End User Focus: Talc in coated paper

By
Published: Monday, 23 July 2012

We often hear our bosses talking about working in a paperless office. Yet everyone knows that no office can ever be completely paperless. It is hard to conceive of any life without paper, whether in the office or at home. Since its invention, paper has become inseparable from our daily lives. Closely connected to the cultural development of mankind, it can be considered a socio economic indicator of any nation.

We often hear our bosses talking about working in a paperless office. Yet everyone knows that no office can ever be completely paperless. It is hard to conceive of any life without paper, whether in the office or at home. Since its invention, paper has become inseparable from our daily lives. Closely connected to the cultural development of mankind, it can be considered a socio economic indicator of any nation.

Paper is normally manufactured from vegetable fibers such as wood pulp, old rags, bagasse, agro-waste, recycled paper and other ingredients including inorganic non-metallic minerals such as calcium carbonate, talc, kaolin. Some of these minerals play a very important role and are termed as “functional fillers”. With the latest advancements in technologies such as nano technology, the role of minerals is expected to increase even more.

 
Talc mine belonging to Golcha
Associated
Group, southern Rajasthan, India.

A brief history of papermaking

The basic idea of papermaking is bringing single natural fibers together into a writable sheet. It began as a hand-crafted system in the ancient world, gradually spreading across the globe and becoming a mechanised process as different countries applied their own systems and technological know-how.

Although paper is known to have been used as far back as 3500 (BC), when it appeared in the form of papyrus in Egypt, the first real papermaking, as we understand it, is believed to have been attempted by Ts’ai Lun in 105 AD. He was working for the Chinese Emperor, Ho Ti, and succeeded in making paper from a combination of mulberry bark, tow and old linen.

Further developments occurred in Mexico, where the early Mayans and Aztecs produced a similar material from the inner bark of the fig tree. The bark was treated with water and lime to remove a latex- like sap, before being beaten on stones and then ‘felted’ on a board where it was left to dry.

Papermaking in Europe appears to have been introduced by the Moors who had mills in Spain at Xativa, Valencia and Toledo appearing around the middle of the 12th century. The oldest recorded document on paper is a deed of King Roger of Sicily in the year 1102.

Italian mills were first set up in 1276 in Fabriano, and the first known watermarked paper is said to have come from a mill in Bologna about 1282, or possibly from the Fabriano mill in 1293 or 1294.

The earliest German mills were established in Cologne in 1320. Stromer’s mill at Nuremberg was established in 1390 with the aid of Italian workmen, and other early factories were located at Ratisbon and Augsburg. The industry also progressed well in France, from where it expanded into the Netherlands.

English papermaking lagged behind the French and Dutch industries, with the first English mill established by John Tate early in the 16th century.

Once the process was established in Europe, different types of paper began to appear: blotting paper, a coarse, gray product, is mentioned as early as 1465, while the use of brown paper is first recorded 1570.

In America the first paper mill was built in 1690 by William Rittenhouse and William Bradford in the city of Philadelphia.

In 2004 global production of paper touched 300m tpa, comprising of over 3,000 qualities. Some of the more widely used examples are: writing and printing paper; photocopying paper; art paper; coated paper; tissue paper; newsprint; brown paper; matt finish coated; duplex; board and specialty paper.

The per capita consumption of paper in 2010 is shown in table No 1.

 


Talc as a functional filler

There are a number of fillers which are used in paper manufacturing. These fillers may be functional or non-functional in character.

Non-functional fillers include gypsum, powdered cuttlebone, wax and zinc oxide. The purpose of using non-functional fillers is to decrease costs without having any adverse effect on the properties of the paper.

Functional fillers, on the other hand, are critical to achieving various properties of paper. Such fillers include non-micronised or micronised talc, ground calcium carbonate, wet ground calcium carbonate and processed clays.

Of the functional fillers, talc is the world’s softest mineral; its ores are soft, platy, water repellant and chemically inert. Commercially the terms steatite, soapstone and talc are applied to the same mineral commonly referred to as talc.

Soapstone is a broad term, which is composed essentially of talc with accessory minerals such as dolomite, magnesite, quartzite and, rarely, with marble and limestone. Steatite is pure form of industrial talc, which contains CaO not more than 1.5%, MgO, 28-32%, SiO, 58-60%, grit not more than 0.5%, Al2O3, 2-3% and other impurities in traces.

Talc is the purest form, which is rarely available. Talc is a hydrated magnesium sheet silicate with the chemical formula (Mg3SiO4 (OH)2. The elementary sheet is composed of a layer of magnesium-oxygen hydroxyl octahedral, sandwiched between two layers of silicon oxygen tetrahedral. The main or basal surfaces of this elementary sheet do not contain hydroxyl groups or active ions, which explains talc’s hydrophobicity and inertness.

Talc is practically insoluble in water and in weak acids and alkalis. It is neither explosive nor flammable. Although it has a very little chemically reactivity, talc does have a marked affinity for certain organic chemicals (i.e. it is organophilic). At above 900⁰C talc progressively losses its hydroxyl groups and above 1,050ºC, it re-crystallises into different forms of enstatite (anhydrous magnesium silicate). Talc melts at 1,500ºC.

The paper industry is the largest consumer of talc and generally uses 300 mesh (53µ) in normal writing and printing varieties with the range of brightness varying from 80-96% in normal uncoated paper.

Talc is used in three stages of paper making:

- As a filler. In this stage, talc generally makes up 17-20% of every 1 tonne of paper. Use of talc increases the opacity, porosity, smoothness and brightness of the paper. Talc is chemically inert and remains in the paper as a cost-effective, infinitely recyclable filler.

- To control pitch and stickiness during the papermaking process. Oleo resinous droplets, micro or colloidal in nature, get added to the paper stream. These micro impurities can clog the paper machinery. Talc, being hydrophobic absorbs these micro droplets or, if the droplets are big, then talc lamellae stick to their surface thus controlling their stickiness.

 


- In coating formulations. Due to the platy nature of talc particle, it functions as a soft, printable and non-abrasive material in paper and paper coating. For high brightness finishes, talc leads to reduction in usage of expensive whitening and helps to control the gloss of the coat. Talc also provides smoothness to the surface and imparts opacity to the coat. Due to its hydrophobicity it gives printing runnability, porosity and a barrier effect in coated paper.

Additionally, talc reduces the friction on paper manufacturing machinery thus decreasing wear. However, the use of filler also reduces the physical strength of the sheet of paper, and limits sizing.

Kaolin can be used instead of talc, although the nature of the difference between these two fillers is mainly reflected in the whiteness achieved. While the whiteness of talc is 90% - 96.8%, the whiteness of kaolin is 80-86%. Thus, paper types requiring a relatively high whiteness of paper types tend to use talc. However, talc is generally used less in the production of thin sheets of paper, such as tissue paper.

The development of coated paper

Despite the early developments in paper manufacture noted above, the first paper coating was not developed until 1935.

Normal base paper needs value addition to satisfy consumer tastes and preferences by applying different types of coating layers. Duplex, board, coated and specialty art and matt finish papers are some of the forms of value added coated paper products.

A coating mixture is made up of different adhesives and pigments. Talc is the mineral pigment which is widely used in coating mixtures. Different properties of paper after coating like liquid penetration, surface tension and resistance to mechanical stress are enhanced and leads to better surface for printing, processing, or for specific applications.

A comprehensive definition of a coated paper might be ‘a base paper to which a coating of any kind has been applied.’ This would include a wide variety of papers, used for a multitude of purposes. The majority of the demand for coated paper generally comes from books, magazines and advertising matter.

Coated paper is generally believed to have originated in China, but no dates are given, and it must be assumed that any early Chinese coated papers were of a crude type and used for wall coverings.

European wallpapers were at first printed on uncoated paper, and an English journal states that this kind of paper first came into use around 1650.

These early hangings were quite crude and a marked improvement was made when a flat ground-coat was applied to the paper which was then printed with a decorative design. Such papers were made in the United States in 1824, and this may be assumed as the approximate date at which coated paper was first manufactured in North America.

It seems highly probable that the great improvement facilitated by printing wallpaper over a ground coat was responsible for the coating of other types of paper to obtain similar improvements in printing.

Early coated papers were made sheet by sheet, brushing on the coating mixture by hand. Many of the papers were used for highly glazed labels or for colored box coverings, and the mixture applied became known as “colour.” In many plants this name persists to this day, even when the coating contains only enough colouring matter to give the paper some tone of white.

It appears that the first paper to be coated on both sides was made in either 1874 or 1875 and this is also apparently the first application of coated paper for book printing. This paper was coated on each side separately. In later years the coating was applied on both sides of the paper in a single operation using machines.

Today, coated paper is made in two different ways. The first method to be developed was carried out by transferring the base paper, or ‘raw stock,’ as it is sometimes called, to a separate plant and completing the work there. Coated papers of the highest quality are still produced in this way, which is generally called ‘conventional coating.’

The second method is called ‘machine coating’ because it is carried out as a part of paper machine operation and at the regular speeds of paper production. Machine coated paper was at first quite inferior to that made by conventional methods, but with the skill and experience developed during the last fifteen years its quality is becoming much nearer to that of the good grade conventional coateds

The essential feature of a coated paper which makes it superior for printing is the smoothness of its nearly plane surface.

No matter how heavily uncoated paper is calendared, or how smooth its surface appears to be, there are always roughnesses which prevent perfect contact of the halftone dots of the printing plate and which injure the appearance of the illustration.

When a coating is applied it fills in the irregularities of the base paper surface and, after calendaring, presents a much more uniform surface to the printing plate, thus permitting a far more accurate reproduction of the dots of the halftones.

Talc, with or without other minerals combined with one or more adhesives, is used in coating mixture. Adhesives cause the pigments to stick to the surface of the paper firmly enough to withstand the pull of the printing ink.

The selection of pigment and adhesive combinations is responsible to a great extent for the resulting characteristics of the finished paper and its suitability for printing. It can influence such properties as colour, brightness, gloss, opacity, and surface smoothness, ink receptivity, rate of ink drying and the folding characteristics of the paper.

Among the oldest mineral components used in coating pigments are ground bone ash, white lead, calcium carbonate, gypsum, powdered cuttlebone, wax, zinc oxide, titanium dioxide, acrylic gesso, clay, talc and satin white (aluminium hydroxide, calcium hydroxide and calcium sulphate), barium sulphate, and calcium sulphate, sulphite and carbonate.

These have been combined with the traditional binders such as starch, mucilage and gums, gelatin and animal glues (such as hide glue), and casein, as well as modern latexes and synthetic resins (like acrylic emulsions) and peanut and soybean proteins.

In modern times, kaolin, calcium carbonate, whiting, talc and titanium dioxide are some of the preferred coating pigments.

Talc as a coating pigment

Talc is a very unique coating pigment that helps improve the tactile properties and printability of rotogravure paper, producing a silky, soft feel to the finished product. Since talc is a soft and platy pigment, it increases compressibility and maximises ink transfer, thus reducing missing dots and print mottle and improving the opacity of the printing.

The improved ink setting results in a lower smearing tendency. It also lowers the surface friction of the paper, and significantly reduces core bursts and crepe wrinkles at the paper mill and the printing plant, and improves runnability in rotogravure printing.

Any waste due to breaks during reel changes is reduced. The soft talc also prevents the blades of roll cutting and finishing machines from wearing out. All these factors improve efficiency, leading to cost savings.

In matt and silk offset papers, some grades of talc products improve delta gloss. This attractively emphasises the difference between printed and unprinted areas. Talc coatings also support lower print and varnish mottle.

In addition, talc can serve as a carrier for optical brighteners. These are used in many papers, and result in a reflection of light with a bluish tinge that suppresses yellow. Talc prevents the optical brightening agents from soaking into the sheet. It remains on the surface for maximum effectiveness.

In barrier applications, the platy structured, hydrophobic talc improves the barrier coating’s sealing effect by blocking water vapor transmission. The coating is oxygen, aroma and oil resistant - even liquid resistance is possible. With the use of talc, the product can be made printable, compostable and recyclable. Processing efficiency is increased due to improved runnability - for example, by preventing the blocking of reels.

For food packaging board and paper, talc may amount to a significant share of the coating dispersion. As regards cost efficiency, this means a lower demand for binder in the formulation.

High brightness talc pigments, including calcium carbonate, are also used in coating formulations in paper mills operating under alkaline conditions. Studies comparing the affinity of different minerals such as talc, ground calcium carbonate, bentonite, clay and diatomaceous earth to different contaminants, showed talc to have the best affinity.

In fact, talc was the only mineral to have an affinity to all the contaminants studied: adhesive, wax, toner ink and hot melt.

Talc therefore cannot be termed as just filler in the paper coating: it has been well established that it is a functional filler. Its bluish tone improves whiteness, and the right level of gloss, matt or silk finish can be achieved by simply fine-tuning talc particle size.

Expected increase in coated paper consumption

Worldwide consumption of paper and board is set to grow by approximately 2-2.5 % for at least the next two decades. The development and fast worldwide expansion of electronic media has led to a certain shift in paper qualities and challenged the development of new paper and board qualities. In particular, pigment coated grades have participated most in this new competition in the area of communication and packaging.

Because of their significantly enhanced printability, added aesthetic attractiveness and more valuable feel, the growth of coated paper and board will be twice as high as that of paper and board in general. Additionally, printed paper is a cost efficient medium with a high capacity for information, easily available and fully recyclable.

Most recently, pigments have become significantly cheaper than chemical pulps. Therefore, increasing the proportion of coating layers compared to fibers is an important economic factor in cost-efficient papermaking.

According to the production figures in 2004, coated paper and board together contributed about 17% of total global paper production. This is expected to increase to 21% by 2014. The volume of paper will be much higher compared to the weight per area by 2014 due to the increased coating to pulp ratio.

In coated paper, the proportion of coated layers per cm² of the base paper is between 30-65%, which is significantly higher than board for which coated layers make up roughly 5-20%.

The magnified image above shows the base paper and two coating layers.

Global trends in paper manufacturing

Future trends for coated papers will involve more specific paper products. Further advances in coating and printing technology, environmental factors, the development of coating colour, availability of raw materials and the globalisation of paper companies will all contribute to the evolution of the industry.

According to the Confederation of European Paper Industries (CEPI), Germany is the largest paper producer in Europe, followed by Finland, Sweden, Italy and France. The main pulp-producing countries are Finland and Sweden.

Over the last 15 years, paper and board consumption in Europe has increased by an average of 2.6% per annum. In 2006, CEPI member countries produced more than 100m tonnes of paper and board, of which 18m tonnes were exported. In volume terms, graphic paper grades account for around 48% of European paper production while packaging paper grades account for 40%, and hygiene and speciality papers for 12%.

Europe’s papermaking economic cluster generates an annual turnover of more than Û375bn ($461), or 6.5% of the European manufacturing industry’s total annual turnover. This figure is dwarfed by the industry’s presence in other parts of the world, however: in North America papermaking accounts for 28% of manufacturing turnover, and for Asia it is 35%.

One of Asia’s notable success stories in the field of papermaking has been Asia Pulp & Paper Co. Ltd (APP), one of the Sino-Indonesian Widjaya family of companies. Before 1995, APP was a relatively modest sized company but has risen to become the seventh largest paper producer in the world, exporting final products to 65 countries, according to Alex Goh, head of the firm’s finance department.

Perhaps unsurprisingly, China is poised to become the world’s largest paper producer. But China’s ambitions for this industry are causing serious deforestation in the country and across South East Asia. Modern paper production in China only developed during the last decade, but has grown aggressively in line with the rest of China’s economic boom.

Nick Harambasic, manager of the US natural solutions and paper producer Penford Products Corporation said: “At first, China had many small factories of low quality, which produced for the domestic market. In the nineties, it started installing paper mills of international standard. The growth has been phenomenal.”

He predicted that China would become the world’s largest producer of paper, a position currently occupied by the US.

Contributor:
Ajay Kulshreshtha, consultant, AK Minerals Consultancy Services, India.