Specifying Plasterboard Liner for Moisture-Resistant (MR) Applications

'''## Introduction: The Critical Role of Liner in MR Boards

Moisture-resistant (MR) gypsum boards, often identified by their green or blue paper liner, are a standard building material for environments with intermittent exposure to humidity, such as bathrooms, kitchens, and basements. According to the EN 520 standard, these boards are classified as Type H, with varying levels of performance (H1, H2, H3). Achieving the stringent requirements for a Type H1 board—defined by a total water absorption of less than 5% and surface water absorption of less than 180 g/m² after a 2-hour immersion—depends critically on the synergy between the gypsum core and the paper liner. While the core contains additives like silicones or waxes to reduce water uptake, the paper liner serves as the first line of defence. An improperly specified liner can compromise the entire board's performance, leading to field failures and reputational damage.

This article details the key technical parameters for specifying and qualifying plasterboard liner for MR applications. It is intended for plant managers, quality control personnel, and procurement teams involved in the production of high-performance gypsum boards. We will cover water absorption targets, the chemistry of water resistance, fibre furnish considerations, on-line performance, and essential questions for supplier audits.

The Primary Metric: Water Absorption and Cobb 60 Targets

The single most important property of an MR liner is its resistance to water penetration. The industry-standard method for quantifying this is the Cobb test, as defined in ISO 535. The test measures the mass of water absorbed by 1 square meter of paper in a specified time. For plasterboard liner, this is typically the "Cobb 60" value, denoting a 60-second test duration.

A standard ivory or greyback liner used for regular wallboard might have a Cobb 60 value in the range of 25–35 g/m². This level of absorbency is necessary to ensure good adhesion of the gypsum slurry and proper bonding. In contrast, for a high-performance MR liner intended for H1-type boards, the Cobb 60 value must be significantly lower.

Typical specification targets for a quality MR liner are **< 10 g/m²**, with many top-tier producers aiming for **< 8 g/m²**. It is crucial to specify not just the maximum value but also the acceptable statistical distribution (e.g., Cpk > 1.33) to ensure process consistency. A liner with high variability, even if the average is within spec, can lead to unpredictable board performance, with some boards passing QC while others from the same batch fail.

Lowering the Cobb value creates a more water-repellent surface, but it comes with manufacturing trade-offs. Overly low Cobb values can impede the penetration of starch-based adhesives used to bond the liner to the gypsum core, potentially leading to delamination issues on the board line. Therefore, the specification must balance water resistance with the physical necessity of achieving a strong, permanent bond.

Sizing Chemistry: The Science of Water Resistance

The low water absorption of MR liners is achieved through the use of sizing agents that make the paper hydrophobic. These chemicals are applied either internally during the stock preparation phase ("internal sizing") or on the surface of the paper sheet at the size press ("surface sizing").

**Internal Sizing:**

• **Alkyl Ketene Dimer (AKD):** This is one of the most common internal sizing agents for MR liner. AKD is a wax-like emulsion added to the pulp slurry. During the drying process on the paper machine, the AKD molecules form covalent ester bonds with the hydroxyl groups of the cellulose fibres. This chemical reaction creates a stable, hydrophobic matrix within the paper sheet. The reaction is relatively slow and continues to "cure" for days or even weeks after production, meaning the final Cobb value may not be reached until after the paper has left the mill.
• **Alkenyl Succinic Anhydride (ASA):** ASA is another highly effective internal sizing agent. It is more reactive than AKD and forms ester bonds with cellulose much faster, often achieving full cure on the paper machine itself. While highly efficient, ASA is more challenging to handle in the paper mill as it hydrolyzes quickly when in contact with water, requiring specialized emulsification and dosing equipment.

**Surface Sizing:** In addition to internal sizing, most MR liners receive a surface treatment at the paper machine's size press. This typically involves applying a solution of cooked starch mixed with a synthetic co-polymer (e.g., styrene-maleic anhydride or styrene-acrylate). This treatment forms a thin, water-resistant film on the paper surface, plugging pores and further reducing the Cobb value. It also improves the printability and surface strength of the liner. The combination of robust internal sizing and a well-formulated surface size is what allows paper mills to consistently achieve very low Cobb 60 values.

Procurement teams should inquire about the sizing system used, as it can affect performance. For example, issues like "sizing reversion"—a loss of water resistance over time due to hydrolysis of the sizing agent—can occur if not managed correctly, especially in hot and humid storage conditions.

Fibre Selection and Paper Machine Operations

The structural backbone of the liner is its fibre composition, or "furnish." Plasterboard liners are typically made from a blend of recycled fibres (often from old corrugated containers or mixed office waste, processed into Deinked Pulp/DIP) and virgin kraft fibres.

For MR liners, the furnish recipe is critical. While a high percentage of recycled fibre is economical, it can introduce variability. The presence of contaminants and shorter, weaker fibres from repeated recycling can make it harder to achieve consistent sizing and strength. Therefore, MR liners often contain a higher percentage of virgin softwood kraft fibres. These long, strong fibres provide excellent tensile and tear strength (measured by Elmendorf tear test) and create a more uniform sheet structure, which is more receptive to sizing agents.

The degree of mechanical treatment the fibres receive, known as "refining," is also a key parameter. Refining slightly "frays" the fibres, increasing their surface area and promoting better fibre-to-fibre bonding and sheet strength. However, over-refining can crush the fibres, reducing porosity (measured by Gurley or Bendtsen air permeability) too much and making the sheet brittle. For an MR liner, the paper maker must strike a balance: enough refining to produce a strong, well-formed sheet, but not so much that it closes the sheet to a point where adhesive penetration is compromised.

Board-Line Behaviour and Process Adjustments

A perfectly specified MR liner can still fail if it is not handled correctly on the plasterboard production line. The hydrophobic nature of the paper introduces several challenges:

• **Adhesion and Gluing:** As mentioned, the low surface energy and porosity of MR liner makes it difficult for water-based starch adhesives to wet the surface and penetrate. This can lead to a weak bond between the paper and the gypsum core, resulting in delamination, especially at the board edges or during the scoring and snapping process. The board plant must often work with its adhesive supplier to formulate a specific starch mixture with better wetting agents or a higher solids content for use with MR liners.
• **Creasing and Folding:** The higher density and sizing level can make MR liners stiffer and more prone to cracking at the edges when folded to form the board envelope. The creasing wheels on the forming table must be precisely adjusted and maintained to create a deep, clean crease without fracturing the paper's surface sizing or fibre structure.
• **Drying and Delamination:** During passage through the multi-stage kiln, massive quantities of water must be driven out of the gypsum core as steam. This steam must escape through the paper liner. The low permeability of an MR liner can trap this steam, increasing the internal vapour pressure. If the drying curve (time and temperature profile) is too aggressive, this pressure can cause "blows" or large-scale delamination between the core and the liner. Board manufacturers often need to run MR boards at a slightly slower line speed or with a modified, more gradual drying profile to allow for the controlled release of water vapour.

Supplier Qualification: Key Questions for Procurement

To ensure a consistent and reliable supply of MR liner, procurement and QC teams should engage in a thorough technical dialogue with potential paper mills. Sitting down with a supplier's technical team is non-negotiable. Key questions include:

• **Quality Control Data:** Can you provide historical data for key parameters on this grade, including mean and standard deviation for Cobb 60, basis weight (g/m²), caliper (µm), and key strength properties (MD/CD tensile, Elmendorf tear)?
• **Sizing System:** What specific sizing chemistry (AKD, ASA, surface additives) do you employ? What is your process control for ensuring consistent application and cure?
• **Furnish Composition:** What is the typical percentage of virgin softwood, virgin hardwood, and recycled fibre in this grade? How do you manage variability in your recycled fibre stream?
• **Traceability:** What is your system for reel and batch traceability? In the event of a an on-line performance issue, can you trace the specific paper reel back to its production date and parent roll?
• **Claim Handling:** What is your formal procedure for investigating and resolving a customer claim related to liner performance (e.g., delamination, staining, cracking)?
• **Trial Reels:** Are you able to provide a dedicated trial reel for evaluation on our board line, and can your technical service representative be present during the trial?

By asking these detailed questions, a plasterboard manufacturer can move beyond a simple price-per-tonne discussion and build a partnership with a supplier who understands the technical demands of moisture-resistant board production. As a specialized trader, WeePaper facilitates these technical dialogues, ensuring that the specified paper meets the precise performance requirements of the end application. '''