EP1816260A2

EP1816260A2 - Process for the production of wood-free gloss coated paper

Info

Publication number: EP1816260A2
Application number: EP06016172A
Authority: EP
Inventors: Paolo Mattei
Original assignee: Cartiere del garda SpA
Current assignee: Cartiere del garda SpA
Priority date: 2006-01-04
Filing date: 2006-08-03
Publication date: 2007-08-08
Also published as: ITRM20060004A1; EP1816260A3

Abstract

A process for the production of wood-free gloss coated paper with good printability including a stage of preparation of the fibrous substrate and a stage of coating, characterised by the fact that said coating stage includes the application of at least four successive layers of coating to both sides the paper; and by the fact that subsequently to the application of the last layer of coating, the paper goes to a preparation stage, omitting a calendering stage.

Description

The present invention concerns a process for the production of wood-free gloss coated paper, with high specific volume and improved printability.
In particular, the invention refers to a process for the production of wood-free gloss coated paper, having comparable mechanical characteristics (thickness, rigidity) and paper gloss >/= 65 (Tappi 480) of a conventional gloss paper, but a higher grammage (10% to 15%), without having recourse to the use of machines for glazing (calender, brushing machine). The process solves the problem of loss of thickness and rigidity of paper because of the mechanical action to which it is subjected to obtain the glazing.
Moreover, the process of the invention requires a lesser amount (by 20-25%) of virgin cellulose fibres, and allows to set comparable or even greater production speeds than normal gloss coated papers.
The standard process for the production of gloss coated paper generally requires, besides the usual stages of preparing mixtures and fabrication of the fibrous substrate, other two subsequent stages, the coating stage and the finishing stage.
To obtain an effect of gloss finish, the paper is generally glazed by calendering.
It, is known from the state of the art that the coating stage has the purpose of smoothing the surface of the fibrous substrate through the application of various layers of coating in order to raise the print quality to the maximum, while the subsequent and last stage of processing depends on the desired finish of the paper itself.
The coating therefore assumes a primary role and consists of a dispersion in water of a pigment and an adhesive able to bind the particles of pigment to each other and anchor them to the paper substrate.
The raw materials generally used are calcium carbonate and kaolin (pigments), synthetic latexes and starches as binders, thickening substances, optical whiteners, colourants and antifermentatives.
Once the mixture is prepared, it is applied to the fibrous substrate, by means of coating machines commonly called "coaters" - either mounted in direct succession with the continuous machine, (i.e. the piece of equipment which produces the fibrous substrate), "on-line" coaters, or mounted separately, "off-line coaters" - which allow one or more layers of coating to be applied to each side of the strip of paper.
Gloss coated paper is currently used principally in virtue of its properties of surface uniformity and gloss, which give prominence to particular printed details which are present especially in images of photographic type.
Among the procedures already known for the production of gloss paper it should be distinguished a process defined as "cast coating" which allows a gloss coated paper to be obtained by applying the coating and subsequent, immediate, drying by contact with a heated chromed cylinder. This method is substantially slower in production than conventional processes of coating, and is applied usually on only one face of the paper. The method allows obtaining very high paper glosses (out of the normal range within which gloss coated paper falls), but with gloss after printing at best equal to that of unprinted paper, therefore with an effective print performance of mediocre level.
Another method consists of subjecting the paper, on which the layer(s) of coating have been applied, to the action of brushing rolls whose surface is made up of horsehair. This allows a particularly glossy surface to be obtained (generally superior to the normal market target for wood-free gloss coated papers) while limiting the loss of thickness while passing through the calender (though this operation cannot be eliminated), to get a surface that is not too delicate (a delicate surface might be damaged by the bristles of which the cylinders of the brushers are made up).
The productivity of such machines is very limited as the speeds allowed by this technique are restricted to between 200 and 500 m/min.
An alternative system, better known as 'soft calender' consists of a series of rolls (from 2 to 6), alternately deformable (soft) and undeformable (hard), arranged vertically or horizontally, of which the hard rolls are made of metal, while the soft rolls may be of elastomer or compressed fibre. The passage of the paper through the rolls produces compression of the material and leads to a loss of thickness and rigidity in the paper, and it does not achieve the gloss values required for gloss coated papers.
It is a technique comparable to the "supercalenders" with the advantage of being on-line and the disadvantage of not allowing adequate gloss values to be reached for wood-free gloss coated papers.
The system which uses the so-called "supercalender" is undoubtedly the most widely-used method for the production of wood-free gloss coated paper.
It consists of a series of rolls (from 10 to 14) being alternately deformable (soft) and undeformable (hard), arranged vertically. The hard rolls are made of metal, while the soft rolls may be of elastomer or compressed fibres.
They exert pressure and mutual slippage, and allow the particles of the material applied to the surface of the paper during the coating stage to be oriented, with the result that a notably improved specular reflection of the light and increased smoothness is obtained.
However the process is carried out off the machine, as the rolls are easily damaged and require the machine to be frequently stopped. The productivity of such processes is inferior to what is obtainable from a modem coater, as the speed of calendering is inferior to that of the coating process. The passage of the paper through the rolls produces an increase in the gloss and excellent smoothness. However, the major disadvantage of this procedure consists in the loss of thickness, rigidity, opacity and whiteness which is actually caused by the action of compression exercised by the rolls.
To reduce the intensity of the pressure necessary for obtaining the desired effect on the particles constituting the layer of coating, alternative systems have been identified over the years which make use of particular thermoplastic materials (e.g. plastic pigments) which, having greater glossing properties than classical inorganic pigments (kaolin and calcium carbonate), allow higher values to be achieved for paper gloss, or in the last analysis require lower pressures for the same gloss obtained, and thus allow the negative effect of passage through the calender to be limited, without however eliminating the use of the calender altogether.
The use of these materials is, furthermore, very onerous both economically and technologically because of the difficulty of re-using the offcuts and scrap from production within the same productive cycle. It is therefore an unsolved problem to have available a gloss paper, within the normal scale of commercial grammage, which guarantee levels of thickness, rigidity, opacity and whiteness comparable to the characteristics conferred by coating (but normally prejudiced in the passage through the calender), and which will contribute, by avoiding losses of thickness and rigidity, to a saving in the use of fibrous substrate material, and therefore to a reduction in the environmental impact on forest resources, by comparison with classic gloss papers.

Description of the invention

To solve the above problem a process is proposed, according to the invention, to obtain wood-free gloss coated paper, with high specific volume, in which the calendering stage, and therefore the disadvantages connected with it, is eliminated, specifying instead the application to the substrate of many successive layers of coating, having a specific composition and requiring a particularly careful application system from the third layer of coating onwards.
The present invention consists of a process for the production of wood-free coated paper, of gloss type, including the stages of preparing the fibrous substrate and coating characterised by the fact that said stage of coating specifies the application of four successive layers for each side of the paper and that said successive layers constitute the final surface of the paper itself, not requiring further passes through the calender to obtain the level of surface gloss required.
It is a specific object of the invention a process for the production of wood-free gloss coated paper with good printability including a stage of preparation of the fibrous substrate and a stage of coating, characterised by the fact that said coating stage includes the application of at least four successive layers of coating to both sides the paper; and by the fact that subsequently to the application of the last layer of coating, the paper goes to a preparation stage, omitting a calendering stage. Preferably the application is of four successive layers of coating.
Preferably the first layer of coating is applied either by means of a roll coater or by blade with applicator roll, with a levelling stage either in bent blade or stiff blade configuration.
Preferably the second layer of coating is applied either by means of a blade coater with applicator roll, or with a jet flow applicator with a levelling stage either in bent blade or stiff blade configuration.
Preferably the third layer of coating is applied either by means of a blade coater with applicator roll, or with a jet flow applicator (e.g. Jet-Flow F®, Beloit S-Flow®) with a levelling stage either in bent blade or stiff blade configuration.
Preferably the fourth layer of coating is applied either by means of a blade coater with applicator roll, or with a jet flow applicator with a levelling stage either in bent blade or stiff blade configuration.
Preferably the first layer of coating requires a combination of ingredients consisting of a mixture of pigments of the type 100 parts % of C60+ .
Preferably the second layer of coating requires a combination of ingredients consisting of a mixture of pigments of the type 100 parts % of C60+ .
Preferably the third layer of coating requires a combination of ingredients consisting of a mixture of pigments of the type 70 to 100 parts % of C90+ and of 30 to 0 parts % of ultrafine clay.
Preferably the fourth layer of coating requires a combination of ingredients consisting of a mixture of pigments of the type 70 to 100 parts % of C99+ and of 30 to 0 parts % of ultrafine clay.
It is a further object of the invention a paper producible according to the process of the invention having the characteristics of grammage, thickness, apparent specific volume, longitudinal rigidity, transverse rigidity, PPS smoothness, paper gloss, print gloss and dry pick as described in Table 3.
In the formulations, "parts %" are used which correspond to the dry quantity of the ingredient in relation to the dry total of pigments present.
Example: if in a formula there are 1000 kg (dry) of pigments (carbonates + kaolins), these correspond to 100 parts %. If 100 kg (dry) of latexes are added, these will correspond to 10 further parts %.
The procedure according to the invention specifies the application of successive layers (up to 4) of coating for each of the faces of the fibrous substrate (fibrous support) of the paper, with particular regard to the last two layers.
According to a preferred embodiment of the invention, in Table 2 the following meanings are intended:

C60+: family of products with a base of calcium carbonate with particle distribution in which at least 60% (starting from 60% and upwards) of the weight of the particles have a hydrodynamic equivalent diameter lower than 2 µm, e.g. Hydrocarb60®, Hydrocarb75®.
C90+: a family of products is intended with a base of calcium carbonate with particle distribution in which at least 90% (starting from 90% and upwards) by weight of the particles have a hydrodynamic equivalent diameter lower than 2 µm, e.g. Hydrocarb90®, Hydrocarb99®.
C99+: a family of products is intended with a base of calcium carbonate with particle distribution in which at least 99% (starting from 99% and upwards) of the weight of the particles have a hydrodynamic equivalent diameter lower than 2 µm, e.g. HydrocarbHG®, F-14115®.
Ultrafine clay: a family of kaolins is intended characterised by a particle distribution in which at least 95% by weight of the particles have a hydrodynamic equivalent diameter lower than 2 µm, e.g. Hydragloss 90®, Amazon 90®.

In fact, according to the invention, the penultimate layer is characterised by the presence of a mixture of "fine" pigments belonging to class C90+ or C99+ in proportions varying from 70 to 100 parts %, preferably Hydrocarb 90®, plus percentages varying from 30 to 0% of kaolin belonging to the class "ultrafine clay", preferably Hydragloss 90®, Amazon 90®.
Still according to the invention, advantageously, the final layer is characterised by the presence of a mixture of "ultrafine" mineral pigments belonging to class C99+ in proportions varying from 70 to 100 parts % (natural calcium carbonate, calcium carbonate precipitate), commercial example Omya F-14115® plus percentages varying from 30 to 0% of kaolin belonging to the class "ultrafine clay", preferably Hydragloss 90® which make the surface of the paper sufficiently glossy and smooth to make it fall within the commercial type "gloss" without having to resort to the conventional means of glossing described above.
According to the invention the coating procedure can be performed using either roll coaters (e.g. GRIC®) or blade coaters (e.g. Beloit S-Matic®, Jagenberg CombiBlade®), with coating applicators either of roll type (applicator roll) or of jet flow type (e.g. Jet-flow F®, S-Flow®). The stage of levelling the coating (doctoring) can be achieved either with a bent-blade configuration, characterised by working angles of 0-15°, or in stiff blade configuration, characterised typically by an angle greater than 30°.
The production of this paper requires a revision of the production parameters and settings (targets) of the fibrous substrate, and of the procedures, quality and quantity of the coating applied.
In fact to obtain the product according to the invention of a final grammage of 305 g/m², equivalent to a "standard" product of 350 g/m² grammage, the fibrous substrate must be reduced from 256 g/m² to 205 g/m²; the invention specifies carrying out a succession of 4 layers of coating instead of three, and the total quantities of coating applied in the various passes are different. In the product according to the invention, about 90 g/m² of coating are applied in total, while previously in the old type 76 g/m² of coating were applied, and the individual quantities applied for each layer of coating are consequently altered.
This global revision is necessary in order to confer on the paper not only the necessary qualitative characteristics but also good machinability, free from defects such as for example the presence of lines due to the increasing smoothness of the paper from one pass to the next, particularly in the application of the last layer. It should be noted that the degree of finish of the third layer of coating is already equivalent to a normal matt coated paper and therefore capable of being marketed "as it is". The choice of ingredients for the last two layers of coating and particularly of the pigments, is fundamental for obtaining the final characteristics.
It is a further object of the invention a paper producible with the process referred to above, having the characteristics of grammage, thickness, apparent specific volume, longitudinal rigidity, transverse rigidity, PPS smoothness, paper gloss, print gloss, and Dry Pick resistance documented in the following examples and given in Table 3.
The present invention will now be described by way of illustrative but not limiting purposes, with reference also to the tables and the examples given below.
The process according to the invention consists of preparing a suitable fibrous substrate in reels, complying with the new grammage standards (see Tables 1, 2 and 3 and the examples given below) and configuring it for the coating stage which, as stated in Table 2, in a preferred embodiment adopts a roll system (e.g. GRIC®) and/or a blade system (e.g. Beloit S-Matic® or Jagenberg Combi-Blade®) and a regulating system of scraping/smoothing type.

According to a preferred form of embodiment, the ingredients used in the four successive stages of coating are those given in Table 2, bearing in mind that the procedure according to the invention is applied substantially only to examples 1 to 7, while STD 1, 2, 3 and 4 are descriptions of the parameters of conventional gloss paper of different grammages and thicknesses treated with a calendering process and used for comparison of the parameters as given in Table 3.

Table 1: sequence and types of processes of coating and calendering

	1 st layer of coating	2nd layer of coating	3rd layer of coating	4th layer of coating	Calender
STD1	P1	T6			11 nips
STD2	P1	P2	T1		11 nips
STD3	P1	P2	T1		11 nips
STD4	P1	P2	T1		11 nips
Es1	P1	P2	T3		no
Es2	P1	P2	T2	T4	no
Es3	P1	P2	T2	T3	no
Es4	P1	P2	T1	T3	no
Es5	P1	P2	T2	T3	no
Es6	P1	P2	T2	T3	no
Es7	P1	P2	T2	T5	no

Table 2: procedure and quantity and quality of the coatings applied

	Type of application	Method of regulation	Application (g/m²)	Pigment parts % - type	Starch parts %	Latex parts %
P1	Roll	-	12-13	100 C60+	15-17	2-4
P2	Blade	stiff blade - bent blade	28-31	100 C60+	1-3	9-11
T1	Blade	stiff blade - bent blade	30-34	50-90 C90+ 50-10 fine clay	0	8.5
T2	Blade	stiff blade - bent blade	22-26	50-90 C90+ 50-10 fine clay	0	7-9
T3	Blade	stiff blade - bent blade	22-26	80-C99+ 20-fine clay	1-3	14-16
T4	Blade	stiff blade- bent blade	22-26	100- C99+	1-3	14-16
T5	Blade	stiff blade- bent blade	22-26	80- C99+ 20-fine clay	1-3	8-10
T6	Blade	stiff blade-bent blade	26-28	80-C90+20-fine clay	0	9-11

Comparison parameter 1 , designated STD1

Corresponds to a conventional (normally sold) gloss triple-coated paper in which a cellulose fibre substrate of suitable grammage is repeatedly coated on both faces using both roll coaters and blade coaters (see Table 2) with coatings specially designed to achieve the final characteristics of gloss, thickness, rigidity and printability.
Final grammage (5.2% moisture) 150 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 98 -

First layer 113 15 Roll

Second layer 143 30 Blade
The pass through the calender requires the following settings:

No. of rolls 12
No. of nips 11
Max speed 500 m/min
Max pressure 130 kg/cm
Temperature 40 °C

Comparison parameter 2 , designated STD2

Corresponds to a conventional (normally sold) gloss triple-coated paper in which a cellulose fibre substrate of suitable grammage is repeatedly coated on both faces using both roll coaters and blade coaters (see Table 2) with coatings specially designed to achieve the final characteristics of gloss, thickness, rigidity and printability.
Final grammage (5 % moisture) 250 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 162 -

First layer 174 12 Roll

Second layer 207 33 Blade

Third layer 237 30 Blade
The pass through the calender requires the following settings:

No. of rolls: 12
No. of nips: 11
Max speed: 500 m/min
Max pressure: 130 kg/cm
Temperature: 40 °C

Comparison parameter 3 , designated STD3

Corresponds to a conventional (normally sold) gloss triple-coated paper in which a cellulose fibre substrate of suitable weight is repeatedly coated on both faces using both roll coaters and blade coaters (see Table 2) with coatings specially designed to achieve the final characteristics of gloss, thickness, rigidity and printability.
Final grammage (5 % moisture) 300 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 206 -

First layer 219 13 Roll

Second layer 251 45 Blade

Third layer 285 79 Blade
The pass through the calender requires the following settings:

Comparison parameter 4 , designated STD4

Corresponds to a conventional (normally sold) gloss triple-coated paper in which a cellulose fibre substrate of suitable grammage is repeatedly coated on both faces using both roll coaters and blade coaters (see Table 2) with coatings specially designed to achieve the final characteristics of gloss, thickness, rigidity and printability.
Final grammage (5% moisture) 350 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 256 -

First layer 269 13 Roll

Second layer 298 42 Blade

Third layer 331 33 Blade
The pass through the calender requires the following settings:

Example 1, designated Ex 1

Final grammage (4.2% moisture) 250 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 162 -

First layer 174 12 Roll

Second layer 207 33 Blade

Third layer 237 30 Blade

Example 2, designated Ex 2

Final grammage (4.2% moisture) 215 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 119 -

First layer 129 10 Roll

Second layer 156 27 Blade

Third layer 182 26 Blade

Fourth layer 206 24 Blade

Example 3, designated Ex 3

Example 4, designated Ex 4

Example 5, designated Ex 5

Final grammage (4.5% moisture) 265 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 164 -

First layer 176 12 Roll

Second layer 204 28 Blade

Third layer 229 25 Blade

Fourth layer 254 25 Blade

Example 6, designated Ex 6

Final grammage (4.5% moisture) 305 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 205 -

First layer 215 10 Roll

Second layer 240 25 Blade

Third layer 265 25 Blade

Fourth layer 292 27 Blade

Example 7, designated Ex 7

Final grammage (4.2% moisture) 215 g/m²

Dry grammage Coating applied Type of coater

g/m² g/m²

Substrate 119 -

First layer 129 10 Roll

Second layer 156 27 Blade

Third layer 182 26 Blade

Fourth layer 206 24 Blade
The data given in the examples were examined and processed by the following methods:

Grammage: the method used was ISO 536:1995
Dry grammage: the method used was ISO 287-1985
Thickness: the method used was ISO 534:-1988 paragraph 8.2 "sheet thickness"
Apparent specific volume: the method used was ISO 534:1988 paragraph 10.3.2 "apparent sheet density" calculating the reciprocal, in other words δ₂/g where δ₂ is the thickness of a single sheet in µm and g the weight in g/m².
Rigidity: the method used was TAPPI 489-om92
Gloss paper: the method used was TAPPI 480om-92, by means of a Lehman instrument at 75°
Gloss printing: the method used was TAPPI 480om-92, by means of a Lehman instrument at 75° on paper which had been printed 24 hours earlier with NERO HORIZON MACH2 ICIFICIS ink on a Prufbau machine.
Smooth (PPS): the method used was ISO 8791-4
Pick resistance: this test gives indications on the resistance of the surface of the paper to stresses during the printing stage. The printing was done on a Prufbau printing machine using NERO HORIZON MACH2 ICIFICIS ink. The level of picking was assessed visually.
Assessment criteria: A= no picking; B= light picking; C= high level of picking.

Table 3

		Grammage	Non calendere	Final thicknes	Final apparent	Longitudin al rigidity	Transvers e rigidity	PPS smoothnes	Paper gloss	Print gloss	Dry Pick
		ISO536:199	ISO	ISO	ISO	ISO	ISO	ISO 8791-	Tappi	Tappi	Rank
	Description	g/m²	µm	µm	cm³/g	gf:c	gf:cm	µm	%	%	B
STD1	Standard	150	148	117	0.78	4.5	2.8	1.29	70-74	86	B
STD2	Standard 250 gloss	250	249	193	0.77	29.0	19.3	0.70	70-74	86	B
STD3	Standard 300 gloss	300	309	242	0.81	52.0	33.0	0.70	70-74	86	B
STD4	Standard 350 gloss	350	376	290	0.83	88.0	48.5	1.20	70-74	86	B
Es1	Tri-coat 250	250	-	-	-	-	-	-	50-54	-	-
Es2	Quad 215	215	194	194	0.90-	-	-	-	61-63	-	-
Es3	Quad 215	215	194	194	0.90	22.5	14.2	0.98	64-66	89	B
Es4	Quad 215 top	265	250	250	0.94	24	16	0.88	64-66	89	B
Es5	Ouad 265	265	250	250	0.94	53	33	0.88	64-66	89	B
Es6	Ouad 305	305	308	308	1.01	83	53	0.88	64-66	89	B
Es7	Ouad 215	215	194	194	0.90	-	-	-	66-70	86	B

From analysis of the data given in Table 3, it is clear that:

1. Calendering of STD1, STD2, STD3 and STD4 entails a reduction in thickness of a little over 20% by comparison with the initial value, irrespective of the initial grammage and the number, procedure and quality of the layers of coating applied.
As a result of calendering, as is well-known from the literature, both the rigidity and the opacity of the calendered paper reduce significantly.
2. Prototype Ex1 demonstrates that just the pass with the final top layer T3 without suitable preparation by means of the previous layers, as in prototypes Ex 3, Ex 4 and Ex 5, does not succeed in achieving the gloss values required (>=65%), therefore the final layer alone does not allow the achievement the objectives if not combined with suitable preparation of the previous layers.
3. Prototype Ex2 demonstrates that just the pass with the top layer of coating T5 with just pigment F-14115 without the presence of other ingredients, suitably metered, does not succeed in satisfying the requirements.
4. The prototype in example Ex3 reaches all the requirements demanded, in fact:
- the thickness of Ex3 is equal to that of STD2 but with a weight 35 g/m² less
- the rigidity of Ex3 is equal to that of STD2 but with a weight 35 g/m² less
- the paper gloss of Ex3 without passing through the calender is 65%
- the paper gloss of Ex3 of 89% is better in absolute terms and clearly better in relative terms of Delta gloss in comparison to STD2
- the test of dry pick resistance shows an equivalence between Ex3 and STD2
- the PPS smoothness value of Ex3 is within the normal range relative to ordinary gloss papers (0.70 -1.0 µm).
5. The prototype in example Ex4 reaches all the requirements demanded, in fact:
- the thickness of Ex4 is equal to that of STD2 but with a weight 35 g/m² less
- the rigidity of Ex4 is equal to that of STD2 but with a weight 35 g/m² less
- the paper gloss of Ex4 without passing through the calender is 65%
- the paper gloss of Ex4 of 89% is better in absolute terms and clearly better in relative terms of Delta gloss in comparison to STD2
- the test of dry pick resistance shows an equivalence between Ex4 and STD2
- the PPS smoothness value of Ex4 is within the normal range relative to ordinary gloss papers (0.70 -1.0 µm).
6. The prototype in example Ex5 reaches all the requirements demanded, in fact:
- the thickness of Ex5 is equal to that of STD3 but with a weight 35 g/m² less
- the rigidity of Ex5 is equal to that of STD3 but with a weight 35 g/m² less
- the paper gloss of Ex5 without passing through the calender is 65%
- the paper gloss of Ex5 of 89% is better in absolute terms and clearly better in relative terms of Delta gloss in comparison to STD3
- the test of dry pick resistance shows an equivalence between Es5 and STD3
- the PPS smoothness value of Ex5 is within the normal range relative to ordinary gloss papers (0.70 -1.0 µm).
7. The prototype in example Ex5 reaches all the requirements demanded, in fact:
- the thickness of Ex6 is equal to that of STD4 but with a weight 45 g/m² less
- the rigidity of Ex6 is equal to that of STD4 but with a weight 45 g/m² less
- the paper gloss of Ex6 without passing through the calender is 65%
- the paper gloss of Ex6 of 89% is better in absolute terms and clearly better in relative terms of Delta gloss in comparison to STD4
- the test of dry pick resistance shows an equivalence between Ex6 and STD4
- the PPS smoothness value of Ex6 is within the normal range relative to ordinary gloss papers (0.70 -1.0 µm).
8. The prototype in example Ex7 reaches all the requirements demanded, in fact:
- the thickness of Ex7 is equal to that of STD2 but with a weight 35 g/m² less
- the rigidity of Ex7 is equal to that of STD2 but with a weight 35 g/m² less
- the paper gloss of Ex7 without passing through the calender is 69%
- the paper gloss of Ex7 of 87%
- compared with Ex3 has a better level of paper gloss but a worse level of print gloss (in virtue of the smaller quantity of latex used), therefore it is possible to adjust the relative ratio between paper gloss and print gloss by varying the quantity of latex between 10 and 16%.
- the test of dry pick resistance shows an equivalence between Ex7 and STD2
- the PPS smoothness value of Es7 is within the normal range relative to ordinary gloss papers (0.70 -1.0 µm).

Claims

Process for the production of wood-free gloss coated paper with good printability including a stage of preparation of the fibrous substrate and a stage of coating, characterised by the fact that said coating stage includes the application of at least four successive layers of coating to both sides the paper; and by the fact that subsequently to the application of the last layer of coating, the paper goes to a preparation stage, with no calendering stage.
Process according to claim 1 wherein the application is of four successive layers of coating.
Process according to any of previous claims characterised by the fact that the first layer of coating is applied either by means of a roll coater or by blade with applicator roll, with a levelling stage either in bent blade or stiff blade configuration.
Process according any of previous claims characterised by the fact that the second layer of coating is applied either by means of a blade coater with applicator roll, or with a jet flow applicator with a levelling stage either in bent blade or stiff blade configuration.
Process according any of previous claims, characterised by the fact that the third layer of coating is applied either by means of a blade coater with applicator roll, or with a jet flow applicator (e.g. Jet-Flow F®, Beloit S-Flow®) with a levelling stage either in bent blade or stiff blade configuration.
Process according to any of previous claims, characterised by the fact that the fourth layer of coating is applied either by means of a blade coater with applicator roll, or with a jet flow applicator with a levelling stage either in bent blade or stiff blade configuration.
Process according to any of previous claims in which said first layer of coating requires a combination of ingredients consisting of a mixture of pigments of the type 100 parts % of C60+ .
Process according to any of previous claims in which said second layer of coating requires a combination of ingredients consisting of a mixture of pigments of the type 100 parts % of C60+ .
Process according to any of previous claims in which said third layer of coating requires a combination of ingredients consisting of a mixture of pigments of the type 70 to 100 parts % of C90+ and of 30 to 0 parts % of ultrafine clay.
Process according to any of previous claims in which said fourth layer of coating requires a combination of ingredients consisting of a mixture of pigments of the type 70 to 100 parts % of C99+ and of 30 to 0 parts % of ultrafine clay.
Paper producible according to the process of any of previous claims having the characteristics of grammage, thickness, apparent specific volume, longitudinal rigidity, transverse rigidity, PPS smoothness, paper gloss, print gloss and dry pick as described in Table 3.