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Sustainable and Green Composite Functional Hydrogels: Synthesis, Characterization and Performance Evaluation

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I am thankful to my seniors (Dr. Ritesh Malani, Dr. Arup Bora, Dr. Kuldeep Roy, Dr, Amit Batghare, Dr. Neha Singh and Dr. Niharika Kashyap) for their valuable suggestions. I acknowledge and thank my colleagues (Dr. Bhaskar, Udangshree, Karan, Aradhana, Pushpita, Rishiraj, Avinash, Komal, Umesh) for their help and enthusiastic companionship.

Abstract

Furthermore, the swelling and water retention capacity of the fire retardant hydrogels were measured to check the water encapsulating and retaining strength. The network parameter of the hydrogels (mesh size, crosslink density and molecular weight between crosslinks) was also estimated using the results of the swelling test and elastic moduli (estimated by rheological studies).

C ONTENTS

  • Introduction and Literature Review
  • Extraction of lignin and Cellulose from Arundo Donax
  • ULTRASOUND ASSISTED LIGNIN-DECORATED MWCNT DOPED FLEXIBLE PVA‒CHITOSAN
  • DEVELOPMENT OF NaOH-BORAX CROSSLINKED PVA-XANTHAN GUM-LIGNIN HYDROGEL
  • KAOLIN EMBEDDED CELLULOSE HYDROGEL WITH TUNABLE PROPERTIES AS GREEN FIRE

106 4.2.4 Characterization of lignin nanoparticles and hydrogels Swelling and water retention test Rheological properties of hydrogel samples Experiments to determine flammability.

L IST OF T ABLES

L IST OF F IGURES

PX3B0.4N2L5 hydrogels have a smaller pore size due to a higher degree of cross-linking between PVA, XG and LNP. PX3B0.4N2 and PX3B0.4N2L20 hydrogel coated cotton fabric samples (a) Heat release rate (HRR) and (b) Total heat release (THR).

Figure 4.6  (a) Swelling ratios and (b) water retention ability of the  formulated hydrogels
Figure 4.6 (a) Swelling ratios and (b) water retention ability of the formulated hydrogels

Introduction and Literature Review

I NTRODUCTION AND L ITERATURE R EVIEW

  • Sustainable and green hydrogel
  • Fundamentals of hydrogel
    • Crosslinking of hydrogel
    • Structure of hydrogel
    • Swelling behavior of hydrogel
  • Classification of hydrogel
    • Based on source
    • Based on the synthesis methods
    • Based on the charge of polymeric network
    • Based on hydrogel response to stimuli
    • Based on polymeric compositions
  • Characterization techniques
    • Swelling ratio and water retention of hydrogel
    • Rheology
    • Estimation of structural parameter
    • Thermal analysis
    • Fourier Transform Infrared (FTIR) analysis
    • Morphological analysis
  • Literature review
  • Thesis objectives

As discussed in detail in the previous section, the degree of cross-linking of the polymer is the most important parameter of the hydrogel. Thus, depending on the application, the properties of the hydrogel can be tuned and synthesized accordingly.

Figure 1.1 Fire retardant mechanism  and different char inducing materials
Figure 1.1 Fire retardant mechanism and different char inducing materials

This chapter reports synthesis, characterization, and testing of hydrogels comprising a biodegradable PVA matrix with two biomaterials additives, viz.,

This chapter summarized the findings of all the chapters of the thesis. The thesis has presented investigations in formulation of three types of hybrid composite

Open fire test (OFT) and calculation of the limiting oxygen index (LOI) of the hydrogel samples. Investigation of flammability and thermal stability of halogen-free intumescent system in biopolymer composites containing bio-based carbonizing agent and mechanism of their char formation. Enzymatically cross-linked hydrogels based on a linear poly(ethylene glycol) analogue for controlled protein release and 3D cell culture.

Characterization of the microstructure of hydrazone cross-linked polysaccharide-based hydrogels through rheological and diffusion studies. Self-healing antifouling zwitterionic hydrogel based on synergistic photoactivated dynamic disulphide metathesis reaction and ionic interaction. Synthesis and characterization of novel carboxymethylcellulose/acrylic acid based gels prepared by electron beam irradiation.

Characterization of the network structure of dextran glycidyl methacrylate hydrogels by studying the rheological and swelling behavior1". Preparation and characterization of Artocarpus Heterophyllus waste-derived lignin added to chitosan biocomposites for wound dressing application. Fire retardant and ductile biocomposites of mamori and basalones in Mamorri biocomposites. Cellulose nanofibers and carboxymethylcellulose.

EXTRACTION OF LIGNIN AND CELLULOSE FROM Arundo Donax

E XTRACTION OF L IGNIN AND C ELLULOSE FROM

  • Introduction
  • Experimental
    • Collection and processing of biomass
    • Compositional analysis of AD
    • Extraction of lignin and cellulose from Arundo donax
    • Characterization techniques
  • Results and discussion .1 Compositional analysis
    • Extraction of lignin and cellulose
    • XRD to determine crystallinity of cellulose
    • FTIR of lignin and cellulose
    • TGA of lignin and cellulose
    • Estimation of molecular weight
    • FESEM of lignin and cellulose
  • Conclusions

Another study by Seca et al.7 extracted lignin from nodes and internodes of AD and compared the chemical composition of the extracted lignins. The weight of the sample with the pan was recorded and then placed in an oven at 105± 3ºC for a minimum of four hours. 10 mL of the sample was aliquoted for further processing and the remaining autoclaved sample was vacuum filtered to separate the filtrate and solids.

FESEM [Sigma 300, Zeiss (USA)] allows to visualize the surface morphology of the extracted lignin and cellulose. Gel permeation chromatography (GPC) [Agilent, G7820A] was used to estimate the molecular weight of the extracted components. Tables 2.4 and 2.5 show the yield and purity of the extracted lignin and cellulose under different extraction conditions.

A small hump is observed at 234ºC due to degradation of phenylpropane side chains of lignin.20 At the end of the analysis, 40%. The molecular weight of the extracted lignin and cellulose was determined by GPC and represented in Table 2.5 and Figure 2.7. The morphological features of the extracted lignin and cellulose are revealed by the FESEM images (Figure 2.8).

Figure 2.1 Arundo donax in IIT Guwahati campus
Figure 2.1 Arundo donax in IIT Guwahati campus

ULTRASOUND ASSISTED LIGNIN-

DECORATED MWCNT DOPED FLEXIBLE PVA‒

CHITOSAN COMPOSITE HYDROGEL

U LTRASOUND A SSISTED L IGNIN -D ECORATED MWCNT

  • Introduction
  • Experimental .1 Materials
    • Aqueous dispersion of MWCNT
    • Hydrogel formulation
    • Investigation of MWCNT dispersion by lignin
    • Characterization of PVA ‒CS hydrogel
    • Measurement of conductivity of the formulated hydrogel
  • Results and discussion
    • MWCNT dispersion by lignin
    • Characterization of formulated hydrogels .1 Physical crosslinking of hydrogel
  • Conclusions

Thermal stability of the freeze-dried hydrogels was characterized using TGA [TG 209 F1 Balance, Netzsch (Germany)]. Moreover, the increased intensity of the peak at 1118 is due to C‒O stretching (Figure 3.3b).42 These LPs, along with SU and acetone, aided the dispersion of MWCNT in water. The dissolution of lignin due to addition of acetone is evident from the change in color of the suspension from light brown to dark brown (Figure 3.1a).

Furthermore, FETEM was used to validate the homogeneous dispersion of the suspension and the effect of US and acetone on lignin and MWCNTs (Figure 3.5). Furthermore, it can be seen from Figure 3.7a that the presence of lignin in PVA‒CS improved the thermal stability of the hydrogel. The mechanical strength and viscoelastic behavior of PVA–CS composite hydrogels were evaluated by performing oscillatory rheological measurements, i.e.

The internal structure and morphology of the freeze-dried hydrogels were evaluated using FESEM micrographs. The degree of cross-linking and the internal structure of the hydrogel have a great influence on the swelling ability of hydrogels (Figure 3.11). The swelling ability of the hydrogel helps determine the time required to achieve the hydrogel.

Figure  3.1  (a)  Process  followed  for  dispersion  and  formulation  of  hydrogels  (b)  Illustrative representation of physical crosslinking of the formulated hydrogel
Figure 3.1 (a) Process followed for dispersion and formulation of hydrogels (b) Illustrative representation of physical crosslinking of the formulated hydrogel

DEVELOPMENT OF NaOH-BORAX

CROSSLINKED PVA-XANTHAN GUM-LIGNIN HYDROGEL AS GREEN FIRE RETARDANT

COATING

D EVELOPMENT OF NaOH- BORAX C ROSSLINKED

AS G REEN F IRE R ETARDANT C OATING

Introduction

However, PVA is highly flammable when dehydrated and has a low water holding capacity, limiting its use as a flame retardant. A self-healing flame retardant coating on the fabric was produced by combining water-soluble chitosan (WC), PVA, acrylic acid and Cu2+.12 It was found that increasing the WC content and incorporating Cu2+ significantly improved the thermal properties of the fabric. . Similarly, agarose, a naturally occurring algal gel, is a self-flame retardant that provides low heat release rate (HRR) and total heat release (THR) composites.

2 formulated a triple network hydrogel that could be laminated to cotton fabric to prevent burning of the highly flammable cotton fabric. Lignin is known for its flame retardant character due to carbonization and high thermal stability. A 5 wt% TP-g-L met industry standards of UL94 V0 flame retardancy rating due to the balanced charring nature of the additive.

However, nano-lignin incorporation has been observed to yield better thermal stability than pristine lignin.25 Lignin nanoparticles also offer easier chemical modification and tunable morphological structure.26,27 In summary, the previous literature clearly demonstrates the potential of xanthan gum and lignin biomaterials nanoparticles as additives for fire-retardant hydrogels. The nature of cross-linking and stability of the hydrogels were characterized by evaluating the swelling ratio, water retention capacity and finally calculating the network parameters. The fire retardant nature of the synthesized hydrogels was assessed using vertical flammability test (UL94), TGA, limiting oxygen index (LOI) and cone calorimeter test (CCT).

Figure  4.1  Schematic  representation  of  hydrogel  synthesis  and  hydrogel  coating  on  cotton cloth
Figure 4.1 Schematic representation of hydrogel synthesis and hydrogel coating on cotton cloth

Experimental .1 Materials

  • Synthesis of lignin nanoparticles
  • Synthesis of PBXN, PBXNL hydrogel and hydrogel coated cotton cloth To begin with, NaOH solution of required concentration and PVA were mixed
  • Characterization of lignin nanoparticles and hydrogels
    • Swelling and water retention test
    • Rheological properties of the hydrogel samples
    • Experiments for determination of flammability and flame retardancy Cone calorimeter test (CCT) and Limiting Oxygen Index (LOI) test were

All experiments for the synthesis of hydrogels were repeated 3x to assess the reproducibility of the results. The particle size distribution of LNPs was determined using dynamic light scattering as described in Chapter 3. The zeta potential of LNPs was evaluated using a zeta potentiometer as described in Chapter 3.

The structural and surface morphological analysis of the LNPs and freeze-dried hydrogel samples was performed using field-emission scanning electron microscopy (FE-SEM) [Sigma 300, Zeiss (USA)] and field-emission transmission electron microscopy (FE-TEM) [2100F , Jeol (Japan)]. Similarly, the water retention capacity of the freshly prepared composite hydrogels (1 cm × 1 cm) was determined gravimetrically at an interval of 1 hour for 24 hours in an ambient atmosphere (25±2⁰C, humidity 45±10%) according to equation 4.4. Where Wi is the weight of hydrogel in swollen state (at specified time) kept for dehydration in ambient atmosphere and Wd is the weight of the dehydrated hydrogel.

However, these parameters are strongly influenced by both the composition and the method of preparation of the hydrogels. The flammability of the hydrogel-coated cotton fabric was tested by an open flame test and vertical combustion (UL94). The char length of the samples was recorded at the end of the second burn time.

Table 4.1 Compositions of the hydrogels
Table 4.1 Compositions of the hydrogels

Results and discussion

  • Characterization of lignin nanoparticles
  • Structural characterization of the developed hydrogel
  • Rheological properties .1 Effect of xanthan gum
    • Effect of borax and NaOH
    • Effect of LNPs
  • Network parameter
  • Thermal analysis of hydrogels
  • Thermal kinetics analysis
  • Combustion studies of hydrogel coated cotton cloth

The result of the XPS analyzes was confirmed by the FTIR spectra of pristine lignin and LNP. Thermogravimetric (TG) and derived thermogravimetric (DTG) curves of pristine lignin and LNP under nitrogen atmosphere are shown in Figure 4.3c. Further frequency sweep was performed in this region without damaging the structure of the hydrogels.

Also, the increase of the elastic moduli of the PX hydrogel samples with XG concentration can be seen in Figure A1.2 (Appendix 1). The trends in Figure 4.5a clearly show the superiority of G over G over the entire frequency range, confirming the dominant elastic nature of the hydrogel. Similar to the strain sweep, frequency sweep shows the viscoelastic behavior of the PX3B0.4 hydrogel, which was determined by a gel-like or elastic behavior.

Furthermore, the porous structure (shown in Figure 4.7) and large mesh size of the hydrogel facilitates water absorption. However, higher incorporation of LNP causes rapid burning of the sample and formation of charred layer. These results essentially show that incorporation of LNP into the PXBN hydrogel leads to significant improvement in the fire retardant behavior of the hydrogel.

Table 4.2 Functional groups recognized in pristine lignin and LNP.
Table 4.2 Functional groups recognized in pristine lignin and LNP.

Conclusions

Synthesis of an effective flame retardant hydrogel for skin protection using xanthan gum and resorcinol bis(diphenylphosphate) coated starch. Formation of self-healing, fire-retardant, water-soluble chitosan/chemically cross-linked polyvinyl alcohol/Cu(II) gel. Poorly crystallized poly(vinyl alcohol)/carrageenan matrix: highly ionic conductive and flame retardant gel polymer electrolytes for safe and flexible solid state supercapacitors.

Experimental and simulation study of solvent effects on the intrinsic properties of spherical lignin nanoparticles. One-pot synthesis of environmentally friendly lignin nanoparticles with compressed liquid carbon dioxide. Green and scalable preparation of a colloidal suspension of lignin nanoparticles and its application in an environmentally friendly sunscreen.

Investigations on the Interactions between Xanthan Gum and Poly(Vinyl Alcohol) in Solid State and Aqueous Solutions. An Easy and Green Strategy to Simultaneously Improve the Flame Retardant Properties and Mechanical Properties of Poly(Vinyl Alcohol) by Introducing a Bio-Base. A green approach for the construction of multilayer nanocoating for flame retardant treatment of polyamide 66 fabric from chitosan and sodium alginate.

KAOLIN EMBEDDED CELLULOSE HYDROGEL WITH TUNABLE PROPERTIES AS GREEN FIRE

RETARDANT

K AOLIN E MBEDDED C ELLULOSE H YDROGEL WITH T UNABLE

  • Introduction
  • Experimental section
    • Synthesis of cellulose-kaolin hydrogel
    • Structural characterization and adhesion strength of composite hydrogels To gain insights into the interactions between various components of the
    • Swelling and water retention studies
    • Performance evaluation of hydrogels in flame retardancy
  • Results and discussion
    • Structural characterization of cellulose, kaolin and composite hydrogel Figure 5.2a presents the XRD patterns of kaolin, cellulose, and cellulose‒
    • Swelling and water retention study
    • Influence of MBA and kaolin on crosslinking of the hydrogel

Finally, the flame retardancy characteristics of the hydrogels were determined using the limiting oxygen index (LOI), cone calorimeter test (CCT), vertical flammability test (VFT) and open fire test (OFT). We think that the synergistic effect of water encapsulated in the hydrogel matrix and kaolin would delay the ignition of the flammable material. The KAS isoconversion method was used to obtain the kinetic parameters of the thermal conversion (or decomposition) of the hydrogels as described in chapter 4 section 4.2.4.

In addition, an open fire test (OFT) was performed to verify the performance of the hydrogel-coated samples under dehydrated and hydrated conditions. The proposed mechanism of the MBA cross-linked cellulose hydrogel is shown in Figure 5.3c. a) Schematic illustration of the overlay shear test specimen. A comparison of hydrogels formulated with and without kaolin (Figures 5.5a, 5.5b and 5.5c) shows that the presence of kaolin reduced the hydrogel's ability to absorb water.

Therefore, a higher dosage of kaolin in the hydrogel resulted in a small swelling of the hydrogels. Measuring the moduli of hydrogels gives an idea of ​​its crosslinking and quantifies the degree of crosslinking. A strong influence of MBA and kaolin was seen in the overall frequency shift of the hydrogel.

Figure 5.1 Synthesis process of kaolin cellulose hydrogel coating on cotton fabric
Figure 5.1 Synthesis process of kaolin cellulose hydrogel coating on cotton fabric

Figure

Figure 4.6  (a) Swelling ratios and (b) water retention ability of the  formulated hydrogels
Figure A2.2  FTIR spectrum of cellulose, (a)  MBA and kaolin and (b)  CMK hydrogels with C:M ratio 1:2
Table 1.1 Conditions for a material to be elastic/solid or liquid.
Table 1.3 Summary of representative literature on PVA, CS, lignin/LNPs composites.
+7

References

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