The influence of monobutyl itaconate and β-carboxyethyl acrylate on acrylic latex pressure sensitive adhesives

International Journal of Adhesion and Adhesives 84 (2018) 387–393

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The influence of monobutyl itaconate and β-carboxyethyl acrylate on acrylic latex pressure sensitive adhesives

T

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Cheng Fang , Qiming Yan, Zhuangzhuang Liu, Yifeng Lu, Zhongxiang Lin College of Chemical Engineering, Nanjing Forestry University, Nanjing, 210037, People's Republic of China

A R T I C LE I N FO

A B S T R A C T

Keywords: Monobutyl itaconate β-carboxyethyl acrylate Acrylic pressure sensitive adhesive Emulsion polymerization Adhesive property

The application of two different kinds of carboxylic monomers, monobutyl itaconate (MBI) and β-carboxyethyl acrylate (β-CEA), in acrylic pressure sensitive adhesive (PSA) latexes is presented in this article. The hydrophiliclipophilic balance (HLB) values of carboxylic monomers were calculated to determine their water solubility, which was further confirmed by their partition coefficients between organic and water phases. Influences of the functional monomers on particle size and zeta potential of the latexes, as well as on water resistance, thermal stability, gel content, sol molecular weight (Mw, Mn) and adhesive properties of the polymer were investigated in this work. In addition, the application of commonly used methacrylic acid (MAA) was also determined as a benchmark. The results indicated that for equiweight amounts of acid comonomer, latex particle sizes fall in the order: MBI > MAA > β-CEA, depending on water solubility of the corresponding monomer. Besides, the latex film prepared with MBI showed the best water resistance when compared with other two counterparts. Results also indicated that the PSA prepared with β-CEA exhibited the highest gel content among the three acid monomers. Moreover, the thermal stability of the three carboxylated polymers follow the order P(nBA+MAA) > P(n-BA+β-CEA) > P(n-BA+MBI). Finally, the effects of carboxylic monomers on the adhesive properties of the PSAs were also evaluated.

1. Introduction Functionalized latexes are often synthesized by emulsion polymerization via the addition of a small amount of functional carboxylic acids into the comonomer mixture. These latexes are typically used in the production of paper coatings, textile coatings, and adhesives [1,2]. The application of unsaturated carboxylic acids as functional comonomers at low concentrations (usually < 20 wt% of the total monomer) in emulsion polymerization can enhance the colloidal and freeze-thaw stability, improve adhesion to various substrates, improve compatibility with pigments and allow the introduction of reactive groups on the particle surface for posterior reactions of chemical modification [3,4]. The most frequently studied carboxylic acid monomers used in emulsion polymerization are itaconic acid (IA) [5,6], acrylic acid (AA) [7,8], methacrylic acid (MAA) [9,10], and fumaric acid (FA) [5], listed in order of increasing hydrophobic nature. Oliveira [5] and coworkers discussed the effect of IA and FA in the emulsion copolymerization of methyl methacrylate (MMA) with n-butyl acrylate (BA) and found that IA and FA were distributed differently throughout the three phases of the emulsion, with these differences depending on the solubility of the corresponding monomer. Koh et al. [9] reported the influence of

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Corresponding author. E-mail address: [email protected] (C. Fang).

https://doi.org/10.1016/j.ijadhadh.2018.05.007 Accepted 7 May 2018 Available online 26 May 2018 0143-7496/ © 2018 Elsevier Ltd. All rights reserved.

copolymerization with methacrylic acid on poly(butyl acrylate) film properties. They found that the presence of small amounts of MAA enhanced the tensile and adhesive properties of the film, and the distribution of the MAA in the latexes was predominantly inside the particles regardless of the polymerization temperatures. In this article, we employed two unsaturated carboxylic acids, monobutyl itaconate (MBI) and β-carboxyethyl acrylate (β-CEA), instead of those traditional acid monomers, into the application of emulsion polymerization. Itaconic acid esters are obtained by introducing one or two ester groups into the molecular structure of itaconic acid. Moreover, the monoesters were found more active in polymerization than corresponding itaconic acid and diesters and therefore play a significant role in the production of a series of polymers and copolymers with free carboxyl groups that offer excellent properties as adhesives, cleaning media, surfactants, and plastic additives, etc [11]. Hence, monoitaconate esters, especially monobutyl itaconate (MBI), which contains both ester and carboxylic groups, are in high market demand. An important achievement obtained by copolymerizing monoitaconates is a possibility of balancing hydrophilic/hydrophobic properties at different pH levels [12]. Lu et al. [13] discussed the function of three different carboxylic monomers in acrylate emulsion

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copolymerization, where they found that the latex prepared with MBI has better water resistance than latex with AA. β-CEA is a carboxylic monomer of the acrylic or methacrylic acid type. A significant difference of β-CEA compared to common carboxylic acids is the greater separation of the carboxylic acid functional group from the unsaturated vinyl functionality in the molecule. It can be polymerized in solution or emulsion to produce acrylic, vinyl-acrylic or styrenic-acrylic polymers with improved adhesive properties. Other major characteristics of βCEA monomer are: (1) it promotes flexibility in the copolymers owing to the relative low glass transition temperature (Tg) of its homopolymers (< 30 °C). (2) It provides improved adhesion and stability in the emulsion polymers, due to its -COOH groups being more available than those in the conventional carboxylic acids. (3) β-CEA can be incorporated into the copolymer at high levels over a wide pH range, due to its more reactive salt form compared to AA. (4) β-CEA is more compatible with other monomers, hence reducing polymerization in the aqueous phase and producing more uniform copolymers because of its long chain [14]. Although these two carboxylic acid monomers have so many benefits, only a few fundamental studies have been carried out concerning the behavior of MBI [13,15] and β-CEA [16] in both emulsion polymerization and acrylic PSAs. The partitioning behavior of MBI and βCEA between organic and water phases was studied in our previous work [17]. In the present study, carboxylated acrylic emulsion PSAs have been synthesized with various vinyl acids. HLB values of different carboxylic monomers were calculated to determine their water solubility, which was further confirmed by their partition coefficients between organic and water phases. Besides, the effect of the functional comonomers on the particle size and zeta potential of the latexes, as well as on the water resistance, thermal stability, gel content, sol molecular weight (Mw, Mn) and adhesive properties of the polymer films were investigated. Results were compared with those obtained when using methacrylic acid (MAA).

Scheme 1. The molecular structures of (a) MBI, (b) β-CEA and (c) MAA. Table 1 Emulsion polymerization pressure sensitive adhesive recipe. Component Monomers

a

Amount (g) BA MMA carboxylic monomer HEA

CO-436 APS NaHCO3 Deionized water a

91 5 2 2 2.1 0.5 0.2 102

The total monomer weight was set 100 g.

mixture through a constant pressure funnel over a period of 20 min. After that, the pre-emulsion was stirred for a further 30 min. The polymerization was carried out by a monomer seeded semicontinuous emulsion polymerization process in a 500 mL four-neck round-bottom flask equipped with an electromotive stirrer, thermometer, two separated addition funnels, and a reflux condenser. The stirring speed was maintained at 270 rpm throughout the experiments. First, a homogeneous aqueous solution containing 47 g DI-H2O, 0.1 g CO-436, and 0.2 g NaHCO3 was charged into the reactor. When the temperature reached 78 °C, a monomer mixture containing 5.69 g BA and 0.31 g MMA and an initiator solution containing 0.15 g APS and 5 g water were charged to the flask to form the seed latex. The temperature was then raised to 83 ± 2 °C and the seed polymerization was continued for an additional 30 min. Subsequently, the pre-emulsion and initiator solutions containing 0.35 g APS and 25 g water were added slowly to the reactor using two separate constant pressure funnels. The feeding times for the pre-emulsion and the initiator solutions were 3.5 and 4.0 h, respectively. After the feed was completed, the reaction was allowed to proceed for an additional 1 h to increase monomer conversion. The latex was then cooled to room temperature and poured into a glass bottle to be used for further characterization.

2. Experimental 2.1. Materials Butyl acrylate (BA; technical grade, Shanghai Huayi Acrylic, China), methyl methacrylate (MMA; reagent grade, Shanghai Lingfeng Chemical, China), methacrylic acid (MAA; reagent grade, Shanghai Lingfeng Chemical, China), monobutyl itaconate (MBI; technical grade, Qingxin Hanerchem, China), β-carboxy ethyl acrylate (β-CEA; technical grade, Rhodia, France), 2-hydroxy ethyl acrylate (HEA; technical grade, Shanghai Huayi Acrylic, China), ammonium persulfate (APS; technical grade, Shanghai Aijian Modern Reagent Factory, China) and sodium bicarbonate (NaHCO3; reagent grade, Shanghai Lingfeng Chemical, China) were used as the initiator and the buffer agent, respectively. All these materials were used without further purification. The conventional ammonium nonyl phenol ethoxylate sulfate surfactant (Rhodapex CO-436), which contains 4 poly(ethylene oxide) (PEO) groups, was supplied by Shanghai Honesty Fine Chemical (China) and used as received. Distilled deionized water (DI-H2O) was used throughout the study. Ammonia (25 wt% in H2O) was obtained from Nanjing Chemical Reagent Co. The solvent used in the polymer characterization, tetrahydrofuran (THF, HPLC grade, Shanghai Lingfeng Chemical) was also used as supplied by the manufacturer. The molecular structures of MBI, β-CEA and MAA are shown in Scheme 1.

2.3. Characterization 2.3.1. HLB value calculation Griffin's method [18–20]: HLB=20×(Mh/M). Mh=molecular weight of hydrophilic groups, M=molecular weight of the whole molecule. An HLB value of 0 corresponds to a completely hydrophobic molecule, and a value of 20 corresponds to a completely hydrophilic molecule. The HLB value can be used to predict the properties of a molecule: A value < 10: Lipid soluble, A value > 10: Water soluble.

2.2. Emulsion polymerization The recipes for water-based PSA latexes synthesized with different carboxylic monomers are described in Table 1. 25 g deionized water and 2.0 g CO-436 were added to a 500 mL four-neck round-bottom flask and were stirred rapidly to aid dissolution of the emulsifier. The monomer mixtures were then slowly added into the water-emulsifier 388

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from the gel content test was concentrated and analyzed for sol molecular weight. THF was used as the eluent and the flow rate was set at 1 mL/min. The internal temperature was set at 30 °C. Narrow polystyrene standards having a molecular weight range from 400 to 1 × 106 were used for calibration.

2.3.2. Partition coefficient The partition coefficient of carboxylic acid monomers between organic and water phases was determined in our previous research [17]. 2.3.3. Conversion and coagulation Conversion values were measured by gravimetric analysis. About 2 g of latex was weighed into an aluminum foil dish and dried at 105 °C to a constant weight. The solid content and the final conversion were calculated by the following equations, respectively:

Solid content (wt%) =

W3 − W1 × 100 W2 − W1

2.3.9. Water absorption ratio of the latex film The weighed latex films were immersed in distilled water at 25 °C and weight monitored over time (24 h). Prior to weighing, films were removed from the water environment and free water on the surface of the films was quickly removed using filter paper. The water absorption ratio of the films was calculated using Eq. (4):

(1)

where W1 is the weight of the aluminum foil dish, W2 and W3 are the weights of latex before and after drying, respectively.

Conversion (wt%) =

Solidcontent (wt%) × W4 × 100 W5

Water absorption ratio (wt%) =

(4)

Where W6 and W7 are the weight of the films before and after the films absorb water, respectively.

(2)

where W4 is the total weight of all the materials added into the glass bottle before polymerization and W5 is the total weight of monomers. The coagulum was collected from the bottle cap and 100-mesh filter screen and then dried at 105 °C to a constant weight. The coagulation was described as the weight of coagulum per total weight of monomer added.

2.3.10. Contact angle on the latex film Contact angle (CA) measurements were performed on a JC2000D contact angle goniometer (Shanghai Zhong Chen Powereach Co., China) by the sessile drop method using a microsyringe at room temperature. Static contact angles were obtained from water droplets (4 μL) on the surface of emulsion films. Typically, three drops of the liquid were placed on the surface of the emulsion films and five readings of contact angles were taken for each drop. The average of fifteen readings was used as the final contact angle of each sample.

2.3.4. Particle size and zeta potential Particle sizes and zeta potential values of latexes were measured using a dynamic light scattering (DLS) instrument (Malvern NanoS Zetasizer). The analyses were carried out at 25 °C, and every result was an average of three parallel measurements. The latex was diluted until the solid content was about 1%. The reported diameter is an intensityweighted average particle size.

2.3.11. PSA testing Loop tack, peel strength and shear holding power were measured according to the Pressure Sensitive Tape Council standards PSTC-6, PSTC-1, and PSTC-7, respectively [21]. Further details of these methods were given previously [22].

2.3.5. DSC analysis A differential scanning calorimeter (DSC) model 214 polyma from NETZSCH Instruments was used to determine glass transition temperatures (Tg). 5–15 mg of dry polymer was weighed into a standard DSC hermetic alumina crucible. High-purity nitrogen was used as the purge gas with a gas flow rate of 20 mL/min. The sample was first cooled to −70 °C and then raised from −70 to 100 °C at a heating rate of 10 °C/min. The Tg was calculated from the inflection point in the reversed heat flow curve using the software provided.

3. Results and discussion 3.1. Hydrophilicity and lipophilicity of carboxylic monomers The hydrophilic-lipophilic balance (HLB) was generally used to describe the hydrophilicity and hydrophobicity of a surfactant, determined by calculating values for the different regions of the molecule, as described by Griffin in 1949 [18] and 1954 [19]. It could be observed from the molecular structural formula in Scheme 1 that these three kinds of carboxylic monomers consist of a molecule that combines both hydrophilic and hydrophobic groups (or polar and nonpolar groups), and it should also be noted here that without addition of buffer or base, the ionized portion of these vinyl acid monomers is very little [23]. So HLB values for carboxylic acids might be measured by Griffin's method, described in the experimental section, which was generally applicable for nonionic surfactants. The result of HLB values for different carboxylic monomers in the decreasing order is: β-CEA (12.36) > MAA (10.46) > MBI (9.57). From this trend, it can be concluded that (1) β-CEA is very hydrophilic due to its very large HLB value. (2) The HLB value of MAA is right close to 10, indicating that the hydrophilicity of MAA is a little larger than its hydrophobicity. (3). The HLB value of MBI is left close to 10, indicating that the lipophilicity of MBI is a little larger than its hydrophilicity. A Key question is whether this HLB method can be used to accurately compare the water solubility of different carboxylic acids? In attempt to answer this question the partition coefficient of carboxylic monomers between the organic and water phases is discussed below to determine the accuracy of the HLB method. The results are shown in Fig. 1. It can be found that the partition coefficient for MBI is much larger than β-CEA's and MAA's due to its very strong lipophilic property. This can be further confirmed by its solubility phenomenon in water which is: a small amount of MBI could be dissolved into the water, but

2.3.6. Thermal gravimetric analysis (TGA) TGA was carried out to demonstrate the thermal stability of the latex PSA polymers using SHIMADZU DTG-60AH, Japan. The polymer films (around 5~10 mg) were heated from ambient temperature to 600 °C at a rate of 10 °C/min under a 100 mL/min nitrogen flow. 2.3.7. Gel content The gel content of the acrylic PSA polymers was measured via the solvent-extraction method. Three samples (around 0.2 g) of the dried latex film were weighed and sealed in a PTFE coated membrane pouch. This was then placed in a Soxhlet extractor with tetrahydrofuran (THF), followed by refluxing for 24 h. After the extraction process, the membrane pouch was removed and first dried in a fume hood for 3 h and then in a vacuum oven at 70 °C until it reached a constant weight. The weight of the remaining dry gel was taken and the gel content was calculated using Eq. (3):

Gel content = mass of the dry gel/mass of the initial dry polymer

W7 − W6 × 100% W6

(3)

2.3.8. Molecular weight determination The molecular weight, number average molecular weight (Mn) and weight average molecular weight (Mw) and molecular weight distribution (Mw/Mn) of the soluble fraction of polymer were determined by using Agilent HPLC 1200 Infinity Series. The THF solution remaining 389

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pH -10

14.77 0.046

14

2

6

8

10

12

-20

12 10

zeta potential (mV)

partition coefficients

4

8 6

3.26 0.108

4

-30

-CEA MBI MAA

-40

-50

1.04 0.035

2

-60

0

Fig. 2. Effect of various carboxylic monomers on zeta potential of the acrylic latex.

carboxylic acid monomer Fig. 1. The partition coefficient of different carboxylic acids between organic and water phases.

the other two acid monomers. This is mainly caused by the strong lipophilic property of MBI, which has little effect on the homogeneous nucleation and secondary particle formation. On the other hand, for equiweight amounts of acid comonomer, the latex particle sizes fall in the order: MBI > MAA > β-CEA.

still in the form of monomer droplets dispersing into the water phase instead of forming a continuous phase. Nevertheless, β-CEA and MAA are both soluble in water, with the hydrophilicity of β-CEA larger than MAA's. It can be obviously seen that the results of partition coefficients of carboxylic acid monomers between organic and water phases are totally consistent with the trend in HLB values.

3.3. Zeta potential analysis Zeta potential value, as an important measure standard for the stability of the latex, indicates the thickness of electrical double layer, which plays a decisive role in repellency between particles. If all the particles have a large negative or positive zeta potential, they would repel each other, and there would be no tendency for them to aggregate. Generally, the particles with zeta potential values more positive than 30 mV or more negative than −30 mV are regarded as stable, and the latex would have a reasonable long-term stability when the absolute values of zeta potential are higher than 40 mV [28]. The effects of various carboxylic monomers on the zeta potential of the acrylic latex are shown in Fig. 2. It can be seen that the zeta potential values of the three carboxylated latexes without neutralization were almost the same. This phenomenon can be explained by the fact that electrostatic repulsion between the particles was mainly dependent on the amounts of electrolytes (surfactant, initiator, buffer), which was kept constant in the system. However, with increasing pH, the absolute values of zeta potential of the three carboxylated latexes increased and showed a decreasing order: β-CEA > MAA > MBI. The reason was that after the latexes were neutralized, the COOH group of the carboxylic monomer on the surface of the particles would dissociate to COO-, increasing the repulsive force between the particles. Furthermore, the stronger the hydrophilicity of the carboxylic acid monomer, the larger amounts of -COOH and -COO−1 on the surface of the particle [5,13].

3.2. Particle size analysis Conversion of monomers and polymerization stability as a function of carboxylic monomers are shown in Table 2 with the results indicating that all the experiments were completed successfully with conversions exceeding 96% and negligible coagulums. It is well known that in conventional emulsion polymerization, polymer particles are generally formed by the entry of radicals into the micelles, precipitation of growing oligomers in the aqueous phase and radical entry in monomer droplets [24]. Furthermore, the high surfactant concentration helps produce small particles [25]. From Table 2, we can see that with the incorporation of carboxylic monomer into the system, the latex particle size becomes smaller, which was mainly caused by the homogeneous nucleation and secondary particle formation. Qualitatively, the ability of acid monomers to induce particle formation can be explained by the theory of Fitch and Tsai [26], Ugelstad and Hansen [27]. They clarified that persulfate radicals induce polymerization in the aqueous phase producing highly carboxylated oligomers which combine and nucleate to form particles. Particle growth continues inside the particle. The particle maintains its stability from the hydrophilic shell provided by the carboxyl groups which concentrate at the particle-water interface. Besides, it is of interest to comment that the carboxylic monomer, MBI, does not produce any significant effect on the particle size as do

3.4. Water absorption and contact angle analysis

Table 2 Some parameters of the latexes prepared with different carboxylic acid monomers. Runs

Particle size (Dz, nm)

Polydispersity index (PDI)

Conversion (wt%)

Coagulation (wt%)

β-CEA MAA MBI Blank samplea Seed

190.6 195.0 205.8 207.1 102.6

0.013 0.016 0.018 0.035 0.007

96.44 98.06 96.45 97.82 –

1.38 0.86 1.52 1.42 –

a

The carboxylic acid monomers could improve the stability of the latex, but it would result in poor water resistance of the latex film which is, to a large extent, related to the types and amounts of the carboxylic monomers. The water absorption and contact angle values of the carboxylated latex films as a function of carboxylic monomer are shown in Fig. 3. It can be seen that the water absorption of the latex films prepared with these three carboxylic monomers showed an increasing order: MBI (7.6%) < MAA (9.8%) < β-CEA (11.3%), while the contact angle demonstrated a decreasing order: MBI (102.3°) > MAA (92.0°) > β-CEA (86.7°). In other words, the latex film prepared with MBI showed better water resistance (lower water absorption and higher contact angle) than that of conventional carboxylic acid (MAA)

means latex without carboxylic monomers. 390

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existence of labile hydrogen (see Fig. 5 below) in the molecular structure due to β-CEA participating in the polymerization as a comonomer, which leads to an increment in chain transfer to polymer and branching [32], thus increasing the gel content of the polymer. The effects of various carboxylic monomers on sol molecular weight (Mw, Mn) and its distribution are shown in Table 3 and Fig. 6, respectively. It can be observed that all PSAs prepared with these three vinyl acids had wide molecular weight distributions (see Fig. 6), and their sol molecular weights fell in the order: MAA > MBI > β-CEA, opposite to the trend of gel content aforementioned, which was mainly due to the transformation of sol polymer to gel.

water absorption 14

contact angle

110

100 95

10

90 85

8

)

12

contact angle (

water absorption (wt%)

105

80

6

75

3.6. DSC analysis

70

Glass transition temperature (Tg) is one of the most important factors affecting PSA performance. A higher Tg commonly shows the higher rigidity of the PSA chains, which might manifest itself in better elasticity and cohesive behaviour. On the contrary, PSAs with lower Tg will demonstrate better fluidity or deformability, which makes for better tack but might cause cohesive failure. Generally, PSAs with Tg values ranging from −30 to −50 °C are favored, which will show good end-use performance. Thus, in this work, Tg was determined for each final polymer latex prepared with different carboxylic monomers. Fig. 7 shows the DSC curves for P(n-BA), P(n-BA+MAA), P(n-BA+β-CEA) and P(n-BA+MBI). It is evident that all samples show similar curves with one characteristic endothermic peak indicating the presence of a homogeneous polymer. The Tg of PMAA is reported to be 162 °C [33] which is much higher than that of P(n-BA) (−43 °C [34]), resulting in the Tg of copolymer increasing greatly with the incorporation of MAA. On the other hand, β-CEA has no significant effect on the Tg of the copolymer. This may be due to its relatively low glass transition temperature (Tg) of its homopolymers(< 30 °C) [14]. For MBI, it can increase slightly the Tg of the copolymers, but there is no Tg data for MBI presented in the literature, because DSC scans on pure poly(mono-nalkyl itaconates) do not show any Tg [35], which is a common characteristic for most poly(monoalkyl itaconates) [36,37].

different carboxylic monmers

Fig. 3. Effect of carboxylic monomers on water absorption and contact angle of latex films.

counterpart. This was mainly due to the high lipophilicity of MBI which was discussed in detail before. On the other hand, the latex film synthesized by β-CEA showed the worst water resistance among the three acid monomers due to its strongest hydrophilicity. Taking into account the results presented above, it can be concluded that the carboxylic monomers have significant effects on the water resistance of the latex films. The higher the hydrophobicity of the acid monomer, the better is the water resistance of the resultant latex film. 3.5. Gel content and sol molecular weight analysis Gel content is an important parameter contributing to the adhesion performance of PSAs. As we know, when polymerizing a BA-rich monomer mixture in the absence of crosslinker, gel is formed by a chain transfer mechanism which involves two steps: (1) branch radical formation via either intramolecular chain transfer by backbiting or intermolecular chain transfer to polymer, due to the presence of labile hydrogen in the BA unit of the polymer chains [29,30]. (2) gel formation through combination termination between the branched polymer radicals [31]. Fig. 4 displays the final gel content of the acrylic polymers produced with various carboxylic monomers. It can be seen that the gel contents of the polymers prepared with different carboxylic acids fall in the order: β-CEA > MBI > MAA. In other words, the gel content of the latex film prepared with β-CEA was slightly larger than the ones prepared with MBI and MAA. These results may be attributed to the

3.7. TGA analysis The thermal stability of the obtained PSAs with different carboxylic monomers were evaluated using thermogravimetric analysis (TGA). The weight loss curves for the acrylic latex PSAs are shown in Fig. 8. It is evident that the thermal degradation process for all polymer samples proceeds in one step. Ozlem [38] proposed that for P(n-BA), thermal degradation proceeds through simultaneous and subsequent processes, γ-hydrogen transfer from main chain to carbonyl group, transesterification reactions causing loss of butanol, and generation of six-membered products stabilized by cyclization reactions being among the major decomposition routes. As can be seen from Fig. 8, the thermal stability of the polymers was improved with the incorporation of the carboxylic acid monomers into polymeric backbone. The reason may be that the anhydride units are formed during the polymer decomposition process with the addition of carboxylic acid monomers [39], which may retard the degradation process, thus improving the thermal stability of the polymers. Furthermore, the thermal stability of the three carboxylated polymers follow the order P(n-BA+MAA) > P(n-BA+β-CEA) > P(nBA+MBI). This was consistent with the work of Malhotra et al. [40] who found that as the bulk size of the substituent increases, the thermal stability of the polymer decreases. On the other hand, the inductive effect between the carboxylic acid and the ester group in β-CEA was weaker when compared with MBI. The electron withdrawing effect of the ester group will weaken the ester bond and make this link prone to scission [41]. Thus, P(n-BA+β-CEA) shows higher thermal stability than that exhibited by P(n-BA+MBI).

gel content of the polymer films (wt%)

62

59.41 0.282

60

58

56

55.14 0.400 54.18 0.106

54

52

50

48

carboxylic monomer Fig. 4. Effect of carboxylic acids on the gel content of polymer films. 391

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Fig. 5. Structure diagram of different carboxylic monomers after copolymerization.

Table 3 Effect of different carboxylic monomers on sol molecular weight of the acrylic latex PSA. β-CEA

MBI

154.8 27.5 5.64

112.0 31.1 3.61

144.7 33.4 4.33

0.7

MAA -CEA MBI

0.6

P(n-BA) P(n-BA+MBI) P(n-BA+ -CEA) P(n-BA+MAA)

80

Weight loss (%)

Mw(kg/mol) Mn(kg/mol) MWD

MAA

100

60

40

20

dwt/d(logM)

0.5

0 0

0.4

100

200

300

400

Temperature ( 0.3

500

600

)

Fig. 8. TGA curves of acrylic latex PSAs with different carboxylic monomers added.

0.2

Table 4 Effect of different carboxylic monomers on adhesive properties of acrylic emulsion PSAs.

0.1

0.0 3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

log (Mw) Fig. 6. The curves of molecular weight distributions of the soluble polymer with various carboxylic monomers.

Heat Flow (mW/mg)

MAA

β-CEA

MBI

Loop tack(N/25 mm) 180° peel strength (N/25 mm)

8.52 ± 1.175 10.67 ± 0.351 CF 329 ± 18.6

9.62 ± 0.387 10.99 ± 0.240 AT 113 ± 9.6

8.87 ± 0.852 10.71 ± 0.389 AF 459 ± 25.8

Shear strength (min)

CF: cohesive failure; AF: adhesive failure; AT: adhesive transfer.

0.0

P(n-BA) P(n-BA+MBI) P(n-BA+MAA) P(n-BA+ -CEA)

-0.1

counterpart prepared with MAA, which was mainly attributed to the lower Tg [42]. It is well known that shear strength increased with the increase in Tg and gel content of the PSA [43]. However, as can be seen from Table 4, the shear strength of the β-CEA adhesive with higher gel content is lower than the MAA adhesive with lower gel content. The reason might be that the negative effect on the shear strength, caused by the decrease in Tg of the PSA, counterbalances the positive effect caused by the increase in gel content of the PSA. Moreover, PSAs prepared with MAA and β-CEA had the same peel strength value. Meanwhile, adhesive transfer (AT) and cohesive failure (CF) were observed during the peel test of PSAs obtained with β-CEA and MAA, respectively. On the other hand, the application of MBI can improve the comprehensive performance of PSA, as shown in Table 4. Adhesive failure (AF) was observed for the peel test of PSA prepared with MBI. It should be concluded here that cohesive strength of the PSAs synthesized with these three carboxylic monomers showed a decreasing order: MBI > MAA > β-CEA, which is corresponding to their shear strength results.

Tg: -33.2

-0.2

Tg: -30.6

-0.3

Tg: -34.6 Tg: -34.1 -0.4 Exo Up

Adhesive properties

-60

-40

-20

Temperature (

0

20

)

Fig. 7. DSC curves of acrylic latex PSAs with different carboxylic monomers added.

3.8. Adhesive properties analysis

4. Conclusions

The results of experimental measurements of loop tack, peel strength, and shear strength of acrylic emulsion PSAs with different carboxylic monomers are presented in Table 4. It can be seen that PSA synthesized with β-CEA have higher loop tack when compared with the

In this paper, carboxylated acrylic pressure sensitive adhesive (PSA) latexes were synthesized via a monomer seeded semi-continuous emulsion polymerization process with three different kinds of carboxylic monomers. The results showed that water solubility of the three 392

International Journal of Adhesion and Adhesives 84 (2018) 387–393

C. Fang et al.

acid monomers fall in the order: β-CEA > MAA > MBI, based on the HLB value and partition coefficient methods. With the incorporation of the carboxylic monomers, latex particle size decreased, moreover, for equiweight amounts of acid comonomer, latex particle sizes fall in the order: MBI > MAA > β-CEA, depending on the water solubility of the corresponding monomer. Besides, water absorption of the latex films prepared with these three carboxylic monomers showed an increasing order: MBI (7.6%) < MAA (9.8%) < β-CEA (11.3%), while the contact angle demonstrated a decreasing order: MBI (102.3°) > MAA (92.0°) > β-CEA (86.7°). PSA prepared with β-CEA exhibited the highest gel content among the three acid monomers. The thermal stability of the polymer films was enhanced with the addition of carboxylic monomers. Moreover, the thermal stability of the three carboxylated polymers follow the order P(n-BA+MAA) > P(n-BA+β-CEA) > P(nBA+MBI). When comparing to MAA, the application of MBI can improve the cohesive strength of PSA, while the application of β-CEA lead to reduction in the cohesive strength of PSA.

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Acknowledgment We would like to thank the financial support from the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, China. The test supports from the teachers of Advanced Analysis & Testing Center of Nanjing Forestry University are also greatly appreciated.

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