Study on Shrinkage of UV-cure Adhesives (3)

3 Shrinkage Research Methods

The study of the shrinkage of a polymer is generally based on the measurement of the volume shrinkage or the linear shrinkage before and after the curing, and the investigation on the dynamics generally follows the measurement of the shrinkage during curing of the resin.

The method of measuring the volume shrinkage can be performed according to the relevant regulations in the international standard ISO3521. One method is to measure the density before and after the resin is cured, and then calculate the shrinkage according to the density. Shrinkage ratio = (p post-p pre) / post post x 100%, where p post and p pre are the densities after curing and before curing, respectively. The density before and after curing can be measured with a pycnometer method using water as a reference. The liquid before curing was also measured with a pycnometer, and the solid density after curing was measured by a buoyancy method. Another method is to calculate the volumetric shrinkage by measuring the volume of the mold cavity and the cured sample. In order to reduce system errors and the impact of different methods, the same volume or density measurement method should be used to determine the volumetric shrinkage ratio, so that the data can be comparable.

Line shrinkage refers to the ratio of the difference in length before and after curing between the two end faces corresponding to the centerline of the resin casting body and the length before curing. Its determination can refer to ISO2577 thermosetting molding compound shrinkage method and ASTMD2566 standard according to different requirements. In China, there are experimental methods for the shrinkage of national standard GB1404 phenolic plastic powders, but these methods are all based on standards established for thermosetting resins. UV resin curing is an uneven curing process from the outside to the inside, so these methods are easy to produce irregular deformation, such as the shrinkage of the resin line in ASTM D2566 is a semicircular groove, with a Teflon film in the groove On the surface, the change in the length of the end face was measured, and it was difficult to ensure irregular deformation during the curing process. In recent years, cylindrical molds have been designed based on the characteristics of the resin light curing process. When curing, the mold is rotated upright and the upper section is blocked first. After the main body is completely cured, the upper section is cured. This eliminates irregular deformation of the resin and maintains good measurement accuracy.

The shrinkage tracking test of the resin cure process can be performed with a dilatometer. The dilatometer method has long been a method of studying the kinetics of polymerization and can be used to monitor the over-curing process in real time. Early dilatometers were not suitable for studying the entire process of polymerization or systems with early gelation reactions. To overcome these shortcomings, many improvements have been made to the structure of the dilatometer. L.C. Rubens reported on a dilatometer suitable for studying photopolymerization. The main improvement was to put monomers in polyvinyl fluoride film bags. This can overcome bubbles or cracks inside the system due to large viscosity and the polymerization rate is very high. It is difficult to maintain constant temperature and other disadvantages when it is large. On this basis, an automated recording dilatometer was also designed for the detection of photo-curing reactions. This dilatometer studied the photopolymerization process of MMA with an error of ±2% compared with the mass method.

4 Relationship between shrinkage and shrinkage stress

During the curing of UV adhesives, there is a gelation process. Before gelation, although the system shrinks, it still has fluidity. After gelation, molecular motion is hindered. Further curing reaction is the main cause of shrinkage stress. Therefore, the shrinkage stress is not proportional to the volumetric shrinkage of the entire curing process. It depends on the volume shrinkage after gelation. The higher the functionality of the reactants, the less reactive the functional groups are when gelling occurs. Therefore, a highly functional reaction system will produce higher shrinkage stress after curing.

The contraction rate has the same tendency as the contraction stress, but it is not proportional. Pang Zhengzhi et al. used diffused polarimetry to measure the shrinkage stress of the photocured coating and the substrate. Combined with the determination of the photocurable volume, the functional and structural properties of the acrylate monomer were investigated. The law of the effect of shrinkage. The results showed that (Table 4) with the increase of the degree of functionality, the volume shrinkage of the photocured and the relative internal stress both showed an increasing trend, but the two did not show a positive relationship.

5 Ways to reduce shrinkage

5.1 Low Shrinkage Monomers and Oligomers

According to the principle of shrinkage generation, the use of low shrinkage monomers and oligomers with branched or cyclic structures in the formulation of adhesives can effectively reduce the curing shrinkage, so difunctional and trifunctional single Although the body is commonly used in paints, in UV adhesives, the diluting monomers are generally dominated by monofunctional monomers. On the other hand, excessive use of low-functionality monomers often decreases the cross-linking density, which results in a decrease in the solvent resistance of the adhesive. Therefore, cross-linking density should be taken into account in practical applications. Among the difunctional monomers, NPGDA or alkoxylated NPGDA has a relatively low shrinkage rate and provides an appropriate crosslink density, which is superior to other difunctional monomers in reducing shrinkage.

The use of low-viscosity acrylate oligomers and the use of less diluted monomers are also effective ways to reduce shrinkage.

5.2 Adding Polymer or Inorganic Fillers

Both of these methods can reduce the content of functional groups in the system, thereby reducing shrinkage. In the process of adding the polymer, the newly formed system and the pre-added polymer sometimes have a phase separation during the curing process and can also cancel out part of the volume shrinkage. However, the disadvantage of this phase separation is that it will reduce the transparency of the cured product. It has been reported that the addition of thermoplastic polymers such as polyvinyl acetate, polyvinyl acetal, and polyester to the radical polymerization system can significantly reduce the volume shrinkage.

The addition of inert fillers such as quartz glass microspheres or glass fibers can also reduce cure shrinkage. Karrer et al. used porous polystyrene and quartz powder to modify the light-curable resin system, and the volume shrinkage of the resin reached 2%. These two methods usually cause the viscosity of the adhesive to increase, which reduces the curing speed and affects the processability.

In order to overcome the above shortcomings, the structural design of the polymer should have a ring structure and the relative molecular mass should not be too large, and it is not suitable to use the conventional thermoplastic resin type. According to reports, the United States Dymax company uses functionalized cellulose added to the UV adhesive, while adding filler, effectively reducing the shrinkage rate, the shrinkage rate of 0.05% to 0.1%, not other performance. This adhesive can be used for bonding high-precision glass instruments.

5.3 Using cationic or hybrid curing methods

Cationic UV adhesives, the monomers used are epoxy resins and vinyl ethers, with oxime salt as the photoinitiator. Since the shrinkage of the cationic curing reaction of the epoxy resin is much lower than that of the radical reaction, the cure shrinkage can be reduced by using a cationic formulation. For example, Norio et al. used alicyclic epoxy as the main resin and diluent, and micrometer-sized spherical quartz as the filler. The prepared adhesive had good adhesive force and its volume shrinkage was only 1.2% at room temperature. Curing for 1 to 5 minutes, suitable for bonding of precision optical structures. However, the curing speed of cationic adhesives is generally slow, easily affected by alkali and moisture, and the types of oligomers and diluents are small, and the cost of raw materials is relatively high. Therefore, the current UV adhesives are still mainly free radicals, accounting for more than 95%.

Hybrid UV adhesives are a mixture of free radicals and cationics, usually in the form of a mixture of acrylates and epoxy compounds. This hybrid UV curing system can increase the initiation efficiency when light is triggered, fully exert the advantages of both, reduce the volume shrinkage, and have a good synergy effect in other aspects.

In addition, depending on the process requirements, it is also possible to use a combination of UV and other curing methods, such as UV and thermal curing or an anaerobic curing method, which can also reduce the curing shrinkage.

5.4 Add Expansion Monomer

It has been known from the cationic polymerization of epoxy resins that the volume shrinkage of the ring-opening polymerization reaction is smaller than that of the addition polymerization reaction. For monocyclic monomers, there is a covalent distance converted to the van der Waals distance for each van der Waals distance converted to a covalent bond distance. The resulting shrinkage and expansion can be partially offset. Further studies have found that the volume shrinkage of the ring-opening polymerization reaction is related to the size of the ring. As the ring increases, after the covalent bond is opened between the carbon and oxygen atoms, the distance becomes closer to the van's distance, and the volume shrinkage thus decreases. For the bicyclic monomers, due to the double ring-opening polymerization reaction, there are two single-bond bonds that break into near van der Waals distances, and it is possible to make the volume shrink smaller after polymerization, without shrinkage or even expansion. In addition, at the same time as the open loop, the entropy of the system also becomes larger, that is, the free volume becomes larger. So far, the expanded monomers that have been found follow the mechanism of this ring-opening polymerization. The most studied expanding monomers are basically spiro compounds, such as spiro orthoesters (SOE), bicyclic orthoesters (BOE), spirocyclic orthoesters (SOC), and cyclic lactones, which can be classified as cations and Free radicals and other reaction mechanisms are aggregated. For example, the volume expansion coefficient of the spirocyclic monomer cation synthesized by Pan et al. was 10.4% after curing, and the effect of lowering the curing shrinkage was obtained in the epoxy resin. At present, the cost of the spirocyclic monomer is very high, so its practical application is still very small, but it is a very promising fundamental method to eliminate shrinkage.

In addition, it is worth mentioning that the method of appropriately lowering the Tg of an adhesive cured product can reduce the residual stress and increase the adhesive strength without reducing the shrinkage rate. This method actually relies on the toughness of the cured resin itself to absorb the volumetric shrinkage stress and is suitable for non-structural adhesives.

Source: 21st Century Fine Chemicals Network