LAYERED SMOOTH SURFACE ARAMID PAPERS OF HIGH STRENGTH AND PRINTABILITY Background of the Invention This invention relates to an improved layered aramid paper having a smooth surface and good tensile and tear strengths. The smooth surface provides for better print clarity and makes such papers particularly useful for high temperature label applications. Prior art techniques that improve on surface smoothness often lead to a reduced level of mechanical strength and/or thermal stability. Moreover, synthetic papers which have been pressed or calendered at high temperature and pressure will generally have fibers on the surface which cause roughness or snagging when the surface of the paper is worked during end use processing.

Summary of the Invention

This invention provides a multi-layered, smooth surface aramid paper containing from 40 to 55% by weight of fibrids and comprising a substrate layer which consists essentially of aramid fibrids and floe and one or two surface layers each intimately bonded to the substrate layer, said surface layer(s) consisting essentially of from 65 to 90% by weight aramid fibrids and from 10 to 35% by weight aramid floe and comprising from 10 to 67% of the weight of the paper. Preferably, the paper has a density of 0.8 to 1.0 g/cc with thickness of 1 to 30 mils (0.025 to 0.762 mm) .

Detailed Description of the Invention

The multi-layered aramid papers of the invention are comprised of layers of different compositions to provide desired properties. The surface layer(s) provide a smooth surface and contain from 65 to 90% aramid fibrid and from 10% to 35% of aramid floe. The surface layer(s) constitutes from 10 to 67% of the weight of the paper. The substrate layer provides high tear strength. In order for

the multi-layered paper to behave as a unitary structure, it is preferred that the fibrous materials at the interface between layers be intermingled. This is achieved by depositing a layer of furnish, i.e., a paper- making aqueous dispersion of floe and fibrid on an undried, previously formed layer of furnish in a paper making machine or by simultaneously depositing the layers of different composition on the screen of the paper making machine using a 2 or 3 layer hydraulic type headbox. The paper coming off the machine is dried and calendered, preferably to a thickness of from 1 to 30 mils. The density of the layered paper is preferably from 0.8 to 1.0 g/cc for use as labels.

It has been found that the multi-layered papers of this invention have excellent mechanical properties. The smooth surface retains a high degree of smoothness even after the necessary working to prepare it for end use applications. This quality is important if print clarity and color density is to be achieved. Aramid floe is high temperature resistant floe or short fiber cut from longer aramid fiber, such as those prepared by processes described in U.S. Pat. Nos. 3,063,966; 3,133,138; 3,767,756 and 3,869,430. It refers to short fibers typically having a length of 2 to 12 mm and a linear density of 1-10 deeitex, made of aromatic polyamide which is non-fusible.

The aramid fibrids can be prepared using a fibridating apparatus where a polymer solution is precipitated and sheared in a single step as described in U.S. Pat. No. 3,756,908.

Tests and Measurements

Total Break Strength. The tensile break strength of paper is determined based on ASTM method D 828-87 for "Standard Test Method for Tensile Breaking Strength of Paper and Paperboard". Specimens are 2.54 cm wide and 20.3. cm long and the jaws of the tensile testing machine are initially separated by 12.7 cm. Ten paper samples are

tested in the machine direction (MD) and ten are tested in the cross direction (CD) and the values for each direction are averaged. The total of the MD and CD strengths is divided by paper density and paper basis weight to obtain the Total Break Strength.

Thickness. Thickness of papers is determined using calipers in accordance with ASTM D 374-79 (1986) .

Density. Density of papers is determined by determining the weight per unit area of the paper (Basis Weight) in accordance with ASTM D 646-86 and dividing by the thickness.

Abraded Fiber Count.

In order to further investigate the abrasion qualities of these papers, the papers were folded and the edge of the fold was viewed against a dark background.

The number of fibers extending greater than about 0.5 mm above the solid paper surface was taken as the Abraded Fiber Count (per centimeter) and indicates the degree of roughness of the sample. The following examples are illustrative of the invention and are not to be construed as limiting.

EXAMPLES Example 1 A two layered structure was made by combining fibrids of poly(m-phenylene isophthalamide) prepared as described in Example 1 of U.S. Pat. No. 3,756,908 and floe prepared by dry spinning poly(m-phenylene isophthalamide) from a solution containing 67% dimethyl acetamide (DMAc) , 9% calcium chloride and 4% water. The spun filaments were flooded with an aqueous liquid and contained about 100% DMAc, 45% calcium chloride and 30-100% water based on dry polymer. The filaments were washed and drawn 4X in an extraction-draw process in which the chloride and DMAc contents were reduced to about 0.10% and 0.5%, respectively. The filaments had a denier of 2 (2.2 dtex) and typical properties were: elongation to break, 34%, and tenacity, 4.3 grams/denier (3.8 dN/tex) . The filaments

were then cut to floe length of 0.27 inch (0.68 cm) and slurried in water to a concentration of about 0.35%.

Blends of fibrids and floe were separately fed to a 2-layer hydraulic type headbox which maintains each blend as a distinct layer until the slice exit where limited mixing of the layers occurs. This allows good bonding between the layers while still maintaining the individual nature of each layer. The formed sheet is then processed as is normally done on a fourdrinier paper machine by pressing and drying.

The papers are dried completely using infrared heaters before being calendered at 320"C at a line speed of 30 feet per minute (9 meters per minute) using a pressure of 725 pounds per linear inch (130 kg/cm) . The composition of the layers varied from 35 to

65% fibrid, the remainder being floe. The basis weight of each layer was adjusted so that the high fibrid layer (65% fibrid) ranged from 33 to 67% of the total basis weight of the final sheet. The total fibrid content of the test papers ranged from 45 to 55% of the sheet versus 53% for the single layer control papers (Cl-1) . Table 1 gives the basis weight of each layer and its composition.

Table 1

Total Sheet Substrate Layer Surface Layer

Run BW aim % % BW aim % % BW aim % % Number g/mi. Fibrid Floe g/mf- Fibrid Floe g/mfi Fibrid Floe

1-1 42 45 55 28 35 65 14 65 35

1-2 42 50 50 21 35 65 21 65 35

1-3 42 55 45 14 35 65 28 65 35

Cl-1 42 53 47 42 53 47 _ — _

The amount of loose fibers on the surfaces of the sheet as a result of mechanical working of the calendered paper was measured (Table 2) . Side 1 is the substrate layer (low fibrid content layer) and Side 2 the surface (high fibrid content) layer.

Table 2 Abraded Fiber Count

Fiber Count

Sample (per 5 cm) Number Side 1 Side 2

1-1 20 0

1-2 12 2

1-3 14 0

Cl-1 14 _

Even with the significant reduction in the number of loose fibers on the surface of the high fibrid content papers, superior mechanical properties are maintained versus a control paper of similar average composition but with no layering (Table 3) .

Table 3 Calendered Paper Properties

Sample Number 1-1 1-2 1-3 Cl-1

B.W.*, oz/yd ² 1.3 1.5 1.4 1.3

(g/m ² ) (44.1) (50.9) . (47.5) (44.1)

Thickness, mils 2.0 2.5 2.2 2.4

(mm) (0.051) (0.064) (0.056) (0.061)

Density, g/cc 0.82 0.89 0.86 0.72

B.S.**, lb/in MD/CD 15/7 21/10 18/7 20/8

(N/cm) (26/12) (37/18) (32/12) (35/14)

Eb***, MD/CD 4/3 6/3 5/2 6/3

Elmendorf Tear, 108/191 120/193 87/166 127/215 g MD/CD

(N) (1.06/1.87) (1.18/1.89) (0.85/1.63)1 (1.25/2.1

Shrinkage @ 300°C, 2/0 2/0 2/0 2/0 % MD/CD

* Basis Weight

** Break Strength

*** Break Elongation

Example 2

Layered structures, 4.0-4.5 oz/yd ² (135.6- 152.6 g/m ² ) were produced with high fibrid layers on both top and bottom of the structure. The top and bottom plies (outer layers) had equal basis weight. The top and bottom layers contain 65% fibrid and 35% floe. The top layer was applied using a secondary headbox jetting the furnish onto an already formed sheet which was prepared using the headbox of Example 1. The control (C2-1) was a single layer paper.

Table 4

Total Sheet Each Outer Layer Inner (Substrate) Layer

Run BW aim % % BW aim % % BW aim % % Number g/mi Fibrid Floe gm Fibrid Floe g/m Fibrid Floe

2-1 132 46 54 24 65 35 84 35 65

2-2 132 55 45 44 65 35 44 35 65

C2-1 137 47 137 53 47 _ — — _

Improvement in the amount of loose fibers on the surface as a result of mechanical working of the paper is obvious from Table 5.

Table 5 Abraded Fiber Count

Sample Fiber Count

Number (per 5 cm.

2-1 5 2-2 7

C2-1 12

Even with the major reduction in the number of loose fibers on the surface of the papers superior mechanical properties are maintained versus a control paper of similar average composition but with no layering

(Table 6). The low shrinkage at 300°C along with the high tear and tensile properties as compared with the control is especially noteworthy.

Table 6 Calendered Paper Properties

Sample Number 2-1 2-2 C2-1

Basis Weight, oz/yd ² 4.3 4.3 4.1 (g/m ² ) (145.7) (145.8) (139.0)

Thickness, mils 7.5 6.7 6.8

(mm) (0.191) (0.170) (0.173)

Density, g/cc 0.77 0.87 0.80

B.S. , lb/in MD/CD 55/30 61/39 54/33 (N/cm) (96/53) (107/68) (95/58)

Eb, % MD/CD 6/4 9/6 7/5

Elmendorf Tear, g MD/CD D 695/762 421/598 504/662

(N) (6.82/7.48) (4.13/5.87) (4.94/6.49)

Shrinkage §300°C, 1/1 1/1 1/1 % MD/CD