Formaldehyde crosslinking

A range of self crosslinking formaldehyde free acrylic resins which impart different degrees of stiffness and improve fabric stability. Cured in a relatively short time above 150C to a wash resistant finish without catalyst. 

Under acidic conditions, furfuryl alcohol polymerizes to black polymers, which eventually become crosslinked and insoluble in the reaction medium. The reaction can be very violent and extreme care must be taken when furfuryl alcohol is mixed with any strong Lewis acid or Brn nstad acid. Copolymer resins are formed with phenoHc compounds, formaldehyde and/or other aldehydes. In dilute aqueous acid, the predominant reaction is a ring opening hydrolysis to form levulinic acid [123-76-2] (52). In acidic alcohoHc media, levulinic esters are formed. The mechanism for this unusual reaction in which the hydroxymethyl group of furfuryl alcohol is converted to the terminal methyl group of levulinic acid has recendy been elucidated (53). 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

It has been reported that tertiary aromatic amines can be encapsulated in formaldehyde-crosslinked microbeads, and used in an adhesive [54]. These microbeads burst when the joint is mated, and cure is accelerated. 

The term novolac refers to the early use of phenolic to replace expensive shellac-based coatings. Novolacs are now those resins made at formaldehyde-to-phenol molar ratios of less than one-to-one. They are generally, though not always, manufactured under acidic conditions. Sulfuric or oxalic acids are most often chosen as catalyst though aromatic sulfonic acids and phosphoric acid are also quite common. Many other acids are used for special purposes. The finished novolac resin is incapable of further polymerization or crosslinking and therefore... 

Forced by the necessity to limit the subsequent formaldehyde emission, the UF-resin molar ratio, F/U, has been progressively decreased to very low values. The main differences between UF-resins with high and with low content of formaldehyde, are (1) the reactivity of the resin due to the different content of free formaldehyde, and (2) the degree of crosslinking in the cured network. 

For example, a UF-resin for particleboard at the end of the 1970s would have had a F/U molar ratio of approx. 1.6-1.8. To day a UF-resin for the same application has a molar ratio of between 1.02 and 1.08, but the requirements for the boards, as given in the quality standards, are still the same. The degree of crosslinking of the cured resins as well as the reactivity of the hardening reaction depends on the availability of free formaldehyde in the system. 

Possible formaldehyde crosslinkers are paraformaldehyde [16,17,144,145], methylolurea mixtures such as urea-formaldehyde precondensate or formurea [145], or urea and phenol methylols with longer chains to overcome steric hindrance. 

The autocatalytic hardening of tannins without addition of formaldehyde or another aldehyde as crosslinker is possible, if small traces of alkaline Si02 are present as catalyst and also a high pH is used, or with certain tannins just by the catalytic action induced by the wood surface [152-160]. 

Depending on the formulations various grades of water resistance can be achieved according to EN 204 (D1-D4) [172], For the two-component PVAc adhesives crosslinking and hence a duroplastic behavior is effectuated by addition of hardening resins (e.g. on basis of formaldehyde), complex forming salts (based... 

An alternative copolymerization is illustrated by the method of Blasius. In this preparation, a phenol-formaldehyde (novolac) type system is formed. Monobenzo-18-crown-6, for example, is treated with a phenol (or alkylated aromatic like xylene) and formaldehyde in the presence of acid. As expected for this type of reaction, a highly crosslinked resin results. The method is illustrated in Eq. (6.23). It should also be noted that the additional aromatic can be left out and a crown-formaldehyde copolymer can be prepared in analogy to (6.22). ... 

Plastics and fibers have been produced from regenerated proteins obtained from a number of sources [17]. The process involves dispersing the proteins in dilute sodium hydroxide followed by extrusion through a spinneret into an acid bath to form the fibers that are then crosslinked with formaldehyde to improve strength. The fibers are used along with silk and wool. 

A series of phosphorus- and bromine-containing FRs were synthesized and studied to understand their role, especially their combined effects. Thus, monocar-danyl phosphoric acid, its bromo derivatives and their formaldehyde condensates and crosslinked products [28,188] were prepared and their properties compared with analogous products made from phenol [28,189]. Table 14 gives the LOI values, char yields (Cy at 600°C), and thermal stability at 50% (T6o) decomposition. 

Crosslinking may occur during the polymerization reaction when multifunctional groups are present (as in phenol-formaldehyde resins) or through outside linking agents (as in the vulcanization of rubber with sulfur). 

In these reactions, the monomers have two functional groups (whether one or two monomers are used), and a linear polymer results. With more than two functional groups present, crosslinking occurs and a thermosetting polymer results. Example of this type are polyurethanes and urea formaldehyde resins (Chapter 12). 

The acid-catalyzed reaction occurs by an electrophilic substitution where formaldehyde is the electrophile. Condensation between the methylol groups and the benzene rings results in the formation of methylene bridges. Usually, the ratio of formaldehyde to phenol is kept less than unity to produce a linear fusible polymer in the first stage. Crosslinking of the formed polymer can occur by adding more formaldehyde and a small amount of hexamethylene tetramine (hexamine. 

CH2)6N4). Hexamine decomposes in the presence of traces of moisture to formaldehyde and ammonia. This results in crosslinking and formation of a thermoset resin ... 

After a de-nib, spray surfacer is applied to build up the film thickness before top-coating. The surfacer contains a high level of pigment and extender (at least 35% by volume) and frequently a saturated polyester resin with a melamine —or urea —formaldehyde crosslinker. The coating is applied at thicknesses up to 35 ftm and stoved for 20 min at 150-165°C. 

Sanding is carried out at this stage and, after clean-up, the final colour or top-coat is applied. There is some variation in the resin chemistry used. Alkyds crosslinked with melamine-formaldehyde are widely used for non-metallic pigmentation. Metallics are usually based on acrylics for better durability. The acrylic may be thermoset with melamine-formaldehyde or a thermoplastic lacquer (plasticised copolymer of methyl methacrylate). A thickness of about 50ftm is applied and stoved for 20 min at 130°C (lacquers receive a bake-sand-bake process for a smoother appearance). 

Used for crosslinking novolacs or catalyzing resole syntheses, HMTA is prepared by reacting formaldehyde with ammonia (Fig. 7.5). The reaction is reversible at high temperatures, especially above 250°C. HMTA can also be hydrolyzed in the presence of water. 

The most common crosslinking agent for novolac resins is HMTA which provides a source of formaldehyde. Novolac resins prepared from a phenol-formaldehyde (F/P) ratio of 1/0.8 can be cured with 8-15 wt % HMTA, although it has been reported that 9-10 wt % results in networks with the best overall performance.3... 

Since a small amount of water is always present in novolac resins, it has also been suggested that some decomposition of HMTA proceeds by hydrolysis, leading to the elimination of formaldehyde and amino-methylol compounds (Fig. 7.15).42 Phenols can react with the formaldehyde elimination product to extend the novolac chain or form methylene-bridged crosslinks. Alternatively, phenol can react with amino-methylol intermediates in combination with formaldehyde to produce ortho-or para-hydroxybenzylamines (i.e., Mannich-type reactions). 

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