Bisphenol A Product Yield

The yield of monomeric diglycidyl ether can be increased by conducting the reaction in two stages, the first being the reaction between epichlorohydrin and bisphenol A in the presence of a catalityc amount of a lithium compund to form the chlorohydrin ether, and the second the reaction wherein dehydrochlorination of this is effected, giving a product consisting of up to 87% of the monomeric of this is effected, giving a product consisting of up to 87% of the monomeric diglycidyl ether when using three moles epichlorohydrin per phenolic hydroxyl group.

It will be seen from the molecular structures shown that whereas the pure monomeric diglycidyl ether contains no hydroxyl groups, as the value of n increases the percentage of hydroxyl groups, as the value of n increases the percentage of hydroxyl content, epoxide content, physical state (liquid or solid) is shown in the table below.




These properties are important in considering adhesives use: for example De Bruyne has shown that there is a correlation between hydroxyl concentration and adhesion (as measured by the joint strength of oxide coated aluminum joints), although recognizing the influence of hydroxyl groups on properties that can indirectly affect adhesion, for example, shrinkage.

Mole/ratio of combined epichloro- hydrin to bisphenol A	Value of n (2)	Epoxy Content (eq/kg)	Hydroxyl content (eq/kg)	Viscocity at 21 oC	Softening point (ASTM)
2:1	0	5.88	0	Crystalline solid at R.T
1.95:1	0.05	5.65	not det.	118 P (3)
1.84:1	0.2	5.05	0.86	318 P
1.32:1	2.15	2.10	2.62		70 OC
1.24:1	3.12	1.63	3.09		75 OC
1.16:1	5.44	1.06	3.36		90 OC

Notes:

1. Calculated from n by using the formula: ratio = (n+2)/(n+1)
2. Calculated from the epoxy content by using the formula n = (2000/284.4E) – 340.4/284.4  Where E = epoxy content in eq/kg
3. (In this calculation the assumption is made that each molecule is terminated by two epoxy group)
4. Determinates on the super cooled liquid.

The Curing of Bisphenol A Epoxy Resin

As all users of bisphenol A epoxy adhesives will know, the uncured (solid) resins have low mechanical strength, and by themselves are of little value simply as hot-melt adhesives. It is only when cured that they make really useful adhesives.

The valuable polymers are those by reaction with crosslinking substances, referred to as accelerators strength, curing agents or hardeners. Certain polymerization catalysts, in conjunction with curing agents, sometimes have a use in adhesives.




The normal method of crosslinking epoxy resins entails a resinification reaction whereby molecular growth takes place throught the linking of dissimilar molecues which, as in the case of the polyurethanes, involves a poly addition reaction with compounds having active hydrogen atoms.

There is, of course, no difference between the curing of an epoxy resin for adhesive use and any other application; nevertheless, one or two curing agents appear to be especially good for loading certain materials, in particular dicyandiamide for the bonding of certain metals.

If the curing agent is a substance of high molecular weight, or more correctly, if its equivalent weight is high with respect to the functional groups involved, a large mass of it will be required to cure the resin; the cured product will therefore contain a large percentage of it, perhaps more than one half. As a result, the mature of the curing agent itself will be built into the final resin molecule and contribute its own part towards physical (including mechanical) and chemical properties. Thus the subject of curing agents is at least as important as the subject of epoxy resins themselves.

As well as the physical properties of the curing agent, the number and the arrangement of its functional groups is important; on these depend the nature and speed of the reaction with the epoxide groups, the nature of the cured polymer, its hot-strength and so on. Equally of course, the epoxy compound itself has a similar influence. Collectively, a number of factors determine the properties of the cured resin; the functionalities of the epoxide and the hardener, the nature of the functional groups of the hardener, the overall structure of both the epoxy resin and the hardener molecules, and the ratio of hardener to resin.

Bisphenol A Epoxy Adhesive

The bisphenol A. Epoxy adhesives stick to most things, and are probably the most versatile of all high strength adhesives. Since the time the early nineteen forties when the commercial importance of epoxy resins became recognized in Switzerland and the USA, an enormous number of patents has been taken out relating to their composition and use; while most of these patents do not deal specifically with adhesive use, many embrace it in a general way. Also numerous papers and three or four books have been published on the subject of epoxy resins and their applications.

Among the early patent are those of Trey Fretes, and Ciba of Switzerland, and Devoe & Reynolds and Shell of USA. Some of these describe use as adhesives while others relate to methods of preparing the resin and hardening it, the glycidyl compounds themselves having been described some years earlier.

Preparation of the Glycidyl Compounds

By reacting epichlorohydrin with a substance having a molecular structure containing replaceable hydrogen atoms such as hydroxyl group, a glycidyl ether R-O-CH2 -CH-O-CH 2 may be obtained, the glycidyl compounds of bisphenol A are the outstanding examples. The molecular structure of this is commonly represented as complete formula that is shown aromatic structure.

The monomeric diglycidyl ether, for which the value of n is 0, is of lower viscosity than higher members of the series, although if sufficiently pure it may be a crystalline solid. As the value of n increases the resin becomes progressively, a more viscose liquid, a solid of low softening point, and a solid of high softening point. The special advantage of a liquid (solvenless) adhesive needs no emphasing; nevertheless, the solid polymers also make useful adhesives, they are in fact together but they are in fact tougher but have lower hot-strengths.

Adhesive Contents

Content of March 2008 - Urea Formaldehyde

• Urea Formaldehyde Adhesive
• Adhesive Scope Discussion

Content of April 2008 - Melamine Adhesive

• Hot Setting Phenolic Glues
• Conversion of Resol to Thermoset Resin (Resite)
• Novolak Formation
• Phenol Formaldehyde Adhesive
• Hardening of Melamine Adhesive
• Stability of Melamine Formaldehyde Resin
• Melamine Formaldehyde Adhesive
• Addition of Polyvinyl Acetate
• Incorporation of the Hardener in the Resin
• Curing Agents for UF Adhesive
• Water Toleration of Urea Formaldehyde Adhesive
• Chemicals Composition

Content of May 2008 - Phenol Formaldehyde

• Resorcinol Formaldehyde Adhesive
• The properties of phenol formaldehyde
• Phenolic Adhesive from Vegetable Tannins
• Thickening Agents
• Fillers and Extenders
• Sulphonated Phenolic Resin Adhesives
• Curing of Cold Setting Phenolic Glues
• Cold Setting Phenolic Glues
• Phenolic Epoxy Adhesive for Metal
• Curing Temperature Phenolic Glue

Content of June 2008 - Oak Adhesive

• The Gluing of Oak
• Wood to Metal Adhesive
• The Application and Properties of Resorcinol Glue
• Addition Filler to Resorcinol Resins
• Curing at Elevated Temperature
• Correlation PH and Mole of Formaldehyde
• The Hardening of Resorcinol Adhesives.
• Resorcinol Phenol Formaldehyde Adhesive

Content of July 2008 - Epoxy Adhesive

• Curing Agents of Epoxy Resin
• Epoxy Resin Adhesives
• Bisphenol A Epoxy Adhesive
• General Procedure of Bisphenol A Reaction
• Curing of Bisphenol Epoxy Resin
• Other Bisphenol Curing Factors

Adhesive Contents of August 2008 - Polyamine Adhesives

• Other Factors Curing of Bisphenol A
• Aliphatic Polyamine
• Polyamines Hardeners
• Aromatic Polyamines Hardeners
  

Curing Agents of Epoxy Resin

Epoxy adhesives comprise a liquid or a fusible solid containing epoxies groups, and a curing agent containing functional groups with which the epoxies groups combine to form a cross linked polymer. A type of so-called curing agent, of little importance in adhesive, is that which catalyses homo polymerization of the epoxy compound.

When the reaction between resin and hardener is capable of taking place at room temperature the two component are mixed immediately before use, but if the mixture is relatively inactive under ordinary storage conditions the components may be mixed together and marketed as a 'single package' adhesive. The 'B-stage' adhesive systems also comprise a single component, but in this case the resin and hardener have been reacted to an intermediate, but still fusible stage.




The curing that takes place when the resin and hardener are brought into intimate contact, for example be melting the solid under appropriate conditions, is an irreversible reaction that result in a thermoset resin. This reaction does not involve condensation polymerization and therefore no low-molecular weight substance such as water is split off. For this reason the shringkage that takes place during cure is slight, and this is one of the properties that makes epoxy resins especially valuable as adhesives. Furthermore, a wide range of end products having a correspondingly wide range of properties can be obtained by use of different resin/hardener adhesive systems.

Epoxy Resin Adhesives

The development of epoxy resins has been directed towards many different field of application, one important application being adhesive bonding. It follows that much of the information which is available does not relate specially to adhesive use, but at the same time can't be separated from it; this, as will be seen from the follow article, applies particularly to methods of curing.

In the unhardened state the chemical structure of an epoxy resin (less well known also as an ethxyline resin) is characterized by the epoxy group.

---C-O-C---

There are two broad methods of adding an epoxide group in the preparation of epoxy resins or epoxy compounds leading to resins. In the first method a substance already containing an epoxide group such as epichlorohydrin (or other epihalohydrin) is reacted with a substance having repleacable hydrogen atoms, and in the second, peracitic acid (or other appropriate oxygen-donating compound) is used to effect epoxidation of an olefinic compound. The product of the first method is a glycidyl compound, which in the most important cases is a glycidyl ether, amine or ester. The glycidyl ether include the bisphenol. A epoxy resins, the type that is usually mean when reffering to epoxy resins in
a general way.



In view of the much larger scale on which the bisphenol. A resins are used, especially in the field of adhesion, they will be deal with separately as Part I of this section. Part II will deal with other glycidyl resin and also with the epoxy resins produced by the second method referred to above. None of the resins discussed in Part II is at present widely used as an adhesive, but some of the polyfunctional glycidyl resins have resistance. Other types have great potentially, and after further development valuable adhesive may be evolved.

Epoxy adhesives comprise a liquid or fusible solid containing epoxide groups, and a curing agent containing functional groups with which the epoxide groups combine to form a cross linked polymer. A type of so-called curing agent, of little importance in adhesives, is that which catalylises homo-polymerization of the epoxy compound.