Process for preparing a laminated composite of addition type...

Details Process for preparing a laminated composite of addition type... Process for preparing a laminated composite of addition type...

1999-11-29

2001-09-18

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

To produce composite, plural part or multilayered article

C264S331110, C264S337000, C264S338000

Reexamination Certificate

active

06290892

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to a technique of bonding or adhering polyurethane to an addition type silicone rubber, and in particular to a technique of surface modification of an addition type silicone rubber to facilitate the bonding between the addition type silicone rubber and polyurethane.
BACKGROUND OF THE INVENTION
The unusual heat, electrical weather, and low-temperature resistance properties of silicones, i.e., room temperature-vulcanizable (RTV) silicone rubber, have manifested themselves in a wide variety of applications. They are used as engineering materials for manufacturing electrical apparatus and machines. Strong adhesion of silicones to the target matrix is a requirement for long product life. The inherently hydrophobic nature of silicones, coupled with their ability to segregate to the surface, facilitates their use as a surface modifier for other materials. This property has been exploited in the preparation of silicone-modified polyesters; the synthesis of polydimethylsiloxane (PDMS) and polystyrene copolymers; polyimidesiloxane segmented copolymers triblock siloxane copolymers used as surface modifying additives; blood-compatible polyurethane (PU)-polysiloxane graft copolymers [Y. Ezuka, T. Ono, and K. Imai, J.
Colloid Interface Sci.,
136, 408 (1990)]: and the surface treatment of fumed silica for use as antifoammg compounds. Small amounts of the copolymers (ca. 1~4 by weight) could be added to different polymers to alter their surface properties. The air-polymer surface of the resulting polymer system is usually dominated by the low surface-energy siloxane, preventing adhesion to polar materials. This phenomenon of surface segregation of a component with a low surface energy in multi-component polymeric materials attracts attention to the analysis of polymer surfaces and the modifications of their properties [G. H. Frederickson,
Macromolecules,
20, 2535 (1987); T. P. Russell, G. Coulon, V. R. Deline, and D. C. Miller,
Macromolecules,
22, 4600 (1989); K. R. Shull,
Macromolecules,
25, 2122 (1992)].
Surface modification of polymers by chemical, photochemical, corona, plasma, and high-energy radiation treatments are currently used to increase the polarity of polymer surfaces, thus enhancing their adhesion to immiscible polymers [C. M. Chan,
Polymer Surface Modification and Characterization,
Hanser Publishers, Munich, 1993; W. H. Waddall, L. R. Evans, J. G. Gillick, and D. Shuttleworth, Rubber Chem.
Technol.,
65, 687 (1992)]. An alternative means of modifying polymer surface is by blending with diblock copolymers, where one block interacts favorably with the base matrix and the other block with the target matrix [T. P. Russell, V. R. Deline, V. S. Wakharkar, and G. Coulon,
MRS Bull.,
Oct., 33 (1989)]. The polymeric surface can form ordered, oriented layers at the surface during subsequent curing. The part of such copolymers with a low critical surface tension is readily miscible with the silicone base matrix, whereas the other part interacts favorably with the more polar matrix. Both blocks can anchor the copolymer into the respective substrate, which ensures permanency of the surface modification and increases adhesion between two immiscible polymers.
The low surface energy of the cured silicone rubber of addition type causes the polyurethane have insufficient wetability to adhere to the surface thereof.
A primary objective of the present invention is to provide a process for preparing a laminated composite of an addition type silicone rubber and polyurethane.
Another objective of the present invention is to provide a process for preparing an addition type silicone rubber having an increased surface energy by induce surface reconstruction.
Still another objective of the present invention is to provide a modifier for increasing a surface energy of an addition type silicone rubber.
SUMMARY OF THE INVENTION
In order to accomplish the objectives of the present invention, a process for preparing a laminated composite of an addition type silicone rubber and a polyurethane carried out in accordance with the present invention comprises the following steps:
(a) mixing a modifier, a silicone gel of addition type and a curing agent for said silicone gel to form a mixture, said modifier having the following formula (I):
wherein m=25~50; R
1
, R
2
, and R
3
, and R
4
, independently are —CH
3
or —CH
2
CH
3
; R′ is R or OR, wherein R is a polymer backbone having a molecular weight of 1000~20000;
(b) introducing said mixture from step (a) into a cavity of a mold, in which a surface of the cavity is formed by a material having a critical surface tension greater than that of a polymer having a repeating unit of said R;
(c) curing said mixture in said cavity to form a shaped article;
(d) removing said shaped article from said mold;
(e) placing a polyurethane precursor on a surface of said shaped article which contacts said surface of said cavity; and
(f) curing said polyurethane precursor to form an cured polyurethane bonding to said surface of said shaped article, so that a laminated composite is formed.
In step (e), a suitable polyurethane precursor is a liquid mixture comprising a polyol, and a polyisocyanate, and optional a catalyst, wherein OH group: NCO group=0.7:1 to 0.9:1 (molar), preferably 0.8:1.
Alternatively, the polyurethane precursor is a solid polyurethane prepolymer having a tensile strength of 20~40 kg/cm
2
and an elongation ratio at break of 200~400%.
The curing in step (f) of the process of the present invention is preferably carried out at a temperature of 60° C. to 80° C. for a period longer than 120 hours.
Preferably, the process of the present invention further comprises (d′) subjecting said shaped article to a heat treatment between step (d) and step (e). Said heat treatment is preferably carried out at a temperature of 30° C. to 200° C. for a period of 10 minutes to 24 hours.
The present invention also discloses a process for preparing an addition type silicone rubber having an enhanced critical surface tension, which comprises the steps (a) to (c) defined as above, and preferably further comprises step (d) and (d′) defined as above following step (c).
Said silicone gel of addition type used in step (a) can be any polysiloxane containing a vinyl group known in the prior art, and preferably said silicone gel of addition type is a silicone having the following formula
wherein k is a value so that the silicone (II) has a molecular weight of 1000~100000; and R
5
, R
6
, R
7
, and R
8
, independently are —CH
3
or —CH
2
CH
3
.
Preferably, R in the formula (I) is
or a combination of each other, wherein a is an integer of 30~100, preferably a is an integer of 40~60; b is an integer of 20~80, preferably b is an integer of 40~50; y is an integer of 30~60;
z/
(
x+y
)=0.01~0.25, preferably
z
/(
x+y
)=0.01~0.15; and p, q and r independently are an integer of 1~20; preferably p=1, q=3, and r=2.
A suitable curing agent for said silicone gel in step (a) includes (but not limited thereto) compounds comprising a repeat unit of the following formula:
wherein n is a positive integer.
A suitable amount of said modified used in step (a) ranges from 0.5 to 10.0 phr (per hundred parts by weight of said silicone gel of addition type), and preferably from 1.0 to 3.0 phr.
Preferably, said curing in step (c) is carried out at a temperature from 60° C. to 80° C. for a period of 6 to 10 hours.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
In this invention we describe a specific bond between silicone rubber of addition type and polyurethane (PU) using an induced surface reconstruction approach. This was realized by blending small amounts of the modifier (I) copolymer with the silicone gel of addition type, then casting them on appropriate molding plates during curing to induce the formation of desired surface property. The modifier (I) copolymers at the surface of the silicone rubber can serve a

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