basics of wood - sika group · tyloses season: in the large earlywood vessels. in other hardwoods...

BASICS OF WOOD

NATURE OF WOOD

• Trees are Primary Producers, meaning that they use natural resources to grow. Everything in a tree is built from raw materials gathered from soil and air and that is the secret to renew-ability.

• Learning about wood, its properties and its uses is an endlessly fascinating study.

• More you know about wood, more you want to know.

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HOW TREES WORK

The roots

• Anchor the tree into the soil

• Absorb water, minerals and oxygen

The tree-stem

• Static

• Water and food transport

• Production of new cells

The crown

• Photosynthesis

• To afford shade

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THE TREE-STEM

The outer bark (A):

• Protection from the outside world.

• Helps keep out moisture in the rain (continually renewed from within) and prevents from losing moisture when the air is dry.

• Insulation against cold and heat

• Wards off insect enemies

The inner bark, or “phloem” (B):

• Pipeline through which food is passed to the rest of the tree. It lives for only a short time, then dies and turns to cork to become part of the protective outer bark

The cambium cell layer (C):

• Growing part of the stem. It annually produces new bark and new wood in response to hormones that pass down through the phloem with food from the leaves.

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THE TREE-STEM

Sapwood (D):

• Is the tree’s pipeline for water moving up to the leaves. Sapwood is new wood. As newer rings of sapwood are laid down, inner cells lose their vitality and turn to heartwood.

Heartwood (E):

• Is the central, supporting pillar of the tree. Although dead, it will not decay or lose strength while the outer layers are intact. A composite of hollow, needle like cellulose fibers bound together by a chemical glue called lignin, it is in many ways as strong as steel. A piece 12” long and 1” by 2” in cross section set vertically can support a weight of twenty tons.

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GROWING OF A TREE

• Each year a tree grows in height from its tip, although new wood is added along the length of the stem, no previous growth rings are present at the top of the stem.

• The number of growth rings increases down the stem according to the number of annual height increments.

• The appearance of growth rings is due to changes in the structure of wood produced through the growing season. Cells produced at the beginning of the season are commonly larger, and so this early wood appears less dense than the latewood produced towards the end of the season.

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GROWTH RINGS

• In the north temperate zones, cutting any stem surface will show the wood to be composed of a series of concentric bands.

• These bands are referred to as growth rings, and in temperate trees commonly one ring is formed each year.

• Growth rings actually extend vertically along the stem as a series of concentric cylinders.

• If the numbers of growth rings on the two ends of a log are counted, more rings will be found on the lower end of the log than on the upper.

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GROWTH RINGS

• Not all trees produce visible growth rings, neither are all growth rings necessarily annual.

• In some trees seasonal changes in wood structure may be so slight that growth rings are not evident.

• Under conditions of severe drought an annual growth ring may not be produced. On the other hand under continuously favorable conditions, such as the tropics, several growth rings may be produced in a year.

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TRANSPIRATION

• On a warm, windy day up to 100 litres of water can be removed from the soil, transported through the tree’s xylem system and moved into the atmosphere.

• The water evaporates out of the needle or leave pores. This water loss is called transpiration

• Transpiration maintains the supply of water in the cells. A tree takes up a lot more water in it’s cells as he needs. The extra water is used to carry the dissolved minerals from the roots to the leaves. This watery solution is called the sap. Once the minerals have been removed from the sap, the remaining water is no longer needed and is removed from the plant through transpiration. This results in more water and minerals being taken in through the plant roots and moved into the xylem (a system of tubular cells linking the roots with the top of the tree) to move up through the plant to the leaves.

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PHOTOSYNTHESIS

• In a process called photosynthesis, trees use free water, carbon dioxide and sunlight to produce cellulose.

• Together with the lignin that binds it together, cellulose is the main component of this miraculous fiber called wood.

• It’s the only raw material that is at the same time renewable, biodegradable, recyclable, durable, versatile, energy efficient and extremely beautiful.

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SOFTWOOD - HARDWOOD

• Hardwoods do not always have harder wood as softwoods (Balsa is the softest wood but it still a hardwood)

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Softwood trees have needles

and are mostly evergreen (e.g.

spruce, pine). Some species

shed there needles in fall (e.g.

larch, douglas)

Hardwood trees have leaves

but may also be evergreen

(e.g. eucalyptus) or shed their

leaves in fall (e.g. oak, maple)

SOFTWOOD - HARDWOOD

In softwoods, the cells (tracheids) that serve to transport water also provide the mechanical support for the stem.

In hardwoods, some division of labor has evolved, with some cells specializing in water transport (pores or vessels), and others in providing mechanical support.

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Cross section 20 x Cross section 20 x

As a result hardwoods are commonly referred to as porous woods, and softwoods as nonporous woods.

MICROSCOPIC STRUCTURE OF WOOD

Softwood: Most cells run longitudinally but some cells run horizontally. The big hole is called a resin canal (rc). The majority of the cells shown here are called "longitudinal tracheids".

Hardwood: More cell types exists. The large holes represent vessels, the small ones are fibers. The lines between the vessel elements on the top of the block are bundles of ray cells called multiseriate rays.

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Electron micrograph scanning from redwood and birch


• This diagram shows some of the cell types in softwoods and hardwoods.

• a) Longitudinal tracheids accounts for over 90% of the wood volume of softwood. They are approximately 3-5 mm in length and 30-50 micrometers in diameter. These long cells, (fibers) are the main cell type which make up writing paper and brown paper bags.

• b) Hardwood vessels is earlywood and: d) is latewood

• c) Represents a hardwood fiber, while

• e) is a hardwood tracheid. Hardwood fibers are similar to softwood tracheids, but are much shorter. The fibers are approximately 1-2 mm in length and 20-30 micrometers in diameter. Kodak color paper is mainly made of maple and beech fibers. Toilet paper, napkins, and Kleenex are made of poplar fibers.

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• SOFTWOODS Here is a close look at pine wood. Most of the cells run vertically and resemble long, straight tubes, these are the tracheids.

• The circles with a hole at the center (side of specimen) are bordered pit-pairs. These pits are channels through which materials can flow from or into neighboring cells.

• The block-like holes on top of the specimen are cross sectional views of tracheids. Wood rays are perpendicular to the tracheids.

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bordered pit-pairs tracheids rays

Resin canal


In redwood the zones from the earlywood to latewood changes distinctively. This is called an abrupt transition.

In balsam fir, the transition zone between the earlywood and the latewood displays a gradual change similar to white spruce.

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Enlarged 100 times through a light microscope

The earlywood is produced at the beginning of a growing season with a

relatively thin cell wall and a large diameter. The latewood is formed late in the

growing season with a relatively thick cell wall and small cavity.


•In this view southern yellow pine earlywood is light in color and latewood is darker brown. The large holes are resin canals. Abrupt transition.

•Sugar pine has large resin canals. The growth ring in this sample is primarily earlywood with narrow bands of latewood. Gradual

•transition.

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Cross section 20 x Cross section 20 x


• Hardwoods This micrograph shows a cross sectional view of red oak (20x). The largest diameter holes in the earlywood zone are cross sectional views of vessel elements.

• In latewood, these vessel elements are small and sometimes grouped together. Because of this distinctive size and arrangement of the vessel elements, the growth ring is very clear and distinctive.

• This type of hardwood is called a ring-porous wood.

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Earlywood vessels

Latewood vessels


White oak is another good example of a ring-porous wood. Large vessels are located in the earlywood zone, small vessels are in the latewood. Tyloses in the large earlywood vessels.

In other hardwoods (walnut), the size of the vessels change gradually from the early growing season to the late growing season: semi-ring-porous wood.

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late

wo

od

e

arl

yw

oo

d


In some hardwoods size of vessels does not change very much during a growing season (sugar maple). The large circles are the vessels. This type of even sized vessels is called diffuse porous wood.

Another diffuse porous wood is sweet birch. The vessel elements in this micrograph are solitary (individual vessels) or grouped in "multiples" of two.

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White oak is a hardwood that has distinctive wide rays amongst narrow rays. Note the large diameter early wood vessels and the small latewood vessels, that are arranged in "flame shaped groups” parallel to the rays. (20x)

In this cross section of ash, several rays may be seen, but they are much narrower than those in oak. The latewood vessels are often circled by fine whitish areas which are groups of longitudinal parenchyma cells. (20x)

Birch is a diffuse porous hardwood that also has fine rays. (20x)

Microscopic Structure of Wood

DIRECTION IN WOOD

Lumber is solid wood that has been cut into different shapes.

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radial tangential

pith

ANISOTROPIC

• Anisotropic means that structure and properties vary in different directions.

• Due to the way cells are formed in a tree-truck, wood has 3 structural directions-the radial, the tangential, and the longitudinal direction.

• The tangential direction is parallel to the growth rings.

• The radial direction is parallel to the wood rays.

• The longitudinal direction is parallel to the wood cells (the grain).

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R = Radial direction

T = Tangential direction

L = Longitudinal direction

R

T

L

Many materials have mechanical properties which are the same in all directions. Such a material is said to be isotropic. Examples of isotropic materials are steel, concrete, and plastic. The mechanical properties of wood, vary with the direction in which a load or force is applied. Wood, therefore, is described as an anisotropic material.

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R

T

L

Anisotropic

vertical

HYGROSCOPIC

• Adsorption is a process by which the surface of a material attracts moisture from the air. It differs from absorption which is the uptake of liquid due to physical contact with the liquid like a sponge.

• Wood is hygroscopic. That’s way it changes in moisture content as the moisture environment in the air around it changes. Wood is able to attract moisture from the air. This is important because as the moisture content of wood changes, it changes in dimensions (swells or shrinks), the mechanical properties change, and other physical properties are affected.

• Therefore: proper use of wood products requires an understanding of their interaction with moisture.

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WOOD AND MOISTURE

Water moves through wood as liquid or vapor through several kinds of passageways.

These are:

• cell cavities of fibers and vessels, ray cells, pit chambers (microscopic openings on the sides of cell walls) and their pit membrane openings,

• the cell walls themselves.

• Water movement along the grain is many times faster than across the grain.

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WOOD AND MOISTURE

Water is contained in wood as either bound water or free water.

• Free water is contained in the cell cavities and is not held by these forces – it is comparable to water in a pipe (liquid water or water vapor)

• Bound water is held within cell walls by physical / chemical forces of attraction within the cell walls (water remaining in the cells)

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H2

O H2

O

H2

O

H2

O H2

O

H2

O

H2

O

H2

O

H2

O

WOOD AND MOISTURE

Free water moves through cell cavities and pit openings (microscopic openings on the sides of cell walls). During drying it is moved by capillary forces that exert a pull on the free water deeper in the wood. This is similar to the movement of water in a wick.

Bound water moves as vapor through empty cell cavities and pit openings as well as directly through cell walls. The basic cause of bound water movement is differences in water vapor pressure caused by relative humidity, moisture content, and temperature differences

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H2

O

H2

O

H2

O

H2

O

H2

O

H2

O

H2

O

H2

O

H2

O

H2

O

WOOD AND MOISTURE

• Oven dry 105°C, 214-221 F / >24 hours (= 0%)

• Equilibrium Moisture Content (EMC) = dependent RH – Wood loses or gains bound water until the amount it contains is in balance with that of the

surrounding atmosphere. The amount of water at this point of balance is called the equilibrium moisture content (EMC)

• Faber Saturation Point (= 22-30%)

• Free Water / Green Wood (>22% up to 600%) – Wood is dimensionally stable when the

moisture content is greater than the fiber saturation point. (Weight-%)

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MOISTURE CONTENT IN WOOD

• The amount of moisture adsorbed by wood can be expressed as the moisture content. Moisture content, calculated as a percentage of the oven-dry weight of wood is:

• % MC = wood weight + water (wet weight) – weight of dry wood x 100 weight of oven dry wood

• Oven dry weight of the wood only, no water (determined by drying the wood in an oven at 221 F for >24 hours until no water remains and the weight of the block no longer changes).

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SHRINKING AND SWELLING

Wood changes dimension as it gains or loses moisture below the fibre saturation point.

• It shrinks when losing moisture from the cell walls and

• swells when gaining moisture in the cell walls (Bound Water)

• This shrinking and swelling can result in warping, splitting, and loosening of tool handles, gaps in strip flooring, or performance problems that detract from the usefulness of the wood product. Therefore it is important that these phenomena be understood and considered when they can affect a product in which wood is used.

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R 1/2

T 1/1

L 1/10

+ H2O - H2O

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Shrinking and Swelling

Wood changes the volume with the moisture content mainly in tangential direction.

lon

git

ud

inal

Sapwood

Core


• Wood shrinks most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially), and only slightly (one-tenth to one-hundredth) along the grain (longitudinally).

• The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because of the difference in shrinkage and the curvature of annual rings.

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after shrinking initial form after swelling

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branch knot

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Beech panel 9 % moisture content 13 % moisture content

length in metre 5,0 5,013

width in metre 5,0 5,130

All wood species have different shrinking and swelling behaviours


Types of wood: moisture content Oak Beech

Average volume change per 1 % moisture content 0,25 % 0,32 %

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Activity medium high high low medium high

Bonding

difficult

easy

Wood tropical characteristics oak beech maple pine bamboo woods

Shrinking and swelling low high medium low medium medium

Wood Characteristics

All wood species have different behaviours in activity (bending, curving) and different contents of resins, oil what has an influence on bonding properties

EQUILIBRIUM MOISTURE CONTENT

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Table according NWFA Association USA

Example: 26.7°C + 50% r.h. = EMC 9.1%

WOOD FLOORING SPECIES

Most popular Hardwood Species

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Wood-Bondingcharacteristics(activity + adhesionproperties)

Hardness (JankaHardness Test),higher = harder

Stability (reactivityto ambientvariations such asrelative humidity)*

Relative cost (Pricefactor incomparison to redoak)

Red Oak easy ~1290 0,00369 1,00

White Oak easy ~1360 0,00365 0,95

Hard Maple difficult ~1500 0,00353 1,30

Yellow Birch easy ~1260 0,00338 1,30

White Ash easy ~1320 0,00274 1,20

Black Cherry medium ~950 0,00248 1,70

American Beech difficult ~1300 0,00431 1,20

Black Walnut medium ~1010 0,00274 3,00

WOOD FLOORING SPECIES

Most popular Tropical Wood Species

Some Softwood Species (Imported Engineered Wood)

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Wood-Bondingcharacteristics(activity + adhesionproperties)

Hardness (JankaHardness Test),higher = harder

Stability (reactivityto ambientvariations such asrelative humidity)*

Relative cost (Pricefactor incomparison to redoak)

Bamboo difficult ~1800 ? ?

Mahogany difficult ~2200 0,00238 1,55

Teak difficult ~1000 0,00186 2,50

Cypress difficult ~1375 0,00162 1,30

Wenge difficult ~1630 0,00201 5,50

Hickory difficult ~1820 0,00411 1,20

Cherry Brazilian difficult ~2350 0,00300 1,30

Douglas fire medium ~660 0,00267 1,70

Southern Yellow Pine easy ~690-870 0,00265 0,95

CALCULATION SOLID WOOD

The numbers in the chart above were developed by the Forest Products Laboratory of the U.S. Department of Agriculture. They reflect the dimensional change coefficient for the various species, measured as tangential shrinkage or swelling within normal moisture content limits of 6-14 percent.

Calculation:

Multiply the change in moisture content by the change coefficient, then multiply by the width of the board

Sample: 13 cm (5-inch) red oak plank board:

1. Interior temperature of 21°C (70 F) and a relative humidity of 40%, the board has a moisture content of 7.7%

2. If the relative humidity falls to 20%, the moisture content of the board will be 4.5% (difference 3.2%)

3.2% x 0.00369 x 13 cm (5 inch) = 1.5 mm (0.059 inch)

The 13 cm (5-inch) board will shrink by 1.5 mm (0.059 inch). Across 3 m (10 feet) of flooring. that could translate to as much as 3.5 cm (1.4 inch) of shrinkage

3. If the humidity rises to 60%, the board’s moisture content would be 12% (difference 4.3%)

4.3% x .00369 x 13 cm (5 inch) = 2 mm (0.079 inch)

The 13 cm (5-inch) board would expand by 2 mm (0.079 inch). Across 3 m (10 feet) of flooring, this could translate to 4.8 cm (1.9 inch) of expansion

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CALCULATION SOLID WOOD

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Board width: 130mm, species: red oak, room width: 3000mm

Initial conditions: Interior temp. =21°C (70°F) / Relative Humidity =40% =EMC of 7.7% (table)

A) Drop in R. H. Interior temp. =21°C (70°F) / Relative Humidity =20% =EMC of 4.5% (table)

Impact on wood floor:

Change in EMC: 7.7% to 4.5% =-3.2% difference

Difference in dimension: Formula: Difference in EMC [%] x shrinking/swelling coefficient [-] x dimension of plank [mm] Board: -3.2% x 0.00369 x 130mm = -1.53mm (shrinking) Room: -3.2% x 0.00369 x 3000mm = -35mm (shrinking)

B) Increase in R. H. Interior temp. =21°C (70°F) / Relative Humidity =60% =EMC of 12% (table)

Impact on wood floor: Change in EMC: 7.7% to 12% =+4.3% difference Difference in dimension: Board: +4.3% x 0.00369 x 130mm = +2.0mm (swelling) Room: +4.3% x 0.00369 x 3000mm = +47mm (swelling)

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Pictures: • Forest Products and Wood Science • J.G. Haygreen and J.L. Bowyer (c) 1996 by Iowa State University Press, Ames, IA • Brown Center for Ultrastructure Studies of S.U.N.Y. - College of Environmental

Science and Forestry, Syracuse • Audrey Zink-Sharp, Va Tech, Blacksburg, VA. • Chip Frazier, Va Tech, Blacksburg, VA. • Society of Wood Science and Technology, One Gifford Pinchot Drive Madison, WI

53726, http://www.swst.org

Books: • Bowyer, J.L., R. Shumlsky, J. G., Haygreen. 2003. Forest Products and Wood Science -

An Introduction, 4th ed. Iowa State Univ Press, Ames, Iowa. • Hoadley, R. B. 1980. Understanding Wood: A Craftsman’s Guide to Wood

Technology. Taunton Press, Newtown, CT. • Panshin, A.J. and C. deZeeuw. 1980. Textbook of Wood Technology, 4th ed. McGraw

Hill Book Company, New York.

WOOD FLOOR TYPES

Solid Wood Floors

are one solid piece of wood that have tongue and groove sides. Solid wood floors are more sensitive to moisture.

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Engineered Wood Floors

are generally 2-, 3 or Multi-ply of wood,

glued on top of each other in the

opposite direction. The wood floor is

dimensionally more stable and less

affected by moisture.

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Mosaic Parquet

• Only small movements:

• Small wood elements

• Frequently thin joints

2-, 3-ply:

Production: same moister content and change coefficient (stability) per layer!

Production

Dry

Wet

MECHANICAL PROPERTIES OF WOOD

• Just as shrinking and swelling of wood vary in the radial, tangential, and longitudinal directions, so do various mechanical properties of wood.

• So, wood is anisotropic in hygroscopic behavior as well as mechanical behavior.

• Three important mechanical properties of wood are used as a measure of its strength. These properties are compression, tension, and bending.

• The hardness / softness is an important indicator for the mechanical resistance of a wood floor

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DENSITY ()

• Density () of a material is defined to be the mass per unit volume and is calculated using the following equation:

• = mass / volume

• Density of the substance that makes up a wood cell wall has been found to be about 1.5 g/cm3. But a wood contains air in the cell lumens, so most woods have a density less than 1 g/cm3. And therefore it swims in water

• Density of a sample of wood is usually calculated as the weight density instead of mass:

• Wt density = weight of wood with moisture volume of wood with moisture

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SPECIFIC GRAVITY

• Specific gravity is a measure of the amount of solid cell wall substance and is also known as “relative density”. It is a ratio of the density of a substance to the density of water. In our case, the oven dry (OD) weight of a wood sample is used as the basis and comparison is made with the weight of the displaced volume of water. In equation form this is:

• SG = OD weight of wood Weight of an equal volume of water

• Most of commercial woods fall between 0.35 and 0.65. Exotic woods exhibit a much greater range, with SG’s reported as low as 0.04 and as high as 1.40

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NATURE OF WOOD

Never forget………Wood is watching you

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basics of wood - sika group · tyloses season: in the large earlywood vessels. in other hardwoods...

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