WO2008009607A1

WO2008009607A1 - Concentrated gypsum suspensions for coating colour of paper

Info

Publication number: WO2008009607A1
Application number: PCT/EP2007/057101
Authority: WO
Inventors: Riccardo Vago; Stefano Fumagalli; Giuseppe Li Bassi
Original assignee: Lamberti Spa
Priority date: 2006-07-18
Filing date: 2007-07-11
Publication date: 2008-01-24

Abstract

Gypsum aqueous suspensions having Brookfield viscosity at 25°C and 10 rpm comprised between 100 and 1,000 mPa*s containing: a. from 60 to 85% by weight of calcium sulfate dihydrate (Ca2SO4.2H2O); b. from 0.1 to 2.0% by weight of a polynaphthalene sulfonate or of an acrylic sulfonated polymer; c. from 0.1 to 2.0% by weight of an organic polyphosphonate are useful for the preparation of coating colour compositions and of matt coated paper.

Description

CONCENTRATED GYPSUM SUSPENSIONS FOR COATING

COLOUR OF PAPER.

[0001] The present invention relates to concentrated gypsum suspensions for coating colour of paper and paperboard. [0002] It is known that a coating colour, i.e. a layer of pigments and additives, is usually applied on the surface of printing paper to improve its characteristics; the coating colour renders the paper smooth and confers it a good printability. [0003] Aqueous suspensions of gypsum (Ca₂SO₄2H₂O), calcium carbonate (CaCO ₃), kaolin

(Al₂O₃.SiO₂.H₂O) and their mixtures, are usually employed to formulate coating colours. [0004] In the case of gypsum, many problems are encountered when preparing concentrated suspensions, because the mixture of gypsum and water does not maintain its fluidity over time and has the tendency to rapidly set. [0005] To improve the viscosity and stability of gypsum suspensions, many solutions have been proposed, mainly based on the use of specific complexing agents and dispersants. [0006] We cite, among the several patents published on this issue:

• DE 3203067, where the effect of the addition of two different polymers, at least one of which is a polysaccharide, on the preparation of aqueous gypsum suspension is described;

• DE 36086370, where aqueous suspensions of gypsum containing amines, ammonium salts or potassium salts are described;

• EP 216 516, where the aqueous suspensions contain at least 55% of a mixture of gypsum with a different pigment and the dispersing agent is a phosphoric ester or a water soluble partially esterified polyacrylic acid;

• DE 3815281, where the stabilisers are phosphoric esters of alcohols having at least 11 carbon atoms;

• GB 2173781, where gypsum aqueous suspensions containing a surfactant agent and a complexing agent are described;

• US 4927463, reporting the use of organic phosphoric esters acting as surfactants to stabilise gypsum aqueous suspensions.

[0007] All the above cited patents are related to the preparation of concentrated gypsum suspensions for use in coating colour: in this applicative field, low viscosity, high concentration (better if higher than 70%), absence of foam during preparation and use, and stability are requested.

[0008] With the term stability we mean easily re-dispersability of the suspension by simple stirring, and not lack of sedimentation.

[0009] All proposed solutions suffer from some disadvantage; aqueous gypsum suspensions having concentrations higher than 70% by weight, by way of example, are described in GB 2173781 and US 4927463; nonetheless these suspensions contain foaming surface active components.

[0010] It is therefore highly desirable to provide the operators of this field with stable, low viscous, no foaming aqueous concentrated gypsum suspensions which are mixable with suspensions based on other pigments.

[0011] It has now been found that it is possible to obtain low viscous, highly concentrated aqueous gypsum suspensions by adding gypsum in the form of hemihydrate or anhydrous salt to an aqueous solution containing: a) a polyacrylic sulfonated dispersant or a polynaphthalene sulfonate dispersant and b) an organic poly- phosphonate.

[0012] In the building industry it has been known for a long time to use several setting retarders for aqueous gypsum compositions.

[0013] The possibility of using a non sulfonated polyacrylate having low molecular weight for this scope is cited in US 6,805,741; nonetheless, this patent is referred to stuccoes or mixtures having very high viscosity, varying from 5,000 to 48,000 mPa _*s, which are not suited for the use in paper production.

[0014] It is therefore a fundamental object of the present invention a gypsum aqueous suspension having Brookfield viscosity at 25⁰C and 10 rpm comprised between 100 and 1,000 mPa_*s containing: a. from 60 to 85% by weight of calcium sulfate dihydrate (Ca₂SO₄^H₂O); b. from 0.1 to 2.0% by weight of a polyacrylic sulfonated dispersant or of a polynaphthalene sulfonate; c. from 0.1 to 2.0% by weight of an organic poly- phosphonate.

[0015] The viscosity value at 10 rpm is particularly relevant because high Brookfield viscosity values at 10 rpm have a detrimental effect on the return of the coating colour in the recycles after the coating heads that occurs by simple gravity drop.

[0016] Preferably, the Brookfield viscosity at 25⁰C and 100 rpm of the aqueous gypsum suspensions is less than 500 mPa_*s, more preferably it is comprised between 100 and 500 mPa_*s when a polyacrylic sulfonated dispersant is used and from 50 to 200 mPa _*s when a polynaphthalene sulfonate dispersant is used.

[0017] The acrylic sulfonated polymers useful for the preparation of the suspensions of the present invention are obtained by polymerisation of acrylic and/or methacrylic acid (otherwise indicated with the expression (meth)acrylic acid), in acidic or salt form, with a monomer containing a sulfonic functional group and have a molecular weight from 5,000 to 40,000 dalton (measured with an acrylic acid standard); they are well known polymers, available on the market, and sold by way of example in the form of aqueous slurries by Lamberti SpA, Italy; they can be used in such form for the preparation of the gypsum suspensions of the invention.

[0018] Preferably the molar ratio between (me th) acrylic acid and monomer containing a strongly acidic functional group is from 3 to 30, preferably from 3 to 10.

[0019] More preferably, the sulfonated acrylic polymer is a copolymer of acrylic acid, methacrylic acid and 2-acrylamido-2-methyl-l-propanesulfonic acid, where the molar ratio between acrylic and methacrylic acid is from 2 to 8.

[0020] The polynaphthalene sulfonates useful for the realisation of the present invention are condensation products obtained from formaldehyde and sulfonated aromatics; they act as dispersants, and are normally available in the market.

[0021] The polynaphthalene sulfonates are generally prepared by reacting aromatic sulfonated compounds (such as naphthalene sulfonic acids, naphthol sulfonic acids, alkylated naphthalene sulfonic acids, alkylated naphthol sulfonic acids, and also tolu- enesulfonic acid, benzenesulfonic acid, phenolsulfonic acid and similar compounds) with formaldehyde, to form a condensation product which is usually neutralised or al- kalinised by adding a sodium hydroxide solution.

[0022] Among the aromatic formaldehyde sulfonated condensation products which are commercially available as dispersants and are useful for the realisation of the present invention we cite TAMOL NN 9104, sold by BASF, Germany.

[0023] With the term "organic polyphosphonate" we mean organic phosphonates containing two or more phosphonic groups, in the form of acid or salt thereof.

[0024] Polyphosphonates useful for the realisation of the present invention are: aminotri(methylene-phosphonic acid), aminotri(methylene-phosphonic acid)pentasodium salt, l-hydroxyethylidene-l,l-diphosphonic acid, l-hydroxyethylidene-l,l-diphosphonic acid tetrasodium salt, diethylenetriamine penta(methylene phosphonic acid)pen1asodium salt, diethylenetriamine penta(methylene phosphonic acid)trisodium salt, hexamethylene diamine tetra(methylene phosphonic acid), hexamethylene diamine tetra(methylene phosphonic acid)po1assium salt, and combinations thereof.

[0025] Other utilisable polyphosphonate are the polyphosphonate described in EP 1713568. [0026] The aqueous suspensions of the present invention are prepared by adding hemihydrate or anhydrous gypsum to an aqueous basic solutions, preferably at pH about 9, containing the sulfonated acrylic polymer and the organic polyphosphonate, or they may be prepared by adding the hemihydrate or anhydrous gypsum under vigorous stirring to water containing the organic polyphosphonate and subsequently adding the sulfonated acrylic polymer.

[0027] When a polynaphthalene sulfonate is used as the dispersant, the aqueous suspensions of the present invention are prepared by adding hemihydrate or anhydrous gypsum under vigorous stirring to an aqueous solution containing the polynaphthalene sulfonate and the organic polyphosphonate, or by adding the hemihydrate or anhydrous gypsum under vigorous stirring to water containing the organic polyphosphonate, and subsequently adding the polynaphthalene sulfonate.

[0028] Between 10 and 20% by weight of the dihydrate gypsum obtained by performing this procedure has a particle size of a less than 2 microns.

[0029] This particle size distribution is obtained directly, without the need of grinding and independently from the particle size distribution of the hemihydrate gypsum used to prepare the suspensions of the invention.

[0030] The hemihydrate or anhydrous gypsum useful for the preparation of the aqueous suspension of the invention is commercially available and is normally obtained by calcination of dihydrate gypsum

[0031] The suspensions of the invention may further contain a thickener which avoid the sedimentation of gypsum

[0032] Thickeners utilisable for this scope are carboxymethylcellulose, hydroxypropyl guar, hydroxypropylmethyl cellulose, xanthan, ASA polymers (i.e. "Alkali Swellable Acrylic" polymers) and are added in an amount of from 0.1 to 1% by weight on active matter, which is meant to be the dry matter of the suspension.

[0033] According to a particularly advantageous aspect of this invention, it is observed that the possible presence of sedimentation in the suspension can be eliminated by simple mechanical stirring, even after months from its preparation.

[0034] The aqueous gypsum suspensions of the present application are useful to prepare composition for the coating colour of printing paper and paperboard (in the present description the term "paper" also means paperboard).

[0035] It is therefore a further object of the present invention a composition for the coating colour of paper containing: a. 100 parts by weight of pigments, among which from 20 to 100% by weight consisting of gypsum having a particle size from 10 to 20% of less than 2 microns; b. from 4 to 12 parts by weight of a polymeric binder; c. from 0.1 to 1 parts by weight of a sulfonated acrylic polymer or of a polynaphthalene sulfonate; d. from 0.1 to 1 parts by weight of an organic polyphosphonate; from 30 to 60 parts of water. [0036] The coating colour composition of the invention may further contain, beside gypsum, other pigments, preferably having particle size from 40 to 90% of less than 2 microns; example of these pigments are kaolin, calcium carbonate, talc, titanium dioxide, barium sulphate. [0037] The preparation of the composition for the coating colour of paper of the invention is made by adding the binder to the aqueous suspension of gypsum, possibly adding the suspensions of other pigments and regulating the amount of water. [0038] Other ingredients, such as biocides and thickeners, can indifferently be added before or after the addition of the binder. [0039] Among the polymeric acrylic binders useful for the realisation of the present invention we cite those which are normally employed in the field, such as polymers from acrylic or methacrylic acid esters, copolymers of acrylic monomers with vinyl acetate, styrene, butadiene, and mixtures thereof. [0040] It was observed that the utilisation of the coating colour composition of the present invention provides matt coated paper of particular interest. [0041] Matt coated paper (also called "matt satin") is usually manufactured by using in the coating colour mixtures of pigments having relatively coarse particle size (by way of example calcium carbonate having 75% particle size <2 microns, kaolin 80% < 2 microns, talc 50% < 2 microns) and soft calendering. [0042] Matt coated paper generally suffers from two disadvantages: it has a more porous surface than glossy coated paper and this feature makes printing more difficult and lowers the printing yield; furthermore, matt coated paper has the tendency to easily loose its matt appearance, becoming glossy when rubbed with hard surfaces. [0043] Undesired "glossy spots" can be formed during printing and re-manufacturing

(cutting, setting, packaging) by mere friction of one sheet with another, originating defects that can be particularly relevant. [0044] Surprisingly, the coated paper obtainable by using the coating colour compositions of the present invention has characteristics of printability (smoothness), glossy and matt resistance to rubbing which cannot be obtained with the prior art coating colour. [0045] In particular, the matt resistance to rubbing makes the paper obtained with the coating colour compositions of the present invention very appreciable. [0046] Matt resistance is measured by determining the gloss of paper, measured according to Tappi standard method T 480 om - 05, before and after calendering at temperature and pressure conditions typical of glossy paper calendering, that is with cylinders at 5O⁰C, linear pressure = 67.5 Kg /cm, 4 nip.

[0047] The lower the increase of gloss, the higher is the matt resistance to rubbing.

[0048] Matt coated paper obtained by using the coating colour composition of the present invention shows a much lower increase of gloss after calendering than matt paper prepared with traditional formulations (increase of gloss less than 20), and it represent therefore a further object of the present invention.

[0049] The paper according to the invention further exhibits a high level of smoothness, which is correlated with good printability, and a particularly soft touch.

[0050] The paper obtainable with the coating colour compositions of the present invention has a surface layer of coating of from 8 to 16 g/m ², preferably from 10 to 14 g/m².

[0051] EXAMPLE 1

[0052] 6 gypsum suspensions are prepared with the following ingredients:

• Hemihydrate calcium sulfite

• Sequion 50 K33 (33% by weight hexamethylene diamine tetra(methylene phosphonic acid) potassium salt from Bozzetto SpA, Italy)

• AA= 45% aqueous solution of sodium aery late homopolymer dispersant

• AA-MA- AMPS=30% aqueous slurry of acrylic acid, methacrylic acid and 2-acrylamido-2-methyl-l-propanesulfonic acid sodium salt ter-polymer (monomers molar ratio: 4: 1 : 1).

PNS= Tamol NN 0104, polynaphthalene sulfonate from BASF (Germany) [0053] in the quantities reported in Table 1 and with the following procedure. [0054] Suspension 1 is prepared by adding in the order 3 g of Sequion 50 K33 and 252 g of calcium sulfite hemihydrate to 176 g of water, under vigorous stirring; the dispersion is prepared by caowles stirring at 1000 rpm for 30 minutes. [0055] After those 30' the pH is regulated at 9 with NaOH and 3 g of AA-MA-AMPS are added. [0056] Suspension 2, 3, 4, all comparative, are prepared in the same way but with the following variations: Suspension 2 was prepared without AA-MA-AMPS, Suspension

3 by adding AA instead of AA-MA-AMPS and Suspension 4 without Sequion 50 K

33. [0057] Suspension 5 is prepared by adding in the order 3 g of Sequion 50 K33 and 252 g of calcium sulfite hemihydrate to 176 g of water, under vigorous stirring; the dispersion is prepared by caowles stirring at 1000 rpm for 30 minutes.

[0058] After those 30' the pH is regulated at 9 with NaOH and 3 g of PNS are added. [0059] Suspension 6, comparative, is prepared like Suspension 5 but without Sequion 50 K 33.

[0060] In Table 1 the Brookfield viscosity at 25⁰C, 10 rpm and 100 rpm and the stability of the obtained suspensions is also reported. [0061] The suspension stability was evaluated by placing it in a 250 ml closed glass jar. The suspension was then kept standing in a conditioned room at 23⁰C and 50% r.h. for seven days.

[0062] Stability is considered good if, after the above reported length of time, the suspension returns to its original viscosity and homogeneity by gently manually stirring it for 10", even if the separation of supernatant liquid and the formation of a precipitate had occurred. [Table 0001] [Table ] Table 1- Suspensions 1-6

[0063] **calcium sulfite dihydrate concentration [0064] * comparative [0065] Suspension 6 sets, it is not possible to measure its viscosity or its stability [0066] EXAMPLE 2 [0067] Three paper coating colour formulations are prepared with the following ingredients:

Suspension 1 from Example 1

Hydrocarb 75 (calcium carbonate, 77% aqueous dispersion from Omja, Switzerland)

- CoCoat (calcium sulfite dihydrate, 68% aqueous suspension from Kemira, Finland)

- Dow Latex 935 (styrene butadiene copolymer, 50% latex from Dow Chemicals, United States)

- Lamovil CI 5/99 (99% hydrolysed polyvinylalcohol, 20% aqueous solution from Lamberti SpA)

- Lampac Supreme UL (poljanionic cellulose from Lamberti SpA)

- Lamkote (calcium stearate, 50% aqueous dispersion from Lamberti SpA) 25% NaOH

[0068] in the amounts reported in Table 2 and with the following procedure.

[0069] For the Coating colour 1, 66 g of Dow Latex 935 are added under stirring with caowles at 500 rpm to 400 g of Suspension 1; the pH is adjusted at 9.0 with NaOH and

2.5 of Lamovil CI 5/99 are added. [0070] The stirring speed is increased to 1000 rpm and 2.4 g of Lampac Supreme UL are added, maintaining stirred for 30 minutes, until complete dissolution. The stirring speed is reduced to 700 rpm and 3.0 g of Lamkote are added. [0071] The Coating colours 2 and 3 are prepared analogously, by substituting the

Suspension 1 with Hydrocarb 75 (Coating colour 2) and with CoCoat (Coating colour

3). [0072] In Table 2 the main features of the coating colours are reported too.

[Table 0002]

[Table ]

Table 2 - Coating colours 1-3

[0073] Coating colour and evaluation of Gloss.

[0074] The paper coating is performed with the Coating colours 1, 2 and 3 on industrial paper offset sheets of 40 g/m². [0075] A bar coating machine is used, suitable for the plane coating of A4 sheets; the machine has a set of bars wound by threads having different diameters, allowing to vary the volume of the coat by changing the dosing bar; it is also possible to vary the speed of the moving bar to modify the amount of coat applied. [0076] The coating machine, after a series of tests made to optimize the procedure, is regulated to dose 12 g/m ² of dry coat on the desired support. [0077] The setting of the coating machine is adjusted for each coating colour to constantly apply 12 g/m² of dry coat (amount always checked on every produced sheet). [0078] Immediately after the coating, the sheets are dried with air for 15" at 12O⁰C, and then are maintained for 2 minutes at 11O⁰C.

[0079] The coated sheets are allowed to slay in a conditioned room for 24 hours at 21⁰C and 50% of relative humidity; then they are calendered with the temperature of the rolls set at 5O⁰C, linear pressure = 67.5 Kg/cm, 4 nip, and contacting the coated side of the sheets on the steel roll.

[0080] After being calendered the sheets are again conditioned at 21⁰C and 50% of relative humidity. [0081] Gloss was measured before and after calendering (Tappi standard T480 om-05) and smoothness was measured after calendering (ISO 8791). [0082] The results are reported in Table 3. [Table 0003] [Table ] Table 3

[0083] [0084] The obtained results show that the paper coated with the calcium sulfite of Suspension 1 maintains gloss characteristics typical of "matt" paper even after relevant calendering, while the two other samples become significantly glossy.

[0085] The smoothness is comparable for all samples. [0086] Evaluation of the "silky hand". [0087] To evaluate the "silky hand", that is the tactile characteristics of the coated samples, we operated as follows.

[0088] 5 paper sheets were prepared for each coating colour formulation (grammage of base paper = 95 g/m²); the sheets were coated on both sides with 15 g/ m² of coating colour. [0089] Immediately after coating the sheets were dried for 15" with an air jet at 12O⁰C and stored in an oven at 11O⁰C for 2'. [0090] The coated sheets are kept for 24 h in a conditioned room at 21⁰C and 50% r.h.; they are then calendered (temperature of the cylinders=50°C, linear pressure=67.5 Kg/cm, 4 nip), keeping the coated side and the steel cylinder in contact. [0091] A blind test with 5 people (testers) was performed on the samples; the arbitrary scale gave score 1 to minimum and 5 to maximum of sliding/silkiness of the tested paper.

The results are reported in Table 4.

[0092] The thickness, bulk and smoothness of the sample were measured too. [0093] The results are reported in Table 5. [0094] The test results clearly show that, when calendering and smoothness are the same, the paper coated with the calcium sulfite dehydrate prepared according to the invention is lightly superior in bulk than paper coated with calcium carbonate, while its tactile characteristics are easy distinguishable, thanks to a particularly silky and sliding touch.

[Table 0004]

[Table ]

Table 4

[Table 0005] [Table ] Table 5

Claims

[0001] A gypsum aqueous suspension having Brookfield viscosity at 25⁰C and 10 rpm comprised between 100 and 1,000 mPa _*s containing: a. from 60 to 85% by weight of calcium sulfite dihydrate (Ca ₂SO₄.2H₂O); b. from 0.1 to 2.0% by weight of a polynaphthalene sulfonate or of an acrylic sulfonated polymer; c. from 0.1 to 2.0% by weight of an organic polyphosphonate.

[0002] The gypsum aqueous suspension of claim 1, where between 10 and 20% by weight of the dihydrate gypsum particles have a size of a less than 2 microns.

[0003] The gypsum aqueous suspension of claim 2, having Brookfield viscosity at 25⁰C and 100 rpm of less than 500 mPa_*s.

[0004] The gypsum aqueous suspension of claim 3, where the acrylic sulfonated polymer is obtained by polymerisation of acrylic and/or methacryϊc acid, in acidic or salt form, with a monomer containing a sulfonic functional group and have molecular weight from 5,000 to 40,000 dalton (measured with an acrylic acid standard).

[0005] The gypsum aqueous suspension of claim 4, where the molar ratio between the

(meth)acrylic acid and the monomer containing a strongly acidic functional group is from 3 to 30.

[0006] The gypsum aqueous suspension of claim 5, where the sulfonated acrylic polymer is a copolymer of acrylic acid, methacryϊc acid and 2-acrylamido-2-methyl-l-propanesulfonic acid, where the molar ratio between acrylic and methacrylic acid is from 8 to 2.

[0007] The gypsum aqueous suspension of claim 3, where the polynaphthalene sulfonate is prepared by reacting aromatic sulfonated compounds selected from naphthalene sulfonic acids, naphthol sulfonic acids, alkylated naphthalene sulfonic acids, alkylated naphthol sulfonic acids, toluenesulfordc acids, ben- zenesulfoiic acids, phenolsulfoiic acids with formaldehyde to form a condensation product.

[0008] The gypsum aqueous suspension of claim 3, where the polyphosphonate is selected among: aminotri(methylene-phosphordc acid), aminotri(methylene-phosphordc acid)pentasodum salt, 1 -hydroxy ethyϋdene- 1 , 1 -dphosphoiic acid, l-hydroxyethyBdene-lJ-dphosphonic acid tetrasodum salt, dethylenetriamine penta(methylene phosphoric acid)pentasodum salt, dethylenetriamine penta(methylene phosphoric acid)trisodum salt, hexamethylene damine tetra(methylene phosphoric acid), hexamethylene damine tetra(methylene phosphoric acid)potassium salt, and combinations thereof.

[0009] The gypsum aqueous suspension of claim 3, further comprising from 0.1 to 1% by weight of a thickener, the percentage being referred to the active dry substance.

[0010] A composition for the coating colour of paper containing: a. 100 parts by weight of pigments, among which from 20 to 100% by weight consisting of gypsum having a particle size from 10 to 20% of less than 2 microns; b. from 4 tol2 parts by weight of a polymeric binder; c. from 0.1 to 1 parts by weight of a sulfonated acrylic polymer or a polynaphthalene sulfonate; d. from 0.1 to 1 parts by weight of an organic polyphosphonate; from 30 to 60 parts of water.

[0011] The composition of claim 10, where the sulfonated acrylic polymer is obtained by polymerisation of acrylic and/or methacryϊc acid, in acidc or salt form, with a monomer containing a sulfonic functional group and have a molecular weight from 5,000 to 40,000 dalton (measured with an acrylic acid standard).

[0012] The composition of claim of claim 11, where the molar ratio between

(meth)acrylic acid and monomer containing a strongly acidic functional group is from 3 to 30.

[0013] The composition of claim 12, where the sulfonated acrylic polymer is a copolymer of acrylic acid, methacryϊc acid and

2-acrylamido-2-methyl-l-propanesulforic acid, where the molar ratio between acrylic and methacryϊc acid is from 8 to 2.

[0014] The composition of claim 10, where the polynaphthalene sulfonate is prepared by reacting aromatic sulfonated compounds selected from naphthalene sulfonic acids, naphthol sulfonic acids, alkylated naphthalene sulfonic acids, alkylated naphthol sulfonic acids, toluenesulfonic acids, benzenesulforic acids, phenol- sulfonic acids with formaldehyde to form a condensation product

[0015] The composition of claim 10, where the polyphosphonate is selected among: aminotii(methylene-phosphoric acid), aminotri(methylene-phosphoric acid)pentasodum salt, l-hydroxyethyBdene-l,l-dphosphoric acid, l-hydroxyethyBdene-l,l-dphosphoric acid tetrasodum salt, dethylenetriamine penta(methylene phosphoric acid)pentasodum salt, dethylenetriamine penta(methylene phosphoric acid)trisodum salt, hexamethylene damine tetra(methylene phosphoric acid), hexamethylene damine tetra(methylene phosphoric acϋ)potassium salt, and combinations thereof. Mitt coated paper having a surface layer of coating of from 8 to 16 g/m ² and showing an increase of gloss of less than 20 when calendered with cylinders at 5O⁰C, linear pressure = 67.5 Kg /cm, 4 rip.