POSS Modified Cellulose for Reduced Polymer Flammability

Jieun LeeFaculty Advisor: Dr. Douglas M. Fox

ABSTRACT

A study was initiated to prepare a flame retardant formulation using a modified cellulose as a charring agent. Nanofibrilated Cellulose (NFC) was modified by alkaliating and reacting with a polyhedral oligomeric silsesquioxane (POSS) that contained an epoxide. After soxhlet extraction and a series of rinses, the modified cellulose was dried in a vacuum. Thermal analysis was used to determine the thermal stability and the amount of char produced from the modified cellulose. Additionally, FTIR was used to determine the extent of the reaction with the POSS by comparing the modified cellulose with a standard. SEM images have shown that the char of the modified NFC still preserves some of the morphology of the NFC. Not only has this modified cellulose been shown to increase char yield without reducing the thermal stability of the fibers, but also toxicological studies have indicated that POSS may be non-toxic enough to use as a drug delivery material. Furthermore, the POSS-cellulose composite is insoluble in water and absorbs less water than other carbonization agents, such as pentaerythritol, which could reduce flame retardant migration and hydrolysis problems.

BACKGROUND

Due to public perceptions and increased environmental regulations, the need for nontoxic polymer flame retardants based on renewable materials or with low environmental impacts is growing rapidly. To proactively help industry address the current lack of suitable materials, we have been investigating the use of natural products, such as montmorillonite clay and cellulose, modified with polyhedral oligomeric silsesquioxanes (POSS) as potential flame retardant replacements. Cellulose is a non-toxic, abundant, and underutilized resource material that has the potential to form char. However, the char yield is typically low, which reduces its potential applicability as a carbonization agent in intumescent flame retardants. As a result, surface modification has been performed on nanofibrilated cellulose (NFC). There are several potential benefits to using NFC. First, even if the cellulose fibers do aggregate, the nanofibrils can extend into the polymer to disperse and increase polymer-cellulose interactions. Second, the nanofibrils increase the surface area of the cellulose for more efficient chemical modification. Finally, some modifications can cause the swelled alkaliated cellulose to collapse during epoxidation, and nanofibrils may help retain the swollen state. Polyhedral oligomeric silsesquioxanes (POSS) have been used as fillers to produce polymer nanocomposites with improved thermal characteristics, surface hardness, and processability over pure polymers. These modified polymers exhibited an improvement in thermal stability and char yield. Cellulose, which normally suffers from lower thermal stability, may also benefit from the modification using POSS molecules.

EXPERIMENTAL

Lyocell fibers with a precursor length of 6 mm and low degree of nanofibrillation, referred to here as NFC was used in this reaction. The NFC contained some residuals from the production process and were neutralized with 0.5 M acetic acid, then thoroughly washed with deionized water and ethanol to a neutral pH.

Cellulose was alkaliated using 15 wt% NaOH for 1 hour. Ethanol of approximately equal volume was then added to the Na-cellulose and was filtered by using a 41 size filter paper until its final wet uptake was about 400 – 500%. The filtered Na-cellulose was then placed in toluene and Glycidyl Phenyl POSS was added. The reaction mixture was heated to 95 °C for 24 hours. The reaction mixture was filtered and washed with 4-6 10 mL aliquots of ethanol followed by deionized water. The product was then neutralized with 0.5 M acetic acid and thoroughly washed with water to remove excess acetate. Then the fibers were thoroughly washed with toluene using soxhlet extraction. The final product was dried in a vacuum at room temperature.

RESULTS

Cellulose was successfully modified using Glycidyl Phenyl POSS in toluene. According to TGA analysis, reaction efficiencies were slightly better using toluene than any other solvents, as shown in Figure 1.

Figure 1. Solvent effects on Reaction Efficiencies of NFC

Thermal Gravimetric Analysis (TGA) has shown that POSS-modified fibers tend to have an increase in the char yield. Even when soxhlet extraction was added to the procedure to remove excess POSS remaining. The effects of washing on char yield is shown in Figure 2 below.

Figure 2. Comparison Among L040-6, Glycidyl Phenyl POSS-NFC without Soxhlet Extraction, and Glycidyl Phenyl POSS-NFC with Soxhlet Extraction

Furthermore, Scanning Electron Microscope (SEM) images of the char formed from the modified cellulose has shown that even after it has been pyrolyzed, the char still retains some of the morphology of the NFC as shown in Figure 3.

Figure 3. SEM Images of Glycidyl Phenyl POSS-NFC and Char

As shown in Figure 2, the reaction of NFC and Glycidyl Phenyl POSS was not only successful as a modification procedure but also successful in improving thermal properties, even when the fibers were put in soxhlet extraction to remove excess POSS. Moreover, the nanofibrilations of NFC were still mostly preserved in the char of the modified cellulose.

CONCLUSION

The cellulose was successfully modified using Glycidyl Phenyl POSS. TGA has shown that in terms of reaction efficiency, the toluene solvent system works best (Fig 1). In addition, SEM images have shown that the fibrilation in NFC is still somewhat preserved when using the toluene as a solvent in the modification procedure. Also, according to the TGA graph in Figure 2, these modified fibers showed improvements in thermal stability and char yield, even after the soxhlet extraction process removed any excessive POSS remaining in the fibers. Scanning Electron Microscope (SEM) images have also shown that the morphology of the cellulose is still preserved in the char.

REFERENCE

George, J.; Sreekala, M.S.; Thomas, S. Polym. Eng. Sci. 2001, 41, 1471.

Rowell, R.M.; Ellis, W.D. Reaction of Epoxides with Wood Research Paper FPL 451, U.S. Dept. Agric., Madison, WI, 1984.

Zheng, L.; Farris, R.J.; Coughlin, E.B. Macromol. 2001, 34, 8034.

Fox, D.M.; Maupin, P.H., Harris Jr., R.H.; Gilman, J.W.; Eldred, D.V.; Katsoulis, D.; Trulove, P.C.; De Long, H.C.; Langmuir 2007, 23, 7707.

D. M. Fox, J. Lee, E. Ford, E. Balsley, M. Zammarano, S. Batko, and J. W. Gilman, “POSS modified cellulose for improving flammability characteristics of polystyrene,”

in Proc. 10th Int. Conf. Wood Biofiber Plastic Composites, Forest Products Society, Madison, WI, 2010, 337-342.

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Solvent Effects on Product Thermal Stability

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L040-6 Char of L040-6