Abstract
To improve the chemical, physical, mechanical and thermal properties of polyvinyl acetate (PVAc), N-hydroxymethyl acrylamide (NMA) was used to polymerize with vinyl acetate (VAc). During the emulsion polymerization of VAc using polyvinyl alcohol as protective colloid and ammonium persulfate as initiator, PVAc-NMA was prepared under the conventional conditions in laboratory. In the polymers, larger PVAc-NMA particles and other smaller NMA-NMA particles randomly dispersed together. Their dispersion was good. PVAc-NMA was a pseudo-plastic non-Newtonian fluid, also possessed the normal stress effect (or Weissenberg effect), the pole-climbing phenomenon. With the change of different synthesis processes and the increase in NMA content in the synthesis system, the molecular weight of PVAc-NMA increased. NMA could lower the glass transition temperature of PVAc, and then improved PVAc’s low temperature resistance, so PVAc-NMA could be applied in more harsh or much lower temperature conditions. The static tensile of PVAc-NMA was better. PVAc and PVAc-NMA all had the cold crystallization. The pyrolysis of PVAc and PVAc-NMA was similar. It was found to consist of eight phases. NMA had no obvious effect on the pyrolysis temperature but mainly delayed the thermal degradation process.
Introduction
Polyvinyl acetate (PVAc) is considered to be a kind of acceptable environmentally friendly adhesive which is usually prepared using vinyl acetate (VAc) monomer in a mixture of protective colloid, nonion emulsifier, initiator and water. It has many characteristics, such as being nonpoisonous, nonharmful, easily produced, low price, convenient in application, economizing resources and so on. Because of these reasons, PVAc is widely used in bonding of many porous materials, such as wood processing, furniture packaging, building decoration, texture bonding and print bonding. However, it also has some defects, its cold tolerance, mechanical stability, thermal stability and water resistance are poor. 1,2 These defects directly affect its integrated value of utilization and processing. Till now, many studies have been done on this topic, including the preparation methods, co-polymerization, co-blending, radiation preparation and so on.
In this study, to improve the chemical, physical, mechanical and thermal properties of PVAc, an active monomer named N-hydroxymethyl acrylamide (NMA) was introduced to modify PVAc based on our previous work. 3 –9 Here, NMA is an amide derivative from unsaturated carboxylic acid, it could polymerize with VAc, and the cross-linkable thermosetting copolymer of PVAc-NMA could be obtained. The molecules could further get cross-linked and cured under suitable conditions. The chemical, physical, mechanical and thermal properties were studied, mainly including the chemical structure, dispersion of particles, rheology, static tensile, solid content, storage time, dynamic mechanical properties and pyrolysis properties.
Experimental
Twenty-five grams of VAc, 0.50 g of NMA, 70.00 g of 10% polyvinyl alcohol (PVA) solution, 0.50 g of alkylphenol polyoxyethylene (10) ether, 3.75 g of 10% ammonium persulfate solution, 6.25 g of sodium lauryl sulfate and 250 g (200 g, 150 g, 100 g and 50 g) of water were mixed together by stirring vigorously until it became a homogeneous emulsion, and the temperature rose to 70°C. While stirring vigorously, 3.75 g of 10% ammonium persulfate solution and 25.00 g of VAc were gradually added into the homogeneous emulsion every 6 h (5 h, 4 h, 3 h and 2 h) for polymerization. Subsequently, the temperature rose to 85–90°C, the emulsion further polymerized for 0.5–1 h. After polymerization, the temperature dropped below 50°C, then 4.00 g of ethanol, 3.00 g of water, 0.30 g of sodium benzoate, 0.18 g of sodium bicarbonate and 6.00 g of di-n-butyl phthalate were added to the emulsion. Finally, PVAc-NMA-A (PVAc-NMA-B, PVAc-NMA-C, PVAc-NMA-D and PVAc-NMA-E) was obtained. Here, we used A, B, C, D and E to, respectively, correspond to the different synthesis processes with different polymerization time and different proportion of water. PVAc-A, PVAc-B, PVAc-C, PVAc-D and PVAc-E were also synthesized by the same processes as PVAc-NMA but without NMA. Here, VAc and NMA were the main polymerization monomers, PVA whose polymerization degree and alcoholysis degree were 1700 and 88% respectively was the protective colloid, ammonium persulfate was the initiator, alkylphenol polyoxyethylene (10) ether was the emulsifier, sodium lauryl sulfate was the anionic surfactant, di-n-butyl phthalate was the plasticizer, and the mixture of ethanol, water, sodium benzoate and sodium bicarbonate was the pH regulator. All the analytical reagents were obtained from Sigma-Aldrich in Shanghai, China. Then, based on the specifications in our previous work, 3 –12 their chemical, physical, mechanical and thermal properties were studied, mainly including the chemical structure, dispersion of particles, rheology, static tensile, solid content, storage time, dynamic mechanical properties and pyrolysis properties.
Results and discussion
Structure
To test the chemical structure of PVAc and PVAc-NMA, first the obtained polymers were purified. After being washed in 30°C warm water and filtering for about 5 times, they were tested by the NICOLET 380 Fourier transform infrared spectroscopy (FTIR; Figure 1). The comparison among FTIR spectra of NMA, PVAc and PVAc-NMA revealed that the absorption peak of PVAc-NMA near 3000 cm−1 became weaker, even disappeared, those near 1600 cm−1 showed a platform from PVAc’s originally slightly sharp absorption peak and those near 1100 cm−1 showed two sharp absorption peaks from PVAc’s originally wide absorption peaks. These differences showed that the absorption band of PVAc-NMA was formed by NMA and PVAc, the polymer of PVAc-NMA (Figure 2(a)) was indeed produced, so did PVAc and NMA-NMA (Figure 2(b)). The grafted polymer of PVAc-PVA might also be produced according to El-Aasser et al.’s work. 13 –15 In this study, PVA was mainly used as the protective colloid, and the polymerization mainly occurred between the monomers of VAc and NMA. Even if PVAc-PVA was produced in the polymerization, it would not affect the polymers that we require, at least it would not bring any bad effect on their properties. Obviously, the different synthesis processes had little effect on the interaction between PVAc and NMA. FTIR spectra of PVAc-A to PVAc-E were the same, and those of PVAc-NMA were the same, too.
Fourier transform infrared spectroscopy (FTIR) spectra.
Molecular structures of (a) PVAc-NMA and (b) NMA-NMA. PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide.
From the visual, all PVAcs, including PVAc-A, PVAc-B, PVAc-C, PVAc-D and PVAc-E, and all PVAc-NMAs, including PVAc-NMA-A, PVAc-NMA-B, PVAc-NMA-C, PVAc-NMA-D and PVAc-NMA-E, were the same. They were viscous, milk-white, homogeneous, and had fine emulsions. All had no coarse particles, no foreign bodies and no delamination. Moreover, 3–5 drops of them were added to 20 ml of water in a glass dish (diameter 90 mm); when they became homogeneous, their dispersion was little different (Figure 3). Comparing with water, no coarse particle was found in the dispersion of all PVAcs and all PVAc-NMAs, their dispersion were good.
Dispersion of (a) PVAc and (b) PVAc-NMA. PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide.
From the results described above, we can see that the different synthesis processes had little effect on the chemical structure of PVAc and PVAc-NMA. From PVAc-A to PVAc-E and from PVAc-NMA-A to PVAc-NMA-E, their FTIR spectra were almost the same. And from the dispersion in water, for all PVAc and all PVAc-NMA, their dispersion was good. Therefore, to further study their dispersion, the liquid samples of PVAc-A and PVAc-NMA-A were chosen and observed by JEM-3100F transmission electron microscopy (TEM) (JEOL Ltd., Japan) with a magnification of ×20,000. By comparing the images of TEM, the differences were very clear. PVAc particles were larger, their diameter ranged from 250 to 500 nm (Figure 4(a)). PVAc-NMA particles were also larger, and their diameter was about 250 nm (Figure 4(b)). Besides this, other smaller particles were also found, whose diameter was about 50–100 nm, they may be the condensed polymer of NMA-NMA. These particles randomly dispersed together, they absorbed others or were absorbed by others.
Transmission electron microscopic (TEM) images of (a) PVAc-A and (b) PVAc-NMA-A. PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide.
Rheology
The liquid samples of PVAc and PVAc-NMA were tested using a NDJ-1 rotary viscometer, respectively, under the shear rate (γ, s−1) of 0.63 s−1, 1.26 s−1, 3.14 s−1 and 6.28 s−1. The apparent viscosity (η, mPa s; Table 1) varied with the different synthesis processes. From process A to E, the apparent viscosity increased for PVAc and PVAc-NMA. It just was the different synthesis processes making the great differences on the apparent viscosity. As described above, PVAc and PVAc-NMA synthesized in different weight of water, respectively, were 250 g, 200 g, 150 g, 100 g and 50 g at different polymerization time, respectively, of 6 h, 5 h, 4 h, 3 h and 2 h. These would significantly affect the microstructure of final products, so the apparent viscosity was different.
Apparent viscosity.
PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide; γ, shear rate.
The apparent viscosity of PVAc-NMA was higher than PVAc synthesized by the same process. This is because (1) NMA was added into the polymerization, NMA-NMA and PVAc-NMA were produced and (2) the intermolecular and intramolecular hydrogen-bond-driven self-assembly (Figure 5) 16 –20 might appear in PVAc-NMA. Moreover, they also showed that η decreased with the increase in shear rate. This phenomenon indicated that both PVAc and PVAc-NMA were pseudo-plastic non-Newtonian fluids. As we know, a pseudo-plastic fluid is one of the most common non-Newtonian fluids. Rubber, most polymers and their plastic melt and concentrated solution all are pseudo-plastic fluids. Their characteristic is that the apparent viscosity decreases with the increase in shear rate. The orientation of the slender molecular chains for a polymer in the flow direction causes the decrease in viscosity, so it is also often called as the shear thinning fluid. This rapid shear thinning behavior of PVAc-NMA was much greater than PVAc.
Hydrogen-bond-driven self-assembly.
Based on the apparent viscosity and according to our previous work, 5 –9 the rheology of PVAc and PVAc-NMA was further studied by the power-law function equation, the Newtonian fluid flow equation and the Cross-Williamson model viscous equation. Their fluid consistency (Ï, mPa s), flow index (i), zero shear viscosity (η0, mPa s), limit viscosity (η∞, mPa s), characteristic time (ζ, s), weight average molecular weight (M w) and number average molecular weight (M n) were obtained (Table 2). i is used to judge the difference between fluid and Newtonian fluid. If i is farther away from 1, the non-Newtonian behavior is more obvious. For the Newtonian fluid, i = 1, and i < 1 for the pseudo-plastic fluid. These of PVAc and PVAc-NMA were less than 1, so they all were pseudo-plastic non-Newtonian fluids which coincided with the results of shear thinning fluids obtained from the apparent viscosity. Ï, η0, M w and M n increased from process A to E, and those of PVAc-NMA all were much higher than PVAc. These trends were similar to the apparent viscosity. η∞ was 0 mPa s at all time whether for PVAc or PVAc-NMA. ζ is a characteristic constant for materials, it is always expressed by time, mainly as second. The variation trend of PVAc and PVAc-NMA was not similar; this may be because PVAc and PVAc-NMA were two different polymers. From process A to E, the molecular weight increased, and PVAc-NMA’s was much higher than PVAc synthesized by the same process. As in our synthesis processes, NMA was 0.50 g, about 1% of VAc, which was a reasonable addition; and from process A to E, the water weight in the reaction decreased, respectively, to 250 g, 200 g, 150 g, 100 g and 50 g. In other words, in the synthesis from process A to E with the decrease in water, the NMA content increased, so much more grafting were carried out between VAc monomers, NMA monomers or between VAc and NMA, and much more macromolecular chains were produced. It was the same for other polymers, PVAc and PVAc-NMA both possessed the normal stress effect which is the pole-climbing phenomenon, it is also called as the Weissenberg effect. This is closely related to the shear rate distribution when the fluid flows.
Rheological results.
PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide; i, flow index; Ï, fluid consistency; ζ, characteristic time; η0, zero shear viscosity; η∞, limit viscosity; M w, average molecular weight; M n, average molecular weight.
Static tensile
In the static tensile test, the liquid samples of PVAc and PVAc-NMA were made into films of 50 mm × 10 mm × 0.5 mm. After drying them to constant weight at room temperature, they were tested by SANS CMT 5000 computer-controlled electronic universal testing machine under a tensile rate of 10 mm min−1 and a test temperature of 25°C. The test should be finished in 10 min. Based on the specifications in our previous work, 5 –9 the static tensile strength (σ, MPa), break strain (∊, mm mm−1), vertical strain (∊x, mm mm−1), horizontal strain (∊y, mm mm−1), break elongation (∊t, %), elastic modulus (E ′, MPa) and Poisson’s ratio (ι) of PVAc and PVAc-NMA were calculated (Table 3). PVAc’s σ ranged from 2.07 (PVAc-A) to 3.11 MPa (PVAc-E) and that of PVAc-NMA ranged from 3.77 (PVAc-NMA-A) to 4.33 MPa (PVAc-NMA-D), NMA greatly improved the σ of PVAc, and it seemed that the different synthesis processes had no significant influence on σ of PVAc and PVAc-NMA, neither others. Their ∊(∊x), ∊y, ∊t, E ′ and ι were little affected by the different synthesis processes, but anyway, the static tensile of PVAc-NMA was much better than PVAc, its σ was higher, its ∊t was greater, its E' was larger and its ι was more reasonable. PVAc-NMA contained the cross-linkable monomer of NMA which has the active functional group of hydroxymethyl and the double bonds conjugated with the carbonyl group, under heat or in an acidic condition, NMA would play a cross-linking role, making the molecules further cross-linked, and so the thermosetting polymer of PVAc-NMA was generated, which could improve the films’ tensile strength and the adhesion to the bonded substrate in a great extent. In PVAc-NMA, the system itself had the weak acid surroundings and/or by the help of the heat during the curing or hardening, this self-crosslinking reaction naturally occurred (Figure 6). The intermolecular and intramolecular hydrogen-bond-driven self-assembly in PVAc-NMA also contributed to the static tensile.
Static tensile.a
PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide; σ, static tensile strength; ∊, break strain; ∊x, vertical strain; ∊y, horizontal strain; ∊t, break elongation; E', elastic modulus; ι, Poisson’s ratio.
aEach data in this table was averaged from 20 sets of results.
Self-crosslinking reactions of (a) PVAc-NMA and (b) NMA-NMA. PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide.
In addition, the solid content (O, %) and storage time (S, days) of PVAc and PVAc-NMA were also tested (Table 4). PVAc-NMA’s O was higher than PVAc synthesized by the same process, so NMA could improve the solid content of PVAc. From process A to E, O gradually increased, especially process D and E, their O was much higher, so they will be helpful for the development of high solid content emulsions. The S both for PVAc and PVAc-NMA was good at more than 180 days. This showed their storage stability was very great.
Solid content and storage time.
PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide.
Dynamic mechanical properties
Here, PVAc-A and PVAc-NMA-A were chosen and tested by dynamic mechanical analysis (DMA; Figure 7). Their liquid samples were made into circular films, their diameter was 10 mm and their thickness was 0.5 mm. After drying them to constant weight at room temperature, they were tested by NETZSCH DMA 242C through the compression model under 10 Hz of frequency, rising the temperature from 0°C to 200°C at a heating rate of 5°C min−1, 120 μm of the maximum amplitude, 8 N of the maximum dynamic force and 2 N of the minimum static force. From these DMA spectra, we could obtain the elastic modulus (E ′, MPa), loss modulus (E ′′, MPa), loss tangent (tan δ) and glass transition temperature (T
g, °C) of PVAc and PVAc-NMA. T
g and tan δ are two important indexes of damping materials. tan δ determines the damping properties (good or bad), and T
g determines the applied temperature and frequency range of damping materials. T
g is a physical parameter depending on the test methods and test conditions. In a DMA spectrum, it is always the temperature that corresponds to the maximum tangent loss. Moreover, for the damping materials, its effective damping temperature zone (Z
0, °C) can be calculated by:
Dynamic mechanical analysis (DMA) spectra of (a) PVAc-A and (b) PVAc-NMA-A. PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide.
So for PVAc-A, its T g was 72°C, its Z 0 was 72 ± (10–15)°C, and those of PVAc-NMA, respectively, were 26°C and 26 ± (10–15)°C. That is to say that NMA could lower the T g of PVAc, and the low temperature resistance of PVAc was greatly improved by NMA, so PVAc-NMA could be applied in more harsh or much lower temperature conditions.
Also from these DMA spectra, the cold crystallization appeared. As we know, the rise in temperature, a homogeneous amorphous polymer generally experiences three different mechanical states: the glass state, the high elastic state and the viscous flow state. But some amorphous polymers that possess the crystallization ability but cannot crystallize at the beginning because of the inappropriate conditions are more likely to appear cold crystallized above T g when temperature is rising; as a result, E′ will recover and increase after falling around the end of the glass transition until the crystallization phase is melt, then it will sharply fall (Figure 8). Obviously, PVAc and PVAc-NMA had cold crystallized, and they were all homogeneous amorphous polymers. This has very important practical significance that makes the material’s structure easy to control in the processing and molding. 21,22
Cold crystallization.
Pyrolysis properties
While heating from 25°C to 800°C at a heating rate of 10°C min−1 and a nitrogen flow rate of 60–80 ml min−1, the solid samples of PVAc-A and PVAc-NMA-A were pyrolyzed by NETZSCH TG209 F1 thermogravimetric analysis (Figure 9). The pyrolysis of PVAc-A and PVAc-NMA-A was found to consist of eight phases, phase 1 from 25°C to 150°C, phase 2 from 150°C to 230°C, phase 3 from 230°C to 290°C, phase 4 from 290°C to 350°C, phase 5 from 350°C to 410°C, phase 6 from 410°C to 510°C, phase 7 from 510°C to 710°C and phase 8 from 710°C to 800°C. Their weight loss curves were similar. The total weight loss was more than 90%, and only less than 10% was left, which include burnt residues such as some fire retardant materials and some other inorganic additives added in the polymerization. The pyrolysis mainly occurred in phase 2, phase 3, phase 4 and phase 6. In these phases, small molecules and macromolecules all were completely burnt, that is to say that with the rising temperature, PVAc-A and PVAc-NMA-A were gradually pyrolyzed and finally was completely burnt. However, as a polymer, the pyrolysis process of PVAc-NMA-A was somewhat different from that of PVAc, which could be reflected from the weight loss rate curves, their weight loss rates were different. From the pyrolysis valley temperature (T v) and the maximum pyrolysis rate (v v) of these weight loss rate curves, we can see that the pyrolysis mainly occurred in phase 2, phase 4 and phase 6; especially in phase 4, its v v was the maximum among these three phases, followed by phases 2 and 6. In phases 2 and 6, the pyrolysis was relatively complex; they were 2 or 3, even 5 T v and v v. Maybe many small molecules were pyrolyzed in phase 2, and many macromolecules were pyrolyzed in phases 4 and 6. They showed that NMA had no obvious effect on the pyrolysis temperature but mainly delayed the thermal degradation process.
Thermogravimetric analysis (TGA) curves of (a) PVAc-A and (b) PVAc-NMA-A. PVAc, polyvinyl acetate; NMA, N-hydroxymethyl acrylamide.
Conclusions
In this study, PVAc and PVAc-NMA were prepared through five different synthesis processes, their chemical, physical, mechanical and thermal properties were mainly investigated, which can be concluded as follows: PVAc-NMA was synthesized by VAc and NMA, its dispersion was good. PVAc-NMA particles were larger, and their diameter was about 250 nm. Besides this, other smaller particles were also found whose diameter was about 50–100 nm, they may be the condensed polymer of NMA-NMA. These particles randomly dispersed together, they absorbed others or were absorbed by others. The different synthesis processes significantly affected the apparent viscosity of PVAc and PVAc-NMA. They all were pseudo-plastic non-Newtonian fluids, also possessed the normal stress effect (or Weissenberg effect), that was the pole-climbing phenomenon. And by the help of the apparent viscosity analyzed by the power-law function equation, the Newtonian fluid flow equation and the Cross-Williamson model viscous equation, the different synthesis processes and NMA had great effect on the rheology of PVAc and PVAc-NMA. With different synthesis processes and the increase in NMA content in the synthesis system, the molecular weight of PVAc-NMA increased. The static tensile of PVAc-NMA was better than PVAc. NMA could lower T
g of PVAc, greatly improved PVAc’s low temperature resistance, so PVAc-NMA could be applied in more harsh or much lower temperature conditions than PVAc. PVAc and PVAc-NMA all had cold crystallized. The pyrolysis of PVAc and PVAc-NMA were similar, it was found to consist of eight phases. NMA had no obvious effect on the pyrolysis temperature but mainly delayed the thermal degradation process.
Footnotes
Funding
The authors are grateful for the financial support from the Program for New Century Excellent Talents in University of the Educational Ministry of China (NCET-06-0825).