Water-soluble polymers, methods of making the same, and melt-processable water-soluble polymer compositions

Abstract

A method of producing processable water-soluble polymers, the method comprising the steps of: extruding a water-soluble polymer composition from an extruder barrel without the use of a die, wherein the extruder barrel is not vented except through an extruder outlet of the extruder barrel, to produce irregularly shaped polymer extrudates; directing the irregularly shaped polymer extrudates onto a cold conveyor belt to cool the irregularly shaped polymer extrudates to below 60°C; and pelletizing the irregularly shaped polymer extrudates to form pellets.

Description

Technical Field

[0001] This invention relates to a polymer composition, particularly but not limited to polyvinyl alcohol polymers, and a method for manufacturing the same. Background Technology

[0002] Currently, there is a growing demand for water-soluble, biodegradable polymers to replace a large amount of non-biodegradable polymers on the market. Non-biodegradable polymers, due to their need for landfill or incineration, place significant demands on resources.

[0003] Polyvinyl alcohol (PVA) is considered one of the very few water-soluble vinyl polymers and is also readily biodegradable in the presence of properly acclimated microorganisms. Therefore, increasing attention is being paid to the development of widely applicable, environmentally compatible PVA-based materials. PVA exhibits excellent film and thin-wall container molding properties and high impermeability to a variety of gases, making it ideal for packaging products for release in aqueous environments. PVA also possesses high adhesive strength and is non-toxic. However, these properties are humidity-dependent, as water absorption by the polymer reduces its tensile strength but increases its elongation and tear strength. Successfully extruding PVA or PVA-containing compositions is also challenging, further limiting its potential applications. In particular, no formulation currently allows for easy molding to the required minimum thickness (less than 200 micrometers) to ensure product release at low water temperatures, typically within 2 minutes at or below 5°C. Such films are ideal for applications such as packaging laundry products requiring the release of detergents at low temperatures and short wash cycles.

[0004] One of the properties that polymers need to improve is the melt flow index (MFI). This refers to the ease with which a polymer melts and is defined as the mass (in grams) of polymer that flows through a capillary of a specific diameter and length in ten minutes under a specified pressure applied by alternative gravity and a specified alternative temperature. One such method is described in standard ISO 1133. High MFI is required to provide thin-walled molding.

[0005] It is known in the art to use internal lubricants within PVA to increase its melt flow index. For example, EP1112316B1 (PVAXX Technologies Ltd.) includes up to 5% by weight of fatty acid amides. Fatty acids provide lubrication between polymer chains, thereby increasing the polymer's melt flow. However, fatty acids are insoluble and melt and coat the polymer during processing, which can hinder polymer dissolution. Furthermore, the amount of lubricant that can be used is limited due to the excess lubricant separated from the mixture (typically more than 5% by weight of PVA), thus limiting their ability to improve the polymer's MFI.

[0006] The polymer is made from polyvinyl acetate (PVAc), formed by the polymerization and subsequent hydrolysis of vinyl acetate. PVA and PVA-derived polymers are soluble in water, and their solubility depends on the polymer's molecular weight and degree of hydrolysis, i.e., the percentage of acetic acid groups replaced by OH groups in the starting polymer (PVAc). A higher degree of hydrolysis results in lower solubility and a lower dissolution rate. Due to the formation of crystalline regions within the polymer, the difference is more pronounced at lower dissolution temperatures than at higher dissolution temperatures.

[0007] It is also known in the art that PVA requires the removal of volatiles from the composition prior to melt processing, as this removal leads to processing difficulties due to vapor formation and subsequent polymer foaming. Depending on the brand, model, and formulation, drying is typically performed using standard drying equipment at 90°C for 4–9 hours.

[0008] Despite attempts to mold articles with PVA or PVA-derived compositions, the desired solubility characteristics have not yet been achieved due to the lack of melt flow required to mold thin-walled articles that will dissolve in aqueous solutions within the desired timeframe.

[0009] The object of the present invention is to provide a water-soluble polymer composition, particularly but not limited to, a polyvinyl alcohol composition, that solves or at least mitigates the aforementioned problems encountered by prior art polymer compositions.

[0010] Another object of the present invention is to provide a method for producing and extruding and / or molding water-soluble polymer compositions. Summary of the Invention

[0011] According to the present invention, a method for generating a water-soluble polymer is provided, the method comprising the steps of: extruding a water-soluble polymer composition from an extruder barrel without using a die, wherein the extruder barrel does not discharge except through its extruder outlet, to produce an irregularly shaped polymer extrudate; guiding the irregularly shaped polymer extrudate onto a cold conveyor belt to rapidly cool the irregularly shaped polymer extrudate to below 60°C; and granulating the irregularly shaped polymer extrudate to form granules.

[0012] Alternatively, the granulation of irregularly shaped polymer extrusions can occur immediately after the cold conveyor belt without the need for an additional drying step.

[0013] Preferably, the method may further include the step of forming the particles into a water-soluble polymer product.

[0014] Preferably, the water-soluble polymer may comprise a water-soluble polymer composition having: at least 15% by weight of a hygroscopic salt for use as a lubricant to make the polymer extrudable and / or moldable; and a solvent polymer plasticizer, wherein the solvent polymer plasticizer is monopropylene glycol or dipropylene glycol, and the water content of the composition is less than 10% by weight of the total components.

[0015] Preferably, the hygroscopic salt can be provided in a higher amount by weight than the solvent polymer plasticizer.

[0016] Preferably, the hygroscopic salt can be anhydrous or hydrated salt selected from the group consisting of sodium chloride, sodium citrate, magnesium chloride, calcium chloride, potassium chloride, sodium sulfate, sodium carbonate, potassium carbonate, and ammonium carbonate.

[0017] Preferably, the hygroscopic salt is a water-soluble salt that dissolves in water at 90°C at a rate of at least 10% by weight within 10 minutes.

[0018] Preferably, the moisture content of the hygroscopic salt can be less than 10% of the total weight of the composition.

[0019] Preferably, the content of the hygroscopic salt can be at least 25% of the total weight of the composition.

[0020] Preferably, the content of the hygroscopic salt can be at least 30% of the total weight of the composition.

[0021] Preferably, the hygroscopic salt can be in anhydrous form.

[0022] Preferably, the water-soluble polymer and the hygroscopic salt can be provided in solid form.

[0023] Preferably, the polymer may include a polyvinyl alcohol polymer.

[0024] Preferably, the method may further include the step of extruding the water-soluble polymer composition without using a mold.

[0025] Preferably, the method may further include the step of extruding the water-soluble polymer composition onto a cold conveyor belt.

[0026] Preferably, the temperature of the cold conveyor belt may be between 10°C and 30°C.

[0027] Preferably, the method may further include a step of granulating the water-soluble polymer composition immediately after the cold conveyor belt, without the need for an additional drying step.

[0028] Preferably, the water-soluble polymer may contain wax to improve moisture resistance.

[0029] Therefore, the present invention provides a melt-processable water-soluble polymer composition comprising a mixture of water-soluble polymers having a hygroscopic salt comprising at least 15% by weight of the total weight of the composition, the hygroscopic salt acting as a lubricant to enable the polymer to be extruded and / or molded, wherein the composition has a water content of less than 10% by weight.

[0030] Preferably, a hygroscopic salt is provided, comprising at least 20% by weight of the total weight of the composition. Furthermore, the water-soluble polymer may be solid at ambient temperature. More preferably, the polymer comprises a polyvinyl alcohol polymer. The PVA used in this invention is not limited to any particular degree of hydrolysis. Partially or fully hydrolyzed PVA may be used in this invention. Similarly, PVA is not limited to a specific molecular weight. PVA can have a relatively low molecular weight of about 20,000 up to and exceeding 150,000.

[0031] PVA preferably has a maximum moisture content of 5% by weight. Surprisingly, it has been found that hygroscopic salts not only absorb water from PVA, but also act as an internal lubricant for PVA, increasing its melt flow index.

[0032] Preferably, the hygroscopic salt is an anhydrous or hydrated salt selected from alkali metal salts or alkaline earth metal salts. Depending on the end use of the composition, it may be advantageous to use a salt approved for food and / or pharmaceutical use and / or with other properties (e.g., water softeners) that can impart additional benefits to the final product. More preferably, the salt is selected from the group consisting of sodium chloride, sodium citrate, magnesium chloride, calcium chloride, potassium chloride, sodium sulfate, sodium carbonate, potassium carbonate, and ammonium carbonate, especially sodium chloride or citrate.

[0033] A preferred embodiment of the first aspect of the present invention comprises a mixture of a water-soluble polymer and a certain amount of sodium chloride, wherein the sodium chloride is used to make the mixture extrudable.

[0034] The salt has a water content of less than 10% by weight, preferably less than 1% by weight, more preferably less than 0.5% by weight, and especially less than 0.2% by weight. Ideally, the salt is in anhydrous form. The salt can be micronized, for example, with an average particle size of less than 100 micrometers, preferably in the range of 0.03 to 75 μm, especially 60 to 70 μm. The salt can also be coated with, for example, sodium aluminosilicate, silica, and / or sodium ferrocyanide to improve its properties. One such example is a sodium chloride salt coated with 0.5% sodium aluminosilicate (E554), 0.75% silica (E551), and sodium ferrocyanide (E535) as an anti-caking agent, which is available from CustomPowders (www.custompowders.co.uk).

[0035] The composition may contain salts comprising up to 75% of the total weight of the formulation. The salt content shall comprise at least 15% of the total weight of the formulation, preferably at least 20%, more preferably at least 25%, more preferably at least 30%, more preferably at least 35%, more preferably at least 40%, more preferably 45%, and especially at least 50%.

[0036] The composition can be blended with optional additives to improve its processability, such as with plasticizers to enhance flexibility and / or lower the melt temperature of the polymer during extrusion or molding, with stabilizers to increase heat resistance, and / or with pigments to add color. Preferably, a heat stabilizer (e.g., metal stearate) is included in an amount of up to 0.5%, preferably up to 0.3% by weight. However, the composition preferably does not contain any fatty acid amides or esters.

[0037] In one aspect of the invention, a water-treated salt from other components (e.g., the polymer itself) within the surface of the composition is used to enable it to function as an internal lubricant. The role of this "internal" lubricant is to improve lubrication between polymer chains. Using salt as a lubricant eliminates the need for other types of prior art internal lubricants, such as fatty acid amides or esters.

[0038] Example compositions that do not contain solvent plasticizers may be as follows:

[0039]

[0040] The melt flow index, measured over 10 minutes using a 2.16 kg weight at 190°C, was 21.5.

[0041] Sodium chloride is more soluble in water than the polymer, but it is not a thermoplastic resin. Therefore, one would expect the angular crystalline structure of the salt to not mix with the polymer and, in fact, to impede melt flow. Surprisingly, this is not the case.

[0042] Preferably, the salt is provided in a higher weight percentage than the solvent polymer plasticizer. More preferably, the ratio of salt to solvent polymer plasticizer is 1.25 to 12:1, especially 1.25 to 7:1, and ideally 4 to 5:1.

[0043] Alternatively, the composition may include a solvent polymer plasticizer, preferably a hygroscopic organic solvent, more preferably selected from glycerol (also known as glycerol) and propylene glycol. Preferably, the solvent polymer plasticizer is monopropylene glycol, and in a more preferred embodiment, the solvent polymer plasticizer is dipropylene glycol.

[0044] According to the present invention, a melt-processable water-soluble polymer composition is provided, comprising a mixture of water-soluble polymers having at least 15% by weight of a hygroscopic salt, the hygroscopic salt acting as a lubricant to enable the polymer to be extruded and / or molded.

[0045] Another aspect of the invention provides a soluble polymeric internal lubricant comprising a mixture of a hygroscopic salt and a solvent polymeric plasticizer thereof.

[0046] The lubricant according to the invention can be mixed with a water-soluble polymer for its processing. In this respect, another aspect of the invention provides a melt-processable water-soluble polymer composition comprising a mixture of a water-soluble polymer and an internal lubricant to make the polymer extrudable and / or moldable, wherein the lubricant comprises a hygroscopic salt, preferably an anhydrous or hydrated metal salt, mixed with a solvent polymer plasticizer. Preferably, the lubricant conforms to the foregoing aspects of the invention.

[0047] Preferably, the salt has a minimum water content, more preferably a water content of less than 10% by weight, and the weight percentage of the salt to the total weight of the preparation is at least 15%, more preferably at least 40%, and particularly at least 50%.

[0048] When mixed with water-soluble polymers, the lubricant according to the invention preferably has a melt flow index of at least 20 g (10 minutes / 190°C / 2.16 kg, according to ISO 1133), more preferably at least 40 g, and particularly 60 g.

[0049] The compositions of the present invention can be used in food and / or pharmaceuticals. Therefore, it should be understood that, where possible, the lubricants and other components of the compositions are permitted for food and / or pharmaceutical use.

[0050] The compositions according to the invention can be provided in any suitable form for further processing, but are preferably provided in the form of powder, tablets, or pellets for extrusion and / or molding of extruded and / or molded products, such as extruded filaments containing a soluble polymer. The compositions can be manufactured by any conventional method, such as by melt mixing or cold working, in the latter case including calendering, suitable calendering, and / or compaction. Cold pressing, more preferably suitable calendering, may be an alternative technique.

[0051] The compositions of the present invention may further comprise a plasticizer to reduce the melt temperature of the polymer during extrusion and / or molding. The plasticizer may be selected from glycerol, ethylene glycol, triethylene glycol, low molecular weight polyethylene glycol, and low molecular weight amides. A preferred plasticizer is glycerol, more preferably monopropylene glycol, or most preferably dipropylene glycol. The plasticizer may also be used as a solvent polymer plasticizer for internal salt lubricants.

[0052] Another aspect of the invention provides a method for preparing a water-soluble polymer composition, the method comprising mixing a water-soluble polymer with a hygroscopic salt comprising at least 15% by weight of the total weight of the composition, the hygroscopic salt acting as a lubricant to thereby make the polymer extrudable and / or moldable, wherein the composition contains less than 10% by weight of water, the method optionally comprising adding a solvent polymer plasticizer.

[0053] Another aspect of the invention provides a method for extruding and / or molding a water-soluble polymer composition, comprising softening the composition according to the foregoing aspects of the invention to form a melt flow. Preferably, the composition is softened by heating and / or pressurizing to produce the melt flow.

[0054] The melt flow preferably has a melt flow index of at least 20 g (10 min / 190 °C / 2.16 kg, according to ISO 1133), more preferably at least 40 g, and especially 60 g. Preferably, the polymer composition is molded into a form with a thickness of less than 200 micrometers, preferably less than 100 micrometers, so that this form can dissolve in an aqueous solution within 80 seconds at 5 °C. The molded form can be any thin-walled molded article, such as a container or film. Extrusion is also possible, although the compositions of the present invention are particularly advantageous for molded articles.

[0055] Another aspect of the present invention provides a water-soluble polymer product formed according to the foregoing aspects of the present invention.

[0056] Another aspect of the invention provides a melt-processable water-soluble polymer composition comprising: a mixture of a water-soluble polymer and a hygroscopic salt, the hygroscopic salt acting as a lubricant to enable the polymer to be extruded and / or molded; a solvent polymer plasticizer; and a wax, wherein the wax content of the composition is at least 0.3% by weight of the total weight of the composition; wherein the water content of the composition is less than 10% by weight of the total weight of the composition; and the composition contains at least 15% by weight of the hygroscopic salt.

[0057] Preferably, the melt-processable water-soluble polymer composition may have a wax content of at least 1.0% by weight of the total weight of the composition.

[0058] Preferably, the wax can be glycerol monostearate. Detailed Implementation

[0059] The melt-processable composition of the present invention can be processed by any known thermal processing method, including but not limited to injection molding, compression molding, rotational molding, and film extrusion. This composition is particularly suitable for thin-walled molded articles.

[0060] The melt-processable compositions of this invention are suitable for manufacturing any articles currently made from extrudable and / or moldable polymers, including films, containers, and bottles. The compositions are suitable for manufacturing filaments and fibers for spunbond, nonwoven, and meltblown fabric applications. The compositions are also suitable for manufacturing articles such as: bags and containers for detergents and agricultural chemicals, mulch films, flower pots, household bags, diapers, straws, feminine hygiene products, hangers, incontinence pads, pouches, six-link pouches, disposable clothing, foam plastics, gloves, film canisters, golf tees, shotgun cartridges, bedpans, bottles, bowls, cotton swabs, hospital curtains, "disposable" sterile products, and packaging materials, etc.

[0061] PVA typically has a moisture content of up to 5% by weight. This must be reduced to below 1% to avoid processing problems on standard thermoplastic equipment, such as the generation of volatiles that cause foaming. Typically, the polymer is dried in a standard polymer dryer at 90°C for 4–8 hours. It has been found that the addition of a specific amount of hygroscopic salt (preferably with low or no moisture content) removes water from the surrounding PVA. More surprisingly, the absorption of water by the salt provides a self-lubricating coating on the salt, which can then be used as an internal lubricant for the PVA. The desiccant effect of the salt reduces drying time to 2–4 hours, resulting in significant energy savings in polymer production. Furthermore, the lubricating effect of the salt greatly improves the melt flow index of the composition, allowing the PVA to be easily extruded and / or molded into products (especially thin profiles less than 200 micrometers), making it suitable for applications requiring the dissolution of films and / or molding at temperatures as low as 5°C within a short time (less than 2 minutes). The self-lubricating effect remains effective even at low moisture contents below 1% (i.e., even during the drying process). It is feasible to further reduce the temperature and thus approach 0°C, and the time can also be reduced.

[0062] Hygroscopic salts (e.g., sodium chloride) have been found to have enhanced lubrication effects when compositions contain polymer plasticizers (e.g., hygroscopic salt solvents, including glycerol or propylene glycol, and particularly monopropylene glycol or most preferably dipropylene glycol). The absorption of water by the salt appears to act as a surface treatment, allowing non-solvent plasticizers to be used in the formulation when necessary. Anhydrous salts are generally considered unsuitable for internal lubrication of water-soluble polymers. In this regard, small amounts (2–3%) of precipitated calcium carbonate (PCC) have been used. Alcohol plasticizers are not solvents for PCC, therefore high loading must be used to produce high melt flowability, resulting in very good product ductility because PVA is encapsulated around PCC particles. Melt flow index is also disappointing. In contrast, the present invention uses glycerol, monopropylene glycol, or dipropylene glycol as solvents for anhydrous sodium chloride to partially dissolve the outer surface of the sodium chloride to provide lubrication within the polymer chain. This produces a relatively high MFI and increases the solubility of the polymer while reducing its ductility (an ideal property).

[0063] Sodium chloride is one possible hygroscopic salt that can be used here. Sodium citrate and magnesium chloride are other possible alternatives, as are calcium chloride, potassium chloride, sodium sulfate, sodium carbonate, potassium carbonate, and ammonium carbonate. An essential characteristic of the hygroscopic salt used in this invention is that the hygroscopic salt is water-soluble, has the ability to absorb atmospheric moisture, and / or has a solubility of at least 10% by weight in water at 90°C within 10 minutes.

[0064] It should be understood that even if the composition contains a high level of water (as a plasticizer or binder) along with the hygroscopic salt, the benefits of the present invention will not be achieved. Water will cause reversible dissolution of some or all of the salt present. Furthermore, reducing the water content for successful processing will result in the removal of the self-lubricating layer and the reformation of salt crystals of unpredictable size and shape. This will impede melt flow and increase drying time. Therefore, it is preferred in the present invention that no water is added to the formulation except for the low water content contained in the various components constituting the composition (e.g., PVA).

[0065] The invention will now be described with reference to the following non-limiting examples. These examples illustrate the high melt flow values ​​and reduced drying times achieved using compositions according to the invention, and compare these properties with compositions that fall outside the scope of protection.

[0066] Manufacturing method

[0067] PVA (polymer), sodium chloride (lubricant), glycerol (plasticizer), and thermoplastic plasticizer are mixed for 3 minutes in a bell-shaped drum, low-shear mixer. The mixture is then fed through a screw to a mixer and granulated using a suitable calendering process. As the PVA passes between the rollers and the die, the suitable calendering process causes the PVA to partially or completely melt as a result of frictional shearing, leading to agglomeration before extrusion through the die. The temperature of the pre-extruded material is varied between 110°C and 140°C, and the resulting granules are then placed in a tray polymer dryer at 90°C for 3 hours.

[0068] Melt Flow Analysis

[0069] The MFI of a 5-gram sample prepared according to the formulation of the present invention was tested at 190°C using a 2.16 kg weight. The MFI of each sample was tested and compared according to ISO 1133. The test was repeated 10 times, and the average results were recorded.

[0070] The samples were formed using a 50-ton molding press in automatic mode with a mold equipped with a hot runner system at 180°C to 200°C, with a cycle time of 7–10 seconds. The screw temperature profile (in °C) from hopper to tip was 160, 170, 180, and 180–190. The part wall cross-section was measured between 600 and 350 micrometers.

[0071] Example 1

[0072] Formulations were prepared according to the method described above, having the components shown in Table 1 below, mixed in a given weight percentage. Each formulation used 88% hydrolyzed PVA, and the heat stabilizer was calcium stearate. The melt flow index (MFI) was determined based on the analysis given above.

[0073] Table 1

[0074]

[0075] Formulation 2 shown in Table 1 above was found to be white / cream-colored and possessed the following characteristics:

[0076] Density 1.68 g / cm³

[0077] The melt density at 200℃ is 1.52 g / cm³ (according to ISO 1183).

[0078] These results illustrate the importance of having a high salt to plasticizer ratio in the formulation to achieve the desired high MFI, but peak MFI values ​​were obtained in the above formulations with a salt to plasticizer ratio of 3.5 to 5:1 (more preferably 4 to 4.4:1).

[0079] Example 2

[0080] Formulations were prepared according to the method described above, having the components shown in Table 2 below, mixed in a given weight percentage. 88% hydrolyzed PVA was used in each formulation. The melt flow index (MFI) was determined based on the analysis given above. The part wall cross-section measurements of the molded formulations ranged from 600 micrometers to 100 micrometers.

[0081] Table 2

[0082]

[0083] Table 2 demonstrates that the type of plasticizer has no significant impact on the achieved MFI.

[0084] Example 3

[0085] Formulations were prepared according to the above method, having the components mixed in the weight percentages shown in Table 3 below. Formulations 10–13 used 98% hydrolyzed PVA, and formulations 14–16 used 80% hydrolyzed PVA. These formulations were molded using a 50-ton molding process in automatic mode with a 20-second cycle using a mold equipped with a cold runner system. The screw temperature profiles (in °C) from hopper to tip were 160, 170, 180, 180, and 220. The measurement range of the part wall cross-section was 600 to 2000 micrometers.

[0086] Table 3

[0087]

[0088]

[0089] Example 4

[0090] Formulation 17 is prepared as a mixture in a manner similar to Formulation 2 in Example 1, but with sodium citrate instead of sodium chloride:

[0091]

[0092] The formulation was found to have the following characteristics:

[0093] Density 1.67 g / cm³

[0094] The melt density is 1.40–1.42 g / cm³ at 190°C (according to ISO 1183).

[0095] MFI 38g.

[0096] The processing temperature is 190℃~200℃, and the residence time is up to 30 minutes. Drying time at 90℃ is 4 hours. The MFI is significantly higher when the composition contains salt.

[0097] The extrudability of formulation 1 and formulation 2 was examined in injection molding machines manufactured by Boehringer Ingelheim, Demag, and Albemarle. The extrusion process was performed using a single fully threaded screw with a constant pitch. The barrel temperature profile ranged from 160°C to 200°C, and the screw speed typically varied between 20 and 150 rpm. The unit was shut down by maintaining the temperature at 100°C and stopping the screw rotation. A complete shutdown was then achieved by turning off the machine.

[0098] Formulations 2 and 17 can be molded into containers of various sizes and colors and are suitable for injection molding. Using sodium citrate as a polymer lubricant offers additional advantages when used in packaging laundry products, as sodium citrate can also be used as a water softener.

[0099] Example 5

[0100] An investigation was conducted to study the requirement for low water content in the formulation of the present invention.

[0101] Table 4 below lists the composition of the formulations, as well as their MFI and drying time.

[0102] Table 4

[0103]

[0104] Table 4 clearly illustrates the importance of salt and water content in the formulation for drying time and MFI. The formulation has a high percentage of salt (at least 15%, preferably at least 20%, more preferably at least 40%) and minimal or no water content. Formulations C and D, containing 13% and 17% water respectively, are very viscous and cannot flow freely, thus unsuitable for mixing. Furthermore, excessively long drying times result in undesirable loss of glycerol vapor.

[0105] Example 6

[0106] The ratio (usually expressed as a percentage) of the partial pressure of water vapor in a given environment to the equilibrium vapor pressure of pure water is called relative humidity (RH). The RH at which a given material begins to deliquesce (i.e., when water adsorbed onto the material begins to solvate the material's molecules) is called the material's deliquescence point (RHo; also known in the art as "critical RH"), and is an important parameter characterizing the structural stability of polymer materials. RHo is temperature-dependent and is usually expressed as the RHo value at a given temperature. For example, at 20 degrees Celsius (0°C), the RHo of crystalline NaCl is approximately 77% RH; that is, when the ambient temperature is 20 degrees Celsius, when the ambient RH is equal to or higher than 77%, atmospheric water vapor will spontaneously adsorb onto NaCl crystals and solvate them.

[0107] Materials with low RHo values ​​tend to absorb moisture more readily and can be used effectively as desiccants. In contrast, materials with high RHo values ​​tend to absorb moisture less readily. In the case of polymer compositions, a higher RHo value advantageously reduces the need for secondary packaging and provides a more structurally stable product when exposed to unprotected environments.

[0108] One problem with products containing water-soluble polymers is that they tend to absorb moisture and become structurally unstable in high-RH environments. For example, at an ambient temperature of 20 degrees Celsius, PVA begins to deliquesce when the RH reaches above 50%.

[0109] One problem with formulations containing PVA, sodium chloride, and glycerol as described above is that while the lubricating effect of the combination of glycerol and salts aids in melt processing, the resulting molded products are prone to moisture absorption, and secondary packaging requires high moisture and gas barrier properties.

[0110] Various water-soluble polymer compositions containing 88% hydrolyzed polyvinyl alcohol (PVA) and NaCl are produced using one of the following three different solvent polymer plasticizers: glycerol, monopropylene glycol, and dipropylene glycol. Table 5 provided below shows formulations for nine different formulations, with formulations 1–3 containing glycerol, formulations 4–6 containing monopropylene glycol, and formulations 7–9 containing dipropylene glycol.

[0111] Table 5

[0112]

[0113] As shown in Table 5, it was found that using propylene glycol as a solvent polymer plasticizer unexpectedly resulted in lower water absorption and therefore more stable PVA-based polymer products compared to comparable formulations using glycerol as a solvent polymer plasticizer. Formulations containing glycerol 7–9 showed RH at 20°C. O The RH content is 62% to 66%, while all formulations containing propylene glycol have a RH content of 62% to 66% at 20°C. O Significantly higher, between 76% and 83%.

[0114] Furthermore, formulations containing dipropylene glycol were found to be more stable than comparable formulations containing monopropylene glycol; formulations 1–3 containing dipropylene glycol showed better stability at 20°C with RH. O The RH content is 80% to 83%, while formulations containing monopropylene glycol 4–6 have lower RH at 20°C. O The percentage is 76% to 77%.

[0115] Sodium chloride has a deliquescence point of approximately 77% RH at 20°C. O However, molded articles made using formulations containing PVA, sodium chloride, and glycerol exhibit a RH exceeding 60% at 20°C. O The tendency to deliquinate.

[0116] The remarkable effect of replacing glycerol with dipropylene glycol is that the deliquescence point of sodium chloride increases by more than 82 RH at 20°C. O This reduces the specifications required for secondary packaging and produces products that are more dimensionally stable when opened in an unprotected environment.

[0117] Example 7

[0118] Formulations containing dipropylene glycol and sodium chloride were prepared to compare melt flow indices.

[0119] Table 6

[0120]

[0121]

[0122] As the concentration of hygroscopic salt increases, the melt flow index shows a significant improvement.

[0123] Other production methods

[0124] Traditionally, in this field, extrusion is achieved through either a channel die or a wire die to manufacture PVA resin. Both channel and wire dies provide sufficient back pressure to produce a uniform polymer melt flow from the extruder to the point where the extrudate needs to be cooled. This is typically performed by immersion in water or by cooling with ambient or cold air.

[0125] The problem with water immersion is that PVA is water-soluble, so it needs to be dried further after immersion.

[0126] When using ambient air for cooling, the problem is that the plasticizer is lost from the formulation before the extrudate cools to below 60°C.

[0127] The PVA formed according to this method, mixed with lubricating and hygroscopic salts, is more soluble and brittle, making it unsuitable for wire extrusion and water cooling. When cooled in water, the particles become viscous due to dissolution and clump together, requiring further drying. The wire also becomes brittle due to polymer reduction, high salt content, and plasticizer loss of up to 0.75% during air cooling. The process is unstable due to wire breakage and material loss, leading to process downtime. Volatile loss of plasticizer also results in inconsistent product outcomes, depending on environmental conditions and cooling time. The melt flow index can vary significantly.

[0128] Slit dies produce strips for extrusion, without forming granules for further processing.

[0129] Air cooling is preferred over the conventional water bath. However, auxiliary air cooling can be achieved using fans supported on a moving conveyor belt. The extruded material leaves the die at 165°C to 200°C and then reaches the vertical granulator at a maximum temperature of 60°C. The aforementioned plasticizer evaporation occurs on this moving conveyor belt. In addition to its adverse effect on the melt flow index, this reduction in quality also leads to economic losses.

[0130] Therefore, a new extrusion cooling method was developed to address the improved polymer compositions. For ease of description, the new method can be referred to as the cooling roller method.

[0131] In the cooling roller process, the polymer is extruded without additional vents in the barrel or without a channel or wire die. Without back pressure, the barrel releases steam and produces extrudates that are (1) irregular and inconsistent in size and shape; and (2) have a very low moisture content, preferably less than 1% (v / v). This is significantly different from conventional processes that extrude polymers into channel or wire dies. Channel or wire die methods require extrudates with regular and consistent size and shape to maintain operation, and due to the presence of steam in the barrel leading to irregularities, additional vents must be provided for the barrel in channel or wire die methods.

[0132] Irregular extrudates are allowed to fall onto cooling rollers or a conveyor belt, where the temperature rapidly cools the extrudate to below 100°C but above 10°C, preferably below 60°C. The cooling process is almost instantaneous, occurring within 30 seconds, more preferably less than 20 seconds, even more preferably less than 10 seconds, and typically less than 5 seconds. Cooling rollers are used to achieve instantaneous cooling, thereby preventing plasticizer loss. In fact, if the extrudate is slightly cooled before being frozen in an instantaneous manner, it will no longer have sufficient extensibility to pass through the cooling rollers. It must pass through immediately to maintain sufficient extensibility for rolling. Once cooled, the irregular extrudates are fed into a commercial polymer mill to be granulated. These granules can be stored for subsequent processing, typically by being sealed in vacuum bags.

[0133] The calendering process utilizes (preferably water-cooled) cooling rollers to adapt to inconsistent extrudates, enabling rapid cooling of the extrudates to 60°C and below. Furthermore, the high thermal transfer properties of PVA containing at least 15% by weight of hygroscopic salts further enhance cooling through this system.

[0134] If no salt is used in the formulation, a lower temperature roller (i.e., below 10°C) is required to achieve the same cooling rate and prevent plasticizer loss. This can lead to condensation on the cooling roller, causing process problems and extrudate sticking to the roller. This renders the process ineffective. Water temperatures between 10°C and 30°C have been found to be most effective for PVA and salt compositions, while salt-free compositions require temperatures below 10°C, although condensation has proven to be a problem here.

[0135] Below 60°C, the plasticizer becomes locked in the polymer composition, below the plasticizer evaporation temperature, preventing a total weight loss of 0.75% to 1.25% of the polymer composition due to evaporation. The resulting polymer also has a very low water content, as most of the water present is lost as vapor during extrusion. Thus, the water content can be maintained below 1% by weight, which is the maximum possible water content required for successful reprocessing of irregular extrusions. This also eliminates the need for further polymer drying steps.

[0136] The following Tables 7 and 8 summarize example products of embodiments of the cooling roller method described above and compare them with conventional cooling air rack production line processes (i.e., processes that extrude regular extrudate directly into strands for air cooling).

[0137] Table 7 – Cooling Roller Method

[0138]

[0139] Table 8 – Extruders and Frame Lines

[0140]

[0141] It can be seen that, under otherwise identical conditions, the extrudate produced by the cooling roller method has a higher melt flow index. Furthermore, in the embodiments of the cooling roller method, the extrudate temperature remains low when it reaches the granulator, thus reducing plasticizer evaporation.

[0142] Once cooled, the irregular extrudate can be fed into a commercial polymer mill or granulator, where the polymer is ground until the particulate material falls through a sieve to determine the maximum particle size. However, the irregularity of the extrudate results in variable polymer particle sizes, typically ranging from 2 mm to 4 mm.

[0143] Surprisingly, this specific dimensional inhomogeneity facilitates polymer melting during subsequent reprocessing. Therefore, the combination with cooling rollers is well-suited for use with pelletizers. Compared to wire-die pelleting, smaller pellets melt faster, require less energy, and exhibit a significant 10°C reduction in barrel temperature. The lower processing temperature also results in a lower plasticizer loss rate, which could otherwise be converted into gas and generated during reprocessing, typically through injection molding or extrusion. Outgassing can lead to burn marks during molding, part scrap, and mold contamination during processing; therefore, reducing outgassing is a significant improvement.

[0144] Plant waxes are commonly used to impart moisture resistance to polymers. Typically, 0.3% to 1.0% glyceryl monostearate or stearamide is used as the plant wax. However, the use of plant waxes during the melting process can also have the undesirable effect of causing "screw slippage." Screw slippage occurs when, during the melting process, the extruder or injection molding machine is unable to convey the molten material due to the smoothness of the polymer melt; the plant wax acts as a lubricant applied to the barrel.

[0145] For salt-free PVA, a wax content of 0.3% is typically the maximum permissible level due to screw slippage, but higher wax contents, up to 2% to 3%, can be provided with small amounts of salt. Adding these plant waxes can reduce water absorption in PVOH formulations containing monopropylene glycol and dipropylene glycol by up to 50% compared to salt-free formulations.

[0146] Surprisingly, when formulating polymer compositions containing amounts of plant waxes that are typically expected to cause screw slippage, the irregularity of the particle shape also advantageously resulted in a reduction in screw slippage.

[0147] In this invention, the irregular size and shape of the particles before melting more gradually form a mixture, allowing for better adhesion during polymer transfer via a screw. This allows for the addition of more wax components.

[0148] Therefore, the compositions of the present invention provide a melt-processable PVA-containing polymer that typically has a flexural modulus similar to other extrudable polymers. This allows the soluble and biodegradable polymer to be used to process a wide variety of articles without the processing problems encountered in the prior art, such as thermal degradation and high-temperature crosslinking. The known advantageous properties of PVA (e.g., its high tensile strength and good barrier properties) are retained in this melt-processable composition, and it can be extruded on current extrusion lines, blow molding machines, and injection molding machines without modification.

Claims

1. A method for generating a water-soluble polymer, the method comprising the following steps: A water-soluble polymer composition is extruded from an extruder barrel without the use of a die, wherein... Apart from the extruder outlet of the extruder barrel, the extruder barrel does not provide an exhaust port to discharge steam only from the extruder outlet, thereby producing polymer extrudates with irregular shapes; The irregularly shaped polymer extrusion is guided onto a cold conveyor belt to rapidly cool it to below 60°C; and The irregularly shaped polymer extrudate is granulated to form particles; The polymer includes a polyvinyl alcohol polymer, and after the irregularly shaped polymer extrusion is cooled to below 60°C using the cold conveyor belt, the irregularly shaped polymer extrusion on the conveyor belt is immediately granulated without further drying.

2. The method of claim 1, further comprising the step of forming the particles into a water-soluble polymer product.

3. The method according to claim 1, wherein, The water-soluble polymer composition comprises a water-soluble polymer having a hygroscopic salt and a solvent polymer plasticizer, wherein the hygroscopic salt comprises at least 15% by weight of the total weight of the composition as a lubricant, thereby enabling the polymer to be extruded and / or molded, wherein the solvent polymer plasticizer is monopropylene glycol or dipropylene glycol, and wherein the water content of the composition is less than 10% by weight of the total weight of the composition.

4. The method according to claim 3, wherein, The hygroscopic salt is provided in a higher amount by weight than the solvent polymer plasticizer.

5. The method according to claim 3, wherein, The hygroscopic salt is an anhydrous or hydrated salt selected from the group consisting of sodium chloride, sodium citrate, magnesium chloride, calcium chloride, potassium chloride, sodium sulfate, sodium carbonate, potassium carbonate, and ammonium carbonate.

6. The method according to claim 3, wherein, The hygroscopic salt is a water-soluble salt that will dissolve in water at 90°C at a rate of at least 10% by weight within 10 minutes.

7. The method according to claim 3, wherein, The moisture content of the hygroscopic salt is less than 10% by weight of the total weight of the composition.

8. The method according to claim 7, wherein, The hygroscopic salt content accounts for at least 25% of the total weight of the composition.

9. The method according to claim 8, wherein, The hygroscopic salt content accounts for at least 30% of the total weight of the composition.

10. The method according to claim 3, wherein, The water-soluble polymer and the hygroscopic salt are provided in solid form.

11. The method according to claim 1, wherein, The cold conveyor belt is at or below a temperature of 10°C to 30°C.

12. The method according to claim 1, wherein, The water-soluble polymer includes waxes to improve moisture resistance.

13. The water-soluble polymer product formed by the method according to claim 1.

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