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What Is Wood Pulp?

Wood pulp is a lignocellulosic fibrous material that’s primarily produced by separating cellulose fibers from wood. It’s made from breaking down the fibrous part of plants, primarily trees, and is the main ingredient in the paper making process. Pulp is commonly used to produce paper for all sorts of products. You can find wood pulp in napkins, toilet paper, cups, newspaper and just about every other paper product. There are two ways that pulp is produced: chemically or mechanically. Wood is transformed into pulp which results in a liquid combination of wood fiber, lignin, and water. Chemical by-products can also be left behind by the pulping process if chemicals were used during manufacturing. Currently, wood is the most widely used material for pulp production worldwide.

The mechanical pulping process involves grinding wood chips into mix of short cellulose fibers and a lot of lignin. This method generally yields weak paper that’s best suited for thin products like newspaper.

Chemical pulping, also known as the kraft process, is the more common method used to produce wood pulp. Once the raw wood is turned into “kraft pulp,” chemicals are used to separate lignin and cellulose fibers. These raw cellulose fibers can be used to produce stronger paper. However chemicals are often left behind as well which can work it’s way into the paper. The most common chemicals used in pulping are sulfurous acid and limestone which creates calcium bisulfite when blended. When wood chips are cooked using the sulfite process, lignin dissolves and leaves behind raw cellulose fibers.

Besides wood, companies also use other materials such as bamboo, straw, flax, cotton, hemp, etc., to make pulp.

Ahead we’ll discuss more about wood pulp. What it is? How it’s made? And what it’s used for?

What is Wood Pulp?

Pulp made from trees, a.k.a. wood fiber, is the most common source of fiber used for making paper. Wood pulp is made from breaking down either wood chips or paper recovered for recycling. There are a few methods used to produce wood pulp which each result in a different type of paper. But the two main methods used to separate fibers from wood to produce pulp are chemical and mechanical.

Chemically Produced Wood Pulp

Pulp is obtained by processing the wood with solutions made with a variety of chemicals. The paper products made from this type of pulp results in stronger paper that resists discoloring. The principal chemical processes used are sulfate (kraft), sulfite, and soda processes.

Chemical pulps are used to make thicker paper products like paper bags, writing papers, cups, dishes and other products that require strength. The kraft pulping process is the most common process used to produce paper.

The term “kraft” is commonly used interchangeably with “sulfate.” It’s derived from a German word which means “strong.” Kraft pulps are typically broken down into two types:

  • Hardwood Kraft Pulps: Made from trees with leaves.
  • Softwood Kraft Pulps: Made from trees with needles.

There are a few other types of chemically produced pulps as well.

  • Dissolving Pulps: A special grade of chemical pulp typically made from wood. It’s used in the manufacture of regenerated or cellulose derivatives such as acetate. The final product is used to make non-paper products such as film, fabrics, eyeglass frames and some plastic products.
  • Fluff Pulp: Wood pulp made during a chemical, mechanical or combination chemical/mechanical process from long fiber soft woods. It’s usually bleached and used as an absorbent material to make disposable diapers, bed pads and personal hygiene products. It’s also known as “fluffing” or “comminution” pulp.

Mechanically Produced Wood Pulp

Any wood pulp manufactured entirely or partially by a mechanical process. This includes stone ground wood and mechanical chip pulping.

Mechanical pulp has a higher yield than chemical pulping but it results in a weaker paper that discolors in sunlight easily. This is due to a high residual lignin or glue content left behind once pulping is complete. It’s uses include newspaper, specialty papers, tissue, and other types of thin paper.

Recovery Produced Wood Pulp

Pulp created from paper that was recycled is considered recovery pulp.

Sources of recovered pulp include newspapers, magazines, cardboard boxes, and just about any other type of recycled paper or box. Recovered pulp is commonly made by combining recycled paper with water in a pulper. The pulper operates as a blender which separates and mixes the fibers into a wet slurry that resembles oatmeal or cottage cheese.

The recovered paper may then be further processed to remove inks, coatings or other impurities. This process is called de-inking.

Recovered paper pulp can be used to produce new products made entirely of recycled pulp or mixed with other pulps.

During the paper production process, different pulps are often combined together to create products with specific properties. For example, high quality writing papers typically use a large percentage of bleached Kraft pulp with smaller amounts of mechanical or recovered paper pulps mixed in. This saves money and helps achieve a different feel for the paper.

Components of Wood

The principal components of wood are cellulose, hemicellulose, lignin, and extractives. Cellulose is a long chain of carbohydrates with repeated units of sugar or glucose. Hemicellulose is a short chain of carbohydrates with five different types of sugars that act as building blocks. Different species of wood have varying amounts and types of these ingredients. This means not all wood makes the same quality or type of paper.

In addition, another principal component of wood is lignin, a polymeric substance with excellent cementing properties. Extractives refer to fatty acids, oils, and resins.

Pulp and paper product manufacturers are very specific about the type of wood they want they’re pulp to come from as well as the process used to produce it.

Let us now discuss hardwood and softwood pulps.

Hardwood and Softwood Pulp

The properties of pulp depend on a wide range of actors, such as species, age, moisture content, density, etc. Generally, scientists classify each species into two primary categories or families: hardwoods and softwoods.

Hardwoods are deciduous, leafed and softwoods are needle-bearing conifers. There are different variations in the wood chemistry and fiber structure within these two families. Depending on the variation, the pulp produced has different properties.

Research shows that hardwoods contain more cellulose than softwoods. On the other hand, softwoods have more lignin than hardwoods. On average, softwood fibers for pulp purposes have three times the length of hardwoods.

Remember, increased fiber length is directly proportional to improved inter-fiber bonding, meaning softwood pulps offer more strength. For example, bleached softwood Kraft is stronger and durable than bleached hardwood Kraft.

Different Trees Produce Different Pulp

The type of wood and species helps determine the quality and properties of the pulp. This dictates what type of product can be used by the pulp that’s made. For example, softwood produces a very different pulp than a hardwood and is used to make very different end products.

Conifers like pine and spruce are considered soft wood trees. They have long fibers with a low density. Long fibers give them strength, so softwood pulp is used to create strong products. Softwood pulp also increases absorbency which makes it great for products such as paper towels, toilet paper, diapers and other hygiene products.

Leafy trees are generally considered a hard wood. Their fibers are shorter which makes the pulp softer. Because of this, hardwood pulp is often used for tissue papers.

Softwood and hardwood fibers are typically blended in different proportions which alters the end product. Think of it as a recipe that adjusts the qualities of the paper your making. Want a little more absorbancy, add more pulp from soft wood trees. But if you want a bit more softness, add more pulp from a hardwood tree.

In the pulp and paper industry, softwood pulp is prized for its strength. This is why it’s used to make boxes and packing papers.

Eucalyptus wood creates a very soft pulp that’s great for making soft tissue paper.

Each tree produces a different type of wood pulp with different properties. Those pulps can then be mixed together, like a recipe, to alter the paper product your making.

Conversion of Wood into Pulp

Previously, companies would carry out two distinct pulping procedures: mechanical and chemical. Thanks to advanced technology, companies can combine several aspects of both methods. Generally, the pulping processes have the following categories:

Mechanical Pulps

Mechanical pulps are highly effective because it contains 95% of original wood components. The products made of mechanical pulps have a high percentage of wood components, offering an economic advantage. However, this process can cause inevitable damage to the wood. As a result, the pulp produced has a lower strength.

Stone Ground-Wood

It is another crucial process that involved forcing debarked logs against a massive abrasive stone. The stone shreds the fibers from the wooden material. Next, the staff carries out the screening process to remove oversized slivers or shivers and other undesirable components from the wood. Some companies also carry out the bleaching process to make the pulp appear brighter.

Refiner Mechanical Pulp

Refiner mechanical pulp uses wood chips as its primary input. Companies use large metal discs in the refiner machinery to make this type of pulp. The process involves separating fibers from the wood and producing a mixture with a longer length than stone ground wood pulp. Bear in mind that refiner mechanical pulp contains a lower portion of small fiber pieces.

Thermomechanical Pulp

Thermomechanical pulp is produced through a unique process involving softening the wood chips by steam before entering a pressurized refiner. The purpose is to separate the cellulose mechanically and reduce the risk of damage to individual fiber pieces. As a result, the pulp produced is more potent than stone ground wood and refiner mechanical pulp. However, this process requires a lot of energy to get the job done.

Chemical Thermomechanical Pulp

It is an advanced procedure that involves treating the wood chips before carrying out the heating of fibers. The process produces pulp with strong physical properties. Companies also alter different parameters, including chip length, chemical concentration, temperature, etc., to customize the output. Therefore, the chemical thermomechanical pulp can be used for a wide range of applications.

What Kinds Of Products Are Made From Wood Pulp?

Pulp is the base that’s used to produce almost all paper products. Just about any paper product you use started out as wood pulp.

Paper towels, writing or print paper, cups, dishes, boxes, cardboard, craft paper and pretty much every other paper product is made from wood pulp.

Lots of other items you use every day contain wood pulp too. Fluff pulp is used to make absorbent and personal care items like diapers and personal hygiene products because it’s good at absorbing liquids.

Dissolving pulp can be dissolved in a solvent and made into textile fibers like rayon or Lyocell. These spun fibers are then used to make shiny types of cloth.

Used be surprised to know that wood pulp is even used in a lot of our foods and other products like toothpaste as a filler.

Green Pulp Making

Chemical processing pulp involves cooking wood chips with chemicals under high pressure. This removes lignin from the wood and separates it into cellulose fibers. The fiber is then washed, dried and finally shipped in bales to manufacturers who turn it into a final product.

The lignin that’s separated out can be used to create new products or burned to produce bio energy. In fact, many modern pulp mills can actually produce more energy than they consume to make the pulp by burning lignin. They can then provide electricity to local homes and businesses. This conservation makes modern pulp making a much greener process than it used to be.

Byproducts of the pulp making process, such as turpentine and crude tall oil, can be used to create renewable diesel or biochemicals. This is another way manufacturers limit waste and create a greener industry.

About 70-80% of pulp-based paper and board products are recycled in Europe and the US. And not just once. Wood fibers can be recycled 7-10 times. This makes products made from pulp much more renewable and green than other types of manufacturing.

The Future Of Pulp

Wood pulp is a strong, reliable, and versatile material. Companies use it for manufacturing a wide range of products, such as tissues, printing/writing papers, and board papers. Lastly, wood pulp is a primary fiber obtained through hardwood and softwood tree harvesting.

But these days there are lots of non-traditional uses for pulp. Technological advances mean products made from pulp, or the residue of pulp making, that you can eat, wear, power your car or conduct medical research with.

Biochemicals, for example, are not made of standard pulp but of variations of the pulp making process. The same raw materials or residuals of the pulping process can also be used for medical purposes. Who knows what creative uses for pulp scientists will come up with in the future. Here are some other areas you probably didn’t realize wood pulp was being used for.

  • Clothing
  • Bioplastics
  • Liquid Packaging Boards used in the coatings for products such as milk cartons
  • Medicine
  • Energy

Wood pulp is as a great alternative to fossil raw materials. Some day Bioplastics may replace fossil based plastics and biochemicals may replace oil based chemicals. Almost anything can be made out of raw wood materials and scientists are conducting research into new products and applications right now.Who know what they’ll come up with in the future. Wood pulp is definitely not just for making paper any more.

Summary: What Is Wood Pulp?

Wood pulp is a lignocellulosic fibrous material that’s primarily produced by separating cellulose fibers from wood. It’s made from breaking down the fibrous part of plants, primarily trees, and is the main ingredient in the paper making process. Pulp is commonly used to produce paper for all sorts of products. You can find wood pulp in napkins, toilet paper, cups, newspaper and just about every other paper product. There are two ways that pulp is produced: chemically or mechanically. Wood is transformed into pulp which results in a liquid combination of wood fiber, lignin, and water. Chemical by-products can also be left behind by the pulping process if chemicals were used during manufacturing. Currently, wood is the most widely used material for pulp production worldwide.

The mechanical pulping process involves grinding wood chips into mix of short cellulose fibers and a lot of lignin. This method generally yields weak paper that’s best suited for thin products like newspaper.

Chemical pulping, also known as the kraft process, is the more common method used to produce wood pulp. Once the raw wood is turned into “kraft pulp,” chemicals are used to separate lignin and cellulose fibers. These raw cellulose fibers can be used to produce stronger paper. However chemicals are often left behind as well which can work it’s way into the paper. The most common chemicals used in pulping are sulfurous acid and limestone which creates calcium bisulfite when blended. When wood chips are cooked using the sulfite process, lignin dissolves and leaves behind raw cellulose fibers.

Besides wood, companies also use other materials such as bamboo, straw, flax, cotton, hemp, etc., to make pulp.

If you have any questions or comments about wood pulp E-mail any time.

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