Geopolymer composite and expandable vinyl aromatic polymer granulate and expanded vinyl aromatic polymer foam comprising the same

Abstract

A process for the production of a geopolymer composite. The disclosure further relates to a geopolymer composite, and the use of a geopolymer, a geopolymer in combination with an athermanous additive, or the geopolymer composite in expanded vinyl polymer, preferably vinyl aromatic polymer. Furthermore, the disclosure relates to a process for the production of expandable vinyl aromatic polymer granulate, and expandable vinyl aromatic polymer granulate. Finally, the disclosure relates to expanded vinyl foam, preferably vinyl aromatic polymer, and to a masterbatch comprising vinyl polymer and a), b), or c).

Claims

1. Expanded vinyl polymer foam, comprising vinyl polymer and a geopolymer composite derived from geopolymer and comprising athermanous additive, wherein the athermanous additive comprises one or more athermanous additives selected from carbon black, petroleum coke, graphitized carbon black, graphite oxides, graphite and graphene, rutiles, chamotte, fly ash, fumed silica, hydromagnesite/huntite mineral, and mineral having perovskite structure, the foam having a density of from 8 to 30 kg/m.sup.3, and a thermal conductivity (as measured according to ISO 8301) of from 25 to 35 mW/K.Math.m, wherein the geopolymer composite has a particle size of from 0.01 to 200 μm.

2. The expanded vinyl polymer foam of claim 1, wherein the foam comprises vinyl aromatic polymer.

3. The foam of claim 2, having a density in a range of from 8 to 14 kg/m.sup.3 and a thermal conductivity (as measured according to ISO 8301) of from 31 to 34 mW/K.Math.m.

4. The foam of claim 2, having a density in a range of from 17 to 21 kg/m.sup.3 and a thermal conductivity (as measured according to ISO 8301) of from 28 to 31 mW/K.Math.m.

5. The foam of claim 2, wherein the geopolymer composite is produced in a process comprising a) mixing of an aluminosilicate component with an alkaline silicate solution, to form a gel, b) adding of an athermanous additive component to the gel, to form a filled gel, c) mixing of the filled gel, to form filled geopolymer, d) curing, drying and milling, to give filled geopolymer particles, e) removal of cations from the filled geopolymer particles, and f) obtaining the geopolymer composite, wherein the athermanous additive comprises one or more athermanous additives selected from carbon black, petroleum coke, graphitized carbon black, graphite oxides, graphite and graphene, rutiles, chamotte, fly ash, fumed silica, hydromagnesite/huntite mineral, and mineral having perovskite structure.

6. The foam of claim 5, wherein the aluminosilicate component comprises one or more materials selected from the group consisting of metakaolin, metakaolinite, metafly ash, furnace slag, silica fume, mine tailings, pozzolan, kaolin, and building residues.

7. The foam of claim 5, wherein the aluminosilicate component comprises one or more materials selected from the group consisting of metakaolin or metakaolinite, metafly ash, silica fume.

8. The foam of claim 5, wherein the aluminosilicate component is metakaolin or metakaolinite, or a mixture thereof.

9. The foam of claim 5, wherein the athermanous additive component is carbon black, graphite, or a mixture thereof.

10. The foam of claim 5, wherein the alkaline silicate comprises one or both of sodium silicate and potassium silicate.

11. The foam of claim 5, wherein the alkaline silicate is potassium silicate.

12. The foam of claim 5, wherein silane is added to the aluminosilicate component, prior to mixing with the alkaline silicate solution in step a).

13. The foam of claim 12, wherein the silane is selected from aminopropyltriethoxysilane, aminopropyltrimethoxysilane, phenyltriethoxysilane, and mixtures thereof.

14. The foam of claim 5, wherein silane is added to the geopolymer composite, after step e).

15. The foam of claim 14, wherein silane is added to the geopolymer composite after step f).

16. The foam of claim 14, wherein the silane is selected from aminopropyltriethoxysilane, aminopropyltrimethoxysilane, phenyltriethoxysilane, and mixtures thereof.

17. The foam of claim 12, wherein the concentration of silane is in the range of from 0.01 to 10 wt. %, based on the weight of geopolymer composite.

18. The foam of claim 5, wherein step e) comprises removal of cations with an acid solution, and subsequent drying.

19. The foam of claim 5, wherein step e) comprises removal of cations with an acid solution, washing with water, and subsequent drying.

20. The foam of claim 17, wherein the concentration of silane is in the range of from 0.05 to 5 wt. %, based on the weight of geopolymer composite.

21. The foam of claim 20, wherein the concentration of silane is in the range of from 0.1 to 3 wt. %, based on the weight of geopolymer composite.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows SEM micrographs of Ranco 9895 (left-hand column) and Geopolymer Composite 1 (right-hand column) at different magnifications.

(2) FIG. 2 shows a microscopic picture (at a magnification 30×) and the cell size distribution histograms for expanded beads made by an extrusion process (left-hand column) and by a suspension process (right-hand column).

(3) FIG. 3 shows the structure of an ideal cubic perovskite (ABO.sub.3), where A and B represent cations and O represents oxygen anions forming an octahedron.

(4) FIG. 4 shows the thermogravimetric analysis (TGA) of three geopolymer composite samples.

EXAMPLES

Example 1

(5) This example shows the advantages of the invention for the foam derived from extrusion based polymer granulate, with addition of 16 wt. % of geopolymer composite, and its preparation, and for the suspension based composite foam with addition of 10 and 5 wt. % of geopolymer composite, respectively.

(6) 1. Geopolymer Composite Preparation

(7) The components: 572 kg of a mixture which comprises metakaolinite and calcium silicate in an optimum weight ratio (product Baucis L160 from České lupkové závody, a.s.) and 457 kg of sodium water glass were charged into a planetary mixer having a volume 2 m.sup.3 and mixed over 15 min, to obtain a thixotropic pulp. Then, the athermanous additive, namely petroleum coke (Ranco 9895 from Richard Anton KG having a mean diameter particle size of 5 μm, a BET surface area of 28.7 m.sup.2/g, and a sulphur content of 10,100 ppm) was added in an amount of 250 kg, and 163 l of water was added subsequently to the pulp and mixed during the next 15 min. After that, the highly viscous, homogenous pulp was discharged from the mixer do the hermetic moulds made of polypropylene (each portion was 100 kg). The moulds were transported then to the hall (polycondensation hermetic room) and placed on the racks. After 24 hours, the ready blocks (each approx. 100 kg) were removed from the moulds and again placed on the racks and warm air having a temperature of about 70° C. was pumped from the ground floor to the roof of the polycondensation hall. Under these conditions, the geopolymer composite dried over 24 h, and 25 wt. % of water excess was evaporated from the material.

(8) The dried geopolymer composite blocks were then placed into a crushing mill to obtain the ballast form of the material. The ballast with a mean diameter of 10 mm was dried in a rotary cylindrical dryer for 30 min at a temperature of 140° C. In the next step, the ballast was milled, to obtain fine particles (powder) with a mean diameter size of 6 μm, containing D90=10 μm, D99=15 μm, D100=20 μm.

(9) The fine powder (amount of approx. 1000 kg) was then placed in a 5 m.sup.3 tank equipped with a turbine blade agitator. Immediately thereafter, 2000 l of processing water were charged into the vessel and mixing was started simultaneously. An amount of 250 kg of concentrated aqueous hydrochloric acid (37%) was then “drop wise” added to the tank over 30 min. (8.3 kg/min.). The starting pH, as measured before acid addition, was 13, after 60 min. of mixing and elution the final pH was 7.5. The water was filtrated from the powder of geopolymer composite, and a second portion of water was added to dilute remaining sodium chloride, and the slurry was mixed for 30 min. After this time, the pH increased to 7.7.

(10) The slurry (water and powder) was then filtrated partially in filtration press and transferred to a powder drying process to remove the water and to dry the powder. After filtration, the pH of the powder was 7.2. After the powder-drying process, the fine powder possessed the same particle size distribution as after the milling process.

(11) To improve the adhesion of petroleum coke or other carbon based athermanous additive to the geopolymer, 1 wt. % of silane (aminopropyltriethoxysilane or phenyltriethoxysilane) was added to the mixture of metakaolinite and calcium silicate (1 wt. % of silane per amount of mixture) before addition of sodium glass water, and mixed for several min. Special equipment for silanization of powders can be used, for example a twin-cone blender or a vacuum tumble dryer. Alternatively, the silanization can be performed in the slurry reactor, by using for example toluene as liquid medium.

(12) To further improve adhesion and thus dispersion of the final geopolymer composite powder in the expandable vinyl aromatic polymer as obtained by the extrusion process, one can perform silanization of the final powder. The silane phenyltriethoxysilane can be used for this purpose, in a concentration of 1 wt. % calculated per geopolymer composite powder amount.

(13) The mechanical properties, in particular the strength according to standard ISO 679 as used for the measurement of mechanical properties of concrete, were measured to study the influence of silane addition to the geopolymer composite on the cohesion of the composite matrix.

(14) The geopolymer's or geopolymer composite's high ability for absorption of the blowing agent (a typical hydrocarbon) was confirmed by measurement according to standard ASTM C830-00.

(15) 2. Expandable Vinyl Aromatic Polymer Preparation Via an Extrusion Process

(16) A mixture of vinyl aromatic polymer in the form of granules, containing 1.5 wt. % of polymeric brominated flame retardant (Emerald 3000) and 0.3 wt. % of bicumyl, were dosed to the main hopper of the main 32D/40 mm twin-screw co-rotating extruder. The melt temperature in main extruder was 180° C.

(17) The geopolymer composite powder in a concentration of 16 wt. % (containing 25% of Ranco 9895, having a mean particle size of 6 μm and a BET surface area of 20.5 m.sup.2/g) was dosed to the side arm (54D/25 mm) twin-screw co-rotating extruder via two side feeders and the vinyl aromatic polymer (in the form of granules) was dosed to the main hopper of this extruder. The melt containing 40 wt. % of concentrated geopolymer composite was transported to the main extruder. The melt temperature inside the extruder was 190° C.

(18) The blowing agent (n-pentane/isopentane mixture 80/20%) was injected to the main 32D/40 mm extruder downstream from the injection of the melt from the side twin-screw extruder. The concentration of blowing agent was 5.5 wt. %, calculated on total mass of product.

(19) The melt of vinyl aromatic polymer containing flame retardant, bicumyl, geopolymer composite and blowing agent was transported to the 30D/90 mm cooling extruder and pumped through a 60 mm length static mixer, melt pump, screen changer, diverter valve and extruded through the die head with 0.75 mm diameter holes, and underwater pelletized by the rotating knifes. Downstream, the rounded product, a granulate with a particle size distribution of 99.9% of the fraction 0.8-1.6 mm was centrifuged to remove the water, and was finally coated by the suitable mixture of magnesium stearate with glycerine monostearate and tristearate. The melt temperature in the cooling extruder was 170° C.

(20) The coated beads were expanded to measure the final general properties of expanded foam composite: thermal conductivity according to standard ISO 8301. mechanical properties (compressive and bending strength) according to standard EN 13163. flammability according to tests methods: EN ISO 11925-2 and DIN 4102 B1, B2. dimensional stability under specified temperature and humidity conditions of expanded foam were determined according to standard PN-EN 1604+AC, which is normally used for XPS materials. the total water content in the expandable polymer particles was determined by the standard Karl Fischer titration method according to ASTM E203.

(21) The expandable granulate with a particle size distribution 0.8 to 1.6 mm was in the pre-expander vessel treated for 50 sec. with steam having a pressure of 0.2 kPa, and was then dried in a connected fluid bed drier. The obtained beads' density was 17 kg/m.sup.3. Then the expanded beads were conditioned in a silo for h and introduced to the block mould with dimensions of 1000×1000×500 mm. Steam having a pressure of 0.7 kPa was used to weld the beads, and to obtain moulded blocks having a density of 17.5 kg/m.sup.3. The mould cooling time in this case was 70 sec. The ready block was cut into plates and then specimens after 6 days of conditioning at room temperature.

(22) 3. Expandable Vinyl Aromatic Polymer Preparation Via a Suspension Process

(23) To a 50 l, 20 bar reactor vessel, equipped with frame agitator, 20 kg of styrene monomer were charged. Geopolymer composite (as produced according to Example 1 but silanized with 1 wt. % of vinyltriethoxysilane) was introduced into the reactor part-by-part in equal portions of 4 kg each. The composition was mixed, to obtain a homogenous paste, and 0.5 wt. % of benzoyl peroxide were added, as well as dicumyl peroxide in the same 0.5 wt. % concentration. The reactor was closed and a nitrogen pressure of 1 bar was established. The intensively mixed mass was heated to 100° C. and kept at that temperature for 40 min. After that, a melt pump transferred the melt to the 54D/25 mm co-rotating twin-screw extruder with the speed to provide 10 wt. % concentration of geopolymer composite in the melt. An amount of 30 wt. % per total mass of masterbatch of geopolymer composite powder (silanized with 2 wt. % of vinyltriethoxysilane) was introduced to the extruder by the side feeder before the melt from the extruder was injected. High molecular weight vinyl aromatic polymer (Mn of 80 kg/mol) was dosed to the main hopper at the first zone of extruder. In this way, the copolymer with geopolymer composite (copolymerized with styrene and grafted with polystyrene) was obtained in the form of masterbatch granules, containing 40 wt. % of geopolymer composite in the polymer matrix. High shear force and processing temperature of 190° C. set-up for all zones in extruder provided good reaction capability. A masterbatch with mean granules size of 3 mm was produced by an underwater pelletization method.

(24) An amount of 2.15 kg of the 40 wt. % concentrated masterbatch (10 wt. % of geopolymer composite) was then placed into 6.2 kg of styrene, combined in a 20 l reactor equipped with 4-blade turbine agitator, and mixed slowly, and 0.002 wt. % of divinylbenzene, 1 wt. % of Emerald 3000, 0.3 wt. % of Polywax 1000 and 0.5 wt. % of dicumyl peroxide were then charged into the mixture.

(25) The mixture was heated relatively quickly to a temperature of 70° C. and mixed at this temperature for 30 min with 275 rpm. Then, the temperature was increased to 90° C. and 9 l of demineralised water (temperature of 60° C.) were added. The mixing force immediately created a suspension of prepolymer and the suspension was heated to 82° C. Immediately, 0.3 wt. % of Peroxan PO and 0.5 wt. % of TBPEHC were added. The radical polymerization was started and the following surfactant composition was introduced: potassium persulfate—0.0001 wt. % Poval 205-0.18 wt. % of 5% concentrated water solution Poval 217 (alternatively Poval 224)—0.09 wt. % of a 5% concentrated water solution DCloud 45—0.1 wt. % Arbocel CE 2910HE50LV—0.1 wt. % (hydroxypropylmethylcellulose supplied by J. RETTENMAIER & SÖHNE GMBH)

(26) The polymerization was then continued for 120 min. at a temperature of 82° C., and the temperature was then increased to 90° C. The suspension was kept at this temperature for 120 min. to achieve particle identity point of suspension. A further portion of Poval 217 (in a concentration of 0.3 wt. % of a 5 wt. % concentrated solution in water) was introduced and the reactor was filled with 0.5 l of demineralised water. In this step, the sodium chloride can be added in an amount of 0.5 wt. % per water phase, to reduce the water content in the polymer. Alternatively, the surfactant (sodium dodecylbenzenesulfonate, SDBS) can be used in an amount of 0.2 wt. %.

(27) The reactor was closed and an n-pentane/isopentane 80/20% mixture in amount of 5.5 wt. % was added over 60 min. Simultaneously, the temperature was increased to 125° C. Then the polymerization was continued for 120 min. and after that time the suspension slurry was cooled down to 25° C.

(28) The product was removed from the reactor and water was removed in a basket centrifuge. The particles were then dried in a fluid bed drier at a temperature of 40° C. for 30 min. and fractionated on 80% of particles fraction 0.8-1.6 mm, 15% of 0.3-1.3, 4% of 1.0-2.5 mm and 1% of upper and lower size. Fractions were then coated the same way as the product as obtained in the extrusion process, and then expanded to foam.

Example 2

(29) This example is comparable to Example 1 but with a lower amount of geopolymer composite in the foam, and with a higher content of carbon based athermanous additive in the composite.

(30) An expandable granulate was produced with the same conditions and process as in Example 1, except that 10 wt. % of geopolymer composite containing 40 wt. % of Ranco 9895 and having a mean particle size of 6 μm and BET surface of 10 m.sup.2/g was used.

Example 3

(31) This example shows the influence of silanization on the cohesiveness of geopolymer composite and the mechanical properties of the foam. The example is comparable to Example 1.

(32) Expandable particles were produced with the same conditions, ingredient concentrations and process as in Example 1, except that aminopropyltriethoxysilane was added in a concentration of 2 wt. % per amount of used Baucis L160. Additionally, the ready geopolymer composite powder was silanized with phenyltriethoxysilane in a concentration of 1 wt. % per dry weight of powder.

Example 4

(33) In this example, a different athermanous additive was used. This example shows that the same or very similar foam properties can be obtained independently.

(34) Expandable particles were produced with the same conditions and process as in Example 1, except that, instead of petroleum coke, 20 wt. % of Monarch 460 carbon black having a BET surface of 71.8 m.sup.2/g and 5600 ppm of sulphur was used to prepare the geopolymer composite. Moreover, the ready composite powder with a mean diameter 7 μm and BET surface 21.3 m.sup.2/g in concentration of 15 wt. % was added to the total composition.

Example 5

(35) This example compares the influence of pure geopolymer addition on the structure and foam composite properties and shows that properties are very similar to those examples were geopolymer composites were used.

(36) Expandable particles were produced with the same conditions as in Example 1, except that pure geopolymer (without addition of any athermanous filler) was prepared and added to the expandable vinyl aromatic polymer in a concentration of 10 wt. % calculated per total mass of expanded foam composite.

Example 6

(37) This example shows that carbon-based athermanous additives (which would otherwise deteriorate the self-extinguishing properties of expanded foams made of expandable vinyl aromatic polymers) are completely inert when they are well encapsulated in the geopolymer matrix, thus a reduction of the concentration of flame retardant is advantageously possible in accordance with the invention.

(38) Expandable particles were produced under the same conditions as in Example 1, except that the Emerald 3000 flame retardant was added in a concentration of 1 wt. %.

Example 7

(39) This example is further focused to show that an even lower amount of flame retardant may be added when thermal and thermo-oxidative stabilizers are incorporated into the mixture.

(40) Expandable particles were produced with the same conditions as in Example 1, except that the thermo-oxidative stabilizers were added in a concentration of 0.04 wt. % of Irgafos 126 and 0.04 wt. % of Irganox 1010, and 0.08 wt. % of Epon 164 as HBr acid scavenger, and 0.32 wt. % of F-2200 HM as thermal stabilizer for Emerald 3000 (according to ICL recommendation) were used. The flame retardant concentration was decreased down to 0.8 wt. %.

Example 8

(41) In this example, a complex geopolymer composite was prepared and used in the preparation of expanded foam. The example was performed in particular to show the influence of geopolymer composite based on a mixture of ilmenite, rutile and carbon black on the thermal conductivity reduction of the foam.

(42) Expandable particles were produced with the same conditions as in Example 1, except that 10 wt. % of Monarch 460, 10 wt. % of synthetic rutile from Iluka with a mean particle size of 5 μm, and 10 wt. % of ilmenite (standard grade from Titania AS, Norway with a mean particle size of 5 μm) were used to prepare the geopolymer composite with a mean particle size of 6 μm, and the geopolymer composite was used in an amount of 15 wt. %. Emerald 3000 concentration was reduced to 1.25%.

Example 9

(43) In this example, a geopolymer composite with perovskite, a barium titanate (BaTiO.sub.3), with small inclusion of hydromagnesite/huntite (product UltraCarb 1250) and chamotte was prepared. A very small amount of flame retardant was used then too (0.7 wt. %).

(44) Expandable particles were produced with the same conditions as in Example 1, except that 30 wt. % of barium titanate with a mean particle size of 5 μm and 5 wt. % of hydromagnesite/huntite with a mean particle size of 2.6 μm and 5 wt. % of chamotte with a mean particle size of 6 μm from České lupkové závody, a.s. were used to prepare the geopolymer composite with a mean particle size of 6 μm, which was added in a concentration of 10 wt. %.

Example 10

(45) In this example, the suspension process specified in Example 1, point 3 was used to prepare the expanded foam; 10 wt. % of geopolymer composite were incorporated. The organic stabilization system for suspension was used.

Example 11

(46) In this example, the suspension was prepared according to Example 10, except that the geopolymer composite concentration was reduced to 5 wt. % and the inorganic stabilization system was based on potassium persulfate and tricalcium phosphate.

(47) The following table (Table 1) lists the compositions of the seven different compositions of geopolymer composite used in Examples 1 to 11. Properties of the resultant geopolymer composites are shown below in Table 5.

(48) TABLE-US-00001 TABLE 1 Geopolymer composite compositions. No. 1 2 3 4 5 6 7 Unit wt. % Silanes YES Geopolymer matrix 75 60 75 80 100 70 60 (%) Ranco 9895 25 40 25 Monarch 460 20 10 Synthetic rutile 10 Ilmenite 10 Barium titanate 30 Hydromagnesite 5 Chamotte 5

(49) TABLE-US-00002 TABLE 2 Examples summary-foam from an extrusion process. Examples 1 2 3 4 5 6 7 8 9 Unit wt. % Synthos PS 585X YES YES YES YES YES YES YES YES YES Aminopropyltriethoxysilane — — 1/ — — — —     powder Phenyltriethoxysilane — — 2/ — — — — — — powder Geopolymer composite 16 10 16 15 10 16 16 15 10 (type) (1) (2) (3) (4) (5) (1) (1) (6) (7) Emerald 3000 1.5 1.5 1.5 1.5 1.5 1.0 0.8 1.25 0.7 Bicumyl 0.3 0.3 0.3 0.3 0.3 0.3 0.18 0.28 0.16 F 2200 HM — — — — — — 0.32 — — Irganox 1010 — — — — — — 0.04 — — Irgafos 126 — — — — — — 0.04 — — Epon 164 — — — — — — 0.08 — — Polywax 2000 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Pentane/Isopentane 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 80/20

(50) TABLE-US-00003 TABLE 3 Examples summary-foam from a suspension process. Ex. Ex. Examples 10 11 Unit wt. % Aminopropyltriethoxysilan 1/powder 1/powder Phenyltriethoxysilane 2/powder 2/powder Geopolymer composite (type) 10 (1) 5 (1) Emerald 3000 0.7 0.7 Polywax 1000 0.3 0.3 Pentane/Isopentane 5.5 5.5 80/20 Other components According to Example 1, point 3

(51) In general, various mixtures are possible, and for all possibilities, the same or similar structure will be obtained, as well mechanical properties, foaming, and block moulding parameters. The difference will be only in thermal conductivity, as shown below in Table 4.

(52) TABLE-US-00004 TABLE 4 Expanded foam composite parameters at ca. 17.0 kg/m.sup.3. Examples 1 2 3 4 5 6 7 8 9 10 11 Cell size 30/ 40/ 30/ 40/ 30/ 30/ 50/ 40/ 50/ 40/ 40/ distribution 90 80 100 90 100 90 110 120 120 90 90 (μm) Dimensional 0.1 0.5 0.0 0.2 0.1 0.0 0.5 0.3 0.0 0.5 0.4 stability at temp. 70° C. and humidity 50 ± 5 % (% of shape change) Thermal 30.2 30.0 30.3 29.9 33.5 30.1 30.5 30.0 32.0 30.5 31.0 conductivity (mW/m .Math. K) Flammability + + + + + + + + + + + (EN standard) Flammability +/+ +/+ +/+ +/+ +/+ +/+ +/+ +/+ +/+ +/+ +/+ (DIN B1/B2 standard) Compressive 102 89 111 98 91 100 98 101 125 76 87 strength at 10% def. (kPa) Bending strength 193 175 199 186 172 201 195 188 221 171 184 (kPa) Water content 0.1 0.07 0.05 0.15 0.11 0.1 0.12 0.09 0.1 0.1 0.1 (%) Passed (+ or B2 or B1); Not passed (− or B2 or B1)

(53) TABLE-US-00005 TABLE 5 Geopolymer and geopolymer composite parameters. Geopolymer composite 1 2 3 4 5 6 7 Density (g/cm.sup.3) 2.58 1.48 2.60 2.23 3.22 4.2 4.1 Compresive 26 17 71 30 44 53 62 strength (MPa) Water content (%) 2.5 1.0 2.0 4.0 1.9 2.1 3.1 Blowing agent 13.1 8.0 12.2 14.9 19.4 11.0 14.2 absorption (%)