AU2002344714A1 - Apparatus and method for improving paper strength - Google Patents

Description

APPARATUS AND METHOD FOR IMPROVING PAPER STRENGTH

This invention relates to an apparatus and a method for improving the strength of paper.

Paper and paper products perform an enormous role in today's society. They provide the means for recording, storage and dissemination of information, it is the most widely used material for packaging and wrapping and is also used in structural applications. The uses and opportunities for paper and paper products are ever expanding and in order to pursue these opportunities, paper strength is a quality that is critical.

Paper is traditionally manufactured in a paper mill from pulp. Pulp is the fibrous raw material used to make paper and is usually of vegetable origin. Pulp is generally produced from wood or from recycled paper. It can be prepared mechanically, thermally or chemically or by a combination of these means.

The pulp, once prepared and processed is then subject to beating and/or refining in order to develop the optimal papermaking properties for the product being manufactured. Any necessary wet end additives are added before the pulp is fed into a headbox (or flowbox) or vat (see below) in the form of a slush or slurry. The headbox distributes the paper stock onto the moving forming wire or between two wires. The moving wire forms the fibres into a sheet, with more plys being added as required, from which some of the water is removed as the paper is formed. Paper can also be formed on cylinder which rotates in a vat which contains a pulp slurry or the pulp is again applied to the cylinder via a headbox. The pulp drains into the mesh-covered cylinder, leaving a layer of fibre on the surface to become a sheet of paper. The paper sheet is removed from the cylinder surface onto a felt which then goes to another cylinder to collect another ply. This process continues until the desired equivalent dry weight of fibre is achieved. The paper is then processed through a press section where more water is removed by pressing and this process also assists in pressing the fibres closer together. The majority of the remaining water is then evaporated in the drying section, which also increases the paper strength as it dries.

Finally, the paper can be calendered to smooth the surface, which will reduce thickness, and it is then wound into a roll.

It has been an ongoing challenge to improve the basic paper manufacturing technique to increase paper strength. An increase in paper strength also allows new applications to be developed for paper and paper products. It is also desirable to increase process speed and to thus provide a product at a lower price or increased volume.

In the past, various attempts have been made to increase paper strength. In particular, additives have been used to increase physical strength, generally by assisting the fibres to bond together to a greater degree. Typically, starch has been added to the product at various points in the process to improve the strength of the paper. Starch can also provide improved surface characteristics and improved physical properties to the sheet of paper. Other additives such as PVA can also be used.

Starch is commonly added in liquid form to the papermaking process at a "size press". This is usually located between half way and three quarters along the length of the dryer train. Here the liquid penetrates the sheet and hence transfers the starch into the sheet of paper where the solvent (usually water) is dried leaving the starch to bond to the fibres. Starch is generally added in a converted or modified form - typically oxidized or enzyme converted. This method of starch addition whilst making an improvement in paper strength has limitations dictated by the penetration rate of starch into the paper at the size press. It is not possible to distribute increasing amounts of starch through the z- direction (thickness direction) of the paper to achieve the optimum profile for strength development. This is most evident on thicker papers. Use of this process also requires the rewetting of the paper web and consequent redrying adding to operating cost and limiting production rate.

Chemically modified starch can also be added to the pulp slurry at the flowbox. The chemical modification involves adding functional groups to the starch molecule to alter its characteristics, i.e. it may become charged (i.e. cationic) which assists it to adhere to fibres as the sheet is being formed. This starch is converted before being added to the pulp prior to its entry into the flowbox (headbox). The presence of starch attached to the fibres in the sheet improves the characteristics of the paper particularly with respect to strength properties. Attempts have also been made to add unmodified or partly modified starch to paper, where the starch is further converted by the heat in the drying process. This process requires a good retention system to ensure the retention of the starch in the sheet as it forms. As the paper is dried, the starch granules in the sheet will expand and bond surrounding fibres together. This increases the strength properties of the paper if added in moderate amounts, but if increasing amounts are added, the strength properties decline or stabilise at a moderate level due to the disruption to the structure caused by the expansion of the starch as it is converted.

Starch has also been added by spraying unconverted (uncooked) starch into the forming sheet of paper. Although this method is often used to bond two plies of paper together, it can produce variable results depending upon the spray method used.

Thus, there is an ongoing need to find a method of manufacturing paper which increases paper strength compared to current techniques. The invention therefore relates to a method of manufacturing paper having improved strength characteristics, comprising: combining paper fibre and unconverted starch entering a headbox such that the starch is retained; forming a paper web; drying said paper web to a moisture content of about 60% to about 48%; converting said starch within said paper web; and passing said paper web through a dryer train which includes pressing said paper web.

More particularly, a dry press is normally positioned in the dryer train of the mill to press the forming paper when it has a water moisture content of about 55% to about 20%. More preferably, the moisture content of the paper when dry pressed is about 45% to about 30%, most preferably about 40%. Furthermore, the invention relates to the provision of a dry pressing process to be applied to the paper after conversion of the raw (wet end) starch in the paper web and whilst the paper web is relatively moist.

It is highly desirable that the paper fibre and starch be pretreated before the headbox with polyacrylamide and bentonite for retention of the starch. The resultant floes from the addition of polyacrylamide are sheared before bentonite is added. Other additives such as converted cationic starches can also be added as required.

The invention further relates to a paper mill for manufacturing paper having improved strength characteristics, comprising: a headbox for distributing a paper fibre and starch slurry evenly; a paper forming section for forming a paper web; a pressing section which reduces moisture content to about 60% to about 48%; a section of a dryer train to heat and to convert said starch within said paper web; and said dryer train also including a dry press for said paper web.

The invention is applicable to all types of paper, for example tissue, printing paper, paper/cardboard and any other packaging, tubewinding grades, wrapping paper, and others. It is particularly relevant to paper making processes which include starch addition to the pulp.

DESCRIPTION OF FIGURES

Figure 1 shows a representation of a paper mill. Figure 2 shows a representation of a paper mill designed to produce paper of increased strength.

DETAILED DESCRIPTION

The pulp is prepared from wood or recycled paper and as appropriate is cleaned, screened and refined. It is then diluted to a concentration of about 1%

(which can vary depending on machine type). The highest concentration is expected to be about 4% and the lowest about 0.2% of fibre in water. The fibre can be added as a single ply or as a multi-ply. Starch is added in a large range of concentrations (i.e.% starch in final product) with lower concentrations, about 1 %, in some fine paper or wrapping grades to some very high strength grades such as coremaking paperboards levels where levels of over 12% are possible. The starch is the native or partly converted or partly modified starch. It is added at the desired level to achieve the required physical properties of the product.

Chemicals can also be added to retain the starch during forming of the sheet and for other desired properties (such as increased retention of fibre and additives, and increased water removal). Further starch can also be added for other desired products and this starch is added in a converted (cooked) liquid form, a chemically modified liquid form or as a granular (unmodified or partly modified) suspension in water at various points in the process. The starch addition points are well known to a person skilled in the art of paper-making and, for example, would include wet-end addition at many possible points in the approach to the machine, spraying to the wet web and at a size press. Product demands and equipment design will determine the points used. The starch can be modified (by cooking, with or without chemical addition, at elevated temperatures) or modified by chemical additives to develop specific properties. Typical levels of chemicals added for this purpose are bentonite (2 to 5 kg per tonne), polyacrylamide (0.2 to 0.5 kg per tonne) and modified starch (0 to 40 kg per tonne). "Per tonne" means per tonne of finished product. The bentonite and polyacrylamide form the basis of the Ciba Hydrocol system where the polyacrylamide is added to the dilute pulp then the mixture is sheared to break the floes that form. The bentonite is then added to re-floc the fibre in smaller "microflocs" which results in better paper making properties (being formation, retention, etc.). The use of the Hydrocol system for starch retention is the basis of the Ciba Hyforce system (available from Ciba). Again this starch can be native or modified, and can be unconverted or partly converted.

Figure 1 shows the headbox (1) (sometimes named a "flowbox") where the fibre containing the starch is added in a traditional mill that is using chemically modified starch. A multi-ply machine can have more than one headbox. The starch can be added to the diluted pulp prior to entering the headbox (thin stock), or alternatively, the starch can be added to the undiluted pulp (thick stock) further back in the machine approach system. The starch can be modified in some way, either converted or chemically modified (for example, into cationic starch, or amphoteric starch). The pulp becomes a sheet of paper as it passes through the forming section (2) and once formed, the paper web passes through the press section (3). This section presses water out of the paper web and reduces moisture content to about 50% to about 60%. The paper web then enters the dryer train (4) where further moisture is evaporated out of the paper web after which it may be further processed by calendering before rolling onto reels (not shown). A size press (5) can also be present in the dryer train. The size press is used to apply chemical additives in a wet form to the paper to enhance some of its properties such as starch to increase compressive strengths or dyes and coatings for appearance.

In the mill and method of the invention, the process occurs as demonstrated in Figure 2. The process commences by the introduction of fibre, starch, bentonite and polyacrylamide to the water circulating to the headbox in the following concentrations: fibre (0.2% to 4 %, preferably about 1 %), starch (0 to 8%, addition rate is variable dependent of product specification), polyacryamide (100 to 500 g/tonne, preferably 250 g/tonne, but this will vary depending on fibre quality). Polyacrylamide is currently the material of choice but other polymer types can also be used. The starch selected will depend upon the nature of the paper mill, but it is necessary that the starch selected has a gel point (temperature) that will be achieved by the temperature of the dryers. The polyacrylamide produces floes of fibre, which are then broken up by shearing. Bentonite is then added in a concentration of 0.5 to 6 kg/tonne of product (the addition rate being determined by the quality of the fibre used to make the product) to re-floc the fibre. The pulp treated in this way is then passed through the forming section (2) and once formed, passes through the press section (3). The press section presses water out of the paper web and reduces water content to about 60% to 48% (depends on machinery used, settings, grade made, and fibre quality).

In a preferred embodiment, the forming paper enters the dryer train (4) with a moisture content of about 60% to 48%, preferably about 54%. The pre- dryers then heat the sheet to the gel point (temperature) of the starch, the conversion of starch occurs causing it to expand, disrupting the sheet structure. The dryers then reduce the moisture content to about 40% to 20% preferably about 35%, and the sheet is subsequently pressed by a further press (6) provided in the dryer train (4) for strength development.

It is preferred that the dry press takes the form of a breaker stack or hot press (e.g. marking press), but a size press or a band press could also be used. The principal function of the dry press is to press the sheet, as distinct from a size press for which the principal function is to add chemical additives to the paper. The dry press may be treated with additives (i.e. cleaning chemicals) that improve its performance as a dry press but do not detract from or compromise its principal function as a compaction press.

In particular, it is believed that this further pressing assists to redistribute and compress the expanded starch particles to produce a paper product with superior strength properties. The press ensures that the paper fibres and expanded starch particles are compressed more closely together. It is then further dried to a moisture content of about 10% to 4%, preferably about 8% before being reeled into large rolls until further processed. Calendering is optional.

In general, the set up of the paper mill required to achieve the invention will be similar to the set up required when raw starch is sprayed onto a forming paper web, with the exception of the inventive addition of a further dry press.

The exact location of the dry press in the drying section of the plant will vary depending on a number of parameters including sheet temperature, starch characteristics, moisture, runnability of the machine, sheet compaction and the strength development required. It is possible to provide a heating device designed specifically to reach the gel temperature of the starch converting it in the sheet ahead of a conventional dryer train and/or pressing process (for strength improvement). The term "dryers" and "dryer train" in this text include this heating section as the start of the heating and drying process.

The process described above uses a press to redistribute starch that has been converted (completely or partially) whilst in the formed sheet of paper. The starch has been added as native starch, or in various stages of partial conversion. Modified starches can also be used.

The process has been demonstrated to date on several products. Examples are:

■ A high strength coremaking board (380 gsm) made with this process has a compressive strength 30% higher than a similar product made in a conventional process. Internal bond of this board is also 20% higher using 3% to 4% total starch. ^■ A medium weight test liner (190 gsm) is now commercially manufactured that is 20% less weight than the (conventionally manufactured) product it replaced, for the same compressive strength specification. 3% to 4.5% total starch was used.

^■ A mid-weight fluting medium (145 gsm) is currently being made to existing specifications (as conventional product) but at a 20% increased machine output rate. 3% to 4.5 % total starch was used. The following graph illustrates data from a trial to quantify (ring crush) strength development (using a dryer section press, 35 PLI (pounds per linear inch), 35 to 40% moisture) on a conventional 233 gsm paperboard (non-size press treated) with various levels of native tapioca starch addition in accordance with the above description, incorporating the addition of polyacrylamide and bentonite with cationic starch. The results in this trial were achieved with 5 kg/tonne of cationic starch, which is not included in the % Starch calculation on this graph. (This cationic starch was not further converted or modified during the paper making process and was hence not redistributed by the dry press.) RING CRUSH Vs STARCH

Starch %

This data shows how ring crush increases as the percentage of starch added is increased.

The process can be used on a wide range of paper types, hence the range of conditions under which this process operates must be varied considerably depending on the specifications for the final product, quality of fibre, design of the paper machine being used, and operating conditions such as retention, temperature, etc. Process knowledge will determine the specific conditions required for each application and each type of starch used but the critical issues such as those covered above (i.e. press location, moisture levels) must be operated within the specified ranges.

At the start and end of trials, the process has been observed to operate in conjunction with a conventional size press to produce paper board of exceptionally high compressive strength. Further increases in the ring crush index of 135 gsm paper of 15% (above size press (only) strengths) have been observed.

Claims (13)

1. A method of manufacturing paper having improved strength characteristics, comprising: combining paper fibre and unconverted starch entering a headbox, such that the starch is retained; forming a paper web; drying said paper web to a moisture content of about 60% to about 48%; converting said starch within said paper web; and passing said paper web through a dryer train which includes pressing said paper web.

2. A method as claimed in claim 1 , wherein starch retention is achieved by use of the addition of one or more polymers and bentonite.

3. A method as claimed in claim 2, wherein polyacrylamide, starch and bentonite are added sequentially to said paper fibre.

4. A method according to any one of claims 1 to 3, wherein conversion of starch occurs by application of heat.

5. A method as claimed in claim 4, wherein the heat causes said starch to reach its gel point.

6. A method as claimed in claim 5, wherein pressing said paper web causes starch converted in the web to be redistributed and/or compressed within said paper web.

7. A paper mill for manufacturing paper having improved strength characteristics, comprising: a headbox for distributing a paper fibre and starch slurry evenly; a paper forming section for forming a paper web; a pressing section which reduces moisture content to about 60% to about 48%; a section of a dryer train to heat and to convert said starch within said paper web; said dryer train also including a dry press for said paper web.

8. A paper mill as claimed in claim 7, wherein starch retention is achieved by use of the addition of one or more polymers and bentonite.

9. A paper mill as claimed in claim 8, wherein polyacrylamide, starch and bentonite are added sequentially to said paper fibre.

10. A paper mill as claimed in any one of claims 7 to 9, wherein conversion of starch occurs by application of heat.

11. A paper mill as claimed in claim 10, wherein the heat causes said starch to reach its gel point.

12. A paper mill as claimed in claim 11 , wherein pressing said paper web causes starch converted in the web to be redistributed and/or compressed within said paper web.

13. A paper product having improved strength prepared by the method of any one of claims 1 to 6.