Biermann Ch. Handbook of Pulping and Papermaking

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394

17.

BLEACHING AND PULP PROPERTIES CALCULATIONS

A 50.0 ml aliquot of

CIO2

solution consumed

19.96 ml of 0.152 N thiosulfate solution.

What is the concentration of

CIO2

in g/L?

In the Solvay process for making CIO2, how

much methanol is theoretically required per

ton of sodium chlorate?

A softwood, unbleached kraft pulp has a

kappa number of 34. During the bleaching

process, all of the lignin and 2% of the

carbohydrates are removed, what is the pulp

yield of the bleach plant? If the yield from

the pulp plant is 46%, what is the overall

bleached pulp yield from wood?

6. Describe how to prepare and standardize 5

gallons of KMn04 solution of 0.1000 N

used in the pulp mill for quality control. The

laboratory has 2 M

H2SO4,

1 N

H2SO4,

2 M

KI,

0.1000 N Na2S204, and starch indicator

solution.

Paper machine calculations

A paper machine with a width of 2.5 m has

a speed of 10 m/s and produces a paper with

a basis weight of 50 g/m^. Calculate the

weight of paper produced per second. From

this,

and assuming a consistency of 0.5% in

theheadbox, calculate the volumetric flow

rate necessary through the headbox. Since

one knows the width of the slice, and the

length per second going through the slice,

you should be able to easily solve the slice

height. Assume this is an open headbox with

100%

efficiency of conversion of potential

energy to kinetic energy (unlikely in the first

case;

impossible in the second case). Calcu-

late the height of the water above the slice

from the equations for potential energy (PE

= mgh, ^ = 9.8 m/s^) and kinetic energy

(KE = 0.5

mv^)

to obtain the equation.

8. A paper machine operates at 20 m/sec and is

10 m

wide.

Calculate

the

required equivalent

height of water in the headbox assuming all

of the potential energy of water is converted

to kinetic energy. For paper of basis weight

50 g/m^ what is the required slice height?

(The headbox consistency is

0.5%.)

What is

the maximum annual production in tons?

9. A paper machine has a speed of 5 m/s with

a basis weight of 100 g/m^ and stock consis-

tency of 0.6% at the headbox. What should

the slice height be?

Strength of

wet

fiber mats

10.

In Fig. 17-6, where are the strongest

bonds formed between fibers?

Hydrogen bond

Fiber or

surface

As water is removed, it helps draw hydroxyl groups

together and align them for hydrogen bonding.

Fig. 17-6. Hydrogen bonding and water remov-

al. ®1991 James E. Kline. Reprinted from

Paper and Paperboard vfith permission.

POLYMER CHEMISTRY

18.1 INTRODUCTION AND TYPES OF

POLYMERS

Introduction

This chapter is an introduction to polymer

chemistry with emphasis on topics related to pulp

and paper. Polymers are very important to many

aspects of pulp and paper, including wet end

chemistry, surface sizing, and coating. Of course,

the principal constituents of pulp fibers are all

polymers. Like other chapters in this book, many

of the introductory details are not included; for

further information, one should consult a textbook

on polymer science.

History of polymers

One of the earliest industrial developments

was the use of natural, soft rubber (poly-cw-

isoprene) in the early 1800s; the development of

the vulcanization process (reaction of the carbon

double bonds with sulfur to form crosslinked

chains) by Goodyear in 1839 led to hard rubber.

The first human-made plastic may be consid-

ered to be cellulose nitrate, which was discovered

in 1846 by Schonbein of Switzerland by the action

of a mixture of nitric and sulfuric acids on cotton.

Cellulose nitrate was used as a propel-

lant/explosive by the Austrian army in 1852 (now

called guncotton or smokeless powder), with amyl

acetate solvent as the first modern lacquer in the

United States in 1882, and in early photographic

films (plasticized with camphor to form Celluloid,

and as the first artificial silk by Count de

Chardonnet in France in 1884. Cellulose acetate

(discovered in 1865), being much safer since it is

less flammable, soon replaced cellulose nitrate for

most uses. Regenerated cellulose (see cellulose

xanthate) was invented by Cross and Bevan in

1892;

this is the viscose rayon process and was

used by Brandenberger to make cellophane,

marking the beginning of modern packaging with

transparent, plastic films in 1924 when the first

cellophane plant started operation in Buffalo, New

York. Indeed the cellulose-based plastics domi-

nated the synthetic plastics field for about 50

years.

The development of purely synthetic poly-

mers began with the discovery of the phenol-

formaldehyde resins by Baekland with the trade

name of Bakelite, small scale production of which

began in 1907. Other developments included the

use of styrene in synthetic rubbers in the 1930s,

the appearance of nylon (invented by Carothers) in

1939,

and the appearance of Teflon in 194L

Polymers

Polymers are high molecular weight chemi-

cals made of repeating units, called monomers,

which are linked by covalent

bonds.

The physical

properties of polymers depend on 1) the chemical

composition of the monomeric units, 2) stereo-

chemistry, if present, between the monomeric

units,

3) the mechanical configuration of the

polymer chain (that is, is it coiled or linear), and

4) the chain length of the polymer, that is, the

number of monomers, known as the degree of

polymerization (DP), of the polymer. Typical DP

values of commercial polymers range from several

hundred to many tens of thousands.

Polymers may be grouped according to their

component monomers. The simplest type of

polymer is the

homopolymer,

a polymer containing

only one type of monomer. Fig. 18-1. For conve-

nience, letters of the alphabet are assigned to

different types of monomers so that a homopoly-

mer may be described as one having the configu-

ration of: A-A-A-A-A-A-A-A-A-A. This can also

be written: A-(A)^-A; if « = 8 the DP of these

polymers is 10. A simple example of this is

polyethylene, made from the ethylene monomer

CH2=CH2; the structure of the polymer is CH3-

(CH2-CH2)n-CH3. Copolymers are polymers

containing two types of monomer, and terpolymers

contain three types of monomers. A copolymer

with an alternating structure is of the form

ABABABABABABABABABABABABA; a

copolymer with a random structure is

ABBAAABAABBABBBABABAABBBABA; a

395

396 18. POLYMER CHEMISTRY

HOMOPOLYMER: A-A-A-A-A-A-A-A-A-A

or AAAAAAAAAA or A(A)„A

COPOLYMERS:

ABABABAB

alternating

ABBAAABAABBABBBABABAABBBABBA

random

A(AUB(B)^

block (diblock)

A(AUB(B)^A(AU

triblock

GRAFT

COPOLYMER:

-AAAAAAAAAAAAAAAAAAAAAAAAAA-

B B B

B B

CROSSLINKED

HOMOPOLYMER:

-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA-

A A A

-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA-

A A

-AAAAAAAAAAAAAAAAAAA-

etc.

in all

directions.

Fig. 18-1. Representative structures of various classes of polymers.

block copolymer has the structure A(AXAB(B);3

if it is diblock or A(AXAB(B)„BA(A)„A if it is

triblock. Block copolymers are made by anionic

polymerization techniques since monomers can be

added consecutively with this method as the anion

is stable indefinitely. The structure of a graft

copolymer is shown in Fig. 18-1.

A branched homopolymer is like a graft

copolymer except all of the monomer units are

identical. Polymers may be cross linked to give

additional strength and rigidity. A crosslinked

homopolymer is depicted in Fig. 18-L

Many plastics or adhesives form crosslinks

during their manufacture or curing phase. These

plastics are called thermosets because once set in

position they will not soften with heating. Epoxy

resins,

phenol formaldehyde resin, and polystyrene

crosslinked with divinyl benzene are several

examples of thermosetting materials. Other

polymers consisting of linear chains or linear

chains with little or no branching are called ther-

moplastics, which soften when heated and harden

when cooled to ambient temperatures.

18.2 ADDITION POLYMERS

There are two categories for types of poly-

merization reactions used to form polymers:

condensation and addition mechanisms to form

condensation and

addition

polymers, respectively.

The addition mechanism is used to make

polymers from monomers with ring structures or

double bonds by a chain reaction. The "extra"

bond of the monomer is used to form the bond

between monomers; this means that no molecules

are lost during polymerization, that is, there is no

change in the molecular weight of the monomer

incorporated into the polymer. These polymers

are usually formed by free radical reactions;

however, anionic and cationic mechanisms may

also be used, but require special solvents and

reaction conditions. Free radical reactions are

started using initiators. Initiators are compounds

that form free radicals, such as peroxides, to start

the reaction. The free radical is always carried by

the terminal carbon atom between propagation

steps.

Initiation may also be carried out with high

ADDITION POLYMERS

397

energy radiation, photolysis, or thermal energy.

The reaction of a vinyl monomer, which has the

form CH2=CHR, is shown in Fig. 18-2.

Monomers almost invariably add head to

head, -CH2CHR-CH2CHR-, as opposed to head to

tail,

which is -CHjCHR-CHRCHj-. Head to head

addition can form three types of polymers which

differ in rotation around a carbon-carbon single

bond: in atactic polymers the R groups are ran-

domly oriented around the longitudinal axis of the

polymer; in isotactic polymers the R groups

branch out on one side of the polymer chain; and

in syndiotacticipo\ymtxs the R groups alternate on

one side of the axis to the opposite side of the

axis.

Atactic polymers are usually amorphous and

less dense than their often crystalline syndiotactic

or isotactic counterparts. Their structures are:

—

CH^

ISOTACTIC

—

CH^

-CHo

SYNDIOTACTIC H

-CHo

Addition polymers are characterized by high

molecular weight averages, rubbery or brittle

solids, and, usually, amorphous structures. The

chain reaction goes very quickly and the entire

polymer is built in a matter of seconds. It should

be noted that polymerization proceeds until all of

the monomer is removed or the free radicals are

all terminated. It is quite possible that residual

monomer will be present after polymerization.

Fig. 18-3 shows a variety of vinyl diene struc-

tures.

Fig. 18-4 shows a variety of vinyl poly-

mers,

and Fig. 18-5 shows a variety of acrylic

polymers all of which are usually formed by the

addition mechanism. Two other polymers include

-CH2CCI2- [poly(vinylidene chloride), Saran] and

-CF2CF2- [poly(tetrafluoroethylene), Teflon]. The

diene polymers are very elastic because the double

bonds cause the backbone to assume a kinked,

random coil shape; these are rubber materials.

Tensile forces allow these coils to be partially

straightened; relaxation of the forces allow the

original structure to be assumed.

One other type of acrylic polymer is

polyacrylic acid where R and R' in Fig. 18-5 both

consist of H atoms. The so-called superabsorbent

STEP

Initiation:

EXAMPLE REACTION

12-^21-

DESCRIPTION

The initiator forms two free radi-

cals.

I- +H2C=CHR-^ICH2C-

The free radical reacts with the

first monomer.

Propagation: I(CH2CHR)„CH2CHR- + H2C=CHR

-> I(CH2CHR)„+iCH2CHR-

Monomers are added to the growing

chain.

Chain transfer:

-CH2CHR- + H2C=CHR'

->-CH=CHRH-CH3C-

One possibility.

Termination:

2 -CH.CHR- -* -CH.CHRCHRCHy

Termination by coupling or

combination.

2 -CH2CHR- -* -CH2CH2R + -CH=CHR Termination by disproportionation.

Fig. 18-2. The steps involved in polymerization by the addition mechanism.

3<)8 18. POLYMER CHEMISTRY

H H R

Polymer Name

H Poly(butadiene) (synthetic rubber)

CHT

Poly(isoprene) (natural rubber, cis conflg*)

CI Poly(chloroprene) (Neoprene)

Fig. 18-3. Polymerization of dienes. The

position of the double bond is variable; it may

be a side group.

polymers that are used in diapers and incontinence

products are based on polyacrylic acid, where

some of the carboxylic acids occur as the sodium

salt. Sometimes the polyacrylic acid polymers are

reacted with starch to form the superabsorbent

polymers.

Another

type

of addition polymerization

the

ring opening reaction. One example is the poly-

merization of ethylene oxide used to form polyeth-

ylene glycols and ethoxylated molecules used as

surfactants.

/ \

n CH2CH2

HOCH2CH20CH2CH2^-

H R

1 1

n Cz=:C

1 1

i i

CH3

OAc

C=ti

CONH2

COOH

Fig.

—• -CH2-CHR-CH2-CHR-CH2-CHF

Polymer Name

Polyethylene, PE

Poly(propylene), PPE

Poly(vlnyl chloride), PVC

Poly(vinyl alcohol), PVA

Poly(vinyl acetate), PVAc

Poly(acrylonitrile), PAN

Poly(acrylamfde), PAM

Poly(acrylic acid)

Polystyrene, PS

18-4. Vinyl polymer structures.

H COOR

„u —

COOR

CHo

H R'

-CH^

Polymer Name

CH,

12 Poly(methacrylate)

H CH2CH3 Poly(ethacrylate)

Poly(methyl methacrylate)

(Luclte, Plexiglas)

CH3 CH3

Fig. 18-5. Acrylic ester structures.

18.3 CONDENSATION POLYMERS

The condensation mechanism is used to make

polymers from monomers with two (or more for

crosslinked polymers) reactive functional groups

by a step-wise reaction. Each reaction step be-

tween two monomers produces a simple molecule,

often water, but sometimes NH3 or another mole-

cule,

as a side product. These reactions are

common, well known organic chemistry reactions.

They are easily controlled reactions, unlike free

radical reactions. The most common reactions of

synthetic polymers are esterification (formation of

ester linkages) and amidation (formation of amide

linkages). Fig. 18-6 gives an example of

each.

the case of nylon 6,6 (6 is the number of carbon

atoms in each of the two monomers if a

dicarboxylic acid and diamine were used to form

it) it is possible to use a single compound to make

the polymer, and that is H2N(CH2)5COOH.

It is apparent that monomers will react

together initially and small polymers (oligomers)

will react later. This means that after a short time

essentially all of the monomers will be gone and

oligomers will exist. These oligomers will then

react to form short chain polymers, then the short

chained polymers will react to form moderately

long chain polymers. The number average DP„

can be predicted as a fiinction of the extent of

reaction. The extent of reaction is the fraction of

fiinctional groups which have participated in the

polymerization reaction. For bifimctional reac-

tants present in equal proportions, such as in Fig.

CONDENSATION POLYMERS

399

HCX:H2CH20H

ETHYLENE GLYCOL

HOC-

•-^L - -t^^L^,-<L^.

TEREPKHHAUC ACID

COCH2CH2-

POLYCETHYLENE TEREPHTHALATE)

-COCH2CH2O—

+ n

H2O

H2N(CH2)eNH2

H00C(CH2)4C00H

1,6-HEXANEDIAMINE ADIPIC ACID

—. -NH(CH2)6NH-C0(CH2)4C0-

+ n

H2O

NYLON

6.6

EPICHLOROHYDRIN

Fig.

18-6.

Formation

polyester and polyamide polymers.

CH3

0^H0^<"

I2C

CH—CH2--'

CH3

bisPHENOL

(DIPHENYL PROPANE)

CH,

HCI

UNEAR

POLYMER WITH EPOXIDE GROUPS

RCHCH2NHR'

(CURING STEP)

Fig.

18-7.

Formation

epoxy resins.

18-6,

the DP„

l/(l-p).

For

example,

50%

the functional groups reacted/?

0.5

and DP^

this would occur

each monomer reacted with

one other monomer

that only dimers were left.

p =

75%

(if

these dimers reacted

form

tetramers), then DP^

= 4. It

apparent tiiat if p

99%,

DP„ = 100; if

p_=

99.9%,

DP„ =

1000;

and

ifp =

99.99%,

DP„

10,000.

It is

very difficult

obtain such high extent

reac-

tions;

therefore, condensation polymers tend

have relatively

low

molecular weights unless

the

monomers have,

the

average, more than

two

functional groups each. Two other useful relation-

ships include

DP^

(l+p)/(l-p)

and

DPJDP„

-i-p;

consequently,

for

high extents

reaction

the polydispersity approaches

Condensation polymers

are

characterized

polar linkages, relatively

low

molecular number

averages, unless crosslinking agents are used,

and,

often, crystalline polymers. Other examples

condensation polymers include polysaccharides

(Chapter

19),

proteins,

and

polyurethanes. Pro-

teins

are

made from amino acids, which have

the

structure HOOC-CHR-NH2

and are

joined

amide linkages

form:

-COCHRNH-COCHRNH-COCHRNH-

Polyurethanes

are

formed

by the

reaction

diisocyanates (O=C=N-R-N=C=O) and glycols

with the linkage -NH-CO-0-

by the

reaction:

OCNCH2CH2CNO

-h

HOCH2CH2OH

-[OCONHCH2CH2NHCOOCH2CH2-];^

Epoxies are polyethers but retain the name

one

the reactants.

The

most common epoxies

involve reaction

excess epichlorohydrin with

bisphenol A;

the

excess epoxide groups

are

cured

by reacting with polyamines

shown

Fig.

18-

Epichlorohydrin

used

numerous resins

supplied

to the

pulp

and

paper industry.

18.4 MOLECULAR WEIGHTS OF POLYMERS

Synthetic polymers consist

individual

polymers with varying molecular weights.

The

properties

the polymers depend

not

only

on the

average molecular weight but on the distribution of

molecular weights

well.

example

of a

molecular weight distribution typical

free

radical reactions

shown

Fig.

18-8.

Many

the methods used

characterize polymers give

average molecular weights that depend

on the

molecular weight distribution

well

as the

num-

ber average molecular weight.

becomes useful,

as will

demonstrated shortly,

define several

types

molecular weight distributions.

400 18. POLYMER CHEMISTRY

The first is a simple average where each

molecule contributes equally to the average^ (a

number average molecular weight depicted as M^,

Let Ni equal the number of the molecules (or

moles or other measure of number of molecules)

with mass M, (or a measure of mass such as DP).

Using summation terminology for each fraction

from / = 1 to 00, abbreviated here as E, the

number of molecules in each fraction multiplied by

the molecular weight (or other measure of molecu-

lar weight such as DP) for each fraction are

summed and the entire sum is divided by the total

number of molecules as shown in Eq. 18-1.

M„ = -^^ k DP. =

(18-1)

A number average molecular weight is

obtained by methods that depend on the colligative

properties of polymer solutions. One such method

is vapor phase osmometry that indirectly measures

vapor pressures of polymer solutions. Such

methods are Ihnited to low molecular weight

polymers (several thousand grams per mole or

less).

This method has the drawback that it does

not represent the behavior of polymer mixtures

very well.

Consider an extreme, hypothetical example;

suppose during kraft pulping that 10 individual

glucose units are cleaved from a cellulose chain

with a DP of 1000. We now have 11 "polymers",

one with a DP of 990 and 10 with a DP of 1. The

number average DP is (1 x 990 + 10 x

1)/(10+1) = 90.9; in reality, however, the only

effect one would notice is a slight loss of yield

(1 %)

with identical strength properties. In fact, if

every cellulose chain had a DP of 91, the pulp

would be useless for structural papers.

A second type of average gets aroimd these

difficulties; it is the weight average, M^ where

the weight of each molecule is considered in

importance not the number. We introduce the

term W;., which is the weight of the fraction of

with molecular weight

M,.

The weight of fraction

/ can be substituted as follows: W; = A/; x M^,

</)

-J

—1

LLI

*s.

MyV

MOLECULAR WEIGHT

Fig. 18-8. Typical distribution of molecular

weights in polymers produced by free radical

polymerization.

Using

this

substitution, the following relation-

ships are obtained:

(18-2)

EPF,

TN.XM.

DP„

EN.XDP^

EN.XDP,

A weight average molecular weight is deter-

mined by light scattering methods but is limited to

relatively high molecular weights, above several

tens of thousand g/mol. One can calculate the

weight average DP in the example above where

the number average DP was 91. The DPj^= (1

X 9902 + 10 X P)/(1000) =

980.1;

MJM^ is a

measure of the polydispersity, that is, how dis-

perse the molecular weight distribution is. Since

the weight average molecular weight is always

higher than the number average molecular weight,

except for the special case when they are equal if

all of the polymers have the exact same molecular

weight (a monodisperse sample), the ratio is

greater than one. The polydispersity of this

hypothetical example is extremely high. It is

980.1/91 = 10.8

MOLECULAR WEIGHTS OF POLYMERS 401

EXAMPLE 1.

SOLUTION:

Imagine a cellulose chain which has a DP of 5000. In case "A" it is split into two

equal parts of 2500 each. Incase

"B"

it is split into one part of 1000 DP and one

part of 4000 DP. Calculate DPn, DP^, and DP^/DP„ in each of the two cases.

Case A: DP„ = (2 x 2500)/2 = 2500

pP^

=J2 X 25002)7(2 X 2500) = 2500

DP,/DP„ = 2500/2500 = 1

Case B: DP„ = (1000 + 4000)/2 = 2500

DP^

=jl X 1000^ + 1 X 40002)7(1000 + 4000) = 3400

DPJDPn = 340072500 = 1.36

EXAMPLE 2. What is

and

for a polymer mixture containing 0.5 moles of each of three

polymers where M = 20,000; 40,000; and 50,000; respectively.

SOLUTION: M„ = (0.5 x 20,000 + 0.5 x 40,000 + 0.5 x 50,000)7(0.5 + 0.5 + 0.5) = 36,700

M^ = (0.5 X 20,0002 + 0.5 x 40,000^ + 0.5 x 50,000^)7(0.5 X 20,000 +

0.5 X 40,000 + 0.5 X 50,000) = 37,500

EXAMPLE 3. Repeat Example 2, except instead of 0.5 moles each assume you have 0.5 parts by

weight of each component.

SOLUTION: Use Eq. 18-2 directly and find:

M^ = (0.5 X 20,000 + 0.5 x 40,000 + 0.5 x 50,000)7(0.5 + 0.5 + 0.5) = 36,700

In order to solve for the number average we need a relative measure of moles of

the different materials. Assume we have 0.5 kg of each polymer: 500 g720,000

g7mol =

0.025

mol; 500 g740,000 g7mol = 0.0125 mol; and 500 g750,000 g7mol

= 0.01 mol. Now:

M„ = (0.025 X 20,000 + 0.0125 x 40,000 + 0.01 X 50,000)70.0475 mol = 31,600

A molecular weight profile of soluble poly-

mers may be obtained by a chromatographic meth-

od known as size exclusion chromatography

(SEC),

which is also known as gel permeation

chromatography (GPC). The chromatography

colunm consists of a "gel" such as polystyrene

crosslinked with divinyl benzene. The gel effec-

tively has "tunnels" of various sizes on the order

of the size of the molecules to be separated.

Large molecules cannot enter these tunnels,

whereas small molecules can. Therefore the

volume of

the

chromatography colunm is effective-

ly larger for the small molecules. The large

molecules come off

the

colunm in the solvent first,

and the smallest molecules come off

last.

The fact

that large molecules of similar structure come off

the colunm first is unique to tiiis method that

separates by size. In all other forms of chroma-

tography, large compounds elute later than small

molecules of similar structure.

402

18.

POLYMER CHEMISTRY

Solution viscosity

Strength^

Degree of polymerization —>

Fig. 18-9, Properties of polymers as a function

of average DP.

The mechanical properties of polymers and

polymer solutions are greatly dependent on their

molecular weights. Fig. 18-9 shows the effect of

the DP on plasticity, strength properties, and

solution viscosity.

EXERCISES

Describe the mechanism whereby GPC sepa-

rates compounds and indicate the unique

characteristic, in regards to the order of

elution of similar compounds, of GPC.

A polymer contains 1 mole with DP 200 and

1 mole with DP 400. What are the number

average and weight average DPs. If the

polymer is polystyrene, what is the number

average molecular weight?

What are some differences between addition

polymers and condensation polymers?

Draw the general structure of vinyl polymers

and show the structures of three.

A polymer has an average molecular weight

of 6x

10^

and is monodisperse. How was it

made?

CARBOHYDRATE CHEMISTRY

19.1 INTRODUCTION

The field of carbohydrate chemistry has seen

many advances in the past thirty years. The

interest in carbohydrate chemistry is not surprising

considering the importance of carbohydrates in

most aspects of our environment. It is only in the

last few decades that the significance of carbohy-

drates, in their many forms, has truly been recog-

nized. Carbohydrates have long been recognized

for their roles as structural materials and sources

of energy in the biological world, but their role as

informational molecules has only relatively recent-

ly been understood, and we have much more to

learn. Some interesting facts regarding carbohy-

drates only hint at their remarkable properties.

Cellulose is the most abundant organic chem-

ical on the face of the earth.

Some 400 billion tons of carbohydrates are

produced annually by photosynthesis.

The typical diet consists of more than 60%

(dry weight) of carbohydrates.

The major blood group types are determined

by sequences and branching of carbohy-

drates.

Clearance of erythrocj^es from the blood

stream by the spleen is determined by the

structure of oligosaccharides on the erythro-

cyte membrane.

Generally, carbohydrates may be defined as

polyhydroxy aldehydes or polyhydroxy ketones,

occurring in their open chain forms or in their

heterocyclic ring forms (the acetal or ketal forms).

The simplest types of carbohydrates are called

neutral sugars. Sugars may be chemically modi-

fied to form derivatives. Some examples include,

reduction of the carbonyl group to give alditols,

oxidation of either terminal carbon to form sugar

acids,

acetylation or methylation of hydroxy 1

groups. The chemistry of carbohydrates is pre-

sented here to the extent that it is of importance to

the understanding of cellulose and hemicelluloses

during pulping and papermaking. For further

information, introductory biochemistry, wood

chemistry, and organic chemistry texts should be

consulted. Many of these texts contain chapters

devoted to carbohydrate chemistry.

19.2 NOMENCLATURE

The monosaccharides, simple sugars that

cannot be easily hydrolyzed into smaller

units,

are

classified according to the number of carbon atoms

in the molecule. This classification is used for

carbohydrates with three to seven carbon atoms;

that is, with trioses, tetroses, pentoses, hexoses,

and heptoses. Aldoses are monosaccharides that

have an aldehyde when in the acyclic form (in the

absence of the hemiacetal form); ketoses are

monosaccharides with a ketone when in the acyclic

form (absence of the hemiketal bond). Glucose is

an example of an aldohexose, and fructose is an

example of a ketohexose or hexulose, a six-carbon

ketose, as shown in Fig. 19-1.

If the terminal R-CHjOH (at the C-6 posi-

tion) of an aldose is oxidized to a carboxylic acid,

then the monosaccharide is known as an uronic

acid;

if the aldehyde is oxidized to a carboxylic

acid, the compound is referred to as an aldonic

acid; and if both terminal carbon atoms are oxi-

dized to carboxylic acids, the compound is re-

ferred to as an aldaric acid. Monosaccharide

constituents of particular importance in woody

plant cell wall polysaccharides are the pentoses

arabinose and xylose; the hexoses glucose,

mannose, and galactose; and the uronic acid (4-0-

methyl) glucuronic acid; these structures are

shown with the hemicelluloses in Section 2.6.

19.3 FORMS OF MONOSACCHARIDES

Monosaccharides may be represented, as in

Fig.

19-1,

by the Fischer projection, introduced by

Emil Fischer in the late 19th century. The Fischer

projection and the ball-and-stick model of D-

glucose are presented in Fig. 19-2, as well as the

Fischer projection of L-glucose. Horizontal lines

of the Fischer projection represent H and OH

403