Wasting paper?

Materials World magazine
,
1 Jan 2015

Waste paper can be transformed into longer-lasting products. Dr Anthony Crabbe and Dr Anton Ianakiev of Nottingham Trent University, UK, describe research into a new moulding compound. 

The remarkable qualities of paper as a construction material are well demonstrated by the papier-mâché furniture compression, moulded by Jennens and Bettridge in the 1840s. An investigation into how that technology might be updated at a time when sustainability is high on the agenda led to coating dry, shredded wastepaper with water-soluble binders and hot pressing it into a tough material. It is called paplam, because it comprises a matrix of randomly orientated paper strands laminated together. Since it is made using dry paper rather than wet pulp, it is a faster, cleaner way of transforming waste paper into durable product forms.

A non-pulping method of converting wastepaper into a moulding material offers both economic and ecological benefits. The ecological benefits increase when the secondary material is used to make products that have a longer lifetime than ones made of paper. Paplam has similar properties to MDF and can be compression-moulded into a variety of product forms found in the furniture and automotive industries.


Better than recycling

The UK recycles around 64% of its wastepaper, but one sheet of paper can only be recycled 5–6 times. Therefore, recycling converts 20–30% of all recovered paper into a problem waste called sludge. Paper-making also impacts heavily on local water resources. The final production pulp comprises 99.5% water and some grades of recycled paper must first be de-inked while immersed – paper is one of the most expensive forestry-derived products in both economic and ecological terms. Paplam could help mitigate undesirable recycling costs by transforming appropriate grades of wastepaper into a moulding aggregate. Its woody characteristics make it suited to moulding larger components and products for use in interior environments.

Paplam has a piebald appearance, which, like that of chipboard, requires applied surface coatings to make it acceptable to consumers. The potential variety of surface treatments and inlays for moulded paper products is amply demonstrated by Victorian furniture. The fact that the strands in paplam are laminated 20% w/w by water-soluble binders means that it needs to be coated for use in damp areas, as do most engineered woods. Such coatings can either be applied after moulding, or moulded in by inserting pre-cut layers against chosen mould tool surfaces. Research to date has been confined to producing this basic matrix in the lab and will later investigate how best to apply suitable coatings.

Water-soluble resins were chosen to make a material that can be relatively easily broken down at the end of the product’s life, either for composting or recycling. Despite its poor reputation for emissions, the urea formaldehyde (UF) resin widely used to bind engineered woods quickly breaks down when soaked in water. That breakdown allows the component fibres to be recovered and reused in making more of the same. Alternatively, one research group has spread chipboard waste as a mulch on agricultural land, to which the urea is slowly returned as a fertiliser. Both approaches are ones that might be applied to paplam, providing the coatings chosen are removable and, ideally, recyclable.

The principle resins used in the research project are low-emission UF, SSi, starch and new soy adhesives. Of these, UF still gives the best all round performance and workability, while SSi significantly improves fire resistance. There is currently a great deal of research into new kinds of environmentally friendly binders and these could prove important to any future development.


Improving strength

Other researchers have already tried using paper as an aggregate on the two-dimensional rolling presses used to make particle board. The relative weakness of the resulting sheets shows that the material can only be formed by compressing resin-coated paper strands in all three dimensions. Because compression moulding is a much slower process than roller pressing, this cannot be a competitor to particle boards.

However, the highly foldable nature of damp paper means that it can be moulded into the sort of product forms first manufactured in the 19th Century. Compression moulding also makes it possible to press the compound at pressures higher than the 4.5MPa used for particle board. As shown in the table (opposite, top), this makes it possible to mould paplam into a product as strong as rigid thermoplastics such as ABS.

Paplam exhibits the remarkable property of elongating without fully breaking after reaching its yield strength. The graphs produced by the four-point bending tests show the differences recorded between a paper and woodchip sample prepared and moulded in exactly the same way. The sample uses 18–28mm paper strand, and the wood uses 2–5mm softwood chips. The graphs show how woodchip samples snap at yield strength, but paplam samples deform without breaking. This is because strands on the compression surface fold, those on the strain surface tear, while those close to the linear axis remain largely intact.

These differences help to explain the greater toughness of paplam compared to both natural and engineered woods. The foldable nature of paper means that this material forms a denser matrix than wood fibres when pressed at lower pressures. This greater density and hardness is shown in the table. Because it comprises a matrix of concatenated laminated strands, it has no grain and fewer voids through which fractures can propagate. 

If a nail is driven into it, displaced material tends to be forced out from the surface rather than compacted into it. Its hardness is greater than that of many endangered hardwoods, such as teak and sapele, which, respectively, have a Janka hardness of 525 and 685kgf.

Samples produced from longer strands have around 30–45% better flexural modulus (3.3–3.9GPa for long strands and 2.2–2.7GPa for short strands) and better flexural strength. Long strands have a larger contact area than short strands and this provides better bonding strength within the matrix. The hardness and flexural strength of paplam also makes it highly suited to construction sector products, such as floor and wall panels. The correct choice of resins can also help to improve the water and fire resistance of these products.

The choice of paper stock is also an important consideration. So far, office paper has mostly been used as stock, because many tonnes of this clean material are produced daily for security reasons. Using this kind of pre-shredded paper obviates one expensive manufacturing step. However, office paper is a premium scrap material and less valuable types are also being investigated. The most promising of these is a new flake material derived from cleaned domestic waste by a major European waste handler. Since the flakes are smaller and more absorbent than shredded printing papers, they make a more workable aggregate at the expense of a small loss of strength.

The use of shorter strands of lower grade wastepaper may also open the way to producing engineered lumber profiles by extrusion moulding. The extruder type we are considering is that used in the production of fuel briquettes. They can mould at pressures up to 100MPa, which would produce a high-density lumber with properties comparable to those of endangered forest hardwoods. As with thermoplastic extruders, it is possible to use a variety of die tools to produce various lumber profiles. However, this is currently just one idea in a research programme that is still in its infancy.

This preliminary research into updating a once booming, but now almost forgotten, technology has produced encouraging results that warrant further work. Paper is an indispensable material, but that does not mean it should always be recycled at all costs. There are gaps in the paper recycling economy that make this approach a more feasible alternative. The strength and toughness of this new material could make it a competitor not only to many forest timbers, but also certain thermoplastics. In other words, paplam’s own unique properties could add value to manufactured products as well as alleviate environmental problems that competing materials do not.

For further information, contact Chris Davison, chris.davison@ntu.ac.uk