Stock material or miscellaneous articles – Structurally defined web or sheet – Including aperture
Reexamination Certificate
1997-07-28
2002-07-23
Watkins, III, William P. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including aperture
C428S105000, C428S134000, C428S135000, C428S136000, C428S910000, C264S288800, C264S290200, C264SDIG004, C264SDIG008, C405S258100, C405S016000, C405S262000, C405S284000
Reexamination Certificate
active
06423394
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of producing a biaxially-stretched plastics material mesh structure, suitable for geotechnical and other civil engineering uses; a mesh structure employed for such uses can be referred to as a geogrid. The mesh structure is unbalanced, ie has a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD. The method comprises providing a starting material which has a thickness (defined below) of at least about 2 mm and has a pattern of holes on a notional substantially square or rectangular grid whose axes are substantially parallel to the PD and to the SD respectively, the sides of PD end portions of at least some of said holes being defined by crotch-forming zones. The starting material is of square construction; ie the PD and the SD are aligned with the two orthogonal axes of the eventual product. PD stretch is applied to form oriented PD strands and to apply some orientation to the notional junction zones (defined below) so that orientation extends into and through the notional junction zones, from the end of one oriented PD strand to the adjacent end of the aligned oriented PD strand. SD stretch is applied to form oriented SD strands. At least part of the length of the edges of crotches connecting adjacent sides of adjacent PD and SD strands is oriented in the direction running round the crotch. The mid-point of the notional junction zone (defined below) in the mesh structure is significantly thicker than the mid-point of any oriented strand entering the notional junction zone.
The present invention also relates to a biaxially-molecularly-oriented integral plastics material mesh structure made from a starting material which has a thickness of at least about 2 mm and comprising oriented PD strands and oriented SD strands, interconnected by junctions with respective sides of adjacent oriented PD and oriented SD strands interconnected by crotches at least part of whose length is oriented with orientation running in the direction around the crotch. The orientation of the PD strands extends into and through the junctions. The junction mid-point thickness is significantly greater than the mid-point thickness of any oriented strand entering the junction.
The mesh structures of the invention were designed for applications in which the main tensile force will be applied in the PD, and the mesh structures have significantly greater strength in the PD than in the SD; for instance, when the mesh structure is attached to a vertical wall facing in a retained wall geoengineering construction, the main tensile force will be at right angles to the facing and can be aligned with the PD of the mesh structure.
U.S. Pat. Nos. 4,374,798 and 5,053,264 disclose mesh structures of the general type with which the present invention is concerned. U.S. Pat. Nos. 5,267,816 and 5,269,631 disclose mesh structures which have greater strength in the PD than in the SD.
From the point of view of PD strength and failure mode, the best structure is found to be a uniaxially-oriented (uniax) structure, as for example strengths of up to at least 160 kN/m width can be achieved with a 5.5:1 PD stretch ratio. However, in order to reduce the time taken to build a geotechnical construction whilst maintaining the strength of the construction, it is desirable to increase the width of the geogrid without reducing the PD strength per strand or the economy rating in the PD more than about 5% or 10%. As there is a limitation on the width of starting materials, this can only be achieved in manufacture by stretching in the transverse direction (TD). Thus although the improvements of U.S. Pat. Nos. 5,267,816 and 5,269,631 are primarily directed towards significantly increasing the machine direction (MD) strength by enabling one to apply greater PD stretches, the present invention is more concerned with being able to apply SD stretches without greatly reducing the PD strength per strand or the economy rating, whilst providing a satisfactory creep strain behaviour with adequate long term strength. Nevertheless, it is very difficult to produce biax products whose PD strength per strand or economy rating is similar to the PD strength per strand or economy rating of the intermediate uniax material if reasonable stretches are applied in the SD. This is because the SD stretch affects the PD strength of the junction. Also the SD strands must be sufficiently robust eg to reinforce soil.
It is desirable to have a low SD punch-out in order to leave as much material as possible in the PD strand-forming zones and produce a high strength product using not too thick a starting material. However, if the SD punch-out is decreased, the PD orientation through the junction becomes more difficult to control.
It is desirable that any alteration in the manufacturing technique should produce a mesh structure which appears robust to the eye (as this is important for sales), and that the mesh structure should have an acceptable failure mode. As a general principle, it is desirable to have failure in the PD strand and not in the junction as this enables better prediction of the long term performance of the product.
THE INVENTION
According to the method of the invention, crotch-forming zones in the starting material have protuberances. Each protuberance may be in the plan view shape of the hole, namely a portion of the crotch-forming zone which projects into the respective hole beyond a base line which is tangent to each end of the projecting portion. Alternatively or in addition, the protuberance may be out of the plane of the starting material, being a thickening when approached from the edge of the PD strand-forming zone and/or when approached from the edge of the SD strand-forming zone. In the special case where there is no thickening when approaching from the edge of the PD strand-forming zone (defined below) and also no protuberance as seen in the plan view shape of the hole, there is a tendency for the orientation to proceed too far around the crotch in the PD stretch. This tendency is avoided by ceasing PD stetching before the orienation has reached the thickening (which considered as one progresses from the PD strand-forming zone will be a decrease in thickness).
The size of said protuberance is such that during the stretch, at least part of the edge of the crotch interconnecting adjacent sides of adjacent oriented PD and oriented SD strands is oriented in the direction running around the crotch, but the orientation ratio decreases significantly as one passes around the crotch edge, either from the oriented PD strand or from the oriented SD strand. In the biax product, the crotch edge either a) has an unoriented part, or b) the thickness of the least oriented part of the crotch edge is reduced (or the length of the least oriented part of the crotch edge is increased) by no more than about 20% (or than about 15%, 10% or 5%, as alternatives) of its thickness (or length) prior to stretching. The action of stretching does not reduce the thickness of any point along notional ridge lines of maximum thickness on the product from the junction mid-point to the crotch edges to such an extent that the ratio of finished thickness to starting thickness at that point is less than about 80% (or than about 85%, 90%, 95%, as alternatives) of the ratio of finished thickness to starting thickness of the notional junction zone mid-point.
In the biaxially-stretched mesh structure of the invention, the junctions comprise a central zone which is thicker than thinner zones adjacent the ends of oriented PD and oriented SD strands. If the junction interconnects two oriented PD strands entering a junction from opposite sides and two oriented SD strands entering the junction from two other opposite sides, the junction central zone will be generally square or rectangular; the central zone need only be generally square or rectangular and its sides could be concave or convex. The central zone has a narrow pro jection at the corner extending outwar
Green Stuart
Martin Keith Fraser
Mercer Frank Brian
Wrigley Nigel Edwin
Jacobson & Holman PLLC
Netlon Limited
Watkins III William P.
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