Polyterpenes

Isoprenoids with more than eight isoprene units are classified as polyterpenes Natural rubber (caoutchouc), formerly an important raw material for the rubber industry, is primarily obtained by coagulating the milk juice (latex) of Hevea bra-siliensis (Euphorbiaceae) growing in the Amazonian area of Brazil and southeastern Asia. It consists essentially of c -polyisoprene. The milky juice is an emulsion of this polyterpene in water stabilized by proteins as protecting colloids. 

Various traws-oligoterpenols isolated from the birch Betula verrucosa (Betulaceae) are known as betulaprenols, labeled according to the number of isoprene units that their molecules contain. Betulaprenol-9 also occurs in tobacco Nicotiana tabacum, Solanaceae). Betulaprenol-11 and -12 are found in the leaves of Morus nigra (Moraceae) and in the feces of silk-worms Bombyx mori) eating these leaves. 

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A rather different type of macromolecule is generated from the polymerization of certain terpenes such as limonene and a-pinene. The polymers generated from these compounds retain a double bond in the structure of the molecule, and it can be argued that they have a mixed saturated and unsaturated carbon chain backbone as shown below  

Polymers with saturated carbon chain backbone 

17 similar to a dimer of a diterpene (one less double bond) 74.89 274 6.95 

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Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

Terpenes are characterized as being made up of units of isoprene in a head-to-tail orientation. This isoprene concept, invented to aid in the stmcture deterrnination of terpenes found in natural products, was especially useful for elucidation of stmctures of more complex sesquiterpenes, diterpenes, and polyterpenes. The hydrocarbon, myrcene, and the terpene alcohol, a-terpineol, can be considered as being made up of two isoprene units in such a head-to-tail orientation (1). 

Natural rubber displays the phenomenon known as natural tack. When two clean surfaces of masticated rubber (rubber whose molecular weight has been reduced by mechanical shearing) are brought into contact the two surfaces become strongly attached to each other. This is a consequence of interpenetration of molecular ends followed by crystallisation. Amorphous rubbers such as SBR do not exhibit such tack and it is necessary to add tackifiers such as rosin derivatives and polyterpenes. Several other miscellaneous materials such as factice, pine tar, coumarone-indene resins (see Chapter 17) and bitumens (see Chapter 30) are also used as processing aids. 

The other class of acrylic compatible tackifiers includes those based on ter-penes. Terpenes are monomers obtained by wood extraction or directly from pine tree sap. To make the polyterpene tackifiers, the monomers have to be polymerized under cationic conditions, typically with Lewis acid catalysis. To adjust properties such as solubility parameter and softening point, other materials such as styrene, phenol, limonene (derived from citrus peels), and others may be copolymerized with the terpenes. 

Common rosin and polyterpene tackifiers typically are fairly low in molecular weight (commonly in the neighborhood of 1000-2000 Da) and they have softening points in the range of 60-120°C. 

In this section the rosins and rosin derivative resins, coumarone-indene and hydrocarbon resins, polyterpene resins and phenolic resins will be considered. The manufacture and structural characteristics of natural and synthetic resins will be first considered. In a second part of this section, the characterization and main properties of the resins will be described. Finally, the tackifier function of resins in rubbers will be considered. 

I.4. Polyterpene resins. Terpene resins are obtained from natural terpene monomers obtained from naval stores, paper pulp production, and citrus juice production. Terpenes are found in almost all living plants, and the turpentine oil from pine trees is the most important source. 

Tackifiers. Resins are generally added to adjust the desired tack. In general, resins must be used with plasticizers to obtain a good balance between tack and cohesive strength. Typical tackifiers are polyterpenes, although hydrocarbon resins and modified rosins and rosin esters can also be used. In some cases, terpene-phenolics or phenol-formaldehyde resins are added to increase adhesion. 

Polyterpenes. Polyterpenes is one of the first classes of non-polar tack-ifiers to be developed. Terpene monomers are a by-product in the extraction of rosin from wood stumps or tree sap, and from the extraction of oils from citrus fruits. The latter is the dominant source. As such, polyterpene prices generally mirror those of citrus fruits, which fluctuate substantially from one growing season to the next. Terpenes like rosin are cyclic, see Fig. 6, which is partly responsible for their excellent solvent properties. 

Polyterpenes enjoy a number of FDA approvals. They are not only suitable for adhesives with indirect food contact, but also for use in both chewing gum and in films that will have direct food contact. Their higher price compared to petroleum-derived equivalents has resulted in a significant decline in usage over the past 10 years, except where FDA approvals dictate their usage. 

Acyclic C5. The C5 petroleum feed stream consists mainly of isoprene which is used to produce rubber. In a separate stream the linear C5 diolefin, piperylene (trans and cis), is isolated. Piperylene is the primary monomer in what are commonly termed simply C5 resins. Small amounts of other monomers such as isoprene and methyl-2-butene are also present. The latter serves as a chain terminator added to control molecular weight. Polymerization is cationic using Friedel-Crafts chemistry. Because most of the monomers are diolefins, residual backbone unsaturation is present, which can lead to some crosslinking and cyclization. Primarily, however, these are linear acyclic materials. Acyclic C5 resins are sometimes referred to as synthetic polyterpenes , because of their similar polarity. However, the cyclic structures within polyterpenes provide them with better solvency power and thus a broader range of compatibility than acyclic C5s. 

Tackifiers. The tackifiers usually are hydrocarbon resins (aliphatic C5, aromatic C9) or natural resins (polyterpenes, rosin and rosin derivates, tall oil rosin ester). They improve hot tack, wetting characteristics and open time and enhance adhesion. The content on tackifiers in a hot melt can be in the region of 10-25%. 

A class of hydrocarbons occurring in many essential oils of plants. They can be regarded as low-molecular weight polymers of isoprene (C5I I8)n. Dipentene is a terpene, while natural rubber, gutta percha and balata have been termed polyterpenes. 

Polyterpenes, coumarone-indene resins, and so-called petroleum resins are produced commercially using cationic polymerization. These are used as additives for rubber, coatings, floor coverings, and adhesives. 

Hydrocarbon resins comprise a range of low-molecular-weight products (M < 3000) used as adhesives, hot-melt coatings, tackifying agents, inks, and additives in rubber. These include products based on monomers derived from petroleum as well as plant sources. The petroleum-derived products include polymers produced from various alkenes, isoprene, piperylene, styrene, a-methylstyrene, vinyltuolene, and dicyclopentadiene. The plant-derived products include polyterpenes obtained by the polymerization of dipentene, limonene,... 

Terpenes are built out of Cs-isoprene units. There is a variety of terpenes ranging from Cs-terpenes which are called hemiterpenes to tetraterpenes with C40 atoms. There are also terpenes with more Cs-units, such as natural rubber, which is a polyterpene. Terpenes are also classified by the number of carbon cycles in the molecule, e.g., monoterpenes can have up to two cyclic systems in one molecule. Examples of some monoterpenes are shown in Scheme 2 myrcene is acyclic,... 

P-Pinene is an important raw material for various perfumes and polyterpene resins.7 In contrast to a-pinene, only the (-)-isomer of P-pinene is isolated from nature in large quantities. The (+)-isomer of P-pinene (6) is produced mainly by chemical means from (+)-a-pinene (1) through the use of various isomerization methods 5 17 One of these methods is to heat Ipc2BH (3) from 1 with high optical purity to about 130°C, followed by treatment with 1-hexene and benzaldehyde to release the (+)-P-pinene (6) (>99.5% ee) (Scheme 5.3) °... 

The abovementioned materials can be mixed with one another. A series of other polymers and resins can also be added if the substances listed in 1 to 4 form the bulk of the material. Additional materials are PE, PP, low molecular weight polyolefins, polyterpenes (mixtures of aliphatic and cycloaliphatic hydrocarbons produced by polymerisation of terpene hydrocarbons), polyisobutylene, butyl rubber, dammar gum, glycerine and pentaerythritol esters of rosin acid and their hydration products, polyolefin resins, hydrated polycyclopentadiene resin (substance mixtures manufactured by thermal polymerization of a mixture mainly composed of di-cyclopentadiene with methylcyclopentadiene, isoprene and piperylene which is then hydrogenated). 

Polyterpenes are compounds comprising several hundred isoprene units and give rise to natural rubber. Poly means many, so that rubber is made up of many repeating isoprene units. The name for a compound made up of many such repeating units is a polymer. There are many both naturally occurring and synthetically produced polymers of importance with many applications. 

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