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“Non-food Crops-to-Industry schemes in EU27”
WP1. Non-food crops
D3.3 Resins that can be produced in EU27
Lead beneficiary: CHIMAR Hellas SA
Authors: Eleftheria Athanasiadou
The project is a Coordinated Action supported by
Grant agreement no. 227299
Table of contents
According to data from Europen Panel Federation, the consumption of wood-based panels in Europe accounts for 12.5% of the total consumption related to products from the wood working industry .
Wood-based panels or Composite wood panel products are made from wood-based materials bonded together with an adhesive under heat and pressure. The wood-based materials include veneers, strands, particles and fibres. Wood-based panels find application primarily in construction and furniture. The adhesives used for the manufacturing of wood-based panels are thermosetting polymers. In their majority they are formaldehyde based (like urea-formaldehyde, melamine- formaldehyde, phenol-formaldehyde) and isocyanate based (pMDI). The nature of the wood raw material and the adhesive essentially determine the differentiated characteristics of the products. These include mechanical properties, water resistance, dimensional stability, surface quality and machinability.
Traditionally, the adhesives used in the industrial sector of wood-based panels were products synthesized from petrochemical raw materials, although the last few years intense efforts have been exerted for their replacement by materials of natural resources. This concept is not new, since the first man-made adhesives were derived from biomass resources (animal bones, horns and hooves, often modified; celluloid; casein plastics, shellac). Nevertheless, they were abandoned due to their inferior performance and increased cost compared to synthetic adhesives. Nowadays, the need for protection of human health and environment, the finite of the fossil fuel resources as well as the technological advancements rekindled the scientific interest for the production of chemicals and products.
Figure 1: Short-term growth prospects of bio-based materials 
All materials derived from agricultural, forestry, marine and rural biomass exploitation as well as their development in products has generated special industrial and economical sectors called bio-industry and bio-economy accordingly. The bio-industry sector has significant input in the general economy as according to the data presented in figure 1, by 2010 as much as 10% of the chemicals produced in Europe could be biobased, increasing from a value of €77 billion in 2005 to €125 billion in 2010. This represents a 62% increase within 5 years.
The activities of wood-based panels industry benefits from the developments in bio-industry as agricultural and forestry biomass exploitation offers a variety of raw materials useful either for the development of natural-based adhesives or the replacement of wood.
2. Natural materials useful for the synthesis of polymers and adhesives
Agricultural and forestry biomass consists basically of:
and at a minor extend from:
Most of the above materials are used as such or after treatment and/or fragmentation for the synthesis of polymers suitable to be used as adhesives in the wood-based panels industry. Till today these natural materials have been effectively used only in partial replacement of petrochemical raw materials or as additives to improve the performance of the existed synthetic polymers. However research is carrying out for the development of effective adhesives with materials totally derived from renewable resources.
Till today, the materials that have been mostly studied and utilized in the synthesis of polymers and adhesives are: cellulose, starch, tannin, lignin, bio-oil and its phenolic fractions derived from the fractionation of biomass, proteins, natural oils and their derivatives.
Cellulose is one of the main cell wall constituents of all major plants, both non-lignified (such as cotton) and lignified (such as wood) ones. It is also found in the cell walls of green algae and the membranes of most fungi. Chemically, cellulose (Figure 2) is a complex polysaccharide (C6H10O5)n with crystalline morphology. It is a polymer of glucose in which the glucose units are linked by β-1,4-glucosidic bonds . Cellulose yields only glucose on complete hydrolysis by acid . Cellulose is resistant to hydrolysis due not only to the primary structure based on glucosidic bonds but also, to a great extent, to the secondary and tertiary configuration of the cellulose chain bonds (strong hydrogen bonds may form between neighbouring chains), as well as its close association with other protective polymeric structures in the plant such as lignin, starch, pectin, hemicellulose, proteins and mineral elements . For this reason, cellulose modification is costly, requiring quite harsh processing conditions .
Figure 2: The structure of cellulose
Cellulose was first used as a basis for polymer production in the mid- to late-19th century, when applications in both films and fibres were developed. Today, cellulosic polymers (or cellulosics) are produced by chemical modification of natural cellulose.
Cellulosics have good mechanical properties but are moisture sensitive. As the theoretical melt temperature is above the degradation temperature, cellulose is not thermoplastic and therefore cannot be heat sealed . On the other hand, cellulose esters and cellulose ethers are thermoplastic.
Cellulose derivatives posses excellent film-forming properties but are too expensive for bulk use. Cellulose acetate, cellulose butyrate and cellulose propionate have antistatic properties despite high electrical resistance, are crystal clear, tough, hard, scratch-resistant, insensitive to stress cracking, readily dyeable with brilliant colours, but are not permanently weather resistant. They are used to make a wide range of products including knobs, appliance housings, handles, toys, packaging, consumer products, and automotive parts , as well as electric insulation films, lights and casings. Cellulosics - in particular, acetate and xanthate esters for fibres - can technically partially replace polyester, nylon, and polypropylene, but when compared to them are found of a lower strength to weight ratio and less resistance to rot, mildew, burning, and wrinkling . In the future, another possible substitution route will be bacterial cellulose substituting for standard cellulosics and for non-cellulosics in high-end applications.
Starch is the major storage carbohydrate (polysaccharide) in higher plants and is available in abundance surpassed only by cellulose as a naturally occurring organic compound. It is composed of a mixture of two polymers, an essentially linear polysaccharide – amylose (Figure 3) and a highly branched polysaccharide-amylopectin (Figure 4). The building block for both constituent polymers of starch is the glucose monomer. A starch chain is typically made up of between 500 and 2000 glucose units linked in the 1,4 carbon positions.
Figure 3: A section of the amylose molecule showing the repeating
Figure 4: A section of the amylopectin molecule showing the two different
types of chain linkages.
Native starch exhibits hydrophilic properties while its melting point is higher than its thermal decomposition temperature; hence the poor thermal processability of native starch leads to the need for its conversion to a starch polymer which has a much improved property profile.
The starch crops used include corn, wheat, potato, tapioca and rice. Currently, the predominant raw material for the production of starch polymers is corn. Other sources of starch can also being utilised where price and availability permit it. Examples include the use of potato starch by BIOP Biopolymer Technologies in Germany and a process based on a potato starch waste stream at Rodenburg Biopolymers in the Netherlands.
Polymers based on starch are an attractive alternative to polymers based on petrochemicals, because of their relatively low cost. A starch polymer is a thermoplastic material resulting from the processing of native starch by chemical, thermal and/or mechanical means. When starch is complexed with other co-polymers, the result can vary from a plastic as flexible as polyethylene to one as rigid as polystyrene.
Starch polymers commercialized during the last few years and today dominate the bio-based polymer market.
The majority of starch polymers are produced via extrusion and blending of pure or modified starch. Starch polymers can be converted into finished product on slightly modified standard thermoplastic resins machinery. Conversion technologies in use include film blowing, extrusion, thermoforming, injection moulding and foaming .
Starch polymers find many applications. Packaging is now the dominant application area for Modified Starch Polymers, amounting to 75% of the total market share for starch polymers. Starch-PCL blends are used to laminate paper, cardboard, cotton and other natural fibres. Starch blends are also used for packaging films, shopping bags, strings, straws, tableware, tapes, technical films, trays and wrap film . Further novel applications include materials for encapsulation and slow release of active agents such as agrochemicals . Other small-volume or emerging applications include starch-PVOH blends for diaper backsheets, soluble cotton swabs and soluble loose fillers. Other starch blends are used for cups, cutlery, edge protectors, golf tees, mantling for candles and nets.
In the transportation, complexed starch is used as a bio-polymeric filler to substitute partially carbon black in tyres (between 5-10 % w/w; replacing carbon black and silica: 10-20% w/w) . Apart from other applications, this technology has been jointly developed by Goodyear and Novamont and currently it is being applied by Goodyear for the production of a certain type of tyre.
Benefits from the use of starch polymers include lower rolling resistance, noise reduction, reduced fuel consumption and CO2 emissions, and reduced manufacturing energy requirements .
In 2002, about 30,000 metric tonnes per year of starch polymers were produced and the market share of these products was about 75-80% of the global market for bio-based polymers . A 75% of starch polymers are used for packaging applications, including soluble films for industrial packaging, films for bags and sacks, and loose fill. Leading producers with well established products in the market include Novamont, National Starch, Biotec and Rodenburg.
Today, co-polymers used for blending or complexing may constitute up to 50% of the total mass of the starch polymer product . These co-polymers are generally derived from fossil feedstock. It is envisaged by Novamont that by 2020 it will be possible to produce a polymer based 100% on starch having a similar property profile as these blends of thermoplastic starch and petrochemical copolymers. It is expected that this will be achieved by the development of more efficient chemical and biological starch modification processes .
The word tannin has been loosely used to define two different chemical compounds of mainly phenolic nature, the hydrolysable tannins and the condensed tannins.
Figure 5: Structures of tannin
The former, are mixtures of simple phenols and esters of a sugar mainly glucose with gallic and digallic acids. Their chemical behaviour is analogous to that of simple phenols and can easy substitute phenol in PF adhesives having numerous of disadvantages such as high viscosity, low strength and poor water resistance. The need of modification and the limited worldwide production decrease their chemical and economical interest (Figure 5).
Condensed tannins, on the other hand, constituting more than 90% of the total world production are both chemically and economically more interesting for the production of adhesives. Condensed tannins and their flavonoid precursors are known for their wide distribution in nature and particularly for their substantial concentration in the wood and bark of various trees, acacia (wattle and mimosa), hemlock, quebracho, pine etc. They consist of flavonoid units, which have undergone varying degree of condensation, carbohydrates, amino and imino acids. The simple carbohydrates and hydrocolloid gums are often present in sufficient quantities to influence viscosity and reactivity, and the variation in their percentages would alter the physical properties of tannins (Figure 5). In wattle tannin up to 70% of the main polyphenolic pattern is represented by resorcinol A rings and pyrogallol B rings. The quebracho tannin presents similar composition as the wattle extract. Completely different patterns and relationships exist in pine tannin with main flavonoid analogs based on phloroglucinol A rings and catechol B rings (Figure 6).
Figure 6: Structures of mimosa (quebracho) and pine tannin
The use of tannin as adhesive in wood industry is known from the literature since 1958.
Tannins, being phenolic in nature, undergo the same well-known reaction of phenol with formaldehyde either base or acid catalysed, mainly weakly base catalysed for industrial applications, however, 30-50 times faster than the phenol. Thus, formaldehyde reacts with tannins to produce methylene bridge linkages mainly with flavonoid A rings, having reactivity comparable though slightly lower than the resorcinol. Furfural aldehyde is also found to be very good cross-linker and plasticiser when coupled with formaldehyde.
Co-polymerisation of urea, formaldehyde and tannin is reported at a pH 5.5-6.5, where the probability of copolymerisation and self-polymerisation could occur at ratio 50:50. However, the compositions applied industrially are in fact mixtures of UF resin with tannin solutions rather than co-polymers.
The phenolic nature of tannins favours the co-polymerisation reaction of tannin with formaldehyde and phenol or resorcinol (Figure 7). Nevertheless, the rapid viscosity development of the adhesive system is still an issue and separate application is required.
Figure 7: co-polymerisation reaction of tannin with formaldehyde.
Tannin can also react with glyoxal and benzaldehyde. Since the tannin molecules are generally large, the rate of molecular growth is high, so that the tannin adhesives tend to have short pot lives. The viscosity of the tannins is strongly dependent on the concentration and increases very rapidly above a concentration of 50%.
Sulfitation of tannins, introduction of sodium sulphonate group, affords tannins of lower viscosity and better solubility (Figure 8).
Figure 8: sulfitation of tannin
The addition of 3% Carboxymethyl cellulose (CMC) also will result in low viscosity tannin adhesive.
Pizzi and Scharfetter  have shown that furfuraldehyde is an efficient cross-linking agent and excellent plasticizer for tannin adhesives when coupled with formaldehyde.
Glutaraldehyde has been shown to react with tannins to produce a slow-forming precipitate whereas precipitates with formaldehyde form much faster.
Reports [15, 16, 17] on the use of condensed tannin-furan systems as wood adhesives have suggested that the combination of these two renewable resources may hold promise as an approach that would permit the forest products industry to be more self-sufficient.
Recently, a newly conceived aldehyde that is not colored, is water clear, nontoxic, and nonvolatile has been developed and produced, namely dimethoxy ethanal (DME), a derivative of glyoxal . This alternative aldehyde could be a promising path for the production of tannin aldehyde binder system.
K. Li et al. have proposed a tannin–PEI mixture performed successfully as a formaldehyde-free wood adhesive [19, 20]. Due to their ease of mixing, tannin–PEI adhesives appeared to be well suited for in situ applications.
Resent studies performed by S. Kim  are focused on hybrid adhesives formed from mixture of tannin with different portion (5%, 10%, 20% and 30%) of PVAc. Tannin/PVAc hybrid adhesives showed better bonding than the commercial natural tannin adhesive with a higher level of wood penetration.
Resins comprised from pure tannin have been synthesised in laboratories all over the world but currently none of these routes are commercially exploited. These resins are produced by tannin autocondensation using catalysts such as hydroxymethylated nitroparaffins and hexamethylene tetramine , silica or other Lewis acids .
Generally there were three main obstacles to the successful commercialization of tannin-based resins: