hardwood vessel picking

Download Hardwood Vessel Picking

Post on 08-Oct-2014

35 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

Hardwood Vessel Picking

Hardwood vessel pickingq

q

q

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!

One annual ring from a hardwood

Small HW fibres - small lumens and relatively thick walls

Large HW vessels

Pictures courtesy PPRIC

Hardwood vessels Ray cells

Vessels

Fibres

Pictures courtesy PPRIC

Annual rings

Mixed Tropical Hardwood - Fibres and Vessels (x110)

Mixed Tropical Hardwood - Vessel (x280)

Mixed Tropical Hardwood - Vessel (x280)

Eucalyptus vessels

Pictures courtesy PPRIC

Hardwood vessel pickingq

q

q

q

q

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.

Picking propensity for some HW species

By vessel By weight

Beech

Poplar

Maple

Birch

Oak

MTH

Heintze & Shallhorn, P&P Canada 96:11

How papermaking variables affect picking tendencyFine, 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 that may 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.

q

q

q

q

How papermaking variables affect picking tendencyLiterature 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.

q

q q

q

q

Vessel problems in coated papersq

q

q

q

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.

Two mechanisms of coating picking (after Colley) Vessel pick Base paper Coating pick

Printing

Film split during printing

A vessel pick

Pictures courtesy PPRIC

A picked vessel

Pictures courtesy PPRIC

Effect of refining on vessel count100 90IGTVessel Pick count

80 70 60 50 40 30 20 10 0 550 450 350 CSF 250 150 10% Cons 20% Cons 30% Cons 5% Cons

Picking reduces as pulp is refined Picking reduces at higher refining consistenciesHeintze & Shallhorn, P&P Canada 96:11

Summaryq

q

q

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

q

Control z direction distribution of filler These conclusions are found in the literature, and are borne out by customer experience.

q

Useful ReferencesColley, 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 Gifford Pinchot Drive, Madison, Wisconsin 53705

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

Thank you!For more information or if you have further questions, please contact your Canfor representative or email to mbradley@mail.canfor.ca