WO1996037363A1

WO1996037363A1 - Epoxidize oil and cycloaliphatic epoxy resin coating

Info

Publication number: WO1996037363A1
Application number: PCT/US1996/007805
Authority: WO
Inventors: Ramkrishna Ghoshal; James V. Crivello
Original assignee: Polyset Company, Inc.
Priority date: 1995-05-26
Filing date: 1996-05-28
Publication date: 1996-11-28
Also published as: AU5804696A

Abstract

A thermally-cured coating for containers such as cans provides reasonable storage viscosity and practical shelf life and comprises and epoxidized vegetable oil, a cycloaliphatic epoxy resin, a diaryliodonium salt thermal initiator and a wax.

Description

EPOXIDIZE OIL AND CYCLOALIPHATIC EPOXY RESIN

COATING

Background of the Invention

Although largely unappreciated, coatings have made possible the tremendous growth in the use of aluminum cans that has occurred in recent years. In this role, coatings not only serve an aesthetic purpose in providing a highly attractive, clear, glossy consumer product, but they must possess many functional properties as well. For example, the coatings must protect the underlying printed surface from abrasion and from scratching during distribution and handling. Successful coatings must possess excellent adhesion, toughness, lubricity and hardness. The coating must especially possess excellent abrasion resistance for protection against abrasive failure of the container and loss of its contents. Coatings should also possess sufficient hydrolytic stability and adhesion to survive sterilization for those situations in which the container will be sterilized after coating. In addition, the coating materials in the uncured state should have good flow and leveling properties and be capable of high speed application and cure. Further, the components of the coating should have a low level of oral, skin, and eye toxicity and should not contribute to or change the taste of any food contents in the container. Lastly, the cost of a coating material designed for one-time use must be inherently low.

Virtually all major manufacturers of aluminum cans coat the cans with solvent-based polymer coatings. In a conventional thermal cure process, the polymer, in solution, is applied to the container and the container is transferred to a long pin chain which travels through a thermal dryer. The conventional thermal drying oven used to accommodate speeds of 1500 cans per minute (cpm) is approximately 50 feet long, 25 feet high and 8 feet wide. The ink and coating chemistry generally require a peak metal temperature of about 220°C to initiate cross-linking. Conventional thermally cured resin mixtures contain 30 to 40% solvents, which, under the curing conditions, are volatilized to leave behind a solid film. Solvent-based thermal processes have the serious drawback that they generate significant VOC (volatile organic compound) emissions which are coming under more stringent regulation.

One alternative to thermal curing is UV curing. In the UV process, cans are coated in the same manner as in the thermally cured system, but instead of a pin chain taking them into a thermal oven, the cans are conveyed inside the UV chamber where they are cured using a bank of UV lamps. Despite some apparent advantages of a UV-curable coating system, it constitutes only a small fraction of the present market because the only UV-curable resins presently in commercial use have poor adhesion to metal, the UV-curable monomers are potent irritants and sensitizing agents, and they are polymerized by a free-radical mechanism, which is inhibited by oxygen (air) . Moreover their abrasion resistance is only marginally acceptable, and initial capital investment for the changeover to a UV system is high.

An alternative to solvent-based thermal coatings and to UV coatings is a solventless, thermally cured polymer coating. Solventless thermal coatings to date have been attractive in theory (no VOC's), but disappointing in practice. Single component systems have to be refrigerated in storage, and even so, they commonly do not retain reasonable viscosity for more than about a month. They must be warmed just before use and then quickly used, because they have a pot life at working temperature on the order of a couple of hours. On the other hand, two-component systems have reasonable shelf life at room temperature, but are a nuisance to work with because the components must be precisely measured and uniformly mixed immediately before coating.

There is a real need for an inexpensive, single component, thermally curable coating resin that has good adhesion, is non-toxic and non-sensitizing, is rapidly cured, results in a coating that is attractive and durable, has a practically useful shelf life and releases no VOCs on curing. A further important consideration is that the coating not exhibit excessive weight loss on thermal curing. We have found that coating compositions based on pure cycloaliphatics [e.g. 3,4-epoxycyclohexylmethyl 3',4- epoxycyclohexanecarboxylate] lose more than half their mass under conditions employed for commercial thermal curing.

Summary of the Invention

The present invention, in one aspect, relates to a composition for a thermally curable coating comprising:

(a) from 40 to 90 parts of an epoxidized vegetable oil or mixture of epoxidized vegetable oils having an overall oxirane content from about 3% to about 11%; (b) from 5 to 40 parts of a low molecular weight cycloaliphatic epoxy resin or mixture of such resins;

(c) from 1 to 6 parts of a diaryliodonium salt thermal initiator; and

(d) from 1 to 5 parts of a wax or mixture of waxes.

Preferred vegetable oils are epoxidized soybean, linseed, sunflower, meadowfoam, safflower, canola, crambe, vernonia, lesquerella, corn, rapeseed, castor and cashew oils.

A preferred low molecular weight cycloaliphatic epoxy resin is 3,4-epoxycyclohexylmethyl 3',4- epoxycyclohexanecarboxylate.

The preferred thermal initiators are diaryliodonium hexafluoroantimonate and diaryliodonium tetra(perfluorophenyl)borate.

Preferred waxes are paraffin wax, polyethylene wax, polypropylene wax, ethylene-propylene copolymers, and powdered polytetrafluoroethylene. The term "wax" in the art is variously defined as hydrocarbons or additionally as including long chain esters of monohydroxylic alcohols. In the compositions of the present invention, it has been found that ester-type waxes are not well-suited, and the term "wax" as used herein refers to hydrocarbon, fluorinated hydrocarbon and related polymeric waxes only.

The composition may additionally comprise from 5 to 20 parts of an epoxy resin diluent having a viscosity less than 100 cps; Q!-olefin oxides and methyl epoxy esters of vegetable oil acids are preferred for this purpose.

In a more particular aspect, the compositions of the invention comprise: (a) from 60 to 85 parts of epoxidized linseed oil;

(b) from 8 to 32 parts of 3,4- epoxycyclohexylmethyl 3' ,4' -epoxyeyelohexane- carboxylate; (c) from 1 to 2 parts of diaryliodonium hexafluoroantimonate; and

(d) from 2 to 4 parts of a wax or mixture of waxes chosen from the group consisting of polyethylene wax and polytetrafluoroethylene wax.

The composition may additionally comprise about 0.01 parts of a color-correcting pigment or dye. The composition may also additionally comprise from 0.1 to 1.0 parts of an adhesion promoter.

In a process aspect, the invention relates to a process for providing a clear, glossy, adherent, abrasion-resistant coating on a substrate comprising the steps of:

(a) mixing from 40 to 90 parts of an epoxidized vegetable oil or mixture of vegetable oils having an overall oxirane content from about 7% to 11%, from 5 to 40 parts of a low molecular weight cylcoaliphatic epoxy resin, from 1 to 6 parts of a diaryliodonium salt thermal initiator, and from 1 to 5 parts of a wax or mixture of waxes to provide a coating mixture; (b) applying said coating mixture to said substrate to provide a coated substrate; and

(c) heating said coated substrate at a temperature of 175° to 260°C for 10 to 1000 seconds whereby said coating is substantially completely polymerized. The application step is preferably accomplished by roll coating. The preferred heating conditions are at 210 to 230°C for a total of 2 to 4 minutes.

In yet a further aspect, the invention relates to a container, preferably a can, coated with a clear, glossy, adherent, abrasion-resistant coating comprising from 2 to 4 parts of a hydrocarbon or fluorinated hydrocarbon wax or mixture thereof, less than 6 parts of a residue from a diaryliodonium salt thermal initiator for cationic polymerization, and a copolymer of from 40 to 90 parts of an epoxidized vegetable oil and from 5 to 40 parts of a low molecular weight cycloaliphatic epoxy resin.

Description of Preferred Embodiments

The container coating formulations of this invention contain the following four major components.

Component A.

From 40 to 90 parts of an epoxidized vegetable oil or mixture of epoxidized vegetable oils having an overall oxirane content of from 3 to 11%.

Component B.

From 5 to 40 parts of a low molecular weight cycloaliphatic epoxy resin or mixture of such resins. Component C.

From 1 to 6 parts of a diaryliodonium salt as a thermal initiator for cationic polymerization.

Component D. From 1 to 5 parts of a wax or mixture of waxes.

Epoxidized oils which may be employed in component A are typically derived from unsaturated vegetable oils by standard epoxidation techniques as described by H. Lee and K. Neville in The Handbook of Epoxy Resins (1967) pages 3-9 to 3-11. Among such oils are for example, epoxidized soybean, linseed, sunflower, meadowfoam, safflower, canola, crambe, vernonia, lesquerella, corn, rapeseed, castor, cashew, etc. The preferred epoxidized oil of this invention is epoxidized linseed oil . While the epoxidation of vegetable oils may be carried out to various levels, for this application the total epoxidation level of the oil or mixture of oils is preferably above 5% (oxirane content) and more preferably above 7%. The epoxidized vegetable oil content in the final coating formulation can range from 40 to 90 parts but is preferably in the range of from 60 to 85 parts.

Component B can consist of a wide variety of low molecular weight epoxy resins. These resins serve to control viscosity, to modify the flow properties of the coating formulation and to modulate the speed of curing. They also contribute to the mechanical properties of the final cured coating. Compositions having epoxidized vegetable oils as the only polymerizing component tend to cure more slowly, do not provide a coating with enough abrasion resistance and are subject to yellowing, particularly during any heating processes that may occur subsequent to coating. By low molecular weight is meant that the molar mass of the resin should be under 1000 mass units. The quantity of Component B in the coating formulation may range from 5 to 40 parts, but preferably lies in the range from 8 to 32 parts. The term "cycloaliphatic epoxy resin" as used herein refers to epoxy resins in which the reactive epoxide functionality is attached to a 5, 6 or 7-membered ring so as to form an oxabicyclo [n.1.0] alkane, where n is 5, 6 or 7. Examples of those resins which can be employed in compositions of the invention include 3,4-epoxycyclohexylmethyl 3' ,4' -epoxycyclohexane- carboxylate; bis (3,4-epoxy-6-methylcyclohexylmethyl) adipate; bis (3,4-epoxycylohexylmethyl) adipate; vinyl cyclohexene dioxide; bis (2, 3-epoxycyclopentyl) ether and other members of the ERL series of cyclo- aliphatics available from Union Carbide.

The diaryliodonium salt thermal initiators of component C are described in US patents 4,882,201 and 5,073,643 which are incorporated herein by reference. They are generally known as UV photoinitiators, but are not commonly employed as thermal initiators, although US patent 4,842,800 suggests that they may be so used, particularly in combination with copper salts. The diaryliodonium salts which are primarily useful as thermal initiators have the generic formula I :

IV wherein R is hydrogen or -hydroxyalkoxy; M is an element from group Ilia, IVa or Va; X is a halogen and n is an integer equal to one more than the valence of M.

In the above formula the counter-ion of the onium salt initiators is usually the SbF₆ ^" anion. These are the most effective thermal initiators with respect to achieving the most rapid cure speed; however, other thermal initiators bearing such anions as PF₆ ^", AsF₆ ^", BF₄ ^", B(C₆F₅)₄ ^", and CF₃S0₃ ^" are equivalents for some purposes. The tetra(perfluorophenyDborate salts may be preferred in some instances where no residue of antimony can be tolerated. The solubility of the onium salt photoinitiator in epoxidized vegetable oils and in mixtures of these oils with other resins is a critical feature in the choice of an initiator. The preferred thermal initiators discussed above show excellent compatibility with many resin mixtures.

The range of the concentration of the onium salt initiator in the formulation can be from 1-10 parts but is preferably from 1-6 parts to achieve the high cure speeds required for container coatings.

The coating formulation must contain waxes as denoted in Component D, which serve to modify the abrasion, lubricity and cure characteristics of the final, cured coating. If the wax component is omitted, the coating produced upon curing shows a greatly diminished abrasion resistance. Waxes which function in the invention include hydrocarbon waxes, such as paraffin; polymeric hydrocarbon waxes, such as polyethylene wax, polypropylene wax, or ethylene- propylene copolymers; fluorinated hydrocarbon or polymer waxes, such as powdered polytetrafluoroethylene; and mixtures of the foregoing. The amount of such waxes which may be employed in the coating is in the range of 1 to 5%. The preferred amounts are from 2 to 4%.

In the practice of this invention, the above thermally curable coating mixtures may be modified by the addition of various types of additives and modifiers. Among these are adhesion, wetting, flatting and flow control agents, pigments, dyes and fillers. Although the coatings of the invention exhibit excellent adhesion, there may be occasions when a combination of high adhesion and very short processing time are desired. In these circumstances there may be some advantage to adding from 0.1 to 1.0 parts of an epoxy functional silane adhesion promoter such as A186 or A187 (glycidoxypropyl- trimethoxysilane) , available from Union Carbide.

The above mentioned coatings may be cured at temperatures from 175° to 260°C; 210° to 230° appears an optimal range. The cure times are generally on the order of a few minutes and cure may be effected by continuous or discontinuous exposure to heat. We have found that two two-minute cycles are fully effective in achieving adequate hardening, but the combination of time and temperature can be varied according to the needs of the process and apparatus, as will be obvious to the person of skill.

The containers which may be coated using the coating materials of this invention include those made of aluminum, steel, tin coated steel, glass and plastic. Application can be achieved by means of roll, gravure, flow, curtain or knife coating. The techniques are well known in the art and are described, for example, in The Encyclopedia of Polymer Science and Engineering Vol. 3 p 550 to 605 Wiley-Interscience, New York, which is incorporated herein by reference.

Although the coatings have been designed with application to cans in mind, it will be obvious to those of skill that the coatings could be used in any application that requires an inexpensive, durable, attractive coating on a substrate that will withstand an elevated temperature (>200°C ) for a few minutes.

General Procedure

In a plastic container were placed 40-90 parts of epoxidized vegetable oil, 5-40 parts of cycloaliphatic epoxy resin and 2 parts of 50% solution of cationic thermal initiator in the low molecular weight epoxy resin. For the purposes of correcting the color balance in the resulting coating, 1 part of a blue dye was added, but this component is not necessary for the functioning of the invention. The components were mixed with a high speed mechanical mixer for two minutes and 2.5 parts of wax were added. The mixture was further blended for an additional 5 minutes. Zero to ten parts of Vikolox™ 14 (an α-olefin oxide) were added to correct the viscosity to 250 cps at 25°C so that the mixture could be roll coated on the cans. Compositions containing 40 or more parts of cycloaliphatic (ERL- 4221) were too viscous to roll coat without the addition of 10 parts of α-olefin oxide. Atochem™

9010 (methyl ester of linseedate) may also be used to lower viscosity. Additives (1.5-2.0 parts) were provided to improve adhesion and wetting. Table 1

Component Control Expt. Expt. Expt. A B C

Epoxidized linseed oil¹ (Vikof lex 7190) 0 43 60 85

3,4-epoxycyclohexyl- methyl-3',4'-epoxy cyclohexane carboxylate (ERL-4221) 83 40 32 8

Additives 1 .1 1 .3 1 .0 1 .0

Diluent (Vikolox 14) 10 10 0 0 mixture wax and PTFE² (commercially available) 2.5 2.5 3.0 3.0 diaryliodonium salt 1 1 1.5 1

1 % phthaloblue dispersion in ERL-4221

1 0.8 1 1

¹Epoxide content 9.02%. ²ratio unknown.

The formulations were roll coated by a reverse roll coating process (see Encyclopedia of Polymer Science and Engineering p 561-562) onto aluminum cans, which had been previously printed with graphics, and were thermally cured on a commercial can line at about 218° C for two 2-minute cycles.

The amounts of the formulations applied were in the range of 65-120 mg/can, which gives a coating about 5μm thick. After coating, the cans were subjected to various tests to determine the properties of the coatings. Some or all of the following tests were performed on each of the cured coatings:

Hardness - A pencil hardness test is performed according to the procedure of ASTM specification D-3363-84. The result of the test is a number relating to hardness. Generally, the higher, the better. Abrasion - A so-called G-Cat test is performed. This is a beverage container industry - 13 -

standard test which simulates the "rocking" motion that the cans would experience in a rail car or semi trailer during shipping when the cans are packaged. The test employs a machine which evaluates the abrasion of the ink and/or overcoat on the finished cans, based upon the rubbing of six cans against each other at fixed pressure settings. The settings used in the G-Cat machine are: 40 psi side pressure, 60 psi top pressure, the amplitude of the stroke is 1 inch and the frequency is 3 strokes per second. The result of the test is a number of strokes that the coating survives without sufficient wear to expose the underlying metal. The minimum requirement for beverage containers is 2000 strokes. Thermally cured coatings show scores in the range of 5000.

Appearance - a simple visual observation is made and the appearance is rated either acceptable or unacceptable.

Mobility - An angle of inclination test is performed. Beverage cans are transported by gravity during coating, processing and filling. Therefore, they must roll smoothly down the automated processing lines. The mobility test measures the angle of inclination at which the cans slide. The test is a measure of the combined properties of coating hardness, smoothness and lubricity. Three cans coated with the coating to be tested are weighted to represent their approximate fill weight (e.g. 350 g) . They are placed on their sides in a tray with one on top of the other two. The sides of the tray prevent the lower two from moving but allow the upper can to slide. The tray is gradually elevated at one end so that the cans are tilted along the top to bottom axis of the cans . The angle at which the top can moves is measured and reported.

Beverage cans should slide at 15° or less.

Heat Stability - A pasteurization test is conducted to determine the adhesion of the coating to the container during and after sterilization of its contents. The test is performed by immersing the coated can in 180° F deionized water containing 0.5% detergent for 30 minutes. The can is then scribed with a stylus to give a 1 inch Crosshatch in the necked region of the can.

(The coating is applied prior to the necking of the can. During necking the coating is stretched along with the metal of the can to form the top of the can. Thus, the test in the neck portion of the can is conducted at the point of highest stress in the coating.) After scribing, Scotch Brand No. 898 tape is applied to the crosshatched area and the tape is pulled off. The tape should remove none of the coating during this process.

The foregoing tests, relating to the physical properties of the film are carried out after curing. The following test was carried out on the polymerization mixture before curing:

Weight loss - An ASTM test is employed to quantify weight loss on curing. An approximately 3 gram sample of the test composition is placed in a pan measuring 5 cm x 5 cm and the pan is heated at 110°C for 60 minutes. The weight before and after are compared and expressed as % loss. Thirteen percent weight loss on the ASTM test is considered the upper limit for acceptable compositions because this correlates with about 50% weight loss when the composition is a thin film on a can at 220°C. A weight loss under 5% is preferred.

The results of tests on examples of coatings of the invention are shown in Table 2.

Table 2

Property Control Expt. A Expt. B Expt. C

Pencil hardness 4H 3H 2H

Abrasion G-Cat (strokes) >4000 >4000 >4000

Gloss very good good good

Mobility (degrees) 5 6 6

Pasteurization Test pass pass pass

Flexibility (spin neck) pass pass pass

Time to dryness (sees) 5-10 10-20 15-25

Weight loss (ASTM) 14.0% 12.9% 2.9% 1.6%

Viscosity (cps) 350 525 950 The formulations of the invention are comparable to those of the commercially available solvent-based systems in adhesion under sterilization conditions and in cure speed. In addition, toxicity and irritancy of the components used in the formulations of this invention are not a problem in the work environment, no VOC's are released, and the weight loss is minimal during cure. This stands in marked contrast to the solvent-based coatings. In the areas of hardness, gloss, mobility, and flexibility, the coatings of this invention are competitive with commercially available solvent-based coatings. Moreover, unlike known single-component, solventless monomer mixes, the compositions of the invention do not darken or become highly viscous on standing at room temperature for extended periods [up to 6 months] .

Claims

We claim :

1. A process for providing a clear, glossy, adherent, abrasion-resistant coating on a substrate comprising the steps of:

(a) mixing from 40 to 90 parts of an epoxidized vegetable oil or mixture of vegetable oils having an overall oxirane content from about 7% to 11%; from 5 to 40 parts of a low molecular weight cycloaliphatic epoxy resin; from 1 to 6 parts of a diaryliodonium salt thermal initiator, and from 1 to 5 parts of a wax or mixture of waxes to provide a coating mixture;

(b) applying said coating mixture to said substrate to provide a coated substrate; and (c) heating said coated substrate at a temperature of 175° to 260°C for 10 to 1000 seconds whereby said coating is substantially completely polymerized.

2. A process according to claim 1 wherein said epoxidized vegetable oil is chosen from the group consisting of epoxidized soybean, linseed, sunflower, meadowfoam, safflower, canola, crambe, vernonia, lesquerella, corn, rapeseed, castor and cashew oils.

3. A process according to claim 1 wherein said diaryliodonium salt is a hexafluoroantimonate or a tetra(perfluorophenyl)borate salt.

4. A process according to claim 1 comprising the steps of:

(a) mixing from 60 to 85 parts of epoxidized linseed oil, from 8 to 32 parts of 3,4-epoxycyclohexylmethyl 3' ,4'epoxycyclohexane- carboxylate, from 1 to 2 parts of diaryliodonium hexafluoroantimonate thermal initiator, and from 2 to 3 parts of a wax or mixture of waxes to provide a coating mixture; (b) applying said coating mixture to said substrate to provide a coated substrate; and (c) heating said coated substrate at a temperature of 175° to 260°C for 10 to 1000 seconds whereby said coating is substantially completely polymerized.

5. A process according to claim 1 wherein said applying said mixture to said substrate is accomplished by roll coating.

6. A process according to claim 1 wherein said substrate is heated at 210 to 230°C for a total of 2 to 4 minutes.

7. A container coated with a clear, glossy, adherent, abrasion-resistant coating comprising from 2 to 4 parts of a hydrocarbon or fluorinated hydrocarbon wax or mixture thereof, less than 6 parts of a residue from a diaryliodonium salt thermal initiator for cationic polymerization, and a copolymer of from 40 to 90 parts of an epoxidized vegetable oil and from 5 to 40 parts of a low molecular weight cycloaliphatic epoxy resin.

8. A can according to claim 7.

9. A container according to claim 7 coated with a coating comprising from 2 to 3 parts of a hydrocarbon or fluorinated hydrocarbon wax or mixture thereof, less than 2 parts of a residue from a diaryliodonium hexafluoroantimonate or tetra (perfluorophenyDborate, and a copolymer of from 60 to 85 parts of epoxidized linseed oil and from 8 to 32 parts of 3,4-epoxycyclohexylmethyl 3' ,4'epoxycyclohexanecarboxylate.

10. A composition for a thermally curable coating comprising:

(a) from 40 to 90 parts of an epoxidized vegetable oil or mixture of epoxidized vegetable oils having an overall oxirane content from about 3% to about 11%;

(b) from 5 to 40 parts of a low molecular weight cycloaliphatic epoxy resin or mixture of such resins; (c) from 1 to 6 parts of a diaryliodonium salt; and

(d) from 1 to 5 parts of a wax or mixture of waxes .

11. A composition according to claim 10 wherein said epoxidized vegetable oil is chosen from the group consisting of epoxidized soybean, linseed, sunflower, meadowfoam, safflower, canola, crambe, vernonia, lesquerella, corn, rapeseed, castor and cashew oils.

12. A composition according to claim 10 wherein said diaryliodonium salt is a hexafluoroantimonate or a tetra(perfluorophenyl)borate salt.

13. A composition according to claim 10 additionally comprising from 5 to 20 parts of an epoxy resin diluent having a viscosity less than 100 cps.

14. A composition according to claim 13 wherein said epoxy resin diluent is chosen from the group consisting of cn-olefin oxides and methyl epoxy esters of vegetable oil acids.

15. A composition according to claim 10 comprising:

(a) from 60 to 85 parts of epoxidized linseed oil; (b) from 8 to 32 parts of 3,4- epoxycyclohexylmethyl 3' ,4' - epoxycyclohexanecarboxylate;

(c) from 1 to 2 parts of diaryliodonium hexafluoroantimonate; and (d) from 2 to 4 parts of a wax or mixture of waxes chosen from the group consisting of polyethylene wax and polytetrafluoroethylene wax.

16. A composition according to claim 15 additionally comprising about 0.01 parts of a color- correcting pigment or dye.

17. A composition according to claim 16 additionally comprising from 0.1 to 1.0 part of an adhesion promoter.