阅读说明：本技术 一种高抗冲击性防腐轻质pe实壁管及其制备工艺 (High-impact-resistance anticorrosive light-weight PE solid-wall pipe and preparation process thereof ) 是由 冯圆圆 于 2021-09-15 设计创作，主要内容包括：本发明公开了一种高抗冲击性防腐轻质PE实壁管及其制备工艺,涉及PE管件技术领域,包括内层和外层,所述内层和外层的层厚比为1:1.5-3；其中,所述外层由外层共聚物5-8份,聚乙烯80-100份,三元乙丙橡胶10-15份,抗氧化剂4-8份,增粘剂3-5份,二氧化硅0.5-1份组成；内层内层共聚物2-6份,高密度聚乙烯15-30份,聚氨酯3-4份,流平剂0.5-1份,消泡剂0.5-1份,增粘剂1-1.5份,硫酸钡1-2份,氧化钙0.5-1份组成。本发明的优点在于：本发明提出的PE管的机械性能和抗冲击性能有着极大的提升,在实际的使用过程中可有效的抵御外力冲击,能够有效的提高其防止其自身的损坏,提高了PE管的使用寿命。(The invention discloses a high-impact-resistance anticorrosive light PE solid-wall pipe and a preparation process thereof, relating to the technical field of PE pipe fittings and comprising an inner layer and an outer layer, wherein the layer thickness ratio of the inner layer to the outer layer is 1: 1.5-3; wherein the outer layer consists of 5-8 parts of outer layer copolymer, 80-100 parts of polyethylene, 10-15 parts of ethylene propylene diene monomer, 4-8 parts of antioxidant, 3-5 parts of tackifier and 0.5-1 part of silicon dioxide; 2-6 parts of inner layer copolymer, 15-30 parts of high-density polyethylene, 3-4 parts of polyurethane, 0.5-1 part of flatting agent, 0.5-1 part of defoaming agent, 1-1.5 parts of tackifier, 1-2 parts of barium sulfate and 0.5-1 part of calcium oxide. The invention has the advantages that: the PE pipe provided by the invention has greatly improved mechanical property and impact resistance, can effectively resist external force impact in the actual use process, can effectively prevent the PE pipe from being damaged, and prolongs the service life of the PE pipe.)

1. The high-impact-resistance anticorrosive light PE solid-wall pipe is characterized by comprising an inner layer and an outer layer, wherein the layer thickness ratio of the inner layer to the outer layer is 1: 1.5-3;

wherein the outer layer comprises the following components in parts by weight:

5-8 parts of outer-layer copolymer, 80-100 parts of polyethylene, 10-15 parts of ethylene propylene diene monomer, 4-8 parts of antioxidant, 3-5 parts of tackifier and 0.5-1 part of silicon dioxide;

the inner layer comprises the following components in parts by weight:

2-6 parts of inner layer copolymer, 15-30 parts of high-density polyethylene, 3-4 parts of polyurethane, 0.5-1 part of flatting agent, 0.5-1 part of defoaming agent, 1-1.5 parts of tackifier, 1-2 parts of barium sulfate and 0.5-1 part of calcium oxide.

2. The PE solid-wall pipe with high impact resistance and corrosion resistance as claimed in claim 1, wherein the outer layer copolymer is prepared by mixing propylene glycol adipate polyester, perfluoroallyl benzene, polybutyl acrylate, 2-thiol benzimidazole, and diformate according to a ratio of 1:2-2.5:3-3.5:0.8-1: 10-15.

3. The high-impact-resistance anticorrosion light PE solid-wall pipe as claimed in claim 1, wherein the inner layer copolymer is prepared by mixing phthalate, fatty acid zinc, 5- (vinyloxy) -1, 3-adamantane diol, azobisisobutyronitrile and dimethyl sulfoxide according to a ratio of 1:1.5-2.5:1-1.5:0.1-0.15: 5-10.

4. The anticorrosive light PE solid-wall pipe with high impact resistance according to claim 1, wherein the tackifier is prepared by mixing terpene phenolic resin, aromatic petroleum resin, benzoyl peroxide, 2, 5-dimethyl-2, 5-bis (tert-butylperoxy) hexane and silane coupling agent KH560 according to a ratio of 1:0.6-0.8:0.1-0.15:0.3-0.4: 0.5-0.65.

5. The high-impact-resistance anticorrosion light PE solid-wall pipe as claimed in claim 1, wherein the leveling agent is prepared by mixing polydimethylsiloxane, polyester modified organosiloxane and acrylic resin in a ratio of 1:2-3: 5-8.

6. A preparation process of a high-impact-resistance anticorrosive light PE solid-wall pipe is characterized by comprising the following steps:

preparation of copolymer: preparing an inner layer copolymer and an outer layer copolymer according to a principle component ratio;

mixing materials: mixing the inner layer copolymer with high-density polyethylene, polyurethane, a flatting agent, a defoaming agent, a tackifier, barium sulfate and calcium oxide in proportion to prepare an inner layer raw material, and mixing the outer layer copolymer with polyethylene, ethylene propylene diene monomer, an antioxidant, a tackifier and silicon dioxide in proportion to prepare an outer layer raw material;

and (3) extrusion molding: respectively adding the inner layer raw material and the outer layer raw material into a double-head extruder for co-extrusion molding;

cooling and shaping: introducing the PE pipe extruded and molded by the double-head extruder into a cooling and shaping groove, quickly opening a spray pump, and cooling and shaping for 3-5 hours;

cutting: and cutting the cooled and shaped PE pipe according to a preset size to obtain the PE solid-wall pipe.

7. The preparation process of the high-impact-resistance anticorrosive light PE solid-wall pipe according to claim 6, wherein the preparation step of the inner layer copolymer is as follows:

weighing raw materials: weighing phthalate, fatty acid zinc, 5- (vinyloxy) -1, 3-adamantanediol, azobisisobutyronitrile and dimethyl sulfoxide according to the weight ratio of 1:1.5-2.5:1-1.5:0.1-0.15: 5-10;

mixing: adding the phthalate, the fatty acid zinc, the 5- (vinyloxy) -1, 3-adamantane diol and the azobisisobutyronitrile which are weighed according to the proportion into dimethyl sulfoxide, stirring and reacting for 4-7 hours at 65-75 ℃ in a nitrogen atmosphere, and then precipitating in water;

cleaning: washing the precipitate with ethanol for 3-5 times;

and (3) drying: and (3) putting the cleaned polymer in a vacuum drying oven, heating to 80-90 ℃ under vacuum, and drying to constant weight to obtain the inner layer copolymer.

8. The preparation process of the high-impact-resistance anticorrosive light PE solid-wall pipe according to claim 6, wherein the outer layer copolymer is prepared by the following steps:

weighing raw materials: weighing propylene glycol adipate polyester, perfluoroallyl benzene, polybutyl acrylate, 2-thiol benzimidazole and diformate according to the weight ratio of 1:2-2.5:3-3.5:0.8-1: 10-15;

mixing: adding propylene glycol adipate polyester, perfluoro allyl benzene, polybutyl acrylate and 2-thiol benzimidazole weighed in proportion into diformate, stirring and reacting for 5-9 hours at 60-80 ℃ in the atmosphere of nitrogen, and then precipitating in water;

cleaning: washing the precipitate with ethanol for 3-5 times;

and (3) drying: and (3) putting the cleaned polymer in a vacuum drying oven, heating to 80-90 ℃ under vacuum, and drying to constant weight to obtain the outer-layer copolymer.

Technical Field

The invention relates to the technical field of PE (polyethylene) pipe fittings, in particular to a high-impact-resistance anticorrosive light PE solid-wall pipe and a preparation process thereof.

Background

PE is polyethylene plastic, the most basic plastic, plastic bags, preservative films and the like are PE, and HDPE is a nonpolar thermoplastic resin with high crystallinity. The appearance of the original HDPE is milky white, and the micro-thin section is semitransparent to a certain degree. PE has excellent resistance to most domestic and industrial chemicals, and therefore many pipes and tubes are produced using PE as a base material.

When the existing PE pipe is used, due to the fact that the self impact resistance is poor, the pipeline is prone to being damaged under the condition that the existing PE pipe is impacted by external force, pipeline leakage accidents are prone to occurring, and the safety of the surrounding environment is affected, so that the PE pipe with the good impact resistance is designed, the safety performance of the PE pipe can be effectively improved, and the service life of the PE pipe can be effectively prolonged.

Disclosure of Invention

In order to solve the technical problems, the technical scheme provides the high-impact-resistance anticorrosive light PE solid-wall pipe and the preparation process thereof, and solves the problems that when the conventional PE pipe provided in the background art is used, the pipeline is easily damaged under the condition of external force impact due to poor self impact resistance, so that the pipeline leakage accident is easily caused, and the safety of the surrounding environment is influenced.

In order to achieve the above purposes, the technical scheme adopted by the invention is as follows:

a high-impact-resistance anticorrosion light PE solid-wall pipe comprises an inner layer and an outer layer, wherein the layer thickness ratio of the inner layer to the outer layer is 1: 1.5-3;

wherein the outer layer comprises the following components in parts by weight:

5-8 parts of outer-layer copolymer, 80-100 parts of polyethylene, 10-15 parts of ethylene propylene diene monomer, 4-8 parts of antioxidant, 3-5 parts of tackifier and 0.5-1 part of silicon dioxide;

the inner layer comprises the following components in parts by weight:

2-6 parts of inner layer copolymer, 15-30 parts of high-density polyethylene, 3-4 parts of polyurethane, 0.5-1 part of flatting agent, 0.5-1 part of defoaming agent, 1-1.5 parts of tackifier, 1-2 parts of barium sulfate and 0.5-1 part of calcium oxide.

Preferably, the outer layer copolymer is prepared by mixing propylene glycol adipate polyester, perfluoroallyl benzene, polybutyl acrylate, 2-thiol benzimidazole and diformate according to the proportion of 1:2-2.5:3-3.5:0.8-1: 10-15.

The inner layer copolymer is prepared by mixing phthalic acid ester, fatty acid zinc, 5- (vinyl oxy) -1, 3-adamantane diol, azobisisobutyronitrile and dimethyl sulfoxide according to the proportion of 1:1.5-2.5:1-1.5:0.1-0.15: 5-10.

Preferably, the tackifier is prepared by mixing terpene phenolic resin, aromatic petroleum resin, benzoyl peroxide, 2, 5-dimethyl-2, 5-di (tert-butylperoxy) hexane and a silane coupling agent KH560 according to the proportion of 1:0.6-0.8:0.1-0.15:0.3-0.4: 0.5-0.65.

Preferably, the leveling agent is prepared by mixing polydimethylsiloxane, polyester modified organosiloxane and acrylic resin according to the proportion of 1:2-3: 5-8.

And further. The preparation process of the high-impact-resistance anticorrosive light PE solid-wall pipe is characterized by comprising the following steps of:

preparation of copolymer: preparing an inner layer copolymer and an outer layer copolymer according to a principle component ratio;

mixing materials: mixing the inner layer copolymer with high-density polyethylene, polyurethane, a flatting agent, a defoaming agent, a tackifier, barium sulfate and calcium oxide in proportion to prepare an inner layer raw material, and mixing the outer layer copolymer with polyethylene, ethylene propylene diene monomer, an antioxidant, a tackifier and silicon dioxide in proportion to prepare an outer layer raw material;

and (3) extrusion molding: respectively adding the inner layer raw material and the outer layer raw material into a double-head extruder for co-extrusion molding;

cooling and shaping: introducing the PE pipe extruded and molded by the double-head extruder into a cooling and shaping groove, quickly opening a spray pump, and cooling and shaping for 3-5 hours;

cutting: and cutting the cooled and shaped PE pipe according to a preset size to obtain the PE solid-wall pipe.

Optionally, the preparation steps of the inner layer copolymer are as follows:

weighing raw materials: weighing phthalate, fatty acid zinc, 5- (vinyloxy) -1, 3-adamantanediol, azobisisobutyronitrile and dimethyl sulfoxide according to the weight ratio of 1:1.5-2.5:1-1.5:0.1-0.15: 5-10;

mixing: adding the phthalate, the fatty acid zinc, the 5- (vinyloxy) -1, 3-adamantane diol and the azobisisobutyronitrile which are weighed according to the proportion into dimethyl sulfoxide, stirring and reacting for 4-7 hours at 65-75 ℃ in a nitrogen atmosphere, and then precipitating in water;

cleaning: washing the precipitate with ethanol for 3-5 times;

and (3) drying: and (3) putting the cleaned polymer in a vacuum drying oven, heating to 80-90 ℃ under vacuum, and drying to constant weight to obtain the inner layer copolymer.

Optionally, the preparation steps of the outer layer copolymer are as follows:

weighing raw materials: weighing propylene glycol adipate polyester, perfluoroallyl benzene, polybutyl acrylate, 2-thiol benzimidazole and diformate according to the weight ratio of 1:2-2.5:3-3.5:0.8-1: 10-15;

mixing: adding propylene glycol adipate polyester, perfluoro allyl benzene, polybutyl acrylate and 2-thiol benzimidazole weighed in proportion into diformate, stirring and reacting for 5-9 hours at 60-80 ℃ in the atmosphere of nitrogen, and then precipitating in water;

cleaning: washing the precipitate with ethanol for 3-5 times;

and (3) drying: and (3) putting the cleaned polymer in a vacuum drying oven, heating to 80-90 ℃ under vacuum, and drying to constant weight to obtain the outer-layer copolymer.

Compared with the prior art, the invention has the advantages that:

the PE pipe provided by the invention adopts a double-layer structure design, wherein the outer layer takes polyethylene resin as polymerization aggregate, and polybutyl acrylate is added into a copolymer of the outer layer, the polybutyl acrylate is soft in texture, so that the capability of the outer layer pipe for absorbing impact force can be effectively improved, and meanwhile, the added ethylene propylene diene monomer rubber can form a thermoplastic elastomer in the processing process, so that the outer layer structure has good elasticity, and can deform per se when the outer layer is impacted, so that the internal impact stress is reduced, and meanwhile, the outer layer has good toughness, so that the self structure can be maintained without damage under impact, and the impact resistance of the PE pipe is effectively improved;

the phthalic acid ester and the fatty acid zinc are added into the inner layer copolymer, the corrosion resistance of the inner layer can be effectively improved, the phthalic acid ester and the fatty acid zinc can soften the PE pipe in certain process when the PE pipe is molded, the tear resistance of the inner layer can be improved by adding the fatty acid zinc, when the outer layer is impacted, the inner layer can also elastically deform, the impact resistance of the PE pipe can be further improved while the corrosion resistance of the inner layer is ensured, the probability that the PE pipe is damaged by itself when being impacted is reduced, and the use safety of the PE pipe is improved.

Drawings

FIG. 1 is a flow chart of the preparation process of the present invention.

Detailed Description

The following description is presented to disclose the invention so as to enable any person skilled in the art to practice the invention. The preferred embodiments in the following description are given by way of example only, and other obvious variations will occur to those skilled in the art.

Example 1

Preparation of the inner copolymer: weighing raw materials: weighing phthalate, fatty acid zinc, 5- (vinyloxy) -1, 3-adamantane diol, azodiisobutyronitrile and dimethyl sulfoxide according to the weight ratio of 1:1.5:1:0.1:5, adding the phthalate, the fatty acid zinc, the 5- (vinyloxy) -1, 3-adamantane diol and the azodiisobutyronitrile which are weighed according to the weight ratio into the dimethyl sulfoxide, stirring and reacting for 7 hours at the temperature of 65 ℃ in the nitrogen atmosphere, precipitating in water, washing precipitates for 3-5 times by using ethanol, placing the washed polymer into a vacuum drying box, heating to 80 ℃ in vacuum, and drying to constant weight to obtain an inner layer copolymer;

preparation of the outer copolymer: weighing raw materials: weighing the propylene glycol adipate polyester, the perfluoroallyl benzene, the polybutyl acrylate, the 2-thiol benzimidazole and the diformate according to the weight ratio of 1:2:3:0.8:10, and mixing: adding propylene glycol adipate polyester, perfluoro allyl benzene, polybutyl acrylate and 2-thiol benzimidazole weighed according to a certain proportion into diformate, stirring and reacting for 9 hours at 60 ℃ in a nitrogen atmosphere, then precipitating in water, and cleaning: washing the precipitate with ethanol for 3-5 times, and drying: putting the washed polymer in a vacuum drying oven, heating to 80 ℃ under vacuum, and drying to constant weight to obtain an outer layer copolymer;

mixing materials: 2 parts of inner layer copolymer, 15 parts of high-density polyethylene, 3 parts of polyurethane, 0.5 part of leveling agent, 0.5 part of defoaming agent, 1 part of tackifier, 1 part of barium sulfate and 0.5 part of calcium oxide are mixed to prepare an inner layer raw material, 5 parts of outer layer copolymer, 80 parts of polyethylene, 10 parts of ethylene propylene diene monomer, 4 parts of antioxidant, 3 parts of tackifier and 0.5 part of silicon dioxide are mixed to prepare an outer layer raw material, wherein the leveling agent is prepared by mixing polydimethylsiloxane, polyester modified organosiloxane and acrylic resin according to a ratio of 1:2:5, and the tackifier is prepared by mixing terpene phenolic resin, aromatic petroleum resin, benzoyl peroxide, 2, 5-dimethyl-2, 5-bis (tert-butyl peroxy) hexane and silane coupling agent KH560 according to a ratio of 1:0.6:0.1:0.3: 0.5;

and (3) extrusion molding: respectively adding the inner layer raw material and the outer layer raw material into a double-head extruder for co-extrusion molding;

cooling and shaping: introducing the PE pipe extruded and molded by the double-head extruder into a cooling and sizing groove, quickly opening a spray pump, and cooling and sizing for 3 hours;

cutting: and cutting the cooled and shaped PE pipe according to a preset size to obtain the PE solid-wall pipe.

Example 2

Preparation of the inner copolymer: weighing raw materials: weighing phthalate, fatty acid zinc, 5- (vinyloxy) -1, 3-adamantane diol, azobisisobutyronitrile and dimethyl sulfoxide according to the weight ratio of 1:2.5:1.5:0.15:10, adding the phthalate, the fatty acid zinc, the 5- (vinyloxy) -1, 3-adamantane diol and the azobisisobutyronitrile which are weighed according to the weight ratio into the dimethyl sulfoxide, stirring and reacting for 4 hours at 75 ℃ in a nitrogen atmosphere, precipitating in water, washing precipitates for 3-5 times by using ethanol, placing the washed polymer in a vacuum drying box, heating to 90 ℃ in vacuum, and drying to constant weight to obtain an inner layer copolymer;

preparation of the outer copolymer: weighing raw materials: weighing propylene glycol adipate polyester, perfluorinated allyl benzene, polybutyl acrylate, 2-thiol benzimidazole and diformate according to the weight ratio of 1:2.5:3.5:1:15, and mixing: adding propylene glycol adipate polyester, perfluoro allyl benzene, polybutyl acrylate and 2-thiol benzimidazole weighed according to a certain proportion into diformate, stirring and reacting for 5 hours at 80 ℃ in a nitrogen atmosphere, then precipitating in water, and cleaning: washing the precipitate with ethanol for 3-5 times, and drying: putting the washed polymer in a vacuum drying oven, heating to 90 ℃ under vacuum, and drying to constant weight to obtain an outer layer copolymer;

mixing materials: mixing 6 parts of an inner layer copolymer, 30 parts of high-density polyethylene, 4 parts of polyurethane, 1 part of a leveling agent, 1 part of a defoaming agent, 1.5 parts of a tackifier, 2 parts of barium sulfate and 1 part of calcium oxide to prepare an inner layer raw material, mixing 8 parts of an outer layer copolymer, 100 parts of polyethylene, 15 parts of ethylene propylene diene monomer, 8 parts of an antioxidant, 5 parts of a tackifier and 1 part of silicon dioxide to prepare an outer layer raw material, wherein the leveling agent is prepared by mixing polydimethylsiloxane, polyester modified organosiloxane and acrylic resin according to a ratio of 1:3:8, and the tackifier is prepared by mixing terpene phenolic resin, aromatic petroleum resin, benzoyl peroxide, 2, 5-dimethyl-2, 5-bis (tert-butyl peroxy) hexane and a silane coupling agent KH560 according to a ratio of 1:0.8:0.15:0.4: 0.65;

and (3) extrusion molding: respectively adding the inner layer raw material and the outer layer raw material into a double-head extruder for co-extrusion molding;

cooling and shaping: introducing the PE pipe extruded and molded by the double-head extruder into a cooling and shaping groove, quickly opening a spray pump, and cooling and shaping for 5 hours;

cutting: and cutting the cooled and shaped PE pipe according to a preset size to obtain the PE solid-wall pipe.

Example 3

Preparation of the inner copolymer: weighing raw materials: weighing phthalic acid ester, fatty acid zinc, 5- (vinyloxy) -1, 3-adamantane diol, azobisisobutyronitrile and dimethyl sulfoxide according to the weight ratio of 1:2.5:1.2:0.1:8, adding the phthalic acid ester, the fatty acid zinc, the 5- (vinyloxy) -1, 3-adamantane diol and the azobisisobutyronitrile which are weighed according to the weight ratio into the dimethyl sulfoxide, stirring and reacting for 4 hours at 75 ℃ in a nitrogen atmosphere, precipitating in water, washing precipitates for 3-5 times by using ethanol, placing the washed polymer into a vacuum drying oven, heating to 90 ℃ in vacuum, and drying to constant weight to obtain an inner layer copolymer;

preparation of the outer copolymer: weighing raw materials: weighing the propylene glycol adipate polyester, the perfluorinated allyl benzene, the polybutyl acrylate, the 2-thiol benzimidazole and the diformate according to the weight ratio of 1:2.2:3.25:0.8:12, and mixing: adding propylene glycol adipate polyester, perfluoro allyl benzene, polybutyl acrylate and 2-thiol benzimidazole weighed according to a certain proportion into diformate, stirring and reacting for 5 hours at 80 ℃ in a nitrogen atmosphere, then precipitating in water, and cleaning: washing the precipitate with ethanol for 3-5 times, and drying: putting the washed polymer in a vacuum drying oven, heating to 90 ℃ under vacuum, and drying to constant weight to obtain an outer layer copolymer;

mixing materials: 5 parts of inner layer copolymer, 20 parts of high-density polyethylene, 3 parts of polyurethane, 1 part of flatting agent, 1 part of defoaming agent, 1 part of tackifier, 1 part of barium sulfate and 1 part of calcium oxide are mixed to prepare an inner layer raw material, 7 parts of outer layer copolymer, 85 parts of polyethylene, 12 parts of ethylene propylene diene monomer, 6 parts of antioxidant, 4 parts of tackifier and 1 part of silicon dioxide are mixed to prepare an outer layer raw material, wherein the flatting agent is prepared by mixing polydimethylsiloxane, polyester modified organosiloxane and acrylic resin according to the proportion of 1:2.5:7, and the tackifier is prepared by mixing terpene phenolic resin, aromatic petroleum resin, benzoyl peroxide, 2, 5-dimethyl-2, 5-bis (tert-butyl peroxy) hexane and silane coupling agent KH560 according to the proportion of 1:0.7:0.15:0.4: 0.5;

and (3) extrusion molding: respectively adding the inner layer raw material and the outer layer raw material into a double-head extruder for co-extrusion molding;

cooling and shaping: introducing the PE pipe extruded and molded by the double-head extruder into a cooling and shaping groove, quickly opening a spray pump, and cooling and shaping for 5 hours;

cutting: and cutting the cooled and shaped PE pipe according to a preset size to obtain the PE solid-wall pipe.

And (3) performance testing: taking a PE pipe of a sample of each example, and taking a conventional PE pipe or plate in the current market at the same time, and marking as a comparative example;

the tensile yield stress (MPa) of each sample was tested according to GB/T1042 and 1992, respectively, wherein the tensile speed was 100mm/min and the test temperature was 25 ℃;

testing the flexural modulus of elasticity (MPa) of each sample according to GB/T9341-2000, wherein the speed is 2mm/min, and the testing temperature is 25 ℃;

each sample was tested for notched Izod impact strength (kj. m) according to GB/T1843-^-2) The test temperature is 25 ℃;

a10 m long sample was then laid flat on the ground while water was passed through the sample at 2m/s across the entire pipe. The impact claw is used to act downwards on the top of the middle part of the pipeline at the speed of 0.3m/s, the height of the claw which continuously descends after acting on the pipeline is 30cm, the acting speed and the acting depth of the excavator are simulated when the excavator acts on the pipeline, and the times (times) of claw tests before the pipeline starts to seep water are recorded.

The results are as follows:

according to the test result, the mechanical property and the impact resistance of the PE pipe are greatly improved, the PE pipe can effectively resist external force impact in the actual use process, the damage of the PE pipe can be effectively prevented, and the service life of the PE pipe is prolonged.

The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are merely illustrative of the principles of the invention, but that various changes and modifications may be made without departing from the spirit and scope of the invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.