User:Senbtech2000/Polyurethane

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References

1. “An Introduction To The Principles Of Leather Manufacture” by “Mr. S S Dutta”.
2. “Theory and Practice of Leather Manufacture” by “Mr. K.T. Sarkar”.
3. “Chemistry of Tanning Processes” by “Mr. K.H. Gustavson”.
4. “Chemistry & Technology of Leather” by “Roddy, O` Flahorty & Lollar, Vol. 2& 3. Robert E. Kreiger Publishing Co.,N. Y”.
5. “Comprehensive Footwear Technology” by “Mr. Somenath Ganguly”.
6. “Manual of Shoe Making” by “Clarks”.
7. “Adhesives in Shoe Manufacturing” by “Mr. B. Venkatappiah”.

External links

• example.com

[[P O L Y U R E T H A N E S' ]]

Urethane is the trivial name of carbamic acid NH₂.CO.OH - amino derivative of carbonic acid, HO.CO.OH (ie.H₂CO3). For polyurethanes (PU), this name is a misnomer because it is not made by the polymerization of urethane. In common with condensation polymers, the name refers to the presence of urethane groups in PU’s as inter-unit linkage. The group is obtained in the polymer in the following manner:

                                  OCN-R-NCO    +     HO-R’-OH  ---->  OCN-R-NH.COO-R’-OH
                              (Di-isocyanate)         (Diol)          (urethane)

Though apparently polyurethane formation is an addition reaction, the mode of formation, however, involves step growth mechanism only. That is, the polymeric chains are not formed all at once as in the case of true addition polymers; but only gradually as dimmers, trimers, tetramers, pentamers etc.

RAW MATERIALS

I. Isocyanates and
II. H- donor components

I. ISOCYANTES
Isocyanates in order to form polymers should be poly-isocyanates. Iso-cyanates being highly reactive, are capable of abstracting H from all kinds of proteins forming part of animal body and hence, are as a class highly toxic. In fact, there are only hand–full of reputed firms like (Dupont) engaged in the manufactures of iso-cyanates, in the entire world over.
The following are currently the most important isocyanate monomers used to make industrial products:
1. TDI (Toluene Di-Isocyanate):
TDI was the first monomer used to make PU’s. It exist in 2forms - the para (2, 4) TDI and the ortho (2, 6) isomers. However, actual products are mixtures of the two. The mixtures again are available in 2 compositions – a) 80:20 and b) 65:35. The former (80:20) is more important of the two. Nearly pure (2, 4) isomer is also available at a premium prize.
2. NDI (Naphthalene Di-Isocyanate)
3. MDI (diphenyl Methane Di-Isocyanate).
4. H₁₂MDI
5. HDI (Hexa methylene Di-Isocyanate) - This was the first aliphatic isocyanate used in 1961 to make non-yellowing PU’s. But, this is necessarily less reactive than aromatic isocyanates.
6. IPDI (Iso-Phorone Di-Isocyanate)
7. TMXDI (meta Tetra Methyl Xylene Di-Isocyanate)
8. Various Vinyl Isocyanates
II. H DONOR - COMPONENTS
Isocyanates react with compounds containing active hydrogen. These are essentially of amino or OH compounds. But, amines give polyureas only and therefore only hydroxyl compounds are useful to make PU’s. First, glycols (e.g. 1, 4, butylene glycol) were used as H-donors to make plastics and fibres. Subsequently, OH-ended polyester and polyether came to be used as OH kind of H-donors. Because of differences in the properties of the products made using these 2 classes of H-donors, they are separately designated as polyester PU’s and polyether PU’s described more fully below.
a) Polyester PU’s:
These were first made in 1954-55 to form flexible foams. But, they suffered from 2 counts –
1. High cost of ester and
2. Susceptibility (of alkali sensitive ester bonds) to hydrolysis.
b) Polyether PU’s:
The first PU from polyether was made in 1957 as 1-pot pre-polymer. About 90% of the polyols used in PU manufacture belong to polyether type. But, for coating purposes, they are not prepared because of their high moisture susceptibility and low Tg (Glass Transition Temperature).

Range of Uses and Fields of Applications of PU’s:
Uses of PU’s range from adhesives, surface coatings, films, fibres, foams, elastomers, plastics, thermosetting resins, ion exchangers membranes for dialysis etc.
Fields of application vary from surface coatings to automobiles and to binders for rocket fuels. No other polymer has lent itself to make so many products this is because of the unique combination of properties to be discussed elsewhere.

'''CLASSIFICATION OF PU’S'''

1. The first principle of classification of PU’s is the chemical nature of the polymeric molecules which is the fundamental feature of any chemical.
2. The second principle of classification adopted, here, is the vehicle / medium which is the point that has assumed enormous significance in these days of great Environmental consciousness.
3. Since the Purpose / End –use such as Retanning, Impregnation, Sealer/ base coat etc., though of great importance to the users, vary from to classification and hence comes third in the scheme of things.
A general classification of PU’s based on the first two considerations is shown in the chart below:

PU’S IN LEATHER INDUSTRY

• 
  Caption1
• 
  Caption2

Fully Reacted Reactive Ionomeric

                                           2 – Component                   	1- Component		   	         Emulsion   
      

Anionic Cationic

    True 2- Component        Catalyzed      Blocked       Moisture cure	  Aqueous		  Lacquer     	    	

Moisture cure Emulsion Emulsion

1. Patent coat 6. Easy-care 8. Impregnant 10. Grain filler 13. Reactive lacquer 14.Retanning 16.Retanning

   (1954-55)              (1968)                                   		 (1959)                      (1971)          	Emulsion (1974)           (1979)                    (1970)

2. Aliphatic PU 7. Wet-look 9. Anti-crock 11. General base coat 15.Topcoat 17.Topcoat

   Lacquer (1965)      (1974)                               		           agent for Suedes       (2/3 PU + 1/3 Acrylic)    		             (1981)                    (1981)                            

3. Aliphatic PU 12. Base coats for splits

   Emulsion (1977)                        						         (1973 / 1977).                                                               				     	                                                               							             

4. Transfer Foil (1972)

5. Lamination foils (1972)

POLYURETHANE IN LEATHER INDUSTRY

The contribution of polyurethane to leather industry is tremendous. Today one is able to fabricate full shoe out of polyurethane. PU is next only to NC and Acrylics in leather manufacture in respect of volume. But, as for versatility PU is unique and finds use as
1. Retanning agent,
2. Impregnant,
3. Sealing coat & Base coat binder,
4. Top coat agent,
5. Transfer film,
6. Lamination foil,
7. Adhesives,
8. Soling materials,
9. PU Uppers

1. RETANNING AGENT:
Retanning PU auxiliaries are anionic and cationic products capable of self-emulsification in water. The anionic products are more popular. The pendant free carboxyl groups solublise the PU. The characteristics of these products are as follows:
1. Since the ionizing groups are carboxylic, this agent fixed on the positive charge ammonium groups of collagen.
2. Stable over a wide pH range and hence can be used in Chrome, vegetable and combination tanned leather and with polyacrylates can be stabilized also.
3. Alone does not increase Ts (Shrinkage
Temperature), but as it fixes more chrome, chrome and combination tanned leather can have higher Ts (Shrinkage Temperature). 4. Increases area yield more than polyacrylate
5. Increases softness.
6. Fixes more dye and hence a lower amount of dye will for the same colour strength and the levelness of dyeing is more for the same amount of dye. PU retanned leather given 10-20% colour strength. The method of application is the same as the used for polyacrlylate (pH 5-6).
In 1940, skins were treated with HDI (Hexa methylene Di-Isocyanate) in the presence of emulsifiers at pH 5-6 and the leather obtained was white run-fading chamois type.
[[ 2. IMPREGNANATS:]]
Loose leathers have poor break and poor scuff resistance. Their upgradation requires invariably impregnation with either acrylic or PU resins.
Upgradation with PU requires the use of a 2-component system comprising urethane grade polyester and polyisocyanates. The lacquer can have a solids content of 35-50%. Because of their low surface tension, they spread well in to the pores of leather. But, the leather taken for impregnation should not have a moisture content > 5%. The urethane lacquer entering into the gaps between corium and grain bind them together by reaction through their free NCO groups. The oligomeric PU molecule, being somewhat elastomeric in nature, ensures flexibility in the filled regions. About 5.6gms of lacquers of 35% solids may be required for a foot2 of leather. And the solids deposited however, is only about 2gms / foot2. Impregnated leathers can be piled. They should be dried so that they are free from all solvents, before plating. Plating is done after 24 - 48 hrs.
The impregnated leathers have the following properties:
1. The crusts are white with a green cast.
2. Pull-up is smoother.
3. Scuff resistance is superior (to that of acrylic impregnated leather).
4. Since the leather is filled, no sealer coat is necessary and also less finish will do.
5. All overlying coats adhere better.
In short, PU impregnation is an excellent upgradation technique and it makes a good leather better.
Use of PU for impregnation was suggested in 1951 itself. But, only after the discovery at Batelle institute of the fact that NCO ended PU improved the break, the PU impregnation was seriously attempted. But, unfortunately, the first impregnated leathers cracked. The problem was solved only in 1964. The moisture-cure resins developed in 1959 and curtain coating made PU impregnation easier. Nowadays, aqueous PU emulsions have also become available for impregnation. Whether lacquer or emulsion, the PU should be of low Tg and low molecular weight, so that it can get into the gaps between corium and grain. Impregnation with lacquers however requires the use of curtain coater.
The advantages of impregnation is gently
1. Retanning can be wholly dispensed with or reduced, Sealer coat can be totally avoided and fewer number of finish coats will do.
2. Improved grain break, scuff resistance and cutting value are the immediate and perceptible benefits of impregnation.
3. It can be plated at high temperature with improved smoothness and lay-out.
4. Dyed impregnants improve dye leveling and increased dye stability to light.

3. SEALER and BASE COATS
Porous leathers require a sealing coat. Lacquer sealer coats of PU were the first to appear in the market. However, eco-friendly PU emulsions also were available for the purpose, since 1971.
The base coat developed in later times was a mixture of PU and acrylics. Thus, this is a uracrylic system. The PU content, however, should not exceed 66%. The ability of a NCO to react with a variety of groups ensures good adhesion of the base coat along with increased scuff resistance. In fact, incorporation of a small amount NCO ended PU even in acrylics is desirable, as good bonding can be effected even if the leather surface is a little bit greasier.

4. TOP COATS:
The superiority of PU’s over others for top coating purposes is easily attributable to their following 10 merits of PU.
1. Good adhesion and scuff resistance.
2. Insolubility in water and most organic solvents except CH2CH2 and Isophorone.
3. Low temperature flexibility and high temperature toughness.
4. Good scratch resistance.
5. Good abrasion resistance.
6. Superior impact resistance.
7. Superior soil resistance.
8. Superior weather resistance.
9. Excellent gloss.
10. Good Penetration, good fastness properties

The only Demerits are:
1. Difficulty of renovation.
2. Plastic look and feel
3. High cost.
4. Toxic hazards.

5. TRANSFER FILM:
A variant of film transfer finish is coating with two- component polyurethane mixtures on a coating machine, either by a direct or reversing process.
The finishing method is not performed by the application of finishing floats, but by means of transfer films. The transfer films are manufactured in specialized factories by lamination of different colored aluminium bronzes at high temperatures. They are made in uni-colors or with various patterns. The best known products are gold and silver leathers. These leathers were formerly produced with real gold leaf and silver foils.
Colorless or dyed PVC foils are also sealed onto the surface in order to achieved high gloss effects.
6. LAMINATION FOIL:
A ready-made finish film which is on a carrier foil is applied to the leather surface. For this purpose an adhesive, mostly a polymer binder, is sprayed onto the foil and this side of the foil is placed smoothly onto the leather. It is then plated at about 80ºC, after which the carrier foil is drawn off. Uni-color laminating or any effect is possible this method. Firm colored or dyed polyvinyl chloride foils can also be applied. As this foil is about 0.2mm thick this method is classified as coating.

7. ADHESIVES:
PU Adhesives are produced when a di-isocyanate having two isocyanate groups is reacted with a diol having two hydroxyl groups. Properties:
1. High resistance to PVC plasticizer, oil, grease, heat.
2. Strong Adhesive bond, highly reactive.
3. Good wetting & penetrating properties.
4. Melting Temperature: PU Adhesive – 80 to 90⁰C
Ingredients:
Polyols like polyether, polyester or polybutediene
Di-isocyanate like Toluene diisocyanate (TDI),
Methyl bis (4, 4’ – phenyl isocyanate) (MDI)
Curing Agent like diol, triol, aromatic diamine
Classification of PU Adhesives:
1. One part PU adhesives
2. Two Part PU adhesives
Sl. No One part PU adhesives Two Part PU Adhesives 1 These are higher molecular weight & requires special type of Organic Solvent to dissolve. Low Molecular Weight polyester resin dissolved in solvents like MEK & Isocyanate 2 Pot Life is longer & exposure to health hazard of solvent & isocyanate is less Pot Life is shorter & more exposure to health hazard of solvent & isocyanate. 3 Colorless Liquid appearance Colorless Liquid appearance 4 Low to Medium Viscosity ( MPa at 20 ºC) Low to Medium Viscosity ( MPa at 20 ºC) 5 Shelf Time is 6 month Shelf Time is 12 month 6 Solid content is 16.5 -18.5% Solid content is 19-20% 7 Specific Gravity is 0.875 Specific Gravity is 0.859 8 Activation Temp is 80-90ºC Activation Temp is 80-90ºC 9 Drying Time 6 -10 min Drying Time 6 -10 min 10 Green Strength is 3 Kg/cm (min) Green Strength is 3 Kg/cm (min) 11 Wear Performance is generally good Wear Performance is generally good 12 Heat resistance is poor Heat resistance is Good

Typical PU adhesive used in industry are
1. 10% solution of triphenylmehtane triisocyanate
2. 4% solution of diphenylmethan isocyanate.
3. Polymethylene polyphenyl isocyanate

PU Dispersion:
The water based PU dispersion are solvent free adhesives, environmentally free from polluting the atmosphere and safe to use and are costlier than other types of PUs. These are prepared by re-acting an isocyanate terminated prepolymer in a water miscible solvent with a diamine carboxylate. The resultant solution is ionomer and is mixed with water, the organic solvent is removed by distillation, then fillers are added and viscosity is adjusted with an acrylic thickener or partially hydrolyzed polyvinyl alcohol or cellulosic thickener.

8. SOLING MATERIALS:

Since 1960 PU is being used as a soling material in the shoe industry.	 Due to its light weight and good abrasion resistance it creates a very good position in footwear industry as a soling material. PU solings are produced from the reaction of polyhydroxyl compound and a di-isocyanate in a process called reaction moulding. It is a sort of polymerization process, compounding process, and moulding process rolled into one.
	 

A PU Sole is made by measuring the ingredients of polyhydroxyl compound and di-isocyanate in liquid form, cross-linking agents, catalysts, silicone, blowing agent, & pigment into a mixing chamber, mixing them, and in the mould, the liquids react to form a polyurethane. The gas is produced during the reaction, and this makes the polyurethane to assume a cellular structure.
The two basic polyhydroxyl compounds are polyester and polyether. The PU cell structures are available in two forms.
a) Micro cellular PU unit structure (polyester)
b) Integral skin PU unit structure (polyether)

a) Micro cellular PU unit structure (polyester):
This cell structure is generally uniform and similar to microcellular rubber, although the cells are, on average, larger. The surface skin is very thin, is not usually interconnected unless the density is very low (below 0.40 mg/m³) and sucking up water is not a problem.
This structure is associated with polyester based Pus, but micro cellular polyether Pus are also produced. This Polyester PUs which can perform well, superior wear and physical proerties but it is difficult to process and expensive.

b) Integral skin PU unit structure (polyether):
This structure is usually consists of polyether based PUs that have comparatively thick outer skin and a low density middle layer. But, the density is low enough, the cells are large and merge into one another, causing them to suck up water. Polyether PUs which can perform reasonably well, these are liquid at room temperature and so are easier to process and is more resistant to hydrolysis.
Advantages of PU soling material:
1. Light in weight
2. Lower wearer fatigue
3. Good insulation
4. Good wear & durability properties
5. Good slip resistance
6. Good resilience
7. Good resistance to flex cracking at low temperature (-10ºC)
8. Better breathability than many of the other synthetic soling material
9. It has low specific gravity (0.4 -0.6 g/cm³). As such of light in weight sit has been chosen for an ideal material for sports shoe’s bottom materials
10. Good oil and solvent resistance
11. Good abrasion resistance and high co-efficient of friction
12. As it is a self adhesive material, the bond failure is very meager (1%)

9. PU UPPERS:

PU Uppers are known as Poromeric (leather like material) and are man-made shoe upper materials that are generally similar to leather and, in particular, have a comparable permeability to water vapour. They are available as PU coated fabric & Solid PU. The surface preparation on uppers consists of scouring until the base fabric is exposed, the material is scoured up to 3mm from the feather edge to prevent peeling back of the upper during water.
Polyurethane Coated Fabrics (PUCFs) consist of a thin solid PU film, an adhesive tie coat, and a raised fabric base, usually 4 x 1 twill weave. PUCFs have more attractive appearance and are permeable.