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Development and evaluation of non-fogging silicone sealant for automotive lamp assembly


发布时间:

2018-07-04

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1. Preface

The function and quality of automotive lamps and lanterns are related to the performance and operational safety of the whole vehicle, and the stable working and function of the lamps and lanterns not only depend on the design of the lamps and lanterns, but also on the processing and assembly technology and supporting materials of the lamps and lanterns. Along with the development of automobile industry, the technology of automobile lamps and lanterns is also constantly updated. The current design of automobile lamps and lanterns has set more components in the lamp chamber, increased reflection chamber, higher heat emission, improved lamp design and function, and higher technical requirements for lamp materials.

There are four kinds of general lamp sealant: butyl hot melt adhesive, polyurethane hot melt adhesive (PUR), one-component room temperature vulcanized silicone rubber (RTV-1), two-component room temperature vulcanized silicone rubber (RTV-2). Among them, butyl hot melt adhesive because of poor heat resistance and weak adhesion, has been basically eliminated from the game; polyurethane hot melt adhesive bonding strength, once affirmed and used by the industry, but in the current conditions that attach importance to labor protection and environmental protection, because of its material toxicity, increasingly questioned; silicone rubber sealant because of its excellent weather resistance, high and low temperature resistance, water repellent and moisture-proof and excellent electrical insulation properties, making Silicone rubber sealant is gradually replacing other rubber types as the material of choice for headlight sealant.

Since 2013, Zhejiang Kosta New Materials Co., Ltd. has been working closely with Zhejiang Tian Chong Automotive Lighting Group Co., Ltd. to research and develop high-performance silicone sealants according to the assembly process of automotive lamps and lanterns and the requirements for the use of headlights. Through the testing and evaluation of commercially available sealants, the causes of commercially available sealant silicone rubber defects are analyzed item by item, especially the mechanism of fogging of certain commercially available brands of headlamp rubber is clearly analyzed, and on this basis, the best process and formula are determined to produce headlamp sealant, and through close monitoring, the stable and reliable performance of the developed and produced sealant silicone rubber products is ensured to provide a solid technical guarantee for improving the quality of automotive lamps and lanterns.


2 High performance sealing silicone rubber for headlight assembly development and evaluation

2.1 Requirements and corresponding technical measures for the use of automotive lamp sealing silicone rubber

In order to meet the assembly process and use requirements of automotive lamps and lanterns, the silicone rubber used for lamp sealing requires proper and rapid curing, and the cured sealant maintains stable adhesive strength to polypropylene plastic (PP) and polycarbonate (PC), long-term resistance to environmental temperature change and high temperature of lamp work, and aging resistance, and for the lamps with anti-fog coating film, the sealant is especially required to be compatible with the anti-fog coating film without fogging.

For the headlights and other larger lamp assembly, the production line occupies a larger space, in order to improve production efficiency, should be used to cure relatively fast two-component condensation type room temperature vulcanization silicone rubber for sealant, the system with the preferred filling materials, fast cross-linking vulcanizing agent, the introduction of silicone efficient viscosity enhancement additives, by kneading and mixing to fully meet the above requirements for the use of lamp sealing silicone rubber.

For the smaller shape of the lamp assembly, it is appropriate to use the larger packaging specifications of the single-component room temperature vulcanization silicone rubber for sealant. The system in addition to the application of single-component room temperature vulcanization of silicone rubber conventional mature technology, but also requires a faster curing speed and high initial bonding strength.

2.2 The mechanism of lamp sealant fogging and preventive measures

Automotive lamps and lanterns anti-fog is an important technical requirement, in order to improve the anti-fog performance of the lights, the interior of the lights with anti-fog coating is one of the practical measures. If the lamp adhesive sealing material and anti-fog coating can not be good compatibility, that may appear fogging and other phenomena. There are indeed some brands of silicone rubber on the market with serious defects, and these silicone rubber is used to seal the lights with anti-fog coating, and some lights appear fogged. The reason for the fogging is that the brand's own products have technical defects.

The direct cause of transparent material surface fogging, and the substrate surface material condensation agglomeration state, the material in the substrate surface adsorption cohesion, such as the formation of droplets discontinuous uneven spread, due to the different material light transmission or refractive index differences, resulting in fogging or fog shadow. Substances in the substrate surface adsorption and spreading state and the polarity of the two materials or surface tension related. Liquid adsorption and spreading state on the solid surface can be characterized by its contact angle size, liquid contact angle is small, indicating that the liquid and solid substrate polarity similar, liquid in the solid substrate surface can be uniformly spread into a uniform film; conversely, the liquid contact angle is large, liquid and solid substrate polarity differences, liquid in the solid substrate surface into droplet distribution, when the droplets are finely dispersed into a non-uniform film distribution, that is, showing the fogging state If the material gathered on the anti-fog coating reacts with the anti-fog coating to form a new solid film, it will form an irreversible fog film.

(1) Contact angle test

In order to analyze the various factors associated with the fogging phenomenon between the substrate material of the lamp component and the sealant component, the contact angle of the possible volatile condensation components of the current general lamp sealant on the lamp component material was tested, and some of the measured contact angle images are listed in the following figure set, and the relevant contact angle test data are listed in Table 1.

The possible volatile components in the headlight sealant are indicated by code A and B for the luminaire material polycarbonate (PC) and the addition of anti-fog coating polycarbonate (C-PC), respectively.

The contact angle measurement images of micro-fine droplets of water and volatile condensation components on uncoated anti-fog coating film (PC) and plus coated anti-fog coating film (C-PC) are as follows.

 

  

H2O/PC  θ=76.25°     H2O/C-PC  θ=13.34      B/PC  θ=16.58°      B/C-PC  θ=38.47°
 
 
 

Above (left), the contact angle of water on the surface of PC substrate was originally 76.25°, and the contact angle of water on the surface of C-PC substrate coated with anti-fog coating was reduced to 13.34°, which shows the enhanced hydrophilic effect of anti-fog coating film, that is, water (fog) can be adsorbed by the anti-fog coating film, based on which the lights coated with anti-fog coating film are not easy to appear water fog.

The above figure (right) is the contact angle image of the volatile substance B of sealant. If the amount of aggregation is so much that it can converge into a liquid film, then B can be evenly dispersed on the surface of ordinary PC without significantly affecting the transparency of PC, but B is difficult to be evenly dispersed on the surface of C-PC coated with anti-fog coating, and the result will show slight fogging.

Table 1 Contact angle of organic substances related to silicone rubber on polycarbonate substrates

 
 
Base material

PC

C-PC

 

Contact angle

Water

 76.25゚

 13.34゚

Volatiles A

 8.86゚

 37.87゚

Volatiles B

 16.58゚

 38.47゚

 

 

(2) Certain volatiles condense on the surface of the lampshade when it is cold

In addition to the fogging phenomenon due to the large contact angle of volatiles, there is also fogging due to the reaction coagulation property of the volatiles themselves, which will also occur when they encounter hydrolysis precipitation on the surface of the lampshade after volatilization. We conducted a cold condensation fogging test on the possible volatile substances in the lamp seal silicone rubber, and the experimental result pictures are as follows.

 

 

The active use of high concentrations of volatiles experimentally verified that can cause fogging phenomenon, it can be seen that some volatiles in a large enough concentration will form an opaque film on the surface of the lamp cover, causing another type of fogging phenomenon.

(3) Analysis of the internal fogging mechanism of the lamp

In general, the formation of fogging requires two factors at the same time: the existence of condensable "fog source" material and the environmental conditions to promote the condensation of condensable material. The internal fogging of the lights, in fact, there are two types of condensable substances that may fog: one is the moisture that is prevalent in the natural environment, mainly from the moisture in the air, and the other is the condensable volatiles that escape from materials such as lamp sealant, the residual volatiles in the sealant and the volatile by-products released from the sealant curing process.

Usually, there is always more or less moisture in the natural ambient air. The headlight vent is connected to the atmosphere, the headlight work makes the internal temperature of the headlight rise, the headlight cools down after the headlight is turned off, the air cooling contraction in the headlight cavity will lead to the external air carrying moisture into the interior of the headlight, if it reaches the dew point, the headlight will appear fogging. The fogging caused by moisture entering the interior of the headlight may occur repeatedly due to the humid environment; after the headlight works for a period of time, the water fog will be eliminated due to the rising temperature. Improve lamp design, lamp into the exhaust hole to facilitate the exclusion of moisture in the lamp cavity, lamp cavity without airflow exchange dead ends, add hydrophilic anti-fog coating and other measures, can slow down or avoid the fogging caused by moisture.

The fogging caused by the volatiles released from the sealant is caused by the volatiles coalescing on the lamp components into tiny dispersed droplets that are not wetted with the substrate, such as the application of sealant containing more volatiles, may appear soon after the lamp assembly that slight fogging, the lamp first work after heating up and then turn off the power, to be cooled down will aggravate the fogging phenomenon; for the fogging caused by volatiles, the lamp components are coated with For the fogging caused by volatiles, the fogging situation after the hydrophilic anti-fog coating is often more serious than the fogging of lights without the anti-fog coating, which is because the volatiles of the sealant are not infiltrated on the hydrophilic anti-fog coating, thus making it easier to coalesce to show fogging. In addition, because the sealant volatiles in the hydrophilic anti-fog coating film agglomeration may also be followed by hydrolysis precipitation reaction, and then the formation of solid fog film, in the subsequent work of the lamp process, the fog film can not be eliminated.

(4) Assessment of different manufacturers of silicone headlight adhesive

A variety of brands of headlight sealants are currently on the market, with varying quality. In order to preferably select the applicable headlight sealant, Tian Chong Automotive Lighting has conducted an assessment and analysis of the sealants sold by different manufacturers since 2016. The main test sealant volatiles content, and combined with the corresponding sealant package lamp fogging conditions and work stability analysis and evaluation.

Sealant performance assessment includes three: ① according to the supply of rubber manufacturers to provide mixing and curing conditions, the measurement of mixed rubber curing reaction process at room temperature (24 hours) weight reduction; ② cured sealant by 120 ℃ heating 2 hours of weight reduction; ③ 200 ℃ heating 2 hours of weight reduction and appearance changes. The test results of the sealant samples sold by different manufacturers are listed in Table 2.

Table 2 Comparison of silicone rubber volatile matter content determination, heat aging and fogging

 
 
Sealant sample

RTV-2

RTV-1

GCompany

ZCompany

9521

9622

TCompany

XCompany

DCompany

9611

Room temperature 24h curing weight loss %

1.98

0.96

1.06

0.85

4.99

5.19

1.06

2.26

120℃×2h weight loss

2.69

1.32

1.53

1.29

5.94

5.60

1.63

2.73

200℃×2h weight loss

3.39

1.80

1.95

1.80

6.60

5.76

2.01

1.99

200℃×2h change Brittle cracking

 

No change No change Become brittle Soften Soften Slightly softened
Headlight fogging situation Severe fogging

 

Normal No fog Normal No fog Fogging Fogging

 

Normal No fog

 

* Among them, G, Z and T are products of domestic companies; X and D are products of foreign companies; 9521, 9622 and 9611 are sealants of Kosta.

The test results of sealant volatile matter content further verify the analysis judgment of positive correlation between lamp fogging and sealant volatile matter. It is confirmed that the application of high volatile content of sealant assembly lights, lights will appear fogging phenomenon, especially the addition of anti-fog coating lights, the application of high volatile content of sealant assembly lights fogging more obvious.

Inherent quality defects in silicone rubber, subjected to 200 ℃ × 2h accelerated thermal aging test brittle cracking damage, so the loss of adhesive sealing performance, which will lead to serious damage such as headlight leakage or moisture fogging.

The fogging of lights sealed with silicone rubber is not a common problem of silicone rubber materials, it is only an individual mistake of some brands of silicone rubber. The application of scientific and rigorous technology production of silicone rubber assembly lights, not only will not fog, and silicone rubber sealed lights run more stable and reliable.

(5) Polyurethane sealant assessment and evaluation

Individual auto factories are concerned that silicone rubber assembled lights will fog up, and have asked Zhejiang Tian Chong to switch to reactive polyurethane (PUR) for lamp sealing. Long-term application of silicone rubber assembly lamp operators in the switch to polyurethane rubber, nausea and headache and other discomfort. Safety information provided by the polyurethane manufacturer also confirmed that the polyurethane sealant product contained substances harmful to health, such as diphenylmethane diisocyanate. In order to confirm whether polyurethane adhesives are indeed more stable than silicone rubber and do not fog. We conducted parallel comparative evaluation tests of polyurethane and silicone rubber using a special device for intensive testing of sealant fogging.

The same test device was used to evaluate the sealant silicone rubber to test the fogging of a sample of polyurethane hot melt adhesive from company H and a sample of KSD9622 headlight adhesive from Kosta together under enhanced conditions. The results of the polyurethane hot melt adhesive tests are shown in Table 3 below.

Table 3 Polyurethane hot melt adhesive heating weight loss and fogging test

 
 
Test conditions Thermal Weight Loss % Fogging condition

90℃×2h

0.59

--

120℃×2h

0.87

Fogging

140℃×2h

1.44

 

150℃×2h

 

Fogging

 

Under the same conditions (120℃×2h, 150℃×2h), the same headlight PC translucent cover was used to compare and test the fogging condition of polyurethane rubber of H company and KSD-9622 silicone rubber of Kosta company, and the test results are shown as follows.

 

 

Test results: PC corresponding to the sample part of polyurethane hot melt adhesive showed obvious translucent white fog film; PC corresponding to the sample part of silicone sealant of Kosta did not show obvious changes.

4. High-performance lamp sealing silicone rubber application

Zhejiang Kosta company in depth according to the working conditions and use of automotive lamps and lanterns sealant requirements, based on scientific theory, to develop a rigorous development program, the use of a number of effective new technologies, the production of headlights in line with the assembly of silicone sealant. The silicone sealant material of KOSDA has low volatile matter content, fast curing speed after mixing with two-component silicone rubber material, high adhesive strength of cured silicone rubber, good resistance to high and low temperature impact and weather aging performance; single-component sealant silicone rubber curing speed, solid and reliable bonding. The products are used in the bonding and sealing of many types of headlights, and have experienced more than five years of long-term operation assessment, proving that the quality of the silicone rubber products of KOSDA is stable, especially suitable for the application of the assembly of headlights coated with anti-fog coating, and indeed achieve the excellent and reliable effect of assembling headlights without fog.

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