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Synthesis and Characterization of Water and Poly(vinyl acetate) Based Wood Adhesives

A. Sarac1, C. Elgin2 and P. Yeşilbaş Şen2 1 Department of Chemistry, University of Yıldız Technical, 34220 Esenler, İstanbul, Turkey 2 Bes Kimya Kauçuk Research Laboratory, 34555 Hadımköy, İstanbul, Turkey

Poly(vinyl acetate) (PVAc) and vinyl acetate-acrylic copolymer latices are very important in industrial applications such as adhesives, paint, binders for nonwovens, paper and textile additive industries, and many related industries. Their production is growing steadily in both actual quantities and different applications. PVAc emulsion is environmentally friendly and exhibits superior prospects due to its waterborne nature, having lower volatile organic compounds and better film-forming property. Nevertheless, its inherent shortcomings, such as poor heat-, water- and creep-resistances, make its applications limited in specific fields. In order to improve the drawbacks of PVAc emulsion, great effort has been made through various experimental techniques and modified approaches. Among them, it is considered that core-shell (CS) manufacturing technique is a promising approach to improve disadvantages of PVAc-based emulsion. The fundamental concept to obtain desirable performances through PVAc-based core-shell (PVAc-CS) emulsion could be materialized by introducing various components into the core or/and shell, thus utilizing composite effect of CS structure to meet the requirements under the precondition of keeping single-component PVAc-based emulsion [1-4]. A systemic and comprehensive study with regard to formation, morphology and application can play a significant role in better understanding and facilitating the development of PVAc-CS emulsion both in theory and practice. Therefore, in this chapter, we summarized the synthesizing and characterization of PVAc latexes. Additionally, formation and morphological control mechanism, the factors affecting the formation, as well as the properties and applications of PVAc-CS emulsion were discussed with the fundamental aspects, especially in practical and end use applications.

Keywords: vinyl acetate; emulsion polymerization; wood adhesives; water based adhesives

1. Adhesives

Adhesives are defined as nonmetallic substances capable of joining materials by surface bonding (adhesion), the bond itself possessing adequate internal strength (cohesion). With the developments in chemistry in the early 1900s, the technology of adhesives began to successful. Phenolic resins, melamine resins, and urea resins, polymer dispersions, epoxy resins, and cyanoacrylates largely superseded the natural adhesives and, with a multitude of bindings agents, form the basis for modern adhesive technology, which is one of the most advanced joining processes. An adhesive is composed of basic raw materials, which are called binders and which determine its adhesiveness and its internal strength, and of frequently necessary auxiliaries, which establish particular end-use and processing characteristics. The binders used for adhesives are primarily high polymers having high molecular weights and optimal strength properties. High internal strength is essential if the adhesive in an adhesive joint is to be able to transmit forces from one adherent to the other. Most adhesives contain high molecular weight organic substances as their basic raw materials or reactive organic compounds that are preliminary stages of polymers and that react during the bonding process to form polymers. Practically any standard polycondensate, homopolymer, and copolymer and also polyadducts may be used, provided they can be applied as solutions, dispersions, emulsions, or melts. In addition to these raw materials, auxiliaries such as plasticizers, fillers, resins, thickeners, solvents, antiagers, protectives, hardeners, or setting retarders, are required, depending on the end use of adhesives [5]. Basic raw materials are natural and synthetic polymers or monomers and prepolymers which can form such polymers. The required application properties frequently can be obtained by using different basic raw materials or different kinds of formulations. Accordingly, the raw materials often cannot be assigned unequivocally to specific types of adhesives. In principle, almost every polymer and resin and also many other substances may be used for the production of adhesives. But it is not possible to benefit from every possible raw material. Poly(vinyl esters) are one of the most important groups of raw materials for adhesives. Poly(vinyl acetates) (PVAc) are important in solvent adhesives. Synthetic resin emulsions, based on poly(vinyl esters) with a solids content of 50 - 70 wt % are the principal raw materials for emulsion-based adhesives. Besides PVAc emulsions, emulsions of vinyl acetate (VAc) copolymers with ethylene, (meth)acrylates, vinyl chloride, maleic esters and vinyl laurate are of considerable importance as internally plasticized synthetic resin emulsions.

Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) _______________________________________________________________________________________________

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PVAc emulsions for adhesives contain preferably poly(vinyl alcohol) (PVOH) as a protective colloid. Spray-dried PVAc emulsions, known as redispersion powders, are used in combination with cellulose ethers as binders in floor-leveling compositions and for increasing solids content in emulsion-based adhesives. PVAc are used as solid polymers for formulating solvent adhesives. PVOH is used primarily as a protective colloid for PVAc emulsions. Relatively small quantities are added to modify emulsion-based adhesives. Aqueous solutions of PVOH, in some cases combined with fillers and emulsions, are used as paper glues and as labelling adhesives. Aqueous emulsions of polymeric compounds, e.g., homopolymers of VAc and vinyl propionate, VAc copolymers with ethylene or maleic esters; polyacrylic esters, styrene copolymers, have industrial importance, and they can be used in paper, wood industries, also bonding to plastics [5]. The most proper and widely used polymerization process to produce adhesives is emulsion polymerization.

2. Emulsion Polymerization

Emulsion polymerization is scientifically, technologically and commercially important reaction. It was developed during the World War II because of the need to replace the latex of natural rubber. The synthetic rubbers were produced through radical copolymerization of styrene and butadiene. Today, emulsion polymerization is the large part of a great global industry. It produces high molecular weight colloidal polymers and no or negligible volatile organic compounds. The reaction medium is usually water and this facilitates agitation and mass transfer, and provides a fundamentally safe process. Moreover the process is environmentally friendly. Other domains justifying also a huge production are that of the versatility of the reaction and the ability to control the properties of the emulsion polymers produced. Because of these unique properties, the industry including waterborne polymers produced by emulsion polymerization continues to expand incrementally. The wide range of the products which include synthetic rubbers, paints, adhesives, paper coatings, toughened plastics, floor polishes, sealants, cement and concrete additives, and nonwoven tissues can be produced by emulsion polymerization. More sophisticated applications are also found in cosmetics, biomaterials and high-tech products in last decade. Main characteristics of emulsion polymerization, advantages and disadvantages, are listed as follows, respectively: - Efficient heat transfer - Low viscosity even at high solids - High monomer conversion - High rates of polymerization - Achievement of high molecular weight - Direct application of the final product - High surface area of particles - Contamination of product with additives, emulsifiers, e.g. - Not easy to control in the case of hydrophilic monomers In the emulsion polymerization products, PVAc emulsion homopolymer and VAc based emulsion copolymers have a great importance in industrial aspect as well as scientific aspect. They account for 28% of the total waterborne synthetic latexes. PVAc emulsion homopolymer was the first synthetic polymer latex to be made on a commercial scale. Its production is growing steadily in both actual quantities and different applications. The largest volume applications are in the area of coating and adhesive. It offers good durability, availability at low cost, compatibility with other materials, excellent adhesive characteristic, and ability to form continuous film upon drying of the emulsions. In addition, VAc can mostly be copolymerized with ethylene, acrylic esters, versatic ester, or vinyl chloride. So it is possible to overcome some poor properties of the VAc homopolymer such as weak resistance against alkaline and water, being hydrolysis, and impractical values of glass transition temperature (Tg) and minimum film forming temperature (MFFT) for many applications by these copolymerizations. The reaction variables play a determinative role on the emulsion copolymerization reactions and the properties of the resulting copolymers due to the significant differences between the properties of VAc and other comonomers. The emulsion polymerization of VAc possesses the rather typical properties in comparison the emulsion polymerizations of the comonomers. VAc has high water solubility, a high monomer-polymer swelling ratio, and a high chain transfer constant. Thus, the type of emulsion polymerization process (batch, semi-continuous or continuous) is a very important factor affecting the polymerization mechanism and the final properties of the copolymers [5]. Semi-continuous type emulsion polymerization process is widely used in industrial applications, due to easily remove reaction heat from the reactor. The other advantage is coming from the mechanism itself; as there will be no formation of new particles after the first feed stage, the final particle size will be determined by the first emulsifier concentration. Water and PVAc based adhesives can have easily certain application properties by varying the first feed (i.e emulsifier, initiator, and monomer (type and concentration)).


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In batch polymerization, all the ingredients except initiator/s are charged into the reactor. Until adequate time is given for proper emulsification and suitable jacket temperature is reached, an initiator solution is added to start the reaction. Reaction goes on until % 99.9 of VAc is consumed. Experiments show that there is a great deal of increase in heat resistance and setting speed (for wood glues and paper adhesives) properties, this clearly shows that the resulting polymer has high molecular weight and broad bimodal particle size distribution. PVOH or protective colloid concentration in the aqueous phase can significantly be increased to yield high solids content with very low dilatancy. There may be severe risks based on worst case scenarios but, with slightly higher initial investment all these risks can be minimized. There are also many different variables such as agitation speed, initiator type and concentration, emulsifier type and concentration, feeding policy, feeding rate and temperature. Eventually, the production of vinyl acetate based copolymer latexes in a wide range of molecular, particle-morphological, colloidal, physical and film properties can be possible for use in wide variety of applications by change in molecular structure of the comonomer, copolymer composition and the emulsion polymerization variables.

3. Water and Poly(vinyl acetate) Based Adhesives

Today for the assembly gluing of wood, for gluing veneers, plastic sheets, and films, and for the manufacture of wood-based materials (chipboard, plywood, hardboard, profiles), synthetic adhesives are used almost exclusively. The conventional use of adhesives based on natural products (glutin and casein glues) is confined to a few special cases only [5]. PVAc emulsion “white glues” are used widely for the gluing of wood and wood-based materials. Wood panels, chipboard, and similar wooden materials can be fixed with aqueous adhesives based on PVAc or polyacrylate emulsion. The adhesives must be pasty and have high initial strength [5]. They generally contain emulsifier/s, protective colloid, small quantities of solvents and/or plasticizers to adjust the film-forming temperature and chalk as a pH buffer and filler. Emulsifier in the emulsion polymerization are almost one of the most important component, and anionic or/and non-ionic types of it is generally used in this polymerization process. Cationic and amphoteric types are very rarely used in this process, because of low stability, high cost, and incompatible with the monomers, etc. The type of emulsifier affects the stability mechanism, and also properties of latex. Some important point of emulsion polymers are listed in Table 1. Table 1 Some characteristics of the anionic and non-ionic emulsifiers.

Emulsifier Type Anionic Nonionic Stability mechanism Electrostatic stability Steric stability Supported property Control of Particle Size Enhances electrolyte stability

Contribution to final property Controlled PSD Enhances shear, freeze-thaw & storage stabilities

Critic micelle concentration (CMC) is the most important parameters of an emulsifier for emulsion polymerization, because of forming micelle and beginning the polymerization. CMC and preferable polymerization temperature of some most common emulsifiers are given in Table 2. Table 2 CMC and polymerization temperature of some commercially available emulsifiers.

Emulsifier CMC [g/L] T [°C] Sodium-n-alkyl sulfate, C12H25SO4Na 2.60 35-60

Sodium-n-alkyl sulfonate, C14-16H29-33SO3Na 0.65 25 Ether-alcohol sulfate, C12-14H25-27-(OCH2CH2)2-OSO3Na 0.86 25

Sodium-di-n-alkyl succinate, C8H19-O2C-CH-SO3Na 0.19 50 Tri-tert.-butylphenylether sulfate, (C4H9)3-(C6H4)-O-(CH2CH2O)7-SO3Na 0.50 50

Alkylphenolpolyglycolether, C9H19-(C6H4)-O-(CH2CH2O)30-H 0.12 55 They have a solids content of approximately 45 – 60 wt %, a film-forming temperature of 0 to 15°C, and glass transition temperature of 35 to - 70°C. These final properties are depending on monomer types used in polymerization, water solubility of monomer, and also type of latex (homopolymer, copolymer, core-shell, etc.). Some values of the monomers are listed in Table 3. Special components may be added to increase the setting rate or to prolong the open time. The application weight is 100 – 200 g/m2, the open time 5 – 20 min, and the clamping time 5 – 45 min for a compressive force of 5 – 20 N/cm2. The exact figures strongly depend on the glue coating weight and the type of substrate [5].


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Table 3 Some parameters of monomers used in latexes.

Homopolymer of Tg (°C) Water solubility (g/100 mL) Vinyl acetate + 29 2.5 Vinyl propionate + 7 6.0 VeoVa 10 - 2 <0.1 Vinylidene chloride + 80 0.11 Styrene + 100 0.027 Methyl methacrylate +105 1.3 2-Ethyl hexyl acrylate -85 0.01 n-Butyl acrylate -54 0.16 Acrylic acid (crystalline) + 166 miscible Methacrylic acid + 185 miscible Acryl amide (crystalline) + 153 204 Hydroxy ethyl methacrylate + 55 -

3.1 Synthesis and Characterization of Wood Adhesives

3.1.1 Synthesis

Emulsion polymers (latexes) are increasingly used in adhesives, inks and coating materials due to its benign environmental and safety advantages. However, the main challenge with those polymer latexes is how to obtain good film formation and high performance films without aid of organic coalescing agent. A typical approach to solve this problem is based on the incorporation of some functional groups in the polymer latex for post-cross-linking. The presence of post-cross-linking in polymer latex offers major advantages with regard to the quality and the ultimate properties of the latex film such as increasing surface hardness and low solvent sensitivity of latex film. The frequently employed functional groups are mainly from various self-condensable monomers such as N-methylol acrylamide (NMA), N-(iso-butoxymethyl) acrylamide, glycidyl methacrylate, dimethyl meta-isopropenylbenzyl isocyanate, alkoxysilanes, and t-butylcarbodiimidoethyl methacrylate. Among all these functional monomers, alkoxysilane monomers are becoming increasingly interesting subjects in recent years since alkoxysilane-functionalized latexes not only have the benefits associated with post-cross-linking as mentioned above, such as enhanced mechanical properties, but also are advantageous for especially the outdoor durability or weatherability, the stain and water resistance of the latex films. Moreover, alkoxysilane functionalized latexes have unique advantages in application: it can yield self-cross-linkable latexes at ambient temperature and has the facile mode of cross-linking afforded by alkoxysilane groups [6]. In contrast, limited synthetic polymers as homopolymer possess excellent adhesion characteristics. PVAc homopolymers combined with external plasticizers show high adhesion, sufficient cohesion, and are easy to use [5]. In industrial application preferred using in copolymer form of them, because of some excellent properties, such as water, acid and/or alkali resistance, mechanical strength, hydrophobicity, etc.

3.1.2 Characterization

In practice the following adhesive properties are mainly determined for all types of adhesives including wood adhesives. – Solids content: solvent evaporate after keeping in room temperature and a drying oven for a certain time – Viscosity or Particle size: it is certain to the field of application and should lie in a certain range. The particle diameter of wood glue lattices suitable for direct application should have an average particle size between 0.3-2 micrometers. Viscosity of the latex can be verifying according to application requirements. If the emulsion itself will be used for edge gluing with a roller, the viscosity value has to be between 6.000-12.000 cP. If the application is through a nozzle system and finger joint gluing is targeted, the value should be between 2.000-4.000 cP. – Glass transition temperature: the temperature at which the polymer goes from hard to rubbery state. It is unique for polymers and shows how the polymer will behave at the service temperature. – Shelf life or storage temperature range: almost all the lattices should be stored between 5-30°C for a maximum of one year. The major restriction is coming from the water content of the product. – Mechanical properties: tensile strength, modulus, Shore hardness, and elongation to break. Besides emulsifier and/or initiator type and concentration, crosslinkers play a critical role on the above. This parameter is application specific and be studied before scaleup. - Discoloring: In wood glues, neutralization is important for it prevents tannin containing woods from discoloring on contact with iron. This does not apply to water resistant wood glues for it thoroughly distorts the properties.


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- Heat resistance, the relevant test is called WATT 91. Brief information of this test was given as follows. • preparation of test specimen according to DIN EN 205. • after 7 days of storage in standard climate 1 h in a heated chamber at 80°C. • measure of bond strength as described in DIN EN 204, but using the heated (80°C) test specimen. • determination of bond strength at 80°C in N/mm² as the average of the measured bond strength values.

- Open time (manual finger test): • Wet emulsion film is applied to wood surface • Chronometer is started just after the film is applied and stopped until there is no wet feeling MFFT • Having high boiling temperature solvents and plasticizers are known to decrease MFFT, but has an adverse

effect on the strength of adhesion so, suitability tests should be made beforehand. • The amount of agents mentioned above should lie between 1-2 wt % based on the environmental conditions.

– Rheological properties – Softening point – Flammability and flash point: most of the lattices in question are waterborne and flammability/flash points do not apply. This is the point what makes the lattices environmentally friendly and nonhazardous. – Health and environmental classification: unlike solvent based polymers latexes are health and environmental concerns are less severe. Glues, all can be obtained by emulsion polymerization of VAc, can be classified after determining some above properties as indicated in Table 4. Table 4 The classification of wood glues.

In general, D3 type wood adhesives can directly be applied to the relevant substrate, but to reach D4 standards the latex itself has to be mixed with a suitable isocyanides at very low ratios. Trials should be made to see the suitability and pot life (a time of 12-24 h should be considered) of the resulting dispersion. The half time of initiator (peroxydisulphate) solutions is specifically important to the synthesizing emulsion polymers of VAc. To predict half time of peroxydisulphates (in minutes of 10 wt % solution) in polymerization for the first seed formation and the post dosing period times can use Table 5. The residues of initiator solution in the final latex may deactivate the biocide and reduce the shelf life.

Table 5 Half time (min.) of the commonly used initiators per peroxydisulphates depending on the polymerization temperature.

T [°C] APS NaPS KPS 20 2100 2200 3300 30 656 688 1031 40 205 215 322 50 64 67 101 60 20 21 32 70 6.3 6,6 9.8 80 2 2.1 3.1 90 0.61 0.64 0.96 100 0.19 0.2 0.3

Ammonium persulphate (APS), Sodium persulphate (NaPS), Potassium persulphate (KPS)


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3.1.2.1 Colloidal Characterization of Latexes

Particle size affects the appearance (color) of the latex. Depending on particle size and polymer concentration the dispersions show optical effects due to light scattering at the polymer particles. These relations are given in Table 6. Table 6 Changing latex color by particle size.

Particle Size Aspectabove 1 µm milky white dispersion 1 µm - 0.1 µm bluish white or brownish white dispersion 0.1 µm - 0.05 µm grey white semi transparent less than 0.05 µm almost transparent

On the other hand, a comparison chart for polymer solution and polymer dispersion can be found in Table 7. Table 7 Comparison of polymer solution and polymer dispersion in some properties.

Features Polymer Solution Polymer Dispersion Appearance clear Opaque Character coiled or stretched macromolecules discrete particles consisting of coiled

macromolecules Molecular weight < 20 000 > 100 000 Particle size < 0.01 µm > 0.1 µm Viscosity high, strongly depending on molecular

weight low, independent from molecular weight

Solids content relatively low High

3.1.2.2 Film preparation and characterization

Film formation process in emulsion polymerization can be described mainly four (Figure 1a) or five (Figure 1b) stages which consist of i) evaporation of solvent, ii) concentrated and particle ordering, iii) sintering and particle deformation iv) coalescence, and/or v) maturation and obtaining mechanically stabile film. Solvent or water evaporates at a constant rate until the latex particles come into contact and form an ordered structure. Coalescence is defined as the step where the particles lose their individuality by interdiffusion of polymer chains. The initial dispersion has a concentration of about 50 wt % polymer (generally between 20 and 50) and is prevented from agglomeration by electrostatic and steric stabilization depending on emulsifier type (stabilization mechanism) using in the emulsion polymerization (in Table 1). During drying, the particles come closer and arrange in a more or less ordered way. After a certain drying time, the proximity of the particles and the increased concentration of ions in the remaining dispersion overcome particle repulsion which leads to irreversible particle contact and deformation. The particles ideally adopt a densest lattice packing and deform into space-filling dodecahedrons. The deformability of the particles depends on the temperature difference between the ambient temperature and MFFT. If the temperature is higher than MFFT, this stage (film formation) can be occurred. At this stage, the deformed particles are surrounded by a more or less thick layer of hydrophilic, emulsifier that prevents direct particle contact. The latex particles will deform as a result of surface tension and capillary forces. The rupture of the emulsifier layer is pre-requisite for polymer interdiffusion which gives mechanical strength to the film. The polymer chain mobility which controls coalescence is again a function of MFFT. If drying is well above the MFFT, particle deformation and polymer diffusion are strong enough to destroy the network of hydrophilic emulsifier material to form a non-porous film. Complete particle deformation will occur only well above MFFT. The final film properties are obtained long after water has been removed from the film. Although the final film appears transparent, it is a porous structure of individual particles with a network of hydrophilic emulsifier material present at the particle-particle interface [7-8]. The film forming mechanism of latexes illustrated in Figure 1a [7]. This mechanism can be detailed with additional one step which given in Figure 1b. The latexes are cast on glass dishes of well-controlled depth, and dried at ambient temperature at least for 24 h, or/and under vacuum for another 24 h to obtain clear (transparent) polymer films.


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a) b)

Fig. 1 Two different film forming representations of latex at: a) four steps [7] b) five steps.

References

[1] Lovell PA, El-Aasser MS, Editors, Emulsion Polymerisation and Emulsion Polymers, John Wiley and Sons, Inc., New York, 1997.

[2] Erbil HY, Vinyl Acetate Emulsion Polymerization and Copolymerization with Acrylic Monomers, CRC Pres, Florida, 2000. [3] Berber Yamak H, Emulsion Polymerization: Effects of Polymerization Variables on the Properties of Vinyl Acetate Based

Emulsion Polymers, Polymer Science: InTech., 2013. [4] Bai L, Gu J, Huan S, Li Z, RSC Adv. 2014; 4: 27363–27380. [5] Gerhartz W, Executive Editor, Ullmann's Encyclopedia of Industrial Chemistry, Adhesives, Wiley-VCH Verlag GmbH & Co.

KGaA; VCM Vol. A1, 221-268, 1985.Available from: DOI: 10.1002/14356007.a01_221 [6] Zhang SW, Liu R, Jiang JQ, Bai HY, Film formation and mechanical properties of the alkoxysilane-functionalized poly

(styrene-co-butyl acrylate) latex prepared by miniemulsion copolymerization, Progress in Organic Coatings, 2009; 65: 56–61. [7] Ludwig I, Schabel W, Kind M, Castaing JC, Ferlin P, Drying and Film Formation of Industrial Waterborne Latices, AIChE

Journal, 2007; 53: 549-560. [8] Ludwig I, Schabel W, Ferlin P, Castaing JC, Kind M, Drying, film formation and open time of aqueous polymer dispersions,

Eur. Phys. J. Special Topics, 2009; 166: 39–43.


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