Question

In: Chemistry

Describe the stage of polymerization through which a phenolic resin passes to become thermoset.

Describe the stage of polymerization through which a phenolic resin passes to become thermoset.

Solutions

Expert Solution

In aqueous solution, formaldehyde exists in equilibrium with methylene glycol.

Depending on the pH of the catalyst, these monomers react to form one of two general resin types: NOVOLAC RESINS and RESOL RESINS.



Novolac Resins

An acidic catalyst and a molar excess of phenol to formaldehyde are conditions used to make novolac resins. The following simplified chemistry illustrates the wide range of polymers possible. The initial reaction is between methylene glycol and phenol.The reaction continues with additional phenol, and splitting off of water.

The reaction creates a methylene bridge at either the ortho position or the para position of the phenolic aromatic rings. The "rule of thumb" is that the para position is approximately twice as reactive as the ortho position, but there are twice as many ortho sites (two per phenol molecule) so the fractions of ortho-ortho, para-para and ortho-para bridges are approximately equal.

Branching occurs because reaction can occur at any of three sites on each ring. As the reaction continues, the random orientations and branching quickly result in an extremely complex mixture of polymers of different sizes and structures. The reaction stops when the formaldehyde reactant is exhausted, often leaving up to 10% of un-reacted phenol. Distillation of the molten resin during manufacturing removes the excess phenol and water.

The final novolac resin is unable to react further without the addition of a cross-linking agent. Plenco novolac resins come with and without a curing agent. The resins having the curing agent incorporated cure or "thermoset" to the desired degree when processed by the customer.

Because an additional agent is required to complete the resin's cure, the industry commonly refers to novolac resins as "two-stage" or "two-step" products. The most common phenolic resin cross-linking agent is hexamethylenetetramine, also known as hexa, hexamine, or HMTA. Ground and blended with the resin, hexa serves as a convenient source of formaldehyde when heated to molding and curing temperatures. A special attribute of hexa is that it reacts directly with resin and phenol without producing appreciable amounts of free formaldehyde. Hexa cures the resin by further linking and polymerizing the molecules to an infusible state. Due to the bond angles and multiple reaction sites involved in the reaction chemistry, the resulting polymer is not a long straight chain but rather a complex three-dimensional polymer network of extreme molecular weight. This tightly cured bonding network of aromatic phenolics accounts for the cured materials' hardness, and heat and solvent resistant properties.

Certain catalysts can affect the orientations of the methylene linkages. Catalysts that preferably promote ortho-ortho linkages tend to preserve the more reactive para positions:


Novolac resins made with these catalysts tend to cure more rapidly than the standard randomly linked resins. Novolac resins are amorphous (not crystalline) thermoplastics. As they are most typically used, they are solid at room temperature and will soften and flow between 150° and 220°F (65°C - 105°C). The number average molecular weight (Mn) of a standard phenol novolac resin is between 250 and 900. As the molecular weight of phenol is 94 grams per mole, a Mn of 500 corresponds to a resin where the average polymer size in the entire distribution of polymers is five linked phenol rings. Novolac resins are soluble in many polar organic solvents (e.g., alcohols, acetone), but not in water.

Characteristics


Bonding Strength

The primary use of phenolic resin is as a bonding agent. Phenolic resin effortlessly penetrates and adheres to the structure of many organic and inorganic fillers and reinforcements, which makes it an ideal candidate for various end uses. A brief thermal exposure to complete the cross-linking or "thermoset" process results in attainment of final properties. The unique ability of phenolic resin to "wet out" and to cross-link throughout the fillers and reinforcements provides the means to engineer the desired mechanical, thermal, and chemically resistant properties.

Applications benefiting from the hardness, and heat and chemical resistance properties afforded phenolic resins include abrasive grinding wheels, friction linings, refractory products, and other molded parts used in high temperature or aggressive environments. For years, phenolic resin's exceptional compatibility with cellulose fillers has been used to great benefit for particleboard, plywood, hardboard, oriented strand board, substrates for melamine laminates and decking applications. Composites for demanding applications such as on oil platforms, missile components, and heat shields are produced using phenolic resins along with process technologies such as resin transfer molding (RTM), pultrusion, or filament winding.

Liquid phenolic resins penetrate and saturate paper and other substrates to provide good mechanical strength, electrical properties, or filtration capabilities. Typical examples of these applications include NEMA electrical laminates, decorative laminates, clutch and transmission papers, and filtration products.


High Temperature Performance

A key characteristic of thermoset phenolic resin is its ability to withstand high temperature under mechanical load with minimal deformation or creep. In other words, cured phenolic resin provides the rigidity necessary to maintain structural integrity and dimensional stability even under severe conditions. For this reason, phenolic resin binders meet the challenges of high temperature environments in demanding applications such as refractory, friction, foundry and aerospace products. Examples of applications that take special advantage of the dimensional stability of phenolic molding compounds are natural gas valves, automotive brake pistons, pulleys, and hydraulic and water pump housings and seals.


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