Below, briefly describe the causes and solutions of common problems in PBT modification.
1. Gap sensitivity
The reason:
The benzene ring and the ester group in PBT molecule form a large conjugated system, which reduces the flexibility of the molecular chain and increases the molecular rigidity. Moreover, the presence of polar ester group and carbonyl group increases the intermolecular force and further enhances the molecular rigidity, resulting in poor toughness.
The solution:
a) polymerization modification
Polymerization modification is to introduce new flexible chain segments into PBT molecules by means of copolymerization, grafting, block, crosslinking, etc., so that PBT molecules have good toughness.
b) blending modification
Blending modification is to blend or compound the modifier or high impact strength material with PBT, so that it is distributed in the PBT matrix as a dispersed phase, and use the partial compatibility of the two components or appropriate interface bonding to improve the notch impact performance of PBT. For example, the reactive capacitive agent POE-g-GMA is added to PBT, and the interface force is strengthened through the in-situ capacitive reaction of GMA and the end carboxyl group of PBT, so as to achieve the toughening effect.
![MODIFIED PBT COMMON PROBLEMS AND SOLUTIONS! 1]()
2. PBT thin-wall products need higher mobility
In the field of electronic appliances, automotive electronics industry, thinner components are the trend, which requires higher fluidity of materials to achieve mold filling with the smallest possible filling pressure or closing force of the corresponding pouring instrument. Shorter cycle times are also often achieved using thermoplastic polyester compositions with low viscosity. In addition, good flow ability is also very important for high filling thermoplastic polyester compositions such as glass fiber and/or minerals with a mass fraction of more than 40%.
![MODIFIED PBT COMMON PROBLEMS AND SOLUTIONS! 2]()
The solution:
A: Low molecular weight PBT is selected, but the reduction of molecular mass will affect the mechanical properties.
B: PBT fluidity can be improved with the help of flow accelerators such as stearate or lignite ester, but such low molecular weight esters can seep out during product processing and use.
C: For PBT materials that need to be toughened, the addition of toughening agents will certainly lead to reduced fluidity, so it is necessary to choose a toughening agent that has a smaller impact on the fluidity.
D: The addition of similar low molecular polyester with a specific structure, such as CBT, CBT is a functional resin with a macrocyclic oligopolypet structure, and has good compatibility with PBT, a very small amount of addition, can greatly improve the fluidity of the resin, and almost does not affect the mechanical properties.
E: When nanomaterials are added, the ideally dispersed nanomaterials play a role similar to internal lubrication in PBT, which can improve the fluidity of PBT, but the dispersion of nanomaterials is a major difficulty in the blending modification process.
![MODIFIED PBT COMMON PROBLEMS AND SOLUTIONS! 3]()
3. glass fiber reinforced PBT material is easy to warp
The reason:
Warping is the result of uneven shrinkage of the material. The orientation and crystallization of the components in the material, the improper process conditions used in the injection molding, the incorrect shape and position of the gate during the mold design, and the uneven wall thickness during the product design will cause the warping of the product.
The warping of PBT/GF composites is mainly because the orientation of the glass fiber in the flow direction limits the shrinkage of the resin, and the induced crystallization of PBT around the glass fiber strengthens this effect, making the longitudinal (flow direction) shrinkage of the product less than the horizontal (perpendicular to the flow direction), and this uneven shrinkage leads to the warping of PBT/GF composites.
The solution:
A: Add minerals and use the shape symmetry of mineral fillers to reduce the anisotropy caused by the glass fiber orientation;
B: Add amorphous materials to reduce the crystallinity of PBT and reduce the uneven shrinkage caused by crystallization, such AS adding ASA or AS, but they have poor compatibility with PBT, and appropriate compatibilizers need to be added;
C: Adjust the injection molding process, such as appropriately increasing the mold temperature and appropriately increasing the injection molding cycle.
4. Glass fiber reinforced PBT surface floating fiber problem
The reason:
The causes of floating fiber are more complex, in short, there are mainly the following aspects:
a: PBT and glass fiber compatibility is poor, resulting in the two can not effectively bond together;
b: The viscosity of PBT and glass fiber is very different, resulting in a tendency of separation between the two in the flow process. When the separation effect is greater than the adhesive force, the separation will occur, and the glass fiber will float to the outer layer and leak out;
c: The presence of shear force will not only lead to local viscosity differences, but also destroy the interface layer of the glass fiber surface melt viscosity is smaller, the interface layer is damaged, the glass fiber by the bonding force is also smaller, when the viscosity is small to a certain degree, the glass fiber will get rid of the PBT resin matrix bondage, gradually accumulated to the surface and exposed.
d: Influence of mold temperature. Due to the low temperature of the mold surface, the glass fiber with light weight and fast condensation is frozen instantaneously, and if it is not fully surrounded by the melt in time, it will be exposed and form a "floating fiber".
The solution:
Compatibilizers, dispersants and lubricants are added to improve the floating fiber problem. For example, the use of special surface treatment glass fiber, or the addition of compatibilizers (such as: SOG, a good flow PBT modified compatibilizer), through the "bridge" effect, increase the bond between PBT and glass fiber.
Optimization of molding process to improve the floating fiber problem. Higher injection temperature and mold temperature, larger injection pressure and back pressure, faster injection speed, lower screw speed, can improve the floating fiber problem to a certain extent.
5. Glass fiber reinforced PBT injection molding process produces more mold scale
The reason:
The formation of mold scale is caused by the high content of small molecules or the poor thermal stability of the material. PBT is easy to produce mold scale compared with other materials because of its oligomer and small molecule residue rate is usually 1% ~ 3%. And after the introduction of glass fiber, more obvious. This will lead to the need to clean the mold regularly during continuous processing, resulting in low production efficiency.
The 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 molecule products produced by thermal degradation during processing;
6. PBT has poor thermal hydrolysis
The reason:
The main factor affecting the hydrolysis of PBT is the concentration of terminal carboxyl group. Because PBT contains ester bonds, the ester bonds will break when placed in water at a temperature higher than its glass transition temperature, and the acidic environment formed by hydrolysis will accelerate the hydrolysis reaction, and the performance will decline sharply.
The solution:
Adding a hydrolysis stabilizer, such as carbonized diimide, the hydrolysis stabilizer will consume the carboxyl group generated by hydrolysis, slow down the acid hydrolysis rate of PBT, and improve the hydrolysis resistance of PBT resin.
By blocking the end carboxyl group of PBT, the concentration of end carboxyl group is reduced and the hydrolysis resistance of PBT is improved. For example, an additive with epoxy functional group is added (such as SAG series, a random copolymer of styrene - acrylonitrile -GMA), and the end is sealed by the reaction of functional group GMA with the end carboxyl group of PBT, so as to improve the hydrolysis resistance of PBT.
