Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2000-05-12
2002-08-13
Berman, Susan W. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S074000, C522S081000, C522S109000, C522S120000, C522S121000, C523S115000, C523S116000, C524S439000, C524S502000, C524S560000, C525S178000, C525S183000
Reexamination Certificate
active
06433037
ABSTRACT:
FILED OF THE INVENTION
This invention relates to the preparation of thermosetting molding compositions with an amorphous
on-crystalline acrylic resin thickener, the nature of which permits fiber reinforcement and other additives to be easily incorporated therein. These thermosetting compositions can be dispensed as premixed doughs (bulk molding compounds) or sheets (sheet molding compounds) and can be molded using low pressure molding techniques and conditions (temperature/pressure) to form articles ranging from automotive parts to dental appliances to bathroom shower stalls.
A. Acrylic Resins
Acrylic resins include polymer and copolymer formulations whose major monomeric components belong to two families of esters: acrylates and methacrylates. Acrylics are well known and are commercially available from ICI and others. Trade names such as Elvacite®, Lucite®, Plexiglas®, PERSPEX™ acrylic resins denote these polymer resins. Methyl-methacrylate, ethylacrylate and acrylic acid are common acrylic monomers. Acrylic monomers can polymerize by a free-radical, addition reaction. Two commercially used initiators for free-radical polymerization of acrylic are the peroxide initiator, benzoyl peroxide (BPO) and the aliphatic azonitrile initiator, azobisisobutyronitrile (AIBN). Both BPO and AIBN will decompose into free-radical activators at ambient temperature.
Peroxide initiated polymerizations tend to be more vigorous. Peroxides produce a higher polymerization exotherm which induces polymer chain decomposition with subsequent crosslinking between chain fragments. Crosslinking produces three dimensional network polymers that are said to be thermoset. AIBN and other azonitrile initiators produce lower polymerization exotherms, little polymer chain decomposition, minimal crosslinking and thermoplastic acrylic resins. In addition, azo initiators irreversibly decompose when heated. Therefore, residual azo initiator, AIBN, can be eliminated from acrylic resin by the deliberate heating of the resin.
That acrylic monomers can be polymerized in liquid suspensions using azo initiators such as AIBN is well known. Such azo polymerized vinyl polymers have minimal molecular crosslinking, are thermoplastic and are more or less soluble in various vinyl monomers. Since these thermoplastic vinyl polymers are easily molded using injection molding operations, they are available from commercial manufacturers. But linear thermoplastic vinyl polymers easily absorb surrounding liquids, including water, with resultant swelling and sometimes dissolution. So where absorbed liquids or thermoplasticity may be a problem, crosslinked, thermoset, acrylic polymers are preferable. Most dental appliances are molded of thermoset resin. Thermosetting acrylic resins are the plastic of choice in many industries since the crosslinked polymers resist penetration by many liquids and resist distortion by heat or mechanical stress.
B. Acylic Thickened Molding Compositions/Doughs
Both suspension and emulsion polymerization processes are used to produce commercial acrylic resins. Suspension polymerization generally produces larger beads of resin while emulsion polymerization generates a very, fine powder. Acrylic beads of linear polymers with their high volume and small surface area are well suited for use in thermoplastic molding processes and equipment, but acrylic powders are typically used in forming acrylic thermosetting molding doughs (bulk molding compounds and sheet molding compounds). Those used in dentistry typically comprise acrylic resin powder, liquid acrylic monomer and an initiator for free-radical polymerization. Thermosetting molding doughs used for industrial applications typically contain a thickener such as an alkali metal oxide. Three types of thermoset dough formulations are commonly used: those incorporating heat activated initiator; those with chemically activated initiator; and those with radiation (light) activated initiator. A mixture of initiators is used in, “dual cure”, resins. Dual cures are typically first light initiated, then heat cured.
Industrial molding doughs, also called premix, are prepared and sold as bulk molding compound (BMC) and sheet molding compound (SMC). Industrial mass producers mix perishable doughs at the job site for high volume production runs, or purchase another premix for molding within a few weeks of its preparation. The alkaline earth thickeners, peroxide catalysts and promoters in the dough cause gelation that limits shelf life. Commonly used industrial, alkaline earth thickeners, CaO or MgO, polymerize and build dough viscosity indefinitely.
Thermoset plastic dental fillings, dental crowns and dental prosthesis are most often made by low pressure compression molding of the acrylic dough. To maintain viscosity, the thermosetting acrylic thickened doughs used in dentistry or other small or occasional applications is typically prepared on site due to the short shelf life characteristic.
The thermosetting acrylic molding doughs contain powdered acrylic resin rather than large beads because 1) their rapid solubility in acrylic monomer quickly dissolves the resin into the monomer; and 2) the soluble resin powder contains enough residual initiator to trigger the polymerization of a lightly inhibited monomer liquid. In dental practice, these finely powdered polymers are also of low molecular weight of about 60,000 so as to be readily soluble in the acrylic monomer liquid.
C. Fiber Reinforcement
Although the acrylic doughs provide useful molded articles, the physical properties are not ideal for all applications and have been manipulated by blending methyl methacrylate with other resins, forming copolymers with the methyl methacrylate monomer and/or increasing the degree of crosslinking between polymer chains. Adding fiber reinforcement is desirable for some applications.
Long fibrous fillers, such as glass, carbon, aramid, etc., are known to greatly enhance strength, stiffness and toughness of plastic materials. Long fibers being defined as lengths equal to or exceeding the critical aspect ratio of the fiber matrix combination. Plastics reinforced with such long fibrous inclusions, i.e., composites, exhibit physical and chemical properties that are a composite of the properties of the fibrous fillers and plastic matrix. Typically, the included fiber has tensile strength much higher than the resin matrix, is insoluble in the resin matrix and is chemically, or physically bonded to the resin matrix in such a way as to deflect a crack propagating through the resin matrix along the length of the fiber-matrix interface. Fibers turn the crack, absorb the energy of fracture, reduce the incidence of through-and-through-fracture, and give composites their characteristic properties of high strength, high stiffness, toughness and light weight. The properties of some conventional polymeric materials and composites are disclosed in
CRC Practical Handbook of Materials Science
, Ed. Charles T. Lynch, 1994, pp. 547-548 (vinyls, ASA resins), 327-328 (glass fiber, organic fiber) and 342 (organic matrix composites). While the use of long fibrous fillers can provide advantageous physical properties, fiber is difficult to incorporate into a resin matrix, particularly where the matrix resin is highly viscous.
The thermosetting acrylic thickened doughs of powdered, low molecular weight acrylic resin and lightly inhibited acrylic monomer liquid are highly viscous. The mixture quickly passes from a wet slurry, to a viscous paste and then to a moldable dough as the resin particles first absorb and then dissolve in the monomer. Unfortunately, this otherwise convenient, rapid transition from slurry to paste to dough produces a very abrupt rise in viscosity. In addition, residual BPO initiator in the acrylic resin powder, intended to thermoset the mix, begins spontaneous decomposition, initiates polymerization and can reduce the shelf life of the fresh dough to only a few hours at 80° F.
Early efforts to bring a thermoset, fiber reinforced, organic polymer composite to the dental market have foundered on th
Berman Susan W.
Millen White Zelano & Branigan P.C.
Reinforced Polymers, Inc.
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