In Synthetic Leather

J. Adhesion Sci. Technol., Vol. 21, No. 8, pp. 705–724 (2007)  VSP 2007. Also available online - www.brill.nl/jast

Taguchi-based optimization of adhesion of polyurethane to plasticized poly(vinyl chloride) in synthetic leather

S. NOURANIAN, H. GARMABI ∗ and N. MOHAMMADI Polymer Engineering Department, Amir Kabir University of Technology, P.O. Box: 15875-4413, Tehran, Iran

Received in ﬁnal form 25 February 2007

Abstract—In this investigation, the effects of formulation and processing factors on the adhesion between polyurethane (PU) and plasticized poly(vinyl chloride) (pPVC) layers was studied using the Taguchi method for experimental design. Among the many factors, nine parameters were screened and tested at two or three levels, taking advantage of the Taguchi L27 orthogonal array. The factors studied were PVC type, PVC K-value, plasticizer type and content, filler type and content, fusion temperature and fusion time of PVC, and PU type. Using the results of T-peel adhesion test at 60◦C as a response, the data were analyzed by appropriate software based on the ANOVA technique. The effect of the various factors on the adhesion was found to be in the following descending order: PU type, PVC fusion temperature, PVC type, plasticizer content, PVC K-value, PVC fusion time, interaction between plasticizer type and PVC fusion temperature, plasticizer type, filler type, filler content, interaction between filler type and PVC fusion temperature, and interaction between PVC K-value and filler type.

Keywords: Adhesion; polyurethane; poly(vinyl chloride); Taguchi method; synthetic leather.

1. INTRODUCTION Adhesion between dissimilar polymers and relevant phenomena have challenged today’s technologists and researchers. The complexity of the mechanisms involved in the achievement of a satisfactory bond has added to the research efforts needed to solve the problems faced in industrial applications. From the theoretical point of view, two main mechanisms of adhesion are responsible for the interfacial adhesion between polymeric materials: adsorption and diffusion [1]. Though different in nature, neither of them can be accepted as the sole mechanism of adhesion in polymers. Although somewhat controversial, the diffusion mechanism introduced

∗To whom correspondence should be addressed. Tel.: (98-21) 6454-2428; e-mail: [email protected] 706 S. Nouranian et al. by Voyutskii in 1949 [2] explains with good accuracy the phenomena relevant to the interfacial adhesion of high polymers with similar solubility parameter (δ). The thermodynamic work of adhesion (WA) is another concept based on adsorption theory of adhesion [1], which relates the work needed to separate two adhered surfaces to their surface free energies (equation (1)). = D D 1/2 + P P 1/2 W A 2(γa γs ) 2(γa γs ) , (1) where γ is the surface free energy, subscripts a and s stand for adhesive and substrate, and superscripts D and P stand for dispersion and polar components of surface free energy, respectively. In the industrial practice, adhesion plays an important role. In the synthetic leather industry, where a lack of a systematic study of the phenomena involved in different applications has been felt for a long time, so much concern has been focused on the problems relative to adhesion, especially as more new products are being manufactured. Among these products is a hybrid leather composed of a polyurethane topcoat and plasticized poly(vinyl chloride) (pPVC) skin layer. Here, two dissimilar polymers with a close solubility parameter (δ = 9.7 cal1/2/cm3/2 for PVC and 9.5–10 cal1/2/cm3/2 for PU) are brought in contact and processed to the final product. Experience has revealed that the selection of inappropriate layer materials and formulations and the use of inappropriate process conditions leads to weakness in the final product in many cases, showing this weakness as a diversity of failure phenomena such as delamination (debonding), non-uniform properties in the tensile and flexure tests, etc. In this study, we conducted a thorough investigation of the parameters affecting the adhesion between polyurethane (PU) and pPVC layers in synthetic leather in order to reach some resolutions on the aforementioned problems. Due to the complexity of physical and chemical factors involved in the achievement of a satisfactory bond between these layers, an initial differentiation was made between formulation and process-related parameters. The Taguchi method for the design of experiments (DOE) [3] was chosen as a tool for the organization of the experiments, evaluation of the effects of various parameters and interpretation of the results. It has to be mentioned that so far no reports have been published on the use of the Taguchi method in the transfer coating industry. Transfer or spread coating is the main method for the manufacturing of synthetic leather. Problems with the adhesion of polyurethane to PVC were recognized previously in many applications. Several patents have been granted to inventors of adhesion promotion methods between PU and PVC [4–6]. In the work of Leriche and Turin [4], a non-pretreated PVC layer was covered with a two-component polyurethane film in the presence of a plasticizer compatible with both polymers (e.g., diisooctyl phthalate or DIOP) with a content of 2–12 wt% based on PU resin. Curing of the PU film was conducted while pressure was applied to the surface. According to the inventors, the adhesion improved through the diffusion- promoting effect of the plasticizer in the interfacial region. Study of the parameters affecting adhesion of PU to pPVC 707

In an attempt to introduce chemical bonds of the covalent type at the interface of a PU/PVC joint, Boba and co-workers [5] used a completely different approach. They added a chemical containing active hydrogen groups (e.g., hydroxy, carboxy, amino, etc.) to the PVC layer, while making available free isocyanate groups in the polyurethane layer. Of said chemical 0.5–20 wt% was added. The amount of free isocyanate was in the range of 0.5–10 wt%. After completion of chemical reaction between active hydrogens and isocyanate groups in the two layers (during the curing process), a strong chemical bond was achieved, resulting in high adhesion strength. Fogle and Cooley [6] used N-substituted 2-pyrrolidone and/or ethoxylated alkyl phenol as an effective adhesion-promoting agent for PU and PVC. Based on their reports, an amount of 1–15 vol% of the said agents would sufﬁce for the achievement of acceptable adhesion strength. After an adequate contact time between the layers, these agents were removed from the interface through reaction with an organic isocyanate. In the present study, our main objective was to investigate the parameters affecting adhesion between PU and plasticized PVC at elevated processing temperatures. Both formulation and processing conditions were taken into account.

2. EXPERIMENTAL 2.1. Selection of parameters In order to conduct a systematic assessment of interfacial adhesion between pPVC and PU layers, an initial study was made on the physical and chemical factors likely to have inﬂuence on the adhesion. In this connection, the role of processing and formulation conditions in achieving a satisfactory bond between the said layers was taken into account. Nine factors were screened for the study as shown in Table 1.

2.2. Materials The materials and processing conditions chosen for the sample preparation were selected based on the most common practice in the synthetic leather industry.

Table 1. Factors screened for the study of adhesion

Factor No. Factor Relationship 1 PVC type PVC formulation 2PVCK-value PVC formulation 3 Plasticizer type PVC formulation 4 Plasticizer content PVC formulation 5 Filler type PVC formulation 6 Filler content PVC formulation 7 PVC fusion temperature Process condition 8 PVC fusion time Process condition 9 PU type PU formulation 708 S. Nouranian et al.

2.2.1. Ingredients of PVC and PU layers. TwomainPVCtypeswereusedin the formulation of PVC plastisols, namely emulsion and microsuspension. Other normal components of the PVC formulation were heat stabilizers, co-stabilizers, plasticizers and ﬁllers. For the PU layer, polyester-type aromatic and aliphatic polyurethane solutions were used as the pre-skin to be coated on top of PVC layer. Dimethyl formamide (DMF) was used as the solvent for aromatic-type, while a 1:1 ratio of toluene (TOL) to isopropyl alcohol (IPA) was used for aliphatic-type PU. The solvent is a regular component of any PU formulation used in the transfer coating process and is completely removed during processing leading to a thin ﬁlm. In order to obtain a better contrast between PU and PVC layers, the PU layer was prepared in black color, while PVC layers were colorless. The material grades and suppliers are shown in Table 2. In addition, some general characteristics of PU and PVC grades are summarized in Table 3.

2.2.2. Release paper. A semi-matt plain release paper, Alfakote CR (Cartiera Di Crusinallo Favini, Italy), was used for the coating of PVC paste and sample preparation.

2.3. Equipment 2.3.1. Sample preparation equipment. For the preparation of PVC pastes and PU solutions, a laboratory-scale mixer with variable speed (200–2000 rpm) was used. Samples were coated on the release paper using a laboratory coating blade and fused (or dried) in the oven of a laboratory-scale, two-head coating machine (Colombo, Italy).

2.3.2. Sample testing equipment. A tensile testing machine with a load capacity of 5 metric tons (Santam, Iran) equipped with a heating chamber with temperature regulation in the range of 50–450◦C was used for the acquisition of peel test data. For the calculation of sample surface energy, a contact angle measuring instrument was used (Krüss Model G10, Germany).

2.4. Design of experiments The Taguchi method for the design of experiments [3] was chosen for the organization of the experiments and analysis of the results. Using the L27 orthogonal array, a mixed-level design (six factors at three levels and three factors at two levels) was made. The use of more than two levels for factors makes it possible to study some eventual non-linear effects. The interactions between the factors were considered negligible. The factor levels were chosen in accordance with the common industrial practice and in a range suitable for preparing the maximum number of samples in all 27 “treatments”. It has to be mentioned that each experimental setup is called a treatment. The factor designations and levels are shown in Table 4. Study of the parameters affecting adhesion of PU to pPVC 709

Table 2. Material trade names and speciﬁcations

Ingredient Trade name Speciﬁcation Supplier PVC layer PVC powder E 7012 Emulsion Vestolit, Germany PVC powder P 1345 K 80 Emulsion Vestolit, Germany PVC powder P 1430 K 90 Emulsion Vestolit, Germany PVC powder B 7021 Microsuspension Vestolit, Germany Heat stabilizer CBZ 277 Ba/Cd/Zn stabilizer Dahin Group, Taiwan Co-stabilizer ESBO-132 Epoxidized soybean oil Dahin Group, Taiwan Plasticizer DOP Dioctyl phthalate Farabi Petrochem., Iran Plasticizer DOA Dioctyl adipate Dahin Group, Taiwan Plasticizer (Unimoll BB) (BBP) Butylbenzyl phthalate Bayer, Germany Filler – Calcium carbonate Alborz Carbonate, Iran Filler – Talc Towlipoudr, Iran PU layer PU pre-skin Larithane AL 233N Aliphatic Novotex Italiana, Italy PU pre-skin Larithane MS 128 Aromatic Novotex Italiana, Italy Solvent DMF Dimethyl formamide Caldic, The Netherlands Solvent TOL Toluene Isfahan Petrochem., Iran Solvent IPA Isopropyl alcohol – Leveling agent Noresil S 900 Liquid Novotex Italiana, Italy Pigment Tecnopur TP 3035 Paste Rohm & Haas, Italy

Table 3. Some general characteristics of PVC and PU grades

Speciﬁcation Grade E 7012 P 1345 K 80 P 1430 K 90 B 7021 PVC K-value 67 80 90 70 Viscosity number (cm3/g) 112 170 227 125 Particle dimensions (Sieve analysis – retained on 0.063 mm sieve) (%) <1 <1 <3 <1 AL 233 N MS 128 PU Type Aliphatic, polyester Aromatic, polyester Molecular weight (g/mol) 50 000 50 000 Isocyanate structure Blend of IPDIa and HMDIb MDIc a Isophorone diisocyanate. b Dicyclohexyl-4,4-methylene diisocyanate (hydrogenated MDI). c 4,4-methylene diphenyl diisocyanate.

The factors were assigned to the columns of an array as designated in Table 4. Note that factor A has three K-values (67, 80 and 90) corresponding to its ﬁrst level and only one K-value (70) corresponding to its second level. Therefore, 710 S. Nouranian et al.

Table 4. Designations and levels of factors

Factor Factor No. of Level 1 Level 2 Level 3 designation levels A PVC type 2 Emulsion Microsuspensiona – BPVCK-value 3 67 80 90 C Plasticizer type 3 DOP DOA BBP D Plasticizer content (phr) 3 50 53 57 E Filler type 2 Calcium carbonate Talc – F Filler content (phr) 3 7 10 13 G PVC fusion temperature (◦C) 3 170 175 180 H PVC fusion time (min) 3 3 3.5 4 I PU type 2 Aliphatic Aromatic – a The microsuspension type of PVC has a K-value of 70. a branched design had to be chosen as follows: (1) factors A and B were designated to columns 1 and 2 of the orthogonal array (Table 5), (2) the interaction columns [3] for columns 1 and 2 (i.e., columns 3 and 4) were left blank and (3) level 1 of factor B (K-value = 67), corresponding to level 2 of factor A (microsuspension type PVC), was replaced by the correct K-value of 70. Other factors were assigned to columns 5–11 (Table 5). The mixed-level nature of the orthogonal array required “dummy treatment” for all two-level factors, i.e., A, E and I [3]. In dummy treatment, a third level is assumed for two-level factors using either the value for level 1 or 2. Based on this approach, the value of level 1 for factor A was chosen for the third level of this factor. In the same manner, the third level of factor E acquired the value of level 1 for this factor, and the third level of factor I acquired the value of level 2 for this factor. The ﬁnal design is shown in Table 5.

2.5. Sample preparation Table 6 shows the formulations of PVC and PU layers used for the preparation of samples.

2.5.1. Preparation of the pastes. For the PVC part, solid ingredients (PVC and ﬁller) were added gradually to the liquids (plasticizer, heat stabilizer, and epoxidized soybean oil) while the paste was mixed with an initial speed of 600 rpm. The speed was then increased to 1600 rpm. After 5 min, a homogeneous paste was obtained, which was kept unstirred for 24 h in order to eliminate air bubbles introduced during the mixing process. PU solutions were mixed gently using a steel bar for approximately ﬁve minutes. The total rpm for the mixing was around 300. The order of addition of ingredients was PU solution, solvent, leveling agent, and black pigment. The deaeration (degassing) of PU solutions was complete in 2 h. Study of the parameters affecting adhesion of PU to pPVC 711

Table 5. The ﬁnal design of experiments and the corresponding factor levels

Treatment A B C D E F G H I 1 2 345 6 78 910111213 1111111111 2112222222 3113313332 4121112222 5122223331 6123311112 7131113332 8132221112 9133312221 10 2 (1) 1 2 1 1 2 3 1 11 2 (1) 2 3 1 2 3 1 2 12 2 (1) 3 1 2 3 1 2 2 13 2 (1) 1 2 1 2 3 1 2 14 2 (1) 2 3 1 3 1 2 1 15 2 (1) 3 1 2 1 2 3 2 16 2 (1) 1 2 1 3 1 2 2 17 2 (1) 2 3 1 1 2 3 2 18 2 (1) 3 1 2 2 3 1 1 19 1 1 1 3 2 1 3 2 1 20 1 1 2 1 1 2 1 3 2 21 1 1 3 2 1 3 2 1 2 22 1 2 1 3 2 2 1 3 2 23 1 2 2 1 1 3 2 1 1 24 1 2 3 2 1 1 3 2 2 25 1 3 1 3 2 3 2 1 2 26 1 3 2 1 1 1 3 2 2 27 1 3 3 2 1 2 1 3 1

The Taguchi L27 orthogonal array can accommodate 13 different factors, indicated in this table by 13 columns. The total number of treatments (experimental setups) is 27. In this study nine factors were investigated, indicated by letters A to I (Table 4). These factors were designated as columns 1–11, intentionally leaving columns 3 and 4 blank. Numbers 1, 2 and 3 indicate levels of the factors as described in Table 4. Level 1 in parentheses (column B) indicates K-value 70 replacing K-value 67 as per the branched design described in the text.

2.5.2. Preparation of the test samples. Samples were prepared in a sandwich form on the release paper support, where a thin PU layer was sandwiched between two thicker PVC layers using a laboratory-scale coating knife. This was done in order to minimize the unwanted effects of non-uniform elongation of PU and PVC layers during the peel test. PVC fusion time and temperature were set in accordance with the factor levels for each treatment in the design. First, a 1.00- mm-thick layer of PVC plastisol was coated on the release paper with the aid of coating knife and heated in an oven at the temperature speciﬁed by the treatment setup (Table 4). Coating was obtained by pouring the plastisol on the release paper behind the coating knife and pulling the release paper with a constant speed through 712 S. Nouranian et al.

Table 6. PVC and PU formulations used for the preparation of the samples

Ingredient phra Formulation of PVC layers PVC powder 100 Plasticizer (level 1/2/3) 50/53/57 Filler (level 1/2/3) 7/10/13 Heat stabilizer 2 Epoxidized soybean oil 2 Formulation of PU layers PU solution 100 Solvent for aliphatic PU (1:1 ratio of TOL to IPA) 20 Solvent for aromatic PU (DMF) 25 Leveling agent 0.6 Black pigment paste 3 a part per hundred resin. the knife gap. Then, a thin PU layer was coated on top of the fused PVC layer in the same manner. For the PU layer, drying of the coated film was conducted in an oven in two successive heating cycles: (1) 2 min at 80◦C and subsequently 2 min at 100◦C for aliphatic PU and (2) 2 min at 90◦C and subsequently 2 min at 130◦C for aromatic PU. After completion of film drying, another layer of PVC plastisol with a thickness of 1.00 mm was coated over the dried PU layer and fused in the oven, again at the temperature specified by the treatment setup (Table 4). The final samples were cut into standard dimensions (2 × 200 mm) as specified in the ASTM D 1876-95 standard.

2.6. Sample testing The T-peel test (ASTM D 1876-95) was used for the acquisition of peel force data. The peeling was conducted at 60◦C, because at room temperature, several samples experienced cohesive failure in the PVC layer resulting in no data. Peel force measurements were conducted on three separate samples for each treatment. The peel rate was 200 mm/min and the average peel force was calculated using the obtained peel force curves for these samples. Some samples did not yield peel force curves because of the early cohesive failure in the PVC layer even at the elevated temperature. For eight selected samples, the thermodynamic work of adhesion (WA) [1] was calculated using the Kaelble–Rabel–Wendt–Owens and Wu methods [9–11].

3. RESULTS AND DISCUSSION As mentioned before, peel force was chosen as the response (y), in order to analyze the interfacial adhesion between PU and pPVC layers. Among the 27 samples, Study of the parameters affecting adhesion of PU to pPVC 713

Table 7. Response values (average peel forces) for the treatments

Treatment Peel force (kN/m) 1– 22.5 32.0 42.3 53.4 60.9 72.2 81.6 9– 10 – 11 2.9 12 3.1 13 3.3 14 – 15 1.9 16 2.0 17 2.0 18 – 19 – 20 1.9 21 2.1 22 1.5 23 – 24 1.9 25 1.9 26 2.4 27 –

19 tests gave a set of data adequate for the calculation of average peel force. Due to the strong adhesion between the layers, eight samples could not be peeled and, therefore, no experimental results were obtained for these samples. As mentioned previously, this was due to the cohesive failure in the PVC layers during peeling. Table 7 shows the response values measured for the treatments.

3.1. Linear regression modeling and data analysis As the Taguchi method for experimental design is a fully saturated design, treatments without response affect negatively the determination of the signiﬁcance of factors. Therefore, a semi-empirical model was derived using the response values of 19 successful samples. A linear regression method [7, 8] was used for this purpose. The derived equation was:

y = 2.6382 + 0.1509XA − 0.2932XB − 0.1648XC − 0.3033XD

+ 0.1456XE + 0.1216XF + 0.4072XG − 0.2289XH − 0.5071XI, (2) 714 S. Nouranian et al.

Table 8. Calculated response values (average peel forces) for the treatments with no actual peel force data

Treatment Peel force (kN/m) 13.3 92.1 10 3.3 14 2.9 18 4.5 19 3.6 23 3.5 27 1.8

Table 9. Results of data analysis using the ANOVA technique

Factor Factor Degrees of Sum of Mean of % designation freedom (DOF) squares squares Contribution I PU type 1 6.15 6.15 35.65 G PVC fusion temperature 2 3.08 1.54 17.88 A PVC type 1 2.12 2.12 12.27 D Plasticizer content 2 1.68 0.84 9.73 BPVCK-value 2 1.04 0.52 6.01 H PVC fusion time 2 0.97 0.49 5.65 C × G Interaction between C and G 4 0.62 0.15 3.57 C Plasticizer type 2 0.61 0.30 3.52 E Filler type 1 0.51 0.51 2.94 F Filler content 2 0.30 0.15 1.74 E × G Interaction between E and G 2 0.12 0.032 0.72 B × E Interaction between B and E 2 0.022 0.011 0.13

where y is the response and XA,XB ...XI are variables indicating the respective levels of factors A, B ...I. Using the above equation, the response values for eight samples without peel force data were calculated (Table 8). Then, through implementation of analysis of variance (ANOVA) technique with the aid of DesignExpert software version 6.0.6 trial edition, ﬁnal analysis was made on peel force data. The results are shown in Table 9.

3.2. Results of thermodynamic work of adhesion (WA) The thermodynamic work of adhesion is calculated based on the adsorption theory of adhesion [1]. In this theory, no consideration is given to the diffusion phenom- enon, and only the secondary bonds are taken into account. Two main methods for determination of surface energy of solids are harmonic-mean and geometric-mean methods [11]. The former is proposed by Wu [9] and the latter by Owens, Wendt, Kaelble and Rabel [10]. Both methods use data from contact angle measurements Study of the parameters affecting adhesion of PU to pPVC 715 to calculate the dispersion and polar components of solid surface energy. The ex- pressions for these methods are shown in equations (3)–(6). Harmonic-mean method: d d P P γ1 γS γ1 γS (1 + cos θ1)γ1 = 4 + , (3) γ d + γ d γ P + γ P 1 S 1 S γ dγ d γ Pγ P (1 + cos θ )γ = 4 2 S + 2 S . (4) 2 2 d + d P + P γ2 γS γ2 γS Geometric-mean method: + = d d + P P (1 cos θ1)γ1 2 γ1 γS γ1 γS , (5) + = d d + P P (1 cos θ2)γ2 2 γ2 γS γ2 γS , (6) where θ is contact angle and γ is surface energy. Subscripts 1 and 2 refer to test liquids 1 and 2 and subscript S refers to solid. Superscripts d and P refer to dispersion and polar components, respectively. The harmonic-mean method is mainly used for low-energy surfaces, while the geometric-mean method is preferred for high-energy surfaces [11]. The results, however, are quite close to each other. In this study, we used both methods for the determination of thermodynamic work of adhesion. This was done for comparison purposes only. The test liquids were water and diiodomethane, and their contact angles on the surfaces of PU and PVC were measured using an appropriate apparatus described in Section 2.3.2. Using the bundled software, the dispersion and polar components of the surface energy for PU and PVC were calculated and input into equation (1), whereby the thermodynamic work of adhesion was calculated. These results for eight selected samples are shown in Table 10. By determining the thermodynamic work of adhesion for selected samples, a comparison between peel force results and the results obtained through harmonic-mean or geometric- mean methods was performed. The peel force measurement includes a signiﬁcant

Table 10. Thermodynamic work of adhesion (WA) results for eight selected samples using the (a) Kaelble, Rabel, Wendt and Owens method and (b) Wu method

2 Treatment WA (mJ/m ) (a) (b) 1 839 904 2 966 1050 6 753 828 7 761 831 9 891 954 11 1070 1150 14 968 1050 23 757 822 716 S. Nouranian et al. viscoelastic loss in the polymeric layers, which will result in a major difference between the thermodynamic work of adhesion and the peel force results. This relationship has been discussed previously by several researchers [12–16].

3.3. Main effects of factors The main effects of nine factors and three interactions were determined by the ANOVA technique, which are given as follows.

3.3.1. PVC type. As indicated in Fig. 1, a stronger interfacial adhesion resulted for microsuspension-type PVC compared to emulsion-type PVC. Contribution of this factor to the adhesion between layers is 12.27%, which ranks it as the third important factor among others (Table 9). The observed behavior could be attributed to the lack of emulsiﬁer in microsuspension-type PVC, which results in better and faster fusion of PVC at the same temperature and time conditions compared to emulsion-type PVC. This results in a better adhesion between the layers. It should be mentioned that a higher and faster fusion of PVC provides better macromolecular diffusion into the substrate and increases the degree of polymer chain entanglements. This is in accordance with the diffusion theory of adhesion proposed by Voyutskii [2].

Figure 1. Main effects of PVC type (factor A) and PVC K-value (factor B) on peel force (adhesion). The numbers on the x-axis correspond to factor levels. Note that PVC type is a two-level factor; thus, for this factor, number 3 on the x-axis is an indication of level 2. Study of the parameters affecting adhesion of PU to pPVC 717

3.3.2. PVC K-value (molecular weight). The K-value is a measure of PVC molecular weight. Figure 1 shows that peel force decreases with increasing K-value and then levels off. The contribution of this factor to the adhesion is 6.01%, which makes it as the ﬁfth important parameter. Voyutskii [2] has described a similar trend for cellophane tapes bonded to poly(isobutylene) with different molecular weights. Likewise, he observed the leveling off of the adhesion strength at higher molecular weights. Based on the diffusion theory, with increasing molecular weight, the number of polymer chain free ends at the interface capable of penetrating into the substrate is reduced. There- fore, at a relatively high molecular weight, the middle segments of macromolecular chains (normally with limited diffusion due to steric hindrance) are the only source for diffusion. This results in reduced peel force. Above a certain molecular weight, the diffusion of middle chain segments is the only dominant diffusion mechanism and this will be independent of chain length; therefore, the adhesion strength will not be affected noticeably by increasing the molecular weight.

3.3.3. Plasticizer type. The contribution of this factor to the adhesion strength of PVC and PU layers is 3.52%, meaning it is the sixth important parameter inﬂuencing the interfacial adhesion properties. As shown in Fig. 2, butylbenzyl phthalate (BBP) resulted in a higher peel force than dioctyl phthalate (DOP) and dioctyl adipate (DOA). This could be attributed to the high compatibility of this low-molecular-weight plasticizer (BBP) with

Figure 2. Main effects of plasticizer type (factor C) and plasticizer content (factor D) on peel force (adhesion). Numbers 1, 2 and 3 indicate factor levels as designated in Table 4. 718 S. Nouranian et al.

Table 11. Clear-point (solid–gel) temperatures of plasticizers [17]

Plasticizer Clear-point temperature (◦C) Butylbenzyl phthalate (BBP) 102 Dioctyl phthalate (DOP) 117 Dioctyl adipate (DOA) 138

PVC or in other words, its strong solvating power compared to other plasticizers. BBP results in faster fusion of PVC at lower temperature and, thus, provides a higher diffusion rate. Based on the diffusion theory, a higher intermolecular diffusion between polymeric layers provides a stronger adhesion. Clear-point (solid–gel transformation) temperature is a good indication of fusion characteristics of plasticizers. Table 11 shows the clear-point temperatures for the three plasticizers used in this study [17]. As shown in Table 11, BBP has the lowest clear-point temperature and, thus, provides the fastest fusion of PVC.

3.3.4. Plasticizer content. This factor has an average contribution of 9.73%, which makes it as the fourth important parameter. Figure 2 shows a reduction of peel force with increasing plasticizer content, which was expected. Voyutskii [2] observed the same trend for the effect of plasticizer content on adhesion. It has been reported that at low plasticizer content in a highly viscous adhesive, the addition of plasticizer would lead to a relative increase in adhesion due to the facilitation of molecular motion and thereby interlayer molecular diffusion [2]; however, at higher plasticizer levels, “molecular extraction” or removal of adhesive macromolecule segments from the substrate at the interface increases and, therefore, adhesion strength is reduced sharply.

3.3.5. Filler type and content. The filler type has a minor effect on the adhesion strength (2.94%) of pPVC and PU layers. Talc powder results in a slightly stronger adhesion compared to calcium carbonate powder. According to Voyutskii [2], addition of filler to the interface of bonded layers generally decreases the adhesion strength. This is mainly due to the attachment of macromolecules on the surface of filler particles and the resulting decrease in motion, which in a way leads to decreased diffusion. Furthermore, the presence of filler particles at the interface reduces the effective contact area of the molecules of the two layers, leading again to decreased adhesion. However, this is the case when cohesive strength of the polymer is higher than its adhesion strength. Otherwise, the addition of a surface-active filler, like carbon black, may actually lead to a stronger interfacial adhesion. Filler content showed a minor effect on the adhesion, with a contribution of 1.74%. It seems that, because the three levels chosen for filler content are so close to each other, the effect of this factor is not noticeable in this study. Study of the parameters affecting adhesion of PU to pPVC 719

3.3.6. PVC fusion temperature. The sharp increase in peel force with increasing fusion temperature (Fig. 3) is a strong indication of the significance of PVC fusion temperature on the final adhesion strength. The contribution of this factor is 17.88%, which makes it the second most important parameter. With reference to the diffusion theory, the increase in adhesion strength with increasing fusion temperature is mainly due to the facilitation of macromolecular motion and, more specifically, polymer chain segmental motion. At higher temperatures, the steric hindrance for macromolecular motion is minimized.

3.3.7. PVC fusion time. As indicated in Fig. 3, adhesion strength is reduced with increasing fusion time. The contribution of this factor is 5.65%, which means it is a moderately important parameter in the overall adhesion property. The reason for this behavior is thought to be the possible thermal deterioration of polyurethane or PVC surface during long exposure times at high temperatures. The loss of plasticizer in PVC layer and possible polyurethane degradation at the interface deteriorates the intermolecular diffusion between the layers and leads to a weaker interfacial adhesion. In order to conﬁrm this hypothesis, thermogravimetric analysis was conducted on a PVC sheet sample with 57 phr DOP as the plasticizer and an aromatic polyurethane ﬁlm (Larithane MS 128). The test was conducted isothermally at temperature of 180◦C corresponding to level 3 of PVC fusion

Figure 3. Main effects of PVC fusion temperature (factor G) and PVC fusion time (factor H) on the peel force (adhesion). Numbers on the x-axis indicate factor levels. 720 S. Nouranian et al.

Figure 4. Isothermal TGA graphs for PVC sheet with DOP as plasticizer and aromatic PU (Larithane MS 128) ﬁlm. The temperature was 180◦C. temperature as per the Taguchi design described earlier. As shown in Fig. 4, the weight loss initiation was observed after 3 min of exposure time for PVC and 4 min of exposure time for polyurethane. This is in accordance with the fusion time of PVC during sample preparation step in this study.

3.3.8. Polyurethane type. Among all the factors studied, polyurethane type has the highest percentage contribution, namely 35.65%, on the adhesion strength. As shown in Fig. 5, aliphatic-type PU results in considerably higher adhesion strength in comparison to the aromatic type. The polyol part of both PU grades is a polyester type and, therefore, molecular structure, and intermolecular forces could not be responsible for the enormous difference in adhesion characteristics. The reason should be attributed to the difference in diisocyanate groups attached to the polymer chains in the PU resins. The aromatic diisocyanate possesses a much lesser diffusion rate compared to the aliphatic diisocyanate, because of the steric hindrance. This may lead to signiﬁcantly lower adhesion strength. The conclusion can be conﬁrmed by a comparison between Tg values of the two polymers. As shown in Figs 6 and 7, the ◦ ◦ Tg for aliphatic-type PU (Larithane AL 233N) is around −50 C, while it is −20 C for aromatic-type PU (Larithane MS 128). Thus, the macromolecule chain mobility of aliphatic PU is greater than that of aromatic one.

3.3.9. Interactions of factors. The contributions of the interactions to the total adhesion strength are generally low and negligible. However, one noticeable interaction was discovered in this analysis, which is C × G interaction, i.e., an interaction between plasticizer type and PVC fusion temperature. Study of the parameters affecting adhesion of PU to pPVC 721

Figure 5. Main effect of polyurethane type (factor I) on the peel force (adhesion). Numbers on the x-axis indicate factor levels.

Figure 6. DMTA diagram for aromatic polyester-type PU (Larithane MS 128). The onset of decrease in modulus is considered as the Tg for the polymer.

This interaction has a contribution of 3.57% to the adhesion strength. As shown in Fig. 8, BBP results in the highest adhesion at the lowest temperature (170◦C) among the three plasticizers used for the study. However, at higher temperatures, 722 S. Nouranian et al.

Figure 7. DMTA diagram for aliphatic polyester-type PU (Larithane AL 233N).

Figure 8. Main effects of C × G interactions (interaction between plasticizer type (C) and PVC fusion temperature (G)) on the peel force (adhesion). Numbers on the x-axis correspond to the levels of factor C and numbers on the graphs correspond to the levels of factor G. Study of the parameters affecting adhesion of PU to pPVC 723 this behavior is reversed. The reason probably lies in the fact that BBP has a high volatility rate at elevated temperatures. Thus, a high proportion of BBP is evaporated leading to a considerably weaker adhesion. The other two interactions, i.e., B × E interaction (PVC K-value and filler type) and E × G interaction (filler type and PVC fusion temperature), showed only minor influences on the interfacial adhesion of pPVC and PU layers.

4. CONCLUSIONS The purpose of this study was an in-depth evaluation of the parameters affecting adhesion of two dissimilar polymers, namely polyurethane and plasticized poly(vinyl chloride). The results were best interpreted using the diffusion theory of adhesion proposed by Voyutskii [2]. This study is in a way a reconﬁrmation of the basics of Voyutskii’s diffusion theory for high polymers using the Taguchi method for experimental design. Comparing the results with the thermodynamic work of adhesion data based on adsorption theory of adhesion suggests that the main difference is attributed to the viscoelastic loss during sample peeling [12–16], as discussed earlier. Based on the ANOVAresults in the Taguchi method for the design of experiments, the optimum level of factors, i.e., levels corresponding to the highest achieved adhesion strength (peel force), was determined as follows:

A2, B1, C3, D1, E2, F2, G3, H2 and I1, where the capital letters indicate factors and subscripts point to the factor levels. This study reveals that four main factors inﬂuencing the adhesion strength between PU and pPVC layers are PU type, PVC fusion temperature, PVC type and plasticizer content. The combined contribution of these four factors to the adhesion strength is more than 70%.

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