Polyvinyl Alcohol Coated Cellulosic Products

Abstract

A method of manufacture of a coated material is provided. A polyvinyl alcohol polymer, where the polyvinyl alcohol polymer has a degree of hydrolysis of 90% or greater and a melting point in the range of 180° C. to 225° C. is melted on a cellulosic substrate and extruded to form a molten polyvinyl alcohol polymeric film which is then applied directly to the surface of the substrate to form the coated material.

Claims

1. A method of manufacture of a coated material comprising the steps of: providing a cellulosic substrate; melting a polyvinyl alcohol polymer, wherein the polyvinyl alcohol polymer has a degree of hydrolysis of 90 wt % or greater and a melting point in the range of 200° C. to 225° C.; extruding the melted polyvinyl alcohol polymer to form a molten polyvinyl alcohol polymeric film; applying the molten film directly to the surface of the substrate; and allowing the film to solidify on the surface to form a coated material.

2. A method as claimed in claim 1, wherein the molten film is applied directly to the surface of the substrate without use of an adhesive or intermediate bonding layer.

3. A method as claimed in claim 1, wherein the polyvinyl alcohol polymer is formed by hydrolysis of polyvinyl acetate homopolymer.

4. A method as claimed in claim 1, wherein the polyvinyl alcohol has a degree of hydrolysis of 93 wt % or greater, preferably 95 wt % or greater.

5. A method as claimed in claim 4, wherein the degree of hydrolysis is in the range of 93 wt % to 96 wt %.

6. A method as claimed in claim 1, wherein the polyvinyl alcohol polymer includes one or more plasticizer selected from the following compounds and mixtures thereof: (a) sugar alcohols selected from the group consisting of: diglycerol, triglycerol, fructose, ribose, xylose, D-mannitol, triacetin, and mixtures thereof; (b) polyols selected from the group consisting of: pentaerythritol, dipentaerythritol, and mixtures thereof; (c) diols selected from the group consisting of: methyl pentanediol, 1,2-propanediol, 1,4-butanediol, 2-hydroxy-1,3-propanediol, 3-methyl-1,3-butanediol, 3,3-dimethyl-1,2-butanediol, and mixtures thereof; (d) glycols selected from the group consisting of: polyethylene glycol 300, polyethylene glycol 400, alkoxylated polyethylene glycol, and mixtures thereof; (e) caprolactam, cyclic trimethylolpropane formal, rosin esters, euricamide, and mixtures thereof.

7. A method as claimed in claim 1, wherein the cellulosic material is paper, board or card.

8. A method as claimed in claim 1, wherein the polymer comprises a blend of two or more polyvinyl alcohol polymers.

9. A method as claimed in claim 1, wherein the two or more polyvinyl alcohol polymers have the same hydrolysis degree (HD) value.

10. A method as claimed in claim 8, wherein the blend comprises a first grade having a molecular weight in the range of 13,000 to 27,000 and a degree of polymerization of 300 to 600 and a second grade having a molecular weight in the range of 107,000 to 120,000 and a degree of polymerisation of 2,400 to 2,600.

11. A method as claimed in claim 1, wherein the polymer composition includes one or more reactive stabilizers selected from the group consisting of: calcium stearate, stearic acid, sodium stearate, potassium oleate, potassium sorbate, sodium benzoate and mixtures thereof.

12. A method as claimed in claim 1, wherein a reduced viscosity is maintained when shear is applied to the molten polymer, the reduced viscosity being maintained if the shear is applied for a period up to one hour.

13. method as claimed in claim 1, wherein the viscosity of the molten polymer decreases with increasing shear rate from a viscosity at a shear rate of 80 l/s to a viscosity at a shear rate of 1000 l/s, the viscosities being measured at steady states following application of shear to the polymer for up to one hour.

14. A method as claimed in claim 1, wherein the polymer is pseudoplastic.

15. A coated material comprising a cellulosic substrate directly coated with a polyvinyl alcohol polymer, wherein the polymer has a degree of hydrolysis of 90 wt % or greater and a melting point of 180° C. to 225° C.; and wherein the polymer dissolves within 10 minutes in water at a temperature from about 30° C. to about 60° C. to allow release of 100% cellulosic fibres from the coated substrate.

16. A coated material as claimed in claim 15, where the temperature is from about 35° C. to about 45° C., preferably about 40° C.

17. A coated material as claimed in claim 15, comprising: a cellulosic substrate; and a coating of polyvinyl alcohol polymer applied to the substrate; wherein the polyvinyl alcohol polymer comprises plasticized polyvinyl alcohol having a degree of hydrolysis of about 93 wt % to about 98 wt %.

18. A coated material as claimed in claim 12, wherein the degree of hydrolysis is about 93 wt % to about 95 wt %.

19. A coated material as claimed in claim 12, wherein the plasticized polymer includes a plasticizer selected from the group consisting of: selected from the following compounds and mixtures thereof: (a) sugar alcohols selected from the group consisting of: diglycerol, triglycerol, fructose, ribose, xylose, D-mannitol, triacetin, and mixtures thereof; (b) polyols selected from the group consisting of: pentaerythritol, dipentaerythritol, and mixtures thereof; (c) diols selected from the group consisting of: methyl pentanediol, 1,2-propanediol, 1,4-butanediol, 2-hydroxy-1,3-propanediol, 3-methyl-1,3-butanediol, 3,3-dimethyl-1,2-butanediol, and mixtures thereof; (d) glycols selected from the group consisting of: polyethylene glycol 300, polyethylene glycol 400, alkoxylated polyethylene glycol, and mixtures thereof; (e) caprolactam, cyclic trimethylolpropane formal, rosin esters, euricamide, and mixtures thereof.

20. A coated material as claimed in claim 12, wherein the amount of the plasticizer is from 1 wt % to 10 wt %.

21. A coated material as claimed in claim 12, wherein the plasticizer includes glycerol.

22. A coated material as claimed in claim 12, further comprising a reactive stabilizer selected from the group consisting of: calcium stearate, stearic acid, sodium stearate, potassium oleate, potassium sorbate, sodium benzoate, and mixtures thereof.

23. A coated material as prepared by the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0089] This invention is further described by means of example but not in any limitative sense with reference to the accompanying drawing, of which:

[0090] FIG. 1 is a schematic diagram of an extrusion-coated apparatus in accordance with this invention; and

[0091] FIG. 2 is a schematic diagram showing details of the extrusion head.

DETAILED DESCRIPTION

EXAMPLE 1

Polymer Compositions

[0092] The formulation used in an extrusion coating run comprised a blend of two polyvinyl alcohol (PVOH) polymers having different degrees of hydrolysis and viscosity.

[0093] PVOH grade 1—14.31% (HD 91-93 wt % with viscosity in 4 wt % water solution at 20° C. of 14.5-19.5 cP)

[0094] PVOH grade 2—57.26% (HD 87-89 wt % with viscosity in 4 wt % water solution at 20° C. of 3.5-4.5 cP)

[0095] The composition was:

TABLE-US-00001 dipentaerythritol 6-9%, preferably 7.5%; caprolactam 2-3%, preferably 2.5%; sodium benzoate 0.25%; polyvinyl alcohol balance to 100%.

[0096] The extruder used to prepare the polymer formulation was a Zeppelin RHC 25 twin screw extruder with L:D 56:1; vented at zones 7 and 10. The gross feed rate to the extruder was 5.0 Kg/h and the torque generated was 19+/−4.0%. The extruder was fitted with a strand pelletizer at the die exit. The extrusion temperature profile settings were as follows:

TABLE-US-00002 zone 0 1 2 3 4 5 6 7 8 9 10 11 Die Temp. x 10 10 75 200 235 250 250 250 230 230 225 225 (° C.)

[0097] Samples of the pellets produced from the run were characterised by DSC, TGA, capillary rheometry and other laboratory methods. A key test was that the pellets produced good coatings using standard commercial extrusion coating methods.

[0098] The viscosity of the polymeric material was determined on a Dynisco LRC 7000 Series capillary rheometer at three temperatures and different shear rates. The polymer was allowed to equilibrate for 1 hour to reach a steady state at the specified shear rate before the viscosity was measured.

TABLE-US-00003 TABLE 1 Viscosity range Viscosity Viscosity Viscosity Shear rate (η [Pa .Math. s]) (η [Pa .Math. s]) (η [Pa .Math. s]) (γ [1/s]) @200° C. @210° C. @220° C. 80 500-900 300-700 200-600  150 300-800 200-600 100-500  250 250-700 150-500 80-450 400 200-500 100-400 70-400 650 150-400  90-300 60-300 1000  90-350  70-250 50-250 3000  60-200  40-200 30-150 5000  40-150  30-150 20-100 8000  10-100  10-100 10-80 

TABLE-US-00004 TABLE 2 Viscosity values Viscosity Viscosity Viscosity Shear rate (η [Pa .Math. s]) (η [Pa .Math. s]) (η [Pa .Math. s]) (γ [1/s]) @200° C. @210° C. @220° C. 80 700 500 380 150 550 400 300 250 450 350 250 400 350 250 200 650 250 200 150 1000 200 150 120 3000 100 80 70 5000 70 60 50 8000 50 45 35

EXAMPLE 2

Properties of the Molten Polymer

[0099] Melt strength testing of the polyvinyl alcohol polymer was performed on a RHEOTENS 71.97 extensional rheometer in combination with a Goettfert Rheograph 20 Model Capillary Rheometer using a 2000 bar transducer. The material equilibrated in the test barrel for 2 minutes before testing started.

[0100] Testing was performed at 210° C. using a 30 mm long die and 2 mm diameter, and entrance angle of 180° C.

TABLE-US-00005 TABLE 3 Melt strength test results Elongation Speed at Stress at Break, Force at Draw Ratio at Break, σ Vb (mm/s) Break, F (N) Break (λ) (MPa) 267.87 0.0266 7.195 0.13473

EXAMPLE 3

Coating of the Substrate

[0101] The apparatus shown in FIG. 1 comprises a drive unit (1) arranged to drive a single screw extruder, the screw having a diameter of 25 mm and a length/diameter (L/D) ratio of 30. A gravity feed hopper (2) serves as an inlet into the extruder of pellets of the polyvinyl alcohol feed stock. A slot die (4) provides a 200 mm wide film of molten polymer. A web path is provided by rolls (5) to support the cellulosic substrate during application of the polyvinyl alcohol coating. Uncoated substrate (6) is supplied from the unwound roll (9) and passes over an idler roll (8) to a nip roll assembly (5) at the coating station. The coated substrate (7) passes over an idler roll (11) to rewind roll (10).

[0102] FIG. 2 is an enlarged partial view showing detail of the extrusion head (4) and nip roll assembly (5). Uncoated substrate (12) passes over the middle roll beneath the slot of die (4). A curtain of molten polyvinyl alcohol polymer (13) passes vertically from the die into contact with the surface of the substrate to form a coated substrate (7). The temperature of the molten polymer (13) when it leaves the die may be in the range 180° C. to 225° C. and the temperature of the polymer when it contacts the substrate (6) is greater than 70° C.

[0103] The extruder barrel (3) has five heated zones and a heated adapter nozzle where polymer is transported due to back pressure from the screw into the back of the coating dye.

[0104] The extruder temperature profile was as follows:

TABLE-US-00006 Zone 1 (inlet) Zone 2 Zone 3 Zone 4 Zone 5 70° C. 150° C. 185° C. 190° C. 200° C.

[0105] The coating die has 4 heated zones, top, bottom and sides, with the following temperature settings:

TABLE-US-00007 top bottom sides 205° C. 205° C. 210° C.

[0106] The cellulosic substrate was a clay coated kraft paper having a basis weight of 45 g/m.sup.2. The paper was coated with a layer of polyvinyl alcohol polymer composition as described in example 2, having a basis weight of 20 g/m.sup.2. The temperature of the polymer melt was 202° C., the rotational screw speed was 30 rpm and the pressure was 5,000 to 8,000 kPa (50 to 80 bar). The line speed was set to 10 m/min.

[0107] Polymer pellets were fed into the extruder via the gravity fed hopper. The polymer material was fed directly into the screw and was transported along the length of the barrel where it was subjected to heat and pressure to form molten polymer.

[0108] The molten polyvinyl alcohol was then forced by the extruder screw through the narrow slit of the extrusion coating die. The slit is linear. The melt emerged as a thin molten film.

[0109] A variety of coating dies with different internal geometries may be used. The die used in this example has a so called “coat hanger” geometry. The coat hanger name refers to the internal shape of the flow path which allows polymer to enter from a single point and is then distributed evenly along the width of the die. The shape and profile of the die varies in depth so as to maintain an even parallel flow of material.

[0110] The thickness of the molten polymer resin was drawn down from the die gap thickness of (details) to the coating thickness of (detail) by the nip roll assembly.

[0111] At the point of coating, the polymer was dropped vertically into a nip point between a chill roll and a backing or pressure roll. The paper substrate was fed over the backing roll and around the underside of the chill roll. When the polymer contacts the paper, it was bonded through heat and pressure applied by the nip. The chill roll ensured crystallisation of the polymer so that it would not stick to any rolls or to itself once on the rewind roll where the composite polyvinyl alcohol coated paper was collected.

EXAMPLE 4

Solubility Test

[0112] The solubility of a film or coating of a polyvinyl alcohol composition may be determined by the following method.

[0113] A glass beaker was set up on a stirred hot plate with a calibrated thermometer.

[0114] Six slide frames were split into upper and lower faces. One was used as a template to cut six replicate samples from a sample of polymer composition film having a thickness of 25-30 microns. The sample was sealed between the upper and lower slide frames and the frames were snapped closed.

[0115] Water (400 g) was placed in the beaker with a magnetic stirrer bar and placed on the hot plate. The stirrer rate was adjusted to give a vortex of 80% and the water was heated to the required temperature.

[0116] The framed sample was clamped and placed in the heated water and a timer was started. The time of breakdown of the film was recorded.

[0117] The framed film sample was left in the water until the film had completely dissolved and there were no remaining visible particles. The time was recorded.

[0118] The measurement was repeated with two other replicate samples and fresh water.

[0119] For measuring the time and temperate at which the film is no longer soluble, a framed film sample was clamped below the water line and any changes in the structure of the film were observed. The time of any film breakdown was recorded.

[0120] The measurement was repeated with two replicate samples and fresh water.