Describe the functions of four- tier layer vegetation in 2 full paragraph??...
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Rain forests are divided into four layers, or stories: emergent layer, canopy, understory, and forest floor. Each layer receives a different amount of sunlight and rainfall, so different types of animals and plants are found in each layer....
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. The higher the degree of condensation during the production process (the higher the viscosity), the shorter is thThe limits of the increase of the degree of condensation in the production process of the resin are given by (1) the viscosity of the resin (the resin must be able to be pumped, a certain storage stability as well as a proper distribution of the resin on the particles during blending is required), and (2) the flow behavior of the resin under heat, guaranteeing wetting of the unglued second wood surface and a sufficient penetration into the wood surface. Decreasing the solid content of the resin is limited by a possible too high moisture content of the glued particles.
Alkaline PF-resins contain free reactive methylol groups in sufficient number and can harden even without any further addition of formaldehyde, of a formaldehyde source or of catalysts. The hardening reaction is initiated by heat. The methylol groups thereby react to methylene and methylene ether bridges. Under high temperatures, ether bridges can be retransformed to methylene bridges. The lowest possible temperature for a technically sufficient gelation rate is approximately 100°C. In some cases potash in the form of a 50 mass% solution is added in the core layer resin mix in an amount of approximately 3–5% potash solid based on resin solid content.
Pizzi and Stephanou [81] investigated the dependence of the gel time on the pH of an alkaline PF-resin. Surprisingly they found an increase in the gel time in the region of very high pH values (above 10). Such pH values, however, are obtained in the case of the usual PF-resins at a content of NaOH of 5 to 10 mass%. A decrease of the pH in order to accelerate the hardening process is not possible, because spontaneous precipitation would occur. A change of the pH of the resin, however, might occur when the resin comes into contact with the acid wood surface. Especially with rather acidic wood species, the pH of the resin could drop significantly [82].
Lu and Pizzi [83] showed that lignocellulosic substrates have a distinct influence on the hardening behavior of PF-resins, whereby the activation energy of the hardening process is much lower than for the resin alone [84]. The reason is a catalytic activation of the PF-condensation by carbohydrates like crystalline and amorphous cellulose and hemicellulose. Covalent bonding between the PF-resin and the wood, especially lignin, does not play any role [84].
Acid-induced gelling reactions of PF resins can cause severe deterioration of the wood substrate and therefore have lost any importance in the wood adhesives field. Pizzi et al. [85] describe a procedure for the neutralization of acid-hardened PF glue lines by partly using as hardener a mix of p-toluene sulfonic acid with a complex of morpholine and a weak acid in order to partly prevent the acid-induced deterioration of the wood substance. Several other attempts, which however failed, have been done by Christiansen [86], incorporating the acid by chemical means into the resin or fixing the hardeners (high molecular polystyrenesulfonic acids) physically within the glue line.
Acceleration of the hardening reaction is possible by using an as high as possible degree of condensation. Another way is the addition of esters such as propylene carbonate [81,87–92] or, even better, glycerol triacetate (triacetin) [81,93,94] or guanidine carbonate [94]. The mechanism of this acceleration, however, is not yet clear. It might be due to the hydrogen carbonate ion formed after hydrolysis of the propylenecarbonate [92], or due to the formation of hydroxybenzyl alcohols and aromatic carbonyl groups in the reaction of the propylene carbonate with the aromatic ring of the phenol [87,88]. The higher the addition of ester, the faster the gel time of the PF-resin [81].
Other accelerators for PF-resins are potassium carbonate, sodium carbonate [91,92] or sodium and potassium hydrogen carbonate. Also wood-inherent chemicals might have an accelerating influence on the hardening reactivity of PF-resins [91].
Since phenolic resins harden only thermally, post-curing during hot-stacking is very important. Different from UF-bonded boards, PF-bonded boards should be stacked as hot as possible to guarantee a maximum post-curing effect. On the other hand, very high temperatures during stacking might already cause partial deterioration (seen as discoloration) of the wood.
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Phenol–Formaldehydes
A. Pizzi, C.C. Ibeh, in Handbook of Thermoset Plastics (Third Edition), 2014
Reactivity and Hardening Reactions of PF Wood Adhesive
Alkaline PF-resins contain free reactive methylol groups in sufficient number and can harden even without any further addition of formaldehyde, of a formaldehyde source or of catalysts. The hardening reaction is initiated by heat. The methylol groups thereby react to methylene and methylene ether bridges. Under high temperatures, ether bridges can be retransformed to methylene bridges. The lowest possible temperature for a technically sufficient gelation rate is approximately 100°C. In some cases potash in the form of a 50 mass% solution is added in the core layer resin mix in an amount of approximately 3–5% potash solid based on resin solid content.
Pizzi and Stephanou [81] investigated the dependence of the gel time on the pH of an alkaline PF-resin. Surprisingly they found an increase in the gel time in the region of very high pH values (above 10). Such pH values, however, are obtained in the case of the usual PF-resins at a content of NaOH of 5 to 10 mass%. A decrease of the pH in order to accelerate the hardening process is not possible, because spontaneous precipitation would occur. A change of the pH of the resin, however, might occur when the resin comes into contact with the acid wood surface. Especially with rather acidic wood species, the pH of the resin could drop significantly [82].
Lu and Pizzi [83] showed that lignocellulosic substrates have a distinct influence on the hardening behavior of PF-resins, whereby the activation energy of the hardening process is much lower than for the resin alone [84]. The reason is a catalytic activation of the PF-condensation by carbohydrates like crystalline and amorphous cellulose and hemicellulose. Covalent bonding between the PF-resin and the wood, especially lignin, does not play any role [84].
Acid-induced gelling reactions of PF resins can cause severe deterioration of the wood substrate and therefore have lost any importance in the wood adhesives field. Pizzi et al. [85] describe a procedure for the neutralization of acid-hardened PF glue lines by partly using as hardener a mix of p-toluene sulfonic acid with a complex of morpholine and a weak acid in order to partly prevent the acid-induced deterioration of the wood substance. Several other attempts, which however failed, have been done by Christiansen [86], incorporating the acid by chemical means into the resin or fixing the hardeners (high molecular polystyrenesulfonic acids) physically within the glue line.
Acceleration of the hardening reaction is possible by using an as high as possible degree of condensation. Another way is the addition of esters such as propylene carbonate [81,87–92] or, even better, glycerol triacetate (triacetin) [81,93,94] or guanidine carbonate [94]. The mechanism of this acceleration, however, is not yet clear. It might be due to the hydrogen carbonate ion formed after hydrolysis of the propylenecarbonate [92], or due to the formation of hydroxybenzyl alcohols and aromatic carbonyl groups in the reaction of the propylene carbonate with the aromatic ring of the phenol [87,88]. The higher the addition of ester, the faster the gel time of the PF-resin [81].
Other accelerators for PF-resins are potassium carbonate, sodium carbonate [91,92] or sodium and potassium hydrogen carbonate. Also wood-inherent chemicals might have an accelerating influence on the hardening reactivity of PF-resins [91].
Since phenolic resins harden only thermally, post-curing during hot-stacking is very important. Different from UF-bonded boards, PF-bonded boards should be stacked as hot as possible to guarantee a maximum post-curing effect. On the other hand, very high temperatures during stacking might already cause partial deterioration (seen as discoloration) of the wood.
View chapter
Purchase book
Phenol–Formaldehydes
A. Pizzi, C.C. Ibeh, in Handbook of Thermoset Plastics (Third Edition), 2014
Reactivity and Hardening Reactions of PF Wood Adhesive
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