MINH-TAN TON-THAT, TRI-DUNG NGO, and BOUCHERVILLE

ABSTRACT

The incorporation of renewable resources in composite materials is a viable means to reduce environmental impact and support sustainability develop­ment in the composites industry. Cellulosic fiber polymer composites have received very much attraction for different industrial applications because of its low density and its renewable ability. However, the uses of cellulosic fibers in the composite are limited in many applications that require fire re­sistance due to their flammability and their low thermal resistance.

This chapter reports an innovative and sustainable treatment approach to retard the burning of cellulosic fibers for composite production in which a minimum amount of nontoxic and low cost inorganic chemicals have been used. Different types of reacting minerals and different treatment parameters have been investigated in order to determine the most cost-effective treat­ment solution. The cellulosic fibers obtained from this approach become self-extinguished while there is no negative effect on fiber strength. The composite with the treated cellulosic fibers also shows their good fire resis­tance with minor effect on the mechanical properties. Thus this solution will open the door for the use of the cellulosic fibers in composites for applica­tions where fire resistance is an important issue, particularly in aerospace, transportation, and construction.

“Fire Resistance Cellulosic Fibers for Biocomposites” by Minh Tan Ton-That and Tri Dung Ngo was originally published with the National Research Council of Canada (© by the authors).

11.1 INTRODUCTION

Cellulosic materials (natural and synthetic) in different forms (fiber, film, powder, particle, pellet, chip, etc.) at different sizes (nano, micro or macro) are often flam­mable and have low thermal resistance. They can be burned and also can spread the fire in the presence of oxygen. Thus, their use either in direct or nondirect form is limited in applications requiring fire resistance. Due to their flammability, the use of cellulosic materials in polymer composites is also limited in certain applications.

Cellulosic materials are treated with different flame retardants depending on the application, for example in furniture, textiles or composites. The most com­monly used flame retardants are based on halogen, phosphorous, boron, ammonium, graphite, alkaline-earth metallic compounds or mixtures thereof. To improve fire re­sistance of organic polymer composites, the incorporation of flame retardants based on halogen, phosphorous, metallic hydroxide (magnesium hydroxide, aluminum hydroxide, calcium hydroxide, layer double hydroxide), metallic oxide (antimony oxide, boron oxide), silicate (clay, talc), etc., in the polymer matrix has been widely used.

Among the compounds listed above, halogen based flame retardants are well known to be the most efficient as they can be used at a low concentration in the final composition thus limiting their impact on other properties of the product. How­ever, halogen compounds are considered to be harmful to the environment. Boron compounds are supposed to be efficient, however, they tend to be washed off due to their good solubility in water. Less harmful flame retardants based on phospho­rous, graphite or alkaline-earth metallic compounds are much less efficient, thus a large amount of those additives must be used in the formulation. The use of flame retardant incorporated in a polymer matrix alone does not satisfactorily resolve the flammability problem in cellulose-polymer composites, especially when the con­centration of cellulose is quite significant in the formulation of the composite.

It is generally known that metal hydroxides, including barium hydroxide, can be used as a flame retardant for cellulosic materials1-4 and for polymer materials.5 Fur­ther, Herndon6 used a flame retardant composition for cellulosic material compris­ing sodium hydroxide and a metal salt of boron among other ingredients. The metal salt of boron is defines as borax, which is a sodium tetraborate. De Lissa7 suggested a flame-proofing composition comprising potassium hydroxide and/or potassium carbonate and possible a small amount of sodium hydroxide and/or sodium carbon­ate and may include another potassium salt. Musselman8 proposed inorganic addi­tives to impart flame resistance to polymers. The additives include hydroxides and metal salts that evolve gas. One such metal salt is barium chloride dihydrate. The use of a mixture of a polycondensate of a halogenated phenol and an alkaline earth metal halide in a flame retarding composition has also been suggested.9

Flame retardant compositions in which ancillary flame retardant additives may be used alone or in combination, such as metal hydroxides and metal salts, including alkaline earth metal salts, has also been reported10.

Fukuba11 discloses the use of “alkali compounds” for use in flame resistant plas­ter board. The “alkali compounds” are defined as at least one of an alkali metal hy­droxide, alkali metal salt, alkaline earth metal hydroxide or alkaline earth metal salt. It is preferred to use a mixture of alkali metal salts and alkaline earth metal salts, for example a mixture of sodium and calcium formate.

Yan demonstrate the use of a flame retardant composition which initially in­volves the step of making magnesium hydroxide from the reaction of magnesium sulfate and sodium hydroxide.12

It is known that treatment of cellulosic materials with alkaline earth metal car­bonates (e. g., barium carbonate) imparts fire resistance to the cellulosic material13. Here, the alkaline earth metal carbonate is applied to the cellulosic material by first coating the cellulosic material with an alkaline earth metal chloride and then treating the so-coated material with sodium carbonate. It is also known to use both a clay and a metal hydroxide in a fire retarding composition comprising a polymer material.1415

However, there is no disclosure treating a cellulosic material with an aqueous reaction mixture of an alkali metal hydroxide and alkaline earth metal salt simulta­neously with or shortly after mixing the alkali metal hydroxide with alkaline earth metal salts.

There remains a need for an environmentally friendlier, effective approach to producing fire-resistant cellulosic materials. This chapter presents an innovative method for improving fire resistance of cellulosic materials, especially when the cellulosic material is to be used in polymer composites, which is simple, cost-effec­tive and environmentally friendly.