How is Paper Recycled?

1. Sorting

Successful recycling requires clean recovered paper, so you must keep your paper free from contaminants, such as food, plastic, metal, and other trash, which make paper difficult to recycle. Contaminated paper which cannot be recycled must be composted, burned for energy, or landfilled. 

2. Storage

Paper mill workers unload the recovered paper and put it into warehouses, where it is stored until needed. The various paper grades, such as newspapers and corrugated boxes, are kept separate, because the paper mill uses different grades of recovered paper to make different types of recycled paper products. When the paper mill is ready to use the paper, forklifts move the paper from the warehouse to large conveyors. 

3. Re-pulping and Screening

The paper moves by conveyor to a big vat called a pulper, which contains water and chemicals. The pulper chops the recovered paper into small pieces. Heating the mixture breaks the paper down more quickly into tiny strands of cellulose (organic plant material) called fibers. Eventually, the old paper turns into a mushy mixture called pulp. The pulp is forced through screens containing holes and slots of various shapes and sizes. The screens remove small contaminants such as bits of plastic and globs of glue. This process is called screening. 

4. Cleaning

Mills also clean pulp by spinning it around in large cone-shaped cylinders. Heavy contaminants like staples are thrown to the outside of the cone and fall through the bottom of the cylinder. Lighter contaminants collect in the center of the cone and are removed. This process is called cleaning. 

5. Deinking

Sometimes the pulp must undergo a "pulp laundering" operation called deinking (de-inking) to remove printing ink and "stickies" (sticky materials like glue residue and adhesives). Papermakers often use a combination of two deinking processes. Small particles of ink are rinsed from the pulp with water in a process called washing. Larger particles and stickies are removed with air bubbles in another process called flotation. During flotation deinking, pulp is fed into a large vat called a flotation cell, where air and soap-like chemicals call surfactants are injected into the pulp. The surfactants cause ink and stickies to loosen from the pulp and stick to the air bubbles as they float to the top of the mixture. The inky air bubbles create foam or froth which is removed from the top, leaving the clean pulp behind. 

6. Refining, Bleaching, and Color Stripping

During refining, the pulp is beaten to make the recycled fibers swell, making them ideal for papermaking. If the pulp contains any large bundles of fibers, refining separates them into individual fibers. If the recovered paper is colored, color stripping chemicals remove the dyes from the paper. Then, if white recycled paper is being made, the pulp may need to be bleached with hydrogen peroxide, chlorine dioxide, or oxygen to make it whiter and brighter. If brown recycled paper is being made, such as that used for industrial paper towels, the pulp does not need to be bleached. 

7. Papermaking

Now the clean pulp is ready to be made into paper. The recycled fiber can be used alone, or blended with new wood fiber (called virgin fiber) to give it extra strength or smoothness. The pulp is mixed with water and chemicals to make it 99.5% water. This watery pulp mixture enters the headbox, a giant metal box at the beginning of the paper machine, and then is sprayed in a continuous wide jet onto a huge flat wire screen which is moving very quickly through the paper machine.

On the screen, water starts to drain from the pulp, and the recycled fibers quickly begin to bond together to form a watery sheet. The sheet moves rapidly through a series of felt-covered press rollers which squeeze out more water.  

The sheet, which now resembles paper, passes through a series of heated metal rollers which dry the paper. If coated paper is being made, a coating mixture can be applied near the end of the process, or in a separate process after the papermaking is completed. coating gives paper a smooth, glossy surface for printing.

Finally, the finished paper is wound into a giant roll and removed from the paper machine. One roll can be as wide as 30 feet and weigh as much as 20 tons! The roll of paper is cut into smaller rolls, or sometimes into sheets, before being shipped to a converting plant where it will be printed or made into products such as envelopes, paper bags, or boxes. 

sumber: 

 http://www.tappi.org/paperu/all_about_paper/earth_answers/earthAnswers.htm

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Physical and Optical Properties of Paper

Physical Properties

1. Basis weight/gramature, yaitu perbandingan berat suatu lembaran kertas per satuan luas, dengan satuan g/m2.
2. Thickness, yaitu ketebalan lembaran kertas, dengan satuan mm.
3. Bulk density, yaitu perbandingan gramature kertas terhadap ketebalannya, dengan satuan g/cm3.
4. Roughness (kekasaran permukaan kertas), yaitu volume udara yang mengalir keluar per satuan waktu pada tekanan tertentu diantara permukaan kertas, dengan satuan ml/min.
5. Smoothness, yaitu kehalusan permukaan kertas yang dihitung secara tidak langsung berdasarkan waktu yang dibutuhkan untuk mengalirkan 10 ml udara pada permukaan kertas berbentuk lingkaran seluas 10 ± 0,05 cm2 pada kondisi standar, dalam satuan sec/10 ml.
6. Bursting strength (kekuatan jebol kertas), yaitu besarnya tekanan hidrostatik yang dapat menjebol kertas melalui diaphragma karet pada daerah lingkaran berdiameter 30,5 mm, dalam satuan kPa.
7. Tensile strength, yaitu daya tahan maksimum dari lembaran kertas terhadap gaya tarik yang bekerja pada kedua ujung lembaran kertas tersebut, dalam satuan kN/m.
8. Tearing strength (ketahanan sobek), yaitu gaya yang diperlukan untuk menyobek selembar kertas yang dinyatakan dalam gram gaya (gf) atau mili Newton (mN).
9. Surface strength (kekuatan permukaan kertas), yaitu daya tahan permukaan kertas terhadap kekuatan cabut (picking) dari wax.
10. Cobb value (daya serap air), yaitu jumlah air yang diserap oleh lembaran kertas pada kondisi tertentu dalam kondisi standar, dengan satuan g/m2/min.
11. Moisture content (kadar air), yaitu kandugan air dalam kertas yang merupakan perbandingan antara berat air dalam kertas dengan berat kertas tersebut, dinyatakan dalam satuan persen (%).
12. Ash content (kadar abu), yaitu perbandingan antara berat abu hasil pembakaran dengan berat kering semula, dinyatakan dalam satuan persen (%).
13. Porosity (daya tembus udara), yaitu waktu yang diperlukan oleh 100 ml udara untuk menembus selembar kertas berbentuk lingkaran seluas 645 mm2, dinyatakan dalam satuan sec/100 ml.
14. Internal bounding, yaitu kekuatan antar serat dalam selembar kertas, dinyatakan dalam satuan J/m2.
15. Dirt count (noda kertas), yaitu benda asing (kotoran) pada lembaran kertas yang mempunyai warna kontras dan mempunyai luas tertentu yang diukur dengan dirt estimation chart, dinyatakan dalam satuan mm2/m2.
16. Stiffness, yaitu kelenturan (momen lentur) kertas terhadap getaran pada frekuensi konstan yang dinyatakan dalam mNm.
17. Water resistance, yaitu ketahanan kerats terhadap penetrasi cairan yang diukur berdasarkan waktu yang dibutuhkan cairan untuk menembus lembaran kertas, dinyatakan dalam satuan second.
18. Paper formation, yaitu derajat keseragaman penyebaran serat-serat dalam lembaran kertas.
19. Defect, yaitu cacat yang terdapat pada lembaran kertas dengan pemeriksaaan secara visual.

Optical Properties 

1. Brightness, perbandingan antara intensitas cahaya biru dengan panjang gelombang 457 nm yang dipantulkan oleh permukaan kertas terhadap cahaya sejenis yang dipantulkan permukaan lapisan Magnesium Dioksida. 
2. Opacity, perbandingan antara faktor pantul pencahayaan (Ro) dengan faktor pencahayaan intrinsic (R2) yang dinyatakan dalam persen (%). 
3. Whiteness, angka ang menunjukkan derajat putih kertas. 
4. Yellowness, angka yang menunjukkan derajat kuning kertas. 
5. L*, parameter yang menunjukkan peningkatan lightness (terang/gelap). 
6. a*, parametar yang menunjukkan tingkat merah/hijau. 
7. b*, parametar yang menunjukkan tingkat kuning/biru

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