A Publication on World Pulp, Paper & Allied Industry

April-June'2001 - Vol. 5 Issue 3

Tech Watch

  Continued......

PAGE 3 OF 5

Physical Strength Properties of Standard test sheets made from bagasse and their Blends with Bamboo pulp
by Sushma Mahajan*, O. P Bhatt** and Rajiv Pandey***

 
Statistical Analysis and Results:

Strength properties (tensile index, burst index and tear index of paper sheets were regressed with intensity of pressure applied as independent variable. In all cases the quadratic model (Y = a + bx + cx2, where a,b & c are parameters to be estimated, Y is strength properties and x is applied pressure) was found to fit well to data, although different models were also tried. The criteria of model selection were coefficient of determination (R2), together with model error. The quadratic model reflects the behavior of applied pressure on optimum strength properties.

To investigate the effect of wet pressing on sheet properties in case of short fibred pulp, bamboo (Dendrocalamus strictus) unbleached sulphate pulp and depithed bagasse unbleached soda pulp were used. Detailed investigations on wet pressing under increased intensities of pressure have been carried out on bagasse pulp and results are recorded in Table-1. Detailed investigation in case of bamboo (D. strictus) was carried out earlier in this laboratory. The data available has been used in this study.

The results of evaluation of standard test sheets prepared from pulp blend containing 80 parts bagasse soda pulp and 20 parts of bamboo sulphate pulp and subjected to increasing intensity of pressure are recorded in Table-2. The above results have been plotted in Fig- 6 to 10.

Effect of increasing intensity of pressure during wet pressing on physical strength properties of standard test sheets prepared from blend have been depicted in Fig-7 to10.

The bagasse pulp was beaten to three levels of freeness and unbeaten pulp was also used as described above. The bamboo pulp was beaten to about 400 ml.(CSF) to get the best results of wet pressing as indicated in earlier study. For blending studies both bamboo and bagasse pulps were beaten to about same freeness 400 ml (CSF) and blended to nullify any effect of freeness on sheet formation. The effect of wet pressing under increasing intensities of pressure or pulp blend was studied and results recorded in Table.1 & 3 are discussed below.

 
Discussion:

While discussing the results graphs based on statistically derived relationship have been used for interpretation of results of this investigation.

A. Bagasse Pulp

 

Solid Content

 

From Fig.1, it is observed that as the intensity of pressure increases from 2.04 Kg/cm2 to 42.96 Kg/cm2, the initial solid content of the sheet increases from 36.08% to 67.31% for unbeaten pulp, from 36.60% to 69.57% for lightly beaten, from 31.27% to 65.43% for well beaten pulp and 29.82% to 60.83% for over beaten pulp. From Fig.1, it can also be observed that once the solid content of the sheet has reached its maximum, any further increase in intensity of pressure, irrespective of freeness, slowly decreases the solid content.

 

Apparent density

 

Study of Fig.2 indicates that the maximum apparent density which could be achieved is of the order of 0.98g/cm3 where sheets (made from highly beaten pulp ) were pressed at intensity of pressure between 30.68 to 36.82 kg/cm2. This shows an increase in apparent density relative to that under standard conditions (0.93 g/cm3) by 5.3 percent. So by increasing the intensity of pressure during wet pressing the apparent density can be increased by 5.3 percent of the standard condition. However, further examination of Fig.2, it can be observed that apparent density of the sheet decreases with increase in intensity of pressure. The reason of the decrease is the same as already explained in case of solid content.

This depiction of the Fig.1 is due to the trend of the quadratic model, which is best suited for the available experimental data.Theoretically solid content should not decrease once it has reached its maximum and any further increase in intensity of pressure should not decrease it. So the model is being utilized for the limited purpose of studying the pattern up to the pressure where maximum solid content percentage is attained.

Tensile index:

Under standard conditions a tensile index of 47.5 N.m/g is achieved (Table 4). Tensile index of more than 49-52 N.m/g of could be obtained using higher intensity of pressure (20-24 kg/cm2) for pulps beaten to a lesser degree than the standard 250 ml CSF i.e., 300 and 450 ml CSF. From Fig.3, it can be seen that further beating to a lower degree of freeness of 150 ml CSF did not bring about appreciable further increase in tensile index even using pressure intensity around 24.55 kg/cm2. The maximum tensile index attained was 63.40 N.m/g, and increase of 33.47 percent in tensile index relative to standard conditions. It can also be observed that on further increasing the intensity, value of tensile index starts decreasing gradually.

 

Burst index

Under standard conditions a burst index of 2.85Kpam2/g is achieved (Table-4). A higher burst index up to 3.51 to 3.36 Kpam2/g even less beaten pulp obtained with less beaten pulps (300 and 450ml CSF) were pressed at higher intensities of around 24.55kg/cm2. From Fig.4, it can be observed that an increase of burst index up to 3.81Kpam2/g, further beating of pulp to 150 ml (CSF) and pressing at identical intensity of pressure around 24.55kg/cm2. The maximum increase in burst index value is 25.6% as compared to standard conditions of pressing (Table-4). The fig.4 also indicates that by further increasing the intensity of pressure, the burst index gradually decreases. The burst index and tensile index show the same trend. The decrease in tensile and burst index after reaching maximum value 


by further increasing the intensity of pressure might be due to sheet crushing which has just started but is not visible.

Tear index

Under standard conditions a tear index of 5.6 mNm2/g has been recorded (Table-4). From Fig.5, it can be observed that a tear index of 5.6mNm2/g could be attained at 450 ml/CSF level of freeness and at intensity of pressure of 18.41 Kg/cm2, There is a general trend that after attaining the maximum value of tear index, it decreases sharply with further increase in intensity of pressure.

Blend of Bagasse Pulp and Bamboo Pulp
Solid Content

In case of blend, the solid content (Fig.6) of the sheet increased from 32.56% to 64.72% in almost linear manner as the intensity of pressure increased from 2.04 kg/cm2 to 18.41 kg/cm2. On further examination of the figure it can be observed that solid

content decreases on further increase in  pressure intensity. This might be due to the same reason as already explained in case of bagasse pulp.

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