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防水型胶粘剂

来源：林中祥胶粘剂技术信息网2011年02月01日

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Waterproofing Adhesives

Despite the kind of "oily" organic compounds that many might consider inherently water-repellent being the primary constituents of adhesives, dealing with moisture remains a major, severe challenge. For example the polymers on which most mainstream construction sealants are based, including polysulphides, polyurethanes, epoxides and acrylics, are sensitive to moisture.

These polymer backbones are all polar, meaning that water can diffuse in and cause them to swell and deform. The absorbed water can also hydrolyse the backbone, breaking bonds and causing substrates to fall apart. However, some adhesive systems are inherently more waterproof.

Bitumen and Silicone

For construction adhesive applications where waterproofing is the major concern engineers deploy highly viscous organic liquids, modified with polymers to improve bond strength. Major applications using this approach include roofing, where the base liquid is bitumen, and bridge building, which largely exploits asphalt. Both are commonly modified with styrene-butadiene-styrene (SBS) rubber to enhance adhesion, and roofing adhesives are also often modified with atactic polypropylene (APP) for similar purposes.

While the thick organic adhesive is highly hydrophobic it is not easy to apply, usually needing high temperatures to be adequately spread. Conventional application approaches, including hot-mopping and flame torches, pose health and safety hazards. Formulators can avoid this by producing a solvent-based cold adhesive, but these typically have a curing time that may last weeks, during which the layer is not waterproof. Another more convenient approach has been developed that adds styrene-isoprene-styrene rubber to the mix, giving it enough tack to be self-adhesive. In this way, peel-and-stick roofing membranes can be produced.

Most people will be more familiar with silicone sealants that are commonly used in bathrooms and kitchens to produce water-tight seals. The inherently hydrophobic nature of silicone makes it uniquely useful amongst construction sealants for waterproofing purposes. The cheapest sealants are produced by reacting hydroxy-terminated linear polysiloxanes with triacetoxysilane crosslinkers, which produce acetic acid in the process of curing, generating a characteristic vinegar smell. Typically a silicone formulation will struggle to match other construction adhesives simultaneously on both bonding performance and cost, but if a readily-applied waterproof bond is needed, they should be a clear first choice.

Figure 1: The acetoxy-crosslinker curing method that produces the characteristic vinegar smell associated with these types of silicones.

Reinforcing Conventional Adhesives Against Water

To reduce the moisture sensitivity problem of other adhesives the structure should be modified to make it more hydrophobic, or lower in surface energy. One way to do this is by using halogenated polymers, which have low surface energies, making them less wettable and more hydrophobic. For example, in epoxy resins using fluorinated monomers or fluorinated curing agents lowers water absorption. Cheaper halogenated epoxides and curing agents, which are less expensive than fluorinated compounds, has been shown to lower water absorption by 50 percent.

Figure 2: Fluorinated monomers for more hydrophobic adhesives

Another approach in increasing moisture resistance is to introduce siloxane linkages into the polymer chain. Several polymers, for example siloxyimides, fluorosilicones, phenylated silicone and silastyrene, are considered to be relatively more hydrophobic than regular hydrocarbon polymers.

If the adhesive does absorb water, physical swelling and contraction can exert stresses and cause bond separation. Excessive and non-uniform swelling of the sealant can also break the substrate. To improve the stress resistance of an adhesive, the material should be toughened with an elastomer to increase its fracture energy. Nitrile rubbers have been the m