PROCESSES FOR MAKING

Background of the Invention

Field of the Invention

This invention pertains to the use of synthetic fibers in an interlaminar adhesive layer of a laminar wood beam, and more particularly this invention pertains to the use of discontinuous aramid fibers in the inter- laminar adhesive layer of a laminar wood beam to improve interlaminar shear strength and to reduce creep.

Prior Art

A laminated wood beam is comprised of multiple laminae of wood joined together with adhesive. When placed into service wood beams can span distances of up to 500 feet and support loads of many tons. The beams are subjected to tension, compression and shear stress. When a laminated beam is loaded, the load causes tensile forces in some laminae and compressive forces in other laminae. For example, in a simple loading of a laminated beam with a uniform load the lower laminae are subjected to a tensile load between support points and the upper laminae are subjected to a compressive load between support points. This loading of the beam causes stress in the interlaminar layers of adhesive which, over time, causes creep. Creep is defined as slow, plastic defor¬ mation (inelastic or permanent deformation) under a constant load. Creep causes a vertical displacement of the beam which is referred to as sag.

Thus, although a cured interlaminar layer of adhesive may be quite rigid, sustained loading of a laminar wood beam over time causes creep in the layers of adhesive between laminae, which causes the beam to sag under its load. This is an undesirable property and beam designers try to prevent sag by such measures as over- designing the beam and adding anti-sag agents to the

adhesive. Over-designing the beam is an expensive solu¬ tion to the creep problem. Typically, anti-sag agents are added to the adhesive.

A typical anti-sag additive for adhesives in the wood beam industry is cellulose, commonly used in a granular form known as wood flour. Cellulose is added to the adhesive used between laminae to improve the shear strength of the adhesive. The shear strength of the adhesive is thus related to the interlaminar shear strength of the beam.

Any additive to an interlaminar adhesive will ideally neither interfere with the application of the adhesive, nor with the future use or processing of the laminate. The use of fibers as an additive to the adhe- sive has not been effective because the fibers interfere with the applicators used to apply the adhesive to a lamina of wood. Thus, the use of anti-sag agents in the wood industry has been confined to adhesive additives that do not interfere with the adhesive applicators such as cellulose, described above. However, because of the increasing scarcity of wood as a resource, and the related demand for higher performance wood beam struc¬ tures, a more suitable solution to the problem of creep in the adhesive layer of a wood laminate is required. Gagliani, et al., U.S. Patent No. 4,444,823 discloses the use of an adhesive-soaked fiber mat or tow as filler and reinforcing additive in a modified poly- imide adhesive. The adhesive is used to bond metal to metal, glass to glass, or ceramics to ceramics. Schnabel, U.S. Patent No. 3,755,067 discloses the use of processed asbestos fibers to improve viscosity and thixotropic properties of a phenolic wood laminating adhesive resin.

Schijve, et al., U.S. Patent No. 4,500,589 discloses the use of yarns of endless filaments of poly(paraphenylene terephthalamide) arranged to lie in a

straight line within a resin matrix between metal sheets to create a strong composite article.

European Patent No. 00 013 146 discloses the fabrication of a composite article having thin aluminum layers over a relatively thick thermoplastic matrix and teaches modification of the thermoplastic matrix with various short, discontinuous fibers.

However, none of these patents discloses the use of a discontinuous synthetic fiber in an interlaminar adhesive for improving interlaminar shear strength of a wood laminate.

Summary of the Invention

The present invention solves the problems described above by the use of discontinuous aramid fibers in the adhesive layer of laminar wood beams. It has been found that adding aramid fibers to the adhesive layer between laminae improves the shear strength of the adhe¬ sive, reduces creep of the adhesive and thus reduces sag of the wood beam. The present invention comprises the use of discontinuous aramid fibers having a length up to about 3 cm added to a layer of adhesive that has been applied to a laminae of wood. A second lamina of wood is then placed over the adhesive layer and pressure is applied to adhere together the two laminae.

The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.

Brief Description of the Drawings

FIG. 1 is a perspective view of a laminar wood beam of the present invention. FIG. 2 is an enlarged, perspective view of a wood beam of the present invention having the upper

laminae partially removed to show features of the invention.

Detailed Description of a Preferred Embodiment Referring generally to FIGS. 1 and 2, a laminar beam 10 of the present invention comprises a plurality of laminae 24 of elongate planar wood boards or planks that have been joined together by an adhesive 26. Although only six laminae 24 are shown, it is to be understood that the laminar wood beam 10 could comprise fewer or many more laminae of wood. Furthermore, in most embodi¬ ments of laminar wood beams a single plank would not extend the entire length of the beam, but would abut the end of another plank. The region between laminae of wood, where the adhesive 26 is applied, is a glue line 28. Within the glue line 28 there is a plurality of short, discontinuous fibers 30 that are randomly arranged in the adhesive 26. As shown in FIG. 2, the fibers 30 are applied to each glue line 28. In a preferred embodi- ment of the present invention, the glue line 28 has an approximate thickness of 0.05 to 0.1 mm (0.002 to 0.004 inches) .

A beam is fabricated according to the present invention by the following steps. A layer of adhesive is applied to a surface of a first wood lamina. After application of the adhesive, chopped fibers 30 are added to the adhesive layer such as by sprinkling the fibers over the adhesive. To date, good results have been obtained by applying the fibers by hand, but it is contemplated that during full scale production the fibers will be applied by a specially devised machine. Ideally, the fibers will be applied at a uniform density along the length and width of the beam. Preferably, the fibers are added at a weight ratio of fiber-to-adhesive between 0.25% and 1.5%, and in no event should the ratio exceed 5%. The fibers are 0.0254 to 2.54 cm (0.010 to 1.000

inches) long, and in a preferred embodiment of the present invention, are 3 to 5 mm long.

A preferred embodiment of the invention uses aramid fibers of a polyparaphenylene terephthalamid (PPTA) available under the trade name "TWARON" from AKZO Fibers, Inc. of Conyers, Georgia. Alternative embodi¬ ments of the invention could use other fibers such as glass, carbon or any other suitable high strength, high modulus fiber. In addition, it is envisioned that alter- native embodiments of the invention may use different fibers in different glue lines. For example, carbon fibers may be used in the glue lines that will be subject to the greatest compressive forces and aramid fibers may be used in those glue lines that receive the greatest tensile forces.

After the fibers have been applied to the adhesive, a second wood lamina is placed on top of the adhesive. The process is repeated until the desired number of laminae 24 are in place. The beam can be left for 40 to 115 minutes open assembly time before it is removed, loaded into a press and the press is closed. The beam is subjected to a minimum of 8 hours at full pressure of 125 to 200 psi after which the pressure is relieved. Alternatively, the aramid can be added to the adhesive in the form of a PPTA polymer, herein referred to as a microfibril. The PPTA microfibril has a length of about 50 to 600 microns. A preferred microfibril length is about 50 to 300 microns, and an average microfibril length of 75 microns is presently most preferred.

It is noted that the microfibril form of the aramid and the aramid fibers of length 3 to 5 mm can be also added to the adhesive prior to applying the adhesive with standard applicators. When the aramid is added to the adhesive prior to application, it has been found that the adhesive has excellent gap filling properties.

The present invention has been found suitable for use with all adhesives currently in use in fabricat¬ ing wood beams, including, but not limited to epoxies, polyesters, mela ines, urea resins and phenolic resins such as phenol-formaldehyde resins. A preferred adhesive is a phenol-resorcinol-for aldehyde resin system such as that sold by Borden Chemicals of Columbus, Ohio as LT 75 with hardener FM 260.

Example 1 A specimen of Douglas fir was prepared for a standard block shear test with LT 75 with hardener FM 260 adhesive applied at a spread rate of 74 lbs/1,000 ft ² . "TWARON" 1056 fibers of length 3 to 5 mm were added to the adhesive at approximately 0.25 wt% fiber/adhesive. The measured ambient air temperature was 63°F and the relative humidity was 42%. The lumber temperature was 60 ^β F and the adhesive temperature was 58°F. The moisture content of the wood was 12% or below. The specimen was subjected to a minimum full pressure close time of 8 hours at 130 psi. The specimen was tested to failure according to the American Institute of Timber Construc¬ tion (AITC) Test T107 shear test. The results are presented in Table 1.

According to industry standards, the required average shear strength of adhesive joints in laminated construction of Douglas fir at 12% moisture content or below is 1130 psi, and at 14% moisture content the required shear strength is 1080 psi.

Table 1

Average Shear Average Strength % Wood Number of

Example psi Failure Measurements

1 1,695 95 16

2 1,520 97 16

3 1,254 93 16

4 1,224 90 16

5 1,769 94 16

6 1,314 99 8

7 1,487 99 8

8 1,473 96 8

Example 2 The procedure of Example 1 was followed except the fibers were added to the adhesive at approximately 0.50 wt% fiber/adhesive. The specimen was tested to failure according to the AITC Test T107 shear block test. The results are presented in Table 1. Example 3

A specimen of Douglas fir was prepared for a standard block shear test with LT 75 with hardener FM 260 adhesive applied at a spread rate of 72 lbs/1,000 ft ² . "TWARON" 1056 fibers of length 3 to 5 mm were added to the adhesive at approximately 0.75 wt% fiber/adhesive. The measured ambient air temperature was 61*F and the relative humidity was 43%. The lumber temperature was 59°F and the adhesive temperature was 58"F. The moisture content of the wood was 14% or below. The specimen was subjected to a minimum full pressure close time of 8 hours at 130 psi. The specimen was tested to failure according to the AITC Test T107 standard shear block test. The results are presented in Table 1.

Example 4 The procedure of Example 3 was followed except the fibers were added to the adhesive at approximately 1.0 wt% fiber/adhesive. The specimen was tested to failure according to the AITC Test T107 shear block test. The results are presented in Table 1.

Example 5 The procedure of Example 3 was followed except the fibers were added to the adhesive at approximately 5.0 wt% fiber/adhesive. The specimen was tested to fail¬ ure according to the AITC Test T107 shear block test. The results are presented in Table 1.

Example 6 A specimen of Douglas fir was prepared for a standard block shear test with LT 75 with hardener FM 260 adhesive applied at a spread rate of 74 lbs/1,000 ft ² . PPTA microfibril with an average length of 75 microns was added to the adhesive at approximately 0.25 wt% microfibril/adhesive. The room air temperature was 6 *F and the relative humidity was 42%. The lumber tempera¬ ture was 61*F and the adhesive temperature was 58*F. The moisture content of the wood was below 12%. The specimen was subjected to a minimum full pressure close time of 8 hours at 130 psi. The specimen was tested to failure according to the AITC Test T107 shear block test. The results are presented in Table 1.

Example 7 The procedure of Example 6 was followed except the PPTA microfibril was added to the adhesive at approx- i ately 0.75 wt% microfibril/adhesive. The specimen was tested to failure according to the AITC Test T107 shear block test. The results are presented in Table 1.

Example 8 The procedure of Example 6 was followed except the PPTA microfibril was added to the adhesive at approx¬ imately 5.0 wt% microfibril/adhesive. The specimen was

tested to failure according to the AITC Test T107 shear block test. The results are presented in Table 1.

As shown in Table 1 the interlaminar adhesive layers of the present invention demonstrate improved shear strength with wood failure well above the minimum 80% required for wet-use and dry-use adhesives.

The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expres¬ sions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention, is defined and limited only by the claims which follow.