Thermosetting acrylics a r e the first new species
of coating resins since the epoxies which have
had
real industrial significunce.
Production
in 7960 was more than 5 million pounds
Thermosetting Acrylic Resins IN
ADDITION to the home appliance market in which thermosetting acrylics have gained major acceptance, another industry of great importance for these coatings is the precoating of coil stock, steel, or aluminum, for building sheets, residential home sidings, and a host of other end uses. Recent progress in the chemistry of thermosetting acrylics shows promise for use in automotive top coats, and these are now under development in our research department. T h e excellent qualities of coatings formulated from the homo- and copolymers of methyl methacrylate have been known for several decades. Specific development work during 1950-54 led to the large-scale application by 1957 of such thermoplastic acrylic polymers as the basis for automotive lacquer-style topcoats having these good qualities.
Nonyellowing Superior gloss and color holding on exterior exposure Inertness to salt, gasoline, oil, antifreeze, cleaners, and polishes
For more general industrial uses, a relatively low service temperature was recognized as a shortcoming. This has now been corrected by several mechanisms of cross linking which have increased the fusion point and have extended the use of acrylic coatings into the area once held almost exclusively by alkyd-nitrogen resin types. T h e initial identification of commercial products labeled Thermosetting Acrylic Coatings can be attributed to independent research in the laboratories of Canadian Industries Ltd. in Toronto, and Pittsburgh Plate Glass
c o . a t Springdale, Pa. A short time later, third and fourth mechanisms for cross linking were developed in the laboratories of the Du Pont and American Cyanamid Companies. Because the four cross-linking reactions had been accomplished by entirely different means, it is difficult to associate products categorized under the heading Thermosetting Acrylic with a single chemical reaction, as is the case in the other resins employed in surface coating technology. There has been a tendency to oversimplify the categorization of these new coatings by labeling all polymers as Acrylic even when the parent monomers are esters of methacrylic acids. I t is regrettable to find that in current usage, the public or the trade has come to designate as Thermosetting Acrylic those resins which react on heating, becoming highly insoluble, and which, in one way or another, contain derivatives of acrylic or methacrylic acid. Fortunately, it is possible to find some order in the apparent chaos which might have developed, for the resins in this new family do have these common chemical characteristics : @ A resinous backbone is deliberately modified with pendant reactive groups generously spaced along the copolymer chain. These groups are: Carboxylic 11'-Methylol and i\J-alkoxymethyl carboxamide Hydroxyl Allylic double bond-a plastic product; surface coating uses not developed Epoxy groups
e The pendant groups are dormant in resinous solution and in the solid state at room temperature.
These five articles are condensed versions of papers from the Symposium on Thermosetting Acrylic Resins, presented before the Division of Organic Coatings and Plastic Chemistry, 138th Meeting, ACS, New York, N. Y . , September 11-16, 1960. Complete papers will be published in the June issue of the Oficial Digest of the Federation of Societies for Paint Technology.
458
INDUSTRIAL AND ENGINEERING CHEMISTRY
QbSuchpendant groups may be either self- or coreactive. Coreactants (usually resinous) contained in the solution are appropriately chosen so that cross linking proceeds at elevated temperatures. The commonly used coreactive groups for the respective pendant reactive groups are: Carboxyl-die poxides Alkoxymethyl carboxamide-self-reactive, epoxy, alkyd, functional vinyl, etc. Hydroxyl-epoxy and urea or melamine condensation products Allylic double bond-peroxide catalyzed Epoxy-dibasic acids, anhydrides, polyamines
c
During the past year, as the Thermosetting Acrylic designation became accepted in the trade, products were produced which contain a n appreciable ratio of monomers unrelated to acrylic chemistry-such as styrene or vinyl toluene. While substantial amounts of styrene may be included in the otherwise acrylic backbone for economy sake, it continues to be true that the crosslinking reaction depends upon the presence of one or more derivatives of acrylic (or methacrylic) acids. This single fact serves to distinguish the thermosetting acrylic family and can be considered in any expansion of the family to be the criterion for properly naming the various members. Thus, it has become possible to prepare backbone polymers of polystyrene and to cross link these by one or more of the mechanisms as indicated. Such coatings might, in one sense, be called Thermosetting Styrene, but the trade has not accepted this designation and it thus appears that no attempt will be made to be absolutely definitive with regard to the monomer components of the backbone. I t can only be hoped that sufficient honor can remain so that the designation Thermosetting Acrylic will be reserved for those reactions which do, in fact, use derivatives of acrylic and methacrylic acids in the cross-linking steps. This suggestion is highly recommended in view of the fact that the chemical coatings industry regularly creates a certain image of each resin species in terms of durability, chemical resistance, color, and adaptability to specific uses. By whichever of the above mechanisms of cross linking, the total results have provided new coatings-and also one plastic material-which offer some advantages over
the earlier types. Sometimes, these advantages are major, and a t other times, are improvements in minor but important degree, as will be apparent in the following articles. T h e improved properties over alkyd-amine coatings are acknowledged to be in the areas of: Surface hardness Color retention One-coat performance equivalent to primer plus topcoat Stain resistant Adhesion to chemically treated metals Easy fabrication combined with surface hardness Hardness associated with film toughness High temperature service
Having established this new resin series a relatively few years ago, the industry is busy providing tailor-made properties through permutation studies involving variations of monomers, cross-linking density, and molecular weight, and by polyblending with other film formers or plasticizers. T h e spectrum of properties is already broad. Many of the thermosetting acrylic resins are easily compatible with other resins. T h e solvents used are often quite economical. Grinding and pigment wetting properties are good. With such a promising and sound beginning much can be expected in the near future from this welcome new coating polymer series. HOWARD Chairman
L. GERHART
Pittsburgh Plate Glass Co. Springdale, Pa.,
Thermosetting Compositions from Reacting Acrylamide lnterpolymers with Formaldehyde T H E USE of soluble addition polymers in coating applications has generally been a compromise between good properties and reasonable application concentrations. Very soluble low molecular weight polymers have had poor film properties, while increasing the molecular weight to secure good film properties has resulted in low application solids, especially when compared to alkyd resins or blends of alkyd resins and melamine and urea resins. The use of latex compositions has been one method of overcoming these limitations. Another method is to design highly soluble polymers containing functional groups capable of cross linking upon baking to give improved properties to the film. During an investigation of methods of obtaining thermosetting polymers suitable for use in coating application, the use of acrylamide containing addition polymers followed by subsequent reaction with formaldehyde was considered.
Earlier work had been done with polyacrylamide and polymethacrylamide in aqueous systems, but no literature existed on the reaction of solvent soluble interpolymers of acrylamide and other monomers with formaldehyde to give nongelled and stable methylol or methylol ether derivatives of the solution polymer. The interpolymers of acrylamide with other monomers are prepared by simple solution free radical induced polymerization, usually in an alcohol such as
CH-CH2
butanol or a mixture of an aromatic hydrocarbon solvent and butanol, and are reacted with formaldehyde dissolved in butanol in the presence of a mild acidic catalyst. The functional groups that are present in a typical styrene, ethyl acrylate, acrylamide interpolymer are represented in the following structure. The numbers of each monomer unit and their arrangement, of course, depend on the charge and the course of polymerization.
CH-CHp
c=o
I c=o
NH
0
--
I I
C2Hs
0
VOL. 53, NO. 6
Z
0
JUNE 1961
459
