Polybutylene terephthalate ( PBT) , which has good comprehensive properties, such as high crystallinity, fast forming speed, weather resistance, low friction coefficient, high heat distortion temperature, excellent mechanical properties, fatigue resistance and Ultrasonic welding, etc. However, the notched impact strength is low, the molding shrinkage rate is large, the hydrolysis resistance is poor, and it is easily eroded by halogenated hydrocarbons. After being reinforced by glass fiber, the longitudinal and transverse shrinkage of the product is inconsistent, and the product is warped. The following Jiuzhi Plastics Network briefly introduces the common problems and solutions in PBT modification:
First, the gap sensitivity
the reason:
The benzene ring and the ester group in the PBT molecule form a large conjugated system, which reduces the flexibility of the molecular chain and increases the rigidity of the molecule, and the presence of the polar ester group and the carbonyl group increases the intermolecular force and the molecular rigidity. Further enhancement, resulting in poor toughness.
Solution:
a, polymerization modification
Polymerization modification is to introduce a new soft segment into the PBT molecule during the polymerization process by copolymerization, grafting, block, cross-linking, etc., so that it has good toughness.
b, blending modification
Blending modification is to blend or compound the modifier or high impact strength material with PBT, and distribute it as a dispersed phase in the PBT matrix, and improve PBT by partial compatibility of two components or proper interfacial bonding. Notched impact performance. For example, in the PBT, the reactive compatibilizer POE-g-GMA is added, and the in-situ compatibilization reaction of the terminal carboxyl groups of GMA and PBT is used to strengthen the interfacial force to achieve the toughening effect.
Second, PBT thin-walled products require higher liquidity
Thin-walled electronic components
In the field of electronic appliances and automotive electronics, the thinner component is the trend, which requires the material to require higher fluidity, in order to achieve the filling of the mold with the filling pressure or clamping force of the corresponding casting device as small as possible. Shorter cycle times are often achieved with low viscosity thermoplastic polyester compositions. In addition, good flowability is also very important for high filled thermoplastic polyester compositions such as glass fibers and/or minerals having a mass fraction of more than 40%.
Solution:
Low molecular weight PBT is chosen, but reduced molecular mass can affect mechanical properties.
PBT fluidity can be improved by means of flow promoters such as stearate or montanate, but such low molecular weight esters can bleed during processing and use of the product.
For PBT materials that require toughening, the addition of toughening agents must result in a decrease in fluidity, so it is necessary to choose a toughening agent that has less influence on fluidity.
Adding a similar low molecular weight polyester with a specific structure, such as CBT, CBT is a functional resin with a macrocyclic oligoester structure, which has good compatibility with PBT, and can be greatly increased with a small amount of addition. Improve the fluidity of the resin with little effect on mechanical properties. Adding nano-materials, the ideally dispersed nano-materials play a role similar to internal lubrication in PBT, which can improve the fluidity of PBT, but the dispersion of nano-fillers is a major difficulty in the process of blending modification.
Third, glass fiber reinforced PBT material is easy to warp
the reason:
Warpage is the result of uneven shrinkage of the material. The orientation and crystallization of the components in the material, improper process conditions during injection molding, incorrect gate shape and position during mold design, and uneven wall thickness during product design can cause warpage of the product.
The warpage of the PBT/GF composite is mainly because the orientation of the glass fiber in the flow direction limits the shrinkage of the resin, and the induced crystallization of the PBT around the glass fiber reinforces this effect, so that the longitudinal (flow direction) shrinkage of the article is less than the transverse direction. (The direction perpendicular to the direction of flow), this uneven shrinkage causes warpage of the PBT/GF composite.
Solution:
Adding minerals and utilizing the shape symmetry of the mineral filler to reduce the anisotropy caused by the orientation of the glass fibers;
Adding amorphous materials, reducing the crystallinity of PBT, reducing uneven shrinkage due to crystallization, such as adding ASA or AS, but they are poorly compatible with PBT, and it is necessary to add a suitable compatibilizer;
Adjust the injection molding process, such as properly increasing the mold temperature, and appropriately increase the injection cycle.
Fourth, glass fiber reinforced PBT surface floating fiber problem
the reason:
The reasons for the production of floating fiber are more complicated. In short, there are mainly the following aspects:
PBT has poor compatibility with glass fiber, resulting in the inability to bond effectively together;
The viscosity difference between PBT and glass fiber is very large, which leads to the separation of the two in the flow process. When the separation is greater than the adhesion, the detachment occurs, and the glass fiber floats to the outer layer and leaks out;
The existence of shearing force will cause the local viscosity to be different, and the interface layer of the glass fiber surface will be weaker, the interface layer will be damaged, and the adhesion of the glass fiber will be smaller. When the viscosity is small to a certain extent, the viscosity will be small. At the same time, the glass fiber will get rid of the bond of the PBT resin matrix and gradually accumulate to the surface and be exposed.
Mold temperature effects. Due to the low temperature of the mold profile, the glass fiber with light weight and fast condensation is instantaneously frozen. If it is not fully surrounded by the melt in time, it will be exposed to form a "floating fiber".
Solution:
Compatibilizers, dispersants and lubricants are added to improve the problem of floating fibers. If special surface treated glass fiber is used, or a compatibilizer (such as SOG, a good flow PBT modified compatibilizer) is added, the adhesion of PBT to glass fiber is increased by the function of “bridgeâ€.
Optimize the molding process to improve the floating fiber problem. Higher injection temperature and mold temperature, larger injection pressure and back pressure, faster injection speed and lower screw speed can all improve the floating fiber problem to some extent.
Five, glass fiber reinforced PBT injection process produces more mold scale
PBT+GF surface floating fiber, mold scale phenomenon
the reason:
Mold scale production is caused by excessively small molecular content of the material or poor thermal stability of the material. PBT has a residual rate of 1% to 3% due to its oligomer and small molecule, and is relatively easy to produce mold scale with other materials. After the introduction of glass fiber, it is more obvious. This will result in the need to periodically clean the mold during continuous processing, resulting in inefficient production.
Solution:
Reduce the amount of small molecule additives (such as lubricants, coupling agents, etc.), try to choose polymer additives;
Improve the thermal stability of PBT and reduce the small molecular products produced by thermal degradation during processing;
6. PBT has poor heat hydrolysis resistance.
the reason:
The main factor affecting the hydrolysis of PBT is the terminal carboxyl group concentration. Since PBT contains an ester bond, ester bond cleavage occurs in water at a temperature higher than its glass transition temperature, and the acidic environment formed by hydrolysis accelerates the hydrolysis and the performance is drastically lowered.
Solution:
A hydrolysis stabilizer such as carbodiimide is added, and the hydrolysis stabilizer consumes the carboxyl group produced by hydrolysis, slows the acid hydrolysis rate of PBT, and improves the hydrolysis resistance of the PBT resin.
By blocking the carboxyl group of the PBT terminal, the concentration of the terminal carboxyl group is lowered, and the hydrolysis resistance of the PBT is improved, such as adding an auxiliary agent having an epoxy functional group (such as a SAG series, a random copolymer of styrene-acrylonitrile-GMA), The hydrolysis resistance of PBT is improved by blocking the functional group GMA by reacting with the carboxyl group of the PBT terminal.
Formic Acid has strong reducing ability and is a by-product of coking. Adding Formic ACID is more effective than adding inorganic acids such as H2SO4 and HCL, because inorganic acids have only acidification effect, and formic acid can not only reduce the pH value of silage, but also inhibit Plant respiration and poor microbial fermentation.
Formic Acid
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