PIPD PAPER AND COMPONENTS MADE THEREFROM

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. Application No. 60/753,060 filed December 21, 2005, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention concerns papers containing polypyridobisimidazole fiber and a binder material and structural components made therefrom.

BACKGROUND OF THE INVENTION

[0003] High performance papers have been made from aramid fibers. See, for example, U.S. Patent Nos. 2,999,788, 3,756,908, and 4,472,241 . Some aramid papers have been impregnated with resins. See, for example, U.S. Patent Nos. 4,698,267, 4,729,921, 5,223,094, and 5,314,742.

[0004] Fibrous articles which function as a structural core between exterior skins is commonly known as a type of a "sandwich panel." One commonly used sandwich panel is a corrugated panel used in light-weight box construction. Another core structure is the

honeycomb core. Honeycomb cores have a plurality of hollow, columnar honeycomb cells separated by cell walls adhered linearly to each other. Aramid fibers have been used in some core structures to impart flamc-rcsistancc, stiffness, strength and toughness to the structures. See, for example, U.S. Patent Nos. 5,137,768 and 6,544,622.

[0005] There is a need for papers useful in structural components with improved properties.

SUMMARY OF THE INVENTION

[0006] In some embodiments, the invention concerns a paper useful as an structural component, the paper comprising 20 to 100 percent by weight of polypyridobisimidazole fiber and optionally a binder material, wherein the paper has (i) a density of 0.08 to 1.5 grams per cubic centimeter and (ii) a basis weight of from 10 to 150 grams per square meter, and the paper, when impregnated with at least 15 % by weight of phenolic resin, has a specific tensile stiffness of at least 100 (N/cm)/(g/m ² ), wherein the weight % is based on the total weight of the paper and resin.

[0007] In certain embodiments, binder material is phenolic resin. Other resins, as well as fibrids, pulps from other than polypyridobisimidazole polymers, thermoplastic fioc and thermoplastic powder can be used as a binder material for this papers.

[0008] One preferred polypyridobisimidazole fiber is PIPD. In some embodiments, the paper has 70 to 90 percent by weight PIPD fiber

[0009] Some papers have a specific tensile stiffness after impregnation with phenolic resin of 110-150 (N/cm)/(g/m ² ).

[0010] Some papers have a Gurley Hill porosity of from 1 to 20 seconds. Other papers have a Gurley Hill porosity of from 5 to 12 seconds.

[0011] Certain paper have a density of 0.6 to 1.2 grams per cubic centimeter. Some papers have a basis weight of from 15 to 65 grams per square meter.

[0012] Some papers further comprise at least one of conductive fillers, antioxidants, pigments, and catalysts.

[0013] Tn some embodiments, the PTPD fiber is in the form of the floe. Tn other embodiments, the PIPD fiber is in the form of the pulp. In other embodiments, the PIPD fiber is a blend of PIPD floe and PIPD pulp.

[0014] Also provided are structural cores comprising a paper described herein. Some core structures are honeycomb core structures.

[0015] The invention also relates to folded paper cores, printed wiring boards, or a sandwich panel comprising a paper described herein.

[0016] Further embodiments concern a method of making a paper comprising forming a slurry of polypyridobisimidazole fiber and removing at least a portion of the water in one or more steps to form a paper; where the paper has (i) a density of 0.08 to 1.5 grams per cubic centimeter; (ii) a basis weight of from 10 to 150 grams per square meter; and (iii) a specific tensile stiffness after impregnation with phenolic resin of at least 100 (N/cm)/(g/m ² ).

[0017] In some embodiments, the paper is made by removing a portion of the water from the slurry to form a wet paper; and then drying the wet paper.

[0018] In some embodiments, a method of making the paper involves calendering of the formed sheet at ambient or increased temperature.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0019] In some embodiments, the invention concerns a paper useful as an structural component, the paper comprising 20 to 100 percent by weight of polypyridobisimidazole fiber and, optionally, a binder material, wherein the paper has (i) a density of 0.08 to 1.5 grams per cubic centimeter and (ii) a basis weight of from 10 to 150 grams per square meter, and the paper, when impregnated with at least 15 % by weight of phenolic resin, has a specific tensile stiffness of at least 100 (N/cm)/(g/m ² ), wherein the weight % is based on the total weight of the paper and resin.

[0020] In some embodiments, the paper comprises 30 to 97 percent by weight of polypyridobisimidazole fiber.

[0021] For the purpose of this invention, "Papers" are flat sheets producible on a paper machine, such as a Fourdrinier or inclined-wire machine. In preferred embodiments these sheets are generally thin, fibrous sheets comprised of a network of randomly oriented, short fibers laid down from a water suspension and bonded together by their own chemical attraction, friction, entanglement, binder, or a combination thereof.

[0022] The floe of this invention means short lengths of fiber, shorter than staple fiber. The length of floe is about 0.5 to about 15 mm and a diameter of 4 to 50 micrometers, preferably having a length of 1 to 12 mm and a diameter of 8 to 40 micrometers. Floe that is less than about 1 mm does not add significantly to the strength of the material in which it is used. Floe or fiber that is more than about 15 mm often does not function well because the individual fibers may become entangled and cannot be adequately and uniformly

distributed throughout the material or slurry. Floe is generally made by cutting continuous spun filaments or tows into specific-length pieces using conventional fiber cutting equipment. Generally this cutting is made without significant or any fibrillation of the fiber.

[0023] Fibrids are non-granular, fibrous or film-like, particles that can be used as binders in papers. Preferably, they have a melting point or decomposition point above 320°C. Fibrids are not fibers, but they are fibrous in that they have fiber-like regions connected by webs. Fibrids have an average length of 0.2 to 1 mm and a ratio of maximum to minimum dimension of 5:1 to 10:1. The thickness dimension of the fibrid web is less than 1 or two microns and typically on the order of a fraction of a micron. Fibrids, before being dried, can be used wet and can be deposited as a binder physically entwined about other ingredients or components of a product. The fibrids can be prepared by any method including using a fϊbridating apparatus of the type disclosed in U.S. Patent No. 3,018,091 where a polymer solution is precipitated and sheared in a single step.

[0024] The instant invention utilizes polypyridobisimidazole fiber. This fiber is made from a rigid rod polymer that is of high strength. The polypyridobisimidazole polymer of this fiber has an inherent viscosity of at least 20 dl/g or at least 25 dl/g or at least 28 dl/g. Such fibers include PIPD fiber (also known as M5® fiber and fiber made from poly[2,6-diimidazo[4,5-b:4,5-e]- pyridinylene-l,4(2,5-dihydroxy)phenylene). PIPD fiber is based on the structure:

[0025] Polypyridobisimidazole fiber can be distinguished from the well known commercially available PBI fiber or polybenzimidazole fiber in that that polybenzimidazole fiber is a polybibenzimidazole. Polybibenzimidazole fiber is not a rigid rod polymer and has low fiber strength and low tensile modulus when compared to polypyridobisimidazoles.

[0026] PIPD fibers have been reported to have the potential to have an average modulus of about 310 GPa (2100 grams/denier) and an average tenacity of up to about 5.8 Gpa (39.6 grams/denier). These fibers have been described by Brew, et al., Composites Science and Technology 1999, 59, 1109; Van der Jagt and Beukers, Polymer 1999, 40,

1035; Sikkema, Polymer 1998, 39, 5981; Klop and Lammers, Polymer, 1998, 39, 5987; Hageman, etal., Polymer 1999, 40, 1313.

[0027] One method of making rigid rod polypyridoimidazolc polymer is disclosed in detail in United States Patent 5,674,969 to Sikkema et al. Polypyridoimidazole polymer may be made by reacting a mix of dry ingredients with a polyphosphoric acid (PPA) solution. The dry ingredients may comprise pyridobisimidazole-forming monomers and metal powders. The polypyridobisimidazole polymer used to make the rigid rod fibers used in the fabrics of this invention should have at least 25 and preferably at least 100 repetitive units.

[0028] For the purposes of this invention, the relative molecular weights of the polypyridoimidazole polymers are suitably characterized by diluting the polymer products with a suitable solvent, such as methane sulfonic acid, to a polymer concentration of 0.05 g/dl, and measuring one or more dilute solution viscosity values at 30 ⁰ C. Molecular weight development of polypyridoimidazole polymers of the present invention is suitably monitored by, and correlated to, one or more dilute solution viscosity measurements. Accordingly, dilute solution measurements of the relative viscosity ("V _ro i" or "η _rcl " or "n _rc i") and inherent viscosity (" ^" VW or "ηinh" or "ninh") are typically used for monitoring polymer molecular weight. The relative and inherent viscosities of dilute polymer solutions are related according to the expression

V _1n J ₁ = ^ (V ₁₆₁ ) / C, where In is the natural logarithm function and C is the concentration of the polymer solution. V _τel is a unitless ratio of the polymer solution viscosity to that of the solvent free of polymer, thus Vmh is expressed in units of inverse concentration, typically as deciliters per gram ("dl/g"). Accordingly, in certain aspects of the present invention the polypyridoimidazole polymers are produced that are characterized as providing a polymer solution having an inherent viscosity of at least about 20 dl/g at 30°C at a polymer concentration of 0.05 g/dl in methane sulfonic acid. Because the higher molecular weight polymers that result from the invention disclosed herein give rise to viscous polymer solutions, a concentration of about 0.05 g/dl polymer in methane sulfonic acid is useful for measuring inherent viscosities in a reasonable amount of time.

[0029] Exemplary pyridobisimidazole-forming monomers useful in this invention include 2,3,5,6-tetraaminopyridine and a variety of acids, including terephthalic acid, bis- (4-benzoic acid), oxy-bis-(4-benzoic acid), 2,5-dihydroxyterephthalic acid, isophthalic acid, 2,5-pyridodicarboxylic acid, 2,6-napthalenedicarboxylic acid, 2,6-quinolinedicarboxylic

acid, or any combination thereof. Preferably, the pyridobisimidazole forming monomers include 2,3,5,6-tetraaminopyridine and 2,5-dihydroxyterephthalic acid. In certain embodiments, it is preferred that that the pyridoirnidazolc-forming monomers arc phosphorylated. Preferably, phosphorylated pyridoimidazole-forming monomers are polymerized in the presence of polyphosphoric acid and a metal catalyst.

[0030] Metal powders can be employed to help build the molecular weight of the final polymer. The metal powders typically include iron powder, tin powder, vanadium powder, chromium powder, and any combination thereof.

[0031] The pyridobisimidazole-forming monomers and metal powders are mixed and then the mixture is reacted with polyphosphoric acid to form a polypyridoimidazole polymer solution. Additional polyphosphoric acid can be added to the polymer solution if desired. The polymer solution is typically extruded or spun through a die or spinneret to prepare or spin the filament.

[0032] PIPD pulp has a high affinity for water, meaning the pulp has a high moisture content. This is believed to help eliminate static effects that cause clumping and defects normally associated with other high performance pulps that do not absorb water to the same degree and are afflicted with static problems. In addition, both PIPD pulp and PIPD floe have the surprising attribute of self-bonding; that is, papers formed solely from the pulp or solely from the floe have a surprisingly higher strength than would be anticipated by the prior art papers made from high performance fibers. While not wanting to be bound by theory, it is believed that this higher strength is due to hydrogen bonding between the surfaces of the pieces of pulp and floe.

[0033] PIPD pulp can be made from conventional pulp making process well known to those skilled in the art. See, for example, Handbook for Pulp & Paper Technologists, Smook, Gary A.; Kocurek, M.J.; Technical Association of the Pulp and Paper Industry; Canadian Pulp and Paper Association, and US 5,171,402 and US 5,084,136 to Haines et al.

[0034] In addition to PIPD fiber, the paper composition of this invention includes a binder material chosen from the group consisting of thermoset and thermoplastic resins in the form of solutions, emulsions, suspensions, powders, flakes, and fibers; fibrids; and pulps from other polymers outside of PIPD. The binder material can be introduced to the paper composition in the papermaking process or later. Thus, formed and calendered papers can be impregnated with resin solutions, emulsions, suspensions or powders after the papermaking process is completed.

[0035] In preferred embodiments, phenolic, epoxy or other resins easily impregnate the papers of this invention. One measure of the ability to be impregnated by resins is represented by the porosity of the paper as measured by a "Gurlcy Hill porosity, which measures how fast a set volume of air passes through the paper. The papers of this invention that have a Gurley Hill porosity of from 1 to 20 seconds are believed to be easily impregnated by resins. In other embodiments papers of this invention that have a Gurley Hill porosity of from 5 to 12 seconds are especially preferred.

[0036] Likewise, papers of this invention have a density of 0.08 to 1.5 grams per cubic centimeter, and in some embodiments have a density of 0.6 to 1.2 grams per cubic centimeter.

[0037] Paper from PIPD pulp, for example, can be made by a process comprising of the following steps: (a) preparing an aqueous dispersion of PTPD pulp; (b) diluting the aqueous dispersion in a paper making mold cavity: (c) draining the water from the aqueous dispersion to yield a wet paper; (d) dewatering and drying the resultant paper; and (e) conditioning the paper for physical property testing.

[0038] The papers described herein are useful for structural composites- Construction of such structures from papers are well known by those skilled in the art. Structures using the papers described herein include honeycomb core structures, folded paper cores, printed wiring boards, and sandwich panels.

[0039] For the usage in structural composites, the papers of this invention can be additionally impregnated with different resins. If some resin was already used as a binder for the paper structure on the step of the papermaking or after that, an additional resin can be the same or different but compatible with the first one.

Test Methods

[0040] In the non-limiting examples that follow, the following test methods were employed to determine various reported characteristics and properties. ASTM refers to the American Society of Testing Materials. TAPPT refers to Technical Association of Pulp and Paper Industry.

[0041] Thickness and Basis Weight of papers were determined in accordance with ASTM D 645 and ASTM D 646 correspondingly. Thickness measurements were used in the calculation of the apparent density of the papers.

[0042] Density (Apparent Density) of papers was determined in accordance with ASTM D 202.

[0043] Tensile Strength and Modulus were determined for papers and composites of this invention on an Instron-type testing machine using test specimens 2.54 cm wide and a gage length of 18 cm in accordance with ASTM D 828.

[0044] Canadian Standard Freeness CCSF) of the pulp is a measure of the rate, at which a dilute suspension of pulp may be drained, and was determined in accordance with TAPPI Test Method T 227.

[0045] Fiber length was measured in accordance with TAPPI Test Method T 271 using the Fiber Quality Analyzer manufactured by OpTest Equipment Inc.

[0046] Gurley Air Porosity (Resistance) for papers was determined by measuring air resistance in seconds per 100 milliliters of cylinder displacement for approximately 6.4 square centimeters circular area of a paper using a pressure differential of 1.22 kPa in accordance with TAPPI T 460.

[0047] Tensile Stiffness was determined for papers of this invention on an Instron- type testing machine using test specimens 2.54 cm wide and a gage length of 18 cm in accordance with ASTM D 828. Specific Tensile Stiffness was determined as a mathematical quantity calculated by dividing tensile stiffness by basis weight of the paper.

Examples

[0048] Examples 1-3 demonstrate the preparation and properties of papers containing PIPD fiber in the form of the pulp. The tensile strength of all papers, which composition includes PIPD pulp, satisfied boundary equation:

Tensile strength > or = 0.00057X * Y,

where X is the volume portion of PIPD pulp in the total solids of the paper in % and Y is basis weight of the paper in g/m ² . The high strength of PTPD-pulp-based papers gave them significant advantage in the paper manufacturing and in the further processing of the paper into the final application (it is possible to go to lighter basis weight and/or to use more simple and cheaper equipment). It is still possible to use an additional binder on the papermaking step, as shown in example 3, where meta-aramid fibrids are working as such

binder. However, in the case of PIPD fiber based papers it is optional, not absolutely necessary thing.

[0049] Examples 4-6 demonstrate the preparation of calendered papers based on the formed papers from Examples 1-3. For many composite applications, a high density paper is desired, and calendering is a method of achieving that density.

[0050] Examples 7-9 demonstrate resin impregnated papers (with relatively small resin content) based on PIPD pulp and its composition with para-aramid floe. In comparative Example A, resin impregnated paper based on the commercial composition of para-aramid floe and meta-aramid fibrids is described. It can be seen that, at about the same resin content, PIPD pulp based papers provide the same or higher stiffness and much higher strength.

[0051] Comparative example A shows that even at higher para-aramid floe content in the paper composition in this case vs. example 9, stiffness and strength of the resin impregnated sheet is still lower if to compare to example 9.

Example 1

[0052] 3.2 g (dry weight) of the PIPD pulp with CSF of about 200 ml was placed in a Waring Blender with 300 ml of water and agitated for 1 min. The dispersion was poured into an approximately 21x21 cm handsheet mold and mixed with additional 5000 g of water. A wet-laid sheet was formed. The sheet was placed between two pieces of blotting paper, hand couched with a rolling pin, and dried in a handsheet dryer at 190 C. The composition and properties of the final paper are shown in Table 1.

Example 2

[0053] 1.6 g (dry weight) of PIPD pulp with CSF of about 300 ml was placed in a Waring Blender with 800 ml of water and agitated for 1 min. 1.6 g of para-aramid floe were placed with about 2500 g water in the laboratory pulp disintegrator and agitated for 3 minutes. The para-aramid fioc was poly (para-phenylene terephthalamide) floe having a linear density of about 0.16 tex and cut length of about 0.67 cm (sold by E. T. de Pont de Nemours and Company under trademark KEVLAR® 49).

[0054] The both dispersions were poured together into an approximately 21x21 cm handsheet mold and mixed with additional 5000 g of water. A wet-laid sheet was formed. The sheet was placed between two pieces of blotting paper, hand couched with a rolling pin,

and dried in a handsheet dryer at 190 C. The composition and properties of the final paper are shown in Table 1.

Example 3

[0055] 0.64 g (dry weight) of PIPD pulp with CSF of about 300 ml was placed in a Waring Blender with 800 ml of water and agitated for 1 min. 2.24 g of para-aramid fioc and 0.32 g (based on the dry weight) of never dried meta-aramid fϊbrids with CSF about 40 ml were placed with about 2500 g water in the laboratory pulp disintegrator and agitated for 3 minutes. The para-aramid floe was poly (para-phenylene terephthalamide) floe having a linear density of about 0.16 tex and cut length of about 0.67 cm (sold by E. I. de Pont de Nemours and Company under trademark KEVLAR® 49). The meta-aramid fϊbrids were made from poly(metaphenylene isophthal amide) as described in U.S. Pat No. 3,756,908.

[0056] The both dispersions were poured together into an approximately 21x21 cm handsheet mold and mixed with additional 5000 g of water. A wet-laid sheet was formed. The sheet was placed between two pieces of blotting paper, hand couched with a rolling pin, and dried in a handsheet dryer at 190 C. The composition and properties of the final paper are shown in Table 1.

Examples 4-6

[0057] After drying, the paper samples produced in Examples 1-3, respectively, were calendered in the nip of metal-metal calender at temperature of about 300 C and linear pressure of about 1200 N/cm. The properties of the final papers are shown in Table 1.

Examples 7-9

[0058] Resin impregnated papers were prepared by the impregnation of the papers from Examples 4-6 with a solvent-based phenolic resin (PLYOPHEN 23900 from the Durez Corporation) following by removing any excess resin from the surface with blotting paper and curing in an oven by ramping up the temperature as follows: heating from room temperature to 82°C and holding at this temperature for 15 minutes, increasing the temperature to 121 ⁰ C and holding at this temperature for another 15 minutes and increasing the temperature to 182°C and holding at this temperature for 60 minutes. Properties of the final impregnated papers are shown in Table 2.

Comparative Example A

[0059] 0.64 g (of the dry weight) of meta-aramid fibrids with CSF of about 40 ml and 2.56 g of para-aramid floe were placed with about 2500 g water in the laboratory pulp disintegrator and agitated for 3 minutes. The mcta-aramid fibrids were made from poly(metaphenylene isophthalamide) as described in U.S. Pat No. 3,756,908. The para- aramid floe was the same as in Example 2. The dispersion was poured into an approximately 21x21 cm handsheet mold and mixed with additional 5000 g of water. A wet- laid sheet was formed. The sheet was placed between two pieces of blotting paper, hand couched with a rolling pin, and dried in a handsheet dryer at 190 C. After that, the paper was calendered in the nip of metal-metal calender at temperature of about 300 C and linear pressure of about 1200 N/cm. And impregnated with phenolic resin as described in examples 7-9. The composition and properties of the final impregnated paper are shown in Table 2.

Table 1 - Properties of the paper samples with basis weight 68 g/m ² .

Table 2 - Properties of the resin impregnated composite sheets based on 68 g/m ² calendered papers

Example Paper composition, wt.% Resin Specific tensile Tensile

PIDP pulp p-aramid m- content in stiffness, strength, aramid floe fibrids the (N/cm)/(g/m ² ) N/cm composite, wt.%

7 100 — — 15 74 114

8 50 50 — 26 98 109

9 20 70 10 23 113 87

Comp. A — 80 20 21 77 58