HRC is superior to other cellulose derivatives because of its high viscosity, water holding capacity, and surface area (Lun et al, 1995). The water holding capacity and surface area of HRC has been shown to be much higher than microcrystalline cellulose (MCC), or powdered cellulose (Ruan et al, 1998). Because the viscosity, water holding capacity, and surface area correspond to better performance in HRC's applications, it is the primary measurement to determine the HRC's quality.

A new and improved process for converting corn cobs and husks into HRC has been developed and is shown in Figure 1. Compared to the old alkaline cooking process, the new process is relatively cheap and environmentally friendly. The chemical treatments are necessary to attain high HRC viscosity, water holding capacity, and surface area. Additionally, the bleaching step is necessary since HRC is often blended into food products. However, although there are ideas on how to do the alkali soaking and bleaching, minimal testing has been done to optimize these processes (Ruan, 1999).

The objective of this project is to optimize the alkali and bleaching processes of HRC production. Attempts will be made to minimize the amount of chemicals used while maximizing HRC's viscosity, water holding capacity, surface area, and aesthetic appeal.

Additional Experiments

My main project was for the bleaching, however there was a lot of work to do besides the bleaching treatment. There were five other procedures, which also had to be optimized. These included the pretreatment soaking of sodium hydroxide, refining, emulsifying, and homogenization. I helped in all of these experiments also.

For the pretreatment soaking with sodium hydroxide, there were many variables, including temperature, volume of water compared to the mass of corn stalks, the amount of sodium hydroxide relative to both volume of water and mass of the corn stalks, the method of stirring, and the size and shape of the container. The grad student, whom I was working with, Brock Lundberg, designed the experiments. I usually carried them out by myself while he was doing something else, or did one set while he did the other.

Next came the bleaching treatment, which I did entirely by myself except for the occasional asking of advise from Brock. I also would have him check to make sure that the procedure I had planned to use was one with as few flaws as possible.

The next part was refining. This was another part mainly done by Brock, but usually took two people to finish a normal group of samples in under a day. All samples needed to be refined to do the final analysis on them. There were also variables in refining that had to be optimized. The main two were the flow rate of the pulp in to the refiner and the distance between the plates that refined the pulp.

The last two steps purpose was to reduce the size of the cellulose particles even more. In emulsifying we would mix HRC in a one percent solution with water. The emulsifier would make this into a solution that could then be put through the homogenizer to make a solution that thick sort of like gravy, only green if it was not bleached or tan if it was bleached. The variables on these were flow, and number of passes.

The testing flowing all of the experiments were ISO brightness tests for the bleaching, water holding capacity tests, and viscosity test for everything else. These tests were another time consuming activity and towards the end of summer and on into the weekends of September and October is what I spent most of my time doing. In my report there are no mentions of water holding capacity and viscosity because they we used only as a quality check to make sure that I had not lost any quality from the bleaching. Since there was never any evidence of quality loss, there was no need to mention the results of the experiments since they were relatively similar.

Data Collected:

The bleaching process of HRC is a modified version of one used in the paper industry. The bleaching process requires four chemicals. They are Hydrogen Peroxide, Sodium Silicate, Magnesium Sulfate, and Sodium Hydroxide. Other than these four chemicals there are only variables of time, number of times, and temperature. First a experiment must be designed in order to test each of the variables. The best way to do any test is to eliminate the variables except for the one we want to test. Each was done with that in mind. All have the same initial pH and the same moisture content, and had one of the variables tested.

Sodium hydroxide purpose in the bleaching formula was to activate the hydrogen peroxide. When too much is present a yellow color is given to the pulp which is being bleached. Since our pretreatment involves being soaked in sodium hydroxide we do not need to add any because there is enough left in the pulp to activate the hydrogen peroxide, while excess sodium hydroxide hinders the reaction. For our purpose, there is already excess sodium hydroxide in the pulp without adding any to the bleaching recipe.

The next test was the effect of the concentration of hydrogen peroxide . The pulp was bleached with one, three, five, seven and a half, and ten percent peroxide relative to the mass of the pulp. The pulp was mixed in a five gallon pale with 500g of corn stalks, which end up to 415g dry mass after washing and soaking in sodium hydroxide and then straining. Dividing it down into 5 bags. This experiment was done on pulp that was not rinsed after the sodium hydroxide soaking because the pH would effect brightness and the pH would vary because of the number of variables in rinsing the pulp. In the percentage of hydrogen peroxide graph you can see the results of the experiment. All of these sat in 80-degree Celsius water bath for one hour. This led us to believe that to get the brightest result with the least amount of peroxide would be about 8% to the dry mass of the pulp.

The next variable tested was the time of bleaching. To do this again we started with a large amount of pulp with known moisture content. This was then evenly divided among five bags. The time durations were 15 minutes, 30 minutes, 45 minutes, 60 minutes, and 75 minutes. The bleaching solution had 7.5% hydrogen peroxide and sodium silicate relative to the percentage of solids the sample contained. The graph shows that it brightens significantly from the 24.4 to 31.94. Then it stayed right a round that only moving up to 33.82. In the next period the brightness just to 37.26 then only went up to 38.43. The results can be seen in figure 2.

The third experiment was brightness compared to the number of times that it was bleached. A large sample of pulp was made and the percentage of solids was calculated. This sample was washed before the first bleaching step occurred. The samples were bleached with a 7.5% hydrogen peroxide and sodium silicate relative to the dry mass of the pulp samples. This experiment showed significant brightening after each bleaching. Four bleaches were done, and after the initial brightening of the first bleaching, all went up by five points of brightness. How long will it keep brightening at this rate is the question that will now have to be examined. We knew that the pulp would get brighter with continual bleaching but had figured it would have started to taper off at three or four bleachings and not go beyond that point. The results are pictured in figure 3.

Next we tested temperature. This was a very time consuming experiment because the water bath, which we were using for our reactions had to be constantly watched, and then we had to allow warm up time as well as waiting the hour for individual bleachings. Using the same technique as before, we made five bags with equal portions of the same basic pulp. Using the basic bleaching recipe we allowed each bag to sit in its assigned temperature. The samples were all refined and had hand plates made of them to test their ISO brightness. The results from this test are in Figure 4.

The test that was performed next was the effect of silicate on the brightness with a constant percentage of peroxide. The percentage of peroxide used was 6.1% approximately. The Silicate was varied, at 0%, 1.5%, 3%, 6%, and 12%. All percentages refer to the dry weight of the solids in the sample. The results of the experiment were what we had expected to see. The brightness went up with a linear relationship to the amount of sodium silicate used. Figure five shows the brightness relative to the amount of silicate.

Seeing that there was a linear relationship between brightness and silicate, a matrix type experiment was done with varying percentages of hydrogen peroxide and of sodium silicate. Three percentages of Sodium silicate were used and five different percentages of hydrogen peroxide were used. The idea of this experiment was to try to see what percentage of silicate worked the best, along with what percentage of hydrogen peroxide. From the test of using only hydrogen peroxide, we knew that there should be a jump of brightness between five and eight percent of hydrogen peroxide, the answer we were looking for is what percentage of sodium silicate increased that jump the most.

The tests for magnesium sulfate in the bleaching formula took many runs to find a small enough dosage to have an effect. This effect was constant and noticeable, but not overly large. The first experiment showed that with a large concentration the brightness was decreased. This did not make sense due to because magnesium sulfate was contained in the original formula for bleaching wood pulp. The second experiment was designed at using much smaller concentrations. These went from no magnesium sulfate to one milli-Liter. The stock solution was a one molar solution. The test showed that it worked best around two milli-Liters, as shown in figure six.

Both magnesium sulfate and sodium silicate are silght poisons however and while they both have beneficial attributes, the possibility of them being a poison is much greater. The grad student whom I was working with was going to check with the FDA about the toxic levels of these two chemicals to see if we were putting others in danger by using them in our bleaching formula. Since magnesium sulfate had much less effect oon the brightness of our pulp then sodium silicate had, we opted to eliminate it form the bleaching treatment.

We wanted something to compare our general brightness tests to, so we bleaching other agricultural by-products after the same pretreatment. The different types of the materials used in the bleaching were oat hulls, ground oat hulls, pith, ground soy, and corn. These were all bleached in 7.5% by mass hydrogen peroxide and silicate, with .1% by mass Magnesium sulfate. With our bleaching treatment the oat hulls bleached the best. We also ran several other test on these products to test how they compared to cornstalks with other traits that we were looking for, such as viscosity and water holding capacity. The only one the showed any results close to the cornstalks was pith. It had much higher water holding capacity and similar viscosity.

Conclusions:

The final conclusions that were drawn took into consideration how bright we wanted to get our product. The purpose of bleaching was to make it light enough to color and to make it light enough so it did not give other things it's color when used as an additive. We found that to meet these criteria it had to be around 38 or above ISO brightness. This was usually reached with 7.5% H2O2 and 7.5% Sodium silicate by mass, or with 12% H2O2 with out any additives.

References

Glass, J.E. and Swift, G. 1990. Agricultural and synthetic polymers: biodegradability and utilization. American Chemical Society, Washington DC.

Lun, Y., Zhang, J., Ruan, R., Addis, P., and Chen, L. 1995. Study of structure-function and relationships of microfibrillated cellulose made from agricultural wastes fibrous materials: proceedings of International Agricultural mechanization Conference. Beijing China

Ruan, R. 1999. Personal Communications. University of Minnesota Department of Biosystems and Agricultural Engineering. St. Paul, Minnesota.

Ruan, Roger, Ling Chen, Jack Edgar Johnson, Paul Bradley Addis, and Li Xu. 1998. Cellulose Fiber Based Compositions and Film and the Process for Their Manufacture. U.S. Patent: approved.