Common problems and solutions of modified PBT, practical!

Time:2022-09-29 08:36:36 / Popularity: / Source:

Polybutylene terephthalate (PBT) has excellent comprehensive properties, such as high crystallinity, rapid prototyping, weather resistance, low friction coefficient, high heat distortion temperature, good electrical properties, excellent mechanical properties, fatigue resistance, and ultrasonic welding. However, its notched impact strength is low, molding shrinkage rate is large, hydrolysis resistance is poor, and it is easily attacked by halogenated hydrocarbons. After glass fiber is reinforced, product will be warped due to inconsistency of longitudinal and transverse shrinkage of product.
Below, briefly explain causes and solutions of common problems in PBT modification.

1. Gap sensitivity

Benzene ring and ester group in PBT molecule form a large conjugated system, which reduces flexibility of molecular chain and increases rigidity of molecule. Presence of polar ester groups and carbonyl groups increases intermolecular forces and makes molecules rigid. It is further strengthened, resulting in poor toughness.
a) Polymerization modification
Polymerization modification is to introduce new flexible segments into PBT molecule during polymerization process by means of copolymerization, grafting, block, crosslinking, etc., to make it have good toughness.
b) Blending modification
Blending modification is to blend or compound modifiers or high-impact strength materials with PBT so that they are distributed in PBT matrix as a dispersed phase, and partial compatibility of the two components or appropriate interfacial bonding is used to improve notched impact performance of PBT. For example, reactive compatibilizer POE-g-GMA is added to PBT, through in-situ compatibilization reaction of GMA and terminal carboxyl group of PBT, interface force is strengthened to achieve toughening effect.

2. PBT thin-walled products need higher fluidity

Polymerization modification 
Thin-walled electronic components
In the fields of electronics and automotive electronics industries, thinner components are a trend, which requires materials to have higher fluidity in order to achieve filling of molds with smallest possible filling pressure or clamping force of corresponding casting equipment. Use of low-viscosity thermoplastic polyester compositions can often achieve shorter cycle times. In addition, good flowability is also very important for highly filled thermoplastic polyester compositions with glass fibers and/or minerals with a mass fraction of more than 40%.
Choose low molecular weight PBT, but decrease in molecular weight will affect mechanical properties.
With the help of flow promoters such as stearate or montanate, fluidity of PBT can be improved, but this type of low molecular weight ester will ooze out during product processing and use.
For PBT materials that need to be toughened, addition of a toughening agent will definitely cause a decrease in fluidity, so it is necessary to choose a toughening agent that has a smaller impact on fluidity.
Adding similar low-molecular-weight polyesters with a specific structure, such as CBT, CBT is a functional resin with a macrocyclic oligopolyester structure, which has good compatibility with PBT, a very small amount of addition can be greatly improve fluidity of resin without affecting mechanical properties.
Adding nano-materials, ideally dispersed nano-materials play a role similar to internal lubrication in PBT, which can improve fluidity of PBT, but dispersion of nano-fillers is a major difficulty in process of blending modification.

3. Glass fiber reinforced PBT material is easy to warp

Warpage is result of uneven shrinkage of material. Orientation and crystallization of components in material, improper process conditions used in injection molding, incorrect gate shape and position during mold design, uneven wall thickness during product design, etc. will cause product warpage.
Warpage of PBT/GF composites is mainly because orientation of glass fiber in flow direction limits shrinkage of resin. Induced crystallization of PBT around glass fiber strengthens this effect, making longitudinal (flow direction) shrinkage of product smaller than transverse direction (direction perpendicular to flow direction). This uneven shrinkage causes warpage of PBT/GF composite material.
One is to add minerals to reduce anisotropy caused by glass fiber orientation by using shape symmetry of mineral filler;
Second is to add amorphous materials to reduce crystallinity of PBT and reduce 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;
Third is to adjust injection molding process, such as appropriately increasing mold temperature, and appropriately increasing injection cycle.

4. Glass fiber reinforced PBT surface floating fiber problem

Causes of floating fiber are more complicated. To put it simply, there are mainly following aspects:
Compatibility of PBT and glass fiber is very poor, so that the two cannot be effectively bonded together;
Viscosity of PBT and glass fiber is very different, which leads to a tendency of separation between the two in flow process. When separation effect is greater than adhesive force, it will detach, glass fiber will float to outer layer and leak out;
Presence of shear force will not only cause differences in local viscosity, but also damage interface layer on the surface of glass fiber. The lower melt viscosity, interface layer will be damaged, bonding force on glass fiber will be smaller. When viscosity is small to a certain extent, glass fiber will get rid of constraints of PBT resin matrix, gradually accumulate and expose to surface.
Influence of mold temperature. Due to low mold surface temperature, light-weight and fast-condensing glass fiber is instantly frozen. If it is not fully surrounded by melt in time, it will be exposed to form "floating fiber".
Add compatibilizer, dispersant and lubricant to improve problem of floating fiber. For example, using glass fiber with special surface treatment, or adding a compatibilizer (such as SOG, a kind of good-flow PBT modified compatibilizer), through "bridge" function, increase bonding force of PBT and glass fiber.
Optimize molding process to improve floating fiber problem. Higher injection temperature and mold temperature, higher injection pressure and back pressure, faster injection speed, and lower screw speed can all improve floating fiber problem to a certain extent.

5. Glass fiber reinforced PBT injection molding process produces more mold scale

Polymerization modification 
PBT+GF surface floating fiber, mold fouling phenomenon
Mold fouling is caused by high content of small molecules in material or poor thermal stability of material. Because of its residual rate of oligomers and small molecules, PBT is usually 1% to 3%, it is prone to mold scale compared with other materials. After introduction of glass fiber, it is more obvious. This will result in the need to clean mold regularly during continuous processing, resulting in low production efficiency.
Reduce addition of small molecule additives (such as lubricants, coupling agents, etc.), and try to choose polymer additives;
Improve thermal stability of PBT and reduce small molecule products produced by thermal degradation during processing;

6. PBT has poor thermal hydrolysis resistance

Polymerization modification 
Main factor affecting hydrolysis of PBT is concentration of terminal carboxyl groups. Since PBT contains ester bonds, it will break when placed in water at a temperature higher than its glass transition temperature. Acidic environment formed by hydrolysis accelerates hydrolysis reaction, and its performance drops sharply.
Adding a hydrolysis stabilizer, such as carbodiimide, will consume carboxyl group produced by hydrolysis, slow down acid hydrolysis rate of PBT, and improve hydrolysis resistance of PBT resin.
By blocking PBT terminal carboxyl group, concentration of terminal carboxyl group is reduced, and hydrolysis resistance of PBT is improved, such as adding an epoxy functional group auxiliary agent (such as SAG series, a random copolymer of styrene-acrylonitrile-GMA), functional group GMA reacts with PBT terminal carboxyl group to block end, thereby improving hydrolysis resistance of PBT.

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