Renewable (waste) material based polyesters as plasticizers for adhesives

International Journal of Adhesion & Adhesives 46 (2013) 56–61

Contents lists available at SciVerse ScienceDirect

International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh

Renewable (waste) material based polyesters as plasticizers for adhesives Edita Jasiukaitytė-Grojzdek a,b, Matjaž Kunaver a,b,n, Dolores Kukanja a,c, Darko Moderc c a

Center of Excellence for Polymer Materials and Technologies, Tehnološki Park 24, SI-1000 Ljubljana, Slovenia National Institute of Chemistry, Hajdrihova 19, SI-1000, Ljubljana, Slovenia c Mitol, d.d. Partizanska 78, SI-6210, Sežana, Slovenia b

art ic l e i nf o

a b s t r a c t

Article history: Accepted 7 May 2013 Available online 1 June 2013

The aim of the presented work was to replace phthalate based plasticizers with environmentally friendly materials to provide similar properties for poly(vinyl acetate) (PVAc) adhesives. Polyesters synthesized from the liquefied wood (PE-LW) and depolymerized polyethylene terephthalate (PE-PET) were used as renewable raw materials and evaluated as plasticizers used in PVAc dispersion adhesives for flooring applications. As a reference plasticizer, 1,2,3-triacetoxypropane was used. PVAc adhesives were evaluated with respect to solids content, viscosity, glass transition temperature (Tg), tensile shear strength and binding strength. TGA analysis showed significant differences between the thermal stability of added polyesters and the commercial plasticizer. Samples prepared with PE-PET exhibit the best thermal stability even with an increase of 25% PE-PET. The addition of coalescing agents or plasticizers leads to a temporary softening of the PVAc polymer and a decrease in the glass transition temperature. The type and content of plasticizer have great influence on wood–wood binding strength, tensile strength and elongation. The requirements for the mechanical properties of adhesives were fulfilled by the compositions containing 8.8% (w/w) of PE-PET and 20% (w/w) of PE-LW. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Liquefied wood polyester PET recycling PET polyester Adhesive

1. Introduction Liquefaction of biomass based materials is considered as an effective method for the utilization of renewable resources in polymer chemistry. Such a liquefaction process is usually carried out in polyhydroxy alcohols or phenol, at elevated temperature and in the presence of an acid catalyst. Biomass as the main precursor in the liquefaction reaction can be any kind of lignocellulosic material. The most common sources are different types of wood and wood wastes, as well as the lignocellulosic parts of agricultural wastes. Pure cellulose such as cotton textiles and waste paper can also be used. The liquefaction of wood and other lignocellulosic materials has been conducted in phenol or polyhydroxy alcohols in the presence of an acid catalyst. The process is carried out at temperatures between 140 1C and 180 1C. Microwave irradiation or ultrasound may also be used as an additional source of energy to shorten the reaction time. The liquefied biomass contains the depolymerized products of cellulose, lignin n Corresponding author at: Center of Excellence for Polymer Materials and Technologies, Tehnološki Park 24, SI-1000 Ljubljana, Slovenia. Tel.: +386 1 476 0363; fax: +386 1 476 0300. E-mail address: [email protected] (M. Kunaver).

0143-7496/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijadhadh.2013.05.015

and hemicelluloses. Considerable efforts have been given to the characterization of the products and to define the reaction pathways. Liquefied biomass has high reactivity due to the large number of hydroxylic groups present. These functional groups can be used in the creation of polyurethanes, polyesters, and epoxides after reaction with epichlorohydrin. Attention has been given to the preparation of environmentally friendly products from liquefied biomass. Chen and Lu [1] prepared rigid polyurethane foams from liquefied wheat straw. Juhaida et al. [2] liquefied kenaf core and from the liquefied product synthesized polyurethane adhesive. Kunaver et al. [3] applied a mixture of liquefied wood and melamine–urea–formaldehyde resin as an adhesive in wood particle board production. The same authors used liquefied wood as a polyol in polyester synthesis [4]. Poly(ethylene terephthalate) (PET) plastic packaging is produced in considerable amounts and represents a significant portion of household waste. Consequently, the growing production of PET waste has led to the development of efficient recycling procedures such as mechanical recycling, where PET wastes are separated from contaminants and reprocessed into granules by melt extrusion, and chemical recycling where PET is totally or

E. Jasiukaitytė-Grojzdek et al. / International Journal of Adhesion & Adhesives 46 (2013) 56–61

partially depolymerized to oligomers or other chemical products. According to the information published by the European trade association of plastic recyclers, 1.59 million tons of PET wastes were collected in 2011. That represents 51% of all PET bottles on the market. More than 50% of the resulting recycled PET was used for new packaging applications. Awaja and Pavel [5] have described the synthesis, processing and applications of virgin PET and recycled PET. They also focused on the chain extension processes. Billiau-Loreau et al. reported the structural effects of diacidic and glycolic moieties and studied the physicochemical properties of aromatic polyesterdiols from glycol-depolymerized PET wastes [6]. They found that the viscosity increases greatly with decreasing hydroxyl value or with an increasing proportion of aromatic diacidic residues. Storage stability is also reduced when the amount of PET units in glycolic and acidic moieties overcome a critical value. Depolymerization of PET with ethylene glycol or higher glycols and converting into products such as unsaturated polyesters, polyester polyols, and polyurethanes is the most common industrial process used nowadays. Atta et al. [7] used glycoldepolymerized PET with epichlorohydrine to prepare a series of di- and tetraglycidyl epoxy resins with different molecular weights. The obtained resins showed excellent chemical resistance as organic coatings. The glycolysis process involves utilization of small amounts of glycol and a transesterification catalyst such as zinc acetate. The process is conducted at temperatures between 180 1C and 220 1C for 0.5–8 h. The glycolysis can be conducted at atmospheric pressure or under pressure and can also be carried out under microwave irradiation. The latter can speed up the process and thus save energy [8]. Cakić et al. [9] synthesized polyurethane dispersions from glycol-depolymerized PET waste. Such dispersions could be used in coatings. Poly(vinyl acetate) (PVAc) adhesives are an important type of thermoplastic adhesive, especially in flooring applications, furniture manufacturing, and carpentry. They are ready to use, have a short setting time, and give invisible joints. As a water based system, they are also easy to clean and have a long storage life. Due to their high glass transition temperature (42 1C), PVAc adhesives have limited application at room temperature [10]. In order to extend its applicability, the flexibility and softening of PVAc are achieved either by (a) addition of plasticizer or (b) copolymerization with an appropriate comonomer such as the acrylic acid esters (butylacrylate, 2-ethylhexylacrylate), dialkylfumarates, ethylene, and others. Among these, ethylene is the most efficient comonomer that gives internal plasticity to vinyl acetate–ethylene (VAE) polymers. Therefore, VAE copolymers have been extensively studied in for some time [11–17]. On the other hand, plasticizers soften the film and increase both the adhesion and setting rate. The most common plasticizers are phthalates, adipates, and benzoates. The amount added can be up to 50% by weight [18]. They affect the swelling and softening of the PVAc particles and hence ensure film-forming capabilities at room temperature and a better water and moisture resistance. The disadvantages of plasticizers are the lower resistance of the bondline to heat and possible migration. Unfortunately, the removal of plasticizers can have a negative impact on the quality of PVAc adhesives. The increasing demand for more environmental friendly materials has led to increasing research into new plasticizers and coalescing agents that improve the PVAc adhesive performance. Not much literature exists on the topic of poly(vinyl acetate) dispersion adhesives with increased functional properties that are also environmentally friendly. Choi et al. [19] studied the influence of dialkyl ester, acetyl tributyl citrate, and pentandiol-di-isobutyrate as

57

new plasticizers on characteristics of PVAc adhesives. A PVAc emulsion modified by starch in order to achieve better film toughness and water resistance was studied by Zhang et al. [20]. The improved adhesion strength of a PVAc emulsion modified with SiO2 nanoparticles was investigated by Liu et al. [21]. Kaboorani and Riedl [22] enhanced the PVAc adhesive properties by adding melamine–urea–formaldehyde or melamine–formaldehyde resins in different proportions. The addition of small amounts of these resins improved the water resistance and shear strength of the wood joints. The same authors also studied the addition of nano-clay to PVAc adhesive and its influence on the shear strength and thermal stability of wood joints [23]. Both properties were improved depending on nano-clay loading and type. Brown and Frazer [24] synthesized three poly[(vinyl acetate)co-N-methylolacrylamide] (VAc–NMA) latex adhesives. The influence of different additions of NMA on crosslinking was studied by 15 N-NMR and 13C-NMR spectroscopy. The influence of dibutyl phthalate (DBP) plasticizer on poly (vinyl acetate) (PVAc) degradation using a multi-analytical approach combining FTIR, fluorescence spectroscopy, NMR and DSC analyses was investigated by Toja et al. [25]. The aim of the presented work was the full replacement of phthalate-based plasticizers with environmentally friendly materials in order to reduce Tg and the minimum film formation temperature (MFFT) and provide good properties for the PVAc adhesives. Polyesters synthesized from liquefied wood (PE-LW) and from depolymerized PET (PE-PET) were used as renewable raw materials and tested as substitutes for commercial plasticizers used in PVAc dispersion adhesives for flooring applications. The PVAc based on 1,2,3-triacetoxypropane as a plasticizer was used as a reference. 1,2,3– Triacetoxypropane is commonly used as a commercial substitute for phthalates based plasticizers, which due to their toxicity are on the SVHC list and should be removed from formulations. The prepared PVAc adhesives were evaluated with respect to solids content, viscosity, glass transition temperature (Tg), tensile shear strengths, and binding strength of the bondline between two wood specimens.

2. Experimental 2.1. Materials Polyvinyl acetate dispersion Mekolit H45 (Mitol), calcium carbonate (Calcit), 1,2,3-triacetoxypropane (Sigma Aldrich), adipic acid (Merck), di-n-butyl tin oxide (Fluka). Wood meal (flours) of poplar (Populus spp.), oak (Quercus spp.), spruce (Picea spp.), and beech (Fagus sylvatica spp.) were sieved through 2 mm screens and dried at room temperature to a constant water content. Polyethylene terephthalate (PET) was obtained from a local recycling company and was then milled to particles less than 5 mm in diameter. All chemicals and solvents were of synthesis grade and were used without further purification. 2.2. Liquefaction of wood (LW) The polyhydroxy alcohol mixture of glycerol (150 g), diethylene glycol (150 g), and p-toluene sulfonic acid (9 g) were placed into a 1000 cm3 three-necked glass reactor equipped with mechanical stirring. The mixture was heated to 160 1C and was stirred constantly. After the wood (100 g) was added to the preheated

E. Jasiukaitytė-Grojzdek et al. / International Journal of Adhesion & Adhesives 46 (2013) 56–61

reaction mixture, the liquefaction reaction was carried out for 120 min. 2.3. Synthesis of polyester from LW Three hundred grams of LW were used in combination with 30 g of adipic acid, and the resulting polyester was identified as PS-LW. The LW was introduced into the four-necked 1000 cm3 reactor, equipped with a water condenser and mechanical stirrer. The reactor was placed in an electric heater. Adipic acid was added when the liquefied wood reached 180 1C. Dibutyl tin oxide (0.1% w/w) was added as the esterification/transesterification catalyst. The mixture was gradually heated up to 200 1C, with constant stirring, and was held at this temperature. Water was continuously distilled from the reaction system. A slight stream of nitrogen was introduced into the reactor for easier transport of water vapor into a condenser. The total reaction time was 160 min. After completion of the reaction, when the acid value was reduced to less than 30 mg KOH/g, the reaction mixture was cooled to ambient temperature. The hydroxyl value of the polyester was 334 mg KOH/g. The high hydroxyl value was the result of the depolymerization of cellulose, hemicelluloses and lignin during the liquefaction process. The amount of the adipic acid for the polyester synthesis was optimized in order to obtain the product with free OH groups and with suitable viscosity for application [4]. 2.4. Glycolysis of PET waste and synthesis of polyester from depolymerized PET PET waste flakes (10 g) together with (31.25 g) of polyethylene glycol (PEG) 400 (molar ratio of PET:PEG 400¼ 1:1.5) and transesterification catalyst (0.1% w/w) were added into the reactor. The glycolysis of PET waste was carried out at 220 1C for 6 h. The reaction mixture was cooled to 190 1C and adipic acid (4.12 g) was added in two portions. The reaction temperature was raised to 225 1C and held at this temperature for 2 h until the end of the reaction. Water was continuously distilled from the reaction system. A slight stream of nitrogen was introduced into the reactor for easier transport of water vapor into the condenser. After the completion of the reaction, when the acid value was reduced to less than 5 mg KOH/g, the reaction mixture was cooled to ambient temperature. The resulting polyester was identified as PS-PET. The hydroxyl value was 88 mg KOH/g. The reactant for the glycolysis reaction, PEG, has lower OH functionality (OH value is 281 mg KOH/g) than those used in the wood liquefaction reaction (diethylene glycol OH value is 1057 mg KOH/g). The products of wood liquefaction have high OH value due to their free OH groups. 2.5. Adhesive preparation Water based adhesives were prepared from a commercially available PVAc dispersion Mekolit H45 (Mitol) as a binder, calcium carbonate (Calcit) as a filler, and a plasticizer. Commercial plasticizer (CL; 1,2,3-triacetoxypropane), PE-LW and PE-PET were used to prepare adhesives by varying the plasticizer amount from 0% to 25% (w/w). Thin adhesive films were prepared with a special stencil on silicon-impregnated paper. Prior to testing, adhesive films were dried for 7 days at 23 72 1C and 5075% relative humidity.

apparatus at a heating rate of 20 1C/min under nitrogen atmosphere. Intrinsic viscosity measurements of polyesters were performed using a Brookfield DV-E rotational viscometer according to the International Standard (ISO) 2555. Tensile strength (N/mm2) and elongation (%) were determined using a Zwick/Roell Z 010. Testing was performed using three samples with the dimensions of 100 mm (length) and 15 mm (width). (A modification of the ASTM 882-95a standard method.) Binding strength (N/mm2; wood–wood) was measured using a Zwick 1435 mechanical testing machine. For this purpose three samples with an adhering surface of 2 cm2 were kept under a pressure of 7.5 kg/cm2 for at least 24 h. Binding strength was determined after 72 h of sample conditioning at room temperature with a constant crosshead speed of 50 mm/min. (A modification of the EN 14293:2006 standard method.)

3. Results and discussion Polyesters synthesized from liquefied wood (PE-LW) and from depolymerized PET (PE-PET) were tested as substitutes for plasticizers used in PVAc dispersion adhesives. The influence of different additions of plasticizers on the mechanical properties of PVAc adhesives for flooring applications was studied. The thermal properties of the novel dispersion adhesive were investigated by TGA and DSC. Fig. 1 shows the thermal decomposition temperature dependence on the percentage of plasticizers in the adhesive. The highest thermal stability was exhibited by samples prepared with PE-PET independent of the amount of the added plasticizer. As a result, with the addition of PE-PET up to 25% (w/w), the 5% weight loss temperature (Tdec-5%) was reduced by only 10 1C, while due to the addition of PE-LW and CL, Tdec-5% was reduced by 90 1C and 150 1C, respectively (Fig. 1). It seems that PE-PET and PE-LW are more compatible with the PVAc polymer, and they stay with the polymer for a considerably longer time than the commercial plasticizer based on 1,2,3triacetoxypropane. In the TGA traces recorded under nitrogen atmosphere, a threestep decomposition was observed for the pure PVAc adhesive without plasticizer. The polymer was stable up to 200 1C and started losing mass above this temperature. In the first step (temperature range of 200–370 1C) acetic acid is eliminated from vinyl acetate. The observed mass loss in the second step

340 320 T-decomposition-5%, °C

58

PE-PET

300 280 260 240

PE-LW 220 200

Commercial plastificizer

180 2.6. Methods Thermogravimetrical analysis (TGA) was performed on a Mettler Toledo TGA/DSC1 apparatus at a heating rate of 10 1C/min under nitrogen atmosphere. Differential scanning calorimetry (DSC) measurements were performed on a Mettler Toledo DSC1

160 0

5

10 15 % of plasticizer

20

25

Fig. 1. Five percent decomposition temperature with respect to percentage of plasticizer.

E. Jasiukaitytė-Grojzdek et al. / International Journal of Adhesion & Adhesives 46 (2013) 56–61

50 40 30

Tg, °C

20

PE-LW

10 0 PE-PET

-10 -20 -30

Commercial plasticizer 0

4

8

12 16 % of plasticizer

20

24

Fig. 2. Glass transition (Tg) temperature dependence on the percentage of plasticizer.

200000 % (CL) % (PE-LW) % (PE-PET)

180000

Viscosity, mPa*s

160000 140000 120000 100000 80000 60000 40000 20000 0

5

10 15 % of plasticizer

20

25

Fig. 3. Viscosity (mPa s) dependence on the percentage of plasticizer.

CL % (PE-LW) % (PE-PET)

12 Binding strength, N/mm2

(temperature range of 370–480 1C) is due to the fragmentation and breakdown of the polymer backbone. Similar results were observed by Gupta et al. [11] when they studied vinyl acetate– ethylene copolymers, although the temperature range of the second step (350–450 1C) was lower due to ethylene incorporation. The mass loss in the third step (temperature range of 600– 850 1C) is a result of calcium carbonate decomposition. The addition of commercial plasticizer caused additional decomposition in the temperature range of 100–250 1C. The mass loss is greater with larger amounts of plasticizer. This was not observed with the PE-PET addition in the PVAc adhesives. The addition of coalescing agents or plasticizers leads to temporary softening of the PVAc polymer and a decrease in the minimum film formation temperature and glass transition temperature (Tg). DSC measurements revealed glass transition peaks for all DSC scans except the ones prepared with more than 10% (w/ w) of commercial plasticizer. Increasing amounts of CL and PE-PET in the adhesive formulation greatly reduced Tg, while the least effect on Tg was observed for samples prepared with PE-LW (Fig. 2). According to the obtained results, the commercial plasticizer usually used in PVAc adhesives could be replaced with up to 5% (w/w) of PE-PET and still fulfill the requirements for parquet adhesives. The PE-LW was not as efficient as PE-PET. The lowest Tg of 12.36 1C was reached in the adhesive with an addition of 24.1% PE-LW in the formulation. Both polyesters are firmly incorporated into the adhesive and because of the high hydroxyl group content, they are bonded by intermolecular forces. The effect of hydrogen bonds and strong intermolecular forces is the most obvious in mixtures with PE-LW. Higher hydroxyl value of the PE-LW (334 mg KOH/g) in comparison with PE-PET (88 mg KOH/g) enables stronger hydrogen bonding and, consequently, influences higher Tg due to hindered relative motions of segments. The optimal required viscosity (below 85,000 mPas) was exhibited by adhesives with the addition of up to 5% (w/w) of PE-PET and up to 23.5% (w/w) of PE-LW (Fig. 3). The graph designating the viscosity of the adhesive with added PE-PET is different from the others because PE-PET is less polar than PE-LW and CL. Therefore, we hypothesize that initial additions form secondary structures and viscosity reaches a maximum at 10% added plasticizer. At higher loadings, the structures breakdown due to the dilution effect. The addition of commercial plasticizer up to 3.6% and PE-LW up to 4.9% increases the wood–wood binding strength. Above those

59

10 8 6 4 2 0 0

5

10 15 % of plasticizer

20

25

Fig. 4. Binding strength (N/mm2) dependence on the percentage of plasticizer.

values, increasing the content of plasticizer in the adhesive composition tends to reduce wood–wood binding strength. A decline in binding strength in the whole concentration region is obvious for PE-PET. The requirement for the parquet adhesives is binding strength higher than 5 N/mm2. This criterion was fulfilled by compositions containing up to 8.8% (w/w) of PE-PET and up to 20% (w/w) of PE-LW (Fig. 4). The adhesive ductility dependence on the amount of plasticizer was revealed by examination of the mechanical properties (Table 1). As evident in Fig. 5, the increasing amount of plasticizer in the adhesives based on commercial plasticizer and PE-LW increases elongation at breakage significantly. On the other hand, addition of PES-PET in concentrations up to 10% greatly increases the elongation of the adhesive film; with higher concentrations the elongation begins to fall. The reason is limited compatibility between the PVAc and PES-PET. At higher loadings than 7.5% the mixture during drying begins to separate and results are not longer reliable. Measurements were repeated at least three times and this effect was observed in all experiments. A similar phenomenon was observed in Fig. 3, which shows the viscosity dependence on the percentage of plasticizer. As evident in Fig. 6, the maximum tensile strength has the film of the adhesive with 5% of PE-LW in the composition. At higher concentrations the increasing amount of plasticizer reduces tensile strength significantly.

60

E. Jasiukaitytė-Grojzdek et al. / International Journal of Adhesion & Adhesives 46 (2013) 56–61

Table 1 Mechanical and physical properties of PVAc adhesive prepared with PE-PET and PE-LW. Required values Plasticizer (%)

0 3.6 4.9 9.7 14.4 19.2 24.1

Viscosity (mPa s) o 85,000

Binding strength (N/mm2) 4 5

Elongation (%) o 10

Tensile strength (N/mm2) 4 1.5

CL

PE-LW

PE-PET

CL

PE-LW

PE-PET

CL

PE-LW

PE-PET

CL

PE-LW

PE-PET

23,800 27,600 29,400 36,400 49,200 78,000 79,000

23,800 30,000 33,200 22,000 20,600 31,000 89,000

23,800 48,400 72,400 155,400 160,200 93,000 73,200

8.85 11.4 9.89 6.07 3.68 2.2 1.19

8.85 9.11 11 8.02 7.27 5.21 2.01

8.85 8.32 7.14 4.53 2.74 1.68 0.50

0.3 4.4 7.3 11.4 16.7 41.1 72.2

0.3 0.6 0.9 2.1 4.4 8.5 28.9

0.3 6.1 18.6 31.3 13.6 9.6 –

2.91 3.21 2.22 1.29 0.63 0.43 –

2.91 5.05 5.39 1.63 0.63 0.20 –

2.91 1.69 0.65 0.15 0.15 0.06 –

80 % (CL) % (PE-LW) % (PE-PET)

Elongation at break, %

70 60 50 40 30 20 10 0 0

5

10

15

20

25

% of plasticizer Fig. 5. Elongation (%) dependence on the percentage of plasticizer.

Fig. 7. Solid content of adhesives (%) dependence on the percentage of plasticizer.

6 % (CL) % (PE-LW) % (PE-PET)

Tensile strength, N/mm2

5

4

3

2

1

strength is reduced in a similar manner as with PE-PET and CL (Fig. 7). The solid content for all adhesives was measured according to the ISO 1625 standard. Weight loss is very high for the samples prepared by using commercial plasticizer. We think that the migration and evaporation of the plasticizer from the PVAc polymer while drying at 105 1C might have caused the weight loss. In the case of PE-LW and PE-PET, the solid content slightly increases with the percentage of plasticizes used in the formulation. Both polyesters have higher molar mass and polymer structure and have reduced migration properties because average molecular molar mass for PE-PET is 2500 Da, for PE-LW is 19.000 Da and for CL is 258 Da.

0 0

5

10 15 % of plasticizer

20

25

Fig. 6. Tensile strength (N/mm2) dependence on the percentage of plasticizer.

Hydrogen bonding of high OH functional PE-LW has a similar effect on tensile strength as on viscosity. Due to the higher Tg and the intermolecular bonding, a more rigid structure is built at lower concentrations. After the maximum at 5% of PE-LW, the tensile

4. Conclusions

 We replaced the commercial plasticizer in a PVAc adhesive



with environmental friendly polyesters synthesized from liquefied wood and from depolymerized PET. The results were compared to adhesives where a 1,2,3-triacetoxypropane as a commercial plasticizer was used. The influence of the added polyesters on the solids content, viscosity, glass transition temperature (Tg), tensile shear

E. Jasiukaitytė-Grojzdek et al. / International Journal of Adhesion & Adhesives 46 (2013) 56–61











strengths, and binding strength of the bondline between two wood samples was studied. TGA analysis showed significant differences between the thermal stability of added polyesters and commercial plasticizer. Samples prepared with PE-PET exhibit the highest thermal stability even with the addition of 25%. PE-LW was less stable although still better than commercial plasticizer. We think that PE-PET and PE-LW are more compatible with the PVAc polymer, have stronger interactions with the polymer, and therefore stay in the mixture for a considerably longer time. The addition of coalescing agents or plasticizers leads to a temporary softening of the PVAc polymer. The DSC measurements revealed a decrease in glass transition peak temperature with increasing amounts of CL and PE-PET. The least affected Tg was determined for the samples prepared with PE-LW, where the lowest Tg was reached in the adhesive with 24.1% added PELW. Commercial plasticizer in concentrations above 3.6% and PELW in concentrations above 4.9% in the dispersion adhesive composition tends to reduce wood–wood binding strength and also the tensile strength of the adhesive film. Samples based on PE-PET show a decreasing tensile and binding strength in the entire concentration area from 0% to 25%. A significant difference is observed in the solid content of the adhesive when samples were dried at elevated temperatures. While the commercial plasticizer slowly evaporates, both polyesters remain in the mixture since both have a polymer structure and are completely stable at those temperatures. The requirements for parquet adhesives were fulfilled by compositions containing up to 8.8% (w/w) PE-PET and up to 20% (w/w) PE-LW.

Acknowledgments The authors wish to gratefully acknowledge the support for the presented work received from the Ministry of Higher Education, Science and Technology of the Republic of Slovenia through the contract No. 3211-10-000057 (Center of Excellence for Polymer Materials and Technologies) and for the analysis of products within the Program P2-0145 (National Institute of Chemistry). References [1] Chen E, Lu Z. Liquefaction of wheat straw and preparation of rigid polyurethane foam from the liquefaction products. J Appl Polym Sci 2009;111:508–16. [2] Juhaida MF, Paridah MT, Hilmi MM, Sarani Z, Jalaludin H, Zaki ARM. Liquefaction of kenaf (Hibiscus cannabinus L.) core for wood laminating adhesive. Biores Technol 2010;101:1355–60. [3] Kunaver M, Medved S, Čuk N, Jasiukaitytė E, Poljanšek I, Strnad T. Application of liquefied wood as a new particle board adhesive system. Biores Technol 2010;101:1361–8.

61

[4] Kunaver M, Jasiukaitytė E, Čuk N, Guthrie JT. Liquefaction of wood, synthesis and characterization of liquefied wood polyester derivatives. J Appl Polym Sci 2010;115:1265–71. [5] Awaja F, Pavel D. Recycling of PET. Eur Polym J 2005;41:1453–77. [6] Billiau-Loreau M, Durand G, Tersac G. Structural effects of diacidic and glycolic moieties on physicochemical properties of aromatic polyesterdiols from glycolysis/esterification of poly(ethylene terephthalate) wastes. Polymer 2002;43:21–8. [7] Atta AM, El-Karawy AF, Aly MH, Abdel-Azim AA. New epoxy resins based on recycled poly(ethylene terephthalate) as organic coatings. Prog Org Coatings 2007;58:13–22. [8] Achilias DA, Redhwi HH, Siddiqui MN, Nikolaidis AK, Bikiaris DN, Karayannidis GP. Glycolyitic depolimerization of PET waste in a microwave reactor. J Appl Polym Sci 2010;118:3066–73. [9] Cakić SM, Ristić IS, Marinović-Cincović M, Nikolić NČ, Ilić OZ, Stojiljković DT, BSimendić JK. Glycolytic products from PET waste and their application in synthesis of polyurethane dispersions. Prog Org Coatings 2012;74:115–24. [10] Dunky M. Adhesives in the wood industry. In: Pizzi A, Mittal KL, editors. Handbook of adhesive technology. 2nd ed.. Taylor & Francis; 2003 [Revised and expanded]. [11] Gupta N, Srivastava RK, Choudhary V, Varma IK, Patnaik S. Thermal characterization of vinyl acetate–ethylene copolymers. J Thermal Anal Calorimetry 1999;58:509–15. [12] I. Poljanšek, E. Fabjan, K. Burja, D. Kukanja, Emulsion copolymerization of vinyl acetate–ethylene in high pressure reactor - influence of different parameters. In: Proceedings of the international conference on Slovenski kemijski dnevi 2011, Slovenia; 2011, p. 1–8. [13] Scot PJ, Penlidis A, Rempel GL, Lawrence AD. Ethylene-vinyl acetate semibatch emulsion copolymerization: Experimental design and preliminary screening experiments. J Polym Sci Part A Polym Chem 1993;31:403–26. [14] Scot PJ, Penlidis A, Rempel GL, Lawrence AD. Ethylene-vinyl acetate semibatch emulsion copolymerization: Use of factorial experiments for improved process understanding. J Polym Sci Part A Polym Chem, 31; 2205–30. [15] Scot PJ, Penlidis A, Rempel GL. Reactor design consideration for gas–liquid emulsion polymerization: the ethylene-vinyl acetate example. Chem Eng Sci 1994;49:1573–83. [16] Scot PJ, Penlidis A, Rempel GL, Lawrence AD. Ethylene-vinyl acetate semibatch emulsion copolymerization: use of factorial experiments for process optimization. J Polym Sci Part A Polym Chem 1994;32:539–55. [17] Scot PJ, Penlidis A, Rempel GL. Ethylene-vinyl acetate emulsion copolymerization: monomer partitioning and preliminary modeling. Polym React Eng 1995;3:93–130. [18] M. Dunky, T. Pizzi, M. Van Leemput, Wood adhesion and glued products, state of the art – report, COST Action E13, 1st ed; February 2002. [19] Choi YM, Lee BH, Park JW, Kim HJ, Eom YG, Jang SW, Lee YK. Adhesion properties of eco-friendly PVAc emulsion adhesive using nonphthalate plasticizer. J Adhes Sci Technol 2012:1–15. [20] Zhang YH, Gu JY, Tan HY, Di MW, Li ZG, Si Q. Study on film of polyvinyl acetate emulsion modified by starch. Adv Mater Res 2011;403-408:80–3. [21] Liu HY, Gu JY, Di MW. Performances of polyvinyl acetate hybrid emulsions with nanoparticles SiO2. Adv Mater Res 2011;183–185:2258–62. [22] Kaboorani A, Riedl B. Improving performance of polyvinyl acetate (PVA) as a binder for wood by combination with melamine based adhesives. Int J Adhes Adhes 2011;31:605–11. [23] Kaboorani A, Riedl B. Effects of adding nano-clay on performance of polyvinyl acetate (PVA) as a wood adhesive. Composites Part A: Appl Sci Manuf 2011;42:1031–9. [24] Brown NR, Frazier CE. Cross-linking poly[(vinyl acetate)-co-N-methylolacrylamide] latex adhesive performance part I: N-methylolacrylamide (NMA) distribution. Int J Adhes Adhes 2007;27:547–53. [25] Toja F, Saviello D, Nevin A, Comelli D, Lazzari M, Levi M, Toniolo L. The degradation of poly(vinyl acetate) as a material for design objects: a multianalytical study of the effect of dibutyl phthalate plasticizer. Part 1. Polym Degradation Stability 2012;20 [online article available].