Product Polymers from Plant Oils

Ideally, producing environmentally friendly polymers directly in plants would be the most energy efficient process (one-step process) (Figure 7.6), provided that suitable technologies are available for the extraction and downstream purification processes of the polymers from plant materials. At present however, plant derivatives such as sugars and oils are the most popular carbon sources for the production of PHA by microbial fermentation. 

Unanswered questions persist that influence the potential of PHA production in plants. Achieving control over the final composition of the polymer may be more difficult in plants than in bacterial fermentation. Isolation and quality of the purified polymer from plants is also a hurdle. A very important consideration in commercialisation is the level of PHA achieved in the plant. Monsanto has achieved PHA levels as high as 5% in plants [31]. The long-term goal is to produce a level of PHA comparable to the oil content in soybeans of 20% to achieve commercial viability. Monsanto is no longer actively researching PHA Metabolix licensed its technology in 2001. 

Photosynthetic products are synthesized by plants from water and carbon dioxide, and therefore renewable monomers obtained from plants are very attractive. Though natural polymers from plants are also very attractive for the same reason, monomers are preferentially focused on in this chapter due to limitations of space (monomers obtained by digestion of natural polymers are included here). Types of renewable monomers obtained from plants without any chemical, enzymatic, and microbial conversion are limited, since plant cells are mainly composed of polymers. The principal monomer compounds that can be directly extracted from plants are oils triglycerides of fatty acids and essential oils. 

There has been only one major use for ozone today in the field of chemical synthesis the ozonation of oleic acid to produce azelaic acid. Oleic acid is obtained from either tallow, a by-product of meat-packing plants, or from tall oil, a byproduct of making paper from wood. Oleic acid is dissolved in about half its weight of pelargonic acid and is ozonized continuously in a reactor with approximately 2 percent ozone in oxygen it is oxidized for several hours. The pelargonic and azelaic acids are recovered by vacuum distillation. The acids are then esterified to yield a plasticizer for vinyl compounds or for the production of lubricants. Azelaic acid is also a starting material in the production of a nylon type of polymer. 

Pyrolysis of scrap tires was studied by several mbber, oil, and carbon black industries [14]. Pyrolysis, also known as thermal cracking is a process in which polymer molecules are heated in partial or total absence of air, until they fragment into several smaller, dissimilar, random-sized molecules of alcohols, hydrocarbons, and others. The pyrolysis temperature used is in the range of 500°C-700°C. Moreover, maintenance of partial vacuum during pyrolysis in reactors lowered the economy of the process. Several patents were issued for the pyrolysis of worn out tires to yield cmde oil, monomers, and carbon black in economic ways [15-18]. The major drawback of chemical recycling is that the value of the output is normally low and the mixed oils, gases, and carbon black obtained by pyrolysis cannot compete with similar products from natural oil. Pyrolyzing plant produces toxic wastewater as a by-product of the operation [19]. 

Primary organics are emitted to the atmosphere by industrial sources (oil refineries, chemical plants, producers and users of solvents and plasticizers), vehicles (as a result of incomplete fuel combustion, oxygenated degradation products of lubricating oil, polymers from tires), and agricultural activities (use of pesticides). An exhaustive literature survey is beyond the scope of this section, but can be found in Air Quaiity Criteria for Particulate Matter many useful references are also available. 

Biodegradable polymers can also be made from mineral oil based resources such as the aliphatic-aromatic co-polyester types. Mixtures of synthetic degradable polyesters and pure plant starch, known as starch blends, are also well-established products on the market. 

The most important monomers for the production of polyolefins, in terms of industrial capacity, are ethylene, propylene and butene, followed by isobutene and 4-methyl-1-pentene. Higher a-olefins, such as 1-hexene, and cyclic monomers, such as norbornene, are used together with the monomers mentioned above, to produce copolymer materials. Another monomer with wide application in the polymer industry is styrene. The main sources presently used and conceivably usable for olefin monomer production are petroleum (see also Chapters 1 and 3), natural gas (largely methane plus some ethane, etc.), coal (a composite of polymerized and cross-linked hydrocarbons containing many impurities), biomass (organic wastes from plants or animals), and vegetable oils (see Chapter 3). 

It has been accounted that, on a production scale of PHB of 100,000 tons per year, the production costs will decrease from US 4.91 to US 3.72 kg , if hydrolysed com starch (US 0.22 kg ) is chosen as the carbon source instead of glucose (US 0.5 kg ) [33]. But this is still far beyond the cost for conventional polymers, which in 1995 was less than US 1 [32]. Lee et al. estimated that PHB and mcI-PHA can be produced at a cost of approximately US 2 kg [36]. The precondition therefore would be attaining high productivity and the use of inexpensive carbon sources. Among such substrates, molasses [37], starch [38], whey from the dairy industry [37-42], surplus glycerol from biodiesel production [39, 43], xylose [44, 45], and plant oils [46] are available. 

Solvent Extraction with Oilseeds—Extraction of the bleaching earths in a mixture with oilseeds is practiced by some extraction plants with processing capabilities, but the potential problems for this type of recovery may outweigh the savings for example, the mineral content of the meal may be increased beyond the acceptable limits, and the recovered oil may decrease the quality of the new oil extracted. The oxidation products and polymers from the recovered oil could contaminate the fresh oil. 

An alternative to a flash devolatilization unit is the oil heated thin film or WFE. In this equipment, the molten polymer/solvent solution is throttled to the WFE comprising a rotating set of blades that draws the melt into a thin film. In this manner, very good heat transfer from the oil heated surface is obtained and the thin film minimizes diffusion distances and allows rapid mass transfer of volatiles out of the melt. Both vertical and horizontal WFE units are in commercial production and are effective for small-to-medium-sized plants with moderate viscosity melts. Larger units require very large motors to strip viscous resins. Like flash devolatilization units, bubble formation and collapse are essential to effective mass transport of solvent from the polymer melt. 

Generally speaking, numerous synthons can be extracted from biomass. One known example is that of ethylene, produced from the dehydration of ethanol which is a very common product of fermentation. Another example is 1,3-propanediol, which is a monomer used as a building block for the production of polymers such as polyesters and polyurethanes. Several industrial processes have studied its production by fermentation with the aim of producing it directly from inexpensive plant raw materials (starch or sucrose). To synthesize polyamides and polyesters, we also aim to produce a,(o-dicarboxylic acids by the biological conversion of esters from vegetable oils. 

Plants grown as crops have provided a variety of raw materials for plastics. The cellulose from cotton has always been the quality raw material for cellulosic plastics. The cellulose from other plants is used in a variety of building materials. The oils from plants provided glycerol, and mono- and di-basic acids, for many plastics and plasticizers. Fermentation was used successfully to produce acetone and a number of alcohols and acids useful in polymers and plasticizers. Even the protein in plants, and in animal by-products, has been used in a number of plastics applications. 

Plant oils and their derivatives are attractive for building renewable polymers due to their renewability, worldwide availability, and their comparatively low price (65). In order to achieve a more sustainable production of polymers from these resources the recent methods of synthesis go in the direction of catalytic transformations. In particular, olefin metathesis and thiol-ene additions have been used for synthetic purposes. These methods have been reviewed recently (65,66). 

Metathesis Reactions Applied to Plant Oils and Polymers Derived from the Ensuing Products... 

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