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 which includes one or more plasticizers, wherein the polyvinyl alcohol polymer is a blend of two or more homopolymeric polymer grades having low and high respective molecular weights; each homopolymeric 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 polyvinyl alcohol polymeric film directly to the surface of the substrate; and allowing the molten polyvinyl alcohol polymeric film to solidify on the surface to form a coated material.

2. The method of claim 1, wherein the molten polyvinyl alcohol polymeric film is applied directly to the surface of the substrate without use of an adhesive or intermediate bonding layer.

3. The method of claim 1, wherein the polyvinyl alcohol polymer is formed by hydrolysis of polyvinyl acetate homopolymer.

4. The method of claim 1, wherein the polyvinyl alcohol polymer has a degree of hydrolysis of 93 wt % or greater.

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

6. The method of claim 1, wherein the one or more plasticizers are selected from the following plasticizers 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-,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; and (e) caprolactam, cyclic trimethylolpropane formal, rosin esters, euricamide, and mixtures thereof.

7. The method of claim 1, wherein the cellulosic substrate is paper, board or card.

8. The method of claim 1, 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 polymerization of 2,400 to 2,600.

9. The method of claim 1, wherein the polyvinyl alcohol polymer 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.

10. The method of claim 1, wherein a reduced viscosity is maintained when shear is applied to the molten polyvinyl alcohol polymer, the reduced viscosity being maintained if the shear is applied for a period up to one hour or longer.

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

12. The method of claim 1, wherein the polyvinyl alcohol polymer is pseudoplastic.

13. The method of claim 1, wherein the polyvinyl alcohol polymer has a degree of hydrolysis of 95 wt % or greater.

14. The method of claim 8, wherein the two or more polyvinyl alcohol polymer grades have the same hydrolysis degree (HD) value.

15. The method of claim 1, wherein the one or more plasticizers are diglycerol, triglycerol, xylose, D-mannitol, triacetin, dipentaerythritol, 1,4-butanediol, 3,3-dimethyl-,2-butanediol, caprolactam and mixtures thereof.

16. The method of claim 1, wherein the polyvinyl alcohol polymer dissolves in 10 minutes in water having a temperature from about 30 C. to about 60 C. and 100% of the undissolved cellulose fibers are released from the coated material.

17. The method of claim 16, wherein the coated material is processable in a conventional paper mill.

18. The method of claim 9, wherein the polyvinyl alcohol polymer includes sodium benzoate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a schematic diagram of an extrusion-coated apparatus in accordance with this invention; and

(3) FIG. 2 is a schematic diagram showing details of the extrusion head.

DETAILED DESCRIPTION

Example 1Polymer Compositions

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

(5) The composition was: dipentaerythritol 6-9%, preferably 7.5%; caprolactam 2-3%, preferably 2.5%; sodium benzoate 0.25%; polyvinyl alcohol balance to 100%.

(6) 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:

(7) TABLE-US-00001 zone 0 1 2 3 4 5 6 7 8 9 10 11 Die Temp. ( C.) x 10 10 75 200 235 250 250 250 230 230 225 225

(8) 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.

(9) 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.

(10) TABLE-US-00002 TABLE 1 Viscosity range Viscosity Viscosity Viscosity Shear rate ( [Pa .Math. s]) ( [Pa .Math. s]) ( [Pa .Math. s]) ( [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

(11) TABLE-US-00003 TABLE 2 Viscosity range Viscosity Viscosity Viscosity Shear rate ( [Pa .Math. s]) ( [Pa .Math. s]) ( [Pa .Math. s]) ( [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 2Properties of the Molten Polymer

(12) 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.

(13) Testing was performed at 210 C. using a 30 mm long die and 2 mm diameter, and entrance angle of 180 C.

(14) TABLE-US-00004 TABLE 3 Melt strength test results Elongation Speed at Force at Draw Stress at Break, Vb Break, Ratio at Break, (mm/s) F (N) Break () (MPa) 267.87 0.0266 7.195 0.13473

Example 3Coating of the Substrate

(15) 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).

(16) 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.

(17) 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.

(18) The extruder temperature profile was as follows:

(19) TABLE-US-00005 Zone 1 (inlet) Zone 2 Zone 3 Zone 4 Zone 5 70 C. 150 C. 185 C. 190 C. 200 C.

(20) The coating die has 4 heated zones, top, bottom and sides, with the following temperature settings:

(21) TABLE-US-00006 top bottom sides 205 C. 205 C. 210 C.

(22) 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.

(23) 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.

(24) 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.

(25) 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.

(26) 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.

(27) 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 4Solubility Test

(28) The solubility of a film or coating of a polyvinyl alcohol composition may be determined by the following method.

(29) A glass beaker was set up on a stirred hot plate with a calibrated thermometer.

(30) 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.

(31) 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.

(32) 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.

(33) 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.

(34) The measurement was repeated with two other replicate samples and fresh water.

(35) 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.

(36) The measurement was repeated with two replicate samples and fresh water.