increasing Cat/EO, but decreases after reaching a certain level. Therefore, a Cat/EO ratio of 1.1–1.3% (molar) is more suitable.
(2) Effect of Polymerization Solvent Amount
The heterogeneous catalytic ring-opening polymerization of ethylene oxide is a slurry solution polymerization, that is, ethylene oxide is dissolved in the polymerization solvent, and the resulting polymer precipitates out as a precipitate. The results of changing the amount of polymerization solvent are shown in Figure 2. Figure 2. Effect of Polymerization Solvent Amount
As shown in Figure 2, the polymerization yield decreases with increasing solvent amount. The polymer molecular weight increases with increasing solvent amount, but excessive solvent leads to a decrease in catalyst concentration, resulting in a slight decrease in polymer molecular weight. Therefore, a weight ratio of approximately 3.0/1.0 between the polymerization solvent and ethylene oxide is suitable.
(3) Effect of Polymerization Temperature
Polymerization temperature is a crucial factor affecting polymer molecular weight and polymerization yield. The effects of different temperatures in the 20-liter reactor polymerization experiment are shown in Figure 3.
Figure 3. Effect of Polymerization Temperature
As shown in Figure 3, the polymer molecular weight decreases with increasing polymerization temperature, while the polymerization yield increases. To obtain high molecular weight polyethylene oxide and improve the polymerization yield, we used a lower temperature (10–20℃) in the early stage of polymerization and a higher temperature (35–40℃) in the later stage, with good results.
(4) Polymerization Experiment in a 20-liter Reactor
The experimental results under favorable process conditions are listed in Table 1. Table 1. Polymerization Test Results in a 20L Reactor
As can be seen from Table 1, the polymerization yield of most experiments was higher than 90%, and the molecular weight of the polymer was higher than 3.70x106. The molecular weight of the polymer in some experiments was higher than 4X106.
(5) Degradation of Polymer
Polyethylene oxide resin undergoes oxidative degradation under the action of oxidants, ultraviolet light and heat, resulting in chain scission and a decrease in molecular weight. To understand the degradation of the polymer, we measured the molecular weight of a portion of the polyethylene oxide resin produced in a 20-liter reactor every month. The results are listed in Table 2.
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Due to sampling and measurement errors, the data in Table 2 fluctuate slightly. However, it can still be seen that after six months, the molecular weight of polyethylene oxide resin is higher than 3 x 10⁶, with most samples higher than 3.5 x 10⁸, and some samples around 4 x 10⁶. The monthly degradation rate within six months is less than 5%.
IV. Application
1. In the paper industry, it is used as a long fiber separator. High molecular weight polyethylene oxide resin has been tested at Shanghai Limin Paper Mill, Shanghai Songjiang Pulp Mill, and Beijing No. 11 Paper Mill. All agree that the dispersion effect is very good, approaching the level of Japanese PEO-PF products.
(1) Experiment at Shanghai Limin Paper Mill: The polyethylene oxide concentration was 0.05%. Products numbered C-tw-4, 5, 6, and C-tw-10 dissolved completely within 24 hours. Intermittent stirring was used for the first few hours, after which stirring ceased.
1. C-tw-4, 5, and 6 were used on a short-wire paper machine. The raw material was 100% cotton pulp, with a taper of 36·SR, a thermogravimetric index of 10 g/m², and a paper machine speed of 110 m/min. Originally, it produced 18±1 g/m² crepe toilet paper. After using C-tw-4, 5, and 6, paper uniformity significantly improved, and the basis weight decreased to 16 g/m² (compared to 19 g/m² without PEO). The paper had a soft feel, with a softness of 85 mm/150 g (compared to approximately 78 mm/150 g without PEO). Paper with a basis weight of 16 g/m³ exhibited excellent uniformity, with a PEO dosage of 0.44 kg/ton of paper. 2. C-tw-10 was used on a cylinder wire machine with 100% paper scraps as raw material. The beating conditions were based on a small sample with no pulp spots. After using PEO, paper uniformity significantly improved, and the basis weight decreased from 22 g/m² to 19 g/m³. If the felt and copper wire are in good condition, the basis weight can be further reduced. The PEO dosage was approximately 0.4 kg/ton of paper.
Shanghai Limin Paper Mill believes that the PEO dosage and its quality effect on paper achieved by our institute are close to those of Japanese PEO-PF. (2) Shanghai Songjiang Pulp Mill
This mill conducted a large-scale trial production of PEO produced by our institute and compared it with Japanese PEO-PF products. Under the same conditions of dissolution, filtration, dilution, and addition, each test lasted 24 hours, producing crepe-textured toilet paper with basically the same appearance and physical properties.
2. As a coagulant
High molecular weight polyethylene oxide resin was used as a coagulant. It was found that PEO is very effective in coagulating semi-soluble and suspended solids in solutions, especially soluble and colloidal silica. Adding 0.2 mg of PEO to 100 ml of solution can immediately coagulate and precipitate almost all the silica, and the process is fast, generally taking only 5-10 minutes. It can be carried out at room temperature, making it very convenient to use.
3. As a binder
Polyethylene oxide resin with a molecular weight of 3-5 x 10⁵ was used as a binder. It was found that PEO has low ash content, low decomposition temperature, low content of alkali metal impurities that significantly affect glass properties, and good adhesion when used in combination with other binders. In addition, polyethylene oxide resin can also be used as a liquid drag reducer, thickener, water-soluble packaging material, etc., and its application fields are very wide.
