Technical Field of the Invention

The present invention relates to a system and process for manufacturing plasterboard liner. The plasterboard liner is preferably light weight Kraft based liner paper for the production of plasterboa rd .

Background of the Invention

As shown in Figure 1, the manufacture of plaster boa rd has traditionally involved the steps of:

1. rolling out a layer of face plasterboard liner (face PBL) ;

2. pouring a slurry of Gypsum over the face PBL;

3. rolling out a layer of back plasterboard liner (back PBL) (also referred to as a base) over slurry of gypsum;

4. cutting the plasterboa rd into sheets; and

5. drying the plasterboard sheets.

To be suitable for use in plasterboard, plasterboard liner (PBL) has previously had the following characteristics:

1. a high degree of dimensional stability with cross-direction wet expa nsion not exceeding 0.7%; and

2. high degree of tensile strength machine direction (MD) to cross-direction (CD) tensile ratio in the range of 2.4 to 2.8.

It is a lso desirable for face PBL to have a light coloured outer surface in anticipation of the plasterboa rd being later painted .

A known system for ma nufacturing PBL is shown schematically in Figures 2a to 2d . As shown, the system uses recycled paper fibre (RCF) as the main source of paper fibre. PBL manufactured using a majority of RCF can achieve the necessary end product (plasterboard) market characteristics of: smooth, flat, ripple free surfaces evident by visual inspection ;

high bending / flexural strength with breaking force MD:CD tensile ration not less than 2.4; and

face having a light colour with diffuse blue reflectance factor (also known as ISO brightness) not less than 40.

Over the past 15 years, PBL weight has been reduced from greater than 200 gsm to 150 gsm. This has been achieved through various incremental improvements that have allowed for reduced RCF volume and hence weight. These improvements include:

General increase in RCF strength (due to increased Kraft fibre production from virgin wood and subsequent OCC fibre increase) ;

General improvements to equipment, such as lower ply paper machines (i.e. from 7 ply to 3 ply);

Better process knowledge and quality control measures which, for example, has allowed for improved fibre formation; and

Additives, such as starch, which are used to increase paper tensile strength. PBL of weight greater than 150 gsm manufactured using 100% RCF has the advantages of high dimensional stability and tensile strength which is necessary for the subsequent manufacture of plasterboard sheet. However, it is generally desirable to produce a plasterboard liner that is lighter in weight and contains an appreciable content of virgin fibre. To this end, the lighter PBL would :

1. Reduce fibre input per PBL unit length;

2. Increase volume of virgin Kraft fibre product per unit extracted fibre;

3. Reduce transport cost per PBL unit length ;

4. Increase plasterboard unit length per PBL unit weight; and

5. Reduce plasterboard drying thermal energy per unit length .

PBL of weight less than 130 gsm manufactured using 100% RCF is not marketable as it does not meet tensile strength requirements for subsequent plasterboard manufacture. PBL with an appreciable content of virgin Kraft fibre is not marketable as it does not meet the dimensional stability requirements for subsequent plasterboard manufacture.

Tensile strength for paper is expressed as MD:CD ratio. This is the ratio of the maximum pulling stress before breaking of the paper sheet in both the machine direction (MD) and the cross direction (CD). In a paper sheet, fibres oriented in the MD lay in the longitudinal direction of travel of the paper sheet through the paper machine. Fibres oriented in the CD lay perpendicular to the MD. Traditional PBL of greater than 150 gsm has a high MD:CD ratio for both back and face PBL. This is because there is sufficient fibre available to allow non-specialised fibre orientation in the PBL compared with general paper making in which the predominant fibre orientation is in the MD.

Dimensional stability for paper means its resistance to change in size as the result of a change in moisture content. Paper fibres have a natural tendency to absorb moisture and hydro expand. A measure of dimensional stability is wet expansion, which is the percentage difference between a paper sheet's dry and wet size. Traditional PBL of greater than 150 gsm has a high dimensional stability in part because it does not contain an appreciable content of virgin Kraft fibre. Cellulose fibre used for paper making is recyclable. However, each time a fibre is recycled, it loses strength until it is no longer able to be used. Typically, a cellulose fibre can be recycled for paper making up to about 8 times. The term Kraft fibre is derived from the Kraft process technology for converting wood into wood pulp and extracting virgin cellulose fibres for paper manufacture. This process produces virgin Kraft fibre that has not previously been used for paper manufacture. 'Kraft' is a German word for 'strength'. However, the term Kraft fibre also is used to more broadly refer to cellulose fibres that retain strength similar to that of virgin cellulose fibre and so it can encompass fibres that have been recycled up to about twice. Kraft fibre is stronger than RCF and its inclusion in PBL can allow for reduced weight whilst maintaining the necessary paper tensile strength. However, the inclusion of virgin Kraft fibre in PBL reduces the dimensional stability as virgin Kraft fibre is more reactive in contact with moisture than recycled fibre. This is because the fibre is damaged through each recycle, which reduces its strength and also makes it less reactive to contact with moisture. The reactivity of cellulose fibre to contact with water is a known phenomenon to which the paper industry worldwide invests substantial resea rch . This is important in the manufacture of plasterboard sheet as the process involves contact with moisture via the gypsum slurry. As above-mentioned, it is also desirable for face PBL to have a light coloured outer surface in anticipation of the plasterboa rd being later painted . A light coloured surface for PBL face has previously been achieved through the addition of a Night colour fibre' layer during the manufacture process. The process to produce RCF light colour fibre imposes an ongoing operational expense and its feedstock of recovered white paper is becoming less available. Figures 2a to 2d show schematically how this 'light colour fibre' layer is produced from recovered white paper feedstock via a de-inking process during the stock preparation stage.

It is generally desirable to overcome or ameliorate one or more of the above mentioned difficulties, or at least provide a useful alternative.

Summary of the Invention

In accordance with the invention, there is provided a process for manufacturing plasterboard liner (PBL) for plaster boa rd, including the steps of:

(a) receiving virgin Kraft fibre ( VKF) ;

(b) receiving recycled paper fibre (RCF) ;

(c) blending the VKF and the RCF to produce multiple fibre blend streams;

(d) forming said streams into paper plys;

(e) forming the plys into a multi ply paper sheet;

(f) chemically sizing the paper sheet to increase its hydrophobicity; and

(g) drying the multi ply paper sheet over drying drums to produce the PBL, wherein the PBL is no more than 140 grams per square meter (gsm) . Preferably, the PBL is no more than 130 grams per square meter (gsm) .

Preferably, a Cobb (60 second) value of the PBL is less than 30 gsm.

Preferably, a cross direction wet expansion of the PBL is no g reater than 0.8%. Preferably, the step of blending results in multiple fibre blend strea ms each having a substantially even mixture of RCF and VKF. For example, the substantially even mixture includes greater than 35% VKF. Preferably, the PBL is for a base of the plasterboard and the process includes the step of manipulating fibres of the fibre blend streams so that a machine direction to cross direction tensile ratio (MD : CD) of the PBL is not more than 1.4.

Preferably, the PBL is for a face of the plasterboard and the process includes the step of manipulating fibres of the fibre blend streams so that a machine direction to cross direction tensile ratio (MD : CD) of the PBL is not less than 2.2. Further, the process advantageously includes the step of applying a pigment coating to the face PBL. The pigment coating preferably provides a light colour surface with diffuse blue reflectance factor not less than 30. Alternatively, the pigment coating preferably provides a light colour surface with diffuse blue reflectance factor not less than 40.

The present invention also provides plasterboard liner for use in manufacturing plasterboard formed from the above-described process. The present invention also provides base plasterboard liner for use in manufacturing plasterboard formed from the above-described process.

The present invention also provides face plasterboard liner for use in manufacturing plasterboard formed from the above described process.

The present invention also provides plaster board, including gypsum interposed between the above described face plasterboard liner and the above described base plasterboard liner. The present invention also provides a system for manufacturing plasterboa rd liner (PBL) for plaster board, including :

(a) fibre prepa ration and blending apparatus, said apparatus for performing the steps of:

(i) receiving virgin Kraft fibre ( VKF) ;

(ii) receiving recycled paper fibre (RCF) ; and blending the VKF and the RCF to produce multiple fibre blend streams; and

(b) paper making apparatus, said paper making apparatus for perfo

the steps of:

(i) receiving the multiple fibre blend streams from the fibre preparation and blending apparatus;

(ii) forming said streams into paper plys;

(iii) forming the plys into a multi ply paper sheet;

(iv) chemically sizing the paper sheet to increase its hydrophobicity; and

(v) drying the multi ply paper sheet over drying drums to produce the PBL,

wherein the PBL is no more than 140 grams per square meter (gsm). Preferably, the PBL is no more than 130 grams per square meter (gsm) . Preferably, a Cobb (60 second) value of the PBL is less than 30 gsm. Preferably, a cross direction wet expansion of the PBL is no greater than 0.8%.

Preferably, the step of blending results in multiple fibre blend streams each having a substantially even mixture of RCF and VKF. For example, the substantially even mixture includes greater than 35% VKF. Preferably, the PBL is for a base of the plasterboard and the system incl udes the step of manipulating fibres of the fibre blend streams so that a machine direction to cross direction tensile ratio (MD:CD) of the PBL is not more than 1.4.

Preferably, the PBL is for a face of the plasterboard and the system includes the step of manipulating fibres of the fibre blend streams so that a machine direction to cross direction tensile ratio (MD:CD) of the PBL is not less than 2.2. Further, the system advantageously includes the step of applying a pigment coating to the face PBL. The pigment coating preferably provides a light colour surface with diffuse blue reflectance factor not less than 30. Alternatively, the pigment coating preferably provides a light colour surface with diffuse blue reflectance factor not less than 40. Brief Description of the Drawings

Preferred embodiments of the present invention are hereafter described, by way of non-limiting example only, with reference to the accompanying drawing in which :

Figure 1 is a schematic diagram showing steps performed by a known system for manufacturing plaster board from plasterboa rd liner and gypsum slurry;

Figure 2a is a schematic diagram showing steps performed by a known system for manufacturing plasterboard liner from mainly recycled paper fibre;

Figure 2b is a schematic diagram showing steps performed for stock prepa ration by the known system shown in Figure 2a ;

Figure 2c is a schematic diagram showing steps performed for paper making by the known system shown in Figure 2a ;

Figure 2d is a schematic diagram showing steps performed for paper strengthening by the system shown in Figure 2a ;

Figure 3a is a schematic diagram showing steps performed by a system for manufacturing plasterboard liner;

Figure 3b is a schematic diagram showing steps performed for fibre prepa ration and blending by the system shown in Figure 3a ;

Figure 3c is a schematic diagram showing steps performed for paper making by the system shown in Figure 3a ;

Figure 3d is a schematic diagram showing steps performed for pigment coating by the system shown in Figure 3a ; and

Figures 4 to 9 show test data for plasterboard liner produced by the system shown in Figure 3a .

Detailed Description of Preferred Embodiments of the Invention The system 10 shown in Figures 3a includes apparatus 12 for manufacturing plasterboard liner (PBL) 14 of less than 140 grams per square meter (gsm) . For example, the system 10 can advantageously produce PBL ranging between l lOgsm and 130gsm. Advantageously, the system 10 achieves PBL of less than 140 gsm with one or more of the following : Back PBL MD : CD tensile ratio not greater than 1.4;

Face PBL MD : CD tensile ratio not less than 2.2;

CD wet expansion less than 0.8%;

Cobb value (60 seconds) less than 40 gsm; and

Face PBL light colour surface not less than ISO brightness 30.

The PBL thus achieves the necessary characteristics of dimensional stability, and tensile strength for the combined back and face to make it suitable for subsequent plasterboard sheet ma nufacture. The system 10 achieves these results using a substantially even ratio of recycled paper fibre (RCF) 16 and virgin Kraft fibre (VKF) 18, for example. Alternatively, the system uses any other suitable ratio of RCF and VKF to achieve a PBL 14 of less than 140 gsm.

The system 10 is also suitable for manufacturing plasterboard liner (PBL) 14 of less than 130 grams per square meter (gsm) . For example, the system 10 can advantageously produce PBL ranging between l lOgsm and 125gsm. Advantageously, the system 10 achieves PBL of less than 130 gsm with one or more of the following :

Back PBL MD : CD tensile ratio not greater than 1.4;

Face PBL MD : CD tensile ratio not less than 2.2;

CD wet expansion less than 0.8%;

Cobb value (60 seconds) less than 30 gsm; and

Face PBL light colour surface not less than ISO brightness 40. The PBL thus achieves the necessary characteristics of dimensional stability, and tensile strength for the combined back and face to make it suitable for subsequent plasterboard sheet ma nufacture. The system 10 achieves these results using a substantially even ratio of recycled paper fibre (RCF) 16 and virgin Kraft fibre (VKF) 18, for example. Alternatively, the system uses any other suitable ratio of RCF and VKF to achieve a PBL 14 of less than 130 gsm.

In either case, the lighter PBL 14 is advantageous across a number of manufacturing and commercial aspects including :

The market units of lighter PBL are increased for the same weight of paper produced and this has a commercial advantage for transporting the PBL 14;

• The paper product volume is increased for the same weight virgin Kraft fibre by mixing with RCF and this has a manufacturing advantage for a pulp mill in reducing the bottle neck imposed by the rate and cost of pulp production to output of paper products; and

• The plasterboard sheet manufacture drying energy is reduced for the same unit length of plasterboard sheet due to the reduced weight of the PBL 14.

The system 10 also advantageously solves the problem of a light coloured PBL face without additional fibre layer. The system 10 includes an option to apply a pigment coating 24 to PBL to achieve an ivory coloured surface with ISO brightness not less than 30, for example. Alternatively, the system 10 includes an option to apply a pigment coating 24 to PBL to achieve an ivory coloured surface with ISO brightness not less than 40, for example. This is achieved using an application process to the PBL surface. The advantage of the solution is that PBL face with a light coloured surface is manufactured without the need to use light coloured RCF and this avoids the need for RCF recovered white paper feedstock and the stock preparation de-inking process or the purchase of white fibre feedstock. The apparatus 12 includes:

1. fibre preparation and blending apparatus 20;

2. paper making apparatus 22; and

3. pigment coating apparatus 24.

By way of non limiting example, a detailed description on the operation of each apparatus 20 to 24 is set out below.

1. Fibre Preparation & Blending Apparatus 20

The fibre preparation and blending apparatus 20 receives RCF 16 and VKF 18 from preceding co-located manufacturing processes. The VKF 18 is manufactured using the Kraft process technology and refined to achieve paper properties using known process technology. For example, Visy's Tumut Kraft Mill in Australia produces high quality kraft paper for domestic and international markets using woodchips from softwood plantations in southern NSW supplemented by recyclable wastepaper. The RCF 16 is manufactured using recovered paper/cardboard from which recyclable cellulose fibres are extracted using known process technology. RCF 16 and water 32 are added to the RCF pulper tank 26. The contents of the pulper tank 26 is then passed through contaminant screens 30 to remove contaminant materials such as plastic and metal in a known step common to paper making using RCF. The RCF is then sent to the blend tank 34. VKF 18 and water 32 are added to the Kraft fibre pulper & pre-treatment tank 28. Charge control chemicals are added to maximise reactivity of the VKF 18 with sizing chemicals later added by the chemical additive apparatus 38 (described below in further detail with reference to the paper making apparatus 22) .. For example, adding anionic trash collector (ATC) at 1 to 2 KG per tonne to achieve 500 to 1000 uEq/l. The pre-treated VKF is then sent to the blend tank 34.

The blend tank 34 takes the pre-treated VKF 18 and the screened RCF 16 and generates multiple fibre blend streams of substantially even mixtures. As above mentioned, it is anticipated that other mixtures of RCF 16 and VKF 18 can be used with a view to obtaining PBL of less than 140 gsm or 130 gsm. For example, the mixture may be 40% VKF. However, for ease of description, the system 10 is described below with reference to the mixture being substantially even.

Each stream forms a ply of the multi ply paper machine 36 of the paper making apparatus 22 with a substantially even fibre loading on the total PBL of RCF 16 and VKF 18.

All paper products have a natural tendency to absorb moisture and hydro expand (called wet expansion). Even with internal and surface applied chemical sizing technology, wet expansion results in dimensional changes on the paper relative to the original paper in the dry state.

Certain fibres have a greater potential to reabsorb moisture than others. In the example of PBL, RCF has a lower level of wet expansion than VKF. However, RCF has a lower tensile strength than VKF. The blend tank 34 maintains a balance between RCF 16 and VKF 18 content of the fibre streams for each ply of the multi ply paper machine 36. It uses fibre slurry consistency and flow rate to achieve a specified RCF 16 and VKF 18 composition of the multiple outflow fibre streams. Each outflow fibre stream forms a ply and the multiple plys are combined through the paper making apparatus 22 to form a multi ply paper sheet. The distribution of RCF and VKF is very important within the respective paper plys as well as the ply split within the composite PBL. The blending of RCF 16 and VKF 18 to achieve the necessary fibre blend of the fibre streams is identified schematically in Figure 3b.

An advantage of RCF is than it has a lower level of wet expansion and significantly cheaper raw material to manufacture PBL. A disadvantage with RCF is that manufactured PBL has a lower tensile strength than, for example, an equivalent weight PBL using virgin Kraft fibre. The advantage of virgin Kraft fibre is that manufactured PBL has a vastly superior tensile strength. The disadvantage of virgin Kraft fibre is that manufactured PBL has a high level of wet expansion.

To obtain an optimal balance between the benefits of using VKF 18 to obtain higher tensile strength and minimising wet expansion, the blend tank 34 provides a substantially even mixture of RCF 16 and VKF 18 in multiple streams for the paper making apparatus 22. This results in an even fibre ratio across the total multi ply PBL sheet, for example. 2. Paper Making Apparatus 22

The paper making apparatus 22 includes: a. a multi ply paper machine 36, including :

i. paper fibre formation apparatus 37 (also known as paper machine wet end);

ii. chemical additive apparatus 38; and

iii. speed control apparatus 40; and

b. drying drums 42. The operation of each one of the above is below described in further detail. a. Multi Ply Paper Machine 36 The multi ply paper machine 36 manufactures paper sheet by first forming multiple fibre plys that are then combined into a multi ply paper sheet prior to drying. The formation of paper sheet using a 2 ply paper machine is shown schematically in Figure 3c, for example. i. Paper Fibre Formation Apparatus 37

The paper machine wet end 37 receives fibre from the blend tank 34 in multiple streams and this is carefully manipulated to achieve the required basis weight of the combined ply paper sheet, for example 130gsm, and composition of RCF 16 and VKF 18, for example, greater than 35% virgin Kraft fibre.

Importantly, the paper machine wet end 37 jet-to-wire ratio is carefully manipulated to control the physical fibre orientation of the paper sheet. This ratio, which is the difference between the speed of the fibre forming section jet and that of the forming section wire, is used to fine-tune a PBL's fibre structure. It also effectively determines the dominant directional tensile strength of the paper sheet in either the machine direction (MD) or cross direction (CD).

Traditionally the jet-to-wire ratio is less than unity, and the fibre content is high, and this 'draws' or 'drags' the fibres out in the MD and achieves a high MD:CD tensile ratio. For PBL the jet-to-wire ratio is carefully controlled to either 'drag' or 'rush' (when jet-to-wire ratio is greater than unity) the paper sheet to manipulate the proportion of fibre alignment in the CD. Combined with the low fibre content, this achieves MD:CD tensile ratios that differ substantially between back and face PBL. For example:

PBL Jet-to-wire Fibre content MD:CD tensile ratio

ratio

Back 1.000 120 gsm 1.1

Face 1.015 125 gsm 2.6 Importantly the jet-to-wire ratio and resultant fibre orientation also affects the dimensional stability through the degree of wet expansion. The advantages of very low MD:CD tensile ratio for back PBL is that it attains a lower CD wet expansion, for example 0.8%. ii. The Chemical Additive Apparatus 38

The chemical additive apparatus 38 chemically sizes the multiple paper substrate plys to increase hydrophobicity of both RCF 16 and VKF 18 thereby reducing the multi ply paper sheet's tendency to absorb liquid . The following steps are performed in that regard : a. the VKF 18 is pre-treated at the fibre preparation and blending apparatus 20 with charge control chemical technology to maximize the virgin Kraft fibre's ability to react with sizing chemicals;

internal sizing chemical technology is applied to the multi ply paper sheet by the chemical additive apparatus 38 using for example the following cationic rosin > 12 kg/t, aluminum sulphate (Alum) >24 kg/t, and Alkyl Ketene Dimer >3 kg/t

In addition, surface sizing chemical technology is applied at the coating machine 44 by the surface sizing apparatus 46, as shown in Figure 3d .

The steps performed by the multi ply paper machine 36, as described above, are shown schematically in Figure 3c, using a 2 ply paper machine for example. They result in improved dimensional stability and tensile strength and of PBL containing substantial content of VKF 18.

The PBL manufactured by the system 10 includes the use and sizing of VKF 18 to achieve the necessary dimensional stability and tensile strength suitable for the subsequent manufacture of plasterboard sheet. This, in turn, enables the plasterboard sheet to achieve its required market characteristics of flat, ripple free surfaces and high blending strength.

The system 10 solves the problem of refined VKF 18 which is reactive to contact with moisture and this makes it unsuitable for use in PBL as the required dimensional stability is not achieved. The manufacture of plasterboard sheet requires the PBL to be in contact with gypsum slurry as it is sandwiched between the face and back PBL. Too much wet expansion through water absorption from the slurry during pressing to adjust thickness can result in subsequent rippling of the end product surfaces following drying of the slurry to produce the plasterboard sheet.

The system 10 can use various functional sizing chemical technology for PBL . A combination of sizing chemicals is used, for example : a. cationic rosin > 12 kg/t;

b. aluminum sulphate (Alum) >24 kg/t;

c. Alkyl Ketene Dimer >3 kg/t; and

d. surface sizing agent 3-12 kg/t. Both VKF 18 and RCF 16 are treated with internal sizing chemical technology to manufacture PBL grades.

Functional sizing chemical technology reduces the natural tendency of fibres to reabsorb moisture after paper has been dried. Internal sizing chemical technology reduces the paper tendency to absorb water by attaching hydrophobic molecules to the fibres. These molecules effectively repel water before water is able to penetrate into the fibre structure.

To further enhance hydrophobicity, surface sizing chemical technology is also applied at the surface sizing chemical apparatus 46 of the coating machine 44. Here a limited amount of sizing chemical is applied as a layer to create surface barrier to reduce water penetration into the PBL.

2(a)(iii) & 2(b). Speed Control Apparatus 40 & Drying Drums 42

The speed control apparatus 40 acts to reduce the draw between successive processes on the formed paper sheet through its pressing and drying . This reduces the tendency for formed paper sheet to react when contacted with water thereby reducing the hydro expansion. This feature is identified schematically in Figure 3c. Successive paper machine sections (press, dryer and reel) relative speed controls are carefully controlled by the speed control apparatus 40 to minimize fibre draw in the MD before application to the drying drums 42 to minimise sheet shrinkage to less than 2% for example.

3. Pigment coating apparatus 24

The pigment coating apparatus 24 includes: a. a coating machine 44;

b. surface sizing chemical apparatus 46;

c. pigment application apparatus 48; and

d. drying drums 50. Traditional Ivory PBL utilizes de-inked RCF from recovered white paper feedstock to manufacture the face (Ivory) grades. With the incorporation of the coating machine 44, for example a film press, there is the option of pigment coating the PBL 14 to resemble the ivory sheet appearance for face PBL, or any other desired colour. The coating refers to a layer of material from a single stage application, for example including pigment formulation, starch and sizing agent.

This offers the benefit of improved strength gains associated with starch to offset the negative effect of the pigment coating and an overall benefit of lower cost of pigment. Figure 3d shows schematically how the coating machine 44 is used for this feature.

The advantage of pigment coating is that coating can be formulated to achieve the desired surface appearance, for example light coloured face PBL of ISO brightness not less than 30. Alternatively, the coating can be formulated to achieve the desired surface appearance, for example light coloured face PBL of ISO brightness not less than 40. Pigment coating is significantly cheaper than light coloured RCF used in traditional Ivory grades. A disadvantage is that pigment coating chemicals do not have any tensile strength enhancement of the PBL. As such, the tensile strength must be derived from the VKF 18 and the RCF 16. The coating machine 44 works by enabling a finite amount of pigment coating film to be metered onto one (or two) rolls through which paper runs. The pigment coating film effectively transfers onto the sheet once the paper passes through the rolls. The amount of pigment coating is carefully controlled by the coati ng recipe, its solid content, the applicator rod selection and contact time on the rolls.

The pigment coating recipe includes a mixture of different clay and/or calcium carbonate pigments and binders.

Coating machines for single stage applications have previously been installed on paper machines to enhance dry strength with the application of modified starches. In the example shown in Figure 3d, the coating machine 44, for example a film press, includes the pigment application apparatus 48 and the surface sizing chemical apparatus 46. The incorporation of the pigment application apparatus 48 allows the coating machine 44 to add a coating pigment in addition to the applied starch. The reason for the addition of pigment is to affect the surface appearance of the face PBL.

The addition of the surface sizing chemical apparatus 46 to the coating machine 44 allows for blending surface sizing chemical technology with starch, water or coating pigment to enhance hydrophobicity of the paper and achieve a coating thickness of 7- 8 ml/m2 for example. In this application the surface sizing chemical is 100% retained on the face PBL sheet. Figure 3d shows schematically how the coating machine 44, for example a film press, is used for this feature.

The advantages of the coating machine 44 is that cheaper alternative pigments eg. clay and calcium carbonate can be precisely metered onto the PBL sheet to replicate an Ivory ply for face PBL. Other advantages are that the appearance can be modified with recipe of coating and application loading.

Alternatives to a coating machine 44 are: a. For light colour: to use a de-inked RCF from recovered white paper feedstock on the face PBL (Ivory ply);

b. For dry strength enhancement: to use specialized dry strength chemicals in the fibre recipe; and

c. For surface sizing : there is no effective alternative as only internal sizing chemicals can be applied using the paper machine.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention. To this end, the apparatus for manufacturing plasterboard liner 12 has been above described, by way of non-limiting example, with reference to the following being separate apparatus:

1. fibre preparation and blending apparatus 20;

2. paper making apparatus 22; and

3. pigment coating apparatus 24.

However, in practice, the above is alternatively formed as one unit, or many separate units. Sizing for paper means the use of chemicals to reduce its tendency to absorb liquid. Internal sizing chemicals are applied during paper formation to be incorporated throughout the paper sheet through bonding to cellulose fibres to increase their hydrophobicity. Surface sizing chemicals are applied as a thin coating on the paper sheet and have a hydrophillic end that joins to the cellulose fibre and a hydrophobic end that faces away to make the paper surface more resistant to the penetration of liquid. A measure of resistance to liquid absorption is the Cobb value, which is the surface water absorption in grams per square meter over 60 seconds. As the manufacture of plasterboard sheet requires PBL to be in contact with moisture via the gypsum slurry, PBL is 'hard sized' paper. This means it is in the category of papers with the highest water resistance.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that the prior art forms part of the common general knowledge in Australia

In this specification and the claims that follow, u nless stated otherwise, the word "comprise" and its variations, such as "comprises" and "comprising", imply the inclusion of a stated integer, step, or group of integers or steps, but not the exclusion of any other integer or step or group of integers or steps. References in this specification to any prior publication, information derived from any said prior publication, or any known matter are not and should not be taken as an acknowledgement, admission or suggestion that said prior publication, or any information derived from this prior publication or known matter forms part of the common general knowledge in the field of endeavour to which the specification relates.

Summary of Paper Trials:

Trial 1 Initial base line study to determine quality at normal ply loading at 30% RCF substitution to the top wire (TW) (also referred to as the "bottom ply") .

Trial 2 Second baseline study based on 50 : 50 ply loading . RCF loading increased to 40% and refining study to understand impact on porosity. Secondary objective to establish the impact of the BW FormMaster.

Moderate water absorption test performed on paper (COBB) 25-30 at maximum CRS size dosage for normal wet end chemistry. The test resulted in significant rippling associated with Wet Expansion .

Trial 3 Trial to test the baseline to understand %RCF and Tensile ratio on Wet Expansion . Also TOPKraft j umbo. The trial also included manufacture K110PB at low Tensile ratio and TOPKraft jumbo.

The trial resulted in significant improvement in back rippling . That is, it was less defined and finer. The TopKraft paper did not show any benefit. That is, it appeared easier to delaminate.

Lower tensile ratio 260-280 N MD breaking strength MD (AS/NZ minimum 360N ) . Trial 4

Implement trial 3 machine set up at lowest Tensile ratio. Focus on improving wet end chemistry for maximum or HIGH SIZE cationic rosin size efficiency. That is, the lowest COBB capability.

· Incorporating anionic trash collection application • Establish effect on Wet Expansion at extreme Tensile ratio set-up

• Rewettinq paper at VPC to reduce internal strain with water uptake on gypsum application

• Third Plasterboard trial 14/06/2013 at Boral Port Melbourne

· Rippling for low Tensile ratio was as good as second trial

• High Tensile ratio paper was not tested

Trial 5

• ASA (Alkyl Succinic Anhydride) size conversion trial - achieve as low COBB as possible

• Substitute 60% RCF for 110 gsm and 115 gsm

• Rewetting paper at VPC to reduce internal strain with water uptake on gypsum application

• Fourth Plasterboard trial 11/07/2013 at Boral Pinkenba

· General consensus hard sized ASA compared with hard sized Rosin sized paper for rippling

• 60% RCF substitution reel gave the best OVERALL result on rippling

• Confirmed by the lowest Wet Expansion < 0.8%

Trial 6

Objective to establish the max. and min. Tensile MP at 110 gsm and 125 gsm. Standardize 50% RCF for 115 gsm and 125 gsm. Rewetting paper at VPC to reduce internal strain with water uptake on gypsum application . · Combination max. and min. Tensile ratio was ran for FACE and BASE

• Max. Tensile MD paper acceptable for FACE break strength

• MD Break Strength K110PB 280 N ; K125PB 380 N (Spec 360N)

• BASE rippling is almost acceptable but more work required to achieve PN 150 flatness Process Capability and Expected Quality:

Minimum Wet Expansion (BASE) K110PB K125PB a. Tensile MD 50-52 N.m/g ! 5.5-5.7 kN/m 6.25-6.5 kN/m b. Tensile CD 40-42 N.m/g 4.4 - 4.6 5.0-5.25 kN/m c. Tensile ratio 1.0 - 1.1

d. Porosity 250 - 300 ml/min

e. ASA Sizing. COBB does not correlate with wet expansion. EMCO retests on Trial 4 and Trial 5 indicates a significant difference in initial water hold out (40-80%) with ASA hard sized paper.

f. Higher RCF substitution UP to 60%

g. Minimum refining levels

h. Lowest risk of rippling at Wet Expansion 0.7- 1.0%

Balanced Tensile Strength (BASE) K110PB K125PB

a. Tensile MD 55-60 N.m/g 6.0-6.6 kN/m 6.9-7.5 kN/m b. Tensile CD 45 N.m/g 5.0 kN/m 5.6 kN/m c. Tensile ratio 1.2- 1.5

d. Porosity 230 - 250 ml/min e. ASA Sizing.

f. Higher RCF substitution up to 50%

g. Moderate refining levels CSF 500-550 CSF

h. Higher risk of rippling with Wet Expansion 1.0-1.5%

Highest Tensile Strength (FACE) K110PB K125PB

a. Tensile MD 70-73 N.m/g 7.7-8.0 kN/m 8.8-9.12 kN/m b. Tensile CD 23 - 30 N.m/g 2.5-3.3 kN/m 2.9-3.75 kN/m c. Tensile ratio >2.7

d. Porosity 200 - 220 ml/min e. High Size Cationic Rosin High Size.

f. RCF substitution up to 50%

g. Max refining levels < CSF 500 CSF

h. Highest risk of rippling with Wet Expansion > 1.5% Chemical additives

Fibre preparation sizing :

• Anionic trash collector 3kg/t 500-1000uEq/l

Paper making internal sizing :

• Cationic rosin > 12 kg/t

• Aluminum sulphate > 24 kg/t

• AKD (Alkyl Ketene Dimer) > 3 kg/t

Film press surface sizing

• Surface sizing agent 3-12 kg/t

General :

1. BASE application best option is Capability 1 (lowest tensile ratio) K125PB: a. Expect plasterboard MD breaking strength 300 N ( in combination with PV170) b. ASA hard sized 4.5-5 kg/Adt

c. Lowest risk of rippling

2. FACE application best option is Capability 3 (highest tensile ratio) K125PB and high sized ROSIN :

a. Rosin hard sized 10-11 kg/Adt

b. Expect plasterboard MD breaking strength >360 N ( in combination with PV170)

c. Lowest risk of board break on the gypsum line

3. Although COBB does not correlate well with Wet Expansion, EMCO testing indicates a difference between hard sized ASA vs ROSIN :

a. ASA appears to have superior initial water hold-out Glossary

TW Top Wire (Bottom ply)

GSM Grams per square meter

BW Bottom Wire (Top ply)

FormMaster Name of papermaking Equipment

COBB Water absorptiveness (cobb value) is the mass of water absorbed in a specific time by one square meter of paper, board, or corrugated fibreboard under one centimetre of water.

CRS cationic Rosin Size (rosin is a chemical added to achieve water holdout).

jumbo jumbo / master reels of paper

K110PB Paper grade

MD Machine Direction - paper direction along the length of the paper machine.

VPC Visy Paper Coatings

K125PB Paper grade

CD Cross Direction - paper direction across the width of the paper machine.

EMCO Water absorption test equipment name.

CSF Canadian Standard Freeness. A measure of the drainage of paper pulp linked to refining and pulp quality.

PV170 170gsm Ivory Face paperboard liner

TSI Tensile Stiffness Index. Ultrasonic, non-destructive test method for determining the strength of paper, and plaster board liner.

ABB AWP Moisturising bar from ABB used at VP9

furnish paper going to pulping stage of papermaking.

WIS Web Inspection System - hole detector

WW White Water - papermaking water removed from wet sheet and recycled back into the process water.

PSD Paper machine shutdown

WRV Water Retention Value - laboratory test for delivering how much water a pulp will hold under controlled conditions.

Mutec Laboratory equipment for measuring charge in water.