Lecture 3

Cellulose DerivativesCellulose Derivatives

BSE 4514: Industrial Processing

Lecture 3

UV StabilityUV StabilityUV stability is relative, i.e., all polymers are highly susceptible to UV degradation:

“with the exception of fluoropolymers, most polymers are susceptible to oxidation, particularly at high temperature and UV,” J.R. Fried, Polymer Science and Technology, Prentice Hall, 1995, p. 239.

Aliphatic polyesters are more UV stable than aromatic polyesters and chains of hydrocarbons b/c:

1) Ester groups absorb less UV radiation than phenyl rings, C=C, or C-C (I was told this and I still do not understand what it means physically, but there are other ways to rationalize!)

2) UV degradation is an oxidative process, i.e., it runs in the presence of O2. Esters already contain oxygen and when C=C and C-C are oxidatively degraded, they degrade to C=O, which esters already contain so esters can be though of as oxidized olefins. Phenyl groups, like in PS or PETE, absorb lots of UV radiation and transfer it to the main polymer chain so these polymers are highly susceptible to UV degradation.

Lecture 3

UV StabilityUV Stability3) C=O is a polar group and the polarity stabilizes it relative to non-polar groups

like C-C and C=C4) The bond energy of C=O is 0.75 MJ/mol, C=C is 0.61 MJ/mol and C-C is 0.33

MJ/mol so this is a much more stable bond5) PE and PP, for instance, are polymerized using free radicals (LDPE) or

transition metal catalysts (HDPE, PP). Residual catalyst usually remains following polymerization. Although free radicals can combine, they can be re-initiated with UV light. So UV will activate residual catalyst and depolymerize (or in some cases cross-link or continue to polymerize) the polymer.

6) PHA is polymerized through enzyme catalyzed bacterial synthesis so there is no residual catalyst to be re-activated by UV radiation.

7) PHA and PLA are of very high crystallinity and crystalline regions UV degrade slower than amorphous regions.

From Pol. Deg. Stab., 91, 1128-1137, 2006.

Lecture 3

CelluloseCellulose

textile fibers (cotton, flax, etc.)paper (pulping)building (wood)polymers: renewable resource

but not benign processing (we only want to mention but not concentrate on)

composites: good app, “green” materials

Lecture 3

CellulosCellulosee

You remember this structure!

Reaction of the OH group: esters, ethers, etc.

In these rxns, DS=degree of substitution is important: DS=0-3/glucoseThe DS is dependent on the availability of OH groups so it is a function of the H-bonding in cellulose!

Lecture 3

Cellulose DerivativesCellulose Derivatives

Derivatize or functionalize hydroxyls (OH)

Similar rxns can be done on starch (glucose subunit in both) but these materials were abandoned b/c of inferior properties relative to cellulose derivatives

Three examples: 1) viscose, 2) cellulose acetate (esters), 3) carboxy methyl cellulose (ethers)

All cellulose derivatives are variations on these reactions

Lecture 3

Viscose Viscose ProcessProcessTo make Rayon fiber or cellophane film: classic products from a classic process

cellulose + NaOH + CS2C-O-C

=-

S

S-Na+

spin fiber

acid bath

C-OH based fiber

salt

Used to make a lot of, now not so much in US b/c requires strong base, strong acid, and CS2

Need base to break up cellulose H-bond structure to make C-OH available

Lecture 3

Viscose Viscose ProcessProcessWhile CS2 is naturally produced in small quantities from the enzymatic degradation

of proteins in the environment, in concentrated quantities it is lethal (and you don’t need much):

Toxicology

Poison - may be fatal if swallowed or inhaled. Serious health hazard, affecting the Central Nervous System (CNS). Readily absorbed through the skin. Sufficient material may be absorbed through the skin to be fatal. May cause reproductive damage, including imparing fertility. Chronic exposure may cause liver, kidney and CNS damage, or impaired vision. Causes burns. Severe eye and respiratory irritant. Skin irritant. Typical PEL 7 ppm.Toxicity dataIHL-HMN LCLO 4000 ppm/30m IPR-GPG LDLO 400 mg kg-1 IHL-MAM LCLO 2000 ppm/5m ORL-RAT LD50 3188 mg kg:-1 ORL-MUS LD50 2780 mg kg-1 ORL-RBT LD50 2550 mg kg-1 IHL-MUS LC50 10000 mg/m3/2h0 100

from CS2 MSDS

Lecture 3

Cellulose Cellulose AcetateAcetateEsters commercially made by Eastman.Mostly short fibers for non-wovens like cigarette butts!

Relies on acetylated OH groups

-C

- -CO-

= O

cellulose + acetic acid + acetic anhydride + H2SO4

partial

hydrolysis

dissolve in acetone and spin fiber

This is another > 100 yr old process like viscose and had not changed much.Cellulose will react under anhydrous conditions in the presence of acid catalyst w/ acetic anhydride to form cellulose tri-acetate. Cellulose acetate is the partially acetylated rxn product of high purity cellulose and acetic anhydride. It is obtained by acid catalyzed hydrolysis of the tri-acetate to a DSav~2.4 (cellulose tri-acetate would be DS~3.0)

from Nexant Outlook on Chemical Industry

ester linkages!

Lecture 3

Cellulose Cellulose AcetateAcetate

H2SO4

C-OH + (CH3CO)2OOH group on

celluloseacetic anhydride

C-OOCCH3 + CH3COOHcellulose tri-

acetate=esteracetic acid (H from

cellulose OH)

acetic anhydride is split in 1/2

hydrolysisC-OOCCH3

cellulose tri-acetate=ester

+ 0.2H2O

C-(OOCCH3)0.8

cellulose acetate=ester

(OH)0.2

+ 0.2CH3COOH

acetic acid

DS=3(0.8)=2.4/glucose can be v. high!

This material is soluble in acetone and other organic solventsProperties, like biodegradability, are f(DS): remember, the biggest impediment to this rxn is the availability of OH’s from H-bonding

this is fully acetylated cellulose

(sulfuric acid)

Lecture 3

Carboxy methyl cellulose Carboxy methyl cellulose (CMC)(CMC)mostly food and drug applications; some packaging, adhesives, paints, paper binder

cellulose + chloracetic acid pH>7

C-O-CH2-C

=-

O

OH

Organic acid (polar) solubilizes the cellulose by breaking-up H-bonding.

Acid groups functionalize hydroxyls to form ether.

DS~0.6-0.9some cellulose chain cleavage b/c of strong acid and base so Mw

This makes a H2O-soluble material!

ether bond

Lecture 3

Cellulose Derivative Cellulose Derivative PropertiesProperties tex=g/1000 m (a measure

of fiber diameter)

,E N

tex*1x109 *

in g/cm3