Breathability studies of electron beam curable polyurethane pressure ...

Radiation Physics and Chemistry 103 (2014) 75–83

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Breathability studies of electron beam curable polyurethane pressure sensitive adhesive for bio-medical application Anil Kumar Singh a,n, Dayal Singh Mehra a, Utpal Kumar Niyogi a, Sunil Sabharwal b, Gurdeep Singh c a

Shriram Institute for Industrial Research, 19-University Road, Delhi 110007, India International Atomic Energy Agency, Vienna, Austria c Centre of Mining Environment, Department of Environmental Science & Engineering, Indian School of Mines, Dhanbad, Jharkhand-826004, India b

H I G H L I G H T S

On increasing e-beam dose from 5 kGy to 60 kGy the MVTR decreased continuously. Increasing crosslinker concentration from 2% to10% resulted in decrease of MVTR. IPDI has the highest and TAC has the least MVTR in PU-PSA system among the crosslinkers. MVTR/peel adhesion/shear adhesion/initial tack: IPDI4MDI 4CMDI 4PMDI 4 TAIC4TAC. Gel content/Tg/crystallinity: TAC 4TAIC4PMDI 4 CMDI 4MDI4 IPDI.

art ic l e i nf o

a b s t r a c t

Article history: Received 12 August 2013 Accepted 10 May 2014 Available online 17 May 2014

Polyurethane (PU) based pressure sensitive adhesive (PSA) commonly used in surgical dressing has been made by electron beam (e-beam) irradiation. In contact with biological substrate like skin, PSAs generally lose their adhesive strength due to very low moisture vapor transmission rate (MVTR). In the present study, effects of varying e-beam dose and different crosslinkers on the MVTR of the PU-PSA have been investigated. A comparative study of effects of different crosslinkers showed that PU-PSA with IPDI has the least while that with TAC has the highest gel content and crystallinity and a reverse trend was observed for the MVTR. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Polyurethane Pressure sensitive adhesive Crosslinker Electron beam curing MVTR

1. Introduction Pressure sensitive adhesives (PSAs) are unique in properties and applications. They can be applied instantly on any surface with a slight finger pressure (Satas, 1999). Because of their visco-elastic nature they easily wet the surface and make a bond of measurable strength almost immediately upon contact. PSAs are generally used for a range of applications in tapes, drapes, labels and decals (Webster, 1997; Singh et al., 2011, 2012). They have a major market share in health care and biomedical applications. Different classes of materials are used for making PSAs for biomedical applications. The early PSAs were based on natural or synthetic rubbers, subsequently followed by polyacrylates, silicones, hydrocolloids and polyurethanes. All these materials have inherent advantages and disadvantages. Among all of them polyurethanes have maximum advantages because of their unique

n

Corresponding author. Tel.: þ 91 1127667267; fax: þ91 1127667676. E-mail address: [email protected] (A.K. Singh).

http://dx.doi.org/10.1016/j.radphyschem.2014.05.015 0969-806X/& 2014 Elsevier Ltd. All rights reserved.

chemistry and presence of a segmented structure consisting of hard (isocynate) and soft (polyol) segments which can be tailor-made as per the requirement (Fries, 1982). Polyurethane (PU) has excellent strength, elongation, shape memory, low temperature flexibility, solvent resistance, biocompatibility and inherent anti-microbial properties which make it unique in outclassing all other materials in performance. Unlike other PSAs, the ones used for bio-medical applications have to meet certain requirements; they should have good adhesion in terms of longer stay on skin without debonding, should be skin friendly and breathable, i.e. should be able to transfer the moisture vapor through it without accumulating at the interface with skin so that it stays for a long time (Laschke et al., 2009). The transfer of moisture from skin through PSA is measured as MVTR and is an essential requirement of medical grade PSAs besides retaining their adhesion strength. MVTR is the amount of moisture that is passed (transported) through a membrane such as, a dressing during a given period of time (http://www.smith-nephew.com/global/assets/pdf/products/ wound/v1-young_does-allevyns-foam-management_bjcn2007.pdf).

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A.K. Singh et al. / Radiation Physics and Chemistry 103 (2014) 75–83

The higher the MVTR, the more effective the dressing in removing moisture from the skin and preventing the accumulation of moisture under the membrane. This not only helps in lowering the cutaneous colonization of pathogens, thereby reducing the risk of infection but also keeps the skin underneath the dressing dry and fresh. The transportation of molecules through a homogeneous polymer matrix can be described by a diffusion mechanism which involves the condensation of the penetrant at one surface of the adhesive membrane, followed by diffusion, in the form of a liquid, through it under the influence of a concentration gradient and finally evaporation at the other surface to the gaseous state (Naylor, 1989). The quantity of penetrant (Q), which passes through the polymeric membrane within a unit time (t) and unit area (A), is called the diffusive flux (J) given by: J¼ Q/At. There is a linear relationship between the flux of substance diffusing through a membrane and the concentration gradient between both sides of the membrane (Fick, 1855; Crank, 1975). The transport behavior of the given penetrant varies from one polymer to another. It depends on free volume within the polymer and on the segmental mobility of the polymer chains. The segmental mobility of polymer chains is affected by the extent of unsaturation, degree of crosslinking, degree of crystallinity and nature of substituents. The glass transition temperature (Tg) of polymer has a profound influence on transport properties. Polymers with low Tg possess greater segmental mobility and will have higher MVTR (George and Thomas, 2001). Numerous theoretical models have been proposed by several authors to describe the transport mechanism of species in polymers (Crank and Park, 1968; Comyn, 1985; Stern and Trohalaki, 1990; Stern, 1994). Such models provide the coefficients of diffusion and permeability from statistical mechanical considerations (free volume theory) and energetic or structural considerations. The formulation of these coefficients is complicated by the fact that the transport phenomena are due to different mechanisms in rubbery and glassy polymers, i.e. at temperatures respectively above or below their Tg. Besides, parameters such as the degree of crystallinity of the polymer, its swelling by the sorbed molecules as well as its thermal history are significant (Crank and Park, 1968; Comyn, 1985). The present PU-PSA has been made keeping the MVTR in view and it does not use plasticizers which may leach in contact with the biological fluids and does not contain solvents which are generally used in making PSAs that may be emitted to environment during thermal curing. It has an edge over the UV curing as it does not involve costly photoinitiators which remain in the

product and may be reactive towards the skin. E-beam curing is a clean and green technology having many advantages, viz. a clean and environment friendly process, better control on curing characteristics, uniform and consistent product quality, cold process, does not use heat, photoinitiators for gaining strength of the adhesive bond, minimum possible residual monomer and high throughput. The present study involves development of an e-beam curable PU-PSA with breathability and adhesion strength properties suitable for use in biomedical applications. In the study, the effects of increasing dose and types of crosslinkers, i.e. TAC, TAIC, PMDI, CMDI, MDI and IPDI on the MVTR and adhesion properties of PU-PSA and also the effects of gel content and crystallinity of the cured PU-PSA compositions on MVTR and adhesion properties have been investigated.

2. Experimental 2.1. Materials Different materials used in this study are given in Table 1, while structure of different crosslinkers is given in Table 2. All these materials were used as such without further purification in the adhesive formulations. 2.2. Method 2.2.1. Preparation of PU-PSA composition and analysis PU-PSA compositions were made using various combinations of crosslinkers with the base resin and system was homogenized by the rapid mechanical rotatory homogenizer till the homogenous composition for 20 min before coating without any solvent or diluent. Finally, homogenized PSA composition made for e-beam curing was investigated for the equal dispersion with the help of a Polarizing Microscope (Nikon Eclipse E200 POL) and Transmission Electron Microscopic (TEM) analysis. Polarizing microscope Nikon Eclipse E200 POL was used for taking photomicrographs of PU-PSA with the optical system CFI60 infinity optics, magnification 500 for observation, 3600 rotary dial analyzer, 2 μm coarse/fine focusing and 6 V/20 W halogen lamp illumination for taking the photomicrographs of PU-PSA samples. TEM studies were performed at All India Institute of Medical Sciences, New Delhi, India, (Morgagni 268D Fei Electron Optics). A drop of the sample diluted in tetrahydrofuran (THF) was placed on a carbon-coated copper grid to form a thin film. The grid was allowed to air dry and the samples were viewed and photographed.

Table 1 List of raw materials. Name

Properties

Function

Source

Genomer 4269 (aliphatic urethane polyester acrylate in combination with 2-[{(butylamino)carbonyl}oxy]ethyl ester and 2-propenoic acid) Genomer 6043 (inert modified saturated polyester resin with a combination of 2-[{(butylamino)carbonyl}oxy]ethyl ester and 2-propenoic acid) Fumed silica (Cab-O-Sil) Triallyl cyanurate (TAC)

Tg 15 1C, viscosity 21,000 cP s, density 1.1 g/ml, water solubility o1 g/l Tg 18 1C, flash point 4100 1C, viscosity 19,000 cP s, density 1.13 g/ml, water solubility o 1 g/l Particle size 5–50 nm, surface area 5–600 m2/g Density 1.105 g/cm3, M.P 26–28 1C, B.P 360.4 1C, functionality 3 Molar mass 249.27 g/mol, density 1.11 g/cm3, M.P 26 1C, B.P 120 1C MDI content 43%, NCO content 32%, viscosity 50 cP s, mol.wt 340, density 1.234 g/cm3, functionality 2.7 MDI content 73%, NCO content 29.2%, viscosity 33 cP s, functionality 2.1 Molar mass 250.25 g/mol, density 1.23 g/cm3, M.P 40 1C, B.P 314.4 1C, flash point 212 1C Molar mass 222.3 g/mol, density 1.06 g/cm3, M.P 60 1C, B.P 158 1C, flash point 155 1C

Base resin

Rahn Corp., USA.

Tackifier

Rahn Corp., USA.

Triallyl isocynurate (TAIC) Polymeric methylene diphenyl diisocyanate (PMDI) Carbodiimide modified methylene diphenyl diisocyanate (CMDI) Methylene diphenyl di-isocynate (MDI) Isophorone di-isocynate (IPDI)

Filler Cabot, USA Crosslinker Sigma-Aldrich, USA Crosslinker Acros Organics, Belgium Crosslinker Dow Chemical Company, USA Crosslinker Dow Chemical Company, USA Crosslinker Sigma-Aldrich, USA Crosslinker Sigma-Aldrich, USA


77

TAC

2.2.4. Determination of MVTR MVTR of PU-PSA tape was determined as per specifications (ASTM D3833M, 1988). A cup shaped aluminum disc with a diameter of 8 cm was filled with weighed quantity of dry CaCl2 and the top of cup was covered with PSA tape and sealed from peripheries by hot paraffin wax. Initial weight of disc containing CaCl2 and tape was taken before placing in the incubation chamber at a RH 70% and temperature 30 1C for 24 h. The MVTR was calculated as follows:

TAIC

MVTRðg=h m2 Þ ¼

Table 2 Structure of crosslinkers. Name

Structure

W2 W1 24 T A

where W1 is initial weight (g) of system, W2 is final weight (g) of system, T is time of exposure (hours) between W1 and W2, and A is area of cup opening (m2). PMDI

CMDI

MDI

IPDI

2.2.2. Preparation of PU-PSA tape In order to evaluate the PSA for its MVTR a PSA tape was made by coating 0.2 mm thick and 30 g/m2 adhesive layer with the help of bar-coater having adjustable thickness blade on a release paper. More than 0.2 mm thick coating was removed by the sharp blade maintaining the equal thickness throughout the coating process. This system was left for air drying at room temperature for 30 min. The adhesive layer thus formed was ultimately transferred to the non-woven polypropylene fabric to make the PSA tape. The thickness of non-woven fabric was 0.32 mm while its weight was 90 g/m2.

2.2.3. Irradiation of PU-PSA tape The dried tape samples were irradiated in air, covering its surface with release paper avoiding any kind of surface oxidation at Bhabha Atomic Research Centre, Mumbai, India, by an e-beam accelerator (ILU-6). The accelerator has 2.0 MeV energy level with a conveyor speed of 3 cm/s for 10 kGy dose/pass and 6 cm/s for 5 kGy dose/pass, with a pulse current of 300 mA, average current 2 mA and pulse frequency of 15 Hz. The samples were irradiated at different doses starting from 5 kGy to 60 kGy. The irradiated samples were further kept in a vacuum oven at 50 1C for 1 h to avoid any kind of entrapment before packaging. E-beam dosimetry and calibration of thin radiachromic/CTA films using graphite calorimetry and current density measurements were standardized as part of developing reliable dose and dose assurance program.

2.2.5. Determination of adhesion properties Studies of adhesion properties of dry as well as wet samples (by immersing in 1% saline solution for 8 h) that include, peel and shear adhesion were carried out using a Universal Testing Machine (Star Testing System, Mumbai). The samples for peel adhesion were prepared as per ASTM D3330, 1993. The sample size was 300 mm 20 mm, contact area of the adhesive on the glass sheet was 100 mm 20 mm, grip length 150 mm and speed of the UTM was 500 mm/s. The shear adhesion test of both dry and wet samples was carried out as per ASTM D3654, 1988 by preparing samples of size 200 mm 20 mm with contact area of 50 mm 20 mm on the glass sheet; grip length was 150 mm and speed of the UTM was 200 mm/s. The initial tack of both dry and wet samples was carried out as per ASTM D3121, 1994. The samples of size 400 mm 50 mm with contact area of 380 mm 50 mm were prepared. The diameter of the steel ball was 10.96 mm while the weight of the ball was 5.47 g. 2.2.6. Determination of gel content The gel content of PU-PSA was determined by immersing 100 mg of cured sample in tetrahydrofuran (THF) at 50 1C for 24 h. The sample was filtered and dried in a vacuum oven at 50 1C till constant weight. The gel fraction was calculated by the following equation (Park et al., 2009): Gel fraction ð%Þ ¼

Wt 100 W0

where W0 is the original weight and Wt is the weight after drying. 2.2.7. Thermal analysis The thermal properties of various samples were determined using an SDT 2960 (TA Instruments, USA) Differential Scanning Calorimeter over a temperature range of 100 1C to –250 1C at a scan rate of 5 1C/min under inert atmosphere. The dry samples were sealed in a crucible inside the glove box. The sealed samples were taken out of the glove box only at the time of DSC experiments. Samples were kept at 250 1C for 3 min to remove the heat history and cooled up to 50 1C at the cooling rate of 5 1C/min. Finally, it was reheated at the rate of 5 1C/min up to 250 1C to obtain heat flow change with increasing temperature. All the thermograms were base line corrected and calibrated against Indium metal. Glass transition temperature (Tg) was reported at the midpoint of the transition process and melting temperature (Tm) was the peak temperature. 2.2.8. X-ray diffraction (XRD) analysis of PU-PSA The XRD analysis was done by using a Philips analytical X-ray, Philips X’pert XRD System at a voltage of 40 V, 30 mA current with


a radiation wavelength 1.542 Å. Spectra were recorded in the range of Bragg's angle 2θ ¼ 10–801 with scanning step size 0.021 and time per step 1 s. The percent crystallinity (Xc) of PU-PSA was calculated using the method described by (Young and Lovell, 1991) ∑i Aci 100 Xc ¼ ∑i Aciþ ∑i Aaj where Ac is the area of the X-ray diffraction curve due to scattering from the crystalline phase, and Aa is the area of the X-ray diffraction curve due to scattering from amorphous phase.

The decrease in MVTR of PU-PSA can be attributed to the crosslinking of polyurethane acrylate chains by e-beam; higher the e-beam dose, higher the extent of crosslinking 100

4%

6%

8%

10%

90 85 80 75

65 60

0

5

10

15

20

25

30

35

40

45

50

55

60

65

Dose (kGy) Fig. 2. Effect of IPDI and e-beam dose on MVTR of PU-PSA. 100

Control

2%

4%

6%

8%

10%

MVTR (g/hr-m2)

95 90 85 80 75 70 65 60

0

5

10

15

20

25

30

35

40

45

50

55

60

65

Dose (kGy) Fig. 3. Effect of MDI and e-beam dose on MVTR of PU-PSA.

100

Control

2%

4%

6%

8%

10%

95

MVTR (g/hr-m2)

The homogeneity of the developed composition was investigated and is shown in Fig. 1a and b. From the figure, it was found that ingredients added are equally distributed in the form of tiny spots in adhesive matrix, required for the development of PSA tape and shows a uniform film structure without any aggregation. The results for MVTR of PU-PSA at different doses and different IPDI concentrations along with control sample are shown in Fig. 2. The e-beam dose varied from 5 to 60 kGy while the concentration of IPDI varied from 2% to 10%. It was observed that with increasing the dose from 5 kGy to 60 kGy the MVTR decreases continuously at a particular IPDI concentration and decreases with increasing IPDI concentration. The control sample (0% crosslinker), however, has the maximum MVTR. Uncertainty in the e-beam dose was determined to be 71.3% based on uncertainty associated with the dosimetry system (intrinsic), calibration procedure, curve fitting and instruments (systematic as well as random), projected in results as error bars. Similar studies for MVTR were also carried out with other crosslinkers, viz. MDI (Fig. 3), CMDI (Fig. 4), PMDI (Fig. 5), TAIC (Fig. 6) and TAC (Fig. 7); a trend in properties similar to that of IPDI was observed for them, i.e. with increasing the e-beam dose from 5 to 60 kGy and crosslinkers concentration from 2% to 10% the MVTR decreases. The highest MVTR was observed at 5 kGy dose and 0% crosslinker concentration.

2%

70

2.2.9. Scanning electron microscopic (SEM) analysis of PU-PSA SEM studies were performed by using EVO 18 Special Edition (Zeiss, Germany) for imaging. The samples were mounted on aluminum stub with the help of double-sided adhesive carbon tapes (Agar Scientific, UK). The mounted samples were then exposed to 20 nm gold coating at 20 mA for 165 s by a Sputter coater SC7620 (Quorum Technologies Ltd., UK) to make them electrically conductive. The magnification of the microscope was selected as required for the detailed study the samples.

3. Results and discussion

Control

95 MVTR (g/hr-m2)

78

90 85 80 75 70 65 60 0

5

10

15

20

25

30

35

40

45

50

55

Dose (kGy) Fig. 4. Effect of CMDI and e-beam dose on MVTR of PU-PSA.

NIKON 500X 6V/20W PSA Fig. 1. Investigation of adhesive composition under a) polarizing microscope; b) TEM.

60

65


100

Control

2%

4%

6%

8%

10%

1.3

95 90

0.8

85

Heat Flow (W/g)

MVTR (g/hr-m2)

79

80 75 70 65 60

TAC TAIC PMDI CMDI MDI IPDI Control

0.3 -0.2 -0.7 -1.2

55 50 0

5

10

15

20

25

30

35

40

45

50

55

60

-1.7 -100

65

Dose (kGy)

-80

-60

-40

-20

0

20

40

60

80

100

Temperature (0) Fig. 5. Effect of PMDI and e-beam dose on MVTR of PU-PSA. Fig. 8. DSC curves at 25 kGy dose and 6% crosslinker concentration.

100

Control

2%

4%

6%

8%

10%

95

Table 3 Summary of results of DSC analysis.

MVTR (g/hr-m2)

90 85

TAC

TAIC

PMDI

CMDI

MDI

IPDI

Control

Tm (1C) Tg (1C) ΔH (J/g) Crystallinity (%)

45.4 5.3 22.0 11.2

44.6 6.4 20.8 10.6

44.1 7.2 20.0 10.2

43.6 8.0 19.3 9.8

43.2 9.2 18.6 9.5

42.7 10.0 18.1 9.2

42.3 11.0 16.4 8.3

4500

Control

IPDI

MDI

4000

PMDI

TAIC

TAC

80 75 70 65 60 55 50

0

5

10

15

20

25

30

35

40

45

50

55

60

65

Dose (kGy)

100

Control

2%

4%

6%

8%

Relative Intensity

Fig. 6. Effect of TAIC and e-beam dose on MVTR of PU-PSA.

10%

95 MVTR (g/hr-m2)

90 85 80

3500 3000 2500 2000 1500 1000

75

500

70

0

65 60

0

20

40

60

80

Double of Diffraction (0)

55 50

CMDI

0

5

10

15

20

25

30

35

40

45

50

55

60

65

Dose (kGy) Fig. 7. Effect of TAC and e-beam dose on MVTR of PU-PSA.

(Dutta and Sikdar, 1991). Crosslinking induces crystallinity in the adhesive chains, which can be defined in terms of sorption and diffusion across a polymeric membrane. The above mentioned results have also been verified by gel content measurements, DSC and XRD analyses of the PU-PSAs. The DSC curves of the PU-PSA are shown in Fig. 8 and the related thermal property data for different crosslinkers are summarized in Table 3. All the samples show a melting transition at around 42–45 1C. The PU-PSA with TAC shows higher melting transition temperature and crystallinity than that of control and other systems. This is due to the higher reactivity of TAC than the other crosslinkers, which could enhance the crosslinking leading to more gel content and crystallinity (Mondal and Hu, 2006). The percent crystallinity calculation from the DSC analysis was done by considering the enthalpy of crystallization of a 100% crystalline material (H100% ¼196.8 J/g) (Pistor et al., 2012). This is reflected in the DSC thermograms where higher reactivity of the crosslinkers leads to higher extent of crosslinking thus enhancing values of Tg. In Fig. 8, the control sample has Tg as 11.0 1C which increased after reacting with the crosslinkers in PU-PSA samples.

Fig. 9. XRD analysis at 25 kGy dose and 6% crosslinker concentration.

The Tg increased as per the reactivity order of the crosslinkers. Increased crosslinking restricts the chain flexibility and increases the cohesive energy among the molecular chains thus increasing gel content Tg and crystallinity with consequent reduction in MVTR. The increased Tg increases the activation energy of diffusion and correspondingly the diffusion is decreased (Crank and Park, 1968). In order to validate the DSC results, XRD analysis was done to measure the percent crystallinity of PU-PSA. The patterns are shown in Fig. 9. The peaks of XRD curves were separated and areas of amorphous phase broad peak and crystalline phase sharp peak were integrated using the Philips X’pert XRD system software. The calculated crystallinities are given in Table 4. From the study it was found that permeability of a gas through a semicrystalline polymer depends on the free volume within the polymers and on the segmental mobility of the polymer chains. The segmental mobility of semicrystalline polymer chains is affected by the degree of crosslinking, degree of crystallinity and Tg. Polymers with low Tg possess greater segmental mobility and will have higher permeability (Michaels and Parker, 1959; Michaels and Bixler, 1961a, 1961b; George and Thomas, 2001). This may be due to the fact that sorption and diffusion take place in the amorphous regions, while crystalline zones act as excluded volumes for the sorption process and are impermeable barriers

80


Table 4 Percent crystallinity by XRD analysis.

Crystallinity (%)

TAC

TAIC

PMDI

CMDI

MDI

IPDI

Control

14.0

12.4

11.3

11.0

10.4

9.8

9.2

for the diffusion process. The dispersed crystalline phase presents a resistance to the permeant passage by increasing effective path length of diffusion through a more irregular tortuous diffusive path around the crystallites for penetrant molecules. In addition, they seem to reduce the polymer chains’ mobility in the amorphous phase (because chain ends are trapped in the neighboring crystalline lamellae) leading to higher activation energy of diffusion (Pace and Datyner, 1979a; Klopffer and Flaconneche, 2001). The permeability of the gas through semi crystalline polymer thus decreases with increase in crystallinity. With the increase in degree of crosslinking the diffusion coefficient decreases and reduction in polymer chain's mobility and local segmental motion of polymer chains take place due to increase in activation energy of diffusion, which represents the energy necessary for the separation of the polymer chains by cooperative motions of sufficient amplitude to allow the penetrant to undergo its diffusional jump (Pace and Datyner, 1979a, 1979b, 1979c; Klopffer and Flaconneche, 2001). Thus the net effect is with the increase in degree of crosslinking the permeability of permeant decreases. In the present study it was found that the Tg, crystallinity and gel content of the e-beam cured PU-PSA systems decreased in the order IPDIo MDIoCMDI o PMDIoTAIC o TAC systems for the corresponding increase in MVTR in the order IPDI 4MDI4 CMDI 4PMDI 4TAIC 4TAC systems. These observations support the above explanations of effects of glass transition temperature, percent crystallinity and extent of crosslinking (percent gel content) on permeability of permeant like moisture vapor. In order to observe any morphological change in the PU-PSA tape, the PSA tapes (adhesive side) were analyzed by SEM. Fig. 10a presents the photomicrograph of control PU-PSA and shows a spongy and porous film structure with flexible adhesive domain in the adhesive matrix. Fig. 10b–g indicates the presence of medium to tiny pores which are randomly distributed throughout the matrix of PU-PSA. In the control sample, i.e. without crosslinker the molecular domains are loosely attached to each other showing large gaps/pores distributed in the matrix due to the less interaction and aggregation of molecular chains during irradiation and the presence of these structures are responsible for more MVTR in the control sample vis-à-vis the crosslinked PU-PSA samples. In the crosslinked adhesive (Fig. 10b–g) there is no such large pore/ hole like structure because of higher extent of crosslinking among molecular chains involving the crosslinkers indicating greater interaction and reactivity of the crosslinkers with PU acrylate matrix. For TAC the molecular domains are very close, dense and well defined, indicating close interaction of TAC with the matrix. This is due to higher reactivity of TAC vis-à-vis to other crosslinkers resulting in higher gel-content and crystallinity with clearly defined molecular domains supporting its lower MVTR. The molecular domains are interactive and closely packed for TAC, followed by TAIC, PMDI, CMDI, MDI and IPDI. The higher permeability of the control sample can be ascribed to the interaction of the water vapor molecules with the –R–NH– COO– of the urethane linkage and modification of the chains due to plasticization resulting in the increase of frequency of co-operative molecular motions. The permeant vapor reduces intermolecular forces among the polymer chains and thus decreases the activation energy necessary for separation of chains (Pace and Datyner, 1979c). The increased permeant concentration

induces enhanced mobility of the chains. Moreover, during permeation the hydrostatic pressure is increased resulting in the increase of polymer density due to polymer compaction, thereby, reducing the permeation path inside the polymer. The pressure increase leads to an increase of penetrant concentration in the membrane, thereby higher permeation. In case of less crosslinking, the segmental motion is higher which results in the higher diffusion and sorption, thus leading to higher permeation in the control sample. In higher crosslinking the chain segmental mobility is restricted and more and more activation energy is required for segmental mobility, thus, for lower MVTR. Fig. 11 presents the comparative MVTR values for all the six crosslinkers, i.e. IPDI, MDI, CMDI, TAIC and TAC at 25 kGy dose. From the figure, it is clear that the control sample, i.e. PU-PSA without crosslinker has the maximum MVTR due to higher segmental motion of molecular chains and reduced free energy of sorption which leads to higher permeation of the molecules. In the case of crosslinkers, IPDI has the minimum crosslinking, while TAC has the maximum due to their different chemical reactivities. TAC is highly reactive with three allylic groups attached to highly electronegative oxygen atom in the cyanurate ring, resulting in maximum gel content and lowest MVTR among all the six crosslinkers, i.e. TAIC, PMDI, CMDI, MDI and IPDI. TAIC is less reactive than TAC because the allylic groups in TAIC are attached to the less electronegative nitrogen atom in the cyanurate ring. Therefore, maximum crosslinking was achieved with TAC, which is also evident from the gel contents’ measurements, and DSC and XRD analyses. Among the isocynates, PMDI has the functionality of 2.7 followed by that of CMDI 2.1 MDI 2.0 and IPDI 2.0, which is primarily responsible for reactivity. MDI has two symmetrical NCO groups, which are equal in reactivity, and reaction rate. Even after reaction of one of the NCOs, the other NCO has the same reaction rate as before. But in case of IPDI the two NCO groups are asymmetrical and have different reactivities, and the secondary NCO group is more reactive than the primary NCO. The more efficient or reactive a crosslinker, the lower the MVTR. As the order of reactivity increases among the crosslinkers towards the urethane acrylate chain, the MVTR is consequently decreased and thus TAC has the lowest MVTR due to its highest reactivity. The increased degree of crosslinking among various crosslinkers is also substantiated by their gel content (Table 5). TAC has the highest gel content while, IPDI has the lowest due to their varied reactivity towards urethane acrylate chains. The adhesion properties, viz. peel adhesion, shear adhesion and initial tack of the PU-PSA for different crosslinkers are shown in Figs. 12–14 both in dry and wet conditions. Since the optimum adhesion properties were observed for 6% crosslinker concentration and 25 kGy dose, comparative adhesive properties reported in this study are carried out at 6% crosslinker concentration and 25 kGy dose. The peel adhesion strength of a pressure sensitive adhesive is the measure of the amount of force required to remove the adhesive from the substrate. The comparative peel adhesion strength of various crosslinkers is shown in Fig. 12 which reveals that IPDI has the maximum peel adhesion strength, while TAC has the least among the crosslinkers. The higher peel strength of IPDI can be attributed to its better wetting of the surface due to linear crosslinking and higher segmental motion for spreading on the substrate among the crosslinkers. The peel adhesion strength of different crosslinkers has the reverse order of its reactivity, i.e. the higher reactive the crosslinker the lesser the adhesion strength. This is because of the fact that higher crosslinking reduces the wetting of the substrate thereby decreasing the adhesive strength. The shear adhesion strengths of both dry and wet samples are shown in Fig. 13, which is defined in terms of its cohesive strength;


81

Fig. 10. SEM micrographs of control (a), IPDI (b), MDI (c), CMDI (d), PMDI (e), TAIC (f) and TAC (g) at 6% crosslinkers concentration and 25 kGy dose.

the more cohesive the system, the more the shear strength. Though crosslinking improves cohesion among the molecular chains, but in our study we found that shear strength is highest

with IPDI and not with TAC which has the highest crosslinking; this may be due to the lack of required balance between cohesion and adhesion properties. Though TAC gives higher crosslinking, it


4. Conclusion The following conclusions can be drawn from the above study: i. With increasing e-beam dose from 5 kGy to 60 kGy the MVTR decreased continuously in PU-PSA. ii. Increasing crosslinker concentration from 2% to 10% resulted in decrease of MVTR in PU-PSA. iii. The PU-PSA without crosslinker has the maximum MVTR. iv. IPDI has the maximum and TAC has the least MVTR in PU-PSA system among the crosslinkers. v. The decreasing order of MVTR, adhesion properties and gel content for the e-beam cured PU-PSA for different crosslinkers is given as below: IPDI

MDI

CMDI

PMDI

TAIC

MVTR (g/hr-m2)

85 80 75 70

Dose 25kGy

Dry

Wet

2000 1500 1000 500 0 TAC

TAIC

PMDI

CMDI

MDI

IPDI

Crosslinker Fig. 12. Comparative peel adhesion of dry and wet samples for different crosslinkers at 6% concentration and 25 kGy dose.

16000

6% Crossliker Conc.

Dose 25kGy

Dry

Wet

14000 12000 10000 8000 6000 4000 2000 0 TAC

TAIC

PMDI

CMDI

MDI

IPDI

Crosslinker Fig. 13. Comparative shear adhesion of dry and wet samples for different crosslinkers at 6% concentration and 25 kGy dose.

25

TAC

90

6% Crossliker Conc.

2500

Initial tack (N/m)

95

3000

Peel adhesion (N/m2)

imparts the lowest tack or wetting property due to restriction of the segmental motion of chains by crosslinking; consequently, adhesion failure occurs during deformation. The tack of a pressure sensitive adhesive is its ability to conform to and wet out a given substrate with the application of only minimal external force. An adhesive with high tack will wet out and stick efficiently to a substrate, while low tack will cause poor immediate adhesion. Each pressure sensitive adhesive is unique, and takes specific time to completely wet out a substrate. The tack is affected mainly by the crosslink density of the adhesive or functionality of the reactants. A lower crosslink density will generally result in a higher tack value. This will allow a greater flexibility of the adhesive and assist in its conforming to and wetting out substrates (Roches and Murphy, 2006). Another vital component of tack involves Tg of the adhesive. As a general rule, the lower the overall Tg, the higher the tack value. Higher crosslinking gives higher Tg and correspondingly the gel content is also higher (Singh et al., 2013). These are the reasons that can be ascribed to the higher value of tack in case of IPDI having lowest crosslinking among the crosslinkers and lower value in case of TAC having highest crosslinking of the PU acrylate chains (Fig. 14).

Shear adhesion (N/m2)

82

65

6% Crossliker Conc. Dose 25kGy

Dry

Wet

20 15 10 5

60 55

0 TAC

50 Control

2

4

6

8

TAIC

PMDI

10

CMDI

MDI

IPDI

Crosslinker

Crosslinker Conc. (%) Fig. 14. Comparative initial tack of dry and wet samples for different crosslinkers at 6% concentration and 25 kGy dose.

Fig. 11. Comparative MVTR of control and different crosslinkers at 25 kGy dose.

Table 5 Summary of results at 6% crosslinkers concentration and 25-kGy dose. Crosslinker

TAC TAIC PMDI CMDI MDI IPDI

MVTR (g/h m2)

Gel content (%)

Peel adhesion (N/m2)

Shear adhesion (N/m2)

Initial tack (N/m)

Dry

Dry

Dry

Wet

Dry

Wet

Dry

Wet

68 7 0.88 727 0.94 747 0.96 787 1.01 807 1.04 827 1.07

927 1.22 907 1.17 887 1.14 867 1.12 837 1.08 817 1.05

987 7 12.83 11607 15.08 18177 23.62 22407 29.12 23107 30.03 24407 31.72

554 77.20 653 78.49 922 711.99 1460 718.98 1654 721.50 1850 724.05

8540 7 111.02 9425 7 122.53 10,220 7 132.86 11,926 7 155.04 12,6917 164.98 13,205 7 171.67

64127 83.36 71247 92.61 8234 7 107.04 92457 120.19 10,2357 133.06 11,0247 143.31

14 70.18 15 70.20 17 70.22 19 70.25 22 70.29 23 70.30

87 0.10 107 0.13 137 0.17 157 0.20 177 0.22 197 0.25


MVTR/peel adhesion/shear adhesion/initial tack:

IPDI4MDI 4CMDI 4PMDI 4TAIC 4 TAC Gel content/Tg/percent crystallinity: TAC 4TAIC 4 PMDI4 CMDI 4MDI4 IPDI

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