US3503770A - Hydrocarbon wax coatings and their process of preparation - Google Patents

Description

United States Patent 3,503,770 HYDROCARBON WAX COATINGS AND THEIR PROCESS OF PREPARATION James E. Guillet, Toronto, Ontario, Canada, and Harry W. Coover, Jr., Kingsport', Tenn., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Continuation of application Ser. No. 309,391, Sept. 17, 1963. This application Aug. 7, 1967, Ser. No. 658,946 The portion of the term of the patent subsequent to Oct. 8, 1980, has been disclaimed and dedicated to the Public Int. Cl. C08h 9/00 US. Cl. 106270 11 Claims ABSTRACT OF THE DISCLOSURE Hydrocarbon waxes capable of forming wax coated substrates. These waxes have improved tack time and find particular use in curtain coating operations.

This application is a continuation of application Ser. No. 309,391 filed Sept. 17, 1963, which is a continuationin-part application Ser. No. 79,872 filed Dec. 30, 1960, now U.S. Patent 3,106,485 issued Oct. 8, 1963.

This invention relates to hydrocarbon wax coated sub strates and their process of preparation. More particularly, this invention relates to substrates coated with a synthetic wax which exhibits a substantially improved tack time in comparison to prior art waxes. In a specific aspect, this invention relates to substrates coated with butene polymer waxes and a process for preparing the same.

For a number of years, various substrates, including paper, metal foil, fabrics, foodstuffs and the like, have been coated with synthetic waxes prepared from a-monoolefins such as butene to preserve and protect the coated surface. In recent years, melt coating procedures, such as curtain coating, have been employed to coat various substrates, particularly those of irregular shape. In this method, the substrate to be coated is passed through a curtain of molten wax which is provided by a spray head or other suitable means. The wax curtain, except when broken by the passage through it of a substrate, falls directly into a collector and is returned to a molten wax reservoir for reuse. Where a substrate to be coated is regular in shape such as paper, fabric or the like, the wax is generally applied to the substrate using a gravure roll immersed in a fountain containing the molten wax or it can be applied from a slit die onto the surface and smoothed with a doctor blade. The coated substrate can then be taken up on a rewind roll. Where the substrate has an irregular contour, for example, ham, bacon, nuts, bolts, or the like, it is curtain coated by placing it on a moving belt or roller and passing it rapidly through a curtain of molten wax. The molten wax is cooled substantially simultaneously upon contact with the solid substrate which is usually at a temperature below the melting point of the wax.

The prior art synthetic wax coatings prepared by the above melt coating techniques often exhibit gOOd physical properties including good clarity and sparkle, tensile strength and toughness. However, these coatings are tacky for periods of about 120 to about 135 seconds which is much too long for commercial coating operations. During the period when the wax coating is tacky, hereinafter referred to as tack time, coated substrates will adhere to one another and to surrounding materials which gives rise to problems in handling these materials and often damages the coating. Furthermore, substrates such as paper are generally melt coated in commercial operations at high rates, for example, rates of 200 feet per minute and generally rates in excess of 500 feet per minute so that such a prolonged tack time requires slower rates or additional equipment such as conveyors and rollers to run the coating in air until it becomes substantially free of tack. For most melt coating commercial operations tack times no greater than about 45 seconds can be tolerated without any substantial delay in processing.

It is evident, therefore, that the state of the art will be greatly enhanced by providing synthetic wax coatings which have tack times which do not exceed about 45 seconds and often are not in excess of 30 seconds. Likewise, a significant contribution to the art would be substrates coated with such materials and a process for their preparation.

Accordingly, it is an object of this invention to provide synthetic wax coatings exhibiting improved properties.

Another object of this invention is to provide coated substrates which exhibit improved tack times.

Another object of this invention is to provide synthetic wax coatings which can be applied by melt coating techniques, particularly curtain coating procedures, to substrates without exhibiting the poor tack times characteristic of prior art synthetic wax coatings available heretofore.

Still another object of this invention is to provide a method for obtaining the aforementioned improved synthetic wax coatings and coated substrates.

Other objects and advantages of this invention will become apparent from an examination of the specification and claims that follow.

It has now been found that a particular type of hydrocarbon wax, as hereinafter described, when contacted at a temperature up to about 180 C., in the molten amorphous state, with a substrate to form a film having a thickness no greater than about 60 mils will crystallize and form a coating having a tack time no greater than about 45 seconds and generally no greater than about 30 seconds. In order to obtain these results the synthetic wax must be a hydrocarbon polymer of an u-monoolefin containing at least 4 carbon atoms, have a density in the range of about .83 to about .94, an inherent viscosity in Tetralin at 145 C. in the range of about .2 to about .5, a melt viscosity at 190 C. in the range of about 1,500 to about 20,000 cp., a brittle point no lower than about C. and be able to crystallize at a temperature at least 40 C. below its D.T.A. melting point within about 5 to about 45 seconds after contact with the substrate upon which it is coated.

The discovery that the aforementioned synthetic waxes would form substantially tack-free coatings in 45 seconds or less was quite surprising and could not have been predicted from the prior art. Thus, it is known that hydrocarbon waxes can be made by thermal degradation of high-molecular-weight polyolefins such as polypropylene and polyethylene as shown by British Patent No. 569,043 and US. Patent No. 2,835,659, respectively. However, these hydrocarbon waxes are hard, brittle materials with melt viscosities ranging from about 50,000 to about 30 cp. at 190 C. and when coated on substrates, particularly paper or foil in thin layers, form brittle coatings which are substantially free of tack but crack when flexed. Furthermore, hydrocarbon polymers of a-monoolefins containing at least 4 carbon atoms, unless they have the properties set forth hereinabove, form coatings which exhibit tack times of the order of -135 seconds. It was completely unexpected, therefore, that polymers prepared from these same monomers, but have the specific properties set forth above, would form coatings that have tack times which do not exceed about 45 seconds and generally are of the order of about 30 seconds, or less.

In practicing this invention, the hydrocarbon waxes in the molten amorphous state are contacted with the substrate to be coated at temperatures up to about 180 C. In general, coating temperatures in the range of about to about 150 C. are employed, although it is desirable to operate at temperatures of about 20 C. It should be understood, however, that the most desirable results will be achieved at coating temperatures which will vary with the nature and composition of the particular hydrocarbon wax within the limits set forth. Temperatures substantially in excess of about 180 C. are not satisfactory because the prolonged heating employed in melt-coating operations can cause excessive oxidation of the synthetic wax which deleteriously affects the coating. Furthermore, if the substrate to be coated is bacon or some foodstuff, temperatures above about 180 C. generally results in cooking the material which obviously should be avoided. At temperatures up to about 180 C., the wax is formed into a film having a thickness not greater than about 60 mils. In general, the film thickness will be in the range of about 1 to 10 mils, although film thickness of less than 1 mil can be prepared in practicing this invention. If the hydrocarbon wax is coated in a film having a thickness in excess of 60 mils the coating exhibits a tack time well in excess of 45 seconds, for example, tack times of 100 or even 130 seconds.

The hydrocarbon waxes are in the molten amorphous state when contacted with the substrate to be coated, for example, paper, glass, wood, ham, bacon, nuts, bolts, and the like. It is obvious, therefore, that the wax employed must be one having a melting point no greater than about 180 C. Upon contact with the substrate, which is below the melting point of the wax, the amorphous polymer cools below its melting point and is allowed to crystallize to form a coating which is substantially tack free in 45 seconds or less. In order to achieve this result, the hydrocarbon Wax must be able to crystallize at a temperature at least 40 C. below its D.T.A. melting point within about 5 to about 45 seconds after contact with the substrate upon which it is coated. Generally, the hydrocarbon wax employed will be capable of crystallizing at a temperature in the range of about 50 to about 125 C. below the D.T.A. melting point of the wax. Furthermore, the wax must be able to crystallize at these temperatures within about 5 to about 45 seconds, preferably about 5 to 30 seconds, after contact with the sub strate, which is at a temperature within the above ranges The crystallizability of the Wax can be shown by X-ray or infrared analysis, as is obvious to those skilled in the art. The D.T.A. melting point is determined by differential thermal analysis (D.T.A.) and, is reported in C. This method has been used extensively for determining polymer melting points and is described in detail in Organic Analysis, vol. 4, Interscience Publishing Co. (1960), page 361. Certain hydrocarbon waxes will exhibit two D.T.A. melting peaks, as, for example, hydrocarbon waxes that contain stereo-regular blocks of two different polymerized u-monoolefins. The D.T.A. melting point which is referred to hereinabove is the highest of two D.T.A. melting points where a particular polymer wax exhibits two such melting points.

It has been found that synthetic hydrocarbon waxes having the properties set forth above can be melt coated according to the process of this invention and will form coatings that are tack free in 45 seconds or less and exhibit other god physical characteristics. Any low molecular weight hydrocarbon polymer of an a-olefin containing at least 4 carbon atoms having a density in a range of about 0.83 to about 0.94, preferably about 0.88 to about 0.93, an inherent viscosity in the range of about .2 to about .5, preferably about .25 to about .4, in Tetralin at 145 C., a melt viscosity in the range of about 1,500 to about 20,000 cp., preferably about 5,000 to about 10,000 cp. at 190 C., and brittle points no lower than about 80 0., preferably in the range of about to about 60 C., can be employed in the process of this invention if they will crystallize as described hereinbefore. Although any of the aforementioned waxes can be employed in the process of the invention, it is preferred that the waxes be low molecular weight hydrocarbon homo or copolymers of one of the well-known polymerizable aliphatic a-olefins containing 4-10 carbon atoms. When copolymers of a-olefins are employed in the process of this invention, it is generally most desirable to use copolymers containing at least 40%, and more preferably about 40-45%, by weight, of one of the aliphatic a-olefins containing 4-6 atoms, the preferred being butene. In addition, copolymers of a-olefins containing at least 4 carbon atoms with a lower aliphatic a-olefin such as propylene, should usually contain no more than about 60%, preferably no more than about 40 to 50%, by weight, of the propylene for best results. The ot-olefins that are used to form the hydrocarbon waxes employed in the process of this invention can be generally described as having the formula CH;, :CH-R where R is an alkyl radical containing at least 2 carbon atoms. Examples of the a-olefins include butene, 4-methyl-1-pentene, 4-methyl-1-hexene, S-methyl-l-hexene, 4,4-dimethyl-l-pentene, 3-methyl-lbutene, pentene, hexene, octene, decene, and the like.

The physical properties exhibited by the hydrocarbon waxes employed in the process of this invention can be determined using any of the procedures generally employed for this purpose. For example, the melt viscosity can be determined using a standard Brookfield viscometer or a Capillary Melt Method. The inherent viscosity of the hydrocarbon waxes employed can be determined in Tetralin at C., a typical procedure being described by Schulken et al. in The Journal Of Polymer Science, volume 26, page 227 (1957) and the brittle point can be determined using the procedure described in ASTM D 746-57T employing a sample which is in the order of 5 to 10 mils in thickness. As already indicated, a wax will be suitable in most commercial melt coating applications if it forms a coating having a tack time that is no greater than about 45 seconds, preferably below 30 seconds. Tack time is that period when a synthetic wax coating is tacky or sticky to the touch and tends to adhere to other materials. A simple test for establishing the tack time of a wax coating is merely to touch the coating with the finger. If the coating tends to adhere to the finger upon touching after a period of 45 seconds it is not suitable for most commercial operations.

In practicing this invention, any of the conventional methods of melt coating can be employed. For example, the hydrocarbon wax can be applied to the substrate using a gravure roll immersed in a fountain containing the molten wax or it can be applied from a slit die onto the surface of the substrate and smoothed with a doctor blade. The wax coating can also be applied using the curtain coating technique which is particularly advantageous where the substrate to be coated is of irregular form. Furthermore, this method provides a simple, direct, and economical method for packaging irregularly shaped articles which are extremely difficult to package in other ways. A wax coating prepared according to the process of this invention using curtain coating adheres to an object and follows the contour of irregularly shaped articles to form clear coatings with improved tack times. In addition, if articles to be coated are placed upon a support and both support and article are passed through the curtain of molten wax this results in an attractive package in which the wax coating adheres to the article and secures it to the support. Such packages are particularly attractive because the coatings do not bridge or balloon from the article to the support but rather, follow the contour of the article and support. Using this method, it is possible, therefore, to package such irregularly shaped articles, as nuts, bolts, screw drivers, ham

slices and bacon slices in an attractive manner. The most significant aspect of this invention is that the hydrocarbon wax coating is prepared from a wax having the particular characteristics set forth in detail herein. When such a wax is contacted with a substrate, at least 40 C. below the D.T.A. melting point of the wax, it cools and crystallizes to form a substantially tack free coating in about 5 to about 45 seconds.

In general, the process of this invention can be used to coat any substrate and the coating can be applied to one or more sides of the substrate. In addition to coating paper for packaging as above described, this invention can be used in coating substrates such as foil, glass, fabric, wood, ham, bacon, bologna, nuts, bolts, screw drivers, pliers, and the like.

This invention can be further illustrated by the following examples of preferred embodiments thereof although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

EXAMPLE 1 The novel polymer waxes of this invention are particularly valuable coating materials and can be coated on a number of substrates such as paper, foil, food, etc. A particularly etfective method for this purpose is to employ a curtain coater. To illustrate, 50 lbs. of a propylene l-butene copolymer wax containing approximately 40%, by weight, l-butene is charged to the reservoir of a machine designed for curtain coating. Said wax has a density of 0.90, an inherent viscosity of 0.4 in Tetralin at 145 C., a melt viscosity of 9,000 cp. at 190 C. and is able to crystallize at a temperature at least 40 C. below its DTA melting point within 5 to about 45 seconds. The polymer wax is melted in the reservoir and raised to a temperature of 175 C. It is then pumped thr ugh a spray-head which gives a curtain of molten wax approximately 12 in. wide and 5 mils in thickness. The wax curtain falls directly into a collector and is returned to the melt reservoir except when an object is passed through the curtain. Objects to be coated are placed on moving belts or rollers and passed rapidly through the curtain. The melt is cooled substantially simultaneously upon contact with the solid object which is at a temperature of about 120 C. below the DTA melting point of the wax. Coatings on bacon placed on a support are made by the following procedure:

A piece of paper board approximately 6" x 12" is passed through the curtain of molten wax, as described above. This results in a thin, uniform coating of the wax on the paper surface. One-half pound of sliced bacon is then placed on the coated surface of the board and passed through the curtain. This results in a smooth, tough, transparent coating which completely encloses the bacon. After cooling, the coating can be removed from the package by peeling it away from the bacon in a single continuous film. The resulting coating having a thickness of about 5 mils shows a tack time of 19 seconds and a brittleness temperature of 38.5 C.

When the melt temperature of the hydrocarbon wax is raised to 185 C. the raw bacon coated with the wax is undesirably discolored due to scorching or cooking. In addition, excessive degradation of the wax occurs during continuous coating operations resulting in a severe loss of film toughness.

EXAMPLE 2 The procedure of Example 1 is followed except that the hydrocarbon wax is a copolymer of propylene and l-butene containing approximately 50% by weight 1- butene, and having a density of 0.885, an inherent viscosity of 0.45 in Tetralin at 145 C., and a melt viscosity of 11,500 cp. at 190 C. The resulting coating shows a tack time of 30 seconds and a brittleness temperature of 32 C.

When a similar propylene butene copolymer wax, but having a melt viscosity of 23,000 cp. at 190 C. is substituted in the above procedure, the temperature for coating must be increased to above 180 C. and there is obtained a coating which exhibits a tack time substantially in excess of the desired 45 seconds.

EXAMPLE 3 A propylene l-butene copolymer wax containing approximately 45% l-butene by weight and having a density of 0.89, an inherent viscosity of 0.38 and a melt viscosity of 8,700 cp. at 190 C. is melted, placed in the reservoir of a single-roll kiss-coater for hot-melt coating operations and maintained at a temperature of 180 C. A roll of 40-pound bleached kraft paper is coated with the wax to a depth of 1 mil at a speed of 65 feet per minute. The resulting coating shows a tack time of 12 seconds.

When a propylene l-butene copolymer wax which is unable to crystallize at a temperaure of at least 40 C. below its D.T.A. melting point within 45 seconds is used in the kiss-coating operation, the resulting wax coating has a tack time in excess of 45 seconds and the coating operation must be carried out at prohibitively low rates to prevent the coated paper from sticking together in the roll. At the very high coating rates necessary in commercial operations this is, of course, a significant disadvantage. To illustrate, a propylene l-butene copolymer wax containing 65% l-butene, by weight, and showing a brittleness temperature of C. in the form of an oriented, quenched film of 5 mils thickness, gives a wax coating having a tack time of seconds. Similar results are obtained when the above procedure is used to prepare a film having a thickness in excess of 60 mils.

EXAMPLE 4 The procedure of Example 1 is followed using a propylene l-hexene copolymer Wax containing 35% l-hexene by weight, having a density of 0.88, an inherent viscosity of 0.45 in Tetralin at C., a melt viscosity of 12,000 cp. at 190 C. and a brittle point of 25 C., and being able to crystallize at a temperature 65 C. below its D.T.A. melting point within 30 seconds. The resulting wax coating shows a tack time of less than 15 seconds.

Similar results are obtained when the above hydrocarbon wax is replaced with a propylene l-octene copolymer wax containing 40% l-octene, by weight, and having a melt viscosity of 3,500 cp. at 190 C., an inherent viscosity of 0.30 and a brittle point of 36 C.

EXAMPLE 5 The procedure of Example 1 is followed using a propylene l-decene copolymer containing 33%, by weight, 1- decene and having an inherent viscosity in Tetralin at 145 C. of 0.38, a melt viscosity of 5,300 cp. at 190 C., and a brittle point of 60 C. The resulting wax coating has a tack time of 12 seconds.

When propylene l-decene copolymer wax containing about 50% l-decene by weight and having a melt viscosity of 11,500 cp. at 190 C., and a brittle point of about 87 C. is used in the coating operation in place of the above hydrocarbon wax, the resulting wax coating has a tack time of 90 seconds. This tack time is too long for commercial operations, since coated packages on an assembly line tend to stick together and cannot be processed satisfactorily.

EXAMPLE 6 The results obtained with a hydrocarbon Wax which does not crystallize within 5 to 45 seconds at a temperature at least 40 C. below its D.T.A. melting point are highly unsatisfactory. To illustrate, a l-butene l-hexene copolymer containing 20% l-hexene and having a melt viscosity of 1,800 cp. at 190 C. and an inherent viscosity of about 0.2 is coated onto cold bacon using a curtain coater. The resulting coating has a tack time of greater than seconds. A sample of the above l-butene 1-hexene copolymer, when melted and cooled rapidly to 7 80 C. below its D.T.A. melting point, will not crystallize within 45 seconds after cooling.

Thus, by means of this invention there is provided novel hydrocarbon wax coatings exhibiting excellent physical properties including a substantially reduced tack time in comparison to hydrocarbon wax coating now available. Substrates coated according to the process of this invention can be used in packaging food, dry goods and other articles which require protection from moisture or air.

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variety and modifications can be effected without departing from the spirit and scope of the invention as described hereinabove and as designed in the appended claims.

We claim:

1. The method of providing an improved wax coating which comprises contacting, at a temperature up to about 180 C., a synthetic wax in the molten amorphous state, with a substrate to form a film having a thickness no greater than about 60 mils, said synthetic wax being a hydrocarbon polymer of an a-monoolefin containing at least 4 carbon atoms having a density in the range of about .83 to about .94, an inherent viscosity in Tetnalin at 145 C. in the range of about .2 to about .5, a melt viscosity at 190 C. in the range of about 1,500 to about 20,000 cp., a brittle point no lower than about -80 C. and being able to crystallize at a temperature at least 40 C. below its D.T.A. melting point within about to about 45 seconds after contacting said substrate.

2. The method of claim 1 in which the synthetic wax is a hydrocarbon polymer of an a-monoolefin containing 4 to carbon atoms.

3. The method of claim 1 in which the synthetic wax is a hydrocarbon copolymer of an ot-monoolefin containing 4 to 10 carbon atoms.

4. The method of providing an improved wax coating which comprises contacting, at a temperature in the range of about 0 toiabout 150 C., a synthetic wax in the molten amorphous state, with a substrate to form a film having a thickness in the range of about 1 to about 10 mils, said synthetic wax being a hydrocarbon polymer of an a-monoolefin containing at least 4 carbon atoms, having a density in the range of about .88 to about .93, an inherent viscosity in Tetralin at 145 C. in the range of about .25 to about .4, a melt viscosity at 190 C. in the range of about 5,000 to about 10,000 cp., a brittle point in the range of about l0 to about 60 C. and being able to crystallize at a temperature in the range of about 50 to about 125 C. below its D.T.A. melting point within about 5 to about 30 seconds after contacting said substrate.

5. The method of claim 4 in which the synthetic wax is a butene propylene copolymer containing about 40 to about 45%, by weight, of butene.

6. The method of claim 5 in which the substrate is bacon.

7. The method of claim 5 in which the substrate is ham.

8. The method of providing an improved wax coating which comprises contacting, at a temperature of about 175 C., a synthetic wax in the molten amorphous state, with a substrate, to form a film having a thickness of about 5 mils, said synthetic wax being a propylene lbutene copolymer containing about 40%, by weight, of l-butene, having a density of about .90, an inherent viscosity in Tetralin at C. of about .4, a melt viscosity at 190 C. of about 9,000 cp., a brittle point of about 38.5 C. and being able to crystallize at a temperature at least 40 C. below its D.T.A. melting point within about 5 to about 45 seconds after contacting said substrate.

9. The method of providing an improved wax coating which comprises contacting, at a temperature of about C., a synthetic wax in the molten amorphous state, with a substrate, to form a film having a thickness of about 5 mils, said synthetic wax being a propylene 1- hexene copolymer containing about 35% by weight, of l-hexene, having a density of about .88, an inherent viscosity in Tetralin at 145 C. of about .45, a melt viscosity at C. of about 10,000 cp., a brittle point of about 25 C. and being able to crystallize at a temperature of about 65 C. below its D.T.A. melting point within about 30 seconds after contacting said substrate.

10. The method of claim 4 in which the synthetic wax is a propylene decene copolymer containing about 33%, by weight, l-decene having an inherent viscosity in Tetralin at 145 C. of .38, a melt viscosity of about 5,300 cp. at 190 C. and a brittle point of about 60 C.

11. The product obtained by the process according to claim 1.

References Cited UNITED STATES PATENTS 3,243,395 3/1966 Guillet et a1 26028.5

JULIUS FROME, Primary Examiner JOAN B. EVANS, Assistant Examiner US. Cl. X.R.

99l69; 106271, 285; 1l7-l4l; 26033.6, 93.7