Phenol-formaldehyde polymers cross-linking

Polymers. AH nitro alcohols are sources of formaldehyde for cross-linking in polymers of urea, melamine, phenols, resorcinol, etc (see Amino RESINS AND PLASTICS). Nitrodiols and 2-hydroxymethyl-2-nitro-l,3-propanediol can be used as polyols to form polyester or polyurethane products (see Polyesters Urethane polymers). 2-Methyl-2-nitro-l-propanol is used in tires to promote the adhesion of mbber to tire cord (qv). Nitro alcohols are used as hardening agents in photographic processes, and 2-hydroxymethyl-2-nitro-l,3-propanediol is a cross-linking agent for starch adhesives, polyamides, urea resins, or wool, and in tanning operations (17—25). Wrinkle-resistant fabric with reduced free formaldehyde content is obtained by treatment with... 

Figure 1.14. Portion of a cross-linked phenol-formaldehyde polymer.

Role of activator is restricted mainly or only to polymer cross-linking. In this case the action of the adhesive becomes equivalent to that of phenol-formaldehyde or urea- formaldehyde. 

Each of these monomers has more than two reactive sites and can react with up to three others to form a cross-linked polymer that is much stronger and more impact-resistant than the linear polymer. The very first synthetic plastic, Bakelite, was made in 1907 from cross-linked phenol and formaldehyde. Modern phenol-formaldehyde polymers are used as adhesives for plywood more than a billion kilograms are produced per year in the United States. 

There are two types of phenol-formaldehyde condensation polymers resoles and novolacs (117). Phenol-formaldehyde polymers prepared from the base-catalyzed condensation of phenol and excess formaldehyde are called resoles. In most phenolic resins commonly used with epoxies, the phenolic group is converted into an ether to give improved alkali resistance. At elevated temperatin-es (>150°C), resole resins react with the hydroxyl groups of the epoxy resins to provide highly cross-linked polymers. 

Phenol-formaldehyde polymers are usually cast in lead molds. The polymer is poured while it is liquid, that is, before extensive cross-linking has occurred. The liquid has the consistency of a syrup. Filled molds are cured at a temperature controlled somewhere in the 60-80 C range. The cure time can range from hours to days. 

A phenol-formaldehyde polymer synthesized under acidic conditions (molar ratio formaldehyde to phenol less than 1) is known as novolak. The novolak can be cross-linked by addition of curing agents. 

As indicated previously, phenol-formaldehyde polymers find practical utilization mainly in the form of network polymers. The polymerization is normally carried out in two separate operations. The first operation involves the formation of a low molecular weight fusible, soluble resin and the second operation involves curing reactions which lead to the cross-linked product. Various types of initial low molecular weight resins are produced commercially and are considered in this section. 

Some commercially important cross-linked polymers go virtually without names. These are heavily and randomly cross-linked polymers which are insoluble and infusible and therefore widely used in the manufacture of such molded items as automobile and household appliance parts. These materials are called resins and, at best, are named by specifying the monomers which go into their production. Often even this information is sketchy. Examples of this situation are provided by phenol-formaldehyde and urea-formaldehyde resins, for which typical structures are given by structures [IV] and [V], respectively ... 

In the case of phenoHcs, it is possible to make linear thermoplastic polymers called novolaks, but this is done by reaction of less than one mole of formaldehyde with one mole of phenol the resulting resin has a large excess of free phenol. Usually in appHcation hexamethylene tetramine (HEXA) is added to the novolak. When heated, the HEXA breaks down into ammonia and formaldehyde and enters the reaction to form a light degree of cross-links in the final product. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Chcmically, Bakelite is a phenolic resin, produced by reaction of phenol and formaldehyde. On heating, water is eliminated, many cross-links form, and the polymer sets into a rocklike mass. The cross-linking in Bakelite and other thermosetting resins is three-dimensional and is so extensive that we can t really speak of polymer "chains." A piece of Bakelite is essentially one large molecule. 

The first widely used synthetic polymer was phenol formaldehyde (Bakelite). It is made by heating phenol (C6H5OH—hydroxybenzene) together with formalde-hyde (H2CO).These react to yield a three-dimensionally cross-linked polymer. To reduce the brittleness of Bakelite, it is usually filled with fibers or platelets of an inert solid. It is a good electrical insulator, relatively hard, and thermally stable to a few hundred degrees Centigrade. Its hardness is 50-60 kg/ mm2 (Mott, 1956). 

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. 

Highly cross-linked condensation materials form the basis for a number of important adhesives and bulk materials, especially phenolic and amino plastics. Most of these products have formaldehyde as one of their starting reactants. These materials are thermosets that decompose prior to melting, and are therefore more difficult to recycle than most condensation polymers that are thermoplastics and do melt prior to decomposition. 

In the early 1930 s, a second type of resin prepared from formaldehyde was introduced to the market—namely, urea-formaldehyde resins. A few years later, melamine-formaldehyde resins also appeared. The same basic process is employed in polymerization of all these resins it consists of the catalyzed reaction of formaldehyde with the second ingredient—phenol, urea, or melamine—to evolve water and produce three-dimensional, cross-linked thermosetting polymers. 

The first synthetic plastics were the phenol-formaldehyde resins introduced by Baekeland in 1907 [1], Melamine and urea also react with formaldehyde to form intermediate methylol compounds which condense to cross-linked polymers much like phenol-formaldehyde resins. Paper, cotton fabric, wood flour or other forms of cellulose have long been used to reinforce these methylol-functional polymers. Methylol groups react with hydroxyl groups of cellulose to form stable ether linkages to bond filler to polymers. Cellulose is so compatible with these resins that no one thought of an interface between them, and the term reinforced composites was not even used to describe these reinforced systems. 

A procedure based on condensation with phenol and paraform (used as formaldehyde source) was developed to convert spent UNEX solvent (CCD, PEG-400, Ph2-CMPO, and FS-13) into a solid infusible resin for disposal. The resulting material is insoluble in aqueous alkali and acidic solutions and organic solvents. Incorporation of FS-13 in the cross-linked polymer was confirmed by physicochemical methods. Resistance of the cured resin to high temperatures was proven by thermogravimetry... 

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