WO1995006777A1

WO1995006777A1 - Method for improvement of paper properties such as wet strength and resistance to light induced ageing

Info

Publication number: WO1995006777A1
Application number: PCT/SE1993/000712
Authority: WO
Inventors: Rune Simonson; Magnus Paulsson; Mats Westin
Original assignee: A-Cell Acetyl Cellulosics Ab
Priority date: 1993-08-28
Filing date: 1993-08-28
Publication date: 1995-03-09
Also published as: AU5287593A

Abstract

The present invention relates to a method for improvement of paper properties such as wet strength and resistance to light induced ageing by acetylation of paper. The acetylation medium is acetic anhydride and the method is characterized by no use of any catalyst or subjection of the paper to any chemical pretreatment.

Description

TITLE: Method for improvement of paper properties such as wet strength and resistance to light induced ageing.

FIELD OF INVENTION:

The present invention relates to the chemical treatment of any kind of paper and paperboard made from cellulosic materials by acetylation with acetic anhydride in the absence of any added catalyst or chemical pretreatment of the paper. More particularly the invention relates to the treatment of paper and paperboard to obtain increased mechanical and optical properties of said products.

PRIOR ART:

Paper and paperboard are formed from lignocellulosic fibres that normally originate from wood although non-wood plants and their derivatives (eg. bagasse) are also being used. The lignocellulosic fibres are liberated from the wood or non-wood plant structure by chemical or mechanical means to yield a fibre pulp. This fibre pulp can be further treated with bleaching chemicals to improve its brightness and by beating to improve strength properties.

Paper (in higher basis weights called paperboard) is formed in a paper machine in which the incoming dilute stock solution consisting of fibres suspended in water is successively dewatered to allow the formation of a fibre web. The dewatering starts in the wire net section where water passes through the wire net leaving fibres on the net. In a second step, the press section, water is pressed out from the fibre web which at this stage is mainly held together by capillary forces. In the final drying section, the water content in the formed papersheet is reduced to 5- 10% by heating and evaporation of water. The fibres are now

SUBSTITUT E held together by hydrogen bonds formed between hydroxyl groups in the fibre constituents such as cellulose and hemicellulose.

THE TECHNICAL PROBLEM:

Although paper possesses many unique and desirable pro¬ perties that can be tailored to some extent, it also has undesirable properties which have limited its use in many applications. One example is the low strength shown in wet condition which results from the splitting of hydrogen bonds between the fibres and the formation of new hydrogen bonds to water. For paper grades where strength is an essential property (eg. kraft paper and liner) this is a serious disadvantage. Improved wet strength properties and a less hydrophilic behaviour of the paper are, however, also required in many other applications.

Another undesirable property of paper is the light induced darkening or yellowing shown by papers containing high ^' yield pulps such as thermo echanical (TMP) and chemithermo- mechanical (CTMP) pulps with their high lignin content.

As a means to solve the above problems acetylation of paper and lignocellulosic materials has been proposed.

Acetylation of lignocellulosic materials using a catalyst free process and supplying the acetylation medium in the form of acetic anhydride free from cosolvents or diluents has proved to be a superior way of improving dimensional stability and biological resistance of lignocellulosic materials in the form of solid wood, chips, flakes or fibres (European patent 0 213 252) . Boards and moulded products made from wood particles or fibres which are glued together by means of an added adhesive to form the final product have been shown to possess a high degree of dimensional stability and resistance to degrading fungi and

SUBSTITUTESHEET bacteria. By replacing hydroxyl groups originally present in the lignocellulosic material with acetyl groups the swelling of the material caused by moisture as well as the equilibrium moisture content of the material are greatly reduced. However, the reduced swelling in water makes the material in the form of fibres less flexible and acetylated fibres cannot be suspended in water in a proper way to yield a dilute fibre stock solution suitable for formation of paper in a paper machine.

Another method of acetylation was proposed by Manchester et al. (Svensk Papperstidn. 63(20) :699, 1960) who used acetic anhydride in the vapour phase to acetylate ground wood pulp and sulphite pulp. Handsheets made from acetylated pulps showed considerable reduction in breaking length, the reduction being increased with increased degree of acetyla¬ tion. This was ascribed to a decrease in fibre bonding ability due to acetylation.

Stamm and Beasley [Tappi 44(4) :271] reported in 1961 on the acetylation of saturating papers with acetic anhydride in the presence of different catalysts. Prior to the acetyla¬ tion step the paper to be acetylated was impregnated with an aqueous solution containing zinc chloride or sodium acetate and potassium acetate and then dried to 50% relative humidity. Pyridine was also used as a catalyst mixed with the acetic anhydride. The wet breaking length was reported to be considerably increased at longer acetylation times (>90 in.). This acetylation process has not had commercial acceptance because of its complexity, long treatment times and the difficulty in removing the pyridine and the smell from it.

Leary [Tappi 51(6) :257(1968) ] studied the yellowing of wood by light using wood panels and newspaper sheets. A complex method for acetylation was used including soaking of the newspaper for 10 min. in sodium hydroxide solution (IN) prior to transferring the paper into liquid acetic anhydride.

Mechanical pulps were acetylated using acetic anhydride vapour [Loras, Pulp Paper Mag. Can. 1968. T49, Loras and Rengard, Norsk Skogind.23(1969) :239] . The acetylation made the pulps difficult to disintegrate and a real loss in strength was found when the pulps were tested according to British standard. The strength loss which was difficult to regain by any mechanical treatment was ascribed to a serious reduction in the possibility for hydrogen bonds between the fibres due to the replacement of hydroxyl groups for acetyl groups. However, the pulps showed increased stabilization towards yellowing.

Acetylated ground wood pulps were used to prepare hand- sheets which were subjected to accelerated light induced ageing [Ek et al. (1992), Nordic Pulp Paper Res. J. 7:3, 108]. A stabilization was reported but no reference was given to the strength development due to acetylation.

Although several methods of acetylation have been used to study the effect of light induced ageing of different pulps no commercial utilization of the methods has been achieved. The main reason for this is the difficulty in making paper from acetylated fibre pulps and thereby the low strength properties obtained as a result of the decreased ability of the acetylated fibres to form hydrogen bonds in the dried state. Another reason is the complexity of the acetylation methods adopted and the difficulty and cost involved in the removal of chemicals and catalysts such as pyridine from the fibre product after the chemical modification.

An improved photostabilization or resistance to yellowing in combination with increased strength under wet or

SUBSTITUTESHEET moisture condition is therefore a highly required and desired improvement.

THE SOLUTION: The primary objective of this invention is accordingly to develop a method for improving mechanical and optical properties of paper and paperboard. According to the invention one has therefore brought about a method for improvement of paper properties such as wet strength and resistance to light induce ageing which is characterized in that paper is subjected to acetylation with acetic anhydride without the use of any catalyst or subjecting the paper to any chemical pretreatment. The expression "paper•' includes any kind of paper, paperboard and other types thereof.

According to the invention it is suitable that the acety¬ lation process yields an acetyl content of up to 15% by weight of acetylated paper.

The acetic anhydride can according to the invention be supplied to the paper in the form of a liquid, vapour or gas.

According to the invention a suitable reaction temperature is between 80°C to 180°C.

The acetylation method according to the invention is suitably carried out as an intermediate or final step in the drying section in a paper machine or as a post opera¬ tion e.g. before conditioning.

The paper during the acetylation step should according to the invention have a water content not exceeding 20%, preferably not exceeding 15%. It is suitable according to the invention that the acetyla¬ tion medium is sprayed on the paper or the paper is passed through the acetylation liquid medium.

The acetylation medium can according to the invention also be supplied to the paper in the form of vapour in a com¬ partment.

According to the invention the paper is suitably heated by electromagnetic radiation, preferably microwaves, by heat transfer from the drying cylinders of the paper machine or by hot gas.

It is further suitable according to the invention that excess chemicals are removed after the acetylation by the steps of heating applying vacuum, introducing hot steam or gas or by rinsing with water or a combination of any of these steps before final conditioning or in combination with final conditioning.

DETAILED DESCRIPTION OF THE INVENTION:

The substance of this invention is chemical modification of paper to obtain increased mechanical and optical properties of the paper by acetylation with acetic anhydride as the acetylation medium. The acetylation medium consists of acetic anhydride which is allowed to contain acetic acid in an amount of up to 30% calculated on basis of the weight of the acetylation medium. No cosolvents, diluents or cata¬ lysts are included in the acetylation medium.

In paper, the lignocellulosic fibres that constitutes the paper are bonded together with hydrogen bondings. These bondings are formed at a late stage of the paper forming process when the paper web is dried by supplying heat in the drying section of the paper machine. Surprisingly, these bonds which are responsible for paper strength are

SUBSTITUTESHEET not broken during the acetylation of paper according to the invention, and the acetylated paper shows about the same tensile strength in dry condition as does the unmodified control paper. The acetylation step is consequently applied to the paper at a stage of its production where the strength giving hydrogen bondings between fibres are already developed to the main extent. This means that in the industrial production of paper, the acetylation treatment according to the invention is performed as an intermediate or final step in the drying section, or as a separate step in a post treatment operation of the paper. The paper to be acetylated is allowed to contain moisture. However, since acetic anhydride reacts with moisture to form acetic acid thereby increasing the consumption of acetylation medium and the cost of the treatment, the moisture content should not exceed 20%, preferably not 15%. The pH of the paper before addition of the acetylation medium can range from pH 4 to pH 11, approximately.

The acetic anhydride allowed to contain acetic acid can be supplied to the paper in the liquid form or in the form of vapour or gas. The reaction temperature can be chosen within a broad range (approximately 80°C to 180°C) . When the temperature is increased, the reaction rate is increased, and the reaction time to a proper acetyl content is decreased. By choosing the way of adding acetic anhydride to the paper, and the temperature during reaction, the process design of the acetylation step can be adjusted to the paper production. This renders the method according to the invention a high degree of flexibility. It should, however, be borne in mind that too high temperature may lead to thermal degradation and discolouration of the paper and should therefore be avoided.

The beneficial effects gained by the acetylation of paper according to the invention are noticed already at low

SUBSTITUTESHEET degree of acetylation expressed as acetyl content of the paper. Thus, acetyl contents of the chemically modified paper well below 10-15% calculated on weight basis give a substantial increase in paper properties which for many applications of paper are fully satisfactory. This means a low consumption of acetylation medium and short treatment times as compared with the conditions required to obtain acetyl contents around 20%, which otherwise are recommend- able in achieving dimensional stability of e.g. solid wood and moulded products.

The chemical modification of paper by acetylation according to the invention can be performed as a batch process as well as a continuous process. Incorporated in the paper machine, it is located to the drying section and at a stage where hydrogen bonds have been developed between the fibres thereby providing the necessary paper strength in dry condition. The acetylation step can also be performed as a post operation, e.g. before calendering the paper if performed. Surprisingly, it has been found that the acetylation according to the invention does not lead to a lowering of the tensile strength in dry condition. This finding can be interpreted as an indication that hydrogen bonds already formed between hydroxyl groups of the fibre constituents such as cellulose and hemicellulose are not affected by the treatment to a great extent. The consider¬ ably increased wet strength achieved by acetylation according to the invention is believed to be a result of the lowered swelling and equilibrium moisture content which accompany the substitution of hydroxyl groups in cellulose, hemicellulose and remaining lignin for acetyl groups.

The acetylation medium consisting of acetic anhydride allowed to contain acetic acid (<30%) can be added to the paper having a moisture content below about 20% either in liquid form or as a vapour or gas. If added in the liquid

SUBSTITUTESHEET form, the liquid acetylation medium which might be preheated to a proper temperature can be sprayed on the paper in a regulated way, or the paper can be allowed to pass through the liquid acetylation medium thereby soaking up liquid. The reaction between paper and the acetylation medium requires the paper and the acetic anhydride to be heated to a temperature of 80°C or more. At lower tempera¬ ture, the reaction rate for the acetylation reaction can be considered too low to be applicable in an industrial process. The heating can be performed in an indirect way, e.g. by letting the paper pass over heated cylinders, or by direct heating of the added acetylation medium by means of microwaves. The addition of heat needed is in its turn dependent on the temperature of the incoming paper and of the acetylation medium. The heat of reaction developed in the exothermic reactions between acetic anhydride and hydroxyl groups on one hand and moisture on the other should also be taken into consideration.

The acetylation medium can also be added to the paper in the form of vapour or gas which can be saturated or superheated. A rapid heating of the paper to be acetylated can be obtained in this way, and the reaction temperature can be kept high resulting in a more rapid acetylation reaction. The temperature should not be allowed to be too high (>180°C) for a longer period of time, since this might result in thermal degration and discolouration of the paper.

From the compartment where the acetylation reaction takes part, the acetylated paper is directly transferred to a compartment where remaining acetic anhydride and acetic acid added and formed are removed from the paper. This removal and recovery of chemicals can be performed in different ways from a technical point of view, and has to be decided with respect to the economics of the treatment.

SUBSTITUTESHEET Alternative methods for the removal of chemicals are by heating by applying vacuum, by introducing hot steam or gas, by rinsing with water, or a suitable combination of any of these treatments. Finally, the paper can be condi- tioned with respect to moisture content and temperature, this step being performed separately or integrated in the step constituting the recovery of chemicals.

The invention will now be further described through the following examples 1-4.

Example 1 (Table 1)

Three different kinds of liner (kraft paper) were subjected to acetylation using liquid acetic anhydride containing about 5% acetic acid, and without any added catalyst or organic cosolvent. Liner A with a basis weight of 140 g/m² (denoted A140) contained about 80% of virgin kraft softwood fibre, the remaining part being recirculated chemical fibre pulp. Liner B with basis weights of 186 and 400 g/m² (denoted B186 and B400, respectively) contained about 85% of virgin kraft softwood fibre, the rest being composed of recirculated fibre pulp containing also mechanical fibre pulp. Liner D with a basis weight of 145 g/m² (denoted D145) consisted of virgin kraft softwood pulp.

Liner papersheets with a moisture content of about 6% were impregnated with commercial grade liquid acetic anhydride and heated in a closed stainless steel reactor submerged in an oil bath. The reaction temperature was 80, 100 or 120°C as given in table 1. The reaction time was chosen in such a way that only a low degree of acetylation was obtained. After reaction, excess acetic anhydride and byproduct acetic acid were removed by washing the sheets with distilled water. The acetylated sheets were then pressed according to SCAN-standard 26:76 and dried and conditioned according to SCAN-standard P2:75.

TE SHEET Dried and conditioned sheets were tested for dry tensile strength according to SCAN-standard P38:80, and for wet tensile strength according to TAPPI standard T 456-87. The water soaking times were 4 and 24 hours, respectively.

The acetyl content, expressed as percentage of acetyl groups based on the weight of acetylated liner, as well as the tensile strength in dry and wet conditions are given in table 1. As can be seen from the figures presented, a substantial increase in wet tensile strength was obtained even at low acetyl contents as compared with the wet tensile strengths found for non-acetylated control sheets.

Example 2 (Table 2) Liner sheets of the kinds examined in example 1 but with different basis weight in two cases (A125 and B200) were acetylated and tested as described in example 1 with the exception that the heating of the acetic anhydride impreg¬ nated sheets was performed by microwaves. The reaction temperature was within the range 110-130°C.

The acetyl contents and the tensile strengths obtained are given in table 2.

Example 3 (Table 3)

Liner sheets of the different grades used earlier (A125, B186 and D145; of example 1) as well as a quality of testliner containing recirculated fibre pulp with a basis weight of 140 g/m² (denoted C140) were treated with acetic anhydride vapour in a stainless steel reactor. The reactor was connected to a storage tank for commercial grade acetic anhydride containing about 5% acetic acid which was kept at about 175°C.

Liner sheets were placed in the preheated reactor (about 150°C) which was then evacuated. Anhydride vapour was

SUBSTITUTE SHEET introduced from the storage tank and the vapour pressure in the reactor was recorded as given in table 3. The reaction time was 5 minutes or shorter. After reaction, the pressure was released and the sheets were removed from the reactor. The liner sheets were then treated and tested as described in example 1.

The acetyl contents and tensile strengths obtained are given in table 3. Acetylated liner sheets, A125 and C140, were also water soaked for 21 and 7 days, respectively, before testing of wet tensile strength. As shown in the table, the tensile strength in wet condition remained at a high level even after this prolonged water soaking.

Example 4 (Table 4)

Paper sheets with a basis weight of about 60 g/m² were handmade from unbleached thermomechanical pulp (denoted TMP, unbleached; cf. table 4) , bleached thermomechanical pulp (denoted TMP, bleached) and bleached chemithermo- mechanical pulp (denoted CTMP, bleached) , respectively. Sheets of the three kinds of paper were acetylated with liquid acetic anhydride to various acetyl contents, treated and tested for strength properties as described in example 1. As shown in table 4, the tensile strengths in dry condition were slightly improved for the acetylated samples within an acetyl content range up to 10-12% by weight as compared with the strength shown by the corresponding control. In wet condition, the tensile strength increased gradually with increased degree of acetylation, the non- acetylated control samples showing no measurable wet strength.

The tear strength was determined according to SCAN-standard Pll:73. The water soaking time was 30 s. The tear strength in dry condition was essentially unaltered, or in some cases slightly lowered, by the acetylation.

SUBSTITUTESHEET The different papersheets were subjected to accelerated light induced ageing using an Original Hanau Suntest CPS apparatus equipped with a Xenon lamp and filters giving irradiation above 310 nm. The paper brightness expressed as %ISO was measured according to SCAN-standard after differ¬ ent irradiation times (cf. table 5) . As can be seen from table 5, the papers made from the different kinds of mechanical pulps all gained in photostabilization due to the acetylation. This is especially noticeable in the case of unbleached TMP where increased brightness was achieved during irradiation.

Through the present invention one has obtained several advantages over the prior art:

(1) Retains the paper strength in dry condition at a high level.

(2) Renders the paper a high strength in wet condition.

(3) Reduces the light induced ageing of paper. (4) Eliminates the need for an added catalyst.

(5) Eliminates the use of any cosolvents or diluents.

(6) Allows the use of acetic anhydride containing acetic acid.

(7) Allows for a low degree of acetylation. (8) Reduces the time of treatment.

(9) Allows the acetylation to be performed with acetic anhydride in the liquid, vapour or gas state.

The invention is not limited to the examples and embodi- ents shown but can be varied in different ways within the scope of the succeeding claims.

SUBSTIT T Table 1. Liquid phase acetylation.

1) Water soaking before testing for 4 and 24h, respectiv¬ ely.

SUBSTITUTE SHEET Table 2. Liquid phase acetylation; microwave heating (110- 130°C) .

1) Water soaking before testing for 4 and 24h, respective¬ ly.

SUBSTITUTE SHEET Table 3. Vapour phase acetylation.

1) Water soaking before testing for 4 and 24h, respective¬ ly.

2) 53,1 N after 2 days of water soaking.

3) 26,1 N after 7 days of water soaking.

EET Table 4. Paper sheets made from mechanical pulps.

Table 5. Photostabilization of paper made from mechanical pulps.

Brightness, %ISO after different irradiation times (min).

Acetyl

Paper grade con¬ tent 0 5 15 60 120 240 480 960 1200 %

TMP, unbleached

Control 1.4 60,2 59,5 59,1 58,1 57,5 55,8 53,9 50,3 49,1

A 6,1 63,4 63,8 64,2 64,6 64,8 64,4 63,6 61,1 60,2

B 9,6 63,9 64,6 65,1 65,7 66,1 66,1 65,9 64,5 63,9 c 10,7 64,1 64,7 65,2 65,9 66,2 66,4 66,3 65,1 64,7

D 12,8 64,1 64,6 65,1 66,1 66,5 67,1 67,1 66,6 65,9

E 16,3 64,7 65,2 65,7 66,4 66,8 67,3 67,5 67,1 66,6

TMP, bleached

Control 0 76,9 75,5 74,1 70,7 68,5 65,3 60,9 56,1

A 4,6 75,8 75,3 74,6 72,9 71,7 70,1 65,7 61,5

B 6,4 75,5 75,3 75,1 74,2 73,2 72,1

C 11,4 74,5 74,2 73,9 73,2 72,5 72,1 70,8 69,6

D 12,6 73,9 73,8 73,8 73,6 73,1 72,4

CTMP, bleached

Control 0 79,9 77,8 73,5 71,6 69,1

A 4,4 78,6 77,5 74,3 72,9 70,9

B 9,1 78,2 77,8 76,5 75,8 75,1

SUBSTITUTE SHEET

Claims

1. Method for improvement of paper properties such as wet strength and resistance to light induced ageing c h a r a c t e r i z e d i n, that paper is subjected to acetylation with acetic anhydride without the use of any catalyst or subjecting the paper to any chemical pre- treatment.

2. Method according to the claim ^ c h a r a c t e r ¬ i z e d in, that the acetylation process yields an acetyl content of up to 15% by weight of acetylated paper.

3. Method according to the claim l or 2, c h a r a c ¬ t e r i z e d i n, that the acetic anhydride is supplied to the paper in the form of a liquid, vapour or gas.

4. Method according to any of the claims 1-3, c h a r a c t e r i z e d i n, that the reaction tempera¬ ture is maintained at 80°C-180°C.

5. Method according to any of the claims 1-4, c h a r ¬ a c t e r i z e d i n, that the acetylation is carried out as an intermediate or final step in the drying section in a paper machine or as a post operation.

6. Method according to any of the claims 1-5, c h a r a c t e r i z e d i n, that the paper before the acetylation has a water content not exceeding 20%, prefer¬ ably not exceeding 15%.

SUBSTITUTESHEET

7. Method according to the claims 1 and 6, c h a r a c t e r i z e d in, that the acetylation medium is sprayed on the paper or the paper is passed through the acetylation liquid medium.

8. Method according to the claims 1-6, c h a r a c t e r i z e d i n, that the acetylation medium is supplied to the paper in the form of vapour or gas in a compartment.

9. Method according to any of the claims 1-8, c h a r a c t e r i z e d i n, that the paper is heated by electromagnetic radiation preferably microwaves, by heat transfer from the drying cylinders of the paper machine or by hot gas.

10. Method according to any of the claims 1-9, c h a r a c t e r i z e d i n, that excess chemicals are removed after the acetylation by the steps of heating applying vacuum, introducing hot steam or gas or by rinsing with water or a combination of any of these steps before final conditioning or in combination with the final conditioning.