Carbon fiber precursor acrylic fiber bundle, method for thermally oxidizing part thereof, thermal oxidation oven, and process for producing carbon fiber bundle

1. A carbon fiber precursor acrylic fiber bundle comprising a plurality of high density parts as a portion thereof, wherein each high density part has a preceding density increase start point and a trailing density increase start point, wherein the high density parts satisfy requirements A and B:

A: the high density parts have a maximum fiber density ρ max of 1.33 g/cm 3 or higher, and

B: an increase in fiber density between an intermediate density point and a maximum density region arrival point is 1.3×10 −2 g/cm 3 or less per 10 mm of a fiber bundle length;

with the proviso that,

the term “intermediate density point” is defined as the site which has a density ρ m (=(ρ 0 +ρ max )/2) that is intermediate between the fiber density ρ 0 of a non-high density part and the maximum fiber density ρ max ,

the term “maximum density region arrival point” is defined as the site P r at which the increase in fiber density per 10 mm of the fiber bundle length becomes 1.0×10 −3 g/cm 3 or less, the increase in fiber density being represented by (ρ r+1 −ρ r )/5 determined from a measurement in which density measurement points beginning with the density increase start point and located at intervals of 50 mm (P 1 , P 2 , . . . , P r , P r+1 , . . . , P n ) are successively examined for fiber density (ρ 1 , ρ 2 , . . . , ρ r , . . . , ρ n ), and

the term “density increase start point” is defined as a site at which the fiber density is higher by 0.01 g/cm 3 than the fiber density ρ 0 of the non-high density part.

2. The carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the high density parts further satisfy requirement C:

C: the fiber density monotonously increases from the preceding density increase start point to the maximum density region arrival point.

3. The carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the high density parts further satisfy requirement D:

D: the increase in fiber density per 10 mm of the fiber bundle length is 2.0×10 −2 g/cm 3 or less from the density increase start points to the intermediate density points.

4. The carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the high density parts further satisfy requirement E:

E: a length of the part in which the fiber density is 1.33 g/cm 3 or higher is 50 mm or longer.

5. The carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the high density parts further satisfy requirement F:

F: a length from the density increase start points to the maximum density region arrival points is 150 mm or longer.

6. The carbon fiber precursor acrylic fiber bundle according to claim 1 obtained by

(1) arranging a part of a carbon fiber precursor acrylic fiber bundle inside a thermal oxidation oven having at least one opening and arranging the remaining part of the carbon fiber precursor acrylic fiber bundle outside the thermal oxidation oven,

(2) heating the part of the carbon fiber precursor acrylic fiber bundle arranged inside the thermal oxidation oven at a position corresponding to the longitudinal direction thereof, with hot wind at a high temperature and hot wind at a low temperature, in which part of the carbon fiber precursor acrylic fiber bundle present near at least one opening is heated by the hot wind at a low temperature,

wherein for the temperature of the hot wind at a high temperature, the maximum temperature from the start to the end of heating is in the temperature range of 200° C. to 300° C., and

the heating is performed until the maximum fiber density ρ max of the high density parts of the carbon fiber precursor acrylic fiber bundle reaches 1.33 g/cm 3 or higher.

7. The carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the high density parts have a maximum fiber density ρ max of 1.36 g/cm 3 or higher.

8. The carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the high density parts have a maximum fiber density ρ max of from 1.33 g/cm 3 to 1.42 g/cm 3 .

9. The carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the non-high density parts have a fiber density of from 1.18 g/cm 3 to less than 1.33 g/cm 3 .

10. The carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the high density parts comprise a plurality of entangled end-cut fibers.

11. The carbon fiber precursor acrylic fiber bundle according to claim 10 , wherein the length of entanglement of end-cut fibers is from 150 to 400 mm.

12. A boxed precursor acrylic fiber bundle comprising the carbon fiber precursor acrylic fiber bundle according to claim 1 , wherein the carbon fiber precursor acrylic fiber bundle has a first end, a second end and a length, wherein at least the length of the carbon fiber precursor acrylic fiber bundle is in a box.

13. A method for producing the carbon fiber precursor acrylic fiber bundle according to claim 1 , the method comprising

(1): arranging a part of a carbon fiber precursor acrylic fiber bundle inside a thermal oxidation oven having at least one opening and arranging the remaining part of the carbon fiber precursor acrylic fiber bundle outside the thermal oxidation oven,

(2): heating the part of the carbon fiber precursor acrylic fiber bundle arranged inside the thermal oxidation oven at a position corresponding to the longitudinal direction thereof, with hot wind at a high temperature and hot wind at a low temperature, in which part of the carbon fiber precursor acrylic fiber bundle present near at least one opening is heated by the hot wind at a low temperature,

wherein for the temperature of the hot wind at a high temperature, the maximum temperature from the start to the end of heating is in a range of from 200° C. to 300° C., and

heating is performed until the maximum fiber density ρ max of the high density parts of the carbon fiber precursor acrylic fiber bundle reaches 1.33 g/cm 3 or higher.

14. The method according to claim 13 , wherein the thermal oxidation oven has a nozzle for spraying hot wind to the carbon fiber precursor acrylic fiber bundle and a wind-blocking plate for blocking the hot wind and hot wind at a low temperature is formed by the wind-blocking plate.