What are the defects of FRP hand paste process and the solutions?

Glass fiber reinforced plastic production, China only began in 1958, the main molding process are hand paste method, according to incomplete statistics more than 70% of glass fiber reinforced plastic is hand paste molding; With the development of the domestic FRP industry, the introduction of foreign advanced technology and equipment, such as large automatic winding machine, continuous waveform plate production units, pultrusion molding units, etc., has greatly shortened the gap with foreign countries. Even if the large equipment has the advantages of high production efficiency, high quality and low cost; However, hand paste FRP in the construction site, special occasions, low investment, simple and convenient, small customization is still large equipment can not be replaced. In 2021, China's glass fiber reinforced plastic production of 5 million tons, a large part of which is hand paste glass fiber reinforced plastic products. In the construction of anti-corrosion engineering, most of the on-site FRP production is also made by hand paste process methods, such as sewage tank FRP lining, acid-alkali storage tank FRP lining, acid-resistant FRP floor production, buried pipeline FRP external corrosion prevention and so on. Therefore, the resin FRP produced by the on-site anti-corrosion engineering is a hand paste process. Glass fiber reinforced plastic composites account for more than 90% of the total number of composite materials, is the largest use of composite materials today, which is mainly made of glass fiber reinforced materials, synthetic resin adhesives and auxiliary materials through a specific molding process, hand paste glass fiber reinforced plastic process is one of them. Hand paste FRP has more quality defects than mechanical molding, which is the main reason why modern FRP production prefers mechanical equipment. Hand paste FRP mainly depends on the experience and operation level and maturity of construction personnel to control the quality, so for the hand paste FRP construction personnel skill training and experience summary, as well as the use of failure cases for education, to avoid repeated quality defects of hand paste FRP, resulting in economic losses and social impact; Hand paste FRP defects and treatment solutions should become the anti-corrosion construction of FRP construction personnel in the technology, the application of these technologies to ensure the anti-corrosion service life and life and excellent corrosion resistance effect has a positive significance.

There are many large and small quality defects of hand paste FRP, summed up the important and directly cause damage or failure of FRP there are some of the following defects, for these defects in addition to avoid in the construction operation, but also through the maintenance treatment of follow-up remedial measures to meet the same quality requirements as the whole FRP. If the defects that cannot meet the requirements of use can not be repaired and treated, they can only be reworked and rebuilt. Therefore, the hand paste FRP as far as possible to eliminate defects in the construction process is the most economic solution and way.

First, glass fiber cloth "exposed"

The glass fiber cloth should be fully saturated with resin adhesive material, which means that some of the cloth has no adhesive material or little adhesive material. The main reason is that the glass cloth is contaminated or contains wax, and dewaxing is not complete; The viscosity of the resin glue is too large, the glue is difficult to apply or the resin glue is overhead on the glass cloth holes; The mixing and dispersing of the resin binder is not good, the filler is not good or the filler particles are too thick; The coating resin adhesive material is not uniform, there is leakage coating or too little coating resin adhesive material. The solution is to use wax-free glass cloth or fully dewaxed cloth before construction to keep the cloth clean and not contaminated; The viscosity of the resin binder configuration should be appropriate, and the viscosity of the resin binder should be adjusted in time for the construction under high temperature environment. When mixing and dispersing resin, mechanical mixing must be used to disperse evenly, without clumping or caking phenomenon; The fineness of the selected filler must be greater than 120 mesh and fully dispersed evenly in the resin compound.

Second, the glue content of FRP is too low or too high

In the process of FRP production, the glue content is too low, which is easy to make the glass cloth produce white spots, lamination, shell and other defects, the interlayer strength is greatly reduced, and the mechanical properties of FRP are decreased. If the amount of glue is too high, there will be a "flow hanging" flow defect. The main reason is the leakage of coating, less coating caused by "low glue", when the amount of glue is too thick, resulting in "high glue"; The viscosity of the resin binder is improper, the viscosity is large, the glue content is high, the viscosity is small, and the glue content is too low after the diluent is cured. The solution is to effectively control the viscosity, adjust the viscosity of the resin adhesive at any time, take multiple gluing method to ensure the content of the resin adhesive when the viscosity is small, and use diluent to dilute it properly when the viscosity is large or in high temperature environment; Pay attention to the uniformity of the coating during the gluing operation, do not overcoat the amount of resin glue or too little, too thin and too thick.

Third, the surface of FRP is sticky

In the process of FRP construction, the surface of the product is prone to sticky phenomenon after contact with the air, and it lasts a long time. The main reason for this sticky defect is that the humidity in the air is too high, especially for the curing of epoxy resin and polyester resin has the effect of delaying and blocking polymerization, and may also cause the surface of FRP sticky or long-term curing incomplete defects; The ratio of curing agent or initiator is not accurate, the dosage does not meet the specified requirements or the failure causes the surface sticky phenomenon; Oxygen in the air can inhibit the curing of polyester resin or vinyl resin, and the use of benzoyl peroxide is more obvious. The crosslinking agent volatilizes too much in the surface resin of the product, such as the styrene volatilizes too much in the polyester resin and vinyl resin, resulting in imbalance and not curing. Solution, the relative humidity of the construction environment must be lower than 80%, and about 0.02% paraffin wax or 5% isocyanate can be added to the polyester resin or vinyl resin; The surface is covered with plastic film, so that the surface is isolated from the air; The resin can not be heated before gelling, avoid high temperature, maintain a good ventilation environment, and reduce the volatilization of active ingredients.

Fourth, there are more bubbles in FRP products

Glass fiber reinforced plastic products produce many bubbles, the main reasons are that the use of resin glue is too much or the resin glue contains more bubbles; The viscosity of the resin compound is too large, and the air brought into the configuration mixing process is not driven out, and it remains in the resin compound; Improper selection or contamination of glass cloth; Improper construction operation, leaving bubbles; The surface of the base is uneven, there is no leveling or the arc of the equipment bend is large. For the solution of more bubbles in FRP products, control the glue content of resin binder and mixing method; Add diluent appropriately or improve the ambient temperature to reduce the viscosity of the resin compound; Choose non-twist glass cloth that is easy to be soaked by resin adhesive material, clean and dry; Keep the base level, potholes and uneven places filled with putty; According to the different types of resin adhesive and reinforcement material selection of dipping, brushing, rolling process.

Five, plastic flow defects

The main reason for the glue flow of FRP products is that the viscosity of resin glue is too small; Uneven ingredients, resulting in inconsistent gel and curing time; The curing agent corresponding to the resin binder is not enough. The solution is to add active silica powder appropriately, add 2%-3%, the resin compound must be fully stirred when preparing, and the amount of curing agent should be adjusted appropriately.

Six, FRP lamination defects

There are many reasons for the lamination defect of FRP, in summary, there are several main points, the glass cloth has wax or dewaxing is incomplete, the glass cloth is polluted or damp; The viscosity of resin glue is too large, and it does not penetrate the cloth eyes; During construction, the glass cloth is too loose, not tight, and there are too many bubbles; When the resin compound is prepared, the formula is not suitable, resulting in poor bonding performance, and it is easy to cause the curing speed to be too slow or too fast during field construction; Improper curing temperature, premature heating or excessive heating temperature will affect the interlayer bonding property. Solution, use wax-free glass fiber cloth; Keep enough resin adhesive material, brush vigorously; Compacting the glass cloth, driving away the bubbles, and adjusting the formulation of the resin compound; The resin compound shall not be heated before gelling, and the temperature control of FRP requiring post-curing treatment needs to be determined by testing.

Seven, poor solidification of FRP, incomplete defects

Fiberglass often has poor curing, incomplete phenomenon, such as the surface of fiberglass products is soft, sticky, low strength, the main reason for these defects is not enough curing agent use or failure; During construction, the ambient temperature is too low or the air humidity is too high, and the water absorption is serious. The solution is to use qualified and effective curing agent, adjust the amount of curing agent, when the temperature is too low, take the heating method to increase the ambient temperature, when the humidity is higher than 80%, strictly prohibit FRP construction; It is recommended that there is no need to repair the quality defects of poor curing and long-term non-curing, and only rework and re-paste by hand. In addition to the above typical cases, there are many, whether it is a large defect or a small defect, will affect the quality and service life of FRP products, especially in the anti-corrosion engineering will affect the anti-corrosion resistance years. From a safety point of view, heavy anticorrosive FRP defects will directly lead to major accidents, such as the leakage of acid, alkali or other strong corrosive media. Fiberglass steel is a special composite material composed of various materials. The molding of this composite material is restricted by various factors in the construction process. Therefore, the hand paste FRP molding process looks simple and convenient, without many equipment and tools; However, the molding process is strict, the need for skilled operation technology, but also need to understand the causes of the formation of defects and solutions, in the actual construction to avoid the formation of defects, in fact, the hand paste glass fiber reinforced plastic is not the traditional "hand work" people imagine, but a not simple construction process method with higher operating technology.

The author expects the domestic hand paste FRP practitioners in the spirit of "craftsman spirit" will be each construction looks to do a beautiful "handicraft"; Then the defects of FRP products will be greatly reduced, so as to achieve the goal of "zero defects" of hand paste FRP, and create a piece more than a beautiful and flawless FRP "crafts".