A tube having an elongated core composed of a flexible narrow molecular weight distribution first polymeric composition, a sheath composed of a broader molecular weight second polymeric composition co-extruded over the outer core surface, and a liner composed of a broader molecular weight distribution second polymeric composition co-extruded over the inner core surface. The core may be composed of a composition exhibiting relative poor processability, and the sheath and liner of a composition exhibiting better processability.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention described in this application relates generally to tubing, 
and more particularly to a multilayer tube featuring beneficial extrusion 
and flow properties and a pleasing appearance. 
2. Background Information 
Tubing carrying a flow of fluid under pressure, such as irrigation tubing 
and the like, should exhibit properties of flexibility, strength, and 
durability. It should be lightweight and present minimal resistance to 
fluid flow, and it should employ inexpensive materials and fabrication 
techniques. In addition to these attributes, many users prefer tubing with 
a pleasing appearance. 
A relatively narrow molecular weight distribution polymeric composition, 
such as a narrow molecular weight distribution ethylene copolymer, 
exhibits many of the desired attributes. However, such a composition is 
difficult to extrude, and this is important because extrusion is the 
preferred fabrication technique, enabling relatively convenient and 
inexpensive fabrication of the miles of tubing in demand. A narrow 
molecular weight distribution polymeric composition tends to stick to the 
extrusion die and become pitted. This results in increased resistance to 
fluid flowing within the finished tubing, as well as an unpleasant 
appearance. Thus, it is desirable to have a new and improved tubing 
structure that overcomes these concerns. 
SUMMARY OF THE INVENTION 
This invention recognizes the problems associated with the prior art and 
provides a new and improved tube with the desired attributes. 
Briefly, the above and further objects of the present invention are 
realized in a tube structure comprising a core composed of a relatively 
narrow molecular weight distribution first polymeric composition having an 
inner liner and an outer sheath. At least one of the sheath and the liner 
is composed of a second polymeric composition having a molecular 
distribution that is broader than the molecular weight distribution of the 
first polymeric composition. 
The tube is preferably produced by co-extruding the first and second 
polymeric compositions. 
The tube structure, including one or more relatively broad molecular weight 
distribution second polymeric compositions, has improved extrusion 
properties, and presents less resistance to fluid flow and a more pleasing 
appearance relative to a tube structure composed of a single, narrow 
molecular weight distribution polymeric composition. This is achieved 
while maintaining many of the desired attributes of a tube structure of a 
relatively narrow molecular weight distribution polymeric composition. 
Thus, the tube of this invention overcomes many drawbacks of existing tube 
structures by producing, preferably co-extruding, the desired tube 
configuration from a combination of selected materials. 
The above mentioned and other objects and features of this invention and 
the manner of attaining them will become apparent, and the invention 
itself will be best understood, by reference to the following description 
taken in conjunction with the accompanying illustrative drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, there is shown a new and improved tube 10 
constructed according to the invention. Although the illustrated tube 10 
is a length of irrigation tubing, the inventive concepts are equally 
applicable to other tubing utilized for carrying a flow of liquid under 
pressure. 
Generally, tube 10 includes an elongated, tubular core 11 having an outer 
core surface 12 and an inner core surface 13. Core 11 is composed of a 
narrow molecular weight distribution first polymeric composition, a type 
of linear, low density polyethylene, that exhibits desired characteristics 
of strength, durability, flexibility, light weight, and relative 
inexpense. 
A sheath 15 is co-extruded over outer core surface 12 and a liner 16 is 
co-extruded over inner core surface 13. The sheath 15 and the liner 16 are 
composed of a relatively more broad molecular weight distribution second 
polymeric composition, another type of linear, low molecular weight 
polyethylene. The sheath l5 and the liner 16 allow the tube 10 to be more 
easily manufactured, e.g., extruded, to be less resistant to fluid flow, 
and to have a more pleasing, e.g., less pitted, appearance. 
The core 11 preferably comprises at least about 90%, more preferably about 
94%, e.g., about 99%, by volume of the total volume of the core 11, the 
sheath 15 and the liner 16. The thickness of each of the sheath 15 and the 
liner 16 is preferably in the range of about 0.0001 inch (2.5 microns) to 
about 0.01 inch (250 microns), more preferably about 0.0002 inch (5 
microns) to about 0.0008 inch (20 microns). 
The first and second polymeric compositions useful in the present tube 
structure, e.g., the tube 10, may be selected from any such compositions 
capable of functioning as described herein. One important characteristic 
of the present tube structure is that the core or center of the structure, 
e.g., the core 11, be made of a first polymeric compositions having a 
given molecular weight distribution and that the outer sheath, e.g., the 
sheath 15, and inner liner, e.g., the liner 16, be made of one or more 
second polymeric compositions having broader molecular weight 
distributions. The outer sheath and inner liner can be made of different 
compositions, although it is preferred to employ the same second polymeric 
composition for both of these components. 
Materials useful as the first and second polymeric compositions in the 
present invention include polymeric mixtures thereof, such as homopolymers 
and copolymers of olefins containing two to about eight or more carbon 
atoms per molecule, and the like. Since the presently useful polymeric 
compositions can be produced using conventional and well known 
polymerization processes and techniques, a detailed discussion of such 
processes and techniques need not be presented here. 
In one embodiment, the first polymeric composition preferably has a 
molecular weight distribution, i.e., defined as the ratio of the weight 
average molecular weight, M.sub.w, of the composition to the number 
average molecular weight, M.sub.n, of the composition, in the range of 
about two to about six, more preferably about two and one-half to about 
four and one-half. 
The first polymeric composition may advantageously be selected from the 
group consisting of homopolymers of olefins containing two to about eight 
carbon atoms per molecule, copolymers of such olefins and mixtures 
thereof. Such copolymers are preferably derived from ethylene and at least 
one olefin containing three to about eight carbon atoms per molecule. 
Particularly useful first polymeric compositions are selected from the 
group consisting of linear homopolymers of ethylene, linear copolymers of 
a major amount, i.e., greater than about 50 mol percent, and preferably 
greater than about 90 mol percent, of ethylene and a minor amount, i.e., 
less than about 50 mol percent, and preferably less than about 10 mol 
percent, of one or more C.sub.3 to C.sub.8 alpha olefins, and mixtures 
thereof. 
The second polymeric compositions are chosen so as to have a broader 
molecular weight distribution than the first polymeric composition and to 
provide at least one desired property, e.g., outlined herein, to the 
present structure. In one particular embodiment, the second polymeric 
composition is selected from homopolymers of olefins containing two to 
about eight carbon atoms per molecule and mixtures thereof. Polyethylene 
and polypropylene, especially polyethylene, are preferred second polymeric 
compositions. 
Thus, the tube structure has improved extrusion properties. It presents 
less resistance to fluid flow and a more pleasing appearance. This is 
achieved while maintaining many of the desired attributes of the core 
composition to overcome many drawbacks of existing tube structures by 
producing, preferably co-extruding, the desired tube configuration from a 
combination of selected materials. 
From a different perspective, the invention provides a tube that includes a 
core composed of a composition exhibiting relatively poor processability 
disposed between a sheath layer and a liner layer, at least one of which 
is composed of a composition exhibiting better processability. In this 
regard, processability refers to the extrudability of the composition, 
i.e., the degree to which the composition can be extruded without 
undesirable consequences such as pitting. Thus, according to one aspect of 
the invention, a sheath layer and a liner layer are used advantageously to 
enable co-extrusion of a tube that exhibits many desirable attributes of 
the core. And, this may be done with sheath and liner layers as thin as 
approximately 0.00025 to 0.020 inch. 
Although an exemplary embodiment of the invention has been shown and 
described, many changes, modifications, and substitutions may be made by 
one having ordinary skill in the art without necessarily departing from 
the spirit and scope of this invention.