Yang Huan, Zhang Jide, Zhu Yangrong, Yang Mujie
(Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China)

Abstract Self-crosslinkable core-shell latex was synthesized, and the influence of the hydrophilic core T_g on the latex structure and the film properties was studied, then the film formation mechanisms were suggested. Lower core T_g helps the migration of the core polymer to the outer crosslink layer and increases the gel content and compatibility of the film with two phases. The latex with higher core T_g have more obvious core-shell structure.
Keywords latex particles, core-shell structure, self-crosslink, glass-transition temperature, particle structure, IPN

1 INTRODUCTION
Recently, the coatings industry has been forced to develop environmentally friendly coatings with less or ultimately zero VOCs (volatile organic compounds). Latex coatings have been promised to reduce the VOCs emitted from the coatings. However, normal latex coating, such as acrylic latexes, has relatively poor water resistance and unsatisfactory film hardness etc., which limit their practical application in many fields. In order to improve their properties, the applications of preparation technology of the core-shell structured latex and self-crosslink technology in emulsion polymerization have been attracting more and more interest in recent years. For the core-shell structure latex, the optimum glass-transition temperature (T_g) of the latex particle could either improve latex film hardness or favor formation of film ^[1-3]. For the self-crosslink technology in emulsion polymerization, introducing the reactive groups onto latex polymer, self-crosslinkable system could be obtained, which not only enhances the physical and chemical integrity of latex films, but also makes the film formation process complicated ^{[4 -7]}. Diacetone acrylamide (DAAM) has got a lot of attentions recently, because that it can be copolymerized with acrylate easily and react with adipic dihydrazide (ADH) by weak acidity or alkalescence as catalysts at room temperature to form self-crosslinkable system ^[8], so DAAM and ADH were used in this study.
    In this paper, self-crosslinkable core-shell latex was synthesized through two-stage emulsion polymerization with acrylic monomers and DAAM. The influence of the hydrophilic core T_g on the latex structure and the film properties was investigated. The formation mechanisms of latex film so obtained were suggested.

2 EXPERIMENTAL
2.1 Materials
Styrene (St), n-butyl acrylate (BA), methyl methacrylate (MMA), ethylhexanoic acid (EHA), and acrylic acid (AA) were obtained from Shanghai Wulian Co., China. Hexanediol diacrylate (HDDA), DAAM, and ADH were purchased from Hangzhou Xinghua Fine Chemical Co., China. Sodium dodecylsulfate (SDS), polyoxyethylene (10) octylphenyl ether (OP-10) as composite emulsifuers, and APS as initiator were bought from Wenzhou Dongsheng Chemical Co., China. Sodium hydrogencarbonate (NaHCO₃) as buffer was got from Shanghai Hongguang Chemical Plant, China. All the above materials were used without further purification. Distilled and deionized water was used throughout the work.
2.2 Preparation of self-crosslinkable core-shell latex
Emulsion polymerization was performed at 75ºC in a conventional thermostated three-necked glass reactor equipped with a mechanical stirrer and a reflux condenser using a two-stage semi-continuous addition of stable emulsions of the core and shell monomers. The first-stage copolymerization monomers include EHA、MMA and DAAM. According to Fox equation, we modified the ratio of MMA to EHA to vary the T_g of the core polymer ^[9]. A constant amount of DAAM was introduced to make the latex cross-linkable with ADH and hydrophilic. St, BA, AA and HDDA were copolymerized during the second-stage and the quantity of each monomer was constant. The ratio of first-stage monomer/second-stage monomer is 1/1(wt). The interval of the two stage polymerization was 30min. Starve-feed condition was applied (~10g monomer per 1h) throughout the second-stage polymerization. When monomers addition was completed, the temperature was increased to 80ºC and reacted for 30min, then 5g of initiator solution (1 wt %) was fed. After 1 hour, another 5g of the initiator solution (1 wt %) was added and polymerization was continued for a further 30min at 85ºC. The obtained system was cooled to 40ºC to adjust the pH value to ~7.5 by adding ammonia. The molar quantity of ADH added to latex was half amount of that of DAAM.
2.3 Characterization of the latex and film
Particle morphology was investigated by transmission electron microscopy (TEM).The emulsion was dispersed in the pure water, dropped on the holey copper net, and then treated with a 2 wt % aqueous solution of phosphotungstic acid as a negative stain for TEM (JEM 1200EX).
    The particle size of the latex was determined by dynamic light scattering (DLS) using a 90 Plus particle size analyzer (Brookhaven Instruments Corp.) equipped with diode laser operating at 658.0 nm at 20°C after the latex was diluted by deionized water.
    The latex viscosity was measured by a NDJ-79 rotational viscometer (Tongji Mechanical & Electronic Plant).
    Gel content (%) of the latex film is calculated from Eq. (1).
          Gel content (%) = (w₁/ w₀) × 100                       (1)
    Where w₀ and w₁ are the weights of the latex film before and after swollen in acetone at room temperature for 24h and then carefully dried. The latex films were formed with the aid of Texnaol at 40ºC for 36h in a poly (tetrafluoroethylene) mold after blended with ADH, then at -0.1MPa for 24h, and the thickness is ~0.5mm. The films were also used for turbidity experiments.
    The turbidity t is defined as the light intensity reduction per unit penetration length in the sample, L can be determined from the ratio of the transmitted light intensity, I_t to the incident intensity, I₀:
   t = (-1/L) ln(I_t/I₀)                                                 (2)
    The absorbance, I_t/I₀ of the films was performed with a Cary 100 Bio Ultraviolet-visible light spectroscopy at 400nm. The conversion from absorbance to turbidity was performed by using Eq. (2).

3 RESULTS AND DISCUSSION
3.1 Effect of core T_g to particle morphology and viscosity of latex
The design of latex particle T_g is an important factor for obtaining the core-shell structured latex particles. Figure 1 shows the TEM image of latex particle with different core T_g. It is seen that the latex particles with higher core T_g (30ºC) exhibit obvious core-shell structure in comparison to that with lower core T_g (-30ºC). Moreover, its shell layer is more integrated and more uniform in the thickness. The latex particles with lower core T_g (-30ºC) have not more integrated shell layer, which may be due to the migration of the hydrophilic core polymer to the water-latex interface ^[10].
    Figure 2 shows latex particles size and latex viscosity as the function of the core T_g. No more change of the size of the latex particles was observed with increasing the core T_g. The values of volume mean diameters are between 96nm~103nm, and the polydispersity values are below 0.042. However, at the same test temperature (11ºC), the viscosity of the latexes with 44% solid content value decreases obviously along with increase of the core T_g. For this case, when the core T_g decreases, the polymer segment motion ability increases, and the migration of the hydrophilic core polymer to the outer layer may become more and more easily, at the same time, the concentration of hydrophilic secondary amine which is from DAAM on the water-latex interface would rise too, thus increases the volume fraction that latex particles occupy because of the hydration effect. According to Einstein's viscosity law ^[11], the higher volume fraction that particles occupy increases the system viscosity.
     
Figure 1 TEM micrographs of latex particle with different core T_g: (a) -30ºC, (b) 30ºC

Figure 2 Influence of core T_g of the latex particles on viscosity of latex (at 11ºC) and particle size

3.2 Effect of core T_g to the film properties
Latex films formed from latex prepared blended with ADH have network structure, because film formation process is accompanied with the crosslinking reaction between ketone carbonyl and hydrazide. The quantity of DAAM introduced into emulsion polymerization was consistent, so the invariable value of the gel content was expected. But the practical value varies along with the core T_g. The results of the swelling experiments are shown in Figure 3. When T_g of the core polymer is the lowest (-30ºC), the gel content of the film is the highest (81.4%), and with the increase of the T_g, the value decreases to 76.5% at 15ºC. This decreasing trend should be related to the core polymer's migration to the outer layer. This migration may result the formation of semi interpenetrating network (semi-IPN) structured latex particle because of the crosslink of the shell layer, and after addition of ADH, interpenetrating network (IPN) could be obtained finally. The lower interpenetration degree between core and shell layer, the less contribution the network of the original shell would be made to the final IPN, and lower gel content value would be obtained. The gel content increases at 30ºC of the core T_g, which might be explained by that the embedment of the ketone carbonyl in the inner latex helps to make the polymer diffusion across the intercellular preceding cross-linking^[12], so the further reaction would not be hindered.

Figure 3 Gel content (%) of the film formed from latex with different core T_g blended with ADH

    Formation of the IPN could reduce the contrast between different phases in material, and helps to have better compatibility. The turbidity t has been used to characterize the compatibility of the two-phase materials because of the phases with different refractive indexes ^[13]. Higher turbidity shows lower compatibility. Along with the increase of the core T_g, t of the film presents the same trend which increases from 1.428mm^-1 at -30ºC to 3.709mm^-1 at 30ºC (Figure 4). The lower turbidity value should be related to the higher interpenetration degree between the core with the lower T_g and the outer layer, and when core T_g becomes higher, more original shell network could not play a part in the formation of the final IPN and might be separated in the network in small phases, so higher turbidity value is obtained. For core-shell structured latex, which is with two phases, more uniform film with IPN structure could be obtained by proper design of the latex particle, which is meaningful for improvement of the performance of film.

Figure 4 Turbidity of the film formed from latex with different core T_g blended with ADH

3.3 Suggestion of film formation mechanism
According to the discussion on the structure of the latex and the film properties, two film formation mechanisms were suggested, which is depicted schematically in Figure 5. For lower T_g of the core, typically -30ºC, semi-IPN structured latex particle could be obtained because of the migration of hydrophilic core polymer to the outer cross-linked layer. A part of ketone carbonyl would be located at the water-latex interface. With the evaporation of water, particles coalesce and film is formed, being accompanied with the crosslinking reaction and IPN film was obtained finally. For higher T_g of the core, typically 30ºC, it may be difficult for the core polymer migration to the water-latex interface, and the particle is core-shell structure. Cross-linkable ketone carbonyl is embedded in the core. During the process of film formation, particles come into contact and deform, and the original shell would fragment and separate in the final network in small phases.

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具有不同玻璃化转变温度乳胶粒子的结构与性能
杨欢，张继德，朱杨荣，杨慕杰
(浙江大学高分子科学研究所，中国杭州310027)
摘要 本文合成了可自交联的具有核壳结构的乳胶粒子，研究了亲水性核的玻璃化转变温度对于乳胶粒子结构和膜性能的影响，并提出了成膜机理。较低的核的玻璃化转变温度有利于核高分子向交联的壳层的迁移，增加了具有两相结构的膜的凝胶含量，提高了其相容性。当核的玻璃化转变温度较高时，乳胶粒子具有更为明显的核壳结构。
关键词 乳胶粒子，核壳结构， 自交联， 玻璃化转变温度， IPN