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Technical papers

Adhesive Mechanism between Polyvinyl Chloride and Urethane Foam

Chen Kai* Masahiko Oonishi**

AbstractUrethane foam covered with PVC (polyvinyl chloride) skin is one of the most widely used combinations on the soft interior products. But this combination may lead to weak peeling strength. The aim of this study is to clarify the adhesion mechanism between PVC skin and urethane foam. The adhesive interface was examined by scanning electron microscope (SEM) and energy dispersive X-ray, and the component analysis was examined by Fourier transform infrared spectroscopy (FT-IR) and gas chromatography-mass spectrometer (GC-MS). The adhesion is created by the diffusion of polyvinyl chloride molecular chain into the urethane foam. The peeling strength depends on the diffusion depth and diffusion concentration of polyvinyl chloride molecular chain, and it is induced with the diffusion of plasticizers.

Key Words : Adhesive / Polyvinyl chloride, Urethane foam, Plasticizer, Peeling strength, Diffusion

1. Introduction To offer occupant comfort in the automotive cockpit, recent instrument panels have been widely adopting a laminate structure where urethane foam is formed on a substrate and a soft skin is layered over them at the out-ermost surface. This structure with the soft materials has advantages in visual and tactile impression (so called “soft instrument panel”). On the other hand, this struc-ture requires the skin to firmly adhere to the urethane foam, otherwise the skin might float or peel off during vehicle usage. To solve this issue, material and part suppliers had been striving to improve adhesion of the skin to the urethane foam. Regardless of the efforts, however, the mechanism of how the skin adheres to the urethane foam in the soft instrument panel was not precisely proven. This report presents our elucidation analysis of the adhesive interfac-es on the urethane foam and the of the adhesion mech-anism through precise observation polyvinyl chloride (PVC) skin, which is applied to many soft instrument panels.

2. Laminate Structure of Soft Instrument Panel Fig. 1 shows the laminate structure of the PVC skin and the urethane foam. As a test sample specification in this study, the PVC skin was not treated with primer that contributes to effective adhesion. Table 1 shows material composition of the test samples.

The two suppliers’ PVC skin materials were selected as test samples because they had a significant difference in peeling strength at the adhesive interface during the preliminary study.

Fig. 1 Schematic cross-sectional diagram of urethane foam.

Table 1 Materials (PVC skins and Urethane foam)

3. Basic Adhesion Principle Identification3.1. IdentificationProcedure

For elucidation of the adhesion mechanism, the basic principle was identified through the following procedure:

(1) Presuming the basic action principle in the adhesion(2) Observing the fracture state and microstructure

on the adhesive interfaces (3) Measuring the peeling strength of the adhesive

interfaces* Calsonic Kansei (Shanghai) Automotive Technology, Fundamental Technology Division** Global Technology Division, Material Engineering Group

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Adhesive Mechanism between Polyvinyl Chloride and Urethane Foam

We took this procedure to clarify the basic adhesion principle for an approach to a further precise phenom-enon analysis.

3.2. FundamentalStudy In general, the adhesion mechanism is typically classi-fied into the four types below:

(1) Chemical bond (chemical reaction by functional groups)

(2) Intermolecular force (hydrogen bond)(3) Mechanical joint (anchor effect)(4) Molecular diffusion (formation of interface layer

with entanglement of diffused molecular chain) On actual adhesive interfaces, however, since the ad-hesion mechanism is not easily identified, it is said that diverse approaches are required in the analysis (1). The general vinyl chloride resin (homo-polymer) is chemically stable and does not contain a functional group (e.g. hydroxyl group) that reacts with the isocy-anate group in the urethane foam. This theory deduces that the possibility of the chemical bond is low as an adhesion mechanism in the interfaces. In some vinyl chloride resins, however, copolymer can be blended and grafted with a compound having hydroxyl, carboxyl, es-ter, and such functional groups. Thus, isocyanate group in the urethane foam might react with these functional groups. In this respect, according to the two suppliers' specifications, none of these functional groups is grafted in the provided PVC skins. Therefore, it is conceivable that the chemical bond is unlikely to act significantly in the adhesion. In regard to intermolecular force, although a hydrogen bond may form between the urethane group bond in the urethane foam and the aforesaid functional groups in the vinyl chloride resin, it was excluded from the analysis subject due to difficulty in its verification process. As a result of the fundamental study, we focused on the subjects of “(3) Mechanical joint” and “(4) Molecular diffusion” as presumable adhesion mechanisms.

3.3. ObservationofFractureState Firstly, a study was conducted on the mechanical joint by observing the adhesive interfaces and verifying the peeling strength in the test sample which was made of the material combinations shown in Table 1. The test sample was produced in the following order. The PVC skin was in advance set at the bottom of a polypropylene tray of 80 mm width, 125 mm length, and 15 mm depth. After the proper amounts of polyol and poly isocyanate

were prepared according to Table 2, they were agitated by a handheld mixer to form a urethane compound uni-formly. Then, the urethane was poured into the tray for foaming. Fig. 2 shows the produced test sample.

Table 2 Mixing ratio of Urethane foam

Fig. 2 PVC leather and urethane foam adhesion sample.

The PVC skin was manually peeled from an edge of the test sample to investigate qualitative peeling strength of the urethane (manual peeling test) and to observe the fracture state on the adhesive interfaces. Fig. 3 shows the peeled sample. The test result shows that Adhesive Failure (AF) occurred in the Supplier A skin because the peeling strength is weak. In contrast, Cohesive Failure (CF) occurred in the Supplier B skin because the peeling strength is relatively strong.

Fig. 3 Manual peeling test.

According to JIS K6866 (Adhesives - Designations of main failure patterns), the fracture state is formally de-fined as Special Cohesive Failure (SCF). In this report, it is simply called CF for clarity.

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3.4. MeasurementofPVCSkinPeelingStrength Secondly, the peeling strength was quantitatively iden-tified by peeling the PVC skin from the urethane foam perpendicularly on the test bench. The peeling strength of the PVC skin was measured with the test jig and the tensile tester shown in Fig. 4. The peeling width of the PVC skin was 20 mm while the tensile head speed is 300 mm/min in this test.

Skin

Double sided tapeUrethane

Fig. 4 90 degree peeling test apparatus.

Since the soft urethane foam is not easily fixed by the gripping jig, it was pasted to an ABS plastic plate with a pressure sensitive tape, and the plate was fixed on the baseplate. Then, an edge of the PVC skin was held by the gripping jig. Under the aforesaid conditions, the 90 degree peeling strength was measured. Fig. 5 shows the test results of Supplier A and B skins. The test results indicate that there is a significant difference in the 90 degree peeling strength between the Supplier A and B skins. As the manual peeling test results, AF occurred in the Supplier A skin while CF occurred in the Supplier B skin in this bench test. Fig. 6 shows the fracture state on these tested samples.

Fig. 5 Peeling strength.

Fig. 6 Peeling test specimen and its failure mode.

In the Supplier A skin, we deduced that the peeling strength is expressed by a mechanism of chemical bond, mechanical joint, or molecular diffusion. In the deduction, furthermore, the peeling strength differs by shapes or material compositions of the adhesive interfaces on the Supplier A and B skins. Therefore, the study proceeds to observation of micro-structure on adhesive interfaces and to analysis of the PVC skin constituent elements.

4. Observation and Analysis of Adhesive Interface (Microstructure observation)

4.1. ObservationofAdhesiveInterface(SEM) The cross sections of adhesive interfaces on the PVC skin and the urethane foam were observed with a Scan-ning Electron Microscope (SEM) to identify if there is any effect of the mechanical joint (anchor effect). From the SEM photographs shown in Fig. 7, no obvious me-chanical joint was identified on the both Supplier A and B skins because the entire interfaces are almost flat. For reference of the photographs, the left region is the PVC skin while the right region is the urethane foam. Since no obvious mechanical joint appeared in the ob-servation, we presumed that the molecular diffusion acts on expression of the adhesion between the PVC skin and the urethane foam. Therefore, we mapped constituent elements of the adhesive interfaces as follows.

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Adhesive Mechanism between Polyvinyl Chloride and Urethane Foam

Fig. 7 Observation of cross section.

4.2. EnergyDispersiveX-ray(EDX)Analysis For revelation of molecular diffusion on the adhesive interfaces, the EDX analysis was conducted on their cross-sections, and the elemental distribution was ob-served through mapping. The EDX analysis is an X-ray technic used to identify constituent elements by detect-ing characteristic X-ray (X-ray fluorescence) in reflection of an electron or X-ray beam and measuring the energy/quantity as intensity. Since the constituent elements differ between the PVC skin and the urethane foam, we mapped the element distribution (chlorine, oxygen, and nitrogen) on those adhesive interfaces and observed how the urethane constituents are distributed on the PVC skin near the adhesive interface. Table 3 shows the differences of the constituents between the PVC skin and the urethane foam.

Table 3 PVC and Urethane constituent element

The EDX analysis results on the PVC skin and the urethane foam are shown in Fig. 8. In this analysis, the urethane constituent of the nitrogen (N) was not detect-ed from the PVC skin due to low detection sensitivity. Moreover, although the chlorine (Cl) and the oxygen (O) were identified on adhesive interfaces of the both Sup-plier A and B skins, molecular diffusion of those atoms was not clarified.

Fig. 8 Energy dispersive X-ray analysis

To identify depthwise diffusion of those constituent elements, distribution maps shown in Fig. 9 were cre-ated from the O and the Cl detected on the adhesive interfaces. The created maps of the both Supplier A and B skins indicate that the O did not diffuse into the skin from the urethane foam. On the other hand, the Cl diffused to the urethane foam by approximately 10 micron meters depth from only the Supplier B skin.

Fig. 9 Mapping image of detected constituent elements.

This phenomenon implies that the PVC molecular chain in the skin diffused into the urethane foam.

4.3. ElucidationofAdhesiveFactor(1) Presumablefactorofthedifference

Further consideration was conducted on the EDX anal-ysis results. From the element maps in Fig. 9, it is evident that the Cl diffusion was denser and deeper in the Supplier B skin than that in the Supplier A skin. This phenomenon

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is conceivably caused due to differences in the additive, especially plasticizer, which is the highest content in the skin materials. In details, solid PVC does not generally diffuse into solid urethane foam; however, depthwise diffusion of the liquid plasticizer into the solid urethane foam may also make selective diffusion of the PVC mo-lecular chains.

(2) Quantitativeanalysisofplasticizercontent Chemical composition and content rate of the plasticizer were analyzed to proof the presumption of the plasticizer diffusion from the PVC skin. The plasticizer was extract-ed from the PVC skin by using a Soxhlet extractor with diethyl ether. After the extraction, the constituents were identified with Fourier transform infrared spectroscopy (FT-IR) and gas chromatography-mass spectrometry (GC-MS). The FT-IR is a constituent identification technic that distinguishes functional groups by obtaining an infrared spectrum penetrated or reflected from the chemical constituents. On the other hand, the GC-MS is an an-alytical technic where chemical constituents are firstly separated in the gas chromatograph based on differences in molecular adsorbent to the column packing or in their partition coefficients, and then the separated molecules are identified/quantified in the mass spectrometer. As a result of these analyses, benzenetricarboxylic acid tris (2-ethylhexyl) ester (called TOTM) was commonly detected as a main plasticizer content in the Supplier A and B skins. Additionally, dioctyl sebacate (2-ethylhexyl) (called DOS) was detected from the Supplier A skin, and an unknown plasticizer substance was slightly detected from the Supplier B skin(2). Fig. 10 shows the GC-MS results of the plasticizer in the PVC skins before adhesion to the urethane foam.

Not identified

Fig. 10 Identification of plasticizer by GC-MS spectrometer

Since one plasticizer substance was unidentifiable due to unknown chemical structure, it is provisionally referred to as “plasticizer α” in this report. Table 4 shows the

identified plasticizer contents in PVC skins.

Table 4 Plasticizer in PVC

Further analyses were conducted to compare the plas-ticizer content rates in the PVC skin before adhering to the urethane foam and those in the PVC skin peeled immediately after adhesion. Table 5 shows the FT-IR and GC-MS results.

Table 5 Content of plasticizer

The total plasticizer content in the Supplier A skin decreased by 0.5% after adhesion, whereas that in the Supplier B skin decreased by 1.5%. From this fact, we inferred that the decrease of the plasticizer promotes the diffusion of the PVC molecular chain. In details of the inference, the plasticizer in the PVC skin starts diffusing to the urethane foam concomitantly with their contact. As the plasticizer diffuses to the urethane foam, the PVC molecular chains also selectively diffuses there in depthwise direction. An increase in solid phase transition of the plasticizer promotes diffusion of the PVC molecular chain to the urethane foam. It results in formation of the molecular diffusion layer at the adhesive interface as described in the Section 4.2 and expresses the peeling strength on the adhesive interfaces.

(3) Influence of urethane foam mixture (verifyingexpressionofchemicalbond)

Those analysis results strongly suggest that the molec-ular diffusion acts on the adhesion mechanism. Howev-er, it still remains provability of the chemical bond as mentioned in the Section 3.2. Therefore, we verified the provability by intentionally changing the mixing ratio of the urethane foam.

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Adhesive Mechanism between Polyvinyl Chloride and Urethane Foam

The urethane foam is generally formed by mixture of polyol (base polymer) and polyisocyanate (curing agent). Based on the hydroxyl groups and isocyanate groups reaction expressed in the equation below, water content reacts with the isocyanate group through the cross-link-ing process so that the urethane is foamed with gener-ation of carbon dioxide: nOCH-R-NCO + nHO-R’-OH → [-CO-NH-R-NH-CO-O-R’-O-]n In this process, if the isocyanate amount exceeds the chemical equivalent, the functional groups in the PVC skin conceivably reacts with the isocyanate group. This chemical behavior allows verification of the chemical bond by mixing excessive/insufficient isocyanate amounts and analyzing influences on the peeling strength. Thus, the urethane foams were prepared with different mixtures of the polyol and the isocyanate. Table 6 shows the mixing ratio of the urethane foams prepared for testing. As a reference, the urethane foam material supplier recommended that 46 pts.wt. of the polyisocyanate be mixed with 100 pts.wt. of polyol (the sample c). The sample a and b were composed of insuf-ficient isocyanate, whereas the sample d and e were of excessive isocyanate.

Table 6 Mixing ratio of urethane foam

The 90 degree peeling test described in the Section 3.3 was conducted on the Supplier A PVC skin which was bonded on those urethane foam samples. Fig. 11 shows the test results of each sample with the mixing ratio above. Regardless of different mixing ratio, all the test results were almost the same in those test samples at initial ad-hesion state. Furthermore, the fracture state was AF on all the adhesive interfaces. It is evident from these facts that the mixing ratio of the urethane foam hardly affects the initial adhesiveness of the PVC skin and the chem-ical reaction does not occur on the adhesive interfaces.

Fig. 11 Peel strength depend on mixing ratio

5. Conclusion This report introduced our elucidation of the adhesion mechanism between the urethane foam and the PVC skin, which are used for instrument panels inside auto-motive cabins. This elucidation revealed that the skin peeling strength is expressed by the selective diffusion of the PVC molecular chains along the transition of the plasticizer from the PVC skin.

(1) An increase of the PVC skin plasticizer transition promotes the molecular diffusion into the urethane foam.

(2) The Cl diffuses by approximately 10 micron meters depth. This depthwise diffusion causes the fracture of CF state on the adhesive interfaces and expresses peeling strength.

Reference(1) Ying Hu: The Journal of Physical Chemistry, Volume

1, Issue 6, People’s Education Press (2014)(2) Tadashi Hamanaka, Takashi Aoki, and Shin Horiu-

chi: Analysis of Polymer-Adhesion Interfaces, Denso Technical Review, Volume 7, No.2, P.58-63 (2002)

ChenKai MasahikoOonishi