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Debnath, M., Sarder, R., Pal, L., and Hubbe, M. A. (2022). "Molded pulp products for sustainable packaging: Production rate challenges and product opportunities," BioResources 17(2), 3810-3870.

Abstract

Molded cellulosic pulp products provide eco-friendly alternatives to various petroleum-based packaging systems. They have a long history of reliable usage for such applications as egg trays and the shipping of fruits. They have recently become increasingly used for the packaging of electronics, wine bottles, and specialty items. Molded pulp products are especially used in applications requiring cushioning ability, as well as when it is important to match the shapes of the packed items. Their main component, cellulosic fibers from virgin or recycled wood fibers, as well as various nonwood fibers, can reduce society’s dependence on plastics, including expanded polystyrene. However, the dewatering of molded pulp tends to be slow, and the subsequent evaporation of water is energy-intensive. The article reviews strategies to increase production rates and to lower energy consumption. In addition, by applying chemical treatments and processing approaches, there are opportunities to achieve desired end-use properties, such as grease resistance. New manufacturing strategies, including rapid prototyping and advances in tooling, provide opportunities for more efficient form factors and more effective packaging in the future.


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Molded Pulp Products for Sustainable Packaging: Production Rate Challenges and Product Opportunities

Mrittika Debnath, Roman Sarder, Lokendra Pal, and Martin A. Hubbe *

Molded cellulosic pulp products provide eco-friendly alternatives to various petroleum-based packaging systems. They have a long history of reliable usage for such applications as egg trays and the shipping of fruits. They have recently become increasingly used for the packaging of electronics, wine bottles, and specialty items. Molded pulp products are especially used in applications requiring cushioning ability, as well as when it is important to match the shapes of the packed items. Their main component, cellulosic fibers from virgin or recycled wood fibers, as well as various nonwood fibers, can reduce society’s dependence on plastics, including expanded polystyrene. However, the dewatering of molded pulp tends to be slow, and the subsequent evaporation of water is energy-intensive. The article reviews strategies to increase production rates and to lower energy consumption. In addition, by applying chemical treatments and processing approaches, there are opportunities to achieve desired end-use properties, such as grease resistance. New manufacturing strategies, including rapid prototyping and advances in tooling, provide opportunities for more efficient form factors and more effective packaging in the future.

DOI: 10.15376/biores.17.2.Debnath

Keywords: Molded fiber products; Alternative cellulosic fibers; Production rate; Containers; Cushioning; Barrier properties

Contact information: North Carolina State University, Dept. of Forest Materials, Campus Box 8005, Raleigh, NC 27695-8005, USA; Corresponding author: hubbe@ncsu.edu

INTRODUCTION

Molded pulp products, which often are made with recovered cellulosic fibers, are being increasingly used for the packaging and shipping of three-dimensional objects (Didone et al. 2017; Su et al. 2018). Such products, which are formed by the dewatering of cellulosic pulp suspensions, can have a wide range of three-dimensional shapes (Ma et al. 2004). Since about 1890, molded pulp systems have been used for egg cartons (Keyes 1890). More recently, there has been growing usage of molded pulp systems for packaging of electronics and specialty items (Marcondes 1997, 1998). Not only can molded pulp products be made from photosynthetically renewable and/or recycled materials, but they also can be easily recycled. Molded pulp products also face some important challenges. The focus of the present review article is on such challenges as relatively slow rates of production. Much of the time required to produce each molded pulp item involves its dewatering on a screen surface, followed by the energy-intensive process of drying. Publications describing ways to speed up molded pulp processing, including relevant strategies used in papermaking operations, are reviewed in this work. Further challenges are related to meeting a range of specifications for such properties as strength and water resistance. Accordingly, published approaches including chemical treatments of fiber suspensions are reviewed with respect to product goals.

Packaging is an important aspect of the economy. Along with providing basic protection of the goods inside, packaging helps in promoting its value. It can offer convenience in handling and display (Gutta et al. 2013). The global packaging market is set to reach over $1 trillion by 2021 (Smithers 2018). However, packaging has also been the subject of concerns about environmental sustainability. Every year, large amounts of packaging materials are being used with the expectation of single usage and disposal. Large portions of them are made of non-biodegradable and non-renewable materials such as plastics, glass, and metals in packaging applications. Even their incineration methods affect the environment.

Due to their low cost, light weight, high performance, and ease of processing, consumers and businesses have become used to disposable plastics. Usage of these materials has resulted in the accumulation of plastic products in the environment during the last several decades (MacArthur 2017; Schneiderman and Hillmyer 2017; Horejs 2020; MacLeod et al. 2021). Of the 8.3 billion metric tons plastics that has been produced since 1950, about 60% has ended up in either a landfill or the natural environment (Ashraf et al. 2021).

The small plastic fragments with less than ≤ 5 mm dimensions are defined as microplastics (Duis and Coors 2016). The accumulation of these microplastics, often due to mechanical breakdown, poses a great threat to marine organisms and also indirectly affects the ecosystem by absorbing hydrophobic pollutants due to its large area to volume ratio (Chatterjee and Sharma 2019). In light of such challenges, the focus is to find renewable, recyclable, biodegradable alternatives that can replace non-biodegradable and non-renewable packaging materials, such as expanded polystyrene (Ting and Chang 2014). Many businesses currently are moving from linear to circular economy by developing a new processing chain of materials and manufacturing; by such means the waste, emissions, and energy usage can be minimized by narrowing and closing energy and material loops (Garcia and Robertson 2017; Zhang et al. 2021). Considering all these factors, paper and cellulose-based materials are becoming more attractive, sustainable options for packaging materials (Didone 2017). Among these materials, molded fiber and its products have attracted increasing amount of attention, specifically to replace expanded polystyrene (EPS), which is also known as Styrofoam™.

A typical molded pulp packaging item generally can be described as having a relative low density, moderate resistance to compression, and good insulating properties, especially when dry (Anon. 2011; Abhijith et al. 2018). It generally has good shock absorbing ability (Wever and Twede 2007). Unlike expanded polystyrene (EPS), the cushioning or shock absorption property is often achieved by the inclusion of a number of dimples on the pulp sheet; more specifically, it was related to the perimeter of the dimples rather than their area (Eagleton and Marcondes 1994). Recently, the emphasis on cost-effective and eco-friendly packaging has been accelerated by increasing levels of electronic commerce, which has increased the amounts of packages individually sent to people’s homes (Abukhader and Jönson 2003). In such applications, packaging can play a pivotal role in brand’s reputation and the consumer’s expectation. As more and more consumers embrace online shopping, the amount of related waste is also set to increase.

BACKGROUND

Before considering research related to the challenges and opportunities in molded pulp products, this section will provide some essential background. Published review articles and chapters have considered some aspects of molded pulp production and attributes (Emery and Emery 1966; Paine 1991; Anon 2011; Didone et al. 2017; Abhijith et al. 2018; Su et al. 2018; Zhang et al. 2021). For instance, Didone et al. (2017) place emphasis on the potential of hot-press processes, which can be important when the goal is to achieve increased mechanical strength. Abhijith et al. (2018) focused on potential use of fungal mycelia for molded pulp products. Su et al. (2018), in their brief review, covered a broader area that included not only molded pulp products but also bio-based foams. Zhang et al. (2021), in another brief review, focused on emerging applications and environmental sustainability. Relative to some other technologies, such as papermaking in general, the amount of scientific publication related to molded pulp products has been small. Though the present article has a limited focus on production rates and product attributes, the authors hope to encourage others to help fill the gap of information related to this important area of technology.

Molded Pulp Product Classes

During the many years of industrial production of molded pulp products, there have been some advances with respect to both the processing methods and the types of products that can be achieved (Keyes 1890; Twede and Wever 2007; Didone et al. 2017). For a long time the technology was limited to low-cost retail products, e.g. egg trays and boxes, and disposable food service wares (Chiang 1993; Cullen Packaging 2012). With advancement of technologies and considering its environmental benefits, its demand and market are presently increasing (Xiao and Shao 2015). Molded pulp products are being used in the packaging of industrial products such as electrical and electronic appliances, e.g. printers and computers (Noguchi et al. 1997; Ma et al. 2004; Wever and Twede 2007). Companies including Seventh Generation™, Pangea Organics®, and Paper Water Bottles have launched packages such as laundry-detergent bottles, packages of soap, and prototypes of water bottles to replace plastics (Didone et al. 2017). Molded pulp can also be used as a cushioning material for protecting the product during transportation, e.g. molded honeycomb paperboard (Ma et al. 2010).

According to the International Molded Fiber Association (IMFA), molded pulp products can be categorized in various groups based on the manufacturing process and quality of materials. The first one, which is made out of kraft and recycled paper and has a typical wall thickness ranges from 5 to 10 mm, is known as “thick wall.” The procedure for making such products is illustrated in Fig. 1. As shown, the process starts with an aqueous suspension of fibers in a vat. A porous mold is inserted into the vat, and vacuum is applied to draw fibers (and any other additives) toward a screen surface. The mold is then withdrawn from the vat, still with the damp molded product attached. The molded pulp product is then released from the mold and subjected to oven drying. Thick-wall products mainly serve as support packaging for non-fragile, heavy items (e.g. furniture and vehicle parts) during shipping.

Fig. 1. Schematic illustration of thick-wall process for molded pulp production, involving vacuum forming, with separate drying in an oven

The second type is known as “transfer molded.” These have thinner walls ranging from 3 to 5 mm with relatively smooth surfaces on both sides and better dimensional accuracy. These are typically used as egg trays and packaging for electronic equipment. The third type is the most recent approach known as “thermoformed” (“thin-wall”). Such products having thickness ranging from 2 to 4 mm with good dimensional accuracy, and smooth, rigid surfaces. This type of product utilizes heated molds, within which the initially formed product is pressed, densified, and dried. Such products can be used as a substitute for thermoformed plastic items. This process is illustrated schematically in Fig. 2. As shown, the process using for such items begins in essentially the same way as was shown in Fig. 1. However, for transfer-molded products the mold – with the damp molded product attached – is manipulated so that it gets pressed against an opposing surface that matches the intended shape of the product. As shown, the transfer mold surface is often heated, as implied by the term thermoformed. A key difference, in comparison to thick-walled molded pulp products, is that transfer-molded products tend to be relatively smooth on both the inside and the outside.

Fig. 2. Schematic illustration of transfer molded process, incorporating thermoforming conditions for initial drying

The final type is the “Processed,” which refers to the previous items that require some further or special treatment such as additional printing, coatings, or additives (“International Molded Fiber Association,” n.d.).

Transfer molded and thermoformed products

By transferring the initially formed item to a mating surface, the structure then can be pressed. Not only does the side initially facing away from the screen become much smoother, but also there is an opportunity to squeeze more water from the material. The improvements in accuracy and dimensional stability can expand application range of the molded pulp products (Didone et al. 2017). When using a thermoforming procedure, the goal is to at least partially dry the molded pulp product while it is still being pressed (Didone et al. 2017). In principle, such a process can achieve higher strength and higher rates of production.

Paine (1991) describes an alternative forming procedure in which pressurized air is used to dewater fiber suspensions against a screen. High temperatures can be used, such that solids contents of over 50% can be achieved during the forming step. In a related technology, a flexible diaphragm can be forced into the interior of a specialized mold (Kucherer 1995; Buxoo and Jeetah 2020; Saxena et al. 2020). The water is thus forced outwards through a set of detachable mold segments. Hollow molded pulp products, such as bottles, can be made in this way.

Property Ranges of Molded Pulp Products

Differences relative to flat papermaking

In light of characteristic differences in their forming processes, one might expect that the properties of molded pulp products will differ from those of flat paper products prepared from similar fibers. As a result of a higher typical solids content (consistency) of the fiber slurry, in comparison to typical flat papermaking conditions, one can expect that fibers will be oriented in all directions, rather than being mainly oriented in a plane. Figure 3 illustrates the likely mechanism whereby formation of paper from a dilute suspension tends to give a paper sheet in which fibers are mainly oriented in the plain of the sheet.

Fig. 3. Concept of why cellulosic fibers tend to become oriented in the plane of the sheet when formed from a dilute (< 1%) consistency suspension (A), whereas they tend to have more out-of-plain orientation when formed at higher consistencies (B)

Other differences between typical molded pulp products and paper grades, such as printing papers, relate to the types of fibers that are utilized. For instance, as will be discussed later, the fibers employed in molded pulp production are likely to be stiffer (less refined), compared to when making such products as printing papers or linerboard. Recycled kraft fibers, which can be considered for molded pulp products, will have experienced hornification, which results in an increase in stiffness of the wet fibers, leading to a bulkier mat structure (Zhang et al. 2002; Hubbe et al. 2007b). Recent work has shown promise for decreasing reliance on traditional refining of recovered kraft pulp fibers, thus making it possible to maintain higher drainability (Debnath et al. 2021). High-yield fibers, which are available from a wide range of woody materials (Salem et al. 2020), likewise are widely used for molded pulp products. Such fibers have a lower conformability in the wet state compared to refined kraft fibers due to their high content of lignin. The lignin is inherently rigid and it helps to resist internal delamination of the cell walls of high-yield fibers during refining.

Another important comparison one can make is with glass fibers. Westman et al. (2010) found that structures prepared with cellulosic fibers were much weaker than what could be achieved with glass fibers, which are individually very strong.

Differences relative to expanded polystyrene and related issues

The most critical properties of molded pulp products are mechanical strength and cushioning ability. From this perspective, molded pulp products can have similar properties to expanded polystyrene. One of the main jobs of a typical molded pulp packaging structure is to cushion the contained items against breakage. To accomplish this, the packaging needs not only to hold the packaged item, but it also needs to provide somewhat reversible deformation. Ideally, the function of a cushioning property is to minimize the damage of a product from any physical impact or vibration by absorbing the shocks arising during transportation. The cushion curves obtained by Pacific Pulp showed that molded pulp was actually a better protective cushion than the expanded polystyrene. The performance shown on a cushion curve is dependent upon the manufacturer of the material and the thickness, size, and shape of the cushion. The drop test showed that if a package is exposed to shock, then the molded pulp package will be capable of absorbing that shock and the product would be protected (Eagleton and Marcondes 1994). It also provides excellent blocking and bracing functionality (Mabie and Camuel 2010).

Molded products often are designed with dimples, which are able to crush upon impact, thus absorbing some of the shock (Eagleton and Marcondes 1994). The cited article assigned molded pulp products only an intermediate rating in terms of shock-absorbing ability, noting that expanded polystyrene has a much greater ability to recover after crushing. Manufacturers can adjust the structure of molded pulp products, such as the addition of ribs, in an effort to improve the cushioning performance (Marcondes 1997). However, further testing showed that the transmission of shock though molded pulp material can be relatively high (Marcondes 1998). According to Paine (1991), molded pulp packaging products can be effective because they are able to fail sacrificially by distortion and crushing, ideally such that the contents of the package remain undamaged.

The apparent density within molded pulp structures falls within a wide range, which is understandable based on the wide range of process conditions. For example, a bulky, low-density structure is appropriate for thick-walled biocontainers intended for planting of seedlings (Aguerre and Gavazzo 2016). The cited study found apparent densities in the range of 0.28 to 0.69 g/cm3. Values of about 0.6 g/cm3 were judged as optimum. Howe (2010) noted that the densities of typical molded pulp products are higher than that of expanded polystyrene, which is unfavorable from the perspective of the mass than needs to be transported. Paine (1991) reported a range of apparent density of 0.2 to 1.0 g/cm3 for various molded pulp products.

Evaluation of elastic modulus and stretch

The mechanical strength characteristics of molded pulp products can be revealed by stress-strain testing. Jacobson (2017) compared the strength characteristics of molding pulp structures formed with three kinds of pulp, namely kraft fibers, thermomechanical pulp (TMP), and recycled fibers. The unrefined kraft fibers, when formed into molded pulp sheets, achieved a tensile breaking stress of only about 5 MPa, whereas after refining (called “activation” in the article), the strength reached 37 to 47 MPa. These differences are shown in Fig. 4. Meanwhile, the TMP achieved breaking stresses in the range 25 to 27 MPa, and the recycled pulp reached 15 to 18 MPa (apparently with no additional refining having been applied). Hua et al. (2020) evaluated the Poisson’s ratios and determined the failure mechanisms of molded paper materials by means of digital images correlations and uniaxial tensile testing. They found that the drying of molded pulp under pressure within a mold led to higher Poisson’s ratio. In other words, stretching of the material in one direction led to a greater contraction in the other direction when the extent of bonding had been increased. The most important modes of tensile failure were found to be cracking and buckling. The mechanical properties also can be expected to depend on the presence of any defects, such as post-forming instabilities, cracks, wrinkles, or water-pockets (Jacobsen 2017).

Fig. 4. Effect of mechanical refining on the stress-strain curves of molded pulp structures prepared from refined and unrefined kraft fibers. Figure redrawn based on more detailed plots reported by Jacobson (2017)

Jacobsen (2017) also compared the stress-strain curves for kraft sheets prepared under papermaking conditions, in comparison to when the same material was prepared as a molded pulp item. As shown in Fig. 5, the paper sheets exhibited a much higher initial modulus of activity within the plane of the sheet.

Fig. 5. Effect of slurry consistency (i.e. whether the sheets are formed as paper or as a molded pulp product) on the stress-strain curves of molded pulp structures prepared from refined and unrefined kraft fibers, respectively. Figure redrawn from more detailed plots reported by Jacobson (2017)

Such a result is consistent with the predominantly planar orientation of the majority of fibers in a typical paper structure. By contrast, the higher consistency that is used when forming a molded pulp structure is expected to lead to a more random distribution of fiber orientations with respect to the three orthogonal directions. In addition, restraint of sheet contraction during drying of a conventional paper sheet (as in the standard drying of paper handsheets) is expected to have a straightening effect on the fibers, which makes a further contribution towards enhancing the within-plane elastic modulus.

Compression strength is expected to be important for the performance of molded pulp products (Anon. 2011). Ji and Wang (2011) conducted short-span compression tests of molded pulp material (apparent density ca. 0.54 g/cm3), studying the effects of apparent density and loading rate. Peak compression stresses in the range of 2 to 2.4 MPa were observed. The peak stress increased from about 2.4 to about 3.0 MPa with the increase in rate of compression from 0.008 to 0.08 reciprocal seconds. This observation is an indication of a somewhat visco-elastic response of the molded pulp material. Xiao and Shao (2015) found increasing peak compression stress of molding pulp with increasing temperature. Since equilibrium moisture content tends to decrease with increasing temperature (even at constant 50% relative humidity), the higher compression strength results at higher temperature can probably be attributed to lower moisture contents. These results are consistent with those of Sørensen and Hoffmann (2003), who observed strong decreases in static compression strength of molded pulp sheets with increasing relative humidity at controlled temperatures.

Viscoelastic effects of molded pulp material have been considered further. Ji et al. (2008) used the term emplastic (or adhesive) to characterize the material they studied. In other words, the material appeared to have a component of glue-like behavior (agglutinizing) when subjected to tensile stress.

Materials

In order to make a molded pulp product, one starts with a fiber slurry that may contain mostly water and about 3 to 5% fibers. Though the pulp molding industry uses similar fibers as the paper industry, the forming processes, fiber forming characteristics, densities, and structural functionality are different (Hunt 1998). Both virgin and primary fiber derived from wood or non-wood plants and recycled or secondary fiber, derived from waste paper and paperboard are used. The most used sources of natural fiber from plants include sugarcane bagasse, jute, flax, pineapple leaf, kenaf, bamboo, sisal, abaca, oil palm, rice husk, coir, coconut, and hemp (Westman et al. 2010). Both the physical and chemical properties of some of these fibers, such as high specific strength and stiffness, impact resistance, flexibility, and modulus, make them an attractive alternative over the traditional materials. For example, sugarcane bagasse fiber exhibits high specific strengths and modulus, cost-effectiveness, low density, and low weight, which make it a promising choice of raw materials by the industry (Sabdin 2014). In general, relatively long fibers provide strength, toughness, and structure, whereas shorter fibers often can provide high bulk (low density), closeness of texture, and smoothness of surface. Fiber length distribution influences a number of properties as well. For example, a short fiber length distribution will form a finer structure compared to a distribution with long fibers. Furnishes with a fiber distribution having more long fibers can increase the thickness of product. In the case of recycled fiber, furnishes with more old corrugated container (OCC) pulp content tend to be thinner than those prepared from old newspaper (ONP) pulp, even if their fiber length distributions are similar. This is because OCC fibers have a significant portion of kraft fibers that are more flexible and conformable when wet, and these tend to be compressed more, thus forming a thinner product (Kirwan 2013). Considering the strength properties, with each subsequent reprocessing cycle of kraft fiber, there is a decrease in mechanical properties attributed to the resistance of fiber swelling when rewet after being dried (Hubbe et al. 2007b). Wet end addition of lignin-containing micro and nano fibrillated cellulose in containerboard was found to offset the decreased strength associated with this by increasing the relative bonded area (RBA) of the paper (Tarrés et al. 2017; Starkey et al. 2021). Chemical compositions of some wood and nonwood fibers are given in Table 1 (Salem et al. 2021).

Table 1. Chemical Composition of Feedstock

Cellulose, the major component of fiber, has a high tendency to form intra-and intermolecular hydrogen bonds (Fengel and Stoll 1989). As a hydrophilic polymer, it has a high capacity for water absorption as well as limits to its barrier properties (Alavi et al. 2015). Hemicelluloses contribute to many intrinsic fiber properties, such as the swelling, fibrillation, bonding ability, and resistance to a hornification tendency (Pere et al. 2019). Resistance to hornification means that the material is more capable of swelling again after it has been dried, then placed back into water. Lignin is a rigid hydrophobic polymer, which is mostly removed during the kraft pulping process. During hot-pressing processes, the retention of a certain amount of lignin may contribute to the stiffness and water resistance (van de Wouwer et al. 2016). Lignin can also improve the mechanical properties, waterproofness, thermal stability, and adhesive properties of fiber materials in the polysaccharide-based materials represented by wood. Hence, light delignification has been developed to produce higher bonding. When the content of lignin was decreased from 24.9% to 11.45%, the density increased by 6.0%, the tensile strength increased by 22.0%, the bending strength increased by 23.9%, and the water contact angle increased from 64.3°–72.7° to 80.8°–84.3° (Wang et al. 2018).

Since the range of targeted properties is very broad, a wide range of fibrous materials have been considered for molded pulp production in various studies. Researchers have prepared molded pulp items from kraft pulps (Aguerre and Gavazzo 2016; Jacobsen 2017; Dislaire et al. 2021), high-yield wood or bamboo pulps (Jacobsen 2017; Wang et al. 2021), and chemi-mechanical pulps (Kirwan 2013; Zhao et al. 2020; Dislaire et al. 2021). As noted earlier, recycled pulps are very widely used by the molded pulp industry. Types of recycled fibers that have been studied for production of molded pulp items include old newspaper (Corwin 1972; Ahn 1994; Gavazzo et al. 2003, 2005; Dislaire et al. 2021), recycled cardboard (Dislaire et al. 2021), and old corrugated containers (Kirwan 2013; Aguerre and Gavazzo 2016).

Dislaire et al. (2021) compared six different pulp types in parallel tests, but for unknown reasons did not apply any refining action to three types of kraft pulp that were considered. Thus, they found the highest elastic modulus (ca. 1.2 GPa) for tensile tests of bleached chemithermomechanical pulp, which was followed by recycled cardboard pulp, recycled newspapers, and then the three unrefined kraft pulps.

A particular challenge when using recycled paper for production of molded products is that there may be a mismatch between the pH conditions of different waste paper types. Modern printing papers often are prepared under alkaline (pH 7.5 to 9) papermaking conditions with the presence of 20% or more of calcium carbonate filler (Hubbe and Gill 2016). At the same time, the mixture of recovered fibers also is likely to contain paper that has been prepared under acidic papermaking conditions, with aluminum sulfate as the primary buffering agent (Ehrhardt and Leckey 2020). Acidification of the calcium carbonate will result in its partial or complete dissolution, with the release of Ca2+ ions, and this can contribute to water hardness and various deposit problems in the mill. When there is sufficient calcium carbonate present to dominate the pH, the alkaline pH conditions may interfere with the action of rosin and aluminum sulfate additives, which have traditionally been used in molded pulp products (Kucherer 1995). When the amount of calcium carbonate is substantial, then the best approach often is to control the pH in the alkaline papermaking range and employ hydrophobic agents other than rosin (see later discussion).

Issues related to the choice between acidic and neutral-alkaline pH conditions of processing are illustrated in Fig. 6.

Fig. 6. Snapshot view of some key issues of concern when utilizing fiber sources that may contain acidic fiber materials (extract pH 4 to 5.5) and other materials that contain substantial amounts of calcium carbonate (extract pH 7.5 to 9)

As shown, the manufacturer is faced with some difficult compromises. Merely using the acidic and alkaline recovered fiber materials without pH adjustment can be expected to result in serious foaming issues. This is because the calcium carbonate mineral filler in the alkaline paper will be dissolving, especially if the amount of acidic material has a dominant effect on pH. Dissolution of CaCO3 releases carbon dioxide gas (contributing to foam). It also dissolves Ca2+ ions, contributing to water hardness. In cases where the amount of acidic material is minor, a reasonable approach would be to add base (usually NaOH) to the water used to dilute the pulp. The rosin size already present in the fibers will no longer contribute to hydrophobization. Rather, an alkaline sizing such as alkylketene dimer (AKD) will need to be used. On the other hand, if the amount of alkaline paper in the mixture is minor, then one might opt to add some sulfuric acid to the dilution water used in pulping the recovered fibers. Large amounts of foam can be expected, at least initially, as the calcium carbonate dissolves. The very high levels of dissolved calcium carbonate can be expected to make it more difficult to hydrophobically size the paper with rosin and alum. Lab work and production-scale trials may be needed in order to decide which option makes more sense.

Non-wood fibers

Considerable research attention has been focused on the potential usage of non-wood fibers. These have included sugarcane bagasse (Jeefferie et al. 2011; Waranyou 2014), various types of straw (Curling et al. 2017; Hart 2020; Prasertpong et al. 2021), invasive grass (Chen et al. 2012), hemp (Buxoo and Jeetah 2020), and mycelium (fungal biomass) (Abhijith et al. 2018). As the price of global wood pulp is rising and the advancement of electronic media are decreasing the recycling of fibers from paper waste, the demand of alternative sources of raw material for creating the molded pulp packages has been continually increasing (Gouw et al. 2017; Johnston 2016). Based on the data of 80/20 straw pulp/kraft pulp mix based molded material, significantly better tensile properties were obtained compared to expanded polystyrene (modulus of 0.47 MPa for an 80% straw mix compared to 0.16 MPa for EPS). Along with strength properties, molded pulp has shown great biodegradability (20% mass loss after only 4 weeks covered in unsterile soil) (Curling et al. 2017). Be Green Packaging has introduced material made of six different resources, including bulrush, wheat straw, sugar cane, and bamboo (OAS Cataloging-in-Publication Data, 2016). Non-wood plants such as Spartina alternifolia, an invasive species, have been utilized recently. Thermo-mechanical pulping was utilized for this wild grass, and it shows good mechanical and cushioning property after mixing with other chemical pulps such as bamboo (Chen et al. 2012). Another study has shown the potential of fruit pomace (FP) as a source of fiber for partially replacing recycled newspaper (NP) to create molded pulp products. Along with certain amount of CNF, the FP based molded pulp board showed better or similar properties to 100% NP-based board (Gouw et al. 2017). A molded cup made out of composite of hemp–pineapple peels in 40:60 combination showed good strength property. A coating of beeswax having thickness of 0.70 mm on that cup was adequate to retain cold water for 30 min (minimum) without any leakage. The cup can degrade in both active soil and damp sand environments within 5 and 6 weeks, respectively. This means that fiber isolation from fruit peel wastes and hemp leaves to produce eco-friendly, biodegradable disposable paper cups is a viable approach (Buxoo and Jeetah 2020). Another vegetative part, the mycelium, can be grown in a mold to form different shapes for different items. It can grow quickly into a desired density. In addition, it can be dehydrated to stop further growth. After its useful life as a packaging material, it can be left out in the backyard for decomposition within a few weeks. However, there are challenges of maintaining a consistent density with a raw material that is a living organism and acceptancy by consumers (Abhijith et al. 2018).

Residues from forestry, agriculture, or discarded materials

The term residues has been used for lignocellulosic materials such as solid waste, forestry and agroindustrial waste, newspaper (ONP), office paper (OWP), corrugated cardboard (OCC), pine sawdust, eucalyptus sawdust, and sugar cane bagasse (Sengupta et al. 2020). These have been utilized to prepare molded products, and the properties that were evaluated were density, tensile, bursting, tearing, compression, stiffness, wet tensile, permeability, and water retention. It was shown that the OWP pulps increased strength properties, OCC pulps increased tear and wet tensile, ONP pulps increased stiffness, and reinforcement materials increased permeability. By adjusting the proportions of different pulp types, it becomes possible to reach the objectives of different product grades. For instance, a mixture of pulp OWP/OCC in a 50/50 proportion was been found to be optimum for some products (Aguerre and Gavazzo 2016). Mushroom root or mycelium-based materials have the potential as well to become the material of choice for a wide variety of applications, with the advantage of low cost of raw materials and disposal of polystyrene posing an environmental issue. The mycelium can be grown in a mold to form different shapes for different items. It can grow quickly into a desired density. In addition, it can be dehydrated to stop further growth. After its useful life as a packaging material, it can be left out in the backyard for decomposition within a few weeks. However, there are challenges of maintaining a consistent density with a raw material that is a living organism and acceptancy in consumers (Abhijith et al. 2018).

Fiber development

Processing technology, including pulping and refining operations, can affect the chemical composition of fiber. Chemical pulp has reduced lignin and can be almost free of lignin (Su et al. 2018). The contrast between the two main forms of pulping is illustrated in Fig. 7. As emphasized in the figure, although mechanical pulping retains almost all the raw material in the prepared pulp (i.e. “high yield”), the costs of electricity are generally high. That is because it takes electricity to run the mechanical refiners. By contrast, the fibers come apart easily after chips of wood are cooked in a digester with a mixture of NaOH and Na2S (i.e. kraft pulping). The pulping can dissolve much or almost all of the lignin, as well as a substantial fraction of the hemicellulose, sometimes reducing the yield to values as low as 40%. Kraft pulps have excellent bonding ability, especially after an initial cycle of mechanical refining. However, some of the strength advantage of the kraft pulp fibers is lost with each cycle of paper production and recovery (Hubbe et al. 2007b). As mentioned earlier, the greater stiffness of both recycled kraft pulp and high-yield fibers can contribute to bulkier, less dense fiber structures, which can contribute better to the cushioning of packaged goods.

Fig. 7. Contrasts between the two major classes of cellulosic fibers that can be used in molded pulp production

The mechanical refining of pulps is well known to play a dominant role with respect to achieving various specified properties of paper products (Gharehkhani et al. 2015). Though the same general trends are likely to hold true with respect to molded pulp products, there are some important distinctions. First, typical apparent densities of molded pulp items (especially when not hot-pressed) are lower than those of common paper and paperboard products, such as printing grades and linerboard. As refining energy is applied, typically the resulting apparent density increases. Second, the combination of relatively high mass per unit area and the fact that the molded pulp items are individually formed places a high premium on the ease with which water can be removed by vacuum.

Nevertheless, refining can be a straightforward way to achieve the strength specifications, including the elastic modulus values needed for different molded pulp products. For example, Prasertpong et al. (2021) judged that a freeness value in the range 348 to 423 mL would give suitable tensile properties in typical cases when refining delignified rice straw. Such numbers represent a relatively low level of refining treatment.

The mechanism by which refining of a kraft fiber affects structural properties, and thereby affects dewatering as well as inter-fiber bonding ability, is represented in Fig. 8. The left-hand image represents the cross-section of an unrefined kraft fiber, which keeps the shape of a native fiber in wood, except that the fibers have been separated and most of the lignin has been removed. The repeated shearing and compression of wet bunches of fibers cause internal delamination of the fiber cell wall, especially the S2 layer, which accounts for most of the thickness of a typical fiber from wood. The delamination process is accompanied by swelling of the cell wall, as well as an increase in the conformability of the refined wet fibers. At the same time, the outer parts of the fiber, i.e. the primary (P) layer, the S1 sublayer, and outer parts of the S2 layer, become fibrillated. As shown, some of those fibrils break off and become part of the fines content of the suspension. In addition, the increased conformability of the cell wall leads to a tendency towards a ribbon-like cross-section of refined kraft fibers (Molin and Daniel 2004; Debnath et al. 2021).

Fig. 8. Depiction of changes in the cross-sectional view of a kraft fiber subjected to mechanical refining action, leading to delamination within the cell wall (especially the S2 layer), external fibrillation, peeling off of cellulosic fines, and development of a ribbon-like shape

When cellulosic fibers are subjected to a very high level of mechanical refining, the extensive shearing and compression eventually will produce micro- or nanofibrillated cellulose (NFC) (Lavoine et al. 2012; Abdul Khalil et al. 2016). Klayya et al. (2021) esterified NFC with lactic acid. The treatment gave more favorable dewatering rates and strength. Because nanofibrillated cellulose is known to greatly increase the time required to remove water from pulp mats, such findings are worth noting.

Additives to the suspension

In addition to the fiber materials, the suspension that is used to prepare molded pulp properties often will contain chemical additives. Three main categories of such additives can be called binders (or strength aids), hydrophobic substances, and flocculants.

Starch varieties are widely used as binders to increase the strength properties of molded pulp products (Noguchi et al. 1997; Jeefferie et al. 2011; Waranyou 2014). Though relatively little has been reported pertaining to optimal usage of starch in molded pulp products, a great deal is known based on general paper industry applications of starches. In particular, cationic starch products are effective in increasing product strength when added to the pulp slurry at levels of about 0.5 to 1.5%, based on solids relative to fiber solids (Howard and Jowsey 1989; Chemelli et al. 2020). In cases where it is important to retain a substantial portion of the initial strength even when the material becomes wet, it can be advantageous to add wet-strength agents. For example, polyamidoamine-epichlorohydrin resins, which are ideal for neutral to moderately alkaline pH conditions, can be used (Su et al. 2012). Such resins cure and provide some covalent cross-linking within the structure when dried under hot conditions. Crosslinked glyoxylated polyacrylamide (GPAM) polymers have the combined advantages of crosslinked polymers and glyoxal groups. The glyoxal content of the polymer produces a hemiacetal structure between the aldehyde group and the hydroxyl groups in cellulose in the papermaking process, which reduces the expansion and deformation of rewetted paper. Addition of GPAM not only considerably improved the dry strength of paper sheets, but it also significantly enhanced their wet strength (Yuan and Hu 2012). In the case of coated papers, the coating layer can include either starch products or synthetic latex (e.g. styrene-butadiene resins) to serve as a binder for mineral pigment particles, including clay or calcium carbonate. Such materials often become incorporated into molded pulp products due to recycling of the coated paper or paperboard. Some of the most prominent strengthening and binding agents are represented in Fig. 9.

Fig. 9. Representation of various binding agents likely to be present in recovered paper, including dry-strength agents, wet-strength agents, and coating binders

Hydrophobic substances

Many molded pulp items, especially those used for food shipping or service, can benefit from hydrophobic treatment. Some of the major hydrophobic agents used in molded pulp applications can be classed as sizing agents, lignin, and waxes. Rosin products have traditionally been used in molded pulp products, in combination with aluminum sulfate (papermaker’s alum) (Kucherer 1995). However, as noted earlier, complications can arise when the main fiber supply is recovered paper. The calcium carbonate filler that is present in a large proportion of recovered paper renders the pH too high for effective usage of rosin and alum. For this reason, alkylketene dimer (AKD) is likely to be the most practical sizing agent to be added to the pulp slurry before forming and drying the product (Ehrhardt and Leckey 2020). Waxes, when needed, are typically applied to the surface of a molded pulp product (Waranyou 2014; Buxoo and Jeetah 2020).

Lignin, one of the three main components of wood, can be described as a natural phenolic resin. Due to its low price, generally hydrophobic nature, and its potential contribution to bonding when sufficiently heated, it has been considered as an additive for thermoformed or heat-cured molded pulp products. Liu et al. (2021) achieved favorable results when adding enzymatic hydrolysis lignin to the fiber mixture when forming molded pulp products. The system was treated with ferric hydroxide and hydrogen peroxide to activate the lignin. Strength increases were observed. Because of the relatively high basis weights of many molded pulp products, it can be expected that the lengths of time and temperature conditions will be enough to cause the lignin to flow (Back and Salmén 1982). Such flow, especially when combined with pressure, can be expected to contribute to bonding within the structure.

Cationic emulsions of maleic anhydride derivatives of fatty acids such as oleic acid, as well as abietic acid, can be used as a potential hydrophobic sizing agent as they are low cost, ecofriendly, abundant and most importantly the presence and self-assembly between the hydrophobic tails of fatty acids, that are responsible for the sizing effect may be considered. Addition of aluminum sulfate to the fatty acids will help to attach to the fiber surfaces and contribute to making the paper hydrophobic (Bildik Dal et al. 2020).

Flocculants

As noted by Gavazzo et al. (2003), flocculating agents can have a large influence on the success of a pulp molding process. Two classes of flocculant have been employed routinely in such production. When using virgin fibers, such as mechanical pulps or chemi-mechanical pulps, it can be cost-effective to use aluminum sulfate as the main flocculant. In addition to its role in the fixing of rosin size (see earlier), the aluminum sulfate can contribute to a bulky, porous mat structure, which will tend to drain well and to retain the fine particles during the vacuum application. Within a pH region of about 4.5 to 6, the ratio of aluminum ion to OH ions can be optimized to form oligomeric species of alum, in addition to Al(OH)3 precipitate; these conditions can be effective to flocculate the mixture (Strazdins 1986, 1989). Part A of Fig. 10 represents the three aluminum species that are likely to contribute to the flocculation of cellulosic suspensions under weakly acidic conditions.

The other main class of flocculating agent includes very-high-mass copolymers of acrylamide. When such additives are prepared with about 3 to 10% of cationically charged monomeric groups, they can be used directly as flocculants for cellulosic materials, which typically have a negative surface charge (Hubbe et al. 2009). Such additives are believed to function by bridging of the macromolecular chains between the adjacent surfaces in the suspension. The polymeric flocculating agents are a favored choice when the fiber suspension has a pH in the range of about 7 to 8.5, i.e. the neutral to alkaline pH range. Later sections will focus on the role of flocculants in promoting dewatering and in decreasing the rate at which the wetted surfaces of production equipment become contaminated with deposits.

Fig. 10. A. Three species of aluminum that are likely to contribute to flocculation of cellulosic fiber suspensions; B. Representation of a very-high-mass cationic copolymer of acrylamide (flocculant or retention aid). Note that the cationic groups are randomly distributed in such polymers.

Manufacturing Processes

Reviews of manufacturing processes

Having discussed product classes, typical properties, and key components and additives for molded pulp products, the remaining background topic to be covered here is the manufacturing processes. As described in earlier review articles (Emery and Emery 1966; Porteous 1977; Ebmeyer 1994; Anon. 2011; Didone 2020), although not all molded pulp items are made in the same way, there are some common features. For instance, molded pulp operations start with the mixing a fiber suspension, often with a consistency of about 4% (Hunt 1998). The fiber suspension is supplemented with selected additives from among those mentioned in the previous subsection. The suspension is then dewatered in a mold, usually by suction, but sometime by use of pressurized air (Paine 1991). Next, there is an option of mating the mold with an oppositely matching mold structure, so that the molded pulp structure becomes squeezed between the two, expelling more water (Didone et al. 2017). The final step is drying, which can either be within the mold (press drying or thermoforming) or after the product is released (unconstrained drying) (Anon. 2011). The steps just mentioned will be considered below in more detail.

Vacuum forming in three dimensions

Vacuum is the most common means by which fiber solids are drawn toward a screen surface during production of molded pulp items (Eagleton and Marcondes 1994; Anon. 2011). Figure 1, shown earlier in this article, provides a schematic illustration of such a process. Porteous (1977) reports that as much as 85% moisture content may remain in the molded pulp item at the end of the vacuum application. Though the consistency (filterable solids content) of the suspension can fall in a wide range, its value is determined by the desired mass per unit of area, as well as the volume provided within the mold equipment. Thus, whereas paper machines often operate with a headbox consistency of about 0.5%, the consistency during molded pulp manufacturing is often about four to ten times higher. Another key difference is that the wet web of paper will pass over multiple dewatering units, such as hydrofoils or forming blades, as well as suction boxes having increasing levels of vacuum (Hubbe et al. 2020). By contrast, a typical molded pulp production operation entails a single application of vacuum at a single level. The implications of this difference will be considered in a later section, when considering factors affecting the production rate.

Pressurized air molding

Different designs of equipment can make it possible to achieve nearly equivalent results with pressuring air in place of vacuum (Porteous 1977; Paine 1991). Kucherer (1995) describe a Swiss company that used such processing in the period between 1925 and 1969 to make such molded pulp items as cans, tubes, and bottles, which were called “blow-molded”. By usage of hot air, such a method can achieve at least partial drying of the formed object (Paine 1991; Anon. 2011). Paine (1991) reports that solids contents in the range of 50 to 55% could be achieved in such operations, with the remainder of the water being removed in a secondary drying step. Kumamoto and Otani (2001) and Kumamoto et al. (2002) patented a related technology in which air pressure is used to inject a balloon into the interior space of a multi-part, detachable mold system, thus forcing a fiber suspension outwards to the screen surfaces. Because the flexible membrane presses against the interior, that surface becomes much smoother than would have been achieved by the other methods described so far in this section.

Precision forming with pressure

The terms transfer molded and precision forming are used in cases where, after the application of vacuum, part of the mold rotates so as to mesh with an oppositely-shaped structure that presses against the layer of molded pulp (Didone et al. 2017; Hurter 2017). Such a process was shown schematically earlier in this article in Fig. 2. As implied by the word “precision”, such a procedure provides an opportunity to achieve more exacting replication of structural details, including smoothness of the side facing away from the initial screen surface (Saxena et al. 2020).

Drying and curing

Challenges associated with the drying of molded pulp items include the three-dimensional shapes, non-uniform thicknesses, and non-uniform densities of the formed objects. Conventional steam-heated cylinders, as used in production of most ordinary paper, are unsuited to the job of evaporating water from molded pulp products. That is unfortunate, since such technology has become highly optimized, with efficient recovery of latent heat from the evaporated vapors (Hubbe 2021). Unconstrained drying means that heat is applied to the item after it has been released from the mold (Anon. 2011). This can be done in an oven, typically with circulation of air to promote even drying.

Terms including press-drying and thermoforming can be used when heat is applied during the transfer operation from the initial vacuum forming to the mated surface (Sutton 1978; Anon. 2011; Sridach 2014; Waranyou 2014; Wang et al. 2017; Didone et al. 2020; Klayya et al. 2021). As the molded pulp item is pressed between the two surfaces, it becomes drier and denser simultaneously as the water continues to be expelled through microchannels in the mold surfaces. Didone et al. (2020) estimated that the drying operation requires about 8 to 20 times as much energy in comparison to the initial forming process with vacuum. A particular challenge that faces manufacturers when employed press-drying procedures is the likelihood of undesirable delamination of the product (Didone et al. 2020). The mechanism of such delamination has been studied in detail in connection with earlier efforts to speed up the drying or ordinary paper (Lucisano and Martinez 2001). In principle, such delamination can be minimized by slowing down the process, giving the vapors more time to escape through the porous openings of the mold before the mold surfaces are separated.

Processing challenges

Though the manufacturing steps outlined above are being effectively used to produce a wide range of molded pulp items, there are some important challenges that seem to stand in the way of rapid expansion in manufacturing. These include issues related to production efficiency, the relatively high amounts of energy needed to evaporate water, and the relatively slow processes involved in individually forming, dewatering, and drying each molded pulp item. The next section addresses factors affecting the production rate.

FACTORS AFFECTING THE PRODUCTION RATE

Cost issues

Factors related to the drainage rates of water, the evaporation of water, as well as operational efficiency issues can be expected to have a major impact on the production rate of a molded pulp operation. Such issues have been considered in various articles on the topic (Porteous 1977; Luan et al. 2019; Klayya et al. 2021), but there has been a need to discuss such issues in an integrated manner.

As illustrated in Fig. 11, a factory based on continuous processing needs to cover different kinds of costs in order to remain economically viable. Fixed costs can include insurance, lighting, salaried labor, and debt payments on capital items related to the production. Semi-variable costs can include some contract labor. The most important variable costs will be the materials used in manufacturing (Hubbe and King 2009).

An important topic to consider, while discussing the various factors affecting water removal rates, is whether some of what has been learned in studies of the papermaking process can be borrowed and applied in the field of molded pulp production. A key finding from such studies is that more rapid dewatering, along with less stratification of the density of the wet fiber mat, often can be achieved by applying repeated pulses having gradually increasing intensity (Hubbe et al. 2020). Gentle suction, by means of hydrofoils, can be quite effective while the wet web of paper consists of mostly water and lacks strength. Increasing vacuum can be applied as the wet web reaches higher solids contents, along with greater ability to resist applied forces.

Fig. 11. Illustration of how production rate is necessary to cover different kinds of costs and maintain profitability in a manufacturing business. The bold dotted lines indicate an assumed “normal” operating condition. The bold red lines indicate what would happen if slow dewatering made it necessary to decrease the speed of production. The downward arrows represent before-tax income for the “normal” operations and the slower operations.

Various versions and implications of this hypothesis will be considered in the discussion that follows, while keeping in mind that testing or implementation of the hypothesis may be limited by the available equipment in specific cases. These issues are highlighted in Fig. 12. As shown, a typical flat paper machine system will include several hydrofoils, each providing a separate pulse of very low vacuum, followed by low-vacuum flat boxes (often with increasing vacuum), then high-vacuum boxes the progressive vacuum levels, and finally two to four felted wet-press nips, which again typically have increasing intensities.

Fig. 12. Contrasting the number of vacuum or pressure instances in typical papermaking vs. typical molded pulp production

Kozeny-Carman Analysis

Imagine a molded pulp operation in which the screen surface of a mold has just come into contact with the fiber suspension but no vacuum has yet been applied. At that instant, the mixture adjacent to the screen surface will be relatively uniform and most of it will consist of water. At that point, the water can rush quickly toward the screen as soon as the vacuum is applied. Almost immediately, a mat begins to form, and the resistance to flow increases. According to Darcy’s law, the rate of flow is expected to be proportional to the applied pressure, times a permeability coefficient, and divided by the fluid viscosity, i.e. the viscosity of water (Darcy 1856). Kozeny and Carman (Kozeny 1927; Carman 1937, 1938; Carrier 2002) are credited with the first rudimentary attempts to account for the permeability coefficient. The theory assumes a uniform bed comprised of incompressible, sphere-like particles. The model is illustrated conceptually in Fig. 13. Notably, when predicting the flow rate, the adjustable parameters include the applied pressure, the fluid viscosity, and the radius of the particles, and the porosity (fractional void space) of the bed. Based on such a model, it is easy to propose that the resistance to flow would be expected to increase in proportion to the thickness of the mat than has accumulated on the screen surface of the mold.

Fig. 13. Conceptual illustration of assumptions made in the Kozeny-Carman and Xu-Yu models to predict the flow rate through a packed bed, based on the applied pressure, the fluid viscosity, a measure of particle size, and the porosity of the bed

When attempting to explain the resistance to the flow of water through a suspension of particles, several important points can be learned from the Kozeny-Carman equation (Pal et al. 2006). The first is that the resistance to flow tends to increase with increasing specific surface area (area per unit mass) of the particles. If one assumes that the particles are identical in shape and non-porous, then it follows that smaller particles will contribute to greater resistance to flow. High levels of fine particles are known to slow down the dewatering of cellulosic fiber suspensions (Kullander et al. 2012). Luan et al. (2019) reported that fines contributed to longer dewatering times during production of molded tableware. The Kozeny-Carman theory also explains why heating up of process water will tend to increase the dewatering rate. That is because water’s viscosity decreases markedly with increasing temperature. The Kozeny-Carman theory also provides a rationalization of why the resistance to flow would increase with increasing mat consistency.

In an attempt to make the Kozeny-Carman analysis more realistic and to apply it in more situations, some aspects of fractal theory have been used (Costa 2006; Xu and Yu 2008). Costa (2006) derived a model in which the two fitting parameters were the Kozeny-Carman coefficient and a fractal exponent. The model gave an improved fit to data from the permeability through beds of pumice particles having differing porosity. Likewise, Xu and Yu (2008) found that a fractal model corresponding to a square geometric model gave the best fit to practical data. This approach is illustrated, in an approximate way, in Fig. 13. Note that the cubes shown in the illustration are meant to represent cubic in a fractal sense, rather than representing actual cubes. The original Kozeny-Carman model assumed spherical particles of equal size, and such a model did not fit the data as well.

Mat Structure and Permeability

The Kozeny-Carman analyses, as described above, tend to give misleading predictions for wet cellulosic fiber mats. The particles in the mat or bed are assumed to be incompressible and are assumed to remain in fixed, uniformly distributed positions. Real mats of cellulosic fibers exhibit compression. Such compression implies that the fibers can become deformed and pressed together, possibly sealing off some of the flow channels. Ingmanson (1952, 1953), when applying the Kozeny-Carman approach to the dewatering of paper, introduced a way to account for compressibility. Further progress, related to incorporating issues of compressibility and different shaped particles into the prediction of flow resistance, are covered in an earlier review article (Hubbe and Heitmann 2007).

The structure within a fiber mat can be expected to have a large effect on the flow resistance. In principle, relatively stiff fibers can be expected to contribute to a bulky mat structure having relatively open channels available for flow. Support for this concept includes the fact that mechanical refining of fibers tends to render them less stiff in the wet state (Paavilainen 1993), and it also tends to slow rates of dewatering (Gharehkhani et al. 2015). Based on the theoretical work of Philipse and Wierenga (1998), there is a huge range of packing that can be achieved by a random assembly of rod-like particles, depending on the assumptions that one makes in how they interact.

Lindström (1989) proposed that a bulking effect is associated with frictional effects among fibers during the formation of a fiber mass during the dewatering process. It is known that such frictional effects tend to produce a greater volume of sediment when particles or fibers settle from a suspension and form a cake (Kline 1967; Alince and Robertson 1974; Gruber et al. 1997). That is because the fibers each tend to stick to each other upon contact rather than sliding into a densely packed form. In practice, such effects can be achieved in molded pulp production by the use of flocculants, i.e. retention aids or alum treatment (Gavazzo et al. 2003). Such effects will be discussed when considering factors affecting the rate of release of water from pulp suspensions and mats.

Flow Resistance through the Screen

In addition to the main resistance to flow provided by the fiber mat itself, it is also worth considering the flow resistance provided by the screen and other structural aspects of the mold. Though one can expect a wide diversity is the details of mold construction, a metal wire mesh screen often faces the fiber suspension (Cullen Packaging Ltd. 2012). Such a screen is illustrated in Part A of Fig. 14. The cited article describes a metal screen having 50 μm wires with 50 μm spacing between the wires. These details are in a similar range to a 200-mesh screen, as has been used in the preparation of standard handsheets (TAPPI Method T205). The pressure loss during flow of incompressible fluid (such as water) through such a screen can be estimated using equations obtained by Brundrett (1993) over a range of Reynolds numbers.

The mold structure itself, which supports the screen, generally has much larger but far less numerous perforations, allowing for release of water, in response to the applied vacuum or pressure. Although a simple cylindrical drilled hole could be considered, other hole profiles may be preferred. For example, conical-profile holes are used in the type of screen that is used for the Dynamic Drainage and Retention Jar, which has been widely used for many years for evaluation of retention and drainage aid for papermaking (Britt 1981; TAPPI Method T 261 cm-94). Part B of Fig. 14 illustrates both the Britt jar apparatus and a profile of the screen detail. By presenting the smallest part of the openings towards the fiber suspension, such a format minimizes the chances that material will get stuck in a passage. In addition, it seems likely that flow resistance would be less compared to when the liquid has to pass through cylindrical holes matching that minimum diameter.

Fig. 14. A: Plain-woven screen for which calculations of flow resistance are available;

B: Illustration of the Britt Dynamic Drainage/Retention Jar, showing also a cross-sectional view of the conical holes of the screen

Fig. 15. Concept of how the size of opening at a screen surface may influence the degree to which fibers landing on the screen tend to droop into the throats of the openings

In addition to the flow resistance of the screen itself, many papermakers have experienced a phenomenon that has been called sealing. This happens when individual cellulosic fibers are able to droop into the openings of a forming fabric, thus slowing down the flow. A pictorial theory to explain this phenomenon was advanced by Kufferath (1983). He envisioned openings within a forming fabric as each being like a funnel. Fibers able to get into the throat of the funnel were anticipated to have a large adverse impact on dewatering, whereas those lying across the rims of the funnel did not contribute greatly to the sealing of flow through the fabric. Such a concept leads logically to an expectation that a relatively small size of opening might be ideal – small enough to discourage entrance by drooping parts of fibers. This concept is illustrated in Fig. 15.

Deposits of pitch, stickies, or scale, etc., within a screen or multiply perforated surface of a mold also can be expected to decrease flows. Such issues will be considered later, when discussing deposition and deposit-control strategies.

Effects of Fines

When evaluating the freeness of papermaking pulps, the rate of dewatering is profoundly sensitive to the presence and amounts of cellulosic fines (Hubbe 2002; Cole et al. 2008; Chen et al. 2009). As noted by Gess (1991), the adverse effect of fines on dewatering tends to be greatest when both the content of fines and the basis weight are high. These are expected to be issues of great concern when preparing molded pulp products from recovered paper. Not only are the basis weights generally at the higher end of what papermakers often deal with, but also the recycled pulp may include fines-rich components, such as mechanically pulped fibers. Not only is the fines content often relatively high in recycled pulp mixtures, but there may be large hour-to-hour or day-to-day variations in fines content.

The types of fines having the greatest adverse effect on dewatering are the fibrillar fines produced by delamination of the outer layers of fibers during their refining (Hubbe 2002; Cole et al. 2008; Hubbe et al. 2008). Not only are these the most slender fines, contributing to a high specific surface area, but they are also long enough to easily become stuck as they are drawn through a wet mat of fibers.

Fig. 16. Illustration of the choke-point mechanism to explain the adverse effect of cellulosic fines on dewatering rates, especially during low-shear dewatering of relatively heavy basis weight fiber mats when the content of cellulosic fines is relatively high

A mechanism to explain the adverse effect of fines on dewatering is illustrated in Fig. 16. To be able to show both the fibers (with lengths 1 to 3 mm) in the same view as the fines (defined to be less than 76 μm according to a screen separation assay (TAPPI Method T 261), the fibers are shown in a cross-sectional view. The idea implied by Fig. 16 is that unattached fines will tend to be drawn through the fiber mat by the flow. Some of the fines may get stuck in the choke-points of the mat, especially in parts of the mat that are denser. In addition to cellulosic fines, suspensions of fibers in a mill environment may contain substantial amounts of entrained air. A study by Lorz (1987) compared the drainage times when paper handsheets were formed with different amounts of air present in the mixture just before starting the drainage. Just 0.4% of air, on a volume basis, was enough to double the dewatering time. The effect can be attributed to bubbles blocking the drainage channels within the fiber mat. Problems related to entrained air can be addressed, in general, by using a multi-part strategy. One begins by minimizing unnecessary addition of anything likely to contribute to the stabilization of foam bubbles, e.g. surface-active agents and dissolved polyelectrolytes not attached to surfaces. The next step is likely the judicious addition of a low and steady dosage of a defoamer (Pelton 1989; Rekonen et al. 1990; Wilson and Wittich 2022). These products are typically emulsions of water-insoluble surfactants, sometimes formulated with hydrophobic particles, such as hydrophobized silica. Upon addition to the pulp suspension, the active ingredients have an affinity for the surface-active materials already associated with the walls of bubbles. Upon reaching those interfaces, the defoamer molecules spread rapidly, leading to the coalescence of adjacent bubbles. Although defoamers can be highly effective, their overuse can contribute to deposit problems and difficulties with the effectiveness of hydrophobic sizing agents. Further progress against the adverse effects of entrained area can be achieved by mechanically removing air from the fiber suspension (Matula and Kukkamäki 1998). This can be accomplished by devices in which vacuum is applied. Dissolved air thereby comes out of solution, and the entrained bubbles become greatly enlarged, so that they float to the water’s surface and break.

Densification Dynamics and Rewetting

Densification of layer adjacent to screen

Whenever a wet mat of fiber is dewatered under pressure or vacuum, the structure becomes densified adjacent to the screen or felt into which water is flowing (Hubbe et al. 2020). The situation is represented, pictorially, in Fig. 17.

Fig. 17. Representation of how a densified layer can be expected to form adjacent to a screen from which water is allow to leave the material

The phenomenon depicted in Fig. 17 is particularly noticeable when the wet-web of paper passes through a single-felted wet-press nip (Paulapuro 2000, 2001; McDonald 2020). Chang (1978) noted that the effect tends to be strongest when there is strong resistance to flow, e.g. in the case of well-refined kraft fibers. Campbell (1947) was apparently the first to propose that such densified layers would be a key point of resistance to dewatering.

To explain the stratification phenomenon, is has been proposed that pressure within a wet web of paper during pressing is supported by two main components, namely mechanical pressure and hydraulic pressure (Wahlström 1969; MacGregor 1989). On the side of the paper opposite to the felt, any possibility of net flow of water is completely blocked by the presence of a solid press roll, and the hydraulic component of pressure can be expected to be dominant in that location. By contrast, on the side facing the highly permeable felt, there is continuous leakage of water from the fiber mat, and hence the solid material is forced to take up more and more of the applied load.

Though there are some key differences, it can be argued that a very similar situation results when a fiber mat is dewatered against a screen by vacuum or applied pressure, as in the case of molded pulp production. The flow of water, in response to the applied vacuum, dynamically pulls upon the fibers in the mat, squeezing them in the direction of the screen surface (Didone et al. 2017). So even though such densification may be difficult to measure, it is proposed here that it is likely to explain some key aspects of both molded paper production and high-vacuum dewatering of paper associated with suction boxes and couch rolls (Hubbe et al. 2020). Because molded pulp operations typically employ a single vacuum step, drawing the fiber susp