METHOD OF MAKING POLYOLEFIN-PAPER LAMINATE WITH FLAME TREATMENT OF THE PAPER
United States Patent 3640788
The disclosure relates to a method of priming paper web for its firm and even bond with subsequently applied molten polyolefin as a coating. The method provides for continuously feeding a paper web at relatively high speed to and past a station at which there is directed against the face to-be-coated of the passing web stiff flame which is fed by a combustible mixture of gas and air at a ratio to obtain substantially complete combustion of the mixture at the flame tip, thereby to cause a decrease in the polarity of the cellulose structure of the web face for enhanced adhesion of the subsequently applied molten polyolefin to this web face.
US Patent References:
Method of making jointed polyolefincoated paperboard products
Flynn - January 1967 - 3300350
Method of heat bonding panels of paperboard having a thermoplastic coating
Hittenberger et al. - December 1968 - 3416411
Method of making jointed polyolefincoated paperboard products
Flynn - January 1967 - 3300350
Method of heat bonding panels of paperboard having a thermoplastic coating
Hittenberger et al. - December 1968 - 3416411
Application Number:
04/888954
Publication Date:
02/08/1972
Filing Date:
12/29/1969
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
156/497, 156/244.230
International Classes:
B05D1/26; B05D3/08; B05D7/26; B29C59/08; B29C65/10; B32B27/00; D21H19/22; B29C59/00; D21H19/00; B29C25/00
Field of Search:
156/82,244,306,497
Primary Examiner:
Quarforth, Carl D.
Assistant Examiner:
Lechert Jr., Stephen J.
Parent Case Data:
This application is a continuation-in-part of my copending application Ser. No. 643,147, filed June 2, 1967, which was a continuation-in-part of my prior application Ser. No. 325,055, filed Nov. 20, 1963, both now abandoned.
Claims:
What is claimed is
1. In a method of making polyolefin-coated paper web, involving at any time after the hereinafter specified flame treatment of a face of a paper web extruding molten polyolefin onto said web face, that improvement which comprises continuously feeding the web to and beyond a station, feeding to a first high-capacity burner at said station a mixture of gas and air at a ratio to produce a first stiff flame with substantially complete combustion of the mixture at the flame tip, and directing the flame with its tip against said web face transversely of the feed direction of the web to cause a decrease in the polarity of the cellulose structure of said web face and thereby promote adhesion of the subsequently extruded polyolefin on said web face.
2. The improvement in a method of making polyolefin-coated paper web as in claim 1, in which the web is fed at a rate within a range from approximately 400 feet per minute to approximately 1,200 feet per minute at which there clings to said web face an air layer, and the velocity of said flame is regulated so that the same penetrates said air layer at any web speed within said range.
3. The improvement in a method of making polyolefin-coated paper web as in claim 1, in which the web is fed at a rate above 1,200 feet per minute, there is fed to a second high-capacity burner at said station a mixture of gas and air at a ratio to produce another stiff flame with substantially complete combustion of the mixture at the flame tip, and said other flame is spaced from said first flame and directed with its tip against said web face transversely of the feed direction of the web.
1. In a method of making polyolefin-coated paper web, involving at any time after the hereinafter specified flame treatment of a face of a paper web extruding molten polyolefin onto said web face, that improvement which comprises continuously feeding the web to and beyond a station, feeding to a first high-capacity burner at said station a mixture of gas and air at a ratio to produce a first stiff flame with substantially complete combustion of the mixture at the flame tip, and directing the flame with its tip against said web face transversely of the feed direction of the web to cause a decrease in the polarity of the cellulose structure of said web face and thereby promote adhesion of the subsequently extruded polyolefin on said web face.
2. The improvement in a method of making polyolefin-coated paper web as in claim 1, in which the web is fed at a rate within a range from approximately 400 feet per minute to approximately 1,200 feet per minute at which there clings to said web face an air layer, and the velocity of said flame is regulated so that the same penetrates said air layer at any web speed within said range.
3. The improvement in a method of making polyolefin-coated paper web as in claim 1, in which the web is fed at a rate above 1,200 feet per minute, there is fed to a second high-capacity burner at said station a mixture of gas and air at a ratio to produce another stiff flame with substantially complete combustion of the mixture at the flame tip, and said other flame is spaced from said first flame and directed with its tip against said web face transversely of the feed direction of the web.
Description:
This invention relates to a method of treating paper stock for subsequent coating with a polyolefin.
In coating paper or boardstock with a polyolefin in conventional manner by direct extrusion of the same on the stock, it is customary to "prime" the stock, for example, by applying to the face to be coated a bonding solution which must be permitted to dry before applying the polyolefin coating. It is only by virtue of thus priming the stock that the subsequently applied polyolefin coating will firmly and evenly adhere to the stock, for it is well known that the adherence of polyolefin to nonprimed stock is so superficial as to be wholly inadequate for most practical purposes. However, while a polyolefin coating applied to primed stock is satisfactory in most respects, including its firm adherence to, and reliable sealing of, the stock, the cost of priming the stock prior to coating adds so much to the overall cost of polyolefin-coated stock as to prohibit its use for many purposes for which it would otherwise be eminently suited.
It is, therefore, among the objects of the present invention to provide polyolefin-coated paper stock which must have at least as firm and even a bond between stock and coating as that between primed stock and its coating thereon, yet its cost must be sufficiently low to make it pricewise attractive for many purposes for which it was hitherto too expensive.
It is another object of the present invention to devise a method according to which polyolefin-coated paper stock is formed with the aforementioned firm and even bond between stock and coating and at the aforementioned low cost.
It is a further object of the present invention to eliminate in the method of making polyolefin-coated paper stock the aforementioned priming of the stock and, instead, provide for treatment of the stock which not only will afford a bond between stock and coating that is entirely satisfactory in all respects, but the cost of which is also so much smaller than that of stock priming as to make possible the aforementioned low cost of coated stock.
Another object of the present invention is to provide, in the method of making polyolefin-coated paper stock, for treatment of the stock pursuant to a discovery of mine according to which brief impingement of certain high heat-intensity flame on the stock will lend the latter the characteristic of directly bonding with a subsequently applied polyolefin coating with great firmness and evenness. The method of making polyolefin-coated paper stock, which features the stock treatment just mentioned that amounts to "flame-priming" of the stock, thus permits a highly efficient and low-cost, and also continuous, operation, involving no more than continuously feeding paper stock past a station at a rate at which a polyolefin coating may thereat be extruded onto the stock, and on the continuous pass of the stock to the station directing high heat-intensity flame against the face to be coated of the stock.
Other objects and advantages will appear to those skilled in the art from the following, considered in conjunction with the accompanying drawings.
In the accompanying drawings, in which certain modes of carrying out the present invention are shown for illustrative purposes:
FIG. 1 is a diagrammatic illustration of an installation in which to coat paper web stock with a polyolefin in accordance with a method embodying the present invention;
FIG. 2 is a section through polyolefin-coated paper web stock made according to the featured method and also embodying the present invention;
FIGS. 3 and 4 are copies of photomicrographs of identically magnified cut edges of polyolefin-coated identical paper webs, of which the coated web of FIG. 3 embodies the present invention, and the coated web of FIG. 4 is the result of direct polyolefin application to the web without prior priming of any kind of the latter; and
FIG. 5 is a fragmentary diagrammatic illustration of an installation in which to coat paper web stock with a polyolefin in accordance with a method also embodying the invention.
Referring to the drawings, and more particularly to FIGS. 1 and 2, thereof, the reference numeral 10 designates an installation for making polyolefin-coated paper stock s by a featured method of the present invention. The installation 10 comprises, in this instance, a stationary extruder 12, a stationary flame burner 14, and suitable rotary guide rolls 16 to 22 over which exemplary paper web stock w to be coated is led past a flame F from the burner 14 and onto an application roll 24 from which the coated stock is withdrawn. The roll 22 serves also as a pinch roll which holds the web stock directly against the application roll 24, with the extruder 12 discharging molten polyolefin p in sheet form into the bite of the rolls 22, 24 and, hence, against the face f to be coated of the web stock w.
Except for the burner 14, the installation 10 is entirely conventional and is used in continuously coating web stock which has previously been primed by the application of a bonding agent to its face to be coated as previously described, with the continuously fed web stock receiving its polyolefin coating at the bite of the rolls 22, 24 where its primed face contacts the extruded polyolefin. The application roll 24 and also the pinch roll 22 may in this instance be power-driven for the feed of the web stock w in the direction of the arrows 26 to and beyond the coating application station A.
In accordance with an important aspect of the present invention, the paper web stock w is "primed" for a firm and even bond between its face f to be coated and the molten polyolefin applied at the station A, by subjecting the stock face f on the web's pass to the station A to the flame F from the burner 14. The flame F, which is "special" in that it must have certain characteristics to be described, is in this instance a single flame sheet that extends, and impinges, more or less, with its tip on the stock face f, throughout the width of the web w, and the burner 14 may be, and preferably is, an industrial gas burner of high-capacity type shown in my prior U.S. Pat. No. 2,647,569, dated Aug. 4, 1953, having in a burner slot a plurality of burner ribbons from which issue a multitude of piloted utility flames of regulatable heat-intensity that combine to form the flame sheet F, with the burner being fed with a combustible fuel mixture, this being a gas-air mixture.
In operation of the present installation 10, paper web stock w is continuously fed and led over the rolls 16 to 20 into the bite of the application and pinch rolls 24 and 22 and withdrawn from the application roll 24, with the guide roll 18 leading the web stock into sufficiently close proximity to the flame sheet F to be evenly subjected thereto. The coated stock s thus continuously passing from the application roll 24 is shown enlarged in FIG. 2, with the polyolefin coating c being bonded to the face f of the web stock w.
In thus treating the paper web stock w with the special flame F prior to coating the same, the web stock, and more particularly its face f to be coated, may be said to be "flame-primed," for it has been found that a paper web face thus flame-treated will bond with the subsequently applied polyolefin coating at least as firmly and evenly as does previous "solution-primed" web stock bond with a subsequently applied polyolefin coating. It has also been found that such flame treatment renders the web face immeasurably more receptive to the subsequently applied polyolefin coating than a web face not previously primed at all, i.e., neither flame-primed nor solution-primed.
Polyolefin-coating paper web stock in the exemplary operation shown in FIG. 1 is thus according to a method which involves coating a surface of the continuously fed web by contacting molten polyolefin therewith, and which features, prior to coating the web surface priming the web for a firm and even bond of its surface directly with the subsequently applied polyolefin coating, by subjecting the face to be coated of the continuously fed web to the special flame.
To gain a better understanding of the method of coating web stock with a polyolefin according to the present invention, there is given, by way of example and not by way of limitation, the following pertinent data of an actual stock coating operation in an installation like that shown in FIG. 1. Thus, web stock in the form of a paper web of approximately 4 mil thickness was fed at a rate of approximately 1000 feet per minute past the sheet F of special flame and into the bite of the application and pinch rolls 24, 22 where molten polyethylene at normal extrusion temperature was applied to the face to be coated of the paper web, with the polyethylene coating being about one-half mil in thickness. The flame sheet impinged on the passing web stock throughout its width. The resulting polyethylene-coated paper web had such a strong bond between paper and coating as practically to defy all efforts at peeling the coating from the paper web, wherefore at the given feed rate of the paper web the exceedingly brief impingement of the flame sheet of the given temperature on the web primed the latter quite satisfactorily.
To further demonstrate the firm and even bond between the paper and polyethylene of the exemplary coated paper web just described, clean cut edges of the same were inspected under the microscope at 100× magnification and a photomicrograph made of one of them of which FIG. 3 is a copy in accurate outline and in the nearest possible proximation of the demarcation between the paper and polyethylene at the bond between them which despite this high-power magnification was difficult to observe on the actual photomicrograph. Thus, FIG. 3 shows the paper web w and the polyethylene coating c thereon, with the demarcation between them being roughly along the jagged line L. However, all microscopic inspections of cut edges of the coated paper web, including the one photographed and depicted in FIG. 3, clearly indicated that, except for very few and very minute as well as entirely inconsequential pockets between the paper stock and polyethylene, the paper stock and polyethylene was remarkably free from pockets therebetween, for no pockets, except the few mentioned, were discernible at 100× magnification, and the photomicrograph copied in FIG. 3 did not even show a single one of these very few pockets. This, coupled with the described great resistance of the polyethylene coating to being peeled from the paper stock, points clearly at adhesion of the polyethylene coating to the paper stock which to all practical intents and purposes amounts to 100 percent adhesion, and further points equally clearly at the remarkable firmness and evenness of the bond between the paper stock and coating thereon.
To further demonstrate the remarkable effectiveness of flame-priming stock for its subsequent polyolefin-coating, exactly the same paper stock of 4 mil thickness was, without prior flame priming or any other previous priming, coated with polyethylene in the same manner as the described flame-preprimed paper web. Clean-cut edges of samples of the thus coated but not preprimed stock were inspected under the microscope at 100× magnification and a photomicrograph made of which FIG. 4 is a true copy in every respect. The photomicrograph copied in FIG. 4 affords a typical demonstration of what microscopic inspection of all sample edges showed. Thus, FIG. 4 shows the paper stock w and the applied polyethylene coating c of which the latter is bonded or tacked to the former at the isolated places p', with the detached polyethylene coating therebetween forming typical bubbles b. Further investigation brought to light that there were about 54 of these bubbles per inch of linear edge of the coated stock and that about 38 percent of the polyethylene coating was attached to, and 62 percent thereof detached from, the surface of the paper stock. While it is hardly necessary to mention that the coated stock of FIG. 4 is useless for most practical purposes, for its coating has no peel resistance to speak about and the bubble formations of the coating will rupture on the slightest rubbing touch therewith, this coated stock, with but 38 percent of its coating adhering to the paper stock, points unmistakably at the remarkable effectiveness of flame-priming the base stock prior to coating the same which is solely responsible for just about 100 percent adherence of the subsequently applied coating to the base stock, as described.
It has been mentioned heretofore that relatively high heat-intensity of the special flame is essential in flame-priming paper stock for its firm and even bond with the subsequently applied polyolefin coating. More particularly, not only must the high heat-intensity of the flame be at its tip which is that part of the flame that is nearest, and more often than not impinges on, the paper stock and, hence, acts to flame-prime the same, but it has been found that in order to flame-prime paper stock the flame must be fed with a gas-air mixture at a ratio at which preferably most, and even all, of the mixture in the flame tip undergoes combustion, with such flame having the characteristic appearance and relatively high velocity and also tip temperature of a more or less stiff or "sharp" flame.
Tests have been conducted to find out the exact change in a paper web face pursuant to its subjection to sharp flame, and in these tests methods were employed involving surface microscopy, infrared reflectance spectrometry and surface adsorption. These tests have indicated and proved that in a paper web subjected to such a sharp flame the polarity of the cellulosic structure of the flame-subjected web face has notably deceased which, of course, promotes the adhesion of the subsequently extruded, relatively nonpolar, polyolefin to this web face.
As already mentioned, such a sharp flame has a characteristic appearance, wherefore it is merely necessary to vary the ratio of gas and air until a flame has the characteristic appearance of sharp flame which thereupon is entirely suited for the purpose of flame-priming a paper web. Of course, there is no single exact ratio of gas to air which must be maintained for a sharp flame suited for the purpose, for this ratio may vary even for the same gas and will vary for different gases such as natural gas and produced gas, for example, yet as long as the ensuing flame has the appearance of a more or less sharp flame, combustion of the air-gas mixture in the flame tip is sufficiently complete to flame-prime a paper web face, i.e., will cause a decrease in the polarity of the cellulosic structure of the web face and thereby promote adhesion of the subsequently applied polyolefin to this web face. Just to give some indication of air-gas ratios of sharp flames, there is given, by way of example and without any implied limitation, one example of such an air-gas ratio used for a flame suited for the purpose, this being a ratio of 9.6 to 1 for a certain natural gas, i.e., 9.6 parts by volume of air to 1 part by volume of the particular gas.
These same tests have also indicated and proved the surprising fact that sharp flame suited for flame-priming a paper web face, i.e., causes a decrease in the polarity of its cellulosic structure, left intact cellulose fibers that protruded from the web face. These same tests further indicated and proved that such protruding cellulose fibers burned off or fused only on subjection to a flame which was widely different from a sharp flame and is best described as a "soft" flame. Such a soft flame is in its appearance markedly different from a sharp flame, and is further characterized by noncombustion of a quite substantial proportion of the air-gas mixture in the flame tip. It was found that soft flame which did burnoff or fuse protruding cellulose fibers on a paper web face caused an increase in the polarity of the cellulosic structure of the face and, hence, made for even more inferior adherence of an extruded polyolefin coating to the web face. Just to give an example, without any implied limitation, of such a soft flame which burned-off and fused protruding cellulose fibers on a paper web face with ensuing increase of the polarity of the cellulosic structure of the web face, a flame was fed with a mixture of gas and air at a ratio of 6 parts by volume of air to 1 part by volume of gas, with the gas being the same natural gas mentioned above for a sharp flame.
It is imperative for satisfactory priming of a paper web that the sharp flame substantially impinges on the web face. In this connection, at the practical high web speeds contemplated for flame-priming web according to the invention, this being web speeds within a range starting at a minimum of approximately 400 feet per minute and extending at present to 2,500 feet per minute, a layer of air clings to and travels with the web. This air layer increases in thickness with increasing web speed and acts as an effective heat insulator on the web, and this air layer must be penetrated by the flame tip before the latter reaches the web face to prime the same. Accordingly, at different web speeds the velocity of the flame will have to be varied in order to achieve the foregoing. This is done by simply varying the pressure of the combustible air-gas mixture for the burner in well-known manner, without ordinarily changing the air-gas ratio of the mixture so as to retain the sharp characteristic of the flame.
It has been found that for web speeds from approximately 400 feet per minute to approximately 1,200 feet per minute, flame from a single burner satisfactorily flame-primes the web on varying the flame velocity as needed. At web speeds above 1,200 feet per minute, the tip of a single flame apparently is cooled quite considerably by the clinging air layer on the web, and it has been found appropriate for good flame-priming of webs to employ the flames from two burners.
In initial operations of making polyolefin-coated paper stock according to the invention pinholes in the coating were noted. However, pinholes in these coatings were quickly eliminated in toto on first finding that they were caused by breaks or ruptures therein owing to insufficient extrusion speed of the polyolefin and, hence, without compensating in the applied polyolefin for its stretch tendency on cooling to room temperature. The simple remedy for eliminating pinholes altogether was a slight increase in the extrusion speed of the polyolefin onto the passing web.
The web stock is flame-primed according to the present invention preferably and advantageously in the same continuous operation in which the polyolefin coating is applied thereto, as in the installation in FIG. 1 and in the described exemplary operation thereof. However, it is not imperative that the paper web be flame-primed and polyolefin-coated in the same operation, for flame-primed web has been laid aside for several days and then coated with polyolefin, with the bond between them being as firm and even as that achieved by flame-priming the web and coating it in the same operation.
Reference is now had to FIG. 5 which shows an installation 10' for flame-priming a paper web w' with flame F from two burners 14' at the hereinbefore mentioned web velocities of over 1,200 feet per minute.
While in the foregoing description of the invention reference is made to the flame-priming of paper, the term "paper" as used herein and in the appended claims is meant to include stock commonly referred to as "cardboard" or "boardstock" or any other stock having the characteristic properties of paper.
In coating paper or boardstock with a polyolefin in conventional manner by direct extrusion of the same on the stock, it is customary to "prime" the stock, for example, by applying to the face to be coated a bonding solution which must be permitted to dry before applying the polyolefin coating. It is only by virtue of thus priming the stock that the subsequently applied polyolefin coating will firmly and evenly adhere to the stock, for it is well known that the adherence of polyolefin to nonprimed stock is so superficial as to be wholly inadequate for most practical purposes. However, while a polyolefin coating applied to primed stock is satisfactory in most respects, including its firm adherence to, and reliable sealing of, the stock, the cost of priming the stock prior to coating adds so much to the overall cost of polyolefin-coated stock as to prohibit its use for many purposes for which it would otherwise be eminently suited.
It is, therefore, among the objects of the present invention to provide polyolefin-coated paper stock which must have at least as firm and even a bond between stock and coating as that between primed stock and its coating thereon, yet its cost must be sufficiently low to make it pricewise attractive for many purposes for which it was hitherto too expensive.
It is another object of the present invention to devise a method according to which polyolefin-coated paper stock is formed with the aforementioned firm and even bond between stock and coating and at the aforementioned low cost.
It is a further object of the present invention to eliminate in the method of making polyolefin-coated paper stock the aforementioned priming of the stock and, instead, provide for treatment of the stock which not only will afford a bond between stock and coating that is entirely satisfactory in all respects, but the cost of which is also so much smaller than that of stock priming as to make possible the aforementioned low cost of coated stock.
Another object of the present invention is to provide, in the method of making polyolefin-coated paper stock, for treatment of the stock pursuant to a discovery of mine according to which brief impingement of certain high heat-intensity flame on the stock will lend the latter the characteristic of directly bonding with a subsequently applied polyolefin coating with great firmness and evenness. The method of making polyolefin-coated paper stock, which features the stock treatment just mentioned that amounts to "flame-priming" of the stock, thus permits a highly efficient and low-cost, and also continuous, operation, involving no more than continuously feeding paper stock past a station at a rate at which a polyolefin coating may thereat be extruded onto the stock, and on the continuous pass of the stock to the station directing high heat-intensity flame against the face to be coated of the stock.
Other objects and advantages will appear to those skilled in the art from the following, considered in conjunction with the accompanying drawings.
In the accompanying drawings, in which certain modes of carrying out the present invention are shown for illustrative purposes:
FIG. 1 is a diagrammatic illustration of an installation in which to coat paper web stock with a polyolefin in accordance with a method embodying the present invention;
FIG. 2 is a section through polyolefin-coated paper web stock made according to the featured method and also embodying the present invention;
FIGS. 3 and 4 are copies of photomicrographs of identically magnified cut edges of polyolefin-coated identical paper webs, of which the coated web of FIG. 3 embodies the present invention, and the coated web of FIG. 4 is the result of direct polyolefin application to the web without prior priming of any kind of the latter; and
FIG. 5 is a fragmentary diagrammatic illustration of an installation in which to coat paper web stock with a polyolefin in accordance with a method also embodying the invention.
Referring to the drawings, and more particularly to FIGS. 1 and 2, thereof, the reference numeral 10 designates an installation for making polyolefin-coated paper stock s by a featured method of the present invention. The installation 10 comprises, in this instance, a stationary extruder 12, a stationary flame burner 14, and suitable rotary guide rolls 16 to 22 over which exemplary paper web stock w to be coated is led past a flame F from the burner 14 and onto an application roll 24 from which the coated stock is withdrawn. The roll 22 serves also as a pinch roll which holds the web stock directly against the application roll 24, with the extruder 12 discharging molten polyolefin p in sheet form into the bite of the rolls 22, 24 and, hence, against the face f to be coated of the web stock w.
Except for the burner 14, the installation 10 is entirely conventional and is used in continuously coating web stock which has previously been primed by the application of a bonding agent to its face to be coated as previously described, with the continuously fed web stock receiving its polyolefin coating at the bite of the rolls 22, 24 where its primed face contacts the extruded polyolefin. The application roll 24 and also the pinch roll 22 may in this instance be power-driven for the feed of the web stock w in the direction of the arrows 26 to and beyond the coating application station A.
In accordance with an important aspect of the present invention, the paper web stock w is "primed" for a firm and even bond between its face f to be coated and the molten polyolefin applied at the station A, by subjecting the stock face f on the web's pass to the station A to the flame F from the burner 14. The flame F, which is "special" in that it must have certain characteristics to be described, is in this instance a single flame sheet that extends, and impinges, more or less, with its tip on the stock face f, throughout the width of the web w, and the burner 14 may be, and preferably is, an industrial gas burner of high-capacity type shown in my prior U.S. Pat. No. 2,647,569, dated Aug. 4, 1953, having in a burner slot a plurality of burner ribbons from which issue a multitude of piloted utility flames of regulatable heat-intensity that combine to form the flame sheet F, with the burner being fed with a combustible fuel mixture, this being a gas-air mixture.
In operation of the present installation 10, paper web stock w is continuously fed and led over the rolls 16 to 20 into the bite of the application and pinch rolls 24 and 22 and withdrawn from the application roll 24, with the guide roll 18 leading the web stock into sufficiently close proximity to the flame sheet F to be evenly subjected thereto. The coated stock s thus continuously passing from the application roll 24 is shown enlarged in FIG. 2, with the polyolefin coating c being bonded to the face f of the web stock w.
In thus treating the paper web stock w with the special flame F prior to coating the same, the web stock, and more particularly its face f to be coated, may be said to be "flame-primed," for it has been found that a paper web face thus flame-treated will bond with the subsequently applied polyolefin coating at least as firmly and evenly as does previous "solution-primed" web stock bond with a subsequently applied polyolefin coating. It has also been found that such flame treatment renders the web face immeasurably more receptive to the subsequently applied polyolefin coating than a web face not previously primed at all, i.e., neither flame-primed nor solution-primed.
Polyolefin-coating paper web stock in the exemplary operation shown in FIG. 1 is thus according to a method which involves coating a surface of the continuously fed web by contacting molten polyolefin therewith, and which features, prior to coating the web surface priming the web for a firm and even bond of its surface directly with the subsequently applied polyolefin coating, by subjecting the face to be coated of the continuously fed web to the special flame.
To gain a better understanding of the method of coating web stock with a polyolefin according to the present invention, there is given, by way of example and not by way of limitation, the following pertinent data of an actual stock coating operation in an installation like that shown in FIG. 1. Thus, web stock in the form of a paper web of approximately 4 mil thickness was fed at a rate of approximately 1000 feet per minute past the sheet F of special flame and into the bite of the application and pinch rolls 24, 22 where molten polyethylene at normal extrusion temperature was applied to the face to be coated of the paper web, with the polyethylene coating being about one-half mil in thickness. The flame sheet impinged on the passing web stock throughout its width. The resulting polyethylene-coated paper web had such a strong bond between paper and coating as practically to defy all efforts at peeling the coating from the paper web, wherefore at the given feed rate of the paper web the exceedingly brief impingement of the flame sheet of the given temperature on the web primed the latter quite satisfactorily.
To further demonstrate the firm and even bond between the paper and polyethylene of the exemplary coated paper web just described, clean cut edges of the same were inspected under the microscope at 100× magnification and a photomicrograph made of one of them of which FIG. 3 is a copy in accurate outline and in the nearest possible proximation of the demarcation between the paper and polyethylene at the bond between them which despite this high-power magnification was difficult to observe on the actual photomicrograph. Thus, FIG. 3 shows the paper web w and the polyethylene coating c thereon, with the demarcation between them being roughly along the jagged line L. However, all microscopic inspections of cut edges of the coated paper web, including the one photographed and depicted in FIG. 3, clearly indicated that, except for very few and very minute as well as entirely inconsequential pockets between the paper stock and polyethylene, the paper stock and polyethylene was remarkably free from pockets therebetween, for no pockets, except the few mentioned, were discernible at 100× magnification, and the photomicrograph copied in FIG. 3 did not even show a single one of these very few pockets. This, coupled with the described great resistance of the polyethylene coating to being peeled from the paper stock, points clearly at adhesion of the polyethylene coating to the paper stock which to all practical intents and purposes amounts to 100 percent adhesion, and further points equally clearly at the remarkable firmness and evenness of the bond between the paper stock and coating thereon.
To further demonstrate the remarkable effectiveness of flame-priming stock for its subsequent polyolefin-coating, exactly the same paper stock of 4 mil thickness was, without prior flame priming or any other previous priming, coated with polyethylene in the same manner as the described flame-preprimed paper web. Clean-cut edges of samples of the thus coated but not preprimed stock were inspected under the microscope at 100× magnification and a photomicrograph made of which FIG. 4 is a true copy in every respect. The photomicrograph copied in FIG. 4 affords a typical demonstration of what microscopic inspection of all sample edges showed. Thus, FIG. 4 shows the paper stock w and the applied polyethylene coating c of which the latter is bonded or tacked to the former at the isolated places p', with the detached polyethylene coating therebetween forming typical bubbles b. Further investigation brought to light that there were about 54 of these bubbles per inch of linear edge of the coated stock and that about 38 percent of the polyethylene coating was attached to, and 62 percent thereof detached from, the surface of the paper stock. While it is hardly necessary to mention that the coated stock of FIG. 4 is useless for most practical purposes, for its coating has no peel resistance to speak about and the bubble formations of the coating will rupture on the slightest rubbing touch therewith, this coated stock, with but 38 percent of its coating adhering to the paper stock, points unmistakably at the remarkable effectiveness of flame-priming the base stock prior to coating the same which is solely responsible for just about 100 percent adherence of the subsequently applied coating to the base stock, as described.
It has been mentioned heretofore that relatively high heat-intensity of the special flame is essential in flame-priming paper stock for its firm and even bond with the subsequently applied polyolefin coating. More particularly, not only must the high heat-intensity of the flame be at its tip which is that part of the flame that is nearest, and more often than not impinges on, the paper stock and, hence, acts to flame-prime the same, but it has been found that in order to flame-prime paper stock the flame must be fed with a gas-air mixture at a ratio at which preferably most, and even all, of the mixture in the flame tip undergoes combustion, with such flame having the characteristic appearance and relatively high velocity and also tip temperature of a more or less stiff or "sharp" flame.
Tests have been conducted to find out the exact change in a paper web face pursuant to its subjection to sharp flame, and in these tests methods were employed involving surface microscopy, infrared reflectance spectrometry and surface adsorption. These tests have indicated and proved that in a paper web subjected to such a sharp flame the polarity of the cellulosic structure of the flame-subjected web face has notably deceased which, of course, promotes the adhesion of the subsequently extruded, relatively nonpolar, polyolefin to this web face.
As already mentioned, such a sharp flame has a characteristic appearance, wherefore it is merely necessary to vary the ratio of gas and air until a flame has the characteristic appearance of sharp flame which thereupon is entirely suited for the purpose of flame-priming a paper web. Of course, there is no single exact ratio of gas to air which must be maintained for a sharp flame suited for the purpose, for this ratio may vary even for the same gas and will vary for different gases such as natural gas and produced gas, for example, yet as long as the ensuing flame has the appearance of a more or less sharp flame, combustion of the air-gas mixture in the flame tip is sufficiently complete to flame-prime a paper web face, i.e., will cause a decrease in the polarity of the cellulosic structure of the web face and thereby promote adhesion of the subsequently applied polyolefin to this web face. Just to give some indication of air-gas ratios of sharp flames, there is given, by way of example and without any implied limitation, one example of such an air-gas ratio used for a flame suited for the purpose, this being a ratio of 9.6 to 1 for a certain natural gas, i.e., 9.6 parts by volume of air to 1 part by volume of the particular gas.
These same tests have also indicated and proved the surprising fact that sharp flame suited for flame-priming a paper web face, i.e., causes a decrease in the polarity of its cellulosic structure, left intact cellulose fibers that protruded from the web face. These same tests further indicated and proved that such protruding cellulose fibers burned off or fused only on subjection to a flame which was widely different from a sharp flame and is best described as a "soft" flame. Such a soft flame is in its appearance markedly different from a sharp flame, and is further characterized by noncombustion of a quite substantial proportion of the air-gas mixture in the flame tip. It was found that soft flame which did burnoff or fuse protruding cellulose fibers on a paper web face caused an increase in the polarity of the cellulosic structure of the face and, hence, made for even more inferior adherence of an extruded polyolefin coating to the web face. Just to give an example, without any implied limitation, of such a soft flame which burned-off and fused protruding cellulose fibers on a paper web face with ensuing increase of the polarity of the cellulosic structure of the web face, a flame was fed with a mixture of gas and air at a ratio of 6 parts by volume of air to 1 part by volume of gas, with the gas being the same natural gas mentioned above for a sharp flame.
It is imperative for satisfactory priming of a paper web that the sharp flame substantially impinges on the web face. In this connection, at the practical high web speeds contemplated for flame-priming web according to the invention, this being web speeds within a range starting at a minimum of approximately 400 feet per minute and extending at present to 2,500 feet per minute, a layer of air clings to and travels with the web. This air layer increases in thickness with increasing web speed and acts as an effective heat insulator on the web, and this air layer must be penetrated by the flame tip before the latter reaches the web face to prime the same. Accordingly, at different web speeds the velocity of the flame will have to be varied in order to achieve the foregoing. This is done by simply varying the pressure of the combustible air-gas mixture for the burner in well-known manner, without ordinarily changing the air-gas ratio of the mixture so as to retain the sharp characteristic of the flame.
It has been found that for web speeds from approximately 400 feet per minute to approximately 1,200 feet per minute, flame from a single burner satisfactorily flame-primes the web on varying the flame velocity as needed. At web speeds above 1,200 feet per minute, the tip of a single flame apparently is cooled quite considerably by the clinging air layer on the web, and it has been found appropriate for good flame-priming of webs to employ the flames from two burners.
In initial operations of making polyolefin-coated paper stock according to the invention pinholes in the coating were noted. However, pinholes in these coatings were quickly eliminated in toto on first finding that they were caused by breaks or ruptures therein owing to insufficient extrusion speed of the polyolefin and, hence, without compensating in the applied polyolefin for its stretch tendency on cooling to room temperature. The simple remedy for eliminating pinholes altogether was a slight increase in the extrusion speed of the polyolefin onto the passing web.
The web stock is flame-primed according to the present invention preferably and advantageously in the same continuous operation in which the polyolefin coating is applied thereto, as in the installation in FIG. 1 and in the described exemplary operation thereof. However, it is not imperative that the paper web be flame-primed and polyolefin-coated in the same operation, for flame-primed web has been laid aside for several days and then coated with polyolefin, with the bond between them being as firm and even as that achieved by flame-priming the web and coating it in the same operation.
Reference is now had to FIG. 5 which shows an installation 10' for flame-priming a paper web w' with flame F from two burners 14' at the hereinbefore mentioned web velocities of over 1,200 feet per minute.
While in the foregoing description of the invention reference is made to the flame-priming of paper, the term "paper" as used herein and in the appended claims is meant to include stock commonly referred to as "cardboard" or "boardstock" or any other stock having the characteristic properties of paper.