Extensible cellulosic fibre-polyurethane composites prepared via the papermaking pathway

Abstract

Formable papers can be used as an alternative to rigid plastics for making 3D shapes for packaging applications. However, commercial use of formable paper is currently limited, due to its poor extensibility. Cellulosic fibres can be combined with polyurethanes to improve the deformability of resulting fibre-polymer composites. This work describes the effect of spray and wet-end addition of polyurethane dispersions to paper to enhance the extensibility and formability of paper. The increase in extensibility was directly proportional to the amount of polyurethane retained in the paper. Absolute improvements in extensibility were as high as 4 to 6 percentage points. Improved extensibility resulted in better formability of paper, which eventually could allow it to compete with plastic packaging in certain applications.

Full Article

 Extensible Cellulosic Fibre-polyurethane Composites Prepared via the Papermaking Pathway

Alexey Vishtal,^a Alexey Khakalo,^b and Elias Retulainen ^c,*

Formable papers can be used as an alternative to rigid plastics for making 3D shapes for packaging applications. However, commercial use of formable paper is currently limited, due to its poor extensibility. Cellulosic fibres can be combined with polyurethanes to improve the deformability of resulting fibre-polymer composites. This work describes the effect of spray and wet-end addition of polyurethane dispersions to paper to enhance the extensibility and formability of paper. The increase in extensibility was directly proportional to the amount of polyurethane retained in the paper. Absolute improvements in extensibility were as high as 4 to 6 percentage points. Improved extensibility resulted in better formability of paper, which eventually could allow it to compete with plastic packaging in certain applications.

Keywords: Extensibility; Deformation; Formability; Bonding; Packaging; Fibre; Polyurethane composite

Contact information: a: VTT Technical Research Centre of Finland, now at Yash Papers Ltd., Yash Nagar, Faizabad, Uttar Pradesh – 224135, India; b: Aalto University, Department of Forest Products Technology, now at VTT Technical Research Centre of Finland; c: VTT Technical Research Centre of Finland, Koivurannantie 1, 40400 Jyväskylä, Finland; *Corresponding author: elias.retulainen@vtt.fi

INTRODUCTION

Paper and paperboard packaging materials have proved their usability and importance over the last century. Deservedly, paper and paperboard are the most-utilized consumer and industrial packaging materials in the world (Smithers PIRA 2015). This is due to the advantageous features of paper, such as recyclability, renewability, special haptics, printability, and its excellent stiffness and strength per weight ratio in the dry state. However, paper and paperboard lack barrier properties against water vapour, oxygen, and grease permeation. Paper is also limited in terms of convertibility, i.e., how many shapes of packages can reasonably be considered as designs in comparison to common plastics used in packaging. Overcoming these two principal issues would increase the competitiveness of paper in comparison to plastics. If the barrier properties of paper can be improved via the introduction of barrier films and coatings, which can also be made of renewable material and would not impair paper’s recyclability (Vartiainen et al. 2014), this would eventually lead to an increased share of recyclable and renewable packaging on the market (Andersson 2008). Another drawback of paper is that its limited convertibility originates from its insufficient formability, which cannot be improved in the same way as the barrier performance. Due to this, paper and paperboard come in the form of rectangular boxes, tubes, and pouches, while complex 3D shapes cannot be formed from paper. The potential to utilize form-fill-seal type of packaging lines in producing modified atmospheric tray of blister type of packages for food and pharmacy products would be of great benefit.

Previously, the mechanical treatment of fibres, addition of natural polymers, increased drying shrinkage of paper, and compaction of the fibre web have been employed as strategies to improve the formability of paper (Zeng et al. 2013; Vishtal and Retulainen 2014a, 2014b; Vishtal et al. 2015). The addition of elastoplastomers, such as polyurethanes (PU) to the paper furnish or to the already formed paper web, is another method of modifying the deformation characteristics of paper towards the higher extensibility and formability required for 3D forming. Such attempts were made in the 1970s (Alince 1977, 1979); however, the results have not led to further applications in packaging. Paper-plastic composites can be manufactured via several approaches including the addition of polymer dispersions in the pulp slurry, impregnation of the formed paper web, or lamination of the dry paper web with plastic film.

Based on the work of Li and Ragauskas (2011), it can be deduced that the main challenges faced in the combining of cellulosic material and polymers, such as polyurethanes, are the even distribution of the polymer and maintaining the adhesion between cellulose and polymers while preserving the web-like structure of paper. PU is an interesting and versatile material due to its mechanical and chemical properties. It has a high elongation capability, in the range of 400 to 800% (Kojio et al. 2010). The properties can be controlled by the relative proportion of the constituent monomers, so that the product can be thermoplastic, compostable, applied as a waterborn adhesive, and presumably it has some compatibility with cellulose.

In this work, the effect of the addition of PU on the extensibility and formability of fibre networks was studied and evaluated using tailor-made testing equipment, conventional tests, and structural analysis performed using SEM and light microscopy. Two different commercial polyurethanes were used in this study, and they were introduced to paper either as a furnish additive to the pulp suspension or sprayed on the paper after wet pressing. These methods were deemed to be compatible with modern board machine environments. Composite structures with a polyurethane content from 10 wt% to 50 wt% were prepared.

EXPERIMENTAL

Materials

Pulp

The bleached, once-dried softwood kraft pulp that was used in this study was kindly provided by Stora Enso Oy and originated from their pulp mill in Imatra, Kaukopää, Finland.

Polyurethane dispersions

Based on preliminary tests, two different polyurethane dispersions were used in this study. The Impranil® DL519 was kindly supplied by Bayer AG (Leverkusen, Germany) as the 40 wt% dispersion (the average PU particle size was 110 nm), hereafter referred to in the text as PU “A”. The Epotal® P 100 Eco was kindly supplied by BASF SE (Ludwigshafen, Germany) as a 40 wt% (particle size was below 100 nm), hereafter referred to in the text as PU “B”.

Fixing polymer

The water solution of cationic coagulation and fixing polymer, Fennofix® 50 (Polydiallydimethylammoniumchloride), was kindly supplied by Kemira Oyj (Helsinki, Finland) as 40 wt%.

Methods

Mechanical treatment of fibres

The pulp was subjected to the sequential high- (Wing refiner) and low-consistency (Valley beater) mechanical treatments to improve the extensibility of the fibres and paper made of such pulp. High-consistency treatment creates micro-compressions and dislocations in the fibres, while low-consistency refining straightens the fibres and improves bonding (Zeng et al.2013). The detailed effects of combined high- and low-consistency treatments on fibre and paper properties can be found in Khakalo et al. (2017).

Handsheet preparation

Handsheets were prepared according to ISO 5269-1 (2005) with a target grammage of 60 g/m². The handsheets were dried with and without drying restraint. Due to drying shrinkage and the addition of PU, the basis weights of the handsheets were in the range of 61 g/m² to 113 g/m². High basis weight (300 g/m²), A4-sized sheets for 3D forming were prepared using the “Juupeli” sheet former developed by VTT Technical Research Centre of Finland (Jyväskylä, Finland) and used in several studies (inter alia Oksanen et al. 2011). The types of the handsheets prepared in this study, their grammages, and densities are summarized in Table 1.

Table 1. Handsheet Type, Grammage, and Density: “A”- Bayer Impranil DL 519 and “B”- BASF Epotal P100

Note: rstr – restrained dried, N/A not available, BW- basis weight, DS*- two-side addition, **- high basis weight (HBW)

Wet-end addition of polyurethanes

Due to electrostatic repulsion between pulp fibres and PU particles (both anionic), in the absence of some kind of fixative or retention aid, the PU is not well retained in the paper sheet during drainage. Therefore, to retain PU on fibres, a cationic high-charge density, low molecular weight polymer was added to the pulp-PU suspension. The schematic representation of this approach is shown in Fig. 1.

Fig. 1. Schematic representation of the method for addition and retention of polyurethane particles in paper furnish

Pulp fibres were diluted to form an approximately 2.5% consistency suspension having a volume of 1 L after this PU dispersion was added. This suspension was mixed for 10000 revolutions in a British pulp disintegrator, and a sample of filtrate was taken for turbidity analysis (see below). Subsequently, a fixing polymer was added in a proportion of 500 g per 100 kg of PU (0.05 g per 1 g of PU). After this, mixing continued for another 30000 rpm and another sample was taken for turbidity analysis. Additionally, a benchmark sample was prepared to evaluate the retention of PU in the handsheet mould (i.e., dilution to 9.5 L), where the amount of retained PU was gravimetrically measured.

Evaluation of retention of PU in paper

The weight increment of the handsheets prepared from pulp-PU dispersion, in comparison with reference handsheets, was used for gravimetric retention evaluation. The value was derived from averaging the 10 handsheet measurements.

Turbidity measurement

Turbidity was measured for the undiluted filtrates, which were filtered through fine screen (300 mesh) to separate the pulp fibres. Turbidity was measured before and after the addition of the fixing polymer. The HACH 2100N (HACH company, Loveland, CO, USA) turbidity meter was used for the measurements.

Spray addition of polyurethanes

The PU was added to the wet fibre network after wet pressing by spraying. For the low-grammage handsheets, undiluted PU suspension (40 wt%) was added from top side, or in one case (50% addition) from both sides. A universal electrospray gun (Wagner W 140P, J. Wagner GMBH, Germany) was used. To enhance the penetration of PU into the paper, it was placed on a vacuum suction box.

The amount of the sprayed PU dispersion was gravimetrically controlled by weighing wet paper samples after spraying. Preparation of the restrained dried handsheets was not possible due to the high adhesion of PU to the drying plate.

Light microscopy

The images were taken using the Nikon Microphot microscope (Nikon Corporation, Tokyo, Japan) equipped with a CCD (Charge-Coupled Device) camera. The light strength was adjusted in the range of 4.3 to 10.7 in accordance with the magnification used. In each case, the exposure time was set as 10 ms.

Scanning electronic microscopy

Imaging was carried out with a Zeiss Sigma VP (Carl Zeiss NTS Ltd., Oberkochen, Germany) field emission scanning electron microscope (SEM) using an acceleration voltage of 3 kV to 4 kV. Prior to the imaging, the samples were attached to aluminium SEM stubs with carbon tape followed by sputter-coating (Emitech K100X, Emitech SAS, Paris, France) with platinum, forming a thin layer of 10 nm to 15 nm to avoid charging. The cross-sectional images of sheets were taken after resin embedding.

Dynamic mechanical analysis (DMA)

A dynamic mechanical thermal analysis was conducted for the polyurethanes used. Film samples made of the PUs were tested in shear mode using the Mettler Toledo DMA/STDA 861e instrument (Greifensee, Switzerland). The testing frequency was 1 Hz, and temperature range was -50 °C to 120 °C. Some sheet samples were tested in tensile mode under the same conditions.

Formation (grammage uniformity)

Formation was measured using beta radiation and a storage phosphor screen as done by Lappalainen et al. (2010).

Stress-strain measurements, formability strain, and 3D forming of paper

Tensile strength and strain at break of the paper samples were determined in accordance with the ISO 1924-2 (2008) standard. The ‘formability strain’ term refers to the highest strain that the paper experiences during the forming process in the 2D formability tester (VTT Technical Research Centre of Finland, Jyväskylä, Finland) before it breaks.

The formability strain is calculated from the position of the pressing die. Details of this measurement can be found in Vishtal and Retulainen (2014a). The 3D shapes were prepared using the 3D forming device at VTT. The device utilizes the hemispherical non-heated forming die (65-mm diameter) and the respectively shaped female forming cavity. The diameter of the samples was adjusted to 130 mm. Blank holding force of 2.5 kN was applied on the rim area of the sample (~100 cm²). The resulting pressure prevented any sliding and wrinkle formation under the circular blank holder.

RESULTS AND DISCUSSION

The beneficial effect of PU addition on formability depends on the evenness of its distribution in the paper and on the formation of adhesive PU-PU and PU-fibre-PU bonding. Two different addition methods were applied in this study: wet-end addition and spraying.

Wet-end Addition of PU

The addition of synthetic polymers to paper furnish is a known method for improving the dry strength of paper (Mihara and Yamauchi 2008); however, this technique had not been used before with polyurethanes. Polyurethane particles are negatively charged, as are cellulosic fibres, causing electrostatic repulsion and impairing the retention of PU. By adding a highly charged, low molecular weight cationic polymer in the thick stock, this adverse effect can be mitigated by “fixing” polyurethane particles on the surface of fibres. This approach is similar to that used for mineral fillers (Cadotte et al. 2007).

Without the addition of the cationic fixing agent, the retention of PU in the paper furnish was almost non-existent. However, at the same time, a high amount of fixing agent additions did not necessarily improve the ultimate retention of PU. A certain optimal addition level did exist. It was found that at the addition level of the cationic fixing agent of 1 kg per 100 kg of PU added to 1 ton of pulp, the clarity of the filtrates considerably increased, which indicated improved retention of PU on the fibres (Fig. 2).

Fig. 2. Turbidity of the undiluted filtrates obtained from the pulp + PU suspension with (right cuvette) and without addition (left cuvette) of the fixing agent for the 50%, 40%, 30%, 20%, 10% addition of PU to fibres

The increase in clarity of the filtrates was likely to be associated with the electrostatic attachment of the PU particles to the pulp fibres; this was visually confirmed by the light microscopy images shown in Fig. 3.

As shown in Fig. 3, polyurethane particles were attached to the fibres irrespective of the amount of PU added. The PU particles tended to attach to fines, at places where the fibre was fibrillated, or where the cell wall structure was somewhat damaged. This could be explained by the higher surface area and higher density of accessible carboxylate groups in these areas of fibre and consequent higher attraction to the positively charged coagulant. The increase in the PU load on fibres led to an extensive agglomeration of PU particles, and at a PU load of 50% the agglomeration mechanism dominated and the attachment of PU to fibres was almost absent. In this case, the mechanism of retention was likely to be filtration based. It was concluded that the addition of more than 30% PU on fibres may not have been feasible due to the unevenness of distribution and unevenness in mechanical properties, as well as the negative effect on recyclability of such structures.