METHODS FOR MODIFYING THE OPTICAL APPEARANCE OF POLYMERS

Abstract

The disclosure relates to a method for modifying the optical appearance of a polymer, the method comprising extruding a molten polymer comprising polypropylene and a composition comprising acid scavengers consisting of zinc oxide and zinc carbonate.

Claims

1. A method for modifying the optical appearance of a polymer, the method comprising extruding: a molten polymer comprising a polypropylene; and a composition comprising acid scavengers consisting of zinc oxide and zinc carbonate, wherein the zinc oxide and the zinc carbonate are present in the same particle.

2. The method of claim 1, wherein modifying the optical appearance of a polymer comprises suppressing color, maintaining or stabilizing color with time and/or with the number of extrusion steps, reducing haze, increasing gloss and combinations thereof.

3. The method of claim 1, wherein the composition has a BET surface area from 10 m.sup.2/g to 100 m.sup.2/g.

4. The method of claim 1, wherein the composition comprises 90% to 99% by weight of zinc oxide, and 1% to 10% by weight of zinc carbonate.

5. The method of claim 1, wherein the composition comprises a carbon content from 0.1 wt % to 1.0 wt % carbon with respect to the total weight of the composition, the carbon content being determined as described on page 14, lines 24-29 of the description.

6. The method of claim 1, wherein the composition further comprises at least one antioxidant.

7. The method of claim 1, wherein the at least one antioxidant is selected from the group comprising phenolic antioxidants, phosphite antioxidants, amine antioxidants, hydroxylamine antioxidants, phosphonite antioxidants, benzofuranone antioxidants, thiodipropionate antioxidants, acryloyl antioxidants and combinations thereof.

8. The method of claim 1, the method further comprising adding the composition to the polymer so that the amount of the composition added to the polymer ranges from 1000 ppm to 5000 ppm with respect to the amount of the polymer.

9. The method of claim 1, the method comprising a plurality of extrusion steps.

10. The method of claim 1, wherein the polypropylene is selected from the group comprising polypropylene homopolymers, random copolymers comprising propylene, heterophasic copolymers comprising propylene and combinations thereof.

11. A polymeric composition comprising a polymer modified by the method of claim 1.

12. (canceled)

13. (canceled)

14. (canceled)

Description

BRIEF DESCRIPTION OF DRAWINGS

[0087] FIG. 1 illustrates the yellowness index of a polymer composition obtained by a method according to one embodiment disclosed herein compared to comparative examples.

[0088] FIG. 2 illustrates the gloss of an article comprising a polymer composition obtained by a method according to one embodiment disclosed herein compared to comparative examples.

[0089] FIG. 3 illustrates the haze of an article comprising a polymer composition obtained by a method according to one embodiment disclosed herein compared to comparative examples.

[0090] In each figure, error bars are shown indicating empirical standard deviations.

DESCRIPTION OF EMBODIMENTS

[0091] The following examples of methods for modifying the optical appearance of polymers, in particular polypropylene, are given for illustrating but not limiting purposes.

[0092] The examples show properties of polymers obtained by performing methods for modifying the optical appearance of polymers according to embodiments of the present disclosure. In particular, the examples show the improved optical appearance obtained by the methods according to embodiments of the present disclosure compared to three conventional methods using conventional calcium stearate, conventional zinc oxide and conventional zinc carbonate, respectively. Indeed, the examples of methods according to embodiments of the present disclosure show lower yellowness index both after compounding and upon several extrusion steps of the polymer as determined according to ASTM D6290, higher gloss as determined according to ISO 2813 and lower haze as determined according to ISO 14782 with respect to polymers treated in accordance with conventional methods.

[0093] In the following examples, a method for modifying the optical appearance of isotactic polypropylene homopolymer under extrusion conditions will be described. However, different polymers comprising polypropylene may be treated by the method of the present disclosure. Also, compositions will be described comprising zinc oxide, zinc carbonate and two antioxidants. However, different antioxidants and further common additives may be used in accordance with one or more embodiments of the method of the present disclosure. By way of example, also stabilizers and/or additional additives may be used in accordance with one or more embodiments of the method of the present disclosure.

[0094] Each exemplary composition was fed with an exemplary polymer powder through a hopper directly into an extruder. Together with the composition and the polymer powder, any stabilizers and/or additional additives may be also fed through the hopper into the extruder. The exemplary polymer and compositions were extruded in a Brabender lab twin-screw extruder KEDSE 20/40 D with an L/D (extruder Length/screw Diameter) of 40, at an extrusion temperature of 230° C., at a screw speed of 120 rpm, and at an extrusion pressure of bar. The polymer and the composition were mixed by the screw of the extruder.

[0095] The following methods were used to determine the properties reported in the examples.

[0096] Melt Flow Rate (MFR) was measured according to ISO 1133 with a load of 2.16 kg at 230° C.

[0097] The carbon content was determined by detecting the release of carbon dioxide upon treatment of the respective powders with phosphoric acid (25% v/v), washing/drying the released gas with sulfuric acid/potassium permanganate/silver nitrate/silver wool in consecutive wash and dry towers, and by using a nondispersive infrared sensor (NDIR). The employed carrier gas was nitrogen and the powder samples were encapsulated in gelatin prior to measurements.

[0098] The theoretical zinc carbonate content was estimated by assuming that all carbon stemmed from zinc carbonate (theoretical zinc carbonate content=carbon content×molar mass of zinc carbonate/molar mass of carbon).

[0099] The BET surface areas were determined using a Micromeritics ASAP 2420 instrument at the pressure range p/p.sub.0 from 0.06 to 0.2 by acquiring five points and performing a linear regression according to Brunauer-Emmett-Teller (BET) theory (Micromeritics Operator's Manual, Brunauer, S.; Emmett, P. H.; and Teller, E., J. Am. Chem. Soc. 60, 309 (1938)). The analysis adsorptive was nitrogen gas, the analysis bath temperature was 77.350 K, the ambient temperature was 22.00° C., automatic degassing was turned on, thermal correction was turned off and the equilibration interval was 10 s.

[0100] For each point designated for surface area calculations, the quantity of adsorbed gas Q was measured, and f(Q, p.sub.0/p) was calculated:

[00001]$f\left(Q,p_0/p\right)=\frac{1}{Q\times \left(p_0/p-1\right)}$

[0101] where f(Q, p.sub.0/p) is in the units of g/cm.sup.3 STP and Q is in the units of cm.sup.3/g STP.

[0102] A least square-fit was performed on the (p/p.sub.0, f(Q, p.sub.0/p)) designated pairs where p/p.sub.0 was the independent variable and f(Q, p.sub.0/p) was the dependent variable. The following were determined: [0103] a) Slope S in g/cm.sup.3 STP, [0104] b) Y-intercept Y.sub.INT g/cm.sup.3 STP,
Using the results of the above calculations, the BET surface areas SA.sub.BET were calculated:

[00002]$S{A}_{BET}=\frac{CSA\times \left(6.023\times 10^{23}\right)}{\left(22414{\mathrm{cm}}^{3}STP\right)\times \left(10^{18}{\mathrm{nm}}^2/m^2\right)\times \left(S+Y_{INT}\right)}$

[0105] where CSA=0.1620 nm.sup.2 (molecular cross-sectional area (nm.sup.2) of nitrogen).

[0106] The particle size distributions were determined using a Malvern Mastersizer 3000 with a Malvern Aero S dry dispersion unit. The pressure of the dispersing unit (Venturi) was set to 3.0 bar, and the dosing system was adjusted to achieve an obscuration value of 0.5-10%. The experimental diffraction data was analyzed according to Fraunhofer theory as described in ISO 13320. Particle size distribution percentiles D.sub.10 (equivalent to ×10), D.sub.50 (equivalent to ×50), D.sub.90 (equivalent to ×90) were calculated according to ISO 9276.

[0107] Color formation after compounding and extrusion of pellets of the polymer was determined by the yellowness index (YI) of the polymer pellets. To determine the yellowness index, a color determination according to ASTM D6290 with a Group I Spectrophotometer, the LabScan XE from Hunterlab, in the range of 400-700 nm, with a resolution of 10 nm, and with a D65/10° arrangement of Illuminant/Observer was performed. A sample cup was filled to the top with pellets, placed on the sensor port and covered with an opaque and light excluding cover. The measurement delivered the Tristimulus values X, Y and Z. The calculation of the yellowness index was done according to ASTM E313 by the following equation: YI=100 (Cx X−Cz Z)/Y, where the coefficients Cx and Cz were selected according to the setting of Illuminant and Observer used for the measurement of the Tristimulus values. For Illuminant D.sub.65 and Observer 10°, Cx is 1.3013 and Cz is 1.1498.

[0108] Haze measurements were performed on a BYK Haze-gard plus according to ISO 14782.

[0109] Gloss measurements were performed on a BYK micro-TRI-gloss at angles of 20° and 60° according to ISO 2813.

[0110] For haze and gloss measurements, injection molded plaques having a thickness of 1 mm were prepared according to ISO 294, and were measured directly upon preparation.

Comparative Examples 1-3 and Example 4 According to the Disclosure

[0111] Different polymers were prepared by compounding isotactic polypropylene homopolymer powder (MFR 12 g/10 min, ISO 1133) with different compositions including respective additives acting as acid scavengers as well as additives acting as antioxidants. The isotactic polypropylene homopolymer was prepared by a vertically stirred gas-phase polymerization process using a Ziegler-Natta-catalyst. The compositions are identified in Table 1.

TABLE-US-00001 TABLE 1 Example Polymer matrix Composition Comparative Isotactic Calcium stearate (500 ppm), Example 1 polypropylene Phosphite-168 (500 ppm), homopolymer Phenolic AO-1010 (1000 ppm) Comparative Isotactic Zinc oxide (500 ppm), Example 2 polypropylene Phosphite-168 (500 ppm), homopolymer Phenolic AO-1010 (1000 ppm) Comparative Isotactic Zinc carbonate (500 ppm), Example 3 polypropylene Phosphite-168 (500 ppm), homopolymer Phenolic AO-1010 (1000 ppm) Example 4 Isotactic Composition of zinc oxide according to the polypropylene and zinc carbonate (500 ppm), disclosure homopolymer Phosphite-168 (500 ppm), Phenolic AO-1010 (1000 ppm)

[0112] The components used were the following commercially available products:

[0113] Phosphite-168: tris(2,4-di-tert-butylphenyl) phosphite (Irgafos 168® commercially available from BASF)

[0114] Phenolic AO-1010: pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (Irganox 1010® commercially available from BASF)

[0115] Calcium stearate: Ligastar CA350® commercially available from Peter Greven

[0116] Zinc carbonate: Zinc Carbonate RAC® commercially available from Bruggemann,

[0117] Zinc oxide: White Seal S® commercially available from Bruggemann,

[0118] Composition of zinc oxide and zinc carbonate: Zinc Oxide AC 45® commercially available from Bruggemann.

[0119] Prior to compounding, the components of each composition were tumble mixed with the polymer powder for 2 h to ensure good dispersion thereof. The mixture of polypropylene powder and composition of each example was fed in the hopper of the extruder at a feed rate of 3 kg/h. The obtained extrudates were pelletized (MFR of 13 g/10 min to 14 g/10 min of the final pellet) and subsequently subjected to all further tests including multiple extrusion steps, yellowness, gloss, and haze measurements.

[0120] Carbon content, (theoretical) zinc carbonate content, BET surface area, and particle size distribution parameters of representative batches of the zinc-based components are summarized in Table 2.

TABLE-US-00002 TABLE 2 Composition of zinc oxide and Parameter Zinc oxide Zinc carbonate zinc carbonate Carbon content [wt %] 0.0 3.0 0.5 (theoretical) zinc 0.1 31.4 4.7 carbonate content [wt %] BET Surface area 6.1 135.0 49.6 [m.sup.2/g] D.sub.10 [μm] 0.4 1.3 0.8 D.sub.50 [μm] 6.2 24.1 3.7 D.sub.90 [μm] 28.1 65.4 11.2

[0121] In FIG. 1 and Table 3, results of yellowness index measurements on the polymeric compositions of Comparative Examples 1-3 and Example 4 upon initial compounding (Y-0) and the corresponding increase of the yellowness index upon further extrusion steps (YI-1 to YI-5) are shown.

TABLE-US-00003 TABLE 3 Example YI-0 [—] YI-1 [—] YI-3 [—] YI-5 [—] Comparative Example 1 0.3 11.1 24.9 37.9 Comparative Example 2 1.9 13.0 21.3 27.0 Comparative Example 3 1.8 11.4 19.1 24.2 Example 4 according to −0.1 6.4 13.8 20.6 the disclosure

[0122] The composition of Example 4 shows the lowest initial yellowness index after compounding (Y-0) and the lowest increase in the yellowness index upon one, three and five extrusion steps. For example, YI-5 of Example 4 is 45% lower than YI-5 of Comparative Example 1, and 24% lower than YI-5 of Comparative Example 2. Hence, Example 4 shows the best initial color and color-hold stability of all tested compositions.

[0123] In FIG. 2 and Table 4, results of gloss and haze measurements on the polymeric compositions of Comparative Examples 1-3 and Example 4 are shown.

TABLE-US-00004 TABLE 4 Gloss at Gloss at Example 20° [—] 60° [—] Haze [%] Comparative Example 1 87.0 102.1 41.3 Comparative Example 2 82.5 94.0 56.1 Comparative Example 3 87.1 99.3 45.0 Example 4 according to 90.6 102.9 41.1 the disclosure

[0124] As shown, the use of the composition of Example 4 led to significantly higher gloss values compared to both Comparative Example 2 and Comparative Example 3. Accordingly, the composition of Example 4 has an optical appearance and properties which are particularly suitable for manufacturing articles such as slit tapes or BOPP films.

[0125] Furthermore, the composition of Example 4 showed gloss values comparable to the gloss values obtained with calcium stearate, but without phenomena of surface migration. Accordingly, the composition of Example 4 is particularly suitable for printability and raffia applications.

[0126] The results of haze measurements on the polymeric compositions of Comparative Examples 1-3 and Example 4 are also shown in FIG. 3. As shown, the use of the composition of Example 4 led to improved properties also in terms of haze value, which was 25% lower than the value obtained in the case of Comparative Example 2. Accordingly, the composition of Example 4 is particularly suitable for high transparency applications, such as thermoformed articles.

[0127] With reference to the high gloss and low haze values of Example 4, these results were particularly unexpected. Indeed, enhancing both these properties is surprising, among others, because the composition of Example 4 contains inorganic additives which are insoluble in the polymer matrix.

[0128] The results of the modification of the optical appearance of polypropylene show an improved optical appearance with respect to each of the conventional methods using conventional calcium stearate, conventional zinc oxide and conventional zinc carbonate, respectively, in terms of yellowness index after compounding and upon several extrusion steps, gloss and haze.

[0129] While the different aspects of the present disclosure have been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.