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Study On Modification Of Acrylate Emulsion By VTMS

2007/12/3 17:01:00 41980

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Abstract: the modification effect of cross-linking agent vinyl trimethoxy silane (VTMS) on acrylate emulsion polymerization and emulsion properties was studied.

Through orthogonal experiment, the optimum conditions for synthesizing elastomeric emulsion were obtained: 3% of silicone, 0.5% of initiator, 4.8% of emulsifier and 78 of reaction temperature.

The morphology and structure of the polymer were characterized by laser particle size analyzer, infrared spectrometer and differential thermal analyzer.

Keywords: Vinyl trimethoxy silane (VTMS); acrylate; emulsion polymerization; modification.

0 Preface

Acrylate polymers have excellent film formation and adhesion, light retention, weatherability, corrosion resistance and flexibility.

However, it itself is thermoplastic, and there is no crosslinking point on the linear molecule, so it is difficult to form a three-dimensional network cross linking coating. Therefore, its water resistance and stain resistance are poor, it is brittle at low temperature and easy to stick at high temperature.

The Si - O bond energy of silicone resin (450 kJ / mol) is much larger than that of C C bond energy (345 kJ / mol) and C O bond energy (351 kJ / C), which has good UV resistance, weatherability, water resistance, stain resistance and chemical resistance. However, it has the disadvantage of poor white air at normal temperature, which greatly limits its application in building exterior wall coatings.

Organosilicon modified acrylic resin is a siloxane or polysiloxane which is introduced onto the main chain of acrylic polymer, which has both the advantages of acrylic resin and silicone resin.

The coating of the elastic building coating not only has the water resistance, weatherability and corrosion resistance of the conventional coatings, but also maintains resilience, durable toughness and excellent elongation in a wide range of temperature changes, so as to adapt to the generation and movement of the "live" cracks on the building surface.

In this paper, organosiloxane containing vinyl was used as crosslinking agent to prepare silicone modified acrylate elastic emulsion. The best technological conditions of polymerization were studied, and the factors affecting the emulsion performance were analyzed.

1 experimental part

1.1 raw materials

Methyl methacrylate (MMA), butyl acrylate (BA), methacrylic acid, twelve alkyl sulphate (DBS), sodium bicarbonate, 1, 2 propylene glycol, anhydrous sodium sulfite and ammonium persulfate (APS).

They are analytically pure, vinyl trimethoxysilane (VTMS), octyl phenyl polyoxyethylene ether (OP - 10), industrial products.

1.2 emulsion platform

The emulsifier was fully dissolved in three flasks, and the mixture of 1 / 10 monomer was added to the emulsion, pre emulsifying O.5 h, heating up to the test temperature and adding initiator.

After the seed emulsion is formed, the remaining monomer mixture is started to drop, and the drops are controlled at 2.5 ~ 3 h (fractional addition initiator).

Finally, add the remaining initiator, continue to react 1 h at 90 C, cool down to 60 C, add sodium sulfite, continue to react 0.5 h, cool down to 3O C, add ammonia water, adjust pH value to 7~8, filter and discharge.

The amount of fixed acrylate monomer was selected, and the amount of silicone, initiator and emulsifier were selected. The reaction temperature was used as a variable to investigate their effects on gel rate and mechanical properties of film.

Factors and levels are shown in Table 1.

Table

Table 1 Factors and levels

1.3 test and characterization

According to the conventional method, the solid content and conversion rate of the synthetic emulsion were tested.

The calcium stability and mechanical stability of the emulsion were determined according to the conventional method.

Determination of gel rate.

Mechanical properties: the emulsion in the PTFE mold at room temperature film, blade with the preparation of the diaphragm into a test strip, measuring its elongation.

The elongation and fracture strength were 200 mm /dmin.

The particle size and distribution of emulsion were determined by MS-2000 laser particle size analyzer.

Degree of crosslinking: pour the emulsion into the Teflon mold, dry the film at room temperature, extract 6 h with acetone as solvent by Soxhlet extractor, and calculate crosslinking degree by pressing formula.

S%= (W1 - W2) / W1 * 100%

Medium: W1 - extract the quality of the pre adhesive film, G;

W2 - the quality of the adhesive film after extraction, G

Differential thermal analysis: differential scanning calorimetry (SCRKZ202) - type 5 differential thermal analysis (DTA) was used to analyze the synthetic emulsion film. The temperature was measured from room temperature to 500 C, heating rate was 6 C / min, and air atmosphere.

Infrared analysis: the residual emulsion was extracted from the synthetic emulsion film using acetone as solvent, and the structure of the polymer was determined by AVATAR360 infrared spectrometer.

2 results and discussion

2.1 gel rate analysis

According to the analysis of the orthogonal results, it is found that the effects of various factors on gel rate are as follows: reaction temperature > emulsifier dosage > initiator dosage > silicone dosage, which has a specific effect on Figure 1.

It can be seen from Fig. 1 that the gel rate increases with the increase of silicone dosage, increases with the increase of the amount of initiator, increases with the decrease of emulsifier dosage, and the suitable polymerization temperature is 78 C.

Fig.

Fig. 1 Effect of reaction conditions on gelation rate

Effect of 2.1.1 silicone dosage on gel rate

Organosiloxane is easily hydrolyzed in aqueous phase under emulsion polymerization.

The increase of siloxane will accelerate the hydrolysis rate, resulting in the conversion of siloxane into active silanol and then condensation and crosslinking, resulting in the formation of a large number of condensates, resulting in an increase in gel rate.

Effect of 2.1.2 initiator dosage on gelation rate

With the increase of initiator dosage, the rate of free radical formation increases and the chain growth rate increases, so the polymerization rate increases and the monomer conversion rate increases.

According to the theory of oligomers generated by latex particles, polymerization can also be initiated when the free radicals generated and the monomer in the water phase meet.

Because the concentration of monomer in aqueous phase is very low and the solubility of polymer in water decreases sharply with the increase of relative molecular mass, the free radical chain is precipitated when it has not grown up to a relatively large molecular weight. The oligomer precipitated from it absorbs the emulsifier branch from around to stabilize the suspension in the water phase. It absorbs monomer molecules and free radicals from water phase and initiates polymerization.

New latex particles are produced, but when the polymerization rate is too high, the oligomers precipitated above may not be enough to absorb the emulsifier molecules, and their molecular chains continue to grow in the water phase, and finally the gel appears in the system.

Therefore, if the initiator is too large, the gel rate will increase.

However, the low initiator dosage will reduce the reaction rate and make the monomer conversion rate too low.

Effect of 2.1.3 emulsifier dosage on stability of emulsion polymerization

The composition and amount of emulsifier have great influence on the stability of emulsion polymerization.

A composite emulsifier consisting of anionic emulsifier and nonionic emulsifier was selected in the experiment.

The amount of emulsifier is too small, and the micelles are not enough to contain all the monomers. A large number of monomers are dispersed in the form of droplets in the form of polymer droplets. The oligomers adhere to each other in the polymerization process, resulting in excessive gel or glue on the reactor wall, resulting in an increase in gel rate. Increasing the amount of emulsifier can improve the stability of emulsion polymerization.

Effect of 2.1.4 reaction temperature on gelation rate

According to the theory of emulsion polymerization, the higher the reaction temperature is, the greater the decomposition rate constant is, the higher the rate of free radical formation and the higher the chain growth rate, the higher the polymerization rate.

Through the orthogonal experiment, it is found that the mixed monomer dripping into the reactor can not participate in polymerization at any time when the temperature is too low.