question: Why are recycled paper products often a darker colour than original white paper?
comment: White paper is white because it's bleached. Paper is made from wood pulp, and wood pulp isn't white. Any paper that isn't white is either unbleached or dyed, whether or not it's recycled. In the case of recycled paper, it's mix of different papers of different colors, so it becomes a blend of the colors all the paper that went into it, and then it's not bleached.
passages: Pulp used in the manufacture of paperboard can be bleached to decrease colour and increase purity. Virgin fibre pulp is naturally brown in colour, because of the presence of lignin. Recycled paperboard may contain traces of inks, bonding agents and other residue which colors it grey.
  
  Although bleaching is not necessary for all end-uses, it is vital for many graphical and packaging purposes. There are various methods of bleaching, which are used according to a number of factors for example, the degree of colour change required, chemicals chosen and method of treatment. There are three categories of bleaching methods:
   Section::::Paper and newsprint.
  
  Paper and newsprint can be recycled by reducing it to pulp and combining it with pulp from newly harvested wood. As the recycling process causes the paper fibres to break down, each time paper is recycled its quality decreases. This means that either a higher percentage of new fibres must be added, or the paper down-cycled into lower quality products. Any writing or colouration of the paper must first be removed by deinking, which also removes fillers, clays, and fibre fragments.
   BULLET::::- Recycled: Used paper is collected and sorted and usually mixed with virgin fibres in order to make new material. This is necessary as the recycled fibre often loses strength when reused; the added virgin fibres enhance strength. Mixed waste paper is not usually deinked (skipping the deinking stage) for paperboard manufacture and hence the pulp may contain traces of inks, adhesives, and other residues which together give it a grey colour. Products made of recycled board usually have a less predictable composition and poorer functional properties than virgin fibre-based boards. Health risks have been associated with using recycled material in direct food contact. Swiss studies have shown that recycled material can contain significant portions of mineral oil, which may migrate into packed foods. Mineral oil levels of up to 19.4 mg/kg were found in rice packed in recycled board.
   Recycling as an alternative to the use of landfills and recycled paper is one of the less complicated procedures in the recycling industry. Although there is not a landfill crisis at this point in time, it is commonly believed that measures should to be taken in order to lower the negative impacts of landfills, for many hazardous elements are produced and spread because of this enclosure of trash. Most recycled paper is priced higher than freshly made paper, and this tends to plays a deciding factor for the consumer. Because most of the recycled pulp is purchased in an open market, virgin paper is produced cheaper with the pulp that was made by the specific paper mill. Virgin paper contains no recycled content and is made directly from the pulp of trees or cotton. Materials recovered after the initial paper manufacturing process are considered recycled paper. Because that original standard was so vague, some “recycled papers” contained only mill scraps that would have been included in virgin paper anyway. Standards have recently been set to prevent companies from making it seem like they were selling recycled paper. The collection and recycling industries have fixated on the scraps of paper that is thrown away by customers daily in order to increase the amount of recycled paper. Different paper mills are structured for different types of paper, and most “recovered office paper can be sent to a deinking mill”. A deinking mill serves as a step in the recycling paper process. This type of mill detaches the ink from the paper fibers, along with any other excess materials which are also removed from the remaining paper. In the deinking mill, after all of the unwanted coatings of paper are stripped, the refurbished paper is sent to the paper machine. The old scraps are now constructed into new paper at the paper machine. Many papers mills have recycled business papers by transforming the old business papers into beneficial letters and envelopes. The production process for recycled paper is more costly than the well-developed paper mills that create paper with the use of trees. This process in making recycled paper is also much more time-consuming. However, recycled paper has a multitude of benefits from an environmental perspective. “For all the state-of-the-art technology now incorporated into modern paper mills, the industry's underlying structure is still based upon a worldview that was transformative in the 19th-century but is out-of-date as the 21st century approaches”.
   There are three categories of paper that can be used as feedstocks for making recycled paper: mill broke, pre-consumer waste, and post-consumer waste. Mill broke is paper trimmings and other paper scrap from the manufacture of paper, and is recycled internally in a paper mill. Pre-consumer waste is material that was discarded before it was ready for consumer use. Post-consumer waste is material discarded after consumer use such as old magazines, old telephone directories, and residential mixed paper.
   Sometimes recyclers ask for the removal of the glossy paper inserts from newspapers because they are a different type of paper. Glossy inserts have a heavy clay coating that some paper mills cannot accept. Most of the clay is removed from the recycled pulp as sludge, which must be disposed of. If the coated paper is 20% by weight clay, then each ton of glossy paper produces more than 200 kg of sludge and less than 800 kg of fibre.
  
  The price of recycled paper has varied greatly over the last 30 or so years.
   Wood pulp produced primarily by grinding wood is known as "mechanical pulp" and is used mainly for newsprint. These mechanical processes use fewer chemicals than either kraft or sulfite mills. The primary source of pollution from these mills is organic material such as resin acids released from the wood when it is processed. Mechanical wood pulp is "brightened," as opposed to bleached, using less toxic chemicals than are needed for chemical pulps.
  
  Section::::Inks.
   BULLET::::- In 2006-2007, Australia 5.5 million tons of paper and cardboard was used with 2.5 million tons of this recycled.
  
  BULLET::::- Newspaper manufactured in Australia has 40% recycled content.
  
  Section::::By region.
  
  Section::::By region.:European Union.
   The two major classifications of pulp are chemical and mechanical. Chemical pulps formerly used a sulphite process, but the kraft process is now predominant. Kraft pulp has superior strength to sulphite and mechanical pulps and kraft process spent pulping chemicals are easier to recover and regenerate. Both chemical pulps and mechanical pulps may be bleached to a high brightness.
   Section::::Papermaking.:De-inked pulp.
  
  Paper recycling processes can use either chemically or mechanically produced pulp; by mixing it with water and applying mechanical action the hydrogen bonds in the paper can be broken and fibres separated again. Most recycled paper contains a proportion of virgin fibre for the sake of quality; generally speaking, de-inked pulp is of the same quality or lower than the collected paper it was made from.
  
  There are three main classifications of recycled fibre:.
   All of the processes imparted an adequately high pH in studies conducted by the European Commission on Preservation and Access, the Library of Congress, and a team of scientists from the Centre de Recherches sur la Conservation des Documents Graphiques in the early and mid-nineties. BookKeeper produced a pH of 9-10. CSC Book Saver gives a pH of 8.78-10.5. Wei T'o gives 7.5 to 10.4, and Papersave gives a pH of 7.5-9. 
   The United States Environmental Protection Agency (EPA) has found that recycling causes 35% less water pollution and 74% less air pollution than making virgin paper. Pulp mills can be sources of both air and water pollution, especially if they are producing bleached pulp. Modern mills produce considerably less pollution than those of a few decades ago. Recycling paper decreases the demand for virgin pulp, thus reducing the overall amount of air and water pollution associated with paper manufacture. Recycled pulp can be bleached with the same chemicals used to bleach virgin pulp, but hydrogen peroxide and sodium hydrosulfite are the most common bleaching agents. Recycled pulp, or paper made from it, is known as PCF (process chlorine free) if no chlorine-containing compounds were used in the recycling process. However, recycling mills may have polluting by-products like sludge. De-inking at Cross Pointe's Miami, Ohio mill results in sludge weighing 22% of the weight of wastepaper recycled.
   Bales of recycled paper (normally old corrugated containers) for unbleached (brown) packaging grades may be simply pulped, screened and cleaned. Recycling to make white papers is usually done in a deinking plant, which employs screening, cleaning, washing, bleaching and flotation. Deinked pulp is used in printing and writing papers and in tissue, napkins and paper towels. It is often blended with virgin pulp.
   BULLET::::- Preconsumer waste – This is offcut and processing waste, such as guillotine trims and envelope blank waste; it is generated outside the paper mill and could potentially go to landfill, and is a genuine recycled fibre source; it includes de-inked preconsumer (recycled material that has been printed but did not reach its intended end use, such as waste from printers and unsold publications).
   The share of ink in a wastepaper stock is up to about 2% of the total weight.
  
  Section::::Rationale for recycling.
  
  Industrialized paper making has an effect on the environment both upstream (where raw materials are acquired and processed) and downstream (waste-disposal impacts).
   Coloring enthusiasts use coloring notebooks for stress relief. The pages in coloring notebooks contain different adult coloring pages. Students take notes in notebooks, and studies suggest that the act of writing (as opposed to typing) improves learning.
  
  Notebook pages can be recycled via standard paper recycling. Recycled notebooks are available, differing in recycled percentage and paper quality.
  
  Section::::Electronic successors.
   In 2005, elemental chlorine was used in 19–20% of kraft pulp production globally, down from over 90% in 1990. 75% of kraft pulp used ECF, with the remaining 5–6% using TCF. A study based on U.S. Environmental Protection Agency (EPA) data concluded that "Studies of effluents from mills that use oxygen delignification and extended delignification to produce ECF (elemental chlorine free) and TCF pulps suggest that the environmental effects of these processes are low and similar."
   The brightness gains achieved in bleaching mechanical pulps are temporary since almost all of the lignin present in the wood is still present in the pulp. Exposure to air and light can produce new chromophores from this residual lignin. This is why newspaper yellows as it ages.
  
  yellowing also occurs due to the acidic sizing
  
  Section::::Bleaching of recycled pulp.
  
  Hydrogen peroxide and sodium dithionite are used to increase the brightness of deinked pulp. The bleaching methods are similar for mechanical pulp in which the goal is to make the fibers brighter.
  
  Section::::Bleaching chemical pulps.
   Three main issues with the environmental impact of printing inks is the use of volatile organic compounds, heavy metals and non-renewable oils. Standards for the amount of heavy metals in ink have been set by some regulatory bodies. There is a trend toward using vegetable oils rather than petroleum oils in recent years due to a demand for better sustainability.
  
  Deinking recycled paper pulp results in a waste slurry which may go to landfill. De-inking at Cross Pointe's Miami, Ohio mill in the United States results in sludge weighing 22% of the weight of wastepaper recycled.
   There are three categories of paper that can be used as feedstocks for making "recycled paper": mill broke, pre-consumer waste, and post-consumer waste. "Mill broke" is paper trimmings and other paper scrap from the manufacture of paper, and is recycled in a paper mill. "Pre-consumer waste" is a material which left the paper mill but was discarded before it was ready for consumer use. "Post-consumer" waste is material discarded after consumer use, such as old corrugated containers (OCC), old magazines, and newspapers. Paper suitable for recycling is called "scrap paper", often used to produce moulded pulp packaging. The industrial process of removing printing ink from paper fibres of recycled paper to make deinked pulp is called deinking, an invention of the German jurist Justus Claproth.
   Bleaching of wood pulp
  
  Bleaching of wood pulp is the chemical processing of wood pulp to lighten its color and whiten the pulp. The primary product of wood pulp is paper, for which whiteness (similar to, but distinct from brightness) is an important characteristic. These processes and chemistry are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.
  
  Section::::Paper brightness.
   Kraft pulp is darker than other wood pulps, but it can be bleached to make very white pulp. Fully bleached kraft pulp is used to make high quality paper where strength, whiteness, and resistance to yellowing are important.
  
  Section::::Manufacture.
   Kraft pulp is darker than other wood pulps, but it can be bleached to make very white pulp. Fully bleached kraft pulp is used to make high quality paper where strength, whiteness and resistance to yellowing are important.
  
  The kraft process can use a wider range of fiber sources than most other pulping processes. All types of wood, including very resinous types like southern pine, and non-wood species like bamboo and kenaf can be used in the kraft process.
  
  Section::::Byproducts and emissions.
   The de-inking process during paper recycling is also a source of emissions due to chemicals released in the effluent. The European Recovered Paper Council has developed the ‘deinkability scorecard’ so that the printed paper products which have the best recyclability when they are deinked can be identified.
  
  Worldwide consumption of paper has risen by 400% in the past 40 years, with 35% of harvested trees being used for paper manufacture. Plantation forests, from where the majority of wood for pulping is obtained, is generally a monoculture and this raises concerns over the ecological effects of the practice.
   BULLET::::- Mill broke or internal mill waste – This incorporates any substandard or grade-change paper made within the paper mill itself, which then goes back into the manufacturing system to be re-pulped back into paper. Such out-of-specification paper is not sold and is therefore often not classified as genuine reclaimed recycled fibre, however most paper mills have been reusing their own waste fibre for many years, long before recycling became popular.
  