Formulation design of NASC

The uniform design method can be used to further decrease test number in formulation optimization. The approximate formulation of composites can be obtained by uniform design, but the principal and subordinate factors which affect material performances cannot be analyzed by the design table. Furthermore, the regression design can also be adopted to optimize the formulation of NASC. In the mixing regression design, a small quantity of test points will be selected to obtain enough experimental data. According to these data, the regression equation can be derived from the relationship between the test index and the different constituent contents. Finally, the optimization point can be obtained.

1.2.1 Formulation design of aramid fiber reinforced NASC

The formulation design of the aramid fiber reinforced NASC can be defined as the design problem with three components which include reinforcing fiber (aramid fiber), elastic binding material (NBR/NR) and filling material (compatibilization fiber). In consideration of the attribute for processing of the material, the content of components in NASC is limited by Eq. (1).

7% < x < 22%, 40% < x2 < 55%, 10% < x3 < 25% (1)

where x1, x2 and x3 are the weight contents of aramid fiber, compatibilization fiber (brucite fiber or sepiolite fiber) and elastic biding material, respectively.

The contents of other ingredients, such as accessory ingredient, can be determined according to the contents of three components mentioned above. By selecting 5 factor levels for each component, the uniform design scheme for test points is listed in Table 1.

Test points

x1 (wt%)

x2 (wt%)

x3 (wt%)

7

43

19

®

10

49

25

®

13

55

17

®

16

40

23

19

46

15

©

22

52

21

Table 1. Experiment scheme according to the uniform design method

According to the test points in Table 1, aramid fiber reinforced materials were developed by the molding process. It can be seen in Table 2 that the transverse tensile strength of the materials developed according to, and test points meets the strength requirement (>7.0 MPa). The strength of the materials prepared according to and test points are obviously higher than that of scheme. Too much reinforcing fiber in the material of formulation and led to the poor processing manufacturability and low production efficiency, and therefore, formulation was chosen to be the preliminary scheme.

Test points

Transverse tensile strength (MPa)

3.55

®

5.26

®

8.81

®

6.75

®

10.69

©

10.87

Table 2. Transverse tensile strength of aramid fiber reinforced NASC

Results of the single factor experiment about the usage of elastic binding material and aramid fiber are given in Tables 3 and 4, respectively. The aramid fiber and the rubber contents are suggested to be 13 wt% and 20 wt%, respectively.

Performances

Rubber content (wt%)

15%

17%

20%

23%

26%

Transverse tensile strength (MPa)

9.22

8.89

8.7

6.87

6.54

Compression rate (%)

4.75

10.06

11.2

13.66

11.4

Resilience rate (%)

43.20

52.15

52.23

51.04

45.7

Stress relaxation rate (%)

28.9

29.4

32.35

37.06

36.15

Table 3. The effect of rubber content on performances of NASC (The amount of aramid fiber is 13 wt%)

Performances

Aramid fiber content (wt%)

7%

10%

13%

15%

Transverse tensile strength (MPa)

6.57

7.77

8.70

8.86

Compression rate (%)

15.40

11.36

11.20

9.08

Resilience rate (%)

37.78

48.48

52.23

50.36

Stress relaxation rate (%)

41.42

37.80

32.35

29.42

Table 4. The effect of aramid fiber content on performances of NASC (The amount of elastic binding material is 20 wt%)

Formulation design of NASC

The optimization formulation of aramid fiber reinforced NASC is listed in Table 5.

Gradient

Weight content (wt%)

Gradient

Weight content (wt%)

NBR-26

13.0

Filling

materials

Calcium carbonate

7.9

NR

7.0

Kaolin clay

3.4

Aramid fiber

13.0

White carbon black

3.4

Compatibilization fiber

45.3

Soap stone powder

2.3

Vulcanizing agent

2.5

Other accessory ingredients

1.8

Curing catalyst

0.4

Table 5. Formulation of aramid fiber reinforced NASC