Aluminum silane treatment before spraying

2021-04-19

[China Aluminum Industry Network] 1. Preface

Aluminum alloy building profiles are the main structural materials of today's doors and windows and curtain walls, and are widely used throughout the world. Aluminum alloy extruded profiles without surface treatment have a single appearance and are prone to corrosion in a humid atmosphere, making it difficult to meet the high decorative and corrosion resistance requirements of building materials. In order to improve the decorative effect, enhance the corrosion resistance and prolong the service life, aluminum profiles are generally subjected to surface treatment. Organic coated aluminum profiles have been used extensively due to their superior surface quality, soft texture, rich colors, beautiful appearance, corrosion resistance, and good weather resistance. They account for about 35% of the surface treatment of aluminum profiles and have been developed in China in recent years. Faster.

In order to ensure and improve the adhesion of the organic polymer sprayed film and the corrosion resistance under the organic polymer film, the powder sprayed layer or the sprayed paint layer usually needs to be performed on the bottom layer of the aluminum alloy chemical conversion film, and generally it will not be bare. Spray directly on the metal surface. Currently widely used in the industry is a chromate-treated yellow chromate film, and also a phosphoric acid-treated green chromate film. It is an imperative goal to avoid the severe environmental pollution of hexavalent chromium. The industrialization of chromium-free chemical conversion coatings is very important and urgent. The current non-chromium chemical conversion treatment methods include molybdate conversion treatment, cobalt-containing complex conversion treatment, titanium/zirconium fluoride conversion treatment, rare earth compound conversion treatment, organosilane conversion treatment, among which the latter three are shown. With the prospect of industrialization. Silane is a kind of potential. The surface treatment process mainly based on organosilane is simple, non-toxic, non-polluting, and widely applicable. After treatment, the metal surface has excellent corrosion resistance and good adhesion. [1]

2 pre-processing commonly used process [2]

Aluminum pre-treatment before spraying, chromate, phosphorous chromate chemical treatment widely used in industry.

Commonly used pre-treatment process flow:

(1) Degreasing—washing—alkaline etching—washing—neutralization—washing—chemical conversion (chromatization)—washing—washing—drying (60°C-85°C).

(2) "3 in 1" cleaning - washing - washing - chemical conversion (chromatization) - washing - washing - drying (60 °C -85 °C).

Chromium is a commonly used chemical conversion pretreatment method for aluminum materials. The chromed film improves the adhesion between the aluminum and the coating and improves the corrosion resistance of the coating. Its thickness reaches 600mg/m2-1500mg/m2.

3. Silane treatment technology

3.1 Silane treatment mechanism

The general structural formula of the silane molecule is YR-SiX3, wherein X is a hydrolyzable group such as a chloro group, a methoxy group, an ethoxy group, an acetoxy group, etc. bonded to a silicon atom; Y is an organic functional group such as an amino group, an epoxy group Etc.; R is a carbon chain with saturated or unsaturated bonds. When distributed at the interface between the metal and the organic coating, it acts as an "emulsifier" between interfaces that have no affinity for each other and are difficult to be compatible, and connects two materials with different properties.

There are many theories of the behavior of silanes on metal surfaces. The theoretical model proposed by Arkies [3] is considered to be a more practical theory. This theory proposes a four-step reaction process: 1 The silane contacts the water in the air to undergo hydrolysis, and Silicon-linked Si-X groups are hydrolyzed to Si-OH; 2Si-OH dehydration synthesizes Si-OH-containing oligosiloxanes; 3 Si-OH in these oligomers forms hydrogen with hydroxyl groups on the surface of the matrix metal Key; 4 Heat drying A dehydration reaction occurs to form a partially covalent bond so that the metal surface is covered with silane.