THERMOSETTING ACRYLIC RESINS is a good solvent for acrylamide as well as the less polar monomer. The solvent should also be one which is acceptable as a thinner in the ultimate coating formulation. Alcohols, especially l-butanol, have proved the most useful. If the polymerization is made in 100% butanol, the practice has been to distill about 50% of butanol after the completion of the reaction and to replace this material with a cheaper hydrocarbon solvent such as toluene, xylene, or an aromatic naphtha. The choice of diluent depends on the ultimate use of the resin in spray, dip, or roll coat applications. The distillation of the butanol accomplishes the removal of traces of unconverted acrylate monomer which has an offensive odor and also economizes on the consumption of butanol as the recovered butanol can be recycled. Another important function of an alcohol solvent is as a medium for the reaction of the base interpolymers with formaldehyde. The etherification of methylol groups takes place with butanol rather than giving intermolecular reaction with polymer chains to give premature gelation. A mixture of butanol and a hydrocarbon also has been used very successfully as a reaction medium. In this case distillation is not required if acrylate free systems are used or if conversion is pushed to near completion.
Catalysts and Chain Transfer Agents Several commonly available peroxide catalysts have been used in preparing the interpolymer. Benzoyl peroxide is used with lower boiling solvents where its lower decomposition temperature is advantageous, but most polymerizations have been made with cumene hydroperoxide. I t is often desirable to add catalyst in increments during the polymerization to push the conversion toward completion. The control of molecular weight is accomplished to some degree by the initial charge of catalyst and by the solvent medium, but the principal control is through the use of chain transfer agents such as mercaptans. A large initial charge of catalyst, or polymerization in dilute solutions tend toward lower molecular weights. Dodecyl mercaptan is generally used to secure low viscosity polymers, although satisfactory products have been prepared, especially for cancoating application, without using mercaptan. The general objective is to secure low viscosity or molecular weight products which will spray, dip, or roll coat at relatively high solids without application difficulties when compared to old established materials such as alkyd resins, phenolic resins. or epoxy resins or esters.
Formaldehyde Reaction The formaldehyde reaction may be represented as follows : 0
+
-~ N H ZCH20
O H
II
I
-C-N-CHzOH O H
/I
I
-C-N-CH*OH O H
-
+ CIHQOH
+ HzO
-~-A-~H~--Oc4H~
The reaction is conveniently accomplished by using alcoholic solutions of formaldehyde. The reaction mixture is usually made acidic to prepare the resins described here, although if methylolation without etherification is desired, basic conditions can be employed. The amount of formaldehyde ordinarily used is two times the theoretical required for the formation of methylol or methylol ether groups on each pendant amide group on the polymer chain. The excess amount helps to force the reaction to completion and also compensates for a side reaction of formaldehyde and butanol to form dibutyl formal. Evidence of substantial etherification of methylol groups can be derived from gain in weight of nonvolatile material during the reaction, infrared spectra, and hydroxyl value determinations of the solvent-free polymer.
Resin Cure The cure of a resin system is very striking. Uncured films will have fairly high Sward hardnesses, but tend to be weak, easily shattered, solvent sensitive, poor in adhesion and chemical resistance and, in general, exhibit the properties of addition polymers where molecular weight has been decreased to achieve maximum solubility. When cured the films tend to become tough,
flexible, impact resistant, very hard, very insoluble, and chemically resistant. These properties vary with the composition of the polymer. During the cure of the resins, the methylolated amido group or its alkyl ether can react by splitting of the elements of water, butanol, and formaldehyde to form interchain linkages. If a hydroxyl containing resin such as an alkyd resin with excess hydroxyl content, an epoxy resin such as Shell’s Epon 1001, or any of several resinous polyols is used with the thermosetting polymer, the coreaction can be obtained as follows :
O
H
+ HO-
-C-N-CHn--OH O
R ------+
H
+ HzO
!-&--I!-CH2-0-X O H --C--N-CHZOBU O
+ HO-
R
A
H
-C-N-CHz-0--R
+ BuOH
R represents the residue portion of the hydroxyl containing resin. Each of several hydroxyl groups can react to form an intimate bond and a cross-linking reaction between the thermosetting acrylate resin and the hydroxyl containing resin. Related equations can be written for the coreaction of amide, amine, phenolic, urea, and melamine resins with the thermosetting acrylate resin. Cure temperatures usually range from 300’ to 350’ F. for uncatalyzed systems. The polymers are responsive to acid catalysts, both organic and inorganic, and with such catalysts will cure at temperatures as low as 275’ F. R. M. CHRISTENSON and D. P. HART Pittsburgh Plate Glass CO. Paint Division, Research Center Springdale, Pa.
Coatings Based on Acryla mide lnterpolymers P R E P A R A T I O N AND PROPERTIES of acrylamide interpolymer resins have been discussed by R. M. Christenson and D. P. Hart (page 459). Baked finishes made from these resins, as sole resinous binder and when modified by the additional use of several other types of resins, are described in this article. The choice of monomers incorporated into these resins will give rise to a wide
variety of resins differing in physical and chemical properties. I n turn, coatings can be formulated which are specifically useful for various end uses, ranging from highly resistant one-coat appliance and home-laundry equipment enamels to roll-coatable formulations with the excellent fabrication required in prefinished aluminum or tinplate which will be roll-formed or die-punched VOL. 53, NO. 6
0
JUNE 1961
461
after coating. The properties which are closely controllable by use of various monomers include specifically exterior durability, hardness, flexibility, corrosion and chemical resistance, stain resistance, and color retention on overbake and in ultraviolet light exposure. Cure temperatures for these coatings are in general somewhat higher than those required for alkyd or alkyd-melamine finishes, although they may be reduced with acidic catalysts. Such catalysts include citric acid, phosphoric acid, alkyl ortho phosphates, and p-toluene sulfonic acid derivatives. Internal modification of the acrylamide interpolymer by use of unsaturated acids is also a useful tool for lowering the cure requirements. I n general, pigmented films of these resins require a somewhat higher curing schedule than clear films of the same resin. Pigmentation of acrylamide interpolymer resins presents no problems. These resins are polar in nature and are good wetting vehicles which permit heavy pigment loading during dispersion for economical grinding. Choice of pigments is not limited except as dictated by the conditions of use for the coating. The acrylamide interpolymers are compatible with a wide range of natural and synthetic resins. Among these are chemical plasticizers such as phthalate and phosphate esters, resinous plasticizers such as adipate polyester, vinyl resins, epoxies, epoxidized oils, and certain nitrogen, acrylic. and silicone resins. They are also compatible to varying degree with oil-modified alkyd resins of the drying, semidrying, and nondrying types. Shorter oil length alkyds have better compatibility than longer oil length types. Increased polarity in a n alkyd increases its compatibility, as
% by Weight Resin A Monomer Composition 15 Acrylamide Ethyl acryl60 ate Styrene 25 Resin Solution Solids, % Solvent
50
Resin B
Resin C
15 45
None
40
85
50
50
Butanol- Butanol- Butanolaromatic xylene xylene naphtha W-Y
U-W
X-Z
Weight per gallon, lb.
8.0
8.0
7.9
tion, and ultraviolet yellowing, and is very good in exterior durability. These characteristics are attributable to the high level of ethyl acrylate in the resin as well as the cross-linked toughness derived from the acrylamide level. No other modifying resin has been used in this enamel. An enamel of this type is useful for roll-coat and litho finishes requiring post-fabrication.
Enamels from Resins B and C Each of these two resins was pigmented with titanium dioxide (rutile) at a level of 0.6 part per 1.0 part by weight of total binder solids. The binder solids were adjusted by addition of a solution of epoxy resin (Epon 1001 or its equivalent) to give 91% of the resin solids as the acrylamide interpolymer and 9% as the epoxy resin. The two enamels were reduced to spray viscosity with xylol, applied to phosphated steel a t 1.5-mil dry film and cured 30 minutes at 300' F. Phosphoric acid catalyst was used at 0.570 level on binder solids.
Enamel Compositions The following three resins are discussed in terms of enamel properties obtainable as the resin composition is varied quite widely.
Properties Film evaluations were made and compared with properties obtained from typical alkyd-amine appliance white enamels based upon coconut alkvd melamine and DCO (dehydrated castor oil) alkyd-melamine. The properties are tabulated in detail in the table. Enamel from Resin B has the desirable properties of flexibility, adhesion. and recoat adhesion of the DCO alkydmelamine system coupled with hardness, mar resistance, resistance to grease stains, detergenr solutions, and salt spray
Enamel from Resin A The resin was pigmented with rutile titanium dioxide at a level of 0.89 part per 1.0 part binder solids, catalyzed with 0.5yo phosphoric acid on resin solids, roll-coated to give 1-mil dry film, baked 30 minutes at 275' F. on electrolytic tinplate and also on phosphated steel. This composition is extremely flexible and impact-resistant, and possesses excellent resistance to overbake discolora-
These Film Evaluations Are Compared with Properties Obtained from Typical Alkyd-Amine Appliance White Enamels Test Procedure
15
Viscosity, GardnerHoldt
462
will certain alkyd modifiers which decrease functionality of the alkyd system. The compatibility varies depending upon the composition of the acrylamide interpolymer and its modification. Carboxyl modification of the interpolymer will generally lead to wider range of compatibility. Several specific coatings using epoxy, vinyl, and epoxidized oil modification are described. These coatings may be applied by any of the techniques used for other industrial finishes. These include brushing, airspraying, clectrostatic spra) ing, hot spray, airless spray, roll-coat, dipping, flow-coating, and curtain coating. Certain of these application methods require solvent changes in the acrylamide interpolymer resin, and base resin-t) pes may be prepared in several solvent variations. The most generally useful solvents are toluene, xylene, aromatic naphthas. and alcohols. The solvent tolerance of individual resins varies when attempting to use borderline solvents such as aliphatic naphthas.
Gloss, 60' meter Pencil hardness Mar resistance Adhesion Recoat Adhesion Impact resistancea Vegetable oil immersionb 5 % Salt spray creepage' Humidity (1000 hours) Detergent resistanced
Enamel from Resin B
+
Coconut Alkyd Melamine Enamel
Dehydrated Castor Alkyd hIelamine 80 f
90 2H Excellent Excellent Excellent
S5+
F Good Poor Very poor
HB Fair Excellent Excellent
6-12
2 2B-B
24+
H
Nil
1/18
No change
No change Medium no. 8 (Fair)
Few, No 6 (Good)
inch
Less than 3B 1/8
inch
Minute blisters Dense No. 8 (Poor)
Enamel from Resin C
+
90
3H Excellent Excellent Excellent 2 2H Nil No change Nil (Excellent)
Stain resistance Heavy stain No change Very faint mark Stain Mustard No change Heavy stain Stain No change Lipstick No change Heavy stain Slight stain No change Ink Rated Rated after 100 hours a t looo F. as pencil hardness. a Measured in inch-pounds. Rated after 100 hours contact. after 250 hours in 5% NaCl fog at loo0 F.
INDUSTRIAL AND ENGINEERING CHEMISTRY
THERMOSETTING ACRYLIC RESINS which is superior to either alkydmelamine system. Enamel from Resin C is about equal to the coconut alkydmelamine enamel in impact resistance but is much harder and, in general, has better resistance properties then either alkyd finish. I t is more resistant to stains and detergents than the enamel from Resin B, but does not equal the flexibility of the former. Both experimental enamels are significantly harder and more resistant than the enamel from Resin A previously cited. Enamel from Resin B has also been compared to the two conventional alkyd appliance whites in overbake color and gloss retention and shows definite superiority when overbaking for 1 hour at 350” F. The superiority becomes even more marked when testing 1 hour a i 450’ F.
a
The usefulness of the acrylamide interpolymers as sole resins in enamels, and when modified with epoxy resin has been illustrated. The following is an example using Resin B with a vinyl chloride copolymer resin (Vinylite VM-
CH). P a r t s by Weight Titanium dioxide Resin B solution Xylol Vinylite VMCH solution (25% Solids in 1 to 1 isophorone and methyl isobutyl ketone)
155.0 433.7 23.3 373.0
This composition calculates to be 70Yo by weight acrylamide interpolymer and 3oY0 by weight vinyl copolymer. An enamel film baked 30 minutes at 300O F. is thermoset, highly adhesive to steel,
Parts by Weight Bake 0.5% HaPOa Epoxidieeda Temp., Resin C Oil Minutes O F. 90 10 30 300 75 25 30 300 50 50 30 300 Epoxidieed oil was Admex 710.
Sward Hardness 44 42 Soft
Impact Resistance, Inch-Pound 2 48 48
tinplate, and aluminum foil, tough, and flexible. The film is resistant to alkali, acid, and solvents such as alcohols, aromatic naphthas, and ketones. The compatibility of epoxidized oils with acrylamide interpolymer resins was previously mentioned. Resin C is one of the hardest, least flexible interpolymer compositions. The following shows the plasticizing effect of epoxidized oil interblending testing as clear films. With 10% epoxidized oil, there is little or no reduction in the hardness of the film over using the straight interpolymer resin, whereas there is slight improvement in impact resistance. Other features such as light stability of the film would be improved by the addition of the epoxidized oil. When 25Y0to 50% epoxidized oil is used there is a marked improvement in the flexibility of films. From these comparative properties the general adaptability of acrylamide interpolymer resins for industrial finishing has been briefly indicated.
H. A. V O G E L and H. G. BITTLE Pittsburgh Plate Glass Co. Research and Development Center Paint Division Springdale, Pa.
Epoxy Resins in Thermosetting Acrylics T H E COATINGS INDUSTRY has experienced several recent trends which have encouraged investigation of thermosetting acrylic resins. Some of these important trends are:
1
Naturally occurring polymers and oils continue to give way to “custombuilt” polymers synthesized from pure, highly reactive monomers and intermediates. e As a result of the raw material suppliers’ very large-scale production economics, synthetic resins and monomers continue their almost yearly downward shift in prices but upward improvement in purity. 0There is a constant trend away from large quantities of volatile, hazardous solvents. Synthetic latexes, watersoluble vehicles, high solids, one-coat enamels, and solventless coatings are the results. 0 I n the finishing industry, an important factor is the further expansion of techniques and materials for coating metals prior to fabrication, so that application and curing conditions can be better controlled. 0 To be consistent with these developments, polymer chemistry is offered new challenges. Now, relatively low molecular weight reactive polymer
“building blocks” are needed. Traditionally, a great Portion of Polymer synthesis has been directed toward achieving highest possible molecular weight without cross linking. I n the newer industrial coatings and plastics chemistry, the molecular weight buildthrough up is done by our cross linking [‘in situ” after the material is applied.
A review of the Chemical Abstracts patent literature u p to April 10, 1960 gives the unlimited number of different polymer building blocks and coreactive cross linkers adaptable to thermosetting For our initial studies, our interest was narrowed to the more recent solventborne thermosetting resin art which in-
Reactive lnterpolymer Patents Group I Hydroxylfunctional
U. S.2557266 U. S.2681897 U. S. 2853462 U. S. 2853463
Group I1 N-Methylolfunctional
U. S.28701 16 U. S. 2870117 Belg. 554183 Can. 573728
Group I11 Carboxylfunctional
Group IV Epoxidefunctional
u. s. 2324739
E;!: ;::;;;
1
U. S. 2604464
{iiitT*;:7y7Y8 U. S.2897174 U. S.2900359 Brit. 590035
Can. 491115 Can. 534002 U. S. 2662870 Can. 534001 JU.S.2798861 \Can. 534261 U. S. 2810706 Can. 569430
{
Group V Miso.
u. s. 2524432
U. S. 2604463
U. S. 2580901 U. S. 2687405 U. S. 2692876 U. S. 2723971
U. S. 2899404
U. S. 2729625 U. S.2849418
Can. 567165 Brit. 482897
Brit. 681031
U. S. 2866767
u. s.2857354
U. S. 2868760
VOL. 53, NO. 6
JUNE 1961
463
