Cellulose nonwoven fabric

1. A cellulose nonwoven fabric having a porosity of not less than 40% and not more than 99% which comprises not less than 50 wt % of cellulose fibers derived from natural cellulose having a maximum fiber diameter of 1,500 nm or less, a number average fiber diameter of 200 nm or less and a crystallinity determined by solid state NMR techniques of 60% or more wherein the nonwoven fabric has an average transmittance Tr,av of 0.70 or more, which is defined by the following formula (1) and determined by scanning a light with a wavelength of 850 nm perpendicularly to the nonwoven fabric immersed in toluene,

(wherein Tr,av is defined by the following formula (1) with a ratio between Tr,1, an average value of transmittances obtained by filling toluene in a test tube in a condition that the nonwoven fabric is stuck to an inside wall thereof, irradiating a light with a wavelength of 850 nm to the test tube in a direction perpendicular to the nonwoven fabric, and scanning each 40-μm length along the test tube in a lengthwise direction thereof to the total length of 30,000 μm (number of data: 750), and Tr,2, an average value of transmittances obtained by performing the same measurements except that the nonwoven fabric is removed and only toluene is placed in the test tube)

Tr,av=Tr, 1 /Tr, 2  (1),

wherein the dried nonwoven fabric has a sheet non-uniformity parameter H′ of 0.20 or less, which is defined by the following formula (3):

H′=T′r,sd/T′r,av   (3)

(wherein T′r,aV and T′r,sd, respectively, mean an average value and a standard deviation of all Tr values obtained by irradiating a light with a wavelength of 850 nm perpendicular to a dried sheet surface, and measuring Tr for each 40-μm length along the sheet surface in a linear direction thereof to the total length of 30,000 μm (number of data: 750)).

2. The cellulose nonwoven fabric according to claim 1 , wherein the average transmittance defined by the above formula (1) is 0.80 or more.

3. The cellulose nonwoven fabric according to either claim 1 or claim 2 , wherein the nonwoven fabric has a sheet uniformity parameter H of 0.040 or less, which is defined by the following formula (2),

(wherein H is defined by the following formula (2) with Tr,sd defined by a difference between Tr,sd 1 , a standard deviation of transmittances obtained by filling toluene in a test tube in a condition that the nonwoven fabric is stuck to an inside wall thereof, irradiating a light with a wavelength of 850 nm to the test tube in a direction perpendicular to the nonwoven fabric, and scanning each 40-μm length along the test tube in a lengthwise direction thereof to the total length of 30,000 μm (number of data: 750), and Tr,sd 2 , a standard deviation of transmittances obtained by performing the same measurements except that the nonwoven fabric is removed and only toluene is placed in the test tube, and Tr,av obtained by the above formula (1) from the measurements)

H=Tr,sd/Tr,av   (2)

wherein Tr,sd=Tr,sd 1 -Tr,sd 2 .

4. The cellulose nonwoven fabric according to either claim 1 or claim 2 , which comprises cellulose fibers having a nitrogen content of 0.4% by weight or less and a total organic carbon (TOC) content of 60 ppm or less, which is freely present in water 24 hours after 10 g of the fiber is immersed in 100 g of the water at 4° C.

5. The cellulose nonwoven fabric according to claim 4 , which comprises cellulose fibers having a chlorine content of 40 ppm or less.

6. The cellulose nonwoven fabric according to claim 4 , wherein the natural cellulose is a fine cellulose fiber derived from cotton.

7. The cellulose nonwoven fabric according to claim 4 , wherein the natural cellulose is a bacterial cellulose.

8. The cellulose nonwoven fabric according to claim 1 , wherein the ingredients other than the cellulose fiber are less than 20% by weight.