Poplar is a typical fiber material for wood pulp making. Poplar has a short fiber length of about 1/3 of softwood fiber, which is intermediate in hardwood. However, Italian poplar grows in the southern area has long fiber. In addition, the poplar fiber cells are thin, the breast diameter is large, and the wall cavity ratio is small. Relative studies have shown that the fiber materials with wall cavity ratio less than 1 are flexible and easy to combine with each other, and it is a good raw material for papermaking. The fiber length distribution of poplar is uniform. It is suitable for manufacture uniform paper with high strength.
The cellulose content of poplar is higher than that of softwood and other hardwoods. Cellulose is the main composition required for papermaking. The content of lignin and extraction is low. Therefore, the yield of poplar wood pulp is high, and its bleaching performance is good.
Development of poplar pulp making
Poplar is distributed in the countries and regions of northern hemisphere, such as Eurasia, North America and North Africa. It is the native tree species of China and the main tree species in the northern region. Poplar grows rapidly, with light color, it has similar density with softwood, more pores, specific gravity and hardness is small. The characters of poplar are very beneficial for the absorption of chemical liquid and mechanical refining. It is widely used as raw material for hardwood chemical pulp and high yield pulp in northern Europe, North America and Northeast China.
Poplar and other hardwoods can be applied to chemical pulping and mechanical pulping. From now on, chemical pulping is still the main processing for hardwood pulp. Hardwood chemical pulp can be made by the sulfate process and the sulfite process, which is the same as the softwood pulp.
Poplar pulp process
In recent decades, the technology of poplar pulp process and other hardwood pulp process has developed rapidly. Except for poplar wood, hardwood is dense and hard, and generally not suitable for making high-yield pulp, especially mechanical pulp. In the 1980s, chemical thermomechanical pulp (CTMP) was developed. In the 1990s, a new type of alkaline peroxide mechanical pulp (APMP) appeared. For Hardwood, especially poplar and eucalyptus, after the chemical pretreatment, the strength properties of the mechanical pulp have been significantly improved. It is possible to use 100% poplar wood pulp to make newsprint and variety kinds of paper.
As a type of wood fiber materials, the poplar pulp making requires a variety of paper pulp making machines according to specific pulping process, including pulp digester, blow tank, vacuum drum washer, twin roll press, and etc.
The advantages of CTMP as following:
- High content of long fiber, low fragment content even at high freeness;
- Increase softness and tightness of fiber;
- high tearability at low or high freeness;
- low resin content;
- Higher whiteness of bleached pulp;
- Both apply for softwood and hardwood.
Due to a variety of advantages, CTMP can be used to manufacture tissue paper with high freeness and printing paper and writing paper with low freeness. Besides, CTMP is also used for paper board, gravure printing paper, low weight coating paper and etc. The combinative ability of poplar and CTMP is excellent, the CTMP made from poplar wood has 90% yield and 9000m breaking length.
APMP is a new type of pulp developed from CTMP. It differs from the CTMP production method in some aspects, firstly, chemical pretreatment is carried out by replacing NaOH and Na2SO3 with NaOH and H 2 O 2. Secondly, pulp is bleached while refining Peroxide, no need of a separate bleaching section. Thirdly, to prevent high temperature decomposition of peroxides, adopt two-stage refining at atmospheric pressure, no need of a heat recovery system. Therefore, compared to the CTMP, APMP is simpler, easy to operate, less area required, and low-cost. According to reports, equipment can save more than 25% of investment.
Poplar APMP can be used to manufacture most of the paper, such as high-grade low-quantity offset printing paper, low-quantity offset coated base paper, printing and writing paper, tissue paper and cardboard. The APMP provides another effective way to make full use of poplar to develop multi-variety and high-grade paper.
APMP vs CTMP
The chemical pretreatment of APMP is treated with NaOH and H2O2. It uses atmospheric pressure refining instead of pressurized hot grinding. In fact, APMP belongs to chemical mechanical pulp (CMP) pulping. It can be regarded as a new development of cold soda chemical mechanical pulp. However, it utilizes pre-steaming deoxygenation in CTMP pulping process, efficient impregnation by extrusion expansion, multi-stage disc refining, interstage washing and alkaline peroxide and other advanced processes and equipment, therefore it can be considered as a new development in the CTMP pulping process.
The results of the comparison of bleached chemical thermomechanical poplar pulp (BCTMP) and poplar APMP are shown in Table 1. At the same freeness, CTMP and AMPM have similar strength properties and optical properties, and APMP is slightly stronger. In terms of energy consumption, APMP is nearly 30% lower than CTMP. In addition, APMP is easy to be treated because it does not need Na2SO3. Compared to softwood APMP, tensile index of Poplar is higher, the fracture resistance index is equal, as for the tearability, poplar is still less than the softwood.
|caustic alkali (%)||1.8-4.3||5.8|
|Energy consumption (kW #h#t– 1)||1715||1220|
|Freeness (CSF/ ml)||77||77|
|compactness (kg#m– 3)||555||558|
|burst index (kPa#m2# g-1)||2.9||3.0|
|tearing index (mN#m2# g – 1)||6.3||6.5|
|tensile index (N#m#g-1)||53||60|
|light scattering coefficient (m2# kg-1)||39||43|