Grafting yield

Misra et al. have utilized the ceric-amine redox system for grafting MM A onto wool [60] and gelatin [61], The graft yield was explained in terms of basicity, nu-cleophilicity, and steric requirements of amines. A complex of ceric ion and amine (AH) decomposes to generate free radical species, which produce additional active sites onto the polymeric backbone where grafting can occur. 

The radiation sensitivity of a substrate is measured in terms of its GR value or free radical yield, which is the number of free radicals formed per 100 eV energy absorbed per gram. The highest grafting yields will occur for polymer monomer combinations in which the free radical yield of the polymer is much greater than for the monomer. It also follows that the grafting yield will increase at a lower monomer concentration. 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

The NH2 groups can be diazotized and reduced in the presence of thiosulphates and different metal ions. The effect of some metal ions, namely Fe ", Sn, Cu +, and Co on the graft yield of cotton modified with aryl diazonium groups via its reaction with 2,4-dichloro-6-(p-nitroaniline)-5-triazine in the presence of alkali and followed by reduction of nitro group was studied [4]. 

If polyvinyl alcohol is used as the reducing agent and the oxidation is conducted in the presence of vinyl monomer, grafting occurs. This method of grafting yields substantially pure graft copolymers since the free radicals are formed exclusively on the polymer backbone. 

The effect of Fe(II) on grafting of 2-hydroxyethyl methacrylate onto polyester fibers in the presence of benzoyl peroxide was investigated [59]. It was found that increasing the iron ion concentration decreases the graft yield. This suggest that excess Fe(ll) ions participate in the generation of free radical species and the iron ions seem to contribute to the termination and, consequently, decrease the graft yield. 

Figure 3 Effect of residual lignin percent in the pulp on total conversion percent and graft yield percent. LR 40 1, reaction time 1 h, reaction temperature 27°C, monomer concentration 1 mL/g pulp, initiator concentration 1%, and acid conversion 1% — = total conversion (%), O—O = graft yield (%).

Figure 4 Effect of monomer pulp ratio on grafting yield and grafting efficiency. = cotton linters O = wood pulp.

Figure 6 Effect of radiation dose on the graft yield (%) of grafted cotton linters.

Compared with untreated pulp, a relatively high liquor ratio is necessary to attain maximum graft yield (43.5%) because of the small particle size of the ground pulp. For this reason, ground pulp needs a higher liquor ratio than the untreated pulp to help the mobility of the particles. On the other hand, comparing the graft yield... 

Of samples swollen with ethylene diamine, the graft yield at a 50 1 liquor ratio increases as the concentration of ethylene diamine increases. This is due to the increase of decrystallization of swollen samples, which helps the penetration velocity of the chemicals through the cellulosic chains. Graftability of the samples treated with 100% ethylene diamine is lower that of the sample treated with 75%. This is due to the dissolution of low DP chains and some of the hemicelluloses, which is detectable by the increase in DP of the sample teated with 100% ethylene diamine. 

From the preceding results, it is seen that there is a large difference in graft yield between grafted swollen pulp treated with ethylene diamine and untreated pulp at a liquor ratio of 50 1 at all monomer concentrations. 

It has been found that substitution of cellulose hydroxyls of cotton by acetyl groups, greatly affects the graft yield [45],... 

Cotton linters and viscose grade wood pulp were partially acetylated and carboxymethylated. The samples were grafted under the same conditions with acrylamide. Grafting yield and efficiency depend on several factors, such as the kind of pulp, chemical and physical structure, type of the introduced substituent, and degree of substitution (DS). 

By further increasing the methanol in the grafting medium, the graft yield decreases. This can be related to the lower solubility of the initiator in the grafting medium and a reduced formation of free radicals, which... 

Table 1 Effect of Ratio of Different Solvents in Water System on Polymer Loading and Grafting Yield...

A mixture of methanol and butanol or isobutanol in water with a ratio of 4 4 22 in the grafting process produces a higher graft yield than a mixture of butanol or isobutanol in water with a ratio of 8 22. 

The effect of the addition of acetone to the grafting medium has been investigated. The acetone-water system produces a high-grafting yield, especially at low concentrations of acetone in the grafting medium. This is explained by the inhibition of the formation of homopolymer. By increasing the ratio of acetone-water up to 8 22, the grafting yield is lower than that in case of the 8 22 methanol-water medium. 

Grafting of methylmethacrylate onto cellulose using ceric ammonium nitrate (Ce ) as the initiator in a benzene-water system is also demonstrated. The grafting yield in a benzene-water system is much lower than in the case of the methanol-water system and decreases by increasing the ratio of benzene to water. This can be due to the lower polarity and wetting power of benzene, which leads to poor swelling of the cellulose. 

More elaborate maleate and maleimide derivatives have provided a route to grafting various functionalities onto PP. Examples, include antioxidants (29 and 30) 1 11 and the oxazolinc derivative (31 )H2J33 for which very high grafting yields were reported. 

Various (meth)acrylic monomers have been successfully grafted onto polyolefins. Most studies deal with functional monomers. Grafting yields obtained with PP are usually low (<20%) and are dependent on the particular monomer. Liu et al.jM carried out a comparative study on the grafting of various functional methacrylates onto PP. The experiments were performed in a batch mixer at 180 °C with 7 wt% monomer and 0.05 wt% 22 as an initiator. Grafting levels (wt%) obtained under these conditions were as follows HPMA (I), TBAEMA (1), GMA (0.8), IIEMA (0.4), DMAEMA (0.3), 32 (0.2). Grafting yields to PE appear generally higher. 

A1 Malaika et a/.35" 754 have reported on the grafting of antioxidant moieties onto PP as mono- (e.g. 33) or bis-(meth)acrylic derivatives (34). Moderate grafting yields (10—40%) and some homopolymerization was observed in the case of the monoacrylate. However, with the bis-acrylate (34) close to 100% grafting yield was reported. 

---