Method for dewatering a web comprising microfibrillated cellulose and a film produced from the dewatered web
The present invention relates to a method for dewatering a web comprising microfibrillated cellulose, wherein the method comprises the steps of: providing a suspension comprising between 50 weight-% to 100 weight-% of microfibrillated cellulose based on total dry weight, forming a fibrous web of said suspension on a support wherein said web has a dry content of 1-25% by weight, applying a dewatering felt into direct contact with the fibrous web, conducting said fibrous web, arranged between said dewatering felt and said substrate, through a pressing equipment. The invention further relates to a film produced from said method.
1. A method for dewatering a web comprising microfibrillated cellulose, wherein the method comprises the steps of:
providing a suspension comprising between 50 weight-% to 100 weight-% of microfibrillated cellulose based on a total dry weight,
forming a fibrous web of said suspension on a support, wherein said fibrous web has a dry content of 1-25% by weight and wherein said support is a metal support wherein said fibrous web is formed by cast coating on the metal support so that a surface on the metal support is smooth, wherein the metal support is heated to a temperature between 30-150° C. before said fibrous web is applied to the support,
applying a dewatering felt into direct contact with said fibrous web after said fibrous web has been formed,
conducting said fibrous web, arranged between said dewatering felt and said support, through a pressing equipment to provide a dewatered web, and
drying the dewatered web to form a film which has an Oxygen Transmission Rate (OTR) value (23° C., 50% RH) below 400 cc/m 2 /24 h according to ASTM D-3985.
2. The method as claimed in claim 1 , wherein the pressing equipment comprises an extended nip.
3. The method according to claim 1 , wherein the pressing equipment is a belt press.
4. The method according to claim 3 , wherein the fibrous web is treated in the belt press for a distance of at least 20% of a diameter of a roll of the belt press.
5. The method according to claim 1 , wherein the dewatering felt is applied to the fibrous web at least 20 cm before being conducted through the pressing equipment.
6. The method according to claim 1 , wherein a pressure used in the pressing equipment is between 1-100 bar.
7. The method according to claim 1 , wherein a pressure used in the pressing equipment is gradually increased.
8. The method according to claim 1 , wherein said fibrous web has a dry content of 15-50 weight-% after dewatering in the pressing equipment.
9. The method according to claim 1 , wherein the microfibrillated cellulose of the suspension has a Schopper-Riegler (SR) value above 90.
10. The method according to claim 1 , wherein said fibrous web is conducted through the pressing equipment at a speed of at least 50 m/min.
11. The method according to claim 1 , wherein said fibrous web is heated before the dewatering felt is applied into contact.
12. A film comprising:
microfibrillated cellulose and obtained by the method of claim 1 , wherein said film has a density above 700 kg/m 3 and an Oxygen Transmission Rate (OTR) value (23° C., 50% RH) below 400 cc/m 2 /24 h according to ASTM D-3985.