Methods of controlling haze in orthodontic appliances including semi-crystalline polymers
A dental appliance includes a polymeric shell with a plurality of cavities for receiving one or more teeth, including an interior region with a core layer of a first thermoplastic polymer A with a thermal transition temperature of about 70° C. to about 140° C. and a flexural modulus greater than about 1.3 GPa, and first and second interior layers of a second thermoplastic polymer B with a glass transition temperature of less than about 0° C., a flexural modulus less than about 1 GPa, and an elongation a break of greater than 150%; and first and second exterior layers of a third thermoplastic polymer C with a thermal transition temperature of about 70° C. to about 140° C. and a flexural modulus greater than about 1.3 GPa. The dental appliance demonstrates enhanced optical properties, with enhanced light transmission and low haze.
1 . A method of controlling haze in a thermoformed article, the method comprising:
providing a sheet of film comprising an interior region and an exterior region, the interior region comprising at least three alternating layers including:
a core layer having a first major surface and a second major surface, the core layer including a first thermoplastic polymer A having a thermal transition temperature from about 70° C. to about 140° C. and a flexural modulus greater than about 1.3 GPa;
a first interior layer adjacent to the first major surface of the core layer;
a second interior layer adjacent to the second major surface of the core layer, wherein the first interior layer and the second interior layer each independently comprise a second thermoplastic polymer B, different from the first thermoplastic polymer A, the second thermoplastic polymer B having a glass transition temperature of less than about 0° C. and a flexural modulus less than about 1 GPa;
wherein at least one of the layers of the interior region comprises a semi-crystalline polymer, and
wherein the exterior region comprises a first exterior layer on a first side of the interior region and a second exterior layer on a second side of the interior region, each exterior layer independently comprising a third thermoplastic polymer C, the third thermoplastic polymer C being the same as or different from the first thermoplastic polymer A;
providing a first positive model;
drawing the sheet over the first positive model at a molding temperature, the molding temperature being at least about 5° C. lower than an upper end temperature of a primary melting transition of the semi-crystalline polymer, as determined by differential scanning calorimetry (DSC); and
cooling the sheet and the first positive model to atmospheric temperature to form an article, wherein the formed article has an Expected haze, as measured according to ISO 14782 or ASTM D1003, of less than 10%.
2 . The method of claim 1 , wherein the article is a tray aligner, and wherein the first positive model is representative of a patient's dentition.
3 . The method of claim 1 , wherein the semi-crystalline polymer is a polyester or copolyester.
4 . The method of claim 3 , wherein the polyester or the copolyester is chosen from copolyester ether, copolymers of ethylene and (meth)acrylates, polyethylene terephthalate (PET), polyethylene terephthalate glycol (PETg), polycyclohexylenedimethylene terephthalate (PCT), 2,2,4,4-tetramethyl-1,3-cyclobutanediol modified PCT, PEN, and mixtures and combinations thereof.
5 . The method of claim 1 , wherein the semi-crystalline polymer is a copolyester ether elastomer having an elongation at break of greater than about 150%.
6 . The method of claim 1 , wherein the first thermoplastic polymer A is semi-crystalline and has an elongation at break of greater than about 150%.
7 . The method of claim 1 , wherein the molding temperature is no greater than 220° C.
8 . The method of claim 1 , wherein at least 80% of crystalline structures present in the sheet prior to thermoforming are melted.
9 . The method of claim 1 , and further comprising:
identifying, by modulated DSC (MDSC), the primary melting transition of the semi-crystalline polymer; and
selecting the molding temperature based at least in part on the primary melting transition.
10 . The method of claim 1 , wherein the formed article has an Expected light transmission of greater than 80%.
11 . The method of claim 1 , wherein the at least one layer includes a nucleating agent.
12 . The method of claim 11 , wherein the nucleating agent is a sorbitol.
13 . The method of claim 11 , wherein the nucleating agent is present at a concentration of between about 0.1 wt % and 1.0 wt %, based on a total weight of the semi-crystalline polymer.
14 . The method of claim 1 , wherein the sheet is a multilayer sheet including a core layer, a skin layer, and at least one inner layer disposed between the core layer and the skin layer, and wherein the at least one inner layer includes the semi-crystalline polymer.
15 . The method of the claim 14 , wherein at least one of the core layer or the skin layer includes amorphous elastomer.
16 . A dental appliance comprising:
a polymeric shell comprising a plurality of cavities for receiving one or more teeth, wherein the polymeric shell has an Expected haze of less than 10%, as measured according to ISO 14782 or ASTM D1003, and
wherein the polymeric shell comprises an interior region and an exterior region,
the interior region comprising at least three alternating layers, the interior region comprising:
a core layer having a first major surface and a second major surface, the core layer comprising a first thermoplastic polymer A having a thermal transition temperature from about 70° C. to about 140° C. and a flexural modulus greater than about 1.3 GPa;
a first interior layer adjacent to the first major surface of the core layer;
a second interior layer adjacent to the second major surface of the core layer,
wherein the first interior layer and the second interior layer each independently comprise a second thermoplastic polymer B, different from the first thermoplastic polymer A, the second thermoplastic polymer B having a glass transition temperature of less than about 0° C. and a flexural modulus less than about 1 GPa, and wherein at least one of the layers of the interior region comprises a semi-crystalline polymer; and
wherein the exterior region of the polymeric shell comprises a first exterior layer on a first side of the interior region and a second exterior layer on a second side of the interior region, each exterior layer independently comprising a third thermoplastic polymer C, the third thermoplastic polymer C being the same as or different from the first thermoplastic polymer A.
17 . The dental appliance of claim 16 , wherein the polymeric shell includes a multilayer film including a core layer, a skin layer, and at least one inner layer disposed between the core layer and the skin layer, and wherein the at least one inner layer includes the semi-crystalline polymer.
18 . A multilayer composite sheet for thermoforming a dental appliance, the sheet comprising:
an interior region having at least three alternating layers, the interior region comprising:
a core layer having a first major surface and a second major surface, the core layer including a first thermoplastic polymer A having a thermal transition temperature from about 70° C. to about 140° C. and a flexural modulus greater than about 1.3 GPa;
a first interior layer adjacent to the first major surface of the core layer;
a second interior layer adjacent to the second major surface of the core layer,
wherein the first interior layer and the second interior layer each independently comprise a second thermoplastic polymer B, different from the first thermoplastic polymer A, the second thermoplastic polymer B having a glass transition temperature of less than about 0° C. and a flexural modulus less than about 1 GPa, and wherein at least one of the layers of the interior region comprises a semi-crystalline polymer configured to control haze in a formed article; and
external layers on opposing sides of the interior region, each external layer independently comprising a third thermoplastic polymer C, the third thermoplastic polymer C being the same as or different from the first thermoplastic polymer A.
19 . The multilayer composite sheet of claim 18 , wherein the first thermoplastic polymer A has an elongation at break of greater than about 150%.
20 . The multilayer composite sheet of claim 18 , wherein the multilayer composite sheet has an overall flexural modulus in a range of about 0.8 GPa to about 1.5 GPa.