LOW HAZE, SPECKLE FREE POLYESTER FILM

Abstract: A low haze, speckle free polyester film suitable for use in solar windows is disclosed. The film comprises a mixture of calcined silicone particles and fumed silica agglomerates in which essentially all of the calcined silicone particles have a particle size below about 7 microns; and essentially all the fumed silica agglomerates have a discrete agglomerate size below 1 micron. The film very clear and has a minimum amount of haze.

Description: TITLE LOW HAZE. SPECKLE FREE POLYESTER FILM FIELD OF THE INVENTION This invention relates to polyester films. In particular, this invention is related to low haze, speckle free polyester film containing calcined silicone particles and fumed silica particles. BACKGROUND OF THE INVENTION Films of polymeric linear polyesters have excellent draw orientation and have proved especially well suited for biaxial film orientation. These polymeric films, especially those of polyethylene terephthalate (PET), are strong and have excellent inherent chemical and thermal properties. In addition, they have good optical clarity, toughness, and static properties, which makes them extremely suitable for use a myriad of uses. Control of surface slip is a prime requirement for the commercial use of polyester film. Slip is critical to the processability of the film, especially thin films. Slip is typically controlled by incorporating fillers to enhance surface roughness. These additives include inert particles of materials such as silica, china clay, aluminum silicate, calcium phosphate, and glass. The addition of these fillers improves the winding and slitting properties of the film. However, these additives may produce haze and loss of clarity, which make the film unsuitable for certain applications, such as solar windows. Siddiqui, U.S. Pat. No. 5,132,356, discloses films of linear polyester containing glass spheres and fumed silica agglomerates. Addition of these materials improves the dynamic coefficient of friction and the static coefficient of the film. Although this film is suitable for solar window applications, a small amount of haze and speckle, an optical distortion of light at the film surface, is present. Mills, Siddiqui, and Rakos, WO 96/01739 (PCT/GB95/01589), disclose a polymeric film consisting of a layer of polymeric material having on the surface a layer of polymeric material comprising silicone resin particles and/or calcined silicone resin particles in combination with particles of china clay. Although the film had improved optical and handling properties, it is more expensive to manufacture than a single layer film. In addition, the particles had a volume distributed median particle diameter of 1.5 to 12.5 microns. The presence of particles larger than about 7 microns can cause speckle. Thus, there is a continuing need for polyester films with good slip properties and decreased haze and speckle for use in solar window applications. SUMMARY OF THE INVENTION The invention is an orientated and heat-set polyester film with good slip properties and decreased haze and speckle. The film comprises: (a) about 2 parts per million to about 20 parts per million, based upon the weight of the film, of calcined silicone particles, wherein about 100 percent of the calcined silicone particles have a particle size below about 7 microns; and (b) about 20 parts per million to about 6000 parts per million, based upon the weight of the film, of fumed silica agglomerates, wherein about 100 percent of the agglomerates have an agglomerate size below about 1 micron. The film has good slip properties as well as the optical properties required for solar window applications. Because the refractive index of both the fumed silica agglomerates and the calcined silicone particles is close to that of biaxially orientated polyethylene terephthalate that contains no additives, the film is very clear and has a minimum amount of haze. BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows the particle size distribution for Tospearl 130 silicone resin particles after calcining at 300°C for 45 min. DETAILED DESCRIPTION OF THE INVENΗON Calcined Silicone Particles Silicone particles comprise a three-dimensional polymer chain of the formula: R x Si0 2-(x/2) in which x is a positive number greater than or equal to 1 , preferably 1 to 1.9, more preferably 1 to 1.5, and most preferably 1 to 1.2, and R is an organic group, such as an aliphatic hydrocarbon group, e.g., methyl, ethyl, or butyl, or an aromatic hydrocarbon, e.g., phenyl, an unsaturated group, e.g., vinyl, or a mixture of two or more of these groups. R is preferably a hydrocarbon group having 1 to 8, more preferably 1 to 5, carbon atoms. R is most preferably methyl. Particularly preferred silicone resin particles comprise methyl sesquioxane. Silicone particles are described in detail in WO 96/01739, mentioned above. Silicone particles have a cross-linked network of siloxane linkages, comprising a mixture of the following structures: RSi(O-) 3 and (R) 2 Si(O-) 2 in which R is as defined above. Suitable silicone particles are commercially available from Toshiba Silicone Co., Ltd., Tokyo, Japan, under the name of "Tospearl" silicone resin particles. These particles have a three-dimensional network structure in which each silicon atom is bonded to one methyl group. Calcining eliminates some or all or the R group, reducing the value of x. If all of the organic group is eliminated (i.e., x is 0), the particle is converted to silica (SiU 2 ). To form the calcined silicone particles, the particles are calcined at about 300°C to about 400°C for about 30 min to about 3 hr, preferably at about 300°C for about 45 min. Calcining may be carried out in air or in a suitable inert atmosphere, such as nitrogen. Elimination of some or all of the organic material during calcination reduces the weight of the particle. The particles typically lose about 3% to about 5%, preferably about 2% to about 4%, of their original weight when calcined under these conditions. The porosity of the calcined silicone particles enhances the adhesion of the particles to the polymer. Because the calcined silicone particles are softer (mob hardness of 3) than glass particles (moh hardness of 6), they are less prone to scratch the film during the winding and slitting operations. Fumed Silica Agglomerates Fumed silica is formed when silicon tetrachloride (SiC^) reacts in a hydrogen flame to form single, spherical droplets of silicon dioxide, which grow through collision and coalescence to form larger droplets. As the droplets cool and begin to freeze, but continue to collide, they stick together but do not coalesce, forming solid aggregates. The aggregates continue to collide to from clusters, known as agglomerates. The particle size for the fumed silica particle given by the manufacturer refers to the particle size of a single cooled spherical droplet, not to the size of the agglomerate. Composition The film comprises about 2 to about 20 parts per million, preferably about 4 to about 10 parts per million, based on the weight of the film, of calcined silicone particles. About 100 percent (i.e., essentially all) of the calcined silicone particles have a particle size below about 7 microns. Preferably, about 100 percent of the particles have a particle size below 6 microns, more preferably below 5.5 microns. Even more preferably, about 100 percent of the calcined silicone particles have a particle size below about 6 microns, at least about 95 percent have a particle size below about 5 microns, and 50 percent have a particle size below about 2.2 microns. The calcined silicone particles typically have an average particle size of about 2 to about 3 microns, preferably about 2 to about 2.4 microns, more preferably about 2.2 microns. Because the presence of particles larger than about 7 microns can cause speckle, elimination of the larger particles reduces speckle. In the glass particles disclosed in Siddiqui, U.S. Pat. No. 5,132,356, for example, about 3% of the glass particles have a particle size above about 7 microns. About 100 percent (i.e.. essentially all) of the fumed silica agglomerates are below about one micron is size. The average agglomerate size is preferably about 0.10 to about 0.50 microns, more preferably about 0.25 to 0.35 microns. The discrete fumed silica particles that form these agglomerates generally have a particle size of about 0.05 micron. However, these particles are typically present as agglomerates of two or more particles. The concentration of fumed silica agglomerates should be adequate to help impart good winding properties to the film, but not so high that the optical properties of the film are adversely affected by excessive haze. The film comprises about 20 parts per million (0.002%) to about 6000 parts per million (0.6001), preferably about 500 parts per million (0.005%) to about 3000 parts per million (0.300%), more preferably about 1000 parts per million (0.100%) to about 2000 parts per million (0.200%), even more preferably about 1400 parts per million (0.140%) to about 1800 parts per million (0.180%), based upon the weight of the film, of fumed silica agglomerates. The polyester film preferably has a thickness of about 8 microns to about 75 microns, more preferably about 12 microns to about 15 microns. Manufacture Polyester films are well known to those skilled in the art. The polymer preparation and film manufacturing processes are well known to those skilled in the art and are disclosed in many texts, such as Encyclopedia of Polymer Science and Engineering, 2nd. Ed., Vol. 12, Wiley, New York, pp. 1-313, as well as in numerous patents, such as UK Patent 838,708. Polyester may be obtained by condensing one or more dicarboxylic acids or their lower alkyl diesters with one or more glycols. Preferred polyester films are those of the group consisting of polyethylene terephthalate (PET) film and polyethylene naphthanate film. These polymers are typically obtained by condensing the appropriate dicarboxylic acid or its lower alkyl diester with ethylene glycol. Polyethylene terephthalate is formed from terephthalate acid; polyethylene naphthanate is formed from 2,7-naphthalene dicarboxylic acid. The most preferred polyester film is polyethylene terephthalate. The calcined silicone particles and fumed silica agglomerates can be added to the polyester precursors at any point in the manufacturing process prior to extrusion of the polymer. Addition during the polymerization step is preferred. A convenient procedure, which is especially preferred in the production of polyethylene terephthalate film, is to add the particles and agglomerates to the polycondensation mixture used to produce the polymer. It is particularly desirable to add the particles as a slurry after the ester interchange reaction in which the monomer is formed. The particles can be added, for example, as slurry in the glycol from which the polyester is formed, prior to commencement of the polycondensation. In the production of polyethylene terephthalate, for example, the particles are added after formation of bishydroxyethylene terephthalate (BHET). A conventional polyester polymerization catalyst is added and the polymerization carried out in the conventional manner to produce a polyester resin. To produce polyester film, the polyester resin is melted and extruded as an amorphous sheet onto a polished revolving casting drum to form a cast sheet of the polymer. Thereafter, the cast sheet of polymer is heated to just above its glass transition temperature, 80°C to 100°C for polyethylene terephthalate, and is generally stretched or drawn in one or more directions. The film is typically stretched in two directions, the direction of extrusion (longitudinal direction) and perpendicular to the direction of extrusion (transverse direction) to produce a biaxially orientated film. The first stretching, which imparts strength and toughness to the film, conventionally ranges from about 2.0 to about 4.0 times its original length. Subsequent stretchings each also increase the size of the film about 2.0 to about 4.0 times. Generally, it is preferred to stretch first in the longitudinal direction and then in the transverse direction. The film is then heat set, generally at a temperature in the range of about 190°C to 240°C for polyethylene terephthalate, to lock in the strength, toughness, and other physical properties. The film may contain any of the additives known in the art, such as dyes, pigments, lubricants, anti-oxidants, anti-blocking agents, surface active agents, slip aids, gloss improvers, ultra-violet stabilizers, viscosity modifiers, dispersion stabilizers, etc. However, in the case of polyethylene terephthalate, in which, to prevent haze, the refractive index of the film is closely matched to that of the fumed silica agglomerates and calcined silicone particles, the additives should not greatly affect the refractive index of the film. A conventional coating may be applied to the film. Such coatings are customarily applied to improve the adhesive or anti-static properties of the film. The composition of such coatings and the procedures for applying them are well known to those skilled in the art and are described in numerous patents and publications, such as for example, Siddiqui, U.S. Pat. No. 5,132,356, incorporated herein by reference. The coating may be applied to a uniaxially orientated or to a biaxially orientated film. In a simultaneous biaxially orientated stretching process, the coating can be applied either before or after the stretching process. In a sequential stretching process, the coating is preferably applied between the two operations, i.e. between the longitudinal and transverse stretching operations. Preferably, the film is stretched in the longitudinal direction over a series of rotating rollers, coated, stretched transversely in a stenter oven, and, preferably, heat set to produce the coated film. The film can be metalized with, for example gold or aluminum, by techniques well known in the art. INDUSTRIAL APPLICABILITY The films can be used in solar window applications. For solar windows, the film may be metalized and dyed or coated to reduce light transmission. To reduce ultra-violet transmission and improve film aging characteristics, ultraviolet absorbers may be coated onto the film or incorporated into the polymer as described above. A hard coating, which can contain a dye and/or an ultra-violet absorber, can be applied to the outer surface of the film to improve resistance to surface scratching. A pressure sensitive adhesive and a silicone-coated release liner are applied to the opposite surface for use in application of the film to glass. When a metalized film is used, a laminate comprised of at least two laminated layers of polyester film is used to protect the metalized surface. The laminating adhesive also can contain a dye and/or an ultra-violet absorber. The advantageous properties of this invention can be observed by reference to the following examples which illustrate, but do not limit, the invention. EXAMPLES Glossary Aerosil® OX-50 Amorphous fumed silica, discrete particle size 0.050 micron (Degussa, Ridgefield Park, NJ) Tosprearl 130 Silicone resin particles, mean particle size 3.0 microns, specific surface area m 2 , linseed oil absorption 75 mL/100 g (Toshiba Silicone Co., Ltd., Tokyo, Japan) Preparation of Calcined Silicone Particles Tosprearl 130 silicone resin particles were calcined in air at 300°C for 45 min. The particles were analyzed by FTIR before and after calcining. Sharp peaks at 1288 cm - 1 and 2991 cm -1 disappeared when the particles were calcined. The particle size distribution was measured by a Microtrac particle size analyzer. The particle size distribution for the calcined particles is shown in Fig. 1. All the particles were below 5.28 microns in diameter; 99% were below 4.84 microns; 95% were below 4.07 microns; 90% were below 3.73 microns; and 50% were below 2.22 microns. The analysis is shown in Table 1. Table 1 CLE SIZE DISTRIBUTION FOR CALCINED S IILICON] Particle Size (microns) % Pass 5.28 100.00 4.84 99.44 4.44 98.44 4.07 96.68 3.73 93.87 3.42 89.14 3.14 83.13 2.88 76.09 2.64 68.33 2.42 60.29 2.22 52.00 2.03 43.92 1.87 36.44 1.71 29.78 1.57 24.02 1.44 19.08 1.32 14.86 1.21 11.33 1.11 8.46 1.02 6.21 0.93 4.51 0.86 3.30 0.78 2.50 0.72 1.94 0.66 1.50 0.60 1.05 0.55 0.62 0.51 0.25 0.47 0.00 EXAMPLE 1 This example illustrates preparation of a filled biaxially oriented polyethylene terephthalate film containing calcined silicone particles and fumed silica agglomerates. Two slurries were prepared. Calcined Tosprearl 130 silicone particles, prepared as described above, were mixed with ethylene glycol to form a slurry having 1.0% calacined silicone particles in ethylene glycol. Aerosil ® OX-50 amorphous fumed silica agglomerates were mixed with ethylene glycol to form a slurry containing 4.0% fumed silica agglomerates in ethylene glycol. Each slurry was mixed under shear in a 5 gal Ross mixer for 1 hr. Each slurry was added to molten bishydroxyethylene terephthalate (BHET) in a quantity such that the desired final concentration of each additive was produced. After the slurries were added, a conventional polymerization catalyst was added. The molten monomer was polymerized at 285°C to 290°C at a pressure of about 0.5 mmhg. The resulting polyethylene terephthalate (PET) resin was cooled and converted to chips. The dried chips of PET resin were extruded into a film at 285°C and then biaxially oriented by stretching in sequence in mutually perpendicular directions at draw directions of about 2.9: 1 in each direction. The film was heat set at 225°C. The resulting film had a thickness of 12 microns. The film contained 6 ppm of calcined silicone particles and 50 ppm (0.005%) of fumed silica agglomerates. The film had excellent optical and winding properties. Clarity was excellent. Haze was 0.5%, as measured by a Gardner Hazemeter. Surface roughness, measured over a 235 x 312 micron area at a magnification of 20X with a Wyko 3D interferometer, was: R- j = 10.4 nm; Rq = 12.3 nm; and R t = 704.3 nm. Using techniques well known in the art, this film, as well as the other films described, in the Examples, can be metalized with, for example gold or aluminum, for use in solar window applications. EXAMPLE 2 The procedure of Example 1 was repeated except that the film contained 6 ppm of calcined silicone particles and 1650 ppm (0.165%) of fumed silica agglomerates. The film had excellent optical and winding properties. Clarity was excellent. Haze was 1.0%. Surface roughness was: R a = 20.6 nm; R„ = 27.6 nm; and R t = 908.3 nm. EXAMPLE 3 The procedure of Example 1 was repeated except that the film contained 3 ppm of calcined silicone particles and 830 ppm (0.083%) of fumed silica agglomerates. The film had excellent optical and winding properties. Clarity was excellent. Haze was 0. 8%. Surface roughness was: R a = 17.6 nm; R q = 19.5 nm; and R t = 807.7 nm. EXAMPLE 4 The procedure of Example 1 was repeated except that the film contained 6 ppm of calcined silicone particles and 1250 ppm (0.125% ) of fumed silica agglomerates. The film had excellent optical and winding properties. Clarity was excellent. Haze was 1.1%. Surface roughness was: R a -= 19.7 nm; R q = 22.3 nm; and R t = 870.3 nm. EXAMPLE 5 The procedure of Example 1 was repeated except that the film contained 20 ppm of calcined silicone particles and 2200 ppm (0.22%) of fumed silica agglomerates. The film had excellent optical and winding properties. Clarity was excellent. Haze was 1.4%. Surface roughness was: R a = 26.3 nm; R q = 30.7 nm; and R t = 1061.8 nm. Having described the invention, we now claim the following and their equivalents.

Claims: CLAIMS What is claimed is: 1. A film comprising: (a) about 2 parts per million to about 20 parts per million, based upon the weight of the film, of calcined silicone particles, wherein about 100 percent of the calcined silicone particles have a particle size below about 7 microns; and (b) about 20 parts per million to about 6000 parts per million, based upon the weight of the film, of fumed silica agglomerates, wherein about 100 percent of the agglomerates have an agglomerate size below about 1 micron; wherein the film is an orientated and heat-set polyester film. 2. The film of Claim 1 in which the polyester film is a polyethylene terephthalate film. 3. The film of Claim 2 in which the film comprises about 500 parts per million to 3000 parts per million, based upon the weight of the film, of fumed silica agglomerates. 4. The film of Claim 2 in which about 100 percent of the calcined silicone particles have a particle size below about 6 microns. 5. The film of Claim 4 in which the film comprises about 500 parts per million to 3000 parts per million, based upon the weight of the film, of fumed silica agglomerates. 6. The film of Claim 5 in which the film comprises about 4 parts per million to 10 parts per million, based upon the weight of the film, of calcined silicone particles. 7. The film of Claim 6 in which the film comprises about 500 parts per million to 3000 parts per million, based upon the weight of the film, of fumed silica agglomerates. 8. The film of Claim 7 in which about 100 percent of the calcined silicone particles have a particle size below about 5.5 microns. 9. The film of Claim 8 in which the film comprises about 1400 parts per million to 1800 parts per million, based upon the weight of the film, of fumed silica agglomerates. 10. The film of Claim 2 in which about 100 percent of the calcined silicone particles have a particle size below about 6 microns, at least about 95 percent of the calcined silicone particles have a particle size below about 5 microns, and 50 percent have a particle size below about 2.2 microns. 11. The film of Claim 10 in which the film comprises about 500 parts per million to 3000 parts per million, based upon the weight of the film, of fumed silica agglomerates. 12. The film of Claim 1 1 in which the film comprises about 4 parts per million to 10 parts per million, based upon the weight of the film, of calcined silicone particles. 13. The film of Claim 12 in which the film comprises about 1000 parts per million to 2000 parts per million, based upon the weight of the film, of fumed silica agglomerates. 14. The film of Claim 13 in which the film comprises about 1400 parts per million to 1800 parts per million, based upon the weight of the film, of fumed silica agglomerates. 15. The film of Claim 2 in which the calcined silicone particles are produced by calcining silicone resin particles at about 300°C to about 400°C for about 30 min to about 3 hr. 16. The film of Claim 15 in which (a) about 100 percent of the calcined silicone particles have a particle size below about 6 microns, at least about 95 percent of the calcined silicone particles have a particle size below about 5 microns, and 50 percent have a particle size below about 2.2 microns, and (b) the film comprises about 500 parts per million to 3000 parts per million, based upon the weight of the film, of fumed silica agglomerates. 17. The film of Claim 2 additionally comprising a metalized coating on a surface of said film. 18. The film of Claim 17 in which about 100 percent of the calcined silicone particles have a particle size below about 6 microns, at least about 95 percent of the calcined silicone particles have a particle size below about 5 microns, and 50 percent have a particle size below about 2.2 microns. 19. The film of Claim 18 in which the film comprises about 1000 parts per million to about 2000 parts per million, based upon the weight of the film, of fumed silica agglomerates and about 4 parts per million to about 10 parts per million, based upon the weight of the film, of calcined silicone particles. 20. The film of Claim 18 in which the metalized coating comprises gold or aluminum.

Inventors: SIDDIQUI JUNAID A, SIDDIQUI, JUNAID, A.
Applicants: DU PONT, SIDDIQUI JUNAID A, E.I. DU PONT DE NEMOURS AND COMPANY, SIDDIQUI, JUNAID, A.
Application year: 1998
Main classification: C08K3
IPC codes: C08J5, C08J7, C08K3