Chemical Engineering Science 56 (2001) 4191–4196www.elsevier.com/locate/ces

Improved adhesion of polyethylene by copolymerisation of ethylenewith polar monomers

J. M. Santosa, M. R. Ribeiroa, M. F. Portelaa, J. M. Bordadob; ∗aDepartamento de Engenharia Qu��mica, Grupo de Estudos de Cat�alise Heterog�enea, Instituto Superior T�ecnico,

Av. Rovisco Pais, 1049-001 Lisboa, PortugalbDepartamento de Engenharia Qu��mica, ICTPOL=Instituto Superior T�ecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

Abstract

The inertness of polyethylene restricts its compatibility with other materials. Thus, the adhesion between polyethylene andvarious substrates usually requires surface pretreatments on the polyole/n in order to obtain satisfactory results. One di1erentapproach is the production of ethylene copolymers with small amounts of polar comonomers that can act as adhesion promoters.Using metallocene based catalysts, ethylene copolymers with 10-undecenoic acid (UA) and with 5; 7-dimethylocta-1; 6-diene(5; 7DMO) were synthesised. The adhesion strengths of the copolymers in direct tension were measured. The obtained resultsshow that UA=ethylene and 5; 7DMO=ethylene copolymers exhibit higher adhesion strengths with increasing comonomer contents.UA=ethylene copolymers are more e1ective in improving adhesion properties than 5; 7DMO=ethylene copolymers. This di1erenceis easily understandable on the basis of the polarities of both comonomers. ? 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Films; Polymers; Elasticity; Materials; Adhesion; Polyethylene

1. Introduction

For the last 50 years the consumption of polyole/nshas been growing at around 10% per year. Today theyrepresent about 60% of the thermoplastics market (Al-bizzati & Galimberti, 1998). In the thermoplastics worldmarket, linear high density polyethylene (HDPE) is oneof the most important commodity polyole/ns, with anannual production of about 16 Mt (Murphy, 1999).Most of the polyethylene is produced with supported

transition metal catalysts based on titanium chloride withalkylaluminium chlorides (Ziegler–Natta catalysts). Overthe years, several generations of Ziegler–Natta catalystshave been developed with improvement of the produc-tion processes. However, the lack of reactive groups inpolyethylene still limits the scope of uses and applica-tions, especially when some polymer properties such asdyeability, adhesion and compatibility are required.

∗ Corresponding author. Tel.: +351-1-355-2587; fax: +351-1-355-2588.E-mail address: ach@esoterica.pt (J. M. Bordado).

The adhesion between polyethylene and various sub-strates usually requires surface pretreatments on thepolyole/n in order to achieve usable binding resistance.Many of these treatments are oxidative: chromic acidetc., corona and glow discharge, Game oxidation, im-mersion in an aqueous solution of chlorine and exposureto chlorine gas in the presence of UV light. Other treat-ments include solvent wipe, exposure to solvent vapoursand priming. Generally the chosen treatment depends onthe form of the parts to be bonded and on the requiredadhesion strength (Minford, 1990; Park, 1969; Renfrew& Morgan, 1960). However, this modi/cation route maybe accompanied by undesirable side reactions such asdegradation of the polymer.Another approach to obtain polyethylene with im-

proved adhesion properties is the direct incorporation offunctional groups on the backbone of the polymer. Infact, a small percentage of functionality can dramaticallychange the properties of the polymer. However, the directcopolymerisation of ethylene with a polar comonomer isvery diIcult because of catalyst poisoning by Lewis basecomponents. Alternatives include: (a) copolymerisation

0009-2509/01/$ - see front matter ? 2001 Elsevier Science Ltd. All rights reserved.PII: S 0009-2509(01)00093-8

4192 J. M. Santos et al. / Chemical Engineering Science 56 (2001) 4191–4196

Scheme 1.

with protected functional monomer and subsequentmodi/cation of the polymer or (b) postpolymerisationmodi/cation of pro-functional groups in the polymer.The discovery of metallocene catalysts combined with

methylaluminoxane in 1975 has brought a renewed in-terest on the synthesis of functional ole/ns (Kaminsky,1996). In 1998 the production of polyethylene withmetallocenes was about 820 kt (Murphy, 1999). Thesecatalysts present remarkable catalytical performancesand o1er higher versatility and Gexibility for the syn-thesis and the structure control of polyole/ns, whencompared with conventional Ziegler–Natta systems.In addition, metallocene based catalysts show a bet-

ter tolerance to functional monomers. Still the directcopolymerisation of ethylene with an oxygen contain-ing comonomer is usually accompanied by a great fallon catalytic activity. Some approaches to minimise theproductivity loss are the use of spacer groups in themonomer and use of less reactive polar monomers (Aal-tonen & Lofgren, 1995).Our study focused metallocene catalysed ethylene

copolymerisation with an ole/n containing a car-boxyl group, 10-undecenoic acid (UA), and witha non-conjugated diene, 5; 7-dimethyl-1; 6-octadiene(5; 7-DMO). The catalyst systems used were Cp2ZrCl2and (Me4Cp)SiMe2(N-t-Bu)TiCl2 (CGC) respec-tively, associated with methylaluminoxane (MAO)(Scheme 1). The structure characteristics of the chosendiene help preventing cyclisation and crosslinking duringthe polymerisation, which may arise when common lin-ear dienes such as 1; 5-hexadiene are used (Pietikainen,Seppala, Ahjopalo, & Pietila, 2000).

2. Experimental

2.1. Reagents

Cp2ZrCl2 (Aldrich) and (Me4Cp)SiMe2(N-t-Bu)TiCl2(Dow) were used as such. Methylaluminoxane (Witco)was purchased as a 10%(m=v) toluene solution. Toluene(Petrogal), 10-undecenoic acid 98% (Aldrich), nitrogenand ethylene (Air Liquide) were dried with molecularsieves. 5; 7-dimethyl-1; 6-octadiene (DRT) was puri/edwith appropriate drying agents and distilled. Triethylalu-minium (Aldrich), methanol and HCl (Merck) were usedas received.

2.2. Polymerisations

All polymerisations were carried out in a 1:0 dm3

magnetically stirred BNuchi autoclave. 10-Undecenoicwas precontacted with MAO for 15 min. Toluene and thecomonomer were charged into the reactor and the nitro-gen was substituted for ethylene. The polymerisation wasstarted with the introduction of the MAO preactivatedcatalyst. The consumption of ethylene was followed atconstant pressure with an on-line mass Gow controller.The polymerisation mixture was quenched with a diluteHCl solution in methanol. The polymer was washedtwice with methanol and dried under vacuum overnight.

2.3. Polymer characterisation

1H-NMR spectra were performed in 1; 1; 2; 2-tetrachloroethane at 380 K with a Bruker AC200 spec-trometer. Di1erential scanning calorimetry (DSC)measurements were obtained with a TA InstrumentsDSC2920 with MDSCJ option, recording the secondheating scan at a rate of 10◦C=min.

2.4. Adhesion testing

Films of ethylene homo- and copolymers with thick-nesses of 40–60 �mwere prepared bymelt pressing poly-mer samples. The /lms were then bonded to aluminiumdollies and to glass, PVC or stainless steel subtracts withan epoxy adhesive. Adhesion strengths in direct tensionwere measured with an Elcometer 106 Mechanical Ad-hesion Tester.

2.5. Hardness testing

Hardness tests were performed on 2 mm thick meltpressed pellets of the polymers. An Equotip HardnessTester based on resilience evaluation was used.

J. M. Santos et al. / Chemical Engineering Science 56 (2001) 4191–4196 4193

Fig. 1. 1H-NMR spectrum of an ethylene=UA copolymer.

Fig. 2. DSC thermograms for HDPE (a) and ethylene UA copolymerswith di1erent contents of UA: (b) 0.3, (c) 1.0, (d) 1:7 mol%.

3. Results

Homo- and copolymers of ethylene with severalconcentrations of UA in feed were prepared with thecatalyst system Cp2ZrCl2=MAO. The inGuence of thepolar monomer and its protection on polymerisationactivity was studied previously (Santos et al., 2001).The UA contents in the polymer were determined by1H-NMR spectroscopy (Fig. 1). The DSC thermogramsobtained for HDPE and for the copolymers show amoderate decrease in the melting temperature and inthe heat of fusion (associated to the polymer crys-tallinity) as the UA content in the polymer is increased(Fig. 2).In spite of the small incorporation values, the adhe-

sion of the polyethylene is greatly improved, as seen inTable 1. For UA incorporations as low as 1:7 mol%; a/ve-fold increase in the adhesion strength is observed.The ruptures were found to be between the polyethy-

lene /lm and aluminium for the lowest UA incorpora-tions. However, for higher UA contents, ruptures were ofthe cohesive type. In addition, when /lms were bondedto glass or PVC, the accurate measurement of the adhe-sion strength could not be done for values higher than3:5 MPa, since the materials would break or deteriorate.Also, the homopolyethylene produced with Cp2ZrCl2

shows better bonding properties than a commercialHDPE, which is probably due to impurities in the lattercase.Hardness tests were also performed with rebound re-

silience type equipment (Table 1). These values are notabsolute measurements of the hardness of a polymerpellet, because a correspondence between resilience andhardness is only true for metals. Nevertheless they givestraightforward relative values. The results show that,up to UA contents of about 2%, the hardness levels aremaintained. The loss in crystallinity and the decrease inthe melting temperature are also not paramount. Thus,we have greatly improved the adhesion of high densitypolyethylene while maintaining the same level of theremaining properties of the polymer.The catalyst system (Me4Cp)SiMe2(N-t-Bu)TiCl2=

MAO was chosen for the homo- and copolymerisationof ethylene with 5; 7-DMO (Santos, Ribeiro, Portela,Cramail, & DeIeux, 2001). Again, 1H-NMR spec-troscopy was used for the quanti/cation of the polarmonomer content in the polymer (Fig. 3). The presenceof 5; 7-DMO in the polymer causes a strong decrease inthe melting temperatures and in the heat of fusion, asshown by DSC analysis (Fig. 4).Since 5; 7-DMO is not so detrimental to the catalyst,

the incorporations obtained are higher than for UA.Thus, the ethylene=5; 7-DMO copolymers show thusa stronger fall in hardness and, particularly in crys-tallinity (Table 2 and Fig. 4). However, the adhesion

4194 J. M. Santos et al. / Chemical Engineering Science 56 (2001) 4191–4196

Table 1Ethylene homo- and copolymerisations with UA: adhesion and hardness results

Comonomer OHm Tensile adhesion strengths Hardness value Lcontent (J=g) (MPa)(mol%)

Al=glass Al=PVC Al=steel

Commercial HDPE 0 n.d. 0.8a 0:7a 0:7a n.d.

HDPEc 0 161 1:5a 1:0a 1:0a 655

0.3 140 1.3a 2.0a 1.3a n.d.Ethylene=UA 1.0 151 ¿3:5b 3.0a 2.2a n.d.Copolymersc 1.7 138 ¿3:5b ¿3:5b 5b 645

aRupture between polyethylene and aluminium.bCohesive rupture.cPolymers prepared with the catalyst system Cp2ZrCl2=MAO at 25◦C.

Fig. 3. 1H-NMR spectrum of an ethylene=5; 7-DMO copolymer.

Fig. 4. DSC thermograms for HDPE (a) and ethylene=5; 7-DMOcopolymers prepared at 25◦C with di1erent contents of 5; 7-DMO:(b) 1.4, (c) 4.0, (d) 5:8 mol%:

improvement is not so large as for UA copolymers.Again, for higher incorporations the ruptures becamecohesive.

Copolymers prepared at a higher temperature were alsotested, since they show much higher activities with sim-ilar 5; 7DMO contents. A greater fall of hardness withcomonomer content than at room temperature is observed.The loss in crystallinity and the improvement in adhe-sion are comparable, despite the apparently more homo-geneous distribution of comonomer units in the copoly-mers prepared at 60◦C (Table 3 and Fig. 5).

4. Conclusions

For ethylene=UA copolymers a great improvementon adhesion, even for low incorporation values of thecomonomer is obtained, while hardness and crystallinitylevels are maintained.Ethylene=5; 7-DMO copolymers show a good improve-

ment on adhesion, only for reasonable incorporation val-ues. There is a greater hardness and crystallinity fall thanfor ethylene=UA copolymers (elastomeric behaviour). No

J. M. Santos et al. / Chemical Engineering Science 56 (2001) 4191–4196 4195

Table 2Ethylene homo- and copolymerisations with 5; 7-DMO at 25◦C: adhesion and hardness results

Comonomer OHm Tensile adhesion strengths Hardnesscontent (J=g) (MPa) value L(mol%)

Al=glass Al=PVC Al=steel

Commercial HDPE 0 n.d. 0.8a 0:7a 0:7a n.d.

HDPEc 0 201 0:8a 0:5a 0:9a 680

1.4 114 1.5a 1.0a 1.3a n.d.Ethylene=5; 7-DMO 4.0 59 1.5a 1.2a 1.2a 630Copolymersc 5.8 32 ¿2:5b ¿2:4b ¿2:5b n.d

aRupture between polyethylene and aluminium.bCohesive rupture.cPolymers prepared with the catalyst system (Me4Cp)SiMe2(N-t-Bu)TiCl2=MAO at 25◦C.

Table 3Ethylene homo- and copolymerisations with 5; 7-DMO at 60◦C: adhesion and hardness results

Comonomer OHm Tensile adhesion strengths Hardnesscontent (J=g) (MPa) value L(mol%)

Al=glass Al=PVC Al=steel

Commercial HDPE 0 n.d. 0.8a 0:7a 0:7a n.d.

HDPEc 0 207 n.d n.d n.d 705

Ethylene=5; 7-DMO 2.9 91 0.8a 0.6a 0.7a 590Copolymersc 4.4 63 1:4b 1:4a 1:0a 552

aRupture between polyethylene and aluminium.bCohesive rupture.cPolymers prepared with the catalyst system (Me4Cp)SiMe2(N-t-Bu)TiCl2=MAO at 60◦C.

Fig. 5. DSC thermograms for HDPE (a) and ethylene=5; 7-DMOcopolymers prepared at 60◦C with di1erent contents of 5; 7-DMO:(b) 2.9, (c) 4:4 mol%.

signi/cant di1erences between copolymers prepared atroom temperature or at 60◦C were found.The di1erences between the e1ects of both polar

monomers, particularly with respect to adhesion im-provement of polyethylene are easily understood, con-sidering the chemical nature of the polar groups.

Acknowledgements

This work was supported by FundaPcao para a Cienciae a Tecnologia—PRAXIS XXI (Projecto PRAXIS=PCEX=C=QUI=75=96). JMS acknowledges the Sub-Programa Ciencia e Tecnologia do 2◦ Quadro Comu-nitVario de Apoio for a doctoral fellowship.

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