DEGREE PROJECT IN CHEMICAL ENGINEERING, FIRST CYCLE, 15 CREDITS
STOCKHOLM, SWEDEN 2021
Reaction mechanism between
chitosan and cerium(IV)
ammonium nitrate for
production of a greener
poly(vinyl acetate) adhesive.
MÅRTEN SCHOLLIN
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF ENGINEERING SCIENCES IN CHEMISTRY,
BIOTECHNOLOGY AND HEALTH
www.kth.se
.
Author
Marten Schollin, [email protected] of Fibre and Polymer Technology
KTH Royal Institute of Technology
Place for Project
Stockholm, SwedenKTH Royal Institute of Technology
Examiner
Prof. Eva Malmstrom Jonsson, Division of Coating TechnologyKTH Royal Institute of Technology
Supervisors
Tijana Todorovic, Linda Fogelstrom and Eva MalmstromDivision of Coating Technology
KTH Royal Institute of Technology
Abstract
Poly(vinyl acetate) (PVAc) has a major application as an indoor woodadhesive. Low water stability is however, one of the greatest drawbacksof PVAc. By grafting PVAc from a chitosan (CS) backbone (CS-graft-PVAc) water stability of adhesive is increased while good mechanicaland adhesive properties are retained. Simultaneously the percentage ofbio-based content is increased. This work investigates the proposed re-action mechanisms between chitosan and cerium(IV) ammonium nitrate(CAN) which is used as an initiator for the grafting reaction. Litera-ture studies showed one dominating reaction mechanism and some notas common. The reaction mechanisms and their shortcomings are pre-sented and discussed in the report.
KEYWORDS: Chitosan, Poly(vinyl acetate), Cerium(IV) ammoniumnitrate), Grafting, Reaction mechanism.
Sammanfattning
Poly(vinyl acetat)(PVAc) har ett stort anvandningsomrade som ett tralimfor mobler som ska anvandas inomhus. Den daliga vatten stabilitetenar ett av de storsta problemen for anvandning av PVAc. Genom attympa PVAc med chitosan(CS) (CS-graft-PVAc) kan vatten stabilitetenokas samtidigt som en god limfunktion finns kvar och delen fossilbaseradmonomer blir mindre och byts ut mot en biobaserad polymer. I dettaarbete undersoks de foreslagna reaktionsmekanismerna mellan CS ochcerium(IV) ammonium nitrat(CAN) som anvands som en katalysatorfor att grafta PVAc med CS. Litteraturstudier visade en domineradereaktionsmekanism och nagra mindre forekommande. Reaktionsmekanis-merna och eventuella tillkortakommanden som finns gallande hur defortloper presenteras och diskuteras i detta arbete.
NYCKELORD: Chitosan, Poly(vinyl acetate), Cerium(IV) ammonium-nitrate,) Ympning, Reaktionsmekanism.
Acknowledgements
This thesis was conducted at the coating division at KTH Royal Insti-tute of Technology. Firstly I would like to thank my supervisors EvaMalmstrom and Linda Fogelstrom for guiding me through this project.Secondly a special thanks to Tijana Todorovic for the support, help andall the laughs throughout this project. Finally I would like to thank myfamily and friends for the motivation and helpful words.
Mårten Schollin
Contents
1 Introduction 11.1 Aim of thesis . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Background 32.1 Wood adhesives . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Theory of adhesion . . . . . . . . . . . . . . . . . 32.2 Poly(vinyl acetate) . . . . . . . . . . . . . . . . . . . . . 52.3 Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.4 Cerium(IV) ammonium nitrate . . . . . . . . . . . . . . 72.5 Grafting polymerisation . . . . . . . . . . . . . . . . . . 8
2.5.1 Emulsion polymerisation . . . . . . . . . . . . . 82.5.2 Grafting principles . . . . . . . . . . . . . . . . . 92.5.3 CS-graft-PVAc reaction mechanism . . . . . . . . 10
3 Experimental 123.1 Materials and method . . . . . . . . . . . . . . . . . . . 12
3.1.1 Characterisation . . . . . . . . . . . . . . . . . . 133.1.2 Instrumental . . . . . . . . . . . . . . . . . . . . 14
4 Experimental results 15
5 Discussion 17
6 Conclusion 19
7 References 20
Chapter 1
Introduction
Wood adhesives are frequently used in carpentry and other wood work-ing, both by professionals and private users. Poly(vinyl acetate) adhe-sive or white adhesive are of interest for this thesis. Wood adhesivesmade with other components such as epoxy, cyanoacrylate and polyurethane are some of them. As society looks for more environmentallyfriendly products, there is a need for wood adhesives made, completelyor partially, from renewable resources. One approach is to include abiopolymer into the adhesive mixture, e.g. by grafting poly(vinyl ac-etate) (PVAc) from chitosan (CS). Chitosan is a water stable biopoly-mer made by deacetylating chitin, which is found in the shells of crus-taceans. The grafting reaction of monomers on glucose chains withmetallic ion complexes, such as cerium(IV)ammonium nitrate (CAN),has been around for the majority of the twentieth century in wood re-search [1, 2, 3]. It has been used to alter the properties of cellulose suchas water and thermal stability. The same technique has been utilisedwhen grafting PVAc from CS.
1.1 Aim of thesis
By reviewing existing literature and their claims of how the initiation re-action take place, it is apparent that different ideas of what is occurringon the molecular level are present. This work investigates the proposedreaction mechanisms of CS grafted with PVAc in the presence of CAN,derived from the graft copolymerisation reaction of cellulose-monomerwith CAN as an oxidising agent. The use of CS as a component in woodadhesive is the topic of the two studies that this thesis is based on [4,
1
5]. However, this thesis does not have application of this adhesive as itsprimary subject of interest, therefore this part will be presented briefly.Experiments were conducted to try to shed some light on the mechanismby analysing pure and grafted CS. The synthesis method for graftingPVAc from CS used for this work is based on the master thesis of Xiut-ing Zhang [4], and on the doctoral thesis of Emelie Norstrom[5]. Thiswork is the final report for a Degree of Bachelor of Science in Chemi-cal Engineering. The project was conducted at KTH Royal Institute ofTechnology in Stockholm.
2
Chapter 2
Background
2.1 Wood adhesives
Wood adhesives are homopolymers or combinations of different poly-mers. The wood adhesive is used to bind two or more substrates to-gether. The use of these polymers has become an integral part of oursociety as wood adhesives is found everywhere from to go coffee cupsto carpentry. Wood adhesives can be divided up into major and minorgroups with respect to consumption. Synthetic, fossil-based, polymersare the dominating building blocks, whereas semi-synthetic and natu-ral sourced polymers are not as frequently used. As society emphasisesustainability more and more, paving a way for these less explored poly-mer systems, semi-synthetic and natural sourced polymers are becomingmore important [6].
2.1.1 Theory of adhesion
There are four major mechanisms concerning adhesion [7], mechanicalinterlocking theory and adsorption theory illuistrated in figure 2.1, elec-trostatic attraction theory and diffusion theory illustrated in figure 2.2.The first works, as the name suggests, by the adhesive migrating intothe irregularities of two surfaces and locking them together when it so-lidifies. This makes the properties and structure of the surfaces verycritical as to how easy it is for the adhesive to migrate into the surfacecavities together with the cavities depth and shape. The second theoryfocuses on the intermolecular interaction of the substrate surface andthe adhesive, their capability to form bonds (covalent and ionic) whichis strong enough to keep them locked together [7].
3
Substrate
Adhesive
Substrate
Adhesive
Substrate
Substrate
Figure 2.1: Mechanical interlocking theory to the left. Absorption the-ory to the right, the green lines illustrates covalent/ionic bonds betweensubstrate and adhesive.
The third theory, is the adhesion by electrostatic interaction by whichthe surface-adhesion interface needs to be able to have a free exchangeof electrons. The surface and the adhesive form an acid-base interface,an electrostatic double layer, which hold them together. Lastly thediffusion theory, where chains from polymeric materials ”wander” intothe adjacent material binding them together [7].
+ + + + + + + + + + + +
------------
+ + + + + + + + + + + +
------------
Figure 2.2: Electrostatic adhesion theory to the left. Diffusion theoryto the right. The blue and green lines illustrates polymers migratinginto the adjacent material.
4
2.2 Poly(vinyl acetate)
Poly(vinyl acetate)(PVAc) adhesives or carpenters glue as it is alsocalled, is frequently used when producing furniture for indoor use. PVAcis classified as a thermoplastic making it possible to alter the polymermatrix after drying. As the polymers are not ”locked” completely into place PVAc will be susceptible to creep. It is cheap to produce, nontoxic, has fairly short drying time, good adhesion properties and whendried it turns almost completely transparent [8].
Figure 2.3: Poly(vinyl acetate)
One of the biggest drawbacks of PVAc is its low water stability, failingin high humidity environments making, indoor use its major field of use.
As a standalone adhesive, it is made through emulsion polymerisation[9]. How emulsion polymerisation works is described in 2.5.1. To addressthe low water stability of PVAc, grafting techniques has been utilised[10, 11, 5]. this is of interest for outdoor use were fluctuating humidityand rain poses a problem compared to indoor use.
2.3 Chitosan
Chitosan (CS) is a polysaccharide-based biopolymer made from chitinwhich is found in the shell of crustaceans for example crabs, shrimps, andlobsters. The function of chitin in the exoskeleton is among other thingsto make it stiff and stable [12] Chitin has good mechanical properties butis not soluble which makes chitin a difficult polymer to work with. CS onthe other hand is soluble and retains most of the mechanical propertiesof chitin which makes it easier to utilise for other purposes. To produceCS from chitin both biological and chemical treatments can be applied[13].Chitin is deacetylated by introducing an alkaline substance such assodium hydroxide. This process is conducted in rather mild conditionswhich makes the production of CS somewhat environmentally friendly.
5
The deacetylation occurs according to figure 2.4 and produces the B-(1-4) N-acetyl-D-glucosamine linked polymer.
Figure 2.4: Deacetylation of chitin to chitosan
After deacetylation CS can be dissolved under mild acidic conditionswith pH lower than 6.3 [14]. The protonation of CS will in turn increasewith the increasing pH of the solution that is, more acidic environmentswill produce a higher degree of protonation on the amino groups [15][16].With higher degree of deacetylation solubility increases [14]. CS can bepurchased with deacetylation degrees ranging from 60-100%. Chitosan,as aforementioned can be used as a backbone in grafting polymerisationreactions[17, 4, 5]. Together with stability in water, environmentallyfriendly nature and good adhesive properties make it a very interest-ing polymer for deeper investigation. Chitosan (CS) can be used as agrafting site for PVAc for the production of wood adhesives. By includ-ing CS in the adhesive mixture, a higher stability towards moisture hasbeen achieved while simultaneously making the adhesive more environ-mentally friendly by reducing the amount of fossil-based monomer inthe mixture. Performance of CS-based wood adhesive have been shownto retain the adhesive properties as that of an adhesive containing onlyPVAc [4][5].
6
2.4 Cerium(IV) ammonium nitrate
Cerium(IV) ammonium nitrate (CAN) is a high valent metal salt ca-pable of one electron oxidation. The structure of CAN is illustrated infigure 2.5. CAN has a high solubility in a variety of organic solventsand low toxicity. This in combination with the cerium being about ascommon as copper makes CAN easy to handle as well as highly usablein numerous chemical reactions [18, 19, 20].
NH4+
Ce4+ O−
N+
O−
OO−
N+O−
O
O−N+
O−
O O−
N+
O−
O O−
N+
O−
O
O− N+
O−
O
NH4+
Figure 2.5: Structure of CAN.
CAN is very useful for the reaction of carbon-carbon bonds. This andthe other aforementioned properties of CAN are some of the reasonsfor using can in grafting reactions of aliphatic carbon chains. CANinduces a radical on the carbon chain producing a highly reactive site.This radical is highly useful for chemical alteration. CAN has beenwidely used to graft a variety of monomers from cellulose [21, 1, 3, 2,22], enabling the production of cellulose based polymers with tailoredproperties. As chitosan has backbone fairly similar to that of celluloseCAN can be used to graft monomers such as vinyl acetate from chitosan[23, 21].
7
2.5 Grafting polymerisation
When producing the CS containing adhesive, PVAc is grafted to theCS backbone through emulsion polymerisation with cerium (IV) am-monium nitrate (CAN) as an initiator.
2.5.1 Emulsion polymerisation
An emulsion polymerisation reaction mixture contains four major com-ponents, monomer, emulsifier, dispersion medium containing mostly wa-ter and initiator. The monomer is emulsified in the dispersion mediumforming a two phase mixture. To stabilise the emulsion, emulsifier isadded forming micelles with a hydrophilic surface and hydrophobicbulk. Emulsion polymerisation occurs through free radical reactionchain polymerisation. This reaction goes through three steps, initia-tion, propagation and termination, scheme proposed by Ghosh[24].
InitiationThe initiator I, decomposes most commonly by heat and produces rad-icals, equation 2.1. The newly formed radicals react with the monomerproducing a radical on the monomer where the chain growth reactioncan proceed, equation 2.2. The reaction is governed by the decomposi-tion speed of the initiator.
Ikd−−→ 2 R• (2.1)
R• + Mki−−→ RM• (2.2)
PropagationNew monomers react with the monomer-radicals producing a polymerchains, equation 2.3 and 2.4.
RM• + Mkp−−→ RMM•
RMM• + Mkp−−→ RMMM•
RMMM• + Mkp−−→ RMMMM•
}(2.3)
RMn−1• + M
kp−−→ RMn• (2.4)
TerminationTwo mechanism are possible in the termination step, recombination anddisproportionation, top and bottom path in equation 2.5 respectively.The dominant path is recombination, where two propagating chains
8
react and together form a new polymer chain, which ends the chaingrowth.
RMn• + RMm
• RM(n+m)R
RMn + RMn
ktd
ktc
(2.5)
Very hydrophobic monomers as styrene undergo nucleation inside themicelles by so called micellar nucleation, where the hydrophobic monomerdiffuses from the emulsion droplets into the micelles. Radicals formedin the continuous phase diffuses into the monomer filled micelles andstart the reaction. For PVAc which is used used in this project, watersolubility is higher (<3%), compared to styrene (<0.04%). The mainnucleation occurs in the continuous phase as the concentration of sol-ubilized monomer is sufficiently high to start the nucleating processwithout diffusing into micelles. [25, 26, 27].
2.5.2 Grafting principles
Grafting polymerisation is frequently used to produce copolymers, byeither ”graft to” or ”graft from” reaction. In the ”graft to” reactionan already formed polymer is attached by reacting with active siteson another polymer backbone [28]. ”Graft to” and ”graft from” areillustrated in figure 2.6 In ”graft from” reaction, a monomer propagateson the polymer backbone and then polymerises into a polymer. Graftcopolymers can be produced in many different conformations dependnot only above-mentioned reaction routes, but multiple polymers canalso be used, and the shape of the graft-polymer can differ giving amultitude of properties.
Figure 2.6: ”Graft to” and ”graft from” polymerisation
9
2.5.3 CS-graft-PVAc reaction mechanism
The exact mechanism for CS-graft-PVAc has not found consensus inthe scientific community; there are one major, and some minor waysproposed. The most frequently used reaction mechanism to produceCS-graft-PVAc is based on the same reaction with cellulose. The groundwork for grafting various monomers from cellulose is based on the revisedmechanism proposed by Gaylord in 1976 [1]; other studies have agreedwith the mechanism proposed by Gaylord [2, 23, 29, 30, 21, 31]. Thismechanism focuses on a ring-opening between the C2 and C3 carbon.This has been shown to be one of the possible ways of CAN forminga complex with a cellulose substrate to induce a grafting site on thepolymer backbone. The C2-C3 ring-opening mechanism is transferred tothe chitosan system producing a mechanism scheme illustrated in figure2.7. The complex is formed between the Ce4+ ion, amino group and C3
Figure 2.7: Most frequently used mechanism between chitosan andCAN.
carbon producing a radical either on the C2 or C3 carbon through theaforementioned ring opening that occurs.
10
A less common reaction proposed by E. Yilmaz et al.[32], is illustratedin figure 2.8.
O
OH
OH
O
NH2
O
OO
OH
OH
N H
O
CH3
C
OHO
OH
OH
N H
O
CH3
O
O
C
OH
OH
O
NH2
. .+Ce4+
Figure 2.8: Chain scission mechanism proposed by E. Yilmaz et al. [32]
This mechanism is not as detailed concerning the interaction betweenthe cerium ion and chitosan compared to the C2-C3 ring-opening mech-anism. Instead of the ring opening reaction, this mechanism proposesa chain scission between C1 carbon and oxygen. Another possible reac-tion site is the opening of the glucose ring between C1 and C2 carbon onthe reducing end of the cellulose chain. The reaction rate of D-glucosehas been measured by Doba [3], showing high rates for the C2-C3 ring-opening but even higher rates for the ring-opening between the C1 andC2 carbon on the reducing end illustrated in figure 2.9.
Figure 2.9: C1-C2 ring opening at reducing chain end. [3]
The reaction path has not been proposed for the CS-CAN system butcould be a possibility at the reducing ends.
Another possible reaction site is the OH-group on the C6 carbon for acellulose based system proposed by Mzinyane et al. [33].This reactionsite is less reactive compared to the C2-C3 ring opening sites (figure2.7) but could be a possible route for when the more reactive sites arealready grafted or blocked sterically.
11
Chapter 3
Experimental
There is no consensus on the mechanism for cerium(IV) ammoniumnitrate (CAN) initiated grafting polymerisation of poly(vinyl acetate)(PVAc) on chitosan (CS). The mostly used mechanisms explains thereaction by CAN forming a complex with the CS and inducing radicalseither on the C2 or C3 carbon on the pyranose ring. This seems unlikelysince the proposed mechanism does not takes the protonation of CSthat occurs when solubilised in acidic conditions in into account [34].CS was analysed in its pure and oxidative state in an attempt to seechanges in the polymer backbone. A full grafting reaction with PVAcwas conducted to familiarise with the process and to try to shed somelight on the mechanism.
3.1 Materials and method
The recipe developed by Zang [4] and Norstrom [5] was used.Materials used:
• Low molecular weight chitosan (CS) (50,000-190,000 Da), productnumber: 448869, viscosity = 92 mPas, degree of deacetylation =75-85 %, Sigma Aldrich
• Cerium(IV) ammonium nitrate (CAN), (≥ 99.99%) Acros organics
• Vinyl acetate (VAc)(≥ 99 %, 3-20 ppm hydroquinone), SigmaAldrich
• Acetic acid acid (AA) (≥ 90%, ReagentPlus R©), Sigma Aldrich
The targeted dry content of the polymer was 20%. The recipe includesmixing chitosan (0.6 g) in 10 wt.% of AA solution (15 mL). This mixture
12
is left over night on a shaking table to make sure that the CS is dis-solved. The set-up used for the reaction included a three-necked roundbottom flask halfway submerged in an oil bath with magnetic stirrer. Acondenser was connected to one of the necks and the other where twoblocked by septums. The CS-AA mixture (7.8 g) was thereafter trans-ferred to the round bottom flask. VAc (1.7 g, 1.82 mL) was added to themixture and stirred for 30 minutes at room-temperature with moderateto moderate-high rpm:s. This was followed by adding CAN (40 mg)dissolved in deionized water (1 mL). The temperature was increased to60◦Cand the flask was left to stir for 2 hours. To ensure polymerisa-tion of all VAc a burnout step where included as the last step; CAN (7mg) was dissolved in deionized water (0.1 mL), added to the reactionmixture and left to react for 30 minutes at 75◦C. This reaction was alsoperformed in an oxygen free environment, by using argon and with aneedle in condenser opening as an outlet. The reaction mixture waspurged with argon gas for several hours before adding CAN, and duringthe polymerisation reaction after adding CAN.
3.1.1 Characterisation
The produced polymer was weighted and left to dry overnight at roomtemperature. The following day, polymer was additionally dried for 1hour at 105◦Cto ensure complete solvent evaporation.Dry content andconversion were calculated by equation 3.1 and 3.2 respectively.
Weight of wet polymer−Weight of dry polymer
Weight of wet polymer(3.1)
Weight of dry polymer−Weight of chitosan
Weight of monomer(3.2)
To calculate the grafting yield, the same procedure for drying was per-formed, and film was thereafter immersed in acetone. Because PVAchomopolymer is soluble in acetone [35], the film was immersed in ace-tone and left on a shaking table for one hour. Acetone solution wascentrifuged to separate homo- and graft-polymer. This procedure wasrepeated 4 times.Grafting efficiency (GE) (%) and grafting ratio (GR) (%) where calcu-lated according to equation 3.3 and 3.4
GE =Wgraft−PV Ac −WCS
Wpol −WCS∗ 100(%) (3.3)
GR =Wgraft−PV Ac −WCS
WCS∗ 100(%) (3.4)
13
were Wpol is the weight of the dried polymer before washing with ace-tone (mg), WCS the weight of CS (mg) and Wgraft−PV Ac the weightof the dried polymer after washing with acetone and removing PVAchomopolymer (mg).
3.1.2 Instrumental
Nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy (1H-NMR) of CS and CS con-taining CAN was performed in 10 wt% AA solution with D2O on aBruker AM NMR. The spectra were recorded at 400 MHz. A spectralwindow of 20 ppm and a relaxation delay of 1 second and 632 scanswere used. Spectra was analysed by using software MastReNova.
Fourier-transform infrared spectroscopy
Fourier-transform infrared spectroscopy (FTIR) was preformed using aPerkin-Elmer Spectrum 100 equipped with a single reflection (attenu-ated total reflection (ATR)) accessory unit (Golden Gate) from GrasebySpecac LTD (Kent, England) and a triglycine sulphate (TGS) detector.Spectra were based on 16 scans averaged at 4.0 cm−1 resolution rangeof 600–4000 cm−1. Spectra were analysed with software from Perkin-Elmer.
14
Chapter 4
Experimental results
The targeted for dry content of polymer of 20% was archived, which isthe same as the work of Zhang [4]. Grafting efficiency and grafting ra-tio were calculated as 30% and 83% respectively. It was concluded thatoxygen is needed for the reaction to take place. This was confirmed byrunning the reaction in an oxygen free environment, which did not giveany polymer. Analysis of pure and oxidised chitosan (CS) in the sameand double the ratio as the grafting recipe by 1H-NMR (figure 4.1) andFTIR (figure 4.2) gave unconclusive results. The changes in the NMRare nominal and the disturbance of water makes the output difficult toanalyse. Figure 4.1 shows the output of the H1NMR analysis of chi-tosan oxidised with cerium (IV) ammonium nitrate (CAN). Some newpeaks appear in the oxidised sample but the intensity of these are soweek. Assigning these peaks as alterations in the CS backbone as proofthat advocates for certain reaction paths is not possible.
FTIR was used to try to see some changes in the oxidised CS. FTIRspectra which is illustrated in figure 4.2.The FTIR results were unable to produce clear enough data of thestructural changes in the CS backbone attributed to specific reactionpaths. The results from FTIR, figure 4.2, do however indicate thatoxidation has occurred. There is a shift of amide I from 1648 to 1630and a shift of amide II from 1552 to 1525 cm−1. Similar results havebeen obtained by Yilmaz et al. [32]. The chain scission that theyobserved by the loss of the the C–O–C linkage peaks at 1067 and 1030cm−1, and the addition of two new pekas at 995 and 1044 cm−1 form,which the interpret as oxidative degradation at the C-1 and C-4 sites.
15
Figure 4.1: 1H-NMR spectra of CS with CAN (black line) and withoutCAN (red line)
Figure 4.2: FTIR spectrum of chitosan with CAN (red line) and withoutCAN (black line)
16
Chapter 5
Discussion
The reaction mechanism found in the litterateur favours ring openingbetween the C2 and C3 carbon as the site of grafting. This might bethe case for the cellulose system and for some novel molecules. Theproblem that no one addresses is the protonation of the amino groupfrom NH2 to NH+
3 on chitosan. This is induced by the acidic environ-ment needed to dissolve chitosan in water. How the cerium ion interactswith a protonated amino group is in need of investigation. The alterna-tive mechanism (figure 2.8) does not take the protonation into accounteither, but proposes a chain scission instead. The C1-C2 carbon ringopening reaction at the reducing ends could be another possible path.However, if this occurs in the middle of the polymer, was not found inthe literature survey. Grafting from the OH-group of the C6 carbon,even though less reactive, could serve as grafting sites if the repulsionbetween cerium(IV) ammonium nitrate and protonated chitosan can beshown strong enough to make this reaction unfavourable. To solve thequestion of how this mechanism actually looks like is in dire need ofinvestigation.
chitosan is problematic to analyse when it is dissolved due to its highviscosity. Solid state NMR seems to be a good way to circumvent thisproblem. Some of the articles used as background for this thesis utilisesthis technique. It should however be stated that the result of the char-acterisation in the reviewed articles give rise to different mechanisms[21, 32, 23, 29, 30, 31]. This could indicate that the obtained results aredifficult to interpret as multiple reaction mechanisms are proposed evenif similar or identical analytical procedures are used (NMR and FTIR).Some of the articles assign the C2-C3 ring-opening mechanisms to thegrafting reaction without results supporting their claim. Their result
17
indicates that the grafting reaction has occurred and then refer back tostudies using most often the cellulose based system. There is perhapsa possibility that the authors is in one way or another affected by theoverwhelming amount of articles proposing the ”standard” mechanism(figure 2.7), and therefore no notice is taken of the overseen protonation.
Analysis of novel molecules with the characteristics of chitosan dissolvedin an acidic environment so to induce a protonation of the amino groupcould be a possible way to reduce the viscosity problems present withdissolved chitosan.
The experiments preformed for this project unfortunately produced in-conclusive data both for NMR and FTIR. The viscosity problems en-countered in combination with limited time in the lab became a majorhindrance. The peaks are too weak or to blurry to assign them to specificstructural changes. A question not treated in this thesis is why oxygenneeds to be present for the reaction to happen, only that it needs tobe present in order for the reaction to occur. This could potentially bea point of great importance to understand how cerium(IV) ammoniumnitrate interacts with chitosan.
18
Chapter 6
Conclusion
Chitosan shows a significant potential for increasing the water resistanceas well as replacing parts of the fossil-based materials used in wood ad-hesives without affecting the mechanical properties. Today wood adhe-sives with about 20 weight% of chitosan can be produced. To increasethe amount chitosan to poly(vinyl acetate) ratio, further improvementsof the adhesive mixture have to be accomplished. For instance, the largeincrease of solution viscosity with increasing amount of chitosan have tobe addressed without decreasing the mechanical properties of the finalproduct.
Reaction between chitosan and cerium(IV) ammonium nitrate still haselements that need to be further investigated. All parameter of thereaction needs to be addressed to understand what is occurring on amolecular level. As of now the vast majority of scientific papers onthe subject of the mechanism does not account for the protonation ofchitosan in the acidic environment needed to dissolve the polymer. Rig-orous analysis by NMR, FTIR and ESR (electron spin resonance), ofintermediates as well as the final polymer is required to fully understandthe reaction mechanism.
19
References
[1] Norman G. Gaylord. “Cellulose-Monomer Interaction and a Re-vised Mechanism for Graft Copolymerization”. In: Journal of Macro-molecular Science: Part A - Chemistry 10.4 (May 1976), pp. 737–757. doi: 10.1080/00222337608061213.
[2] Tomasz Graczyk and Vladimir Hornof. “Graft copolymerizationof cellulose initiated by ceric salts: Effect of reaction conditionson the consumption of ceric ion”. In: Journal of Polymer SciencePart A: Polymer Chemistry 26.8 (Aug. 1988), pp. 2019–2029. doi:10.1002/pola.1988.080260803.
[3] Takahisa Doba, Cenita Rodehed, and Bengt Ranby. “Mechanismof graft co-polymerization onto polysaccharides initiated by metalion oxidation reactions of model compounds for starch and cel-lulose”. In: Macromolecules 17.12 (1984), pp. 2512–2519. doi:10.1021/ma00142a009. eprint: https://doi.org/10.1021/
ma00142a009. url: https://doi.org/10.1021/ma00142a009.
[4] Xiuting Zhang. “Synthesis and characterization of a greener Poly(vinylacetate) adhesive”. MA thesis. KTH, 2019.
[5] Emelie Norstom. “Hemicelluloses and other Polysaccharides forWood Adhesive Applications”. PhD thesis. KTH, 2018.
[6] A.H. Conner and M.S.H. Bhuiyan. “Wood: Adhesives”. In: (2017).doi: https://doi.org/10.1016/B978-0-12-803581-8.01932-9.
[7] M. Dawood. “Durability of steel components strengthened withfiber-reinforced polymer (FRP) composites”. In: (2014), pp. 96–114. doi: https://doi.org/10.1533/9780857096654.1.96.
[8] A. Kaboorani and B. Riedl. “13 - Mechanical performance ofpolyvinyl acetate (PVA)-based biocomposites”. In: Biocomposites.Ed. by Manjusri Misra, Jitendra K. Pandey, and Amar K. Mo-
20
hanty. Woodhead Publishing Series in Composites Science andEngineering. Woodhead Publishing, 2015, pp. 347–364. isbn: 978-1-78242-373-7. doi: https : / / doi . org / 10 . 1016 / B978 - 1 -
78242- 373- 7.00009- 3. url: http://www.sciencedirect.
com/science/article/pii/B9781782423737000093.
[9] Manfred Amann and Oliver Minge. “Biodegradability of Poly(vinylacetate) and Related Polymers”. In: (2011), pp. 137–172. doi:10.1007/12_2011_153.
[10] Xiao Zhang, Long Bai, Jiaxing Sun, Zhiguo Li, Zhao Jia, and JiyouGu. “Design and fabrication of PVAc-based inverted core/shell(ICS) structured adhesives for improved water-resistant wood bond-ing performance: II. Influence of copolymerizing-grafting sequen-tial reaction”. In: International Journal of Adhesion and Adhe-sives 99 (June 2020), p. 102571. doi: 10.1016/j.ijadhadh.
2020.102571.
[11] Fatemeh Ferdosian, Zihe Pan, Guchuhan Gao, and Boxin Zhao.“Bio-Based Adhesives and Evaluation for Wood Composites Ap-plication”. In: Polymers 9.12 (Feb. 2017), p. 70. doi: https://doi.org/10.3390/polym9020070.
[12] Helge-Otto Fabritius, Christoph Sachs, Patricia Romano Triguero,and Dierk Raabe. “Influence of Structural Principles on the Me-chanics of a Biological Fiber-Based Composite Material with Hi-erarchical Organization: The Exoskeleton of the LobsterHomarusamericanus”. In: Advanced Materials 21.4 (Jan. 2009), pp. 391–400. doi: DOI:10.1002/adma.200801219.
[13] Wassila Arbia, Leila Arbia, Lydia Adour, and Abdeltif Amrane.“Chitin Extraction from Crustacean Shells Using Biological Meth-ods - A Review”. In: Food Technology and Biotechnology 51 (Apr.2012).
[14] S.H. Lv. High-performance superplasticizer based on chitosan. El-sevier, 2016, pp. 131–150. doi: 10.1016/B978-0-08-100214-8.00007-5.
[15] Bozena Tyliszczak, Anna Drabczyk, Sonia Kud lacik-Kramarczyk,and Agnieszka Sobczak-Kupiec. Sustainable Production of Chi-tosan. Springer International Publishing, Jan. 2019, pp. 45–60.doi: https://doi.org/10.1007/978-3-030-11274-5_4.
[16] Y.N. Sudhakar, M. Selvakumar, and D. Krishna Bhat. “Methodsof Preparation of Biopolymer Electrolytes”. In: (2018), pp. 35–52.
21
doi: https://doi.org/10.1016/B978-0-12-813447-4.00002-9.
[17] Chen Yu, Xiao Kecen, and Qu Xiaosai. “Chapter 7 - GraftingModification of Chitosan”. In: Biopolymer Grafting. Ed. by Vi-jay Kumar Thakur. Elsevier, 2018, pp. 295–364. isbn: 978-0-323-48104-5. doi: https://doi.org/10.1016/B978-0-323-48104-5.00007-X. url: http://www.sciencedirect.com/science/article/pii/B978032348104500007X.
[18] Vijay Nair, Lakshmi Balagopal, Roshini Rajan, and Jessy Mathew.“Recent Advances in Synthetic Transformations Mediated by Cerium(IV)Ammonium Nitrate”. In: Accounts of Chemical Research 37.1 (Jan.2004), pp. 21–30. doi: https://doi.org/10.1021/ar030002z.
[19] Vellaisamy Sridharan and J. Carlos Menendez. “Cerium(IV) Am-monium Nitrate as a Catalyst in Organic Synthesis”. In: ChemicalReviews 110.6 (June 2010), pp. 3805–3849. doi: https://doi.org/10.1021/cr100004p.
[20] Vijay Nair and Ani Deepthi. “Cerium(IV) Ammonium Nitrate AVersatile Single-Electron Oxidant”. In: Chemical Reviews 107.5(May 2007), pp. 1862–1891. doi: https://doi.org/10.1021/cr068408n.
[21] M. Jalal Zohuriaan-Mehr. “Advances in chitin and chitosan modi-fication through graft copolymerization: A comprehensive review”.In: Iranian Polymer Journal (English Edition) 14 (Mar. 2005),pp. 235–265.
[22] Biranchinarayan Tosh and Chittaranjan Routray. “Grafting ofCellulose Based Materials: A Review”. In: Chemical Science Re-view and Letters 3 (Jan. 2014), pp. 74–92.
[23] A. Pourjavadi, G. R. Mahdavinia, M. J. Zohuriaan-Mehr, andH. Omidian. “Modified chitosan. I. Optimized cerium ammoniumnitrate-induced synthesis of chitosan-graft-polyacrylonitrile”. In:Journal of Applied Polymer Science 88.8 (Mar. 2003), pp. 2048–2054. doi: 10.1002/app.11820.
[24] Premamoy Ghosh. Polymer Science and Technology: Plastics, Rub-bers, Blends and Composites. Tata McGraw-Hill Education, 2001.
[25] Hale Berber. “Emulsion Polymerization: Effects of Polymeriza-tion Variables on the Properties of Vinyl Acetate Based EmulsionPolymers”. In: (Jan. 2013). doi: DOI:10.5772/51498.
22
[26] V.N Kislenko. “Emulsion graft polymerization: mechanism of for-mation of dispersions”. In: Colloids and Surfaces A: Physicochem-ical and Engineering Aspects 152.1-2 (July 1999), pp. 199–203.doi: https://doi.org/10.1016/S0927-7757(98)00687-6.
[27] Ignac Capek. Preparation of polymer-based nanomaterials. Else-vier, 2019, pp. 175–265. doi: https://doi.org/10.1016/B978-0-444-63748-2.00003-1.
[28] S.G. Roy, S. Banerjee, and P. De. “Cationic Polymerization ofNonpolar Vinyl Monomers for Producing High Performance Poly-mers”. In: Reference Module in Materials Science and MaterialsEngineering. Elsevier, 2016. doi: https://doi.org/10.1016/B978-0-12-803581-8.01357-6.
[29] Wei Li, Zhaoyang Li, Wenshen Liao, and Xin-De Feng. “Chemicalmodification of biopolymers-mechanism of model graft copolymer-ization of chitosan”. In: Journal of Biomaterials Science, PolymerEdition 4.5 (Jan. 1993), pp. 557–566. doi: https://doi.org/10.1163/156856293X00203.
[30] M. Pati and P. L. Nayak. “Graft copolymerization of methyl acry-late on chitosan: Initiated by ceric ammonium nitrate as the initiator-characterization and antimicrobial activity”. In: Advances in Ap-plied Science Research 3.3 (2012), pp. 1646–1645. issn: 0976-8610.
[31] Jigar M. Joshi and Vijay Kumar Sinha. “Ceric ammonium nitrateinduced grafting of polyacrylamide onto carboxymethyl chitosan”.In: Carbohydrate Polymers 67.3 (Feb. 2007), pp. 427–435. doi:10.1016/j.carbpol.2006.06.021.
[32] E. Yilmaz, T. Adali, O. Yilmaz, and M. Bengisu. “Grafting ofpoly(triethylene glycol dimethacrylate) onto chitosan by ceric ioninitiation”. In: Reactive and Functional Polymers 67.1 (Jan. 2007),pp. 10–18. doi: 10.1016/j.reactfunctpolym.2006.08.003.
[33] N. Mzinyane, H. Chiririwa, A. Enakpodia Ofomaja, and E. BobbyNaidoo. “EFFECT OF AMMONIUM CERIC NITRATE AS INI-TIATOR IN GRAFTING OF ACRYLIC ACID ONTO PINECONE POWDER”. In: Cellulose Chemistry and Technology 53.9-10 (Nov. 2019), pp. 971–979. doi: 10.35812/CelluloseChemTechnol.2019.53.95.
[34] M. Rinaudo, G. Pavlov, and J. Desbrieres. “Influence of aceticacid concentration on the solubilization of chitosan”. In: Polymer
23
40.25 (Dec. 1999), pp. 7029–7032. doi: 10.1016/S0032-3861(99)00056-7.
[35] J.Y. Olayemi and A.A. Adeyeye. “Some properties of polyvinylacetate films cast from methanol, acetone and chloroform as sol-vent”. In: Polymer Testing 3.1 (1982), pp. 25–35. doi: https:
//doi.org/10.1016/0142-9418(82)90010-1.
24
