Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones
Reexamination Certificate
1998-11-27
2001-07-24
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in two or more physically distinct zones
C526S206000, C526S220000, C526S245000, C526S251000, C526S279000, C526S303100, C526S320000, C526S328000, C526S329700, C526S330000
Reexamination Certificate
active
06265499
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a shaped product. More particularly, it relates to a process for producing a shaped product, which is useful for the production of molded products, for example, three-dimensional shaped products having specific shapes including ocular lenses such as contact lenses or intraocular lenses, camera lenses, pickup lenses for compact discs (CDs) or lenses for spectacles, or for the production of compound products such as bone cement for substitution of artificial hip joints.
2. Discussion of the Background
Heretofore, ocular lenses such as contact lenses or intraocular lenses, have been produced by a so-called mold polymerization method wherein a monomer is charged into a mold and polymerized in the mold by using azobisisobutyronitrile or a peroxide as a polymerization initiator.
However, when such a mold polymerization method is employed, there is a drawback that polymerization shrinkage occurs as the polymerization of the monomer progresses in the mold, whereby it is difficult to prepare a molded product corresponding precisely to the shape of the mold.
Further, a technique so-called “Stereo Lightgraphy”, has recently been employed in which a polymer is laminated by photopolymerization to obtain a three-dimensional shaped product. In such a method, it is common in many cases to employ an epoxy resin which is less susceptible to polymerization shrinkage. However, if a resin susceptible to polymerization shrinkage, such as an acryl monomer, is used, dimensional precision of the shaped product tends to be low, and it tends to be difficult to obtain a shaped product having a desired shape. Thus, this technique has a problem that the types of useful resins are substantially limited.
As is apparent from the above examples, it is important to reduce polymerization shrinkage in many fields where polymerization curing of a monomer is utilized.
It is usually considered that polymerization shrinkage occurs at the time of polymerization of a monomer, because the van der Waals distance among the monomer molecules changes to a covalent bond distance by the polymerization. Accordingly, the smaller the amount of the monomer involved in the polymerization, the less the polymerization shrinkage. In a case where polymerization is resumed from a partially polymerized state, the degree of polymerization shrinkage decreases to a level corresponding to the amount of the remaining monomer. For example, in radical bulk polymerization of methyl methacrylate, it is known that when polymerization is carried out from the monomer, the polymerization shrinkage will be about 21%. In a case where a mixture partially polymerized to a level of 50%, is further polymerized, the polymerization shrinkage will be reduced to a level of 10%.
Paying an attention to such a point, it has been proposed to fill a partially polymerized material in a predetermined mold or cavity and to polymerize it again to reduce the polymerization shrinkage.
In such a proposal, it has been common to use, as a polymerization initiator, an azo compound such as azobisisobutyronitrile, a thermal polymerization initiator such as a peroxide compound such as benzoyl peroxide, or a photopolymerization initiator such as benzophenone, benzil or a benzoin compound. In the polymerization with these polymerization initiators, once active free radicals are dissociated from the polymerization initiator, they immediately attach to the monomer, and the polymer chains will continue to grow, unless deactivation due to a chain transfer or termination reaction takes place during the polymerization. Accordingly, once polymerization is initiated, it is not possible to terminate the polymerization unless the growing species are deactivated. Once growing species are deactivated, the polymerization ability will not be regained. Such a characteristic makes it difficult to obtain a partially polymerized mixture in a desired state quantitatively. For example, it is difficult to completely terminate polymerization even if the material wherein the polymerization is once initiated, is transferred to a low temperature environment to reduce the apparent reaction rate, whereby there will be a problem that polymerization gradually proceeds during the storage, and the material is thereby cured and becomes useless. Further, from the viewpoint of resuming polymerization from a partially polymerized state, it is impossible to resume polymerization of a partially polymerized mixture which has once been deactivated, from such a deactivated state, and an additional mixing operation will be required to incorporate a fresh polymerization initiator, and such a method can hardly be useful for a common user who has no equipment required for such an operation.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-described prior art, and it is an object of the present invention to provide a process for producing a shaped product by a method of reducing polymerization shrinkage to prepare a molded product highly precisely in the shape of the mold or to prepare a compound product free from forming a space, wherein the method for reducing the polymerization shrinkage comprises preparing a partially polymerized mixture and resuming polymerization by an optional method, whereby the partially polymerized mixture can quantitatively be obtained, polymerization will not proceed during the storage, and polymerization can be resumed from the partially polymerized state without necessity of adding any active agent.
The present invention provides a process for producing a shaped product, which comprises polymerizing a mixture comprising a radical scavenger, a polymerization initiator and a radical polymerizable monomer to obtain a partially polymerized mixture having the conversion of the monomer into the polymer of from 5 to 90 wt %, filling the partially polymerized mixture in a predetermined cavity and then applying polymerization energy again to the partially polymerized mixture to complete polymerization.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the present invention, the radical scavenger means what can temporarily terminate polymerization reaction by binding propagation radicals and can generate active radicals by reversibly dissociating. The radical scavenger may or may not be a radical when the mixture is prepared.
In the present invention, the polymerization initiator is what generates radicals having a polymerization initiation function upon exposure to energy such as light and heat.
In the present invention, the partially polymerized mixture means a mixture of a polymer and an unpolymerized monomer having the conversion of the monomer into the polymer below 100%.
Now, the present invention will be described in detail with reference to the preferred embodiments.
According to the present invention, a shaped product can be prepared highly precisely, as mentioned above, by polymerizing a mixture comprising a radical scavenger, a polymerization initiator and a radical polymerizable monomer to obtain a partially polymerized mixture having the conversion of the monomer into the polymer of from 5 to 90 wt %, filling the partially polymerized mixture into a predetermined cavity and then applying polymerization energy again to the partially polymerized mixture to complete polymerization.
The process for producing a shaped product of the present invention has a feature that a radical scavenger and a polymerization initiator are used in combination.
In a case where a conventional radical polymerization initiator which initiates polymerization by forming active free radicals upon thermal cleavage, such as azobisisobutyronitrile, or by forming active radicals upon photolysis, such as benzil or benzophenone, is used alone as a polymerization initiator, the progress of the polymerization reaction at growing terminals of the polymer will stop due to various elementary reactions such as a chain transfer reaction which transfers the chain to the initiator fragment, other
Ando Ichiro
Nagino Haruko
Lipman Bernard
Menicon Co. Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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