Page 24 - Handbook of Adhesion Promoters

2.5 Acid-base and electrostatic interactions 17

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be interpolated. The whole procedure for this method of determination is well described
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in literature.
Here are some examples of application of acid-base properties in the development
and understanding of the adhesive properties of materials.
Carbon fibers unsized and sized using polyimide and polyurethane were used in the
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epoxy composites to study their adhesion to the composite matrix. Epoxy matrix is basic
2 23
(basic component of surface energy = 66.8 mJ/m ). The unsized and sized fibers are
acidic (an acidic component of surface energy = 32.4 (unsized), 20.5 (polyimide-sized),
2
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and 15.3 mJ/m (polurethane-sized), respectively). This shows that sizing decreased the
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acidic properties of fibers. Composites containing unsized, polyimide-sized, and poly-
urethane-sized had tensile strengths of 3.2, 2.6, 2.6, MPa and interfacial shear strengths of
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28, 19.4, 13.5 MPa, respectively. This shows that the decrease in acidic component of
surfaces energy (decease of acid-base interaction) caused proportional decrease in
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mechanical performance of composite.
The effect of acid-base properties on performance of adhesive dental polymers was
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studied. Three resins (polymethylmethacrylate, PMMA, polyhydroxyethylmethacrylate,
PHEMA, and bis-GMA/triethyleneglycoldimethacrylate, TGD) used in adhesive interac-
tions with tooth hard tissues were evaluated according to the Fowkes acid-base theory of
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interfacial interactions. Acid-base work of adhesion of these resins was as follows: 5.3
2
(PMMA), 5.6 (TGD), and 15.3 mJ/m (PHEMA). PHEMA was the most basic out of the
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three. The inorganic component of tooth enamel and dentin (hydroxyapatite) is calcium
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phosphate having acidic properties. The strong affinity of PHEMA toward tooth hard tis-
sues is based on the the Lewis acid-base interaction between PHEMA − the electron donor
and hydroxyapatite − the electron acceptor. PHEMA molecules can displace water from
the acidic sites of hydroxyapatite, and polymerized PHEMA can establish strong acid-
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base interactions with the hydroxyapatite surface.
The correlation between the acid-base properties and the tensile strengths of adhe-
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sive joints were evaluated for low-pressure plasma-treated polymer surface. A good cor-
relation between the mechanical strength and the acidity parameters were found for
surface treated polyetheretherketone and polyacetal bound with epoxy and polyurethane
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adhesives.
2.5.2 ELECTROSTATIC INTERACTIONS
Incompatible materials (for example, polymeric and metal surfaces) may form an electri-
cal double layers which are the acting elements of the electrostatic attraction theory
(known also as a parallel plate capacitor theory). The mechanism treats an adhesive-sub-
strate system as a plate capacitor having the strength of interface dependent on the charge
8
density.
Commercially available reverse osmosis membranes are polyamide composite mem-
branes composed of a polyamide rejection layer and polysulfone support membrane. 26
The performance of membranes is affected by bacterial growth onto the membrane surface
to form biofilms containing extracellular polymeric substances such as proteins and poly-
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saccharides − the phenomenon called “biofouling”. The adhered matter can be removed
by the chemical treatment but it requires additional operation and amide bonds in the poly-
amide membranes are cleaved by a chlorine treatment, causing membrane performance to
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deteriorate. In the alternative method, the membrane is immersed into an aqueous solu-