Synthesis of melamine-formaldehyde microcapsules containing oil-based fragrances via intermediate polyacrylate bridging layers☆

Yanping He,Shunzhi Yao,Junzheng Hao,Hong Wang,Linhua Zhu,Tian Si,Yanlin Sun,*,Jianhao Lin,*

1 School of Chemical Engineering,Kunming University of Science and Technology,Chenggong Campus,Kunming 650504,China

2 Faculty of Science,Kunming University of Science and Technology,Chenggong Campus,Kunming 650504,China

Keywords:Melamine formaldehyde Microcapsule Oil-based fragrance Particle Polymers Synthesis

ABSTRACT A general and versatile strategy to prepare melamine-formaldehyde(MF)microcapsules encapsulating oil-based fragrances by combining solvent evaporation and in situ polymerization was proposed in this work.The oil-based fragrance was pre-encapsulated by an inner polyacrylate membrane via solvent evaporation,followed by in situ polymerization of MF precondensates as an outer shell.The polyacrylate membrane is used as an intermediate bridging layer to stabilize the oil-based fragrance,and to provide driving forces for in situ polymerization of MF precondensates through electrostatic attractions between carboxyl groups and ammonium ions.It was demonstrated that MF microcapsules containing clove oil were prepared successfully.The amount and the composition of the intermediate polyacrylate bridging layer were critical.Smooth and sphere-shaped MF-clove oil microcapsules were prepared when the weight ratio of polyacrylate to clove oil was over 60 wt%and the concentration of acrylic acid(AA)increased to 10 wt%in polyacrylate.In addition,MF microcapsules containing sunflower oil and hexyl salicylate were prepared by using this method.The work suggests that this new approach can be potentially used to encapsulate various core materials,tuning the shell properties of microcapsules such as thickness,mechanical strength and release properties.

1.Introduction

Microencapsulation has been widely used to encapsulate various core materials to protect them from the outside environment and provide long-lasting releases in the end use of the products[1-6].Perfume oil,as an odor supplier in personal care and household products,has been encapsulated to form perfume oil-filled microcapsules[7-9].In order to encapsulate various perfume oils,the encapsulation technology is critical,since it depends on the properties of the wall material and the core oil[10].

Melamine formaldehyde(MF)resin,a thermosetting polymer material,which can form different crosslinking network structures,is the most common used shell material because of its superior mechanical strength and thermal stability[11,12].In situ polymerization has been always adopted to prepare MF microcapsules[13].By using in situ polymerization,high molecular copolymers,such as poly(acrylamide-acrylic acid sodium)[14]and styrene-maleic anhydride polymer[15]have been used as emulsifiers to stabilize oil droplets and also provide driving forces for capturing MF precondensates to the oil droplets'surfaces.In situ polymerization of MF has been applied to encapsulate various essential oils including sunflower oil[14],tea tree oil[16],thyme oil[17]successfully.However,this method is not workable for those essential oils containing high active unsaturated bonds on the structure such as clove oil[18],Litsea cubeba oil[19]and lemongrass oil[20].The high active double bonds on the structure are susceptible to the environment or may react with the shell materials.Therefore,it may change the physical and chemical properties of the core materials,which may also influence the formulation of microcapsules' shell,leading to the failure in preparation of perfume microcapsules.

So far,the core materials with high active double bounds were reported to be encapsulated via physical methods,such as spray drying[21],emulsion-curing(mutual neutralization of two oppositely charged polymers)[22]and solvent evaporation[23,24]etc.Among them,the solvent evaporation method is simple with high efficiency,which encapsulates the core oil through phase separation between polymer and the core oil during the solvent evaporation process.Phase separation can be controlled by adjusting the molecular weight of polymer,the solution viscosity and the evaporation rate of solvent[25].It has been used to encapsulate soy lecithin,cholesterol,colve oil[26],polylactide[27],and so on[28].However,microcapsules prepared by solvent evaporation have certain limitations,e.g.poor heat and solvent resistances.Since the polymer used is no-crosslinking,resulting in the microcapsule wall of being no-crosslinked.Therefore,there is a need to develop a versatile method which can encapsulate various perfume oils with good heat and solvent resistances,especially those ones with high active unsaturated bonds.

Since the core oil with high active unsaturated bonds can be preencapsulated with polyacrylate via solvent evaporation[28,29],where a polyacrylate shell is formed as a barrier.The barrier can not only separate the core oil from the shell materials,but also provide electrostatic attractions to MF precondensates as driving forces because polyacrylate contains carboxylate ions.Therefore,in this article we reported the promising and general strategy to encapsulate various essential oils,by combining solvent evaporation and in situ polymerization of MF precondensates as shell materials and polyacrylate as bridging layers.The main purpose of our work is to demonstrate that this approach is capable of encapsulating multifarious essential oils.Moreover,we examined the influence of the amount and the composition of the intermediate polyacrylate bridging layer on the preparation of perfume microcapsules.

2.Experimental

2.1.Materials

Clove oil(99 wt%,analytical pure),was purchased from Josiah Chemical Regent(Chengdu,China).Sunflower oil(98 wt%,analytical pure)was purchased from Mayan Reagent(Jiaxing,China)and hexyl salicylate(98 wt%,analytical pure)was purchased from Aladdin Reagent(Shanghai,China).They were used as the core materials.Melamine(analytical grade),methanol(analytical grade),sodium hydroxide(analytical grade),and glacial acetic acid(analytical grade)were purchased from Zhiyuan Chemical Regent(Tianjin,China),and formaldehyde solution(37 wt%-40 wt%,analytical grade)was purchased from Kelon Chemical Regent(Chengdu,China).They were used to prepare MF precondensates.Polyvinyl alcohol(PVA,1788,analytical pure)was purchased from Kelon Chemical Regent(Chengdu,China)and it was used as the dispersant.Dichloromethane(DCM)(analytical pure)was purchased from Aladdin Regent(Shanghai,China)and it was used as the volatile solvent.Methyl methacrylate(MMA,chemically pure),n-butyl acrylate(BA,chemically pure)and acrylic acid(AA,chemically pure)were supplied by Sinopharm Chemical Reagent(Shanghai,China)to prepare polyacrylate.Azobisisobutyronitrile(AIBN,analytical pure)was supplied by GuangFu Fine Chemical Research Institute(Tianjin,China)as the initiator and ethyl alcohol(analytical pure)was purchased from Zhiyuan Chemical Regent(Tianjin,China).

2.2.Preparation of MF fragrance microcapsules

2.2.1.Preparation of polyacrylate

In a typical experiment,a mixture of MMA,BA,AA was decanted into a 250 ml three-neck round-bottom flask,where the mixture was admixed with 3 wt%AIBN.The resulting mixture was immersed in a water bath at room temperature and thoroughly vortexed at 300 r·min-1for 10 h to obtain polyacrylate.Table 1 shows the experimental reagents to prepare polyacrylate with different concentrations of AA.

2.2.2.Preparation of MF precondensates

MF precondensates were prepared by mixing melamine and formaldehyde solution at a molar ratio of 1:3.5.20.36 g formaldehyde solution was added into a 250 ml three-neck round-bottomed flask with a refluxcondenser and a mechanical stirrer.20 wt%aqueous NaOH was added to adjust pH of formaldehyde solution to 8.5-9.0,followed by addition of 8.5 g melamine.Afterwards,the temperature of the mixture was increased to 90 °C under stirring until melamine was dissolved completely.The solution was then cooled to 72°C,followed by addition of a reaction medium of 33 g methanol and kept reaction for 60 min;and then acetic acid was added to adjust pH to 5.5,and kept agitation at 50 °C for 45 min afterwards.Finally,20 wt% aqueous NaOH was added until the pH of the mixture was increased to 7.0.At last,MF precondensates was obtained by distilling methanol and water under atmospheric pressure at 90°C for 20 min,following the method described in patent[30].

Table 1 Preparative conditions of polyacrylate

2.2.3.Pre-encapsulated essential oil by polyacrylate

In a typical experiment,3 g polyacrylate and 5 g clove oil were dissolved in 30 ml DCM to form a homogeneous solution;and then the solution was added into 100 g deionized water containing 1 wt%PVA and mechanically emulsified at 550 r·min-1for 15 min,followed by heating up to 70°C slowly to obtain the pre-encapsulated fragrance oil droplets.In order to study the effect of the intermediate bridging layer‘s amount on the preparation of microcapsules,the weight of polyacrylate was varied from 2 g to 10 g.Polyacrylate with different concentration of AA prepared in §2.2.1 was used to study the concentration of AA on the preparation of microcapsule.Hexyl salicylate and sunflower oil were used to replace clove oil to prepare MF microcapsules encapsulated various essential oils.

2.2.4.Polymerization

The pre-encapsulated microcapsules suspension was cooled down to 25°C at 550 r·min-1.2.5 g MF precondensates(solid content of 40 wt%)were added and the pH was adjusted to approximately 4.5 with glacial acetic acid.The temperature was increased to(65±1)°C,allowing the polymerization of MF precondensates for 3 h.MF microcapsules in suspension were cooled down to 25°C and the pH of the suspension was adjusted to 7.0-8.0 by using 20 wt%aqueous NaOH.Finally,the suspension was filtered and rinsed with an excess volume of deionized water to remove the residual PVA.

2.3.Characterization

2.3.1.Morphology observation

The MF fragrance microcapsules were characterized by an optical microscope(OM)(Leica-DM4000,Leica,Shanghai,China).The surface morphologies and the core-shell structure of the microcapsules were observed by a scanning electron microscope(SEM)(TESCAN,Shanghai,China).Before observation,microcapsules were vacuum sputter coated with a thin layer of gold to provide electrical conduction.

2.3.2.Structure behavior analysis

The chemical structures of MF resin,MF microcapsules,polyacrylate were analyzed by a Fourier transform infrared spectra(FTIR)(BRUKERI27000,Germany).The encapsulation of oil-based fragrance was monitored using an ultraviolet-visible spectrophotometry(UV-vis)(UV-5300PC,Metash Instruments,Shanghai,China).The thermal stabilities of MF resin,MF microcapsules,polyacrylate were tested by the thermal gravimetric analysis(TGA)(STA 449 F3 Jupiter,NETZSCH,Germany)at a scanning rate of 5°C·min-1in a flow of N2from 25°C to 600°C.

3.Results and Discussion

3.1.Fabrication of clove oil microcapsules

Fig.1.Sketched mechanism of the fabrication process of microcapsules containing essential oil by combining the solvent evaporation process and in situ polymerization.

Clove oil,an essential oil with high active unsaturated bonds,was used as a representative core oil.MF microcapsules containing clove oil were fabricated by using polyacrylate as an intermediate bridging layer.The preparation process of the MF-clove oil microcapsules is illustrated in Fig.1.At first,clove oil,polyacrylate and DCM solution was dispersed into 1 wt% PVA aqueous phase via high speed agitation to produce oil in water(O/W)droplets(Fig.1a).By increasing the temperature,phase separation occurred gradually between polyacrylate and oil through DCM evaporation(Fig.1-I),followed by accumulation of polyacrylate at the interface of the O/W droplets(Fig.1b),leading to the formation of the polyacrylate pre-encapsulated oil droplets(Fig.1-II).MF precondensates were added afterwards.The polyacrylate shell worked as a barrier to separate the core oil from MF precondensates to avoid reactions between the shell material and the core oil.Additionally,since acrylic acid was a functional component in polyacrylate,the functional carboxylic acid groups presented on the surface of the polyacrylate pre-encapsulated oil droplets(Fig.1-II).The amino groups on MF precondensates were towards forming ammonium ions in acetic condition.In the meanwhile,carboxylic acids were ionized.The electrostatic attractions between ionized carboxyl groups on polyacrylate layer and ammonium ions on MF precondensates drove polyacrylate-stabilized oil droplets capturing MF precondensates(Fig.1c and-III),and then formed MF microcapsule shell under reaction condition(Fig.1d).Finally,MF microcapsules were fabricated successfully(Fig.1-IV).

Optical microscope was used to monitor the preparation process as showing in Fig.2.O/W droplets were produced after the mixture of clove oil,polyacrylate and DCM was dispersed into the aqueous phase via high speed agitation(Fig.2a).After increasing the temperature,the intermediate bridging polyacrylate layer stabilized oil droplets were formed(Fig.2b).In Fig.2b,the polyacrylate-stabilized oil droplets was sphere-shaped in wet condition,however they were breaking up in dry condition.Therefore,the mechanical strength of polyacrylate shell was weak and it was only acting as an intermediate bridging layer to stabilize the colve oil droplet.After adding MF precondensates and reaction,MF microcapsules containing clove oil were fabricated as shown in Fig.2c.Both in wet and dry conditions,the prepared MF microcapsules were sphere-shaped(Fig.2c).

SEM was used to observe the surface morphologies and the coreshell structure of microcapsules,as shown in Fig.3.Microcapsules are sphere-shaped with smooth surfaces,and the average diameter is approximately 30 μm;and they have core-shell structures and the thickness of the microcapsule shell is approximately 2.5 μm,in which the theoretical intermediate polyacrylate bridging layer and MF thickness is 1.8 μm and 0.9 μm respectively by assuming the average particle size is 30 μm.The slight inconsistence between the result and the theoretical value is mainly because the particle size of microcapsules is scattering.

Fig.2.OM images of MF-clove oil microcapsules in preparation process;the images in the top right corner are in dry condition.(a)O/W droplets;(b)Polyacrylate pre-encapsulated oil droplets;(c)MF-clove oil microcapsules.

Fig.4.The UV-visible spectrum of MF-clove oil microcapsules.

Fig.5.The FTIR spectrum of MF resin(a);MF-clove oil microcapsules(b);Polyacrylate(c).

UV-vis spectra were used to confirm the encapsulation of clove oil(Fig.4).The main component of clove oil is eugenol[31].Here,a peak of λmax～281 nm is observed for pure clove oil,which is consistent with the value reported in literature[32].Meanwhile,the peak value of the adsorption of MF-clove oil microcapsule is also observed in this region,which indicates that clove oil was encapsulated successfully.The structures of shell components were further confirmed by FTIR,as shown in Fig.5.From Fig.5a,curve a is MF resin,it could be seen that the absorption peaks at about 812 cm-1and 1355 cm-1corresponding to the bending vibrations of triazine ring of MF condensation polymer are detected.These peaks were reported as the characteristic peaks of MF resins[33].In addition,the absorption peak of 1557 cm-1is another characteristic peak of triazine ring[33].In curve c,the characteristic peaks of 2968 cm-1for CH stretching absorbance of methyl group,1727 cm-1forstretching vibration,1451 and 1394 cm-1for CH2distortion vibration and 1066 cm-1possible forflexing vibration are observed,which are in correspondence to the functional groups of acrylate copolymer[34,35].For curve b,all these absorption peaks observed in curve a and curve c are detected in the spectrum of the MF microcapsules.The results indicate that clove oil was encapsulated by polyacrylate and MF resin.

TGA results of polyacrylate,MF resin and MF-clove oil microcapsules are presented in Fig.6.For polyacrylate,there is an obvious decrease in mass from 300°C to 420°C.The mass loss in this process is ascribed to the decomposition of polyacrylate.Similar result was observed by Cao et al.[25].MF resin in curve c has three mass loss stages.Below 350°C,the mass loss is about 35 wt%,which is mainly due to the loss of the bound water and small molecules of the uncured MF prepolymer.The mass loss in the second stage(350°C-400°C)and the third stage(＞400 °C)is about 34 wt%.It is due to the degradation of the threedimensional melamine-formaldehyde network structures.The analogous phenomena were also observed by Salaün et al.and Fei et al.[33].Compared with curve a and c,curve b has the same loss in the range of 300 °C-400 °C,which is due to the decomposition of polyacrylate and MF resin.From 120 °C to 300 °C,the mass loss is about 27 wt%,and it is attributed to the evaporation of clove oil whose boiling point was 251°C[31].The microencapsulation efficiency of clove oil was calculated and it is about 49%,in which the theoretical loading amount of clove oil in MF microcapsules is approximately 55 wt%.The mass loss of clove oil may be due to the release of clove oil during storage.The results also confirm that clove oil was encapsulated successfully.

Fig.6.TGA diagrams of essential oil microcapsule.

Fig.7.SEM images of MF-clove oil microcapsules prepared by changing the mass ratio of polyacrylate to clove oil:(a)20 wt%;(b)40 wt%;(c)60 wt%;(d)100 wt%.

3.2.The effect of the amount of polyacrylate on the preparation of microcapsules

Fig.8.SEM&OM images of MF-clove oil microcapsules;the concentration of AA in the acrylate polymer is:(a)0 wt%;(b)5 wt%;(c)7 wt%;(d)10 wt%.

Fig.9.OM images of fragrance microcapsules:(a)Hexyl salicylate microcapsules;(b)Sunflower oil microcapsules.

Polyacrylate was used as an intermediate bridging layer to preencapsulate clove oil and provide driving forces for in-situ polymerization of MF precondensates.Therefore,the intermediate bridging layer is critical to the preparation of MF-clove oil microcapsules.The amount of polyacrylate is relevant to the thickness of the intermediate bridging layer and further correspondent to the success of pre-encapsulation process,which is shown in Fig.7.The mass ratio of polyacrylate to clove oil varied from 20 wt% to 100 wt%,in which the corresponded theoretical thickness of intermediate bridging layer varied from 0.7 μm to 2.5 μm by assuming the average particle size of clove oil droplets as 30 μm.From Fig.7,we could see that when the mass ratio of polyacrylate to clove oil is at 20 wt%and 40 wt%,the sphere shape of pre-encapsulated clove oil is deformed.This is probably because of the swelling effect of clove oil to polyacrylate.After swelling,the mechanical strength of the intermediate polyacrylate bridging layer decreases and it is not strong enough to support the reaction force of polymerization of MF,leading to the shrinkage and collapse of the final microcapsule(Fig.7a and b).When the mass ratio of polyacrylate to clove oil increases to 60 wt%and 100 wt%,the theoretical thickness increases to 1.8 μm and 2.5 μm.Polyacrylate swelling occurred at the inner wall of the intermediate polyacrylate bridging layer,and it is negligible for the outer intermediate polyacrylate bridging layer.Therefore,the sphere shape of the final microcapsules(Fig.7c and d)is maintained well.

3.3.The effect of the concentration of AA on the preparation of microcapsules

The main purpose to introduce an intermediate polyacrylate bridging layer between clove oil droplets and MF shell is to provide driving forces between them,since MF precondensates are water soluble while clove oils are water insoluble.Here,AA with carboxyl groups is introduced to polyacrylate and the driving forces are mainly via electrostatic attractions between carboxyl and amine groups.Therefore,the concentration of AA in polyacrylate is significant to the preparation of MF-clove oil microcapsules,and its effect was studied as shown in Fig.8.In Fig.8,it could be seen that when there is no AA,nearly all MF precondensates polymerize in aqueous phase and form a block of MF resin(Fig.8a);since there is no driving force,resulting in no MF microcapsules.By increasing AA concentration,more carboxyl groups present on the surface of the preencapsulated clove oil droplets and MF precondensate molecules are attracted to the surface followed by polymerization of MF to form microcapsule shell.When AA increases to 5 wt%and 7 wt%,the non-sphereshaped and incomplete MF-clove oil microcapsules are observed(Fig.8b and c);however,MF block polymer is still detected.This is mainly because the number of the active reaction sites is not enough and part of MF precondensates polymerize in water phase.Therefore,the concentration of MF on the surface of the oil droplets is limited,leading to the defect of MF microcapsules.When AA is further increased to 10 wt%,the prepared MF microcapsules are in good spheres and their surfaces are smooth(Fig.8d).Therefore,in order to prepare MF-clove oil microcapsules,the optimum AA concentration is 10 wt%in polyacrylate.

3.4.The effect of the various properties of oil-based fragrances on microcapsules

The versatile method by introducing an intermediate bridging layer to prepare MF-clove oil microcapsules was extended to encapsulate other oil-based core materials including hexyl salicylate and sunflower oil.As shown in Fig.9,MF microcapsules containing hexyl salicylate and sunflower oil in good sphere shape and with smooth surfaces are prepared by using this method.Fig.10 shows the UV-visible of the core oils and the related microcapsules.The correspondence between the core oils and the related microcapsules on the adsorption peaks indicated that core oils were encapsulated successfully by using this method.The results suggest that the method proposed in this work can be potentially used to encapsulate various core materials by using different shell materials,only by choosing the right intermediate bridging layer to provide the driving forces between core and shell materials.

Fig.10.The UV-visible spectrum of different essential oils and fragrance microcapsules.(a)Hexyl salicylate microcapsules;(b)Sunflower oil microcapsules.

4.Conclusions

In this work,MF microcapsules containing clove oil were synthesized by combining the solvent evaporation method and in situ polymerization.MF-clove oil microcapsules were prepared successfully and they were characterized by using an optic microscope,SEM,UVvis spectra,FTIR and TGA.The polyacrylate membrane was used as an intermediate bridging layer to pre-encapsulate the oil-based fragrances.It worked as a barrier to separate the core oil from MF precondensates to avoid reactions between the shell material and the core oil,and provided driving forces for in-situ polymerization of MF precondensates as electrostatic attractions presenting between negative charged carboxyl groups and positive charged ammonia ions.Therefore,the amount and composition of polyacrylate were critical.The sphere-shaped MF fragrance microcapsules with smooth surfaces were prepared when the weight ratio of polyacrylate to clove oil was over 60 wt%and the concentration of acrylic acid(AA)increased to 10 wt%in polyacrylate.Moreover,the developed approach was further extended to encapsulate various oil-based fragrances including sunflower oil and hexyl salicylate successfully.The proposed method is versatile and can be potentially used to encapsulate various core materials,especially the essential oil with high active unsaturated bonds on the structure.It can be further used to tune the shell properties of microcapsules such as thickness,mechanical strength and release properties.