WO1989010902A1

WO1989010902A1 - Coatings for reducing coefficient of friction on glass/metal working surfaces of glass forming machines

Info

Publication number: WO1989010902A1
Application number: PCT/US1989/001956
Authority: WO
Inventors: Ronald T. Myers; Lesley G. Hampson; Thomas Bell
Original assignee: International Partners In Glass Research
Priority date: 1988-05-05
Filing date: 1989-05-05
Publication date: 1989-11-16
Also published as: AU3764489A

Abstract

The use of a ceramic based composition is disclosed for reducing frictional forces between moving molten glass and working surfaces of a glass forming machine. The ceramic based composition is applied as a dispersion and cured on the working surfaces, such as the blank cavity of a glass container mold. The preferred ceramic based composition is sold under the trademark "XYLAR" R and has been found to be particularly effective in reducing frictional engagement over extended periods of time in the operation of glass forming machines.

Description

COATINGS FOR REDUCING COEFFICIENT OF FRICTION ON GLASS/METAL WORKING SURFACES OF GLASS FORMING MACHINES

FIELD OF THE INVENTION

This invention relates to a ceramic based composition which is particularly useful in lowering the sliding coefficient of friction of molten glass as it moves over a working surface of a glass forming machine.

BACKGROUND OF THE INVENTION

Glass forming machines are used extensively in forming molten glass into a variety of shapes. In forming the glass and working it into various shapes, surface imperfections can be created in the glass such that the resultant glass object is inherently weak and will readily break when impacted. There are, of course, a variety of glass objects in which the strength is not of particular concern, such as ornamental articles. However, by far glass forming machines are used to manufacture a variety of glass containers some of which are designed to contain pressurized liquids. It is important that glass containers possess inherent structural strengths which exceed those of normally manufactured glass articles. All ware receives impacts of some degree during bottle plant inspection and packing and during a customer's filling operation. Also thermal stresses are applied to bottles which are washed and filled, often with hot product. So, all glass containers receive stresses during their service life and hence strength is a near universal concern. Considerable work has been conducted therefore in strengthening glass containers to withstand impact in the normal handling, conveying, packaging and use of the containers. An aspect which assists in strengthening the glass container is to reduce as much as possible the sliding coefficient of friction of the molten glass as it is being formed into the glass object, particularly glass containers. A high coefficient of friction will result in marks which cause cosmetic and strength defects, unfilled finishes and rough bottoms which are not accepted by the customer and machine malfunctions which increase manufacturing downtime. At present, the most effective form of reducing the coefficient of friction is to use water-based graphite containing precoatings which are applied to metal mold parts, principally the blanks, before the equipment is placed on the glass forming machine. The graphite coating is periodically replenished on the mold surfaces by the application of swab dopes which are oil-based graphite mixtures. If one chooses not to replenish the precoating of water-based graphite, the time that the mold equipment can function correctly is significantly diminished. Examples of known procedures to coat mold surfaces are disclosed in United States patents 3,052,629, 3,874,862 and 4,526,600.

Principal precoatings used are those manufactured and sold under the trademarks "RENITE" (a sodium silicate binder with graphite) , "MICROSEAL" (an acid aluminum phosphate binder with graphite) and "GLASSLUBE" (a phosphoric acid binder with graphite) . All of these compositions include suspending agents in one form or another. None of the above binders in these compositions promote glass against metal lubrication. It has been experienced that the sliding coefficient of friction of these binders can cause an increase in the sliding coefficient of friction on cast iron compared to untreated cast iron. SUMMARY OF THE INVENTION

According to an aspect of this invention, a ceramic based composition has been discovered which lowers the sliding coefficient of friction compared to untreated base metal and, when loaded with graphite or carbon, it has a relatively long lifetime compared to prior materials.

According to an aspect of the invention, in a glass forming machine having a multiplicity of working surfaces which come into sliding contact with heated glass the improvement comprises one or more of the working surfaces being coated with a porous low density ceramic based composition which includes ceramic constituents of chromium, magnesium and phosphorous.

In accordance with another aspect of the invention, a blank cavity for use in a mold for forming glass objects from molten glass has its interior surfaces, which contact the molten glass, coated with a porous low density ceramic based composition which includes ceramic constituents of chromium, magnesium and phosphorous.

In accordance with another aspect of the invention, a process for reducing sliding coefficient of friction generated by moving molten glass over a forming work surface of a glass forming machine comprises coating the forming work surface with a dispersion of a low density ceramic composition in a ,suitable carrier which includes ceramic constituents of aluminum, chromium, magnesium and phosphorous. The composition is heat cured to remove the carrier and to provide a cured porous ceramic coating composition. Optionally, the composition may include elemental carbon. Brief Description of the Drawing

Figure 1 is a perspective view of components of the apparatus used in testing the sliding friction- coefficient of selected coating materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Glass forming machines are of a variety of configurations as used worldwide in forming a number of glass objects. In such glass forming machines, molten glass is delivered in the form of a gob to a distributor which directs the gob into a chute. The chute in turn directs the travelling gob into a mold. In one variant of the machine operation, the plunger enters the mold to press the glass into the shape of a parison or the shape of the desired glass object. In situations where the parison is formed, it is then transferred with or without treatment to a second mold in which it is blown to the internal shape of the second mold. The blown object is then removed from the second mold and optionally treated before subsequent heat treatment. It is apparent that in such glass forming machines, the molten glass, as it is being worked into a variety of shapes, comes into contact with various working surfaces of the machine. It has been discovered that, in lowering the sliding coefficient of friction of the glass relative to the working surface characteristics in the resultant glass object, such as a container or the like, are attained. Such improved surface characteristics normally result in an increase in the overall strength of the glass container. Components of a glass forming machine, which are commonly in sliding contact with the glass, include items such as mold parts, blank cavities, plungers, chutes, distributor troughs and the like. In accordance with this invention, one or more of these working surfaces of the glass forming machine may be coated with a ceramic based material in particulate form which is porous or has absorptive surfaces to carry selective additives. The composition includes effective amounts of ceramic constituents of chromium, magnesium and phosphorous and optionally aluminum. The composition is in a suitable carrier which can be evaporated when the material is applied as a coating.

The preferred form of the ceramic based composition is that sold under the trademark "XYLAR" by Whitford Corporation of Pennsylvania. The low density ceramic based composition is normally supplied in an aqueous dispersion which includes metallic and/or non-metallic fillers. The dispersion with the aqueous carrier or other suitable carrier may be applied to the working surface of the glass forming machine, such as the interior surface of the cavity blank. The coating is then cured at a sufficient temperature to remove the carrier and set the ceramic based composition. For most compositions sold under the trademark "XYLAR", a cure of approximately 30 minutes at 345°C (650°F) is required. It is appreciated that cure times will vary depending upon the temperature used. For example, a higher temperature which is at a level which does not ' degrade the coating will result in less cure time, but in any event constitute a functional equivalent in terms of curing. The composition includes chromium, magnesium, phosphorous and optionally some aluminum. The composition may be formulated into a base coat and a top coat. The top coat may additionally include titanium dioxide. The base coat can comprise a "XYLAR 1" coating which may be applied to a dried thickness in the range of 25 to 30 μm. The top coat can also comprise a "XYLAR" coating which may be applied to a, dried thickness in the range of 5 to 10 μm. Graphite may be used in combination with the "XYLAR 1" base coat or one of the "XYLAR" top coats. "XYLAR" top coats are available under the numeral designations of 101, 201 and 251. The material sold under the trademark "XYLAR" is a new proprietary product of Whitford Corporation. At this time, detailed physical and chemical characteristics of the product are not described in any scientific journals, nor in any published patents nor patent applications. For purposes of this invention, the "XYLAR" material is described in terms of a porous low density ceramic based composition which includes ceramic constituents of chromium, magnesium and phosphorous. It is understood that any other form of porous low density ceramic based composition, which behaves the same way as "XYLAR" in reducing the coefficient of friction for glass or metal contact, is included within the scope of the invention.

It has been discovered that elemental carbon, when added to the "XYLAR" composition, improves the lubricity of the surface. The porosity of the ceramic material is such that the composition carries the elemental carbon after the coating is cured. The , elemental carbon is therefore added to the composition before the applied coating is cured. It is understood that the term elemental carbon, as used herein, refers to amorphous carbon such as lamp black, petroleum cokes which are semi-crystalline, graphite or mixtures thereof. Preferably, one or more of the above may be used with the "XYLAR" ceramic composition to improve lubricity of the coating composition.

It is generally accepted that the study of friction and wear is called tribology. One of several engineering objects of this study is to minimize the sliding coefficient of friction between a pair of materials in contact. In the field of tribology, it is acceptable to use a tribometer to carry out the tests. The tribometer is designed to measure the sliding coefficient of friction (μ) between a heated body of glass and a test material, usually metal or metal coated with a test coating kept at an elevated temperature different from the glass temperature. The measurement is made by measuring the torque across a glass containing refractory and test cup contact about an axis collinear with the axis of rotation of the test cup rotated with constant angular velocity, normally in the range of 390 rpm. The test cup of the desired metal or coating starts at an initial temperature T_m, the glass starts at a temperature T_g and, a known force is applied normally to the contact surface, all with a fixed geometry. The contact time may be in the range of 3 to 4 seconds.

The tribometer test technique may be described generally as follows. A rotating metal test specimen (a cup) preheated to a controlled temperature is brought into contact with a 10 mm deep pool of glass also preheated to a controlled temperature. This pool , of glass is mounted on a torque sensor such that when contact is made, the axis of rotation of the metal test cup is collinear with the axis of detection of the torque sensor.

Further details of the system are shown in Figure 1. The motor 10 rotates the test cup 12 up to the desired RPM. The test cup 12 has an annular or ring-shaped contact surface 14. The ring 14 is coated with the composition to be tested or a ring of material to be tested may be mounted on the annular portion. 'A pool of molten glass at the desired temperature is contained in the cast refractory having a cavity 18. The cast refractory 16 is mounted on a spindle 20 which, in turn, is connected to the torque sensor 22. The torque sensor is adapted to provide an output electrical signal representative of the torque generated in rotating the disk at a desired rp . In operation, the test cup is heated to a temperature in the range of 900°F. The pool of glass, which can be contained in a platinum or platinum gold holder, is at a temperature in the range of 1900"F. The preferred dimensions for the testing mechanism of Figure 1 are as follows. The contact surface is a ring with an outside diameter of 2.000 inches (50.80 mm) and an inside diameter of 1.500 inches (38.10 mm). The well inside the cup should be at least .250 inches (6.35 mm) deep. It can, however, be up to .75 inches deep. The mounting socket must be .625 inches (15.88 mm) in diameter.

The cup need not be a monolithic object; a composite cup can be made and used. In particular, a ring of material of interest can be mounted on a base made of less valuable or more durable material.

The glass pieces produced will be 3 inches ' (76 mm) in diameter and 1/4 to 3/8 inches thick (6 to 10 mm) . The glass will be annealed. The weight of the glass will be determined during the preliminary work and held constant thereafter.

To bring about this measurement, the glass and the test cup must be brought to their respective temperatures in different furnaces and then the glass moved into the test cup furnace so contact between the glass and the test cup can be made. The glass is heated to about 1900"F in a globar furnace and the test cup is heated to 900°F in a muffle furnace. The former is near the gob temperature, the latter is near the actual blank surface temperature. The globar furnace temperature, in accordance with this embodiment, is controlled by an Omega Model 49-K proportional time controller in conjunction with a variac and solid state relay.

The globar furnace is split so that the globars and the upper portion of the brick is mounted rigidly to the frame of the tribometer. The top part of the muffle furnace is also mounted rigidly to the tribometer frame. The bottom portions of both furnaces are mounted on a cart to which the torque sensing device is also mounted. The bottom part of the globar furnace has the refractory for containing a pool of molten glass. The refractory is mounted on the torque sensing column. The cart is mounted on a track which allows travel from a heat-up position in which the glass refractory is under the globar furnace to a test position in which the glass refractory is under the muffle furnace. The bottom of the muffle furnace has a heat seal in the heat-up cart position. An extra bottom mounted on the cart fills the position vacated by the globar furnace bottom when the cart goes to the , test position. This extra bottom prevents excessive heat loss by the globar furnace when the cart is in the test position. While the cart is being moved to the test position, the bottom of the muffle furnace cools but this heat loss is more than offset by the heat transferred to the muffle furnace and test up by the relatively hotter glass.

The above type of methodology is therefore used to test various materials as coated onto the test cup to determine its effect on reducing the sliding coefficient of friction. The following Examples outline the test procedures and the results, where such Examples demonstrate various embodiments of the invention.

EXAMPLE 1

Using the standard apparatus for tribology investigations employing the above method, a cast iron sample was prepared with the ceramic based composition coating applied to the surface. The sample was then tested over a series of 30 contacts, each contact being in the range of 3.5 seconds. The initial contacts generated torques in the range of 0.3 to 1 N-m. By the sixth contact, there was an initial spike in the torque of 0.45 N-m and shortly thereafter the torque dropped to 0.36 to 0.38 N-m. Contacts 10 through 20 were typical of this behavior. By contact #28, the initial spike had almost disappeared. The average torque was in the range of 0.3 N-m. In this test, the contact force of the glass disc and holder was about 8 Newtons.

The decrease in torque after the first few contacts is significant in indicating the extended use of this coating- material in reducing the sliding , coefficient of friction. With the fresh glass contacts, which occurred in contacts 29 and 30, reasonable recovery was demonstrated which is indicated by the reduction in torque compared to contact 28. EXAMPLE 2

The sample of cast iron was coated with a graphite impregnated "XYLAR" composition and tested in accordance with the procedure of Example 1. The graphite, after mixing into the composition, had a grain size in the range of lOμ. In the initial contacts, the torque generated was in the range of 0.150 N-m. The torque required in each contact climbed slowly. By the 28th run, the torque was in the range of 0.225 N-m.

The first four contacts were essentially the same with the beginning of a small spike in the torque and then dropping off by about 0.03 N-m to the median of approximately 0.5 N-m. By contact #5, a pattern was established through to contact #25. There was a low initial torque which then rose to a maximum value by mid-contact where it remained for the balance of the contact time. Surprisingly after contact #25, the torque began to show a slight downward trend after the mid-contact time.

EXAMPLE 3

An aluminum bronze alloy sample was coated with a graphite impregnated "XYLAR" the same as in Example 2. The sample was then tested in accordance with the procedure of Example 1. The behavior of this coated material followed very closely the behavior of the coated cast iron. The torques in this series of , tests were slightly lower by approximately 15%. The initial contacts developed torques in the range of 0.15 to 0.18 N-m. Subsequent contacts resulted in somewhat lower torques generated during the contact stage in the range of 0.10 to 0.15. The resting load of glass disc and holder was 37.9 NT. These torque values for the samples tested in Examples 1, 2 and 3 are comparable to or better than the known coating compositions such as "RENITE" in reducing the coefficient of friction. It is appreciated that other forms of elemental carbon may be used with the "XYLAR" coatings to provide equally acceptable results.

Hence, according to this invention, the ceramic based compositions, such as those based on using "XYLAR", either by themselves or mixed with elemental carbon produce an acceptable level of sliding coefficient of friction to be of significant value in minimizing the amount of surface imperfections produced in the molded glass article, as caused by this frictional engagement. The composition, as cured on cavity blanks, is particularly useful in molding of glass containers.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims

WE CLAIM

1. In a glass forming machine having a multiplicity of working surfaces which come into sliding contact with heated glass, the improvement comprising one or more of said working surfaces being coated with a porous ceramic based composition, said composition including ceramic constituents of chromium, magnesium and phosphorous.

2. In a glass forming machine of claim 1, said working surfaces are selected from mold parts, blank cavities, plungers, chutes and distributor troughs.

3. In a glass forming machine of claim 1, said working surface coated with said ceramic based composition is the interior surfaces of a blank cavity of a glass container mold.

4. In a glass forming machine of claim 1, said working surface coated with said ceramic based composition is a plunger for insertion in a closed mold for forming a glass container.

5. In a glass forming machine of claim 1, said porous ceramic based composition includes elemental carbon.

6. In a glass forming machine of claim 3, said porous ceramic based composition includes elemental carbon.

7. In a glass forming machine of claim 4, said porous ceramic based composition includes elemental carbon.

8. In a glass forming machine of claim 5, said porous ceramic based composition includes aluminum as an additional ceramic constituent.

9. A blank cavity for use in a mold for forming glass objects from molten glass, said blank cavity having an interior surface for contacting molten glass as it is formed into a desired glass object, said interior surface being coated with a porous ceramic based composition, said ceramic composition including ceramic constituents of chromium, magnesium and phosphorous.

10. A blank cavity of claim 9, wherein said porous ceramic based coating includes elemental carbon.

11. A blank cavity of claim 9, wherein said porous ceramic based composition includes aluminum as an additional ceramic constituent.

12. A process for reducing sliding coefficient of friction generated by moving molten glass over a work surface of a glass forming machine, said process comprising coating said forming-jwork surface with a dispersion of a ceramic composition, said ceramic composition including ceramic constituents of chromium, magnesium and phosphorous, in a carrier and heat curing said dispersion to remove said carrier and to provide a cured porous ceramic coating composition.

13. A process of claim 12 wherein said forming work surface is selected from mold part, blank cavity, plunger, chute and distributor trough of a glass container forming machine.

14. A process of claim 12 wherein said porous ceramic composition includes elemental carbon.

15. A process of claim 13 wherein said porous ceramic composition includes elemental carbon.

16. A process of claim 14, wherein said porous ceramic based composition includes aluminum as an additional ceramic constitutent.

17. A process of claim 15, wherein said porous ceramic based composition includes aluminum as an additional ceramic constituent.

18. A process of claim 12, wherein said ceramic based composition is cured at a temperature of approximately 650°F for thirty minutes or a functional equivalent thereof.

19. In a glass forming machine of claim 1, said ceramic constituents being present in amounts effective to reduce a sliding coefficient of friction of molten glass as it moves over said working surface during a glass forming operation in which the molten glass comes into sliding contact with said ceramic composition, the coefficient of friction being such that average torque measured by a torque sensor of a tribometer supporting a pool of the molten glass at about 1900^βF and a test specimen of the tribometer rotating at about 390 rpm and having a coating of sa'id ceramic composition thereon at about 900°F is about 0.3 N-m when a contact force between the molten glass and the coated test specimen is about 8 Newtons.

20. In a glass forming machine of claim l, said ceramic composition comprising a base coat on said working surface and at least one top coat on said base coat, said top coat including titanium dioxide.

21. In a glass forming machine of claim 1, said ceramic composition comprising a coating formed by heat curing an aqueous dispersion of said ceramic constituents.