The Synthesis of VinyloxyacetateâSiloxane Copolymer and Its

Chapter 29

The Synthesis of Vinyloxyacetate-Siloxane Copolymer and Its Absorption onto Ε Glass Fibers

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Rosalind P. M a , Christopher Le-Huy, Leanne G. Britcher, and Janis G . Matisons Ian Wark Research Institute, University of South Austraila, Mawson Lakes Campus, SA 5095, Australia

Polyvinyl acetate is used as a film former in sizing solutions used to coat glass fibers and is often chosen for its availability, price, and the ability to modify its properties. The 1-[(2vinyloxy)ethoxy]ethylacetate monomer was synthesised by Zhang, which could be polymerized at room temperature in the presence of the Lewis acid, zinc chloride, to form a highly branched polymer with remnant vinyl groups. The polymer was then hydrosilated with triethoxysilane and a polysiloxane. The triethoxysilane/vinyloxyactate hyperbranched polymer was applied to the Ε glassfiberand SEM, DRIFT and XPS was used to analyze the adhesion.

Fiber sizes play a critical role in the manufacture of glassfiberreinforced composites. The size is applied to the glassfiberfilamentsonline after they have been formed and cooled to protect them during subsequent processing as well as meeting various requirements set by both the glass fiber and the composite manufacturers. A size usually consists of a coupling agent, film former(s), lubricant, surfactant and an antistatic agent (see Table I). Further additives may be required depending on the characteristics of the final composite (/). Among the components of the size, coupling agents and film formers are considered to be critical to the success of, not only the manufacture of the glassfiber,but also of the composite. The film former must bind the glass fiber filaments into strands, impart the required handling characteristics, and protect both the filaments and the strandsfromdamage during the manufacturing process.

© 2003 American Chemical Society

In Synthesis and Properties of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

341

342


Table L Silane Agent Ingredients Amount in Water Ingredients 3.5-15.0% Polymer Film Former(s) incl. plasticizer 0.1-0.3% Lubricants 0-0.3% Antistatic Agents 0.3-0.6% Silane Coupling Agent

Polyvinyl acetate (PVAc) is commonly used as the film former in size formulations (l).lt is chosen for its cost, availability and ease of modification of its properties to give the required final effect. PVAc is formed by free radical polymerization of emulsified vinyl acetate in water and the molecular weight or particle size can be controlled. Generally the higher the molecular weight the greater the cohesive strength and the higher the softening point of the film former. However, at the same time the particle size can not be too large as this will make it difficult for the particles to penetrate thefiberbundles. Additives in the emulsion as well as in the size formulation can also affect the characteristics of the film former. For example, plasticizers, which are generally added to improve itsflexibilityand thermoplasticity, can also affect the softening point of the film former. Development in film former technology over the years has resulted in a vast range of film formers, including copolymers with PVAc, to meet the needs of customers. When glass fibers are incorporated into polymer resins the film former chosen should also be soluble in it. During impregnation of the resin the film former dissolves into the resin, leaving behind the silane coupling agent on the fiber. The coupling agent, on the other hand, plays a very different role in the glassfibersizing process. Glass fiber reinforced composites in the early days suffered from delamination, due to diffusion and reaction of water at the glass-matrix interface (2-4). Coupling agents were introduced to eliminate this problem by generating a water-resistant interface between the inorganic reinforcing fiber and the organic matrix. If designed properly, coupling agents can also react with groups of the polymeric resin in the composite, and become strongly bonded during the curing process (5-9). For more than a decade, we have investigated the adhesions of silanes and siloxanes onto Ε glass fibers (10-13). Silanes may interact with the glass fiber surface initially through hydrogen bonding after which, condensation reactions at the glass surface or with each other, generates a siloxane structure. Functionalised siloxanes, like silanes, are also capable of adhering to the glass surface (11). Britcher, et. al. successfully hydrosilated vinyltrimethoxy silane or vinyltris(2-methoxyethoxy)silane onto siloxanes containing SiH groups (12). The adhesion of these siloxane coupling agent analogues to glass fibers was compared with its silane counterpart as well as the OH terminated polydimethylsiloxane (PDMS). They proved that the interactions of the



343

siloxanes were similar to that of the silane, and both showed greater interaction than that of the PDMS. Additionally, Provatas (1998) showed that siloxanes, functionalized with amino groups, also helped improve adsorption of siloxanes to glass surfaces, despite having no hydrolyzable groups (13). Conventional silane coupling agents form bonds to the oxide surface which are hydrolyzable, and the rate of hydrolysis increases dramatically with both increasing temperature and pH (12). A long flexible polymer backbone, such as a siloxane however, may help to prevent water ingress, and the concomitant rupture of hydrolyzable silane oxide bonds. Siloxanes, unlike silanes, can be multifunctional, with groups along the polymer chain, which are, not only used to attach the polymer to the oxide or glass surface, but also to improve the hydrophobicity and polarity of the treated surface and thereby improve the strength and durability of the coating. Additionally, the long flexible siloxane backbone allows for a continuous coverage of the surface, as the siloxane polymer is flexible enough to adjust to the flaws of the oxide surface. 3ROH

3ROH

)

)

XSi(OR) —

RSi(OH)

3

S i - O - S i - 0 ~ Si-

3

H 0

2

OH

OH +

OH 3H 0

X

X

X

2

OH

OH

OH

Substrate X

X

HO-Si-0

Α Η

2H 0 2

A

λ HH

O

X S i - O — -SSii--O OH H

O

HH

V W

X HO-Si-0

X

J

Η — — •

?

?

O

Substrate

X

S i - O — Si-OH

! ! Η--Ο

L Substrate

Figure L Hydrolysis of silanes on glass surfaces Silane hydrolysis (see Figure 1) is thought to proceed in two steps. The first, the displacement of alkoxy groups from Si-OR bonds (or alternatively halogen groups from Si-X, where X = CI, Br, or I) by water to form Si-OH bonds. Condensation of Si-OH bonds, forming Si-O-Si (siloxane) bonds, then occurs in a second slower step.



344 Nevertheless, as most silane coupling agents are applied from aqueous solution, some condensed 'siloxane type species' are thought to be present. Given that perspective, we thought to examine whether the functions of both film former and coupling agent could be combined by using a suitable multifunctional siloxane polymer, containing some functional groups that can attached to the glass surface, and other functional groups that can associate with the composite matrix. Since earlier work has shown that siloxanes can adhere to glass surfaces if appropriately functionalized, the focus of this paper is to look at attaching the film former, PVAc to the coupling agent. Two products were synthesized, PVAc polymer was attached to triethoxysilane and similarly the same PVAc polymer was also attached to polydimethylsiloxane. The triethoxysilane product was applied onto the glassfiberand the coating was then characterized by Scanning Electron Microscopy (SEM), Diffuse Reflectance Fourier Transform Infrared (DRIFT) spectroscopy, and X-ray Photoelectron Spectroscopy (XPS).

Experimental Materials Ethylene glycol divinyl ether (Aldrich), triethoxysilane (Aldrich) and zinc chloride in 1.0M solution (Aldrich) was used as received. Acetic acid (Aldrich) was dried azeotropically with toluene, and toluene (Ace Chemicals) was dried over sodium and benzophenone. Hexachloroplatinate hexahydrate (IV) (Sigma, 40wt.% Pt) was made to a 1% solution in diried THF (distilled from sodiun^enzophenone).

Instrumentation Nuclear Magnetic Resonance (NMR) Spectroscopy. l

13

H nuclear magnetic resonance (NMR) and C NMR spectra of all products was obtained with a Varian Gemini Fourier Transform NMR Spectrometer (200MHz). Samples were prepared in dilute deuterated chloroform (Cambridge Isotope Laboratories) with an internal standard at 7.25ppm for Ή NMR and 77ppm for C NMR. ,3

Gel Permeation Chromatography (GPC). A Waters 2690 Separation Model equiped with a differential refractometer, Model Rl 2410, detector was used to determine molecular weight (M ) of the w


345

polymer. The column used was a Waters Styragel HR 5E, 7.8 χ 300mm column, with THF as the solvent and polystyrene was used as linear standards.

Diffuse Reflectance Fourier Transform Infrared (DRIFT) Spectroscopy. DRIFT spectra were measured at room temperature analyzed with a single beam Nicolet Magna Model 750 Spectrometer in the wavenumber region 4000650cm" , with the use of a MCT-A liquid nitrogen cooled detector. The interferogram was apodized using the boxcar method installed in the FTIR software (OMNIC FTIR software version 2.0). Signal to noise ratio is generally better than 100 - 1, using 256 scans. The fibers were mounted parallel to each other in a specially constructed sample holder, which was placed in a Spectra Tech diffuse reflectance apparatus [10]. Each glassfibersample was scanned 256 times, with the fibers at an angle of 90°C, with respect to the direction of the infrared beam, for maximum signal to noise ratio. The background spectrum was taken from high purity (IR) grade KBr powder (Merck), placed in a sample cup and leveled to the top of the cup using a spatula. No pressure was applied to the KBr powder in the cup, when packing or leveling.


1

Scanning Electron Microscopy (SEM). A Cambridge stereoscan 100 SEM was used for scanning electron micrographs. The treated fibers were coated with a thin (~ 200 Â) evaporated carbon layer to reduce the effects of charging.

X-ray Photoelectron Spectroscopy (XPS). The XPS data were analyzed using a Perkin-Elmer PHI 5100 XPS system with a concentric hemispherical analyzer and a MgKcc X-ray source functioning at 300W, 15kV, and 20mA. High vacuum pressure achieved during analysis varied from 10* to 10' Torr. The angle between the X-ray source and the analyzer was fixed at 54.6°. Surface charging was corrected to the adventitious carbon Is peak (284.6eV). The glassfiberswere carefully cut and placed on a metallic sample holder with a molybdenum cover plate securing thefibers.Care was taken such that the X-ray beam was only on the fibers and not on the molybdenum cover plate. 8

9


346

Method Synthesis of l~[(2-vinyloxy)ethoxy]ethyl acetate


Acetic acid (99%, 4.20g, 70mmol) was added dropwise to a stirred solution of ethylene glycol divinylether (9.14g, 80mmol) at 60°C under nitrogen. After addition was completed, the mixture was refluxed for 5hrs. The required l-[(2vinyloxy)ethoxy]ethyl acetate was isolated by vacuum distillation ( 85-90°C; 10 Torr). Yield 75%

Synthesis of poly l-[(2-vinyloxy)ethoxy]ethyl acetate 3

ZnCl (2cm of 1.0M solution in diethyl ether) was added dropwise (30min) to a stirred solution of l-[(2-vinyloxy)ethoxy]ethyl acetate from above (5g, 29mmol) and toluene (15cm ) and stirring was maintained for 24hours under nitrogen. The reaction was quenched with methanol containing 25vol. % ammonia (5cm ), then washed with 10% aqueous sodium sulfate, followed by water, and then brine, before being dried over MgS0 . Finally, the solvent removed in vacuo to obtain poly l-[(2-vinyloxy)ethoxy]ethyl acetate in 95 % yield. GPC data: M = 945g/mol. 2

3

3

4

w

Synthesis of poly l-[(2~vinyloxy)ethoxy]ethyl acetate triethoxysilane A mixture of poly l-[(2-vinyloxy)ethoxy]ethyl acetate (5g, 29mmol), triethoxysilane (4.7g, 890mmol), toluene (10cm ), and the hexachloroplatinate hexahydrate catalyst (0.1cm ) was refluxed under nitrogen at 80°C for 24 hours. The resultant mixture was placed under high vacuum to remove excess reagents and the solvent. Poly l-[(2-vinyloxy)ethoxy]ethyl acetate triethoxysilane was isolated in 43% yield as a clear colourless liquid. 3

3

Synthesis of poly l-[(2-vinyloxy)ethoxy]ethyl acetate polydimethylsiloxane. A mixture of the poly l-[(2-vinyloxy)ethoxy]ethyl acetate (l,06g) Si-H terminated polydimethyl siloxane (5g, Mw=1688g/mol), toluene (100cm ) and the hexachloroplatinate hexahydrate catalyst (0.1cm ) was refluxed under nitrogen for 32 hours and then allowed to cool. The solvent was removed in vacuo leaving the product, in 41% yield, as a clear colourless oil. 3

3


347

Application ofpoly l-[(2-vinyloxy)ethoxy]ethyl acetate triethoxysilane to Ε glass fibers The poly l-[(2-vinyloxy)ethoxy]ethyl acetate triethoxysilane (8.12g), synthesized above, was mixed with toluene (262g) to form a 3wt% silane solution. Glassfibers(15g) were soaked in this solution for 16 hours and then allowed to dry at room temperature. The glass fibers were then washed with a sequence of solvents (40 - 50cm ) as follows: - dichloromethane; toluene; hexane; wet acetone; and then dichloromethane again. The washed fibers were then allowed to dry at room temperature again Downloaded by UNIV OF IOWA on October 22, 2015 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch029

3

Results and Discussion Synthesis of polyvinylacetate hyperbranched polymer. The polyvinylacetate hyperbranched polymer were first was synthesised by Zhang (1997) [14], through the self condensation of the monomer, l-[(2vinyloxy)ethoxy]ethyl acetate, see Scheme 1. This monomer was initially synthesised by the addition of equimolar amounts of dry acetic acid and divinylethylene glycol. Our own resultant mixture was vacuum distilled to remove any excess reagents and any solvent from the reaction, as the presence of any excess divinylethylene glycol will lead to gelation during the polymerisation stage. A 75% product yield was recovered based on H NMR data. The H NMR (Figure 4) revealed, a doublet at 1.4ppm due to the C H next to the CH, a singlet at 2.05ppm from the C H next to the carbonyl carbon, a multiplet at 3.6 - 4.0ppm from the CH -CH sequence of the ethylene glycol, a doublet of doublets at 4.1ppm due to the C H of the vinyl group, a quartet at 5.9ppm from the CH attached to the CH group, and a doublet of doublets from the CH of the vinyl groups at 6.4ppm. !

!

3

3

2

2

2

3

Polymerization was initiated by the addition of a Lewis acid, zinc chloride, to a dilute solution of the monomer. The living cationic polymerisation reaction proceeds at room temperature and is readily terminated by quenching with methanol. The mechanism of the reaction is thought to involve the 'RCOOH/ZnCl ' system [15], where the ZnCl complexes with the ester group thereby creating a stabilized carbocation (see Scheme 2.). 2

2


348

Ο CH =CH- O 2

(CH )2- O - C H - O - C - CH3 2

ZnQ

2

o---zna


Q^C-CH

2

3

The nucleophilicity of the RCOO- group depends on the electron withdrawing power of the R group. Generally, the more acidic the R group, such as R = CF3 or CC13, the faster the polymerisation reaction and the narrower the molecular weight range of the product (15). Once the carbocation is created, it reacts with another monomer at the vinyl end in a 'living manner' until the reaction is terminated eg. by adding methanol. Every addition of a monomer to the chain, brings in an additional initiating site that can be activated by more ZnC12 resulting in multiple branches, characteristic of hyperbranched molecule (19). It is also important to note that there always remains one vinyl group on every hyperbranched molecule formed. The molecular weight of our synthesized polymer was determined by GPC calibrated against polystyrene linear standards to be 945g/moL, which equates to a hyperbranched tetramer. The *H NMR (Figure 3.) shows changes, which can be seen, compared to that of the monomer. Firstly, the CH3 attached to the CH has divided into two different peaks, differentiating the internal ÇH3 and the peripheral CH3 (labelled A and Β in Scheme 1. and Figure 3.) on the polymer branch. Secondly, the peaks from the vinyl groups at 4.1ppm and 6.4ppm decrease significantly due to the fact that the majority of the vinyl groups are consumed in the reaction.

Synthesis of Polyvinyloxyacetate/Triethoxysilane, Hyperbranched Polymer Both the Si-Η terminated polydimethyisiloxane (PDMS) and triethoxysilane (TEOS) were separately hydrosilated onto the vinyl terminated polyvinyloxyacetate in the presence of a platinum catalyst (H2PtC16) (see Scheme 3). Hydrosilation involves the addition of a Si-Η group to an unsaturated bond. Two possible adducts can be formed according to the 'antiMarkonikov's rule'(see Scheme 4.), however, the proportion of the major a-



2

2

2

2

2

2

2

2

2

2

o

ZnCI

a

CH*

i

2

2

2

o

2

2

2

CH

3

b

Ο

CH

3

I.I

2

ο

II

2

3

2

II

Ο

^

3

b

Ο—CH CH OCHOCCH

3

O — CH CH OCHOCCH

I

2

3

3

O — CH CH OCHOCCH

Ο—CH CH OCHCH CHOCCH

I

CHj

I l

CH =CHOCH CH OCHCH CHOCCH

CH =CHCX:H CH OCHCH CHCH CHOCCH3

a = Internal methyl groups b = Peripheral methyl groups

I

Ο

CH Scheme 2. Synthesis of the vinyloxyacetate hyperbranched polymer

2

CHOCH CH OCHOœH

ZnCl,


3


JbL

H

2

2

2

3

I

O- -

2

2

i - *

2

C H O

3

b

3

3

F

CH

E

2

D

CH 3

•

of Polyvinyloxyacetate

2

0

O—CH CH OCHOC|CM,

3

^ _

O — CH CH OCHCH CHOCCH

2

O—CH CHoOCHOCCH

The Synthesis of VinyloxyacetateâSiloxane Copolymer and Its

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