hardwood vessel picking

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Hardwood Vessel Picking

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Page 1: Hardwood Vessel Picking

Hardwood Vessel Picking

Page 2: Hardwood Vessel Picking

Hardwood vessel picking

● Canfor has often been asked what suggestions we can make to help our customers overcome a problem with vessel picking.

● This presentation is a summary of the advice we give, mostly gleaned from the literature or the experience of papermakers.

● We hope you find some useful ideas here!

Page 3: Hardwood Vessel Picking

One annual ring from a hardwood

Small HW fibres - small lumensand relatively thick walls Large HW vessels

Pictures courtesy PPRIC

Page 4: Hardwood Vessel Picking

Hardwood vessels

Pictures courtesy PPRIC

Ray cells

Annual rings

Vessels

Fibres

Page 5: Hardwood Vessel Picking

Mixed Tropical Hardwood - Fibres and Vessels (x110)

Page 6: Hardwood Vessel Picking

Mixed Tropical Hardwood - Vessel (x280)

Page 7: Hardwood Vessel Picking

Mixed Tropical Hardwood - Vessel (x280)

Page 8: Hardwood Vessel Picking

Eucalyptus vessels

Pictures courtesy PPRIC

Page 9: Hardwood Vessel Picking

Hardwood vessel picking

● Vessels are present in all HWs, they are one of the mechanisms by which liquids flow through the tree.

● Size varies enormously between species, Oak and MTH have some of the largest.

● Vessels are normally very much wider than the fibres, and this leads to many of the problems they cause.

● The ability of any fibre or vessel to �collapse� is controlled by its lumen to fibre diameter ratio. Wood density affects this ratio.

● Vessel picking problems are influenced by vessel width, length and number per unit weight.

Page 10: Hardwood Vessel Picking

Picking propensity for some HW species

Beech Poplar Maple Birch Oak MTH

By vesselBy weight

Heintze & Shallhorn, P&P Canada 96:11

Page 11: Hardwood Vessel Picking

How papermaking variables affect picking tendency

● Fine, well bonded reinforcing fibre gives Z direction strength, and helps hold vessels in place. Better bonding reduces vessel picking.

● Good refining is essential, probably in the range of 100kWhr/t. This means that HW should ideally be refined separately from the SW.

● Refining lowers the freeness, so try to avoid other factors thatmay affect drainage, ie keep broke out of the HW refining line.

● At least one N. American coated paper producer thickens their HW and then puts it through a HCR refiner. This applies lots of specific energy without lowering the freeness by much.

Page 12: Hardwood Vessel Picking

How papermaking variables affect picking tendency

● Literature reports that hydrocyclone cleaning can remove tropical hardwood vessels. They can then be refined separately and returned to the main stream. This does not work with other hardwoods.

● Vessel picking decrease as wet pressing is increased. ● Surface sizing is extremely important for coated or uncoated

papers. Use a good quality starch, and check its quality regularly.

● High filler contents are positively correlated with vessel picking, they interfere with bonding.

● Z-direction distribution of filler affects vessel picking.

Page 13: Hardwood Vessel Picking

Vessel problems in coated papers

● Vessels on the surface of the base sheet prevent coating penetration into the base sheet. If the vessel is not well bonded to the base sheet, then a pickout can occur during offset printing. The base of the pit left in the paper will be free of coating.

● Coating can pick during offset printing for reasons other than the presence of vessels. In these cases the pit left in the coated surface will have traces of coating remaining.

● Microscopic examination can resolve the differences between the two types of defect (see Colley, 1984).

● Base paper surface strength was found to be the chief papermaking factor influencing both types of coating defects.

Page 14: Hardwood Vessel Picking

Two mechanisms of coating picking (after Colley)

Base paper

Printing

Film splitduring

printing

Vessel pick Coating pick

Page 15: Hardwood Vessel Picking

A vessel pick

Pictures courtesy PPRIC

Page 16: Hardwood Vessel Picking

A �picked� vessel

Pictures courtesy PPRIC

Page 17: Hardwood Vessel Picking

Effect of refining on vessel count

�Picking reduces as pulp is refined�Picking reduces at higher refining consistencies

0102030405060708090

100

150250350450550

CSF

IGTV

esse

l Pic

k co

unt

10% Cons20% Cons30% Cons5% Cons

Heintze & Shallhorn, P&P Canada 96:11

Page 18: Hardwood Vessel Picking

Summary

● With tropical hardwoods, consider removal of vessels by cleaning hydrocyclone cleaning systems.

● Refine the HW separately, applying significant specific energy, ca 100 kW/hr/t.

● Maximise the sheet surface strength:❖ ensure a good surface strength, ie use high quality starch❖ use fine, collapsible softwoods

● Control z direction distribution of filler

● These conclusions are found in the literature, and are borne out by customer experience.

Page 19: Hardwood Vessel Picking

Useful References

Colley, J., de Jong, J and Higgins, H.G., Surface Properties of hardwood papers in relation to fibres and vessels, in Fundamental Properties of Paper Related to its Uses, Transactions of the symposium held at Cambridge, BPBIF, 394 (1973)

Colley, J., Cross, P.,Tabart, J., Overcoming the vessel picking tendency of coated papers,Appita Vol. 39, No.1

T.W. Jeffries, Enzymatic Treatments of Pulps: Opportunities for the Enzyme Industry in Pulp and Paper Manufacture, Thomas W. Jeffries, USDA, FS, Forest Products Laboratory, One GiffordPinchot Drive, Madison, Wisconsin 53705

H. Heintze & P. Shallhorn, Hardwood vessel picking and manufacturing process, P&P Canada 96:11.

Page 20: Hardwood Vessel Picking

Thank you!

For more information or if you have further questions, please contact your Canfor representative or email to

[email protected]