New Durable Sealant of Telechelic Polyacrylate
ABSTRACT: In this paper, we discuss the application possibilities of a novel telechelic acrylic polymer (NTP) which we have been developing as a versatile base polymer for sealants and adhesives. Due to its unique features, our NTP is expected to offer a variety of applications, including the one for sealants applied in contact with self-cleaning glass (SCG). Our discussion here focuses on the possibilities of this particular application. SCG possesses a photocatalyst layer which enables two reactions, i. e., lessening the contact angle of the glass surface and decomposing organic compounds on the glass surface, by reacting with sunlight. The reactions help wash off dust from the glass surface, mainly by rainwater, to maintain the clearness and beauty of the glass used for building windows in particular. Conventional sealants for this application often lack sufficient weatherability or risk, or a combination thereof contaminating the glass surface causing hydrophobicity. The NTP sealants, on the other hand, has no surface-polluting substances such as a low-molecular-weight silicone often contained in silicone sealants, while maintaining high weatherability. We have conducted our research to study the weatherability of the NTP sealants on SCG and the possible contamination of the SCG surface by the sealants. Our studies have found that the NTP sealants retain good adhesiveness after more than 10 000 h of exposure to UV irradiation (in a Super Xenon Weather Meter) at the interface between the sealant and the glass. We also have found almost no contamination on the SCG surface by the NTP sealants, demonstrated by the results of our tests conducted to measure the contact angles of sealant samples to the glass, through outdoor exposure for about two months.
KEYWORDS: telechelic polyacrylate, self-cleaning glass, contact angle, silyl-terminated polyacrylate
Telechelic polymers with cross-linkable silane terminal groups are well-known as base polymers for elastic sealants and adhesives. Cross-linkable silane terminal groups such as alkoxy silane groups on the polymers react with moisture in the air, generating a silanol, and silanol groups then react with each other in a condensation reaction to form three-dimensional network structures. This cross linking and the design of polymer backbone form an elastic rubber with high elongation and strength.
Since 1978, Kaneka has launched several types of telechelic polymers. The first polymer was a silyl-terminated polyether which has been used for a wide range of applications in many parts of the world, such as construction sealants and industrial sealants and adhesives, because of the polymer’s good workability, high weatherability, good adhesion to various substrates, high durability, and environmental friendliness [1,2]. Now silyl-terminated polyether-based sealants have a leading share in the Japanese elastic sealant market. As a second backbone type, a silyl-terminated polyisobutylene was launched in 1997. This polymer is in a liquid form at room temperature and is manufactured by means of living cationic polymerization technology. This polymer’s features include such functions as low-gas permeability, excellent weatherability, high durability, and high-heat resistance. The silyl-terminated polyisobutylene is a base
polymer for two-part sealants due to its low moisture permeability, and has been used for high-rise building sealants as its major applications.
Recently buildings and constructions with a large amount of glass have increased. To reduce the cleaning cost and to improve the visibility from the interior during rain, self-cleaning glass (SCG) has been introduced onto the world market. SCG is a titanium dioxide coated glass with photocatalytic and hydrophilic properties for easy cleaning of windows. As displayed in the mechanism shown in Fig. 1, the photocatalyst coated on SCG first decomposes the organic compounds, which promote the collection of dust, and decreases the contact angle of the SCG’s surface at the same time. Running water over the SCG surface then flushes away dust and decomposed organic compounds, making the SCG surface clean. A silicone sealant is generally used as a glass glazing sealant, but is not suitable for self-cleaning glass, because low-molecular-weight silicone components can migrate onto the coating with the result that the glass surface becomes hydrophobic and that the self-cleaning effect is destroyed. Even a photocatalyst cannot decompose silicone contamination, thus the self-cleaning capability is lost.
The weatherability of a silyl-terminated polyether is sufficient as a construction joint sealant, but is not applicable for the window glazing application where the adhesion interface can be attacked by the UV irradiation from sunlight. For the above glazing situation, the silyl-terminated (telechelic) polyacrylate has been developed as the third backbone type. This paper discusses the abilities and application possibilities of our novel telechelic polyacrylate (NTP), as a sealant for SCG in particular, based on a variety of general and durability evaluations.