Biobased materials: Part I

Biomass vs biobased materials vs biodegradable polymers

• biomass

  • organic substances produced in nature based on carbon fixation (CO2) through photosynthesis

• biobased chemcicals:

  • organic functional materials made from biomass as carbon source

  • but almost all carbon found in products made by chemical industry are fossile based

• biodegradable polymer

  • polymeric material that is readily degradable under regular composting conditons

  • But their is a difference between home composting and industrial composting

  • industrial composting is much harser and more efficient than ordinary environmental degradation

  • so it may be called biodegradable because it degrades under controlled conditions but does not degrate in normal outdoor environments

renewable and sustainable

• renewable

  • resoruce can be regenerated on the human timescale; biomass is renewable because plants can regrow and fix new CO2. fossil sources are not because they take millions of years to form

• Sustainable

  • broader term

  • material or process can be maintained long-term with acceptable environmental, economic and social impact including its full life cycle.

preparation of biobased materials

They can be prepared by normal organic chemsitry or via bioprocesses:

• enzymatic synthesis = using well defined isolated enzymes

  • synthesis is assisted by enzymes that were isolated and might even be immobilized on solid support to facilitate recovery

  • but expensive: excraction, purification, immobilization

• fermentation

  • uses whole cells

  • let bacteria do the job for us, but they only produce the product in low amounts and also produce a lot of other stuff

  • this leads to a complicated mixture and leads to a difficult purification

the name matters

• biobased does not mean biodegradable

  • biobased plastic can still be persistent in the environment

• biobased does not mean sustainable

  • bioethanol is made from sugar which compets with food supply which is not always acceptable

• biodegradabledoes not mean sustainable

• biodegradable is not necessarily advantageous

  • biodegradable cars wouldn’t be a good thing

• renewable does not mean sustainble

  • production of bioethanol is can only be considered sustainable in certain parts of latin america, since they can harvest sugarcane from the same piece of land 2 times a year.

  • so reneawable sources can still be used in an unsustainable way, if biomass causes deforestation, high water use, high energy consuption, competition with food industry,…

biobased does not mean sustainable

• only 5% of fosile oil is used as carbon source for chemicals

  • although additionla E used in the process should be accounted for as well

  • the rest is burned for energy

  • So much more efficient to generate biomass for energy production than replacing fossile fuel based chemicals

• biomass needs to be grown

  • limited surface on land to grow biomass and not all land is suitable for agriculture

    • biomass can also be grwon in the sea like algea (this are in some cases rich in vegetable oils which can be usefal as biofule for planes)

    • greening of aviatory travel is difficult, electric flying is limited by the weight of batteries, so biofules might be an easier green alternative

• Need fertilizers and pesticides

  • comes with large environmental impact and lowers sutainability

• Waste streams should be efficiently used

  • although for some high value crops this is justified

  • eg. limited amount of crops can be used to exctract sucrose, so production of waste streams for its extraction can be justified for the production of chemicals

Factors determining Biodegradability (5)

• FG’s in backbone

  • esters and amides are readily cleaved by enzymes

  • if only C-C bonds in backbone there’s limited biodegradability

  • polythioesters although less stable than ester, are less common in biology and are less readily cleaved by enzymes

• copolymer composition

  • if copolymer = PET, these regions have dense packing due to stifness of the chains and pi-pi stacking

  • this limits access for enzymes, leading to almost no biodegradability

  • whil for aliphatic polyesters, are much less densely packed and are more readily cleaved

• chain fleixibility Tg

  • Tg is a measure of chain flexiblity and mobility

  • introducting side chains lowers flexiblity, increasing Tg and lowers the biodegradability

  • 
    

  

• crystalinity

  • nylon 6 and nylon6,6 can form stable crystal structure due to amide H-bond formation

  • but still faster degradation than PET

• molecular weight

  • the higher the molecular weight the more difficult biodegradation becomes because less bioavailable

  • above 20,000 g/mol becomes more difficult

Use of biomass

= closed circle for materials but not for energy

• it contains a lot of steps so it can be advantageous to shorten it (green arrows)

  • directly turn plant constituents into polymer materials

  • or even use biomass directly as polymer material

    • Wood for instance is a polymer material

need to use more wood in construction:

construction requires a lot of E and involves a lot of CO2

• concret from cement industry - a lot of CO2

• Steel production also has high CO2 emission

—> solution

• can use biofules for these processes, but there still is a limited capacity of it

• can use more wood in construction, this can lead to an overall negative CO2 output (as wood is biomass made from CO2)

composition of biomass and biopolymers

biomass

1. cellulose

   1. regular and well defined

2. other carbohydrates

   1. starch

3. lignin

   1. polymer of aromatic building blocks, more diverse and branched so more difficult to process

4. others

   1. fats, proteins

biopolymers

1. cellulose

2. hemicellulose

   1. short-chain, branched, substituted polymer of sugars

   2. more irregular nephew of cellulose

3. lignin

traditional use of some major biopolymers

Cellulose

• extracted from wood

  • having hemicellulose and lignin as side products

  • drawback

    • very low solubility in common solvents

    • due to strong h-bonds difficult to break down or chemically modify

    • only creatures able to digest it are cows or related animals through their microbiome (bacteria living in their gastrointestinal tract)

amylose

• extracted form patatoes, corn, peas

• only difference from cellulose is configuration of the anomeric carbon

• it is water soluble, easily degradable and digested

‘Direct’ conversion of cellulose

(direct refers to the shortened biomass cycle)

• 90% of cellulose is used to make paper

  • paper is a biobased biodegradable polymer

  • can enhance properties by making a composite material eg. with PE increases strength and water resistance but not biodegradable anymore

• Viscose process

  

  • chemical reprecipitation of cellulose

  • cellulose is treated with NaOH and CS2, the free OH’s attack on CS2 forming anionic charges

  • if this occured to a certain extent, the charges repel, H-bonds are broken and it becomes soluble

  • cellulose solution is pumpted through spinneret (a lot of tiny holes)

  • in acidic medium it will be insoluble and reprecipitate as fine viscose fibers that are collected washed and assembled into clothes

  • disadvantages: high HSE cost

    • CS2 highly toxic and flammable

    • need equiv amount of NaOH, which is harmful and requires a lot of energy to produce

    • need equiv amount of H2SO4

    • this all leads to a lot of aqueous waste which is worse than organic solvents becaue organic solvetns can be burned to recover energy

• treatment with HNO3 forms nitrate esters formign plastics

• treatmetn whith acetic anhydride forms acetyl groups, making it soluble and from which biobased plastics can be made

Alternative solvents for cellulose

NMO = N-methylmorpholin-N-oxide

• need a strong H-bond acceptor to break the cellulose intramolecular H-bonds

  • NMO does this with the negatively charged O

  • so NMO can dissolve cellulose without the need of additional chemical modification

• spinning from NMO/cellulose solution into water as a non solvent produces tencel or lyocell

  • similar yarns to viscose but doesn’t have the disadvantages of the viscose process

• NMO can be recovered quite efficiently

  • but still rather hazardous due to irritating character

ionic liquids as alternative solvents for cellulose

= liquids consisting entirely of ions

• most compounds known consisting ions are high metling crystal solids (NaCl)

  • due to strong attraction between the ions and thus stable crystal packing

• Thus need lower attraction between ions, disrupting crystal lattice and leading to lower melting points:

  • make ions larger

  • make charge delocalised

  • include irregular shapes (further disrupts crystal packing)

• Some combinations of cations and ions are able to dissolve cellulose such as EMIM (1-ethyl-3-methylimidazolium) or EEIM with acetate

  • How? acetate with negatively charged oxygen is a better H-bond acceptor than the alcohols in cellulose —> disrupted

Pro’s and cons of ionic liquids

advantages:

• designer solvents

  • practically infinite combinations of cations and anions

  • can tailor solvent properties for a specific application by combining the right cations and anions

  • thus you can use solvent to optimize the process = good for sustainability

• low volatility (due to strong attraction between cations and anions)

  • no inhalation (major cause of toxicity of organic solvents)

  • not flammable (Et2O eg. is so flammable due to its high volatility)

  • minimizes losses due to evaporation

disadvantages

• still rather toxic

• difficult to recover and purify

  • organic solvents can be easily recovered and purified by desitlation (due to low bp) and requires less E

  • purificaton of ionic liquids requires a lot of E and is complicated

    • due to high T needed

    • and E need for high vacuum

    • —> high cost and not sustainable

purification of ionic liquids

= short path distillation:

1. a screw is heated in a strong vacuum chamber surrounded by a cooling mantel

2. the contaminated liquid is injected onto the screw

3. the ionic liquid vapourizes and quickly condenses onto the wall due to cooling liquid

   1. very short path between hot evaporation surfacse and cold condensor due to low volatility so vapoour does not have to travel far

   2. also reduces time of compounds spent at high T limiting decomposition

4. the ionic liquid are collected

5. the less volatile or decomposed impurities stay behind on the screw as impurities

are ionic liquids really ‘green’ solvents?

Contra:

• stil rather toxic

  • LD50 = dose (mg/kg bodyweight) at whcih 50% of the animals on which it is tested on die

  • lower LD50 is more leathal

  • some combinations are very leathal

  • although EMIM OAc which can be used as solvent for cellulose has a higher LD50

Pro

• sustainability involves optimization of current chemistry

  • this is possible with ionic liquids using different ion combinations

• cellulose is one of the most abundant biomasses available but insoluble

  • ionic liquids can solubilize it

• can replace viscose process

  • no toxic CS2, base, acid or aqueous waste

  • only need ionic liquid

  • and water is added to dilute the ionic liquid, so the polymer will precipitate again

  • However to recover ionic liquid, you have to distill excess water which is hard and energy demanding

  • but this viscose replacement has been scaled up and under research for industrial apllicability

Direct conversion: Use of Amylose

= one of the main components of starch

• amylose

  • linear polymer of glucose

• amylopectine

  • brached amylose variant

  • not interesting for material chemistry which prefers linear polymers

  • these are more likely to entangled and these entanglements are the main cause for plastic polymer properties

the mixture is treated with a plasticizer (glycerol)

• glycerol inserts between starch chains and disrupts the H-bonds this makes it more processable and turns it into a thermoplastic starch (TPS) which is water soluble

• glycerol has high bp so will not evaporate upon mixing with amylopectine

TPS can be blended with biodegradable polyesters to improve properites

• applications: biodegradable packaging (cheeto like packaging foam)

Engineered biomass

= biomass produced by living organisms like bacteria that have been genetically engineered to increase the yield of the polymer produced

example: PHB

• high Tm, biocompatible —> good for flexible materials

• could be made better by making it more flexible and decreasing its Tm —> copolymerization

  • feed the bacteria a second monomer that will be copolymerized

• This creates PHBV which is a high value polymer with high cost for high value application:

  • than be used for surgical threads (that degraded over time)

  • not for packaging (too expensive)

Synthesis of poly(lactic acid) (PLA)

= very common biopolymer, and a polymer of polylactic acid obtained from bacterial fermentation

• glucose is obtained from enzymatic hydrolysis of starch which is fed to bacetria (Lactobacillus) and produce enantiomerically pure LA (efficiently 90% yield)

• this is important as the polymer composed of pure (S)-LA has much better material properties than form a entionmeric mixture

But

• bacetria produce LA in complicated mixture of other fermentation products, which is a difficult and energy intensive separation

Separation:

• treatment with Ca(OH)2 producing Ca-lactate, which is liberated by treatment with sulfuric acid

  • this produces CaSO4, not a useful side product (gypsum)

• treatment with NaOH is better productin Na-lactate salt

  • ca(OH)2 is produced from CaCO3 + heat —> CaO + CO2 / CaO + H2O —> Ca(OH)2

  • = energy intensive an liberates CO2

  • NaOH can be obtianed from electrolysis of aqueous NaCl, producing NaOH and Cl2 = both useful products and requires less E

• LA is then liberated by electrodialysis +HCl

Polymerization of PLA

Step growth polymerization:

• 1 step

• requires high T and long reaction time

  • so a lot of energy

• because of high T, need high boiling solvent

  • use diphenyl ether (not green solvent)

  • need high T for azeotropic removal of water to shift the equilibirum towards high Mw polymers

  • without water removal, small polymers with bad material properties

Chain growth polymerization:

• mulitple steps

  • but this approach is still aplied in industry due to higher Mw and less E demand

• LA is polymerized by increase of T to short prepolymers

• these are cyclised to lactide dimers under Sn catalysis

  • uses Sn catalyst which is rather toxic and bad for sustainability

  • it is not removed, so it stays in the polymer material and ends up with the consumer and it is also not recovered

  • Sn is a lewis acid which can cause epimerization

  • 3 isomers are produced, only need L-lactide so need additional purification step - via recrystallization (much more E- efficient than destilation)

• L-lactide is then polyerized with a Sn catalyst to PLA

  • still high T needed but for shorter time, so much less E needed

  • higher molecular weight

energy balance PLA vs PE

PLA

• Also requires E from fossile fuels

  • monomer synthesis, purification, plymerization

  • transport

  • insecticedes and fertilizers needed to grow the biomass where we get the starch from

• assuming the polymer is fully biodegradable it converts to CO2 and H2O (if it is fully composted)

  • it cannot be burned to recover internal energy so this is lost

• in total 57 MJ equivalents of E

PE

• polymer is made from crude oil

  • 40 MJ for production

• also uses fossile fuels for the production

  • 40 MJ

• polymer can be incinerated to recover internal energy which has ben taken into account

• 80 MJ in total

  • more than PLA when ONLY foccusing on E-demand and assuming internal E of PLA is not recovered

BUT MORE DETAILS AER NECESSARY

More detailed analysis

• Cradle to gate (mostly done)

  • consider only the production of the material until the moment it leaves the production plant

• Cradle to grave

  • also consider use and disposal phase

  • take into behavior of consumer with respect to disposal after use

  • becomes more difficult

• Cradle to Cradle

  • also consider reuse (plastic bags) of the material and take recycling into account

  • now really complicated

The results of such an analysis will depend on assumption you make that will strongly influence the outcome

—> have to be clear about the assumption you make

is PLA better compared to other plastics

Cannot answer question, more research necessary

main message

• how you decide to look at a problem strongly influences the outcome - people will try to convince you their approach is better, and it is our job to decide to that’s true

concusionf of graphs

These graphs are the result of cradle to gate analysis

• PLA second to worst compared the 5 most common petroleum based plastics in terms of CO2 emission

  • due to high energy demand

  • 
    

    

• PET carcinogenic

  • polyethylene part is made from ethylene glycol which is made from oxiranes which are mutagenic

  • 
    

    

• TPS (thermoplastic starch) is bad for human health (not considering carcinogens)

  • considers environmental impact of the plants grown needed to provide the starch

  •