17-07-2012, 04:27 PM
IMPACT MODIFIED NYLON
[attachment=27848]
TECHNICAL FIELD
This invention relates to nylon molding and extrusion resins and, more particularly, to type 6 nylon that is modified to provide higher impact strength and improved melt forming properties.
BACKGROUND
Nylon 6, a polymer of caprolactam, is a well-recognized engineering plastic. As is characteristic of these materials, nylon is strong, it is capable of withstanding considerable abuse, it has low coefficient of friction, long wear life and makes an excellent bearing element. However, due to its stiffness, nylon 6 can fracture under high impact forces and, accordingly, it is a principal object of this invention to provide a modified nylon 6 with a higher impact strength.
A secondary object of this invention is to provide a modified nylon 6 that has a higher melt viscosity and melt strength that makes it easier to melt form as by extrusion and injection molding.
DISCLOSURE OF THE INVENTION
Briefly, these and other objects of this invention are achieved by melt blending from 5 to 50 weight percent of a polyester elastomer with nylon 6. It has been found that these polymers are misceable and compatible above their melting point and, further, the melt viscosity of a melt blend is higher than that of either polymer. This later observation implies that intermolecular reactions take place between the polymers, as by chain branching or chain extension, resulting in an apparent increase in molecular weight.
The addition of polyester elastomers improves the impact properties of the nylon and the increased melt viscosity facilitates melt forming processes. The polymeric blend is easier to extrude, the extrudate has improved ovality and is less subject to cracking during extrusion and annealing, and the surface of the extrudate is unusually smooth. Because of the concentricity and smoothness of the extrudate, it is easier to extrude to desired dimensional tolerances and secondary operations, such as centerless grinding and planing, may often be eliminated. If the blend is extruded in the form of small diameter rods which are chopped to make molding pellets, the smooth surface makes it easier to feed the pellets to extruders and injection molding machines.
The properties of the polymer blend, such as its ability to resist wear, can be improved by heat treating after a shape has been formed which suggests that polymerization can be continued after melt forming an article. Temperatures approaching the melting point, e.g., above about 200° C., are effective to increase the molecular weight over a period of 6 to 24 hours.
It should be noted that the above improved properties are not obtained when a polyester elastomer is added to type 6/6 nylon, the principal type of nylon with which type 6 nylon competes in bearing and wear applications.
EXAMPLE
85 weight percent of a nylon 6 resin was mixed with 15 weight percent of a polyester elastomer (Hytrel 5556 -duPont). The resins were fed through a mixing extruder, melted and extruded at 220° C. in the form of 1/8" strands. The strands had an extremely smooth surface and were chopped into 1/8" lengths to form a molding resin. This molding resin, in turn, was then extruded into a 1" rod. The polymeric blend was much easier to extrude than an unmodified nylon 6 due to its higher melt strength. It not only extruded more readily with very little of the stick-start type of operation usually observed with nylons but also the extruded rod had improved ovality and smoothness and had less tendency to crack during extrusion and subsequent annealing. The smoothness and dimensional tolerance of the rod was sufficiently good that centerless grinding was not required.
[attachment=27848]
TECHNICAL FIELD
This invention relates to nylon molding and extrusion resins and, more particularly, to type 6 nylon that is modified to provide higher impact strength and improved melt forming properties.
BACKGROUND
Nylon 6, a polymer of caprolactam, is a well-recognized engineering plastic. As is characteristic of these materials, nylon is strong, it is capable of withstanding considerable abuse, it has low coefficient of friction, long wear life and makes an excellent bearing element. However, due to its stiffness, nylon 6 can fracture under high impact forces and, accordingly, it is a principal object of this invention to provide a modified nylon 6 with a higher impact strength.
A secondary object of this invention is to provide a modified nylon 6 that has a higher melt viscosity and melt strength that makes it easier to melt form as by extrusion and injection molding.
DISCLOSURE OF THE INVENTION
Briefly, these and other objects of this invention are achieved by melt blending from 5 to 50 weight percent of a polyester elastomer with nylon 6. It has been found that these polymers are misceable and compatible above their melting point and, further, the melt viscosity of a melt blend is higher than that of either polymer. This later observation implies that intermolecular reactions take place between the polymers, as by chain branching or chain extension, resulting in an apparent increase in molecular weight.
The addition of polyester elastomers improves the impact properties of the nylon and the increased melt viscosity facilitates melt forming processes. The polymeric blend is easier to extrude, the extrudate has improved ovality and is less subject to cracking during extrusion and annealing, and the surface of the extrudate is unusually smooth. Because of the concentricity and smoothness of the extrudate, it is easier to extrude to desired dimensional tolerances and secondary operations, such as centerless grinding and planing, may often be eliminated. If the blend is extruded in the form of small diameter rods which are chopped to make molding pellets, the smooth surface makes it easier to feed the pellets to extruders and injection molding machines.
The properties of the polymer blend, such as its ability to resist wear, can be improved by heat treating after a shape has been formed which suggests that polymerization can be continued after melt forming an article. Temperatures approaching the melting point, e.g., above about 200° C., are effective to increase the molecular weight over a period of 6 to 24 hours.
It should be noted that the above improved properties are not obtained when a polyester elastomer is added to type 6/6 nylon, the principal type of nylon with which type 6 nylon competes in bearing and wear applications.
EXAMPLE
85 weight percent of a nylon 6 resin was mixed with 15 weight percent of a polyester elastomer (Hytrel 5556 -duPont). The resins were fed through a mixing extruder, melted and extruded at 220° C. in the form of 1/8" strands. The strands had an extremely smooth surface and were chopped into 1/8" lengths to form a molding resin. This molding resin, in turn, was then extruded into a 1" rod. The polymeric blend was much easier to extrude than an unmodified nylon 6 due to its higher melt strength. It not only extruded more readily with very little of the stick-start type of operation usually observed with nylons but also the extruded rod had improved ovality and smoothness and had less tendency to crack during extrusion and subsequent annealing. The smoothness and dimensional tolerance of the rod was sufficiently good that centerless grinding was not required.