Natural fibre (surface) treatments

Lecture PowerPoint

Review papers

Subject Index

After extraction from the plant stem, bast natural fibre textiles may be subjected to further treatments [1-4] to promote physical and/or chemical adhesion to the matrix for composites, which may include:

Biological methods

• bacterial cellulose or nanocellulose [2, 3].
• enzyme treatment [5-11]: enzymes are natural chemicals produced by bacteria, fungi, protozoans, termites, plants and animals with microbial enzymes having more stability.  They catalyse selective chemical synthesis and/or decomposition at low concentations under mild (temperature, humidity, pH) conditions (Table 1) and consume less energy and water than conventional chemical processes.  Tailored enzymes are highly specific.  Enzyme, like true catalysts, are not consumed by the reactions they facilitate.  An enzyme facilitates a molecule, termed the substrate, to undergo a reaction by forming a temporary complex with that substrate.  The active site on the enzyme must have the correct chemical nature and perfect conformation for the specific substrate. For example, cellulose is the substrate for the cellulase enzyme.  Enzyme function depends on the precision of the activated enzyme-substrate complex (AESC).  A substrate and its enzyme are bound together like a key in a lock (known as the Lock and Key Analogy).  The AESC forces the substrate into a stressed form making the bonds more susceptible to reaction (the induced fit hypothesis) when the required energy exists.
Table 1: Enzymes, bacteria and fungi associated with retting, and their activity (images of the organisms)

Source	System	Enzyme	Activity  (EC = Enzyme Commission number)	Reference
Acidobacteria	bacterium			12
Actinobacteria	bacterium			12
Alternaria alternate	fungus		isolated from dew retted plants	9, 13
Ascomycetes ophiostomn floccosum	fungus			14
Aspergillus awamori	fungus	Xylanase I		15
Aspergillus niger	fungus	PGase 31660 (Serva Feinbiochemica GmbH, Germany) polygalacturonase (PAn, Sigma-Aldrich)	CAS 9032-75-1 EPA Risk Assessment.  Most commonly used fungal species in industrial production of pectolytic enzymes.	9, 16-19
Aspergillus sojae	fungus			20
Aspergillus (selected strain)	fungus	Viscozyme L (Novo Nordisk, Franklinton NC)	acts against branched pectin-like compounds.  Also contains a wide range of carbohydrases including arabanase, cellulase, β-glucanase, hemicellulase and xylanase.	16, 21
Aureobasidium pullalans	fungus		isolated from dew retted plants	9, 13
Bacillus sp.	bacterium			22
Bacillus licheniformis	bacterium			23
Bacillus macerans	bacterium		active in aerobic phase	9, 17
Bacillus mesentericus	bacterium		active in aerobic phase	9, 17
Bacillus mojavensis		Pectinase		24
Bacillus polymyxa	bacterium		active in aerobic phase	9, 17
Bacillus subtilis	bacterium		active in aerobic phase	9, 17, 23, 25
Basidomycetes schizophyllum commune (S.com) white rot	fungus			14
Basidomycetes phanerochaete sordida (D2B) white rot	fungus			14
Basidomycetes pycnoporus sp. (Pyc) white rot	fungus			9, 14
Cerrena unicolor	fungus	laccase		DOI
CFB: Cytophaga - Flexibacter - Bacteroidetes	bacteria			12
Chaetomium sp.	fungus			17
Chaetomium globosum	fungus			11
Chlorobiales	bacterium			12
Cladosporium sp.	fungus		isolated from dew retted plants	9
Cladosporium herbarum	fungus		isolated from dew retted plants	9, 13
Clostridium acetobutyricum	bacterium		active in anaerobic phase	9, 17, 23, 25
Clostridium aurantibutyricum	bacterium		active in anaerobic phase	9
Clostridium felsinium	bacterium		active in anaerobic phase	9, 17, 23, 25
Clostridium saccharobutylicum	bacterium		active in anaerobic phase	25
Clostridium tertium	bacterium		active in anaerobic phase	9, 17
Epicoccum nigrum	fungus		isolated from dew retted plants	9, 13, 26
Firmicutes	bacterium			12
Fusarium sp.	fungus		isolated from dew retted plants	9
Fusarium culmorum	fungus			13
Fusarium lateritium	fungus			18, 26
Lactobacillus	bacterium		effective during fermentation to produce pectolytic enzymes	9
Leuconostoc	bacterium		effective during fermentation to produce pectolytic enzymes	9
Macrophomia phaseolina	fungus			17
Mucor sp.	fungus		isolated from dew retted plants	9, 13, 17
Ochrobactrum anthropi	bacterium		identified with seawater retting treatments	27
Pediococcus	bacterium		effective during fermentation to produce pectolytic enzymes	9
Penicillium sp.	fungus		isolated from dew retted plants	9
Penicillium chrysogenum	fungus	pectinolytic		28
Phoma sp.	fungus		isolated from dew retted plants	9, 13, 17
Proteobacteria	bacterium			12
Rhizomucor pusillus	fungus		thermotolerant zygomycete fungal isolate identified as the best retting organism for producing a soft and well retted flax fiber.	9, 18, 26, 29
Rhizopus sp.	fungus	PGase I P-4300 (Sigma Chemical Co., St. Louis MO)		13, 16
Rhizopus sp.	fungus	PGase II 76285 (Fluka Chemie GmbH, Switzerland)		13, 16
Rhizopous oryzae	fungus	isolated from dew retted plants		9, 30
Rhodotorula sp.	yeast	isolated from dew retted plants	basidiomycetous yeasts	9
Stenotrophomonas maltophilia	bacterium		identified with seawater retting treatments	27
Thermomyces lanuginosus	fungus	XTl, Sigma-Aldrich	CAS 37278-89-0, xylanase	18, 19
Trichoderma viride	fungus	Endoglucanase V	xyloglucan, xylan and carboxymethylcellulose	15
Verrucimicrobia	bacterium			12
Zygomycetes absidia (B101)	fungus			14
		Bioprep® 3000 L (Novozymes)	An alkaline pectate lyase isolated and produced for the unique ability to degrade the pectin layer between the waxy cuticle and cellulosic fabric cotton	14, 31
		BioPrep® L (Ciba Geigy AG)	alkaline pectinase (CAS 9032-75-1)	32, 33
		Cellulosoft Ultra L (Nono Nordisk)	cellulase (CAS 9012-54-8)	32, 33
		Flaxzyme® (Novo Nordisk)	mixture of pectinases, hemicellulases and cellulases developed specifically for enzyme retting	9
		Novamix (lipase, protease and amylase–xylanase)		34, 35
		Novozyme (xylanase, laccase and lipase)		36
		Pectinex® Ultra SP-L (Novozymes)	highly active pectolytic enzyme from Aspergillus aculeatus with pectolytic and hemicellulolytic activities that disintegrate plant cell walls	9, 31
		Pulpzyme (Pz, Novozymes CH)	CAS 9025-57-4	18, 19
		Rohapect MPE (AB Enzymes, Germany)	CAS 9025-98-3	18, 19
		Scourzyme® L (Novozymes)	A bioscouring alkaline pectate lyase which degrades the pectin primary cell wall without significant degradation of the cellulose fibre	9, 18, 19, 31, 37
		SIHA-Panzym®DF (Novozyme A/S)	pectinase (EC 3.2.1.15)	38
		Texazyme® (Inotex)	multicomponent product without cellulase activity developed for elementarisation of bast fibres through the degradation of pectin layers	9, 31, 39
		Tinozym CEL (Ciba Geigy AG)	CAS 651044-47-2	17, 18
		Viscozyme® L (Novozyme)	multienzyme solution containing a wide range of carbohydrases, including arabase, cellulose, b-glucanase, hemicellulose, and xylase	9, 31

Chemical methods:

• bleaching: defined as the "procedure, other than by scouring only, of improving the whiteness of textile material by decolorizing it from the grey state, with or without the removal of natural colouring and/or extraneous substances. Note: removal of colour from dyed or printed textiles is usually called 'stripping'".
• coupling agents [40]: coating with molecules in which one end normally reacts with chemical groups on the fibre surface and the opposite end has chemical functionality appropriate to the matrix resin system.  Typical silane coupling agents include APS (3-aminopropyltriethoxysilane), MPS (γ-methacryloxypropyltrimethoxy silane) and VTMO (vinyltrimethoxysilane).
• esterification involves a chemical reaction normally with anhydrides (of organic acids).  The most common form is acetylation which is defined as the "process of introducing an acetyl radical into an organic molecule" and the term "is used to describe the process of combining cellulose with acetic acid", CH₃COOH.  The reaction with propanoic (a.k.a propionic) acid, CH₃CH₂COOH, is propionylation.
• grafting is the incorporation of monomers (e.g. maleation: reaction with maleic anhydride or cyanoethylation: reaction with acrylonitrile) or oligomers (short chain polymers) by chemical reaction at the fibre surface.
• mercerisation: defined as the "treatment of cellulosic textiles in yarn or fabric form with a concentrated solution of caustic alkali [soda], whereby the fibres are swollen, the strength and dye affinity of the materials are increased, and their handle is modified. The process takes its name from its discoverer, John Mercer (1844)" [41].  The treatment removes practically all non-cellulose components except waxes and changes the crystal structure of the cellulose.  Mercerisation results in "hydrated cellulose" which is chemically identical to the precursor material but has different physical properties [42]. For rapeseed straw, Paukszta [43] has shown optimal conditions for the process to be solution concentrations in the range 12.5-20% for >5 minutes. Mercerisation converts cellulose I to 50±15% cellulose II with a consequent expansion of the crystal lattice (Table 2) as adjacent chains move further apart. This change explains the greater chemical reactivity and increased water absorption of mercerised cellulose [44].
Table 2: Unit cell dimensions for cellulose crystals [45]


Cellulose	Crystal unit cell	a (nm)	b (nm)	c (nm)	α (°)	β (°)	γ (°)
Iα	one-chain triclinic P1	67.17	59.62	104.0	118.08	114.80	80.37
Iβ	two-chain monoclinic P21	77.84	82.01	103.8	90	90	96.5
II	monoclinic P21	81.0	90.3	103.1	90	90	117.10

Cellulose has four crystal structures (I-IV), although the III and IV forms are infrequent.  The cellulose I crystal has a theoretical elastic modulus of 134 GPa (similar to aramid fibres), but after solution treatment it recrystallises as cellulose II with a modulus of 90 GPa [46].
• scouring (solvent treatment): defined as the "treatment of textile materials in aqueous or other solvents in order to remove natural fats, waxes, proteins and other constituents, as well as dirt, oil, and other impurities".
• surface oxidation: may use chemicals (e.g. KMnO₄), or physical methods (see below), and may be combined with grafting.

Physical methods:

• plasma treatment: plasma is one of the four fundamental states of matter (the others are solid, liquid and gas) and consists of ionised gas. It is normally formed by heating a gas or subjecting it to a strong electromagnetic field.  Plasma can be divided into:
  • low pressure plasma under vacuum conditions or at atmospheric pressure (a.k.a., atmospheric pressure glow discharge (APGD) or atmospheric plasma pressure technology (APPJ)) can be generated at powers around 60-100 W.
  • corona discharge (> 500°C).
  • thermal torches, arcs, etc (> 10,000°C).
  although only the low pressure methods above can be used without damage to natural fibres.  Li et al [47] have reviewed the use of plasma technologies on reinforcement fibres for use in fibre-reinforced polymer composites.
• ultraviolet irradiation: Khan et al [48] subjected bleached jute cloth moving at 4 m/min to 2 kW UV (133-254 nm wavelength) radiation before incorporation into polymer composites subjected to gamma irradiation curing.  Water uptake was significantly reduced relative to untreated jute.
• ionic liquids (IL) [49]: ionic liquids are combinations of organic cations, and inorganic or organic anions, which are molten salts below 100°C.  They have potential to be more sustainable and environmentally responsible alternatives to organic solvents for many industrial applications.  IL can dissolve and transform cellulose dependent on the size of the two ions, the Kamlet-Taft [50] parameters (KT, α hydrogen bond acidity/ability to donate a hydrogen bond, β: hydrogen bond basicity/ability to accept a hydrogen bond), polarisability (π^*), viscosity (η) and surface tension (σ).  The anion β attracts hydroxyl protons of the cellulose, disrupting the hydrogen bonding interactions and thus opening the crystal lattice.  Treatment efficacy is influenced more by β, with β >0.72 resulting in higher yields of glucose, than by viscosity with lower viscosities resulting in more dissolved cellulose.  Some IL which have been studied in the context of cellulose treatment are listed in Table 3.
  Table 3:  Cations and anions in IL used for the treatment of cellulose

  Cation	Anion	Reference
  1-ethyl-3-methylimidazolium [C2mim]+	acetate [OAc]-	44
  1-butyl-3-methylimidazolium [C4mim]+	chloride [Cl]-	44
  	tetrafluoroborate [BF4]-	44

  The size of the anion falls in the order [OAC]^- < [Cl]^- < [BF4]^-.
  The effect of the anion on surface tension of the butyl-cation IL is [OAC]^- > [Cl]^- > [BF4]^-.
  The corresponding glucose yields are in the order [OAC]^- > [Cl]^- > [BF4]^-.

Compatibilisers

There are a variety of chemicals which can be compounded to enhance the miscibility of polymers with very different characteristics.  They are generally referred to as compatibilisers [e.g. 51-54]. To improve the interface between hydrophilic cellulose fibres and hydrophobic polypropylene matrix, the most common compatibiliser is grafted poly(MaleicAnhydride-co-ProPylene), commonly referred to as MA-g-PP or simply MAPP.

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