Electroless plating of nickel–phosphorous on surface-modifiedpoly(ethylene terephthalate) films

S.C. Domenecha,b, E. Lima Jr.c, V. Dragoc, J.C. De Limac,N.G. Borges Jr.a, A.O.V. Avilaa,b, V. Soldid,*

aLaboratorio de Biomecanica, CEFID, Universidade do Estado de Santa Catarina, 88080-350 Florianopolis, SC, BrazilbCentro Tecnologico de Couros, Calcados e Afins, Araxa 750, 93334-000 Novo Hamburgo, RS, Brazil

cDepartamento de Fısica, Universidade Federal de Santa Catarina, Campus Trindade, 88040-900 Florianopolis, SC, BrazildGrupo de Estudo em Materiais Polimericos (Polimat), Departamento de Quımica, Universidade Federal de Santa Catarina,

Campus Trindade, 88040-900 Florianopolis, SC, Brazil

Received 2 May 2003; received in revised form 26 May 2003; accepted 10 June 2003

Abstract

Chemical surface preparation for Ni–P electroless metallization of poly(ethylene terephthalate) (PET) films without using

Chromium-based chemicals, was studied. The applicability of this method was verified by a subsequent metallization process.

Thermal analysis was conducted to observe the main thermal transitions and stability of the polymer and metallized films.

Contact angle analysis was performed to assess the surface hydrophilicity so as to optimize the substrate preparation process.

X-ray diffraction, EDAX and SEM analysis were used to understand the composition and morphology of the polymeric substrate

and Ni–P coat growing process. Adherence strength, contact sheet resistivity and optical diffuse reflection were measured on the

metallized films. The time of chemical etching affects the polymer surface hydrophilicity, polymer/metal adherence strength,

surface resistance and optical diffuse reflection, while Ni coating morphology is controlled by the pH of the electroless bath.

High wettability of the polymer surface, adherence strength of 800 N cm�2, high optical diffuse reflection and low surface

resistivity of the Ni coating, were found for films etched for 60 min. Metallizations performed at pH 7.5 produce Ni–P coatings

with 12.0 wt.% phosphorous content, which were amorphous and flexible. The contact sheet resistivity of the plated films is

sensitive to roughness variations of the substrate. The method proposed in this work allows the production of metallized films

appropriate for the fabrication of flexible circuits.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Electroless nickel; Plating on plastics; Poly(ethylene terephthalate); Flexible circuits

1. Introduction

The metallization of polymer materials has

attracted attention in recent years due to its wide

range of technological applications. Many polymer

films, fibers and plastics are metallized for food

packaging, microelectronics, computer technology

and the automotive industry to provide an electro-

magnetic shielding property [1]. Metal foil and lami-

nates, conductive paints and laquers, sputter-coating,

vacuum deposition, flame, arc spraying and electroless

metal plating are recently developed metal-coating

Applied Surface Science 220 (2003) 238–250

* Corresponding author. Tel.: þ55-48-331-9219;

fax: þ55-48-331-9711.

E-mail address: vsoldi@qmc.ufsc.br (V. Soldi).

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0169-4332(03)00815-8

techniques [1–3]. Among them, electroless metal

plating is a preferred way to produce metal-coated

materials [2,4–6]. Because electroless plating has

advantages in terms of coherent metal deposition,

excellent conductivity and applicability to compli-

cated-shaped materials or nonconductors, it can be

applied to most polymer substrates. The electroless

process has been reported to occur due to a combina-

tion of partial electrode oxidation and reduction pro-

cesses. The driving force for these reactions arises

from the potential difference that exists between the

metal-solution interface and the equilibrium electrode

potential for these (cathodic and anodic) half-reac-

tions [2]. Generally, plating is initiated upon a cata-

lyzed surface and the plating reaction is sustained by

the catalytic nature of the plated metal surface itself,

so the adhesion at the metal/polymer interface and the

mechanical and electrical properties of the coating are

important considering the use of such materials in

technological applications.

Since the surface activity of the polymer substrate

with metals is generally low, to enhance adhesion the

polymer surface must be formed with a suitable irre-

gularity for a mechanical anchorage of the coating [7].

Some treatments already reported in the literature are

the photo-etching by far-ultraviolet excimer laser radia-

tion [8], plasma treatment [1] and chemical etching

[2,3,7]. The electroless plating method consists basi-

cally of a series of pretreatments, such as pickling,

sensitization and activation. The former, is an essential

process which provides the substrate with cavities and

in some cases, a modified chemistry which improves the

wettability of the surface and sometimes renders a

hydrophobic surface hydrophilic. Chemical etching is

a commonly known pickling process that has been

applied to several kinds of polymers for metallization,

such as acrylonitrile–butadiene–styrene (ABS) copo-

lymer, polypropylene, polysulfone, polyethersulfone,

polyetherimide, Teflon1, polyarylether, polycarbonate,

polyphenylene oxide, polyacetal, mineral reinforced

polyamide, among others [2,3,7,9]. However, in most

of these materials, the metal coating has little or no

adhesion to the substrate, or/and unacceptable appear-

ance. Only ABS has found wide acceptance in the

plating industry, due to this polymer being formed by

an acrylonitrile–styrene matrix with butadiene rubber

phases uniformly distributed in it. This structure makes

it unique for plating, as butadiene can be selectively

etched out of the matrix leaving microscopic holes, used

as bonding sites in electroless plating.

Considering the difficulty to obtain adherent metal

coatings by electroless plating on commonly used

polymers and the lack of research describing the

processing of flexible metallized polymer films by

electroless methods, the present work was focused

on the electroless metallization of poly(ethylene ter-

ephthalate) (PET) films with nickel–phosphorous

alloys. In addition, the pretreatment solutions usually

contain chromic acid, which can generate Cr6þ, a very

toxic and mutagenic ion. Therefore, the pretreatment

of the polymer surface by chemical etching without

using chromium-based chemicals proposed in this

work is an interesting route from an environmental

point of view. The excellent adherence and flexibility

obtained with the metallized films makes this simple

and low-cost method viable for the fabrication of

flexible circuits.

2. Experimental

2.1. Sample preparation

Poly(ethylene terephthalate) biaxially stretched

films (type 19.10 donated by Terphane Inc., Cabo,

Brazil), 12 mm thick, containing 600 mg g�1 of SiO2

(purity 99.9 wt.%) as a filler were rinsed with ethylic

alcohol (Sigma), distilled water and dried in air. The

metallization process of the polymer films consisted of

the following steps: (i) surface treatment; (ii) surface

activation; (iii) electroless metallization with nickel–

phosphorous. Surface treatment was carried out by

immersion of the PET film in a solution containing

0.17 mol l�1 of KMnO4, 1.24 mol l�1 of HNO3 and

tensoative CR (Atotech GmbH, Berlin, Germany)

aqueous solution at 65 8C. The time of acid etching

was controlled from 30 to 90 min in order to observe

its influence on the hydrophilicity and surface rough-

ness. After, samples were rinsed with distilled water

and dried in air. Reagents for chemical etching were of

analytical purity, purchased from Sigma. Surface acti-

vation was conducted by immersion of the samples in

an aqueous solution containing HCl (Sigma) and a Pd-

based activator (Noviganth AKI, Atotech GmbH,

Berlin, Germany) at 25 8C for 1 h. The specimens

were then rinsed in distilled water and immersed in a

S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250 239

solution containing the accelerator (ABS AKII, Ato-

tech GmbH, Berlin, Germany) at 25 8C. Afterwards,

the specimens were immersed in the electroless Ni

metallization bath (Noviganth Ni AK Atotech GmbH,

Berlin, Germany) containing: (A) complexing agent;

(B) sodium hypophosphite-based reducing agent; and

(C) NiCl2-based solution; at 65 8C for 300 s at pH 7.5.

Temperature was controlled with a thermostatized

bath (model 8TZ99-20, Microquımica Ltd., Floriano-

polis, Brazil). Some specimens were metallized at pH

8.5 to observe the influence of this parameter on the

coating properties. Finally, the samples were rinsed

with distilled water, ethylic alcohol and dried in an

oven at 50 8C.

2.2. Characterization

The main thermal transitions and thermal stability

of the PET films were observed by differential scan-

ning calorimetry and thermogravimetry with DSC-50

and TGA-50 (Shimadzu, Kyoto, Japan) facilities,

using 0.005 g samples, aluminum and platinum cells,

respectively, under N2 flow rate 50 ml min�1 and

heating rate 10 8C min�1. DSC and TGA experiments

were conducted from 25 to 300 8C, and from 25 to

900 8C, respectively. Metallized PET films (and an

uncoated film, for comparison) were analyzed by DSC

from 25 to 400 8C, N2 flow rate 50 ml min�1 and

heating rate 20 8C min�1.

The influence of the acid etching on the surface

hydrophilicity was observed by contact angle mea-

surements [7,10–16]. The sessile drop experiment

[7,10–14] consists of placing a known volume of a

probe liquid (in this work, 10 ml of distilled, deionized

water were employed) on the polymer surface. The

formed drop spreads until it attains an equilibrium state,

in which a three-phase system is formed (Fig. 1A).

Under these conditions, a finite contact angle (static

angle, ys) formed by the static drop with the surface can

be determined. With respect to the solid phase, the

liquid may be either wetting (ys ¼ 08) non-wetting

(ys ¼ 1808) or partially wetting (0 < ys < 1808) [14].

The Young’s equation [15,16] describes the energy

balance of the phases in equilibrium:

gsg ¼ gsl þ glg cosY (1)

where g represents surface tension (expressed in units

of energy per units of interfacial area) and the sub-

scripts s, l and g, denote the solid, liquid and gaseous

phases, respectively.

If additional liquid is added to the drop (in this case,

2 ml) the contact line advances and stops. Each time

motion ceases, the drop exhibits an advancing contact

angle (ya). Alternatively, if liquid is removed from the

Fig. 1. (A) Contact angle of a sessile drop on a solid surface; (B) contact angle measurement facilities.

240 S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250

drop the contact angle decreases without movement of

the contact line. If enough liquid is removed (2 ml) the

contact line retreats. When motion ceases the drop

exhibits a receding contact angle (yr). Three measure-

ments were made on each sample and the data are

presented as a mean with a standard deviation. Image

data were captured with a video micrometer (Javelin

model JV 600 T, Syosset, NY, USA) and Computer

Eyes 5.12 software. Image analysis was performed

with the Image Tool 1.28 software. A scheme of the

contact angle measurements is shown in Fig. 1B.

Morphological analysis was conducted on the sur-

faces before and after etching the PET films and after

electroless metallization with nickel–phosphorous

using a scanning electron microscope (model XL

30, Philips, The Netherlands). Chemical analysis

was conducted with a EDAX (model New XL 30,

Philips) facility. Additionally, the morphological and

chemical analysis of the cross-section of metallized

polymer films was conducted. These specimens were

placed between two glass sheets and embedded with

an acrylic resin (Atotech Inc., Fountain Hills, LA,

USA). Afterwards, the cross-section of the samples

was sanded (with sandpaper of different grain size),

followed by polishing with Al2O3 (grain diameter:

1 mm) and diamond paste (grain diameter: 1 mm). All

specimens were coated with gold prior to the analysis.

X-ray diffraction (XRD) analysis of metallized PET

films was performed with an X-ray diffractometer

(model Miniflex, Rigaku, Tokio, Japan), operating

at 30 kV and 0.015 A. The incident radiation was

Cu Ka, operating without monochromator. Data were

collected from 2y ¼ 10 to 408, in a y/2y geometry.

Adhesion tests were performed as described in the

literature [17]. The metallized PET sample (1 cm wide

and 1 cm long) was attached between two AISI 1020

steel bars (1 cm wide, 1 cm thick and 5 cm long) using

an epoxi resin/polyaminoamide adhesive (Araldite1,

Vantico AG, Switzerland) and dried in air for 48 h.

After the preparation, stress-strain tests were con-

ducted with 10 samples of each composition in tensile

mode, at deformation rate 0.5 mm min�1, at 20 8Cusing an extensometer (model DL 500, EMIC Ltd.,

Sao Paulo, Brazil) facility equipped with a 5000 N cell

(maximum force: 4413 N).

In order to get information on the influence of the

chemical etching on the surface roughness of the PET

films, and consequently, on the optical properties of

the Ni–P coating, optical diffuse reflection of the

metallized surfaces was measured using a Na vapor

lamp, a digital camera CCD (light sensor, model

CI-6504A, Pasco, Roseville, CA, USA) and an opti-

cally polished pure Ni sheet as a calibrator, as shown

in Fig. 2.

Contact sheet resistivity measurements were carried

out on the surface of the metallized PET films by the

four-point-probe method [18] using a homemade cur-

rent source, at 25 8C and applied current of 10 mA.

3. Results and discussion

3.1. Characterization of the polymer substrate

3.1.1. Thermal analysis

The DSC thermograms (second heating run) and

TGA obtained for a PET film (before etching) are

shown in Fig. 3. The glass transition temperature (Tg)

of the polymer film is observed at 74 8C (Fig. 3A). The

presence of a melting peak (Tm) with maximum at

258 8C and DCp 12.61 cal g�1 indicates that the poly-

mer presents a certain degree of crystalinity. For

semicrystalline poly(ethylene terephthalate) the Tg

and Tm transitions are described in the literature

[19] as occurring at 81 and 280 8C, respectively.

The shift of these transitions to lower temperatures

and the presence of a shoulder at the onset of the

melting process is probably caused by the presence of

SiO2 in its composition. The TGA curve (Fig. 3B)

indicates that thermal degradation occurs in one step,

with onset at 410 8C.

3.1.2. Contact angle measurements

The aim of the acid etching (surface treatment)

is to increase the hydrophilic character and surface

Fig. 2. Equipments for optical diffuse reflection measurements.

S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250 241

roughness of the polymer films, allowing anchorage of

the Ni coating during the metallization process [7].

The effects on the polymer surface after different time

periods of chemical etching were therefore assessed

by contact angle measurements performed by the

sessile drop method (Fig. 4). The data are presented

in Table 1 as static, advancing and receding angles.

Comparing the results of the analyzed surfaces, the

non-etched PET sample (shown in Fig. 4A), presented

the highest values of static and advancing angle,

0 50 100 150 200 250 300

1 mW

exo

endoH

eat

Flu

x

T (oC)

0 200 400 600 800 10000

20

40

60

80

100MassLoss(%)

T (oC)

(A)

(B)

Fig. 3. Thermal analysis of a PET film: (A) DSC (second heating run); (B) TGA thermograms. Analysis conditions: N2 flux, 50 ml min�1;

heating rate, 10 8C min�1.

242 S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250

indicating that under these conditions, the surface is

preferentially hydrophobic. After 30 min of acid etch-

ing (Fig. 4B), the values of these parameters decrease

substantially, indicating an increase in the hydrophilic

character of the surface. This behavior is more pro-

nounced as the treatment time increases up to 60 min

(Fig. 4C and D). Acid etching promotes the formation

of active sites and anchorage points at the polymer

surface and the modification of the surface roughness,

increasing the interaction between water molecules

and polymer chains at the surface.

After longer acid etching times (Fig. 4E and F) the

values of ys and ya increase, suggesting that after this

period, hydrolysis reactions (decomposition) occurred,

affecting the surface roughness and impairing the

surface hydrophilicity. The yr parameter presents rela-

tively high values in all specimens, except for that

submitted to 60 min of acid etching, indicating that the

optimal time of etching for obtaining a hydrophilic

surface, is close to this value.

3.1.3. Scanning electron microscopy

Fig. 5 shows the micrographs of the PET surface

before and after different acid etching times. The non-

etched sample (Fig. 5A) presents a surface morphol-

ogy preferentially amorphous, with particles of SiO2

(white-colored spots in the picture) homogeneously

distributed in the polymer matrix. After 30 min of acid

etching (Fig. 5B) an interesting step pattern is

observed, which resembles the plan of orientation

of the polymeric film (due to the biaxial stretching

of the polymer films after extrusion). It is important to

note that an increase in the micro-roughness occurred

in this sample (see Fig. 5C which represents a mag-

nification of the square drawn in Fig. 5B). The mor-

phology of the polymer surface changes for specimens

submitted to longer acid etching times (Fig. 5D),

which suggests that after 60 min, acid etching pro-

motes the removal of SiO2 particles and reaches

deeper layers of the polymeric substrate, increasing

Fig. 4. Profile of a water drop (10 ml) on PET film surfaces submitted to acid etching for: (A) 0 min; (B) 30 min; (C) 45 min; (D) 60 min; (E)

75 min; (F) 90 min.

Table 1

Values of contact angle calculated for PET samples after acid

etching

Acid etching

time (min)

ys (8) ya (8) yr (8)

0 77.4 � 2.1 79.2 � 0.8 55.1 � 1.4

30 61.4 � 2.9 62.6 � 2.8 58.3 � 1.7

45 47.2 � 1.3 52.3 � 0.9 45.8 � 1.1

60 27.3 � 1.8 35.4 � 1.7 21.9 � 0.2

75 55.9 � 1.9 56.3 � 6.4 41.5 � 5.5

90 52.9 � 0.4 58.1 � 1.5 42.8 � 3.8

S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250 243

its surface roughness. These results are in a good

agreement with those obtained by the above-described

contact angle measurements.

3.2. Characterization of the metallized polymer

substrates

3.2.1. Thermal analysis

Fig. 6 shows the DSC thermograms obtained for

PET films metallized with nickel–phosphorous at

different pH 7.5 and 8.5. A thermogram of a non-

etched, uncoated PET film was obtained under the

same conditions for comparison. The thermogram of

the uncoated polymer film, shows a melting peak

(endothermic) with a maximum at 255.8 8C. The

curves were plotted in the range of 150–350 8Cbecause the glass transition process of PET was not

clearly observed during the first heating run.

The thermogram of the sample metallized at

pH 8.5, depicts two thermal transitions clearly

observed in this temperature range: an endothermic

peak with maximum at 242.6 8C (related to the

melting of the polymer substrate) and an exothermic

process with maximum at 258.7 8C (related to the

crystallization of the Ni coating). On the other hand,

the DSC curve of the PET film metallized at pH 7.5,

presents a melting peak with maximum at 254.3 8Cfollowed by a broad exothermic process occurring in

the range of 261–300 8C. Compared to the uncoated

sample, a slight shift in the melting peak of PET (Tm)

to lower temperatures was observed in both metal-

lized samples.

It is important to note that the exothermic peak

observed as a narrow peak for the sample metallized at

pH 8.5, was broadened and shifted to higher tempera-

tures for the sample metallized at pH 7.5. The broad-

ening of the crystallization peak suggests that ordered

structures are not completely form. This behavior

indicates that the crystallization process of the nickel

coating is markedly influenced by the pH of the

metallization bath. Metallizations performed at pH

7.5 resemble polymer films covered by coatings with

low degree of crystallinity (which will be flexible). On

the other hand, metallizations carried out at pH 8.5

produce crystalline nickel films (which will be brittle).

It is expected that samples coated at different pH

will present different polymer/metal interactions

which can also influence the thermal processes and

Fig. 5. Scanning electron micrographs of PET film surfaces

submitted to acid etching for: (A) 0 min; (B) 30 min; (C) 30 min

(at higher magnification); (D) 60 min.

244 S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250

as a consequence, the adhesion strength between

components.

3.2.2. Scanning electron microscopy

Fig. 7A and B shows the micrographs of the surface

of Ni coatings deposited at pH 7.5 on PET films

submitted to different acid etching times, which show

that the nickel–phosphorous films are homogeneous

with preferentially amorphous appearance under these

conditions. The white specks observed are related to

the formation of some nucleus associated with the

Ni–P deposit.

The morphological analysis of PET metallized films

were observed in Fig. 8A and B (cross-section) and C

(angled view) revealed that a decrease in the surface

roughness occurs after metallization. The leveling

action promoted by the Ni coating on the polymer

surface produced on a sample etched for 30 min can be

observed in detail in Fig. 8A. This figure depicts a

metallic film (�0.5 mm thick) homogeneously distrib-

uted onto the polymer substrate covering the super-

ficial defects. A similar effect can be observed for a

nickel coating generated on a PET film etched for

60 min (Fig. 8B). As described before, the interactions

between the nickel coating and the PET film could be

impaired at surfaces etched for time periods longer

than 60 min, due to the decrease in the hydrophilic

character of the polymer surface.

The influence of the pH of the metallization bath on

the crystallinity of the nickel–phosphorous coating

can be observed in Fig. 8C, which shows that

Ni coatings deposited at pH 8.5 present a brittle

appearance and low adherence to the polymer sub-

strate. In general, nickel–phosphorous coatings are

classified into three categories: low (1–5 wt.%), med-

ium (5–8 wt.%) and high (9 wt.% and above) phos-

phorous deposits [4]. Studies have shown that as-

coated low-phosphorous deposits are either crystalline

[20] or consist of microcrystalline nickel [21]. As-

coated medium phosphorous deposits are reported to

be either fully amorphous [20] or mixtures of micro-

crystalline nickel and amorphous phases [22]. Studies

have also indicated that the crystallization behavior of

nickel–phosphorous deposits depends on the level of

phosphorous content. A study using DSC and trans-

mission electron microscopy [4] revealed that deposits

with less than 7 wt.% P transform directly into final

stable NiP3 phase in a matrix of crystalline nickel.

Fig. 6. DSC curves for PET films metallized with Ni–P in solutions of different pH. Analysis conditions: N2 rate flow, 50 ml min�1; heating

rate, 20 8C min�1.

S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250 245

Deposits with higher phosphorous content, however,

transform from amorphous to some metastable phases

such as Ni5(P, Ni)2, and Ni3(P, Ni) before final stable

Ni3P formation. On the other hand, high phosphorous

deposits (12.1 wt.%), crystallize to metastable Ni12P5

phase before transforming to nickel and the stable

Ni3P phase.

In this work, the EDAX analysis (shown in Table 2)

reveals that the coating is constituted by a Ni–P alloy.

The low-phosphorous content presented by the coat-

ing generated at pH 8.5 is probably the cause of the

crystallinity and the fragile aspect of the film, which

also alters the growing regime of the Ni–P alloy from a

layer-type to a columnar-type [23]. On the other hand,

the metallization performed at pH 7.5 promoted the

formation of amorphous alloys with high P content,

which are flexible metallic coatings with a low degree

of crystallinity.

The XRD analysis of a PET specimen etched for

60 min, metallized at pH 7.5 (Fig. 9) reveals a wide

band at approximately 128, a narrow peak with

maximum at 268 (representing the amorphous and

Fig. 7. Scanning electron micrographs of PET film surfaces submitted to acid etching for: (A) 60 min; (B) 90 min, metallized with Ni at

pH 7.5.

246 S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250

crystalline portions of the polymer substrate, respec-

tively) and a wide band with low intensity in the range

50–588 (related to the nickel–phosphorous alloy) indi-

cating that under these conditions, the coating presents

a low degree of crystallinity. These results are in good

agreement with the above mentioned EDAX analysis,

which confirms that metallizations carried out at pH 7.5

produce Ni films with high phosphorous content,

which are amorphous (and flexible).

As will be further discussed, the adherence strength

between the coating and the polymer substrate is a

function of the acid etching time. However, morphol-

ogy and flexibility of the nickel coating are dependent

of the P content in the alloy, which can be controlled

by the pH of the electroless deposition bath [23].

3.2.3. Adherence tests

The adherence strength of the Ni–P coating on the

PET films as a function of the acid etching time is

shown in Fig. 10. As expected, the nickel films formed

on PET surfaces etched for 30, 75 and 90 min pre-

sented lower adherence strength in the case of the

polymer substrate than that etched for 60 min.

Therefore, the optimal time of surface treatment for

obtaining adherent metallic films seems to be in the

range of 50–60 min. Visual inspection of the Ni coat-

ings also revealed that the specimen etched for 60 min

presented higher metallic shine. The lower adherence

observed in specimens etched for time periods other

than 60 min should be related to the physicochemical

anchorage of the nickel layer to the polymer surface,

given by the hydrophilic character and surface rough-

ness of the substrate.

3.2.4. Optical diffuse reflection and contact

sheet resistivity

The results of optical diffuse reflection and contact

sheet resistivity as a function of acid etching time for

PET films metallized with nickel at pH 7.5 are shown

in Fig. 11A and B.

The behavior of the optical diffuse reflection curve

(Fig. 11A) is in agreement with that observed in

the contact angle measurements and adherence tests:

initially, values increase with acid etching time, reach

Fig. 8. Scanning electron micrographs of PET films submitted to

acid etching for: (A) 30 min; (B) 60 min (A and B representing the

cross-section of films prepared at pH 7.5); (C) 75 min (angled view

of a film prepared at pH 8.5).

Table 2

EDAX analysis for PET samples metallized with Ni–P at different

pH values

Sample Element

contents (wt.%)

Structure

P Ni

PET film metallized at pH 8.5 8.5 91.5 Crystalline

PET film metallized at pH 7.5 12.0 88.0 Amorphous

S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250 247

a maximum in the range 50–60 min, decreasing for

longer time periods. It is therefore believed that the

optical diffuse reflection of the film is related to the

surface roughness (see Fig. 5). The optical diffuse

reflection values were obtained in relation to those of

an optically polished pure nickel calibrator. All nickel

coatings obtained at pH 7.5 presented higher reflec-

tivity values than those of the calibrator itself. This

fact was not observed with the films produced at pH

8.5 possibly due to their crystalline (and fragile)

characteristics. The straight line in Fig. 11A shows

that there is a good fitting of the values to a Gaussian

curve, which seems to indicate that an adequate rough-

ness is obtained for samples etched for approximately

50 min.

Fig. 11B shows the behavior of the contact sheet

resistivity of the nickel films as a function of the acid

etching time of the corresponding PET substrate.

0 20 40 60 80 100

0

200

400

600

800

1000

Inte

nsity

(cp

s)

Diffraction angle 2θ

Fig. 9. XRD analysis of a PET film etched for 60 min, metallized with Ni–P at pH 7.5.

Fig. 10. Tensile strength (maximum) vs. time of acid etching for PET films metallized with Ni–P at pH 7.5.

248 S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250

Since the nickel deposits have an amorphous structure

it is expected that the electrical mean free path should

be at the nanometric scale. It can be observed that for

up to 60 min of etching time, the substrate roughness

does not influence the resistivity values. This behavior

is expected because for this time period, the topogra-

phy of the PET films is at the micrometric scale, as

shown in the SEM analysis (Fig. 5B). The significant

increase in the contact sheet resistivity values for

etching times longer than 60 min indicates a marked

influence from the surface topography [24].

4. Conclusions

PET films present a low degree of crystallinity and

are thermally stable up to 410 8C. The hydrophilic

character of the polymer surface depends on the acid

etching time, reaching a maximum at approximately

60 min. Contact angle measurements demonstrate that

the surface of PET is hydrophobic. The hydrophilic

character of the surface increases with acid etching

time up to 60 min, decreasing for longer time periods.

Morphological analysis of the PET films shows a

morphology preferentially amorphous, containing

SiO2 particles homogeneously distributed in the film.

Acid etching promotes a drastic increase in the surface

roughness and the removal of the oxide particles from

the polymer matrix.

Polymer films metallized at different pH values

present different characteristics. Thermal analyses

show that the nickel–phosphorous coatings produced

at pH 7.5 present a low degree of crystallinity. These

films are homogeneous and flexible, as was confirmed

by the morphological analysis. In addition, a decrease

in the surface roughness was observed after metalliza-

tion (due to the leveling action promoted by the Ni

coating on the polymer surface). On the other hand,

coatings produced at pH 8.5 are crystalline (and

fragile) and thermal transitions are unaltered by the

presence of the polymer substrate. EDAX analysis

revealed that the coating is constituted by a Ni–P alloy.

The differences in the crystallinity of the coatings

produced at different pH values is related to the

phosphorous content in the alloy. The metallization

performed at pH 7.5 promoted the formation of alloys

with high P content, which were flexible and prefer-

entially amorphous, as confirmed by XRD analysis.

Morphology and flexibility of the nickel coating are

dependent on the P content in the alloy, which can be

controlled by the pH of the electroless deposition bath.

Additionally, the adherence strength between the

coating and the polymer substrate is a function of the

acid etching time. Excellent adherence strength

(800 N cm�2) for the substrate was observed for spe-

cimens etched for 60 min. The lower adherence

observed in specimens etched for different time per-

iods should be related to the hydrophilic character

and to the surface roughness of the substrate. The

optical diffuse reflection was a function of acid etch-

ing time and consequently of the topography of the

polymeric surface (reaching maximal values in the

Fig. 11. Curves of: (A) optical diffuse reflection; (B) contact sheet resistivity as function of the acid etching for PET films metallized with Ni–

P at pH 7.5.

S.C. Domenech et al. / Applied Surface Science 220 (2003) 238–250 249

range 50–60 min of etching). Only the nickel coatings

obtained at pH 7.5 presented higher reflectivity values

than those of the calibrator, due to their amorphous

(and smooth) surface. The contact sheet resistivity

presented two different regimes in relation to the acid

etching time. The transition between the two regimes

occurred around 60 min and was also dependent on the

surface roughness. Up to 60 min of etching, the sub-

strate roughness did not influence the resistivity

values. For longer time periods, contact sheet resis-

tivity values had a significant increase.

The methods of surface treatment and nickel plating

proposed in this work, allow the production of nickel

coatings on poly(ethylene terephthalate) films with

appropriate structures for the fabrication of flexible

plastic circuits. A detailed study on the thermal and

chemical degradation mechanisms of the PET sub-

strates (by XRD, AFM and TGA) and the use of

this technology for sensor applications is still under

study.

Acknowledgements

We thank Terphane Inc. (Brazil) for donating the

poly(ethylene terephthalate) films, CNPq (Brazil) for

a research fellowship and Prof. Nito A. Debacher for

the contact angle measurements.

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