Preparation and Characterization of Soap-Free Vinyl Acetate/Butyl Acrylate Copolymer Latex

materials

Article Preparation and Characterization of Soap-Free Vinyl Acetate/Butyl Acrylate Copolymer Latex

Yifu Zhang, Wenkai Bei and Zhiyong Qin *

School of Resources, Environment and Materials, Guangxi university, Nanning 530000, China; [email protected] (Y.Z.); [email protected] (W.B.) * Correspondence: [email protected]

Received: 7 January 2020; Accepted: 13 February 2020; Published: 14 February 2020

Abstract: The soap-free emulsion of vinyl acetate (VAc)/butyl acrylate (BA) copolymer was prepared by a semi-continuous and pre-emulsification polymerization method, using ammonium sulfate allyloxy nonylphenoxy poly(ethyleneoxy) (10) ether (DNS-86) as a reactive emulsifier. The effects of DNS-86 on the stability of the emulsion and the properties of the latex film were investigated. The infrared spectrum, thermal stability, glass transition temperature and micromorphology of latex were also studied. The results showed that the emulsion had the best stability and the conversion rate reached a maximum of 98.46% when the DNS-86 amount was 4 wt % of the total amount of monomers. Compared with the PVAc latex synthesized with octylphenol polyoxyethylene ether (10) (OP-10), the latex prepared with DNS-86 has higher thermal stability and ionic stability, whereas the latex film has better water resistance.

Keywords: reactive emulsiﬁer; vinyl acetate; soap-free emulsion

1. Introduction Polyvinyl acetate emulsion (PVAc) is a high molecular polymer synthesized by free radical polymerization, which is widely used in industries such as light industry, textile and wood processing [1–3]. Generally, the polyvinyl acetate homopolymer has poor water resistance and weather resistance, and the stability of the emulsion is also poor without a protective colloid, so it is often modified with certain methods to improve emulsion performance [4–6]. Butyl acrylate (BA) has a long-chain alkyl group which provides a certain steric hindrance. When BA copolymerizes with the VAc monomer, it can protect the ester group of the VAc chain and reduce the hydrolysis rate of the ester group. Thereby, the shortcomings such as water resistance and stability of the polyvinyl acetate emulsion are improved [7–9]. The emulsifier has an important influence on the morphology distribution of the latex particles, the polymerization stability of the emulsion, the surface energy and the water resistance of the film. At present, the VAc-BA copolymer emulsion is basically prepared with a conventional emulsifier (ionic or nonionic) in the industry. The emulsifier is mainly composed of a hydrophilic polar group and a lipophilic non-polar group that reduce the interfacial tension of the oil–water system [10–12], making the incompatible oil–water system a uniform and stable emulsion system. However, conventional emulsifiers have some disadvantages [13–15]. First, since the emulsifier is distributed on the surface of the latex particles by physical adsorption, emulsifier desorption is likely to occur with external action such as low-temperature freezing or mechanical shearing, causing the stable state of the emulsion to be destroyed. In addition, some of the emulsifiers migrate to the surface of the latex film during the drying process of the emulsion, affecting the surface properties and water resistance of the latex film [16,17]. The reactive emulsifier contains a C=C bond which finally can be bonded to the polymer

Materials 2020, 13, 865; doi:10.3390/ma13040865 www.mdpi.com/journal/materials MaterialsMaterials 20202020,, 1313,, 865 865 2 2of of 11 11 bonded to the polymer chain by free radical polymerization. It not only achieves the emulsification effect,chain bybut free also radical solves polymerization. the problem of mi It notgration only of achieves convention the emulsificational emulsifiers [18]. effect, but also solves the problemAt present, of migration researchers of conventional have used emulsifiers reactive [18em].ulsifiers to carry out emulsion synthesis of polyvinylAt present, acetate. researchers Schoonbrood have [19] used found reactive in emulsifiersVAc/Veova-10/AA, to carry out VAc/BA/AA emulsion synthesis and VAc/MMA/BA of polyvinyl systemsacetate. Schoonbroodthat some of reactive [19] found emulsifier in VAc/ Veova-10will be buried/AA, VAcinside/BA the/AA particles, and VAc affecting/MMA/BA the systems stability that of thesome emulsion. of reactive Sarkar emulsifier [20] will studied be buried the insideeffect the of particles, different aff ectingemulsifiers the stability on VAc/BA/AA/AM of the emulsion. multi-componentSarkar [20] studied copolymer the effect of emulsi differentons. emulsifiers Experiments on VAc showed/BA/AA that/AM reactive multi-component emulsifiers copolymer help to prepareemulsions. emulsions Experiments with showedbetter water that reactive resistance emulsifiers and bonding help to properties. prepare emulsions Zhu [21] with synthesized better water a fumaricresistance acid and reactive bonding properties.emulsifier, Zhuwhich [21 ]can synthesized produce aa fumaric VAc/BA/Veova-10/HFBMA acid reactive emulsifier, quaternary which can copolymerproduce a VAcemulsion/BA/Veova-10 with a better/HFBMA stable quaternary state and copolymerwater resistance. emulsion Sun with [22] ahas better prepared stable a state cationic and reactivewater resistance. emulsifier Sun that [22 significantly] has prepared improves a cationic the reactive water emulsifierresistance thatand significantlystability of the improves polyvinyl the acetatewater resistance copolymer and emulsion. stability ofZhang the polyvinyl [23] obtained acetate the copolymer soap-free emulsion.VAc/BA latex Zhang by [ 23adding] obtained 1 wt the% 2-acrylamido-2-methylpropanesoap-free VAc/BA latex by adding sulfonic 1 wt %acid 2-acrylamido-2-methylpropane (AMPS) as emulsifier with sulfonic smaller acid particle (AMPS) size. as However,emulsifier studies with smaller on the particle synthesis size. of However, polyvinyl studies acetate on emulsion the synthesis using ofallyloxy polyvinyl reactive acetate emulsifiers emulsion haveusing not allyloxy been reported. reactive emulsifiers have not been reported. InIn thisthis study,study, based based on on the the core–shell core–shell structure struct designure design and self-crosslinking and self-crosslinking technology, technology, the VAc /theBA VAc/BAcopolymer copolymer emulsion wasemulsion successfully was successfully prepared using prepared ammonium using sulfateammonium allyloxy sulfate nonylphenoxy allyloxy nonylphenoxypoly(ethyleneoxy) poly(ethyleneoxy) (10) ether (DNS-86) (10) asether reactive (DNS-8 emulsifier.6) as reactive PVA was emulsifier. used as aPVA protective was used colloid as toa protectiveimprove emulsion colloid to stability; improve diacetone emulsion acrylamide stability; (DAAM),diacetone adipylacrylamide hydrazide (DAAM), (ADH) adipyl and acrylichydrazide acid (ADH)(AA) were and used acrylic as aacid crosslinking (AA) were monomer used as to a enhancecrosslinking polymer monomer cohesion. to enhance The structure polymer of octylphenol cohesion. Thepolyoxyethylene structure of octylphenol ether (10) (OP-10) polyoxyethylene and DNS-86 ether are (10) shown (OP-10) in Figure and 1DNS-86. The e ffareects shown of DNS-86 in Figure on the 1. Theproperties effects ofof theDNS-86 emulsion on the were properties investigated. of the emulsion were investigated.

FigureFigure 1.1. The chemical structures of emulsifiers: emulsiﬁers: ( (aa)) octylphenol octylphenol polyoxyethylene polyoxyethylene ether ether (10) (10) (OP-10) (OP-10) and (b) ammoniumand (b) ammonium sulfate allyloxy sulfate allyloxy nonylphenoxy nonylpheno poly(ethyleneoxy)xy poly(ethyleneoxy) (10) (DNS-86). (10) (DNS-86).

2.2. Materials Materials and and Methods Methods 2.1. Materials 2.1. Materials Ammonium sulfate allyloxy nonylphenoxy poly(ethyleneoxy) (10) ether (DNS-86), diacetone Ammonium sulfate allyloxy nonylphenoxy poly(ethyleneoxy) (10) ether (DNS-86), diacetone acrylamide (DAAM) and adipyl hydrazide (ADH) were supplied from Guangzhou Shuangjian Trading acrylamide (DAAM) and adipyl hydrazide (ADH) were supplied from Guangzhou Shuangjian Co., Ltd. (Guangdong, China). Poly(vinyl alcohol) (PVA-1788) and octylphenol polyoxyethylene ether Trading Co., Ltd. (Guangdong, China). Poly(vinyl alcohol) (PVA-1788) and octylphenol (10) (OP-10) were supplied by Beijing Chemical Works (Beijing, China). Vinyl acetate (VAc), butyl polyoxyethylene ether (10) (OP-10) were supplied by Beijing Chemical Works (Beijing, China). Vinyl acrylate (BA) and acrylic acid (AA) were obtained from the Xilong Chemical Co., Ltd. (Shantou, China). acetate (VAc), butyl acrylate (BA) and acrylic acid (AA) were obtained from the Xilong Chemical Ammonium persulfate (APS) and phosphotungstic acid (HPWA) were purchased from Tianjin Kemiou Co., Ltd. (Shantou, China). Ammonium persulfate (APS) and phosphotungstic acid (HPWA) were Chemical Reagent Co., Ltd. (Tianjing, China). The deionized water was obtained by ion exchange. purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjing, China). The deionized water was2.2. Preparationobtained by of ion Emulsion exchange.

2.2. PreparationPVA and deionizedof Emulsion water were added to a flask and raised to 90 ◦C with stirring; after the PVA was completely dissolved, it was cooled to room temperature to obtain PVA solution. PVAThe emulsifier, and deionized deionized water water, were VAc added and PVAto a solutionflask and were raised added to to90 the°C flaskwith and stirring; stirred after for 0.5 the h PVAat room was temperature completely todissolved, obtain a componentit was cooled I. The to room emulsifier, temperature deionized to obtain water andPVA VAc-BA-AA-DAAM solution. mixedThe monomer emulsifier, was deionized added to the water, flask VAc and stirredand PVA for solu 0.5 htion at room were temperature added to the to flask obtain and component stirred for II. 0.5 h at room temperature to obtain a component I. The emulsifier, deionized water and

Materials 2020, 13, 865 3 of 11

Component I and APS solution were added to a four-necked flask equipped with a thermometer, condenser and electric stirrer, and heated up to 70 ◦C. After the system displayed a blue light, the reaction started. Until the reflux in the condenser tube disappeared, component II and the remaining APS solution were added dropwise in 2–3 h for copolymerization. After the completion of the dropwise addition, the temperature was heated up to 78 ◦C and maintained for 1 h for full reaction, and then cooled to room temperature. Then, ADH was added with an equimolar ratio with DAAM, and finally, the latex was uniformly stirred and discharged. The component amount is shown in Table1. The emulsion using OP-10 was also prepared by the above method.

Table 1. Amounts used in the emulsion polymerization.

VAc BA AA DNS-86 DAAM Water APS PVA ADH Component I/g 30 0 0 0.3–1.2 0 75 0 6 0 Addition/g 0 0 0 0 0 5 0.24 0 0 Component II/g 60 30 1.2 0.9–3.6 2.4 110 0 0 0 Addition/g 0 0 0 0 0 10 0.48 0 2.4 Total/g 90 30 1.2 1.2–4.8 2.4 200 0.72 6 2.4

2.3. Characterization

2.3.1. Conversion Rate The ﬁnal conversion rate was calculated by gravimetric analysis. A certain quantity of latex was cast into a petri dish and dried at 120 ◦C for 1 h. The conversion rate was calculated by the following formulas.

m2 m0 m3 − 100% m4 m solid content(%) m m1 m0 Conversion rate (wt %) = 3× − 4 100%= × − × − 100% (1) m5 × m5 × where m0 is the weight of the petri dish; m1 and m2 are, respectively, the weight of latex before and after being dried to a constant weight. m3 is the total weight of all the materials put in the ﬂask before polymerization, m4 is the weight of materials that cannot volatilize during the drying period and m5 is the total weight of monomers.

2.3.2. Polymerization Stability Coagulate in latex was ﬁltered by ﬁlter meshwork (100 M), washed by deionized water and dried to a constant weight at 120 ◦C. m Coagulation ratio (wt %) = a 100% (2) mb × where ma is the weight of dried coagulate and mb is the total weight of all the monomers.

2.3.3. Water Absorption Rate The latex films were naturally dried at room temperature. The weighted latex films were dipped in deionized water at 25 ◦C for 48 h. Then, the water on the surface of the films was quickly erased by filter papers and the films were weighed again. The water absorption ratio of the films was calculated by the following formula:

W W Water absorption ratio (wt %) = 1 − 0 100% (3) W0 × where W0 and W1 are the weight of the ﬁlms before and after the ﬁlms absorb water. Materials 2020, 13, 865 4 of 11

2.3.4. Mechanical Stability The mechanical stability of the latex was tested by the centrifugal machine with the rotational speed of 4000 rpm for 30 min. If ﬂocculation, precipitation and delamination occur, the test is considered to have failed.

2.3.5. Ionic Stability

Calcium ion stability was tested by adding 2 mL CaCl2 aqueous solutions (0.5 wt %) into 8 mL latex in a test tube and observed after 48 h. If ﬂocculation, precipitation and delamination occurred, the test was considered to have failed.

2.3.6. Contact Angle Contact angle was measured by the sessile drop method at room temperature, using a contact angle meter (DSA100E, KYUSS, Germany).

2.3.7. FTIR A thin latex ﬁlm dried at room temperature for ATR-IR was directly ﬁxed on a sample frame and 1 measured in the range from 4000 to 400 cm− by using an FTIR spectrometer (Nicolet iS50, Nico-let Instrument Corporation, Madison, WI, USA). The number of scans was 16; scans resolution was 1 4.0 cm− .

2.3.8. TEM The morphology of the latex particles was observed via a high-resolution transmission electron microscope (TECNAI, FEI, Portland, OR, USA). The emulsion was slightly diluted to 1 wt %. The diluted emulsion was dripped onto a special copper mesh and dried, then the membrane on the copper mesh was stained with 2% phosphotungstic acid (HPWA) and dried again.

2.3.9. DSC The glass transition temperature (Tg) of the latex film was measured using a DSC (404C/3/G, NETZSCH, Bavaria, Germany) in the nitrogen atmosphere. A certain amount of freeze-dried samples (approximately 6–8 mg) was weighted for measuring. The nitrogen flow rate was 40 mL/min, the heating rate was 10 C/min and the temperature from 40 to 60 C. ◦ − ◦ 2.3.10. TG The thermal properties parameter of the samples were tested via thermogravimetric analyzer (TGA) (DTG-60, Shimadzu, Kyoto, Japan). A certain quality of samples (approximately 3–5 mg) was obtained for testing. The heating rate was 10 ◦C/min from 20 to 600 ◦C, and the nitrogen flow rate was 50 mL/min.

2.3.11. Latex Particle Size Measurement After the emulsion was diluted to 2 wt %, the latex particle size and particle size distribution were measured using a laser particle size analyzer (Beckman LS320, Beckman Coulter, UK).

3. Results and Discussion

3.1. Effect of the DNS-86 Amount on Polymerization Figure2 shows the e ffect of reactive emulsifier DNS-86 amount on conversion rate and emulsion polymerization stability. It can be seen that the conversion rate increases first and then decreases with the increase of the DNS-86 amount. When the amount of DNS-86 increases from 1 wt % (accounting for the mass fraction of the monomer, the same below) to 4 wt %, the monomer conversion rate increases Materials 2020, 13, 865 5 of 11 from 83.67% to 98.64%. This is because the emulsifier will form micelles in the system which provide a free radical reaction place for the monomer and catalyst [24,25]. While high conversion cannot be achieved with an insufficient number of active species, with the emulsifier amount increased, more

Materialsmonomers 2020, polymerized13, 865 in the micelles and the ﬁnal conversion rate of the monomer is increased.5 of 11

FigureFigure 2. 2. EffectEﬀect of of the the DNS-86 DNS-86 amount amount on on conv conversionersion rate rate and and coagulation coagulation ratio.

FigureThe coagulation 2 shows ratiothe effect was usedof reactive to judge emulsi the polymerizationfier DNS-86 amount stability on of conversion the emulsion rate [26 and,27]. emulsionWith the increasepolymerization of the DNS-86stability. amount, It can be the seen coagulation that the conversion ratio decreases rate increases first and thenfirst increases.and then decreasesWhen the with amount the increase of DNS-86 of th ise 1 DNS-86 wt %, the amount. coagulation When the ratio amount is 5.95%. of DNS-86 This is increases due to that from the 1 low wt %emulsifier (accounting concentration for the mass in the fraction system of was the insu monomefficientr, to the the same stable below) latex particles to 4 wt during %, the the monomer reaction, conversionwhich enhances rate increases the collision from chance83.67% ofto each98.64%. particle, This is resulting because the in moreemulsifier coagulates. will form The micelles superior in thepolymerization system which stability provide is obtaineda free radical when reaction the DNS-86 place amount for the ismonomer 4 wt %. and catalyst [24,25]. While high conversion cannot be achieved with an insufficient number of active species, with the emulsifier3.2. Effect ofamount DNS-86 increased, on Mechanical more and monomers Ionic Stability polymerized of Emulsion in the micelles and the final conversion rate ofEmulsifiers the monomer maintain is increased. the stability of the emulsion mainly by electrostatic repulsion (ionic emulsifier) and stericThe coagulation hindrance (nonionicratio was emulsifier)used to judge in the the system polymerization [28–30]. It stability can be seen of the from emulsion Table2 that [26,27]. the Withemulsion the increase prepared of by the the DNS-86 OP-10 emulsionamount, systemthe coag hasulation poor ratio stability decreases of Ca2+ first, and and flocculation then increases. occurs Whenshortly the after amount the addition of DNS-86 of the is 1 CaCl wt %,2 solution. the coagulation In the DNS-86ratio is system,5.95%. This the emulsionis due to that stability the low test emulsifierfailed when concentration the DNS-86 amount in the wassystem 1 wt was %. When insufficient the DNS-86 to the amount stable was latex too particles low to cover during the latexthe reaction,particles interface,which enhances collision the between collision the chance latex particles of each inparticle, the system resulting was intensifiedin more coagulates. by an external The superiorforce and polymerization the stable state stability of the emulsionis obtained was when destroyed. the DNS-86 With amount the emulsifier is 4 wt %. amount increasing, the emulsifier coverage ratio also increases such that the mechanical and ionic stability of the emulsion 3.2.is improved. Effect of DNS-86 When theon Mechanic emulsifieral and amount Ionic exceededStability of 3 Emulsion wt %, superior stability was obtained. Emulsifiers maintain the stability of the emulsion mainly by electrostatic repulsion (ionic emulsifier) and steric Tablehindrance 2. Effect (nonionic of the DNS-86 emulsifi amounter) in on the stability system of emulsion. [28–30]. It can be seen from 2+ Table 2 that the emulsion prepared by theAmount OP-10 of emulsion DNS-86/wt system % has poor stability of Ca , and (1) flocculation occurs Sampleshortly after the addition of the CaCl2 solution. In the5% OP-10DNS-86 system, the 1 2 3 4 5 emulsion stability test failed when the DNS-86 amount was 1 wt %. When the DNS-86 amount was Mechanical stability √ √ √ √ too low to cover the latex particles interface,× ×collision between the latex particles in the system was Ca2+ stability √ √ √ intensified by an external force and× the stable × state of the emulsion was destroyed.× With the Note: √ means that the test is passed. means that the test did not pass. (1)—5% is the best amount of experiment. emulsifier amount increasing, the emulsifier× coverage ratio also increases such that the mechanical and ionic stability of the emulsion is improved. When the emulsifier amount exceeded 3 wt %, superior stability was obtained.

Table 2. Effect of the DNS-86 amount on stability of emulsion.

Amount of DNS-86/wt % Sample 5% OP-10(1) 1 2 3 4 5 Mechanical stability × × √ √ √ √ Ca2+ stability × × √ √ √ ×

Materials 2020, 13, 865 6 of 11

Materials 2020, 13, 865 6 of 11 Note: √ means that the test is passed. × means that the test did not pass, and 1%–5% is the best amount of experiment. 3.3. Effect of the DNS-86 Amount on Water Resistance of the Latex Film 3.3. Effect of the DNS-86 Amount on Water Resistance of the Latex Film Table3 is the water absorption rate data of the latex film with a di fferent emulsifier. It can be seen that as the DNS-86Table amount 3. Effect increases of the fromDNS-86 1 wt amount % to 4on wt water %, the resistance water absorption of the latex ratefilm. of the latex film decreases from 20.18% to 6.84%. This is because the size of the latex particles decreases as the emulsifier Amount of DNS-86/wt % amount increases,Sample which leads to a denser film with less water penetration. In addition,5% the OP-10 reactive 1 2 3 4 5 emulsifier is bonded to the polymer molecular chain, which avoids the emulsifier migration during the Water absorption rate/% 20.18 16.92 13.47 6.84 11.49 26.81 film formation. When the amount of DNS-86 is higher than 4 wt %, the water absorption rate of the latex film increased,Table 3 is Partly the water because absorption some of therate emulsifiers data of the fail latex to connectfilm with to a the different polymer emulsifier. chain through It can the be Cseen=C bond that andas the migrate DNS-86 to theamount surface increases of the latex from film 1 wt after % to film 4 wt formation, %, the water the hydrophilic absorption properties rate of the oflatex the latexfilm decreases film is changed from 20.18% by the to hydrophilic 6.84%. This sulfonic is because acid the group size ofof DNS-86.the latex particles However, decreases the water as absorptionthe emulsifier rate ofamount the film increases, made ofDNS-86 which isleads always to lowera denser than film that with of OP-10. less Inwater addition, penetration. the same In resultsaddition, were the obtained reactive from emulsifier the contact is angle bonded data to of thethe latexpolymer filmof molecular Figure3 (when chain, the which DNS-86 avoids amount the increasedemulsifier from migration 1 wt % during to 4 wt the %, thefilm contact formation. angle When of the the film amount increased of DNS-86 from 61.71 is higher◦ to 77.49 than◦). 4 wt %, the water absorption rate of the latex film increased, Partly because some of the emulsifiers fail to connect to the polymerTable 3. E ffchainect of through the DNS-86 the amount C=C bond on water and resistance migrate of to the the latex surface film. of the latex film after film formation, the hydrophilic properties of the latex film is changed by the hydrophilic Amount of DNS-86/wt % sulfonic acid groupSample of DNS-86. However, the water absorption rate of the film5% OP-10made of DNS-86 is always lower than that of OP-10. In addition,1 the 2 same 3 results 4 were obtained 5 from the contact angle data of the Waterlatex absorptionfilm of Figure rate/ %3 (when 20.18 the 16.92DNS-86 13.47 amount 6.84 increased 11.49 from 1 26.81wt % to 4 wt %, the contact angle of the film increased from 61.71° to 77.49°).

FigureFigure 3. 3.E Effectﬀect of of the the DNS-86 DNS-86 amount amount on on water water contact contact angle angle of of the the latex latex ﬁlm. film.

3.4.3.4. E Effectffect of of the the DNS-86 DNS-86 Amount Amount on on Size Size of of Latex Latex Particles Particles FigureFigure4 illustrates4 illustrates the the e ff effectsects of theof the DNS-86 DNS-86 amount amou onnt theon the average average particle particle size ofsize latex. of latex. With With the DNS-86the DNS-86 amount amount increasing, increasing, the the average average particle particle size size of of latex latex decreases decreases first first and and then then increases. increases. TheThe average average particle particle size size of of latex latex reduces reduces to to 90 90 nm nm when when the the DNS-86 DNS-86 amount amount is is 4 4 wt wt % % and and the the polydispersitypolydispersity index index is is 1.01, 1.01, which which signifies signifies that that a narrow a narrow distribution distribution of the of latexthe latex particle particle size. size. As the As DNS-86the DNS-86 amount amount continues continues to increase, to increase, latex latex particle particle size size increases increases extremely. extremely. This This is is because because of of thethe self-polymerization self-polymerization of of DNS-86, DNS-86, which which produces produces a a water-soluble water-soluble homopolymer, homopolymer, and and the the lack lack of of emulsifieremulsifier on on the the interface interface of of latex latex particles particles reduces reduces particle particle stability. stability. The The particles particles will will agglomerate agglomerate to raiseto raise the surfacethe surface charge charge density density to enhance to enhance particle particle stability, stability, resulting resulting in the increasein the increase of particles of particles finally. finally.

Materials 2020, 13, 865 7 of 11 Materials 2020, 13, 865 7 of 11 Materials 2020, 13, 865 7 of 11

Figure 4. Effects of the DNS-86 amount on particle size.

3.5. FT-IR FigureFigure 4. 4. EffectsEﬀects of of the the DNS-86 DNS-86 amount amount on on particle particle size. size. 3.5. FT-IRFrom the ATR spectrum of the latex film shown in Figure 5, there is no C=C absorption peak at 3.5.1680 FT-IR cm− 1, indicating that all monomer and the reactive emulsifier successfully participated in the From the ATR spectrum of the latex film shown in Figure5, there is no C =C absorption peak at reaction.From The the deformationATR spectrum vibration of the latex of -CH film3 appears shown inat aFigure wavenumber 5, there isof no 1432 C=C cm absorption−1. The asymmetric peak at 1680 cm 1, indicating that all monomer and the reactive emulsifier successfully participated in the reaction. stretching−−1 and bending vibration of -CH2- are at 2936 cm−1 and 1373 cm−1, respectively. At 1733 cm−1, 1680 cm , indicating that all monomer and the reactive emulsifier successfully1 participated in the The deformation vibration of -CH3 appears at a wavenumber of 1432 cm− . The asymmetric stretching and it is a stretching vibration peak of C=O. The3 stretching and bending vibration absorption−1 peaks of the reaction. The deformation vibration of -CH1 appears at a 1wavenumber of 1432 cm .1 The asymmetric bending vibration of -CH2- are at 2936 cm−−1and 1373 cm− ,−1 respectively. At 1733 cm− , it is a stretching stretchingsecondary and amide bending N-H vibrationbond at of3332 -CH cm2- are and at 29361538 cm cm−1 and and 1373 the cm C-N−1, respectively.bond stretching At 1733 vibration cm−1, vibration peak of C=O. The stretching and bending vibration absorption peaks of the secondary amide itabsorption is a stretching peak vibration at 1239 peak cm− 1of indicate C=O. The that stretching DAAM and entered bending the vibrationpolymer absorptionbackbone peaksthrough of thethe N-H bond at 3332 cm 1 and 1538 cm 1 and the C-N bond stretching vibration absorption peak at 1239 cm 1 secondaryreaction. In amide addition, N-H− the bond N-H(NH at 3332−2) absorption cm−1 and 1538peak cmat 3310−1 and cm the−1 was C-N not bond observed, stretching indicating vibration that− indicate that DAAM entered the polymer backbone through the reaction. In addition, the N-H(NH ) absorptionADH had reacted.peak at The 1239 absorption cm−1 indicate peak appearingthat DAAM at 1669entered cm− 1th ise the polymer stretching backbone vibration through absorption the2 1 absorption peak at 3310 cm− was not observed, indicating that ADH had reacted. The absorption peak reaction.peak of Inthe addition, C=N bond, the N-H(NH indicating2) absorption that the ketopeakne at carbonyl3310 cm− 1 groupwas not of observed, DAAM reactsindicating with that the appearing at 1669 cm 1 is the stretching vibration absorption peak of the C=N bond, indicating that the ADHhydrazide had reacted. group of The ADH.− absorption peak appearing at 1669 cm−1 is the stretching vibration absorption ketone carbonyl group of DAAM reacts with the hydrazide group of ADH. peak of the C=N bond, indicating that the ketone carbonyl group of DAAM reacts with the hydrazide group of ADH.

Figure 5. FT-IR spectrum of the latex ﬁlmfilm withwith 4%4% DNS-86.DNS-86.

3.6. DSC 3.6. DSC Figure 5. FT-IR spectrum of the latex film with 4% DNS-86. Figure6 6 is is a a DSCDSC curvecurve ofof thethe latexlatex film.film. ItIt cancan bebe seenseen thatthat thethe latexlatex filmfilm hashas twotwo glassglass transitiontransition 3.6.temperatures DSC (Tg): Tg1 = 5.05 ◦C and Tg2 = 34.8 ◦C. According to the design of the sample, it is known temperatures (Tg): Tg1 =− −5.05 °C and Tg2 = 34.8 °C. According to the design of the sample, it is thatknown the compositionsthat the compositions of the core of phase the core polymer phase and polymer the shell and phase the polymershell phase are different.polymer are According different. to Figure 6 is a DSC curve of the latex1 film.w It1 canw be2 seen thatw ithe latex film has two glass transition the Gibbs–Dimarzio formula [31–33]: Tg = Tg + Tg + + Tg (w1, w2, ... , wi—mass percentage of temperatures (Tg): Tg1 = −5.05 °C and Tg2 = 134.8 °C.2 According··· i to the design of the sample, it is known that the compositions of the core phase polymer and the shell phase polymer are different.

Materials 2020, 13, 865 8 of 11 Materials 2020, 13, 865 8 of 11 AccordingMaterials 2020 ,to13 ,the 865 Gibbs–Dimarzio formula [31–33]: ⋯ (w1, w2, …, wi—mass8 of 11 According to the Gibbs–Dimarzio formula [31–33]: ⋯ (w1, w2, …, wi—mass percentage of specific monomers in the polymer composition; Tg1, Tg2, …, Tgi—glass transition percentage of specific monomers in the polymer composition; Tg1, Tg2, …, Tgi—glass transition temperaturespecific monomers of inspecific the polymer monomer composition; homopolyme Tg , Tg r/°C;, ... , TgTg—glass—glass transition transition temperature temperature of specific of temperature of specific monomer homopolyme1 2r/°C; Tg—glassi transition temperature of copolymer),monomer homopolymer the theoretical/ C; value Tg—glass of the transition shell phase temperature is −2.15 °C of and copolymer), the core phase the theoretical is 30 °C. value Therefore, of the copolymer), the theoretical◦ value of the shell phase is −2.15 °C and the core phase is 30 °C. Therefore, itshell can phase be inferred is 2.15 that◦C andTg1 theis the core glass phase transition is 30 ◦C. Therefore,temperature it can of bethe inferred shell phase that Tg copolymer,1 is the glass and transition Tg2 is it can be inferred− that Tg1 is the glass transition temperature of the shell phase copolymer, and Tg2 is thetemperature glass transition of the shell temperature phase copolymer, of the andcopolymer Tg is the of glass the transitioncore phase. temperature This shows of the that copolymer the latex of the glass transition temperature of the copolymer2 of the core phase. This shows that the latex particlesthe core phase.have Thistwo showsphase thatstructures. the latex While particles crosslinking have two phase between structures. molecular While crosslinkingchains restricts between the particles have two phase structures. While crosslinking between molecular chains restricts the movementmolecular chains of molecular restricts thesegments, movement so ofthe molecular values of segments, Tg1 and soTg the2 are values different of Tg 1fromand Tgthe2 aretheoretical different movement of molecular segments, so the values of Tg1 and Tg2 are different from the theoretical value.from the On theoretical the other value.hand, Ontwo the Tg other indicated hand, the two phase Tg indicated separation the phasebetween separation the core between phase polymer the core value. On the other hand, two Tg indicated the phase separation between the core phase polymer andphase the polymer shell phase and polymer. the shell phaseHowever, polymer. the final However, latex film the finalis visibly latex transparent, film is visibly which transparent, indicates which that and the shell phase polymer. However, the final latex film is visibly transparent, which indicates that theindicates phase that separation the phase existing separation is only existing a microphase is only a microphaseseparation. separation. the phase separation existing is only a microphase separation.

Figure 6. DSC curve of the latex film with 4% DNS-86. Figure 6. DSC curve of the latex film film with 4% DNS-86. 3.7. TGA 3.7. TGA Figure 7 shows the TG and DTG (Derivative Thermogravimetric) curves of the latex films Figure7 shows the TG and DTG (Derivative Thermogravimetric) curves of the latex films synthesizedFigure 7with shows two theemulsifiers. TG and DTGTable (Derivative4 shows the Th specificermogravimetric) data of the thermogravimetriccurves of the latex test films of synthesized with two emulsifiers. Table4 shows the specific data of the thermogravimetric test of eachsynthesized sample. withIt can two be seenemulsifiers. from Figure Table 7 4and shows Table the 4 specificthat the datainitial of decomposition the thermogravimetric temperature test ofof each sample. It can be seen from Figure7 and Table4 that the initial decomposition temperature of theeach latex sample. film preparedIt can be seenwith fromDNS-86 Figure was 7 293.5 and Ta°C.ble It is4 that12.6 the°C higherinitial decompositionthan the initial decompositiontemperature of the latex film prepared with DNS-86 was 293.5 C. It is 12.6 C higher than the initial decomposition temperaturethe latex film (280.9 prepared °C) withof the DNS-86 latex filmwas 293.5prepared ◦°C. It with is 12.6 OP-10. ◦°C higher The temperaturesthan the initial of decomposition the 50 wt % temperature (280.9 C) of the latex film prepared with OP-10. The temperatures of the 50 wt % thermal thermaltemperature mass (280.9 loss◦ °C)of theof thefilm la texobtained film prepared by DNS-86 with and OP-10. OP-10 The were temperatures 340.1 °C ofand the 333.5 50 wt °C, % mass loss of the film obtained by DNS-86 and OP-10 were 340.1 C and 333.5 C, respectively, and the respectively,thermal mass and loss the of residual the film carbon obtained amounts by wereDNS-86 3.2 wtand % OP-10and◦ 1.5 were wt %. 340.1 ◦The data°C andshow 333.5 that the°C, residual carbon amounts were 3.2 wt % and 1.5 wt %. The data show that the latex film prepared by latexrespectively, film prepared and the residualby the DNS-86carbon amounts has better were thermal 3.2 wt %stability. and 1.5 wtThis %. isThe because data show DNS-86 that theis the DNS-86 has better thermal stability. This is because DNS-86 is polymerized into the molecular polymerizedlatex film prepared into the molecularby the DNS-86 chain. Thehas large-volumebetter thermal flexible stability. side groupThis itselfis because can play DNS-86 a role ofis chain. The large-volume flexible side group itself can play a role of steric hindrance and shielding stericpolymerized hindrance into and the shielding molecular effect, chain. delaying The large-volume the decomposition flexible side reaction group of itself the side can chainplay a at role high of effect, delaying the decomposition reaction of the side chain at high temperature. temperature.steric hindrance and shielding effect, delaying the decomposition reaction of the side chain at high temperature.

FigureFigure 7. 7. TGTG ( (aa)) –DTG –DTG ( (bb)) curve curve of of the the latex latex film ﬁlm with with 4% 4% DNS-86 DNS-86 and and 5% 5% OP-10. OP-10. Figure 7. TG (a) –DTG (b) curve of the latex film with 4% DNS-86 and 5% OP-10.

Materials 2020, 13, 865 9 of 11 Materials 2020, 13, 865 9 of 11

TableTable 4. 4.Thermal Thermal degradation degradation temperature temperature of of films. films. Secondary Degradation Temperature SampleSample First FirstDegradation Degradation Temperature Temperature ( C) (°C) Secondary Degradation Temperature ( C) ◦ (°C) ◦ T0-1 T0-1 Tp-1 Tp-1 Tf-1 Tf-1 T0-2 T0-2 Tp-2Tp-2 Tf-2Tf-2 4%DNS-864%DNS-86 293.5293.5 327.9327.9 340.1340.1 375.3375.3 445.3445.3 510.7510.7 5%OP-10 280.9 325.1 333.5 374.8 444.1 509.5 5%OP-10 280.9 325.1 333.5 374.8 444.1 509.5 T0: initial degradation temperature; Tp: maximum degradation rate temperature; Tf: terminated temperature; T1: T0: initial degradation temperature; Tp: maximum degradation rate temperature; Tf: terminated temperature; T1: first degradation; T2: second degradation. first degradation; T2: second degradation. In addition, the DTG curve shows that there are two peaks in the decomposition process of the film. ThisIn addition, information the DTG demonstrates curve shows that that the thermalthere are decomposition two peaks in the of the decomposition material is by process two stages. of the Thefilm. first This stage information is mainly demonstrates the elimination that of the polymer thermal branches, decomposition and the of formation the material of small is by molecules two stages. suchThe as first acetic stage acid. is mainly The second the elimination stage is mainly of polymer the thermal branches, degradation and the of theformation C-C backbone of small by molecules random breaks,such as which acetic eventually acid. The forms second a carbonized stage is mainly layer [the34,35 thermal]. degradation of the C-C backbone by random breaks, which eventually forms a carbonized layer [34,35]. 3.8. TEM 3.8. TEM Figure8 shows TEM images of the latex particles at di fferent scales. Latex particles colored by HPWAFigure can be 8 clearlyshows seenTEM on images the graph. of the The latex core particles phase isat completely different scales. stained Latex with particles HPWA. colored The shell by phaseHPWA exhibits can be di clearlyfferent seen colors on due the tograph. the inclusion The core of phase other is monomers completely such stained as BA. with The HPWA. latex particles The shell arephase spherical exhibits or ellipsoidaldifferent colors and randomlydue to the dispersedinclusion of together. other monomers The particle such diameter as BA. The was latex about particles 90 to 100are nm, spherical complying or ellipsoidal with the particle and randomly size analyzer dispersed results. together. In addition, The distinguishableparticle diameter phase was separationabout 90 to of100 the nm, bright complying and shadow with layers the particle can be observed,size analyzer which results. confirmed In addition, the existence distinguishable of the core–shell phase structureseparation latex of particles.the bright and shadow layers can be observed, which confirmed the existence of the core–shell structure latex particles.

FigureFigure 8. 8.TEM TEM images images of of emulsion emulsion particles particles with with 4% 4% DNS-86: DNS-86: (a ()a 500nm,) 500nm, (b ()b 200nm.) 200nm.

4.4. Conclusions Conclusions 1.1. InIn this this study, study, a a soap-free soap-free VAc VAc/BA/BA copolymer copolymer was wa successfullys successfully prepared prepared by by the the semi-continuous semi-continuous andand pre-emulsification pre-emulsification polymerization polymerization method. method. When When the DNS-86 the DNS-86 amount amount is 4 wt %, is the 4 monomerwt %, the conversionmonomer isconversion 98.64%, the is coagulation98.64%, the ratiocoagulation is 0.34% ratio and theis 0.34% latex has and better the latex ionic has stability. better ionic 2. Comparedstability. with the PVAc latex prepared with OP-10, the latex film prepared with DNS-86 has a 2. lowerCompared water with absorption the PVAc rate, latex which prepared means thatwith theOP-10, addition the latex of reactive film prepared emulsifier with significantly DNS-86 has improvesa lower water the water absorption resistance rate, of which the latex means film. that the addition of reactive emulsifier significantly 3. FTIRimproves demonstrated the water that resistance each monomer of the latex takes film. part in copolymerization. A distinguishable phase 3. separationFTIR demonstrated of core phase that andeach shell monomer phases takes was part observed in copolymerization. obviously by TEM. A distinguishable DSC showed phase that theseparation copolymer of core had phase two Tg, and which shell phases is consistent was observed with the obviously design of by the TEM. core–shell DSC showed structure. that TGthe indicated copolymer that had the two large-volume Tg, which flexibleis consistent side group with the of DNS-86 design raisedof the thecore–shell thermal structure. stability ofTG theindicated latex. that the large-volume flexible side group of DNS-86 raised the thermal stability of the latex.

Materials 2020, 13, 865 10 of 11

Author Contributions: Conceptualization, Y.Z.; Investigation, W.B.; Project administration, Z.Q.; Resources, Y.Z.; Software, Z.Q.; Supervision, Z.Q.; Visualization, Y.Z. and W.B.; Writing—original draft, W.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Innovation-Driven Project Funds of Guangxi (AA17204087-15). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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