Improvement of thermal stability of UV curable pressure sensitive adhesive by surface modified silica nanoparticles

https://doi.org/10.1016/j.mseb.2013.08.005Get rights and content

Highlights

  • ā€¢

    Silica nanoparticles were modified to carry the vinyl groups for photo-crosslinking.

  • ā€¢

    Acrylic copolymer was modified to have the vinyl groups for photo-crosslinking.

  • ā€¢

    Strong and extensive interfacial bondings were formed between polymer and silica.

  • ā€¢

    Thermal stability of PSA was improved by forming nanocomposite with modified silica.

Abstract

Pressure sensitive adhesives (PSAs) with higher thermal stability were successfully prepared by forming composite with the silica nanoparticles modified via reaction with 3-methacryloxypropyltrimethoxysilane. The acrylic copolymer was synthesized as a base resin for PSAs by solution polymerization of 2-EHA, EA, and AA with AIBN as an initiator. The acrylic copolymer was further modified with GMA to have the vinyl groups available for UV curing. The peel strength decreased with the increase of gel content which was dependent on both silica content and UV dose. Thermal stability of the composite PSAs was improved noticeably with increasing silica content and UV dose mainly due to the strong and extensive interfacial bonding between the organic polymer matrix and silica.

Introduction

Pressure sensitive adhesives (PSAs) are semi-solid phase materials with viscoelastic properties and adhesion strength on solid substrates after applying a light contact pressure in a short contact time. PSAs are generally used in a wide range of fields including medical products, electronic devices, and the construction and automobile industries [1]. The use of acrylic polymers in PSAs is growing because of their resistance to oxidation and transparency under exposure to sunlight [2], [3]. The PSA films for silicon wafer require both higher reliability and thermal stability to adapt the rapid development of integrated semiconductor technology.

The linear acrylic chains in PSAs are not generally crosslinked with chemical bonds but connected physically by the hydrogen bonding interactions with the carboxylic groups. The reversible hydrogen bonding of acrylic PSAs results in the relatively poor thermal stability [2], [4]. Therefore, the mobility of molecular chains in PSA has to be restricted intentionally in order to improve the thermal stability. One of the effective methods to restrict the molecular mobility is to crosslink the acrylic chains in PSAs.

Compared to the traditional thermal curing method, UV curing method has various advantages such as faster curing speed, lower power consumption, and lower curing temperature, resulting in the proper application to thermally sensitive substrate. The UV curable materials are widely used in the fields such as coatings, adhesives, paper, microelectronics, three-dimensional precision molding processing, and laser recording materials [5], [6], [7]. Acrylic copolymer, epoxy resin, and polyurethane are usually employed as important main components in the UV curing system. In addition, the properties of the UV-cured acrylic copolymer could be further improved in many aspects [8].

The combination of both organic polymers and inorganic fillers into nanocomposites has attracted considerable attention in recent years, as these materials offer the prospect of new synergetic properties that originate from their organic and inorganic components [9], [10], [11], [12]. The silica/polymer nanocomposites have attracted considerable interest among various organic/inorganic nanocomposites because of the potential uses as aerospace materials, structural materials in electronics, sensors, and materials in other various industries [13], [14], [15], [16], [17], [18], [19], [20]. The shape-memory polyurethane-silica nanocomposites prepared with fumed silica and hydrolyzed 3-amino-propyltriethoxysilane showed the increase in both modulus and tensile strength although some aggregation of fumed silica particles in the polyurethane matrix was found [21], [22].

Many preparation methods for nanoparticles/polymer composites via UV curing have been studied recently. The in situ synthesis of noble metal nanoparticles in a polymer matrix has been performed by the simultaneous photopolymerization and photoinduced electron transfer triggered by UV irradiation resulting in the unique properties and structural characteristics [23], [24], [25], [26]. Another approach to prepare the nanoparticle/polymer composite involves the uniform dispersion of inorganic nanoparticles in the liquid monomer formulation followed by photopolymerization in order to trap the nanoparticles in the photocrosslinked network and thus to avoid their macroscopic agglomeration. A variety of hybrid polymer composites have been thus prepared by photoinduced polymerization of multifunctional acrylic monomers in the presence of various nanoparticles with spherical, rod or plate-like shapes for the highly potential applications in advanced technologies [27], [28], [29]. Photopolymerizable silane coupling agent Rā€²Si(OR)3, where Rā€² included an unsaturated carbon double bond susceptible to UV polymerization, was employed as precursors for the solā€“gel reactions to prepare hybrid polymer composites. The polysiloxane network was formed inside the polymer matrix after a preliminary solā€“gel stage implying a series of hydrolysis and condensation reactions. The crosslinked organic/inorganic hybrid composites were finally formed by UV irradiation [30], [31], [32], [33].

Despite of the many researches performed on the preparation of silica/polymer nanocomposites, acrylic copolymer/silica nanocomposites have not been studied in depth yet for the PSA applications in terms of thermal stability and adhesive property improvement to our best knowledge. The improvement in thermal stability of temporary bonding PSAs is needed strongly in the thin wafer handling and fabrication processes especially for the multichip packaging. In this study, silica nanoparticles were surface modified to have unsaturated carbon double bonds for the possible UV curing with acrylic copolymers as presented schematically in Fig. 1. The improvement in properties of acrylic copolymer/silica composite PSAs was evaluated depending on the dispersion of silica, silica content, and UV dose.

Section snippets

Materials

Ethyl acrylate (EA, Samchun Pure Chemical, Korea), 2-ethylhexyl acrylate (2-EHA, Samchun Pure Chemical, Korea), and acrylic acid (AA, Samchun Pure Chemical, Korea) were used as monomers for radical polymerization to synthesize the acrylic copolymer. N,Nā€²-azobisisobutyronitrile (AIBN, Daejung Chemicals & Metals, Korea) and ethyl acetate (Junsei Chemicals, Japan) were used as a radical initiator and a solvent, respectively. Glycidyl methacrylate (GMA, Mitsubishi Rayon, Japan) bearing a reactive

Surface modification of silica nanoparticles

The anchoring of MPS on the surface of the silica nanoparticles was carried out via condensation reaction between silanol groups present on the silica surface and silanol groups formed by the hydrolysis of alkoxysilanes in MPS. Therefore, the progress of condensation reaction could be easily identified by the simultaneous disappearance of characteristic bands assigned to the methoxysilane and silanol groups [34]. The FTIR spectra are useful to a better understanding of the formation of anchored

Conclusions

The silica nanoparticles were modified with MPS to carry the UV curable vinyl groups and the acrylic copolymer was also modified with GMA to have the UV curable vinyl groups. The modified silica nanoparticles were dispersed more uniformly than unmodified silica in the acrylic copolymer matrix to form the UV curable acrylic copolymer/silica composite PSAs. The gel content of UV-cured acrylic copolymer/silica composite PSAs increased with increasing UV dose and silica content due to the extensive

Acknowledgment

This study was supported from a grants for ā€œDevelopment and Performance Control of Bonding and Debonding PSAs for MCP Semiconductorā€ by the Ministry of Knowledge Economy and Korea Research Council for Industrial Science & Technology.

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