Process for manufacturing a composite article comprising cellulose pulp fibers and a thermoplastic matrix

Abstract

The present invention relates to a process for manufacturing a composite article comprising cellulose pulp fibers and a thermoplastic matrix, wherein said process comprises the steps of: a) mixing a refined aqueous pulp suspension with a water suspension of thermoplastic fibers into a composition, b) forming the composition into a fiber web, c) dewatering the fiber web, d) drying the fiber web, and e) heating and pressing the dried fiber web from step d) to melt said thermoplastic fibers into a thermoplastic matrix and form a composite article.

Claims

1. A process for manufacturing a composite article comprising cellulose pulp fibers and a thermoplastic matrix, wherein said process comprises the steps of: a) mixing a refined aqueous pulp suspension with a water suspension of crimped non-refined thermoplastic fibers into a composition, b) forming the composition into a fiber web, c) dewatering the fiber web, d) drying the fiber web, and e) heating and pressing the dried fiber web from step d) to melt said crimped non-refined thermoplastic fibers into a thermoplastic matrix and form a composite article.

2. A process according to claim 1, wherein the aqueous pulp suspension has a concentration from 0.1 weight % to 4 weight %.

3. A process according to claim 1, wherein the pulp of the pulp suspension is wood pulp.

4. A process according to claim 1, wherein the pulp of the pulp suspension is never-dried wood pulp.

5. A process according to claim 4, wherein the never-dried wood pulp is never-dried softwood pulp.

6. A process according to claim 1, wherein the water suspension of the crimped non-refined thermoplastic fibers has concentration from 2 to 5 weight %.

7. A process according to claim 1, wherein the crimped non-refined thermoplastic fibers are fibers of a crimped non-refined thermoplastic selected from the group consisting of a polyolefin, a polyester, a polycarbonate, a polyvinyl and a copolymer or mixture thereof.

8. A process according to claim 7, wherein the crimped non-refined thermoplastic fibers comprises fibers of the polyolefin, wherein the polyolefin is selected from the group consisting of polyethylene and polypropylene.

9. A process according to claim 7, wherein the crimped non-refined thermoplastic fibers comprises fibers of the polyester, wherein the polyester is selected from the group consisting of polyhydroxybutyrate (PHB) and polylactic acid (PLA).

10. A process according to claim 1, wherein the crimped non-refined thermoplastic fibers are PLA fibers.

11. A composite article made by the process of claim 1.

12. A composite article according to claim 11, characterized in that said composite article has a formation of from 6 to 14, an air permeability from 10 to 250 m/Pas, and a wet strength equal to or above 2 Nm/g.

13. A composite article according to claim 11, wherein said composite article comprises from 5 to 70 weight % of the thermoplastic matrix.

14. A composite article according to claim 13, wherein said composite article comprises from 10 to 30 weight % of the thermoplastic matrix.

15. A composite article according to claim 11, wherein the thermoplastic matrix is a PLA matrix.

16. A composite article according to claim 11, wherein the composite article is present in a label, a billboard, a paper board, a fiber board, a furniture, a container, a food container, a laminate, a package or a composite.

17. A product comprising a composite article manufactured in accordance with claim 1.

18. A process for manufacturing a composite article comprising cellulose pulp fibers and a thermoplastic matrix, wherein said process comprises the steps of: a) mixing a refined aqueous pulp suspension with a water suspension of non-refined thermoplastic fibers into a composition, b) dewatering the composition into a composite material, c) drying the composite material, d) suspending the composite material in water, e) forming the composite material obtained from step d) into a fiber web and drying said fiber web, and f) heating and pressing the fiber web from step e) to melt said thermoplastic fibers into a thermoplastic matrix and form a composite article.

19. A process according to claim 18 further comprising a refining step between the step of suspending the dried composite material in water and the step of forming the composite material into a fiber web and drying said fiber web.

20. A process for manufacturing a composite article comprising cellulose pulp fibers and a thermoplastic matrix from a composite material that has been formed by the steps of: mixing a refined aqueous pulp suspension with a water suspension of non-refined thermoplastic fibers into a composition, dewatering the composition into a composite material, and drying the composite material, wherein said process comprises the steps of: a) suspending the dried composite material in water, b) forming the composite material obtained from step a) into a fiber web and drying said fiber web, and c) heating and pressing the fiber web from step b) to melt said thermoplastic fibers into a thermoplastic matrix and form a composite article.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The processes disclosed herein will be described in more detail with reference to the appended drawing wherein:

(2) FIG. 1 shows a process diagram of a process for manufacturing a composite article.

(3) FIG. 2 shows a further process diagram of a process for manufacturing a composite article.

(4) FIG. 3 shows the crimp angle of a fiber.

(5) FIG. 4 shows the properties of paper sheets produced by different refining procedures and different PLA concentrations.

(6) FIG. 5 shows the properties of paper sheets comprising crimped and non-crimped PLA fibers, respectively.

(7) It should be noted that the drawings have not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION

(8) The present process for manufacturing a composite article such as a paper or paper sheet comprising a thermoplastic matrix and cellulosic fibers as described herein will provide composite articles having appropriate formation, controlled air permeability, excellent mechanical properties and excellent wet strength.

(9) During the preparation of the composition (stock preparation), the refined cellulosic fibers and the thermoplastic fibers are mixed, pH may be adjusted and the obtained composition may be diluted with e.g. white water or water, to a concentration of 0.15%. After that the composition may be pumped to the headbox and the composition may be uniformly distributed at the forming wire. Optionally, the cellulose fibers may be never-dried wood pulp as described herein.

(10) The forming wire may be an endless cloth of polyurethane or similar material wherein the fiber suspension may be filtrated and the thermoplastic and cellulosic fibers may be retained at the top of the wire during the dewatering, while solvent/white water passes through the permeable cloth web. Dewatering occurs by gravity and vacuum. The solids content of the wet web may be about 20% prior to being transferred to the press felt and enters the pressing section. Ultra sound equipment may be used to improve the formation and dewatering of the wet web.

(11) The pressing section may comprise one or more presses, such as, but not limited to, roll and/or shoe presses. The main task of press section may be to increase the solids content and thereby reduce the required drying energy. Dewatering may occur by mechanical impact on the wet web and the water may be pressed out to the press felt. The drying section may comprise of a few hundred meters long drying path through a drier. Drying may be performed by hot air (150 C.), which may be heated by hot steam. The drier may comprise several levels, wherein the wet web may be transferred above the plates and hot air may be blown through narrow nozzles. Alternatively, the drying may be conducted by several hot-surfaced rolls whereby the paper web is dried by conductive heating.

(12) After the drying step, the temperature of the obtained web may be reduced and the web may be cut into individual sheets, which may be piled up to stacks until final height of the stack has been achieved. In a following step, the stacks are pressed and bales may be produced. Alternatively to sheet, the dried composition may be transferred into small rolls.

(13) One alternative process for providing the composition comprising fibers and cellulosic fibers to be used in the composite article comprising a thermoplastic matrix and cellulosic fibers as defined herein is to use flash drying. The composition (the stock) will be dewatered and pressed. The solids content may thereby be increased and the composition as defined hereinabove or hereinafter may be shredded and flakes may be produced. These flakes may then be transferred to a flash dryer by airflow wherein the pulp flakes may be dried in several steps. After drying, the composition flakes may enter the first press by using screw feeders wherein bales may be formed. Thereafter, the bales may be taken to a second press and thereafter packaged.

(14) FIG. 1 shows a process for manufacturing an activated paper sheet. The composition comprising thermoplastic fibers and cellulosic fibers, which may be in the form of e.g. a sheet or a bale, is then suspended in water or other suitable solvent. The obtained suspension has a temperature of about 30 C. and a concentration of about 4 weight %, such as from 2 to 5 weight %. Additional process chemicals and fillers, such as kaolin and chalk may be added to the obtained suspension and also hydrophobic agent and other polymers may be added to increase the strength of the sheet (such as starch and CMC). Furthermore, pigments and defoaming agents and also retention agents may be added. The suspension may be purified using sieves. Solvent, such as water, may then be added to the suspension, which will have a concentration of from 0.7 to 1.5 weight %. The composition is then sprayed on the paper wire. The thickness of the paper sheet may be controlled by controlling the speed of the wire. The paper sheet may then be dewatered on a flat wire and may leave the wire with a solid content of 25%.

(15) The heat treatment may be performed by hot pressing and may be performed on dry as well as wet paper sheets. If the pressing is performed between two hot plates, then the paper sheets have to be dried in an oven until the paper sheet is completely dry, e.g. during 15 minutes at about 105 C., this in order to avoid steam expansion and delamination in the paper sheet. Hot pressing of wet paper sheets may be performed between two steam permeable wires which will allow steam to be released from the paper sheet without causing rupturing of the web by delamination. Additionally, the press level may be adjusted and may vary during the process cycle in order to release steam and protecting structure of the sheet.

(16) Non-limiting examples of conditions for hot pressing of one (1) dry paper sheet incorporating thermoplastic fibers with a melting point of about 170 C. are listed below.

(17) Pressing temperature: 1805 C.

(18) Pressing load: >100 N/cm.sup.2

(19) Pressing time: 1 minutes

(20) If the number of paper sheets increases or if the basis weight of the paper sheets increases, then the pressing time has to be adjusted. For a stack containing multiple layers, e.g. 20 dry paper sheets with a basis weight of 240 g/m.sup.2, the pressing time needs to be longer than 2 minutes, such as 7 minutes. The pressing time depends on the pressing device used and on the heat conduction from the hot plate to the web and also on the heat transfer within the paper sheet and on the composition of the paper sheet. When the pressing cycle is due, the hot pressed paper sheets are cooled down. Optionally, the paper sheets may be stacked together in wet condition, hence using steam permeable wires that allow evacuation of steam from the composite article.

(21) Paper sheets with higher moisture content require longer pressing time. Pressing sequences with different pressure levels and pressing temperatures might be used in order to reduce the pressing time or to achieve particular sheet properties.

(22) Crimp Measurement:

(23) The equipment used were: Ruler (mm), Tweezers (2 pair), Specimen Board (velveteen covered board), Magnifier with light, Tape Ruler (mm) on black velveteen board, Crimp Conversion Chart.

(24) The procedure was the following: 10 clumps of fibers from samples were randomly selected and placed on black/white specimen board. Tweezers were used for removing 1 fiber from a clump of fiber. The number of primary crimp in fiber was counted and recorded. Using same filament, one tweezers was placed on zero and carefully measured and the relaxed length was recorded. This was a relaxed stage with all the primary crimp intact, secondary crimp was removed. Using same filament, one end of filament was hold with tweezers on zero and the other end was carefully extended with the tweezers in order to remove all crimp without stretching filament. The extended length was measured to the nearest mm.

(25) The calculation was performed accordingly:

(26) Staple length or extended length: The sum of extended lengths was divided by number of results to get average extended length or average staple length (convert to inches by dividing average extended length (mm) by 25.4)

(27) Crimp per inch: The average of crimps was divided by average extended length

(28) Crimp angle: The average of relaxed lengths was divided by (mm) average of extended lengths (mm) and a crimp angle table was used to get crimp angle.

(29) FIG. 3 shows the waviness of a fiber. The fiber is depicted as continuous line. The fiber has a crimp angle .

(30) FIG. 4 shows the properties of paper sheets manufactured by three different refining procedures and three different PLA concentrations. The physical testing was performed as described in Example 2 below. The formation was measured as described in this document.

(31) The three different PLA concentrations refer to aqueous suspensions of PLA having a concentration of 10 weight %, 20 weight % and 30 weight %. The procedure denominated Pulp mill refers to the trial where the refining was performed only on cellulosic pulp during the production of the pulp/PLA mixture, while Paper mill refers to the trial where the refining was performed only on the pulp/PLA mixture.

(32) Pulp Mill:

(33) The procedure denominated Pulp mill refers to refining of never-dried softwood kraft pulp at a concentration of 4 weight % after which it is mixed with an aqueous solution of PLA fibers.

(34) Dry 4 mm PLA fibres were used and diluted in the water where 4% suspension of PLA fibres was produced. Similarly the 4% pulp suspension was produced of a never dried cellulose pulp. The refining was performed only with cellulose pulp at the laboratory refiner LR 40. Energy input for refining was 100 kWh/t.

(35) The suspensions were mixed in a blender and dewatered by centrifugation. The whole wet mixture was first transformed to form approximately 1 cm wide flings which were stored in a large drying basket so that the layer of fling was 5 cm high. The basket with fling-pulp was then placed in a drying chamber and dried. Drying of fling pulp in the drying chamber was performed with the hot air at 90 C. and 50% Rh. The hot air was circulated within the drying chamber and the temperature and the Rh were controlled automatically. The hot air was blown from underside of the drying chamber, passing through the basket and wet pulp/PLA flings. The drying was ongoing until the moisture content of the pulp/PLA mixture was reduced to 10%. After drying the pulp/PLA mixture was disintegrated in the water to consistency of 4% and the wet paper sheets were produced at Finnish sheet former according standard procedure described in SCAN-CM 26:99. The basis weight of produced paper sheets with dimension 16.516.5 cm were 100 g/m.sup.2. The wet paper sheets were pressed between two blotters to a solids content about 40-50% depending on refining and swelling. Wet pressed paper sheets were dried restrained at 23 C. and 50% Rh 48 hours and after that prepared for hot pressing. Prior the hot pressing the paper sheets were dried 15 minutes in an oven at 105 C. After this drying the sheets were pressed in 7 minutes in a plane press at a temperature of 180 C. and pressure of 200 N/cm.sup.2. The activated paper sheets with melted PLA were cold down and left in a testlab at 23 C. and 50% Rh until they were analysed.

(36) Paper Mill:

(37) Dry 4 mm PLA fibres was used and diluted in water there 4% suspension of PLA fibres were produced. Similarly the 4% pulp suspension was produced of a never-dried cellulose pulp.

(38) The suspensions were mixed in a blender and dewatered by centrifugation. The whole wet mixture was first transformed to form approximately 1 cm wide flings which were stored in a large drying basket so that the layer of fling was 5 cm high. The basket with fling-pulp was then placed in a drying chamber and dried. Drying of fling pulp in the drying chamber was performed with the hot air at 90 C. and 50% Rh. The hot air was circulated within the drying chamber and the temperature and the Rh were controlled automatically.

(39) The hot air was blown from underside of the drying chamber, passing through the basket and wet pulp/PLA flings. The drying was ongoing until the moisture content of the pulp/PLA mixture was reduced to 10%.

(40) After drying the pulp/PLA mixture was disintegrated in the water to consistency of 4% and the whole mixture was refined at the laboratory refiner LR 40. Energy input for refining was 100 kWh/t.

(41) After refining the PLA/pulp suspension was diluted and the wet paper sheets were produced at Finnish sheet former according standard procedure described in SCAN-CM 26:99. The basis weight of produced paper sheets with dimension 16.516.5 cm were 100 g/m.sup.2. The wet paper sheets were pressed between two blotters to a solids content about 40-50% depending on refining and swelling. Wet pressed paper sheets were dried restrained at 23 C. and 50% Rh 48 hours and after that prepared for hot pressing. Prior the hot pressing the paper sheets were dried 15 minutes in a oven at 105 C. After this drying the sheets were pressed in 7 minutes in a plane press at a temperature of 180 C. and pressure of 200 N/cm.sup.2. The activated paper sheets with melted PLA were cooled down and left in a testlab at 23 C. and 50% Rh until they were analysed.

(42) The process to be used will depend on the desired properties of the paper sheet. For instance, when high wet strength is desired the Pulp mill procedure may be used in combination with a PLA concentration of 10 weight %.

(43) FIG. 5 shows the properties of a paper sheet manufactured in accordance with Example 2 described herein. Crimped and non-crimped fibers were used, respectively. Contrary to expectation, the paper sheet comprising crimped PLA fibers exhibited substantially the same properties as the paper sheet comprising non-crimped, i.e. straight, PLA fibers.

(44) Further Aspects

(45) Also the present disclosure relates to a process for manufacturing a paper sheet comprising a thermoplastic matrix and cellulose pulp fibers and to paper sheets manufactured by said process. The process comprises the following steps: a) refining pulp; b) mixing the refined pulp from step a) with non-refined thermoplastic fibers; c) dewatering the obtained composition from step b); d) drying the obtained composition from step c); e) suspending the obtained composition from step d) in a solvent; f) forming the composition obtained from step e) into a paper sheet and drying; and g) heating the obtained paper sheet from step f) to the melting temperature of the thermoplastic fiber.

(46) The paper sheet obtainable by the process as defined hereinabove or hereinafter has adequate formation, wet strength and controlled air permeability. It has surprisingly been found that the magnitude of these properties may be determined by selecting the degree of refining. Additionally, it has surprisingly been found that the homogeneity of the composition as defined hereinabove or hereinafter is better if the crimp angle is high and crimp number is low.

(47) Furthermore, in the process for manufacturing a paper sheet comprising a thermoplastic matrix and cellulose pulp fibers as defined hereinabove or hereinafter, step d) may comprise both drying the composition obtained from step c) or forming the composition obtained from step c) into a sheet and drying said sheet. The solvent used in step e) may be selected from water. According to the present disclosure, it is also possible to dry the composition as flakes and these flakes are then formed into a bale.

(48) The process for manufacturing a paper sheet comprising a thermoplastic matrix and cellulose pulp fibers as defined hereinabove or hereinafter may comprise an additional step which is that the composition obtained from e) is refined before it is formed into a paper sheet and dried. Optionally, step g) comprise that the paper sheet obtained from step f) may be heated and pressed at the same time.

(49) Additionally, the thermoplastic fibers may have a length of from 2 to 6 mm and said a crimp angle which is 98 or more.

(50) The present invention is further illustrated by the following non-limiting examples.

EXAMPLES

Abbreviations

(51) kg kilogram m.sup.3 cubic meter rpm revolutions per minute kWh kilowatt hour mm millimeter J/m joule/meter LDPE low-density polyethylene LLDPE linear low-density polyethylene MDPE medium-density polyethylene HDPE high-density polyethylene TEA tensile energy adsorption TSI tensile stiffness index Nm/g Newton meter per gram mNm.sup.2/g milli Newton square meter per gram m/Pas micro meter per Pascal second kPa kilopascal kJ/kg kilo Joule per kilogram MNm/kg mega Newton meter per kilogram Log 10 decimal logarithm dtex linear mass density of a textile fibre, which is defined as the mass in grams per 10 000 meters CTMP Chemi-Thermo-Pechanical Pulp TMP thermomechanical pulp Rh relative humidity RH relative humidity

Example 1Manufacture of a Paper Sheet

(52) PLA fibers denominated Trevira D260 were purchased from Trevira GmbH, Hattersheim, Germany.

(53) Never-dried softwood kraft pulp was washed and refined in a Voith LR40 disk refiner at a concentration of 4 weight %. Specific refining energy was ranged from 0 to 150 kWh/ton and specific edge load was 2 J/m. After refining, the pulp slurry was mixed with from 0 to 50% PLA fibers (0 means that the slurry comprised cellulosic fibers only and this sample was used as reference sample) and dried at 90 C. and 50% RH until a 90% solid content was obtained. In this document, RH or Rh stands for relative humidity. After drying, small flake bales were made by compressing the dry composition comprising cellulosic fibers and coated PLA fibers into a steel mold (1020 cm) having in the 14-ton press and the latter with 35-ton press.

(54) The bale was dissolved in water and refined again in a Voith LR40 disk refiner at a concentration of 4 weight %. Prior to the refining, the pulp was disintegrated by circulating for 10 minutes with no load. The refining was adjusted to a specific refining energy of from 0 to 200 kWh/ton and the specific edge load was 2 J/m.

(55) All paper sheets (pulp mill and paper mill refined) were made in a Finnish sheet former according to ISO 5269-1. Before the paper sheets were produced, the composition comprising the cellulosic fibers and coated PLA fibers was disintegrated in a pulper at 30 000 rpm at a concentration of 3%. After pulping, the composition was diluted and the appropriate amount was added to the sheet form, wherein wet sheets (16.516.5 cm) having a basis weight of 100 g/m.sup.2 were produced. The wet sheets were covered and removed from the paper wire. After the forming, the wet sheets were pressed in two rounds of 400 kPa (5.2 and 2.2 minutes) between blotters that were exchanged to dry blotters between pressings. The solids content of wet paper sheet after pressing varied and was from 50 to 60 weight %. These paper sheets were dried for at least 3 days in a conditioned room at 23 C. and 50% RH.

(56) After drying at room conditions, the paper sheets were dried in an oven in 15 minutes at 105 C. followed by hot pressing in a plane press at temperature 180 C. and pressure 200 N/cm.sup.2. The obtained paper sheets comprising cellulosic fibers and a thermoplastic matrix were cooled in room temperature.

(57) Paper sheets comprising cellulose pulp fibers and polylactide (PLA) matrix were used for physical testing.

(58) Physical testing was conducted according to standard procedures for paper testing. The thickness was determined according to the standard EN20534. Measurements were carried out with a precision micrometer with a static pressure of 100 kPa and on a surface of 200 mm.sup.2. With these data and the sample weight and surface density was calculated, and the paper grammage. Tear strength was made with Lorentzen & Wettre tester in accordance with standard method EN 21 974 which used samples 6240 mm. Tensile strength (tensile index, TSI, TEA, elongation at break (stretch)) in accordance with ISO 1924-3, within 15 mm paper strips were analyzed by strain rate 100 mm/min and the distance between the clamps was 100 mm. The wet strength measurements were made according to EN 12625-5 with 10 strips and a Roel Zwick tensile tester. Soaking the samples was performed during 15 sec, elongation was 50 mm/min and the distance between the clamp and the rod was 50 mm. The air permeability was determined in accordance with ISO 5636-5 (Gurly). Water retention value WRV, was measured according to SCAN-C 62:00, wherein 2 g of bone dry pulp were centrifuged for 10 minutes in 10 000 rpm. Z-strength was analyzed according to SCAN P90. Folding strength and folding endurance (Khler-Molin) were analyzed according to ISO 5626:1993 and beta formation according to the FA 11 701.