Acid-stage resin

Carbohydrate-phenolic-based resins have shown promise for partial replacement of phenol and formaldehyde in exterior plywood adhesives (7,2). Such resins are produced in a two-stage reaction sequence. First, the carbohydrate is reacted with phenol, and sometimes urea, under acid catalysis at elevated temperatures (up to 150 °C), to produce an acid-stage resin. The acid-stage resin is then made basic, formaldehyde added, and the reaction continued at lower temperatures to produce a resol-type resin. Adhesives formulated from these resins have curing speeds consistent with present-day plywood production needs in the western United States, veneers are typically dried to 0 to 7% moisture content and the adhesive cured by hot pressing the panels at approximately 140 to 150 °C and 1.2 MPa. 

Figure 2. 13C-NMR spectrum (in CDCI3) of peracetylated glucose-urea-phenol-based acid stage resin.

Stopping the polymer at this point requires the ratio of formaldehyde to phenol to be less than unity. Both methylene and ether bridges are known to be present. The reaction is either acid or base catalyzed, and branching is uncommon at this stage. The products are variously known as A stage resins, novolacs, or resole prepolymers. 

Elforts have been made to characterize the nature and content of individual components that are present in the low-molecular-mass fraction of the total mill effluents, which include the spent chlorination and alkali extraction stage liquors [2,4]. Approximately 456 types of compounds have been detected in the conventional bleach effluents, of which 330 are chlorinated organic compounds [22]. The compounds may be lumped into three main groups, namely, acidic, phenolic, and neutral (Table 2). Acidic compounds are further divided into the five categories of acids fatty, resin, hydroxy, dibasic, and aromatic acids. The most important fatty acids are formic and acetic acids. The dominant resin acids are abietic and dehydroabietic acids. Among the hydroxy acids identified, glyceric acid predominates. Dibasic acids such as oxalic, malonic, succinic, and mafic acids are derived from the lignin and carbohydrate fraction... 

Baekeland found that a relatively stable resole prepolymer could be obtained by the controlled condensation of phenol and formaldehyde under alkaline conditions. These linear polymers of phenol-formaldehyde (PF) may be converted to infusible cross-linked polymers called resites by heating or by the addition of mineral acids. As shown in structure 4.80, the initial products obtained when formaldehyde is condensed with phenol are hydroxybenzyl alcohols. The linear resole polymer is called an A-stage resin, and the cross-linked resite is called a C-stage resin. 

Leo Baekeland obtained thermoplastic resoles (one-stage resins) by adding stoichiometric quantities of formaldehyde to phenol and heating gently under alkaline conditions. He also produced thermoplastic novolacs (two-stage resins) by using less than stoichiometric quantities of formaldehyde and heating under acid conditions. 

Two-stage resins (novolacs) are produced by the acid-catalyzed reaction of phenol and a portion of the required formaldehyde. Tire resin product is brittle at room temperature. It can be melted, but it will not crosslink. Novolacs can only be cured by the addition of a hardener, almost always formaldehyde supplied as hexamethylene tetramine. Upon heating, the latter compound decomposes to yield ammonia and formaldehyde. 

Tall oil, derived from the Swedish tallolja meaning pine oil, is recovered from the black liquor of softwood pulping. It is taken out at an intermediate stage of the multiple-effect evaporation when the liquor contains about 30% total solids, after it is allowed to stand [21]. The soaps (sodium salts of fatty acids present) are insoluble, cream to the top of the vessel, and are skimmed off. The residual black liquor is returned to the evaporators to continue chemical recovery. The soap yield, which can range from 10 to 200 kg/tonne of pulp (or even higher for pine), is then acidified and the free fatty acids and resin acids obtained are separated by distillation. The fatty acids recovered consist mainly of oleic and linoleic acids and are employed in soap manufacture and as the drying oil components of paints and varnishes [22] (Chap. 19). Resin acids consist of terpene acids such as abietic acid and its positional and reductive variants, and are mainly employed in paper sizing. 

Simple PF resins are readily attacked by sodium hydroxide. However, cresol-formaldehyde, and especially xylenol formaldehyde resins, are much less susceptible to attack. Resins are often more resistant to strong alkaline solutions (i.e., 15 to 20%) than to dilute solutions (i.e., 5%). The filler has a considerable influence on the chemical resistance of the resins. Inert mineral fillers have a better resistance than cellulosic fillers. C-stage resins are resistant to most acids, except sulfuric acid stronger than 50%, formic acid, and oxidizing acids such as nitric and chromic acids. The insolubility of hardened resins in acetone is used to test the degree of cure of the resin. The curing temperature influences the... 

One-stage resin. See Phenolic resin Oniachlor. See Trichloroisocyanuric acid Onion (Allium cepa) extract CAS 8054-39-5 EINECS/ELINCS 232-498-2 Synonyms Allium cepa Allium cepa extract Onion extract... 

Phenol-formaldehyde n. A condensation product of reaction of phenol and formaldehyde. If the reaction is carried under acidic conditions, the product formed is Novolac. Under alkaline conditions, with an excess formaldehyde, an C-stage resin is formed. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. Salamone JC (ed) (1996) Polymeric materials encyclopedia. CRC Press, Raton, FL. See image)... 

The novolac (two-stage) resins are made with an acid catalyst, and only part of the necessary formaldehyde is added to the reaction kettle, producing a mole ratio of about 0.81. The rest is added later a hexamethylenetetramine (hexa), which decomposes in the final curing step, with heat and moisture present, to yield formaldehyde and ammonia, which act as the catalyst for curing. 

The solution-based polyamide-acid process limits the type and speed at which articles can be made from these polyimides. This limitation is true of many polyimides that have very rigid backbones. These polyimides, which have very little flexibility in the polymer chain and a high Tg, are very hard to melt process into complex shapes, especially on a commercial scale. Such intractable polymers are processed into articles in the more flexible polyamide-acid form, often with solvent present. The solvent needs to be removed and recovered to form the final polyimide article. This is true of polyimides based on PMDA and BPDA with ODA, as discussed previously. They have only one flexible hnk in the polymer chain— the ether group of the ODA— and can only be melt-formed in the amide-acid stage or by machining from a sohd block of resin. 

These materials are produced by heating phenol and formaldehyde in the presence of either an acid or a base catalyst. With a base catalyst and an excess of formaldehyde, a composition called resole resins are prepared. With an acid catalyst and an excess of phenol, a two-stage resin called novolacs are made. In both cases high-melting or viscous oligomers are made, which react further at elevated temperatures to produce high modulus but brittle materials (9). 

One-stage resins or resoles contain an adequate number of methylol groups to be cured either thermally (160°-200°C) or by acids. Novolac resins (two-stage resins), which are prepared with an acidic catalyst and less than one mole of formaldehyde per mole of phenol, are permanently soluble and fusible and cure only upon the addition of a curing agent (heat and paraformaldehyde or more preferably hexamethylenetetramine) [13, 78]. 

The thermal hardening of one-stage resins involves simultaneous formation of methylene and ether linkages [87]. Further thermal reactions involve loss of formaldehyde from dibenzyl ether linkages [88]. The latter reaction is catalyzed by either acids or bases. 

When one mole of phenol is reacted with 0.8-0.9 mole of formaldehyde in an acidic medium, the reaction product is a soluble, fusible resin which can be converted into an insoluble, infusible product only upon the addition of excess formaldehyde. These resins are therefore termed two-stage resins known as novolacs. On the other hand, when one mole of phenol is reacted with one or more moles of formaldehyde at a pH of 8 or above (i.e. alkali-catalyzed medium), then insoluble, infusible products are directly formed. These resins are termed one-stage resins and known as resols, which are linear or branched low molecular products. 

Sulfuric acid hydrolyzed samples were neutralized with CaO and then approximately 10% of monomeric phenol was added. The mass was "cooked", dried and pressed producing a strong board resistant to six weeks water immersion (Table 2). This result was most encouraging since phenol costs less than half the selling price of commercial phenol/formaldehyde B-stage resins. Combinations of nitric and sulfuric acids were at least no better than sulfuric acid alone (Table 2). 

NASA-Langley Research Center. The characteristics of the samples in the two sets are given in Table I. The first set of samples were lap-joints of Pasa-Jell cleaned Ti-6-4 panels bonded with one polyimide resin adhesive. The resin adhesive was prepared from benzophenone tetracarboxylic acid dianhydride (BTDA) and m,m -diaminobenzophenone (m,m DABP). The structures of these compounds are given in Table II. The uncured adhesive was applied on the adherend in the polyamic acid stage from either diglyme or DMAC solution and then heat cured to the polyimide resin form. This condensation polymerization reaction is shown below. 

The uncured adhesive was applied on the adherend in the polyamic acid stage from the solvent diglyme and then heat-cured to the polyimide resin form. Tensile lap shear sandwich specimens were prepared by bonding 13 x 2.5 x 0.1 cm Ti-6-4 coupons with a 1.3 cm overlap. Typically, the coated coupons were air dried for 30 min at room temperature and then for 30 min at 60 C. Five successive coats were applied. The panels were overlapped at room temperature, placed under a constant pressure of 50 psi, and heated to 300°C at a rate of 5°C min""l. The specimen was held at 300°C for... 

The major disadvantage of solid-phase peptide synthesis is the fact that ail the by-products attached to the resin can only be removed at the final stages of synthesis. Another problem is the relatively low local concentration of peptide which can be obtained on the polymer, and this limits the turnover of all other educts. Preparation of large quantities (> 1 g) is therefore difficult. Thirdly, the racemization-safe methods for acid activation, e.g. with azides, are too mild (= slow) for solid-phase synthesis. For these reasons the convenient Menifield procedures are quite generally used for syntheses of small peptides, whereas for larger polypeptides many research groups adhere to classic solution methods and purification after each condensation step (F.M. Finn, 1976). 

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