Isocyanate treatment

The mechanical properties of composites reinforced with wood fibers and PVC or PS as resin can be improved by an isocyanate treatment of those cellulose fibers [41,50] or the polymer matrix [50]. Polymethylene-polyphenyl-isocianate (PMPPIC) in pure state or solution in plasticizer can be used. PMPPIC is chemically linked to the cellulose matrix through strong covalent bonds (Fig. 8). 

Only the epoxide and isocyanate treatments have been reported to add to wood cell wall components to such a degree that they cause the wood structure itself to break apart (2, 80), Other chemical substitution treatments of wood components add to wood up to about 35% weight gain with no cell wall rupture. The epoxide and isocyanate systems seem to swell the cell wall, react with it, and continue to swell and react to the point of cell wall rupture and beyond. 

Two groups have independently explored dipolar cycloadditions of oxiranes with chlorosulfonyl isocyanate. Treatment of oxiranes with C1S02N=C=0 in benzene <84JHC1721, 84SC687) or in CH2C12 <86SC123> gives rise to either cyclic carbonates or 2-oxazolidones, or both after hydrolytic workup (Equation (27)). 

The formahon of hydantoins represents an early application of the cyclahve cleavage strategy. The group of Hobbs DeWitt presented a synthesis starhng from Merrifield resin-bound amino acid derivahves 1 (Scheme 1). These were N-deprotected and subsequently converted to ureas 2 by reachon with an isocyanate. Treatment with 6 M HCl (83-100 °C, 2 h) gave hydantoins 3 in yields of 4-81%, with two dimensions of diversity [1]. 

George et al. (1996) treated pineapple leaf fiber with polymethylene-polyphe-nyl-isocyanate (C15H10N2O2) solution at 50°C for 30 min to improve the fiber-matrix interfacial adhesion. Comparing silane and isocyanate-treated wood fiber-PS composites, it was reported that isocyanate treatment was more effective than silane treatment in enhancing the mechanical properties of cellulose fiber-PS composites (Maldas et al. 1989). 

To improve the adhesion between natural fiber and polymer matrix, chemical modification of natural fibers was investigated by a number of researchers. The mechanism and utilization of selected chemical treatments is discussed in this section. There are many different methods to improve the interfacial adhesion between fiber and matrix by modifying fiber surface such as acetylation, benzoyla-tion, acrylation, permanganate, and isocyanate treatment. These treatments are described in detail by Kalia et al. [69]. 

Isocyanate is very susceptible to reaction with the hydroxyl group of cellulose and lignin in the fibers and forms strong covalent bonds, hence increases fiber-matrix interface adhesion. In isocyanate treatment, the isocyanate group acts as a coupling agent [71, 72], and fiber is treated with the polymethylene-polyphenyl-isocyanate (C15H10N2O2) solution at 50°C for 30 min duration [72]. 

Eor pretreatment of sisal fiber chemicals such as alkali (NaOH), permanganate (KMn04), acetylation, peroxide (BPO and DCP), isocyanate treatment, stearic acid, silane, benzoylation [79], maleic anhydride modification, PPG-TDI [34, 60, 61], and cold-plasma treatment are used [76, 118]. 

Other less frequently used treatments for improving moisture resistance are benzo-ylation treatment, peroxide treatment, sodium chlorite treatment and isocyanate treatment [37]. 

Fig. 25.6 Proposed reactions during the isocyanate treatment of GO where organic isocyanates react with the hydroxyl left oval) and carboxyl groups right oval) of graphene oxide sheets to form carbamate and amide functionalities, respectively (Reprinted with permission from Stankovich et al. 2006a, Copyright 2006 Elsevier)...

Isocyanate treatment In this treatment, —N=C 3, the functional group of isocyanate, reacts with the cellulose and lignin s hydroxyl groups and forms strong... 

Investigations of variation in fiber began some years ago. Using polymeric isocyanate treatment, Maldas, Kokta, and Daneault reported the differences in mechanical properties between aspen, spruce, and birch, and between sawdust, cotton fiber, and fiber from paper pulp processing." Typically, aspen and birch provided better properties than softwood spruce. At 30 percent loading, the highest modulus at 1 percent strain was found with two types of treated pulp, followed by cotton fiber, and then treated sawdust. The effects of isocyanate treatment were more dramatic with sawdust. 

---