In the production of plastic-lined steel pipes, the majority function of the hot-melt adhesive resin is to bond the non-polar polyethylene layer and the steel strip layer. However, in the production of processing, storage and using, the adhesive resin is often aged by external environmental factors such as light, heat, oxygen, moisture, toxic and harmful gases, and microorganisms. Aging will not only occur the deterioration of its appearance quality, but also cause irreversible changes in physical and chemical properties, which is manifested in the decline of mechanical properties, and eventually failed to use due to decline in performance until impact the structure of plastic-lined steel pipes, which cause invisible threat for safety production.
TThere are many aging causes of the adhesive resin, which can be divided into internal factors and external factors by type.
For the internal composition, the main component of hot melt adhesive resin comes up with high polymer material.The molecular structure of polymers under certain conditions will occur the changes of twisting, breaking, coupling, etc. The stability of the substructure determines how fast the polymer material ages. The molecular structure of some polymer materials contains special structures, such as long chain branches, unsaturated bonds, end groups, etc., which will seriously affect the aging of the material.
When influenced by external factors, these structures will become the starting point of the aging for adhesive resin. In addition, during the production process of the adhesive resin, high temperature, oxygen and other factors will affect the change of the molecular structure of the high molecular polymer. Functional additives contain relatively active functional groups, which are prone to chemical reactions during processing, which may accelerate the aging of the bonding resin, thereby affecting the performance of the bonding resin.
In terms of external factors, environmental factors may also the reason for the hot melt adhesive aging. Simply heating can degrade or cross-link the adhesive resin, while in the atmospheric environment, much more complex thermo-oxidative aging occurs due to the intervention of oxygen and heat.
When the adhesive resin is exposed under the sunlight, its light-absorbing groups stimulated to generate free radicals. If there is still oxygen, it will be very easy to cause photo-oxidative aging. If photo-oxidative aging occurs, the scission or cross-linking of the polymer chain will become very easy.
Oxygen-containing groups are also generated during the aging process, which eventually leads to a serious decline in the toughness and strength of the material. During the wet-heat aging process, water penetrates the resin matrix, which increases the distance between the polymer chains and causes the matrix to swell, resulting in plasticization, which eventually leads to aggravation of the relaxation movement of the polymer chain segments and a decrease in the intermolecular force, resulting in chain scission and degradation.
Different chemical media, such as acid, alkali, salt, etc., can not only corrode the adhesive resin, but also accelerate the aging process of the adhesive resin such as light, oxygen and heat.According to the actual application of plastic-lined steel pipes, the operating environment includes high temperature, high humidity, thermal oxygen, photo-oxygen, etc.; the transmission medium of the pipeline includes water, acid, alkali, salt, hot water, corrosive fluid, etc. Hot melt adhesive resin is the key material in the production of composite pipes, and its aging resistance is directly related to the long-term application stability of plastic-lined steel pipes under different environmental conditions.
Adhesive resin with excellent aging resistance is not only the guarantee of the quality of plastic-lined steel pipe products, but also the key to prolong the service life of plastic-lined steel pipe products. Banzan, we concentrate on high quality adhesive resin
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