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Influence of Diisocyanate Structure on the Synthesis and Properties of Ionic Polyurethane Dispersions
M. Barikani a ; M. Valipour Ebrahimi a ; S. M. Seyed Mohaghegh a a
Department of Polyurethane, Fibers and Nanomaterials, Faculty of Science, Iran Polymer and Petrochemical Institute, Tehran, Iran Online Publication Date: 01 November 2007
To cite this Article Barikani, M., Ebrahimi, M. Valipour and Mohaghegh, S. M. Seyed(2007)'Influence of Diisocyanate Structure on the
Synthesis and Properties of Ionic Polyurethane Dispersions',Polymer-Plastics Technology and Engineering,46:11,1087 — 1092To link to this Article: DOI: 10.1080/03602550701525214URL: /10.1080/03602550701525214
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Influence of Diisocyanate Structure on the Synthesis and Properties of Ionic Polyurethane Dispersions
雅思英语M.Barikani,M.Valipour Ebrahimi,and S.M.Seyed Mohaghegh
Department of Polyurethane,Fibers and Nanomaterials,Faculty of Science,Iran Polymer and Petrochemical Institute,Tehran,Iran
A number of aqueous polyurethane dispersions bad on polyte-tramethylene glycol (PTMG),1,4-butanediol (1,4-BDO),dimethy-lol propionic acid (DMPA)and diisocyanates of differing structures such as toluene diisocyanate (TDI),diphenylmethane diisocyanate (MDI),isophorone diisocyanate (IPDI),and dicyclohexylmethane diisocyanate (H 12MDI)were prepared.IR Spectroscopy was ud to check the end of polymerization reaction and also the polymer characterization.The effects of diisocyanate structure on the par-ticle size,contact angle,mechanical and thermal properties of the emulsion-cast films were studied.Average particle size of prepared polyurethane emulsions change by different diisocyanate bad poly-urethane.TDI bad PU shows higher average particle size and contact angle than the others.Tensile strength,hardness,and elon-gation at break were higher in the ca of MDI bad polyurethane.Thermal property and thermal stability is also affected by variation of diisocyanate molecular structure.
Keywords Diisocyanates;Ionomers;Particle size;Polyurethane
dispersions;Thermal propertiesbruna
1.INTRODUCTION
Waterborne polyurethanes have been studied for over 40years and their performance improved has
continu-ously [1–4].The polymers are very interesting industrial products,which have gained substantial commercially importance in recent years due to their wide range of appli-cations.This is becau of environmental concerns regard-ing volatile organic compounds as solvents;the economical aspect,as aqueous polyurethanes do not contain the expensive solvents;and modified aqueous polyurethanes are comparable or better than the conventional solvent–bad polyurethanes for many applications [4–10].
Aqueous polyurethane dispersions can be divided into ionic and nonionic types.Nonionic types contain hydro-philic soft gment pendant group such as polyethylene oxide [11,12],whereas the ionic types contain anionic or
cationic centers in the polymer chains.The centers could be pendant acid or tertiary nitrogen groups which are neutralized to form salts [13].rebel
lect
It is now generally accepted that the properties of PU ionomers are primarily due to the pha behavior of soft and hard gments as well as their ionic character.Since ions are introduced into either hard or soft gments and are imparting many properties to the polyurethane matrix;the attention is being drawn to the polyurethane ionomers [14,15].For the synthesis and preparation of
polyurethane dis-persions veral process have been developed [16–19].All of the process are common in the first step,in which a medium molecular weight prepolymer is formed by the reaction of suitable polyols with a molar excess of diisocya-nates.The prepolymer is then chain-extended with an inter-nal emulsifier either before or more usually after dispersion in water.The internal emulsifier becomes part of the main chain of the polymer.One of the most important process for the synthesis of polyurethane dispersion is the melt dis-persion process.In the melt dispersion process,the hydro-philically modified chain extended polymer is directly mixed with water.There are a number of diisocyanates that have been ud in the preparation of such PUs [20–23].Micro-pha paration due to soft polyol and hard urethane quences can contribute to the unique properties of PU ionomers.Factors affecting this micropha paration include the symmetry and type of diisocyanate,the molecular weight and type of polyol,and ionic group content [24].
In continuation of our interest to prepare ionic poly-urethane dispersion [25],we report the synthesis and proper-ties of ionic polyurethane dispersions containing different diisocyanate structures in the polymer backbone.The diisocyanates ud in this study were toluene diisocyanate (TDI),diphenylmethane diisocyanate (MDI),isophorone diisocyanate (IPDI),and dicyclohexylmethane diisocya-nate (H 12MDI).The influence of diisocyanate structure on the physical,mechanical,and ther
mal properties of emulsion-cast films,contact angle,glass transition tem-perature,and particle size distribution were studied.
Address correspondence to M.Barikani,Department of Polyurethane,Fibers and Nanomaterials,Faculty of Science,Iran Polymer and Petrochemical Institute,P.O.Box 14965=115,Tehran,Iran.E-mail:m.barikani@ippi.ac.ir
Polymer-Plastics Technology and Engineering ,46:1087–1092,2007Copyright #Taylor &Francis Group,LLC ISSN:0360-2559print/1525-6111online DOI:
10.1080/03602550701525214
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2.EXPERIMENTAL
2.1.Materials
Polytetramethylene glycol(PTMG),(Mn¼2000)(Arak Petrochemical Company)was dried and degasd at80 C, and1–2mm Hg for5h before u.1,4-butanediol(1,4-BDO)(Merck)was dried and degasd for2h at50 C and1–2mm Hg.Dimethylol propionic acid(DMPA) (Aldrich)was dried at100 C for2h in an oven.Dimethyl-formamide(DMF)(Merck),and triethylamine(TEA) (Merck),were dried over molecular sieves(4A˚),Toluene diisocyanate(TDI)(Merck),diphenylmethane diisocyanate (MDI)(Aldrich),isophorone diisocyanate(IPDI)(Merck), and dicyclohexylmethane diisocyanate(H12MDI)(Aldrich) was ud as received.
2.2.Preparation of Polymer
The polymer reaction apparatus consisted of a250-ml round bottom,four-necked flask with a mechanical stirrer, thermometer,condenr,nitrogen inlet,and pipette outlet. Reaction was carried out
in a constant temperature oil bath.PTMG was weighed out into a clean,dry reaction flask.The reaction temperature was t at90 C.The diiso-cyanate was added slowly to the reaction mixture by a dropper funnel with stirring of PTMG.The mixture was heated at90 C for2h to obtain NCO terminated prepoly-mer.After the solution of DMPA in DMF was added and stirring continued for1h,the temperature was lowered to 60 C.1,4-butanediol was added at this temperature and after15min the neutralizing solution TEA dissolved in DMF was charged into the reactor.The reaction scheme for the prepolymer preparation and the process of disper-sion and chain extension are shown in Fig.1.Samples were prepared by different diisocyanate structure with constant ionic content and chain extender as shown in Table1. 2.3.Dispersion
Aqueous dispersion of PU was obtained by adding deio-nized water(30 C)to the mixture(60 C).The temperature was gradually decread by addition of water.The rate of water addition is very important and is a critical parameter to obtain stable dispersion.Water should be added with
a
FIG.1.Formation of PU dispersion with anionic center.
1088M.BARIKANI ET AL.
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dropping funnel or dosing pump at a constant flow rate.The pha inversion occurred and the dispersion was obtained.The dispersion was then allowed to stir for a further 30min.The emulsion was stable nearly for 5months at room temperature after preparation.
2.4.Preparation of Films
Films were prepared by casting the aqueous dis-persion onto a Teflon plate at room temperature,followed by drying at 40,60,70and 85 C (one day for each tem-perature).Then the films were stored in a desiccator at room temperature for further characterization and measurements.
2.5.Measurementsgfy
中国历任第一夫人Particle size and its distribution were measured by lar light scattering (SemaTech,SEM-633,He-Ne lar)at room temperature.The samples were first diluted by deionized water to 0.5%and the dispersions were homogenized.
The IR spectrum of the PU polymer films were obtained with an EQUINOX55Fourier transform infrared (FT-IR)spectrometer from Bruker by H.ATR accessories equipped with ZnSe crystal over the range 4000–400cm À1at room temperature.Contact angle was measured by Gl0(KRUSS)instrument through ssile drop method with water as a liquid.
The thermogravimetric analys (TGA)was carried out by using a thermogravimetric analyzer (Perkin Elmer Pyris 1)at a heating rate of 10 C min À1in a nitrogen atmosphere from room temperature to 700 C.The sample weight was 7–9mg in all cas.
The tensile properties of the emulsion cast films were measured by using MTS 10=M tensile testing machine according to ASTM412at a crosshead speed of 50mm min À1.An average of at least five measurements was taken and the 1-KN load cell was ud.
3.RESULTS AND DISCUSSION
Preparation of aqueous polyurethane dispersion bad on polytetramethylene glycol (PTMG),dimethylol propi-onic acid (DMPA),1,4-butanediol (1,4-BDO)and different diisocyanate structure was achieved.The polyurethane was prepared by reaction of stoechiometric amount of polyol =diisocyanate =chain extender with block ratio of 1:3:2according to synthetic route as depicted in Fig.1.The polyol molecular weight (Mn)of 2000was ud with differ-ent diisocyanates such as TDI,MDI,IPDI and H 12MDI.DMPA,a carboxylic group containing diol,was ud to form water dispersible urethane prepolymer without any significant reaction between the carboxylic and isocyanate group becau the hydroxyl group is much more reactive than the carboxylic relative to isocyanate component.3.1.IR Spectroscopy
IR spectroscopy obtained from the cast film is shown in Fig.2.This analysis was ud to check the ending of polymerization reaction,verifying the disappearance of the v NCO at 2265cm À1and the appearance of N ÀH at 3000–3400cm À1.Prence of expected peaks implies that the reaction is completed and the predesigned polyurethane is formed.IR spectra also contained all the information relating to the primarily structure of the final polymer.For of the N ÀH stretching mode an absorption band at 3291cm À1was obrved.Aliphatic C ÀH stretching mode at 2795–2938cm À1and carbonyl (C =O)stretching absorp-tion band was obrved at 1726cm À1.N ÀH bending vibra-tions at 1532cm À1,C ÀO ÀC stretching absorption band corresponding to the ether oxygen of soft-gment was obrved at 1000–1150cm À1.The vibrations are strong evidence for the formation of PU.
TABLE 1
Feed compositions (g)of dispersion polyurethanes with various diisocyanate
Diisocyanate
Sample PTMG (Mn)(2000)
催眠培训TDI MDI IPDI H 12MDI DMPA 1,4BDO TEA SI129.5–12.5–– 3.350.75 2.52SI229.58.84––– 3.350.75 2.52SI329.5––11.59– 3.350.75 2.52SI4
29.5
–
–
–
13.10
3.35
0.75
2.52
TABLE 2
The variation of particle size and contact angle with different diisocyanate structures in dispersion polyurethane Sample Diisocyanate Particle size
(nm)
Contact angle
( )
SI1MDI 6583SI2TDI 7486SI3IPDI 3964SI4
H 12MDI
37
77
INFLUENCE OF DIISOCYANATE ON POLYURETHANE DISPERSION
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The N ÀH group in polyurethane could form hard-hard gment H-bonding with the carbonyl oxygen and hard-soft H-bonding with the ether oxygen.The stronger hard-hard gment H-bonding acts as p
hysical cross-link leading to difficult gmental motion of the polymer chain which results in a more significant pha paration between hard and soft gments.The pha paration improves mechan-ical properties of polyurethanes but reduces the flexibility and solubility of the polymer [4,5].
3.2.Effect of Diisocyanate Structure 3.2.1.Particle Size
The effect of diisocyanates structures on particle size of the PU dispersions are shown in Fig.3.For the samples,the NCO =OH ratio was kept constant.The particle size decreas from 74nm for TDI to 37nm for H 12MDI.The other diisocyanate-bad polyurethanes remained in between the two values (Table 2).This may be due to intimate pha mixing of TDI bad PU compared with
pha paration of symmetrical H 12MDI bad poly-urethane.Pha mixing of polyurethanes make it difficult for anionic centers to leave the surface during emulsifi-cation.Each particle in the dispersion is surrounded by a thin layer of water due to the prence of hydrophilic ÀCOO ÀHN þ(C 2H 5)3groups on the surface of the particles.With a decrea of the size of the polymer disper-sion particles,the relative size of the water layer to total particle size is incread.With decrea in particle size,there is also an increa in the number of particles [26,27].The aqueous polyurethane dispersions are two pha systems,in which water is the continuous pha and the disperd pha
is a solid polymer,although the obtained aqueous polyurethane dispersions emed to be clean solu-tions.It is well known that the particle size has a direct effect on the polyurethane dispersion stability and that the larger average particle size (>1000nm)is generally unstable with respect to dimentation [4].It is reasonably presumed that the anionic centers are hydrophilic in nature and predominantly located on the surfaces of PU particles in aqueous media.However,deformation of hydrophobic-hydrophilic spherical structure in water is less possible with decrea in pha paration and the lead to a larger par-ticle size at the stage of emulsification.
3.2.2.Contact Angle
The contact angle measurements of polyurethanes with different molecular structure of diisocyanates are shown in Fig.4.There is a difference in contact angle between differ-ent diisocyanate bad PUs,decreasing from 86for TDI to 64for IPDI bad polyurethane (Table 2).The carboxylic ion of DMPA in the polymer is a hydrophilic group and rves as an internal emulsifier for polyurethane dispersions.Coulom-bic forces between ionic centers and the decread contact angle is ascribed to the incread hydrophilicity of
the
FIG.2.IR spectrum of polyurethane
ionomer.
FIG.3.Particle size of PU ionomers as a function of diisocyanate
structure.FIG.4.Contact angle of PU ionomers as a function of diisocyanate structure.
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