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Development of polyurethane adhesive for reflective body

November 07, 2019

With the increasing emphasis on road traffic safety, the use of reflective stickers has increased year by year, and the quality requirements for reflective stickers have gradually increased. Generally, the reflective sticker should have the characteristics of good reflection effect and excellent aging resistance. Reflective stickers generally consist of a substrate, oriented glass beads, adhesives and titanium dioxide, and the quality of the adhesive directly determines the quality of the reflective stickers. The commonly used adhesives for reflective stickers are mainly acrylate and PU (polyurethane) adhesives. PU adhesive has the advantages of good bonding strength, low temperature resistance, wear resistance, water resistance, oil resistance, solvent resistance, chemical resistance, ozone resistance and bacteria resistance [1], which can be used as the preferred reflective Apply adhesive.

At present, domestic reflective stickers generally have shortcomings such as poor aging resistance, which seriously affects their service life. This is mainly due to the low bonding strength of the adhesive, poor light resistance and thermal properties. Therefore, the development of PU adhesives with high bond strength and excellent aging resistance has a good market application prospect.

Shandong Huacheng Gaoke Adhesive Co., Ltd. specializes in the research and development and manufacture of adhesives, coatings and raw materials. It has many high-quality products based on water-based acrylate emulsion polymerization, Polyester polyol synthesis and polyurethane synthesis.

1 test part

1.1 test materials

Polyadipate-1,4-butanediol ester [relative molecular mass (Mr) 2000, 3000 and 4000], industrial grade, Qingdao Xinyutian Chemical Co., Ltd.; 1,4 butanediol (BDO), 1 ,6-hexanediol (HDO), industrial grade, BASF, Germany; IPDI (isophorone diisocyanate), HDI (1,6-hexamethylene diisocyanate), MDI (diphenylmethane diisocyanate), industrial grade , Bayer, Germany; gas phase white carbon black, industrial grade, Degussa company; stannous octoate, dibutyltin dilaurate, BiCAT8108 (organic bismuth catalyst), industrial grade, Shanghai Shanji Chemical Co., Ltd.; butanone, industrial grade , marketed. PET (polyester) film (3 cm wide), commercially available.

1.2 Test equipment

Viscosity meter, Beijing xxxx Technology Co., Ltd.; HS-3000A type tensile machine, Shanghai Heyi Instrument Technology Co., Ltd.; ultraviolet (UV) irradiation device, homemade.

1.3 Preparation of PU adhesive [2]

In a three-necked flask equipped with a stirrer, polyester polyol, linear diol chain extender and nano-additive are added separately, and vacuum-dehydrated at 110 ° C for several times while stirring; when the moisture in the system is less than 0.05% , the vacuum device is removed, the catalyst is added, and the mixture is uniformly stirred; then the isocyanate is added, and the mixture is rapidly stirred and vacuum degassed for 5 min; then the above materials are poured into a polytetrafluoroethylene tray, aged at 130 ° C for 5 h; cooled, and a small amount of solid is used for methyl ethyl ketone. Dissolved into a solid content of 20%, that is, a PU adhesive.

1.4 Testing or characterization

(1) Viscosity: According to the GB/T2794-1995 standard, the viscosity is measured (the solid content is 20%).

(2) Peel strength: According to the GB/T2791-1995 standard, the test is carried out by a tensile machine (the substrate is 3 cm wide PET, measured after 12 h; the test temperature is 25 ° C, the test humidity is 55%, and the tensile rate is 100 mm. /min).

(3) Tensile strength: According to the GB/T6329-1996 standard, the measurement was carried out by a tensile machine (test temperature was 25 ° C, test humidity was 55%, and tensile rate was 100 mm/min).

(4) Anti-aging performance: According to GB/T528-2009 standard for aging test (UV irradiation for 1000h, adjust the distance between the UV lamp and the sample to be tested, so that the temperature of the sample to be tested is 70 ° C), and then determine the pull Stretch strength.

(5) Gel time: The tray with the glue solution is placed in a constant temperature oven for a certain period of time to start the heat preservation until the time period when the glue liquid appears obvious drawing phenomenon is used as a measurement index.

2 Results and discussion

2.1 Polyol polyol types and their choice of Mr

Since the reflective adhesive is required to have a high bonding strength, a polyester polyol having a relatively high crystallinity is usually used as one of the main raw materials. Commonly used crystalline polyester polyols include polyadipate-1,4-butanediol ester and polyadipate-1,6-hexanediol, which are rarely used because of their high price. Therefore, this study selected the former as the main raw material [3]. Table 1 lists the effect of different Mr. polyadipate-1,4-butylene glycol ester on the peel strength of PU adhesives. It can be seen from Table 1 that as the polyester polyol Mr increases, the peel strength of the PU adhesive increases first and then decreases; when the Mr of the polyester polyol is 3000, the peel strength of the PU adhesive is relatively largest.


Development of polyurethane adhesive for reflective body


This is because the larger the Mr of the polyester polyol, the stronger the crystallinity of the corresponding PU matrix (the higher the cohesive strength), so the peel strength of the PU adhesive increases, but when the Mr of the polyester polyol is too large, the corresponding The crystallization rate of the PU matrix is too fast. During the curing process, the PU adhesive does not have time to penetrate into the substrate, that is, it has crystallized (forms a glue layer), so the effective adhesion to the substrate is not good, and the peel strength does not rise and fall. In general, it is preferable to select a polyester polyol having a Mer of 3000.

2.2 Selection of isocyanate species

The effect of the type of isocyanate on the properties of the PU adhesive was examined under the premise that other conditions remained unchanged. The results are shown in Table 2.


Development of polyurethane adhesive for reflective body


It can be seen from Table 2 that the aliphatic isocyanate does not contain a rigid structure, and the strength and thermal aging properties of the corresponding PU adhesive are not as good as those of the aromatic isocyanate [4], but it does not yellow after being exposed to light, so it can effectively improve the light aging resistance of the PU adhesive. Performance; IPDI is higher than HDI, the latter is more expensive, and the latter imparts good bond strength to PU adhesives due to its high methylene structure (excluding the alicyclic ring that destroys the crystallinity of PU), and has excellent aging resistance. . Therefore, this study chose HDI as the main raw material for PU adhesives.

2.3 Selection of chain extender types and their contents

In order to improve the bonding strength and heat resistance of PU adhesives, a linear chain extender diol chain extender is generally used (chain extenders containing side chains reduce the crystallinity of PU and affect its bond strength). The effect of different diol chain extenders on the performance of PU adhesives is shown in Table 3 under the premise that other conditions remain unchanged.


Development of polyurethane adhesive for reflective body


It can be seen from Table 3 that the peeling strength and the tensile strength of the PU adhesive made of the two chain extenders are not much different; when the chain extender is HDO, the aging resistance of the corresponding PU adhesive is relatively good. This is because the amount of methylene contained in HDO is relatively large, which is beneficial to improve the thermal stability of PU adhesives. Therefore, this study selected HDO as a chain extender and examined the effect of its content on the performance of PU adhesives. The results are shown in Table 4.


Development of polyurethane adhesive for reflective body


It can be seen from Table 4 that as the content of HDO increases, the viscosity of PU adhesive increases, the aging resistance increases, and the peel strength increases first and then decreases. This is because the higher the content of HDO, the higher the content of hard segment in PU and the larger the relative molecular mass of Mr. Therefore, the viscosity and aging resistance of PU adhesives are on the rise; however, when the content of HDO is too high, PU The solubility is deteriorated, so the peel strength of the PU adhesive is declining. In general, it is preferred to select n (polyester polyol): n (HDO) = 1:0.3.

2.4 Selection of catalyst types and their contents

The effect of catalyst type and its content on the performance of PU adhesives is shown in Table 5 under the condition that other conditions remain unchanged.


Development of polyurethane adhesive for reflective body


Since the reactivity of HDI is low, the catalyst added during the reaction can both increase the reaction rate and limit the occurrence of side reactions. The commonly used catalysts are mainly organotin and tertiary amine catalysts. The latter has a greater influence on the catalytic activity of the H2O/-NCO reaction system than the -OH/-NCO reaction system [5], and it is easy to generate a large amount of CO2 gas during the reaction. Therefore, organotin catalysts are generally used. In recent years, many PU products have restrictions on the content of organotin. Considering that the suitable gel time in the PU reaction process is 3.0~5.0min (the reaction is relatively stable and the side reactions are less), BiCAT8108 is selected as the catalyst and controlled in this study. When the content is 0.03%, it is suitable.

Effect of 2.5 nanometer fumed silica on properties of PU adhesive

In order to effectively improve the bonding strength of the PU adhesive, a nano-additive is usually added as a reinforcing agent. The commonly used nano-additive is gas phase white carbon black, because the gas phase white carbon black molecule is a three-dimensional structure, the surface of the siloxane group can adsorb moisture, and the PU molecules act through hydrogen bonding and intermolecular forces. Thereby, the peel strength of the PU can be effectively enhanced. The effect of fumed silica content on the properties of PU adhesives is shown in Table 6 under the premise that other conditions remain unchanged.


Development of polyurethane adhesive for reflective body


It can be seen from Table 6 that as the content of the fumed silica increases, the viscosity, peel strength and tensile strength of the PU adhesive increase first and then decrease, and the gel time gradually increases. This is because the gas phase white carbon black is acidic, and when the content is too large, the acid value of the reactant is increased, thereby suppressing the progress of the reaction (restricting the degree of reaction), so the content thereof is not excessive. Considering comprehensively, it is more suitable to select w (gas phase white carbon black) = 0.2% to 0.3%.

3 Conclusion

(1) A PU adhesive for body reflective paste having high adhesion strength and aging resistance is prepared by using a crystalline polyester polyol, a linear diol chain extender, an isocyanate, and a nano-additive as main raw materials.

(2) When the crystalline polyester polyol is polyadipate-1,4-butanediol ester of Mr=3000, the diol chain extender is HDO, the isocyanate is HDI, the catalyst is BiCAT8108, w(catalyst)= When 0.03% and n (polyester polyol): n (HDO) = 1:0.3, the overall performance of the PU adhesive is relatively best.

(3) When an organic ruthenium catalyst (BiCAT8108) is used and w (catalyst) = 0.03%, the reaction rate is moderate, and the obtained product meets environmental protection requirements.

(4) The addition of fumed silica can effectively increase the strength of the PU adhesive, but when the amount of addition is too large, the reaction rate is affected, so the suitable addition amount is 0.2% to 0.3%.

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