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==== Resin and gum ducts ====
Resin and gum ducts can be a normal feature in plant tissue, for instance the resin ducts in ''Pinus '' wood and in ''Canarium '' bark. Production from existing resin ducts is low, however, and exudates are usually formed by secondary resin and gum flow, i.e. by newly formed resin and gum ducts and associated tissues. Their formation is induced by microorganisms, insects, mechanical injury (e.g. tapping), and physiological disturbances. "Gummosis", as this process is known, is thought to be a result of metamorphosis of the organized cell-wall materials to unorganized amorphous substances such as resins and gums.
Secondary resin flow is generally produced in newly formed wood after injury of the cambium. Its resin acid content is different from resin in unwounded tissue (primary resin). Numerous ducts are formed in one, sometimes more, concentric circles in the wood. These are also known as "traumatic ducts" or "traumatic canals" (see Photo 1). After injury, the cambium forms special groups of parenchyma cells instead of normal wood elements. At first, these cells develop schizogenously: they grow intrusively between existing cells. Soon after, the central cells of these parenchymatous groups start to disintegrate and to produce resin and this process proceeds to the periphery. In each cell the disintegration of cell walls starts in the primary wall and then proceeds towards the innermost lamella of the secondary cell wall (lysigenous development). The resulting cavity is filled with resin. As both processes are closely interlinked, this type of resin duct formation is called "schizo-lysigene" (Tschirch & Stock, 1933).
The resin ducts of ''Coniferae '' have been extensively studied. In this taxon the traumatic ducts are located in the wood and are lined with epithelial cells. In some genera (e.g. ''Abies, Tsuga '' (Endl.) Carrière) epithelial cells die in the year of formation, whereas in others (e.g. ''Pinus'') they remain active for several years. In the tangential plane the resin ducts anastomose extensively to form a network of ducts. They have an open connection to the wound and the resin exudes between the renewed bark and the wood exposed by the wound. More importantly, the wound stress reaches the tissues above the wound, where it induces resin duct formation in the wood for up to several centimetres above the wound. The number of resin ducts formed depends directly on the size of the wound (Hillis, 1987; Tschirch & Stock, 1933). Normally-formed resin ducts in ''Pinus '' are elongated structures built of thin-walled epithelial cells surrounding an intercellular space (see Photo 2). The cross-section of this space is round, unlike that of the traumatic ducts, which are much more irregular in shape. On the outer side of the latter there is a sheath of cells of one or more layers with relatively thick walls, very rich in pectic substances. In secondary tissues, both vertical and horizontal ducts occur. The inner end of each radial duct is connected to a vertical duct of the secondary xylem, and the lumina of the two types of ducts are continuous. The vertical ducts have a larger diameter (100-200 μm) than the horizontal ones (30-50 μm) (Hillis, 1987). Radial ducts continue in the phloem, the outer end is enlarged into a cyst-like vesicle. There is no connection between the resin system of the different organs of the primary body (needles, shoots), between them and the duct systems of the xylem and between the separate systems of the xylem itself. This may explain why differences in composition of terpene fractions of resin could be detected at different heights of the trunk and between different organs and tissues. No vertical ducts occur in the phloem of ''Pinus '' (Fahn, 1979). Epithelial cells produce the resin which passes into the lumen of the duct where it collects. In ''Pinus'', the lipophilic droplets have been observed in the plastids, in the periplastidal and cytoplasmic endoplasmic reticulum, in the mitochondria and in Golgi vesicles of the secretory cells, suggesting that these are the locations of formation of the resin (Fahn, 1978). When the volume of resin in a duct increases it compresses the epithelial cells which, because of the reduction of their size, increases their osmotic potential. This encourages absorption of water and when this occurs cells grow larger and pressure is exerted on the resin in the duct. If the latter is ruptured, the resin is forced out, and the epithelial cells may then produce more resin to refill the duct. Obviously this procedure of emptying and refilling can best be performed in ducts lined by thin-walled cells, as found in pines (Hillis, 1987).
The amount of resin exuded when a duct is ruptured is also influenced by the length of the duct. In pines the ducts are much longer than in the other conifers and can average from about 10 cm in the centre to about 50(-100) cm in the outer sapwood layers. Pines are more sensitive to injuries than other conifers; moreover, this sensitiveness increases with the age of the tree. Many of the injuries made by insects result in greater development of ducts than do purely mechanical wounds of the same size. Wounding, pressure, and the auxins indoleacetic acid (IAA), naphthalene acetic acid (NAA), and 2,4-dichlorophenoxyacetic acid (2,4-D) induce the formation of vertical ducts, but radial resin ducts are formed after a layer of ductless wood has first been laid down (Hillis, 1987).
Gums are formed in this way in the bark of ''Acacia senegal'' and other ''Acacia'' spp. Occasionally, gums are formed in the wood, e.g. in the ''Prunoideae'' (''Rosaceae''), where vessels are frequently filled with gum. Here, the gum production most probably results from secretion from vessel-associated parenchyma cells. ==== Laticifers====
A laticifer is a specialized cell or a row of cells containing latex. Two types of laticifers are distinguished based on their formation. Non-articulated laticifers arise from single initial cells in the embryo or seedling. Articulated laticifers consist of chains of cells whose adjoining walls may sometimes break down, forming tubes or vessels (Metcalfe, 1967). Both types can branch, giving rise to anastomosing laticifers, or to non-anastomosing laticifers.
Non-articulated laticifers are formed in the primary tissues and these cells extend by intrusive apical growth into intercellular air spaces. Successive mitoses which are not followed by the formation of cells make the laticifer a multinucleate cell. In the lower portion of the cell the nuclei soon disappear. Growth of the laticifer occurs only in tissues in which the cells have not lost their ability to divide. In lianas the non-articulated laticifers can become several metres long.
Articulated laticifers are formed by anastomosis of the cells in a chain which are of primary or secondary origin. The laticifers of rubber, for instance, occur in the bark, which is a secondary tissue. The end wall of such cells remains entire or becomes porous or disappears. In the latter case the final result is a large multinucleate structure which cannot be differentiated from the non-articulated laticifers. However, all laticifers of secondary origin formed by the cambium of stem or roots are articulated laticifers (Fahn, 1979).
Laticifers in wood (secondary xylem) usually extend radially in the rays (in Apocynaceae, Asclepiadaceae, Campanulaceae, Caricaceae, Euphorbiaceae and Moraceae) or axially (only known in Moraceae).
Articulated laticifers are formed in concentric rings in the bark of the stem of Hevea brasiliensis and in the phloem of the root of Taraxacum kok-saghyz L.E.Rodin. Branching and subsequent anastomosis in these plants only occurs tangentially between laticifers of the same ring.
Non-articulated laticifers are found in the ''Apocynaceae, Asclepiadaceae, Euphorbiaceae '' (excluding ''Hevea '' Aubl. and ''Manihot '' Mill.) and ''Moraceae'', whereas articulated laticifers occur in ''Caricaceae, Compositae, Hevea '' and ''Manihot '' (''Euphorbiaceae''), ''Papaveraceae '' and ''Sapotaceae''. In ''Parthenium argentatum '' A.Gray ("guayule", ''Compositae''), latex occurs in parenchymatous cells. This is, however, the only example of laticifers that are not morphologically differentiated. In a few species, notably ''Guajacum officinale '' ("lignum vitae", ''Zygophyllaceae'') there is rubber in the xylem.
Exudate-producing plants are managed either in plantations, in agroforestry systems or in natural forests (see Table 6). Apart from rubber, the only exudate for which a plantation has been established is gutta-percha: a plantation of ''Palaquium gutta '' was established in Cipetir, West Java, in 1885 and is still functional despite the falling demand for the product. Due to the present low demand for gutta-percha, management of the plantation is at a low level, as the plantation is sufficiently large to supply the present annual demand. It is not economically viable to intensify harvesting, which would increase production. For the same reason, old and/or poorly productive trees are not being replaced and natural regeneration seems satisfactory in the present situation. Finally, note that in South-East Asia no indigenous exudate-producing plants are currently being managed intensively in plantations, primarily for their exudate.There are plantations from which exudates are harvested, but which have another primary goal: the production of timber. This is the case of Pinus and Agathis, although pine resin is of particular economic importance in Indonesia. A century ago both genera were exclusively exploited in natural forest. Since then, exploitation of natural forest has gradually decreased because of dwindling resources and the advent of forest plantations for timber and raw material for pulp and paper manufacture. In these plantations, tapping increasingly proved to be a viable option, especially in the case of tapping Pinus trees a few years before felling. There are now fairly large plantations of both Pinus merkusii Jungh. & de Vriese and Agathis dammara (Lambert) Rich. in Java, where the bulk of pine resin and "Manila copal" is produced in South-East Asia.It is interesting to draw a parallel with the developments in rubber cultivation. Here, plantations have always been primarily geared towards latex production. A fairly recent development is that in these plantations, increased attention is being paid to another product: wood. Rubberwood is now a well-known and a well-accepted product in international trade. The parallel between this development and trends in forest plantations of the other exudate-producing trees (Agathis and Pinus) is that the plantations are becoming more multifunctional, with greater interest being shown in the various products which these trees can yield. Whether this is a general trend in landuse in South-East Asia caused by an increasing pressure on land resources cannot be confirmed here.The natural forest in South-East Asia is generally owned by the state. In most national legislations it is considered a resource from which non-timber products may be collected by all people living in or near the forest. In the case of the harvesting of exudates, it is customary for those persons who started tapping particular trees to exert use rights over these trees for the period of tapping. Only rarely are use rights exerted over individual exudate-yielding trees which are currently not tapped. The appropriation of the rights to individual trees guarantees the sustainable use of the trees. Towards the end of the 19th Century, when many exudates were rapidly becoming important as an economic commodity, exploitation of natural forest was the rule. At that time, some exudates (e.g. gutta-percha) were harvested by felling the tree whereas others (e.g. jelutong) were tapped in such a destructive way that the trees died. The financial gain from natural forest proved to be more important than the sustainability of exudate collection. Hence the exudate-yielding species in certain tracts of forest, especially those which were easily accessible, were rapidly depleted. This was the case for Palaquium trees at the end of the 19th Century. As a result national legislation was endorsed to counteract this depletion. In the Philippines, for instance, exploitation permits specifying the area and the amount of resin which may be collected are mandatory for the tapping of Agathis philippinensis Warb. (Callo, 1995).The case of Styrax in the Tapanuli region (Sumatra, Indonesia) is an instance of natural forest that has always been maintained for the collection of benzoin. Instead of natural forest being depleted, there has been a well-managed transition from natural forest to a forest with a larger proportion of Styrax trees. During the transition, there was both natural regeneration and sowing or planting. Most of the production of benzoin from Sumatra currently comes from agroforests in which a number of annuals and perennials, mainly fruit trees such as durian (Durio zibethinus Murray), jengkol (Archidendron jiringa (Jack) Nielsen), petai (Parkia speciosa Hassk.) and kemiri (Aleurites moluccana (L.) Willd.) have been planted. The plantation and management of several (perennial) crops reduces the risk for farmers, as benzoin prices fluctuate (Achmad, 1998).Another interesting case is the damar forests of Lampung Province (Sumatra, Indonesia), which are an agroforestry system producing the resin from Shorea javanica. In this region all the damar forests have been planted and there has been no gradual transition from natural forest to agroforest. As seed availability is irregular and seeds cannot be stored, the farmers "store" the seedlings instead, using them both in existing damar forests as well as for establishing new ones. The management of existing damar forests is aimed at maintaining a minimum level of different development stages of the damar trees, ensuring a more or less continuous production of these individually owned agroforests. Natural regeneration is probably too unpredictable to rely on when managing the damar forests (Michon & Bompard, 1985), so only the natural regeneration of a number of other tree species yielding timber is maintained. Here too, fruit trees, mainly durian and "duku" (Lansium domesticum Correa) are being introduced as a risk-minimizing strategy of the farmers (de Foresta & Michon, 1994).
It is interesting to draw a parallel with the developments in rubber cultivation. Here, plantations have always been primarily geared towards latex production. A fairly recent development is that in these plantations, increased attention is being paid to another product: wood. Rubberwood is now a well-known and a well-accepted product in international trade. The parallel between this development and trends in forest plantations of the other exudate-producing trees (''Agathis'' and ''Pinus'') is that the plantations are becoming more multifunctional, with greater interest being shown in the various products which these trees can yield. Whether this is a general trend in landuse in South-East Asia caused by an increasing pressure on land resources cannot be confirmed here. The natural forest in South-East Asia is generally owned by the state. In most national legislations it is considered a resource from which non-timber products may be collected by all people living in or near the forest. In the case of the harvesting of exudates, it is customary for those persons who started tapping particular trees to exert use rights over these trees for the period of tapping. Only rarely are use rights exerted over individual exudate-yielding trees which are currently not tapped. The appropriation of the rights to individual trees guarantees the sustainable use of the trees. Towards the end of the 19th Century, when many exudates were rapidly becoming important as an economic commodity, exploitation of natural forest was the rule. At that time, some exudates (e.g. gutta-percha) were harvested by felling the tree whereas others (e.g. jelutong) were tapped in such a destructive way that the trees died. The financial gain from natural forest proved to be more important than the sustainability of exudate collection. Hence the exudate-yielding species in certain tracts of forest, especially those which were easily accessible, were rapidly depleted. This was the case for ''Palaquium'' trees at the end of the 19th Century. As a result national legislation was endorsed to counteract this depletion. In the Philippines, for instance, exploitation permits specifying the area and the amount of resin which may be collected are mandatory for the tapping of ''Agathis philippinensis'' Warb. (Callo, 1995). The case of ''Styrax'' in the Tapanuli region (Sumatra, Indonesia) is an instance of natural forest that has always been maintained for the collection of benzoin. Instead of natural forest being depleted, there has been a well-managed transition from natural forest to a forest with a larger proportion of Styrax trees. During the transition, there was both natural regeneration and sowing or planting. Most of the production of benzoin from Sumatra currently comes from agroforests in which a number of annuals and perennials, mainly fruit trees such as durian (''Durio zibethinus'' Murray), jengkol (''Archidendron jiringa'' (Jack) Nielsen), petai (''Parkia speciosa'' Hassk.) and kemiri (''Aleurites moluccana'' (L.) Willd.) have been planted. The plantation and management of several (perennial) crops reduces the risk for farmers, as benzoin prices fluctuate (Achmad, 1998). Another interesting case is the damar forests of Lampung Province (Sumatra, Indonesia), which are an agroforestry system producing the resin from Shorea javanica. In this region all the damar forests have been planted and there has been no gradual transition from natural forest to agroforest. As seed availability is irregular and seeds cannot be stored, the farmers "store" the seedlings instead, using them both in existing damar forests as well as for establishing new ones. The management of existing damar forests is aimed at maintaining a minimum level of different development stages of the damar trees, ensuring a more or less continuous production of these individually owned agroforests. Natural regeneration is probably too unpredictable to rely on when managing the damar forests (Michon & Bompard, 1985), so only the natural regeneration of a number of other tree species yielding timber is maintained. Here too, fruit trees, mainly durian and "duku" (''Lansium domesticum'' Correa) are being introduced as a risk-minimizing strategy of the farmers (de Foresta & Michon, 1994). === Harvesting === ==== Resin==== Nowadays, resins like copal, damar and elemi which are intended for international markets, are obtained by tapping the tree, rather than collecting fossil resin (in the case of copals) from the ground. To obtain the copal and elemi from the living inner bark of the trunk, the bark of the trunk is incised. Fresh cuts are made at suitable intervals a few days or a week or more, gradually moving up the tree, and the exudate is collected. The size and shape of the cut and its frequency of application vary according to the country or region in which tapping is undertaken, or the tradition of the communities involved. The recommended way of tapping ''Agathis'' and ''Canarium'' trees is shown in Figure 1(1), in which a narrow horizontal strip of bark is removed above the panel without damaging the cambium (Ella & Tongacan, 1992; Ella et al., 1998a, 1998b).
Present practice in Indonesia, specifically in Java (the biggest producer and exporter of copal), is for the tapper to return to the tree to make fresh incisions every 3-4 days; up to 4 or more small tin cups may be in place at different points on the tree at any one time, depending on the size of the tree (Coppen, 1995b). In the Philippines, the second biggest producer of copal, research has been conducted on tapping methods very similar to those used in tapping pine trees (involving the use of sulphuric acid as chemical stimulant) but these methods are not yet being used commercially. Experiments have also been done on using Ethrel as a source of ethylene to stimulate resin flow, but these too have not yet been applied commercially (Ella, 2000).
Crude resin from ''Pinus'' trees is collected in a variety of ways worldwide. Some traditional methods, as used in Indonesia for example, entail making a shallow cut into the wood every 3-4 days. The panel opened is enlarged at every consecutive tapping by removing a narrow strip of bark and a little sapwood above the panel as indicated in Figure 1(2). In many other countries, "bark chipping" methods are used, which involve removing a small strip of bark from the stem and applying an acid stimulant, either as a liquid spray or as a paste. The stimulant does not increase the biosynthesis of resin but simply keeps the resin ducts open for longer. In this way the tapper only has to return to the tree to repeat the treatment every 1-2 weeks. In China an alternative method is used in which small channels are cut into the xylem in a V-shape or "herringbone" pattern, while in India a similar "rill" method of tapping is used, involving the use of an acid stimulant. In all cases, a small container is fixed to the tree, into which the soft resin runs and accumulates. If carried out correctly none of the tapping methods used, or the use of acid, are damaging to the tree. Damar is obtained from the wood of ''Dipterocarpaceae'' (e.g. ''Dipterocarpus, Shorea'') which are tapped by cutting holes in the stem ("boxing") in which the resin can collect (Figure 1(3)). Once again, the number of holes and the frequency of harvesting, scraping the holes and enlarging them vary with species and region (Torquebiau, 1984). Often, fire is used to stimulate the flow of the resin. Sindora wood oil is harvested in the same way. Figure 1(4) shows the particular way in which ''Styrax'' is tapped: a small tongue of bark is loosened up to the cambium. The resin exudes from the rays in the wood and collects behind the tongue, which is cut off after the resin has solidified. The same tree can be tapped again by gradually moving upward along the stem. To counter the detrimental effects of traditional tapping methods, the Forest Products Research and Development Institute (FPRDI) in the Philippines has produced guidelines for the proper tapping of "almaciga" resin (''Agathis philippinensis'') and elemi (''Canarium'' spp.) (Ella et al., 1998a, 1998b). The FPRDI group has already conducted a number of training sessions for resin tappers all over the Philippine archipelago. The tappers learn improved tapping and harvesting techniques to prevent too deep tapping, overtapping by opening too many and/or too large panels, too frequent tapping, and resin contamination with e.g. bark and dirt. The training sessions have been welcomed by both almaciga and ''Canarium'' resin licensees, and the tappers are keen to learn the improved techniques that enable them to preserve the resin-producing trees which have for so many years been providing them with a source of livelihood. ==== Latex====
The techniques for collecting latex differ according to the plant sources. Most latex-producing species, however, are tapped by making series of cuts around the trunk and removing the bark, as is the case for chicle, jelutong, sorva, and gutta-percha. This practice however, is indiscriminate and inflicts great damage to the trees, especially in the case of gutta-percha. The traditional method of harvesting the gutta-percha latex depicted in Figure 1(5) shows a felled tree with rings cut into the bark and with cups placed underneath to collect the exuding latex. With the threat of extinction of the gutta-percha trees and disappearance of the gutta-percha industry, attention has finally focused on more effective and less destructive methods, such as the one evolved in Singapore, Sumatra, Java, and Borneo which involves extracting gutta-percha from the leaves with a chemical or mechanical process. The superior quality of the product obtained from the leaves led to the establishment of a large gutta-percha plantation in Java, which is now the principal source of high-grade gutta-percha.
The latex of the para rubber tree is harvested by tapping the bark without damaging the cambium. Using a V-shaped tapping knife, grooves are cut at an angle of 30° with the horizontal. At each consecutive tapping a thin slice of bark is removed. The latex runs along the groove into a vertical groove which transports the latex via a metal spout to a cup (Figure 1(6)). After 5-6 years of tapping the panel is 150 cm high; in smallholdings, however, bark consumption is much greater. Stimulants are increasingly being applied in rubber plantations, to decrease the dependency on labour which is in short supply.
The tapping of ''Dyera '' trees for their latex is shown in Figures 1(7) and 1(8): in both cases only bark is removed, without damage to the cambium. The tapping panel in Figure 1(7) is enlarged horizontally until the edges meet up. The V-groove in Figure 1(8) is enlarged vertically in both directions and a cup is needed to collect the latex.''Ficus '' and ''Madhuca'', ''Palaquium '' and ''Payena '' trees are not retapped, because the laticifers are not anastomosing and the latex that is harvested is what drains from the immediate vicinity of the place where the bark has been removed. Ficus elastica used to be tapped similar to the techniques indicated in Figure 1(7) and 1(8), but without enlarging the tapping panel.
As can be clearly seen from Figures 1(6),(7),(8), techniques for tapping latex entail removing the bark, but never up to the cambium, thus pathogens do not get easy access to the wood. The undamaged cambium can form new bark tissue which may be tapped again.
The relationship between the tissues tapped and the type of exudate is given in Table 7 for the South-East Asian trees dealt with in this book.
==== Gum==== Exudate gums are usually, but not always, obtained by tapping the trees. In Sudan, ''Acacia senegal'' (in natural stands or in "gum gardens") is tapped, but the gum arabic from ''A. seyal'' is collected from natural exudation. In Sudan, tapping methods have been developed which do not damage the trees and normally begin when the trees are just starting to shed their leaves. Traditional methods of making small incisions into the tree with an axe have largely been replaced by one which uses a specially designed tool, a "sunki". This has a metal head fixed to a long wooden handle. The pointed end of the head is pushed tangentially into the stem or branch so as to penetrate just below the bark, and then pulled up so as to strip a small length of bark longitudinally from the wood. Damage to the wood should be minimal. Several branches are treated in a similar manner at one tapping. In subsequent years, the other branches on the opposite side of the previously treated branch are tapped (Coppen, 1995b).
Drops of gum accumulated on the exposed surfaces after tapping are left to dry and harden. It is recommended that hard pieces of gum be picked off by hand and not by knocking them off onto the ground, where they can pick up dirt. Whenever possible, gums should be placed in an open container; the use of plastic bags increases the risk of mould formation.
In the case of ''Astragalus'', gum is produced in the taproot and when the root is cut, it exudes rapidly and the exudate hardens into the characteristic ribbons of tragacanth. The process of tapping entails clearing the earth away from the taproot and making one or two cuts into the upper part of the root.
Unlike exudate gums obtained from the trunk of trees, where the sunlight shining on the tree increases gum flow, most exudation of tragacanth occurs at night, under conditions which minimize the drying out of the gum and maintain the outward flow under high osmotic pressure.
=== Resin===
The primary processing of collected resin entails a preliminary cleaning (picking over by hand and, less commonly, sieving to remove foreign matter such as pieces of bark). Sorting and grading is common practice and is carried out by hand and/or sieving. When grading benzoin, special care is taken not to break the whole "tears" of the resin, which are the most highly valued form of benzoin. Crude pine resin cannot be used in its original form and is rarely exported as such. The distillation of crude pine resin to obtain rosin and turpentine can be considered part of "primary processing". The oleoresin of ''Dipterocarpus '' is filtered locally in gunny sacks to clean it and to partially remove the non-volatile fraction which is more viscous and remains behind.
Much of the Sumatra benzoin which enters international trade has undergone secondary processing. "Block benzoin" consists of pale pieces of damar embedded in a much darker matrix of low quality benzoin. Occasionally, pieces of high-quality benzoin ("almonds") are used instead of damar. The blocks are made according to a well-tried formula that involves cooking the mixture briefly in hot water. After cooling, the mass solidifies and is made into blocks that are easy to transport and handle. Damar acts as a binder, it improves the burning quality as does the presence of powdered bark, although the scent of damar is inferior (Coppen, 1995b; van de Koppel, 1950).
The value-added processing of resin may consist of purification by ethanol, or hydrocarbon solvent extraction to improve its quality for lacquer and varnish manufacture. Distillation of resin or oleoresin may yield valuable essential oil, as is the case for pine resin and elemi.
In the Philippines, the major application of almaciga resin is in the manufacture of varnish and paint by the cottage industry and small furniture makers. Value-added processing entails the pulverization of the raw almaciga resin, which is then dissolved in 95% ethanol. The solution is filtered to remove impurities and insolubles. The clean resin solution is heated at 60°C for 30 minutes, cooled, and is then ready for packing (Tavita & Palanginan, 1999). Modifying almaciga resin chemically or through thermal processing significantly reduces its acid number, usually by the preparation of copal esters, and renders it oil-soluble (Gonzales et al., 1991).
Value-added processing of "pagsahingin" (resin from ''Canarium asperum '' Benth.) for paint production involves heating the raw product with xylene. The solution is screened and filtered to remove impurities. The clean solution is distilled to separate the solvent and the "modified pagsahingin resin".
=== Latex===
Primary processing of the latex like gutta-percha entails pressing the partially formed coagulum into blocks after first softening it in hot water and removing larger pieces of foreign matter. The blocks are then transported to the factory for further processing; if they need to be stored for any length of time before transportation they are best kept under water to avoid spoilage by aerial oxidation (van Gelder, 1950; Williams, 1963).
Preparation of purified gutta-percha involves chopping the blocks of crude material into small pieces, removing the resinous (non-gutta) fraction by dissolution in cold petroleum spirit, and then dissolving the remaining, separated gutta fraction in hot petroleum spirit. The hot extract is drained from any insoluble and foreign matter and then allowed to cool, whereupon the purified gutta-percha separates out. After separation and distillation of residual solvent the hot, plasticized gutta is rolled into sheets and stored, either in the dark in well sealed tins, or in water.
Solvent extraction of gutta from harvested leaves follows the same principles as above, but involves pulverized leaf material instead of chopped crude gutta-percha. Bleaching earth is added to the hot mixture to remove unwanted leaf pigments.
An alternative method of processing the leaves involves digesting the leaf pulp in hot water, and collecting the coagulated latex which separates out and pressing it into blocks.
Rubber latex is filtered and bulked on arrival at the factory. Generally, it is coagulated with formic acid in tanks. To produce sheet rubber the coagulated latex is then milled through pairs of rollers, the last pair of which are ribbed. The milled sheets are dried in a smoke house for several days to produce ribbed smoked sheets ("RSS"). If crêpe rubber or air-dried sheets are required, the coagulated latex is milled using crêpers to produce a well-knitted thin crêpe. After milling, the crêpe can be dried in hot air rooms.
After being softened by grinding or by dissolving in a suitable solvent, the rubber is compounded with other ingredients. The compounds added are for instance carbon black, a filler to increase wearing resistance, whiting for stiffening, antioxidants, plasticizers (usually oils, waxes, or tars), accelerators and vulcanizing agents. After the compounded rubber has been sheeted, applied as coating or moulded, it is vulcanized.
Most commercial rubbers are diene polymers. These rubbers deform readily when the randomly coiled chains extend due to rotation about C-C bonds in the polymer backbone. However, slippage of chains gives unlimited extension, the sample gets thinner and eventually breaks. Vulcanization involves the formation of chemical crosslinks between neighbouring chains, thereby preventing chain slippage, limiting extension and ensuring that the original dimensions are recovered on removal of the load. Vulcanization is effected by heating the rubber up to 140-170°C with sulphur, one or more organic accelerators, a metal oxide (ZnO) and an organic acid (stearic acid). It comprises a complex sequence of parallel and consecutive reactions. Many different formulations are used, the properties of the product being a function of the particular recipe. Vulcanization can be applied in moulds (e.g. in car tyre manufacture) where the transfer of heat is more favourable than in the case of "boiler vulcanization" (e.g. with hot air). Non-vulcanized rubber is sticky and rapidly becomes hard and brittle (Compagnon, 1986).
Although the invention of the vulcanization process is ascribed to Charles Goodyear in the year 1839, in Ancient Mesoamerica (by 1600 B.C.) people were already making rubber articles like solid balls, important in religious ball games, and hollow human figures and bindings. They applied the juice of "morning glory" (''Ipomoea alba'' L.) to the latex of ''Castilla elastica'' Sessé to make rubber articles elastic and tough, but still workable. Morning glory also contains sulphur compounds that are capable of cross-linking the latex polymers and introducing rigid segments into the polymer chains. Today, local communities in Chiapas, Mexico still process rubber using the same methods as recorded by the Spanish observers in the 16th Century, i.e. mixing the latex with the juice of ''Ipomoea alba'' (Hosler et al., 1999). === Gum===
Gums such as gum arabic are often further processed into kibbled and powdered forms after they have been imported into the consumer countries. Kibbling entails passing whole or large lumps of gum through a hammer mill and then screening it to produce smaller granules of more uniform size. These pieces dissolve more easily in water, and under more reproducible conditions, than the raw gum and so are preferred by the end-user.
Powdered gum may be produced kibbled, or by a process known as spray drying. This results in a high-quality, free-flowing powder with even better solubility characteristics than kibbled gum. The gum is dissolved in water, filtered and/or centrifuged to remove impurities and, after pasteurization to remove microbial contamination, the solution is sprayed into a stream of hot air to promote evaporation of the water. By altering atomizing conditions, powder can be produced with varying particle sizes and bulk densities, according to the end-user's requirements. Spray drying is an energy-intensive process and this, together with the requirements for large quantities of pure water, puts it beyond the reach of most gum arabic producers. The difficulty of handling large volumes of aqueous solution of gum in a producer country - where ambient temperatures are high - without suffering unacceptable increases in the microbiological load aggravates the problem (Coppen, 1995b).
Around 1910, at the time of the "rubber boom" in South-East Asia, breeding activities were started to improve rubber latex production. The first step was clonal propagation by budding, as it was realized that the yield per tree was very variable. Today, there are many clones that yield 6 times what the first plantation trees in South East Asia yielded. It is increasingly being recognized that rubber breeding should focus not only on yield but also on other desirable characteristics, such as vigour, quality of virgin and renewed bark, colour and stability of latex, resistance to leaf and bark diseases and to wind damage and timber volume produced (Webster & Baulkwill, 1989).
The main organizations conducting research and development on exudate-producing plants are:
=== Indonesia===
=== Malaysia===
- Forest Research Institute Malaysia (FRIM), Kepong
