Patent Application
20120168068
The present disclosure relates to fabrics generally, and more specifically to a scrim suitable for use in a contourable core.
Composite core structures are used in wind turbine blades, airplanes, boat hulls and other structures that are very stiff, but light. Within these composite materials are cores made out of expanded polyvinyl chloride or other types of polymer foams or balsa wood. Such cores and their general construction are described in U.S. Pat. Nos. 3,376,185, 3,540,967 and 4,536,427, which are incorporated by reference herein in their entireties.
The core materials are sold in sheet form, and they can be cut to any shape. The core material starts out as a panel, such as a solid 4×8 sheet of PVC foam, of varying thickness depending on the application. The sheet is sent through a machine which has a steel roller. The roller is heated with hot oil and over top of the panel a fabric is fed from a roll. The fabric has a water based adhesive coating on one major face of the fabric. The fabric is an open mesh that weighs about 60 grams per square meter.
The fabric and panel are fed through the rotating roller that applies heat and pressure to activate the fabric onto the core.
Once the fabric is laminated to the panel, the panel is fed through another machine that scores it down the length and across to turn the board into a mosaic pattern of evenly spaced blocks.
From there the board goes to a bending machine that breaks the scores all the way through and then, at that point, the panel is held together by the fabric.
Improved contourable cores and methods of fabricating the cores are desired.
In some embodiments, a method comprises providing an open weave fiber glass scrim and printing a discontinuous pattern of an ethylene vinyl acetate (EVA) hot melt adhesive on a major surface of the open weave fiber glass scrim, the adhesive having sufficient viscosity at an application temperature used during the printing step to avoid wicking through the scrim.
In some embodiments, a product comprises an open weave fiber glass scrim; and a discontinuous pattern of EVA hot melt adhesive applied on one side of the open weave fiber glass scrim without bleeding through the scrim.
In some embodiments, a method of using the product comprises laminating the scrim to at least one panel from the group consisting of expanded polymer foam and wood, with the adhesive joining the scrim to the panel; and slitting the foam or wood in two directions without slitting the scrim to form a contourable core material having a plurality of blocks attached to the scrim.
This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.
The completed fabric 100 is suitable for use in contourable cores 200.
Fibers
Many types of fiber may be used to make the yarns of scrim 102. The following fibers may be used to form a fabric for a contourable core backing
In some embodiments, the scrim 102 is a greige scrim, without any sizing applied to it prior to the application of the adhesive 108. Other embodiments include glass fibers that are supplied with a starch/oil based size on the fibers to facilitate weaving of the fibers on a loom. This can avoid damage to fiberglass in the weaving process. However, adhesion of resins (epoxy, phenolic; unsaturated polyester, etc) to the glass fibers may be better without the starch/oil size. In some embodiments, the glass fibers are “Direct Sized,” i.e., the size on the glass is polymer based and contains silane coupling agents such that resins will bond covalently to the glass fibers and improve the strength properties of the composite structure.
In the example, the weave is a plain weave, with the weft yarns 106 alternately passing over and under the warp (machine direction) yarns 104. Table 1 provides an example of a fiberglass scrim fabric construction suitable for this purpose.
Such a scrim is available from San Gobain Technical Fabrics, Grand Island, N.Y., as product No. 4586.2/1208. Such a fabric has been woven by the assignee using a dry warped beam. Compared to a slashed beam, the dry warp beam resulted in a cost savings as well as a flatter profile to the warp yarns. The flatter profile provides a higher surface area on the warp yarns for the application of adhesive 108. In some embodiments, the warp yarn count is 49.48+/−4 per 10 cm.
Adhesive
The adhesive 108 is applied in a discontinuous pattern using a gravure process, which can apply either a 100% solids or water based hot melt adhesive to print a discontinuous pattern of hot melt adhesive. For example, EVA adhesive may be provided in an aqueous carrier, or in pellet or bead form, in which case the solid EVA is melted and picked up by the gravure roll for printing the adhesive on the fabric 102. Other methods may apply a discontinuous film with a water based hot melt adhesive such as rotary screen printing, flexographic printing, or the like.
The discontinuous pattern is a pattern that repeats in the machine direction, as can be applied using a gravure roll 404 (
In other embodiments, the application method may include 100% hot melt spray systems; extruded lines; hot melt adhesive webs laminated on the fabric, rotary screen printing, flexographic printing, or the like. For example,
In some embodiments, the adhesive 108 is applied in the form of dots by a gravure roll such as the roll 600 shown in
The fabric 100 produced by the dot gravure process is free of adhesive covering the openings 114 in the fabric 102. This process has low probability of adhesive 108 saturating or wicking into the fibers 104, 106. It provides a reliable way to meet adhesive level specifications consistently. The dot gravure process also avoids distorting the weft yarns 106 in the greige fabric 102. The dot gravure approach allows the machine to be run at very high speeds, and the adhesive pickup is independent of line speed. The discrete nature of the dots 108 may yield an advantage in terms of resin penetration through the fabric and perhaps increased shear properties in the sandwich structure. The extra height of the dots may result in more efficient adhesion of the fabric to the cores with potential raw material savings resulting. The dot process can also uses relatively low cost raw materials, such as EVA hot melt in pellet form.
In other embodiments, the discontinuous pattern comprises a plurality of lines and/or curves.
A discontinuous pattern of hot melt EVA adhesive has several benefits. The scrim 100 with this adhesive pattern resists “blocking”. At the end of the line, the center winder 412 winds the fabric 100 into a hard, square ended, adhesive coated roll suitable for shipping in containers world wide without damage in transit. If a fabric can adhere to itself, this condition is referred to as “blocking”, which makes it very difficult to dispense the fabric from the roll. Using the discontinuous pattern of hot melt EVA adhesive, blocking is avoided, without the need for a release liner.
Further, using the discontinuous pattern of hot melt EVA adhesive, both the weight and cost of the adhesive are reduced relative to a continuous film of adhesive. Further, the gaps in between regions of the adhesive 108 permit ingress of a second adhesive 305 (
In some embodiments, the adhesive has an average area density of 5 grams/meter2 to 28 grams/meter2. In some embodiments, the discontinuous pattern of adhesive includes adhesive with an average area density of 20 grams/meter2 to 27 grams/meter2, arranged in line segments lying along lines oriented in two different directions to form a grid of lines. In some embodiments, the gravure roll is a cross hatch roll having line segments spaced at from 10 to 55 (preferably 25 to 55) parallel lines per inch (4 to 22—preferably 10-22—parallel lines per centimeter) in at least one direction.
In some embodiments, the adhesive has a softening point in a range from 60° C. to 90° C. A softening temperature from 65° C. to 90° C. it preferred and a softening temperature from 70° C. to 90° C. is more preferred. In some embodiments, the softening temperature of the EVA adhesive material is about 75° C. In general, the higher the softening temperature of the hot melt EVA adhesive, the less likely blocking is at any given storage temperature. If the softening temperature is greater than the highest expected storage temperature and the highest expected shipping temperature, then the risk of blocking is greatly reduced, and costly blocking avoidance measures (e.g., refrigeration or a release layer) are avoided.
In general, higher viscosity prevents wicking into the loom-state fabric during application and during the customers nip rolling process (to attach the skins 302 and 304 to the core material 204 and scrim 100). However providing a distinct, clean print of adhesive that is free of hairs favors a low viscosity adhesive. In some embodiments, the viscosity at the application temperature is in a range from 10,000 to 60,000 centipoises at 191° C. Experiments showed that for adhesive viscosity below 10,000 centipoises at 191° C., wicking through the fabric is more likely to occur, despite the use of a chilled press roller. When the adhesive 108 passes through the open areas 114 in the fabric 102, or wicks through the fabric, it contaminates the rubber press roller 405 (
In some embodiments, for applying the hot melt EVA to the scrim 100 at 191° C., the adhesive should have a viscosity from 5700 cps to 35,000 cps.
The selected adhesive should have good reactivation behavior. The adhesive should be free of cross linking or other behavior that inhibits its melting a second time in the customer's process to laminate the skins 302, 304 to the core material 204 and scrim 100, respectively.
Preferably, the adhesive has a short open time, such as three seconds or less, which reduces the chance of blocking Coatings grade polymer adhesive provides a shorter open time than adhesive grade polymer for this use. An adhesive providing high hot tack has good grab to the core in its hot state, to provide good fundamental adhesion to PVC foam or balsa wood panel 202. Tack is not a requirement once adhesive is cooled.
Ethylene vinyl acetate or EVA is available as aqueous dispersions but more readily available as hot melt solids. In some embodiments, hot melt solid EVA pellets are melted to provide the adhesive which is picked up by the gravure roll 404 and deposited on the scrim 102.
Alternative adhesive are available, which are reactivatable via melting with heat. The heat reactivatable aspect is characteristic of “thermoplastic” polymers. Further, such polymers may be available as aqueous dispersions that are applied to the fabric and then dried with heat in an oven; or applied as 100% solids materials that are first melted to liquefy them so that they can be applied and then simply cooled so that they become solid again. Some examples of alternative adhesive options include, but are not limited to:
Preferably, the selected adhesive is not humidity sensitive; as such sensitivity can lead to tackiness, and increase risk of blocking. Preferably, the adhesive has a shelf life at least one year from date of manufacture in dry and cool (15° to 25° C.) conditions. Preferably the adhesive has good oxidation and color stability characteristics in the melt.
In some embodiments, a hot wire may be used for cutting any strings that may be formed normal to the fabric surface.
The fabric 100 including the adhesive 108 may optionally be drawn from the gravure roll 404 by the nip rolls 406, which also press the cooling hot melt adhesive into the scrim 102 of the fabric 100. The nip rolls 406 may also smooth out the surface of the adhesive on the coated fabric. The web 100 is then fed around chill rolls 408 to chill the adhesive below its softening point. For example, the chill roll temperature may be set so that the temperature of the fabric leaving the chill rolls 408 is less than 90° C., preferably less than 75° C., and more preferably less than 60° C. The chilled fabric 100 is then wound tightly into a square ended roll by center winder 412. The web of fabric 100 wound into a single roll may include 1000 meters of fabric, for example.
In experiments, when the 55 lpi gravure roll 404 was used, an adhesive add on level reached about 20 g/m2, and no strike through was observed. The result was achieved over a wide range of adhesive viscosity, which yields greater freedom in adhesive selection and a larger window of capability for the process. Samples made at 20 g/m2 with the 55 line per inch roller showed the fabrics to have excellent adhesion to the foam substrates and the fabric met the tensile and thickness requirements.
In some embodiments, a hot air knife is directed against the roller gap (between the roller and the fabric) is used for eliminating adhesive “curlicues” 130 (
In some embodiments, a surface winder 412 (
An advantage with the fabric 102 having a hot melt EVA adhesive is that the fabric can be laminated to dry balsa core material 204 (
Shear tests showed that 20 g/m2 of qualified adhesive on the fabric provides sufficiently strong bonds that the core material failed before the scrim with a discontinuous pattern of hot melt EVA delaminated from the core material 204. A fabric with a much lower level of adhesive will adhere well to foam. An adhesive area density of 5 g/m2 or more should provide sufficient adhesion. Further an adhesive area density of 12 g/m2 or more enhances material handling material handling characteristics. Using 20 g/m2 or more has the added function of providing stability of the fabric, relative to an adhesive level below 12 g/m2. With 20 g/m2 or more of adhesive, the fabric is less likely to distort during handling and is easily cut with scissors, because the adhesive volume helps to hold the weave intact. The gravure process is believed to be capable of control to within +/−1 g/m2.
During testing, if the roller passed over the adhesive area of the strip for 2 seconds (2.54 cm/s) or longer, adhesion was initiated with most adhesives. At 5 seconds (1.02 cm/s), all of the adhesives were securely fastened without the adhesive flowing away from the bond line. Thus, times of two to five seconds are suitable. Note that the 2 and 5 second durations were suitable for identifying differences in adhesion during lab testing. However, on an actual contourable core production line, the speeds used were between 4 and 10 times faster than those used in the lab tests.
Lap shear tests were performed using a variety of adhesives, including several steps:
1) Preparation of the fabric test specimens and foam or balsa substrate
2) Assembly of the fabric specimens to the substrate
3) Measurement of the maximum shear force to remove the fabrics from the substrate
4) Logging and analysis of the results
The control specimens used a commercially available Gavazzi (Gav) fabric, sold by GAVAZZI TESSUTI TECNICI S.p.A. of Calolziocorte, Italy. In addition to testing of several Jowat EVA adhesives (sold by Jowat, AG of Detmold, Germany), EMS copolyester adhesives (sold by EMS-CHEMIE North America Incorporated, Sumter, S.C.) were tested.
The approach in the lab was to mimic the final product 100 as closely as possible in terms of the location and amount of adhesive on the fabric. A style 0004 fabric was chosen for its areal weight and openness, and it was coated with 281 Finish to stabilize it for cutting and handling. Using coated fabric also eliminated the possibility of adhesive wicking through so that it would be inclined to stay at the bond line and not confound the results.
As long as the lap shear failure occurred in the foam itself; between the adhesive and the foam; or in the adhesive itself, conclusions could be drawn about the ability of the adhesive to bond to the substrate. If the failure occurred between the coated fabric and the adhesive, this would be noted and taken under consideration. The 281 Finish was chosen since it did not contain releases agents that could lead to failure of the bond between the fabric and the adhesive.
A roll of 0004/281 fabric was procured from inventory and strips of 25.4 cm×4.8 cm in size were cut across the width of the fabric. The 25.4 cm dimension was parallel to the warp direction. The strips were numbered in sequence and weighed on an analytical balance to 0.0001 grams.
The following procedure was used to apply adhesive to the above strips. An aluminum plate of 43 cm×30 cm in area and 1.3 cm thick was placed in the lab oven at 150° C. and left to equilibrate for several hours. About 1 tablespoon of a given adhesive, usually in pellet form, was sprinkled on the hot plate in a lengthwise direction down its center and left for at least 10 minutes to melt. A draw down bar was made by using a flangeless test tube of about 2 cm diameter and wrapping either masking tape or scotch tape or both around its circumference at two places separated by a distance of 5.1 cm from tape edge to tape edge. The test tube was placed in the oven along with the adhesive to equilibrate. The tube was drawn lengthwise down the hot plate by hand and over the adhesive to make a film that was 5.1 cm across and about 35 cm long. The fabric strips were quickly placed onto the film at 90° to the length of the film, with the end of the fabric lined up with the edge of the film, to form an area of intersection of 5.1 cm×4.8 cm. The fabric was not pressed into the adhesive film, rather it was left until the adhesive wet out the fabric, which was obvious by a change in the opacity of the fabric. The strips were then peeled off the film fairly quickly and placed outside the oven to cool. At this point a quantitative determination could be made of the open time of the adhesive by measuring how long it took for the tack to disappear to the touch.
Once cool the strips were reweighed on the analytical balance. By subtracting the original weight of the strip from the coated weight the amount of adhesive added on to the fabric could be determined very accurately. By dividing that weight in grams by the adhesive coated area (0.051 m×0.048 m) the areal weight of the adhesive could be determined and compared to the target level. By varying the thickness of the tapes and the number of wraps around the test tube the adhesive target level could be approached in a reasonably repeatable fashion.
The core material substrate consisted of Diab PVC foam board or balsa board that was 1.27 cm thick and cut to 10.16 cm×7.62 cm in size.
The method used to attach the fabric to the substrate was to heat the standard 2 kg steel Fibatape roller in the lab oven along with the hot plate to 150° C. Two rollers were heated so that they could be used in an alternating fashion so that 6 or 12 specimens could be attached to substrates in batch wise fashion such that the rollers did not cool too far.
The adhesive covered area of the fabric specimen was placed on the substrate parallel to the long direction of the substrate and centered on the narrower side such that such that 1.26 cm of space was present on each side of the strip and 20.3 cm of strip overhung the substrate. The roller could then be passed over the fabric and substrate intersection with constant pressure as long as no down force was applied to the handle to activate the adhesives in a repeatable fashion without it flowing away from the bond line or melting the PVC substrate, so that the bond of different adhesives could be assessed and compared.
If the time was 2 seconds (2.54 cm/s), adhesion was initiated with most adhesives. At 5 seconds (1.02 cm/s), all of the adhesives were securely fastened without the adhesive flowing away from the bond line. Given the sensitivity of the adhesion to the rolling rate, all of the testing was conducted at two rolling durations: 2 seconds and 5 seconds.
The test specimens made above were inserted vertically into an Instron tester equipped with upper and lower jaw facings of dimensions 5.1 cm×7.6 cm and a gap of 20.3 cm. The lower jaw overlapped the foam or balsa substrate by 5.1 cm or just up to but not covering the fabric strip. The upper end of the fabric strip was clamped in the top jaw for the height of the jaw or 5.1 cm. The air pressure fed to the pneumatic jaws is to adequately clamp the foam but not crush it; the pressure was set to 40 psig. The top of the strip was clamped first; the specimen was pulled taught; and then the lower jaw was clamped on the foam. Every effort was made by the operator to make sure the specimen was centered in the jaws and was parallel to the direction of travel of the jaws.
The cross head speed of the Instron was set to 100 mm/min. Once clamped, the Instron was started and the test progressed until the specimen ruptured.
The data are reproduced in a Table 2 below for foam core substrate. Table 2 includes data for fabric prepared by hand in the lab; the adhesives were applied on 0004 fabric manually. The lap shear specimens in Table 2 were strips of fabric welded to an area of foam using the hot steel roller in the lab. Thus, the data of Table 2 are more relevant for comparing adhesives to each other, and are not intended to represent absolute values to be achieved using mass production line equipment.
Jo 299.75
580
18.7
Foam break
75
35,000
Small hairs in open.
Jo 299.75 LV
498
19.5
Foam break
90
6,100
Excellent
Jo 297.90
566
21
Foam break
108
6,500
Small hairs in open.
Jo 297.90 HV
546
19
Foam break
115
20,000
Small hairs in open.
Jo 211-009
541
19.9
Foam break
90
5,700
Excellent; best
Jo 299.75
542
18.7
100% foam break
75
35,000
Small hairs in open.
Jo 299.75 LV
496
19.5
66% foam break
90
6,100
Excellent
Jo 297.90
527
21
30% foam break
108
6,500
Small hairs in open.
Jo 297.90 HV
521
19
17% foam break
115
20,000
Small hairs in open.
Jo 211-009
506
19.9
50% foam break
90
5,700
Excellent; best
Jo 299.75
Balsa 534
18.7
Balsa break
75
35,000
Small hairs in open.
In the data, the softening point is an important variable that one would want to maximize as it reduces the risk of roll blocking. Eliminating the risk of blocking would open the door to shipping fabric on its side; maximizing the number of rolls that can be stored on a container or truck; and therefore reduce the product cost component due to shipping. Viscosity is an important variable that one would want to maximize to prevent adhesive wicking during the adhesive coating step and through the customers nip process.
Referring now to the data of Table 3, under the conditions of the 5 second roll on PVC foam, all Jowat adhesives failed by foam break or to a very small extent, fabric rupture. In other words, the adhesive bond between the foam and the scrim did not fail, and was stronger than the foam itself.
Under the conditions of the 5 second roll on PVC foam the Gavazzi fabric failed adhesively, with the warp yarns pulling off the specimen and leaving the weft yarns behind on the foam
Under the conditions of the 5 second roll on PVC foam the EMS copolyesters failed by adhesion to the foam but at a load well above the Gavazzi fabric. The adhesive add-on level was very low however and if it was closer to 20% the performance would likely be similar to the Jowat samples.
Under the conditions of the 2 second roll on PVC foam the Jowat 299.75 failed by foam break every time.
Under the conditions of the 2 second roll on PVC foam the other Jowat samples showed a combination of failure modes that was in proportion to the softening point of the adhesive; the lower the softening point, the higher the percentage of specimens that failed by foam fracture. In any case, the loads encountered in adhesive failure overlapped with those of foam failure and far exceeded the Gavazzi sample
Under the conditions of the 2 second roll on PVC foam the Gavazzi sample failed adhesively at a load that was lower than that seen in the 5 second roll test
Under the conditions of the 2 second roll on PVC foam the EMS samples failed adhesively at a load below that seen in the 5 second roll test but still well above the Gavazzi fabric
The EMS Copolyester type adhesives can outperform EVA in some applications on an adhesive weight basis. This would mean that the copolyester adhesive might be effective to form a bond at a lower level (areal weight). When this is done the EMS copolyester adhesives are better than the Gavazzi mesh. However, the EVA adhesives adhered just as well, and could be used at a greater add-on level and lower cost to give the fabric superior handling and cuttability. Further, the EVA adhesive provides superior stringing behavior (i.e., less stringing) than copolyester.
The performance of five Jowat EVA adhesives (bold font in Table 3; also shown in
The Jowat 299.75 adhesive has the advantage of very high viscosity and a softening point of 75° C. It is very easy to activate the adhesive even at short roll times and the adhesive is very aggressive to foam and balsa. There was a small amount of the adhesive protruding into the opening of the fabric. The adhesive did not bleed through the fabric in testing, nor was it picked up by the steel roller.
A lower viscosity version of 299.75 was made called EXP 211-005. Although it has a much lower viscosity it has a higher softening point of 90 C. The quality of the fabric made from this adhesive was extremely high. This adhesive in terms of performance and quality was very close to the 211-009.
Some embodiments described herein can be used at a lower application temperature than a mesh having a water based adhesive. For example, printed hot melt EVA adhesives may be applied at 217° F. versus 300° F. for some water based EVA adhesives. Reducing the roll temperature allows the adhesives to stay viscous enough to remain at the bond line under the pressure of the roll without wicking through the fabric. The process described herein still results in an excellent bond of fabric to contourable core material at the lower temperature. Lower application temperatures will result in energy savings for the contourable core manufacturer.
Some embodiments described herein permit printing the hot melt adhesive at faster line speeds than are possible with water based adhesives. For example, a gravure type printing machine can be run at very high speeds and the adhesive pickup is independent of line speed. The rapid activation behavior of the hot melt adhesives would also allow for higher line speeds at the contourable core manufacturers.
One of ordinary skill can employ a number of variations from the details of the examples described above. For example:
a. Textured fiberglass yarns may be used to increase the surface contact area of the mesh to the core material 204.
b. More open constructions may be used to increase penetration of resin 305 through the fabric 100 to the core material 204.
c. Higher performing fibers such as Aramid or carbon or the like may be used.
d. The hot melt adhesives 108 may be a higher performing such as copolyesters or polyamide, or the like may provide better affinity for the resins 305 that join the skin 304 to the scrim 100.
e. Different weave patterns may be used. The example described above is a plain woven fabric but other weaves, such as weft inserted warp knit (WIWK) fabrics have generally higher strength than plain woven fabrics. Other weaves, such as, but not limited to multi-axial fabric may also be used.
Although the subject matter has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments, which may be made by those skilled in the art.
