This application claims priority to U.S. Provisional Patent Application Serial ^' No. 61/728,915, which is hereby incorporated by reference herein.

Background and Summary

The use of thermoplastic pipe systems for gas distribution has been very successful

These materials, especially polyethylene f'PE") including medium density polyethylene ("MDPB" (density in a range of 0.926-0,941 g/cm ³ )) and high density polyethylene ("HOPE" (density equal or greater than 0.941 g cm*)) resins in particular, have become major matrixes for low pressure gas systems, solving the corrosion and reliability issues of steel and ductile iron systems, HDPE is a tough, flexible, lightweight piping product, which can be butt-fused into long, continuous lengths. These unique performance properties combined with exceptional chemical resistance and long term durability make PE pipe a preferred product for a variety of demanding applications.

Today, shipments of PE pressure pipe account for approximately one billio pounds per year for a variety of applications, including gas distribution (see, e.g., htt r.'Vm w ene s,^

E pressure pipe is produced throughout North America and shipped in sizes ranging from ½ ^* ' CTS to 63" IPS. PE profile pipe for low pressure applications can be provided in sizes up to 144" in diameter, impro ving the following properties of the PE material will lead to wider use and new applications: long term strength, slow crack growth resistance, rapid crack resistance, and tensile strength. Historically, with each advancement in long term strength and tensile strength of HDPE, the gas pressure capability of pipe made from this material lias also improved.

Slow crack, growth ("SCO"} is one principal failure mode in PE ressu e pipe applications (see, e.g., Mm e, Shibing Bai, Qi Wang, "High-density polyethylene pipe with high resistance to slow crack growth prepared via rotation extrusion," Polymer Bulletin 65(6), pp. 609-621 (2010)}. This property, also commonly referred to as Environmental Stress Crack Resistance ("ESCR"), is an indicator of the ability of the PE piping material to resist the initiation of slow, slit-type cracks over time in response to long terra stress. These cracks, which could, ultimately lead to failure of the piping system, are associated with stresses imposed on the piping product by such phenomena as extreme chemical exposure, excessive growing or scraping, severe temperatures, or irregular loading conditions. If the damage is not detected and repaired, premature failure can occur in the material. Cracks or deiaminatioiis also provide sites for ingress of contaminants such as micro-organisms and moisture. Conditions such as moisture ingress significantly reduce the strength of composite structures over time. Thus, the use of PE pipes is limited due to the difficulty in damage detection and repair as well as lack of extended fatigue and impact resistance (see, e.g., Eyassu Woldesenbet and Roehe!le Williams, "Self-healing of a single fiber-reinforced polymer matrix composite ' Experimental Analysis of Nano and Engineering Materials and Structures 1.9, pp. 737-738 (2007)),

Brief ^' Description of the Drawings

FIG. 1 illustrates a schematic diagram of an embodiment of the present invention. FIG. 2 illustrates a schematic diagram of an embodiment of the present invention.

Detailed Description

Embodiments of the present invention protect against degradation of the PE matrix through the incorporatio of self-healing abilities. Induced by thermal and mechanical fatigue, niierocracking is a long-standing problem in PE pipes. If the PE pipes integrated microcapsules filled with a self-healing agent and catalyst, the polymeriz tion of the healing agent, triggered by contact with the embedded catalyst, ca bond the crack faces to recover the original mechanical properties.

The first use of self-healing for a polymer composite was in 1996 (see, e.g., C, Dry, "Procedures Developed for Self-Repair of Polymeric Matrix Composite Materials," Composite Structures 35, pp. 263-269 ( 1996)), Dry showed positive results in the feasibility of developing polymer matrix composites that have the ability to self-repair internal cracks caused by mechanical loading. The study focused on the cracking of hollow repair fibers dispersed in a matrix and the subsequent timed release of repair chemicals, which resulted in the sealing of matrix cracks, the restoration of strength in damaged areas, and the ability to retard crack propagation. ^' These materials, capable of passive, smart self-repair, comprise several parts:

(I ) an agent of internal deterioration that induces cracking, such as dynamic loading. (2) a stimulus to release the repairing chemical such as the crackmg of a fiber,

(3) a fiber,

(4) a repair chemical carried inside the fiber (e.g., either a partial polymer or a monomer), and

(5) a .method of hardening the chemical in the matrix in the ease of cross linking polymers, or a method of drying the matrix in the case of a monomer.

It was found that cracking of the repair fiber and subsequent release of the repair chemicals could he achieved. Dry's work is considered by most to be a pioneer in the field of self-healin polymer composites and has paved the way for several other mechanisms of autonomic healing in composites.

Microcracks in engineering materials are common and are often the initial sites of failure of a structure, in composite materials, fatigue and impact damage can lead to matrix cracking and delaminatioii in the material structure, thereby reducing the structural capability of the composite (see, e.g., B. Stavrinidis, D.G. Ho Ho way, "Crack Healing in Glass ' Phys. C!iem. Glasses 24, ( 1983),. pp. 19-25). The concept of self-healing composites relies on a healing agent stored in a container that breaks open when damaged.

A breakthrough in the study of self-healing materials was reported in 2001 by a research group at the University of Illinois (see, S. R. White, , N. . Sottos, P. H. Geubelle, J. S. Moore, M, R. Kessler, S. R. Srirarn, E. N, Brown, and S. Vis anathan, "Autonomic healing of polymer composites," Nature 409, pp. 794-797 (2001 )),. White et at. first introduced the incorporation of microcapsules containing a polymer precursor into the matrix materia! of a non-fiber reinforced polymer composite for self-healing purposes. The polymer precursor was contained in microcapsules and embedded into the matrix. The matrix contained a randomly dispersed catalyst, which was supposed to react with the precursor flowing through any crack formed due to damage, and initiate polymerization. The polymer was then supposed to bond the crack face closed. The researchers overcame several challenges In developing microcapsules that were weak: enough t be ruptured by a crack but strong enough not to break during manufacture of the composite system. The researchers showed that it was possible to recover up to 75% of the maximum tensile strength of the v ^" i.rgin compo ^' si tes .

If successful incorporation of the self-healing material into the fiber reinforced composites FRP") can be achieved, the benefit is quite obvious. Those composites can serve longer with better performance. Self-healing materials embedded in a F P composite or !aminaie have shown considerable restoration of mechanical properties, such as flexural strength, compressive strength, impact resistance, and a highly efficient recovery of matrix strength (see, G. Williams, . S. Trask, and I. R Bond, "Self-healing sandwich panels:

Restoration of compressive strength after impact," Composites Science and Technology 68, pp. 3171-3177 (2008); and G. Williams, R. S. Trask, and I. P. Bond, "A self-healing carbon, fiber reinforced polymer for aerospace application," Composites 38(6), pp. 1525-1532 (2007)).

However, the properties (especially mechanical properties) of the polymer materials may be degraded when a self-healing system is introduced (see, G, Williams, R. S, Trask, and I . P. Bond, "Self-healing sandwich panels: Restoration of compressive strength after impact," Composites Science and Technology 68, p, 3171 -3177 (2008); and O. Williams, R. S. Trask, and 1, P. Bond, "A sell-healing carbon fiber reinforced polymer for aerospace application," Composites 38(6), pp. 1525-1532 (2007)). By adding reinforcing ingredients in the polymer material, properties such as mechanical, thermal, and chemical properties can be potentially recovered or even improved.

Embodiments of the present invention introduce self-healing technology (a microencapsulated self-healing agent with catalyst) into a PE matrix to solve the problems previously mentioned. Embodiments of the present invention also improve the mechanical properties of a PE matri using nanofiller-remfbrcemen

A self-healing system utilizing a microencapsulated dkyclopentadiene ("DCPD") monomer and a solid phase Gruhbs's catalyst has been successfully employed in thermosetting polymers (see, S. . White, N. Sottos. P. I L Geubelle, S. Moore, M. . essler, S. R. Sriram, E. N. Brown, and S. Viswanaihan, "Autonomic healing of polymer composites," ^' Nature 409, pp, 794-797 (2001 )). However, these microcapsules are easil ruptured under stress, force, or pressure. Larger sized microcapsules are especially difficult to handle in such situations without them rupturing. In such situations, the self-healing agent prematurely flows out, endering the self-healing effect ineffective at a later time when a crack does occur. And, therefore, as a result, self-healing microcapsules have only been used in thermosettin matrixes tor self-healing purposes, as the microcapsules are not exposed to the forces that would prematurely rupture them during the thermosetting manufac ring stages. However, the manufacturing of thermoplastic matrices requires a melt-compounding process, such as an extrusion process, to be utilized. With such processes, the microcapsules will easily rupture, negating any future self-healing properties to be available in the resultant material

Embodiments of the present invention are able to implement self-healing microcapsules in thermoplastic matrices. The integrity of the microcapsules is preserved if they are of a small enough diameter. It was discovered ihat an average microcapsule diameter of 50 μ-m or less allows for the safe manufacture of thermoplastic matrices withoiu prematurely rupturing the self-healing microcapsules. As a result, embodiments of the present invention incorporate such .self-healing microcapsules with thermoplastics, such as

IM :.

In embodiments of the present invention, a microencapsulated dicyclopentadiene ("DCPD") monomer and a solid phase Grubbs's catalyst is embedded in a PE matrix to achieve self-healing properties,

There are other self-healing systems, such as tin catalyzed polycondensation of silanol fimciionalized polyCdsmethy] siloxane) ("PDMS"), whic has very good compatibility with a polymer matrix (see, Michael W. Keller, Scott R. White, and Nancy R. Sottas, "A. self- healing poly(dimethyl siloxane) elastomer " Adv. Fund Mater. 17, pp. 2399-2404 (2007)),. Depending on. results, other candidates of a self-healing agent and catalyst may be used for this effort. Microcapsules filled with a self-healing agent may be prepared by an in situ polymerization in an oi!~in-water emulsion. The sizes of the microcapsules may be in a range of 5-2000 μιη. Smaller microcapsules also have a greater chance of rupturing under stress and therefore healing cracks in a PE matrix.

Additionally, a nanoteehnology approach is utilized m embodiments of the present invention to improve the properties of the previously disclosed PE matrix incorporating a self-healing system. Nanocomposltes are composite materials that contain particles in the size range of 1 - 100 mrt. These materials bring into play the submkron structural properties of molecules. These particles, such as clay and carbon nanotubes ("CNTs") (e.g., including single, double, and multiwa!l carbon nanotubes), generally have excellent physical properties (see, e.g., XJ. He, J. H, Du, Z. Ymg, H. . Cheng, X. J. He, "Positive temperature coefficient effect in raultiwalled carbon nanotube/lngh-density polyethylene composites," AppL Phys. Lett 86, 0621 12 (2005 )), including a high aspect ratio and a layered structure that maximizes bonding between the polymer and particles. Funetionalized CNTs (such as fimctionaKzed with COOH-, NH2-, and/or OH~functional groups) can further improve the properties of the PE matrix. Adding a small quantity of these nano filler additives (0.5-5%) can increase man of the properties of polymer materials, including higher strength, greater rigidity, higher heat resistance, higher IJV resistance, lower water absorption rate, lower gas permeation rate, and other improved properties;

1. The majority of nano.fille.rs, such as nanoclay, ceramic, carbon nanotubes, carbon nanofibers, mineral particles (CaC<¼), and oxide nanoparticles are able to improve the mechanical, properties, such as tensile strength and modulus, of a PE matrix;

2, Carbon nanotubes _. , carbon nanofibers, carbon black, graphite, and graphene are effective fillers for improving the electrical conductivity of a PE matrix;

3. Carbon nanotubes and carbon black are able to improve the UV damage resistance of a PE matrix;

4. ^' Nanoclay and carbon nanofibers are able to improve the resistance of slow crack growth;

5. Nanoclay, carbon nanofibers, and iro oxide nanoparticles are able to improve the magnetic properties of a PE matrix.

Various combinations of the above-mentioned nanofi!Sers maybe used to co-reinforce a PE matrix. And, forthemwe, a nielt-cornpounding (extrusion) process may be used to synthesize PE composites with microcapsules filled with a self-healing agent and such naiio illers. For example, a twin screw extruder may be used to blend PE pellets with self- healing microcapsules and the corresponding catalysl, and, optionally, any one or more of the above-disclosed nanofii!ers. Following are parameters used in an exemplary process. However, these parameters may be customized to achieve desired final results.

Screw zone 1 temperature - .!60 ^¾ C;

Screw zone 2 temperature - 1 S0°C;

Screw zone 3 temperature - 180°C;

Die temperature - I 80°C.

Screw speed - Approximatel 100 rpm.

FIG. 1 schematically illustrates a PE .matrix manufactured t include self-healing microcapsules, an appropriate catalyst for the self-healing microcapsules, and one or more of any of the nanofs.llers disclosed herein. FIG, 2 schematically illustrates a PE matrix manufactured to include self-healing microcapsules and an appropriate catalyst for the self-healing microcapsules, but without any additional nanofillers.