Copolymerization graft

Graft copolymerization treatments using silane or maleated agents, form a surface coating via chemical bonding. This improves the interfacial adhesion between the fiber and the matrix polymer also by impeding fiber s hydrophilic character [15]. 

Ashori and Nourbakhsh [ 16] obtained tensile, flexural and impact property increases as high as 50% of corn stalk, sunflower stalk and bagasse fiber-reinforced polypropylene composite due to incorporation of maleic anhydride polypropylene compatibilizer. 

Panthapulakkal and Sain [10] reported that incorporation of compatibilizer resulted in 15-25% increase in tensile strength of wheat straw, corn stalk or corncob-filled HDPE composites and 20% increase in flexural strength of wheat straw-filled HDPE composites. 

This review investigated agro-based fibers as potential reinforcement elements in biocomposites. Methods of fiber extraction from agricultural residues have been explained, characteristics of fibers with resources of different agricultural byproducts have been compared, fiber modification techniques have been presented and research efforts devoted to utilize these agro-based fibers have been listed. 

Kavelin, Investigation of Natural Fiber Composites Heterogeneity with Respect to Automotive Structures, Delft University of Technology, The Netherlands, PhD Dissertation, (2005). 

We shall consider here graft copolymerization only by free-radical processes. There are three main techniques for preparing graft copolymers via a free-radical mechanism. All of them involve the generation of active sites along the backbone of the polymer chain. These include (i) chain transfer to both saturated and unsaturated backbone or pendant groups (ii)radiative or photochemical activation and (iii) activation of pendant peroxide groups. 

Unsaturated rubber polymers are especially important grafting vehicles. Consider, for example, the polymerization of styrene in the presence of l,4-poly(l,3-butadiene). A method consists of dissolving the polybutadiene rubber (about 5 to 10%) in monomeric styrene containing benzoyl peroxide initiator and applying heat. Polymer radicals, formed by (a) chain transfer between the propagating radical and polymer or (b) addition (copolymerization) of the propagating radical to the -double bonds of the polymer. 

Polymer radicals, that have radical centers on the backbone chain to initiate grafting, can also be produced by irradiation of a polymer-monomer mixture with ionizing radiation. Most radiation graft polymerizations are carried out as heterogeneous reactions. The typical reaction system involves equilibration (swelling) of polymer with monomer followed by irradiation of the monomer-swollen polymer while immersed in excess monomer. For example, to produce poly(ethylene-gra/t-styrene), 

Most graft polymerizations, irrespective of the initiation process, yield mixtures of the graft copolymer, ungrafted backbone copolymer, and homopolymer of the monomer. The relative amounts of the three species depend on the monomer-polymer combination and the initiation process. 

When high-energy radiation is used, the most important factor affecting the grafting efficiency is the radiation sensitivity of the monomer to be 

Some polymers already contain reactive groups that can initiate a graft polymerization. The hydroxyl groups in cellulose initiate the polymerization of ethylene imine  

With poly(vinyl alcohol), it is possible to produce free radicals which start the polymerization of vinyl monomers  

In many cases, however, it is impossible to start a graft copolymerization by including or forming specific groups in the chain. Reactive sites 

One universally applicable method of grafting (with respect to substrate) is by chain transfer. A radical P (polymer free radical) or R (initiator free radical) abstracts, for example, an H or Cl atom and forms a macroradical, which initiates the polymerization of the added monomer  

The transfer constants of the polymer free radicals P are relatively low, however, so that the graft yield will be very small. The macroradicals are therefore formed through an addition of initiator radicals R . 

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Gross-Linking. A variety of PE resins, after their synthesis, can be modified by cross-linking with peroxides, hydrolysis of silane-grafted polymers, ionic bonding of chain carboxyl groups (ionomers), chlorination, graft copolymerization, hydrolysis of vinyl acetate copolymers, and other reactions. 

Another use is of minor proportions of polyfunctional aHyl esters, eg, diaHyl maleate, ttiaHyl cyanurate, and ttiaHyl isocyanurate, for cross-linking or curing preformed vinyl-type polymers such as polyethylene and vinyl chlotide copolymers. These reactions ate examples of graft copolymerization in which specific added peroxides or high energy radiation achieve optimum cross-linking (see Copolymers). 

Issues to be considered in selecting the best stabilizing system are polymeric chain branching which increases with high temperature and the presence of some stabilizers, polydispersity of the particles produced, and grafting copolymerization, which may occur because of the reaction of vinyl acetate with emulsifiers such as poly(vinyl alcohol) (43,44). 

Thermoplastic elastomers are often multiphase compositions in which the phases are intimately dispersed. In many cases, the phases are chemically bonded by block or graft copolymerization. In others, a fine dispersion is apparentiy sufficient. In these multiphase systems, at least one phase consists of a material that is hard at room temperature but becomes fluid upon heating. Another phase consists of a softer material that is mbberlike at RT. A simple stmcture is an A—B—A block copolymer, where A is a hard phase and B an elastomer, eg, poly(styrene- -elastomer- -styrene). 

A waterborne system for container coatings was developed based on a graft copolymerization of an advanced epoxy resin and an acryHc (52). The acryhc-vinyl monomers are grafted onto preformed epoxy resins in the presence of a free-radical initiator grafting occurs mainly at the methylene group of the aHphatic backbone on the epoxy resin. The polymeric product is a mixture of methacrylic acid—styrene copolymer, soHd epoxy resin, and graft copolymer of the unsaturated monomers onto the epoxy resin backbone. It is dispersible in water upon neutralization with an amine before cure with an amino—formaldehyde resin. 

Graft Copolymerization Onto Natural and Synthetic Polymers... 

Figure 1 Components of rough copolymer products in graft copolymerization.

A number of metal chelates containing transition metals in their higher oxidation states are known to decompose by one electron transfer process to generate free radical species, which may initiate graft copolymerization reactions. Different transition metals, such as Zn, Fe, V, Co, Cr, Al, etc., have been used in the preparation of metal acetyl acetonates and other diketonates. Several studies demonstrated earlier that metal acetyl acetonates can be used as initiators for vinyl polymeriza-... 

In its simplest form the direct grafting method involves the irradiation of polymeric substrate in the absence or presence of oxygen. Graft copolymerization of the monomer to the polymer is then initiated through the free radicals generated in the latter. The reaction can be schematically written as ... 

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]. 

Two parameters such as percentage of grafting (%G) and grafting efficiency are usually determined as functions of different variables that influence graft copolymerization. Percent grafting is usually determined from the increase in weight of the backbone polymer after grafting after complete removal of the homopolymer. It is expressed as follows ... 

There are a considerable number of methods available for effecting graft copolymerization onto preformed polymers, each with its own particular advantages and disadvantages. 

The hydrogen abstraction from —SH groups is faster than from —OH groups. Hebeish et al. [9] and Misra et al. [10,11] reported the chain-transfer method of initiation of graft copolymerization onto cellulosic substrates with azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) as initiators. 

Graft copolymerization of acrylonitrile with various vinyl comonomers such as methyl acrylate, ethyl acrylate, vinyl acetate, and styrene onto cellulose derivatives using ceric ion was studied [24]. The results showed that... 

In aqueous solutions the persulphate ion is known as a strong oxidizing agent, either alone or with activators. Thus, it has been extensively used as the initiator of vinyl polymerization [43-47]. However, only later, Kulkarni et al. [48] reported the graft copolymerization of AN onto cellulose using the Na2S203/K2S20s redox system. 

During mutual graft copolymerization, homopolymerization always occurs. This is one of the most important problems associated with this technique. When this technique is applied to radiation-sensitive monomers such as acrylic acid, methacrylic acid, polyfunctional acrylates, and their esters, homopolymer is formed more rapidly than the graft. With the low-molecular weight acrylate esters, particularly ethyl acrylate, the homopolymer problem is evidenced not so much by high yields as by erratic and irreproducible grafting. 

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