Silicone Modified Polyesters Vulcanizable at Room Temperature for Anti-corrosion Coatings on Tinplate

GAN Qimao ,WANG Li ,HUANG Long ,HUANG Ronghua＊

(1.School of Power &Mechanical Engineering,Wuhan University,Wuhan 430072,China;2.Engineering Center of New Tobacco Product,China Tobacco Hubei Industrial,Wuhan 430040,China;3.College of Civil Engineeringand Architecture,Zhejiang University,Hanzhou 310058,China)

Abstract: Trimethoxysilyl Polyester (TMOS-PET) was prepared via hydrosilylation of trimethylsilane with acryl-terminated polyester.Copolymerization of TMOS-PET with trimethylmethoxysilane,tetramethoxysilane,and/or dimethyldimethoxysilane give silicone modified polyesters (Si-PET) containing=(SiO2)=,-(CH3SiO3/2)=,and/or -((CH3)2SiO)-units.Coatings from Si-PET were formed on tinplate surface via room temperature vulcanization with applicable curing time (the quickest tact free time and completely drying time are 2 h and 15 h,respectively).Mechanical properties,thermal stability,chemical resistance,and anti-corrosion of coatings were detected.Although the flexibility of the coatings were lowered by=(SiO2)=and-(CH3SiO3/2)=units,adhesion and thermal stability of coatings were both improved.On the contrary,(CH3)2SiOunits seem to improve the flexibility,while decrease the adhesive force for less crosslink points.Generally,the silicone modified polyesters act as an effective shielding for tinplate.According to electrochemical impedance spectroscopy (EIS),the electric resistance of coatings (Rc) were improved about 25 times and open circuit potential (OCP) increased about 0.10 V compared to the control.The chemical resistance and anti-corrosion of the coatings were decreased slightly after=(SiO2)=and -(CH3SiO3/2)=units were added,but they could be greatly improved by -(CH3SiO3/2)=units.Specially,after adding -(CH3SiO3/2)=units,coatings give doubled Rc,and its OCP is about 0.20 V higher than the control.

Key words: silicone modified polyester;coatings;electrochemical impedance spectroscopy;mechanical property;thermal stability

1 Introduction

Silicone materials,which is usually regarded as organic-inorganic hybrid polymers,contain high thermal resistance,high insulation against high electric voltage,high chemical resistance,and high hydrophobicity,

etc

,and are widely adopted as coatings.But for its low molecular interactions,its mechanical properties are considerately poor comparing to other organic polymers.And its giant -Si(CH)groups endow it with high permeation of water and air.On the contrary,polyesters,usually from o-phthalic anhydride and fatty diol,contain distinguished mechanical properties and low permeation of water moisture and air,are applied as package or coatings.Meanwhile,its thermal resistance and electric insulation are relatively poor comparing to silicone.Furtherly,silicone materials contain high film formability and low surface tension that polyesters is devoid of.Then,with the purpose of obtaining perfect coatings with the merits of both,silicone modified polyester have been deeply investigated.There’re many methods for silicone to modify polyester,and the prevailing one is introducing silicone component to polyester by Si-O-C bonds.This is useful in high temperature vulcanization for polyester component will vulcanized over 150 ℃ based on its own method.But in some case that high temperature vulcanization is not convenient,such as large scale of outdoor coatings,or furniture coatings that high temperature treatment usually resulting dimensional instability.In these cases room temperature vulcanization is desired.Silicone can realize room temperature vulcanized via dihydroxy condensation of Si-OH groups.The above prevailing silicone modified polyester could also be cured and form coatings because polyesters could also be involved in the vulcanization system for the chemical bonds (Si-O-C) between silicone and polyester component.But polyester components itself will not be cured.The Si-O-C bonds would be broken under the attack of water moisture,and the polyesters then be dissociated from the vulcanized matrix.According to our experiences,the polyesters dissociated from the vulcanized matrix would aggregate on the surface of coatings,resulting in a sticky surface,accompanied by mechanical deterioration.Early in 1978 it was found that silicone modified polyesters via Si-C bonds are stable against water,and the silicone components was introduced by ring opening of epoxy groups from silanes by carboxyl groups from polyesters.More often condensation of carbon-hydroxyl groups with carboxyl groups is selected to form Si-C modification.Cristiana A

et al

reported that vinyl silicone polymers could form copolymers with unsaturated polyesters(polymers with terminal acryl groups) by means of N,N-dimethylaniline,and they suggested that formation of Si-C links between silicone and polyesters are positive for mechanical properties and thermal stability of blends.But all these researches adopted silicone components as property improving additives,not focusing on room temperature vulcanization,which is necessary for large scale outdoor paitings.

Scheme 1 Synthesis of silicone modified polyester via hydrosilylation reaction

Here we introduce a new method for silicone modification of polyester that could form coatings at room temperatures.The silicone component is introduced to polyester via Si-C bonds by hydrosilylation reaction as Scheme 1.Coatings from this type of silicone modified polyester could be cured on steel surface at room temperature catalyzed by organic tin and a suitable vulcanization time are obtained,which should be applicable for outdoor steel protection.Mechanical properties of coatings,including flexibility (by broken bending radius and broken impact strength),adhesion force,and thermal resistance (by TG),hydrophilicity(by static water contact angle),chemical resistance(acid,salt,and deionized environment) are investigated.Furtherly,we also investigated its possibility as room temperature vulcanized anti-corrosion coatings for tinplate (by electrochemical impedance spectroscopy,EIS).The influences of silicone content on them are also discussed.

2 Experimental

2.1 Materials

Trimethoxysilane,dimethyldimethoxysilane,methyltrimethoxysilane,and tetramethoxysilane,are industrial grade (Hubei Wuda Silicone New Material Co.,Ltd.).o-Phthalic anhydride,methyl methacrylate and 2,2-Dimethyl-1,3-propanediol are kindly granted by Guangzhou Huayu Chem.CO.,LTD.with >99%purity.Platinum catalyst was prepared from commercial chloroplatinic acid hexahydrate with a platinum content not less than 37%.Other chemical agents like vulcanization accelerator dibutyltin dilaurate,hydrosilylation accelerator phenothiazin，methyl methacrylate polymerization inhibitor pyrocatechol,sulfuric acid,sodium chloride,ethyl alcohol,n-Butyl acetate and p-toluenesulfonic acid

etc

,are all analytical reagents(Sinopharm Chemical Reagent Co.,Ltd.).

2.2 Preparation of polyester with terminal acrylate groups

Terminal acrylate polyester was prepared according to literature with minor modifications.Summary,o-Phthalic anhydride (160.26 g,1.1 mol),2,2-dimethyl-1,3-propanediol (104.15 g,1.0 mol) and p-toluenesulfonic acid (1.0 g) placed in a 250 mL 4-neck round type reactor fitted with a condenser for removing the water from esterification were heated to 120 ℃ for 1 h under N.The carboxy-excess pre-polymer was obtained until no water could be found.Then methyl methacrylate (10.10 g,0.11 mol) and pyrocatechol (5 g) were added to the pre-polymer and kept on heating at 90 ℃ and condensing for about 1.5 h to completely remove the generated methanol.Finally the system was distilled at vacuum to remove the residue methyl methacrylate and obtain terminal acrylate polyester (250.02 g,labelled as Acry-PET).

2.3 Preparation of terminal trimethoxysilylated polyester

Acry-PET (100 g in 70 mL n-Butyl acetate,about 0.04 mol acrylate groups),Pt catalyst (50 ppm of Pt)and phenothiazine (0.5 g) were mixed together and heat to 85 ℃ under N,then trimethoxysilane (6.11 g,0.05 mol) was added dropwise within 1.0 h.The system was kept at this temperature for 2.0 h,and then another pyrocatechol (5.0 g) was added before the residue trimethoxysilane and solvent n-Butyl acetate were removed by vacuum distillation to obtain trimethoxysilylated polyester (104.3 g,labeled as TMOS-PET).

2.4 Preparation of silicone modified polyester

TMOS-PET (40 g,about 0.020 mol terminal trimethoxysilyl groups),determined quantity of methytrimethoxysilane and tetramethoxysilane were stirred at room temperature.Then deionized water with 0.375 mol ratio to the whole methoxy groups (including not only that of TMOS-PET,but also that of methytrimethoxysilane and tetramethoxysilane) were added within 0.5 h to partly hydrolyze methoxy groups under catalyzing of HCl acid.Finally the system was vacuum distilled at 40 ℃ to remove the generated methanol.The product was labelled as Si-PET-1 to 4 with different feed ratio of TMOS-PET,methytrimethoxysilane and tetramethoxysilane (Table 1).Furtherly,-Si(CH)units,usually regarded as a typical components of organic silicon materials,were also fed to silicone modified PET,for the improvement of anti-corrosion of coatings(labelled as Si-PET-DM).

2.5 Coatings on tinplate surface from Si-PET

Tinplate was polished and then cleaned with ethanol according to Chinese standard GB/T1927-1992.10 g of Si-PET solution in n-Butyl acetate (about 50 wt%)was mixed with dibutyltin dilaurate (0.5 g in 2 mL n-Butyl acetate) and a more additional of 2.0 wt% aminopropyl triethoxysilane as accelerator,then solutions are casted by brushing in tinplate surface for two times to obtain about 0.2 mm thickness.The tinplate was placed in oven with controlled relative humidity (40%-70%) at room temperature to obtain cured coatings.The tack free time and completely drying time were obtained according to GB/T1728-79 (88).

2.6 Methods

FT-IR of samples were measured on a NICOLET 5700 FT-IR spectrometer (Thermo Electron Corporation,America);The specimens were scanned at a wave number range within 4 000-400 cm;flexibility of coatings were evaluated by broken bending radius(according to Chinese standard GB/T1731-93 by QTX painting film flexibility analyzer from Shanghai Meiyu Instrument Co.,LTD) and broken impact strength(according to Chinese standard GB/T1732-93 by QCJ painting film impact analyzer from Tianjin Kexin Instrument Co.,LTD).Adhesion force of coatings were analyzed according to Chinese standard GB/T7124-1986 by CMT6503 mechanical test machine from MTS,Shenzhen,China.Thermal stability were evaluated by thermal gravity analysis on a Shimadzu DSC-40 Differential Scanning Calorimeters from room temperature to 600 ℃ at a heating rate 10 K/min under both nitrogen and air.Chemical resistance of coatings to different environment were carried out according to different Chinese Standard (GB/T9274-88 for 3.0%HSO,GB/T9274-88 for 3.0% NaCl,and GB/T1733 for deionized water environment).Summarily,coatings were immersed in different environment,the broken immersing time (time when they are broken) were recorded for evaluation of their resistance to the chemical environment.Static water contact angles measurement was carried out in an OCA201 unit from Dataphysics Co.with 2.000 uL water drops,and for each sample,the contact angle data obtained from five individual drops were averaged.

Anti-corrosion of coatings were evaluated with electrochemical impedance spectroscopy (EIS) by CS electrochemical system from Wuhan Corrtest Instrument CO.,LTD.The EIS analysis was done in a conventional three-electrode cell including saturated Pt|Hg(l)|HgCl(s)|KCl as reference electrode,Platinum as counter electrode and coated tinplate specimen as working electrode.The measurements were done at open circuit potential (OCP) in the frequency range of 10 kHz to 10 mHz (peak to zero) and at 10 mV amplitude sinusoidal voltage.EIS analysis was carried out on 1.0 cmof coated tinplate immersed in the 3.0 wt%NaCl solution with stable OCP.The measurements were implemented 3 times to ensure the repeatability of the measurements.

3 Results and discussion

3.1 Structure of reaction products

Fig.1 FT IR spectra of Acry-PET,its silylated product TMOSPET,copolymers Si-PET-3 and Si-PET-DM with methoxysilanes as co-polymerized component

Fig.1 presents the FT IR spectra of Acry-PET,TMOS-PET,and Si-PET-3.The strong absorptions at 3 400 and 1 720 cmare assigned to the stretching vibration of -OH and C=O.Absorption at 983 cmand 1 640 cmare assigned to the bending vibration of -CH=CH.After silylated by trimethoxysilane,absorptions at 983 and 1 640 cmare obviously weakened.Simultaneously,new absorptions at 2 840 cm(assigned to the stretching vibration of Si-OCH) and 2 160 cm(assigned to the stretching vibration of Si-H) could be observed.These indicate that trimethoxysilyl groups are indeed bonded to Acry-PET via hydrosilylation reaction,even there’s residue of Si-H,acryl,and -OH in the TMOS-PET.In case of -OH groups,few variations at 3 400 cmcould be observed.Moreover,double absorption at 1 075 and 1 125 cm(usually assigned to the C-O-C of PET) become broad,which is usually assigned to the formation of Si-O-C.After copolymerization with silanes,this absorption becomes broader for the formation of Si-O-C as reported.DMSi-PET is feed with dimethyldimethoxysilanes,whose FT IR spectrum clearly presents a new absorption at 802 cm.This absorption could be assigned to antisymmetric stretching Si-C and rocking of CHin polydimethylsiloxanes.

3.2 Curing of Si-PET coatings

With organic tin as catalyst,Si-PET could be cured by absorbed moisture from air at room temperature.Fig.2 presents the tact free time and completely drying time for Si-PETs with pure PET as reference.As Fig.2,the curing time of Si-PET is greatly dependent on Si-OCHgroups.Pure PET contains no Si-OCH,so it could not be cured even for a long time (we observed for about 3 month).Curing of Si-PET become quicker from Si-PET-1 to 4,in consistent with that more methoxy-silane are fed and more Si-OCHare left.The fast quick curing sample Si-PET-3 and Si-PET-4 gives tact free times at 3.5 and 2 h,completely drying time at 21 and 15 h,which is applicable for coatings.For Si-PET-DM,which contains -Si(CH)units and then is more hydrophobic,moistures are difficult to be absorbed,whose curing speed are lower even than Si-PET-1.

Fig.2 Tact free time and completely drying time for Si-PET catalyzed by dibutyltin dilaurate,aminopropyl triethoxysilane as accelerator,at room temperature and 40%-70% humidity.Acry-PET will not be cured as founded.

3.3 Mechanical properties of coatings

Fig.3 presents the mechanical properties of coatings from Si-PET-1 to 4.The flexibilities of coatings are evaluated by both broken bending radius and broken impact force,and tinplate surface bonding strength are evaluated by adhesion force.It could be found that the flexibility and bonding strength are both dependent on Si-OCH.From Si-PET-1 to 4,as Si-OCHcontents increase,the broken bending radius increases and broken impact force bending decreases.They mutually conform that the flexibility of coatings are deteriorated.But the adhesion force of coatings to tinplate increases with Si-OCHcontent (Fig.3).Silanes copolymerized with TMOS-PET in this research are three or four functionalized,and after curing,they will form a great deal of crosslink points,making the coatings not so flexible.But these Si-OCHgroups can be hydrolyzed to form Si-OH groups,which react with surface Sn-OH or Fe-OH groups of tinplate.The reaction between Si-OH and Sn-OH or Fe-OH will generate chemical bonds,Si-O-Sn or Si-O-Fe,between coatings and tinplate.So adhesion force increases with content of Si-OCHgroups.(CH)SiO-units seems to improve the flexibility,while decreases the adhesive force for less crosslink points.

Fig.3 Mechanical properties of coatings from Si-PET,with broken bending radius and broken impact force for flexibility evaluation,and adhesion force for tinplate bonding strength

3.4 Thermal stability of coatings from Si-PET

Fig.4 and Table 2 present the thermal gravity of coatings from Si-PET-1 and Si-PET-3.Si-CHis reported to be degraded over 300 ℃.Polyester also give obvious thermal gravity loss over 350 ℃.The minor gravity loss below about 250 ℃ might be assigned to the condensation of residue active groups like Si-OH and Si-OCH.It could be found that no matter in nitrogen or air environment,the high silicone content coatings that from Si-PET-3 give high

T

,

T

,and

Y

,which indicates that it shows high thermal stability.Silicone materials are regarded as organic-inorganic hybrid and with high thermal stability than PET,and then introducing silicone to polyester is reasonable to improve its thermal stability.

Fig.4 Thermal gravity analysis of coatings from TMOS-PET and Si-PET-3 under air or nitrogen from 40 to 600 ℃

Table 2 5%weight loss temperature (),maximum weight loss temperature () and 600 ℃ char yield () of Si-PET-1 and Si-PET-3 drawn from Fig.4

3.5 Chemical resistance of coatings from Si-PET

Fig.5 Chemical resistance of coatings in different environment for Si-PET-1 to 4,as well as for Si-PET-DM.Static water contact angles also presented.

Fig.6 Nyquist plot of tinplates coated with silicone modified PET,comparing to that of uncoated tinplate.The tinplates were immersed in 3.0 wt% NaCl solutions for 30 min before test.

Chemical resistance,as well as static water contact angles,of coatings from Si-PET-1 to 4 are presented in Fig.5.It could be observed that no matter in what kind of environment,the broken immersing time decreases with content of Si-OCH.This indicates that chemical resistance of coatings become weaker.Usually organic-silicon materials contain high resistance against water,salts,and diluted acid or base for its high hydrophobicity,though they give high chemical permeation.But in this research,silanes for copolymerization is not that typical organic-silicon which contains mainly dimethylsiloxane units.The typical organic-silicon materials contains high hydrophobicity because of the low polarity of Si-CHgroups and high rotatability of Si-O backbone that is convenient for Si-CHgroups to modulate its conformation to cover the polar Si-OSi.The three or four functionalized silanes in this research contains less hydrophobic Si-CHgroups,and it rotatability is limited for the high crosslink points,so,the hydrophobicity of coatings is not so distinguished as the typical organic-silicon materials,and more additional silanes does not increase the water contact angles(Fig.5).Organic-silicon materials contain large intermolecular space for chemicals permeation,then in aqueous environment,the chemical resistance decreases with silane content.A simple method that introducing dimethylsiloxane units in to the coatings will well resolve this problem as reported.As Fig.6 confirms,after -Si(CH)were introduced,the static water contact angle of Si-PET-DM were greatly increased to near 90°.Subsequently,its resistances against HSO,NaCl salt,and water greatly increase for broken immersing time also increases.

3.6 Anti-corrosion of coatings from Si-PET

Nyquist plot of tinplate with coatings from Si-PET-1 to 3 are obtained through three-electrode cell in 3.0 wt% NaCl solution (Fig.6).Before tests,the tinplate cells were immersed in solution for 3.0 h.

Most coated tinplate just presents one,not two capacitive arc,which means that the corrosion medium cannot penetrate the coatings to the surface of metal.Then,electrochemical impedance spectroscopy was modelled with the following equivalent circuit in Fig.6 (

R

,resistance related to the coating;

R

,resistance related to solution;

CPE

,the constant phase element).The line is that fitted by Autolab software according to electric equivalent circuit (Fig.1) with Adj.R-Square>99.0%,except that of Si-PET-DM,with Adj.R-Square>95.0%Parameters like

CPE

(

CPE-T

and

CPE-P

) and

R

c from modelling are present in Table 2,as well as

OCP

from testing.The values of

R

s obtained are considerably low comparing to

R

c,which means that solution resistance could be neglected,so we do not give its value in Table 2.Comparing to the control,real and imaginary impedance (Z′ and Z′′),

OCP

and

R

c of coated tinplate (in Table 2 and Fig.6) are greatly increased.

R

c of coated tinplates were improved about 25 times and

OCP

increased about 0.10 V.This means that the coatings is an effective shielding for tinplate.

Fig.7 Equivalent electric circuit for electrochemical impedance spectroscopy model

As Table 3 and Fig.6 indicate,the coating shielding effectiveness declines with Si-OCHcontent.

Z

′,

Z

′′,and

R

c and impedance decrease as more tetramethoxysilane and methyltrimethoxysilane were fed.This is similar to that of chemical resistance.As touched,silanes used for copolymerization in this research is not that of typical organic silicon materials,more additional of them will make the coatings more hydrophilic and increase the permeation of chemicals,then lower the

R

c.More Si-(CH)units to the coatings could solve this problem like the same method for chemical resistance.Si-PET-DM contains silane with the samecontent to Si-PET-3.But it was fed with dimethyldimethoxysilane,not methyltrimethoxysilane or tetramethoxysilane (Table 1).This will introduce -Si(CH)units to the coatings and increase the hydrophobicity and chemical resistance of coatings as Fig.5 indicates.Comparing to Si-PET-1,even with high silane contents,Si-PET-DM gives doubled impedance and

R

c,higher

OCP

.

Fig.8 Time dependences of Nyquist plots for Si-ET-1 and SI-PETDM and OCP for tinplate coated with Si-PET-1 and Si-PET-DM in 3.0 wt% NaCl solutions

Furtherly,we check the time dependence of anti-corrosion of coatings from Si-PET-1 and Si-PET-DM(Fig.8).OCP of Si-PET-DM are about 0.10 V higher than Si-PET-1,and its impedance are higher at all testing time.Moreover,this phenomenon become more and more obvious as immersing time increases.This means that additional -Si(CH)units could considerably improve the anti-corrosion of silicone modified PET coatings.

4 Conclusions

Silicone modified polyesters (Si-PET) containing=(SiO)=,-(CHSiO)=,and/or -((CH)SiO)-units were prepared.Coatings from Si-PET were formed on tinplate surface via room temperature vulcanization with applicable curing time (completely drying with less than 20 h).Generally,the silicone modified polyesters acted as an effective shielding for tinplate.Although the flexibility of the coatings were lowered by=(SiO)=and -(CHSiO)=units,adhesion and thermal stability of coatings were both improved.Even chemical resistance and anti-corrosion performance of coatings were slightly lowered by=(SiO)=and -(CH-SiO)=units,they could be greatly improved by -((CH)SiO)-units.(CH)SiO-units also improve the flexibility,while decrease the adhesive force for less crosslink points.