CA1138168A - Biaxially stressed fluorinated polymer roll cover and method for making same - Google Patents

Abstract

Abstract of the Invention A biaxially heat shrinkable sleeve, a prism which is usually a roller having its lateral area covered with a biaxially heat shrunken sleeve and the method for covering the lateral area of a prism such as a pro-cess roller with a biaxially heat shrunken sleeve.

Description

~ D42-36 BIAXIALLY STRESSED FLUORINATED POLYMER ROLL COVER

BACKGROUND OF THE INVENTION
A) Field of the Invention This invention relates to the installation of a heat shrinkable polymer roll cover over the'lateral area of a process roll. The invention further relates to the heat shrinkable sleeve and the process roller covered with the sleeve.
B) Histor~ of the Prior Art Process rollers or rolls are used in industry to press, roll or convey sheet or laminar materials.
Examples of such rolls are nip rolls for the production of plastic sheets and films, paper machine dryer drums used in the production of paper and textile dyeing and drying drums used in the production of textiles.
Originally, such process rolls had a metallic surface. These surfaces were sometimes covered with a polymer film to obtain better corrosion resistance or better abrasion resistance. In addition, such metallic surfaces are now known to have high surface energy characteristics which frequently causes the material being processed to stick to the roll. In most processes, such sticking or adhesion cannot be tolerated.
In order to reduce the surface energy of such process rolls, the rolls were frequently covered with a fluorinated polymer as soon as such polymers became available for such applications.
One of the better methods for covering such rolls in the prior art consisted of placing the roll within a radially elongated fluorinated polymer sleeve which was then heat shrunk to provide a tight fitting roll cover.
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Such covers, however, continued to have serlous problems since, especially in large rollers, the cover expanded in the axial direction when the roll was heated thus causing uneven roller surface char-acteristics. Furthermore, especially in larger rolIers, the sleeve was able to move on the roller, par-ticularly at high temperatures, which also caused uneven roller surface characteristics.
In order to avoid axial elongation when it was required to heat a roller, the sleeves were axially stretched at the time of application of the sleeve to the roller. Such axial stretching at the time of applying the sleeve to the roller made the appli-cation of the sleeve exceedingly difficult and re-quired complicated processes and apparatus. Examples of such processes and apparatus to axially stretch a sleeve during application to a roller are disclosed in U. S. Patents 3,426,119 and 3,749,621. Furthermore, such sleeves continued to be free to move on the roller thus creating uneven surface characteristics.
In order to prevent such movement, various ad-hesives were tried which, while being somewhat success-ful at low temperatures, were not completely successful at high processing temperatures such as those encountered in paper drying. Certain adhesives were, for example, tried and were found to become brittle at high temperatures, i.e., above about 150C, and were found to have insufficient adhesion to the fluorinated polymer sleeve.
U. S. Patent 3,481,805 discloses that a silicone resin adhesive could be used when the inside surface of a sleeve was preliminarily etched to aid the adhesive.
The patent does not, however, disclose the axial stretch-ing of the sleeve during its application to a roller.

Furthermore, the silicone resin adhesive was still in-sufficiently adherent to a fluorinated polymer sleeve to prevent the sleeve from moving under high temperature conditions.
As a result of the foregoing disadvantages, no com-mercially available prior art fluoropolymer covered roller was able to operate for an extended period at temperatures in excess of 120C.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with this invention, there is therefore provided a polymer sleeve for a prism which is usually a cylinder such as a process roller, which can be readily applied to the cylinder, a process for applying this sleeve to the prism, eg., a cylinder which prevents movement of the sleeve upon the cylinder even at tem-peratures as high as 260C (500F) and a combined prism and sleeve manufactured in accordance with the process.
The cylinder is usually a process roller, having its lateral area covered with the biaxially heat shrunken polymer sleeve wherein the sleeve is preferably adhesively bound to the lateral surface of the cylinder with a high temperature elastomeric adhesive which is desirably a polydimethylsiloxane. The novel sleeve is biaxially stretched beyond its room temperature elastic limit. The interior surface of the sleeve is desirably slightly abraded, eg. with 0~ steel wool or with a very fine abrasive, eg. 600 grit or smaller, and etched with a composition selected from the group consisting of sodium dissolved in liquid ammonia; sodium dissolved in a mixture of naphthalene and tetrahydrofuran; and molten sodium hydroxide. The method for covering the lateral area of a cylinder with a heat shrinkable polymer sleeve comprises placing the biaxially heat 1~.3R~

shrinkable sleeve over the lateral area of a cylinder and biaxially heat shrinking the sleeve into a tight fitting relationship with the cylinder by applying heat to the sleeve. The sleeve is selected so that at least 1%, preferably 2% and most preferably at least 3% re-sidual biaxial stress remains in the sleeve after heat shrinking. Desirably, a silicone adhesive is applied between the lateral surface of the cylinder and the interior surface of the sleeve and the interior surface 0 i9 desirably slightly abraded and etched as previously described.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an end perspective view of a roller covered with a fluorocarbon polymer roll cover in accordance with the invention.
Figure 2 is a cross sectional view of the roller shown in Figure 1 taken along line 2-2 of Figure 1.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, there is pro-vided a prism such as a cylinder having its lateralarea covered with a biaxially heat shrunken polymer, preferably a fluoropolymer, sleeve.
"Prism" as used herein means any prism having from three to an infinite number of sides, i.e., a cylinder.
The circumference of the prism need not be completely uniform along its entire length but for a tight sleeve fit should generally not have a circumference which - varies by ove~ 1007a~d th~ pJ-iS~ s~uld n~t have axial concave portions which are greater than 50~ of the radius of the prism. The prism should have no radial concave portions for a tight sleeve fit. Whenever "cylinder"
is used herein, it is to be understood that other prisms can be substituted for a cylinder.
"Lateral Area" as used herein means the surface 1~1.3~
area of 8 prism such as a cyllnder excluding the end surface areas.
"Sleeve" as used herein means a tube formed from a flexlble polymeric material which has a wall thickness which is thin relative to the overall tube diameter, eg. usually not thicker than about 0.01 times the overall diameter. Relatively thicker walls can be used when very small diameter cylinders are being covered. Examples of such cylinders are small tubes such as 1/8 inch diameter heat exchange conduit.
The prism is almost always a cylinder which is usually a process roller such as those encountered in plastic film, paper, and textile industries. The cylinder or .
roller can be made of any suitable material such as natural rubber, silicone rubber, steel or combinations thereof such as rubber covered steel.
"Axial" as used herein means along the longitudinal axis of the sleeve or cylinder.
"Biaxially" as used herein means along both the axial and radial axes of the cylinder or sleeve or in directions perpendicular to each other along the surface of the flexible plastic material from which the sleeve is manufactured.
The polymer from which the sleeve is manufactured may be any flexible polymer which can be used at tem-peratures of about 100C or above and which can be bi-axially stretched beyond its room temperature elastic limit yet biaxially shrink upon the application of heat.
The polymer is usually an alkene polymer such as poly-ethylene, polypropylene and substituted polyethylenesand polypropylenes such as polyvinyl chloride, chloro-vinylidene and polytetrafluoroethylene. The thickness of the sleeve is usually from about 0.001 to about 0.5 cm., and most frequently from about 0.025 to about 1~.3~

0.25 cm. for most applications. The polymer should have low surface energy to prevent material being processed, such as polyethylene film, textiles or paper, from sticking to the roller. Examples of suitable fluorinated polymers are polytetrafluoro-ethylene (PTFE), fluorinated ethylene-propy~lene - copolymer (FEP) and perfluoroalkoxy fluoroalkane polymers (PFA).
PFA and PTFE are particularly good under tem-perature conditionq as high as 260C whereas FEP i8a melt processable material which is suitable for use at temperatures as high as 200C.
The interior surface of the sleeve is desirably adhesively bound to the lateral surface of the cy-linder to prevent movement of the sleeve about the cylinder while the cylinder is in use. The adhesive should be a high temperature elastomeric adhesive; i.e., an adhesive stable at use temperatures without becoming brittle and without losing its ad-hesive properties. Examples of such adhesives areusually specific adhesives selected for their pro-perties which comprise a polymer selected from poly-sufones, silicones, polyurethanes, and silicone and fluorine modified epoxies. The most preferred adhesive is a silicone elastomer adhesive which is preferably a polydimethylsiloxane since such an adhesive is able to bind the fluorinated polymer sleeve to the cylinder yet function at a temperature as high as 260C without becoming brittle or de-composing. An example of a suitable polydimethyl-siloxane silicone elastomer adhesive is a polydi-methylsiloxane with pendant hydroxy groups available ~.3~

from Dow Corning Corporation under the designation RTV
140. Similar adhesives are also available from General Electric Company.
Another very desirable adhesive is a silicone elastomer adhesive comprising a chain which contains polydimethylsiloxane, alkane and phenylmethylsilane groups. Such an adhesive is available from Dow Corning under the designation 96-083 and is cured by means of a catalyst comprising platinum. This adhesive is particularly desirable, not only due to its performance at high temperatures but due to its ability to cure without emitting undesirable gases or vapors. The RT~ 140 type adhesive has been found to emit gases or vapors which can cause bubbling under the sleeve.
It has, however, been unexpectedly found that very - small holes or perforations can be placed in a pro-cess roller sleeve to permit venting of such gases without causing surface imperfections in material which is subsequently processed by the roller. The size of the holes can be up to about 4 mm for certain applications but desirably should have a diameter of from about 0.1 to about 0.5 millimeters. The distance or interval between holes should be from about 0.1 to about 5 cms.
- In order to further enhance the adhesion of the sleeve to the lateral surfac~ of the cylinder, the in-ternal surface of the sleeve is desirably etched. The etching composition is usually selected from a com-position comprising sodium dissolved in liquid ammonia;
a composition comprising sodium dissolved in tetra-hydrofuran and napthalene; and a composition com-prising molten alkali metal hydroxide such as sodium hydroxide.

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Adhesion is even further enhanced if prior to etching the interior surface of the sleeve is slightly abraded with very fine steel wool or about 600 grit or finer and preferably 1,000 grit or finer polish-ing abrasive such as silicon carbide or alumina. The abrasive is preferably a loose abrasive powder or steel wool but may be a coated or bonded abrasive product.
The method in accordance with the invention com-. prises placing the biaxially heat shrinkable sleeve 10 over a cylinder and biaxially shrinking the sleeve into a tight fitting relationship with the cylinder by applying heat to the sleeve. As previously discussed, the cylinder is usually a process roller. Any suitable means for placing the sleeve over the cylinder may be used. The sleeve may be preformed and slipped over the cylinder or a biaxially stretched heat shrinkable polymer sheet may be wrapped over the lateral surface of the cylinder and seamed to form a sleeve. Heat i8 i then applied to shrink the sleeve. After shrinking, usually at least 1% and preferably at least 3% butusually less than 25% residual biaxial stress remains in the sleeve at operating temperature and pressure, that is, the sleeve would further contract the addi-itional percentage in both the axial and radial directions at operating temperature if it were not restrained by the cylinder.
The biaxially heat shrinkable fluorinated polymer sleeve, again as previousl.y discussed, is manufactured from a polymer, preferably a fluorinated polymer, which can be biaxially stretched beyond its room temperature elastic limits yet biaxially shrink upon the application 3~
of heat. The sleeve has a finished thickness of from about 0.05 to about 0.15 centimeters for most appli-cations. The sleeve is prepared by biaxially expand-ing a smaller sleeve or the film from which it is made, in both the radial and axial directions beyond its room temperature elastic limits. That is, at room temperature it retains a dimension larger than its unexpanded di-mension for a sufficient time to slip the sleeve over the roll. For ease of processing, such expansion may take place at an elevated temperature, eg., about 100C, followed by cooling of the sleeve to room temperature.
Once it was determined, in accordance with this invention, that biaxial expansion of the sleeve was desirable, numerous methods for obtaining such biaxial expansion become apparent to those skilled in the art.
In one embodiment for obtaining such biaxial stretch-ing or expansion, end plugs are installed in two ends of a piece of fluoropolymer tubing such as FEP tubing and tightly banding the tubing to the plug to prevent loss of air and slippage. The plugs are connected in the center by means of a pipe that can be extended such as a hydraulic ram. The assembly is then place'd in a sizing pipe of a larger diameter. The assembly is then heated as with application of steam inside the sizing pipe and on the outside of the inserted assem-bly to a temperature of about 105C . The tubing is then extended axially by lengthing the pipe between the end plugs and circumferentially or radially by air pres-sure inside the tubing applied through one of the plugs.
The tube is thereby distended to the length of the 3~

pipe between the end plugs and radially to the inside diameter of the sizing pipe. The sleeve is then cooled thus producing a roll cover which becomes smaller in circumference and shorter in length when subsequently heated to a temperature of above about 110~C.
Other methods and apparatus for biaxially stretch-ing the sleeve beyond its room temperature elastic limits are possible. For example, a formed tube or sleeve can be drawn over a tapered mandrel to stretch it radially, i.e., to lengthen the radius as well as the circumference, and the force applied against the friction to pull it over the mandrel would be used to stretch it axially.
It is also possible to biaxially stretch a polymer film before it is made into a sleeve. For example, a film could be biaxially calendered or could be placed in or on an expandable frame or rack and pulled in biaxial di~
rections. Similarly, a finished tube or sleeve can be placed in or on an expandable frame or rack or calender-~' 20 ed in biaxial directions. The film or sleeve in or on a frame or rack can be gripped at its edges by clamps attached to the frame or rack or the film or sleeve be wrapped around the frame or rack, eg., pipes which are then separated. A particularly desirable method for biaxially stretching a sleev~ or tube is to radially ex-pand it with a compressed gas within a retaining tube which can be readily adjusted to different diameters, eg., a screen or canvas which is rolled at its edges to change the diameter of a cylinder formed by the screen or canvas, while axially stretching the sleeve by any suitable means. When the tube or sleeve is radially ex-panded with compressed air or other gas, it is desirable 1~.3~68 to expand the end~ of the sleeve or tube first, eg., by end calendering or placing an expandable sphere with-in each end and blowing up the spheres with compresged air. Another method for biaxially stretching or ex-pandlng a film or sleeve is to hammer the film or sleeve, eg., with a reciprocating hammer on one surface of the film or sleeve and an anvil on the opposing surface of the film or sleeve. The area struck by the hammer should be small relative to the total surface of the film or sleeve and the film or sleeve should move relative to the hammer and anvil so that essentially the entire film or sleeve i9 struck between the hammer and anvil.
Many other possibilities for biaxially distend-ing or expanding the sleeve beyond its cold tem-perature biaxial elastic limits can be envisioned by those skilled in the art.
Further, in accordance with the invention, and as previously discussed with respect to the resulting cylinder, the adhesive can be applied between the cylinder such as a roller and the interior surface of the sleeve. The adhesive may be applied by any suitable method as by brushing the adhesive on the cylinder before applying the sleeve or by injecting adhesive beneath the sleeve with a long hyperdermic type needle after the sleeve is applied to and shrunken upon the cylinder.
The sleeve, as previously discussed, may be slightly abraded and etched to further enhance a &esion of the sleeve to the lateral surface or area of the cylinder such as a process roller. Another method for increasing adhesion is by applying colloidal silica to the internal surface of the sleeve, i.e., the sur-face facing the cylinder and heating the silica coated ~.3~8 surface before using the adhesive.
The resulting process roller is provided with a polymer sleeve, desirably a fluorinated polymer sleeve, which has a smooth surface and which does not move about the lateral surface or area of the roller at tem-peratures above 150C, as high as 200C and even as high as 260C when appropriate fluorinated polymer sleeves and silicone elastomer adhesives are used.
Referring now to the drawings, Figure 1 shows an end perspective view of a cylinder 10 having a lateral area 12 covered with a biaxially heat shrunk-en fluorinated polymer sleeve 14. As best seen in Figure 2, sleeve 14 is secured to lateral area 12 by means of a silicone adhesive 16. Desirably, the interior surface 18 of sleeve 14 is slightly abraded and etched prior to securing sleeve 14 to lateral area 12 by means of adhesive 16.
The following examples are provided to more fully illustrate and not limit the invention. Unless other-wise indicated, all parts and percentages are by weight.EXAMPLE I
An FEP polymer film having a thickness of about 0.5 millimeters is slightly abraded on onè side with 00 steel wool and etched with a composition comprising 11.8 weight percent metallic sodium dissolved in 8 mix-ture of 11.8 weight percent napthalene and 78.4 weight percent tetrahydrofuran. The film is then formed into a sleeve in accordance with methods well known to those skilled in the art. The resulting seam is then abraded with 00 steel wool and etched with the composition com-prising sodium dissolved in a mixture of napthalene and tetrahydrofuran. The sleeve is formed so that it has an inside diameter of about 54 centimeters and a length of 1~.3~

- about 163 centimeters. The sleeve is then biaxially stretched by plugging the ends wlth aluminum plugs and banding the ends of the sleeves to the plugs.
Compressed air is forced into the sleeve through one of the plugs at a pressure of about 2.45 kilograms per square centimeter. The plugs are simultaneously pulled in opposite directions with a force of about 1000 kilograms over and above the force being applied against the plugs by the compressed air. The resulting sleeve has an inside diameter of about 61 centimeters and a length of about 178 centimeters. The sleeve is then slid over a steam heated textile drying drum having an outside diameter of about 58 centimeters and a lateral surface length of about 152 centimeters., Each end of the sleeve is then heated to a tem-perature of about 95C and banded to the end of the cylinder. The entire sleeve is then heated to about 120C to shrink the sleeve into a tight fitting re-lationship with the cylinder. The cylinder and sleeve are then cooled to room temperature. About 158 grams of Dow Corning 96-083 adhesive is then injected be-neath the sleeve along the length of the cylinder at several locations using a hyperdermic type syringe.
The adhesive is then uniformly spread beneath the sleeve by moving a rubber squeegee about the external surface of the sleeve. The resulting sieeve and cylinder are then cured at 160C.
The cylinder and sleeve are then placed into textile drying service at a temperature of 165C.
After a period of 18 months, the sleeve continues to have a ~mooth wrinkle-free appearance.
EXAMPLE II
The procedure of Example I is repeated except 1~ 3~

that the cylinder is a heating roll utilized for laminating plastic underlayment for carpet. The drum has an outside diameter of 40 centimeters and a length-of about 221 centimeters. The inside di-ameter of the unstretched sleeve is about 37 cen-timeters-and the length of the unstretched sleeve is 238 centimeters. The stretched sleeve has an inside diameter of about 42 centimeters and a length of 258 centimeters.
The finished roll and sleeve are utilized to laminate plastic to foam at a temperature of about 230 DC . After a time period of 4 months, the sleeve has a smooth wrinkle-free appearance.

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A biaxially heat shrinkable fluorinated polymer sleeve, having an internal surface which is etched with a com-position selected from the group consisting of sodium dis-solved in liquid ammonia, sodium dissolved in a mixture of napthalene and tetrahydrofuran, and molten sodium hydroxide.

2. The invention of Claim 1 wherein the fluorinated polymer sleeve is essentially a composition selected from the group consisting of polytetrafluoroethylene, fluorinated ethylene-propolene copolymer, and perfluoroalkoxy fluoralkane polymer.

3. A biaxially heat shrinkable fluorinated polymer sleeve having an internal surface which is etched with a com-position selected from the group consisting of sodium dissolved in liquid ammonia, sodium dissolved in a mixture of napthalene and tetrahydrofuran, and molten sodium hydroxide, said sleeve which is also abraded with steel wool of 00 grade or finer.

4. The invention of Claim 3 wherein the fluorinated polymer sleeve is essentially a composition selected from the group consisting of polytetrafluoroethylene, fluorinated ethylene-propolene copolymer, and perfluoroalkoxy fluoroalkane polymer.

5. A biaxially heat shrinkable fluorinated polymer sleeve having an internal surface which is etched with a composi-tion selected from the group consisting of sodium dissolved in liquid ammonia, sodium dissolved in a mixture of napthalene and tetrahydrofuran, and molten sodium hydroxide, said sleeve which is also abraded with a polishing abrasive of 600 grit or finer.

6. The invention of Claim 5 wherein the fluorinated polymer sleeve is essentially a composition selected from the group consisting of polytetrafluoroethylene fluorinated ethylene-proplene copolymer, and perfluoroalkoxy fluoroalkane polymer.

7. A biaxially heat shrinkable fluorinated polymer sleeve, having an internal surface which is etched with a com-position selected from the group consisting of sodium dissolved in a liquid ammonia, sodium dissolved in a mixture of napthalene and tetrahydrofuran, and molten sodium hydroxide, said sleeve which also has said internal surface essentially coated with a high temperature elastomeric adhesive which is essentially a composition selected from the group consisting of polydimethyl-siloxane silicone elastomer, with pendant hydroxy groups, and a chain silicone elastomer containing polydimethylsiloxane, alkane and phenylmethylsilane groups.

8. The invention of Claim 7 wherein the fluorinated polymer sleeve is essentially a composition selected from the group consisting of polytetrafluoroethylene, fluorinated ethylene-propolene copolymer, and perfluoroalkoxy fluoroalkane polymer.

9. The invention of Claim 7 wherein a plurality of apertures is placed through the surface of said sleeve, each of said apertures being about 4 millimeters or less in diameter and the distance between any two or more of said apertures being in a range of about 0.1 to about 5 centimeters.

10. The invention of Claim 7 wherein said internal surface is coated with said high temperature elastomeric adhesive following said sleeve being heat shrunk.

11. The invention of Claim 9 wherein said internal surface is coated with said high temperature elastomeric adhesive following said sleeve being heat shrunk.

12. The invention of Claim 9 wherein said plurality of apertures is placed through the surface of said sleeve following said sleeve being heat shrunk.

13. The invention of Claim 11 wherein said plurality of apertures is placed through the surface of said sleeve following said sleeve being heat shrunk.