CA1080419A

CA1080419A - Reticulated web structures

Info

Publication number: CA1080419A
Application number: CA275,049A
Authority: CA
Inventors: Chia-Seng Liu
Original assignee: Hercules LLC
Current assignee: Hercules LLC
Priority date: 1976-04-08
Filing date: 1977-03-29
Publication date: 1980-07-01
Also published as: JPS52124984A; FR2347171A1; SE7703990L; IT1078040B; GB1544608A; AU502499B2; NO771220L; DE7710067U1; AU2407977A; FR2347171B1; DE2714190A1; BR7702246A

Abstract

RETICULATED WEB STRUCTURES
Abstract of the Disclosure It has been found that reticulated web structures having a uniform thickness, soft hand and high strength-to-weight ratio comprising a first set of oriented substantially parallel flat filaments running in the machine direction intersecting a second set of oriented substantially parallel flat filaments running in the cross-machine direction, where the said filaments all have a rectangular cross-section, with the width of the filaments being at least 1.5 times their thickness, the filament intersections be-ing biaxially oriented and the first set of filaments running in the machine direction being perpendicular to the second set of filaments running in the cross-machine direction, can be prepared by drawing from about 3 to 6 times in both the machine and cross-machine directions a thermoplastic polymer film having a thickness of from about 3 to 25 mils and a pattern of round discontinuous openings aligned uniformly longitudinally and transversely in a square pattern while heating said film at an elevated temperature appropriate to obtain orientation of the filaments along their long axis and in both directions at their intersections, provided said round discontinuous openings in said film are situated such that the distance between the circumference of any opening and the circumference of any neighboring opening at the narrowest point is at least 1.5 times the thickness of the film and the ratio of the distance between the circumference of any opening and the circum-ference of any neighboring opening at the narrowest point to the diameter of the opening is from about 1 to 4 to about 1.5 to 1.

Description

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This invention relates to thermoplastic reticulated web structures having a sot hand and high strength and their proces~
of preparation. More particularly, this invention relates to thermoplastic reticulated web structures having a uniform thick ness across the web comprising a first set of oriented substanti-ally parallel flat filaments running in the machine direction intersecting a second set of oriented substantially parallel flat filaments running in the cross-machine direction, where the said ilaments all have a substantially rectangular cross-section, 10 with the width of the filaments being at least 1.5 times their thickness and the filament intersections being biaxially oriented and having a thickness substantially equal to the thickness of the filaments and the first set of filaments being perpendicular to the second set of filaments; and their process of preparation.
The term "oriented" or "orientation" as used throughout the specification and claims means "molecularly oriented" ox "molecu-lar orientation".
It is known from the prior art to make thermoplastic retic-ulated web structures. For example, U.S. patent 3,386,876 dis-20 closes a thermoplastic reticulated web structure having substant-ially round or oval filaments and unoriented filament intersect-ions which are much thicker than the connecting filaments. Be-cause of the nonuniform thickness across the web struc~ure and its low strength due to unoriented thick intersections it is not as desirable for use in reinforcing, such as in reinforcing paper sheets. Another type of thermoplastic reticulated web structure ; is disclosed in U S. patent 3,666,609. This structure differs from the structure of this invention particularly in having fila-ments with round or oval cross-sections. The round or oval cross-30 section of the filaments contribute to an undesirable stiffness, giving the structures a stiffer hand than the structures of this invention. Still another type of thermoplastic reticulated web structure is disclosed in U,S. patent 3,365,352. This structure differs from the structure of this invention particularly in

- 2 ~

having the filaments intersect in diagonal directions and wherein both the intersections and filaments are biaxially oriented. Because of the diagonal alignment of the filaments and its low strength it is not as desirable for use in reinforcing~
The relative stiffness or conversely softness (sometimes referred to as soft hand) of a reticulated web structure is primarily the result of the cross-sectional configuration of the ilaments. For a given basic weight and gi~en spacing between filaments the softness of a given thermoplastic reticu-lated web structure may be judged by the flexural rigidity o~ its filaments;
thè.smaller the flexural rigidity, the softer the web. The flexural rigidity ~D) of a rectangular filament may be expressed by the following formula:

D = wt where w is the width of the filament, tl is the thickness of the filament and E is the tensile modulus of the rnaterial. The above formula clearly indicates that the filament thick~ess has a stronger influence on softness than the filament width. Thus, a web with a rectangular filament cross-section with width greater than thickness will be softer than one with a circular or square filament cross-section of the same polymeric material and weight.
It is an object of this invention to provide a reticulated web structure of therrnoplastic polymer material having a soft hand and a high strength-to-weight ratio.
; According to the invention there is provided a thermoplastic retic-ulated web structure having a uniform thickness, comprising a first set of substantially parallel flat filaments running in the machine direction inter-secting a second set of substantially parallel flat filaments running in the cross-machine direction, said filaments all being uniaxially oriented along their respective longitudinal axes and having a substantially rectangular cross-section with the width of the filaments being at least 1.5 times their

- 3 -thiclmess and the filament intersections being biaxially oriented with the filaments of the first set being perpendicular to the filaments of the second set at the intersections.

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Also according to the invention, a process for producing a thermo-plastic reticulated web structure comprising: forming in a thermoplastic polymer film having a thickness of from about 3 mils to about 25 mils a pattern of round openings aligned uniformly longitudinally and transversely in a square patte~n, the openings in said film being situated such that the distance between the circumference of any openings and the circumference of any neigboring opening at the narrowest point is at least 1.5 times the thickness of the film and the ratio of the distance between the circumference of any opening and the circumference of any neighboring opening at the nar-rowest point to the diameter of the openings is from about 1 to 4 to about 1.5 to 1; drawing said film from about 3 to 6 times in both machine and cross-machine directions at an elevated temperature to form a structure hav-ing a first set of substantially parallel flat filaments of rectangular cross-section running in the machine direction intersecting a second set of substantially parallel flat filaments of rectangular cross-section running in the cross-machine direction, the filaments each being oriented uniaxially along its longitudinal axis and the intersections of said filaments being biaxially oriented.
The specific elevated temperatures at which the perforated film should be drawn to obtain the desired orientation will depend upon the specific polymer from which thefilm is made. For example, polypropylene film will be drawn at a temperature of from about 100 C. to about 140 C.
The exact temperature required for other polymers will be apparent to those skilled in the art.
The sequence in which the film is drawn is not critical. The machine direction draw may be carried out before, after or simultaneous with the transverse ~i.e., cross-machine) direction draw. ~or good proress con-inuity in commercial production, "

transverse direction draw using a tenter followed by machine di-rection draw on a differential speed multi-roll stretcher is pre-ferred.
- As mentioned above, the size, shape and spacing of the open-ings in the thermoplastic polymer film is critical to obtaining a reticulated web structure having a soft hand and a high strength-to-weight ratio. Accordingly, care must be taken in forming the openings. There are many ways in which the openings can be formed; as for example by punching, drilling, molding, etc. The 10 bes~ method to use in each specific case will be obvious to those skilled in the art.
The starting polymeric material which may be used to pro-duce the reticulated web structure of this invention will be any ~- ;
readily orientable thermoplastic polymer material; such as the polyolefins, polyesters~ polyamides, eth~lene--vinyl acetate co-- polymers, etc. The most preferred starting materials are the homopol~mers and copolymers of olefins because of their low cost and ease of handling.
The present invention will become more apparent when taken 20 in conjunction with the accompanying drawings wherein:
Fig. 1 is a top view of a portion of an undrawn thermoplas-tic polymer film which has been perforated in accordance with the present invention preparatory to drawing the same;

:~ B Fig. 2 is a cross~sectional view along line 2-2 of Fig. l;
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Fig. 3 is an ~e~etri~ c~i~ of a portion o the re-ticulated web structure of this invention~
For a more detailed description, reference is made to Fig.
1 of the drawing which illustrates a portion of an undrawn film of thermoplastic polymer material ~a) having a series of punched 30 round openings ~c~. The opening~ are all of equal diameter (d), have equally spaced dis~ance~ at the narrowest poin~ between the circumferences of neighboxing openings ~b) and are uniformly spaced longitudinally and transversely in a square pattern.
The cross-sectional view of the undrawn film in Fig. 2 ~ 5 ~

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hows the thickness (t~ o the film (a) in relation to the diame-ter of khe openings (d) and the distance at the narrowest point between the circumferences of neighboring openings (b).
p~pec~
. The ~m~nrnJ-pro~e~t-ron of the reticulated web structure of this invention in Fig. 3 shows the sets of oriented substanti-ally parallel flat filaments ~f) running in the machine and cross-machine directions intersecting at oriented intersections (i), with the filaments (f) perpendicular one to the other and the sub-stantially rectangular cross-section of the ilaments (f) with 10 the width (w) being at least 1,5 times its thickness (tl). ~`
Because of its soft hand and high strength/weight ratio, the reticulated web structures of this invention can be used in a variety of applications, such as for example in reinforcing tissue paper, film, foam or non~oven fabrics.
The following examples are presented to illustrate the uni-~ue reticulated web structures of this invention and their process of production.
Example 1 A 10 mil thick cast film of ethylene--propylene copolymer, 20 containing approximately 5% of units derived from ethylene, pro-vided with a uniform square pattern o round holes each having a 0.045" diameter, positioned so that the distance between the cir-cumference ~i.e., edges) o~ neighboring holes is 0.022" and the width-to-thickness ratio at the narrowest portion between two neighboring holes is 2.2, is stretched in a tenter rame 4.5 times in the cross-machine direction in an oven at a temperature of 110C. and then stretch~d on a differential speed multi-roll stretcher 4.5 times in the machine direction with the rolls heated to a temperature of 100C. The resulting reticulated web ~truc-30 ture has approximately 3.5 filaments per inch running in bothmachine and cross-machine directions and a basic weight of 0.25 oz./yd.2. The filaments are approximately 0.012" wide, 0.004"
thick and rectangular in cros~-section. The re~ulting structure is flat and of relatively uniform thickness across the web. It _ 6 -

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has a soft hand, a tensile strength of 6.7 lbs./in. in the machine direction, 6.1 lbs./in. in the cross-machine direction and a tongue tear of approximately 1 lb. in both directions. Because of its soft hand and high strength/weight ratio this reticulated web structure is well suited for reinforcing plastic films.
Example 2 A 5 mil thick cas~ film of ethylene-propylene copolymer, containing approximately 5% of units derived from ethylene, provided with a uniform square pattern of round holes each having a 0.045" diameter, positioned so that the distance between the circumferences ~i.e.~ edges) of neighboring holes is 0.022" and the width-to-thickness ratio at the narrowest portion be-tween two neighbor~ng holes is 4.4, is first stretched on a differential speed multi-roll stretcher ~.1 times in the machine direction with the rolls heated to 100C. and then stretched in a tenter frame 5 times in the cross-machine direction in an oven at a temperature of 110C. The resulting reticulated web structure has approximately 3.7 filaments per inch in the machine direction and ; 3 filaments per inch in the cross-machine direction and a basic weight of 0.13 oz./yd. . The filaments are rectangular in cross-section with a width of 0.012"
and a thickness of 0.002". The resulting structure is flat and uniform in ~ 20 thickness. It has a soft hand, a tensile strength of 3.3 lbs./inch in the `~ machine direction, 3.4 lbs./inch in the cross-machine direction and a tongue tear of approximately 0.6 lbs. in both directions. The reticulated web struc-ture is well suited or reinforcing tissue paper.
Example 3 The reticulated web structure described in Example 2 is com-pared to commercial web structures made from the same copolymer and having simi-lar basis weight. The physical properties including stiffness (i.e., flexural r~- g tc~j~f~
~ med~s, determined using the for~ula discussed above divided by the tensile ! modulusntimes the number of filaments per inch) are recited in Table I.

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p5 F~ ~ o5 9 ~' - 8 -Example 4 Thus, the relative stiffness is defined as 1 times the number of filaments per inch.
A 5 mil thick cast film Of.the ethylene--propylene copoly-mer described in Example 2, provided with the same pattern of round holes having the same diameter and positioned at the same distance between the circumferences as described in Example 2 is first stretched on a differential speed multi-roll stretcher 4.7 times in the machine direction with the rolls heated to 125C.
10 and then stretched in a tenter frame 5 times in the cross-machine direction in an oven at a temperature of 130C. The resulting reticulated web structure has approximately 3.5 filaments per inch running in both ~he machine and cross-machine directions and a basic weight of 0.11 oz./yd.2. The filaments are rect-angular in cross-section with a width of 0.012" and a thickness of 0.002". The resulting structure is flat and uniform in thickness with a soft hand.
The thus prepared net with the rectangular configuration of filaments is compared with a net having the same basic 20 weight, essentially the same number and size openings, and shape of filaments except the filaments run diagonally. The modulus, tensile strength and tensile elongation of the two types of nets is recited in Table II.
Table II

Modulus-Machine Direction Tensile Strength Tensile Elongation (lb./in. at Machine Direction Machine Direction, Net4% elon~ation) (lb./in.) %
30 Rectangular 3.5 2.4 35 Diagonal 1.8 0.9 18

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A thermoplastic reticulated web structure having a uniform thick-ness, comprising a first set of substantially parallel flat filaments running in the machine direction intersecting a second set of substantially parallel flat filaments running in the cross-machine direction, said filaments all being uniaxially oriented along their respective longitudinal axes and having a substantially rectangular cross-section with the width of the filaments being at least 1.5 times their thickness and the filament intersections being biaxially oriented with the filaments of the first set being perpendicular to the filaments of the second set at the intersections.

2. The thermoplastic reticulated web structure of claim 1 wherein the thermoplastic is a polyolefin.

3. The reticulated web structure of claim 2 wherein the polyolefin is an ethylene--propylene copolymer.

4. A process for producing a thermoplastic reticulated web structure comprising: forming in a thermoplastic polymer film having a thickness of from about 3 mils to about 25 mils a pattern of round openings aligned uni-formly longitudinally and transversely in a square pattern, the openings in said film being situated such that the distance between the circumference of any openings and the circumference of any neighboring opening at the narrowest point is at least 1.5 times the thickness of the film and the ratio of the distance between the circumference of any opening and the circumference of any neighboring opening at the narrowest point to the diameter of the openings is from about 1 to 4 to about 1.5 to 1; drawing said film from about 3 to 6 times in both machine and cross-machine directions at an elevated temperature to form a structure having a first set of substantially parallel flat fila-ments of rectangular cross-section running in the machine direction intersect-ing a second set of substantially parallel flat filaments of rectangular cross-section running in the cross-machine direction, the filaments each being oriented uniaxially along its longitudinal axis and the intersections of said filaments being biaxially oriented.

5. The process of claim 4 wherein the thermoplastic polymer film is first drawn in the cross-machine direction using a tenter and then drawn in the machine direction using a differential speed multi-roll stretcher.

6. The process of claim 5 wherein the thermoplastic polymer film is drawn an equal amount in both the machine and cross-machine directions.