This lecture is given in the memory of Professor Dr. Tadcusz Perkitny, the senior of wood research at the Agricultural University Poznan, who passed away on August 8, 1986 at the age of 84 years after long and successful life for wood science. Among the various lines of work Professor Perkitny was concerned with the up-grading of wood quality by technological processes.
The prospects of wood improvement have been further developed and extended by his successor, Prof. Dr. Lawniczak since 1977 scientific symposia deal every second year with the many facts of wood modification. The most recent one, the 9th September 1993, has covered a wide range of topics for the quality improvement of timber in Europe.
However, the chances of technological developments have to be considered and applied not only for timber, but also for other biological materials which have economic importance in other parts of the world. Much concern exists about the destruction of the tropical forests, about the decreasing availability of timber in these regions and about the lack of employment in rural areas.
One of the consequences is a greater consciousness of renewable local resources, which are plentiful and utilised since ever. I may stress especially for bamboo and rattan as two major biological assets, which give hundreds of millions of people material, shelter, housing, food and employment. Thus, already Professor Perkitny praised the comfort of his bamboo home during his stay in Laos.
The general appreciation of rattan furniture in Poland was demonstrated by at least five companies at the Furniture Fair, April 2, 1994, in Poznan. The wider and wiser utilisation of these often neglected natural goods is the target of quite a number of international, national, institutional and also commercial activities. To mention especially are the International Network for Bamboo and Rattan (INBAR), the IUFRO Project Group P5-04 projects in Costa Rica and on Bali.
Since, about 15 years nearly, countless investigations have dealt with the properties and improved utilisation of bamboo and rattan. Both materials are used for a wide range of commodities, but as a disadvantage both have a low natural resistance against biological deterioration. Since, they do not develop ‘heartwood’ a chemical preservation is often considered as the only solution for increasing their service life. Also certain species of bamboo and rattan are classified as inferior material either, due to cracking, swelling or breaking or, because of their lower technological properties.
Among the many investigations carried out for improving their behaviour, only a few have considered, so far the possibilities of polymer impregnation with the aim to increase technological properties and durability against deterioration by organisms.
Due to the increasing environmental concern about the use of toxicants for the protection of timber and bamboo and the awareness about the safe disposal of treated residues, all possibilities for non-toxic treatments are to be considered.
Investigations in Taiwan have dealt with the manufacture of bamboo plastic combinations with a thermoplastic monomer and with the resistance of bamboo-polymer composites against insects attack. Also rattan has been a material for related studies, as investigations with phenolic resin impregnation have shown.
These efforts are now extended to lesser popular palm, in order to produce value added products. Also the stem of oil-palm, considered for a long time as a waste after the rotation time for harvesting the seed bunches, is subject for improvement by polymer impregnation.
Such innovative approaches are not confined to the bamboo and rattan growing countries, since Prof. Lawniczak has contributed by applying the experience and technology developed in Poznan to the needs of bamboo improvement.
Thus, a paper was delivered at the 4th International Bamboo Workshop 1991 in Chiangmai, Thailand on “Bamboo-Polymer composite-new construction material” and two further ones at the recent 1st National Bamboo Convention, November 1993, in Bandung, Indonesia on “Method of production and properties of the composite bamboo-polystyrene elaborated in Poland”. Also the possibility of cocoas wood polymer composites have been studied in Poznan.
These contributions originated from mutual impregnation of bamboo and rattan tends to apply and established technology with known monomers on the lesser well known material bamboo and rattan. Both have a quite different structure from timber, experimented with so far.
The anatomical make-up of these monocotyledons bears principle dissimilarities compared with timbers, which influence much the results of a treatment and the expected quality of the products generally, any liquid penetration into timber, into bamboo or rattan depends on suitable anatomical pathways.
The considerable influence of the completely different structure of monocotyledons on monomer impregnation has not been outlined so far. The purpose of this treatise is therefore to present the structural parameters of bamboo and rattan for the penetration of monomers. On this basis prospects and also limitations can be evaluated and possible fields of application identified.
Basic Considerations for the Polymer Impregnation of Wood Material:
Numerous investigations have dealt with the production of wood polymer composites the results have shown that:
Monomers do penetrate through the lumina of the cells and through openings in the connecting pits, but much less through pit membranes.
Infiltration of the cell is limited to only few substances with low molecular weight.
Material to be treated must be air-dry, so that no liquid water hinders the flow of the monomer. The presence of water inhibits the polymerisation of some monomers.
To overcome the structural barriers and the hindrance by compressed air due to the invading monomer, an evaporation of the woody material is necessary, followed by a pressure forced penetration, thus requiring a substantial technical instrumentation.
For the impregnation of timber is well established, that great differences exist in the penetration of the monomers according to the species, thus only permeable species can be successfully treated. Treatability also varies within species and within trees. Softwoods are generally less suitable than hardwoods.
This is explained in the sense that softwoods are made-up by tracheids 40-50 m. For penetration any liquid has to pass the connecting bordered pits with their small openings in the membrane. Only the more permeable sapwood can be treated. The influence of the radial ray cells for permeation depends on species.
In hardwood the axial flow is much facilitated through the larger vessels. In comparison with the softwoods their greater size and length ease the penetration considerably. The vessels in diffuse porous wood have a tang, diameter about 100 m like Fagus sp. and Populus sp. and length of around 30 cm and more.
The vessels of certain species have scalar form perforations and they are connected with each other by numerous pits with a solid membrane structure. Both can have a limiting effect on penetration.
The percentage of vessel area on a cross section varies between 8% in Quarcus roobur 1., 12% in Fraxinus excelsior 1, 24% in Betula pendual Ehrh., 31% in Fagus silvatica 1., and in 34 % Populus nigra 1., The porous structure facilities much an easy penetration.
The radial penetration in hardwoods, in contrary, is almost nil, during impregnation the vessels system by dissolving the woody tissue, like heating in concentrated nitric acid, whereby only a resin casts of the vessel system remains. For an efficient penetration the porous structure almost be free of occlusions, which can develop as tylosis or slime during storage and seasoning of the hardwoods. Their influence in the penetration of Populus tremula, by a polystyrene composite was shown by Lawniczak.
Structure of Bamboo and Rattan:
The structure of, dicotyledons as above differ principally to those of monocotyledons. The stem of monocotyledons consists only of a primary tissue due to apical growth. There is no secondary growth due to the absence of a cambium, so that the stem elongates with its final diameter.
The transport system for the water and assimilates is only axially oriented with no radial pathways, like the rays in wood. There is a bark for outside protection of the stem. A very hard skin, the epidermis safeguards the water saturated tissue of the stem, but prevents also any sideways penetration of liquids.
This principle structural make-up becomes modified, if one considers in detail the anatomy of the two plant groups, here is question, namely bamboo and rattan. Bamboo is a giant grass, but rattan belongs to the climbing palms.
In the following for both groups those structural features will be briefly outlined, which are significant of the penetration of monomers in order to obtain polymer composites.
The Pathways of Bamboo:
There are about 1,200 bamboo species of 70 genera, but their growth follows the pattern of a simple grass. The culm elongates only during few weeks until its final height has been reached, which can differ between few meters up-to about 30 m. The young culm will “mature” during the following two years, by which increasing lignification will harden the tissue. Its size, however, will not change during life time.
The culm of bamboo is divided into internodes with a length of about 10 cm upto 100-150 cm and nodes between. The outermost cells of the internodes are covered by a cutinized layer with a waxy coating. The stomata appear occulted.
The inner lining of the culm wall consists of a continuous zone of sclerenchymatic or parenchymatous cells. Pathways for penetration are only the cross-ends of culm segments and hardly the sheath scars around the nodes. Detailed information exists about the anatomical structure of a bamboo culm and its implications for the treatability with preservatives.
Water and assimilates as transported in vascular bundles, consisting of two larger metaxylem vessels, smaller protoxylem elements and surrounds fibre sheaths resp. bundles. The vascular bundles are embedded in a ground parenchyma.
Their location and also size changes from large the bundles with larger vessels at the inner side of the culm to smaller and more crowed bundles with smaller vessels, but often larger fibre sheaths towards the periphery. Within an internode the vessels are strictly axially orientated with no branching or deflection; they provide easy pathways for liquid”‘ movement, especially in fresh condition.
The vessels themselves consist of short cell elements connected by simple openings, so that any liquid can pass freely within the internode. At the nodes, however, only few vessels ran straight through into the following internode. Most turn into the nodal plate, the diaphragm, and become structurally modified by shortening the individual vessel segments and by branching. Any liquid movement through these twisted structures is considerably hindered. Even waterborne preservatives are often stopped at the nodes.
The possible flow of the different monomers through the nodal area has not been investigated yet. Such information is needed for evaluating the passage of the various liquids for a treatment of culm segments with nodes.
Most bamboos culms are hollow within the internode and have tight cell layers wall around a center cavity. Only few species are solid like Dendrocalamus strictus and Chusquea sp. In the vertical, the thickness of the culm wall becomes smaller.
As a result the inner part of the wall with the larger vessels will be reduced, so that the upper culm portion contains smaller vessels and also more fibres, providing a higher strength and flexibility. Consequently, the percentage of the various cell types varies not only across the wall but also with the height of the culm.
Very general, a bamboo culm is composed of 50% parenchyma, 40% fibre and 10% conducting cells, e.g. vessels and phloem. Considerable differences exist between species. For liquid/monomer penetration the two metaxylem vessels of a vascular bundle are most important.
They occupy a rather low percentage of the total area of about 3-5% at lower internodes and 6-12% at higher internodes. Although, these figures differ much according to species and the pattern within a culm less that 10% of a cross-section are available for the penetration of monomers.
This underlines the restricted penetration of monomers into a bamboo culm. Also Liap and Peng (1992) have stated a close relation between size of vessel and monomer loading. The same limitation applies also to the axial treatment of fresh bamboo- culms by sap displacement with waterborne preservatives.
In order to protect the 90 % remaining tissue outside the treated vessels, the preservative must have a good diffusion capacity, so that mostly Boron containing mixtures are applied. In case of monomers, however, it has to be clarified, to what extent diffusion outside of the vessels can take place.
A penetration of only the vessels will certainly reduce water uptake and oxygen supply for a fungal attack, but the effect on technological properties will be less. In addition, the particular species of bamboo and the position of the culm segment to be treated will influence the possible monomer loading and its related effects.
As for wood, the bamboo can also develop tylosis and gum like substances as cellular reactions during ageing, and. after harvesting during seasoning and dying of the culm. The consequences of such occlusions on the penetration of liquids have not been considered so far, even not for the long-term practice of bamboo treatment with preservatives. Since, the origin of the material to the treated, like age and storage condition is hardly known, these cellular reactions could influence considerable the results and their interpretation.
The Pathways of Rattan:
A rattan stem exhibits a certain similarity with a bamboo culm, but it is quite different by its origin and structure. Rattans are climbing spiny palms of the tropical rain forest of the paleotropics, consisting of about 13 genera with ca 600 species.
The plant has a continuous apical growth and reaches a length between half a meter upto 300 m and more. Unlike bamboo the stem is covered with layers of spiny leaf sheaths, which have to be removed before any processing. All rattan stems are solid and nearly cylindrical. Their diameter varies with the species from about 0.5 cm to 15 cm.
Detailed anatomical studies were undertaken mainly in a comprehensive anatomical investigation of all 13 genera by Wainer & Liese (1991, 1981, 1992, and 1993) and for species of the genera Calamus by Bhat et al. (1990, 1993).
A cross section of a stem can be divided into an outer part, termed cortex and a central one. The cortex is covered by an epidermis, consisting of one layer of un-lignified cells with only few stomata between the outer walls is either impregnated with SiO2 or covered by a layer of wax. The sub-epidermal zone is composed of lignified parenchyma cells with small fibre bundles.
This zone provides considerable strength and also a tight sealing of the central part against loss of moisture of vice versa the sideways penetration of liquids during the treatment of a cane. The central corpus presents the typical structure of monocotyledons with vascular bundles embedded in a parenchymatous ground tissue.
Average figures for the various cells type can hardly be accurate due to the variation that exists between species and within a culm. Cum grano salis 20-25% fibres, 30-35%’parenchyma and around 45% vascular bundles give a rough estimate. For the purpose of this presentation the vessels as pathways for penetration are of interest. The vessels are part of the vascular bundles with the additional phloem surrounded by a fibre sheath and parenchyma.
They comprise about 15-20% of a stem, which has consequently a more porous structure than a bamboo culm with 5-10%. The make- up of the vascular bundles is unique among all the many palms, since there are distinct differences among all the 13 genera, which have even generic characters.
Thus the metaxylem consists of one or two vessels accompanied by one or more phloem fields. Rattans with two vessels have thus a greater capacity of liquid uptake. A cross section of a culm reveals that the outer vessels are somewhat smaller than the inner ones. But with less differences than present a bamboo culm. Longitudinally, only a very small increasing trend can be noticed in the size of metaxylem vessels at the central part of the stem.
The diameter of vessels ranges from 150 to 500 m. Noteworthy, for considering the pathways of penetration is the orientation of the vessels, which follows the typical palm pattern. Whereas, in bamboo, the vessels run straight through an internode with connections only in the nodal part, in rattans, the vessels exhibit a slight spiral orientation form the inner part outwards to the leaf sheaths with branching by anastomoses. A penetrating liquid within a rattan stem is consequently more dispersed than within a bamboo culm.
To be mentioned is, also the ground parenchyma with about 30-35%. It shows among the various genera three different types of cellular arrangement (27-30). The so-called type ‘A’ consists of parenchyma cells with larger intercellular spaces between type ‘B’ and even more type ‘C’ with smaller ones.
They could be pathways for penetration, but their permeability has not been investigated so far. Common for both rattan and bamboo are possible cellular reactions by ageing or by wounding. Thus, also a rattan stem can develop tyloses and gum-like substances, which hinder penetration. The possible consequences of such structural alterations in permeability studies have not been considered yet.
Practical Consequences:
Bamboo and Rattan are plat resources of greatest importance for the producing countries and the welfare of their rural populations. Every effort to overcome the wide spread impression of cheap materials with low quality appears justified. Investigations to produce value added products by polymer modifications are very timely. Such efforts must be based on some considerations.
The permeability of bamboo and rattan is restricted to an axial penetration. The vessels comprise only a small area of the cross section. The possible polymer loading will much on their porous structure since the diffusion of monomers into the surrounding tissue is much restricted. Even, for wood, a polymer impregnation does not provide complete resistance towards biological deterioration.
The bio-protective action of those substances depends mainly on limiting the access of water to the wood tissue for fungal invasion as a prerequisite. The bamboo and rattan samples treated so far have a rather short length, hardly suitable for practical use. The depth of axial penetration has to be investigated, according to species and sample origin as well as the influence of nodes in bamboo and anatomises its palms.
The treatment itself requires air-dry material and a closed cylinder for applying the necessary vacuum and pressure which needs special handling and investment. For certain purposes only the lower cross ends have to be treated which is endangered due to soil contact. For such purpose a vertical cylinder filled at a low level with the monomer would be suitable, as proposed by Lawniczak (1993, 1994). Similar pressure plants are in use for food preservation.
Bamboo and Rattan grow fast and are generally available at lower costs, so that even preservation with preservatives for increasing the service life finds only a restricted application due to economic reasons. The production of bamboo/rattan polymer composites would increase the material costs considerably. Therefore, only selected fields of application would merit such a quality improvement.
For rattan a stabilization of furniture parts could be considered and generally an application for wider end uses. Also the improvement of those rattan species which are disregarded and neglected as non-commercial due to their inferior technological properties and unsuitable anatomical make-up should be evaluated.
As for bamboo the quality and service life of poles for horticulture and vineyards could be improved. It is estimated that in Europe, more than 20 million bamboo poles (210-240 cm) are imported annually for fruit trees. Also 100 million grapes are planted in Europe annually, and of those 40 million could be supported by bamboo poles (1.50 m).
These high figures result from the necessary replacement due to a restricted service life at the base. Finally, bamboo splinters are needed to support flower plants, alone for Begonia about 250 million pieces annually in Germany an even more in Denmark or the Netherlands. These sticks have the disadvantage of an easy infestation by the mould fungus Botrytis. Again an improvement without toxic substances may provide an interesting niche.
Supposed, an adequate treatment technology will be available for the quality improvement of bamboo and rattan, the decisive factor for such practical application will be the economy of the final product. Bamboo and also Rattan are still available at relatively low costs, although, the situation will change with the diminishing resources and the increasing wages. This pattern may also have influence on the prospects of bamboo and rattan polymer composites.