WATER-SOLUBLE POLYMER COMPOSITION AND METHOD OF MAKING A WATER-SOLUBLE POLYMER COMPOSITION

Abstract

A method of forming a melt-processable water soluble polymer composition having a high Melt Flow Index comprising blending a water soluble polymer with at least 15% by weight of the total weight of the composition of a hygroscopic salt to act as a lubricant to render the polymer extrudable and/or mouldable and with a solvent polymer plasticizer, the hygroscopic salt being partially dissolved in the solvent polymer plasticizer, wherein the solvent polymer plasticizer is mono-propylene glycol, di-propylene glycol or a combination thereof.

Claims

1. A method of making a water soluble polymer composition comprising: blending a water soluble polymer with at least 15% by weight of the total weight of the composition of a hygroscopic salt to act as a lubricant to render the polymer extrudable and/or mouldable and with a solvent polymer plasticizer, the hygroscopic salt being partially dissolved in the solvent polymer plasticizer, wherein the solvent polymer plasticizer is mono-propylene glycol, di-propylene glycol or a combination thereof, wherein the water content of the composition is of less than 10% by weight of the total weight of the composition.

2. The method according to claim 1, wherein the solvent polymer plasticizer is di-propylene glycol.

3. The method according to claim 1, wherein the hygroscopic salt is provided in a higher amount by weight than the solvent polymer plasticizer.

4. The method according to claim 1, wherein the polymer comprises a polyvinyl alcohol polymer.

5. The method according to claim 1, 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 sulphate, sodium carbonate, potassium carbonate, and ammonium carbonate.

6. The method according to claim 1, wherein the hygroscopic salt is a water-soluble salt which will dissolve at a rate of at least 10% by weight in 90 C. water in ten minutes.

7. The method according to claim 1, wherein the hygroscopic salt has a water content of less than 10% by weight of the total weight of the composition.

8. The method according to claim 1, wherein the hygroscopic salt is included in an amount of at least 25% by weight of the total weight of the composition.

9. The method according to claim 8, wherein the hygroscopic salt is included in an amount of at least 30% by weight of the total weight of the composition.

10. The method according to claim 1, wherein the hygroscopic salt is in the anhydrous form.

11. The method according to claim 1, further comprising the step of extruding the water soluble polymer composition without using a die.

12. The method as claimed in claim 1, wherein the water soluble polymer and hygroscopic salt are provided in solid form.

13. A melt-processable water soluble polymer composition comprising a blend of a water soluble polymer with a hygroscopic salt to act as a lubricant to render the polymer extrudable and/or mouldable, the composition further comprising a solvent polymer plasticizer selected from the group of mono-propylene glycol, di-propylene glycol, and a combination thereof, wherein: the water content of the composition is of less than 10% by weight of the total weight of the composition; and the hygroscopic salt is included in the composition in an amount of at least 15% by weight of the total weight of the composition.

14. The melt-processable water soluble polymer composition according to claim 13, wherein the solvent polymer plasticizer is di-propylene glycol.

15. The melt-processable water soluble polymer composition according to claim 13, 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 sulphate, sodium carbonate, potassium carbonate, and ammonium carbonate.

16. The melt-processable water soluble polymer composition according to claim 13, wherein the hygroscopic salt is provided in a higher amount by weight than the solvent polymer plasticizer.

Description

DETAILED DESCRIPTION

[0037] The melt-processable compositions of the present invention may be processed by any known thermoprocessing method, including but not limited to, injection moulding, compression moulding, rotational moulding and film extrusion. The compositions are particularly suitable for thin-walled mouldings.

[0038] The melt-processable compositions of the invention are suitable for the manufacture of any article currently made from extrudable and/or mouldable polymers, including films, containers and bottles. The compositions are suited to the manufacture of filaments and fibre, for use in spunbond, non-woven and melt-blowing applications. The composition is suitable also for manufacture of such articles as detergent and agro chemical sachets and containers, mulch films, plant pots, dome tic bags, diapers, drinking straws, fem care products, hangers, incontinence pads, sachets, six pack rings, disposable clothing, expanded foams, gloves, film canisters, golf tees, shot gun cartridges, bed pans, bottles, bowls, cotton buds, hospital curtains, one-use sterile products and packaging materials.

[0039] PVA generally has moisture content up to 5% by weight. This has to be reduced to below 1% to avoid processing issues on standard thermoplastic equipment, such as the generation of volatiles which causes foaming. Conventionally, the polymer is dried in a standard polymer dryer for 4-8 hours at 90 C. It has been found that the addition of a hygroscopic salt in a particular amount, preferably having a low water content or being anhydrous, removes water from the surrounding PVA. More surprisingly, the absorption of water by the salt provides a self-lubricating coating on the salt which is then able to act as an internal lubricant for the PVA. The desiccant effect of the salt reduces drying time to 2-4 hours enabling a substantial energy saving in the polymer production and furthermore, the lubrication brought about by the salt greatly increases the Melt Flow Index of composition such that the PVA may be readily extruded and/or moulded into products and, in particular to thin forms of less than 200 microns making it suitable for applications where dissolution of the film and/or moulding is required at temperatures as low as 5 C. within a short time period (under 2 minutes). The self-lubricating effect is still active at low moisture levels of less than 1%, i.e. even during the drying process. It is feasible that the temperature may be decreased further and therefore closer to 0 C., and the time may also be decreased.

[0040] The hygroscopic salt, such as sodium chloride, has been found to have an enhanced lubricating effect if polymer plasticizers are included in the composition, such as hygroscopic salt solvents including glycerine or propylene glycol, and particularly mono-propylene glycol or most preferably di-propylene glycol. Absorption of water by the salt appears to act as a surface treatment enabling non solvent plasticizers to be used in the formulation, if desired. Anhydrous salts would not normally be considered as suitable for internal lubrication of water soluble polymers. In this respect, precipitated calcium carbonate (PCC) has been used in small amounts (2-3%). Alcohol plasticizers are not solvents of PCC so high loadings have to be used to produce high melt flow resulting in very ductile products as the PVA becomes encapsulated around the PCC particles. Melt flow indexes are also disappointing. In comparison, the present invention uses glycerine, mono-propylene glycol, or di-propylene glycol, which are solvents of anhydrous sodium chloride, to partially dissolve the outer surface of the sodium chloride to provide a lubricant within the polymer chains. This produces a comparatively high MFI and increases the solubility of the polymer, whilst decreasing its ductility (a desirable trait).

[0041] Sodium chloride is one possible hygroscopic salt which can be used here. 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 requisite characteristics of the hygroscopic salt for the present invention are that the hygroscopic salt is water-soluble, having the ability to absorb atmospheric water, and/or which will achieve a dissolution rate of at least 10% by weight in 90 C. water in 10 minutes.

[0042] It is to be appreciated that including high levels of water in the composition (either as a plasticizer or binder) even with a hygroscopic salt would not achieve the benefits of the present invention. The water would result in reversible dissolution of some or all of the salt present. Furthermore, reduction of the water content for successful processing would result in removal of the self-lubricating layer and the reformation of salt crystals of unpredictable size and shape. This hampers melt flow and would add to the drying time. Thus, it is a preference of the present invention that water is not added to the formulation beyond the low water content contained in the various constituents, such as the PVA, making up the composition.

[0043] The present invention will now be described with reference to the following non-limiting examples. The examples illustrate the high melt flow values achieved with the compositions according to the present invention, their reduced drying times and compare these properties with compositions falling outside the scope of protection.

[0044] Method of Production

[0045] PVA (polymer), sodium chloride (lubricant), glycerin (plasticizer) and thermal plasticizers were mixed in a bell tub, low-shear mixer for 3 minutes. The mix was then fed into a compounder via a screw and formed into pellets via an adapted calendaring process. The adapted calendaring process causes partial or complete melting of the PVA as a result of frictional shear as it is passed between the roller and die, causing agglomeration before extrusion through the die. The temperature of the pre-extrudate varied from 110 C. to 140 C. and the formed pellets were then placed in a tray polymer dryer for 3 hours at 90 C.

[0046] Melt Flow Analysis

[0047] 5 gram samples of the formulations prepared according to the invention were tested for MFI at 190 C. using a 2.16 kg weight. Each sample was tested and compared for MFI according to ISO 1133. The test was repeated by a factor of 10 and the mean result was recorded.

[0048] The samples were moulded using a 50 tonne moulding press in automatic mode with a cycle time of 7-10 seconds using a mould with a hot runner system at 180 C. to 200 C. The screw temperature profile (in C.) from the hopper to tip was 160, 170, 180, 180-190. The part wall section was measured between 600-350 microns.

Example 1

[0049] Formulations were made according to the method above having the ingredients shown in Table 1 below mixed in the given percentage by weight. 88% hydrolysed PVA was used in each formulation and the thermal stabiliser was calcium stearate. The Melt Flow Index (MFI) was determined according to the analysis given above.

TABLE-US-00002 TABLE 1 Thermal PVA Glycerin NaCl stabilizers Ratio MFI/g Formu- (% by (% by (% by (% by of salt: 10 lation weight) weight) weight) weight) glycerin minutes 1 84.7 10.0 5.0 0.3 0.5:1 7.0 2 39.7 8.0 52.0 0.3 6.5:1 44.0 3 38.7 10.0 51.0 0.3 5.1:1 56.0 4 36.7 12.0 51.0 0.3 4.25:1 77.0 5 25.7 14.0 60.0 0.3 4.28:1 78.0
Formulation 2 shown in Table 1 above was found to have a white/cream colour with the following properties:

TABLE-US-00003 Density 1.68 g/cm Melt density 1.52 g/cm at 200 C. (under ISO 1183).

[0050] These results illustrate the importance of having a high ratio of salt to plasticizer in the formulation to achieve the desired high MFI but that peak MFI values are obtained in the above formulations where the salt to plasticizer ratio is 3.5-5:1, more preferably 4-4.4:1.

Example 2

[0051] Formulations were made according to the method above having the ingredients shown in Table 2 below mixed in the given percents by weight. 88% hydrolysed PVA was used in each formulation. The Melt Flow Index (MFI) was determined according to the analysis given above. The part wall section of the moulded formulations was measured from 600 microns to 100 microns.

TABLE-US-00004 TABLE 2 Propylene Thermal Ratio of PVA glycol NaCl stabilizers salt: Formu- (% by (% by (% by (% by propylene lation weight) weight) weight) weight) glycol MFI/g 6 84.7 10.0 5.0 0.3 0.5:1 9.0 7 39.7 8.0 52.0 0.3 6.5:1 40.0 8 36.7 10.0 51.0 0.3 5.1:1 51.0 9 38.7 12.0 51.0 0.3 4.25:1 75.0
Table 2 demonstrates that the type of plasticizer does not have a significant effect on the MFI achieved

Example 3

[0052] Formulations were made according to the method above having the ingredients mixed in the percent by weights shown in Table 3 below. 98% hydrolysed PVA was used in formulation 10-13 and 80% hydrolysed PVA was used in formulations 14-16. The formulations were moulded using a Boy 50 tonne moulding process in automatic mode with a cycle time of 20 seconds using a mould with a cold runner system. The screw temperature profile (in C.) from the hopper to tip was 160, 170, 180, 180, 220. The part wall section was measured from 600 to 2000 microns.

TABLE-US-00005 TABLE 3 Thermal PVA Glycerin NaCl stabilizers Ratio of Formu- (% by (% by (% by (% by salt: lation weight) weight) weight) weight) glycerin MFI/g 10 89.0 10.0 0.0 0.3 0:1 1.9 11 84.5 9.5 5.0 0.3 0.52:1 3.48 12 80.1 9.0 10.0 0.3 1.1:1.sup. 2.75 13 67.2 7.5 25.0 0.3 3.33:1 1.91 14 20.0 15.7 64.0 0.3 4:1 73.5 15 15.0 11.7 73.0 0.3 6:1 1.9 16 12.0 9.3 78.4 0.3 8:1 0.00

Example 4

[0053] Formulation 17 was prepared as a blend in a similar way to Formulation 2 of Example 1 but having sodium citrate in place of sodium chloride, as follows:

TABLE-US-00006 PVA (88% hydrolysed) 39.0% by wt. Sodium citrate 51.0% by wt. Glycerol 9.70% by wt. Calcium stearate 0.3% by wt

[0054] This formulation was found to have the following properties:

TABLE-US-00007 Density 1.67 g/cm Melt density 1.40-1.42 g/cm at 190 C. (under ISO 1183) MFI 38 g.

[0055] Processing temperature was 190-200 C. with a residence time of up to 30 minutes. Drying time was 4 hours at 90 C. The MFI is again substantially higher with the salt included in the composition.

[0056] This formulation and that of Formulation 2 were examined for their extrudability in injection moulding machines made by Boy, Demag and Arburg. Extrusion processing was carried out using a single full flight screw with constant pitch. The barrel temperature had a profile of 160-200 C. and the screw speed varied typically between 20-150 rpm. Shut down of the apparatus was carried out by maintaining the temperature at 100 C. with screw rotation stopped. Complete shutdown was then carried out by switching off the machine.

[0057] Formulations 2 and 17 were capable of being moulded into a range of containers of various sizes and colours and were suitable for injection moulding. The use of sodium citrate as the polymer lubricant provides additional advantages if it is used for packaging laundry products as it acts as a water softener.

Example 5

[0058] A study was carried out to investigate the requirement to have a low water content in the formulations of the present invention.

[0059] Table 4 below sets out the composition of the formulations, together with their MFI and drying time.

TABLE-US-00008 TABLE 4 Thermal PVA Glycerin NaCl Stabilizers Water Moisture Moisture Drying Formu- (% by (% by (% by (% by (% by content content time/hr lation weight) weight) weight) weight) weight) pre-drying post drying at 90 C. MFI/g A 73.0 11.7 15.0 0.3 0.00 2.4 0.9 5.0 22.0 B 37.0 11.7 51.0 0.3 0.00 2.0 0.9 3.0 77.0 C 71.3 10.2 5.2 0.3 13.0 24 0.9 9.0 10.0 D 60.5 9.7 12.5 0.3 17.0 23 0.9 13.0 7.0

[0060] Table 4 clearly shows the importance of the amount of salt and water contained within the formulation on drying time and on MFI. The formulation has a higher percentage of salt (of at least 15%, preferably at least 20%, and more preferably at least 40%) with minimal or no water content. Formulations C and D which contained 13 and 17% water respectively were very sticky formulations that were not free-flowing making them unsuitable for compounding. Additionally, the excess drying times resulted in undesirable glycerine vapour loss.

Example 6

[0061] The ratio (typically expressed as a percentage) of partial pressure for water vapor in a given environment compared to the equilibrium vapor pressure of pure water is known as relative humidity (RH). The RH at which a given material begins to deliquesce, that is, when water adsorbed onto the material starts to solvate molecules of the material, is termed the material's deliquescence point (RH.sub.O; also referred to in the art as critical RH), and is an important parameter characterizing structural stability of a polymer material. The RH.sub.O is temperature-dependent and is typically expressed as a RH.sub.O value at a given temperature. By way of example, the RH.sub.O of crystalline NaCl at 20 degrees Celsius (deg. C) is approximately 77% RHthat is, when ambient temperature is at 20 deg. C, water vapor from the atmosphere will spontaneously be adsorbed onto and solvate NaCl crystals when ambient RH is at or above 77%.

[0062] A material with a low RH.sub.O value tends to absorb moisture more readily and may serve well as desiccants. By contrast, a material with a high RH.sub.O tends to absorb moisture less readily. In the case of polymer compositions, having a higher RH.sub.O advantageously reduces requirements for secondary packaging, and provides a more structurally stable product when opened to an unprotected environment.

[0063] One issue with products comprising water soluble polymers is that they tend to be hygroscopic and structurally unstable in environments with a high RH. By way of example, at an ambient temperature of 20 deg. C, PVA begins to deliquesce when RH reaches above 50%.

[0064] One issue found with formulations comprising PVA, sodium chloride and glycerine as outlined above is that, although the lubricating effect of the glycerine and salt combination aids melt processing, the resultant mouldings are susceptible to moisture uptake, and secondary packaging is required with high moisture- and gas-barrier properties.

[0065] Various water-soluble polymer compositions comprising 88% hydrolyzed Polyvinyl alcohol (PVA) and NaCl were produced using one of three different solvent polymer plasticizers: glycerine, mono-propylene glycol and di-propylene glycol. Table 5, provided herein below, shows the formulation of nine different formulations, formulas 1-3 with glycerine, formulas 4-6 with mono-propylene glycol and formulas 7-9 with di-propylene glycol:

TABLE-US-00009 TABLE 5 Di-Propylene Mono-Propylene PVA PVA Glycol Glycol Glycerine NaCl Deliquiescent hydrolysis (% by (% by (% by (% by (% by Point (%) weight) weight) weight) weight) weight) (RH.sub.0 at 20 C.) 88.0 74.0 8.0 15.0 83% 88.0 62.0 10.0 25.0 82% 88.0 37.0 10.0 50.0 80% 88.0 74.0 8.0 15.0 77% 88.0 62.0 10.0 25.0 77% 88.0 37.0 10.0 50.0 76% 88.0 74.0 8.0 15.0 66% 88.0 62.0 10.0 25.0 66% 88.0 37.0 10.0 50.0 62%

[0066] As shown in Table 5, it was found that using propylene glycol as the solvent polymer plasticizer surprisingly resulted in a less water-absorbent and thus more stable PVA-based polymer product relative to comparable formulations using glycerine as the solvent polymer plasticizer. Whereas formulations 7-9 with glycerine demonstrated an RH.sub.O at 20 deg. C of 62% to 66%, all formulations with propylene glycol had a substantially higher RH.sub.O at 20 deg. C, between 76% and 83%. Moreover, formulations with di-propylene glycol were found to be even more stable than comparable formulations with mono-propylene glycol: formulations 1-3 with di-propylene glycol had an RH.sub.O at 20 deg. C of 80% to 83%, whereas formulations 4-6 with mono-propylene glycol had a lower RH.sub.O at 20 deg. C of 76% to 77%.

Example 7

[0067] Formulations involving di-propylene glycol and sodium chloride were made to compare melt-flow indexes

TABLE-US-00010 TABLE 6 PVA Di-Propylene hydrolysis PVA (% by Glycol (% by NaCl (% by MFI/g (%) weight) weight) weight) 10 minutes 88.0 74.0 8.0 15.0 10.0 88.0 69.0 8.0 20.0 19.0 88.0 64.0 8.0 25.0 20.0 88.0 57.0 10.0 30.0 33.0 88.0 52.0 10.0 35.0 33.0 88.0 47.0 10.0 50.0 34.0 88.0 42.0 10.0 45.0 40.0 88.0 45.0 12.0 50.0 44.0

[0068] Appreciable improvements in melt-flow index are demonstrated as the hygroscopic salt concentration increases.

[0069] The compositions of the present invention thus provide a melt-processable PVA containing polymer typically having a flexural modulus similar to other extrudable polymers. This enables a soluble and biodegradable polymer to be used for the processing of a wide variety of articles without the processing problems experienced in the prior art, such as thermal degradation and high temperature cross-linking. The known advantageous properties of PVA, such as its high tensile strength and good barrier characteristics, are retained in the melt-processable composition, which may be extruded on current extrusion lines, blow-moulders and injection moulders without modification.

[0070] There is therefore provided a method of making a water soluble polymer composition, the method comprising blending a water soluble polymer with at least 15% by weight of the total weight of the composition of a hygroscopic salt to act as a lubricant to render the polymer extrudable and/or mouldable and with a solvent polymer plasticizer, the hygroscopic salt being partially dissolved in the solvent polymer plasticizer, wherein the solvent polymer plasticizer is mono-propylene glycol, di-propylene glycol or a combination thereof, wherein the water content of the composition is of less than 10% by weight of the total weight of the composition.

[0071] Optionally, the solvent polymer plasticizer is di-propylene glycol.

[0072] Optionally, the hygroscopic salt is provided in a higher amount by weight than the solvent polymer plasticizer.

[0073] Optionally, the polymer comprises a polyvinyl alcohol polymer (PVA).

[0074] Optionally, 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 sulphate, sodium carbonate, potassium carbonate, and ammonium carbonate.

[0075] Optionally, the hygroscopic salt is a water-soluble salt which will dissolve at a rate of at least 10% by weight in 90 C. water in ten minutes.

[0076] Optionally, the hygroscopic salt has a water content of less than 10% by weight of the total weight of the composition.

[0077] Optionally, the hygroscopic salt is included in an amount of at least 25% by weight or at least 30% by weight of the total weight of the composition.

[0078] Optionally, the hygroscopic salt is in the anhydrous form.

[0079] Optionally, the method further comprises the step of extruding the water soluble polymer composition without using a die.

[0080] Optionally, the water soluble polymer and hygroscopic salt are provided in solid form.

[0081] There is also provided in an embodiment of the disclosure a melt-processable water soluble polymer composition comprising a blend of a water soluble polymer with a hygroscopic salt to act as a lubricant to render the polymer extrudable and/or mouldable, the composition further comprising a solvent polymer plasticizer selected from the group of mono-propylene glycol, di-propylene glycol, and a combination thereof, wherein: the water content of the composition is of less than 10% by weight of the total weight of the composition; and the hygroscopic salt is included in the composition in an amount of at least 15% by weight of the total weight of the composition.

[0082] Optionally, the solvent polymer plasticizer is di-propylene glycol.

[0083] Optionally, 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 sulphate, sodium carbonate, potassium carbonate, and ammonium carbonate.

[0084] Optionally, the hygroscopic salt is provided in a higher amount by weight than the solvent polymer plasticizer.

[0085] In the description and claims of the present application, each of the verbs, comprise, include and have, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb.

[0086] Descriptions of embodiments of the invention in the present application are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments, will occur to persons of the art. The scope of the invention is limited only by the claims.