HK1085165B - Formation of aramid paper laminate - Google Patents

Description

Method for making aramid paper laminate

Technical Field

The present invention relates to a process for making an improved laminate having aramid paper and polymer layers, and preferably, to a process for making a laminate of two aramid papers separated by a polymer layer.

Background

Japanese patent publication 8-99389 discloses the production of a laminated sheet of m-aramid paper and polyester film by a process of calendering and rapid cooling to form a laminate.

British patent 1486372 discloses a metal layer adhered to a nonwoven web of a blend of different staple fibers which have been consolidated and held together by a binder material of a film-forming high molecular polymer.

U.S. 5320892 to Hendren et al discloses a laminate for honeycomb structures composed of a core containing poly (m-phenylene isophthalamide) fibrids and outer layers of poly (m-phenylene isophthalamide) flocs and fibrids.

US 5948543 to Ootuka et al discloses a laminated base material made of aramid fiber nonwoven fabric by bonding para-aramid and meta-aramid fibers using a resin binder.

Laminates made from one or more layers of aramid sheet or paper and one or more layers of polyester polymer are used in transformers where such laminates are used as dielectric insulation. Any improvement in the internal adhesion of such laminates or the tear or elongation at break properties of such laminates is desirable.

Disclosure of Invention

The present invention relates to a process for producing a laminate comprising, in order, a layer of aramid paper, a layer of polymer and a layer of aramid paper, the process comprising the steps of:

a) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

b) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

c) applying a polymer to the more porous surface of the aramid paper from step a), and

d) laminating the aramid paper from step b) to the polymer wherein the more porous surface of the aramid paper is in contact with the polymer.

Another embodiment of the present invention relates to a process for producing a laminate comprising, in order, a layer of aramid paper and a layer of polymer, the process comprising the steps of:

a) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

b) the polymer was applied to the more porous surface of the calendered aramid paper.

Other embodiments of the invention relate to a process for producing a laminate wherein the temperature difference between the two calendering rolls is at least 50 c, preferably at least 100 c.

Detailed description of the invention

The starting material of the present invention is an aramid paper. The term paper as used herein is used in its normal sense and paper can be produced using conventional papermaking processes and equipment and methods. Aramid fiber material, i.e., fibrids and short fibers, can be slurried together to form a mixture that is converted into paper, for example, by a Fourdrinier machine or by hand using handsheet mold with a forming screen. See US 3756908 to Gross and US 5026456 to Hesler et al for methods of making aramid into paper.

The thickness of the aramid paper is not critical and depends on the final laminate used and the number of aramid layers used in the final laminate. While the present invention may be used with two layers, one aramid and one polymer layer, it is preferred to use three layers, two aramid papers and one polymer layer, it being understood that there is no upper limit to the number of layers or other materials that may be present in the final article. For purposes of illustration, a five-layer laminate may be made, namely a first aramid paper layer, a first polymer layer, an intermediate paper layer or a second polymer layer, a third polymer layer, and a second aramid paper layer. The aramid layers, intermediate layers and polymer layers can have different compositions.

The term aramid, as used herein, refers to polyamides wherein at least 85% of the amide (-CONH-) linkages are directly attached to two aromatic rings. Additives may be used with the aramid, up to 10 weight percent of other polymeric materials may be blended with the aramid, or copolymers may be used that replace the aramid diamine with up to 10 weight percent of other diamines, or copolymers may be used that replace the aramid diacid chloride with up to 10 weight percent of other diacid chlorides. In the practice of the present invention, the most commonly used aromatic polyamides are: poly (p-phenylene terephthalamide) and poly (m-phenylene isophthalamide), with poly (m-phenylene isophthalamide) being the preferred aramid.

Aramid paper can be calendered between two heated calendering rolls with the high temperature and pressure of the rolls increasing the non-bond strength of the paper. Calendering aramid paper in this manner also reduces the porosity of the paper, which is also believed to result in poor adhesion of the paper to the polymer layer in the laminate. Good layer adhesion is very important to avoid delamination of the electrically insulating laminate when in use. The present invention solves this adhesion problem of aramid paper using a calendering process that allows each of the opposing surfaces of the aramid paper to be of different porosity. In producing one laminate of the present invention, two aramid papers are first calendered and then laminated with one polymer inner layer. Such a polymer layer is not considered critical to the present invention because of the porosity of the surface of the paper. While thermoplastic and thermoset polymers may be used, thermoplastic polymers that have some degree of flow at the elevated temperatures of the lamination operation are preferred. In addition, the use of thermoplastic polymers in lamination facilitates application because they can be applied in liquid form to the aramid paper surface during or prior to lamination. However, it is also possible to use a composition comprising a preformed polymer layer, for example having thermoplastic or thermosetting properties. The preformed layer generally has memory from its formation which may negatively affect the mechanical properties of the laminate, and therefore it is preferred to use a liquid polymer.

The preferred polymer for application to the aramid paper of the present invention is polyester, particularly poly (ethylene terephthalate) (PET). The PET used may include a variety of comonomers including diethylene glycol, cyclohexanedimethanol, poly (ethylene glycol), glutaric acid, azelaic acid, sebacic acid, isophthalic acid, and the like. In addition to these comonomers, branching agents such as trimesic acid, pyromellitic acid, trimethylolpropane, trimethylolethane and pentaerythritol may be used. PET can be obtained using terephthalic acid or its lower alkyl esters (e.g., dimethyl terephthalate) and ethylene glycol or blends or mixtures of these compounds using known polymerization techniques.

Typical polymers that may be used in the present invention also include polyethylene naphthalate, polybutylene terephthalate, Polyetheretherketone (PEEK), Polyetherketoneketone (PEKK), polyethersulfone, polyimide, polyetherimide and aromatic liquid crystalline polyesters, or mixtures or blends of any of these compounds.

In the calendering of aramid paper, it should further be understood that the pressure of the calender rolls against the paper is not critical. Higher roller pressures generally result in a denser overall article. The desired degree of densification will depend on the aramid selected and the end use application of the final laminate.

However, in the present invention, when aramid paper is made into a laminate and two calender rolls are kept at different temperatures, there is a criticality. It should be understood that calendering the papers between two rolls as used in the present invention means that the papers can be produced using a single calendering step between heated rolls having different temperatures, or using a single calender step that first calendars one surface of the sheet at one temperature and then calendars the opposite surface at a second temperature. This temperature difference can directly cause a difference in porosity of the opposite surfaces of the aramid paper. A temperature difference of at least 20 c is essential to obtain the advantages of the invention. More preferably, the temperature difference should be at least 50 c and in many cases at least 100 c.

The temperature of the two rolls will depend on the aramid or aramids used to produce the final laminate. Although in many cases the same aramid can be used to produce both outer surfaces of the laminate, different aramids are suitable for these outer surfaces. The choice of aramid will depend on the end use of the laminate. For purposes of illustration, it will be apparent that the characteristics of the surfaces of these laminates will help dictate the temperature and temperature differential during calendering of the paper. If the outer surface of the aramid paper is desired to be denser, the temperature applied to the roll contacting this surface is higher during calendering. It is understood that the temperature of the heated rolls can be below the glass transition temperature of the aramid component of the paper. However, in a preferred mode, the temperature of the at least one heated roller should be near or above the glass transition temperature of the aramid component.

In the following examples, all parts and percentages are by weight and degrees are in degrees Celsius unless otherwise indicated. Initial tear strength was determined by elongation at break according to ASTM D1004. Tear propagation resistance was determined by mean tear load according to ASTM D1938. In the examples below, the failure mechanism was determined by examining and observing how these samples failed in the test method described above.

Example 1

This example illustrates the improved calendered aramid papers for laminates of the present invention and the effect of these improved papers on the adhesion of polyester polymers to paper by comparison to prior art laminates.

Four laminates were produced to illustrate the effect of paper calendering conditions on the final laminate of the invention. Aramid paper consisting of 45% poly (m-phenylene isophthalamide) floc and 55% poly (m-phenylene isophthalamide) fibrids can be produced using conventional Fourdrinier paper making methods and equipment. This paper was then calendered between two rolls at 800 dram and four different sets of elevated temperatures to yield different calendered papers useful in laminates.

Paper a was differentially calendered between two rolls, with one roll operating at a surface temperature of 360 ℃ and the other roll operating at a surface temperature of 300 ℃, thus resulting in a differentially calendered aramid paper having two different surfaces, one surface being more porous (because of contact with the lower temperature roll) and the other surface being less porous (because of contact with the higher temperature roll).

Paper B was differentially calendered between two rolls, with one roll operating at a surface temperature of 360 ℃ and the other roll operating at a surface temperature of 250 ℃, thus resulting in a differentially calendered aramid paper having two different surfaces, one more porous and the other less porous. Paper C was differentially calendered between two rolls, with one roll operating at a surface temperature of 360 ℃ and the other roll operating at a surface temperature of 200 ℃, thus resulting in a differentially calendered aramid paper having two different surfaces, one more porous and the other less porous. Paper D was calendered between two rolls, both of which were operated at the same surface temperature of 360 ℃, thus resulting in calendered aramid paper having substantially the same surface and substantially the same porosity.

A hot press operating at 288 ℃ has been used to laminate a poly (ethylene terephthalate) polyester polymer having an intrinsic viscosity of 0.60dl/g between two sheets, each of calendered paper A, B, C and D. For the differently calendered papers A, B and C, the laminate was produced such that the polymer contacted the more porous surface of the calendered papers. The three-layer laminate was held in the press for five minutes and then cooled. The resulting polymer layer was 0.01 inch thick. The paper surface is not subjected to any chemical, flame, heat or corona treatment or similar activation treatment. Examination of laminates containing paper A, B and C showed that the polymer melted and flowed between the calendered papers and when tested, these laminates had acceptable adhesion. These laminates failed adhesively, i.e., the plane of failure was not within the aramid paper. Investigation of the laminate containing paper D showed that the polymer melted and flowed between the calendered papers and when tested, this laminate did not have acceptable adhesion. This laminate failed adhesively, i.e. the plane of failure was between the aramid paper and the polymer layer.

Example 2

This example illustrates the effect of extruded polymer intrinsic viscosity on final laminate properties.

An illustrative laminate of the invention was prepared as in example 1, with four different sets of calendered papers prepared in example 1, however, the polymer was prepared by extruding molten poly (ethylene terephthalate) (PET) polyester polymer between two papers, wherein the polymer contacted the more porous surface of the papers, and the polymer was PET having an intrinsic viscosity of 0.65dl/g or 0.80 dl/g. The polymer layer thickness of all the resulting laminates was 0.005 inches. The study of laminates containing paper D (prior art) and PET polymer having an intrinsic viscosity of 0.65dl/g showed that such laminates had unacceptable adhesion defects, i.e. the paper could be separated from the polyester polymer with a slight force. Studies of laminates made with different calendered papers a and the same polymer showed that such laminates had acceptable adhesion, but had adhesion defects after moderate force application. Investigation of laminates made with different calendered papers A, B and C with PET polymers having an intrinsic viscosity of 0.80dl/g showed that the higher viscosity polymer had an effect on the way the laminate breaks. In particular, laminates made using differentially calendered paper a, which has been calendered with the lowest amount of temperature difference, have unacceptable adhesion, requiring only a slight amount of force to lose adhesion between the polymer and the paper. Laminates made using the different calendered papers C that had been calendered with the greatest amount of temperature difference had acceptable adhesion, no bonding, i.e., in the plane of the aramid paper. Laminates made using differentially calendered paper B, which had been calendered at moderate amounts of temperature difference, had acceptable adhesion, failing in a balanced and acceptable manner between adhesion and bond failure.

Example 3

This example illustrates one embodiment of the present invention. A laminate was made by extruding a poly (ethylene terephthalate) polyester polymer between two differently calendered sheets as was done in example 2, however, the polymer was extruded in such a way that the different intrinsic viscosity polymers were layered between the two aramid papers. In particular, the polymer layer consists of three layers, which are, in order, one layer of PET polymer with an intrinsic viscosity of 0.65dl/g, one layer of PET polymer with an intrinsic viscosity of 0.80dl/g and a second layer of PET polymer with an intrinsic viscosity of 0.65 dl/g. The two outer layers 0.65dl/g polymer each represents 15% of the total polymer layer thickness, while 0.80dl/g polymer represents 70% of the total polymer layer thickness. Two laminates were made using different calendered papers C that had been calendered with the greatest amount of temperature difference. The first laminate was made using a PET polymer having an intrinsic viscosity of 0.65dl/g, and the second laminate was made using the above three layer polymer, with each laminate having a total polymer layer thickness of 0.005 inches. Studies of these laminates showed that both behave very similarly with acceptable adhesion, no adhesion, i.e. in the plane of the aramid paper. An attempt was made to produce a similar 3-polymer layer laminate using prior art paper D, but this molten polymer did not bond to prior art paper D and therefore did not produce an acceptable laminate.

Claims (20)

1. A process for producing a laminate comprising, in order:

a layer of aramid paper, and

a layer of a polymer, wherein the polymer,

the method comprises the following steps:

a) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

b) the polymer was conformed to the more porous surface of the calendered aramid paper.

2. A process for producing a laminate according to claim 1, which comprises, in order:

one layer of an aramid paper is provided,

a layer of a polymer, and

one layer of an aramid paper is provided,

the method comprises the following steps:

a) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

b) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

c) applying a polymer to the more porous surface of the aramid paper from step a), and

d) laminating the aramid paper from step b) to the polymer wherein the more porous surface of the aramid paper is in contact with the polymer.

3. The method of claim 2, wherein the aramid paper resulting from step a) and step b) is the same.

4. The method of claim 2 wherein the aramid paper resulting from step a) and step b) is not the same.

5. The method of claim 2, wherein polymer conforming in step c) uses a molten polymer.

6. The method of claim 2, wherein at least one of the papers of step a) and step b) comprises poly (metaphenylene isophthalamide).

7. The process of claim 2 wherein the polymer of step c) comprises poly (ethylene terephthalate) or a copolymer thereof.

8. The method of claim 2 wherein the polymer is simultaneously applied to the paper surface between a pair of roll nips.

9. The method of claim 2, wherein the temperature difference of at least one of step a) and step b) is at least 50 ℃.

10. The method of claim 2, wherein the temperature difference of at least one of step a) and step b) is at least 100 ℃.

11. The process of claim 2 wherein at least one of the rolls in step a) or step b) has a temperature above the glass transition temperature of the aramid.

12. The method of claim 2, wherein the polymer of step c) comprises polyethylene naphthalate, polybutylene terephthalate, polyetheretherketone, polyetherketoneketone, polyethersulfone, polyimide, polyetherimide, aromatic liquid crystalline polyester, or a mixture or blend of any of these compounds.

13. A process for producing a laminate according to claim 1, which comprises, in order:

one layer of an aramid paper is provided,

a layer of a polymer, and

one layer of an aramid paper is provided,

the method comprises the following steps:

a) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

b) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

c) conforming the molten polymer to the more porous surface of the aramid paper of step a) and step b), and

d) the two layers of paper and polymer are laminated together.

14. The method of claim 13 wherein the polymer is simultaneously applied to the paper surface between a pair of roll nips.

15. The process of claim 13 wherein the polymer of step c) comprises poly (ethylene terephthalate) or a copolymer thereof.

16. The method of claim 13, wherein the polymer of step c) comprises polyethylene naphthalate, polybutylene terephthalate, polyetheretherketone, polyetherketoneketone, polyethersulfone, polyimide, polyetherimide, aromatic liquid crystalline polyester, or a mixture or blend of any of these compounds.

17. A process for producing a laminate according to claim 1, which comprises, in order:

one layer of an aramid paper is provided,

the first layer of a polymer is a first layer,

at least one intermediate layer,

a second layer of polymer, and

a second layer of aramid paper was formed,

the method comprises the following steps:

a) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

b) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

c) applying a polymer to the more porous surface of the aramid paper from step a), and

d) conforming the polymer to the more porous surface of the aramid paper from step b),

e) laminating the aramid paper from steps (c) and (d) and the intermediate layer together.

18. A process for producing a laminate according to claim 1, which comprises, in order:

one layer of an aramid paper is provided,

the first layer of a polymer is a first layer,

at least one intermediate layer of a polymer,

a second layer of polymer, and

a second layer of aramid paper was formed,

the method comprises the following steps:

a) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

b) calendering an aramid paper between two rolls having a temperature difference of at least 20 ℃, wherein the surface of the paper exposed to the lower roll temperature has more pores than the opposite surface exposed to the higher roll temperature,

c) applying a first and a second layer of polymer with an intermediate polymer layer therebetween to the more porous surface of the aramid paper from step a) and step b), and

d) the two layers of paper and polymer layer are laminated together.

19. The laminate production method of claim 18, wherein the temperature difference of the two rolls is at least 50 ℃.

20. The laminate production method of claim 18, wherein the temperature difference of the two rolls is at least 100 ℃.