CN115812026A

CN115812026A - Water-soluble polymer, method for producing same, and melt-processable water-soluble polymer composition

Info

Publication number: CN115812026A
Application number: CN202180049541.1A
Authority: CN
Inventors: 彼得·莫里斯
Original assignee: Peter Morris Research and Development Ltd
Current assignee: Peter Morris Research and Development Ltd
Priority date: 2020-05-26
Filing date: 2021-05-11
Publication date: 2023-03-17
Also published as: EP4153392A1; AU2021279371A1; KR20230016192A; WO2021240130A1; CA3184903A1; JP2023528600A; BR112022024229A2

Abstract

A method of forming a processable, water soluble polymer comprising the steps of: extruding the water-soluble polymer composition from the 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 an irregularly-shaped polymer extrudate; directing the irregularly-shaped polymer extrudate onto a cold conveyor belt to cool the irregularly-shaped polymer extrudate to below 60 ℃; and pelletizing the irregularly shaped polymer extrudate to form granules.

Description

Water-soluble polymer, method for producing same, and melt-processable water-soluble polymer composition

Technical Field

The present invention relates to a polymer composition, particularly but not exclusively a polyvinyl alcohol polymer, and a method of making the same.

Background

There is an increasing demand for water-soluble, biodegradable polymers to replace the large number of non-biodegradable polymers on the market. Non-biodegradable polymers place a great demand on resources due to the need for disposal in landfills or incineration.

Polyvinyl alcohol (PVA) is considered to be one of the few water-soluble vinyl polymers and is also subject to eventual biodegradation in the presence of appropriately domesticated microorganisms. Accordingly, increasing attention has been devoted to the preparation of environmentally compatible materials based on PVA for a wide range of applications. PVA has excellent film and thin-walled container forming properties, exhibiting a high degree of impermeability to a variety of gases, which makes it well-suited for use in packaging products for release in aqueous environments. PVA also has high adhesive strength and is non-toxic. However, these properties are dependent on humidity, since absorption of water by the polymer reduces its tensile strength but increases its elongation and tear strength. Successful extrusion of PVA or PVA-containing compositions is also difficult, further limiting its potential use. In particular, there is currently no formulation that can be easily molded to the minimum thickness required (less than 200 microns) to ensure that the product releases at low water temperatures, typically 5 ℃ or less than 5 ℃ in 2 minutes. Such films are ideal for applications such as packaging of laundry products that require the release of laundry detergent at low temperatures and short wash cycles.

One of the properties that polymers need to be improved is the Melt Flow Index (MFI). This relates to the ease of melt flow of the polymer, defined as the mass of polymer (in grams) flowing through a capillary of a particular diameter and length in ten minutes at a pressure applied by a specified alternate gravity and a specified alternate temperature. One such method is described in standard ISO 1133. A high MFI is required to provide thin wall molding.

It is known in the art to use internal lubricants in PVA to increase its melt flow index. For example, EP1112316B1 (PVAXX technologies ltd) comprises up to 5% by weight of fatty acid amide. The fatty acid provides lubrication between the polymer chains, thereby increasing the melt flow of the polymer. However, fatty acids are insoluble and melt and coat the polymer during processing, which may hinder the dissolution of the polymer. Furthermore, the amount of lubricant that can be used is limited due to the separation of excess lubricant from the mixture (typically greater than 5% by weight of PVA), thereby limiting their ability to improve the MFI of the polymer.

The polymer is made from polyvinyl acetate (PVAc) formed by the polymerization of vinyl acetate followed by hydrolysis. PVA and PVA-derived polymers are soluble in water, the solubility of which depends on the molecular weight and the degree of hydrolysis of the polymer, i.e. the percentage of acetate groups substituted by OH groups in the starting polymer (PVAc). The higher the degree of hydrolysis, the lower the solubility and dissolution rate. The difference is more pronounced at low dissolution temperatures than at high dissolution temperatures due to the formation of crystalline regions within the polymer.

It is also known in the art that PVA requires removal of volatiles from the composition prior to melt processing because without such removal processing difficulties result due to the formation of steam and subsequent foaming of the polymer. Drying is usually carried out at a temperature of 90 ℃ for 4-9 hours, depending on the brand, model and formulation, using standard drying equipment.

Although attempts have been made to mold articles with PVA or PVA derived compositions, the desired solubility characteristics have not been achieved due to the lack of melt flow required to mold thin walled articles that will dissolve in aqueous solution within the desired time frame.

It is an object of the present invention to provide a water soluble polymer composition, in particular but not exclusively a polyvinyl alcohol composition, which solves or at least mitigates the above-mentioned problems encountered with polymer compositions of the prior art.

It is a further object of the present invention to provide a process for producing and extruding and/or molding a water-soluble polymer composition.

Disclosure of Invention

According to the present invention, there is provided a method of forming a water-soluble polymer, the method comprising the steps of: extruding the water-soluble polymer composition from the extruder barrel without the use of a die, wherein the extruder barrel is not vented except through its extruder outlet, to produce an irregularly-shaped polymer extrudate; directing the irregularly-shaped polymer extrudate onto a cold conveyor belt to rapidly cool the irregularly-shaped polymer extrudate to below 60 ℃; and pelletizing the irregularly shaped polymer extrudate to form granules.

Alternatively, pelletization of irregularly shaped polymer extrudates can occur immediately after the cold conveyor belt without the need for an additional drying step.

Preferably, the method may further comprise the step of forming the particles into a water-soluble polymer product.

Preferably, the water soluble polymer may comprise a water soluble polymer composition having: a hygroscopic salt in a weight percentage of at least 15% of the total weight of the composition to act as a lubricant to render the polymer extrudable and/or moldable; and a solvent polymer plasticizer, wherein the solvent polymer plasticizer is monopropylene glycol or dipropylene glycol, and the composition has a water content of less than 10% by weight based on the total weight of the components.

Preferably, the hygroscopic salt may be provided at a higher content by weight than the solvent polymer plasticizer.

Preferably, the hygroscopic salt may be 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.

Preferably, the hygroscopic salt may be a water-soluble salt that dissolves in water at 90 ℃ at a rate of at least 10% of the weight dissolved in 10 minutes.

Preferably, the hygroscopic salt may have a water content of less than 10% by weight of the total weight of the composition.

Preferably, the hygroscopic salt may be present in an amount of at least 25% by weight based on the total weight of the composition.

Preferably, the hygroscopic salt may be present in an amount of at least 30% by weight of the total weight of the composition.

Preferably, the hygroscopic salt may be in anhydrous form.

Preferably, the water soluble polymer and hygroscopic salt may be provided in solid form.

Preferably, the polymer may comprise a polyvinyl alcohol polymer.

Preferably, the method may further comprise the step of extruding the water-soluble polymer composition without using a die.

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

Preferably, the cold conveyor belt may be at or below 10 ℃ to 30 ℃.

Preferably, the process may further comprise the step of granulating the water-soluble polymer composition immediately after the cold conveyor belt without an additional drying step.

Preferably, the water soluble polymer may contain a wax to improve moisture resistance.

Accordingly, the present invention provides a melt processable, water soluble polymer composition comprising a mixture of water soluble polymers having at least 15% by weight of the total weight of the composition of hygroscopic salt which acts as a lubricant to render the polymer extrudable and/or mouldable, wherein the composition has a water content of less than 10% by weight.

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

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

Preferably, the hygroscopic salt is an anhydrous or hydrated salt selected from an alkali metal salt or an alkaline earth metal salt. Depending on the end use of the composition, it may be beneficial to use salts that are approved for food and/or pharmaceutical use and/or have other properties (e.g., water softeners) that may impart additional benefits to the end 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.

A preferred embodiment of the first aspect of the invention comprises a mixture of a water-soluble polymer and an amount of sodium chloride for rendering the mixture extrudable.

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, especially less than 0.2% by weight. Desirably, the salt is in anhydrous form. The salt may be micronised, for example with particles having a mean size of less than 100 microns, preferably in the range 0.03 to 75 μm, especially 60 to 70 μm. The salt may also be coated with, for example, sodium aluminosilicate, silica, and/or sodium ferrocyanide to improve its performance. 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, available from Custom Powders (www.

The salt is contained in the composition in a weight ratio of up to 75% of the total weight of the preparation. The salt is present in an amount of at least 15%, 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%, especially at least 50% by weight based on the total weight of the formulation.

The composition may be mixed with optional additives to improve the processability of the composition, for example with plasticizers to enhance flexibility and/or reduce the melting temperature of the polymer under extrusion or molding, stabilizers to increase heat resistance and/or pigments to add color. Preferably, the heat stabilizer (e.g. a metal stearate) is included in an amount of at most 0.5%, preferably at most 0.3% by weight. However, the composition is preferably free of any fatty acid amide or ester.

In one aspect of the invention, bound water from other ingredients within the surface of the composition (e.g., the polymer itself) treats the salt to enable it to be used as an internal lubricant. The role of the "internal" lubricant is to improve lubrication between the polymer chains. The use of a salt as a lubricant eliminates the need for using other types of prior art internal lubricants such as fatty acid amides or esters.

An exemplary composition without solvent plasticizers may be as follows:

melt flow index =21.5 tested at 1900C using a 2.16kg weight in 10 minutes

Sodium chloride is more soluble in water than polymers, but is not a thermoplastic resin. Thus, the horn-like crystalline structure of the salt is not expected to mix with the polymer and actually impede melt flow. Surprisingly, this is not the case.

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 from 1.25 to 12, especially from 1.25 to 7, ideally from 4 to 5.

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

According to the present invention there is provided a melt processable, water soluble polymer composition comprising a mixture of water soluble polymers having at least 15% by weight of a hygroscopic salt which acts as a lubricant to render the polymer extrudable and/or moldable.

Another aspect of the invention provides a soluble polymeric internal lubricant comprising a mixture of hygroscopic salt and a solvent polymeric plasticizer for the salt.

The lubricant according to the invention can be mixed with water-soluble polymers for its processing. In this regard, another aspect of the present invention provides a melt-processable, water-soluble polymer composition comprising a mixture of a water-soluble polymer and an internal lubricant to render the polymer extrudable and/or moldable, the lubricant comprising a hygroscopic salt, preferably an anhydrous or hydrated metal salt, mixed with a solvent polymer plasticizer. Preferably, the lubricant is in accordance with the aforementioned aspect of the invention.

Preferably, the salt has a minimum moisture content, preferably a moisture content of less than 10% by weight, and the weight percentage of salt to the total weight of the formulation is at least 15%, more preferably at least 40%, especially at least 50%.

The lubricant according to the invention preferably has a melt flow index of at least 20g (10 min/190 ℃/2.16kg according to ISO 1133), more preferably at least 40g, in particular 60g, when mixed with a water-soluble polymer.

The composition of the present invention can be used for food and/or medicine. Thus, it will be appreciated that the lubricants and other ingredients of the composition are permitted for food and/or pharmaceutical use, if possible.

The composition according to the invention may be provided in any suitable form for further processing, but is preferably provided in the form of a powder, tablet or pellet for extrusion and/or moulding of extruded and/or moulded products, such as extruded filaments, containing the soluble polymer. The composition may be manufactured by any conventional method, for example by melt mixing or cold working, which in the latter case may include calendering, suitable calendering and/or compaction. Cold pressing, more preferably adapted to calendering, may be an alternative technique.

The compositions of the present invention may also contain a plasticizer to lower the melting temperature of the polymer under extrusion and/or molding. The plasticizer may be selected from the group consisting of glycerol, ethylene glycol, triethylene glycol, low molecular weight polyethylene glycols and low molecular weight amides. The preferred plasticizer is glycerol, more preferably monopropylene glycol or most preferably dipropylene glycol. Plasticizers may also be used as solvent polymer plasticizers for the salt internal lubricants.

In another aspect the present invention provides a process for the preparation of a water-soluble polymer composition, which process comprises mixing a water-soluble polymer with at least 15% by weight of the total weight of the composition of a hygroscopic salt, which salt acts as a lubricant, thereby rendering said polymer extrudable and/or mouldable, wherein the water content of said composition is less than 10% by weight, optionally including the addition of a solvent polymer plasticiser.

Another aspect of the invention provides a method of extruding and/or molding a water-soluble polymer composition comprising softening a composition according to the preceding aspect of the invention to form a melt stream. Preferably, the softening composition is heated and/or pressurized to produce a melt stream.

The melt stream preferably has a melt flow index of at least 20g (10 min/190 ℃/2.16kg according to ISO 1133), more preferably at least 40g, in particular 60g. Preferably, the polymer composition is molded into a form having a thickness of less than 200 microns, preferably less than 100 microns, such that the form is capable of being dissolved in an aqueous solution at 5 ℃ within 80 seconds. 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 invention are particularly beneficial for molded articles.

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

Another aspect of the present 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 so that the polymer is extrudable and/or moldable; 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 composition has a water content of less than 10% by weight based on the total weight of the composition; and at least 15% by weight of hygroscopic salt based on the total weight of the composition is included in the composition.

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.

Preferably, the wax may be glycerol monostearate.

Detailed Description

The melt processable compositions of the present invention may be processed by any known thermal processing method including, but not limited to, injection molding, compression molding, rotational molding and film extrusion. The compositions are particularly suitable for thin-walled moldings.

The melt processable compositions of the present invention are suitable for use in the manufacture of any article currently made from extrudable and/or moldable polymers, including films, containers, and bottles. The composition is suitable for making filaments and fibers for spunbond, non-woven and meltblown applications. The composition is also suitable for use in the manufacture of: bags and containers for detergents and agrochemicals, mulching films, flower pots, household bags, diapers, straws, feminine care products, clothes hangers, incontinence pads, pouches, six-links, disposable garments, foams, gloves, film canisters, golf tees, shotgun cartridges, bed pans, bottles, bowls, cotton swabs, hospital curtains, "disposable" sterile products and packaging materials, and the like.

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 lead to foaming. Typically, the polymer is dried in a standard polymer dryer at 90 ℃ for 4 to 8 hours. It has been found that the addition of a specific amount of hygroscopic salt, preferably having a low water content or being anhydrous, removes water from the surrounding PVA. Even more surprising is that the absorption of water by the salt provides a self-lubricating coating on the salt which can then act as an internal lubricant for the PVA. The salt dewetting effect reduces the drying time to 2-4 hours, resulting in significant energy savings in polymer production, and in addition, the salt-borne lubrication effect greatly increases the melt flow index of the composition, allowing PVA to be easily extruded and/or molded into products (particularly thin, less than 200 microns), making it suitable for applications requiring dissolution of films and/or molding at temperatures as low as 5 ℃ in a short period of time (less than 2 minutes). The self-lubricating effect is still effective at low moisture levels below 1% (i.e., even during drying). Further reduction of the temperature and thus approaching 0 ℃ is feasible and the time can also be reduced.

It has been found that hygroscopic salts (e.g. sodium chloride) have enhanced lubricating effect where polymeric plasticizers (e.g. hygroscopic salt solvents including glycerol or propylene glycol, and especially monopropylene glycol or most preferably dipropylene glycol) are included in the composition. The absorption of water by the salt appears to act as a surface treatment enabling the use of non-solvent plasticizers in the formulation if 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. Alcoholic plasticizers are not solvents for PCC and therefore high loads must be used to create high melt flow resulting in very good ductility of the product because PVA is encapsulated around the PCC particles. Melt flow index is also disappointing. In contrast, the present invention uses glycerol, monopropylene glycol or dipropylene glycol as a solvent for anhydrous sodium chloride to partially dissolve the outer surface of the sodium chloride to provide a lubricant within the polymer chain. This results in a relatively higher MFI and increases the solubility of the polymer while reducing its ductility (desirable characteristics).

Sodium chloride is one possible hygroscopic salt that may be used herein. Sodium citrate and magnesium chloride are other possible alternatives, as are calcium chloride, potassium chloride, sodium sulphate, sodium carbonate, potassium carbonate and ammonium carbonate. The essential features of the hygroscopic salt used in the present invention are that the hygroscopic salt is water-soluble, has the ability to absorb atmospheric moisture, and/or has a dissolution rate of at least 10% by weight in 90 ℃ water within 10 minutes.

It will be appreciated that even high levels of water (as a plasticizer or binder) in the composition together with the hygroscopic salt will not achieve the benefits of the present invention. Water will result in reversible dissolution of some or all of the salt present. Furthermore, reducing the water content for successful processing will result in removal of the self-lubricating layer and reformation of salt crystals that are unpredictable in size and shape. This can impede melt flow and can increase drying time. Therefore, in the present invention, it is preferable that water is not added to the formulation except for the low water content contained in the various components (e.g., PVA) constituting the composition.

The invention will now be described with reference to the following non-limiting examples. These examples illustrate the high melt flow values achieved with the composition according to the invention and its reduced drying time and compare these properties with compositions falling outside the protective range.

Manufacturing method

PVA (polymer), sodium chloride (lubricant), glycerol (plasticizer) and thermoplastic plasticizer were mixed in a bell jar, low shear mixer for 3 minutes. The mixture is then fed to a compounder by a screw and formed into pellets by a suitable calendaring process. As the PVA passes between the roller and the die, a suitable calendering process causes the PVA to partially or fully melt as a result of frictional shearing, resulting in clumping prior to extrusion through the die. The temperature of the pre-extrudate was varied between 110 ℃ and 140 ℃ and the formed granules were then placed in a tray polymer dryer at 90 ℃ for 3 hours.

Melt flow analysis

The MFI of a 5 gram sample of a formulation prepared according to the invention was tested at 190 ℃ using a 2.16 kilogram weight. MFI testing and comparison were performed according to ISO1133 for each sample. The test was repeated 10 times and the average results were recorded.

The samples were molded using a 50 ton molding press in an automatic mode using a mold with a hot runner system at 180 ℃ to 200 ℃ for a period of 7 to 10 seconds. The screw temperature profile (in c) from hopper to tip was 160, 170, 180-190. The part wall cross-section measures between 600 and 350 microns.

Example 1

The formulations were prepared according to the above method with the ingredients shown in table 1 below mixed in the given weight percentages. 88% hydrolyzed PVA was used in each formulation and the heat stabilizer was calcium stearate. The Melt Flow Index (MFI) is determined according to the analysis given above.

TABLE 1

Formulation 2, shown in table 1 above, was found to have a white/cream color with the following characteristics:

density 1.68g/cm

Melt density 1.52g/cm (according to ISO 1183) at 200 ℃.

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 are obtained in the above formulation with a salt to plasticizer ratio of 3.5 to 5 (more preferably 4 to 4.4).

Example 2

The formulations were prepared according to the above method with the ingredients shown in table 2 below mixed in the given weight percentages. 88% hydrolyzed PVA was used in each formulation. The Melt Flow Index (MFI) is determined according to the analysis given above. The part wall cross-section of the molded formulation measures 600 microns to 100 microns.

TABLE 2

Table 2 demonstrates that the type of plasticizer has no significant effect on the MFI achieved.

Example 3

Formulations were prepared according to the above method with the ingredients mixed in the weight percentages shown in table 3 below. 98% hydrolyzed PVA was used for formulations 10 to 13, and 80% hydrolyzed PVA was used for formulations 14 to 16. These formulations were formed using a gambling 50 ton molding process in an automated mode for a 20 second cycle using a mold with a cold runner system. The screw temperature profile (in c) from hopper to tip was 160, 170, 180, 220. The part wall cross section is measured in the range of 600 to 2000 microns.

TABLE 3

Example 4

Formulation 17 was prepared as a mixture in a similar manner to formulation 2 of example 1, but with sodium citrate replacing sodium chloride, as follows:

the formulation was found to have the following properties:

density 1.67g/cm

Melt density at 190 ℃ of 1.40 to 1.42g/cm (according to ISO 1183)

MFI 38g.

The processing temperature is 190-200 ℃, and the retention time is 30 minutes. The drying time was 4 hours at 90 ℃. The MFI is also significantly higher when salt is included in the composition.

The extrudability of the formulation and formulation 2 was examined in an injection molding machine manufactured by gambling, demagard and yabao. The extrusion process is performed using a single full flight screw with a constant pitch. The barrel temperature profile is 160-200 ℃ and the screw speed is typically varied between 20-150 rpm. The shut-down of the device was performed by keeping the temperature at 100 ℃ and stopping the screw rotation. Then, a complete shutdown is performed by shutting down the machine.

Formulations 2 and 17 can be molded into containers of various sizes and colors, and are suitable for injection molding. The use of sodium citrate as a polymeric lubricant may provide additional advantages when used in packaging laundry products, as sodium citrate may be used as a water softener.

Example 5

Investigations were carried out to investigate the requirement of having a low water content in the formulations of the present invention.

Table 4 below lists the formulation compositions, as well as their MFI and drying times.

TABLE 4

Table 4 clearly shows the importance of the salt and water content of the formulation on 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 has minimal or no water content. Formulations C and D, which contained 13% and 17% water, respectively, were very viscous formulations, not free flowing and therefore not suitable for mixing. In addition, excessively long drying times lead to undesirable glycerol vapor losses.

Example 6

The ratio of the partial pressure of water vapor to the equilibrium vapor pressure of pure water (usually expressed as a percentage) in a given environment is referred to as the Relative Humidity (RH). The RH at which a given material begins to deliquesce (i.e., when water adsorbed onto the material begins to solvate molecules of the material) is referred to as the deliquescence point (RHo; also referred to in the art as "critical RH") of the material and is an important parameter in characterizing the structural stability of a polymeric material. RHo is temperature dependent and is typically expressed as a RHo value at a given temperature. For example, the RHo of crystalline NaCl is about 77% RH at 20 degrees celsius (0C), i.e., when the ambient RH is equal to or higher than 77% at 20 degrees celsius, atmospheric water vapor spontaneously adsorbs onto and solvates NaCl crystals.

Materials with low RHo values tend to absorb moisture more readily and may function well as desiccants. In contrast, materials with high RHo tend to be less prone to absorb moisture. In the case of polymer compositions, having higher RHo advantageously reduces the need for secondary packaging and provides a more structurally stable product when opened to an unprotected environment.

One problem with products comprising water soluble polymers is that they tend to absorb moisture and are structurally unstable in high RH environments. For example, PVA begins to deliquesce when RH reaches 50% or more at an ambient temperature of 20 degrees celsius.

One problem with the formulation comprising PVA, sodium chloride and glycerol as described above is that although the lubricating effect of the glycerol and salt combination aids melt processing, the resulting molded article is prone to moisture absorption and secondary packaging requires high moisture and gas barrier properties.

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

TABLE 5

As shown in table 5, it was found that the use of propylene glycol as a solvent polymer plasticizer unexpectedly resulted in a PVA-based polymer product that was less water absorbent and thus more stable, relative to a comparable formulation that used glycerin as a solvent polymer plasticizer. Formulations containing Glycerol 7-9 RH at 20 deg.C _O 62% to 66%, and all formulations containing propylene glycol had RH at 20 deg.C _O Significantly higher, between 76% and 83%.

In addition, the formulation containing dipropylene glycol was found to be more stable than a comparable formulation containing monopropylene glycol; formulations with dipropylene glycol 1-3 RH at 20 deg.C _O 80% to 83%, while formulations 4 to 6 with monopropylene glycol had lower RH at 20 deg.C _O And is 76% to 77%.

The deliquescence point of sodium chloride is about 77% RH at 20 deg.C _O (ii) a However, the molded article prepared by using the formulation comprising PVA, sodium chloride and glycerin has a 60RH or more at 20 ℃ _O Tendency to deliquesce.

The surprising effect of replacing glycerol with dipropylene glycol is that the deliquescence point of sodium chloride rises above 82RH at 20 deg.C _O . This reduces the specification requirements for secondary packaging and yieldsA more dimensionally stable product when opened in a protected environment.

Example 7

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

TABLE 6

A significant improvement in melt flow index was shown with increasing hygroscopic salt concentration.

Alternative production methods

Conventionally, in the art, extrusion is carried out by a slot die or a wire die to manufacture PVA resins. Both the slot die and the line die provide sufficient back pressure to produce a uniform polymer melt flow from the extruder to where the extrudate is cooled. This is typically performed by immersion in water or by ambient or cold air cooling.

The problem with water immersion is that the PVA is water soluble and therefore requires further drying after immersion.

When cooled with ambient air, the problem is that the plasticizer runs off from the formulation before the extrudate cools below 60 ℃.

PVA mixed with a lubricious hygroscopic salt formed according to the present method is more soluble and brittle and therefore not suitable for wire extrusion and water cooling. When cooled in water, the granules become sticky due to dissolution, sticking together and therefore requiring further drying. The wire also becomes brittle due to polymer reduction and high salt content, and plasticizer loss during air cooling of up to 0.75%. Due to wire breakage and material loss, the process is unstable, resulting in process downtime. The loss of volatility of the plasticizer can also lead to inconsistent product results depending on ambient conditions and cooling time. The melt flow index may vary greatly.

Slot dies produce tapes for extrusion without forming granules for further processing. .

Air cooling is preferred over the conventional water bath described. However, fan-assisted air supported on a moving conveyor belt can be used to cool the extrudate. The extruded strands exited the die at 165 ℃ to 200 ℃ and then reached the vertical pelletizer at a temperature of up to 60 ℃. The above-mentioned plasticizer evaporation takes place on the moving conveyor belt. In addition to having an adverse effect on melt flow index, this reduction in quality also results in economic losses.

Thus, a new extrudate cooling process has been developed to address the improved polymer compositions. For convenience of presentation, the new process may be referred to as a chill roll process.

In the chill roll process, the polymer is extruded without an additional discharge port in the barrel, nor a slot die or a wire die. Without back pressure, the barrel vented the steam and produced the following extrudate: (1) irregular and inconsistent in size and shape; and (2) very low water content, preferably below 1% (v/v). This is a significant difference from conventional processes where the polymer is extruded into a slot or wire die. The slot die or wire die method requires the extrudate to have a regular and consistent size and shape to maintain operation, and additional vents must be provided for the barrel for the slot die or wire die method because the presence of steam in the barrel can cause irregularities.

The irregular extrudate is allowed to fall onto a chill roll or conveyor belt where the temperature rapidly chills the extrudate to less than 100 ℃ but above 10 ℃, preferably below 60 ℃. The chilling 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. The chill roll was set to obtain instantaneous cooling, thereby avoiding plasticizer run-off. Indeed, if the extrudate is cooled slightly before freezing in a transient manner, it will no longer have sufficient ductility to pass through the chill roll. Must immediately pass through to maintain sufficient ductility to roll. Once cooled, the irregular extrudate is directed to a commercial polymer mill to form granules which can be stored for subsequent processing, typically by packaging in sealed vacuum bags.

Calendering is accommodated by the use of (preferably water-cooled) chill rolls to accommodate inconsistent extrudate so that the extrudate is rapidly cooled to 60 ℃ and below 60 ℃. In addition, the high heat transfer characteristics of PVA having at least 15 weight percent of hygroscopic salts results in further improved cooling by the system.

If no salt is used in the formulation, a lower temperature roller (i.e. less than 10 ℃) is required to achieve the same cooling rate to prevent loss of plasticizer. This can lead to condensation on the chill roll, which can lead to process problems and extrudate sticking to the roll. This makes the process ineffective. It has been found that water temperatures between 10 ℃ and 30 ℃ are most effective for PVA and salt compositions, whereas salt-free compositions require temperatures below 10 ℃, although condensation has proven to be a problem here.

Below 60 ℃, the plasticizer becomes locked in the polymer composition, below the plasticizer evaporation temperature, preventing the total weight of the polymer composition from losing by evaporation in the range of 0.75% to 1.25%. The water content of the resulting polymer is also very low, since most of the water present is lost as steam during extrusion. The water content can then be kept below 1% by weight, which is the maximum possible water content required for successful rework of irregular extrudates. This also eliminates the need for a further polymer drying step.

A comparison between the example products of the embodiments of the chill roll method described above and a conventional chilled air frame line process (i.e., a process in which a regular extrudate is extruded directly into strands for air cooling) is summarized below in tables 7 and 8.

TABLE 7 chill roll method

TABLE 8 extruder and Rack line

It can be seen that the chill roll process produces extrudates with higher melt flow index under otherwise identical conditions. Furthermore, in the embodiment of the chill roll process, the extrudate temperature is always lower when it reaches the pelletizer, thus reducing plasticizer evaporation.

Once cooled, the irregular extrudate can be directed to a commercial polymer mill or pelletizer and the polymer is ground until the particulate matter falls through a screen to determine the maximum particle size. However, irregularities in the extrudate can produce variable polymer particle sizes, typically in the range of 2 mm to 4 mm.

Surprisingly, this particular size heterogeneity aids in the melting of the polymer during subsequent reprocessing. The combined chill roll method is therefore well suited for use with a pelletizer. Compared to the wire-die pellet process, the smaller pellets melt faster, require less energy, and the barrel zone temperature drops significantly by 10 ℃. Lower processing temperatures also result in lower plasticizer loss rates that might otherwise be converted to gas and generate gas during reprocessing, typically by injection molding or extrusion. Outgassing can lead to burn marks during molding, part scrap, and mold contamination during machining, and reducing outgassing is a significant improvement.

Vegetable waxes are commonly used to impart moisture resistance to polymers. Typically, from 0.3% to 1.0% of glyceryl monostearate or stearamide will be used as the vegetable wax. However, the use of vegetable waxes during the melting process also has the adverse effect of causing "screw slippage". Screw slippage refers to the situation: during the melting process, the extruder or injection molding machine cannot deliver the molten material due to the smoothness of the polymer melt, and vegetable wax is applied as a lubricant to the barrel.

For salt-free PVA, a wax content of 0.3% is generally the maximum allowable due to screw slippage, but with small amounts of salt, higher wax contents can be provided, up to 2% to 3%. The addition of these vegetable waxes can reduce the water uptake of PVOH formulations containing monopropylene glycol and dipropylene glycol by as much as 50% compared to formulations without salt.

Surprisingly, when formulating polymer compositions comprising vegetable waxes in amounts that would normally be expected to cause screw slippage, the irregularities in particle shape also advantageously result in reduced screw slippage.

In the present invention, the irregular size and shape of the particles prior to melting more gradually form a mixture, allowing for better adhesion during transfer of the polymer through the screw. This allows more wax component to be added.

Thus, the compositions of the present invention provide a melt-processable PVA-containing polymer that generally has a flexural modulus similar to other extrudable polymers. This enables soluble and biodegradable polymers to be used in the processing of 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 (such as its high tensile strength and good barrier properties) are retained in the melt processable composition and can be extruded on current extrusion lines, blow molders and injection molders without modification.

Claims

1. A method of forming a water-soluble polymer, 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 an irregularly shaped polymer extrudate;

directing the irregularly-shaped polymer extrudate onto a cold conveyor belt to rapidly cool the irregularly-shaped polymer extrudate to below 60 ℃; and

pelletizing the irregularly shaped polymer extrudate to form granules.

2. The method of claim 1, wherein the pelletizing of the irregularly-shaped polymer extrudate occurs immediately after the cold conveyor belt without an additional drying step.

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

4. The method of any preceding claim, wherein the water-soluble polymer composition comprises a water-soluble polymer having a hygroscopic salt and a solvent polymer plasticizer, the hygroscopic salt comprising at least 15% by weight of the total weight of the composition to act as a lubricant such that the polymer is extrudable and/or moldable, 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.

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

6. The method according to claim 4 or 5, 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.

7. The method of any of claims 4 to 6, wherein the hygroscopic salt is a water-soluble salt that will dissolve in water at 90 ℃ at a rate of at least 10% of the weight dissolved in 10 minutes.

8. The method according to any one of claims 4 to 7, wherein the hygroscopic salt has a water content of less than 10% by weight relative to the total weight of the composition.

9. The method of claim 8, wherein the hygroscopic salt content is at least 25% by weight of the total weight of the composition.

10. The method of claim 9, wherein the hygroscopic salt content is at least 30% by weight of the total weight of the composition.

11. The method of any one of claims 4 to 10, wherein the hygroscopic salt is in anhydrous form.

12. The method of any one of claims 4 to 11, wherein the water soluble polymer and hygroscopic salt are provided in solid form.

13. The method of any one of the preceding claims, wherein the polymer comprises a polyvinyl alcohol polymer.

14. A method according to any preceding claim, wherein the cold conveyor belt is at or below a temperature of 10 ℃ to 30 ℃.

15. The process according to any one of the preceding claims, further comprising the step of granulating the water-soluble polymer composition immediately after the cold conveyor belt without an additional drying step.

16. The method of claim 15, wherein the water soluble polymer includes a wax to improve moisture resistance.

17. A water-soluble polymer product formed according to the method of any one of the preceding claims.

18. A melt processable, water soluble polymer composition comprising:

a mixture of a water-soluble polymer and a hygroscopic salt, said hygroscopic salt acting as a lubricant to render said polymer extrudable and/or moldable,

a solvent polymer plasticizer; and

a wax, wherein the composition has a wax content of at least 0.3% by weight of the total weight of the composition;

wherein the composition has a water content of less than 10% by weight of the total weight of the composition;

the hygroscopic salt is included in the composition in an amount of at least 15% by weight based on the total weight of the composition.

19. A melt processable, water soluble polymer composition according to claim 18 wherein the wax is present in an amount of at least 1.0% by weight based on the total weight of the composition.

20. A melt processable, water soluble polymer composition according to claim 18 or 19 wherein the wax is glycerol monostearate or stearamide.